Handbook of Polymer Foams

Handbook of Polymer Foams

Editor: David Eaves

Handbook of Polymer Foams

Editor: David Eaves C. Vasile

Rapra Technology Limited Shawbury, Shrewsbury, Shropshire, SY4 4NR, United Kingdom Telephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118 http://www.rapra.net

First Published in 2004 by

Rapra Technology Limited Shawbury, Shrewsbury, Shropshire, SY4 4NR, UK

©2004, Rapra Technology Limited

All rights reserved. Except as permitted under current legislation no part of this publication may be photocopied, reproduced or distributed in any form or by any means or stored in a database or retrieval system, without the prior permission from the copyright holder. A catalogue record for this book is available from the British Library.

Every effort has been made to contact copyright holders of any material reproduced within the text and the authors and publishers apologise if any have been overlooked. Cover photograph reproduced with permission from EVC

ISBN: 1-85957-388-6

Typeset, printed and bound by Rapra Technology Limited Cover printed by The Printing House, Crewe, UK

Contents

Contents Preface .................................................................................................................. ix 1

Foam Fundamentals ........................................................................................ 1 1.1

Introduction ........................................................................................... 1

1.2

Foam Structure ...................................................................................... 1

1.3

Foam Properties ..................................................................................... 3 1.3.1

Compression Properties ............................................................. 3

1.3.2

Energy Absorption Properties .................................................... 5

1.3.3

Thermal Properties .................................................................... 6

References ....................................................................................................... 8 2

Blowing Agents ............................................................................................... 9 2.1

Introduction ........................................................................................... 9

2.2

Physical Blowing Agents ...................................................................... 10

2.3

2.2.1

Selection Criteria for Physical Blowing Agents ........................ 10

2.2.2

Halogenated Hydrocarbons ..................................................... 13

2.2.3

Hydrocarbons (HC) ................................................................. 20

2.2.4

Inert Gases ............................................................................... 22

2.2.5

Other Physical Blowing Agents ................................................ 24

2.2.6

Blends of Physical Blowing Agents ........................................... 24

2.2.7

Encapsulated Physical Blowing Agents .................................... 25

2.2.8

Physical Blowing Agent by Foam Type and Application .......... 25

Chemical Blowing Agents .................................................................... 25 2.3.1

Selection Criteria for Chemical Blowing Agents ...................... 29

2.3.2

Exothermic CBA ...................................................................... 30

2.3.3

Endothermic CBA .................................................................... 32

2.3.4

Endo/Exo Blends ...................................................................... 33 i

Handbook of Polymer Foams

References ..................................................................................................... 34 3

Expanded Polystyrene: Development, Processing, Applications and Key Issues ............................................................................................... 37 3.1

Introduction ......................................................................................... 37 3.1.1

Development of Expanded Polystyrene (EPS) .......................... 37

3.2

Manufacture of Expanded Polystyrene Mouldings .............................. 38

3.3

Applications for Expanded Polystyrene Packaging ............................... 41

3.4

3.5

3.6

3.3.1

Packaging ................................................................................ 41

3.3.2

Construction ............................................................................ 42

3.3.3

Other Applications .................................................................. 43

3.3.4

Novel Applications .................................................................. 44

Properties of EPS ................................................................................. 44 3.4.1

Mechanical Performance .......................................................... 45

3.4.2

Thermal Insulation .................................................................. 45

3.4.3

Chemical Properties ................................................................. 46

3.4.4

Recent Research on Properties of EPS: Value for Fruit and Vegetables ................................................................ 46

Global Structure of Markets and Companies ....................................... 47 3.5.1

Europe ..................................................................................... 47

3.5.2

Asia .......................................................................................... 48

3.5.3

USA ......................................................................................... 49

Key Issues Facing the EPS Industry ...................................................... 50 3.6.1

Fire .......................................................................................... 50

3.6.2

Recycling ................................................................................. 51

3.6.2

Alternatives to Mechanical Recycling ...................................... 53

Further Information ...................................................................................... 54 4

Rigid Polyurethane Foams ............................................................................ 55 4.1

ii

Introduction ......................................................................................... 55

Contents

4.2

Materials.............................................................................................. 57 4.2.1

Polyols ..................................................................................... 57

4.2.2

Isocyanates .............................................................................. 60

4.2.3

Blowing Agents ........................................................................ 62

4.2.4

Other Additives ....................................................................... 71

4.3

Manufacturing Processes for Rigid Polyurethane Foam ....................... 73

4.4

Recycling Processes for Rigid Polyurethane Foam ............................... 75

4.5

Properties of Rigid Polyurethane Foams .............................................. 76

4.6

Applications ......................................................................................... 78 4.6.1

Applications in Construction ................................................... 78

4.6.2

Applications in the Appliance Industry .................................... 80

References ..................................................................................................... 82 5

Flexible Polyurethane Foam .......................................................................... 85 5.1

Introduction ......................................................................................... 85

5.2

Chemistry ............................................................................................ 85

5.3

Starting Materials ................................................................................ 87 5.3.1

Isocyanate ................................................................................ 88

5.3.2

Polyol ...................................................................................... 88

5.3.3

Water ....................................................................................... 90

5.3.4

Surfactant ................................................................................ 90

5.3.5

Catalyst ................................................................................... 91

5.3.6

Colorants ................................................................................. 92

5.3.7

Antioxidants ............................................................................ 92

5.3.8

Light Stabilisers ...................................................................... 93

5.3.9

Flame Retardants ..................................................................... 93

5.3.10 Adhesion Promoters ................................................................. 94 5.3.11 Other Additives ....................................................................... 94 5.4

The Foaming Process ........................................................................... 94

iii

Handbook of Polymer Foams

5.5

5.6

5.4.1

Raw Material Conditioning ..................................................... 95

5.4.2

Mixing ..................................................................................... 95

5.4.3

Growth .................................................................................... 96

5.4.4

Cell Opening ............................................................................ 97

5.4.5

Cure ......................................................................................... 98

Manufacturing Equipment ................................................................... 99 5.5.1

Storage and Delivery .............................................................. 100

5.5.2

Mixing ................................................................................... 101

5.5.3

Foam Rise and Cure .............................................................. 101

5.5.4

Innovations ............................................................................ 103

Foam Characterisation ....................................................................... 104 5.6.1

Density .................................................................................. 104

5.6.2

Hardness ................................................................................ 104

5.6.3

Resilience ............................................................................... 105

5.6.4

Porosity ................................................................................. 106

5.6.5

Strength Properties ................................................................. 106

5.6.6

Cell Structure ......................................................................... 107

5.6.7

Environmental Stability ......................................................... 107

5.6.8

Fatigue ................................................................................... 108

5.6.9

Compression Set .................................................................... 108

5.6.10 Flammability .......................................................................... 108 5.7

FPF Markets ...................................................................................... 109 5.7.1

Transportation ....................................................................... 110

5.7.2

Comfort ................................................................................. 110

5.7.3

Carpet Cushion ...................................................................... 110

5.7.4

Packaging .............................................................................. 111

5.7.5

Specialty Applications ............................................................ 111

5.8

Environmental Issues ......................................................................... 111

5.9

Organisations .................................................................................... 113

References ................................................................................................... 114 iv

Contents

6

Rigid PVC Foam ......................................................................................... 123 6.1

Introduction ....................................................................................... 123

6.2

Foam Extrusion ................................................................................. 124

6.3

6.2.1

Basic Principles ...................................................................... 125

6.2.2

Extrusion Processes ................................................................ 126

6.2.3

Effect of Processing Conditions ............................................. 131

Foam Formulation Technology .......................................................... 132 6.3.1

Blowing Agents ...................................................................... 133

6.3.2

Processing Aids ...................................................................... 138

6.3.3

Type of PVC .......................................................................... 140

6.3.4

Stabilisers ............................................................................... 140

6.3.5

Lubricants .............................................................................. 141

6.3.6

Typical Formulations ............................................................. 141

6.4

Properties ........................................................................................... 143

6.5

Novel Processes and Applications ...................................................... 145

6.6

6.5.1

Recycling ............................................................................... 145

6.5.2

Microcellular Foam ............................................................... 146

6.5.3

Foamed Composites ............................................................... 147

Summary ............................................................................................ 147

References ................................................................................................... 148 7

Flexible PVC Foams .................................................................................... 155 7.1

Introduction ....................................................................................... 155

7.2

Flexible Foam Types and PVC Types ................................................. 155 7.2.1

Flexible Foams Based on Suspension PVC ............................. 155

7.2.2

Flexible Foams Based on Dispersion or Paste Resins ............. 156

7.2.3

Chemically Blown Foams from PVC Plastisols: Fundamentals ........................................................................ 156

7.2.4

PVC Resins used in Plastisol Foam Formation ....................... 160

v

Handbook of Polymer Foams

7.3

7.2.5

Mineral Fillers ....................................................................... 161

7.2.6

Pigments ................................................................................ 162

7.2.7

Liquid Plasticiser .................................................................... 162

7.2.8

Blowing Agent Type and Level ............................................... 165

Products Utilising Foamed Plastisols .................................................. 166 7.3.1

Floorings and Carpet Backings .............................................. 166

7.3.2

Wallcoverings ......................................................................... 168

7.3.3

Synthetic Leather ................................................................... 169

7.3.4

General Foams ....................................................................... 170

References ................................................................................................... 171 8

vi

Polyolefin Foams ......................................................................................... 173 8.1

Introduction ....................................................................................... 173

8.2

Manufacturing Processes and Materials ............................................. 174 8.2.1

Extruded Non-Crosslinked Foam .......................................... 174

8.2.2

Expanded (Non-Crosslinked) Polyolefin Beads ...................... 177

8.2.3

Extruded Crosslinked Foam - Processes ................................. 179

8.2.4

Press Moulded Crosslinked Foam Process ............................. 186

8.2.5

Injection Moulded Foam Process ........................................... 189

8.2.6

The Nitrogen Autoclave Process ............................................ 189

8.2.7

Recycling Processes ................................................................ 193

8.2.8

Post Manufacturing Operations ............................................. 194

8.3

Properties of Polyolefin Foams .......................................................... 195

8.4

Applications ....................................................................................... 197

8.5

Foam Specifications ........................................................................... 200 8.5.1

Packaging .............................................................................. 200

8.5.2

Automotive ............................................................................ 201

8.5.3

Furnishings ............................................................................ 201

8.5.4

Buoyancy ............................................................................... 201

Contents

8.6

8.5.5

Aerospace .............................................................................. 201

8.5.6

Construction .......................................................................... 202

8.5.7

Toys ....................................................................................... 202

8.5.8

Food contact .......................................................................... 202

Markets ............................................................................................. 202

References ................................................................................................... 203 9

Latex Foam ................................................................................................. 207 9.1

Introduction ....................................................................................... 207

9.2

Dunlop Process .................................................................................. 208 9.2.1

Batch Process ......................................................................... 209

9.2.2

Selecting a Formulation for Latex Compounds ...................... 215

9.2.3

Selection of Other Compounding Ingredients ........................ 217

9.2.4

Continuous Process for Latex Foam Production .................... 229

9.3

Talalay Process ................................................................................... 230

9.4

Troubleshooting in Latex Foam Manufacture .................................... 235

9.5

Testing ............................................................................................... 236

9.6

9.5.1

Compression Set .................................................................... 236

9.5.2

Indentation Hardness ............................................................. 236

9.5.3

Flexing Resistance .................................................................. 238

9.5.4

Density .................................................................................. 238

9.5.5

Metallic Impurities ................................................................. 238

Important Uses of Latex Foam........................................................... 238 9.6.1

Transportation ....................................................................... 238

9.6.2

Furniture ................................................................................ 239

9.6.3

Special Uses ........................................................................... 239

References ................................................................................................... 239 10 Microcellular Foams ................................................................................... 243

vii

Handbook of Polymer Foams

10.1 Introduction ....................................................................................... 243 10.2 Processing of Microcellular Foams .................................................... 244 10.2.1 The Solid-State Batch Process ................................................ 244 10.2.2 The Semi-Continuous Process ................................................ 247 10.2.3 Extrusion and other Processing Methods ............................... 248 10.3 Properties of Microcellular Foams ..................................................... 249 10.4 Current Research Directions .............................................................. 254 10.4.1 Microcellular Materials for Construction .............................. 254 11.4.2 Open-Cell (Porous) Microcellular Foams ............................... 255 10.4.3 Sub-Micron Foams and Nanofoams ...................................... 256 10.5 Commercial Opportunities ................................................................ 261 References ................................................................................................... 262 Abbreviations .................................................................................................... 269 Contributors ...................................................................................................... 275 Index ................................................................................................................. 277

viii

Preface

Preface

The use of polymer foams is extremely widespread. Indeed, it is difficult to think of a single industry where polymer foams do not have a significant application. They can be found for example in sports and leisure products, in military applications, in vehicles, in aircraft, and in the home. Most people will encounter polymer foams every day in one form or another, whether it be in furniture, in packaging, in their car, in refrigerator insulation, or in some other common application. Although naturally occurring polymer foams have been known for a long time, (e.g., sponges, cork), synthetic polymer foams have only been introduced to the market over the last fifty years or so. The development of a new polymer has usually been quickly followed by its production in an expanded or foam form owing to the unique and useful properties which can be realised in the expanded state. Compressibility, thermal insulation and, of course, low density/lightness are all enhanced in a foam as compared with the original polymer. Such properties can be tailored to meet particular requirements and can increase substantially the potential applications of a polymer. Foaming technology and the properties of expanded materials should therefore be of interest to all those working in the polymer industry. This Handbook aims to review the chemistry, manufacturing methods, properties and applications of the synthetic polymer foams used in most applications. In addition, a chapter is included on the fundamental principles which apply to all polymer foams. There is also a chapter on the blowing agents used to expand polymers, blowing agents having undergone considerable change and development in recent years in order to meet the requirements of the Montreal Protocol in relation to the reduction and elimination of chloroflurocarbons (CFC) and other ozone depleting agents. A chapter is also included on microcellular foams - a relatively new development where applications are still being explored. Each chapter has references to facilitate further exploration of the subject. I am very much indebted to my co-authors for their contributions. They all have substantial expertise on their particular topics and have given considerable time to the production

ix

Handbook of Polymer Foams of their chapters. I am also indebted to the staff at RAPRA for their work in pursuing and assembling the various contributions to produce the final composite, especially Frances Powers (Commissioning Editor), Claire Griffiths (Editorial Assistant) and Sandra Hall (Graphic Designer).

David Eaves Harbury, UK November 2003

x

Foam Fundamentals

1

Foam Fundamentals David Eaves

1.1 Introduction Although polymer foams are based on a wide variety of materials and are manufactured in many different forms, there are some principles and concepts which apply to all foams. These have been reviewed in depth elsewhere [1, 2]. Since these concepts are useful in understanding the relationships between polymer properties and foam structure and can facilitate the prediction of the physical behaviour of foams in typical applications, this chapter provides a summary of some of these fundamentals.

1.2 Foam Structure Most foams are formed by a process involving nucleation and growth of gas bubbles in a polymer matrix (exceptions being syntactic foam where micro-beads of encapsulated gas are compounded into a polymer system, and latex foam). As the bubbles grow, the foam structure changes through stages which may be described as follows: •

Initially, small dispersed spherical bubbles are generated in a liquid matrix, with a small reduction in density.

•

Whilst the bubbles grow but still remain spherical, the lowest foam density is achieved when bubbles reach close packed structure.

•

Further growth and lower foam density then involves distortion of cells to form polyhedral structures, sometimes idealised as pentagonal dodecahedrons.

•

Viscous and surface tension effects subsequently cause material to flow towards intersecting cell elements to form junctions of tricuspid cross-section.

•

A final stage may involve rupture of cell walls to result in an open cell foam.

1

Handbook of Polymer Foams Depending on the degree of expansion and the particular formulation, foams can have any or a combination of these structures. A comprehensive characterisation may therefore include a measure of: a) Foam density - usually expressed as kg/m3. A more fundamental measure is relative density (φ), i.e., density of foam (ρf) compared with density of the original polymer (ρs), or φ = ρf/ρs Relative density may also be regarded as the volume fraction of polymer in a foam. Low density foams are generally regarded as those with volume fraction of polymer <0.1. b) Cell size which may be measured directly by inspection of a foam cross-section. A convenient method is that described in the US Standard ASTM D3576 [3] in which a count is made of the number of cells intersecting a specific length. The average chord length t is determined and the average cell diameter d is calculated from d = t / 0.616. In practice, there is usually a distribution of cell sizes, sometimes a very wide distribution. Most foam manufacturers quote either an average cell size, or a range of cell sizes for their foams. The production of cells of different sizes is caused partly through cells being nucleated randomly in space and time, and partly through large cells growing by diffusion of gas from small cells. A uniform cell size is not a stable structure. Diffusion of gas from small to large cells can be understood from the classic equations giving the pressure inside a bubble enclosed by liquid: P = 2γ/r where P is the pressure differential between gas and surrounding liquid, γ is the interfacial surface tension and r is the bubble radius. Small bubbles contain gas at a higher pressure than large ones; hence, diffusion is always from small to large. c) Open cell/closed cell ratio - determined for example by water absorption or permeation. This ratio is important in determining properties, especially in low density foams. In flexible polyurethanes (PU), it is also important in the manufacturing process as, unless the cells open at the end of the expansion step, the rapid diffusion of carbon dioxide from the cells, much faster than air can diffuse in to replace it, will cause foam collapse. d) Anisotropy - unless foams are made in conditions allowing essentially free expansion in three dimensions, they can exhibit substantial structural (and property) anisotropy.

2

Foam Fundamentals This is frequently the case with foams produced by extrusion processes where cells have some lateral distortion. This results in differences in physical properties measured in the length and the width directions. Manufacturers will sometimes quote properties measured in these two directions. e) Cell shape - the topology of cells in foams has been studied extensively, perhaps first by Robert Hooke in 1664 who observed and described the porous structure of cork using his recently perfected microscope. Low density polymeric foams formed by the growth of nucleated expanding gas bubbles are often considered to have cells in the form of pentagonal dodecahedra. Five-sided faces are frequently identified in microscopical observation, and the faces meet in tetrahedral junctions which closely approximate the angles necessary to maintain a stable structure. However, this geometry is not quite space filling, and a form which more closely fits many observations is the β-tetrakaidecahedron, which includes some four sided faces. Cell struts however are generally curved, more so as the foam density increases.

1.3 Foam Properties

1.3.1 Compression Properties All foams show a compression stress-strain curve (Figure 1.1), which may be split into three regions: •

Region 1 - linear ‘Hookian’ behaviour

•

Region 2 - collapse plateau

•

Region 3 - densification

In region 1, linear elastic behaviour is controlled by cell wall bending and, in closed cell foams, by cell wall stretching due to the contained gas pressure. In region 2, the cells collapse through cell wall buckling, or in brittle foams, by cell crushing and cell wall fracture. In region 3, densification occurs. As foam density increases, Young’s modulus increases in region 1, the plateau stress value increases, and the strain at which densification starts reduces. In closed cell foams, the compression of gas in the cells also contributes, more so in regions 2 and 3 than 1. At large compressive strains the stress-strain curve increases sharply due to densification, and tends towards a limiting slope (very considerably higher than the initial modulus) at

3

Handbook of Polymer Foams

Figure 1.1 Schematic compression stress-strain curve for a foam

a limiting strain. This strain is lower than can be calculated by assuming it is the point at which all porosity has been eliminated, as the cell walls join together at a somewhat lower strain. Using idealised cell structures, equations have been derived relating foam properties to the foam structure, the matrix polymer properties, and the relative density of the foam. Initial (Young’s) compression modulus, shear modulus, elastic collapse (plateau) stress, and Poisson’s ratio (the negative ratio of lateral to axial strain) have all been considered. However, for practical purposes and because in many foams the initial polymer properties are modified by reactions during foam manufacture such as crosslinking, foam properties are always measured directly and given by manufacturers in tables or figures.

4

Foam Fundamentals

1.3.2 Energy Absorption Properties Many foam applications make use of the energy absorption capability of foam structures which perform much better in this regard than the solid polymers from which they are made. This arises from the ability of foams to keep the peak force (or deceleration G) below the limit which results in damage to a packaged object, whilst absorbing the energy of impact. Energy is absorbed essentially in the plateau region of the stress-strain curve where cells deform by elastic buckling, plastic yielding or brittle crushing. In elastomeric foams, such as flexible PU, the plateau stress is determined by elastic buckling of the cells and much of the external work is stored during impact deformation and released after impact. However, some energy is dissipated by hysteresis effects due to viscous flow of the contained fluid (air) in open cell foams and by hysteresis in polymer deformation. In plastic foams, the energy is absorbed by plastic flow of cells and there is often little if any immediate recovery. Brittle foams also absorb energy with little recovery, energy absorption occurring by cell fracture and crushing. Polystyrene foams are an example of the latter showing good impact absorption with little recovery, and hence are useful only for the first impact. Common applications therefore use polystyrene foams for single transit packaging. Closed cell polyethylene foams are an example of a material which absorbs energy mainly by plastic deformation of the cells. Whilst there is no instant recovery after impact, such foams do show substantial recovery longer term due to the restoring pressure of gas in the cells and, in crosslinked foams, network restoring effects. These foams offer good performance in multiple impacts and have applications in packaging designed for more than one journey (with consequent environmental benefits). Various methods have been put forward to enable the packaging, or cushioning, ability of foams to be characterised. Most manufacturers of foams intended for cushion packaging applications publish cushioning curves which are generated by measuring the peak deceleration of a series of falling weights dropped onto the foam from a given height. The peak deceleration, G, is plotted against cushion loading, i.e., the static load exerted on the foam by the weight when at rest, to obtain a cushion curve. These always show a minimum G at a particular static stress. At high static stress, i.e., greater falling weights - the foam becomes densified and generates higher G. At low static stress, the foam is not compressed sufficiently to reach the plastic deformation region, resulting again in higher G. Cushion packaging design aims to create conditions at or near the minimum in the cushion curve so as to provide the maximum protection. Some further information on cushion packaging is given in Chapter 8, Polyolefin Foams, and mathematical treatments of energy absorption in foams are given in [1, 2].

5

Handbook of Polymer Foams

1.3.3 Thermal Properties

1.3.3.1 Thermal Conductivity A major application for polymer foams is thermal insulation in areas including building and construction (wall and floor insulation, pipe insulation), transportation (refrigerated food trucks, liquefied gas tankers) and appliances (refrigerator linings). Although in construction applications mineral wool remains the insulating material most used, the superior properties of polymer foams, better insulation, easier handling, no water absorption (in closed cell foams), compensates for their generally higher cost and polystyrene foam with rigid PU foam are used in very significant amounts [4]. The thermal conductivity of a foam is governed by four factors: •

conduction of heat through the solid polymer

•

conduction of heat through the gas

•

convection of heat through the cells

•

radiation through cell walls and across voids

Convection is only significant in foams having extremely large cell sizes greater than some 10 mm, and may therefore be neglected in most commercial foams which have cell sizes in the range 0.1–2 mm. The other factors all contribute significantly, with conduction through the gas generally the major contributor, with radiation and conduction through the solid together amounting only to about one-third of the gas conduction. An example given by Gibson and Ashby [1] which shows the relative contributions of these factors is for a closed cell polystyrene foam with a relative density of 0.025, containing air at 0.1 MPa. This has a thermal conductivity of 0.04 W/mK. The contribution from the polystyrene solid was calculated as 0.003 W/mK, and that from the contained air 0.024 W/mK, which leaves 0.013 W/mK as the radiative contribution, convection being assumed to be insignificant. In this case the relative contributions amount to: Solid

62.5%

Gas

7.5%

Radiation

32.5%

Work by Glicksman and Cunningham in the 1980s also showed contributions from radiation to be about 30% of the total.

6

Foam Fundamentals As the foam density reduces, the amount of thermal conduction through the solid polymer reduces correspondingly and conductivity of the foam falls to a minimum at (for closed cell foams) a relative density of around 0.05. Below this value the radiative contribution increases steeply as the foam density approaches zero due to the increasing transparency of the cell walls to radiation. Above this value, conduction through the solid begins to dominate. At the minimum, foam conductivity is not much greater than that of the air contained in the cells. The most used method of reducing conductivity further is to replace air with a contained gas of lower conductivity, such as trichlorofluoromethane (TCFM). Thermal conductivity values for air and TCFM are: Air

0.025 W/mK

TCFM

0.08 W/mK

This has been done for some while, for example in insulating foams based on rigid PU, development considerations then being the rate of diffusion of TCFM out of the foam and replacement by air (with a resulting deterioration in insulation ability), and more recently the replacement of chlorofluorocarbons (CFC) by alternative materials which are less environmentally damaging but which generally have an inferior insulation performance. The effect of cell size on thermal conductivity in foams with cell diameters less than 2 mm is relatively small, but the tendency is for conductivity to reduce as the cell size decreases. This is due not so much to lower convection effects as to reduced radiation owing to the greater number of internal reflections from cell walls. Some minor improvements (reductions) in thermal conductivity can be made by modifying the cell structure to increase the reflectivity of cell walls and hence reduce radiation conduction. Recent work on rigid PU foams for insulation applications has concentrated on CFC replacements and consequent work to recover CFC insulation performance by reducing radiation effects (by cell structure modification) and improving long term ageing [5]. Heat transfer through foams decreases sharply as the temperature is lowered, since the thermal conductivity of solids and gases decreases with temperature and radiation is also less at low temperatures. Thermal conductivities of foams are measured and quoted at a standard temperature, usually 10 ºC.

1.3.3.2 Other Thermal Properties Melting point, or softening point, of a foam is essentially that of the solid polymer. This may be a relatively sharp transition, as for foam produced from high density polyethylene

7

Handbook of Polymer Foams (melting transition), or a wider transition, as with foams from low density polyethylene (melting transition) or polystyrene (glass transition). The specific heat of a foam is essentially that of the solid from which it is made, since the contribution from the contained gas is very small. Thermal expansion coefficient is, like specific heat, essentially that of the solid polymer, and most polymer foams have an expansion coefficient of about 10-4/K. However, closed cell plastic or elastic foams of low density tend to have higher expansion coefficients due to the pressure exerted on the foam by the contained gas as temperature increases. The initial expansion in such low density foams is often followed by a reduction in volume as the gas diffuses out of the foam and, if the temperature is sufficiently high approaching the softening point of the polymer, the volume can reduce to less than the original as the foam cells begin to collapse.

References 1.

L.J. Gibson and M.F. Ashby, Cellular Solids, 2nd Edition, Cambridge University Press, Cambridge, UK, 1999.

2.

Low Density Cellular Plastics, Physical Basis of Behaviour, Eds., N.C. Hilyard and A. Cunningham, Chapman and Hall, London, UK, 1994.

3.

ASTM D3576-98, Standard Test Method for Cell Size of Rigid Cellular Plastics, 1998.

4.

D.E. Eaves, Polymer Foams, Trends in Use and Technology, Rapra Technology, Shrewsbury, UK, 2001.

5.

M.A. Schütz and L.R. Glicksman, Journal of Cellular Plastics, 1984, 20, 2, 114.

8

Blowing Agents

2

Blowing Agents Sachchida N Singh

2.1 Introduction A substance that produces a cellular structure in a polymer mass is defined as a blowing agent. Alternatively, the gaseous phase in most polymeric foam material derives from the blowing agent(s) used in the foam manufacturing process. Blowing agents include gases that expand when pressure is released, liquids that develop cells when they change to gases, and chemical agents that decompose or react under the influence of heat/catalyst to form a gas. The blowing agent plays a very important role in both the manufacturing and performance of polymer foam. The blowing agent is the dominant factor controlling density of the foam. Besides density, it affects the cellular microstructure and morphology of the foam, which in turn define the end-use performance. In some applications, such as insulation, the blowing agent properties play a central role in the overall long-term performance of the foam. In these and many other cases, the foam is closed cell and the blowing agent is retained in the cellular structure of the foam, at least until it diffuses out, which in some cases can be decades. In many others, such as packaging and cushioning, the cellular structure of the foam is such that the blowing agent escapes almost immediately after the foam is formed. In such cases, referred to as open cell foam, though the cellular structure and morphology imprint of the blowing agent impact on the performance of the foam, the blowing agent per se does not. Of course there are many applications, such as buoyancy, impact resistance and load bearing where the role of blowing agents is intermediate. During manufacturing of foam, the choice of blowing agent and the choice of processing conditions/steps are inter-linked, i.e., each drives the other. In the case of foaming of high molecular weight polymers, such as polystyrene (PS), polyolefins, polyamides and polyester, the blowing agent modifies the melt viscosity and the thermal history of the polymer and thus rheology during foam formation and shaping. In cases where polymerisation, foaming and shaping all happens in one step, e.g., during the formation of typical polyurethane (PU), epoxy or phenolic foam, the blowing agent not only affects the liquid viscosity and heat history but also the compatibility, reactivity and phasemixing of the components. As the role of blowing agents is somewhat different in the

9

Handbook of Polymer Foams two cases, henceforth we will refer to the first foaming of high weight polymers, as thermoplastic foaming. The other, foaming of polyurethane, etc., will be referred to as thermoset foaming. Undoubtedly, there is much in common among blowing agents used for any polymer but there are also many unique requirements specific to a polymer used in a specific application in a specific part of the world. Carbon dioxide (CO2) generated by isocyanate-water reaction was the primary blowing agent for PU foam in the early years of their use whereas low boiling point liquids, such as methyl chloride or butylene were the primary blowing agent for thermoplastic foam such as PS [1-3]. Over the years, the number of polymer foams in commercial use has increased, each with its own set of unique performance and processing requirements. There has been a corresponding increase in the range of blowing agent technology in use. Currently, there are many different blowing agents and many more blends in use. The blowing agents are generally classified as physical or chemical. Chemical blowing agents are generally a solid at standard temperature and pressure (STP) and undergo a chemical transformation when producing gas, while physical blowing agents, generally a liquid or gas at STP, undergo either a reversible change of state or expansion. One exception to this classification would be water, a liquid used extensively to make polyurethane foam that reacts with isocyanate to liberate CO2 gas.

2.2 Physical Blowing Agents Physical blowing agents (PBA) provide gas for the expansion of polymers by undergoing a change in physical state. The change may involve volatilisation (boiling) of a liquid, or the release of a compressed gas to atmospheric pressure after it has been incorporated into a polymer, generally at elevated temperature and/or pressure. Common liquid physical blowing agents are low boiling liquids and include short-chain (C5 to C7) aliphatic hydrocarbons and halogenated C1 to C4 aliphatic hydrocarbons. Common gaseous blowing agents include: CO2, nitrogen (N2), short-chain (C2 to C4) aliphatic hydrocarbons and halogenated C1 to C4 aliphatic hydrocarbons. Physical blowing agents are used in the production of all types of foamed plastics, both thermoplastics and thermosets over the full range of density. They are almost the only types of blowing agent used when the foam density is low (less than 50 kg/m3). PBA are relatively low in cost but may require special equipment for use in some cases.

2.2.1 Selection Criteria for Physical Blowing Agents Many factors must be considered prior to selecting the PBA. Some such characteristics are common across all applications, polymers, and geographic location, whereas others

10

Blowing Agents are specific. Environmental acceptability is one such common factor and includes considerations of stratospheric ozone depletion, global warming, ground level air pollution, tropospheric degradation, long-term breakdown products, halogen content, acidification potential, etc. The stratospheric ozone depletion potential (ODP) of a blowing agent is an index defined as stratospheric ozone depleted per unit mass of a given product compared to that of trichlorofluoromethane (CFC-11). As discussed in Section 2.2.2, the ODP of a blowing agent intimately affects the production and use of blowing agent in a given application or at a given geographical location. Global warming and resultant climate change is a consideration because it is thought that some of the blowing agents when present in the lower atmosphere reflect infrared radiation (heat) back to earth and thereby raise the earth’s surface temperature. How much a given mass of a blowing agent contributes to global warming over a given time period, usually 100 years, compared to the same mass of carbon dioxide is referred to as its global warming potential (GWP). The GWP of a blowing agent is a function of its atmospheric lifetime and its ability to absorb infrared radiation. Atmospheric lifetime characterises the overall stability of the blowing agents in the atmosphere. Some of the blowing agents may undergo photochemical reactions in the lower atmosphere and contribute to smog formation and are classified as volatile organic compounds (VOC). Their use is strictly regulated in some parts of the world, e.g., the USA, and may require monitoring and control measures to limit emission during foam manufacturing. In cases where a foam product is exported, environmental requirements of both the manufacturing and the country of use needs to be considered. Toxicity, flammability, compatibility with materials of construction, a safe and economic manufacturing process are some of the other common factors considered while choosing blowing agents. Toxicity concerns over human health impacts of blowing agents include worker and consumer exposure of blowing agent and possible decomposition products formed in foams. Both acute and chronic effects need to be considered. Many blowing agents present varying degrees of flammability. To safely use flammable blowing agents, it is necessary to evaluate manufacturing risks from ignition, storage and transportation of blowing agent, and fire performance of foam products and finished product. Similar considerations apply to compatibility with materials of construction. Boiling point, molecular weight, vapour pressure in the temperature range used, heat of vapourisation, solubility in raw material and finished foam, compatibility with materials of construction are among the many performance attributes of a compound that must be considered while choosing a blowing agent. Though some performance attributes such as non-reactivity and compatibility of the blowing agents with materials of construction are common to all, most depend on the final application of the foam product. There are still some guidelines about preferred attributes across the applications. For example, lower molecular weight, or perhaps lower cost per mole of a blowing agent that meets all the

11

Handbook of Polymer Foams other performance criteria is a more desirable blowing agent. This is because generally, the lower the molecular weight, the higher the gas volume that can be generated per unit weight of the blowing agent. Good solubility of the blowing agents in polymer/oligomer under foam formation conditions and poor solubility in the finished foam is another common desired attribute though the specific reasons are somewhat different in thermoplastics compared with thermoset foaming. For thermoplastics, good solubility of the physical blowing agents in the melt means relatively lower minimum melt pressure to get and keep the blowing agent in solution and more plasticisation of the melt by the blowing agents. This allows the melt temperature to be reduced which makes it easier to cool the melt to the optimum foaming temperature. Conversely, if the blowing agent has poor solubility, a higher melt pressure and temperature is required to force the blowing agents into solution. This can degrade the polymer and make it more difficult to cool the melt to the optimum foaming temperature, leading to poor cell structure, loss in blowing efficiency, surface imperfection, non-optimal closed cell content etc. The lowest density that can be obtained with a given blowing agent depends on the amount of gas that can be dissolved in the molten polymer. The solubility of blowing agents in most commonly used polymers is found to follow Henry’s Law, i.e., amount of gas dissolved is directly proportional to the gas pressure [4]. Henry’s Law constant depends strongly on the blowing agent and polymer under consideration. In thermoset foams, poor solubility in the reactive liquid component means limited shelf life for the resin components, larger cell size, etc. In either case, addition of additives such as surfactant and compatabiliser is used to circumvent problems associated with poor solubility. Low solubility of the physical blowing agent in the finished foam is important to prevent weakening of the polymer by the solvent effect. In general, high heat of vapourisation while still getting a high blowing efficiency is desired. For thermoset foam, this reduces the maximum exotherm temperature thus reducing any thermal degradation of foam and residual stress gradient in composite products. In thermoplastics, it makes it easier to cool the melt to the optimum foaming temperature, e.g., in the extrusion die. For closed cell foam, the vapour pressure of the blowing agent(s) at room temperature needs to be sufficient to withstand atmospheric pressure during usage conditions to avoid foam shrinkage. Also the permeation rate of the blowing agent out of the foam needs to be slower than that for air into the foam to avoid lowering the total pressure inside the cells and thus foam shrinkage. This is obviously more critical with flexible foam such as low-density polyethylene (LDPE) than more rigid foams, like PS. Vapour pressure of the blowing agents also affects processing conditions. The higher the vapour pressure, the

12

Blowing Agents higher the pressure required to keep it in the liquid phase in the polymer melt. Thus, for example, use of butane will require a higher melt pressure than use of pentane. The blowing agents used to make closed cell insulation foam have some additional, very specific, requirements. The ideal blowing agent is a fluid with a low thermal conductivity and a low diffusion coefficient through the foam. There are other application dependent, very specific requirements for blowing agents, e.g., products used in food packaging applications require appropriate FDA approval. Ultimately, the choice of a blowing agent is dependent upon the performance, costeffectiveness and competitiveness of the finished product in a particular application. The market price of the finished products is often the determining factor as to whether a blowing agent can be used and sold competitively in the market. A description follows of the different physical blowing agents currently in use or consideration.

2.2.2 Halogenated Hydrocarbons Many chlorinated and/or fluorinated aliphatic hydrocarbons (C1 to C4) have unique attributes that make them very suitable blowing agents for all types of polymers. Halogenated hydrocarbons were one of the earliest blowing agents used to make foam. Though methyl chloride was used to make PS foam in the 1930s, large-scale use of polymer foam was greatly accelerated by discovery and use of chlorofluorocarbons (CFC) as physical blowing agents in late 1950s.

2.2.2.1 Chlorofluorocarbons (CFC) The use of CFC, in particular trichlorofluoromethane (CFC-11) to make rigid PU foam, chlorofluorocarbon-12 (CFC-12) to make extruded polystyrene (XPS) and 1,1dichloro,1,2,2,2-tetrafluoroethane (CFC-114) to make LDPE and phenolic foam, led to the production of closed cell rigid foam with low densities, good mechanical properties and extremely good thermal insulation performance. It led to the attainment of low density and tailored load-bearing properties for many cushioning products using flexible PU. As shown in Table 2.1, CFC have near ideal characteristics to be blowing agents: low molecular weight, boiling point around room temperature, low toxicity, nonflammability, and low thermal conductivity. This along with excellent chemical and thermal stability and low cost made CFC the blowing agents of choice for most polymer

13

Handbook of Polymer Foams

Table 2.1 Properties of CFC and other blowing agents CFC-11

CFC-12

CFC-114

Methyl Chloride

Methylene Chloride

CCl3F

CCl2F2

CClF2CClF2

CH3Cl

CH2Cl2

75-69-4

75-71-8

76-14-2

74-87-3

75-09-2

Molecular weight

137.4

120.9

170.9

50.5

85

Boiling point, °C

23.8

-29.8

3.8

-24.2

40

Critical temperature, °C

19 8

112

145.7

14 3

235

Critical pressure, MPa

4.41

4.11

3.39

6.71

6.35

Liquid specific gravity at 25 °C

1.477

1.31

1.456

1.10

1.33

Heat of vapourisation at boiling point (BP), kJ/mole

24.8

20.0

23.2

21.5

28.0

Gas conductivity, mW/m°K at 10 °C at 25 °C

7.4 7.9

9.2 9.9

10.4

10.6

N/A 8.4

Vapour pressure, kPa at 10 °C at 25 °C

60 106

418 644

213

567

31 57

Flammable limit in air (vol.%)

None

None

None

8.1-17.2

12-19

TLV (ACGIH) or OEL, ppm

1000

1000

1000

50

35-100

ODP (with CFC-11 = 1)

1

1

1

0.02

0.007

GWP (100 y, CO2 = 1

4600

10600

9800

16

10

Atmospheric lifetime, years

45

100

300

1.3

0.5

Chemical formula CAS number

TLV: Threshold Limit Value as determined by the American Conference of Governmental Industrial Hygienists OEL: Occupational Exposure Limit as measured by manufacturer

properties of methyl chloride methylene chloride

14

Blowing Agents foams, but especially for rigid thermal insulation foam, both thermoset and thermoplastic. Solubility of CFC were high in many thermoplastics and thus they were used to make foam from many different thermoplastics. The only other halogenated hydrocarbon in wide use until the 1980s was methylene chloride as a blowing agent for flexible and integral skin PU foam. The chemical stability of CFC, however, led a number of scientists to question their ultimate environmental fate as it was recognised that almost all usage of CFC resulted in release to the atmosphere. In 1974, Rowland and Molina published their now famous ozone depletion hypothesis in which they claimed that CFC would diffuse into the stratosphere where they break down to release chlorine atoms which would catalytically destroy ozone [5]. Rowland and Molina shared the Nobel Prize for Chemistry in 1995 for this work. Destruction of stratospheric ozone leads to an increase in UV-B radiation in the 290-320 nm region at the earth’s surface with consequent implications for human health, and other biological systems. This resulted in the development of an international protocol, known as the Montreal Protocol, which required a sharp curtailment in production and use of substances that deplete the ozone layer. The Montreal Protocol profoundly changed the direction and the pace of technology development in polymer foam industry. The choice of blowing agent in many different applications of polymer foam across the globe continue to be intricately affected by the Montreal Protocol. The Montreal Protocol and subsequent amendment sets out a time schedule for freeze and reduction of ozone depleting substance (ODS) based on whether a country is deemed developed (referred as to non-Article 5(1) parties) or developing (referred as Article 5(1) parties), based on their annual per capita calculated consumption level of ODS. Australia, Canada, Czech Republic, France, Germany, Greece, Italy, Japan, Netherlands, Russia, Spain, UK and USA are the non-Article 5(1) parties. Brazil, Chile, China, India, Indonesia, Mexico, Saudi Arabia, South Africa and Zimbabwe are among the 113 countries listed as Article 5(1) party. Table 2.2 shows the phaseout schedule of CFC [6].

