Acid-Base Cements: Their Biomedical and Industrial Applications (Chemistry of Solid State Materials)

Although acid-base cements have been known since the mid 19th century, and have a wide variety of applications, there has been a failure to recognize them as constituting a single, well-defined class of material. This book remedies the situation by unifying the subject and treating this range of materials as a single class. These cements are defined as materials that are formed by mixing a basic powder with an acidic liquid, and offer an alternative to polymerization as a method for forming solid substances. They are quick-setting materials, with unusual properties, which find diverse applications as biomaterials and in industry.

Chemistry of Solid State Materials Acid-base cements Their biomedical and industrial applications

Chemistry of Solid State Materials Series Editors A. R. West, Department of Chemistry, University of Aberdeen H. Baxter, formerly at the Laboratory of the Government Chemist, London 1 Segal: Chemical synthesis of advanced ceramic materials 2 Colomban: Proton conductors 3 Wilson & Nicholson: Acid-base cements

Acid-base cements Their biomedical and industrial applications Alan D. Wilson formerly Head, Materials Technology, Laboratory of the Government Chemist Senior Research Fellow, Eastman Dental Hospital

John W. Nicholson Head, Materials Research, Laboratory of the Government Chemist

m CAMBRIDGE

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UNIVERSITY PRESS

CAMBRIDGE UNIVERSITY PRESS Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, Sao Paulo Cambridge University Press The Edinburgh Building, Cambridge CB2 2RU, UK Published in the United States of America by Cambridge University Press, New York www.cambridge.org Information on this title: www.cambridge.org/9780521372220 © Cambridge University Press 1993 This book is in copyright. Subject to statutory exception and to the provisions of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press. First published 1993 This digitally printed first paperback version 2005 A catalogue recordfor this publication is available from the British Library Library of Congress Cataloguing in Publication data Wilson, Alan D. Acid—base cements: their biomedical and industrial applications /Alan D. Wilson, John W. Nicholson p. cm. - (Chemistry of solid state materials; 3) Includes bibliographical references and index. ISBN 0-521-37222-4 1. Adhesives. 2. Dental cements. I. Nicholson, John W. II. Title. III. Series. TP968.W54 1993 620.1'35-dc20 91-38946 CIP ISBN-13 978-0-521-37222-0 hardback ISBN-10 0-521-37222-4 hardback ISBN-13 978-0-521-67549-9 paperback ISBN-10 0-521-67549-9 paperback

Dedicated to the past and present members of the Materials Technology Group at the Laboratory of the Government Chemist

Contents

1

Preface Acknowledgements Introduction References

xvii xix 1 4

2

Theory of acid-base cements 2.1 General 2.2 The formation of cements 2.2.1 Classification 2.2.2 Requirements for cementitious bonding 2.2.3 Gelation 2.3 Acid-base concepts 2.3.1 General 2.3.2 History of acid-base concepts 2.3.3 Acid-base concepts in AB cement chemistry 2.3.4 Relevance of acid-base theories to AB cements 2.3.5 Acid-base strength 2.3.6 Acid-base classification 2.3.7 Hard and soft acids and bases (HSAB) References

5 5 7 7 8 10 12 12 12 14 19 20 22 24 26

3

Water 3.1 3.1.1 3.1.2 3.2 3.2.1 3.2.2 3.3 3.3.1 3.3.2 3.3.3 3.4

30 30 30 30 31 31 33 34 34 35 36 40

and acid-base cements Introduction Water as a solvent Water as a component Water Constitution Water compared with other hydrides The structure of water The concept of structure in the liquid state The structures of ice Liquid water Water as a solvent

IX

Contents 3.4.1 Hydrophobic interactions 3.4.2 Dissolution of salts 3.4.3 Ion-ion interactions in water 3.4.4 Dissolution of polymers 3.5 Hydration in the solid state 3.5.1 Coordination of water to ions 3.6 The role of water in acid-base cements 3.6.1 Water as a solvent in AB cements 3.6.2 Water as a component of AB cements 3.6.3 Water as plasticizer References

40 41 44 45 47 47 48 48 48 51 52

4

Polyelectrolytes, ion binding and gelation 4.1 Polyelectrolytes 4.1.1 General 4.1.2 Polyion conformation 4.2 Ion binding 4.2.1 Counterion binding 4.2.2 The distribution of counterions 4.2.3 Counterion condensation 4.2.4 Effect of valence and size on counterion binding 4.2.5 Site binding - general considerations 4.2.6 Effect of complex formation 4.2.7 Effect of the polymer characteristics on ion binding 4.2.8 Solvation (hydration) effects 4.2.9 Hydration of the polyion 4.2.10 Hydration and ion binding 4.2.11 Desolvation and precipitation 4.2.12 Conformational changes in polyions 4.2.13 Interactions between polyions 4.2.14 Polyion extensions, interactions and precipitation 4.3 Gelation References

56 56 56 58 59 59 59 63 65 67 69 70 72 73 76 77 79 82 82 83 85

5

Polyalkenoate cements 5.1 Introduction 5.2 Adhesion 5.2.1 New attitudes 5.2.2 The need for adhesive materials 5.2.3 Acid-etching 5.2.4 Obstacles to adhesion 5.2.5 The nature of the adhesion of polyalkenoates to tooth material 5.3 Preparation of poly(alkenoic acid)s 5.4 Setting reactions

90 90 92 92 92 93 93 94 97 98

Contents 5.5 Molecular structures 5.6 Metal oxide polyelectrolyte cements 5.7 Zinc polycarboxylate cement 5.7.1 Historical 5.7.2 Composition 5.7.3 Setting and structure 5.7.4 Properties 5.7.5 Modified materials 5.7.6 Conclusions 5.8 Mineral ionomer cements 5.9 Glass polyalkenoate (glass-ionomer) cement 5.9.1 Introduction 5.9.2 Glasses 5.9.3 Poly(alkenoic acid)s 5.9.4 Reaction-controlling additives 5.9.5 Setting 5.9.6 Structure 5.9.7 General characteristics 5.9.8 Physical properties 5.9.9 Adhesion 5.9.10 Erosion, ion release and water absorption 5.9.11 Biocompatibility 5.9.12 Modified and improved materials 5.9.13 Applications 5.10 Resin glass polyalkenoate cements 5.10.1 General 5.10.2 Class I hybrids 5.10.3 Class II hybrids 5.10.4 Properties References

99 101 103 103 103 104 106 112 113 113 116 116 117 131 133 134 143 146 147 152 156 159 162 166 169 169 170 171 173 175

Phosphate bonded cements

197

6.1 6.1.1 6.1.2 6.1.3

197 197 198

6.1.4 6.1.5 6.2 6.2.1 6.2.2 6.2.3 6.2.4 6.2.5

General Orthophosphoric acid solutions Cations in phosphoric acid solutions Reactions between oxides and phosphoric acid solutions Effect of cations in phosphoric acid solutions Important cement-formers Zinc phosphate cement General History Composition Cement-forming reaction Structure

201 203 204 204 204 204 205 207 212 XI

Contents 6.2.6 6.2.7 6.2.8 6.2.9 6.2.10 6.3 6.4 6.4.1 6.4.2 6.4.3 6.4.4 6.4.5 6.4.6

xn

Properties Factors affecting properties Biological effects Modified zinc phosphate cements Hydrophosphate cements Transition-metal phosphate cements Magnesium phosphate cements General Composition Types Cement formation and properties Cement formation with phosphoric acid Cement formation with ammonium dihydrogen phosphate 6.4.7 Cement formation with diammonium hydrogen phosphate 6.4.8 Cement formation with ammonium polyphosphate 6.4.9 Cement formation with aluminium acid phosphate 6.4.10 Cements formed from magnesium titanates 6.5 Dental silicate cement 6.5.1 Historical 6.5.2 Glasses 6.5.3 Liquid 6.5.4 Cement-forming reaction 6.5.5 Structure 6.5.6 Physical properties 6.5.7 Dissolution and ion release 6.5.8 Biological aspects 6.5.9 Conclusions 6.5.10 Modified materials 6.6 Silicophosphate cement 6.7 Mineral phosphate cements References

214 218 219 219 220 220 222 222 222 222 223 223

Oxysalt bonded cements 7.1 Introduction 7.1.1 Components of oxysalt bonded cements 7.1.2 Setting of oxysalt bonded cements 7.2 Zinc oxychloride cements 7.2.1 History 7.2.2 Recent studies 7.3 Magnesium oxy chloride cements 7.3.1 Uses 7.3.2 Calcination of oxide 7.3.3 Setting chemistry

283 283 284 284 285 285 286 290 290 290 291

223 231 232 232 235 235 235 237 241 243 249 253 255 260 261 262 263 265 265

Contents

8

9

7.3.4 Kinetics of cementation 7.3.5 Phase relationships in the MgO-MgCl2-H2O system 7.3.6 Consequences for practical magnesium oxychloride cements 7.3.7 Impregnation with sulphur 7.4 Magnesium oxy sulphate cements 7.4.1 Setting chemistry 7.4.2 Phase relationships in the MgO-MgSO4-H2O system 7.4.3 Mechanical properties of magnesium oxysulphate cements 7.5 Other oxy salt bonded cements References

293 294

Miscellaneous aqueous cements 8.1 General 8.2 Miscellaneous aluminosilicate glass cements 8.3 Phytic acid cements 8.4 Poly(vinylphosphonic acid) cements 8.4.1 Metal oxide polyphosphonate cements 8.4.2 Glass polyphosphonate cements 8.5 Miscellaneous copper oxide and cobalt hydroxide cements References

307 307 307 309 310 311 314

Non-aqueous cements

318

9.1 9.2 9.2.1 9.2.2 9.2.3 9.2.4 9.2.5 9.2.6 9.2.7 9.2.8 9.2.9 9.2.10 9.2.11 9.3 9.3.1 9.3.2 9.4 9.4.1 9.4.2 9.4.3

General Zinc oxide eugenol (ZOE) cements Introduction and history Eugenol Zinc oxide Cement formation Setting Structure Physical properties Biological properties Modified cements Impression pastes Conclusions Improved ZOE cements General Reinforced cements 2-ethoxybenzoic acid eugenol (EBA) cements General Development Setting and structure

295 297 299 299 300 302 304 305

315 316

318 320 320 321 321 322 323 331 333 334 334 335 335 336 336 336 337 337 337 339 Xlll

Contents

10

xiv

9.4.4 Properties 9.5 EBA-methoxyhydroxybenzoate cements 9.5.1 EBA-vanillate and EBA-syringate cements 9.5.2 EBA-divanillate and polymerized vanillate cements 9.5.3 EBA-HV polymer cements 9.5.4 Conclusions 9.5.5 Other zinc oxide cements 9.6 Calcium hydroxide chelate cements 9.6.1 Introduction 9.6.2 Composition 9.6.3 Setting 9.6.4 Physical properties 9.6.5 Biological properties 9.6.6 The calcium hydroxide dimer cement References

340 342 342 344 345 346 347 347 347 348 348 350 350 351 352

Experimental techniques for the study of acid-base cements 10.1 Introduction 10.2 Chemical methods 10.2.1 Studies of cement formation 10.2.2 Degradative studies 10.3 Infrared spectroscopic analysis 10.3.1 Basic principles 10.3.2 Applications to AB cements 10.3.3 Fourier transform infrared spectroscopy 10.4 Nuclear magnetic resonance spectroscopy 10.4.1 Basic principles 10.4.2 Applications to AB cements 10.5 Electrical methods 10.6 X-ray diffraction 10.6.1 Basic principles 10.6.2 Applications to AB cements 10.7 Electron probe microanalysis 10.7.1 Basic principles 10.7.2 Applications to dental silicate cements 10.7.3 Applications to glass-ionomer cements 10.8 Measurement of mechanical properties 10.8.1 Compressive strength 10.8.2 Diametral compressive strength 10.8.3 Flexural strength 10.8.4 Fracture toughness 10.9 Setting and rheological properties 10.9.1 Problems of measurement 10.9.2 Methods of measurement

359 359 360 360 361 361 361 362 364 364 364 365 366 367 367 368 369 369 369 369 370 371 372 372 373 374 375 375

Contents 10.10 Erosion and leaching 10.10.1 Importance in dentistry 10.10.2 Studies of erosion 10.11 Optical properties 10.11.1 Importance in dentistry 10.11.2 Measurement of opacity 10.12 Temperature measurement 10.13 Other test methods References

Index

378 378 379 379 379 380 380 381 382

386

xv

Preface

The senior author first became interested in acid-base cements in 1964 when he undertook to examine the deficiencies of the dental silicate cement with a view to improving performance. At that time there was much concern by both dental surgeon and patient at the failure of this aesthetic material which was used to restore front teeth. Indeed, at the time, one correspondent commenting on this problem to a newspaper remarked that although mankind had solved the problem of nuclear energy the same could not be said of the restoration of front teeth. At the time it was supposed that the dental silicate cement was, as its name implied, a silicate cement which set by the formation of silica gel. Structural studies at the Laboratory of the Government Chemist (LGC) soon proved that this view was incorrect and that the cement set by formation of an amorphous aluminium phosphate salt. Thus we became aware of and intrigued by a class of materials that set by an acid-base reaction. It appeared that there was endless scope for the formulation of novel materials based on this concept. And so it proved. Over the years, from 1964 to date, a team at the LGC, with its expertise in Materials Chemistry, has studied many of the materials described in this book, elucidating structures, setting reactions and behaviour. This experience has formed a strong experimental background against which the book was written. In addition we have maintained contact with leaders in this field throughout the world. We should mention Professor Dennis Smith of Toronto University, who amongst his many achievements invented the adhesive zinc polycarboxylate cement (Chapter 5); Dr G. M. Brauer, who was for many years at the Institute for Materials Research, National Bureau of Standards, Washington, D.C., and is the acknowledged authority on cements formed by the reaction between zinc oxide and phenolic bodies (Chapter 9); and Dr J. H. Sharp of the University of Sheffield, who has developed magnesium phosphate cements (Chapter 6). xvu

Preface

In particular we thank Dr J. H. Sharp for supplying original photographs for use in the section on magnesium phosphate cements and for critically reading the draft manuscript and making constructive suggestions. On clinical matters we have benefited from a 20-year collaboration with Dr J. W. McLean OBE. Our own research at the LGC, while not confined to, has centred on, cements formed by the reactions between acid-decomposable glasses and various cement-forming acids (Chapters 5, 6, 8, 9). One of these materials invented at the LGC, the glass polyalkenoate or glass-ionomer cement, has proved of immense importance. Indeed, so successful has this material been in general dentistry, that the Materials Technology Group earned the Queen's Award for Technology in 1988. This material illustrates the useful combination of properties that can be found in the acid-base cements, for it has the aesthetic appearance of porcelain, the ability to adhere to teeth, and also the ability to releasefluoridewith its beneficial effect of reducing caries. We hope that this work will encourage, stimulate and assist others choosing to work in this interesting field. Alan D. Wilson John W. Nicholson

xvin

Acknowledgements

We make a particular acknowledgement to the late Dr John Longwell CBE, Deputy Government Chemist in 1964, who encouraged the Laboratory to enter the field, and to the line of Government Chemists who supported the work over the long years; the late Dr David Lewis CB, the late Dr Harold Egan, Dr Ron Coleman CB (who became Chief Scientist of the Department of Trade and Industry), Mr Alex Williams CB and Dr Richard Worswick. We note the particular contributions of Brian Kent, present Head of the Materials Technology Group, as co-inventor of the glass polyalkenoate cement way back in 1968, and of Dr John McLean OBE in developing clinical applications. It was Surgeon Rear Admiral Holgate CB, OBE, Chief Dental Officer at the Ministry of Health in 1964, who introduced Dr McLean to the Laboratory of the Government Chemist (LGC) to initiate a collaboration that proved so fruitful. Since then there has been constant support from the Department of Health and its various officers and also from the British Technology Group, particularly from G. M. Blunt and R. A. Lane. Most importantly we acknowledge the contribution of those who worked at that essential place, the laboratory bench, on which everything depends. Our colleagues in the Materials Technology Group (formerly the Dental Materials Group) who have worked with one or other of us since 1964 are: R. F. Batchelor, B. G. Lewis, Mrs B. G. Scott, J. M. Paddon, G. Abel, Dr S. Crisp, A. J. Ferner, Dr H. J. Prosser, M. A. Jennings, Mrs S. A. Merson, M. Ambersley, D. M. Groffman, S. M. Jerome, D. R. Powis, Mrs P. J. Brookman (nee Brant), R. P. Scott, J. C. Skinner, Dr R. G. Hill, G. S. Sayers, Dr C. P. Warrens, Miss A. M. Jackson, Dr J. Ellis, Miss E. A. Wasson, Miss H. M. Anstice, Dr J. H. Braybrook, Miss S. J. Hawkins and A. D. Akinmade. xix

Acknowledgements In addition we have received support from members of other divisions at the LGC: Dr R. J. Mesley, M. A. Priguer, D. Wardleworth, Dr I. K. O'Neill, B. Stuart, R. A. Gilhooley, Dr C. P. Richards, Dr O. M. Lacy and Dr S. L. R. Ellison. Guest workers to the Materials Technology Group who have contributed include Professor P. Hotz (Klinik fur Zahnerhaltung der Universitat, Bern), Ms T. Folleras (NIOM, Scandinavian Institute of Dental Materials). Workers in other Government Research Stations and the Universities who have collaborated with us are: R. P. Miller, D. Clinton, Dr T. I. Barry, Dr I. Seed (National Physical Laboratory); K. E. Fletcher (Buildings Research Station); Miss D. Poynter (Warren Spring Laboratory); Professor L. Holliday, Dr J.H.Elliott, Dr P. R. Hornsby, Dr K. A. Hodd, Dr A. L. Reader (Brunei University); R. Manston, Dr B. F. Sanson, Dr W. M. Allen, P. J. Gleed (Institute for Research on Animal Diseases); Professor Braden (London Hospital); A. C. Shorthall (Birmingham University), I. M. Brook (University of Sheffield); and R. Billington (Institute of Dental Surgery, London). We thank Dr L. J. Pluim of the Rijksuniversiteit te Groningen for drawing our attention to the early and neglected work of E. van Dalen on zinc phosphate cements. We thank Mrs Margaret Wilson for her help in checking the proofs and the indexing. We acknowledge the stoic forbearance of our wives in putting up with the disturbances and neglect of domestic routines and duties occasioned by the writing of a book. Alan D. Wilson John W. Nicholson

xx

1

Introduction

Acid-base (AB) cements have been known since the mid 19th century. They are formed by the interaction of an acid and a base, a reaction which yields a cementitious salt hydrogel (Wilson, 1978) and offers an alternative route to that of polymerization for the formation of macromolecular materials. They are quick-setting materials, some of which have unusual properties for cements, such as adhesion and translucency. They find diverse applications, ranging from the biomedical to the industrial. Despite all this there has been a failure to recognize AB cements as constituting a single, well-defined class of material. Compared with organic polymers, Portland cement and metal alloys, they have been neglected and, except in specializedfields,awareness of them is minimal. In this book we attempt to remedy the situation by unifying the subject and treating this range of materials as a single class. Human interest in materials stretches back into palaeolithic times when materials taken from nature, such as wood and stone, were fashioned into tools, weapons and other artifacts. Carving or grinding of a material is a slow and time-consuming process so the discovery of pottery, which does away with the need for these laborious processes, was of the greatest significance. Here, a soft plastic body, potter's clay, is moulded into the desired shape before being converted into a rigid substance by firing. Pottery is but one of a group of materials which are formed by the physical or chemical conversion of a liquid or plastic body, which can be easily shaped by casting or moulding, into a solid substance. Other examples of this common method of fabrication are the casting of metals and the injection moulding of plastics. Into this category come the water-based plasters, mortars, cements and concretes which set at room temperature as the result of a chemical reaction between water and a powder. Some of these have been known 1

Introduction since antiquity. The AB cements are related to these materials except that water is replaced by an acidic liquid. ThefirstAB cement, the zinc oxychloride cement, was reported by Sorel in 1855. It was prepared by mixing zinc oxide powder with a concentrated solution of zinc chloride. Its use in dentistry was recommended by Feichtinger in 1858 but it did not prove to be a success (Mellor, 1929). However, other AB cements have proved to be of the utmost value to dentistry, and their subsequent development has been closely associated with this art (Wilson, 1978). The AB cements, developed against the backcloth of the severe demands of dentistry, have interesting properties. Some possess aesthetic appeal and the ability to adhere to base metals and other reactive substrates. Most have superior properties to plasters, mortars, and Portland cements, being quick-setting, stronger and more resistant to erosion. These advantageous properties make them strong candidates for other applications. In fact, one of these cements, the magnesium oxychloride cement of Sorel (1867), is still used to surface walls and floors on account of its marble-like appearance (Chapter 7). In the 1870s more effective liquid cement-formers were found: orthophosphoric acid and eugenol (Wilson, 1978). It was also found that an aluminosilicate glass could replace zinc oxide, a discovery which led to the first translucent cement. Thereafter the subject stagnated until the late 1960s when the polyelectrolyte cements were discovered by Smith (1968) and Wilson & Kent (1971). In recent years Sharp and his colleagues have developed the magnesium phosphate cements - Sharp prefers the term magnesia phosphate cement - as a material for the rapid repair of concrete runways and motorways (Chapter 6). These applications exploit the rapid development of strength in AB cements. This cement can also be used for flooring in refrigerated stores where Portland cements do not set. Interestingly, this material appears to have started life as an investment for the casting of dental alloys. The glass polyalkenoate, a polyelectrolyte cement, of Wilson & Kent (Chapter 5), was originally developed as a dental material but has since found other applications. First it was used as a splint bandage material possessing early high-strength and resistance to water. Currently, it is being used, as a biocompatible bone cement, with a low exothermicity on setting and the ability to adhere to bone, for the cementation of prostheses (Jonck, Grobbelaar & Strating, 1989). Outside thefieldof biomaterials it has been patented for use as a cement for underwater pipelines, as a foundry sand and as a substitute for plaster

Introduction in the slip casting of pottery. Quite often it appears as a substitute for plaster of Paris, for it is stronger, less brittle and more resistant to water. There are other possibilities. Its translucent nature suggests that it could be used for the production of porcelain-like ceramics at room temperature. Phosphate and polyelectrolyte AB cements are resistant to attack by boiling water, steam and mild acids and this suggests that they could be employed in technologies where these properties are important. The ability of the polyelectrolyte-based AB cements (Chapter 5) to bond to a variety of substrates, combined with their rapid development of strength - they can become load-bearing within minutes of preparation suggests that they have applications as rapid-repair and handyman materials. A current area of interest is the use of AB cements as devices for the controlled release of biologically active species (Allen et aL, 1984). AB cements can be formulated to be degradable and to release bioactive elements when placed in appropriate environments. These elements can be incorporated into the cement matrix as either the cation or the anion cement former. Special copper/cobalt phosphates/selenates have been prepared which, when placed as boluses in the rumens of cattle and sheep, have the ability to decompose and release the essential trace elements copper, cobalt and selenium in a sustained fashion over many months (Chapter 6). Although practical examples are confined to phosphate cements, others are known which are based on a variety of anions: polyacrylate (Chapter 5), oxychlorides and oxysulphates (Chapter 7) and a variety of organic chelating anions (Chapter 9). The number of cements available for this purpose is very great. A recent development has been the incorporation of a bioactive organic component into the AB cement during preparation. Since AB cements are prepared at room temperature, this can be done without causing degradation of the organic compound. In this case, the AB cement may merely act as a carrier for the sustained release of the added bioactive compound. Another development has been the advent of the dual-cure resin cements. These are hybrids of glass polyalkenoate cements and methacrylates that set both by an acid-base cementation reaction and by vinyl polymerization (which may be initiated by light-curing). In these materials, the solvent is not water but a mixture of water and hydroxyethylmethacrylate which is capable of taking dimethacrylates and poly(acrylic acid)-containing vinyl groups into solution. In the absence of light these materials set slowly and

Introduction have extended working times, but they set in seconds when illuminated with an intense beam of visible light. These hybrids are in their infancy but have created great interest. From this account we are to expect diversification of these AB cements both for biomedical and for industrial usages. There should be further developments of the glass polyalkenoate cements both as bone substitutes and as bioadhesives. We also expect more types of AB cements to be formulated as devices for the sustained release of bioactive species. These materials would have applications in agriculture, horticulture, animal husbandry and human health care. In industrialfieldswe expect that there will be continued interest in developing AB cements as materials for the rapid repair of constructural concrete, as materials for the surfacing of floors and walls, and as adhesives and lutes for cementation in aqueous environments. The hybrid light-cured cements also appear to be a promising new line of development which may give us entirely novel classes of materials. References Allen, W. M., Sansom, B. F., Wilson, A. D., Prosser, H. J. & Groffman, D. M. (1984). Release cements. British Patent GB 2,123,693 B. Jonck, L. M., Grobbelaar, C. J. & Strating, H. (1989). The biocompatibility of glass-ionomer cement in joint replacement: bulk testing. Clinical Materials, 4, 85-107. Mellor, J. W. (1929). A Comprehensive Treatise on Inorganic and Theoretical Chemistry, vol. IV, p. 546. London: Longman. Sorel, S. (1855). Procede pour la formation d'un ciment tres-solide par 1'action d'un chlorure sur l'oxyde de zinc. Comptes rendus hebdomadaires des seances de T Academie des sciences, 41, 784-5. Sorel, S. (1867). On a new magnesium cement. Comptes rendus hebdomadaires des seances de VAcademie des sciences, 65, 102—4. Wilson, A. D. (1978). The chemistry of dental cements. Chemical Society Review, 7, 265-96.

2

Theory of acid-base cements

2.1

General

From the chemical point of view AB cements occupy a place in the vast range of acid-base phenomena which occur throughout both inorganic and organic chemistry. Like Portland cement they are prepared by mixing a powder with a liquid. However, this liquid is not water but an acid, while the powder, a metal oxide or silicate, is a base. Not surprisingly, the cement-forming reaction between them is extremely rapid and a hardened mass is formed within minutes of mixing. AB cements may be represented by the defining equation Base + Acid = Salt + Water (powder) (liquid) (cement matrix) The product of the reaction, the binding agent, is a complex salt, and powder in excess of that required for the reaction acts as the filler. Each cement system is a particular combination of acid and base. The number of potential cement systems is considerable since it is a permutation of all possible combinations of suitable acids and bases. Cement-forming liquids are strongly hydrogen-bonded and viscous. According to Wilson (1968), they must (1) have sufficient acidity to decompose the basic powder and liberate cement-forming cations, (2) contain an acid anion which forms stable complexes with these cations and (3) act as a medium for the reaction and (4) solvate the reaction products. Generally, cement-forming liquids are aqueous solutions of inorganic or organic acids. These acids include phosphoric acid, multifunctional carboxylic acids, phenolic bodies and certain metal halides and sulphates (Table 2.1). There are also non-aqueous cement-forming liquids which are multidentate acids with the ability to form complexes. Potential cement-forming bases are oxides and hydroxides of di- and

Theory of acid—base cements Table 2.1. Examples of acids used for cement formation Protonic acids (used in aqueous solution)

Aprotic acids (used in aqueous solution)

Phosphoric acid Poly(acrylic acid) Malic acid Tricarballylic acid Pyruvic acid Tartaric acid Mellitic acid Gallic acid Tannic acid

Magnesium chloride Zinc chloride Copper(II) chloride Cobalt(II) chloride Magnesium sulphate Zinc sulphate Copper(II) sulphate Cobalt(II) sulphate Magnesium selenate Zinc selenate Copper(II) selenate Cobalt(II) selenate

Protonic acids (liquid non-aqueous) Eugenol 2-ethoxybenzoic acid

Table 2.2. Examples of bases used for cement formation Copper(II) oxide Zinc(II) oxide Magnesium oxide Cobalt(II) hydroxide Cobalt(II) carbonate Calcium aluminosilicate glasses Gelatinizing minerals

trivalent metals, silicate minerals and aluminosilicate glasses (Table 2.2). All cement-forming bases must be capable of releasing cations into acid solution. The best oxides for cement formation are amphoteric (Kingery, 1950a,b) and the most versatile cement former is zinc oxide, which can react with a wide range of aqueous solutions of acids, both inorganic and organic, and liquid organic chelating agents. Gelatinizing minerals, that is minerals that are decomposed by acids, can act as cement formers, as can the acid-decomposable aluminosilicate glasses. In this chapter the nature of the cementitious bond and the acid-base reaction will be discussed.

The formation of cements 2.2

The formation of cements

2.2.1 Classification Before proceeding further it is well to consider the term cement, for its definition can be the source of some confusion. Both the Oxford English Dictionary and Webster give two alternative definitions. One defines a cement as a paste, prepared by mixing a powder with water, that sets to a hard mass. In the other a cement is described as a bonding agent. These two definitions are quite different. The first leads to a classification of cements in terms of the setting process, while the second lays emphasis on a property. In this book the term cement follows the sense of thefirstof these definitions. Cements can be classified into three broad categories: (1) Hydraulic cements. These cements are formed from two constituents one of which is water. Setting comprises a hydration and precipitation process. Into this category fall Portland cement and plaster of Paris. (2) Condensation cements. Here, cement formation involves a loss of water and the condensation of two hydroxyl groups to form a bridging oxygen: R-OH + HO-R = R-O-R + H2O One example is silicate cement where orthosilicic acid, chemically generated in solution, condenses to form a silicic acid gel. Another is refractory cement where a cementitious product is formed by the heat treatment of an acid orthophosphate, a process which again involves condensation to form a polyphosphate. (3) Acid-base cements. Cement formation involves both acid-base and hydration reactions (Wilson, Paddon & Crisp, 1979). These cements form the subject of this book. This classification differs from that given by Wygant (1958), who subdivides cements into hydraulic, precipitation and reaction cements. The advantage of the present classification is that it clearly differentiates phosphate cements formed by condensation from those formed by an acid-base reaction (Kingery, 1950a). Wygant includes these in the same category, which can be confusing. Moreover, he puts silicate cements and the heat-treated acid phosphate cements into separate categories, although both are condensation cements.

Theory of acid—base cements 2.2.2

Requirements for cementitious bonding

The essential property of a cementitious material is that it is cohesive. Cohesion is characteristic of a continuous structure, which in the case of a cement implies an isotropic three-dimensional network. Moreover, the network bonds must be attributed to attractions on the molecular level. Increasingly, recent research tends to show that cements are not bonded by interlocking crystallites and that the formation of crystallites is incidental (Steinke et al., 1988; Crisp et al., 1978). The reason is that it is difficult to form rapidly a mass which is both cohesive and highly ordered. Cement formation requires a continuous structure to be formed in situ from a large number of nuclei. Moreover, this structure must be maintained despite changes in the character of the bonds. These criteria are, obviously, more easily satisfied by aflexiblerandom structure than by one which is highly-ordered and rigid. Crystallinity implies well-satisfied and rigidlydirected chemical bonds, exact stoichiometry and a highly ordered structure. So unless crystal growth is very slow a continuous molecular structure cannot be formed. In random structures, stoichiometry need not be exact and adventitious ions can be incorporated without causing disruption. Bonds are not highly directed, and neighbouring regions of precipitation, formed around different nuclei, can be accommodated within the structure. Continuous networks can be formed rapidly. Thus, random structures are conducive to cement formation and, in fact, most AB cements are essentially amorphous. Indeed, it often appears that the development of crystallinity is detrimental to cement formation. The matrices of AB cements are gel-like, but these gels differ from the tobermorite gel of Portland cement. In AB cements, setting is the result of gelation by salt formation, and the cations, which cause gelation, are extracted from an oxide or silicate by a polyacid solution. The conversion of the sol to a gel is rapid and the cements set in 3 to 5 minutes. Two basic processes are involved in cement formation: the release of cations from the oxide or silicate and their interaction with polyacid. This interaction involves ion binding and changes in the hydration state which are associated with gelation and structure formation (Section 4.3). Thus, there are two reaction rates to be considered: the rate of release of cations and the rate of structure formation. These two reaction rates must be balanced. If the rate of release of cations is too fast a non-coherent precipitate of crystallites is formed. If too slow the gel formed will lack strength.

The formation of cements During cement formation, domains are formed about numerous nuclei and there must be bonding between the domains as well as within them. In AB cements bonding within the domains is mainly ionic, with a degree of covalency. The attractive forces between domains are those of a colloidal type. In random structures, residual forcefieldsexist which act in a similar fashion to polar forces and serve to bond domains. These forces must include hydrogen bonds, for the addition offluorideions always enhances cement strength and the fluoride-hydrogen bond is a strong one. The structures of cement gels bear some relationship to the structure of glasses. Spatially, the O2~ ion is dominant. The matrices are based on a coordinated polyhedron of oxygen ions about a central glass-forming cation (Pauling, 1945). In effect, these are anionic complexes where the cations are small, highly charged, and capable of coordinating with oxygen or hydroxyl ions. Examples of these polyhedra are [SiOJ, [POJ and [A1OJ. Thus, wefindthat there are silicate, phosphate and aluminosilicate glasses and gels. There are, however, differences which are best illustrated by reference to the simple example of silica glass and silica gel. In silica glass, Si4+ is fourcoordinate and the polymeric links are of the bridging type: -«>Si—O—Si<^-

In aqueous solution, coordination increases to 6, Si-OH links are possible as well as Si-O-Si, and H2O is a possible ligand. In silica cements the condensation of silicic acid, Si(OH)4, to SiO2 is only partial. Silica gel therefore contains both bridging oxygen and non-bridging hydroxyl linkages. Again, in contrast to the situation in glasses the possibility of hydrogen bond formation will also exist. In AB cements the gel-forming cations are frequently Zn2+, Mg2+, Ca2+ or Al3+. As Kingery (1950b) has pointed out, it is the amphoteric cations, for example Zn2+ and Al3+, that possess the most favourable cementforming properties. Their oxides are capable of glass formation, not by themselves, but in conjunction with other glass formers. Kingery also indicated that weakly basic cations, for example Mg2+, are less effective, and more strongly basic cations, for example Ca2+, even less effective. The nature of the association between cement-forming cation and anion is important. As we shall see from theoretical considerations of the nature of acids and bases in section 2.3, these bonds are not completely ionic in character. Also while cement-forming cations are predominantly a-

Theory of acid-base cements acceptors and the anions cr-donors, both have weak ^-capabilities also. This topic is treated in more detail in the next section. Complex formation is clearly important and this view is supported by the anomaly that B2O3 forms cements with acids, not as a result of salt formation, but because of complex formation (Chapters 5 and 8). A final point needs to be made. Theory has indicated that AB cements should be amorphous. However, a degree of crystallization does sometimes occur, its extent varying from cement to cement, and this often misled early workers in the field who used X-ray diffraction as a principal method of study. Although this technique readily identifies crystalline phases, it cannot by its nature detect amorphous material, which may form the bulk of the matrix. Thus, in early work too much emphasis was given to crystalline structures and too little to amorphous ones. As we shall see, the formation of crystallites, far from being evidence of cement formation, is often the reverse, complete crystallinity being associated with a noncementitious product of an acid-base reaction.

2.2.3

Gelation

The formation of AB cements is an example of gelation, and the matrices may be regarded as salt-like hydrogels. They are rigid and glass-like. A gel has been defined by Bungenberg de Jong (1949) as 'a system of solid character, in which the colloidal particles somehow constitute a coherent structure'. A more exact definition is not possible, for gels are easier to recognize than define; they include a diversity of substances. Coherence of structure appears, however, to be a universal criterion for gels. Flory (1974) classified gels into four types on the basis of their structures: (1) Well-ordered lamellar structures. The lamellae are arranged in parallel, giving rise to long-range order. Examples are soaps, phospholipids and clays. (2) Covalent polymeric networks which are completely disordered. Continuity of structure is provided by an irregular threedimensional network of covalent links, some of which are crosslinks. The network is uninterrupted and has an infinite molecular weight. Examples are vulcanized rubbers, condensation polymers, vinyl-divinyl copolymers, alkyd and phenolic resins. 10

The formation of cemen ts (3) Polymer networks formed through physical aggregation; these are predominantly disordered, but have regions of local order. Linear structures of finite length are connected by multiplestranded helices, which may be crystalline. Examples are gelatin and sodium alginate gels. (4) Particulate, disordered structures. These include flocculent precipitates where particles generally consist of fibres in brush-heap disarray or connected in irregular networks. Since the matrices of AB cements bear some similarity to alginate gels they most probably fall into type 3. The classical theory of gelation, due to Flory (1953, 1974), sees gelation as the result of the formation of an infinite three-dimensional network. According to Flory, the theory can be applied without ambiguity to the type 2 (covalent) gels and is also applicable to type 3 gels. The conditions for the formation of such an infinite network are critical. Flory conceives the growth of a random network as a sequential condensation process between difunctional and multifunctional units involving a branching process. During growth, the probability of branching (a) at each potential branching point has to reach a critical value (ac) for an infinite network to be formed. In the case of condensation between di- and trifunctional groups, the probability has to be more than 50 % for an infinite network to be formed. If it is 50 % or less then an infinite network is not formed. This theory explains why gelation occurs suddenly. In general, the critical value for a, ac, is given by the expression where / i s the degree of functionality of the multifunctional group. The most investigated examples are to be found in the precipitation of polyelectrolytes by metal ions. Here, networks are formed by the random crosslinking of linear polymer chains, and the theory requires some modification. The condition for the formation of an infinite network is that, on average, there must be more than two crosslinks per chain. Thus, the greater the length of a polymer chain the fewer crosslinks in the system as a whole are required.

11

Theory of acid-base cements 2.3 2.3.1

Acid-base concepts General

The cement-forming reaction is a special case of an acid-base reaction so that concepts of acid, base and salt are central to the topic. In AB cement theory, we are concerned with the nature of the acid-base reaction and how the acidity and basicity of the reactants are affected by their constitution. Thus, it is appropriate at this stage to discuss the various definitions and theories available. Although acids and bases have been recognized since antiquity, our concepts of them are still the subject of debate and development (Walden, 1929; Hall, 1940; Bell, 1947, 1973; Luder, 1948; Kolthoff, 1944; Bjerrum, 1951; Day & Selbin, 1969; Jensen, 1978; Finston & Rychtman, 1982). The history of these concepts is a long one and can be seen as a prolonged and continuous refinement of inexact and commonsense notions into precise scientific theories. It has been a long and difficult journey and one that is by no means ended. There are various definitions of acids and bases, and in discussing them it should be emphasized that the question is not one of validity but one of utility. Indeed, the problem of validity does not arise because of the fundamental nature of a definition. The problem is entirely one of choosing a definition which is of greatest use in the study of a particular field of acid-base chemistry. One point that needs to be borne in mind is that a concept of acids and bases is required that is neither too general nor too restrictive for the particular field of study.

2.3.2

History of acid-base concepts

From early times acids were recognized by their properties, such as sourness and ability to dissolve substances, often with effervescence. The story of Cleopatra's draught of a pearl dissolved in vinegar illustrates this point (Pattison Muir, 1883). Vinegar, known to the Greeks and Romans, was associated with the concept of acidity and gives its name to the term acid which comes from the Latin acetum. Boyle (1661) observed that acids dissolve many substances, precipitate sulphur from alkaline solution, change blue plant dyes to red and lose these properties on contact with alkalis. It also has been known since antiquity that aqueous extracts of the ash 12

Acid-base concepts of certain plants have distinctive properties: slipperiness, cleansing power in the removal of fats, oils and dirt from fabrics, and the ability to affect plant colours (Day & Selbin, 1969; Pattison Muir, 1883). These substances were called alkalis, a name which comes from the Arabic for plant ash, al halja (Finston & Rychtman, 1982) or algali. The term alkali applies only to the hydroxides and carbonates of sodium and potassium, and it was Rouelle in 1744 who extended the concept to include the alkaline earth analogues and used the term base to categorize them (Walden, 1929; Day & Selbin, 1969). Salt formation as a criterion for an acid-base interaction has a long history (Walden, 1929). Rudolph Glauber in 1648 stated that acids and alkalis were opposed to each other and that salts were composed of these two components. Otto Tachenius in 1666 considered that all salts could be broken into an acid and an alkali. Boyle (1661) and the founder of the phlogistic theory, Stahl, observed that when an acid reacts with an alkali the properties of both disappear and a new substance, a salt, is produced with a new set of properties. Rouelle in 1744 and 1754 and William Lewis in 1746 clearly defined a salt as a substance that is formed by the union of an acid and a base. It can be seen that these definitions are derived from experimental observation and are no more than classifications based on a set of properties shared by a group of substances. They are scientifically inadequate for the interpretation of results, which requires a definition based on concepts. Historically, the attempt to provide a model rather than a classification comes in the form of a search for underlying universal principles. It seems that the alchemists recognized vague principles of acidity and alkalinity, and in the 17th century the iatrochemists made these the basis of chemical medicine. Disease was attributed to a predominance of one or other of these principles (Pattison Muir, 1883). Boyle (1661) attempted to provide a more definite concept and attributed the sour taste of acids to sharp-edged acid particles. Lemery, another supporter of the corpuscular theory of chemistry, had similar views and considered that acid-base reactions were the result of the penetration of sharp acid particles into porous bases (Walden, 1929; Finston & Rychtman, 1982). However, the first widely accepted theory was that of Lavoisier who in 1777 pronounced that oxygen was the universal acidifying principle (Crosland, 1973; Walden, 1929; Day & Selbin, 1969; Finston & Rychtman, 1982). An acid was defined as a compound of oxygen with a non-metal. 13

Theory of acid-base cements After this theory was disproved, other acidifying principles were proposed. The most significant was the recognition, by Davy & Dulong early in the 19th century, of hydrogen as the acidifying principle (Walden, 1929; Finston & Rychtman, 1982). During this period no such search was made for a basic principle. Bases were merely regarded as a motley collection of antiacids with little in common apart from the ability to react with acids. The first substantial constitutive concept of acid and bases came only in 1887 when Arrhenius applied the theory of electrolytic dissociation to acids and bases. An acid was defined as a substance that dissociated to hydrogen ions and anions in water (Day & Selbin, 1969). For thefirsttime, a base was defined in terms other than that of an antiacid and was regarded as a substance that dissociated in water into hydroxyl ions and cations. The reaction between an acid and a base was simply the combination of hydrogen and hydroxyl ions to form water. This theory was a milestone in the development of acid-base concepts: it was the first to define acids and bases in terms other than that of a reaction between them and the first to give quantitative descriptions. However, the theory of Arrhenius is far more narrow than both its predecessors and its successors and, indeed, it is the most restrictive of all acid-base theories. Since Arrhenius, definitions have extended the scope of what we mean by acids and bases. These theories include the proton transfer definition of Bronsted-Lowry (Bronsted, 1923; Lowry, 1923a,b), the solvent system concept (Day & Selbin, 1969), the Lux-Flood theory for oxide melts, the electron pair donor and acceptor definition of Lewis (1923, 1938) and the broad theory of Usanovich (1939). These theories are described in more detail below. 2.3.3

Acid—base concepts in AB cement chemistry

We now review the various concepts of acids and bases in order to see how appropriate and useful they are in the field of AB cements. The definition of Arrhenius This definition of acids and bases is of restricted application. The reaction between acids and bases is seen as the combination of hydrogen and hydroxyl ions in aqueous solution to form water. 14

Acid-base concepts An acid is defined as a species that dissociates in aqueous solution to give hydrogen ions and onions, and a base as a species that dissociates in aqueous solution to give hydroxyl ions and cations.

Thus, acids and bases are defined as aqueous solutions of substances and not as the substances themselves. It follows that ionization is a necessary characteristic of Arrhenius acids and bases. Another restriction of this definition is that acid-base behaviour is not recognized in non-aqueous solution. The Arrhenius definition is not suitable for AB cements for several reasons. It cannot be applied to zinc oxide eugenol cements, for these are non-aqueous, nor to the metal oxychloride and oxysulphate cements, where the acid component is not a protonic acid. Indeed, the theory is, strictly speaking, not applicable at all to AB cements where the base is not a water-soluble hydroxide but either an insoluble oxide or a silicate. The protonic Bronsted-Lowry theory The theory of Bronsted (1923) and Lowry (1923a, b) is of more general applicability to AB cements. Their definition of an acid as' a substance that gives up a proton' differs little from that of Arrhenius. However, the same is not true of their definition of a base as' a substance capable of accepting protons' which is far wider than that of Arrhenius, which is limited to hydroxides yielding hydroxide ions in aqueous solution. These concepts of Bronsted and Lowry can be defined by the simple equation (Finston & Rychtman, 1982): Acid = Base + H+

[2.2]

Thus, the relationship between acid and base is a reciprocal one and an acid-base reaction involves the transfer of a proton. This concept is not restricted to aqueous solutions and it discards Arrhenius' prerequisite of ionization. This concept covers most situations in the theory of AB cements. Cements based on aqueous solutions of phosphoric acid and poly(acrylic acid), and non-aqueous cements based on eugenol, alike fall within this definition. However, the theory does not, unfortunately, recognize salt formation as a criterion of an acid-base reaction, and the matrices of AB cements are conveniently described as salts. It is also uncertain whether it covers the metal oxide/metal halide or sulphate cements. Bare cations are not recognized as acids in the Bronsted-Lowry theory, but hydrated 15

Theory of acid-base cements cations are. Thus, in the case of the group III elements, the octahedral [M(H2O)6]3+ aquo ions are quite acidic (Cotton & Wilkinson, 1966): [M(H2O)6]3+ = [M(H2O)5 (OH)]2+ + H+

[2.3]

However, although both zinc and magnesium ions, the cations of the oxycements, are hydrated as [M(H2O)6]2+ ions, these hydrated ions hydrolyse only slightly (Baes & Mesmer, 1976). Thus, in magnesium chloride solutions the aquo ions, in contrast to beryllium aquo ions, are not perceptibly acidic. So there must be some doubt as to whether these hydrated ions can be regarded as protonic acids. But for this, the Bronsted-Lowry theory would almost exactly define AB cements. Aluminosilicate glasses are used in certain AB cement formulations, and the acid-base balance in them is important. The Bronsted-Lowry theory cannot be applied to these aluminosilicate glasses; it does not recognize silica as an acid, because silica is an aprotic acid. However, for most purposes the Bronsted-Lowry theory is a suitable conceptual framework although not of universal application in AB cement theory. The solvent system theory Although the protonic theory is not confined to aqueous solutions, it does not cover aprotic solvents. The solvent system theory predates that of Bronsted-Lowry and represents an extension of the Arrhenius theory to solvents other than water. It may be represented by the defining equation: Acid + Base = Salt + Solvent

[2.4]

This theory is associated in its early protonic form with Franklin (1905, 1924). Later it was extended by Germann (1925a,b) and then by Cady & Elsey (1922,1928) to a more general form to include aprotic solvents. Cady & Elsey describe an acid as a solute that, either by direct dissociation or by reaction with an ionizing solvent, increases the concentration of the solvent cation. In a similar fashion, a base increases the concentration of the solvent anion. Cady & Elsey, in order to emphasize the importance of the solvent, modified the above defining equation to: Acidic solution + Basic solution = Salt + Solvent

[2.5]

Thus, acids and bases do not react directly but as solvent cations and anions. Since emphasis is placed upon ionization interactions, inherent acidity and basicity is neglected, as are interactions in the non-ionic state. The theory is a simple extension of the Arrhenius theory and suffers from 16

Acid-base concepts the same drawbacks. The definition cannot be applied directly to the reaction between a basic solid and acidic liquid characteristic of AB cements. The Lux-Flood theory The Lux-Flood theory relates to oxide melts. Geologists have often used acid-base concepts for the empirical classification of igneous silicate rocks (Read, 1948). Silica is implicitly assumed to be responsible for acidity, and the silica content of a rock is used as a measure of its acid-base balance: Rock type Acid Intermediate Basic Ultra-basic

Silica content (SiO2) % >66

52-66 45-52 <45

Lux (1939) developed an acid-base theory for oxide melts where the oxide ion plays an analogous but opposite role to that of the hydrogen ion in the Bronsted theory. A base is an oxide donor and an acid is an oxide acceptor (Lux, 1939; Flood & Forland, 1947a,b; Flood, Forland & Roald, 1947): Base = Acid+ O2~

[2.6]

Thus an acid-base reaction involves the transfer of an oxide ion (compared with the transfer of a proton in the Bronsted theory) and the theory is particularly applicable in considering acid-base relationships in oxide, silicate and aluminosilicate glasses. However, we shall find that it is subsumed within the Lewis definition. The Lewis theory This theory was advanced by G. N. Lewis (1916, 1923, 1938) as a more general concept. In his classic monograph of 1923 he considered and rejected both the protonic and solvent system theories as too restrictive. An acid-base reaction in the Lewis sense means the completion of the stable electronic configuration of the acceptor atom of the acid by an electron pair from the base. Thus: A base has the ability to donate a pair of electrons and an acid the ability to accept a pair of electrons to form a covalent bond. The product of a Lewis acid—base reaction may be called an adduct, a coordination compound or a coordination complex (Vander Werf 1961). Neither salt nor conjugate acid—base formation is a requirement. 17

Theory of acid-base cements Although Lewis and Bronsted bases comprise the same species, the same is not true of their acids. Lewis acids include bare metal cations, while Bronsted-Lowry acids do not. Also, Bell (1973) and Day & Selbin (1969) have pointed out that Bronsted or protonic acids fit awkwardly into the Lewis definition. Protonic acids cannot accept an electron pair as is required in the Lewis definition, and a typical Lewis protonic acid appears to be an adduct between a base and the acid H+ (Luder, 1940; Kolthoff, 1944). Thus, a protonic acid can only be regarded as a Lewis acid in the sense that its reaction with a base involves the transient formation of an unstable hydrogen bond adduct. For this reason, advocates of the Lewis theory have sometimes termed protonic acids secondary acids (Bell, 1973). This is an unfortunate term for the traditional acids. Lewis (1938) was not content with a purely conceptual view of acids and bases, for he also listed certain phenomenological criteria for an acid-base reaction. The process of neutralization is a rapid one, an acid or base displaces a weaker acid or base from its compounds, acids and bases may be titrated against each other using coloured indicators, and both acids and bases have catalytic effects. The Lewis definition covers all AB cements, including the metal oxide/metal oxysalt systems, because the theory recognizes bare cations as aprotic acids. It is also particularly appropriate to the chelate cements, where it is more natural to regard the product of the reaction as a coordination complex rather than a salt. Its disadvantages are that the definition is really too broad and that despite this it accommodates protonic acids only with difficulty. The Usanovich theory

The Usanovich theory is the most general of all acid-base theories. According to Usanovich (1939) any process leading to the formation of a salt is an acid-base reaction. The so-called' positive-negative' definition of Usanovich runs as follows. An acid is a species capable of yielding cations, combining with onions or electrons, or neutralizing a base. Likewise a base is a species capable of yielding anions or electrons, combining with cations, or neutralizing an acid.

When developed, this theory proved to be more general than the theory of Lewis, for it includes all the above acid-base definitions and also includes oxidation-reduction reactions. 18

Acid—base concepts

It is better than the Lewis theory for describing acid-base cements, for it avoids the awkwardness that the Lewis definition has with protonic acids. However, as Day & Selbin (1969) have observed, the generality of the theory is such that it includes nearly all chemical reactions, so that acid-base reactions could simply be termed 'chemical reactions'. 2.3.4

Relevance of acid-base theories to AB cements

The various acid-base definitions are summarized in the Venn diagram (Fig. 2.1). From this it can be seen that the Usanovich definition subsumes the Lewis definition, which in turn subsumes all other definitions (i.e. Arrhenius, Bronsted-Lowry, Germann-Cady-Elsey, Lux-Flood). Also shown is how the topic of AB cements relates to these definitions. An ideal definition for a subject should be one that exactlyfitsit. It should cover all aspects of the subject while excluding all extraneous topics. Thus, a theory should be neither too restrictive nor too general. The Arrhenius and Germann-Cady-Elsey definitions do not relate to the subject at all as USANOVICH

LEWIS E l e c t r o n - p a i r acceptor

BR0NSTED proton-donor any solvent

ARRHENIUS proton-donor in water

GERMANN sol vent-cation donor

Figure 2.1 Venn diagram showing the relationship between the various definitions of acids and bases.

19

Theory of acid-base cements the basic component of an AB cement is a powdered solid. The Bronsted-Lowry definition is not broad enough to include all AB cements and excludes the concept of salt, which is unfortunate since the matrices of AB cements are salts. Both the Lewis and Usanovich definitions cover all aspects of AB cement theory at the cost of including topics not relevant to this subject. From this discussion it can be seen that there is no ideal acid-base theory for AB cements and a pragmatic approach has to be adopted. Since the matrix is a salt, an AB cement can be defined quite simply as the product of the reaction of a powder and liquid component to yield a salt-like gel. The Bronsted-Lowry theory suffices to define all the bases and the protonic acids, and the Lewis theory to define the aprotic acids. The subject of acid-base balance in aluminosilicate glasses is covered by the Lux-Flood theory. 2.3.5

Acid-base strength

Ever since the formulation of the Bronsted-Lowry theory, efforts have been made to develop a general approach to acid-base strength. The influence of ionic charge and size of the central atom on acidity and basicity is important. In 1926, Bronsted found that an increase in acidity corresponded to an increase in positive charge or a decrease in negative charge on an ion. Cartledge (1928a,b), against the background of the protonic theory, proposed to correlate acidity or basicity with a function he called ionic potential, by considering acids and bases to be hydroxides of non-metals and metals, respectively. He defined ionic potential, (/>, as = Z/r

(2.1)

where Z is the charge on the central atom and r its ionic radius. Cartledge (1928b) then used values of ^ 0 5 to define acidity and basicity of a species.

f5 value >3-2 2-2-3-2 <2-2

Acid-base status acidic amphoteric basic

Thus, highly charged smaller cations are highly acidic. This point is illustrated for the series Na + , Mg2+, Al3+, Si4+, P5+, S6+ and Cl7+ in Table 2.3a. Note, however, that Zn(OH)2 is not classified as amphoteric as it should 20

Acid-base concepts Table 2.3a. Effect of cation on acidity-basicity (Cartledge, 1982a,b) Cation

Ionic potential \

Species

Acidity-basicity

Na + Ca 2+ Zn 2+ Mg 2+ Al 3+ Si4+

102 1-42 1-64 1-76 2-45 313 3-83 4-55 5-20

NaOH Ca(OH) 2 Zn(OH) 2 Mg(OH) 2 A1(OH)3 Si(OH) 4 H 3 PO 4 H 2 SO 4 HC1O4

Strong base Weak base Weak base Weak base Amphoteric Weak acid Intermediate acid Strong acid Strongest acid

p5+ S

6

+

Cl 7+

Table 2.3b. Effect of cation on acidity-basicity Cation

Ionization potential In

Species

Acidity-basicity

Na + Ca 2+ Mg 2+ Cd 2+ Zn 2+ Cu 2+ Bi3+ Al 3+ Si4+

5-14 11-87 1503 16-84 17-96 20-20 25-42 28-45 45-14 6502 88-05

NaOH Ca(OH) 2 Mg(OH) 2 Cd(OH) 2 Zn(OH) 2 Cu(OH) 2 Bi(OH) 3 A1(OH)3 Si(OH) 4 H 3 PO 4 H 2 SO 4

Strong base Weak base Weak base Amphoteric Amphoteric Amphoteric Amphoteric Amphoteric Weak acid Intermediate acid Strong acid

p5+

s6+

In is the nth ionization potential.

be. Clearly, ionic potential alone is not a sufficient criterion for classification. As will be shown, unlike other cations in Table 2.3a which are classified as hard acids, Zn2+ is an intermediate because of the presence of d orbital electrons. The effect of d electrons in increasing the polarizing power of the cations, because of ineffective screening, has been demonstrated by Hodd & Reader (1976). They found that Cd2+ was a more effective cement-former than Ca2+, because although both have a similar ionic radius, Ca2+ has no d electrons. For these reasons, ionization potential is a better criterion than ionic potential. As Table 2.3b shows, Zn2+ is ranked correctly by this criterion and can be classified as 21

Theory of acid-base cements amphoteric. Inspection of this table throws some light on the requirements for cement formation. If judged by strength and hydrolytic stability of cements formed with orthophosphoric acid, poly(acrylic acid) and poly(vinylphosphonic acid), the common cement-forming cations can be ranked in the following order of decreasing effectiveness. Al3+> Cu2+ > Zn2+ > Mg2+ > Ca2+ The first three form amphoteric oxides and are distinctly superior, as cement-formers, to the latter two which form weakly basic oxides. Data from Table 2.3b indicate that optimum cement formation occurs with cations that have In values lying between 18 and 29. 2.3.6

Acid-base classification

The strength of a Lewis acid or base depends on the particular reaction, and for this reason there is no absolute scale for the strengths of Lewis acids and bases. However, certain qualitative features have been observed. Ahrland, Chatt & Davies (1958) divided metal ions (which are Lewis acids), on the basis of the stability of their complexes, into what they termed class (a) and class (b) acceptors (Table 2.4). They stated that class (a) acceptors form their most stable complexes with ligands of the lightest member of a non-metal group. By contrast, class (b) acceptors form their most stable complexes with heavier members of each group. Thus, complex stability can be ranked according to the ligand as follows. For class (a) acceptors O P S and for class (b) acceptors O <^ S. Class (a) metal ions are small and non-polarizable, whereas class (b) metal ions are large and polarizable. The class of a given element is not constant and depends on oxidation state; class (a) character increases with increase in the positive charge. Chatt (1958) considers that the important feature of class (b) acids is the presence of loosely held outer d orbital electrons which can form nbonds to certain ligands. These ligands would contain empty d orbitals on the basic atom; examples are P and As. In the context of AB cements, Al3+, Mg2+, Ca2+ and Zn2+ are in class (a) while Cu2+ is in the border region. Zn2+ contains a completed 3d shell and forms stronger complexes with O than with S ligands, as do other class (a) cations.

22

Table 2.4. Classification of acceptor atoms in their normal valent states (Ahrland, Chatt & Davies, 1958)

Class (a)

H Li

Be

Na Mp K

a/b border Class (a)

Ca I >c Ti

a/b border

N

0

F

Al

Si

P

s

Cl

Zn

Ga

Ge

As

Se

Br

Ru

Rh Pd Ag

Cd

In

Sn

Sb

Te

I

Os

Ir

Pt Au

Hg

TI

Pb

Bi

Po

At

Mn Fe

Rb Sr ^ft Zr Nb

Mo

Tc

Cs Ba 1^a Hf Ta

W

Re

Class (a)

C

Ni Cu

Cr

V

B

a/b border

Co

Class (b)

a/b border

Theory of acid-base cements 2.3.7

Hard and soft acids and bases (HSAB)

This concept of Chatt and his coworkers was developed further by Pearson (1963, 1966, 1968a,b) in his theory of hard and soft acids and bases. Hard acids correspond with class (a) acceptors and soft acids with class (b) acceptors. Hard acids prefer to react with hard bases and soft acids prefer to react with soft bases. Hard acids are characterized by small size, high positive charge and absence of outer electrons which are easily excited to higher states; they are thus of low polarizability. In this class are the common protonic acids, HA, the hydrogen-bonding molecules in the Lewis scheme and Mg 2+ , which are all acids of relevance to AB cements. The soft acids have low or zero positive charge, large size and several easily excited outer electrons (often d orbital electrons). These properties lead to high polarizability. The division between these two classes is not sharp; amongst the intermediate class are Zn 2+ and Cu 2+ . Pearson (1966) defines a soft base as 'one in which the donor atom is of high polarizability and low electronegativity and is easily oxidized or associated with empty, low-lying orbitals'. A hard base has opposite properties. 'The donor atom is of low polarizability and high electronegativity, is hard to reduce, and is associated with empty orbitals of high energy.' The underlying theory for hard-hard and soft-soft preferences is obscure and no one factor is responsible (Pearson, 1966). Pearson (1963, 1968b) advanced several explanations. He stated that the ionic-covalent theory provides the most obvious explanation. Hard acids are assumed to bind bases primarily by ionic forces and soft acids by covalent bonds. High positive charge and small size favour strong ionic bonding, and bases of large negative charge and small size would be most strongly held. Soft acids bind to bases by covalent bonding, and the atoms should be of similar size and electronegativity for good bonding. The classification of Lewis acids and bases relevant to AB cements is shown below. Hard acids: Borderline acids: Hard bases: 24

H A , H + , Ca 2 + , Mg 2 + , Al 3 + , Si 4+ Zn 2+ , Cu2+ H 2 O, O H , F", POJ", SO2", RCOO"

Acid-base concepts Table 2.5. YatsimirskiVs hardness indices {Yatsimirskii, 1970) Base

Indices

Acid

Indices

OHF-

6-3 1-7 1-7 0-8 0-5

H+

9-0 1-2 10 0-2 01

HPOJCH3COO-

sor

H2O

In 3 + Cu 2 + Zn 2 + La 3 +

zero

Extension of HSAB theory Yatsimirskii (1970) attempted to quantify HSAB theory and produced hardness indices (S) for acids and bases. These indices were obtained by plotting the logarithms of the equilibrium constants for the reactions of bases with the proton (the hardest acid) against similar values for the reactions with CH 3 Hg + (one of the softest acids). For acids, the hydroxyl ion (the hardest base) and the chloride ion (a soft base) were chosen. These S indices for cations and anions relevant to AB cements are shown in Table 2.5. Bases which add on through F or O and do not form Tr-bonds have similar hardness values; they are hard bases. Soft bases form dative 7r-bonds with many cations. They have high-energy-level occupied orbitals with unshared electron pairs. Yatsimirskii considered that the hard and soft classification was too general and proposed instead a more detailed approach. He classified Lewis acids and bases into six groups, based on the nature of the adduct bonding. Group (1) Cations and anions which are incapable of donor-acceptor interactions. These are the large univalent ions. Bonding is purely by Coulomb and Madelung electrostatic interactions. From the Lewis point of view these are not acids or bases. They have no cement-forming potential. Group (2) Strong a-acceptor acids and donor bases. Included here are protonic acids, which are relevant to AB cements. Their adducts can only contain one coordinate bond. Group (3) G- and n-acceptor acids and donor bases with o-interactions predominating. In this group acceptors are capable of adding on electron pairs of donors in both types of interactions. Includes cations with stable closed electron shells: Al 3+ , Mg 2+ , Ca2+ and 25

Theory of acid-base cements Zn 2+ . Donors are ligands coordinated through oxygen atoms or fluoride ions: RCOO", PO*~, OH", F" and H 2 O. These acceptors and donors are of relevance to AB cements. Group (4) Strong a- and n-acceptor acids and donor bases. Bi3+, In 3+ and Sn2+ are of some relevance to AB cements. Group (5) Acids that are o-acceptors but capable of n-donation in backbonding. This group includes cations with mobile d electrons e.g. Cuw+, Co w+ , Fe w+ . Group (6) Bases that are a-donors but n-acceptors. According to Yatsimirskii, group (2) and (3) species are equivalent to Pearson's hard acids and bases, and group (4), (5) and (6) species correspond to Pearson's soft acids and bases. This classification is of more value than HSAB theory to our subject. It can be seen that all cementforming anions come from group (3) and cations from groups (3), (4) and (5). Thus, the bonding in cement matrices formed from cation-anion combinations is not purely a but contains some n character.

References Ahrland, S., Chatt, J. & Davies, N. R. (1958). The relative affinities of ligand atoms for acceptor molecules and ions. Quarterly Reviews, 12, 265-76. Baes, C. F. & Mesmer, R. E. (1976). The Hydrolysis of Cations. New York: John Wiley. Bell, R. P. (1947). The use of the terms 'acid' and 'base'. Quarterly Reviews, 1, 113-25. Bell, R. P. (1973). The Proton in Chemistry. Ithaca, New York: Cornell University Press. Bjerrum, J. (1951). Die Entwickhmgsgeschichte des Saure-Basenbegriffes und iiber die ZweckmaBigkeit der Einfuhrung eines besonderen Antibasenbegriffes neben dem Saurebegriff. Naturwissenschaften, 38, 461-4. Boyle, R. (1661). The Sceptical Chymist. Everyman Library Edition, 1911. Brensted, J. N. (1923). Einige Bemerkungen iiber den Begriff der Sauren und Basen. Recueil des Travaux chimiques des Pays-Bas et de la Belgique, 42, 718-28. Bronsted, J. N. (1926). The acid-base function of molecules and its dependency on the electronic charge type. Journal of Physical Chemistry, 30, 777-90. Bungenberg de Jong, H. G. (1949). In Kruyt, H. R. (ed.) Colloid Science II, p. 2. Amsterdam: Elsevier Publishing Co. Inc. Cady, H. P. & Elsey, H. M. (1922). A general conception of acids, bases and salts. Science, 56, 27 (Lecture abstract). Cady, H. P. & Elsey, H. M. (1928). A general definition of acids, bases and salts. Journal of Chemical Education, 5, 1425-8. 26

References Cartledge, G. H. (1928a). Studies on the periodic system. I. The ionic potential as a periodic function. Journal of the American Chemical Society, 50, 2855-63. Cartledge, G. H. (1928b). Studies on the periodic system. II. The ionic potential and related properties. Journal of the American Chemical Society, 50, 2863-72. Chatt, J. (1958). The stabilisation of low valent states of the transition metals. Journal of Inorganic & Nuclear Chemistry, 8, 515-31. Cotton, F. A. & Wilkinson, G. (1966). Advanced Inorganic Chemistry, 2nd edn. New York, London & Sydney: Wiley Inter science. Crisp, S., O'Neill, I. K., Prosser, H. J., Stuart, B. & Wilson, A. D. (1978). Infrared spectroscopic studies on the development of crystallinity in dental zinc phosphate cements. Journal of Dental Research, 57, 245-54. Crosland, M. P. (1962). Historical Studies in the Language of Chemistry. London: Heinemann. Crosland, M. (1973). Lavoisier's theory of acidity. Isis, 64, 306-25. Day, M. C. & Selbin, J. (1969). Theoretical Inorganic Chemistry. New York: Reinhold. Finston, H. L. & Rychtman, A. C. (1982). A New View of Current Acid-Base Theories. New York: John Wiley & Sons. Flood, H. & Forland, T. (1947a). The acidic and basic properties of oxides. Ada Chemica Scandinavica, 1, 592—604. Flood, H. & Forland, T. (1947b). The acidic and basic properties of oxides. II. The thermal decomposition of pyrosulphates. Acta Chemica Scandinavica, 1, 781-9. Flood, H., Forland, T. & Roald, B. (1947). The acidic and basic properties of oxides. III. Relative acid-base strengths of some polyacids. Acta Chemica Scandinavica, 1, 790-8. Flory, P. J. (1953). Principles of Polymer Chemistry, Chapter 11. Ithaca, New York: Cornell University Press. Flory, P. J. (1974). Introductory lecture. In Gels and Gelling Processes. Faraday Discussions of the Chemical Society, No. 57, pp. 7-18. Franklin, E. C. (1905). Reactions in liquid ammonia. Journal of the American Chemical Society, 27, 820-51. Franklin, E. C. (1924). Systems of acids, bases and salts. Journal of the American Chemical Society, 46, 2137-51. Germann, A. F. O. (1925a). What is an acid? Science, 61, 71. Germann, A. F. O. (1925b). A general theory of solvent systems. Journal of the American Chemical Society, 47, 2461-8. Hall, N. F. (1940). Systems of acids and bases. Journal of Chemical Education, 17, 124^8. Hodd, K. A. & Reader, A. L. (1976). The formation and hydrolytic stability of metal ion-polyacid gels. British Polymer Journal, 8, 131-9. Jensen, W. B. (1978). The Lewis acid-base definitions: a status report. Chemical Reviews, 78, 1-22. Kingery, W. D. (1950a). Fundamental study of phosphate bonding in refractories. I. Literature review. Journal of the American Ceramic Society, 33, 239-41. 27

Theory of acid-base cements Kingery, W. D. (1950b). Fundamental study of phosphate bonding in refractories. II. Cold setting properties. Journal of the American Ceramic Society, 33, 242-7. Kolthoff, I. M. (1944). The Lewis and Bronsted-Lowry definitions of acids and bases. Journal of Physical Chemistry, 48, 51-7. Lewis, G. N. (1916). The atom and the molecule. Journal of the American Chemical Society, 38, 762-85. Lewis, G. N. (1923). Valence and the Structure of Atoms and Molecules. New York: Chemical Catalog Co. Lewis, G. N. (1938). Acids and bases. Journal of the Franklin Institute, 226, 293-337. Lowry, T. M. (1923a). The uniqueness of hydrogen. Chemistry & Industry, 42, 43. Lowry, T. M. (1923b). Co-ordination and acidity. Chemistry & Industry, 42, 1048-52. Luder, W. F. (1940). The electronic theory of acids and bases. Chemical Reviews, 27, 547-83. Luder, W. F. (1948). Contemporary acid-base theory. Journal of Chemical Education, 25, 555-8. Lux, H. (1939). 'Sauren' und 'Basen' im Schelzfluss: Die Bestimmung der Sauerstoffionen-Konzentration. Zeitschrift fur Elektrochemie, 45, 303-9. Pattison Muir, M. M. (1883). Heroes of Science-Chemists, Chapter IV, pp. 171-89. London: Society for Promoting Christian Knowledge. Pauling, L. (1945). The Nature of the Chemical Bond. Ithaca, New York: Cornell University Press. Pearson, R. G. (1963). Hard and soft acids and bases. Journal of the American Chemical Society, 85, 3533-9. Pearson, R. G. (1966). Acids and bases. Science, 151, 172-7. Pearson, R. G. (1968a). Hard and soft acids and bases, HSAB. Part I. Fundamental principles. Journal of Chemical Education, 45, 581-7. Pearson, R. G. (1968b). Hard and soft acids and bases, HSAB. Part II. Underlying theories. Journal of Chemical Education, 45, 643-8. Read, H. H. (1948). Rutle/s Elements of Mineralogy, 24th edn. London: Thomas Murby & Co. Smith, D. C. (1968). A new dental cement. British Dental Journal, 125, 381-4. Steinke, R., Newcomer, P., Komarneni, S. & Roy, R. (1988). Dental cements: investigation of chemical bonding. Materials Research Bulletin, 23, 13-22. Usanovich, M. I. (1939). On acids and bases. Journal of General Chemistry (USSR), 9, 182-92. Vander Werf, A. (1961). Acids, Bases, and the Chemistry of the Covalent Bond. New York: Reinhold. Walden, P. (1929). Salts, Acids and Bases: Electrolytes, Stereochemistry. New York: McGraw-Hill. Wilson, A. D. (1968). Dental silicate cements: VII. Alternative liquid cement formers. Journal of Dental Research, 47, 1133-6. Wilson, A. D. & Kent, B. E. (1971). The glass-ionomer cement: a new

28

References translucent cement for dentistry. Journal of Applied Chemistry and Biotechnology, 21, 313. Wilson, A. D., Paddon, J. M. & Crisp, S. (1979). The hydration of dental cements. Journal of Dental Research, 58, 1065-71. Wygant, J. F. (1958). Cementitious bonding in ceramic fabrication. In Kingery, W. D. (ed.) Ceramic Fabrication Processes, pp. 171-88. New York: John Wiley & Sons. Yatsimirskii, K. (1970). Acid-base and donor-acceptor properties of ions and molecules. Theoretical and Experimental Chemistry (USSR), 6, 376-80.

29

3

Water and acid-base cements

3.1

Introduction

The setting reaction for the great majority of acid-base cements takes place in water. (The exceptions based on o-phenols are described in Chapter 9.) This reaction does not usually proceed with formation of a precipitate but rather yields a substance which entrains all of the water used to prepare the original cement paste. Water thus acts as both solvent and component in the formation of these cements. It is also one of the reaction products, being formed in the acid-base reaction as the cements set. 3.1.1

Water as a solvent

It is widely recognized that the solvent in which any chemical reaction takes place is not merely a passive medium in which relevant molecules perform: the solvent itself makes an essential contribution to the reaction. The character of the solvent will determine which chemical species are soluble enough to enter solution and hence to react, and which species are insoluble, and thus precipitate out of solution, thereby being prevented from undergoing further chemical change. In the case of water, as will be seen, polar and ionic species are the ones that most readily dissolve. But even so, mere polarity or ionic character is not sufficient to ensure solubility. Solubility depends on a number of subtle energetic factors, and the possible interactions between water and silver chloride, for example, do not fulfil the requirements despite the ionic nature of the silver salt. Hence silver chloride is almost completely insoluble in water. 3.1.2

Water as a component

In AB cements water does not merely act as solvent for the setting reaction. It also acts as an important component of the set cement. For example, 30

Water glass-ionomer dental cements as generally formulated include at least 15% by mass of water, all of which becomes incorporated into the complete cement (Wilson & McLean, 1988). Indeed, great importance is attached to the retention of water by these cements, since if they are allowed to dry out by storage under conditions of low humidity, they shrink significantly, and develop cracks and crazes. Another class of AB cement, the oxychloride cements of zinc and magnesium, are also formulated in aqueous solution and retain substantial amounts of water on setting (Sorrell & Armstrong, 1976; Sorrell, 1977). Water may have a number of roles in the set versions of these cements. It is capable of solvating the cement-forming ions, such as Ca2+ or Zn2+, depending on the cement. It also contributes a sheath of solvating molecules around polyelectrolytes such as poly(acrylic acid) in glassionomer and zinc polycarboxylate cements. Significant amounts of water are known to be retained by metal polyacrylate salts at equilibrium and this water contributes to reducing the glass transition temperature of such materials by acting as a plasticizer (Yokoyama & Hiraoko, 1979). These various aspects of water in AB cements are covered in the present chapter. Its solvent character, structure and hydration behaviour are described, and the chapter concludes with a more thorough consideration of the precise role of water in the various AB cements.

3.2 3.2.1

Water Constitution

Water has a deceptively simple chemical constitution, consisting as it does of molecules containing two atoms of hydrogen and one of oxygen. It was viewed by the ancients as one of the four 'elements', following Aristotle's classification, the others being air, fire and earth. The modern view that it is a compound composed of hydrogen and oxygen was first established in 1789 by two amateur chemists, Adriaan Paets van Troostwijk (1752-1837), a merchant, and Jan Rudolph Deiman (1743-1808), a pharmacist (Hall, 1985). They were able to show by synthesis which elements combine to make water, forming it from reaction of hydrogen gas with oxygen. Their work was important historically for the part it played in undermining the phlogiston theory of combustion. It was left to the great Swedish chemist J. J. Berzelius (1779-1848) to determine that the ratio of hydrogen to oxygen is 2:1. 31

Water and acid-base cements Table 3.1. Molecular dimensions of normal and isotopic water in the vapour phase {Benedict, Gailar & Plyler, 1956) Molecule

Bond length, pm

Bond angle, degrees

D2O H2O HDO

95-75 95-718 95-71

104-474 104-523 104-529

As a compound water is remarkable. It is the only inorganic liquid to occur naturally on earth, and it is the only substance found in nature in all three physical states, solid, liquid and vapour (Franks, 1983). It is the most readily available solvent and plays a vital role in the continuation of life on earth. Water circulates continuously in the environment by evaporation from the hydrosphere and subsequent precipitation from the atmosphere. This overall process is known as the hydrologic cycle. Reports estimate that the atmosphere contains about 6 x 1015 litres of water, and this is cycled some 37 times a year to give an annual total precipitation of 224 x 1015 litres (Franks, 1983; Nicholson, 1985). The bond lengths and bond angle for the water molecule are known very precisely following studies of the rotation-vibration spectra of water vapour, and also the vapour of the deuterated analogues of water, D2 O and HDO (Eisenberg & Kauzmann, 1969). The data for these compounds are shown in Table 3.1. The nuclei of the water molecule, regardless of the isotopes involved, form an isosceles triangle having a slightly obtuse angle at the oxygen atom. All of the data in Table 3.1 refer to the equilibrium state of the water molecules, which is formally acceptable, but is actually a hypothetical state, since it assumes neither rotation nor vibration in the molecule. The equilibrium bond lengths and bond angles can be seen to differ little between the different isotopic molecules. Such a finding agrees with the predictions of the Born-Oppenheimer approximation, that the electronic structure of a molecule is independent of the mass of its nuclei, it being the electronic structure of a molecule alone which determines the geometry. The bond angle in water is slightly less than the ideal tetrahedral angle of 109-5°.This is attributed to the presence of lone pairs of electrons on the oxygen atom which repel more strongly than the bonding pairs of electrons between the oxygen and hydrogen nuclei (Speakman, 1975). The valence32

Water Table 3.2. Properties of hydrides of first row elements (Weast, 1985-6)

Compound

Relative molar mass

Melting point, °C

Boiling point, °C

Gas phase dipole moment, debyes

CH 4 NH 3 H2O HF

16 17 18 19

-182-0 -11-1 00 -834

-1640 -33-4 1000 19-5

000 1-47 1-85 1-82

shell electron-pair repulsion concepts of Gillespie & Nyholm (1957) show that such increased repulsion by lone pairs closes the angle between the bonding pairs slightly but significantly for the water molecule. The O-H bond energy of water is taken as half the energy of formation of the molecule, since water has two such bonds. This gives a value of 458-55 kJ mol"1 at 0 K (Eisenberg & Kauzmann, 1969). Related to the bond energy is the dissociation energy, i.e. the energy required to break the bond at 0 K. Neither of the O-H bonds in water has a dissociation energy equal to the O-H bond energy. Instead, the first O-H dissociation energy has been found experimentally to be 424-27 kJ mol"1. From conservation of energy considerations which lead to the requirement that the sum of the two dissociation energies must equal the energy of formation, it is found that the second O-H dissociation energy has to take a value of 492-83 kJ mol"1. This has been explained (Pauling, 1960) by postulating an electronic rearrangement on the oxygen atom of the O-H fragment left behind after scission of the first O-H bond, and that breaking the bond between oxygen in this new electronic configuration and the remaining hydrogen requires greater energy. 3.2.2

Water compared with other hydrides

Water shows properties that are interestingly different compared with hydrides of the neighbouring elements of thefirstrow of the periodic table. Some of these properties are given in Table 3.2. From this table, water can be seen to have a very high melting point and a very high boiling point for its relative molar mass. Indeed, it is the only one of the hydrides of the 33

Water and acid-base cements elements from this portion of the periodic table to be liquid at room temperature and atmospheric pressure. In the gas phase it has a dipole moment that, while only slightly greater than that of hydrogenfluoride,is the highest for this group of hydrides. All of these properties point to water having a structure in which its constituent molecules are more highly associated and interact more strongly than the molecules of the closely related hydrides.

3.3

The structure of water

At first sight the concept of a 'structure' for liquid water appears strange. In the solid state atoms are relatively fixed in space, albeit with some vibrational motion about equilibrium positions, and no difficulty is associated with the idea of locating these equilibrium positions by some appropriate physical technique, and thereby assigning a structure to the solid.

3.3.1

The concept of structure in the liquid state

With water or any other liquid, molecules do not occupy even reasonably fixed locations but have considerably more freedom for movement than in the solid state. What then do we mean by the term structure applied to a liquid? To answer this question we need to consider the kind of physical techniques that are used to study the solid state. The main ones are based on diffraction, which may be of electrons, neutrons or X-rays (Moore, 1972; Franks, 1983). In all cases exposure of a crystalline solid to a beam of the particular type gives rise to a well-defined diffraction pattern, which by appropriate mathematical techniques can be interpreted to give information about the structure of the solid. When a liquid such as water is exposed to X-rays, electrons or neutrons, diffraction patterns are produced, though they have much less regularity and detail; it is also more difficult to interpret them than for solids. Such results are taken to show that liquids do, in fact, have some kind of long-range order which can justifiably be referred to as a 'structure'. In considering the structure of a liquid, two possible conceptual approaches exist. One is to begin from an understanding of the gaseous 34

The structure of water state, characterized as it is by gross translational movement of the constituent molecules and substantial disorder. The liquid is then viewed as a gas that has been condensed and in which translational motion has become constrained. Alternatively, consideration can start from the solid state, with its well-characterized structure, having little or no translational motion, but some vibrational motion of the constituent atoms or molecules. The liquid state is then viewed as a solid in which some degree of translational motion has become allowed, but with a structure still recognizable as being derived from that existing in the solid state (Franks, 1983). With the growth in application of the techniques of X-ray and neutron diffraction to the study of the liquid state, the latter approach has become increasingly favoured in recent years. In this section, rather than give a detailed account of theories of the liquid state, a more qualitative approach is adopted. What follows includes first a description of the structure of ice; then from that starting-point, ideas concerning the structure of liquid water are explained. 3.3.2

The structures of ice

Water is capable of solidifying into a number of different structural states or polymorphs depending, for example, on the external pressure applied during solidification. The simplest and most common of these polymorphs is known as ice I, whose structure was first determined by W. H. Bragg (1922). In this structure, every oxygen atom occupies the centre of a tetrahedron formed by four oxygen atoms, each about 0-276 nm away. The water molecules are connected together by hydrogen bonds, each molecule being bonded to its four nearest neighbours. The O-H bonds of a given molecule are oriented towards the lone pairs on two of these neighbouring molecules, and in turn, each of its lone pairs is directed towards an O-H bond of one of the other neighbours. This arrangement gives an open lattice in which intermolecular cohesion is large. The arrangement of oxygen atoms in ice I is isomorphous with the wurtzite form of zinc sulphide, and also with the silicon atoms in the tridymite form of silicon dioxide. Hence, ice I is sometimes referred to as the wurtzite or tridymite form of ice (Eisenberg & Kauzmann, 1969). Location of the hydrogen atoms in ice I has caused more problems. This is because hydrogen is less effective at scattering X-rays or electrons than oxygen. For a long time, arguments about the position of hydrogen were based on indirect evidence, such as vibrational spectra or estimates of 35

Water and acid-base cements residual entropy at 0 K (Eisenberg & Kauzmann, 1969). Since the advent of neutron diffraction the positions of the hydrogen atoms have become clearer. These studies have shown that the water molecules have very similar dimensions in ice I to those in the isolated molecule: the O-H bond length is 0-101 nm and the bond angle 104*5°. Ice I is one of at least nine polymorphic forms of ice. Ices II to VII are crystalline modifications of various types, formed at high pressures; ice VIII is a low-temperature modification of ice VII. Many of these polymorphs exist metastably at liquid nitrogen temperature and atmospheric pressure, and hence it has been possible to study their structures without undue difficulty. In addition to these crystalline polymorphs, socalled vitreous ice has been found within the low-temperaturefieldof ice I. It is not a polymorph, however, since it is a glass, i.e. a highly supercooled liquid. It is formed when water vapour condenses on surfaces cooled to below -160°C. It is not appropriate in this chapter to give a detailed review of the solidstate behaviour of water in its various crystalline modifications. However, there are some general structures which are relevant and worth highlighting. Firstly, water molecules in these various solids have dimensions and bond angles which do not differ much from those of an isolated water molecule. Secondly, the number of nearest neighbours to which each individual molecule is hydrogen-bonded remains four, regardless of the ice polymorph. The differences in structure between the polymorphs, particularly the high-pressure ones, lie in (a) the distances between the non-hydrogen bonded molecules, and hence the amount of' free volume' in the structure, (b) the angles of the hydrogen bonds, which may differ markedly from the 180° of ice I, and (c) the distance between nearest neighbouring oxygen atoms, which may fall to well below the 0-276 nm value in ice I. All of these are consistent with closer packing of the water molecules, and a closing up of the cage structure by comparison with that found for ice I. 3.3.3

Liquid water

Before considering the details of the structure of liquid water, it is important to define precisely what is meant by the term structure as applied to this liquid. If we start from ice I, in which molecules are vibrating about mean positions in a lattice, and apply heat, the molecules vibrate with greater energy. Gradually they become free to move from their original 36

The structure of water lattice sites and acquire significant translational energy. However, translational energy is not confined to molecules in the liquid state. There is a finite possibility of any molecule in ice I moving from its lattice site, thus acquiring translational energy. In principle, a given molecule can move through the solid structure in a process that is essentially diffusion. From this model of ice I we derive three meanings of the term structure for the solid. We may refer to the positions of the molecules at an instant of time. We may allow some averaging of the positions, i.e. we may have a vibrationally averaged structure, considered over a short time-period, during which molecules have time to undergo only minor vibrational reorientations. Finally we may have a diffusionally averaged structure, considered over longer time-periods, in which the minor translational motion has been allowed to proceed to such an extent as to be significant. These three possible structures, the instantaneous, the vibrationally averaged and the diffusionally averaged, are referred to as I-, V- and Dstructures respectively. Let us now turn our attention to liquid water. Just as in ice I, molecular motions may be divided into rapid vibrations and slower diffusional motions. In the liquid, however, vibrations are not centred on essentially fixed lattice sites, but around temporary equilibrium positions that are themselves subject to movement. Water at any instant may thus be considered to have an I-structure. An instant later, this I-structure will be modified as a result of vibrations, but not by any additional displacements of the molecules. This, together with the first I-structure, is one of the structures that may be averaged to allow for vibration, thereby contributing to the V-structure. Lastly, if we consider the structure around an individual water molecule over a long time-period, and realize that there is always some order in the arrangement of adjacent molecules in a liquid even over a reasonable duration, then we have the diffusionally averaged D-structure. No experimental technique exists for determining I-structures in either the liquid or the solid state. Techniques do exist for obtaining information on both the V- and D-structures of liquid water; the results of applying these techniques are considered next. Spectroscopic studies have established that for liquid water, the Vstructure has the following features. (a) Considerable local variation between the environments of the individual water molecules, compared with the relatively uniform 37

Water and acid-base cements molecular environments in a crystal of ice I. The frequency spans of the uncoupled O-H and O-D spectral bands indicate that some nearest neighbours are as close as 0-275 nm, while others are separated by 0.310 nm or more. The most probable equilibrium separation is about 0.285 nm (Eisenberg & Kauzmann, 1969). (b) The differences between the various molecular environments are continuous. In other words, the V-structure does not contain discrete types of molecular environment. (c) The frequency of the stretching band indicates that hydrogen bonds in the V-structure are weaker than those in ice I, though still distinctly present. Ideas about the D-structure have come mainly from two sources, namely a consideration of the underlying reasons for the values of certain physical properties, such as heat capacity or compressibility, and a study of radial distribution functions that arise from X-ray diffraction work on liquid water. The D-structure represents the average arrangement of molecules around an arbitrary central water molecule. This average is either the 'space average' for several central molecules in different V-structures, or the 'time average' for a single molecule over very long periods of time. Near the freezing point, the D-structure is found to have relatively high concentrations of neighbours at distances 0-29, 0-50 and 0*70 nm from the central water molecule. This suggests that a substantial hydrogen-bonded network is discernible, even in the liquid state. As the temperature is raised, so the distinct concentrations at 0-50 and 0-70 nm disappear. Thermal agitation thus distorts or destroys the hydrogen-bonded networks, and the amount of observable long-range order decreases significantly. Structural studies on liquid water reveal that the majority of molecules are effectively tetrahedral, since the O-H bonds and the lone pairs are used in hydrogen-bonding. Questions remain about the nature of these hydrogen-bonds (Symons, 1989). Specifically: on average, how many such hydrogen bonds are formed per molecule, how strong and how linear are they, and what is their lifetime? One recent approach has been to consider the possibility that, because of their weakness, some of the hydrogen bonds in liquid water will break. This then gives concentrations of free O-H bonds, OHfree, and free lone pairs, LPfree, on certain molecules which are bonded to only three others (Symons, 1989). Symons (1989) also suggests that the chemical properties of liquid water depend on the relative concentrations of these species. Fully hydrogen-bonded water can be 38

The structure of water considered as inert; reactions requiring attack by O-H depend on the concentration of OHfree molecules; and those requiring nucleophilic attack by lone pairs depend on the concentration of LPfree molecules. Evidence for OHfree and LPfree molecules has been obtained spectroscopically using monomeric deuterated water, HOD, in inert solvents such as dimethyl sulphoxide, though debate continues over interpretation of the results obtained in such studies. An important phenomenon when considering the differences between ice I and liquid water is that water achieves its maximum density not in the solid state, but at 4 °C, i.e. in the liquid state. The reasons for this were first discussed by Bernal & Fowler (1933). They noted that the separation of molecules in ice I is about 0-28 nm, corresponding to an effective molecular radius of 014 nm. Close packing of molecules of such radius would yield a substance of density 1*84 g cm"3. To account for the observed density of 10 g cm"3, it was necessary to postulate that the arrangement of molecules was very open compared with the disordered, close-packed structures of simple liquids such as argon and neon. The increase in density on melting is assumed to arise from two competing effects that occur as water is heated. First, increasing translational freedom for the water molecules weakens the hydrogen-bonded network that exists in ice I. This network thus collapses, and reduces the volume. Second, increased vibrational energy for the molecules causes an effective increase in the volume occupied by any one molecule, thus enlarging the overall volume of the liquid. The first effect is considered to predominate below 4 °C, the second above 4 °C. Overall, the main conclusions that are to be drawn concerning the structure of liquid water are as follows. (a) Water has a degree of long-range order that is appropriately described as structure; it is possible to measure detailed parameters for either a vibrationally averaged or a diffusionally averaged structure. (b) The force between molecules that sustains this order in the liquid state is the hydrogen bond. (c) The bond lengths and angles of individual water molecules are almost independent of whether they occur in ice I or liquid water.

39

Water and acid-base cements 3.4

Water as a solvent

The general criterion for solubility is the rule that 'like dissolves like'. In other words polar solvents dissolve polar and ionic solutes, non-polar solvents dissolve non-polar solutes. In the case of water, this means that ionic compounds such as sodium chloride and polar compounds such as sucrose are soluble, but non-polar compounds such as paraffin wax are not. In general, solubility depends on the relative magnitudes of three pairs of interactions, namely solute-solute, solvent-solvent and solute-solvent (Robb, 1983). For a substance to be soluble in a given liquid, the solute-solvent interactions must be greater than or equal to the other two interactions. Insolubility does not only result from the kind of energetic considerations outlined above. It can also be the result of essentially kinetic barriers. For example, the naturally occurring macromolecule cellulose is not soluble in water, yet its monomer, D( + )-glucose is extremely watersoluble (Morrison & Boyd, 1973). This is because cellulose adopts a wellordered structure, in which individual hydroxyl groups are aligned via hydrogen bonds; the overall structure simply has too great an integrity to allow water molecules to enter and hydrate the individual molecules in order to carry them off into solution. 3.4.1

Hydrophobic interactions

The qualitative discussion of solubility has focussed so far on the attractive forces in solute-solvent interactions. However, where water is concerned, it is also important to consider the forces of repulsion due to the so-called 'hydrophobic' interactions that may arise in certain cases (Franks, 1975). These hydrophobic interactions may be explained in terms of thermodynamic concepts. Measuring enthalpy changes for the dissolution of hydrocarbons, such as alkanes, in water shows that heat is evolved, i.e., AH is negative and energetically water and alkanes attract each other. However, such attraction does not make alkanes soluble in water to any appreciable extent. This is because the free energy change AGsolution opposes the process and is positive. From the Gibbs equation, Absolution — Absolution

40

1 Absolution

Water as a solvent it follows that the ^ASsolution term (and hence A*Ssolution itself) must be negative. This means that the proposed solution has lower entropy and is more ordered than pure water, which is a striking conclusion, since entropy is usually increased by mixing. It occurs because the relatively ordered structure of the liquid water, based as it is on a hydrogen-bonded array of water molecules, actually becomes more ordered when alkane molecules enter it. This result is attributed to the formation of a 'cage' structure of water molecules around the non-polar alkane molecule, in which water has less vibrational and translational freedom than in the pure liquid (Franks, 1983). In cases where the solvation energies are large, as for example when ionic compounds dissolve in water, these hydrophobic effects, based on adverse changes in entropy, are swamped. Dissolving such compounds can be readily accomplished due to the very large energies released when the ions become hydrated.

