Surface Coatings

SURFACE COATINGS

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SURFACE COATINGS

MARIO RIZZO AND

GIUSEPPE BRUNO EDITORS

Nova Science Publishers, Inc. New York

Copyright © 2009 by Nova Science Publishers, Inc. All rights reserved. No part of this book may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic, tape, mechanical photocopying, recording or otherwise without the written permission of the Publisher. For permission to use material from this book please contact us: Telephone 631-231-7269; Fax 631-231-8175 Web Site: http://www.novapublishers.com NOTICE TO THE READER The Publisher has taken reasonable care in the preparation of this book, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained in this book. The Publisher shall not be liable for any special, consequential, or exemplary damages resulting, in whole or in part, from the readers’ use of, or reliance upon, this material. Any parts of this book based on government reports are so indicated and copyright is claimed for those parts to the extent applicable to compilations of such works. Independent verification should be sought for any data, advice or recommendations contained in this book. In addition, no responsibility is assumed by the publisher for any injury and/or damage to persons or property arising from any methods, products, instructions, ideas or otherwise contained in this publication. This publication is designed to provide accurate and authoritative information with regard to the subject matter covered herein. It is sold with the clear understanding that the Publisher is not engaged in rendering legal or any other professional services. If legal or any other expert assistance is required, the services of a competent person should be sought. FROM A DECLARATION OF PARTICIPANTS JOINTLY ADOPTED BY A COMMITTEE OF THE AMERICAN BAR ASSOCIATION AND A COMMITTEE OF PUBLISHERS. LIBRARY OF CONGRESS CATALOGING-IN-PUBLICATION DATA Surface coatings / editors, Mario Rizzo and Giuseppe Bruno. p. cm. Includes index. ISBN 978-1-61668-992-6 (E-Book) 1. Surface sealers. 2. Protective coatings. I. Rizzo, Mario, 1958- II. Bruno, Giuseppe, 1959TA418.9.C57S88 2009 667'.9--dc22 2009003249

Published by Nova Science Publishers, Inc.

New York

CONTENTS Preface

vii

Chapter 1

State of the Art Bioactive Titanium Implant Surfaces Anna Göransson Westerlund

Chapter 2

Antimicrobial Surface Coatings in Packaging Applications Jari Vartiainen

45

Chapter 3

Environmentally Friendly Conversion Coating Applications for Hot Rolled Steel (HRS) Prior to Powder Coating Application Bulent Tepe

93

Chapter 4

Precise Synthesis of Amphiphilic Polymeric Nano Architectures Utilized by Metal-Catalyzed Living Ring-Opening Metathesis Polymerization (Romp) Kotohiro Nomura

123

Chapter 5

Atmospheric Pressure Plasma Polymerisation R. Morent, N. De Geyter and C. Leys

153

Chapter 6

Interface Research on Films and Coatings Xiaolu Pang and Kewei Gao

177

Chapter 7

A Study on Inorganic Metallic and Dielectric Thin Films Grown on Polymeric Substrates at Room Temperature by PVD and CVD Techniques P. Mandracci, R. Gazia, P. Rivolo, D. Perrone and A. Chiodoni

189

Chapter 8

Sonochemical Coatings of Nanoparticles on Flat and Curved Ceramic and Polymeric Surfaces A. Gedanken and N. Perkas

213

Chapter 9

Post-Consumer PET and Post-Consumer PET-Containing Materials for Flame Spray Coatings on Steel: Processing, Properties and Use V.F.C. Lins , J.R.T. Branco and C.C. Berndt

237

1

vi Chapter 10 Index

Contents Coating of Carbon Nanotubes with Insulating Thin Layers Martin Pumera

259 265

PREFACE This book presents current research on thin films and coatings. The mechanical properties of films and coatings, which are highly affected by their microstructure and their adhesion to substrates, are reviewed. Furthermore, electronic semiconductor devices and optical coatings, which are the main applications benefiting from thin film construction are looked at. This book discusses antimicrobial surface coatings as promising applications of advanced active food packaging systems. Ways in which they effectively control the microbial contamination of various foodstuffs are analyzed. Research that has been done in the last decade using ultrasonic waves for coating surfaces is also examined. Finally, since coatings and films mechanical properties are highly affected by their microstructure, and their adhesion to substrates, this book includes research on interface microstructure and the important role that bond formation plays on coatings and films. Materials that have the ability to bond to living tissue are defined as “bioactive” and the first possibly bioactive material Bio-glass was described in the 1970s by Hench and coworkers. Furthermore, Jarcho and co-workers were the first to present indications of a direct bone bonding to hydroxyapatite (HA). The mechanism proposed was ion exchange resulting in an apatite layer requested not only by the bone cells but also because proteins that serve as growth factors preferentially adsorb to this layer. The “bioactive” properties of these materials were based on morphological observations of the tissue coalescence by TEM and apatite formation in vitro and in vivo. Poor mechanical properties of these materials make them unsuitable for load-bearing, clinical applications. Therefore, experiments were made to coat titanium surfaces with calcium phosphates by plasma spraying technique. The surfaces indeed showed rapid tissue response initially, but in later stages biodegradation and delaminating of the thick coating was frequently observed. Additionally, the line-of sight problem made the technique unsuitable to use for the coating of complex shapes. To avoid these problems, alternative techniques have been used to make commercially pure (CP) titanium “bioactive”. Chapter 1 reviews recent research on “bioactive” titanium implant surfaces, focusing on five specific modifications:(I) etching with fluoride containing acids, (II) alkali-heat treatment, (III) anodization and (IV) ultra-thin coatings of calcium phosphates in sol-gels. Another possible approach to enhance the bone response is to (V) immobilize organic bio-molecules to the surface. These five CP titanium surface modifications will be reviewed separately with a short background, suggested mechanism of action and performance in simulated body fluids (SBF), in vitro and in vivo. Clinical evaluations will be discussed briefly. Each section is followed by

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an appendix with a list of references of importance for the area of interest. The references are presented as short abstracts with similar information providing a quick overview and easy comparison of the studies. As explained in Chapter 2, antimicrobial packaging materials are interesting and promising applications of advanced active food packaging systems. They can effectively control the microbial contamination of various solid and semisolid foodstuffs by inhibiting the growth of micro-organisms on the surface of the food, which normally comes into direct contact with the packaging material. Recently, a lot of efforts has been put on the development of antimicrobial packaging, which can considerably prolong the shelf lives of packed food products and/or decrease the need of preserving agents in foods. Some promising results have been obtained of which the surface activation and coating treatments seem to offer the most applicable solutions. Antimicrobial surface treatment can be done by several ways such as coating, printing, grafting or covalent binding. Other surface pre-activation methods such as physical, chemical or enzymatic treatments or their combinations may be necessary to produce permanently coupled antimicrobial agents. By using surface treatments the harmful effects on valuable bulk properties of packaging materials can be minimized. Also the safety aspects should be easier to fulfil as migration of substances can be kept at very low level. Antimicrobial surface treatments can be completely separated from the highvolume production lines of bulk materials. They can be done with smaller scale equipment immediately before the packaging is formed ensuring the maximum antimicrobial efficiency. Development of antimicrobial packaging materials, which can be produced at commercial scale, is a challenging and promising area, where intensive research is still needed. They can be exploited in direct contact with certain foods only and each food system must be investigated separately. Hot rolled steel (HRS) is extensively used in a wide range of applications by many different industries such as automotive, domestic appliances, defence etc. It is common knowledge that hot rolled steel comes with oxide scale, often called mill scale, on the surface, due to the hot rolling process. Despite the disadvantage of oxide scale on HRS, it is still one of the most popular materials used in industry due to its availability, cost and ease of profiling properties. One of the most important coating applications for HRS is powder coating, which has a number of advantages over its favourability to wet coating, therefore it is widely used for HRS components in industry, prior to powder coating, to increase corrosion and blister resistance and enhance adhesion pre-treatment systems are used. Pre-treatment systems usually contain five or more stages: cleaning, rinsing, conversion coating, rinsing and passivation. Conversion coating is the most important stage in the pre-treatment process and it is usually phosphating. Phosphating offers many advantages, however it is considered as a hazardous material to human health and the environment. The phosphating process creates sludge, which results in pipe and pump blockages and sludge built up in the phosphating tank. These concerns have driven chemical companies to conduct research aimed at finding a conversion coating that meets the requirements of health and safety and is environmentally friendly. Some companies have already developed environmentally friendly conversion coating systems which are promoted as ecological material and an alternative to the phosphating process. The main objective of Chapter 3 is to evaluate the ability of commercially available environmentally friendly pre-treatment systems as a metal pre-treatment in finishing operations, to eliminate or reduce the amount of environmentally hazardous and toxic

Preface

ix

chemicals. This objective must be accomplished whilst maintaining equal or better product performance properties, with economic benefit or no significant economic penalty to the metal finishing companies who would like to change their pre-treatment system to an environmentally friendly pre-treatment system. The evaluation focuses on technical performance and economics while validating the laboratory tests and environmental benefits. In order to evaluate the conversion coatings’ performance studies on: corrosion behaviour, adhesion and blister resistance, salt spray, prohesion test, Electrochemical Impedance Spectroscopy (EIS) measurement, cross hatch test, conical bend test, pull-off test, humidity test and surface morphology were performed. In this chapter the most popular environmentally friendly conversion coatings were evaluated. Environmentally friendly coatings are usually Silane and Zirconium based. Chapter 4 summarizes recent examples for precise synthesis of amphiphilic block copolymers by adopting transition metal-catalyzed living ring-opening metathesis polymerization (ROMP). In particular, unique characteristics of the living ROMP initiated by molybdenum alkylidene complexes (so-called Schrock type catalyst), which accomplish precise control of the block segment (hydrophilic and hydrophobic) as well as exclusive introduction of functionalities at the polymer chain end, enable us to provide the synthesis of block copolymers varying different backbones by adopting the “grafting to” or the “grafting from” approach. Moreover, use of the “grafting through” approach (polymerization of macromonomers) by the repetitive ROMP technique, using the molybdenum alkylidene catalysts, offers precise control of the amphiphilic block segments. Plasma polymerisation is a unique technique for modifying material surfaces by depositing a thin polymer film. Plasma polymerised films have received a great deal of interest due to their unique characteristics. These coated films are pinhole-free and highly cross-linked and are therefore insoluble, thermally stable, chemically inert and mechanically though. Furthermore, such films are often highly coherent and adherent to a variety of substrates including conventional polymer, glass and metal surfaces. Due to these excellent properties, plasma polymerised films can offer many practical applications in the field of mechanics, electronics and optics. Plasma polymerisation at low pressure is already a well established technology. However, the NECESSITY of expensive vacuum systems is the biggest shortcoming of this technology in industrial applications besides the limitation to batch processes. Therefore, to overcome these disadvantages, considerable efforts are made in developing alternative techniques. Atmospheric pressure plasmas are one of the most promising methods to deposit polymer films in a more flexible, reliable, less expensive and continuous way of treatment. In the last two decades, a lot of effort has been put into the development of plasma polymerisation at elevated pressure. Chapter 5 attempts to review this research and its applications in a broad perspective. Coatings and films mechanical properties are highly affected by their microstructure, and their adhesion to substrates, which sustains their mechanical integrity, and consequently improves their properties. Interfaces with high adhesion are also known to ensure prolonged coatings lifetime. Research on interface microstructure and bond form plays a very important role on coatings and films. In Chapter 6, interfaces between chromium oxide coating deposited by reactive radio frequency (RF) magnetron sputtering technique, chromium interlayer and steel substrate are examined with scanning electron microscopy (SEM), high resolution electron microscopy

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(HREM) and atom force microscopy (AFM) focusing on the interfacial structure properties affecting the adhesion performance and surface roughness. This examination revealed the presence of several Cr–Fe phases, which may ensure good adhesion of the interlayer to the underlying steel. Furthermore, amorphous chromium and chromium oxide layers about 100 nm thick were detected at each interface, which may have some effect on corrosion resistance and growth of columnar coating microstructure. The amorphous interfacial layer detected may give novel thought when deposited thick film but small size column grains. The deposition of both metallic and dielectric inorganic thin films on polymeric substrates is of great interest for several industrial and research applications. The growth of metallic coatings on polymers is of raising usage in order to impart specific functionalities, such as electrical, aesthetic and chemical-resistance properties, to polymeric substrates. Some examples are the substitution of chromium electroplating processes on plastics by PVD deposition in several industrial fields and the use of aluminum or silver coatings for the fabrication of hybrid fabrics. Dielectric thin films are also commonly grown on polymeric materials for several aims, including the protection of polymeric substrates from scratch, the attribution of barrier coatings to food packaging films, the incorporation of new functionalities to artificial fabrics, and the increase of biocompatibility of some kind of polymeric dental materials and prostheses. Unfortunately, the growth of thin films on polymeric substrates suffers of several constraints, due to the peculiar properties of polymers, such as the low heat resistance, the high elasticity and the low hardness. These limitations lead to the necessity of very low processing temperatures (often as low as room temperature) in order to avoid substrate damage, and the deposition of films of very low thicknesses, in order to reduce the interface stress. Plasma-assisted PVD and CVD techniques are suitable to satisfy these requirements, since they allow very low deposition temperatures, they are suitable for the deposition of composite materials, and provide a very good control on a wide range of process parameters. Chapter 7 deals with an experimental study of the interaction between the surface of different polymeric substrates, such as ABS, polyester, polyamide and some dental resins, with metal and dielectric coatings, such as Cr, Al, a-SiOx, grown by RF sputtering or PECVD. Different types of surface modifications, such as plasmaassisted surface activation and deposition of interlayers, were also applied to some of the polymeric substrates in order to study their effect on the growth process of the inorganic coatings. Several characterization techniques were used in order to analyze the materials involved in the study. The polymeric substrates and the inorganic coatings were characterized against their surface morphology by means of high resolution mechanical profilometry, optical microscopy and field emission scanning electron microscopy (FESEM), while some of the film chemical characteristics were analyzed by Fourier transform infrared spectroscopy (FTIR). Some chemical resistance tests were also performed to investigate some properties of the polymer-dielectric multilayer structures. Chapter 8 will review the research that has been done in the last decade using ultrasonic waves for coating surfaces. Sonochemistry is a field of research in which chemical reactions occur due to a collapse of an acoustic bubble. The review will present examples limited to coating nanoparticles on ceramic bodies and polymeric surfaces. However, the same technique works also on metallic, glass, and textile surfaces. The excellent adherence of the nanoparticles to the substrate is reflected, for example, in the lack of bleaching of the nanoparticles from the polymeric substrate when deposited by the sonochemical process.

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Sonochemistry is a research field where waves in the frequency range of 20 kHz - 1 MHz are the driving force for the chemical reactions. The reaction is dependent on the development of an acoustic bubble in the solution. Extreme conditions (temperature >5000 K, pressure >1000 atm and cooling rates >1011 K/sec) are developed when this bubble collapses, thus causing the chemical reactions to occur. The current review will introduce to the reader what kind of surfaces serve as the substrates for the coating. It will present the variety of nanoparticles that have been anchored sonochemically to the surface, and finally it will explain the role of the ultrasonic waves in depositing nanoparticles onto solid surfaces. The review will compare the deposition of newly formed nanoparticles with that of nanoparticles purchased from a commercial source. The first chapter of this review will introduce the reader to the field of sonochemistry. The current review is a continuation of a series of previous reviews published by our group. These reviews introduced the sonochemical technique as a new means for the fabrication of nanomaterials [1], for the use of ultrasonic waves for the doping of nanoparticles into ceramic and polymer bodies [2], and for the microspherization of proteins by a sonochemical process [3]. Other review articles on similar topics have also been published [4-6]. However, no review on using the sonochemical technique for coating surfaces was found in our literature search. In our literature search we will scan for papers published until May 2008. We will try to avoid duplication and the review will not include examples presented in previous reviews. As presented in Chapter 9, yet with the generation of large quantities of thermoplastics, the use of the thermal spray method is a logical and efficient means of recycling thermoplastics, thereby reducing the accumulation of polymer residues. Poly (ethylene terephthalate), PET, has excellent mechanical and chemical properties, and is a potential corrosion barrier since it presents low permeability to gases and solvents. Solutions of polymer recycling using the post-consumer PET to produce polymeric and composite coatings on steels in order to improve the tribological and chemical properties of steels are reported. Thermal sprayed and re-fused PET coatings, blend coatings of PET and the copolymer of ethylene and methacrylic acid, EMAA, and PET-based composite coatings were produced. Quenched PET blends with 80% PET and 20% EMAA and quenched PET coatings showed corrosion resistance in a salt spray chamber, small friction coefficient, and adhesion, which are necessary for the application of polymeric films as protective coatings against corrosion and wear. Peeling and swelling of the thermally sprayed PET coatings did not occur in the immersion tests in gasoline, diesel oil, and alcohol for a period of 60 days. The higher corrosion resistance in H2SO4 solution was observed for the composite PET coatings with 0.1% of glass powder and flakes, and zinc powder. The aim of Chapter 10 is to discuss the problematic of coatings of carbon nanotubes with thin and ultrathin layers with insulating properties.

In: Surface Coatings Editors: M. Rizzo and G. Bruno, pp. 1-44

ISBN: 978-1-60741-193-2 © 2009 Nova Science Publishers, Inc.

Chapter 1

STATE OF THE ART BIOACTIVE TITANIUM IMPLANT SURFACES Anna Göransson Westerlund1 Dept of Biomaterials, Institute of Surgical Science, Sahlgrenska Academy at Göteborg University, Sweden Dept of Orthodontics, Institute of Odontology, Sahlgrenska Academy at Göteborg University, Sweden

Abstract Materials that have the ability to bond to living tissue are defined as “bioactive” and the first possibly bioactive material Bio-glass was described in the 1970s by Hench and coworkers. Furthermore, Jarcho and co-workers were the first to present indications of a direct bone bonding to hydroxyapatite (HA). The mechanism proposed was ion exchange resulting in an apatite layer requested not only by the bone cells but also because proteins that serve as growth factors preferentially adsorb to this layer. The “bioactive” properties of these materials were based on morphological observations of the tissue coalescence by TEM and apatite formation in vitro and in vivo. Poor mechanical properties of these materials make them unsuitable for load-bearing, clinical applications. Therefore, experiments were made to coat titanium surfaces with calcium phosphates by plasma spraying technique. The surfaces indeed showed rapid tissue response initially, but in later stages biodegradation and delaminating of the thick coating was frequently observed. Additionally, the line-of sight problem made the technique unsuitable to use for the coating of complex shapes. To avoid these problems, alternative techniques have been used to make commercially pure (CP) titanium “bioactive”. This article reviews recent research on “bioactive” titanium implant surfaces, focusing on five specific modifications:(I) etching with fluoride containing acids, (II) alkali-heat treatment, (III) anodization and (IV) ultra-thin coatings of calcium phosphates in sol-gels. Another possible approach to enhance the bone response is to (V) immobilize organic bio-molecules to the surface. These five CP titanium surface modifications will be reviewed separately with a short background, suggested mechanism of action and performance in simulated body fluids (SBF), 1

E-mail address: [email protected]. Phone +46 31 786 2962 Fax +46 31 7732962 Correspondence to: Anna Westerlund PhD, Specialist Orthodontist, Department of Biomaterials, Göteborg University, Box 412, SE 405 30 Göteborg, Sweden.

2

Anna Göransson Westerlund in vitro and in vivo. Clinical evaluations will be discussed briefly. Each section is followed by an appendix with a list of references of importance for the area of interest. The references are presented as short abstracts with similar information providing a quick overview and easy comparison of the studies.

1. Introduction In the 1960s a new system for permanent anchorage of artificial teeth was discovered when the Brånemark group studied bone marrow cells in bone chambers. The concept of “osseointegration” was defined in 1977 in conjunction with a 10-year follow-up study of titanium implants for edentulous jaws [1]. The initial definition “a material in intimate contact with living bone without intervening fibrous tissue” has during the years been redefined to adapt to current knowledge. The Brånemark system was for a long time the gold standard based mainly on good clinical records [2]. However, in parallel implant parameters were evaluated for predicting good osseointegration and in the 1980s Albrektsson proposed six parameters as being important for the implant performance—material compatibility, implant design and surface quality, status of implant bed, surgical trauma at installation and prosthetic loading [3]. There are several methods by which the titanium surface quality can be modified [4]; physical turning, blasting), chemical (acid etching, alkali), electrochemical (electropolishing anodizing), deposition (plasma-spraying, sol-gel) and biochemical [simulated body fluids (SBF), proteins] methods. The different techniques will result in a surface quality with different topographical, chemical, physical and mechanical properties. Since osseointegration depends on biomechanical bonding, i.e. ingrowth of bone into small irregularities of the implant, the topography and especially the roughness of the implants has been an area of interest and has been the subject of numerous research efforts. Guidelines of how to perform and present the measurements of surface topography in a standardized way have been suggested by Wennerberg and Albrektsson [5]. Furthermore, based on experimental evidence from the mid 1990s a surface roughness of about 1.5 µm Sa (average deviation in height from a mean plane) has been defined as optimal for osseointegration [6]. This is rougher than the original, turned Brånemark implant that demonstrated a surface roughness of about 0.5 µm. Titanium surface roughness has also demonstrated to affect protein absorption [7], inflammatory cell [8-13] and bone cell [14-27] responses in vitro. Furthermore, there have been indications that surface orientation may be of importance [28, 29] for implant bone integration, however, not evaluated in a scientifically controlled manner. Except for the concomitant change in chemical composition when changing the surface topography, attempts have been made to intentionally modify chemical composition to add a biochemical bonding to the biomechanical bonding. The theoretical benefit of a chemical bond would be earlier attachment, since it is hypothesized to occur more rapidly than bony ingrowth. Materials that have the ability to bond to living tissue are defined as “bioactive” and the first possibly bioactive material Bio-glass was described in the 1970s by Hench and coworkers [30]. Furthermore, Jarcho and co-workers were the first to present indications of a possible direct bone bonding to hydroxyapatite (HA)[31].

State of the Art Bioactive Titanium Implant Surfaces

3

The mechanism proposed was ion exchange resulting in an apatite layer requested not only by the bone cells but also because proteins that serve as growth factors preferentially adsorb to this layer. The “bioactive” properties of these materials were based on morphological observations of the tissue coalescence by transmission electron microscopy (TEM), apatite formation in SBF in vitro and in vivo. However, it must be pointed out that bioactivity or chemical bonding are difficult to prove and that the presented evidence is of an indirect nature. Poor mechanical properties of these materials make them unsuitable for loadbearing, clinical applications. Therefore, experiments were made to coat titanium surfaces with calcium phosphates (CaP) by the plasma spraying technique. The surfaces indeed showed rapid tissue response initially, but in later stages biodegradation and delaminating of the thick coating was frequently observed [32]. Additionally, the line-of-sight problem made the technique unsuitable to use for the coating of complex shapes. To avoid these problems, alternative techniques have been used to make commercially pure (cp) titanium possibly bioactive; I) etching with fluoride containing acids (fluoridated surfaces), II) alkali-heat treatment (alkali-heat treated surfaces), III) anodic oxidation with specific ions (anodized surfaces) and IV) sol-gel processing in calcium phosphate solutions (nano HA surfaces). Another possible approach to enhance the bone response is to V) immobilize organic bio-molecules to the surface (protein covalent immobilized surfaces). These five CP titanium surface modifications will be reviewed in the following sections with a short background, suggested mechanism of action and performance in SBF, in vitro and in vivo. Clinical evaluations will only be concluded briefly. Each section is followed by an appendix with a list of references of importance for the area of interest. The references are presented as short abstracts with similar information providing a quick overview and easy comparison of the studies.

2. State of the Art CP Titanium Implant Surfaces 2.1. Fluoridated CP Titanium Surfaces Etching of titanium surfaces with different acids to modify surface roughness has been extensively studied during the last decades [33]. The idea of using fluoride-containing acids in low concentrations for the purpose of incorporating fluoride ions on titanium implants in small amounts was presented by Ellingsen and co-workers [34]. The action of the fluoride ion has mostly been evaluated in the area of caries research, where the beneficial effect because of its high attraction for calcium and phosphate is of great clinical importance, when the ion is brought in contact with the enamel. Fluoride has also specific attraction for skeletal tissues, e.g. trabecular bone density can be increased by the presence of fluoride ions during remodeling [35]. The proposed effects of the fluoride ion in bone are increased proliferation of bone cells by increasing intracellular levels of the ion, increased differentiation of mesenchymal cells into bone cells and stimulation of endogenous growth factor production [36]. Fluoridated titanium implant surfaces have been studied both in SBF [37], in vitro [3742], in vivo [34, 38, 43] and clinically. OsseoSpeed™ (Astra Tech, Gothenburg, Sweden) is a commercially available dental implant system that has been evaluated in approximately 5-10 articles since the launch in 2004. The longest follow up period is 1 year [55]. The surface has

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mainly been used in poor bone and in early loading situations where it in general has demonstrated good results. In addition there is an orthopedic hip implant available with some clinical documentation [44]. The possible bioactivity of titanium implant surfaces is based on its ability to give rise to early apatite formation in SBF [37], where the fluoride-modified surface demonstrates a Ca/P ratio of 2 [45]. When adding proteins to the SBF, the fluoride-modified surface demonstrate an increased apatite formation and protein adhesion compared to a blasted control [46]. Furthermore, in vivo studies have demonstrated increased bone response by means of increased bone implant contact [38, 43, 47], bone area [47] and stability [43, 48, 49] at shorter healing times than turned and blasted surfaces [50]. The mechanisms for the faster healing time of the implants are not fully understood. A possible explanation is that fluoride ion modification seems to augment the thrombogenic properties of titanium [51], another possibility is that fluoride modified surfaces demonstrate increased proliferation [38] and differentiation [38-41] of bone cells. However, results have shown decreased cell number [40], differentiation and protein production compared to blasted controls [37]. According to other studies, the amount of fluoride ions in the surface also seems to be of importance for the bone retention [52]. Appendix - Fluoridated CP Titanium Surfaces

SBF Arvidsson et al -07 [45] compared four types of possibly bioactive surfaces; a blasted surface prepared by alkali heat treatment, anodization (Mg ions incorporated), fluoridation or hydroxyapatit coating, where the blasted surface served as control. Surfaces were analyzed by weight, Profilometry, SEM/EDX and XPS after immersion in SBF for 1, 2, 3, 4 and 6 weeks. The results demonstrated that the Ca/P mean ratio of all the surfaces was approximately 1.5 after 1 week except for the fluoridated specimens that displayed mean ratio of approximately 2. All surfaces showed the presence of hydroxyapatite after 4 and 6 weeks of immersion, but a higher degree of crystallinity at 6 weeks. It was concluded that differences appeared at the early SBF immersion times of 1 and 2 weeks between controls and bioactive surface types, as well as between different bioactive surface types. Franke-Stenport et al -08 [46] compared four types of possibly bioactive surfaces; a blasted surface prepared by alkali heat treatment, anodization (Mg ions incorporated), fluoridation or hydroxyapatit coating, where the blasted surface served as control. Surfaces were analyzed by Profilometry, SEM/EDX and XPS after immersion in SBF with 4.5 mg/ml albumin for 3 days, 1, 2, 3 and 4 weeks. The results demonstrated that all the bioactive surfaces initiated an enhanced calcium phosphate (CaP) formation and a more rapid increase of protein content was present on the bioactive surfaces compared to the blasted control surface. It was concluded that this might be an advantage in vivo.

In Vitro Eriksson et al -01 [42] compared smooth (polished) and rough (HF etched) surfaces with thick (annealed 700ºC) and thin (HNO3) oxide. The surfaces were characterized by SEM,

State of the Art Bioactive Titanium Implant Surfaces

5

Optical Profilometry and AES. After exposure to whole blood for 8 minutes to 32 hours, immunofluorescence and chemiluminescence techniques were used for evaluation of cell adhesion, expression of adhesion receptors and the stimulated respiratory burst, respectively. PMN cells were the dominating cell on all surfaces followed by monocytes. While cells on rough surfaces demonstrated increased expression of adhesion receptors, earlier maximum respiratory burst occurred on the smooth surfaces. It was concluded that surface topography had greater impact on most cellular reactions, while oxide thickness often had a dampening effect. Cooper at al -06 [38] compared grit-blasted (25 and 75 µm) titanium implants with and without fluoride ions (various fluoride concentrations). Cell attachment, proliferation and osteoblastic gene expression were measured by SEM, Tritiated thymidine incorporation and RT-PCR, respectively. There were no differences in human mesenchymal stem cell (hMSCs Osiris) attachment between the differently modified surfaces but cells on the fluoride ion modified implants demonstrated an increased proliferation and differentiation (BSP, BMP-2) compared to grit-blasted implants. Masaki et al -05 [39] compared grit-blasted titanium implants with and without fluoride ions and grit-blasted etched surfaces (OsseoSpeed, TiOBlast, SLA-1 and SLA-2). Cell morphology, attachment, and osteoblastic gene expression were measured by SEM, Coulter counter (electrical conduction) and RT-PCR, respectively. There were no differences in mesenchymal pre-osteoblastic cell (HEPM 1486, ATCC) attachment, while cell morphology differed between the differently modified surfaces. Furthermore, cells demonstrated increased ALP gene expression on the SLA-2 surface, while cells on TiOBlast and OsseoSpeed demonstrated increased expression of Cbfa1/RUNX-2. It was concluded that implant surface properties might contribute to the regulation of osteoblastic differentiation by influencing the level of bone-related genes and transcription factors. Isa et al -06 [40] compared blasted titanium implants with and without fluoride ions. Cell proliferation, alkaline phosphatase specific activity and gene expression were evaluated by Coulter counter, Spectrophotometry and RT-PCR, respectively. The number of cells human embryonic palatal mesenchymal (HEPM) were decreased on the fluoride surface compared to the blasted control. The gene expression was similar, except for Cbfa1, a key regulator for osteogenisis that was up regulated after 1 week on the fluoridated surface. Stanford et al -06 [41] compared blasted titanium implants with and without fluoride ions. Platelet attachment and activation were evaluated by immunofluorescence technique, while human palatal mesenchymal (HEPM 1486, ATCC) morphology and gene expression were evaluated by SEM and RT-PCR, respectively. The number of attached platelets was decreased, while activation was increased on the fluoride surface compared to the blasted control. The gene expression was similar for the surfaces, except for Cbfa1 and bone sialoprotein that were increased on the fluoride modified surfaces. Thor et al -07 [51] compared hydroxyapatite, machined, grit- blasted and fluoride ion modified grit- blasted surfaces. The trombogenic response, platelet activation, generation of thrombin-antithrombin complex where evaluated in a slide chamber model with blood, platelet-rich and platelet poor plasma after 60 min. The results demonstrated that whole blood was necessary for sufficient thrombin generation and that the fluoride ion modified surface augmented the thrombogenic properties of titanium compared to the other surfaces.

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Göransson et al -08 [37] compared four types of possibly bioactive surfaces; a blasted surface prepared by alkali heat treatment, anodization (Mg ions incorporated), fluoridation or hydroxyapatit coating, where a blasted surface served as control. Surfaces were analyzed by Profilometry, SEM and XPS after immersion in SBF for 12, 24 and 72 hours. Cells Primary (human mandibular osteoblast-like cells) were cultured on the various surfaces subjected to SBF for 72 h. Cellular attachment, differentiation (osteocalcin) and protein production (TGF-beta(1)) was evaluated after 3 h and 10 days respectively. The results demonstrated that the possibly bioactive surfaces gave rise to an earlier CaP formation than the blasted surface. Subsequent bone cell attachment was correlated to neither surface roughness nor the amount of formed CaP. In contrast, osteocalcin and TGF-beta(1) production were largely correlated to the amount of CaP formed on the surfaces.

In Vivo Ellingsen et al -95 [48] compared turned titanium implants with and without fluoride ions (various fluoride concentrations NaF). The surfaces were characterized before installation and after push out test by SEM. It was demonstrated that fluoride modified surfaces had increased push out values in rabbit ulna after 4 and 8 weeks compared to untreated implant surfaces. Furthermore, on the fluoride modified surfaces fractures occurred in bone, while for the turned surface it occurred in the bone-implant interface. Ellingsen et al -04 [43] compared blasted titanium implants with and without fluoride ions (HF). The surfaces were characterized by Optical Profilometry. It was demonstrated that fluoride modified surfaces had an increased amount of bone-implant contact in a rabbit model after 1 and 3 months compared to untreated implants. Additionally, the fluoride modified surfaces demonstrated increased RTQ and shear strengths between bone and implant after 3 months. It was concluded that fluoridated implants achieved greater bone integration after short healing time compared to blasted controls. Cooper at al -06 [38] compared blasted surfaces with and without fluoride ions (HF). The surfaces were characterized by SEM. The results demonstrated improved bone formation by means of bone-implant contact in a rat tibia model for the fluoridated surface compared to the blasted surface after 3 weeks. Berglundh et al -07 [50] compared implants with a grit-blasted (TiOblast) and gritblasted fluoride modified (OsseoSpeed) surfaces. Histological analyses were made in a dog model after 2 and 6 weeks. It was demonstrated that the amount of new bone formed in the voids after 2 weeks of healing was larger at fluoride-modified implants. Furthermore the amount of bone-to-implant contact that had been established after 2 weeks in the macrothreaded portion of the implant was significantly larger at the test implants than at the controls. Abrahamsson et al -08 [47] compared implants with a grit-blasted (TiOblast) and gritblasted fluoride modified (OsseoSpeed) surfaces. Histological analyses were made in a dog model after 2 and 6 weeks. The histological analysis demonstrated a larger area of osseointegration and degree of bone-to-implant contact within the defect at fluoride-modified implants after 6 weeks of healing.

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Lamolle et al -08 [52] compared fluoride ion modified titanium implants prepared in various HF concentrations (0,1, 0,01, 0,001 vol%). The surface topography and chemistry were characterized by AFM, SEM, and tof-SIMS respectively. Bone response was evaluated in a rabbit model by using a pull out test method after 4 weeks. The group of 0,01% HF demonstrated the highest retention in bone. Furthermore, fluoride and hydride content in the surface as well as the surface skewness, kurtosis and core fluid retention were positively correlated to implant retention. Monjo et al -08 compared grit-blasted and of fluoride-modified titanium implants. The attachment to cortical bone, [49] its association with gene expression of osteoblast (runx2, osteocalcin, collagen-I and IGF-I), osteoclast (TRAP, Hþ-ATPase and calcitonin receptor) and inflammation (TNF-a, IL-6 and IL-10) markers from peri-implant bone tissue and bone density were evaluated after 4 and 8 weeks by using pull-out test, real-time RT–PCR and micro -CT respectively. The results demonstrated lower LDH and TRAP mRNA activity for fluoride modified implants after 4 weeks, however no differences in pull-out force. After 8 weeks pull out force, bone density and gene expression for osteocalcin-, runX2-, collagen typ I were increased compared to grit-blasted surfaces.

Clinic OsseoSpeed™ (Astra Tech, Gothenburg, Sweden) is a commercially available dental implant system that has been clinically evaluated in approximately 5-10 articles since their launch in 2004. The longest follow up period is 1 year [53]. The surface has mainly been used in poor bone and in early loading situations where it in general has demonstrated good results.

2.2. Alkali-Heat Treated CP Titanium Surfaces The Kokubo group introduced the alkali-heat treated surface in the middle of the 1990s[54]. NaOH treatment results in a sodium titanate hydrogel, and the subsequent heat treatment at 600 degrees result in an amorphous sodium titanate surface layer [55, 56]. The possibly bioactivity of the surfaces are based on its ability to give rise to apatite formation in SBF and has been thoroughly investigated [37, 45, 54-63] also when adding proteins [46]. The apatite formation process on the surfaces has been carefully described [58, 59] and is attributed to TiOH groups exchanging sodium ions from the material and hydronium ions from the solution. Thereafter, adsorption of calcium ions from the fluid takes place to form calcium titanate. This calcium titanate surface then causes adsorption of phosphate as well as calcium ions to apatite nucleation layers. Once this layer is formed bone like apatite growth follows spontaneously. Furthermore, studies have demonstrated an increased [64, 65] differentiation and decreased proliferation, differentiation and protein production of bone cells compared to untreated controls in vitro [37, 63]. In vivo studies have shown increased bone response by means of bone-implant contact, detachment load and tensile failure load compared to untreated surfaces [66-70]. However, the bonding strength seems to be time dependent with an initial high bonding strength and no further increase or difference compared to controls at later time points [67]. If the surface were pre-immersed in SBF, the apatite layer on the surface significantly increases the bone response resulting in increased failure loads [69, 70].

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Increased bone response in vivo by means of enhanced bonding strength has additionally been demonstrated after sodium removal in hot water immersion or, as reported lately, by immersion in HCl [71]. If the bulk is a porous titanium material, the surface has been shown to induce ectopic bone formation in vivo in dog soft tissue model [72, 73]. This surface has so far not been applied to dental implants. However, clinical trials of seventy hip arthroplasty patients have been successfully concluded. Appendix - Alkali-Heat Treated CP Titanium Surfaces

General Kim et al -97 [55] evaluated bonding strength of the apatite layer formed in SBF on alkali treated implant surfaces with and without subsequent heat treatment (500, 600, 700, 800ºC) and compared it to bonding strengths of apatite formed on Bioglass 45S5-type glass, glassceramic AW and dense sintered HA. The results showed the highest bonding strengths of the apatite layer to the alkali treated titanium surfaces that were maximized after a subsequent heat treatment in 500-600ºC. It was concluded that bioactive titanium metal was useful as bone substitutes, even under load-bearing conditions. Kim et al-99 [56] compared the structure of alkali-heat treated titanium surfaces (5M NaOH 60ºC 24h) prepared with various hydrothermal treatment (600 or 800ºC). Furthermore, the bonding strengths of the apatite layer formed on the various surfaces after soaking in SBF. The surfaces were characterized by SEM, AES, Raman spectroscopy, TF-XRD, XPS and ICP. At 600ºC an amorphous sodium titanate layer with a smooth graded surface was formed, while at 800ºC a crystalline rutile sodium titanate with an intervening thick oxide was formed. The apatite layer prepared in 600ºC demonstrated the tightest bond to the surface.

SBF Kim et al -96 [54] evaluated apatite formation in SBF (1-4w) on titanium and titanium alloy surfaces subjected to alkali (NaOH or KOH) and heat treatment (5º C/min to 400-800º C). The surfaces were characterized by SEM-EDX, TF-XRD, ICP and pH- metry. Apatite was formed on the SBF treated titanium and titanium alloy surfaces, though, not on cobalt chromium and stainless steal surfaces. Kim et al -00 [54] subjected alkali-heat treated (5M NaOH 60ºC 24h+ 600ºC 1h) macroporous titanium (plasma-spraying method) to SBF. The surfaces were characterized by SEM-EDX and TF-XRD. The induction period for apatite formation was 3 days, which is comparable to bioactive glass-ceramics A/W. It was concluded that alkali-heat treatment is an effective method for preparation, irrespective of the surface macro-texture. Wang et al -01 [62] compared heat-, H2O2-, and NaOH treated titanium surfaces. The surfaces were characterized by SEM, FTIR and XRD. Dense oxide layer, titania gel and sodium titanate gel was formed on the surfaces, respectively. Some of the specimens were pre-immersed in distilled water up to 5 days before SBF. The discs were arranged with (contact surface) and without (open surface) contact with the bottom of the container. It was concluded that bioactivity of titania gel originated from the favorable structure of the gel itself because it formed apatite on open surface and after water immersion, while the sodium

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titanate was dependent of ion release and therefore was unable to produce apatite on open surfaces and after water immersion (decreased ion concentration). Subsequent heat treatment decreased the apatite forming ability of the treated surfaces, but not the untreated titanium surfaces. Takadama et al -01 [58] carefully described the apatite forming process on alkali-heat treated titanium surfaces by TF-XRD, ICP, pH-metry and XPS. It was stated that ”Bioactive titanium metal with a surface sodium titanate layer forms a bone-like apatite layer on its surface in the SBF by the following process; The Na+ ions were released from the surface sodium titanate via the exchange with H3O+ ions in the SBF to form Ti-OH groups. These TiOH groups induce the apatite nucleation indirectly, by forming a calcium titanate. The initial formation of the calcium titanate may be attributable to the electrostatic reaction of the negatively charged Ti-OH groups and the positively charged calcium ions in the SBF. Takadama et al -01 [59] further described the structure of apatite formation on alkali-heat treated titanium (5M NaOH 60ºC 24h + 600ºC 1h) subjected to SBF by TEM-EDX, ICP and pH-metry. The Ca/P ratios of the apatite were 1.4, 1.62 and 1.67 after 36, 48 and 72 hours in SBF, respectively. Uchida et al -03 [61] compared apatite forming ability of Ti-OH with different structural arrangements in SBF after 14 days by SEM, TF-XRD and ICP. Gels with anatase and rutile structures induced more apatite on their surfaces compared to amorphous surfaces. It was concluded that crystalline planar arrangement in anatase structure was superior to rutile structure for apatite formation. Lu et al -04 [57] subjected an alkali-heat treated titanium (10M NaOH 60ºC 24h + 600ºC 1h) surface to SBF for 1 month. The apatite formed was characterized by Profilometry, SEM, TEM-EDS and TF-XRD. The study showed that octacalcium phosphate (OCP), not apatite, was formed on the surface after immersion in SBF. Arvidsson et al -07 [45] compared four types of possibly bioactive surfaces; a blasted surface prepared by alkali heat treatment, anodization (Mg ions incorporated), fluoridation or hydroxyapatit coating, where the blasted surface served as control. Surfaces were analyzed by weight, Profilometry, SEM/EDX and XPS after immersion in SBF for 1, 2, 3, 4 and 6 weeks. The results demonstrated that the Ca/P mean ratio of all the surfaces was approximately 1.5 after 1 week except for the fluoridated specimens which displayed mean ratio of approximately 2. All surfaces showed the presence of hydroxyapatite after 4 and 6 weeks of immersion, but a higher degree of crystallinity at 6 weeks. It was concluded that differences appeared at the early SBF immersion times of 1 and 2 weeks between controls and bioactive surface types, as well as between different bioactive surface types. Franke-Stenport et al -08 [46] compared four types of possibly bioactive surfaces; a blasted surface prepared by alkali heat treatment, anodization (Mg ions incorporated), fluoridation or hydroxyapatit coating, where the blasted surface served as control. Surfaces were analyzed by Profilometry, SEM/EDX and XPS after immersion in SBF with 4.5 mg/ml albumin for 3 days, 1, 2, 3 and 4 weeks. The results demonstrated that all the bioactive surfaces initiated an enhanced calcium phosphate (CaP) formation and a more rapid increase of protein content was present on the bioactive surfaces compared to the blasted control surface. It was concluded that this might be an advantage in vivo.

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In Vitro Nishio et al -00 [65] compared titanium, alkali-heat treated titanium (5M NaOH 60ºC 24h + 600ºC 1h) and alkali-heat treated titanium subjected to SBF for 2 weeks. The surfaces were characterized by SEM, TF-XRD and XPS. Cell number (Primary rat bone marrow cells), differentiation and gene expression (OC, OP, ON COL) were evaluated by DNA content, ALP activity and Northern blot, respectively. Results demonstrated that cell differentiation increased on the apatite prepared surfaces, while cell number was similar for the differently modified surfaces. It was concluded that apatite formed on the surfaces favored osteoblast differentiation and that alkali-heat treatment favored apatite formation. Muramatsu et al -03 [74] compared thrombus resistance of alkali-heat treated titanium (5M NaOH 60ºC 24h + 600ºC 1h), alkali-water treated titanium (distilled water 40ºC 48h) and alkali-heat treated titanium subjected to SBF. The surfaces were characterized by AFM, XRD and contact angle measurement. Platelet attachment and protein adsorption were evaluated and it was concluded that SBF treated alkali-heat treated titanium behaved thrombus resistant probably because heparin was preferentially adsorbed to its surface. Chosa et al -04 [64] compared TCP, titanium and SBF treated (8 days) alkali-heat treated titanium (5M NaOH 60ºC 24h + 600ºC 1h). The surfaces were characterized by SEM, TFXRD, FTIR and XPS. Cell (Human osteoblast SaOS-2) differentiation-related gene expression (ALP, COL, OPN, BSP, OSC) was evaluated by RT-PCR after 1, 2, 3 and 4 weeks. The results indicated that the treated implants accelerated middle (OPN, BSP) and late (OSC) stage differentiation, while early differentiation was down-regulated (ALP, COL). Maitz et al -05 [75] compared bioactivity of titanium following sodium plasma immersion, ion implantation and deposition (alkali) in SBF for 7 days. The surfaces were characterized by AES. In a parallel experiment, cell (rat bone marrow cells) viability, proliferation and differentiation was evaluated by LDH test, Alamar blue test and ALP activity, respectively. It was concluded that ion implantation and deposition could well substitute alkali treatment. Göransson et al -08 [37] compared four types of possibly bioactive surfaces; a blasted surface prepared by alkali heat treatment, anodization (Mg ions incorporated), fluoridation or hydroxyapatit coating, where a blasted surface served as control. Surfaces were analyzed by Profilometry, SEM and XPS after immersion in SBF for 12, 24 and 72 hours. Cells Primary (human mandibular osteoblast-like cells) were cultured on the various surfaces subjected to SBF for 72 h. Cellular attachment, differentiation (osteocalcin) and protein production (TGF-beta(1)) was evaluated after 3 h and 10 days respectively. The results demonstrated that the possibly bioactive surfaces gave rise to an earlier CaP formation than the blasted surface. Subsequent bone cell attachment was correlated to neither surface roughness nor the amount of formed CaP. In contrast, osteocalcin and TGF-beta(1) production were largely correlated to the amount of CaP formed on the surfaces.

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In Vivo Yan et al -97 [70] compared titanium, alkali-heat treated titanium (5M NaOH 60ºC 24h + 600ºC 1h) and SBF treated (4 weeks) alkali-heat treated titanium implants. Tensile testing demonstrated that both treated surfaces showed significantly increased failure loads after 4, 8 and 16 weeks in the rabbit tibia compared to the control. Furthermore, both treated surfaces demonstrated direct bone contact with no intervening soft tissue capsule in a histological evaluation after 4 weeks, whereas untreated implants formed direct contact with bone only at 16 weeks. Yan et al -97 [69] compared titanium and SBF (4weeks) treated alkali-heat treated (10M NaOH 60ºC 24h + 600ºC 1h) titanium implants. The surfaces were characterized by SEMEPMA and TF-XRD. Tensile testing demonstrated that the treated surfaces showed significantly increased failure loads after 6, 10 and 25 weeks in the rabbit tibia compared to the control. Histologic examination demonstrated that the treated surfaces demonstrated more immediate bone contact compared to the control titanium surface at all evaluation times. Nishiguchi et al -99 [68] compared titanium, alkali-treated titanium and alkali-heat treated titanium implants (5M NaOH 60ºC 24h + 600ºC 1h). The surfaces were characterized by SEM. Mechanical and histomorphometrical evaluations were performed after 8 and 16 weeks in the rabbit tibia. The alkali-heat treated surfaces demonstrated direct bone-implant contact after 8 weeks, while alkali treated implants demonstrated an intervening fibrous capsule. Additionally, the alkali-heat treated surfaces demonstrated significantly increased failure load after 8 and 16 weeks. It was concluded that heat treatment is essential for preparing a bioactive surface, even though the alkali surface had previously demonstrated apatite formation in SBF, since implants with gel surfaces are unstable and difficult to preserve and install. Nishiguchi et al.-01 [76] compared macroporous titanium (plasma-spraying method), macroporous titanium coated with AW-glass ceramic and alkali-heat treated macroporous titanium (5M NaOH 60ºC 24h + 600ºC 1h). Mechanical and histomorphometrical evaluations were performed after 4 and 12 weeks in dog femur. Bone-implant contact was significantly increased on alkali-heat treated implants at 4 and 12 weeks. Push out test revealed increased shear strengths for the alkali-heat treated surfaces compared to the other surfaces after 4 weeks. It was concluded that alkali-heat treated implants provided earlier stable fixation than control implants. Nishiguchi et al -01 [67] compared titanium and titanium alloy implants with and without alkali-heat treatment (5M NaOH 60ºC 24h + 600ºC 1h). Histomorphometric evaluations and push out tests were performed after 4 and 12 weeks in dog femur. Alkali-heat treated implants showed direct bone-implant contact; while alkali treated, implants demonstrated an intervening fibrous capsule. After 4 weeks, the heat-treated surfaces demonstrated increased push out shear strengths compared to untreated surfaces. However, after 12 weeks the untreated implants demonstrated a catch up compared to the treated implants. Nishiguchi et al -03 [66] compared titanium and alkali-heat treated implants (5M NaOH 60ºC 24h + 600ºC 1h). Mechanical and histomorphometrical evaluations were performed after 3, 6 and 12 weeks in the rabbit femur. Alkali-heat treated implants demonstrated increased bone-implant contact and increased bonding strengths (pull out test) compared to untreated surfaces at all evaluation times.

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Fujibayashi et al -01 [71] evaluated the effectiveness of sodium removal from alkali-heat treated titanium surfaces, where CP titanium were used as controls. The in vivo detaching failure load was evaluated after 4, 8, 16 and 24 weeks in rabbit tibia. Thereafter, the surfaces were evaluated by SEM. It was concluded that sodium removal accelerated bone bonding because of the anatase structure. However, the adhesive strengths decreased for the sodium free surfaces. Fujibayashi et al. -04 [72] compared ectopic bone formation of porous (plasma-spraying) and mesh titanium surfaces with and without alkali-heat treatment (sodium removed). Evaluations were performed in dog muscle after 3 and 12 months. In a parallel experiment, the surfaces were immersed in SBF for 7 days. The surfaces were evaluated by SEM and micro-CT/3D reconstruction. The porous alkali-heat treated surfaces demonstrated osteoinductive ability after 12 months. Takemoto et al -05 [60] compared macroporous titanium (plasma-spraying method) with and without alkali-heat treatment (5M NaOH 60ºC 24h + 600ºC 1h). The surfaces were characterized by micro-CT/3D reconstruction and SEM. Mechanical tests by means of compression strengths, four-point binding strengths and compressive fatigue strengths were performed of the surface. In vitro bioactivity was evaluated in SBF for 3-7 days and in vivo histomorphometric evaluation was performed after 2, 4, 8 and 16 weeks in rabbit femur. Apatite formation in vitro was apparent after 3 days on the alkali-heat treated surfaces, while no apatite could be detected after 7 days on the control surfaces. Bone-implant contact and bone-area in growth were significantly higher on alkali-heat treated implants at all evaluation times. In addition, the surface had mechanical properties sufficient for clinical use in load bearing conditions Takemoto et al. -06 [73] compared ectopic bone formation of alkali-heat treated porous titanium, alkali-heat treated (sodium removed by hot water) porous, and alkali-heat-treated (sodium removed by HCl and hot water) titanium surfaces. The surfaces were characterized by SEM-EDX and TF-XRD and evaluated in dog muscle after 3, 6 and 12 months. In a parallel experiment, the surfaces were immersed in SBF for 1, 3 and 7 days. The porous sodium free alkali-heat treated surfaces demonstrated osteo inductive ability after 3 months, while apatite formation could be seen on all surfaces after 1 day. Isaac et al -08 [63] compared titanium and alkali-heat treated implants (5M NaOH 60ºC 24h + 600ºC 1h). SBF and a bone explant model (immunohistochemical staining, alkaline phosphatase histoenzymatic localization and SEM after) were used to evaluate the surfaces after 3 and 15 days respectively. Results demonstrated bone-like apatite layer on the modified surface in simulated body fluids. Furthermore, that cells from frontal and parietal bones from 21-day-old rat fetuses can migrate from the explants and subsequently differentiate to form a mineralized nodular structure. The cells expressed alkaline phosphatase, bone sialoprotein, osteocalcin and the transcription factor, Runx2.

Clinic So far, there are no commercially alkali-heat treated dental implant systems available.

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2.3. Anodized CP Titanium Surfaces Electrochemical modification of titanium surfaces related to implant research has been performed since the 1970s. The process called anodic spark discharge (ASD) was proposed by Kurze and co-workers and was further described by Ishizawa and co-workers [77-79]. Anodized titanium surfaces have been extensively evaluated in vitro [37, 72, 80-86], in vivo [77, 78, 87-113]. There are some commercially available implant systems as well, with TiUnite™ (Nobel Biocare, Gothenburg, Sweden) so far dominating the market. Anodized TiUnite™ implants have been clinically evaluated in approximately 50 articles since their launch in 2001 where the longest follow up period is 5 years [130]. This implant system is however not claimed to be bioactive, instead the good results is explained by the topography. Since the oxide properties can be controlled by anodic forming voltage, current density, electrolytes, electrolyte concentrations and temperature, agitation speed etc., the resulting surfaces present heterogeneous characteristics by means of surface chemistry, oxide thickness, morphology, surface roughness, pore configurations (pore size, porosity, pore density and crystal structure) [114, 115]. In vitro studies have demonstrated various results with either increased [72, 80] or decreased [37, 81, 82, 85] bone cell attachment, increased [82, 85, 86] or decreased [37, 81] differentiation and decreased protein production [37] compared to control surfaces. In vitro inflammatory response show increased cell adherence despite similar cytokine production and differentiation [83]. In general but with some exceptions [88, 90, 97, 100], the anodized surfaces demonstrate increased bone response compared to control titanium surfaces in vivo [87, 93, 98, 105, 106, 108, 112, 116]. This is attributed to the changes of topography, but also the oxide thickness, pore configurations and crystal structure of the oxide layer, where an oxide thickness of > 600 nm has demonstrated to be favorable [105, 106, 108]. When incorporating certain ions i.e. calcium [104] and magnesium [101-103, 109, 111, 117], the increased bone response has been attributed to chemistry and a potential biochemical bond. Indications of biochemical bonding (bioactivity) has been proposed on the basis of ultrastructural analysis of interfacial fracture (scanning electron microscopy-SEM), ion movement/exchange at the interfacial tissue (X-ray microanalysis-EDS), speed and strength of implant integration to bone (removal torque-RTQ) [101, 102, 111, 117] and increased bone implant contact (BiC) [113]. Calcium [118, 119] and magnesium [37, 45] incorporated anodized surfaces have additionally increased apatite formation in SBF [120] and when adding proteins to the SBF the apatite formation and protein content increased on the possibly bioactive titanium surfaces compared to blasted control [46]. Furthermore, an additional hot water treatment could contribute to increased apatite formation, enhanced bonding strengths between apatite layer and metal [121], increased differentiation and protein production in vitro [84]. Recently Biolin AB (Gothenburg, Sweden) launched OsPol™, an implant system with a calcium reinforced possibly bioactive surface.

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Appendix – Anodized CP Titanium Surfaces

General Ishizawa et al. -95 [79] compared anodized titanium surfaces prepared with different anodic voltage 150-400 V (50mA/cm2), electrolytes and concentrations. Spark discharge occurred at 200V. The surfaces were characterized by SEM, EDX and XRD. Calcium acetate monohydrate and ß-glycerophosphate turned out to be suitable electrolytes, since the resulting Ca/P had a ratio equivalent to HA. HA crystals were precipitated by an additional heat treatment. Hall and Lausmaa -00 [122] introduced an anodized surface that later resulted in the commercially available TiUnite. The surfaces were characterized by Optical Interferometry, SEM, AES and XRD. The surface had a roughness of 1,2 µm (Ra), an oxide thickness of 1-2 µm at the cervical part and 7-10 µm at the apical part, a pore size in the range of 1-2 µm. The surface contained 15% Ti, 55% O, 20% C, 5% P, 1% S and 1% Si. Furthermore, it was demonstrated that the oxide layer strongly adhered to the underlying metal. Sul et al. -01 [114] compared the oxide growth behavior on titanium surfaces in acid and alkaline electrolytes with different electrolyte concentrations, temperature (14-42°C), anodic forming voltage (20-130V), current forming density (5-40 mA/cm2), and agitation speed (250-800 rpm). The formed oxide surfaces were thoroughly characterized by AES and a Spectrophotometry system. It was concluded that colors were useful for thickness determination of titanium oxide and that each electrolyte presented an individual growth constant nm/V. Furthermore, a general trend that increased electrolyte concentration and temperature decreased anodic forming voltage, anodic forming rate and the current efficiency, while an increased current density and surface area ratio anode/cathode increased anodic forming voltage, anodic forming rate and current efficiency. The effects of electrolyte concentration, temperature and agitation speed were explained by the electrical double layer. Sul et al. -02 [115] prepared anodic oxides by galvanostatic mode in acetic acid up to dielectric break down and spark formation (100-400V). The surfaces were characterized by Profilometry, AES, SEM, XPS, TF- XRD and Raman Spectroscopy. The results demonstrated a well characterized surface regarding surface roughness, oxide thickness, poresize and distribution, chemical composition and crystal structure. Crawford et al. -07 [123] prepared titanium surfaces with nanotubes by anodic oxidation using NaF electrolyte. The surface was characterized by field-emission scanning electron microscope (FE-SEM) and mechanical properties of the coatings were probed by nanoindentation. Results demonstrated that increased anodization time had no effect on tube diameter or tube wall thickness. However, coating thickness increased with time up to 2 h of anodization, at which point an equilibrium thickness was established. Progressively higher values of elastic modulus were obtained for thinner films.

SBF Yang et al. -04 [119] compared anodized titanium surfaces prepared in an electrolyte (H2SO4) with different concentrations (0,5-3M) anodic forming voltage (90-180V), with and without subsequent heat treatment (600ºC 1h). The surfaces were characterized with SEM and TFXRD. A simulated body fluid was used to evaluate the CaP nucleation capacity of the

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surfaces after 3 and 6 days. Apatite forming ability could be attained at 3 and 6 days by anodic oxidation > 90V and < 90V co-joined with heat treatment. Both the anatase and rutile was effective for apatite formation. No apatite formed on the surfaces without spark discharge (<90V) and heat treatment indicating that a certain thickness of the titanium oxide was required for apatite formation. Vanzilotta et al. -06 [118] compared CaP nucleation capacity in SBF of three surface modifications; etching and etching followed by either anodization or heat treatment. The surfaces were characterized by Profilometry, SEM-EDX and AAS, XPS before and after SBF soaking, respectively. The Ca ion concentration decreased in the SBF solution for all surfaces from day 1 to day 7. The heat treated and anodized surfaces demonstrated increased CaP nucleation capacity compared to the etched surfaces, while no differences were detected between the anodized and heat treated surfaces. Arvidsson et al. -07 [45] compared four types of possibly bioactive surfaces; a blasted surface prepared by alkali heat treatment, anodization (Mg ions incorporated), fluoridation or hydroxyapatit coating, where the blasted surface served as control. Surfaces were analyzed by weight, Profilometry, SEM/EDX and XPS after immersion in SBF for 1, 2, 3, 4 and 6 weeks. The results demonstrated that the Ca/P mean ratio of all the surfaces was approximately 1.5 after 1 week except for the fluoridated specimens which displayed mean ratio of approximately 2. All surfaces showed the presence of hydroxyapatite after 4 and 6 weeks of immersion, but a higher degree of crystallinity at 6 weeks. It was concluded that differences appeared at the early SBF immersion times of 1 and 2 weeks between controls and bioactive surface types, as well as between different bioactive surface types. Cui X et al. -08 [120] prepared titanium surfaces by anodic oxidation in four different electrolytes: sulfuric acid, acetic acid, phosphoric acid and sodium sulfate solutions with different voltages (1min at room temperature). The surfaces were immersed in SBF for 1, 3 and 7 days and analyzed in TF-XRD and; FE-SEM. Results demonstrated that the anodic films consisted of rutile and/or anatase phases with porous structures on titanium metal after anodizing in H(2)SO(4) and Na(2)SO(4) electrolytes, while amorphous titania films were produced after anodizing in CH(3)COOH and H(3)PO(4) electrolytes. Moreover, titanium metal with the anatase and/or rutile crystal structure films showed excellent apatite-forming ability and produced a compact apatite layer covering all the surface of titanium after soaking in SBF for 7d, but titanium metal with amorphous titania layers was not able to induce apatite formation. Cui X et al. -08 [121] prepared titanium surfaces by anodic oxidation in acetic acid followed by hot water and heat treatments to transform titania layers from an amorphous structure into a crystalline structure. The apatite-forming ability of titania layers in simulated body fluid was investigated by XRD and SEM. Results indicated that hot water and/or heat treatment transformed the crystal structure of titania layers from an amorphous structure into anatase, or a mixture of anatase and rutile. It was suggested that abundance of Ti-OH groups formed by hot water treatment could contribute to apatite formation on the surface of titanium metals, and subsequent heat treatment would enhance the bond strength between the apatite layers and the titanium substrates. It was concluded that bioactive titanium metals could be prepared via anodic oxidation and subsequent hot water and heat treatment made them suitable for applications under load-bearing conditions.

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Franke-Stenport et al. -08 [46] compared four types of possibly bioactive surfaces; a blasted surface prepared by alkali heat treatment, anodization (Mg ions incorporated), fluoridation or hydroxyapatit coating, where the blasted surface served as control. Surfaces were analyzed by Profilometry, SEM/EDX and XPS after immersion in SBF with 4.5 mg/ml albumin for 3 days, 1, 2, 3 and 4 weeks. The results demonstrated that all the bioactive surfaces initiated an enhanced calcium phosphate (CaP) formation and a more rapid increase of protein content was present on the bioactive surfaces compared to the blasted control surface. It was concluded that this might be an advantage in vivo.

In Vitro Takabe et al.-00 [80] compared anodized-heat treated titanium surfaces (CA/ß-GP) and titanium controls. The surfaces were characterized by Profilometry and Contact Angle Measurements. Initial cell (Rat Bone Marrow Stromal Cells) attachment, morphology and cytoskeleton were evaluated after 30, 60 and 120 minutes by Coulter counter (electrical conduction), SEM and CLSM, respectively. The anodized-heat treated surfaces were rougher, more hydrophilic and demonstrated increased cell attachment after 60 and 120 minutes compared to the controls. The cells showed a flattened surface with irregular edges and extended filipodium-like processes intimately adapted to the crystals of the surface. The actin filament was arranged parallel to the long axis of the cells and localized in the periphery. Rodriguez et al. -03 [84] compared 3 groups of anodized titanium surfaces (CA/ß-GP); with and without additionally heat treatment for 2 and 4 hours. The surfaces were characterized by SEM, XRD, Profilometry and EPMA. Cells (human embryonic palatal mesenchymal cells- HEPM) were used to evaluate differentiation (ALP, osteocalcin) and protein production over an 8-day period (0, 4, 8 days). Results demonstrated that mineralization, differentiation and protein production increased after hydrothermal-treatment. Li et al. -04 [82] compared anodized-heat treated titanium surfaces (CA/ß-GP) with various anodic forming voltage (190-600V). The surfaces were characterized by Optical Interferometry, SEM-EDS, XTEM and XRD. Cell adhesion (MG-63 and Human Osteosarcoma Cells/HOS) after 3 days, proliferation after 7 days and differentiation after 10 days were evaluated by SEM, Hemocytometry and Spectrophtometry (ALP activity), respectively. The surface roughness, oxide thickness and concentration of Ca and P ions increased with increasing voltage. In addition, there was a phase change from anatase to rutile. As a result the differentiation increased (>300 V), while the proliferation decreased (>190V). Preliminary results in vivo indicated increased removal torque values after 4 weeks for the anodized surfaces (270V). Zhu et al. -04 [81] compared anodized titanium surfaces prepared in different electrolytes (CA/ß-GP and H2PO4) and anodic forming voltage (140-350V). The surfaces were characterized by SEM, Profilometry, XPS and Contact Angle Measurements. Cells attachment and spread (SaOS-2) after 1 and 2 hours, proliferation and differentiation after 1, 2 and 4 days were evaluated by immunohistochemistry (vinculin, phalloidin), Hemocytometry and Spectrophotometry (ALP activity), respectively. Cell attachment and proliferation increased with increasing voltage, while differentiation was similar or decreased. The cells on the anodized surfaces demonstrated a polygonal growth and lamellipodia, reflecting high motility, while the control demonstrated thick stress fibers and intense focal contacts.

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Kim et al. -04 [124] compared turned and anodized titanium surfaces (CA/ß-GP, 270V). The surfaces were characterized by Optical Interferometry, SEM and XRD. Cell (MG-63) adhesion and gene expression were evaluated after 12, 24 and 48 hours by Spectrophotometry (Crystal Violet) and Microarray technique, respectively. The anodized surfaces were rougher and displayed increased attachment of MG-63 osteoblast like cells without significantly affecting the gene expression. Kim et al. -06 [86] prepared titanium surfaces by anodic oxidation (CA/ß-GP. The surfaces were characterized by scanning electron microscopy, X-ray diffraction, and electron probe microanalysis. Osteoblast were used to evaluate the cell differentiation. Results demonstrated that osteoblast differentiation (ALP), increased on the anodized surfaces. It was concluded that the phenotypic expression of osteoblast was enhanced by the presence of Ca phosphate and higher roughness on anodized surfaces. Vanzilotta et al. -06 [118] compared CaP nucleation capacity in SBF of three surface modifications; etching and etching followed by either anodization or heat treatment. The surfaces were characterized by Profilometry, SEM-EDX and AAS, XPS before and after SBF soaking, respectively. The Ca ion concentration decreased in the SBF solution for all surfaces from day 1 to day 7. The heat treated and anodized surfaces demonstrated increased CaP nucleation capacity compared to the etched surfaces, while no differences were detected between the anodized and heat treated surfaces. Göransson et al. -06 [83] compared titanium surfaces prepared by a turned, blasted, anodized and anodized surface with Mg ions incorporated. The surfaces were characterized by Optical Interferometry. The inflammarory response was evaluated by cellnumber (human mononuclear cells), viability (LDH), cytokineproduction (TNF-α, IL-10) and differentiation were analyzed after 24h and 72 hours. The result demonstrated that the anodized surfaces with and without Mg ions incorporated increased cell adherence, despite the anodized Mg ion incorporated surface having a smoother character, however no differences in cytokine production and differentiation between the surfaces. For all surfaces the viability was good at both 24 and 72 hours and cytokine IL-10 production remained over time while TNF-α and cellnumber decreased. Das et al. -07 [125] prepared titanium surfaces with; nanotubes by anodic oxidation using different electrolyte solutions, H[3)PO(4), HF and H(2)SO(4). The surface were characterized by field-emission scanning electron microscope (FE-SEM) fitted with an energy dispersive spectroscopy (EDS), Glancing angle X-ray diffraction (GAXRD), profilometry and contact angle measurement. Bone cells (osteoblastic precursor cell line -OPC1) were used to study cell adhesion (Vinculin, - confocal scanning laser microscopy) and proliferation (MTT assay) and differentiation (alkaline phosphatase) after 3, 5 and 11 days. The surfaces were additionally immersed in simulated body fluids for 3, 7, 14, and 21 Results demonstrated distinctive cell-to-cell attachment in the HF anodized surface, cellular adherence with extracellular matrix extensions in between the cells was noticed for samples anodized with H(3)PO(4) electrolyte. The TiO(2) layer grown in H(2)SO(4) electrolyte did not show significant cell growth on the surface, and some cell death was also noticed. Cell adhesions and differentiation were more anodized surfaces.

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Das et al. -08 [126] prepared titanium surfaces with; nanotubes by anodic oxidation citric acid, sodium fluoride, and sulfuric acid as electrolyte solution with and without an additional anodic oxidation in silver nitrate solutions The surface were characterized by field-emission scanning electron microscope (FESEM) fitted with an energy dispersive spectroscopy (EDS), Glancing angle X-ray diffraction (GAXRD), profilometry and contact angle measurement. Bone cells (osteoblastic precursor cell line -OPC1) were used to study cell adhesion and proliferation (MTT assay) after 5 and 11 days. The antibacterial effect was studied using Pseudomonas aeruginosa. Results demonstrated that silver-treated titania nanotube surfaces provided antibacterial properties to prevent implants against postoperative infections without interference to the attachment and proliferation of bone tissue on titanium Bose et al. -08 [127] prepared titanium surfaces with; nanotubes by anodic oxidation (citric acid, sodium fluoride, and sulfuric acid as electrolyte solution with and without an additional anodic oxidation in silver nitrate solutions), Tricalcium phosphate (TCP) coatings by LENS™ processing with different laser power and with powder having particle size ranging from 45 to 150 μm. A titanium surface served as control. The surfaces were characterized by (FE-SEM) (EDS), (GAXRD), profilometry and contact angle measurement. Bone cells (OPC1) were used to study cell adhesion (Vinculin) and proliferation (MTT assay) and differentiation (alkaline phosphatase) after 3, 7 and 11 days. Additionally microhardness of the coating was analysed. Results demonstrated that anodic oxidation and laser processed TCP-coated Ti surface showed enhanced cell adhesion, higher proliferation and early differentiation in comparison to the control-Ti surface. The TCP coating hardness was significantly increased from the base metal and further increased as the volume fraction of TCP increased in the coating Das et al. -08 [128] prepared nanotubes on titanium surfaces by anodic oxidation using citric acid, sodium fluoride, and sulfuric acid as electrolyte solution (20 V for 4 h). The surface were characterized by field-emission scanning electron microscope (FE-SEM) fitted with an energy dispersive spectroscopy (EDS), Glancing angle X-ray diffraction (GAXRD), profilometry and contact angle measurement. Bone cells (osteoblastic precursor cell line -OPC1) were used to study cell adhesion (Vinculin, - confocal scanning laser microscopy) and proliferation (MTT assay) and differentiation (alkaline phosphatase) after 3, 7 and 11 days. The surfaces were additionally immersed in simulated body fluids for 3, 7, 14, and 21 days. The anodized nanoporous sample surfaces demonstrated increased cell adhesion, proliferation and differentiation. Apatite layer formation was non-uniform on the nanotube surface even after 21 days in SBF. De Angelis et al. -08 [85] compared three surfaces; titanium surfaces prepared with anodic sparc oxidation in (Ca/P –Ca electrolytes), alkali etched titanium and nontreated titanium. Cell (SaOS-2) attachment, morphology, viability, proliferation, metabolic activity, differentiation and mineralization were analysed by SEM (6, 24, 48 h and 4 days), Immunohistochemistry (1, 2, 4, 7days) and RT-PCR (4 and 7 days). Results demonstrated the

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prepared surfaces supported cell attachment, cell proliferation, and mineralization, revealing no cytotoxicity effects. The expression of differentiation markers on the anodized surface demonstrated that genes related to the proliferation phase (Collagen type I, Coll I; Cbfa-1) were early expressed, whereas genes related to the mineralization phase (alkaline phosphatase, osteopontin, bone sialo protein) increased with time. Furthermore, mineralization was increased on the anodized surface. Göransson et al. -08 [37] compared four types of possibly bioactive surfaces; a blasted surface prepared by alkali heat treatment, anodization (Mg ions incorporated), fluoridation or hydroxyapatit coating, where a blasted surface served as control. Surfaces were analyzed by Profilometry, and XPS after immersion in SBF for 12, 24 and 72 hours. Cells Primary (human mandibular osteoblast-like cells) were cultured on the various surfaces subjected to SBF for 72 h. Cellular attachment, differentiation (osteocalcin) and protein production (TGF-beta(1)) was evaluated after 3 h and 10 days respectively. The results demonstrated that the possibly bioactive surfaces gave rise to an earlier CaP formation than the blasted surface. Subsequent bone cell attachment was correlated to neither surface roughness nor the amount of formed CaP. In contrast, osteocalcin and TGF-beta(1) production were largely correlated to the amount of CaP formed on the surfaces

In Vivo Larsson et al. -94 [96] compared machined titanium surfaces and machined electropolished with and without anodization (1M acetic acid 10 and 80V). The surfaces were characterized by SEM, AES and AFM. The surfaces differed with respect to surface oxide thickness (17200 nm) and topography, although were similar with respect to surface composition. Boneimplant contact was evaluated in cortical bone in a rabbit model after 7 and 12 weeks. The results demonstrated decreased bone around the smooth electropolished surfaces compared to the machined surfaces with similar oxide thickness and anodized implants with thicker oxides after 7 weeks. It was concluded that a high degree of bone contact and formation were achieved by surface modifications with respect to oxide thickness and surface roughness. Furthermore, that a reduction in surface roughness influenced the rate of early bone formation. Ishizawa et al. -95 [77] compared anodized titanium surfaces prepared in an electrolyte (CA/ß-GP, 350V) with different concentrations and with and without a subsequent heat treatment (300ºC , 2h) in a rabbit model. The surfaces were characterized by SEM. Turned titanium and a solid HA surface were used as positive and negative controls, respectively. The push out strengths and bone apposition increased after 8 weeks on the anodized-heat treated surface and were equivalent to HA ceramics. Furthermore, the anodized implants without heat-treatment showed increased push out strengths and bone apposition compared to the turned control surfaces. It was concluded that the good hard tissue compatibility of the implant surfaces might be attributed to the surface roughness and the possibly inhibition of titanium ion release. Larsson et al. -96 [95] compared electropolished (smooth) and machined (rough) surfaces with (thick oxide) and without (thin oxide) anodization (1M acetic acid, 80V) after 1, 3 and 6 weeks. The surfaces were characterized by SEM, AES and AFM. At early stages, the smooth implants demonstrated decreased bone-implant contact compared to the machined implants

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irrespective of oxide layer thickness. At later stages, the thicker oxide layer increased the bone formation around the smooth surface, but not on the rougher machined surfaces. It was concluded that both topography, on the submicrometer scale, and the oxide thickness influenced the bone response to titanium surfaces. Furthermore, that reduction of surface roughness in the initial phase decreases the rate of bone formation. Larsson et al. -97 [94] compared electropolished (smooth) and machined (rough) surfaces with (thick oxide) and without (thin oxide) anodization (1M acetic acid, 80V) after 1 year. The surfaces were characterized by SEM, AES and AFM. It was demonstrated that there were no significant differences between the differently prepared implant groups after 1 year. It was concluded that a reduction of surface roughness from Rq 30 nm to 3 nm, which in the initial phase decreases the rate of bone formation, had no influence on the amount of bone after 1 year in rabbit cortical bone Ishizawa et al. -97 [78] compared anodized (CA/ß-GP) machined, grit-blasted and plasma-sprayed surfaces. A plasma-sprayed titanium surface and a solid HA surface were used as controls. The surfaces were characterized by SEM and XRD.Bone response was evaluated after 4 weeks in a dog model. The anodized blasted implant showed increased bone formation compared to the smooth surface. Furthermore, the thin HA layer demonstrated quantitatively the same osteoconduction as the solid HA surface, however, with differed qualitatively. Fini et al. -99 [88] compared etched (HF) titanium implants and anodized titanium implants (CA/ß-GP) prepared with and without heat-treatment. The surfaces were characterized by Profilometry, SEM, XRD and GD-OES. Histomorphometric analysis demonstrated increased bone contact for the etched and anodized-heat treated surfaces compared to the anodized surfaces after 4 weeks, while the anodized-heat treated surfaces showed the highest values after 8 weeks in a rat femoral model. Albrektsson et al. -00 [87] compared turned and anodized (TiUnite) titanium implants. The anodized implants demonstrated increased bone-implant contact and increased RTQ compared to the turned surfaces after 6 weeks in rabbit tibia and femur. Gottlow et al. -00 [92] compared double etched (Osseotite) and anodized (TiUnite) implants. The anodized surfaces demonstrated increased bone-implant contact and stability by means of RFA and RTQ measurements compared to Osseotite after 6 weeks in rabbit femur and tibia. Gottlow et al. -00 [91] compared double etched (Osseotite) and anodized (TiUnite) implants. The anodized implants demonstrated increased stability by means of RTQ after 10 weeks in dog mandible; however, there were no differences in bone-implant contact compared to the Osseotite implants. Sennerby et al. -00 [99] compared insertion torque and stability of double etched (Osseotite) and anodized (TiUnite) implants. The anodized surface demonstrated an increased insertion torque, however, no differences in stability (RFA) after 3 weeks in rabbit tibia. Henry et al -00 [93] compared stability of anodized (TiUnite) and turned implants after 10 weeks in dog mandible. The anodized implants demonstrated a significantly increased RTQ compared to turned implants. Rompen et al. -00 [98] compared stability of anodized (TiUnite) and turned surfaces after 3 and 6 weeks in dog mandible. It was concluded that the anodized implants maintained higher primary stability during 6 weeks of healing compared to the turned controls.