2.2.2.2 Hydrochloroflurocarbons (HCFC) Featuring at least one carbon hydrogen bond in the molecule, HCFC are chemically less stable than CFC and tend to breakdown in the lower atmosphere into simple inorganic species, such as hydrogen halides and formyl fluoride. Consequently, the ability of HCFC to migrate to the atmosphere and to decompose into ozone-damaging chlorine is much lower than CFC. Also the breakdown products do not contribute significantly to the photochemical smog formation in urban areas or to acid rain. Thus, HCFC have low ODP, generally between 0.01 – 0.13 and are not VOC. Nevertheless, use of HCFC is severely restricted, e.g., in USA, these are allowed only in applications where the thermal

15

Handbook of Polymer Foams

Table 2.2 Montreal Protocol phase out schedule for CFC1 Date

Control Measure (ODP Weighted % Reduction) Non-Article 5(1) Parties

Article 5(1) Parties

July 1, 1989

Freeze at 1986 level

-

January 1, 1994

75% of 1986 level

-

January 1, 1996

Phased out

-

July 1, 1999

-

Freeze at 1995-97 average level

January 1, 2005

-

50% of 1995-97 average level

January 1, 2007

-

85% of 1995-97 average level

January 1, 2010

-

Phased out

1

Up to and including 1999 Beijing Amendment and applicable to production and consumption

insulation value of the foam is of critical importance. In most other, non-essential applications, such as automotive interiors, furniture, flotation, packaging, etc., use of HCFC is not permitted in USA. Table 2.3 lists the properties of most widely used HCFC. Generally speaking, thermal insulation foam using CFC-11 switched to HCFC-141b (CH3CCl2F), those using CFC-12 switched to HCFC-22 (CHClF2) and HCFC-142b (CH3CClF2) and those using CFC-114 to HCFC-124 (CF3CHClF). However, there are some notable exceptions to this general guideline. For example, flexible faced polyurethane thermal insulation laminate producers in Europe switched from CFC-11 to pentanes. However, many PU insulation foam in many part of the world still use HCFC-141b and HCFC-22. For production of XPS insulation board, HCFC-142b is preferred in USA whereas a non-flammable blend of HCFC-142b/HCFC-22 (60:40) is preferred in Europe. In some cases, ethyl chloride or ethanol is used as a co-blowing agent with HCFC-142b. Due to their ozone depleting potential, HCFC are viewed as only the transitional alternatives to be used till zero ODP alternatives are available. In 1992, the Parties to the Montreal Protocol added the Copenhagen Amendment outlining allowable HCFC consumption (see Table 2.4) from 1996 to 2040. In addition to the restrictions imposed by Montreal Protocol, other bodies such as the European Union (EU) and national governments have imposed more strict regulations and phase out schedule for HCFC.

16

HCFC blowing agents properties of Blowing Agents

Table 2.3 Properties of HCFC blowing agents HCFC-141b

HCFC-22

HCFC-142b

HCFC-124

Chemical formula

CH3CCl2F

CHClF2

CH3CClF2

CHFClCF3

CAS number

1717-00-6

75-45-6

75-68-3

2837-89-0

Molecular weight

116.9

86.5

100.5

136.5

Boiling point, °C

32.9

-40.8

-9.8

-12.0

Critical temperature, °C

210.3

96

137.1

122.2

Critical pressure, MPa

4.64

4.97

4.12

3.57

Liquid specific gravity at 25 °C

1.233

1.19

1.12

1.36

Heat of vapourisation at BP, kJ/mole

25.8

20.2

22.4

Gas conductivity mW/m°K at 10 °C at 25 °C

8.8 10

9.9 10.7

8.4 9.5

12.9

Vapour pressure, kPa at 10 °C at 25 °C

46 79

665 934

209 337

220 385

7.6-17.7

None

6.4-14.9

None

TLV or OEL, ppm

500

1000

1000

500

ODP (with CFC-11 = 1)

0.11

0.055

0.065

0.02

GWP (100 y, CO2 = 1)

700

1900

2300

470

Atmospheric lifetime, years

9.2

11.8

18.5

6.1

Flammable limit in air (vol%)

22.9

Effects of the mandated US and EU regulations are shown in Table 2.4 [7-8]. There are numerous other, often evolving, restrictions on use of HCFC, many specific to a country, corporation, application, and blowing agent. It is best to consult the environmental protection agency of the country where the foam will be manufactured and used along with any corporate policy of manufacturer and user before using HCFC.

17

Montreal Protocol HCFC phase out schedule Handbook of Polymer Foams

Table 2.4 Phase out schedule for HCFC Date

Montreal Protocol Reduction1

US Reduction2

EU Reduction3

Non-Article 5(1) Parties

Article 5(1) Parties

January 1, 1996

Freeze at cap4

-

-

-

January 1, 2003

-

-

141b – 100%5

100% appliances, and lamination6

January 1, 2004

35% of cap4

-

-

Phased out

January 1, 2010

65% of cap4

-

142b and 22 – 100% 7

-

January 1, 2015

90% of cap4

-

Phased out8

-

January 1, 2016

-

Freeze at 2015 level

-

-

January 1, 2020

99.5% of cap9

-

142b and 22 – 100%

-

January 1, 2030

Phased out

-

Phased out

-

January 1, 2040

-

Phased out

-

-

1

Up to and including 1999 Beijing Amendment and applicable to production only As of May 1, 2003 3 As of September 1, 2002 4 ODP weighted HCFC consumption cap = 100% 1989 HCFC + 2.8% (Montreal Protocol) or 2.6% (EU) of 1989 CFC consumption 5 Production only for domestic foam use 6 Except in refrigerated transport 7 Except for use in equipment manufactured before 1/1/2010 8 Except for use in equipment manufactured before 1/1/2020 9 Phased out except for service of existing refrigeration and air-conditioning equipment 2

2.2.2.3 Hydrofluorocarbons (HFC) These are compounds with no chlorine in them and thus have zero ODP. Two liquid HFC, HFC-245fa (CF3CH2CHF2) and HFC-365mfc (CF3CH2CF2CH3) and two gaseous HFC, HFC-134a (CH2FCF3) and HFC-152a (CHF2CH3) are most widely used [2]. Table 2.5 lists the properties of these HFC. The flammability limit of HFC-365mfc in air

18

HFC blowing agents properties of Blowing Agents

Table 2.5 Properties of HFC blowing agents HFC-134a

HFC-245fa

HFC-365mfc

HFC-152a

Chemical formula

CH2FCF3

CF3CH2CHF2

CF3CH2CF2CH3

CHF2CH3

CAS number

811-97-2

460-73-1

406-58-6

75-37-6

Molecular weight

102

134

14 8

66

Boiling point, °C

-26.5

15.3

40.2

-24.7

Critical temperature, °C

100.6

157.5

189.7

113.5

Critical pressure, MPa

4.06

3.62

2.75

4.48

Liquid specific gravity at 25 °C

1.20

1.32

1.25

0.90

Heat of vapourisation at BP, kJ/mole

22.1

28

26.2

21.7

Gas conductivity mW/m°K at 10 °C at 25 °C

12.4 13.8

12.5 13.3

10.6 11.6

14.7

Vapour pressure, kPa at 10 °C at 25 °C

425 670

85 145

26 59

273 500

Flammable limit in air (vol%)

None

None

3.8-13.3

3.9-16.9

TLV or OEL, ppm

1000

500

N/A

1000

ODP (with CFC-11 = 1)

0

0

0

0

GWP (100 y, CO2 = 1)

1600

990

910

140

14

7.4

10.8

1.7

Atmospheric lifetime, years

has caused introduction of a non-flammable blend with HFC-227ea (CF3CHFCF3) in the weight ratio of 94:6 [9]. Zero ODP of HFC means that it is permissible to use them in all applications including non-insulation. However, use of HFC as a PBA is not as widespread as that of CFC in the 1980s and 1990s or of HCFC in the 1990s or 2000s. This is primarily because of their

19

Handbook of Polymer Foams high cost, relatively smaller benefit in insulation performance as compared to alternatives, (e.g., hydrocarbons, CO2), and high GWP. High GWP is an especially controversial issue and regulations related to global warming are divergent and emerging. Thus use of HFC is very application, geographical location and time dependent (as related to HCFC phaseout schedule shown in Table 2.4) and somewhat tentative. At the present time, HFC-245fa is the leading HCFC replacement candidate blowing agent in the USA for use in many PU foam especially domestic appliance, spray and integral skin foam [2]. HFC-152a, HFC-134a and their blends are the leading candidates in USA for use in XPS insulation board and polyolefin pipe-wrap insulation [10-12]. HFC-134a is currently being used in many applications, such as PU flotation, extruded PS sheet in USA and many European countries [12]. HFC-134a is also a leading HCFC replacement candidate blowing agent for use in many PU foams. Use of HFC-365mfc as a replacement for HCFC-141b is primarily limited to Europe due to some patent related issues. Other than the previously mentioned insulation and integral skin applications, HFC are not being pursued as a blowing agents for polymer foam.

2.2.3 Hydrocarbons (HC) Many low boiling aliphatic hydrocarbons have desirable characteristics, namely, low cost, high specific volume, zero ODP, nearly zero GWP, halogen-free, compatibility with common polymers, universality and easy availability. Properties of a few HC are listed in Table 2.6. Such HC do present one big hazard though, fire. For example, the flammability limit of pentanes in air is about 1.4 to 8.0 volume %, the energy of ignition is extremely low and the density of pentane vapour is about 2.5-times that of air. Such flammability characteristics blocked serious consideration of HC as blowing agents in the early years of making foam but this changed more recently. A careful consideration of equipment and procedures used in storage, handling, manufacturing, monitoring and shipping of both the HC and the finished foams, made it possible to use HC as blowing agents in a wide range of foams. Though the extent of plant modifications required to use HC may vary significantly, depending on local codes and regulations, in general, improved ventilation, explosion-proofing and alarm systems are required. Hydrocarbons are classified as VOC and are subject to emissions control in many urban areas, especially in USA. HC are used to blow foam using a large range of polymers, in a wide range of densities for use in a large variety of applications. They are used to make insulation from PU, PS and polyolefins, foams for floral arrangements from phenolics, energy absorbing (acoustical and impact) and food packaging foam from polyolefins, etc. This is because the solubility of many HC is high in many thermoplastics [13]. Though many HC are suitable for most polymer/application, preferences have emerged. Pentanes have emerged

20

HC blowing agents properties of Cyclo-pentant n-pentane Iso-pentane n-butane Isobutane Propane Blowing Agents

Table 2.6 Properties of HC blowing agents

Chemical formula CAS number

Cyclopentane

npentane

Isopentane

nbutane

Isobutane

Propane

(CH2)5

C5H12

C5H12

C4H10

C4H10

C3H8

78-78-4

106-97-8

75-28-5

74-98-6

287-92-3 109-66-0

Molecular weight

70.1

72.0

72.0

58.1

58.1

44.1

Boiling point, °C

49.3

36.2

27.8

-0.5

-12

-42.1

Critical temperature, °C

238.6

196.7

188

149.9

134.6

96.8

Critical pressure, MPa

4.51

3.36

3.38

3.8

3.65

4.25

Liquid specific gravity at 25 °C

0.75

0.63

0.62

0.57

0.55

-

Heat of vapourisation at BP, kJ/mole

27.3

25.7

24.6

22.4

21.3

18.8

Gas conductivity mW/m°K at 10 °C at 25 °C

11.4 12.8

13.7 15.0

12.8 14.3

16.1

14.8 16.2

17.9

Vapour pressure, kPa at 10 °C at 25 °C

24 43

40 69

54 91

243

220 350

951

Flammable limit in air (vol.%)

1.4 – 8.0 1.3 – 8.0 1.4 – 7.8

1.8-8.4

1.8 – 8.4 2.9 – 9.5

TLV or OEL, ppm

60 0

600

600

600

N/A

2500

ODP (CFC-11 = 1)

0

0

0

0

0

0

GWP (with CO2 = 1)

11

11

11

<10

11

11

Atmospheric lifetime, years

Few days Few days Few days Few days Few days Few days

as the dominant blowing agents for use in many PU foam insulation and expanded polystyrene (EPS) foams [12]. Butanes and propane are the dominant blowing agents in use to make protective wrapping and cushioning foam from polyolefins.

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Handbook of Polymer Foams

2.2.4 Inert Gases Inert gases especially carbon dioxide and nitrogen are among the most widely used blowing agents. This is partly because nitrogen is cheap, abundant and by far the most environmentally acceptable as it is simply borrowed from the atmosphere. The same is true for CO2 even though it is a greenhouse gas. This is because it can be deemed ‘greenhouse neutral’ whether produced as a by-product of ammonia manufacture or fermentation processes, or extracted from air or other natural resources. Table 2.7 lists the properties of CO2, N2 and O2. Oxygen is not used as a blowing agent but is still relevant as it diffuses into the cellular space of closed cell foams.

Table 2.7 Properties of inert gases used as blowing agents Carbon Dioxide

Nitrogen

Oxygen

C O2

N2

O2

124-38-9

7727-37-9

7782-44-7

Molecular weight

44

28

32

Boiling point, °C

-78.3

-195.8

-183.0

31

-146.9

-118.3

Critical pressure, MPa

7.38

3.4

5.0

Liquid specific gravity at 25 °C

N/ A

N/A

N/A

Heat of vapourisation at BP, kJ/mole

6.8

-

-

Gas conductivity mW/m°K at 10 °C at 25 °C

15.3 16.4

24.6 25.8

25.2 26.6

Vapour pressure, kPa at 10 °C at 25 °C

4502 6434

Very high

Very high

Flammable limit in air (vol.%)

None

None

None

TLV or OEL, ppm

N/A

N/A

N/A

ODP (CFC-11 = 1)

0

0

0

GWP (with CO2 = 1)

1

N/A

N/A

120

N/A

N/A

Chemical formula CAS number

Critical temperature, °C

Atmospheric lifetime, years

22

Blowing Agents One reason CO2 and N2 are widely used as blowing agents is their relatively moderate critical temperature and pressure, especially for CO2. As is well known, a material is in a supercritical fluid state when it is maintained at a temperature and pressure exceeding its critical temperature and pressure. In the supercritical state, the material becomes dense like a liquid yet maintains a gas like ability to flow with almost no viscosity or surface tension. Its solubility in a polymer increases significantly and this lowers the glass transition temperature (Tg) of most polymers. When a polymer melt saturated with supercritical fluid is depressurised rapidly, the polymer becomes supersaturated with the gas, nucleation of cells occurs at a very high concentration and growth of these cells continues until the polymer vitrifies. This phenomena is used to make cellular (typical cell size >70 μm), microcellular (cell size between 1-70 μm) and super-microcellular (cell size below 1 μm) with a variety of polymers using CO2 and N2 [14]. Though many gases exhibit supercritical behaviour within the typical ranges of temperature and pressure used for processing thermoplastics, CO2 is in a supercritical state when its temperature exceeds a modest 31 °C and pressure exceeds a modest 7.38 MPa. In the supercritical state, CO2 is a supersolvent for many polymers but especially PS where the Tg can be lowered to near ambient conditions. This makes the rapid depressurisation step relatively easy for the PS/CO2 system and many insulation and impact energy absorption foams are made using these materials [1]. For essentially the same reasons, CO2 is being used to make low density foam with many other plastics including, high density polyethylene (HDPE), and polypropylene (PP), for use in a whole range of applications [15]. In addition, CO2 is being used to make foam from many different thermoplastics using conventional CO 2 injection along with use of a heterogeneous nucleating agent. CO2 is also being evaluated as an alternative to chemical blowing agents (see Section 2.3) to make high density foams [16]. Carbon dioxide in all its physical states, i.e., gas, liquid and supercritical fluid is the single largest blowing agent in use today to make polymer foam. In addition to the use described previously to make thermoplastic foam, CO2 is used extensively to make PU foam. Here it is either generated chemically from the reaction of water and isocyanate, henceforth referred as CO2(water), or added as a liquid, referred as CO2 (LCD) or as a gas, referred to as CO2 (GCD). All rigid and flexible PU foams made today are at least partially blown with CO2(water). Many flexible foams such as slabstock use CO2 (LCD) and others, such as moulded foams use CO2 (GCD) in addition to CO2(water) [17, 18]. It is the ability to handle CO2 in liquid form under moderate conditions that has made it the auxiliary blowing agent of choice in flexible slabstock PU foam. Nitrogen is also used to make microcellular foam [15] and conventional foam using many different thermoplastic polymers [19]. Compressed N2 is used in specialised injection moulding processes for structural foam, and in a unique high pressure gas solution process

23

Handbook of Polymer Foams for polyethylene packaging foam [19]. The solubility of N2 in most common polymers is lower than that of CO2 and thus higher melt pressure is required to get to same density when using N2 than CO2 [4]. Generally N2 is used for plastics with higher melt viscosities or for injection moulding parts that are difficult to fill. Nitrogen or air are occasionally used as nucleating agents for some thermoset foams but are rarely used as the primary blowing agent. Other inert gases such as argon and helium have been mentioned as blowing agents in many patents, but they are rarely, if at all, used commercially [20, 21].

2.2.5 Other Physical Blowing Agents As indicated in Section 2.2.4, water is used extensively as a blowing agent in the polyurethane industry where it reacts with isocyanate to give CO2. Lately, water is also being investigated as a physical blowing agent, especially for thermoplastic elastomers (TPE) such as PP/ethylene-propylene diene monomer and styrene-ethylene/butylenestyrene [22]. Besides low cost and environmental benefits, water brings some unique characteristics as a PBA. It has low volatility, low solubility and low foaming pressure compared to other PBA, especially CO2 and N2. Methyl chloroform (CCl3CH3, ODP =0.1, GWP = 140, boiling point = 74 °C), acetone (CH3COCH3, ODP = 0, boiling point = 56.1 °C) and alcohol are some other physical blowing agents that have been used to make primarily non-insulation foams. Perfluorocarbons, e.g., perfluoropentane (C5F12) and perfluorohexane (C6F14), have been evaluated as blowing agents and as a co-blowing agent with HCFC [23]. They are not considered to be a viable blowing agent option as they are characterised by very long atmospheric lifetimes of the order of hundreds to thousands of years and are very infraredactive and thus have high GWP. Fluorinated ethers, such as HFE-245 (CF3CH2OCF2H), HFE-356 (CF3CHFCF2OCH3), and HFE-254mf (CF3CH2OCHF2) have been evaluated in the laboratory in rigid PU foam applications but their commercial viability is dubious because of cost, toxicity for some of them and performance [24]. Similarly fluoroiodocarbons such as heptafluoro-2iodopropane and hydrogen containing fluoromorpholine have been evaluated in the laboratory but high cost has meant no commercial use [25].

2.2.6 Blends of Physical Blowing Agents Blends of two or more PBA are often used to meet all the performance requirements of the end use and/or the processing requirements of foaming. For example, in PU foam, mixtures of a liquid and a gaseous blowing agent is often used to balance the flow,

24

Blowing Agents dimensional stability and thermal resistance requirements for use as insulation for appliances, construction, etc. Low boiling blowing agents such as HCFC-22, HFC-134a, and isobutane have been used to improve flow and dimensional stability of PU foam blown with more insulating gases HCFC-141b, HFC-365mfc and pentane, respectively [26]. Similarly, blends of blowing agents are often used to solve processing issues. For example, extrusion of polyethylene terephthalate (PET) foam to give a density reduction of over 60% using a single blowing agent such as CO2, HCFC-22 or butane leads to cell collapse due to the high volatility of the blowing agents before crystalline PET can be cooled from melt temperature to it’s Tg. A blend of blowing agents, one having a high boiling point, such as n-octane, with a low boiling point blowing agent, such as n-pentane, leads to over 80% reduction in density with no cell collapse [27]. This is because noctane provides plasticisation required for foaming and n-pentane provides the vapour pressure needed to prevent foam cell collapse during the cooling.

2.2.7 Encapsulated Physical Blowing Agents While a number of physical blowing agents are widely used, their use typically requires special storage, handling, and processing equipment. Physical blowing agent encapsulated in a thermoplastic shell is used to avoid such equipment and to make a very controlled cell structure. Generally referred to as expanding fillers, the thermoplastic shell and the physical blowing agent, generally a hydrocarbon, are chosen to meet the processing and application requirements [28]. They are generally used to expand thermoplastic polymers to a density range of 300-900 kg/m3.

2.2.8 Physical Blowing Agent by Foam Type and Application The choice of blowing agents depends on the end product performance requirements, manufacturing process, country of manufacturing and country of use. Table 2.8 lists the PBA currently in use in different foams and the anticipated blowing agent in year 2007 to 2010 based on current understanding of market dynamics and regulatory environment. Though various sources have been used to compile this list, periodic reports written by the Flexible and Rigid Foams Technical Options Committee of the United Nations Environmental Programme (UNEP) has been a key source [12].

2.3 Chemical Blowing Agents Chemical blowing agents (CBA) are compounds that liberate gas(es) under the foam processing conditions, either due to thermal decomposition or due to chemical reaction.

25

physical foam type application Handbook of Polymer Foams

Table 2.8 Physical blowing agents by foam type and application1 Foam type

Blowing Agent in use (2003/2004)

Anticipated blowing agents in 2007-2010 Non-Article 5(1) parties

Article 5(1) parties

Rigid Polyurethane2 Domestic refrigerators and freezers

HC, HFC-245fa, HFC-134a, HCFC141b, HCFC141b/22, HCFC142b/22

HC, HFC-245fa, HFC-134a

HCFC-141b, HC

Water heater, Picnic cooler

HC, HCFC-141b, HCFC-141b/22, HFC-245fa, HFC134a, CO2 (water)

HC, HFC-245fa, HFC-365mfc/227ea, HFC-134a, CO2 (water)

HCFC-141b, HC, CO2 (water)

Boardstock (flexible faced lamination)

HC, HCFC-141b

HC

N/A

Metal panel continuous

HC, HCFC-141b, HCFC-22, HCFC142b/22

HC, HFC-134a, HFC-365mfc/227ea, HFC-245fa

HCFC-141b, HC

Metal panel discontinuous

HFC-134a, HCFC141b, HFC-134a

HC, HFC-134a, HFC-365mfc/227ea, HFC-245fa

HCFC-141b

Spray

HCFC-141b

CO2 (water), HFC245fa, HFC365mfc/227ea

HCFC-141b

Blocks

HC, HCFC-141b, HFC-245fa, HFC365mfc/227ea

HC, HFC-245fa, HFC-365mfc/227ea

HCFC-141b

Pipe

HC, CO2 (water), HCFC-141b

HC, CO2 (water), HFC-245fa, HFC365mfc/227ea

HCFC-141b

One component foam

Dimethylether/ propane/butane (DPB), HCFC-22

DPB, HFC-134a, HFC-152a

DBP, HFC-134a, HFC-152a

HFC-134a, HCFC-22

HFC-134a

HFC-134a

CO2 (water, LCD)

CO2 (water, LCD)

CO2 (water)

Flotation Packaging, floral

26

physical foam type application Blowing Agents

Table 2.8 Continued Foam type

Blowing Agent in use (2003/2004)

Anticipated blowing agents in 2007-2010 Non-Article 5(1) parties

Article 5(1) parties

Flexible Polyurethane Slabstock and box

CO2 (water, LCD), Methylene chloride

CO2 (water, LCD)

CO2 (water, LCD), Methylene chloride

Moulded

CO2 (water, LCD, GCD)

CO2 (water, LCD, GCD)

CO2 (water, LCD, GCD)

CO2 (water), HFC-134a, HC, HFC-245fa

CO2 (water), HFC134a, HC, HFC245fa, HFC365mfc/227ea

CO2 (water), HFC-134a, HC

Phenolic foam floral

HC

HC

HC

Phenolic foam insulation

HCFC-141b, HFC245fa, HFC365mfc/227ea, HC, 2-chloropropane

HFC-245fa, HFC365mfc/227ea, HC, 2-chloropropane

N/A

Extruded sheet/dunnage

HC, HFC-134a, HFC-152a, CO2 (LCD), N2

HC, HFC-134a, HFC-152a, CO2 (LCD), N2

HC

Extruded board

HCFC-142b, HCFC-142b/22

CO2 (LCD), HFC152a/134a, HC

HC, HCFC-142b

HC, water

HC, water

HC

Polyolefins, pipewrap

HCFC-142b, HCFC-142b/22

HFC-152a, HFC-134a

HCFC-142b/22

Polyolefins, all others

HC, CO2 (LCD), CO2, N2

HC, CO2, N2

HC

Polycarbonate (PC)

CO2 (LCD), HC

CO2 (LCD), HC

N/A

Integral skin

Polystyrene

Expanded

1

Blowing agents specified here cover ~98% of total commercial production of foam, remainder may use other blowing agents 2 All PU rigid foams are blown partially with CO2 (water) along with the specified blowing agents

27

chemical Handbook of Polymer Foams In general, use of CBA results in the decomposition of the original molecule, one or more gases for polymer expansion, and one or more solid residues that remain in the foamed polymer. Table 2.9 lists the common CBA along with their decomposition temperature range, gases evolved and gas yield. As can be seen from Table 2.9, the gases produced are generally N2 and CO2 and once produced, they behave much like a PBA but with some effects due to presence of the decomposition products. Thus much of the discussions in Section 2.2.4 on inert gas, especially about solubility in polymers applies here too. CBA are widely used to make high and medium density foam plastics and rubbers. Typical densities of foams produced using CBA are in the range of 400-800 kg/m3, i.e., a density reduction of about 20-45% from full polymer density. CBA are rarely used to make foam with densities below 400 kg/m3 because they are essentially too expensive. For example, CO2 and N2 liberated from CBA cost about 10 times that used from a cylinder

Table 2.9 Properties of chemical blowing agents Description

Type

Decomposition temperature, °C

Gas yield @STP x 10-3 m3/kg

Gases

Azodicarbonamide (ADC)

Exo

200-230

220-245

N2, CO, NH3, O2

4,4-Oxybis(benzenesulfonylhydrazide) (OBSH)

Exo

150-160

120-125

N2, H2O

p-Toluenesulfonylhydrazide (TSH)

Exo

110-120

110-115

N2, H2O

p-Toluenesulfonylsemicarbazide (TSS)

Exo

215-235

120-140

N2, CO2

Dinitrosopentamethlenetetramine

Exo

19 5

190-200

N2, NH3, HCHO

Polyphenylene sulfoxide (PPSO)

Exo

300-340

80-100

SO2, CO, CO2

Sodium bicarbonate

Endo

120-150

130-170

CO2, H2O

Zinc carbonate

Endo

Citric acid derivatives

Endo

200-220

110-150

CO2, H2O

5-Phenyltetrazole

Endo

240-250

190-210

N2

28

CO2

Blowing Agents [4]. The primary reason many CBA are used is because their use requires little modification to an existing thermoplastics processing line. This makes it the preferred approach for many smaller processors as use of PBA generally requires additional investment in specialised extrusion equipment. Certain foam processing methods are especially well suited for the use of CBA. For example, the low-pressure (atmospheric) nature of the rotational foam moulding process requires use of only CBA [29]. Other benefits of CBA include broader operating window, self-nucleation and finer cell size. Some of the disadvantages of CBA include difficulty of recycling non-conforming products or contamination due to unreacted or solid residue from the reacted CBA. Almost all CBA are finely divided solids and no special storage or handling equipment is generally needed to utilise them in plastics processing. They are either blended with the plastics before processing or fed directly into a hopper. CBA may be incorporated into virtually any thermoplastic process to produce foam, such as extrusion, injection moulding, calendering, coating, expansion casting, and rotational moulding. Almost half of all CBA are used to blow polyvinyl chloride (PVC) followed by polyolefins and rubber [30, 31]. PVC applications include foam core pipes, profiles and sheets, textured wall covering and cushion vinyl flooring. Polyolefin sheets and foamed rubber profiles for the automotive industry, thermoformed polyolefin foam for food packaging industry are some other key applications. CBA are widely used for structural foaming, as an additive for the elimination of sink marks during plastic moulding, and as a nucleating agent for the physical foaming process. CBA are being evaluated to foam wood-plastics composites [32]. CBA still plays a relatively minor role in foaming many thermosets. The high cost of CBA compared to PBA has discouraged research into PU foam applications or for that matter any low density (<30 kg/m3), foam applications.

2.3.1 Selection Criteria for Chemical Blowing Agents A number of factors must be considered when selecting a CBA. The first and foremost is that the gas release temperature closely matches the processing temperature of the polymer. If the CBA decomposition temperature is significantly above the polymer process temperature, little or no foaming will occur. If the CBA decomposition temperature is significantly below the polymer process temperature, poor (overblown, ruptured) cell structure and surface skin quality is likely to result. Along with the correct decomposition temperature, the CBA must release the gas at a controllable but rapid rate. Gas yield must be commensurate with the target density. Most CBA are stable under normal storage conditions and generally easy to handle, mix and dose. A key consideration while selecting CBA is that the reaction products and

29

Handbook of Polymer Foams residue of CBA must be compatible with the material to be foamed and have little or no detrimental effect on properties or colour of the end product. The decomposition products must not interfere with the flame retardant, stabiliser or other additives. In addition, the decomposition products should not cause corrosion of equipment and tools, i.e., no plate-out deposits on screws and moulds. For applications that involve contact with food, the toxicity of both the CBA and the decomposition products must be considered. Certain CBA yield water upon decomposition which can be problematic for moisture sensitive polymers such as PC, thermoplastic polyesters, polyamide, and acetals. Such polymers can chemically degrade in presence of water causing noticeable loss of properties. The same can be true for CBA that generate ammonia or other alkaline or acidic gases. Like PBA, the choice of a CBA depends upon the performance, cost-effectiveness and competitiveness of the finished product in a particular application. Table 2.9 gives a description of the different chemical blowing agents currently in use or consideration. CBA are generally subdivided into two major categories: exothermic, and endothermic.

2.3.2 Exothermic CBA By definition, exothermic CBA generate heat during their decomposition. This manifests itself not in terms of significantly changing the polymer melt temperature but once the decomposition starts, it is difficult to stop it before it reaches full decomposition. This results in rapid decomposition in a narrow temperature range. Generally speaking, the exothermic CBA are associated as those giving N2 as the main blowing gas. Thus, much of the discussion about N2 in Section 4.2.4 will apply here too. The level of usage for exothermic CBA (100% active) is usually 0.3-0.5% by weight for foam applications [33]. For the elimination of sink marks, the best starting level is 0.1%. Given in Section 2.3.2.1 are some of the common exothermic CBA in use today.

2.3.2.1 Azo Compounds Azodicarbonamide (ADC), also known as azodicarbonic acid diamide or 1,1´azobisformamide, and its blends continue to be the world’s most commonly used chemical blowing agent. For example, it accounts for around 90% of the chemical blowing agents consumed in Western Europe [30]. The pure material decomposes at temperatures of 205-215 oC, evolving ~220 x 10-3 m3/kg of gas, primarily N2, with lesser amounts of CO, CO2 and NH3. Its decomposition temperature can be reduced to as low as ~150 oC through the addition of activators or kickers, to match the desired processing method

30

Blowing Agents and polymer to be foamed. Common activators include zinc oxide, zinc stearate, many tin or zinc-containing PVC stabilisers, urea, alcohol amines, and some organic acids such as stearic acid. In it’s most basic form, ADC is a fine powder with a range of particle diameters of 3-30 μm corresponding to specific surface areas from 3 to 0.5 x 106 m2/m3 and apparent densities of 300 to 700 kg/m3. It is also available as formulated material in powder form, or basic or modified material in pre-dispersion or masterbatch form. Modifications of ADC include flow conditioned grades which disperse more easily, damped grades to reduce dust, and nonplateout grades which eliminate plating out on die and mould tooling [30]. ADC is a yellow to yellow-orange material which generates white to off-white residue upon decomposition. The solid decomposition products of ADC include biurea, cyamelide, cyanuric acid and urazol, and may cause plateout in extrusion and injection moulding [31]. Virtually all rubber and plastics grades can be foamed with ADC. The exceptions are the ammonia-sensitive polymers such as PC and thermoplastic polyester that undergo molecular weight degradation during the foaming process. Rigid PVC pipe, sheet and profile; vinyl flooring, earplugs, coated-fabric, sealing tape, HDPE wire insulation and crosslinked polyolefin foams are just some of the many products which use ADC as a blowing agent [33]. Other azo compounds investigated as CBA include azobisisobutyronitrile which yields N2 upon decomposition and diisopropyl azodicarboxylate which yields N2, CO, and CO2 [34]

2.3.2.2 Hydrazine Derivates Hydrazine derivates generally break into N2 and H2O at relatively low decomposition temperature in the range of 110-160 oC. OBSH, a white powder, is the most common hydrazine derivative as it forms non-volatile and non-toxic oligomeric decomposition residues and yields 120-150 x 10-3 m3/kg of gas at STP [30]. Another product, TSH, was widely used at one time but its use has severely declined in recent years in favour of OBSH and azo compounds [30]. Hydrazine derivates are most widely used to blow polymers requiring relatively low temperature processing such as rubber, PVC plastisols, LDPE wire and cable insulation and epoxy foams.

2.3.2.3 Sulfonyl Semicarbazides TSS is the most commercially significant semicarbazide. It decomposes in the temperature range of 215-235 oC into a mixture of N2 and CO2 [33]. It is a white powder and

31

Handbook of Polymer Foams generates 120-140 x 10-3 m3/kg of gas at STP. It is considered as an intermediate high temperature CBA and is used in PS, PP, polyamide and modified polypropylene oxide.

2.3.2.4 N-nitroso Compounds Dinitrosopentamethlenetetramine is the best known of such compounds which yield ammonia, formaldehyde and nitrogen, in the temperature range of 190-200 oC. Used widely in the past due to relatively low cost per unit of gas yield, its use has largely disappeared due to toxicity and odour concerns [30].

2.3.2.5 Other Compounds PPSO is a relatively high temperature CBA used especially where the evolved sulfur dioxide can act also as a crosslinker to the polymer matrix [35].

2.3.3 Endothermic CBA These absorb heat during their decomposition which leads to broader decomposition temperature and time range. Most commercially used endothermic CBA generate CO2 as the main blowing gas. Thus much of the discussion about CO2 in Section 2.2.4 will apply here too. They are generally white in colour with a white residue. As the components of endothermic CBA are essentially food additives, they are ‘generally regarded as a safe’ (GRAS) as regards toxicity considerations [33]. The use level of endothermic products (100% active) is usually twice that of exothermic CBA [33]. Endothermic CBA are often used in applications where a rapid diffusion rate of CO2 gas through polymers allows post-finishing of foamed parts right out of the mould without the need for a degassing period. They are the preferred CBA type for extruding rigid PVC foam profiles where their cooling effect and slower rate of decomposition helps in the formation of a thick skin. Nucleation of physically foamed materials, especially those used for food packaging, has become a well established application area for endothermic CBA.

2.3.3.1 Bicarbonates/carbonates Generically referred as inorganic CBA, these decompose endothermically into CO2 and in some cases into water. Sodium bicarbonate and zinc carbonate are the most common

32

Blowing Agents such products though other bicarbonate and carbonate salts have been used over the years [31].

2.3.3.2 Polycarboxylic Acid Derivatives Bicarbonates and carbonates are often used with a selected polycarboxylic acid such as citric acid, or a selected salt or ester of a polycarboxylic acid. Sodium citrate and the trimethyl ester of sodium citrate are examples of the latter. Such combinations allow the decomposition temperature to be tailored to polymer processing temperature [3, 36].

2.3.3.3 Tetrazoles Tetrazoles decompose into N2 alone and are thus especially suited for ammonia and moisture sensitive polymers. 5-Phenyltetrazole, a white powder, is the primary product in this class and it decomposes in the temperature range of 240-250 oC yielding 190210 x 10-3 m3/kg of gas at STP. Considered a high temperature CBA, it is used in polycarbonates and thermoplastic polyesters [33].

2.3.3.4 Other Compounds Other compounds such as dihydrooxadiazinone, which generates nitrogen, have been used to blow/nucleate polycarbonates [33].

2.3.4 Endo/Exo Blends Often the variety of demanded properties are such that that no single type of chemical blowing agents can satisfy all of the requirements. Blends of exo- and endothermic CBA have been used in many such cases [36]. Because of favourable price/performance balance, ADC is the key exothermic CBA component. In most cases, selected endothermic systems have been physically blended with ADC to provide a boost in the expanding ability of the endothermic components, while taking advantage of their cooling, stabilising effect and faster degassing characteristic. Some examples are blends of sodium bicarbonate and ADC for extrusion of rigid PVC and PVC/wood composite and zinc carbonate and ADC to foam crosslinked PE and plasticised PVC [37]. Most of these products are available in various physical forms including powder, pellets, and often proprietary dispersions.

33

Handbook of Polymer Foams

References 1.

K.W. Suh, C.P. Park, M.J. Maurer, M.H. Tusim, R. de Genova, R. Broos and D.P. Sophiea, Advanced Materials, 2000, 12, 23, 1779.

2.