3.4.2

Dissolution of salts

Salts dissolve in water with dissociation of the constituent ions, this concept having been proposed originally by S. Arrhenius in 1887. His first idea was that all salts, including those of what would now be regarded as weak acids or bases, are completely dissociated at extreme dilution (Hall, 1985). It was eventually realized that substances such as NaCl, KC1, etc, are effectively completely dissociated at all concentrations. Dissolution of an ionic salt is essentially a separation process carried out by the interaction of the salt with water molecules. The separation is relatively easy in water because of its high dielectric constant. Comparison of the energies needed to separate ions of NaCl from 0-2 nm to infinity shows that it takes 692-89 kJ mol"1 in vacuum, but only 8-82 kJ mol"1 in aqueous solution (Moore, 1972). Similar arguments have been used to try to estimate solvation energies of ions in aqueous solution, but there are difficulties caused by the variations in dielectric constant in the immediate vicinity of individual ions. In order to dissolve ionic solutes so readily, water molecules must solvate the ions as they enter solution. Consequently, water molecules lose their translational degrees of freedom as a result of their association with specific ions. It is possible to estimate the number of water molecules in clusters of the type M+ (H2O)W using mass spectrometry (Kebarle, 1977). 41

Water and acid-base cements The number of water molecules in such a cluster, the hydration number, varies with ionic size; it is four for Li+, three for Na+, but only one for Rb+. Mass spectrometry has been used to study the energetics of solvation and has shown that the enthalpies of attachment of successive water molecules to either alkali metal or halide ions become less exothermic as the number of water molecules increases (Kebarle, 1977). The Gibbs free energies of attachment for water molecules have also been found to be negative. The different hydration numbers can have important effects on the solution behaviour of ions. For example, the sodium ion in ionic crystals has a mean radius of 0-095 nm, whereas the potassium ion has a mean radius of 0133 nm. In aqueous solution, these relative sizes are reversed, since the three water molecules clustered around the Na+ ion give it a radius of 0-24 nm, while the two water molecules around K+ give it a radius of only 0-17 nm (Moore, 1972). The presence of ions dissolved in water alters the translational freedom of certain molecules and has the effect of considerably modifying both the properties and structure of water in these solutions (Robinson & Stokes, 1955). The precise orientation of water molecules around cations is not clear, though two models have been proposed for the possible structures that occur (Vaslow, 1963). In one, the water molecules are arranged so that the dipole moments are aligned with the centres of the ions. In the other, water molecules are arranged so that interaction between the lone-pair orbitals on the oxygen atoms and orbitals on the cation is maximized. This latter model is supported by molecular dynamics calculations (Heinzinger & Palinkas, 1987; Heinje, Luck & Heinzinger, 1987). Less uncertainty surrounds the structure of hydrated anions: the hydrogen atoms are almost collinear with the oxygen atoms and the centres of the ions (Briant & Burton, 1976). Monte-Carlo calculations have shown that F~ is surrounded by four hydrogen atoms each 017 nm away (Watts, Clementi & Fromm, 1974). Neutron scattering has been used for studying the state of solvation of ions in aqueous solution (Enderby et ai, 1987; Salmon, Neilson & Enderby, 1988). These studies have shown that a distinct shell of water molecules of characteristic size surrounds each ion in solution. This immediate hydration shell was called zone A by Frank & Wen (1957); they also postulated the existence of a zone B, an outer sphere of molecules, less firmly attached, but forming part of the hydration layer around a given ion. The evidence for the existence of zone B lies in the thermodynamics of 42

Water as a solvent

the hydration process, and may be appreciated by considering the isoelectronic species KCl and two moles of argon. The standard enthalpy of hydration is significantly more exothermic for KCl than for the two moles of argon; however, the corresponding entropy of hydration is less for KCl than for argon. The results for the values of enthalpy can be readily understood in terms of the greater intensity of the interactions between water molecules and the ions of KCl than between those of water molecules and the uncharged argon atoms. At first sight the greater loss of freedom in the water molecules involved in hydrating the ions of KCl would be expected to reduce disorder in such solutions. In other words the entropy of hydration for KCl ought to be greater than the entropy of hydration for two moles of argon. To explain the fact that the opposite is found experimentally, Frank & Evans (1945) suggested that there is a compensating gain in entropy which can be attributed to disruptions in the water-water interactions within zone B. As a result of these electrostatic effects aqueous solutions of electrolytes behave in a way that is non-ideal. This non-ideality has been accounted for successfully in dilute solutions by application of the Debye-Huckel theory, which introduces the concept of ionic activity. The Debye-Huckel limiting law states that the mean ionic activity coefficient y± can be related to the charges on the ions, z+ and z_, by the equation Iog 10 y ± =-0-509z + z_ Ionic activity essentially represents the concentration of a particular type of ion in aqueous solution and is important in the accurate formulation of thermodynamic equations relating to aqueous solutions of electrolytes (Barrow, 1979). It replaces concentration because a given ion tends not to behave as a discrete entity but to gather a diffuse group of oppositely charged ions around it, a so-called ionic atmosphere. This means that the effective concentration of the original ion is less than its actual concentration, a fact which is reflected in the magnitude of the ionic activity coefficient. Debye-Hiickel theory assumes complete dissociation of electrolytes into solvated ions, and attributes ionic atmosphere formation to long-range physical forces of electrostatic attraction. The theory is adequate for describing the behaviour of strong 1:1 electrolytes in dilute aqueous solution but breaks down at higher concentrations. This is due to a chemical effect, namely that short-range electrostatic attraction occurs 43

Water and acid-base cements either between ion-pairs or between solvent-separated ion-pairs (Russo & Hanania, 1989); this effect becomes important in concentrated aqueous solutions of the type used to form AB cements. 3.4.3

Ion-ion interactions in water

Two ions are particularly important in the chemistry of water, namely H+ and OH~ (Clever, 1963). Hydrogen ions do not exist as discrete entities. This is because ionization of the hydrogen atom leaves behind a proton, which is very small compared with a typical ion. Thus the local charge density developed around the proton is very high. The polarizing effect of such a high charge density is such that the resulting system is simply too unstable to form to any detectable extent in aqueous solutions. When water undergoes self-ionization, a range of cationic species are formed, the simplest of which is the hydronium ion, H3O+ (Clever, 1963). This ion has been detected experimentally by a range of techniques including mass spectrometry (Cunningham, Payzant & Kebarle, 1972), as have ions of the type H+ (H2O)W with values of n up to 8. Monte-Carlo calculations show that H3O+ ions exist in hydrated clusters surrounded by three or four water molecules in the hydration shell (Kochanski, 1985). These ions have only a short lifetime, since the proton is highly mobile and may be readily transferred from one water molecule to another. The time taken for such a transfer is typically of the order of 10~14 s provided that the receiving molecule of water is correctly oriented. Several other discrete species have been found to arise from the selfionization of water. These include H5Og (Kearley, Pressman & Slade, 1986), H4O2+ (Bollinger et al., 1987), H9O+ (Robinson, Thistlethwaite & Lee, 1986) and hydrated electrons (Hart & Anbar, 1970). Intense ion-ion interactions which are characteristic of salt solutions occur in the concentrated aqueous solutions from which AB cements are prepared. As we have seen, in such solutions the simple Debye-Huckel limiting law that describes the strength goes up so the repulsive force between the ions becomes increasingly important. This is taken account of in the full Debye-Hiickel equation by the inclusion of a parameter related to ionic size and hence distance of closest approach (Marcus, 1988). For concentrated solutions, there are approaches that are more sophisticated than that of Debye & Hiickel. A particularly successful method of describing such solutions is that due to McMillan & Mayer (1945) which has subsequently been developed by Ramanathan & 44

Water as a solvent Friedman (1971). This approach is described as the hypernetted chain procedure and in it ion-ion pair potentials are expressed as the sum of four terms. These are: COUL U , the charge-charge interactions between two ions, / and j , as a function of their separation, COR U , a repulsive core potential term, CAVU, arising from the dielectric cavity effect, and GUR U , the so-called Gurney potential (Gurney, 1953), which describes the effect of co-sphere overlap. Using this approach, calculations can be made of volumetric, entropic and energy parameters taking account of the effect of overlapping cospheres. Some indication of the organization in the solution is also possible. The properties of a number of concentrated salt solutions have been analysed by this procedure, including simple 1:1 salts, alkaline earth salts and alkylammonium salts. A number of other attempts have been made to account for the properties of concentrated aqueous solutions of ionic compounds by procedures that represent further improvements on the simple DebyeHiickel approach. However, they lie outside the scope of the present chapter. The important point to emphasize is that the concentrated aqueous solutions that are generally employed in the preparation of AB cements tend to exhibit significant ion-ion interactions; such interactions lead to significant deviations from ideality which may be accounted for by substantial extension of the ideas of simple dilute solution theory. 3.4.4

Dissolution of polymers

AB cements are not only formulated from relatively small ions with well defined hydration numbers. They may also be prepared from macromolecules which dissolve in water to give multiply charged species known as polyelectrolytes. Cements which fall into this category are the zinc polycarboxylates and the glass-ionomers, the polyelectrolytes being poly(acrylic acid) or acrylic acid copolymers. The interaction of such polymers is a complicated topic, and one which is of wide importance to a number of scientific disciplines. Molyneux (1975) has highlighted the fact that these substances form the focal point of 'three complex and contentious territories of science', namely aqueous systems, ionic systems and polymeric systems. 45

Water and acid-base cements In polyelectrolytes, the ionic charges are carried by groups which are themselves attached covalently to the macromolecular backbone. When all of the groups are negatively charged, as with polyacrylate, the polyelectrolyte that results is referred to as a polyanion. Polyelectrolytes are of high solubility in water, especially when compared to most organic macromolecules. This increased solubility may be attributed not only to the more favourable interactions between the charged groups and the water molecules, but also to the fact that entropy strongly favours dissolution and dissociation of these molecules (Molyneux, 1975). The conformations adopted by polyelectrolytes under different conditions in aqueous solution have been the subject of much study. It is known, for example, that at low charge densities or at high ionic strengths polyelectrolytes have more or less randomly coiled conformations. As neutralization proceeds, with concomitant increase in charge density, so the polyelectrolyte chain uncoils due to electrostatic repulsion. Eventually at full neutralization such molecules have conformations that are essentially rod-like (Kitano et ai, 1980). This rod-like conformation for poly(acrylic acid) neutralized with sodium hydroxide in aqueous solution is not due to an increase in stiffness of the polymer, but to an increase in the so-called excluded volume, i.e. that region around an individual polymer molecule that cannot be entered by another molecule. The excluded volume itself increases due to an increase in electrostatic charge density (Kitano et al.9 1980). In a study of the transition in conformation from random coil to stiff rod by poly(acrylic acid), it was found that the point of transition depended on a number of factors, including the nature of the solvent, the temperature, the particular counterion used and the degree of dissociation (Klooster, van der Trouw & Mandel, 1984). Methanol was used in this study, though in terms of Flory-Huggins solution theory it is not a good solvent for poly(acrylic acid) at room temperature. In other words, the polymer adopts a tightly coiled conformation that excludes solvent, thereby approaching the point of precipitation. This phenomenon may be responsible for the observation that the addition of methanol to poly(acrylic acid) solutions intended for use in glass-ionomer cements prevents gelation (Wilson & Crisp, 1974). This was originally attributed to methylation of the polymer, leading to a reduction in the stereoregularity of the poly(acrylic acid) and hence to a lessening of the readiness with which stable hydrogen-bonded links could be formed. However, there is the alternative possibility that the presence of 46

Hydration in the solid state methanol altered the conformation of the polymer and that this conformational change prevented the development of the hydrogen-bonded network necessary for gelation to occur. 5.5

Hydration in the solid state

Many ionic compounds contain what used to be referred to as 'water of crystallization'. For example, magnesium chloride can exist as a fully hydrated salt which was formerly written MgCl2. 6H2O, but is more appropriately written Mg(OH2)6Cl2, since the water molecules occupy coordination sites around the magnesium ions. This is typical. In most compounds that contain water of crystallization, the water molecules are bound to the cation in an aquo complex in the manner originally proposed by Alfred Werner (1866-1919) in 1893 (Kauffman, 1981). Such an arrangement has been confirmed in numerous cases by X-ray diffraction techniques. 3.5.1

Coordination of water to ions

The ions that tend to be involved in AB cements include such species as Al3+, Mg2+, Ca2+ and Zn2+. These are all capable of developing a coordination number of six, and hexaquo cations are known to be formed by each of these metal ions (Hiickel, 1950). The typical requirements for an ion to develop such coordination characteristics are that the ion should exist in the + 2 or +3 oxidation state, and in this state should be of small ionic radius (Greenwood & Earnshaw, 1984). Another feature of the metal ions that are typically involved in cementitious bonding in AB cements is that most of them fall into the category of hard in Pearson's Hard and Soft Acids and Bases scheme (Pearson, 1963). The underlying principle of this classification is that bases may be divided into two categories, namely those that are polarizable or soft, and those that are non-polarizable or hard. Lewis acids too may be essentially divided into hard and soft, depending on polarizability. From these classifications emerges the useful generalization that hard acids prefer to associate with hajd bases and soft acids prefer to associate with soft bases (see Section 2.3.7). Of the ions most often implicated in cementitious bonding in AB cements, Ca2+, Mg2+ and Al3+ are classified as clearly hard; Zn2+ by contrast falls into the category that Pearson designated 'borderline', as 47

Water and acid-base cements does Cu2+ (Pearson, 1963). This means that most of these ions form particularly stable complexes with hard bases, i.e. those which are not readily polarizable. This requirement demands that the bases be those of first row elements, such as oxygen or nitrogen. Water is thus a hard base, and the complex that it forms with these ions involves coordination by the oxygen atoms. As predicted by the Hard and Soft Acids and Bases concept, the aquo complexes of the cement-forming metal ions are extremely stable and do not readily lose their coordinated water. Hence, one of the functions of water in fully set AB cements is coordination to the metal ions. 3.6

The role of water in acid-base cements

Water has three possible roles in acid-base cements. First, it acts as the medium for the setting reaction of these materials, and second, it is one of the components of the set cement, actually becoming incorporated into the cement as it hardens. Third, water may act as plasticizer in these cements. All of these roles are reviewed here. 3.6.1

Water as solvent in AB cements

Water as the solvent is essential for the acid-base setting reaction to occur. Indeed, as was shown in Chapter 2, our very understanding of the terms ' acid' and' base', at least as established by the Bronsted-Lowry definition, requires that water be the medium of reaction. Water is needed so that the acids may dissociate, in principle to yield protons, thereby enabling the property of acidity to be manifested. The polarity of water enables the various metal ions to enter the liquid phase and thus react. The solubility and extent of hydration of the various species change as the reaction proceeds, and these changes contribute to the setting of the cement. 3.6.2

Water as a component of AB cements

Water is also a component of set AB cements. In glass-ionomer cements, for example, it may serve to coordinate to certain sites around the metal ions. It also hydrates the siliceous hydrogel that is formed from the glass after acid attack has liberated the various metal ions (Wilson & McLean, 1988). Such reactions continue long after the initial hardening of the cement is complete, and for this reason water must be retained as far as possible during the first hours and days after formation of the cement. If water is lost from the cement and desiccation occurs, these post-hardening 48

The role of water in acid-base cements reactions stop, and the cement will not achieve maximum possible strength. Moreover, if desiccation is excessive, the material will also shrink and crack. Water occurs in glass-ionomer and related cements in at least two different states (Wilson & McLean, 1988; Prosser & Wilson, 1979). These states have been classified as evaporable and non-evaporable, depending on whether the water can be removed by vacuum desiccation over silica gel or whether it remains firmly bound in the cement when subjected to such treatment (Wilson & Crisp, 1975). The alternative descriptions 'loosely bound' and 'tightly bound' have also been applied to these different states of water combination. In the glass-poly(acrylic acid) system the evaporable water is up to 5 % by weight of the total cement, while the bound water is 18-28 % (Prosser & Wilson, 1979). This amount of tightly bound water is equivalent to five or six molecules of water for each acid group and associated metal cation. Hence at least ten molecules of water are involved in the hydration of each coordinated metal ion at a carboxylate site. It has been suggested by Ikegami (1968) that the carboxylate groups of a polyacrylate chain are each surrounded by a primary local sphere of oriented water molecules, and that the polyacrylate chain itself is surrounded by a secondary sheath of water molecules. This secondary sheath is maintained as a result of the cooperative action of the charged functional groups on the backbone of the molecule. The monovalent ions Li+, Na+ and K+ are able to penetrate only this secondary hydration sheath, and thereby form a solvent-separated ion-pair, rather than a contact ion-pair. Divalent ions, such as Mg2+ or Ba2+, cause a much greater disruption to the secondary hydration sheath. The effectiveness with which divalent ions cause gelation of poly(acrylic acid) has been found to follow the order Ba2+ > Sr2+ > Ca2+ (Wall & Drenan, 1951) and this has been attributed to the formation of salt-like crosslinks. Gelation has been assumed to arise in part from dehydration of the ion-pairs (Ikegami & Imai, 1962), and certainly correlates with precipitation in fairly dilute systems. Indeed, the term precipitation has sometimes been applied to the setting of AB cements derived from poly(acrylic acid) as they undergo the transition from soft manipulable paste to hard brittle solid. At the molecular level, a number of features are associated with the phenomenon of gelation or precipitation. In particular the disruption of the secondary hydration sheaths around the polyacrylate chains appears 49

Water and acid-base cements important (Prosser & Wilson, 1979). In glass-ionomer cements the bound water has been assumed to be associated with the intrinsic water spheres around the carboxylate anion-metal cation units, while evaporable water is associated with the secondary hydration sheath around the polyacrylate chain. As these cements age, the ratio of tightly bound to loosely bound water increases. This is accompanied by an increase in strength and modulus of the cement and by a decrease in plasticity (Paddon & Wilson, 1976; Wilson, Crisp & Paddon, 1981). The loosely bound water in glass-ionomer cements is labile, and is easily lost or gained. Indeed, such cements are stable only in an atmosphere of 80% relative humidity (Hornsby, 1980). In higher humidities the cements absorb water and the resulting hydroscopic expansion can exceed the shrinkage that usually accompanies setting, which is a distinct clinical advantage for the use of these cements in dentistry. By contrast, the cement can lose water under drying conditions leading to shrinking, crazing and failure to develop full strength. Glass-ionomer cements become less susceptible to desiccation as they age, because a greater proportion of the water in older cements has become 'tightly bound'. Early contact with moisture is also damaging, and this problem is overcome clinically to some extent by using some sort of protection such as clear nail varnish to seal the cement during its early life (Wilson & McLean, 1988). However, this does not give perfect results, and as yet there is no ideal barrier material for this purpose (Earl, Hume & Mount, 1985). The role of water in dental silicate cements was studied by Wilson et al. (1970), and they found that the properties of these materials including setting time, compressive strength and resistance to attack by water and acids were markedly affected by the amount of water in the original cement paste. Water in these materials was also found to fall into the two categories of evaporable and non-evaporable. In this case non-evaporable water was defined as that water remaining in the cement after heating at 105 °C for 24 hours. With increasing acid concentration (i.e. decreasing amounts of water in the initial cement paste), the amount of nonevaporable water went down, until at 75 % phosphoric acid concentration nearly all of the water in the cement was found to be evaporable. Moreover the cements containing almost no non-evaporable water were found to be extremely weak. Hence the non-evaporable (bound) water could be equated with bonding water. Infrared analysis had previously shown that non-bonding water was associated with the water-soluble hydrated salt 50

The role of water in acid-base cements sodium dihydrogen phosphate, NaH 2 PO 4 .H 2 O (Wilson & Mesley, 1968). The presence of this compound was known to have a deleterious effect on cement properties and the water associated with it was known to be readily removed. Hence in cements prepared from aqueous solutions having high concentrations of phosphoric acid this salt was assumed to be present in quantity and to be responsible for the relatively high levels of evaporable water (Wilson et aL, 1970). A series of AB cements can be prepared from aqueous solutions of oxides and halides (or sulphates) of magnesium or zinc. These cements are described in detail in Chapter 7. For the moment we will confine our discussion to a consideration of the role of water in these cements. In the cements of this type a number of phases are known to be present. For example, in the zinc oxychloride cement two discrete phases, corresponding to the composition ZnO. ZnCl 2 . H 2 O in the ratios 4:1:5 and 1:1:2 respectively, are known to occur (Sorrell, 1977). Similarly, in the magnesium oxychloride cement, phases corresponding to Mg(OH) 2 . MgCl 2 . H 2 O in the ratios 5:1:8 and 3:1:8 have been shown to exist and have been studied by X-ray diffractometry (Sorrell & Armstrong, 1976). The precise structural role played by the water molecules in these cements is not clear. In the zinc oxychloride cement, water is known to be thermally labile. The 1:1:2 phase will lose half of its constituent water at about 230 °C, and the 4:1:5 phase will lose water at approximately 160 °C to yield a mixture of zinc oxide and the 1:1:2 phase. Water clearly occurs in these cements as discrete molecules, which presumably coordinate to the metal ions in the cements in the way described previously. However, the possible complexities of structure for these systems, which may include chlorine atoms in bridging positions between pairs of metal atoms, make it impossible to suggest with any degree of confidence which chemical species or what structural units are likely to be present in such cements. One is left with the rather inadequate chemical descriptions of the phases used in even the relatively recent original literature on these materials, from which no clear information on the role of water can be deduced. 3.6.3

Water as plasticizer

An additional possible role for water in AB cements is plasticization. Water is known to act as plasticizer for a number of polymeric materials, whether synthetic or natural, and whether or not they are predominantly 51

Water and acid-base cements polar. Thus water has been found to affect the properties of poly(methyl methacrylate) (Turner, 1982) and of alkyd resins used in surface coatings (Mayne & Mills, 1982). As plasticizer, the principal effect of water in these systems is to reduce the glass transition temperature, Tg, and this in turn affects a number of other properties of the materials, including rigidity, dimensional stability and diffusion coefficients within the bulk. Given the polar nature of the components of AB cements, the known water content of set cements, and the fact that water has been shown to act as plasticizer in pure metal poly(acrylate) salts (Yokoyama & Hiraoko, 1979) it seems probable that one of the roles of water in the solid state of AB cements is that of plasticizer. References Barrow, G. M. (1979). Physical Chemistry, 4th edn. Tokyo: McGraw-Hill Kogakusha Ltd. Benedict, W. S., Gailar, N. & Plyler, E. K. (1956). Rotation-vibration spectra of deuterated water vapour. Journal of Chemical Physics, 24, 1139-65. Bernal, J. D. & Fowler, R. H. (1933). A theory of water and ionic solutions with particular reference to hydrogen and hydroxyl ions. Journal of Chemical Physics, 1, 515-48. Bollinger, J. C , Faure, R., Yvernault, T. & Stahl, D. (1987). On the existence of the protonated dication H4 O2+ in sulfolane solution. Chemical Physics Letters, 140, 579-81. Bragg, W. H. (1922). The crystal structure of ice. Proceedings of the Physical Society of London, 34, 98-103. Briant, C. L. & Burton, J. J. (1976). Molecular dynamics study of the effects of ions on water microclusters. Journal of Chemical Physics, 64, 2888-95. Clever, H. L. (1963). The hydrated hydronium ion. Journal of Chemical Education, 40, 637^41. Cunningham, A. J. C , Payzant, J. D. & Kebarle, P. (1972). A kinetic study of the proton hydrate H+(H2O) and equilibria in the gas phase. Journal of the American Chemical Society, 94, 7627-32. Earl, M. S. A., Hume, W. R. & Mount, G. J. (1985). Effect of varnishes and other surface treatments on water movement across the glass-ionomer cement surface. Australian Dental Journal, 30, 298-301. Eisenberg, D. & Kauzmann, W. (1969). The Structure and Properties of Water. Oxford: Oxford University Press. Enderby, J. E., Cummings, S., Herdman, G. H., Neilson, G. W., Salmon, P. S. & Skipper, N. (1987). Diffraction and study of aqua ions. Journal of Physical Chemistry, 91, 5851-8. Frank, H. S. & Evans, M. W. (1945). Entropy in binary liquid mixtures; partial molal entropy in dilute solutions; structure and thermodynamics in aqueous electrolytes. Journal of Chemical Physics, 13, 507-32. 52

References Frank, H. S. & Wen, W-Y. (1957). Structural aspects of ion-solvent interactions in aqueous solutions-water structure. Discussions of the Faraday Society, 24, 133^0. Franks, F. (1975). The hydrophobic interaction. In Franks, F. Water. A Comprehensive Treatise, vol. 4, Chapter 1. London and New York: Plenum Press. Franks, F. (1983). Water. London: Royal Society of Chemistry. Gillespie, R. J. & Nyholm, R. S. (1957). Inorganic stereochemistry. Quarterly Reviews of the Chemical Society, 11, 339-80. Greenwood, N. N. & Earnshaw, A. (1984). The Chemistry of the Elements. Oxford: Pergamon Press. Gurney, R. W. (1953). Ionic Processes in Solution. New York: McGraw-Hill. Hall, V. M. D. (1985). In Russell, C. A. (ed.) Recent Developments in the History of Chemistry. London: Royal Society of Chemistry. Hart, E. J. & Anbar, M. (1970). The HydratedElectron. New York: Wiley. Heinzinger, K. & Palinkas, G. (1987). In Kleeberg, H. (ed.) Interactions of Water in Non-ionic Hydrates. Berlin: Springer-Verlag. Heinje, G., Luck, W. A. P. & Heinzinger, K. (1987). Molecular dynamics simulation of an aqueous sodium perchlorate solution. Journal of Physical Chemistry, 91, 331-8. Hornsby, P. R. (1980). Dimensional stability of glass-ionomer cements. Journal of Chemical Technology and Biotechnology, 30, 595-601. Hiickel, W. (1950). Structural Chemistry of Inorganic Compounds, vol. 1. New York: Elsevier. Ikegami, A. (1968). Hydration of polyacids. Biopolymers, 6, 431-40. Ikegami, A. & Imai, N. (1962). Precipitation of polyelectrolytes by salts. Journal of Polymer Science, 56, 133-52. Kauffman, G. B. (1981). Coordination Chemistry. New York: Heyden. Kearley, G. J., Pressman, H. A. & Slade, R. C. T. (1986). The geometry of the H5OJ ion in dodecatungstophosphoric acid hexahydrate, (H5O£)3 (PW^O^), studied by inelastic neutron scattering vibrational spectroscopy. Journal of the Chemical Society Chemical Communications, 1801-2. Kebarle, P. (1977). Ion thermochemistry and solvation from gas phase ion equilibria. Annual Reviews in Physical Chemistry, 28, 445-76. Kitano, T., Taguchi, A., Noda, I. & Nagasawa, M. (1980). Conformation of polyelectrolytes in aqueous solution. Macromolecules, 13, 57-63. Klooster, N. Th. M., van der Trouw, F. & Mandel, M. (1984). Solvent effects in polyelectrolyte solutions. 3. Spectropho tome trie results with (partially) neutralised poly(acrylic acid) in methanol and general conclusions regarding these systems. Macromolecules, 17, 2087-93. Kochanski, E. (1985). Theoretical studies of the system H3O+(H2O)n for n = 1—9. Journal of the American Chemical Society, 107, 7869-73.

Marcus, Y. (1988). Ionic radii in aqueous solution. Chemical Reviews, 88, 1475-98. Mayne, J. E. O. & Mills, D. J. (1982). Structural changes in polymer films. Part 1. The influence of the transition temperature on the electrolytic resistance 53

Water and acid-base cements and water uptake. Journal of the Oil and Colour Chemists' Association, 65, 138^2. McMillan, W. G. & Mayer, J. E. (1945). The statistical thermodynamics of multicomponent systems. Journal of Chemical Physics, 13, 276-305. Molyneux, P. (1975). Synthetic polymers. In Franks, F. Water. A Comprehensive Treatise, vol. 4, Chapter 7. London and New York: Plenum Press. Moore, W. J. (1972). Physical Chemistry, 5th edn. London: Longman Group Ltd. Morrison, R. T. & Boyd, R. T. (1973). Organic Chemistry, 3rd edn. New York: Allyn and Bacon. Neilson, G. W., Schioberg, D. & Luck, W. A. P. (1985). The structure around the perchlorate ion in concentrated aqueous solutions. Chemical Physics Letters, 111, 475-9. Nicholson, J. W. (1985). Waterborne Coatings. OCCA Monograph No. 3. London: Oil and Colour Chemists' Association. Paddon, J. M. & Wilson, A. D. (1976). Stress relaxation studies on dental materials. 1. Dental cements. Journal of Dentistry, 4, 183-9. Pauling, L. (1960). The Nature of the Chemical Bond, 3rd edn. Ithaca, New York: Cornell University Press. Pearson, R. G. (1963). Hard and soft acids and bases. Journal of the American Chemical Society, 85, 3533-9. Prosser, H. J. & Wilson, A. D. (1979). Litho-ionomer cements and their technological applications. Journal of Chemical Technology and Biotechnology, 29, 69-87. Ramanathan, P. S. & Friedman, H. L. (1971). Refined model for aqueous 1-1 electrolytes. Journal of Chemical Physics, 54, 1086-99. Robinson, R. A. & Stokes, R. H. (1955). Electrolyte Solutions. London: Butterworth Scientific Publications. Robinson, G. W., Thistlethwaite, P. J. & Lee, J. (1986). Molecular aspects of ionic hydration reactions. Journal of Physical Chemistry, 90, 4224-33. Robb, I. D. (1983). Polymer-small molecule interactions. In Finch, C. A. (ed.) Chemistry and Technology of Water Soluble Polymers. New York: Plenum Press. Russo, S. O. & Hanania, G. I. H. (1989). Ion association solubilities and reduction potentials in aqueous solution. Journal of Chemical Education, 66, 148-53. Salmon, P. S., Neilson, G. W. & Enderby, J. E. (1988). The structure of Cu2+ aqueous solutions. Journal of Physics C, 21, 1335-49. Sorrell, C. A. & Armstrong, C. R. (1976). Reactions and equilibria in magnesium oxychloride cements. Journal of the American Ceramic Society, 59, 51-4. Sorrell, C. A. (1977). Suggested chemistry of zinc oxychloride cements. Journal of the American Ceramic Society, 60, 217-20. Speakman, J. C. (1975). The Hydrogen Bond. London: Chemical Society. Symons, M. C. R. (1989). Liquid water- the story unfolds. Chemistry in Britain, 25, 491-4. 54

References Turner, D. T. (1982). Poly(methyl methacrylate) plus water. Sorption kinetics and volumetric changes. Polymer, 23, 197-202. Vaslow, F. (1963). The orientation of water molecules in the field of an alkali ion. Journal of Physical Chemistry, 67, 2773-6. Wall, F. T. & Drenan, J. W. (1951). Gelation of polyacrylic acid by divalent ions. Journal of Polymer Science, 1, 83-8. Watts, R. O., Clementi, E. & Fromm, J. (1974). Theoretical study of the lithium fluoride molecule in water. Journal of Chemical Physics, 61, 2550-5. Weast, R. C. (ed.) (1985-6). Handbook of Physics and Chemistry. Ohio: Chemical Rubber Company. Wilson, A. D. & Crisp, S. (1974). Unpublished data cited in Wilson & McLean, 1988. Wilson, A. D. & Crisp, S. (1975). Ionomer cements. British Polymer Journal, 1, 279-96. Wilson, A. D., Crisp, S. & Paddon, J. M. (1981). The hydration of a glass-ionomer (ASPA) cement. British Polymer Journal, 13, 66-70. Wilson, A. D., Kent, B. E., Batchelor, R. F., Scott, B. G. & Lewis, B. G. (1970). Dental silicate cements. XII. The role of water. Journal of Dental Research, 49, 307-14. Wilson, A. D. & McLean, J. W. (1988). Glass-ionomer Cement. Chicago, London, etc.: Quintessence Publishers. Wilson, A. D. & Mesley, R. J. (1968). Dental silicate cements. VI. Infrared studies. Journal of Dental Research, 47, 644—52. Yokoyama, T. & Hiraoko, K. (1979). Hydration and thermal transition of poly(acrylic acid) salts. Polymer Preprints of the American Chemical Society, Division of Polymer Chemistry, 20, 511-13.

55

4

Polyelectrolytes, ion binding and gelation

4.1

Polyelectrolytes

4.1.1

General

The setting of AB cements is an example of gelation, and gelation is related to ion binding. A theoretical examination of the various phenomena associated with ion binding and gelation finds its clearest exposition in the field of polyelectrolytes. Moreover, this field may be wider than it seems at first. Polyelectrolytes form the basis of those modern cements which are distinguished by their ability to adhere to reactive surfaces. At present the main use of such cements lies in the medical field, principally in dental surgery. They adhere permanently to biological surfaces where they have to withstand adverse conditions of wetness, chemical attack, the stress of biological activity, and chemical and biological changes within the substrate. Nevertheless, adhesive bonds are maintained. Polyelectrolytes are polymers having a multiplicity of ionizable groups. In solution, they dissociate into polyions (or macroions) and small ions of the opposite charge, known as counterions. The polyelectrolytes of interest in this book are those where the polyion is an anion and the counterions are cations. Some typical anionic polyelectrolytes are depicted in Figure 4.1. Of principal interest are the homopolymers of acrylic acid and its copolymers with e.g. itaconic and maleic acids. These are used in the zinc polycarboxylate cement of Smith (1968) and the glass-ionomer cement of Wilson & Kent (1971). More recently, Wilson & Ellis (1989) and Ellis & Wilson (1990) have described cements based on polyphosphonic acids. There is also the question of whether there are inorganic polyelectrolytes within the field of AB cements. A number of cements are based on orthophosphoric acid, and in the two most important ones aluminium is 56

Polyelectrolytes known to be essential for cement formation. Aluminium forms complex aluminophosphoric acids with orthophosphoric acid. The solutions of these complexes are markedly viscous and there is some NMR spectroscopic evidence that these aluminophosphoric acids form linear polymers based on the Al-O-P linkage (Sveshnikova & Zaitseva, 1964; Akitt, Greenwood & Lester, 1971; O'Neill et al., 1982). Callis, Van Wazer & Arvan (1954), Salmon & Wall (1958) and Wilson et al. (1972) consider that aluminophosphate polymers are formed in which [POJ tetrahedra are linked by aluminium atoms. Polyanion chains containing many linked charged groups exert a considerable electrostatic effect on the orientation of dipolar solvent molecules and on the counterions. The counterions are constrained to remain in the neighbourhood of the charged polymer chains, a phenomenon known as ion binding. This phenomenon is supported by a wealth of experimental evidence (Morawetz, 1975; Wilson & Crisp, 1977; Rymden & Stilbs, 1985a, b) and an early illustration of it is found in the work of Huizenga, Grieger & Wall (1950a, b) who observed that, in an electric field, cations were sometimes transported with the polyanion.

r

CH2

2

CH2

CH—COOH

T

CH—COOH Poly (acrylic acid)

CH2

1

1

CH—SO2OH CH2 CH—SO 2 OH

Poly (vinyl sulphonic acid)

C H — COO" 1

C H — COO~

1

Polyacrylate

1 1 CH—PO(OH)

CH 2

2

T2

CH—PO(OH) 2

Poly (vinyl phosphonic acid)

Figure 4.1 Some typical anionic polyelectrolytes.

57

Polyelectrolytes, ion binding and gelation 4.1.2

Polyion conformation

The shape, configuration or morphology of a polyion is usually known as its conformation. There are very many possible conformations available to a polyion because of the flexibility of the main chain due to the free rotation of bonds. The particular conformation adopted will be the one with the lowest free energy. This free energy has two components, one arising from chainflexibilityand the other from electrical interactions. The intrinsic free energy of rotation is a function of the relative position of neighbouring bonds. There are energy minima, at the trans position, which corresponds to the stretched form of the chain, and at the two gauche positions, corresponding to the contracted form. The difference in energy between the trans and gauche positions is one important factor determining theflexibilityof the chain. The other component of free energy arises from interactions between charged groups on the polyion, counterions and solvent molecules. There are two broad kinds of polyion conformation; the random coil and the ordered helix. In a helix there are regularly repeated structures along the coil; there are none in the case of a random coil. In this book we are concerned with the latter where there are often several conformations with approximately equal free energies and, thus, conformational changes occur readily. Random coil conformations can range from the spherical contracted state to the fully extended cylindrical or rod-like form. The conformation adopted depends on the charge on the polyion and the effect of the counterions. When the charge is low the conformation is that of a contracted random coil. As the charge increases the chains extend under the influence of mutually repulsive forces to a rod-like form (Jacobsen, 1962). Thus, as a weak polyelectrolyte acid is neutralized, its conformation changes from that of a compact random coil to an extended chain. For example poly(acrylic acid), degree of polymerization 1000, adopts a spherical form with a radius of 20 nm at low pH. As neutralization proceeds the polyion first extends spherically and then becomes rod-like with a maximum extension of 250 nm (Oosawa, 1971). These pHdependent conformational changes are important to the chemistry of polyelectrolyte cements. The situation is more complex in the reactions found in AB cements because neutralization is accompanied by ion binding. Although a polyion chain extends as the number of ionized groups increases, the binding of 58

Ion binding counterions has the reverse effect because intrachain repulsive forces are decreased. An increase in the concentration of polyions in solution has the same effect, for an increase in interchain repulsion will inhibit the unwinding of polymer chains. Thus, the effects predicted by dilute solution theory will be much less in the concentrated conditions found in AB cements. From this it can be seen that the effect of ion binding on conformation change is complex. Conversely, conformation affects the binding of counterions to polyions (Jacobsen, 1962). In the compact spherical conformation some ionized groups on polymer chains will be inaccessible for ion binding. 4.2

Ion binding

4.2.1

Counterion binding

Oppositely charged ions are attracted to each other by electrostatic forces and so will not be distributed uniformly in solution. Around each ion or polyion there is a predominance of ions of the opposite charge, the counterions. This cloud of counterions is the ionic atmosphere of the polyion. In a dynamic situation, the distribution of counterions depends on competition between the electrostatic binding forces and the opposing, disruptive effects of thermal agitation. The phenomenon has been studied by a number of techniques: titration (Gregor & Frederick, 1957; Kagawa & Gregor, 1957); viscosity and electrical conductance measurements (Gregor, Gold & Frederick, 1957; Bratko et al., 1983); determination of counterion activity (Kagawa & Katsuura, 1955); measurement of transference (Ferry & Gill, 1962); dilatometry (Strauss & Leung, 1965; Begala & Strauss, 1972); and NMR spectroscopy (Rymden & Stilbs, 1985a, b). Ion binding is affected by the size and charge of the counterion, the charge and conformation of the polyion, and states of hydration. We will examine these effects in some detail. 4.2.2

The distribution of counterions

The potential distribution around the polyion is important to any discussion of counterion binding and hydration effects. Oosawa (1971) has distinguished four regions of potential about a polyion (Figure 4.2): (1) a localized potential hole around each charged group, (2) a cylindrical potential valley or tube along the polyion chain, (3) a spherical trough in 59

Poly electrolytes, ion binding and gelation

the apparent volume occupied by the whole of the coiled chain, and (4) the region outside the polyion. Counterions are distributed between these four potential regions and may be classified as free, bound but mobile (atmospheric) and localized {site-bound). Free ions remain outside the volume of the polyion (in region 4); the remaining ions are bound to the polyion. Of the bound ions, the mobile atmospheric ions occupy the potential trough or valley around each polyion (regions 2 and 3). Localized binding occurs in the potential holes at the sites of the individual charged groups of the polyion, and ion-pairs are formed. Oosawa (1971) used a simple calculation to illustrate the effect of a highly charged polyion on the binding counterions. The distribution between free ions and bound ions depends on the ratio between potential energy and kinetic energy. In the case of a random coil, containing n ionized groups of charge — e0, and of spherical conformation, radius/?, the potential drop, Si//, for a counterion of charge + e0 at the edge of the polyion is given by neo/eop

\ \

r

Region of free counterions

Figure 4.2 The four regions of potential about a polyion. Based on Oosawa (1971).

60

(4.1)

Ion binding

where the dielectric constant of the solvent is equal to e0. Hence the potential energy is nel/eop

(4.2)

The ratio of the potential energy to the kinetic energy, kT, is ne*/eopkT

(4.3)

This ratio is related linearly to the degree of polymerization n. In the case of a poly(acrylic acid) where n = 1000 and p = 20 nm, this ratio works out at 35. Thus, many of the counterions must enter the region of the polyion. Even when 90 % of the counterions are within the polyion this ratio is still high with a value of 3-5. A similar calculation for the rod-like random coil gives an energy ratio of 26 and similar arguments apply. Oosawa (1971) developed a simple mathematical model, using an approximate treatment, to describe the distribution of counterions. We shall use it here as it offers a clear qualitative description of the phenomenon, uncluttered by heavy mathematics associated with the Poisson-Boltzmann equation. Oosawa assumed that there were two phases, one occupied by the polyions, and the other external to them. He also assumed that each contained a uniform distribution of counterions. This is an approximation to the situation where distribution is governed by the Poisson distribution (Atkins, 1978). If the proportion of site-bound ions is negligible, the distribution of counterions between these phases is then given by the Boltzmann distribution, which relates the population ratio of two groups of atoms or ions to the energy difference between them. Thus, for monovalent counterions nJK = (nJV^xpi-e^/kT)

(4.4)

where Sy/ is the average potential difference between the two phases, nh is the number of bound ions contained in a total volume Vh9 nt is the number of free ions contained in a total volume Vt and n is the total number of counterions in a total volume V. This case can be rewritten In {(1-/?)/# = ln{0/(l-0)}-e o
(4.5)

where p is the apparent dissociation constant, i.e. the ratio of free to total counterions, nt: n, and 6 is the volume concentration of the polyion, Vh: V. For a rod-like or cylindrical polyion, the potential difference Sy/ between 61

Polyelectrolytes, ion binding and gelation the inside and outside of a cylindrical polyion of length / and radius r, with an average distance between the polyions of 2R, is given by dy, = -2(nie0/s0)\n(R/r)

(4.6)

If v is the mean effective volume occupied by a single polyion and N is the number of polyions then v = nrH and V/N = nR2l; thus r*/R2 = Nv/V=0 and (4.7)

Hence equation (4.5) becomes = ln{0/(l-0)}-/fein0

(4.8)

where Q is the charge density along the polyion and equals nel/sokTl. The equation for a spherical polyion conformation of radius a is similar: ln{(l -fi)/fi = In{0/(1 -0)}-/?P(l -0 1/3 )

(4.9)

where P is the charge density along the polyion and equals nel/skTa. Thus /?, and hence the extent of the ion binding, depends both on the volume concentration of the polyion, 9, and the charge density, Q. The consequences of these equations, are not easy to see, because they cannot 1.0

0 =1 0.5

Q=2

•

•

Q=3

•

1 0.1

I 0.2

I

0.3

Figure 4.3 Variation of the apparent degree of dissociation, /?, with v and Q. Based on Oosawa (1971).

62

Ion binding be solved. However, when 9 is sufficiently small the equations can be simplified and rendered easy to discuss. Although the conditions in very dilute solutions are far removed from practical reality, the simplified situation can be used to illustrate certain basic points. Thus, for a rod-like configuration where 9^0, equation (4.8) reduces to In {(1-/?)//?} = ( l - / ? 0 In 0

(4.10)

or (\-P)/P=8a-pQ)

(4.11)

The apparent degree of dissociation, /?, varies in a complex way with 9 and depends on the value of Q (Figure 4.3). Apart from the case Q = 1, where /? decreases with 9, fi shows little variation with 9. It slowly decreases when Q = 2, and when Q = 3 or 4 it increases slightly to a plateau. Consequently, in practical cases, /? is unaffected by increases in the charge on the polyion associated with ionization. This conclusion is supported by the results of Nagasawa & Kagawa (1957). 4.2.3

Counterion condensation

The theory of counterion condensation is implicit in Oosawa (1957) but the term was coined later (Imai, 1961). The phenomenon was demonstrated by Ikegami (1964), using refractive index measurements of the interaction between sodium and polyacrylate ions. It has since been confirmed for many mono-, di- and trivalent counterions and polyionic species (Manning, 1979). Manning (1969) suggested that there is a critical charge density above which counterions condense on the surface of the polyion. This phenomenon is most clearly illustrated by the simple case of infinite dilution. As 9^0 in equation (4.11), the graph of j$ against Q falls into two parts about the critical point Q = 1: P^\

andy->l for Q ^ 1

(4.12)

£-> \/Q and y-> \/Q for Q ^ 1

(4.13)

where y is the activity coefficient and equals /?/(l — 0). The consequences of these solutions are shown in Figure 4.4. The abscissa is n, the total number of counterions or charged groups on the polyion, and is proportional to Q. Along the ordinate are the numbers of counterions bound, nh, and free, nt, equal to n(\—fi) and n^respectively. The increase of counterion binding with the charge on the polyion has 63

Polyelectrolytes, ion binding and gelation

been termed counterion condensation as it is analogous to the condensation of a vapour. This point is illustrated by Figure 4.4. As the charge on the polyion increases from zero there is a proportional increase in the number of counterions. At first, all the counterions are free and none are bound. This continues until a plateau is reached at a critical value of Q = 1. Above this point, all additional counterions are bound to the polyion and the number of free ions remains constant. Thus, PQ remains constant and /? decreases as Q increases. The simple situation depicted in Figure 4.4 is a limiting one, and the discontinuity does not occur when 6 > 0; however, for large values of g, PQ increases only slowly. Nor does the discontinuity appear in the case of the spherical conformation, but again, for large values of P, PP increases only logarithmically. Thus, the situation is similar to that for the rod-like configuration although there is no specific critical value for P. The above treatment is based on the assumption that 9 is small. However, as Figure 4.3 shows, /? does not greatly change with concentration so that counterion condensation is probably insensitive to concentration. The delayed binding of counterions is of some importance to the onset of gelation.

bound / counterions, / n

b

a

7

H

C r—t

a c

/ free / counterions, / n f / / / / :Q = i / / / / / : / • / ;

/

Total number of counterions, n[Q] Figure 4.4 The abscissa is n, the total number of counterions or charged groups on the polyion, and is proportional to Q. Along the ordinate are the number of counterions bound, nh, and free, «f, equal to n(\ —f$) and «/? respectively. Based on Oosawa (1971).

64

Ion binding Theories of counterion condensation have been reviewed by Manning (1979, 1981); and Satoh, Komiyama & Iijima (1984) have extended the theory.

4.2.4

Effect of valence and size on counterion binding

Cations of small ionic radius and high charge are more firmly bound than monovalent ions of large ionic radius (Ikegami & Imai, 1962; Strauss & Leung, 1965; Begala & Strauss, 1972). Divalent ions are more strongly bound than monovalent ions and the interaction is often localized. This can be examined theoretically by applying Oosawa's two-phase model to counterions with a valence z and charge of + e0, and a polyion with a total charge — ne. Equations (4.5) and (4.8), which were developed for univalent ions, can be rewritten, thus: In {(1-/?)//?} = \n{e/(\-e)}-zeQ5¥/kT

(4.14) (4.15)

The critical value for Q is 1/z. There is a proportional increase in the number of free counterions, njz, as Q increases from zero, reaching a plateau when Q = 1/z. Also, below this value the degree of dissociation, /?, increases as the concentration decreases, and tends to unity as v tends to zero. When Q > 1/z, f5 decreases with 9 and tends to 1/zQ as 6 tends to zero. The number of free ions cannot exceed n/z2Q. Note that this number is inversely proportional to the square of the valence. The condensation of ions is thus very sensitive to valence; for multivalent counterions it takes place at a lower value of Q and the number of free ions is much smaller (l/* a ). Imai (1961) has observed that multivalent counterions are more strongly bound than are monovalent ones. This phenomenon can be demonstrated theoretically by considering equilibrium conditions for two counterions with valencies zx and z2 (z2 > zx) and degrees of dissociation fix and /?2. For a cylindrical model the equilibrium equations are ^-^/.A)^

(4.16)

fflwJi+fMW (4.17) where fx and/ 2 represent the proportions of the total charge carried by the counterions, i.e./i+/ 2 = 1. 65

Polyelectrolytes, ion binding and gelation As 9^0 then the solutions to these equations fall into four groups depending on the value of Q. A->1, &^1,

Px-> 1, P1->l/f1z1Q,

&"•!

forg
A^l//2 2 2 - / i / / 2

p2->0 02^O

for l z

(4.18) l

z

/ 2 < Q < /fi 2

(4.19)

for I/ft z2 ^ Q ^ I/ftz x (4.20) for I / f t z ^ e (4.21)

These equations are represented graphically in Figure 4.5. Increase in the binding of counterions as Q increases is reflected as a decrease inftvalues. No binding occurs until Q reaches l/z 2 , when the binding of the highervalence ions begins. This process is complete when Q attains a value of l/ftz 2 . There is no further binding of counterions until Q reaches l// 1 z 1 when the binding of the lower-valence ions commences. Figure 4.5 shows that when there is a mixture of counterions then those of the higher valence are preferentially bound. Lower-valence ions can completely suppress the dissociation of those of higher valence.

Figure 4.5 The effect of Q on the dissociation {fix /?2) of ions of two valencies. Note the suppression of the dissociation ( / y of ions of higher valence zx by those of lower valence z2. Based on Oosawa (1957).

66

Ion binding The selective binding of cations is not as sensitive to size as to valence. The value of Q for the condensation of counterions of the same valence is unaffected. In the case of monovalent cations, the dissociation of all counterions is complete at infinite dilution, when Q ^ 1. When Q ^ 1 the dissociation of the smaller counterion is always greater than that of the larger one and increases relatively as Q increases. A number of workers have observed that the strength of binding of monovalent counterions depends on ionic radius. However, the effect of ionic radius is somewhat obscure as it depends on hydration phenomena and whether the size of the bare ion or that of the hydrated ion is the significant parameter (Wilson & Crisp, 1977). 4.2.5

Site binding - general considerations

Not all ions are mobile within the ionic atmosphere of the polyion. A proportion are localized and site-bound-a concept apparently first suggested by Harris & Rice (1954). Localized ion binding is equivalent to the formation of an ion-pair in simple electrolytes. Experimental evidence comes mainly from studies on monovalent counterions. This concept is due to Bjerrum, who in 1926 suggested that in simple electrolytes ions of the opposite charge could associate to form ion-pairs (Szwarc, 1965; Robinson & Stokes, 1959). This concept of Bjerrum arose from problems with the Debye-Hiickel theory, when the assumption that the electrostatic interaction was small compared with kT was not justified. Bjerrum considered the case of spherical ions in a solvent of dielectric constant e. The probability of finding two ions of opposite charge at a distance A from each other is calculated from the number of ions surrounding a central ion of opposite charge in a spherical shell of thickness dA and radius A. This probability, W{A), is given by W(A) dA = (4nA2 dA/v) exp (e2/sAkT)

(4.22)

for monovalent ions. This distribution has a minimum, Am, at Am = e'/2ekT

(4.23)

For a cation of charge z+ and anion of charge z_, this minimum becomes Am = z+z_e2/2ekT

(4.24)

When A > Am the ions are free and the Debye-Hxickel theory applies. When A < Am the two ions tend to approach each other and form an ionpair, and there is no contribution to the electrostatic energy from the interaction between an ion and its atmosphere. 67

Poly'electrolytes, ion binding and gelation

The high dielectric constant of water normally militates against the formation of ion-pairs for simple salts because a high dielectric constant reduces the strength of the electrostatic forces. The phenomenon is more readily observed in solvents of low dielectric constant; for a typical monomonovalent salt, ion-pair formation takes place only when the dielectric constant is less than 41 (Fuoss & Kraus, 1933). The fraction of all ions forming ion-pairs is W(A)dA

(4.25)

J2c

where a is the radius of the central ion. This distribution has some inconsistencies - for example it diverges when R is large - and was modified by Fuoss (1934); see Figure 4.6. These arguments for simple electrolytes can be extended to the relationship between the two types of bound counterion in polyelectrolytes: the bound but mobile (atmospheric) and the localized (site-bound). Under equilibrium conditions, the relationship between sitebound and atmospheric ions is (4.26)

2

I

Con tact distance

Ion-pair range Inter-ion distance

Figure 4.6 The Fuoss (1934) distribution function.

68

Ion binding

where ns is the number of site-bound ions, «a is the number of atmospheric ions and K is the equilibrium constant. For monovalent cations in dilute solution (0-1 M) the degree of localized ion binding is negligible; in more concentrated solutions some site binding does occur. In general, localized ion binding can be expected only with multivalent cations. When site binding occurs, the equations which relate the numbers of free and bound ions require some modification. The relationship is then between free ions and those bound ions that are mobile. The equations are similar to equations (4-8), (4-9), (4-14), and (4-15), but the number of sitebound ions has to be discounted in all calculations for P, Q, /? etc. 4.2.6

Effect of complex formation

In a discussion of papers by Rice & Harris (1954) and Harris & Rice (1954), Van Wazer (1954) suggested that there could be covalent binding as well as electrostatic interaction and that cations could be held at specific sites by complex formation. This is a reasonable inference, because site binding is significant only with multivalent cations and strong electrostatic interactions. Under these conditions ion polarization occurs and bonds have some covalent character (Cotton & Wilkinson, 1966). This is illustrated by the data of Gregor, Luttinger & Loebl (1955a,b). They measured the complexation constants of poly(acrylic acid), 0-06 N in aqueous solution, with various divalent metals, which, as it so happens, are of interest to AB cements (Table 4.1). The order of stability was found to be Mg < Ca < Co < Zn < Mn < Cu Mandel & Leyte (1964) found a similar order for the complexes of poly(methacrylic acid): Mg < Co < Ni < Zn < Cd < Cu Some of these divalent cations form part of the Irving-Williams series: Mn, Fe, Co, Ni, Cu and Zn. Irving & Williams (1953) examined the stability constants of complexes of a number of divalent ions and found that the order Mn < Fe < Co < Ni < Cu > Zn held for the stability of most complexes irrespective of the nature of the coordinated ligand. The stability constants of metal-poly(alkenoic acid) 69

Polyelectrolytes, ion binding and gelation Table 4.1. Metal PAA complexes (Gregor, Luttinger & Loebl, 195 5 a,b) Metal ion

Crystal ionic radius A

Complexation constant

Cu 2+ Mn 2+ Zn 2+ Co 2+ Ca 2+ Mg 2+

0-72 0-80 0-80 0-72 0-99 0-66

6-0 xlO 3 2-3 x 103 2-1 x 103 4-0 x 102 1-OxlO2 6-0 x 101

complexes, for the most part, follow the Irving-Williams series as do the stabilities and strengths of poly(alkenoic acid) cements. The complexation constant for copper(II) is particularly high and Wall & Gill (1954) have suggested that chelate formation takes place with two carboxyl groups:

From all of this discussion it is apparent that, as Manning (1979) said, the binding between counterion and polyion can range from atmospheric to covalent site binding. 4.2.7

Effect of the polymer characteristics on ion binding

The extent of ion binding depends on a number of characteristics of the polyion: degree of dissociation, acid strength, conformation, distribution of ionizable groups and cooperative action between these groups (Wilson & Crisp, 1977; Oosawa, 1971; Harris & Rice, 1954, 1957). The hydration state of the macromolecule, which is in turn dependent on conformation, also affects ion binding (Begala, 1971). 70

Ion binding There are differences in ion binding between different polyacids. Thus, alkali metal ions are bound more strongly to poly(acrylic acid) than to the weaker poly(methacrylic acid) (Wilson & Crisp, 1977). Again, the ranking order for the binding strength of alkali metal ions depends on the nature of the polyanion, and the order is different for poly(acrylic acid) than for poly(maleic acid) or poly(itaconic acid). Thus, for poly(acrylic acid) the binding strength increases in descending order of the ionic radius of the bare cation: K+ < Na+ < Li+ For poly(maleic acid) and poly(itaconic acid), the binding strength increases in descending order of the size of the hydrated metal ion, which is the reverse of that for the bare ion (Muto, Komatsu & Nakagawa, 1973; Muto, 1974). This observation has been explained by postulating the formation of a stable ring structure with a hydrogen bridge between ionized and non-ionized carboxylate groups. The strength of ion binding is enhanced when the arrangements of the functional groups permit chelate formation (Begala & Strauss, 1972). Thus, magnesium is more firmly bound to poly(vinyl methyl ether-maleic acid) than to either poly(acrylic acid) or poly(ethylene maleic acid). The charge or number of dissociated groups on a poly acid chain depends on the degree of neutralization and is reflected by the pH of the solution. Behaviour is determined by the site binding of hydrogen ions; in the case of a weak polyacid the number of free hydrogen ions may be neglected. It follows that decrease of site binding of hydrogen ions is directly proportional to the amount of added alkali. In the case of poly(acrylic acid) its polymer chain can be regarded as a copolymer containing pendant COOH and COO" groups, the relative amounts of each depending on the degree of neutralization. When the degree of neutralization is small the charge on the polyion and the number of counterions will also be small and the majority of counterions will be free. As the degree of neutralization, a, increases, the polyion charge, Q, will increase. This observation follows from the following equations: Q = nel/eokTl

(4.27)

where n = number of ionized groups on the polyion. It follows that Q = anoel/sokTl

(4.28) 71

Poly electrolytes, ion binding and gelation

where n0 = the potential number of ionizable groups on the polyion. When Q is low, most of the counterions are free, but as neutralization increases a point is reached at which the counterions condense; above this point, additional counterions are bound. This follows from the discussion in Section 4.2.3.

4.2.8

Solvation (hydration) effects

The solvation (hydration) and desolvation of ions is important to the gelation process in AB cement chemistry. The large dipole moment of ionpairs causes them to interact with polar molecules, including those of the solvent. This interaction can be appreciable. Much depends on whether the solvent molecule or molecules can intrude themselves between the two ions of the ion-pair. Thus, hydration states can affect the magnitude of the interaction. The process leading to separation of ions by solvent molecules was perceived by Winstein et al. (1954) and Grunwald (1954). Consider two ions in contact. As they are pulled apart the potential energy of the two ions increases. At some critical point the separation becomes sufficient for a polar solvent molecule to occupy the space between them, which reduces the energy of the system. Further separation increases the energy of the system again. These changes demonstrate that two types of ion-pair exist: contact and solvent-separated. This distinction is meaningful if the resultant distribution function is of the type shown in Figure 4.7 (Szwarc, 1965). This figure shows that there is a high probability that the cation and anion are either in contact, separated by a solvent molecule or far apart (Szwarc, 1965). Intermediate positions are improbable. The structure of solvated ion-pairs has been studied by Grunwald (1979) using dipole measurements. Winstein & Robinson (1958) used this concept to account for the kinetics of the salt effects on solvolysis reactions. They considered that carbonium ions (cations) and carbanions could exist as contact ion-pairs, solvated ion-pairs and as free ions and that all these forms participated in the reactions and were in equilibrium with each other. These equilibria can be represented, thus: X : Y = X+Y" = X+SY~ = X+ contact solvent-separated free ions ion-pair ion-pair 72

Ion binding

where X is a carbonium radical and X+ the carbonium ion, Y~ the carbanion and Y the associated radical. S represents a solvent molecule. Eigen & Tamm (1962a,b) and Atkinson & Kor (1965, 1967) envisage a more complex situation and consider that there are two kinds of solventseparated ion-pairs: those with one intervening molecule of solvent and others where the ion-pair is fully solvated (Wilson & Crisp, 1977). 4.2.9

Hydration of the polyion

The electric potential around a polyion can aflfect the structure of water. There are three regions of potential about a polyion to consider: the potential holes at the site of the individual charged groups, cylindrical regions along the polymer chain, and the outlying region. In the outlying region the potential is small and the water molecules have a normal structure. In the other two regions there are strong electricfields,and water molecules are oriented and have special structures. Oriented water is denser and has a higher refractive index than normal water (Begala & Strauss, 1972; Ikegami, 1964, 1968). The structure of this water can be affected by ion binding. If the counterions are tightly bound at the sites of individual charged groups, the

Solvent ion-pair Inter-ion distance

Figure 4.7 Distribution function for contact and solvent-separated ion-pairs.

73

Poly electrolytes, ion binding and gelation structure of the water around them will be profoundly modified. If the counterions are not localized but mobile, the influence on water structure may be small. Thus, the state of the binding of a counterion will be reflected in changes in water structure which in turn can be measured by changes in refractive index or density. The effect has been studied experimentally by Ikegami (1964, 1968) who measured changes in refractive index, Asai (1961) employing an ultrasonic method, Begala & Strauss (1972) who measured changes in molar volume, and Grunwald (1979) using dipole moment measurements. When an acid is neutralized by a base the refractive index of the salt solution formed is less than the weighted mean of the refractive indices of the acid and base solutions from which it is formed. Likewise, the density increases. By these means, the progress of neutralization may be followed. At low degrees of neutralization, the average distance between ionized groups is great, so that the rearrangement of neighbouring water molecules induced by the ionization of a carboxyl group is solely due to the charge on that individual group. Individual hydration spheres of oriented water, intrinsic water, are formed at each charged site. In the case of poly(acrylic acid) when the degree of neutralization, a, is 0-3 the radius of these spheres is 031 nm (Ikegami, 1964). As a increases, the average distance between ionized groups decreases so that these neighbouring groups begin to have an effect. When a exceeds 0-3, individual water spheres begin to overlap and eventually coalesce into a cylindrical form. With further increases in a, a second outer cylindrical sheath of water appears in which water molecules are oriented by the cooperative effect of two or more carboxyl groups. When neutralization is complete, the inner layer of intrinsic water assumes a cylindrical form along the length of the polyion with a diameter of 0-5-0-7 nm (Ikegami, 1964). The outer second cylindrical hydration region has a diameter of 0-9-1-3 nm (Figure 4.8). The explanation for this volume increase is as follows. For a cylindrical model of uniform charge density the electric field around the cylinder is 2cm0e0/e0rl

(4.29)

where a is the degree of neutralization, r the radius of the cylinder and /its length. When the magnitude of the electric field exceeds a certain value, water molecules are reoriented; the above expression shows that the radius, r, of the cylinder increases as the degree of neutralization, a, increases. 74

Ion binding

According to Ikegami (1968) the presence of hydrophobic groups, for example the methyl group in poly(methacrylic acid), can induce an additional hydration region around neighbouring charged groups. The arrangement of carboxyl groups on the polyacid is also important. Thus, poly(ethylene maleic acid), PEMA, which is an 'isomer' of poly(acrylic acid), PAA, has a different hydration structure. Whereas in PAA the COOH groups are pendant on alternate chain C atoms, those in PEMA are paired on adjacent chain C atoms. These structural differences affect hydration (Begala, 1971). The separation of the hydrophilic carboxyl groups by a pair of hydrophobic chain C atoms effectively prevents the cooperative effect between ionizable groups. Thus, by contrast with PAA, as the degree of ionization increases, the hydration regions around PEMA never coalesce to form a cylindrical sheath. In the fully ionized state there is a spherical region of intrinsic water around each carboxyl group and an outer spherical region of water which encloses each pair of carboxyls. The formation of a stable hydrogen-bonded ring structure as in poly(itaconic acid) and in poly(maleic acid) has also been shown to affect hydration states (Muto, Komatsu & Nakagawa, 1973; Muto, 1974). 0.9-1.3nm 0.5-0.7 nm

a<0.3 a=0.3 a=1.0 Figure 4.8 Cylindrical and spherical hydration regions around poly(acrylic acid) at various degrees of neutralization (or charge densities). Based on Ikegami (1964).

75

Poly electrolytes^ ion binding and gelation 4.2.10 Hydration and ion binding Counterions can affect the structure of hydration regions, and conversely hydration regions can affect ion binding. We have already touched on this subject in discussing contact and solvent-separated ion pairs in Section 4.2.8. Large bound monovalent cations, e.g. tetrabutylammonium ions, are too large to penetrate any of the hydration regions. However, the smaller lithium, sodium and potassium ions are able to penetrate the outermost hydration region of the neutralized polyacid and this is accompanied by volume increases (Figure 4.9). These cations are probably not site-bound but are mobile in the outer cylindrical region of hydration (Figure 4.10). Divalent cations cause a much greater disruption of the hydration

0.5

1.0

Figure 4.9 Volume increases associated with the binding of various counterions to poly(acrylic acid). Based on Ikegami (1964).

76

Ion binding

regions. These ions completely penetrate the outer hydration region and partly penetrate the inner one (Figure 4.10). Such effects manifest themselves in much greater changes in molar volume than are the case for monovalent ions (Figure 4.9). Divalent ions may be considered to be partly mobile and partly site-bound. Even greater disruption is encountered in the case of trivalent cations (Figures 4.9, 4.10). They completely penetrate both hydration regions and destroy the structure of water around the polyion. This amounts to complete desolvation. The same is true of bound hydrogen ions which are localized. 4.2.77

Desolvation and precipitation

Divalent and trivalent ions can precipitate PAA, and this phenomenon is related to the loss of a hydration region. Such precipitation is to be distinguished from salting-out effects which occur with high concentrations of monovalent ions.

Figure 4.10 The effect of monovalent, divalent and trivalent counterions on the hydration state of neutralized poly (acrylic acid). Based on Ikegami (1964).

77

Poly electrolytes, ion binding and gelation

Ikegami & Imai (1962) made a study of precipitation and hydration using turbidity, conductance, refractive index and viscosity measurements. The following account is based on their description. Although all divalent ions precipitate PAA when the degree of dissociation, a, approaches 1-0, there are differences when a = 0-25 (Figure 4.11). Small amounts of barium and calcium ions precipitate PAA at this low a value, whereas magnesium ions do not. These differences are not to be attributed to differences in the amounts of counterions bound, for condensation theory (Section 4.2.3) predicts that all divalent counterions are bound to polyanions to the same extent (Imai, 1961). Therefore, differences must arise from differences in solubility between the various polyacrylates. At low degrees of neutralization barium polyacrylate has low solubility, while magnesium polyacrylate is very soluble. This is related to the extent of disruption of hydration regions as cations are bound to polyions. Ikegami & Imai (1962) explained their results by assuming that divalent ions can be bound to PAA in two forms, which they represented as (I) COO-Me+ + COO" (II) COO-Me-OOC 1.2 -

0.9 -

0.6 •

0.3

0.25

0.50

0.75

Figure 4.11 The effect of a on the precipitation of PAA by divalent ions. Cs is the critical salt concentration. Based on Ikegami & Imai (1962).

78

Ion binding According to these workers the formation of COO-Me+ causes a small degree of dehydration, while that of COO-Me-OOC is accompanied by considerable dehydration. The experimental results showed that, when a = 1*0, divalent ions are bound as COO-Me-OOC, a form which favours precipitation. However, as a decreases, the COO-Me+ form becomes more apparent. The ratio of the two forms depends on the cation as well as on a. Ba2+ has a greater tendency to make linkages of the COO-Me-OOC type than Mg2+ and this difference is accentuated when the density of COO~ in the polyanion is low. Thus, at a = 0*25 more Ba2+ ions are in the COO-Ba-OOC form than in the COO-Ba+ form, while the reverse is true for Mg2+ ions. Moreover, the structure COO-Mg+ is more stable and soluble than COO-Ba+ because Mg2+ is more hydrophilic than Ba2+. For these reasons, Ba2+ is precipitated at a = 0-25 while Mg2+ is not. This interpretation is supported by titration experiments in the presence of divalent cations (Jacobsen, 1962). Magnesium forms very stable hydrates and would be expected to be more difficult to desolvate. It is, perhaps, more in line with other thinking to represent form (I) as a solvent-separated ion-pair: COO" H2O Me2+ H2O OOC, and form (II) as a contact ion-pair: COO~ Me2+ OOC. Thus, precipitation occurs when a solvent-separated ion-pair is desolvated.

4.2.12 Conformational changes in polyions The conformation of macro- or polyions has been defined and discussed briefly in Section 4.1.1. The conformation of a polyion is determined by a balance between contractile forces, which depend on conformation free energy, and extension forces, which arise from electrical free energy. The extent of conformational change is determined by several factors. Changes are facilitated by the degree of flexibility of the polyion, and conformational change is greatest at low concentration of polyions. Conformation depends on the degree of ionization and concentration of the polyion, the type and concentration of the counterion and the interaction between counterion and polyion. Extension is favoured by low concentrations of counterion and polyion. Conformational change is also affected by the extent of the charge on the polyion. As the charge on a polyion increases, the chain uncoils and expands under the influence of repulsive forces. Thus, the neutralization of a polyacid is accompanied by 79

Polyelectrolytes, ion binding and gelation

chain expansion as carboxyl groups ionize. The distribution of ionized groups is labile and depends on conformation; an extended polyion has a larger number of ionized groups than a contracted one. Ionization of the carboxyl groups is accompanied by binding of the cations. But if counterions are site-bound the charge on the carboxyl groups is neutralized and chain contraction results. A special case is that of the polyacid which adopts a contracted form because the close association of hydrogen ions with carboxyl groups results in a neutral chain. Extensive forces arise from the electrical interaction between counterions and polyions. There are two repulsive forces which act to extend a polyion. One results from coulombic repulsion between the charged groups on the polyion and the other from osmotic pressure of the counterions within, which seek to increase the space in which they can move. The coulombic force is proportional to the square of the effective charge on the polyion, i.e. n\. (The effective charge is equivalent to the number of free counterions, nv) When the charge along the polyion, Q, is small the extensive forces involved are those of purely coulombic repulsion. The most important factor determining the sensitivity of the conformation to the concentration of polyions is the change in ion activity or osmotic pressure with conformation. If the activity coefficient of the counterions is sensitive to conformation then conformational change resulting from concentration changes of polyions becomes large. Osmotic pressure results from the difference in concentration between the bound but mobile counterions within the polyion and the free counterions outside it. The concentration of counterions is greater within the polyion so that solvent molecules tend to enter this region. The osmotic force is proportional to the difference n — nt, where n equals the total number of counterions or the number of ionizable groups on the polyion. The predominant force is that of osmotic pressure, unless both the charge density and the concentration are low. These statements may be substantiated by a simple and approximate mathematical treatment which applies for very dilute solutions. Coulombic forces oc n\ oc (nflf oc (/?g)2 where ji is the degree of dissociation; Osmotic force oc (n-nt) oc n{\~P) oc Q(\-p) These relationships apply for both forces along and perpendicular to the chain, although the proportionality constants differ. These simple 80

Ion binding

expressions are represented graphically in Figure 4.12. As the figure shows, when Q is low the extensive force depends solely on coulombic repulsion, thus when Q ^ 1, p = 1 Coulombic force oc Q2, Osmotic force = 0 At higher Q values, when

Q>l,PQ=l

Osmotic force oc 1 — 1 /Q and the Coulombic force is constant. At high Q values the contribution from osmotic pressure predominates. This is shown by the ratio of the two forces which is given by Coulombic force: Osmotic force oc /?2/(l —/?) These considerations apply to dilute solutions. In concentrated solutions the extensive forces will be diminished. Also if the bound counterions become site-bound then both extensive forces are diminished. These are important factors to consider in the theory of acid-base gelation in AB cements, where solutions are concentrated and many counterions are sitebound.

Osmotic force

Figure 4.12 The effect of Q on the extensive forces, coulombic and osmotic, acting on a cylindrical and a coiled polyion. Based on Oosawa (1971).

81

Polyelectrolytes, ion binding and gelation 4.2.13 Interactions between poly ions Repulsive coulombic forces exist between charged polyions. These are attenuated by the bound counterions; conversely they are stronger for polyions having a higher concentration of free counterions. When the charge along the polyion, Q, is small the forces involved are purely coulombic repulsion forces. However, when Q exceeds a certain value, counterions condense on the polyions and reduce the repulsive forces. Attractive forces arise from dipole interaction, a result of the fluctuations in the cloud of counterions. Although the mean distribution of counterions is uniform along the length of the polyion, there are fluctuations in the cloud of counterions which induce transient dipoles. When two polyions approach each other counterionfluctuationsbecome coupled and enhance the attractive force. Since polyions have a high polarizability these attractive forces can be considerable. The repulsive force between polyions, calculated for the mean equilibrium distribution of the counterions, is ell/eQz*

(4.30)

where / is the length of the polyion. The attractive forces are kTl/D2

(4.31)

where D is the average distance between polyions. If D
(4.32)

then the attractive force predominates over the repulsive one. This occurs for monovalent ions when D is 0*7 nm and for divalent ions when D is 2-8 nm. This means that attraction before direct contact between two rodlike macroions will occur only in the case of multivalent counter ions. The attractive force is important at high charge densities because it continues to increase with charge density whereas the repulsive forces become constant. 4.2.14 Polyion extensions, interactions and precipitation The precipitation of polyelectrolytes by the addition of multivalent counterions may be explained in these terms. When there are no multivalent ions in solution there is a strong repulsive force between polyions and the osmotic pressure is large. The solubility of polyions is a result of these repulsive forces. 82

Gelation

The binding of multivalent counterions decreases the repulsion and causes attraction between polyions. This attraction is the result of the fluctuation of the counterion distribution and is equivalent to a multivalent counterion bridge between polyions. 4.3

Gelation

The theory of gelation (Flory, 1953,1974) has been summarized in Section 2.2.3. This theory regards gelation as the consequence of the random crosslinking of linear polymer chains to form an infinite three-dimensional network. The phenomenon is, of course, well illustrated by examples drawn from the gelation of polycarboxylic acids by metal ions. Since chemical gelation occurs only when the cations have a valency greater than one, an early view was that it resulted from the formation of ionic crosslinks, a concept which is useful when applying Flory's theory of gelation. Thus, in early studies, the gelation of alginates and pectinates by Ca2+ ions was attributed to the crosslinking of COO~ groups by Ca2+ bridge formation. Wall & Drenan (1951) had a similar view in their study of the gelation of poly(acrylic acid) with various divalent alkaline earth ions. However, they noted that the concentration of cations required to produce gelation differed widely between cations and so concluded that the phenomenon could not be explained in terms of simple ionic equilibrium. Implicitly their mechanism assumes that chain extension occurs during ion binding. The concept of ionic crosslinking is in accord with the idea that a gel must possess a coherent structure. However, although crosslinking may be essential to gel formation it does not necessarily have to be a simple ionic salt bridge. Michaeli (1960) opposed these views. He concluded that whatever the exact mechanism was, the binding of divalent cations caused contraction and coiling of the polyelectrolyte as was the case with acids. He disagreed with the concept of ionic crosslinking. The phenomenon of precipitation could be explained simply in terms of reduced solubility. From this he concluded that precipitation took place in an already coiled molecule and the matrix consisted of spherical macromolecules containing embedded cations. These early views are, perhaps, too simplistic to explain in full the rheological changes that occur in polyelectrolyte cement pastes before and at gelation. There are several physicochemical processes that underlie 83

Poly electrolytes^ ion binding and gelation

such changes: ionization, ion binding, desolvation of the ion-pair, conformational changes in the polymer chain and interpolyion attraction. The extent and rate of interaction between hydrated counterion and polyanion depends on polymer structure and conformation, acid strength, degree of dissociation, and distribution and density of ionic charge on the polymer chain. The underlying physicochemical process leading to gelation in AB cements may be summarized as follows. The interaction between metal oxide or silicate and the polyacid solution involves a neutralization process. As neutralization proceeds and the charge on the polymer chain grows, the polymer chain, which is initially in random coil form, unwinds, a process which causes the cement paste to thicken. The forces which cause this unwinding are osmotic pressure and coulombic repulsion between the charged groups on the polymer chain. The cations released become bound by electrostatic forces to the polyanionic chain. These counterions can be either mobile (atmospheric) or site-bound at specific centres. Ion binding reduces the repulsive forces between the charged groups on the polyanion but, unless the counterions are site-bound, the repulsive osmotic forces are not affected. At full neutralization the coulombic forces along the polymer chain become zero. However, the polymer does not contract, because the osmotic forces remain; unless, of course, all the cations become site-bound. (Of course, in the case of a free weak acid the concentration of mobile hydrogen ions is very small and the polymer adopts a compact form.) Ion binding by reduction of repulsive forces also causes the attractive forces between polyions to increase, and the cement paste thickens. This interaction between polyions may be regarded as a kind of bridge formed by multivalent ions located between the polyions. At this stage the cement paste has the characteristic of a lyophilic sol - high viscosity. It is well known that lyophilic sols are coagulated by the removal of a stabilizing hydration region. In this case, conversion of a sol to a gel occurs when bound cations destroy the hydration regions about the polyanion, and solvated ion-pairs are converted into contact ion-pairs. Desolvation depends on the degree of ionization, a, of the polyacid, and the nature of the cation. Ba2+ ions form contact ion-pairs and precipitate PAA when a is low (0-25), whereas the strongly hydrated Mg2+ ion disrupts the hydration region only when a > 0*60. More than one type of site binding is possible. There is the simple 84

References Bjerrum ion-pair formation based on purely coulombic attraction. There is also complex formation and, if the ligand is bidentate, chelate formation enhances this effect, as in the case of Cu2+ (Wall & Gill, 1954). In cement gels Crisp, Prosser & Wilson (1976) found that the binding of Na+, Mg2+ and Ca2+ was purely ionic, whereas Al3+ binding had some covalent character. There was a suggestion, too, that the binding of Zn2+ might not be purely ionic. Nicholson et al. (1988a,b) found positive evidence that the binding of Zn2+ and Al3+ involved some covalent character. It is difficult to avoid the view, which is consistent with gelation theory, that crosslinking is involved in gelation. Simple ionic bridgesfitin with this view, but there are alternatives. Networks of type 3 (see Section 2.2.3) can be formed by crosslinks consisting of bundles of chains or multistranded helices (Flory, 1974). In gelatin, triple helices are involved, i.e. three chains are joined at a point (Peniche-Covas et al., 1974). In alginates, gelation is believed to result from the formation of a junction zone where there is local chain dimerization with cavities formed capable of holding calcium ions (Reid, 1983). This complicated junction of chain association and ion binding is known as the 'egg-box' model. Although this account of gelation is made with reference to organic polyelectrolytes, it is of wider application and may be applied to phosphoric acid cements. Orthophosphoric acid solutions used in these cements contain aluminium, and soluble aluminophosphate complexes are formed. Some appear to be multinuclear and there is evidence for polymers based on the bridging Al-O-P unit. These could be termed polyelectrolytes (Akitt, Greenwood & Lester, 1971; Wilson et al, 1972; O'Neill et al., 1982). References Akitt, J. W., Greenwood, N. N. & Lester, G. D. (1971). Nuclear magnetic resonance and Raman studies of aluminium complexes formed in aqueous solutions of aluminium salts containing phosphoric acid and fluoride ions. Journal of the Chemical Society, A, 2450-7. Asai, H. (1961). Study of the hydration-dehydration in polyelectrolyte solutions by the ultrasonic technique. Journal of the Physical Society of Japan, 16, 761-6. Atkins, P. W. (1978). Physical Chemistry, p. 338. Oxford: Oxford University Press. Atkinson, G. & Kor, S. K. (1965). The kinetics of ion association in manganese sulphate solutions. I. Results in water, dioxane-water mixtures, and methanol-water mixtures at 25 °C. Journal of Physical Chemistry, 69, 128-33. 85

Poly electrolytes, ion binding and gelation Atkinson, G. & Kor, S. K. (1967). The kinetics of ion association in manganese sulphate solutions. II. Thermodynamics of stepwise association in water. Journal of Physical Chemistry, 71, 673-7. Begala, A. J. (1971). Interactions of cations with polycarboxylic acids. PhD Dissertation. Rutgers University, The State University of New Jersey. Begala, A. J. & Strauss, U. P. (1972). Dilatometric studies of counterion binding by polycarboxylates. Journal of Physical Chemistry, 76, 254-60. Bratko, D., Dolar, D., Godec, A. & Span, J. (1983). Electric transport in poly(styrenesulfonate) solutions. Makromolekulare Chemie Rapid Communications, 4, 697-701. Bungenberg de Jong, H. G. (1949). In Kruyt, H. R. (ed.) Colloid Science II, p. 2. Amsterdam: Elsevier Publishing Co. Inc. Callis, C. F., Van Wazer, J. R. & Arvan, P. G. (1954). The inorganic phosphates as polyelectrolytes. Chemical Reviews, 54, 777-96. Cotton, F. A. & Wilkinson, G. (1966). Advanced Inorganic Chemistry, 2nd edn, Chapter 2. New York: Wiley Inter science. Crisp, S., Prosser, H. J. & Wilson, A. D. (1976). An infra-red spectroscopic study of cement formation between metal oxides and aqueous solutions of poly (acrylic acid). Journal of Materials Science, 11, 36-48. Eigen, M. & Tamm, K. (1962a). Schallabsorption in Elektrolytlosungen als Folge chemischer Relaxation. 1. Relaxationtheorie der mehrstufigen Dissoziation. Zeitschrift fur Elektrochemie, 66, 93-107. Eigen, M. & Tamm, K. (1962b). Schallabsorption in Elektrolytlosungen als Folge chemischer Relaxation. 2. Messergebnisse und Relaxationmechanismen fur 2-2-wertige Elektrolyte. Zeitschrift fur Elektrochemie, 66, 107-21. Ellis, J. & Wilson, A. D. (1990). Polyphosphonate cements: a new class of dental materials. Journal of Materials Science Letters, 9, 1058-60. Ferry, G. V. & Gill, S. (1962). Transference studies of sodium polyacrylate under steady state electrolysis. Journal of Physical Chemistry, 66, 999-1003. Flory, P. J. (1953). Principles of Polymer Chemistry. Ithaca, New York: Cornell University Press. Flory, P. J. (1974). Introductory lecture. In Gels and Gelling Processes. Faraday Discussions of the Chemical Society, No. 57, pp. 7-18. Fuoss, R. M. (1934). Distribution of ions in electrolyte solutions. Transactions of the Faraday Society, 30, 967-80. Fuoss, R. M. & Kraus, C. A. (1933). Properties of electrolytic solutions. IV. The conductance minimum and the formation of triple ions due to the action of Coulomb forces. Journal of the American Chemical Society, 55, 2387-99. Gregor, H. P. & Frederick, M. (1957). Titration studies of polyacrylic acid and polymethacrylic acids with alkali metals and quaternary ammonium bases. Journal of Polymer Science, 23, 451-65. Gregor, H. P., Gold, D. H. & Frederick, M. (1957). Viscometric and conductometric titrations of polymethacrylic acids with alkali metals and quaternary ammonium bases. Journal of Polymer Science, 23, 467-75. Gregor, H. P., Luttinger, L. B. & Loebl, E. M. (1955a). Metal-polyelectrolyte

86

References complexes. I. The polyacrylic acid-copper complex. Journal of Physical Chemistry, 59, 34-9. Gregor, H. P., Luttinger, L. B. & Loebl, E. M. (1955b). Metal-polyelectrolyte complexes. IV. Complexes of polyacrylic acid with magnesium, calcium, cobalt and zinc. Journal of Physical Chemistry, 59, 990-1. Grunwald, E. (1954). Interpretation of data obtained in nonaqueous media. Analytical Chemistry, 26, 1696-701. Grunwald, E. (1979). Structure of solvated ion pairs from electric dipole moments. Journal of Pure and Applied Chemistry, 51, 53-61. Harris, F. E. & Rice, S. A. (1954). A chain model for polyelectrolytes. I. Journal of Physical Chemistry, 58, 725-32. Harris, F. E. & Rice, S. A. (1957). A model for ion binding and exchange in polyelectrolyte solutions and gels. Journal of Physical Chemistry, 58, 725-32. Huizenga, J. R., Grieger, P. F. & Wall, F. T. (1950a). Electrolytic properties of aqueous solutions of polyacrylic acid and sodium hydroxide. I. Transference experiments using radioactive sodium. Journal of the American Chemical Society, 72, 2636-42. Huizenga, J. R., Grieger, P. F. & Wall, F. T. (1950b). Electrolytic properties of aqueous solutions of polyacrylic acid and sodium hydroxide. II. Diffusion experiments using radioactive sodium. Journal of the American Chemical Society, 72, 4228-32. Ikegami, A. (1964). Hydration and ion binding of polyelectrolytes. Journal of the Polymer Society, A2, 907-21. Ikegami, A. (1968). Hydration of polyacids. Biopolymers, 6, 431-40. Ikegami, A. & Imai, N. (1962). Precipitation of polyelectrolytes by salts. Journal of Polymer Science, 56, 133-52. Imai, N. (1961). Interaction between polyions and low molecular weight ions. Journal of the Physical Society of Japan, 16, 746-60. Irving, H. & Williams, R. J. P. (1953). The stability of transition-metal complexes. Journal of the Chemical Society, 3192-210. Jacobsen, A. (1962). Configurational effects of binding of magnesium to polyacrylic acids. Journal of Polymer Science, 57, 321-36. Kagawa, I. & Gregor, H. P. (1957). Theory of the effect of counter ion size upon titration behavior of polycarboxylie acids. Journal of Polymer Science, 23, 477-84. Kagawa, I. & Katsuura, K. (1955). Activity of counterions in polyelectrolyte solutions. Journal of Polymer Science, 17, 365-74. Mandel, M. & Leyte, J. C. (1964). Interactions of poly(methacrylic acid) and bivalent counterions. Journal of Polymer Science, A2, 2883-99. Manning, G. S. (1969). Limiting laws and counterion condensation in polyelectrolyte solutions. 1. Colligative properties. Journal of Chemical Physics, 51, 924-33. Manning, G. S. (1979). Counterion binding in polyelectrolyte theory. Accounts of Chemical Research, 12, 443-9. Manning, G. S. (1981). Limiting laws and counterion condensation in

87

Poly electrolytes, ion binding and gelation polyelectrolyte solutions. 6. Theory of the titration curve. Journal of Physical Chemistry, 85, 870-7. Michaeli, I. (1960). Ion-binding and the formation of insoluble polymethacrylic salts. Journal of Polymer Science, 48, 291-9. Morawetz, H. (1975). Macromolecules in Solution, 2nd edn, Chapter 7. New York: Wiley. Muto, N., Komatsu, T. & Nakagawa, T. (1973). Counterion effect on the titration behaviour of poly(maleic acid). Bulletin of the Chemical Society of Japan, 46, 2711-15. Muto, N. (1974). Counterion effect on the titration behaviour of poly(itaconic acid). Bulletin of the Chemical Society of Japan, 47, 1122-8. Nagasawa, M. & Kagawa, I. (1957). Colligative properties of polyelectrolyte solutions. IV. Activity coefficient of sodium ion. Journal of Polymer Science, 25, 61-76. Nicholson, J. W., Brookman, P. J., Lacy, O. M., Sayers, G. S. & Wilson, A. D. (1988a). A study of the nature and formation of zinc polyacrylate cement using Fourier transform infrared spectroscopy. Journal of Biomedical Materials Research, 22, 623-31. Nicholson, J. W., Brookman, P. J., Lacy, O. M. & Wilson, A. D. (1988b). Fourier transform infrared spectroscopic study of the role of tartaric acid in glass-ionomer cements. Journal of Dental Research, 67, 1450-4. O'Neill, I. K., Prosser, H. J., Richards, C. P. & Wilson, A. D. (1982). NMR spectroscopy of dental materials. 1.31P studies on phosphate-bonded cement liquids. Journal of Biomedical Materials Research, 16, 39-49. Oosawa, F. (1957). A simple theory of thermodynamic properties of polyelectrolyte solutions. Journal of Polymer Science, 23, 421-30. Oosawa, F. (1971). Polyelectrolytes. New York: Marcel Dekker. Peniche-Covas, C. A. L., Dev, S. B., Gordon, M., Judd, M. & Kajiwara, K. (1974). The critically branched state in covalent synthetic systems and the reversible gelation of gelatin. In Gels and Gelling Processes. Faraday Discussions of the Chemical Society, No. 57, pp. 165-80. Reid, D. S. (1983). Ionic polysaccharides. In Wilson, A. D. & Prosser, H. J. (eds.) Developments in Ionic Polymers-1, Chapter 6. London and New York: Applied Science Publishers. Rice, S. A. & Harris, F. E. (1954). A chain model for polyelectrolytes. II. Journal of Physical Chemistry, 58, 733-9. Robinson, R. A. & Stokes, R. H. (1959). Electrolyte Solutions, 2nd edn, Chapter 14. London: Butterworths. Rymden, R. & Stilbs, P. (1985a). Counterion self-diffusion in aqueous solutions of poly (aery lie acid) and poly (methacry lie acid). Journal of Physical Chemistry, 89, 2425-8. Rymden, R. & Stilbs, P. (1985b). Concentration and molecular weight dependence of counterion self-diffusion in aqueous poly(acrylic acid) solutions. Journal of Physical Chemistry, 89, 3502-5. Salmon, J. E. & Wall, J. G. L. (1958). Aluminium phosphates. Part II. Ion-

References exchange and pH-titration studies of aluminium phosphate complexes in solution. Journal of the Chemical Society, 1128-34. Satoh, M., Komiyama, J. & Iijima, T. (1984). Counterion condensation in polyelectrolyte solutions: a theoretical prediction of the dependences on the ionic strength and degree of polymerization. Macromolecules, 18, 1195-2000. Szwarc, M. (1965). Ions, ion-pairs, and their agglomerates. Die Makromolekulare Chemie, 89, 44-80 (in English). Smith, D. C. (1968). A new dental cement. British Dental Journal, 125, 381-4. Strauss, U. P. & Leung, Y. P. (1965). Volume changes as a criterion for site binding of counterions by poly electrolytes. Journal of the American Chemical Society, 87, 1476-80. Sveshnikova, V. N. & Zaitseva, S. N. (1964). Aluminophosphates as poly electrolytes. Russian Journal of Inorganic Chemistry, 9, 672-5. Van Wazer, J. R. (1954). In Rice, S. A. & Harris, F. E. (1954). A chain model for polyelectrolytes. II. Journal of Physical Chemistry, 58, 739. Wall, F. T. & Drenan, J. W. (1951). Gelation of polyacrylic acids by divalent cations. Journal of Polymer Science, 7, 83-8. Wall, F. T. & Gill, S. J. (1954). Interaction of cupric ions with polyacrylic acid. Journal of Physical Chemistry, 58, 1128-30. Wilson, A. D. & Crisp, S. (1977). Organolithic Macromolecular Materials, Chapters 2 & 4. London: Applied Science Publishers. Wilson, A. D. & Ellis, J. (1989). Poly-vinylphosphonic acid and metal oxide or cermet or glass-ionomer cements. British Patent Application 2, 219, 289A. Wilson, A. D. & Kent, B. E. (1971). The glass-ionomer cement: a new translucent cement for dentistry. Journal of Applied Chemistry and Biotechnology, 21, 313. Wilson, A. D., Kent, B. E., Clinton, D. & Miller, R. P. (1972). The formation and microstructure of dental silicate cement. Journal of Materials Science, 1, 220-38. Winstein, S., Clippinger, E., Fainberg, A. H. & Robinson, G. C. (1954). Salt effects and ion-pairs in solvolysis. Journal of the American Chemical Society, 76, 2597-8. Winstein, S. & Robinson, G. C. (1958). Salt effects and ion-pairs in solvolysis and related reactions. IX. The //zre0-3-/>-anisyl-2-butyl system. Journal of the American Chemical Society, 80, 169-81.

89

5

Polyalkenoate cements

5.1

Introduction

Poly(acrylic acid) and its salts have been known to have useful binding properties for some thirty years; they have been used for soil consolidation (Lambe & Michaels, 1954; Hopkins, 1955; Wilson & Crisp, 1977) and as aflocculant(Woodberry, 1961). The most interesting of these applications is the in situ polymerization of calcium acrylate added to soil (de Mello, Hauser & Lambe, 1953). But here we are concerned with cements formed from these poly acids. The polyelectrolyte cements are modern materials that have adhesive properties and are formed by the cement-forming reaction between a poly(alkenoic acid), typically poly(acrylic acid), PAA, in concentrated aqueous solution, and a cation-releasing base. The base may be a metal oxide, in particular zinc oxide, a silicate mineral or an aluminosilicate glass. The presence of a polyacid in these cements gives them the valuable property of adhesion. The structures of some poly(alkenoic acid)s are shown in Figure 5.1. The polyelectrolyte cements may be classified by the type of basic powder used to form the cement. (1) (2) (3) (4)

The metal oxide cements (Section 5.6) The zinc polycarboxylate cement (Section 5.7) The mineral ionomer cements (Section 5.8) The glass-ionomer or glass polyalkenoate cement (Section 5.9)

Only two of these materials are of practical importance: the zinc polycarboxylate cement of Smith (1968) and the glass-ionomer cement of Wilson & Kent (1971). Both are used in dental applications and both have been used as bone cements. The glass-ionomer cement is, perhaps, the most versatile of all AB cements. It has many applications in dentistry: a 90

Introduction filling material for the restoration of anterior (front) teeth, a cementing agent for the attachment of crowns and bridges, a cavity liner and a base under amalgams and composite resins, and a general repair material. Outside dentistry it is marketed as a splint bandage material and as a bone cement. It has also been considered as an underwater cement for North Sea pipelines, as a replacement for plaster of Paris in slip casting, and as a model material. The invention and development of the zinc polycarboxylate and glass-ionomer cements was brought about by a change in basic attitudes in materials science in dentistry. This largely revolved around the necessity of inventing materials which would adhere to tooth enamel and dentine.

Acrylic acid unit

I

CH—COOH

CH2 .COOH

Itaconic acid unit

-CH 2 COOH

T CH—COOH

Maleic acid unit

CH—COOH

7

CH 9

.COOH XH—COOH

3-Butene 1,2,3-tricarboxylic acid unit

CH2COOH Figure 5.1 The structure of poly(alkenoic acid)s containing acrylic, itaconic, maleic and 3butene 1,2,3-tricarboxylic acid units.

91

Polyalkenoate cements 5.2

Adhesion

5.2.1

New attitudes

Up to the 1950s the quality of a dental material was judged entirely by its physical and mechanical properties (Wilson, 1991). This proved to be a concept which hampered development. We may take the amalgam as representing the traditional dental restorative material with all its advantages and disadvantages. The amalgam is strong and resistant to abrasion, but it is essentially a foreign body in the tooth, an unattractive black mass of metal that does not bond to tooth structure. In order to ensure its mechanical retention, cavities have to be cut which are wasteful of sound tooth material. It does nothing for the tooth and, despite its excellent mechanical properties, is little more than a mechanical plug. In the late 1940s a reaction against this idea of a dental material took place. Increasing attention was paid to problems of compatibility between the restoration and the tooth. We now believe that a restorative should be at one with the tooth material in all respects. It should possess identical properties. Its thermal characteristics should be the same as those of the tooth and its appearance should match that of the enamel. It should provide some therapeutic action. In fact, a restorative material should no longer be regarded as a 'filling' but as an 'enamel or dentine substitute'. 5.2.2

The need for adhesive materials

To achieve such compatibility the primary requisite is that the restorative adheres to tooth material. This concept of adhesion is hardly to be found in the literature of the 1920s and 1930s. For that reason wefindno attempt at developing tooth adhesives in that period. Adhesion was, apparently, only recognized as a desirable property in the 1950s. It seems for some reason to be associated with the introduction of simple resins as dental restorative materials. Although they were not a great success, attempts were made to bond them to tooth material. The Conference on Adhesive Restorative Dental Materials held in Indianapolis in 1961 (Phillips & Ryge, 1961) may be considered as ushering in the era when dental adhesives were actively sought. Buonocore (1961) summed up the new thinking. The lack of adhesion of available filling materials to tooth structure is considered as one of their shortcomings. A solution to this problem would indeed represent a milestone in dentistry. 92

Adhesion Thus, thought became directed towards developing adhesive dental materials, an approach that has led to considerable successes and has revolutionized restorative dentistry.