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Sul et al. -02 [108] compared anodized (acetic acid) and turned titanium implants with various oxide thicknesses (600-1000 nm and 17-200 nm, respectively) in rabbit tibia. The surfaces were characterized by Laser Scanning Profilometry, SEM, XPS, TF-XRD and AES. There were no differences in ALP and ACP activity between the surfaces with different oxide thickness. However, implants with an oxide thickness > 600 nm demonstrated increased bone-implant contact compared to the control surfaces. The increased bone response was ascribed the oxide properties including oxide thickness, pore size distribution, porosity and crystal structure. Sul et al. -02 [106] compared anodized (acetic acid) and turned titanium implants with various oxide thickness (20-1000 nm) in rabbit tibia. The surfaces were characterized by Laser Scanning Profilometry, SEM, XPS, Raman spectroscopy, TF-XRD and AES. Implants with an oxide thickness > 600 nm demonstrated increased RTQ values compared to thinner layers, though there were no significant differences in RFA between the surfaces. The increased bone response were ascribed the oxide properties including oxide thickness, micropore configuration and crystal structure. Sul et al -02 [104] compared calcium ion incorporated anodized implants and turned titanium implants in rabbit tibia and femur. The surfaces were characterized by Laser Scanning Profilometry, SEM, XPS, TF-XRD, AES. The surfaces varied with respect to chemical composition, crystal structure and porosity but demonstrated similar surface roughness. The anodized calcium reinforced surface demonstrated increased RTQ values bone-implant contact and mineralization of the new bone after 6 weeks compared to turned implants. The results were ascribed the chemical composition (the Ca ions) of the implant. Giaveresi et al -03 [90] compared etched (HF) titanium surfaces and anodized (CA/ß-GP) titanium surfaces prepared with and without heat treatment. Machined surfaces and a plasmasprayed HA surfaces were used as negative and positive control, respectively. The surfaces were characterized by Profilometry and SEM. After 8 weeks in sheep cortical bone the anodized and heat-treated surfaces showed increased push out force compared to the turned surface, while the etched surface showed decreased values compared to the machined surface. Highest values was demonstrated for the HA surface. Histomorphometric evaluation after 8 and 12 weeks revealed significantly decreased rates for the etched surface compared to the other surfaces. They concluded that there were no specific differences in behavior between the machined, anodized-heat-treated and HA surfaces. Giaveresi et al -03 [89] compared etched (HF) titanium surfaces and anodized (CA/ß-GP) titanium implant surfaces prepared with and without heat treatment. Machined surface s and a plasma-sprayed HA surfaces were used as negative and positive control, respectively. The surfaces were characterized by Profilometry and SEM. Histomorphometrical and micro hardness evaluations in sheep cortical bone after 8 and 12 weeks revealed that the anodizedheat treated surfaces had osteoconductive properties but it did not affect the surrounding bone in terms of bone remodeling or micro hardness. Liang et al -03 [97] evaluated bone bonding ability of anodized-heat treated implants (H2SO4, 155V + 600°C) in rabbit tibia after 4, 8, 16 and 24 weeks. The surface was characterized by FIB and FE-SEM. High bone bonding ability in early stages by means of deattaching tests was observed for the anodized titanium surfaces compared to sodium free alkali-heat treated surfaces used in another study. The lack of improvement of bone bonding ability at later stages compared to the alkali-heat surface was explained by the low porosity of the anodized surface and furthermore, superficial apatite deposition into the pores.

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Zechner et al -03 [110] compared machined, anodized (TiUnite) and HA coated (Replace) implants. The anodized implants demonstrated similar histomorphometrical results in mandible of mini-pigs 3, 6 and 12 weeks as the HA coated implants, and furthermore, an increased bone response compared to the turned implant. Son et al -03 [100] compared anodized titanium implants (CA/ß-GP, 350V, 70A/m2) prepared with and without heat treatment. The surfaces were characterized by SEM EPMA and XRD. Stability test and histomorphometrical evaluations were made after 6 and 12 weeks. There were no differences in bone-implant contact between the implants, however the anodized implants showed increased RTQ values at 6 weeks compared to the controls. Sul et al -05 [102] Compared anodized surfaces prepared with and without reinforced magnesium ions (anodized/Mg), although, with similar morphology. The surfaces were characterized by Optical Interferometry, SEM-EDS, XPS, TF-XRD. The anodized/Mg surfaces demonstrated increased RTQ values, fracture lines distant from the bone implant interface and ion concentrations gradient in rabbit femur after 6 weeks. It was concluded that this was positive evidence for the biochemical bonding theory. Sul et al -05 [109] compared magnesium ion incorporated anodized implants (anodized/Mg), and turned controls. The surfaces were characterized and evaluated by Optical Interferometry, SEM-EDS, XPS, TF-XRD. The anodized/Mg surface demonstrated increased RTQ and RFA values in rabbit femur after 6 weeks. The results were ascribed the chemical composition (the Mg ions) of the implants. Sul et al -05 [103] compared magnesium ion incorporated anodized implants (anodized/Mg), with various oxide thickness porosity, crystal structure and surface roughness. The implants were characterized by Optical Interferometry, XRD, XPS, SEM and AES. The highest removal torque values in rabbit tibia after 6 weeks were achieved with an oxide thickness of 1000-5000 nm, porosity of about 24%, surface roughness of about 0,8 µm Sa and 27-46 % Sdr and relative atomic Mg concentration of 9 %. Salata et al -06 [116] compared turned and oxidized titanium implants when placed in experimental bone defects with autogenous bone graft, with and without BMP-2. Results demonstrated no statistically significant differences between control and treated sites, for neither turned nor for oxidized implants by means of histomorphometry and implant stability tests (RFA) after 4 and 12 weeks in dog model. However, the oxidized implants demonstrated a significantly higher stability after 4 weeks compared to turned implants. It was concluded that oxidized implants gained stability more rapidly and integrate with more bone contacts than implants with a turned surface when placed in bone defects. Sul et al -06 [101] compared magnesium ion incorporated anodized implants (anodized/Mg), TiUnite and Osseotite implants in rabbit tibia after 3 and 6 weeks. The implants were characterized by Optical Interferometry, XRD, XPS and FE-SEM. The anodized/Mg surfaces demonstrated increased RTQ values compared to the SLA surface after 3 weeks, while the Osseotite implants demonstrated significantly deceased values after 6 weeks compared to the other surfaces. Furthermore, histomorphometrical evaluations demonstrated increased new bone formation for the anodized/Mg surfaces compared to the others after 3 and 6 weeks. It was concluded that the comparatively rapid and strong osseointegration of the anodized/Mg implants enhanced the possibility of immediate/early loading of clinical implants. Sul et al -06 [117] compared turned and magnesium ion incorporated anodized implants (anodized/Mg), in rabbit tibia after 3 and 6 weeks. The implants were characterized by

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Optical Interferometry, XRD, XPS, SEM and AES. The anodized/Mg implants demonstrated increased RTQ values compared to the turned surfaces after 3 and 6 weeks. Additionally, the rate of osseointegration was increased for the andodized/Mg surface compared to the turned surface at both evaluation times. Bonding failure mainly occurred at the interface of the turned surfaces and in the immature bone for the anodized/Mg surfaces. It was concluded that the rapid and strong integration of bioactive anodized/Mg implants might encompass immediate/early loading of clinical implants. Sul et al -08 [111]compared turned, blasted and anodized implants (anodized/Mg). The surfaces chemistry and topography were characterized by XPS, LV-SEM and Optical Interferometry respectively. The RTQ value was evaluated after 3 and 6 weeks in a rabbit tibiae model. The results demonstrated that in spite of a smoother surface the anodized Mg implants demonstrated significantly higher osseointegration strength compared with turned and blasted implants, whereas blasted implants showed significantly higher osseointegration than turned implants at 6 weeks but not at 3 weeks. It was concluded that this provided evidence for the biochemical bonding theory. Franco et al -08 [112] compared turned and anodized (anodized/Ca/P) implants. The surfaces were characterized by SEM-EDX. A dog model was used and histologic and histomorphometric analyses were performed after 8 weeks. Results demonstrated increased (although not statistically significant) bone-toimplant contact for the anodized (Ca/P) compared to turned controls. Fröjd et al -08 [113] compared titanium implants prepared by anodic oxidation with and without Ca ions incorporated, where a blasted titanium implants were used as control. The isurfaces were topographically characterized using an optical interferometer. Histomorphometric evaluation was made in a rabbit model after 12 weeks. Results demonstrated an increased bone contact for smooth but more densely peaked calciumincorporated oxidized implants when compared to slightly rougher oxidized or blasted implants.

Clinic There are some commercially available implant systems, with TiUnite (Nobel Biocare, Gothenburg, Sweden) so far dominating the market. Anodized TiUnite implants have been clinically evaluated in approximately 50 articles since their launch in 2001 where the longest follow up period is 5 years [129]. The surface has been used in poor bone and in early loading situations where it has demonstrated good results in general with a success rate of about 95%. Recently Biolin AB (Gothenburg, Sweden) launched an implant system Ospol, with this calcium reinforced possibly bioactive surface.

2.4. Thin HA Sol-Gel Coated CP Titanium Surfaces There are several techniques to produce thin (<10µm) HA coatings on titanium surfaces i.e. physical and chemical vapor deposition, covalent immobilization etc. An alternative approach to prepare HA coatings on titanium surfaces is sol-gel processing, which has been regularly used since 1950s. It has potential advantages such as reduced thickness, uniform composition and lower processing temperatures [130]. Furthermore, it is a simple and cheap method and efficient for coating complex shapes [130].The sol-gel process involves the transition of a

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system from a liquid, the (colloidal “sol") into a solid (the "gel") phase. With further drying and heat-treatment, the "gel" is converted into dense ceramic i.e. ceramic is built starting from its molecular components in solution by a carefully controlled condensation reaction. Thin films can be produced on a piece of substrate by spin-coating or dip-coating [130]. Titania sol-gel ceramics were the first coatings to be extensively studied for biomedical applications [131]. Studies of HA sol-gel coated CP titanium surfaces have so far mainly been performed in vitro [132-139]. HA has either been coated as a composite [134, 139] or as a double layer [136] with titania to obtain an increased bonding strength to the under lying substrate and furthermore to increase the corrosion resistance. Other approaches have been to coat the titanium surface with a composite [132] or double layer [135] of HA and FHA with the intention to improve integrity and longevity of the coating, since FHA has decreased solubility compared to HA. In vitro HA composites have demonstrated increased bone cell differentiation [132-137] and occasionally increased bone cell proliferation [135, 137, 138]. An in vivo study has demonstrated increased bone response by means of increased BiC and increased RTQ [140]. The thickness of HA coatings made by sol-gel so far has been in the range of microns (1-10µm). Furthermore, the HA crystals have been obtained by change in phase on the surface after an additional hydrothermal treatment. Recently a group at Chalmers University modified the technique to produce HA coatings of nanometer thickness (<150 nm). The coating was obtained from a micro-emulsion that contained highly crystalline HA particles in the size range of 10-15 nm. In order to control the crystal size and crystal structure, i.e. its apatite structure, surfactant self-assembly was utilized, and by changing parameters such as temperatures and surfactant concentrations it was possible to produce the desired phase of HA. The surface has demonstrated similar or increased apatite formation in SBF [37, 45, 46] and increased bone cell proliferation, differentiation and protein production [37] in vitro compared to other possibly bioactive surfaces. In vivo the nano HA surface demonstrated increased bone response (BiC, BA) compared to electropolished surface [141] however failed to do the same in a “gap model” [142] and compared to a titania coated surface with similar nanotopography [143]. So far, there is no commercially available implant system on the market with this sol-gel processed surface. In 2006 Biomet 3i launched a surface produced by ion beam assisted deposition of Calcium Phosphate NanoTite™ (Palm Beach Gardens, Florida), where clinical follow up studies have just started. Appendix - Thin HA Sol-Gel Coated CP Titanium Surfaces

SBF Arvidsson et al -07 [45] compared four types of possibly bioactive surfaces; a blasted surface prepared by alkali heat treatment, anodization (Mg ions incorporated), fluoridation or hydroxyapatit coating, where the blasted surface served as control. Surfaces were analyzed by weight, Profilometry, SEM/EDX and XPS after immersion in SBF for 1, 2, 3, 4 and 6 weeks. The results demonstrated that the Ca/P mean ratio of all the surfaces was approximately 1.5 after 1 week except for the fluoridated specimens which displayed mean ratio of approximately 2. All surfaces showed the presence of hydroxyapatite after 4 and 6 weeks of immersion, but a higher degree of crystallinity at 6 weeks. It was concluded that differences appeared at the early SBF immersion times of 1 and 2 weeks

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between controls and bioactive surface types, as well as between different bioactive surface types. Franke-Stenport et al -08 [46] compared four types of possibly bioactive surfaces; a blasted surface prepared by alkali heat treatment, anodization (Mg ions incorporated), fluoridation or hydroxyapatit coating, where the blasted surface served as control. Surfaces were analyzed by Profilometry, SEM/EDX and XPS after immersion in SBF with 4.5 mg/ml albumin for 3 days, 1, 2, 3 and 4 weeks. The results demonstrated that all the bioactive surfaces initiated an enhanced calcium phosphate (CaP) formation and a more rapid increase of protein content was present on the bioactive surfaces compared to the blasted control surface. It was concluded that this might be an advantage in vivo.

In Vitro Ramires et al -01 [139] compared sol-gel coated titanium surfaces with different titania/HA ratios (1:2, 1:1, 2:1). Titanium and TCP were used as controls. The surfaces were characterized by SEM-EDS. Cytotoxity tests performed demonstrated that the surfaces were biocompatible. The coated surfaces, especially titania/HA 1:1, demonstrated increased differentiation (ALP, collagen and osteocalcin) of MG-63 compared to the control, although with similar proliferation. It was concluded that titania/HA coating resulted in a bioactive surface due to presence of hydroxyl groups promoted Ca/P precipitation that improved the interactions with osteoblastic cells. Kim et al -04 [132] compared sol-gel coated (5µm) titanium surfaces with different FHA/HA ratios (FHA/HA ratios = 0, 0.25, 0.5, 0.75). TCP and titanium were used as controls. The surfaces were characterized by SEM, XRD and ICP-AES. Increased concentration of FHA demonstrated lower dissolution rate after 14 days in SBF. An increase in FHA demonstrated a slight decrease in proliferation of MG-63 cells after 5 and 7 days. However, FHA/HA coatings increased differentiation (ALP) of HOS cells after 10 days compared to the control surfaces. It was concluded that the sol-gel coated surfaces improved the cell functions. Kim et al -04 [136] compared HA sol-gel coated (1 µm) titanium surfaces with and without an intervening titania layer hypothesized to increase bonding strengths to the underlaying substrate and decrease the corrosion rate. Titanium and Thermanox were used as controls.The surfaces were characterized by SEM and XRD. Adhesion tests demonstrated that the double layer with titania increased the bonding strengths compared to a single layer of HA. Titania/HA coated surface demonstrated similar proliferation and increased differentiation (ALP activity) of HOS cells compared to control surfaces and titania coatings. Kim et al -04 [135] compared titanium surfaces coated (0,6-8 µm x2) with double layer of FHA (550ºC 30 min) and HA (550ºC 30 min) prepared by sol-gel treatment Titanium and HA were used as controls. The surfaces were characterized by XRD, FT-IR, ICP-AES, SEM and Laser Scanning Profilometry. The FHA/HA surface demonstrated biphasic dissolution behavior. The FHA/HA, FHA and HA surfaces demonstrated similar attachment after 2 and 6 hours and increased proliferation of HOS cells after 1, 3, 5 and 7 days compared to controls. Furthermore, differentiation (ALP,osteocalcin) was significantly increased after 5, 10 and 14 days on the coated surfaces compared to the CP titanium controls.

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Kim et al -05 [133] compared HA sol-gel coated (1 µm) titanium prepared at different temperatures (400, 500, 600ºC) and with different rate of heating (1 or 50ºC/min). The surfaces were characterized by XRD, FT-IR, ICP-AES, SEM-EDS and Laser Scanning Profilometry. The HA films treated with higher temperature increased crystallinity, lower dissolution rate compared to controls and surfaces prepared with lower temperature, however similar roughness. Cell (HOS) attachment after 2 and 6 hours and proliferation after 2, 4 and 7 days were similar on control and coated surfaces. There was a slight increase in differentiation after 7 and 14 days (ALP, Osteocalcin) on the HA films compared to the controls. Increased heating rate demonstrated a 4-6 times rougher surface with a slightly higher dissolution rate, enhanced cell attachment, but no difference in differentiation. Kim et al -05 [134] compared sol-gel coated (<1 µm) titanium surfaces with different titania/HA ratios (10, 20, 30, 40 mol %). HA and titanium were used as controls. The surfaces were characterized by XRD, SEM-EDS and Laser Scanning Profilometry. Adhesion tests showed that the HA/titania composites showed increased binding strengths (56 MPa) compared to HA (35 MPa). The composites demonstrated similar proliferation of HOS cells after 1, 3 and 5 days compared to control and HA. Furthermore, increased differentiation (ALP) after 7 days compared to the other surfaces with the highest concentrations obtained by 20% titania/HA. Sato et al-05 [138] compared PLGA/titania sol-gel coated titanium prepared with and without HA and subsequent heat treatment. The surfaces were characterized by XRD, EDS, AFM, ICP-AES SEM and Contact angle measurements. The adhesion of MG-63 cells were similar on the sol-gel HA compared to traditional HA surface and heat-treatment improved the adhesion. Li et al -05 [137] compared anodized titanium surfaces with and without an additional HA so-gel coating with different concentrations. The surfaces were characterized by XRD, SEM-EDS and Laser Scanning Profilometry. Sol-gel treatment increased Ca and P concentration, while roughness did not change. The HA coated surfaces demonstrated an increased proliferation after 5 days and differentiation after 5 days (ALP) compared to the anodized surfaces. Göransson et al -08 [37] compared four types of possibly bioactive surfaces; a blasted surface prepared by alkali heat treatment, anodization (Mg ions incorporated), fluoridation or hydroxyapatit coating, where a blasted surface served as control. Surfaces were analyzed by Profilometry, SEM and XPS after immersion in SBF for 12, 24 and 72 hours. Cells Primary (human mandibular osteoblast-like cells) were cultured on the various surfaces subjected to SBF for 72 h. Cellular attachment, differentiation (osteocalcin) and protein production (TGF-beta(1)) was evaluated after 3 h and 10 days respectively. The results demonstrated that the possibly bioactive surfaces gave rise to an earlier CaP formation than the blasted surface. Subsequent bone cell attachment was correlated to neither surface roughness nor the amount of formed CaP. In contrast, osteocalcin and TGF-beta(1) production were largely correlated to the amount of CaP formed on the surfaces

In Vivo Ramires et al -03 [140] compared titanium surfaces coated (<10 µm) HA/titania and BG/titania by sol-gel treatment (550ºC 30 min).

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The surfaces were characterized by a Laser Scanning Profilometry. In vitro and in vivo results were in good agreement. In vitro results showed increased differentiation of MG-63 cells (ALP, osteocalcin , collagen) compared to control, while in vivo evaluation showed increased RTQ and bone-implant contact compared to the titanium control in rabbit femur and tibia after 12 weeks. Meirelles et al -08 [141] compared electropolished titanium implants with and without nanosize hydroxyapatite particle coatings. The electropolishing removed the microstructures from the surface to ensure that bone response observed was dependent only on the nanotopography and/or chemistry of the surface. The surfaces were characterized topographically and chemically by an optical interferometer, AFM and XPS, respectively A rabbit model (tibia) was used and histological evaluation demonstrated significantly increased bone formation to the coated compared to uncoated implants after 4 weeks of healing. It was concluded that early bone formation is dependent on the nanosize hydroxyapatite structures, however questioned remained whether it was an isolated effect of the chemistry or nanotopography or a combination of both. Meirelles et al -08 [142] compared electropolished titanium cylinders with and without nanosize hydroxyapatite particle coatings. The electropolishing removed the microstructures from the surface to ensure that bone response was dependent only on the nanotopography and/or chemistry of the surface. The surfaces were characterized topographically and chemically by an optical interferometer, AFM and XPS, respectively. A gap-healing model in rabbit tibia was used and the surgical site was 0.7 mm wider than the implant diameter. Histological evaluation (BiC, BA) demonstrated similar bone formation for the nano HA and electropolished implants after 4 weeks of healing. It was concluded that the results do not support that nano-HA chemistry and nanotopography will enhance bone formation when placed in a gap-healing model. Meirelles et al -08 [143] compared electropolished titanium cylinders with nano-HA coating or nano-titania coating. The electropolishing removed the microstructures from the surface to ensure that bone response was dependent only on the chemistry of the surface. The surfaces were characterized topographically and chemically by an optical interferometer, AFM and XPS, respectively. A rabbit model was used and histological evaluation (BiC, BA) was made after 4 weeks. The results demonstrated that nano-titania-coated implants showed an increased BA a tendency to increased BiC compared to nano-HA implants. It was concluded that there were no evidence of enhanced bone formation to nano-HA-modified implants compared to nanotitania-modified implants.

Clinic So far, there are no commercially available Nano HA coated dental implant systems. In 2006 Biomet 3i launched a surface produced by ion beam assisted deposition of Calcium Phosphate NanoTite™ (Palm Beach Gardens, Florida), where clinical trials have just started.

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2.5. Proteins Covalently Immobilized to CP Titanium Surfaces The aim of biochemical surface modifications is to induce specific cell and tissue response, i.e. to control the tissue implant interface. The biomolecules are administrated to the surface either by adsorption or by covalent immobilization. Covalent immobilization is hypothesized to provide an opportunity to align and expose the appropriate active site for a more rapid physiological response [144]. Furthermore, the coupling and cross linking is suggested to suppress the fast in vivo diffusion and degradation of the coating and thereby decrease the quantity required [144]. Polymeric substrates have been extensively studied because they possess abundant reactive functional groups for coupling, studies have been performed mainly in cardiovascular models, where the impact of interactions may be more obvious, with implications e.g. for the blood contact activation. Early surface modifications of titanium surfaces were concerned with improvement of adhesion of soft tissue to provide a stable seal towards bacterial invasion. Approaches in hard tissue implant research involve immobilization e.g. of the peptides (RGD), extra cellular matrix proteins (collagen) and growth factors (BMPs). Modifications of metallic implants with RGD containing peptides have been performed by covalent immobilization, while proteins like collagen and growth factors have mostly been carried out by adsorption immobilization or embedding in degradable polymers such as PLGA. A commonly used technique to covalently attach peptides and proteins to CP titanium surfaces is silanization (amino-functionalization) [145, 146] with additional coupling agents such as glutaraldehyde [147-149], IPN [150, 151] or Star-PEG [152]. Other techniques presented are based on polymers (PLL-g-PEG) [153, 154], multimeric phophonates [155] and gold-thiol chemistry [156, 157]. RGD is an amino acid sequence (Arg-Gly-Asp) in many ECM proteins that has been identified to mediate cell attachment to the integrin receptors. Binding activates signaling pathways, which are able to stimulate different cell functions like migration, proliferation, differentiation or matrix mineralization. Some in vitro studies have demonstrated an increased differentiation and matrix mineralization with RGD coated titanium implants [150, 151, 157] , while others have failed to demonstrate convincing results [152, 153, 158]. Furthermore, RGD coated titanium implants have demonstrated early increased bone formation but no additional increase in stability in vivo [154, 156]. Collagen is the main component of ECM of bone and serves as scaffold for mineralization. Covalently immobilized collagen has been shown to increase bone cell response in vitro [159] and bone response in vivo [160-163]. On the contrary, it has been shown that native ECM produced by bone cells in vitro on titanium surfaces increased cell densities compared to covalently immobilized peptides and protein coating [164]. Few studies has dealt with covalently immobilized growth factors to titanium surfaces and there is no [165] or weak [166] evidence of increased bone response in vitro and in vivo. Bone cell attachment, collagen and growth factor production, however, appear in later stages of the healing phase.

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In the present thesis attempts were made to intervene in earlier stages i.e. the inflammatory phase. Covalent immobilization of blood proteins such as fibrinogen and blood plasma on titanium surfaces has so far only been studied in vitro [167] and in vivo soft tissue models [147-149]. Appendix – Proteins Covalently Immobilized to CP Titanium Surfaces

In Vitro Peptides Xiao et al -97 [145] were among the first to covalently immobilize (APTES, SMP) the RGDC peptide to CP titanium. The individual steps were semi-quantitatively characterized by XPS, Radio Labeling Techniques and Ellipsometry. It was concluded that the achieved surface coverage was expected to be enough for specific surface-cell interactions. Barber et al -03 [151] covalently immobilized (APTES/Interpenetrating polymer network - IPN) RGD from Rat bone sialoprotein. The surfaces were characterized by Contact Angle Goniometry and XPS. Primary rat calvarial osteoblasts showed decreased attachment and spreading after 4, 24, 68 and 142 hours compared to TCPS and titanium, but significantly higher mineralization after 28 days compared to control titanium and the negatrive control surfaces IPN-RGE. It was concluded that the RGD surfaces to a greater extent promoted a more mature osteoblast than the titanium controls. Barber et al -06 [150] covalently immobilized (ATC/IPN) RGD from Rat Bone Sialoprotein with different concentrations. The surface was characterized by Fluorometry and XPS. Primary rat calvarial osteoblast attachment, spreading and differentiation were evaluated. The results demonstrated that surfaces with higher RGD densities (> 0.1 pmol/cm2) showed significantly higher mineralization (ALP) at later stages compared to titanium, RGD with lower densities and IPN-RGE. It was concluded that RGD peptide coated surfaces with certain densities enhanced the kinetics of differentiation. Huang et al -03 [157] compared two types of covalently immobilized gold SAM) peptides, RGDC and RDGC. XPS and FTIR were used for surface characterization. Primary calvarial osteoblasts were cultured and cell attachment, morphology, proliferation, and gene expression were assessed by Coulter counter, Immunofluorescence, Coulter counter and Northern blot, respectively. Four and 8 hours after culture, cell attachment was enhanced on the peptide surfaces compared to the control. Furthermore, osteoblasts on RGDC surfaces showed earlier osteocalcin mRNA expression (day 15) compared with controls (day 21). It was concluded that osteoblasts functions were enhanced on the RGDC coated surface, which might be effective in improving osseointegration for dental implants. Senyah et al -05 [158] compared attachment and morphology of MC3T3 –E1 osteoblast like cells on different immobilized (APTES) RGD peptides after 48 hours. The surfaces were characterized by AFM, SEM and CLSM.

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Results demonstrated that the RGD sequence is not necessarily required to enhance adhesion of cells. Instead, the attachment was correlated to surface hydrophobicity caused by the different RGD peptides. Auernheimer et al -05 [155] presented a method for covalently immobilizing tailor made cRGD peptides through multimeric phophonates to titanium surfaces. Characterization was made by Radio labeling and ELISA. Several advantages of the technique were presented; stable against enzymatic degradation, no risk of disease transmission, no risk of thrombosis formation. Groll et al -05 [152] evaluated covalently immobilized (Star-PEG) linear RGD coatings with different concentrations. The surfaces were characterized by Scanning force microscopy, Ellipsometry and Contact angle measurements. Adhesion and spreading of fibroblasts, SaOS and hMSC cells were recorded up to 30 days on the RGD surfaces, whereas no cell adhesion could be detected on unmodified Star PEG surfaces. Furthermore, increased RGD concentrations increased the cell amount and spreading. Since differentiation of the hMSC cells after 14 days were comparable with TCPS, it was concluded that the PEG/RGD films did not negatively influence the differentiation process. Schuler et al -06 [153] compared cell adhesion and spreading patterns of epithelial cells, fibroblasts and osteoblasts on covalently immobilized (PLL-g-PEG) RGDSP with different concentrations, RDG and titanium control (SLA). The surfaces were characterized by AFM, SEM and XPS. Fibroblast attachment increased on the smooth surfaces, while there was an opposite tendency for the osteoblasts. The epithelial cells did not follow any regular pattern. In general, attachment and spread increased on the RGD containing surfaces compared to RDG and PEG. Furthermore, osteoblast attachment increased with increasing RGD concentrations resulting in similar attachment as control titanium at densities ≥ 0,67 pmol/cm2.

In Vitro Proteins Nanci et al -97 [144] presented a method to covalently immobilize (APTES, glutaraldehyde) ALP and albumin to titanium surfaces. The surfaces were characterized by SEM, XPS and AFM. Evaluation by immunohistochemistry revealed that the proteins were linked at biologically relevant densities, and retained their enzymatic activity and antigenicity. Jennissen et al -99 [146] presented a method to covalently immobilize (APS/CDI) labeled model proteins (125I-ubiquitin) to CP titanium powder and polished as well as anodized disc surfaces. On the anodized surfaces model protein were coupled in 2-3 fold concentrations compared to polished surfaces. Furthermore, the biocoating technique was applied to rh-BMP-2 on titanium implants with the aim of constructing specific juxtracrine bone cell-reactive interfaces (Jennissen, H.P et al -95 and -97). Pham et al -04 [164] compared titanium surfaces coated with covalently immobilized RGD peptid (APTES), fibronection (APTES) or native ECM secreted by the SaOS-2 cells for 4 days. CP titanium was used as control. The removal of cells by NH4OH was verified by LDH and ALP. The surfaces were characterized by SEM-EDS and AFM. An increased cell density was demonstrated for the native ECM surfaces after 4 hours. However, it was concluded that it could be excluded that cells adhered to remnants of cell surface proteins.

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Muller et al -06 [159] evaluated collagen covalently immobilized (APS/EDC/NHS) on surfaces prepared with different oxides (etched, anodized). The surfaces were characterized by XPS, AFM and Profilometry. The concentrations of interfacial bonds and the stability of cross-linked proteins increased with increasing oxide layer (OH groups). Furthermore, the cross-linked collagen layer improved cellular response (Human osteoblast-like cells/MG-63) in vitro.

In Vivo Peptides Ferris et al -99 [156] compared implants with covalently immobilized (gold-thiol chemistry) RGD peptides and control gold coated titanium surfaces in rat femur. The study did not include any non specific control. No surface characterization was available. Pull out tests and histomorphometry were performed after 2 and 4 weeks. There were no significant differences between the surfaces when tested mechanically after 2 and 4 weeks. However, after 4 weeks the RGD implants demonstrated a significant increase in bone thickness. It was concluded that the RGD coated surface might enhance osseointegration. Germanier et al -04 [154] compared the SLA surface with and without covalently immobilized (PLL-G-PEG) RGD, RDG in a mini-pig model. Bone implant contact was increased after 2 weeks, while the surface after 4 weeks showed decreased bone implant values compared to the other surfaces. It was concluded that the RGD coated SLA surface promoted enhanced osseointegration during early stages. Schliephake et al -05 [163] compared machined titanium surfaces, covalently immobilized (anodic polarization/EDC) collagen and collagen coated implants with high and low concentrations of an additional immobilized (acrylat) RGD peptide in dog mandible. After 1 month, BIC was significantly enhanced only in the group of implants coated with the higher concentration of RGD peptides. Volume density of the newly formed bone was significantly higher in all implants with organic coating. No significant difference was found between collagen coating and RGD coatings. After 3 months, BIC was significantly higher in all implants with organic coating compared to implants with machined surfaces. Bernhardt et al -05 [168] compared titanium implants and titanium implants covalently immobilized (anodized) with either collagen type I, type III, or immobilized (phosphonate) RGD peptide in femur of goats. Histomorphometry and Micro CT were used for evaluation. All three coatings demonstrated a significant increase in bone volume compared to the uncoated controls after 5 and 12 weeks with the highest results for the collagen coatings. The coating with the RGD-sequence showed only a slight improvement compared to the control surfaces. While collagen type I demonstrated increased response in denser bone, collagen type III, appeared to be the more effective coating in areas of lesser bone density.

In Vivo Proteins Jansson et al -01 [147] compared machined and alkali-heat treated implant surfaces with and without a covalently immobilized (APTES/EDC/NHS) blood plasma coating in soft tissue rat model after 7 and 28 days. The thickness of the fibrous capsule, the fluid space width between implants and fibrous capsule, and formation of blood vessels were evaluated. The results demonstrated the thinnest fluid space for the plasma clot coated porous titanium surface compared to the untreated surfaces controls. The number of vessels and proportion of vessels

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in the fibrous capsule increased with time for the treated implant surfaces compared to the control titanium surfaces. It was concluded that the healing process around titanium could be modulated by porosity and thin pre-prepared plasma coatings. Jansson et al -02 [148] compared the release of inflammatory reactions around machined and alkali-heat treated implants with and without a covalently immobilized (APTES/EDC/NHS) blood plasma coating inserted subcutaneously in rats after 3 and 24 hours. Ex vivo PMA-stimulated oxygen radical production, cell recruitment, TNF-alpha secretion and cell-type pattern in the exudates around the porous plasma-coated implant were evaluated. It was concluded that the healing process around blood plasma coated titanium surfaces were more sham-like than references without blood plasma coating. Morra et al -03 [160] evaluated covalently immobilized (acrylic acid grafting/EDC/NHS) collagen coated implants. The surfaces were characterized by XPS, ATR-IR and AFM. In vitro cell (SaOS-2) demonstrated decreased proliferation and no difference in differentiation compared to the untreated surface after 1, 4 and 10 days. However, treated surfaces showed increased bone response after 4 weeks in rabbit femur and no adverse effect in rabbit soft tissue after 12 weeks. Morra et al -05 [162] compared bone micro-hardness around covalently immobilized (plasma deposited acrylic acid grafting/EDC/NHS) collagen coated implants in rabbit femur after 4 weeks. The surfaces were characterized by XPS. The micro-hardness measurements demonstrated improved bone maturation and mineralization at the interface of collagen coating compared to untreated CP titanium. Morra et al -06 [161] evaluated covalently immobilized collagen (acrylic acid grafting/EDC/NHS) coated anodized titanium implants. The surfaces were characterized by XPS and SEM. The collagen coated surfaces showed increased bone-implant contact after 4 weeks in rabbit femur and enhanced cell growth of human mesenchymal cells after 12, 24 and 72 hours. Becker et al -06 [165] compared bone formation around sandblasted acid etched titanium implants prepared with and without covalently and non-covalently bound rhBMP-2 in a dog model. The bone-implant contact and bone densities were evaluated after 4 weeks. The covalently and non-covalently bound rhBMP-2 implant surfaces demonstrated a similar but increased bone response compared to control implants. It was concluded that rhBMP-2 covalently and non-covalently immobilized by this method seemed stable and promoted direct bone apposition in a concentration dependent manner and that an optimal method for BMPs still is lacking. Seol et al -06 [166] evaluated implants with covalently immobilized (APTES/EDH/NHS) synthetic receptor motif mimic BMP-2 [10 amino-acids). The surface was characterized with XPS and Gamma counting (radioactivity). In vitro test were conducted by osteoblast like MC3T3-E1 cells to evaluate proliferation after 1, 7, 14, 21, 30 days and differentiation after 10 days. In vivo tests were conducted in a dog mandible model at 4 weeks. The results showed increased cell activity and bone response on the BMP-coated surfaces compared to the control.

Clinic So far, there are no commercially available covalently immobilized protein coated dental implant systems

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3. Conclusion Five possibly bioactive/bone-bonding (fluoridated, alkali-heat treated, anodized and nano HA) and biomimetic (covalent protein immobilization) CP titanium implants surfaces have been presented in this review. The different surface modifications have reached different levels of commercialization. While some implant surfaces (alkali heat-treated, covalent immobilized biomolecules, anodized Mg) seem promising in the experimental stadium, others have become commercial products with ongoing clinical trials [anodized Ca (Ospol), nano HA (NanoTite™)]. Yet others have some years of clinical follow-up protocols with generally good results in bone with poor quality [fluoridated (Osseospeed™)].

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In: Surface Coatings Editors: M. Rizzo and G. Bruno, pp. 45-91

ISBN: 978-1-60741-193-2 © 2009 Nova Science Publishers, Inc.

Chapter 2

ANTIMICROBIAL SURFACE COATINGS IN PACKAGING APPLICATIONS Jari Vartiainen VTT Biotechnology Tietotie 2, Espoo, P.O. Box 1500 FIN-02044 VTT Finland

Abstract Antimicrobial packaging materials are interesting and promising applications of advanced active food packaging systems. They can effectively control the microbial contamination of various solid and semisolid foodstuffs by inhibiting the growth of microorganisms on the surface of the food, which normally comes into direct contact with the packaging material. Recently, a lot of efforts has been put on the development of antimicrobial packaging, which can considerably prolong the shelf lives of packed food products and/or decrease the need of preserving agents in foods. Some promising results have been obtained of which the surface activation and coating treatments seem to offer the most applicable solutions. Antimicrobial surface treatment can be done by several ways such as coating, printing, grafting or covalent binding. Other surface pre-activation methods such as physical, chemical or enzymatic treatments or their combinations may be necessary to produce permanently coupled antimicrobial agents. By using surface treatments the harmful effects on valuable bulk properties of packaging materials can be minimized. Also the safety aspects should be easier to fulfil as migration of substances can be kept at very low level. Antimicrobial surface treatments can be completely separated from the high-volume production lines of bulk materials. They can be done with smaller scale equipment immediately before the packaging is formed ensuring the maximum antimicrobial efficiency. Development of antimicrobial packaging materials, which can be produced at commercial scale, is a challenging and promising area, where intensive research is still needed. They can be exploited in direct contact with certain foods only and each food system must be investigated separately.

1. Introduction The value of global packaging business is some 400 billion euros. Food accounts for half of the consumer packaging end use. Plastic is the most widely used and the fastest growing packaging material, followed by paper/board (about 30%) [1]. The value of packaging

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materials produced in Finland is some 1.8 billion euros of which paper/board accounts for almost 75% and plastics about 20% [2]. Whilst food packaging is required to fulfil a number of different functions, its primary role is to prevent loss of quality and to give protection against environmental contamination. During primary production, manufacturing, transport, processing and storage, food may deteriorate in quality through one or more of the following mechanisms: 1) contamination by growth of micro-organisms, e.g. bacteria, fungi and yeasts, 2) chemical reaction brought about by enzymes, through oxidation or hydrolysis, 3) physical changes, e.g. loss of moisture, ingress of gases, taint, radiation and mass transfer, and 4) depredation by macro-organisms, e.g. pests and vermin [3]. In recent years lots of efforts have been aimed at developing new active, intelligent, biobased and high barrier packaging materials. Active packaging materials are defined as materials that change the conditions of the packed food to extend shelf life or to improve food safety or sensory properties, while maintaining the quality of the food. Intelligent systems monitor the conditions of packaged foods to give information about the quality of the food during transport and storage [4]. Biobased packaging materials are defined as materials from renewable sources. There is increasing interest in functional packaging materials that are also biobased, biodegradable and recyclable. The use of waste by-products of agriculture and the food industry to make 100% biodegradable but still feasible packaging materials will be of particular value in the future. The majority of food applications for transparent barrier films are for protective atmosphere packaging (PAP) including [5]: • •

•

•

Vacuum packaging – a packaging method where air is removed from the pack at the time of closure. Modified atmosphere packaging (MAP) – a method in which a combination of gases (oxygen, nitrogen and carbon dioxide) are introduced into the packaging at the time of closure, by a process known as gas flushing. Equilibrium modified atmosphere packaging (EMAP) – a packaging method where the EMAP atmosphere is created by tailoring the gas permeability of the packaging film to match the packed product’s respiration rate. Controlled atmosphere packaging (CAP) – a packaging method where the modified atmosphere is constantly monitored to ensure correct composition of gases.

Active packaging materials have been widely studied and reviewed in recent years [615]. They interact with packed foodstuffs by actively modifying the conditions inside the package. These interactions can affect on physiological (e.g. respiration of fruits), physical (e.g. desiccation, softening, dripping), chemical (e.g. oxidation of lipids, pigments, vitamins) or microbiological phenomenon (e.g. spoilage micro-organisms). The active mechanisms depend on the individual requirements of the each different food type. Active packaging materials include various types of functional systems, for example, oxygen, carbon dioxide and ethylene scavengers, carbon dioxide and ethylene emitters, moisture scavengers, ethanol emitters, flavour releasing and absorbing systems as well as antimicrobial packaging materials. Oxygen and moisture scavengers currently have the most commercial significance, but also the other techniques have been predicted to play an important role in the near future. Intelligent packaging, on the other hand, include packaging systems that are capable of carrying out so called intelligent functions (such as detecting, sensing, recording, tracing,

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communicating etc.). Novel barcodes, radio frequency identification tags (RFID), timetemperature indicators, gas indicators, toxin indicators, spoilage detectors/ freshness indicators and other biosensors are typical types of modern intelligent packaging [15,16]. Table 1. Active packaging technologies and mechanisms [6,8,9,10,12,14]. Active packaging Oxygen scavenger

Carbon dioxide scavenger Carbon dioxide emitter Ethylene scavenger Moisture scavenger

Ethanol emitter

Active substances and mechanisms Iron powder, ferrous carbonate, ascorbic acid, polyunsaturated polymers, ethylene methacrylate and cyclohexene methacrylate, benzyl acrylate, ascorbate, MXD-6 (aromatic polyamide) oxidation, enzymes (glucose oxidase, catalase, alcohol oxidase, superoxide dismutase), sulphites, unsaturated fatty acids (oleic, linoleic, linolenic), immobilization of yeasts in solid holders, photosensitive dyes, butylated hydroxytoluene (BHT), vitamins E and C, transition metal catalysts (platinum, cobalt, copper), rice extracts, catechol Calcium hydroxide, sodium hydroxide, potassium hydroxide, zeolites, active carbon Ferrous carbonate, ascorbic acid, sodium bicarbonate Potassium permanganate, palladium catalyst, zeolites, clays, Oya-stone, active carbon, silica gel, metallic oxides, tetrazine Silica gel, zeolites, propyleneglycol, calcium, barium and magnesium oxides, calcium sulphate, natural clays (such as montmorillonite), different salts, polyvinyl alcohol, molecular sieves, glycerol, polyacrylate salts, graft copolymers of starch Encapsulated ethanol, ethanol vapour generators

The management of the microbiological safety of food has become increasingly important for a number of reasons, including the following: 1) the increasing globalisation of the food supply chain, 2) a consumer population that is more knowledgeable on issues associated with the food production chain and particularly those related to food safety, and 3) highly sophisticated innovations in product development [17]. In the industrialised world the spoiled food related diseases cost about 8-12 billion euros per year [289]. Up to one-third of the populations of developed countries are affected by foodborne illnesses each year. Food and waterborne diarrhoeal diseases, for example, are leading causes of illness and death in less-developed countries, killing an estimated 2.2 million people annually [18]. In the US an estimated 76 million cases of foodborne illness occur each year, causing about 325,000 hospitalizations and 5,000 deaths. Since 1990, over 400 produce-related outbreaks have occurred across North America [347]. The microbial safety and stability as well as sensory and nutritional quality of most foods is based on an application of combined preservative factors (called hurdles). The most important hurdles used in food preservation are temperature, water activity, acidity, redox potential, preservatives, and competitive micro-organisms (e.g. lactic acid bacteria). It is well known for some time that different hurdles in food might not have an additive effect only on microbial stability, but they could act synergistically. It is more effective to use different hurdles, as well as different antimicrobial substances, of small intensity than one of larger intensity, as different factors might have a synergistic effect [19].

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Antimicrobial packaging system is a promising active food packaging application. It uses the antimicrobial substances combined with the packaging materials. It can control the microbial contamination by reducing the growth rate and maximum growth population, and extending lag period of the target micro-organism depending on the mechanism and kinetics of used antimicrobial agent. The antimicrobial packaging materials can inhibit the growth of micro-organisms on the food product. Therefore, they can prevent the microbial spoilage, improve the microbial safety and may prolong the shelf life of the packed food products [20]. By using antimicrobial packaging materials, the total amounts of preservatives added into food matrix can be minimized. Although the most active packaging materials will probably have a bright future ahead some important aspects should be considered before they can be fully exploited. The safety aspects must be ensured and the legislative requirements should be completely fulfilled. Novel active systems must meet the reliability and effectiveness requirements in real applications and conditions. Added new functionalities should not have any harmful effects on the technical properties of the base material. Processing of new materials should be carried out in most economical way preferably with the existing equipment and methods. Costs and amounts of active substances must fulfil both economical and efficiency requirements. Consumers need and acceptance is absolutely necessary, as well as the positive attitude of food producers and retail networks. Antimicrobial packaging materials have been studied worldwide and especially in Japan for decades [7]. Great amount of creditable publications, patents and review articles have been published [21-25,348,349]. However, the antimicrobial packaging materials have not yet commercially broken through. There are four possible reasons for that. First, the present legislation has restricted the use of antimicrobial packaging materials releasing antimicrobial substances to foodstuffs. Secondly, antimicrobial packaging materials developed so far have not been effective enough in preventing the microbial spoilage or prolonging the shelf life of the food products. They do have shown the antimicrobial activity against several individual micro-organisms in laboratory conditions, but in many cases lost most of their activity when used as a packaging material in direct contact with real foodstuffs and their compounds. In addition, many of the used antimicrobial substances have turned out to be too sensitive to processing conditions (temperature etc.) or too expensive, preventing the successful industrial scale production. It is presumable that the easiest way to get legislative approval of antimicrobial substances to be used in contact with foodstuffs is to choose additives classed as traditional food preservatives (substances with E numbers). The other way is to make sure and scientifically show that no migration will occur during the contact with foods [268]. According to Pira International, the demand for active packaging is set to increase over the next five years. The consumers still want to have healthier and more natural foods which are only minimally processed. This trend will foster the demand for active packaging systems, which are designed to maintain the quality of these less processed foods on a high level. Packaging converters indicate an interest in antimicrobial technology, but Pira predicts the widespread commercialisation of antimicrobial films is unlikely before the end of 2007 [26]. According to Pira’s market forecast, the antimicrobial packaging sales will start to growth in Europe but still remain relatively small as compared to oxygen and moisture scavenger markets. This is due to concerns among European Union authorities with antimicrobial resistance problems and the limitations set by the current food packaging legislation. World sales of antimicrobial films, however, are expected to rise by a round 70% in the period to

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2010 [27]. According to research society the breakthrough of antimicrobial packaging materials will be 2-5 years away [28]. Table 2. Market trends of active packaging [27]. Packaging type Active packaging Moisture scavengers -desiccants -pads, other moisture absorbers Oxygen scavengers Carbon dioxide scavengers/ emitters -combination linesa -non-combination lines Ethylene scavengers Antimicrobial films Ethanol emitters Flavour/ odour absorbers -combination linesb -non-combination lines Total active packaging

2005 (million $)

2010 (million $)

722 454 268 588 121 94 27 62 99 37 46 23 23 1558

1286 823 463 924 182 136 46 121 169 65 68 30 38 2649

Note: combination line sales subtracted to avoid double-counting a includes combination oxygen/carbon dioxide scavenger and oxygen scavenger/ carbon dioxide emitter lines b includes activated carbon desiccant-style lines with flavour absorption capabilities

2. Antimicrobial Packaging Materials 2.1. Food/ Packaging Interaction Most food packaging systems consist of the packaging material, the food itself and the headspace in the package. The volume of the headspace depends on the type of food and packaging material as well as the packaging method. By using vacuum methods with flexible packaging films the headspace volume can be minimized. In this case we have a direct contact between the packaging material and outer surfaces of the foodstuff. In the case of liquid packaging the contact can be even more profound. Individually vacuum wrapped cheese and deli products as well as aseptic brick packages or bottles are good examples. Diffusion between the packaging and the food is the main migration phenomena involved in this type of packaging system. Active substances may be incorporated into the packaging materials and let to migrate into the food through diffusion. Alternatively, they can be immobilized onto surface of the packaging resulting activity but no migration. The solid food items packed in the rigid boxes may have a large headspace volume and a minimal contact area. Typical examples of this type of packaging with a considerable headspace volume are trays, cans and cartons. Evaporation or equilibrated distribution of an active substance among the headspace, packaging material and food is considered as a part of main migration mechanisms. Addition to non-volatile substances which have capability to

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migrate only through contact surfaces, it is be possible to utilize volatile active substances having access through the headspace and air gaps between the package and the food [20].

Package

Food

Package

Headspace

Equilibrium evaporation Equilibrium evaporation

Migration

Immobilization

Migration Immobilization

Figure 1. Food packaging systems and migration phenomena [20].

Controlling the release rates and migration amounts of active substances from packaging into food is very important in terms of both optimal functionality and legislative compatibility. For this purpose the diffusion of many active substances in various packaging matrices has been studied and the legislative and safety aspects of many novel active and antimicrobial packaging systems have been widely considered. Migration is a mass-transfer process by which normally low molecular weight substances initially incorporated into package are released into packed food. Migration has a critical influence in the food packaging since the transfer of substances may result in organoleptic changes and can even promote toxicity. For these reasons the migration behaviour of all food packaging materials has to be determined properly. Analysis of all the mass transport processes in a real food packaging system is very complex due to the numerous components migrating, permeating or being retained by the package. In addition some components may, while not transferred, have special effects on these processes. For this reason the system is simplified and each transferable substance is analysed separately. The package is assumed to be homogeneous and the food is substituted by a suitable simulant. Moreover, environmental conditions are assumed to be constant and in many cases accelerated tests with higher temperatures are used [29]. From a theoretical point of view, migration in plastic packaging is a result of diffusion and equilibrium processes involving typically low molecular weight compounds being transferred from a polymeric medium into a contacting food or headspace. The migrating substances diffuse through the amorphous portion of the polymer matrix towards the surface where they are partitioned between the package and the food (or headspace) until the equilibrium is reached [29]. The migration actually starts at the moment the packaging material is manufactured. Volatile substances start to evaporate immediately in the surrounding atmosphere. Therefore the values of migration depend on the time interval between the manufacture and actual use with foods. Besides migration, some active packaging systems can be based on non-leachable, permanently immobilized antimicrobials attached on the surface of packaging materials. In this case no diffusion or mass transfer will take place. These types of antimicrobials typically

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require a molecular structure large enough to retain activity towards microbes even though bound to the surface of packaging material. Mass transport properties are often described by three common coefficients: diffusion, solubility and permeability. The diffusion coefficient describes the movement of migrating molecule through a polymer and thus represents a kinetic property of the system. Activated diffusion is described as the opening of a void space among polymer chain segments followed by translational motion of the migrant within the void space before the segments return to their initial state. The solubility coefficient describes the dissolution of a migrant in a polymer and thus represents a thermodynamic property of the system. The solubility coefficient is a function of temperature and may be a function of the concentration of dissolved migrant. The permeability coefficient incorporates both kinetic and thermodynamic properties of the system and thus provides a gross mass transport property. Material’s structure has a great influence on mass transport. The ability of polymer chains to relax and shift their structure is essential for activated diffusion. Several structural properties influence mass transport: chemical structure, method of preparation, processing conditions, free volume, crystallinity, polarity, tacticity, crosslinking and grafting, orientation, presence of additives, and use of blends. It has been shown that an increase in crystallinity, density, orientation, molecular weight or crosslinking results in decreased mass transport [30].

2.2. Migration of Antimicrobial Substances There is a wide selection of antimicrobial substances, for example, organic acids, bacteriocins, spice extracts, thiosulfinates, enzymes, proteins, isothiocyanates, antibiotics, fungicides, chelating agents, parabens and metals, which all are considered to have possible antimicrobial activity when incorporated in or coated onto food packaging materials [6]. Most of these substances are also normally considered to be safe enough for human beings, even if all of them are not currently allowed, by legislation, to be incorporated in foodstuffs or packaging materials intended to come into contact with foodstuffs. Table 3. Antimicrobial substances of potential use in food packaging [31]. Type of substance Organic acids Bacteriocins Essential oils Natural phenols Enzymes Proteins Antioxidant phenolics Isothiocyanates Antibiotics Antimicrobial peptides Fungicides Chelatin agents Inorganics Metals Parabenes

Example of substance Propionic, sorbic, acetic, lactic, benzoic acid Nisin, pediocins Thymol, eugenol, cinnamic acid p-Cresol, hydroquinones, catechins Lactoperoxidase, lysozyme, lactoferrin Conalbumin, cathepsin BHT (butylated hydroxytoluene), BHA Allyl isothiocyanate, hypothiocyanite Natamycin Defensins, cecropins, attacins, magainins Imazalil, benomyl EDTA, pyrophosphate, citrates Sulphites, sulphur dioxide Silver, copper Methyl, propylparaben

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Benzoic anhydride and ethyl and propyl parabens have been incorporated into LDPE films in concentrations of 1% and 0.5% and migration into distilled water, olive oil, tomato ketchup, salad dressing and mayonnaise was determined at 6 °C and 25 °C. All antimicrobial substances migrated easily into foods and simulants. 30-40% of total amount of incorporated benzoic anhydride migrated into water, ketchup or dressing, but the level of migration into olive oil and mayonnaise was lower ranging from 10% to 20%. In the case of parabens the level of migration into both distilled water and olive oil clearly exceeded 70%. More than 80% of all mass transfer took place during first 24 hours. Only the migration of benzoic anhydride into olive oil and mayonnaise was slower thus 80% was transferred within first 5 days. There was no significant influence of different temperatures on total level of migration. The levels of antimicrobials migrating ranged from 2 to 3.5 mg/g of LDPE film corresponding to approximately 1-2 mg/dm2 of packaging material. As the recommended concentration of benzoic acid and parabens for various foods are 0.03-0.1% (0.3-0.1 mg/g) the quantity of antimicrobial substance released seemed to be efficient only in a thin layer of food in direct contact with packaging film [32]. 5% of propyl paraben has been incorporated into styrene-acrylate copolymer binder and applied onto clay-coated paper. Release kinetics of propyl paraben into Saccharomyces cerevisiae culture medium was determined at 30 °C. An equilibrium concentration was observed at 150 mg/L after 2 days. It was shown that slow release was not as antimicrobially effective as direct addition of propyl paraben when the initial microbial concentration was rather high. Slow release could be more favourable as compared to direct addition in continuous microbial inhibition of low cross-contaminated foods during use and storage rather than in sterilization [33]. Heat-pressed, NaOH and HCl modified poly(ethylene-co-methacrylic acid) PEMA films have been incorporated with benzoic acid and sorbic acid by using acetone as a swelling agent. Migration into phosphate (pH 3.5, 0.2 M) buffer was measured. The release of benzoic acid from NaOH-pretreated PEMA films reached equilibrium at 75 mg/g of film after 2 weeks. The maximum benzoic acid migration from untreated and HCl treated films were about 5 mg in the same period of time. Similar results were obtained with films containing sorbic acid. About 55 mg of sorbic acid was released from NaOH-treated films, but only about 0.5 mg from other film types. Higher polarity of NaOH-pretreated films may explain the larger amount of antimicrobial substances absorbed and released by the films [34]. Sodium propionate, sodium sorbate, sodium benzoate and nisin have been incorporated into paper coating binder solutions (acrylic polymer or vinyl acetate-ethylene copolymer) in concentration of 2% and applied on a virgin paperboard. Migration into distilled water was measured during 30 days storage at 10 °C. Nisin had a significantly lower equilibrated release level (7-28%) as compared to other antimicrobials (76-100%). High proportion of nisin seemed to be immobilized on the binder matrix probably due to its chemical nature for adsorption and high molecular size relative to the pore size of the swollen coating matrixes which resulted in very low level of equilibrated migration. In all cases the higher diffusion was obtained with vinyl acetate-ethylene copolymer. Swelling properties played an important role in diffusion process [35]. Benzoic acid and sodium benzoate have also been incorporated into silicone coatings resulting minimal effects on the bulk and surface properties with concentrations below 2%. Benzoic acid formed large crystals in silicone which led to fast and uncontrolled leaching.

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Sodium benzoate, on the other hand, exhibited slow and controllable leaching resulting enhanced antibacterial properties [291]. 3% of lysozyme, nisin or sodium benzoate has been incorporated into polyvinylalcohol (PVOH) films crosslinked with 7.7%, 2.0%, 0.77% and 0.077% glyoxal. The release kinetics into water was dependent on water diffusion, macromolecular matrix relaxation and diffusion of the active substance through the swollen polymeric network. The amount of active substance released at equilibrium decreased as the degree of crosslinking of the polymeric matrix increased. The release kinetics also strongly depended on the molecular weight of the antimicrobial substances [36]. For lysozyme and nisin increasing the crosslinking increased the time required to reach equilibrium conditions, suggesting that the release kinetics of these substances can be adjusted by modifying the degree of crosslinking. In contrast the crosslinks did not have influence on the release kinetics of sodium benzoate indicating that the molecular size of sodium benzoate is smaller than the mesh size of the investigated crosslinked PVOH films [37]. Nisin has been incorporated into vinyl acetate-ethylene copolymer binder at a concentration of 3% and applied onto paperboard. The migration of nisin from the coating into a model emulsion composed of 66% water and 32% praffin oil with 2% emulsifier (Tween 20) was measured at 10 °C during 12 days storage time. The migration was complete in 8 days and the maximum equilibrium concentration of nisin released into the solution reached 8.6% of the total nisin content incorporated in the coating layer. The migration did not change on the addition of 3% α-tocopherol [38]. Nisin has also been incorporated into protein films (cast or heat-pressed, corn-zein or wheat-gluten) at concentration of 7%. Nisin diffusion into water was studied at temperatures of 5, 25, 35 and 45 °C. The cast corn-zein films had the lowest nisin diffusion and highest nisin retention. Diffusivities of other films were not significantly different. It was proposed that nisin was more profoundly incorporated into corn zein in ethanol based cast solution, whereas only suspended as particles in heat-pressed films. This may have caused the difference of the nisin diffusivity between cast and heat-pressed films. In all cases the diffusion increased as a function of temperature [39]. 0.3% of natamycin and 1.6% of potassium sorbate were incorporated into whey protein isolate films in order to investigate diffusion into 20% glycerol solution as a function of added glycerol or beeswax at 24 °C. Almost all potassium sorbate was released from the films in 8 min. As the amount of glycerol decreased, the films became less flexible and a significant decrease in sorbate diffusion was seen. This decrease was related to the increased film stiffness. As the glycerol amount decreased, the free volume available for sorbate diffusion decreased, reducing both the film flexibility and the sorbate diffusion coefficient. The release of natamycin was significantly slower than the release of potassium sorbate due to larger shape and size of natamycin molecules. Also the diffusion of natamycin decreased as the amount of glyserol decreased. The amount of beeswax did not have significant effects on sorbate migration [40]. This however was in contrast to previous studies which indicated that addition of beeswax or other lipids decreased the diffusion of sorbic acid in wheat gluten films [41]. In addition, the methylcellulose and hydroxypropyl methylcellulose films mixed with lauric, palmitic, stearic and arachidic acids have been shown to lower significantly the potassium sorbate permeation rate as compared to cellulose ether films containing no fatty acids [42].

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Crosslinked gelatin films have been loaded with lysozyme dosages varied from 0.2% to 1.0% in order to investigate the release properties into solidified agarose medium at 37 °C. After 50 hours, most of the lysozyme was released. Free carboxylic acid content of crosslinked gelatin films had a considerable effect on release profiles, thus the samples with low free carboxylic acid content released 50% of their total lysozyme within 8 hours, while the samples having high amount of carboxylic acids needed 32 hours to release 50% of their total lysozyme content. Thus, the electrostatic interactions between lysozyme and gelatine played an important role in releasing process [43]. 0.2% or 0.5% of hexamethylenetetramine (HMT) were incorporated into LDPE films and used as vacuum package for fresh orange juice and distilled water stored at 6 °C for 39 days. For both migration experiments a clear difference was obtained between two concentrations, thus the higher concentration (0.5%) produced higher migration. In orange juice the maximum migration was reached after 24 hours. After that period the amount of HMT decreased slowly over time. A possible explanation for this was a reaction of produced formaldehyde with proteins and micro-organisms present in the juice. In distilled water the migration reached its maximum after 1 week and the amount of released HMT remained constant as no reactions occurred between formaldehyde and distilled water. The total migration of HMT into orange juice was about three times higher as compared to distilled water [44]. A coating made of a styrene-acrylate copolymer containing triclosan (87 ± 9 mg/ cm3 of coating) was evaluated as an antimicrobial layer for packaging materials. Using pure water, no release of triclosan was observed at 30 °C, because of its insolubility. In 10% ethanol about 1.2% of triclosan was quickly released and equilibrium was observed within 20 h. Using n-heptane, about 65% of the triclosan was quickly released, which suggest the coating was strongly affected by n-heptane. The coating may not be suitable for fatty food applications because of the quick release of a large amount of triclosan [329]. Antimicrobial coating systems may offer much faster performance as compared with compounding technology. For example, incorporation of some antimicrobials into PVC or PP polymers resulted in plastics that provided substantial reduction in bacterial growth over a period of 24 hours. However, using the same product in a coating gave similar reductions within five minutes [268]. The release processes of antimicrobial substances from the packaging materials towards foodstuffs can be controlled using combinations of materials having different diffusion properties. Multilayer structures of which individual layers have different diffusion coefficients may be used for optimizing the delivery of antimicrobials in most effective way. In the case of synthetic plastics, LDPE can be utilized in the layer containing the antimicrobial substance whereas the other layers (HDPE, PP, PET etc.) may be useful for very slow diffusion layers or even barriers in the multilayer structure. Co-extrusion processing is recommended for multilayer materials because it’s expected to cause less interfacial resistance at the layer interface than the adhesive lamination processing [45]. Recently, the controlled release of active substances has been accomplished using smart blending technology, where different film morphologies, such as multilayer, selective permeable, interpenetrating, network-like, platelet, fibrous or droplet morphologies can be produced in separate smart blender unit. By manipulating the film morphologies, the release rates of active substances can be predictably controlled [269]. Multilayered structures can also be exploited in biodegradable hydrophilic films where releasing is mainly based on the

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swelling of film matrix. By using multilayers it’s possible to slow down the release rate of antimicrobials from the swellable PVOH film and to control the amount of antimicrobials released at equilibrium [49]. The multilayer structures having both synthetic and biobased layers and possibly further treated with functional printing may offer new possibilities for optimizing the delivery of antimicrobial substances.

