Update on Mouldable Particle Foam Technology

Update on Mouldable Particle Foam Technology

Robin Britton

Smithers Rapra Update

Update on M

ouldable Particle Foam Technology

Robin Britton

Update on Mouldable Particle Foam Technology

Robin Britton

iSmithers – A Smithers Group Company

Shawbury, Shrewsbury, Shropshire, SY4 4NR, United Kingdom Telephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118

http://www.rapra.net

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ISBN: 978-1-84735-406-8

First Published in 2009 by

iSmithersShawbury, Shrewsbury, Shropshire, SY4 4NR, UK

©2009, Smithers Rapra

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

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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.


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Contents

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

2. Expandable Polystyrene (EPS) – A Mature Technology? ....... 7

2.1 Brief Description of the Product and Processes .....................................................7

2.2 Developments in (Non-Halogen) Fire Retardation of EPS ....................................................................... 12

2.3 Developments in Insulation Performance of EPS ....... 16

2.4 Modification of Other Properties of EPS ................... 21

2.4.1 Enhanced Temperature Resistance .............21

2.4.2 Enhanced Cushioning Performance ...........23

2.5 Reduced Levels of EPS Blowing Agents and Alternatives .............................................................. 27

2.5.1 The Need to Reduce Volatile Organic Compound (VOC) Emissions ....................27

2.5.2 ‘Low-pentane’ EPS Developments .............28

2.5.3 Alternative Blowing Agents .......................31

2.6 Developments in Making or Processing EPS Beads ... 36

2.6.1 Extrusion Processes for Bead Making ........36

2.6.2 Foam Nucleation .......................................38

2.6.3 Moulding of EPS .......................................38

2.6.4 Recycling of EPS ........................................39

3. Expanded Polyolefin Moulded Foams (EPE and EPP) – Materials with Growing Application .................................. 51


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Update on Mouldable Particle Foams

3.1 Key Features of Polyolefin Moulded Foams, Compared with EPS .................................................. 51

3.2 Developments in Materials for EPP and EPE Manufacture ............................................................. 55

3.3 Developments in Processing of EPP and EPE ............ 59

3.4 Development of Applications for Expanded Polyolefin Bead Products .......................................... 62

4. ‘Sustainable’ Polymers – The Future? ................................. 69

4.1 Why Sustainable Polymers? ...................................... 69

4.2 Polylactic Acid (PLA) ................................................ 75

4.2.1 Production and Properties of PLA Polymers ............................................75

4.2.2 Processing of PLA-based Beads – A Trend Towards Carbon Dioxide ..........................78

4.2.3 Processing of PLA-based Beads – Interactions Between PLA and Carbon Dioxide ............80

4.2.4 Processing of PLA-based Beads – Review of Recent Patents and Patent Applications .....83

4.2.5 End-of-life Aspects of PLA Foam Products ....................................................89

4.3 Starch and Starch-based Foams ................................ 91

4.3.1 Production and Properties of Starch-based Polymers ....................................................91

4.3.2 Processing of Starch-based Foamable Beads .........................................................92

4.3.3 End-of-life Aspects of Starch-based Foam Products ....................................................95

4.4 Polyhydroxyalkanoates (Including Polyhydroxybutyrate (PHB) and Copolymers) .......... 96

4.4.1 Production and Properties of PHA Polymers ...........................................96


Contents

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4.4.2 Blowing Agents and Processing of PHA Foamable Beads .........................................98

4.4.3 End-of-life Aspects of PHA Foam Products ....................................................99

4.5 Cellulosic and Other Sustainable Polymers ............... 99

5. Concluding Remarks – What Forces Will Drive Development in this Field? ............................................... 107

Abbreviations ........................................................................... 111

Author Index ............................................................................ 113

Subject Index ............................................................................ 125


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vii

Preface

This Update has been written for technologists active in the mouldable particle foam industry, who require an up-to-date account of the status of the field, including areas with which they may not be so familiar. It is also written for those who may have only recently come into the industry, and though acquainted with polymer technology, are not yet fully immersed in the specifics of particle foams.

This Update attempts to place the technology in context, beginning with an account of expandable polystyrene and how its evolution has responded to the demands of the market. This is followed by a description of the expanded polyolefins and their development trajectory, examining the way the differences between polystyrene and the polyolefins have influenced their technology and applications. Thirdly, there is a review of the emerging field of particle foams based on renewable or sustainable polymers. Because these polymers are not as familiar to many technologists, each has a short account of how they are made and of their key properties, before going on to examine their development status and the way their individual natures may affect their market potential. Finally come a few thoughts on the future and the influence of the critical factors we can envisage being important in the way the field continues to develop.

I must first acknowledge the help and advice I have received from my colleagues in the Synbra Group, especially Jan Noordegraaf and Jürgen de Jong at Synbra Technology bv, who have been very helpful in reviewing the text and suggesting improvements. I have to thank all those within this industry who have helped me to explore what, less than ten years ago, was to me an esoteric branch of polymer


viii


knowledge and one I had never expected to understand. I must also acknowledge the assistance and encouragement I have received from the editorial team at Smithers Rapra Technology, particularly Dr. Stuart Fairgrieve. Finally, and most importantly, I am grateful for the continuing support and patience of my wife, Rosemary, who has tolerated my unavailability for more important or interesting activities through much of the winter months, while this book has taken shape.

Robin Britton,

Spring 2009


1

1 Introduction

Particle foams form a significant part of the plastics foam industry, and have their own technologies and markets. While there exist good general reviews of polymer foam markets and technologies [1, 2], these are now a few years out of date and do not go into the detail of particle foams to the depth that a technologist working in that industry would desire. This update is intended to fill that gap and provide an overview of the most important technical developments in this field.