5.2.3 Acid-etching The first experimental study on adhesion appears in a paper by Kramer & McLean (1952). They reported on the use of glycerol phosphoric acid dimethacrylate as a dentine bonding agent. They achieved some success but, unfortunately, the bond deteriorated with time (Buonocore, 1961). More significant was thefindingreported by Buonocore in 1955. This was his innovative technique for the acid-etching of enamel for the micromechanical attachment of dental resins. Resins are capable of penetrating an etched surface and, when polymerized, are bonded to the enamel by resin tags. Buonocore's significant innovation proved to be far ahead of its time because the simple restorative resins then available were not a clinical success. Buonocore's invention remained unnoticed until the arrival of the composite resin some ten years later. This technique has ensured the lasting success of the composite resin. The dental surgeon now has the means to achieve the aesthetic restoration of damaged incisal edges on anterior teeth. Formerly such damaged teeth would have had to be crowned. Although the importance of Buonocore's discovery cannot be overemphasized, micromechanical attachment cannot be regarded as true adhesion. True adhesion must be on the molecular level and must involve chemical or physicochemical bonds.

5.2.4

Obstacles to adhesion

There are many obstacles to permanent adhesion under oral conditions. The substrate is a biological tissue and subject to change, and the presence of moisture represents the worst kind of situation for adhesion. Water is the great barrier to adhesion. It competes for the polar surface of tooth material against any potential polymer adhesive. It also tends to hydrolyse any adhesive bond formed. These twin obstacles gave rise to considerable doubt as to whether materials adhesive to tooth material could be developed at all (Cornell, 1961). Nevertheless adhesive materials were developed, for in 1968 Dennis Smith announced the zinc polycarboxylate cement (Smith, 1968,1969) and 93

Polyalkenoate cements this material was followed by the glass-ionomer cement of Wilson and Kent in 1969 (Wilson & Kent, 1971, 1972, 1973, 1974). These inventions demonstrated that materials based on poly(acrylic acid) or similar poly acids are effective dental adhesives. Even today, these materials are the only ones known with certainty to form a permanent bond to tooth material - that is a bond that does not deteriorate with time; if anything the bond strength increases (Tyas et al., 1988).

5.2.5

The nature of the adhesion of polyalkenoates to tooth material

Dentine forms the bulk of a tooth and is covered by a harder material, enamel. Enamel is almost entirely inorganic, with only 0-25-0-45 % protein and 0-60% lipids (Hess, 1961). Enamel is composed of rod-shaped structural units known as enamel prisms c. 5 jam in diameter (Silverstone, 1982). It is generally accepted that the mineral is hydroxyapatite (Posner, 1961; Silverstone, 1982), although the evidence is not entirely conclusive. Dentine is composed of 70 % inorganic material, 20 % organic material and 10 % water (Ten Cate & Torneck, 1982). The mineral portion is largely a hydroxyapatite-like mineral and the organic portion is largely the protein collagen. The precise nature of the adhesion of the polyelectrolyte cements to untreated dental enamel and dentine has yet to be established. The earliest theory was due to Smith (1968) who speculated that the polyacrylate chains of the cement formed a chelate with calcium ions contained in the hydroxyapatite-like mineral in enamel and dentine. Beech (1973) considered this unlikely since it involved the formation of an eight-membered ring. Beech studied the interaction between PAA and hydroxyapatite, identified the formation of polyacrylate and so considered that adsorption was due to ionic attraction. Wilson (1974) emphasized the importance of wetting the substrate surface. Later, as the reaction proceeded, these hydrogen bonds would be replaced by ionic salt bridges. Wilson stressed the importance of the polymeric nature of these cements in adhesion. Their polymeric nature allowed interfacial gaps between cement and substrate to be bridged and also provided a multiplicity of bonds. Under oral conditions, where the substrate is subject to change, adhesive bonds will be broken, but if there are a multiplicity of these, attachment of the cement to the substrate will endure and allow broken bonds to be re-established. It is significant that 94

Adhesion

the related phosphate cements based on monomeric [POJ units do not have this adhesive property. Wilson, Prosser & Powis (1983) studied the adsorption of polyacrylate on hydroxyapatite using infrared and chemical methods. They observed an exchange of ions and concluded that polyacrylate displaced surface phosphate and calcium, and entered the hydroxyapatite structure itself (Figure 5.2). They postulated that an intermediate layer of calcium and aluminium phosphates and polyacrylates must be formed at the cement0"

0 0 "

o-

0"

O

O

Hydroxy apatite Surface

Ca 2+

0"

Ca

2+

Ca 2+

COO"

POf

o-

ooCa2+

o Ca2+

Figure 5.2 The adsorption of polyacrylate on hydroxyapatite.

95

Polyalkenoate cements hydroxyapatite interface. This layer has actually been observed by Mount (1990) who observed debonding at the interface between this intermediate layer and the body of the cement when the cement was dehydrated. Adhesion in vivo appears to be dynamic. Bonding to bone was observed to be disrupted as extensive bone remodelling took place, and then reestablished once damage had been repaired (Brook, Craig & Lamb, 1991b). Adsorption studies In order to elucidate the mechanism of adhesion of ionomer-carboxylate cements, Wilson and his coworkers have carried out several studies on the adsorption of carboxylates - aliphatic, aromatic and polymeric-on hydroxyapatite (Skinner et ai, 1986; Scott, Jackson & Wilson, 1990; Ellis et al9 1990). Aliphatic monocarboxylates are not adsorbed at all (Skinner et al, 1986). The extent of the adsorption of aliphatic dicarboxylates depends on the spacing between the carboxyl groups, and is greatest when the number of carbon atoms in the molecule is three or four (malonate and succinate). Adipate (six carbon atoms) is not adsorbed at all. Adsorption is, therefore, dependent on a cooperative effect between pairs of carboxyl groups. Adsorption cannot occur at OH sites in hydroxyapatite for these are 0-69 or 0-94 nm apart and, remembering that the length of the C-C bond lies between 014 to 016 nm, could only be bridged by the pairs of carboxyls in adipate, which is not adsorbed. However, attachment to H2PC>4 sites is possible via hydrogen bonds. Similar results were found in a study of aromatic carboxylates with one to six carboxyl groups (Scott, Jackson & Wilson, 1990). Adsorption increased with the number of carboxyl groups and was also dependent on the spacing between the carboxyl groups. With the benzene dicarboxylates, maximum permanent adsorption was obtained with the 1,3-dicarboxylate, while the 1,4-dicarboxylates was not adsorbed at all. This is again evidence of the cooperative effect between carboxyl groups. Polymeric aliphatic carboxylates, the poly(alkenoic acid)s, were very much more strongly adsorbed than the difunctional carboxylates (Ellis et al., 1990). Results showed that adsorption depended on the conformation of the polyanion. When extended, as in dilute solutions, a polyanion is adsorbed onto a relatively large number of sites and further adsorption is hindered. Thus, increases in acidity (and concentration) were found to result in greater adsorption because the polyanion adopted a more compact 96

Preparation of poly(alkenoic acid)s

conformation. Above a certain concentration, further adsorption had a reversible rather than an irreversible (permanent) character. High levels of adsorption were achieved under conditions of high chain entanglement, that is with polyanions of high molecular weight. In all studies it was noted that calcium and phosphate ions were displaced, the amount generally increasing with the degree of carboxylate adsorption. It would appear that negatively charged carboxylate groups disrupt the hydroxyapatite surface, upsetting the equilibrium between phosphate in solid phase and solution phase thus allowing phosphate to be exchanged by carboxylate.

5.3

Preparation ofpoly(alkenoic acid)s

The most common poly(alkenoic acid) used in polyalkenoate, ionomer or polycarboxylate cements is poly(acrylic acid), PAA. In addition, copolymers of acrylic acid with other alkenoic acids - maleic and itaconic and 3-butene 1,2,3-tricarboxylic acid - may be employed (Crisp & Wilson, 1974c, 1977; Crisp et al, 1980). These polyacids are prepared by freeradical polymerization in aqueous solution using ammonium persulphate as the initiator and propan-2-ol (isopropyl alcohol) as the chain transfer agent (Smith, 1969). The concentration of poly(alkenoic acid) is kept below 25 % to avoid the danger of explosion. After polymerization the solution is concentrated to 40-50% for use. Poly(alkenoic acid)s may be prepared as follows. 200 cm3 of a solution containing between 0-5 and 2-5 g of ammonium persulphate contained in a flask is heated to a controlled temperature, lying between 80 and 95 °C, while purging with nitrogen to displace dissolved oxygen. Two solutions, Solution (I) and Solution (II) are added, in the ratio 3-4:1-0, to the flask charge, with continuous stirring, over a period of two hours. These solutions are: Solution (I) 100 g redistilled inhibitor-free alkenoic acid, 20 g propan2-ol in 100 cm3 water Solution (II) 0-5-2-5 g ammonium persulphate in 60 cm3 water. After the addition is completed the contents of the flask are heated for a further two hours. The reaction mixture is then concentrated by vacuum distillation at 40-45 °C until the desired concentration is attained. 97

Polyalkenoate cements The molecular mass of the poly acid obtained lies between 10000 and 55000. Increasing the temperature of polymerization and the concentration of ammonium persulphate serves to decrease the molecular mass of the poly(alkenoic acid). Poly(acrylic acid) is very soluble in water as are its copolymers with maleic and itaconic acids. Solutions of 50% by mass are easily obtained. The 'isomer' of PAA, poly(ethylene maleic acid), is not so soluble. However, solutions of PAA tend over a period of time to gel when their concentration in water approaches 50 % by mass (Crisp, Lewis & Wilson, 1975); this is attributed to a slow increase in the number of intermolecular hydrogen bonds. Copolymers of acrylic acid and itaconic acid are more stable in solution and their use has been advocated by Crisp et al. (1975, 1980). 5.4

Setting reactions

The cement-forming reaction of the polyelectrolyte cements may be considered to take place in a number of overlapping stages. These are the attack by the acid on the oxide or glass, the migration of the liberated ions from the oxide or glass into the aqueous phase, the ionization of the polyacid with consequent unwinding of the polymer chain, the interaction between the charged chains and oxide or glass cations leading to ion binding and gelation, and lastly the hardening phase represented by the continuation of ion binding. Setting results from the gelation of the poly(alkenoic acid) by metal ions liberated from the metal oxide or silicate by acid attack. The gelation of polysalts, which has been discussed in Sections 2.2.3 and 4.3, occurs as the pH of the cement increases. As pointed out in those chapters there are several physicochemical processes that underlie these rheological changes. Amongst these are conformational changes in the polymer chain, binding of the cations to the polymer chains, and hydration changes. As reaction proceeds, the polymer chain (which is in random coil form) unwinds as the charge on it grows as a result of neutralization and ionization. This contributes to thickening of the cement paste. Cations released become bound to the polymer chain. Countercations can either be bound to a polyanionic chain by general electrostatic forces or be sitebound at specific centres. More than one type of site binding is possible. Complex formation and, if the ligand is bidentate, chelate formation enhance the effect. 98

Molecular structures The extent and rate of interaction between hydrated counterion and polyanion depend on polymer structure, acid strength, conformation, degree of dissociation, and distribution and density of ionic charge on the polymer chain. This interaction between the cations - the counterions and the polyanion chain disrupts the hydration regions surrounding both. Desolvation of the ion-pair, which depends on the nature of the cation and the degree of neutralization, results in gelation. Gelation itself occurs suddenly when the critical condition for the formation of an infinite random network is met, that is when there are more than two crosslinks per polymer chain (Flory, 1953, 1974).

5.5

Molecular structures

The molecular structure of the polyelectrolyte cements has been examined by a number of workers using infrared spectroscopy (Crisp et ah, 1914; Crisp, Prosser & Wilson, 1976; Wilson, 1982; Nicholson et al., 1988a,b). The asymmetrical COO" stretching modes in particular can be used to

0

0

C«0Zn

2 +

0iC

0 (a)

\ ^ ZrC

C,

Zn

0 IONIC

^—•-" P I \ \

' c ———^ / /

( c ) CHELATING BIDENTATE

(b) BRIDGING BIDENTATE \ CH-

\^

CH,

Zn

CH

(d) ASYMMETRIC UNIDENTATE

C ^ \ 0 (e) CHELATE BIDENTATE 8-membered ring /

Figure 5.3 Metal polyacrylate molecular structures: (a) purely ionic, (b) bridging bidentate, (c) chelating bidentate, (d) asymmetric unidentate, (e) chelate bidentate.

99

Polyalkenoate cements obtain structural information; if the metal-carboxylate bond is not purely ionic and coordination complexes are formed then there are frequency shifts. The types of structure are given in Figure 5.3. These structures are (a) purely ionic, (b) bridging bidentate, (c) chelating bidentate, (d) asymmetric unidentate, (e) chelate bidentate (Nakamoto, 1963; Mehrotra, Bohra & Gauer, 1978; Mehrotra & Bohra, 1983). Infrared spectroscopy is not able to distinguish between all these structures but asymmetric stretching bands can be used to distinguish between COOH (c. 1700 cm"1), ionic COO" (c. 1540 cm"1) and certain coordination complexes. Infrared spectroscopy shows complexes with asymmetric bands both above and below the ionic COO" band. The bonds between PAA and Na+, Mg2+, Ca2+ are purely ionic, but the absorption bands of other cation-PAA interactions - Zn2+ (15401560 cm"1), Cu2+ (1605 cm-1) and Al3+ (1600 cm" 1 )-show evidence of complex formation. The strength and stability of cements parallel the I

I

o

|

\

/

I

H20

/

/ \

CH 2

|

CH—C — 0"—Ca2—"0—C CH2

CH2

H20

H20

CH 2

H20

CH2

H20

\

CH — C — 0

CH

o

H20

0

/ Ca

H20

H20

CH2

A

CH2

CH 2

A'

I 8 \ /

H20

CH 2

CH—C—0'—Al 3 — 0 — C — CH

I CH I

2

/ \ H0 2

H20

I

5 ICH

2

CH—C

H20 0"| /

CH 2

Ca2

CH — C

H20

I °

H20

CH2

Figure 5.4 Hypothetical molecular structures in polyalkenoate cements, where A represents OH" or F-.

100

Molecular structures magnitude of the complexation constants of the cations and are in the order Al3+ > Cu2+ > Zn2+ > Ca2+ > Mg2+. The detailed molecular structure of the polyelectrolyte cements remains a subject for conjecture. The structure is determined basically by the charge and coordination number of the cation. Firstly, we must consider the question of coordination and examine it in respect of the three most important cations in these cements: Zn2+, Ca2+ and Al3+. Of the divalent cations, Zn2+ can assume a coordination number of 4, 5 or 6 and Ca2+ of 6 or 8. If we assume a coordination number of 6, then an electrically neutral coordination complex would have to contain two ligands with a single negative charge and four neutral ligands. The coordination of Al3+ in aqueous solution is 6 and for an electrically neutral complex this requires that there should be three single-charged ligands and three neutral ligands. In AB cements the ligands available are COO~, F", OH~ and H2 O; there is a possibility of chelate formation and there are a number of possible complexes. Chelate formation and bridging between chains cannot be excluded. Cations can be seen as acting as ionic crosslinks between polyanion chains. Although this may appear a naive concept, crosslinking can be seen as equivalent to attractions between polyions resulting from the fluctuation of the counterion distribution (Section 4.2.13). Moreover, it relates to the classical theory of gelation associated with Flory (1953). Divalent cations (Zn2+ and Ca2+) have the potential to link two polyanion chains. Of course, unlike covalent crosslinks, ionic links are easily broken and re-formed; under stress there could therefore be chain slipping and this may explain the plastic nature of zinc polycarboxylate cement. A trivalent cation, for example Al3+, has the potential to link three chains. Sterically, this is improbable; Mehrotra & Bohra (1983) assert that simple aluminium tricarboxylates are not known in solution. Nevertheless we consider it probable that a small proportion of Al3+ ions link three chains, in which all three charged ligands are COO". More probable molecular structures would contain one or two F~ ions; with a single F", an [A1F(H2O)3]2+ unit could bridge two polyanion chains, while an [A1F2(H2O)2]+ would have no crosslinking ability. Some possible molecular structures are depicted in Figure 5.4.

101

Polyalkenoate cements Table 5.1. Compressive strength of metal oxide-poly{acrylic acid) cements {Elliott, Holliday & Hornsby, 1975; Hornsby, 1977) Wet compressive

Oxide

ZnO CuO HgO PbO MgO Bi2O3

5.6

liquid, gem" 3

Strength,

Modulus,

Strain at failure,

MPa

GPa

%

1-4 20 10 20 10 2-0

76 83 29 26 58 32

1-40

102 0-66 0-61 0-45 0-97

5-4 81 4.4 4-3 12-9

3-3

Trup

porosity, % 18 24 25 26 — 12

Metal oxide poly electrolyte cements

Many divalent and trivalent oxides form cements with PAA (Crisp, Prosser & Wilson, 1976; Hodd & Reader, 1976; Hornsby, 1977). Cement formation was observed using infrared spectroscopy and physical and chemical tests. Of these cements that of ZnO (Smith, 1968) was thefirstand remains by far the most important; it is given detailed treatment in Section 5.7. Certain oxides of divalent metals, those of ZnO, CuO, SnO, HgO, and PbO, form cements that are hydrolytically stable; in addition MgO, CaO, BaO and SrO form cements that are softened when exposed to water. Compressive strengths of these materials range from 26 to 83 MPa, the strongest being the copper(II) and zinc polyacrylate cements (Table 5.1). Crisp, Prosser & Wilson (1976) found that for divalent oxides the rate of reaction increased in the order CuO < ZnO < CaO < MgO. This is in descending order of the stability constants of the cations. Trivalent oxides A12O3, La2O3, Bi2O3 and Y2O3 are also capable of cement formation but the reaction is only partial (Hornsby, 1977). Hornsby also made the interesting observation that B2O3 forms a cement with poly(acrylic acid), but, since B2O3 is acidic, an acid-base reaction does not take place. Although the cement is hydrolytically unstable it is of theoretical interest; it is to be presumed that cement formation takes place by the formation of hydrogen-bonded complexes rather than by salt formation. 102

Zinc polycarboxylate cement The nature of the poly(alkenoic acid) can affect the hydrolytical stability of metal oxide cements (Hodd & Reader, 1976). For example the B2O3-poly(ethylene maleic acid) cement, unlike its poly(acrylic acid) counterpart, is not hydrolytically stable.

5.7

Zinc poly carboxy late cement

5.7.1

Historical

The zinc polycarboxylate cement was the first of a new generation of dental cements. It is based on the gelation of concentrated solutions of a poly(alkenoic acid) by zinc ions provided by a zinc oxide powder (Wilson, 1975a,b, 1978a). It was invented as a result of a search by Smith (1968, 1969) for a luting cement that would, unlike the traditional zinc phosphate dental cement, adhere to tooth material. It was the first adhesive dental cement discovered and represented a considerable advance in dental cement technology. It combined the strength of the zinc phosphate cement with the bland qualities of the zinc oxide eugenol cement. It is used for luting, lining and as a periodontal pack. Indeed, it can be used to replace the zinc phosphate dental cement in all applications with the possible exception of post crowns (crowns which are placed on a metal post placed in the tooth root) and cantilever bridges (Smith, 1982a). There are a number of brands on the market and, as far as can be ascertained, development of this cement has virtually ceased since the mid 1970s. 5.7.2

Composition

In their original form these cements came as a zinc oxide powder and a concentrated solution of poly(acrylic acid) (Wilson, 1975b). Since then they have been subject to a number of chemical modifications. The liquid is usually a 30-43 % solution of a poly(alkenoic acid) which is a homopolymer of acrylic acid or a copolymer with itaconic acid, maleic acid, or 3-butene 1,2,3-tricarboxylic acid (Smith, 1969; Bertenshaw & Combe, 1972a; Jurecic, 1973; ESPE, 1975; Wilson, 1975b; Suzaki, 1976; Crisp, Lewis & Wilson, 1976a; Crisp & Wilson, 1974c, 1977; Crisp et aL, 1980). The method of preparation has already been given in Section 5.3, and the structures of these alkenoic acid units are shown in Figure 5.1. The molecular mass of these polyacids varies from 22000 to 49000 103

Polyalkenoate cements Table 5.2. Composition of zinc polycarboxylate cements (Bertenshaw & Combe, 1972a, b, 1976) Zinc oxide powder: 85-2-96-8% ZnO; 4-73-10-06% MgO Poly(acrylic acid) solution: 32-4-42-9 % Molecular weight: 15000-50000 Copolymers of acrylic acid with maleic or itaconic acid are sometimes substituted for poly (acrylic acid). (Smith, 1969; Bertenshaw & Combe, 1976). There is an optimum molecular mass. A high molecular mass gives high-strength cements but leads to difficulties in manipulation of the cement paste. Two methods are available for the preparation of the powder (Smith, 1969). In one, zinc oxide is ignited at 900 to 1000 °C for 12 to 24 hours until activity is reduced to the desired level. This oxide powder is yellow, presumably because zinc is in excess of that required for stoichiometry. Alternatively, a blend of zinc oxide and magnesium oxide in the ratio of 9:1 is heated for 8 to 12 hours to form a sintered mass. This mass is ground and reheated for another 8 to 12 hours. The powder is white. Altogether the powder is similar to that used in zinc phosphate cements. Commercial powders are composed chiefly of a deactivated zinc oxide containing up to 10% magnesium oxide (Bertenshaw & Combe, 1972b; Kohmura & Ida, 1979). In addition they may contain silica, alumina or bismuth salts. The most important additive is stannous fluoride (4-5 %), which strengthens the cement although it was originally added as a fluoride release agent (Foster & Dovey, 1974). In some brands the polyacid is in dry form and blended with the zinc oxide powder (Baumann & Gerhard, 1970; Jurecic, 1973; Bertenshaw & Combe, 1972a). The cement is formed by mixing this powder blend with water. In early examples, sodium dihydrogen phosphate was added to the liquid (Bertenshaw & Combe, 1972a); as a result the viscosity of the cement paste was lowered and setting was retarded, possibly because of the slow dissolution of the solid polyacid (Bertenshaw, Combe & Grant, 1979). Typical compositions of zinc polycarboxylate cements are given in Table 5.2.

104

Zinc polycarboxylate cement 5.7.3

Setting and structure

The cement sets as the result of an acid-base reaction between a zinc oxide dental powder and a poly(alkenoic acid). The pH increases and an insoluble amorphous salt is formed which acts as the cement matrix. A general account of the gelation processes is given in Section 5.4. Wilson (1982) studied the setting reaction of a stoichiometric cement and a cement containing 100% excess of zinc oxide, using infrared spectroscopy and physical and chemical methods. The setting of the cement, measured by an oscillating rheometer, was paralleled by the loss of bands associated with COOH groups (1700 cm"1 asymmetric C-O stretch) and the appearance and growth of carboxylate bands at 1540-1560 cm"1 (asymmetric C-O stretch). Using deuterated cements, two bands were observed in the young cement paste (1550 and 1560 cm"1), but only one in the set cement (1550 cm"1). Nicholson et al (1988a) clarified this picture using Fourier transform infrared spectroscopy. An initial fast ionic reaction, associated with a band at 1562 cm"1, was attributed to a purely ionic structure (Figure 5.3a). Later, as the cement matured, bands at 1554 and 1548 cm"1 became predominant; these were tentatively assigned to chelated structure (Figure 5.3c). Finally, when the cement had set there was one band at 1537 cm"1 (Figure 5.3d). This was attributed to a change in bond type during setting and hardening. Of course, assignment of bands to bond type is rendered more difficult by hydration and dehydration processes. Wilson, Paddon & Crisp (1979) and Wilson, Crisp & Paddon (1981) noted that as the cement matured the proportion of bound water to total water increased. X-ray diffraction shows that both the cement matrix and the salt are amorphous (Wilson, 1982; Smith, 1971; Steinke et al, 1988). On the basis of chemical analysis, Wilson (1982) assigned the following empirical formula to the zinc polyacrylate salt: Zn0.98H0.004(CH2. CH. COO)2 ,{H,O\

61

He compared the infrared spectra of cements with that of zinc polyacrylate salt and found differences. Inspection of his data shows that, unlike the cements, the salt was purely ionic, so that it seems here that cement formation is associated with the formation of coordination complexes. There are no ligand field stabilization effects with the Zn2+ ion because it has a completed d shell (Cotton & Wilkinson, 1966). For this reason the 105

Polyalkenoate cements stereochemistry of Zn2+ compounds is determined solely by size, electrostatic forces and covalent bonding forces. Zinc can be four-,five-or sixcoordinate. Most commonly it is four-coordinate, although six-coordinate compounds are known. Five-coordination is rare. Scanning electron microscopy shows the cement to consist of zinc oxide particles embedded in an amorphous matrix (Smith, 1982a). As with the zinc phosphate cement, a separate globular water phase exists since the cement becomes uniformly porous on dehydration. Porosity diminishes as the water content is decreased. Wilson, Paddon & Crisp (1979) distinguish between two types of water in dental cements: non-evaporable (tightly bound) and evaporable (loosely bound). They found, in the example they examined, that the ratio of tightly bound to loosely bound water was 0-22:1-0, the lowest for all dental cements. They considered that loosely bound water acted as a plasticizer and weakened the cement. Most practical cements contain Mg2+ which is less strongly bound to the polyacrylate than Zn2+ (Gregor, Luttinger & Loebl, 1955a). Magnesium oxide forms a paste with PAA which sets to a plastic mass; this is not hydrolytically stable, for when placed in water it swells and softens (Hornsby, 1977; Smith, 1982a). Moreover, if ZnO powder contains more than 10% MgO, the resultant cement deteriorates under oral conditions. Evidence for the firm binding of Zn2+ comes from studies using labelled zinc polyacrylate containing 65Zn and 14C. Only small amounts of these ions were lost to a saline solution over a three-month period, even in the presence of calcium (Peters et al., 1972; Peters, Jackson & Smith, 1974). There is some evidence, from leaching studies, that Zn2+ is more firmly bound to a copolymer of acrylic and itaconic acids than to poly(acrylic acid), and less firmly bound to a copolymer of maleic and acrylic acids. 5.7.4

Properties

Setting The zinc polycarboxylate cement sets within a few minutes of mixing and hardens rapidly. Strength is substantially developed within an hour. However, even when fully hardened the cement exhibits marked plastic behaviour. Its most important property is its ability to bond permanently to untreated dentine and enamel. The early zinc polycarboxylate cement did not possess the ease of mixing characteristic of the zinc phosphate and zinc eugenolate cements. It suffered because it was expected to mix exactly as a traditional zinc 106

Zinc polycarboxylate cement Table 5.3. Properties of zinc polycarboxylate cements (Jendresen & Trowbridge, 1972; Plant, Jones & Wilson, 1972; Paddon & Wilson, 1976; Powers, Johnson & Craig, 1974; Powers, Farah & Craig, 1976; Chamberlain & Powers, 1976; Levine, Beech & Garton, 1977; 0ilo & Espevik, 1978; Bertenshaw, Combe & Grant, 1979; Peddy, 1981; Hinoura, Moore & Phillips, 1986) Working time, 23 °C Setting time, 37 °C Compressive strength (wet), 24 h Tensile strength (wet), 24 h Compressive modulus (wet), 24 h Adhesion to enamel, 24 h (tensile) Adhesion to dentine, 24 h (tensile)

2-5 minutes 3-12 minutes 48-80 MPaa 4-8-15-5 MPab 3-2-6-2 GPa 4-1-6-9 MPa 2-2-5-1 MPa

a

Omitting atypical high and low values of 10 and 100 MPa Measured by the diametral compression method, omitting an atypical low value of 1-5 MPa

6

phosphate cement and the viscous nature of the polyacid liquid could deceive the operator as to the actual fluidity of the cement paste (McLean, 1972). Frequently, the cement was mixed too thinly in a misguided attempt to make it appear as fluid as a zinc phosphate cement paste. This led to poor properties. In fact, the fluidity of the cement is greater than the apparent consistency of the cement paste would suggest, because it is pseudo-plastic. Thus, it exhibits shear thinning when a restoration is seated on it, and it flows as readily as a zinc phosphate cement (Mortimer & Tranter, 1969; McLean, 1972). An unfortunate characteristic of early zinc polycarboxylate cements was the early development of elastomeric characteristics - 'cobwebbing' - in the cement pastes as they aged, thus shortening working time (McLean, 1972). Improvements in cement formulation, the addition of stannous fluoride to the oxide powder (Foster & Dovey, 1974, 1976) and modifications in the polyacid have eliminated this defect. However, the cements have to be mixed at quite a low powder/liquid ratio, 1*5:1-0 by mass, when used for luting. The properties of these cements - the fluidity of the mix, the working and setting times of the cement paste, and the strength of the set cement - are affected by a number of factors. These include the composition of the powder, the concentration, molecular mass and type of the polyacid, the 107

Polyalkenoate cements powder/liquid ratio and the presence or absence of metalfluorides(Smith, 1971; Foster & Dovey, 1974, 1976). Working time varies from 2 to 5 minutes (at 23 °C) and setting time from 3 to 12 minutes (at 37 °C) (Plant, Jones & Wilson, 1972; Jendresen & Trowbridge, 1972; Chamberlain & Powers, 1976; Powers, Johnson & Craig, 1974) (Table 5.3). These ranges are suitable, at the lower end, for the cementation of single crowns and, at the upper end, for bridges. As with other cements, working time can be prolonged by refrigerating the mixing slab (McLean, 1972; Chamberlain & Powers, 1976). Bertenshaw, Combe & Grant (1979) found thefilmthickness of cements to vary widely from 20 |am to 110 |im, but this property depends on the plasticity of the paste which changes rapidly with time. Thus, 0ilo & Eyje (1986) found for one cement that film thickness increased from 15 \mi at 1 min, to 25 jim at 3*5 min, and to 60 \xm at 5 min. In practice,filmthicknesses lower than 25 jim, the specification upper limit for luting agents, can be obtained (Jendresen & Trowbridge, 1972; 0ilo & Evje, 1986). There is a hardening stage after set when the cement rapidly becomes stronger and less plastic (Plant & Wilson, 1970; Bertenshaw, Combe & Grant, 1979; Paddon & Wilson, 1976; Wilson, Paddon & Crisp, 1979). Mechanical properties All properties are time-dependent. Smith (1982a) reported one example that developed 80% of its ultimate tensile strength in one hour and maximum strength in 24 hours. Watts, Combe & Greener (1979) noted little change in strength in seven days while Smith (1971) reported a slight decline which he attributed to water sorption. Small increases in strength have been recorded after 30 days (Osborne et al.9 1978; Smith, 1971) and 228 days (Smith, 1977). Paddon & Wilson (1976) found little increase in either strength or modulus after 24 hours. When cements are mixed to a luting consistency, compressive strength varies, typically, from 48 to 80 MPa, compressive modulus from 3-2 to 6-2 GPa and tensile strength from 4-8 to 15-5 MPa (Table 5.3), all measurements being made on 24-hour-old cements. Thesefiguresare not absolute as they depend on test conditions. Thus, 0ilo & Espevik (1978) found a temperature dependency; an increase in temperature from the usual 23 °C to 37 °C reducing the compressive strength of a cement from 48 MPa to 36 MPa and modulus from 3-2 GPa to 1-9 GPa. The cement shows marked viscoelastic properties. Thus, measured strength is affected by the crosshead speed of the testing machine and this 108

Zinc polycarboxylate cement effect is particularly noticeable if the crosshead speed is less than O^mmmnT 1 (0ilo & Espevik, 1978; Wilson & Lewis, 1980). Wilson & Lewis (1980) recorded a compressive strength of 65 MPa with crosshead speed of 0-05 mm min"1 which increased to 100 MPa when the crosshead speed was increased to 20 mm min"1. Unlike other aqueous dental cements, the zinc polycarboxylate retains plastic characteristics even when aged and shows significant stress relaxation after four weeks (Paddon & Wilson, 1976). It creeps under static load. Wilson & Lewis (1980) found that the 24-hour creep value for one cement, under a load of 4-6 MPa, was 0-7 % in 24 hours, which was more than that of a zinc phosphate cement (0-13 %) and a glass-ionomer cement (0-32%), but far less than that of the zinc oxide eugenol cement (2-2%). Plastic deformation is observed when the freshly set cement is subjected to a slowly increasing load at 37 °C (Plant & Wilson, 1970; Hertet et al., 1975; Paddon & Wilson, 1976; 0ilo & Espevik, 1978; Wilson, Paddon & Crisp, 1979). 0ilo & Espevik (1978) recorded strain at failure of 1-7%, at 23 °C, and 4-3%, at 37 °C, values which are greater than that of a zinc phosphate cement and far less than that of ZOE and EBA cements (Chapter 9). The plastic strain at fracture decreases markedly with time as the cement ages; also the elastic modulus increases (Wilson, Paddon & Crisp, 1979; Barton et al., 1975). There is an increase in dynamic modulus with time (Barton et al., 1975). Properties are affected by temperature. Compressive strength is reduced from 48 MPa at 23 °C to 36 MPa at 37 °C. Strain at failure increases from 1-7% at 23 °C to 4-3% at 37 °C. But these are nothing like the massive changes encountered with the ZOE and EBA cements. Although these appear to be about as strong as the zinc polycarboxylate cement when measurements are made at 23 °C, they are far weaker when tested at 37 °C, the temperature of the mouth. Erosion Erosion depends on the solubility of the powder (the filler) and the matrix in the aqueous medium. Here, acidity and complexing power of the solution for metal ions compared with the stability of the metal PAA complexes are important. The solubility of these cements in water (when aged from one to 24 hours) is small and ranges from 0-1 to 0-6% (Gourley & Rose, 1972; Bertenshaw, Combe & Grant, 1979; Crisp, Lewis & Wilson, 1976a; Smith, 109

Polyalkenoate cements 1971; Chamberlain & Powers, 1976; Jendresen & Trowbridge, 1972). The addition of stannous fluoride to the cement increases dissolution, but this is an advantage rather than a disadvantage, for the fluoride released is taken up by neighbouring enamel (Bitner & Weir, 1973). Once the cement has aged, dissolution occurs mainly at the site of the oxide particles rather than at the matrix (Crisp, Lewis & Wilson, 1976a; Anzai et al., 1977). The addition of high levels of magnesium oxide to zinc oxide (for the purpose of densification) is undesirable. Early commercial examples, containing 10% or more MgO added to the ZnO powder, absorbed water, swelled and showed high dissolution. Crisp, Lewis & Wilson (1976a) found that both zinc and magnesium are steadily eluted from these cements with magnesium predominating. This observation led them to recommend the omission of magnesium oxide from cement formulations. The nature of the poly(alkenoic acid) was found to affect the rate of elution of zinc. This elution was highest for cements based on a copolymer of maleic and acrylic acids and lowest for those based on a copolymer of acrylic and itaconic acids. Values for cements based on poly(acrylic acid) lay between these two extremes. Water absorption varied, according to brand, from 1-2 to 3-4 % and appeared to increase with the ratio of COO to total C in the poly(alkenoic acid). It was also affected by powder/liquid ratio. More important is the behaviour of these cements in solutions approximating to conditions in the mouth. Calcium does not affect the stability, but phosphate, also a constituent of saliva, increases dissolution (Peters et al, 1972; Peters, Jackson & Smith, 1974). Acidic conditions greatly increase the erosion of the cement, to an extent depending on the nature of the acid. Using the impinging jet method with lactic acid/lactate solutions, Wilson et al. (1986b) found, for one cement, erosion rates of 0-4% per hour at pH = 5-0, 5-3 % at pH = 4-0 and 16-2% at pH = 2-7. For a group of different cements, Wilson et al. (1986a) found erosion rates in solutions of pH = 2-7 in the range 8-5 to 19-8 %, and in one exceptional case 0-1 %. These workers found that the zinc polycarboxylate cement was markedly less resistant to acid erosion than the aluminosilicate glass cements, the glass-ionomer and dental silicate cements. They also found that, with one exception, zinc polycarboxylate cements were somewhat less resistant to acid erosion than the zinc phosphate cement. These results have been confirmed by Smink & Arends (1980) and Beech & Bandyopadhyay (1983) using a similar method. In vivo studies show that zinc polycarboxylate cements are much less 110

Zinc polycarboxylate cement resistant to erosion than aluminosilicate cements, but there is no consensus on their durability vis-a-vis the zinc phosphate cement (Ritcher & Ueno, 1975; Mitchem & Gronas, 1978, 1981; Osborne et al9 1978; Pluim & Arends, 1981, 1987; Mesu & Reedijk, 1983; Theuniers, 1984; Pluim et al., 1984; Pluim, Arends & Havinga, 1985). Adhesion

An important property of the zinc polycarboxylate cement is its ability to bond to untreated dentine and enamel. It also adheres to bone. This adhesion can be observed visually (Mizrahi & Smith, 1969b; Smith, 1975; Abramovich et al., 1977) and there is, of course, mechanical and chemical evidence as well. After fracture, areas of cement remained attached to the substrate (Smith, 1975; Eick et al., 1972). The principles and mechanism of adhesion have already been discussed in Section 5-2. The bond strength to tooth material develops rapidly in a matter of hours (Mizrahi & Smith, 1969a) and is maintained over many months (Mizrahi & Smith, 1969b, 1971). The tensile bond strength of the zinc polycarboxylate cement to untreated dentine is 2-2 to 5-1 MPa (Bertenshaw, Combe & Grant, 1979; Peddy, 1981; Levine, Beech & Garton, 1977; 0ilo, 1981; Hinoura, Moore & Phillips, 1986). The bond strength to enamel is somewhat higher at 4-1 to 6-4 MPa (Peddy, 1981; Levine, Beech & Garton, 1977). Jemt, Stalblad & 0ilo (1986)findthat in vivo bond strengths are much lower and reported tensile values as low as 1-7 and 2-8 MPa. The lower bond strength to dentine is significant and emphasizes that the bonding is to apatite. Thus, demineralization of tooth material by acids reduces bond strength (Smith, 1975). Both fluoride and calcium ions lead to an increase in bond strength. Fluoride-containing cements bond more strongly to dentine, with a shear bond strength of 7-0 MPa against 5-2 MPa for others (Causton, 1982); bond strength can be increased by increasing the calcium concentrations at the dentine surface (Beech, 1973). Such observations led to successful attempts to improve bond strength by pre-treating dentine with specially formulated solutions. Calcifying solutions were developed to pre-treat the tooth surface and so improve bond strength. Levine, Beech & Garton (1977) used a solution containing calcium hydrogen phosphate, sodium fluoride and disodium hydrogen phosphate to pre-treat dentine and so raised tensile bond strength from 2-4 to 5-5 MPa. Similarly, Causton & Johnson (1982) used their so-called IT-S solution, a calcifying isotonic solution buffered with carbonate and phosphates to pH of 7-4, and 111

Polyalkenoate cements improved the shear bond strength of two examples of cement from 5-2 and 7-0 MPa to 7-2 and 12-9 MPa respectively. The cement also bonds to metals. Saito et al. (1976) found that the bond strength of zinc polycarboxylate cement to alloys decreased in the following order of substrate: copper alloy, nickel-chromium alloy, silver-palladium alloy, type III gold alloy. The bond strength to stainless steel has been reported as varying from 6 to 9 MPa (Moser, Brown & Greener, 1974; Jendresen & Trowbridge, 1972; Mizrahi & Smith, 1969a,b) with similar values for the cobalt-chromium alloy (Moser, Brown & Greener, 1974). The cement does not bond to the inert surfaces of porcelains. Biological properties

The biocompatibility of these materials is in general excellent (Beagrie, Main & Smith, 1972; Peters et al, 1972; Peters, Jackson & Smith, 1974; Beagrie et al, 1974; Lawrence, Beagrie & Smith, 1975; Eames, Hendrix & Mohler, 1979; Main et al., 1975). Results from in vitro experiments are conflicting, but it would appear that cytotoxic effects (inhibition of cell growth or cell death) are low unless test conditions permit the release of toxic concentrations of zinc ions, fluoride ions and free poly(acrylic acid) (Peters et al., 1972; Leirskar & Helgeland, 1977; Spangberg, Rodrigues & Langeland, 1974; Welker & Neupert, 1974). It is probable that these cements do not cause cytotoxic effects in use. However, the possible release of zinc rules them out as bone cements. The effects of zinc polycarboxylate cements on calcified tissue appear minimal and they are as bland as the zinc oxide eugenol cement is towards dental pulp (Smith, 1969; Plant, 1970; von Klotzer et al., 1970; Barnes & Turner, 1971; Beagrie, Main & Smith, 1972; Jendresen & Trowbridge, 1972); the traditional zinc oxide eugenol is recognized as exceptional in this respect. Reactions were mild in the teeth of monkeys even in deep cavities (El-Kafrawy et al., 1974). In clinical practice these cements give little pain and there are no long-term adverse effects (McLean, 1972). They are less irritating than the zinc oxide eugenol cement when used in implants in soft tissue and bone (Beagrie, Main & Smith, 1972; Lawrence, Beagrie & Smith, 1975). There are several reasons why these cements are bland. Acid irritation is probably minimal. Poly(acrylic acid) is a weak acid and, in addition, because of its high molecular weight will not readily diffuse along dentinal tubules and is also immobilized by phosphatic material in these tubules. Moreover, once set these cements rapidly become neutral. 112

Zinc polycarboxylate cement Another cause of inflammation is leakage of bacteria from the mouth at the interface between the cement and tooth material. Adhesion at the interface reduces this effect. Full accounts of the biological responses of these cements are to be found in reviews by Smith (1982b), Helgeland (1982) and Granath (1982). 5.7.5

Modified materials

The most important modification of these materials was the discovery of the effect of adding stannous fluoride (Foster & Dovey, 1974, 1976). Originally added to provide fluoride release, it was found to improve the mixing qualities of the cement and to increase strength by about 50 %. This is reflected also in improved adhesion to enamel and dentine (Section 5.7.4). Attempts have been made to improve the mechanical properties of these cements by adding reinforcing fillers (Lawrence & Smith, 1973; Brown & Combe, 1973; Barton et al, 1975). Lawrence & Smith (1973) examined alumina, stainless steel fibre, zinc silicate and zinc phosphate. The most effective filler was found to be alumina powder. When added to zinc oxide powder in a 3:2 ratio, compressive strength was increased by 80% and tensile strength by 100 % (cements were mixed at a powder/liquid ratio of 2:1). Because of the dilution of the zinc oxide, setting time (at 37 °C) was increased by about 100 %. As far as is known, this invention has not been exploited commercially.

5.7.6

Conclusions

The cement is to be regarded as a replacement for the zinc phosphate cement. Its advantages over that traditional cement are its adhesion to tooth substance and a more bland reaction towards living tissues. Its disadvantages are that it is less user-friendly than the zinc phosphate cement and requires greater care in preparation. Unlike other aqueousbased dental cements, it retains distinct plastoelastic properties even after ageing for months. Thus, it is less rigid and more liable to creep. Whether this constitutes an advantage or disadvantage is a matter of opinion. The current view is that cement properties should match those of dentine, in which case it is too plastic. On the other hand, the zinc phosphate cement is too rigid. 113

Polyalkenoate cements 5.8

Mineral ionomer cements

Cements can be formed by reacting silicate minerals with solutions of a poly(alkenoic acid) but they are weaker than zinc polycarboxylate and glass polyalkenoate cements and so far have found little practical use. They are, however, of theoretical interest for they give some insight into the silicate structures required for cement formation. Broadly, silicates that are capable of cement formation fall into the class of naturally occurring silicates known as gelatinizing minerals. These are minerals decomposed by acids with the formation of silica gel; this behaviour is related to silicate structure. Larsen & Berman (1934) recorded the effect of acids on a large number of minerals and their work has been used by others to classify gelatinizing minerals (Murata, 1943; Mase, 1961; Petzold, 1966). It is helpful in the discussion to describe silicate structures using the Qn nomenclature, where Q represents [SiOJ tetrahedra and the superscript n the number of Q units in the second coordination sphere. Thus, isolated [SiO4]4~ are represented as Q° and those fully connected to other Q units as Q4. In general, minerals based on Q°, Q1, and Q2 units are decomposed by acids. Such minerals are: those containing isolated silicate ions, the orthosilicates, SiO4^ (Q°); the pyrosilicates, Si2C>7~ (Q1); ring and chain silicates, (SiO3)2w~ (Q2). Certain sheet and three-dimensional silicates can also yield gels with acids if they contain sites vulnerable to acid attack. This occurs with aluminosilicates provided the Al/Si ratio is at least 2:3 when attack occurs at Al sites, with scission of the network (Murata, 1943). Mase (1961) explains the reactivity of silicate minerals towards acids in terms of the polarizing ability of the cation and the bond energy of the mineral. Clearly, too, the reactivity of minerals towards acids is connected with their basicity, and one notes that the orthosilicates are basic minerals. According to the ideas of Flood & Forland (1947a,b) (Section 2.3.3) basicity is related to the residual polarizability of the oxygen atom. If this is large, as will be the case if the associated cation has little polarizing power, then the oxygen atom is basic. Thus, sodium silicates are the most basic of the silicates, and silica itself is acidic and so resistant to acid attack. Basicity is also related to the silicate structure: orthosilicates are more basic than pyrosilicates which, in turn, are more basic than ring and chain silicates. Using this information, Crisp et al. (1977, 1979) and Hornsby et al. (1982) selected candidate minerals for cement formation with poly (acrylic acid) and found a number of minerals that formed cements (Table 5.4). 114

Table 5.4. Properties of mineral ionomer cements {Crisp et al, 1979; Hornsby et aL, 1982)

Powder: liquid, gem" 3

Setting time, min

Compressive strength (24 h), MPa Humid

Sohibil

Water

Orthosilicates Willemite Gadolinite

Zn 2 [SiOJ Be 3 Fe(YO) 2 [SiOJ 2

2-0 2-1

26 140

19 40

21 40

0-35 002

Pyrosilicates Gehlenite Hardystonite

Ca2Al[AlSiO7] Ca 2Zn[Si 2O7]

1-0 10

48 6

1 9

6 12

2-4 0-4

Chain silicate Wollastonite

Ca3[(SiO3)3]

1-0

14

18

3

3-4

Sheet silicate Thuringite

(Fe(II), Fe(III), Mg, Al)12[(Si, Al)8O20](O, OH, F ) r

1-5

29

28

35

10

Zeolite Scolecite

Ca[Al2Si3O10]3H2O

0-5

20

160

30

1-35

Ultramarine Hackmanite

Na 8 [Al 6 Si 6 O 2 J(Cl 2 ,S)

10

13

89

37

2-4

Feldspar Labradorite

(Ca,Na)[Al1_2Si2_3O8]

1-0

59

134

2

3-6

Polyalkenoate cements While some formed hard, rigid cements that were stable in water, others yielded rubbery or plastic masses that were hydrolytically unstable. Minerals with cement-forming capability were found in the following classes: (1) Island silicates containing discrete ions. Orthosilicates, SiO4~ (Q°); pyrosilicates, Si2O*~ (Q 1 ); and ring silicates, Si3OJ-, S&.O"- (Q2). Most orthosilicates reacted completely with poly(acrylic acid) solution; an exception was andradite, Ca 3 Fe 2 [SiO4]3. Even so, the cements of gehlenite and hardystonite were very weak and affected by water. Only gadolinite and willemite formed cements of some strength which were unaffected by water, probably because one contained beryllium and iron and the other zinc. (2) Chain silicates, consisting of connected metasilicate units, (SiO3)2w~ (Q2), and of an open structure. Wollastonite Ca(SiO3) reacted completely with poly(acrylic acid), but the cement was much affected by water. (3) Sheet silicates (Q3) with significant isomorphic replacement of Si4+ by Al3+ or Fe 3+ . These were decomposed by poly (aery lie acid) to silica gel. The chlorite, thuringite, formed a strong cement but was much affected by water. (4) Aluminosilicates with a three-dimensional network (Q 3 and Q4) where the Al/Si ratio was 2:3. These reacted with poly(acrylic acid), but none reacted completely. The zeolite, scolecite, the feldspar, labradorite, and the ultramarine, hackmanite, gave high-strength cements but all were much affected by water - the strength of the labradorite cement disappeared almost entirely - possibly because of the presence of free acid. 5.9

Glass polyalkenoate (glass-ionomer) cement

5.9.1

Introduction

The glass polyalkenoate cement, formerly known as the glass-ionomer cement, was invented by Wilson and Kent in 1969 (Wilson & Kent, 1973) and is now well established as a material that has an important role in clinical dentistry. It has proved to have considerable development potential and has been subjected to continuous development, improvement and 116

Glass polyalkenoate (glass-ionomer) cement diversification. It is the most versatile of all dental cements and currently accounts for most of the research and development on them. There are other applications of the cement as a biomaterial. It is used as a splint bandage material and as a bone cement. Glass polyalkenoate cement has a unique combination of properties. It adheres to tooth material and base metals. It releases fluoride over a long period and is a cariostat. In addition it is translucent and so can be colourmatched to enamel. New clinical techniques have been devised to exploit the unique characteristics of the material. The material originated from the general dissatisfaction with the clinical performance of the dental silicate cement. Wilson and his coworkers made extensive studies on the dental silicate cement (Section 6.5) and drew the conclusion that this cement could not be further improved. Wilson (1968) examined several alternatives to orthophosphoric acid, including organic chelating agents, as a liquid cement-former, but none of these were successful. Finally, after considerable research, the glass polyalkenoate cement was developed (Wilson & Kent, 1971, 1972, 1973). The cement is formed by mixing an ion-leachable glass powder with an aqueous solution of a poly(alkenoic acid). The glass is generally a fluoride-containing calcium aluminosilicate but calcium may be replaced by strontium or lanthanum. The cement was originally known as ASPA, an acronym of Aluminosilicate Polyacrylic Acid. The term ASPA is now applied to materials developed by the Laboratory of the Government Chemist in the UK, and was once also the brand name of an early commercial material. For many years it was known as the glass-ionomer cement - indeed, that is still the term in common use-but the International Standards Organization officially adopted the name glass polyalkenoate cement. The term glass-ionomer cement is now used as a generic term to cover these cements and the new glass polyphosphonate cements invented by Ellis and Wilson in 1987-9 (Ellis & Wilson, 1990). 5.9.2

Glasses

General The powders used in glass polyalkenoate cement formulations are prepared from glasses and not opaque sintered masses. In this they resemble the traditional dental silicate cement from which they are descended. The glass plays several roles in the chemistry and physics of the glass polyalkenoate 117

Polyalkenoate cements cement. It acts as a source of ions for the cement-forming reaction, controls the setting rate and strength of the cement and imparts the property, unusual in a cement, of translucency. Chemically these are special aluminosilicate glasses. Until quite recently, all were calcium aluminosilicates, but now calcium is sometimes wholly or partly replaced by strontium and lanthanum. Most glasses also contain fluorides which, besides lowering the temperature of glass fusion, play a role in cement formation and affect cement properties. Provided the Al/Si ratio is high enough, these glasses are decomposed by acids to release cement-forming ions (Wilson & Kent, 1973, 1974; Crisp & Wilson, 1978a,b, 1979; Kent, Lewis & Wilson, 1979; Wilson et al., 1980; Hill & Wilson, 1988a). They are similar to the glasses used for dental silicate cements, although the Al/Si ratio is higher. Types of glass There are a great number of potential glasses and some can be extremely complex. All contain silica and alumina and an alkaline earth or rare earth oxide or fluoride. The two essential glass types are SiO2-Al2O3-CaO and SiO2-Al2O3-CaF2, from which all others are derived. Oxide glasses have been reported by Crisp & Wilson (1978a,b, 1979), Wilson et al. (1980), and Hill & Wilson (1988a). The fusion mixtures contain silica, alumina and calcium carbonate to which sodium carbonate or calcium orthophosphate may be added. They may be represented thus, with fusion temperature given in parentheses: SiO2-Al2O3-CaO (1350-1550 °C) SiO2-Al2O3-CaO-P2O5 (1370-1450 °C) SiO2-Al2O3-CaO-Na2O (1200-1350 °C) Influorideglasses, calciumfluorideis an essential constituent, but generally cryolite, Na3AlF6, is also added as a flux to lower the temperature of fusion. Aluminium orthophosphate is also generally added to the fusion mixture for various reasons. Of course, the various elements may be added in different ways. Thus, calcium orthophosphate, aluminium fluoride and sodium carbonate are often used in the preparation of fluoride glasses. Apart from lowering the temperature of glass fusion, fluoride improves the handling qualities of the cement paste, increases cement strength and translucency, and has a therapeutic quality when used as a dental filling material. In fluoride glasses the ratio of alumina to silica controls the setting time of the cement; fluoride tends to slow setting while aluminium 118

Glass polyalkenoate (glass-ionomer) cement orthophosphate improves the mixing of the paste. Sodium in the glass improves the translucency of the cement but can affect its hydrolytic stability. In addition, glasses have been reported where calcium is replaced by strontium or lanthanum (Akahane, Tosaki & Hirota, 1988) which impart radio-opacity to the cement. Fluoride glasses are difficult to classify because the various constituents can be added to the fusion mixture in several ways. However, glasses of the Laboratory of the Government Chemist (Wilson & Kent, 1973; Kent, Lewis & Wilson, 1979; Wilson et ai, 1980; Hill & Wilson, 1988a), which form the basis of many commercial cements, can be represented as SiO2-Al2O3-CaF2 (1150-1350 °C) SiO2-Al2O3-CaO-CaF2 (1320-1450 °C) SiO2-Al2O3-CaF2-AlPO4 (1150-1300 °C) SiO2-Al2O3-CaF2-AlPO4-Na3AlF6-AlF3 (1100-1300 °C) where again the temperature of fusion is given in parentheses. After fusion the molten glass is shock-cooled by pouring it onto a metal plate and then into water. The glass fragments are then finely ground to pass either a 45-|im sieve for afillingmaterial, or a 15-jim sieve for a finegrained luting agent. The glass powders may be annealed after preparation by heating at 400 to 600 °C; in general, the effect is to slow down the setting reaction. Sometimes the powder is acid-washed to improve the mixing qualities of the cement. Structure of aluminosilicate glasses The formation of a cement is dependent on the ability of the glass to release cations to acid solutions. It is not sufficient for network-modifying cations to be exchanged for hydrogen ions as this would restrict the attack to the glass surface only. It is required that the glass structure itself be completely decomposed if all the glass ions are to be available for release. Aluminosilicate glasses have this property. To discuss why this is so it is useful to have an appropriate conceptual framework, and one can be developed from the Random Network model of Zachariasen (1932). Zachariasen (1932) conceived of a glass structure as a random assembly of oxygen polyhedra, these polyhedra consisting of a central glass-forming cation surrounded by a small number of oxygen atoms, e.g. [SiOJ tetrahedra. These polyhedra were considered to be linked at corners only, via 2-coordinate oxygen atoms. This concept amounts to regarding a glass as a type of highly crosslinked polymer based on -Si-O-Si- linkages. This 119

Polyalkenoate cements idea, although much criticized (Rawson, 1967), has proved to be a fruitful one. Zachariasen (1932) added another criterion, namely that the random network was three-dimensional, and, therefore, in modern terminology, composed only of Q4 and Q3 units. Hagg (1935) considered that this requirement was not always necessary and a glass might contain large irregular anionic groups. The work of Trap & Stevals (1959) supported this view, for they prepared so-called invert glasses containing only Q2 and Q1 units, that is glasses with no crosslinking. In these glasses at least half the oxygen atoms are non-bridging -O" groups, so the -Si-O-Si- chains are anionic and are held together by network-modifying cations (these do not form part of the glass structure). Today, following Ray (1975, 1983) we would call these ionic polymers. We are now in a position to discuss requirements for ionomer glasses further. Consider the case of the simple silica (SiO2) glass where we can represent the network diagrammatically thus: O

I

O

I

O

I

— Si—O—Si—O—Si—O—Si — O

I

I

I

O

I

I

This infinite three-dimensional network is electrically neutral and impervious to acid attack. If so-called network-modifying cations are introduced then this network must acquire a negative charge leading to the breaking of an Si-O-Si bridge to form non-bridging oxygens: \ / — Si—O—Si— / \

Ca2+ \ „ • — S i — Or Ca 2+ "O — S i — /

/ \

This is a type of ionic polymer where the negative charge on the network is balanced by the positively charged network modifier. Statistically all types of [SiOJ tetrahedra, Q1, Q2, Q3 and Q4, will be present in varying proportions, depending on the ratio of bridging to non-bridging oxygens. Aluminosilicates are more complex as aluminium can be either a network modifier in sixfold coordination or a network former in fourfold coordination. In the latter case, Al3+ is able to replace Si4+ in the glass network because it has a similar ionic radius, but the network then acquires a negative charge. If this charge becomes sufficiently high then the network becomes susceptible to acid attack. Again this charge on the network has to be balanced by positively charged network-modifying cations. Thus, we 120

Glass polyalkenoate (glass-ionomer) cement can regard an aluminosilicate glass structure as consisting of linked [SiOJ and [A1OJ~ tetrahedra. There are restrictions on the replacement of Si4+ by Al 3+ . The Al/Si ratio apparently cannot exceed 1:1 (Lowenstein, 1954). Nor can all the aluminium go into the network if there are insufficient network-modifying cations to balance the network charge. Under such conditions aluminium adopts a sixfold coordination. We must note that recently Ellison & Warrens (1987), using 27 A1NMR spectroscopy, have found evidence for the existence of aluminium in pentacoordination in asymmetric or distorted sites using previously established assignments (Kirkpatrick et al. 1986; Risbud et al., 1987; Cruikshank et al., 1986). A negatively charged network of non-bridging oxygens and aluminium sites renders these glasses susceptible to acid attack. Overall the introduction of network-modifying cations and aluminium ions increases the polarizability of oxygen ions and, therefore, vulnerability to acid attack. The mode of acid decomposition of an aluminosilicate glass is depicted in Figure 5.5. It can be seen that attack by hydrogen ions involves exchange of network-modifying cations (Ca2+, Na + ) and rupture of the aluminosilicate network at aluminium sites to yield silicic acid and aluminium ions (Wilson, 1978b; Prosser & Wilson, 1979). Glasses used in glass polyalkenoate cements have been observed to release cations, fluoride if present and silicic acid (Crisp & Wilson, 1974a; Wasson & Nicholson, 1990,1991). Similar observations have been made for the related dental silicate cement

polymerizes silica gel Figure 5.5 The mode of acid decomposition of an aluminosilicate glass.

121

Polyalkenoate cements Table 5.5. Glass compositions and acid extracts {Crisp & Wilson, 1974a; Wasson & Nicholson, 1990) G-200

G-338

Mole ratio

Glass

Cement extract

Glass

Si:Al Ca:Al Na:Al F:A1

0-98 0-89 014 2-46

1-17 0-25 0-97

0-67 0-26 0-44 1-67

Cement extract 0-63 0-36 0-65

Table 5.6. Composition of oxide glasses used in studies on polyalkenoate cements, parts by mass {Wilson et al., 1980; Crisp, Merson & Wilson, 1980)

SiO2 A12O3 CaO Ca 3 (PO 4 ) 2 Appearance Crystallites

G-273

G-275

G-287

G-255

G-247

120 102 168 —

240 102 112 —

180 102 56 —

120 102 56 —

160 100 — 140

clear

clear

clear

opaque

opal

—

—

—

An

—

Properties

Powder .liquid, g cm"3 Setting time (37 °C), min Strength (24 h), MPa

20 2-75

95

30 40 35

30 8-25

29

30 40 56

— 5-25

72

An = Anorthite

(Wilson & Kent, 1970). The release of each glass species is roughly governed by the amount contained in the glass and so varies with glass type (Table 5.5). The reactivity of a glass towards acids depends on its acid-base properties and both the Bronsted-Lowry and Lewis theories have been applied to oxide glasses (Volf, 1984). Basic components of a glass are the metal oxides, and acidic ones are silicon, boron or phosphorus oxides. The important factor is the state of the oxygen atoms. In purely oxide glasses the basicity of a glass depends on the ability of the oxygen atoms to give up 122

Glass polyalkenoate (glass-ionomer) cement electrons. This is greatest when the oxygen atoms are associated with cations of low electrostatic field strength, for example Na+ and Ca2+, and least when the cations have a high electrostatic field strength, for example the highly charged, small Si4+ ion. Lux (1939) introduced the symbol pO (note it is not an exponent like pH) to quantify the acid-base balance in a glass, and various attempts have been made to obtain values for this parameter. All are based on the electronegativity of the cation or a related characteristic, such as electrostatic field strength (Volf, 1984). SiO2-Al2O3-CaO glasses In these glasses (Table 5.6) the coordination state of aluminium depends on its chemical environment and can only be entirely fourfold when the Ca/Al SiO * D A o 9 1:3

•

Non-setting Slow setting Moderate setting Fast setting Fast setting crystalline mass Ultra-fast setting

1:2 Al /Si mole ratio

C9S

C,S

CaO

A1 2 O 3

Figure 5.6 Triangular composition diagram for SiO2-Al2 O3-CaO glasses, showing that glasses with cement-forming ability fall within the gehlenite and anorthite composition region, and that only glasses with less than 61 to 62 % by mass of silica have the potential to form a cement (Hill & Wilson, 1988a).

123

Polyalkenoate cements Table 5.7. Properties of cements formed from glasses corresponding to the generic formula xSi02.Al203.Ca0 Cement properties

moles(x)

mass %

setting time, minutes

strength, MPa

10 20 40 60

21-9 35-9 52-8 62-7

3-5 2-25 40 non-setting

104 74 35 zero

SiO * D • o •

Zero strength Unworkable Weak Low strength Moderate strength

1:3 1:2 Al /Si mole ratio

C2S C,S

CaO

A12O3

Figure 5.7 Triangular composition diagram for SiO2-Al2 O3-CaO glasses. Glasses in the gehlenite region yield stronger cements (95 to 104 MPa in compression) than those in the anorthite region (29 to 56 MPa) (Hill & Wilson, 1988a).

124

Glass polyalkenoate (glass-ionomer) cement Table 5.8. Composition of fluoride glasses used in studies on polyalkenoate cements, parts by mass (Kent, Lewis & Wilson, 1979; Wilson et al., 1980)

SiO2 A12O3 CaF 2 Na 3 AlF 6 A1F3 A1PO4 SiO 2 :Al 2 O 3 bymass Appearance Crystallites

G-200

G-307

175 100 207 30 32 60 1-75 opaque Fl

133 67 100 100 100 100 — — — — — — 1-33 0-67 clear opaque Fl,Co

175 100 166 — — 60 1-75 opal Fl

175 100 117 — — 60 1-75 opaque Co

175 100 90 135 32 170 1-75 opal Ap

30 3-5 107

3-5 3-0 149

30 30 199

1-8 3-75 149

Properties Powder: liquid, g cm"3 30 Setting time (37 °C), min 5-2 Strength (24 h), MPa 185

G-309

30 6-5 166

G-235 G-237

G-338

Fl = Fluorite, Co = Corundum, Ap = Apatite

ratio > 1:2 and Al/Si ratio < 1:1 (Isard, 1959; Lowenstein, 1954). Thus, aluminium is in fourfold coordination in anorthite glass (Ca: Al = 1:2, Al:Si = 1:1); the glass is composed of Q4 and Q 3 units, i.e. it is threedimensional. Gehlenite glass must contain some aluminium in sixfold coordination (Ca: Al > 1:2, Al:Si > 1:1) and is composed of paired Q 1 units, i.e. [AlSiO7]. A study by Ellison & Warrens (1987) on two of these glasses, using 27A1 and 29Si NMR, produced results not too dissimilar from theoretical predictions. In glass G-273, Ca3Al2Si2O9 (Table 5.6), aluminium was found to be mainly in tetrahedral coordination with a minor amount in octahedral coordination. Similar results were found for glass G-275, Ca2Al2Si4O13 (Table 5.6), but, in addition, some aluminium was found to be pentacoordinate. Possible structural units were considered to be Q 3 (1A1) and Q 2 (0A1) with some Q 4 (3A1). The number of Al replacing Si in the second coordination sphere is given in parentheses. Glasses that have cement-forming ability fall within the gehlenite and anorthite composition regions of this system, and only glasses with less than 61 to 62 % by mass of silica have potential to form a cement (Figure 5.6). Cements are not formed if the Si/Al mole ratio exceeds 3:1. When the 125

Polyalkenoate cements ratio is less than 2:1 fast-setting cements are obtained with setting time of 2 to 10 minutes. There is only a very small region for slow-setting cements and as Table 5.7 shows there is a critical region between setting and nonsetting. Glasses in the gehlenite region yield stronger cements (95 to 104 MPa in compression) than those in the anorthite region (29 to 56 MPa) (Figure 5.7). SiO2-Al2O3-CaF2

glasses

These (Table 5.8) are the basic type from which most biomedical glass polyalkenoate cements are derived. Although thefluoridecontent is high, many of these shock-cooled glasses are clear. Clear glasses are confined to a narrow central compositional range at the centre of the phase diagram where the Al2O3/CaF2 ratio is around 1:1 by mass and the SiO2/Al2O3 ratio exceeds 1-33:1 by mass (Figure 5.8). Outside this regionfluorite,and sometimes corundum, phase-separate. Even the clear glasses can be SiO2 • • A • o •

Clear, non-setting Opal, non-setting Clear, slow setting, low strength Opal, slow setting, low strength Clear, fast setting, high strength Opal, fast setting, high strength

1:3

CaF 2 /Si0 2 ratio by mass

ratio by mass

3:1

CaF2

AI2O3

ratio by mass

Figure 5.8 Clear glasses are confined to a narrow central compositional range at the centre of the phase diagram where the Al 2 O 3 /CaF 2 ratio lies in the region 1:1 by mass and the SiO 2 /Al 2 O 3 ratio exceeds 1-33:1 (Hill & Wilson, 1988a).

126

Glass polyalkenoate (glass-ionomer) cement induced to phase-separate when heated to 450 °C, and this reduces their reactivity. The ability of a glass to form a cement is governed by the SiO 2 /Al 2 O 3 ratio which represents the acid-base balance in these glasses. If this ratio is 3-0 or more by mass then the glass will not form a cement. If it is below 2-0 then the cements formed are rapid-setting (2-5 to 5-0 minutes). Glasses in a very narrow band around a ratio of about 2-0 are slower-setting (6-5 to 18 minutes). The critical ratio for non-setting lies somewhere between 2-0 and 3-0. The effect of SiO 2 /Al 2 O 3 ratio on setting time and compressive strength is shown in Figure 5.9. Note that compressive strength increases steadily as the SiO 2 /Al 2 O 3 ratio decreases. Setting time decreases as the SiO 2 /Al 2 O 3 ratio decreases until a point is reached when phase separation 200 - | 1 -

10.0i

0.8

8.0 150

|6.0

o> cb <

^3

100

if)

essi

%

Z 4.0

Q."

£ o

I/)

0.6 o

o ••5 a 0

0.4

u 50

0.2

2.0

Opal glasses I Clear glasses G-309 I 0.5

G-308 I

|G-307 J

1.0 1.5 SiO2 • AI2O3 by mass

G-379 2.0

Figure 5.9 The effect of SiO 2 /Al 2 O 3 mass ratio on setting time, compressive strength and opacity (Hill & Wilson, 1988a).

127

Polyalkenoate cements occurs in the glass. Phase separation has the effect of deactivating the glass. In these glasses the main phase is depleted in calcium and fluoride, which reduces its reactivity. Acid attack occurs selectively at the phase-separated droplets which are rich in calcium and fluoride. This selective attack is shown in Figure 5.10. Phase-separated glasses produce stronger cements than clear glasses. The strongest cements produced from a clear glass have a compressive strength of 130 MPa and a flexural strength of 20 MPa, whereas phaseseparated glasses produce cements with compressive strength exceeding 200 MPa and flexural strengths exceeding 35 MPa. Note that fluoride glasses produce much stronger cements than oxide glasses. The strongest cement produced from an oxide glass has a compressive strength of only 104 MPa. Ellison & Warrens (1987) have reported NMR results on an atypical phase-separated glass of extreme composition G-309 (Table 5.8) finding

Figure 5.10 In these glasses, the main phase is depleted in calcium and fluoride, which reduces its reactivity. Acid attack occurs selectively at the phase-separated droplets which are rich in calcium and fluoride (Hill & Wilson, 1988a).

128

Glass polyalkenoate (glass-ionomer) cement Table 5.9. Composition (%) of oxide/fluoride glasses used in studies on polyalkenoate cements, parts by mass. Generic composition: 2SiO2.Al2O3. (2-x)CaO.xCaF2 (Kent, Lewis & Wilson, 1979; Wood & Hill, 1991a) G-241

G-278 G-276 G-279 G-280 G-281 G-282

SiO2 A12O3 CaO CaF 2 Tg Appearance below

0 120 102 112 0 nd clear

0-20 120 102 101 15-6 745 clear

Crystallites below

—

—

—

0-33 120 102 93 26 nd clear

0-5 120 102 84 39 717 clear

—

—

Properties Powder: liquid, 2-5 2-5 20 20 gcnr3 Setting time (37 °C), 2-25 2-25 2-5 u/w min Strength (24 h), 74 125 120 u/w MPa

10 120 102 56 78 642 clear —

1-5 1-8 120 120 102 102 11-2 28 117 140 nd 636 clear opaque Fl

2-0

20

u/w

u/w

u/w

u/w

2-5 3-0 165

Fl = Fluorite, nd = not determined u/w = unworkable pastes that set during mixing that the Al is mainly in sixfold coordination, not surprising in view of the high alumina content. The structural units are mainly Q 4 (3A1). The role of fluoride is a matter of speculation and debate (Kumar, Ward & Williams, 1961). According to Weyl and Marboe (1962), in addition to [SiOJ and [A1OJ, such glasses contain tetrahedra such as [SiO 3 F], [A1O3F] etc. The replacement of O2~ by F~ reduces the screening of the central cation and so strengthens the remaining cation-oxygen bonds, but fluoride is non-bridging and so structure-breaking. This role of fluoride may be represented, thus Q4

Q3

Another view of the role of fluoride is that metal fluorides occupy holes in the major glass network (Rabinovich, 1983). There is experimental 129

Polyalkenoate cements evidence to support both views. However, these views are not necessarily mutually exclusive. SiO2-Al2O3-CaO-CaF2 glasses These glasses (Table 5.9) with generic composition 2SiO2.Al2O3. (2-x)CaO.xCaF 2 , first described by Wilson et al (1980) and recently studied by Wood & Hill (1991a), are of theoretical interest. In this series, starting with the oxide glass G-241, O atoms are progressively replaced by F atoms (Table 5.9). Fluoride is often considered to be a structure-breaker and this is reflected in the reduction of the glass transition temperature, T g, as x increases (Wood & Hill, 1991a). Another indication of structure breaking is shown by a decrease in setting time of cements as x increases (Wilson et al, 1980). When x reaches 0-5 (G-279) the reaction between glass powder and polyacid liquid becomes very vigorous and cement pastes set during mixing. Glasses with x values of 1-0 and 1-5 behave similarly. However, when x reaches 1-8 (G-282) phase-separation of fluorite (CaF2) occurs, fluoride is removed from the main phase, and the reactivity of the glass is reduced. The cement of G-282 has the longest setting time of the series. Wood & Hill (1991b) induced phase-separation in the clear glasses by heating them at temperatures above their transition temperatures. They found evidence for amorphous phase-separation (APS) prior to the formation of crystallites. Below the first exotherm, APS appeared to take place by spinodal decomposition so that the glass had an interconnected structure (Cahn, 1961). At higher temperatures the microstructure consisted of distinct droplets in a matrix phase. When x was 1-0, fluorite and anorthite crystallites were formed. With glasses of lower fluoride content (x < 1-0) gehlenite crystallites were also found. As x decreased, increasingly more gehlenite was formed at the expense of anorthite and fluorite. In connection with this observation it should be noted that the chemical composition of the glass corresponds to gehlenite when x = 0 and to a mixture of anorthite and fluorite when x=\. Wood and Hill consider that the role of fluoride in these glasses is uncertain. Phase-separation studies suggest that the structure of the glass might relate to the crystalline species formed, in which case a microcrystallite glass model is appropriate. But other evidence cited above on the structure-breaking role of fluoride is compatible with a random network model. 130

Glass polyalkenoate (glass-ionomer) cement Table 5.10. Examples of practical glasses used in glass polyalkenoate cement {Crisp, Abel & Wilson, 1979; Wilson & McLean, 1988; Brook, Craig & Lamb, 1991)

SiO2 A12O3 A1F3 CaF 2 CaO NaF Na 2 O A1PO4 SiO 2 : A12O3 by mass

ASPA IV G-200

ASPAX G-338

GC Fuji

ESPE Ketac

301 19-9 2-6 34-5

24-9 14-2 110 12-8

41-9 28-6 1-6 15-7

34-9 201 8-8 20-8

3-7

12-8

9-3

3-6

100 1 51

24-2 1-75

3-8 146

11-8 1-74

Pilkington MP4 30-8 38-5 28-6 21 0-80

G-200 and G-338 are LGC formulations used in certain commercial materials GC Fuji and ESPE Ketac are dental cements MP4 is used in splint bandage materials

SiO2-Al2Oz-CaF2-AlPO^ glasses Not much needs to be said on these glasses, except that one, G-235 (Table 5.8) has been subjected to solid state NMR (Ellison & Warrens, 1987). It would appear that aluminium is mainly in tetrahedral coordination with some octahedral and pentahedral coordination and that the structural units are mainly Q 3 (1A1) and Q 4 (2A1). glasses These are practical glasses and some examples are given in Table 5.10. One of the most important of these is G-200, an unusual glass in that it forms a practical cement without the need for ( + )-tartaric acid. It was used in early commercial materials and in fundamental studies on setting and cement structure. The glass must now be regarded as atypical as it is very high in fluoride and low in sodium. Barry, Clinton & Wilson (1979) examined the glass structure. They found that it was heavily opal and phase-separated with droplets rich in calcium and fluoride of complex morphology. These droplets were of average size 1-7 \im and volume fraction 20%. There were also massive inclusions of fluorite. When the glass was fused at a higher temperature, 1300 °C as against 1150 °C, 131

Polyalkenoate cements Table 5.11. Effect of ( + )-tartaric acid on glass polyalkenoate cement properties

Tartaric acida

Absent

Working time (23 °C), 10 minutes Setting time (37 °C), 4-3 minutes Setting rate, arbitrary 50 units Compressive strength, 121 MPa

G-200

G-309

G-237

Present Absent

Present Absent Present

10

1-2

1-7

1-7

2-7

2-7

3-5

2-8

60

3-8

165

65

100

20

55

140

125

136

100

120

Cement-forming liquid: 45 % poly(acrylic acid) Powder: liquid ratio of 3:1 a Added in 5 % concentration fluoride was lost, the morphology of the glass changed to one with smaller droplets, and it became more reactive towards poly(acrylic acid). 5.9.3

Poly(alkenoic acid)s

The poly(alkenoic acid)s used in glass polyalkenoate cement are generally similar to those used in zinc polycarboxylate cements. They are homopolymers of acrylic acid and its copolymers with itaconic acid, maleic acid and other monomers e.g. 3-butene 1,2,3-tricarboxylic acid. They have already been described in Section 5.3. The poly (aerylie acid) is not always contained in the liquid. Sometimes the dry acid is blended with glass powder and the cement is activated by mixing with water or an aqueous solution of tartaric acid (McLean, Wilson & Prosser, 1984; Prosser et al., 1984). Increase in concentration of the polyacid increases solution viscosity, quite sharply above 4 5 % by mass (Crisp, Lewis & Wilson, 1977). The strength of glass polyalkenoate cements also increases, almost linearly, with polyacid concentration. This is achieved at the cost of producing overthick cement pastes and loss of working time. The molecular weight of the polyacid affects the properties of glass polyalkenoate cements. Strength, fracture toughness, resistance to erosion and wear are all improved as the molecular weight of the polyacid is

132

Glass polyalkenoate (glass-ionomer) cement Table 5.12. Effect of the various tartaric acids on glass polyalkenoate cement properties {Crisp, Lewis & Wilson, 1979)

None (+ )-tartaric acid ( —)-tartaric acid Meso-tartaric acid Racemic tartaric acid

Setting time, minutes (37 °C)

Compressive strength, MPa (24 hours)

8-5 50 4-75 10-25 5-25

104 112 120 72 109

Powder: G-200 Liquid: 47-4% poly(acrylic acid), 5-3% tartaric acid increased (Wilson, Crisp & Abel, 1977; Hill, Wilson & Warrens, 1989; Wilson et al.91989). Setting is accelerated and working time is lost, and this places a restriction on improving cement properties by this route. The maximum molecular weight of the polyacid that can be used would appear to be 75000. 5.9.4

Reaction-controlling additives

The glass polyalkenoate cement system was not viable until Wilson and Crisp discovered the action of ( + )-tartaric acid as a reaction-controlling additive (Wilson & Crisp, 1975,1976, 1980; Wilson, Crisp & Ferner, 1976; Crisp & Wilson, 1976; Crisp, Lewis & Wilson, 1979). It may be regarded as an essential constituent and is invariably included in glass polyalkenoate cements as a reaction-controlling additive. It affects the nature of the setting reaction profoundly and this subject is discussed in Section 5.9.5. It sharpens set and increases the hardening rate, without decreasing, and even sometimes increasing, working time. Strength is also increased (Table 5.11). Crisp, Merson & Wilson (1980) found moreover that the addition of ( + )-tartaric acid conferred the setting property on a glass (G-288) that otherwise did not form a cement. No other additive has the same effect although many alternatives were examined by Wilson, Crisp & Ferner (1976) and Prosser, Jerome & Wilson (1982). Other multifunctional carboxylic acids, including citric acid, had little effect, apart from a slight tendency to shorten working time and increase the setting rate. That the effect is a subtle one is shown by the fact 133

Polyalkenoate cements Table 5.13. Effect of fluorides on glass polyalkenoate cement compressive strength, MPa {Crisp, Merson & Wilson, 1980) ( + )-tartaric acid Fluoride

Absent

Present

None A1F3 MgF 2 SnF 2 ZnF.

72 91 101 128 111

112 158 145 128

Powder: G-247 Liquid: 50 % acrylic-itaconic acid copolymer, 5 % tartaric acid that raeso-tartaric acid does not have the effect of sharpening the set (Table 5.12). Ethanolamines and polyphosphates slow the reaction down as a whole. Both tetrahydrofurantetracarboxylic acid and polyphosphates are sometimes to be found in commercial examples. Crisp, Merson & Wilson (1980) found that the addition of metal fluorides to formulations had the effect of accelerating cement formation and increasing the strength of set cements; the effect was enhanced by the presence of ( + )-tartaric acid (Table 5.13). Strength of cements formed from an SiO 2 -Al 2 O 3 -Ca 3 (PO 4 ) 2 glass, G-247, can be almost doubled by this technique. 5.9.5

Setting General

The setting and hardening reactions are first outlined in general terms. They can be considered to take place in a number of overlapping stages. (1) On mixing the cement paste, the calcium aluminosilicate glass is attacked by hydrogen ions from the poly(alkenoic acid) and decomposes with liberation of metal ions (aluminium and calcium), fluoride (if present) and silicic acid (which later condenses to form a silica gel). (2) As the p H of the aqueous phase rises, the poly(alkenoic acid) ionizes and most probably creates an electrostatic field which aids the migration of liberated cations into the aqueous phase. 134

Glass polyalkenoate (glass-ionomer) cement (3) As the poly(alkenoic acid) ionizes, polymer chains unwind as the negative charge on them increases, and the viscosity of the cement paste increases. The concentration of cations increases until they condense on the polyacid chain. Desolvation occurs and insoluble salts precipitate, first as a sol which then converts to a gel. This represents the initial set. (4) After gelation or initial set, the cement continues to harden as cations are increasingly bound to the polyanion chain and hydration reactions continue. Recent evidence suggests that a siliceous hydrogel may be formed in the matrix. In considering the setting reaction in more detail, cognizance must be taken of the nature of the glass and the presence of reaction-controlling additives. These affect both the nature of the cement-forming reaction and setting characteristics. These effects stem from complex formation; the two most important complexing agents are fluoride, derived from the glass, and (4- )-tartaric acid, by far the most important of the reaction-controlling additives. We distinguish four possible systems: (1) oxide glasses, (2) oxide glasses with added (+ )-tartaric acid, (3) fluoride glasses and (4) fluoride glasses with added (H-)-tartaric acid. Cement formation with oxide glasses Little more can be said concerning the cement-forming reactions between oxide glasses and poly(alkenoic acid)s because they have not been studied. It is almost impossible to prepare cements from oxide glasses and a polyacid, because the paste formed on mixing is intractable. The evidence points toward the premature bonding of aluminium ions as the cause (Ellis & Wilson, 1987). This does not occur in the related dental silicate cement, which is formed using a concentrated solution of orthophosphoric acid; here aluminium forms complexes with orthophosphoric acid, which delays precipitation. Practical cements can be formed from oxide glasses if ( + )-tartaric acid is added to the system. Since aluminium forms soluble complexes with ( + )-tartaric acid it is reasonable to suppose that the formation of these complexes prevents the premature ion-binding of aluminium to poly(alkenoic acid) and so allows workable cement pastes to be formed. This view is supported by the solution studies of Ellis & Wilson (1987).

135

Polyalkenoate cements Table 5.14. Infrared spectroscopic bands of reference carboxylates {Nicholson et aL, 1988b) C-O stretch of salt, cm"1 Salt

asymmetric

symmetric

Ca-PAA Al-PAA Ca-tartrate Al-tartrate

1550 1599 1595 1670

1410 1460 1385 1410

Cement formation with fluoride glasses - no tartaric acid Cements can be formed using fluoride-containing glasses in the absence of ( + )-tartaric acid, but high-fluoride glasses have to be used to obtain cements with sufficient working time (Crisp & Wilson, 1976; Kent, Lewis & Wilson, 1979). Only very few glasses, those very high in fluoride, can be used in practical cements. Fluoride clearly has a considerable effect on the reaction, probably because it forms strong soluble complexes with aluminium such as A1F2+ and A1F+; these have been reported by Connick & Poulsen (1957), O'Reilly (1960), and Akitt, Greenwood & Lester (1971). These complexes probably prevent the premature gelation of the polyanions by aluminium ions. In this system there is a useful cooperative effect between aluminium, fluoride and calcium, which has been demonstrated by the solution studies of Ellis & Wilson (1987). In the absence of aluminium, calcium precipitates as the fluoride at all pHs. Aluminium has the effect of preventing the precipitation of calcium as fluoride, again because it forms strong soluble complexes with fluoride. Detailed studies of the cement-forming reaction using fluoride-containing glasses and aqueous solutions of poly(acrylic acid) have been carried out; mainly by Wilson and his coworkers (Crisp & Wilson, 1974a,b, 1976; Crisp et aL, 1974; Barry, Clinton & Wilson, 1979; Prosser, Richards & Wilson, 1982; Hill & Wilson, 1988b; Nicholson et aL, 1988b), but the work of Cook (1982, 1983a) should also be noted. The following account is based mainly on the early studies of Crisp, Wilson and coworkers (Crisp & Wilson, 1974a,b, 1976; Crisp et aL, 1974) who used glass G-200 and 50 % solutions of poly(acrylic acid), and is supported by the later studies of Nicholson et aL (1988b) using glass G309. The compositions of materials used are shown in Table 5.8. 136

Glass polyalkenoate (glass-ionomer) cement These workers applied chemical and infrared spectroscopic methods to study cement-formation. Infrared spectroscopy exploits the fact that calcium polyacrylate and aluminium polyacrylate give rise to different carboxylate bands (Table 5.14). On mixing the powder and liquid, hydrogen ions from the poly(acrylic acid) solution rapidly attack the glass particles, which are decomposed to silicic acid, and Al3+, Ca2+, Na+ and F~ ions are released (Crisp & Wilson, 1974a,b; Cook, 1983c; Crisp, Lewis & Wilson, 1976d; Wasson & Nicholson, 1990). Originally, it was supposed that this attack occurred only at the surface layer of the glass particles (Barry, Clinton & Wilson, 1979) but later observations using 29Si NMR by Ellison & Warrens (1987) suggest that attack occurs throughout the glass particles. In the case of glass G-235 pentacoordinated aluminium disappeared and Q4 (2A1) units were converted to Q3 (1A1) units, i.e. aluminium was lost from the glass structure. Since hydrogen ions are six to twelve times more mobile than other cations, there will be a delay between loss of hydrogen ions from solution and migration of glass cations into the aqueous phase. Presumably, this electrical imbalance results in an electricfieldwhich acts as a driving force for the migration of cations. Aluminium and fluoride are almost certainly transported as cationic aluminofluoride complexes, A1F2+ and A1FJ, mentioned above. Figure 5.11 (Crisp & Wilson, 1974b) shows the time-dependent variation of the concentration of soluble ions in setting and hardening cements. Note that the concentrations of aluminium, calcium andfluoriderise to maxima as they are released from the glass. After the maximum is reached the concentration of soluble ions decreases as they are precipitated. Note that this process is much more rapid for calcium than for aluminium and the sharp decline in soluble calcium corresponds to gelation. This indication is supported by information from infrared spectroscopy which showed that gelation (initial set) was caused by the precipitation of calcium polyacrylate. Thisfindingwas later confirmed by Nicholson et al. (1988b) who, using Fourier transform infrared spectroscopy (FTIR), found that calcium polyacrylate could be detected in the cement paste within one minute of mixing the cement. There was no evidence for the formation of any aluminium polyacrylate within nine minutes and substantial amounts are not formed for about one hour (Crisp et al., 1974). Crisp & Wilson (1974b, 1976) attributed the slowness of binding in the case of aluminium to several effects: preferential leaching of calcium ions, lack of mobility of the hydrated or multinuclear aluminium species 137

Polyalkenoate cements (Aveston, 1965; Akitt, Greenwood & Lester, 1971; Waters & Henty, 1977) and steric problems because of the triple charge on the ion. However, when it is remembered that the Al3+ ion is responsible for the premature gelation of the oxide glass system, it may be that the true explanation lies in the stability of the fluoride complexes or the formation of large multinuclear aluminium complexes. Such complexes have been reported by a number of workers - Aveston (1965), Akitt, Greenwood & Lester (1971) and Waters & Henty (1977) - and may be represented by the generic formula [Alx(OH,F)y(H2O)z]{3xy). The presence of such complexes may explain the slow reactivity of the Al3+ ion with EDTA in slightly acid solutions. Of course, this argument does not apply to the premature binding of Al3+ ions in the dental glass system when the pH is low. Gelation involves an extended structure and some type of linking between chains. The concept of salt-like crosslinks has already been described (Section 5.5). Other possibilities may be considered. Hill, Wilson & Warrens (1989) examined the possibility that chain entanglements might account for the strength of polyelectrolyte cements. They used in particular

-0-v

6 -

5 -

\

3

t 1 A. 1

•• F

••

• • • • • • • • • • . . .

y

•

*•

*'

2 -

1 -

PA 1

1

1 1 1 Mil

10

1

1

1 1 1 1 1 1 1

time, min

100

1

1

1 1 1 1 1 1

J

1000

Figure 5.11 The time-dependent variation, in setting and hardening cements, of the concentration of soluble ions Al 3+ , Ca 2+ , F~ and VO\~ (expressed as P 2 O5). These ions are released from the glass powder into the cement matrix (Crisp & Wilson, 1974b).

138

Glass polyalkenoate (glass-ionomer) cement the reptation (chain pull-out) model described by de Gennes (1979) and Edwards (1969). Here a polymer chain is considered to be trapped by a tube of entanglements formed by neighbouring chains, and strength is related to the forces required to pull out the trapped chain. They found, however, that this model did not account for the effect of polyacid molecular mass on cement strength (Nicholson, 1992). After initial set the cement continues to harden and strengthen (Crisp, Lewis & Wilson, 1976b) as cations are increasingly bound to the polyanion chain (Cook, 1983c) and hydration processes continue (Wilson, Paddon & Crisp, 1979; Wilson, Crisp & Paddon, 1981). There are still free COOH groups present after 24 hours of reaction (Nicholson et al., 1988b). Cook (1983c) observed that the transfer of aluminium and calcium from the glass to the matrix continued for at least five weeks, during which time both strength and modulus increased (Paddon & Wilson, 1976). The increase in strength after set is illustrated by the results of Elliot, HoUiday & Hornsby (1975) presented in Figure 5.12. The reaction probably never ceases entirely, for Crisp, Lewis & Wilson (1976b) have observed that strength continues to increase, logarithmically, for at least a year if specimens are stored in kerosene. This slow increase in strength has been attributed to hydration processes and the slow diffusion of cement-forming cations, especially aluminium, seeking anionic sites. There remains the question of the role of silica. This aspect of setting 200 r o

\ 150 o

I

«» 100

a 50 E o

1.0

10

100

1000

Time, hours Figure 5.12 The time-dependent increase in strength after set (Elliott, HoUiday & Hornsby, 1975).

139

Polyalkenoate cements chemistry has been neglected and there are some ambiguities in the literature. Recently, Wasson & Nicholson (1990, 1991) have revived interest in this topic. The release of orthosilicic acid, which accompanies the release of glass ions, has been observed and is implicit in the theory of acid decomposition of the glass (Section 5.9.2). During the setting reaction orthosilicic acid is converted to silica gel (Crisp et aL, 1974). Crisp, Lewis and Wilson (1976d) found that the reduction in the amount of silicic acid eluted as a cement aged was matched by similar reductions in amounts of cations and anions eluted. Age of cement, minutes Silica (SiO2) eluted mg/g cement Aluminium (A12O3) eluted mg/g cement Sodium (Na2O) eluted mg/g cement Fluoride (F) eluted mg/g cement

7 3-39 2-07 3-31 2-59

60 0-55 0-24 1-27 0-73

1440 010 003 0-49 0-73

The insolubilization of cations and anions during the setting and hardening process is thus paralleled by that of silica. Under acid conditions orthosilicic acid condenses first to form polymeric silicic acid and then silica gel (Her, 1979; Andersson, Dent Glasser & Smith, 1982). These processes are discussed more fully in Section 6.5.4. Gelation of silica, like the formation of salt gels, is enhanced by a reduction in the acidity of solutions. Much of the siliceous hydrogel formed is found enveloping the glass particles, and the attacked glass particles maintain their original morphology (Barry, Clinton & Wilson, 1979). The mechanism of this process is obscure and it is uncertain whether it results from an ion depletion process or deposition of a siliceous gel on the site of the decomposed glass network. Recently, Wasson & Nicholson (1990, 1991) have suggested that a hydrated silicate is formed in the matrix and may contribute to the hardening process. The composition of this species is at present unknown. It could be a silicate gel, similar to tobermorite gel found in Portland cement (Taylor, 1966) which could account for the long term increases in strength of glass-ionomer cement. Alternatively, it could be a type of silica gel. Silica gel cements are known: they are weak with a compressive strength of 14 MPa, but are acid-resistant (Trautschold, 1940). In this connection, it may be noted that glass-ionomer cements are much more resistant to acid attack than are the related zinc polycarboxylates. Clearly, further studies are required into this aspect of glass-ionomer cement chemistry. 140

Glass polyalkenoate (glass-ionomer) cement Cement formation with fluoride glasses - ( + )-tartaric acid

The presence of ( + )-tartaric acid in a cement formulation exerts a profound effect on the cement-forming reaction. The nature of the underlying chemical reaction is changed and this is reflected in timedependent changes in viscosity. Both Cook (1983a,b) and Hill & Wilson (1988b) have studied the effect of (+ )-tartaric acid on the development of viscosity. Hill & Wilson used two glasses in their study, an oxide glass, G-287 (Table 5.6), and a fluoride glass, G-307 (Table 5.8). They noted that in the absence of ( + )-tartaric acid there was an almost exponential increase in apparent viscosity with time and that this effect was exaggerated when small amounts of ( + )tartaric acid, 0-3 %, were added to the liquid (Figure 5.13). However, when added in larger amounts ( + )-tartaric acid reduced the apparent viscosity of the pastes and also delayed the increase in apparent viscosity for a period of time which depended on the amount of ( + )-tartaric acid added; this plateau is very evident in the curves of apparent viscosity against time. Clearly, there are competing reactions taking place. The early reaction leading to gelation has been studied by Prosser, Richards & Wilson (1982) who used 13C NMR to examine the interaction between glass G-200 and poly(acrylic acid) solutions, and by Nicholson et

Viscosity

(+) tartaric acid no (+) tartaric acid

Time

1

Figure 5.13 Effect of tartaric acid on viscosity development (Hill & Wilson, 1988b).