2.3. Antimicrobial Activity Antimicrobial activity is possible to accomplish by several ways including addition of sachets or pads containing antimicrobial volatiles, incorporation of antimicrobials into matrix of packaging material, coating or adsorbing antimicrobials onto packaging surface, using inherently antimicrobial substances as packaging raw materials or immobilization of antimicrobial substances onto packaging surface [21]. 2.3.1. Leachable Antimicrobials Over 50 years ago, sorbic acid and its potassium salts were studied as preservatives for the packaging of cheese products [50-52]. More recently, sorbic acid, potassium sorbate, nisin, lactoferrin and sodium diacetate have been incorporated into PVDC films produced using a solvent casting method. Films containing nisin, sorbic acid and potassium sorbate inhibited Listeria monocytogenes at concentrations of 1.0%, 1.5% and 2.0%. Films containing sorbic acid were the most compatible with the plastic solution and had the best physical appearance and the most homogenous structure [53]. Films extruded using LDPE with potassium sorbate powder decreased the growth rate and maximum growth of yeast. Tensile properties were not affected significantly, but the transparency decreased as a concentration of potassium sorbate increased [54]. Sorbates incorporated into tapioca starch films resulted in more than two times increase in strain and over 50% decrease in tensile strength after two weeks of storage [280]. In addition, sorbic acid (2 g/m2) coated butter paper has been shown to extend the shelf life of wrapped paneer to 36 days at ambient temperature [55]. During the years several studies have been carried out to determine the antimicrobial activity of nisin incorporated into various packaging materials. Nisin has been shown to inhibit the growth of spoilage bacterium Brochothrix thermosphacta on vacuum packaged meat [56]. Nisin has been dissolved in a binder solution (acrylic polymer or vinyl acetateethylene copolymer), which was used as antimicrobial coating material for paper against Micrococcus flavus [35]. It has also been dissolved in zein solutions, which have been further formed into biodegradable films with activity against Lactobacillus plantarum [57,58] and L. monocytogenes [59,444]. Nisin incorporated into hydroxypropyl methylcellulose (HPMC) [60], sorghum starch [61] and polyvinylidene copolymer [62] films have been reported. Nisin coated paper and plastic (70:30, PE:PA) films [63], as well as cellulose casings [64] and PVC, LLDPE and nylon films [65] have been studied as potential antimicrobial packaging materials. Recently, 0.5% nisin together with 5% EDTA have been used as additives in chitosan films resulting in antimicrobial activity against Bacillus subtilis [66]. Nisin and /or chitosan have also been coated, in 3% concentrations, onto paper with a binder medium of vinyl acetate-ethylene co-polymer (pH 4.4) to provide antimicrobial activity against L. monocytogenes and/or Escherichia coli [67]. LDPE films have been coated with nisin in

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methylcellulose and hydroxypropyl cellulose solutions. Films were effective in inhibiting L. monocytogenes, but the release of nisin was not controlled and did not provide consistent inhibition throughout the 8 days study [68]. Nisin containing cellophane-based coating reduced significantly the growth of the total aerobic bacteria on chopped meat through 12 days of storage at 4 °C [69]. The inhibitory effects of in-package pasteurization combined with a nisin containing wheat gluten film were tested over an 8-week storage period against L. monocytogenes and salmonella inoculated on refrigerated bologna. Combining both treatments significantly reduced the growth of L. monocytogenes over the 2 month storage period, but provided no additional inhibitory effect against salmonella as compared to only pasteurization [70]. Nisin coated onto LDPE films showed antimicrobial activity against Micrococcus luteus and inhibited the total bacterial flora in raw cow milk, pasteurized milk and ultra-high temperature (UHT) milk. The release was favoured by low pH and high temperature [266]. Nisin has been incorporated into polyethylene (PE)/biopolymer composite films made from methylcellulose, hydroxypropyl methylcellulose, κ-carrageenan and chitosan. Films were prepared either by heat-pressing with PE powder or casting onto preformed PE films. Cast films exhibited larger inhibitory zones as compared to heat-pressed films using M. luteus as test strain. Among all nisin containing films, the antimicrobial activity was most effective in methylcellulose films as compared to other heat-pressed films and in chitosan films as compared to other cast films [278]. Nisin, lysozyme and sodium benzoate have also been incorporated into polyvinylalcohol (PVOH) films crosslinked with glyoxal. All substances released from the films and were effective in inhibiting the microbial growth of M. luteus, Alicyclobacillus acidoterrestris and S. cerevisiae [37]. Na-alginate- and κ-carrageenan-based antimicrobial films have been prepared using active substances such as lysozyme, nisin, grape fruit seed extract (GFSE) and EDTA. Na-alginate-based films exhibited larger inhibitory zones as compared to κ-carrageenan-based films even within similar combinations and levels of antimicrobial agents. All films treated with antimicrobials were antimicrobially effective, however, GFSE-EDTA combination was the only one having activity against all test organisms (M. luteus, listeria, salmonella, E. coli and Staphylococcus aureus) in both Naalginate- and κ-carrageenan-based films [277]. Natamycin-incorporated cellulose-based films have been used as packaging materials for Gorgonzola cheese. Films with 2 and 4% natamycin presented satisfactory results for Penicillium roqueforti inhibition and the amount of natamycin released to the cheese was below that allowed by the legislation [315]. The antimicrobial effects of alginate, zein and polyvinyl alcohol films containing enterocins (Enterococcus faecium CTC492) were determined against L. monocytogenes. Films with two concentrations of enterocins (200 and 2000 AU/cm2) were used as packaging material for cooked ham. The most effective treatment for controlling L. monocytogenes during storage at 6 ˚C was vacuum-packaging of sliced cooked ham with alginate films containing 2000 AU/cm2 of enterocins [320,321]. The antilisterial bacteriocin produced by Pediococcus parvulus has been incorporated into zein and whey protein isolate films. Antimicrobial zein films were more effective in reducing L. innocua as compared to WPI films at the same bacteriocin concentrations [322]. Incorporation of lysozyme and EDTA into crude exopolysaccharide (59% pullulan) films produced antimicrobial activity against E. coli. In films lysozyme retained up to 70% of its initial enzymatic activity. Enzymatic activity in films showed sufficient stability during 3

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weeks of storage at 4 °C [267]. Zein films incorporated with lysozyme showed antimicrobial activity against B. subtilis and L. plantarum. By the addition of disodium EDTA, the films also became effective against E. coli [71]. Gelatin films formulated with lysozyme at concentrations as low as 0.001% were effective against B. subtilis and Streptococcus cremoris [295]. Extruded pea starch containing 1% lysozyme had activity against B. thermosphacta. The high shear and temperature of extrusion process had a negative effect on the residual activity of lysozyme [310]. Benzoic anhydride has been incorporated into LDPE films, which exhibited antimycotic activity in contact with media and cheese. 1% of benzoic anhydride completely inhibited Rhizopus stolonifer, Penicillium spp. and Aspergillus toxicarius growth on potato dextrose agar. Levels of 0.5-2% benzoic anhydride delayed mould growth on cheese [72]. 1% benzoic anhydride has also been incorporated into LDPE films and used as vacuum package for cheese and toasted bread stored at 6 °C. Significant inhibition of mould growth was obtained on the surfaces of both foods [32]. Poly(ethylene-co-methacrylate acid), PEMA, has been combined with benzoic acid and sorbic acid to form an antimicrobial food packaging material. The films did not only absorb benzoic and sorbic acids into the structure, but also inhibited the microbial growth of Aspergillus niger and Penicillium sp. [73]. Sodium propionate, sodium sorbate, sodium benzoate and nisin were incorporated into paper coating binder solutions (acrylic polymer or vinyl acetate-ethylene copolymer) in concentration of 2% and applied on a virgin paper board. All coatings were antimicrobially active against M. flavus. The binder of vinyl acetate-ethylene copolymer gave higher diffusion and thus resulted in a higher antimicrobial inhibition zones compared to acrylic polymer [35]. Also propyl paraben has been incorporated into styrene-acrylate copolymer emulsion and used as antimicrobial coating material for paper against S. cerevisiae [33]. Various antimicrobial materials, in which silver-zeolite is incorporated in plastics have been developed. Zeolites are hydrated aluminosilicates of the alkaline and alkaline-earth metals. They are traditionally used because of their unique adsorption, ion-exchange, molecular sieve and catalytic properties [74]. As silver-zeolite is expensive, it is typically laminated as a thin layer of around 2 μm onto top of the film. The antimicrobial activity is demonstrated by the silver ions contained in zeolite particles on the film surface. As silver ions are taken into microbes, they react and bond to the cellular enzyme microbes. This inhibits enzyme activity and multiplication of microbes, thus extinguishing the microbes. The amount of added silver-zeolite may influence the heat sealing strength and other physical properties like transparency of the packaging films. The normal incorporation level is 1-3 % [7]. In the presence of moisture, the silver-zeolite acts as an ion pump, providing the controlled release of the silver ions into the food system in exchange for sodium ions from the environment. The controlled release provides continuous antimicrobial protection of food surface. Silver-coated plastic films can be of weak effectiveness because the hydrophobic polymer matrix may limit the silver ion concentration near food surface. However, if silver compounds are incorporated in a hydrophilic layer that provides greater aqueous diffusion, the antimicrobial activity can be improved. The silver ion release characteristics depend on the nature of the silver antimicrobial used and the polymer matrix. It has been shown that some of the commercial silver based antimicrobials are more efficient than the elementary silver powder owing to the presence of porous structures used as carriers of silver. Crystallinity and water uptake play an important role in antimicrobial releasing. Composites having higher water uptake properties showed

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early antimicrobial activity whereas PP and PA containing elementary silver powder having activity against E. coli and S. aureus showed better long-term silver ion release behaviour [282, 285]. The catalytic action of silver in the presence of light and/ or water at polar surfaces may result in a conversion of gaseous oxygen into active oxygen, which causes structural damage to microbes [75]. In the case of silver ions incorporated into zirconium phosphate ceramic matrix, the antimicrobial properties against E. coli and S. aureus were found to be caused by, not silver ions but photochemical reactions taken place in ceramic particles instead [76]. Chitosan has antimicrobial activity against different groups of micro-organisms, both bacteria, yeasts and moulds [77,346]. As the amino group of chitosan is positively charged at below pH 6, it has a better antimicrobial activity than chitin and many other biopolymers [78]. In addition, due to its good film-forming properties, chitosan has been successfully used in food packaging materials [79]. The antimicrobial properties of chitosan seem to be a result of two mechanisms; (1) carboxylate groups in the form of evaporated acetic acid solvent and (2) diffused protonated glucosamine fractions [323]. The preparation of chitosan films [80-85,301] and chitosan laminated with pectin [86] or polyethylene [87] or mixed with lipids [88] and gliadins [308] have been reported. Chitosan has been mixed with methylcellulose as well as 4% of sodium benzoate or potassium sorbate to form antimicrobial films. Investigations revealed that the film possessed significant antimicrobial properties against Penicillium notatum and Rhodotorula rubra [81]. Films have been formed by dissolving chitosan into hydrochloric, formic, acetic, lactic and citric acid solutions [89]. Diffusivity of acetic and propionic acids incorporated in packaging films for processed meats has been determined [90] and the strongest inhibition was observed when the acid release rates from chitosan matrix were slower [91]. The browning and water loss in cut apple slices were inhibited by coatings of chitosan and lauric acid [92]. Chitosan has also been used as an edible, invisible film for shelf life extension of seafoods [93]. Chitosan films with or without essential oils (anise, basil, coriander, oregano) showed antimicrobial activity against both L. monocytogenes and E. coli. Films also reduced L. monocytogenes on bologna meat stored at 10 °C for 5 days [94]. Sensory evaluation suggested that addition of 45 ppm or less of oregano oil to bologna would be acceptable to consumers [326]. Addition to strong inhibition against A. niger, chitosan films were found to have potential in limiting dehydration of food products [95]. Lysozyme-chitosan composite films have been developed for enhancing the antimicrobial properties of chitosan. Excellent biocompatibilities between chitosan and lysozyme and homogenous distribution of lysozyme throughout the chitosan matrix were obtained. The films with 60% lysozyme incorporation increased the inhibition efficiency of chitosan films against both S. faecalis and E. coli [96]. Antimicrobial films have also been prepared by incorporating acetic or propionic acid into a chitosan matrix, with or without addition of lauric acid or cinnamaldehyde, and were applied onto bologna, reguler cooked ham or pastami. Regardless of film composition or meat type, propionic acid was nearly completely released from the chitosan matrix within 48 h of application, whereas release of acetic acid was more limited, with 2-22% of the acid remaining in chitosan after 168 h of storage. The growth of Enterobactericeae including Serratia liquefaciens was delayed or completely inhibited as a result of film application. Strongest inhibition was observed on drier surfaces (bologna), onto which acid release was slower, and with films containing cinnamaldehyde [91]. Trans-cinnamaldehyde has also been added to polyamide solution and applied onto

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LDPE films having antimicrobial activity against Listeria innocua. Films were further used as wrappings for fresh-cut romaine lettuce together with electron beam irradiation treatment. Synergistic effects were obtained with low-dose irradiation (up to 1.0 kGy) and transcinnamaldehyde containing films against aerobic micro-organisms including mold and yeast growth [292]. Active agent release rate on irradiated films decreased as compared to the nonirradiated films, thus electron beam irradiation can be exploited as a controlling factor for release of antimicrobial compounds [319]. Trans-cinnamaldehyde solutions sprayed on the surface of ready-to-use carrots increased the radiation sensitivity of L. monocytogenes [293]. Calcium caseinate coating containing trans-cinnamaldehyde was necessary to reduce the L. innocua growth when carrots were packed under air, whereas under modified atmosphere the antimicrobial coating was not necessary to inhibit bacterial growth [294]. The idea of using chitosan combined with paper is not new. It has been used as a papermaking additive or for surface treatment of paper for decades. Chitosan graft copolymers have been exploited for making paper products of improved dry strength [97] and chitosan has been added to α-cellulose and unbleached sulfite to increase burst, dry tensile and wet tensile properties of handsheets [98]. Its use has been recommended in the manufacture of electric insulation papers and various types of technical papers, particularly wet strength papers [99]. Chitosan has been proved to meet the criteria for wet strength agents [100]. Effects of chitosan treatments on 17 varieties of paper have been studied, and found out that surface treatment with 1% chitosan solution improved all strength properties of paper [101]. Treatment with 0.05-0.3% solution enhanced mechanical as well as printing properties and reduced the consumption of sizing agents [102]. Soaking in chitosan solution improved surface strength, softness and permeability of the machine-made paper [103]. Chitosan dissolved in acetic acid was mixed with pulp, and converted to sheets, which could then be used as a food wrapping [104,105]. Water-insoluble, biodegradable food packaging composite materials of chitosan and cellulose have been developed and patented [106,107]. Cationic chitosan acetate blends with poly(vinyl alcohol) and gelatinous starch have been used as fillers for paper [108]. Chitosan has also been precipitated onto wood pulp and glass fibers to be formed into paper sheets [109]. Chitosan curtain-coated onto paperboard showed good adhesion and high oxygen barrier properties [110]. As chitosan is a cellulose-binding polysaccharide with a strong adhesion to cellulose, it is an interesting component to mix with cellulose into composites [111]. Chitosan oligosaccharide and chitosan polysaccharide have been used to impart the antimicrobial properties to paper for food packaging against S. aureus. Chitosan oligosaccharide showed the greatest activity and gave promising results even at the concentration of 0.005%. It also improved the tensile strength of the paper [112]. Nisin and /or chitosan have been coated, in 3% concentrations, onto paper with a binder medium of vinyl acetate-ethylene copolymer (pH 4.4) to provide antimicrobial activity for use in food packaging. The prepared coating solutions were coated on one side of the virgin paperboard after which the paper was dried at 60 °C. The paper coated with nisin was more effective than the chitosan-coated paper in inhibiting L. monocytogenes, whereas the latter was more effective against E. coli. Combined inclusion of nisin and chitosan in the coating gave antimicrobial activity against both bacterial strains [67]. Paperboard coated with nisin and/or chitosan in a binder of vinyl acetate ethylene copolymer has also been used as antimicrobial packaging material for pasteurized milk and orange juice. Coated paperboard significantly improved the microbial stability of milk and juice stored at 3 °C and 10 °C, but not so

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noticeably at 20 °C. Combination of nisin and chitosan gave the highest microbial inhibition [284]. Chitosan and nisin have been incorporated into konjac glucomannan films resulting activity against S. aureus, L. monocytogenes and B. cereus. Chitosan and nisin was more effective combination as compared to konjac glucomannan and nisin combination at each corresponding concentration [302]. Antimicrobial activity of chitosan incorporated with garlic oil has been compared with potassium sorbate and nisin at various concentrations. Incorporation of garlic oil up to levels at least 100 μL/g, potassium sorbate at 100 mg/g and nisin at 50 mg/g of chitosan were found to be effective against S. aureus, L. monocytogenes and B. cereus. Garlic oil did not have any effects on the physical and mechanical properties of chitosan films as it did not have any interaction with the functional groups of chitosan [281]. Chitosan/ layered silicate (rectorites) nanocomposites have been shown stronger antimicrobial activity particularly against gram-positive bacteria as compared to pure chitosan without silicates [338]. Also silver containing chitosan-based nanocomposite films have been exhibited increased activity against various microbes [341]. A grapefruit seed extract (GFSE) at concentrations of 0.5% or 1.0% has been incorporated in the food contact surface of multilayered PE film by a co-extrusion or solution coating process. GFSE coated with the aid of a polyamide binder resulted in more efficient antimicrobial activity on the agar plate medium than did its incorporation by a co-extrusion process. The film co-extruded with a 1.0% GFSE layer showed antimicrobial activity only against M. flavus, while the film coated with 1.0% GFSE also showed activity against E. coli, S. aureus, B. subtilis, Leuconostoc mesentroides, S. cerevisiae and B. cereus. Both types of GFSE-incorporated multilayer PE films exhibited a reduction of the growth rates of aerobic and coliform bacteria on the ground beef [113]. LDPE films have been produced with addition of antimicrobial extracts of Rheum palmatum and Coptis chinensis as well as sorbic acid and silver-substituted inorganic zirconium matrix in 1% concentration. The extracts or silver-substituted zirconium did not show any antimicrobial activity on the disk tests, while film with sorbic acid had antimicrobial against E. coli, S. aureus and L. mesenteroides. For the packed curled lettuce and cucumber stored at 5 °C and 10 °C, all the films showed the reduced growth of total aerobic bacteria in the vegetables as compared to control films without any additives [114]. Linear low density polyethylene (LLDPE) films containing either linalool or methyl chavicol (constituents of basilika) have been prepared by absorption, coating and blown-film extrusion in concentrations varying from 0.05% to 1.5%. All films were antimicrobially active against E. coli [115]. Linalool (0.54 and 1.19%) containing extruded LDPE films have also inhibited the growth of E. coli and L. innocua on packed cheddar cheese [317]. Antimicrobial activity in the vapour-phase of PP/EVOH and PE/EVOH films incorporated with essential oils such as cinnamon, oregano, clove and cinnamon fortified with cinnamaldehyde was evaluated against wide range of microbes. Films with a concentration of 4% of fortified cinnamon or oregano essential oil completely inhibited the growth of the fungi. Higher concentrations (8 and 10%) were required to inhibit the gram-positive bacteria and higher concentrations still were necessary to inhibit the gram-negative bacteria [332]. Cellulose ether films containing cinnamaldehyde have shown stronger activity against grampositive bacteria, such as L. monocytogenes and S. aureus as compared to gram-negative bacteria, such as E. coli and salmonella [339]. Calcium caseinate and whey protein isolate (WPI) films containing oregano and/or pimento essential oils have been applied on beef

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muscle slices to control the growth of pathogenic bacteria and increase the shelf life during storage at 4°C. Films containing oregano were the most effective against both E. coli and Pseudomonas spp. Pimento-based films presented the highest antioxidant activity [116]. Oregano, rosemary and garlic essential oils (1-4%) incorporated into (WPI) films have been produced in order to create antimicrobial activity against E. coli, S. aureus, salmonella, L. monocytogenes and L. plantarum. Films containing oregano essential oil were the most effective against tested strains at 2% level. The use of rosemary did not exhibit any antimicrobial activity whereas the inhibitory effects of garlic were observed only at 3% and 4% levels [275]. Oregano, cinnamon and lemongrass oils have been incorporated into apple puree solutions and cast into edible films. Films had activity against E. coli and oregano oil had significantly greater activity as compared to cinnamon oil and lemongrass oil [334]. Lemongrass oil has also been incorporated into films made from partially hydrolyzed sago starch and alginate. Antimicrobial activity against E. coli was enhanced in the presence of glycerol as compared to films without glycerol [337]. Soy protein isolates and octenylsuccinate modified starch have been used as paper coating and inclusion matrices for cinnamaldehyde and carvacrol having antimicrobial activity against E. coli and Botrytis cinerea [336]. Heat-pressed starch-casein films containing neem extract had antimicrobial activity against E. coli, S. aureus, B. cereus, L. monocytogenes, Pseudomonas spp. and salmonella [313]. Thymol and eugenol have been incorporated into ethylene vinyl acetate (EVA) and coated onto LDPE films using the solution-coating method. Films were effective against L. monocytogenes, B. cereus, S. aureus and E. coli [342]. Thymol and carvacrol have also been blended into masterbatches containing LDPE and EVA and further compression moulded into films having antimicrobial activity [343]. Bacteriocin-like inhibitory substance produced by Pediococcus parvulus has been incorporated into protein film matrices of corn zein and whey protein isolate. Corn zein films were more effective in reducing L. monocytogenes as compared with whey protein isolate at the same concentrations of active agent [117]. Volatile essential oils and oleoresins from spices and herbs have been added to paper and tested against a range of fungi commonly found on bread. Mustard essential oil of which active component is allyl isothiocyanate (AITC) showed the strongest effect. Cinnamon, garlic and clove also had high activity, while oregano oleoresin only inhibited growth weakly. 3.5 μg/ mL of AITC in gas phase was fungicidal to all tested fungi [118]. Although AITC is not yet approved as a preservative in foods in Europe, it’s well-known as a flavouring substance. It has a GRAS status in the USA and it is also permitted in Japan as a preservative, provided that the compound is extracted from natural sources [210]. Also the sorption and permeation behaviour of AITC vapour in polyamide film has been studied. The barrier of the polyamide against AITC can be weakened by moisture uptake in high humidity, thus activating the release of AITC vapour [119]. Microencapsulated AITC has been shown to inhibit the growth of E. coli in refrigerated, nitrogen packed, finely chopped beef [297]. AITC in combination with modified atmosphere packaging was more effective than conventional MAP to prevent salmonella, Listeria, and total aerobic bacteria on chicken at 4 ˚C [303]. Polylactide (PLA) and polylactide-polycaprolactone (PLA-PCL) copolymer films have been compounded with cyclodextrin (CD) complexes designed to provide slow release of encapsulated antimicrobials such as AITC, thymol, citral and imazalil for control of mould growth on packaged cheese. In direct contact the CD-citral and CD-thymol complexes

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exhibited very little inhibitory effect against moulds tested whereas by using the CD-AITC and CD-imazalil the growth of Penicillium commune, P. roqueforti, P. nalgiovense, P. verrucosum, P. caseifulvum, P. camemberti and Kluyveromyces marxianus was completely inhibited. The CD-AITC was effective even in indirect contact with P. commune, P. roqueforti and P. nalgiovense whereas the CD-imazalil in this case generated very little inhibitory effects [210]. A combination of both modified atmosphere packaging and volatile mustard oil has been proved to be effective resulting in total inhibition of molds with 2 μL mustard oil/ rye bread slice and 2-3 μL/ wheat bread during the storage period of 30 days [120]. Sachets containing encapsulated ethanol have been shown to prevent microbial spoilage of intermediate moisture foods, cheese products and sweet bread [121]. N-vinylacetamide polymer has been used to absorb ethanol and the resulting gel was packed in a heat-sealable bag made from a laminate of nylon and EVA. The ethanol releasing system inhibited the growth of mold in bread [250]. To mask the odour of ethanol or AITC, some sachets containing small amounts of flavours could be added into packages. Natural occurring antifungal plant volatile, 2E-hexenal, has been inserted into βcyclodextrins and further mixed with PLA resin. Pre-forms were placed in a hydraulic press where antifungal films were obtained. Storage of blueberries along with these films showed an improvement in fruit shelf life due to lower weight losses, delayed respiration rates and reduction in infected blueberries [306]. Also acetaldehyde and hexanal incorporated into βcyclodextrins and further mixed with PLA exhibited antifungal activity against Colletotrichum acutatum, Alternaria alternate and Botrytis cinerea [316]. 0.5% of hexamethylenetetramine (HMT) has been incorporated into LDPE films and used as antimicrobial package for cooked ham stored at 6 °C. After 20 days a significant reduction of total aerobic count and lactic acid bacteria was observed. The shelf life of orange juice, however, could not be prolonged significantly [44]. Lactoperoxidase system is of interest for use in foods and packaging due to its naturally occurring antimicrobial components such as H2O2, hypothiocyanite ion and hypothiocyanous acid. Antimicrobial activity of alginate films incorporated with lactoperoxidase system was obtained against E. coli and L. innocua. H2O2 was a critical limiting factor in lactoperoxidase system and inhibitory effect was generally ended due to exhaustion of H2O2 and reactive thiocyanate oxidation products [305]. Imazalil has been used as an antimycotic agent incorporated into LDPE film. Films containing 1000 ppm imazalil inhibited both Penicillium sp. and A. toxicarius moulds growing on Cheddar cheese [122]. To prevent mould growth on the surface of cheese and the possible formation of mycotoxins, cheese coatings containing imazalil have also been applied. The coatings were as emulsions of synthetic materials which were dried to a film when applied to the cheese surface. They were composed of a co-polymer (vinyl acetate – maleic acid-di-n-butylester), annatto and water. Coatings were proved to be antimicrobially active against moulds. Additional safety issues were not raised by the use of imazalil, given that the residue levels will not exceed 3 mg/kg of cheese [123]. Imazalil has also been incorporated into LDPE and used as shrink-wrapping film for peppers [124]. Chlorine dioxide releasing antimicrobial tags can be effective with highly perishable foods such as berries etc. or where bacteria caused odour is an issue. Tags activate when exposed to moisture. For example, sodium chlorite has been incorporated into plastics and

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converted into chlorine dioxide which is highly soluble in water. Within the plastic, water reacts with a hydrophobic phase component to produce an acid that migrates to a hydrophilic phase, converting ionomeric chlorite to chlorine oxide. Chlorine oxide reacts with organic matter such as bacteria, and produces hypochlorous molecules and oxygen ions. The hypochlorous molecules penetrate the cell membranes of bacteria and react with metabolic enzymes, while the oxygen ions trigger albuminoid degeneration in bacteria [304]. Chlorine dioxide is highly active against a broad spectrum of microbes functioning at very low concentrations down to 0.05-5 ppm. The end products of the reaction, chloride ions, are harmless [75]. Also powdered chlorine dioxide can be moisture activated and released as gas inside the sealed food package. It has ability to kill of a full range of spoilage microbes including yeasts, molds, E. coli, listeria, salmonella and campylobacteria. Chlorine dioxide source can be incorporated into the structure of films, for example lid stocks. Such films may be available in clear form and approved for use in direct contact with foods [125]. Chlorine dioxide combined with modified atmosphere packaging reduced the population of salmonella in fresh chicken, but it was less effective against the natural microflora and had an adverse effect on the colour of the chicken placed close to the sachets [290]. The increased grape quality maintenance has been achieved by combining the use of generators of SO2 with a slightly CO2 enriched atmosphere. Below 10% of grapes were affected by Botrytis cinerea during 60 days at 2 °C followed by 4 days at 20 °C. Over 50% of grapes stored at same temperatures in normal atmosphere were spoiled by B. cinerea [272]. TiO2 has been incorporated into polyhydroxybutyrate (PHB) films in order to improve the photocatalytic antimicrobial activity against E. coli. Under UVA illumination, the number of surviving bacteria decreased exponentially before reaching non-detectable level. Illumination time up to 6 h resulted in more than 99% inactivation as compared to the samples stored in dark condition [249]. TiO2-coated film has also been prepared by spraying TiO2 solution onto PP film and dried in air for 72 h prior to use as wrappings for Penicillium expansum inoculated lemon fruits. Packed fruits were stored at 25 °C for 14 days under UVA irradiation. TiO2-coated films exhibited photocatalytic reaction against P. expansum, and no surface damage was caused by a coating on lemon fruit [271]. Triclosan (2,4,4´-trichloro-2´-hydroxydiphenyl ether), a non-ionic, broad-spectrum, antimicrobial agent has been used in many personal hygiene products, including e.g. hand soaps, deodorants, shower gels, mouthwashes and toothpastes. Triclosan can also be added during the extrusion of plastic and fibers. Some products that are manufactured with this technology include cutting boards, garbage bags, carpet, surgical gauze and bathroom fixtures. It has also been used as coated onto plasma–modified PE with excellent long-term antimicrobial properties against E. coli and S. aureus [126]. A coating made of styreneacrylate copolymer containing triclosan inhibited the growth of E. faecalis [329]. Despite its apparent effectiveness, it is not a substitute for conventional antimicrobial systems because its action in package films for food has not been demonstrated [75]. Photosensitization requires the presence and interaction of light, oxygen and photosensitizer. The initiating step is the absorption of a light photon by the sensitizer resulting in an extremely unstable excited singlet state or longer lived triplet excited state. The interaction of the triplet sensitizer with surrounding molecules results in photo-oxidative reactions forming peroxides, superoxide ions, hydroxyl radicals or singlet oxygen [273]. Chlorophyllins are semi-synthetic porphyrins which can be activated by visible light. The

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photodynamic effect of chlorophyllins incorporated into gelatine films produced considerable antimicrobial activity against S. aureus and L. monocytogenes [340]. 2.3.2. Non-Leachable (Immobilized) Antimicrobials Besides incorporated antimicrobials which will eventually be released, some novel packaging systems utilise immobilized active agents. This type requires typically a long-chain molecular structure large enough to retain activity on the microbial cell wall even though bound to the plastic. Active enzymes can be immobilized onto food packaging surfaces. For example, naringinase for reducing bitterness, lactase and cholesterol reductase for reducing lactose and cholesterol, glucose isomerase for increasing sweetness, lysozyme, chitinase or lactoferrin for producing antimicrobial activity and glucose oxidase, lactoperoxidase or sulfhydril oxidase for absorbing oxygen and/ or producing antimicrobial activity. Various immobilization methods, such as ionic and covalent immobilization, crosslinking, graft copolymerization and entrapment, have been described [129]. Most of them require the presence of suitable functional groups on both the antimicrobial substance and the packaging surface. Examples of antimicrobial substances with functional groups are peptides, enzymes, polyamines and organic acids [21]. Plasma treatment can be utilized for modifying surface properties of food packaging materials to improve both safety and quality of foods [128]. In addition, it can be used to generate suitable anchor groups for active substances. The functional groups that can be utilized in the covalent immobilization of antimicrobial substances include for example amino, carboxyl, hydroxyl and phenolic groups. NH3 and CO2 -plasmas have been used to incorporate amine groups [128,130-134] and carboxyl groups [135,136] at polymer surfaces. Bifunctional reagents like carbodiimides and glutaraldehyde can be used as covalent binding agents between the suitable functional groups. Carbodiimides are generally utilized as carboxyl activating agents for amide bonding with primary amines whereas glutaraldehyde is used as coupling agent between amines. Covalent binding often exhibits the highest stabilization of enzyme activities because the active conformation of the immobilized enzyme is stabilized [129]. The total activity of covalently immobilized enzyme is typically lower than the activity of free enzyme. However, for example the immobilized glucose oxidase has been shown to retain its activity over a wider temperature and pH range than soluble enzyme [137]. Also the storage stability of immobilized glucose oxidase was notably higher than that of free enzyme [138,139]. During the years glucose oxidase has been used as immobilized with many different substrates [137,140-142]. Recently it has been immobilized onto electrochemically prepared poly(aniline-co-fluoroaniline) [143] and chemically modified acrylonitrile copolymer [138] films as well as in chitosan gel beads [144]. Immobilization of glucose oxidase can be done for example by covalent binding of the amino group of the enzyme and the activated surface via suitable bifunctional reagents. Also lysozyme has been immobilized onto cellulose, polyacrylamide, chitosan, silica gel, glass, stainless steel, polystyrene, polyvinylalcohol, cellulose triacetate and polyamide matrices [145-149, 270,312]. Lysozyme immobilised onto polyvinylalcohol and polyamide showed low activity, while on cellulose triacetate (CTA) it had strong efficiency against M. luteus. The amount of enzyme had a clear effect on activity and the highest activities were

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obtained with CTA films formed by adding 150-250 mg lysozyme/g polymer [148]. Recently, lysozyme has been completely immobilized onto the glyoxal crosslinked PVOH films using glutaraldehyde as a binding agent. Films were effective in inhibiting the growth of M. luteus and Alicyclobacillus acidoterrestris and the activity increased as the amount of lysozyme increased [150,298,299, 328]. Chemically modified chitosan can be attached to various substrates e.g. sugars, dendrimers, cyclodextrins, crown ethers, and glass beads, forming interesting multifunctional materials [151]. Recently, plasma treatments have been exploited to improve the adhesion between chitosan and other polymers like PP [152] or PLLA [153]. Chitosan has also been immobilized onto acrylic acid grafted HDPE [154], PP [155-157], PET [158,159], glass [160], PHB [161] and polysulfone (PSF) [283] using carbodiimide [155,159,160] and onto Nvinylformamide (NVF) grafted PET [158] or silk fibroin [162] via glutaraldehyde [158]. Laccase-enzyme catalysed grafting of different phenol compounds to unbleached kraft liner paper has been shown to increase antimicrobial activity. Among the tested essential oil compounds, covalently bound isoeugenol was the most effective having activity against S. aureus, E. coli, B. subtilis, E. hirae, S. epidermidis, P. aeruginosa and K. pneumoniae [309]. Quaternary ammonium salts (QAS) have been known for a long time as antimicrobially active agents. They provide broad spectrum antimicrobial activity to the surface of a wide variety of substrates. They are leach-resistant and non-migrating and not consumed by microbes. They are allowed to be used in several non-food contact applications, but not in direct contact with food. However potatoes have been packed in plastic films first coated with adhesive containing 1% of silicon quaternary ammonium salt. In this case the treatment had no apparent advantage for wrapped potatoes stored at 24 °C [127]. Octadecylammoniumtrimethoxysilane (ODAMO) has also been applied onto plasma activated BOPP films in order to produce antimicrobial packaging surfaces. High activity was obtained against E. coli, B. subtilis and M. luteus. QAS have been chemically coupled to the various plastic surfaces resulting activity against e.g. E. coli, M. luteus and A. niger. Coatings formed thin layers of only 2 nm having no influence on the bulk properties. Immobilized QAS have also been exploited in manufacturing of antimicrobial textiles and non-wovens for many years. Light activated porphyrins have been grafted to polyamide fibers and films [163,164] as well as regenerated cellulose films [165]. The fibers were shown to be active against S. aureus at light exposures of 10.000 lux and greater and against E. coli at 60.000 lux. Without light they were ineffective against both strains. Regenerated cellulose films were antimicrobial against S. aureus, E. coli, P. vulgaris and B. subtilis. The effects were considered to be caused by singlet oxygen generated at the film surface. Interestingly, the singlet oxygen did not need a direct contact with bacteria, thus it operated through an air gap of 1 mm causing direct cell damage after diffusion to the microbial wall. The photosensitization methodology might be used as decontamination tool for different raw materials, foodstuffs or even food packaging surfaces [274]. LDPE film has been activated using a UV/TiO2/O3 method which created polar functional groups onto surface. Silver nanoparticles were then linked on the surface using peptide linker. Antimicrobial activity against E. coli and airborne microbes was obtained [318]. Bacteriocins and especially nisin have been adsorbed or immobilized onto several substrates, such as ionomer films [166-167], silica surfaces [168,169], paper and PE [170] as well as ion-exchange resins and adsorbents [171]. The antimicrobial properties of halogens

66

Jari Vartiainen

have been known for years, and they have been used as sterilizing agents in the food industry. Fluorine based (CF4, C2F6, C3F6, C4F8 etc.) plasmas can be used to form fluorinated surface layers on food packaging materials [128]. Also an imazalil-bound ionomer film with antifungal properties has been reported [172]. Table 4. Antimicrobial packaging materials studied during the years. Antimicrobial substance Sodium benzoate Potassium sorbate Nisin + EDTA Imazalil Sorbic acid

Packaging material Caseinate films Chitosan films Keratin films

Casein films Carnauba wax films

Target microbe B. subtilis

A. niger S. rouxii, A. niger

Food product

Reference

-

[173]

Papaya and apricot cubes

[174]

Tocopherol

Gelatin films

-

Margarine

Citric acid

Pectinate, pectate, zein films Paper/ plastic bag

-

Nuts

Penicillium spp., A. flavus, Endomyces fibuliger S. aureus, L. monocytogenes, P. aeruginosa

Rye and wheat bread

[120]

Cheese

[175]

[176]

Mustard oil

Chitosan

Chitosan coating

Chitosan

Chitosan coating

L. innocua, L. monocytogenes

Emmental cheese

Chitosan

Chitosan coating

Alternaria sp, Penicillium sp, Cladosporium sp.

Precooked pizza [177]

Chitosan

Chitosan coatings and films

A. niger

-

[95]

Lysozyme

Chitosan films

S. faecalis, E. coli

-

[96]

Antimicrobial Surface Coatings in Packaging Applications

67

Table 4. Continued Antimicrobial substance Benzoic acid

Benzoic acid Sorbic acid

Nisin

Potassium sorbate Acetic acid Nisin

Packaging material

Target microbe

Food product

Reference

Methylcellulose Zygosaccharomyces coatings rouxii, Zygosaccharomyces mellis Methylcellulose P. notatum, Rhodotorula coatings

Fruit preserves

[178]

-

[81]

-

[179]

Salmonella

Tomatoes

[180]

L. innocua, S. aureus

-

[60]

[181] Turkey breast (fresh poultry), ham (processed meat), beef (fresh meat) Chicken breast [182] Lean beef muscle [183]

Methylcellulose/ chitosan coatings Methylcellulose M. luteus coatings Hydropropyl methylcellulose coatings Hydropropyl methylcellulose films Hydropropyl methylcellulose films

Pediocin

Cellulose casings Plastic barrier bags

L. monocytogenes

Potassium sorbate Lactic acid

Starch films Alginate films

Glucose oxidase p-Aminobenzoic acid Potassium sorbate Acetic acid Lactic acid Nisin

Alginate films Whey protein based (WPI) films

E. coli, salmonella E. coli, salmonella, L. monocytogenes L. monocytogenes, E. coli, salmonella

Whey protein based (WPI) films

L. monocytogenes

Fish Bologna slices Sausage slices Hot dogs

[184] [185-187]

-

[188]

68

Jari Vartiainen Table 4. Continued Antimicrobial substance

Packaging material

Whey protein Nisin + EDTA Lysozyme + EDTA based (WPI) films Propyl paraben

Target microbe B. thermosphacta, salmonella, E. coli, L. monocytogenes, S. aureus L. plantarum, E. coli

Food product

Reference

-

[188-189]

Nisin Lysozyme + EDTA Nisin Nisin

Soy protein isolate films Corn zein films Corn zein films L. monocytogenes Corn zein films L. monocytogenes

-

[57]

Milk Chicken

[190] [191]

Potassium sorbate Sorbic acid

Corn zein films Corn zein films Wheat gluten films LDPE films Chitosan films

S. aureus L. monocytogenes

Cheese Cooked sweet corn

[46] [192]

L. monocytogenes Enterobacteriaceae, Lactobacillus sakei, Serratia liqueficiens

Bologna, cooked ham, pastrami

[68] [193]

Moulds

[194] Lettuce, turnip greens, fresh ground mince, kiwifruit, strawberries, rock cake, bread rolls Cheese [195] [38]

Nisin (adsorbed) Acetic acid Propionic acid Lauric acid Cinnamaldehyde Allyl isothiocyanate Labels

Silver Nisin Tocopherol

Nisin Sorbic acid Potassium sorbate Benzoic anhydride

Benzoic acid Sorbic acid

Plastics Vinyl acetate- M. flavus ethylene copolymer binder (Elvace) on paper PVDC films L. monocytogenes

LDPE films

Rhizopus stolonifer, Penicillium spp., A. toxicarius Poly(ethylene- Penicillium spp., A. co-methacrylic niger acid) PEMA

-

[53]

Cheese

[72]

-

[73]

Antimicrobial Surface Coatings in Packaging Applications

69

Table 4. Continued Antimicrobial substance

Packaging material

Target microbe

Food product

Reference

Grapefruit seed extract

LDPE films

S. aureus, E. coli

Curled lettuce, [196] Soybean sprouts

Potassium sorbate Nisin Citric acid Lactic acid Malic acid Tartaric acid

LDPE films Soy protein films

Yeasts L. monocytogenes, E. coli, salmonella

-

[54] [197]

Hexamethylenetetramine

LDPE films

Yeasts, lactic acid bacteria

Orange juice, Cooked ham

[44]

Benzoic anhydride

LDPE films

-

Cheese, Toasted bread

[32]

Nisin (adsorbed)

PVC films LLDPE films Nylon films Polyamide coatings on LDPE films

Salmonella

Broiler

[65]

Total aerobic bacteria, coliform bacteria

Fresh oyster, Ground beef

[198]

Benzoic acid Benzoyl chloride

Ionomer films

Penicillium spp., A. niger

-

[199] [296]

Nisin (adsorbed)

L. innocua, S. aureus Plastic film (70:30, PE:PA)

Sliced cheese and ham

[170, 200]

Nisin, Lacticin from Lactococcus lactis

Imazalil

Silicon quaternary ammonium salt

Greaseproof and moisture resistant packaging papers Vinyl acetate copolymer coatings

L. innocua, S. aureus -

Molds, Penicillium echinulatum

Adhesive layer Helminthosporium on plastic film solani (Cryovac D955)

Hard and semihard cheeses

[123]

Potatoes

[127]

70

Jari Vartiainen Table 4. Continued

Antimicrobial substance

Packaging material

Target microbe

Food product

Reference

Extracts (Rheum palmatum, Coptis chinensis) Ag-substituted zirconium

LDPE films

-

Fresh curled lettuce cucumber

[114]

Cheddar cheese

[115,317, 324]

-

[61]

-

Sorbic acid

E. coli, S. aureus, Leuconostoc mesenteroides E. coli, L. innocua

Basil (linalool, methyl chavicol)

LDPE films

Nisin

Sorghum starch and flour films

Nisin Lauric acid EDTA

Corn zein films L. monocytogenes, salmonella

-

[59]

Butylated hydroxyanisole (BHA)

Corn zein films -

Turkey breast

[201]

Lauric acid Nisin

Soy films

L. monocytogenes

Turkey bologna

[202]

Grapefruit seed extract (Citrex)

LLDPE films

M. flavus

Ground beef

[113]

Polyamide coatings on LDPE films

M. flavus, E. coli, S. aureus, Leuconostoc mesenteroides, B. cereus, B. subtilis, S. cerevisiae

Nisin

LDPE films

L. helveticus, B. thermosphacta

Beef carcass surface tissue

[56]

Nisin Lauric acid

Corn zein films L. plantarum

-

[58]

Chitosan

Chitosan coatings

Fresh fillets of herring and Atlantic cod

[93]

Lactobacillus delbrueckii

Total aerobic psychrotropic bacteria

Antimicrobial Surface Coatings in Packaging Applications

71

Table 4. Continued Antimicrobial substance

Packaging material

Allyl isothiocyanate Paper

Nisin (adsorbed) Chitosan

Cellophane films Chitosan coatings

Chitosan Lactic acid/ sodium lactate

Chitosan coatings

Nisin

Wheat gluten films PVOH films

Lysozyme Nisin Sodium benzoate

Target microbe

Food product

Reference

A. flavus, Endomyces Rye bread, fibuliger, Pichia Hot-dog bread anomala, Penicillium commune, P. corylophilum, P. discolor, P. palitans, P. polonicum, P. roqueforti, P. solitum. Total aerobic bacteria Chopped meat

[118]

Fungi

Clementine mandarin fruit

[203]

Total aerobic psychrotrophic bacteria, lactic acid bacteria, yeast L. monocytogenes

Strawberry Lettuce

[204]

Turkey bologna

[70]

M. luteus, Alicyclobacillus acidoterrestris, S. cerevisiae Salmonella

-

[37,150]

-

[205]

[69]

Chitosan

Yam starch films

Herbs (oregano, marjoram, thyme)

Plastic film

-

Hamburger meat [206] patties, pizza

Essential oils (oregano, pimento)

Casein/ whey protein isolate films

E. coli, Pseudomones spp.

Beef muscle

[116]

Nisin

Hydroxypropyl methylcellulose films

M. luteus

-

[207]

Potassium sorbate Sorbic acid

Celluse-based coating

Salmonella

Eggs

[208]

Benzoic acid Sorbic acid

Varnish on plastic films

-

Fresh meat and cheese

[209]

72

Jari Vartiainen Table 4. Continued Antimicrobial substance

Allyl isothiocyanate, imazalil

Packaging material Cyclodextrin + PLA and PLA/PCL films

Butylhydroxytoluen HDPE films e (BHT) Sorbic acid Butter paper Lysozyme

Propyl paraben

Potassium sorbate Citric acid Calcium sorbate Benomyl Imazalil Imazalil Ethanol vapour

PVOH, nylon, cellulose triacetate (CTA) films Styreneacrylate copolymer coating on paper Starch-based coatings CMC/ paper Ionomer films LDPE films LDPE films Silicon oxide sachet (Ethicap) Paper wraps

Iodine potassium iodide, Sodium orthophenylphenate, Biphenyl, Diphenylamine, Dichloronitroaniline Chitosan BOPP films Glucose oxidase BOPP films BOPP films Chitosan Octyl gallate Dodecyl gallate

Target microbe

Food product

Penicillium commune, P. Cheese roqueforti, P. nalgiovense, P. verrucosum, P. caseifulvum, P. camemberti and K. maxianus Fresh oat flaked cereal Bacteria, yeasts, molds Paneer from milk M. luteus -

Reference [210]

[211] [55] [148]

S. cerevisiae

-

[33]

Bacteria, molds and yeasts Fungi Molds -

Strawberry

[212]

Bread Bell pepper Cheese Bakery

[213-214] [172] [124] [122] [215]

Trichothecium roseum, Glomerella cingulata, Penicillium expansum, Rhizopus stolonifer

Golden [216] Delicious apples

E. coli, B. subtilis E. coli, B. subtilis S. aureus, L. innocua

-

[217] [218] [360]

Antimicrobial Surface Coatings in Packaging Applications

73

Table 4. Continued Antimicrobial substance

Packaging material

Target microbe

Food product

Reference

Lactic acid

Chitosan coated paper

B. subtilis

-

[219]

Sodium benzoate Potassium sorbate Sodium nitrite Sodium lactate

LDPE films PS PET Poly(maleic acid-co-olefine) Cellulose casings Corn zein films

B. subtilis, A. niger

-

[220]

L. monocytogenes

Frankfurters

[221]

B. subtilis, L. plantarum, E. coli L. monocytogenes

-

[71]

Smoked salmon

[222]

Salmonella, E. coli, P. commune

Roasted turkey

[223,224, 300]

Moulds L. monocytogenes

Marmelade -

[225] [68]

Alicyclobacillus acidoterrestis

Apple juice

[226]

M. flavus

-

[35]

Nisin Lysozyme EDTA Lactoperoxidase system Lactoperoxidase system Cinnamon Nisin

Silver

Sodium propionate Sodium sorbate Sodium benzoate Nisin

Whey protein films and coatings Whey protein films and coatings PET container Methylcellulose and hydroxypropyl methylcellulose coatings on LDPE Polyethyleneoxide coating on PE films Acrylic polymer or vinyl acetateethylene copolymer binder on paper

Imazalil EDTA

LDPE films

B. subtilis, A. niger

-

[227]

Nisin

LDPE films

M. luteus

Milk

[266]

TiO2

PP films

Penicillium expansum

Lemon fruit

[271]

SO2 + CO2

Cardboard box

Botrytis cinerea

Grape fruit

[272]

Sorbic anhydride

PE films

A. niger, Penicillium sp.

-

[276]

74

Jari Vartiainen Table 4. Continued Antimicrobial substance

Packaging material

Target microbe

Lysozyme, nisin, grape fruit seed extract (GFSE), EDTA Nisin

Na-alginate, κ-carrageenanbased films

M. luteus, listeria, salmonella, E. coli, S. aureus

-

[277]

PE/biopolymer (methylcellulose, hydroxypropyl methylcellulose, κ-carrageenan or chitosan) Chitosan films

M. luteus

-

[278]

S. aureus, L. monocytogenes, B. cereus Aerobic bacteria and yeasts

-

[281]

Garlic oil, potassium sorbate, nisin Nisin Chitosan

Lysozyme

Nisin + grape seed extract (GSE) green tea extract (GTE) Bacteriocin from Lactobasillus curvatus (adsorbed) Transcinnamaldehyde

Reference

Vinyl acetate ethylene copolymer coating on paperboard Whey protein L. monocytogenes isolate (WPI) films and coatings Soy protein films L. monocytogenes and coatings

Pasteurized milk [284] and orange juice

PE films

L. monocytogenes

Polyamide coating on LDPE films

L. innocua

+ e-beam irradiation L. monocytogenes, Chlorine dioxide Sachets in Allyl isothiocyanate multilayer barrier salmonella tray Cyclodextrin + PLA Fungi 2E-hexenal Ion-exchanged zeolite Lysozyme

Food product

PE film Chitosan films

Cold-smoked salmon

[286]

Turkey frankfurters

[287]

Pork steak, ground beef

[288]

[292] Fresh-cut romaine lettuce

Chicken breast

[303]

Blueberries

[306]

E. coli, salmonella, S. Iceberg lettuce typhimurium L. monocytogenes, E. Mozzarella cheese coli, P. fluorescens, molds, yeasts

[307] [311]

Antimicrobial Surface Coatings in Packaging Applications

75

Table 4. Continued Antimicrobial substance

Packaging material

Starch coating Antimicrobial peptide dermaseptin on PE films K4K20-S4 Natamycin Celluse-based film Enterocins Alginate, zein and PVA films Chitosan Nisin, sodium coatings on lactate, sodium diacetate, potassium Surlyn film sorbate, sodium benzoate Allyl isothiocyanate Labels in packaging Sorbic acid Cellulose polymer film 2-nonanone Cinnamaldehyde – enriched cinnamon essential oil Oregano Nisin

Target microbe

Reference

Molds, aerobic bacteria

Fresh cucumber

[314]

Penicillium roqueforti

Gorgonzola cheese Sliced cooked ham Ham steaks

[315]

[325]

Cheese

[327]

Pastry dough

[330]

Strawberries Strawberries

[331] [333]

Beef meat Cooked ham

[335] [345]

L. monocytogenes L. monocytogenes

P. commune, P. roqueforti, A. flavus Mesophilic and psychotropic bacteria, Staphylococcus spp. PP/EVOH cups Botrytis cinerea Paraffin coated Fungi paper PE film PP/PA/PP interleavers

Food product

Yersinia enterocolitica Salmonella

[320,321]

Traditionally a large gap has been existing between that which has been reported in the literature and the realities of functionality in real packaging applications. The gap is narrowing for some antimicrobials, but for many, the differences between theory, laboratory scale, and commerce in some cases are still wide [228]. It should not be forgotten that in addition to antimicrobials the microbes are also affected by other factors, such as temperature, pH, chemicals, moisture content, water activity, oxygen, nutrient content and biological structures of foods etc. Also the release of antimicrobial substances from polymeric materials is affected by factors like fat, alcohol, trace metal, and organic acid content of foods [38]. For example nisin has been shown to be more active in hydrophilic environments. In real food systems nisin efficiency can be reduced by food characteristics such as high pH, high fat content, large particle size and non-uniform distribution of nisin in food [39]. Although nisin has been shown to inhibit the growth of spoilage bacterium B. thermosphacta on vacuum packaged meat, [56], there are also evidences that especially glutathione in fresh meat inactives nisin [229]. In contrast with the strong antimicrobial effect of triclosan in agar diffusion tests against L. monocytogenes, S. aureus, S. enteritidis, E. coli and B. Thermosphacta as well as in invitro simulated vacuum packaged conditions against L. monocytogenes, 0.1% of triclosan

76

Jari Vartiainen

containing LDPE film did not effectively reduce spoilage bacteria and growth of L. monocytogenes in real packaging application on refrigerated vacuum packaged chicken breasts stored at 7 °C [230]. Triclosan incorporated plastic was also found to have strong antimicrobial activity against B. thermosphacta, salmonella, S. aureus, B. subtilis, S. flexneri and several strains of E. coli measured with plate overlay assays. In experiments with vacuum packed and refrigerated beef meat, however, the bacteria were not sufficiently reduced. The possible interaction with fatty acids or adipose may be responsible for this inactivity against bacteria on real meat [231]. UV irradiated polyamide has been shown to exhibit strong antimicrobial activity at laboratory scale against S. aureus, P. fluorescens, E. faecalis, E. coli and Klebsiella pneumoniae [232-234] although all the results did not correlate perfectly with each other. It was however also seen that protein and salt inhibited the activity of these films [234]. Also the effectiveness of silver-zeolite was found to be dependent on the nutrient level, presence of salts and pH of the food. Many foods have substances that weaken the activity of silver ions, for example, sulfates, hydrogen sulfide and sulphur containing amino acids [7]. The microbial action of most antimicrobial substances is based on their reaction with proteins and enzymes in the microbial cell. Therefore, foods containing proteins may often impair the antimicrobial efficiency of the package. It has been shown, that for example starch, whey proteins and NaCl have negative effects on the antimicrobial activity of chitosan [204]. In addition, some antimicrobial substances are simply not allowed to be used with certain kinds of foods. For example HMT may not be used in combination with nitrite and nitrate (often present in meat) because this combination increases the risk to the formation of dimethylnitrosamines [44]. For the above mentioned reasons every food application needs to be investigated separately. Before the packaging material is ready for an industrial production it must be storage tested carefully with real foodstuffs. The regulatory aspects of antimicrobial packaging materials in general must be clarified as well.

3. Conclusion The efficiency of active packaging materials based on leachable antimicrobials is always diffusion dependent and therefore problematic to control. The releasing rates and total amounts of active substances should always meet the requirements set by the each individual food system as well as the current packaging legislation. Addition to material parameters such as chemical structure, free volume, crystallinity, polarity, tacticity, crosslinking and orientation the diffusion is also time and temperature dependent. Surface interactions between food and packaging material may also have effects on diffusion. The properties, such as mechanical, optical, barrier and sealability of commercial packaging materials are highly optimised during the years of material development phase. Processing methods and equipment are designed to produce these materials at industrial scale and cost-effectively. New antimicrobial packaging materials which are prepared by mixing or incorporation of active substances are often problematic to be integrated into existing production lines and raw materials. In addition, due to the complexity of diffusion phenomena their effectiveness and safety may be difficult to assure. Adding antimicrobial substances into well-optimised commercial packaging materials may have drastic effects on the base

Antimicrobial Surface Coatings in Packaging Applications

77

properties and quality. It may even be impossible to produce the doped material without making major changes to process parameters or even to equipment. This all will naturally cost a lot of time and money. Thus the surface treatment of existing commercial packaging materials seems to be a better option. Antimicrobial surface treatment can be done by several ways such as coating, printing, grafting or covalent binding. Other surface pre-activation methods such as physical, chemical or enzymatic treatments or their combinations may be necessary to produce permanently coupled antimicrobial agents. By using surface treatments the harmful effects on valuable bulk properties of packaging materials can be minimised. Also the safety aspects should be easier to fulfil as migration of substances can be kept at very low level. Antimicrobial surface treatments can be completely separated from the high-volume production lines of bulk materials. They can be done with smaller scale equipment immediately before the packaging is formed ensuring the maximum antimicrobial efficiency. Development of antimicrobial packaging materials, which can be produced at commercial scale, is a challenging and promising area, where intensive research is still required. They can be exploited in direct contact with certain foods only and each food system must be investigated separately.

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In: Surface Coatings Editors: M. Rizzo and G. Bruno, pp. 93-122

ISBN: 978-1-60741-193-2 © 2009 Nova Science Publishers, Inc.

Chapter 3

ENVIRONMENTALLY FRIENDLY CONVERSION COATING APPLICATIONS FOR HOT ROLLED STEEL (HRS) PRIOR TO POWDER COATING APPLICATION Bulent Tepe University of Ulster, Jordanstown, Co. Antrim, UK

Abstract Hot rolled steel (HRS) is extensively used in a wide range of applications by many different industries such as automotive, domestic appliances, defence etc. It is common knowledge that hot rolled steel comes with oxide scale, often called mill scale, on the surface, due to the hot rolling process. Despite the disadvantage of oxide scale on HRS, it is still one of the most popular materials used in industry due to its availability, cost and ease of profiling properties. One of the most important coating applications for HRS is powder coating, which has a number of advantages over its favourability to wet coating, therefore it is widely used for HRS components in industry, prior to powder coating, to increase corrosion and blister resistance and enhance adhesion pre-treatment systems are used. Pre-treatment systems usually contain five or more stages: cleaning, rinsing, conversion coating, rinsing and passivation. Conversion coating is the most important stage in the pre-treatment process and it is usually phosphating. Phosphating offers many advantages, however it is considered as a hazardous material to human health and the environment. The phosphating process creates sludge, which results in pipe and pump blockages and sludge built up in the phosphating tank. These concerns have driven chemical companies to conduct research aimed at finding a conversion coating that meets the requirements of health and safety and is environmentally friendly. Some companies have already developed environmentally friendly conversion coating systems which are promoted as ecological material and an alternative to the phosphating process. The main objective of this chapter is to evaluate the ability of commercially available environmentally friendly pre-treatment systems as a metal pre-treatment in finishing operations, to eliminate or reduce the amount of environmentally hazardous and toxic chemicals. This objective must be accomplished whilst maintaining equal or better product performance properties, with economic benefit or no significant economic penalty to the metal finishing companies who would like to change their pre-treatment system to an environmentally friendly pre-treatment system. The evaluation focuses on technical performance and economics while validating the laboratory tests and environmental benefits.

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Bulent Tepe In order to evaluate the conversion coatings’ performance studies on: corrosion behaviour, adhesion and blister resistance, salt spray, prohesion test, Electrochemical Impedance Spectroscopy (EIS) measurement, cross hatch test, conical bend test, pull-off test, humidity test and surface morphology were performed. In this chapter the most popular environmentally friendly conversion coatings were evaluated. Environmentally friendly coatings are usually Silane and Zirconium based.

Introduction Chemical conversion coatings containing phosphate are widely used in the engineering industry for pre-treatment of aluminium, alloys and steel [4.[1] ]. Phosphate conversion coating is known to furnish a moderate degree of corrosion protection to the unpainted substrates and provides an excellent base for adhesion of organic coatings [4.[2] ]. Phosphating offers many advantages; however it is considered as a hazardous material and is costly to operate, as it is uneconomical to process in waste water treatment because it has a high concentration of metal ions and acid and an extremely reactive treating agent. Treatment with a phosphating agent results in water-insoluble salts, which are deposited as a precipitate. Such a precipitate is generally referred to as sludge and considered problematic due to the cost of removal. Sludge creates many issues such as: • •

• •

•

•

Creation of high volumes of waste. Clogging of spray nozzles: Sludge blocks spray nozzles in the spray cabinet, reducing contact between conversion solution and substrate. This creates quality and maintenance issues such as weekly acid cleaning of spray nozzles. Scaling on the inside of the washer: Sludge built-up in the spray cabinets, requires acidic cleaning, which introduces health and safety related issues. Blocking of pipes and pumps. Blocked pipes and pumps affect the quality and performance of the conversion coating and require weekly maintenance therefore, both cost and health and safety related issues increase. Unexpected pump failure due to sludge is a common problem in phosphating plants. Figure 1 shows spray pipes and pumps blocked by sludge. High operating temperature: Sludge inside the tank covers the heat exchanger preventing transfer of heat to the tank solution. Therefore heating energy cost for conversion coating solution increases. Figure 2 shows sludge built-up in the zinc phosphate tank. Excessive sludge built-up in the phosphating tank: Removal of phosphate sludge build-up from the tank is required in addition to the scheduled “acid cleans” normally performed during plant shut down. Health and safety risks are associated with this operation i.e. handling of dangerous chemicals for operators and contractors. There is an additional cost for the handling and removal of special waste as well as the cost of acid solution. There is also risk associated with effluent treatment plant having to cope with additional rinse water from phosphate tank. Figure 2 shows before and after cleaning of phosphate tank.

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Figure 1. Blocked Spray Pipe and Pump.

a

b

Figure 2. Zinc Phosphate Tank a) Before Cleaning the Sludge Built-up b) After Cleaning.

Since a phosphate ion may burden the environment by eutrophication, it takes efforts to treat liquid waste and therefore it is preferable not to use it [4.[3] ]. Zinc phosphating sludge, in general, has 20 wt.% iron, 10 wt.% zinc, 1-3 wt.% manganese, <1 wt.% nickel and 50-55 wt.% phosphate (composition on dry basis) [4.[4] ]. As a consequence, there is a risk that the use of such coatings will be restricted in the future [4.[5] ]. Furthermore, there is the occupational health and safety issues arising from the risk to workers exposed to these chemicals, the cost and potential liabilities resulting from accidental leakage into the environment and waste disposal issues from normal finishing operations. These make the use of phosphating conversion coatings unattractive to the metal finishing industry. Phosphating conversion coatings are currently subject to regulation under the Clean Air Act, Clean Water Act, and the Resource Conservation and Recovery Act. Control of Substances Hazardous to Health (COSHH) Regulations require a risk assessment for any substance to determine the potential hazard, and other regulations relate to water discharge and its impact on watercourses. Legislation includes Water Order and Water Act, which restrict discharge of certain substances into watercourses. Energy cost related to phosphating is another burden issue for industries. Most of the zinc phosphating baths in current use require high temperatures and long treatment times, which is a luxury for energy conscious industries [4.[6] ]. The traditional phosphating tank works in a temperature range between 45 °C to 60 °C. The phosphating tanks are usually heated with gas

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burners. In addition the requirement for a passivation stage on the phosphating process is unfavourable because of the cost of chemicals, energy, and maintenance. To allay these concerns, many surface finishing chemical companies are attempting to qualify an environmentally friendly conversion coating that meets the requirements of engineering companies. These products may be named by specific trade identification or by general descriptions such as nano-technology, silane technology or phosphorus-free pre-treatment. Most of these developing technologies use similar key components for creating a surface conversion. Fluoro-based chemistries operate in low or ambient temperature, but they also generate a small amount of sludge during production of a corrosion resistant conversion layer, which is promoted as improved technology over conventional phosphating. However all these newly developed pre-treatment systems have not been widely used due to the systems being new and unproven. This is despite the fact that they may be able to meet all of the specifications that phosphating conversion coatings offer. The overall objective of this section was to evaluate the ability of commercially and environmentally available pre-treatment systems, as a metal pre-treatment in finishing operations, which can be used to eliminate or reduce the amount of environmentally hazardous and toxic chemicals. This objective must be accomplished while maintaining equal or better product performance properties, with economic benefit or no significant economic penalty to the surface finishing companies seeking to change their pre-treatment system to an environmentally friendly pre-treatment system. The evaluation focused on technical performance and economics while validating the laboratory tests and environmental benefits. In order to evaluate the conversion coatings’ performance tests such as: corrosion behaviour, adhesion and blister resistance, Salt spray, prohesion test, Electrochemical Impedance Spectroscopy (EIS) measurement, cross hatch test, conical bend test, pull-off test, humidity test and surface morphology study were performed.

Experimental Description of Conversion Coatings Used Phosphate Coatings Phosphating can be defined as the treatment process of a metal surface to give a reasonably hard, electrically non-conducting surface coating of insoluble phosphate, which is contiguous and highly adherent to the underlying metal and is considerably more absorptive than the metal. The coating is formed as a result of a chemical reaction, which causes the surface of the base metal to integrate itself as a part of the corrosion resistant film. Basically, phosphating transforms basis metal into a nonmetallic crystalline coating. The chemical reaction occurs in an acidic solution containing Zn 2+ ions, phosphoric acid, other metal ions, and accelerators such as nitrate and viscosity control agents [4.[7] ]. Because of the loss of hydrogen at the metal/solution interface, localised pH rises and deposition occurs on the metal surface. Phosphating could be categorized into three main types: zinc, iron and manganese. The phosphate coating is usually applied in a five stage pre-treatment system. Additional stages

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may be added depending on the material and process. In this chapter commercially available iron and zinc phosphate have been used in the tests. Phosphating solvent tank temperature has been adjusted to 50°C. Total acid for iron phosphate has been adjusted to 7.5 and pH level to 5.8 by using caustic soda solution. For activation of iron phosphate solution, prior to testing, the spray pump was commenced and operated for about 1 hour with scrap steel in the spray cabinet. Silane Nanocoating Investigation into silane pre-treatment technology began because of environmental concerns over chromating of metals. A chrome film can give good adhesion and corrosion resistance to metal. However, the use of chromates has recently been regulated, as they have been found to be toxic and carcinogenic [4.[8] ]. This knowledge has opened an intensive search to replace the chromate process with other materials such as silane. The term “silanes” is often used for organofunctional silanes. Basic silane structure contains a silicon atom combined with an organic molecule. Complex silanes, such as organofunctional silanes have a group of molecules that reacts with metals or metal hydroxides and paint resins to form bonds. Packham [4.[9] ] explains typical formation of silane based pre-treatment on metal surface as shown on Figure 3. Surface

Film 0.3 µm

Metal

Figure 3. Typical Silane Based Pre-treatment Formation on Metal Surface [4.[9] ].

There are many commercially available silane based conversion coatings, however such coatings are complicated and difficult to analyse as with all newly developed technologies,

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especially those not designed for steel substrates; therefore the most successful and well known products have been chosen for this study. Zirconium Chemistries Most of the newly developed, environmentally friendly and commercially available conversion coatings use zirconium as a base. The zirconium based conversion coat system works, as it is free of regulated heavy metals and phosphate. Eliminating heavy metals in the solutions minimises wastewater effluent treatment issues with virtually no need for a sludge removal system. Zirconium based conversion coatings are frequently applied to metal components, and usually do not require heat. Eliminating the heating process saves energy and reduces water volume. Reduced waste disposal and sludge are additional cost saving measures. In most cases, zirconium based conversion coatings can reduce the number of pre-treatment stages required; this is led by a shortened conversion process and reduced maintenance requirements. Despite their lighter weight zirconium oxide pre-treatments may provide sufficient corrosion and adhesion performance when used with a good substrate and paint system. Corrosion performance may however still be somewhat inferior to zinc phosphate, but it does pass many specifications. In this study two types of zirconium based conversion coatings were used: zirconium phosphate and zirconium nanocoating. Zirconium phosphate conversion coating was developed in 1976 [4.[10] ]. Zirconium coating film is mainly zirconium oxide and hydrated zirconium phosphate. This type of coating has been used on the aluminium beverage can body for the last two decades [4.[11] ]. The conversion coating solution contains fluorozirconium acid, phosphoric acid, nitric acid and hydrofluoric acid. The pH of the solution is between 2.6 and 3.3. The temperature of the solution is usually 20 °C to 45 °C and contact time ranges from 10 to 90 seconds. It is normally applied by spray method [4.[12] ]. Zirconium nanocoating consists of at least one type selected from the group comprising of: zirconium, titanium and hafnium, contained in the chemical conversion coating agent. Supply sources of zirconium are not particularly limited and examples of them are alkaline metal fluoro-zirconate such as K2ZrF6, fluoro-zirconate such as (NH4)2ZrF6, soluble fluorozirconate like fluoro-zirconate acid such as H2ZrF6 zirconium fluoride, zirconium oxide and the like. Table 1. List of Conversion Coatings Used Description

Main Component

Zirconium Phosphate Silane Nanocoating Iron Phosphate Zirconium Nanocoating Zinc Phosphate

Zirconium Phosphate Silane Iron Phosphate Zirconium Zinc Phosphate

Required Pre-treatment Stages 4 Stages 5 Stages 5 Stages 5 Stages 5 Stages

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A list of conversion coatings and coating systems used in this study is shown in Table 1. The descriptions of the coatings are detailed in this study. Phosphate conversion coatings were used as a reference to other newly developed conversion coatings. Phosphating is the most popular conversion coat used in the metal finishing industry and it is known to be one of the best conversion coatings developed in the last two centuries; therefore zinc phosphate and iron phosphate has been used in the tests to correlate newly developed systems.

Sample Preparation Oxide scale on the steel surface is unwanted prior to any coating application because of its brittle and unsteady properties. It is usually removed prior to coating by a mechanical method such as shot blasting or a chemical method such as acid pickling, or a combination of both. Both hot rolled steel (HRS) and hot rolled steel, pickled and oiled (HRS-P), were used for the experiment. Test panels were prepared according to ASTM D609 [4.[13] ] and profiled in turret punching machines. Figure 4 shows test panels. HRS test panels were cut from 3 mm and HRS-P test panels were cut from 2 mm thickness sheet.

Used for SEM, XPS

a)

b)

c)

d)

Figure 4. Test Panels Used in the Study a) Hot Rolled Steel (HRS), b) Hot Rolled Steel after Shot Blasting c) Hot Rolled Steel Pickled and Oiled (HRS-P), d) Hot Rolled Steel Pickled and Oiled after Shot Blasting.

Some of the surface investigations such as SEM and XPS experiments were conducted on test panels measuring 10mm x 10mm. These are illustrated in Figure 4. After pre-treatment in different conditions, a side cutter was used to cut small test panels suitable for SEM and XPS study. Table 2. Legend for Conversion Coatings Material HR4

ZrPO

Silane Nanocoating

FePO

Zirconium Nanocoating

ZnPO

A1

B1

C1

D1

E1

HR4 Blasted

A2

B2

E2

A3

B3

C2 C3

D2

HR4-P

D3

E3

HR4-P Blasted

A4

B4

C4

D4

E4

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All the test panels were marked according to the legends in Table 2. The test panels, which were shot-blasted were marked after blasting; this was because during the blasting operation markings on the test panels became less visible. The test panels were blasted in a monorail type blasting machine using S 280 grade steel shots. Surface roughness (Ra) was measured after blasting and noted as shown in Table 3. Note that Ra is the arithmetic mean of the absolute departures of the roughness profile from the mean line. It was established that the profile of the shot blasted test panels was lower than that of larger shot blasted components; this was due to some of the energy being lost through moving the panels rather than impacting them. Adhesion to the basis metal is mechanical, being achieved by prior shot blasting to Sa 2.5 – 3 of the substrate, i.e., to a “near white” or “white” surface (removing all oxide scale and rust) on the scale originally developed in Sweden and now widely used (ISO 8501-1, [4.[14] ]) (ASTM D2200 [4.[15] ]) Table 3. Blast Profile Ra Measurement (μm) Test Panel

Reading 1 (Ra)

A2 A4 B2 B4 C2 C4 D2 D4 E2 E4

7.3 6.6 6.4 7 6.6 8.1 6.3 6 6.5 7.5

Reading Reading Reading Reading 2 (Ra) 3 (Ra) 4 (Ra) 5 (Ra) 5.8 4.3 9.4 7.3 6.3 6.3 7.4 8.2 6.4 7.2

7.3 5.7 9.3 6.5 7.1 6.2 6.5 7.5 6.6 6.4

6.9 7.1 8.1 6.2 6.9 7.5 5.5 7.2 7.1 6.4

6.7 6.2 8.1 6.7 6.9 7.5 6.6 7.5 6.2 6.8

Average Reading (Ra) 6.8 5.98 8.26 6.74 6.76 7.12 6.46 7.28 6.56 6.86

Standard Deviation 0.62 1.07 1.21 0.43 0.31 0.83 0.68 0.80 0.34 0.49

Pre-Treatment Prior to the powder coating phase, a pre-treatment phase was administered. The pretreatment phase has 5 stages and is widely used in phosphating in industry. The conventional five stages are: cleaning, rinsing, conversion coating, rinsing and passivation. During this study the average size of a pre-treatment plant was taken as a reference and it was considered that newly developed conversion coating replacement to conventional iron or zinc phosphate should take place in the same pre-treatment plant. Therefore typical phosphating pretreatment plant zone length and spray contact times have been calculated according to line speed. Table 4 shows pre-treatment stages and zone lengths. It has been considered that it was a conveyor line and line speed is 2400 mm/min. According to this line speed, contact time of each stage was calculated and simulated in a small spray cabinet. 100 litres of solution was prepared for conversion coating. According to the supplier’s instruction each pre-treatment solution was diluted with tap water or DI water to make pre-treatment solution. pH level and tank temperature was adjusted according to manufacturers instructions. The solutions pH level was adjusted accordingly using caustic soda.

Table 4. Pre-treatment Stages

Process Type

Stage 1

Stage 2

Stage 3

Stage 4

Stage 5

Dry Off Oven

Zone Length (m)

6.00

1.50

6.00

1.50

2.40

12

Contact Time (sec)

150

38

150

38

60

300

ZrPO

Alkaline Cleaning (45°C)

Tap Water Rinse (Ambient temp.)

Conversion Coat Zirconium Phosphate (30°C)

Tap Water Rinse (Ambient temp.)

Silane Nanocoating

Alkaline Cleaning (45°C)

Tap Water Rinse (Ambient temp.)

DI Water Rinse (Ambient temp.)

Conversion Coat Nano Structured (30°C)

DI Water Rinse (Ambient temp.)

Drying (135°C)

FePO

Alkaline Cleaning (45°C)

Tap Water Rinse (Ambient temp.)

Conversion Coat Iron Phosphate (50°C)

Tap Water Rinse (Ambient temp.)

Passivation (45°C)

Drying (135°C)

Zirconium Nanocoating

Alkaline Cleaning (45°C)

Tap Water Rinse (Ambient temp.)

DI Water Rinse (Ambient temp.)

Conversion Coat Nano Structured (Ambient temp.)

DI Water Rinse (Ambient temp.)

Drying (135°C)

ZnPO

Alkaline Cleaning (45°C)

Tap Water Rinse (Ambient temp.)

Conversion Coat Zinc Phosphate (50°C)

Tap Water Rinse (Ambient temp.)

Passivation (45°C)

Drying (135°C)

Drying (135°C)

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Powder Coating Powder coatings on the differently pre-treated test panels were applied by automatic electrostatic spray method. RAL 9001 colour, TGIC (Triglycidyl Isocyanurate) free polyester powder coating was used. Coated test panels were placed for 20 minutes in a curing oven at 200 °C. After the coating process some of the panels were tested according to ASTM 5402 [4.[16] ] for curing, using the MEK solvent rub test. Powder coating dry film thickness (DFT) on the test panels was determined in accordance with test method ASTM D 1186 [4.[17] ], averaging six readings for each panel. DFT for salt spray and adhesion tests was kept between 60 and 80 μm. DFT for Electrochemical Impedance Spectroscopy (EIS) study was kept between 40 and 45 μm. Prior to any test DFT was measured to ensure there was not a big difference in the coating thickness on the test panels. In this section the aim of the tests was to evaluate the conversion coatings under the powder coating. Therefore, the aim was to use the same coating thickness for all test panels. Table 5 shows average DFT readings for test panels. Table 5. Average DFT Readings for Test Panels Legends A1 A2 A3 A4 B1 B2 B3 B4 C1 C2 C3 C4 D1 D2 D3 D4 E1 E2 E3 E4

Adhesion 76 85 68 65 74 85 79 86 84 65 81 74 66 77 85 81 68 62 82 75

DFT Readings of Test Samples (μm) Salt Spray EIS Prohesion Test 81 40 75 68 42 82 66 44 82 60 40 74 85 43 62 75 46 66 67 41 68 81 45 72 63 42 81 82 41 80 69 43 66 71 39 72 75 43 79 80 45 75 61 42 86 65 42 74 74 45 73 82 44 82 66 40 65 80 43 61

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Findings Coating Weight To develop a suitable replacement for the traditional phosphating process, many surface finishing chemical suppliers have evaluated nano scale coatings using silane, zirconium and other metal ions, either alone or combined. Coating weights can be different for each supplier, but generally they are much lower than zinc and iron phosphate. Newly developed environmentally friendly technologies have a tendency to have a lower coating weight and thinner coatings; this could reduce the contact time necessary to deposit a full coating on the substrate. The coating weight for zinc-phosphate and iron-phosphate was measured according to Equation 1. The weights of phosphated test panels were recorded in gram (W1) and then they were cleaned using 6 % volume hydrochloric acid (HCl) and afterwards their weights were recorded in gram (W2). The weight of the coating per unit area (g/m2) was calculated from the change of panels’ weights. Table 6 shows coating weights for test panels used in this study. Because the coating layers obtained using silane technology are thin, weight measurements are not required for silane and zirconium nanocoating conversion coatings. Coating weight (g/m2) = W2 -W1/coated area (m2)

(1)

Table 6. Coating Weights Coating Type Zirconium Phosphate Silane Nanocoating Iron Phosphate Zirconium Nanocoating Zinc Phosphate

Coating Weight 0.25 g/ m2 Unknown 0.65 g/ m2 Unknown 2.54 g/ m2

Adhesion Test Adhesion testing was conducted according to ASTM D4541 [4.[18] ] pull-off and ASTM D 3359 [4.[19] ] tape test. A Zwick tensile testing instrument was used for the pull-off method in order to measure the lift force required to pull a small area of coating away from the base metal. A dolly was attached by adhesive (epoxy araldite) to the coating. After curing of the adhesive, the coating was cut around the base of dolly. Figure 5 shows the pull-off test. The dolly and test panels were attached by jig to the upper and lower jaw of the tensile test machine, 0.5 mm/min speed was applied and maximum force was recorded on breakage point. The percentage of paint lifting and maximum force was recorded. Table 7 shows result of the pull-off test. The tests were performed with the same instrument and condition for all test panels.

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a

b

Test Samples

Figure 5. Pull-off Test a) Test Samples b) Test on Zwick Tensile Test Instrument.

Pull-off tests showed that adhesion tests for all types of conversion coatings did not diverge. It is known that powder coating has good adhesion properties compared to wet paint [4.[20] ]. The aim of the test was to find premature adhesion failure caused by conversion coatings. It was mentioned earlier that one of the purposes in the application of conversion coating is to increase adhesion between coating and substrate. The maximum forces at breakage point of the pull-off test were recorded at between 4.3 and 8 MPa. It was noticed that the average maximum force for shot blasted test panels was higher than other test panels, which were not shot blasted. This could be attributed to the shot blasted surface providing mechanical anchoring, enabling increased adhesion between substrate and coatings. The percentage of paint lifting at maximum force for all test panels was between 35 % to 60 %. From pull-off test results, it was found that all types of conversion coatings used in this study were good adhesion promoters for polyester types of powder coatings. Adhesion by tape ratings for all test panels was 4B and 5B, indicating that none of the lattice became detached from the cross hatch. Both pull-off and cross hatch tests did not reveal any significant differences between each of the conversion coatings. For tape testing, cross-cut technique was used as stated in ASTM 3359 [4.[19] ] method B. Elcometer™ cross-cut blades were used in this test. Classifications of adhesion tape test results were recorded according to the test method. 1mm spacing cross hatch cutter 6 cutting edges was used for this test. Figure 6 shows cross hatch test on test panels. All test panels permuted very good cross hatch adhesion test. Most of the panels had less than 5% of their area removed after the cross hatch test. This test also showed that all types of conversion coatings were good at promoting adhesion between the substrate and the powder coatings.

Figure 6. Cross Hatch Test Samples.

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Table 7. Pull-off Adhesion and Cross Hatch Test Results

Test Samples A1 A2 A3 A4 B1 B2 B3 B4 C1 C2 C3 C4 D1 D2 D3 D4 E1 E2 E3 E4

Pull-off Adhesion Test Percentage Adhesion Area MPa Removed 6.2 50 % 4.3 60 % 5.2 60 % 7.5 60 % 6 35 % 6.4 50 % 5.6 40 % 6.8 50 % 6.6 50 % 6.2 50 % 4.3 60 % 7.5 60 % 5.1 60 % 6 35 % 5.4 50 % 8 40 % 6.8 50 % 6.2 50 % 4.3 60 % 7.5 60 %

Cross Hatch Test Classification

Percentage Area Removed

5B 3B 4B 4B 5B 4B 4B 4B 5B 5B 3B 4B 4B 5B 4B 4B 4B 5B 5B 4B

0% 5-15 % Less than 5 % Less than 5 % 0% Less than 5 % Less than 5 % Less than 5 % 0% 0% 5-15 % Less than 5 % Less than 5 % 0% Less than 5 % Less than 5 % Less than 5 % 0% 0% Less than 5 %

Scanning Electron Microscopy (SEM) SEM was used to observe both the coating and the oxide cleaning process in this study. Figure 7. SEM Study–A illustrates the chemical cleaning of oxide scale with 10 % v/v hot sulphuric acid. Acid pickling is an effective chemical cleaning process for the removal of oxide scale. Figure 7-B and C show the test samples after ZnPO pre-treatment. The HRS samples were pre-treated as delivered from the supplier and did not have their oxide scale removed. The SEM study showed that it is necessary to remove oxide scale prior to coating as it was found that it prevents ZnPO deposition on the surface and in the dry-off after the pre-treatment process the oxide scale cracks causing poor protection between coating and substrate. The oxide scale did not provide protection. This was clearly seen in the salt spray, prohesion and EIS studies. Pretreatments on HRS are shown in Figure 7-B and C. The mechanical oxide cleaning method and shot blasting, removed oxide scale on HRS fairly successfully. However, 20-25 % of oxide scale still remained on the surface. Figure 7-E shows that shot blasted test samples still have some oxide scale on the surface. Figure 7-D and F show typical zinc and iron phosphate coating. Due to nanoscale characteristics of the ZrPO, silane nanocoating and Zirconium nanocoating conversion coats used in this study did not show significant difference during SEM study. Figure 7-E shows a nanoscaled conversion coat on the HRS substrate.