The term ‘mouldable particle foams’ describes an industry which manufactures small beads of thermoplastic polymers such as polystyrene and polyolefins, expands those beads, usually by heating a physical blowing agent (a volatile fluid) dissolved in the beads, and subsequently moulds the expanded beads into shaped products. These products can be as small as drinking cups or as large as blocks for wall insulation or ground stabilisation. The purpose of foaming is to reduce the weight of those products to obtain properties which cannot be achieved in the solid form, such as thermal insulation or impact protection. The particle foam industry is therefore defined by its use of discrete foamed particles to mould into useful shapes. Because such foams have found many and varied applications, their technology has developed to meet the specific requirements of those applications, as this update will show.

The foaming of thermoplastics, particularly polystyrene, has been practised for many years – the first commercial process was patented by Dow Chemical in 1935, and the buoyancy and water resistance of their foamed ‘logs’ much exploited during World War II. Processes


2


were later developed for making sheets of foamed polystyrene, and, by the 1950s, for mouldable expandable beads, the subject of this update. ‘EPS’ (the abbreviation serves equally for expanded as well as expandable polystyrene) has become a familiar product met with at every turn of modern life – and when it has been discarded, or crumbled into small white lightweight beads, a very obvious sign of environmental degradation out of proportion to its quantity.

Surveys of the market for EPS suggest that 2007 saw worldwide consumption of some 5 million tonnes [3, 4], compared with only 35,000 tonnes in 1960 [5]. Growth continues to be strong, especially in Asia, which now produces around half the world total, the majority for protective packaging. The second largest market is in Europe, where construction (principally insulation) takes more than 70%, while NAFTA countries are the major users of EPS for drinking cups. The strength of manufacturing of consumer goods in Asia is clearly the reason for the high proportion used for packaging there (and the low proportion in Europe).

Although the technology of EPS manufacture and moulding can reasonably be described as mature, there remains a steady flow of innovation, and there are increasingly new challenges to its position. Some of those challenges arise as a result of the perceived environmental disfigurement alluded to above, the use of volatile organic compounds in the production of the foam, the presence of brominated compounds in fire-retarded EPS, and inevitably the prospect of rising costs as the primary source of styrene monomer, crude oil, becomes more scarce. The industry is addressing these as well as the inevitable pressures of competitive products in its major markets, and continues to develop new solutions.

Other polymers have also been produced as mouldable beads, principally polyethylene and polypropylene, for applications where EPS is inadequate, and there have also been successful efforts to modify EPS in various ways to improve particular aspects of its performance. There is now increasing interest in more ‘environmentally friendly’ foamed materials, made from ‘sustainable’ or ‘bio’ polymers, and


Introduction

3

in foam products which degrade quickly in the environment, should they be discarded or composted. This interest applies just as much to mouldable bead foams as to sheet products.

The main applications of moulded expandable foams are in protective packaging and in insulation (mainly thermal). As beads can be moulded into relatively convoluted shapes, they can be used to hold and cushion irregularly shaped products within cartons or boxes, with a minimal effect on the package weight. Moulded foams are also used in impact-absorbing situations such as liners for cycle helmets, while expanded, but not moulded, beads are used as loose fill in cushions such as bean bags. There are also markets in food packaging and in horticulture, where the lightness and rigidity of EPS allows delicate products or plants to be handled without risk of damage.

The construction industry uses a great deal of EPS insulation within roof structures, as interlayers within walls and also in floor structures, as well as in very lightweight concrete shuttering systems which can double up as moisture barriers. There has also been increasing use of large EPS blocks to stabilise poor ground for construction of roads, railways, etc. [4]. Loose expanded beads can also be blown into wall cavities to improve their insulation. The long life and water resistance of EPS are important in such applications, where legislation also demands flame retardation.

Other applications exploit the water resistance and low density of foamed polymers, such as in flotation aids for people, boats and even buildings. The sound-deadening ability of polyolefin foams leads to applications in vehicles (such as door liners). A low-volume application is in a ‘lost wax’ process for metal casting, where the desired product shape is first moulded in foam, then embedded in sand. In the casting process, molten metal is poured onto the foam which decomposes and leaves a precise cavity for the metal to fill.

The applications of mouldable bead foams are therefore many and varied. This update reviews the most recent innovations in the field, divided between polystyrene, polyolefins and ‘bio’ polymers. In the


4


course of addressing the issues that are currently driving development, the discussion will highlight the key features that have made EPS so successful, and show how the differences between polystyrene and the other expandable polymers have affected their development (and will certainly shape the development of new expandable bead systems). Chapter 2, therefore, describes how the technology of EPS is continuing to develop in the face of the challenges to its position as the low-cost, large-volume market leader. Chapter 3 considers polyolefin particle foams, competitive in particular niches, but always held back by their relatively higher cost and complexities of processing. Chapter 4 looks to the future for particle foams made from ‘sustainable’ polymers (where the feedstock for the polymers is plant materials rather than crude oil). There are a number of candidate polymers, and their specific features, strong points and weaknesses are reviewed; then the literature on the way the technology for each is developing is summarised. Chapter 5 draws the update together with a few thoughts on how particle foam technology is likely to develop in the near future.

I have mostly drawn on ‘hard copy’ publications such as journal articles and patents, although there are increasingly useful sources of information on company web pages, etc. The difficulty with these is that they are always likely to change, rendering the reference outdated, though the reader is recommended to make use of these sources in addition to the printed archive. Patents present another challenge to the reader – the applicant is under no obligation to make it easy for the researcher either to find the patent in the first place or to work out the optimum embodiment of the invention. As a result, it is quite possible that relevant patents have not come to light, or that important points have been missed, and for any such failings I apologise in advance. I have often referred to US patents because they are in English and the other family members are in other languages, but US patents are often less informative because they tend to have a slighter discussion of the background and prior art than, for example, European filings. The reader is encouraged to review those other patent family members where more detailed insight is needed. To add further words of caution, the existence of a patent does not


Introduction

5

guarantee that the product or process has been commercialised, or is even commercially viable – there are undoubtedly examples in the references of this update.