S.N. Singh, Blowing Agents for Polyurethane Foams, Rapra Review Report, Volume 12, No.10, No.142, Rapra Technology, Shawbury, UK, 2002.

3.

T. Pontiff in Foam Extrusion: Principles and Practices, Ed., S-T. Lee, Technomic Publishing, Lancaster, PA, USA, 2000, 251.

4.

M. Gale, Proceedings of Rapra Blowing Agent Systems: Formulations and Processes Conference, 1998, Shawbury, UK, Paper No.6.

5.

F. Rowland and M. Molina, Nature, 1974, 249, 8.

6.

The Montreal Protocol on Substances that Deplete the Ozone Layer, Ozone Secretariat, UNEP, Nairobi, Kenya, www.unep.ch/ozone/pdf/montrealprotocol2000.pdf

7.

Ozone Depletion, US Environmental Protection Agency, Washington DC, USA, www.epa.gov/ozone

8.

Ozone Layer, Europa, EU, Brussels, Belgium, http://europa.eu.int/comm/ environment/ozone

9.

P. Dournel and L. Zipfel, Proceedings of Polyurethane Expo 2001 Conference, Columbus, OH, USA, 2001, 325.

10. P. Dournel and L. Zipfel, Proceedings of Rapra Blowing Agents and Foaming Processes Conference, Frankfurt, Germany, 2001, Paper No.24. 11.

C. Bratt and A. Albouy, Proceedings of Rapra Blowing Agents and Foaming Processes Conference, Frankfurt, Germany, 2001, Paper No.15.

12. UNEP Flexible and Rigid Foams Technical Options Reports, available through the Montreal Protocol Technology and Economic Assessment Panel (TEAP) website www.teap.org 13. S.T. Lee and K. Lee, Proceedings of Blowing Agents and Foaming Processes Conference, Frankfurt, Germany, 2001, Paper No.16 14. C.B. Park, N.M. Suh and D.F. Baldwin, inventors; MIT, assignee; US 6,051,174, 1998. 34

Blowing Agents 15. D. Pierick and R. Janisch, Proceedings of Blowing Agents and Foaming Processes Conference, Frankfurt, Germany, 2001, Paper No.19. 16. M. Gale, British Plastics and Rubber, 2000, May, 4. 17. C. Fiorentini, M. Taverna, B. Collins and T. Griffiths, Proceedings of Cellular Polymers III, Coventry, UK, 1995, Paper No.21. 18. I. A. Cella, Proceedings of Polyurethane Expo 1998 Conference, Dallas, TX, USA, 1998, 679. 19. Zotefoams Plc Packaging Technology, Leading The Way With Plastazote And Evazote, Zotefoams plc, Croydon, UK, 2001, 4. 20. S.C. Tan, inventor; no assignee; US 6,232,354, 2001. 21. D.W. Kreiser, A.L. Dinkel and J.P. Weibel, inventors; Armacell, assignee; US 6,245,267, 2001. 22. A. Sahnoune, Journal of Cellular Plastics, 2001, 37, 2, 149. 23. O. Volkert, Proceedings of the Polyurethane World Congress: The Voice of Advancement, 1991, Nice, France, 1991, p740. 24. N. Takada, R. Tamia, Y. Amamoto, A. Sekiya, N. Tsukida, H. Takeyasu, Proceedings of the Polyurethane Expo 1998 Conference, Dallas, TX, USA, 1998, p.663. 25. J. Nimitz, Proceedings of Polyurethanes ‘94, Boston, MA, USA, 1994, p.110. 26. J.R. Gurecki and I.A. Wheeler, Proceedings of Polyurethanes ‘95, Chicago, IL, USA, 1995, p.454. 27. M.E. Reedy and W. Harfmann, Proceedings of the 2nd International Conference on Thermoplastic Foam: Foams 2000, Parsippany, NJ, USA, 2000, p.12. 28. K. Elfving, Proceedings of Blowing Agents ’99, Manchester, UK, 1999, Paper No.6. 29. R. Pop-Iliev, G. Rizvi and C.B. Park, Proceedings of ANTEC 2001 Conference, Dallas, Tx, USA, 2001, Paper No.435. 30. H. Hurnik, Proceedings of Addcon World ’98, London, UK, 1998, Paper No.16. 31. S. Quinn, Plastics Additives & Compounding, 2001, 3, 5, 16.

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Handbook of Polymer Foams 32. G. Luebke, Proceedings of Rapra Blowing Agents and Foaming Processes Conference, Frankfurt, Germany, 2001, Paper No.2. 33. R.L. Heck, Proceedings of Retec on Polymer Additives: What’s New and Review, Ft. Mitchell, KY, USA, 1997, p.277. 34. Plastics Additives: An A-Z Reference, Ed., G. Pritchard, Kluwer, London, UK, 1998. 35. R. Heinz, K. Breyer and W. Michaeli, Proceedings of Blowing Agents ’99, Manchester, UK, 1999, Paper No.4. 36. R.L. Heck, Modern Plastics, 1999, 76, B-26. 37. G. Luebke, Proceedings of Blowing Agent Systems: Formulations and Processes, 1998, Shawbury, UK, Paper No.11

36

Expanded Polystyrene: Development, Processing, Applications and Key Issues

3

Expanded Polystyrene: Development, Processing, Applications and Key Issues Andrew Barnetson

3.1 Introduction Expanded polystyrene (EPS) has rightly taken its place in modern society as an important material in, for example, insulation in construction applications and as a packaging material for a wide range of industrial appliances and foodstuffs. Throughout this chapter we will consider the product in its many forms, looking at the origins of EPS with its development by BASF in 1950 and the manufacturing process, which is similar for both construction and packaging applications. Applications will be considered for both the construction and packaging industries and the broad global structure of the industry will be discussed. Key issues will be addressed for both the construction and packaging industries. It should be noted that throughout this chapter we refer solely to a specific product belonging to the family of polystyrenes recognised within the industry as ‘EPS’. Other expanded polystyrenes such as ‘extruded polystyrene’, which is used solely in the construction industry, and ‘polystyrene paper’, the material used to make trays for wrapping small portions of foodstuffs and which we usually see at the supermarket, will not be covered here. Confusingly, both of these types of polystyrene are referred to by the abbreviation XPS.

3.1.1 Development of Expanded Polystyrene (EPS) Over half a century ago, EPS was first produced experimentally by chemists at BASF. A patent was awarded to Stastny and Gäth in February 1950 for the ‘process of producing porous masses from polymers’ and this became the basis for the production of EPS. The earliest opportunities for EPS were exploited in the construction industry, where it quickly become popular in a wide range of insulating roles due to its excellent thermal insulating properties. It was not long before its advantages as a packaging material were

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Handbook of Polymer Foams also apparent. The ability to protect merchandise from large shocks meant that goods were rarely damaged during the transportation process and straw and wood were gradually replaced by EPS packaging. Big users of the new product included the mechanical typewriter companies, the fish industry, Kodak and the Meccano Toy Company. IBM perhaps the most technologically advanced company of the time - also used EPS packaging when delivering its range of computers. Before considering the manufacturing process for expanded polystyrene foam, there are two specific comments that need to be made on the chemical structure of the material. There are many misconceptions about EPS and it is important to note that: •

EPS is based on the monomer styrene which is not just a synthetic product but occurs commonly in a range of natural foodstuffs such as strawberries and coffee beans.

•

EPS products do not now contain chlorofluorocarbon or hydrochlorofluorocarbon blowing agents, indeed, a large proportion of the industry never used these at all, while other sectors have replaced them with other agents such as hydrocarbons which readily decompose in the atmosphere and as a result, EPS does not have any adverse impact on the ozone layer.

3.2 Manufacture of Expanded Polystyrene Mouldings It is important to differentiate between the product that is manufactured by the raw material manufacturer (commonly referred to in the industry as ‘bead’ supplier) and the product that is produced by the processor/converter. The bead supplier produces an expandable polystyrene that has a spherical shape and takes the form of small grains (similar to sugar in appearance). The expandable polystyrene (bead) is typically purchased in one tonne containers known as octabins. At the point of purchase, expandable polystyrene contains 4-7% (by weight) of a hydrocarbon blowing agent (usually pentane). To make EPS, tiny spherical polystyrene beads are expanded to about 40 times their original size. The manufacturing process, in brief, involves the heating of beads using a flow of steam, which causes the blowing agent to boil and a honeycomb of closed cells is formed. After a maturing stage in silos, these expanded beads are transferred to a mould and heated again with steam, They expand further and fuse together to form the shape required. In this form EPS is made up of 98% air; making it one of the lightest packaging materials available. Bead suppliers offer a range of grades of material and specifications but there are two basic types, flame retardant and non-flame retardant. The former grade is used mainly in the building industry, the latter is used in packaging. 38

Expanded Polystyrene: Development, Processing, Applications and Key Issues During polymerisation the batches follow a ‘normal distribution’ in the range of bead sizes produced. These are then sieved and the companies involved normally market three bead size ranges, which can be referred to as large, medium and small. Large is used exclusively for block manufacture for the construction industry, medium is the major material used in packaging and the small material is used only where the packaging product has very thin sections which are difficult to fill with the medium material. The customer generally specifies only one property, namely the density of the finished product. For packaging electronic items this is generally in the range of 18-25 g/l. Note: it is commonplace in the EPS industry for density to be quoted as g/l rather than kg/m3 but the numbers involved are interchangeable.

3.2.1 Conversion of Bead to Product The conversion process is carried out in three stages: pre-expansion, maturing and final moulding.

3.2.2 Pre-expansion The raw material is heated (in special machines called pre-expanders or pre-foamers) with steam at temperatures between approximately 80 and 100 ºC. The density of the material falls from approximately 630 kg/m3 to values of between 10 and 35 kg/m3. During the process of pre-expansion the compact beads turn into cellular plastic beads with small closed cells that hold air in their interior. The pre-expander is, essentially, a cylindrical container with diameter of 1 m, height of 2 m, and a central stirring mechanism with the facility to introduce low-pressure steam. There are two types of pre-expansion equipment: continuous and batch. Continuous pre-foam equipment is less favoured today by package moulding companies because the accuracy of producing density is insufficient for subsequent moulding. It is however commonly used by the moulding companies where large volumes of pre-foam material are required on a continuous basis. The batch pre-foaming process involves a pre-weighed amount of the raw material, which is dropped into the pre-foamer machine before the steam is introduced. Steam is introduced and the volume is allowed to increase to a predetermined point in the vessel before the steam is shut off. A previous experiment determines this point which is based on the weight of the material that is charged into the pre-foaming machine.

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Handbook of Polymer Foams During the process, the low pressure, low temperature steam is mixed with the raw material, it softens the polystyrene and raises the temperature until the blowing agent boils. The granules expand dramatically by up to 40-50 times. This pre-foam material is then conveyed to large storage hoppers where it is left to stand for approximately 12 hours.

3.2.2 Maturing During the pre-foam process as a result of the rapid bead size expansion a vacuum has been created inside the bead which leaves it with a low mechanical strength. The permeation of air inside the bead to create atmospheric pressure is essential to the quality of the mouldings at the final stage of manufacture. In addition, the pre-foaming stage leaves moisture inside each bead and it is advantageous to the final moulding process if the bead is dry as less energy is required. It is for these reasons that the pre-expanded beads are blown, through pipes, to large silos (hoppers) where they are dried and the internal pressure is equalised. This is how the beads achieve greater mechanical elasticity and expansion capacity is improved, something very useful in the subsequent transformation stage.

3.2.3 Final Moulding In this final stage the stabilised pre-expanded beads are transported to moulds where they are again subjected to steam so that the beads bind together. This enables large blocks to be produced (that are later sectioned to the required shape like boards, panels, cylinders, etc). Alternatively products are moulded in their final finished shape. Moulding machines used for EPS throughout most of the world are predominantly manufactured in Germany and Italy. The machines operate on the basis of the use of aluminium moulds which are fitted as a male and female design, the shape between the two halves of the mould being the shape required. These are fitted into a press which has the facility to introduce steam from behind each half of the tool through small slotted core vents which have been built into the tool. The dimensions of any foam product are difficult to measure precisely and in most instances plus or minus 1.5 mm is no great difficulty as the product can generally take up this difference when it is fitted into a pack.

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Expanded Polystyrene: Development, Processing, Applications and Key Issues

3.3 Applications for Expanded Polystyrene Packaging Over half a century, the uses of EPS have become numerous both within the packaging industry, where it can offer excellent impact resistance, and within the building industry, where its excellent thermal insulating properties mean that it is often used in the insulation of homes and offices. The reasons for this and the broad areas of application concerned will be considered in this section. However, both of these areas are now mature and established markets and real growth is expected from other, more novel applications that are outside the conventional packaging and construction sectors. These applications are addressed separately under a broader category of other applications.

3.3.1 Packaging EPS is an extremely important material for the modern packaging industry offering an excellent combination of protection, cost-effectiveness and environmental performance. In short, there are many reasons to use EPS as a packaging material. The distinct packaging advantages that EPS offers means that it has become the predominant packaging material for industries as diverse as electronic consumer goods, horticultural products and fish, throughout the world. Its versatility means that whether it is moulded or hand cut, EPS will protect the smallest electronic component through to the largest fridge freezer. The outstanding shock absorbency of EPS packaging ensures the protection of a broad range of products. Moreover, its compression resistance means that EPS is ideal for stackable packaging goods. For example most white goods, such as fridges and washing machines, and brown goods, such as TVs and videos come packed in EPS to prevent damage in transit. Since it is such a light material it adds very little to the weight of the package so transport costs and consequent vehicle emissions are kept to a minimum. The thermal insulation properties of EPS help keep food fresh and prevent condensation throughout the distribution chain. It is widely used in the agricultural sector for the packaging of fruit and vegetables. There is no loss of strength in damp conditions, making EPS ideal for cool-chain products. Furthermore, the fact that the material is moisture resistant means that the highest hygiene requirements are met. Another common use for EPS is therefore to package fish for transportation, which can be packed with ice and so remain chilled until reaching the final destination. In this case the EPS primarily provides thermal insulation rather than mechanical protection.

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Handbook of Polymer Foams Garden centres are also using EPS with a number of smaller plants and flowers now coming in EPS containers. It is easier to break plants out of these than from the usual plastic pots and, here again, EPS provides an important level of thermal insulation that protects the plants from frost in the early stages of growth. When safety is paramount, EPS comes into its own. It is used in the manufacture of children’s car seats and cycling helmets, where its protective qualities, strength and shockabsorbency are vital.

3.3.2 Construction The characteristics of EPS make it ideal for use as lightweight filler, insulation, as an element for decorating or adding imaginative finishing touches, as a lightweight filling material in roads, to facilitate land drainage, etc. Visit a construction site or examine the structure of a wide range of buildings and you find products made from EPS carrying out diverse and important functions. Low thermal conductivity: due to its closed air-filled cell structure which inhibits the passage of heat or cold, a high capacity for thermal insulation is achieved. Low weight: densities of between 10 and 35 kg/m3 allow light but safe construction works. Mechanical resistance: although they are light, EPS products enjoy good mechanical properties which are important for certain applications (load-bearing roof insulation, sub-pavement flooring, etc). Low water absorption: this helps in maintaining the thermal and mechanical properties which might otherwise be affected by humidity. Ease of handling and installation: the material can be worked with the usual tools and guarantees perfect finishing and adjustments. At the same time, its low weight makes it easy to transport materials to the site and provides economy of installation. Chemical resistance: Expanded polystyrene materials are perfectly compatible with the materials usually used in the construction industry including cements, plasters, salt or fresh water, etc., but not solvents. Versatility: it can come in many different shapes and sizes, to meet the specific requirements of the specific application.

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Expanded Polystyrene: Development, Processing, Applications and Key Issues Ageing resistance: all of these properties are retained over the whole of the material’s life which can be expected to be as long as the building itself. EPS products are not altered by external agents or by fungi or parasites as these will find no nutritional value whatsoever in the material. The main use of EPS in building applications is for thermal insulation, where EPS is used to insulate wall structures (in the cavity, internally or externally), roofs and floors. It is equally appropriate in new or refurbished buildings. Installing EPS insulation means that less heating fuel is required as the EPS panels that are placed in the cavity of a house wall can reduce heat loss up to five times that of a wall with no insulation. Put another way, heat loss can be reduced by more than 70%. For every kilogram of oil used on the manufacture of EPS insulation board, about 200 kg of oil will be saved in terms of reduced heating demand over the lifetime of the building. EPS is also used as a ‘fill’ material or void former, where it can help to overcome problems caused by poor ground in the construction of bridges and road embankments, dams and dock harbour walls. EPS can also be used to improve impact sound insulation in office blocks with solid concrete floors by damping impact vibrations. Large slabs can be laid quickly with no special equipment. Since it is so buoyant, EPS can also be used to make floating pontoons for yachting marinas. Further applications of EPS in the field of construction are possible and enormously varied. In fact, we can find EPS in all building requirements. What’s more it can be used in building work including large structures like roads, bridges and railway lines. All manufacturers of EPS are aware of the need to consider the impact of the product on the environment. The manufacturers are also aware of the need to minimise the energy used in making the product and to maximise the amount of energy which can be reclaimed.

3.3.3 Other Applications In addition to the conventional application sectors of packaging and construction, there are other markets that fall outside these areas. For the sake of completion, they are worthy of note but most do not currently use significant volumes of material and remain minority applications.

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Handbook of Polymer Foams The use of EPS in the automotive industry has become more widespread in recent years, alongside its ‘sister’ product expanded polypropylene (EPP). However, where EPP is used in structural applications such as front and rear bumpers, EPS is being used as a lightweight voidfill under floors. Avoided for a long time due to the acoustic ‘squeak’, manufacturers have now overcome the problem and the opportunities for use are increasing. The production levels for cycle helmets have undergone a steady increase over recent years as the usage becomes more popular. Incorporating the impact resistance and light weight that has made the material successful for many packaging applications, EPS can now add a level of protection to a new sector. In other applications, surf boards have been a popular leisure tool throughout the world for many years and EPS is chosen for its light weight and durability. The opportunities for aerating soil are also addressed by including small amounts of EPS beads.

3.3.4 Novel Applications One of the most important and most novel construction applications that has come to prominence in recent years is the insulated concrete form (ICF). This enables quick and energy efficient construction of a building by establishing a wall comprising two separate end walls in EPS. The gap between the two layers of EPS is then filled with concrete to give strength. The EPS shapes are custom moulded in the same way as a typical package moulding but a metal or plastic member is inserted into the mould. Each EPS piece then has two opposing, parallel sheets of EPS that are held at a fixed distance by the inserted member. Pieces can be fitted together quickly and, once concrete is added, the resultant structure has an exceptional combination of strength and thermal insulation. Sound transmission tests have also demonstrated that sound transmission is reduced to one-third that of a conventional wall structure.

3.4 Properties of EPS Expanded polystyrene has primarily been developed for use in the construction and packaging industries where the properties of thermal insulation and mechanical protection have been paramount.

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Expanded Polystyrene: Development, Processing, Applications and Key Issues Construction applications such as underfloor or wall insulation, benefit from the very high levels of thermal insulation, while the packaging of fragile electronic goods requires high mechanical (impact) protection. Increasingly though, EPS is used for the transport of food, and packaging (fish, fruit and vegetables) and in these cases, both the mechanical and the thermal insulation play an important role. It should be noted that the properties of an expanded polystyrene foam are intrinsically linked to the density of the material and are often quoted in a range.

3.4.1 Mechanical Performance The closed cell structure of EPS foams – incorporating 98% air – enables it to react to impact by deforming and returning to shape. This process absorbs the energy associated with sudden impact and offers a superb level of protection, refer to Table 3.1. It may be noted that expanded polystyrene foams may absorb very small amounts of water but the product is not hygroscopic and the mechanical properties are unaffected by moisture content.

Table 3.1 Physical properties of EPS foam at varying densities Density (g/cm3)

15

25

40

50

Tensile strength (kPa)

200

350

600

750

Flexural strength (kPa)

200

400

700

900

Compressive stress at 10% compression (kPa)

90

180

320

400

Source: BASF

3.4.2 Thermal Insulation Apart from a vacuum, air is the simplest and lowest cost thermal insulator and thin layers of air (up to 6 mm) have a very low thermal conductivity. However, it is not always practical to rely on air as an insulator and, for packaged goods a material is required to form the package.

physical properties BASF

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Handbook of Polymer Foams Expanded polystyrene is ideal in this case as it has outstanding thermal insulating properties due to its closed-cell foam structure consisting of microscopically small air bubbles. Due to the inert nature, expanded polystyrene foams do not directly affect other substances such as food. It may be noted that thermal conductivity for EPS foams is greater at very low densities (10-15 g/cm3) and falls with an increase in density so that optimum thermal performance occurs in the range 30-50 g/cm3. After that, the thermal conductivity starts to rise again, refer to Table 3.2.

Table 3.2 Thermal properties of EPS foam at varying densities Density (g/cm3) Thermal conductivity (W/mK)

10

20

40

50

60

0.043

0.035

0.033

0.033

0.034

Source: BASF

physical properties BASF

3.4.3 Chemical Properties The performance of expanded polystyrene in the presence of chemicals can vary widely. It is unaffected by prolonged contact with brine, soaps solutions, bleaches and most dilute acids. However it is extremely susceptible to the presence of many organic solvents. EPS foams may undergo the same changes following prolonged exposure to UV radiation that can occur in other polymers. However, by the nature of the applications due to the short life associated with packaging, in most cases this does not feature significantly. Construction applications have a greater life span but are unlikely to be exposed to UV radiation. EPS does not have any nutritional value for animals and does not support mould growth. It does not emit any water soluble substances that might be considered to contaminate groundwater.

3.4.4 Recent Research on Properties of EPS: Value for Fruit and Vegetables Research has demonstrated the value of using expanded polystyrene for the packaging of foodstuffs. It is recognised that carbon dioxide is formed by fruit as a product of

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Expanded Polystyrene: Development, Processing, Applications and Key Issues metabolism and that the release will lead to an increased concentration in the package, slowing the ripening process. EPS packaging retains the nutritional value of fruit and vegetables better than any other packaging, according to new research conducted by the Korean Food Research Institute and confirmed by the Michigan State University School of Packaging. Tests showed that vitamin C levels were preserved in both fruits and vegetables packed in EPS at a higher level than those packed in other materials. Researchers selected three fruits - apples, pears and grapes - along with three kinds of vegetables - squash, cucumbers and tomatoes - and separated, packed and stored the produce at controlled temperatures. They then routinely monitored the freshness and nutritional value of the items using a variety of established tests. On virtually every property measured, EPS outperformed the other materials. One of the primary reasons the study was undertaken was because such a high percentage of agricultural produce harvested today is lost due to spoilage. The research found that EPS could effectively slow the rate of spoilage, which is up to 45% in some countries. The specific findings on vitamin C show that certain vegetables retain up to 44% more vitamin C after a week of storage. The fruits, which were monitored for the study over varying periods of time, were shown to hold between 6.6% and 41% more vitamin C than those packed in corrugated paperboard boxes.

3.5 Global Structure of Markets and Companies 3.5.1 Europe All of the major countries throughout Europe, produce EPS for both packaging and construction applications. However, it can be noted that almost without exception, every country produces significantly more EPS for construction than for packaging. In some few cases, such as Denmark and UK, the difference between the two application sectors is small. However, for most countries there is a pronounced difference and in some cases, most notably Germany and Poland, the production for construction is 5600% that of packaging. The net effect is that throughout Europe, construction is the major user of EPS, refer to Table 3.3. In recent years, packaging manufacturers have seen the production facilities for electronic equipment moving to central Europe and the Far East. EPS manufacture has been forced

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Table 3.3 EPS Production in European countries (Source: EUMEPS)

European EPS production (2001)

Packaging (tonnes)

Construction (tonnes)

200,000

600,000

to follow and several major European moulding companies have established manufacturing sites throughout the Czech Republic, Slovakia and Poland. Thus, while EPS packaging production has declined by a small amount across current EU member states, much of this will be regained when accession states join the EU. There are increasing demands from Building Regulations for the energy efficiency that follows from high levels of thermal insulation and this is complemented by some innovative new approaches to the methods of house assembly. As a result, construction applications are still enjoying a small but significant level of growth throughout Europe and this is expected to continue for some years. When the number of manufacturing companies are considered, both the EPS construction and packaging industries have seen significant consolidation in recent years. Many major producing countries – notably Germany, France and UK – now have a small number of large, influential manufacturers and this has had a bearing on their negotiations with the raw material manufacturers. Other countries, such as Italy and Spain, still have a very large number of small moulding companies and consolidation in the future may be likely.

3.5.2 Asia Unlike other regions of the world, the production of EPS construction products in Asia is extremely unusual and tonnages are very low. By comparison, the production of EPS packaging for consumer electronic goods and household appliances is very significant with much of this being exported to Europe and USA. The production of EPS boxes for fresh fish remains a very significant area of application. In numerical terms, total Asian production of EPS dominates the world market. Some countries such as Japan and Korea have maintained large levels of production over many years while one country in particular – China – has seen very dramatic growth in recent years, refer to Table 3.4.

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Expanded Polystyrene: Development, Processing, Applications and Key Issues

Table 3.4 EPS Production in Asian countries Country

EPS packaging production, tonnes (2001)

China

700,000

Korea

228,000

Japan

201,000

Thailand

37,000

Taiwan

32,000

Malaysia

29,000

India

25,000

Others

50,000

Total

1,302,000

Source: AMEPS

production in Asia

3.5.3 USA In the USA, levels of production for EPS packaging and construction applications are largely similar. However, it may be noted that the production of EPS trays and other containers for food service is much more significant in USA than in Europe, refer to Table 3.5.

Table 3.5 EPS Production in USA Production, 2001 (tonnes) Total EPS sales

394,000

EPS block

180,000

EPS packaging

214,000

Source: EPSMA

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3.6 Key Issues Facing the EPS Industry 3.6.1 Fire The performance of EPS in a fire needs to be well understood because it is commonly used in construction applications. To do this effectively, it is important that an assessment of the material is based on performance in end-use conditions and this depends not only on the chemistry of the material but also on the physical state of the EPS. Almost every organic material is susceptible to fire and polystyrene foam is combustible. However, in practice, the burning behaviour depends on the conditions under which it is used, as well as the inherent properties of the material. These inherent properties differ chemically, depending on whether or not it contains a fire retardant additive but the burning behaviour also depends on the nature of bonding of EPS to other materials, the location of the product and the availability of oxygen (ventilation). The important point to note is that, when installed correctly, expanded polystyrene products do not present an undue fire hazard. The presence of fire retardants leads to significant improvements in the fire behaviour of EPS. In the presence of large ignition sources from fires involving other materials, fire retarded grades will eventually burn but in such cases the building may be considered to be beyond the point of rescue. The presence of the fire retardant has a beneficial effect when EPS is exposed to a smaller fire source. In these cases, the foam shrinks away from the fire source, reducing the likelihood of ignition. Decomposition products of the fire retardant additive cause quenching so that on removal of the ignition source, the EPS will not continue to burn. The most commonly used flame retardant is HBCD (hexabrominated cyclododecane) and there are currently few alternatives. HBCD is currently undergoing a risk assessment but this has a long way to go. However, some labelling of the chemical in relation to aquatic organisms appears likely. The European Chemical Industry Council (CEFIC) has set up a steering committee to review the position. Smoke is an important factor in fires since a high density of smoke can be toxic and can inhibit the search for an emergency exit. In actual fires where smoke is produced, it is often anticipated that this originates from EPS insulation. In fact, most of the smoke originates from materials such as burning wood, asphalt felt and furniture. EPS is flammable, as are many other building materials but this is only relevant if EPS is assessed as an exposed insulating material. Fire safety philosophy correctly considers ‘end use conditions’ and recognises that EPS does not present an undue hazard.

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Expanded Polystyrene: Development, Processing, Applications and Key Issues

3.6.2 Recycling Environmental legislation is developing throughout the world and the polystyrene industry is affected along with all other materials. In Europe, the Packaging industry has been at the forefront of developing legislation (Packaging and Packaging Waste - 94/62/EC). Following the implementation of this Directive into national legislation in each member state (1999), targets were established for the recycling of all packaging materials. It is important to remember that the Packaging and Packaging Waste Directive makes no specific reference to EPS and it is only ever considered under the general plastics target (15% recycling in 2001). Throughout Europe, the expanded polystyrene industry has exchanged information on recycling and promoted best practice and, as a direct result, in most European countries the levels of recycling already exceed targets. Likewise, moves to reduce the presence of EPS in landfill – either through an increasing tax or through localised bans – are being pre-empted by the significant levels of recycling that are being achieved. Further legislation is being developed to tackle other applications such as such as Waste Electrical and Electronic Equipment (WEEE) and End of Life Vehicles but neither of these are expected to have any impact on EPS. It is unclear how additional future legislation will develop but there is no current or pending European legislation that discriminates against the use of EPS packaging. With this vast increase in the amount of environmental legislation across a broad range of industry and in the light of recent legislative developments, some companies became concerned that the use of EPS packaging would make it difficult to meet this legislation. Indeed, a perception has continued in all too many cases that EPS cannot be recycled. This could not be further from the truth as levels for EPS recycling across Europe already exceed legislative requirements and are amongst the highest for any plastic material. The EPS industry throughout Europe has worked for many years to actively encourage the recycling of used packaging material and many large electronics companies are successfully collecting and recycling their EPS. The low weight of EPS is an advantage during its life as a packaging material but can cause complications in recycling, as it becomes difficult to transport over any distance. When uncompacted EPS packaging is placed in a typical (12 m) container, it will be filled with less than half a tonne of material. This is not economically viable for transport. The EPS industry has addressed this point however and a wide range of compaction and densification machinery is now available to ensure that recycling continues to be viable.

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Handbook of Polymer Foams The majority of these compaction machines operate in a mechanical mode with a rotating screw of varying pitch that causes the material to be physically compacted by up 35:1 or even 40:1. Once compacted, material can be transported much larger distances and transport to a recycling site becomes a viable option. It is less common for companies to use a thermal system, which melts the polystyrene, returning it to the original, solid material. These thermal systems tend to be slower and more energy intensive for the waste holder, as well as producing a solid block that may be more difficult for the recycler to process. However, new developments in technology may make this method of size reduction more popular in future. Solvent-based systems have also been proposed whereby the EPS is added to a vat of solvent and dissolved to produce a slurry that can be transported for recycling. Given that EPS is only 2% material and 98% air, the material readily dissolves and there are a wide range of solvents that could be used for this operation but – due to modern concerns about volatile organic compounds (VOC) – are not generally acceptable. Some research has been carried out into the use of a chemical that is extracted from citrus fruits, negating the concerns about VOC. However, the logistics and economics of processing the slurry have still proved complicated and this process has not received widespread acceptance and the physical compaction process remains most common. Once collected by a recycler, EPS is recycled in a number of different ways. Firstly it can go back into new EPS foam and is widely used by the construction industry where recycled packaging is added to insulation board. Studies have demonstrated that, done correctly, the introduction of recycled material to new foam polystyrene has no detrimental effect on the properties of the new product. Throughout Europe, recycled material is also returned to the packaging industry where it is either incorporated into new package mouldings or it is used in the form of ‘loosefill’ packaging. Once the air content is removed, EPS is no different from crystal polystyrene and it can be used in a range of non-foam applications. Thus recycled EPS can be widely used in office equipment, coat hangers, videocassettes and plant pots. One of the most novel uses for recycled EPS is a wood substitute that can be used in a vast range of areas. Recycled polystyrene can be converted into a product that looks and acts like wood and can be sawn, nailed or screwed, just like wood. It can be used for simple products such as park benches and fence posts and is already found in garden furniture and marine walkways. The product costs less than hardwood but really has a

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Expanded Polystyrene: Development, Processing, Applications and Key Issues value when used to replace hardwoods such as mahogany and teak where it can save the impact on the rainforests. A major UK retail store has announced that they will be using the product in their counter tops where it is extremely durable and resistant to impact from trolleys. One UK recycler has now developed the replacement idea further and is now marketing a slate replacement that is made from recycled polystyrene and slate dust. This product closely resembles conventional slate but it is expected to last longer and cost less. The project involves the manufacture of housing roof slate replacement made from a 50:50 blend of slate dust and recycled EPS Packaging. It has the full backing of a major conventional slate manufacturing company and it is expected that once the site is running it will require very large amounts of polystyrene.

3.6.2 Alternatives to Mechanical Recycling When mechanical recycling options are not appropriate, incineration with energy recovery is a viable alternative. This enables the energy content of EPS to be recovered to generate substantial amounts of energy since EPS has a higher calorific value than coal and approximately the same value as fuel oil. Emissions from incineration plants are strictly regulated and control systems have been developed to the level where incineration plants can be run with no adverse effects on the surrounding environment or population. Tests conducted at the SELCHP energy from waste plant in London have shown that combustion can be accomplished within stringent emission limits. EPS in the waste stream can readily be treated in a modern energy from waste plant. This is an environmentally acceptable option for dealing with that portion of plastics waste which cannot be sensibly recycled. In any modern waste management programme, landfill should only be used as a last resort. Well-managed sites should be stable enough not to suffer from subsidence and EPS is an ideal material for disposal in landfill, being mechanically stable and inert. No company or individual should have any concerns about the use of EPS on environmental grounds. There is no European environmental legislation that specifically targets EPS and in many European countries it is already recycled to a higher degree than legislation requires.

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Handbook of Polymer Foams

Further Information There is limited information currently in print about expanded polystyrene foam and the majority of available literature comes from the raw material manufacturers and trade associations. Major manufacturers of expandable polystyrene:

Regional Trade Associations:

BASF AG Carl Bosch Strasse 38 67056 Ludwigshafen Germany www.basf.de

European Manufacturers (EUMEPS) Avenue Marcel Thiry, 204 B-1200 Brussels Belgium www.eumeps.org

Nova Chemicals Corporation 1000 Seventh Avenue SW PO Box 2518, Station M Calgary Alberta Canada T2P 5C6 www.novachemicals.com BP European Customer Service Centre Building A Chertsey Road Sunbury-on-Thames Middlesex TW16 7LL http://www.bpchemicals.com/

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of

EPS

EPS Moulders Association (EPSMA) 1298 Cronson Boulevard Suite 201 Crofton MD 21114 USA www.epspackaging.org Asian Manufacturers of EPS (AMEPS) 6F Showa Akihabara Building Bld 2-20 Kanda Sakuma-Cho Chiyoda-Ku Tokyo 101-0025 Japan www.ameps.org

Rigid Polyurethane Foams

4

Rigid Polyurethane Foams David Eaves

4.1 Introduction The term ‘polyurethane’ covers a wide range of materials produced by the reaction of polyfunctional isocyanates with substances containing at least two hydroxyl groups. The main chemical moiety resulting from this reaction is the urethane group: -NH-CO-OOther groups which may be formed include urea, amide, biuret, allophanate, ether and ester linkages, depending on reaction conditions and the particular catalysts used. Hence, the term ‘polyurethane’ is a very general one covering materials having very different properties and including both expanded and non-expanded products. The materials discussed in this chapter are hard, expanded polyurethanes having low flexibility and a high load bearing capacity in relation to their density. Compression deflection curves of these materials show a pronounced yield point and hence the foams have a significant permanent set after compression. In contrast to the open cell structure of flexible polyurethane foams, rigid foams have a mainly closed cell structure. They are expanded using physical blowing agents which are retained in the closed cells and which have a significant influence on some of the foam properties, particularly thermal insulation. Isocyanate-based rigid foams for insulation applications were introduced in the early 1950s and can be made by both one shot or prepolymer processes using either toluene di-isocyanate (TDI) or di-phenyl methane di-isocyanate (MDI). In the one shot process, isocyanate and polyol are reacted to provide the final foam in a single step. In the prepolymer process, some polyol and excess isocyanate are reacted to provide an isocyanate tipped prepolymer which is then reacted with further polyol in a subsequent step to produce the final polymer. The prepolymer process has the advantage of low free isocyanate levels (and hence low toxicity) together with better regulation and control of the reaction and lower exotherm. The main disadvantage is the extra process of prepolymer production which adds to the cost.

55

Handbook of Polymer Foams The isocyanate/polyol reaction is exothermic generating high temperatures within the reaction mix. Expansion of rigid polyurethane foams results largely from the consequent evaporation of a liquid physical blowing agent which is added as a solution to one of the components. Alternatively, it is possible to use a gas with a boiling point well below room temperature. The gas is dissolved under pressure in one of the components and subsequently comes out of solution when the mixed components are depressurised. Reactions are affected by additional factors including the presence of catalysts, surfactants and fire retardants. Such additives regulate the rate of reaction, the type of chemical structure formed, the foam morphology and ultimately the physical properties of the foam. The general class of rigid polyurethane foam can be divided into rigid polyurethane, or PU foams, and polyisocyanurate, or PIR foams. PU foams are produced by reaction of isocyanates (usually MDI) with short chain polyols whilst PIR foams result from reactions in which MDI is used at levels substantially higher than stoichiometric (around 50% higher) in the presence of special metal salt catalysts. The excess MDI is the key to PIR production as under the reaction conditions it reacts with itself to form cyclic trimeric isocyanurate groups. The final structure contains both urethane and isocyanurate groups and the materials are more correctly referred to as modified polyisocyanurate foams. PIR foams are intrinsically more thermally stable than PU foams and have a superior fire resistance. However, there are some disadvantages with PIR foams such as generally poorer adhesion and friability. The rapid increase in the use of these rigid polyurethane foams following their introduction in the early 1950s was due mainly to their enhanced water resistance (both liquid and vapour) and low thermal conductivity. Hitherto, isocyanate foams had a generally open cell structure through which water or water vapour could easily penetrate, together with thermal conductivities around 32 mW/m.K at 10 °C. The new rigid, closed cell, isocyanate based foams were essentially impervious to water and, using low thermal conductivity expansion gases such as CFC-11 or CFC-12, thermal conductivities were reduced to typically 17 mW/m.K. Hence, insulation thicknesses could be reduced to half that of the old products which depended on the presence of air trapped in cells and foam interstices for their insulation performance. It was soon realised, however, that insulation performance of the new closed cell products decreased with time as the chlorofluorocarbon (CFC) gas diffused out of the cells, but even so a superior thermal performance was maintained. Thermal insulation of appliances, e.g., refrigerators and freezers, and buildings remain the main applications of these foams. The basic chemistry involved in the manufacture of PU and PIR foams has been discussed at length elsewhere [1, 2]. Some trends which have been reported recently are the move

56

Rigid Polyurethane Foams towards the use of polyisocyanurate foams incorporating aromatic polyester polyols [3, 4] due to the higher fire resistance and, most importantly, the move away from chlorofluorocarbon-11 (CFC-11) and chlorofluorocarbon-12 (CFC-12) to blowing agents having greater environmental acceptability to meet Montreal Protocol requirements.