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10 % H2SO4 Acid Pickled HRS

ZnP without any oxide scale cleaning HRS

Etched Surface

A

Cracks in the Oxide Scale after dry-off

B

HRS ZnP Conversion Coat

Zinc Phosphate Conversion Coat on Shot Blasted Panels

Oxide Scale

C

Oxide Scale Free Area

D

Zirconium Nanoscaled Conversion Coatings on Shot Blasted Test Panels

Iron Phosphate Conversion Coat on Shot Blasted Panels

Oxide Scale Free Area

E

F

Figure 7. SEM Study.

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Salt Spray Test The neutral salt spray (fog) test (ASTM B 117 [4.[21] ]) is perhaps the most commonly used salt spray test in existence for testing inorganic and organic coatings, in particular where such tests are used for material or product specifications. The duration of the test can range from 8 to over 3000 hours, depending on the product. A 5% NaCl solution containing not more than 200 parts per million (ppm) total solids, and with a pH range of 6.5 to 7.2, is used. The temperature of the salt spray chamber is controlled so as to maintain 35 + 1.1 or – 1.7 °C within the exposure zone of the closed chamber. All test panels were observed at 250 hour intervals after which they were removed from the salt spray cabinet, washed with DI water, and the failure point defined. For this study it was taken as loss of adhesion. Images after 1000 salt spray hours for all test panels are shown in Figure 9,Figure 10,Figure 11,Figure 12 andFigure 13. The degree of rusting after 1000 hours salt spray test is shown in Table 8. The progress of corrosion on differently pre-treated test samples was evaluated after the salt spray test. The anticorrosive protection achieved with zinc phosphate (E1, E2, E3 and E4 shown on Figure 13) was more predominant than on other types of conversion coatings. The findings show that test panels, which had oxide scale on the surface (A1, B1, C1, D1 and E1), displayed the worst corrosion protection, as oxide scale does not provide a barrier coat. Adhesion failure was also recorded after 500 salt spray hours on all test panels which had oxide scale on their surfaces. SEM study showed that test panels with oxide scale did not react with conversion coating to provide a barrier coating (see Figure 7-C). The worst salt spray results were obtained from zirconium phosphate pre-treated test panels (Figure 9). Test panels A1, A2 and A4 showed that the creepage from scribe was over 3 mm after 500 salt spray hours. Adhesion failure was also recorded after 250 hours of salt spray exposure on the zirconium phosphate pre-treated test panels. Silane nanocoating pre-treated test panels performed second best out of all test panels (B2, B3 and B4). All test panels other than B1 showed less than 3 mm creepage from scribe. Specially blasted test panels (B2 and B4) had very good corrosion performance; the iron phosphate pre-treated test panels had good corrosion resistance up to 500 salt spray hours. However, after the 500 hours limit, adhesion failure was recorded for C1, C2 and C3 test panels. Iron phosphate pre-treated test panels, which had oxide scale on the surface, performed well up to 250 salt spray hours. The creepage for these panels was less than 3 mm. Some of the acid pickled and blasted test panels failed after 250 hours. SEM results, in Figure 7, show that acid pickling and shot blasting are effective cleaning methods for oxide scale, although some oxide scale remains on the substrate. Iron phosphated HRS-P test panels performed well over 500 hours in the salt spray test, whereas results for acid pickled and shot blasted panels were very poor. Many of the panels failed in a 250 hour time span. Zirconium based nano scaled conversion coating performed better than iron phosphate coating, and some of the shot blasted and acid pickled panels reached 750 salt spray hours.

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Coating

ZrPO

Silane Nanocoating

FePO

Zirconium Nanocoating

ZnPO

Average Creepage of Conversion Coats (mm) Legend 250 hrs. 500 hrs. 750 hrs. A1 3 7 15 A2 1 2 10 A3 1 1.5 2 A4 0.5 2 8 B1 2 6 12 B2 0.5 0.5 0.5 B3 1 1 2 B4 0.5 0.5 1 C1 0.5 3 11 C2 3 6 10 C3 2 4 7 C4 1 2 4 D1 0.5 4 10 D2 1 5 9 D3 1 1 5 D4 2 2 3 E1 0.5 8 13 E2 0.5 0.5 0.5 E3 0.5 1 1 E4 0.5 1 1

1000 hrs. 20 20 3 11 12 0.5 2 1.5 13 14 10 4 12 18 7 6 13 0.5 1 1

20 250 hrs.

Average Creepage (mm)

500 hrs. 750 hrs.

15

1000 hrs.

10

5

0 A1

A2

A3

ZrPO

A4

B1

B2

B3

B4

Silane Nanocoating

C1

C2

C3

FePO

C4

D1

D2

D3

D4

Zirconium Nanocoating

Test Panels and Conversion Coats

Figure 8. Salt Spray Test Average Creepage from Scribe.

E1

E2

E3

ZnPO

E4

Environmentally Friendly Conversion Coating Applications

A1

A2

A3

109

A4

Figure 9. Zirconium Phosphate Pre-treated Test Panels after 1000 Hours Salt Spray Fog Exposure.

B1

B2

B3

B4

Figure 10. Silane Nanocoating Pre-treated Test Panels after 1000 Hours Salt Spray Fog Exposure.

C1

C2

C3

C4

Figure 11. Iron Phosphate Pre-treated Test Panels after 1000 Hours Salt Spray Fog Exposure.

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D1

D2

D3

D4

Figure 12. Zirconium Nanocoating Pre-treated Test Panels after 1000 Hours Salt Spray Fog Exposure.

E1

E2

E3

E4

Figure 13. Zinc Phosphate Pre-treated Test Panels after 1000 Hours Salt Spray Fog Exposure.

Prohesion Test Although conventional salt-spray tests (e.g., ASTM B117 [4.[21] ]) are widely used in the coatings industry, it has long been argued that correlation of salt-spray data with outdoor exposure results can be quite poor. Therefore, an alternative accelerated corrosion test ASTM G 85 [4.[22] ] Prohesion test is used by some of the industries. Due to its periodic cycling of test conditions, including temperature and wet/dry cycles, it was preferred by some authors who believed it gave better outdoor correlation [4.[23] ]. In this study ASTM G-85 A5 [4.[22] ] test method was used for all differently prepared and pre-treated test panels. The test solution consists of 0.05 wt % NaCl, 0.35 wt % (NH4)2SO4 and 99.6 % DI water. The test was applied on the test panels as a cyclic method; 1 hour test solution fog (no heating, ambient temperature) followed by 1 hour dry (heating to 35 °C) with air purge. All test panels were cross scribed then evaluated according to ASTM D1654. Images for test panels are shown in Figure 15,Figure 16 Figure 17 and Figure 18. Every 250 hours test panels were checked and reported, as shown in Table 9. Average creepage from scribe after 1000 hours prohesion is shown in Figure 14. All test panels showed good corrosive behaviour at 250 hours. Acid pickled and shot blasted zirconium nanocoating test panels showed an average creepage of 1.4 mm from scribe (Figure 15). The other test panels showed less than 1 mm average creepage from scribes. ZnPO conversion

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coated test panels especially did not show any creepage for all types of panels. After 500 prohesion hours ZrPO test panels started to show creepage for most types of the panels other than shot blasted test panels. The average creepage values for ZrPO test panels were worst in prohesion test condition. This was also recorded in the salt spray test results. The average creepage from scribe for silane nanocoated panels doubled at 500 prohesion hours, however. This stabilised after 500 hours. There was minor change on creepage at 750 hours and 1000 hours. Average creepage values from scribe for zirconium nanocoating test panels also doubled at 500 prohesion hours and continued rising every 250 hours. The prohesion test results for zirconium nanocoating show that the conversion coating did not provide any corrosion inhibition. FePO conversion coated panels showed contradictory results from prohesion test to salt spray test. The test panels displayed a small amount of creepage after the first 250 prohesion hours and this doubled every 250 hours. However, the test panel C1 hot rolled steel, as received, showed good corrosion performance in the prohesion test condition. The HRS test panels A1, B1, C1, D1 and E1 showed better corrosion protection in the prohesion test compared to the salt spray test condition. This could be attributed to the prohesion test solution being different from the salt spray test solution and oxide scale on the test panels not being as soluble as in the salt spray test solution. In addition, the oxide scale on the HRS did not delaminate from the substrates; possibly, because the solution sprays was not continuous. Table 9. Prohesion Test Average Creepage from Scribe Average Creepage of Conversion Coats (mm) Legend 250 hrs. 500 hrs. 750 hrs. A1 0.9 1.1 4.7 A2 0.15 0.2 0.5 ZrPO A3 0.1 2.2 2.5 A4 1.6 3.9 4.2 B1 0.6 1.4 1.5 B2 0.9 1.4 1.5 Silane Nanocoating B3 0 0.05 0.1 B4 0 1.8 1.8 C1 0.5 1.5 1.8 C2 0.2 0.5 0.9 FePO C3 0 1.9 2.4 C4 0 0.6 1.8 D1 0.4 1.5 2.5 D2 0 1 1.5 Zirconium Nanocoating D3 0 1 3.5 D4 1.4 5.9 6.5 E1 0 1.9 1.9 E2 0 0 0 ZnPO E3 0 0.1 0.2 E4 0 0 0 Coating

1000 hrs. 6.2 0.7 3.2 6.7 1.5 1.8 0.3 2.1 1.9 1.6 5.6 3.6 3.1 1.8 6.8 10.3 1.9 0 0.4 0

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10 250 hrs.

Average Creepage (mm)

500 hrs. 8

750 hrs. 1000 hrs.

6

4

2

0 A1

A2

A3

A4

ZrPO

B1

B2

B3

B4

C1

Silane Nanocoating

C2

C3

FePO

C4

D1

D2

D3

D4

Zirconium Nanocoating

E1

E2

E3

ZnPO

Test Panels and Conversion Coats

Figure 14. Prohesion Test Average Creepage from Scribe.

A1

C1

A2

C2

A3

A4

B1

C3

C4

D1

E1

E2

E3

B2

D2

B3

B4

D3

D4

E4

Figure 15. ASTM G-85 A5 Prohesion 250 Hours Test Results.

E4

Environmentally Friendly Conversion Coating Applications

A1

A2

C1

C2

A3

C3

E1

A4

C4

B1

D1

E2

E3

B2

D2

B3

D3

113

B4

D4

E4

Figure 16. ASTM G-85 A5 Prohesion 500 Hours Test Results.

A1

C1

A2

C2

A3

A4

C3

C4

E1

E2

B1

B2

D1

E3

B3

D2

B4

D3

E4

Figure 17. ASTM G-85 A5 Prohesion 750 Hours Test Results.

D4

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Bulent Tepe

A1

C1

A2

C2

A3

C3

E1

A4

B1

C4

E2

B2

D1

E3

B3

D2

B4

D3

D4

E4

Figure 18. ASTM G-85 A5 Prohesion 1000 Hours Test Results.

Electrochemical Impedance Spectroscopy On each differently pre-treated test panel one cylindrical tube of a transparent acrylic tube was fixed by using an epoxy (epoxy araldite) adhesive. The electrochemical tests were conducted with 3.5 % wt. NaCl solution. A three electrode electrochemical cell was used in the study. A platinum mesh counter electrode (CE), a saturated calomel reference electrode (SCE) (4 M KCl), and the working electrode (WE) were used to perform EIS study. The working electrode was attached (WE) to the test panels (pre-treated and powder coated). The working electrode was attached to the powder coating and to the conversion coated free test samples to provide current. The potentials were measured and referred (RE) to a (SCE). The electrochemical experiment set-up is shown in Figure 19. A Solartron 1260 frequency response analyser (FRA), a Solartron 1287 potentiostat and a PC were used for all tests. Impedance data was collected at frequencies in the range 104 Hz to 10-1 Hz. ZPlot and electrochemical impedance software was used to interpret the experimental impedance. The powder coated test samples have a hydrophobic surface. A thin layer of coating was lifted with fine emery paper to allow the surface to absorb NaC1 solution on to the painted surface. The powder coating thickness was aimed to be at approximately 40 μm. The geometrical area (inside diameter of acrylic tube) for impedance measurements was 10 cm2. All tests were performed at 20 °C ±3 °C.

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RE CE

Powder Coating Conversion Coating HRS Substrate

Acrylic Tube WE NaCl Solution

Figure 19. Electrochemical Impedance Experiment Set-up (RE: Reference electrode CE: Counter electrode WE: Work electrode).

Figure 20. XPS Spectra of Oxide Layer.

EIS data was collected from electrochemical impedance measurements, (Figure 21), for each of the test samples shown in Table 2. Bode plots display absolute value of the impedance |Z| (Ωcm2) plotted against the frequency (Hz.). The total impedance of the test panels, defined as |Z| (Ωcm2) value, extrapolated to 10-1 Hz. in the Bode magnitude plots, were used as a comparison for each coating system. Figure 23, Figure 24, Figure 25 and Figure 26 show impedance |Z| (Ωcm2) at 10-1 Hz for all test samples. Low frequency up to 101 Hz. was used to measure capacitive response. It was noted that most of the test panels did not show any Bode magnitude plot in the first few days of immersion in electrolyte. This was due to powder coating and conversion coating resistance to permeability. Therefore, EIS

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readings started from 5th day of the immersion. According to Granata et al [4.[24] ] high performance coating systems can be categorised by high impedance |Z|> 109 at low frequency <10-2 Hz. -200000 A2-5days.z A2-9days.z A2-15days.z A2-20days.z A2-26days.z

Z''

-150000

-100000

-50000

0 0

50000

100000

150000

200000

Z' Figure 21. Nyquist (Z’ Ωcm2 vs. Z” Ωcm2) and Bode (Frequency Hz. vs. |Z| Ωcm2) Magnitude Plots for ZrPO Coated Blasted HRS (A2) Test Panels. -7.5e6

A1-5days.z A1-25days.z

Z''

-5.0e6

One time constant after 25 days immersion

Two time constant after 5 days immersion

-2.5e6

0 0

2.5e6

5.0e6

7.5e6

Z'

Figure 22. Nyquist (Z’ Ωcm2 vs. Z” Ωcm2) Magnitude Plot for ZrPO Coated HRS (A1) Test Panel.

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117

1.0E+09 A1

B1

C1

D1

E1

|Z| at 10-1 Hz (Ohm cm2)

1.0E+08

1.0E+07 E1

1.0E+06

D1 B1

1.0E+05

A1 C1

1.0E+04

1.0E+03 5

10

15

20

25

Time (days)

Figure 23. The Coating Resistance of Differently Pre-treated Powder Coated HRS Samples During 25 Days of Immersion in 3.5 % NaCl Electrolyte. 1.0E+09

A2

|Z| at 10-1 Hz (Ohm cm2)

1.0E+08

B2

C2

D2

E2

1.0E+07

B2 E2

1.0E+06

D2 1.0E+05 A2 C2 1.0E+04

1.0E+03 5

10

15

20

25

Time (days)

Figure 24. The Coating Resistance of Differently Pre-treated Powder Coated HRS Blasted Samples During 25 Days of Immersion in 3.5 % NaCl Electrolyte.

The HRS test samples A1, B1, C1, D1 and E1 displayed readings between |Z| 107 Ωcm2 to Z| 109 Ωcm2 at 10-1 Hz on 5th day of immersion. This shows that coatings on the surface have a high coating resistance response. However, rapid decreasing of the |Z| was recorded

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after 5 days immersion for all differently pre-treated HRS test panels (Figure 23). When the immersion of the test panels reached 25 days A1, B1, C1 and D1 test panels showed very low total impedance values, between |Z| 104 Ωcm2 and Z| 105 Ωcm2 at 10-1 Hz. The rapid decrease of total impedance could be attributed to the test panels with oxide scale on the surface. This creates a barrier preventing movement of ionic species towards the substrate in the first few days of immersion. However, after the 5th day of immersion, oxide scale on the surface lost its protective proprieties and a rapid decrease of total impedance was noted. This was also seen on the Nyquist plot for HRS test panels. The two time constant (coatings and oxide scale) was recorded at the start of immersion and the two time constant was changed to 1 time constant (coatings) towards to end of the test schedules. The ZnPO coated HRS test panels showed the best impedance readings after 25 days immersion at approximately |Z| 107 Ωcm2 at 10-1 Hz. The EIS test results of HRS shot blasted test samples A2, B2, C2, D2 and E2 are shown in Figure 24. The oxide scale for these samples was cleaned by shot blasting leaving a small amount of oxide scale remaining on the surface. The existence of oxide scale for blasted and pickled HRS test panels was analysed by using XPS. Binding energy values of Fe 2p3/2 and Fe 2p1/2 indicated the existence of oxide scale magnetite (Fe3O4) and hematite (Fe2O3) [4.[25] , 4.[26] ]. The XPS survey scan confirmed that oxide scale remained on HRS shot blasted test panels. However, HRS pickled test panels were free from oxide scale. The XPS spectra results for HRS are shown in Figure 20. In the 5th day of immersion the E2 and B2 test panels showed high impedance due to their conversion coatings providing good barrier properties. The salt spray and prohesion test results for these test samples were also better than other conversion coated HRS blasted test samples. Differences between HRS blasted test samples (A2, B2, C2, D2 and E2) and HRS test samples (A1, B1, C1, D1 and E1) were identified, as the blasted HRS test samples showed higher impedance. It was also recorded that rapid decreasing in total impedance after 5 days immersion did not occur as happened with the HRS test samples. Acid pickled test samples A3, B3, C3, D3 and E3 displayed the best corrosion barrier properties in the EIS study. Figure 25 shows the coating resistances of differently pre-treated test samples. ZnPO and silane nanocoated test samples especially exhibited the best corrosion resistance on HRS pickled test panels. Capacitive region for those panels at 10-1 Hz was over 108 Ωcm2 after 25 days immersion in 3.5 % wt NaCl electrolyte solution and resistance to corrosion degraded very little during the immersion period around 101 Ωcm2; despite this decrease the panels still measured the highest resistance. ZrPO conversion coated conversion coating had mediocre barrier properties compared to those of other conversion coatings for HRS acid pickled samples; however, the coating resistance was still over 106 Ωcm2 at 10-1 Hz. It was clearly identified that acid pickled HRS test panels were outperformed by other HRS preparations such as blasting. This could be attributed to the fact that smooth oxide clean surfaces are more receptive to conversion coatings and acid pickled surfaces free of contamination had active surfaces with which the conversion coatings could more easily react. A4, B4, C4, D4 and E4 test samples were prepared with acid pickling and shot blasting followed by pre-treatment. EIS test results |Z| Ωcm2 at 10-1 Hz for them are shown in Figure 26. Zirconium nanocoating conversion coating had superior barrier properties compared to those of other conversion coatings for HRS acid pickled and blasted samples. Zirconium nanocoated samples displayed excellent corrosion protection at the beginning of the test and it

Environmentally Friendly Conversion Coating Applications

119

1.0E+09

B3 1.0E+08

E3

|Z| at 10-1 Hz (Ohm cm2)

D3 1.0E+07 C3 A3 1.0E+06

1.0E+05

A3

1.0E+04

B3

C3

D3

E3

1.0E+03 5

10

15

20

25

Time (days)

Figure 25. The Coating Resistance of Differently Pre-treated Powder Coated HRS Pickled Samples During 25 Days of Immersion in 3.5 % NaCl Electrolyte. 1.0E+09

|Z| at 10-1 Hz (Ohm cm2)

1.0E+08

1.0E+07 E4 D4 B4

1.0E+06

A4 1.0E+05

C4

A4

1.0E+04

B4

C4

D4

E4

1.0E+03 5

10

15

20

25

Time (days)

Figure 26. The Coating Resistance of Differently Pre-treated Powder Coated HRS Pickled and Blasted Samples During 25 Days of Immersion in 3.5 % NaCl Electrolyte.

was recorded at around 109 Ωcm2 at 10-1 Hz. However, as the test progressed there was a rapid decrease to around 107 Ωcm2 at 10-1 Hz. after 25 days immersion. ZnPO based nano structured conversion coating showed consistent results. During the immersion time low

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frequency impedance did not drop as much as with other coating processes. Following the 25 day test a smaller reduction was noted. Acid pickled, shot blasted test samples showed lower resistance to test samples that were only acid pickled. This may have occurred because conversion coatings did not cover the peaks on the shot blasted surface. Some of the test cells showed corrosion media in the acrylic tube, the electrolyte solution’s colour changed to yellow and some rust spots were noted on the surfaces. In particular, HRS test samples (A1, B1, C1 and D1) displayed rust spots on the surface (Figure 27).

Rust Spots and colour changed in electrolyte solution

Figure 27. EIS Test Cells.

Conculusions During the investigations in this study, it was demonstrated that: 1. Corrosion resistance of powder coated hot rolled steel can be enhanced by applying conversion coatings. 2. Oxide scale on the substrate should be removed, prior to pre-treatment and powder coating, to ensure a superior result with the best resistance to corrosion. 3. It was established that zinc phosphate outperforms iron phosphate and nano scaled conversion coatings in corrosion performance, as it displays high impedance values in low frequency. 4. Silane based nano scaled conversion coating was also demonstrated as a favourable barrier against corrosion and generally it is satisfactory where corrosion resistance is required for HRS with a powder coating system.

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References [1] [2] [3]

[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]

N. Liberto (2003), User’s Guide to Powder Coating, SME Publication, Michigan. R. Eichholtz (2001), Non-chromate Conversion Coatings for Weapon Systems Rework and Repair Johnstown: Concurrent Technologies Corporation. K. Makino, A. Hashimoto, M. Futsuhara, T. Shimakura, T. Kobayashi, E. Murakami (2006), Pre-treatment Method for Coating Surface of Metal for Vehicle Chassis and Method of Applying Powder Coating Composition, US Patent Application 2006/0134327. S. Narayanan (2005) “Surface Pre-treatment by Phosphate Conversion coatings - A Review” in Advanced Material Science, Volume 9, Pages 130-177. F. Deflorian, L. Fedrizzi, P. L. Bonora (1997) “Corrosion Behaviour of Fluotitanate Pre-treated and Painted Aluminium Sheets” in Electrochimica Acta, Volume 42, No. 6, Pages 969-978. L. C. Deepa, S. Sathiyanarayanan, C. Marikkannu, D. Mukherjee (2003), Effect of Divalent Cations in Low Zinc Ambient Temperature Phosphating Bath, Anti-Corrosion Methods and Materials, Volume 50, Number 4, Pages 286-290. F. B. Mansfeld (1986), Corrosion Mechanism, Marcel Dekker Publication, New York J. Kim, J. Zhang, R. Yoon, R. Gandour (2004), Development of Environmentally Friendly Nanchrome Conversion Coating for Electrogalvanized Steel in Surface and Coating Technology, Volumes 188-189, Pages 762-767. D. E. Packham (2005), Handbook of Adhesion 2nd Edition, Sussex; John Wiley and Sons Ltd. T. L. Kelly (1979), Coating Solution for Metal Surface, US Patent 4148670. G. E. Totten, D. S. MacKEnzie (2003), Handbook of Aluminium: Volume 2: Alloy Production and Material Manufacturing New York; Markel Dekker Inc. F. Yoshida (1992), Zirconium Coatings Technology, The 3rd Asian Coating Forum. Proceedings, Pages 280-292. ASTM D609, (2005) “Standard Practice for Preparation of Cold Rolled Steel Panels for Testing Paint, Varnish, Conversion Coatings, and Related Coating Products” ISO 8501-1 (1998) “Preparation of Steel Substrates before Application of Paints and Related Products - Visual Assessment of Surface Cleanliness” ASTM D2200, (1995, Re-approved 2001) “Pictorial Surface Preparation Standards for Painting Steel Surfaces” ASTM D 5402 (2006) “Standard Practice for Assessing the Solvent Resistance of Organic Coatings Using Solvent Rubs” ASTM D1186, (2008) “Test Methods for Non-destructive Measurement of Dry Film Thickness of Nonmagnetic Coatings Applied to a Ferrous Base” ASTM D4541 (2002) “Standard Test Method for Pull-off Strength of Coatings Using Portable Adhesion Testers” ASTM D3359 (2002) “Standard Test Methods for Measuring Adhesion by Tape Test” F. C. Porter (1994), Corrosion Resistance of Zinc and Zinc Alloys, CRC Press, New York. ASTM B117 (2007) “Standard Practice for Operating Salt Spray (Fog) Apparatus” ASTM G85 (2002) “Standard Practice for Modified Salt Spray (Fog) Testing”

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[23] N. D. Cremer (1989) Prohesion Compared to Salt Spray and Outdoors, Presented at Federation of Societies for Coatings Technology 1989 Paint show. [24] R. Granata, K. Kovaleski (1993) Electrochemical Impedance analysis and Interpretation, 1993, ASTM Publication, ISBN 0-8031-1861-9. [25] M. Jenko, B. Korousic, D. Mandirano, V. Presern, (2000) Vacuum, Surface Engineering and Surface Instrumentation and Vacuum Technology Volume 57, Page 295. [26] M. Oku, K. Hirokawa (1976) Electron Spectroscopy Phenomenon, Volume 8, Page 475.

In: Surface Coatings Editors: M. Rizzo and G. Bruno, pp. 123-152

ISBN: 978-1-60741-193-2 © 2009 Nova Science Publishers, Inc.

Chapter 4

PRECISE SYNTHESIS OF AMPHIPHILIC POLYMERIC NANO ARCHITECTURES UTILIZED BY METALCATALYZED LIVING RING-OPENING METATHESIS POLYMERIZATION (ROMP) Kotohiro Nomura1 Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), Takayama, Ikoma, Nara, Japan

Abstract This article summarizes recent examples for precise synthesis of amphiphilic block copolymers by adopting transition metal-catalyzed living ring-opening metathesis polymerization (ROMP). In particular, unique characteristics of the living ROMP initiated by molybdenum alkylidene complexes (so-called Schrock type catalyst), which accomplish precise control of the block segment (hydrophilic and hydrophobic) as well as exclusive introduction of functionalities at the polymer chain end, enable us to provide the synthesis of block copolymers varying different backbones by adopting the “grafting to” or the “grafting from” approach. Moreover, use of the “grafting through” approach (polymerization of macromonomers) by the repetitive ROMP technique, using the molybdenum alkylidene catalysts, offers precise control of the amphiphilic block segments.

1. Introduction 1.1. Amphiphilic Block Copolymers (ABCs) Amphiphilic block copolymers (ABCs), consisting of both hydrophilic and hydrophobic segments in the polymer molecules, display substantial prominence owing to their ability to exhibit unique structural characteristics such as the formation of a diverse range of micellar aggregates (e.g. spheres, vesicles, rods, lamellae etc.) in bulk or solution (in both aqueous and 1

E-mail address: [email protected]. tel. +81-743-72-6041, fax +81-743-72-6049.

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Kotohiro Nomura

hydrophobic media).[1,2] An exquisite control over macromolecular architecture and the resulting material properties has been a central goal in the field of synthetic polymer chemistry, and considerable efforts have thus been directed towards accomplishment of the new synthetic methodologies for precise placement of the chemical functionality and control over their molecular weight and composition. The unique architectural as well as functional control achieved during their synthesis, by tuning the initial building blocks, i.e., hydrophobic/hydrophilic segments, results in the preparation of various well-defined phaseseparated microstructures and nano-architectures (spheres, rods, vesicles, lamellae, large compound micelles, nanofibers, nanotubes etc.), which should offer possibilities of potential applications in pharmaceutical, biotechnological, and polymer sciences. Several variations on copolymer topology have thus been probed including linear[3] and double hydrophilic block copolymers,[4] miktoarm star copolymers,[5] graft copolymers,[6] dendrons,[7] and poly(macromonomer)s.[8] The use of polymeric micelle as nano-vehicle is effective due to the core-shell morphology leading to the protection of an active agent in the core by the polymer shell, and has recently been exploited for the encapsulation of gold particles,[9] hydrophilic biofunctional materials,[10] hydrophobic anti-inflammatory agents,[11] chemotherapeutics,[12] and for other pharmaceutical applications.[13,14] The ability of ABCs to serve as delivery agents arises from their unique chemical structures wherein the hydrophobic core segment serves as a reservoir for hydrophobic substances upon micellization, which may be loaded by chemical, physical, or electrostatic means, depending on the specific functionalities of the core-forming block and the solubilizer. Therefore, the current research efforts aim at the preparation of micelles which are capable of responding to the environmental changes, while considerable success has been achieved using external stimuli such as pH,[14e,15] temperature,[16] IR[17] and UV light[18] for implementing programmed functions that respond to the signatures in vivo.

1.2. Living Ring-Opening Metathesis Polymerization (ROMP) for Preparation of ABCs Ring-opening metathesis polymerization (ROMP) has been considered to be a promising polymerization technique because the resultant polymers possess rather linear structures compared to ordinary vinyl polymers such as poly(acrylamide)s,[19] and this thus contributes to better maintain nature of the functionality at the side chain and the polymer chain end, because the functionality should not be (covered and) strongly affected by the polymer main chain. Transition metal carbene (alkylidene) complexes are known to play an essential role as the initiators for this polymerization (Chart 1).[20] It has been well known that the molybdenum alkylidene complexes called Schrock type catalysts are useful initiators for the living ROMP of cyclic olefins, especially substituted norbornenes and norbornadienes (Chart 2).[21] The absence of chain-transfer and termination reactions in such polymerization systems allows synthesis of the homopolymers and the block copolymers with narrow molecular weight distributions, and precise control of the functionality in both the initiation and the termination sites can be thus possible (Scheme 1).[21] Being a particularly powerful synthetic tool, the ROMP has found tremendous utility in preparing macromolecular materials displaying promising biological, electronic, and mechanical properties.

Precise Synthesis of Amphiphilic Polymeric Nano Architectures

R

R1

R1 Cl

N

Ti

R2O Mo CHCMe 2R R2O A

Cl

N Ar Ph Ru

Cl B2 PCy3

Cl

B1 PCy3

Ar = 2,4,6-Me 3C6H2 Cy = cyclohexyl

Mo catalyst（Schrock cat.）

Ti catalyst

Ph

Ru

R = t Bu, R' = i Pr (A1a); R = CMe(CF 3)2, R' = i Pr (A1b); R = CMe(CF 3)2, R' = Me (A2b).

R = H,

Ar N

PCy3

125

Ru catalyst（Grubbs cat.）

Chart 1. Typical transition metal complex catalysts employed for olefin metathesis.

R1

[Molybdenum-Alkylidene Initiators] 1) Living Polymerization 2) Quantitative Initiation 3) High reactivity, Efficient Polymerization (Quantitative conversion in most cases) 4) Various monomer can be used (but protection required)

R1 N

R2O Mo CHCMe R 2 R2O A

R = Me, Ph; R1 = Me, i Pr etc.; R2 = tBu, CMe(CF3)2 , C(CF3 )3 etc.

Chart 2. Summary of unique characteristics seen in molybdenum alkylidene complex catalysts for ring opening metathesis polymerization (ROMP).

M CHR

M

M

M

CHR

M

R

M

R cis/trans

M R

R

R cis/trans

olef in coordination & metathesis

M

R'CHO n

R

M = Mo, W etc.

n

R'

+ M=O

M = Ru

R

OR' [Required Factors for "Living Polymerization]

n

+M R

OR'

1) Exclusive reaction with cyclic olefins 2) No catalyst deactivation, generation of the other catalytically-active species Scheme 1. General scheme for ring-opening metathesis polymerization (ROMP).

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Kotohiro Nomura

Note that the quantitative introduction of reactive functionality into the polymer chain end can be easily achieved by adopting the living ROMP technique especially using the Schrock type molybdenum alkylidene initiator.[3b,3g,8a,22,23] The exclusive preparation of end-functionalized ring-opened polymers realized by a living polymerization with quantitative initiation can be applied not only to prepare block copolymers (ABCs) coupled with another living polymerization techniques,[24] but also for preparation of macromonomers, as described below. In contrast, the initiation efficiency is not always perfect as seen in the molybdenum alkylidene initiators, because dissociation of ligand (PR3 etc.) should be required to generate the catalytically-active species in the ROMP with the ruthenium carbene catalysts (Scheme 2),[25] An equilibrium between coordination and dissociation of PR3 should be present even in the propagation process; and replacement of halogen with the other anionic ligand (and/or replacement of PR3 with the other neutral donor ligands/substrates) can also be considered as the probable side reactions. Importance of using the molybdenum catalysts should be thus emphasized for their precise preparations, although the initiators should be very sensitive to moisture and both monomers and solvent have to be thus purified to avoid the catalyst decomposition. In this article, we thus introduce recent examples for precise synthesis of (i) amphiphilic block copolymers (ABCs) utilized by the living ROMP technique (via so called “grafting from” and/or “grafting to” approach), and (ii) precise synthesis of graft copolymers, poly(macromonomer)s by adopting the repetitive living ROMP technique (“grafting through” approach). Through these examples, we wish to introduce recent trend and update including explanation why the approach using the ROMP seems to be effective for this purpose.

L Cl

Ru CHPh Cl PR3

L

-PR3 k1

Cl

k -1 PR3

L

Ru CHPh Cl

k -2

Ph

Cl

Ru

Ru

Cl

Cl

Cl k -1 PR3

Ru Cl

Ph

CHPh Cl

L Ph

L

Ru

-

L Cl

Cl

k2

-PR3 k1

L Cl

Ru PR3

Cl

Ph

Scheme 2. General scheme for generation of the active species in ring-opening metathesis polymerization (ROMP) by ruthenium carbene catalysts.

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127

2. Precise Synthesis of Amphiphilic Block Copolymers (ABCs) by the Living ROMP Using Molybdenum-Alkylidene Catalysts “Living” polymerizations, such as ring-opening metathesis polymerization (ROMP), group transfer polymerization (GTP), controlled radical polymerization (CRP), and anionic polymerization (AP), generally provide synthesis of polymers with controlled molecular weights with narrow molecular weight distributions. The unique characteristic features of living polymerization (appropriate initiation, moderate/fast polymerization rates, and the absence of undesirable side reactions such as chain transfer, termination, or cross-linking, branching) can also provide synthesis of well-defined block copolymers by simple sequential addition of monomers. However, due to the (severe) limitation of effective monomers that can be employed in certain living polymerization methods, methodology for preparing two or more independent block by combination of different living polymerization techniques should greatly curtailing the diversity of accessible block copolymers. The approach for the precise synthesis adopted by the above methodology can greatly expand potential of the block copolymers, which can be achieved either by the end-functionalization of polymers with complementary groups[26] followed by their reaction/condensation (“graft-to approach”, section 2.1) or by accomplishing the transformation of the polymer chain end into an initiator for a different class of polymerization (“graft-from approach”, section 2.2).

2.1. Precise Synthesis of Amphiphilic Block Copolymers (ABCs) by Grafting Poly(Ethylene Glycol) to End-Functionalized Block ROMP Copolymers The examples of “grafting to” the ROMP polymer by reaction with another polymer chain end had been limited until recently, due to not only the difficulty in achieving the complete conversion, but also concern for separation of the unreacted polymers by fractionation etc., as in a previous case, an excess amount of ω-aldehyde-functionalized polystyrene was used to terminate the living ROMP of norbornene.[27] We recently demonstrated a new synthetic methodology to prepare novel ABCs by adopting the “grafting to” approach.[3b] Therefore, we herein summarizes our results for synthesis of the various ABCs adopting a “grafting to” approach whereby poly(ethylene glycol) (PEG) is attached to the ROMP polymers, prepared by the living technique using the Schrock-type molybdenum initiator.[33d] A molybdenum alkylidene, [Mo(CHCMe2Ph)(N-2,6-iPr2C6H3)(OtBu)2 (A1a)], has been chosen as an initiator due to its ability to prepare multi-block copolymers in a precisely controlled manner.[20-24] Typical procedures for preparation of the end-fuctionalized ringopened poly(norbornene)s are outlined in Scheme 3. Various block copolymers were also prepared by sequential addition of norbornene and its sugar-containing derivative [1,2:3,4-diO-isopropylidene-α-D-galactopyranos-6-O-yl-5-norbornene-2-carboxylate, endo/exo = 87/13] (Table 1, run 5-11). The resultant carbohydrate-containing polymers are expected to exhibit strong specific affinity with cell surface proteins, most probably arising from the clustering and binding of the cells by multivalent arrays, thus leading to a greater affinity and specificity than their mono-valent counterparts. Norbornene was polymerized in toluene at 25 ºC with A1a at different monomer/initiator molar ratio and the termination was effected by the

128

Kotohiro Nomura

addition of 4-Me3Si-C6H4CHO and 3,5-(Me3SiO)2-C6H3CHO to afford poly(1a) and poly(1b), respectively, in high yield (>94 %) (Table 1, run 1–4). This is an established procedure to perform the cleavage of the ROMP polymer-metal bonds as reaction of the Mo living ends with aldehyde yielding a carbon-carbon bond in a Witig-like reaction. X Mo cat.

n

CMe 2Ph

Mo

CHO

CMe2Ph

n

n

X Poly(1)-homo X = 4-Me3 SiO- (a), 3,5-(Me 3SiO)2- (b)

R m equiv. 2nd monomer R=

OSiMe3

X

O2C O O O

=

X

O O

OSiMe3

CHO

OHC a CHO b Terminator

CHO

OSiMe 3

CMe 2Ph m

n

X Poly(1)-block

R

X = 4-Me3SiO- (a), 3,5-(Me3SiO)2- (b)

Scheme 3. End-functionalization of ring-opened poly(norbornene)s prepared by ROMP using the molybdenum alkylidene initiators.[3b]

Table 1. Ring-opening metathesis polymerization (ROMP) of norbornene and its carbohydrate derivative with Mo(CHCMe2Ph)(N-2,6-iPr2C6H3)(OtBu)2 (A1a) in toluene.a Preparation of homo and diblock copolymers, poly(1).[3b] run 1 2 3 4 5 6 7 8 9 10 11 a

n/mb 1st/2nd 50/0 50/0 75/0 100/0 25/25 25/25 50/25 50/25 50/35 50/40 50/50

time/ min 1st/2nd 30/-30/-30/-30/-20/20 20/20 40/20 40/20 40/25 40/30 40/30

poly(1) poly(1a) poly(1b) poly(1a) poly(1a) poly(1a) poly(1b) poly(1a) poly(1b) poly(1a) poly(1a) poly(1a)

Mnc ×10-4 1.03 1.03 1.38 1.77 1.53 1.56 1.88 1.85 2.10 2.28 2.37

Mw/Mnc 1.08 1.12 1.1 1.05 1.11 1.11 1.07 1.16 1.09 1.12 1.31

yieldd /% 99 96 94 99 99 99 96 99 99 97 96

Polymerization conditions: in toluene, at room temperature (25 ºC); bMolar ratio based on Mo (shown in Scheme 3); cGPC data in THF vs polystyrene standards; dIsolated yield.

Table 2. Synthesis of amphiphilic block copolymers, poly(3).[3b] runa 1 1 3 4 5 7 9 10 11 a

n/mb 50/0 50/0 75/0 100/0 25/25 50/25 50/35 50/40 50/50

poly(1a) Mnc ×10-4 Mw/Mnc 1.03 1.08 1.03 1.08 1.38 1.10 1.77 1.05 1.53 1.11 1.88 1.07 2.10 1.09 2.28 1.12 2.37 1.31

Mnc ×10-4 0.99 0.99 1.35 1.82 1.47 1.92 2.05 2.25 2.42

poly(2a) Mw/Mnc 1.09 1.09 1.09 1.05 1.08 1.07 1.08 1.11 1.29

yield / % 98 98 99 98 95 98 98 99 98

PEG Mnc ×10-3 2200 4600 2200 2200 4600 4600 4600 4600 4600

Mnd ×10-4 0.7 0.96 0.93 1.17 1.63 1.86 2.22 2.41 2.75

Mnc ×10-4 1.41 1.61 1.72 2.15 2.30 2.38 2.52 2.69 2.83

poly(3) Mne ×10-4 0.69 0.97 1.07 1.16 1.62 1.85 2.32 2.44 2.79

Mw/Mnc 1.10 1.08 1.12 1.06 1.16 1.06 1.11 1.10 1.22

yieldf / % 86 86 90 82 68 72 77 82 88

Run no. in Table 1 (sample of ROMP polymer); bMolar ratio based on Mo (shown in Scheme 3); cGPC data in THF vs polystyrene standards; dCalculated value based on initial feedstock ratio; eEstimated from 1H NMR spectra; fIsolated yield.

130

Kotohiro Nomura

The Mn value of the ring-opened poly(norbornene)s, determined by GPC, increased linearly upon increasing the monomer/Mo molar ratios while the molecular weight distributions remained narrow (Mw/Mn = 1.05-1.12). The Mn values of the block copolymers also increased linearly upon increasing the added amount of the norbornene containing acetalprotected galactose, indicating livingness of the present polymerization. The quantitative removal of the trimethylsilyl group of both the homopolymer and the block copolymer, poly(1), was achieved by treating the polymers with aqueous HCl solution, yielding the ROMP polymers carrying hydroxy functionality at the chain end, poly(2) (yield 91–99 %, Scheme 4, and Table 2), whereas the cyclic acetal groups of the carbohydrate residue in the polymer remained intact under these weakly acidic conditions. OSiMe3 PhMe2C

OSiMe3

Poly(1a)

PhMe2C

m

n

n

X R

Poly(1b)

(i) Exclusive deprotection of SiMe3 group

0.5N HCl aq. in THF

m

OSiMe3

R

(i) Exclusive deprotection of SiMe3 group

0.5N HCl aq. in THF

OH PhMe2C

OH n

PhMe2C

m

n

Poly(2a)

OH R

Poly(2b) a) KH in THF b) PEGMs 2, 20h

(ii) Attachment of PEG a) KH in THF b) PEGMs2, 20h

PhMe2C

O n

Poly(3)

m

R

O

O

PEGMs2

x

O x

(ii) Attachment of PEG

Ms

Ms

x

O

O PhMe2C

R= Ms

m

R

Ms Ms = MeSO2

m

n

O2C O O O

Poly(4)

R

O

O O

x

O Ms

Scheme 4. Precise synthesis of linear, star shape amphiphilic block copolymers by grafting poly(ethylene glycol) (PEG) to end-functionalized block ROMP copolymers.[3b]

- Precise Synthesis of Amphiphilic Multi-Block Copolymers by Grafting Poly(Ethylene Glycol) (PEG) to the ROMP Polymer The hydroxy group at the polymer chain-end in poly(2a) was treated with KH in THF, and its subsequent coupling with PEGMs2 [MsO(CH2CH2O)nMs, Ms = MeSO2] resulted in the diblock linear ABCs consisting of ring-opened poly(norbornene) and PEG, poly(3), in high yield (Scheme 4, Table 2). The GPC traces for the resulting poly(3) were unimodal and displayed appropriate increment in the Mn values (Figure 1) with narrow molecular weight distribution in all cases (Mw/Mn = 1.06–1.12).

Precise Synthesis of Amphiphilic Polymeric Nano Architectures

131

a) Mn(GPC) = 10 300 Mw/M n = 1.08

poly(1a ) poly(NBE)50

b) poly(3): poly[(NBE)50-bl-(PEG)47]

c) poly(3 ): poly[(NBE)50-bl -(PEG)110]

20

Mn(GPC) = 14 100 Mw/M n = 1.10

M n(GPC) = 16 100 M w/M n = 1.08

25 elution time/min

30

Figure 1. GPC traces of a) poly(1a) (run 1) and PEG-grafted poly(3) [b) poly(1)-bl-PEG47, c) poly(1)bl-PEG110].[3b]

In addition, the Mn values estimated by the 1H NMR spectra (by the integration ratio of olefinic protons to those of PEG) were in good agreement with those calculated on the basis of monomer/initiator ratios. Following the same reaction sequence as described above, the hydroxy functionality at the diblock copolymer chain-end in poly(2a) was condensed with PEGMs2 to afford linear amphiphilic triblock copolymer, poly(3) [Mw/Mn = 1.06-1.22], in relatively high yield. Furthermore, triarm ABCs consisting of ring-opened poly(norbornene) and PEG, poly(4) (Scheme 4, Table 2) [Mw/Mn = 1.11–1.12], and those comprising diblock ROMP copolymers and PEG, (ABC2 type) poly(4) (Scheme 4, Table 2, run 9 and 12) were synthesized in an analogous manner by the coupling of bi-functionalized ring-opened poly(norbornene), poly(2b), and diblock copolymer with PEGMs2, respectively. Moreover, the reaction of poly(2a) with 0.5 equiv. of PEG in the presence of KH afforded ABA or ABCBA (sandwich) type amphiphilic multiblock copolymers, poly(5), in high yield (Scheme 5, Table 3). Cyclic acetals in the ABCs poly(3–5) could be selectively removed, without accompanying any ester-cleavages, by using a mixture of CF3CO2H and water (9/1 v/v) at

132

Kotohiro Nomura

room temperature (for 15 min), as reported previously.[3g] The isolated yields in these hydrolysis procedures were very high. The deprotedcted polymers were identified by the NMR and FT-IR spectra, and the integration ratios estimated from the 1H NMR spectra for PEG/sugar/ring-opened NBE protons of the resultant polymers were very close to the calculated values. OH

PhMe2C n

Poly(3a)

m

R=

R

O2C O

i) KH in THF ii) 0.5 eq. PEGMs 2

PhMe2C

O O

O

O n

O O

m

x

n

CMe2P h

m

R

R

ABCBA (ABA) type amphiphilic block copolymers

Poly(5)

PhMe2C

OH n

Poly(3a)

m

R

R=

O

i) KH in THF ii) 0.5 eq. PEGMs2

PhMe2C

O O

O O CMe2Ph

O

O n

O2C

m

x

n

m

R

R ABCBA (ABA) type amphiphilic block copolymers

Poly(5)

Scheme 5. Precise synthesis of ABA or ABCBA type amphiphilic block copolymers by attachment of poly(ethylene glycol) (PEG) to end-functionalized block ROMP copolymers.[3b]

Table 3. Synthesis of amphiphilic triarm block copolymers, poly(4).[3b] poly(1b) runa

n/mb

Mnc ×10-4

Mw/Mnc

poly(2b) Mnc ×104

Mw/Mnc

PEG yield / %

Mnc

poly(4) Mnd ×10-4

Mnc ×10-4

Mne ×10-4

Mw/Mnc

yieldf / %

2

50/0

1.03

1.12

1.02

1.06

96

2200

0.71

0.72

1.52

1.12

50

6

25/25

1.56

1.11

1.42

1.09

91

2200

1.44

1.48

1.92

1.20

93

8

50/25

1.85

1.16

1.91

1.17

97

2200

1.84

1.87

2.75

1.12

83

a

Run no. in Table 1 (sample of ROMP polymer); bMolar ratio based on Mo (shown in Schemes 3,4); cGPC data in THF vs polystyrene standards; dCalculated value based on initial feedstock ratio; eEstimated from 1H NMR spectra; fIsolated yield.

Since precise control of the repeat units of both norbornene (hydrophobic) and the sugarsubstituted norbornene derivatives (rather hydrophilic after deprotection) as well as the

Precise Synthesis of Amphiphilic Polymeric Nano Architectures

133

attached PEG (hydrophilic) could be possible by using this approach, it can thus be concluded as a promising technique for the preparation of new types of ABCs consisting of ROMP and PEG units in a precise fashion. As demonstrated above, precise control of the amphiphilic block segments in linear (AB or ABC type), triarm (AB2 or ABC2 type), and sandwich (ABA or ABCBA) type polymeric architectures can be achieved by grafting PEG to the chain end of the living ROMP polymer. Taking into account these facts, the exhaustive control over functionality placement, molecular weight and polydispersity of the polymers, can be attained through ROMP, followed by the quantitative attachment of PEG, this approach is expected to serve as an efficient synthetic methodology for the precise designing of unique polymeric architectures for desired properties.

2.2. Precise Synthesis of Amphiphilic Block Copolymers by Combination of ROMP with Other Living Polymerization Techniques Monomers that can be employed for the living ROMP are limited to highly strained cyclic olefins such as norbornene, norbornadiene, dicyclopentadiene etc., because the driving force is to release the ring strains.[28] The range of attainable block copolymers would be thus greatly extended, if a methodology for synthesis of block copolymers by coupling with another living polymerization, by which the mechanism of the propagation is changed to the one best suited for the propagation of the second monomer, can be achieved. [24,29] 1) n OHC

2)

Cp2Ti

OHC

n

CHO

m

O Si ZnCl2

OHC O Si

H

m

O Si

m

OH OH

n

1) NaBH4 2) nBu4NF 3) CH3OH

n

Scheme 6. Pioneering examples for transformations of ROMP propagating titanium species for synthesis of various block copolymers.[30]

134

Kotohiro Nomura

As the pioneering efforts to this approach, the methods consisting of the living ROMP coupled with the other living polymerization techniques were explored. Risse and Grubbs demonstrated a new route to prepare the well-defined AB diblock copolymers by combining olefin metathesis polymerization and aldol condensation polymerization through the transformation of the metathesis-active end group into an initiator for the aldol-group-transfer polymerization (aldol-GTP). Moreover, the chemical modification of the second block endowed the block copolymer with the unique feature of amphiphilicity (Scheme 6).[30] Matyjaszewski et al. reported in the late 90’s the first example of transformation involving the living ROMP and the controlled “living” atom transfer radical polymerization (ATRP) for synthesis of the block copolymers of norbornene and dicyclopentadiene with styrene and methyl acrylate (Scheme 7).[31] A well-defined molybdenum alkylidene initiator, Mo(CHCMe2Ph)(NAr)(OtBu)2 (A1a, Ar = 2,6-iPr2C6H3), was employed to conduct the living ROMP and subsequent termination with p-(bromomethyl)benzaldehyde affording formation of the efficient macro-initiators for homogeneous controlled/living ATRP of styrene and methyl acrylate, catalyzed by CuBr and 4,4′-di(5-nonyl)-2,2′-bipyridine (dNbipy).

1) n N

CMe2Ph

BrH2C

2) OHC

(t-Bu)O Mo CHCMe2Ph (t-Bu)O

n

CH2Br MeO2C

CuBr/dNbipy, 90 oC Ph

Br

CHMe2Ph

m

CO2Me dNbipy: 4,4'-di(5-nonyl)-2,2'-bipyridine

n

Br m

Ph

CMe2Ph n

Scheme 7. Transformation of propagating species: ROMP to ATRP (Atom Transfer Radical Polymerization).[31]

Grubbs et al. reported one pot tandem synthesis of amphiphilic block copolymers by combination of living ROMP and ATRP using a multifunctional Ru catalyst (Scheme 8).[32c] This is because that Ru(CHPh)Cl2(PCy3) (B1 in Chart 1) is known to be an effective catalyst not only for the ROMP but also for the ATRP of methyl methacrylate (MMA).[33] Although the molecular weight distributions in the resultant copolymer were rather broad (Mw/Mn = 1.51.6) due to that the ROMP of 1,5-cylooctadiene afforded poly(butadiene)s with broad molecular weight distributions (Mw/Mn = 2.0), they confirmed that the resultant copolymers are real diblock copolymers, poly(butadiene-bl-MMA)s. The ruthenium complex can be used as the hydrogenation catalyst, and poly(ethylene-bl-MMA) could be thus prepared in the present tandem system by adopting the multifunctional ruthenium catalyst.[32a] Since then

Precise Synthesis of Amphiphilic Polymeric Nano Architectures

135

the versatile combination of living polymerization techniques of ROMP and ATRP for synthesis of block copolymers has been employed by several other research groups, as described below.[6a,24,32,34] ROMP O Cl

PCy 3

Br

m

O

O

Ru Cl PCy 3

2m

n

O

n

OCH3

CO2CH3

O ATRP H2

O 2m

O

n

CO2CH3

Scheme 8. Synthesis of amphiphilic block copolymers, poly(ethylene-bl-MMA), by one pot synthesis by combination of RMOP and ATRP using multifunctional ruthenium catalyst.[32c]

- Selected Examples for Synthesis of Graft Copolymers by Combination of ROMP with Other Living Polymerization Techniques The above methodology (combination of ROMP with the other living polymerization technique like ATRP) was applied to prepare amphiphilic polymer brushes (graft copolymers).[34] For example, Weck et al. reported synthesis of poly(norbornene-gr-acrylic acid) by combination of ATRP and ROMP as shown in Scheme 9.[34h] ROMP of norbornene derivatives using Ru(CHPh)Cl2(PCy3) (B1 in Chart 1) afforded polymers with relatively narrow molecular weight distributions. However, the subsequent ATRP of tert-butyl acrylate using CuBr-dNbipy in toluene afforded polymers with broad molecular weight distributions (Table 4),[34h] suggesting that certain degree of chain transfer and/or termination reaction accompanied under these conditions.[34h] Most probable reason to explain the above fact would be due to coupling of propagating radicals, although the content of radicals should be controlled via an equilibrium between active and dormant species. The fact introduced a utility of combination of these two highly controlled polymerization methods which allows for a modular approach toward the synthesis of graft copolymers, since the backbone length, the graft density, and the graft length can be varied in a highly controlled manner. Table 4. Synthesis of graft copolymers of poly(norbornene)s/poly(tert-butyl acrylate) (Scheme 9).[34h] poly(norbornene)s / mol-% inita Mwb×10-4 4.9 2.3 9.6 2.5 19.6 2.4 a

Mw/Mnb 1.29 1.32 1.31

poly(tert-butyl acrylate) graft copolymersc Mwb×10-4 Mw/Mnb 17.0 1.67 18.1 1.72 16.2 1.68

Mole % initiator as determined by 1H NMR spectroscopy. bMw and PDI values determined by GPC vs polystyrene standards in CH2Cl2.

136

Kotohiro Nomura PCy3

O

Cl O

O

3

O

Ph

Ru Cl PCy3

Br

O

x

1-x

O

1-x

CuBr 2, dNbpy toluene 90 oC

O

t BuO

O O

O

O

O

O

O

CO2tBu

O

Br

3

O

x

O

3

3

3

O

O

Br

n

O

O

t BuO

O O

3

Scheme 9. Synthesis of poly(norbornene-graft-tert-butyl acrylate)s via a combination of ATRP and ROMP.[34h]

Since, as demonstrated above,[32c] Ru(CHPh)Cl2(PCy3) is an effective catalyst not only for the ROMP but also for the ATRP of methyl methacrylate (MMA),[33] one pot tandem synthesis of graft copolymers by controlled ROMP and ATRP was reported by Novak et al. (Scheme 10).[34g] The resultant copolymers also possessed broad molecular weight distributions (Mw/Mn = 1.67-1.89), and no trace of the homo-ROMP or homo-PMMA polymers was present when the initiator concentration was controlled at least 0.04 M. The conversion of MMA is not complete after 24 h (at 65 ºC), and the MMA consumption was found to be a first-order kinetic process, which indicated the absence of chain termination, the major problem often encountered in radical polymerizations.[34g] The results thus suggest that the methodology, one-pot synthesis approach for the preparation of graft copolymers based on a ROMP skeleton with PMMA grafts using a single catalyst, relies on the controlled activity of Ru(CHPh)Cl2(PCy3)2 for two distinct polymerization processes with the selected monomer structures. Cl 1)

PCy3

Ph y

x

Ru Cl

N O

PCy3

O

O

O N

N

2)

O O 10 O

O

O

O N

O

OCH3 O CO2Me O

O 10

O

m

Scheme 10. One-pot tandem synthesis of graft copolymers by controlled ROMP and ATRP reported by Novak et al.[34g]

Precise Synthesis of Amphiphilic Polymeric Nano Architectures

137

Fontaine et al. reported synthesis of polybutadiene-gr-[polystyrene-bl-poly(acrylic acid)] copolymers by ROMP of α-cyclobutenyl macromonomers prepared by ATRP using a cyclobutenyl-functionalized initiator.[34d] Synthesis of cyclobutenyl macromonomers prepared by ATRP possessed narrow molecular weight distributions, however, the attempted polymerization of macromonomer (Mn = 4300, Mw/Mn = 1.17) using another (more active) ruthenium catalyst (B2 in Chart 1, macromonomer/Ru = 10, molar ratio) afforded polymer that possessed rather high molecular weight with rather broad molecular weight distribution (Mn = 8200, Mw/Mn = 1.21). The result suggests that the ROMP of the macromonomer did not proceed into completion. Similar to the methodology reported by Novak et al.,[33g] which Ru(CHPh)Cl2(PCy3) is an effective catalyst not only for the ROMP but also for the ATRP of methyl methacrylate,[32c,33] Wooley et al. also demonstrated a facile one-pot synthesis of the polymer brushes, by tandem catalysis using a Grubbs’ catalyst that is effective for both ROMP and ATRP (Scheme 11).[34e] After separation of low molecular weight polymers (oligomers) using column chromatography (silica, and alumina), the resultant brush polymer macromolecules essentially presented as unimolecular nanoparticles on the surface, measured by tapping-mode atomic force microscopy (AFM). These nanoparticles exhibited ellipsoidal shapes with variable sizes, in agreement with the limited length ratio of the backbone to grafts and somewhat broad molecular weight distributions (Mn = 5.21×105, Mw/Mn = 1.45; Mn = 9.99×105, Mw/Mn = 1.67), and their surface aggregation behaviors are dependent upon the solvent and concentration employed. Cl

PCy3 Ru

O n

O

10

O

Br

CHPh Cl

PCy3

n

CO2Me

n

57 °C O

O 10

10

O

O O

O

Br MeO2H2C Br

m

Scheme 11. One pot tandem synthesis of graft copolymers by controlled ROMP and ATRP reported by Wooley et al et al.[34e]

The polymerization methodology was applied to synthesis of core shell brush copolymers by combination of ROMP and NMP (nitroxide-mediated polymerization), as shown in Scheme 12.[35b] The subsequent NMP using the nitroxide and isoprene (and careful repetitive precipitation using a mixed solution of MeOH-THF for separation) afforded polymer brushes consisting of poly(norbornene)s containing poly(isoprene) side arms with narrow molecular weight distributions [starting macro-initiator: Mn(GPC) = 1.22×105, Mw/Mn(GPC) = 1.13; polymer brush: Mn(GPC) = 3.66×105, Mw/Mn(GPC) = 1.19], although degree of grafting (isoprene repeating units) seems low (DPn = 18.1 by GPC, 20.1 by NMR). The subsequent NMP using the nitroxide and tert-butyl acrylate followed by flash

138

Kotohiro Nomura

O

n

10

O O N PCy 3

Cl

n

Ru CHPh Cl PCy 3 O

10

O O N

O N

n

120 °C O

10

O O a

O

b

N

c m

O N

O

n

122 °C

O

10

O O a

b

c m

N

l

O

O

n

H , 10 % H2O-p-dioxane

O

10

O O a

b

c m

N

l

O

OH

Scheme 12. Synthesis of graft copolymers by controlled ROMP and NMP reported by Wooley et al.[35b]

Precise Synthesis of Amphiphilic Polymeric Nano Architectures

139

O O

n

10

S

O

S

C12H25

S PCy 3 Ru CHPh Cl PCy 3

n

Cl

O O

10

S

O

S

C12H25

S O

O

O

Ph

n

AIBN, 50 °C

O O

10

O

O

O Ph

O a

S

b

Ph

m

l

S

C12H25

S

1) KOH, THF-H2O 2) neutralization H

H

n

O O O O

10

O

O Ph

O a

Ph

S

b m

l

S

C12H25

S

Scheme 13. Synthesis of graft copolymers by controlled ROMP and RAFT reported by Wooley et al.[35a]

chromatography (eluting with 10% CH2Cl2-hexane) afforded core-shell brush copolymer with narrow molecular weight distribution [Mn(GPC) = 1.41×106, Mw/Mn(GPC) = 1.23; DPn = 41 by GPC, 39 by NMR]. Although the procedure requires rather tedious separation process in each step and the conversions for grafting were low (conversion of isoprene, tert-butyl acrylate for grafting were 1.30, 2.25 %, respectively), poly(macromonomer)s with unimodal molecular weight distributions were obtained. The amphiphilic core shell brush polymer was then obtained by hydrolysis using HCl solution of 10% water-p-dioxane (Scheme 12, last step).[35b] These polymer brushes before/after hydrolysis exhibited aggregated structures on mica but presented as collapsed, globular micelles on silicon, as detected by AFM measurement. A peripherally cross-linked brush copolymer was then prepared by treating with 2,2-(ethylenedioxy)bis(ethylamine) and 1-[3-(dimethylamino)propyl]-3ethylcarbodiimide methiodide.[35b]

140

Kotohiro Nomura

One pot synthesis of core shell polymer brushes was also demonstrated by Wooley et al., adopted by combination of ROMP and RAFT (reversible addition-fragmentation chain transfer), as shown in Scheme 13.[35a] After separation of low molecular weight polymer (oligomer) by column chromatography, the resultant polymer possessed rather narrow molecular distribution [Mn(GPC) = 1.20×106, Mw/Mn(GPC) = 1.32]. Although the procedure requires separation procedure, the methodology would be emphasized as a convenient method to prepare various amphiphilic poly(macromonomr)s, polymer brushes in a controlled repeated units. In contrast to the methodologies by combination of ROMP with ATRP, NMP, RAFT, Feast and Khosravi presented an integration of anionic polymerization (AP) with ROMP leading to the synthesis of quite well-defined, narrow-distribution diblock copolymers, whereas norbornene macromonomer containing polystyrene polymerized by anionic manner were polymerized by ROMP using well-defined molybdenum alkylidene initiators (Scheme 14).[36] The methodology was applied to prepare amphiphilic poly(macromonomer)s (Scheme 15), ring opened poly(norbornene)s containing poly(styrene-bl-ethylene oxide).[37]a Incorporation of styrene repeat units are important to avoid strong interaction between molybdenum and oxygen in PEO [poly(ethylene oxide].[37b] Li

nBu n

BuLi

OLi

nBu

n

n

O

COCl COCl COCl O nBu

O

O n

nBu

O O

n n

O Mo cat.

nBu

R' Mo cat. O

nBu

R'

O n

O m

nBu

O O m

R O

n

n

nBu

R

Scheme 14. Synthesis of comb and graft copolymers containing polystyrene. Combination of ROMP and anionic polymerization.[36]

Precise Synthesis of Amphiphilic Polymeric Nano Architectures

n Li

n'

benzene TMEDA

n'

O

1)

O n'

OH

2) MeOH

1)

Om

Li

141

O n'

2) PhCH2Br

OK

k

Mo cat. n'

O

Om

Scheme 15. Synthesis of amphiphilic comb and graft copolymers containing polystyrene and poly(ethylene oxide) segments. Combination of ROMP and anionic polymerization.[37]

3. Precise Synthesis of Amphiphilic Block Copolymers by the Repetitive Living ROMP Approach: Living ROMP of Macromonomers Poly(macromonomer)s (PMMs) containing ABCs in the side chain are of particular interest not only due to the axisymmetric distribution of the side chains from the central polymeric backbone but also because of their ability to exhibit interesting (spherical, cylindrical, star, and worm-like) morphologies in bulk and solution, which are prone to the variation of the side chain and backbone composition.[38] The preparation of amphiphilic PMMs bearing carbohydrates has potential for improved targeting, recognition, and modulation of cell surface processes.[39] The increased density of the sugar moieties and the ability to mediate protein-carbohydrate interactions in three dimensions should confer the PMMs with improved properties in comparison with those of the corresponding monovalent or linear polyvalent displays of carbohydrates, as reported previously.[40] There have been several reports concerning the synthesis of PMMs by radical,[41] anionic,[42] and metallocene-catalyzed polymerizations.[43] We have demonstrated the preparation of amphiphilic PMMs via repetitive ROMP, and this technique allows precise control of the degree of polymerization of the side chain with complete macromonomer conversion.[22b] We have recently extended this strategy by incorporating sugar-containing norbornene derivatives, and efficient preparation of PMMs containing acetal/acetyl-protected sugars has been carried out via repetitive living ROMP.[8a,22a] The molybdenum alkylidene catalysts by Schrock[21] have proven to be powerful synthetic tools as they facilitate living polymerization and enable access to the precise polyvalent arrangements containing a variety of functionalities.[3b,3g,44,45] Therefore, we focused on the repetitive ROMP technique using well-defined molybdenum alkylidene initiators of the type, Mo(CHCMe2Ph)(NAr)(OR)2 (A, Chart 1).[21] The macromonomer

142

Kotohiro Nomura

preparation encompassed the following three key steps: (i) exclusive end-capping of block ROMP copolymer with TMS (SiMe3) protected 4-hydroxybenzaldehyde; (ii) quantitative removal of the TMS moiety to generate OH group on the terminus,[3b,8a,22a,22b] and (iii) thorough esterification of the terminal OH group with norbornene carboxylic acid chloride,[8a,22a,22b] [ex. poly(6) in Scheme 16]. Various poly(macromonomer)s, composed of ring-opened poly(norbornene) backbone and its substituted analogues in the side chain [poly(A)–(D), Chart 3], have been prepared efficiently by using Mo(CHCMe2Ph)(N-2,6-iPr2C6H3)[OCMe(CF3)2]2 (A1b), an effective initiator in order for the polymerization to proceed with complete conversion in a controlled manner [e.g., poly(7) in Scheme 16]. R Mo cat. A1a

n

Me3SiO

CMe2Ph m

Mo n

R

R

R

CHO

R = CH2OSitBuMe 2 Me3SiO

CMe2Ph m

n

R

R

N HO

R1O Mo CHCMe2Ph R1O A1

CMe2Ph m

O C

R 1 = t Bu (A1a), CMe(CF3 )2 (A1b)

O C

n

R Cl

R

in THF NEt3

O n

m

R

poly(6)

k

Ph

CMe 2Ph

R

1/k equiv. Mo cat. A1b O C

O

CMe2Ph n

m

poly(7)

R

R

CMe2Ph Scheme 16. Precise synthesis of amphiphilic poly(macromonomer)s, poly(7), by repeating living ringopening metathesis polymerization by molybdenum catalysts.[22b]

Precise Synthesis of Amphiphilic Polymeric Nano Architectures CMe2Ph

CMe2Ph O

)

)

O O

CMe2Ph

O

Ph

CH2OH

(

HOH2C

k

m

n

n

(

143

HOH2C

k'

Ph

poly(A)

CMe2Ph

CMe2Ph

CH2OH

poly(B)

O

)

)

CMe 2Ph

CH2OH

O

CMe2Ph

x

CH2OH O

)(

)(

n z

CMe2Ph

O

HOH2C

O

CMe2Ph

O

Ph

poly(C)

n'

(

(

n'

y

CH2OH

y

poly(D)

Ph

Chart 3. Various amphiphilic block (graft) copolymers prepared by polymerization of macromonomers by repetitive ROMP technique.

Note that synthesis of various amphiphilic poly(macromonomer)s can be achieved via both the homopolymerization of macromonomers containing amphiphilic segments [poly(A) & poly(B) in Chart 3] and block copolymerization by sequential addition of different macromonomers [poly(D)] or substituted norbornene and macromonomers [poly(C)].[22b] Since use of the molybdenum alkylidenes allows the precise control of the repeating unit as well as the block segment, the present synthetic approach seems to be of particular significance/quite promising for synthesis of a variety of functional group-containing poly(macromonomer)s, especially for precision synthesis of amphiphilic poly(macromonomer) architectures. Moreover, the ROMP of macromonomers bearing galactose [poly(8a)] and ribose [poly(8b)], effected by the Mo(CHCMe2Ph)(N-2,6-Me2C6H3)[OCMe(CF3)2]2 (A2b) catalyst, proceeded to completion (Scheme 17) yielding the PMMs, poly(9a,b), with narrow molecular weight distribution (Mw/Mn = 1.07–1.22). By varying the initial feedstock ratio of poly(8a)/A2b, poly(9a)s having different main chain lengths (DPn, estimated based on Mn values by GPC) were obtained in high yield (Table 5, run 16-18).[8a,22a] These results demonstrate the efficacy of A2b to bring about the ROMP of the macromonomers in a livinglike manner with high initiation efficiency. Furthermore, the complete conversion could also be achieved in the ROMP of the macromonomer containing the triblock copolymer of NBE20b-b20-b-a20, and the GPC traces displayed an increase in the Mn value from 1.82×104 for poly(9b-a) to 17.47×104 for the PMM, which corresponds to a main chain of approximately 10 units while maintaining a narrow molecular weight distribution (Mw/Mn = 1.11, run 23), illustrating the ability to produce a PMM with three different well-defined blocks in the side chain.

Table 5. Preparation of poly(macromonomer)s (PMMs) by repetitive ROMP.a,[8a] run

poly(8) monomer feed ratio in poly(8)b

a

cat.

time

poly(9), PMMs

Mn(Calcd) c ×10-4

Mn(GPC)d ×10-4

Mn(NMR)e ×10-4

Mw/Mnd

(equiv.f/ k)

/h

Mn(calcd)c ×10-4

Mn(GPC)d ×10-4

Mw/Mnd

DPng

yieldh / %

12

NBE25-b-a25

1.18

1.58

1.24

1.1

A1b (10)

1.5

11.95

4.50i

1.08

--i

98

13

NBE25-b-a25

1.18

1.58

1.24

1.1

B (10)

1

11.95

1.27

1.7

--

95

14

NBE25-b-a25

1.18

1.58

1.24

1.1

A2b (5)

2

5.98

8.18

1.15

5.2

98

15

NBE25-b-a25

1.18

1.58

1.24

1.1

A2b (10)

2.5

11.95

16.45

1.13

10.4

96

16

NBE20-b-a20

0.95

1.28

1.02

1.11

A2b (10)

2

9.72

11.76

1.07

9.2

96

17

NBE20-b-a20

0.95

1.28

1.02

1.11

A2b (5)

2

4.83

5.87

1.09

4.6

97

18

NBE20-b-a20

0.95

1.28

1.02

1.11

A2b (3)

1

2.89

3.99

1.19

3.1

97

19

NBE20-b-b30

0.84

1.02

0.92

1.18

A2b (10)

2

11.72

14.33

1.22

10.2

97

20

NBE10-b-a20

0.85

1.18

0.94

1.12

A2b (15)

3

12.77

20.04

1.07

17

>99

21

NBE10-b-a20

0.85

1.18

0.94

1.12

A2b (5)

2

4.27

5.74

1.17

4.9

98

22

NBE20-b-b20

1.15

1.4

1.24

1.16

A2b (10)

2

8.42

8.42

1.12

8.3

96

23

NBE20-b-b20-b-a20

1.56

1.82

1.59

1.08

A2b (10)

3

15.62

17.47

1.11

9.6

99

Conditions: toluene (2.0 g), at 25 ºC; bStarting feedstock ratio; cCalculated from initial feedstock ratios; dCalculated from GPC data; eEstimated from 1H NMR spectra; f Ratio of macromonomer to initiator (Scheme 17); gCalculated from GPC data; hIsolated yield; iPolymerization did not proceed to completion (mixture of macromoner and oligo(macromonomer)s).

Precise Synthesis of Amphiphilic Polymeric Nano Architectures

145

In contrast, Mo(CHCMe2Ph)(N-2,6-iPr2C6H3)[OCMe(CF3)2]2 (A1b) and the Grubbs ruthenium carbine, [Ru(CHPh)(Cl)2(IMesH2)(PCy3)] (B2 in Chart 1, IMesH2 = 1,3-dimesityl4,5-dihydroimidazol-2-ylidene), were not suitable for the complete conversion of the macromonomers, poly(8a-b).[8a,22a] Although the ROMP of poly(NBE)-containing macromonomer by A2b proceeded to complete conversion, however that of poly(8a) resulted in a mixture of the trimer, tetramer and poly(8a) {conditions: [poly(8a)]:[A1b] = [10]:[1], Table 5, run 12}, probably on account of the insufficient reactivity toward the norbornenyl olefins under these conditions. The same attempt with initiator B2 gave a polymer with low Mn value and broad distribution (run 13), suggesting occurrence of the metathesis (degradation) with internal olefins rather than the ROMP. O C

O

CMe2Ph n

m

R

poly(8) Ph k

1/k equiv. Mo cat. A2b O C

O

CMe2Ph n

m

poly(9)

R O2C

CMe2Ph Ar N R'

R' N

Cl Cl

N Ar

R=

O

Ru CHPh

O O O a

O

O O

O

O O b

Mo B PCy 3 RO CHCMe2Ph RO A Ar = 2,4,6-Me3C6H2 R = tBu, R' = iPr (A1a); R = CMe(CF3)2, R' = iPr (A1b); R = CMe(CF3)2, R' = Me (A2b).