The references are up to date at the time of writing (early 2009), but, as always, some may already have been superseded – new publications may be on the way, or further patent grants or refusals. It is worth the effort to follow up any point that is of interest and check for new information – the field does not stand still, and this is particularly true of the materials of Chapter 4, where significant progress can be expected in the coming years.

References

1. Handbook of Polymer Foams, Ed., D. Eaves, Rapra Technology, Shawbury, Shrewsbury, UK, 2004.

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

3. W. Glenz, Kunststoffe, 2007, 10, 82.

4. M. Riethues and E. Klement, Kunststoffe, 2005, 10, 74.

5. E. Klement, Kunststoffe, 2004, 10, 80.


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2 Expandable Polystyrene (EPS) – A Mature Technology?

2.1 Brief Description of the Product and Processes

As described in Chapter 1, processes for making EPS beads were first developed in the 1950s, and there are good descriptions of the conventional suspension polymerisation process in a number of texts [1–3] as well as the subsequent processes of pre-expansion and moulding which are used to deliver EPS products. A brief summary will therefore suffice in order to identify the most important aspects which give EPS its main advantages and disadvantages.

Unexpanded EPS beads are typically 0.1–2 mm in size, and the predominant production route is via a suspension polymerisation which yields a range of sizes of spherical beads. These are charged with a volatile organic blowing agent (often a mix of isomers of pentane) in the final stage before dewatering and drying. They are then given an organic coating (this works as an antistatic to prevent agglomeration and also assists in the later processes) and sieved to different size fractions to suit particular applications – smaller beads are used for thinner-walled products – before they are stored in silos, or filled into gas-proof bags or octabins. Each manufacturing site will have its own set of recipes and process settings for the polymerisation, which depend at least partly on the design and size of the reactors and also on the product desired. There is a great deal of (‘secret’) proprietary information about such aspects as suspending agents, bead size control and cell nucleation additives, initiator and chain extender recipes, process times and temperatures, blowing agent blends, bead coatings, etc., although one suspects there has also been some convergent evolution over the years that EPS has been produced.


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More recently, extrusion processes have been developed which yield ‘microbeads’ of uniform size directly from a melt of polystyrene (which may already contain blowing agent) by the use of an underwater micropelletiser. The polystyrene melt may be supplied from a one- or two-stage extrusion line, or even directly from a polymerisation reaction carried out in a series of static mixers and melt pumps. These extrusion processes offer the potential for efficient incorporation of additives which create difficulties in suspension polymerisation, and the uniform size of the beads reduces the potential for waste when only a specific product size fraction is required. For more discussion of the extrusion process, see Section 2.6.1. The subsequent processing of extruded beads is precisely the same as for polymerised beads.

The pre-expansion process involves using steam to heat and agitate the beads and can be either a batch or continuous process, and may be a single stage or be completed over a series of expansions. As the beads are warmed by the steam to above the glass transition temperature of the material, they soften and the blowing agent boils at a large number of nucleation points, forming cells which grow so that the whole bead is foamed throughout. Typically there will be hundreds of cells formed within each bead, sometimes even more. The key variables in prefoaming are the steam pressure (temperature), the amount of dilution air and time – some pre-expanders have a level sensor which stops the expansion when a desired volume has been reached. Following expansion, the beads (now a ‘prepuff’) are dried and discharged and allowed to mature for a period of several hours (often overnight or even longer). This allows them to cool and the cell walls to become rigid, able to support the negative pressure once the residual blowing agent has condensed. As maturing progresses, air diffuses into the beads, and they become stable enough to be processed further. It is well known that for a given bead type a lower final density can be reached in two or more expansion stages than can be achieved in a single steaming. In the case of multi-stage expansion, maturing is required between expansions – usually these expansions are individually less severe than a single-stage process (lower steam


Expandable Polystyrene (EPS) – A Mature Technology?

9

pressure, or shorter steaming time), but ultimately result in a lower foam density.

Moulding is also effected with steam – the prepuff beads are blown into an aluminium mould and steam applied through a number of small vents. This softens the beads and expands them further, using the residual blowing agent which remains in them after the prefoaming and ageing steps, in order to fuse adjacent beads together. Vacuum may be applied to the mould in the later stages, to help create a well-fused surface on the moulding. The distribution of steam between the mould halves can be adjusted to optimise the moulding process, prevent distortion, etc. Cooling follows before the moulding can be ejected and allowed to dry. Moulding cycles are much longer than, for example, those of injection moulding, as the insulating properties of the foam dictate long heating and cooling times, although section thicknesses are also generally much larger than can be achieved in injection mouldings. Tolerances on product dimensions are usually quite wide – 1.5 mm is not unusual – as these are rarely critical in the end application.

The machinery used for processing of EPS has developed to match the properties of the material, and is quite different from that used for more conventional plastics processing. Most moulded products are (relatively) simple in shape and have wall thicknesses of several to tens of millimetres – when a bead of 0.3 mm unexpanded diameter is reduced to a density of 30 kg/m3, it has a diameter around 1 mm, and it is good practice to design for sections which include several beads in order to optimise bead fusion and product strength. Blocks made for insulation purposes can be more than a metre wide and high, and several metres long – smaller blocks and sheets are then cut from them using hot wires or band saws. These differences have tended to separate EPS and other foam moulding processes from the plastics mainstream, but there remains a great deal of polymer science which is applicable across the divide.