4.2 Materials

4.2.1 Polyols

4.2.1.1 Polyether Polyols Polyols used for the production of polyurethane foams are oligomers, (i.e., low molecular weight polymers), containing at least two hydroxyl groups. They may be polyethers, polyesters, polyolefins or vegetable oils. Polyether and polyester polyols are the main compounds used in both rigid and flexible foams. Polyester polyols were used to a significant extent in the early development period of rigid polyurethane foams but were later discontinued except for special applications owing to their high cost coupled with generally low functionality, high viscosity and low dimensional stability of the resulting foam. More recently however, polyester polyols are being increasingly used in the production of rigid PU foams for the construction industry owing to the enhanced fire retardancy that can be achieved. Polyether polyols are produced by the anionic polymerisation of alkylene oxides, (e.g., propylene oxide, ethylene oxide), in the presence of an initiator and catalyst. The functionality and equivalent weight of the polyol can be controlled within wide limits and the materials are widely used for both rigid and flexible polyurethane foams. Initiators are low molecular weight hydroxylated compounds whose functionality determines that of the resulting polyol. Some commonly used substances are shown in Table 4.1. Polyether polyols for rigid PU foams are produced using high functionality initiators such as glycerol, sorbitol and sucrose. Some polyol characteristics are shown in Table 4.2. The most widely used catalyst is potassium hydroxide although this does result in some side reactions which lead to the production of undesirable monols. Improved catalysts have been developed which eliminate this problem and this is particularly important for the production of high molecular weight diols for the manufacture of flexible polyurethane foam.

57

Handbook of Polymer Foams

Table 4.1 Functionality of some common polyols Hydroxylated Compound

Functionality

Ethylene Glycol

2

Glycerol

3

Trimethylol Propane

3

1,2,6-Hexane Triol

3

Triethanolamine

4

Sorbitol

6

Sucrose

8

Table 4.2 Polyols for polyurethane foams Polyol property

Rigid foam

Semi-rigid foam

Flexible foam

OH number (mg KOH/g)

350-560

100-200

5.6-70

OH equivalent (56,110/OH No)

160-100

560-280

10,000-800

Functionality

3.0-8.0

3.0-3.5

2.0-3.0

>700

700-70

<70

Elastic modulus at 23 °C (MPa)

Reproduced with permission from Handbook of Plastic Foams, Ed., A.H. Landrock, Noyes Publications, Chapter 2, Thermosetting Foams Note: OH Number - mg KOH/g OH Equivalent - 56,110/OH Number

4.2.1.2 Polyester Polyols Polyester polyols for polyurethane foams can be produced by the reaction of di-basic acids, (e.g., adipic acid, phthalic acid) with glycols, (e.g., ethylene glycol, propylene glycol); alternatively, they can be made by the ring opening polymerisation of lactones. These materials are used in the manufacture of flexible polyurethanes. However, for rigid polyurethane foams, aromatic polyester polyols are the preferred polyol type. functionality of Polyols polyols for Polyurethane foams Landrock A.H.

58

Rigid Polyurethane Foams

4.2.1.3 Aromatic Polyester Polyols Aromatic polyester polyols (APP) have in recent years become the materials of choice for rigid polyurethane foams used for insulating board in the construction industry [4]. Initially these materials were used as low cost extenders to replace just part of the polyether polyol, but it was soon realised that such foams had improved fire resistant properties, i.e., low flammability and low smoke production. In Europe, where hydrocarbon blowing agents have, for environmental reasons, largely replaced the original CFC types, the use of APP in conjunction with PIR foam chemistry has counteracted the adverse effects on flammability characteristics resulting from the change in blowing agents. The trimeric isocyanurate structure of PIR foams gives them enhanced thermal stability compared with PUR foams and char formation during thermal decomposition is high - around 50% compared with 20% from PU foams. This char, by virtue of its low thermal conductivity and ability to reflect a high proportion of incident radiation, is the main reason for the better fire resistance of these materials. By incorporation of polyols containing aromatic ring structures, char formation is even further enhanced - hence the additional benefit conferred by APP [4]. The manufacture of APP involves the reaction of carboxylic acids with alcohols. The acids are obtained from three sources, namely: di-methyl terephthalate (DMT); polyethylene terephthalate (PET); and phthalic acid or anhydride (PA). DMT is available as a residue from PET production where, to avoid transport problems, it is generally converted on site to polyesters by glycolysis. Process residues include aromatic esters of di- and tri-carboxylic acids, and the functionality of the APP resulting from glycolysis (with di-ethylene glycol) is increased to 2.2-2.3. This, together with improved viscosity stability attributed to the heterogeneous nature of the mix, gives APP produced from a DMT source an advantage over materials from PET or PA. Production of APP from PET, like the DMT process, involves utilisation of otherwise waste material. PET waste is obtained either from post consumer scrap or as waste from PET production. Environmental initiatives, (e.g., the European Packaging Waste Directive which requires member states to introduce legislation stipulating minimum recycling and recovery rates for packaging waste), together with generally heightened environmental awareness is likely to lead to greater pressure for PET recycling by the glycolysis route and PET-based APP are likely to become increasingly and widely available. Recyclers have developed proprietary methods for dealing with such wastes. These require the contaminated nature and varied source of such waste to be taken into account, together with the necessity of removing catalyst residues which could have an adverse and unpredictable effect on subsequent foam production.

59

melting points of Polyesters Tideswell R.B.

Handbook of Polymer Foams A recently developed process which offers better control over molecular structure has in Europe provided APP with improved consistency and lower viscosity. These ‘second generation’ APP are very suitable for rigid PU foam production. Polyols made from PA, a substance widely used for the production of phthalate plasticisers for polyvinyl chloride (PVC), are pure, clearly defined materials used in formulations where precise functionality and fixed mix ratios are needed. However, for many applications this is unnecessary. The improved fire resistance of PIR foams incorporating APP is maximised using APP based on DMT or PET. This has been attributed to the higher softening temperature and greater heat stability of polymers based on terephthalic acid. The foams also have greater thermal stability. Approximate melting points of phthalic acid ester isomers are shown in Table 4.3.

Table 4.3 Approximate melting points of polyesters from phthalic acid (PA) isomers (°C) Phthalic acid isomer Glycol

Ortho PA

Iso PA

Tere PA

Ethylene

63-65

103-108

25 6

Diethylene

10-11

55-60

65-70

Reproduced with permission from R.B. Tideswell, SPI Annual PU Conference 1988, Philadelphia, PA, USA, p.374 [4]. Copyright SPI, 1988

4.2.2 Isocyanates The two isocyanates used for the production of almost all polyurethane foams, both rigid and flexible, are TDI and MDI. The original production method for isocyanates was based on the phosgenation of aromatic or aliphatic amines: R-NH2 + COCl2 ———> R-NH-COCl + HCl

(4.1)

R-NH-COCl ———> R-NCO + HCl

(4.2)

More recently phosgene free methods have been developed, for example, reductive carbonylation of a nitro compound in the presence of an alcohol to produce a urethane which thermally dissociates to give an isocyanate:

60

Rigid Polyurethane Foams R-NO2 + 3CO + R´-OH ————> R-NH-CO-O-R´ + 2CO2

(4.3)

R-NH-CO-O-R´ ————> R-NCO + R´-OH

(4.4)

Several other phosgene free routes are also available. MDI is obtained initially as a mixture of di-cyclic monomeric MDI (MMDI) and polycyclic polyisocyanates (PMDI) with approximate proportions as shown in Table 4.4. MMDI is separated from the polymeric form by distillation and exists in two isomeric forms: 2,4-MDI and 4,4-MDI. Both are solid materials at room temperature (melting point 38 °C), the latter with a tendency to dimerise. The main applications for MMDI are prepolymers for flexible foams, shoe soles and thermoplastic polyurethanes (TPU). The more important form of MDI in terms of volume is PMDI which is a mixture of isocyanates with two or more aromatic rings remaining in the distillation residue. PMDI is a yellow to dark brown material, liquid at room temperature (melting point less than 10 °C) with a higher functionality than MMDI but, owing to the lower NCO content, of reduced reactivity. The higher functionality of PMDI facilitating the production of a rigid, crosslinked, network coupled with the highly aromatic character which contributes to flame resistance, makes PMDI the isocyanate of choice for most rigid polyurethane foams. TDI, produced from toluene diamine, is a colourless liquid commercially available as a mixture of the 2,4 and 2,6 isomers in the ratio of 20:80 (TDI T-80) or 65:35 (TDI T-65). Its main use is in the production of flexible polyurethane foams. Other isocyanates are available, such as, 1,6-hexamethylene diisocyanate, isophorone diisocyanate and tetra-methyl xylene diisocyanate. These are used for special applications such as light, weather and heat resistant coatings, but have little application as rigid foams.

Table 4.4 Composition of MDI as manufactured 2 ring monomeric MDI

48%

3 ring compounds

27%

4 ring compounds

5%

5 ring compounds

4%

Higher homologues

16%

Source: [5]

61

Handbook of Polymer Foams Isocyanates may be modified by including linkages such as allophanate, carbodiimide and isocyanate to control reactivity and lower vapour pressure. They may also be ‘blocked’ by reaction with compounds containing labile hydrogen atoms, e.g., phenols and nitrophenols. Blocked isocyanates have the advantage of being inert at room temperature but liberate free isocyanate groups for reaction when the temperature is raised. This technology has application in heat activated coatings but again is not commonly used for rigid foam production.

4.2.3 Blowing Agents Rigid polyurethane foams produced in the 1960s used CFC-11 (trichlorofluoromethane) as the principle blowing agent. The type of blowing agent used, which is retained as vapour in the closed cells of the foam, is a major factor controlling the thermal conductivity and such foams had conductivity values around 17 mW/m.K. With the introduction of aromatic-based polyethers and esters, and further work adjusting formulations, conductivities down to 15 mW/m.K were achieved and these later foams also showed good long-term retention of the low conductivity values. Development work at that time aimed for even further improvements in insulation to achieve satisfactory performance from increasingly thinner, and hence more cost effective, sheets. However, by the late 1980s, CFC-11 and CFC-12 (dichlorodifluoromethane) had been recognised as chemicals very effective at depleting the protective ozone layer in the atmosphere and were environmentally unacceptable. This resulted in the introduction in 1987, of the now well known Montreal Protocol in which international action was agreed to combat the effects of these substances. The Protocol came into force in 1989 and provided a timetable whereby, in developed countries, use of CFC would be phased out by 1st January 1996. In Europe, this was advanced to 1st January 1995 with the adoption of regulation EC3093/94. Since it was recognised that products like rigid PU foams made an important environmental contribution through their insulative qualities which reduce energy requirements and hence carbon dioxide emissions, the phase-out arrangements were of a gradual nature with an interim period. During this time, CFC could be replaced with ‘transitional substances’ of much reduced ozone depletion potential (ODP) which would be phased out in the period 2003-2015, or earlier if possible. The phase-out schedule agreed at the Montreal meeting of September 1997 for ozone depleting substances (ODS) is shown in Table 4.5 (developed countries) and Table 4.6 (developing countries). The Annexes refer to various classes of ODS with CFC-11 and CFC-12 in Annex A and hydrochlorofluorocarbon (HCFC) in Annex C, Group 1. There are some exemptions for essential use but these do not include use for blowing agents. 62

consumption phase-out of ODS Rigid Polyurethane Foams

Table 4.5 Consumption phase-out of ODS in developed countries Date

Central Measure

1st July 1989

Freeze Annex A (CFC)

1st January 1992

Freeze of halons

1st January 1993

• Annex B CFC reduced by 20% from 1989 levels • Freeze of methyl chloroform

1st January 1994

• Annex B: CFC reduced by 75% from 1989 levels • Annex A: CFC reduced by 75% from 1986 levels • Halons phased out • Methyl chloroform reduced by 50%

1st January 1995

• Methyl bromide frozen at 1991 levels • Carbon tetrachloride reduced by 85% from 1989 levels

1st January 1996

• HBFC phased out • Carbon tetrachloride phased out ª Annex A and B: CFC phased out • Methyl chloroform phased out • HCFC frozen at 1989 level + 2.8% of 1989 consumption of CFCs (base level)

1st January 1999

Methyl bromide reduced by 25% from 1991 levels

1st January 2001

Methyl bromide reduced by 50% from 1991 levels

1st January 2003

Methyl bromide reduced by 70% from 1991 levels

1st January 2004

HCFC reduced by 35% below base levels

1st January 2005

Methyl bromide phased out

1st January 2010

HCFC reduced by 65%

1st January 2015

HCFC reduced by 90%

1st January 2020

HCFC phased out, allowing for a service tail of 0.5% until 2030 for existing refrigerators and air conditioning equipment

HBFC: Hydro bromo fluorocarbons

Some developed countries including USA, Japan and the European Union (EU) have brought these targets forward. The EU has agreed that from 1st January, 2003, the use of HCFC in appliances and sandwich panels (except for insulation trucks) will be prohibited. From 1st January, 2004, prohibition extends to HCFC in other rigid foams

63

consumption phase-out of ODS Handbook of Polymer Foams

Table 4.6 Consumption phase-out of ODS in developing countries Date

Central Measure

1st July 1999

Freeze of Annex A CFC at 1995-1997 average levels

1st January 2002

• Freeze of halons at 1995-1997 average levels • Freeze of methyl bromide at 1995-1998 average levels

1st January 2003

• Annex B: CFC reduced by 20% from 1998-2000 average levels • Freeze of methyl chloroform at 1998-2000 average levels

1st January 2005

• Annex A: CFC reduced by 50% from 1995-1997 average levels • Halons reduced by 50% • Carbon tetrachloride reduced by 85% from 1998-2000 average levels • Methyl chloroform reduced by 30% from 1998-2000 average levels

1st January 2007

• Annex A: CFC reduced by 85% from 1995-1997 average levels • Annex B: CFC reduced by 85% from 1998-2000 average levels

1st January 2010

• CFC, Halons and CCl4 phased out • Methyl chloroform reduced by 75% from 1998-2000 average levels

1st January 2015

Methyl chloroform phased out

1st January 2016

Freeze of CFC at baseline figure of year 2015 average levels

1st January 2040

HCFC phased out

Source: [6]

such as spray, block and polyisocyanurate. An EU amendment to bring dates forward by another 12 months narrowly failed. The replacement of CFC has been the main development target of the rigid PU foam industry since 1986, and the repercussions of this, in optimising the performance of alternative non CFC formulations and moving away from transitional substances, continue. The success of work to replace CFC can be judged from Table 4.7, which shows the use of ozone depleting substances by the European foam industry since 1986.

64

Ozone depleting substances Rigid Polyurethane Foams

Table 4.7 Use of ozone depleting substances by the European foam industry Year

ODP (000s weighted tonnes)

1986

63

1987

67

1988

70

1989

70

1990

69

1991

51

1992

39

1993

22

1994

4

1995

4

1996

4

1997

4

1998

4

1999

4

Derived from data in [7]

The desirable characteristics of blowing agents for rigid PU foams are well understood, and are: •

Boiling point in the range –20 to +30 °C

•

Good solubility in the foam precursors

•

Poor solubility in the foam polymer

•

Low thermal conductivity of gas/vapour

•

Low diffusion rate of gas/vapour through the polymer (for long-term retention in the cells)

65

Handbook of Polymer Foams •

Zero ODP

•

Low flammability both as liquid and gas

•

Chemically inert

Initial work on alternatives resulted in the development of so-called 50% reduced CFC-11 systems which were adopted in the late 1980s [8]. These systems were very much an interim measure to meet the short term reductions of ODP substances specified by the Montreal Protocol. Development to meet the longer term requirements have resulted essentially in two types of blowing agent - HCFC (such as HCFC-141b) and hydrocarbons (such as n-pentane). Some characteristics of these substances are shown in Table 4.8.

Table 4.8 Characteristics of some alternative blowing agents Property

CFC-11

HCFC-141b

HFC-134a

CO2

n-pentane

cyclopentane

Formula

CCl3F

CH3CCl2F

CH2FCF3

CO2

C5H12

C5H10

Molecular weight

13 7

117

102

44

72

70

Boiling point (°C)

24

32

-26.5

-

36

49

0.082

0.053

Vapour pressure at 30 °C (MPa)

0.08

Vapour conductivity at 20 °C (mW/m.K)

8

9

14

15

13

11

Flash point (°C)

none

none

none

none

< -50

-42

Explosion limits in air (% by volume)

none

7.4-15.5

none

none

1.4-7.8

1.4-8.0

TLV (ppm)

1000

500

1000

-

1000

600

1.0

0.11

0

0

0

0

good

good

good

poor

good

good

ODP (cf CFC-11) Permanency in cells

Adapted from data in [7], with permission from Rapra Technology HFC: Hydrofluorocarbon TLV: Threshold limit value

66

Rigid Polyurethane Foams From Table 4.8 it may be seen that there is no ideal replacement for CFC-11. HCFC141b, whilst giving a very substantial reduction in ODP does not have a zero ODP factor and the Protocol requires eventual phase-out. In addition, thermal conductivity is slightly worse and mixtures with air are explosive (within defined limits). HFC, such as HFC134a, do have zero ODP and are acceptable under the Protocol (although they do have some potential for global warming). They are non-flammable and non-explosive, but thermal conductivity is significantly worse than CFC-11. Carbon dioxide is a very safe gas to work with, but again has worse thermal insulation properties which are exacerbated by the impermanence of the gas in the cells, leading to a gradual deterioration in insulation and some dimensional stability problems. It also contributes to global warming, so is not totally environmentally acceptable (though it may be argued that environmental deterioration due to the use of carbon dioxide as a blowing agent is vastly outweighed by contributions of carbon dioxide to the atmosphere from transport and energy production). Hydrocarbons have zero ODP (and negligible global warming potential) and hence offer a permanent solution to Protocol requirements, but they are flammable and form explosive mixtures with air. Compared with CFC-11 there is also a significant worsening of thermal insulation. In addition to the intrinsic properties of alternative blowing agents and the properties they consequently impart to the foam, effect on processing is also very important. Whereas HCFC-141b is essentially a drop-in agent which can be run with relatively minor adjustments on conventional machines, engineering solutions have had to be found to introduce gases and flammable blowing agents. Worldwide, no single blowing agent technology has been universally adopted [9]. The phase-out of CFC has resulted in a broad diversification of rigid PU foam processes using HCFC-141b, HCFC-22 with HCFC-142b, HFC-134a, pentane isomers and carbon dioxide. HCFC-141b is currently used in the manufacture of metal faced sandwich boards. Whilst it is ‘drop-in’ technology, greater control of processing is needed and some dimensional stability problems require higher foam densities. This technology has been in place for some time producing foams of density 38-43 kg/m3 (continuous process) and 41-43 kg/ m3 discontinuous. This is the main technology adopted in the USA owing to the large emphasis placed on energy conservation and the environmental penalties exacted for units with large energy demands. HCFC-141b has also been used for appliance insulation applications, (e.g., in the Pacific area), where it is also a ‘drop in’ technology in terms of the foaming process. However, the solvent effect of HCFC-141b on acrylonitrile-butadiene-styrene or high impact polystyrene thermoplastics has required the development of special liners to prevent cracking, resulting in extra cost. In both appliance and board insulation, the early phase67

Handbook of Polymer Foams out of HCFC-141b in such countries as USA, Japan and the EU will considerably reduce its use. HCFC-22/142b has been used particularly in Germany and offers advantages of good dimensional stability but with some worsening of thermal conductivity. HFC-134a, in low amount, has been used industrially in both the continuous and discontinuous processes. The substance is dosed into the high pressure polyol stream in continuous operations, or used in a fully saturated polyol component in discontinuous operations. Carbon dioxide water blown foams are also produced successfully by panel manufacturers, with densities ranging from 43 to 48 kg/m3. The inferior long-term ageing (loss of insulation) due to diffusion of carbon dioxide from the cells can be mitigated by use of an impervious facing. It is claimed that with such composites, thermal conductivity remains practically unchanged for a long time in the largest part of the panel [9]. A recent variant is the use of liquid carbon dioxide to partially replace water. The carbon dioxide is metered and fed in the high pressure polyol stream at a level of some 3 pbw. Some process advantages have been described, but engineering modifications are necessary to handle and meter the gas and the technology has not so far been used industrially for sandwich panels due to unclear economics in comparison with all water blown technology. The use of pentane - either n-pentane or cyclopentane - is now widespread in the European industry for the production of metal faced rigid PU panels. Technology has been developed to deal with the problems imposed by flammability, including membrane pumps for dosing and static mixers for blending with the polyol stream. Strict precautions are taken to ensure there is no build-up of explosive vapours, and continuous monitoring ensures vapour levels are maintained below 10% of the lower explosion limit. Ignition sources must be eliminated. With these precautions, pentane can be used safely and the first equipment for production of pentane blown rigid PU was introduced in 1990. Pentane may be partially emulsified (and not solubilised) in the polyol stream depending on the amount used and its solubility in the polyol. Blends of n-pentane and methylene chloride have also been used and have some advantage since methylene chloride in non-flammable. However, concerns over environmental and toxicity issues have prevented any significant use as a blowing agent. Pentane has some advantages over other blowing agents - it has zero ODP, poor solubility and low permeability in the PU matrix, gives acceptable foam insulation values (though inferior to those of CFC foams) and low price. The obvious disadvantage is its flammability. In addition to the necessity of introducing new equipment to provide adequate operating safety there is an adverse contribution of the hydrocarbons to the foam combustion behaviour, and inferior fire performance results. This may be combated by the introduction of additional fire retardants or, as indicated previously, by use of

68

comparative costs Rigid PU foam Rigid Polyurethane Foams polyisocyanurate foam chemistry, but this results in a cost penalty. Thus, the economics of pentane blown foam versus other technologies depends on the fire classification requirements. Some data given by Bertucelli and co-workers are shown in Table 4.9. Cyclopentane and n-pentane (or n-i pentane blends) are sufficiently different in properties to give rise to significant differences in processing and foam properties. Where a homogeneous blend is preferable (discontinuous processes) cyclopentane is preferred, and this also provides optimum thermal properties. N-pentane has lower cost and gives a lower foam density, resulting in better foam economics. There is significant work still ongoing in the polyurethane industry to fully replace HCFC141b in developed countries. Whilst pentane and carbon dioxide technologies are already in place, neither provides the advantageous process and product characteristics given by CFC-11. For a few years now, HFC-245fa (structure CF3-CH2-CHF2) and HFC-365mfc (structure CF3-CH2-CF2-CH3) have been identified as the leading candidates. It has been reported by Zipfel and co-workers [12] that whilst neither of these substances entirely matches CFC-11, a blend of HFC-365mfc with either HFC-134a or HFC-245fa provides a non-flammable blowing agent with good thermal insulation performance and a convenient boiling point in the range 20-25 °C. Such a blend would provide a ‘drop-in’ replacement for HCFC-141b (and CFC-11), but is the most expensive option of those under examination. A further possibility for the future is an azeotropic blend of HFC365mfc with any of the three pentane isomers (n, i, or cyclo). Although these azeotropes are flammable, they do provide a more cost effective solution than use of HFC-365mfc alone, and results show good thermal insulation values for foams based on such systems. Some characteristics of these ‘third generation’ blowing agents are shown in Tables 4.10, 4.11 and 4.12. These recent developments, which have included foaming trials, have focused attention on the possibilities afforded by HFC-365mfc, and Solvay took a decision to develop

Table 4.9 Comparative costs of rigid PU foam Blowing agent

Comparative cost (%) of 1 m3 of foam to meet DIN EN 1363-2 [10]

DIN EN 1363-1 [11]

HCFC-141b

100

107

CO2 - water

111

114

n-Pentane

96

126

Data from [9]. Reproduced with permission of Rapra Technology

69

characteristics of HFC blowing agents

Handbook of Polymer Foams

Table 4.10 Main characteristics of HFC blowing agents Property

HFC-245fa

HFC-365mfc

Boiling point (°C)

15.3

40.2

Vapour pressure (MPa at 20 °C)

0.124

0.047

Vapour thermal conductivity at 25 °C (mW/m.K)

12.2

10.6

Flash point

none

none

Flame limits (% by volume)

none

3.5-9.0

GWP relative to CO2 = 1

820

840

GWP: Global warming potential Reproduced with permission from [12]. Copyright Rapra Technology, 1999

Table 4.11 Main characteristics of HFC-365mfc/HFC-245fa blends Blend ratio

50/50

95/5

93/7

24

37

20

Flash point

none

none

none

Vapour thermal conductivity at 25 °C (mW/m.K)

11.4

10.7

10.8

Boiling point (°C)

Reproduced with permission from [12]. Copyright Rapra Technology, 1999

Table 4.12 Main characteristics of HFC-365 mfc/pentane azeotropes Pentane isomer

cyclopentane

isopentane

n-pentane

HFC/pentane ratio

73/27

46/54

58/42

Boiling point (°C)

32

22.5

27

Flash point

yes

yes

yes

11.1

13

13

Vapour thermal conductivity at 25 °C (mW/m.K) (calculated)

Reproduced with permission from [12]. Copyright Rapra Technology, 1999

70

Rigid Polyurethane Foams production at pilot scale level with a view to making this substance available as an HCFC-141b substitute [12]. The scenario for blowing agents over the next few years is likely to be: The replacement of polycyclic polyisocyanates -141b in the USA by HFC-365mfc, or a blend thereof; The continued domination in Europe of pentane technology, as it meets Protocol requirements and the safety problems involved with production operations have been solved. In Asia, a mixture of technologies, with some countries (such as China and Korea) still relying on CFC-11. Money is being made available by the World Bank and the United Nations Development Program to subsidise the considerable capital costs involved in changing to flammable blowing agents. Whilst some manufacturers in the Pacific region have made the (relatively easy) change to HCFC-141b, most are now looking at pentane technology to comply with the Protocol and to achieve cost reductions which are important in view of the economic turmoil in Asia. In Japan, conversion to pentane technology is well advanced.

4.2.4 Other Additives

4.2.4.1 Catalysts Foam catalysts have a very substantial influence on process rheology, process latitude, dimensional stability and physical properties. They comprise: •

Tin compounds, e.g.,

Stannous 2-ethylhexanoate Di-butyl tin di-laurate

•

Tertiary amines, e.g.,

Polycat 5 (pentamethyldiethylene triamine) Polycat 8 (dimethylcyclohexylamine) Polycat 41 (tris-dimethylaminopropyl hexahydrotriazine)

•

Quaternary ammonium salts, e.g., Dabco TMR series

•

Potassium salts, e.g.,

Dabco K-15 (Potassium octoate) Polycat 46 (Potassium acetate)

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Handbook of Polymer Foams Tin catalysts mainly promote the reaction between isocyanate and hydroxyl groups, whilst tertiary amines promote both this and the water - isocyanate reaction. Potassium salts catalyse the trimerisation of isocyanates to form isocyanurates. A comprehensive list of catalysts is given in reference [13]. The main technical development in rigid PU foams has been the move away from CFC blowing agents, but there has been substantial formulation development to support this, and to mitigate property changes resulting from use of CFC alternatives. The main suppliers have produced new catalysts which minimise the problems of such formulation changes [14].

4.2.4.2 Surfactants By reducing the surface tension of the polyol, surfactants can perform several useful functions in foam manufacture. They can stabilise the initial liquid-air dispersion (controlling the number of cells); they can emulsify and stabilise a blend of incompatible reagents; and they can stabilise the rising foam. Appliance foam generally incorporates highly functional polyols and isocyanates which results in a relatively high liquid phase viscosity and good foam stability. The surfactant is used mainly to control cell size and to enable blend miscibility of incompatible reagents. Surfactants commonly used, e.g., Dabco DC5357, are copolymers of polydimethyl siloxane and polyalkylene oxide. In all rigid polyurethane foams the change to pentane blowing agents to replace CFC has required surfactant development to overcome the poor solubility of pentane in the polyol (resulting in high viscosities) and a generally higher foam density with a coarser cell structure. Recently developed surfactants, e.g., Nitroil’s PC STAB EP 26+, are claimed to overcome these problems, combining the properties of a foam stabiliser and pentane compatibiliser.

4.2.4.3 Cell Openers The normal open cell ratio in rigid polyurethane foams is 10-20% and for the main application of thermal insulation it is advantageous to retain the blowing agent in closed cells to maintain a low thermal conductivity. In contrast to flexible polyurethane foams, cell opening at the end of the reaction is not required to prevent shrinkage. However, where insulation performance is not the prime concern, cell openers are sometimes recommended to improve dimensional stability. Whereas cell opening of flexible foams can be achieved by mechanical crushing, with rigid foams this would result in permanent compressive deformation. Hence, a chemical method is necessary.

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Rigid Polyurethane Foams A cell opener for rigid foams, Dabco DC5000, has been described [15]. It is designed as a separate additive for use with traditional silicone-polyether surfactants to allow good formulation flexibility. The additive is incorporated in small amounts up to 0.25 parts per hundred (pph) polyol to promote cell window rupture immediately prior to the rapid viscosity build up as the polymeric crosslinked network is formed. It acts by replacing the foam-producing surface film with an alternative film with little or no surface elasticity which rapidly thins and breaks as the foam expands.

4.2.4.4 Blend Compatiblisers One of the major limitations of hydrocarbons as blowing agents is their poor solubility in polyol. Use of higher amounts is desirable, particularly in PIR applications, as it would allow an improvement in reaction kinetics by the reduction or elimination of water. Higher levels of hydrocarbon above the solubility limits can result in phase separation during and before the foam reaction leading to foam inhomogeneity. Goldschmidt, for example, have developed Ortegol 410 which helps to solubilise pentane in rigid foam polyols. Using technical cyclopentane (70% cyclopentane plus mainly n and i isomers) in a European appliance polyol system, clarity up to 11 pph pentane is achieved without additive; with 5.0 pph of Ortegol 410, clarity was maintained up to 15 pph; at 10 pph Ortegol 410, clarity was maintained up to 18 pph pentane [16].

4.2.4.5 Glass Fibre There are few examples of the use of non-reactive fillers in rigid polyurethane foams, but glass fibre is one that was originally developed by Celotex Corporation. Incorporation in PIR foams was found to give additional stability to the protective char formed in a fire situation, hence improving fire performance to meet the USA requirements for built up roofs. An additional benefit is a lower thermal expansion coefficient which leads to improved durability and helps to prevent cracking when hot bitumen is poured onto laminate panels for roofing.

4.3 Manufacturing Processes for Rigid Polyurethane Foam The original method for production of rigid PU foam was simply to pour the foam mix into a box mould. The product was subsequently cut to form boards, pipe shells and other complex shapes. This method is essentially that now used for ‘pour in place’ applications such as insulative linings for refrigerators and similar appliances. Increasing

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Handbook of Polymer Foams volumes required more economic production methods, and a continuous process for block foam was developed, followed later by continuous lamination technology for panel production. The latter allows efficient production of rigid PU board with different facing materials like bituminous paper, glass fibre reinforced bitumen sheets, multi layer papers or metal sheets. Currently, both discontinuous and continuous processes operate for production of laminated rigid PU panels, although the continuous process (introduced in the late 1960s) is dominant. In a typical continuous process (CONTIMAT), galvanised steel sheets (0.50.8 mm) are wound off large coils and fed through a profiling unit to provide edge and surface profiles. Lower and upper facings pass through simultaneously and are then preheated to the process temperature required. The reactive PU mix is dispensed onto the lower facing using a linear application method whereby the mix head oscillates across the entire plate width. Sophisticated control ensures uniform distribution giving expansion only in the vertical direction, resulting in uniform density through the sheet and optimum production output. An adjustable gap between upper and lower conveyors controls panel thickness. Conveyor lengths are typically 20 to 30 m. Panels are cut to length some 10 m after leaving the conveyor. Output rates of a double conveyor plant with a 30 m length pressure zone and 1.2 m width are typically 860 m2/h (50 mm thickness) and 390 m2/h (150 mm thickness) [17]. Output rate is directly dependent on conveyor length, being 30% lower for a length of 20 m. The discontinuous process comprises a series of steps linked to provide a manufacturing operation. These steps are: •

Preparation and positioning of metal sheets, either from a coil of metal or by use of stacked sheets.

•

Profiling the edge and surfaces of the sheets.

•

Feeding the sheets to a multiple daylight press characterised by a central pressing plate.

The pressing plate moves so as to close an upper mould in preparation for dispensing the PU foam mix whilst releasing the lower mould for removal of the finished panel and insertion of further facing sheets. Such presses may be configured, e.g., as 1+1, 2+2, 3+3 or 4+4. Optimum output of a discontinuous process is achieved when curing time and operating time are of equal duration and when the platen area of the presses is fully utilised. Typical output rate of a 4+4 daylight press, 6 m long, 1.2 m wide, maximum platen area utilised, is 150 -170 m2/h (50 mm thickness) and 40 -50 m2/h (150 mm thickness) [17].

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Rigid Polyurethane Foams A joint cooperation between Hennecke GmbH and Bayer AG has resulted in the VarioCast process which is claimed to not only reduce the cycle time for discontinuous panel foaming operations but also to improve cell structure, increase dimensional stability and reduce raw material costs by 5%. The equipment is modular, is said to give cycle time savings up to 30% and is compatible with various blowing agents including pentane. The continuous process has higher output rates and there is no technical limitation on panel length (in Europe, currently up to 18 m). Foam densities are generally lower than those of foam produced by the discontinuous process (40-45 kg/m3 versus 45-55 kg/m3). In the discontinuous process, panel length is limited by press dimensions, but there is greater flexibility in the use of facing materials (these may include chipboard, plywood, or light building board, in addition to the usual steel or aluminium sheet). Cell structure and uniformity is inferior in the discontinuous process since the distribution of reaction mixture is non uniform and expansion occurs sideways as well as vertically. Dimensional stability of the foam is adequate but inferior to that from the continuous process. There is, however, more scope for sheet profiling, including ends as well as sides, which is not feasible in the continuous system. It is also possible to spray foam in place for insulative or sealing applications. Formulations are required with high reactivity and low odour. Considerable development has been carried out on rigid polyurethane foam processes in recent years to overcome the problems imposed by the change from CFC blowing agents to hydrocarbons. The main problem is the flammable nature of hydrocarbon blowing agents which requires special precautions to enclose the process to ensure there is no chance of any vapours escaping, and removing any possible source of ignition. The necessary technology is now well developed and equipment capable of handling these substances is now available from all the main manufacturers.

4.4 Recycling Processes for Rigid Polyurethane Foam Whilst rigid PU foam is a stable product with a long lifetime, material for disposal can arise when, for example, appliances are scrapped or when underground insulated pipes are replaced. It naturally arises also from within a process from trimming and from out of specification foam during start-up and change-over. Companies are giving increasing attention to the problems of waste due to environmental concern and the cost of disposal. Many companies use glycolysis to process their relatively pure PU scrap whilst other wastes are dealt with by a mechanical process. Glycolysis turns the PU waste back into polyols for new rigid PU slabstock production and is a process which has been used for many years.

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Handbook of Polymer Foams In a recycling method developed by Puren Schaumstoffe [18], PU waste is mixed with scrap PET from, for example, soft drink bottles. This has the advantage of reducing methylene diphenylamine levels arising from MDI based PU which can reach undesirable levels if only PU waste is used. Contaminated waste, i.e., waste containing PU foam mixed with covering paper, mineral wool, glass wool, aluminium sheet, etc., is first separated and pressed into briquettes which are then granulated and transported to a mixing system. An MDI-based binder is added and the mixture pressed at 140-160 °C at a pressure of 1-3 MPa for about 3060 seconds to form boards which are subsequently cut to size. The product is said to have good mechanical properties, low heat conduction and is moisture resistant. Thicknesses range from 5 to 60 mm and densities from 250 to 850 kg/m3. Various construction applications are envisaged, including floor insulation and sandwich elements. The process was sufficiently successful for Puren to decide in 1997 to build a second plant to increase capacity. BASF have developed a ‘second generation chemistry’ using glycolysis to recover polyols from a variety of PU systems including rigid PU foam. Residual amine is claimed to be less than 1%. The polyols are said to be reusable in polyisocyanurate foam, insulation non-critical foam, structural foam, friable energy absorbing foam, low density void filling foam, elastomeric coatings, rigid non-cellular PU, and moulded semi-rigid foam.

4.5 Properties of Rigid Polyurethane Foams Some of the properties of rigid PU foam important in its applications are: •

High insulating capacity with low thickness

•

Lightweight, with high mechanical strength

•

Chemically and biologically resistant

•

Durable

•

Satisfies fire regulations for particular applications

•

Quickly and easily processed on site. Can be sprayed in place.

Physical properties of a continuously produced pentane blown PIR (insulating panel) are shown in Table 4.13.

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properties of Pentane blown PIR foam Rigid Polyurethane Foams

Table 4.13 Properties of pentane blown PIR foam Property

Value

Density (kg/m3)

32

Compressive strength (MPa) - perpendicular

0.22

Compressive strength (MPa) - in belt direction

0.12

Compressive strength (MPa) - across belt

0.13

Flexural strength (MPa)

0.42

Brittleness (%)

25

DIN EN 1363-1 [11] (cm)

12

Closed cell count (%)

98

Dimensional stability (70 °C/95% RH) (%)

+1.04

Thermal conductivity (mW/m.K) - initial

19.9

Thermal conductivity (mW/m.K) - after 3 months at 70 °C

23.9

Reproduced with permission from [19]. Copyright 1999, Carl Hanser Verlag

In many applications, insulation is the characteristic of most importance, and considerable work has been done to investigate the performance of rigid foams with the newer blowing agents, in particular their long-term performance. Predicting long-term performance from short-term accelerated ageing tests has also received recent attention. Some published data are shown in Table 4.14. Analysis of gas in the cells showed the loss of blowing agent and its replacement by air associated with the deterioration in insulation performance. Thicker foams show a slower loss, as expected. The loss at 70 °C after 39 weeks, whilst substantial, was still less than the value after 5 years at 23 °C. Ball has described a method for predicting long-term (25 years) ageing values based on measuring the increase in thermal conductivity after 25 weeks at 70 °C [7]. It is claimed the method is satisfactory irrespective of rate of ageing and the presence of facings. This work indicates that HCFC-141b and n-pentane blowing agents are similar to CFC-11 and CFC-12 in that a stable thermal conductivity value is reached in less than 25 years,

77

thermal conductivity of Rigid PU foam boards Handbook of Polymer Foams

Table 4.14 Thermal conductivity of rigid PU foam boards after ageing Blowing agent

Product thickness

Thermal conductivity at 10 °C (W/m.K) after ageing at 23 °C or 70 °C 6 weeks

2 years

5 years

at 23 °C

25 weeks

39 weeks

at 70 °C

Pentane

40 mm

0.0237

0.0267

0.0272

0.0266

0.0266

Pentane

80 mm

0.0230

0.0252

0.0263

0.0255

0.0257

HCFC-141b

50 mm

0.0203

0.0243

0.0252

0.0229

0.0231

HCFC-141b

60 mm

0.0192

0.0247

0.0257

0.0229

0.0231

Data used [20] with permission from Rapra Technology

and this value is likely to be maintained well beyond 50 years. the predictive formulae given by Ball for 25 year values (Æ25) are : CFC blown

Æ25 = (Æ1 - 6.5) mW / m.K

Pentane blown

Æ25 = (Æ1 - 5.4) mW / m.K

HCFC-141b blown

Æ25 = (Æ1 - 6.7) mW / m.K

where Æ1 is the initial value (measured after less than two days ageing for thin samples).

4.6 Applications The two major applications for rigid PU foam are in construction and in appliances such as household refrigerator liners.