Scheme 17. Precise synthesis by repetitive ROMP.[8a,22a]

of

poly(macromonomer)s

containing

sugars,

poly(9),

Our extended strategy entails the preparation of a PMM according to the above procedure except that polymerization of the macromonomer was quenched with 4-Me3SiO-C6H4CHO followed by the treatment of PMM with HCl aq. to afford poly(10), as shown in Scheme 18. The quantitative removal of TMS protection under mild acidic conditions accompanied no significant decrease in the Mn values with a narrow molecular weight distribution (Mw/Mn = 1.09). The phenolic terminus was employed for the KH-mediated grafting of methane sulfonyl protected poly(ethylene glycol) [PEG-Ms2, MsO(CH2CH2O)nMs; Ms = MeSO2] to the PMM, and two PEG samples with different molecular weights [PEG47 (Mn = 2200) and PEG110 (Mn = 4600); Mw/Mn = 1.03] were pursued as the hydrophilic segment in the preparation of PMM-block-PEG amphiphilic architectures. The attachment of PEG47 and PEG110 to poly(10) afforded poly(11) (yield, 82% and 85%), and the Mn values measured by

146

Kotohiro Nomura

GPC revealed the increment from 5.85×104 to 6.11×104 (for PEG47) and 6.40×104 (for PEG110) with narrow unimodal dispersity of 1.08 and 1.11, respectively (Table 6). These results clearly demonstrate a facile method of preparing amphiphilic architectures by the careful manipulation of the end groups of PMMs. O C

O

CMe2Ph n

m

poly(8)

R

1/k equiv. Mo cat. A2b

k

CMe 2Ph

Me3SiO O C

O

CHO

CMe2Ph n

m

R (i) Exclusive Deprotection of OH Group 0.5N HCl O C

CMe2Ph

O m

n

poly(10 ) R (ii) Attachment of PEG

k

OH

(i) KH, THF, 3h (ii) PEG-Ms2, 20h Ms

CMe2Ph O C

O

O

Ms

x

Ms = MeSO2 CMe2Ph

O n

m

poly(11) R O2C O x

OSiMe3

k

CMe2Ph

O

R=

O

O O

a

O

O Ms

Scheme 18. Preparation of poly(macromonomer)-graft-PEG, poly(11).

Precise Synthesis of Amphiphilic Polymeric Nano Architectures

147

Table 6. Preparation of poly(macromonomer)-block-(PEG), poly(11).a,[8a] poly(8)/A2b

poly(10) c

/ equiv.b

a

PEG

d

Mn(calcd) Mn(GPC) yield / Mw/Mnd % ×10-4 ×10-4

poly(11)

Mn

Mn(calcd)c -4 ×10

Mn(GPC)d Mn(NMR) e -4 -4 ×10

×10

Mw/Mnd

yieldf / %

14

2.88

3.89

1.11

99

4600

3.33

4.77

3.43

1.12

87

16

4.78

5.85

1.09

98

2200

4.99

6.11

5.07

1.08

82

16

4.78

5.85

1.09

99

4600

5.23

6.4

5.29

1.11

85

4

b

Based on MM (macromonomer) poly(8a20); Mn(GPC) = 1.28×10 , Mw/Mn = 1.11; A2b = F6(Me2); ratio of macromonomer to initiator; cCalculated from initial feedstock ratios; dGPC data in THF versus polystyrene standards; eEstimated from 1H NMR spectra; fIsolated yield.

Poly(11), PMM-block-PEG aggregated to form spherical micelles as observed by TEM (Figure 2), revealing spherical aggregates with a diameter, dTEM = 148.5±7.2 nm, corresponding to a circumference of approximately 467 nm.[8a,46] These well defined micelles are smaller in size to the corresponding PEG-based ABC, poly(3), explained by the more facile packing of the linear chains into the hydrophobic centre in comparison to the bulky core-forming PMM of poly(11). The ability to uptake the hydrophobic dye (Nile Red) into the micellar cores of a variety of amphiphilic polymeric fragments is a significant step towards the production of sugar-coated nano-spheres for cell targeting biomimetic applications.[8a]

Figure 2. TEM images of spherical aggregates of poly(NBE20-b-a20)5-b-PEG110 [poly(11)] at a concentration of 0.05 mg/mL THF at varying magnification.[8a,46] Mn(GPC) = 4.99×104, Mn(calcd) = 6.11×104, Mn(NMR) = 5.07×104, Mw/Mn = 1.08 (shown in Table 6).[8a]

We can demonstrate that precise control of both main and side chain in the new class of amphiphilic poly(macromonomer)s containing sugars can be achieved for the first time by adopting the present repetitive ROMP procedure. Since the present approach should introduce

148

Kotohiro Nomura

a new possibility to prepare various kinds of amphiphilic nano arrangements containing sugars, unique properties such as both strong and specific affinities based on proteincarbohydrate interactions will be thus expected. We are currently exploring other possibilities to prepare another series of amphiphilic nano architectures containing sugars by a combination of this approach with our ‘grafting to’ approach, and these results will be introduced in the near future.

4. Concluding Remarks In this article, we introduced recent examples for preparation of amphiphilic block copolymers utilized by the living ring-opening metathesis polymerization (ROMP) method. Due to the living nature of polymerization with complete conversion, the desired block copolymers can be prepared by simple sequential addition. In particular, unique characteristics of the living ROMP initiated by molybdenum alkylidene complexes (so-called Schrock type catalyst) can provide to accomplish not only precise control of the block segment (hydrophilic and hydrophobic) but also exclusive introduction of functionalities at the polymer chain end. The technique enable us to provide the synthesis of block copolymers varying different backbones by adopting the “grafting to” or “grafting from” approach. Facile one pot synthesis can be used in the combination of ROMP using (so called Grubbs type) ruthenium carbene catalyst, and the catalyst is also effective for atom transfer radical polymerization (ATRP), affording not only amphiphilic block copolymers in a controlled manner but also amphiphilic polymer brushes (graft copolymers) under appropriate conditions (and/or after purification procedures). Moreover, use of the “grafting through” approach (polymerization of macromonomers) by employing the repetitive ROMP technique, using the molybdenum alkylidene catalysts, offers precise control of the amphiphilic block segments. We believe that the approaches employed here should be promising for precise synthesis of amphiphilic polymers displaying controlled nano architectures as well as unique properties.

Acknowledgment K.N. would like to express his thanks to Dr. Fareha Zafar Khan (Nara Institute of Science and Technology, Japan) for her kind help to prepare this manuscript. Our research projects were partly supported by a Grant-in-Aid for Exploratory Research (No. 16656248, 19656215), and K.N. would like to express his heartfelt thanks to former group members who contributed the subjects introduced in the text.

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Kotohiro Nomura Checot, F.; Lecommandoux, S.; Gnanou, Y.; Klok, H.-A. Angew. Chem. Int. Ed. 2002, 41, 1339. Schilli, C. M.; Zhang, M.; Rizzardo, E.; Thang, S. H.; Chong, Y. K.; Edwards, K.; Karlsson, G.; Muller, A. H. E. Macromolecules 2004, 37, 7861. (b) Chung, J. E.; Yokoyama, M.; Okano, T. J. Controlled Release 2000, 65, 93. Goodwin, A. P.; Mynar, J. L.; Ma, Y.; Fleming, G. R.; Fréchet, J. M. J. Am. Chem. Soc. 2005, 127, 9952. Jiang, J.; Tong, X.; Zhao, Y. J. Am. Chem. Soc. 2005, 127, 8290. For example, Gestwicki, J. E.; Cairo, C. W.; Strong, L. E.; Oetjen, K. A.; Kiessling, L. L. J. Am. Chem. Soc. 2002, 124, 14922. Reviews and books for olefin metathesis, (a) Fogg, D. E.; Foucault, H. M. In Comprehensive Organometallic Chemistry III; Crabtree, R. H.; Mingos, D. M. P. Eds.; Elsevier Ltd.: 2007; vol. 11, pp. 623. (b) Buchmeiser, M. R. Ed.; “Metathesis Polymerization”; Springer: Berlin, 2005. (c) Grubbs, R. H. Ed.; Handbook of Metathesis; Wiley-VCH: Weinheim, 2003; vol. 1–3. (d) Buchmeiser, M. R. Chem. Rev. 2000, 100, 1565. (e) Fürstner, A. Ed.; Alkene Metathesis in Organic Synthesis; Springer-Verlag: Berlin Heidelberg, 1998. Schrock, R. R. In Handbook of Metathesis; Grubbs, R.H. Ed.; Wiley-VCH: Weinheim, 2003; vol. 1, p 8. (b) Schrock, R. R.; Hoveyda, A. H. Angew. Chem. Int. Ed. 2003, 42, 4592. (c) Schrock, R. R. In Metathesis Polymerization of Olefins and Polymerization of Alkynes; Imamoglu, Y. Ed.; NATO ASI Series, Kluwer Academic Publishers: 1998; p 1 and p 357. (d) Schrock, R. R. In Alkene Metathesis in Organic Synthesis; Fürstner, A, Ed.; Springer-Verlag: Berlin Heidelberg, 1998; p 1. (e) Feldman, J.; Schrock, R. R. Prog. Inorg. Chem. 1991, 39, 1. (f) Schrock, R. R. Acc. Chem. Res. 1990, 23, 158. Murphy, J. J.; Nomura, K. Chem. Commun. 2005, 4080. (b) Nomura, K.; Takahashi, S.; Imanishi, Y. Macromolecules 2001, 34, 4712. Kitiyanan, B.; Nomura, K. Organometallics 2007, 26, 3461. (b) Nomura, K.; Kuromatsu, Y. J. Mol. Catal. A 2006, 245, 152. (c) Nomura, K.; Ogura, H.; Imanishi, Y. J. Mol. Catal. A 2002, 185, 311. For the synthesis of ROMP copolymers, Khosravi, E. In Handbook of Metathesis; Grubbs, R. H. Ed.; Wiley-VCH: Weinheim 2003; Vol. 3, p 72. For examples, (a) Nguyen, S. T.; Trnka, T. M. In Handbook of Metathesis; Grubbs, R. H. Ed.; Wiley-VCH: Weinheim, 2003; vol. 3, p 61. (b) Sanfold, M. S.; Love, J. A. In Handbook of Metathesis; Grubbs, R. H. Ed.; Wiley-VCH: Weinheim, 2003; vol. 3, p 112. (c) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18. Morita, T.; Maughon, B. R.; Bielawski, C. W; Grubbs, R. H. Macromolecules 2000, 33, 6621. (b) Mitchell, J. P.; Gibson, V. C.; Schrock, R. R. Macromolecules 1991, 24, 1220. Notestein, J. M.; Lee, L.-B. W.; Register, R. A. Macromolecules 2002, 35, 1985. Novak, B. M.; Risse, W.; Grubbs, R. H. Adv. Polym. Sci. 1992, 102, 47. Hadjichristidis, N.; Pispas, S.; Floudas, G. Block Copolymers: Synthetic Strategies, Physical Properties, and Applications; John Wiley & Sons: Hoboken, NJ, 2003. (b) Schue, F. Synthesis of Block Copolymers by Transformation Reactions. In Comprehensive Polymer Science, 2nd ed.; Allen, G.; Bevington, J. C., Eds.; Pergamon Press: Oxford 1989; Vol. 6, Chapter 10.

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[30] Risse, W.; Grubbs, R. H. J. Mol. Catal. 1991, 65, 211. (b) Risse, W.; Grubbs, R. H. Macromolecules 1989, 22, 1558. [31] Coca, S.; Paik, H.-J.; Matyjaszewski, K. Macromolecules 1997, 30, 6513. [32] Castle, T. C.; Hutchings, L. R.; Khosravi, E. Macromolecules 2004, 37, 2035. (b) Li, M.-H; Keller, P.; Albouy, P.-A. Macromolecules 2003, 36, 2284. (c) Bielawski, C. W.; Louie, J.; Grubbs, R. H. J. Am. Chem. Soc. 2000, 122, 12872. [33] Simal, F.; Demonceau, A.; Noels, A. F. Tetrahedon Lett. 1999, 40, 5689. (b) Simal, F.; Demonceau, A.; Noels, A. F. Angew. Chem., Int. Ed. Engl. 1999, 38, 538. [34] Matson, J. B.; Grubbs, R. H. Macromolecules 2008, 41, 5626. (b) Airaud, C.; Héroguez, V.; Gnanou, Y. Macromolecules 2008, 41, 3015. (c) Quémener, D.; Bousquet, A.; Héroguez, V.; Gnanou, Y. Macromolecules 2006, 39, 5589. (d) Morandi, G.; Montembault, W.; Pascual, S.; Legoupy, S.; Fontaine, L. Macromolecules 2006, 39, 2732. (e) Cheng, C.; Khoshdel, E.; Wooley, K. L. Nano Lett. 2006, 6, 1741. (f) Runge, M. B.; Dutta, S.; Bowden, N. B. Macromolecules 2006, 39, 498. (g) Charvet, R.; Novak, B. M. Macromolecules 2004, 37, 8808. (h) Kriegel, R. M.; Rees, Jr., W. S.; Weck, M. Macromolecules 2004, 37, 6644. [35] Cheng, C.; Khoshdel, E.; Wooley, K. L. Macromolecules 2007, 40, 2289. (b) Cheng, C.; Qi, K.; Khoshdel, E.; Wooley, K. L. J. Am. Chem. Soc. 2006, 128, 6808. [36] Feast, W. J.; Gibson, V. C.; Johnson, A. F.; Khosravi, E.; Mohsin, M. A. J. Mol. Catal., A: Chem. 1997, 115, 37. (b) Feast, W. J.; Gibson, V. C.; Johnson, A. F.; Khosravi, E.; Mohsin, M. A. Polymer 1994, 35, 3542. [37] Héroguez, V.; Breunig, S.; Gnanou, Y.; Fontanille, M. Macromolecules 1996, 29, 4459. (b) Héroguez, V.; Gnanou, Y.; Fontanille, M. Macromolecules 1997, 30, 4791. [38] Tsukahara, Y.; Namba, S.; Iwasa, J.; Nakano, Y.; Kaeriyama, K.; Takahashi, M. Macromolecules 2001, 34, 2624. (b) Tsukahara, Y.; Kohjiya, S.; Tsutsumi, K.; Okamoto, Y. Macromolecules 1994, 27, 1662. (c) Tsukahara, Y.; Tsutsumi, K.; Yamashita, Y.; Shimada, S. Macromolecules 1990, 23, 5201. (d) Tsukahara, Y.; Mizuno, K.; Segawa, A.; Yamashita, Y. Macromolecules 1989, 22, 1546. [39] Bes, L.; Angot, S.; Limer, A.; Haddleton, D. M. Macromolecules 2003, 36, 2493. (b) Yasugi, K.; Nakamura, T.; Nagasaki, Y.; Kato, M.; Kataoka, K. Macromolecules 1999, 32, 8024. (c) Yamada, K.; Minoda, M.; Miyamoto, T. Macromolecules 1999, 32, 3553. [40] Kiessling, L. L.; Owen, R. M. In Handbook of Metathesis; Grubbs, R. H. Ed.; Wiley VCH: Weinheim, 2003; vol. 3, p 180. (b) Mann, D. A.; Kiessling, L. L. In Glycochemistry; Wang, P. G.; Bertozzi, C. R. Eds.; Marcel Dekker Inc.: New York, 2001; p 221. [41] Gerle, M.; Schmidt, M.; Fischer, K.; Roos, S.; Muller, A. H. E.; Shieko, S. S.; Prokhorova, S.; Möller, M. Macromolecules 1999, 32, 2629. (b) Dziezok, P.; Shieko, S. S.; Fischer, K.; Schmidt, M.; Möller, M. Angew. Chem. Int. Ed. 1997, 36, 2812. (c) Shieko, S. S.; Gerle, M.; Fischer, K.; Schmidt, M.; Möller, M. Langmuir 1997, 13, 5368. (d) Wintermantle, M.; Gerle, M.; Fischer, K.; Schmidt, M.; Wataoka, I.; Urakawa, H.; Kajiwara, K.; Tsukahara, Y. Macromolecules 1996, 29, 978. (e) Wintermantel, M.; Schmidt, M.; Tsukahara, Y.; Kajiwara, K.; Kohjiya, S. Macromol. Rapid Commun. 1994, 15, 279. [42] Pantazis, D.; Chalari, I.; Hadjichristidis, N. Macromolecules 2003, 36, 3783. [43] Neiser, M. W.; Muth, S.; Kolb, U.; Harris, J. R.; Okuda, J.; Schmidt, M. Angew. Chem. Int. Ed. 2004, 43, 3192. (b) Neiser, M. W.; Okuda, J.; Schmidt, M. Macromolecules

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2003, 36, 5437. (c) Peruch, F.; Lahitte, J.-F.; Isel, F.; Lutz, P. J. Polym. Prepr. Am. Chem. Soc. Div. Polym. Chem. 2002, 43, 140. (d) Ederle, Y.; Isel, F.; Grutke, S.; Lutz, P. J. Macromol. Symp. 1998, 132, 197. [44] Bazan, G. C.; Oskam, J. H.; Cho, H.; Park, L. Y.; Schrock, R. R. J. Am. Chem. Soc. 1991, 113, 6899. (b) Bazan, G. C.; Khosravi, E.; Schrock, R. R.; Feast, W. J.; Gibson, V. C.; O’Regan, M. B.; Thomas, J. K.; Davis, W. M. J. Am. Chem. Soc. 1990, 112, 8378. [45] Miyamoto, Y.; Fujiki, M.; Nomura, K. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 4248. (b) Nomura, K.; Sakai, I.; Imanishi, Y.; Fujiki, M.; Miyamoto, Y. Macromol. Rapid Commun. 2004, 25, 571. [46] These figures are selected from the figures taken independently, and are different from those used in reference 8a.

In: Surface Coatings Editors: M. Rizzo and G. Bruno, pp. 153-175

ISBN: 978-1-60741-193-2 © 2009 Nova Science Publishers, Inc.

Chapter 5

ATMOSPHERIC PRESSURE PLASMA POLYMERISATION R. Morent*, N. De Geyter and C. Leys Research Unit Plasma Technology (RUPT), Department of Applied Physics, Faculty of Engineering, Ghent University, Ghent, Belgium.

Abstract Plasma polymerisation is a unique technique for modifying material surfaces by depositing a thin polymer film. Plasma polymerised films have received a great deal of interest due to their unique characteristics. These coated films are pinhole-free and highly cross-linked and are therefore insoluble, thermally stable, chemically inert and mechanically though. Furthermore, such films are often highly coherent and adherent to a variety of substrates including conventional polymer, glass and metal surfaces. Due to these excellent properties, plasma polymerised films can offer many practical applications in the field of mechanics, electronics and optics. Plasma polymerisation at low pressure is already a well established technology. However, the NECESSITY of expensive vacuum systems is the biggest shortcoming of this technology in industrial applications besides the limitation to batch processes. Therefore, to overcome these disadvantages, considerable efforts are made in developing alternative techniques. Atmospheric pressure plasmas are one of the most promising methods to deposit polymer films in a more flexible, reliable, less expensive and continuous way of treatment. In the last two decades, a lot of effort has been put into the development of plasma polymerisation at elevated pressure. This paper attempts to review this research and its applications in a broad perspective.

1. Introduction Plasma polymerisation is a versatile technique for the deposition of films with functional properties suitable for a wide range of applications[8]. These plasma polymers have different properties than those fabricated by conventional polymerisation: the plasma polymerised films are usually branched, highly cross-linked, insoluble, pinhole-free and adhere well to most substrates including polymer, glass and metal surfaces[2,109]. Due to these excellent

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properties, plasma polymerised films have been utilized in a wide range of applications, such as protective coatings, biomedical materials, electronic and optical devices, adhesion promoters, …[2]. Plasma polymerisation is mostly performed at low pressure (101 – 102 Pa), making its application limited to batch processes[52]. Although vacuum treatment processes afford good control over gas chemistry and provide the possibility of using high energetic species (in the range of several eV to hundreds of eV) in the deposition process, atmospheric pressure processing techniques are offering specific advantages, such as the elimination of expensive vacuum equipment, easier handling of the samples and scalability for industrial in-line processing. Therefore recently, lot of effort has been put into the development of non-thermal plasma reactors for thin film deposition working at or near atmospheric pressure[10,64,104]. Polymer precursors should be administered to the plasma in order to achieve thin film deposition. The uniqueness of non-thermal plasma processes rests in the fact that these permit the conversion of a wide range of organic materials or organic compounds into charged and neutral molecular fragments and atomic species[10]. These fragments can then generate thin layers as a result of recombination of active species on the surfaces that confine the plasma[10]. The functionality of the plasma deposited layer is mainly determined by the nature of the precursors[10]. As stated above, the introduction of a monomer in the active plasma zone is crucial in obtaining plasma polymerised films. These precursors, which lead to the coating formation, are often diluted in a carrier gas, which is usually helium, argon or nitrogen[64]. Some monomers are available in gaseous form, but typically most monomers are commercially available as a liquid. It is easy to mix a gaseous precursor with the carrier gas, but in the case of a liquid precursor, this procedure becomes more complicated. Liquids with high vapour pressure can be inserted in two ways: firstly, directly if the vapour pressure is high enough so that complete evaporation occurs when the monomer liquid enters the system, or secondly through a gas bubbler system. However, traditional bubbler systems cannot handle liquids with low vapour pressure. Therefore until recently, the variety of coating precursors used in deposition was limited to gaseous chemicals and liquid precursors with such high vapour pressures. This considerably limited the coating capabilities. Two new technologies enable to overcome the latter limitations and allow making use of the full variety of chemical compounds available today. One of these technologies uses nano-sized aerosols. An atomiser nebulises a liquid and generates high-concentration aerosols. Another possibility is the use of a liquid delivery system with vapour control (CEM – Bronkhorst®) which delivers a gasified precursor to the plasma. For the completeness, one should also mention the method of immersing the substrates in a monomer solution before plasma treatment leading to polymerisation of the liquid on the surface. This chapter attempts to give an overview of the research on atmospheric pressure plasma polymerisation. Section 2 describes the generation of stable atmospheric pressure plasmas in a broad perspective. Section 3 classifies the literature on deposition of plasma polymers at atmospheric pressure based on the nature of the precursor used. At the same time, various important applications will be outlined. Finally, concluding remarks are given in Section 4.

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2. Non-thermal Atmospheric Pressure Plasmas Plasma is sometimes referred to as the fourth state of matter. The term was introduced by Langmuir in 1929. Plasma is a partly ionized gas and can be defined as a quasi-neutral particle system in the form of gaseous or fluid-like mixtures of free electrons, ions and radicals, generally also containing neutral particles (atoms, molecules). Some of these particles may be in an excited state. Particles in an excited state can return to their ground state by photon emission. The latter process is at least partially responsible for the luminosity of a typical plasma. In plasma, certain electrons are free, rather then bound to molecules or atoms. This means that positive and negative charges can move somewhat independently from each other. Plasmas are frequently subdivided into non-equilibrium (or non-thermal/lowtemperature/cold) and equilibrium (or thermal/high-temperature/hot) plasmas. Thermal equilibrium implies that the temperature of all species (electrons, ions, neutrals and excited species) is the same. This is, for example true for stars, as well as for fusion plasmas. High temperatures are required to form these type of plasmas, typically ranging from 4000 K for easy-to-ionize elements like cesium to 20000 K for hard-to-ionize elements, like helium[9,60]. In contrast, plasmas with strong deviations from kinetic equilibrium have electron temperatures that are much higher than the temperature of the ions and neutrals and are classified as non-equilibrium plasmas. It is obvious that the high temperatures used in thermal plasmas are destructive for a lot of materials. In this chapter about polymer thin films prepared by plasma polymerisation at atmospheric pressure, the scope is limited to nonthermal or cold plasma sources. In plasma technology, non-thermal plasmas are generated by an electrical gas discharge. The application of a strong electric field to a neutral gas ensures ionization in the gas volume and the created charged particles are accelerated in the applied electrical field. Especially the electrons are affected by the field due to their light mass and gain most energy. They achieve high temperatures (105-106 K), while the heavy ions efficiently exchange their energy by collisions with the background gas and thus remain cold. The gas temperature is below 473 K; due to this low gas temperature, plasma surface treatment is applicable to heat-sensitive materials. On collision between energetic electrons and neutral molecules, radicals are created. These radicals play an important role in the chemical activity of the plasma. Different for surface modification developed plasma sources operate at low pressures (10-3-1000 Pa). At low pressure the discharge is more stable and it is easier to control the plasma reactions. A long mean free path of the gas particles guarantees only few collisions and thus only a small reduction in the number of chemically active species. However, the application field of plasma technology is growing very fast. Moreover, increasing demands from industry encourage the continuous development of more efficient and more flexible plasma techniques. Therefore, it can be noticed that in recent plasma technology research large efforts are made to develop atmospheric pressure technology based plasma reactors to overcome the disadvantages of low pressure. Because there is no need for vacuum devices, the investment costs are much lower and atmospheric plasma technology can easily be scaled up to industrial dimensions and integrated in in-line processes. According to the location of the treated sample with respect to the gas discharge chamber, surface treatment with atmospheric plasma technology can be divided into active

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and remote plasma treatment as shown in Figure 1. In the active plasma treatment, the substrate to be treated will pass between the electrodes. In this way, there is direct contact between the substrate surface and the active plasma. In remote plasma treatment, the substrate is located outside the plasma chamber, but passes in the gas stream that runs through the plasma chamber and that is loaded with radicals and other active species. The sample is treated in the afterglow of the plasma. An active plasma treatment has the advantage of a higher concentration of active species near the surface of the substrate, while for remote plasma treatment, this concentration decreases as a function of distance to the plasma chamber and depends on the lifetime of the active species[100]. On the other hand, the active plasma treatment faces a risk of backside treatment and pin-holing, while the remote treatment prevents damages from the discharge as the discharge current does not flow through the sample. Moreover, remote treatment allows the possibility to treat the surface of materials of any thickness and any geometry (3D objects). The active treatment is often limited to thin substrates, due to the dimensions of the interelectrode space.

Figure 1. Active versus remote atmospheric plasma treatment.

The difficulty of working at atmospheric pressure is that instabilities in the discharge rapidly arise, so that transition to a thermal arc discharge is likely to take place. This transition is undesirable because of the loss of homogeneity as the discharge constricts to a narrow current channel. Moreover, the high current density causes an increase in gas temperature, which dispels the non-thermal character of the discharge. Therefore, the major challenge for atmospheric pressure technology is finding a mechanism to prevent this transition. Different solutions are based on limiting the discharge maintenance time by working in a pulsed regime so that the instabilities do not have enough time to develop. In this way, for example pulsed corona and microwave discharges can be employed at atmospheric pressure[20]. Another method to prevent accumulation of charges is to apply a fast gas flow in transverse direction so that instabilities are “blown” away, for example in the DC glow discharge[59].

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Dielectric barrier discharges (DBDs) prevent the transition to an arc discharge by autopulsation of the discharge in an AC arrangement with a dielectric barrier covering one or both electrodes[48,50]. In literature, a large number of excellent reviews on DBDs can be found[23,47,48,50,84,114,117]. It should also be noted that occasionally in literature also the term corona discharge or corona treatment is used in connection with DBDs as in the work on atmospheric pressure plasma polymerisation of Thyen et al.[101], although most authors prefer to use this term only for discharges between bare metal electrodes without dielectric. DBDs have a great flexibility with respect to their geometrical shape, working gas mixture and operating parameters. Depending on the application, the width of the discharge gap can range from less than 0.1 mm to 10 mm and the applied frequency from below line frequency to several gigahertz. Many investigations showed that at elevated pressure, electrical breakdown typically occurs in a large number of short-living current filaments referred to as microdischarges. Most of the industrial applications of barrier discharges operate in this filamentary mode. With uniformity and moderate power density as distinctive advantages, glow discharges are receiving an ongoing attention as an alternative to the “classical” high-pressure nonequilibrium plasmas (corona, dielectric barrier discharge). In particular the generation of continuous, atmospheric pressure glow discharges (APGD) with scale lengths exceeding a few millimetres, continues to be a challenge for present-day plasma engineering[59]. Under special conditions a diffuse mode can be generated in a DBD setup. Different research groups focussed on obtaining such diffuse plasmas in barrier discharges[13,21,38,41,51,62,63,66-70,72,80,87,89,90,105,106,120]. So, depending on the gas mixture, the nature of the dielectric surface properties and the operating conditions, different discharge modes of DBDs, including filamentary and completely diffuse discharges, can exist. It is sometimes difficult to unambiguously categorise different discharges. During the last decades inspired by the search for stable and homogeneous atmospheric pressure discharges, a lot of the described plasma sources in literature are combinations of different techniques. Two general concepts using such combinations are thoroughly studied for the moment and could be stated as emerging trends in plasma source technology. Therefore, a short description of these two techniques will be given.

Microplasmas Spatially confining atmospheric pressure, non-equilibrium plasmas to dimensions of 1 mm or smaller is a promising approach to the generation and maintenance of stable glow discharges at atmospheric pressure[5,19]. Such discharges are called microplasmas and represent a new and emerging field of plasma technology. Microplasmas show a remarkable stability towards arcing. At this point of time, the mechanisms that are responsible for this behaviour are still one of the frontiers of knowledge. One mechanism is the so-called “pd”-scaling. The breakdown voltage of a discharge depends on the product of pressure p and electrode separation d (Paschen curve)[19]. As a consequence, the voltage required to ignite a discharge, can be kept low for essentially all gases even at atmospheric pressure if the electrode separation is below 1 mm. Another mechanism that contributes to the observed stability of microdischarges is the high loss of charge carriers to the surrounding walls.

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Microplasmas are able to selectively generate chemical reactive species which could open the door to a wide range of applications in e.g. surface treatment[49]. Microplasmas are of special interest for the treatment of biomedical products, polymer electronic components and for the functionalisation of the surface of microfluidic devices, because microplasmas are believed to be not useful for large coating processes. However, they can be used for the local functionalisation of surfaces[44]. The most promising application of microplasmas is the creation of patterned surface functionalisation or so-called plasma printing. One main drawback is the erosion of electrodes resulting in fast deterioration of the performance of the microplasma source. Therefore, further research is necessary. To avoid deposition on the electrodes, Raballand et al. kept their argon plasma source running for a few minutes without precursor using the cleaning effect of an argon plasma[86].

Plasma Jet As mentioned before it is desirable for some applications to work with a remote plasma treatment and for this purpose several sources were developed. The plasmas produced are not spatially bound or confined by electrodes and are often referred to as cold plasma jets. Plasma needle, plasma plume or plasma pencil are other designations of plasma jets[55,97]. Some authors even use the term plasma torch, but usually this name is reserved for thermal plasmas employed in industry for cutting and welding applications. Plasma jets normally operate at atmospheric pressure and are widely tested for biomedical applications[97]. Typical plasma jets are launched into the surrounding environment by devices that internally generate atmospheric pressure non-thermal plasmas[55]. The jets are blown outside the source by a gas flow. In this way, the treatment uses the afterglow of the discharge. Plasma jets are generated in different gasses and mixtures and their power sources cover a wide spectrum of frequencies from direct current to microwaves[55,97]. Nevertheless, it should be stressed that a lot of the developed plasma jets at atmospheric pressure still operate at temperatures above 373 K up to 473 K making them unsuitable for deposition of thin polymer films. These temperatures are often too high for e.g. tissue treatment or gas barrier coatings on polyethylene terephthalate (PET).

3. Plasma Polymerisation: Common Monomers and Their Applications In this section, the most common used monomers will be discussed and their applications found in literature will be described.

3.1. Hydrocarbons One of the very first studies on atmospheric pressure plasma polymerisation was published by Donohoe et al.[14] in 1979. They reported on plasma polymerisation of ethylene (C2H4) diluted in helium using an atmospheric pressure pulsed discharge. Films produced in this work were uncoloured and showed good adhesion to glass substrates, but were soft and

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soluble in ethanol indicating a low degree of cross-linking. Other pioneering work was the study of Yokoyama et al.[119] on the deposition of ethylene on different substrates using their newly developed APGD[39] in helium. The deposition rate increased with increasing discharge current, but did not depend on substrate material and ethylene flow rate. More recently, Goossens et al.[26,27] studied ethylene deposition with a DBD using different carrier gasses. In all these experiments the substrate was located in the DBD plasma. They analysed the deposited polymers with ATR-FTIR and 1H-NMR revealing that oxygen or water was incorporated into the coating when the material is exposed to air. Moreover, unsaturated bonds present in the precursor compound remained after plasma polymerisation. It was believed that the double bonds could have a stabilising effect on radicals by expanding their lifetime[27]. These radicals were detected by EPR measurements. The plasma polymerised films obtained in helium and argon plasmas resulted in sticky materials, in contrast to coatings obtained in nitrogen. These observations indicated a difference in polymer structure and were attributed to the density of cross-linking: highly cross-linked polymers appeared non-sticky, while formation and deposition of oligomers gave rise to sticky coatings. The carrier gas also determined the surface morphology as can be seen in Figure 2: ethylene films deposited in helium discharges had very smooth surfaces in contrast to coatings obtained in argon discharges. The study of Mishra et al.[71] contradicts these results. These authors also studied ethylene deposition in a helium APGD and observed no smooth surfaces when the substrate is placed in direct contact with the plasma. For their plasma source, they showed the necessity of a remote treatment in order to obtain excellent surface smoothness. Fanelli et al.[17] studied the influence of several operating parameters on the stability of their APGD in helium-ethylene mixtures. Their work showed that it is possible to obtain stable glow discharges at atmospheric pressure for a large operational range of input power, ethylene concentration and residence time. Similar to the above mentioned results of Goossens et al.[27], the coatings surprisingly contained oxygen despites the absence of oxygen containing functional groups in ethylene. These functionalities are due to O2 and H2O contaminations in the deposition chamber and/or to post-deposition reactions like oxidation of residual free radicals after atmospheric exposure. Due to their extreme reactivity, even low traces of oxygen are often responsible for the incorporation of oxygen containing groups in the plasma polymerised films. Nitrogen-rich surfaces with a high density of primary and secondary amines are chemical reactive and are especially used in biochemistry and other biomedical applications such as promotion of cell adhesion and attraction of proteins or DNA. The coatings must have a high density of amines to become positively charged in aqueous media enabling attraction of negatively-charged biomolecules. In addition, such a coating must be stable in aqueous environments routinely used for cell culture. Truica-Maresescu et al.[103] and GirardLauriault et al.[24] reported on a novel material, plasma polymerised ethylene enriched with nitrogen (PPE:N) coatings. These plasma polymers were obtained in low and atmospheric pressure DBD discharges fed by ethylene/nitrogen mixtures. Results showed that low and atmospheric pressure plasma polymerisation were both economical and efficient methods for producing nitrogen-rich organic coatings. Despite several similar aspects, the deposited layers in low and atmospheric pressure also showed some important differences. The PPE:N coatings obtained at low pressure contained higher concentrations of amine groups, but were partially soluble in water. In contrast, the coatings deposited at atmospheric pressure are

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insoluble in water, but contain less amine groups. Therefore, current efforts aim to reduce water solubility without excessively reducing the amine concentrations.

(a)

(b)

Figure 2. SEM cross-section of a deposition on silicon from (a) a helium-ethylene and (b) an argonethylene plasma at atmospheric pressure (deposition time = 10 min)[124].

Recently, hydrogenated amorphous carbon (a-C:H) films have been used for food packaging purposes due to their attractive gas barrier properties, flexibility, recyclability and biocompatibility. Coating the inside of PET bottles with an a-C:H film of 10-100 nm thickness, drastically improved the gas barrier properties by a factor 20-30[46]. Consequently, the shelf life of bottles becomes longer since they can preserve taste and freshness of the packaged foods for a longer period. Kodama et al.[46] successfully synthesised high gas barrier carbon films on polymer substrates using an APGD with high deposition rates (6070 nm/s) compared to vacuum treatments. Homogeneous and highly stable plasmas were constantly generated under an atmosphere of acetylene gas (C2H2) without using any other carrier gas such as He, Ar or N2 usually necessary to stabilise the plasma. The absence of a dilution gas led to the formation of dense carbon films with rigid carbon-carbon bonds. The gas barrier properties of the coated PET films were 23 times improved compared with the uncoated substrates. a-C:H films were also studied by other authors. Bugaev et al.[11] studied hydrocarbon DBD discharges at atmospheric pressure to deposit protective diamond-like coatings with high transparency on plane dielectric substrates and showed that it is possible to deposit such films in a fast and less expensive way compared to conventional techniques. Benedikt et al.[6,7] demonstrated the possibilities of an atmospheric pressure microplasma jet to deposit homogeneous thin a-C:H films from acetylene/argon mixtures. The presence of argon was beneficiary in creating a protective atmosphere around the plasma jet containing the precursor. In this way, the incorporation of oxygen and nitrogen present in ambient air into the film is prevented[6]. Moreover, the deposition – responsible for electrode erosion – inside the microplasma source was avoided. Using this newly developed plasma device, soft

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polymer-like hydrogenated amorphous carbon films were deposited with rates of a few nm/s on areas of a few square millimetres. Fabrication of biologically active plasma polymers is in most cases a two step process in which the bioactive component is first immobilized on a substrate, followed by a (vacuum) plasma polymerisation step. A technique to immobilise biological macromolecules in one single step without excessive solvent use could be an answer to a variety of applications. In such a process, proteins, enzymes or nucleic acids would be immediately immobilized via plasma copolymerisation with organic precursors. To explore this vision, Heyse et al.[31] used an atmospheric pressure DBD to deposit in mild conditions organic coatings with entrapped proteins. The authors showed that with acetylene as precursor, atmospheric pressure DBD plasma polymerised films were obtained with a homogeneous distribution of immobilised proteins. Also other applications of hydrocarbon monomers are mentioned in literature. For example, Jaššo et al.[33] plasma polymerised butadiene films on PET cords applied in automobile tires to reinforce the rubber matrix. The interfacial adhesion between the rubber matrix and the cord surface should be optimised and is determined by the interactions of the functional groups of the rubber blend and the functional groups on the cord surface. The authors observed a clear amelioration of the adhesion strength between the PET cords and the rubber due to the thin butadiene coating on the cords. In Figure 3 SEM pictures are shown of untreated and butadiene plasma treated PET cords. Another application is the plasma polymerisation of methane (CH4) to deposit hydrophobic coatings on various materials. Kim et al.[42] demonstrated a CH4 atmospheric plasma process for hydrophobic treatments of various substrates including metallic and insulating surfaces such as Si wafers, gold, copper, glass, paper and cotton. This treatment produced a very smooth and stable hydrophobic hydrocarbon coating composed of CH2 and CH3 groups.

3.2. Hydrocarbons with Polar Functional Groups The absence of polar groups on the surface of a polymer often results in a hydrophobic surface which is unfavourable for e.g. printing and adhesion. In the last decade, many efforts have been made to realise surfaces with controllable density of functional groups such as carbonyl, carboxyl, hydroxyl and amine groups. These functional surfaces are essential for different biomedical applications since the reactive polar groups can be used to initiate further chemical graft reactions[99]. In literature, the major part of research on hydrocarbons with polar functional groups is dedicated to the deposition of polyacrylic acid films using acrylic acid as monomer[73,75,78,99,102,116]. Expanded polytetrafluoroethylene (ePTFE) is a flexible and porous polymer with favourable characteristics such as high chemical resistance, high temperature stability, low dielectric constant and a low friction coefficient. However, for biomedical implementation in e.g. catheters and artificial blood vessels, the inherent highly hydrophobic nature of the ePTFE surface retards tissue ingrowth and wound healing. Therefore, an APGD treatment in helium of ePTFE tubes was performed by Njatawidjaja et al.[74,75]. The ePTFE tubes were prior to the treatment immersed in an acrylic acid solution and afterwards treated in the

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helium APGD. The initial hydrophobic inner surface of the tubes showed a hydrophilic behaviour after treatment indicating the formation of a insoluble polymerised acrylic acid layer on the ePTFE surfaces. Ward et al.[116] have deposited structurally well-defined polyacrylic acid films using an APGD deposition reactor operating in helium. FT-IR analysis of the plasma polymerised films confirmed that APGD deposition can lead to the formation of polymeric structures that resemble conventionally polymerised polyacrylic acid. The plasma deposited polyacrylic acid layers exhibited appreciable gas barrier characteristics. A 2300 nm coating reduced oxygen permeation through polyethylene (PE) films by a factor of 7. O’Hare et al.[78] deposited anti-microbial coatings onto fabrics via aerosol precursor injection in an atmospheric pressure discharge in helium. To achieve coatings with specific and more ‘active’ properties such as anti-microbial, enzymatic, pharmaceutical, pesticidal and flame retardant, non-reacting ‘active’ molecules should be mixed with reactive precursors. The authors mixed the precursor acrylic acid with different anti-microbials/disinfectants (quaternary ammonium salts). From this mixture an aerosol was generated, where the ‘active’ molecule was protected from the plasma by the droplet and not directly involved in the plasma reactions. Nevertheless, after polymerisation of the precursor, the ‘active’ molecules were physically entrapped within the resulting plasma polymer without loosing their biological and chemical properties.

(a)

(b)

Figure 3. SEM image of (a) untreated and (b) butadiene plasma treated PET cord[125].

Besides acrylic acid, also a few other monomers containing polar groups are employed to deposit thin films at atmospheric pressure. Kasih et al.[40] focussed on the deposition of methyl methacrylate (MMA) using an atmospheric pressure argon plasma torch. These plasma polymerised MMA films can be used as dry electron-beam resist, membranes for gas seperators, humidity sensors and optical devices such as wavelength transformers. Goossens et al.[27] plasma polymerised MMA films with an atmospheric pressure DBD using different

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carrier gases. They showed that the deposition rate strongly depended on the carrier gas and varied from 15 nm/min if helium was used as carrier gas to over 35 nm/min in nitrogen and to 50 nm/min in argon. Under none of the three discharge conditions pinholes or structural defects were observed. Dubreuil and Bongaers[15] enhanced the hydrophilic properties of different polymer substrates using an atmospheric pressure DBD generated in nitrogen. Two types of monomers (acetic acid and ethyl acetate) were atomised and inserted as an aerosol in the plasma zone. Acetic acid and ethyl acetate in the presence of nitrogen as carrier gas led to the formation of amide functionalities. Also NH and OH groups were present at the film surface enhancing the polarity of the polymer foil. An atmospheric plasma torch was used by Akdogan et al.[1] to modify a piezoelectric quartz crystal for immunosensor preparation with different nitrogen containing monomers: ethylene diamine, 1,4-diaminobutane and n-butylamine. After treatment, the crystals showed an increase in hydrophilicity indicating the presence of an amine group containing thin film coating on the surfaces.

3.3. Hydrocarbons with Halogen Containing Functional Groups Low pressure plasma polymerisation of fluorocarbon thin films has been extensively studied in the last decades due to their remarkable properties such as low dielectric constant and friction coefficients, excellent chemical stability, high tolerance to mechanical stress, thermal stability and high hydrophobic/oleophobic character. More recently, plasma polymerisation of fluorocarbon thin films has also been studied in atmospheric pressure discharges. One of the first investigations of atmospheric pressure deposition of fluorocarbons was reported by Yokoyama et al.[119]. The authors used an APGD with tetrafluoroethylene (TFE) (C2F4) diluted in helium. The deposition rate increased with increasing discharge current and increasing monomer flow rate. The latter behaviour is in contrast to the observation by the same authors that the deposition rate does not depend on the flow rate in the case of ethylene polymerisation, as described in section 3.1. The influence of discharge current and monomer flow rate on the chemical structure of the plasma polymerised TFE films was analysed by XPS. The F/C ratio tended to increase with discharge currents and to decrease with TFE flow rates. Sawada and Kogoma[92] used an APGD to plasma polymerise TFE films on porous granulated silica particles (average diameter: 152 μm). XPS measurements revealed that the films deposited on the particles were composed of highly branched and cross-linked fluorocarbon segments. SEM pictures showed very smooth and uniform films on the silica particles. Thyen et al.[101] also deposited TFE coatings in a filamentary DBD at atmospheric pressure. Soft and smooth fluorocarbon coatings could be deposited with a fairly high deposition rate of about 100-200 nm/min. Multiple articles by Vinogradov et al.[110-113] deal with the deposition of different fluorocarbons (CF4, C2F6, C3HF7, c-C4F8, C2H2F4, C3F8) using DBDs in argon. The surface tension of the deposited films depended on the nature of the fluorocarbon molecule used, deposition time and discharge gap. The effect of hydrogen or oxygen addition to the gas feed on deposition rate and film composition was evaluated in detail[112]. Admixture of hydrogen strongly influenced the deposition rate, the coating morphology and structure of the film.

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(a)

(b) Figure 4. SEM picture of polymer film deposited on Si in (a) an argon/c-C4F8 mixture and (b) an argon/C3HF7 mixture[126].

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Results also showed that an admixture of oxygen drastically reduced the deposition rate. Also the effect of hydrogen content in the precursor was evaluated[110]. In precursors with low hydrogen content as c-C4F8, relatively smooth films could be obtained for an argon/c-C4F8 plasma as shown in Figure 4a. In mixtures with higher hydrogen contents as in argon/C3HF7 plasmas, the deposited polymers had a tendency to form rough surfaces with holes as demonstrated in Figure 4b. APGDs fed with helium/C3F6 and helium/C3F8/H2 were also used to deposit fluorocarbon thin films by Fanelli et al.[16]. Helium/C3F6 mixtures generated fluorocarbon films with an F/C ratio of 1.5 at deposition rates up to 34 nm/min. With helium/C3F8/H2 fed APGDs it is possible to tune the F/C ratio of the coating from 1.5 to 0.6 and to change its cross-linking degree by varying the hydrogen concentration in the gas feed. H2 addition promoted an increase in deposition rate which is maximum for fluorocarbon-to-hydrogen ratio close to 1. Deposition of hydrophobic coatings on various substrates has many important applications such as protective garments, corrosion prevention, micro-device lubrication, water repellent textiles, barrier coatings,… Several other authors used the above mentioned fluorocarbons to plasma polymerise thin hydrophobic layers on different substrates[43,45,52,58]. Prat et al.[85] proposed the utilisation of an APGD to modify the inner surface of commercial polyvinyl chloride (PVC) tubes to enhance biocompatibility for a blood circulating tube. A fluoro-polymer layer on the inner surface of the PVC tube increases its blood compatibility and suppresses the bleeding of plasticisers present in commercially available PVC. It is difficult to coat the inner surface of soft tubular structures by low pressure plasmas due to pressure differences between the tube inside and outside. Using helium/C2F4 gas mixtures, uniform and thick PTFE-like coatings consisting of mainly CF2 groups were obtained, while using helium/C3F6 mixtures coatings with lower F/C ratios were deposited. The concentration of glucose in the blood of a diabetic patient is controlled by insulin. However, this insulin disappears fast from the blood and diabetics are obliged to inject insulin multiple times a day. This drastically decreases their living comfort. Therefore, it would be beneficial if the medicine could be little by little supplied in the blood for some days. A possible approach is the encapsulation of insulin inside poly-DL-lactic-co-glycolic acid (PLGA) microcapsules[98]. However, standard PLGA capsules cannot be used immediately for this purpose, because they tend to unexpectedly burst just after administration due to water penetration. Suppression of this phenomenon was obtained by Tanaka et al.[98] by depositing a thin hydrophobic fluorocarbon layer on the PLGA capsules by means of a C3F6/helium APGD treatment. The most important parameter for an effective hydrophobic coating was the treatment time. At excessive treatment times, the PLGA capsules were destroyed still leading to unexpected burst releases of the medicine. On the contrary, lower treatment times could make PLGA capsules enough hydrophobic to avoid burst release and to obtain controlled and delayed medicine release. Besides fluorine containing monomers, also chlorine containing monomers can be applied for plasma polymerisation at atmospheric pressure as described by Borcia et al.[10] to achieve hydrophobic coatings.

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3.4. Heterocyclic Aromatic Organic Monomers The discovery of electrical conductivity of organic conjugated polymers opened a novel and very important field of modern functional material science[12,25]. Nowadays, conjugated polymers have received a great deal of interest for applications such as polymeric LEDs, organic thin film transistors, photodetectors, (bio)sensors, solar and photovoltaic cells, corrosion protection and antistatic layers. Plasma deposition of conjugated polymer films under atmospheric pressure was studied by Dams et al.[12] using three different thiophene derivates as monomers: thiophene, 3methylthiophene and 3,4-ethylenedioxythiophene. Thiophene and 3-methylthiophene polymerised easily in a nitrogen plasma at atmospheric pressure with high deposition rates of 220 nm/min and 210 nm/min respectively. When a small amount (< 1%) of oxygen was added to the nitrogen carrier gas, the deposition rates increased to 330 nm/min and 260 nm/min respectively. All these polymerisations with thiophene as well as with 3methylthiophene resulted in yellow-brown coatings. Since it appeared more difficult to obtain coatings in a pure nitrogen plasma with 3,4-ethylenedioxythiophene, small oxygen addition and a pulsed power mode was necessary to obtain coatings with this monomer. The best conductivity was accomplished for the 3,4-ethylenedioxythiophene films and conductivities up to 1 x 10-2 S/cm were reached without the use of a doping agent. Heyse et al.[30] obtained smooth coatings with pyrrole as precursor diluted in helium with deposition rates of 102 nm/min. The heterocyclic polymers, such as polypyrrole and polythiophene, are extensively studied due to their high conductivity in doped state and their high stability. However, the possibility of depositing conjugated polymer coatings from furan has attracted relatively less attention due to the difficult synthesising of polyfuran with a regular structure and high conductivity. Gok et al.[25] successfully deposited smooth continuous polyfuran films on glass substrates for application as a support in amperometric biosensors or in redox reactions. It was reported that polyfuran demonstrates some superior properties, such as electrochromic effect and good redox ability.

3.5. Organosilicons Going over literature on atmospheric pressure plasma polymerisation, it is clear that a lot of research is done on precursors containing carbon-silicon bonds, like hexamethyldisiloxane (HMDSO), hexamethyldisilazane (HMDS), tetraethoxysilane (TEOS),… for several applications. HMDSO is one of the most common materials used for coating and is of great interest because of its high deposition rates and the ability to control coating structure and properties by varying deposition conditions. In addition, HMDSO is an easy and safe monomer to handle, especially compared to silane compounds[122,123]. HMDSO plasma polymerised thin films can be assayed for a large number of applications in rather different fields such as protective anti-scratch coatings on plastic substrates, barrier films for food and pharmaceutical packaging, corrosion protection layers, coatings for biocompatible materials and low-k dielectric layers for microelectronic applications[29,94,123]. HMDSO was used to deposit fluorine-free hydrophobic polydimethylsiloxane (PDMS)like coatings by Ji et al.[34,35]. They reported on the formation of super-hydrophobic and

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water-repellent coatings on PET fibres. The thin films were obtained by using an APGD in a mixture of argon and HMDSO. FT-IR measurements showed that the improvement in water repellency was due to the presence of Si-O-Si, Si-(CH3)2 and Si-C chemical functional groups. It is well known that plasma polymerisation of HMDSO can lead to the formation of quartz-like SiOx coatings[18,28,53,64,83,86]. Besides HMDSO, also other organosilicon monomers are used for the formation of glass-like SiOx layers[3,4,32,54,77,88,93,96,107,115,118,121]. These coatings can for example be deposited on metallic surfaces as corrosion protective layers. With an argon DBD plasma jet at atmospheric pressure and HMDSO as precursor, Han et al. plasma polymerised SiOx films on steel and observed a significant reduced corrosion of the coated steel[28]. The film composition depended on the applied treatment power and after immersing the SiOx coated steel in a NaCl solution for 5 days, no rust was found on the surface. Foest et al.[18] tested the corrosion protection of an aluminium sheet covered with a SiOx-like layer deposited from HMDSO in a NaOH solution. The coated aluminium sheet could resist corrosion for 180 s, while an untreated sheet could only resist the test for 30 s. Kuwabara et al.[53] worked with a remote plasma treatment and studied the influence of the distance between a substrate and an atmospheric pressure plasma jet. Short distances resulted in organic HMDSO-like coatings, while larger distances gave rise to inorganic layers of SiOx. Next to deposition on metallic surfaces, plasma polymerisation of organosilicon monomers has been performed on plastics. Polycarbonates for example have very useful bulk properties, such as low density, high elasticity and transparency. They have found their way in industry to replace glasses in many fields, such as headlights, windscreens, lenses and compact discs. However their use is sometimes limited due to low hardness, low scratch resistance and UV-degradation[107]. Therefore to overcome this problem, polycarbonate films are often coated with scratch-resistant films such as SiO2 as done by Ulejczyk et al.[107]. They plasma polymerised thin films from gas mixtures of TEOS, helium and oxygen. The oxygen concentration was the only parameter influencing the deposition rate and roughness of the film: increasing the oxygen content leads to a decrease in deposition rate and an increase in surface roughness due to enhanced etching. Nowling et al.[77] demonstrated that their remote plasma deposition process was able to generate abrasion-resistant glass coatings on plastics with different organosilicon monomers. The properties of the deposited coatings depended on the type and the amount of organosilicon precursor fed to the reactor. High quality films without visible defects were obtained with TEOS, tetramethylcyclotetrasiloxane (TMCTS) and HMDSO at rates ranging from 20 nm/min to 150 nm/min. With tetramethyldisiloxane (TMDSO) substantially higher deposition rates of 910 nm/min could be reached, however the deposited layers showed poor abrasion resistance. Similar to the application mentioned in section 3.2, SiOx layers were deposited on the inner surface of PTFE tubes to enhance biocompatibility for blood by Yoshiki et al.[121]. PTFE tubes with inner diameters of 500 μm were covered with a coating by an atmospheric pressure microplasma generated in a mixture of helium, oxygen and TEOS vapour. Smooth, uniform and transparent SiO2 thin films were fabricated at a deposition rate of 230 nm/min. The wettability of the SiO2-coated PTFE tube was about 3 times larger than an untreated PTFE tube. The plastics and packaging industry is very interested in the deposition of silica coatings for the creation of barrier coatings against oxygen. O’Neill et al.[79] reported on the creation

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of barrier coatings on polypropylene and PET films by aerosol injection of different silicone containing precursors (TEOS, TMDSO, octamethylcyclotetrasiloxane, TMCTS,…) into a helium/oxygen plasma. The major problem was the formation of powder within the plasma which limited the barrier performance. This powder formation strongly depended on the used precursor and could be minimised by the injection of the non-reactive liquid aerosol precursor PDMS into the plasma. Using this latter precursor resulted in the deposition of a clear silica coating with significant barrier properties. Also Thyen et al.[101] and Vangeneugden et al.[108] discussed the problem of powder-like particle formation for several silicon containing precursors. Besides O’Neill et al.[79], also Paulussen et al.[81,82] studied plasma polymerisation for the generation of barrier coatings on PET using an atmospheric pressure DBD with a pure organic precursor (1-hexene) and with a hybrid organic-inorganic precursor (vinyltrimethoxysilane). Figure 5 shows that the plasma polymers obtained from the hybrid organic-inorganic monomer are better oxygen barriers compared to the coating obtained from the pure organic precursor. This was explained by the synergetic effect of the organic and inorganic networks formed during plasma polymerisation.

Figure 5. Oxygen transmission rates of PET foil before and after deposition of different plasma polymer layers with a thickness of 3 μm[127].

Besides organosilicon monomers containing Si-O-Si bonds, a few articles report on plasma polymerisation of organosilicon monomers containing Si-N-Si bonds like HMDS[56,57,91,95,104]. To improve the deep colouring effect of PET fabrics, anti-reflective HMDS coating layers were deposited on the fabric surfaces using atmospheric pressure plasma polymerisation by Lee et al.[56], while Rymuza et al.[91] studied deposition of HMDS for lubrication of micro-electro-mechanical systems.

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3.6. Silane Silane is a chemical compound with chemical formula SiH4. It is the silicon analogue of methane and can be used as monomer for the deposition of silicon containing films. Silicon nitride layers are important in semiconductor industry since these films have high dielectric constants and excellent barrier properties. They are used for encapsulation of fabricated devices, as masks for local oxidation and as diffusion barriers to dopants such as gallium[37,76]. In order to improve such devices and to reduce production costs, it is necessary to deposit uniform SiNx films at high rates. Kakiuchi et al.[37] investigated the deposition of SiNx films at atmospheric pressure from gas mixtures containing helium, H2, SiH4 and N2 or NH3. When NH3 was used, the authors observed the growth of homogeneous SiNx films in contrast to the inhomogeneous ones obtained with N2. It was stated that this remarkable behaviour was due to the large difference in dissociation energy of N2 and SiH4 molecules. They achieved very high deposition rates of 6000 nm/min. Amorphous hydrogenated silicon films are widely used in solar cell technology and thin film transistors (TFT) for flat panel displays. Morajev et al. studied the deposition of these films using an atmospheric pressure helium/hydrogen plasma with addition of silane and observed low deposition rates (7-12 nm/min). Besides organosilicon monomers, also silane can be used to deposit SiOx layers. The research group of Massines[22,36,61,65] studied deposition of such layers with an APGD using mixtures of SiH4 and N2O diluted in nitrogen.

4. Conclusion About two decades ago, the first humble steps on atmospheric pressure plasma polymerisation were taken, nowadays leading to a booming research field in plasma technology. This review shows that already promising results have been obtained with a variety of atmospheric pressure plasmas. For different applications, various coatings derived from all kind of monomers can be deposited with high quality and acceptable deposition rates without the need for vacuum technology. As a result, these techniques use less expensive equipment and can be integrated in in-line processing resulting in an economically attractive approach. However, more fundamental research is necessary to obtain better insights in e.g. deposition mechanisms before industrialisation of atmospheric pressure plasma polymerisation on a large scale can be expected.

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[52] Korotkov, R. Y., Goff, T., and Ricou, P., Surface & Coatings Technology 201(16-17), 7207 (2007). [53] Kuwabara, A., Kuroda, S., and Kubota, H., Plasma Sources Science & Technology 15(3), 328 (2006). [54] Ladwig, A., Babayan, S., Smith, M., Hester, M., Highland, W., Koch, R., and Hicks, R., Surface & Coatings Technology 201(14), 6460 (2007). [55] Laroussi, M. and Akan, T., Plasma Processes and Polymers 4(9), 777 (2007). [56] Lee, H. R., Kim, D. J., and Lee, K. H., Surface & Coatings Technology 142, 468 (2001). [57] Lee, J. H., Pham, T. T. T., Kim, Y. S., Lim, J. T., Kyung, S. J., and Yeom, G. Y., Journal of the Electrochemical Society 155(3), D163-D166 (2008). [58] Leroux, F., Campagne, C., Perwuelz, A., and Gengembre, L., Applied Surface Science 254(13), 3902 (2008). [59] Leys, C., Contributions to Plasma Physics 44(5-6), 542 (2004). [60] M. A. Lieberman and A. J. Lichtenberg, Principles of plasma discharges and materials processing - second edition,(John Wiley & Sons, Inc., Hoboken, New Jersey, 2005). [61] Martin, S., Massines, F., Gherardi, N., and Jimenez, C., Surface & Coatings Technology 177, 693 (2004). [62] Massines, F., BenGadri, R., Decomps, P., Rabehi, A., Segur, P., and Mayoux, C., Phenomena in Ionized Gases (363), 306 (1996). [63] Massines, F., Gadri, R., Decomps, P., Rabehi, A., Segur, P., and Mayoux, C., AIP Conference Proceedings no.363, 306 (1996). [64] Massines, F., Gherardi, N., Fornelli, A., and Martin, S., Surface & Coatings Technology 200(5-6), 1855 (2005). [65] Massines, F., Gherardi, N., and Sommer, F., Plasmas and Polymers 5(3/4), 151 (2000). [66] Massines, F. and Gouda, G., Journal of Physics D-Applied Physics 31(24), 3411 (1998). [67] Massines, F., Mayoux, C., Messaoudi, R., Rabehi, A., and Segur, P., Proc. 10th Int. Conf. on Gas Discharges and Their Applications, 730 (1992). [68] Massines, F., Messaoudi, R., and Mayoux, C., Plasmas and Polymers 3 (1), 43 (1998). [69] Massines, F., Rabehi, A., Decomps, P., Gadri, R. B., Segur, P., and Mayoux, C., Journal of Applied Physics 83(6), 2950 (1998). [70] Massines, F., Segur, P., Gherardi, N., Khamphan, C., and Ricard, A., Surface & Coatings Technology 174, 8 (2003). [71] Mishra, K. K., Khardekar, R. K., Singh, R., and Pant, H. C., Review of Scientific Instruments 73(9), 3251 (2002). [72] Montie, T. C., Kelly-Wintenberg, K., and Roth, J. R., IEEE Transactions on Plasma Science 28(1), 41 (2000). [73] Moravej, M., Babayan, S. E., Nowling, G. R., Iang, X., and Hicks, R. F., Plasma Sources Science & Technology 13(1), 8 (2004). [74] Njatawidjaja, E., Kodama, M., Matsuzaki, K., Yasuda, K., and Matsuda, T., Plasma Processes and Polymers 3(4-5), 338 (2006). [75] Njatawidjaja, E., Kodama, M., Matsuzaki, K., Yasuda, K., Matsuda, T., and Kogoma, M., Surface & Coatings Technology 201(3-4), 699 (2006). [76] Nowling, G. R., Babayan, S. E., Jankovic, V., and Hicks, R. F., Plasma Sources Science & Technology 11(1), 97 (2002).

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plasma discharge in the presence of butadiene/nitrogen gas mixtures, pp. 57-62, Elsevier Science B.V. (2006), with permission from Elsevier. [126] Reprinted from Surface and Coatings Technology, 142-144, Vinogradov, I. P., Dinkelmann, A., and Lunk, A., Deposition of fluorocarbon polymer films in a dielectric barrier discharge (DBD), pp. 509-514, Elsevier Science B.V. (2003), with permission from Elsevier. [127] Reprinted from Surface and Coatings Technology, 142-144, Paulussen, S., Rego, R., Goossens, O., Vangeneugden, D., and Rose, K., Plasma polymerization of hybrid organic-inorganic monomers in an atmospheric pressure dielectric barrier discharge, pp. 672-675, Elsevier Science B.V. (2005), with permission from Elsevier.

In: Surface Coatings Editors: M. Rizzo and G. Bruno, pp. 177-187

ISBN: 978-1-60741-193-2 © 2009 Nova Science Publishers, Inc.

Chapter 6

INTERFACE RESEARCH ON FILMS AND COATINGS Xiaolu Pang and Kewei Gao Department of Materials Physics and Chemistry, University of Science and Technology Beijing, Beijing, China

Abstract Coatings and films mechanical properties are highly affected by their microstructure, and their adhesion to substrates, which sustains their mechanical integrity, and consequently improves their properties. Interfaces with high adhesion are also known to ensure prolonged coatings lifetime. Research on interface microstructure and bond form plays a very important role on coatings and films. In this paper, interfaces between chromium oxide coating deposited by reactive radio frequency (RF) magnetron sputtering technique, chromium interlayer and steel substrate are examined with scanning electron microscopy (SEM), high resolution electron microscopy (HREM) and atom force microscopy (AFM) focusing on the interfacial structure properties affecting the adhesion performance and surface roughness. This examination revealed the presence of several Cr–Fe phases, which may ensure good adhesion of the interlayer to the underlying steel. Furthermore, amorphous chromium and chromium oxide layers about 100 nm thick were detected at each interface, which may have some effect on corrosion resistance and growth of columnar coating microstructure. The amorphous interfacial layer detected may give novel thought when deposited thick film but small size column grains.

Keywords: Interface, microstructure, adhesion, amorphous layers.

Introduction Interfaces research on film/substrate and multilayer films plays a very important role either in industry or in science because interfaces affect the structure and mechanical properties of films which predict the lifetime of film device in some extent. In details, interface can change the stress level, adhesion, and fracture toughness of films and interfaces. Cr2O3 is well suited for wear resistance applications, as it is one of the hardest oxides with 29.5 GPa hardness.[1] Several deposition techniques have been tried out for making

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these coatings. Cr2O3 coating hardness can vary substantially due to compositional and microstructural variations, depending on the deposition method and application, such as corrosion protection, wear resistance, electronics, and optics.[2]-[5] Hardness of a plasmasprayed Cr2O3 coating, 50 μm thick was about 14.7 GPa, while a 200 nm thick RF-sputtered chromium oxide coating, stoichiometrically close to Cr2O3, exhibited 30 GPa hardness combined with good scratch resistance.[6] Even for the bulk Cr2O3, hardness values reported were from 9 GPa to 29.5 GPa. [1, 7, 8] Hones et al. investigated a correlation between the hardness and the sputtering deposition parameters,[3] i.e. oxygen partial pressure and substrate temperature, and found favorable deposition conditions with an oxygen partial pressure of about 15-20% of the total sputtering gas pressure at substrate temperatures exceeding 500 K. Good coating adhesion is required for wear and corrosion resistance applications. Premature failure can occur for many reasons including coating delamination, cracking and plastic deformation. In addition to this, thin ceramic PVD coatings usually have columnar grain structure with micro cracks, pinholes, transient grain boundaries and often high throughcoating porosity, which all lead to accelerated pitting corrosion and failure at the coating/substrate interface, especially in hostile environments.[9-12] On the other hand, several studies showed that coating thickness plays an important role in enhancing both PVDcoated tool cutting performance and resistance to abrasive and erosive wear.[13] Graded systems have been employed to obtain thicker coatings without losing performance in terms of coating adhesion and toughness.[14] It is likely that thicker coatings will improve corrosion resistance in aqueous environments by eliminating through-thickness pin-hole defects. Coating mechanical, adhesion and wear properties are strongly affected by microstructure. Interfaces with high adhesion are known to ensure prolonged coating life and good wear resistance.[15] Sputtered coating microstructure and physical characteristics depend on the deposition parameters.[15-17] Also, substrate surface conditions prior to deposition, characterized by surface roughness, stress and oxidation state, play an important role in controlling coating properties.[16, 17] In this paper SEM and TEM techniques were used to characterize thicker chromium oxide coating interfacial microstructure as a step towards developing a unique method for depositing thicker coatings with small grains, smooth surface and low residual stress.

Experiment Chromium oxide coatings were deposited on polished low carbon steel substrates by unbiased reactive magnetron sputtering from a 50-mm-diameter Cr target (99.95% pure) in Ar/O2 plasma (99.99% pure). The target-substrate separation distance was 60 mm. Deposition was performed at a total pressure of 10−1 Pa in a mixed Ar and O2 atmosphere with 350 W RF power. Argon flow rate was kept at 20 standard cubic centimeters per minute (sccm), while the oxygen flow rate was 3.2 sccm. Prior to coating deposition, low carbon steel substrates were cleaned in acetone and ethanol for 10 minutes in order to remove organic contaminants and then etched for 15 minutes in an Ar plasma at RF power of 100 W. An 800 nm thick chromium interlayer was sputter deposited for 15 minutes, after which oxygen gas was introduced into the sputtering

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chamber for the chromium oxide reactive sputter deposition. The substrate temperature reached 473K during this 1 hour deposition process, which produced a 4 μm thick coating. After coating deposition, cross-sections of the specimen were cut, polished and mounted in bakelite for morphological observation. Coating microstructure characterization was performed by scanning electron microscopy (JSM-6301F), and transmission electron microscopy (Tecnai F30). Chromium oxide coating cross-section TEM specimens were prepared by ion milling.

Results and Discussion Figure 1 shows SEM cross-section of the chromium oxide coating including the Cr interlayer/substrate interface. The coating is dense with no pores or inclusions present. It survived mechanical polishing, so without any obvious stress concentrators in the coating or at the interface, one can expect good coating adhesion.

Figure 1. SEM image of chromium oxide coating cross-section.

Sputter-deposited chromium oxide films can have high residual stresses. Coating failures during deposition are primarily due to high residual stress relief. Residual stresses in PVD films and coatings come from two sources: thermal stress and intrinsic (growth) stress. Thermal stresses arise from the mismatch of coating and substrate thermal expansion coefficients. Intrinsic stresses are affected by deposition parameters, specifically by plasmaforming gas pressure, controlled by the forming gas flow rate. Assuming that coating is stress-free at deposition temperature, one can estimate the magnitude of the thermal stress as:

σ RThermal =

ΔαΔTE 1 −ν

(1),

where Δα is the difference in the coating and the substrate linear thermal expansion coefficients, ΔT is the difference between deposition and room temperature, E is the coating’s

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elastic modulus, and ν is the coating’s Poisson ratio. The thermal expansion coefficient of chromium oxide ranges from 5.4×10-6/K to 7.5×10-6/K, based on its chemical composition. We used 6.5×10-6/K as a mean value. The steel substrate thermal expansion coefficient is 1.2×10-5/K. The chromium oxide coating Poisson ratio,ν, is about 0.25, and its elastic modulus is about 230 GPa. Based on these properties one would estimate 337 MPa compressive residual stress for the chromium oxide coating. This stress level is quite high for ceramic coatings. In order to reduce the amount of thermal stress in the coating, a pure Cr interlayer was deposited prior to the Cr2O3 reactive sputter deposition. Intrinsic stresses develop during sputtering and depend on the deposition conditions that control bombarding ions energy and flux.[17] The resulting residual stress is a sum of the intrinsic and thermal stresses:

σ R = σ Intrinsic + σ Thermal

(2).

A ductile chromium interlayer aids in coating residual stresses relaxation, and allows the growth of thick coatings without delamination. Figure 2 shows a TEM cross-section micrograph of the coating, Cr interlayer and the steel substrate. The substrate, interlayer, coating and the interfaces can be clearly seen. A Cr interlayer has columnar grains, whose size increases with the interlayer thickness. There are some defects present in the substrate, and at the steel/Cr interlayer interface, although both the coating and interlayer are dense. Substrate surface defects can act as stress concentration points and affect coating adhesion.

Figure 2. TEM micrograph of the coating cross-section.

The chromium interlayer has a columnar grain structure (Figure 2 and Figure 3a). A chromium oxide coating also has columnar grains, but they are much smaller compared to the chromium interlayer, as seen in Figure 3b. Figure 3a shows that there is an amorphous layer present at the interface between the substrate and the Cr interlayer. Figure 3b shows an

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amorphous layer at the interface between the Cr interlayer and the coating. These amorphous layers formed naturally during the deposition process. The formation of amorphous layers may be due to oxygen presence and interfacial lattice mismatch. Even in a fully deposited thin film multilayer systems interfacial reactions involving oxygen are possible, and can be thermodynamically favorable.[18]

Figure 3. Interfacial microstructure: a) Substrate and interlayer, b) Interlayer and coating.

There are some benefits to having amorphous layers, as they improve corrosion resistance. There are no grain boundaries, acting as paths of high rate diffusion, leading to premature corrosion failures.[19], [20] Since most thin films and coatings have a columnar grain structure, grain boundaries would lead the corrosive environment directly to the substrate, compromising corrosion protection. An amorphous layer can also block dislocations motion, thus enhancing the coating strength. Another important amorphous layer function is that of blocking columnar coating grain growth. Coatings with smaller grain sizes are harder, so an amorphous layer present in the middle of the coating thickness would reduce the grain size, which scales with the coating thickness.[21] Larger grain sizes have negative effects on optical and mechanical properties.[22] Figure 4 presents AFM images of 4 and 10 μm thick coatings. Deposition parameters were the same for the two samples of different thickness. The thicker coating has higher surface roughness, 40 nm vs. 5 nm for the thinner coating. Amorphous layers also allow relieving growth stress in the coating. Chromium oxide coatings are known to have high residual stresses, sometimes exceeding 2 GPa.[3] It is therefore impossible to produce single-layer chromium oxide coatings thicker than 20-25 μm without encountering adhesion problems on typical substrate materials. It is very difficult to achieve thicker coatings without fracture and delamination because of the high residual stress, as the amount of stored elastic energy per unit area scales with the coating thickness:

G=Z

(1 −ν )σ 2

E

2 R

h (3)

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where Z is a constant on the order of unity, Poisson’s ratio, ν, σR is the coating residual stress, h is the coating thickness, and E is its elastic modulus. Thus, thicker coatings are more likely to fracture. The amorphous middle layer approach will reduce the amount of stored elastic energy in the coating by reducing its residual stress, allowing increasing coating total thickness without fracture failures.

(a)

(b)

Figure 4. AFM analysis of coating surface morphology: a) 4 μm thick coating, b) 10 μm thick coating.

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Figure 5. a) HREM micrograph and b) diffraction pattern of the Cr interlayer.

The HREM micrograph and diffraction pattern obtained from the middle of Cr interlayer are presented in Figure 5. The Cr interlayer appears to be crystalline. Figure 5b shows a diffraction pattern obtained from the Cr interlayer in Figure 5a. This image reveals a presence of Cr and Fe-Cr intermetallic diffraction rings. Table 1 provides a diffraction ring analysis in terms of the corresponding phase and d-spacing. A precise measurement of the lattice spacing from the HREM image gives inter-planar distances of d1 = 0.207 nm, d2 = 0.146 nm, d3 = 0.118 nm d4 = 0.104 nm d5 = 0.0909 nm and d6 = 0.078 nm, which is close to Cr(110), Cr(200), Cr(211), Fe-Cr (220), Fe-Cr (310), and Fe-Cr (222), respectively. Measured dspacing deviation from the theoretical values could be due to the lattice aberration in the interlayer caused by stress. Presence of brittle Fe-Cr intermetallics could affect coating performance. The effect of Fe–Cr phases on the mechanical properties of the coating has to be investigated in more details. Table 1. Experimental and calculated d-spacing values for the diffraction rings of Figure 5b.[23] Ring No. 1 2 3 4 5 6

hkl Cr (110) Cr (200) Cr (211) Fe-Cr (220) Fe-Cr (310) Fe-Cr (222)

d calculated, Å 2.04 1.44 1.18 1.02 0.909 0.83

d experimental, Å 2.07 1.46 1.18 1.04 0.909 0.78

Figure 6 shows high resolution micrographs of the interfaces. There are some defects present on the substrate surface, which act as stress concentrators, so substrate treatment prior to coating depositing is very important for improving coating adhesion strength. While the interfacial layers between the substrate and the Cr, and between the Cr and the coating are amorphous, some nanocrystalline clusters are present in these amorphous layers, as seen in

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Figure 6b. A fast Fourier transform and inverse fast Fourier transform analysis of the nanocluster atomic structure showed that these are Cr2O3 nanocrystals with an atomic spacing of 0.361 nm, which corresponds to the (012) Cr2O3 inter-planar spacing of 0.363 nm.