Key features of the standard EPS process give the material some key strengths and weaknesses, summarised (not exhaustively) next:


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Strengths

• Polymerisation can be carried out economically in relatively small plants, so that the entry cost into the market is not high – this tends to lead to a diversity of suppliers and strong local competition. A few tens of thousands of tonnes per year is a typical plant capacity (although larger producers have capacities much greater than this) – this becomes a very large volume of product when expanded and moulded. The cost of EPS products tends to be kept low as a result.

• The blowing agent is well retained inside the beads at ambient temperatures (provided the packaging has a gas barrier) so that it is economic to transport beads in unexpanded form to dispersed factories where they can be expanded and moulded to the final shape.

• Steam at relatively low pressures is an ideal medium for expanding and moulding EPS, as its glass transition temperature (Tg) lies close to 100 °C. As the blowing agent plasticises the polystyrene, lowering Tg, this helps to stabilise the expansion process – as the beads grow and the blowing agent is driven off, so the material becomes stiffer and more resistant to temperature. Because steam has a high heat capacity, is easily applied and its temperature controlled (via regulation of its pressure), the process is easy to adjust and to optimise.

• Polystyrene is a stiff glassy polymer at ambient temperatures, so that for applications which do not exceed 70 °C or so, it has predictable strength and dimensional stability. It is also unaffected by water and many environments.

• EPS is easily and widely recycled. Foam products may be granulated back to the primary beads which can then be incorporated into new mouldings with very limited effect on properties, while crushed foam can be remelted (in more energy-intensive processes) and recycled into solid applications.



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Weaknesses

• The blowing agents used are mostly flammable organic solvents, with consequent risks of explosion and fire during manufacture, transport, processing and storage. Emissions of volatile organic compounds are closely associated with EPS production and are increasingly limited by environmental legislation, and the costs of reducing the emissions continue to grow as those limits are tightened. There is, however, no ozone-depletion issue as has been the case with the chlorofluorocarbons (CFC) and hydrochlorofluorocarbons (HCFC) used as blowing agents in other polymeric foam systems.

• Both polystyrene and the blowing agent are flammable, and fire retardation is required in many applications. This is particularly important for construction insulation products. Almost all EPS products for construction include brominated hydrocarbon fire retardants often with peroxide synergists. Brominated fire retardants are coming under increasing scrutiny as potential persistent organic pollutants, and although the compound most commonly used in EPS, hexabromocyclododecane (HBCD), has not as yet been so designated, it is now proposed that it should be considered under the European REACH legislation as a Substance of Very High Concern. This issue is potentially very serious for the EPS construction market, and much work has been done to try to find solutions, without dramatic success to date.

• Because polystyrene is a glassy polymer, EPS has quite poor ductility and toughness. Although a well-designed protective package can offer its contents considerable protection against impact, this is achieved at the expense of damage to the foam structure and subsequent impacts are much less well cushioned.

These strengths and weaknesses will be explored further as we consider recent developments in EPS and in other mouldable polymer foam systems – they affect material choice for particular applications and also the approaches which can be deployed to improve foam performance.


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2.2 Developments in (Non-Halogen) Fire Retardation of EPS

As described in Section 2.1, fire retardation in EPS is almost always achieved using brominated fire retardants and peroxides as a means to pass the standard fire tests – the brominated additive releases hydrogen bromide (a free radical scavenger) to quench the flames, and the peroxide accelerates its release. The polystyrene matrix is also attacked and breaks down, allowing the foam to shrink away from the flame. There has long been interest in alternative fire retardants for EPS, as for polystyrene in general, but the effectiveness and economy of the bromine agents has meant that their position has been hard to challenge. It is likely that only legislation (against the bromine compounds) will ultimately drive the use of alternative fire retardant systems, as the alternatives have their own disadvantages – most significantly cost and the amount of fire retardant required to meet test requirements, which can have effects on the quality of the EPS. A recent review article [4] gives a good picture of the current position and development directions.

There is a wide variety of tests used to assess the fire performance of foams, depending on the application and the country in which they are to be used. For automotive applications, for example, the FMVSS302 horizontal burning test [5] can be used to measure a burning rate, and the user can then specify a maximum value. The ISO 3795 [6] and UL94 [7] horizontal burn tests are frequently used in, for example, specifications for domestic appliance insulation. Where the foam is to be used in railway trains, CEN TS 45545 [8] is required. For construction insulation the position is currently complicated by the slow progress of harmonisation of standards within Europe – each country has its own historically preferred test methods and standards. Classification to EN 13501-1 [9], which employs the ‘single burning item’ test (EN 13823 [10]), or a small-scale ignitability test to EN ISO 11925-2 [11], will soon be required throughout Europe, although at present the DIN 4102-1 [12] test (similar to EN ISO 11925-2 [11]) is widely accepted, and EPS foam products with a B2 rating are generally used in construction applications. Each of these many



13

tests is different from the others to some extent, and correlation of performance between them is usually very difficult. A comprehensive review of tests and requirements can be found elsewhere [13]. For the formulator, therefore, it is essential to know the application and to which fire test the EPS must conform – a good product for one application can be overprotected (and therefore costly) for another. HBCD/peroxide recipes work by a combination of flame quenching and foam shrinkage – other fire retardants work in different ways and often respond differently to the precise conditions of the tests they must meet.