4.6.1 Applications in Construction In the construction industry, rigid PU foam is used both as insulating boards and panels, and as an in situ material for sprayed-in-place insulation and as a one component bonding foam. Boards are used in pitched and flat roofs, in suspended ceilings, in floor slabs with or without underfloor heating, in walls for external, internal or cavity insulation (with rendering if required) and in a wide range of buildings from multi-storey car parks to

78

Rigid Polyurethane Foams farms. Load bearing rigid sandwich panels formed by combining the rigid PU foam with rigid wood or metal sheet facings are used for insulating roof and wall cladding. In thicknesses greater than 100 mm, PU sandwich panels are used for cold storage facilities and for refrigerated storage and transport. Roofs and industrial components such as tanks or pipes may be insulated by using polyurethane spray to produce an insitu foam. Doors and window frames can be fixed and joints sealed using one component bonding foams. In roof, wall, floor and ceiling applications, rigid PU foam has much in common with polystyrene foam which is used for similar insulation applications. The insulation market, however, is dominated by mineral or glass fibre wool which, whilst less technically attractive, costs less as a material and more easily meets fire resistance requirements. Some data given by Kaufung, Bayer AG, on the market share of various materials for insulation applications [21] are shown in Table 4.15.

Table 4.15 Insulation material - Western Europe market (1995) Volume (106 m3)

Share (%)

Mineral wool/glass fibre wool

52.6

60.5

Phenolic foam

0.9

1.0

Rigid PU foam

6.4

7.4

Expanded polystyrene foam

24.1

27.7

Extruded polystyrene foam

3.0

3.4

Total

87

100

Material

Data from [21]. Copyright SPI, 1997

The demand in construction has been growing and this growth is expected to continue at a rate greater than that of the construction industry itself, in line with increasing demands for the improved comfort provided by insulation, and the energy savings available when insulation is used. Environmental as well as economic pressures are driving this. Weigand [22] has examined the environmental benefits of rigid PU insulation by carrying out an ‘eco-balance’ calculation for pitched roof insulation and floor insulation. Savings in energy used for heating during use far outweigh the environmental ‘costs’ of production and eventual recovery. The positive environmental effect of using an appropriate level of insulation has also been shown by Ashford [23].

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Handbook of Polymer Foams

4.6.2 Applications in the Appliance Industry The use of rigid PU foam for refrigerator insulation is the other main application for the material. In 1997, the consumption of PU worldwide in refrigerators was 464 tonnes, some 40% of the amount used in construction. A breakdown given by Bayer [24] is shown in Table 4.16.

Table 4.16 Shipments and PU consumption of refrigerators and freezers in main world regions (1997) Region

Shipment (millions of units)

PU consumption (k tonnes)

Share (%)

Europe

18.9

155

33

USA

11.5

98

21

Japan

4.8

28

6

South America and Mexico

10.7

54

12

China

13.2

78

17

Far East and India

12.0

51

11

World Total

71.1

464

100

Data from [24]

The larger markets in Europe, North America and Japan are well developed [24] and although there is a trend towards larger and more sophisticated appliance models to meet customer demand, the growth of the market in these countries is small or nil. Worldwide, these countries account for 60% of the total. The market in less developed countries however, is not yet saturated and still shows significant growth. Between 1993 and 1995, European refrigerator manufacturers moved away from CFC blowing agents, mostly to cyclopentane, although HCFC-141b, HCFC-141b/HCFC-22 blend, and hydrocarbon blends have also been used. In the USA, as with rigid PU foams for construction, HCFC-141b was the preferred blowing agent. In Europe, whilst initially the resulting poorer performance of the non-CFC foams was accepted (higher density and hence higher shot weight, lower insulation value), subsequent development work has aimed to meet the overall properties of the original CFC foams.

80

Rigid Polyurethane Foams Legislation has been one of the driving forces, involving not only the Montreal Protocol, but also regulations on the reduction of appliance energy consumption introduced by the EU, and the voluntary Eco-Label of the EU. The key requirements for the Eco-Label are: •

Sufficient insulation for the refrigerator to be rated A or B according to the 94/2/EC Energy Efficiency Index regulation. For an A rating, energy consumption must be below 55% of the standard value defined by the EU directive, whilst for a B rating energy consumption must be between 55 and 75% of this value.

•

Foam blowing agent must have an ODP of zero and a GWP of no more than 15 CO2 equivalents.

The blowing agent requirements rule out both HCFC and HFC, limiting candidates to hydrocarbons or carbon dioxide. The appliance energy consumption regulations and the Eco-Label arrangements came into force in 1999 (96/57/EC). In the USA, HCFC-141b use is allowed by the US Environmental Protection Agency (EPA) as a transition measure until energy efficient, zero ODP, replacements become available. Formulation development has enabled such foams essentially to match the insulation performance of the CFC foams they replace, enabling refrigerator and freezer manufacturers to remain in compliance with US Department of Energy (DOE) consumption regulations. The deadline for replacing HCFC-141b in the USA was set by the US EPA as 1st January 2003. Additionally, the US DOE issued an energy consumption regulation for refrigerators and freezers sold from July 1st 2001. This calls for up to 30% reduction in energy consumption from the levels permitted under the 1993 regulations. The US DOE has estimated that this will result in significant savings in primary energy consumption and a consequent reduction in carbon dioxide emissions amounting to some 465 million tonnes over 30 years. In Japan, there are no regulations for energy consumption for appliances although the importance of energy saving is well recognised in the industry and strongly supported by the government. The saturation of the market in Japan is illustrated by the figure of 1.2 refrigerators, on average, for each Japanese household. The growth regions of South America and South-East Asia are less advanced in both CFC replacement and energy reduction regulations, but the technologies now available are being put in place and Brazil, for instance, has a regulation to eliminate CFC in household refrigerators which came into force on January lst 2001.

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Handbook of Polymer Foams China and India are areas where the relatively low market saturation offers good market growth potential. The change over from CFC is happening - albeit slowly as use of CFC is still permitted under the Montreal Protocol - the trend being initially to the ‘drop-in’ HCFC systems with hydrocarbons being used in some joint venture foam plants involving European companies. Development trends in the industrial countries are targeted on cost reduction whilst continuing to meet environmental legislation for blowing agents and energy consumption. In Europe and Japan, this focuses on improving hydrocarbon blown foams either by improving existing pentane-based systems or by development of systems based on cyclopentane blends with various butane and/or pentane isomers. The latest systems are claimed to match the properties of CFC-11 foams [24], having lower cabinet fill weights, shorter demould times and improved insulation factors. This has been achieved by tailoring the basic polyol and isocyanate materials to the requirements of the new blowing agents formulated for use with CFC. In the USA, HFC-245fa has been identified as the most suitable replacement for HFC141b in appliances. Work done by ARC and Bayer has shown that appliance insulation made using this blowing agent has an energy consumption matching that of HCFC-141b systems. Vacuum insulation panels are also receiving attention as this technology is able to provide improved k factors (a measure of insulation). However, manufacturing and installation costs are high and only a few manufacturers in USA, Europe and Japan offer appliances with such insulation. This may change with further tightening of energy consumption regulations. It can be expected that development will continue so as to satisfy customer and legislative requirements, and it will be some time before formulations world wide focus on one or a few systems recognised as economically and environmentally optimal.

References 1.

C.J. Benning, Plastic Foams: the Physics and Chemistry of Product Performance and Process Technology, Wiley-Interscience, New York, NY, USA, 1969.

2.

Polyurethane Technology, Ed., P.F. Bruins, Interscience, New York, NY, USA, 1969.

3.

D. Reed, Urethanes Technology, 1999, 16, 1, 32.

4.

R. Brooks, Urethanes Technology, 1999, 16, 1, 34.

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Rigid Polyurethane Foams 5.

Dow Polyurethanes - Flexible Foams, 2nd Edition, Ed., R. Herrington, Dow Chemical Company, Midland, MI, USA, 1997.

6.

UNEP web site, www.unepie.org

7.

G.W. Ball, A. Simpson and H. Fleming, Cellular Polymers, 1997, 16, 2, 110.

8.

A. Cunningham, I.D. Rosbotham, and A.K. Thomas in Proceedings of the SPI 32nd Annual Conference, Polyurethanes ‘89, San Francisco, CA, USA, 1989, p.522.

9.

L. Bertucelli, M. Vreys, F. Pignagnoli, A. Ottens and P. Keller, Proceedings of Utech Asia ‘99, Suntec City, Singapore, 1999, Building and Construction, Paper No.6.

10. DIN EN 1363-2, Fire Resistance Tests - Part 2: Alternative And Additional Procedures, 1977. 11. DIN EN 1363-3, Fire Resistance Tests - Part 1: General Requirements, 1999. 12. L. Zipfel, P. Dournal and W. Kruecke in Proceedings of Utech Asia ‘99, Suntec City, Singapore, 1999, Building and Construction, Paper No.7. 13. K. Ashida and K. Iwasaki in Handbook of Plastic Foams: Types, Properties, Manufacturers and Applications, Ed., A.H. Landrock, Noyes Publications, Park Ridge, PA, USA, 1995, Chapter 2, p.30-38. 14. D.E. Eaves, Polymer Foams, Trends in Use and Technology, Rapra Technology, Shawbury, Shrewsbury, UK, 2001. 15. K.F. Mansfield, J.W. Miller and W. Wong, Proceedings of Utech Asia ‘99, Suntec City, Singapore, 1999, Appliance, Paper No.5. 16. G. Burkhart and M. Klincke in Proceedings of Utech Asia ‘97, Suntec City, Singapore, 1997, Paper No.27. 17. D. Chua, Proceedings of Utech Asia ‘99, Suntec City, Singapore, Building and Construction, Paper No.4. 18. M. Knoedgen, Urethanes Technology, 1998, 15, 6, 34. 19. H. Schmidt and E. Calgua, Kunstoffe Plast Europe, 1999, 89, 2, 36. 20. W. Albrecht and H. Zehendner, Cellular Polymers, 1997, 16, 1, 35.

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Handbook of Polymer Foams 21. R. Kaufung in Proceedings of the SPI Polyurethanes ’97 World Congress, 1997, Amsterdam, The Netherlands, p.2. 22. E. Weigand, Insulation Journal, 1999, March/April, 11. 23. P. Ashford in Proceedings of SPI Polyurethanes World Congress ‘97, Amsterdam, The Netherlands, p.612. 24. Urethanes Technology, 1997, 14, 4, 42.

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Flexible Polyurethane Foam

5

Flexible Polyurethane Foam Tyler Housel

5.1 Introduction Polymer foams that are made primarily from polyol, isocyanate and water are known as flexible polyurethane foams (FPF). In this chapter, we will discuss both slabstock and moulded FPF with a density of less than about 100 kg/m3. The goal is to provide a general introduction to the chemistry, manufacturing, testing, and commercial aspects of FPF. Rigid polyurethane foams are covered elsewhere in this book. Readers are encouraged to use the citations to obtain more detailed information in areas of particular interest. The earliest work on FPF was published in Germany in the 1940s [1]. A viable commercial process was quickly developed, and within a decade there were manufacturing sites in both Europe and North America. By the year 2000, more than three million tonnes of FPF were produced around the globe [2]. The industry continues to grow and expand into new markets, and foam grades are constantly being developed and improved to meet the ever-increasing demands of the end users.

5.2 Chemistry The solid polymer that makes up the struts and membranes of an FPF is called polyurethane by convention, but is really much more than that. It is a polymer with ester and/or ether bonds from the polyol portion, and urethane and urea bonds from the reactions of isocyanate. Polyurethane foam starts out as a liquid mixture of three reactants with a number of additives. Within a few minutes it becomes a solid network of polymer that can have 99% of its volume filled by air. Despite the complexity of the system, the chemistry used to describe it is actually quite simple. Isocyanates are involved in all of the chemistry that occurs during polyurethane foam manufacturing. The two major reactions are known as the ‘gel’ and ‘blow’ reactions. The gel reaction takes place between isocyanate and hydroxyl groups to yield a urethane linkage that builds molecular weight and gels the polymer. The blow reaction occurs in

85

Handbook of Polymer Foams two steps and consumes one water molecule and two isocyanate groups. Water first reacts with isocyanate to produce a carbamic acid intermediate that quickly decomposes to give an amine and carbon dioxide. Carbon dioxide is the blowing gas which fills the cells. The amine reacts with a second isocyanate to form a urea linkage. Both the gel and blow reactions build molecular weight and therefore cause the polymer to gel, but the blowing gas is only produced from the water reaction. These reactions are shown in Figures 5.1 and 5.2.

Figure 5.1 Gel reaction

a)

b)

c)

Figure 5.2 a) Blow reaction - first stage; b) blow reaction - decomposition step; c) blow reaction - urea formation

86

Flexible Polyurethane Foam For completeness, it must be mentioned that isocyanate will react with any active hydrogen compound, including the urethane or urea group formed earlier. It will also react with itself to form dimers and trimers [3]. These crosslinking reactions occur to varying degrees in all foam systems but are generally slower than the gel and blow reactions. Rather than consider each individually, the isocyanate index allows the formulator to account for the minor reactions as a group. The isocyanate index is the percentage of isocyanate groups relative to the total active hydrogen groups from water and hydroxyl in the formula. Remember, each water molecule reacts with two isocyanates so the following formula must be used: Isocyanate index =

equivalents isocyanate x 100 _ equivalents hydroxyl + 2 x equivalents water

An index of 100 means that there are exactly enough isocyanate groups to completely consume the water and hydroxyl groups in the formula. This means that a foam with an index of 110 was made with 10% extra isocyanate. There is no active isocyanate in a cured block of foam [4], so the extra isocyanate must be consumed by the various side reactions. Increasing the isocyanate index makes the foam stiffer, presumably because the extra isocyanate reactions increase crosslinking in the polymer matrix. Most FPF has an index between 90 and 110 although there are special grades outside that range. While the chemistry is easy to describe, it becomes more complicated when considering the number of possible permutations. In most foam systems the reactive groups include three types of active hydrogen (primary hydroxyl, secondary hydroxyl and water) and two types of isocyanate (hindered and unhindered). Even ignoring the minor reactions, this gives six possible reactions that can occur, and each has a different sensitivity to catalyst and temperature. Beyond that, the ingredients are not mutually soluble so some groups may be more available to the isocyanate than others [5]. All of this also changes with time as polymer is formed [6]. A skilled formulator knows how different variables will affect the final product, but inevitably, some trial and error is used to optimise the formula.

5.3 Starting Materials There are five ingredients that are common to all flexible polyurethane foams: isocyanate, polyol, water, catalyst, and surfactant. However, few formulations are actually that simple. There are a number of other additives that can be used to improve processing, performance, appearance, or stability of the foam. By choosing the raw materials and process conditions

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Handbook of Polymer Foams carefully, the manufacturer controls the physical properties, mechanical properties and appearance of the foam [7].

5.3.1 Isocyanate As previously mentioned, all of the reactive chemistry that occurs during the foam manufacturing process involves isocyanate groups. Several isocyanates are commercially used in the polyurethane industry, but the majority of slabstock foams (those made in long, continuous blocks) are made with toluene diisocyanate (TDI) (see Figure 5.3). This is a low viscosity aromatic isocyanate that has hindered and unhindered isocyanate groups that react at different rates. The most common grade is known as TDI-80, which has an 80/20 mole ratio of the 2,4 and 2,6 isomers of TDI. TDI-65 is also popular in Europe, but infrequently used in the rest of the world. With its higher content of the 2,6 isomer, TDI-65 gives a stiffer foam with a more open cell structure.

Figure 5.3 Isomers of TDI Isocyanates derived from methylene diphenyl diisocyanate (MDI) are used in both moulded (those made in a closed mould) and slabstock [8] foams. These are also preferred for low resilience viscoelastic foams [9]. MDI types cover a number of related chemical structures. Pure MDI is not often used in flexible foams because it is a solid that must be used in its molten form. Foam grade liquid MDI derivatives can be unpurified intermediates from the pure MDI process or can be chemically modified to impart desired properties. Besides TDI and MDI types, aliphatic isocyanates are used in a few highly specialised formulas where light stability is an absolute necessity.

5.3.2 Polyol In the polyurethane industry, the term polyol describes any oligomer with two or more active hydroxyl groups that react with isocyanates during the foaming process. Flexible

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Flexible Polyurethane Foam foams are normally made with polyols that have molecular weights from several hundred to several thousand and hydroxyl functionalities from two to three. The polyol gives flexibility to the polymer, so lower molecular weight and/or higher functionality polyols will produce a stiffer foam. In the foam industry, three major types of polyol are prevalent, and each has its own advantages as shown in Table 5.1. The greater use is from polyether polyols. Most of these start with a trifunctional initiator such as glycerine. Ethylene and propylene oxides then react with the initiator to give polyether chains terminated with hydroxyl groups. Moulded foams often work better with more reactive polyols, so ether polyols for moulded foams are terminated with ethylene oxide (EO) to give more reactive primary hydroxyl groups on the ends. These ‘EO capped’ polyethers are less commonly used in slabstock foams. Polyester polyols are made from diacids and diols that react to give polymer chains with repeating ester bonds. Essentially all polyester slabstock polyols are made from diethylene glycol (DEG) and adipic acid. Glycerine or trimethyl propane (TMP) are added to the charge to increase the hydroxyl functionality to the 2.5 to 3.0 range. Flexible moulded polyester foams are made from adipates of short chain glycols such as DEG, ethylene glycol and butanediol. If higher functionality is required, small amounts of TMP are reacted into the polyol. Moulded foams are more prone to shrinkage, so these polyester polyols rarely have functionalities above 2.5. Copolymer polyols are normally derived from polyether polyols. They are sometimes called polymer, graft or filled polyols [10] (note: some sources use the term ‘filled’ to refer to polyols filled with mineral fillers only, excluding the copolymer polyols [11]). These are primarily used to increase strength, hardness and resilience. One type of polymer polyol uses a free radical initiator to polymerise unsaturated monomers such as styrene and acrylonitrile (SAN polyols) that are added to a polyether polyol. The free radical

Table 5.1 Foam polyol property summary Property

Polyether polyol

Polyester polyol

Copolymer polyol

Viscosity

low

high

medium

Hydrolytic stability

excellent

fair

excellent

Oil/solvent stability

fair

excellent

good

Strength

fair

excellent

very good

good

fair

excellent

Resilience

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Handbook of Polymer Foams will occasionally react with the polyether chain, grafting the polymer to the polyol. The other main type of polymer polyols are the PHD polyols. The name comes from ‘PolyHarnstoff Dispersion’ which is German for polyurea. These are made by adding amines and isocyanates to a polyol. The isocyanate will react much more quickly with the amine, so they are essentially dispersions of polyurea in polyol. Either of these techniques gives the polyol a milky appearance and causes a significant increase in viscosity. Most copolymer polyols are based on low viscosity polyethers despite the excellent properties of copolymer polyester polyols [12].

5.3.3 Water Water reacts with isocyanate to produce the carbon dioxide (CO2) that eventually fills the cells and foams the polymer. Flexible foams normally use between 1 and 6 parts of water, with higher water levels leading to a lower density foam. The water-isocyanate reaction is extremely exothermic, so low density water blown foams are prone to thermal discoloration in the centre of the bun where the temperature is hottest for the longest time. In extreme cases, it becomes dangerous to produce very low density water blown foams as they can self ignite. For this reason, most foams below 15 kg/m3 are blown with a combination of water and a non-reactive blowing agent. Variable pressure foaming (VPF) is another way to produce low density foams with moderate water levels.

5.3.4 Surfactant Surfactants (or stabilisers) are essential, multifunctional additives that make polyurethane foam possible. These are often a combination of ingredients that serve three purposes during the foaming process. The first role of the surfactant is to lower the surface tension of the liquid in the mix head so that bubble nuclei can form from the available energy [13]. These nuclei determine the size and structure of the cells in the foam. The second role is to emulsify the water, polyol, isocyanate and additives. If the dissimilar liquid components do not emulsify properly, large scale deformities are produced in the final foam. As the foam rises, cell membranes form between gas bubbles, and the surfactant is also responsible for stabilising these membranes. Yet the surfactant must also allow the thinned membranes to rupture during the cell opening phase. This balance of stability and instability is the key to developing a successful foam formulation. There are many types of foam surfactants and each is usually specific to the type of formulation being poured. For example, surfactants designed for polyester foams will not work in polyether systems and vice versa. Within each category, different surfactants are available depending on the density, porosity, cell structure and other properties desired.

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Flexible Polyurethane Foam Most have a combination of emulsifying, stabilising and destabilising ingredients that are engineered to work together. While it is possible to meter the individual components to the mix head as separate streams, this is rarely done in practice because the surfactant supplier blends them in the appropriate ratio.

5.3.5 Catalyst Most FPF systems reach full rise in less than two minutes, and parts or slabs are handled only a few minutes after that. Not only does a fast reaction improve manufacturing efficiency, it also minimises the critical time between cell opening and strength development that determines whether the product is good. Foams with longer reaction times are more likely to have a poor cell structure and problems with shrinking, settling or collapse. Isocyanates react sluggishly with water and alcohols at room temperature, so catalysts are added to accelerate and control the rise and cure of moulded and slabstock foams. There are several competing chemical reactions that take place during the foaming process, and every catalyst affects each reaction differently. Nothing is completely selective, but gel catalysts primarily speed the reaction between polyol and isocyanate, and blow catalysts are more selective to the reaction between isocyanate and water that generates the blowing gas. A balanced catalyst has similar activity for both reactions. It is important to remember that the type of polyol strongly affects the catalyst requirement. Polyether polyols with a high percentage of secondary hydroxyl groups require much stronger gel catalysts than EO capped or polyester polyols. Catalysts also show varying degrees of activity toward the different side reactions such as allophanate, biuret, and isocyanurate. In rigid foams, these can be critical, but in flexible systems the side reactions are normally ignored. Lastly, temperature affects some catalysts more strongly than others. Delayed catalysts are commonly used in moulded foams to give fast demould times without shortening the time to fill the mould. These become active only after the reaction exotherm increases the temperature. Many organometallic compounds are effective gel catalysts, the most common of which are stannous octoate and dibutyl tin dilaurate. Recent toxicity concerns have reduced organotin usage in Europe. Trialkyl amines such as triethylene diamine are also used as gel catalysts although they are less selective than the organotins. Delayed gel catalysts are usually trialkyl amine salts that will unblock at higher temperatures [14]. Blow catalysts are tertiary amines that have two carbons separating the catalytic amine from another tertiary amine or an ether group. This structure is believed to form a complex with water molecules that enhances their reactivity with isocyanate [15]. Bis (N, N,

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Handbook of Polymer Foams dimethylaminoethyl) ether and the structurally similar pentamethyl diethylene triamine are widely used as strong blow catalysts. Morpholine and piperazine structures are generally weaker and more balanced. Successful foaming requires a balance between the gel and blow reactions. It is not possible to make a universal balanced catalyst, but a number of catalysts are available that are balanced for a particular system. These can be a single compound or a blend of gel and blow components. In practice, most manufacturers use a catalyst system that somewhat favours the blow reaction and then add minor amounts of a strong gel catalyst to allow for daily adjustments. This is also useful for manufacturers that make several different grades of foam because they only need a few catalysts to make many types of foam. Reactive catalysts are frequently used to avoid volatile amine emissions. These have reactive hydroxyl groups that enable them to become part of the chemical network. Dimethyl ethanolamine (DMEA) is an inexpensive gel catalyst, while methyl hydroxyethyl piperazine is a weak blowing catalyst. Triethanolamine is commonly used as a catalyst and crosslinker in moulded foams.

5.3.6 Colorants Natural polyurethane foam is an off-white colour that yellows over time with exposure to air and light. Colorants are added to improve the aesthetic appeal, hide the natural yellowing and give an easy way to tell different foam grades apart. Both pigment and dye-based colorants are available for any type of polyurethane foam. These are usually supplied as a concentrated liquid or paste in a low molecular weight polyol carrier. Colorants with polyester carriers can be used for any type of foam, but polyether carriers can cause pinholes in polyester foams. Carbon black is widely used to make different gray and charcoal grades of foam. Most colours can be made by mixing different ratios from a palette of 3-4 primary colours. White or blue-white pigments are sometimes used to make white foam appear brighter and resist yellowing with age.

5.3.7 Antioxidants Antioxidants are added to protect the colour of a slabstock foam during processing. The foaming reactions are extremely exothermic, and the centre of a large block of foam can reach 170 °C [16] and remain hot for many hours. Under these conditions, thermal oxidation can yellow or ‘scorch’ the foam in the centre. Antioxidants can reduce scorch, but if the block remains too hot for too long, any antioxidant will be overwhelmed and scorch will still be visible. Recent work in exotherm management shows that additives

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Flexible Polyurethane Foam can significantly reduce scorch by lowering core temperature by 25 °C [17]. At the very least, it is important to allow space between curing foam blocks so they can cool as quickly as possible.

5.3.8 Light Stabilisers Most antioxidants provide only a slight benefit in keeping the foam from yellowing over time. Light stabilisers are more effective in reducing long-term yellowing, but again they will slow the process but not prevent it. The only sure ways to make non-discolouring foam is to either make it black or to use an aliphatic isocyanate.

5.3.9 Flame Retardants Some of the largest markets for flexible polyurethane foam are for furniture, bedding and automotive parts. In these applications, the foam is required to meet strict flammability specifications that limit the speed and extent of combustion. To meet these requirements, the manufacturer adds flame retardants that inhibit the combustion process [18]. There are several types of flame retardants, and each has its own benefits and drawbacks. Further, all tend to increase density and reduce the durability and mechanical properties of the foam. The most widely used materials are chlorinated phosphate esters such as tris dichloro isopropyl phosphate (TDCP) and tris monochloro isopropyl phosphate (TCPP). These are very effective in many grades of foam, but their health effects have been questioned, and they contribute to foam discolouration (scorching) [19]. Pentabromo diphenyl oxide gives much less scorch and is widely used in North America but is facing a ban in some European countries because its structural similarity to polychlorinated biphenyls may indicate long-term environmental issues [20]. Other brominated materials are now being introduced that should have a more favourable profile [21]. Powdered melamine is synergistic with chlorinated phosphate esters and brominated materials and is frequently used to improve the performance of flame retarded foams, particularly to meet Cal TB 133 [22] and BS 5852, CRIB 5 Test [23] requirements [24]. Melamine has a favourable toxicological and environmental profile, but doesn’t work universally [25]. It requires high amounts of additive that can make the foam difficult to process [26] and can negatively affect mechanical properties [27]. There are other flame retardant chemicals that are used in minor amounts in FPF. These include aluminium trihydrate, ammonium polyphosphate, antimony trioxide, barium sulfate, graphite and tribromo neopentyl glycol.

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5.3.10 Adhesion Promoters Automotive parts are often made of vinyl with a moulded foam filling. Polyester foams have excellent adhesion to vinyl but insufficient flow properties for this application. Polyether foams have good flow but poor adhesion. To overcome the inadequacies of each system, the moulder can use a polyol blend that contains small quantities of ester polyol as an adhesion promoter to a polyether moulded foam. In this way, both flow and bond strength are acceptable. In a similar manner, ester-ether hybrids are sometimes used for slabstock foams designed for textile lamination. These foams are bonded to textiles to give the strength and aesthetic appeal of the fabric with a padded feel. The lamination process involves passing a thin sheet of foam over a flame and quickly pressing the melted foam against the fabric. Polyester foams give a much better bond, but cannot be used for some applications because of their long-term hydrolytic degradation. It has been found that the lamination properties of polyether foams can be significantly improved by adding small quantities of polyester polyols to the formula. It has been proposed that small quantities of acid in the foam favourably affect flame bonding [28].

5.3.11 Other Additives Polyurethane foam is naturally non-conductive, and it can build up static electricity in certain environments. Anti-static additives help static dissipation for foams that are used in the aircraft of electronics industries. Biocides can be added to protect foam from bacterial or fungal attack if the foam is used where microbial growth is likely. Cell modifiers can be used to make the foam coarser or to give the ‘double cell’ appearance of a natural sponge. Plasticisers can be used to make softer foam. Clickability additives allow the fabricator to cut the foam with a razor edge die without pinching the edges. Mineral fillers increase density which can improve sound absorption, but may reduce durability. Melamine and mineral flame retardant additives act as fillers, while phosphate esters and brominated compounds can plasticise the foam.

5.4 The Foaming Process FPF starts as a mixture of liquid chemicals and within a few minutes becomes a matrix of solid polymer with a uniform, flexible, cellular structure. There are five discrete stages to the foaming process: raw material conditioning, mixing, growth, cell opening and cure. Distinctly different chemical and physical changes occur in each stage, and all affect the final product.

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5.4.1 Raw Material Conditioning A polyurethane foam manufacturer controls many of the chemical and mechanical inputs to the process, but once the liquid leaves the mix head, chemistry and physics determine the fate of the foaming mass. The only way to produce consistent foam is to start with consistent raw materials. The ingredients arriving at the mix head must be at the right temperature and should also have a certain amount of dissolved gas. If these properties vary outside the normal range, defects may result. The main reason for controlling temperature is that some chemical reactions are more thermosensitive than others. The temperature affects the overall rate of reaction, and even the relative rates. This can lead to an imbalance that changes the properties of the final product. Viscosity also changes with temperature, and this affects flowability and mixing. Gas molecules dissolve in polyol and isocyanate as it comes in contact with air during transportation and storage. Dissolved gas is essential for cell nucleation in the mix head. If raw materials are too fresh (not enough dissolved gas) insufficient nucleation can result in a cell structure that is too coarse; conversely, old raw materials can make the cells too fine. Most manufacturers age the major ingredients before use, and some even monitor dissolved gas in the raw materials. Manufacturers can use vacuum or pressurised gas [29] to reduce or increase the dissolved gas in the raw materials, particularly TDI [30]. It is important to recognise that dissolved gas in the liquid ingredients is not the same as bubbles which are undissolved gas. Bubbles can form when pumping viscous raw materials such as polyester polyols, or they can come from air in the raw material lines. This can occur when a filter is changed for example. If air bubbles get into the mix head, pinholes and other defects may result. Occasionally air is intentionally added at the mix head to provide additional nucleation sites. This is most common when an irregular cell structure can be beneficial (high resilience foams), or when a large number of nucleating sites must be present as the mixture leaves the chamber (liquid carbon dioxide blown systems) [31].

5.4.2 Mixing The mix head is where the isocyanate is mixed with water, polyol and other ingredients. Often, some raw material streams are blended prior to entering the mixing chamber, but once the isocyanate, polyol and water are together, the chemistry begins. The mix head serves two critical functions: mixing the ingredients and forming the bubble nuclei that will become cells in the foam.

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Handbook of Polymer Foams Since the residence time in the mixing chamber is normally much less than one second, it must be highly efficient to ensure that the liquid leaving is completely homogeneous. After the mixing stops, dissimilar components will naturally begin to phase separate. However, if there are unmixed regions at the outset, they will lead to large scale defects in the foam. Cell nucleation occurs when dissolved air cavitates into tiny bubble seeds. Tremendous amounts of energy are required to create surface in a bulk liquid and vapourise the dissolved gas. The mix head is the only place where sufficient energy is available for nucleation, so the action in the mix head directly affects the number of cells in the final foam. The manufacturer controls cell size by encouraging or inhibiting nucleation. These examples will describe how to change the manufacturing variables to produce a finer foam. It is possible to make a coarser foam by making the opposite change. The most reliable way to make a finer celled foam is to decrease the pressure in the mix head itself. Cells only nucleate in the mix head, and a low chamber pressure will make it easier to form gas bubbles from dissolved gas. Chamber pressure is normally controlled by the raw material pump settings and the diameter of the outlet pipe. Another way to make finer cells is to use raw materials that have a higher concentration of dissolved gas. These have more gas available to form nuclei, and will result in a finer foam. Surfactants also help create fine celled foam because the energy required to nucleate a bubble is directly proportional to the surface tension of the liquid [32]. Another way the manufacturer can nucleate more cells is to increase the energy input. This can be done by mixing at a higher speed [33] or by increasing the pressure of the line feeding raw materials to the mix head. Both of these changes will increase the pressure drop in the mix head, making it easier to cavitate cell nuclei from the available dissolved gas.

5.4.3 Growth Cell growth begins shortly after the liquid leaves the mix head as gas diffuses out of the liquid and begins to fill the bubble nuclei. The ambient external pressure is much lower than the mix head pressure so the bubbles can grow easily. In the early stages, bubbles are only a small part of the overall volume of the mixture (exception: liquid CO2 foams). They are far apart, and remain spherical. As they grow, the cells come in contact and must distort as they pack in three dimensions. Eventually gas will fill up to 99% of the total volume. Regions where two bubbles converge are called walls or membranes, while the intersections of three or more bubbles are called struts. Foam rises because the gases that are generated are trapped inside closed cells. The pressure inside the cells equals the external pressure plus the overpressure needed to maintain and grow the cells against the forces of gravity, friction, surface tension, viscosity 96

Flexible Polyurethane Foam and elasticity. The overall pressure in the cells drops significantly as the liquid leaves the pressurised mix head and will remain slightly above ambient during the free rise portion of the reaction. In a moulded foam, the pressure rises again as the mould is filled and the foam can no longer expand. In a free rise slabstock foam, the pressure in the cells remains slightly above atmospheric until the cell membranes rupture and gas vents to the atmosphere. If the stress of the expansion is too great, a large tear called a split may form inside the block. There are two potential sources for the blowing gas: chemically produced CO2 from the reaction of water and isocyanate, and volatile additives otherwise known as alternative blowing agents (ABA). Traditional ABA are low boiling compounds such as chlorofluorohydrocarbons (CFC), hydrofluorocarbons (HFC), methylene chloride, acetone [34] and hydrocarbons that evaporate at ambient pressure as the heat of exothermic isocyanate reactions causes the temperature to rise. More recently, machinery has been developed which allows the manufacturer to use liquefied CO2 as an ABA [35]. Of course, CO2 is a gas at ambient temperature and pressure, so it begins to vapourise as soon as the material leaves the mix head and external pressure drops to ambient. These foams form a froth that does not flow like a liquid. Therefore, they are laid on a slab conveyor as a ribbon that expands vertically with only slight growth in the horizontal direction. Bubbles in a liquid are thermodynamically unstable because they have a lot of surface area and therefore high surface energy. Further, the membranes are thinned by both cell growth and surface tension which pulls liquid from membranes into the struts. These factors destabilise the cell walls and make it more likely that they will rupture. It is important that cells do rupture (see Section 5.4.4), but they must remain closed through the growth phase because the pressure of the trapped gas supports the foam and makes it expand. This temporary stability comes from the surfactants and emulsifiers and is aided by the increase in viscosity as the polymer gels. Surfactants lower surface tension and reduce the surface energy needed to maintain the film. Emulsifiers prevent phase separation of dissimilar ingredients that could disrupt thinning membranes. Viscosity makes it more difficult for the liquid to flow, so the membrane is less likely to thin. When the destabilising factors finally overcome the stabilising influences the membranes rupture. Although the blow reaction will produce gas molecules for several minutes, the rise stops when the block vents to the atmosphere. Gas molecules produced after cell opening merely escape out of the foam.

5.4.4 Cell Opening In a flexible foam, cell opening is essential because closed cells lead to poor mechanical properties, low resilience and an undesirable pneumatic feel. In extreme cases, a closed cell flexible foam will shrink. At maximum rise, the cells are filled with hot gas, but as 97

Handbook of Polymer Foams the foam cools, the pressure inside drops. The membranes allow carbon dioxide to diffuse out faster than air diffuses in [36], so this further reduces pressure in cells that remain unopened after curing. The overall result is that a flexible foam with too many closed cells will shrink as it cools. To prevent shrinkage, some foams (notably moulded and high resilience slabstock types) are mechanically crushed to burst any closed cells. Rigid foams are strong enough to maintain a partial vacuum within each cell. These can have closed cells without shrinkage. Cell opening presents an interesting paradox. A microscope shows that cell membranes are often completely missing, indicating that the material had sufficient time and fluidity to flow completely into the strut. Yet at the time of cell opening, the polymer in the struts must be strong enough to maintain the cellular geometry and support the foam against the pull of gravity. Before cell opening, pressure in the cells supports the weight of the foam, but when the foam vents, the support must come from the struts. The foam will lose height (settle or sigh back) if the struts are not strong enough to support the weight above them. Over the years, several mechanisms of cell opening have been proposed, and it is likely that the exact cause of cell opening may be different in different types of foam. Probably the most comprehensive theory [37] begins with the precipitation of urea microdomains that signals the end of membrane stability. As this phase separation occurs, the polymer goes through a period where it exhibits extensional thinning behaviour [38]; that is, the viscosity drops dramatically as stress increases. If the polymer in the membrane is under higher stress, it can flow while the unstressed struts remain intact. There are two probable causes of this stress that will preferentially affect the membranes. One is driven by a pressure gradient known as the Laplace force that thins the membranes of all types of foam. The second is unique to polyurethane foams, where the precipitation of urea probably causes a sudden increase in density of the polymer. The polymer contracts, but the block of foam stays the same size. This gives a uniform force across the foam, but since stress is force per unit area, the membranes feel more stress because the crosssectional area is less.

5.4.5 Cure Once the cells open, the gas pressure is vented to the atmosphere. The foam is no longer supported by pneumatic means, yet gravity continues to pull the foam downward. At this point, the struts must have sufficient strength to support the weight of the foam or it will sag or settle under its own weight. As discussed in Section 5.4.4, it is important to balance the strength in the struts and instability in the membranes to control cell opening and ultimately determine the success of the foaming process.

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Flexible Polyurethane Foam As the polymer cures, the urea groups that form are attracted to each other because of their similar polarity and hydrogen bonding. Urea phase separation begins at cell opening and continues during the curing process. Urethane groups also phase separate as the polymer cures, so that the final polymer has distinct urea and urethane phases [39] dispersed in a ‘soft segment’ comprised mostly of less polar polyol chains. The urea and urethane phases contribute hardness and dimensional stability to the polymer because they are highly crystalline and remain intact well above room temperature. The continuous phase is the non-crystalline soft segment. When an FPF is compressed or elongated, the polyol chains in the soft segment can rearrange in response to the stress. Polyurethane elasticity is a result of the hard and soft segment morphology. It is important to recognise that although the foam achieves mechanical support after cell opening, it is far from having fully developed properties. The temperature continues to rise for several minutes beyond the cell opening, showing that isocyanate groups are still reacting. Both blow and gel reactions build molecular weight and add crosslinks to the polymer backbone. Since the cells have vented, gaseous products from the blow reaction simply escape to the atmosphere without contributing to any further foam rise. The mechanical properties of the foam improve as the curing reactions link together polymer chains and the temperature subsides. Large foam blocks are usually set aside for several hours before further processing, and final quality assurance testing is often delayed until the next day. Moulded foams can usually demould after a few minutes, but these also require several hours, often at elevated temperatures, to fully cure.

5.5 Manufacturing Equipment Over the years, FPF equipment and chemicals have evolved together. Current technology allows manufacturers to produce higher quality foams with less environmental impact and greater economy than ever before. New foam grades are now available that could not be made only a few years before, and that statement is likely to remain true far into the future. There are two main types of manufacturing equipment: slabstock machines that produce large, continuous blocks of foam (see Figure 5.4), and moulding machines that produce discrete parts which are cured in the shape and size of the mould. Both of these have three main systems. The first is the tanks, pumps, and pipes that store, meter and pump the raw materials, the second is the mix head, and the third takes the liquid pre-foam away from the mix head and provides the space for it to rise and cure.