Figure 6. HREM micrographs of the interfaces between a) the substrate and the Cr interlayer, and between b) the Cr interlayer and the coating.

Figure 7 presents HREM and diffraction micrographs of the coating. In Figure 7a nanocrystalline grains are surrounded by amorphous material. Precise measurement of the lattice spacing from the HREM image gives inter-planar distances of d1 = 0.361 nm and d2 = 0.243 nm. These nanocrystalline grains are formed of Cr2O3 due to their similar d-spacing. Actually, the d1 spacing could belong to d012 of Cr2O3, and the d2 to d110 of Cr2O3.[23] In theory, the Cr2O3 d012 distance is 0.363 nm, and d110 is 0.247 nm. The measured d-spacing deviation from the theoretical values is due to the residual stress present in the coating. Figure 8b is a diffraction pattern obtained from the coating HREM image in Figure 8a, and shows Cr2O3 diffraction rings. Table 2 provides diffraction rings analysis. All of the diffraction rings deviate slightly from the standard values for Cr2O3 in PDF cards due to stress.

Figure 7. a) Coating HREM micrograph and b) corresponding diffraction pattern.

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Table 2. Experimental and calculated d-spacing values for the diffraction rings of Figure 7b.[23] Ring No.

hkl

d calculated, Å

d experimental, Å

1

Cr2O3 (110)

2.4796

2.4836

2

Cr2O3 (113)

2.1752

2.1799

3

Cr2O3 (116)

1.6723

1.6974

4

Cr2O3 (300)

1.4315

1.4394

Strain in each crystallographic direction of the Cr2O3 nanocrystals can be calculated as:

ε nanocrystals =

d0 − d d0

(4),

where d is the interplanar distance extracted from high resolution TEM images and d0 is the unstrained value obtained from a corresponding PDF card.[23] One would calculate enormous stresses in GPa range by converting the nanocrystalline cluster stain values into stress using appropriate elastic constants and assuming that these nanoparticles are not deformed plastically due to their small size, which prevents dislocation initiation and propagation.[24] The stress in the nanocrystal comes from the volume change associated with the amorphous-to-crystalline transition and the macroscopic residual stress in the coating. Nanocrystals act as stress concentrators; however, their percentage in the coating total volume is low except close to interfaces. As one of the part of interface research, adhesion is the one of the most important coating properties, and can be assessed with the scratch test. We have investigated the adhesion in our previous research work.[25] We deposited five different samples while the oxygen flow rate was ranged from 2.0 to 3.2 sccm with 0.3 sccm increments. The critical lateral load is easily identified from the increased acoustic emission signal. Figure 8(a) shows the lateral force and acoustic emission signal obtained during a scratch test. The normal fore was increased linearly during the scratch testing, so it was easily calculated at the critical coating delamination. Figure 8(b) shows the critical normal load of interfacial failures at different oxygen flux values. With the oxygen flux increasing, the critical normal load decreased. During sputter deposition oxygen ions energy increases with increased oxygen flux, which can cause a significant temperature rise, leading to higher residual stresses after cooling to room temperature. This effect deteriorated the adhesion strength of the coatings at higher oxygen flux. The challenge lies in developing a method to produce these coatings with high hardness and wear resistance, while at the same time not sacrificing adhesion strength. This challenge can be solved by using the chromium metallic interlayer in the middle of the chromium oxide coating, which can be produced by intermittently turning off the oxygen flow during the reactive sputter deposition.

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28

0

24

Critical load, N

Laterial force, N

(b)

12

Acoustic Emission, V

(a)

Laterial force AE

8

-3

-6

4

20

16

12

-9 15

30

45

60

Time, sec

75

90

0

2.0

2.4

2.8

3.2

Oxygen flux, sccm

Figure 8. (a) Critical lateral force determination for the coating failure during the scratch test; (b) Scratch test critical normal force as a function of the oxygen flux;[25]

Conclusions Dense Cr2O3 coatings were deposited on polished steel substrates by unbiased reactive magnetron sputtering. There is no obvious porosity and defects in the interlayer and the coating, but the substrate surface defects are present and may affect the adhesion strength. Amorphous layers were detected at each interface, which could block coating columnar grains growth, and allow depositing smooth thick coatings. Amorphous layers also allow relieving growth stress in the coating and depositing thick coatings without any delamination. The deposited Cr interlayer is mainly composed of pure chromium with several Fe–Cr phases detected at the interface between the Cr interlayer and the steel substrate. The presence of FeCr phases may affect adhesion between the substrate and the coating. There are highly stressed Cr2O3 nanocrystals present in the amorphous layers of the coatings. The adhesion of coatings and substrates was decreased with the increasing oxygen flow rate.

References [1] [2] [3] [4] [5] [6] [7] [8] [9]

G.V. Samsonov, in The Oxide Handbook, 2nd edition, 1982, 192. U. Rothhaar, H. Oechsner, Thin Solid Film 1997, 302, 266. P. Hones, M. Diserens, F. Levy, Surf. Coat. Technol. 1999, 120/121, 277. E. Sourty, J.L. Sullivan, M.D. Bijker, Trib. Int. 2003, 36, 389. F.D. Lai, C.Y. Huang, C.M. Chang, L.A. Wang, W.C. Cheng, Microelectronic Eng. 2003, 67/68, 17. B. Bhushan, G.S.A.M. Theunissen, X. Li, Thin Solid Films 1997, 311, 67. B. Bhushan, B.K. Gupta, Handbook of Tribology: Materials, Coatings and Surface Treatments, McGraw-Hill, New York, 1991, Ch. 14 W.J. Lackey, D.P. Stinton, G.A. Cerny, A.C. Schaffhauser, L.L. Fehrenbacker, Adv. Ceramic Mater. 1987, 2, 24. J. Creus, H. Mazille, H. Idrissi, Surf. Coat. Technol. 2000, 130, 224.

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[10] M. Fenker, M. Balzer, H.A. Jehn, H. Kappl, J.J. Lee, K-H. Lee, H-S. Lee, Surf. Coat. Technol. 2002, 150, 101. [11] E. Andrade, M. Flores, S. Muhl, N.P. Barradas, G. Murillo, E.P. Zavala, M.F. Rocha, Nucl. Instrum. Methods Phys. Res. B 2004, 219/220, 763. [12] C. Liu, A. Leyland, Q. Bi, A. Matthews, Surf. Coat. Technol. 2001, 141, 164. [13] K.D. Bouzakis, S. Hadjiyiannis, G. Skordaris, I. Mirisidis, N. Michailidis, G. Erkens, Surf. Coat. Technol. 2004, 188/189, 636. [14] J.H.W. Siu, Lawrence, K.Y. Li, Wear 2000, 237, 283. [15] J.D. Olivas, C. Mireles, E. Acosta, E.V. Barrera, Thin Solid Films 1997, 299, 143. [16] A. Leyland, Matthews, Surf. Coat. Technol. 1994, 71, 19 [17] G.S. Kim, S. Y. Lee, J. H. Hahn, B. Y. Lee, J. G. Han, J. H. Lee, S. Y. Lee, Surf. Coat. Technol. 2003, 171, 83. [18] Y. Pauleau, Vacuum 2001, 61, 17. [19] Gebert, U. Wolff, A. John, J. Eckert, Scripta mater. 2000, 43, 279. [20] V. Schroeder, C.J. Gilbert and R.O. Ritchie, Scripta Mater. 1998, 38, 1481. [21] R. Krishnamurthy, D.J. Srolovitz, Acta Mater. 2005, 53, 5189. [22] N.I. Tymiak, A.A. Volinsky, M.D. Kriese, S.A. Downs and W.W. Gerberich, Metallurgical and Materials Transactions A 2000, 31A; 863. [23] PDF Cards #06-0694, #34-0396, #38-1479PCPDFWIN, Version 2.02, JCPDS-ICDD, 1999. [24] W. W. Gerberich, W. M. Mook, C. R. Perrey, C. B. Carter, M. I. Baskes, R. Mukherjee, A. Gidwani, J. Heberlein, P. H. McMurry, S. L. Girshick, Journal of the Mechanics and Physics of Solids 2003, 51, 979. [25] X. Pang, K. Gao, A. Volinsky, Journal of Materials Research 2007, 22, 3531.

In: Surface Coatings Editors: M. Rizzo and G. Bruno, pp. 189-211

ISBN 978-1-60741-193-2 © 2009 Nova Science Publishers, Inc.

Chapter 7

A S TUDY ON I NORGANIC M ETALLIC AND D IELECTRIC T HIN F ILMS G ROWN ON P OLYMERIC S UBSTRATES AT ROOM T EMPERATURE BY PVD AND CVD T ECHNIQUES P. Mandracci1∗, R. Gazia1 , P. Rivolo1 , D. Perrone2 and A. Chiodoni3 Politecnico di Torino 1 Dept. of Materials Science and Chemical Engineering - Materials and Microsystems Lab. (ChiLab) 2 Dept. of Physics 3 Dept. of Land, Environment and Geo-Engineering

Abstract The deposition of both metallic and dielectric inorganic thin films on polymeric substrates is of great interest for several industrial and research applications. The growth of metallic coatings on polymers is of raising usage in order to impart specific functionalities, such as electrical, aesthetic and chemical-resistance properties, to polymeric substrates. Some examples are the substitution of chromium electroplating processes on plastics by PVD deposition in several industrial fields and the use of aluminum or silver coatings for the fabrication of hybrid fabrics. Dielectric thin films are also commonly grown on polymeric materials for several aims, including the protection of polymeric substrates from scratch, the attribution of barrier coatings to food packaging films, the incorporation of new functionalities to artificial fabrics, and the increase of biocompatibility of some kind of polymeric dental materials and prostheses. Unfortunately, the growth of thin films on polymeric substrates suffers of several constraints, due to the peculiar properties of polymers, such as the low heat resistance, the high elasticity and the low hardness. These limitations lead to the necessity of very low processing temperatures (often as low as room temperature) in order to avoid substrate damage, and the deposition of films of very low thicknesses, in order to reduce the interface stress. Plasma-assisted PVD and CVD techniques are suitable to satisfy these requirements, since they allow very low deposition temperatures, they are ∗

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P. Mandracci, R. Gazia, P. Rivolo et al. suitable for the deposition of composite materials, and provide a very good control on a wide range of process parameters. The present work deals with an experimental study of the interaction between the surface of different polymeric substrates, such as ABS, polyester, polyamide and some dental resins, with metal and dielectric coatings, such as Cr, Al, a-SiOx , grown by RF sputtering or PECVD. Different types of surface modifications, such as plasmaassisted surface activation and deposition of interlayers, were also applied to some of the polymeric substrates in order to study their effect on the growth process of the inorganic coatings. Several characterization techniques were used in order to analyze the materials involved in the study. The polymeric substrates and the inorganic coatings were characterized against their surface morphology by means of high resolution mechanical profilometry, optical microscopy and field emission scanning electron microscopy (FESEM), while some of the film chemical characteristics were analyzed by Fourier transform infrared spectroscopy (FTIR). Some chemical resistance tests were also performed to investigate some properties of the polymer-dielectric multilayer structures.

Introduction The deposition of inorganic thin films, such as metals and dielectrics, on polymeric materials is a technological challenge of great interest, due to the continuous rise of the use of plastics in many different industrial application fields, including automotive industry, textile industry, food packaging industry, as well as biomedics and dental research. As an example, in the automotive industry the substitution of metal components with plastic parts is raising in frequency due to the need of reducing production costs. However, most polymeric materials are not able to provide to the automotive components the required properties of moisture and corrosion resistance, as well as the optical properties, that are typically provided by chromed metallic parts. For these reasons the deposition of metal layers over plastic components is a common practice in the automotive industry, and is commonly achieved by means of electroplating processes, but could as well be performed by vapor deposition (PVD or CVD), with the advantage of reducing the ecological impact. For what concerns the textile industry, the increasing demand of high-tech textiles produces a continuous technological effort to the application of surface modification technologies to the natural and artificial fibers and textiles. To this aim, metallic coatings are of great interest since they can imbue fibers and textiles with several functionalities, such as optical properties (e.g. for high visibility jackets) and electrical conductivity properties (e.g. for EM shielding). Moreover, dielectric films are useful to modify the chemical behavior of fibers, e.g. providing hydrophobic properties, and for the protection of fibers from aggressive chemicals used during dye processes. Also in the food packaging industry polymeric films are of wide usage, and it is a common practice to apply metals or dielectric films in order to improve some of the film performances. A typical example is the application of thin silicon oxide or aluminum oxide coatings to reduce the film permeability to oxygen, water vapor, or carbon dioxide. Several kinds of polymeric materials are also used in the biomedical and dental practice for the fabrication of prostheses and implants, and also in this case several kinds of inorganic coatings may be used to improve the performance of these devices, e.g. by providing a barrier against the diffusion of poisonous elements from the implants to the

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living tissues, by reducing the bacterial adhesion on the implants surfaces, by improving the biocompatibility or by accelerating the osteointegration process. The previous examples show how much interest may be present in several and very different industrial fields for the deposition of inorganic coatings on polymeric materials; however, polymeric materials typically present some specific characteristics, that produce several limitations on the types of deposition techniques that may be used on this kind of substrates. One important peculiarity of the great majority of polymeric materials is the low heat resistance, if compared to most of the inorganic substrates on which PVD and CVD depositions are commonly performed, such as metals, glasses or semiconductor materials. As a consequence, there is a strong limitation on the process temperature that can be sustained by a polymeric substrate during the deposition of a thin film, and for this reason not all kinds of deposition techniques are suitable to this aim. As an example, deposition techniques that are based on thermal activation, such as thermal CVD, need high substrate temperatures in order to achieve the deposition process; while other techniques, such as unbalanced magnetron sputtering, may produce a considerable substrate heating at certain process conditions. Another important characteristic of polymeric materials is the low electrical conductivity, that prevents the use of techniques requiring the application of a DC voltage to the substrate surface. Also the substrate reactivity has to be taken into account, since polymeric materials may show a higher chemical reactivity respect to some growth precursors, as compared to more inert materials, like glasses or ceramics. In order to be suitable for the growth of thin films on polymeric substrates, a deposition technique must satisfy some conditions, the first of which is the ability to work at low substrate temperature. Among the various types of vapor deposition techniques, the most promising for the aim of polymeric coating are probably electron beam assisted evaporation, magnetron sputtering and plasma enhanced chemical vapor deposition (PECVD). All these are low-temperature deposition techniques: while the first two belong to the PVD family and are based on the use of a solid source, the latter is a CVD process, that requires the use of reactive gases for the film growth. Both RF sputtering and PECVD rely on the use of a non-thermal plasma: in the former case it is mainly used to produce an ion bombardment on the solid source surface; while in the latter case it is needed to obtain the dissociation of the gas source molecules, avoiding the need of a high substrate temperature to obtain the same effect. When dealing with the deposition of inorganic layers on polymer surfaces, a considerable advantage of plasma-assisted techniques, such as PECVD or sputtering, is the possibility of applying plasma etching processes in situ on the substrate surface, before the film deposition, thus allowing the application of a wide range of surface activation treatments that are often very useful to the aim of improving coating adhesion. The study of the interaction between the surface of a polymer and a coating deposited on its surface is of considerable importance, due to the deep influence exerted by the polymerfilm interface on a wide range of film properties, including the adhesion strength, the morphology, and the structure. A better control of the interface properties can thus allow the introduction of new film functionalities or the improvement of the existing ones, while a wider characterization of the film surface may provide information that are very useful to tune the deposition process parameters. In this chapter we provide some information on the deposition of metallic and dielectric layers on polymeric materials. Metallic layers of Ti and Ni were deposited by means of

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electron beam assisted evaporation on polyester substrates; while Al layers were grown on the same substrate type by means of RF magnetron sputtering. In both cases, some process to improve the film adhesion were tested for some samples, namely the deposition of Ti interlayers in case of evaporation processes, and a plasma-assisted surface etching in case of sputtering deposition. Moreover, the deposition of Cr films on ABS substrates was investigated, also considering the effect of polymer interlayers and oxygen plasma etching on the film morphology. Dielectric layers of silicon oxide (a-SiOx ) were deposited on different polymeric materials, namely polyammide and some types of resins used in dental practice. In some cases plasma-assisted etching was used to improve film adhesion. The quality of films and substrate-film interfaces was investigated by means of different procedures, depending on the typical application of each polymer-coating structure, including surface morphology analysis, surface energy analysis, and chemical resistance tests.

Aluminum and Nickel Coatings Grown on Polyester Substrates Properties Polyester is the common name referred to all those polymeric materials that contain, in the main chain of their branched structure, ester functional groups. The synthesis of polyesters is generally achieved by a polycondensation reaction of two monomers, alcohols and carboxylic acids, which react to form carboxylic esters. In order to propagate the polymerization, the water, formed by the reaction, must be continually removed by azeotrope distillation. Depending on the chemistry of monomeric units, many types of polyesters can be produced, among which the most popular are polyethyleneterephthalate (PET) and polycarbonate (PC). As thermoplastics, these polymers can be modified in shape by heat application [1]. Typically, polyesters offer good mechanical and thermal properties and high chemical resistance, at relatively low cost of production. Due to their properties, polyesters are obtainable in numerous forms such as fibers, sheets and three-dimensional shapes, depending on their final application: textile, bottle resin, packaging films. In particular, fabrics industry is interested in production of dyeable, metal coated, high tenacity and elastic modulus, low water adsorption and minimal shrinkage polyester yarns in order to achieve both aesthetic and functional properties for high-tech textile performance [2].

Film Deposition Evaporation. During this research work, Al and Ni coatings with a thickness of 100 nm were deposited on the top of commercially available polyester films using a multi-crucible, electron-beam assisted, evaporator system (Ulvac EBX 14D), equipped with a quartz-based thickness/rate monitor system (STM-100/MF). Ti films with a thickness of 10 nm were deposited as interlayer between polymer and metal (Al or Ni), with the aim of improving the adhesion of the metallic film on the polymer surface. Thanks to the multi-crucible system, it was possible to perform the interlayer deposition before the top layer growth without breaking the vacuum in the deposition chamber.

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Table 1. Process conditions for deposition of Al films on polyester by sputtering. Process

Gas

Plasma activation Al deposition

O2 Ar

Flow [sccm] 40 40

Pressure [mTorr] 50 5

RF Power [W] 100 100

Duration [min] 10 10

Sputtering. Aluminum coatings with a thickness of 95 nm were grown on the same polyester films by means of radio frequency magnetron sputtering. The deposition system consisted of a cylindrical reactor of stainless steel construction, inside which a magnetron sputtering source was located, which was able of carrying a target of 10 cm diameter. The source was located in the top part of the reactor, facing downward, while the samples were located at the bottom part of the reactor, facing upward, at a distance of 80 mm from the sputtering target. The system was pumped by a turbomolecular pump and a mechanical pump in sequence (nominal pumping speeds 350 l/s and 15 m3 /h respectively) allowing to reach a minimum base pressure of 10−5 Pa. An Ar gas flow was injected in the reactor by means of a mass flow controller and the plasma discharge was lighted applying a radio frequency voltage at a frequency of 13.56 MHz between the target and the grounded substrate holder; the plasma impedance was then tuned by a matching network. Before the deposition of the Al layer, a plasma etching process was applied to the polymeric substrates in order to activate the surface to improve adhesion. The process conditions for Al deposition and plasma etching are shown in tab. 1. Chemical tests. Due to the high importance of the chemical resistance of polyester films and polyester-metal structures for several applications, it was chosen to test the quality of polymer-metal multilayer structures by subjecting them to some selected chemical aggressive processes. Sodium carbonate baths were chosen since they simulate processes that are commonly used in several industrial manufacturing processes, including dye processing in textile industry. The samples were immersed in chemical baths with pH value of about 11, and the chemical resistance tests were performed at room temperature and at a temperature in excess of 95°C. The results of the chemical tests are briefly reported in tab. 2. The polyester-Ti-Ni multilayers were the only structures to show a good resistance at both low and high temperature: after the immersion in the chemical baths for 60 min no evident change was revealed in the morphology of the Ni films. For what concerns the polyester-Ni-Ti structures, they showed a good resistance to the immersion in basic agents at room temperature, but the immersion in a high temperature basic bath resulted in the almost complete removal of the metal film from the polymeric substrate. The worst results were obtained for the polyester-Al structures grown by sputtering with a oxygen plasma pre-deposition activation process, since they did not resist to room temperature baths. It is worth to notice that, when samples are dipped in high temperature sodium carbonate solutions, an abrupt reaction occurs. Probably due to the violent oxidation of Al layer, activated by the high temperature, gas bubbles are formed. Moreover, the phenomenon, that is not observed in the room temperature bath, subjects the substrate-film interface to a noticeable mechanical stress. The resistance to the

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Table 2. Results of chemical tests performed on different multilayer polymer-metal structures by immersion in a N a2 CO3 bath at different temperatures. Material Polyester-Ti-Al Polyester-Ti-Al Polyester-Ti-Ni Polyester-Ti-Ni Polyester-Al Polyester-Al

Growth method Evaporation Evaporation Evaporation Evaporation Sputtering Sputtering

pH 11 11 11 11 11 11

T [°C] RT 98 RT 98 RT 98

Time [min] 60 60 60 60 60 60

Resistance Yes No Yes Yes No No

high temperature basic bath is then a signal of both strong adhesion of the metal film to the polymeric substrate and high film quality (density, purity of chemical composition, etc.).

Chromium Coatings Grown on ABS The aim of this section is to show some physical and aesthetic properties of chromium films deposited on acrylonitrile butadiene styrene (ABS) and to highlight some parameters which can affect the films quality. In particular, the properties of the substrate surface are of great importance in determining the characteristics of the films. The film depositions were carried out by RF magnetron sputtering or DC sputtering, depending on the desired film thickness. The chromium films were deposited on pure ABS, or on polymeric interlayers applied on the substrate surface, and the influence on the film quality of an oxygen plasma treatment, performed on polymeric interlayes, was investigated as well.

Substrates Properties ABS is composed of acrylonitrile, butadiene, and styrene. Acrylonitrile is a monomer derived from propylene and ammonia; butadiene is obtained from butane, and styrene is a monomer obtained from benzene and ethylene. ABS combines the properties of each component: strength and rigidity from acrylonitrile and styrene polymers; toughness from the polybutadiene rubber. This material can be usually found in blends with other polymers, depending on the application [3]. This is a way for accentuating ABS properties or to add new characteristics to the material. Among the possible fields of application for ABS, automotive industry is of great importance, since many vehicle parts are fabricated in ABS and then coated with paint or chrome, for aesthetic reasons. For commercial applications, ABS chrome-plating is currently performed in galvanic baths usually containing hexavalent chromium [4]. Moreover, the deposition of the final chromium layer is often preceded by the deposition of copper and nickel layers [5], in order to improve the adhesion of the chromium layer to the substrate and to reduce the thermal and mechanical stress between the different materials. During the last years, many efforts were made in order to substitute this technique with other environmentally friendly techniques, such as PVD processes [6].

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Untreated ABS substrates. In order to investigate the PVD deposition of metallic films on these polymeric substrates, some measurements and experiments were carried out in our laboratory: in particular, we considered of great importance the determination of the surface properties of the substrate, since they can significantly affect the film adhesion. To this aim, optical contact angle (OCA) measurements were used, since they give important information about the capability of a surface to interact with another one. In this chapter the surface properties of the tested samples will be described on the basis of the results obtained from OCA measurements. Some of the ABS substrates were obtained by removing the metallic layers from commercially available chrome-plated parts, while some ABS slabs were purchased from Goodfellow in order to compare the properties of brand new and old ABS substrates. OCA measurements on new samples revealed a slight hydrophobicity, while old samples showed a hydrophilic surface. However, we have to take into account that the results of contact angle measurements are always affected by the surface roughness of the sample: materials with identical chemical composition can show different values of contact angle when their surface roughness is very different. In particular, a rougher surface gives a lower value of contact angle, which means higher hydrophilicity. In the case of ABS samples, the surface roughness of old samples, subject to many treatments during the electroplating process, was higher than that of the new ABS slabs. Thus, the mismatch of the contact angle values can be attributed to a physical characteristic of the surface rather than to the chemical composition of the two materials. Since most depositions were performed on old samples, the results shown below deal with films grown on a slightly hydrophilic surface. Polymeric interlayers. Film adhesion to the substrate can be improved by increasing the surface energy, and consequently the hydrophilicity, of the substrate. Polymeric interlayers can be used for this purpose [7], and in the present case an UV curing paint was employed: an example of the aspect of the surface of an adhesion layer applied on a polymeric surface is shown in fig. 1. OCA measurements showed a decrease of the hydrophilicity value after the interlayers depositions, unlike what was expected: thus, apparently in the present case the presence of the interlayers deteriorated the capability of creating strong bonds with the deposited films. However, the interlayers depositions could still be considered useful in order to reduce the surface roughness of the substrates. Oxygen plasma treatments. Another way to improve the surface energy of a material is the application of a plasma-assisted surface treatment, that consists in the exposition of the substrate to an oxygen plasma in a plasma reactor, giving rise to the activation of the sample surface, and eventually resulting in a increase of its surface energy. This process was applied to some samples using a PECVD reactor, consisting of a stainless steel chamber pumped by a mechanical pump, able to reach a base pressure of about 10-2 Torr: oxygen gas was injected in the reactor and the plasma was lighted by application of a RF voltage between two electrodes. As a result of the plasma treatment, the surface hydrophilicity was drastically increased as well as the adhesion properties. The first observation, made as soon as the samples were taken out the reactor was that their surface had changed its coloration from nearly white to yellowish.

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Figure 1. Image of a layer of a polymeric layer (UV curing paint) applied on ABS acquired with an optical microscope.

Film deposition The deposition of chromium thin films on ABS substrates was performed employing two different systems: a RF magnetron sputtering system, used for films with a thickness higher than 40 nm, and a DC sputtering system for the others. The polymeric substrates were coated at room temperature, using Ar gas at a pressure of 5 mTorr. In the RF magnetron sputtering system the target-substrate distance was 8 cm and the RF power was 100 W. Before each deposition the chamber was pumped down to a pressure of about 5 × 10−6 Torr. In the DC sputtering system the values of current and voltage applied to the target were 120 mA and 550 V respectively, while the base pressure was ≈ 10−5 Torr. Film deposition on untreated substrates. In the conditions described above, a 1500 nm thick film of chromium was grown on untreated ABS by RF magnetron sputtering. Even to the naked-eye, the appearance of chromium films surface was very rough, mat and non homogeneous. This could be ascribed to the roughness of the underlying surface, which forced the film to follow its structure and grow in a disordered way. FESEM analysis showed at higher magnifications cauliflower-like features responsible of the opacity that characterizes these films, as shown in fig. 2. OCA measurements revealed a hydrophilic surface, characteristic that could allow a good adhesion of the protective finish polymeric layers used in some applications to increase the mechanical resistance of chromed plastics. The surface roughness was measured by a profilometer and was found to be approximately equal to 180 nm, resulting in a very low brightness. Film deposition on polymeric interlayers. Chromium films of different thicknesses were grown on ABS substrates coated with a polymeric interlayer, consisting in a commer-

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Figure 2. FESEM images of a chromium film grown on untreated ABS. cialy available UV curing paint. The interlayers showed a good effectiveness in reducing the roughness of the substrate and, as a consequence, the chromium films showed brighter appearance, compared to the ones grown on untreated ABS. However, with more accurate analysis, the surface homogeneity appeared to reduce with the increase of the films thickness. Optical microscopy analysis revealed on 1500 nm thick films some platform-like structures, separate one from the another by embossed tubular formations. FESEM analysis showed more in details the structure of these features: across the flat areas, the film grew as an alternation of lamellae, all oriented in the same direction, while the tubular structures showed the same cauliflower-like features already observed in the films grown on untreated ABS substrates, as shown in fig. 3. As the film thickness decreased, the tubular formations became less pronounced, and with further decrease in thickness, the films surface showed no trace of embossed structures, but at their place fractures appeared. This evidence could be explained by the strong dependence of the film stress on its thickness: for very low thicknesses, the stress at the film-substrate interface is usually very low and no fractures or other formations should appear on the metallic film. As the thickness increases, fractures may begin to appear on the surface due the higher stress, becoming centers of nucleation for the further growth of the film. With further increase in film thickness, the fractures will turn into embossed structures. Fig. 4 shows FESEM images of thinner samples grown on ABS coated with a polymeric interlayer, showing the change in morphology with the film thickness variation. OCA measurements revealed a slight hydrophobicity for all the samples. As previously asserted, the difference in contact angle between chromium films deposited on untreated ABS and films deposited on ABS coated with the polymeric interlayer could be ascribed to the difference in surface roughness. Profile measurements revealed a roughness between 10 nm and 20 nm for all the samples thick more than 30 nm. Film deposition with oxygen plasma treatments. The above described plasma treatments were applied to some of substrates coated by polymeric interlayers, before performing the film deposition of chromium films. The main desired effects of these surface treatments were the removal of contaminants from the polymer surface and the introduction of reactive chemical groups (such as -OH group [8]) in order to make the substrate surface

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Figure 3. FESEM images of chromium films grown on ABS substrates at different magnifications. In particular (a) and (b) show the flat areas divided by the embossed tubular structures; (c) and (d) represent at high magnifications the “lamellae” of the film constituting tubular structures and flat areas respectively. change from hydrophobic to hydrophilic. Fig. 5 shows optical microscope images of the samples treated in plasma and chromium-coated. Their surface is non uniform and characterized by large dark areas with spots or streaks of a brighter color. The dark areas are the regions where the film grew on the substrate surface, while the bright defects may be due to dust present in the reactor during the activation process which shielded the substrate from the film growth. In fig. 6, FESEM images show the branched sructures, caused by a plasma pre-treatment of the polymeric interlayer: the metallic layer grew following the underlying structure. This kind of behavior proves the formation of stronger bonds between the polymer and the metallic layer deposited on it.

Silicon Oxide Coatings Grown on Dental Resins In the dental practice, polymeric materials are of wide usage for several purposes, among which is of considerable importance the fabrication of dental prostheses (artificial teeth). The fabrication of such kind of medical devices may be performed by different materials, including ceramic and microcomposite resins, the latter showing the advantage of a lower cost and an easier device fabrication. These devices are not in continuous contact with the living tissues, as is the case of orthopedic or dental implants, but are exposed to saliva and to several chemical substances that form in the human mouth as a result of food mastication, many of which are of aggressive nature and may considerably degrade the microcomposite

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Figure 4. FESEM images of chromium films om ABS coated with a polymeric interlayer and thick (a, b) 170 nm; (c, d) 40 nm and (e, f) 30 nm respectively at different magnifications.

Figure 5. Optical microscope images of chromium films grown on ABS substrates coated with a polymer interlayer and activated by oxygen plasma.

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Figure 6. FESEM images at different magnifications of chromium films deposited on ABS substrates coated by a polymeric interlayer and activated via plasma treatment. resins over time [9, 10, 11, 12]. Moreover the surface of these prostheses is suitable to allow the adhesion of bacteria, that may propagate to the natural teeth contributing to the formation of the dental plaque and possibly producing secondary caries [11, 12]. Inorganic barrier coatings have a good potential for the mitigation of at least some of the cited problems, since they can act as a separation between the bulk material and the mouth environment. In fact, the presence of a film coating may prevent the chemical degradation of the prosthesis and also produce considerable changes on the surface characteristics, like roughness and surface energy, eventually leading to a change in bacterial adhesion properties. Among the extremely wide range of inorganic materials, of great interest to the purpose of dental prostheses protection are oxides. In fact, these composite materials usually show high resistance to the action of a wide range of chemicals and good biocompatibility. Moreover they have a high transparency, that is a property of great importance, since the aesthetic features of dental prostheses are carefully tuned in order to reproduce the dental exterior aspect, and may not be changed by the protective layer. For this experimental work we choose to consider silicon oxide (a-SiOx ) films as protective materials, due to their properties of high transparency and inertness, together with their relative easiness of fabrication and low production cost.

Substrates Properties Two types of commercially-available dental materials of composite structure were used as substrates for the deposition of amorphous silicon oxide (a-SiOx ) coatings, and their commercial names and manufacturers are reported in tab. 3. The surface aspect of the

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Table 3. Microcomposite dental materials used for this research. Name Gradia Signum

Producer GC Heraeus-Kulzer

Country Japan Germany

samples is shown in fig. 7 and 8, where some photos taken at the optical microscope are reported.

Figure 7. Surface of a sample of Gradia dental microcomposite resin as seen at the optical microscope. The samples showed a considerable surface roughness, as shown in tab. 4, and a noticeable degree of hydrophobicity, as evidenced by the OCA measurements shown in fig. 9.

Film Deposition The dental materials samples were coated by means of plasma enhanced chemical vapor deposition (PECVD), using silane (SiH4 ) and nitrous oxide (N2 O) as gas precursors. The used PECVD system consisted of a high-vacuum stainless steel chamber, able to reach an ultimate vacuum of the order of 10-8 Torr, in which a radio frequency voltage was applied between two square parallel electrodes, 20 mm distant one from the other. In presence of the specific gaseous reactive mixture, a low pressure plasma discharge was generated in order to produce the dissociation of precursors molecules. Growth processes were carried out at room temperature, since polymeric materials cannot be heated, using the process parameters reported in tab. 5

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Figure 8. Surface of a sample of Signum dental microcomposite resin as seen at the optical microscope. Table 4. Surface roughness of Gradia and Signum microcomposite dental materials.

Path#1 Path#2 Path#3 Path#4 Path#5 Average Max deviation

Gradia RMS [nm] 361 490 392 694 1154 618 396

Signum RMS [nm] 741 606 565 508 384 561 178

Figure 9. Contact angle measurements performed on Gradia (a) and Signum (b) microcomposite dental materials.

Film properties. After the coating procedure, the dental materials were characterized by optical microscopy, profilometry and OCA in order to obtain information on surface properties and make a comparison with the uncoated samples. The exterior aspect of the coated surface, as appeared at the optical microscope, is shown in fig. 10 and fig. 11: the

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Table 5. Optimized process parameters for the deposition of SiOx by RF-PECVD. Process parameter Substrate temperature Total pressure Total gas flow Gas flow ratio ([N2 O]/[SiH4 ]) Applied power density Frequency

Value ˜20 400 61.5 40 208 13.56

Unit °C mTorr sccm W/m2 MHz

images are very similar to the ones related to the uncoated samples (fig. 7 and fig. 8), demonstrating that the SiOx coating does not influence the exterior aspect of the dental resins, as is required in order not to compromise the aesthetic features of dental prostheses.

Figure 10. Surface of a sample of Gradia dental microcomposite resin coated by SiOx , as seen at the optical microscope. The results of profilometry measurements are reported in tab. 6, and show that the presence of the film did not produce relevant changes in the roughness value respect to uncoated resins. On the other side, OCA analysis, shown in fig. 12 evidenced a considerable reduction of the contact angle, revealing an increase of surface energy. It is worth to notice that, according to the literature [13], a high surface energy is desirable in order to reduce bacterial adhesion.

Silicon Oxide Coatings Grown on Polyamide The subject of this section deals with the interaction of amorphous silicon oxides (a-SiOx ) thin films, grown by PECVD on both pure and nanocomposite (blended with nanoclays)

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Figure 11. Surface of a sample of Signum dental microcomposite resin coated by SiOx , as seen at the optical microscope.

Figure 12. Contact angle measurements performed on Gradia (a) and Signum (b) microcomposite dental materials after the deposition of a a-SiOx coating. polyamide 6 (PA6) flexible substrates. The quality of the polymer-SiOx interface, that strongly depends on the deposition parameters, determine the quality of the films for preventing the permeation of gases like oxygen, carbon dioxide or water vapour through the

Table 6. Surface roughness of Gradia and Signum microcomposite dental materials after the deposition of a a-SiOx coating.

Path#1 Path#2 Path#3 Path#4 Path#5 Average Max deviation

Gradia RMS [nm] 627 449 300 406 324 421 164

Signum RMS [nm] 592 594 349 307 436 455 144

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Figure 13. Scheme of nanoclays dispersion in a polymeric matrix. material. In order to explore morphological and compositional properties, surface characterizations were performed on substrates before and after the deposition of coatings. The analyses were also repeated after a long term exposition to some organic fluids, that simulate the typical environment in which food packaging is used.

Substrate properties The proposed substrates were flexible sheets of polyamide 6 (commercial name: Nylon 6 TECHNYL C 302 (Dry) by Rhodia Engineering Plastics SA) obtained by extrusion. This thermoplastic polymer, obtained by polymerization of caprolactam, has good flexibility and impact strength and it is suitable for extrusion. Typical properties are the elasticity and high resistance to continual mechanical and thermal stresses, abrasion and chemicals contact like acids or alkalis. Due to its properties, PA6 is one of the most commonly employed materials in food packaging industry. However, barrier properties needed to preserve food from degradation can be enhanced by reinforcing polymer matrix, embedding nanoparticles [14] constituted by phyllosilicates, that is, sheet silicates. The presence of these synthetic nanoclays do not affect the possibility to process the polymer sheets by co-extrusion. The phyllosilicates fillers [14] are sheets with thickness of one to few nanometers and length of hundreds to thousands of nanometers. The nanosheets can be differently dispersed in the polymer matrix due to the clay ammonium cation organic modifiers and compatibilisers added to the polymeric blend [15, 16, 17]. The both, for a given natural clay, are able to induce the formation of intercalated to exfoliated nanostructures (fig. 13). In the former, few polymer chains are extended among phyllosilicate layers determining an ordered structure where the polymeric phase is alternated to the inorganic phase. In the latter, the inorganic phase is homogenously dispersed in the polymer matrix as nanoparticles. When neither of them is obtained, as the polymer is not able to slide among clay layers, a phase separation occurs and the inorganic content aggregates as micro-structures. This blend structure is simply called microcomposite. A few amount of nanoclays (2-4% wt), properly nano-dispersed in PA6, can increase from twice up to 6 times the gas diffusion barrier as the permeating gas path is incremented

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[18], without reducing mechanical, thermal or transparency properties. Morphological characterization of substrates. A morphological characterization by FESEM analysis was performed on cited substrates before the a-SiOx layer deposition. This technique allows to investigate also non-conductive materials, such as polymers without using metallization. In fact, dielectric materials undergo a surface electronic charging effect when scanned by an electron beam, with thermoionic SEM, so they have to be metallised to allow a correct analysis, hiding in such a way the morphological features.

Figure 14. PA6 FESEM micrograph - top view (a); nanoclays-filled PA6 FESEM micrograph - top view (b). Both the PA6 (fig. 14-a) and the nanoclays-filled PA6 (fig. 14-b) show a slightly rough surface. Surface folding and agglomerates, in a 1x1.5 µm area, are about 100-500 nm in length and diameter, respectively. These features are due the extrusion process, that is, to surface quality of extruder components which transfer their morphological characteristics to polymer sheets. Micrograph clearness and resolution is higher for nanocomposite PA6 sample, as the presence of nanoclays reduces the effect of disperse electronic charges induced by the electron beam. No differences in surface morphology, due to the presence of the embedded nanoclays, are visible. PA6 and nanocomposite PA6 sheets show a thickness in the 200-250 µm range, depending on the extrusion process.

Figure 15. nanoclay-filled PA6 FESEM micrograph - cross section - RT cut (a); nanoclayfilled PA6 FESEM micrograph - cross section - liquid N2 (T=77K) fracture (b). Fig. 15-a shows the cross section of a nanocomposite PA6 sample, obtained by simple

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cutting. Comparing this one with the micrograph of a cross section sample obtained by a liquid nitrogen (T=77K) fracture (fig. 15-b), it results a well-detailed morphology, due to the absence of the micro-fusion areas normally produced by a room temperature cutting, which can induce the change of material appearance. The surface section roughness (fig. 16) is due to the different physical properties between the polymer embedding matrix and the disperse phyllosilicates, constituting the hybrid material. No evident aggregation of inorganic material is observable.

Figure 16. Nanoclay-filled PA6 FESEM micrograph - cross section - liquid N2 (T=77K) fracture: enlargment.

Film Deposition The SiOx thin film coatings were deposited by the same PECVD system described in the previous section, using the process parameters exposed in tab. 5. SiOx layers properties. Process conditions, above described, determine the growth of 240 nm thick SiOx films on both PA6 and nanocomposite PA6 substrate. Layer morphology has been investigated by FESEM analysis.

Figure 17. SiOx FESEM micrograph - top view.

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As shown in fig. 17, a flat nanostructured surface has been obtained. In order to get information about film composition, FTIR spectroscopy has been used. A layer of SiOx has been grown on a Si wafer in the same process conditions.

Figure 18. IR absorption spectrum of a SiOx layer deposited according to cited process conditions. The related absorbance spectrum (fig. 18) shows a main absorption peak centered at 1065 cm-1 due to the stretching vibrational modes of Si-O bonds and a less intense peak at 810 cm-1 , attributed [19] to the bending modes of Si-O species. These features are typical of silicon oxide based materials. The shoulder at 1190 cm-1 is assigned to N-H bending modes [19]. Nitrogen comes from N2 O which is preferable to O2 , as oxygen precursor, excessively reactive with silane. Hydrogen, produced by the dissociation of SiH4 , is easily incorporated during a PECVD growth. This phenomenon can be favoured at low deposition temperatures (e.g. RT), since the hydrogen diffusion from the film is temperature dependent [20]. Properties of SiOx layers after long term immersion in organic fluids. The morphology of a-SiOx coatings was also characterized by FESEM analysis after migration tests, that are usually performed in order to quantify the release of substances from packaging materials to organic liquids, acetic acid and ethanol, which simulate the contact between food and packaging films. In this chapter we do not deal with the results of the migration tests; but only with the analysis of the effect of the immersion in the organic liquids on the film morphology. Fig. 19 and fig. 20 show the micrographs of two samples of nanocomposite PA6 coated with SiOx , after immersion in acetic acid and ethanol, respectively. It is visible that, in both cases, fractures and exfoliation of the inorganic films occurred, producing platform-like structures, though the quality of each platform surface nanostructure does not seem affected by the treatment, so that the chemical resistance of SiOx to simulants is confirmed. The folding of the samples during immersion tests and the low adhesion of

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SiOx coating to the substrates caused these features. Deposition of thinner SiOx layers and adhesion enhancement treatments to carry out before SiOx growth could definitely reduce the problem.

Figure 19. Nanocomposite PA6 coated by an SiOx layer and tested in acetic acid: top view (a); top view enlargement (b).

Figure 20. Nanocomposite PA6 coated by an SiOx layer and tested in Ethanol: top view (a); top view enlargement (b).

Conclusion In this chapter we showed some results concerning the deposition on polymeric substrates of some kinds of metallic and dielectric inorganic films by means of different deposition methods. The polymer-inorganic structures were subject to different quality tests, depending on their typical use, including chemical resistance tests, morphological analysis, roughness measurements, and surface energy analysis. Metal films were grown on polyester substrates by means of electron beam assisted evaporation (Ni and Al) and RF magnetron sputtering (Al), using Ti interlayers or plasma oxygen etching to enhance the film adhesion. According to the experimental results, the polyester-Ti-Ni structures showed a higher resistance to alkali baths at both low and high temperature respect to polyester-Ti-Al and polyester-Al, demonstrating a better quality of the metal-polymer interface. Cr layers were deposited by sputtering on the top of ABS

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substrates, also investigating the effect of polymeric interlayers and of oxygen plasma etching. The morphology of Cr films resulted strongly dependent on films thicknesses, and a considerable influence of both polymeric interlayers and plasma treatments was evidenced. Silicon oxide (a-SiOx ) films were grown on two different types of dental microcomposite resins by PECVD, and the surface characteristics of the polymeric materials were compared to the ones of the coated samples. The coatings left nearly unchanged the surface roughness, while producing a considerable change in surface energy, demonstrating a good potential for the reduction of bacterial adhesion. Silicon oxide barrier coatings of different thicknesses were also grown on polyamide by the same plasma assisted technique and the effect of the long-term exposition to some organic fluids (acetic acid and ethanol) was investigated by morphological analysis, revealing some degree of film exfoliation.

Acknowledgments Some of the research works reported in this chapter were partially funded by the Piedmont Region (Bando per la ricerca scientifica, call 2004). Moreover, the authors wish to thank Sig. Giuliano Giobbio of Marbo s.p.a. for the deposition of the polymeric interlayers on ABS.

References [1] McKeeb, M.G.; Unala, S.; Wilkesb, G. L.; Longa T. E. Progress in Polymer Science, 30, 2005, 507-539. [2] Choudhury, A. K. R.; Textile Preparation And Dyeing; Science Publishers; 2006, p. 244. [3] Utracki, L. A. Polymer Blends Handbook; Springer: Montreal, QC, 2002; Vol. 1, pp 31-34. [4] Brown, H.; Woods, H.; Millage, D. R. Chromium Electroplating; US patent 2750335, 1956. [5] Leininger, M.; Method for applying a decorative metal layer; US patent 7297397 B2, 2007. [6] Goodrich, G. D.; Colahan, P. J. Chrome coating composition; US patent 7150923, 2006. [7] Kloss, T. J. Method for providing a chrome finish on a substrate; US patent 7132130 B1, 2006. [8] Tang, K. C.; Liao, E.; Ong, W. L.; Wong, J. D. S.; Agarwal, A.; Nagarajan, R.; Yobas, L. Evaluation of bonding between oxygen plasma treated polydimethyl siloxane and passivated silicon; Journal of Physics: Conference Series 34 (2006) 155–161. International MEMS Conference 2006.

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[9] Shahal, Y.; Steinberg, Y.; Hirschfeld, Z.; Bronshteyn, M.; Kopolovic, K. J. Oral Rehabil. 1998, 25, p. 52. [10] Suljak, J.P.; Reid, G.; Wood, S.M.; McConnell, R.J.; van der Mei, H.C.; and Busscher, H.J. Journal of Dentistry 1995, 23, p. 171. [11] Stenudd, C.; Nordlund, A.; Ryberg, M.; Johansson, I.; Kallestal, C.; and Stromberg, N. Journal of Dental Research 2001, 80, p. 2005. [12] Montanaro, L.; Campoccia, D.; Rizzi, S.; Donati, M.E.; Breschi, L.; Prati, C. and Arciola C.R. Biomaterials 2004, 25, p. 4457. [13] Grivet, M.; Morrier, J. J.;Benay, G.; and Barsotti, O. Journal of Material Science: Materials in Medicine 2000, 11, 637. [14] Kim, J.; Creasy, T. S. Polymer Testing 2004, 23, pp. 629–636. [15] Jeon, C. H.; Ryu, S. H.; Chang, Y.W. Polymer International 2003, 52, 153-157. [16] Zhang, W.; Chen, D.; Zhao, Q.; Fang Y. Polymer 2003, 44, 7953-7961. [17] Li, X.; Ha, C.; Journal of Applied Polymer Science 2003, 87, 1901-1909. [18] Alexandre, M.; Dubois, P. Materials Science and Engineering R 2000, 28, 1-63. [19] Giorgis, F.; Pirri, C. F. In: Silicon Based Materials and Devices; Nalwa, H.S.; Ed.; Academic Press, London, GB, 2001, Vol. 1, p. 187. [20] Alayo, M.I.; Pereyra, I.; Carreno, M.N.P. Thin Solid Films 1998, 332, 40-45.

In: Surface Coatings Editors: M. Rizzo and G. Bruno, pp. 213-236

ISBN: 978-1-60741-193-2 © 2009 Nova Science Publishers, Inc.

Chapter 8

SONOCHEMICAL COATINGS OF NANOPARTICLES ON FLAT AND CURVED CERAMIC AND POLYMERIC SURFACES A. Gedanken and N. Perkas Department of Chemistry and Kanbar Laboratory for Nanomaterials at the Bar-Ilan University Center for Advanced Materials and Nanotechnology, Bar-Ilan University, Ramat-Gan, Israel

Abstract This article will review the research that has been done in the last decade using ultrasonic waves for coating surfaces. Sonochemistry is a field of research in which chemical reactions occur due to a collapse of an acoustic bubble. The review will present examples limited to coating nanoparticles on ceramic bodies and polymeric surfaces. However, the same technique works also on metallic, glass, and textile surfaces. The excellent adherence of the nanoparticles to the substrate is reflected, for example, in the lack of bleaching of the nanoparticles from the polymeric substrate when deposited by the sonochemical process. Sonochemistry is a research field where waves in the frequency range of 20 kHz - 1 MHz are the driving force for the chemical reactions. The reaction is dependent on the development of an acoustic bubble in the solution. Extreme conditions (temperature >5000 K, pressure >1000 atm and cooling rates >1011 K/sec) are developed when this bubble collapses, thus causing the chemical reactions to occur. The current review will introduce to the reader what kind of surfaces serve as the substrates for the coating. It will present the variety of nanoparticles that have been anchored sonochemically to the surface, and finally it will explain the role of the ultrasonic waves in depositing nanoparticles onto solid surfaces. The review will compare the deposition of newly formed nanoparticles with that of nanoparticles purchased from a commercial source. The first chapter of this review will introduce the reader to the field of sonochemistry. The current review is a continuation of a series of previous reviews published by our group. These reviews introduced the sonochemical technique as a new means for the fabrication of nanomaterials [1], for the use of ultrasonic waves for the doping of nanoparticles into ceramic and polymer bodies [2], and for the microspherization of proteins by a sonochemical process [3]. Other review articles on similar topics have also been published [4-6]. However, no

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A. Gedanken and N. Perkas review on using the sonochemical technique for coating surfaces was found in our literature search. In our literature search we will scan for papers published until May 2008. We will try to avoid duplication and the review will not include examples presented in previous reviews.

1. Introduction Sonochemistry is the research area in which molecules undergo a chemical reaction due to the application of powerful ultrasound radiation (20 KHz - 10 MHz) [7]. The physical phenomenon responsible for the sonochemical process is acoustic cavitation. Let us first address the question of how 20 kHz radiation can rupture chemical bonds (this question also relates to 1 MHz radiation), and try to explain the role of a few parameters in determining the yield of a sonochemical reaction. We will then describe the unique properties that make ultrasound radiation an excellent method for depositing nanoparticles on the flat and curved surfaces of a large variety of substrates. A number of theories have been developed to explain how 20 kHz ultrasonic radiation can break chemical bonds, and they all concur that the main event in sonochemistry is the creation, growth, and collapse of a bubble that is formed in the liquid. The first question is how such a bubble can be formed, considering the fact that the forces required to separate water molecules to a distance of two Van-der Waals radii would require a power of 105 W/cm? On the other hand, it is well known that in a sonication bath with a power of 0.3 W/cm [7], water is readily converted into hydrogen peroxide. Different explanations have been offered; they are all based on the existence of unseen particles or gas bubbles that decrease the intermolecular forces, enabling the creation of the bubble. The experimental evidence for the importance of unseen particles in sonochemistry is that when the solution undergoes ultrafiltration before the application of ultrasonic power, there is no chemical reaction and chemical bonds are not ruptured. The second stage is the growth of the bubble, which occurs through the diffusion of solute vapor into the volume of the bubble. The third stage is the collapse of the bubble, which occurs when the bubble size reaches its maximum value. From here on we will adopt the hot spot mechanism, one of the theories that explains why, upon the collapse of a bubble, chemical bonds are broken. This theory claims that very high temperatures (5,000-25,000 K) [8] are obtained upon the collapse of the bubble. Since this collapse occurs in less than a nanosecond [8, 9], very high cooling rates in excess of 1011 K/sec are obtained. This high cooling rate hinders the organization and crystallization of the products. For this reason, in all cases dealing with volatile precursors where gas phase reactions are predominant, amorphous nanoparticles are obtained. The sonochemical reaction can be a gas phase reaction usually involving the vapors of the volatile reactants. In this case, the reaction takes place at very high temperatures ca. 5000K. If, on the other hand, the precursor is a non-volatile compound, the reaction occurs in a 200 nm ring surrounding the collapsing bubble [10]. In this case, the sonochemical reaction occurs in the liquid phase. The products are sometimes nano-amorphous particles, and in other cases, nanocrystalline. This depends on the temperature in the ring region where the reaction takes place. The temperature in this ring is lower than inside the collapsing bubble, but higher than the temperature of the bulk. In a previous review we mentioned 4 topics in materials science in which we consider the use of ultrasonic waves as superior to other methods [2]. The deposition of nanoparticles on

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substrates usually creates a smooth homogeneous coating layer that is formed on the surface. The immobilization of the nanoparticles and the strong adherence to the substrate can be the result of either the physical embedding of the particles into the surface layer, or, the nanoparticles are anchored to the surface by forming chemical bonds or chemical interactions with the substrate and cannot be removed by washing. It is worth mentioning that the number of collapsing bubbles in the solution in the presence of a solid surface is enhanced. We have found that under the ultrasonication of Mo(CO)6 in a decalin solution in the presence of an Al-MCM-41 support, the amount of nanoparticles of the Mo oxide is 7 times larger than in the absence of the solid substrate [11]. Suslick has demonstrated [12] that microjets and shock waves produced by acoustic cavitation were able to drive metal particles together at sufficiently high velocities to induce melting upon collision. Metal particles that were irradiated in hydrocarbon liquids with ultrasound underwent collisions at roughly half the speed of sound, and generated localized effective temperatures between 2600 °C and 3400 °C at the point of impact of the particles. This approach is adopted in all our experiments. We introduce in the sonication cell the solid substrate and the precursors for a well-known sonochemical reaction leading to known nanoparticles. The microjets formed after the collapse of the bubble throw the just-formed nanoparticles at the surface of the substrate at such a high speed that they strongly adhere to the surface either via physical or chemical interaction, depending on the nature of the substrate. If instead of forming the nanoparticles we purchase them and use ultrasonic radiation just for throwing, a good adherence is still obtained, but the amount of nanopaticles found on the surface is smaller by a factor of 3-4. During the last two decades, the synthesis of nanomaterials has attracted constant attention because it demonstrated a high specific surface area and new size-dependent physical and chemical properties in comparison with the bulk structures. The potential application of the nanoparticles can be significantly extended by their deposition on the different types of substrates. The interest in nanocoatings lies in the possibility to combine the properties of the two (or more) materials involved in the process, namely, the substrate and the coated layer, with emphasis on the fact that one of the materials will determine the surface properties of the composite, while the other can be responsible for other (optical, catalytic, magnetic, etc.) properties of the system [13-15]. One of the unique properties of sonochemistry mentioned above is that under certain conditions the nanoparticles are fabricated [9]. In our work, the sonochemical method was developed for coating the nanoparticles on/into the surfaces of different types of substrates: ceramics, polymers, textile fibers and fabrics, glass, etc.

2. Sonochemical Deposition of Nanoparticles on Ceramic Submicrospheres The first report on the sonochemical anchoring of nanoparticles (NP) on a ceramic body was by Ramesh [16]. The substrate in his study was silica, and later other ceramic bodies such as alumina, titania, and zirconia were also coated sonochemically by nanoparticles. The list of NPs that were coated on the surface of the ceramic microspheres include the NP of metals (Ni, Co, Fe for example), metal oxides (Fe2O3, MoO3, Mo2O5), rare metal oxides (Eu2O3, Tb2O3, Y2O3 stabilized ZrO2), semiconductors (CdS, ZnS), and Mo2C.

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The synthetic procedure is based on the introduction of the ceramic support into the solution of the corresponding precursor. The ultrasonic irradiation was then passed through the sonication slurry under an inert or oxidizing atmosphere for a specified time. This synthetic route is a single-step effective procedure. Ramesh et al. [16] deposited the amorphous nickel particles of 10-15 nm size on silica microspheres by a ultrasound irradiation of a decalin solution of Ni(CO)4 in the presence of silica microspheres, The ceramic spheres were preliminarily synthesized by conventional techniques (e.g., the Stőber method for the silica submicrospheres [17]). Cavitation induced by a high-intensity ultrasound radiation results in the breaking up of the nickel-carbonyl bonds, as well as the activation of the silica surface for the adhesion of nickel through the formation of siloxane links. An alternate mechanism involves the reaction of nickel metal with the surface silanols freed from adsorbed water by cavitation to form positively charged nickel on the silica surface. These Si-O-Niδ+ sites could serve as nucleating centers for the further agglomeration of nickel atoms. Nanophase amorphous nickel was found to be superparamagentic, whereas crystallization resulted in a ferromagnetic material showing a stronger nickel-silica interaction. These materials promise to have potential applications in magnetisms and heterogeneous catalysis. The studies of other magnetic nanometals deposited on silica microspheres were extended with Co and Fe synthesized by the sonochemical decomposition of Co and Fe carbonyls in a suspension of silica in decalin, followed by the crystallization of the resultant amorphous product [18, 19]. The metal nanocrystals that were strongly adhered to the silica core demonstrated a behavior characteristic of the ferromagnetically-ordered materials. Sonochemistry was also used for the deposition of Ni on amorphous and crystalline alumina [20]. Amorphous alumina can provide a great number of active sites for reaction with nickel, and can yield a good coating effect in which most of the nickel is adhered tightly to the alumina surface, while in the case of the crystallized alumina as a substrate, most of the nickel particles are distributed in the free spaces among the alumina submicrospheres. As compared to the unadhered nickel, the adhered nickel has a strong interaction with the alumina core, which can retard the crystallization of elemental nickel and, conversely, promote the formation of the spinel phase, NiAl2O4. Magnetization measurements show that the as-prepared sonication products are superparamagnetic due to the ultrafine nature of nickel particles. Substantial efforts were focused on the fabrication of composites of ceramic materials and rare-earth oxides. One of the reasons for this is the combination of the optical properties of rare-earth ions and the unique qualities of the ceramic materials. The special interest in the NP of these composites relates to the enhanced intensity of the emission detected for smaller size particles. We studied the ultrasound-assisted deposition and doping of the rare-earth oxides of Eu2O3 and Tb2O3 on silica and alumina surfaces [21]. The sonochemical coating of preliminarily prepared Stőber silica [17] resulted in the formation of a thin layer about 5-10 nm in size of europium oxide on the silica spheres, while the doping procedure produced a highly aggregated compound consisting of 20-40 nm spheres. The highest luminescent intensities were observed for europium and terbium doped in the NP of alumina with a dimension of 20-30 nm. The intensities are comparable, or higher, than those in commercial phosphors. These materials may have a significant application potential in optical and luminescent devices and catalysis.

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The work was extended to use different types of ceramics as substrates for deposition. As a ceramic material ZrO2 can resist very high temperatures, and its stabilized form, yttriumstabilized zirconium (YSZ), shows remarkable mechanical properties. Europium oxide was coated on the surface of submicron spherical zirconia and YSZ, which were fabricated by wet chemical methods. The coating process also used ultrasonic waves [22]. Time decay in luminescence measurements of the doped and coated materials was conducted using a pulsed laser source. Lifetimes <1.1 ms, which is the radiative lifetime of the Eu3+ ion, were detected for the doped and coated as-prepared materials. When the doped and coated samples were annealed at 700 °C, longer lifetimes were measured. The shorter lifetimes were attributed to concentration quenching. These experiments were extended for the deposition of europium oxide on titania. The Eu2O3 nanolayer on the submicron-size TiO2 particles was characterized by physical and chemical methods, and the photoluminescence properties of the composite were demonstrated. This composite can be used as a photocatalyst for the degradation of a few aromatic and chloroaromatic compounds [23]. The same sonochemical approach was later used by Guo et al. for the synthesis of another photocatalyst, TiO2/SiO2 [24]. TiO2 clusters in the size range of about 4 nm were deposited on silica particles by the high-intensity ultrasound radiation of a suspension containing TiCl4 as a precursor and silica particles in water. They found that according to the Raman spectra, a mixture of isolated TiO4 species and anatase crystallites on the surface of the composite were found. It was demonstrated that the TiO2/silica photocatalysts exhibited a higher reactivity than bulk TiO2 (P25, Degussa) in the photo-oxidation of methyl orange. Special attention is directed these days to the control of the size and shape of the nanoparticles. We have demonstrated over the years that the control of the particle size is quite easy when using sonochemistry. It is accomplished simply by the variation of the concentration of the precursor in the irradiated solution. The more dilute the solution, the smaller are the particles. We succeeded in depositing the silver and gold nanoparticles with an average size of 5 nm on the surface of silica submicrospheres by a one-step sonochemical procedure [25]. By controlling the atmospheric and reaction conditions, we could achieve a very homogeneous coating of metallic nanoparticles on the surface of the silica spheres, which was confirmed by transmission electron microscopy (TEM) (Fig. 1). The adherence of silver nanoparticles to silica was explained as due to the interaction with the surface silanol groups of the support.

Figure 1. TEM image of silver nanoparticles deposited on silica spheres.

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It was later demonstrated that the level of deposition can be improved, and we are currently able to create a very smooth coating of a monolayer of nanoparticles on the surface. This was achieved by the gradual reduction of the precursor's concentration, thus reducing the thickness of the coated layer. Using the sonochemical method, we obtained a very uniform distribution of gold nanoparticles (about 2 nm in size), also on titania (Fig. 2). In this Au/TiO2 composite, a significant decrease in the melting point of gold NP by ~850 oC was detected by differential scanning calorimetry (DSC) and the TEM method [26]. The nanoparticles coated on a ceramic substrate provide a high surface area and possess chemical and physical properties that are distinct from those of both the bulk phase and individual molecules. They have the potential for application in optics, optoelectronics, catalysis, and chemical engineering, and can be employed for the production of coated particles. It was observed that the gold-coated silica, annealed to 950 °C under a flowing stream of nitrogen, showed two phases in X-ray diffraction (XRD) measurements [27]. They are the fcc Au (powder diffraction files - PDF: 4-784) and tetragonal cristobalite (silicon dioxide, PDF: 39-1425). The crystallization temperature of amorphous silica forming the crystalline phase was reported [28] as 1300 °C. To check whether the low-temperature crystallization is due to a size effect or to the phenomenon of induced crystallization due to the gold nanoparticles, bare silica spheres were annealed at 1000 °C in a furnace under flowing nitrogen gas. XRD measurements confirmed that the silica heated to 1000 °C is amorphous in nature, contrary to the annealed sample, Au/SiO2, which shows the two crystalline phases, namely, fcc Au and tetragonal cristobalite. This means that coated gold nanoparticles induce silica crystallization at a lower temperature. Obviously, the presence of metal disrupts the amorphous network, reducing the kinetic barrier to crystallization. The effect of gold on the crystallization temperature of the support was also detected in the case of deposition on amorphous titania in ethylene glycol, and in a water solution [29], but in water it is assisted by, and combined with, the temperature reached by sonication. Titania has a lower crystallization temperature than silica, and that is why the crystalline phase of anatase can be obtained immediately after the insertion of gold into TiO2.

Figure 2. TEM image of Au-NP of diameter ≤ 2 nm deposited on the titania support.

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We used ultrasound irradiation for the coating of LiMn2O4 spinel with MgO nanoparticles [30]. Solutions of 1 M LiPF6 in a mixture of ethylene, dimethyl, and diethyl carbonates were used. It was possible to obtain LiMn2O4 particles fully covered by porous magnesia films that allow the free transport of Li ions. This composite was studied as an active mass in cathodes of standard Li electrolyte solutions for Li-ion batteries at 60 °C. Electrodes comprising LiMn2O4 modified by MgO showed a higher retention capacity compared to the electrodes comprising an uncoated active mass, especially at elevated temperatures. We suggest that the presence of an MgO film on the surface of the LiMn2O4 particles reduces the detrimental effect of the HF contamination present in LiPF6 solutions, reduces the electrodes’ impedance, and improves their kinetics. The sonochemical approach was used later by other authors for the deposition of nanoparticles on different supports For example, homogeneous Li4Ti5O12 nanoparticles were prepared via the sonochemical method by Shim and coworkers [31]. First, LiOH NP nanoparticles of about 2-5 nm were coated on the surface of TiO2 by ultrasound irradiation. Secondly, the intermediate was thermally treated at 500 oC for 1 h, resulting in the formation of Li4Ti5O12 (LTO) nanoparticles. These LTO nanoparticles had an average grain size of about 30-40 nm with excellent phase purity in good stoichiometric ratios of Li4Ti5O12. These nanoparticles with a uniform size distribution were characterized by infrared spectroscopy, Xray diffraction, and high resolution-transmission electron microscopy. Ling et al. reported on the homogeneous deposition of silver nanoparticles with an average size of 10 nm on bismuth molybdate using the ultrasound-assisted method [32]. Bismuth molybdates are popular and widely explored catalysts of commercial importance. The modification of the surface properties plays an important role in the behavior of these compounds. Sonication of the mixture of α-Bi2Mo3O12 nanorods and Ag2O in pyridine yielded a nanocomposite of bismuth molybdate with deposited Ag nanoparticles. Silver nanoparticles dispersed well in pyridine and were bound to the surface of the nanorods under the influence of the liquid bubbles generated by ultrasound in the pyridine medium. Adsorption and intercalation of pyridine prevents the aggregation and diffusion of Ag nanoparticles. By heating the sample at 450 °C, the formation of a pyridine-free nanocomposite with deposited crystalline Ag-NP nanoparticles was observed. These Agdeposited α-Bi2Mo3O12 nanorods might find applications in catalysis. One of the most advanced developments in the area of nanoparticles is the coating of semiconductor clusters on a solid support [33]. This is associated with the quantum size effect and with the existence of a relatively large percentage of atoms at the surface. Nowadays, many synthetic routes have been developed to control the size and distribution of the semiconductor nanoparticles. However, thermal treatment is necessary in some of the methods, and this is not favorable for the production of quantum-sized semiconductor particles with large surface atoms. We, for the first time, performed the deposition of ZnS semiconductor nanoparticles on the surface of submicron-sized SiO2 by the ultrasound irradiation of a slurry containing SiO2, zinc acetate, and thioacetamide in water, at near room temperature [34]. The TEM image of zinc sulfide-silica (ZSS) showed that the porous ZnS nanoparticles (diameter 1-5 nm) coated the silica (SiO2) surface as thin layers or nanoclusters, depending on the reactant concentration. Infrared spectroscopy illustrated the structural changes that occurred in the siloxane network and surface silanol groups of SiO2 upon the ultrasonic deposition of ZnS. The optical absorption of porous ZnS showed a broad band at around 610 nm, ascribed to unusual surface state transition. The absorption energy of the

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surface state transition was lower than the band gap of the ZnS particles, and probably stems from the dangling surface bonds or defects. On the other hand, the ZSS did not show the surface state transition of ZnS, probably due to the strong surface interaction with SiO2. The classical valence-conduction transition band was observed in the optical reflectance mode, and it showed an absorption edge at around (290-310 nm), which was markedly blue-shifted compared to that of bulk ZnS (345 nm). The photoluminescence spectrum of the porous ZnS and ZSS had a band with a maximum centered on 420 nm, which was similar to that of quantum dot ZnS particles. We propose that the coating process takes place via the ultrasonic cavitation-induced initial grafting of zinc acetate onto the silica surface, followed by the displacement of acetate ion by in situ-generated S2- species. The homogeneous deposition of ZnS on TiO2 nanoparticles was later obtained by Shim and coworkers through a simple one-pot reaction under multibubble sonoluminescence conditions, which can provide a very powerful and efficient coating system [35]. The coating depths of ZnS shell were in the 2–5 nm range in a core/shell type nanostructure, which is very likely to be useful for the development of inorganic dye-sensitized solar cells. The ZnScoating depths on TiO2 in this system were found to be easily controlled in the nanoscale by adjusting the amount of reactants and/or the sonication time. Yang et al. described a simple sonochemical approach for the preparation of PbS of 30 nm sized nanoparticles homogeneously coated on sub-micrometer silica spheres [36]. It was considered that the sonochemical process, in which triethanolamine acted as complex agent, played an important role in the homogeneous coating of PbS nanoparticles on silica spheres. Furthermore, stable PbS hollow structures were fabricated by the dissolution of the silica cores with HF. It was noted that both the PbS/silica core-shell, and hollow PbS structures synthesized by this method might find various applications, such as storage materials and photonic bandgap applications.

3. Ultrasound-Assisted Deposition/Insertion of Nanoparticles on/into the Pores of Mesoporous Supports The discovery of mesoporous materials (MSP) in the nineties in 2007 offered the possibility for the creation of catalysts effective in many technological processes. Their high surface area, large adsorption capacity, and ordered pore structure, make them very useful for oil refining, petrochemistry, and organic synthesis [37, 38]. At the same time, the mesoporous materials have found many applications as supports for metal oxides, organometallic compounds, and other precursors, achieving the high dispersion and functionalization of the active phase [39, 40]. We demonstrated that the sonochemical method can be used for the preparation of a number of mesoporous oxides: mesoporous silica (MCM-41), mesoporous titania, and yttria– stabilized zirconia [41-43]. The main advantage in using sonochemistry for the synthesis of MSP is the drastic shortening of the time involved in the fabrication of the products from days to hours. It has been demonstrated that the wall thickness is also greater when the sonochemical technique is used, which consequently leads to the higher thermal stability of the sonochemically-prepared MSP that was prepared by a conventional hydrothermal method [41].

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The ultrasonication of a slurry containing metal carbonyl and a high-surface-area silicabased mesoporous material (Al-MCM-41) under ambient conditions yielded, for the first time, Mo and Co oxides forming a close-packed monolayer on the inner walls of the support [11]. High resolution electron microscopy (HR TEM) studies demonstrated that the Mo oxide phase is located inside the support's pores and not outside, up to 45 wt% MoO3 loading (Fig. 3).

Figure 3. HR TEM micrographs of an Al-MCM-41 support (a) and a MoO3/Al-MCM-41 composite containing 45 wt% MoO3 (b).

The fixation of the metal oxides at the surface of Al-MCM-41 in the form of a monolayer is the result of an ultrasonically-induced chemical interaction between metal carbonyl (oxide) and the surface silica atomic layer, yielding surface silicates. This study demonstrated that the complete monolayer coverage of an Al-MCM-41 surface by the sonochemical method didn't cause any damage to the mesoporous structure, whereas the conventional methods (e.g., incipient wetness impregnation) resulted in a considerable reduction of the surface area of the support. The activity of the sonochemically-prepared Co-Mo-Al catalysts in the hydrodesulfurization reaction was 1.7 times higher than that of the commercial catalyst. The method of deposition/insertion of the nanoparticles of metal oxides on/into the mesoporous support was further extended for the preparation of Fe2O3/TiO2(MSP) catalysts, which are effective in the oxidation of hydrocarbons [44]. Layered nanoslabs of a WS2 phase with a well-defined hexagonal crystalline structure were inserted into the nanotubular channels of SBA-15, an ordered pure silica material [45]. Sonication of a slurry containing SBA-15 in a W(CO)6–sulfur–diphenylmethane solution first yielded an amorphous WS2 phase inside the mesopores. By sulfidation with 1.5% dimethyldisulfide in toluene under a hydrogen flow at 593K and 5.4MPa, the amorphous phase was transformed into hexagonal crystalline WS2 nanoslabs (as was shown by XRD, HRTEM, and selected area electron diffraction (SAED)). The WS2 nanoslabs with average slab length of 3.6 nm were distributed exclusively inside the mesopores in a uniform manner without blocking the pores, and were oriented with their edge planes towards the support surface. This study constitutes the first report of such a combination of the high loading of a well-defined crystalline catalytic phase into the nanotubular channels of mesoporous silica without blocking them. The first well-resolved HRTEM images of the well-defined

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crystalline catalytic phase (WS2) inside the SBA-15 nanotubes are presented. A Ni component was introduced into the WS2/SBA-15 composite by impregnation from an aqueous solution of nickel acetate. It increased the catalytic activity up to a Ni/W ratio of 0.4. In the hydrodesulfurization of dibenzothiophene and the hydrogenation of toluene, the activity of the optimized Ni-W-S/SBA-15 catalyst was 1.4 and 7.3 times higher, respectively, than that of the sulfided commercial Co-Mo/Al2O3. This finding illustrates the excellent potential of high loading Ni-W-S/SBA-15 catalysts for the deep hydro treatment of petroleum feedstocks. We developed a simple and effective method for the deposition/insertion of Pt, Ru, and Au nanometal catalysts on/into mesoporous TiO2. The synthesis was performed through a one-step, ultrasound-assisted polyol reduction procedure using inorganic acids and chloride salts as precursors [46]. The procedure is fast and lasts only 1 h. The high homogeneity and dispersity of the active metal phase (average size 2-2.5 nm) was revealed by HR TEM (Fig. 4). Another advantage of the sonochemical deposition/insertion of nanoparticles on/into the surface of mesoporous materials is that it takes place without any significant damaging of the host's porous structure and only a slight reduction of the surface area of the support occurs.

Figure 4. HR TEM images of Ru (a), Pt (b) and Au (c) deposited on TiO2(MSP).