As with solid polystyrene compounds, the fire retardants most examined in the literature are the phosphorus compounds, and a number of patents have been filed in the past decade. One of the first was for a combination of HBCD and a phosphorus compound such as triphenyl phosphate (TPP), filed by Dow Chemical in 1999 [14, 15] – the use of up to 4% of TPP allowed a significant reduction in the level of HBCD, although not its elimination. The most active company in the field has been BASF, with patents covering several different combinations of additives filed over the period 1999–2005 [16-22]. There are also patents from GE Plastics (now Sabic Innovative Plastics) covering compositions which include polyphenylene oxide/ether and phosphorus compounds [23, 24]. Dow Global has recently been awarded a patent for a combination of a phosphorus compound and an epoxy [25] and Kanegafuchi has a Japanese patent for a recipe using sulfur and boron compounds [26]. These will now be considered in more detail, as they illustrate the difficulties in finding a good fire retardant system for EPS.

The four BASF patents all cover the use of phosphorus compounds (always claiming for a list of such compounds, although some are obviously more effective than others) but in combination with other additives to improve the material performance. The first patent [16] describes a combination of expandable graphite with a phosphorus compound. The examples mention 5% of either red phosphorus or dimethylphenylphosphonate (DMPP), with 15% of expandable graphite – red phosphorus is preferred for an extrusion process. The


14


second patent [17], which was filed in 1997 but only granted in 2004, takes a different approach, mixing the phosphorus compound with a water-releasing metal hydroxide (such as magnesium hydroxide or alumina trihydrate) at a combined addition rate of greater than 12%. The best-performing examples have 10% TPP and 5 or 10% magnesium hydroxide. The third patent [18-21], which is actually a family of four patents filed in different jurisdictions, brings another phosphorus compound into play, 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPP), in combination with a peroxide such as dicumyl peroxide or di-tert-butyl peroxide. The ignitability test used to assess the samples is, in contrast to the other patents that use the DIN 4102-1 B1 or B2 rating [12], a very ad hoc horizontal burning test with a very short application of the flame, and it is not clear how well any of these formulations would perform in national or international standard test methods. BASF’s most recent patent [22] for non-halogen compositions combines the first two approaches mentioned above [16-21], claiming for recipes which include a filler (chalk in the examples, but metal hydroxides are also claimed), expandable graphite and a phosphorus compound (red phosphorus, TPP and DOPP are all mentioned in the examples), plus optionally carbon black or laminar graphite to improve the insulation performance (see Section 2.3). Clearly the use of phosphorus compounds is not an easy route to fire retardation – the mechanism by which these agents operate, char formation, is less effective in EPS than the flame quenching and shrinking mechanism of the HBCD/peroxide system, and the amounts of additive used are far higher – in a typical current recipe HBCD is present at less than 1% and the peroxide even less, where all of these patents describe recipes with more than 10% of additive, sometimes much more. Such a volume of ‘dead’ (non-expandable) filler can only be detrimental to expandability, mechanical properties, etc.

One aspect that is not much discussed in the BASF patents, but is relevant to the GE Plastics documents, is the strong tendency of phosphate compounds to lower Tg of the polystyrene, reducing the heat resistance of the foam. To overcome this, two patents [23, 24] describe compositions which combine a phosphate such as resorcinol



15

diphosphate (RDP), TPP, DMPP, etc., with polyphenylene oxide (PPO; GE Plastics’ name for polyphenylene ether). The PPO raises Tg (back towards the 100 °C region of EPS) and also provides oxygen to promote char formation.

Dow Global has pursued a slightly different track with their recently granted patent [25], which employs a modified multifunctional epoxy compound together with a phosphate such as DOPP. This patent is not aimed specifically at EPS as the other are, but at polystyrene applications in general, and mentions extruded foamed sheet (XPS) products but not expandable beads. There is, however, no obvious reason why this system could not be used in bead products.

Kanegafuchi Chemical Industries (also known as Kaneka Corporation) filed an application for a Japanese patent [26] in 2003 describing a combination of sulfur with boron compounds (plus, ‘advantageously’, phosphorus compounds also). The fact that this remains only a Japanese patent filing suggests that the approach did not have the necessary performance/cost-effectiveness to justify wider patenting.

There have recently been patents for a quite different approach to the challenge of improving the fire performance of EPS for construction applications. Rather than incorporate a fire retardant package in the beads, both BASF [27] and Ertecee, part of the Dutch Synbra Group [28], have taken the route of applying a fire-resistant coating to pre-expanded beads of EPS, then moulding the coated beads into slabs or other shapes. The common feature to both is the use of sodium silicate (waterglass), but the coatings are otherwise quite different. BASF’s method bonds the waterglass to the beads with an aqueous dispersion of an acrylate adhesive, which can then be used to fuse the beads together in the moulding step, while Ertecee’s process for Xire reacts the waterglass to form a coating on the beads which becomes insoluble after moulding. In both cases the weight of the coating is much greater than that of the EPS – the moulded products have densities of 80–100 kg/m3 or more, with the beads contributing less than 20 kg/m3 – but the fire performance is in a different league from that of a more conventionally retarded EPS. The panels are more


16


fire-resistant and retain their integrity in a fire situation, rather than shrinking away from the flames with a limited degree of burning.

2.3 Developments in Insulation Performance of EPS

The good insulation performance, low weight, moisture resistance and long-term stability of EPS led quickly to its widespread use as a thermal barrier layer within the walls and floors of buildings, and within roof structures. At present, some 70% of EPS produced in Europe is for such applications. The construction industry has in recent years sought improved performance, either for thinner and/or lighter but equally insulating structures, or for higher insulation levels, and as energy efficiency is becoming an ever more central concern, this demand is only likely to grow. The manufacturers of EPS have responded with a series of innovations. ‘System solutions’ such as composite panels for rapid construction of walls or roofs have become popular, but this account will focus on the developments in the foam materials themselves which have brought about improvements in insulation performance.