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Figure 5.4 Typical block of slabstock FPF Source: Beamech

5.5.1 Storage and Delivery Moulded foam equipment is designed to accurately dispense relatively small amounts of liquid into each mould, so the feed tanks and delivery system are normally smaller and less complicated than those seen in a slabstock machine. A moulder may have dozens of machines, with each one running a few formulations. Additives are usually pre-blended and only 2-4 streams actually enter the mix head [40]. This is very different to slabstock machines which pour many different grades of foam. These can have twenty or more additive streams to accommodate the different formulas, each with its own storage, metering and delivery systems as detailed in Figure 5.5.

POLYOL

REQUIRED water catalyst surfactant

ISOCYANATE

OPTIONAL fillers flame retardants antioxidants colorants etc.

Figure 5.5 Schematic of slabstock mix head

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5.5.2 Mixing Years ago, slabstock and moulded foam machines both used a traditional mix head configuration. This is a cylindrical chamber where raw materials are introduced through ports along the sides and the liquid is mixed with a pin mixer on a rotating shaft. This design is ideally suited for slabstock foams because they operate continuously, but most moulded foams require a solvent flush after every shot. As environmental regulations have tightened, the impingement mix head was developed. This design forces together opposing jets of the raw materials and relies on turbulence to mix. Since there is no rotating shaft, the solvent flush is replaced by a plunger that purges the mixing chamber between shots [41].

5.5.3 Foam Rise and Cure The most obvious difference between moulded and slabstock machines is how they handle the liquid after it leaves the mix head. A moulded foam machine usually has many individual moulds that are attached to a carousel. Empty moulds are carried to the mix head, filled, closed and clamped. Most times, the mould is heated to accelerate the cure so that it can be opened sooner and readied for the next use. The foam parts need reasonable strength to demould, but they are often baked to complete the cure. In most slabstock machines, the liquid from the mix head is deposited on a paper covered conveyor with moving sides and an open top. Conventional foam machines (Figure 5.6) pour the liquid on the paper from above and allow it to spread across the conveyor

Figure 5.6 Conventional foam being poured Source: Beamech

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Handbook of Polymer Foams before it begins to froth. The conveyor is lowered through a series of fall plates that help to improve the foam profile. Foam from a conventional machine generally has a very uniform cell structure, but the blocks can have a crown in the middle because less liquid reaches the edges of the conveyor. Several innovations have been developed to eliminate the crown because rectangular blocks can be processed with much less cutting waste. Manufacturers can traverse the mix head to spread the liquid better. They can also use a device to lift the side paper while the foam is rising to raise the shoulders. With these improvements, good conventional foam is still limited to a size of approximately 2 metres wide and 1 metre high. Conventional equipment can also make round blocks of foam by using a conveyor with cylindrical side walls. A Maxfoam machine pumps the liquid into a trough that is almost as wide as the conveyor. As more liquid is pumped in, it spills out over the top so a uniform ribbon of froth is laid on the conveyor. In this case, the foam has a flat top because it expands in the vertical direction without spreading horizontally. As in conventional machines, the flattest blocks are made when fall plates lower the conveyor to nearly match the rise profile. The Maxfoam system can produce large blocks with a good rectangular shape, but the turbulence in the trough adversely affects cell structure. Recently, machines have been introduced that can quickly switch between conventional and Maxfoam configurations to improve operational flexibility [42] (see Figure 5.7).

Figure 5.7 Schematic showing conventional and Maxfoam capabilities Source: Beamech

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5.5.4 Innovations Equipment for large-scale polyurethane foam manufacturing has become much more automated and sophisticated over the years, but in most cases, the difference between machines from the 1960s and today relate to computerisation rather than the engineering principle. All parts of the system are now more precisely controlled by a centralised computer that can store hundreds of different formulations and allows the manufacturer to change between foam grades with a single command [43]. The most notable equipment inventions of the past decade are VPF, CO2 as a blowing agent and ground foam recycling. VPF and CO2 are both aimed at production of lower density slabstock foams without the use of regulated blowing agents. For many years, soft, safe low density foams were made by adding CFC or methylene chloride to the formulation, but environmental regulations severely restricted these additives. Chemical companies developed less harmful blowing agents, but most are flammable or expensive and can still have environmental concerns. Low density, all-water blown foams are not as soft and may be dangerous to produce because the exotherm temperature can exceed the fire point. Rapid cooling strategies were adopted [44, 45] to reduce the fire danger, and more recently, the equipment industry developed two elegant solutions: VPF and CO2. The entire foam conveyor of a VPF machine is enclosed in a large vacuum chamber [46]. Foams made at reduced pressure have a lower density because the same amount of blowing gas will fill a larger volume. Therefore, VPF machines can produce soft, low density foams with a moderate exotherm and without blowing agents [47]. Another advantage is that they collect and treat the toxic gases before releasing them to the atmosphere. Some VPF machines may be operated above ambient pressure to produce unique foam grades. Carbon dioxide blown foam technology was developed at around the same time as VPF, and has been used extensively to make both slabstock and moulded foams. Of course, carbon dioxide is a gas at ambient conditions, and low temperature and high pressure are used so that it can be introduced into the mix head as a liquid [48]. CO2 remains a liquid in the pressurised mix head, but it will vapourise as soon as the mixture exits [49]. The successful implementation of this technology required significant effort to develop hardware that would smoothly transition the pressure to ambient without stressing the liquid [50]. The froth leaves the mix head through a precisely engineered, narrow slot which is almost as long as the conveyor width. The froth flows poorly, so the most of the foam expansion should be in the vertical direction as in a Maxfoam machine. Another recent innovation allows the manufacturer to non-cryogenically grind foam scrap from the manufacturing process. The ultrafine powder is then used to displace more than 20% of the virgin chemicals in the manufacture of new foam. With minor

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Handbook of Polymer Foams formulation adjustment, the resulting foam has properties equal to the original foam, so the economics are driven by the difference between the value of scrap and the price of chemical raw materials. The declining use of scrap foam in North American carpet cushion has improved the economic advantage and led to its rapid adoption. As this technology becomes more widespread, it will also allow the recycling of significant amounts of postconsumer foams that currently end up in landfills [51].

5.6 Foam Characterisation There are many tests used to measure the properties of FPF. The most significant methods are formalised under ASTM D3574 [52] and D3453 [53] and BS EN ISO 1798 [54]. In 1994, the Polyurethane Foam Association (USA) published the Joint Industry Foam Standards and Guidelines [55] which relates the test data to foam performance and value, particularly in the furniture industry. Of course, there are many tests that are specific to a particular customer, manufacturer or application. Some of the more important tests are summarised in the next section.

5.6.1 Density Density and hardness are sometimes thought to be similar, but these are completely different properties, and surprisingly, there is only a weak relationship between them [56]. Adding water to reduce density also increases the amount of urea in the polymer which makes it harder. Many flame retardants increase density while softening the foam, and copolymer polyols will stiffen the foam without affecting density. Density is important to the performance and economics of a foam because it measures how much of a foam is air and how much is polymer. The density is simply the mass divided by the volume and is commonly measured in kilograms per cubic meter (kg/m3) [57]. Most conventional grades of slabstock foams range from about 12 to 40 kg/m3. Foams with fillers, flame retardants and copolymer or polyester polyols usually have a higher density and range from 25 to 100 kg/m3. Flexible moulded foams can be poured in much higher densities, but they can also go down to about 25 kg/m3 [58].

5.6.2 Hardness Hardness is very important because most foams are used in applications where they provide at least some type of cushioning. Therefore it is important to know how much force is required to give a particular deflection or how much weight it can support.

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Flexible Polyurethane Foam Harder foams are produced by increasing the isocyanate index, using copolymer polyols or adding fillers [59]. High density moulded foams can be measured with a hand held durometer which has a small indenter foot attached to a deflection spring. It measures the force required to push the foot into the foam a known depth. The base of the durometer is only a few centimetres across, and the deflection is just a few millimetres so it is useful for small samples but only marginally accurate on slabstock foams. Larger foam samples are usually measured by either the indentation force method (IFD) [60] or the compression force deflection (CFD) [61] methods. Both tests use a large foot (normally 325 cm2). A CFD test uses a foam block smaller than the foot. This eliminates friction effects and allows the foam to flatten horizontally. IFD uses a large 10 cm thick block of foam so that the circular foot is smaller than the sample. In both tests, the force is measured at various deflections. There are many variants of this test depending on the application. For the furniture industry, the most commonly reported values are the 25% IFD and 65% IFD (IFD measured at 25% or 65% compression), and furniture foam is normally specified by density and IFD. There are also other values derived from IFD. For example the guide factor is IFD divided by density. Compression modulus is the ratio of 65% IFD to 25% IFD. In seating applications compression modulus is called support factor because higher values indicate that the foam feels soft yet can support more weight without bottoming out. A hysteresis value is calculated after running 25% IFD, 65% IFD and then a second 25% IFD on the same piece of foam. The loss of 25% IFD after the 65% compression is the hysteresis.

5.6.3 Resilience Resilience is measured by dropping a steel ball on a foam sample and measuring the height of the rebound [62]. The ball rebound test is different from the hardness tests because it indicates the instantaneous feel of the foam. The FPF industry quickly recognised that resilience was important for many applications, and high resilience (HR) foams were defined under ASTM D3770 [63]. Special copolymer polyols for HR foams were introduced in the 1960s [64], and these now constitute a significant part of the overall FPF industry. EO capped polyether polyols with modified polyisocyanates are also used [65, 66]. More recently, low resilience foams have also gained in importance [67, 68]. These are known as viscoelastic, slow recovery or energy absorbing foams and will give a characteristically low value on a ball rebound test. There are many ways to formulate viscoelastic foams, although most are made with MDI. The key is to reduce polymer elasticity around room temperature. Some techniques known to produce viscoelastic

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Handbook of Polymer Foams foams include the use of low molecular weight crosslinkers, blends of incompatible polyols, alkoxylated alcohols, foreign polymers, plasticisers, and variations of isocyanate index [9]. In any of these tests, one must realise that foam will soften as temperature and humidity increase [70]. In slabstock foams, hardness also varies across the height and width of the block, and is even affected by whether the sample was cut in the horizontal or vertical direction relative to the rise.

5.6.4 Porosity Filtering, acoustic and gasketing applications usually require that the foam either permits or blocks the passage of air. End users of other types of foam are not usually concerned about porosity per se, but foam manufacturers have long known that air flow is by far the easiest way to approximate the amount of intact cell windows in the foam, and this affects many other important properties. Increasing porosity normally leads to better resiliency and mechanical properties, but it is more difficult to produce high porosity, low density foam because cell opening occurs earlier in more porous foams, so more of the blowing gas is lost to the atmosphere. The surest way to produce foams with very high air flow is by chemically or physically destroying all residual membranes after the foam is cured. These reticulated foams are excellent for applications such as filtration, or liquid delivery where membranes will interfere with the flow of air or liquid through the foam. The traditional way to determine porosity is to measure the air flow in m3/second needed to maintain a pressure differential of 125 Pa on a standard sized 50 mm x 50 mm x 25 mm [71] piece of foam. It is also possible to measure the pressure drop that occurs when pulling a known volume of air through a cross sectional area of the foam. Open celled foams give high numbers in the first method and low numbers in the second. Like hardness, porosity can be significantly different in the horizontal and vertical directions.

5.6.5 Strength Properties Hardness indicates the force needed to compress a foam. Tensile and tear strength measure the force required to stretch and tear a foam respectively. Therefore, hardness is important in cushioning, and tensile and tear strength are important when the foam is stretched in handling or use. Tensile, tear and elongation are usually mentioned together because they are all run on an extensometer, and differ only in the sample preparation. This device has two jaws which hold the specimen and measure the force while slowly pulling it apart. In principle, these measurements are the same for foams as for any other material. The difference is that foams are not regular on a small scale, so much larger specimens

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Flexible Polyurethane Foam must be used to compensate for the irregularity. The tensile experiment uses a classic ‘dumbbell’ or ‘dog bone’ shaped sample and measures the force per unit area (at the narrow part of the specimen) required to break the sample. The increase in length at this point is the elongation and is reported as a percentage of initial length [72]. Sometimes, the force at 100% elongation is also recorded as the 100% modulus. In the foam industry, tear strength is normally measured on a ‘split tear’ specimen. This requires a rectangular sample with a cut down the centre. The foam is torn apart by pulling opposite sides of the cut apart. Tear strength is the force required to continue tearing the foam. It is measured as force per linear centimetre of the cut [73].

5.6.6 Cell Structure Technical applications often require foams with a particular cell size or structure. It is easy to recognise the difference between large and small cells, but it is more difficult to agree on an exact measurement because a close look reveals the difficulty in measuring a three dimensional cell with a ruler. Further, most cells have a slightly elongated shape. Today, the cell size of most foams is measured by eye using a linear cell count method. Methods that compare images of the foam to circular standards are more accurate and less dependent on the individual running the test, but require more sophisticated equipment [74, 75]. Cell uniformity can be even more subjective than cell size. Customers normally expect the cells in a piece of foam to all be about the same size. Contaminants or air holes can cause obvious non-uniformity, and can immediately turn the block to scrap. Some applications demand a very consistent structure, but if the foam is hidden, there is considerably more leeway. Some types of foams are intentionally contaminated to give a consistent non-regularity to imitate the appearance of natural sea sponge. As with other appearance issues, the manufacturer and customer usually develop an informal understanding about what good foam looks like.

5.6.7 Environmental Stability Stability and resistance tests are designed to determine if the foam will continue to deliver expected performance throughout its working life. Many foams are used in environments that can degrade foam quality. Some examples would be exposure to water [76], microorganisms, oil, solvents, high temperatures [77], sunlight and so on. There are many different stability tests depending on the type of foam and its intended use. In general they involve measuring the properties of interest, (e.g., hardness, tear strength, colour, size) on a foam sample before and after controlled exposure to the degradative element.

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Handbook of Polymer Foams By increasing temperature or severity, it is often possible to estimate the effect of years of ageing in a few weeks [78].

5.6.8 Fatigue Many types of foam, such as those used in carpet cushion and seating applications are expected to provide excellent cushioning properties for years of continuous use. Fatigue resistance is clearly important in carpet padding, upholstered furniture and automotive seating, but the foam thickness, compressive force, and sliding motion in each application is quite different. Literally dozens of fatigue methods have been developed over the years that attempt to mimic a particular end use. All dynamic fatigue tests put a foam sample through thousands of compression cycles and measure softening and thickness loss. Some also add a sliding or rolling element to simulate the other forces that a cushion will see in normal use. ASTM D3574 [52] lists three fatigue methods [79]. One is a static test that measures the decrease in force needed to maintain a constant deflection over time. The other two are dynamic tests which either use a constant force pounding or a constant force compression with a rolling component [80]. Generally, higher density foams perform better in dynamic fatigue tests [81].

5.6.9 Compression Set Compression set tests are similar to static fatigue methods except that they normally run at much higher temperatures and can also include humidity, either as a preconditioning step or during the test [82]. Foams can be permanently compressed to a fraction of their original height if the temperature is high enough. These ‘felted’ foams are useful in certain filtration or liquid delivery applications.

5.6.10 Flammability Numerous disclaimers should precede this discussion of FPF flammability. As a general summary, it is meant to offer an insight into the topic. The information presented herein is believed to be factual, but the reader is warned that regulations and methods change regularly and may not apply in a particular marketplace or geographical region. It is presented without warranty and should only serve to direct interested readers to more definitive sources of information. Flammability testing regulations are exceedingly complex, with numerous governmental and private agencies imposing standards for foams which fall under their auspices.

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Flexible Polyurethane Foam Polyurethane foam flammability has been studied in great detail [83, 84]. While much has been learned since the early days of the industry, fire is almost always accidental, so it is impossible to predict how it will start or how it will spread. The best we can do is come up with tests that are expected to model most likely scenarios. These include different ignition sources, foam sizes and orientations, and may or may not include other materials such as fabric in composite structures [25, 85]. As the industry becomes more global, a few tests have emerged that have broad acceptance. Automotive foams are almost always certified to MVSS 302 [86] or its near equivalent DIN or JIS standards. Furniture foams are usually tested by California TB-117 (residential, component testing) [87] and Cal TB-133 (public occupancy, composite testing) [21] in North America or BS 5852 (furniture) [23] and BS 7177 (mattresses) [88] in Europe [89]. It is likely that Cal TB-117 will move toward composite testing in the near future [87] to harmonise it with the British Standard test. Other regional specifications also exist [90] including the Boston Chair, New York Subway, and Italian Railway tests. Aircraft and other military foams sometimes require a Military Specification (Mil Spec) test, and packaging foam normally falls under UL94HF1 [91, 92]. As a last word, the reader is reminded that no test is completely reliable at predicting fire performance under a particular set of conditions.

5.7 FPF Markets The global market for FPF is estimated at over 3 million tonnes, roughly divided between the Americas, Europe-Africa and Asia-Pacific. Of these, Asia-Pacific has shown the highest growth rate [93]. Table 5.2 breaks down production by foam type and geography. There are thousands of uses for FPF ranging from commodity cushioning products to highly specialised technical foams. The transportation and furniture industries are the two largest

Table 5.2 Estimated foam production in 2001 Tonnes foam

Asia-Pacific

Europe-Africa

Americas

Global

Polyether

530,000

620,000

880,000

2,030,000

HR

90,000

180,000

140,000

410,000

Polyester

60,000

120,000

60,000

240,000

Total Slab

680,000

920,000

1,080,000

2,680,000

Moulded

270,000

380,000

330,000

980,000

Total FPF

950,000

1,300,000

1,410,000

3,660,000

Source: IAL Consultants

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Handbook of Polymer Foams consumers for FPF, and carpet cushion and packaging are also major markets [94]. While all other foam applications only comprise approximately 10% of the foam produced, these cover a broad range of specialty products that tend to have more stringent requirements, and come with concomitantly higher profitability.

5.7.1 Transportation Automotive and other transportation applications are clearly the largest markets for moulded foam, and also use considerable amounts of slabstock foam. Automotive foams use a higher percentage of MDI than the other markets. Within this segment, FPF is used for seating, components (such as head and arm rests), textile lamination, gasketing, filtration, sound absorption, and energy management. As a group, automotive foams tend to require durability, high and low temperature resistance and hydrolytic stability. Volatile compounds such as the antioxidant butylated hydroxy toluene (BHT) must be eliminated from automotive foams because they can condense on the glass and cause a hard-to-clean fog on windows [95]. These foams are often required to meet the fire resistance of MVSS-302 [85] or other similar worldwide standards.

5.7.2 Comfort The combined furniture and bedding industries are the largest user of slabstock foams and also consume considerable amounts of HR foam. These industries are highly dependent on the economy and its effect on disposable income [96]. The foams tend be the lowest density products that give adequate support and fatigue resistance. Polymer polyols are important here as they increase hardness and resilience without changing density [97]. Depending on the location of the cushion, different factors contribute to the overall perception of comfort. For example thick seat cushions and back cushions tend to be softer, while thin seat cushions and bedding foams are usually harder. The bedding industry is now becoming a major user of viscoelastic foams that conform to the weight of the body and offer more uniform support [98]. Flammability is also a major issue in furniture and bedding foams, and a number of appropriate standards may apply.

5.7.3 Carpet Cushion Carpet cushioning foams are a North American phenomenon, as this application is almost non-existent in the rest of the world. These can be high density prime or rebond foam. In North America, most manufacturing remnants are recycled into carpet cushion. This

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Flexible Polyurethane Foam process entails shredding the foam and bonding the bits together with an isocyanate binder. This high density composite gives excellent durability, improves comfort and reduces noise and carpet wear [99].

5.7.4 Packaging In an industry dominated by paper products, cloth, natural materials and other foamed polymers, FPF maintains an important position in the packaging industry. Foam is often considered a high-end choice because it can be shaped to the contours of delicate parts and gives good aesthetics, excellent protection and light weight. Foam hardness, resilience, air flow [100] and sample geometry [101] all affect the cushioning properties of FPF. A packaging engineer will choose an appropriate foam by considering the fragility of the object, anticipated loads, weight and space requirements [102].

5.7.5 Specialty Applications There are thousands of niche FPF products with higher technical requirements and lower production volumes that are grouped together as the technical foams. These often require very specific cell size or cell structure, porosity, hardness, solvent or water resistance, diffusion properties, elongation, strength or compatibility with specific chemicals. Technical foams are more likely to use polyester polyols because they give a uniform cell structure and the cell size is easier to control. They are also stronger and give better solvent resistance. They often go through one or more post processing steps to enhance one or more of these properties. Reticulation refers to the chemical or thermal opening of cell membranes to leave a network of polymer which will pass air with very little resistance. Reticulated foams are commonly used in filtration. Felting is a process which permanently compresses the foam with heat and pressure to give a densified foam that is particularly good at wicking and transporting liquids.

5.8 Environmental Issues In today’s world, most industries have had to answer questions about the environmental impact of their products. The FPF industry is no exception, and despite a long history of significant gains, there is still room for improvement. In fact, more than half of the papers presented at the Polyurethane Foam Association (PFA) meetings since 2000 have dealt with safety or environmental issues [103]. Many fair concerns have been raised, while other claims are based on perception rather than science and the critics aim to try

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Handbook of Polymer Foams the industry in the court of public opinion. Often, spurious claims do more harm than good because they distract the industry and the public from meaningful issues that could be improved. In all, the environmental stakeholders can be divided into four groups: plant workers, neighbours, consumers and the general public. Today, many companies in the FPF industry adhere to Responsible Care® (registered trademark of the American Chemistry Council) guidelines that make safety and environmental considerations an integral part of the decision making process [104]. Plant workers are mainly concerned about the dangers they face during their working lives. This includes acute and chronic effects of the chemicals that are used in the manufacturing process as well as potential hazards from physical dangers such as falling objects, cutting saws, falls, fires and other aspects of the manufacturing environment. Workplace safety is primarily the responsibility of owners, management, and the workers themselves. Organisations such as the Occupational Health and Safety Association, (OSHA) in the USA, establish national safety standards for the workplace. These tend to be stricter in developed nations. Any manufacturing site has unanticipated dangers, but as the industry matures, the hazards become clearer and safer procedures and equipment protect workers better. None of this is unique to the FPF industry. The main specific hazard to FPF workers and people living near factories comes from the vapours of isocyanates and other chemicals used in the process. In the USA, emission standards are established by the Environmental Protection Agency (EPA) [105]. Toxicological information on any chemical should be obtained from a Materials Safety Data Sheet (MSDS), but the industry has generally been most concerned with TDI , and continues to study its properties [106]. A recent study in the USA showed that TDI vapours close to the rising and curing foam occasionally exceed American Conference of Governmental Industrial Hygienists (ACGIH)/OSHA limits, but respiratory protection makes actual worker exposure rare [107]. New technologies more efficiently collect and treat process gases [108-110], and a number of non-fugitive additives have been developed [111-113]. Despite the continuing improvement in air quality both inside manufacturing plants and in nearby areas, neighbourhood complaints have arisen. In 1997, these led to the closure of the Trinity American Corporation’s facility in North Carolina, USA. Later, the diisocyanate panel of the American Chemistry Council reviewed the data and concluded that there was no scientific or legal basis for closing the plant [114]. Consumers expect products to be safe. FPF is lightweight and soft so it does not present any physical hazards. Flammability is certainly a legitimate issue, and was discussed earlier in Sections 5.3.9 and 5.6.10. The industry has extensively studied the potential for toxic chemical exposure through contact with FPF. TDI has not been found in the product, and none of the other emissions occur at concentrations that pose significant health risks to humans under normal use conditions [115]. There is no positive evidence

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Flexible Polyurethane Foam that chemicals emanating from FPF harm consumers, yet advocacy groups occasionally raise concerns. Over the past decade, use of BHT, butyl tin catalysts, volatile amines, nonylphenol ethoxylates and various flame retardant chemicals have all been questioned, and in all cases, the industry has found alternatives. A widely publicised report that the suspected carcinogen toluene diamine is found in aged foam samples [116, 117] has been shown to be the result of an analytical error [118, 119]. The FPF community has continued to develop technologies that reduce the impact of manufacturing on the global environment. In 1987, the Montreal Protocol called for CFC elimination by the year 2000, and this work was completed well ahead of schedule. Other harmful ABA have also been reduced by chemical and mechanical innovations [120]. Nearly all manufacturing trimmings are recycled into rebonded carpet cushion foams [121]. The vast majority of foams that end up in the environment come from consumers. There are many possible fates including litter, landfill, incineration, or recycling. Alliance for the Polyurethane Industry (API) has shown that foams are safe in landfills. Degradation is minimal, and they do not contaminate groundwater [122]. A study by Polyurethane Recycle and Recovery Council (PURRC) demonstrated that municipal incineration is also a safe and viable option for energy recovery [123]. Incineration is also recommended by the European Isocyanate Producers Association (ISOPA) [124]. Recycling is the best option as it uses foam wastes to make new products. There is already a large market for scrap foam as a raw material for rebonded products in North America. European legislation is expected that will preclude disposal of FPF in landfills and mandate recycling, particularly in automotive applications. If enacted, the amount of scrap should significantly outpace demand for rebond. One possibility is to recycle scrap foam by glycolysis which chemically reconstitutes it into new raw materials [125]. Another method grinds the scrap into a fine powder that can be reintroduced into foam production, reducing the need for virgin raw materials. This technology is already used to recycle manufacturing trimmings, and it is likely to be applied to post consumer scraps as the equipment becomes more widespread [53].

5.9 Organisations Four industry trade organisations are specifically dedicated to FPF. The PFA (www.pfa.org) and the Alliance for Flexible Polyurethane Foams (AFPF), (www.afpf.com) are based in North America. Many European countries have national FPF organisations, and the national groups together make up Europur (www.europur.com). The fourth FPF organisation is the Japan Urethane Foam Association (JUFA). Other trade associations

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Handbook of Polymer Foams are involved in the FPF community as part of their broader scope, including ISOPA, API, PURRC and International Isocyanate Institute (III).

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Flexible Polyurethane Foam 14. K. Diblitz and C. Diblitz, Proceedings of the SPI Polyurethanes World Congress, Vancouver, BC, Canada, 1993, p.619. 15. M.L. Listemann, A.L. Wressell, K.R. Lassila, H.C. Klotz, G.L. Johnson and A.C. Savoca, Proceedings of the SPI Polyurethanes World Congress, Vancouver, BC, Canada, 1993, p.595. 16. M. Clauss, S.M. Andrews, J.H. Botkin and L. Macena, Proceedings of the SPI Polyurethanes Expo, Las Vegas, NV, USA, 1996, p.156. 17. J. Sturgeon, Proceedings of the Polyurethane Foam Association Technical Program, Salt Lake City, UT, USA, October 2002, Paper No.16. 18. Personal discussion with Richard Rose of Great Lakes Chemical, 2003. 19. H. Creyf, Proceedings of the Polyurethane Foam Association Technical Program, Arlington, VA, USA, May 2002. 20. B.L. Carson, Toxicological Summary for Selected Polybrominated Diphenyl Ethers, National Institute of Health, Bethesda, MD, USA, 2001. 21. T. Geran, Proceedings of the Polyurethane Foam Association Technical Program, Salt Lake City, UT, USA, October 2002, Paper No.13. 22. Technical Bulletin No.133, Flammability Test Procedure for Seating Furniture for Use in Public Occupations, California Bureau of Home Furnishings and Thermal Insulation, 1991. 23. BS 5852, Methods of Test for Assessment of the Ignitability of Upholstered Seating by Smouldering and Flaming Ignition Sources, 1990. 24. M. Barker, M.P. Hannaby and F.J. Lockwood, Proceedings of the SPI Polyurethanes World Congress, Nice, France, 1991, p.628. 25. Personal discussions with Dave Kelly of William T. Burnett and Dimitri Dounis of Hickory Springs, 2003. 26. H. Stone, Overview of the Combustibility and Testing of Filling Materials and Fabrics for Upholstered Furniture, 1998, PFA, Wayne, NJ, USA. 27. M. Kageoka, Y. Tairaka and K. Kodama, Proceedings of the SPI Polyurethanes World Congress, Chicago, IL, USA, 1995, p.62.

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Handbook of Polymer Foams 28. G. Howard, Proceedings of the Polyurethane Foam Association Technical Program, Scottsdale, AZ, USA, October 1996, Paper No.1. 29. A.B. Lehnert, Proceedings of the SPI Polyurethanes World Congress, Las Vegas, NV, USA, 1996, p.98. 30. L. Jung, Proceedings of the SPI Polyurethanes World Congress, Nice, France, 1991, p.169. 31. Personal discussion with Rob Borgogelli of Goldschmidt, 1999. 32. T. Housel, Proceedings of the Polyurethane Foam Association Technical Program, San Diego, CA, USA, October 1999, Paper No.4. 33. R. Baumhäkel, Journal of Cellular Plastics, 1972, 8, 6, 304. 34. Hickory Springs Manufacturing Company, Proceedings of the SPI Polyurethanes World Congress, Amsterdam, The Netherlands, 1997, p.740. 35. C. Fiorentini, M. Taverna, B. Collins, C. Greaves and T. Griffiths, Proceedings of the Polyurethane Foam Association Technical Program, Newport, RI, USA, October 1994, Paper No.10. 36. M. Olsson, Long-term Thermal Performance of Polyurethane Insulated District Heating Pipes, Chalmers University of Technology, Gothenburg, Sweden, 2001, p.33. [PhD Thesis] 37. T. Housel, Urethanes Technology, 2001, 18, 5, 40. 38. X.D. Zhang, H.T. Davis and C.W. Macosko, Journal of Cellular Plastics, 1999, 35, 5, 458. 39. J. Mertes, H. Stutz, W. Schrepp and M. Kreyenschmidt, Proceedings of the SPI Polyurethanes World Congress, Amsterdam, The Netherlands, 1997, p.46. 40. Mixing Heads, Linden-EMB Technical sales literature, Linden Industries, Inc./ EMB, Cuyahoga Falls, OH, USA, 2002. 41. Introduction to Polyurethane Equipment, Cannon seminar, Mars, PA, USA, 1993. 42. Ultima Felxible Slabstock Foam Equipment, Beamech Ultima literature, Beamech, Manchester, UK, 2000.

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Flexible Polyurethane Foam 43. J. Rayner, Proceedings of UTECH Asia, Suntec City, Singapore, 1999, Paper No.2. 44. M.A. Ricciardi and D.G. Dai, Proceedings of the Polyurethane Foam Association Technical Program, Point Clear, AL, USA, 1992, Paper No.6. 45. H. Stone, E. Reinink, S. Lichvar, G. Rusenko, W. Carlson, and C. Sikorski, Proceedings of the Polyurethane Foam Association Technical Program, Point Clear, AL, USA, 1992, Paper No.8. 46. R. Triolo, Proceedings of the Polyurethane Foam Association Technical Program, Quebec City, Quebec, Canada, 1993, Paper No.10. 47. D. Ramazzotti and S. Carson, Proceedings of the SPI Polyurethanes Conference, Boston, MA, USA, 1994, p.8. 48. M. Taverna, Proceedings of the SPI Polyurethane Expo, Las Vegas, NV, USA, 1996, p.370. 49. B. Blackwell, G. Buckley and W. Blackwell, Proceedings of UTECH Asia, Suntec City, Singapore, 1997, Paper No.48. 50. T. Griffiths, Proceedings of the Polyurethane Foam Association Technical Program, Salt Lake City, UT, USA, 2002, Paper No.3. 51. Personal communication with Martin Dawson of Mobius Technologies, 2003. 52. ASTM D3574, Test Methods for Flexible Cellular Materials – Slab, Bonded and Molded Urethane Foams, 2001. 53. ASTM D3453, Standard Specification for Flexible Cellular Materials – Urethane for Furniture and Automotive Cushioning, Bedding and Similar Applications, 2001. 54. BS EN ISO 1798, Flexible Cellular Polymeric Materials – Determination of Tensile Strength and Elongation at Break, 2000. 55. Joint Industry Foam Standards and Guidelines, PFA, Knoxville, TN, USA, 1994. 56. Joint Industry Foam Standards and Guidelines, Section 1.0, Density Standards and Guidelines, Polyurethane Foam Association, Knoxville, TN, 1994. 57. D3574-01, Standard Test Methods for Flexible Cellular Materials—Slab, Bonded, and Molded Urethane Foams, Test A: Density Test, 2001. 117

Handbook of Polymer Foams 58. K. Usaka, M. Isobe, H. Utsumi and K. Ohkubo, Proceedings of the API Polyurethanes World Congress, Salt Lake City, UT, 2002, p.75. 59. D.R. Gier, R.E. O’Neill, M.R. Adams, R.D. Priester, W.A. Lidy, C.G. Barnes, E.G. Rightor and B.L. Davis, Proceedings of the SPI Polyurethanes World Congress, Dallas, TX, 1998, p.227. 60. D3574-01, Standard Test Methods for Flexible Cellular Materials—Slab, Bonded, and Molded Urethane Foams, Test B1:Indentation Force Deflection Test – Specified Deflection, and Test B2: Indentation residual Gage Load Test – Specified Force, 2001. 61. D3574-01, Standard Test Methods for Flexible Cellular Materials—Slab, Bonded, and Molded Urethane Foams, Test C: Compression Force Deflection Test, 2001. 62. D3574-01, Standard Test Methods for Flexible Cellular Materials—Slab, Bonded, and Molded Urethane Foams, Test H: Resilience (Ball Rebound) Test, 2001. 63. The Furniture Industry’s Guide to Today’s Flexible Polyurethane Foam, AFPF, USA, 1996, p.13. 64. W.C. Kuryla, F.E. Critchfield, L.W. Platt and P. Stamberger, Journal of Cellular Plastics, 1966, 2, 2, 84. 65. D. Hicks, G. Davies, S. Spertini and P. Chaffanjon, Proceedings of UTECH 2000, The Hague, The Netherlands, 2000, Innovations: Flexible Foam Development Session, Paper No.3. 66. S. Murakami, K. Saiki, M. Hayashi, T. Satou and T. Fukami, Proceedings of the API Polyurethanes Conference, Boston, MA, USA, 2000, p.281. 67. S. Narayan and A. Berube, Proceedings of the API Polyurethanes Expo, Columbus, OH, USA, 2001, p.201. 68. P. Farkas, R. Stanciu, and L. Mendoza, Proceedings of the API Polyurethanes Expo, Columbus, OH, USA, 2001, p.143. 69. A. Parfondry in The Polyurethanes Book, Ed. D. Randall and S. Lee, John Wiley & Sons, New York, NY, USA, 2002, Chapter 14, p.220. 70. Joint Industry Foam Standards and Guidelines, Appendix A3.0, Temperature and Humidity Effects on IFD, Polyurethane Foam Association, Knoxville, TN, 1994, Section 3.1.

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Flexible Polyurethane Foam 71. D3574-01, Standard Test Methods for Flexible Cellular Materials—Slab, Bonded, and Molded Urethane Foams, Test G: Air Flow Test, 2001. 72. D3574-01, Standard Test Methods for Flexible Cellular Materials—Slab, Bonded, and Molded Urethane Foams, Test E: Tensile Test, 2001. 73. D3574-01, Standard Test Methods for Flexible Cellular Materials—Slab, Bonded, and Molded Urethane Foams, Test F: Tear Resistance Test, 2001. 74. Visiocell – A New Method to Measure the Cell Diameter of Polyuretahne Foam, Recticel Visiocell Technical literature, Recticel, Wetteren, Belgium, 1999. 75. K.M. Lewis, I. Kijak, K.B. Reuter and J.B. Szabat, Proceedings of the SPI Polyurethanes World Congress, Vancouver, BC, Canada, 1993, p.517. 76. D3574-01, Standard Test Methods for Flexible Cellular Materials—Slab, Bonded, and Molded Urethane Foams, Test L: Wet Heat Aging, 2001. 77. D3574-01, Standard Test Methods for Flexible Cellular Materials—Slab, Bonded, and Molded Urethane Foams, Test K: Dry Heat Aging, 2001. 78. J.T. McEvoy and R. Yamasaki, Proceedings of the API Polyurethanes Expo, Columbus, OH, USA, 2001, p.281. 79. ASTM D3574, Standard Test Methods for Flexible Cellular Materials – Slab, Bonded and Molded Urethane Foams, 2001. 80. Joint Industry Foam Standards and Guidelines, Section 9.0, Standards and Guidelines for Dimensional Tolerances of Polyurethane Foam, Polyurethane Foam Association, Knoxville, TN, 1994. 81. J.E. Knight, Proceedings of the SPI 30th Annual Technical/Marketing Conference, Toronto, Canada, 1986, p.48 82. D3574-01, Standard Test Methods for Flexible Cellular Materials—Slab, Bonded, and Molded Urethane Foams, Test D: Constant Deflection Compression Set Test, 2001. 83. E. Bleuel, P. Boehme, U. Rotermund, M. Reichelt and C. Seitz, Proceedings of the API Polyurethane Conference, Salt Lake City, UT, USA, 2002, p.234. 84. B. Bastin, R. Paleja and J. Lefebvre, Proceedings of the Polyurethane Foam Association Technical Program, Salt Lake City, UT, USA, October 2002, Paper No.15.

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Handbook of Polymer Foams 85. J. David, Assessing the Need for a Federal Small Open Flame/Cigarette Ignition Upholstered Furniture Flammability Standard, National Economic Research Associates, San Francisco, CA, USA, 2001. 86. MVSS 302, Flammability of Interior Material – Passenger Cars, Multipurpose Passenger Vehicles, Trucks and Buses, 1972. 87. Technical Bulletin No. 117, Requirements, Test Procedure and Apparatus for Testing the Flame Retardance of Resilient Filling Materials Used in Upholstered Furniture, California Bureau of Home Furnishings and Thermal Insulation, North Highlands, CA, USA, 2000. 88. BS 7177, Specification for Resistance to Ignition of Mattresses, Divans and Bed Bases, 1996. 89. InTouch, 1999, 7, 1, 4. 90. International Association of Fire Fighters & The National Association of State Fire Marshals, Safety Alert Bulletin, 1999, 1, 1, 1. 91. UL 94HFI, Standard for Tests for Flammability of Plastic Materials for Parts in Devices and Appliances, 1996. 92. Personal discussion with Richard Rose of Great Lakes Chemical, 2003. 93. Data provided by IAL Consultants, London, 2002. 94. End-Use Market Survey on the Polyurethane Industry in the USA, Canada and Mexico in 2000, API, Arlington, VA, USA, 2001. 95. D.J. Grillo, T.L. Housel and F.A. Landis, Proceedings of the Polyurethane Foam Association Technical Program, Newport, RI, October 1994, Paper No.1. 96. B. Poole, Proceedings of UTECH, 2000, The Hague, The Netherlands, Furnishings Session, Paper No.1. 97. InTouch, 1995, 5, 1, 3. 98. L. White, Urethanes Technology, 2001, 18, 6, 22. 99. W. Wald, Proceedings of the Polyurethane Foam Association Technical Program, Arlington, VA, USA, May 1998, Paper No.6. 100. G.L.A. Sims and D. Pentrakoon, Cellular Polymers, 1997, 16, 6, 431.