141

Polyalkenoate cements al (1988b) who used FTIR and glass G-309 (Table 5.8). The addition of ( + )-tartaric acid totally changes the chemistry of the cement-forming reaction. It reacts preferentially with the glass because it is the stronger acid and its complexes are fully formed at pH = 3-4 while those of poly (aery lie acid) only appear at higher pH. The formation of calcium polyacrylate is suppressed, the extent of this suppression being dependent on the amount of ( + )-tartaric acid present. Instead, within the first minute, calcium tartrate makes a transient appearance most probably causing gelation. It disappears within nine minutes as aluminium polyacrylate appears. From these experiments, it appears that ( + )-tartaric acid prolongs working time by preventing the premature formation of calcium polyacrylate, and later sharpens set and accelerates hardening by enhancing the rate at which aluminium polyacrylate is formed. By contrast, while mesotartaric acid delays the premature formation of calcium polyacrylate it does not enhance the rate of formation of aluminium polyacrylate. Thus, it prolongs working time without accelerating hardening. The effects of

Figure 5.14 The microstructure of the set cement is clearly revealed by Nomarski reflectance optical microscopy. Glass particles are distinguished from the matrix by the presence of etched circular areas at the site of the phase-separated droplets (Barry, Clinton & Wilson, 1979).

142

Glass polyalkenoate (glass-ionomer) cement (H-)-tartaric acid and raeso-tartaric acid on the physical properties of glass-ionomer cements described in Section 5.9.4 are explained in terms of the underlying chemistry. 5.9.6

Structure

Barry, Clinton & Wilson (1979) examined the structure of cements prepared from a glass powder from which very fine particles had been removed to improve resolution. The microstructure of the set cement is clearly revealed by Nomarski reflectance optical microscopy (Figure 5.14). Glass particles are distinguished from the matrix by the presence of etched circular areas at the site of the phase-separated droplets. The micrograph

Figure 5.15 More detail than seen in Fig. 5.14 is obtained in a scanning electron image. The reacted glass particles are covered by a distinct reaction layer of silica gel (Barry, Clinton & Wilson, 1979).

143

Polyalkenoate cements is similar to that of the dental silicate cement (Section 6.5.5). More detail can be seen in a scanning electron image (Figure 5.15). The reacted glass particles are covered by a distinct, gel-like reaction layer which is identified as silica gel. This layer has detached itself from the glass core, which,

Figure 5.16 (a) An electron image of a glass-ionomer cement; (b) SL diagrammatic representation of this image: unshaded areas are glass particles, lightly shaded areas are cement matrix, and deeply shaded areas are voids or cracks. The remaining micrographs show the Ka characteristic radiation of (c) Ca, (d) Si, (e) Al, and (/) Si: Al (Barry, Clinton & Wilson, 1979).

144

Glass polyalkenoate {glass-ionomer) cement according to Brune & Smith (1982), is consistent with weak hydrogenbonding between layer and core. Fluorite particles appear as bright highlights because any calcium present gives rise to enhanced electron scattering. Barry, Clinton & Wilson (1979), using a scanning electron microscope fitted with an X-ray analyser, obtained an element distribution map for Ca, Si and Al (Figure 5A6c,d,e). They found that whereas the glass particles contained major amounts of all three elements, the matrix contained only aluminium and calcium in major amounts, with more aluminium than calcium; there were only minor amounts of silicon. Interestingly, element distributions showed that the glass particles appeared slightly larger when judged by silicon radiation than by aluminium radiation. A map of the Si/Al ratio confirmed this observation. An electron image of a glassionomer cement specimen is shown in Figure 5.16a together with an Si: Al distribution map, Figure 5.16/. At the time, this effect was attributed to the depletion of ions from the outer layer of the glass particles, but now we are more inclined to believe that this layer is really due to an outgrowing of silica gel, which, of course, has the capacity to absorb metal ions (Her, 1979; Hazel, Shock & Gordon, 1949). Certainly the siliceous gel layer in Figure 5.15 does have the appearance of a reaction zone rather than that of a relict. Moreover, an aluminosilicate glass matrix is decomposed to silicic acid (Wasson & Nicholson, 1990, 1991), which is detected in significant amounts in young cements (Crisp, Lewis & Wilson, 1976d). The latter interpretation of data is more in accord with the recent 27A1 and 29Si NMR findings of Ellison & Warrens (1987), who found that the structure of an appreciable fraction of the glass changed under acid attack with some loss of aluminium including all in fivefold coordination (see Section 5.9.2). Thus, acid attack was not entirely confined to the surface layer of a glass particle. If this is so then silicic acid as well as ions must migrate from the body of the particle and it is reasonable to suppose that silicic acid deposits as siliceous gel at the particle-matrix interface. The picture of cement microstructure that now emerges is of particles of partially degraded glass embedded in a matrix of calcium and aluminium polyalkenoates and sheathed in a layer of siliceous gel probably formed just outside the particle boundary. This structure (shown in Figure 5.17) was first proposed by Wilson & Prosser (1982, 1984) and has since been confirmed by recent electron microscopic studies by Swift & Dogan (1990) and Hatton & Brook (1992). The latter used transmission electron microscopy with high resolution to confirm this model without ambiguity. 145

Polyalkenoate cements The form of silica in the matrix is at present unknown. In the freshly prepared cement there are appreciable amounts of silicic acid present which decline as the cement ages (Crisp, Lewis & Wilson, 1976d). In the set cement silica could be present as a polymeric silicic acid, a siliceous gel or even a hydrated silicate gel, such as the tobermorite gel present in Portland cements (Taylor, 1966). The molecular structure is mainly a matter of conjecture but the coordination state of aluminium in the matrix is known to be six (Ellison & Warrens, 1987). Therefore, there must be three monovalent anions, COO", F" or OH", and three neutral H2O. This matter has been discussed in detail in Section 5.5. As noted there, Al3+ has the potential to link three chains but this is sterically unlikely. Also, as Mehrotra & Bohra (1983) have pointed out, tricarboxylates tend to hydrolyse into complex structures that involve Al-O-Al bridges. Nevertheless we must note that glass polyalkenoate cement loses its original plastic behaviour and becomes increasingly rigid as it ages over the weeks following set. Such time-dependent changes are not found with other cements and it is tempting to speculate that they may arise from the slow transformation in the nature of the Al bonding.

Rolysalt matrix

Figure 5.17 Diagrammatic representation of a glass-ionomer cement (Laboratory of the Government Chemist: Crown copyright reserved).

146

Glass polyalkenoate (glass-ionomer) cement 5.9.7

General characteristics

The glass polyalkenoate cement uniquely combines translucency with the ability to bond to untreated tooth material and bone. Indeed, the only other cement to possess translucency is the dental silicate cement, while the zinc polycarboxylate cement is the only other adhesive cement. It is also an agent for the sustained release of fluoride. For these reasons the glass polyalkenoate cement has many applications in dentistry as well as being a candidate bone cement. Its translucency makes it a favoured material both for the restoration of front teeth and to cement translucent porcelain teeth and veneers. Its adhesive quality reduces and sometimes eliminates the need for the use of the dental drill. The release of fluoride from this cement protects neighbouring tooth material from the ravages of dental decay. New clinical techniques have been devised to exploit the unique characteristics of the material (McLean & Wilson, 1977a,b,c; Wilson & McLean, 1988; Mount, 1990). This cement also has a low setting exotherm, lower than any other aqueous dental cement (Crisp, Jennings & Wilson, 1978), which means that it can be mixed swiftly as there is no need to dissipate heat. This property also gives it an advantage over bone cements based on modified poly (methyl methacrylate) which have high exo therms. 5.9.8

Physical properties

The glass polyalkenoate cement sets rapidly within a few minutes to form a translucent body, which when young behaves like a thermoplastic material. Setting time (37 °C) recorded for cements mixed very thickly for restorative work varied from 2-75 to 4-7 minutes, and for the more thinly mixed luting agents from 4-5 to 6-25 minutes. Properties are summarized in Table 5.15. Strength develops rapidly and after 24 hours in water (37 °C) can reach 225 MPa (compressive) and 39 MPa (flexural) (Williams & Billington, 1989; Pearson & Atkinson, 1991; Pearson, 1991). Compressive modulus reaches 9 to 18 GPa after 24 hours (Paddon & Wilson, 1976; Wilson, Paddon & Crisp, 1979). Crisp, Lewis & Wilson (1976a) found that for two early types of glassionomer cement (ASPAII and ASPAIV) compressive strength continued to increase for at least a year. Recently, Williams & Billington (1989) have found that this behaviour does not hold for all modern commercial 147

Polyalkenoate cements Table 5.15. Mechanical properties of glass polyalkenoate cement (Prosser et al. 1984; 0ilo, 1988; Paddon & Wilson, 1976; Wilson, Paddon & Crisp, 1979; Pearson & Atkinson, 1991; Williams & Billington, 1989)

Powder .liquid, g cm 3 Conventional Water hardening Consistency disc diameter, mm Maximum particle size, urn Film thickness (2 min), urn Working time (23 °C), minutes Setting time (37 °C), minutes Wet compressive strength (24 h), MPa Wet compressive modulus (24 h), GPa Wet flexural strength (24 h), MPa Wet tensile strength, MPa Creep (24-48 h), % Opacity, C 07 (1 mm) Water leachables (7 min), % Water leachables (1 h), %

Filling materials

Luting agents

20-3-4 6-7-7-2 18-33

1-67-1-83 3-3-3-6 21-31 20-40 24-40 2-3-5-75 4-5-6-25 82-162

— — 1-3-3-8 2-75-4-7 140-225 9-18 8-9-391 9-0-19-3 019-0-33 0-44-0-84 0-29-2-12 0-13-0-70

— 4-1-15-5 5-3-10-9 0-32-1-37 0-67-0-88 0-9-3-2 0-3-1-0

Load: 2-5 Kgf for filling materials, 220 gf for luting agents, applied after 2 minutes

materials. Thus, whereas the compressive strength of Opusfil W increased steadily from 225 MPa (after 24 hours) to 258 MPa (after 115 days), that of Ketac Fil increased from 171 MPa (after 24 hours) to a peak of 262 MPa (after 50 days) and, thereafter, declined to 167 MPa (after 115 days). The young glass polyalkenoate cement is similar to zinc polycarboxylate cement in that it is not as rigid as dental silicate cement and shows marked stress relaxation characteristics (Paddon & Wilson, 1976). However, as the glass polyalkenoate cement ages its properties progressively depart from those of the zinc polycarboxylate and approach those of the dental silicate cement. It loses its viscoelastic properties and becomes increasingly rigid. This is shown by a significant increase in modulus and decline in stress relaxation (Figure 5.18).

148

Glass polyalkenoate {glass-ionomer) cement Fracture toughness

Flexural strength and fracture toughness are clinically more significant than compressive strength. The flexural strength of a glass-ionomer cement can reach 39 MPa after 24 hours (Pearson & Atkinson, 1991) which is a much higher value than that attained by any dental silicate cement. N/mm2

ExiO3

a

Cement Age

Figure 5.18 Thisfigureshows how the properties of a glass polyalkenoate cement change as it ages. S is the compressive strength, E the modulus, a* a stress-relaxation function, and e* a strain-conversion function from elastic to plastic strain (Paddon & Wilson, 1976).

149

Polyalkenoate cements Table 5.16. Strength and fracture toughness of glass polyalkenoate filling materials (Seed & Wilson, 1980; Prosser et ai, 1984; Lloyd & Mitchell, 1984; Goldman, 1985; Prosser, Powis & Wilson, 1986; Lloyd & Adamson, 1987)

Glass polyalkenoate cement Phase-dispersed glass polyalkenoate cement Carbon fibre reinforced glass polyalkenoate cement Silver-glass polyalkenoate cement Dental silicate cement Anterior composite resins Posterior composite resins Dental amalgam

Flexural strength a, MPa

Fracture toughness # l c ,MNm 3 / 2

9-30 38

0-45-0-55 —

53 36 25 29-49 74^129 76

— 0-35 0-12-0-30 0-63-1-84 0-95-2-0 0-74

However, these values are less than those recorded for composite resins used in dentistry. Goldman (1985) reports values of 29 to 49 MPa for anterior composite resins and Lloyd & Adamson (1987) values of 76 to 125 MPa for posterior composite resins. A typical amalgam has a flexural strength of 6 MPa (Lloyd & Adamson, 1987) (Table 5.16). However, the flexural strengths of some glass-ionomer cements increase with time and values as high as 59 MPa (after 3 months) and 70 MPa (after 7 days) have been reported (Pearson & Atkinson, 1991). Fracture toughness values for glass polyalkenoate cement vary from 0-25 to 0-55 MN m3/2 (Lloyd & Mitchell, 1984; Goldman, 1985; Lloyd & Adamson, 1987). The values are generally higher than those found for the traditional dental silicate cement but lower than those found for anterior composite resins (Lloyd & Mitchell, 1984; Goldman, 1985) and much lower than those for posterior composite resins and dental amalgams (Lloyd & Adamson, 1987). These low values for flexural strength and fracture toughness compared with the values for composite resins and dental amalgams make the glass-ionomer cement less suitable than these materials in high-stress situations.

150

Glass polyalkenoate (glass-ionomer) cement Translucency A restorative material can be used for the aesthetic restoration of the front (anterior) teeth only if it is as translucent as tooth enamel. This is because colour matching depends on translucency as well as hue and chroma. The glass polyalkenoate cement is translucent and so can be colourmatched to enamel (Kent, Lewis & Wilson, 1973; McLean & Wilson, 1977a,b,c; Crisp, Abel & Wilson, 1979; Wilson & McLean, 1988); see Figure 5.19. Early glass polyalkenoate cements were significantly deficient in this quality because high-fluoride glasses had to be used before other means of controlling the cement-forming reaction were discovered. These glasses were heavily phase-separated and almost opaque. This has been remedied in recent formulations by employing clear or slightly opalescent glasses in combination with reaction-controlling additives. Another barrier to achieving translucency is mismatch between the refractive indices of the glass and the matrix; the refractive index of the glass is greater than that of the matrix, which causes light-scattering. The dental silicate cement tends to be naturally more translucent than the glass

Figure 5.19 The translucent appearance of glass polyalkenoate cements when placed on a black and white striped background.

151

Polyalkenoate cements polyalkenoate cement because the refractive index of a phosphate matrix is greater than that of a polyacrylate one. However, by incorporating large amounts of leachable phosphate into a polyalkenoate cement glass it has been possible to increase matrix refractive index and to produce fully aesthetic glass polyalkenoate cements (Crisp, Abel & Wilson, 1979). The translucency of dental materials is normally represented as the inverse of opacity, although the scattering coefficient is a more fundamental property (Section 10.11). Opacity is equivalent to contrast ratio, which is the ratio of the light reflected from a disc of cement (1 mm thick) placed on a black background to that when it is placed on a white background. The reflectivity of this background used in the dental context is 70 % and opacity is reported as C0.7 values (Paffenbarger, 1937). The C0.7 values for the enamel of incisor teeth vary from 0-31 to 0*67 (Paffenbarger, Schoonover & Souder, 1938) and it is generally accepted that an aesthetic filling material should have a C0.7 between 0-35 and 0-50. The first glass polyalkenoate cement had a C0.7 of 0-76, which was far too high, but improved modern materials are more acceptable and a value as low as 0-52 has been reported for one of these (Crisp, Abel & Wilson, 1979). Knibbs, Plant & Pearson (1986b) have found that most glass polyalkenoate cements have a good optical match with tooth enamel.

5.9.9

Adhesion

Bonding to tooth material and metals The glass polyalkenoate cement has the important property of adhering to untreated enamel and dentine as many workers have shown (Wilson & McLean, 1988; Lacefield, Reindl & Retief, 1985). It also appears to adhere to bone and base metals (Hotz et al., 1977). The bond strength to enamel (2-6 to 9-9 MPa) is greater than that to dentine (1-5 to 4-5 MPa) (Wilson & McLean, 1988). Bond strength develops rapidly and is complete within 15 minutes according to van Zeghbroeck (1989). The cement must penetrate the acquired pellicle (a thin mucous deposit adherent to all surfaces of the tooth) and also bond to debris of calciferous tooth and the smear layer present after drilling. Whatever the exact mode of bonding to tooth structure, the adhesion is permanent. The principles and mechanism of adhesion have already been discussed in Section 5.2. Various attempts have been made to improve bonding by pre152

Glass polyalkenoate (glass-ionomer) cement conditioning the tooth surfaces with surface conditioners. The earliest conditioner, citric acid solution, was proposed by Hotz et al. (1977). It etches the surface of enamel, revealing its prismatic structure, and removes calciferous debris (Figure 5.20a,&). It removes the smear layer from dentine and opens up dentinal tubules (Figure 5.20c,d). Doubt was cast on its efficacy and since then more effective surface conditioners have been found. These include poly(acrylic acid), which has a less drastic effect on tooth material than citric acid, and tannic acid, which forms reaction layers on both dentine and enamel (Powis et al., 1982). Surface cleansing agents and the fluoride ion also improve adhesion. Prati, Nucci & Montanari

Figure 5.20 The effect of a citric acid solution on tooth structure: (a) enamel surface before application, (b) enamel surface after application showing etching, (c) dentine surface before application, (d) dentine surface after application showing the opening-up of the dental tubules (Powis et al, 1982).

153

Polyalkenoate cements (1989) have reviewed recent studies on surface conditioners using modern glass polyalkenoate cements. They found that bond strength depended on both the cement and the surface conditioner used. The present consensus favours treatment with a solution of poly(acrylic acid). Permanent adhesion is an important attribute in a restorative material for it demands only minimal cavity preparation (i.e. drilling etc.) as there is no need to provide a retentive undercut. In cervical erosion lesions (small cavities at the gum line) it is especially important not to enlarge the lesion by drilling and in this situation the glass polyalkenoate cement is the material of choice. In addition, the glass polyalkenoate cement provides an excellent seal because it is adhesive and shows little or no microleakage compared with composite resins (Hembree & Andrews, 1978; Welsh & Hembree, 1985; Powis, Prosser & Wilson, 1988). This quality accounts for the biocompatibility of glass polyalkenoate cement because a good seal eliminates bacterial invasion at the interface between cavity wall and restoration. The biological consequences of this are described in Section 5.9.11.

Molecular attachment Acid etch attachment

Enamel

Figure 5.21 The laminate restoration, showing the glass polyalkenoate cement as a dentine substitute and a composite resin as an enamel substitute.

154

Glass polyalkenoate (glass-ionomer) cement Bonding to composite resins

An idea with important clinical implications to dentistry was advanced by McLean & Wilson (1977b). This was to use a laminate restoration composed of a glass polyalkenoate cement and a composite resin. In this technique the glass polyalkenoate cement and the composite resin are used together in a laminate restoration - the glass polyalkenoate cement as a dentine substitute and the composite resin as an enamel substitute (Figure 5.21). The glass polyalkenoate cement provides adhesion to dentine and fluoride release. The overlying composite resin provides excellent aesthetics and wear resistance and is bonded both to the enamel and the glass polyalkenoate cement by acid etching. The advantages of both materials are obtained with the laminate restoration. In recent years this concept has been revived and combined with the idea of bonding the composite resin to the glass polyalkenoate cement by using an acid-etch technique to provide micromechanical attachment (McLean et ah, 1985; Wilson & McLean, 1988). In this technique the surface of a set glass polyalkenoate cement is etched using a solution of orthophosphoric acid, and a thin coat of mobile resin is allowed to flow into the etched surface and is then polymerized by light activation. The composite resin is then bonded to this surface. The appearance of the etched cement surface is shown in Figure 5.22. The bond strength appears to be about the same as the cohesive strength of the cement; McLean et al. (1985) reported a value of 10 MPa. There is, however, a considerable variation in the strength of the union, and the combination of glass polyalkenoate cement and composite resin has to be selected with considerable care in order to achieve good results (Hinoura, Moore & Phillips, 1987; Mount, 1989; Prati, Nucci & Montanari, 1989). The strength of the union depends on several factors, including the strength of the cement, its rate of development and the ability of the bonding resin to wet the cement (Mount, 1989). There are problems of adaptation with heavily filled composite resins, and excessive polymerization contraction can destroy the bond. So far in these laminates leakage between the glass polyalkenoate cement and the dentine has not been completely eliminated, but the use of surface pre-treatments has significantly reduced its extent (Prati, Nucci & Montanari, 1989).

155

Polyalkenoate cements 5.9.10

Erosion, ion release and water absorption

Although the glass polyalkenoate cement is the most durable of all dental cements it is susceptible to attack by aqueous fluids under certain conditions. There are three related phenomena to consider: erosion, ion release and water absorption. Ion release and water absorption

When fully hardened, the cement is resistant to erosion provided the solution has a pH above 4. However, the glass polyalkenoate cement is susceptible to erosion immediately after set because some of the matrixforming cations and anions are still in soluble form. In fact, the hardening process is one where these cations and anions continue to precipitate. For this reason these cements have to be protected, temporarily, by a varnish. When immature glass polyalkenoate cements are exposed to neutral solutions, such as normal saliva, they release ions and absorb water. The

Figure 5.22 Effect of acid-etching on the surface of a glass polyalkenoate cement (McLean et aL, 1985).

156

Glass polyalkenoate (glass-ionomer) cement matrix-forming cation Al3+, but not Ca2+, can be lost and then the cement is permanently damaged. Other ions lost are sodium andfluoride,together with silicic acid, but loss of these species does not seem to be significant as far as erosion is concerned. Water is also rapidly absorbed: 3-2% in 24 hours and 3-8 % in 7 days in one example cited by Crisp, Lewis & Wilson (1980). As the cement ages, absorption of water and loss of aluminium ions ceases (after 7 days). Other species - sodium and fluoride ions and silicic acid - continue to be eluted. The release of fluoride is important, for the glass polyalkenoate cement can be seen as a device for its sustained release. Fluoride release

A number of workers have observed the sustained release of fluoride from the glass polyalkenoate cement (Crisp, Lewis & Wilson, 1976d; Forsten, 1977; Maldonado, Swartz & Phillips, 1978; Causton, 1981; Cranfield, Kuhn & Winter, 1982; Swartz, Phillips & Clark, 1984; Tay & Braden, 1988; Wilson, Groffman & Kuhn, 1985). The last-named found that release continued for at least 18 months. Release can be fitted to a log-log plot (Cranfield, Kuhn & Winter, 1982) although it is difficult to give a physical meaning to this expression. Wilson, Groffman & Kuhn (1985) fitted accumulated release of fluoride, sodium and silica to an equation of the following form: Total amount released = C+At*+Bt

(5.1)

The three terms correspond respectively to initial washout, diffusion and erosion. Unfortunately, although the mathematical fit was good (c. 99-9 %), C and B proved to be negative, making it difficult to assign a physical meaning to the equation. Wilson, Groffman & Kuhn (1985) calculated that only about 4-5% of the totalfluorideis available for release, and Meryon & Smith (1984) found that the amount released was not related to the fluoride content of the cement. Wilson, Groffman & Kuhn (1985) observed that release of fluoride was accompanied by the release of sodium, necessary to maintain electroneutrality, and silica (as silicic acid). The release of these species could also be fitted to equation (5.1). Crisp, Lewis & Wilson (1980) found that these same three species were released, in greater amounts but in roughly the same proportions, under acid attack. The association of silica with fluoride suggests, perhaps, that it is principally the glass particles that are attacked rather than the matrix 157

Polyalkenoate cements phase. Cranfield, Kuhn & Winter (1982) also found that the rate of release offluorideis greater in acid than in neutral solution. These results suggest that release of these species may be an erosive rather than a diffusive process, which may explain the perplexity of Cranfield, Kuhn & Winter (1982) in attempting to elucidate the mechanism offluoriderelease in terms of a diffusive process. Kuhn & Jones (1982) suggested that a porous granular monolith as described by Kydonieus (1980) was appropriate to describe fluoride release. More recently, Tay & Braden (1988), in a prolonged study over 2\ years, suggested that there were two processes involved, a rapid surface elution and a slower bulk diffusion. But we must conclude, at present, that the mechanism offluoriderelease is still far from understood. The release offluorideis biologically important because it is taken up by adjacent tooth material (Retief et al, 1984; Shimoke, Komatsu & Matsui, 1987), presumably by the ion exchange of F~ for OH" in hydroxyapatite. Thisfluorideuptake has the effect of improving the resistance of the tooth material to acid attack (Maldonado, Swartz & Phillips, 1978; Wesenberg & Hals, 1980; Kidd, 1978). Maldonado, Swartz & Phillips (1978) have found that the solubility of enamel in acid was reduced by 53 % when in contact with a glass polyalkenoate cement. Also, fluoride adsorption reduces surface energy (Glanz, 1969) making adhesion of caries-promoting plaque more difficult (Rolla, 1977). It also decreases demineralization and increases remineralization of teeth (Wei, 1985) and reduces the fermentation of carbohydrates and the growth of plaque bacteria (Hamilton, 1977; Tanzer, 1989). Of course, it has been known since the early 1940s that fluoride inhibits dental decay (Horowitz, 1973). Acid erosion and clinical durability

The glass polyalkenoate cement is the most durable of all dental cements. In a very early study, Kent, Lewis & Wilson (1973) found that the surface of the glass polyalkenoate cement was much less affected by acids and stained much less than traditional dental silicate cement. This early observation has since been substantiated by both in vivo studies (Mitchem & Gronas, 1978, 1981; Ibbetson, Setchell & Amy, 1985; Mesu & Reedijk, 1983; Pluim et al, 1984; Pluim, Arends & Havinga, 1985) and laboratory tests (Mesu, 1982; Beech & Bandyopadhyay, 1983; Sidler & Strub, 1983; Kuhn, Setchell & Teo, 1984; Theuniers, 1984; Pluim et al.9 1984; Pluim, Arends & Havinga, 1985; Walls, McCabe & Murray, 1985; Wilson et ai, 1986a,b; Gulabivala, Setchell & Davies, 1987). 158

Glass polyalkenoate (glass-ionomer) cement Clinical durability largely depends on resistance to acid erosion, for acid conditions occur in stagnation regions of the mouth where dental plaque accumulates. Plaque contains streptococci and lactobacilli which degrade plaque polysaccharides and sucrose to lactic acid (Tanzer, 1989; Jenkins, 1965), and lactic acid is the most potent driving force in causing demineralization of teeth. The lower extreme of acidity found in the mouth is pH = 40 (Stephan, 1940; Kleinburg, 1961). Glass polyalkenoate cements begin to erode only at this pH (Crisp, Lewis & Wilson, 1980); Walls, McCabe & Murray (1988) and Wilson et al. (1986b) found that one brand did not erode at all at this pH. Susceptibility to acid erosion is low even when the pH is 2-7. Crisp, Lewis & Wilson (1980) made a chemical study of the erosion of a glass polyalkenoate cement under acid attack. They found that the chief species eluted were sodium and fluoride ions and silicic acid suggesting that attack occurred mainly on the glass particles rather than on the matrix. Resistance to acid erosion depends on brand and varies from 0-04 to 0-54% per hour (Setchell, Teo & Kuhn, 1985; Wilson et al., 1986a; Walls, McCabe & Murray, 1988). It would appear that cements based on copolymers of acrylic and maleic acids are less durable than those based on poly (aery lie acid). The extent of erosion varies inversely with the time allowed for the cement to cure prior to exposure (Walls, McCabe & Murray, 1988). McKinney, Antonucci & Rupp (1987) found that the clinical wear of the glass polyalkenoate cement compared favourably with that of the composite resin, but they noted that it was prone to brittle fracture and chemical erosion. Clinical experience shows that these cements are durable. For example, a failure rate as low as 2 % has been reported by Mount (1984) in a clinical trial lasting seven years, and Wilson & McLean (1988) have cited a number of clinical trials attesting to the durability of this cement. 5.9.11 Biocompatibility Dental material The biocompatibility of the glass polyalkenoate cement is good (Wilson & McLean, 1988; Nicholson, Braybrook & Wasson, 1991) and its capacity to release fluoride in a sustained fashion makes it cariostatic (Hicks, Flaitz & Silverstone, 1986; Kidd, 1978). Its ability to provide an excellent seal (Section 5.9.9) is an important attribute because in recent years it has 159

Polyalkenoate cements become generally accepted that pulpal inflammation is caused not so much by chemical toxicity as by the percolation of harmful bacteria between cavity wall and the restorative (Brannstrom & Nyborg, 1969; Paterson, 1976; Browne et al, 1983). The seepage of harmful bacteria beneath a restoration can be the cause of secondary caries (Bergenholtz et al, 1982) and is the cause of much of the failure of dental amalgams (Mjor, 1985). Early studies showed that glass polyalkenoate cement has less of an adverse effect on the dental pulp than has silicate cement (Kawahara, Imanishi & Oshima, 1979; Pameijer, Segal & Richardson, 1981). Apparently, the glass polyalkenoate cement causes only mild pulpal inflammation which reaches a maximum 14 days after placement and then progressively diminishes (Kawahara, Imanishi & Oshima, 1979). The effect is greatly diminished by a layer of 0-5 mm of dentine. More recent materials appear to cause less inflammation than earlier ones (Plant et ai, 1984; Yoshii et al.9 1987). A recent careful study of biocompatibility by Pameijer & Stanley (1988) on primates, using a standard methodology recommended by the American National Standards Institute and the American Dental Association, showed a modern cement to be bland. The glass-ionomer cement is well-tolerated by living cells (Kawahara, Imanishi & Oshima, 1979; Kasten et al, 1989). An important distinction is to be made between the freshly mixed and fully set glass-ionomer cement. The glass-ionomer cement exhibits an antibacterial effect when freshly mixed which diminishes with time (Tobias, Browne & Wilson, 1985). It exhibits some cytotoxicity when freshly mixed, but none when fully set (Tyas, 1977; Kawahara, Imanishi & Oshima, 1979; Meryon & Browne, 1984; Hume & Mount, 1988; Brook, Craig & Lamb, 1991a; Hetem, Jowett & Ferguson, 1989). Both the antibacterial and cytotoxic effects appear to be associated with the leachate from the cements. The precise cause of these effects remains unclear. Some workers have suggested that it is the combination of fluoride and low pH in the young cement (Leirskar & Helgeland, 1987; McComb & Ericson, 1987), while others implicate the release of metal ions and free poly(acrylic acid)s (Kawahara, Imanishi & Oshima, 1979; Nakamura et al., 1983). Recent studies by Brook, Craig & Lamb (1991a,b) on bone substitutes have shed some light on this problem and are described later. No problems arise when the glass-ionomer cement is used to restore abrasion/erosion lesions in primary teeth and as a lining material in shallow cavity preparations (Tobias et ah, 1978, 1987). In deeper 160

Glass polyalkenoate {glass-ionomer) cement preparations, when the dentine layer is very thin it is advisable to use an antibacterial calcium hydroxide liner. Post-operative sensitivity has occasionally been reported when the glass-ionomer cement has been used as a luting agent. This observation is more than anecdotal, but the reason for it is unknown. It is not connected with pulpal irritation but may be related to hydraulic pressures (Pameijer & Stanley, 1988). The indication is that sensitivity is related to clinical technique and is exacerbated if certain slow-setting glass-ionomer cements are used, especially if they are mixed too thinly. Bone cement and bone substitute Not surprisingly, this cement has been studied as a possible replacement for the poly(methyl methacrylate), PMMA, bone cement, a material which is not entirely satisfactory (Jonck, Grobbelaar & Strating, 1989a,b; Jonck & Grobbelaar, 1990). Its use has been linked with the pathogenesis of failure; there is tissue sensitivity and it is not suitable in reconstructive surgery following cancer or replacement procedures following radiation treatment of diseased bone. Jonck, Grobbelaar & Strating (1989a,b) and Jonck & Grobbelaar (1990) have made biological evaluations, using baboons, of glass polyalkenoate cement for use in joint replacement surgery. It was found to be stable within the bone environment and there were no signs of surface dissolution over a period of three years. It was also observed to bond to bone and act as a sealant. The promotion of new bone formation and calcification on the surface of the cement was observed. This reparative process was tentatively attributed to the ion-exchange properties of the cement and, in particular, to the release of fluoride. The glass polyalkenoate cement was found to have no inhibitory effect on bone tissue development. Rather, it appeared to promote the formation of osteoblasts (bone-forming cells) and the normal differentiation of haemopoietic (blood-forming) tissue. The glass-ionomer cement was found to be non-toxic. There were no signs of inflammation or irritation with any of the glass polyalkenoate cement implants even after several months. By contrast a proportion of PMMA cement implants caused swelling and bone reactions. There were also signs of possible hyperaemia (blood congestion) and infarcts (areas deprived of blood supply) and dead tissue. Thus, the glass polyalkenoate cement has distinct biological advantages stemming from its dynamic surface chemistry, which is favourable to bone 161

Polyalkenoate cements mineral precipitation. This process, in turn, promotes normal tissue responses essential for the processes of bone formation. Brook, Craig & Lamb (1991a,b), who studied the use of the cement as an alveolar bone substitute, have confirmed thesefindings.They find that the cement forms an intimate bioactive bond with bone cells and their work has highlighted the complex role offluoride.Their in vitro studies showed that of the cement pastes only the one derived from a non-fluoridecontaining glass (MP4) did not inhibit cell growth. But this cement did not integrate with bone as effectively as did cements based on fluoridecontaining glasses. Jonck, Grobbelaar & Strating (1989a,b) suggested that the slow release of fluoride had a beneficial in vivo effect on osteogenesis (the formation of bone) similar to that of osteoid formation stimulated byfluoridetherapy in the treatment of osteoporosis (abnormal porosity of the bone) (Frost, 1981). There appeared to be an optimum level of fluoride for the stimulation of bone-forming cells (Turner et al, 1989) since mild toxic effects are encountered in the closed in vitro situation (Kawahara, Imanishi & Oshima, 1979; Nakamura et ai, 1983; Hetem, Jowett & Ferguson, 1989; Kasten et al., 1989; Jonck, Grobbelaar & Strating, 1989a,b; Brook, Craig & Lamb, 1991a,b). Brook, Craig & Lamb (1991b) have suggested that in the dynamic in vivo situation the leaching of fluoride may stimulate integration with the bone (osseointegration) thus accounting for the superiority of cements based onfluoride-containingglasses in this respect. Brook, Craig & Lamb (1991a) found that set glass-ionomer cement implants compared favourably with those of other materials. More bone was formed around glass-ionomer cement implants than those of hydroxyapatite. Direct bonding of glass polyalkenoate cement to mineralized collagenous extracellular bone matrix was found without an intervening layer. Behaviour was similar to that of bioglass and glass ceramics. To sum up, glass-ionomer cement forms an intimate bond to living bone, a process which is enhanced by the release of fluoride.

5.9.12 Modified and improved materials Various attempts have been made to improve the glass polyalkenoate cement. We have already described (Section 5.9.5) the most important innovation, the use of (+ )-tartaric acid to improve setting characteristics (Wilson & Crisp, 1976). Before its use was discovered, only one glass, 162

Glass polyalkenoate (glass-ionomer) cement G-200, could be used to form practical cements; afterwards a whole range of glasses became available, including clear ones. The original glass polyalkenoate cement based on G-200 had poor translucency because this highfluoride glass was heavily opal. The use of the new clear glasses yielded a new generation of cements with good translucency. Radio-opacity has also been introduced into glasses by replacing calcium with strontium or lanthanum (Section 5.9.2). All additives, ( + )-tartaric acid and metal fluorides, which improve setting also increase cement strength (Section 5.9.4). The same point can be made about the technique of acid-washing glasses (Schmidt et al., 1981a) which, by removing calcium ions from the surface of glass particles, enhances the mixing and setting qualities of cements. All of which illustrates the general point that strength is related to the working and setting qualities of cements. The realization that cement strength could be increased by increasing the molecular weight of the poly(alkenoic acid) was important (Section 5.9.3). Unfortunately, this also increased the stiffness of the cement mix, necessitating a reduction in powder/liquid ratio. Thus, the benefits gained on the one hand were lost on the other. Fortunately, there was a way round this problem. By drying the poly(alkenoic acid) and blending it with the glass powder cement, water could be used to initiate cement formation, with the benefit of lowering the stiffness of the mix (Prosser et al., 1984; McLean, Wilson & Prosser, 1984). This technique permitted the use of poly(alkenoic acid) of higher molecular weight. A combination of all these technical improvements has led to a significant improvement in the quality of the glass polyalkenoate cement. Reinforced glass polyalkenoate cements More direct attempts at improving cement strength have been made. Wilson et al (1980) and later Prosser, Powis & Wilson (1986) observed that disperse-phase glasses yielded stronger cements. The strongest cement yielded by a clear glass had aflexuralstrength of 21 MPa, whereas a glass containing corundum and fluorite as disperse phase gave a cement with a flexural strength of 33 MPa. The use of reinforcingfillerswas examined by Seed & Wilson (1980). An alumina-fibre cement had a flexural strength of 44 MPa, while one reinforced by carbon fibre had a flexural strength of 53 MPa. Metal reinforcement has also been examined. Seed & Wilson (1980) found that a cement reinforced with silver-tin alloy had aflexuralstrength of 40 MPa. 163

Polyalkenoate cements However, restorations made from this material could not be polished and were aesthetically very poor. Simmonds (1983) has pursued this idea, and a material has been placed on the market. But according to Moore, Swartz & Phillips (1985) such cements have less resistance to abrasion than a simple glass polyalkenoate cement. Recently, Oldfield & Ellis (1991) have examined the reinforcement of glass-ionomer cement with alumina (Safil) and carbon fibres. The introduction of only small amounts of carbon fibres (5% to 7*5% by volume) into cements based on MP4 and G-338 glasses was found to increase considerably both the elastic modulus andflexuralstrength. There was an increase in work of fracture attributable to fibre pull-out. A modulus as high as 12-5 GPa has been attained with the addition of 12% by volume of fibre into MP4 glass (Bailey et ai, 1991). Results using alumina fibre were less promising as there was nofibrepull-out because of the brittle nature of alumina fibres which fractured under load. McLean & Gasser (1985) had the idea of fusing silver with ionomer glass to produce a fused metal-glass (cermet) powder which replaced the glass powder in the conventional cement. The idea was to improve strength, toughness and abrasion resistance. Resistance to abrasion is improved (Moore, Swartz & Phillips, 1985; Swift, 1988b), probably by a lubricating effect since the surface of the restoration can take a polish (McLean & Gasser, 1985; McKinney, Antonucci & Rupp, 1985, 1987). However, Lloyd & Adamson (1987) found that a cermet polyalkenoate cement was no stronger and no tougher than a conventional material. In an in vitro study, Thornton, Retief & Bradley (1986) showed that the bond strength to enamel and dentine was lower, and consequently microleakage has been found to be higher (Robbins & Cooley, 1988). Fluoride release was found to be lower than that of conventional materials (Thornton, Retief & Bradley, 1986). The material is radio-opaque, which is an advantage, for its presence in living tissues can then be detected by X-rays. Obviously, it cannot be used for aesthetic restorations andfindschief use in preparing a core to receive porcelain and gold crowns. One example of this material is available on the market. Very recently, Williams, Billington & Pearson (1992) have examined the effect of reinforcement by silver or silver-tin alloy on the mechanical properties of three glass-ionomer cements. Measurements of compressive, flexural, tensile (measured by the diametral compressive procedure) and shell strength are given in Table 5.17. These results show that the effect of reinforcement varies from cement to cement but, in general, increases it. 164

Glass polyalkenoate (glass-ionomer) cement Table 5.17. Effect of reinforcement on various strength properties of three glass polyalkenoate cements (Williams, Billing ton & Pearson, 1992) Cement

Standard

Reinforced

Powder: liquid ratio, g cm"

A B

Wet compressive strength (24 h), MPa

A B

Wet flexural strength (24 h), MPa

A B

Wet tensile strength, MPa

A B

Wet shell strength (24 h), MPa

A B

3-8 2-4 7-0 146 136 162 21-3 24-7 251 18-6 13-3 16-2 35-7 32-3 46-9

3-2 51 110 137 138 221 290 36-3 670 12-8 14-3 251 44-6 39-6 75-8

C C

C

C C

The effect on cement C is particularly dramatic and the flexural strength of cement C is exceptionally high. In part, this is to be attributed to the high powder/liquid ratio. These results are to be compared with the flexural strengths of early polyalkenoate cements which were c. 10 MPa. Aluminoborate glasses Brief mention may be made of the aluminoborate glasses developed by Bertenshaw et al. (1979). These glasses are prepared by fusing a mixture of boric oxide (replacing silica), alumina and a metal oxide, usually zinc oxide. The fusion temperature is much lower than for the aluminosilicate glasses. The compositional region for glass formation is restricted and glasses are only obtained when the alumina content lies between 1 and 13 mole. Boric oxide content ranges from 35 to 73 mole boric acid and metal oxide content from 27 to 59 mole. The cement-forming reaction will be similar to glass polyalkenoate cement. The cement matrix will consist of metal polyacrylates, but boric acid will be produced instead of silica gel. Since boric acid has a water solubility of 2-7 % compared with the near insolubility of silica gel, it would 165

Polyalkenoate cements be expected that these cements would be less durable than the conventional glass polyalkenoate cements. Compressive strengths of these cements were found by Bertenshaw et al. (1979) to range from 20 to 50 MPa and tensile strengths from 5 to 9 MPa. These values are inferior to those of the conventional glass polyalkenoate cements but similar to those of the zinc polycarboxylate cements. They are reported to have a good translucency and have a low solubility in water. These materials do not appear to be manufactured commercially. Recently, Wilson & Combe (1991) have studied the reactivity of magnesium, zinc, calcium and strontium boroaluminate glasses towards poly (aery lie acid) solutions. The controlling factor would seem to be the alumina content of these glasses which serves to moderate the setting rate of the cements. 5.9.13 Applications The glass polyalkenoate cement is a versatile material and finds use in dentistry and more generally as a biomaterial. There have also been applications outside these fields. Dental The glass-ionomer cement is the most versatile of all the dental cements and has been developed for a variety of applications (McLean & Wilson, 1974, 1977a,b,c; Swift, 1988b; van de Voorde, 1988; Wilson & McLean, 1988; Mount, 1990). Many of its applications depend on its adhesive quality which means that, unlike the non-adhesive traditional filling materials, it does not require the preparation of mechanical undercuts for retention and the consequent loss of sound tooth material. The glass polyalkenoate cement was originally intended as a substitute for dental silicate cements for the aesthetic restoration of front (anterior) teeth (Wilson & Kent, 1972; Knibbs, Plant & Pearson, 1986a; Osborne & Berry, 1986; Wilson & McLean, 1988). It is suitable for restoring anterior cavities in low-stress situations, that is when the restoration is completely supported by surrounding tooth material. These cavities occur on the adjacent surfaces of neighbouring teeth (class III cavities) and at the gum line (class V cavities). Quite early, McLean & Wilson (1977b) found that the glass polyalkenoate cement was particularly effective for restoring cervical lesions 166

Glass polyalkenoate {glass-ionomer) cement small cavities that occur at the gum line. These are often found in middleaged people and are caused by abrasion or erosion rather than by dental decay (caries). These lesions are so small that it is unwise to enlarge them to provide mechanical retention (Wilson & McLean, 1988). For this reason the adhesive glass polyalkenoate cement is the material of choice for the restoration of cervical lesions (Tyas & Beech, 1985). The cement is commonly used to restore primary (children's) teeth since the trauma of drilling may be minimized or avoided altogether (Wilson & McLean, 1988; Walls, Murray & McCabe, 1988). Some recent ingenious applications are of particular interest as they exploit the adhesive properties of the glass-ionomer cement to the full. Clinicians have now developed the concept of minimal cavity preparation (McLean, 1980, 1986; Hunt, 1984; Knight, 1984; Wilson & McLean, 1988). The idea is that since caries is mainly a disease of the dentine, then only a minimal amount of enamel need be removed, just sufficient to allow for the excavation of the carious dentine. A small channel is drilled through the enamel and the carious dentine is removed through it. Sound enamel is thus preserved. The excavated region is thenfilledwith glass polyalkenoate cement, which by virtue of its adhesive nature holds the enamel shell together. Another use of the glass polyalkenoate cement is as a base material which can be placed under dental amalgam or composite resin in the restoration of posterior (molar and semi-molar) teeth (Smith, Ruse & Zuccolin, 1988; Wilson & McLean, 1988). Its role is to adhere to dentine, provide a protective seal against bacteria and release fluoride: functions which prevent caries occurring under the restoration. In the laminate restoration, fully described in Section 5.9.9, it is used, in effect, as a dentine substitute. Base cements used under other restoratives are frequently made radio-opaque so that they can be distinguished from carious dentine. This is achieved by adding zinc oxide, using silver-glass cermets in place of the glass or using glasses in which the calcium is replaced by lanthanum or strontium. Base cements are generally quick-setting. The glass polyalkenoate cement is also used in the fitting of crowns. It is used to build up a substructure, known as a core, if there is insufficient tooth material to take a crown (Wilson & McLean, 1988). Core build-up materials are generally made radio-opaque and the silver cermet is often used. A fine-grained version was developed to be used as a luting agent for the cementation of cement crowns and veneers; this is based on a finegrained glass (Wilson et aL, 1977; Wilson & McLean, 1988). 167

Polyalkenoate cements Another important use for the glass polyalkenoate cement is in preventive dentistry where it can be used to fill and seal naturally occurring pits and fissures in molar teeth which are sites for the initiation of caries (McLean & Wilson, 1974, 1977b; Komatsu, 1981; Wilson & McLean, 1988). Its adhesive quality and ability to act as a long-term fluoridereleasing gel make it particularly suitable for this purpose. Special formulations for this application have been placed on the market. Splint bandage The glass polyalkenoate cement forms the basis of a novel splint bandage that was developed in the early 1970s (Parker, 1974; Potter et ai, 1977, 1979; Hall, 1977) and marketed by Smith & Nephew. In this application, a powder blend of glass and poly(acrylic acid) is applied to a bandage. When required for use the bandage is immersed in water, a technique identical to that used for the conventional plaster bandage. The addition of water activates the cement-forming reaction and the bandage sets hard, but remains more flexible than the plaster bandage. It has other advantages. It is stronger and attains strength more rapidly than the conventional bandage. It is also impervious to water once set. These represent significant advantages for the patient. It can be applied over a normal plaster bandage to protect the latter from the softening effect of water. Bone cement Existing bone cements for orthopaedic use are based on a modified poly(methyl methacrylate) resin. It has disadvantages. The formation of this polymer in situ is accompanied by the marked evolution of heat which can damage tissues (Feith, 1975). The presence of unreacted monomer, which can leach out, also damages tissues (Petty 1980; Pople & Phillips, 1988). The glass polyalkenoate cement has an exceptionally low exotherm and good biocompatibility, which together with its ability to bond to bone give it potential as an improved bone cement. A biological evaluation of the glass polyalkenoate cement as a bone cement has been carried out by Jonck, Grobbelaar & Strating (1989a,b; Jonck & Grobbelaar, 1990) on baboons and is reported in Section 5.9.11; initial findings are promising.

168

Resin glass polyalkenoate cements Alveolar bone substitute In the UK about 20 % of the population over 16 are without natural teeth (Brook, Craig & Lamb, 1991b). Dentures are supported by the alveolar ridge which over the years is subjected to progressive resorption. The reduction of the alveolar ridge gives rise to functional problems. Brook, Craig & Lamb (1991b) have used the glass-ionomer cement successfully to restore this ridge showing that it successfully integrates with bone (see Section 5.9.11). Slip casting mould The glass polyalkenoate cement can also be used to replace plaster as a mould in the slip process for pottery. It possesses the same property as plaster of Paris, of causing material to deposit on its surface from slip suspensions. So far this property has not been exploited in the manufacture of pottery. 5.10

Resin glass polyalkenoate cements

5.10.1 General One of the most interesting recent developments has been the advent of the resin glass polyalkenoate cements (Antonucci, McKinney & Stansbury, 1988; Mitra, 1989; Wilson, 1989, 1990; Mathis & Ferracane, 1989; Minnesota Mining & Manufacturing Company, 1989; Albers, 1990). They are dual-cure hybrids that set by a combination of acid-base and polymerization reactions, and there are several types. Polymerization is effected by either chemical or light initiation. At its most basic, the resin glass polyalkenoate cement can be seen as a glass polyalkenoate cement in which the water component is replaced by a water-HEMA mixture. HEMA is hydroxymethyl methacrylate, its hydroxy group making it water-soluble: CH2 = C — C O — O — CH2-CH2-OH CH3 Also, included in these formulations are water-soluble initiators/activators for the polymerization of HEMA. The resin glass polyalkenoate cements are mixed in the same way as conventional materials. In the case of the light-activated systems they 169

Polyalkenoate cements remain workable for 10 or more minutes unless exposed to light. When light is shone on them they are activated and set hard in 30 seconds. There is a dual setting reaction: the normal glass polyalkenoate cement acid-base reaction and, additionally, a free-radical or photochemical polymerization process, similar to that occurring in composite resins. These may be represented as: (1) Acid—base reaction'. Calcium aluminosilicate glass (base)

Polyacrylic acid

Calcium and aluminium polysalt hydrogel

(2) Polymerization reaction: HEM A + photochemical initiator/activator ^PolyHEMA matrix Two matrices are formed: a metal polyacrylate salt and a polymer. There is a lack of water in the system because some of it has been replaced by HEM A, and lack of water in glass polyalkenoate cements is known to slow down the ionomer acid-base reaction (Hornsby, 1977). Thus, the initial set of these materials results from the polymerization of HEM A and not the characteristic acid-base reaction of glass-ionomer cements. The later reaction serves only to harden and strengthen the already formed matrix. The two matrix-forming reactions are shown in more detail in Figure 5.23. 5.10.2 Class I hybrids In more complex forms of this resin hybrid, other dimethacrylates may be present, such as the ethylene glycol dimethacrylates, and bis-GMA, when HEMA acts as a co-solvent for water and bis-GMA (Antonucci, McKinney & Stansbury, 1988). The general composition of these materials, which we term class I hybrids, is summarized below: Powder component'. Glass (Chemfil II) + poly(acrylic acid) + tartaric acid Liquid component {replaces water): Water/HEMA Other difunctional hydroxy dimethacrylates, e.g. the ethylene glycol dimethacrylates 170

Resin glass polyalkenoate cements

Bis-GMA Initiator/activator In chemically-cured materials, one example of an initiator/activator system is: hydrogen peroxide as initiator, ascorbic acid as activator and cupric sulphate as co-activator. In light-cured materials, camphorquinone is used as a visible-light photochemical initiator, sodium /?-toluenesulphinate as activator and ethyl 4-dimethylaminobenzoate as photoaccelerator. If there is too little water in a composition then the acid-base reaction DUAL CURE 1 ACID — BASE REACTION

I

I

CH2

CH 2

CH-COOH

CH-COO —

' I 2

Calcium aluminosilicate

I

CH-COOH

•

CH 2

A l 3 + - = 0 0 C — CH

CH-,

CH,

CH-COO"—

I

Poly (acrylic acid)

I

F

C a 2 + — 00C—CH

I

Ca, Al polysalt hydrogel

DUAL CURE 2 C=CH2

HEMA POLYMERIZATION

C=0 0 HEMA

Photo or chemical initiator / activator

-C-CH2-

c=o 0

I

CH 2

CH 2

CH 2

CH 2

OH

I

poly HEMA

Figure 5.23 The two matrix-forming reactions in class I resin-based glass polyalkenoate cements.

171

Polyalkenoate

cements

will be completely inhibited and only the polymerization reaction will take place, in which case the material is not strictly speaking a glass polyalkenoate cement. 5.10.3

Class II hybrids

The two matrices in these cements are of a different nature: an ionomer salt hydrogel and polyHEMA. For thermodynamic reasons, they do not interpenetrate but phase-separate as they are formed. In order to prevent phase separation, another version of resin glass polyalkenoate cement has been formulated by Mitra (1989). This is marketed as VitraBond, which we term a class II material. In these materials poly(acrylic acid), PAA, is replaced by modified PAAs. In these modified PAAs a small fraction of the pendant - C O O H groups are converted to unsaturated groups by condensation reaction with a methacrylate containing a reactive terminal group. These methacrylates can be represented by the formula: T—R—C=CH2 CH3 where T is a terminal group, for example: HO—

H 2 N—

OCN—

CH2 — CH —

The condensation reaction can be represented thus: — COOH + H O — R — C = C H 2

•

—CO—O—R—C=CH2

CH3

CH3

Modified PAAs can be represented by the generic formula: —CH 2 —CH—CH 2 —CH—CH 2 —CH — CH 2 —CH—CH CO

CO

CO

CO

OH

OH

O

OH

R—C=CH2 CH3 The liquids used for class II hybrids contain 2 5 ^ 5 % modified PAA and 21-41 % HEM A. The initiator system for light activation contains 172

Resin glass polyalkenoate cements camphorquinone and diphenyliodonium chloride (Mitra, 1989). The glass powder has the following percentage composition: SiO2 27-24; A12O3 0-81; P2O50-95; NH4F3-37; A1F3 20-97; Na3AlF6 10-81; ZnO 20-97; MgO215; SrO 12-74. Formation of matrices When a resin glass polyalkenoate cement, containing a modified PAA and HEMA, is mixed, a paste is formed which sets only slowly in the absence of light. When activated by light the paste sets in 30 s. Several types of polymerization can then take place. Both HEMA and the modified PAA, because it contains unsaturated groups, will polymerize. PolyHEMA and a crosslinked PAA of high molecular weight will be formed. In addition, the modified PAA may copolymerize with HEMA; thus, polyHEMA is chemically linked to the polyacrylate matrix and so cannot phase-separate. The matrix of such a cement contains both ionic and covalent crosslinks (Figure 5.24). Thus, the cement matrix is reminiscent of an ion-exchange resin. 5.10.4 Properties These resin-modified glass polyalkenoate cements have both advantages and disadvantages over conventional glass polyalkenoate cements. However, because of their poor translucency they are recommended only as liners or bases. They have improved setting characteristics. They have a long working time because HEMA slows the acid-base reaction, yet set sharply once the polymerization reaction is initiated by light. They are also resistant to early contamination by water because of the formation of an organic matrix, and so do not require protection by varnish. This combination of properties is bound to appeal to the clinician. The freshly set class II (Vitrabond) resin glass polyalkenoate cement appears to have rubbery characteristics and there is some debate as to

CH2 CH-COO" M CH 2

I

CH 2 2+

"OOC-CH CH 2

II

CH 2

CH2

CH-COO-R-CH-CH2-CH2-CH-R-OOC-CH CH 2

CH 3

CH 3

CH 2

I

Figure 5.24 The matrix of a class II resin-based glass polyalkenoate cement, showing ionic and covalent crosslinks.

173

Polyalkenoate cements Table 5.18. Strength of resin glass polyalkenoate cements (Antonucci, McKinney & Stansbury, 1988; Wilson & McLean, 1988; Mathis & Ferracane, 1989; Mitra, 1989; Minnesota Mining & Manufacturing Company, 1989; Alters, 1990) Resin Wet strength, MPa (24 hours) Compressive Flexural Tensile Adhesion (dentine)

Class I 94-139

— 16-4^33-9

1-6

Conventional Class II

Liner/base

Filling

53-96 25-5 11-2-17-4 9-8-11-3

56-79 5-2-10-3 3-4-9-1 3-4^3-9

140-195 8-9-30-3 9-0-19-3 1-7-6-8

whether this is advantageous or not. The reason for this rubberiness is that the polymer is only lightly crosslinked, and at set the acid-base reaction has not proceeded very far. Most probably, these rubbery characteristics will disappear as the cement ages and the acid-base reaction is completed. But this may take a very long time. Some data have been published on the mechanical properties of these cements (Antonucci, McKinney & Stansbury, 1988; Mathis & Ferracane, 1989; Mitra, 1989; Minnesota Mining & Manufacturing Company, 1989; Albers, 1990), but much of it comes from patents and company reports so it would be unwise to draw firm conclusions from thesefiguresalone. Although both class I and II resin hybrids are stronger than conventional liners and bases, class II materials are not as strong as conventional filling materials (Table 5.18). According to Mathis & Ferracane (1989) a class I material developed 82 % of its 24-hour ultimate compressive strength in one hour, which compares favourably with a figure of about 52 % for a conventional glass polyalkenoate cement. Rapid development of strength is to be expected because of the polymerization process. Both class I and class II resin glass polyalkenoate cements are claimed to bond to dentine. This can be accepted. But that the bond is stronger and develops more rapidly than that of the conventional glass polyalkenoate cement, as is claimed for class II materials (Minnesota Mining & Manufacturing Company, 1989) requires confirmation. It may be, because of the slowness of the acid-base reaction in resin glass polyalkenoate cements, that free poly(acrylic acid) is available for a longer period than in conventional glass polyalkenoate cements, for the 174

References formation of a stronger adhesive bond. However, it seems doubtful that the adhesive bond will be developed more rapidly than in the conventional glass polyalkenoate for, according to van Zeghbroeck (1989), the adhesive bond of a conventional glass polyalkenoate cement develops its maximum strength rapidly (within five minutes). The resin glass polyalkenoate cement has the undoubted advantage of bonding directly to composite resins and this makes it ideal for use in the glass polyalkenoate cement/composite resin laminates. There is bound to be one problem with resin glass polyalkenoate cement. Because the matrix is a mixture of hydrogel salt and polymer, lightscattering is bound to be greater than in the conventional material. Moreover, the zinc oxide-containing glass of class II materials is bound to be opaque. This makes it difficult to formulate a translucent material and is the reason why their use is restricted to that of a liner or base. However, the class II material cited will be radio-opaque because it uses strontium and zinc, rather than calcium, in the glass. A fundamental criticism of the resin-modified glass polyalkenoate cements is that, to some extent, they go against the philosophy of the glass polyalkenoate cement: namely, that the freshly mixed material should contain no monomer. Monomers are toxic, and HEMA is no exception. This disadvantage of composite resins is avoided in the glass polyalkenoate cement as the polyacid is pre-polymerized during manufacture, but the same cannot be said of these new materials. For this reason they may lack the biocompatibility of conventional glass polyalkenoate cements. These materials also absorb excessive amounts of water because of the hydrophilic nature of polyHEMA (Nicholson, Anstice & McLean, 1992). It is far too soon to make a judgement on these materials, which are of considerable interest and only in the very early days of their development. Most probably, much development will take place in this area. References Abramovich, A., Kaluza, J. J., Macchi, R. L. & Ribas, L. (1977). Enamel surface treated with zinc polyacrylate dentine cements. Journal of Dental Research, 56, 471-3. Akahane, S., Tosaki, S. & Hirota, K. (1988). Fluoroaluminosilicate glass powder for dental ionomeric cements. German Patent DE 3,804,469. Akitt, J. W., Greenwood, N. N. & Lester, G. D. (1971). Nuclear magnetic resonance and Raman studies of aluminium complexes formed in aqueous solutions of aluminium salts containing phosphoric acids andfluorideions. Journal of the Chemical Society, A, 2450-7. 175

Polyalkenoate cements Albers, H. F. (ed.) (1990). Light-cured fluoride releasing liners. The Adept Report. 1990; 1, No. 1, l^k Andersson, K. R., Dent Glasser, L. S. & Smith, D. (1982). Polymerization and colloid formation in silicate solutions. In Falcone, J. G. (ed.) Soluble Silicates. ACS Symposium Series No. 194, Chapter 8. Washington, DC: American Chemical Society. Antonucci, J. M., McKinney, J. E. & Stansbury, J. W. (1988). Resin-modified glass-ionomer cement. US Patent Application 160,856. Anzai, M., Hirose, H., Kikuchi, H., Goto, J., Azuma, F. & Higasaki, S. (1977). Studies on soluble elements and solubility of dental cements. I. Solubilities of zinc phosphate cement, carboxylate cement and silicate cement in distilled water. Journal of the Nihon University School of Dentistry, 19, 26-39. Aveston, J. (1965). The hydrolysis of the aluminium ion: ultracentrifugal and acidity measurements. Journal of the Chemical Society, 4438-43. Bailey, J. E., Ellis, B., Howarth, L. G. & Oldfield, C. W. B. (1991). The fracture of glass-ionomer cements. Conference on the Fractography of Glasses and Ceramics II, New York State College of Ceramics, Alfred University, New York, July 1990. American Ceramic Society. Barnes, D. S. & Turner, E. P. (1971). Initial response of the human pulp to zinc polycarboxylate cement. Journal of the Canadian Dental Association, 37,265-6. Barry, T. I., Clinton, D. J. & Wilson, A. D. (1979). The structure of a glass-ionomer cement and its relationship to the setting process. Journal of Dental Research, 58, 1072-9. Barton, J. R., Brauer, G. M., Antonucci, J. M. & Raney, M. J. (1975). Reinforced polycarboxylate cements. Journal of Dental Research, 54, 310-23. Baumann, E. & Gerhard, V. (1970). Vormischung, Verfahren zu ihrer Herstellung und Verwendung. German Patent 1,903,807. Beagrie, G. S., Main, J. H. P. & Smith, D. C. (1972). Inflammatory reaction evoked by zinc polyacrylate and zinc eugenate cements: a comparison. British Dental Journal, 132, 351-7. Beagrie, G. S., Main, J. H. P., Smith, D. C. & Walshaw, P. R. (1974). Polycarboxylate cements as a pulp capping agent. Journal of the Canadian Dental Association, 40, 378-83. Beech, D. R. (1972). A spectroscopic study of the interaction between human tooth enamel and polyacrylic acid (polycarboxylate cement). Archives of Oral Biology, 17, 907-11. Beech, D. R. (1973). Improvement in the adhesion of polycarboxylate cements to human dentine. British Dental Journal, 135, 442-5. Beech, D. R. & Bandyopadhyay, S. (1983). A new laboratory method for evaluating the relative solubility and erosion of dental cements. Journal of Oral Rehabilitation, 10, 57-63. Beech, D. R., Solomon, A. & Bernier, R. (1985). Bond strength of polycarboxylic acid cements to treated dentine. Dental Materials, 1, 154-7. Bergenholtz, G., Cox, C. F., Loesche, W. J. & Syed, S. A. (1982). Bacterial leakage around dental restorations: its effect on dental pulp. Journal of Oral Pathology, 11, 439-50. 176

References Bertenshaw, B. W. & Combe, E. C. (1972a). Studies on polycarboxylates and related cements. I. Analysis of cement liquids. Journal of Dental Research, 1, 13-16. Bertenshaw, B. W. & Combe, E. C. (1972b). Studies on polycarboxylates and related cements. II. Analysis of cement powders. Journal of Dental Research, 1, 65-8. Bertenshaw, B. W. & Combe, E. C. (1976). Studies on polycarboxylates and related cements. III. Molecular weight determinations. Journal of Dental Research, 4, 87-90. Bertenshaw, B. W., Combe, E. C. & Grant, A. A. (1979). Studies on polycarboxylates and related cements. 4. Properties of cements. Journal of Dentistry,!, 117-25. Bertenshaw, B. W., Combe, E. C, Tidy, P. C. & Laycock, J. N. C. (1979). Surgical cement composition containing aluminoborate glass and a polymer glass and a polymer containing recurring carboxylic or carboxylate groups. US Patent 4,174,334. Bitner, T. J. & Weir, S. H. (1973). Fluoride uptake and acid solubility of enamel exposed to carboxylate cement containing MFP. Journal of Dental Research, 52, 157-62. Bitter, N. C. (1986). Glass ionomer-microfil technique for restoring cervical lesions. Journal of Prosthetic Dentistry, 56, 661-2. Bovis, S. C, Harrington, E. & Wilson, H. J. (1971). Setting characteristics of composite filling materials. British Dental Journal, 131, 352-6. Brannstrdm, M. & Nyborg, H. (1969). Comparison of pulpal effect of two liners. Ada Odontologica Scandinavica, 27, 433-51. Brook, I. M., Craig, G. T. & Lamb, D. J. (1991a). In vitro interaction between primary bone organ cultures, glass-ionomer cements and hydroxyapatite/tricalcium phosphate ceramics. Biomaterials, 12, 179-86. Brook, I. M., Craig, G. T. & Lamb, D. J. (1991b). Initial in-vitro evaluation of glass-ionomer cements for use as alveolar bone substitutes. Clinical Materials, 7, 295-300. Brown, D. & Combe, E. C. (1973). Effects of stainless steelfilleron the properties of polycarboxylate cement. Journal of Dental Research, 52, 388. Browne, R. M., Tobias, R. S., Crombie, I. K. & Plant, G. C. (1983). Bacterial microleakage and pulpal inflammation in experimental cavities. International Endodontics Journal, 16, 147-55. Brune, D. & Smith, D. (1982). Microstructure and strength properties of silicate and glass-ionomer cements. Acta Odontologica Scandinavica, 40, 389-96. Buonocore, M. G. (1955). A simple method of increasing the adhesion of acrylic materials to enamel surfaces. Journal of Dental Research, 34, 849-53. Buonocore, M. G. (1961). Test of an adhesive containing glycerophosphoric acid dimethacrylate. In Phillips, R. W. & Ryge, G. (eds.) Adhesive Restorative Dental Materials, pp. 172-6. Spencer, Indiana: Owen Litho Service. Cahn, J. W. (1961). On spinodal decomposition. Acta Metallurgica, 9, 795-801. Causton, B. E. (1981). The physio-mechanical consequences of exposing glass ionomer cements to water during setting. Biomaterials, 2, 112-15. 177

Polyalkenoate cements Causton, B. E. (1982). Primers and mineralizing solutions. In Smith, D. C. & Williams, D. F. (eds.) Biocompatibility of Dental Materials. Volume II. Biocompatibility of Preventive Dental Materials and Bonding Agents, Chapter 7. Boca Raton, Florida: CRC Press Inc. Causton, B. E. & Johnson, N. W. (1982). Improvement of polycarboxylate adhesion to dentine by the use of a new calcifying solution. British Dental Journal, 152, 9-11. Chamberlain, B. B. & Powers, J. N. (1976). Physical and mechanical properties of three zinc polycarboxylate cements. Journal of the Michigan Dental Association, 58, 494-500. Connick, R. E. & Poulsen, R. E. (1957). Nuclear magnetic resonance studies of aluminium fluoride complexes. Journal of the American Chemical Society, 79, 5153-7. Cook, W. D. (1982). Dental polyelectrolyte cements. I. Chemistry of the early stages of the setting reaction. Biomaterials, 3, 232-6. Cook, W. D. (1983a). Dental polyelectrolyte cements. II. Effect of powder/liquid ratio on their rheology. Biomaterials, 4, 21-4. Cook, W. D. (1983b). Dental polyelectrolyte cements. III. Effect of additives on their rheology. Biomaterials, 4, 85-8. Cook, W. D. (1983c). Degradative analysis of glass-ionomer polyelectrolyte cements. Journal of Biomedical Materials Research, 17, 1015-27. Cornell, J. (1961). Adhesion to natural tooth. In Phillips, R. W. & Ryge, G. (eds.) Adhesive Restorative Dental Materials, p. 159. Spencer, Indiana: Owen Litho Service. Cotton, F. A. & Wilkinson, G. (1966). Advanced Inorganic Chemistry. New York: Wiley Interscience. Cranfield, M., Kuhn, A. T. & Winter, G. (1982). Factors relating to the rate of fluoride-ion release from glass-ionomer cement. Journal of Dentistry, 10, 333^1. Crisp, S., Abel, G. & Wilson, A. D. (1979). The quantitative measurement of the opacity of aesthetic dental materials. Journal of Dental Research, 58, 1585-96. Crisp, S., Ferner, A. J., Lewis, B. G. & Wilson, A. D. (1975). Properties of improved glass-ionomer cement formulations. Journal of Dentistry, 3, 125-30. Crisp, S., Jennings, M. A. & Wilson, A. D. (1978). A study of temperature changes occurring in setting dental cements. Journal of Oral Rehabilitation, 5, 139^4. Crisp, S., Kent, B. E., Lewis, B. G., Ferner, A. J. & Wilson, A. D. (1980). Glass-ionomer cement formulations. II. The synthesis of novel polycarboxylie acids. Journal of Dental Research, 59, 1055-63. Crisp, S., Lewis, B. G. & Wilson, A. D. (1975). Gelation of polyacrylic acid aqueous solutions and the measurement of viscosity. Journal of Dental Research, 54, 1173-5. Crisp, S., Lewis, B. G. & Wilson, A. D. (1976a). Zinc polycarboxylate cements. A chemical study of erosion and its relationship to molecular structure. Journal of Dental Research, 55, 299-308. 178

References Crisp, S., Lewis, B. G. & Wilson, A. D. (1976b). Characterization of glass-ionomer cements. 1. Long term hardness and compressive strength. Journal of Dentistry, 4, 162-6. Crisp, S., Lewis, B. G. & Wilson, A. D. (1976c). Characterization of glass-ionomer cements. 2. Effect of powder: liquid ratio on the physical properties. Journal of Dentistry, 4, 287-90. Crisp, S., Lewis, B. G. & Wilson, A. D. (1976d). Glass-ionomer cements: chemistry of erosion. Journal of Dental Research, 55, 1032-41. Crisp, S., Lewis, B. G. & Wilson, A. D. (1977). Characterization of glass-ionomer cements. 3. Effect of poly acid concentration on the physical properties. Journal of Dentistry, 5, 51-6. Crisp, S., Lewis, B. G. & Wilson, A. D. (1979). Characterization of glass-ionomer cements. 5. Effect of tartaric acid concentration in the liquid component. Journal of Dentistry, 7, 304-5. Crisp, S., Lewis, B. G. & Wilson, A. D. (1980). Characterization of glass-ionomer cements. 6. A study of erosion and water absorption in both neutral and acidic media. Journal of Dentistry, 8, 68-74. Crisp, S., Merson, S. A. & Wilson, A. D. (1980). Modification of ionomer cements by the addition of simple metal salts. Industrial & Engineering Chemistry, Product Research & Development, 19, 403-8. Crisp, S., Merson, S., Wilson, A. D., Elliott, J. H. & Hornsby, P. R. (1979). The formation and properties of mineral—poly acid cements. Part 1. Ortho- and pyro-silicates. Journal of Materials Science, 14, 2941-58. Crisp, S., Pringuer, M. A., Wardleworth, D. & Wilson, A. D. (1974). Reactions in glass—ionomer cements. II. An infrared spectroscopic study. Journal of Dental Research, 53, 1414-19. Crisp, S., Prosser, H. J. & Wilson, A. D. (1976). An infra-red spectroscopic study of cement formation between metal oxides and aqueous solutions of poly(acrylic acid). Journal of Materials Science, 11, 36-48. Crisp, S. & Wilson, A. D. (1974a). Reactions in glass ionomer cements. I. Decomposition of the powder. Journal of Dental Research, 53, 1408-14. Crisp, S. & Wilson, A. D. (1974b). Reactions in glass ionomer cements. III. The precipitation reaction. Journal of Dental Research, 53, 1420-4. Crisp, S. & Wilson, A. D. (1974c). Poly(carboxylate) cements. British Patent 1,484,454. Crisp, S. & Wilson, A. D. (1976). Reactions in glass ionomer cements. V. Effect of incorporating tartaric acid in the cement liquid. Journal of Dental Research, 55, 1023-31. Crisp, S. & Wilson, A. D. (1977). Cements comprising acrylic and itaconic acid copolymers and fluoroaluminosilicate glass powder. US Patent 4,016,124. Crisp, S. & Wilson, A. D. (1978a). Poly(carboxylate) cements. British Patent 1,532,954. Crisp, S. & Wilson, A. D. (1978b). Fluoroaluminosilicate glasses. British Patent 1,532,955. Crisp, S. & Wilson, A. D. (1979). Cements. US Patent 4,143,018. Crisp, S., Wilson, A. D., Elliott, J. H. & Hornsby, P. R. (1977). Cement-forming 179

Polyalkenoate cements ability of silicate minerals and polyacid solution. Journal of Applied Chemistry, 27, 369-74. Cruikshank, M. C , Dent Glasser, L. S., Barri, S. A. I. & Pollett, J. F. (1986). Penta-co-ordinated aluminium: a solid-state 27A1 N.M.R. study. Journal of the Chemical Society, Chemical Communications, 23—40. Eames, W. B., Hendrix, K. & Mohler, H. C. (1979). Pulpal responses in rhesus monkeys to cementation agents and cleaners. Journal of the American Dental Association, 98, 40-5. Edwards, S. F. (1969). Theory of crosslinked polymerized material. Proceedings of the Physical Society of London. Solid State Physics, 2 (2), 1-13. Eick, J. D., Johnson, L. N., Fromer, J. R., Good, R. J. & Neuman, A. W. (1972). Surface topography: its influence on wetting and adhesion in a dental adhesive system. Journal of Dental Research, 51, 780-8. El-Kafrawy, A. H., Dickey, D. M., Mitchell, D. F. & Phillips, R. W. (1974). Pulp reaction to a polycarboxylate cement in monkeys. Journal of Dental Research, S3, 15-19. Elliott, J., Holliday, L. & Hornsby, P. R. (1975). Physical and mechanical properties of glass-ionomer cement. British Polymer Journal, 7, 297-306. Ellis, J., Jackson, A. M., Scott, R. P. & Wilson, A. D. (1990). Adhesion of carboxylate cements to hydroxyapatite. III. Adsorption of poly(alkenoic acid)s. Biomaterials, 11, 379-84. Ellis, J. & Wilson, A. D. (1987). Report. Laboratory of the Government Chemist. Ellis, J. & Wilson, A. D. (1990). Polyphosphonate cements: a new class of dental materials. Journal of Materials Science Letters, 9, 1058-60. Ellison, S. & Warrens, C. (1987). Solid-state nmr study of aluminosilicate glasses and derived dental cements. Report of the Laboratory of the Government Chemist. ESPE. (1975). Mixing liquid for dental filling materials. British Patent 1,382,882. Feith, R. (1975). Side effect of acrylic cement implanted into bone. Acta Orthopaedica Scandinavica Supplementum, 161, 1-36. Fields, J. E. & Nielsen, L. E. (1968). Dynamic mechanical properties of some polymeric zinc salts. Journal of Applied Building Science, Yl, 1041-51. Flood, H. & Forland, T. (1947a). The acidic and basic properties of oxides. Acta Chemica Scandinavica, 1, 592-604. Flood, H. & Forland, T. (1947b). The acidic and basic properties of oxides. II. The thermal decomposition of pyrosulphates. Acta Chemica Scandinavica, 1, 781-9. Flory, P. J. (1953). Principles of Polymer Chemistry, Chapter 11. Ithaca, New York: Cornell University Press. Flory, P. J. (1974). Introductory lecture. In Gels and Gelling Processes. Faraday Discussions of the Chemical Society, No. 57, pp. 7-18. Forsten, L. (1977). Fluoride release from a glass ionomer cement. Scandinavian Journal of Dental Research, 85, 503-4. Foster, J. F. & Dovey, E. H. (1974). Surgical cements of improved compressive strength containing stannous fluoride and poly aery lie acid. US Patent 3,856,737. 180

References Foster, J. F. & Dovey, E. H. (1976). Improvements in and relating to surgical cements. British Patent 1,423,133. Frost, H. M. (1981). Coherence treatment of osteoporosis. Orthopedic Clinics of North America, 12, 649-69. de Gennes, P. (1979). Scaling Concepts in Polymer Physics. Ithaca: Cornell University Press. Glanz, P-O. (1969). On wettability and adhesiveness. Odontologisk Revy, 20, Supplement 20. Goldman, M. (1985). Fracture properties of composite and glass ionomer dental restorative materials. Journal of Biomedical Materials Research, 19, 771-83. Gourley, J. M. & Rose, D. E. (1972). Cavity bases under liners. Journal of the Canadian Dental Association, 38, 246. Granath, L. E. (1982). Pulp capping materials. In Smith, D. C. & Williams, D. F. (eds.) Biocompatibility of Dental Materials. Volume II. Biocompatibility of Preventive Dental Materials and Bonding Agents, Chapter 11. Boca Raton: CRC Press Inc. Gregor, H. P., Luttinger, L. B. & Loebl, E. M. (1955a). Metal polyelectrolyte complexes. I. The polyacrylic acid-copper complex. Journal of Physical Chemistry, 59, 34^9. Gregor, H. P., Luttinger, L. B. & Loebl, E. M. (1955b). Metal polyelectrolyte complexes. IV. Complexes of polyacrylic acid with magnesium, calcium, manganese, cobalt and zinc. Journal of Physical Chemistry, 59, 990-1. Gulabivala, K., Setchell, D. J. & Davies, E. H. (1987). An application of the jet test method for the evaluation of disintegration of dental luting cements in marginal gaps analogous to those of crowns and bridges. Clinical Materials, 2, 221-30. Hagg, G. (1935). The vitreous state. Journal of Chemical Physics, 3, 42-9. Hall, J. I. (1977). Orthopedic bandage. US Patent 4,044,761. Hamilton, I. R. (1977). The effects of fluoride on enzymatic regulation of bacterial carbohydrate metabolism. Caries Research, 11 (Supplement 1), 321-7. Hatton, P. V. & Brook, I. M. (1992). Characterization of the ultrastructure of glass-ionomer (glass polyalkenoate) cements. British Dental Journal, 173, 275-7. Hazel, F. M., Shock, R. U. & Gordon, M. (1949). Interactions of ferric ions with silicic acid. Journal of the American Chemical Society, 71, 2256-7. Helgeland, K. (1982). In vitro testing of dental cements. In Smith, D. C. & Williams, D. F. (eds.) Biocompatibility of Dental Materials. Volume II. Biocompatibility of Preventive Dental Materials and Bonding Agents, Chapter 9. Boca Raton, Florida: CRC Press Inc. Hembree, J. H. & Andrews, J. T. (1978). Microleakage of several class V anterior restorative materials: a laboratory study. Journal of the American Dental Association, 91, 179-83. Hertet, R. S., Huget, E. F., de Simon, L. & Cosgrave, J. H. (1975). Short-term stress relaxation behaviour of non-metallic restoratives. Journal of Dental Research, 54, 1149-53.

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References cementum fluoride uptake from a glass ionomer cement. Caries Research, 18, 250-7. Richter, W. A. & Ueno, H. (1975). Clinical evaluation and dental clinical durability. Journal of Prosthetic Dentistry, 33, 294-9. Risbud, S. H., Kirkpatrick, R. J., Taglialavore, A. P. & Montez, B. (1987). Solid-state NMR evidence of 4-, 5- and 6-fold aluminium sites in rollerquenched SiO2-Al2 O3 glasses. Journal of the American Ceramic Society, 70, C10-12. Robbins, J. W. & Cooley, R. L. (1988). Microleakage of Ketac-Silver in the tunnel preparation. Operative Dentistry, 13, 8-11. Rolla, G. (1977). Effects of fluoride on initiation of plaque formation. Caries Research, 11, 243-61. Saito, C, Sakai, Y., Node, H. & Fusayama, T. (1976). Adhesion of polycarboxylate cements to dental casting alloys. Journal of Prosthetic Dentistry, 35, 543-8. Seed, I. & Wilson, A. D. (1980). Poly(carboxylic acid) hardenable compositions. British Patent Application GB 2,028,855A. Schmidt, W., Purrmann, R., Jochum, P. & Glasser, O. (1981a). Calcium aluminosilicate glass powder and its use. European Patent Application 23,013. Schmidt, W., Purrmann, R., Jochum, P. & Glasser, O. (1981b). Mixing compounds for glass-ionomer cements and use of a copolymer for preparing the mixing components. European Patent Application 24,056. Scott, R. P., Jackson, A. M. & Wilson, A. D. (1990). Adhesion of carboxylate cements to hydroxyapatite. II. Adsorption of aromatic carboxylates. Biomaterials, 11, 341-4. Setchell, D. J., Teo, C. K. & Kuhn, A. T. (1985). The relative solubilities of four modern glass-ionomer cements. British Dental Journal, 158, 220-2. Shimoke, H., Komatsu, H. & Matsui, I. (1987). Fluoride content in human enamel after removal of the applied glass ionomer cement. Journal of Dental Research, 66, Special Issue 131, Abstract No. 196. Sidler, P. & Strub, J. R. (1983). In-vivo Untersuchung der Loslichkeit und des Abdichtungsvermogens von drei Befestigungszementen. Deutsche Zahndrztliche Zeitschrift, 38, 564-71. Silverstone, L. M. (1982). The structure and characteristics of human dental enamel. In Smith, D. C. & Williams, D. F. (eds.) Biocompatibility of Dental Materials. Volume I. Characteristics of Dental Tissues and their Response to Dental Materials, Chapter 2. Boca Raton: CRC Press Inc. Simmonds, J. J. (1983). The miracle mixture glass-ionomer and alloy powder. Texas Dentist, October 6-12. Skinner, J. C, Prosser, H. J., Scott, R. P. & Wilson, A. D. (1986). Adhesion of carboxylate cements to hydroxy apatite. I. The effect of the structure of aliphatic carboxylates on their uptake by hydroxy apatite. Biomaterials, 7, 438^0. Smink, L. A. & Arends, J. (1980). Oplosbaarheid en disintegratie van Tandhelkundige cementen in vitro. Nederlands Tijdschrift voor Tandheelkunde, 87, 389-93. 191

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Polyalkenoate cements Wasson, E. A. & Nicholson, J. W. (1990). A study of the relationship between setting chemistry and properties of modified glass-poly(alkenoate) cements. British Polymer Journal 23, 179-83. Wasson, E. A. & Nicholson, J. W. (1991). Studies on the setting chemistry of glass-ionomer cements. Clinical Materials, 7, 289-93. Waters, D. N. & Henty, M. S. (1977). Raman spectra of aqueous solutions of hydrolysed aluminium(III) salts. Journal of the Chemical Society: Dalton Transactions, 243-5. Watts, D. C , Combe, E. C. & Greener, E. H. (1979). Effect of storage conditions on the mechanical properties of polyelectrolyte cements. Journal of Dental Research, 58, Special Issue C, Abstract No. 18. Wei, S. H. Y. (1985). Clinical Uses of Fluoride. Philadelphia: Lea & Febiger. Welker, D. & Neupert, G. (1974). Vergleichender biologischer Test von Polyakrylate- und Phosphatzement an Monolayer-Kulturen. Stomatologie (DDR), 24, 602-10. Welsh, E. L. & Hembree, J. H. (1985). Microleakage of the gingival wall with four class V anterior restorative materials. Journal of Prosthetic Dentistry, 54, 370-2. Wesenberg, G. & Hals, E. (1980). The in vitro effect of a glass ionomer cement on dentine and enamel wall. Journal of Oral Rehabilitation, 7, 35-42. Weyl, W. A. & Marboe, E. C. (1962). The Constitution of Glasses: a Dynamic Interpretation. Volume 1. Fundamentals of the Structure of Inorganic Liquids and Solids. New York: Interscience Publishers. Williams, J. & Billington, R. W. (1989). Increase in compressive strength of glass-ionomer cements with respect to time: a guide to their use in the posterior deciduous dentition. Journal of Oral Rehabilitation, 16, 475-9. Williams, J. & Billington, R. W. (1991). Changes in compressive strength of glass ionomer restorative materials with respect to time periods of 24 h to 4 months. Journal of Oral Rehabilitation, 18, 163-8. Williams, J. A., Billington, R. W. & Pearson, G. J. (1992). The comparative strengths of commercial glass-ionomer cements with and without metal additions. British Dental Journal, 111, 279-82. Wilson, A. D. (1968). Dental silicate cements. VII. Alternative liquid acid formers. Journal of Dental Research, 47, 1133-6. Wilson, A. D. (1974). Alumino-silicate poly aerylie acid and related cements. British Polymer Journal, 6, 165-79. Wilson, A. D. (1975a). Dental cements - general. In von Fraunhofer, J. A. (ed.) Scientific Aspects of Dental Materials, Chapter 4. London and Boston: Butterworths. Wilson, A. D. (1975b). Zinc oxide dental cements. In von Fraunhofer, J. A. (ed.) Scientific Aspects of Dental Materials, Chapter 5. London and Boston: Butterworths. Wilson, A. D. (1975c). Dental cements based on ion-leachable glasses. In von Fraunhofer, J. A. (ed.) Scientific Aspects of Dental Materials, Chapter 6. London and Boston: Butterworths. 194

References Wilson, A. D. (1978a). The chemistry of dental cements. Chemical Society Reviews, 7, 265-96. Wilson, A. D. (1978b). Glass-ionomer cements - ceramic polymers. In Young, J. F. (ed.) Chemical Research Progress. Columbus, Ohio: American Ceramic Society. Wilson, A. D. (1982). The nature of the zinc polycarboxylate cement matrix. Journal of Biomedical Materials Research, 16, 549-57. Wilson, A. D. (1989). Developments in glass-ionomer cements. International Journal of Prosthodontics, 2, 438-46. Wilson, A. D. (1990). Resin modified glass-ionomer cements. International Journal of Prosthodontics, 3, 425—46. Wilson, A. D. (1991). Glass-ionomer cement - origins, development and future. Clinical Materials, 7, 275-82. Wilson, A. D. & Crisp, S. (1975). Ionomer cements. British Polymer Journal, 1, 279-96. Wilson, A. D. & Crisp, S. (1976). Poly(carboxylate) cements. British Patent 1,422,337. Wilson, A. D. & Crisp, S. (1977). Polymer-clay compounds and soil treatment. In Organolithic Macromolecular Materials, Chapter 5. London: Applied Science Publishers. Wilson, A. D. & Crisp, S. (1980). Dental cement containing poly(carboxylic acid), chelating agent and glass ionomer powder. US Patent 4,209,434. Wilson, A. D., Crisp, S. & Abel, G. (1977). Characterization of glass-ionomer cements. 4. Effect of molecular weight on physical properties. Journal of Dentistry, 5, 117-20. Wilson, A. D., Crisp, S. & Ferner, A. J. (1976). Reactions in glass ionomer cements: IV. Effect of chelating comonomers on setting behavior. Journal of Dental Research, 55, 489-95. Wilson, A. D., Crisp, S., Lewis, B. G. & McLean, J. W. (1977). Experimental luting agents based on the glass-ionomer cements. British Dental Journal, 142, 117-22. Wilson, A. D., Crisp, S. & Paddon, J. M. (1981). The hydration of a glass-ionomer (ASPA) cement. British Polymer Journal, 13, 66-70. Wilson, A. D., Crisp, S., Prosser, H. J., Lewis, B. G. & Merson, S. A. (1980). Aluminosilicate glasses for polyelectrolyte cements. Industrial & Engineering Chemistry Product Research & Development, 19, 263-70. Wilson, A. D. & Ellis, J. (1989). Poly-vinylphosphonic acid glass ionomer cement. British Patent Application 8,924,129.3. Wilson, A. D., Groffman, D. M. & Kuhn, A. T. (1985). The release of fluoride and other chemical species from a glass-ionomer cement. Biomaterials, 6, 431-3. Wilson, A. D., Groffman, D. M., Powis, D. R. & Scott, R. P. (1986a). An evaluation of the significance of the impinging jet method for measuring the acid erosion of dental cements. Biomaterials, 7, 55-60. Wilson, A. D., Groffman, D. M., Powis, D. R. & Scott, R. P. (1986b). A study of variables affecting the impinging jet method for measuring the erosion of dental cements. Biomaterials, 7, 217-20. 195

Polyalkenoate cements Wilson, A. D., Hill, R. G., Warrens, C. P. & Lewis, B. G. (1989). The influence of poly(acrylic acid) molecular weight on some properties of glass-ionomer cement. Journal of Dental Research, 68, 89-94. Wilson, A. D. & Kent, B. E. (1970). Dental silicate cements. IX. Decomposition of the powder. Journal of Dental Research, 49, 7-13. Wilson, A. D. & Kent, B. E. (1971). The glass-ionomer cement, a new translucent cement for dentistry. Journal of Applied Chemistry and Biotechnology, 21, 313. Wilson, A. D. & Kent, B. E. (1972). A new translucent cement for dentistry. British Dental Journal, 132, 133-5. Wilson, A. D. & Kent, B. E. (1973). Surgical cements. British Patent 1,316,129. Wilson, A. D. & Kent, B. E. (1974). Poly(carboxylic acid)-fluoroaluminosilicate glasses surgical cement. US Patent 3,814,717. Wilson, A. D. & Lewis, B. G. (1980). The flow properties of dental cements. Journal of Biomedical Materials Research, 14, 383-91. Wilson, A. D. & McLean, J. W. (1988). Glass-ionomer Cement. Chicago, London, etc.: Quintessence Publishing Company Inc. Wilson, A. D., Paddon, J. M. & Crisp, S. (1979). The hydration of dental cements. Journal of Dental Research, 58, 1065-71. Wilson, A. D. & Prosser, H. J. (1982). Biocompatibility of the glass ionomer cement. Journal of the Dental Association of South Africa, 37, 872-9. Wilson, A. D. & Prosser, H. J. (1984). A survey of inorganic and polyelectrolyte cements. British Dental Journal, 157, 449-54. Wilson, A. D., Prosser, H. J. & Powis, D. M. (1983). Mechanism of adhesion of polyelectrolyte cement to hydroxyapatite. Journal of Dental Research, 62, 590-2. Wilson, M. A. & Combe, E. C. (1991). Effects of glass composition and pretreatment on the reactivity of a novel glass polyalkenoate (glass ionomer) dental cement. Clinical Materials, 7, 15-21. Wood, D. & Hill, R. (1991a). Structure-property relationships in ionomer glasses. Clinical Materials, 1, 301-12. Wood, D. & Hill, R. (1991b). Glass ceramic approach to controlling the properties of a glass-ionomer cement. Biomaterials, 12, 164-70. Woodberry, N. T. (1961). A new, anionic, polyacrylamide flocculant. Tappi, 44, 156A-60A. Wygant, J. F. (1958). Cementitious bonding in ceramic fabrication. In Kingery, W. D. (ed.) Ceramic Fabrication Processes, pp. 171-88. New York: John Wiley & Sons. Yoshii, E., Homma, T., Hirota, K. & Tomioka, K. (1987). Cytotoxic evaluation of the improved glass-ionomer cement. Journal of Dental Research, 66, Special Issue 133, Abstract No. 215. Zachariasen, W. H. (1932). The atomic arrangement in glass. Journal of the American Chemical Society, 54, 3841-51. van Zeghbroeck, L. (1989). Bond capacity of adhesive luting cements. Thesis for Doctor in de Tandheelkunde, Leuvan University, Belgium.

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6

Phosphate bonded cements

6.1

General

The phosphate bonded cements described in this chapter are the products of the simple acid-base reaction between an aqueous solution of orthophosphoric acid and a basic oxide or silicate. Such reactions take place at room temperature. Excluded from this chapter are the cementitious substances that are formed by the heat treatment of aqueous solutions of acid metal phosphates. The most important of these are the refractory cements formed by the heat treatment of aluminium acid phosphate solutions. This subject has been well reviewed by Kingery (1950a), Morris et al. (1977), Cassidy (1977) and O'Hara, Duga & Sheets (1972). The chemistry of these binders is extremely complex as the action of heat on acid phosphates gives rise to polymeric phosphates, with P-O-P linkages, and these are very complex systems (Ray, 1979). Here we are concerned with the cement-forming reaction between orthophosphoric acid solutions and basic oxides and silicates where the reaction is much simpler. Polymeric phosphates are not involved, there are no P-O-P bonds, and the structural unit is the simple [POJ tetrahedron. 6.1.1

Orthophosphoric acid solutions

Concentrated solutions of orthophosphoric acid, often containing metal salts, are used to form cements with metal oxides and aluminosilicate glasses. Orthophosphoric acid, often referred to simply as phosphoric acid, is a white crystalline solid (m.p. 42-35 °C) and there is a crystalline hemihydrate, 2H 3 PO 4 .H 2 O, which melts at 29-35 °C. The acid is tribasic and in aqueous solution has three ionization constants (pKa): 2-15, 7-1 and 12-4. 197

Phosphate bonded cements

X-ray diffraction (XRD) studies have shown that the crystalline acid and its hydrate contain tetrahedral [POJ groups (Van Wazer, 1958). In the anhydrous acid, three of the oxygen atoms are bonded to hydrogen atoms and the P-O bonds are 0-157 nm or 0-158 nm in length. The P-O bond of the fourth O has more n character than the others and is shorter (0-152 nm). The [POJ tetrahedra are interconnected by hydrogen bonds and there are also internal hydrogen bonds between pairs of O atoms within the [POJ tetrahedra. Radial distribution curves from XRD studies indicate that the intramolecular hydrogen bonds persist in 86 % phosphoric acid solution. Mostly, a single hydrogen bond connects any two [POJ groups, but double and triple hydrogen bonding occurs to a lesser extent. In more dilute solution (54 %) the [POJ groups are linked to the water lattice rather than to other [POJ groups. Raman spectroscopy supports these structural views. In 75 % solution not all [POJ groups are hydrogen-bonded to water molecules (Wilson & Mesley, 1968). Results from infrared spectroscopy indicate that the only species present in 50 % phosphoric acid are H3PO4, H2PO4 and their oligomers (Wilson & Mesley, 1968). There is evidence that H6P2O8, the phosphoric acid dimer, and H5P2O~, the triple ion HgPO^. H + . H2PO~, are also present (Elmore, Mason & Christensen, 1946; Selvaratnam & Spiro, 1965). Akitt, Greenwood & Lester (1971), on the basis of 31P NMR studies, suggest further that there are oligomers of the type (H3P4O)W. 6.1.2

Cations in phosphoric acid solutions

Cement-forming phosphoric acid liquids nearly always contain cations. The most important of these are aluminium and zinc, but other metals may be used. Manly et al. (1951) have laid down criteria for the choice of modifying metals. They must be moderately soluble in oxide solution, not form coloured sulphides and be non-toxic. To these criteria, following Kingery (1950a,b), may be added another, that the modifying cations must be capable of remaining in vitreous form in the cement gel. This criterion is satisfied only by cations having a low coordination number, multiple charge and small ionic radius, that is having a high ionic potential. These are ions of amphoteric or weakly basic metals such as aluminium, zinc, beryllium and magnesium. More basic metals, for example calcium, barium and thorium, weaken the cement. As noted above, however, aluminium and zinc are the most important and are often found in combination in the liquids used for the zinc 198

General

phosphate and dental silicate cement. The presence of aluminium in phosphoric acid solution serves to retard the setting reaction in zinc phosphate cements and accelerate it in dental silicate cements. Both these metals are soluble to a limited extent in phosphoric acid solutions. Phase diagrams have been constructed for the systems ZnO-P2O5-H2O (Figure 6.1) and A12O3-P2O5-H2O (Figure 6.2). Stable zinc phosphate phases that have been found to exist are Zn3(PO4)2. 4H2O, ZnHPO4. 3H2O, ZnHPO4. H2O, Zn(H2PO4)2. 2H2O and Zn(H2PO4)2.2H3PO4 (Eberly, Gross & Crowell, 1920; Salmon & Terrey, 1950). All are apparently crystalline. Stable aluminium phosphate phases that have been found to exist are 2A1PO4.7H2O, 2A1PO4.4H2O, 2A1PO4.2H3PO46H2O, 2A1PO4.2H3PO4.3H2O; with the possible exception of the first all are crystalline (Jameson & Salmon, 1954). Although these phase diagrams have been used to deduce the course of cement formation, they are of limited use because it is doubtful whether conditions of thermodynamic equilibrium are reached; moreover, many cements are mainly amorphous. H2O

range for cements

ZnO

P2O5

Figure 6.1 The system ZnO-P 2 O 5 -H 2 O (Salmon & Terrey, 1950).