The transfer of heat through a panel of foamed EPS is usually measured (to ISO 8301 [29], using apparatus to EN ISO 8990 [30]) at an average of 10 °C, and the coefficient of heat transfer ( is expressed in units of W/m/k. Heat transfer is controlled by four mechanisms – conduction through the material of the cell walls, conduction through the gas in the cells, radiation through the structure and convection within the cells [31]. The convection contribution is very small if the cells are smaller than around 3 mm in diameter, so unless the foam structure is very coarse it can be ignored for practical purposes. The combined conduction contribution reduces with foam density, as the polymer has a higher conductivity than the air in the cells. There could therefore appear to be an advantage (apart from the economic argument for lower densities) in reducing foam density, but, unfortunately, the radiative transfer (at ambient temperatures it is predominantly due to infrared radiation) rises more rapidly. As a result, the curve of against foam



17

density shows a minimum around 25–30 kg/m3. As it is economically desirable to work at rather lower densities, much effort has been devoted to reduce the minimum value of , and to reduce the rate of rise at densities below that minimum.

A number of additives have been found to improve , by increasing the extinction coefficient of the matrix material of the cell walls, and a large number of patents have been filed for the use of such additives, although only a small number have proved to be cost-effective. The most popular are carbon in various forms – graphite, carbon black and activated carbon – all of which absorb infrared radiation. Other effective additives include metal powders and oxides. As these products are aimed at the building insulation market, all have a fire-retardant additive package, usually HBCD with a peroxide synergist (see Section 2.2). There exists a great deal of intellectual property relating to additives to improve , going back a number of years. Some of it overlaps, and it can be difficult to reliably ascertain whose claims have primacy. It is clear, however, that BASF, with its Neopor product based on graphite, has generally set the pace in this field. Other established products are Silver from Nova Chemicals, which incorporates carbon black [32], and Lambdapor from Sunpor Kunststoff GmbH.

The BASF patent portfolio introduces the term ‘athermanous’, i.e., not permeable to radiant heat, to describe particulate additives for the reduction of heat transfer through EPS, and the first in what is now a large family were filed in the late 1990s. The effectiveness of carbon black and graphite, for example, in reducing the heat transfer through foam blocks (e.g., XPS) was known rather earlier, and there are patents dating from the early 1990s, but there are two [33-35] that are the earliest for an expandable bead product with improved . These patents describe beads made by extrusion, subsequently

suspended in water and impregnated with blowing agent. The composition of the first patent [33, 34] claims the use of 2–8% of a carbon (furnace) black with a primary particle size of 80–120 m, while the second [34] uses 0.5–8.0% of graphite particles, with a size between 2.5 and 12 m. A fire retardant is also incorporated


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and the beads are finally coated with a hydrophobicising coating including a paraffin wax. Other athermanous particulate additives claimed include carbon black, metal powders such as aluminium and metal oxides such as iron or aluminium oxides. A slightly later patent [36, 37] claims extrusion of graphite-containing beads with the blowing agent injected before underwater pelletisation. In the text is a reference to the strong nucleating effect of graphite, requiring the beads to be cooled under pressure. Suspension polymerisation is stated to be possible, but is not mentioned in the claims.

A polymerisation process is claimed [38-40], where the use of 0.5–30% polystyrene dissolved in the styrene monomer is stated to help to increase the viscosity of the reaction mixture (and presumably help the dispersion of the graphite). Another patent [41] covers the use of up to 10% of aluminium powder in a suspension polymerisation process, pointing out the need either to vent hydrogen or to prevent its formation by passivating the metal powder, which should be platy with a maximum dimension of 30 m and is suspended in styrene monomer before adding it to the reactor. Carbon black or graphite may also be added. Aluminium blocks the transmission of infrared radiation by reflecting it (whereas carbon absorbs it) so that it may be advantageous to have both a reflector and an absorber present – more recent patent applications highlight this approach.

Sunpor Kunststoff GmbH of Austria obtained a patent in 2000 [42] which combines additives in this way. The process claimed is either extrusion or polymerisation, and the recipes include (a) up to 4% of platy aluminium particles (greatest dimension 15 m) and (b) 0.3–1.0% of aluminium powder and antimony trisufhide powder (quantity not specified, but size between 10 and 60 m). Additionally, up to 2% of carbon black or graphite may be used. This patent appears to overlap to some extent with BASF’s patent [41], although it was granted earlier.

BASF has continued to file patents covering refinements to the graphite technology, including one [43, 44] that describes the use of



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an ‘expansion assistant’, an olefinic oligomer with unsaturated groups at the chain ends, which is claimed to improve the expandability of the beads (see Section 2.5.3 for more discussion of expansion aids), a second [45, 46] that claims advantage for an extrusion process which results in a fine dispersion of water droplets (up to 1.5%) within the beads as an auxiliary blowing agent and a third [47, 48] that describes the use of surfactants to improve the dispersion of the graphite, in either a suspension polymerisation process (exemplified) or an extrusion process (claimed but not in the examples). There is also a 2007 application [49] for use of a mixture of absorbing and reflecting particles – this document does not appear to have any examples of the invention, and it may be revised before it can be granted.

Synbra Technology BV of The Netherlands obtained two patents in 2007 which cover different additives to reduce [50, 51] – the first claims the use of metal powders, specifically copper powder at an addition level of 2.5–3.0%, while the second claims the use of activated carbon powder (smaller than 5 m) at 3–8%. The possible incorporation of further additives such as graphite, metal powders or metal hydroxides is also mentioned in this patent, although not exemplified.