The catalytic properties of these materials were evaluated in several processes, such as the wet air oxidation of organic pollutants from model waste water [46], partial oxidation of methane to syngas [47], oxidation of CO [48], modification of carbon electrodes applied to the detection of trinitrotoluene [49], and their high catalytic activity and stability were demonstrated. The catalytic properties of these catalysts were attributed to the high homogeneity of the active phase and narrow size distribution of nanoparticles obtained by the sonochemical method. Later on, we reported on an interesting finding in the study of nanoparticle insertion into a mesoporous matrix, where the nanoparticles change the pore structure of the mesoporous matrix. The nanocomposites of Ag and γ- Al2O3 were prepared via sonochemistry and subsequently calcinated under Ar(g) atmosphere at 700 °C for 4 h [50]. The sonicated product consisted of Ag nanoparticles dispersed in the bayerite [Al(OH)3]/boehmite [AlO(OH)] matrix. Upon calcination under argon, the Ag nanoparticles were found to be incorporated in a mesoporous structure of γ-Al2O3. For a solid containing 3.7 wt % Ag nanoparticles, the nanoparticles remained on the surface of mesoporous alumina. Hence, the surface area increased, as compared to pristine γ-Al2O3, whereas for 10.5 wt % Ag

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nanoparticles, the surface area decreased. HRTEM studies corroborated this fact and showed that at higher Ag concentrations, Ag nanoparticles blocked the pores and also increased the diameter of the pores of mesoporous alumina. The products with the 3.7 wt % silver concentration had uniform pores with a narrow pore size distribution, although the TEM pictures indicated wormhole channel motifs. The formation of the mesoporous structure was governed by the templating behavior of the organic groups (mainly formic acid) attached to the alumina nanoparticles. The shape of the pores closely resembled the two-dimensional hexagonal mesoporous structure with the P6mm space group, as observed from small-angle X-ray diffraction experiments. Diffuse reflection optical spectra at 27 °C showed an absorption band (419-424 nm) due to the surface plasmon of Ag inside the ceramic matrix. The 2-D mesostructure is not a common feature, and only a very few such examples are known. The question arose as to what is the mechanism of insertion of the nanoparticles into the mesopores. Since the size that the bubble reaches before its collapse is estimated as ~100 μm [51] and the pore diameter is about 3 nm, the bubble cannot be introduced inside the mesopores. We proposed two possible mechanisms. The first is based on microjets and the shock waves that result when bubbles collapse near the solid: 1) Colloidal nanoparticles are formed due to the collapse of the bubbles. 2) The nanoparticles are pushed inside the mesopores under the forces resulting from the collapsing bubbles. The collapsing bubble is asymmetric and it faces the pore of the mesoporous material. 3) The inserted nanoparticles are precipitated on the inner walls of the mesopores. Macro streaming and acoustic shock waves prevent blockage of nanocapillaries, but they direct the newly created nanoparticles towards the opening of the pore. The alternative variation of the mechanism is: 1) Chemical reactions occur inside mesopores filled with a precursor solution. The driving forces of this reaction are the shock waves created near the opening nanopores. 2) Nanoparticles formed inside the pores are bonded to the inner walls due to chemical interaction. The second mechanism gives a better explanation for the homogeneous spreading of the nanoparticles into the pores. Chen prepared metal/SiO2 MSP nanocomposites of Au, Ag and Pd by the impregnation of mesoporous solids into the corresponding metal salts, followed by sonochemical reduction [52]. From the HR TEM images, Au, Ag and Pd particles in the composites are 4, 7 and 6 nm in their mean diameter, respectively. It was found that the mesoporous silica host fulfilled three roles in the sonication processing: (1) it enabled the dispersion and encapsulation of noble metal nanoparticles, (2) it controlled the particle size, as determined by the pore dimensions, and (3) it prevented particle aggregation and provided a larger surface area for deposition. However, the procedure is rather long: the immersion process of the metal precursors in the support lasted for 3 weeks, followed by sonochemical treatment for 2-4 h.

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The sonochemical approach was also used by Mizukoshi and co-workers for the deposition of noble metal nanoparticles on the surface of maghemite [53]. Aqueous sample solutions containing noble metal ions (HAuCl4, Na2PdCl4 or H2PtCl6), polyethyleneglycol monostearate (PEG-MS), and magnetic maghemite nanoparticles were irradiated with high power ultrasound. Analysis of the products showed that the noble metal nanoparticles were uniformly immobilized on the surface of the maghemite. The authors mentioned that surfactants such as PEG-MS worked as the precursor of the reducing agent in the sonochemical reaction. Under the high concentration of the surfactant in the reaction system, the reducing rate was accelerated, and the generation of the reducing species greatly depended on the type and the concentration of the surfactant. The average diameter of immobilized Au was 7–13 nm, and the diameters of Pd and Pt were several nanometers. The diameters depended upon the concentration of PEG-MS and the concentration of the noble metal ions, but not upon the maghemite concentration, indicating the possibility of a morphological control of the products by adjusting these preparation conditions. The measurements of the average diameters and the number of immobilized Au nanoparticles obtained under various conditions suggest that the nucleation of Au does not occur on the surface of maghemite, but might occur in the homogeneous bulk solution. Yu and coworkers used a combination of the sono- and photochemical approach for the incorporation of highly-dispersed gold nanoclusters into the mesoporous TiO2 films [54]. The first step involved the sonication of a TiO2 film immersed in a gold chloride solution. This effectively removed the air trapped in the porous film matrix and drove the gold chloride into the pore channels, leading to the homogeneous adsorption of the ionic gold Au in the TiO2 mesoporous matrix. The second step takes advantage of the photocatalytic property of titania to reduce the adsorbed Au ions to Au0. As the gold clusters thus produced are stabilized by the TiO2 mesonetwork, no organic capping molecules are required. However, this method required the additional step of irradiation in UV light in methanol vapor. Recently, Zhu reported on the encapsulation of Pd nanoparticles into the channels of modified mesoporous silica SBA-15 via a facile, ethylene glycol (EG)-assisted sonochemical method [55]. The Pd/SBA-15 composite was used for the realization of a direct electron transfer of hemoglobin (Hb). Electrochemical results showed that the Pd nanoparticles in the channels of SBA-15 could enhance the direct electron transfer between Hb and the electrode surface. The composite-modified electrode displayed excellent electrochemical behavior. The sensor fabricated by the composite showed an excellent response to the reduction of hydrogen peroxide.

4. Sonochemical Coating of Polystyrene Spheres The use of a polymer matrix as the environment for in situ nanoparticle growth combines, synergistically, the properties of both the host polymer matrix and the discrete nanoparticles formed within it. The nanoparticles of metals and metal oxides embedded in polymer matrices have attracted increasing interest because of the unique properties displayed by materials containing such nanoparticles. Such composite materials are expected to have novel magnetic, optical, electrical, catalytical, and mechanical properties. In our previous research we studied metal-polymer composites (i.e. Fe-polystyrene [56], Ni-polystyrene [57]) prepared by the sonochemical method. The metal nanoparticles (5 - 10 nm in size) were well dispersed

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in polystyrene (PS) and demonstrated magnetic properties. However, the nanoparticles were sonochemically inserted into polymer, either at the polymerization stage [56] or through the ultrasound irradiation of the polymer solution in a suitable solvent [57]. Breen et al. grew zinc sulfide films on carboxyl-modified PS microspheres through a sonochemical reaction conducted in an aqueous solution containing zinc acetate and sulfide, released through the hydrolysis of thioacetamide [58]. As a result, optically hollow spheres with uniform shapes were obtained. Intact, hollow ZnS shells were formed by heating the particles to remove their polystyrene cores. Interference patterns observed by optical spectroscopy demonstrate that particles consisting of two layers of material with distinctively different refractive indices were produced. The compression of the ZnS lattice from that of the bulk material indicates prominent strain forces which could have important effects on the optical qualities of thin film refractive materials. The authors of [59] reported on the synthesis and characterization of catalytically important noble monometallic colloids prepared by various chemical and sonochemical methods. These metal colloids are then adsorbed onto suitably functionalized PS microspheres. The metal-immobilized microspheres are reacted with a linker such as 4-mercaptobutyl phosphonic acid, and subsequently used for growing multilayers We succeeded in coating the PS microspheres by the sonochemical method [60]. The noble metal nanoparticles (Ag, Au, Pd and Pt) were synthesized and uniformly distributed on the PS surface by a one-step, ultrasound-assisted procedure. In Figure 5 the TEM images of PS coated with gold nanoparticles are demonstrated with different magnifications. The distinct difference between this report and previous reports on the deposition of these nanoparticles on silica spheres and other surfaces is that in all previous depositions the silver, gold, nickel and air-stable iron nanoparticles were deposited on the substrate surface as amorphous products. Carrying out the same sonochemical reaction in the presence of PS spheres yielded Pt, Pd, Ag, and Au nanoparticles in their crystalline form.

Figure 5. TEM images of PS spheres coated with gold nanoparticles (magnification X8000 and X20000, correspondingly).

The mechanism of coating metal nanoparticles on silica spheres and PS spheres might be different due to the differences in their surfaces and the chemical interactions between the particles. As we mentioned previously [16, 25-27], chemical interactions between silica and

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supported metal-like nickel, silver or gold led to the formation of a chemical bond between SiO2 and nanostructured Ni, Ag or Au through the reactive surface silanols, which in turn led to the production of an amorphous product after sonication. On the other hand, when we used the PS spheres as a substrate, chemical interactions between Ag or Au and PS are different. During the sonochemical reaction, PS spheres do not form chemical bonds, but rather become coated as a result of the sintering of the particles and/or interparticle collisions between PS and Au, Ag, Pt, and Pd, which lead only to the melting or softening of the polymer. The metallic particles thus "dissolve" partially in the polymer. This leads to a better heat exchange between the polymer and the metallic particle. Thus, the local temperature is raised and reaches the crystallization temperature. In the coating of the ceramics there is no softening of the outer ceramic layers. The contact time is shorter, which does not allow the local temperature to rise to that of the crystallization temperature.

5. Sonochemical Coating of the Polymer Beads The interest in nanocoatings relies mostly on the combination of the properties of the two (or more) materials involved. In view of the importance of the influence of the surface structure of nanoparticles on their properties, much effort has been invested in order to create new classes of materials through the modification of surface structure by anchoring metals to polymers. Conceptually, the various approaches can be divided into two groups: -

Entrapment of deagglomerated nanoparticles within a forming polymer matrix; Use of solid polymers as substrates for immobilization/anchoring of forming nanoparticles.

The principal requirements for the synthesis of metal-polymer nanocomposites are small dimensions, regular shape, and the uniform size distribution of the metal nanoparticles. However, the dispersion of nanoparticles in a polymer using conventional compounding techniques is a very difficult task because of the strong tendency of nanoparticles to agglomerate. The interface between nanoparticles and the polymer matrix plays an important role in the coating and incorporation of inorganic guests on/into the organic substrates. Thus, the solid-state modification of the polymer with alkoxysilane groups was applied in some advanced studies to provide the covalent bonding of inorganic nanomaterials to the polymer [61, 62]. Nevertheless, the deposition of nanoparticles on the surface of ready state polymers is still a challenge. Recently, we reported on the deposition of silver nanoparticles on Nylon 6,6 solid state polymer beads by a simple and efficient method of ultrasound irradiation [63]. Ethylene glycol was added to the reaction as a polyol reducing agent The incorporation of metallic silver on the 3 mm chips reached the 1 wt% of Ag. The physical and chemical analyses showed that nanocrystalline pure silver of 50-100 nm in size is finely dispersed on the polymer without damage to the Nylon 6,6 structure. The XRD patterns of the product demonstrate that the silver deposited on nylon is of a crystalline nature and the diffraction peaks match well those of cubic Ag phase. The scanning electron microscopy (SEM) method provides an image of the silver particle distribution on the nylon surface (Fig. 6). The

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polymer is uniformly coated by silver nanoparticles 50–100 nm in diameter. Large agglomerates were not observed on the composite surface. The coating is stable, and the concentration of silver in the polymer does not change after 10 washing cycles.

Figure 6. SEM image of a nylon surface coated with silver.

The microtome of the Ag/Nylon composite prepared for the TEM measurement allows the observation of the distribution of silver particles inside the polymeric chip (Fig. 7). It was detected that smaller silver nanoparticles of about 20 nm in size have penetrated the surface and are distributed inside the polymer grains.

Figure 7. TEM image of a silver–nylon composite (cutoff section).

This Ag-Nylon nanocomposite was used as a master batch for the production of nylon yarn by the consequent melting and spinning processes (Fig. 8). The fabric knitted from these yarns demonstrated very good antimicrobial properties against gram positive and gramnegative bacteria.

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Figure 8. Photo of the nylon chips before and after the sonochemical coating with silver, and Ag/Nylon fibers spun from the Ag/Nylon composite.

This method was extended to the deposition of silver nanoparticles on other polymers. Thus, ultrasound irradiation was used for anchoring silver nanoparticles with an average size of ~50 nm onto the surface of poly(methyl methacrylate) PMMA chips and spheres [64]. The microtome technique revealed the penetration of the silver nanoparticles inside the polymer. The crystalline nature of silver nanoparticles in the Ag-PMMA composite was confirmed by XRD. The silver-deposited PMMA chips (loaded with 0.01–1.0 weight percent of silver) were successfully homogenized in melt by extrusion and then injection molded into small, disc-shaped samples. These samples were analyzed with respect to their directional spectral optical properties in UV, VIS, and IR spectroscopy. The results of the spectral optical characteristics, particularly evident in the UV/VIS, show an increasing absorption with an increase in the filling degree of silver nanoparticles at a very low weight percent. We presume that the mechanism by which the Ag nanoparticles are bonded to the PMMA surface is the same as it was with the Ag-Nylon composite. It is related to the microjets and shock waves created near solid surfaces after the collapse of the bubble. These jets push the nanoparticles toward the PMMA surface at very high speeds. When the nanoparticles hit the polymer surface, sintering of the particles and/or interparticle collision between PMMA and Ag particles changes the surface morphology and reactivity, resulting finally in the coating of these particles. It is worth noting that the products of many sonochemical reactions are in the form of amorphous nanoparticles. The reason for the amorphicity of the products is related to the high cooling rates (>1011 K/s) obtained during the collapse of the bubble, which does not allow the products to organize and crystallize. These high cooling rates result from the fast collapse that takes place in less than 1 ns [9]. As mentioned previously, we observed the deposition of amorphous nanoparticles (Ag, Au, Pt, Ru) on the surface of the ceramics [16, 18, 19, 25-27, 29, 46, 47], but observed crystalline nanometals upon the deposition on the polymers/polystyrene [60] and nylon [63, 64] substrates. This difference is due to the different types of interaction between the anchoring species and the substrates. The adhering of metal atoms on the top of the ceramic substrate takes place through covalent bonding with the surface functional groups (silanols in the case of silica and with surface hydroxyl groups in the case of titania), but no chemical interaction was revealed between nanoparticles and polymers. The polymers are coated due to the melting or softening of the polymer surface as a result of the particles and/or interparticle collisions with the substrate surface. The strong

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collision of the particles with the polymeric surface pushes the particles into the polymer body, and thus the particles "diffuse" partially in the polymer. Indeed, 20 nm silver particles were found at the center of the 2 mm polymer bead upon its microtoming. We succeeded in depositing silver nanoparticles on the surface of porous polypropylene (PP) beads by an ultrasound-assisted reduction method [65]. The sonochemically-prepared silver PP composite demonstrated good stability and high antibacterial activity against grampositive and gram-negative microorganisms. The PP is known as a non-polar polymer that does not contain any functional groups. Polypropylene (PP) is one of the most widely used polymer materials. There is a high demand for PP with antimicrobial properties for use in a variety of applications, e.g., in appliances, as filters, in packing, and in the textile industry in various forms such as nonwoven films and fibers, etc. [66] The synthesis and deposition of silver on the PP beads were performed by the one-step sonochemical irradiation of a silver nitrate aqueous solution with the addition of ethylene glycol as a polyol reducing agent. The crystalline structure of nanosilver was confirmed by XRD. To reduce the size of the silver particle on the PP surface, we added water-soluble polymers, poly(ethylene glycol) (PEG), poly(vinyl alcohol) (PVA) and poly(vinyl pyrrolidone) (PVP) to the precursor mixture. With PVP, a highly homogeneous distribution of silver nanocrystals, 50 nm in size, on the PP surface was achieved (Fig. 9).

Figure 9. SEM image of an Ag/PP composite.

To reveal the mechanism of the silver anchoring to the inert polymer, Raman spectroscopy was applied. The Raman spectrum of silver-coated PP shows peaks at 1344 and 1570 cm-1, characteristic for pristine carbon analogous to those of graphite [67] (Fig. 10). These peaks are absent in the original PP beads and in the beads after sonochemical irradiation under similar experimental conditions, but without the addition of the silver ions. It is well known that the surface-enhanced Raman scattering (SERS) obtained by the deposition of silver colloids on the pure carbon is widely used for the characterization of carbon materials because it makes the carbon vibration modes Raman active [68, 69]. Thus, it seems reasonable that the appearance of the high-intensity bands characteristic for carbon

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after coating PP with nanosilver was caused by the localized melting of the polymer at their points of contact with silver nanoparticles.

Figure 10. Raman spectra of silver coated PP

The formation of the carbon moiety on the surface is attributed to sonochemical cavitation and to the microjets mentioned above. The collision of the silver particle with the PP can lead to the local melting of the PP at the contact sites. The locally-developed high temperature can also cause the thermal degradation of polymer chains, resulting in the formation of a small amount of pure carbon. This mechanism of silver deposition on the polymers can also explain the strong anchoring of the silver nanoparticles to the PP surface. We checked the stability of silver coating in water at 70 oC and found that the silver concentration in the polymer did not change, even after 10 washing cycles. The sonochemical method was also applied for the deposition of metal oxide nanoparticles on polymers. For instance, zinc oxide crystals were obtained by the sonochemical irradiation of a mixture containing PMMA, zinc (II) acetate dihydrate, ethanol, water, and 24 wt.% aqueous ammonia for 2 h, yielding a PMMA–zinc oxide composite [70]. By controlling the atmosphere and reaction conditions, we could achieve well-adhered zinc oxide crystals on the surface of the polymer. The resulting zinc oxide–PMMA composite was characterized by physical and chemical methods. The zinc oxide-deposited PMMA chips (loaded with 0.03–1.0 wt.% ZnO) were successfully homogenized in melt by extrusion and then injection molded into small, disc-shaped samples. These samples were analyzed with respect to their directional spectral optical properties in UV, vis and IR spectroscopy. The results of the spectral optical characteristics show an increasing absorption with an increase in the content of zinc oxide particles at a very low weight percent.

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6. Coating of Natural and Synthetic Textile Fibers via Ultrasound Irradiation The growing interest in textile materials with antimicrobial properties has stimulated an extensive search for new technologies for the modification of wool fibers and the production of safety yarns [71-73]. Different types of antimicrobial treatments have been studied for the protection of wool products from damage caused by pathogenic microorganisms. Among these treatments, there is the coating of wool with resinbonded copper-8-quinolinolate, chlorinated phenol and its derivatives, sodium dichloroisocyanurate, quaternary ammonium compounds, metal ions, and organic tin compounds in finishing processes [74-76]. A biotemplate redox technique has been employed for the deposition of silver nanoclusters on another type of natural fiber—silk fibroin fiber [77]. We demonstrated the deposition of small silver nanoparticles on woolen fabrics by the coating of neat fibers with silver nanoparticles via ultrasound irradiation [78]. We suggest that this product can serve as an antimicrobial fabric. The process is performed in a onestep sonochemical procedure with slurry-containing wool fibers, silver nitrate, and ammonia, in an aqueous medium. The produced silver-coated wool fabrics maintained the high flexibility and elasticity typical of wool. The studies of the silver-coated wool fibers by physical and chemical methods have demonstrated the presence of highly dispersed XRD amorphous silver nanoparticles (~5 nm) incorporated into the natural wool (Fig. 11). Some of the silver particles are aggregated into clusters and located mainly at the fiber crossovers. X-ray photoelectron spectroscopy (XPS) studies demonstrated that silver nanoparticles are attached to the keratin fibers as a result of the interaction between both Ag+1 or Ag clusters, and sulfur. The origin of these sulfur atoms is most likely the partial disconnection of the S-S bond in the keratin fibers. The stability of the coating is satisfactory: even after several cycles of thermal treatment and simulating laundering, no change in the silver concentration was detected.

Figure 11. HRTEM images of silver-coated wool fibers: (a) fiber image (scale bar 100 nm) and (b) individual particles on the surface of the wool fiber (scale bar 5 nm).

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The development of new clothing products based on the immobilization of nanophased materials on textile fibers has recently been of increasing interest to both the academic and the industrial sectors [79, 80]. Nanostructured silver deposited on textile substrates can be used to make smart functional textiles, which have great potential for applications ranging from antibacterial materials to conductive shields and electronic sensors [81]. Antibacterial textiles are also in demand for wound-healing. Antibacterial wound-closure products, bandages, and dressing are an important part of the wound care market, which is expected to grow at a compound annual growth rate of 11% through 2011 [82]. Different methods have been used for the deposition of silver nanoparticles on fabrics. For example, a poly(ethylene terphthlate) fabric (meadox double velor) was coated with metallic silver using a patented ion-beam-assisted deposition process developed by the Spire Corporation (Bedford, MA) [83]. Other methods were constant pressure padding [84] and the reduction of silver ions by ethanol or iso-propanol [85]. Some of the methods are based on reactions in the liquid medium and require surfactants, reducing agents or templates for the synthesis of silver nanoparticles, resulting in the presence of impurities in the final products. This method has some disadvantages with regard to the environment. We applied the sonochemical method for the deposition of silver nanoparticles onto the surface of different fabrics (nylon, polyester and cotton) [86]. The advantage of the process is that this is a simple, efficient, one-step synthesis. The process produces a uniform coating of silver nanoparticles on surfaces with different functional end groups. Physical and chemical analysis has shown that nanocrystalline pure silver, 80 nm in size, is finely dispersed on the fabric’s surface without any significant damage to the structure of the yarn (Fig. 12).

Figure 12. (a) HR-SEM images of pristine fibers, (b) fabrics coated with Ag nanoparticles at a low magnification, (c) fabrics coated with Ag nanoparticles at a high magnification.

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The coating is stable on the fabric for at least 20 washing cycles in hot (40 oC) water. The mechanism of the strong adhesion of silver nanoparticles to the fibers is based on the melting point of the substrate due to the high rate and temperature of the silver nanoparticles thrown at the solid surface by sonochemical microjets. The performance of coated fabrics with nanosilver as an antibacterial agent was investigated, and the excellent killing effect of bacteria was demonstrated. The coated fabrics can have potential applications in wound dressing, hospital sheets, doctors apron, and as medicinal bandages. The coated fabrics can also be recommended for the purification of medical and food equipment, domestic cleaning, etc. Furthermore, the materials involved in the preparation are cheap, non-toxic and are commonly available. In light of new demands for environmental protection, the use of nanosilver-containing products is strictly limited by the FDA [87]. It is based on the determination that silver ions can't be easy removed from living organisms and the human body. Thus, we directed our efforts towards the replacement of silver in antibacterial textiles with other antibacterial agents, such as ZnO and MgO. These oxides are generally regarded as materials safe for human beings and animals, and are used extensively in the formulation of personal care products [88]. Using a simple, one-step ultrasound assisted procedure, we succeeded in synthesizing ZnO nanoparticles and depositing them on cotton sheets and bandages. The ZnO cotton composite demonstrated an excellent antibactericidal activity against gram-positive and gramnegative microorganisms. A similar method was also used for the deposition of MgO and CuO nanoparticles on cotton bandages. These materials are now under investigation.

Acknowledgement Part of the research reported in this review was conducted within the framework of the Integrated Project NAPOLYDE, funded by the EUROPEAN Commission, Contract no.: NMP2-CT-2005-515846. Other results were obtained during the research performed as part of the activities of the LIDWINE consortium, contract No. NMP2-CT-2006-026741. LIDWINE is an IP project of the 6th EC program. We also acknowledge the support of the Chief Scientist of the Ministry of Industry, Trade, and Employment through the MAGNET NFM Program.

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In: Surface Coatings Editors: M. Rizzo and G. Bruno, pp. 237-257

ISBN: 978-1-60741-193-2 © 2009 Nova Science Publishers, Inc.

Chapter 9

POST-CONSUMER PET AND POST-CONSUMER PET-CONTAINING MATERIALS FOR FLAME SPRAY COATINGS ON STEEL: PROCESSING, PROPERTIES AND USE V.F.C. Lins1,*, J.R.T. Branco2 and C.C. Berndt3 1

Corrosion and Surface Engineering Laboratory, Engineering School, Federal University of Minas Gerais, Belo Horizonte, Brazil 2 Laboratory of Surface Engineering and Modifications, Technological Center of Minas Gerais Foundation –CETEC, Belo Horizonte, Brazil 3 James Cook University, School of Engineering, Townsville, Austrália

Abstract Yet with the generation of large quantities of thermoplastics, the use of the thermal spray method is a logical and efficient means of recycling thermoplastics, thereby reducing the accumulation of polymer residues. Poly (ethylene terephthalate), PET, has excellent mechanical and chemical properties, and is a potential corrosion barrier since it presents low permeability to gases and solvents. Solutions of polymer recycling using the postconsumer PET to produce polymeric and composite coatings on steels in order to improve the tribological and chemical properties of steels are reported. Thermal sprayed and refused PET coatings, blend coatings of PET and the copolymer of ethylene and methacrylic acid, EMAA, and PET-based composite coatings were produced. Quenched PET blends with 80% PET and 20% EMAA and quenched PET coatings showed corrosion resistance in a salt spray chamber, small friction coefficient, and adhesion, which are necessary for the application of polymeric films as protective coatings against corrosion and wear. Peeling and swelling of the thermally sprayed PET coatings did not occur in the immersion tests in gasoline, diesel oil, and alcohol for a period of 60 days. The higher corrosion resistance in

*

E-mail address: [email protected]. phone + 55 31 34091775, faxsimile + 55 31 34091826, Correspondence to: Corrosion and Surface Engineering Laboratory, Engineering School, Federal University of Minas Gerais, 35 Espirito Santo Street, Zip code 30170-030, Belo Horizonte, Brazil.

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Key words: Recycling, coatings, polymer blends and alloys

1. Introduction The use of thermoplastics in Surface Engineering is increasing, and the process technique commonly known as thermal spray is suited to polymeric coatings on metal. The process of thermal spray is an established technique that has been used for over 100 years in the manufacture of metal and ceramic coatings. However, the application of thermal spray to thermoplastics is a relatively newcomer to the technological field, with the original patent being established in the mid-1980’s [1]. The employment of thermal spray to thermoform appropriate plastics is relatively limited [2-5]. Yet with the generation of large quantities of thermoplastics, the use of the thermal spray method is a logical and efficient means of recycling thermoplastics thereby, reducing the accumulation of polymer residues. Several variations of this technology are described within the patent literature [6-12]. One concern, addressed in this chapter, is thermal decomposition of the thermoplastic feedstock while in contact with the torch of the thermal spray. It has been demonstrated that poly (ethylene terephthalate), PET, has interesting properties that make it a good candidate for further exploration and commercial support as a protective coating for metal against corrosion and wear [13, 14]. The major sections of this chapter deal with thermal processing of post-consumer PETbased coatings, tribology, weathering resistance of PET and PET-EMAA blends, and corrosion resistance of PET-based coatings. This chapter presents a personal view and is based largely on studies conducted by the authors and their co-workers.

2. Material Poly (ethylene terephthalate), PET, is a polyester resin derived from a reaction between terephthalic acid and ethyleneglicol [15]. PET has excellent mechanical and chemical properties [3, 5, 16-19], and is a potential corrosion barrier since it presents low permeability to gases and solvents. PET is a thermoplastic material largely used in the form of fibers, sheets and films. In Europe, in 2000, the production of PET for the manufacture of bottles was 8×105 tons, for fibers it was 2.7×105 tons, for moldings 3×105 tons, and for sheets 2×105 tons. Its consumption for packing applications has increased in the last three decades. As rigid packing for example, in 2007, consumption of PET reached 431 ktons in Brazil, according the data furnished by the Brazilian Association of PET Producers. PET recycling represents one of the most successful and widespread examples of polymer recycling. In 1998, 10.4×104 tons of PET were recycled in Europe compared to 3.6×104 tons in 1995 . In Brazil, in 2007, 231 ktons of the post-consumer PET bottles, 53.5%

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of the production of beverage bottles, was recycled, compared to 18 ktons (25.4%) in 1995 (Figure 1). Recycling, originally introduced in developed countries in the steel and aluminum industries, also created viable economical options in the paper, glass and plastic sectors.

250 200 150

Recycled PET (ktons)

100 50 2006

2004

2002

2000

1998

1996

0 1994

Recycled PET in Brazil (kton)

Recycled PET (ktons)

Year

Figure 1. Recycling of PET in Brazil.

The copolymer of ethylene and methacrylic acid, EMAA, was used to explore its versatility for use in different environments [4, 20-23], and its adhesive properties, already known, have made it useful for application on surfaces that require adhesion, such as in painting, in skin protection, in medical application [23], and in coated metal. PET-EMAA blends were studied as polymeric coatings on carbon steel [24]. The mechanism of interfacial interaction between EMAA and carbon steel consists of the interaction between the hydrogen of the hydroxyls of Fe(OH)2 on the steel surface and the oxygen of the carboxyl [4, 21]. The carboxyl disrupts the linearity of the polyethylene backbone and, as well, interferes with chain alignment and reduces the total crystallinity, as in the case of other ethylene copolymers The acid functionality allows the polymers to bond strongly to polar substrates. Steel coated with composites of PET matrix with the addition of glass powder and flake, and zinc powder were also produced using a re-fusion technique for applications in the acid aqueous media [25]. Glass powder, due to its porous ceramic nature and zinc powder due to its high hydrogen over voltage, can adsorb atomic hydrogen and delay hydrogen access to steel. The substrate used was a carbon steel with 0.20% w/w. Composite coatings with a polymeric matrix were produced with 0.1%, 1.0% e 10% w/w of post-consumer glass powder, zinc powder, and glass flakes. The glass and zinc powders were classified in vibratory sieving to obtain a material with particle sizes smaller than 210, and 50 μm, respectively. Glass flakes were 3.5-5.5 μm in thickness and 50±10 μm in diameter (Figure 2). The PET powder particles used, usually exhibited irregular shapes and the particle size distribution was not homogeneous [26]. These irregularities imply that the heat generated by the flame was insufficient to promote complete coalescence of all the in-flight particles; hence only the small particles were melted completely.

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All thermoplastic materials are, in principle, recyclable by either mechanical or chemical means. Branco et al [16] employed a new recycling route for PET beverage bottles, through their use as feedstock material for thermal spray coatings.

Figure 2. Glass flakes.

One recycling route used for PET beverage bottles was crushing and ball milling. Postconsumer PET bottles were separated by color, and the transparent bottles were washed, cut and submitted to crushing using a reduction crusher with iron rolls. The crushed mass was sieved until 90% of particles under 4 mm were obtained. The ceramic ball mill was 10.5 cm in diameter and 12.5 cm in height, and the ceramic balls filled 67% of the mill volume. The ratio of PET load/ceramic balls was maintained constant during the milling process. The powder was classified in vibratory sieving to obtain a material with particle sizes smaller than 210 μm. Carbon steel (0.2%C w/w) sheets generally used as substrate were 10 cm in length, 8 cm in width, and 3 mm in thickness. The preparation of samples consisted of blasting, chemical cleaning, and pickling of steel samples. The abrasive material used during surface preparation was aluminum oxide, which provided a good roughness and increased the contact area between the substrate and the polymer. The chemical cleaning of steel samples was carried out by using an industrial cleaner. The samples were then submitted to pickling with a hydrochloric acid solution with hexamethylene tetramine used as inhibitor.

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3. Thermal Processing of Post-Consumer PET-Based Coatings Powder Consolidation under Thermal Heat [24] In the re-fusion technique, the steel samples were placed in a furnace at temperatures higher than the melting temperature of polymers, during 10 minutes. The furnace was opened and the polymer powder was scattered on the steel surface to obtain a homogeneous film. The samples then remained in the furnace at the polymer melting temperature during 10 minutes. In cases where the powder was a blend, a lower polymer melting temperature was used. After fusion, the samples were air-cooled, or quenched in an ice-water mixture at 0ºC. Figure 3 shows the re-fused air-cooled 80%PET-20%EMAA (w/w) coating (Figure 3a) and the re-fused quenched 80%PET-20%EMAA coating (Figure 3b). The re-fused air-cooled 80%PET-20%EMAA coating showed a milky color due to the crystallization process. The thickness of the coating is an important parameter in the re-fusion technique. Fast cooling was observed in sheets of high thickness where heat dissipation arose from heat conduction. On the other hand, thin sheets dissipated heat by radiation and a convection mechanism in air, which were less effective in cooling than heat conduction.

Figure 3. Re-fused air-cooled 80%PET-20%EMAA coating (a) and the re-fused quenched 80%PET20%EMAA coating ( b).

Thermal Spraying Thermal spray processes use combustion, electric arc or plasma as a heat source to melt materials produced in the form of powder, wire or rod. The molten particles produced during the spraying are accelerated by the combustion or the plasma gas jet toward a substrate where they solidify and accumulate forming a coating. Thermal spray is used in industrial applications to improve surface protection against corrosion and wear. Some polymers have been tested as coatings [2-4, 16, 17, 21, 27-31] and its commercial use started in the 90’s, with EMAA, a co-polymer between ethylene and methacrylic acid, being the main feedstock. The first openly-published research on the thermal spraying of PET was initiated

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independently in the mid 90’s by Branco and co-workers, at CETEC in Brazil [17] and by Bao and Gawne [31] at South Bank University in London, the UK, 1995. The highly sensitive thermal behavior of polymers implies that special care needs to be taken in comparison to metal and ceramic feedstock. The lower melting and decomposition temperatures, as well as substantially lower thermal conductivities, suggest that the process windows for polymer feedstock may be quite narrow, thereby resulting in a need for more precise process control [32-35]. Results of infrared spectroscopy showed that PET degradation, due to thermal spray processing, may be considered negligible [16]. However, this processing may introduce changes in the polymer structure by promoting an amorphous phase. During combustion thermal spraying, the feedstock particles may observe a torch flame atmosphere temperature as high as 2500°C; therefore, the polymer may be subjected to oxidation and thermal degradation, with or without chain scission. The latter usually promotes the breakage of covalent bonds in the main chain, which maintains the chemical structure but reduces the molecular weight. On the other hand, oxidation can modify the chemical structure of the polymer. A prior study [16] examined the mechanical processing of post-consumer beverage bottles, which were then combined with a thermal spray process, as an option for PET recycling. The thermal spray coatings were examined by Fourier transform infrared spectroscopy and X-ray diffraction while the tribological behavior was investigated by means of pin-on-disc testing. The evolution of tribographic features was monitored by light and scanning electron microscopy; whereas mass loss and friction forces were measured as a function of sliding distance. The coatings, in the as-sprayed condition, were adherent, dense and opaque, without cracks and presented a rough surface. The latter characteristic decreased under quench conditions. Samples of PET, from the beverage bottles, turned from transparent to a milky color during the milling operation, which was interpreted as crystallization of the polymer. X-ray diffraction and thermal analysis of the powder indicated that it exhibited a higher crystalline phase content than the PET of post-consumer bottles, as shown in Table 1 [16]. Table 1 shows glass transition temperature (Tg), melting temperature (Tm), degradation temperature (Td), and crystalline fraction χc (%) of post-consumer PET bottle, PET powder, and thermally sprayed PET coating. Thermal spraying melted and cooled the PET powder, thereby resulting in an increase of the amorphous phase content (Table 1)[16]. Table 1. Thermal analysis results of PET bottles, powder and thermally sprayed coating Tg (ºC)

Tm (ºC)

Td (ºC)

χc (%)

Post-consumer PET bottle

76

247

434

24

PET powder

110

243

444

41

Thermally sprayed PET coating

75

245

445

21

Sample

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This is consistent with the fact that thermal spray has a cooling rate of 104 K/s, leading to a decrease in crystallinity with respect to the powder feedstock. The quenching treatment further increased the amorphous character of the coating. The PET chemical structure is reported in literature [26]. Infra-Red spectrum of a postconsumer PET beverage bottle showed no significant difference with respect to a spectrum of a thermally sprayed coating [16]. In these spectra, the same characteristic wavenumber of PET functional groups can be identified [36] , as shown in Table 2, with the same absorption bands as those presented by virgin PET [37]. There was no chemical degradation of PET, detected by infrared spectroscopy, due to grinding and thermal spraying. Table 2. Infrared spectroscopy peaks of the poly(ethylene terephthalate), PET Wave number (cm-1) 3440 2980-2920 1730 1260 1130 730

Functional group O-H C-H C=O C(=O)O O-C-C C-H

4. Tribology Friction of PET Coatings Friction, a very common phenomenon in daily life and in industry, is governed by the processes occurring in the thin surface layers of bodies in moving contact. The simple and fruitful idea used in studies of friction is that there are two main non-interacting components of friction: adhesion and deformation. Such an approach is correct for all materials including polymers. Behavior of polymers has distinguishing features, some of which were described by Briscoe [38]. Three basic elements involved in friction are interfacial bonds, their type and strength; shearing and rupture of rubbing material inside and around the contact region, and the real contact area [39]. When two surfaces come into contact, their opposing asperities, with maximum height, come into contact. As the load increases, the new pairs of asperities of lesser height make contact forming individual spots. The overall area of these spots is known as the real contact area, RCA. Polymer surfaces have smaller asperities of nanoscale size, which are result of the molecular and supermolecular structure of polymers. In this case, the RCA should be estimated based on a two-level model of Archard type [40]. Analysis of the two-level model has shown that the highest asperities of the first level (roughness) come into contact and form the individual contact spots. Each of the spots consists of a set of smaller spots, and the total area was conditionally named the physical contact area. The area results from the contact of micro asperities and it is less than the real contact area by an order of magnitude. It was shown that in the range of a moderate load (0.02-1N), the friction coefficient decreased with the increase of the load. This behavior may be explained by elastic

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deformation of the surface asperities. On the other side of the proportionality range, the friction coefficient increased with the increase of the load. This is often explained by plastic deformation of asperities in contact. The friction coefficient passes a minimum, which corresponds to transition from an elastic contact to a plastic one. The load can vary the temperature of viscoelastic transitions in polymers and thereby vary the mechanism of friction. It was verified that PET coatings presented low friction coefficients, under 0.15, and negligible wear rates against steel, revealing promising tribological applications [39]. During pin-on-disc testing, the friction coefficient of PET varied between 0.07 and 0.2 (Figure 4). The interruption of sliding, followed by a gas blasting over the contacting surfaces, promoted a significant decrease in friction force [16]. The friction force results showed differences among the different PET materials. In the post-consumer condition the friction force starts at a lower level and slightly increases with time while in the as-sprayed and as-quenched condition the friction force starts at higher values and quickly increases during the first few turns, followed by a sharp fall (Figure 5). During the extent of testing, friction fluctuation was highest for the as-sprayed condition and lowest for the post-consumer PET.

Figure 4. Friction coefficient of PET and quenched PET coatings on carbon steel.

Figure 5. Friction force of PET coating on steel.

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Effect of EMAA Addition on Friction Coefficient of PET Coatings EMAA coating samples, which were produced using the fusion technique and the thermal spraying process, showed the greatest friction coefficient value (Figure 6), with respect to the coatings studied. EMAA is less stiff and more elastic than PET. This elasticity increases the sliding resistance of the pin. The results indicated that addition of EMAA did not significantly increase the PET elasticity, and the blend-coating samples presented a similar behavior as did the PET coating samples.

Figure 6. Friction coefficient of EMAA and PET-based coatings.

Sliding Wear of PET Coatings It is generally recognized that the most common types of sliding wear modes of polymers are abrasion, adhesion, and fatigue. The main aspect of abrasive wear is related to the cutting or plowing of the surface by harder particles or asperities. There are two distinct modes of deformation when an abrasive particle acts on the plastic material. The first mode is plastic grooving, often referring to as ploughing, in which a prow is pushed ahead of the particle, and material is continually displaced sideways to form ridges adjacent to the developing groove. No material is removed from the surface. The second mode is called cutting, because it is similar to micro machining and all the material displaced by the particle is removed as a chip. The fundamental mechanism of this wear is adhesion. This wear process evolves in the formation of an adhesion junction: its growth and fracture. Bely et al [41] noted that the transfer of polymer is the most important characteristic of adhesive wear in polymers. If the transferred polymer film is carried away from a steel surface and is newly formed, the wear rate increases. In the case that the film is held in place, friction occurs between similar

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material that may result in seizure. Another consequence of polymer transfer is a change in the roughness of both surfaces in contact. A friction contact undergoes cyclic stressing at rolling and reciprocal sliding. Each asperity of a friction surface experiences sequential loading from the asperities of countersurface. As a consequence, two varying stress fields are brought about in surface and subsurface regions with different scales from the diameter of apparent contact area in the first case, to that of local contact spot in the second. These fields are responsible for material fatigue in the regions that lead to the generation and propagation of cracks and the formation of wear particles. This process is called friction fatigue, and the loss of material from solid surfaces owing to friction fatigue is referred to as fatigue wear. The fatigue cracks are initiated at points where the maximum tangential stress or the tensile strain takes place. With a low friction coefficient, the point where the shear stress is maximum is located below the surface ( f < 0.3). When the friction coefficient increases ( f > 0.3), the point emerges on the surface. The evolution of wear damage during the pin on disk sliding is also documented by the tribographs taken during testing [16]. The PET surfaces were mechanically deformed by the sliding pin, and the widths of the wear tracks created increased with time. Scratches are the predominant tribographic features inside the wear track, and they are continuous and concentric. Debris was observed in the wear tracks, mainly at their outside edges. This debris accumulated on the pin surface, to where it became, and from where it was periodically released, with flaky shapes, and a globular nature, and with equivalent diameters ranging from 10 to 100 μm . In the groove surface, regions appeared to be smooth , while others were wavy and had rippled like features, with a waviness that resembled the ridged abrasion patterns found in rubber. Discontinuous deep grooves were observed after 2000 revolutions. Scratching is a feature that, in principle, results from abrasion, caused by either a cutting, fracture or ploughing mechanism. The possibility that abrasion was the wear process responsible for scratching is coherent with the fact that the roughness of the pins used was relatively high. Branco et al [16] showed evidence that the pin-on-disk wear develops by an abrasion process, through a ploughing mechanism, even though a fatigue mechanism cannot be disregarded. The low friction coefficient previously observed, between PET and steel was confirmed. It was shown that friction force is very sensitive to the presence of polymer debris at the pin-PET interface, and this force increases as the debris content increases, and decreases once it is released, promoting friction fluctuation during the sliding. In the assprayed condition, the PET coatings showed a higher friction coefficient, due likely to a higher coarse debris production rate during the pin-on-disk testing. Quenching the as-sprayed coating to increase the amorphous PET content improved the sliding behavior by increasing wear resistance (Figure 4).

5. Weathering Resistance of PET and PET-EMAA Blends [24] Ageing is a term used in many branches of polymer science and engineering when the properties of the polymer change over a period of time [42]. The changes may be observed in

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engineering properties such as strength and toughness; in physical characteristics such as density; or in chemical characteristics such as reactivity towards aggressive chemicals [42]. Weathering trials are sometimes referred to as natural ageing. Polymers exposed outdoors become degraded by the action of several agents, including solar ultraviolet radiation (UV); water; pollutants (in gaseous form or as acid-rain);elevated temperatures; and temperature changes [42]. In a majority of cases, the main cause of property deterioration is photo oxidation, which is initiated by ultraviolet irradiation and, as a consequence, much laboratory photo-ageing testing is conducted to determine the weather ability of polymers and to test the effectiveness of stabilizers introduced to improve their weather resistance. Several types of artificial UV sources can be used, the most popular being xenon lamps and fluorescent tubes. As with thermal degradation, a sequence of oxidative reactions follow in which both chain scission and cross linking may occur; both of these molecular changes lead to embrittlement of the material [42]. The re-fused (Figure 7a) and thermally sprayed EMAA coatings did not show visible alterations after the ageing process, but presented a loss of brightness after exposure in the weathering chamber (Figure 7b). The weathering test was performed according to the ASTM G-53 Standard, during 807 hours. In one cycle, the samples were exposed to ultraviolet light at 70 ºC during 8 hours and then they were submitted to condensation condition at 50 ºC during 4 hours [24].

Figure 7. Re-fused EMAA coated steel surface before (a) and after (b) the ageing process, and aircooled 90% PET-10% EMAA coated steel surface before (c) and after (d) the ageing process.

The re-fused 70%PET-30% EMAA and 90%PET-10% EMAA (Figure 7c and 7d) coatings presented a color change after having been exposed in the weathering chamber. The aged surface showed a clear yellow color with brown spots. The air-cooled and quenched re-

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fused 80% PET-20% EMAA coated steels showed a clear yellow color with brown spots and black points on the polymeric surface after the ageing process. The air-cooled 80% PET-20% EMAA coatings also showed more brown and black spots on the surface than the quenched 80%PET coatings. The small black spots are associated to the pores, which allowed corrosion products of steel to reach the surface. The substrate of carbon steel, exposed in the weathering chamber, can generate red- rust, iron oxides/hydroxides, which are visible on the surface [43]. Scanning electron microscopy analysis identified the presence of iron in brown spot regions of surface of air-cooled 80% PET coated steel after the weathering test. The brown spots are not restricted to the pore areas on the polymeric surface and may be due to the chemical alterations of the polymers. Studies carried out by Edge et al [44,45] suggest that color formation starts with hydroxylation of the terephthalic ring producing hydroxylated species, which on further oxidation leads to quinonoid type structures. The 90% PET-10% EMAA coated steels also showed a color change, the appearance of a light yellow color on the surface with a few spots on the polymeric surface. The air-cooled PET coated steels showed cracks after the ageing process [24], with a brownish color in the crack area and in a few points associated to the pores on the surface. The quenched re-fused PET coated steel showed a white superficial layer after the ageing testing, indicating a crystallization process of PET coating during the exposure in the weathering chamber, Figure 8.

Photodegradation of PET-EMAA Coatings on Steel During the Weathering Tests The useful life of the polymeric coatings depends on the rate and type of structural change that the polymer undergoes. If the polymer presents a visible change on its surface after the ageing process, the material fails, even if there was no significant change in the material properties [46]. Degradation by environmental exposure is caused by radiation, temperature, humidity, and pollutants. Photon energy produced by an ultraviolet light is a powerful source and is highly effective in breaking chemical bonds such as C-H and O-H [50]. Photo degradation may generate chemical groups like carbonyl, carboxyl, and hydroperoxides [4749]. PET degradation has been extensively studied and many degradation mechanisms were proposed to account for various types of degradation [44, 45, 51-55], but not many papers have been published about the photodegradation of PET and PET blends [50, 56-58]. The major products of degradation of PET reported were carbon dioxide, acetaldeyde, vinyl benzoate, terephthalic acid, terephthaldeydic acid, and linear dimers [59]. Different types of ultraviolet stabilizers on the photodegradation of poly(ethylene terephthalate) were studied, such as a ultraviolet absorber, carbon black and a mixture of TiO2 and BaSO4 [50]. Blends of the poly (ethylene terephthalate), PET, and the copolymer of ethylene and methacrylic acid, EMAA, showed compatibility [59,60] and toughness in several applications [61]. The photodegradation of the PET-EMAA coated steels was evaluated using weathering tests, and the degradation was evaluated using infrared and visible-ultraviolet spectroscopy, and mechanical and thermal analysis.

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Fourier Transform Infrared Spectroscopy, Ultraviolet-Visible Spectroscopy, Mechanical and Thermal Analysis of Photodegradation of PET-EMAA Coatings on Steel The spectra of the poly(methacrylic acid), and the spectra of poly(ethylene) are reported in literature [62]. Peaks, which are characteristics of the poly(ethylene) and poly(metacrylic acid), were observed in the EMAA powder spectra [24]. The spectrum of the re-fused EMAA and the spectrum of the thermally sprayed EMAA coating on steel were similar to the spectrum of the EMAA powder [24]. After the ageing process, the duplications of the peaks at 1260 cm-1, and 940 and 960 cm1 were observed [24]. The peak at 1260 cm-1 is associated to the C-O bond, which may have been formed due to the cross linking process. Peaks at 960 and 940 cm-1 are associated with the O-H and can appear due to the hydration of the carboxyl group. The intensity of peaks at 1700-1750 cm-1, associated with the C=O functional group, decreased after the ageing process in the weathering chamber [24]. In a previous work [63], a decrease of the strain at break of the PET and EMAA coatings was observed after the ageing process. EMAA coatings produced by re-fusion technique showed the highest elongation, 158% for the coating before ageing and a value of 65% for the aged polymeric coating. Oreski and Wallner [64] cited the strain at break as the most sensitive parameter to evaluate the degradation effects of the ageing test. If a cross linking process occurred with the polymeric molecular chains or the hydrogen bond network occurred, the elongation would decrease after the exposition in the weathering chamber, as observed [63]. In the test with the ZnSe crystal of the refused EMAA before and after the ageing process, peaks at 2800-3000, 1700-1750, 1480, 1260, 940-960, and 720-730 cm-1 were identified [24]. Peaks at 1260 and 940-960 cm-1 of the refused EMAA showed changes before and after the ageing [24]. Before the ageing process, these peaks are broad and after the exposition in the weathering chamber these peaks became narrow. The peak associated with the C=O of the carboxyl acid became weak in the spectrum of refused EMAA after the ageing. During ageing, the UV radiation can break down the chemical bond C=O in the polymer chain and generate a cross linking or an hydration with the carbon linking to hydroxyls [24]. The peak at 3550 cm-1 that is associated with the absorbed moisture, and the peak at 3650 -1 cm appeared in the spectra of PET powder and did not appear in the PET coating spectra [24]. The thermo-oxidative degradation of the polymer induced by heating in air was reflected by the changes observed in the spectra of the degraded samples. After heating in air, which occurs during the re-fusion process, bands were observed in the spectra of quenched and aircooled PET coatings at around 3400 cm-1 and 3200 cm-1 suggesting hydroxylation of the terephthalic ring yielding hydroxylated species. In the spectra of quenched and air-cooled 80% PET-20% EMAA coating on steel, aircooled 90% PET –10% EMAA coating, air-cooled 70% PET-30% EMAA coating on steel (Figure 9), a broad band was observed at about 3270 cm-1 suggesting hydroxylation of the terephthalic ring [24,48]. The hypothesis of the hydroxylation of the degraded PET samples is reinforced by the appearance or the increase of the intensity of peaks at 1371 cm-1 assigned to phenolic –OH, and at 1174 cm-1 attributed to aromatic OH deformation [24]. Similar

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observation was reported by Edge et al [53,54] while investigating the extracts of PET samples degraded in air at 300ºC. The band around 3300 cm-1 was not present when the heating was carried out in nitrogen suggesting that hydroxylation occurs when oxygen is present [44, 45].

Figure 8. Quenched re-fused PET coated steel surface before (a) and after (b) the ageing process.

After ageing, the quenched and air-cooled PET coating on steel did not show a peak at 3200 cm-1, and the peak at 3400 cm-1decreased in intensity, but showed a broad band at around 3300 cm-1 [24].

Figure 9. FTIR spectra of the re-fused 70% PET-30% EMAA before and after the ageing process.

Ciolacu et al [65] reported that after heating in air, a broad band at about 3270 cm-1 was observed in the spectrum of degraded PET samples suggesting hydroxylation of the terephthalate ring. Earlier studies carried out by Edge et al [44, 45] suggest that color formation starts with hydroxylation of the terephthalate ring producing hydroxylated species,

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which on further oxidation leads to quinonoid type structures. Color change on the coating surface was observed for the PET and blend coatings on steel [24]. Quenched PET coating on steel and quenched 80% PET-20% EMAA blend coatings showed loss of brightness and color alteration with the generation of a white color on the surface of the aged samples [24]. The fine white layer produced on the surface of the quenched PET coatings after ageing may be due to a crystallization process. The temperature in the chamber was 70-75ºC, which is lightly higher than the glass transition temperature of PET and PET/EMAA blends measured that occurred in the range from 62ºC up to 69ºC [63]. The time of exposure of 807 hours could also initiate PET crystallization. After the ageing process, the coated steels were also analyzed using DSC [63]. An important result found was the increase in the glass temperature values (Tg) of aged coatings, which can indicate a cross linking process, a hydrogen bond network or a crystallization operating during the exposition in the weathering chamber. After the exposition in the weathering chamber, Tg increased from 68ºC to 76ºC for the quenched PET coating [63]. The crystallization fraction (χc) of the PET powder and the air-cooled thermally sprayed PET coatings was 41%, and 21%, respectively as shown in Table 1 [26]. The deposition process can cause a recrystallization of the PET, and the crystallization fraction decreases [26]. The crystallization grade of the coating depends on the cooling rate and the molecular weight of the polymer [26]. In a previous work [63], the crystallization fraction measured of the PET powder used in this work was 42%, and decreased to 17-21% for the PET-EMAA blend coatings. The crystallization fraction of the quenched 80% PET20% EMAA coatings before and after the ageing process was 19% and 21%, respectively [62], with a probable crystallization of the quenched PET in the weathering chamber. However, the crystallization fraction of the quenched PET coatings did not change, and was 26% before and after the exposure in the weathering chamber [63]. The ultraviolet-visible spectroscopy was realized in all PET-EMAA coatings on steels. Using the visible-ultraviolet spectroscopy, the yellowness index can be estimated by the increase in absorption at 400 nm [63]. This increase is mainly due to quinone and diquinone formed during the photo degradation of PET [44]. The ultra-violet spectra of blend coatings after the ageing process showed a higher value of reflectance than the spectra of the coatings before ageing, Figure 10. In the spectra of EMAA and PET-EMAA blends, a peak at 225 nm, associated to the carboxyl conjugated with a double bond between carbon atoms that produced a dislocation in the maximum wave number, was observed. In the spectra of the re-fused PET coating, there was a peak at 240 nm and a peak at 300 nm that can be associated to the aromatic ring linked to the carboxyl acid ended or ester once the presence of chromophorous groups linked to the benzenic ring produced bands at higher wave numbers. A peak at 490-505 nm in the spectroscopy spectra of the samples before ageing was also observed [63]. The amorphous samples of quenched PET coatings before and after ageing showed a peak at 195 nm associated to the aromatic compounds. In the spectra of the 90%PET – EMAA, 80% PET – EMAA, and 70% PET-EMAA blend coatings and the EMAA coatings, peaks of the both polymers were identified. The peaks at 225 nm of the EMAA and the peak at 240 nm of the PET were overcome. The peak at 300 nm associated to the aromatic ring linked to the carboxyl acid ended and to the ester, which is characteristic of the PET structure, was clearly identified in the blend spectra [63].

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Figure 10. Ultra-violet visible spectroscopy spectra of the air-cooled re-fused 90% PET-10% EMAA before and after the ageing process.

6. Corrosion Resistance of PET-Based Coatings PET Coatings in Fuel Media Thermally sprayed PET coatings, which were produced with a low velocity oxy-fuel torch, and with preheated substrate at 215°C, were immersed totally and partially in gasoline, diesel oil, and alcohol for a period of 60 days [26]. The toughest corrosion conditions provided by these solvents were used. Therefore, immersion tests in gasoline and diesel oil were conducted outside the laboratory to simulate the thermal cycle that cars are subjected to daily usage. The immersion tests in alcohol were carried out at 70°C. The uncovered steel regions in the samples were protected with an adhesive tape [26]. The coating surface and the metal-PET interface of the samples were evaluated before and after the immersion tests by optical microscopy [26]. Peeling and swelling of the coatings did not occur in the immersion tests [26]. However, some reddish spots appeared on the coating surface in areas, which were in contact with the fuel. These spots may be attributed to the deposition of soluble corrosion products, resulting from the action of the fuels on the adhesive protected regions on the steel.

Salt Spray Test of PET and PET-EMAA Blends [24] The salt spray test was performed according to the ASTM D-117 Standard, during 166 hours. The samples were scratched with a tungsten carbide instrument in order to expose the

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substrate. Measurements of the corrosion propagation were taken, which were observed in relation to the scratch. During the salt spray test, some visual observations were made in order to identify corrosion specks on the surface of coated steels. The propagation of corrosion from the scratches was not observed on the samples. PET coating samples, and samples of 80% PET20% EMAA and 90% PET- 10% EMAA coatings showed specks on their surface after 117 hours of testing. No surface alterations on the air-cooled re-fused EMAA (Figure 11a) and on the PET blends, which were quenched after re-fusion were observed after exposure for 166 hours in the salt spray chamber. Quenched PET blends with 80% PET (Figure 11b) showed the highest corrosion resistance after 117 hours of exposition in the salt spray chamber. Aircooled PET coatings showed cracks and rust specks after only 9 hours of testing.

Figure 11. Air-cooled re-fused EMAA coating (a) and quenched re-fused 80%PET-20%EMAA (b) coating after 166 hours of exposure in the salt spray chamber.

PET Composite Coatings in Acid Media [25] Coatings of PET-based composites with the addition of glass powder and flakes, and zinc powder were also produced using a re-fusion technique for applications in the acid aqueous media [25]. Contents of 0.1%, 1% and 10% of glass powder and flake, and zinc powder were mixed with PET powder, and scattered on the surface of a pre-heated steel sheet. In the refusion technique, the steel samples were placed in a furnace at temperatures higher than the melting temperature of PET, during 10 minutes. The furnace was opened and the polymer powder was scattered on the steel surface to obtain a homogeneous film. Samples were immersed in 9.8g.L-1 H2SO4 solutions during 30 days, according to the G 31-72 ASTM Standard. After the corrosion test, the solutions were analyzed using atomic absorption spectroscopy (AAS), with a Thermo Electron Corporation spectrometer. Concentrations of Fe in solution after the immersion tests are shown in Table 3 [25]. The lower contents of Fe were observed for the composite coatings with 0.1% of glass powder and flakes, and zinc powder. The barrier effect was more efficient with lower concentrations of zinc and glass in the polymer matrix. The lowest content of additive can produce the highest homogeneity and dispersion, and a lowest porosity. However, the efficiency of the additives depends on other parameters such as the morphology, granulometry, and ability of dispersion in the polymeric matrix.

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V.F.C. Lins, J.R.T. Branco and C.C. Berndt Table 3. Iron concentration in acid solution after the immersion tests [25] Additive addition Glass flakes 0.1% Glass flakes 1.0% Glass flakes 10% Glass powder 0.1% Glass powder 1.0% Glass powder 10% Zinc powder 0.1% Zinc powder 1.0% Zinc powder 10%

Fe ( mg.L-1) 108.5 663.1 601.5 40.7 571.2 601.5 180.3 571.1 443.2

7. Conclusion The use of the thermal spray method as a deposition technique of post-consumer polymers on steels is a logical and efficient means of recycling thermoplastics, thereby reducing the accumulation of polymer residues. Quenched PET blends with 80%PET and quenched PET coatings showed corrosion resistance in a salt spray chamber, small friction coefficient, and adhesion, which are necessary for the application of polymeric films as protective coatings against corrosion and wear. After heating in air, which occurs during the re-fusion process, bands are observed in the spectra of quenched and air-cooled PET coatings, quenched and air-cooled 80% PET-20% EMAA coatings, air-cooled 90% PET –20% EMAA coatings, 70% PET-30% EMAA coatings at about 3400 cm-1 and 3200 cm-1, suggesting hydroxylation of the terephthalic ring yielding hydroxylated species. The hypothesis of the hydroxylation of the degraded PET samples is reinforced by the appearance or the increase of the intensity of peaks at 1371 cm-1 assigned to phenolic –OH, and at 1174 cm-1 attributed to aromatic OH deformation. Quenched PET coating on steel and quenched 80% PET-20% EMAA blend coatings showed loss of brightness and color alteration with the generation of a white color on the surface of the aged samples. Ultra-violet spectra of PET-EMAA blend coatings, after ageing process, showed a higher value of reflectance than the spectra of the coatings before aging.

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[63] Cury, F.M.: Produção e caracterização de recobrimentos de PET e EMAA depositados em aço carbono. Belo Horizonte, Federal University of Minas Gerais, 2005. M.Sc. Thesis. [64] Oreski, G.; Wallner, G. M. Solar Energy 2005, 79, 612-617. [65] Ciolacu, C.F.L.; Choudhury, N.R.; Dutta, N.K. Polym Degrad Stab 2006, 91(4), 875885.

In: Surface Coatings Editors: M. Rizzo and G. Bruno, pp. 259-263

ISBN: 978-1-60741-193-2 © 2009 Nova Science Publishers, Inc.

Chapter 10

COATING OF CARBON NANOTUBES WITH INSULATING THIN LAYERS Martin Pumera1 International Center for Materials Nanoarchitectonics (WPI-MANA) and Biomaterials Center, National Institute for Materials Science, Namiki, Tsukuba, Ibaraki, Japan

Abstract The aim of this chapter is to discuss the problematic of coatings of carbon nanotubes with thin and ultrathin layers with insulating properties.

Carbon nanotubes (CNT) sheathed with a precise insulating layer exphibit huge potential for a materials nanoarchitectonics. Carbon nanotubes coated with well-defined layer were fabricated for nanowiring of nanoelectronic devices. [1],[2], for nanoelectrodes [3], insulated AFM tips [4] and for biosensing devices [5]. When a coplex nanoelectronics device is build, such as 3D CNT based field effect transistor, efficient electrical insulation of each of its components is a very important feature for function of such a nanoarchitected nanodevice. Ultrathin coating of the individual building blocks of a nanoelectronic device with an insulating layer is a important factor in its fabrication [6]. A wide range of materials that have been used for thin and ultrathin coating of carbon nanotubes with insulating layers can be divided into the following two categories. (i) Polymer-based coatings, which offer uncomplicated approaches to the customized design of sheathing layer with functional groups. (ii) Inorganic thin film metal oxide/hydroxide materials that exhibit very favorable properties, such as long term stability and outstanding electrical insulating properties.

1

E-mail address: [email protected], Fax: +81-29-860-4714.

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Martin Pumera

Figure 1. Schematic of the inductively coupled plasma reactor. Reprinted with permission from Ref. [4].

First, we wish to describe polymer based coatings. There are several ways how to fabricate polymer coated carbon nanotubes. They can be subdivided to (a) gas phase and (b) liquid phase coating methods. In gas phase coating methods, polymer precursor vapors are fed to reactor and under optimal condition deposit on carbon nanotubes and polymerize, creating insulating layer. For example, Esplandiu, et al. fabricated electrically insulated single-wall carbon nanotubes coated with uniform fluorocarbon polymer films. This was done in an inductively coupled plasma reactor (See Figure 1) where C4F8 gas polymerizes under optimal conditions.[4] Liquid phase coating is used in much bigger extent due to its facility and simplicity. For an example, Campbell, et al. described method for coating of multiwalled

Figure 2. Continued on next page.

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Figure 2. Transmission electron microscopy (TEM) images of PPy-coated MWCNT. Detailed view of PPy A) at the side and B) the end of the PPy-coated MWCNT. Inset C shows a TEM image of several PPy-coated MWCNTs at lower magnification. Reprinted with permission from Ref. [7]

carbon nanotubes (MWCNTs) with an insulating layer formed by electrochemical polymerization reactions of polyphenol.[3] We have reported the coating of MWCNTs with an precise and ultrathin polypyrrole layer by in situ chemical deposition of polypyrrole. [7] The undoped polypyrrole layer of thickness about 7 nm effectively insulates MWCNTs (see coating and I-V characteristic in Figure 2). Chen et al. have discussed coating of carbon nanotubes with polyaniline using chemical precipitation method. [8]

Figure 3. Current–voltage curves of MWCNTs (a) and PPy-coated MWCNTs (b) films. The insets separately show the I–V characteristics of the MWCNTs and the PPy-coated MWCNTs. Reprinted with permission from Ref. [7].

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Figure 4. Schematic of gas-phase based coating of carbon nanotubes with metal oxides. Reprinted with permission from Ref. [9]

Figure 5. TEM (a) and high resolution TEM (b) image of Eu2O3 coated multiwall carbon nanotube. Reprinted with permission from Ref. [10].

Second, we wish to discuss selected methods for inorganic oxide coatings. Here again, there are interesting (a) gas phase methods but most methods exist for coating in the (b) liquid phase. As example of gas phase method, we wish to mention classical paper by Rao and coworkers. [9] Oxygen groups functionalized carbon nanotubes are exposed to fumes of metal halide (i.e. TiCl4) which creates metal-carbon bond and releases HCl. The residual metal-halide bonds are hydrolyzed and cycle is repeated, as shown in Figure 4. However, as

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mentioned above, the most of the methods for inorganic oxide coatings of CNTs are liquidbased. For example, Fu, et al. invented a coating of MWCNTs with thin layer of ceramic of europium oxide. They used a supercritical fluid coating with europium nitrate and consequently the deposited europium hydroxide was converted to Eu2O3 by thermal decomposition (see Figure 5). [10] The same authors employed the supercritical fluid method for coating MWCNTs with thin layer of aluminum oxide. [11] We invented method for an insulating europium hydroxide coating of MWCNTs. The method was very simple; it is spontaneous in-situ deposition of europium hydroxide in an aqueous suspension of europium nitrate. Europium nitrate slowly hydrolyzes to europium hydroxide and it bonds on the MWCNT surface by means of dative bonds to oxygen-containing groups on the surfaces of MWCNTs.[12] Carbon nanotubes can be coated with thick (50-100 nm) SiOx layer based on sol-gel method.[13],[14],[15],[16]

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]

Star, J.-C. P. Gabriel, K. Bradley, G. Grußner, Nano Lett. 2003, 3, 459. G. B. Blanchet, S. Subramoney, R. K. Bailey, G. D. Jaycox, Appl. Phys. Lett. 2004, 85, 828. J. K. Campbell, L. Sun, R. M. Crooks, J. Am. Chem. Soc. 1999, 121, 3779. M. J. Esplandiu, V. G. Bittner, K. P. Giapis, C. P. Collier, Nano Lett. 2004, 4, 1873. M. Pumera, S. Sanchez, I. Ichinose, J. Tang, Sens. Actuators B 2007, 123, 1195. M. J. Frampton, H. L. Anderson, Angew. Chem. Int. Ed. 2007, 46, 1028. Pumera, M.; Šmíd, B.; Peng, X.-S.; Golberg, D.; Tang, J.; Ichinose, I.; Chem. Eur. J. 2007, 13, 7644. Liu, Z., Wang, J., Xie, D., Chen, G., Small 2008, 4, 462. Gomathi, S. R. C. Vivekchand, A. Govindaraj, CNR Rao, Adv. Mater. 2005, 17, 2757. L. Fu, Z. Liu, Y. Liu, B. Han, J. Wang, P. Hu, L. Cao, D. Zhu, Adv. Mater. 2004, 16, 350. L. Fu, Y. Liu, Z. Liu, B. Han, L. Cao, D. Wei, G. Yu, D. Zhu, Adv. Mater. 2006, 18, 181. M. Pumera, M. Cabala, K. Veltruská, I. Ichinose, J. Tang, Chem. Mater. 2007, 19, 6513. S. Guo, L. Huang, E. Wang, New J. Chem. 2007, 31, 575. T. Seeger, Ph. Redlich, N. Grobert, M. Terrones, D. R. M. Walton, H. W. Kroto, M. Ruhle, Chem. Phys. Lett. 2001, 339, 41. S. Guo, L. Huang, E. Wang, J. Phys. Chem. C 2008, 112, 2389 M. Olek, T. Bu1sgen, M. Hilgendorff, M. Giersig, J. Phys. Chem. 2006, 110, 12901.