199

Phosphate bonded cements

The actions of zinc and aluminium differ. In general, metal ions such as zinc merely serve to neutralize the acid and are present in solution as simple ions (Holroyd & Salmon, 1956; O'Neill et ai, 1982). But aluminium has a special effect: in contrast to zinc, it prevents the formation of crystallites during the cement-forming reaction in zinc phosphate cements. Aluminium has long been known to form complexes with phosphoric acid (Bjerrum & Dahm, 1931; Jameson & Salmon, 1954; Genge et aL, 1955; Holroyd & Salmon, 1956; Salmon & Wall, 1958; Van Wazer, 1958; Van Wazer & Callis, 1958; Genge & Salmon, 1959). When it is dissolved in phosphoric acid solution the viscosity of the solution increases sharply as, according to Sveshnikova & Zaitseva (1964), aluminophosphoric acids are formed which behave as polyelectrolytes. A number of workers using 31P NMR spectroscopy have found evidence for the formation of aluminophosphoric acid complexes (Akitt, Greenwood, & Lester, 1971; O'Neill et al., 1982). Combining these NMR observations with those from infrared spectroscopy (Wilson & Mesley, 1968) the species present in 50%

range for cements

A12O3

Figure 6.2 The system A1 2 O 3 -P 2 O 5 -H 2 O (Jameson & Salmon, 1954).

200

P2O5

General Table 6.1. Cement-forming oxides Condition of oxide

Modification of liquid

Calcined Calcined

Al salt

Calcined Calcined

Al or NH 4 salt Al salt

BeO Be(OH)2

ZnO CuO Cu 2 O

MgO CaO

Bi2O3

CdO SnO

Pb 3 O 4 Co(OH) 3 Aluminosilicate glass

phosphoric acid solution containing aluminium appear to be H 3 PO 4 , H 6 P 2 O 8 , H 2 P O " , H 5 P 2 O - , A l H 3 P O r , A1H 2 POJ + , A1(H 2 PO 4 )+ and A1(H 3 PO 4 ) W , where n ^ 2, of u n k n o w n protonation. Binuclear aluminium phosphate complexes are also present and Salmon & Wall (1958) consider that bridge structures exist. All this points to the formation of an aluminophosphate polymer in the solution based on - P - O - A l - b o n d i n g .

6.1.3

Reactions between oxides and phosphoric acid solutions

In a classic study, Kingery (1950b) examined a large number of oxides for cement formation with orthophosphoric acid. He observed three types of reaction: no reaction, violent reaction with crystallization, and controlled reaction with cement formation. There was no reaction with oxides of an acidic or inert nature, for example SiO2, A12O3, ZrO 2 and Co 2 O 3 . There was violent reaction with reactive oxides, yielding non-cementitious products, which were crystalline, porous and friable. This type of reaction tended to occur when the oxides were alkaline, although it could be affected by calcining the oxide. Examples cited were calcined CaO, SrO and BaO, and uncalcined MgO and La 2 O 3 . However, calcined CaO did form cements when orthophosphoric acid was partly neutralized by CaO. 201

Phosphate bonded cements Lastly, there was the cementitous reaction which Kingery (1959b) reported with BeO, Be(OH)2, CuO, Cu2O, CdO, SnO and Pb3O4. In addition, calcined ZnO and MgO formed cements. These observations require some comment and amendment. The formation of cements with ZnO, CuO and Bi2O3 has been known for many years and they are found in dental cements (Wilson, 1975a,b; 1978). Also, although Co2O3 does not form a cement, Co(OH)3 does (Prosser et al., 1986). MgO is a doubtful cement-former with orthophosphoric acid, but forms useful cements with ammonium acid phosphate and aluminiumcontaining orthophosphoric acid solutions (Finch & Sharp, 1989). There is also cement formation with calcium aluminosilicate glasses (Wilson 1975c, 1978). This material, the dental silicate cement, is unusual in being translucent. Table 6.1 summarizes known basic cement-formers based on the observations of the workers cited above. Because of its importance and the breadth of the investigation, the work of Kingery (1950b) requires critical examination. He considered that the essential feature of phosphate-bonded cements was an acid phosphate matrix. Extended hydrogen bridges between acid phosphate groups can then be cited as the matrix-forming bond (Wygant, 1958). However, we consider that Kingery was mistaken, for his work has its limitations. Kingery based his conclusions on a literature survey and some experimental studies. The literature survey indicated that in many cases the reaction product of a metal oxide with orthophosphoric acid was an acid phosphate. However, these studies did not relate to cements, where the metal oxide is always present in excess. For example, he cites the phase diagram of Eberly, Gross & Crowell (1920) as showing ZnHPO4. 3H2O as the reaction product. But in the presence of excess ZnO the phase diagram shows Zn3(PO4)2. 4H2O as the stable species. Kingery also neglected to consider the effect of phosphoric acid concentration on the nature of the reaction product. Kingery himself used XRD, but except in one case failed to positively identify crystalline hydrogen phosphates in any of the cements he examined. He found either the neutral orthophosphate or unidentified crystalline species and then the lines were weak. Although an acid phosphate matrix cannot be excluded it is not essential for cement formation. In fact, it must be remembered that when these cements are prepared the oxide or silicate powder is normally in excess of that required for the reaction. Under these conditions most oxides (MgO 202

General

is a notable exception) form neutral orthophosphates. An acid phosphate matrix is to be expected only when there is excess acid in the cement mix; the only practical cements where these conditions obtain are those used for controlled release which are designed to be hydrolytically unstable (Prosser et al., 1986). Hydrated neutral orthophosphates are the normal reaction products in zinc phosphate and dental silicate cements, both of which have been studied in detail. The reaction between magnesium oxide and phosphoric acid is an exception. An acid phosphate is formed, but it is soluble in water (Finch & Sharp, 1989) and, in fact, MgO forms useful cements only with ammonium dihydrogen phosphate. Hydrogen bonds can still be considered as playing an important role, even in the case of a neutral orthophosphate, but they would act via water of hydration. As research progresses over the years it is becoming apparent that the majority of these cements are essentially amorphous, and that crystaUinity is secondary and sometimes very slight. Kingery's arguments based solely on XRD data are, perhaps, not very relevant. 6.1.4

Effect of cations in phosphoric acid solutions

As we have already shown, the presence of cations in orthophosphoric acid solution can have a decisive effect on cement formation. As noted above, Kingery (1950b) found it necessary to modify orthophosphoric acid, by the addition of calcium, to obtain cement formation with calcium oxide. Also, Finch and Sharp (1989) had to modify orthophosphoric acid, with either ammonium or aluminium, to achieve cement formation with magnesium oxide. Even when modifiers are not necessary for cement formation, they can lead to improved cement properties. Kingery (1950b) also examined this effect. He found that optimum bonding was achieved with cations that had small ionic radii and were amphoteric or weakly basic, such as beryllium, aluminium, magnesium and iron. By contrast, cations that were highly basic and had large ionic radii, for example calcium, thorium and barium, had a detrimental effect on bonding. We have noted earlier that aluminium is unusual in forming aluminophosphate complexes in phosphoric acid solution which may be of a polymeric nature. Bearing in mind the analogies between aluminium phosphate and silica structures, it may well be that during cement formation an aluminium phosphate hydrogel is formed. Its character may be analogous to that of silica gel, where a structure is built up by the 203

Phosphate bonded cements

condensation of pairs of hydroxyl groups to form oxygen bridges. Thus, the structure may consist of P in 4- and Al in 6-coordination, linked by oxygen bridges with H2O and OH~ as other ligands. 6.1.5

Important cement-formers

The most important of the phosphate bonded cements are the zinc phosphate, dental silicate and magnesium ammonium phosphate cements. The first two are used in dentistry and the last as a building material. Copper(II) oxide forms a good cement, but it is of minor practical value. In addition, certain phosphate cements have been suggested for use as controlled release agents. The various phosphate cements are described in more detail in the remainder of this chapter. 6.2

Zinc phosphate cement

6.2.1

General

Zinc phosphate cement, as its name implies, is composed principally of zinc and phosphate. It is formed by mixing a powder, which is mainly zinc oxide, with a solution based on phosphoric acid. However, it is not as simple chemically as it appears because satisfactory cements cannot be formed by simply mixing zinc oxide with phosphoric acid solution. The zinc phosphate cement finds use only in dentistry. Here it is used mainly as a 'luting agent' for the attachment of inlays, crowns, bridges, posts and orthodontic bands (Wilson, 1975a,b; Smith, 1982). It is used also as a cavity liner in crowns and bridges (dental prosthesis). Although new types of cement have been introduced in dentistry in the 1970s and 1980s, this traditional cement continues to hold its own, particularly on the continent of Europe. 6.2.2

History

The early history of the material is obscure. According to Palmer (1891) it goes back to 1832, but this statement has never been corroborated. Rostaing (1878) patented a series of pyrophosphate cements which could include Zn, Mg, Cd, Ba and Ca. Rollins (1879) described a cement formed from zinc oxide and syrupy phosphoric acid. In the same paper he mentions zinc phosphate cements recently introduced by Fletcher and Weston. Similar information is given in a discussion of the Pennsylvania 204

Zinc phosphate cement Association of Dental Surgeons (1879), where Peirce describes a cement similar to that of Rollins. Many brands were on the market by 1881 (Miller, 1881). The earliest formulations, as reported by Rollins (1879), Gaylord (1889), Ames (1893), Hinkins & Acree (1901) and Fleck (1902), were variously based on syrupy orthophosphoric acid or unstable mixtures of metaphosphoric acid and sodium metaphosphate in solution. Some used solid pyrophosphoric acid. Many were grossly inferior cements which were hydrolytically unstable. Later, better cements appeared based on c. 50 % solutions of orthophosphoric acid. But even these were far from satisfactory. As always with dental cements, the problems revolved around the control of the setting reaction: the reaction between zinc oxide and orthophosphoric acid was found to be far too fierce. By the time of Fleck's 1902 paper these problems had been solved. The importance of densifying and deactivating the zinc oxide powder to moderate the cement reaction had been recognized. Of equal importance was the realization that satisfactory cements could be produced only if aluminium was incorporated into the orthophosphoric acid solution. The basic science underlying this empirical finding was elucidated only in the 1970s. Comparison of the chemical composition of brands available in the 1960s and 1970s (Axelsson, 1965; Wilson, Abel & Lewis, 1974) shows little variation from those of the 1930s (Paffenbarger, Sweeney & Isaacs, 1933) and it is doubtful whether the composition has changed in essence since the beginning of the century (Table 6.2). 6.2.3

Composition

Powder The powder is principally composed of zinc oxide (Table 6.2). Magnesium oxide is found in all current commercial brands in amounts that range from 3 to 10%. Alumina and silica are sometimes to be found. Present day compositions show less variation than formerly when bismuth, calcium and barium oxides, or sometimes no additives, were to be found in commercial examples (Paffenbarger, Sweeney & Isaacs, 1933). The chief problem with these cements, as with many AB cements, is to moderate the cement-forming reaction. If the reaction is over-vigorous then a crystalline mass rather than a cement is formed (Komrska & Satava, 1970; Crisp et al, 1978). Therefore, the zinc oxide used in these cements 205

Phosphate bonded cements Table 6.2. Chemical composition of commercial zinc phosphate cements (Axelsson, 1965; Wilson, Abel & Lewis, 1974) Powders

Liquids

Species

% by mass

Species

% by mass

ZnO MgO A12O3 SiO2

89-1-92-7 3-2-9-7 0-0-6-8 0-0-2-1

H 3 PO 4 Al Zn

45-3-63-2 1-0-3-1 0-0-9-9

Based on the results of nine examples. has to be deactivated and densified by sintering at temperatures which range from 1000 to 1350 °C to moderate its reaction with aqueous phosphoric acid solutions. The sintering of a non-stoichiometric solid, such as zinc oxide, is affected by its initial physical condition and the surrounding atmosphere. Several processes are involved. Sintering reduces specific surface area and densities zinc oxide. It also reduces surface energy. According to Dollimore & Spooner (1971) freshly prepared zinc oxide has a high surface energy because of its preparation in an oxygen-rich atmosphere. They suggested that during sintering the initial excess of oxygen at the surface is reduced by the diffusion of zinc ions from the bulk to the surface. Magnesium oxide is always blended with the zinc oxide prior to ignition. Magnesium oxide promotes densification of the zinc oxide, preserves its whiteness and renders the sintered powder easier to pulverize (Crowell, 1929). The sintered mixed oxide has been shown to contain zinc oxide and a solid solution of zinc oxide in magnesium oxide (Zhuravlev, Volfson & Sheveleva, 1950). Specific surface area is reduced compared with that of pure zinc oxide and cements prepared from the mixed oxides are stronger (Crowell, 1929; Zhuravlev, Volfson & Sheveleva, 1950). In the presence of silica, zinc silicate is formed, the sintering process is improved and the increase in grain size is enhanced (Zhuravlev, Volfson, & Sheveleva, 1950). Mineralizers, such as fluorite, cryolite and borax, have a similar effect (Zhuravlev, Volfson & Sheveleva, 1950). These mineralizers enhance sintering and promote growth in grain size. As a result the sintering temperature can be reduced from 1350 °C to 1150-1200 °C.

206

Zinc phosphate cement Liquid The liquid is an aqueous solution of phosphoric acid, always containing 1 to 3 % of aluminium, which is essential to the cement-forming reaction (Table 6.2). Zinc is often found in amounts that range from 0 to 10% to moderate the reaction. Whereas zinc is present as simple ions, aluminium forms a series of complexes with phosphoric acid (Section 6.1.1). This has important consequences, as we shall see, in the cement-forming reaction. 6.2.4

Cement-forming reaction

The cement sets rapidly within a few minutes of preparation. The reaction is strongly exothermic and is greater than with any other dental cement (Crisp, Jennings & Wilson, 1978). The excessive heat generated in the reaction has to be dissipated by progressively incorporating the powder into the liquid. Strength develops rapidly. About half the ultimate strength is attained within ten minutes of preparation and 80 % after one hour (Plant & Wilson, 1970; Williams & Smith, 1971). The setting reaction is an acid-base one and the course of the reaction is shown by pH changes in the cement. Two minutes after mixing the pH is as low as 1-6, after 60 minutes it increases to about 4 and reaches between 6 and 7 after 24 hours (Plant & Tyas, 1970). The nature of the setting reaction and the set cement remained imperfectly understood for many years. This is not surprising, for the products of the reaction depend on a number of factors, including the phosphoric acid concentration and the presence or absence of aluminium in the solution. These complexities have caused considerable confusion in the literature. Early workers, and some later ones, ignored the fact that aluminium is always found in the orthophosphoric acid liquid of the practical cement; its presence profoundly affects the course of the cement-forming reaction. It affects crystallinity and phase composition, and renders deductions based on phase diagrams inappropriate. Nevertheless we first describe the simple reaction between zinc oxide and pure orthophosphoric acid solution, which was the system studied by the earliest workers. In the earliest attempt to explain the reaction, Crowell (1929) used, in part, arguments based on the phase diagrams of the ZnO-P2O5-H2O system constructed by Eberly, Gross & Crowell (1920). Later, Darvell (1984) advanced similar arguments using the phase diagrams of Salmon & 207

Phosphate bonded cements Terrey (1950). In the case of the simple zinc oxide-orthophosphoric acid system, but only for this simple system, these phase-diagram arguments are valid. In the presence of excess zinc oxide, the final product of reaction with orthophosphoric acid solution is always hopeite, Zn 3 (PO 4 ) 2 . 4H 2 O, and as early as 1933 Halla & Kutzeilnigg (1933) found that zinc phosphate cements in service in the mouth contain hopeite. But this substance is not responsible for initial set in the absence of aluminium. Crowell (1929) attributed setting to the formation of ZnHPO 4 . 3H2O, which he found slowly converted over the weeks to hopeite, Zn 3 (PO 4 ) 2 . 4H 2 O. Vieira & De Arujo (1963) confirmed this result. The most definitive study in this field was that of Komrska & Satava (1970) who used XRD analysis to identify the crystalline products formed. They found that a cement prepared from ZnO and 82-5% H 3 PO 4 set as the result of the formation of Zn(H 2 PO 4 ) 2 . 2H 2 O crystallites. Under humid conditions at 37 °C the following conversions occurred: Zn(H 2 PO 4 ) 2 . 2H 2 O - ZnHPO 4 . H 2 O -> ZnHPO 4 . 3H2O -> Zn 3 (PO 4 ) 2 .4H 2 O After 7 days all four species were found in the cement. These conversions were speeded up when the cement was placed in water, so that after 7 days only Zn 3 (PO 4 ) 2 . 4H 2 O was found. With 65 % H 3 PO 4 , setting was found to result from the formation of ZnHPO 4 .H 2 O crystallites. On ageing, the same conversions occurred as with the 82*5% H 3 PO 4 cement. All these changes are in accordance with phase-diagram predictions. Unfortunately, Komrska & Satava (1970) did not examine the reaction with orthophosphoric acid solutions having concentrations in the range 45 to 6 3 % H 3 PO 4 which, according to Wilson (1975b), is the range encountered in practical materials. As Darvell (1984) has deduced, when ZnO is added to orthophosphoric acid solutions within this compositional range, ZnHPO 4 . 3H 2 O starts to precipitate. As precipitation continues, the acid concentration declines until it reaches the isoelectric point (22-5 %) at which ZnHPO 4 . 3H 2 O becomes unstable with respect to the liquid and starts to convert to Zn 3 (PO 4 ) 2 . 4H 2 O, the final reaction product. Darvell's deductions for higher initial concentrations of acid are in accordance with the experimental findings of Komrska & Satava (1970). These simple zinc oxide-orthophosphoric acid cements are very weak; indeed, it may be that they are just weakly-bonded aggregates of 208

Zinc phosphate cement crystallites. Nevertheless, they have been used to advance theories of cementation. Kingery (1950b) in his XRD investigations of the reaction reported a crystalline matrix that was not hopeite but was taken to be an acid phosphate. He was apparently unaware of the slow conversion of the matrix to this species. Unfortunately, both he and Wygant (1958) went on to use this and similar observations to construct a theory that attributed cementation to hydrogen bonding between acid phosphate units. This idea can now be seen as dubious. The role of hydrogen bonding in the cement matrix is to be envisaged between particles of colloidal dimensions rather than between molecular units. For many years the picture remained of a cement consisting of zinc oxide particles bonded by a crystalline matrix of hopeite (Dobrowsky, 1942). But these ideas were erroneous for several reasons. Phase-diagram arguments applied only to systems in thermodynamic equilibrium and, clearly, these conditions are not obtained in rapidly setting cements. XRD analysis can be misleading as it ignores amorphous phases. Finally, these early ideas ignored the role of aluminium, although the necessity of having it in the liquid for the production of satisfactory cements had long been known (Section 6.2.2). Komrska & Satava (1970) showed that these accounts apply only to the reaction between pure zinc oxide and phosphoric acid. They found that the setting reaction was profoundly modified by the presence of aluminium ions. Crystallite formation was inhibited and the cement set to an amorphous mass. Only later (7 to 14 days) did XRD analysis reveal that the mass had crystallized directly to hopeite. Servais & Cartz (1971) and Cartz, Servais & Rossi (1972) confirmed the importance of aluminium. In its absence they found that the reaction produced a mass of hopeite crystallites with little mechanical strength. In its presence an amorphous matrix was formed. The amorphous matrix was stable, it did not crystallize in the bulk and hopeite crystals only grew from its surface under moist conditions. Thus, the picture grew of a surface matrix with some tendency for surface crystallization. Surprisingly, the effect of aluminium on the reaction had been anticipated by van Dalen many years before in his thesis of 1933, but had not made its way into the scientific literature. The authors are indebted to Dr L. J. Pluim of the Rijksuniversiteit te Groningen for this information. Van Dalen (1933) was convinced that aluminium had an important role in cement formation and that Crowell had been wrong to ignore it. Van Dalen found that the reaction between zinc oxide and phosphoric acid was 209

Phosphate bonded cements

greatly moderated by the presence of aluminium in the liquid. He attributed this to the formation of a gelatinous coating of aluminium phosphate around each zinc oxide particle. This observation has never been repeated, but it appears entirely reasonable. Van Dalen also found that aluminium inhibited the formation of crystallites and that this inhibition increased with increase in the aluminium content of the liquid. Crisp et al. (1978) were able to follow the course of the cement-forming reaction using infrared spectroscopy and to confirm previous observations. They found that the technique could be used to distinguish between crystalline and amorphous phases of the cement. Hopeite shows a number of bands between 1105 and 1000 cm"1; this multiplicity has been explained by postulating a distortion of the tetrahedral orthophosphate anion. (Twothirds of the zinc ions are tetrahedrally coordinated to four phosphate ions, and the remainder are octahedrally coordinated to two phosphate and four water ligands.) Crisp et al. (1978) were able to detect the formation of crystallites both on the surface and in the bulk of the reaction product. In the absence of aluminium the reaction between zinc oxide and phosphoric acid was very rapid and the cement set in less than two minutes. Hopeite was formed, within minutes, both at the surface and in the bulk of the reaction mass. It was doubted whether this mass constituted a true cement. The addition of aluminium to the liquid slowed down the reaction. An amorphous cement was formed and there was no crystallization in the bulk of the cement. However, after some time crystallites were formed at the surface. Thus, the presence of aluminium exerts a decisive influence on the course of the cement-forming reaction. This effect is to be attributed to the formation of aluminophosphate complexes (see Sections 6.1.2 and 4.1.1). These complexes may delay the precipitation of zinc from solution and also introduce an element of disorder into the structure, thus inhibiting crystallization. It is significant that zinc, which does not form complexes, has little effect on the nature or speed of the reaction. Crisp and coworkers found that the development of surface crystallinity was related to the speed of set. The faster the reaction, the shorter was the inhibition period before surface crystallization took place. When the setting time of a cement was between two and three minutes, surface crystallinity developed in a few minutes. When it was seven minutes, surface crystallinity was delayed by three hours. The reaction rate was affected by the chemical composition and physical state of the cement components. Well-ignited zinc oxide, the presence of magnesium in the 210

Zinc phosphate cement oxide powder, high phosphoric acid concentration, a low powder/liquid ratio and the presence of aluminium in the liquid all served to retard the reaction. It is notable that these workers, like Servais & Cartz, found no evidence for the formation of crystalline acid phosphates in the cements. More recently, Steinke et al. (1988) examined four commercial cements and found that the matrices were mainly amorphous; indeed, they found hopeite in only one of them. This indicates that manufacturers today are able to formulate to prevent crystallization. Wilson, Paddon & Crisp (1979) have shown that the water present in the cement can be divided, somewhat arbitrarily, into bound water of hydration (non-evaporable) and loosely held (evaporable) water. The amount of tightly bound water increases as the cement ages and in one example reached 42 % of the total water. It is interesting that this cement has been known for over 100 years and yet certain features of its chemistry remain obscure. The exact nature of the matrix is still a matter for conjecture. It is known that the principal phase is amorphous, as a result of the presence of aluminium in the liquid. It is also known that after a lapse of time, crystallites sometimes form on the surface of the cement. A cement gel may be likened to a glass and this process of crystallization could be likened to the devitrification of a glass. Therefore, it is reasonable to suppose that the gel matrix is a zinc aluminophosphate and that entry of aluminium into the zinc phosphate matrix causes disorder and prevents crystallization. It is not so easy to accept the alternative explanation that there are two amorphous phases, one of aluminium phosphate and the other of zinc phosphate. This is because it is difficult to see how aluminium could act in this case to prevent zinc phosphate from crystallizing. Summary of experimental evidence A summary of this evidence may be attempted to give a probable reaction mechanism. After mixing, the zinc oxide powder is attacked by the acid solution, water acting as the reaction medium. Zinc ions are extracted and the pH at the powder-liquid interface rises, causing aluminium phosphate or more probably a zinc aluminophosphate to precipitate as a gel at the particle surface. This gel coating moderates the reaction. Zinc ions diffuse through this layer and, as the pH rises, precipitate as an amorphous gel, probably a zinc aluminium phosphate. This reaction mechanism thus postulates both a topochemical and a through-solution reaction. As reaction proceeds, the cement matrix becomes increasingly hydrated. 211

Phosphate bonded cements

Increase in concentration of aluminium and phosphoric acid in the liquid serves to slow the reaction. This observation is in line with the above reaction scheme. Increase in the aluminium content will serve to increase the thickness of the coating formed around zinc oxide particles. Increase in phosphoric acid content implies a decrease in water content and an impairment of the hydration reaction. The phenomenon of surface crystallization could be represented by the equation. H2O

Zinc aluminophosphate -> Zn3PO4. 4H2O + A1PO4. «H2O amorphous hydrogel hopeite amorphous gel 6.2.5

Structure

The set cement is completely opaque and can be regarded as a composite of unconsumed zinc oxide particles, possibly coated with an aluminium orthophosphate gel, bonded together by an amorphous neutral zinc orthophosphate gel. There is some tendency for hopeite crystallites to grow from the surface (Servais & Cartz, 1971; Cartz, Servais & Rossi, 1972; Crisp et al, 1978). Growth is related to the speed of set and the presence of moisture. Under dry conditions the surface is stable and undulating with no sign of crystallites (Figure 6.3a). When the environment is maintained at 100% relative humidity, crystal growth is observed (Figure 63b) and may be compared with the devitrification of a glass. Servais & Cartz (1971) observed that under certain conditions the layer could be 8 |am thick. It is only loosely attached to the body of the cement matrix. The presence of this layer of crystallites may explain why the cement lacks adhesion. The bulk of the cement is extremely porous as the fractured surface of a specimen shows (Figure 6.3c). The pores are 0-5 \im in diameter and more abundant in the depth of the cement. The porosity arises from excess unbound water which separates out as globules in the cement and is trapped by the rapid setting. Subsequent diffusion of these globules leaves the cement porous. This makes the cement permeable to dyes (Wisth, 1972). As mentioned previously, the cement contains both tightly bound and loosely bound water. The set cement can both lose and gain water depending on its environment. Under drying conditions (say 50 % relative humidity) it loses water and shrinks. When placed in water there is an 212

Zinc phosphate cement

(b) Figure 6.3 The effect of environmental conditions on the surface of a zinc phosphate cement: (a) stable and undulating surface with no sign of crystallites observed under dry conditions, (b) crystal growth observed in an atmosphere of 100 % relative humidity, (c) extreme porosity observed in the bulk of the cement; pores are 0-5 urn in diameter (Servais & Cartz, 1971).

213

Phosphate bonded cements Table 6.3. Specification properties of commercial zinc phosphate cements {Wilson 1975b) Value

Specification limits"

Powder: liquid6

20-30

—

Setting time (37 °C), min

3-9-7-5

5-9

Film thickness, urn

24-40

25 maximum

Compressivec strength (24 h), MPa

70-131

70 minimum

Solubility & disintegration (24 h), %

004-3-3

0-2 maximum

a

BS 3364: 1961 Specification for Dental Zinc Phosphate Cement. For a consistency spread of 30 mm diameter for 0-5 cm3 of cement paste under a load of 1-96 N (200 g weight) applied after 3 minutes at 23 °C. c After storage for 24 hours in water at 37 °C, based on the results of nine examples. b

equilibrium water uptake: 9 % for two zinc phosphate cements examined by Eichner, Lautenschlager & von Radnoth (1968). 6.2.6

Properties

Zinc phosphate cement is used as a luting agent for the cementing of crowns and bridges in dentistry. It is not an ideal material. It does not possess the adhesion of modern polyelectrolyte cements, but its clinical performance has been good enough to ensure it a place in dentistry for about 100 years. Despite the challenge of modern polyelectrolyte cements it still holds its own. Undoubtedly this is because it is easy to mix, the fluid paste is easy to manipulate, its working time is long and the paste sets sharply to a hard mass. These are important properties for a dental material. In practice it means that these cements are easier to use than the polyelectrolyte cements and this alone explains their continued popularity with the dentist. Preparation and setting Zinc phosphate cement is prepared by introducing small incremental amounts of powder into the liquid and mixing the paste over a large area on a glass slab in order to dissipate heat because of the excessive exotherm 214

Zinc phosphate cement Table 6.4. Mechanical properties of commercial zinc phosphate cements (Housten & Miller, 1968; Wilson, 1975b; Wilson & Lewis, 1980; Powers, Farah & Craig, 1976; 0ilo & Espevik, 1978) Property"

Value

Compressive strength Compressive modulus Tensile strength Strain Creep (over 24 hours)

70-131 MPa 11-9—13-5 GPa 4-3-8-3 MPa 0-2 %b 0-13 %b

a b

After storage for 24 hours in water at 37 °C. Only one example.

of this cement (Crisp, Jennings & Wilson, 1978). To achieve a consistency suitable for cementing crowns this cement is normally mixed with a powder/liquid ratio that ranges from 2-3 to 2-7 g cm" 3 (Table 6.3) (Wilson, 1975b). The working time (at 23 °C) then varies from 3 to 6 minutes and setting time from 5 to 14 minutes (Plant, Jones & Wilson, 1972; Jendresen, 1973; Wilson, 1975b; Myers, Drake&Brantley, 1978; E a r n e s t a l , 1977). The linear contraction of the cement on setting (0-5 %) can give rise to slits at the cement-tooth and cement-restoration interfaces (0ilo, 1978). Zinc phosphate cement mixes to a paste which is thin and mobile. Under pressure it flows readily to give a film 24 to 40 jim thick (Table 6.3). This film thickness is adequate to seat restorations, especially as McLean & von Fraunhofer (1971) and Dimashkieh, Davies & von Fraunhofer (1974) have shown that in practice the gap between tooth and restoration can be as much as 100 |iim or more. Mechanical properties Fully hardened cements have brittle characteristics (Williams & Smith, 1971; Skibell & Shannon, 1973) and show little creep under load (Wilson & Lewis, 1980). When mixed to a luting (cementation) consistency, their compressive strength reaches 70 to 131 MPa after 24 hours (Wilson, 1975b) depending on brand (Table 6.4). There is little subsequent increase in strength (Paffenbarger, Sweeney & Isaacs, 1933; Smith, 1977). The tensile strength of these cements lies between 4-3 and 8-3 MPa and is thus lower than their compressive strength (Table 6.4) (Williams & 215

Phosphate bonded cements

Smith, 1971; Hannah & Smith, 1971; Powers, Farah & Craig, 1976). The modulus of elasticity in compression is 12 to 13 GPa (Table 6.4) (Powers, Farah & Craig, 1976; Wilson, Paddon & Crisp, 1979). Erosion

Good cements show little dissolution in water, less than 0-1 % using the standard test (see Chapter 10); the amount can be much greater for poorer examples. Dissolution represents the amount of material eluted from a one-hour-old cement as it ages in water for a further 24 hours. As the cement ages further the rate of dissolution falls although it always remains significant (Wilson, Kent & Lewis, 1970; Wilson, 1976; de Freitas, 1973). Wilson, Abel & Lewis (1974), in a detailed chemical study of erosion in aqueous solution, found that in the first 24 hours of the cement's life the ions eluted were Zn2+, Mg2+, HPO2" and H2PC>4. Far more Mg2+ ions were eluted than Zn2+ ions, despite zinc being the major metal constituent of the zinc phosphate cement. These workers deduced that magnesium is far less firmly bound to phosphate than is zinc and that, consequently, its presence in the oxide is a source of weakness. These results were later confirmed by Anzai et al (1977). Wilson, Kent & Lewis (1970) in a long-term study found that most of the soluble phosphate was eluted from the cement in the first 24 hours. By contrast the rate of elution of zinc remained constant for 160 days, the length of the study. They concluded that long-term erosion took place at the surface of the oxide particles rather than in the matrix. Of more significance, however, were their results for the effect of pH. These showed that the cement is at its most stable in neutral solution and that dissolution increases sharply with increasing acidity (Figure 6.4). This is of clinical significance, for pHs as low as 4 can occur in the stagnation regions of the mouth (Stephan, 1940; Kleinburg, 1961), and it is now generally believed that the life of a dental cement is determined by its resistance to acid conditions. The dissolution in lactic acid and especially citric acid is much higher (Norman, Swartz & Phillips, 1957). This effect must be attributed to the complexing effect of these acids. The laboratory impinging jet test for evaluating the acid erosion of dental cements is described in Chapter 10. Using this method with lactic acid-lactate solutions, Wilson et al. (1986b) found, for one cement, that the erosion rate was virtually zero at pH = 5-0, 0-38 % at pH = 4-0 and 5-7 % at pH = 2-7. For a range of cements Wilson et al. (1986a) found erosion rates varying from 3-0 to 5-7 % in lactic acid solutions of pH = 2-7. The 216

Zinc phosphate cement zinc phosphate cement is markedly less resistant to acid erosion than the aluminosilicate glass cements, glass-ionomer cements and dental silicate cements. They also found that, with one exception, zinc phosphate cements were somewhat more resistant to acid erosion than zinc polycarboxylate cement. These results have been confirmed by other workers using similar methods (Beech & Bandyopadhyay, 1983; Kuhn, Setchell & Teo, 1984; Gulabivala, Setchell & Davies, 1987; Mesu, 1982). In vivo studies have indicated that zinc phosphate cements erode under oral conditions. Also, cements based on zinc oxide, including the zinc phosphate cement, are less durable in the mouth than those based on aluminosilicate glasses, the dental silicate and glass-ionomer (Norman et al., 1969; Ritcher & Ueno, 1975; Mitchem & Gronas, 1978,1981; Osborne et al, 1978; Pluim & Arends, 1981, 1987; Sidler & Strub, 1983; Mesu & Reedijk, 1983; Theuniers, 1984; Pluim et al, 1984, Arends & Havinga, 1985). However, there is some disagreement on whether the zinc phosphate cement is more durable than the zinc polycarboxylate cement. Dissolution of the cement has been associated with increased marginal

\P/L26q/ml

silicate cement P/L > 4 2q/ml

2", -

\

I

VJ \

>

I

~r

pH

Figure 6.4 Effect of pH on the elution of phosphate from a zinc phosphate cement mixed at two different consistencies (Wilson, Kent & Lewis, 1970).

217

Phosphate bonded cements

leakage (Andrews & Hembree, 1976) and the penetration of bacteria (Brannstrom & Nyborg, 1974). 6.2.7

Factors affecting properties

Cement properties are affected by a number of factors. Some are determined by the manufacturer, for example the chemical composition of the cement components. Others are under the clinician's control. These include the powder/liquid ratio of the cement mix and the temperature of the surgery. Increase in either of these variables accelerates the reaction and affects properties. Properties are also affected by the composition of the phosphoric acid liquid. We have already pointed out that the liquid has to contain aluminium for satisfactory cement formation. In addition the H 3PO4/H2O ratio exerts a considerable effect on cement properties. This was investigated by Worner & Docking (1958). If the cement liquid is exposed to air, water can be readily gained or lost, depending on humidity, and this clearly affects the H3PO4/H2O ratio and therefore the cement properties (Figure 6.5). If water is lost, then setting time is prolonged, strength increased and

3O °h water in the liquid

4O

Figure 6.5 Effect of water content of the liquid (H 2 O:H 3 PO 4 ) on the properties of a zinc phosphate cement (Worner & Docking, 1958).

218

Zinc phosphate cement resistance to aqueous attack decreased. The converse also occurs, although the reduction in setting time is slight. Both setting time and working time are reduced if the mixing temperature is increased. Longer mixing time gives slower set (Paffenbarger, Sweeney & Isaacs, 1933). As long ago as 1892, Evans advocated the use of a cool slab for cement preparation. Low-temperature preparation extends working time and enables more powder to be incorporated into the cement; this is advantageous for it increases strength and resistance to dissolution, although this varies from brand to brand. This effect has since been confirmed by many workers: Paffenbarger, Sweeney & Isaacs (1933); Henschel (1943); Jendresen (1973); Myers, Drake & Brantley (1978); Windeler (1978); Kendzior, Leinfelder & Hershey (1976); Tuenge, Sugel & Izutsu (1978); Williams et al. (1979). However, there is a danger with this technique, that if the atmosphere is excessively moist, water may condense on the slab and the cement be weakened (Tuenge, Sugel & Izutsu, 1978; Norman et al., 1970). The powder/liquid ratio used in the cement mix affects a number of properties. As it is increased, setting time and working time are reduced. Compressive strength increases almost linearly with powder/liquid ratio (Savignac, Fairhurst & Ryge, 1965). Film thickness is controlled by a number of factors. The grain size of the powder imposes a lower limit on its value and rheological characteristics of the cement affect flow (Jorgensen & Peterson, 1963). An increase in the powder/liquid ratio or a delay in seating a restoration leads to an increase infilmthickness. The geometry of the surfaces to be cemented also affects flow and hence film thickness (Windeler, 1979). 6.2.8

Biological effects

The zinc phosphate cement is not bland towards living tissues. When freshly mixed it is highly acidic with a pH as low as 1-6 (Plant & Tyas, 1970). Even after it has aged one hour the pH may be lower than 4. This can give rise to pulpal irritation and pain. Prolonged pulpal irritation in deep cavities cannot be allowed and some form of pulpal protection is needed in these cases. There are other causes of pain and pulpal irritation. Hydraulic pressure developed during the seating of a restoration can lead to pulpal damage (Hoard et al., 1978). The movement of fluid under osmotic pressure has been cited as a cause of pain (Brannstrom & Astrom, 1972). 219

Phosphate bonded cements If the cement is mixed too thinly it may etch the tooth enamel because of its excess acidity (Docking et al., 1953; Abramovich, Macchi & Ribas, 1976). Of course, etching can promote mechanical attachment to the tooth (Ware, 1971). 6.2.9

Modified zinc phosphate cements

Fluoride is found in some zinc phosphate cements, generally as stannous fluoride. The cements are weaker and have less resistance to dissolution than normal zinc phosphate cements (Myers, Drake & Brantley, 1978; Williams et al., 1979). They releasefluorideover a long period (de Freitas, 1973) and this is taken up by enamel (Wei & Sierk, 1971). This results in reduced enamel solubility (Gursin 1965; Skibell & Shannon, 1973) and increased hardness (Yamano, 1968). Fluoride-releasing cements should reduce the incidence of enamel decalcification under orthodontic bands but this effect has not been recorded. Althoughfluorideis added as the tin salt,fluoriderelease is accompanied by the release of aluminium and not tin (de Freitas, 1973). There is little leaching out of tin apart from an initial wash-out. Of course, aluminium is not released from the normal cement (Wilson, 1976; Wilson, Abel & Lewis, 1974). These cements are of very minor interest in dentistry. 6.2.10 Hydrophosphate cements Interesting attempts have been made to formulate water-setting cements by blending solid acid phosphates with the zinc oxide powder. The cement is then prepared by mixing this powder blend with water. These attempts may be considered to have failed. Nakazawa et al. (1965) used calcium or magnesium dihydrogen phosphate as the acid phosphate. The powders were hygroscopic. The magnesium salt yielded cements which showed excessive contraction on setting and were weak and prone to aqueous attack. The calcium salt was somewhat better. A more successful approach was that of Higashi et al. (1969a,b 1972). They blended solid acid phosphate salts with zinc oxide powder. One acid salt used was a precipitated hydrate of ZnH2PO4. The cement was formed by mixing this powder blend with water. Work progressed to the point where three commercial brands of these so-called 'hydrophosphate' cements appeared on the market. None met the specification requirements 220

Transition-metal phosphate cements (Laswell et ai, 1971; Arato, 1974). All were prone to excessive dissolution and only one had adequate strength and film thickness. Their working characteristics were found to be unduly sensitive to changes in temperature and humidity (Simmons, D'Anton & Hudson, 1968). All were inferior to conventional zinc phosphate cements. No further development of these cements has taken place, nor is it likely that interest in them will be revived. The modern water-activated glass-ionomer cement has filled this niche and has vastly superior properties including adhesion to tooth material. 6.3

Transition-metal phosphate cements

Copper oxides are good cement-formers but copper phosphate cements find little practical use. They have a very minor use in dentistry, the last description of their chemistry appearing in 1940 (Worner, 1940). They have also been proposed for the controlled release of copper as a trace element supplement. They were introduced into dentistry by Ames in 1892 and came into extensive use between 1914 and 1916; Poetschke described them in 1916. The rationale for their use was based on the germicidal action of copper. There appears to be some basis for this belief (Worner, 1940; Babin, Hurst & Feary, 1978). They are, however, excessively acidic and cause pulpal irritation (Worner, 1940; Ware, 1971). At one time they were used for thefillingof pits andfissuresof deciduous (children's) teeth, lining, restoring badly decayed teeth, luting and the placement of orthodontic bands. By 1971 their use was confined to the last two applications (Ware, 1971). There is virtually no knowledge of the setting and structure of copper phosphate cements. Mostly, they are complex materials. The simplest was based on a powder containing 91-5% CuO and 8-4% Co3O4. Others contained respectively 62-2 % CuO and 29-8 % ZnO, and 23-9 % Cu2O and 66-7 % ZnO, with other metal oxides. The strength of these cements is about the same as the zinc phosphate cement (Ware, 1971). There are also pseudo-copper cements, which are zinc phosphate cements coloured by minor amounts of copper(II) oxide. Vashkevitch & Sychev (1982) have identified the main reaction product of the cement-forming reaction between copper(II) oxide and phosphoric acid as Cu3(PO4)2. 3H2O. The addition of polymers - poly(vinyl acetate) and latex - was found to inhibit the reaction and to reduce the compressive strength of these cements. However, impact strength and water resistance were improved. 221

Phosphate bonded cements

More recently copper phosphate cements have been suggested for use as controlled-release agents for supplying trace amounts of copper to cattle and sheep over an extended period (Allen et al, 1984; Manston et al., 1985; Prosser et al., 1986). The cements were prepared with a Cu/P ratio of 1:1 to ensure that the matrix was an acid phosphate and so subject to dissolution in aqueous solutions. They released copper at a constant rate for 90 days. Here we might note that cobalt(II) hydroxide, but not the oxide, also forms cements (Allen et al., 1984; Manston & Gleed, 1985; Prosser et al., 1986). It also is used in controlled-release devices for supplying trace elements to cattle and sheep. Nothing is known of its structure.

6.4

Magnesium phosphate cements

6.4.1

General

Cements based on the reaction between magnesium oxide and phosphates have long been known as investment materials for the casting of alloys (Prosen, 1939, 1941; Earnshaw, 1960a,b). More recently versions have been used as materials for the emergency repair of roads, runways and industrial floors (El-Jazairi, 1982). This is because of their fast-setting properties and ability to set at low temperatures. Setting reactions have only been elucidated in the last decade or so. The important point to note is that although a cementitious mass is formed when magnesium oxide is reacted with phosphoric acid, it is soluble in water (Finch & Sharp, 1989). Practical cements are formed only when ammonium or aluminium phosphates are incorporated into the cement liquid. Interestingly, they appear to occur as a natural cementitious binder in kidney stones (Abdelrazig & Sharp, 1988). Unlike other phosphate-bonded cements that have been fully studied, these appear to have a mainly crystalline matrix. 6.4.2

Composition

Magnesium (or magnesia) phosphate cements are based on the reaction between ignited magnesium oxide and acid phosphates, which are generally modified by the addition of ammonium and aluminium salts. The phosphates may be either in solution or blended in solid form with the magnesium oxide. In the latter form the cement is formed by mixing the powder blend with water. 222

Magnesium phosphate cements Table 6.5. Crystalline phosphate species found in magnesium phosphate cements Crystalline species

Formula

Dittmarite ' Hayesite' Newberyite Phosphorresslerite Schertelite Stercorite Struvite

MgNH4PO4. H2O MgHPO4. 2H2O or MgHPO4. H2O ? MgHPO4. 3H2O MgHPO4. 7H2O Mg(NH4)2(HPO4)2. 4H2O NaNH4HPO4. 4H2O MgNH4PO4. 6H2O

The most important characteristic of the magnesium oxide powder used in these cements is its reactivity (Glasson, 1963). Magnesium oxide needs to be calcined to reduce this, otherwise the cement pastes are too reactive to allow for placement. Surface area and crystal size are important and relate to the calcination temperature (Eubank, 1951; Harper, 1967; Sorrell & Armstrong, 1976; Matkovic et ai, 1977). The lower reactivity of calcined magnesium oxide relates to a lower surface area and a larger crystallite size. 6.4.3

Types

There are several types of cement and mortar (cement plus filler): (1) Those based on the reaction between magnesium oxide powder and orthophosphoric acid solution. (2) Those based on the reaction between magnesium oxide and ammonium dihydrogen phosphate (ADP), often in the presence of sodium tripolyphosphate (STPP), which is mixed with water (El-Jazairi, 1982; Abdelrazig et aL, 1984; Abdelrazig, Sharp & E1Jazairi, 1988, 1989). (3) Those based on the reaction between magnesium oxide and diammonium hydrogen phosphate (DAP) (Sugama & Kukacka, 1983a). (4) Those based on the reaction between magnesium oxide and ammonium polyphosphate (APP) (Sugama & Kukacka, 1983b). 223

Phosphate bonded cements (5) Those based on the reaction between magnesium oxide and aluminium hydrogen phosphate solutions (Finch & Sharp, 1989). 6.4.4

Cement formation and properties

Cement formation between MgO and various acid phosphates involves both acid-base and hydration reactions. The reaction products can be either crystalline or amorphous; some crystalline species are shown in Table 6.5. The presence of ammonium or aluminium ions exerts a decisive influence on the course of the cement-forming reaction. 6.4.5

Cement formation with phosphoric acid

The reaction between magnesium oxide and 85-3 % phosphoric acid has been studied by Finch & Sharp (1989). The reaction products were found to be two highly crystalline phases, one unidentified, and an amorphous phase. One crystalline phase was most probably Mg(H 2 PO 4 ) 2 . 2H 2 O. The other was believed to be another dihydrogen phosphate, Mg(H 2 PO 4 ) 2 .4H 2 O. These findings are in accordance with the phase diagrams of Belopolsky, Shpunt & Shulgina (1950) who reported both these species. These cements are soluble in water and so of no practical significance. 6.4.6

Cement formation with ammonium dihydrogen phosphate

The reaction between MgO and ammonium dihydrogen phosphate (ADP) in aqueous solution yields struvite, MgNH 4 PO 4 . 6H 2 O, and schertelite, Mg(NH 4 ) 2 (HPO 4 ) 2 . 4H 2 O, as the main reaction products. Both may be regarded as cementing species. Only minor amounts of MgO are consumed during these reactions as it is present in excess. The reaction is exothermic. Sodium tripolyphosphate (STPP) or borax may be added to retard the reaction. The main course of the reactions may be represented thus: MgO + 2NH£ + 2H 2 PO 4 + 3H 2 O = Mg(NH 4 ) 2 (HPO 4 ) 2 . 4H 2 O Mg(NH 4 ) 2 (HPO 4 ) 2 . 4H 2 O + MgO + 7H 2 O = 2MgNH 4 PO 4 . 6H 2 O

[6.2] [6.3]

The reaction will not go to completion unless there is sufficient water. Mortars of this system are prepared by blending ignited magnesium oxide, ADP and STPP with a filler, normally quartz sand. On mixing with water a cementitious mass is formed. The reaction has been studied by a number of workers: Kato et al (1976), Takeda et al. (1979), Neiman & 224

Magnesium phosphate cements Sarma (1980), Abdelrazig et al (1984), Popovics, Rajendran & Penko (1987), Abdelrazig, Sharp & El-Jazairi (1988, 1989). The systems used differed in the proportions of MgO: ADP: H2O which did affect some aspects of the reaction. Nevertheless there are common features to all compositions and some broad conclusions can be drawn. On mixing, an exothermic reaction takes place with some loss of ammonia. As the reaction proceeds crystalline phases are formed. All workers are in agreement that these are the tetrahydrate schertelite, or the hexahydrate struvite, or a mixture of both. They also agree that schertelite isfirstformed, which is then hydrated further, if water is available, to form struvite. The relative amounts thus depend on the water available for hydration. Sometimes dittmarite, MgNH4PO4. H2O, and stercorite, NaNH4HPO4. 4H2O, are found in minor amounts. The most extensive studies on these materials are those of Abdelrazig and coworkers, who used XRD, differential thermal analysis, thermogravimetry and scanning electron microscopy to characterize the reaction products. The reaction has been described by Abdelrazig et al. (1984), with subsequent revision (Abdelrazig, Sharp & El-Jazairi, 1988, 1989). On adding water to ADP a solution is immediately formed that contains ammonium and various complex phosphate ions. This solution reacts with the suspended magnesium oxide particles. Hydration products are formed probably by a through-solution mechanism, an idea originally advanced by Neiman & Sarma (1980), rather than by a topochemical reaction. Initially, when the concentration of phosphate is high, the tetrahydrate schertelite (MgO:PO4 = 1:2) is formed. Schertelite can be seen as a reaction intermediate which reacts further with MgO. As reaction proceeds, the phosphate content of the solution drops and the formation of struvite or dittmarite (MgO:PO4 = 1:1) is favoured. The reaction can be frozen by lack of either water or phosphate. If there is an abundance of water then the formation of the hexahydrate struvite is favoured. If, however, water is lacking then some monohydrate dittmarite is formed, and moreover the reaction does not proceed. Schertelite remains in the set material and some ADP remains undissolved. Thus, the reaction is not one of progressive hydration from monohydrate to tetrahydrate to hexahydrate. The presence of colloidal species has been a subject of some debate. Neiman & Sarma (1980) found with their material that crystallinity did not develop for at least the first two hours of setting. However, Abdelrazig et al (1984) and Abdelrazig, Sharp & El-Jazairi (1988, 1989) reported that 225

Phosphate bonded cements abundant amounts of crystallites are formed in only a few minutes. These differences probably relate to the use of different molar ratios of reactants, although this cannot be confirmed because Neiman & Sarma did not give the molar ratios that they used.

(3)

(c)

100 ym

(b)

1 0 0 ]im

1 0 pm

(d)

100

For legend see opposite

226

Magnesium phosphate cements Abdelrazig, Sharp & El-Jazairi (1988, 1989) prepared a series of mortars based on a powder blend of MgO and ADP with a quartz sand filler. They were hydrated by mixing with water. A mortar I (MgO: ADP: silica: water = 17-1:12-9:70-0:12-5), with a water/solid ratio of 1:8, formed a workable paste which set in 7 minutes with evolution of ammonia. The main hydration product, struvite, was formed in appreciable amounts within 5 minutes and continued to increase. Schertelite also appeared, but only in minor amounts, within the first 5 minutes and persisted only during thefirsthour of the reaction. Dittmarite appeared in minor amounts after 15 minutes, and persisted. Microstructural changes were observed during hydration. After the first hour the microstructure appeared to be predominantly crystalline, although the crystallinity was poor. The crystallites appeared to grow between silica grains (Figure 6.6a). After seven hours the crystallinity became well defined (Figure 6.6b); the platy and tabular morphologies observed were tentatively assigned to dittmarite and struvite respectively (Figure 6.6c). After 24 hours there was an abundance of struvite crystallites with a rod-like appearance (Figure 6.6d,e). The compressive strength of the mortar reached 14-9 MPa in 24 hours.

V (e)

10 \im

Figure 6.6 Scanning electron micrographs of magnesium ammonium phosphate mortar I (Abdelrazig, Sharp & El-Jazairi, 1989) after hydration at 22 °C: (a) after 1 hour, low magnification, (b) after 7 hours, low magnification, (c) after 7 hours, high magnification showing platy and tabular morphologies, (d) after 24 hours, low magnification, (e) after 24 hours, high magnification showing rod-like crystallites of struvite. 227

Phosphate bonded cements Total pore (3-75 nm to 7-5 (am) volume was 73-1 mm3 g"1 and coarse pore (1 |im to 7-5 |im) volume was 60-1 mm3 g"1. The addition of STPP (1-7%) acted as a retarder and increased compressive strength (mortar II). Less heat and ammonia were evolved and the cement set more slowly in 10 minutes. The paste hardened in 30 to 60 minutes. Traces of ADP persisted for 30 minutes but no STPP was detected in the reaction products. Struvite, the main hydration product, schertelite and dittmarite all appeared within 5 minutes. Struvite continued to increase in amount as the cement aged; schertelite disappeared after 3 hours and dittmarite after a week. Stercorite was found only during the first 7 hours. The presence of STPP affected the morphology of the cement. After 24 hours struvite with ellipsoidal morphology was observed (Figure 6.1a). Later (7 and 28 days) there was a morphological change with the formation of well-crystalline hexagonal plates (Figure 6.1b). This morphology is quite different from that of mortar I. The addition of STPP improved the compressive strength of the mortar which reached 19-5 MPa in 24 hours. The total pore volume was reduced to 70-4 mm3 g"1 and the coarse pore volume to 55-4 mm3 g"1.

•J 10

10 ym

Figure 6.7 Scanning electron micrographs of magnesium ammonium phosphate mortar II (Abdelrazig, Sharp & El-Jazairi, 1989): (a) after 24 hours, high magnification showing struvite of ellipsoidal morphology, (b) after 28 days, high magnification showing wellcrystalline hexagonal plates of struvite.

228

Magnesium phosphate cements

When a mortar was prepared with insufficient water (mortar III, water: solid = 1:16) some ADP remained even after 3 weeks and the mortar took longer to set (12 minutes). Schertelite was the major reaction product for the first hour; thereafter struvite became the major product, but schertelite remained a significant constituent. The microstructure differed from that of the other two mortars. After one hour there was a dense crystalline microstructure with needle-like and cuboid crystallites growing between silica grains (Figure 6.&a,b). These crystallites were tentatively assigned to struvite and schertelite respectively. These features did not change with time. The compressive strength of the mortar was 27-4 MPa, the total pore volume was reduced considerably to 20-6 mm3 g"1 and the coarse pore volume to 5-3 mm3 g"1. This incompletely hydrated mortar was stronger than the other two fully hydrated mortars. In this, these materials obey the general rule that strength of a cement is increased as the water content is reduced, irrespective of the exact microstructure of the matrix. Abdelrazig et al. (1984) studied the commercial FEB SET-45 cements and mortars (Set Products Inc., Master Builders Division, Martin Marietta Corporation). Their hydration behaviour is similar to those described above. The mortars normally set in 15 minutes and hardened in 30 to 60

100 \m

10 ym

Figure 6.8 Scanning electron micrographs of magnesium ammonium phosphate mortar III (Abdelrazig, Sharp & El-Jazairi, 1989): (a) after 1 hour, low magnification, (b) after 1 hour, high magnification showing needle-like and cuboid crystallites.

229

Phosphate bonded cements

minutes. The compressive strength of an FEB SET-45 mortar (water: solid = 1:16) increases with time. Compressive strength reached 24 MPa in the first hour and 47 MPa after 24 hours. Thereafter, compressive strength increased more slowly to 54 MPa after one week and 56 MPa after one

4U Figure 6.9 The morphology of a commercial mortar, showing well-developed needle-like crystallites. Micrograph span (a) 75 \im, (b) 30 (im (Abdelrazig et aL, 1984).

230

Magnesium phosphate cements month. Strength depends on the amount of water in the system, and compressive strength was drastically reduced when the water/ADP ratio was increased to 1:8. Microstructure is also affected by water content. The 1:16 mortar contained hexagonal plates, while the 1:8 mortar had an ellipsoidal morphology. The 1:5 mortar when prepared at — 5 °C developed striking, well-formed, needle-like crystals (Figure 6.9). The action of heat on these cements is complex (Abdelrazig & Sharp, 1988). The principal sequence based on XRD and thermal analysis is shown in Figure 6.10. 6.4.7

Cement formation with diammonium hydrogen phosphate

Sugama & Kukacka (1983a) described cements based on magnesium oxide and a 40 % solution of diammonium hydrogen phosphate (DAP) liquid. The powder was a fine magnesium oxide that had been calcined above 1500 °C and had a surface area ofc. 1 m2 g"1. These cements set within 3 minutes and developed an early strength of 5-7 MPa after 30 minutes and 19-3 MPa in 15 hours. Sugama & Kukacka, using XRD, considered that Mg3(PO4)2. 4H2O was the main reaction product. They also reported the presence of other hydrates, Mg(OH)2 and struvite. However, Abdelrazig & Sharp (1985) discounted the presence of Mg3(PO4)2. 4H2O and Mg(OH)2 in this system, and we are inclined to agree that it is unlikely that such Mg(NH 4 ) 2 (HP0 4 ) 2 .4H 2 0

MgNH4PO4.6H2O

MgNH4PO4.H2O

l

MgNH 4 PO 4 .H 2 O (amorphous phase) Temperajture increasing

\

[6.4] [6.5] [6.6]

MgHPO 4 (amorphous phase)

[6.7] Mg 21P 2+OMgO 7

Mg3(PO4)2

[6.8]

Figure 6.10 Action of heat on ammonium magnesium phosphate cements (Abdelrazig & Sharp, 1988).

231

Phosphate bonded cements

species are formed in the presence of ammonium ions. Most likely, the reaction products are the hydrates of magnesium ammonium phosphate: schertelite, struvite and dittmarite. 6.4.8

Cement formation with ammonium polyphosphate

Sugama & Kukacka (1983b) described cements based on magnesium oxide and a 56 % aqueous solution of ammonium polyphosphate (APP). The powder was a fine magnesium oxide that had been calcined above 1300 °C and had a surface area of 1 to 5 m2 g"1. The reaction was strongly exothermic; the cements set within 3 minutes and developed an early strength of 13*8 MPa after 1 hour and over 20 MPa after 5 hours. Micrographs taken after 30 minutes reaction showed the matrix to be amorphous (Figure 6.11). After 1-5 hours, crystallites were observed. XRD analysis showed that the most abundant phase was struvite. Sugama & Kukacka also considered that Mg3(PO4)2. 4H2O was another major reaction product and that other hydrates, Mg(OH)2 and newberyite, MgHPO4.3H2O, were also present. Again, Abdelrazig & Sharp (1985)

Figure 6.11 Scanning electron micrographs showing the microstructure of a cement formed from magnesium oxide and ammonium hydrogenphosphate solutions (Sugama & Kukacka, 1983b). 232

Magnesium phosphate cements argued that Mg3(PO4)2. 4H2O, MgHPO4. 3H2O and Mg(OH)2 were unlikely to be present in this system. Thus, it would appear that the reaction products of these cements are struvite and other phases yet to be identified. During the course of the reaction it would appear that P-O-P bridges hydrolyse into orthophosphates. 6.4.9

Cement formation with aluminium acid phosphate

Ando, Shinada & Hiraoka (1974) examined cements formed by the reaction between magnesium oxide and concentrated aqueous solutions of aluminium dihydrogen phosphate. Later, Finch & Sharp (1989) made a detailed examination of the cement-forming reaction and reported that the reaction yielded cements of moderate strength. They considered that cement formation was the result of an acid-base reaction leading to the formation of hydrates by a through-solution mechanism, by nucleation and precipitation from porefluids.Two phases were found in the matrix, one amorphous and the other crystalline. The crystalline phase was newberyite. Finch & Sharp concluded that the amorphous phase was a hydrated form of aluminium orthophosphate, A1PO4, which almost certainly contained magnesium. They ruled out a pure A1PO4.«H2O, for they considered that the reaction could not be represented by the equation 2MgO + A1(H2PO4)3 A 2MgHPO4. 3H2O + A1PO4. «H2O

[6.9]

because cement formation was poor when the MgO/Al(H2PO4)3 mole ratio was 2:1. It followed that the amorphous phase must be a composition within the Al2O3-MgO-P2O5-H2O system. This conclusion was in line with results of an energy-dispersive X-ray microanalysis of the fracture surface using a scanning electron microscope. As we have seen in Section 6.2, there is some evidence for supposing that zinc phosphate cements contain an amorphous aluminium phosphate or zinc aluminophosphate phase. Also, as we shall see in Section 6.5, amorphous aluminium phosphate is the binding matrix of dental silicate cement. Scanning electron micrographs of fracture surfaces revealed the presence of both amorphous and crystalline phases which corresponded to results from XRD analysis (Figure 6.12). What is of interest is that the crystalline phase is MgHPO4. 3H2O and not Mg(H2PO4)2. 2H2O or 233

Phosphate bonded cements Mg(H2PO4)2. 4H2O as was the case in the reaction between MgO and simple phosphoric acid solutions. Inspection of the diagrams of Belopolsky, Shpunt & Shulgina (1950) shows that newberyite is the stable phase at lower concentrations of phosphate. Presumably, in the present case, aluminium locks up some phosphate and so reduces the phosphate available for the magnesium phosphate phase. Finch & Sharp (1989) found the mole ratio of MgO to A1(H2PO4)3 to be an important parameter that affected both the reaction rate and the nature of the reaction products. The critical mole ratio was 2:1. When the ratio was less than 2:1 cements were not formed at all, and when it was exactly 2:1 the paste set slowly and always remained tacky. Further increases in the ratio caused cements to set faster with greater evolution of heat. Finch & Sharp (1989) also found that this ratio affected the proportion of crystalline phase to amorphous phase in the cement matrix. The proportion of newberyite in the matrix reached a maximum when the MgO/A1(H2PO4)3 ratio was 4:1 and decreased to a low level when the ratio was 8:1. When Finch & Sharp (1989) used solutions of lower water content they found an unknown XRD pattern that was distinct from that of MgHPO4.3H2O. This unidentified phase they dubbed hayesite and speculated that it might be a lower hydrate, either MgHPO4.2H2O or MgHPO4.H2O. Infrared spectroscopy showed that hayesite was less well

Figure 6.12 Microstructure of MgO-aluminium hydrogenphosphate cement (Finch & Sharp, 1989). 234

Dental silicate cement

crystalline than newberyite, but did contain bands at c. 3400 cm"1 and 1640 cm"1, showing it to be hydrated. A higher hydrate, phosphorresslerite, MgHPO4. 7H2O, was formed when these cements were exposed to water and dried in air. On ageing, this hydrate readily transformed back to newberyite. 6.4.10 Cements formed from magnesium titanates Cements have been prepared from magnesium titanates (Mg2TiO4, MgTiO3 and Mg2Ti2O5) and phosphoric acid (Sychev et ai, 1982; Sudakas, Turkina & Chernikova, 1982). The reaction product, MgHPO4. 3H2O, is crystalline when Mg2TiO4 is used and amorphous with the other magnesium titanates. The amorphous product gives the stronger cements. 6.5

Dental silicate cement

6.5.1

Historical

Dental silicate cement was once the most favoured of all anterior (front) tooth filling materials. Indeed, it was the only material available for the important task of aesthetic restoration from the early 1900s to the mid 1950s, when the not very successful simple acrylic resins made their appearance (Phillips, 1975). In the mid sixties there were some 40 brands available (Wilson, 1969) and Wilson et al. (1972) examined some 17 of these. Since that time the use of the cement has declined sharply. It is rarely used and today only two or three major brands are on the market. The reason for this dramatic decline after some 50 years of dominance is closely linked with the coming of modern aesthetic materials: the composite resin from the mid 1960s onwards (Bowen, 1962), and the glass-ionomer cement (Wilson & Kent, 1971) from the mid 1970s. Dental silicate cement was also variously known in the past as a translucent, porcelain or vitreous cement. The present name is to some extent a misnomer, probably attached to the cement in the mistaken belief that it was a silicate cement, whereas we now know that it is a phosphatebonded cement. It is formed by mixing an aluminosilicate glass with an aqueous solution of orthophosphoric acid. After preparation the cement paste sets within a few minutes in the mouth. It is, perhaps, the strongest of the purely inorganic cements when prepared by conventional methods, with a compressive strength that can reach 300 MPa after 24 hours (Wilson et al., 1972). 235

Phosphate bonded cements The early history of the cement is obscure. Dreschfeld (1907) and Sanderson (1908) attributed its invention to Fletcher. Fletcher (1878,1879) certainly described cements formed from concentrated orthophosphoric acid solutions and sintered mixtures of oxides which included SiO2, A12O3, CaO and ZnO. One was reported by Fletcher (1879) as being slightly translucent. These cements were not successful in clinical use. Voelker (1916a) reported three early dental silicate cements which appeared in 1895, 1897 and 1902; all proved inadequate. The first successful material was developed by Steenbock (1903,1904) who explicitly sought and formulated a translucent cement (Voelker, 1916a,b). It was marketed by Ascher in 1904 as New Enamel Richters; Harvadid cement followed in the same year. Thereafter development was rapid and eight varieties were reported by Morgenstern in 1905. However, from their chemical composition we doubt whether they were sufficiently translucent. The liquids used in these early formulations were 50% solutions of orthophosphoric acid, often containing aluminium and zinc. Chemical analyses were published between 1904 and 1972 (Voelker, 1916a; Greve, 1913; Watts, 1915; Paffenbarger, Schoonover & Souder, 1938; Axelsson, 1964; W i l s o n s al, 1972). The glasses used by Steenbock in his original compositions were mixtures of calcium aluminosilicates and beryllium silicates; but, as Dreschfeld reported in 1907, subsequent developments moved away from the use of beryllium compounds. Published chemical analyses in the period to 1916 (Voelker, 1916a; Greve, 1913; Watts, 1915) confirmed Dreschfeld's statement. In the following we shall refer to these as oxide glasses. In 1908 Schoenbeck made the most important of all compositional innovations when he introduced the use of fluoride-fluxed glasses into dental silicate cement. In this discussion we shall refer to these as fluoride glasses, although they are, in fact, mixed fluoride and oxide glasses. Over the years fluoride glasses have progressively replaced the purely oxide ones. Although materials based on fluoride-containing aluminosilicate glasses were rare before 1920 (Watts, 1915; Greve, 1913), and Wright (1919) ignored them in his studies, by 1938, Paffenbarger, Schoonover & Souder (1938) reported that most dental silicate cements were of the Schoenbeck type. This development is not surprising because, besides lowering the temperature of fusion, fluoride confers greater translucency and strength on the cement and has beneficial therapeutic effects. There have been no major compositional innovations since. Dental silicate cement is used exclusively for the aesthetic restoration of 236

Dental silicate cement

anterior (front) teeth. In this situation, unlike cements used for cementation, the cement is fully exposed to erosive attack by oral fluids. Thus, considerable interest was shown in the physical, chemical and biological properties of these cements in the years following their introduction. Early workers in the field were Morgenstern (1905), Kulka (1907), Rawitzer (1908, 1909), Proell (1913) and Poetschke (1916). These workers reported a number of faults, finding the cements to be porous, prone to staining, attacked by mouth acids, shrinking under drying conditions and lacking in adhesion. These observations have been confirmed during the course of time. In certain mouths, dental silicate cement stains, erodes and even washes away. For this reason it has never been fully satisfactory and, consequently, now that alternatives have become available, it has fallen out of general use. The first period of development ended with the research of Wright (1919) who published the results of an extensive survey of cements prepared from experimental SiO2-Al2O3-CaO glasses and orthophosphoric acid solutions containing aluminium phosphate. By this time the main cement formulations had been established. Between 1919 and 1950 only minor improvements were attempted; these were of a technological nature and unsuccessful. After 1950 some serious attempts were made to improve dental silicate cement. Notable were those of Manly et al. (1951) and Rockett (1968); also, Pendry reported an acid-resistant cement containing indium (Pendry & Cook, 1972; Pendry, 1973). None of these experimental materials went into production. These attempts came too late in view of progress made in other directions, and the picture of dental silicate cement remains one of an essentially traditional material which has changed little since the original developments made mainly by German chemists prior to 1914. The main line of development now lies with its successor, the glass-ionomer cement, which uses a similar glass, but in which phosphoric acid is replaced by poly(acrylic acid); this cement is more resistant to acid erosion and staining and has the great advantage of adhesion to tooth material. 6.5.2

Glasses

The powders used in dental silicate cement are unusual in being ground opal glasses rather than crushed crystalline clinker. The glassy nature of the powder gives the set cement the unusual property of translucency 237

Phosphate bonded cements which it shares with one other cement, its successor, the glass-ionomer cement. These glasses are calcium aluminosilicates which, in materials available since the 1940s, always contain fluoride. Essentially they are based on the SiO2-Al2O3-CaF2 system. The aluminosilicate glass has a dual role: it acts as afillerand is also the source of ions required to gel phosphoric acid solutions. These glasses act as a source of ions because they are decomposed by acids. This property is dependent on the Al/Si ratio being sufficiently high, approaching 1:1. The reasons and criteria for the decomposition of aluminosilicates are examined fully in Section 5.9.2. The glasses are similar to those used in glass-ionomer cements but their reactivity towards acids has to be less, as orthophosphoric acid is a stronger acid than the poly(alkenoic acid)s. The consequence is that the Al/Si ratio, which determines reactivity, is lower than in the glass-ionomer cement glasses. Typical glass compositions are given in Table 6.6. The preparation of these glasses is given in Section 5.9.2. The essential components are ground silica (quartz), alumina and fluorite (which also acts as a flux). Cryolite is added as an additional flux and minor amounts of aluminium phosphate are present, which apparently improve the mixing qualities of the cement. The ratio of alumina to silica controls the setting time of the cement. Fluoride tends to slow setting while aluminium phosphate improves the mixing of the paste. The temperature of fusion is 1050 to 1300 °C, a somewhat lower range than that for the glass-ionomer cement glasses. The melts are shock-cooled and ground. The median particle size ranges from 8-6 to 11-5 |im. The glasses are slightly opal in appearance and evidence from electron microscopy shows that this arises from phase separation (Wilson et al., 1972). The separated phase appears as droplets although it may be spinodal. An example of one glass (Super Syntrex) showed the segregation of larger droplets of uniform size c. 400 nm in diameter and smaller droplets of 20 to 30 nm (Figure 6.13). Etching with phosphoric acid showed that the droplets were selectively attacked by acids. Effect of glass composition on cement properties As we have indicated previously, two types of glass have been used in dental silicate cements: the obsolete oxide glass and the modern fluoride glass. Only four studies on glass composition and its relationship to cement property have been published (Wright, 1919; Crepaz, 1951; Manly et al., 238

Dental silicate cement Table 6.6. Chemical composition of commercial dental silicate cements {Wilson et al., 1972) Powders

Liquids

Species

% by mass

Species

% by mass

SiO2 A12O3 CaO Na 2 O F

31-5-41-6 (35-9) 27-2-29-1 (290) 7-7-9-0(6-1) 7-7-11-2(14-5) 13-3-22-0(15-2) 3-0-5-3 (4-4) 0-1-2-9 (0-3) 0-0-0-1 (00) 0-0-0-2 (00) 1-6-2-2

H 3 PO 4 Al Zn Mg Be

48-8-55-5 (65-9) 1-6-2-5 (00) 4-2-9-1 (0-0) 0(1-6) 0 (0-65)

PA

ZnO MgO SrO H2O

Based on four typical examples and one atypical example. Composition of the atypical example is in parentheses.

o • O: Figure 6.13 Electron micrograph of a single-stage replica of a dental silicate cement glass, showing phase-separated droplets rich in calcium and fluoride: large droplets 400 nm in diameter and small droplets 20 to 30 nm in diameter (Wilson et al., 1972).

239

Phosphate bonded cements 1951; Rockett, 1968) and these are concerned almost entirely with nonfluoride glasses of the SiO2-Al2O3-CaO and SiO2-Al2O3-CaO-P2O5 types. In their studies on SiO2-Al2O3-CaO glasses Manly et al. (1951) found that the SiO2/CaO ratio controlled the setting rate of the cement pastes. If this ratio was greater than 2-08 by mass then the glass powder-liquid paste did not set; when it was less than 1-74 pastes set rapidly to form cements. Between these extremes there were narrow compositional bands corresponding to slow-setting cements (ratio = 1-95) and moderate-setting cements (ratio = 1-77-1-89). Since the 1940s all commercial dental silicate cements have used fluoride glasses. Fluoride glasses yield more translucent cements than oxide glasses because they have lower refractive indices. It is also probable, by analogy with glass-ionomer cements, that they yield stronger cements (Section 5.9.4). Unfortunately, in the four studies cited above, the workers were concerned almost entirely with non-fluoride glasses and made no systematic studies on fluoride glasses. Only Manly et al. (1951) made even a cursory examination of these glasses, which are the basis of practical cements. Consequently, none of these workers were able to improve on or even equal the performance of the commercial examples. Nor were they able to shed much light on fundamental compositional factors controlling the setting of cements based on fluoride glasses. It was left to Kent & Wilson (1968), in unpublished observations, to discover that in glasses based on SiO2-Al2O3-CaF2 compositions the Al/Si ratio controlled the rate at which the cement paste set. These observations laid the foundation for the development of the glass-ionomer cement, during which most of the work onfluorideglasses was done. This topic is covered in detail in Section 5.9.2. The degree of subdivision of the powder has considerable effect on the properties of cements (Swanson, 1936; Charbeneau, 1961; Kent & Wilson, 1971). Kent & Wilson (1971) showed that the cements prepared from a fine-grain powder when mixed at the same powder/liquid ratio as normalgrain powder had greater strength, but set too fast for practical use and lacked translucency (Table 6.7). Moreover, cements prepared from normal-grain powder could be mixed to a higher powder/liquid ratio, resulting in a notable increase in strength combined with an optimum setting time.

240

Dental silicate cement Table 6.7. Effect of particle size of powder on cement properties (Kent & Wilson, 1971) Powder Fine

Normal

Normal

Surface area, um *

1-62

0-87

0-87

Powder: liquid ratio, g cm"3

3125

3125

Setting time (37 °C), minutes

2-5

90

400 4-0

Wet compressive" strength (24 h), MPa Wet tensile" strength (24 h), MPa a

289 14-7

176 6-2

217 13-6

After storage for 24 hours in water at 37 °C.

6.5.3

Liquid

Dental silicate cement liquids are concentrated aqueous solutions of orthophosphoric acid generally containing aluminium and zinc (Wilson, Kent & Batchelor, 1968; Kent, Lewis & Wilson, 1971a,b; Wilson et al, 1972). The optimum orthophosphoric acid concentration is 48 to 55 % by mass (Wilson et al., 1970a), although higher concentrations are encountered. Aluminium is present as phosphate complexes and zinc as a simple ion (see Section 6.1.2). Examples are given in Table 6.6. Effect of liquid composition on cement properties

The properties of dental silicate cements are affected both by the concentration of phosphoric acid and by the presence of metal salts. The effect of the concentration of orthophosphoric acid on cement properties has long been known (Poetschke, 1916; Ray, 1934; Worner & Docking, 1958; Wilson et al., 1970a). The setting time of a cement paste increases as the orthophosphoric acid concentration increases; this effect is particularly marked above 65% H3PO4 (Figure 6.14). There are several reasons for this sharp change. Water is required to act as a reaction medium and also to hydrate reaction products; a deficiency could therefore retard or even arrest the reaction. There is also the possibility of a change in structure of the orthophosphoric acid solution as its concentration increases (see Section 6.1.1). 241

Phosphate bonded cements

Cement strength and resistance to aqueous attack are also critically dependent on phosphoric acid concentration, and there is an optimum concentration for the development of maximum strength and resistance to aqueous attack (Wilson et ai, 1970a; Wilson, Paddon & Crisp, 1979). The effect is particularly critical when the phosphoric acid liquid contains aluminium and zinc. This sensitivity to water has practical implications. A cement liquid is stable in an atmosphere of 70 % relative humidity. It will gain water in more humid atmospheres and lose it to drier ones, and this will adversely affect cement properties (Paffenbarger, Schoonover & Souder, 1938; Worner & Docking, 1958; Wilson et al., 1970a). All commercial examples of phosphoric acid solutions used in these cements contain metal ions, whose role has been discussed in Section 6.1.2. In the case of the dental silicate cement, aluminium and zinc are the metals added to liquids of normal commercial cements and have a significant effect on cement properties (Table 6.8) (Wilson, Kent & Batchelor, 1968; Kent, Lewis & Wilson, 1971a,b). Aluminium accelerates setting for it forms phosphate complexes and is the principal cation of the phosphatic matrix. Zinc retards setting for it serves to neutralize the acidic liquid - it 3OO

4O

5O 6O 7O liquid composition °/ow/w H3PC

8O

Figure 6.14 Effect of liquid composition on the setting time and strength of dental silicate cements (Wilson et al, 1970a).

242

Dental silicate cement Table 6.8. Effect of metals contained in the phosphoric acid liquid on cement phosphate properties (Wilson, Kent & Batchelor, 1968) Al, %

Zn, %

Powder: liquid, g cm"3

Setting time, minutes

Compressive strength (wet, 24 hours), MPa

00 00 2-5

00 130 00 6-5

3-9 40 3-9 40

3-5 4-5 3-0 3-5

169 267 269 291

1-25

forms simple salts - and thus reduces the rate of extraction of cations from the aluminosilicate powder. When both metals are present these opposing effects tend to cancel out. Both metals, alone or in combination in the liquid, serve to improve the strength of cements, a combination being most effective.

6.5.4

Cement-forming

reaction

The setting reaction of dental silicate cement was not understood until 1970. An early opinion, that of Steenbock (quoted by Voelker, 1916a,b), was that setting was due to the formation of calcium and aluminium phosphates. Later, Ray (1934) attributed setting to the gelation of silicic acid, and this became the received opinion (Skinner & Phillips, 1960). Wilson & Batchelor (1968) disagreed and concluded from a study of the acid solubility that the dental silicate cement matrix could not be composed of silica gel but instead could be a silico-phosphate gel. However, infrared spectroscopy failed to detect the presence of P-O-Si and P-O-P bonds (Wilson & Mesley, 1968). The nature of the setting reaction was finally elucidated by Wilson et al. (1970a), who established that formation of an aluminium phosphate gel was responsible; although siliceous gel was also formed it merely coated the partly reacted glass particles. The following account is based mainly on the studies of Wilson and coworkers, with some re-interpretation of experimental data. The composition of the cement used is given in Table 6.9. In brief, the reaction takes place in several overlapping stages: extraction of ions from the glass, migration of cations into the aqueous phase, precipitation of insoluble salts as pH increases, leading to formation of an aluminium phosphate gel. 243

Phosphate bonded cements Table 6.9. Chemical composition of the dental silicate cement used in reaction and structural studies Powders

Liquids

Species

% by mass

Species

% by mass

SiO2 A12O3 CaO Na 2 O F P2O5 ZnO H2O Less O for 2F

41-6 28-2 8-8 7-7 13-3 3-3 0-3 2-2 -5-6

H 3 PO 4 Al Zn

48-8 1-6 61

Total

99-8

In the subsequent hardening phase, precipitation and hydration continue. The set cement consists, essentially, of partly-reacted glass particles embedded in an aluminium phosphate gel. The morphology of the filler particles is one where a glass core is sheathed by silica gel. The cement-forming reactions may be described as follows. On mixing the powder and liquid, hydrogen ions from the phosphoric acid solution attack the glass particles, which are decomposed to silicic acid (Wilson & Batchelor, 1967a; Wilson & Mesley, 1968). Al 3+ , Ca 2+ , Na + , and F~ ions are released (Wilson & Kent, 1970a), the pH of the aqueous phase increases (Kent & Wilson, 1969) and, as infrared spectroscopy shows, H 3 PO 4 ionizes to H 2 PO 4 (Wilson & Mesley, 1968). An electrical imbalance results and under the influence of an electrostatic field cations migrate into the aqueous phase where they accumulate. Most probably, aluminium and fluoride form cationic A1F2+ and A1F2 complexes, whose existence has been reported by Connick & Poulsen (1957), O'Reilly (1960) and Yamazaki & Takeuchi (1967). The most important cation is aluminium. In the absence of fluoride, aluminium is present in solution as the Al(H2O)g+ complex which hydrolyses to form complex multinuclear species such as [A12(OH)2]4+ and [A113O4(OH)24(H2O)12]7+ (Aveston, 1965; Waters & Henty, 1977). There are also two kinds of phosphate complexes (Akitt, Greenwood & Lester, 1971; O'Neill et al, 1982): those based on the H 3 PO 4 ligand, A1(H3PO4)3+ 244

Dental silicate cement and complexes of unknown protonation, A1(H3PO4)W, where n ^ 2; and those where the ligand is H2PO4, A1(H2PO4)2+ and A1(H2PO4)J. As pH increases and the ionization of H3PO4 to H2PO4 continues (Kent & Wilson, 1969), the formation of those aluminophosphate complexes based on the H2PO4 ligand is favoured. Further increases in the concentration of aluminium and H2PO4 give rise to the formation of binuclear complexes in which aluminium and phosphate are linked by oxygen bridges (Akitt, Greenwood & Lester, 1971; O'Neill et ai, 1982). Most probably, this process continues with the formation of multinuclear complexes and networks based on Al-O-P linkages, which leads to gelation. The presence of fluoride complicates this picture but does not change its essence. Using 31P, 19F and XH NMR Akitt, Greenwood & Lester (1971) found numerous complexes in such solutions. There were 19F NMR peaks associated with A1F2+ and A1F2 complexes and, if the fluoride content was high, a peak corresponding to exchanging AlFjf"w)+, F", HF and HF2. In addition there were peaks corresponding to fluorine-containing aluminophosphate complexes which were similar to the aluminophosphate complexes noted above but with the addition of one, two or three fluoride ligands. The insolubilization of ions during setting and hardening was followed experimentally by Wilson & Kent (1970b) and is shown in Figure 6.15. This figure shows the time-dependent variation of the concentration of soluble ions during setting and hardening. There are two competing processes which govern the concentration of ions: extraction from the glass and removal by precipitation. The extractive process is illustrated by the [Na]/time curve, as Na+ is not precipitated during cement formation. Judging by the concentration of soluble sodium ions extraction is half complete during mixing of the paste and over within 10 minutes, when 20 % of the glass powder is decomposed. The progress of precipitation is revealed by the concentration/time curves for zinc and phosphate, since both these species are present initially in solution. There should be maxima for the soluble aluminium, calcium and fluoride which are extracted from the glass, but because of the early onset of precipitation these are not observed. Precipitation is accompanied by an increase in pH; when it reaches 1-8, at which juncture 50% of both zinc and phosphate have been precipitated, the cement paste gels (5 minutes after preparation). Hardening continues after gelation, rapidly reaching 65 % of its final 245

Phosphate bonded cements

value within 30 minutes, and ceasing after about 72 hours. The pH continues to rise, reaching 5-2 after 48 hours. Precipitation of aluminium and calcium ions appears to be complete within an hour but zinc continues to precipitate. Sodium and fluoride ions do not completely precipitate.

- IOO

30 CEMENT AGE (min)

Figure 6.15 The time-dependent variation of the concentration of soluble ions during the setting and hardening of a dental silicate cement (Wilson & Kent, 1970b).

246

Dental silicate cement

Evidence from electrical conductivity experiments (Wilson & Kent, 1968) indicates that, even after hardening is apparently complete, the reaction continues for at least 7 weeks; indeed it is known from the work of Paffenbarger, Schoonover & Souder (1938) that the cement continues to strengthen for at least a year. The correlation of phosphate precipitation with decrease of conductivity (Wilson & Kent, 1968), increase in pH (Kent & Wilson, 1969) and hardness (Wilson et al., 1972) is shown in Figure 6.16. These results demonstrate the relationship between the development of physical properties and the underlying chemical changes, but there are no sharp changes at the gel point. Evidence from infrared spectroscopy (Wilson & Mesley, 1968) and electron probe microanalysis (Kent, Fletcher & Wilson, 1970; Wilson et al., 1972) indicates that the main reaction product is an amorphous aluminophosphate. Also formed in the matrix were fluorite (CaF2) and sodium acid phosphates. The fate of silicic acid is of some interest. Silicic acid polymerizes, by condensation, and finally a silica gel is formed (Wilson & Mesley, 1968). The insolubilization of silicic acid has been observed to parallel closely the precipitation of phosphate (Wilson & Batchelor, 1967b) and is related to an increase of pH within the cement (Kent & Wilson, 1969). A low concentration of silicic acid must remain in the matrix. All this is in accord with the known aqueous chemistry of silica. Orthosilicic acid at concentrations above 100 ppm in solution condenses to a dimer. At higher concentrations cyclic and polymeric species are formed (Her, 1979; Andersson, Dent Glasser & Smith, 1982). These processes are ones where silanol groups condense to form siloxane linkages: —OH + HO—Si^- = -^Si—O—Si^- + H2O According to Vysotskii et al. (1974) gelation is the result of a complex process, and polymerization does not necessarily ensure gelation. The formation of polymeric particles is followed by their growth. These particles are then linked by siloxane bonds to form branched chains. Networks extend throughout the liquid phase, and finally gelation occurs, provided the pH is below 7. Gelation is most rapid for pH = 5 to 6 and minimal for pH = 1-5 to 3 (Her, 1979). The observations of Wilson & Batchelor (1967b) are in accord with this theory. The role of water is important, for it acts as a reaction medium and 247

Phosphate bonded cements 100r

500

50

40

30

20

10

Conductivity 20

40

OOmin 4h

24h

Figure 6.16 The relationship between the development of hardness and the underlying physicochemical process: decrease in phosphate concentration, increase in pH, and decrease in electrical conductivity (Wilson et al., 1972).

248

Dental silicate cement

serves to hydrate reaction products. During setting and hardening, water becomes progressively bound to the matrix and in the fully hardened cement the ratio of water of hydration (non-evaporable water) to loosely bound (evaporable) water reaches 1:1 (Wilson, Paddon & Crisp, 1979). A deficiency of water can inhibit the reaction. Kent & Wilson (1969) noted that storage conditions affected the rate of reaction as measured by the increase in pH over 18 hours at 37 °C. When water was allowed neither to enter nor to leave the cement the pH rose to 5-15; when cured in water the reaction was enhanced and the pH rose to 5-40; but when stored under desiccating conditions the reaction was retarded and the pH only rose to 4-30. A deficiency of water in the cement liquid has the same effect and this occurs when the H3PO4 content exceeds 60%. Wilson & Mesley (1968) noted that in a cement formed from a solution of 65 % H3PO4 there was evidence of incomplete reaction even after 6 hours. We have noted in Section 6.5.3 that there is a sharp decline in the rate of reaction when the orthophosphoric acid concentration exceeds 65% H3PO4 (Figure 6.14). The avidity of cements to absorb water from humid surroundings also increases sharply when the phosphoric acid in the cement-forming liquid exceeds 60%. It is difficult to avoid the conclusion that these two phenomena are related and that a deficiency of water retards the cementforming reaction.

6.5.5

Structure

Information on the microstructure and molecular composition of the set cement comes almost entirely from the optical, electron microscopic and electron probe microscopic analysis of Kent, Fletcher & Wilson (1970), Wilson et al. (1970a, 1972) and Brune & Smith (1982). There are some differences which require resolution. Although under optical microscopy the set cement appears as an irregular mosaic of angular, and seemingly unattacked, glass particles connected by an apparently structureless matrix, its appearance under the reflectance optical microscope is more revealing (Figure 6.17). Here the glass particles are shown to be pitted with hemispherical craters and to vary in size from 1 to 100 urn. The matrix is seen as particulate. A stereoscan of a fractured surface reveals more detail and shows an angular glass particle which has debonded from a particulate matrix. The surface of the glass particle is spotted where selective acid 249

Phosphate bonded cements attack has occurred at the site of phase-separated droplets (Figure 6.18). Apart from this etching the glass particles appear unattacked, but this is not the case. A combination of infrared spectroscopy and electron probe microanalysis shows that the matrix of the set cement is amorphous aluminium phosphate (Wilson et al., 1970a). Silica gel formed appears to sheathe the partly reacted glass particles. Two crystalline phases have been detected in the matrix: fluorite, CaF 2 , and augelite, A12(OH)3PO4 (Wilson et al., 1972). However, the XRD pattern of the matrix did not quite correspond to that of augelite in published powder diffraction data, and the possibility exists that F replaces OH to give A12F3PO4; this would be in accordance with the formation of fluorine-containing aluminophosphate complexes noted previously. The element distributions of Si, P, F, and Na within a dental silicate cement were recorded by Kent, Fletcher & Wilson (1970) and Wilson et al. (1972) who used cements from which fine particles had been removed, and

Figure 6.17 Phase contrast micrograph of a polished section of a dental silicate cement, showing angular glass particles, size 1 to 100 urn, embedded in a featureless matrix (Wilson et al, 1972).

250

Dental silicate cement

by Brune & Smith (1982) who used a normal powder. The element distribution maps for Si, Al and P are shown in Figure 6.19, where white highlights indicate the positive presence of an element. An optical micrograph of the area of study shows the presence of a distinct outer layer around each reacted glass particle (Figure 6.19a). Silicon is present only in the particles and its concentration is enhanced at the boundary (Figure 6.196). This implies the existence of a layer of silica gel surrounding each glass particle. The gel-like nature of the silica sheath can be clearly seen in Figure 6.19a where it has detached from the glass core. Phosphorus is found substantially only in the matrix (Figure 6.19c). Aluminium is found both in the glass particles and in the matrix, showing that it has migrated. A prominent feature is the depletion of Al at the boundary (Figure 6A9d). Calcium, sodium and fluorine were also shown to have migrated from the glass particles to the matrix. These findings support the view that during the reaction ions are extracted from the surface of the glass particles, migrate to the aqueous phase where they form the matrix, and leave a silica gel relict. This explains why the glass particles appear to be unattacked when examined under the microscope. The presence of both Al and P in the cementing matrix and the

•

Figure 6.18 A stereoscan of a fracture surface of a dental silicate cement. The debonded glass particle is to be identified by its pitted surface, the result of selective acid attack. Note the particulate nature of the matrix (Wilson et al., 1972).

251

Phosphate bonded cements knowledge that infrared spectroscopy had identified the presence of an aluminium phosphate led to the conclusion that the matrix was an aluminium phosphate hydrogel (50 % of the water is bound to the matrix). Some of these conclusions may require revision, since recent findings of Ellison & Warrens (1987) on the related glass-ionomer cement (Section 5.9.6) suggest that acid attack occurs throughout the body of the glass particle and not just at the surface layer. In that case the silica gel layer is not a relict but a zone of gelation. This view is more in accord with ideas on the decomposition of aluminosilicate glasses.

Figure 6.19 Element distribution maps for a scanning electron micrograph of a dental silicate cement: (a) optical micrograph of area of study, (b) Si distribution, (c) P distribution, (d) Al distribution (Wilson et al, 1972).