Polimeri Europa SpA of Italy has recently been active in patenting methods to reduce the heat transmission of EPS, including one [52] for the use in extruded bead manufacture of high-refractive-index infrared reflectors such as titanium dioxide or barium sulfate – this would yield white insulating products, and would therefore be at an advantage in the market on the grounds of distinctiveness. The fact that there does not yet appear to have been a successful commercialisation suggests that either the cost is high or the improvement in is insufficient. More conventional approaches are covered in three further filings [53–55]. The first of these offers an explanation as to why suspension polymerisation with carbon particles present can be problematic – the carbon interferes with the activity of the peroxide initiators, and the patent claims that the way round that interference is to use peroxides without benzene


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rings. The second is at present ungranted and appears to be on the same track as a previous patent [42] in terms of the mix of additives (which also includes talc, hydrotalcites, montmorillonites, etc.), but also claims for a method of heat-treating the beads, after coating them with a lubricant, for about an hour at 50 °C – what change this brings about in the beads is not stated, but it is claimed to improve

by up to 10% – perhaps this treatment affects the nucleation and subsequent cell structure of the foam. The third is also as yet ungranted, and takes a different route to bead manufacture. In this application, the composition incorporates a range of possible additives, including carbon black, graphite, oxides, sulfates or dichalcogenides of various metals and lamellar inorganic silicon derivatives as well as a nucleating agent which is a polyolefin or polyamide wax. This is melt-compounded, blowing agent is injected and the beads extruded in the form of strands which are cut to produce non-spherical particles. The benefit of the nucleator, which is essential to the invention, is to eliminate the need for annealing of the beads after extrusion. Finally, Polimeri has filed a voluminous patent application [56] which claims the use of graphite dispersed (in a masterbatch, therefore) within a polymeric matrix which is not fully miscible with the polystyrene, and therefore forms a heterophase structure within the foam. The foam still has more than 60% closed cells.

Although there has been a great deal of effort devoted to improving insulation performance, it has tended to be along similar lines – that of including within the foam structure additives that block infrared transmission. A more speculative idea, which certainly has not yet been realised, is to reduce the size of the cells within the foam to such a small size that the Knudsen effect comes into play. Here, when the cell size approaches the mean free path of a gas molecule (a few hundred nanometres for air at ambient conditions), heat transfer is significantly hampered and will improve. BASF researchers have been looking into the possibility of producing such ‘nanofoam’ structures, but their realisation appears to be some way off [31, 57].



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2.4 Modification of Other Properties of EPS

2.4.1 Enhanced Temperature Resistance

The relatively high Tg of polystyrene, at around 100 °C, makes EPS a very useful material in many applications, but there are more in which higher temperature resistance is required. These include insulation for water heating appliances, and even in space heating – the long-term temperature limit for EPS (80–90 °C, dependent on how long ‘long term’ is) falls uncomfortably close to service temperatures for such applications, and at present thermoset foams such as polyurethanes capture much of this market. Even higher levels of temperature resistance open up opportunities in, for example, steam-sterilisable items for health care. It is no surprise therefore that EPS producers have sought ways to add value to their products.

One well-established method for enhancing the heat resistance of polystyrene is to blend it with poly(2,6-dimethyl-1,4-phenylene ether), known by the abbreviations PPE or PPO. As PPE has a Tg around 210 °C, and blends with polystyrene are miscible and have Tg values linearly related to their composition, solid moulded or extruded products based on such blends have found wide application – Noryl from Sabic Innovative Plastics (formerly GE Plastics) is probably the best known brand name, but other polymer manufacturers also offer such blends. It was therefore logical to produce foamable grades, and several manufacturers have done so, including GE Plastics (Noryl EEF) and Nova (Dytherm ). It has, however, proved impossible to achieve narrow bead size distributions in polymerisation, and as the market for such products has proved relatively small, it has proved difficult to reduce waste levels by finding customers with applications across the full size range. As a result, polymerised beads with high-temperature performance have been significantly more expensive than standard EPS grades and their market share has shrunk, to the point where most grades have been withdrawn. The introduction of extrusion routes to produce beads with specific sizes has improved the economics to some extent, and


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grades are currently offered by, among others, Sunpor Kunststoff GmbH and Synbra Technology BV. Grades with PPE contents up to 30% (temperature resistance up to 120 °C) have been produced and marketed – this remains, however, a niche and volumes are very small. Fire-retarded grades are often demanded, and the only route to making extruded beads of polystyrene/PPE blends is to use phosphorus-based fire retardants (see Section 2.2) which are very effective plasticisers – as a result the blend formulation even for a 110 °C service temperature is very expensive.

The other feature which handicaps these high-temperature EPS grades is that very heat resistance – pre-expansion and moulding require higher temperatures (high steam pressures) than are achievable with conventional EPS equipment. Although suitable equipment is available, the additional challenges and costs have acted as an additional brake on the appeal of these grades.

The polystyrene/PPE blends are aimed at uplifting the heat resistance significantly, but there are markets where even a few degrees extra performance can be advantageous, for example in drinking cups for hot beverages. Such relatively undramatic improvements in heat resistance have been sought via copolymerisation, and a series of papers from Turkey reports the results of such studies. Copolymerisation with up to 5% of -methylstyrene (which when made into a homopolymer has a Tg value of around 170 °C) raised Tg by a few degrees [58], but as the co-monomer is less reactive than styrene, it was not possible to achieve copolymers beyond that level. The resulting foamed material had acceptable properties. The second paper in the series [59] reported experiments with silicone acrylate as the co-monomer, but targeted more at improved surface properties. Up to 0.5% of a polydimethylsiloxane prepolymer was copolymerised into styrene and foamed beads produced. The siloxane side chains give the foam a smooth glossy surface with a significantly higher contact angle than the control EPS, and Tg was around 2 °C higher. Interestingly, the mean bead size increased with siloxane level, but the spread of sizes reduced. Foam density was higher, although figures are not reported. The third of these papers [60] reports further



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studies with siloxane co-monomer, this time using a ‘macroinitiator’ prepared from the same polydimethylsiloxane prepolymer. The effect of using this longer chain co-monomer was to reduce mean bead size as well as its spread, but increased the foam density to some extent. The increase in Tg was again a few degrees and contact angle was again increased, with a glossy smooth surface – it was remarked that these foam beads flowed better than standard EPS beads and were less prone to blocking the pre-expander.