INDEX 3 3,4-ethylenedioxythiophene, 166

A AAS, 15, 17, 253 ABC, 133, 147 absorption, 2, 49, 60, 63, 208, 219, 220, 223, 228, 230, 243, 251, 253 absorption spectroscopy, 253 academic, 232 access, 50, 141, 239 accidental, 95 acetaldehyde, 62 acetate, 14, 55, 59, 61, 62, 68, 69, 73, 74, 163, 219, 220, 222, 225, 230 acetic acid, 14, 15, 19, 20, 21, 41, 58, 59, 163, 208, 209, 210 acetone, 52, 178 acetylene, 160, 161 acetylene gas, 160 acid, xi, 2, 14, 15, 18, 19, 20, 21, 28, 32, 36, 40, 41, 47, 51, 52, 53, 54, 55, 57, 58, 59, 60, 62, 63, 65, 66, 67, 68, 69, 70, 71, 72, 73, 75, 94, 96, 97, 98, 99, 103, 105, 107, 118, 120, 135, 137, 142, 161, 162, 163, 165, 208, 209, 210, 223, 225, 237, 238, 239, 240, 241, 248, 249, 251, 253, 254 acidic, 94, 96, 130, 145 acidity, 47 acoustic, x, xi, 185, 213, 214, 215, 223 acoustic emission, 185 acrylate, 47, 63, 72, 134, 135, 136, 137, 139 acrylic acid, 32, 65, 137, 161, 162 acrylonitrile, 64, 194 actin, 16 activated carbon, 49 activation, viii, x, 5, 28, 33, 35, 36, 44, 45, 97, 174, 190, 191, 193, 195, 198, 216 active oxygen, 58 active site, 28, 216

Adams, 149 additives, 48, 51, 55, 60, 253 adhesion, vii, viii, ix, x, xi, 4, 5, 16, 17, 18, 26, 28, 30, 33, 36, 42, 43, 59, 65, 93, 94, 96, 97, 98, 102, 104, 107, 154, 158, 159, 161, 177, 178, 179, 180, 181, 183, 185, 186, 191, 192, 193, 194, 195, 196, 200, 203, 208, 209, 210, 216, 233, 237, 239, 243, 245, 254 adhesion properties, 104, 195, 200 adhesion strength, 161, 183, 185, 186, 191 adhesions, 17 adhesive properties, 239 adhesive strength, 12 adipose, 76 administration, 165 adsorption, 7, 10, 28, 52, 57, 192, 220, 224 aerobic, 56, 59, 60, 61, 62, 69, 70, 71, 75 aerobic bacteria, 56, 60, 61, 69, 71, 75 aerosol, 162, 163, 168 aerosols, 154 agar, 57, 60, 75 age, 240, 243 ageing, 247, 248, 249, 250, 251, 252, 254 agent, 48, 52, 59, 61, 62, 63, 64, 65, 94, 98, 124, 166, 220, 224, 226, 229, 233 agents, viii, 28, 45, 51, 56, 59, 64, 65, 66, 77, 91, 96, 124, 193, 232, 233, 247 aggregates, 123, 147, 205 aggregation, 137, 207, 219, 223 aging, 254 agriculture, 46 AIBN, 139 aid, 60 AIP, 172 air, 46, 50, 59, 63, 65, 110, 159, 160, 222, 224, 241, 247, 249, 250, 254 albumin, 4, 9, 16, 25, 30, 36 alcohol, xi, 47, 59, 75, 229, 237, 252 alcohol oxidase, 47 alcohols, 192 Alginate, 67, 75 alkali, 2, 4, 6, 8, 9, 10, 11, 15, 16, 18, 19, 24, 25, 26, 33, 37, 38, 209 alkaline, 5, 12, 14, 17, 18, 19, 40, 57, 98

266

Index

alkaline phosphatase, 5, 12, 17, 18 alloys, 36, 94, 238 ALP, 5, 10, 16, 17, 21, 25, 26, 27, 29, 30 alpha, 34 alternative, vii, viii, ix, 1, 3, 23, 93, 110, 153, 157, 223 alters, 33, 34 aluminium, 94, 98, 167 aluminosilicates, 57 aluminum, x, 189, 190, 192, 193, 239, 240, 263 aluminum oxide, 190, 240, 263 ambient air, 160 amelioration, 161 amide, 64, 163 amine, 64, 159, 160, 161, 163 amines, 64, 159 amino, 28, 58, 64, 76 amino acid, 28, 76 amino acids, 76 ammonia, 194, 230, 231 ammonium, 65, 69, 205, 231 ammonium salts, 65 amorphous, x, 7, 8, 9, 15, 50, 160, 161, 177, 180, 181, 182, 183, 184, 186, 200, 203, 214, 216, 218, 221, 225, 226, 228, 231, 242, 243, 246, 251 amorphous carbon, 160, 161 anatase, 9, 12, 15, 16, 217, 218 animals, 233 anode, 14 antibacterial, 18, 53, 229, 232, 233 antibacterial properties, 18, 53 antibiotics, 51 anticorrosive, 107 antigenicity, 30 anti-inflammatory agents, 124 antioxidant, 61 apatite, vii, 1, 3, 4, 7, 8, 9, 10, 11, 12, 13, 15, 18, 21, 24, 36, 37, 38 apatite layer, vii, 1, 3, 7, 8, 9, 12, 13, 15, 36 appendix, viii, 2, 3 application, xi, 47, 48, 58, 76, 84, 99, 104, 154, 155, 157, 158, 161, 166, 167, 178, 190, 191, 192, 194, 195, 214, 215, 216, 218, 237, 238, 239, 254 aqueous solution, 222, 225, 229 aqueous suspension, 263 argon, 154, 158, 159, 160, 162, 163, 164, 165, 167, 222 arithmetic, 100 aromatic compounds, 251 ART, 1 arthroplasty, 8 ascorbic, 47 ascorbic acid, 47 ASD, 13 aseptic, 49 ASI, 150 Asian, 121 Aspergillus niger, 57

ASTM, 99, 100, 102, 103, 104, 107, 110, 112, 113, 114, 121, 122, 247, 252, 253 ATC, 29 Atlantic, 70 Atlantic cod, 70 Atlas, 256 atmosphere, 46, 50, 59, 61, 62, 63, 160, 178, 216, 222, 230, 242 atmospheric pressure, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 165, 166, 167, 168, 169, 174, 175 atom transfer radical polymerization, 134, 135, 136, 137, 140, 148 atomic force, 137 atomic force microscopy (AFM), x, 7, 10, 19, 20, 26, 27, 29, 30, 31, 32, 137, 139, 177, 181, 182, 259 atoms, 155, 216, 219, 228, 231, 251 attachment, 2, 5, 6, 7, 10, 13, 16, 17, 18, 19, 25, 26, 28, 29, 30, 35, 38, 42, 132, 133, 145 attribution, x, 189 Au nanoparticles, 224, 225 availability, viii, 93 averaging, 102

B Bacillus, 55 Bacillus subtilis, 55, 57, 60, 65, 66, 70, 72, 73, 76 bacteria, 46, 47, 56, 58, 60, 61, 62, 63, 65, 69, 70, 71, 74, 75, 76, 200, 227, 233 bacterial, 28, 54, 56, 59, 191, 200, 203, 210 bacterial strains, 59 bacteriocin, 56 bacteriocins, 51 bacterium, 55, 75 band gap, 220 bandgap, 220 barium, 47 barrier, x, xi, 46, 59, 61, 67, 74, 76, 107, 118, 120, 157, 160, 165, 166, 167, 168, 169, 174, 175, 189, 190, 200, 205, 210, 218, 237, 238, 253 barriers, 54, 168, 169 baths, 95, 193, 194, 209 batteries, 219 beef, 60, 61, 67, 69, 70, 74, 76 behavior, 14, 21, 25, 35, 41, 42, 190, 198, 216, 219, 223, 224, 242, 243, 245, 246 Beijing, 177 Belgium, 153 bending, 208 beneficial effect, 3 benefits, ix, 93, 96, 181 benzene, 141, 194 bicarbonate, 47 binding, viii, 12, 26, 43, 45, 64, 65, 77, 127 biochemistry, 159 biocompatibility, x, 43, 160, 165, 167, 189, 191, 200 biocompatible, 25, 41, 166

Index biocompatible materials, 166 biodegradable, 46, 54, 55, 59 biodegradation, vii, 1, 3 Bioglass, 8 biological macromolecules, 161 biomaterials, 1, 33, 43 biomedical applications, 24, 158, 159, 161 biomimetic, 33, 37, 147 biomolecules, 28, 33, 159 biopolymer, 56, 74 biopolymers, 58 biopsies, 35 biosensors, 47, 166 biotechnological, 124 bismuth, 219 bleaching, x, 213 bleeding, 165 blends, xi, 51, 59, 194, 237, 238, 239, 248, 251, 253, 254 blocks, 94, 143 blood, 5, 28, 29, 31, 32, 33, 36, 42, 161, 165, 167 blood plasma, 31, 32, 42 blood vessels, 31, 161 blot, 10, 29 body fluid, vii, 1, 2, 12, 14, 15, 17, 18, 37 bonding, vii, 1, 2, 3, 7, 8, 11, 12, 13, 21, 22, 23, 24, 25, 37, 64, 210 bonds, 31, 97, 128, 159, 160, 166, 168, 195, 198, 208, 216, 220, 243, 262, 263 bone density, 7, 31 bone graft, 22, 40 bone growth, 44 bone marrow, 2, 10, 37, 38 bone remodeling, 21 Bose, 18, 41 branching, 127 Brazil, 237, 238, 239, 242, 255, 256 Brazilian, 238, 255 breakdown, 40, 157 broad spectrum, 63, 65 bubble, x, xi, 213, 214, 215, 223, 228 bubbles, 193, 214, 215, 219, 223 buffer, 42, 52 building blocks, 124, 259 bulk materials, viii, 45, 77 butadiene, 134, 161, 162, 175, 194 butane, 194 by-products, 46

C cabinets, 94 calcitonin, 7 calcium, vii, 1, 3, 4, 7, 9, 13, 16, 21, 23, 25, 35, 36, 40, 47 CAP, 46 capacity, 14, 15, 17, 219, 220 caprolactam, 205

267

capsule, 11, 31, 32, 42 carbide, 252 carbohydrate, 128, 130, 148 carbohydrates, 141 carbon, xi, 46, 47, 49, 160, 161, 171, 178, 190, 204, 222, 229, 230, 239, 244, 248, 249, 251, 259, 260, 261, 262 Carbon, 47, 49, 171, 235, 240, 259, 261, 263 carbon atoms, 251 carbon dioxide, 46, 49, 204, 248 carbon film, 160, 161 carbon materials, 229 carbon nanotubes, xi, 259, 260, 261, 262 Carbon nanotubes, 259, 263 carbonates, 219 carboxyl, 64, 161, 239, 248, 249, 251 carboxyl groups, 64 carboxylic, 54, 142, 192 carboxylic acids, 54, 192 carcinogenic, 97 carrier, 154, 159, 160, 163, 166 Casein, 66, 71 cast, 53, 56, 61 casting, 55, 56 catalase, 47 catalysis, 137, 216, 218, 219 catalyst, ix, 47, 123, 125, 126, 134, 135, 136, 137, 143, 148, 221, 222 catalyst deactivation, 125 catalytic activity, 222 catalytic properties, 57, 222 catechins, 51 catechol, 47 catheters, 161 cathode, 14 cation, 205, 219 cavitation, 214, 215, 216, 230 C-C, 82, 83 cell, 2, 4, 5, 6, 10, 13, 16, 17, 18, 19, 24, 25, 26, 28, 29, 30, 32, 33, 34, 41, 43, 63, 64, 65, 76, 91, 114, 127, 141, 147, 159, 169, 215 cell adhesion, 17, 18, 30, 43, 159 cell culture, 159 cell death, 17 cell differentiation, 10, 17 cell growth, 17, 32 cell line, 17, 18, 33 cell membranes, 63 cell surface, 30, 127 cellulose, 53, 55, 59, 60, 64, 65, 67, 72, 73, 75, 81 cellulose triacetate, 64 ceramic, x, xi, 8, 11, 24, 35, 58, 178, 180, 198, 213, 215, 216, 217, 218, 223, 226, 228, 238, 239, 240, 242, 263 ceramics, 19, 24, 191, 215, 217, 226, 228 cesium, 155 CH4, 161 chain scission, 242, 247 chain termination, 136

268

Index

chain transfer, 127, 135 channels, 221, 224 charged particle, 155 chelating agents, 51 chemical bonds, 214, 215, 226, 248 chemical composition, 2, 14, 21, 22, 40, 180, 194, 195 chemical degradation, 200, 243 chemical deposition, 261 chemical engineering, 218 chemical interaction, 215, 221, 225, 226, 228 chemical properties, xi, 162, 215, 237 chemical reactions, x, xi, 213 chemical reactivity, 191 chemical stability, 163 chemical structures, 124 chemical vapor deposition, 23, 191 chemicals, ix, 75, 91, 93, 94, 95, 96, 154, 190, 200, 205, 247 chemiluminescence, 5 chemokines, 33 chicken, 61, 63, 76 China, 177 chitin, 58, 80 Chitin, 80 chitosan, 55, 56, 58, 59, 60, 64, 65, 66, 67, 68, 70, 71, 72, 73, 74, 75, 76, 80, 81, 84, 91 chloride, 63, 69, 142, 165, 222, 224 chlorine, 63, 165 cholesterol, 64 chromatography, 137, 139, 140 chromium, ix, x, 8, 177, 178, 179, 180, 181, 185, 186, 189, 194, 196, 197, 198, 199, 200, 210 cis, 125 classes, 226 classical, 157, 220, 262 clay, 205 Clean Air Act, 95 cleaning, viii, 93, 94, 100, 105, 106, 107, 158, 174, 233, 240 cleavage, 128 clinical trial, 8, 27, 33 clinical trials, 8, 27, 33 closure, 46 CLSM, 16, 29 clustering, 127 clusters, 183, 217, 219, 224, 231 CMC, 72 CNTs, 263 CO2, 63, 64, 73 coatings, vii, ix, x, xi, 1, 14, 18, 23, 24, 25, 27, 30, 31, 32, 41, 42, 52, 57, 58, 62, 66, 67, 69, 70, 71, 72, 73, 74, 75, 82, 94, 95, 96, 97, 98, 99, 102, 103, 104, 107, 110, 117, 118, 120, 121, 158, 159, 160, 161, 162, 163, 165, 166, 167, 168, 169, 177, 178, 179, 180, 181, 182, 185, 186, 189, 190, 191, 192, 193, 200, 205, 207, 208, 210, 237, 238, 239, 240, 241, 242, 244, 245, 246, 247, 248, 249, 251, 252, 253, 254, 255, 259, 260, 262, 263

cobalt, 8, 47 collagen, 7, 25, 27, 28, 31, 32, 43 collisions, 155, 215, 226, 228 colloids, 225, 229 colors, 14 combustion, 241, 242 commerce, 75 commercialization, 33 compatibility, 2, 19, 50, 165, 248 complexity, 76 components, viii, 24, 50, 62, 93, 96, 98, 100, 158, 190, 206, 243, 259 composites, 24, 26, 59, 216, 223, 224, 239, 253 composition, 2, 14, 19, 21, 22, 23, 34, 38, 40, 46, 58, 95, 124, 141, 163, 167, 180, 194, 195, 208, 210 compounds, 48, 50, 57, 59, 65, 154, 166, 217, 219, 220, 231 concentration, 9, 14, 15, 16, 17, 22, 25, 26, 31, 32, 51, 52, 53, 54, 55, 57, 59, 60, 94, 136, 137, 147, 156, 159, 165, 167, 180, 217, 218, 219, 223, 224, 227, 230, 231, 254 condensation, 24, 127, 134, 247 conduction, 5, 16, 241 conductive, 232 conductivity, 166, 190, 191 configuration, 21 Congress, 255 constraints, x, 189 construction, vii, 193 consumers, 48, 58 consumption, 59, 136, 238 contact time, 100, 103, 226 contaminants, 178, 197 contamination, vii, viii, 45, 46, 48, 118, 219 contractors, 94 control, vii, viii, ix, x, 4, 5, 6, 9, 10, 11, 12, 13, 15, 16, 18, 19, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 34, 45, 48, 55, 60, 61, 76, 96, 123, 124, 132, 133, 141, 143, 147, 148, 154, 155, 166, 180, 190, 191, 217, 219, 224 convection, 241 conversion, viii, ix, 58, 93, 94, 95, 96, 97, 98, 99, 100, 102, 103, 104, 105, 107, 110, 111, 114, 115, 118, 119, 120, 125, 127, 136, 139, 141, 142, 143, 145, 148, 154 cooling, xi, 185, 213, 214, 228, 241, 243, 251 coordination, 125, 126 copolymer, xi, 52, 53, 54, 55, 57, 59, 61, 63, 64, 68, 69, 72, 73, 74, 124, 130, 131, 134, 139, 142, 143, 237, 239, 248 copolymerization, 64, 91, 143, 161 copolymers, ix, 47, 59, 123, 124, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 143, 148, 150, 239 copper, 47, 51, 161, 194 core-shell, 124, 139, 220 corn, 53, 61, 68 corona, 156, 157 corona discharge, 157

Index correlation, 110, 178 corrosion, viii, ix, x, xi, 24, 25, 93, 94, 96, 97, 98, 107, 110, 111, 118, 120, 165, 166, 167, 177, 178, 181, 190, 237, 238, 241, 248, 252, 253, 254, 255 corrosive, 110, 181 cost saving, 98 costs, 155, 169, 190 cotton, 161, 232, 233 coupling, 28, 64, 130, 131, 133, 135 covalent, viii, 3, 23, 28, 33, 42, 43, 45, 64, 77, 226, 228, 242 covalent bond, 226, 228, 242 covalent bonding, 226, 228 coverage, 29, 221 covering, 15, 157 cow milk, 56 crack, 248 cracking, 178 CRC, 79, 82, 91, 121 crosslinking, 51, 53, 64, 76, 127, 159, 165 CRP, 127 crystal structure, 13, 14, 15, 21, 22, 40 crystalline, 8, 9, 15, 24, 96, 183, 216, 218, 219, 221, 222, 225, 226, 228, 229, 242 crystallinity, 26, 51, 76, 239, 243 crystallites, 217 crystallization, 214, 216, 218, 226, 241, 242, 248, 251 crystals, 14, 16, 24, 52, 163, 230 CTA, 64, 65, 72 CTA films, 65 culture, 29, 34, 44, 52 culture conditions, 44 curing, 102, 103, 195, 196, 197 CVD, x, 189, 190, 191 cycles, 110, 227, 230, 231, 233 cycling, 110 cyclodextrin, 61 cyclodextrins, 65 cyclohexyl, 125 cylindrical reactor, 193 cytokine, 13, 17 cytokines, 33 cytoskeleton, 16 cytotoxicity, 19

D Dallas, 84 death, 47 deaths, 47 decay, 217 decomposition, 126, 216, 238, 242, 263 decontamination, 65 defects, 22, 36, 40, 163, 167, 178, 180, 183, 186, 198, 220 definition, 2 deformation, 41, 178, 243, 244, 245, 249, 254

269

degradable polymers, 28 degradation, 28, 30, 91, 145, 200, 205, 217, 230, 242, 243, 247, 248, 249, 251 degradation mechanism, 248 degree of crystallinity, 4, 9, 15, 24 Degussa, 217 dehydration, 58 delivery, 54, 55, 124, 154 demand, 48, 190, 229, 232 dendrimers, 65 density, 3, 7, 13, 14, 30, 31, 35, 36, 51, 135, 141, 156, 157, 159, 161, 167, 194, 203, 247 dental implants, 8, 29, 36, 42, 43, 198 dental plaque, 200 dental resins, x, 190 deposition, x, xi, 2, 10, 21, 23, 24, 27, 34, 37, 38, 96, 105, 153, 154, 158, 159, 160, 161, 162, 163, 165, 166, 167, 168, 169, 174, 177, 178, 179, 180, 181, 185, 189, 190, 191, 192, 193, 194, 195, 196, 197, 200, 201, 203, 204, 205, 206, 208, 209, 210, 213, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 226, 228, 229, 230, 231, 232, 233, 251, 252, 254, 261, 263 deposition rate, 159, 160, 163, 165, 166, 167, 169 derivatives, 132, 135, 141, 231 desiccation, 46 detachment, 7 detection, 222 developed countries, 47, 239 deviation, 2, 183, 184, 202, 204 dextrose, 57 DFT, 102 Diamond, 171 dielectric constant, 161, 163, 169 dielectric materials, 206 dielectrics, 190 diesel, xi, 237, 252 differential scanning, 218 differential scanning calorimetry, 218 differentiation, 3, 4, 5, 6, 7, 10, 13, 16, 17, 18, 19, 24, 25, 26, 27, 28, 29, 30, 32, 34, 35, 37 diffraction, 183, 184, 185, 218, 219, 226, 242 diffusion, 28, 49, 50, 51, 52, 53, 54, 57, 65, 75, 76, 169, 181, 190, 205, 208, 214, 219 diffusion process, 52 diffusivity, 53 discharges, 156, 157, 159, 160, 163, 172, 174 discs, 8, 167 diseases, 47 dislocation, 185, 251 dislocations, 181 dispersion, 205, 220, 223, 226, 253 dispersity, 146, 222 displacement, 220 dissociation, 126, 169, 191, 201, 208 distillation, 192 distilled water, 8, 10, 52, 54

270

Index

distribution, 14, 21, 49, 58, 75, 130, 137, 139, 140, 141, 143, 145, 161, 218, 219, 222, 223, 226, 227, 229, 239 diversity, 127 DNA, 10, 159 doctors, 233 dogs, 36, 39, 43, 67 donor, 126 dopants, 169 doped, 77, 166, 216, 217 doping, xi, 166, 213, 216 double bonds, 159 drying, 24 DSC, 218, 251 duplication, xi, 214 duration, 107 dust, 198 dyes, 47

E ECM, 28, 30 ecological, viii, 93, 190 economics, ix, 93, 96 ectopic bone, 12 Eden, 170 effluent, 94, 98 elastic constants, 185 elasticity, x, 167, 189, 205, 231, 245 electric arc, 241 electric field, 155 electrical conductivity, 166, 190, 191 electrochemical impedance, 114, 115 electrodes, 156, 157, 158, 195, 201, 219, 222 electrolyte, 13, 14, 17, 18, 19, 115, 118, 120, 219 electrolytes, 13, 14, 15, 16, 18, 40 electron, ix, 14, 17, 18, 34, 35, 59, 155, 177, 179, 191, 192, 206, 209, 219, 221, 224, 248, 261 electron beam, 59, 191, 192, 206, 209 electron microscopy, ix, 3, 13, 17, 177, 179, 217, 219, 221, 226, 242, 248, 261 electrons, 155 electroplating, x, 189, 190, 195 electrostatic interactions, 54 ELISA, 30 elongation, 249 emission, x, 155, 185, 190, 216 emitters, 46, 49 employment, 238 emulsifier, 53 emulsions, 62 encapsulated, 61, 62 encapsulation, 124, 165, 169, 223, 224 energy, 17, 18, 94, 95, 96, 98, 100, 118, 155, 169, 180, 181, 182, 185, 219, 248 enlargement, 209 entrapment, 64

environment, viii, 57, 93, 95, 158, 181, 200, 205, 224, 232 environmental change, 124 environmental contamination, 46 environmental protection, 233 enzymatic, viii, 30, 45, 56, 77, 162 enzymatic activity, 30, 56 enzymes, 46, 47, 51, 63, 64, 76, 161 epithelial cell, 30 epithelial cells, 30 epoxy, 103, 114 EPR, 159 equilibrium, 14, 50, 52, 53, 54, 55, 126, 135, 155, 157 equipment, viii, 45, 48, 76, 77, 154, 169, 233 erosion, 158, 160 Escherichia coli, 55, 56, 57, 58, 59, 60, 61, 62, 63, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76 essential oils, 58, 60, 61 ester, 192, 251 esterification, 142 esters, 192 etching, vii, 1, 2, 3, 15, 17, 167, 192, 193, 209, 210 ethanol, 46, 47, 53, 54, 62, 87, 159, 178, 208, 210, 230, 232 Ethanol, 47, 49, 72, 209 ethers, 65 ethyl acetate, 163 ethylene, xi, 46, 47, 55, 59, 61, 68, 73, 74, 127, 130, 132, 140, 141, 145, 158, 159, 160, 163, 194, 218, 219, 224, 229, 232, 237, 238, 239, 241, 243, 248, 249, 256 ethylene glycol, 130, 132, 145, 218, 224, 229 ethylene oxide, 140, 141 Europe, 48, 61, 238 European Commission, 82 European Union, 48 europium, 216, 217, 263 eutrophication, 95 evaporation, 50, 154, 191, 192, 209 EVOH, 60, 75 evolution, 242, 246 exfoliation, 208, 210 experimental condition, 229 exposure, 5, 107, 109, 110, 159, 247, 248, 251, 253 extracellular matrix, 17, 43 extrusion, 57, 60, 63, 205, 206, 228, 230

F fabric, 168, 227, 231, 232, 233 fabricate, 260 fabrication, x, xi, 189, 190, 198, 200, 213, 216, 220, 259 failure, 7, 11, 12, 23, 94, 104, 107, 178, 186 family, 191 fat, 75 fatigue, 12, 245, 246

Index fatty acid, 47, 76 fatty acids, 47, 76 fax, 123 FDA, 233 feedstock, 129, 132, 143, 144, 147, 238, 240, 241, 242, 243 femur, 11, 12, 20, 21, 22, 27, 31, 32, 40 ferromagnetic, 216 fetuses, 12 FHA, 24, 25 FIB, 21 fiber, 231 fibers, 16, 59, 63, 65, 190, 192, 215, 228, 229, 231, 232, 233, 238 fibrillar, 43 fibrinogen, 29 fibroblasts, 30, 34 fibrous cap, 11, 31, 32, 42 fibrous tissue, 2 field emission scanning electron microscopy, x, 190 field-emission, 14, 17, 18 filament, 16 fillers, 59, 205 film, x, 41, 46, 52, 53, 54, 55, 56, 57, 58, 60, 61, 62, 63, 65, 66, 69, 71, 74, 75, 76, 96, 97, 98, 102, 160, 163, 167, 169, 177, 181, 190, 191, 192, 193, 194, 195, 196, 197, 198, 200, 203, 208, 209, 210, 219, 224, 241, 245, 253 film thickness, 102, 194, 197 films, vii, ix, x, xi, 14, 15, 24, 26, 30, 38, 41, 42, 46, 48, 49, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 84, 91, 153, 154, 159, 160, 161, 162, 163, 165, 166, 167, 168, 169, 177, 179, 189, 190, 192, 193, 194, 195, 196, 197, 198, 199, 200, 204, 207, 208, 209, 210, 219, 224, 225, 229, 237, 238, 254, 261 film-substrate interface, 197 filters, 229 Finland, 45, 46 fixation, 11, 221 flame, 162, 239, 242, 255 flat panel displays, 169 flexibility, 53, 157, 160, 205, 231 flora, 56 flow, 156, 158, 159, 163, 178, 179, 185, 186, 193, 203, 221 flow rate, 159, 163, 178, 179, 185, 186 fluid, 7, 31, 42, 263 fluoride, vii, 1, 3, 4, 5, 6, 7, 18, 35, 36, 98 fluoride ions, 3, 4, 5, 6 fluorinated, 66 fluorine, 165 flushing, 46 focusing, vii, x, 1, 177 folding, 206, 208 food, vii, viii, x, 45, 46, 47, 48, 49, 50, 51, 52, 54, 57, 58, 59, 60, 63, 64, 65, 66, 75, 76, 77, 78, 160, 166, 189, 190, 198, 205, 208, 233 food industry, 46, 66

271

food production, 47 food products, viii, 48, 58 food safety, 47, 78 foodborne illness, 47 foodstuffs, vii, viii, 45, 46, 48, 51, 54, 65, 76, 78 formaldehyde, 54 Foucault, 150 Fourier, x, 184, 190, 242, 249 Fourier transform infrared spectroscopy, x, 208 Fox, 81 FRA, 114 fractionation, 127 fracture, 13, 22, 177, 181, 182, 206, 207, 245, 246 fractures, 6, 197, 208 free radical, 159 free radicals, 159 free volume, 51, 53, 76 friction, xi, 161, 163, 237, 242, 243, 244, 245, 246, 254, 256 fruits, 46, 63 FTIR, x, 8, 10, 25, 26, 29, 132, 162, 167, 190, 208, 250 fuel, 252 functionalization, 220 fungi, 46, 60, 61 fungicidal, 61 fungicides, 51 furan, 166 fusion, 155, 241, 245

G gallium, 169 Gamma, 32 garbage, 63 gas, 46, 47, 61, 63, 95, 154, 155, 156, 157, 158, 159, 160, 162, 163, 165, 166, 167, 169, 175, 178, 179, 191, 193, 195, 196, 201, 203, 205, 214, 218, 241, 244, 260, 262 gas barrier, 158, 160, 162 gas diffusion, 205 gas jet, 241 gas phase, 61, 214, 260, 262 gases, xi, 46, 157, 163, 191, 204, 237, 238 gasoline, xi, 237, 252 gel, 8, 11, 24, 47, 62, 64 gelatin, 54 gelation, 91 gels, 37, 63 gene, 5, 7, 10, 17, 29, 35 gene expression, 5, 7, 10, 17, 29, 35 generation, xi, 5, 36, 125, 126, 154, 157, 168, 224, 237, 238, 246, 251, 254 generators, 47, 63 genes, 5, 19 Geneva, 78 Germany, 201, 256

272

Index

glass, ix, x, xi, 8, 59, 64, 65, 153, 158, 161, 166, 167, 213, 215, 238, 239, 242, 251, 253 glass transition, 242, 251 glass transition temperature, 242 glasses, 167, 191 glow discharge, 156, 157, 159 glucose, 47, 64, 165 glucose oxidase, 47, 64 glutaraldehyde, 28, 30, 64, 65 glutathione, 75 glycerol, 47, 53, 61 glycol, 127, 226, 229 gold, 2, 29, 31, 124, 161, 217, 218, 224, 225, 226 gold nanoparticles, 217, 218, 225 gold standard, 2 GPC, 130, 131, 135, 137, 139, 140, 143, 144, 146, 147 grafting, viii, ix, 32, 45, 51, 65, 77, 123, 126, 127, 130, 133, 137, 139, 145, 148, 220 grafts, 136, 137 grain, 178, 180, 181, 219 grain boundaries, 178, 181 grains, x, 177, 178, 180, 184, 186, 227 gram-negative bacteria, 60 gram-positive bacteria, 60 granulocyte, 33 grapefruit, 60 grapes, 63 graphite, 229 green tea, 74 groups, 7, 9, 15, 16, 20, 25, 28, 31, 58, 60, 64, 65, 127, 130, 135, 146, 157, 159, 160, 161, 162, 163, 165, 167, 192, 197, 217, 219, 223, 226, 228, 229, 232, 243, 248, 251, 259, 262, 263 growth, vii, viii, x, 1, 3, 7, 12, 14, 16, 17, 28, 32, 34, 40, 44, 45, 46, 48, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 65, 75, 76, 161, 169, 177, 179, 180, 181, 186, 189, 190, 191, 192, 197, 198, 207, 208, 209, 214, 224, 232, 245 growth factor, vii, 1, 3, 28 growth factors, vii, 1, 3, 28 growth rate, 48, 55, 60, 232 GTE, 74 guidelines, 33

H hafnium, 98 halogen, 126 halogens, 65 handling, 94, 154 hardness, x, 18, 21, 167, 177, 178, 185, 189 harmful effects, viii, 48, 77 HDPE, 54, 65, 72 healing, 4, 6, 20, 27, 28, 32, 36, 39, 40, 42 health, viii, 93, 94 hearing, 243

heat, x, 4, 6, 7, 8, 9, 10, 11, 14, 15, 16, 17, 19, 21, 22, 24, 25, 26, 37, 38, 41, 57, 94, 98, 189, 191, 192, 226, 239, 241 heating, 26, 94, 98, 110, 191, 219, 225, 249, 250, 254 heating rate, 26 heavy metal, 98 heavy metals, 98 height, 2, 240, 243 helium, 154, 155, 158, 159, 161, 162, 163, 165, 166, 167, 168, 169 hematite, 118 hemoglobin (Hb), 224 herbs, 61 herring, 70 heterogeneous, 13, 216 heterogeneous catalysis, 216 high fat, 75 high resolution, ix, x, 177, 183, 185, 190, 219, 262 high temperature, 95, 155, 161, 193, 194, 214, 217 high-tech, 190, 192 hip, 4, 8 hip arthroplasty, 8 hips, 35 histological, 6, 11, 27, 38, 39 histology, 44 histopathology, 35 hMSC, 30 Holland, 256 homogeneity, 156, 197, 222, 253 homogenized, 228, 230 homogenous, 55, 58 homopolymerization, 143 homopolymers, 124 hospital, 233 hospitalizations, 47 host, 222, 223, 224 hostile environment, 178 hot water, 8, 12, 13, 15, 41 HRS, viii, 93, 99, 105, 106, 111, 115, 116, 117, 118, 119, 120 HRTEM, 221, 223, 231 human, viii, 5, 6, 10, 16, 17, 19, 26, 32, 34, 35, 36, 41, 43, 51, 93, 198, 233 humans, 35 humidity, ix, 61, 94, 96, 162, 248 hybrid, x, 168, 175, 189, 207 hydration, 249 hydride, 7 hydro, ix, 16, 54, 57, 63, 75, 123, 124, 132, 133, 145, 148, 161, 162, 163, 195, 196, 198, 221, 222 hydrocarbon, 160, 161, 215 hydrocarbons, 161, 221 hydrochloric acid, 103, 240 hydrofluoric acid, 36, 98 hydrogen, 76, 96, 163, 165, 169, 208, 214, 221, 224, 239, 249, 251 hydrogen peroxide, 214 hydrogen sulfide, 76

Index hydrogenation, 134, 222 hydrolysis, 46, 132, 139, 225 hydrolyzed, 61, 262 hydroperoxides, 248 hydrophilic, ix, 16, 54, 57, 63, 75, 123, 124, 132, 133, 145, 148, 162, 163, 195, 196, 198 hydrophilicity, 163, 195 hydrophobic, ix, 33, 57, 63, 91, 114, 123, 124, 132, 147, 148, 161, 162, 163, 165, 166, 190, 198 hydrophobic properties, 190 hydrophobicity, 30, 195, 197, 201 hydrothermal, 8, 24, 38, 220 hydroxide, 47, 259, 263 hydroxides, 97, 248 hydroxyapatite, vii, 1, 2, 4, 5, 9, 15, 24, 27, 38, 41, 42 hydroxyl, 25, 63, 64, 161, 228 hydroxyl groups, 25, 228 hydroxylapatite, 35 hydroxylation, 248, 249, 250, 254 hydroxypropyl, 53, 55, 56, 73, 74 hydroxypropyl cellulose, 56 hygiene, 63 hypothesis, 249, 254

I ice, 259 identification, 47, 96 IFT, 89 IGF-I, 7 IL-10, 7, 17 IL-6, 7 illumination, 63 images, 147, 181, 185, 197, 198, 199, 200, 203, 221, 222, 223, 225, 231, 232, 261 immersion, xi, 4, 6, 8, 9, 10, 15, 16, 19, 24, 25, 26, 38, 115, 116, 117, 118, 119, 193, 194, 208, 223, 237, 252, 253, 254 immobilization, 23, 28, 29, 42, 43, 47, 55, 64, 215, 226, 232 immunofluorescence, 5 immunohistochemical, 12 immunohistochemistry, 16, 30 impact strength, 205 Impedance analysis, 122 implants, 2, 3, 4, 5, 6, 7, 10, 11, 12, 13, 18, 19, 20, 21, 22, 23, 27, 28, 30, 31, 32, 33, 35, 36, 37, 38, 39, 40, 41, 42, 43, 190, 191 implementation, 161 impregnation, 221, 222, 223 impurities, 232 in situ, 191, 220, 224, 261 in vitro, vii, 1, 2, 3, 7, 12, 13, 24, 28, 29, 31, 35, 36, 37, 38, 41, 42, 44 in vivo, vii, 1, 2, 3, 4, 8, 9, 12, 13, 16, 24, 25, 27, 28, 29, 35, 36, 43, 124 inactivation, 63

273

inclusion, 59, 61 Indian, 85 indicators, 47 indices, 225 induction, 8 induction period, 8 industrial, ix, x, 48, 76, 153, 154, 155, 157, 189, 190, 191, 193, 232, 240, 241 industrial application, ix, 153, 157, 190 industrial production, 76 industrial sectors, 232 industrialisation, 169 industry, viii, 46, 66, 93, 94, 95, 99, 100, 110, 155, 158, 167, 169, 177, 190, 192, 193, 194, 205, 229, 243 inert, ix, 153, 191, 216, 229 inertness, 200 infections, 18 inflammation, 7 inflammatory, 2, 13, 29, 32, 42 inflammatory cells, 42 inflammatory response, 13 infrared, 124, 190, 219, 208, 228, 230, 242, 243, 248 infrared spectroscopy, x, 190, 219, 228, 230, 242 inhibition, 19, 52, 56, 57, 58, 60, 62, 111 inhibitor, 240 inhibitors, 91 inhibitory, 56, 61, 62 inhibitory effect, 56, 61, 62 initial state, 51 initiation, 124, 126, 127, 143, 185 injection, 162, 168, 228, 230 innovation, 79, 82 inorganic, x, 60, 107, 167, 168, 189, 190, 191, 200, 205, 207, 208, 209, 220, 222, 226, 262, 263 inorganic thin films, x, 189, 190 insertion, 20, 218, 221, 222, 223 instabilities, 156 instruction, 100 insulation, 59, 259 insulin, 165 integration, 2, 6, 13, 23, 35, 131, 132, 140 integrin, 28 integrity, ix, 24, 177 intensity, 47, 216, 249, 250, 254 interaction, x, 41, 60, 63, 76, 190, 191, 203, 215, 216, 217, 220, 223, 228, 231, 239 interactions, 25, 28, 29, 41, 46, 54, 76, 141, 148, 161, 215, 225, 226 intercalation, 219 interface, vii, ix, x, 6, 22, 23, 28, 32, 35, 54, 96, 177, 178, 179, 180, 181, 185, 186, 189, 191, 193, 204, 209, 226, 246, 252 interfacial adhesion, 161 interfacial layer, x, 177, 183 interference, 18 interleukin-1, 44 intermetallics, 183 intermolecular, 214

274

Index

interval, 50 intrinsic, 179, 180 Investigations, 58 investment, 155 Iodine, 72 ion beam, 24, 27 ion implantation, 10, 38 ionic, 64, 118, 224 ionization, 155 Ionomer, 69, 72 ions, 3, 4, 5, 6, 7, 9, 10, 13, 15, 16, 17, 19, 21, 22, 23, 24, 25, 26, 40, 57, 58, 63, 76, 94, 96, 103, 155, 180, 185, 216, 219, 224, 229, 231, 232, 233 iron, 95, 96, 97, 99, 100, 103, 105, 107, 120, 225, 240, 248 irradiation, 59, 63, 74, 216, 219, 224, 225, 226, 228, 229, 230, 231, 247 ISO, 100, 121 isoprene, 137, 139 Israel, 213

J January, 87, 89 Japan, 48, 61, 77, 89, 90, 123, 148, 201, 259 Japanese, 81 Jun, 34, 35, 37, 38, 40, 41, 42, 43, 44

K keratin, 231 killing, 47, 233 kinase, 34 kinetics, 29, 48, 52, 53, 219 KOH, 8, 139 Korean, 82

L labeling, 30 lactase, 64 lactic acid, 47, 62, 69, 71 lactic acid bacteria, 47, 62, 69 Lactobacillus, 55, 68, 70 lactoferrin, 51, 55, 64 lactoperoxidase, 62, 64 lactose, 64 lamellae, 123, 124, 197, 198 laminated, 57, 58 lamination, 54 Langmuir, 87, 149, 151, 155, 171, 174, 234, 235, 236 laser, 17, 18, 35, 217 lattice, 104, 181, 183, 184, 225 laundering, 231 LDH, 7, 10, 17, 30

leaching, 52, 53 lead, x, 154, 162, 167, 178, 181, 189, 226, 230, 246, 247 leakage, 95 legislation, 48, 51, 56, 76 legislative, 48, 50 lemongrass, 61 lenses, 167 lettuce, 59, 60, 69, 70, 74 Leuconostoc, 60, 70 leukocyte, 36 lifetime, ix, 156, 159, 177, 217 ligand, 126 ligands, 126 Li-ion batteries, 219 limitation, ix, 127, 153, 191 limitations, x, 48, 154, 189, 191 Lincoln, 84 linear, 30, 124, 130, 131, 133, 141, 147, 179, 248 links, 216 lipids, 46, 53, 58 liquid nitrogen, 207 liquid phase, 214, 260 liquids, 154, 208, 215 Listeria monocytogenes, 55 localised, 96 localization, 12 location, 155 London, 77, 80, 84, 211, 233, 242, 256 longevity, 24 long-term, 58, 63, 210 losses, 62 low density polyethylene, 60 low molecular weight, 50, 137, 140 low-temperature, 191, 218 lubrication, 165, 168 luminescence, 217 luminescent devices, 216 luminosity, 155 lying, 24 lysozyme, 51, 53, 54, 56, 57, 58, 64, 65

M machines, 99 macromolecules, 137, 161 macrophage, 33 macrophages, 33 maghemite, 224 magnesium, 13, 22, 39, 47 magnetic, 215, 216, 224, 225 magnetic properties, 225 magnetite, 118 magnetron, ix, 177, 178, 186, 191, 192, 193, 194, 196, 209 magnetron sputtering, ix, 177, 178, 186, 191, 192, 193, 194, 196, 209 maintenance, 63, 94, 96, 98, 156, 157

Index management, 47 mandible, 20, 22, 31, 32, 39, 40 mandibular, 6, 10, 19, 26, 34, 35 manganese, 95, 96 manipulation, 146 manufacturing, 46, 65, 193 market, 13, 23, 24, 48, 232 Market trends, 49 markets, 48 Marx, 79, 84, 85 mask, 62 mass loss, 242 mass transfer, 46, 50, 52 mastication, 198 material surface, ix, 153 materials science, 214 matrix, 17, 28, 43, 48, 50, 52, 53, 55, 57, 58, 60, 161, 205, 207, 222, 223, 224, 226, 239, 253 matrix protein, 28 maturation, 32 maxilla, 36 measurement, ix, 10, 17, 18, 94, 96, 139, 183, 184, 227 measures, 98 meat, 55, 56, 58, 67, 71, 75, 76 mechanical properties, vii, ix, 1, 2, 3, 12, 14, 60, 124, 177, 181, 183, 217, 224 mechanical stress, 163, 193, 194 media, 57, 120, 124, 159, 239, 253 medicine, 165 MEK, 102 melt, 228, 230, 241 melting, 215, 218, 226, 227, 228, 230, 233, 241, 242, 253 melting temperature, 241, 242, 253 membranes, 162 MEMS, 210 mesenchymal stem cell, 5 mesoporous materials, 220, 222 metabolic, 18, 63 metal ions, 94, 96, 103, 224, 231 metal nanoparticles, 223, 224, 225, 226 metal oxide, 215, 220, 221, 224, 230, 259, 262 metal oxides, 215, 220, 221, 224, 262 metal salts, 223 metals, 15, 37, 51, 57, 97, 98, 190, 191, 215, 224, 226 methacrylic acid, xi, 239, 241, 248, 249 methane, 145, 161, 169, 222 methanol, 224 methyl methacrylate, 91, 134, 136, 162 methylcellulose, 53, 55, 56, 58, 67, 71, 73, 74 mica, 139 micelles, 124, 139, 147 microbes, 51, 57, 58, 60, 63, 65, 75 microbial, vii, viii, 45, 47, 48, 52, 56, 57, 59, 60, 62, 64, 65, 76, 89, 91 microenvironment, 34 microflora, 63

275

microfluidic devices, 158 microorganisms, viii, 45, 46, 47, 48, 54, 58, 59, 229, 231, 233 microscope, 14, 17, 18, 196, 198, 199, 201, 202, 203, 204 microscopy, ix, x, 17, 18, 30, 137, 177, 179, 190, 197, 202, 219, 221, 248, 252, 261 microspheres, 215, 216, 225 microstructure, vii, ix, x, 177, 178, 179, 181 microstructures, 27, 124 microtome, 227, 228 microwave, 156 microwaves, 158 migrant, 51 migration, viii, 28, 45, 48, 49, 50, 52, 53, 54, 77, 208 milk, 56, 59, 72, 74 mimicking, 43 mineralization, 16, 18, 19, 21, 28, 29, 32 mineralized, 12 mitogen-activated protein kinase, 34 mixing, 76 MMA, 134, 136, 162 models, 28, 29 modulation, 141 modulus, 14, 180, 182, 192 moieties, 141 moisture, 46, 48, 49, 57, 61, 62, 63, 69, 75, 84, 126, 190, 249 moisture content, 75 molar ratio, 127, 130, 137 molar ratios, 130 mold, 59, 62 moldings, 238 molecular changes, 247 molecular structure, 51 molecular weight, 50, 51, 53, 124, 127, 130, 133, 134, 135, 136, 137, 139, 140, 143, 145, 242, 251 molecular weight distribution, 124, 127, 134, 135, 137, 139, 143, 145 molecules, 42, 43, 53, 63, 97, 123, 155, 162, 169, 191, 201, 214, 218, 224 molybdenum, ix, 123, 124, 125, 126, 127, 128, 134, 140, 141, 142, 143, 148 money, 77 monocytes, 5, 33 monolayer, 218, 221 monomer, 125, 127, 128, 130, 131, 133, 136, 144, 154, 161, 163, 166, 168, 169, 194 monomeric, 192 monomers, 126, 127, 154, 158, 161, 162, 163, 165, 166, 167, 168, 169, 175, 192 mononuclear cell, 17 mononuclear cells, 17 montmorillonite, 47 morphological, vii, 1, 3, 179, 205, 206, 209, 210, 224 morphology, ix, x, 5, 13, 16, 18, 22, 29, 38, 39, 41, 43, 94, 96, 124, 159, 163, 182, 190, 191, 192, 193, 197, 206, 207, 208, 210, 228, 253 motion, 51, 181

276

Index

mouth, 198, 200 movement, 13, 51, 118 mRNA, 7, 29 MRS, 254 MSP, 220, 221, 222, 223 multilayer films, 177 multiplication, 57 muscle, 12, 61, 67, 71 mustard oil, 62

non-thermal, 154, 155, 156, 158, 191 non-uniform, 18, 75 norbornene, 127, 128, 130, 131, 132, 133, 134, 135, 137, 140, 141, 142, 143 normal, 57, 63, 95, 185, 186 North America, 47 nucleation, 7, 9, 14, 15, 17, 197, 224 nucleic acid, 161 nutrient, 75, 76 nylon, 55, 62, 72, 226, 227, 228, 232

N NaCl, 76, 107, 110, 114, 115, 117, 118, 119, 167 nanoclusters, 219, 224, 231 nanocomposites, 60, 222, 223, 226 nanocrystal, 185 nanocrystalline, 183, 184, 185, 214, 226, 232 nanocrystals, 184, 185, 186, 216, 229 nanoelectronics, 259 nanofibers, 124 nanoindentation, 14 nanomaterials, xi, 213, 215, 226 nanometer, 24 nanometers, 205, 224 nanoparticles, x, xi, 65, 137, 185, 205, 213, 214, 215, 217, 218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230, 231, 232, 233 nanorods, 219 nanosheets, 205 nanostructures, 42, 205 nanotube, 18, 262 nanotubes, xi, 14, 17, 18, 41, 124, 222, 259, 260, 261, 262 NATO, 150 natural, 47, 48, 61, 63, 91, 190, 200, 205, 231, 247 natural food, 48 Nebraska, 84 neem, 61 Netherlands, 85 network, 29, 53, 193, 218, 219, 249, 251 neutralization, 139 New Jersey, 172 New York, 80, 91, 121, 151, 186, 255, 256 NHS, 31, 32 nickel, 83, 95, 191, 192, 193, 194, 209, 215, 216, 222, 225, 226 Nielsen, 82, 85, 90 Nile, 147 Nile Red, 147 nitrate, 18, 76, 96, 229, 231, 263 nitric acid, 98 nitride, 169 nitrogen, 46, 61, 154, 159, 160, 163, 166, 169, 175, 207, 218, 250 nitrogen gas, 175, 218 nitrous oxide, 201 nitroxide, 137 NMR, 129, 131, 132, 135, 137, 139, 144, 147

O oat, 72 observations, vii, 1, 3, 40, 159, 253 occupational, 95 occupational health, 95 oil, xi, 52, 53, 58, 60, 61, 62, 65, 66, 74, 75, 220, 237, 252 oils, 51, 58, 60, 61, 71 olefins, 124, 133, 145 Olefins, 150 oligomer, 140 oligomers, 137, 159 oligosaccharide, 59 olive, 52 olive oil, 52 opacity, 196 optical, vii, x, 23, 27, 76, 154, 162, 181, 190, 195, 196, 198, 201, 202, 203, 204, 215, 216, 219, 220, 223, 224, 225, 228, 230, 252 optical coatings, vii optical microscopy, 190, 202, 252 optical properties, 190, 216, 228, 230 optics, ix, 153, 178, 218 optoelectronics, 218 oral, 33, 34 orange juice, 54, 59, 74 organ, 220 organic, vii, 1, 3, 31, 51, 63, 64, 75, 94, 97, 107, 154, 159, 161, 166, 167, 168, 178, 205, 208, 210, 220, 222, 223, 224, 226, 231, 256 organic compounds, 154 organic polymers, 256 organization, 214 organoleptic, 50 Organometallic, 150, 220 orientation, 2, 51, 76 osmosis, 84 osteoblastic cells, 25 osteoblasts, 29, 30, 34, 35, 42 osteocalcin, 6, 7, 10, 12, 16, 19, 25, 26, 27, 29 osteoinductive, 12 osteopontin, 19 oxidation, 3, 14, 15, 17, 18, 23, 38, 41, 42, 46, 47, 62, 159, 169, 178, 193, 221, 222, 242, 247, 248, 251 oxidation products, 62

Index oxidative, 247 oxidative reaction, 247 oxide, viii, ix, x, 4, 5, 8, 13, 14, 15, 16, 19, 20, 21, 22, 31, 36, 38, 39, 40, 41, 63, 72, 73, 93, 98, 100, 105, 106, 107, 111, 118, 140, 141, 177, 178, 179, 180, 181, 185, 190, 192, 200, 201, 208, 210, 215, 216, 217, 221, 230, 240, 262, 263 oxide thickness, 5, 13, 14, 16, 19, 20, 21, 22, 36, 39, 40 oxides, 14, 19, 31, 38, 40, 47, 177, 200, 203, 215, 216, 220, 221, 233, 248 oxygen, 32, 46, 47, 48, 49, 58, 59, 63, 64, 65, 75, 140, 159, 160, 162, 163, 165, 166, 167, 168, 178, 181, 185, 186, 190, 192, 193, 194, 195, 197, 199, 204, 208, 209, 210, 239, 250, 262 oxygen plasma, 168, 192, 193, 194, 195, 197, 199, 210 oyster, 69

P packaging, vii, viii, x, 45, 46, 47, 48, 49, 50, 51, 52, 54, 55, 56, 57, 58, 59, 61, 62, 63, 64, 65, 66, 69, 75, 76, 77, 79, 82, 160, 166, 167, 189, 190, 192, 205, 208, 256 palladium, 47 paper, 45, 46, 52, 55, 57, 59, 61, 65, 66, 68, 71, 72, 73, 75, 78, 81, 88, 114, 153, 161, 177, 178, 239, 262 papermaking, 59 parameter, 165, 167, 203, 241, 249 particles, 24, 34, 35, 53, 57, 58, 124, 155, 163, 214, 215, 216, 217, 218, 219, 220, 223, 225, 226, 227, 228, 229, 230, 231, 239, 240, 241, 242, 245, 246 passivation, viii, 93, 96, 100 pasteurization, 56 patents, 48 pathogenic, 61, 231 pathways, 28, 34 patients, 8 PbS, 220 PDI, 135 PDMS, 166, 168 pears, 165 pectin, 58 Pediococcus, 56, 61 penalty, ix, 93, 96 peptide, 29, 31, 42, 43, 65, 75 peptides, 28, 29, 30, 31, 43, 51, 64 performance, vii, ix, x, 1, 2, 3, 36, 38, 40, 41, 54, 93, 94, 96, 98, 107, 111, 116, 120, 158, 168, 177, 178, 183, 190, 192, 233 periodic, 110 permeability, xi, 46, 51, 59, 115, 190, 237, 238 permeation, 53, 61, 162, 204 permit, 154 peroxide, 47, 224 personal, 63, 233, 238

277

personal hygiene, 63 Peru, 42 pests, 46 PET, xi, 54, 65, 73, 158, 160, 161, 162, 167, 168, 174, 192, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255, 256, 257 petroleum, 222 pH, 8, 52, 55, 56, 58, 59, 64, 75, 76, 96, 97, 98, 100, 107, 124, 193, 194 pharmaceutical, 124, 162, 166 PHB, 63, 65 phenol, 65, 231 phenolic, 64, 145, 249, 254 phenotypic, 17 phone, 237 phosphate, 3, 4, 7, 9, 16, 17, 18, 24, 25, 27, 36, 52, 58, 94, 95, 96, 97, 98, 99, 100, 101, 103, 105, 106, 107, 109, 110, 120, 121 phosphates, vii, 1, 3, 36 phosphors, 216 photocatalysts, 217 photochemical, 58, 224 photodegradation, 248, 256 photodetectors, 166 photoelectron spectroscopy, 34, 37 photoluminescence, 217, 220 photon, 63, 155 photonic, 220 photovoltaic, 166 photovoltaic cells, 166 phyllosilicates, 205, 207 physical and mechanical properties, 2, 60 physical properties, 207, 218 physiological, 28, 46 piezoelectric, 163 pigments, 46 pigs, 43 pilot study, 43 planar, 9 plants, 94 plasma, vii, ix, 1, 3, 5, 10, 21, 29, 31, 32, 38, 42, 65, 91, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 165, 166, 167, 168, 169, 172, 175, 178, 179, 190, 191, 192, 193, 194, 195, 197, 198, 199, 200, 201, 209, 210, 241, 260 plasma etching processes, 191 plastic, 50, 55, 57, 63, 64, 65, 66, 69, 71, 76, 166, 178, 190, 239, 244, 245 plastic deformation, 178 plastics, x, 46, 54, 57, 62, 167, 189, 190, 196, 238 platelet, 5, 10, 35, 54 platelets, 5, 33 platinum, 47, 114 play, 46, 57, 124, 155, 178 PLGA, 26, 28, 165 PLLA, 65 ploughing, 245, 246 PMMA, 136, 228, 230

278

Index

poisonous, 190 Poisson, 180, 182 Poisson ratio, 180 polar groups, 161, 162 polarity, 51, 52, 76, 163 polarization, 31 pollutants, 222, 247, 248 poly(ethylene terephthalate), 243, 248, 256 poly(methyl methacrylate), 228 polyacrylamide, 64 polyamide, x, 47, 58, 60, 61, 64, 65, 76, 190, 204, 205, 210 polyamide fiber, 65 polyaniline, 261 polybutadiene, 194 polycarbonate, 167, 192 polycondensation, 192 polydimethylsiloxane, 166 polydispersity, 133 polyester, x, 102, 104, 190, 192, 193, 209, 232, 238 polyesters, 192 polyethylene, 56, 58, 60, 158, 162, 239 polyethylene terephthalate, 158, 192 polyhydroxybutyrate, 63 polymer, ix, xi, 29, 50, 51, 52, 55, 57, 62, 64, 65, 73, 75, 78, 91, 123, 124, 126, 127, 129, 130, 132, 133, 135, 137, 139, 140, 145, 148, 153, 154, 155, 158, 159, 160, 161, 162, 163, 164, 166, 167, 168, 175, 191, 192, 197, 198, 199, 205, 206, 207, 213, 224, 225, 226, 227, 228, 229, 230, 237, 238, 240, 241, 242, 245, 246, 248, 249, 251, 253, 254, 255, 260 polymer blends, 238 polymer chains, 51, 205, 230 polymer film, ix, 75, 153, 158, 164, 166, 175, 245, 260 polymer films, ix, 153, 158, 166, 175, 260 polymer materials, 78, 229 polymer matrix, 50, 57, 205, 224, 226, 253 polymer molecule, 123 polymer structure, 159, 242 polymer-based, 256 polymeric films, xi, 190, 237, 254 polymeric materials, 75, 189, 190, 191, 192, 198, 201, 210 polymeric medium, 50 polymerization, ix, 123, 124, 125, 126, 127, 128, 130, 133, 134, 135, 136, 137, 140, 141, 142, 143, 145, 148, 175, 192, 205, 225, 261 polymerization process, 136 polymerization processes, 136 polymerizations, 127, 136, 141 polymers, x, 28, 47, 54, 65, 124, 126, 127, 130, 132, 133, 135, 136, 137, 148, 153, 154, 159, 161, 165, 166, 168, 189, 192, 194, 206, 215, 226, 228, 229, 230, 239, 241, 242, 243, 244, 245, 247, 248, 251, 254, 256 polymorphonuclear, 33 polypropylene, 168, 229 polysaccharide, 59

polystyrene, 44, 64, 127, 128, 129, 132, 135, 140, 141, 147, 225, 228 polytetrafluoroethylene, 161 polyvinyl alcohol, 47, 53, 56, 64 polyvinyl chloride, 165 poor, 4, 5, 7, 23, 33, 105, 107, 110, 167 population, 47, 48, 63 pore, 13, 14, 21, 52, 220, 222, 223, 224, 248 pores, 21, 179, 221, 223, 248 porosity, 13, 21, 22, 32, 40, 178, 186, 253 porous, 8, 12, 15, 31, 32, 37, 38, 41, 42, 57, 161, 163, 219, 220, 222, 224, 229, 239 porphyrins, 63, 65 postoperative, 18 potassium, 47, 53, 55, 58, 60, 72, 74, 75 potato, 57 potatoes, 65 poultry, 67 powder, viii, xi, 18, 30, 47, 55, 56, 57, 58, 93, 100, 102, 104, 114, 115, 120, 168, 218, 238, 239, 240, 241, 242, 243, 249, 251, 253, 254, 255 powders, 239 power, 18, 157, 158, 159, 166, 167, 178, 196, 203, 214, 224 precipitation, 25, 36, 137, 261 preservative, 47, 61, 87 preservatives, 47, 48, 55 pressure, ix, xi, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 165, 166, 167, 168, 169, 174, 175, 178, 179, 193, 195, 196, 201, 203, 213, 232 prevention, 165 printing, viii, 45, 55, 59, 77, 158, 161 pristine, 222, 229, 232 probe, 17 process control, 242 producers, 48 product performance, 96 production, viii, 3, 4, 6, 7, 10, 13, 16, 17, 19, 24, 26, 28, 32, 34, 45, 46, 48, 76, 77, 96, 147, 169, 190, 192, 200, 218, 219, 226, 227, 231, 238, 239, 246 production costs, 169, 190 program, 233 proinflammatory, 33 proliferation, 3, 4, 5, 7, 10, 16, 17, 18, 19, 24, 25, 26, 28, 29, 32, 34, 35, 38 promote, 50, 216, 239 propagation, 126, 133, 185, 246, 253 property, 51, 200, 224, 247 propionic acid, 58 proportionality, 244 propylene, 194 prostaglandin, 34 prostheses, x, 189, 190, 198, 200, 203 prosthesis, 200 protection, x, 46, 57, 84, 94, 105, 107, 111, 118, 124, 125, 145, 166, 167, 178, 181, 189, 190, 200, 231, 233, 239, 241 protective coating, xi, 154, 237, 238, 254

Index protein, 2, 3, 4, 6, 7, 9, 10, 13, 16, 19, 24, 25, 26, 28, 30, 32, 33, 34, 43, 53, 56, 60, 61, 67, 68, 69, 71, 73, 74, 76, 84, 148 protein films, 53, 74 protein immobilization, 33 protein kinase C, 34 protein synthesis, 34 proteins, vii, xi, 1, 2, 3, 4, 7, 13, 28, 29, 30, 31, 51, 54, 76, 91, 127, 159, 161, 213 protocols, 33 protons, 131, 132 Pseudomonas, 18, 61 Pseudomonas aeruginosa, 18 Pseudomonas spp, 61 PTFE, 167 pumping, 193 pumps, 94 pure water, 54 purification, 148, 233 PVA, 75, 229 PVA films, 75 PVC, 54, 55, 69, 165 PVDC, 55, 68 PVP, 229 pyrophosphate, 51 pyrrole, 166

Q QAS, 65 quality of life, 79, 82 quantum, 219, 220 quantum dot, 220 quartz, 163 quaternary ammonium, 65, 162, 231 quaternary ammonium salts, 162 quinone, 251

R radiation, 46, 59, 214, 215, 216, 217, 241, 247, 248, 249 radical, 32, 127, 136, 141, 148 radical polymerization, 127, 134, 136 radio, ix, 47, 177, 193, 201 Raman, 8, 14, 21, 217, 229, 230, 235, 236 Raman scattering, 229 Raman spectra, 217, 230 Raman spectroscopy, 8, 21 range, viii, x, xi, 14, 24, 60, 61, 63, 64, 93, 95, 107, 114, 123, 133, 153, 154, 157, 158, 159, 185, 190, 191, 200, 206, 213, 217, 220, 243, 244, 251, 259 rat, 6, 10, 12, 20, 29, 31, 38 ratings, 104 rats, 32 raw material, 55, 76 raw materials, 55, 76

279

reactant, 219 reactants, 214, 220 reactivity, 125, 145, 159, 191, 217, 228, 247 reagents, 64 receptors, 5, 28 recognition, 141 recombination, 154 reconstruction, 12 recrystallization, 251 recycling, xi, 237, 238, 240, 242, 254 redox, 47, 166, 231 reduction, 19, 20, 36, 54, 60, 62, 120, 155, 203, 210, 218, 221, 222, 223, 224, 229, 232, 240 refining, 220 reflection, 223 refractive indices, 225 regenerated cellulose, 65 regular, 30, 166, 226 regulation, 5, 95 regulations, 95 reinforcement, 39 relaxation, 53, 180 reliability, 48 remodeling, 3 research, vii, viii, ix, x, xi, 1, 2, 3, 13, 28, 45, 49, 77, 93, 124, 135, 148, 153, 154, 155, 157, 158, 161, 166, 169, 177, 185, 189, 190, 192, 201, 210, 213, 214, 224, 233, 241 reservoir, 124 residues, xi, 237, 238, 254 resin, 62, 192, 201, 202, 203, 204, 238 resins, 65, 97, 192, 198, 200, 203, 210 resistance, viii, ix, x, xi, 10, 24, 48, 54, 93, 94, 96, 97, 107, 115, 117, 118, 120, 161, 167, 177, 178, 181, 185, 189, 190, 191, 192, 193, 196, 200, 205, 208, 209, 237, 238, 245, 246, 247, 253, 254, 255 resolution, ix, x, 177, 183, 185, 190, 206, 221, 262 Resource Conservation and Recovery Act, 95 respiration, 46, 62 respiratory, 5, 42 responsiveness, 34 retail, 48 retention, 4, 7, 35, 36, 53, 219 RFA, 20, 21, 22, 39 RFID, 47 ribose, 143 rice, 47 rigidity, 194 rings, 183, 184, 185 risk, 30, 76, 94, 95, 156 risk assessment, 95 risks, 94 rods, 123, 124 rolling, viii, 93, 246 room temperature, x, 15, 128, 132, 179, 185, 189, 193, 196, 201, 207 roughness, x, 2, 3, 6, 10, 13, 14, 16, 17, 19, 20, 21, 22, 26, 33, 34, 40, 100, 167, 177, 178, 181, 195,

280

Index

196, 197, 200, 201, 202, 203, 204, 207, 209, 210, 240, 243, 246 Royal Society, 77, 233 rubber, 161, 194, 246 rust, 100, 120, 167, 248, 253 ruthenium, 126, 134, 135, 137, 145, 148 rutile, 8, 9, 15, 16 rye, 62

S safety, viii, 45, 46, 47, 48, 50, 62, 64, 76, 77, 93, 94, 95, 231 sales, 48, 49 saliva, 198 salmon, 73, 74 salmonella, 56, 60, 61, 63, 67, 68, 69, 70, 71, 73, 74 75, 76 salt, ix, xi, 65, 69, 76, 94, 102, 105, 107, 111, 118, 237, 252, 253, 254 salts, 47, 55, 76, 94, 162, 222 sample, 18, 129, 132, 155, 156, 195, 201, 202, 203, 204, 206, 207, 218, 219, 224 SBF, vii, 1, 2, 3, 4, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 24, 25, 26 scaffold, 28 scalability, 154 Scanning electron, 248 scanning electron microscopy, ix, x, 13, 17, 105, 177, 179, 190, 226, 242 scattering, 229 scavenger, 47, 48, 49 search, xi, 97, 157, 214, 231 secretion, 32, 33, 42 seed, 56, 60, 69, 70, 74 seizure, 246 selected area electron diffraction, 221 self-assembly, 24 SEM, ix, 4, 5, 6, 7, 8, 9, 10, 11, 12, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 29, 30, 32, 99, 105, 106, 107, 160, 161, 162, 163, 164, 177, 178, 179, 206, 226, 227, 229 semiconductor, vii, 169, 191, 219 semiconductors, 215 sensing, 46 sensitivity, 59 sensors, 162, 166, 232 separation, 127, 137, 139, 140, 157, 178, 200, 205 series, xi, 148, 213 shape, 33, 53, 130, 157, 192, 217, 223, 226 shear, 6, 11, 57, 246 shear strength, 6, 11 sheep, 21, 39 shock, 215, 223, 228 shock waves, 223, 228 shoulder, 208 signaling, 28 silane, 96, 97, 103, 105, 111, 118, 166, 169, 201, 208

silanol groups, 219 silica, 47, 64, 65, 137, 163, 167, 168, 215, 216, 217, 218, 219, 220, 221, 223, 224, 225, 228 silicate, 60 silicates, 60, 205, 221 silicon, 42, 65, 97, 139, 160, 168, 169, 190, 192, 200, 203, 208, 210, 218 silicon dioxide, 218 silk, 65, 231 siloxane, 210, 216, 219 silver, x, 18, 57, 58, 60, 76, 189, 217, 219, 223, 225, 226, 227, 228, 229, 230, 231, 232, 233 simulated body fluid, vii, 1, 2, 12, 14, 15, 17, 18, 37 single-wall carbon nanotubes, 260 sintering, 226, 228 SiO2, 167, 217, 218, 219, 220, 223, 226 sites, 22, 124, 216, 230 skeleton, 136 skewness, 7 skin, 239 SLA, 22, 30, 31, 39 sludge, viii, 93, 94, 95, 96, 98 SME, 121 smoothness, 159 SO2, 63, 73 sodium, 7, 8, 9, 10, 12, 15, 18, 21, 37, 38, 47, 52, 53, 55, 56, 57, 58, 62, 71, 75, 193, 231 sodium hydroxide, 47 software, 114 solar, 166, 169, 220, 247 solar cell, 169, 220 solar cells, 220 sol-gel, 2, 3, 23, 24, 25, 26, 41, 42, 263 solid polymers, 226 solid state, 226 solid surfaces, xi, 213, 228 solubility, 24, 51, 160 solutions, viii, 3, 15, 17, 18, 41, 45, 52, 55, 56, 57, 58, 59, 61, 98, 100, 156, 193, 219, 224, 253 solvent, 55, 58, 97, 102, 126, 137, 161, 225 solvents, xi, 237, 238, 252 Sorghum, 70 sorption, 61 species, 118, 125, 126, 133, 134, 135, 154, 155, 156, 158, 208, 217, 224, 228, 248, 249, 250, 254 specific surface, 29, 215 specificity, 127 spectroscopy, 17, 18, 34, 135, 219, 225, 229, 242, 243, 248, 251, 252 spectrum, 63, 65, 158, 208, 220, 229, 243, 249, 250 speed, 13, 14, 100, 103, 215 spheres, 123, 124, 216, 217, 218, 220, 225, 226, 228 spices, 61 sputtering, x, 178, 180, 190, 191, 192, 193, 194, 196, 209 stability, 4, 20, 22, 28, 31, 47, 56, 59, 64, 157, 159, 161, 163, 166, 220, 222, 229, 230, 231, 259 stabilization, 64 stabilizers, 247, 248

Index stages, vii, viii, 1, 3, 19, 20, 21, 28, 29, 31, 93, 96, 98, 100 stainless steel, 64, 193, 195, 201 standards, 121, 128, 129, 132, 135, 147 Staphylococcus, 56, 75 Staphylococcus aureus, 56 starch, 47, 55, 57, 59, 61, 70, 71, 76 stars, 155 steel, viii, ix, x, 64, 93, 94, 97, 98, 99, 100, 111, 120, 167, 177, 178, 180, 186, 193, 195, 201, 239, 240, 241, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255 sterilization, 52 stiffness, 53 storage, 46, 52, 53, 55, 56, 57, 58, 61, 62, 64, 76, 220 strain, 55, 56, 225, 246, 249 strains, 59, 61, 65, 76, 133 strawberries, 68 strength, 7, 8, 13, 15, 23, 24, 36, 40, 55, 57, 59, 161, 181, 183, 185, 186, 191, 194, 243, 247 stress, x, 16, 163, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 189, 193, 194, 197, 246 stress fields, 246 stress level, 177, 180 stretching, 208 stromal, 38 stromal cells, 38 strong interaction, 140, 216 structural characteristics, 123 styrene, 63, 134, 140, 194 styrene polymers, 194 substances, viii, 45, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 64, 75, 76, 77, 95, 124, 198, 208 substitutes, 8 substitution, x, 189, 190 substrates, vii, ix, x, xi, 15, 28, 64, 65, 94, 98, 111, 126, 153, 154, 156, 158, 159, 160, 161, 163, 165, 166, 177, 178, 186, 189, 190, 191, 192, 193, 195, 196, 197, 198, 199, 200, 204, 205, 206, 209, 210, 213, 214, 215, 217, 226, 228, 232, 239 success rate, 23 sugar, 132, 141 sugars, 65, 141, 145, 147, 148 sulfate, 15 sulfidation, 221 sulfuric acid, 15, 18 sulphate, 47 sulphur, 51, 76, 231 Sun, 236, 255, 263 supercritical, 263 superoxide, 47, 63 superoxide dismutase, 47 suppliers, 103 supply, 47 supply chain, 47 surface area, 14, 218, 221, 222, 223 surface chemistry, 13 surface energy, 192, 195, 200, 203, 209, 210 surface layer, 7, 215, 243

281

surface modification, vii, x, 1, 3, 19, 28, 33, 36, 38, 155, 190 surface properties, 39, 52, 64, 157, 195, 219 surface roughness, x, 2, 3, 13, 14, 16, 19, 20, 22, 33, 34, 40, 167, 177, 178, 181, 195, 196, 197, 201 surface structure, 36, 226 surface treatment, viii, 35, 36, 39, 41, 45, 59, 77, 155, 158, 195, 197 surfactant, 24, 224 surfactants, 224, 232 surgical, 2, 27, 36, 63 surviving, 63 Sweden, 1, 3, 7, 13, 23, 100 swelling, xi, 52, 55, 237, 252 synergistic, 47 synergistic effect, 47 synthesis, ix, 34, 123, 124, 126, 127, 130, 132, 133, 134, 135, 136, 137, 140, 141, 142, 143, 145, 148, 150, 192, 215, 217, 220, 222, 225, 226, 229, 232 systems, vii, viii, ix, 12, 13, 23, 27, 32, 45, 46, 48, 49, 50, 54, 63, 64, 75, 93, 96, 99, 116, 124, 153, 154, 168, 178, 181, 196

T tacticity, 51, 76 tanks, 95 tar, 178, 196 taste, 160 TCP, 10, 18, 25, 29, 30 technology, ix, 48, 54, 63, 96, 97, 103, 153, 155, 156, 157, 169, 238 teeth, 2, 198, 200 temperature, xi, 13, 14, 26, 47, 48, 51, 53, 55, 56, 57, 64, 75, 76, 94, 95, 96, 97, 98, 100, 107, 110, 124, 155, 156, 161, 178, 179, 185, 191, 193, 194, 203, 208, 209, 213, 214, 218, 219, 226, 230, 233, 242, 244, 247, 248, 251 Tennessee, 173 tensile, 7, 55, 59, 103, 246 tensile strength, 55, 59 tension, 163 TEOS, 166, 167, 168 terbium, 216 terephthalic acid, 238, 248 tetraethoxysilane, 166 textile, x, 190, 192, 193, 213, 215, 229, 231, 232 textile industry, 190, 193, 229 textiles, 65, 165, 190, 232, 233 TFE, 163 theory, 22, 23, 75, 184, 214 therapy, 35 thermal activation, 191 thermal analysis, 242, 248 thermal decomposition, 238 thermal degradation, 230, 242, 247 thermal expansion, 179, 180 thermal plasma, 155, 158

282

Index

thermal properties, 192 thermal stability, 220 thermal treatment, 219, 231 thermodynamic, 51 thermodynamic properties, 51 thermoplastic, 205, 238, 240 thermoplastics, xi, 192, 237, 238, 254 thin film, vii, x, 41, 154, 155, 162, 163, 165, 166, 167, 181, 189, 191, 196, 203, 207, 225, 259 thin films, vii, x, 41, 155, 162, 163, 165, 166, 167, 181, 189, 190, 191, 196, 203 three-dimensional, 192 thrombin, 5, 36 thrombosis, 30 thrombus, 10 thymidine, 5 tibia, 6, 11, 12, 20, 21, 22, 27 time, 2, 4, 6, 7, 14, 17, 19, 32, 46, 47, 50, 52, 53, 54, 63, 65, 76, 77, 98, 107, 116, 118, 119, 128, 131, 144, 147, 154, 156, 157, 159, 160, 163, 165, 185, 200, 216, 219, 220, 221, 244, 246, 251 tin, 231 TiO2, 33, 34, 35, 41, 63, 65, 73, 217, 218, 219, 220, 221, 222, 224, 248 tissue, vii, 1, 2, 3, 7, 8, 11, 13, 18, 19, 28, 29, 31, 32, 37, 39, 40, 41, 70, 158, 161 titania, 8, 15, 18, 24, 25, 26, 37, 41, 42, 215, 217, 218, 220, 224, 228 titanium, vii, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 98, 133 titanium dioxide, 35 TNF-alpha, 32, 33, 42 Tocopherol, 66, 68 Tokyo, 89, 90 tolerance, 163 toluene, 127, 128, 135, 136, 144, 221, 222 tomato, 52 topographic, 33 topology, 124 torque, 16, 20, 22, 39, 40 toughness, 177, 178, 194, 247, 248 toxic, viii, 93, 96, 97 toxicity, 50 toxin, 47 trabecular bone, 3, 35, 43 trade, 96 trans, 125 transcription, 5, 12 transcription factor, 5, 12 transcription factors, 5 transfer, 50, 94, 127, 135, 140, 148, 206, 224, 245, 246 transformation, 127, 134 transformations, 133 transistor, 259 transistors, 166, 169

transition, ix, 23, 47, 123, 125, 156, 157, 185, 219, 220, 244 transition metal, ix, 47, 123, 125 transition temperature, 242 transitions, 244 translational, 51 transmission, 3, 30, 168, 179, 217 transmission electron microscopy, vii, 1, 3, 36, 147, 178, 179, 180, 185, 217, 218, 219, 221, 222, 223, 225, 227, 261, 262 transparency, 55, 57, 160, 167, 200, 206 transparent, 46, 114, 167, 240, 242 transport, 46, 50, 51, 219 trauma, 2 trend, 14, 48, 126 tribological, xi, 237, 242, 244 tribology, 238 trimer, 145 tubular, 165, 197, 198 tungsten, 252 tungsten carbide, 252 Turkey, 67, 70, 71, 74 two-dimensional, 223

U ulna, 6 ultrasonic waves, vii, xi, 213, 214, 217 ultrasound, 214, 215, 216, 217, 219, 224, 225, 226, 228, 231, 233 ultra-thin, vii, 1 ultraviolet, 65, 76, 124, 195, 196, 197, 224, 228, 230, 247, 248, 249 ultraviolet irradiation, 247 ultraviolet light, 247, 248 uniform, 23, 163, 165, 167, 169, 198, 218, 219, 221, 223, 225, 226, 232, 260 UV light, 124, 224 UV radiation, 249

V vacuum, ix, 49, 54, 55, 57, 75, 76, 153, 154, 155, 160, 161, 169, 192, 201 values, 6, 14, 16, 20, 21, 22, 23, 31, 50, 111, 118, 120, 130, 131, 132, 135, 143, 145, 178, 183, 184, 185, 195, 196, 244, 251 vapor, 23, 190, 191, 201, 214, 224 variable, 137 variation, 141, 197, 217, 223 vegetables, 60 velocity, 252 versatility, 239 vessels, 31 vibration, 229 vibrational modes, 208 violent, 193

Index viscosity, 96 visible, 63, 100, 167, 206, 208, 247, 248, 252 vision, 161 vitamins, 46, 47 voids, 6

W waste disposal, 95, 98 waste water, 94, 98, 222 water, 8, 9, 10, 47, 52, 53, 54, 57, 58, 62, 63, 75, 94, 95, 98, 100, 107, 110, 131, 159, 160, 165, 167, 190, 192, 204, 214, 216, 217, 218, 219, 222, 230, 233, 247 water diffusion, 53 water vapor, 190, 204 water-soluble, 229 wave number, 251 wear, xi, 177, 178, 185, 237, 238, 241, 244, 245, 246, 254, 256 weathering, 238, 247, 248, 249, 251 weight loss, 62 welding, 158 wet coating, viii, 93 wettability, 167 wheat, 53, 56, 62, 66 whey, 53, 56, 60, 61, 71, 76 windows, 242 wood, 59 wool, 231 workers, 95 World Health Organisation, 78

283

wound healing, 161

X xenon, 247 X-ray photoelectron spectroscopy (XPS), 4, 6, 8, 9, 10, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 29, 30, 31, 32, 99, 115, 118, 163, 218, 221, 223, 226, 228, 229, 231, 242

Y yarn, 227, 232 yeast, 55, 59, 71 yield, 128, 129, 130, 131, 132, 143, 144, 145, 147, 214, 216 YSZ, 217 yttrium, 217

Z zeolites, 47 zinc, xi, 94, 95, 96, 97, 98, 99, 100, 101, 103, 105, 106, 107, 108, 110, 111, 112, 118, 119, 120, 121, 219, 220, 225, 230, 238, 239, 253, 254 zinc oxide, 230, 233 zirconia, 215, 217, 220 zirconium, 58, 60, 70, 98, 103, 107, 110, 111, 217