252

Dental silicate cement A close examination of Figure 6.19&, especially the two closely adjacent particles formed by the splitting of one particle, reveals that the Si regions appear to extend slightly beyond the boundaries of the glass particles. Thus it is possible that the whole of the glass structure is slightly acid-degraded, although remaining essentially glassy, and that silicic acid and ions are released. These species migrate outwards, the ions into the aqueous phase, while the majority of the silicic acid condenses to a shell of silica gel just beyond the particle-matrix boundary. This alternative hypothesis may explain the observational differences between different workers. Brune & Smith (1982), unlike Wilson et al (1972), found Si distributed throughout the cement but were uncertain whether it was due to Si in the matrix or the degradation of fine particles to silica gel. But Brune & Smith (1982) used a normal glass powder while Wilson et al. (1972) removed fine particles to improve resolution. These differing observations are reconciled if the silicic acid which is formed migrates slightly before condensing to silica gel. Summary Overall, the formation of dental silicate cement can be represented in outline as in Figure 6.20. 6.5.6

Physical properties

Dental silicate cement is used for the aesthetic restoration of anterior (front) teeth because it is translucent and so can be made to colour-match tooth enamel. It is prepared by introducing powder into the liquid gradually in order to dissipate heat, although the exotherm is not so great Calcium aluminosilicate glass

+ Phosphoric acid solution

Aluminophosphate complexes

Al phosphate polynuclear complexes-

Calcium fluoride

Sodium acid phosphates (soluble)

Silica gel

Aluminium phosphate gel

Figure 6.20 Formation of dental silicate cement.

253

Phosphate bonded cements Table 6.10. Properties of commercial dental silicate cements

Refs. Powder: liquid*, g cm"3

Value

Specification" limits

[1] [2]

2-70-4-02 3-6

n n

Setting time (37 °C), minutes

[1,2]

3-25-7-0

3-8

Compressivec strength (24 h), MPa

[1,2,3]

68-5-255

166 minimum

Compressivec modulus (24 h), GPa

[4,5]

18-0-21-0

n

Flexuralc strength (24 h), MPa

[2]

24-5

n

Tensilec strength (24 h), MPa

[6]

13-6

n

Fracture toughness0 (24 h), MN m-f

[7]

012-0-30

n

Solubility & disintegration (24 h), %

[1]

0-34-3-8

1-0 maximum

Opacity, C 0 7

[1]

0-42-0-71

0-35-0-55

Working time (23 °C), minutes

a

BS 3365/1: 1969 Specification for Dental Silicate Cement and Dental Silicophosphate Cement. Part 1 Dental Silicate Cement. 6 For a consistency spread of 25 mm diameter for 0-5 cm3 of cement paste under a load of 1-5 Kgf applied after 2 minutes at 23 °C. c After storage for 24 hours in water at 37 °C. n no specification test. [1] Wilson et aL, (1972); [2] 0ilo (1988); [3] Wilson (1975c); [4] Paddon & Wilson (1976); [5] Wilson, Paddon & Crisp (1979); [6] Kent & Wilson (1971); [7] Lloyd & Mitchell (1984).

as that of zinc phosphate cement (Crisp, Jennings & Wilson, 1978). The cement is mixed very thickly and the powder/liquid ratio can be as high as 4 g cm"3 for good examples of this cement (Wilson et aL, 1972). At these thick consistencies the working time is good; an isolated observation indicates that it is 3-6 minutes at 23 °C (0ilo, 1988). Setting time (37 °C) varies from 3-25 to 7-0 minutes (Wilson et aL, 1972; 0ilo, 1988). Properties are summarized in Table 6.10. 254

Dental silicate cement

The dental silicate cements have brittle characteristics which are immediately evident after set (Paddon & Wilson, 1976). The best examples develop a compressive strength of about 250 MPa after 24 hours (Wilson et al., 1972; Wilson, 1975c; 0ilo, 1988), which is higher than that recorded for any other acid-base cement, including the glass polyalkenoate cement (Table 6.10). Strength continues to increase for at least a year (Paffenbarger, Sweeney & Isaacs, 1933). Compressive modulus is 18 to 21 GPa after 24 hours (Paddon & Wilson, 1976; Wilson, Paddon & Crisp, 1979). Modulus changes with time and for one example increased from 18-0 GPa after 24 hours to 30-6 GPa after 30 days. Little information is available on other tests of strength. Isolated measurements give a flexural strength of 24-5 MPa (0ilo, 1988) and a tensile strength of 13-6 MPa (Kent & Wilson, 1971). These values lie within the range of those recorded for glass polyalkenoate cement. Translucency is easily achieved as values for the inverse property of opacity show (Table 6.10). Most of the properties of a dental silicate cement are affected by preparative variables, particularly the powder/liquid ratio (Jorgensen, 1963; Wilson & Batchelor, 1967b). Increase in the powder/liquid ratio accelerates set and increases strength and resistance to erosion (Figure 6.21). Temperature and, to a lesser extent, humidity during mixing have some effect, but chiefly they affect setting time. Although these cements have high compressive strength, their low flexural and tensile strengths coupled with brittleness and lack of toughness makes them suitable only for low-stress anterior (front teeth) restorations. 6.5.7

Dissolution and ion release

The dissolution and ion release from dental silicate cement have been the most investigated characteristics; with good reason, for they are central to its clinical performance. Erosion limits its life but release of fluoride has important clinical consequences. Ion release

In neutral solution when fully hardened, dental silicate cements are resistant to aqueous attack. Before they have fully hardened, set cements contain soluble reaction intermediates - soluble sodium salts, acid phosphates and fluorides - which render them vulnerable to attack even by neutral solutions including saliva (Wilson, 1976). 255

Phosphate bonded cements

Irretrievable loss of matrix-forming cations and anions can result in permanent damage to the cement surface. This is visible as milky or chalky patches or even raised blisters. For this reason it is customary to protect, temporarily, the freshly placed cement by varnish. Once hardened, attack by neutral solutions causes failure only when a cement has been poorly formulated and contains excessive amounts of soluble reaction products. In this case osmotic effects can cause blistering or even disintegration under the action of internal forces, as Figure 6.22 illustrates (Wilson & Batchelor, 1967a). The composition of the leachates does not correspond to the composition of the cement at all (Wilson & Batchelor, 1967a,b). The predominant species eluted are the soluble sodium salts of phosphate and fluorides, although sodium is only a minor constituent of the cement. For one example of cement examined, the leachate contained 0-28 % sodium and 0-20 % phosphate (expressed as a percentage of the amount of the species contained in the cement). For the major constituents of the glass the figures were 0-07 % fluoride, 0-02 % A12O3, 0-01 % SiO2 and 0-003 % CaO.

3.5 4.0 Powder/Liquid Ratio (g/ml)

Figure 6.21 The effect of powder/liquid ratio on setting time and compressive strength of a dental silicate cement (Wilson & Batchelor, 1967b).

256

Dental silicate cement The rate of elution declines sharply with time and the pattern of elution changes. The acid phosphate ions, HgPO^ and HPO|~, are removed by further reaction or by elution, and the release of phosphate changes from predominant to minor. Thereafter, the rate of loss of phosphate is governed by the phosphate concentration of the solution; indeed if the phosphate concentration of the solution is sufficiently high the process is reversed and the cement takes up phosphate. So, clearly, an ion exchange phenomenon is involved (Kuhn & Wilson, 1985; Kuhn, Winter & Tan, 1982). Elution of ions is accompanied by absorption of water and this can amount to as much as 20% by mass in five days (Kuhn et al., 1982). The extent of water uptake is affected by the ionic concentration of the solution. Fluoride release As the cements age, sodium, fluoride and silica become the major species eluted, although the amounts involved are small. Fluoride is released in a sustained fashion over a prolonged period (Wilson & Batchelor, 1967a; de

Figure 6.22 The effect of water on a poor example of a dental silicate cement. An osmotic force causes blistering and disintegration (Wilson & Batchelor, 1967a).

257

Phosphate bonded cements Freitas, 1968; Cranfield, Kuhn & Winter, 1982; Kuhn & Jones, 1982). This is a biologically advantageous property. The release offluorideis governed by the following expression: Rate of release of fluoride = Af~* + B The A term relates to a diffusion-controlled process and the B zero-order term to an erosive process. Kuhn & Jones (1982) examined various models for fluoride release and showed that release did not fit the membrane and homogenous monolith model. Instead, they concluded that the cement behaved as a porous granular monolith, as described by Kydonieus (1980). The release of fluoride appears to be an ion exchange phenomenon, as dental silicate cement takes up rather than releases fluoride from solution if it is present in sufficient concentration (Kuhn, Lesan & Setchell, 1983). Fluoride release is biologically important. Since the early 1940s, fluoride has been known to inhibit dental decay (Dean, Arnold & Elvove, 1942), but the effect is not fully understood and several mechanisms have been suggested (Levine, 1976). Tooth enamel, dentine and bone all possess a mineral phase that has a hydroxyapatite-like substance which takes up fluoride by replacement of the hydroxyl group (Hallsworth & Weatherall, 1969). In fact fluoride released by dental silicate cement is taken up by adjacent tooth enamel (Halse & Hals, 1976). This apparently increases the resistance of enamel to dissolution (McLundie & Murray, 1972) and, if the acidogenic theory of caries is accepted (Levine, 1976) must have a cariostatic effect. Maldonado, Swartz & Phillips (1978) have found that the solubility of enamel in acid was reduced by 39 % when it was in contact with a dental silicate cement. Moreover, the surface energy of fluorapatite is lower than that of hydroxyapatite (Glanz, 1969) making the adhesion of unwanted cariogenic substance, such as dental plaque, more difficult (Rolla, 1977). For these reasons secondary caries is rarely observed under dental silicate cement restorations (Bock, 1971; Hals, 1975) and in this respect the cement is superior to composite resins and dental amalgams (Updegraff, Change & Joos, 1971). Aluminium ions released from the dental silicate cement are also absorbed by hydroxyapatite and have a similar beneficial effect to that of fluoride (Halse & Hals, 1976; Putt & Kleber, 1985). Thus, the dental silicate cement confers protection against caries (dental decay) on surrounding tooth material. 258

Dental silicate cement Acid erosion

Dental silicate cement has always been regarded as suspect in service and has a variable life-span (Paffenbarger, 1940; Wilson, 1969). It was early suspected that this was associated with a susceptibility to acid attack (Kulka, 1907; Rawitzer, 1909; Voelker, 1916b; Poetschke, 1916), a view which has been confirmed by subsequent in vitro and in vivo work (Norman, Swartz & Phillips, 1957, 1959; Jorgensen, 1963; Wilson & Batchelor, 1968; Norman et al., 1969). Thus, although the cement is stable in neutral media, such as normal saliva, it becomes progressively more eroded under acid conditions (Wilson & Batchelor, 1968). This effect is shown in Figure 6.23. Such conditions occur in stagnation regions of the mouth. In these regions, dental plaque containing streptococci and

Figure 6.23 The effect of pH on the removal of ions from a dental silicate cement (Wilson & Batchelor, 1968).

259

Phosphate bonded cements lactobacilli degrades sugars, including plaque polysaccharides, to lactic acid (Jenkins, 1965) and pH values as low as 4-0 have been recorded (Stephan, 1940; Kleinburg, 1961). In fact preferential breakdown of dental silicate cement has been observed in these regions (Henschel, 1949; McLean & Short, 1969). Tay et al. (1974, 1979) have studied the mechanism of erosion of the dental silicate cement in service,findingthat grooving occurs at the margin between the restoration and the tooth. Erosion exposes the cavity and provides sites for the accumulation of food debris and bacteria which can cause inflammation of the gingiva (Larato, 1971). It also leads to staining of the restoration (Bock, 1971; Kent, Lewis & Wilson, 1973). The extent of acid erosion depends on the nature of the acid; acids with strong complexing function, such as citric acid, are particularly erosive (Wilson & Batchelor, 1968; Stralfors & Eriksson, 1969). These acids are found in citrus drinks. Despite the failing of the dental silicate cement under acid conditions it is more resistant to acid attack than all other dental cements with the notable exception of the glass polyalkenoate cement (Norman, Swartz & Phillips, 1959; Walls, McCabe & Murray, 1985; Beech & Bandyopadhyay, 1983; Kuhn, Setchell & Teo, 1984; Wilson et al., 1986a). These studies have been confirmed by in vivo observations (Norman et al., 1969). A clinical study carried out by Robinson (1971) over many years showed that when carefully prepared and placed, the dental silicate cement was capable of giving good performance. Many of the failures of this material must be attributed to faulty preparation.

6.5.8

Biological aspects

The adverse pathological effects of dental silicate cement have been known since Kulka (1911a,b). Since then many workers have observed that this cement causes significant pulpal inflammation (McComb, 1982). Manley (1936, 1943) reported that major histological changes occurred in the pulp 24 hours after placing a silicate restoration, afindingconfirmed by other workers (Zander, 1946; Brannstrom & Nyborg, 1960; Stanley, Swerdlow & Buonocore, 1967; Qvist, 1975). The silicate cement also inflames the gingiva (gum tissues) (Larato, 1971; Trivedi & Talim, 1973) and demineralizes both dentine and enamel (Grieve, 1974). At one time, irritation of the pulp was entirely attributed to the acidity 260

Dental silicate cement

of the cement which is most marked when it is freshly mixed (Manley, 1936; Harvey, Le Brocq & Rakowski, 1944; Roydhouse, 1961; Svare & Meyer, 1965; Matsui et al., 1967). This theory is supported by observations that tissue reactions become less marked with time (Svare & Meyer, 1965). Moreover, it was observed that phosphoric acid penetrated dentine to a considerable depth (Svare & Meyer, 1965; Swartz et al., 1968). Despite this evidence, some workers have doubted this theory (Roydhouse, 1961; Antonioli, 1969; Johnson et al., 1970). In particular, Brannstrom and coworkers have strongly advocated an alternative theory, that bacterial contamination causes pulpal damage (Brannstrom & Nyborg, 1971; Bergvall & Brannstrom, 1971; Brannstrom & Vojinovic, 1976). They considered that virtually all the damage to pulp under a silicate restoration was caused by bacterial infestation. This idea has been amply confirmed by subsequent observations (Qvist, 1975; Brannstrom & Nyborg, 1971;Mjor, 1977). Watts (1979), while agreeing that bacterial contamination plays an important role in causing irritation to tissues, showed that a silicate cement even under germ-free conditions produced tissue damage. Of course, the acidic dental silicate cement does not possess the antiseptic action of the alkaline cements. Cell culture tests show that dental silicate cement is strongly cytotoxic - that is it severely damages cells - even after set (Spangberg et al., 1973). This effect has been attributed to the hydrogen and fluoride ions present (Helgeland & Leirskar, 1972, 1973; Tyas, 1979). Another biological disadvantage is that dental silicate cement does not bond to tooth material, and harmful substances and bacteria can percolate between it and the tooth, giving rise to secondary caries and pulpal irritation (Going, Massler & Dute, 1960). These effects are magnified when dissolution of the cement occurs. One advantageous biological property possessed by dental silicate cement is the sustained release of fluoride; this has been discussed in Section 6.5.7.

6.5.9

Conclusions

Dental silicate cement is solely used for restoring anterior (front) teeth. It is probably the strongest purely inorganic cement and develops its strength rapidly. Although satisfactory in areas of the mouth washed by saliva it is 261

Phosphate bonded cements not quite up to the demands of more severe oral conditions. Its great advantage is that it acts as a fluoride release agent and protects adjacent tooth enamel. 6.5.10 Modified materials A number of innovations made in the 1920s and 1930s may be noted. Several attempts were made to reduce the dissolution of these cements in oral fluids and their adverse effect on the pulp by inclusion of oils and greases (Simon, 1929, 1932; Eberly, 1934). None have been considered beneficial (Paffenbarger, Schoonover & Souder, 1938), a not surprising result because the inclusion of hydrophobic substances is bound to interfere in the setting of an aqueous cement. Poetschke (1925) patented a dental silicate powder prepared by fusing zinc silicate with calciumfluoride.This is a kind of silicophosphate cement (Section 6.6). Thomsen (1931) attempted to formulate a water-setting dental cement. Heynemann (1931) included lithium salts in the flux and Brill (1935) included them in the liquid. During this period a number of attempts at reinforcing these cements were made. Fillers described include carborundum (Salzmann, 1930), cellulosefibres(Schonbeck & Czapp, 1936) and even diamonds (Salzmann, 1930). None of these innovations found their way into commercial materials (Paffenbarger, Schoonover & Souder, 1938). More recently, Stanicioiu, Chinta & Hartner (1959) attempted to reinforce the cement with glassfibres,but this was not successful. The most serious study on the reinforcement of dental silicate cement was made by J. Aveston (in Wilson et al., 1972). Silicon carbide whiskers, carbon fibres and alumina powder were introduced into the cement mix. Unfortunately, the glass powder/liquid ratio had to be reduced, and the strength gained by reinforcement was thereby lost. It is clear that dental silicate cement cannot be strengthened by fibre or particulate reinforcement. Systematic attempts to formulate improved materials have met with no success (Manly et al., 1951; Rockett, 1968). The last and, in some ways, most promising attempt at improving the dental silicate cement was made by Pendry (Pendry & Cook, 1972; Pendry, 1973) who improved its resistance to acid by adding indium to both powder (5*8 %) and liquid (5-65 %). The cement, however, lacked sufficient translucency, and by this time the glass-ionomer cement had arrived with its advantages of translucency and resistance to staining and acid attack. 262

Silicophosphate cement Table 6.11. Specification properties of commercial luting silicophosphate cements {Anderson & Pajfenbarger, 1962)

Powder: liquid,6 g cm

3

Setting time (37 °C), minutes Film thickness (2 min), urn

Value

Specification limits"

2-00-2-70

n

6-14 42-53

50

Compressivec strength (24 h), MPa

101-171

135 minimum

Solubility & disintegration (24 h), %

0-7-2-3

1-0 maximum

a

BS 3365/2: 1971 Specification for Dental Silicate Cement and Dental Silicophosphate Cement. Part 2. Dental Silicophosphate Cement. b For a consistency spread of 25 mm diameter for 0-5 cm3 of cement paste under a load of 220 gf applied after 2 minutes at 23 °C. c After storage for 24 hours in water at 37 °C. n no specification test.

6.6

Silicophosphate cement

The silicophosphate cement has always been a minor and somewhat obscure material. There have been no investigations into its setting reaction and structure. Its chief uses are as a cementing agent and as a temporary posterior filling material in dentistry. It can be regarded as a hybrid of the dental silicate and zinc phosphate cements since the powder is a physical mixture of an aluminosilicate glass and zinc oxide, the amount of zinc oxide varying from 9-3 to 17*9% (Wilson, Crisp & Lewis, 1982). The silicophosphate cement originated with the dental silicate cement, for there is no doubt that early investigators experimented with mixtures of aluminosilicate glass and zinc oxide (Fletcher, 1878,1879; Eberly, 1928). It appears to have no particular advantages. As is often the case with hybrids, it can combine the worst features as well as the best of the parents, and often properties have intermediate values. Nevertheless, it continues to have a small but persistent usage arising from its one advantage over the 263

Phosphate bonded cements Table 6.12. Specification properties of commercial filling silicophosphate cements {Wilson, 1975c) Value

Specification limits"

3-3-4-1

n

Setting time (37 °C), minutes

3-5-5-75

3-6

Compressivec strength (24 h), MPa

179-209

170 minimum

Solubility & disintegration (24 h), %

0-3-0-7

0-7 maximum

Powder: liquid,6 g cm

3

a

BS 3365/2: 1971 Specification for Dental Silicate Cement and Dental Silicophosphate Cement. Part 2. Dental Silicophosphate Cement. b For a consistency spread of 23 mm diameter for 0-5 cm3 of cement paste under a load of 1-5 Kgf applied after 2 minutes at 23 °C. c After storage for 24 hours in water at 37 °C. n no specification test.

zinc phosphate cement: it releasesfluorideand in certain clinical situations this protection against dental caries is invaluable. The flow properties are not as good as those of zinc phosphate cement (Eames et ai, 1978; Hembree, George & Hembree, 1978) and film thickness is greater (Table 6.11). Moreover, it does not have the translucency of dental silicate cement (Wilson, 1975c). The strength of silicophosphate cement lies between that of dental silicate cement and zinc phosphate cement. Anderson & Paffenbarger (1962) have reported properties of luting cements (Table 6.11) and Wilson (1975c) those of filling materials (Table 6.12). Cameron, Charbeneau, & Craig (1963) have confirmed these results. Housten & Miller (1968) reported the properties for silicophosphate cements used for cementing orthodontic brackets, where the consistency of the mix, and consequently cement properties, lie between those of luting agents andfillingmaterials. Silicophosphate cement acts as an agent for the sustained release of fluoride, although different cements behave very differently (Wilson, Crisp & Lewis, 1982). Silicophosphate cement has a durability in the mouth similar to that of dental silicate cement. It is less resistant to oralfluidsthan glass polyalkenoate cement, but more resistant than all other dental cements, as is shown by both in vivo studies (Norman et aL, 1969; Ritcher & Ueno, 1975; Clark, Phillips & Norman, 1977; Mitchem & Gronas, 1978; 264

References

Osborne et al., 1978) and the laboratory impinging jet method (Beech & Bandyopadhyay, 1983; Wilson et ai, 1986a). It is superior to the zinc phosphate cement for bonding orthodontic bands to teeth (Clark, Phillips & Norman, 1977). It has greater durability and there is less decalcification in adjacent tooth enamel. This latter beneficial effect must arise from the release offluoridewhich is absorbed by the enamel, so protecting it in a clinical situation where caries-producing debris and plaque accumulate. 6.7

Mineral phosphate cements

Semler (1976) reported cements formed by reacting ground wollastonite (CaSiO3) with phosphoric acid solution containing aluminium and zinc. The setting times of these cements varied from 4 to 60 minutes. Best results were obtained using a liquid of composition 69% H3PO4, 6-0% Zn, 2-0 % Al with a selected grade of wollastonite. The cement set in 4 minutes and developed a compressive strength of 73 MPa in 24 hours compared with a value of 20 MPa obtained with a fast-setting Portland cement. The matrix of the wollastonite phosphate cement was observed to contain silica, calcium and phosphate. Serpentinite, Mg6Si4O10(OH)8, phosphate cements have been reported by Ter-Grigorian et al (1982, 1984) and Zenaishvili, Bakradze & Chelidze (1984). The reaction products are MgHPO4.3H2O and Mg(H2PO4)2, which are transformed on heating to Mg2P2O7 and Mg3(PO4). Setting time, strength and resistance to aqueous attack are all affected by the concentration of phosphate used to prepare the cements. These materials should be compared with the magnesium phosphate cements described in Section 6.4. Naturally occurring phosphate cements are also known (Krajewski, 1984). References Abdelrazig, B. E. I. & Sharp, J. H. (1985). A discussion of the papers on magnesia-phosphate cement by T. Sugama and L. E. Kukacka. Cement & Concrete Research, 15, 921-2. Abdelrazig, B. E. I. & Sharp, J. H. (1988). Phase changes on heating ammonium magnesium phosphate hydrates. Thermochimica Acta, 129, 197-215. Abdelrazig, B. E. L, Sharp, J. H. & El-Jazairi, B. (1988). The chemical composition of mortars made from magnesia-phosphate cement. Cement & Concrete Research, 18, 415-25. 265

Phosphate bonded cements Abdelrazig, B. E. L, Sharp, J. H. & El-Jazairi, B. (1989). Microstructure and mechanical properties of mortars made from magnesia-phosphate cement. Cement & Concrete Research, 19, 247-58. Abdelrazig, B. E. I., Sharp, J. H., Siddy, P. A. & El-Jazairi, B. (1984). Chemical reactions in magnesia-phosphate cements. Proceedings of the British Ceramic Society, 35, 141-54. Abraham. (1913). Pulpentod unter Silikatfullungen. Deutsche Monatsschrift fiir Zahnheilkunde, 33, 532-6. Abramovich, A., Macchi, R. L. & Ribas, L. M. (1976). Enamel corrosion produced by zinc phosphate dental cement. Journal of Dental Research, 55, 107-10. Akitt, J. W., Greenwood, N. N. & Lester, G. D. (1971). Nuclear magnetic resonance and Raman studies of aluminium complexes formed in aqueous solutions of aluminium salts containing phosphoric acids and fluoride ions. Journal of the Chemical Society, A, 2450-7. Allen, W. M., Sansom, B. F., Wilson, A. D., Prosser, H. J. & Groffman, D. M. (1984). Release cements. British Patent GB 2, 123, 693 B. Ames, W. B. (1892). Discussion - American Dental Association. Dental Cosmos, 34, 926-7. Ames, W. B. (1893). Oxyphosphates. Dental Cosmos, 35, 869-75. Anderson, J. N. & Paffenbarger, G. C. (1962). Properties of silicophosphate cements. Dental Progress, 2, 72-5. Andersson, K. R., Dent Glasser, L. S. & Smith, D. (1982). Polymerization and colloid formation in silicate solutions. In Falcone, J. G. (ed.) Soluble Silicates. ACS Symposium Series No. 194, Chapter 8. Washington, DC: American Chemical Society. Andrews, J. T. & Hembree, J. H. (1976). In vivo evaluation of the marginal leakage of four inlay cements. Journal of Prosthetic Dentistry, 35, 532-7. Ando, J., Shinada, T. & Hiraoka, G. (1974). Reactions of monoaluminium phosphate with alumina and magnesia. Yogyo-Kyokai-Shi, 82, 644-9. Antonioli, C. A. (1969). Etude sur la penetration des ions H + de differents ciments de scellement a travers la dentine. Schweizische Monatsschrift fur Zahnheilkunde, 79, 533-7. Anzai, M., Hirose, H., Kikuchi, H., Goto, J., Azuma, F. & Higasaki, S. (1977). Studies on soluble elements and solubility of dental cements. I. Solubilities of zinc phosphate cement, carboxylate cement and silicate cement in distilled water. Journal of the Nihon University School of Dentistry, 19, 26-39. Arato, A. (1974). Physikalische Eigenschaften von verschiedenen Zementen fur kieferothopadische Zwecke. Schweizische Monatsschrift fiir Zahnheilkunde, 54, 535-58. Asch, W. & Asch, D. (1913). Silicates in Industry and Commerce, pp. 199-336. London: Constable. Aveston, J. (1965). The hydrolysis of the aluminium ion: ultracentrifugal and acidity measurements. Journal of the Chemical Society, 4438-43. Axelsson, B. (1964). Kemisk analys av dentala silikatcement och deras losliga komponenter. Odontologisk Revy, 15, 150-68. 266

References Axelsson, B. (1965). Kemisk analys av dentala zinkfosfatcement. Odontologisk Revy, 16, 126-30. Babin, J. B., Hurst, R. V. V. & Feary, T. (1978). Antibacterial activity of dental cements. Journal of Dental Research, 37, Special Issue B, Abstract No. 214. Beech, D. R. & Bandyopadhyay, S. (1983). A new laboratory method for evaluating the relative solubility and erosion of dental cements. Journal of Oral Rehabilitation, 10, 57-63. Belopolsky, A. P., Shpunt, S. Ya. & Shulgina, M. N. (1950). Physicochemical researches in the field of magnesium phosphates (the system MgO-P2O5-H2O at 80 °C).Journal of Applied Science (USSR), 23, 873-84. Bergvall, O. & Brannstrom, M. (1971). Measurement of the space between composite resinfillingsand cavity walls. Swedish Dental Journal, 64, 217-26. Bjerrum, N. & Dahm, C. R. (1931). Studies on aluminium phosphate. I. Complex formation in acid solution. Zeitschrift fiir physikalische Chemie, Bodenstein Festband 627-37 {Chemical Abstracts, 26, 666). Bock, B. (1971). Klinische Nachuntersuchungen von Silikatzementfullungen. Deutsche Zahndrtzliche Zeitschrift, 26, 665-71. Bowen, R. L. (1962). Dentalfillingmaterial comprising vinyl silane treated fused silica and a binder consisting of a reaction product of bisphenol and glycidyl acrylate. US Patent 3,066,112. Brannstrom, M. & Astrdm, A. (1972). The hydrodynamics of the dentine: its possible relationship to dentinal pain. International Dental Journal, 22, 219-27. Brannstrom, M. & Nyborg, H. (1960). Dentinal and pulpal responses. Odontologisk Revy, 1, 37-50. Brannstrom, M. & Nyborg, H. (1971). The presence of bacteria in cavities filled with silicate cement and composite resin materials. Swedish Dental Journal, 64, 149-55. Brannstrom, M. & Nyborg, H. (1974). Bacterial growth and pulpal changes and inlays cemented with zinc phosphate cement and epoxylite CBA 9080. Journal of Prosthetic Dentistry, 31, 556-65. Brannstrom, M. & Vojinovic, O. (1976). Responses of the dental pulp to invasion of bacteria around threefillingmaterials. Journal of Dentistry for Children, 43, 83-9. Brill, E. (1935). Improvements in or relating to dental cement. British Patent 430,624. Brune, D. & Smith, D. (1982). Micro structure and strength properties of silicate and glass-ionomer cements. Acta Odontologica Scandinavica, 40, 389-96. Cameron, J. C, Charbeneau, G. T. & Craig, R. G. (1963). Some properties of dental cements of specific importance in the cementation of orthodontic bands. Angle Orthodontics, 33, 233-45. Cartz, L., Servais, G. E. & Rossi, F. (1972). Surface structure of zinc phosphate dental cements. Journal of Dental Research, 51, 1668-71. Cassidy, J. E. (1977). Phosphate bonding then and now. American Ceramic Society Bulletin, 56, 640-3. 267

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Phosphate bonded cements Updegraff, D. M., Change, R. W. H. & Joos, R. W. (1971). Antibacterial activity of dental restorative materials. Journal of Dental Research, 50, 382-7. Van Wazer, J. R. (1958). Properties and Chemistry of Phosphorus and its Compounds, pp. 486-91. New York: Interscience Publishers Inc. Van Wazer, J. R. & Callis, C. F. (1958). Metal complexing by phosphates. Chemical Reviews, 58, 1011^6. Vashkevich, N. K. & Sychev, M. M. (1982). Setting of copper phosphate cements in the presence of organic polymers. Chemical Abstracts, 97, 23921. Vieira, D. F. & De Arujo, P. A. (1963). Estudo a cristizacao de cemento de fosfato de zinco. Revista da Faculdade Odontologia da Universidade de Sao Paolo, 1, 127-31. Voelker, C. C. (1916a). The place of silicates in dentistry. The Dental Summary, 36, 177-200. Voelker, C. C. (1916b). Dental silicate cements in theory and practice. The Dental Cosmos, 36, 1098-111. Vysotskii, Z. Z., Galinskaya, V. I., Kolychev, V. I., Strelko, V. V. & Strazhesko, D. N. (1974). The role of polymerization and depolymerization reactions of silicic acid. In Strazhesko, D. N. (ed.) Adsorption and Adsorbents, vol. 1, p. 75. New York: Wiley. Walls, A. W. G., McCabe, J. F. & Murray, J. J. (1985). An erosion test for dental cements. Journal of Dental Research, 64, 1100-4. Ware, A. L. (1971). Properties of cements for orthodontic bonding. Australian Orthodontics Journal, 2, 254-61. Waters, D. N. & Henty, M. S. (1977). Raman spectra of aqueous solutions of hydrolysed aluminium(III) salts. Journal of the Chemical Society: Dalton Transactions, 243-5. Watts, A. (1979). Bacteria contamination and the toxicity of silicate and zinc phosphate cements. British Dental Journal, 146, 7-13. Watts, A. S. (1915). Dental porcelains. Transactions of the American Ceramic Society, 17, 190-9. Wege. (1908). Zur Frage betr. die Ursache des Absterbens der Pulpa unter Silikatzementen, sowie einige Worte iiber Phenakit. Deutsche Zahndrztliche Wochenschrift 346-58. Wei, S. H. Y. & Sierk, D. L. (1971). Fluoride uptake by enamel from zinc phosphate cement containing stannous fluoride. Journal of the American Dental Association, 83, 621-4. Williams, J. I., Gates, G. L., Hembree, J. H. & MacKnight, J. P. (1979). The frozen-aluminium-slab mixing technique: its effect on zinc phosphate cements. Journal of Dentistry for Children, 46, 398-403. Williams, P. D. & Smith, D. C. (1971). Measurement of the tensile strength of dental restorative materials by use of a diametral compressive strength test. Journal of Dental Research, 50, 436-42. Wilson, A. D. (1969). A survey of dental practice in the use of silicate cements. Ministry of Technology Report. British Dental Journal, 127, 7 (abstract). Wilson, A. D. (1975a). Dental cements - general. In von Fraunhofer, J. A. (ed.) 280

References Scientific Aspects of Dental Materials, Chapter 4. London and Boston: Butterworths. Wilson, A. D. (1975b). Zinc oxide dental cements. In von Fraunhofer, J. A. (ed.) Scientific Aspects of Dental Materials, Chapter 5. London and Boston: Butterworths. Wilson, A. D. (1975c). Dental cements based on ion-leachable glasses. In von Fraunhofer, J. A. (ed.) Scientific Aspects of Dental Materials, Chapter 6. London and Boston: Butterworths. Wilson, A. D. (1976). Specification test for the solubility and disintegration of dental cements: a critical evaluation of its meaning. Journal of Dental Research, 55, 721-9. Wilson, A. D. (1978). The chemistry of dental cements. Chemical Society Reviews, 7, 265-96. Wilson, A. D., Abel, G. & Lewis, B. G. (1974). The solubility and disintegration test for zinc phosphate dental cements. British Dental Journal, 137, 313-17. Wilson, A. D., Abel, G. & Lewis, B. G. (1976). The 'solubility and disintegration' test for zinc phosphate dental cements: the use of small specimens. Journal of Dentistry, 4, 28-32. Wilson, A. D. & Batchelor, R. F. (1967a). Dental silicate cements. I. The chemistry of erosion. Journal of Dental Research, 46, 1075-85. Wilson, A. D. & Batchelor, R. F. (1967b). Dental silicate cements. II. Preparation and durability. Journal of Dental Research, 46, 1425-32. Wilson, A. D. & Batchelor, R. F. (1968). Dental silicate cements. III. Environment and durability. Journal of Dental Research, 47, 115-20. Wilson, A. D., Crisp, S. & Lewis, B. G. (1982). The aqueous erosion of silicophosphate cements. Journal of Dentistry, 10, 187-97. Wilson, A. D., Groffman, D. M., Powis, D. R. & Scott, R. P. (1986a). An evaluation of the significance of the impinging jet method for measuring the acid erosion of dental cements. Biomaterials, 1, 55-60. Wilson, A. D., Groffman, D. M., Powis, D. R. & Scott, R. P. (1986b). A study of variables affecting the impinging jet method for measuring the erosion of dental cements. Biomaterials, 1, 217-20. Wilson, A. D. & Kent, B. E. (1968). Dental silicate cements. V. Electrical conductivity. Journal of Dental Research, 44, 463-70. Wilson, A. D. & Kent, B. E. (1970a). Dental silicate cements. IX. Decomposition of the powder. Journal of Dental Research, 49, 7-13. Wilson, A. D. & Kent, B. E. (1970b). Dental silicate cements. X. The precipitation reaction. Journal of Dental Research, 49, 21-6. Wilson, A. D. & Kent, B. E. (1971). The glass-ionomer cement, a new translucent cement for dentistry. Journal of Applied Chemistry and Biotechnology, 21, 313. Wilson, A. D., Kent, B. E. & Batchelor, R. F. (1968). Dental silicate cements. IV. Phosphoric acid modifiers. Journal of Dental Research, 47, 233-43. Wilson, A. D., Kent, B. E., Batchelor, R. F., Scott, B: G. & Lewis, B. G. (1970a). Dental silicate cements. XII. The role of water. Journal of Dental Research, 49, 307-14. 281

Phosphate bonded cements Wilson, A. D., Kent, B. E., Clinton, D. & Miller, R. P. (1972). The formation and microstructure of the dental silicate cement. Journal of Materials Science, 7, 220-38. Wilson, A. D., Kent, B. E. & Lewis, B. G. (1970). Zinc phosphate cements: chemical study of in vitro durability. Journal of Dental Research, 49, 1049-54. Wilson, A. D., Kent, B. E., Mesley, R. F., Miller, R. P. Clinton, D. & Fletcher, K. E. (1970b). Formation of dental silicate cement. Nature, 225, 272-3. Wilson, A. D. & Lewis, B. G. (1980). The flow properties of dental cements. Journal of Biomedical Materials Research, 14, 383-91. Wilson, A. D. & Mesley, R. F. (1968). Dental silicate cements. VI. Infrared studies. Journal of Dental Research, 47, 644-52. Wilson, A. D., Paddon, J. M. & Crisp, S. (1979). The hydration of dental cements. Journal of Dental Research, 58, 1065-71. Windeler, A. S. (1978). The use of film thickness to measure working time of zinc phosphate cements. Journal of Dental Research, 57, 697-701. Windeler, A. S. (1979). Powder enrichment effects on film thickness of zinc phosphate. Journal of Prosthetic Dentistry, 42, 299-303. Wisth, P. J. (1972). The ability of zinc phosphate and hydrophosphate cements to seal space bands. Angle Orthodontics, 42, 395-8. Worner, H. K. (1940). The properties of commercial zinc phosphate cements. Australian Journal of Dentistry, 44, 123-41. Worner, H. K. & Docking, A. R. (1958). Dental materials in the tropics. Australian Dental Journal, 3, 215-29. Wright, J. W. (1919). A study of some dental cements. Journal of Dental Research, 1, 35-60. Wygant, J. F. (1958). Cementitious bonding in ceramic fabrication. In Kingery, W. D. (ed.) Ceramic Fabrication Processes, pp. 171-88. New York: John Wiley & Sons. Yamano, C. (1968). Effect of NaF-phosphate cement on enamel of human tooth. Journal of the Osaka University Dental School, 13, 123-37. Yamazaki, T. & Takeuchi, M. (1967). The Chemical Society of Japan, Industrial Chemistry Section, 10, 656. Zander, H. A. (1946). The reaction of dental pulps to silicate cements. Journal of the American Dental Association, 33, 1233^43. Zenaishvili, N. V., Bakradze, E. G. & Chelidze, D. V. (1984). Serpentinitephosphate mortar containing iron and boron. Chemical Abstracts, 101, 156519r. Zhuravlev, V. F., Volfson, S. L. & Sheveleva, B. I. (1950). The processes that take place in the roasting of zinc-phosphate dental cement. Journal of Applied Chemistry (USSR), 23, 121-8.

282

7

Oxysalt bonded cements

7.1

Introduction

Oxysalt bonded cements trace their origin to studies by Sorel in the third quarter of the nineteenth century. The first of these cements which he studied was zinc oxychloride (Sorel, 1855). Later he described a series of magnesia-based cements which included both the magnesium oxychloride and magnesium oxysulphate types (Sorel, 1867). Of this second group, the magnesium oxychlorides in their hydrated form have been shown to have larger values of modulus of elasticity, microhardness and compressive strength than does Portland cement for a wide range of porosites (Beaudoin & Ramachandran, 1975). The magnesium oxysulphate cements have properties that have led to their being considered for nuclear applications, since they have good fire resistance, low thermal conductivity and above all, in marked contrast to the related oxychloride cements, no potential to initiate corrosion of the reinforcing steel (Beaudoin & Ramachandran, 1978). These cements are also employed as binders in lightweight panels, in insulating materials and in architectural applications (Urwongse & Sorrell, 1980b). Oxysalt bonded cements are formed by acid-base reactions between a metal oxide in powdered solid form and aqueous solutions of metal chloride or sulphate. These reactions typically give rise to non-homogeneous materials containing a number of phases, some of which are crystalline and have been well-characterized by the technique of X-ray diffraction. The structures of the components of these cements and the phase relationships which exist between them are complex. However, as will be described in the succeeding parts of this chapter, in many cases there is enough knowledge about these cements to enable their properties and limitations to be generally understood. 283

Oxysalt bonded cements 7.1.1

Components of oxysalt bonded cements

The three major types of oxysalt bonded AB cement are the zinc oxychloride, the magnesium chloride and the magnesium oxysulphate cements. The bases employed, therefore, are either zinc oxide or magnesium oxide, both of which readily undergo hydration in aqueous solution, behaving as M(OH)2 species and acting as a source of hydroxyl ions. They are thus both clearly bases in the Bronsted-Lowry sense. By contrast, the acidity of the metal salts used in these cements has a less clear origin. All of the salts dissolve quite readily in water and give rise to free ions, of which the metal ions are acids in the Lewis sense. These ions form donor-acceptor complexes with a variety of other molecules, including water, so that the species which exists in aqueous solution is a well-characterized hexaquo ion, either Mg(OH2)g+ or Zn(OH2)g+. However, zinc chloride at least has a ternary rather than binary relationship with water and quite readily forms mixtures of ZnO-HCl-H2O (Sorrell, 1977). Hence it is quite probable that in aqueous solution the metal salts involved in forming oxysalt cements dissolve to generate a certain amount of mineral acid, which means that these aqueous solutions function as acids in the Bronsted-Lowry sense.

7.1.2

Setting of oxysalt bonded cements

The nature of the solidification process in these cements has received little attention. Rather, attention has focussed on the crystalline components that form in cements which have been allowed to equilibrate for some considerable time; the nature of such phases is now quite well understood. Gelation is reasonably rapid for these cements and occurs within a significantly shorter time than does development of crystalline phases. The conclusion may be drawn that initial cementition is not the same as crystallization, but must occur with the development of an essentially amorphous phase. Reactions can continue in the amorphous gelled phase, but are presumably limited in speed by the low diffusion rates possible through such a structure. However, reactions are able to proceed substantially to completion, since in many cases X-ray diffraction has demonstrated almost quantitative conversion of the parent compounds to complex crystalline mixed salts, though several days or weeks of equilibration are required to bring this about. 284

Zinc oxychloride cements 7.2

Zinc oxychloride cements

7.2.1

History

These cements were the earliest of the oxysalt bonded cements to be prepared (Sorel, 1855) and their chemistry has been the subject of numerous investigations over the years. There are considerable difficulties associated with such investigations. Not only does the cement contain a complex mixture of different crystalline precipitates but it is unaffected by boiling water and dissolves only slowly in strong acids. Consequently separation or analysis of any of the phases which may be present is difficult. Nonetheless, as early as 1925 at least 17 crystalline compounds were claimed to occur in the zinc oxychloride cement (Mellor, 1925). Of the early studies, two are still of value in providing an outline of the possible phase compositions and equilibria existing in this material. The first, by Droit (1910), concentrated on measuring the solubility of zinc oxide in zinc chloride solutions at 18 °C, while the second, by Holland (1930), was based on the analysis of saturated solutions and moist residues equilibrated at 25 °C and 50 °C. Droit assigned the compositions 4ZnO. ZnCl2. 6H2O (4:1:6) and 2ZnO. 2ZnCl2. 3H2O (2:2:3 or 1:1:1-5) to solids which he found in equilibrium. The former phase he described as amorphous and reported that five of the six water molecules were lost at 200 °C.The last remaining molecule of water was not lost until a much higher temperature, and then both HC1 and ZnCl2 were lost as well. Droit described the 1:1:1-5 phase as microcrystalline and reported that it lost one of its water molecules at 230 °Cwhile the remaining water, together with HC1, was lost at a higher temperature. Holland, by contrast, reported the existence of three well-defined phases in this system, corresponding to ZnO:ZnCl2:H2O ratios of 5:1:8, 1:1:1 and 1:1:2 respectively (Holland, 1930). More recently it was pointed out that these claims lack any unequivocal support such as X-ray characterization of the phases, so that their validity must remain in doubt (Sorrell, 1977). A number of other workers have reported the existence of an additional phase, corresponding to 4:1:5 (Feitknecht, 1930; Hayek, 1932; Aspelund, 1933), which may or may not be associated with a 1:1:1 phase. Feitknecht (1933) also carried out a detailed study of a phase described as 4:1:4 using X-ray diffraction and concluded that the material had a layer structure with interspersed water molecules. Such a structure would permit the addition and removal of water molecules without altering the interlayer 285

Oxysalt bonded cements distance. Hence all of the phases based on a 4:1 ratio of ZnO to ZnCl2 may be closely related and readily interconverted, depending on the precise conditions of cementition. Droit's original 4:1:5 phase has been studied by X-ray diffraction (Nowacki & Silverman, 1961, 1962) and found to have a rhombohedral layer structure. The 1:1:1 phase was also found to have a layer structure, which consisted of pseudohexagonal layers of zinc atoms separated by ordered layers comprising oxygen and chlorine atoms (Feitknecht, Ostwald & Forsberg, 1959). This fundamental structure was apparently found for both of the crystalline modifications in which this phase has been found to occur, namely the monoclinic and the orthorhombic (Sorrell, 1977).

7.2.2

Recent studies

Sorrell (1977) further elucidated the structure and phase relationships in zinc oxychloride cements and produced a phase diagram for the system (Figure 7.1). Sorrell prepared his solutions of aqueous zinc chloride in one of two ways: either by dissolving reagent-grade ZnCl2 in distilled water, or by dissolving zinc metal cut from an ingot in aqueous hydrochloric acid followed by boiling to low volume to remove the excess acid. Cement samples were prepared by reacting either aqueous HC1 or aqueous ZnCl2 solutions with zinc oxide powder and sealing them in polyethylene containers to equilibrate for at least four days before examining them. Cements were analysed by X-ray diffractometry using Cu Ka radiation. Powdered cement samples were also examined by quantitative differential thermal gravimetry (DTG) from 25 °C to 715 °C at a heating rate of 10 °C per minute. X-ray analysis of the various samples that were produced indicated that the system ZnO-ZnCl2-H2O includes four crystalline phases, two of which, ZnO and ZnCl2.1-5H2O, are essentially the starting materials. Sorrell also found the 4:1:5 phase, reported by Droit, with an identical Xray powder diffraction pattern to that reported by Nowacki & Silverman (1961, 1962), and a 1:1:2 phase. Since neither the 1:1:2 nor the 4:1:5 phase lost or gained weight on exposure to air at about 50% relative humidity and 22 °C and no changes developed in the X-ray diffraction pattern following this exposure, he concluded that the previously reported 1:1:1 phase cannot be formulated from mixtures of ZnO and aqueous ZnCl2. 286

Zinc oxychloride cements The cementition reaction between zinc oxide powder and aqueous zinc chloride was found to be both rapid and extremely exothermic. Although at least four days equilibration was allowed before examining any of the cements in detail, Sorrell found evidence that reaction was complete within 20 to 30 minutes and occurred without observable development of intermediate phases. He also found that, as the concentration of reactants was increased, so reaction rate increased until, at sufficiently high concentrations, reaction occurred too quickly to allow proper mixing of the reactants. Preheating the zinc oxide at 900 °C for 16 hours was found to slow the reaction down, but only slightly. Sorrell showed that the two discrete phases could be readily and reversibly interconverted. For example, the 1:1:2 phase was found to react ZnO

90, O ZnO ond ZnClg Solution* •

H2O

10

20

30

40

50

Wt%ZnCI2

60

ZnO and MCI Solution!

60

90

ZnCU

Figure 7.1 Phase relationships in the ZnO-ZnQ2-H2O system at room temperature (Sorrell, 1977).

287

Oxysalt bonded cements with water to generate the 4:1:5 phase so rapidly that it was not possible to record an X-ray diffraction pattern for the mixture. Samples whose composition fell into the triangle bounded by the phases 4:1:5,1:1:2 and pure water dried in air to give a solid mixture of the two discrete phases. By contrast, samples containing less ZnO did not dry out. Thus all changes caused by variation in composition proved to be predictable from the phase diagram and at no time were there any departures from this, such as formation of Zn(OH) 2 . Thermogravimetric analysis was carried out on both the 4:1:5 and the 1:1:2 phases (Sorrell, 1977). The latter was found to undergo a two-step dissociation beginning at about 230 °C. Above 275 °C, the melting point of ZnCl 2 , weight loss corresponding to removal of half of the constituent water occurred. Above c. 680 °C weight loss ceased, having reached a level corresponding to complete loss of ZnCl 2 and water. Attempts were made to complement this thermogravimetric study by X-ray analysis on the quenched samples but these were unsuccessful due to the deliquescent nature of the dissociation products. Above 275 °C, however, the sample existed as a solid mixed with a liquid and this crystallized on cooling as it absorbed water from the atmosphere. The X-ray patterns which were made shortly after quenching showed the presence of zinc oxide, and repeated scanning indicated a rapid reaction to regenerate the 1:1:2 phase. The 4:1:5 phase was shown by thermogravimetric analysis to dissociate at about 160 °C to zinc oxide and the 1:1:2 phase, a process which was verified using X-ray diffraction (Sorrell, 1977). Once the 1:1:2 phase was formed it underwent characteristic dissociation at temperatures above 160 °C. The equilibrium relationships found by Sorrell (1977) were valid only for room temperature (22 + 2 °C) and, because samples were allowed to cure in sealed containers, for equilibrium water vapour pressures determined by the assembly of phases present. The phases which exist under such conditions were quite unequivocally found to be 4:1:5 and 1:1:2. However Sorrell pointed out that it is entirely possible that lower hydration states of either phase could be stable at higher temperatures or lower humidities. In particular the 4:1:4 phase (Feitknecht, 1933) may well be such a phase, particularly as one of the five waters of hydration is known to be held only loosely in the structure. Indeed, Sorrell reported that he observed a slight shoulder on the larger dehydration peak of the DTG curve of the 4:1:5 phase that might be assigned to the loss of this first water molecule. He did not, however, succeed in isolating or characterizing a 4:1:4 phase. 288

Zinc oxychloride cements Similarly Sorrell (1977) had no success in attempts to isolate the 1:1:1 phase reported by Forsberg & Nowacki (1959), though he conceded that it, too, might exist as a lower hydration state of his well-defined 1:1:2 phase. Unfortunately Forsberg and Nowacki did not provide details of the alleged 1:1:1 phase, so comparison of their results with SorrelPs was not possible. To summarize these findings, the thermal decomposition of the two ternary phases was found to be complex (Sorrell, 1977). On the basis of thermogravimetric data alone, the loss of water appeared to occur in such a way that both a 4:1:4 and a 1:1:1 phase might result. However, what actually occurred was that a mixture of ZnCl2 and ZnO was obtained as a residue; such a result is typical of what is found for aqueous solutions of divalent metal chlorides that have been evaporated to dryness. An additional complexity was the possible reoxidation of zinc as the mixtures containing ZnCl2 were evaporated, thus leading to larger amounts of ZnO in the residue than were originally present in the original mixture. Overall, the thermogravimetric studies gave results that were extremely difficult to interpret, largely because of the highly complicated set of relationships that can exist in this system. In very dilute HC1 solutions, specifically those with a pH above 5-48, the 4:1:5 phase was found to be insoluble. By contrast, addition of concentrated HC1 to the 4:1:5 phase was shown to lead to formation of the 1:1:2 phase (Sorrell, 1977). Below 35wt% HC1, the 4:1:5 phase was found to dissolve congruently. Since the 1:1:2 phase was also found to dissolve congruently in hydrochloric acid solutions with concentrations above 23 wt %, it follows that there is a range of concentrations over which both phases are soluble in aqueous HC1. This behaviour explains why the zinc oxychlorides have proved to be unsatisfactory in attempts to use them as dental cements. The preparation of such cements from concentrated aqueous solutions of ZnCl2 results in the formation either of the 1:1:2 phase alone or of mixtures of the 4:1:5 and 1:1:2 phases, neither of which is stable in the presence of water. Preparing dental cements from less concentrated solutions also results in the formation of mixed phases, unless the bulk composition has excessive amounts of ZnO present. In these latter cases the cement stability is acceptable but it lacks both a workable consistency and a reasonable working time. SorrelFs study of zinc oxychloride cement (1977), in addition to making an important contribution to our understanding of the nature of this material, also highlighted a more general feature of the chemistry of zinc 289

Oxysalt bonded cements chloride. Since the 4:1:5 phase separated from dilute solutions, the ZnCl2-H2O system was shown not to be binary. Instead, it represents a section through the ZnO-HCl-H2O ternary system. Moreover, zinc chloride is known to be one of the most soluble of all substances and is very difficult to prepare in a completely anhydrous condition (Greenwood & Earnshaw, 1984). This feature also presumably derives from the nature of the phase relationships that develop in the ZnO-HCl-H2O system. 7.3

Magnesium oxychloride cements

7.3.1

Uses

Magnesium oxychloride cements are widely used for the fabrication of floors. They find application for this purpose because of their attractive appearance, which resembles marble, and also because of their acoustic and elastic properties and their resistance to the accumulation of static charge. They have also been used for plastering walls, both interior and exterior; for exterior walls the cement often includes embedded stone aggregate (Sorrell & Armstrong, 1976). However, there have been problems with this latter application, since the base cement has been found to be dimensionally unstable and, in certain circumstances, to release corrosive solutions and show poor weather resistance. 7.3.2

Calcination of oxide

The quality of magnesium oxychloride cements is highly dependent on the reactivity of the magnesium oxide used in their preparation. Typically, such oxides are prepared by calcination of the basic carbonate (Eubank, 1951; Harper, 1967), but their reactivity varies according to the conditions under which such calcination is carried out. As the reactivity alters so does the amount of oxide that can be incorporated into a cement relative to the amount of aqueous MgCl2 (Harper, 1967). A detailed study of the effect of calcination conditions on properties of the resulting oxide was carried out by Harper (1967), with the results shown in Table 7.1 Oxides prepared from basic carbonate at 600 °C and 700 °C give weak cements because the reactivity of the oxide is so high that reaction occurs during mixing and the cement is broken in the process. The low strengths of these cements contrasted with the much higher strengths obtained from oxides that had been calcined at higher temperatures. For 290

Magnesium oxychloride cements Table 7.1. Compressive strengths of magnesium oxy chloride cements made from basic carbonate (Harper, 1967) Calcination temperature, °C

Compressive strength, MPa MgO/MgCl 2 , mo 1 Age, days

6

7

8

9

10

600

7 14 28

7-6 7-4 8-3

700

7 14 28

15-9 16-3 17-3

800

7 14 28

49-6 55-9 57-5

55-3 631 60-0

57-4 63-5 50-0

900

7 14 28

47-7 48-6 55-6

58-2 68-6 68-8

67-5 691 65-8

73-8 71-7 67-8

78-1 690 66-6

1000

7 14 28

60-5 65-9 83-2

71-7 85-7 90-4

77-8 87-5 80-6

73-6 87-0 88-2

80-0 68-4 88-5

example, oxides prepared at 1000 °C were much less reactive, which allowed time for adequate mixing with the aqueous magnesium chloride and thus allowed the strength to develop to a greater extent. Such cements went on increasing their strength for at least 28 days after mixing. These lower-reactivity oxides could also be added in greater quantities to the MgCl2 solution, thus allowing for greater variation in the composition of the cement.

7.3.3

Setting chemistry

There have been a number of studies aimed at understanding the chemistry of the curing and setting of magnesium oxychloride cements and at identifying the phases that are present in the final material. Investigations in the first half of the twentieth century revealed that cement formation in the MgO-MgCl2-H2O system involves gel formation and crystallization of 291

Oxysalt bonded cements ternary oxychloride phases of uncertain composition (Robinson & Waggaman, 1909; Bury & Davies, 1932). In the years around 1950, the system was studied by Walter-Levy and coworkers; they succeeded in identifying crystalline phases and determining the structure of one of them (de Wolff & Walter-Levy, 1953). Walter-Levy had previously recognized an oxychlorocarbonate phase in the cement which forms by reaction with carbon dioxide in the atmosphere (Walter-Levy, 1937, 1938). A very extensive study of this system was carried out by Cole and coworkers (Demediuk, Cole & Hueber, 1955; Cole & Demediuk, 1955), who were able to define the temperature ranges over which the various phases are stable, though they confined their studies to cements of low MgO content, e.g. 1-5 g MgO in 75 cm3 MgCl2 solution. Below 100 °C two ternary phases were found, one with an Mg(OH) 2 : MgCl 2 : H 2 O composition of 5:1:8, the other with a compositon of 3:1:8. These phases have been referred to as the 5-form and the 3-form respectively. The 3-form was the reaction product of MgO with solutions of higher MgCl2 concentration than those which tended to yield the 5-form (Cole & Demediuk, 1955). Both forms were found to change gradually with time via slow reaction with atmospheric CO 2 , so that after long periods a basic magnesium carbonate, corresponding to a composition of Mg(OH) 2 . 2MgCO 3 . MgCl 2 . 6H 2 O, had formed (Cole & Demediuk, 1955). Hence the results previously reported by Walter-Levy (1937, 1938) were confirmed. This basic carbonate was found to be much less soluble in water than either of the two oxychloride phases. Above 100 °C, a different set of phases was stable in the simple magnesium oxychloride system (Demediuk, Cole & Hueber, 1955; Cole & Demediuk, 1955); a 2-form 2Mg(OH) 2 . MgCl 2 . 4H 2 O and a 9-form 9Mg(OH) 2 . 5H2O were found to occur. Apart from identifying these phases, these workers were not able to give details on the structures. The phase relationships prevailing in this system have received more attention at temperatures below 100 °C, and there is greater understanding of the equilibria which occur in this temperature range. These equilibria involve a range of solids, namely Mg(OH) 2 , the 5:1:8 and 3:1:8 materials and MgCl 2 . 6H 2 O, depending on the range of concentrations of MgCl2 used in aqueous solution and the ratio of solution to MgO powder employed in fabricating the cement. Of all the studies on this system, that by Sorrell & Armstrong (1976) has provided the most useful information, both on the phase relationships and on the kinetics of interconversion. They used three different grades of 292

Magnesium oxychloride cements MgO, produced by different but well-defined routes and having different reactivities towards aqueous MgCl2; in this way, it was possible to study the cementation reactions in some detail and to ensure a reasonably close approach to equilibrium. To study reaction kinetics, cement batches of total mass 300 g were prepared using ingredients measured to the nearest 0-1 g. Mixing was carried out for 10 minutes using a kitchen blender, after which specimens were cast in slabs 10 x 10 x 1-2-1-5 cm in polyethylene moulds. When the setting reaction had proceeded to a sufficient extent and viscosity had risen to give a reasonably stiff paste, a small portion was removed, placed on a glass microscope slide and immediately examined by X-ray diffraction. The remainder of the sample was allowed to set. 7.3.4

Kinetics of cementation

Sorrell & Armstrong formulated cements in proportions corresponding to the 5:1:8 and 3:1:8 compositions. The initial mixtures were thick slurries with no observable tendency to separate provided a sufficiently reactive oxide was used. They tended to set within about 90 minutes, at which time samples were prepared for X-ray determination. Initially, although the preliminary hardening process was apparently complete, the only crystalline phase that could be found was MgO; moreover, this material was found in amounts that approximated to the quantity in the initial mixture. After some two hours, the X-ray diffraction pattern corresponded to either the 5:1:8 or the 3:1:8 phase; warming of the sample had also occurred. Growth of the crystalline oxychloride phases continued rapidly up to about 15 hours, and more slowly thereafter, until after four days there was no trace of MgO in the diffraction pattern of the cement. The fact that the initial setting process for magnesium oxychloride cements takes place without observable formation of either the 5:1:8 or the 3:1:8 phase is important. It indicates that formation of an amorphous gel structure occurs as the first step, and that crystallization is a secondary event which takes place from what is effectively a supersaturated solution (Urwongse & Sorrell, 1980a). This implies that crystallization is likely to be extremely dependent upon the precise conditions of cementition, including temperature, MgO reactivity, heat build-up during reaction and purity of the components in the original cement mixture. The reactivity of the magnesium oxide in particular was shown to be crucial. A relatively unreactive batch gave thin slurries which showed some 293

Oxysalt bonded cements tendency to settle and took at least 20 hours to set. Reaction was not complete after 14 days and in the mixture corresponding to the 5:1:8 composition it was the 3:1:8 phase which actually crystallized out. 7.3.5

Phase relationships in the MgO-MgCl2-H2O

system

Sorrell & Armstrong (1976) prepared cement slabs in proportions corresponding to the 5:1:8 phase from each of the three magnesium oxide samples that were available. The samples were allowed to set for at least 30 days exposed to air, after which time the phase gradient was determined by X-ray diffraction of surfaces exposed by incremental grinding. These experiments revealed that the most reactive sample of MgO had yielded a material consisting essentially of the 5:1:8 phase uniformly distributed throughout the bulk and containing no more than 2 % residual MgO. The less reactive samples of oxide, by contrast, gave much less homogeneous cements in which the surface layer consisted mainly of unreacted MgO together with MgCl2 solution. The amount of crystalline 5:1:8 phase increased with depth into the sample, thus demonstrating the development of phase gradients for these materials. For studies of the phase equilibria, 20-gram batches of cement were prepared by mixing appropriate solutions of MgCl2 with the most reactive of the three samples of MgO that was available (Sorrell & Armstrong, 1976). Mixing was carried out by stirring with a glass rod and cements were sealed in polyethylene containers for at least four days to allow sufficient time for equilibration. The use of relatively small samples avoided the problems of large temperature increases as the cementition reaction occurred. The studies of phase equilibria resulted in compilation of the phase diagram Figure 7.2. The portion of the phase diagram representing the MgCl2-rich compositions received little attention in the work of Sorrell & Armstrong (1976), but was the subject of a separate study reported a few years later (Urwongse & Sorrell, 1980a). Since it had previously been shown that the MgO-MgCl 2 -H 2 O system is a portion of the MgO-HCl-H 2 O system (Robinson & Waggaman, 1909; Bury & Davies, 1932), and the equilibrium assemblages will be the same whichever reagents are used, Urwongse & Sorrell (1980a) decided to use magnesium oxide and aqueous hydrochloric acid for their work. This approach had the advantage that magnesium oxide is much more soluble in hydrochloric acid solutions than in magnesium chloride solutions, and hence measurements were easier and 294

Magnesium oxychloride cements more reliable. Having obtained data on solubilities for the MgO-HCl-H2O system, these were recalculated to give the compositions in the MgO-MgCl2-H2O phase diagram. This work was of value in constructing that part of the phase diagram involving solutions in water. Of greater value in understanding cement formulations were the results obtained in the earlier study (Sorrell & Armstrong, 1976) for the MgO-rich portion of the phase diagram. 7.3.6

Consequences for practical magnesium oxychloride cements

The results of Sorrell & Armstrong (1976) show clearly that care is needed in selecting the magnesium oxide when practical oxychloride cements are being prepared. In particular, the powder must be of good reactivity with small uniform crystallites and minimum agglomeration. Such an oxide sample is capable of being rapidly dissolved by the aqueous magnesium chloride, thereby forming a thixotropic suspension. This suspension then MgO

Phase assemblages 90

Mgcl2.6H2o

A

Mgo-Mg(OH)2-5:1:8

B

MgO-5:l:8-3:l:8

C

MgO-3:1:8-MgCl2.6H2O

D

3:1:8-MgCl2.6H2O-gel

E

3:l:8-5:l:8-gel

F

5:1:8-Mg(OH)2-gel

G

Mg(OH)2-gel

H

5:l:8-gel

I

3:l:8-gel

J

MgCl2.6H2O-gel

K

gel

L

gel-liquid

Wt% Mgci2 Figure 7.2 Phase relationships in part of the MgO-MgCl2-H2O system at room temperature (Sorrell & Armtrong, 1976).

295

Oxysalt bonded cements develops firstly into a homogeneous gel and finally into a crystalline material consisting of a dense aggregate of the equilibrium ternary oxide phase. Relatively unreactive powders lead to a different sequence of events. First, long periods of time are required for dissolution into the MgCl2 solution, during which water is lost from the liquid surface and a compositional gradient is established. Second, under such conditions, MgCl 2 is able to migrate to the surface where it may either precipitate or remain as a deliquescent layer. The combined effects of the compositional gradient and the presence of unreacted MgCl2 and MgO are almost certain dimensional instability, leaching of corrosive salts, and poor weather resistance. The amount of variation in reactivity which may be tolerated is small, since a reasonable balance has to be struck between rapid and uniform reaction on the one hand and practical working times on the other. Sorrell & Armstrong (1976) found that the mean crystallite diameter could be determined adequately by X-ray diffraction, using line-broadening as an indication of crystallite size, and also by electron microscopy. These techniques were able to distinguish between suitable and unsuitable oxide powders. Both the 5:1:8 and 3:1:8 phases were shown to be unstable in water, dissociating to give Mg(OH) 2 and MgCl2 solution (Sorrell & Armstrong 1976). This clearly has consequences when magnesium oxychloride cements are employed as exterior stuccos on buildings. In fact, these cements do have good weather resistance, but not because of any inherent stability of the parent oxychloride cement. Rather, the slow conversion of the oxychloride to the much less soluble basic magnesium carbonate as a result of reaction with atmospheric carbon dioxide creates a material of inherently good weather resistance. Studies of samples of magnesium oxychloride cements used as exterior structures have been carried out on specimens between one month and 50 years in age (Sorrell & Armstrong, 1976). From these it has been possible to gain a general understanding of the mechanism of the reaction with carbon dioxide and hence the way in which weather resistance develops. If the cement has initially reacted completely to give the 5:1:8 phase, as it will if the magnesium oxide used is sufficiently reactive, then the weather resistance develops due to formation of the chlorocarbonate Mg(OH) 2 .2MgCO 3 .MgCl 2 .6H 2 O. This product arises following interaction with atmospheric carbon dioxide, as first reported by Cole & 296

Magnesium oxychloride cements Demediuk (1955). A surface coating of this carbonate protects the underlying cement from attack by water. The longer-term stability, however, depends on slow leaching of the chloride from this new surface of the cement, resulting in its conversion to hydromagnesite, 5MgO • 4CO2. 5H2O. The precise sequence of these reactions remains to be elucidated. 73.7

Impregnation with sulphur

One method of overcoming some of the instability and loss in strength of oxychloride cements when exposed to water has been to modify them by impregnation with sulphur (Beaudoin, Ramachandran & Feldman, 1977). The resulting material appears to be a composite in which the respective components complement each other. The magnesium oxychloride part has relatively poor resistance to water as initially formed, whereas the sulphur is difficult to wet and is completely insoluble in water. Beaudoin, Ramachandran & Feldman (1977) studied this system in detail, examining both the mechanical properties and water resistance of sulphur-filled cements. Mechanical properties evaluated included porosity, modulus of elasticity and microhardness. Thermal characteristics were determined by DTG, and for certain fractured specimens microstructure was determined by examining fracture surfaces on the scanning electron microscope. Specimens were prepared as 5-1-cm cubes and cured at 50% r.h. for 13 months, after which thin discs were cut from them to enable mechanical properties to be evaluated. Two series of samples were prepared, which were respectively (1) immersed in water for 88 days, impregnated with sulphur, then exposed again to water, and (2) impregnated first, immersed in water for 88 days, followed by further impregnation, then water exposure. Impregnation was carried out in a bath at 128 °C, at which temperature the sulphur was molten and would diffuse thoroughly into all the pores of the native cement. Samples were removed and, after cooling to allow solidification of the sulphur, the excess sulphur was removed by washing in kerosene. For the native cement exposed first to water, there was a dramatic and rapid drop in microhardness, 30^0% in the first hour, and 55-60% at eight hours. Compressive strength was assumed to have undergone a similar decrease, since it is linearly related to microhardness for cementitious materials (Beaudoin & Feldman, 1975). Scanning electron microscopy revealed clearly the differences that occurred on soaking in water. 297

Oxysalt bonded cements These include a change from a non-distinct and amorphous appearance to a more porous structure with larger platy crystals. The reason for this was not completely clear, though it was suggested that some degree of recrystallization might have occurred during immersion in water. It may also be that in the sample that had not been soaked in water the finely divided portion obscured the large platy crystals. The response of magnesium oxychloride cements to water was found to vary according to whether or not they were impregnated with sulphur. For microhardness, the impregnated samples gave initially higher values than the unimpregnated samples. Microhardness decreased after 88 days' immersion in water, but the decline was greater in the unimpregnated samples. Unimpregnated samples had extremely low values of microhardness, 10-20 MPa compared with 400-600 MPa for different formulations prepared by impregnation with sulphur (Beaudoin, Ramachandran & Feldman, 1977). Modulus of elasticity showed similar behaviour. Impregnated samples were found to have initially a higher modulus value than unimpregnated ones; the decrease in modulus was less for the impregnated cements following the 88 days' immersion in water. The mechanism by which sulphur has these observed effects is as follows. Immersion of native magnesium oxychloride cement in water brings about a slow dissolution which creates pores. When those pores are filled with sulphur, sites of possible stress concentration at points of contact between particles are modified. Similar effects occur when sulphur is used to impregnate hydraulic cements based on Portland cement and silica (Beaudoin, Ramachandran & Feldman, 1977). There were, however, important differences between the water-resistance properties of sulphur-impregnated Portland cement and sulphurimpregnated magnesium oxychloride cement. The former showed poor resistance, in one case breaking into small pieces after about two hours' exposure to water vapour. The latter, by contrast, remained completely intact after exposure to water for 88 days, and remained in good condition following reimpregnation with sulphur and a further 28 days of exposure to water. Beaudoin, Ramachandran & Feldman (1977) attributed this result in part to the difference in pore size and distribution in the two cements, which leads to the magnesium oxychloride cement having a smaller surface area exposed to water than Portland cement. Overall, these studies showed that sulphur could be used to impregnate magnesium oxychloride cements thereby yielding materials of superior 298

Magnesium oxysulphate cements properties in terms of both mechanical characteristics and water resistance. The preferred method of preparation was to soak the cement in water prior to impregnation, since of the two impregnation regimes employed this gave cements of marginally superior properties.

7.4

Magnesium oxysulphate cements

7.4.1

Setting chemistry

The magnesium oxysulphate cements are formed by reactions between high-reactivity magnesium oxide powder and aqueous solutions of magnesium sulphate. They have a number of applications in architecture, including use as binders in lightweight panels and as insulating materials. There have been problems in fully characterizing these cements, because often it has not been appreciated that they have not been allowed to equilibrate, and as a result their composition may be uncertain. To rectify this situation, Urwongse & Sorrell (1980b) carried out a study of these materials under conditions which allowed equilibrium to be attained. In this way they were able to gain an insight into the phase relationships that occur in these cements. Previous studies had been carried out on mixtures rich in aqueous magnesium sulphate and maintained at various temperatures between 30 °C and 120 °C (Demediuk & Cole, 1957). These revealed the existence of three crystalline phases at lower temperatures, namely magnesium hydroxide, hydrated magnesium sulphate, MgSO 4 .7H 2 O, and a complex crystalline salt whose composition corresponded to an Mg(OH) 2 : MgSO 4 : H 2 O ratio of 3:1 :8. At higher temperatures, a number of other well-defined crystalline phases formed, including 5:1:3,1:1:5 and 1:2:3. Their relationships to each other and to the starting materials were established. For example, at lower concentrations of magnesium sulphate solution the 5:1:3 phase was found to be stable at temperatures greater than 40 °C in equilibrium with Mg(OH) 2 and liquid. At high concentrations the equilibrium altered, so that the 5:1:3 phase occurred in association with the 1:1:5 phase rather than with Mg(OH) 2 . Above 100 °C, the 1:2:3 phase was stable in equilibrium with the 5:1:3 phase and liquid at lower concentrations, and with the solid MgSO 4 . 7H 2 O and liquid at higher concentrations (Demediuk & Cole, 1957). Other crystalline phases have been reported for the magnesium oxysulphate system. The 2:3:5 phase has been claimed to be formed from 299

Oxysalt bonded cements magnesium sulphate solutions with concentrations exceeding 17-5 wt% (Adomavichiute, Yanitskii & Vektaris, 1962). Unfortunately, no analytical data accompanied this report and so it is not certain how reliable the claim is. Acid salts as well as basic salts have been claimed to occur in this system. In particular, the complex salt MgSO 4 . H 2 SO 4 . 3H2O was found at 12-6 °C (Montemartini & Losana, 1929) arising as the product of reaction between MgSO 4 and aqueous H 2 SO 4 . This acid salt was found to coexist with MgSO 4 .7H 2 O, MgSO 4 .H 2 O and liquid.

7.4.2

Phase relationships in the MgO-MgSO\-H2O

system

A detailed study of the phase relationships in the magnesium oxysulphate cement was carried out by Urwongse & Sorrell (1980b). They used X-ray analysis to examine the phases present in the cement, and established the composition of the invariant liquids after equilibration by measuring specific gravity with the aid of a pycnometer. Specific gravities were related to concentration by means of a calibration exercise in which 30 stock solutions of sulphuric acid at concentrations between 0 and 79-5 wt % were prepared with distilled water. A number of species were found at equilibrium in the solid state in this system, including MgSO 4 .7H 2 O, MgO, MgSO 4 .6H 2 O and MgSO 4 .H 2 O. Only one complex salt, the 3:1:8 phase, was found under the conditions studied, with both H 2 SO 4 and H 2 O occurring as discrete entities in certain of the phases. Non-equilibrium phases were also apparent, including the 1:1:5 phase in certain samples. The non-equilibrium phase MgSO 4 . 7H2O was also found in numerous samples, particularly those prepared from sulphuric acid solutions with concentrations above 20wt%. The occurrence of these non-equilibrium phases, and also uncertainty about the possible existence of the phase MgSO 4 . H 2 SO 4 . 3H 2 O, led to problems in constructing the complete phase diagram for the MgO-H 2 SO 4 -H 2 O ternary system. In particular there were difficulties in establishing phase relationships on the H2SO4-rich side of the diagram. The X-ray diffraction lines corresponding to Mg(OH) 2 were distinctive. The basal line (001) was very broad, the prism line (110) was very sharp and the other lines (hkl) were intermediate in breadth. This was interpreted as implying that Mg(OH) 2 crystallites adopt an exaggerated sheet-like morphology in this system (Urwongse & Sorrell, 1980b). The (001) line 300

Magnesium oxysulphate cements became sharper with time for a series of samples of Mg(OH)2 crystals left in contact with the liquid in sealed containers. By contrast, the prism line (110) remained sharp and unchanged for 64 days. Thesefindingsindicate that not only were the initially formed crystallites in the form of very thin sheets, but that crystal growth continued slowly with time in the c direction. The phase diagram constructed by Urwongse & Sorrell (1980b) for the chemical equilibria in the MgO-H2SO4-H2O system applies also to the MgO-MgSO4-H2O system, and is thus relevant for gaining an insight into the phase relationships which occur in the magnesium oxysulphate cements (Figure 7.3). There is, though, the possibility of non-equilibrium phases appearing. For example the phases 1:1:5 and MgSO4. 4H2O which have been found experimentally in this system have been assumed to arise because of localized temperature increases on mixing. These phases, which

80

O

PREPARED SAMPLES

•

REPORTED COMPOUNDS

©

1-1-3

NON-EQUILIBRIUM

MgS04"4H20 /

NON-EQUILIBRIUM

\ _

Mg(OH) ; SOLUBILITY

40

50

MEASUREMENTS

90

H 2 S0 4

WEIGHT PERCENT

Figure 7.3 Phase relationships in the MgO-H2SO4-H2O system at room temperature (Urwongse & Sorrell, 1980b).

301

Oxysalt bonded cements are known to be stable above room temperature, are then presumably stranded as the mixture cools. Further work over longer periods of time would be necessary to confirm this. The phase diagram of Urwongse & Sorrell (1980b) has important consequences for the formulation of magnesium oxysulphate cements. In particular, it indicates the impossibility of preparing cements at 23 °C which have more than 50 wt % of the 3:1:8 phase in them if the starting materials are MgO and aqueous MgSO4. Attempting to obtain more of the phase by the alternative tactic of reacting the oxide with sulphuric acid solutions is not practical because of the rapid formation of MgSO4. 7H2O from these starting materials. From these and other observations of the phase relationships, the bonding phase in commercial oxysulphate cements is concluded to be 5:1:3. In addition, varying amounts of MgSO4. 7H2O are likely to occur in thefinishedproduct, though this phase is not desirable and minimizing its formation must be part of the aim of formulating practical cements with optimum properties. 7.43

Mechanical properties of magnesium oxysulphate cements

The use of magnesium oxysulphate cement as a binder in building materials has been recognized since the earliest report of it by Sorel (1867). A study of the development of strength in these cements cured under pressure was carried out (Beaudoin & Ramachandran, 1978) in order to determine whether strength could be improved by this means. Other cementitious materials that usually produce weak specimens can undergo significant and technically useful increases in strength when cured under pressure, and the study aimed to discover whether magnesium oxysulphate also fell into this category. For this study a series of magnesium oxysulphate cements was prepared having different initial compositions from those prepared by Urwongse & Sorrell (1980b) in their study of the phase equilibria. Beaudoin & Ramachandran (1978) formulated their cements from solutions of MgO and MgSO4. 7H2O, the latter being dissolved in distilled water to give a saturated solution. Having prepared the cements, a number of physical techniques were employed to study them. Porosity was determined by measuring the apparent volume and the solid volume, the former by straightforward linear measurement, the latter using a helium pycnometer. This technique was used to avoid the problems of dissolution that arise when water is used 302

Magnesium oxysulphate cements in a displacement technique to evaluate porosity. However, because of the water-sensitivity of these cements, care was taken to condition them fully at 11 % r.h. prior to taking any measurements. In addition to porosity, modulus of elasticity and microhardness were determined. Differential thermograms were recorded, at a heating rate of 20 ° min"1. Compaction pressure and resulting porosity were found to be related in a logarithmic way, such that plotting log(compaction pressure) against porosity gave a straight line (Beaudoin & Ramachandran, 1978). This is typical for cementitious materials, though what was not typical was the abrupt change of slope at approximately 7-5 % porosity. Below this value the slope of the graph was found to be much steeper than at higher porosity. This result was held to arise from the porous nature of the particles bound together in the cement matrix. Above the critical value of porosity (7-5 %) compaction occurs when particles slide past each other as the matrix deforms. Below this value, compaction is possible only when the particles themselves are heavily deformed or fractured; this difference in mechanism results in the observed change in relationship between compaction pressure and porosity. The microhardness and modulus of elasticity were found to alter on compaction but in opposite directions. For microhardness, compacted specimens gave higher values than uncompacted, whereas for modulus of elasticity compacted specimens gave lower values. The reason for this was not clear, though a tentative explanation was advanced by Beaudoin & Ramachandran (1978). They suggested that modulus of elasticity is determined predominantly by the bonding between particles, while microhardness is largely dependent on the strength of the solid component. In the case of interparticle bonding, compaction causes bond fracture and particle slippage, and once the compacting force is removed only a fraction of the interparticle bonds will remain unaffected. Hence the property that depends on the strength of these interparticle forces, i.e. modulus of elasticity, will decrease. By contrast, microhardness depends on the extent to which the measurement of hardness reflects the value for the crystalline component, rather than the matrix phase, and this would favour compacted specimens. These explanations are somewhat speculative and qualitative, and it is not clear just how helpful they are in understanding the effect of compaction on these cements. One important finding which did emerge from the work of Beaudoin & Ramachandran (1978) was that, contrary to previous assumptions, 303

Oxysalt bonded cements magnesium oxysulphate cements are not significantly weaker than magnesium oxychloride cements, provided equal porosities are considered. Previous strength measurements had been done on samples at varying porosities, and since the oxysulphate cement more readily develops a porous structure, such a comparison always showed it up unfavourably. However, when specimens of equal porosity were examined, the oxychloride cements proved to be only marginally stronger than the oxysulphate cements. Overall, the major conclusion from this study was that the magnesium oxysulphate cement system responds differently from the Portland cement system to being formed under compaction. The principal difference is that the oxysulphate cements are weakened by compaction. This appears to be because compaction, particularly at higher porosities, alters the morphology of the particulate phase, changes the pore size distribution and reduces the extent of bonding between particles. Nevertheless, from the practical point of view, pressing oxysulphate cements in order to form them is capable of producing specimens of adequate strength for use in architectural applications. 7.5

Other oxysalt bonded cements

The three major oxysalt bonded cements that have already been described in detail in this chapter are not the only ones that have been prepared, though they are the ones that have been the most thoroughly studied. For example, Demediuk, Cole & Hueber (1955) gave some details about the calcium analogues of the magnesium oxychloride cements. Like the magnesium cements, they were fabricated by reaction of powdered metal oxide with aqueous solutions of metal chloride. The resulting calcium oxychloride cements were similar to the magnesium oxychloride cements. However, unlike the latter materials, they have found few or no architectural or similar applications, and as a result there has been hardly any interest in developing an understanding of their setting chemistry or structure. In a similar way there has been a passing reference to a cobalt oxychloride cement (Prosser et ai, 1986). No explicit details of the fabrication or chemical behaviour of this material were provided, but the ingredients were listed among series of acids and bases for forming cements as agents for the sustained release of trace elements to grazing animals. The implication of this paper was that cobalt oxide would function as the base 304

References and aqueous cobalt chloride as the acid, and that these two compounds would form cements that were chemically and structurally similar to the zinc and magnesium oxychloride materials. References Adomavichiute, O. B., Yanitskii, I. V. & Vektaris, B. I. (1962). On the hardening of magnesium cement. Zhurnal Priklandnoi Khimii, 35, 2551-4. Aspelund, H. (1933). Basic salts of bivalent metals: II. Acta Academiae Aboensis, 7 (6), 1-25. Beaudoin, J. J. & Feldman, R. F. (1975). Mechanical properties of autoclaved calcium silicate systems. Cement and Concrete Research, 5 (2), 103-18. Beaudoin, J. J. & Ramachandran, V. S. (1975). Strength development in magnesium oxychloride and other cements. Cement and Concrete, 5 (6), 617-30. Beaudoin, J. J. & Ramachandran, V. S. (1978). Strength development in magnesium oxysulphate cement. Cement and Concrete, 8 (1), 103-12. Beaudoin, J. J., Ramachandran, V. S. & Feldman, R. F. (1977). Impregnation of magnesium oxychloride cement with sulphur. Ceramic Bulletin, 56, 424-7. Bury, C. R. & Davies, E. R. H. (1932). System magnesium oxide-magnesium chloride-water. Journal of the Chemical Society, 2008-15. Cole, W. F. & Demediuk, T. (1955). X-ray, thermal and dehydration studies on magnesium oxychlorides. Australian Journal of Chemistry, 8, 234-51. Demediuk, T. & Cole, W. F. (1957). A study of magnesium oxysulphates. Australian Journal of Chemistry, 10, 287-94. Demediuk, T., Cole, W. F. & Hueber, H. V. (1955). Studies on magnesium and calcium oxychlorides. Australian Journal of Chemistry, 8, 215-33. Droit, A. (1910). Oxychlorides of zinc. Comptes rendus hebdomadaires des seances de V Academie des Sciences, 150, 1426-8. Eubank, W. R. (1951). Calcination studies of magnesium oxides. Journal of the American Ceramic Society, 34, 225-9. Feitknecht, W. (1930). Reaction of solid substances in liquids: 1. Helvetica Chimica Acta, 13, 22-43. Feitknecht, W. (1933). Structure of the basic salts of bivalent metals. Helvetica Chimica Acta, 16, 427-54. Feitknecht, W., Ostwald, H. R. & Forsberg, H. E. (1959). Uber die Struktur der Hydroxidchloride MeOHCl. Chimia, 13 (4), 113. Forsberg, H. E. & Nowacki, W. (1959). Crystal structure of ZnOHCl. Acta Chemica Scandinavica, 13 (5), 1049-50. Greenwood, N. N. & Earnshaw, A. (1984). The Chemistry of the Elements. Oxford: Pergamon Press. Harper, F. C. (1967). Effect of calcination temperature on the properties of magnesium oxides for use in magnesium oxychloride cements. Journal of Applied Chemistry, 17, 5-10. Hayek, E. (1932). Basic salts: I. Zeitschrift fur anorganische undallgemeine Chemie, 207, 41-5. 305

Oxysalt bonded cements Holland, H. C. (1930). The ternary system zinc oxide-zinc chloride-water. Journal of the Chemical Society, 643-8. Mellor, J. W. (1925). A Comprehensive Treatise on Inorganic and Theoretical Chemistry, vol. IV, pp. 535-46. London: Longmans Green. Montemartini, C. & Losana, L. (1929). Equilibria between double sulfates and aqueous solutions of sulfuric acids of various concentrations. Industria Chimica (Rome), 4, 199-205. Nowacki, W. & Silverman, J. N. (1961). Crystal structure of zinc hydroxychloride II, Zn 5 (OH) 8 Cl 2 .1H 2 O. Zeitschriftfur Kristallographie, Kristallgeometrie, Kristallphysik, Kristallchemie, 115, 21-51. Nowacki, W. & Silverman, J. N. (1962). Appendix to the paper 'Crystal structure of zinc hydroxychloride II, Zn 5 (OH) 8 Cl 2 .1H 2 O\ Zeitschrift fur Kristallographie, Kristallgeometrie, Kristallphysik, Kristallchemie, 117, 238-40. Prosser, H. J., Wilson, A. D., Groffman, D. M., Brookman, P. J., Allen, W. M., Gleed, P. T., Manston, R. & Sansom, B. F. (1986). The development of acid-base cements as formulations for the controlled release of trace elements. Biomaterials, 7, 109-12. Robinson, W. O. & Waggaman, W. H. (1909). Basic magnesium chlorides. Journal of Physical Chemistry, 13, 673-8. Sorel, S. (1855). Procedure for the formation of a very solid cement by the action of a chloride on the oxide of zinc. Comptes rendus hebdomadaires des seances de P Academie des sciences, 41, 784-5. Sorel, S. (1867). On a new magnesium cement. Comptes rendus hebdomadaires des seances de TAcademie des sciences, 65, 102^. Sorrell, C. A. (1977). Suggested chemistry of zinc oxychloride cements. Journal of the American Ceramic Society, 60, 217-20. Sorrell, C. A. & Armstrong, C. R. (1976). Reactions and equilibria in magnesium oxychloride cements. Journal of the American Ceramic Society, 59, 51-4. Urwongse, L. & Sorrell, C. A. (1980a). The system MgO-MgCl 2 -H 2 O. Journal of the American Ceramic Society, 63, 501—4. Urwongse, L. & Sorrell, C. A. (1980b). Phase relationships in magnesium oxysulfate cements. Journal of the American Ceramic Society, 63, 523-6. Walter-Levy, L. (1937). Neutral chlorocarbonate of magnesium. Comptes rendus hebdomadaires des seances de VAcademie des Sciences, 204, 1943-6. Walter-Levy, L. (1938). Contribution to the study of halogenocarbonates of magnesium. Comptes rendus hebdomadaires des seances de VAcademie des sciences, 205, 1405—7. de Wolff, P. M. & Walter-Levy, L. (1953). Crystal structure of Mg2(OH)3(Cl, Br), 4H 2 O. Acta Crystallographica, 6, 40-4.

306

8

Miscellaneous aqueous cements

8.1

General

This chapter is devoted to a miscellaneous group of aqueous acid-base cements that do not fit into other categories. There are numerous cements in this group. Although many are of little practical interest, some are of theoretical interest, while others have considerable potential as sustainedrelease devices and biomedical materials. Deserving of special mention as biomedical materials of the future are the recently invented polyelectrolyte cements based on poly(vinylphosphonic acids), which are related both to the orthophosphoric acid and poly(alkenoic acid) cements. 8.2

Miscellaneous aluminosilicate glass cements

In 1968 Wilson published an account of his early search for alternatives to orthophosphoric acid as a cement-former with aluminosilicate glasses. Aluminosilicate glasses of the type used in dental silicate cements were used in the study and were reacted with concentrated solutions of various organic and inorganic acids. Wilson (1968) made certain general observations on the nature of cement formation which apply to all cements based on aluminosilicate glasses. (1) Silica gel is formed in the reaction but is not associated with cement formation. (2) Water is essential to the reaction and cements are not formed when the acid is present in an organic solvent rather than in aqueous solution. Water acts as an effective reaction medium and probably hydrates reaction products. (3) Cements are formed only with acids that are capable of forming complexes with calcium and aluminium. Thus, neither hydrochloric nor acetic acid forms cements. 307

Miscellaneous aqueous cements Table 8.1. Properties of aluminosilicate glass cements prepared with various acids in aqueous solution {Wilson, 1968) Acid aqueous solution, % by mass Tannic acid, 50 % Tartaric acid, 50 % Citric acid, 50 % Pyruvic acid, 50 % Malic acid, 50% Fluoboric acid, 42 % Glycerol phosphoric acid, 35% Conventional silicate cement phosphoric acid liquid

Compressive Powder: Setting strength liquid, time, (24 hour), gem"3 minutes MPa

Water-leachable material," % mass

40 40 40 40 40 3-5 40

8-0 4-5 6-5 5-5 150 2-2 3-2

77 75 42 36 35 106 38

11 15 CD. CD. CD. 6-3 3-3

40

3-5

272

01

a

On 24-hour-old cements. CD. Complete disintegration.

(4) Cements set rapidly. (5) Cement formation with carboxylic acids is associated with carboxylate (salt) formation. (6) The ability of a cement to resist aqueous attack depends on the nature of the acid anion. The properties of these cements are given in Table 8.1. All the cement-forming organic acids were multifunctional and capable of forming strong chelates with aluminium and weaker chelates with calcium (Perrin, 1964; Martell & Calvin, 1952). The cements of citric, pyruvic and malic acids were found to disintegrate completely when placed in water, in accordance with the soluble nature of their metal complexes. The disintegration of the tartrate cement, though substantial, was incomplete - the aluminium salt is soluble but that of calcium is not. The tannic acid cement alone had reasonable hydrolytic stability, but then tannic acid forms an insoluble salt with aluminium (Welcher, 1947). This cement was the strongest, with a compressive strength of 77 MPa. Fluoboric acid was also found to form a cement of some strength (106 MPa) but of poor hydrolytic stability. 308

Phytic acid cements From the practical point of view these results were disappointing, but it was from this unpromising start that the successful glass polyalkenoate cement was eventually developed.

8.3

Phytic acid cements

Phytic acid, myo-inositol hexakis(dihydrogen phosphate), is an abundant constituent of plants (Graf, 1986). Its structure is that of a symmetrical sixmembered ring carrying six dihydrogen phosphate groups (Figure 8.1). This structure suggests that it should form strong complexes, and this is so, with the strength of the complex increasing with the valency of the cation (Graf, 1986). Most of the complexes with polyvalent cations are insoluble, which makes phytic acid a candidate for cement formation. Indeed, it has been found to form cements with zinc oxide and aluminosilicate glasses (Lion Corporation, 1980; Prosser et ai, 1983; Prosser & Wilson, 1986) and these may be compared with phosphate-bonded cements. Cements based on phytic acid set more quickly than their glass polyalkenoate or dental silicate cement counterparts, but have similar mechanical properties (Table 8.2). They are unique among acid-base cements in being impervious to acid attack at pH = 2-7. Unfortunately, they share with the dental silicate cement the disadvantage of not adhering to dentine. They do bond to enamel but this is by micromechanical attachment - the cement etches enamel - and not by molecular bonding. Lack of adhesive property is a grave weakness in a modern dental or bone

Figure 8.1 The structure of phytic acid.

309

Miscellaneous aqueous cements Table 8.2. Properties of cements based on phytic acid (Prosser et al., 1983) G-5

G-200 Phytic acid, 40% Powder: liquid, g cm 3 Working time (23 °C), minutes Setting time (37 °C), minutes Compressive strength" (24 hour), MPa Tensile strength" (24 hour), MPa Opacity, C 0 7 Water-leacnable material 7 minute cure, % mass Water-leachable material 1 hour cure, % mass Acid erosion pitting depth, urn/hour

PAA 50%

Phytic acid, 40 %

40 1-5

30 50

30 3-2

2-7

4-3

3-8

201 12-6

166 140

168 5-7

Cement liquid6 3-5 — 50 183 13-0

0-78 0-88

0-85 21

0-88 0-74

0-55 1-25

0-40

0-45

0-46

0-70

0

11-7

0

12-3

a

Specimens stored for 24 hours in water (37 °C). Cement Liquid: 48-3% H 3 PO 4 ; 2-3% Al; 4-9% Zn. G-200: 29-0% SiO 2 ; 16-6% A12O3; 34-3% CaF 2 ; 3-0% NaF; 2-0% A1F3; 9-9% A1PO4. G-5: 43-7% SiO 2 ; 23-0% A12O3; 12-9% CaF 2 ; 10-5% NaF; 7% A1F3; 2-9% A1PO4. b

cement. Nor do phytic acid cements have the translucency of the dental silicate cement. Unless progress is made in remedying these two disadvantages it is doubtful whether they will find any use despite their excellent resistance to erosion. More promising are the poly(vinylphosphonic acid) cements that are described next. 8.4

Poly(vinylphosphonic acid) cements

Ellis and Wilson studied cements formed from concentrated solutions of poly(vinylphosphonic acid) (PVPA) and oxides and silicate glasses, which they termed metal oxide and glass polyphosphonate cements (Wilson & 310

Poly{vinylphosphonic acid) cements Ellis, 1989; Ellis, 1989; Ellis & Wilson, 1990, 1991, 1992; Ellis, Anstice & Wilson, 1991). They are polyelectrolyte cements related to the polyalkenoate cements and represent an attempt to improve on them. PVPA (Figure 8.2) has a structure similar to that of PAA (Figure 5.2). PVPA was prepared by the free-radical homopolymerization of vinylphosphonyl dichloride using azobisisobutyronitrile as initiator in a chlorinated solvent. The poly(vinylphosphonyl chloride) formed was then hydrolysed to PVPA (Ellis, 1989). No values are available for the apparent pATas of PVPA, but unpolymerized dibasic phosphonic acids have pKal and pKa2 values similar to those of orthophosphoric acid, i.e. 2 and 8 (Van Wazer, 1958). They are thus stronger acids than acrylic acid, which as a pKa of 4-25, and it is to be expected that PVPA will be a stronger and more reactive acid than poly (aery lie acid).