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Flexible Polyurethane Foam 101. G.L.A. Sims and J.A. Bennett, Polymer Engineering and Science, 1998, 38, 1, 134. 102. InTouch, 1996, 5, 2, 2. 103. Polyurethane Foam Association Technical Program Abstracts, May 2000 to October 2002. 104. American Chemistry Council, Responsible Care, http:// www.americanchemistry.com/prodserv.nsf/s?readform&cbes-5hx2vj 105. Code of Federal Regulations Title 40, Parts 9 and 63, Environmental Protection Agency, Washington, DC, USA. 106. P. Conner, Proceedings of the Polyurethane Foam Association Technical Program, Arlington, VA, USA, May 2000, Paper No.2. 107. B. Cummings, Proceedings of the Polyurethane Foam Association Technical Program, Salt Lake City, UT, USA, October 2002, Paper No.6. 108. R. Sack, Proceedings of the Polyurethane Foam Association Technical Program, Salt Lake City, UT, USA, October 2002, Paper No.4. 109. C. Ecob, Proceedings of the Polyurethane Foam Association Technical Program, Salt Lake City, UT, USA, October 2002, Paper No.5. 110. B. Blackwell, Proceedings of UTECH, 2000, The Hague, The Netherlands, Innovations: Flexible Foam Development Session, Paper No.1. 111. R. Milian, Proceedings of the Polyurethane Foam Association Technical Program, New Orleans, LA, USA, October 2001, Paper No.1. 112. J.G. Kniss, L.A. Mercando and M.L. Listemann, Proceedings of the Polyurethane Foam Association Technical Program, Newport, RI, USA, October 2000, Paper No.1. 113. E. Rister, Proceedings of the Polyurethane Foam Association Technical Program, Arlington, VA, USA, May 2002, Paper No.3. 114. S.P. Levine, Proceedings of the Polyurethane Foam Association Technical Program, Arlington, VA, USA, May 2001, Paper No.5. 115. L. Peters, Europur Presentation, Helsingor, Denmark, June 1996, Slides 17 and18. 121

Handbook of Polymer Foams 116. E. Okumus, Arbetarskydd, February 19th, 1999, 6. 117. M. Dalene, G. Skarping and P. Lind, American Industrial Hygiene Association Journal, 1997, 58, 8, 587. 118. K. Hall, F.A.L. van Parys and R.J. Young, Plastics, Rubber and Composites, 2001, 30, 9, 426. 119. K. Hillier, D. King, A. Kronborg-Hansen and T. Schupp, Cellular Polymers, 2001, 20, 4, 279. 120. InTouch, 1997, 6, 2, 2. 121. InTouch, 1994, 4, 1, 3. 122. F. Lichtenberg, Proceedings of the Polyurethane Foam Association Technical Program, May 2002, Arlington, VA, USA, Paper No.2. 123. Sustainability, PURRC Pamphlet, PURRC, New York, NY, USA, 1997. http:// www.utm.edu/~moo/purrc/advanced.html 124. There are a Variety of Ways to Recycle Polyurethane, API Pamphlet, Arlington, VA, USA. http://www.polyurethane.org/polyurethane_recycling/ how_poly_recycled/purrc2.html 125. S.H. Shin, J.H. Chun and B.S. Tae, Proceedings of the SPI Polyurethanes World Congress, Las Vegas, NV, USA, 1996, p.77.

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6

Rigid PVC Foam Noreen L. Thomas

6.1 Introduction Rigid polyvinyl chloride (PVC) foam (flexible PVC Foam is covered in Chapter 7 of this book) was first produced some thirty years ago, when it was heralded as ‘the wood of the future’ [1, 2]. Now this material has a well-established market in Western Europe of about 300 ktonnes. The product range covers diverse applications such as window sills, sewer pipes and advertising boards. The reasons for the success of this material are its ability to compete with wood and also the benefits that it offers over solid PVC. Many of the applications for extruded PVC foam profile (in the density range 500 to 800 kg/m3) are for wood-replacement products. It can be sawn, nailed or screwed without splitting or cracking, thus allowing use of woodworking methods. Foamed extrudates can be produced to resemble wood in terms of structure, colour and appearance. Hence rigid PVC foam profile is often used as a direct substitute for wood. This is being driven [3] by environmental pressure to protect forests, the rising price of timber and the advantages of a maintenance-free product. On a cost/volume basis PVC foam is more expensive than wood but becomes more cost effective when wood finishing operations are taken into account. PVC foam can be extruded into complex shapes (some of which are not possible with wood) without the finishing operations and scrap associated with the manufacture of wood trim. In addition PVC foam offers the following advantages over wood [4, 5]: chemical resistance, good weatherability, (i.e., does not rot), good fire retardancy (Class B1, based on DIN 4102 [6]), and resistance to infestation by vermin. The main obstacles to its use are its reduced toughness and higher cost compared with wood. Apart from its success as a replacement for wood, rigid PVC foam is also being used to replace solid PVC. This is because foamed PVC provides a number of advantages over the solid polymer. Firstly, foaming gives a lower cost per unit volume of product, which in today’s highly competitive and cost conscious environment is a very important consideration. The economics of extruding foamed versus solid PVC are examined by

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Handbook of Polymer Foams Patterson and Hughes [7] and also by Dobrowsky [8]. Other benefits of foaming [4, 9] are lower thermal conductivity (hence improved insulation), reduced thermal expansion, improved acoustic damping properties, higher stiffness (due to increased cross-sectional area for a given weight) and improved resistance to wind-load. There are three major markets for rigid PVC foam: profile, sheet and pipe. Within Western Europe the current sizes of these sectors are estimated to be 60, 50 and 180 ktonnes, respectively. These markets are regional, reflecting the disparate preferences in building design and differences in building regulations throughout Europe. For example, the UK is by far the largest market for foam profile, whereas foam sheet is predominantly produced in Germany, with additional production in Switzerland, UK and Eire. The most important market for foam pipe has always been France, although now significant quantities are also manufactured and used in Germany, the Netherlands and Spain, with some production in Switzerland and the UK. As discussed previously, rigid PVC foam profile is mainly produced for wood-replacement products [5]. These are used in both weathering (exterior) and non-weathering (interior) applications. Examples of weathering applications include cladding, soffit boards, roofing products, windowsills and door-frames. Examples of non-weathering applications are skirting boards, cornices, wall and ceiling panelling, curtain rails, guide rails for shutters, coving and furniture profiles. PVC foam sheet is used in a variety of applications [5, 10] including display panels, signs, advertising boards, partition panels, building panels, garage door panels, table tops and shelves. It is used as the core material of sandwich structures in boat building, and for tanks and refrigerated vehicle bodies. PVC foam sheet is suitable for printing, painting and silk-screening. Sheets can be thermoformed, even after printing, and this opens up an even wider range of possible applications. Foam core pipe is used in applications in which there is no internal pressure [5, 11]. Products include drain pipes, sewer pipes, effluent discharge pipes, ventilation ducts, cable conduits and winding cores for textiles and paper. Apart from its considerable market share in Europe, it also enjoys widespread use in other countries, such as the USA, Australia and China [8].

6.2 Foam Extrusion The principal method for the production of PVC foam profile, sheet and pipe is extrusion. The cellular structure is generated by the decomposition of chemical blowing agents (CBA), which are organic or inorganic compounds that decompose on heating to evolve

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Rigid PVC Foam one or more gases. Physical blowing agents (PBA), which are gases or low boiling point liquids, are not widely used in the production of rigid PVC foam.

6.2.1 Basic Principles The basic principles for the extrusion of polymer foams were first elucidated by Hansen [12, 13] and have also been discussed by Han [14] and Gale [15]. Gas is introduced into the molten polymer in the extruder barrel either by thermal decomposition of CBA or by direct injection of PBA. The principles involved are similar for both types of blowing agent because the gaseous decomposition products from the CBA are generated within the extruder barrel and dissolve in the molten polymer. Dissolution is possible because of the high melt pressure within the extruder. To achieve an optimum foaming operation it is essential that bubble nucleation is delayed until the polymer melt emerges from the die. Hence high pressure must be maintained throughout the barrel and die to keep the gas in solution. This requires appropriate screw and die design, as well as good temperature control. By lowering the die temperature both melt viscosity and pressure are raised, and this will suppress any undesirable, premature foaming. When the melt exits the die, the rapid drop in pressure causes the polymer to become supersaturated with gas. Phase separation occurs and almost instantaneous nucleation of bubbles takes place. Bubbles will nucleate at irregularities in the polymer melt such as CBA solid residues, pigments, fillers, etc. The growth rate of the bubbles is rapid at first and then decreases as the pressure within them diminishes. Bubble growth is also retarded as the polymer cools down and its viscosity increases. To prevent the foam structure from collapsing, it is essential to cool the material very rapidly. This is done by passing the foam through a chilled calibration unit so that the cellular structure is rapidly ‘frozen’ into place. The surface quality, density and thickness of the outer skin are all influenced by the distance between the die and the calibration unit as well as the intensity of the cooling. Many aspects of the foam morphology are controlled by the formulation used. These are discussed in detail in Section 6.3. Cell size and uniformity depend critically on the amount and type of chemical blowing agent. A uniform, closed, fine cell structure is desirable for most applications. Cell size is also a function of the rheological properties of the polymer. If the polymer viscosity is too high, then the bubbles will not be able to expand fully, and it will not be possible to achieve a low density foam. If the polymer viscosity and melt strength are too low, the cells will rupture and/or collapse. In the case of PVC, melt strength is determined by the molecular weight of the polymer and by the use of acrylic processing aids.

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6.2.2 Extrusion Processes Rigid PVC foam is processed from dry blend on parallel or conical, contra-rotating, twin-screw extruders. These are basically the same machines as used for producing solid product with some differences in screw design [5]. The big difference in foam extrusion is in the die and calibration systems used. There are essentially two methods of foaming [5, 9, 11, 16], described as the free-foaming and Celuka methods. The free-foaming method is illustrated in Figure 6.1. The polymer melt is allowed to expand freely after exiting the die, prior to entering the calibration system, which is situated a short distance away. The extrudate has an even foam density distribution across its thickness and a thin higher density skin (Figure 6.2). This process can be used to produce pipe, sheet and simple profiles. The free-foam method is the preferred process for the extrusion of rigid PVC foam sheet [17]. Product thicknesses can be obtained between 2 and 20 mm and widths from 1 to 2 metres. Typical throughput rates are in the range 200 to 600 kg/h. Special temperature controlled die lips can be used to give different processing temperatures for the upper and lower lips of the die. A 3-roll polishing stack is used for calibration and cooling. The free-foam process offers several advantages over the Celuka method for sheet extrusion. The lines are more flexible and do not require new dies and calibrator modifications every time that the sheet thickness is changed. This gives a simpler production process with less investment cost. Figure 6.3 is a scanning electron micrograph of a cross-section through the thickness of a sample of PVC foam sheet made by the free-foam process. This foam has an average density of 600 kg/m3 and shows a fine, even, closed cell morphology.

Figure 6.1 Schematic diagram of free-foaming method Redrawn with permission from G. Beckmann, Proceedings of the PRI PVC ’87 Conference, Brighton, UK, 1987, Paper No.13, Figure 1. Copyright 1987, Institute of Materials

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Figure 6.2 Density distribution across thickness of free-foam product Redrawn with permission from G. Beckmann, Proceedings of the PRI PVC ’87 Conference, Brighton, UK, 1987, Paper No.13, Figure 3. Copyright 1987, Institute of Materials

Figure 6.3 Scanning electron micrograph of cross-section through free-foam sheet

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Handbook of Polymer Foams The Celuka process is also known as the inward-foaming method. The calibration unit is situated adjacent to the die and has the same dimensions as the die, thus controlling foam expansion. This set-up gives very rapid cooling. Bubble formation is quenched in the surface of the product and a solid outer skin is produced. The die contains a mandrel [18], which controls melt pressure decay and allows the melt to foam freely inside the profile (Figure 6.4). The density distribution across the thickness of the Celuka foam profile (see Figure 6.5) is quite different from that produced from the free-foaming process. There is a solid outer skin, usually about 0.5 mm thick, and a lower density core. The outer surfaces of Celuka products are the same as those of solid extrusions, whereas free-foam products have a rougher, more textured surface finish. The Celuka method is used for making pipes, sheet and profiles of complex geometries. Figure 6.6 is a scanning electron micrograph of a cross-section through a Celuka foam profile. This foam has the same average density as that pictured in Figure 6.3, (i.e., 600 kg/m3) but has a coarser cell structure because of differences in formulation (see Section 6.3). The solid Celuka skin, which is formed by quenching of the melt against the chilled calibrator surface, can be seen on the right hand side of this micrograph. Also to the left of the centre of the picture is a vertical line corresponding to the join between the two melt streams that have been separated by the mandrel in the die. This line lies midway through the thickness of the profile and corresponds to the dip in density shown at the centre of Figure 6.5. It is very important that the two melt streams become properly fused: otherwise a plane of weakness will exist at the centre of the profile.

Figure 6.4 Schematic diagram of Celuka method Redrawn with permission from G. Beckmann, Proceedings of the PRI PVC ’87 Conference, Brighton, UK, 1987, Paper No.13, Figure 2. Copyright 1987, Institute of Materials

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Figure 6.5 Density distribution across thickness of Celuka product Redrawn with permission from G. Beckmann, Proceedings of the PRI PVC ’87 Conference, Brighton, UK, 1987, Paper No.13, Figure 4. Copyright 1987, Institute of Materials

Figure 6.6 Scanning electron micrograph of a cross section through a Celuka profile

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Handbook of Polymer Foams The advantages of the Celuka process are that the solid skin gives a product with a harder surface, better appearance, improved impact properties and higher stiffness. Also this process can be used for making profiles with complex shapes, which would be very difficult by the free-foam method. Another important process, which is now widely used, is co-extrusion of a surface layer of solid PVC over the foamed product. This can be used to produce harder, glossier surfaces or different colours. It also has the advantage that unpigmented and/or recycled material can be used in the foamed core. The co-extruder, which can be single or twinscrew, feeds material to form the compact outer skin to an outer die with the same shape as the desired profile [5]. Co-extrusion can be used in conjunction with either Celuka or free-foam processes. An example of a profile produced by co-extruding a skin over a Celuka foam profile is illustrated schematically in Figure 6.7. This is co-extruded foam cladding with a skin covering the top surface. Obviously the co-extruded skin is only required on the surface exposed when the product is in use. Figure 6.8 is a scanning electron micrograph showing a cross-section through co-extruded foam profile, which has an overall density of 500 kg/m3. In the production of foam core pipe, co-extrusion is used to produce the solid skin on both exterior and interior surfaces [8, 19]. Either one or two co-extruders may be used [20, 21]. When only one co-extruder is used, the gelled skin material is fed into the die where it is divided into two concentric layers between which the core material is supplied [21, 22]. Other systems involve the use of two co-extruders to feed inner and outer skins [21]. Proponents of the latter, while acknowledging the additional capital outlay required, claim that there is better control over outer and inner skin thicknesses and improved weld line strength between the layers [20, 21]. No special downstream equipment is

Figure 6.7 Schematic diagram of co-extruded foam cladding

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Figure 6.8 Scanning electron micrograph of a section through a co-extruded foam profile

required, but because foam core pipe is a better thermal insulator than solid PVC (and is therefore harder to cool), it may be necessary to install longer water tanks than those normally used for cooling solid PVC pipes.

6.2.3 Effect of Processing Conditions The choice of processing conditions plays a critical role in determining foam morphology and density - and hence the physical and mechanical properties of the product. The heat and shear imparted to the material will determine the rate of decomposition of the blowing agent, melt viscosity of the polymer and the degree of gelation of the polymer. Kim and co-workers [23] have shown that foam density generally decreases with increasing extrusion temperature up to a certain optimum temperature and thereafter begins to rise. Similar observations have been made by Rabinovitch and co-workers [24]. The explanation is that at low temperatures the foaming process is inefficient because of high polymer melt viscosity coupled with low gas pressure. Foaming efficiency improves with increasing temperature but above the optimum temperature range density begins to rise because too low a melt viscosity causes cell rupture and collapse.

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Handbook of Polymer Foams The effect of volumetric flow rate has also been studied by Kim and co-workers [23], who found that foam density decreased with increasing extrusion rate. This was attributed to better mixing and reduced viscosity. Thomas and co-workers [25] have also investigated how foam density is affected by extrusion screw speed. They reported that foam density decreased by 24% as the screw speed was increased from 6 to 14 rpm and thereafter remained constant (see Figure 6.9). At the highest screw speed investigated (18 rpm) there was evidence of cell collapse and so an intermediate screw speed was recommended. Rabinovitch and co-workers [24] also found that for very high screw speeds there is a substantial increase in foam density due to cell collapse and attributed this to shear heating. Brenis [26] investigated a number of processing variables and concluded that the most influential in controlling foam density were the front cylinder zone temperatures, screw temperature and screw speed. Other variables such as adapter, flange and die temperatures had less effect on density but were more influential over surface finish: running with these temperatures lower produced a smoother surface skin. Work by Thomas and coworkers [25] has also confirmed that the front barrel zone temperatures have the greatest effect on foam density.

Figure 6.9 Effect of screw speed on foam density [25]

6.3 Foam Formulation Technology As in all PVC applications, the formulations for the production of foamed PVC are a complex mixture of thermal stabilisers, lubricants, processing aid, pigment and filler, in addition to the blowing agents required to produce the cellular structure. The type of

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Rigid PVC Foam PVC and formulation ingredients control the thermal stability, gelation behaviour, powder flow characteristics, melt viscosity and melt strength of the material during the production process. Hence all the components of the formulation will influence the subsequent foam density, surface finish and overall properties of the foam [3] and there may be interactions between individual ingredients.

6.3.1 Blowing Agents

6.3.1.1 Chemical Blowing Agents There are many criteria for the choice of a suitable chemical blowing agent [27]. Obviously it is essential that decomposition takes place within the processing temperature range of the polymer. In the case of rigid PVC foam, this processing window is between 160 °C and 190 °C. The blowing agent should not decompose too quickly, but should have a high gas yield and be economical to use. It requires good long-term storage stability and ease of incorporation and dispersion in the polymer. It should have the ability to produce a fine, even cell structure and a stable foam that is not prone to collapse. The decomposition products should be compatible with the polymer and not have an adverse effect on its processing characteristics or thermal stability. Furthermore, both the blowing agent and its decomposition products should be non-toxic and ‘environmentally friendly’. Unfortunately there is no perfect candidate that can meet all these requirements. The blowing agents most commonly used in the production of rigid PVC foam are azodicarbonamide (ADC) and sodium bicarbonate (SBC), and they are often used together because of synergism. ADC is widely used in the plastics industry, particularly in the production of PVC plastisol foams. It has the structure H2N-CO-N=N-CO-NH2 and decomposes exothermically at about 215 °C to give a gas yield of 220 cm3 [5]. The major gaseous decomposition product is nitrogen, which constitutes about 60% by volume of the gases evolved. Other gaseous products are CO2, CO and NH3. The relative amounts of these depends on the decomposition mechanism of ADC, which varies as a function of environmental factors, such as pH [27, 28, 29]. Although the decomposition temperature of ADC lies outside the processing window of rigid PVC foam, there are numerous activators or kickers [30] that can lower the decomposition temperature into the required range. Some examples of these are zinc and lead salts, which are also added as thermal stabilisers for PVC [29, 31]. A blowing agent that is sometimes mixed with ADC to act as a kicker is oxybisbenzenesulfonyl hydrazide (OBSH) [32]. It decomposes in the temperature range 150 to 160 °C and evolves nitrogen and water vapour. The extent to which the decomposition temperature of ADC is reduced depends on the ratio of ADC to OBSH in

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Handbook of Polymer Foams the mixture. Work is still underway to find new kickers for ADC and thereby obtain better control of the foaming process [33]. One of the main advantages of using ADC is that it is a very efficient blowing agent. This is illustrated in Figure 6.10 [34], which shows how the foam density of an extruded freefoam strip varies with the concentration of ADC in the formulation. There is a sharp decrease in density from 1400 to 500 kg/m3 as the ADC level is increased from zero to about 0.5 parts per hundred parts of PVC (phr). The density then stays constant and eventually begins to rise as the ADC concentration is further increased. The explanation for this phenomenon is that with increasing levels of blowing agent above an optimum amount the cells begin to collapse and coalesce [3, 35]. In summary, ADC is a highly efficient exothermic blowing agent, which gives a high rate of gas expansion and a fine, uniform cell structure [34, 36] - as illustrated in Figure 6.3. Its disadvantages are that it can cause cell collapse if used in too high a concentration and may give yellowish foams. Also it is classed as an irritant and may cause allergic respiratory reactions. SBC is a well-known blowing agent because of its use in baking powder. Its decomposition is endothermic and reversible and takes place over a wide temperature range, which fortunately coincides with the processing window for rigid PVC foam. The decomposition

Figure 6.10 Effect of ADC concentration on foam density [34]

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Rigid PVC Foam products are CO2, H2O and Na2CO3 and the gas yield is 125 cm3. Compared with ADC, its decomposition has been described as slow and erratic [36]. SBC is a less efficient blowing agent than ADC. The effect of increasing levels of SBC on the density of extruded free-foam strip was investigated [34] and the results are shown in Figure 6.11. There is an almost linear relationship between increasing SBC concentration and decreasing foam density: cell collapse was not detected in these experiments. To produce a foam with a density of 500 kg/m3 it was necessary to use 2 phr of SBC, compared with 0.5 phr of ADC (Figure 6.10). Also the foams produced using SBC had a coarse, irregular cell structure [34, 37]. In summary, SBC decomposes over a wide temperature range, the reaction does not go to completion and the gas yield and pressure are relatively low compared with those of ADC. The cell structure tends to be coarse and irregular, which can lead to inferior impact properties (see Section 6.4). However, previous problems of variable gas yields and poor dispersion, caused by the hygroscopic nature of SBC, have largely been overcome with current products, which are coated with oil or wax [30]. The advantages of SBC are that it is easier to handle than ADC, produces a whiter foam and is less prone to give problems with cell collapse.

Figure 6.11 Effect of SBC concentration on foam density [34]

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Handbook of Polymer Foams Bearing in mind the complementary properties of the two blowing agents, it is not surprising to find synergistic effects when they are used together. Combined use of ADC and SBC has been reported to give better control of extrudate density, product colour and processability [36]. Overall it is possible to obtain a lower density foam which is less prone to cell collapse. This is illustrated in Figure 6.12, which shows a 3-D plot of foam density as a function of the levels of ADC and SBC in the formulation. It can be seen that the lowest density foams are produced using a combination of the two blowing agents. Another advantage of using a combination of ADC and SBC is that a finer cell structure is achieved than can be obtained with SBC alone [38]. It seems that nitrogen produced from the decomposition of ADC readily nucleates bubbles and these act as nucleation sites for CO2 bubbles from the decomposition of SBC. The propensity for nitrogen bubbles to form more readily than carbon dioxide may be explained by the fact that the solubility of nitrogen in PVC is 20 times lower than that of carbon dioxide [39]. The optimum ratio of ADC:SBC depends on the application and the processing method used [40]. For rigid PVC profiles, which are generally produced by the Celuka process, a high level of SBC is used with a low level of ADC, which acts as a ‘nucleating agent’ to ensure a reasonably fine cell structure. Foam sheet is largely produced by the free-foam process and here it is found that formulations with a high concentration of SBC give a poor surface finish. Hence the best systems for this application are mixtures of exothermic blowing agents either without SBC or with a low level. Similarly for foam core pipe the blowing agent is mainly exothermic with a small amount of SBC, if any.

Figure 6.12 Effect of combined use of ADC and SBC on foam density [34]

136

Rigid PVC Foam

6.3.1.2 Physical Blowing Agents Physical blowing agents are rarely used in the extrusion of foamed PVC. However, the question arises as to why not use direct gassing of carbon dioxide and nitrogen to replace chemical blowing agents: these gases are said to be a factor of ten cheaper when used from cylinders than when obtained from CBA [15]. In fact there was a widespread attempt to use CO2 in thermoplastic foam extrusion [41], but it is difficult to get the same product quality as achieved with other blowing agents both in terms of surface finish and foam consistency. There are problems in delivering the gas to the extruder and sealing the extruder to prevent gas leaking back through the hopper. Nitrogen gas is also used as a blowing agent, but to a lesser extent than carbon dioxide: it is said to be more difficult to work with than CO2. However, in the longer term, the use of inert gases may be the way forward for physical blowing agents in foam extrusion. Gale [15] has described experiments using carbon dioxide, nitrogen and argon as blowing agents for foam extrusion. Carbon dioxide was found to give the best results and was used in further trials of both commodity and engineering polymers [42]. A standard extruder was used with retrofitted parts, including a cavity transfer mixer to mix the CO2 as a supercritical fluid into the polymer melt. The experiments were successful and it was concluded that the commercialisation of direct gas injection of CO2 may depend on capital costs and plant responsibilities for handling high pressure gas as much as any other technical issue. In terms of PVC extrusion, Dey and co-workers [43] have reported a novel method of extruding high density, rigid PVC foam using a commercial PVC compound with inert gas PBA (CO2 or argon). The process was developed on a segmented single screw extruder with L/D ratio of 40. Gas was injected into the barrel through a nozzle in a Dynisco type port. An electronic gas-pressure controller was used to regulate the gas injection pressure, and the mass flow rate of the blowing agent was monitored with a Matheson gas flow meter. Using this set-up they were able to produce free foam rods with densities of the same order as currently produced by conventional technology using chemical blowing agents. It was difficult to get a fine foam structure with CO2 but there was an improvement when talc was added as a nucleating agent. Direct gas injection of CO2 and N2 is also being used in the emerging technology of microcellular foam extrusion [44, 45] - see Section 6.5.2.

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Handbook of Polymer Foams

6.3.2 Processing Aids Processing aids, based on acrylic copolymers of relatively high molecular weight, are widely used in PVC extrusion processes to promote fusion, enhance surface finish, and increase melt extensibility and strength [46, 47]. In formulations for rigid PVC foam there is no doubt that processing aid is a key ingredient [10, 48, 49]: the improvement conferred in both melt elasticity and strength prevents collapse of the cellular structure. The importance of the molecular weight of the processing aid was first demonstrated by the work of Ide and Okano [50]. They showed that the tendency for foam density to increase at high concentrations of blowing agent was arrested if a high molecular weight processing aid was used. With increasing molecular weight, it was possible to achieve progressively lower foam densities before cell collapse took place. This result was confirmed by Pfennig and Ross [35]. Choice of higher molecular weight processing aid also gives foams with improved surface appearance [47]. The effect of processing aid concentration on foam density has been examined by Szamborski and Pfennig [10]. They found that for both tin and lead stabilised PVC foam sheet formulations, increasing the concentration of processing aid up to 8 phr gave significant density reductions. Surface finish was also improved. The influences of both concentration and molecular weight of processing aid on the density of extruded free-foam strip are shown in Figure 6.13 [51]. It can be seen that for all processing aids there is a reduction in foam density as the addition level is increased from 4 to 8 phr, although the effect is less evident at high molecular weights. The corollary is that a desired improvement in melt strength may be achieved either by use of high molecular weight or by high level of processing aid. Measurements carried out using an elongational rheometer have confirmed that melt extensibility and rupture stress are increased with increasing addition level and/or molecular weight of processing aid [51]. The important relationship between processing aid and blowing agent, established by the work of Ide and Okano [50] and Pfennig and Ross [35], is a key factor in the formulation technology of PVC foam. It is important to get the right balance between these ingredients to avoid cell collapse on the one hand and unnecessary use of too much processing aid on the other. This relationship is illustrated in the 2-D contour plot of Figure 6.14 [34], which shows foam density as a function of the level of both blowing agent and processing aid. At processing aid levels of 3 phr and below, the lines of constant foam density run parallel to the blowing agent axis. Hence, increasing blowing agent has no effect on foam density because cell collapse takes place. However, in the bottom right

138

Rigid PVC Foam

Figure 6.13 Effect of processing aid molecular weight and level on foam density [51]

Figure 6.14 Two dimensional contour plot showing effect of concentration of blowing agent and processing aid on foam density [34]

139

Handbook of Polymer Foams hand corner of the plot the lines of constant foam density run parallel to the processing aid axis, showing that density reduction cannot be achieved with high levels of processing aid if there is insufficient blowing agent.

6.3.3 Type of PVC The molecular weight of PVC is usually expressed in terms of K-value, which is a measure of the solution viscosity of the polymer. This is the most important parameter determining the suitability of PVC for foam extrusion. Too low a K-value gives a low melt strength, so that the foam will easily rupture and/or collapse. Too high a K-value gives a highly viscous melt, so that bubbles are unable to expand fully. In both cases the result is a high density foam. Patterson and Szamborski [3] have examined the effect of PVC K-value on foam density. They compared resins of K-values 51, 57, 60, 66 and 68, and found that the lowest foam densities were produced with K57 and K60 resins. It is generally recognised that the best PVC for rigid foam is suspension or mass polymer with a K-value in the range K56 - K62. Higher K-value resins (in the range K65 to K68) are sometimes used for the production of foam core pipe [8]. In this case, melt extensibility is improved by the use of processing aid.

6.3.4 Stabilisers Stabilisers perform a dual role in PVC foam formulations: they prevent thermal degradation of the polymer and function as activators for the decomposition of ADC [29, 30, 31]. Lead and zinc stearates are particularly effective in the latter respect. Lead stabilisers are widely used in foam formulations in Western Europe for both profiles and pipes. They are relatively cheap, have a lubricating effect and give a wide processing window. Calcium/zinc formulations have a small share of the foam profile market. They offer advantages over lead because of their lower toxicity, but they are more expensive and provide a smaller processing window. Organotin stabilisers can be divided into two types: mercaptides and carboxylates. The majority of tin stabilisers used are mercaptides. They are excellent heat stabilisers but give poor weatherability and do not have a kicking action on ADC. Hence formulations based on tin mercaptide, which are used for the majority of non-weathering foam sheet products, require ‘kickers’ for ADC decomposition: otherwise a low gas yield will result [3, 9, 31].

140

Rigid PVC Foam Tin carboxylates are more expensive than tin mercaptides but offer excellent weathering properties and do act as ‘kickers’ for ADC decomposition [52]. These stabilisers are a good choice for free-foam sheets for outdoor applications, (e.g., advertising boards or garage door panels).

6.3.5 Lubricants Lubricants are essential ingredients in any PVC formulation because of their role in controlling gelation, melt viscosity, flow and surface finish. In the foaming process it is important that the PVC blend gels early enough in the extruder so that gaseous decomposition products from the CBA can dissolve in the melt and are not drawn off in the vent or even lost back up the hopper. Also it is essential to generate enough shear to decompose the blowing agent. However, too much shear heating will cause cell collapse. Hence a balance of internal (compatible) and external (incompatible) lubricants is required [5, 9]. Decker [53] has demonstrated that the type of lubricant not only affects the ease of processing, but also determines the effectiveness of the blowing agent and hence the final density of the product.

6.3.6 Typical Formulations The majority of the rigid PVC foam profile produced in Western Europe is lead-stabilised and a typical formulation is given in Table 6.1 [25]. This formulation serves as a guide: it requires tailoring for the particular extruder and product in question. Levels and/or

Table 6.1 Guideline lead-stabilised formulation for rigid PVC foam profile [25] Ingredient

Parts per hundred parts of polymer (phr)

PVC resin - K57

100

Lead stabiliser

3-4

Lead co-stabiliser and lubricant

0.4-0.6

Lubricants (internal and external)

1.0-2.0

Acrylic processing aid Blowing agent (SBC and ADC)

5-8 1.5-2.5

Titanium dioxide pigment

1-5

Filler (calcium carbonate)

4-8

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Handbook of Polymer Foams type of stabiliser, pigment and filler should be adjusted depending on whether the foam is co-extruded and whether the product is intended for outdoor use. Table 6.2 shows a typical formulation for PVC free-foam sheet [5, 10, 49]. The same considerations as discussed previously apply in terms of adjustment of the formulation to the process and product. In particular, the choice of tin stabiliser as well as the level and type of pigment should be governed by the weathering properties required of the finished product. A typical formulation for foam core pipe is given in Table 6.3 [5, 8]. To reduce costs, the maximum amount of filler and minimum amount of processing aid are used commensurate with the properties required in the final product.

Table 6.2 Guideline tin-stabilised formulation for rigid PVC foam sheet Ingredient PVC - resin (K57 - K60)

Parts per hundred parts of polymer (phr) 100

Tin stabiliser

1.0-2.0

Epoxy compound (co-stabiliser)

1.0-2.0

Lubricants (internal and external)

1.5-2.5

Acrylic processing aid

6-10

Blowing agent (ADC)

0.4-0.8

Titanium dioxide pigment

1-5

Filler (calcium carbonate)

4-8

Table 6.3 Guideline lead-stabilised formulation for PVC foam core pipe Ingredient

Parts per hundred parts of polymer (phr)

PVC - resin (K65 - K68)

10 0

Lead stabiliser and co-stabiliser

2-3

Lubricants (internal and external) Processing aid Blowing agent (ADC) Filler (calcium carbonate)

142

0.8-1.5 3-6 0.5-0.7 5-12

Rigid PVC Foam

6.4 Properties The rapid growth in the market for rigid PVC foam products in recent years has been due to the benefits that these materials offer over both wood and solid PVC. Some obvious advantages of foamed PVC over wood are good weatherability, improved chemical resistance, good fire retardancy and excellent resistance to attack by vermin [4, 5]. Over solid PVC, the advantages of foaming are a lower cost per unit volume of product, higher stiffness for the same weight of material, better acoustic damping properties, improved resistance to wind load, lower thermal conductivity and lower thermal expansion [7, 8, 9]. A comparison of the thermal expansion and thermal conductivity coefficients of PVC foam versus solid PVC are given in Table 6.4, showing the reduction produced on foaming. Also included in Table 6.4 is a comparison of the mechanical properties of foamed and solid unplasticised (PVC-U), together with the test methods used for their measurement. It is found that, in common with all other materials, the mechanical properties of the foamed products are reduced in comparison with their solid counterparts. However, it is clear that the method of foaming influences the properties of the material produced. For example, although the Celuka profile has a lower density than the free-foam board, it has a considerably higher impact strength and hardness, due to the integral skin generated in the Celuka process. The characteristics of a foam that determine its mechanical properties [62] are: foam density (including density distribution through the product thickness), skin integrity and thickness, surface texture, and cell morphology, (i.e., cell size and uniformity). However, according to studies of a large number of different polymer foams over a wide range of densities [63], the single most important factor controlling the mechanical properties of foamed materials is the relative density (Ø), i.e., the ratio of the average foam density to the density of the solid material. Throne [64] has reported results of mechanical properties of a number of thermoplastic structural foams (which did not include PVC) over a relative density range of 1.0 to 0.5. Many properties, such as tensile strength, compressive modulus, shear modulus, fatigue strength and creep strength, were found to obey an empirical square law relationship with relative foam density as expressed by the equation: X = Xo.Ø2

(6.1)

where X is the mechanical property of the foamed product and Xo is the mechanical property of the unfoamed material. Young’s modulus of thermoplastic foams [62] has also been shown to depend on the square of the relative density (Equation 6.1).

143

144

Vicat softening temperature

Modulus of elasticity DIN EN ISO 306 [61]

ISO 178 [60]

1,200 63

°C

50

40

16

17.5

12

MPa

-

DIN 53505 [59]

MPa

Shore D hardness

DIN EN ISO 527-1 and 527-2 [57, 58]

Tensile stress at break

MPa

%

DIN EN ISO 527-1 and 527-2 [57, 58]

Tensile stress at yield

kJ/m2

0.062

0.50 x 10-4

K-1 W/mK

600

Free foam profile 120 x 3 mm kg/m3

Units

Elongation to break

DIN EN ISO 179 [56]

DIN 52612 [55]

VDE 0304

DIN 53420 [54]

Test method

Impact strength @ 23 °C

Thermal conductivity

Coefficient of linear thermal expansion @ 20 °C

Density @ 23 °C

Property

67

1,200

80

30

15

17

25

0.062

0.54 x 10-4

550

Celuka profile 140 x 10 mm skin = 0.5 mm

Values

78

3,050

80

27.5

35

58

no failure

0.16

0.70 x 10-4

1,400

Solid

Table 6.4 Comparison of properties of foamed and solid PVC (Reproduced from [16] with permission)

Foamed Solid PVC

Handbook of Polymer Foams

Rigid PVC Foam Flexural properties [64] were found to fit the expression: X = Xo.Ø3/2

(6.2)

or for samples with carefully controlled skin thicknesses and high foam density the linear law of mixtures gave the best fit, as shown in Equation 6.3. X = Xo.Ø

(6.3)

Data [11, 16, 65] for the mechanical properties of rigid PVC foams do not appear to follow the square law relationship. Tensile strength data more nearly fit a law of mixtures (Equation 6.3), whereas flexural properties gave a better fit to Equation 6.2. Yield stress and Young’s modulus data fall somewhere between the values predicted from Equations 6.2 and 6.3, presumably depending on other morphological features in addition to average foam density. For example, density distribution and skin thickness are particularly important in determining flexural properties, Young’s modulus and creep behaviour [62]. Impact behaviour is an important feature of rigid PVC foams. However, there appears to be no theoretical or empirical relationship to describe adequately the impact characteristics of thermoplastic structural foams. The main factors affecting impact properties are overall thickness, foam density and skin thickness [62]. The solid skin is important to eliminate crack-initiation caused by bubbles acting as stress raisers in the surface. A fine, even cell structure will improve impact properties. With this type of foam morphology, surface texture is minimised and the risk of crack initiation from coarse bubbles at the surface is reduced: where crack propagation does occur, it will be much more localised. Hence the method of fabrication and choice of blowing agent level and type are all key factors in determining the impact strength of the foamed product.

6.5 Novel Processes and Applications

6.5.1 Recycling PVC has been the subject of much environmental debate and scrutiny over recent decades, covering many aspects of its lifecycle. One particular criticism is that PVC cannot be recycled. However, this is not the case: PVC is a thermoplastic polymer and like other thermoplastics it can be reground and reprocessed into new products. One of the advantages of producing co-extruded foam profile or pipe is the opportunity that it provides to use recycled material in the foamed core. The recyclate may be industrial

145

Handbook of Polymer Foams scrap recovered from pipe production [7] or foam production processes, or it may be material recovered after consumer use. There have been a number of initiatives to use post-consumer PVC recyclate in co-extruded foamed products [66]. For example, in 1990 the PVC pipe manufacturers in the Netherlands, under the auspices of the FKS (the Dutch Federation of Plastic Pipe System Manufacturers), addressed the problem of collecting and using waste PVC pipes and fittings [67]. These materials are now used in the co-extrusion of non-pressure sewage pipe. The skin layers are of virgin PVC and the middle layer consists of recycled material from the collected waste. The middle layer may be compact or foamed depending on the end-use of the pipe. Another successful initiative has been the collection of PVC bottles in France for use in the foamed core of three-layer pipe [68]. It has also been demonstrated that recycled PVC packaging is suitable for use as the foamed core in co-extruded cellular profiles [69, 70].