8.4.1

Metal oxide polyphosphonate cements

Ellis & Wilson (1991, 1992) examined cement formation between a large number of metal oxides and PVPA solutions. They concluded that setting behaviour was to be explained mainly in terms of basicity and reactivity, noting that cements were formed by reactive basic or amphoteric oxides and not by inert or acidic ones (Table 8.3). Using infrared spectroscopy they found that, with one exception, cement formation was associated with salt formation; the phosphonic acid band at 990 cm"1 diminished as the phosphonate band at 1060 cm"1 developed. The anomalous result was that the acidic boric oxide formed a cement which, however, was soluble in water. This was the result, not of an acid-base reaction, but of complex formation. Infrared spectroscopy showed a shift in the P = O band from 1160 cm"1 to 1130 cm"1, indicative of an interaction of the type

Setting times and hydrolytic stability of these cements are given in Table 8.3. In some cases the speed of reaction was very high, and practical cements could not be formed from ZnO or CaO even when these oxides were deactivated by heating. All the faster-setting cements exhibited good hydrolytic stability. The stability of the complexes between divalent cations and PVPA was found by a titrametric procedure to follow the order Mg ~ Ca < Cu ~ Zn (Ellis & Wilson, 1991). This result was 311

Miscellaneous aqueous cements Table 8.3. Properties of metal oxide phosphonic acid cements {Ellis, 1989; Ellis & Wilson, 1991) Oxide

Setting time

Hydrolytic stability

ZnO, MgO, CaO, Co(OH) 2 , HgO, CdO Bi2O3, PbO

< 1 minute

Stable

1-5 minutes 1-5 minutes 5-20 minutes 20-60 minutes 1-24 hours 1-24 hours 1-24 hours non-setting

stable complete disintegration stable stable stable softened in water complete disintegration

BA

CuO, Pb 3 O 4 , Y 2 O 3 , La 2 O 3 Cu 2 O CoO, SnO, MoO 3 Fe 3 O 4 , MnO 2 In 2 O 3 , Cr 2 O 3 , CrO 3 A12O3, SiO2, ZrO 2 , WO 3

—

expected, since Cu 2+ and Zn 2+ with filled d orbitals tend to form bonds of a covalent character. However, Mg 2+ and Ca 2+ do form stable cements with PVPA, though not with PAA. This is to be attributed to the higher degree of site binding of cations to PVPA compared with PAA (Begala & Strauss, 1972; Strauss & Leung, 1965).

I

CH2

I

OH

CH — CH 2

OH OH

CH — OH Figure 8.2 The structure of poly(vinylphosphonic acid).

I

P-O — M

I

0

I

=

P-0 0— M

o'l\> I o i

I M M Figure 8.3 Possible structures of metal-polyphosphonate complexes. M

312

M

M

Poly(vinylphosphonic acid) cements Table 8.4. Properties of MgO, CuO, Cu2O, Bi2Oz, La2O z phosphoric acid cements (Ellis, 1989; Ellis & Wilson, 1990) Oxide

MgO

CuO

Cu 2 O

Bi2O3

La 2 O 3

Powder:liquid, gcm" 3 Setting time (37 °C), minutes Compressive strength 7 day (water), MPa Flexural strength 7 day (water) 24 hour, MPa Water-leachable material 1 hour cure, % mass Shrinkage, linear %

10 5-7 56-6

50 7-3 54-5

50 — 31*5

50 2-0 18-2

50 — 11-9

4-5

120

—

5-1

—

0-24

00

—

005

—

6-8

1-7

1-3

3-0

31

Table 8.5. Adhesive bond strength of the magnesium phosphonate cement to dentine and enamel (Ellis, 1989) Bond strength, MPa Cement

Dentine

Enamel

MgO-PVPA Zinc polycarboxylate [1] Glass polyalkenoate [2]

3-4 3-2-5-5 1-9-6-8

4-9 4-1-6-9 3-2-9-9

[1] Walls (1986); [2] Wilson & McLean (1988b).

The exact nature of metal-poly(phosphonic acid) interaction is unknown (Ellis, 1989) although a number of structures can be drawn (Figure 8.3). The properties of the most promising cements - those of MgO, CuO, and Bi 2 O 3 -are given in Table 8.4. All had a short and sharp set. Compressive and flexural strengths, although moderate, compare fairly well with those of commercial zinc polycarboxylate cements. All cements shrank when exposed to an atmosphere of 50 % relative humidity. The most promising is the cement based on MgO which adheres to both dentine and enamel with about the same bond strength as the glass polyalkenoate and zinc polycarboxylate cements (Table 8.5).

313

Miscellaneous aqueous cements 8.4.2

Glass polyphosphonate cements

Ellis and Wilson also examined cement formation from aluminosilicate glasses and concentrated solutions of PVPA (Wilson & Ellis, 1989; Ellis, 1989; Ellis & Wilson, 1990). These cements, like the glass polyalkenoate cements, are a type of glass-ionomer cement. Disadvantages of the glass polyalkenoate cements are the susceptibility of the young cement to aqueous attack and problems in achieving sufficient translucency to match that of tooth enamel. PVPA has a higher refractive index than PAA, and should lessen the refractive index mismatch between glass particle and cement matrix which is the cause of light-scattering. PVPA is also a much stronger acid than PAA and the pendant groups are bifunctional. For this reason it should form stronger associations with cations and produce cements that set more rapidly. The reactivity of the PVPA solutions with SiO 2 -Al 2 O 3 -CaO and SiO 2 -Al 2 O 3 -CaF 2 glasses did, as expected, prove greater than that of PAA solutions. Its cements had shorter setting times, as is shown by Figure 8.4 which depicts the setting of cements formed from glasses having the generic composition (mole ratios) zSiO 2 , 1-00 A12O3, 1-30 CaF 2 (Ellis, Anstice & Wilson, 1991). Note that the glass with Si/Al mole ratio of 2-55 forms a cement with PVPA but not with PAA. Decreasing the Si/Al mole ratio accelerates cement formation, and the setting time of cements is corre1000 i -

Clear glasses 0.5

1.0

2.0

2.5

Si / Al mole ratio

Figure 8.4 The effect of Si/Al ratio in the glass on the setting time of glass polyphosphonate and glass polyalkenoate cements (Ellis, 1989).

314

Copper oxide and cobalt hydroxide cements spondingly reduced. This result, which is similar to that found for glass polyalkenoate cements, is to be expected as replacement of Si by Al must weaken the glass network for reasons discussed in Section 5.9.2. When the Si/Al mole ratio reaches 1-1, glass PVPA cements set impossibly fast. Below a mole ratio of 0-57 the effect is reversed as phase separation of fluorite (CaF2) and corundum (A12O3) occurs (glasses with an Si/Al ratio at or above 0-80 are clear). Phase separation reduces the reactivity of the main glass phase as the Si/Al ratio is increased andfluorideis withdrawn. Heat treatment of the glasses at 600 °C for 6 hours reduced their reactivity, by promoting phase separation, and prolonged the setting time of cements. Thus, the setting time of the cement formed from one glass (Si:Al = 1-7) was increased from 2-0 minutes to 5-3 minutes and that of another glass (Si: Al = 2-0) from 3*6 minutes to 25 minutes. These glass polyphosphonate cements are still in an early stage of development. A recent paper by Ellis, Anstice & Wilson (1991) reports a maximum compressive strength of 90 MPa and a maximum flexural strength of 10 MPa. Although more recent data indicate that a compressive strength of 150 MPa is possible these values are still much lower than those recorded for the best dental silicate and glass polyalkenoate cements systems which are, of course, fully developed. The translucency of the glass polyphosphonate cement is good, with an opacity value sufficiently low (C07 = 0-55) to match that of tooth enamel. Its resistance to early contamination by water is very good and much superior to that of the glass polyalkenoate cement. The solubility of a seven-minute-old cement is 0*5 % which compares favourably with values of 1-0-2-1 % reported for glass polyalkenoate cement (Wilson & McLean, 1988a). The solubility of one-hour-old cements is infinitesimal (< 0-05%) and very much lower than that of the glass polyalkenoate cement (0-17-0-33%).

8.5

Miscellaneous copper oxide and cobalt hydroxide cements

Copper(II) oxide and cobalt(II) hydroxide form cements with solutions of many multifunctional organic acids: propanetricarboxylic acid, tartaric acid, malic acid, pyruvic acid, mellitic acid, gallic acid, tannic acid and phytic acid (Allen et al., 1984; Prosser et al., 1986). These have been used mainly in cement devices for the sustained release of copper and cobalt (Manston et al., 1985; Manston & Gleed, 1985). Little is known about 315

Miscellaneous aqueous cements

their structure and mechanical properties. Most devices that have been used in animal husbandry have been based on acid phosphates and have been dealt with in Section 6.3. References Allen, W. M., Sansom, B. F., Wilson, A. D., Prosser, H. J. & Groffman, D. M. (1984). Release cements. British Patent GB 2,123,693 B. Begala, A. J. & Strauss, U. P. (1972). Dilatometric studies of counterion binding by polycarboxylates. Journal of Physical Chemistry, 76, 254-60. Ellis, J. (1989). Materials based on polyelectrolytes. PhD. Thesis (Council for National Academic Awards): Thames Polytechnic and Laboratory of the Government Chemist, London. Ellis, J., Anstice, M. & Wilson, A. D. (1991). The glass polyphosphonate cement: a novel glass-ionomer cement based on poly(vinylphosphonic acid). Clinical Materials, 7, 341-6. Ellis, J. & Wilson, A. D. (1990). Polyphosphonate cements: a new class of dental materials. Journal of Materials Science Letters, 9, 1058-60. Ellis, J. & Wilson, A. D. (1991). A study of cements formed between metal oxides and polyvinylphosphonic acid. Polymer International, 24, 221-8. Ellis, J. & Wilson, A. D. (1992). The formation and properties of metal oxide poly(vinylphosphonic acid) cements. Dental Materials, 8, 79-84. Graf, E. (1986). Chemistry and applications of phytic acid: an overview. In Graf, E. (ed.) Phytic Acid, Chapter 1. Minneapolis: Pilatus Press. Lion Corporation. (1980). Dental cements. Nihon Kokai Tokkyo Koho 80,139,311. Chemical Abstracts, 94, 903,488b. Manston, R. & Gleed, P. T. (1985). Reaction cements as materials for the sustained release of trace elements into the digestive tract of cattle and sheep. II. Release of cobalt and selenium. Journal of Veterinary Pharmacology Therapeutics, 8, 374-81. Manston, R., Sansom, B. F., Allen, W. M., Prosser, H. J., Groffman, D. M., Brant, P. J. & Wilson, A. D. (1985). Reaction cements as materials for the sustained release of trace elements into the digestive tract of cattle and sheep. I. Copper release. Journal of Veterinary Pharmacology Therapeutics, 8, 368-73. Martell, A. E. & Calvin, M. (1952). Chemistry of the Chelate Compounds, p. 415. New York: Prentice-Hall. Perrin, D. D. (1964). Organic Complexing Reagents, p. 269. New York: Interscience Publishers. Prosser, H. J., Brant, P. J., Scott, R. P. & Wilson, A. D. (1983). The cementforming properties of phytic acid. Journal of Dental Research, 62, 598-600. Prosser, H. J. & Wilson, A. D. (1986). The cement-forming properties of phytic acid. In Graf, E. (ed.) Phytic Acid, Chapter 17. Minneapolis, Minnesota: Pilatus Press. Prosser, H. J., Wilson, A. D., Groffman, D. M., Brookman, P. J.? Allen, W. M., Gleed, P. T., Manston, R. & Sansom, B. F. (1986). The development of 316

References acid-base reaction cements as formulations for the controlled release of trace elements. Biomaterials, 1, 109-12. Strauss, U. P. & Leung, Y. P. (1965). Volume changes as a criterion for site binding of counterions by polyelectrolytes. Journal of the American Chemical Society, 87, 1476-80. Van Wazer, J. R. (1958). Phosphorus and its Compounds, pp. 486-91. New York: Interscience Publishers Inc. Walls, A. W. G. (1986). Glass polyalkenoate (glass-ionomer) cements: a review. Journal of Dentistry, 14, 231-46. Welcher, F. J. (1947). Organic Analytical Reagents, vol. 2, pp. 142-3. New York: Van Nostrand. Wilson, A. D. (1968). Dental Silicate Cements: VII. Alternative liquid cement formers. Journal of Dental Research, 47, 1133-6. Wilson, A. D. & Ellis, J. (1989). Poly-vinylphosphonic acid glass ionomer cement. British Patent Application 2,219,289A. Wilson, A. D. & McLean, J. W. (1988a). Glass-ionomer Cement, Chapter 4. Chicago, London and Berlin: Quintessence Publishing Co. Wilson, A. D. & McLean, J. W. (1988b). Glass-ionomer Cement, Chapter 6. Chicago, London and Berlin: Quintessence Publishing Co.

317

9

Non-aqueous cements

9.1

General

The non-aqueous or chelate cements are an exceptionally diverse group of materials (Wilson, 1975a,b, 1978; Smith, 1982b). The term chelate cement is not strictly speaking correct, as a minority of them do not form chelates, and some aqueous AB cements do. However, the term is a convenient one. They are of interest in that the reaction media for the acid-base reaction are non-aqueous, although sometimes water may play a role in cement formation. In these cements water is replaced by an organic acid that is liquid at room temperature and generally has chelating ability. The low permittivity of these liquids compared with water inhibits dissociation of the acids so that cement formation demands much more reactive basic oxides. Oxides and hydroxides that are capable of cement formation are ZnO, CuO, MgO, CaO, Ca(OH)2, BaO, CdO, HgO, PbO and Bi2O3 (Brauer, White & Moshonas, 1958; Nielsen, 1963). In practice these are confined to two: calcium hydroxide and special reactive forms of zinc oxide. Examples of liquid organic acids suitable for cement formation are: (1) Alkoxyphenols, for example the 2-methoxyphenols, which include eugenol, guaiacol and vanillates. Also, 2,5-dimethoxyphenol. (2) /?-diketones, e.g. acetylacetone. (3) /?-keto esters, e.g. jS-keto-ethylate. (4) Keto acids, e.g. acetyl acetic acid. (5) Other difunctional aliphatic carboxylic acids, e.g. lactic acid, pyruvic acid, ethoxyacetic acid. (6) Aldehydic aromatic acids, e.g. salicylaldehyde. (7) Alkoxy aromatic acids, e.g. 2-ethoxybenzoic acid.

318

General

These examples are drawn from the work of Brauer, White & Moshonas (1958), Nielsen (1963), Brauer, Argentar & Durany (1964), Stansbury, Argentar & Brauer (1981), and Brauer, Argentar & Stansbury (1982). All of these organic liquids are capable of forming chelates. They all contain an acidic group (COOH, phenolic OH or enol OH) and a second functional group, such as an ester or an ether, containing an oxygen capable of donating an electron pair. The structural requirement is that a five- or six-membered chelate ring is formed. Typical examples are shown in Figure 9.1. That chelate formation is involved in cement formation is demonstrated by the behaviour of the methoxyphenols. The only methoxyphenols which are capable of cement formation, the 2-substituted, are those which are able to form chelates (Brauer, Argentar & Durany, 1964). Further, Douglas (1978a,b) has shown that magnesium, calcium and zinc form chelates with potassium guaiacol-4-sulphonate, that the zinc chelate is the strongest, and that at pH ~ 6 hydroxy complex formation does not seriously compete with chelate formation. In addition, chelate formation has been reported between /?-diketones and a number of metals including zinc and copper (Graddon, 1968). Some of these complexes are multinuclear. In zinc complexes the preferred coordination number is five although four- and six-coordination are also observed. Related to these cements are the long chain aliphatic acids and arylsubstituted butyric acid (Skinner, Molnar & Suarez, 1964). These materials are on the market as non-eugenol cementing agents but they are unduly

(a)

(b)

Figure 9.1 Chelating agents capable of forming cements, (a) 2-methoxyphenol type. Guaiacol: R, = R2 = H. Eugenol: Rx =-CH 2 -CH=CH 2 , R2 = H. (b) ^-diketone type.

319

Non-aqueous cements weak and can only be used for the temporary cementation of crowns (Powers, Farah & Craig, 1976). Although there are many potential chelate cements only three groups are of any significance: the zinc oxide eugenol (ZOE) cement, the 2-ethoxybenzoic acid (EBA) cements and the calcium hydroxide alkylsalicylate cements. Only cements based on these three groups have been commercially exploited. Their mechanical properties are poor, but all are noted for having therapeutic effects and this is perhaps their most important asset in biological applications. In effect, they are also devices for the sustained release of biologically active agents. They are extensively used for temporary cementation of crowns and as temporaryfillingmaterials because of their therapeutic action. It is claimed that when strengthened some can be used for permanent cementation or as an intermediate restoration, that is a restoration that has a life somewhat greater than a temporary filling material. However, it must be pointed out that these claims have been made on the basis of strength measurements made at room temperature rather than 37 °C (body temperature), and so may be misleading. This is especially true of these materials. When measurements have been made at 37 °C (0ilo & Espevik, 1978), these cements, in contrast to the water-based ones, show a marked decline in all mechanical properties. Flow under load increases sharply and strength decreases by almost an order of magnitude. This point must be borne in mind when reading the following section of this chapter where the reported strength measurements have been made at room temperature.

9.2

Zinc oxide eugenol {ZOE) cements

9.2.1

Introduction and history

The zinc oxide eugenol (ZOE) cement is formed by mixing a reactive form of zinc oxide with eugenol. It is widely used in dentistry for the temporary cementation of crowns, for the temporary filling of teeth, as a root canal sealer, as a sedative cavity liner, as a base in deep cavities and in soft tissue packs for use in oral surgery (Brauer, 1965; Wilson, 1975b; Smith, 1982a). Suitably modified with diluents and fillers it can also be used as an impression paste (Phillips, 1982a,b). The ZOE cement has a long history. Eugenol is the essential constituent of oil of cloves, which has been used medically since the fourth century 320

Zinc oxide eugenol (ZOE) cements (Molnar, 1942). Its use specifically to relieve toothache was recorded by Vigo in the sixteenth century and reactions with metal oxides were reported by Bonastre (1827a,b). The earliest zinc oxide chelate cements used creosote (King, 1872) and later this was mixed with oil of cloves (Chisholm, 1873). Then oil of cloves was used by itself (Flagg, 1875) and finally its essential constituent, eugenol (Wessler, 1894). This cement has retained its popularity to this day, despite its poor mechanical properties, because it is easy to use - it makes no demands on technique - is tolerant towards living tissues and is a palliative or anodyne. It is unlikely to be replaced, because, fortuitously, it is formed from two medicaments and, again fortuitously, is a sustained release agent for eugenol. Cement formation is the result of an acid-base reaction between zinc oxide and eugenol, leading to the formation of a zinc eugenolate chelate. Water plays a vital role in the reaction. Important reviews on these materials have appeared from time to time: Brauer (1965), Wilson (1975a,b, 1978), Smith (1982a). 9.2.2

Eugenol

Eugenol, 4 allyl-2-methoxy phenol, is capable of forming cements with ZnO, CuO, MgO, CaO, CdO, PbO and HgO (Brauer, White & Moshonas, 1958; Nielsen, 1963). Other 2-methoxy phenols are also capable of forming cements with metal oxides, provided the allyl group is not in a 3- or 6-position where it sterically hinders the reaction (Brauer, Argentar & Durany, 1964). These include guaiacol, 2-methoxyphenol, and the allyl and propylene 2-methoxy phenols. Eugenol is a very weak acid with a pK of 10-4 (Brauer, Argentar & Durany, 1964) and occurs as a hydrogen-bonded dimer (Gerner et al., 1966; Wilson & Mesley, 1972). The dimer contains both intra- and intermolecular hydrogen bonds (structure I in Figure 9.2a). The presence of an intramolecular hydrogen bond indicates that eugenol is in the cis form. 9.2.3

Zinc oxide

Active zinc oxide is capable of forming chelate cements with a number of liquid organic chelates. These include the /?-diketones, ketoacids and ketoesters as well as the 2-methoxy phenols (Nielsen, 1963). 321

Non-aqueous cements The zinc oxide used in ZOE cements differs entirely from that used in zinc phosphate cements. Whereas the latter has to be ignited to a very high temperature to deactivate it, the opposite is true of the zinc oxides used in the ZOE cement, which are of an activated variety. They are normally prepared by the thermal decomposition of zinc salts at 350 °C to 450 °C; such oxides are yellow. Zinc oxides prepared by oxidizing zinc in oxygen may also be used; these are white. Further discussion of zinc oxide is deferred until the setting reaction is considered (Section 9.2.5). 9.2.4

Cement formation

The earliest ZOE cements were prepared simply by mixing zinc oxide with eugenol. These cements set under the moist conditions of the mouth and then only slowly. Unlike other dental cements, the cement-forming reaction of the ZOE cement requires acceleration rather than retardation (Smith, 1958; Wilson & Batchelor, 1970; Crisp, Ambersley & Wilson, 1980). Although it is possible to make cements from zinc oxide and plain eugenol by using a very reactive zinc oxide (Smith, 1960), the setting behaviour of these cements is sensitive to variations in humidity and the source of materials (Smith, 1982a). Commercial cements always contain accelerators. The most common accelerators are acetic acid (0-1 % to 2%) dissolved in the eugenol, and zinc acetate or other zinc alkanoate (0-1 % to 8%) blended with the zinc oxide powder (Wilson & Batchelor, 1970; Wilson 1975b). Zinc propionate or succinate is also effective (Phillips, 1982b). Molnar & Skinner (1942) found that many salts acted as accelerators. The most effective acetates were those of silver, sodium and zinc. Chlorides and nitrates were even more effective. Rosin (abietic acid) also produced an accelerating effect. In practice, nearly all commercial materials used either acetic acid in the liquid, a zinc alkanoate, or even zinc eugenolate (20 %) itself. Zinc chloride has been found dissolved in massive amounts in eugenol (10 %); the resulting cement is a cross between a zinc oxychloride and eugenolate cement. The question of 2-ethoxybenzoic acid (EBA) as an accelerator is deferred until the EBA-eugenol cements are discussed (Section 9.4). The setting reaction of all cements is accelerated by increase in temperature.

322

Zinc oxide eugenol (ZOE) cements 9.2.5

Setting

Zinc oxide eugenol cements set and harden as the result of ionic reactions, and physical changes are related to these underlying chemical ones. The setting reaction has been studied systematically first by Copeland et al. (1955) and by a number of workers since. It is still not fully understood. The reaction is essentially an acid-base one with eugenol providing hydrogen ions. Copeland et al. (1955) established that the product of reaction had the empirical molecular formula Zn(C10H11O2)2 and that its XRD pattern corresponded closely to that of zinc eugenolate salt. Wilson & Mesley (1972) confirmed this finding. The overall reaction can be represented by the following equation, where HE represents eugenol and E eugenolate. ZnO + 2HE = ZnE2 + H2O

[9.1]

In fact, the reaction is an ionic one involving zinc and eugenolate ions. A number of infrared spectroscopic studies have been made which have thrown light on the cement-forming reaction (Copeland et al., 1955; Gerner et al., 1966; Wilson & Mesley, 1972). Wilson & Mesley (1972) used ATR spectroscopy to follow the course of the reaction and showed that major spectral changes were almost entirely associated with loss of the CH2 = CH CH2

CH2 = CH CH,

v

,0 - CH3

CH2CH = CH,

\ II Matrix Region

I

CH2 CH = CH2 Water eluted

Figure 9.2a Hydrolysis of the zinc eugenolate bis chelate to the hydrogen bonded eugenol dimer and zinc hydroxide. After Wilson & Mesley (1974).

323

Non-aqueous cements O-H group. In the cement-forming reaction, phenolic hydrogens (O-H stretching bands at 3520 and 3460 cm"1) are replaced by zinc ions, and a weak chelate is formed. In effect, the hydrogen bonds of the eugenol dimer are replaced by stronger Zn2+ bridges. Wilson & Mesley (1972) found little or no free eugenol after 30 minutes, when the reaction was deemed to be complete. The bisligand chelate structure which is formed differs little from that of the parent eugenol dimer (Structure II in Figure 92a). The molecule is an electrically neutral chelate where two eugenolate molecules are attached to a central zinc atom in square planar or tetrahedral configuration (Figure 92b). The CH3O-Zn coordinate bond in the zinc eugenolate chelate is very weak (Gerner et ai, 1966) and the chelate has poor stability thus, the cement-forming reaction [9.1] can be reversed. This occurs when the cement is placed in water, when the matrix is easily hydrolysed to eugenol and zinc hydroxide (Figure 9.2a) (Wilson, 1978; Wilson & Batchelor, H2O CH2=CH.CH

CH2.CH=CHo

CH2=CH.CH

CH2.CH=CH2

CH2=CH.CH

CH2.CH=CH2

CH H2O

Figure 9.2b The bisligand chelate structure of zinc eugenolate, showing bridging water molecules. After Wilson & Mesley (1974).

324

Zinc oxide eugenol (ZOE) cements 1970). Eugenol can also be extracted from the cement matrix by methanol (Molnar, 1967); this is further evidence of the weakness of the chelate, which is decomposed during the extraction. Water was not found in the set cement by Wilson & Mesley (1972), who reasoned that it had entered the matrix. They speculated that zinc was in octahedral coordination with two eugenolate molecules in planar positions and two shared water molecules occupying diametrically opposed sites. These water molecules acted as bridges in the individual chelates. This hypothesis received support from the electrical studies of Braden & Clarke (1974) and Crisp, Ambersley & Wilson (1980), who attributed maxima in curves of permittivity and conductivity against time to the liberation of water and its subsequent reabsorption into the matrix (Figure 93a,b). Crisp, Ambersley & Wilson (1980) also considered that these maxima were due to generation of both water and ionic zinc species. Subsequently, as the reaction proceeds the zinc ions arefixedas insoluble zinc eugenolate.

160

I 120

80

40

20

40

120 140 160 TIME(minutM)

180

200

220

240

260

280~

Figure 9.3a Permittivity/time curves (s) of cements, showing maxima. Prepared from ZnO ignited at 600 °C with: A - - - A dry eugenol; # . . . • eugenol+1% water; O - . - O eugenol + 1 % chloracetic acid; • • • • • eugenol + 1 % acetic acid; • . . . • eugenol + 1 % acetic a c i d + 1 % water. Cement powder/liquid ratio = 2-5 g cm" 3 (Crisp, Ambersley & Wilson, 1980).

325

Non-aqueous cements Eugenol is a very weak acid (pK= 10-4) and will not react with zinc oxide in the absence of promoters. These reaction promoters include water, acetic acid and zinc acetate. The role of water in setting The importance of water as an initiator and catalyst for the reaction between zinc oxide and plain eugenol has been demonstrated by a number of studies (Smith, 1958; Crisp, Ambersley & Wilson, 1980; Batchelor & Wilson, 1969; Prosser & Wilson, 1982). In particular, the reaction is accelerated by the humidity of the atmosphere during mixing (Batchelor & Wilson, 1969; Crisp, Ambersley & Wilson, 1980). As we shall see later, the catalytic effect of water is connected with the

120

140 1M I N Tim* (minute)

Figure 93b Conductivity/time curves (a) of cements, showing maxima. Prepared from ZnO ignited at 600 °C with: A . . . A dry eugenol; * . . . • eugenol+1% water; O - . - O eugenol + 1 % chloracetic acid; • • • • • eugenol + 1 % acetic acid; • . . . • eugenol + 1 % acetic a c i d + 1 % water. Cement powder/liquid ratio = 2-5 g cm"3 (Crisp, Ambersley & Wilson, 1980).

326

Zinc oxide eugenol (ZOE) cements presence of loosely absorbed water on the surface of zinc oxide particles and not with water contained in eugenol (Prosser & Wilson, 1982). The addition of water to eugenol has relatively little effect on the setting rate (Crisp, Ambersley & Wilson, 1980). On the other hand zinc oxide powders that do not have water absorbed on the surface react very slowly with plain eugenol (Prosser & Wilson, 1982). Of course, a reaction will take place even if there is only a trace of water present, for water is generated in the reaction. One might suppose that, since water is essential, the first step in the reaction would be the formation of zinc hydroxide. This is not so, for Prosser & Wilson (1982) found that zinc hydroxide did not react with eugenol. They therefore agreed with the earlier suggestion of Crisp, Ambersley & Wilson (1980) that the reaction was activated by the formation of ZnOH+ on the surface of zinc oxide powders by water. Douglas (1978a,b) has also suggested that the reaction takes place in the water layer or at the water-eugenol interface; this suggestion is based on measurement of zinc chelate formation constraints and the reflectance spectrum of the admittedly water-soluble guaiacol-4-sulphonate. The reaction may be represented as follows. First, the generation of ions: ZnO + H2O ^± ZnOH+ + OH" [9.2] + 2+ ZnOH — Zn + OH" [9.3] HE^±H + + E"

[9.4]

bination of ions: Zn2+ + 2 E — Z n E 2 +

H + OH "^±H 2 O ZnE2 + H2O ^ ZnE 2 . H2O

[9.5] [9.6] [9.7]

The role of additives in setting This scheme applies only to the reaction in simple zinc oxide and eugenol systems. The presence of an accelerator such as zinc acetate profoundly modifies it. The addition of Zn2+ ions or acetic acid (HAc) to the system eliminates the need for water to initiate the reaction. The reactions can then be represented by the following series of equations. ZnO + 2H+^= Zn2+ + H2O HE^± H + + E" Zn2+ + 2E"^± ZnE,

[9.8] [9.9] [9.10] 327

Non-aqueous cements Table 9.1. Effect of zinc oxide type on setting time ofZOE cements {Prosser & Wilson, 1982) Eugenol Cement Type

Dry

Activated"

Indirect process ZnO Thermally decomposed ZnO (500 °C)

15 days 3 hours

5-5 min 90 min

a

1 % by mass of acetic acid added to eugenol. A powder:liquid 20 g cm 3 was used for cement preparation. In this case acetic acid and acetate are reaction promoters. The reaction is greatly accelerated and the setting time of cements is shortened (Table 9.1). The role of zinc oxide in setting The physical and chemical characteristics of zinc oxide powders are known to affect cement formation (Smith, 1958; Norman et ai, 1964; Crisp, Ambersley & Wilson, 1980; Prosser & Wilson, 1982). The rate of reaction depends on the source, preparation, particle size and surface moisture of the powder. Crystallinity and lattice strain have also been suggested as factors that may change the reactivity of zinc oxide powders towards eugenol (Smith, 1958). Zinc oxide is made either by the oxidation of the metal in oxygen (the indirect, IP, or French process), by the direct decomposition of zinc ores in air (the direct or American process) or by the thermal decomposition of zinc salts (TD zinc oxide). IP zinc oxides differ from TD zinc oxides in that their surfaces do not contain absorbed water. Also, whereas TD zinc oxide reacts with plain eugenol, IP zinc oxide hardly reacts unless activated by an acetic acid or zinc acetate accelerator (Table 9.2). Particle size is the rate-controlling factor in the case of cements formed using IP zinc oxide (Smith, 1958; Norman et ai, 1964). Setting time appears to be proportional to the median particle size (Prosser & Wilson, 1982). By contrast, the setting times of cements prepared from TD zinc oxide do not appear to relate to particle size. The heat treatment of zinc oxide powders reduces their reactivity towards eugenol, because of an increase in particle size or a decrease in absorbed water. In the case of zinc oxide powders prepared by the thermal 328

Zinc oxide eugenol (ZOE) cements Table 9.2. Effect ofZnO ignition temperature on cement setting time (Prosser & Wilson, 1982) Ignition temperature

Setting time

400 °c 500 °c 600 °c 800 °c

18 min 3 hours 18 hours 120 hours

Absorbed water

Particle size

1-26%

0-33 jam

0-33% 0-11%

—

—

0-65 urn 0-84 jam

Zinc oxide prepared by ignition of zinc carbonate. A powder: liquid 2-0 g cm"3 was used for cement preparation.

decomposition of carbonate or oxalate, increasing the temperature of heat treatment causes them to react more slowly with eugenol (Smith, 1958; Crisp, Ambersley & Wilson, 1980; Prosser & Wilson, 1982). This is paralleled by a decrease in amount of water physically held on the surface of the powder and an increase in the particle size (Prosser & Wilson, 1982). These effects are shown in Table 9.2. The effect of heating zinc oxide powders in various atmospheres has been studied by several workers (Blackman, 1962, 1963; Lee and Parravano, 1959; Marshall, Enrigh & Weyl, 1952; Dollimore & Spooner, 1971; Prosser & Wilson, 1982). Heating causes sintering, a process of coalescence and densification, which finally leads to the formation of a non-porous body. For example, in air, freshly prepared zinc oxide spheres sinter at 700 °C (Lee & Parravano, 1959). This sintering is shown clearly in electronmicrographs of powders before and after heating at 800 °C (Prosser & Wilson, 1982; Figure 9Aa,b,c,d). Heating results in the loss of oxygen which creates anion vacancies (F). The structure of such zinc oxides may be represented as

Thus, there is excess of zinc over that required by stoichiometry. Sintering was attributed by Lee & Parravano (1959) to the diffusion of Zn2+ ions consequent on the difference in concentration of excess Zn2+ ions between the surface and the bulk. The presence of water on the oxide surface can enhance the sintering of zinc oxide particles (Dollimore & Spooner, 1971). The amount of water reversibly absorbed on zinc oxide surfaces is affected by heat treatment 329

Figure 9.4 The effect of sintering temperature on the morphology of zinc oxide particles. Zinc oxide from zinc oxalate: (a) 400 °C, (b) 800 °C. Carmox zinc oxide (c) 400 °C, (d) 800 °C (Prosser & Wilson, 1982).

Zinc oxide eugenol (ZOE) cements because of the reduction in specific surface area. Nagoe & Morimoto (1969) found that adsorption of water on zinc oxide particles reached a maximum after heating the oxide to 450 °C in vacuo and concluded that both chemisorption and physicosorption occurred. At this temperature, surface hydroxyls are lost from the surface: -Zn-OH + HO-Zn-

> -Zn-O-Zn- + H2O

[9.11]

However, these are rapidly regenerated on exposure to water vapour. This ability is lost on ignition to higher temperatures and must relate to the conversion of surface hydroxyls to oxygen bridges that are resistant to rehydroxylation. 9.2.6

Structure

The set cement consists of zinc oxide particles bonded together by a loose matrix of zinc eugenolate (Wilson, Clinton & Miller, 1973). Electronmicrographs show that the zinc oxide particles are covered by zinc eugenolate (Figure 9.5a). Different workers have reported different results on the nature of the zinc eugenolate matrix. Earlier work and some recent work has indicated

Figure 9.5a Electronmicrograph of a ZOE cement matrix, showing zinc oxide particles covered by zinc eugenolate (Wilson, Clinton & Miller, 1973).

331

Non-aqueous cements that crystallites are present (Copeland et al., 1955; Wilson, Clinton & Miller, 1973; Bayne et aL, 1986). By contrast, Steinke et al (1988) found that the matrix was entirely amorphous. El-Tahawi & Craig (1971), using thermal analysis, came to the following conclusions. The setting of ZOE mixes, containing 0 to 1 % of zinc acetate, does not result in the formation of more than trace amounts of zinc eugenolate crystallites, so that setting has nothing to do with the formation of a crystalline phase. Zinc eugenolate crystallites are formed in appreciable amounts only when the cement formulation contains large amounts of zinc acetate. It is unlikely, therefore, that the classical explanation of Copeland et al (1955), that coherence is due to the interlocking of crystals, is correct. Recently, Bayne & Greener (1985) have cast doubt on El-Tahawi & Craig's interpretation, so the exact nature of the matrix remains to be resolved. On exposure to water the matrix decomposes, with release of eugenol (Figure 9.2a). Wilson, Clinton & Miller (1973) found that the zinc eugenolate matrix was degraded to a weak zinc hydroxide matrix and the zinc oxide particles were washed clean of zinc eugenolate (Figure 9.56).

Figure 9.56 Electronmicrograph of a ZOE cement matrix after aqueous attack. The zinc oxide particles are washed clean of zinc eugenolate and the matrix is degraded to zinc hydroxide (Wilson, Clinton & Miller, 1973).

332

Zinc oxide eugenol (ZOE) cements 9.2.7

Physical properties

The ZOE cement is easy to mix and a greater amount of powder can be incorporated into this cement (5:1 by mass) than any other, where even 4:1 by mass is unusual. Because the ZOE cement is sensitive to moisture it can be formulated to have a long working time under normal room conditions (23 °C, relative humidity 50 %) and a rapid set once placed in the warm and moist conditions of the mouth. This is a considerable clinical advantage, making it convenient to use. The cement can be used in a war pack for use on the battlefield. Nevertheless, sensitiveness to humidity can give rise to problems in use under tropical conditions. The working time of commercial materials varies from 4 to 14 minutes (Plant, Jones & Wilson, 1972; Wilson, 1975a). The International Standard (ISO, 1988) requires that these cements when used for temporary cementation or as a cavity liner should set in 4 to 10 minutes and when used as a base or for temporary restoration should set in 3 to 10 minutes. The setting of commercial ZOE bases can be rapid and so sufficient strength can be developed for an amalgam to be placed over them after 10 minutes (Plant & Wilson, 1970). Linear shrinkage during setting is high, 0-86 % dry and 0-32 % wet (Civjan & Brauer, 1964). Compressive strength is much lower than that of the water-based dental cements and ranges from 13 MPa to 38 MPa (24 hours) for unreinforced materials (Brauer, 1972; Wilson, 1975a). ZOE cements are suitable for use as liners and temporary cements. Gilson & Myers (1970) pointed out that ZOE cements for temporary cementation should be strong enough to ensure retention of devices yet weak enough to allow for ease of removal. They concluded from clinical studies that materials with compressive strengths ranging from 2 to 24 MPa were required. The International Standard (ISO, 1988) requires that for temporary cementation compressive strength should not exceed 35 MPa and for use as liners must be greater than 5 MPa. Tensile strength is much lower, 1-2 to 2-8 MPa (Civjan & Brauer, 1964; Hannah & Smith, 1971). These cements have marked creep characteristics and flow under pressure even when fully set. In this they contrast markedly with the rigid phosphate cements (Wilson & Lewis, 1980). This plastic behaviour explains why such cements provide a good seal despite a high setting shrinkage and thermal expansion of 35 x 10"6 X" 1 (Civjan & Brauer, 1964). The hydrolytic instability of ZOE cements, arising from the weakness of the zinc eugenolate chelate, has been discussed in Section 9.2.5. For this 333

Non-aqueous cements reason these cements easily decompose under oral conditions. They can survive, however, when used as a liner where they are not exposed to aqueous conditions. Otherwise they are strictly temporary materials. Indeed, the weakening of the cement may prove an advantage when they are used for the temporary cementation of crowns. One serious fault of these materials is that the presence of an electronrich phenolic hydroxyl group inhibits free-radical polymerization. Thus, composite resins placed over them do not polymerize completely. 9.2.8

Biological properties

Pulp reaction is minimal: there is a slight reduction in odontoblasts (cells responsible for dentine formation) but these recover in a few weeks (Wilson, 1975b). The sealing ability and bactericidal action appear to facilitate pulpal healing (Beagrie, Main & Smith, 1972). The release of eugenol by hydrolysis of the zinc eugenolate matrix relieves pain in the pulp in deep cavities (Smith, 1982a). There are, however, disadvantages associated with the release of eugenol. Eugenol (and zinc also) is cytotoxic, and causes toxic cell reactions (Coleman, 1962; Roydhouse & Weiss, 1964). Thus, eugenol is an irritant and can cause inflammatory responses in soft tissues (Beagrie, Main & Smith, 1972) with haemolysis and delayed healing (Smith, 1982b). It is also a potential allergen (Smith, 1982a). For these reasons direct contact with connective tissues must be avoided. For full accounts of biological responses the reader is referred to reviews by Brauer (1965), Helgeland (1982) and Smith (1982b). 9.2.9

Modified cements

The ZOE cements are susceptible to modification. Modification by the addition of accelerators has already been discussed in Sections 9.2.4 and 9.2.5. One of the oldest additives is rosin (abietic acid) which improves working, hardening rate and strength (Wallace & Hansen, 1939; Molnar & Skinner, 1942). El-Tahawi & Craig (1971) report that hydrogenated rosin inhibits the formation of crystallites. Some materials intended for temporary cementation and cavity lining are formulated as two pastes. One paste is formed by blending the zinc oxide powder with a mineral or vegetable oil and the other by mixing an inert filler into the liquid. These cements are much weaker, with 334

Zinc oxide eugenol (ZOE) cements compressive strengths between 1-7 and 7 MPa (Gilson & Myers, 1970). This strength is adequate for the temporary cementation of some restorations. Mechanical properties are reduced by water immersion although the effect is much less than with simple cements. Mechanical retention is less than for zinc phosphate cements (Grieve, 1969; Richter, Mitchem & Brown, 1970) but an 83*5% success rate has been reported (Silvey & Myers, 1976). Sometimes antimicrobial agents such as thymol or 8-hydroxyquinoline may be present (Wilson, 1975b). The latter is also capable of forming a cementitious chelate with zinc. 9.2.10

Impression pastes

The ZOE impression paste is used in taking impressions of the mouth prior to constructing a denture. It is used as a corrective impression material after a preliminary impression has been taken (Phillips, 1982a). A preliminary impression lacks detail so it is necessary to take a secondary or corrective impression which is placed on a tray that has the contours of the preliminary impression. The ZOE impression paste is essentially a two-paste ZOE cement. One paste is formed by plasticizing the zinc oxide powder with 13 % of mineral or vegetable oil. The other paste consists of 12 % eugenol or oil of cloves, 50% polymerized rosin, 20% silica filler, 10% resinous balsam (to improve flow) and 5 % calcium chloride (accelerator). 9.2.11 Conclusions The ZOE cement has been in use for over 100 years and is still popular because it is easy to prepare and handle and has palliative and antiseptic properties. Its plastic nature ensures that it adapts well and even its weakness is an advantage in its applications as a temporaryfillingmaterial and cementing agent. In these applications a cement must be sufficiently strong for the restoration to be held in place for a limited period, but not so strong as to make removal difficult. Although not by intention, the cement happens to be prepared from two medicaments, eugenol and zinc oxide, and these impart anodyne and antibacterial properties. Moreover, the cement is a sustained-release device for eugenol and zinc. These accidental benefits make it a difficult material to supplant. Surprisingly, although the material has been in use for over 100 years, the cement structure has yet to be fully elucidated. 335

Non-aqueous cements 9.3

Improved ZOE cements

9.3.1

General

The main disadvantage of the ZOE cements is their low strength. The following approaches have been made to remedy this disadvantage. (1) (2) (3) (4) 9.3.2

Incorporation of reinforcing fillers into the powder Addition of reinforcing polymers to the liquid Replacement of eugenol by other chelating agents Replacement of zinc oxide by other oxides Reinforced cements

Several attempts have been made to improve the strength of ZOE cements either by adding fillers to the powder or by dissolving resin in the liquid (Wilson, 1975b; Smith, 1982a). Examples of fillers used include rosin, hydrogenated rosins, poly(methyl methacrylate), polystyrene, polycarbonate, fused silica and dicalcium hydrogen phosphate. Messing (1961) found that he could improve ZOE cements by adding 10 % polystyrene to the liquid. Strength developed more rapidly than in an unmodified but accelerated cement and after 24 hours reached 42 MPa compared with 36 MPa. The most important approach was to use poly(methyl methacrylate), PMMA, in formulations, either as a particulate filler or as a coating on zinc oxide particles (Jendresen & Phillips, 1969; Jendresen et ai, 1969; Civjan et al, 1972). It is claimed that such materials can be used for permanent as well as temporary cementation. ZOE cements intended for permanent cementation are required by the International Standard (ISO, 1988) to set in 4 to 10 minutes with a film thickness of 25 um. The compressive strength of these reinforced ZOE cements ranges from 36 to 52 MPa (Table 9.3). This compares with a strength of 13 to 38 MPa for unreinforced cements. ZOE cements are required by the International Standard (ISO, 1988) to have a minimum compressive strength of 35 MPa if they are to be used for permanent cementation and 25 MPa if they are intended for use as a base or for temporary restoration. The tensile strength of these materials is very much lower and lies between 2-6 and 4-2 MPa (Table 9.3) and their modulus of elasticity varies from 2-1 to 3-0 GPa (Powers, Farah & Craig, 1976). All these strength 336

2-ethoxybenzoic acid eugenol (EBA) cements Table 9.3. Strength (MPa) ofZOE cements

ZOE simple ZOE accelerated ZOE reinforced ZOE pastes EBA reinforced

Compressive

Tensile

13-38 [1,2] 36-52 [1-4] 1-7-7 [5] 40-70 [4, 8]

1-2-2-8 [1,6] 1-2-2-1 [6] 2-6-^-2 [4, 6, 7] — 5-5-7-0 [4, 6, 7]

[1] Brauer (1972); [2] Wilson (1975b); [3] Jendresen et al (1969); [4] Powers, Farah & Craig (1976); [5] Gilson & Myers (1970); [6] Hannah & Smith (1971); [7] Williams & Smith (1971); [8] 0ilo & Espevik (1978). figures have to be accepted with some reservations, for they represent measurements made at room temperature and it is certain that these cements are very much weaker at oral temperatures (0ilo & Espevik, 1978).

9.4

2-ethoxybenzoic acid eugenol (EBA) cements

9.4.1

General

The term EBA cement is not quite exact, but is convenient to use and has been generally accepted. Originally this cement was a variant of the ZOE cement - its most important variant - and was based on a liquid which was a mixture of 2-ethoxybenzoic acid (EBA) and eugenol. More recently, eugenol has been replaced by other compounds of similar structure. All these cements contain EBA as a major constituent. The structure of EBA has been examined using infrared spectroscopy (Bagby & Greener, 1985) and there are apparently three conformations, two hydrogen-bonded (Figure 9.6). 9.4.2 Development In Section 9.1 it was noted that this cement was the only important zinc oxide cement other than the ZOE cement. Its invention and development is largely associated with Brauer who, with coworkers, has been carrying out a programme of research and development from the late 1950s to the present day. In 1958, in an attempt to improve on the ZOE cement, Brauer, White & 337

Non-aqueous cements Table 9.4. Composition of the EBA cement (Brauer, McLaughlin & Huget, 1968) Liquid:

62-5 % 2-ethoxybenzoic acid 37-5 % eugenol

Powder: 64% zinc oxide 30 % tabular alumina (particle size from < 1 urn to 20 urn, with few particles > 20 urn) 6 % hydrogenated rosin

Moshonas (1958) investigated the reactions between zinc oxide and a large number of chelating agents. Of these, EBA proved to be the most promising. They then examined cement formation between EBA and various metal oxides. Cement formation was found with MgO, CaO, BaO, ZnO, CdO, HgO and PbO. Finding that pure EBA cements tended to be unduly water soluble, these workers went on to study cement formation between zinc oxide and mixtures of EBA and eugenol. Small amounts of EBA added to eugenol produced a marked acceleration in cement formation and reduced the setting time from 2 hours to 3 minutes. Setting time remained short in the range 25 to 75 % EBA. From this basis, Brauer went on to develop what he termed the EBA cement. Later studies showed that the optimum liquid had the composition 62-5 % EBA and 37*5 % eugenol, the EBA-eugenol liquid. This liquid is important for it was used in most of the later experimental studies and in commercial examples (Table 9.4). The weakness of this cement was its tendency to dissolve in water. This was prevented by including rosin (mainly abietic acid) or hydrogenated rosin in the formulation (Brauer, Simon & Sangermano, 1962). Rosin and fused quartz or calcium hydrogen phosphate monohydrate were added to

,0

H—0

0 — H-

C2H5

C2H5

Figure 9.6 The structure of 2-ethoxybenzoic acid, showing the two hydrogen-bonded configurations (Bagby & Greener, 1985).

338

2-ethoxybenzoic acid eugenol (EBA) cements the zinc oxide powder as strengthening fillers. The highest compressive strengths were obtained with the EBA-eugenol liquid and two powders that contained respectively 10% rosin (71 MPa), and 6% rosin and 10 to 30% quartz (74 to 81 MPa). These are stronger than the strongest of the reinforced ZOE cements (55 MPa). In a further attempt to improve properties, Brauer, McLaughlin & Huget (1968) examined the use of alumina as a reinforcing filler. Alumina is considerably more rigid than fused quartz. They achieved a considerable increase in strength. The preferred composition was the powder defined in Table 9.4, which had a compressive strength of 91 MPa. This zinc oxide based powder was the one most commonly used in subsequent studies by Brauer and coworkers. We shall refer to it as the EBA powder for it is the one used in commercial formulations and in a number of experimental studies. Although these materials had a high early (10-minute) compressive strength of 46 MPa, their brittleness limited their use for the temporary restoration of multiple surfaces subject to heavy masticatory forces (Ciyjan & Brauer, 1964). Stress bearing was improved by incorporating powdered polymethacrylate polymers of low elastic moduli into the zinc oxide powder (Brauer, Huget & Termini, 1970). One such cement used a powder containing 58-2% zinc oxide, 27*3% alumina, 5-4% rosin and 9*1% methylmethacrylate copolymer. The compressive strength was 65 MPa and the tensile strength high at 11 MPa. Despite all these endeavours, as Brauer himself admits, the rapid disintegration under oral conditions prevents their use in permanent restorations (Brauer, Stansbury & Argentar, 1983) and subsequent development has taken place in a quite different direction. 9.4.3

Setting and structure

Little is known of the setting reaction and structure of EBA cement. The absence of an infrared band at 1750 cm"1 in the set cement indicates that no unreacted COOH is present (Brauer, 1972). So far, it is not certain whether zinc forms a six-membered chelate or merely a simple salt with EBA. Neither infrared spectroscopy nor solution studies are able to distinguish between these two forms. Eugenol is much less readily extracted and so more firmly bound in the complex than is EBA. The suspicion is that the EBA cement is fundamentally more prone to hydrolysis than the ZOE cement. 339

Non-aqueous cements The only other studies are contained in one brief paper by Wilson & Mesley (1974). The infrared spectrum of the pure EBA cement containing excess zinc oxide was found to correspond to a salt of EBA. The binding matrix was amorphous save for a trace of unidentified crystalline material. Infrared spectroscopy indicated that cements prepared from EBA-eugenol mixtures were not a simple mixture of the two parent cements. The mixed cements contained three crystalline phases: zinc eugenolate and two unidentified phases. Probably, one of these unidentified phases is a zinc 2ethoxybenzoate and the other a zinc eugenolate-2-ethoxybenzoate chelate salt. Different results were obtained when zinc oxide was present only in stoichiometric amounts. Then the evidence was that only two crystalline phases were present: a zinc eugenolate and a zinc 2-ethoxybenzoate. The indications are that there is an equilibrium: ZnO

eugenolate + 2-ethoxybenzoate

• eugenolate-2-ethoxybenzoate

Thus, if zinc 2-ethoxybenzoate is a weak chelate it will be preferentially extracted and the reaction will move to the left. Eventually the matrix will contain only eugenol, as, indeed, Brauer (1972) found. 9.4.4

Properties

These cements have unusual rheological properties (Wilson, 1975b). They can be mixed to higher powder/liquid ratios (6:1 by mass, or more) than any other dental cements and are very fluid. Whereas pastes of other cements behave as plastic bodies, the EBA cement has the characteristics of a very viscous Newtonian liquid and flows under its own weight, even when mixed very thickly (Wilson & Batchelor, 1971). High powder/liquid ratios are required for optimum properties: 3-5 g cm"3 for luting and 5 to 6 g cm"3 for linings and bases. The linear setting shrinkage is 0-24 to 0-52% (dry) and 0-12 to 0-38% (wet), which compares withfiguresof 0-85 % (wet) and 0-31 % (dry) found for a ZOE cement by Civjan & Brauer (1964). The working and setting times of EBA cements range from 7 to 13 minutes (Smith, 1982a) and are dependent on both humidity and temperature. Film thickness ranges from 40 to 70 |im which is greater than the 25 |im required by the International Standard (ISO, 1988). Hembree, George & Hembree (1978) found that under simulated clinical conditions an alumina EBA cement always gave a greater film thickness (c. 50 Jim) than the 20 jim of the unfilled material. 340

2-ethoxybenzoic acid eugenol (EBA) cements Table 9.5. Strength (MPa) of reinforced EBA cements

ZOE EBA EBA-HV EBA-di-HV EBA-poly-HV Glass-EBA-poly-HV EBA polymer cement

Compressive

Tensile

36-52 [1-4] 40-70 [4, 8] 42-60 [9] 48-70 [10] 67-70 [10] 64-73 [10] 73-112 [11]

2-6-4-2 [4, 6, 7] 5-5-7-0 [4, 6, 7] 5-0-6-3 [9] 6-2-7-5 [10] 5-8-6-8 [10] 9-9-11-2 [10] 101-15-8 [11]

[I] Brauer (1972); [2] Wilson (1975b); [3] Jendresen et al (1969); [4] Powers, Farah & Craig (1976); [6] Hannah & Smith (1971); [7] Williams & Smith (1971); [8] 0ilo & Espevik (1978); [9] Brauer, Stansbury & Argentar (1983); [10] Stansbury & Brauer (1985); [II] Brauer & Stansbury (1984b).

EBA cements have marked viscoelastic characteristics. They creep under load to an even greater extent than the ZOE cements (Wilson & Lewis, 1980). When subject to a slowly increasing load they exhibit marked strain at fracture and low strength (0ilo & Espevik, 1978). These characteristics may be the reason why retention values for crowns and orthodontic bands, although better than other ZOE cements, are inferior to those of the zinc phosphate cements (Williams, Swartz & Phillips, 1965; Grieve, 1969; Richter, Mitchem & Brown, 1970). Thermal expansion is 60 to 90 x 10~6 °C-\ which is higher than that of the ZOE cement (Civjan & Brauer, 1964). Compressive strength depends on powder/liquid ratio and test conditions. At the thin cementing consistency, the compressive strength of these materials ranges from 40 to 70 MPa (Powers, Farah & Craig, 1976; 0ilo & Espevik, 1978). Tensile strength is much lower: 4-7 to 7-1 MPa (Williams & Smith, 1971; Powers, Farah & Craig, 1976; Hannah & Smith, 1971). The modulus of elasticity is 5400 MPa (Powers, Farah & Craig, 1976). In vitro studies by Wilson et al. (1986) using the impinging jet method show the EBA cement to be the least resistant of all the cement types to erosion in neutral solution. Clinical studies confirm this result and show that there is greater dissolution in the mouth than for other dental cements (Mitchem & Gronas, 1978; Osborne et al, 1978; Andrews and Hembree, 1976; Shilling, 1977). Despite this, a clinical survey by Silvey and Myers 341

Non-aqueous cements (1976,1977) indicated that the performance of an alumina-reinforced EBA cement over 3 years was only slightly worse than that of zinc phosphate and polycarboxylate cements. Results for cement strengths are summarized in Table 9.5. 9.5

EBA-methoxyhydroxybenzoate cements

9.5.1

EBA-vanillate and EBA-syringate cements

All cements that contain eugenol inhibit the polymerization of acrylates, and those of EBA-eugenol are no exception. In order to remedy this and other defects, Brauer and his coworkers examined alternatives to eugenol (Figure 9.7). These included the esters of vanillic acid (3-methoxy-4hydroxybenzoic acid, HV) and syringic acid (3,5-dimethoxy-4-hydroxybenzoic acid). Both are 3-methoxy-4-hydroxy compounds and are thus chemically related to eugenol and guaiacol. Both are solids and have to be dissolved in EBA where they form satisfactory cements with EBA zinc oxide powder. The vanillate (EBA-HV) cements are the more important. EBA-vanillate cements Using the EBA powder (Table 9.4) and liquids containing 12-5 to 18-3 % of a vanillate ester (either n-hexyl, n-heptyl or n-decyl) Brauer, Stansbury & Argentar (1983) obtained cements that set in 4-5 to 5-5 minutes with compressive strengths from 42 to 60 MPa and tensile strengths from 5-0 to 6-3 MPa (Table 9.5). The preferred liquid was one containing 87-5% eugenol and 12-5% HV. Brauer & Stansbury (1984a) found also that the EBA-HV cements bonded much more strongly to composite resins (5-5 MPa), stainless steel (4 MPa), nickel-chrome (5 MPa) and porcelain (4-1 to 5-5 MPa) than the ZOE cement (Table 9-6). COOR

COOR

OCHq

H 3 CO

(a) Figure 9.7 Structure of (a) vanillates and (b) syringates. 342

OCH3

EBA-methoxyhydroxybenzoate cements Table 9.6. Bond strength (MPa) to substrates

ZOE EBA-HV EBA-poly HV Silica EBA-poly HV EBA polymer cement

Composite resin

Stainless steel

0-3 [12] 5-5 [12] 3-8-6-9 [10] 60-7-9 [10] 4-1-10-3 [11]

0-6 [12] 4-1 [12] 5-9-7-9 [10] 7-1-9-4 [10] 9-8-15-6 [11]

[10] Stansbury & Brauer (1985); [11] Brauer & Stansbury (1984b); [12] Brauer & Stansbury (1984a).

EBA-syringate cements With a liquid containing 7 to 14% n-hexyl syringate, Brauer & Stansbury (1984a) obtained cements that set in 4 minutes with a compressive strength from 54 to 62 MPa and a tensile strength of 5-5 MPa, but they were brittle (Table 9.5). Replacement of n-hexyl syringate by 2-ethylhexyl syringate yielded cements that, depending on powder/liquid ratio, set in 6 to 9-5 minutes with compressive strengths of 40 to 50 MPa and tensile strengths of 5-2 to 5-7 MPa. These cements were less brittle than those of n-hexyl syringate. The best formulation proved to be one based on a liquid containing 88 % EBA, 5% n-ethylhexyl syringate and 7% n-hexyl vanillate. Cements prepared from these liquids set in 5-5 to 6-5 minutes with a compressive strength of 66 MPa and tensile strength of 6 to 7 MPa. All these vanillate and syringate cements are about as strong as those of EBA-eugenol. Setting There is little information available on their setting and structure. Bagby & Greener (1985) used Fourier transform infrared spectroscopy (FTIR) to examine the cement-forming reaction between zinc oxide and a mixture of EBA and n-hexyl vanillate. Although they found evidence for reaction between zinc oxide and EBA, they were unable to find any for reaction between zinc oxide and n-hexyl vanillate because of peak overlaps, the minor concentration of n-hexyl vanillate and the subtle nature of the spectral changes.

343

Non-aqueous cements Advantages Brauer (1988) considers that EBA-HV cements possess a number of advantages over eugenol cements. They bond much more strongly to composite resins and stainless steel than does the ZOE cement. They are compatible with composite resins for, unlike eugenol, vanillates do not inhibit the polymerization of acrylate polymers, because the phenolic hydroxyl is electron-poor. Brauer, Stansbury & Argentar (1983) speculate that they are probably less toxic, as n-hexyl vanillate has been considered as a food preservative. Vanillates should yield cements with bactericidal properties. But all these supposed biological advantages need to be substantiated. One recent evaluation by Keller et al. (1988) has shown that this cement has an acceptable performance when compared with the clinically acceptable zinc oxide eugenol and zinc phosphate cements. Syringate cements possess similar advantages to the vanillate cements. In addition, syringic acid possesses cariostatic properties, so syringates may inhibit the development of caries (dental decay). Again these advantages need to be confirmed. Modifications Brauer, Stansbury & Flowers (1986) modified these cements in several ways. The addition of various acids - acetic, propionic, benzoic etc. accelerated the set. The use of zinc oxide powders coated with propionic acid improved mixing, accelerated set, reduced brittleness and increased compressive strength from 63 to a maximum of 72 MPa. The addition of plasticizing agents such as zinc undecenylate yielded flexible materials. Incorporation of metal powders had a deleterious effect and greatly increased the brittleness of these cements. The addition offluorideswas not very successful, for fluoride release was not sustained. 9.5.2

EBA-divanillate and polymerized vanillate cements

Stansbury & Brauer (1985) investigated the use of divanillates in EBA cements (Figure 9.7). Divanillates consist of two vanillate groups linked by esterifying the COOH groups with diols. Cements based on EBA containing 10 to 11% of divanillates set in 4-5 to 5 minutes with compressive strengths of 48 to 70 MPa and tensile strengths of 6-2 to 7-5 MPa using the EBA zinc oxide powder (Table 9.5). In addition, Stansbury & Brauer (1985), taking advantage of the fact that vanillates do not inhibit free-radical polymerization, incorporated 344

EBA-methoxyhydroxybenzoate cements polymerizable vanillates in cement formulations.They used the methylacryloylethyl, -CH 2 . OOC. C(CH3)=CH2, ester of vanillic acid (Figure 9.7), and added 3 to 10 % to the EBA liquid. In this system the zinc oxide powder contained 1 % benzoyl peroxide as a polymerization initiator and the accelerator, 7V,7V-dihydroxyethyl-/?-toluidine, was added to the liquid. These cements set in 6-5 to 8-5 minutes with compressive strengths of 67 to 70 MPa and tensile strengths of 5-8 to 6-8 MPa (Table 9.6). Thus, these cements are no stronger than other vanillate cements and they are brittle. They possess adhesive properties: bond strengths of 5*9 to 7-9 MPa to stainless steel and 3-8 to 6-9 MPa to composite resins were recorded (Table 9.6). Some improvement in physical properties was obtained by adding a filler of silanized glass (325 mesh) to the EBA zinc oxide powder (1:1). Although the compressive strengths of these reinforced cements were no greater than those of the unreinforced cement (64 to 73 MPa) there were significant improvements in tensile strengths with values of 9-9 to 11-2 MPa being obtained (Table 9.5). There was improved adhesion, with bond strengths of 7-1 to 9-4 MPa to stainless steel and 6-0 to 7-9 MPa to composite resins (Table 9.6). 9.5.3

EBA-HVpolymer cements

The last stage in the development of the EBA cement is represented by the polymer cements. Brauer & Stansbury (1984b), taking advantage of the fact that the EBA-HV liquid does not inhibit vinyl polymerization, included methacrylates into the cement composition. The object was to produce a material that set after mixing, both by polymerization and by salt or chelate formation. The liquids used were 1:1 mixtures of EBA-HV and liquid methacrylate which also contained dihydroxyethyl-/?-toluidine as the accelerator. Both mono- and di-methacrylates were used. The benzoyl peroxide initiator was included in the EBA zinc oxide/silanized (1:1) glass powder. These polymer cements set 5 to 10 minutes after mixing. Since there is a substantial amount of monomer in the liquid (50%) the contribution of the polymer to the strength of the cement must be considerable. Brauer & Stansbury (1984b) suggested that the two matrices, the polymer matrix and the salt matrix, may be interpenetrating; but separation of the two phases is likely. Very high strengths were obtained compared with other ZOE and EBA 345

Non-aqueous cements cements. The most important monomethacrylates studied were methyl-, cyclohexyl- and dicyclophenyl- and mixtures of them. Cements containing these monomers had compressive strengths ranging from 73 to 112 MPa and tensile strengths from 10-1 to 15-8 MPa (Table 9.5). Good bonding was obtained to several substrates under aqueous conditions. Values obtained were 4-1 to 10-3 MPa to composite resins, and 9-8 to 15-6 MPa to stainless steel (Table 9.6). They were also reported as adhering to porcelain. No adhesion was obtained to untreated dentine or enamel. The cements could be bonded to enamel etched with acid (3-5 MPa) and to dentine conditioned with poly(acrylic acid) (1-0 MPa). The mechanism of adhesion to various substrates has not been fully explained. Brauer & Stansbury (1984b) consider that bonding to composite resins occurs by the diffusion of methacrylate polymer chains into the resin. Bonding to base metals is, perhaps, by salt or chelate bridges. Here it is significant that ZOE cements do not bond, so perhaps bonding is due to the action of free EBA on the substrate. The adhesion to porcelain is surprising. Porcelain is inert so that the attachment can hardly be chemical. Also, it would be expected that if a cement adheres to porcelain then it should adhere to untreated enamel and dentine, but this is not so. The use of dimethacrylates led to even greater cement strengths. Compressive strengths ranged from 50 to 199 MPa, although the higher strengths were obtained with pastes that were hardly workable and 132 MPa represents the practical limit of compressive strength. Tensile strength ranged from 8-0 to 15-9 MPa. Unfortunately, the EBAdimethacrylate liquids were unstable and partial polymerization occurred within hours. The biological properties of these materials are unknown, but the presence of methacrylate monomers must adversely affect their biocompatibility. Brauer & Stansbury (1984b) claim that these materials can be used in 'intermediate' restorations which are used in holding-type situations where an extensive cleaning-up regime is required over many weeks prior to the placement of a permanent restorative. 9.5.4 Conclusions We have seen how Brauer and coworkers have developed EBA cements since 1958, during which time they have become increasingly more complex. They are somewhat stronger than reinforced ZOE cements. EBA 346

Calcium hydroxide chelate cements cannot be used alone but always requires the addition of a 2-methoxyphenol. Chemical differences between the different 2-methoxyphenols have little effect on cement strength. Non-eugenol EBA cements have the distinct advantage, however, in not inhibiting vinyl polymerization and are thus compatible with composite resins. This is an important attribute because these cements are used as bases under composite resins. It also means that methacrylate monomers can be mixed with EBA to give polymer cements. These materials are considerably stronger than plain cements. Unfortunately, although EBA cements have been subjected to a considerable amount of development, this work has not been matched by fundamental studies. Thus, the setting reactions, microstructures and molecular structures of these EBA cements are still largely unknown. In addition, the mechanism of adhesion to various substrates has yet to be explained. Such knowledge is a necessary basis for future developments. 9.5.5

Other zinc oxide cements

Skinner, Molnar & Suarez (1964) studied the cement-forming potential of 28 liquid aromatic carboxylic acids with zinc oxide. Twelve yielded cohesive products of some merit. Of particular interest were cements formed with hydrocinnamic, cyclohexane carboxylic, ^-tertiary butylbenzoic, thiobenzoic and cyclohexane butyric acids. One of these cements is on the market as a non-eugenol cement. It is very weak with a compressive strength of 40 MPa, a tensile strength of 11 MPa and a modulus of 177 MPa, and is only suitable as a temporary material (Powers, Farah & Craig, 1976). 9.6 9.6.1

Calcium hydroxide chelate cements Introduction

Pastes of calcium hydroxide with water have been used as pulp-capping materials for many years and it is the material of choice for this application (Granath, 1982). Its favourable tissue responses have been known for many years (Zander, 1939). It has a healing effect, for it induces the formation of hard tissues of reparative dentine when pulp has been exposed (Eidelman, Finn & Koulourides, 1965). This action seems to be associated with its high alkalinity (pH ~ 12-5) and consequent bactericidal and proteinlysing effect (Fisher, 1977). 347

Non-aqueous cements Table 9.7. Composition of a calcium hydroxide chelate cement {American Dental Association, 1977) Basic paste:

51 % calcium hydroxide 9-23 % zinc hydroxide 0-29 % zinc stearate in JV-ethyl toluene sulphonamide A cid paste:

13-8% titanium dioxide 31*4% calcium sulphate 15-2% calcium tungstate in 1-methyl trimethylene disalicylate butane-1-3-diol ester

The manipulation of calcium hydroxide paste is not easy, however, and Dougherty (1962) introduced the calcium hydroxide salicylate cements. These are based on the reaction between calcium hydroxide and salicylate esters and come in two-paste packs which are easy to mix in the dental surgery. 9.6.2

Composition

All commercial materials are based on calcium hydroxide and liquid alkyl salicylates (Prosser, Groffman & Wilson, 1982) and are supplied as a twopaste pack. Zinc oxide is sometimes added to the calcium hydroxide, as are neutral fillers. A paste is formed from this powder by the addition of a plasticizer; examples include Af-ethyl toluenesulphonamide (p- or/?-) and paraffin oil, with sometimes minor additions of polypropylene glycol. The other paste is based on an alkyl salicylate as the active constituent containing an inorganic filler such as titanium dioxide, calcium sulphate, calcium tungstate or barium sulphate. Alkyl salicylates used include methyl salicylate, isobutyl salicylate, and 1-methyl trimethylene disalicylate. An example of one commercial material, Dycal, is given in Table 9.7, but its composition has been subjected to change over the years. 9.6.3

Setting

Prosser, Stuart & Wilson (1979) and Prosser, Groffman & Wilson (1982) examined the setting of a number of these cements using infrared spectroscopy. The infrared spectrum of the alkyl salicylates showed an O-H stretch band at 3190 cm"1 and a C-O stretch band at 1675-95 cm"1, 348

Calcium hydroxide chelate cements which were displaced from the normal frequencies of these groups because of very strong intramolecular hydrogen-bonding conjugate chelation. This conjugate chelation arises from resonance between the ester and its enolized form, and is shown for an alkylsalicylate in Figure 9.8. The essential chemical reaction is an acid-base one between calcium hydroxide and the phenolic group of an alkylsalicylate. Cement formation occurs as calcium replaces phenolic hydrogen. During setting, the O-H stretch band diminished as a carboxylate band appeared at 1540-60 cm"1 (asymmetric stretch). The ester band at 1675-95 cm"1 diminished, showing a conversion of C=O to C~O". The molecular structure can be represented as a chelate containing two bidendate ligands (Figure 9.9). In this case, unlike that of the zinc oxide eugenol cement, infrared spectroscopy can provide proof of a chelate structure, although the setting reaction appears to be very similar. The cement structure consists of an amorphous disalicylate complex filled by the unreacted calcium hydroxide and other inorganic fillers. The case of the disalicylate, 1-methyl trimethylene disalicylate, is interesting. Because of steric hindrance it is unlikely that the two salicylate ligands can chelate to one calcium atom. In theory the disalicylate

Figure 9.8 Alkylsalicylate structure.

Figure 9.9 Chelate of calcium and an alkylsalicylate.

349

Non-aqueous cements structure could bridge calcium ions and so form an ionically linked chain, but this too is unlikely. All the calcium hydroxide cements are weak and friable, indicating that the chelates are bound together by only weak secondary forces. The coordination number of calcium is usually six, and since two water molecules are generated for every calcium ion during the course of the reaction, it is possible that these two water molecules are attached to the central calcium ion to form an octahedral complex. This structure would be similar to that proposed for the zinc eugenolate cement, and water bridges may play a similar structural role. 9.6.4

Physical properties

These materials are prepared by mixing two strips of paste of equal length, and when mixed formfluidpastes. Some set rapidly, but setting depends on the availability of water (Plant & Wilson, 1970; Bryant & Wing, 1976a,b). In contact with water one example set in 2 minutes, and in the absence of water another did not set at all (Bryant & Wing, 1976b). Normally they set in 3-5 to 8 minutes (Plant, Jones & Wilson, 1972). They show marked plastic deformation under compressive load and although this decreases as they harden they still deform rather than fracture when 25 minutes old (Plant & Wilson, 1970). This probably accounts for the observation that there is little incidence of fracture or displacement when amalgams are placed on these cements (Fisher, 1977). The cements are very weak and after 24 hours their compressive strengths only range from 11 to 14 MPa (Bryant & Wing, 1976b). These materials are hydrolytically unstable and weaken when stored in water for a week (Bryant & Wing, 1976b). Prosser, Groffman & Wilson (1982) found that calcium and hydroxide ions and salicylates were released and that the rate of release was controlled by the plasticizer used in the cement formulation. Hydrophilic sulphonamide plasticizers allowed ready ingress of water and promoted decomposition, whereas the hydrophobic hydrocarbon plasticizer repelled water and retarded hydrolytic decomposition. 9.6.5

Biological properties

Hydrolytic decomposition of these cements is clinically advantageous. Free calcium hydroxide is present in excess so that large amounts of calcium are released which, together with high alkalinity, promotes 350

Calcium hydroxide chelate cements sterilization and calcification of carious dentine (McWalter, El-Kafrawy & Mitchell, 1976). There is formation of dentine bridges when they are used for pulp capping. Fisher (1977) found that the bactericidal effects varied from brand to brand and Fisher & McCabe (1978) related this to chemical composition. Only cements which give rise to high alkalinity (pH = 11) are effective. These are the cements which are readily decomposed by water, and this relates to the plasticizer used. Hydrophilic plasticizers are required if these cements are to be clinically effective. Hydrolytic decomposition brings disadvantages. There is continued leakage at the margins where complete dissolution can occur (Gourley & Rose, 1972) and, indeed, these bases have been observed to disappear entirely (Akester, 1979; Barnes & Kidd, 1979). These cements are the materials of choice for pulp capping (a wound dressing for covering an exposed or surgically treated pulp). They are superior to zinc oxide eugenol cements for this purpose (Mjor, 1963; Paterson, 1976). These materials also protect the pulp against invasion by acids from overlying dental cements of the phosphate or polyacrylate type and act as a barrier to the penetration of harmful chemicals such as the unpolymerized methacrylates (Smith, 1982a). 9.6.6

The calcium hydroxide dimer cement

Cowan & Teeter (1944) reported a new class of resinous substances based on the zinc salts of dimerized unsaturated fatty acids such as linoleic and oleic acid. The latter is referred to as dimer acid. Later, Pellico (1974) described a dental composition based on the reaction between zinc oxide and either dimer or trimer acid. In an attempt to formulate calcium hydroxide cements which would be hydrolytically stable, Wilson et al. (1981) examined cement formation between calcium hydroxide and dimer acid. They found it necessary to incorporate an accelerator, aluminium acetate hydrate, A12(OH)2(CH3COO)4. 3H2O, into the cement powder. These cements set in 3-5 to 56 minutes (at 37 °C). Infrared spectroscopy showed that as the cement set there was loss of acid carbonyl groups and OH groups associated with calcium hydroxide, and simultaneously formation of ionic carboxylate groups and hydrogen-bonded OH groups. Although these cements were capable of setting under water and were impervious to aqueous attack they were not a success. They failed to 351

Non-aqueous cements release calcium hydroxide into solution; consequently, there was no alkaline reaction and hence no favourable biological responses. References Andrews, J. T. & Hembree, J. H. (1976). In vivo evaluation of the marginal leakage of four inlay cements. Journal of Prosthetic Dentistry, 35, 532-7. American Dental Association. (1977). Accepted Dental Therapeutics, 36th edn., p. 235. Akester, J. (1979). Disappearing Dycal. British Dental Journal, 146, 369. Bagby, M. & Greener, E. H. (1985). Infrared spectral analysis of EBA-hexyl vanillate-ZnO cement. Dental Materials, 1, 86-8. Barnes, I. E. & Kidd, E. A. M. (1979). Disappearing Dycal. British Dental Journal, 141, 111. Batchelor, R. F. & Wilson, A. D. (1969). Zinc oxide eugenol cements. I. The effect of atmospheric conditions on rheological properties. Journal of Dental Research, 48, 883-7. Bayne, S. C. & Greener, E. H. (1985). ZnO cements: phase identification by thermal analysis. Dental Materials, 1, 165-9. Bayne, S. C , Greener, E. H., Lautenschlager, E. P., Marshall, S. J. & Marshall, jr, G. W. (1986). Zinc eugenolate crystals: SEM detection and characterization. Dental Materials, 2, 1-5. Beagrie, G. S., Main, J. H. P. & Smith, D. C. (1972). Inflammatory reaction evoked by zinc polyacrylate and zinc eugenate cements: a comparison. British Dental Journal, 132, 351-7. Blackman, L. C. F. (1962). Lattice defects and the sintering of oxides. Industrial Chemist, 38, 620-6. Blackman, L. C. F. (1963). Lattice defects and the sintering of oxides. Industrial Chemist, 39, 23-6. Bonastre, J. F. (1827a). De la combinaison des huiles volatiles de girofle et de pimet de la Jamai'que, avec des alcalis et autres bases salifiables. Journal de Pharmacie, 13, 464-76. Bonastre, J. F. (1827b). De la combinaison des huiles volatiles de girofle et de piment de la Jamai'que avec des alcalis. 2. Combinaison d'huiles volatiles de girofle avec les oxides metalliques. Journal de Pharmacie, 13, 513-21. Braden, M. & Clarke, R. L. (1974). Dielectric properties of zinc oxide-eugenol type cements. Journal of Dental Research, 53, 1263-7. Brauer, G. M. (1965). A review of zinc oxide-eugenol type filling materials and cements. Revue Beige de Medecine Dentaire, 20, 323—64. Brauer, G. M. (1972). Cements containing 2-ethoxybenzoic acid (EBA). National Bureau of Standards Special Publication 354, pp. 101—11. Brauer, G. M. (1988). Vanillate or syringate cements. Trends & Techniques, 5, No. 1, 6. Brauer, G. M., Argentar, H. & Durany, G. (1964). Ionization constants and reactivity of isomers of eugenol. Journal of Research of the National Bureau of Standards, 68A, 619-24. 352

References Brauer, G. M., Argentar, H. & Stansbury, J. W. (1982). Cementitious dental compositions which do not inhibit polymerization. US Patent 4,362,510, December 7, 1982. Brauer, G. M., Huget, E. F. & Termini, D. J. (1970). Plastic modified o-ethoxybenzoic acid cements as temporary restorative materials. Journal of Dental Research, 49, Supplement, 1487-94. Brauer, G. M., McLaughlin, R. & Huget, E. F. (1968). Aluminum oxide as a reinforcing agent for zinc oxide eugenol-o-ethoxybenzoic acid cements. Journal of Dental Research, 47, 622-8. Brauer, G. M., Simon, L. & Sangermano, L. (1962). Improved zinc oxide-eugenol type cements. Journal of Dental Research, 41, 1096-102. Brauer, G. M. & Stansbury, J. W. (1984a). Cements containing syringic acid ester-o-ethoxybenzoic acid and zinc oxide. Journal of Dental Research, 63, 137^0. Brauer, G. M. & Stansbury, J. W. (1984b). Intermediate restorative from n-hexyl vanillate-EBA-ZnO-glass composites. Journal of Dental Research, 63, 1315-20. Brauer, G. M., Stansbury, J. W. & Argentar, H. (1983). Development of highstrength, acrylic resin-compatible adhesive cements. Journal of Dental Research, 62, 366-70. Brauer, G. M., Stansbury, J. W. & Flowers, D. (1986). Modification of cements containing vanillate or syringate esters. Dental Materials, 2, 21-7. Brauer, G. M., White, jr, E. E. & Moshonas, M. G. (1958). The reaction of metal oxides with o-ethoxybenzoic acid and other chelating agents. Journal of Dental Research, 37, 547-60. Bryant, R. W. & Wing, G. (1976a). The rate of development of strength in base forming materials for amalgam restorations. Australian Dental Journal, 21, 153-9. Bryant, R. W. & Wing, G. (1976b). The effects of manipulative variables on base forming materials for amalgam restorations. Australian Dental Journal, 21,211-16. Chisholm, E. S. (1873). Discussion. Dental Register, 27, 517. Civjan, S. & Brauer, G. M. (1964). Physical properties of cements based on zinc oxide, hydrogenated resin, o-ethoxybenzoic acid and eugenol. Journal of Dental Research, 43, 281-99. Civjan, S., Huget, E. F., Wolfhard, G. & Waddell, L. S. (1972). Characteristics of zinc oxide eugenol cements reinforced with acrylic resin. Journal of Dental Research, 51, 107-14. Coleman, G. (1962). A study of some antimicrobial agents used in oral surgery. British Dental Journal, 113, 22-8. Copeland, H. I., Brauer, G. M., Sweeney, W. T. & Forziati, A. F. (1955). Setting reaction of zinc oxide and eugenol. Journal of Research of the National Bureau of Standards, 55, 133-8. Cowan, J. H. & Teeter, H. M. (1944). Salts of residual dimerized fat acids: a new class of resinous substance. Industrial and Engineering Chemistry, 36, 148-52. 353

Non-aqueous cements Crisp, S., Ambersley, M. & Wilson, A. D. (1980). Zinc oxide eugenol cements. V. Instrumental studies of the catalysis and acceleration of the setting reaction. Journal of Dental Research, 59, 44-54. Dollimore, D. & Spooner, P. (1971). Sintering studies on zinc oxide. Transactions of the Faraday Society, 67, 2750-9. Dougherty, E. W. (1962). Dental cement material. US Patent 3,047,408. Douglas, W. H. (1978a). The metal oxide/eugenol cements. I. The chelating power of eugenol type molecules. Journal of Dental Research, 57, 800-4. Douglas, W. H. (1978b). The metal oxide/eugenol cements. II. A diffuse reflectance spectrophotometric study of the setting of zinc oxide and magnesium oxide cements. Journal of Dental Research, 57, 805-9. Eidelman, E., Finn, S. B. & Koulourides, T. (1965). Remineralization of carious dentin treated with calcium hydroxide. Journal of Dentistry for Children, 32, 218-25. El-Tahawi, H. M. & Craig, R. G. (1971). Thermal analysis of zinc oxide-eugenol cement. Journal of Dental Research, 50, 430-5. Fisher, F. J. (1977). The effect of three proprietary lining materials on microorganisms in carious dentine. An in vivo investigation. British Dental Journal, 143, 231-5. Fisher, F. J. & McCabe, J. F. (1978). Calcium hydroxide base materials: an investigation into the relationship between chemical structures and antibacterial properties. British Dental Journal, 144, 341-4. Flagg, J. F. (1875). Dental pathology and therapeutics. Dental Cosmos, 27, 465-9. Gerner, M. M., Zadorozhnyi, B. A., Ryabina, L. V. & Batovskii, V. N. (1966). Infrared spectra of eugenol and zinc eugenolate. Russian Journal of Physical Chemistry, 40, 122-3 (translation). Gilson, T. D. & Myers, G. E. (1970). Clinical studies of dental cements. III. Seven zinc oxide eugenol cements used for temporarily cementing completed restorations. Journal of Dental Research, 49, 14-20. Gourley, J. M. & Rose, D. E. (1972). Cavity bases under liners. Journal of the Canadian Dental Association, 38, 246. Graddon, D. P. (1968). Divalent transition metal /Mtetone-enolate complexes as Lewis acids. Coordination Chemistry Reviews, 4, 1-28. Granath, L. E. (1982). Pulp capping materials. In Smith, D. C. & Williams, D. F. (eds.) Biocompatibility of Dental Materials. Volume II. Biocompatibility of Preventive Dental Materials and Bonding Agents, Chapter 11. Boca Raton: CRC Press Inc. Grieve, A. R. (1969). A study of dental cements. British Dental Journal, 127, 405-10. Hannah, C. M. & Smith, D. C. (1971). Tensile strengths of selected dental restorative materials. Journal of Prosthetic Dentistry, 26, 314-23. Helgeland, K. (1982). In vitro testing of dental cements. In Smith, D. C. & Williams, D. F. (eds.) Biocompatibility of Dental Materials. Volume II. Biocompatibility of Preventive Dental Materials and Bonding Agents, Chapter 9. Boca Raton: CRC Press Inc. 354

References Hembree, J. H., George, T. A. & Hembree, M. E. (1978). Film thickness of cements beneath complete crowns. Journal of Prosthetic Dentistry, 39, 533-5. ISO. (1988). International Standard, ISO 3107. Dental zinc oxide/eugenol cements and zinc oxide non-eugenol cements. Jendresen, M. D. & Phillips, R. W. (1969). A comparative study of four zinc oxide eugenol formulations as restorative materials. Part II. Journal of Prosthetic Dentistry, 21, 300-9. Jendresen, M. D., Phillips, R. W., Swartz, M. L. & Norman, R. D. (1969). A comparative study of four zinc oxide eugenol formulations as restorative materials. Part I. Journal of Prosthetic Dentistry, 21, 176-83. Keller, J. C , Hammond, B. D., Kowlay, K. K. & Brauer, G. M. (1988). Biological evaluation of zinc hexyl vanillate cement using two in vivo test methods. Dental Materials, 4, 341-50. King, J. (1872). Treatment of exposed pulps. Dental Cosmos, 14, 193-4. Lee, V. J. & Parravano, G. (1959). Sintering reactions on zinc oxide. Journal of Applied Physics, 30, 1735-^0. McWalter, G. K., El-Kafrawy, A. H. & Mitchell, D. F. (1976). Long term study of pulp capping in monkeys with three agents. Journal of the American Dental Association, 93, 105-111. Marshall, P. A., Enrigh, D. P. & Weyl, W. A. (1952). On the mechanism of sintering and recrystallization of oxides. In The Proceedings of the International Symposium on the Reactivity of Solids, pp. 273-84. Gothenburg. Messing, J. J. (1961). A polystyrene-fortified zinc oxide/eugenol cement. Investigation into its properties. British Dental Journal, 110, 95-100. Mitcham, J. C. & Gronas, D. G. (1978). Clinical evaluation of cement solubility. Journal of Prosthetic Dentistry, 40, 453-6. Mjor, I. A. (1963). The effects of calcium hydroxide zinc oxide/eugenol and amalgam on pulp. Odontologisk Tidsskrift, 71, 94-105. Molnar, E. J. (1942). Cloves, oil of cloves and eugenol. Their medico-dental history. Dental Items of Interest, 64, 521-8. Molnar, E. J. (1967). Residual eugenol from zinc oxide-eugenol compounds. Journal of Dental Research, 46, 645-9. Molnar, E. J. & Skinner, E. W. (1942). A study of zinc oxide-rosin cements. I. Some variables which affect hardening time. Journal of the American Dental Association, 29, 744-51. Nagoe, M. & Morimoto, T. (1969). Differential heat of adsorption and entropy of water absorbed on zinc oxide surface. Journal of Physical Chemistry, 73, 3809-14. Nielsen, T. H. (1963). The ability of 39 chelating agents to form cements with metal oxides, respecting their usability as root-filling materials. Ada Odontologica Scandinavica, 21, 159-74. Norman, R. D., Phillips, R. W., Swartz, M. L. & Frankiewicz, T. (1964). The effect of particle size on the physical properties of zinc oxide-eugenol mixtures. Journal of Dental Research, 43, 252-62. 0ilo, G. & Espevik, S. (1978). Stress/strain behaviour of some dental luting cements. Acta Odontologica Scandinavica, 36, 45-9.

355

Non-aqueous cements Osborne, J. W., Swartz, M. L., Goodacre, C. J., Phillips, R. W. & Gale, E. M. (1978). A method for assessing the clinical solubility and disintegration of luting cements. Journal of Prosthetic Dentistry, 40, 413-17. Paterson, R. C. (1976). The reaction of the rat molar pulp to various materials. British Dental Journal, 140, 93-6. Pellico, H. A. (1974). Settable dental compositions. US Patent 3,837,865. Phillips, R. W. (1982a). Skinner's Science of Dental Materials, Chapter 7. Philadelphia: W. B. Saunders. Phillips, R. W. (1982b). Skinner's Science of Dental Materials, Chapter 29. Philadelphia: W. B. Saunders. Plant, C. G., Jones, I. H. & Wilson, H. J. (1972). Setting characteristics of lining and cementing materials. British Dental Journal, 133, 21-74. Plant, C. G. & Wilson, H. J. (1970). Early strengths of lining materials. British Dental Journal, 129, 269-74. Powers, J. M., Farah, J. W. & Craig, R. G. (1976). Modulus of elasticity and strength properties of dental cements. Journal of the American Dental Association, 92, 588-91. Prosser, H. J., Groffman, D. M. & Wilson, A. D. (1982). The effect of composition on the erosion properties of calcium hydroxide cements. Journal of Dental Research, 61, 1431-5. Prosser, H. J., Stuart, B. & Wilson, A. D. (1979). An infra-red spectroscopic study of the setting reaction of a calcium hydroxide dental cement. Journal of Materials Science, 14, 2894-900. Prosser, H. J. & Wilson, A. D. (1982). Zinc oxide eugenol cements. VI. Effect of zinc oxide type on the setting reactions. Journal of Biomedical Materials Research, 16, 585-98. Richter, W. A., Mitchem, J. C. & Brown, D. (1970). The predictability of retentive value of dental cements. Journal of Prosthetic Dentistry, 24, 298-303. Roydhouse, R. H. & Weiss, M. E. (1964). Tissue reactions in restorative materials. Journal of Dental Research, 43, 807. Shilling, G. (1977). The permanency of EBA cements. Journal of the American Dental Association, 95, 187-9. Silvey, R. G. & Myers, G. E. (1977). Clinical studies of dental cements. V. Recall evaluation of restorations cemented with a zinc oxide-eugenol cement and a zinc phosphate. Journal of Dental Research, 55, 289-91. Silvey, R. G. & Myers, G. E. (1976). Clinical studies of dental cements. VI. A study of zinc phosphate EBA-reinforced zinc oxide eugenol and polyacrylic acid cements as luting agents in fixed prostheses. Journal of Dental Research, 56, 1215-18. Skinner, E. W., Molnar, E. J. & Suarez, G. (1964). Reactions of zinc oxide with carboxylic acids - physical properties. Journal of Dental Research, 43, 915. Smith, D. C. (1958). The setting of zinc oxide/eugenol mixtures. British Dental Journal, 105, 313-21. Smith, D. C. (1960). A quick-setting zinc oxide/eugenol mixture. British Dental Journal, 108, 232. Smith, D. C. (1982a). Composition and characteristics of dental cements. In 356

References Smith, D. C. & Williams, D. F. (eds.) Biocompatibility of Dental Materials. Volume II. Biocompatibility of Preventive Dental Materials and Bonding Agents, Chapter 8. Boca Raton: CRC Press Inc. Smith, D. C. (1982b). Tissue reactions to cements. In Smith, D. C. & Williams, D. F. (eds.) Biocompatibility of Dental Materials. Volume II. Biocompatibility of Preventive Dental Materials and Bonding Agents, Chapter 10. Boca Raton: CRC Press Inc. Stansbury, J. W., Argentar, H. & Brauer, G. M. (1981). Cements from 2,5dimethyloxyphenol and zinc oxide. Journal of Dental Research, 60, 373. Stansbury, J. W. & Brauer, G. M. (1985). Divanillates and polymerizable vanillates as ingredients of dental cements. Journal of Biomedical Materials Research, 19, 715-25. Steinke, R., Newcomer, P., Komarneni, S. & Roy, R. (1988). Dental cements: investigation of chemical bonding. Materials Research Bulletin, 23, 13-22. Wallace, D. A. & Hansen, H. L. (1939). Zinc oxide eugenol cements. Journal of the American Dental Association, 26, 1536-40. Wessler, J. (1894). Pulpol, ein neues medicamentoses Cement. Deutsche Monatsschrift fur Zahnheilkunde, 12, 478-84. Williams, J. D., Swartz, M. L. & Phillips, R. W. (1965). Retention of orthodontic bands as influenced by the cementing media. Angle Orthodontics, 4, 276-85. Williams, P. D. & Smith, D. C. (1971). Measurement of the tensile strength of dental restorative materials by use of a diametral compressive strength test. Journal of Dental Research, 50, 436—42. Wilson, A. D. (1975a). Dental cements - general. In von Fraunhofer, J. A. (ed.) Scientific Aspects of Dental Materials, Chapter 4. London: Butterworths. Wilson, A. D. (1975b). Zinc oxide dental cements. In von Fraunhofer, J. A. (ed.) Scientific Aspects of Dental Materials, Chapter 5. London: Butterworths. Wilson, A. D. (1976). Examination of the test for compressive strength applied to zinc oxide eugenol cements. Journal of Dental Research, 55, 142-7. Wilson, A. D. (1978). The chemistry of dental cements. Chemical Society Reviews, 7, 265-96. Wilson, A. D. & Batchelor, R. F. (1970). Zinc oxide eugenol cements. II. Study of erosion & disintegration. Journal of Dental Research, 49, 593-8. Wilson, A. D. & Batchelor, R. F. (1971). The consistency of dental cements. The specification test for filling materials. British Dental Journal, 130, 437-41. Wilson, A. D., Clinton, D. J. & Miller, R. P. (1973). Zinc oxide eugenol cements. IV. Microstructure and hydrolysis. Journal of Dental Research, 52, 253-60. Wilson, A. D., Groffman, D. M., Powis, D. R. & Scott, R. P. (1986). An evaluation of the significance of the impinging jet method for measuring the acid erosion of dental cements. Biomaterials, 7, 55-60. Wilson, A. D. & Lewis, B. G. (1980). The flow properties of dental cements. Journal of Biomedical Materials Research, 14, 383-91. Wilson, A. D., Prosser, H. J., Paddon, J. M. & Gilhooley, R. A. (1981). Calcium hydroxide dimer (Cal-mer) cements. British Dental Journal, 150, 351-3.

357

Non-aqueous cements Wilson, A. D. & Mesley, R. J. (1972). Zinc oxide eugenol cements. III. Infra-red spectroscopic studies. Journal of Dental Research, 51, 1581-8. Wilson, A. D. & Mesley, R. J. (1974). Chemical nature of cementing matrices of cements formed from zinc oxide and 2-ethoxybenzoic acid-eugenol liquids. Journal of Dental Research, 53, 146. Zander, H. A. (1939). Reaction of the pulp to calcium hydroxide. Journal of Dental Research, 18, 373-9.

358

10 Experimental techniques for the study of acid-base cements

10.1

Introduction

The chief problem in studying the chemical nature of AB cements is that many are essentially amorphous, so that the powerful tool of X-ray diffraction (XRD) analysis cannot be used. Some AB cements do exhibit a degree of crystallinity, but rarely in significant amounts; indeed, complete crystallinity is usually a sign that the reaction product is not cementitious. The literature contains numerous examples of workers being misled by the results of XRD analysis into neglecting the presence and significance of the amorphous phase. A number of techniques have been employed that are capable of giving information about amorphous phases. These include infrared spectroscopy, especially the use of the attenuated total reflection (ATR) or Fourier transform (FT) techniques. They also include electron probe microanalysis, scanning electron microscopy, and nuclear magnetic resonance (NMR) spectroscopy. Nor are wet chemical methods to be neglected for they, too, form part of the armoury of methods that have been used to elucidate the chemistry and microstructure of these materials. In addition to spectrosopic studies of the setting chemistry of AB cements, numerous mechanical tests have been used to measure properties of the set materials. This latter group has included determination of compressive and flexural strengths, translucency, electrical conductivity and permittivity. The present chapter describes each of these techniques in outline, and shows how they have been applied. Results obtained using these techniques are described in earlier chapters which deal more thoroughly with each individual type of AB cement.

359

Experimental techniques 10.2

Chemical methods

Broadly speaking, two groups of purely chemical methods have been employed in the study of AB cements. These have been (1) studies of cement formation and (2) degradative studies on set cements. The first group has generally used the simple approach of attempting to react the candidate acid, usually as an aqueous solution, with the candidate base, which is generally a powder that is only sparingly soluble in water. The success or otherwise of the attempted cementition has then been assessed by a simple criterion, such as stability of the product in water. 10.2.1 Studies of cement formation A number of studies have been carried out using the criterion of water stability to assess the success of cementation. For example, Hodd & Reader (1976) studied a range of metal oxide-poly acid cements by this technique in order to determine the influence of the metal cation and the polymer structure on stability. A number of polymeric carboxylic acids were used in this study, namely poly(acrylic acid), poly(ethylene-maleic acid), poly(methacrylic acid) and poly(vinyl methyl ether-maleic acid). Poly(ethylene sulphonic acid) was also used, but it proved to be a poor cement former and did not form stable cements with many of the cations examined. A large number of metal oxides of both main group and transition metals were examined, and a number of them were found to form water-stable cements with most of the poly carboxylic acids. In rare cases, metal oxides formed stable cements with a few of the acids, but yielded only unstable mixtures with others. For example, bismuth oxide, Bi2O3, gave a stable cement with poly(acrylic acid) that showed no disintegration after 16 hours immersion in water at room temperature, but with ethylene-maleic acid gave a mixture which disintegrated completely. More common, however, was the behaviour of zinc oxide, which gave stable cements with all of the polycarboxylic acids. An equally simple chemical study was carried out on phytic acidaluminosilicate cements (Prosser et al, 1983). Phytic acid, myo-inositol hexakis(dihydrogen phosphate), is a naturally occurring substance found in seeds, and it is a stronger acid than phosphoric acid. Cements were prepared using aqueous solutions of phytic acid, concentrated to 50 wt%, and with 5 wt% zinc dissolved in the acid to moderate the rate of reaction with the glass powder. Discs of cement were prepared and these were 360

Infrared spectroscopic analysis placed in distilled water exactly seven minutes after the start of mixing of the aqueous acid with powder. Soluble material was determined gravimetrically by evaporating the eluates to dryness. This approach demonstrated that these cements had excellent resistance to early attack by water. Other properties of these cements were promising for dental application and a patent protecting this use has been sought (Lion Corporation, 1980). 10.2.2 Degradative studies Degradative methods have been employed on cements that have been allowed to set. In a typical degradative study, Cook (1983) treated glass-ionomer cements with 3-3-molar potassium hydroxide solution. The resulting solution was analysed for release of ions using atomic absorption spectroscopy. This overall degradation technique allowed the measurement of time-dependent concentrations of Al3+, Ca2+ and Na+ ions which had entered the matrix from the glass. Cook concluded that both aluminium and calcium ions were involved in the initial setting reaction of these cements, although this has been disputed by other workers (Nicholson et ai, 1988b; Wilson & McLean, 1988). Cook also concluded that aluminium ions were the least easily extracted of all the cations removed from the glass particles, which is not surprising given that the aluminium is present in the glass initially as part of the aluminosilicate network structure, and is anionic in character (Hill & Wilson, 1988). Extraction studies have also been carried out by grinding the ageing cements and extracting the soluble ions with water (Wilson & Kent, 1970; Crisp & Wilson, 1974). Ion content was determined using atomic absorption spectroscopy. The experiments give different, but complementary, results to those of Cook (1983), since what is extracted are those ions that have been released from the glass powder but not yet insolubilized by reaction with the polyacid. 10.3

Infrared spectroscopic analysis

10.3.1 Basic principles The infrared region of the electromagnetic spectrum lies between the wavelengths 1000 and 15000 nm (Kemp & Vellaccio, 1980). Absorption of radiation in this region by organic compounds has been known since 1866, when Tyndall first conducted experiments on the interaction of radiation with compounds such as chloroform, methyl and ethyl iodides, benzene, 361

Experimental techniques ethyl acetate and so on (Tyndall, 1866). More systematic work in the 30 or so years from 1881 by a number of workers, most notably W. Coblentz, showed that the interaction was attributable to individual groups within the molecule (Cesaro & Torracca, 1988). From this deduction, and the subsequent painstaking acquisition of empirical data, the modern approach to infrared spectroscopy has emerged, in which individual functional groups can readily be identified and structural conclusions can be drawn on the basis of a rapidly prepared infrared absorption spectrum. In the most usual modern form of infrared spectroscopy, absorption is plotted against reciprocal wavelength or wavenumber in cm"1 (Williams & Fleming, 1973). The usual range of such a spectrum is 4000 cm"1 at the high-frequency end to 625 cm"1 at the low-frequency end. Functional groups in organic molecules absorb infrared radiation at well-defined parts of this spectrum. Although this actually occurs due to absorption by the whole of a complex organic molecule, such an absorption can be considered to a reasonable approximation, for a number of spectral bands, to be localized at individual functional groups. These localized absorptions give rise to vibrations of the particular chemical bonds, which may include stretching, bending, rocking, twisting or wagging (Williams & Fleming, 1973). The specific requirement for a vibration to give rise to an absorption in the infrared spectrum is that there should be a change in the dipole moment as that vibration occurs. In practice, this means that vibrations which are not centrosymmetric are the ones of interest, and since the symmetry properties of a molecule in the solid state may be different from those of the same molecule in solution, the presence of bands may depend on the physical state of the specimen. This may be an important phenomenon in applying infrared spectroscopy to the study of AB cements. 10.3.2 Applications to AB cements It is apparent from the preceding discussion that the kinds of molecules generally studied by infrared spectroscopy are organic. This means that the AB cements which have been particularly studied by this technique are those containing organic functional groups, most typically carboxylic acid and carboxylate groups. In particular, extensive studies have been carried out on the setting reactions and final structures of glass-ionomer cements (Wilson & McLean, 1988), zinc polycarboxylates (Wilson, 1982) and other metal oxide-poly(acrylic acid) cements (Crisp, Prosser & Wilson, 1976). 362

Infrared spectroscopic analysis

The range of acids for which infrared spectroscopy can be used is limited. Despite this, the amount of detailed information which can be obtained using the technique is very high. This is because of the extent and comprehensiveness of the data available concerning the effect of subtle changes in the bonding of carboxylate groups on the position of the corresponding infrared absorption bands. Many detailed structures of simple metal carboxylates have been established by complementary techniques, such as X-ray crystallography, and these findings have been used to establish correlations between band position in the infrared spectrum and structure (Mehrotra & Bohra, 1983). There are limits to the extent to which band position and structure can be correlated, however, since there are exceptions to many of the apparently established empirical rules. Moreover, some literature data have been shown to be plainly wrong since they have ignored, for example, possible spectral changes caused by cation and halide exchange due to mounting the sample between alkalimetal halide discs (Deacon & Phillips, 1982). Nonetheless, some valuable conclusions have been drawn using general correlations of carboxylate band position with the nature of the bonding in set or setting cements. Broadly speaking, in the study of AB cements derived from polycarboxylic acids, the band of interest falls in the region 1550-1620 cm"1 (Mehrota & Bohra, 1983; Bellamy, 1975). This band is the asymmetric stretch of the carboxylate group and its exact position depends on both the nature of the bonding involved (i.e. whether purely ionic or partially covalent), and the nature of any chelation by the carboxylate group (Bellamy, 1975). The four possible modes of carboxylate bonding which have been identified are purely ionic, unidentate, bridging bidentate and chelating bidentate, as illustrated in Figure 5.3. The ionic band falls at 1570-1575 cm"1, as it does in sodium and lithium poly(acrylates) (Nicholson & Wilson, 1987) as well as in the simple monomeric carboxylates (Mehrotra & Bohra, 1983). The unidentate covalent binding gives a band close to 1550 cm"1, as does the chelating bidentate, while the bridging bidentate generally gives a band in the region 1600-1620 cm"1. The unidentate mode of binding has been found to be relatively rare in monomeric carboxylate compounds (Mehrotra & Bohra, 1983), and on these grounds was rejected as a probable structure in polycarboxylate materials by Nicholson, Wasson & Wilson (1988).

363

Experimental techniques 10.3.3 Fourier transform infrared spectroscopy In recent years, infrared spectroscopy has been enhanced by the possibility of applying Fourier transform techniques to it. This improved spectroscopic technique, known as Fourier transform infrared spectroscopy (FTIR), is of much greater sensitivity than conventional dispersive IR spectroscopy (Skoog & West, 1980). Moreover, use of the Fourier transform technique enables spectra to be recorded extremely rapidly, with scan times of only 02 s. Thus it is possible to record spectra of AB cements as they set. By comparison, conventional dispersive IR spectroscopy requires long scan times for each spectrum, and hence is essentially restricted to examining fully-set cements. FTIR has been applied to both zinc polycarboxylate cements (Nicholson et al., 1988a) and glass-ionomer cements (Nicholson et al, 1988b), in both cases yielding significantfindings.The zinc polycarboxylate was shown for the first time to become partially covalent with time after setting, while the role of (+ )-tartaric acid in glass-ionomer cements was shown to be to suppress early formation of calcium polyacrylate acid and to enhance later formation of aluminium polyacrylate. These results are discussed in more detail in Chapter 5.

10.4

Nuclear magnetic resonance spectroscopy

10.4.1 Basic principles The NMR spectrum can be recorded for compounds containing those elements whose nuclei have spin values of \ (Williams & Fleming, 1973). A large number of such nuclei exist, including 1H, 13C, 19F, 27A1 and 31P. Unfortunately, many other nuclei of importance in chemistry, such as 12C and 16O, have nuclear spin values of 0 and hence do not give nuclear resonance signals in a magnetic field. To study NMR spectra of compounds, apparatus is required that consists of three sets of components. These are a radio-frequency transmitter, a homogeneous magneticfieldand a radio-frequency receiver. In addition to these, the apparatus includes a unit to sweep the magnetic field over a small range, a mere few parts per million. The earliest NMR technique to gain importance in chemistry was that of proton NMR. Spectra could be obtained for compounds containing the *H nucleus by continuously sweeping the field at constant frequency. This 364

Nuclear magnetic resonance spectroscopy

approach results in so-called continuous wave spectra, which suffer from the general disadvantage that only a small portion of the spectrum is excited at any one time. Such spectra have unduly low signal-to-noise ratios, which is particularly undesirable when studying nuclei of low abundance and/or low sensitivity, such as 13C. As a result continuous wave is not used for 13C NMR spectra but instead pulsing techniques are used together with Fourier transformation of the data thus obtained. For 13C NMR, the radio frequency is applied as a short, powerful pulse which acts like a spread of frequencies. All the NMR-active nuclei are excited by this pulse and then decay back to their equilibrium states. These decays result in a series of complex sine waves which diminish exponentially with time. By the application of Fourier transform techniques, such decay curves can be converted into spectra. By making use of the capability of storing many such pulsed spectra on a computer and adding the signals together prior to applying the Fourier transform, a significant improvement in the signal-to-noise ratio can be obtained. The nuclei studied by NMR spectroscopy are affected by the precise nature of their electronic environments. This means, for example, that protons will resonate at different frequencies according to their position in a molecule. In this way, it is possible to distinguish between protons in methyl groups and methylene groups, in hydroxyl groups or as part of the benzene ring. In addition, protons interact with each other, leading to what is known as spin-spin coupling and giving rise to well-characterized splitting patterns in their NMR spectrum. All of these features may be used to give structural information about molecules containing XH nuclei. Carbon-13 nuclei, due to their low natural abundance, do not interact with each other in a molecule, though they are affected by adjacent protons. In practice, such couplings are removed by irradiation of the whole spectrum as it is recorded, in a technique known as proton noise decoupling. This means that practical 13C NMR spectra exhibit one unsplit signal for each type of carbon atom present in the sample. 10.4.2 Applications to AB cements NMR spectroscopy of various nuclei has been used in the study of AB cements derived from various acids, including phosphoric acid and poly(acrylic acid). For example, 31P NMR has been used in studies of dental silicate cement, i.e. the AB cement made from aqueous phosphoric acid and powdered aluminosilicate glass (Wilson, 1978). In this cement, the 365

Experimental techniques setting reaction is controlled by dissolving small amounts of aluminium and/or zinc metal in the phosphoric acid. Using 31P NMR, O'Neill et al. (1982) were able to distinguish between the various complexes formed by ions of these metals in the presence of phosphoric acid. The chemistry of polyelectrolyte cement liquids has been studied using 13 C NMR. Watts (1979) used this technique to distinguish between the homopolymer of acrylic acid and its copolymer with itaconic acid in various commercial polyelectrolyte dental cements. This was readily achieved because of the ability of 13C NMR to differentiate between carbon atoms in chemical environments that are only slightly different. Prosser, Richards & Wilson (1982) used 13C NMR spectroscopy to study the role of ( + )-tartaric acid in modifying the setting behaviour of glass-ionomer dental cements. For this, a model system was used, with a lower ratio of glass powder to poly (acrylic acid) liquid than in conventional cements; this slowed the reaction, enabling the spectra to be recorded on acceptable time scales. This study showed for the first time that ( + )tartaric acid was an effective additive for controlling the setting characteristics of these cements because it reacts preferentially with Ca2+ ions released from the glass. Hence, (+ )-tartaric acid acts to extend the working time of these cements. The work also showed that there was no difference between the reactivity of the acrylic acid and the itaconic acid segments when the copolymer was used as the acidic component. 10.5

Electrical methods

Changes in electrical conductivity have occasionally been used to study the setting chemistry of AB cements. Conductivity has been particularly used in the study of dental cements, notably the dental silicate (Wilson & Kent, 1968), the zinc polycarboxylate (Cook, 1982), the glass-ionomer cement (Cook, 1982) and the ZOE cement (Crisp, Ambersley & Wilson, 1980). In a typical study of conductivity, Cook (1982) used a cell consisting of two platinum disc electrodes, 12 mm in diameter and 1-5 mm apart. The setting AB cement was examined in this cell which had been calibrated using a standard solution of 0-02 M potassium chloride. Plots were recorded of specific conductance against time for each of the setting cements. For zinc polycarboxylate there was found to be a rapid drop in specific conductance about 10 minutes after the start of mixing. This behaviour was consistent with the replacement of relatively mobile protons by significantly less mobile zinc ions in the polycarboxylate chain. Con366

X-ray diffraction ductivity was found to drop rapidly by a factor of 200 which was held to be consistent with quantitative neutralization of the poly(acrylic acid) and with a very low diffusion coefficient for zinc ions in the set cement. By contrast, the specific conductance of glass-ionomer cements was found to decrease much more gradually, indicating that the setting reaction in these cements is much slower than for the zinc polycarboxylates. Moreover, setting was still not complete even 1000 minutes after the start of mixing. An alternative electrical method that has been used in the study of glass-ionomer cements has been the measurement of dielectric properties. Tay & Braden (1981, 1984) measured the resistance and capacitance of setting cements at various times from mixing. From the results obtained, relative permittivity and resistivity were calculated. In general, as these cements set, their resistivity was found to fall rapidly, then to rise again. Both these results and the results of relative permittivity measurements were consistent with the cements comprising highly ionic and polar structures.

10.6

X-ray diffraction

10.6.1 Basic principles X-ray diffraction is the most accurate and powerful method of both identifying solids and determining their structure. This is essentially because the regular array of atoms or ions in a crystalline solid is spaced at dimensions corresponding to the wavelength of X-rays, and such arrays are consequently able to act as an X-ray diffraction grating. The resulting diffraction pattern is recorded either on photographic film or by an electronic detector. The underlying principle of X-ray diffraction is as follows. When a beam of X-rays passes through a crystalline solid it meet various sets of parallel planes of atoms. The diffracted beams cancel out unless they happen to be in phase, the condition for which is described in the Bragg relationship: nk = 2dsin9 where X is the wavelength of the X-rays, d is the distance between the planes, and 6 is the angle of incidence of the X-rays on the planes. In practice, values of 6 can be measured and, since the wavelength of the X-rays is known in any given experiment, values of d can be calculated. These lvalues are related to the unit cell symmetry and dimensions. If the 367

Experimental techniques intensities of the diffracted beam in each direction are also measured, the complete structure of the solid can be determined (Mackay & Mackay, 1972). Each atom in the lattice acts as a scattering centre, which means that the total intensity of the diffracted beam in a given direction depends on the extent to which contributions from individual atoms are in phase. Relating the underlying structure to the observed diffraction pattern is not straightforward, but is essentially a trial-and-error search involving extensive computer-based calculations. For materials which are available not in the form of substantial individual crystals but as powders, the technique pioneered by Debye and Scherrer is employed (Moore, 1972). The powder is placed into a thinwalled glass capillary or deposited as a thin film, and the sample is placed in the X-ray beam. Within the powder there are a very large number of small crystals of the substance under examination, and therefore all possible crystal orientations occur at random. Hence for each value of d some of the crystallites are correctly oriented to fulfil the Bragg condition. The reflections are recorded as lines by means of a film or detector from their positions, the lvalues are obtained (Mackay & Mackay, 1972). 10.6.2 Applications to AB cements X-ray diffraction has been applied to certain AB cements. For example, Crisp et al. (1979), in a study of silicate mineral-poly(acrylic acid) cements, used the technique both to assess the purity of the powdered minerals employed and to monitor mineral decomposition in mixtures with poly(acrylic acid), in order to indicate whether or not cement formation had taken place. They employed Cu Ka radiation passed through a nickel filter; for most of the samples, a seven-hour exposure time was found to be adequate for the development of a discernible diffraction pattern. Samples were identified by reference to published powder diffraction data. A number of other studies of AB cements have used X-ray diffraction. For example, Sorrell (1977) and Sorrell & Armstrong (1976) employed the technique in the study of oxychloride cements formed in aqueous solution by interaction of oxides and chlorides of either zinc or magnesium. Individual phases were identified, again using Cu Ka radiation, this time comparing results with those previously obtained for pure compounds. Results from these two studies are described in detail in Sections 7.2 and 7.3 respectively. 368

Electron probe microanalysis

10.7

Electron probe microanalysis

10.7.1 Basic principles This technique can be applied to samples prepared for study by scanning electron microscopy (SEM). When subject to impact by electrons, atoms emit characteristic X-ray line spectra, which are almost completely independent of the physical or chemical state of the specimen (Reed, 1973). To analyse samples, they are prepared as required for SEM, that is they are mounted on an appropriate holder, sputter coated to provide an electrically conductive surface, generally using gold, and then examined under high vacuum. The electron beam is focussed to impinge upon a selected spot on the surface of the specimen and the resulting X-ray spectrum is analysed. Analysis is most frequently done qualitatively since there are problems in quantification (Reed, 1973). Although intensity is approximately proportional to mass concentration of a given element there are significant deviations, depending on which other elements are present. There is also a minimum atomic number which can usually be detected, since the practical maximum X-ray wavelength that is used is fixed at 0-12 nm. This in turn fixes sodium (atomic number 11) as the lightest element for which this technique is valid. Special techniques are available to overcome this limitation, but they are not in general use, and have not been applied to AB cements. 10.7.2 Applications to dental silicate cements In a study of dental silicate cements, Kent, Fletcher & Wilson (1970) used electron probe analysis to study the fully set material. Their method of sample preparation varied slightly from the general one described above, in that they embedded their set cement in epoxy resin, polished the surface toflatness,and then coated it with a 2-nm carbon layer to provide electrical conductivity. They analysed the various areas of the cement for calcium, silicon, aluminium and phosphorus, and found that the cement comprised a matrix containing phosphorus, aluminium and calcium, but not silicon. The aluminosilicate glass was assumed to develop into a gel which was relatively depleted in calcium. 10.7.3 Applications to glass-ionomer cements A similar study was carried out on glass-ionomer cements (Barry, Clinton & Wilson, 1979), which showed some interesting similarities to the dental 369

Experimental techniques silicate cements, as well as some differences. Firstly, aluminium and calcium were found to be removed from the glass particles and to reside in the continuous phase. However, by contrast with the dental silicate cement, some silicon was also found in the continuous phase. Attack by the poly(acrylic acid) had apparently occurred preferentially at the calciumrich sites of the glass, a finding that was significant in formulating the theory of the setting chemistry of these materials. 10.8

Measurement of mechanical properties

Strength has been widely measured for AB cements. It is formally defined as the force experienced by a material at the point where fracture occurs (Gillam, 1969). The study of strength is complicated in that fracture is a point of discontinuity, so cannot be readily interpreted in terms of events leading up to it. Strength can be measured in compression, in tension, in shear and transversely (flexural strength). However, if we exclude plastic flow as a means of failure, then materials can only fracture in one of two ways: (1) by the pulling apart of planes of atoms, i.e. tensile failure, or (2) by the slippage of planes of atoms, i.e. shear failure. Strength is essentially a measure of fracture stress, which is the point of catastrophic and uncontrolled failure because the initiation of a crack takes place at excessive stress values. AB cements tend to be essentially brittle materials. This means that when subjected to mechanical loading, they tend to rupture suddenly with minimal deformation. There are a number of different types of strength which have been identified and have been determined for AB cements. These include compressive, tensile and flexural strengths. Which one is determined depends on the direction in which the fracturing force is applied. For full characterization, it is necessary to evaluate all of these parameters for a given material; no one of them can be regarded as the sole criterion of strength. Generally, strength is determined by applying forces uniaxially using an apparatus consisting of a pair of jaws which move either together or apart in a controlled manner. A chart recorder is employed to give a permanent record of results obtained, so that the force at fracture can be determined. Whether such an apparatus measures tensile, compressive or flexural strength depends on how the sample is oriented between the jaws and on the direction that the jaws are set to travel relative to each other. Such tests 370

Measurement of mechanical properties need to be repeated on several samples in order to provide sufficient data for statistical analysis and to allow calculation of both the mean and the scatter of the results. 10.8.1 Compressive strength The most common mechanical property of cements that has been measured routinely is compressive strength (Polakowski & Ripling, 1966). Measurement is easy to carry out but there are several reasons to consider that the results from the technique are unsatisfactory. Interpretation of results is uncertain because of the complexities in the mode of failure. Minor imperfections in the material lead to localized stress concentrations which affect the magnitude of the result. Failure is complex because both the mode and plane of failure are variable. Failure under compressive load can occur by plastic yielding, cone failure (secondary shear forces) or axial splitting (secondary tensile forces) (Kendall, 1978). The mode of failure depends on the size and geometry of the specimen, the nature of the material tested and the rate of loading. The studies of Selenrath & Gramberg (1958) are of interest in showing the effect of the nature of the material on fracture. They found that when cylindrical specimens were placed unrestrained in the testing machine, glass and lithographic limestone exhibited simple vertical fracture or axial splitting, evidence of tensile failure. By contrast, coarse-grained marble and bothfine-grainedand coarse-grained sandstone yielded cones at the ends of the specimen with typical shearing fracture planes. However, when the ends of the lithographic limestone were clamped they exhibited both types of fracture. The variation in the mode of failure makes comparison of different types of cement quite impossible. As Darvell (1990) has pointed out, compressive strength is not a material property under any condition, but can only be used to compare materials of a very similar nature. The compressive strength of AB cements used in dentistry has been widely studied (Wilson & McLean, 1988). It is the method, for example, specified in the British Standard on dental cements. However, there is concern that the result is less clinically relevant than the evaluation of flexural strength. Moreover, the latter is more discriminating (Prosser et al., 1984). Despite this, compressive strength has been used to indicate clinical acceptability; phosphate-bonded cements with low compressive strength tend to be unsatisfactory in other respects such as durability, and 371

Experimental techniques hence there is value in using compressive strength as a criterion of general material quality (Wilson & McLean, 1988). 10.8.2 Diametral compressive strength The diametral compressive strength has been used to estimate the tensile strength of certain AB cements (Smith, 1968). In this test, the load is applied diametrically across a cylinder of cement. Theoretical consideration of the test geometry shows that for a perfectly brittle material the failure that occurs is tensile in character. The difficulty in applying this test to AB cements is that they are not sufficiently brittle for this to hold true. In particular, the zinc polycarboxylate and glass-ionomer cements show sufficient plastic character to make the relationship between diametral compressive and tensile strength vary between AB cements of different types; like the compressive strength test, this test is valid only as a means of comparison between similar materials (Darvell, 1990). For glass-ionomer cements, there have been several studies on the factors affecting strength. For example Crisp, Lewis & Wilson (1977) showed that both compressive and tensile strengths increased linearly with concentration of polyacid in the liquid component, though neither extrapolated to zero at zero acid concentration. Ageing was also shown to influence compressive strength of these materials, older cements being stronger than younger ones. However, the exact development of strength was found to depend on storage conditions (Crisp, Lewis & Wilson, 1976). 10.8.3 Flexural strength Flexural strength is determined using beam-shaped specimens that are supported longways between two rollers. The load is then applied by either one or two rollers. These variants are called the three-point bend test and the four-point bend test, respectively. The stresses set up in the beam are complex and include compressive, shear and tensile forces. However, at the convex surface of the beam, where maximum tension exists, the material is in a state of pure tension (Berenbaum & Brodie, 1959). The disadvantage of the method appears to be one of sensitivity to the condition of the surface, which is not surprising since the maximum tensile forces occur in the convex surface layer. Of all the methods of determining strength, theflexuraltest appears to be the most satisfactory. While not ideal, it does have the advantage of 372

Measurement of mechanical properties measuring a clearly defined parameter. However, very few studies of AB cements involving this test have been carried out. Prosser, Powis & Wilson (1986) studied the influence of various formulation changes on the flexural strength of glass-ionomer cements. They found that the flexural strength of glass-ionomer cements was dependent on the glass and the polyacid used to prepare them. Opaque and opal glasses containing crystallites were found to yield cements of higherflexuralstrength than those prepared from clear glasses; increasing the relative molar mass of the polyacid was also found to improve flexural strength. 10.8.4

Fracture toughness

The use of fracture stress as a measure of resistance to fracture is suspect. In all these tests, whether compressive, tensile or flexural, failure is catastrophic because there is no suitable flaw for crack propagation. A high force is needed to start a crack and as a result the subsequent propagation takes place under too great a stress. Then there is the curious finding that compressive strength values are 10 times those for tensile strength although, in principle, both are measures of cohesion. For these reasons, attention has been paid to energy criteria as a measure of toughness. The following points need to be noted. Materials do not reach their theoretical strength (that is of their primary chemical bonds), because of the presence of minute flaws. Stress is concentrated at these flaws and so is enhanced. In effect this amounts to a weakening of the material. Under load, cracks propagate from these flaws and lead to failure. Propagation of cracks requires energy to create new surfaces but also releases stored energy. Unstable propagation of cracks occurs when the strain energy released exceeds that required to create new surfaces, and occurs when the crack reaches a certain length. This is because strain energy released is proportional to (crack length)2, and the energy to create a new surface is proportional to the crack length. Fracture toughness is the resistance to propagation of cracks through a material and is usually quantified by the stress intensity factor, Kx, defined as Kx = GF(na)*

where a is the flaw size and aF the fracture stress. There are a number of methods of determining fracture toughness, but the one used so far for AB cements is the double torsion method introduced 373

Experimen tal techn iques by Outwater et al. (1974), which appears to have some advantages over other methods. Double torsion test specimens take the form of rectangular plates with a sharp groove cut down the centre to eliminate crack shape corrections. An initiating notch is cut into one end of each specimen (Hill & Wilson, 1988) and the specimen is then tested on two parallel rollers. A load is applied at a constant rate across the slot by two small balls. In essence the test piece is subjected to a four-point bend test and the crack is propagated along the groove. The crack front is found to be curved. The double torsion test specimen has many advantages over other fracture toughness specimen geometries. Since it is a linear compliance test piece, the crack length is not required in the calculation. The crack propagates at constant velocity which is determined by the crosshead displacement rate. Several readings of the critical load required for crack propagation can be made on each specimen. When the load has reached a critical plateau value, the crack continues to propagate at constant load. Crack propagation can be stopped by removing the load, with the implication that several readings can be made on one test specimen. Crack velocity is determined by the crosshead speed, modulus of the material and specimen dimensions. To calculate fracture toughness using the double torsion test piece, the following equation is used: where Kx = fracture toughness, Pc = critical load for crack propagation, Wm = specimen width, t = specimen thickness, tc = crack depth, Wc = distance between the supports and v = Poisson's ratio. 10.9

Setting and rheological properties

The rheological properties of AB cements are important where those cements have been used in dentistry. It is not sufficient simply to have a cement which eventually sets to give a resistant, strong material. The cement must also remainfluidfor a sufficient time to allow placement, and ideally must develop a good degree of hardness very rapidly following placement. Thus the setting chemistry must be such that the reaction which occurs immediately the acid and the base are mixed does not lead too rapidly to the development of a viscous paste that is difficult to manipulate. Ideally, viscosity should be low enough to allow manipulation but high enough so that the fluid cement does not flow appreciably once in place. 374

Setting and rheological properties

Thus, the rheological requirements of materials for this application are demanding, and their evaluation has been important in the study of this group of AB cements. The rheological characteristics of AB cements are complex. Mostly, the unset cement paste behaves as a plastic or plastoelastic body, rather than as a Newtonian or viscoelastic substance. In other words, it does not flow unless the applied stress exceeds a certain value known as the yield point. Below the yield point a plastoelastic body behaves as an elastic solid and above the yield point it behaves as a viscoelastic one (Andrade, 1947). This makes a mathematical treatment complicated, and although the theories of viscoelasticity are well developed, as are those of an ideal plastic (Bingham body), plastoelasticity has received much less attention. In many AB cements, yield stress appears to be more important than viscosity in determining the stiffness of a paste. 10.9.1 Problems of measurement Consistency, working time, setting time and hardening of an AB cement can be assessed only imperfectly in the laboratory. These properties are important to the clinician but are very difficult to define in terms of laboratory tests. The consistency or workability of a cement paste relates to internal forces of cohesion, represented by the yield stress, rather than to viscosity, since cements behave as plastic bodies and not as Newtonian liquids. The optimum stiffness or consistency required of a cement paste depends upon its application. Three useful tests have been used to evaluate the working and setting properties of experimental cements. These are the parallel plate plastometer, the penetrometer and the oscillating rheometer. They are described in the following sections of this chapter. 10.9.2 Methods of measurement Initially, the test that was used to determine setting time was one based on response of the newly mixed AB cements to the application of a weighted needle of known dimensions. This test was originally devised by Gillmore (1864) for his studies on the setting of hydraulic cements and mortars, and the weighted device, known as a Gillmore needle, had a mass of 454 g (1 lb) and a tip diameter of 105 mm (Crisp, Merson & Wilson, 1980). The drawback with the use of the Gillmore needle is that it is not a test of any well-defined rheological property, but of resistance to indentation. 375

Experimental techniques This property does not necessarily correlate with the changes in rheological characteristics undergone by a cement as it sets. A more satisfactory test, developed in the early 1970s, is oscillating rheometry, first described by Bovis, Harrington & Wilson (1971) and subsequently refined slightly by Wilson, Crisp & Ferner (1976). A theoretical treatment of the results of oscillating rheometry was provided by Cook & Brockhurst (1980). The apparatus used for oscillating rheometry consists of a pair of grooved metal plates clamped together, between which the sample of cement is placed. The bottom plate is connected via a spring to a motor, which causes reciprocating motion at the end of the spring furthest from the plate. Initially, motion of the motor causes corresponding motion in the bottom plate. As the cement sets, and the force required to move the plate increases, so the motion in the lower plate diminishes. A trace is produced on a chart recorder which shows the changes in oscillation with time as the cement sets, beginning from the large initial amplitude and declining to negligible, or in some cases, zero oscillation when the cement is fully set. The original definition of working time using this apparatus was the time taken from the start of mixing to reach an oscillation 95 % of the original value (Bovis, Harrington & Wilson, 1971). An alternative method of estimating working time was suggested by Wilson, Crisp & Ferner (1976), which consisted of drawing lines as extensions to the initial straight portion of the rheogram and extending the tangent at the maximum setting rate back to cross these lines. The point of intersection is then taken as the working time. In practice very little difference is observed in the working times from oscillating rheometry obtained by either of these construction methods. Since its development as a technique, oscillating rheometry has been widely applied to the study of AB cements for use in dentistry, most notably to the glass-ionomer cement. For example, it was used to study the effect of chelating comonomers on the setting of glass-ionomers (Wilson, Crisp & Ferner, 1976). This study examined a range of compounds including hydroxybenzoic acids, diketones, and most significantly, hydroxyacids. The technique was useful in identifying the particular advantages which are associated with the use of 5% ( + )-tartaric acid in glass-ionomer cements, namely delayed onset of gelation and eventual increased rate of set. These improvements in handling properties have been crucial in making glass-ionomer cements fully acceptable for use in clinical dentistry (Wilson & McLean, 1988). 376

Setting and Theological properties

In another study, oscillating rheometry was used to examine the effect of adding various simple metal salts to glass-ionomer cements (Crisp, Merson & Wilson, 1980). It was found that cement formation for certain glasses which react only slowly with poly(acrylic acid) could be accelerated significantly by certain metal salts, mainly fluorides such as stannous fluoride and zinc fluoride. Some non-reactive glasses could be induced to set by the addition of such compounds. As a further example of the use of the technique of oscillating rheometry, the work of Crisp et al. (1979) can be cited. This study was of the formation

Figure 10.1 Parallel-plate plastometer for determination of consistency.

377

Experimental techniques of AB cements from poly(acrylic acid) and basic minerals. The minerals examined were ortho- and pyro-silicates, which had been ground into fine powders of sufficiently small particle size to pass through a 38-//m test sieve. With a number of such minerals, including willemite, gehlenite and hardystonite, oscillating rheometry demonstrated that there was a reasonably rapid setting reaction at 23 °C. Infrared spectroscopy was used to confirm reaction in fully hardened cements and mechanical property measurements were carried out, though it was concluded that the cements were too weak and porous to be practically useful. Oscillating rheometry continues to be useful in the study of AB cements, and has recently been used to give further insight into the role of ( + )tartaric acid in glass-ionomer cements (Hill & Wilson, 1988). Further examples of its use are described in earlier chapters of this book. Consistency is tested on a measured volume of freshly mixed cement in the form of a cylinder. This specimen is placed between two horizontal plates using the apparatus illustrated in Figure 10.1 and subjected to a vertically applied load. The cement then flows out rapidly to form a disc. This radial flow ceases almost instantaneously because the applied stress decreases as the disc expands and rapidly reaches the yield stress, at which point outward flow ceases. This is the behaviour expected for a plastic body. The diameter of the disc is measured and this gives an indication of the shear strength of the paste. It is not a measure of viscosity becauseflowhas ceased at this point. The shear strength of the paste can be calculated from the following formula, which was derived by Wilson & Batchelor (1971). Shear strength = 4SPV/TZ2D5 where P = applied load, V = volume of the cement paste, and D = diameter of the disc. The consistency depends on the powder/liquid ratio used to mix the cement, and the parallel plate plastometer can be used to determine the optimum ratio for a particular cement system. 10.10

Erosion and leaching

10.10.1 Importance in dentistry For the various AB cements used in clinical dentistry, erosion and/or leaching of components have been considered important in assessing durability (Wilson & McLean, 1988). In fact, the two aspects are not 378

Optical properties necessarily related to durability. Loss of soluble species from the set cement affects durability only if the species concerned are matrix-formers. If not, such loss has no effect and indeed may be beneficial. Thefluoriderelease by glass-ionomer cements is regarded as clinically advantageous, and apparently takes place by ion exchange, there being no discernible loss of material as the process occurs (Wilson & McLean, 1988). 10.10.2 Studies of erosion Erosion is the result of both chemical attack and mechanical wear. In dentistry the chemical attack comes from acids either generated in the mouth by dental plaque or present in foods and beverages (Pluim & Arends, 1987; Wilson & Batchelor, 1968). To mimic this attack in laboratory testing, a static solubility test was originally carried out, which employed appropriate solutions of eroding acids (Kent, Lewis & Wilson, 1973). More recently, the mechanical aspect has been introduced by using a test in which jets of aqueous acid impinge on a sample of cement (Wilson et al.,1986); this test gives results for erosion that agree with clinical studies of durability (Setchell, Teo & Kuhn, 1985). In particular, using dilute lactic acid as the eroding agent in an impinging jet test, it has been shown (Wilson et al., 1986) that extent of erosion increased in the order glass-ionomer < silicate < zinc phosphate < zinc polycarboxylate Setchell, Teo & Kuhn (1985) observed that glass-ionomer cements prepared from poly(acrylic acid) were more resistant to erosion than such cements prepared from maleic acid copolymers. This has been confirmed by Wilson et al. (1986) and by Billington (1986), even when, as in the latter case, the same glass was used in both cements. The method has been reviewed recently by Billington, Williams & Pearson (1992). 10.11

Optical properties

10.11.1 Importance in dentistry For certain AB cements, used in dentistry, optical properties are important for their overall acceptability as materials. The two particular properties of interest have been colour and translucency, both of which need to match natural tooth material as closely as possible if good aesthetics are to be developed (Wilson & McLean, 1988). Of the AB cements currently used in dentistry, the glass-ionomer cement has the best aesthetics, since it has a 379

Experimen tal techniques degree of translucency. This translucency arises because thefilleris a glass, and unlike zinc oxide used in the zinc polycarboxylate cements, is not opaque. Evaluation of these optical properties may be done by simple observation; this approach is useful clinically (Knibbs, Plant & Pearson, 1986), since acceptability of the colour match to the surrounding tooth material can be readily seen without the need for instrumental measurement. On the other hand, for quantitative evaluation of optical properties, some kind of instrumental measurement is necessary, and the property usually evaluated is opacity. 10.11.2 Measurement of opacity The main technique that has been used for the measurement of opacity has been to prepare a standard disc of AB cement 1-0 mm thick and aged for 24 hours at 37 °C. This disc, contained in a small trough of water to prevent desiccation, is placed in a reflectometer on a black background. It is then illuminated with diffuse light and the amount of light reflected from it, Ro, is measured. The disc is then placed on a white background of 70% reflectivity, and the new amount of reflected light, R07, measured. The contrast ratio R0/R07 is defined as the C0.7 opacity (Crisp et al., 1979). Using this technique, it has been shown that the opacity of glass-ionomer cements decreases as they age; in other words, their translucency increases over this time. This change has been found to be rapid in thefirsthour after mixing, but becomes much slower after this time (Wilson & McLean, 1988). Colour and opacity have been found to be connected for glass-ionomer cements (Crisp et al., 1979; Asmussen, 1983), with darker shades giving increased opacity. However, this is merely a consequence of the underlying physical relationships, and is not thought to be a clinical problem (Wilson & McLean, 1988), mainly because the stained tooth material for which the darker shades are necessary for colour match is itself of reduced translucency. 10.12

Temperature measurement

When AB cements harden there may be a considerably exothermic reaction. For those cements used in dentistry, this may have clinical significance, since excessive temperature rise can cause damage to the 380

Other test methods dental pulp. In a survey of a wide range of AB dental cements, Crisp, Jennings & Wilson (1978) made use of a fairly simple miniature calorimeter. This consisted of a block of expanded polystyrene, into which a hole 6 mm in diameter and 6 mm deep was drilled. This hole could be covered with a lid also made of polystyrene. Cements were mixed and placed in this calorimeter and the temperature rise on setting was monitored using a sealed NiCr-NiAl thermocouple. Of the cements examined, the zinc phosphate cement showed the greatest exotherm on setting, a maximum temperature rise of 22-1 °C being observed. By contrast, glass-ionomer cement gave the smallest exotherm, only 3-9 °C. Dental silicate and silicophosphate cements also showed significant exotherms, at about 8 °C; should such an amount of heat be generated in clinical use, this would be considered potentially harmful to dental pulp (Paffenbarger et al, 1949). Of the remaining cements, the zinc oxide-eugenol and the zinc polycarboxylate gave smaller exotherms of about 5 °C. Difficulties have been found in relating laboratory measurements of exotherm behaviour in these AB cements to what happens when they are used in clinical practice. As a consequence there have been few other studies of temperature rise in the setting of such materials.

10.13

Other test methods

This chapter has covered the more important test methods used in the study of AB cements and their underlying principles, but it is not exhaustive. A number of experimental methods have been used to assess particular properties of certain cements for particular applications. For example, for AB dental cements, and especially glass-ionomer cements, adhesion to tooth material has been studied (Wilson & McLean, 1988). However, this is not a property of general interest for AB cements, nor have satisfactory standard methods been developed. Consequently, adhesion to tooth material has not been covered in the present chapter, but has been left to chapters covering cements for which it is appropriate. Other properties of glass-ionomer cements, such as fluoride-ion release and effect of relative molar mass of the poly(acrylic acid) on strength have been studied, but using techniques that are of interest only in the context of these particular cements. Once again, these tests are discussed in the chapter devoted to the AB cements in question. 381

Experimental techniques References Andrade, E. N. da C. (1947). Introduction to Rheology. London: Oil & Colour Chemists' Association. Asmussen, E. (1983). Opacity of glass-ionomer cements. Acta Odontologica Scandinavica, 41, 151-7. Barry, T. I., Clinton, D. J. & Wilson, A. D. (1979). The structure of a glass-ionomer cement and its relationship to the setting process. Journal of Dental Research, 58, 1072-9. Bellamy, L. J. (1975). The Infrared Spectra of Complex Molecules. London: Chapman and Hall. Berenbaum, R. & Brodie, I. (1959). The tensile strength of coal. Journal of the Institute of Fuel, 32, 320-7. Billington, R. W. (1986). Personal communication, cited in Wilson & McLean (1988). Billington, R. W., Williams, J. A. & Pearson, G. J. (1992). In vitro erosion of 20 commercial glass ionomer cements measured using the lactic acid jet test. Biomaterials, 13, 343-7. Bovis, S. C, Harrington, E. & Wilson, H. J. (1971). Setting characteristics of composite filling materials. British Dental Journal, 131, 352-6. Cesaro, S. N. & Torracca, E. (1988). Early applications of infrared spectroscopy to chemistry. Ambix, 35, 39—46. Cook, W. D. (1982). Dental polyelectrolyte cements. I. Chemistry of the early stages of the setting reaction. Biomaterials, 3, 232-6. Cook, W. D. (1983). Degradative analysis of glass-ionomer polyelectrolyte cements. Journal of Biomedical Materials Research, 17, 1015-27. Cook, W. D. & Brockhurst, P. (1980). The oscillating rheometer-what does it measure? Journal of Dental Research, 59, 795-9. Crisp, S., Abel, G. & Wilson, A. D. (1979). The quantitative measurement of the opacity of aesthetic dental filling materials. Journal of Dental Research, 58, 1585-96. Crisp, S., Ambersley, M. & Wilson, A. D. (1980). Zinc oxide eugenol cements. V. Instrumental studies of the catalysis and acceleration of the setting reaction. Journal of Dental Research, 59, 44-54. Crisp, S., Jennings, M. A. & Wilson, A. D. (1978). A study of temperature changes occurring in setting dental cements. Journal of Oral Rehabilitation, 5, 139^4. Crisp, S., Lewis, B. G. & Wilson, A. D. (1976). Characterisation of glass-ionomer cements. 1. Long term hardness and compressive strength. Journal of Dentistry, 4, 162-6. Crisp, S., Lewis, B. G. & Wilson, A. D. (1977). Characterisation of glass-ionomer cements. 3. Effect of polyacid concentration on the physical properties. Journal of Dentistry, 5, 51-6. Crisp, S., Merson, S. A. & Wilson, A. D. (1980). Modification of ionomer cements by the addition of simple metal salts. Industrial and Engineering Chemistry, Product Research and Development, 19, 403-8. 382

References Crisp, S., Merson, S., Wilson, A. D., Elliott, J. H. & Hornsby, P. R. (1979). The formation and properties of mineral-poly acid cements. Part 1. Ortho- and pyro-silicates. Journal of Materials Science, 14, 2941-58. Crisp, S., Prosser, H. J. & Wilson, A. D. (1976). An infrared spectroscopic study of cement formation between metal oxides and aqueous solutions of poly(acrylic acid). Journal of Materials Science, 11, 36-48. Crisp, S. & Wilson, A. D. (1974). Reactions in glass-ionomer cements: I. Decomposition of the powder. Journal of Dental Research, 53, 1408-13. Darvell, B. W. (1990). Uniaxial compression tests and the validity of indirect tensile strength. Journal of Materials Science, 25, 757-80. Deacon, G. B. & Phillips, R. (1982). Relationship between the carbon-oxygen stretching frequency of carboxylate complexes and the type of carboxylate coordination. Coordination Chemistry Reviews, 33, 227-50. Gillam, E. (1969). Materials under Stress. London: Newnes-Butterworth. Gillmore, Q. A. (1864). Practical Treatise on Limes, Hydraulic Cements and Mortars. New York. Hill, R. G. & Wilson, A. D. (1988). Some structural aspects of glasses used in ionomer cements. Glass Technology, 29, 150-8. Hill, R. G., Wilson, A. D. & Warrens, C. P. (1989). The influence of poly(acrylic acid) molecular weight on the fracture toughness of glass-ionomer cements. Journal of Materials Science, 24, 363-71. Hodd, K. A. & Reader, A. L. (1976). The formation and hydrolytic stability of metal ion-polyacid gels. British Polymer Journal, 8, 131-9. Hondras, G. (1959). The evaluation of Poisson's ratio and the modulus of materials of a low tensile resistance by the Brazilian (indirect tensile) test with particular reference to concrete. Australian Journal of Applied Science, 10, 245-68. Kemp, D. S. & Vellaccio, F. (1980). Organic Chemistry. New York: Worth. Kendall, K. (1978). Complexities of compression failure. Proceedings of the Royal Society of London, A 361, 245-63. Kent, B. E., Fletcher, K. E. & Wilson, A. D. (1970). Dental silicate cements. XL Electron probe studies. Journal of Dental Research, 49, 86-92. Kent, B. E., Lewis, B. G. & Wilson, A. D. (1973). The properties of a glass-ionomer cement. British Dental Journal, 135, 322-6. Knibbs, P. J., Plant, C. G. & Pearson, G. J. (1986). A clinical assessment of an anhydrous glass-ionomer cement. British Dental Journal, 161, 99-103. Lion Corporation. (1980). Dental cements. Nihon Kokai Tokkyo Koho 80,139,311. Chemical Abstracts, 94: 903488, 1981. Mackay, K. M. & Mackay, R. A. (1972). Introduction to Modern Inorganic Chemistry. London: Intertext Books. Mehrotra, R. C. & Bohra, R. (1983). Metal Carboxylates. London and New York: Academic Press. Moore, W. J. (1972). Physical Chemistry, 5th edn. London: Longman Group Ltd. Nicholson, J. W., Brookman, P. J., Lacy, O. M., Sayers, G. S. & Wilson, A. D. (1988a). A study of the nature and formation of zinc polycarboxylate cement 383

Experimental techniques using Fourier transform infrared spectroscopy. Journal of Biomedical Materials Research, 22, 623-31. Nicholson, J. W., Brookman, P. J., Lacy, O. M. & Wilson, A. D. (1988b). Fourier transform infrared spectroscopic study of the role of tartaric acid in glass-ionomer dental cements. Journal of Dental Research, 67, 145-4. Nicholson, J. W., Wasson, E. A. & Wilson, A. D. (1988). Thermal behaviour of films of partially neutralised poly(acrylic acid). 3. Effect of calcium and magnesium ions. British Polymer Journal, 20, 97-101. Nicholson, J. W. & Wilson, A. D. (1987). Thermal behaviour of films of partially neutralised poly (aery lie acid). 1. Effect of different neutralising ions. British Polymer Journal, 19, 67-72. O'Neill, I. K., Prosser, H. J., Richards, C. P. & Wilson, A. D. (1982). Nuclear magnetic resonance spectroscopy of dental materials. 1.31P studies on phosphate-bonded cement liquids. Journal of Biomedical Materials Research, 16, 39-49. Outwater, J. O., Murphy, M. C, Kumble, R. G. & Berry, J. T. (1974). Double torsion techniques as a universal fracture toughness test method. Fracture Toughness and Slow-Stable Cracking, ASTM Special Technical Publication 559, pp. 127-37, American Society for Testing and Materials. Paffenberger, G. C, Swaney, A. C, Schoonover, I. C, Dickson, G. & Glasson, G. F. (1949). An investigation of Diafil, a dental silicate cement. Journal of the American Dental Association, 39, 283. Pluim, L. J. & Arends, J. (1987). The relationship between salivary properties and in vivo solubility of dental cements. Dental Materials, 3, 13-18. Polakowski, N. H. & Ripling, E. J. (1966). The Strength and Structure of Engineering Materials, Chapter 10. Englewood Cliffs, New Jersey: PrenticeHall Inc. Prosser, H. J., Richards, C. P. & Wilson, A. D. (1982). NMR spectroscopy of dental materials. II. The role of tartaric acid in glass-ionomer cements. Journal of Biomedical Materials Research, 16, 431-45. Prosser, H. J., Brant, P. J., Scott, R. P. & Wilson, A. D. (1983). The cementforming properties of phytic acid. Journal of Dental Research, 62, 598-600. Prosser, H. J., Powis, D. R., Brant, P. & Wilson, A. D. (1984). Characterization of glass-ionomer cements. 7. The physical properties of current materials. Journal of Dentistry, 12, 231-40. Prosser, H. J., Powis, D. R. & Wilson, A. D. (1986). Glass-ionomer cements of improved flexural strength. Journal of Dental Research, 65, 146-8. Reed, S. J. B. (1973). In Anderson, C. A. (ed.) Microprobe Analysis. New York: Wiley-Inter science. Selenrath, Th. R. & Gramberg, J. (1958). Stress-strain relations and breakages of rocks. In Walton, W. H. (ed.) Mechanical Properties of Non-metallic Brittle Materials, Chapter 6, pp. 79-105. London: Butterworths. Setchell, D. J., Teo, C. K. & Kuhn, A. T. (1985). The relative solubilities of four modern glass-ionomer cements. British Dental Journal, 158, 220-2. Skoog, D. A. & West, D. M. (1980). Principles of Instrumental Analysis, 2nd edn, Chapter 8. Tokyo: Holt-Saunders Japan Ltd. 384

References Smith, D. C. (1968). A new dental cement. British Dental Journal, 125, 3 8 1 ^ . Sorrell, C. A. (1977). Suggested chemistry of zinc oxychloride cements. Journal of the American Ceramic Society, 60, 217-20. Sorrell, C. A. & Armstrong, C. R. (1976). Reactions and equilibria in magnesium oxychloride cements. Journal of the American Ceramic Society, 59, 51-4. Tay, W. M. & Braden, M. (1981). Dielectric properties of glass-ionomer cements. Journal of Dental Research, 60, 1311-14. Tay, W. M. & Braden M. (1984). Dielectric properties of glass-ionomer cements-further studies. Journal of Dental Research, 63, 74-5. Tyndall, J. (1866). On calorescence. Philosophical Magazine, Series 4, 31, 386-96; 435-50. Watts, D. C. (1979). C-13 NMR spectroscopic analysis of polyelectrolyte cement liquids. Journal of Biomedical Materials Research, 13, 423-35. Williams, D. H. & Fleming, I. (1973). Spectroscopic Methods in Organic Chemistry, 2nd edn. London: McGraw-Hill. Wilson, A. D. (1978). The chemistry of dental cements. Chemical Society Reviews, 1, 265-96. Wilson, A. D. (1982). The nature of the zinc polycarboxylate cement matrix. Journal of Biomedical Materials Research, 16, 549—57. Wilson, A. D. & Batchelor, R. F. (1968). Dental silicate cements: III. Environment and durability. Journal of Dental Research, 41, 115-20. Wilson, A. D. & Batchelor, R. F. (1971). The consistency of dental cements. The specification test for filling materials. British Dental Journal, 130, 437-41. Wilson, A. D., Crisp, S. & Ferner, A. J. (1976). Reactions in glass-ionomer cements: IV. Effect of chelating comonomers on setting behavior. Journal of Dental Research, 55, 489-95. Wilson, A. D., Groffman, D. M., Powis, D. R. & Scott, R. P. (1986). A study of variables affecting the impinging jet method for measuring the erosion of dental cements. Biomaterials, 7, 217-20. Wilson, A. D., & Kent, B. E. (1968). Dental silicate cements. V. Electrical conductivity. Journal of Dental Research, 47, 463-70. Wilson, A. D. & Kent, B. E. (1970). Dental silicate cements. IX. Decomposition of the powder. Journal of Dental Research, 49, 7-13. Wilson, A. D. & McLean, J. W. (1988). Glass-ionomer Cement. Chicago, London, etc.: Quintessence Publishers.

385

Index

abietic acid (rosin) 322, 334, 338-9 acid-base balance in glasses 123-5 in silicate rocks 17 acid-base (AB) cements crystallinity in 8-10, 205-13 formation 7-11, 307-8 theory 1-5, 5-26, 307-8 acid-base concepts 12-26 aprotic acids 6, 17-20 Arrhenius theory 14-5, 19 Bronsted-Lowry theory 15-6, 19-20, 48, 284 Cartledge theory 20-1 classification 22-3 hard acids and bases see HSAB theory history 1 2 ^ HSAB theory 24-6 ionic potential 20-1 ionization potential 21-2 Lewis theory 17-20 Lux-Flood theory 17, 19-20 relevance 19-20 soft acids and bases see HSAB theory solvent system theory 16-7, 19 strength 20-2 Usanovich theory 18-20 acid-decomposable glasses 6, see also aluminosilicate glasses, aluminoborate glasses acid-etching of enamel 93 acids for cement-formation see cementforming acids acrylic acid copolymers see poly(alkenoic acid)s homopolymer see poly(acrylic acid), poly(alkenoic acid)s adhesion 1, 4, 56, 92-7, 107, 152-4, 381 to bone 94-6, 111 to dentine and enamel (tooth) 92-6 of glass polyalkenoate cements 152-4 to hydroxyapatite 95-6

386

obstacles to 93-4 of polyalkenoates 94-6 of zinc polycarboxylates 107 adsorption of carboxylates (alkenoates) 96-7 agriculture 4 A12O3 cements 102 A1 2 O 3 -P 2 O 5 -H 2 O 199-201 aldehydic aromatic acid cements 318, 321 alkanoate adsorption on hydroxyapatite 96-7 alkanoate bonding modes 363 alkanoic acids 5-6, 308, 315, 318, 320-1, 337, 348, 351 alkoxy aromatic acids 318, 320, 337 citric acid 308 dimer (dimerized fatty) acids 351 2-ethoxybenzoic acid 6, 318, 320-1, 337, see also EBA cement malic acid 6, 308, 315 mellitic acid (benzenehexacarboxylic acid) 6, 315 1,2,3-propanetricarboxylic acid 6, 315 pyruvic acid 6, 308, 315 salicylates 318, 348 tartaricacid 6, 308, 315 tricarballylic acid 6 alkenoic acid polymers see poly(akenoic acid)s alkoxy aromatic acid cements 318, 320, 337 allyl-2-methoxyphenol cements 318, 321 aluminium coordination 101-2, 120-1, 123, 125, 129, 131,137-8 alumino complexes 244 fluoride 135-8,244 phosphate 57, 85, 200-1, 210, 244-5 aluminoborate glasses 165-6 aluminophosphoric acids 57, 200-1 aluminosilicate gels 91

Index aluminosilicate glass cements 307-9, see also glass polyalkenoate cement, dental silicate cements aluminosilicate glasses 2, 6, 9, 90,

117-32,236-40,310,314-5

acid-base balance in 123 acid-decomposition of 119-23, 127-8 acid-washing of 163 Al 2 O 3 :SiO 2 123-7 aluminium coordination 120-1, 123-5, 128-9, 130, 137 anorthite in 122, 130 apatite in 125 beryllium-containing 236 Ca:Al 123-5 calcium-containing 6, 118-20, 123-33,236-7,310,314-5 composition and properties 118-9, 122-9, 238^1 coordination polyhedra 119-21 corundum in 125-6 electron micrographs 128, 238-9 fluoride role 118-9, 129^30, 236 fluoride type 117-9, 125-132, 135, 140, 236-40 fluorite in 125-6, 129-30 indium-containing 237 lanthanum-containing 117-9 NMR studies 121, 125, 128, 131 oxide type 117, 122-6, 135, 236-40 phase-separation 126-8, 130-1, 238-9 SiO 2 -Al 2 O 3 -CaF 2 119, 126-9, 238 SiO 2 -Al 2 O 3 -CaF 2 -AlPO 4 119,131 SiO 2 -Al 2 O 3 -CaF 2 -AlPO 4 -Na 3 AlF 6 A1F3 119,131-2 SiO 2 -Al 2 O 3 -CaO 118, 123-6, 237^0 SiO 2 -Al 2 O 3 -CaO-CaF 2 119, 130 SiO 2 -Al 2 O 3 ^CaO-Na 2 O 118 SiO a -Al a O 8 -CaO-P a O 6 118,239-40 spinodal decomposition 130 strontium-containing 117-9 structure 118-131,238-9 transition temperatures 130 types 118-9,235-7 animal husbandry 4 applications agriculture 4 animal husbandry 4 architecture 283 battlefield dental material 333 bone cements 2, 90-1, 117, 147, 161-2, 168 bone substitute 4, 161-2, 169 controlled release devices see sustained release devices dental cements 2, 90-1, 103, 116-7,

147, 166-8, 204, 214, 235-6, 320-1, 333 fire resistant materials 283 floor fabrication 2, 290 floor repair 222 foundry sands 2 handyman materials 3 horticulture 4 human health care 4 impression material 335 insulating materials 283 investment materials 222 nuclear 283 plasters 290 pulp capping 347 road repairs 222 runway repairs 222 slip casting 2-3,91, 169 soil consolidation 90 splint bandage 2, 91, 117, 168 surfacing 4 sustained release devices 3, 4, 157-8, 222, 304 underwater cements 2, 91 architectural applications 283 aromatic carboxylic acid 96-7, 347 attenuated total reflectance (infrared) spectroscopy (ATR) 359; see also infrared spectroscopy B 2 O 3 cements 102, 312 BaO cements 204, 318, 338 bactericides 335 bases for cement formation see cementforming liquids battlefield dental material 333 BeO and Be(OH)2 cements 201-2 Bi2O3 cements 102, 201-2, 312-3, 318 bioactive materials 3 bioadhesion see adhesion bis-GMA 170-1 Bjerrum ion-pairs 67 Boltzmann distribution 61 bone 2 adhesion to 94-6, 111 bone cements 2, 90-1, 117, 147, 161-2, 168 bone substitute 4, 161-2, 169 Born-Oppenheimer approximation 32 Bragg equation 367 Bronsted-Lowry theory 15-6, 19-20, 48, 284 3-butene 1,2,3-tricarboxylic acid/acrylic acid copolymer 91, 1 0 3 ^ , 131-2, see also poly(alkenoic acid)s

387

Index CaO cements 204, 318, 321, 338 Ca(OH) 2 cements see calcium hydroxide chelate cements, calcium hydroxide dimer (dimerized) acid cements calcium aluminosilicate glasses see aluminosilicate glasses calcium hydroxide chelate cements 318, 347-50 applications 347 composition 348 properties 350-1 setting 348-9 calcium hydroxide dimer (dimerized) acid cements 351 carboxylate adsorption on hydroxyapatite 95-7 carboxylate bonding modes 363 carboxylic acids see alkanoic acids, poly(alkenoic acid)s caries, effect of fluoride on 258 CdO cements 201-2, 204, 312, 318, 321, 338 cement classification 7 cement-forming acids 3, 5-6, 308 aldehydic aromatic acids 318, 321 alkoxy aromatic acids 318, 321, 337 allyl-2-methoxyphenol 318-9, 321 aromatic carboxylic acids 347 citric acid 308 cobalt chloride, selenate, sulphate 6 copper chloride, selenate, sulphate 6 creosote 321 yS-diketones 318-9,321 dimer (dimerized fatty) acids 351-2 2,5-dimethoxyphenol 318 2-ethoxybenzoic acid 318, 320-1, 337, see also EBA cements eugenol 318, 320, see also zinc oxide eugenol cements fluoboric acid 308 gallic acid 6, 315 glycerol phosphoric acid 308 guaiacol (2-methoxyphenol) 318-9, 321 ketoacids 318, 321 ketoesters 318, 321 magnesium chloride 6, 2 8 3 ^ , 290, 292-6 magnesium selenate 6 magnesium sulphate 6, 283-4, 299-302 malic acid 6 mellitic acid (benzene hexacarboxylic acid) 6, 315 methoxyhydroxybenzoic acids 342-6 2-methoxyphenols 318-9, 321

388

oil of cloves 320-1 orthophosphoric acid see phosphoric acid phosphoric acid 6, 22, 56, 85, 197-201 phytic acid 3, 5, 309-10 poly(acrylic acid) 6, 22, 56-8, 69, 70-1, 74^5, 78-9, 90-4, 97-8, 1 0 3 ^ poly(alkenoic acid)s 56-8, 69-71, 74-5, 78-9, 90-1, 97-8, 103-5, 360 poly(phosphonic acid)s 310-1 poly(vinyl phosphonic acid) see poly(phosphonic acid)s propylene-2-methoxyphenol 321 pyruvic acid 6 salicylates 348 syringic acid see methoxyhydroxybenzoic acids tannic acid 6,308,315 tartaric acid 6, 308, 315 tricarballylic acid 6 vanillic acid see methoxyhydroxybenzoic acids zinc chloride 6, 283-9 zinc selenate 6 zinc sulphate 6 cement-forming cations 9, 19-22, 198-9, 201-4, 244 cement-forming liquids see cementforming acids cement-forming metal oxides 5-6, 102, 201-2, 312-6, 318, 321 A12O3 102 B 2 O 3 102, 312 BaO 204,318,338 BeO and Be(OH)2 201-2 Bi2O3 102, 201-2, 312-3, 318 CaO 312,318 Ca(OH) 2 204, 311-2, 318, 321, 338 CdO 201-2, 312, 318, 321, 338 CoO 312 Co(OH) 2 6, 202, 222, 312, 315-6 Cr 2 O 3 , CrO 3 312 CuO 6, 102, 201-2, 204, 311-2, 315-6, 318, 321, see also CuO cements Cu 2 O 201-2, 311-2, see also Cu 2 O cements Fe 2 O 3 312 HgO 102,312,318,321,338 In 2 O 3 312 La 2 O 3 102, 312 MgO 6, 102, 201-2, 204, 311-3, 321, 338, see also magnesium phosphate cements MoOo 312

Index MnO 2 312 PbO 102, 312, 318, 321, 338 Pb 3 O 4 201-2, 312 SnO 201-2,312 WO 3 312 Y 2 O 3 102, 312 ZnO 6, 102, 201-2, 204, 311-3, 318, see also zinc polyalkenoate cement, zinc phosphate cement, zinc oxide eugenol cements, EBA cements cement gels 8-10 cementitious bonding 7-11, 307-8 cementitious substances mortars 1 plaster of Paris 1, 7 Portland cement 1, 2, 5, 7 refractory cements 197 silicate/silica gel cements 140 chelate agents 6 chelate cements 318-52 chelate formation 71 citric acid cement 308 cloves, oil of 321 CoO cements 312 Co(OH) 2 cements 202, 222, 312, 315-6 composite resin 154—6, 235 compressive strength 359, 370-2, see also experimental techniques concrete 1 condensation see counterion condensation condensation cements 7 configuration see polymer conformation conformation see polymer conformation consistency 375, 378 controlled release devices see sustained release devices copper phosphate cements 201-2, 221-2 coulombic forces 80-2 counterion binding 7-8, 59-83, 106, see also ion binding distribution 59-63, 82 condensation 63-7, 78 creosote cement 321 Cr 2 O 3 , CrO 3 cements 312 crystallinity in cements 8-10, 205-13 CuO cements 102, 201-2, 221-2, 231, 311-2, 315-6, 318,321 chelate 231 miscellaneous 315-6 phosphate 201-2, 221-2 polyalkenoate 101-2 polyphosphonate 311-2 Cu 2 O cements 6, 201-2, 220-1,311-2 phosphate 201-2, 220-1

polyalkenoate 101-2 polyphosphonate 311—2 Debye-Hiickel theory 43-5, 67 degradative studies (chemical) 105, 136-9, 244-7, 339, 360-1 dehydration and gelation 72, 84 and precipitation 77-9 dental cements 90-1, 103, 106-13, 116-7, 146-7, 166-8, 204, 214, 235-6, 320-1, 333 dental impression material 335 dental materials 2 dental plaque 379 dental silicate cement 235-63, 366, 369, 381 applications 236-7, 249 composition: glasses 238-9, see also aluminosilicate glasses; liquids 218, 241-3, see also phosphoric acid history 235-7 modified materials 237-8: acidresistant 237; indium-containing 237 properties 253-65: acid erosion 259-60; bacterial contamination 261; biological 260-1; erosion 255-8; fluoride release 255-6, 257-8; physical 253-7; powder: liquid effect 256; translucency 255; water absorption 256-7 setting 243-9: electrical conductance 247, 366-7; exotherm 381; hydration 247, 249; ion release 25-7; infrared spectra 243; NMR spectra 245,252,365-6; permittivity 367; pH changes 249; precipitation of ions 243-8; salt formation 244-8; silica formation 247, 250-1; water deficiency effect 249 structure 249-53: electron microscopy 250-1; electron probe analysis 250-2, 369; element distribution 250-3; optical microscopy 250 dentine 91 bonding to 92-5, 111, 152-4 composition 94 treatment 152-^ desolvation and gelation 72, 84 and precipitation 77-9 diffusion in water 37

389

Index /?-diketone cements 318-9, 321 dilatometry 59 dimer acid (dimerized fatty acid) cements 351-2 2,5-dimethoxyphenol cements 318 dipole interactions 82 dissolution of polymers in water 45-7 of salts in water 41 distribution functions for ion-pairs 67-8, 72-3 double-torsion test of fracture toughness 374 dual-cure resin cement see resin glass polyalkenoate cement EBA 6,318,320,337 EBA cements 320, 337-47 EBA divanillate cements 344-5 EBA eugenol cement 337-42 composition 338-9 physical properties 340-2 setting 339-40 structure 339-40 EBA-methoxyhydroxybenzoate cements 342-4 composition 343-3 properties 3 4 2 ^ setting 343-4 EBA polymer cement 344-6 EBA syringate cement see EBAmethoxyhydroxybenzoate cements EBA vanillate cement see EBAmethoxyhydroxybenzoate cements 'egg-box' model for gelation 85 electrical conductance 247, 325-6, 359, 366-7 electron diffraction 33, 35 electron probe microanalysis 105, 144-5, 233, 247, 250, 369-70 electrons, hydrated 44 enamel (tooth) acid-etching 93, 153—4 aluminium uptake 258 bonding to 92-6, 111-2, 1 5 2 ^ composition 94 conditioning 15 3—4 fluoride uptake 158, 258 opacity 152 translucency 152 erosion measurement 378-9 2-ethoxybenzoic acid see EBA ethylene glycol dimethacrylate 170 eugenol 2, 6, 318, 321, see also zinc oxide eugenol cements exotherm measurements 147, 308-1

390

experimental techniques 359-381 adsorption 95-7 compressive strength 359, 370-2 degradative studies (chemical) 105, 136-9, 244-7, 339, 360-1 diametral compressive strength 372 dilatometry 59 dipole interactions 82 double torsion test of fracture toughness 374 electrical conductance 247, 325-6, 359, 366-7 electron diffraction 33, 35 electron probe analysis 105, 144—5, 233, 247, 250 erosion 378-9 exotherm measurements 147, 380-1 flexural strength 359, 370, 372-3 four-point bend test 374 Fourier transform infrared spectroscopy (FTIR) 105, 359, 364, see also infrared spectroscopy fracture toughness 373-4 impinging jet 158-9, 216-8, 341, 379 infrared spectroscopy 99-101, 105-6, 137, 142, 198, 210, 243-4, 247, 250-2, 311, 323-5, 337, 339-40, 343, 348-9, 351, 359, 361-4 leaching studies 106, 378-80 mass spectrometry 42 mechanical properties 370-4 neutron diffraction 33, 35-6, 42 NMR spectroscopy 59, 141, 198, 200-1, 245, 359, 364-66 NMR spectroscopy solid state 121, 125, 131, 145-6, 252 opacity 127, 148, 151-2, 379-80, see also translucency optical microscopy 143, 249 optical properties 127, 146-8, 151-2, 166, 379-80 parallel plate plastometer 377 permittivity 325-6, 359, 367 Raman spectroscopy 198 rheometry 141,374-8 scanning electron microscopy 106, 128, 226-30, 232-3, 329, 331 setting measurements 374-8 tensile strength 370 titrimetric methods 311 transition temperatures 130 translucency 147, 151-2, 166, 359, 379, see also opacity transmission electron microscopy 145 viscosity measurements 141 water analysis 105-6

Index X-ray diffraction (XRD) spectroscopy 9-10, 33, 35, 47, 51, 105, 125-6, 130, 198, 202-3, 208-9, 224^31, 250, 283-6, 293, 323, 359, 367-8 Fe 2 O 3 cement 312 fire-resistant materials 283 flexural strength 359, 370, 372-3 floor fabrication 290 floor repair 222 fluoboric acid cement 308 fluoride additive to glass polyalkenoate cement 133 alumino complexes 135-8, 244 bone remineralization 161-2 and caries 258 cement reaction 106-8 glass polyalkenoate cement reactions 133-41 release from cements 117, 147, 157-8, 255-8, 379 fluoride glasses 136-46 foundry sand 2 four-point bend test 374 Fourier transform infrared spectroscopy (FTIR) 105, 359, 364, see also infrared spectroscopy fracture toughness 373-4 freezing point of water, D-structure 38 Fuoss distribution 68-9 gallic acid cement 6 gelatinising minerals 6, 114-6 gels and gelation 8-11, 49, 56, 64, 83-5, 138 cations, gel-forming 9 * egg-box' model 8 5 hydration 72, 83-4 ion binding 84-5 models 10-1,85 neutralization 84 polyion interaction 84 polymer conformation 77, 84 structures 10-1,85 Gibbs equation 40 Gillmore needle 375 glass-ionomer cement see glass polyalkenoate cement, glass polyphosphonate cement glass polyalkenoate cement 2-4, 56, 90-1, 116-175, 235 applications 117, 147, 160-2, 166-9 composition 123-46, 162-3: additives 133-4, 376; fluoride glasses 136-46; glass effect on properties

123-31; glasses see aluminosilicate glasses; liquids see poly(alkenoic acid)s; metal fluoride additives 134, 163; oxide glasses 135; poly(alkenoic acid), molecular mass effect 163; poly(alkenoic acid) type, effect on properties 132; tartaric acid effect 133-4, 376 history 116—7 light-cured see resin glass polyalkenoate cement properties 94-7, 117, 146-165, 174: acid erosion 158-9; acid-etching of 155-6; adhesion 94-7, 117, 147, 152-6, 164, 174-5; biological 159-61; bonding to composite resins 154-6; bonding to tooth material 152-4; bone remineralization 161-2; consistency of pastes 148; creep 148; erosion 148, 156-9, 165;exotherm 147; fluoride release 157-8, 379; glass composition, effect 123-31; modulus 149, 164; opacity, see translucency; plasticity 147-8; poly(alkenoic acid), molecular mass effect 163; poly(alkenoic acid) type, effect 132; setting behaviour 122-8, 132-3, 165; strength 122, 125, 127, 132-4, 138-9, 147-50, 163-6, 372-3; stress relaxation 148-9; tartaric acid, effect 133-4; translucency 127, 147-8, 151-2, 166, 380; viscoelasticity 148-9; water absorption 156-7; wear 159 resin hybrids see resin glass polyalkenoate cement setting reaction 98-9, 134-43: aluminofluoride complexes in 137-8; coordination changes 145-6; desolvation 135; exotherm 147; fluoride, effect of 13^41, 134, 163; Fourier transform infrared spectra 364; gelation 134-5,137-8; glass ions release 361; hydration 139; infrared spectra 137, 362, 364; ion binding during setting 137-9; NMR spectra 145-6, 366; pH changes 134, 136, 138; polymer configuration 135; precipitation of ions 137-8; release of ions from 134, 137-8; rheometry 376; salt formation 135; silicic acid and silica gel formation 134, 139-40, 145-6; solvation 139; strength increases 139, 148-9; tartaric acid,

391

Index setting reaction (cont.) effect 131-5, 141-3, 162-3; viscosity increases 135, 141 structure 138, 142-6: electron microscopy 143-145; electron probe analysis 145-6, 369-70; element distribution 144-5; molecular structure 99-101; optical microscopy 142-3; reptation model 139; water states 31,49, 146 glass polyphosphonate cement 117, 314-5 glass transition temperatures 52 glasses see aluminoborate glasses, aluminosilicate glasses glycerol phosphoric acid cement 308 guaiacol cement 318, 321 Gurney potential 45 handyman materials 3 hard acids see HSAB theory hard and soft acids and bases see HSAB theory hard bases see HSAB theory HEMA see hydroxyethyl methacrylate hexaquo cations 16, 47, 284 HgO cements 102, 312, 318, 321 horticulture 4 HSAB theory 2 ^ 6 , 4 7 - 8 human health care 4 hybrid light-cured cements see resin glass polyalkenoate cements hydrated electrons 44 hydration 74-9, 139, 247, 249, 307 and gelation 49-50, 72, 77-9, 84 and ion binding 76-7 and ionization 74—7 of ions 31,41-4,47-8 of polyions 31, 73-5 and precipitation 77-9 in solid state 47 hydration number 42 hydration regions see hydration shells hydration shells 42-3, 49-50, 72-7 hydration states 31, 49-50, 59 hydration zones see hydration shells hydraulic cements 7 hydrides 33 hydrogel 1 hydrogen bonding in cements 9, 203 in 2-ethoxybenzoic acid 338 in phenols 321-2 in phosphoric acid 198 of water 38

392

hydrologic cycle 32 hydrophobic interactions 40-1 hydrosphere 32 hydroxyapatite 95-7 adsorption on 95-7 aluminium uptake 258 carboxylate uptake 95-7 fluoride uptake 258 hydroxydimethyl acrylates 170 hydroxyethyl methacrylate (HEMA) 169-173

3,

ice structures 35-6 impinging jet test 158-9, 216-8, 341, 379 In 2 O 3 cement 312 infrared spectroscopy 359, 361-4 attenuated total reflectance (ATR) 359 calcium hydroxide chelate cements 348-9 calcium hydroxide dimer cements 351 dental silicate cement 243, 247, 250-1 EBA (2-ethoxybenzoic acid) cements 339^0 EBA-methoxyhydroxybenzoate cements 343 2-ethoxybenzoic acid (EBA) 337 eugenol 323-^ Fourier transform (FTIR) 105, 359, 364 glass polyalkenoate cement 136-7, 142 metal oxide polyphosphonate cements 311 molecular structure of polyalkenoate (polyelectrolyte) cements 99-101 phosphate bonded cements 198, 210, 244, 247, 250-2 polyalkenoate cements 104-5, 136-7, 142 zinc phosphate cement 210 zinc polycarboxylate cement 104-5 ZOE cements 323-6 insulating materials 283 investment materials 222 ion binding 7-8, 59-83, 106 cation effect 65-67 cements 106 complex formation effect 69-70 density changes 73-4 dipole changes 7 electric conductance changes 59 hydration effects 72-9 molar volume changes 74 polymer type effect 70-2

Index refractive index changes 63, 73-5 turbidity 79 ultrasonic changes 74 viscosity changes 78 ion coordination 47-8, 69, 99-101, 117 ion-ion interactions 44-5, see also ion pairs ion-pairs 49, 72-3, 79 contact 72-3, 79 distribution functions 67-8, 72-3 hydration (solvation) of 72-3 solvent separated 72-3, 79 types 72-3 Irving-Williams series 69-70 itaconic acid/acrylic acid copolymer 56, 91, 97-8, 103-4, 132-3, see also poly(alkenoic acid)s jet test see impinging jet test ketoacid cements 318, 321 ketoester cements 318, 321 La 2 O 3 cements 102, 312 leaching studies 106 Lewis acids 6, 18, 22-4, 47, 284 Lewis theory 17-20 light-cured cements 3-4, see also resin glass polyalkenoate cement liquids for cement formation see cementforming acids Lux-Flood theory 17-20 magnesia cements 283, see also magnesium oxychloride cement, magnesium oxysulphate cement, magnesium phosphate cements magnesia phosphate cements see magnesium phosphate cements magnesium chloride solution 6, 283-4, 290, 292-6 magnesium oxide 103-4, 290-1 cements 283, see also magnesium oxychloride cement, magnesium oxysulphate cement, magnesium phosphate cements deactivation 290-1 magnesium oxychloride cement 2,31, 51, 283, 290-9 applications 290 components 290 phases 294: MgO-MgCl 2 -H 2 O 294-5 setting chemistry 291-3 setting kinetics 293—4

magnesium oxysulphate cement 299-304 phases 300-2: MgO-H 2 SO 4 -H 2 O 301; MgO-MgSO 4 -H 2 O 300-2 porosity 303 properties 302-304 setting chemistry 299-300 magnesium phosphate cements 2, 102, 204, 222-35 aluminum acid phosphate type 233-5 ammonium dihydrogen phosphate type 224-31 ammonium polyphosphate type 232 composition 222-3 diammonium phosphate type 231-2 phosphoric acid type 224 types 223-4 magnesium polyalkenoate cement 235 magnesium polyphosphonate cement 312-3 magnesium titanate phosphate cement 235 maleic acid/acrylic acid copolymer 56, 91, 97-8, 103-4, 132-3, see also poly(alkenoic acid)s malic acid cements 6 mass spectrometry 42 mellitic acid cement 6, 315 metal oxide cements eugenol cements 321; see also zinc oxide eugenol cements oxysalt bonded cements 2-3, 5-6, 304-5, see also magnesium oxychloride cement, magnesium oxysulphate cement, zinc oxychloride cements phosphate cements 201-2, 221-22, see also CuO cements, Cu 2 O cements, dental silicate cement, magnesium phosphate cements, zinc phosphate cement polyalkenoate cements 90, 102-3, see also glass polyalkenoate cement, zinc polycarboxylate cement polyelectrolyte cements see polyalkenoate cements, polyphosphonate cements polyphosphonate cements 311-3 metal oxides for cement formation see cement-forming metal oxides metal poly(acrylic acid) complexes 69-70 methods see experimental techniques methoxyhydroxybenzoate cements see EBA-methoxyhydroxybenzoate cements

393

Index methoxyhydroxybenzoic acids 342-6 2-methoxyphenol cements 318-9, 321, 342 MgO cements 6, 102, 201-2, 204, 311-3, 318, 321, see also magnesium phosphate cements micromechanical attachment 93, 154-5 mineral cements ionomer 90, 114-6 phosphate 265 polyalkenoate 90, 114-6 MnO 2 polyphosphonate cement 312 MoO 3 polyphosphonate cement 312 model materials 91 molecular dynamics 42 mortars 1-2 mouth fluids 216 neutron diffraction 34-6, 42 NMR spectroscopy 59, 141, 198, 200-1, 245, 359, 364-66 NMR spectroscopy solid state 121, 125, 131, 145-6,252 non-aqueous cements 318-52 nuclear applications 283 oil of cloves 320-1 opacity see optical properties optical microscopy 143, 249 optical properties 127, 146-8, 150-2, 165, 379-80 opacity 127, 148, 151-2, 379-80, see also translucency translucency 1, 3, 147, 151-2, 166, see also opacity organic polymers 1 orthophosphoric acid see phosphoric acid orthosilicic acid 7, 121, 134, 139-40, 243-4, 247 osmotic forces 80-1 osteogenesis 162 oxychloride cements 3, 5-6 calcium 304 cobalt 305 magnesium 2, 31, 51, 283, 290-9 zinc 2, 31, 51, 285-90 oxygen polyhedra 9, 120, 123, 125, 128-9 oxysalt bonded cements 2-3, 5-6, 31, 51, 283-305 components 6, 284 setting 284 oxyselenate cements 3, 6 oxysulphate cements 3, 5-6 magnesium 283-4

394

parallel plate plastometer 377 PbO cements 102, 312, 318, 321, 338 Pb 3 O 4 cements 201-2,312 permittivity 325-6, 359, 367 phenolic cement-formers 5-6, 308, 318-21 allyl 2-methoxyphenol see eugenol 2,5-dimethoxyphenol 318 eugenol 318, 321, see also zinc oxide eugenol cements gallic acid 6, 315 guaiacol (2-methoxyphenol) 318, 321 2-methoxyphenols 318-9, 321 propylene-2-methoxyphenol 321 tannicacid 6, 308, 315 phlogiston theory 31 phosphate-bonded cements 3, 7, 197-265 liquids for 218, 241-3 see also copper phosphate cements, dental silicate cement, magnesium phosphate cements, metal oxide phosphate cements, zinc phosphate cement phosphoric acid 2, 5-6, 22, 56, 85, 197-204, 241-3 aluminium complexes 198-200 cations in 198-201, 203-4 cement forming liquids 218, 241-3 concentration effect on cement properties 218, 241-3 dimers H 6 P 2 O 8 198 hydrogen bonding 198 infrared spectra 198 NMR spectra 198, 200 phase diagrams 199-200: A1 2 O 3 -H 2 O-P 2 O 5 199-200; ZnO-H pK 197 properties 197-8 structure 198 triple ion H5P2Og 198 see also dental silicate cement, magnesium phosphate cements, mineral phosphate cements, zinc phosphate cement phytic acid [myo-inositol hexakis(dihydrogen phosphate)] cement 3, 5, 309-10 cements 309-10, 360 pipelines 2, 91 plaster of Paris 1,91 plasters 290 Poisson distribution 61 poly(acrylic acid) 6, 22, 31, 46, 49, 56-9, 69-71, 90-1, 97-8, 103-4, 132-3,

Index 237, 360, 366, see also poly(alkenoic acid)s poly(acrylic acid), modified vinyl containing 3, 170-2 polyalkenoate cements 2, 3, 90-175 molecular structure 99-101 setting 98-9 see also glass polyalkenoate cement, mineral polyalkenoate cements, zinc polycarboxylate cement polyalkenoates adhesion 94-6 adsorption 96-7 complexes 69-70 poly(alkenoic acid)s 56-8, 69-71, 74-5, 90-1, 97-8, 103-5, 132-3, 360 acrylic acid homo and copolymers 56-7, 70-1, 74-5, 91, 97-8, 103—4, 132-3, 360 binding properties 90 3-butene 1,2,3-tricarboxylic acid copolymers 91-2, 103-4, 132 and cement properties 132,162-3 concentration effect on cement properties 132, 162 conformation 46, 58-9 dissolution in water 45-7 ethylene-maleic acid homopolymer 71, 75, 98, 360 gelation 83-5 in glass polyalkenoate cements 132-3 hydration 74-7 ion binding 77-9 itaconic acid polymers 56, 71, 75, 91-2, 103-4, 132-3 maleic acid polymers 56, 71, 75, 91-2, 103^4, 132-3 methacrylic acid polymer 70-1, 75, 360 modified vinyl containing 3, 170-3 molecular mass 98, 103-4 polymer preparation 97-8 preparation 97-8 solid form use of 163 vinyl methyl ether maleic acid polymer 71, 360 zinc polycarboxylate cements 103-4 polycarboxylates see polyalkenoates polyelectrolyte cements 2-3, 90-169, 310-1, see also polyalkenoate cements and polyphosphonate cements polyelectrolytes 31, 45-6, 56-85, 91-8, 200-1, 310-1, see also poly(alkenoic acid)s, polyions, poly(phosphonic acid)

poly(ethylene maleic acid) 71, 75, 98, 360 polyHEMA see poly(hydroxyethyl methacrylate) poly(hydroxyethyl methacrylate) 169-73 polyion-polyion interactions 82-3 polyions attraction 82-3 conformation 58-9 extension 82-3 hydration 73-5 interaction 82-3 ion binding to 56-7, 59-64 repulsion 82-3 poly(itaconic acid) 71, 75 poly(maleic acid) 71, 75 polymer configuration see polymer conformation polymer conformation 58-9, 79-81 coulombic attraction effects 80-1, 84 factors affecting 79-81 gelation and 81, 83^4 neutralization effects 84 osmotic pressure, effect on 80-1, 84 polymer morphology see polymer conformation polymer shape see polymer conformation polymerization 1, 3 poly(methacrylic acid) 70-1, 75, 360 polyphosphonate cements 117,310-5 glass (aluminosilicate) 314-5 metal oxide 311-4 poly(phosphonic acid)s 56, 311 poly(vinyl methyl ether-maleic acid) 71, 360 poly(vinyl phosphonate) cements see polyphosphonate cements poly(vinyl phosphonic acid) 56-7, 311 Portland cements 1, 2, 5, 298 pottery 1 precipitation cements 7 propylene-2-methoxyphenol cement 321 protonic acids 6, 14—6 pulp capping 347 pyruvic acid cement 6 Q nomenclature for silicates 120, 125, 129, 131, 137

114, lib,

Raman spectroscopy 198 rapid repair materials 3, 4 reaction cements 7 refractive index 63, 74—5 refractory cements 197 resin glass polyalkenoate cement 16^-175

3, 4,

395

Index composition 170-3 light activation 171 properties 173 setting reaction 170-2 structure 172-3 rheometry 141, 374-8 road repairs 222 runway repairs 222 salicylate cements 348 salts, dissolution in water 41 scanning electron microscopy 106, 128, 226-30, 233, 329, 331 setting measurements 374-8 silica gel 7, 9, 121, 139^0, 247, 307 silicate cements 7 silicate glass 9 silicate minerals 6, 90, 114—6, 265 silicic acid 7, 9, 121, 140, 244, 247 silicophosphate cement 263-5 slip casting 3,91, 169 SnF 2 113 SnO cements 201-2, 312 soft acids see HSAB theory soft bases see HSAB theory soil consolidation 90 solvation see hydration Sorel's cements see magnesium oxychloride cement, zinc oxychloride cement splint bandage 2, 91, 117, 168 SrO polyacrylate cement 102 sulphur impregnation of cements 297 sustained release devices 3, 4, 157-8, 222, 304 syringate cements see EBA-methoxyhydroxybenzoate cements tannic acid cements 6, 308, 315 tartaric acid cements 6, 308, 315 tartaric acid in glass polyalkenoate cement 132—5 temperature measurements 147, 380-1 tensile strength 370 titrimetric methods 59, 311 tobermorite gel 140 tooth material see dentine, enamel trace element release 3, 4, 156-8, 222, 304 transition temperatures 130 translucency see optical properties transmission electron microscopy 145 tricarballylic acid (1,2,3propanetricarboxylic acid) cements 6,315

396

turbidity measurements

78

ultrasonic methods 74 underwater cement 2, 91 Usanovich theory 18-20 V-structure of water 37-40 vanillate cements see EBA-methoxyhydroxybenzoate cements vinyl polymerization 3 viscoelasticity 148-9, 341, 375 viscosity measurements 59, 141 water 30-55 in AB cements 30-1: hydration 31, 74-9, 139, 247, 249, 307; in cement formation 7, 249; component of 30, 48-51; ligands 101, 137-8; ' loosely bound' (evaporable) 49-50; as plasticizer 31, 51-2; as reaction medium 247, 307; as solvent 30, 48, 249, 307; states 49-50;' tightly bound' (nonevaporable) 49-50 as a base 49 coordination to ions 47-8, 101, 137-8 and gelation 49-50, 72, 77-9, 83-4 hydration shells 50, 74-7 and hydrides 33-4 hydrosphere 32 ion binding effect 73-4 structure 34—40: anomalous density 39; bond angles 32; bond lengths 32; constitution 31-3; D-structure 37-40; density and 7 3 ^ ; hydrogen bonding 38; hydronium ions 44; I-structure 3 7 ^ 0 ; intrinsic 74; nucleophilic attack 39; O-H bond energy 33; orientated 73; refractive index 73-4; translational motion 35; translational energy 37; V-structure 37-8; see also ice as a solvent 30, 40-8: dissolution of salts 41; dissolution of polymers 45; hydrophobic interactions 40-1; solvation energies 41; thermodynamics 40-1 working time 375 X-ray diffraction (XRD) spectroscopy 9-10, 33, 35, 47, 51, 105, 125-6, 130, 198, 202-3, 208-9, 224-31, 250, 283-6, 293, 323, 359, 367-8 Y 2 O 3 cements

102, 312

Index zinc chloride solutions 2, 6, 283-9 zinc oxide 1 0 3 ^ , 205-6, 321-2, 329-31 active forms 321-2, 328-31 deactivation 205-6 defect structures 329 densification 1 0 3 ^ , 205-6 ignition 103-4, 205-6, 329 magnesium oxide in 205-6 mineralizers 206 preparation by thermal decomposition salts 328-9 preparation by zinc oxidation 328 sintering 206, 329 water in 328-9,331 zinc oxide chelate cements 318-20, see also EBA cements, EBA divanillate cements, EBA-methoxyhydroxybenzoate cements, zinc oxide eugenol cements zinc oxide eugenol (ZOE) cements 321-37, 381 applications 320 composition 321-2, 336: abietic acid (rosin) 322, 334; accelerators 322-3; antimicrobial agents in 322, 335; impression paste 335; modified materials 336; reinforced 336-7 history 320-1, 335 properties 333-7: biological 334-5; creep 333; hydrolysis 332—4; mechanical 333, 336-7; setting contraction 333; setting time 328-9; zinc oxide type, effect 328-31 setting reaction 322-31: electrical conductance 325-6; permittivity 325-6; water role 324-9, 331; zinc oxide type, effect 328-31 structure 331-2 zinc oxychloride cements 2, 31, 51, 283, 285-90 history 2,283,285-6 phases 286-90: ZnO-HCl-H 2 O 290; ZnO-ZnCl 2 -H 2 O 51, 286-90 setting 287-8 structure: thermogravimetry 288—9; XRD 286-7 zinc oxysulphate cements 6 zinc phosphate cement 204-21, 381 applications 204, 214 composition 205-6: aluminium role 205, 207, 209-12; fluoride-containing 220; modified 220; liquid 207, see also phosphoric acid; MgO in

205-6; powder (ZnO) 205-6, see also zinc oxide history 204-5 phases: ZnO-P 2 O 5 -H 2 O 199, 207-9 properties 214: biological 219; erosion 216-7; exotherm 207; factors affecting 218-9; film thickness 215; fluidity 215; mechanical 215-6; phosphoric acid:water effect 218; powder: liquid effect 219; preparation 214-5; setting contraction 215; setting time 214-5, 219; strength 215; working time 215 setting reaction 205, 207-12: aluminium role 205, 207, 209-12; crystallite formation 205, 208-12; exotherm 207; hydration 211-2; infrared spectroscopy 210; pH changes 207; phosphoric acid concentration effect 207; XRD 209-10 structure 99-101,212-3: microstructure 212-3; molecular 99-101 zinc phosphates 199-200 zinc polyacrylate cement see zinc polycarboxylate cement zinc polyalkenoate cement see zinc polycarboxylate cement zinc polycarboxylate cement 31, 45, 56, 90-1, 93, 103-116, 362, 366, 372, 380-1 applications 103 composition 103—4: liquids 103—4, see also poly(alkenoic acid)s; modified materials 113; poly(alkenoic acid) liquid 103-4, see also poly(alkenoic acid); powder (ZnO) 103-4, see also zinc oxide; reinforcing fillers 113; stannous fluoride 107 history 103 properties 94-7, 106-18: adhesion 94-7, 111-3; biological 112; erosion 109-11; setting 106-8; mechanical 107-9; metal fluoride effect 108 setting reaction 98-9, 105-6: electrical conductivity and permittivity 366-7; infrared spectroscopy 105, 361, 365 structure 105-6: hydration states 105-6; microstructure 105-6; molecular structure 99-101, 105

397

Index zinc selenate solution 6 zinc sulphate solution 6 ZnO cements 102, 201-2, 204, 311-2, see also EBA cements, zinc oxide chelate cements, zinc oxide eugenol

398

cements, zinc phosphate cement, zinc polycarboxylate cement ZnO-P 2 O 5 -H 2 O phases 199, 207-9 ZOE cements see zinc oxide eugenol cements