6.5.2 Microcellular Foam Microcellular foams are dealt with in detail in Chapter 10 of this book. Development of this technology with respect to rigid PVC is discussed below. Microcellular foams are a novel type of thermoplastic foam with an average cell size of about 10 μm. Relative densities can vary from 0.1 to 0.95 and the bubble density from 108 to 1011 bubbles per cm3 [71]. The concept of microcellular foams was devised by Suh of the Massachusetts Institute of Technology in the early 1980s [72] with a view to reducing material costs of mass produced plastic items. The idea was that if bubbles with a smaller diameter than the critical flaws already present in the polymer could be introduced in sufficient numbers, it would be possible to reduce the density without sacrificing mechanical properties. These novel materials were believed to have the potential to revolutionise the way that thermoplastics are used. The key feature about microcellular foams is that homogeneous nucleation is required to produce the very large number of small cells. Homogeneous nucleation will occur when there is a large thermodynamic instability due to a sudden drop in gas solubility: this is induced by creating a rapid pressure drop during processing [73]. Commercial production of microcellular foams is now underway. The technology is known as the MuCell process and is available for both injection moulding and extrusion [74, 75]. The gas is delivered to the system in the form of a supercritical fluid. The process consists of essentially three stages: (i)

Production of a polymer/supercritical fluid mixture,

(ii)

Formation of a single phase polymer/supercritical fluid solution, and

146

Rigid PVC Foam (iii)

Inducement of thermodynamic instability to produce homogeneous nucleation and hence a foamed material with a large number of micro-voids.

The MuCell process for PVC [44] has been successfully demonstrated on a variety of different extruders: single screw, counter-rotating twin screw and a tandem arrangement in which a counter-rotating twin screw feeds a single screw extruder. At present all the extrusion applications under development have very low wall thicknesses. High output extrusion processes producing profiles of small cross-sectional area are preferred because under these conditions there is a large pressure drop at the die exit, which is an essential criterion to achieve the very small cell size. Standard PVC formulations can be run [44, 45, 76], although some adjustment to the lubrication is necessary to improve the gloss.

6.5.3 Foamed Composites Linear and crosslinked low density PVC foams are used as core materials in composite sandwich structures [77, 78]. These composites are made for applications such as the hulls of catamarans and door assembly units for recreational vehicles. The structure must be optimised for stiffness, strength, weight and cost. The properties that dominate the choice of the foamed core are shear strength and shear modulus. Compressive strength is also important. Although crosslinked PVC has better mechanical properties than linear PVC for a given density, linear PVC foam has excellent thermoforming properties and is the preferred material for fabrication into complex shapes.

6.6 Summary There is growing demand for rigid PVC foam in both Europe and the USA in the three market sectors of profile, sheet and foam core pipe. Many of the applications for PVC foam profile are for wood-replacement products because of its ease of fabrication and the advantages of a maintenance-free product. Foam core pipe is taking a growing share of the non-pressure pipe market because of its cost effectiveness, higher stiffness for a given weight and facility for the incorporation of recycled material. In order to make a product with the minimum weight for the required stiffness, strength and impact performance, it is necessary to control several aspects of the foam morphology. The most significant properties are foam density and skin thickness, although cell size distribution and surface finish are also influential. The means of controlling these factors are threefold. Firstly, there is the choice of processing method, which can be either free-foam or Celuka with the option of co-extrusion. Secondly, the processing conditions play an important role in determining foam morphology and density. The third element is the formulation.

147

Handbook of Polymer Foams Perhaps more so than for other thermoplastic structural foams, there is a complex formulation technology involved in the production of rigid PVC foam products. Formulations contain a mixture of processing aid, thermal stabilisers, lubricants, pigment and filler in addition to the blowing agents required to produce the foamed structure. The type of PVC and other ingredients in the formulation control the powder flow, gelation behaviour, thermal stability, melt rheology and melt strength of the material during the production process, and have an important effect on the properties of the end-product. The formulation and process technology of rigid PVC foam have been well researched. These materials offer the attraction of providing optimum stiffness and strength for the minimum weight of material. Hence, there is a constant drive towards novel applications for PVC foam and also towards new ways of foaming solid products to improve cost effectiveness.

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Rigid PVC Foam 11. R. Brathun and P. Zingsheim in Handbook of Polymeric Foams and Foam Technology, Eds., D. Klempner and K.C. Frisch, Hanser Publishers, Munich, Germany, 1991, Chapter 10, 243. 12. R.H. Hansen and W.M. Martin, Journal of Polymer Science, Part B, 1965, 3, 325. 13. R.H. Hansen, SPE Journal, 1962, 18, 77. 14. C.D. Han, Y.W. Kim and K.D. Malhotra, Journal of Applied Polymer Science, 1976, 20, 1583. 15. M. Gale, Proceedings of Blowing Agent Systems: Formulations and Processes, Rapra Technology Ltd., Shrewsbury, UK, 1998, Paper No.6. 16. G. Beckmann, Proceedings of the PRI PVC ’87 Conference, Brighton, UK, 1987, Paper No.13. 17. Plastics News International, March 1998, 8. 18. P.E. Boutillier, inventor; Kuhlmann Ets, assignee; FP 1,498,620, 1967. 19. Plastics News International, March 1998, 16. 20. M. O’Neill, Modern Plastics International, 1997, 27, 1, 54. 21. A. Odell, Jr, Proceedings of Vinyl Retec ’91, Applying 90s Technology to Vinyl, Fort Michell, KY, USA, 1991, 113. 22. M. Doucat, inventor; Societe Generale de Canalisations (SOGECAN), assignee; EP 19564, 1980. 23. B.C. Kim, K.U. Kim and S.I. Hong, Polymer (Korea), 1986, 10, 215. 24. E.B. Rabinovitch, J.D. Isner, J.A. Sidor and D.J. Wiedl, Proceedings of SPE ANTEC ’97, Toronto, Canada, 1997, Volume III, 3554. 25. N.L. Thomas, R.P. Eastup and T. Roberts, Plastics and Rubber and Composites Processing and Applications, 1994, 22, 115. 26. K.L. Brenis, Proceedings of the 32nd SPE Annual Technical Conference, ANTEC ’74, San Francisco, CA, USA, 1974, 692.

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Handbook of Polymer Foams 27. H. Hurnik in Plastics Additives Handbook: Stabilisers, Processing Aids, Plasticisers, Fillers, Reinforcements, Colorants for Thermoplastics, Ed., R. Gächter and H. Müller, Hanser Publishers, Munich, Germany, 1985, 619. 28. R.A. Marshall, Journal of Vinyl Technology, 1991, 13, 144. 29. R.J. Shute, Proceedings of SPE RETEC PVC Primer, Atlanta, GA, USA, 1985, 99. 30. K. Collington, Proceedings of Cellular Polymers I, Rapra Technology Ltd., Shrewsbury, UK, 1991, Paper No.22. 31. C.F. Tu, Proceedings of SPE ANTEC ’77, Montreal, Canada, 1977, 211. 32. G.L.A. Sims and C.O’Connor, Proceedings of Blowing Agent Systems: Formulations and Processes, Rapra Technology Ltd., Shrewsbury, UK, 1998, Paper No.2. 33. S. Girois and C.A. Bertelo, Proceedings of SPE Vinyl Retec ‘97: Plastic Systems for the Building Industry, Atlanta, GA, USA, 1997, 107. 34. N.L. Thomas, R.P. Eastup and J.P. Quirk, Plastics, Rubber and Composites: Processing and Applications, 1997, 26, 47. 35. J-L. Pfennig and M. Ross, Proceedings of PVC ‘90, PRI, Brighton, UK, 1990, Paper No.21. 36. K.U. Kim, T.S. Park and B.C. Kim, Journal of Polymer Engineering, 1986, 7, 1. 37. B.C. Kim, K.U. Kim and S.I. Hong, Polymer (Korea), 1986, 10, 143. 38. N.L. Thomas and R.J. Harvey, Journal of Vinyl and Additive Technology, 1999, 5, 63. 39. D.W.V. Krevelen, Properties of Polymers: their Correlation with Chemical Structure; their Numerical Estimation and Prediction from Additive Group Contributors, 3rd Edition, Elsevier, Amsterdam, 1990, Chapter 18. 40. G. Luebke, Proceedings of Blowing Agent Systems: Formulations and Processes, Rapra Technology Ltd., Shrewsbury, UK, 1998, Paper No.11. 41. D. Stover, Plastics World, 1994, 52, 7, 33.

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Rigid PVC Foam 42. M. Gale, British Plastics and Rubber, 2000, May, 4. 43. S.K. Dey, C. Jacob and M. Xanthos, Journal of Vinyl and Additive Technology, 1996, 2, 48. 44. K. Blizard, L. Chen, R. Straff, M. Deweerdt and D. Mullie, Proceedings of AMI Profiles Conference 2000, Düsseldorf, Germany, 2000, Paper No.12. 45. M.R. Holl, M. Ma, V. Kumar and R.R. Kwapisz, Cellular Polymers, 1998, 17, 271. 46. F.N. Cogswell, Pure & Applied Chemistry, 1983, 55, 178. 47. K. Kitai, P. Holsopple and K. Okano, Journal of Vinyl Technology, 1992, 14, 211. 48. J. Patterson in Plastics Additives: An A-Z Reference, Ed., G. Pritchard, Chapman and Hall, London, UK, 1998, 526. 49. P.S. Schipper and D. Tanjala, Proceedings of APE Vinyl Retec ‘97: Plastic Systems for the Building Industry, Atlanta, GA, USA, 1997, 137. 50. F. Ide and K. Okano, Pure & Applied Chemistry, 1981, 53, 489. 51. B. Haworth, L. Chua and N.L. Thomas, Plastics and Rubber and Composites Processing and Applications, 1994, 22, 159. 52. M. Cuilleret, Proceedings of Addcon World ‘98, RAPRA, London, UK, 1998, Paper No.11. 53. R.W. Decker, Journal of Vinyl and Additive Technology, 1996, 2, 121. 54. DIN 53420, Testing of Cellular Materials; Determination of Apparent Density, 1978. 55. DIN 52612, Testing of Thermal Insulating Materials. 56. DIN EN ISO 179, Plastics - Determination of Charpy Impact Strength, 1997. 57. DIN EN ISO 527-1, Plastics - Determination of Tensile Properties - Part 1: General Principles, 1996. 58. DIN EN ISO 527-2, Plastics - Determination of Tensile Properties - Part 2: Test Conditions for Moulding and Extrusion Plastics, 1996.

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Handbook of Polymer Foams 59. DIN 53505, Testing Of Rubber - Shore A and Shore D Hardness Test, 2000. 60. ISO 178, Plastics - Determination of Flexural Properties, 2001. 61. DIN EN ISO 306, Plastics; Thermoplastic Materials; Determination of Vicat Softening Temperature, 1997. 62. P.R. Hornsby, Materials in Engineering, 1982, 3, 443. 63. M.F. Ashby, Metallurgical and Materials Transactions A, 1983, 14A, 1755. 64. J.L. Throne, Plastics Design and Processing, 1976, 16, 20. 65. J. Patterson, Journal of Vinyl and Additive Technology, 1998, 4, 26. 66. K.H. Steinbruck, Proceedings of SPE RETEC ARC ‘97: Information to Grow the Plastics Recycling Industry, Chicago, IL, USA, 1997, 219. 67. R.L.J. Pots and P. Benjamin, Proceedings of Plastic Pipes IX, Edinburgh, UK, 1995, 442. 68. S. Dupont, C. Dehennau, P. Benjamin, B. Rijpkema and G. Voituron, Proceedings of Recycle ’91 Conference, Davos, Switzerland, 1991, Paper No.34. 69. N.L. Thomas and J.P. Quirk, Plastics and Rubber and Composites Processing and Applications, 1995, 25, 89. 70. N.L. Thomas, J.P. Quirk and H. Cretney, Progress in Rubber and Plastics Technology, 1997, 13, 56. 71. V. Kumar, Cellular Polymers, 1993, 12, 207. 72. J.E. Martini, F.A. Waldman and N.P. Suh, Proceedings of SPE ANTEC’ 82, San Francisco, CA, USA, 1982, 28, 674. 73. D.F. Baldwin, C.B. Park and N.P. Suh, Polymer Engineering and Science, 1996, 36, 35. 74. European Plastics News, 1998, 25, 8, 107. 75. European Plastics News, 1999, 26, 9, 27. 76.

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V. Kumar, J.E. Weller, M. Ma, R. Montecillo and R.R. Kwapisz, Cellular Polymers, 1998, 17, 351.

Rigid PVC Foam 77. M. Glaskin, Advanced Composites Engineering, 1990, 5, 5, 16. 78. G. Dohn and R. O’Meara, Reinforced Plastics, 1999, 43, 5, 22.

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7

Flexible PVC Foams Christopher J. Howick

7.1 Introduction Chapter 6 of this handbook gives a detailed description of the formulations and techniques involved in the production of rigid polyvinyl chloride (PVC) foams. As far as the end product is concerned, the only difference between a rigid PVC foam and a flexible one is the presence of a plasticising species in the latter. However, this simple chemical difference hides a major difference in the production technologies available to produce flexible foams. The majority of flexible PVC foams are produced via a plastisol route, that is, the products are spread as a liquid dispersion of a speciality PVC resin in liquid plasticiser and gelled and fused in ovens rather than produced via the rigid foam route of extrusion and injection moulding. Additionally there is a large range of plasticising species available on the open market, each with the potential of altering the foaming characteristics of the formulation. All of these will be discussed in detail in this review.

7.2 Flexible Foam Types and PVC Types

7.2.1 Flexible Foams Based on Suspension PVC In order to differentiate further, it should be stated that flexible foams can be formed from PVC resins produced by two distinct technologies. Suspension PVC resins (S-PVC), as used in rigid foams, can be used to form both rigid and flexible articles. For flexible articles, a plasticiser, usually a liquid (see next Section), is added in the dry blend stage of production and this remains in the final product to give flexibility to the resin. The processing technology of the dry blend is the same as those for rigid systems although due account needs to be given to the lower melt viscosity and reduced fusion temperatures of the plasticised formulation. In principle any rigid foam application can be made flexible by this route although applications for flexible foam via this route are somewhat rare. The products themselves tend to have a low melt strength.

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7.2.2 Flexible Foams Based on Dispersion or Paste Resins A very high proportion of flexible PVC foams are based on dispersion or paste resins. These are materials produced from a wide variety of speciality PVC resins - emulsion, seeded emulsion, microsuspension, mini-emulsion - which have the capability of forming a liquid dispersion or plastisol when mixed with a liquid plasticiser. These resins, compared to their S-PVC counterparts, are very fine particle size powders produced by the spray drying and milling of a PVC latex. Plastisols formed from these resins can be spread, rotationally cast, sprayed or have other substrates dipped into them. The wet plastisol is then gelled by various routes but usually this is done thermally with a conventional oven. In simple terms the incorporation of a blowing agent, usually azodicarbonamide (ADC), during the plastisol mixing stage, will produce a foamed article in the oven. The higher the level of blowing agent used the greater the level of expansion in the final product and consequently the lower the density of the final product. This is the production of chemically foamed flexible PVC but the production of mechanically foamed flexible PVC is also possible. This is formed by the deliberate incorporation of air into the plastisol in the mixing stage and if this ‘wet foam’ can be maintained during the fusion stage a foamed product will result. The production of foam by this route is particularly enabled by dispersion resins since in general they contain residual surfactant which can act to stabilise the wet cells, although the deliberate addition of further surfactant is also commonplace. This process is described in more detail in the application sections (Sections 7.3.1 and 7.3.4).

7.2.3 Chemically Blown Foams from PVC Plastisols: Fundamentals Details of applications for plastisol foams are given in Section 7.3 and references [1-3] of this review but all applications of chemically foamed plastisols are governed by the same general principles. The chemical blowing agent is introduced into the formulation during the plastisol mixing stage along with other ingredients. A typical foamed plastisol formulation will contain the following: • • • • • • • •

PVC Dispersion resin Mineral filler (chalk) Pigment Liquid plasticiser Blowing agent Stabiliser/kicker (activator) Viscosity depressant Speciality additives

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Flexible PVC Foams Each of these in turn has a pronounced effect on foam formation and are discussed individually below, but first a description of the normal gelation and fusion processes involved in the formation of a plastisol-based PVC foam is given.

7.2.3.1 Plastisol Foam Formation The formation, gelation and foaming of a plastisol can be divided into a number of key steps: •

Plastisol Formation

The PVC resin and other additives (including chemical blowing agent) are mixed in a planetary or impellor type mixer to form an even dispersion of all the solid ingredients in the liquid phase. For low viscosity pastes it may be necessary to instigate a two fold mix cycle with a reduced amount of liquids in the first cycle to ensure a higher viscosity mix to give adequate dispersion of solid particles in the mix. This is particularly important when the blowing agent in use is in solid form since uneven dispersion of blowing agent will result in uneven expansion in the foam. Chemically foamable plastisols are then usually evacuated to remove excess air. The plastisol may be stored or used immediately. •

Coating

The plastisol is coated onto a substrate or poured into a mould. By far the most common means of application is onto a substrate (for floorings, wallcoverings, coated fabrics, synthetic leathers). Application of the plastisol to the substrate is made by using reverse roll, knife coating or screen coating, each of which place a different viscosity and rheology requirement on the plastisol. A good review of these techniques in the wallcovering sector is given by Niven [4] and the general rheology of PVC plastisols by Sarvetnick [5]. •

Semi-gelation

The coated substrate may then be semi-gelled if subsequent coats and print designs are to be applied. This involves heating of the plastisol to above the glass transition temperature of the PVC polymer (70-85 °C) so that the polymer chains become fully mobile and the plasticiser and other liquid additives can be absorbed into the polymer. This results in a rapid rise in viscosity as the plastisol solidifies and for a typical midmolecular weight resin this state continues until the temperature reaches approximately 140 °C. This is a typical semi-gel temperature although the range varies from 125 °C to 160 °C depending on both polymer and formulation. The state of the semi-gel is a solid

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Handbook of Polymer Foams but as complete fusion of the polymer and all the additives has not yet occurred the material has little strength and flexibility. However if the semi-gel has been produced by passing the plastisol around a heated drum, as is frequently the case, the semi-gel has a very smooth surface which is ideal for printing of designs or the laying down of further plastisol coatings. One key point for semi-gelled plastisol foams is that the semi-gel temperature must be below both the true decomposition temperature and the catalysed decomposition temperature of the blowing agent to prevent foaming and surface texturing of the semi-gel. •

Gelation, Fusion and Expansion

After semi-gel printing and storage the material is then fused by passing through another heating stage. This stage is normally performed using conventional thermal ovens. During fusion the polymer enters the melt and full mixing of formulation ingredients occurs at a molecular level. The result of the melting of the polymer is for a reduction in viscosity to take place. The overall viscosity versus temperature profile for a typical plastisol in shown in Figure 7.1. On cooling from the fused state the product re-establishes the polymer network but with plasticiser molecules intimately mixed. These molecules prevent the establishment of all of the chain - chain interactions that characterise rigid PVC so the flexibility of the final product is greatly enhanced (for detailed descriptions of the plasticisation mechanism see [6-11]).

Figure 7.1 Effect of plasticiser level of plastisol gelation and fusion

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Flexible PVC foam Flexible PVC Foams For the purpose of flexible PVC foams, the key point about the gelation and fusion process is that the temperature of the blowing agent decomposition will be reached and exceeded during this stage. The resulting generation of gas will give rise to the foaming of the melt and this structure will be retained in the final product if cell walls have sufficient strength to survive the thermal treatment after they have formed. The amount of foam expansion depends on a number of factors but in simple terms is governed by the volume of gas generated and the viscosity of the polymer melt at the point of blowing agent decomposition, higher viscosity melts giving a greater level of resistance to the expanding gas. There are many good detailed descriptions of the foaming of PVC plastisols given in the literature [1-3] and the reader is encouraged to read these to gain an indepth understanding.

7.2.3.2 Blowing Agents and Foam Formation A chemical blowing agent is a substance that can be added to a material such that the substance decomposes at a given temperature to release a volume of gas that causes the material to expand. As with rigid foams by far the most common chemical blowing agent is ADC which has a decomposition temperature in the range 200-230 °C. This temperature can be reduced by the incorporation of catalysts or ‘kickers.’ These are typically metal salts or complexes and have chemical features so much in common with the range of metal thermal stabilisers for PVC that the two properties are frequently combined in a ‘stabiliser - kicker’ system. These can be used either separately from the blowing agent or they can be compounded with the blowing agent by the blowing agent manufacturer. Thus in theory it is possible to tailor a blowing agent-kicker combination for an individual formulation and set of processing conditions to give the degree of gas yield and expansion required. In addition to the temperature of blowing agent decomposition the rate of gas generation is also critical and depending on the processing conditions (length of oven, rate of air changes, etc.), a choice of kicker will need to be made. Four basic types are: 1. Low temperature action with rapid gas generation 2. Low temperature action with slow gas generation 3. High temperature action with rapid gas generation 4. High temperature action with slow gas generation and there are also intermediate types available from suppliers. The effect of these various kickers is shown in Figure 7.2. For example a high speed wallcovering formulation based

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Figure 7.2 Effect of different kickers on foam expansion

on a low molecular weight resin would require a low temperature, rapid gas generation type since the low molecular weight of the resin would establish the correct melt viscosity conditions rapidly. In a product which has additional requirements of mechanical strength a higher molecular weight of PVC resin will be specified with the proviso to delay the decomposition of the blowing agent until the plastisol melt viscosity has reduced sufficiently to give the expansion level required. This may result in the need for a high temperature, more controlled gas generation kicker/catalyst type. However raising the molecular weight of the PVC resin can be of limited effect in an attempt to improve the mechanical strength of the foam: greater benefit in mechanical strength can often be obtained by improvements in the structure and uniformity of the foam produced and in many cases this may mean the use of low molecular weight PVC homopolymers.

7.2.3.3 Cell Types Flexible PVC foams are generally described by their cell nature, i.e., closed or open cell. Closed cells tend to give good resistance to compression but if compression occurs they will recover slowly. Open cells, whilst offering very low resistance to compression, will recover quickly from any compressive force. These factors are important where these

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Flexible PVC Foams properties form part of a specification. Generally an open cell structure will result if a large amount of the foam structure has formed before maximum gelation of the plastisol has occurred. When further heat is applied and melt viscosity falls, the pressure within an individual cell increases so that the cell walls break to form an open cell structure. Thus for open cell structure a fast gelling system is desired and this can be made through formulation development and resin choice. Although some statements with regard to preferred resin choice for open and closed cell have been made [1] exceptions to these statements can be found. In addition to the type of cell, i.e., whether open or closed type, the quality of the cell will also need consideration. Cells can range from exceedingly fine to very coarse and this is controlled by a number of factors. Of key importance are the gelation rate, the gas evolution rate of the blowing agents involved and also the surfactant present from the PVC resin manufacturing process. The PVC particles will contain a layer of surfactant on their surface and when the blowing agent decomposes these layers stretch so the surface tension of this surfactant plays an important role in determining the quality of the cell (as defined by structure (open or closed), size and uniformity), as does the solubility of this surfactant in the plasticiser and the ease at which the plastisol can release entrapped air (for chemical foaming formulations) or retain air (for mechanical foam formulations). PVC resin manufacturers will generally develop resin systems for chemical foam formulations of given families of surfactants known to impart good cell structural quality.

7.2.4 PVC Resins used in Plastisol Foam Formation The choice of PVC resin in the chemically foamed formulation has long been a subject which arouses great amounts of discussion and debate. Since these resins are spray dried they tend to have a unique particle size distribution associated with the drying process, which is difficult to match on a different spray drier. Consequently, different resins have different particle size distributions and hence different gelation properties which in turn affect the foam properties. Although end users may deem one resin as an acceptable alternative to another resin, it is rare to find two resins which are indistinguishable. Resins available are made by a variety of routes.

7.2.4.1 Microsuspension Resin Made in a method similar to the suspension resin (in which monomer droplets are suspended in water and then polymerised using a monomer soluble initiator) with the exception that the vinyl chloride:water mixture is mechanically homogenised to produce very fine droplets of monomer in water. These droplets are then stabilised using a surfactant and the system polymerised. This results in a broad particle size PVC latex which is then spray dried and milled.

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7.2.4.2 Emulsion Resins In this case the polymerising medium contains very small droplets of monomer in water which are formed by the use of surfactants. The polymerisation is carried out using water soluble initiators and the resulting PVC latex contains very small PVC particles with a very narrow particle size distribution. This latex can then be spray dried and milled. A number of variants of the process are also available such as seeded emulsion, continuous emulsion and mini-emulsion. The last of these shows similarities to microsuspension polymers in that a broad range of monomer droplet sizes are formed through the use of surfactants. Emulsion and seeded emulsion polymers may be differentiated from mini-emulsion polymers in that for the latter a homogenised mix of vinyl chloride monomer and water is produced prior to polymerisation through the use of surfactants. This combined surfactant system (where surfactants act to both homogenise the monomer:water mixture and to take part in polymerisation) generally leads to polymers possessing relatively high residual surfactant levels and these can play a part in the foaming process since they can lead to a differing surface tension of the expanding cell walls during the formation of the foam. In general terms such polymers exhibit faster gelation properties and a tendency to open cell formation, a property that can be enhanced by careful selection of blowing agents and kickers. The drying and milling phases of the processes - common to all of the polymerisation technologies listed above - play a critical part in the foaming performance of the grade of polymer. The spray drying process forms agglomerates of the primary particles present in the PVC latex. These can range from a few microns in size up to about 50 μm. These agglomerates can be broken down by milling although it is rare for large proportions of them to be broken down entirely. As a result, the final polymer as supplied to the foam processor contains a range of particles from sub-micron primaries to persistent large agglomerates up to 50 μm. The proportion of agglomerates to primaries is set by the precise drying conditions and the severity of the milling used. Essentially the smaller particles melt more quickly on the application of heat which then results in the plastisol entering the true melt phase more rapidly.

7.2.5 Mineral Fillers Fillers such as chalk are used in a formulation for cost reasons since they are normally available at significantly lower costs than the other primary formulation ingredients. The level used is a balance between the cost advantages and the potentially detrimental effects on foaming and viscosity of the plastisol. High filler levels will lead to an increased melt viscosity which will in turn provide more resistance to the evolved gas from the blowing agent decomposition. In turn this will lead to reduced foam expansion. High

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Flexible PVC Foams filler levels will also result in higher densities of the foam (filler density is significantly higher than other formulation ingredients) and result in a deterioration in mechanical properties. At high levels, filler will also have an effect on the cell structure.

7.2.6 Pigments These have a similar effect to fillers but, owing to their significantly higher price and the fact that they have the potential to cause increased plastisol viscosity, their concentration in the final formulation is significantly lower than that of a filler and in normal cases their effect on foaming is negligible.

7.2.7 Liquid Plasticiser This has a pronounced effect on the foaming process, both in terms of type and levels used. Since foaming properties are dependent on the melt viscosity of the plastisol, plasticisers play an important part in the foaming process since they can, by nature of their polarity and concentration, affect the melt viscosity obtained during the fusion process. It needs to be stressed that a conventional viscosity versus temperature plot as recorded using dynamic temperature viscometry, records the change in viscosity with temperature over a period of up to 30 minutes whereas conventional plastisol fusion ovens subject the plastisol to a high temperature, e.g., 200 °C, for a short time period (1-2 minutes). Thus, the time taken for a plastisol to reach a given melt viscosity will be controlled by the fusion characteristics of the formulation. In general, faster fusing formulations will give faster expansion owing to their ability to reach lower melt viscosities during the plastisol fusion stage. In terms of plasticiser types, the effects are summarised next: •

‘Active’ plasticisers: these produce fast fusing systems and are generally based on polar, relatively low molecular weight plasticiser esters such as butylbenzyl phthalate and benzoates. Whilst these produce fast fusion and relatively fast expansion it should be noted that the higher polarity can cause relatively rapid rates of plastisol viscosity increase with time and may lead to undesirable migration and exudation properties in the final product.

•

Dialkyl phthalate esters: these include the industry standards di-2-ethylhexyl phthalate, di-isononyl phthalate and di-isodecyl phthalate (DIDP). Such esters also includes diisoheptyl phthalate.

The effect of these plasticiser types on the foam properties are given in Table 7.1.

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Table 7.1 Effect of plasticiser type on fusion and foaming characteristics Phthalate ester

No of carbon atoms in alkyl chain

Fusion and foam characteristics

4 and 6

Very fast

Dipropylene glycoldibenzoate

3

Very fast

Di-isoheptyl phthalate

7

Fast

Di-2-ethylhexyl phthalate

8

Medium

Di-isononyl phthalate

9

Medium

Di-isodecyl phthalate

10

Slow

Linear 9,11 phthalate

9 and 11

Slow

Di-undecyl phthalate

11

Slow

Di-isotridecyl phthalate

13

Very slow

Butylbenzyl phthalate

The plots shown in Figures 7.1 and 7.3 record the changes in viscosity of the plastisol with temperature and this can be used to find the optimum range of melt viscosity for a given blowing agent system. The key parameter is the precise value for the viscosity of the plastisol melt at the temperature of decomposition of the blowing agent. Figure 7.1 shows the effect of increasing the level of plasticiser on the gelation characteristics with progressive additions resulting in progressive reductions in melt viscosity which in turn will produce greater foam expansion when the blowing agent decomposes. Similar effects can be obtained from increasing mineral fillers (increasing melt viscosity) and decreasing the molecular weight of the resin (K Value) which leads to significant reductions in melt viscosity and therefore greater foam expansion during blowing agent decomposition. Figure 7.3 shows the effect of different plasticiser type on the gelation characteristics with the very active butyl benzyl phthalate giving rapid gelation through to the relatively slow progress of the C10 phthalate ester DIDP. Also shown here is the plot of Tan delta with temperature. This is the ratio of the loss (or viscous) modulus to the storage (or elastic) modulus of the plastisol. Since a PVC plastisol is neither purely viscous (dissipating all energy as heat during flow and not recovering to any degree from the deformation), nor purely elastic (storing all energy input and recovering completely from the applied deformation) this value gives a measure of the change in state of the plastisol as the temperature is raised. In turn this gives a measure of the elasticity of the plastisol at a given temperature as the plastisol is heated and this can give an insight into its ability to

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Figure 7.3 Effect of plasticiser type on plastisol gelation and fusion

be used satisfactorily in certain processes in which the material may be subject to deformation at various temperatures. Figure 7.3 shows the expected increase in Tan delta (increasing viscous behaviour) as the plastisol enters the melt.

7.2.8 Blowing Agent Type and Level Naturally this is a critical feature of foam formation. Whilst the level of foam increases proportionately with the level of blowing agent, higher levels of blowing agent may lead to a disruption in the texture of the foam surface. Birch [12] showed the relationship between blowing agent particle size and speed of expansion: as expected a finer particle size of the blowing agent results in a more rapid decomposition that a corresponding large particle size blowing agent. ADC continues to be the standard for the flexible PVC industry since it is relatively easily activated with foam catalysts (or ‘kickers, see Section 7.2.3.2) so that the decomposition occurs in the melt range for the majority of PVC resins. Where lower plastisol gelation temperatures are used other lower temperature blowing agents such as 4,4´-oxy bis(benzenesulfohydrazide) can be used. Additionally, inorganic hydrogen carbonates can also be used. These are seldom used alone but may be used to augment ADC and in some cases can produce desirable surface effects in fashion goods. There has recently been a move towards very matt finishes for foam surfaces as a desired fashion effect and producers have faced a technical challenge in ensuring that technology and additives available to provide surface disruption on a microscopic level have not led to a deterioration in foam quality.

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7.3 Products Utilising Foamed Plastisols A number of industries utilise flexible PVC foam technology. The principal ones are: 1. Floorings and carpet backings 2. Wallcoverings 3. Synthetic Leather 4. General Foams

7.3.1 Floorings and Carpet Backings Whilst these can be made through a calendered route the foamed products are generally made through a plastisol route. The spread (plastisol) flooring market can be divided into two distinct sectors - domestic and contract. The former places a major emphasis on design and new designs both instigate and follow fashion trends within the sector. There are regular new product launches and the foam can play an important part in the achievement of these design requirements. A typical cushion vinyl flooring consists of four layers (see Figure 7.4). At the top is a wear layer of clear PVC which provides the necessary wear and mechanical resistance. Beneath this there is generally a printed foam layer which utilises chemical inhibition. Both of these layers are spread onto a fibreglass layer impregnated with a solid PVC plastisol. The whole assembly can then be turned and have a backing layer applied and the whole flooring can then be fully fused during which time expansion of the blowing agent occurs.

Figure 7.4 Typical cushion vinyl flooring: 1 = Impregnated fibreglass; 2 = printable foam layer; 3 = clear wear layer and 4 = foamed backing layer (chemical or mechanical)

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Flexible PVC Foams The aim of the process of chemical inhibition is to nullify the effect of the blowing agent. The printable foam coating is spread onto the solidified coated fibreglass and this new layer is then gelled at a temperature below the decomposition temperature of the blowing agent. This stage is performed frequently using a heated metal drum. This solid product is then printed with inks, some or all of which can contain a chemical inhibitor, which either interacts with the blowing agent to prevent its decomposition, (e.g., thiourea), or interacts with the kicker to prevent it from ‘kicking’ the blowing agent (trimellitic anhydride, benztriazole). Whichever route is used the effect is the same - selective prevention of foaming in the layer to give a contour effect, (i.e., areas of foam and areas of no foaming), throughout the flooring. Since these inhibitors may be dispersed or dissolved in selected inks only, this leads to certain colours in the design possessing differential relief to other colours when the remaining PVC layers have been applied and the final product is expanded by fusion - a potentially striking visual effect. An alternative method can involve the inclusion of kickers in the ink so that foaming is generated only in printed areas - again the overall effect is the same. The inks are then dried and other layers applied. The original technology was patented by the Congoleum Corporation [13] although the primary patents have expired and the technique is now used throughout the cushion vinyl flooring industry. Traditional inhibitors were based on trimellitic anhydride although concerns with regard to the toxicity of this material have encouraged the use of benztriazole. More recently technology has moved towards water-based printing systems and derivatised benztriazole and thiourea systems have been studied. Today a number of proprietary inhibition systems are sold. It may be noted that the mechanism of inhibition requires migration of the inhibiting species into the printable vinyl layer. The rate of this migration will depend on both the migration rate of the ink solvent and the thickness of the foam layer. It is possible to dry these inks too rapidly before full penetration has occurred (especially in the case of the generally less polar, water-based systems) and also there is a danger of excessive migration if the unfoamed, semi-finished product is stored for prolonged periods. This is due to the migration of the ink continuing under pressure in the rolled flooring through other layers and into the foamed areas that are not intended to be inhibited. This phenomenon is generally referred to as ‘ghosting’ [4]. The backing layer is frequently foamed, either by a chemical or mechanical means. In this layer foam structure is important since it is this layer which is a major contributor to the ‘cushion’ effect. Contract vinyl flooring possesses many of the features of cushion vinyl flooring. Whilst design and fashion are important, a major consideration is meeting the key technical standards specified by authorities or architects for installation in hotels, schools, hospitals,

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Handbook of Polymer Foams etc. Key areas in which PVC foam has a major contribution are the acoustic properties and the compression and indentation resistance and recovery. The products used in this area have a variety of structures, e.g., printed inhibition type foams, flat printed layers, thick or thin wear layers, compact or highly foamed backings selected to obtain the overall properties required to meet these tests. The foam contributes thickness and a cellular structure, with thick, closed cell foams preferred for sound absorption. For good compression properties, closed cell foams offer better compression resistance although open cell foams perform better on compression recovery. The exact requirements are generally met by a suitable combination of resin type, expansion ratio, fusion conditions and foam thickness. Standards of relevance are EN 424 [14], EN 425 [15] and EN 433 [16]. Acoustic properties of building materials form one of the six essential requirements of the European Construction Products Directive (89/106/EEC) [17]. The production of foamed PVC backings for carpets also takes place although the volumes involved are much lower than that for cushion vinyl floorings. Application technologies are relatively straightforward knife coating technologies and are based on free foaming formulations, (i.e., no inhibition requirement). A range of blowing agents is used. ADC is the most common although lower temperature systems can also be used when either carpet fibres or printing inks used in the carpets are particularly thermally sensitive. Some products in this sector are sold into contract applications and here acoustic and compression recovery is also important although the carpet itself has a major influence on these properties.

7.3.2 Wallcoverings Foamed wallcoverings show many of the foaming technologies described for floorings although the foaming in wallcoverings is almost exclusively through a chemical rather than mechanical means. A variety of foamed wallcoverings are available: 1. Inhibited foamed wallcoverings 2. Free blow foams with flat surface 3. Free blow foams with a textured surface Inhibited foamed wallcoverings are made using the same methodology as described for the inhibition layer of cushion vinyl flooring. In wallcoverings this produces a relief effect and gives the ability for the wallcovering producer to mimic a tile design (hence this particular product being named ‘tiling on a roll’). The technology was again based on the Congoleum patent [13]. This product may also contain a clear PVC coating on top.

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Flexible PVC Foams Relief designs in wallcoverings can also be made by printing foamable plastisols onto paper in a specific design through screen printing. The versatility of PVC resins available on the market enables all types of rheology to be obtained and this includes the highly pseudoplastic rheology, required for high speed screen printing of intricate designs. With good shear recovery plastisol can be laid down onto paper without flowing out of design so that the plastisol is fused with blowing agent decomposition in specific areas to create the desired design effect. With numerous screen printing stations available, each printing a plastisol with a different formulation (often containing a different level of blowing agent), complex designs are possible with different areas having different levels of expansion. Many of these designs have areas of high expansion but with characteristic flat surfaces - an effect requiring complicated PVC formulating involving resin, plasticiser, blowing agent and kicker development. The surface texture can be altered by precise changes in the melt viscosity profile in order to find the optimum melt viscosity at the temperature at which the blowing agent decomposes. Similar designs can also be produced with a textured surface. In this case the surface of the foam is deliberately disrupted through a combination of ‘pre-blow’, caused by the incorporation of low temperature blowing agents, and by the incorporation of a volatile species - frequently iso-propanol - to allow for boiling off during the foam gelation stage. This textured surface is a traditional design feature for certain markets.

7.3.3 Synthetic Leather This is a large market for plastisol-based PVC resins and the foaming of plastisols is an integral part of the technology. The product itself is made up of a number of layers (see Figure 7.5). A plastisol is applied to a backing substrate such as a silicone release paper and gelled. The foam layer is then applied and gelled below the decomposition temperature of the

Figure 7.5 Schematic of PVC leathercloth; 1 = fabric backing; 2 = foam layer; 3 = solid top coat; 4 = release paper or other substrate

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Handbook of Polymer Foams blowing agent. A lamination plastisol is then applied and finally a fabric (either natural or synthetic) is applied and ‘squeezed’ into the lamination plastisol. The whole assembly is then fused in an oven and the blowing agent