A copolymer of styrene that has, in the solid form, a higher heat resistance is styrene-maleic anhydride (SMA) and although there does not appear to be any reference in the literature to foamed beads made from this polymer, there is no obvious reason why it should not be attempted. It is possible that the cost of the polymer would effectively exclude it from the market – SMA grades find somewhat specialised applications and are therefore much more expensive than polystyrenes, but have Tg values some 25 °C higher.

2.4.2 Enhanced Cushioning Performance

The purpose of any packaging is to protect its contents, and should ideally be as cheap and as light as possible, with minimum bulk – it will be discarded (preferably to be reused or recycled) once the contents are unpacked. EPS is widely used to protect fragile items such as electrical or electronic goods against impact damage during transport and handling, because it has good cushioning performance, low weight and low cost, but it does have some drawbacks. In particular, EPS packaging absorbs an impact shock by breakage of the foam cell walls, and is thereby itself damaged – subsequent impacts are less well absorbed. There are many applications where protection against multiple impacts is needed, and this is one where the advantages of moulded polyolefin foams are often exploited. These do have a cost penalty, and EPS manufacturers have long sought to modify their foams to offer better resilience in order to improve their competitiveness.


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The cushioning performance of any protective packaging can be assessed by dropping the assembled package onto a hard surface from a height that is representative of the likely service conditions, then checking the contents for damage. The drop height will depend on the package size and weight – for example, a heavy package (such as a washing machine) is unlikely to be dropped very far, perhaps a metre at most, while lighter packages may well be treated much more roughly. A great deal of expertise has been developed in package design and materials characterisation [61, 62], so only the key features will be described here.

When protective packaging is designed, the first property to be quantified is the tolerance of the contents to impact shock (G-value of acceleration, though usually as a deceleration). This can be an expensive process, but there are good protocols for minimising the number of destructive drop tests to be done. Once the designer knows the maximum G-value the product can tolerate, and can estimate the likely exposure to dropping in transit (e.g., one trip from the factory to the end user, with minimal handling, or shipping by air freight with multiple loading and unloading), a package can be designed to minimise the probability of seeing damaging G-values. This is where the cushioning properties of the packaging material are considered, as the designer seeks to design a protective package with the smallest bulk (to maximise the number of packages that can be shipped in a given volume), lowest weight and lowest cost (as the cost of the packaging is additional to the product cost but adds no value).

Cushioning performance of packaging foams is measured, for example to EN ISO 4651:1995 [63] or ASTM D1596-2003 [64], by using a series of drops of different weights from different heights onto blocks of foam of different thicknesses, and measuring the maximum instantaneous G-value as the weight is brought to rest. Analysis of the results is then required in order to be able to use them in package design. The response of a series of samples of a foam to increasing drop weights from a chosen height yields a ‘cushion curve’. At low drop weights the foam is not much compressed and the G-value is relatively high. As the drop weight is increased, the foam absorbs the



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shock by internal damage and deformation, and the peak G-value falls, before rising again as the shock-absorption capacity of the foam is exhausted and the weight ‘bottoms out’ through the sample. Repeated drops onto the same sample show a steady increase in the G-value, as the foam sustains accumulated damage and is unable to decelerate the weight so efficiently. Traditionally a series of such cushion curves would be required at different drop heights and sample thicknesses, and for five successive drops on each sample, involving a huge number of tests, but it is now possible to characterise a material using a ‘stress-energy’ approach and a relatively small number of tests [65]. Such testing requires specialised test equipment and skills, so can still be expensive to carry out – but the cost of getting a package design wrong can also be very high.

As mentioned, EPS absorbs shock, after the initial compression of the air in the foam and flexing of the cell walls, by crushing of the cells and breaking their walls. After the load is removed, the foam does not recover completely, as its internal structure has been damaged. Subsequent shocks are therefore less well absorbed – the cushion curves show an increase in the minimum G-value for that drop condition – corresponding to a greater likelihood of the package contents being damaged. EPS is also relatively brittle, even when the beads are well fused together, so the protective cushions may become cracked or broken after one or more impacts, further reducing the protection they offer.

Efforts to improve the resilience of EPS were originally focused on the standard polymerisation route to beads, so that any modification to the recipe was required not to interfere with the polymerisation of the styrene. In 1997, Asahi Kasei patented a polymerisation route to an expandable polystyrene bead toughened with dispersed styrene–butadiene copolymer regions [66], but this product has made little impression on the market, probably for reasons discussed next.

A route was found by Sekisui to incorporate polyolefin domains within the beads. The technology was licensed to and taken forward by Arco, later absorbed into Nova Chemicals [67]. This is actually


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accomplished by impregnating a very finely divided polyolefin powder [usually powdered ethylene–vinyl acetate, (EVA)] with styrene monomer, then polymerising the styrene around the olefinic polymer. Typically the styrene content is around 70%. The polymerised beads are then impregnated with pentane before the usual steps of drying, coating, sieving and packing. The presence of the polyolefin domains within the beads brings real improvements in resilience, but has some adverse consequences, which have made Arcel (Nova) and Piocelan (Sekisui) expensive products and limited their market penetration. The first consequence is that the pentane is much less well held in the beads, so a high level (up to 12%) is required to achieve good expansion (much more pentane escapes during pre-expansion than in standard EPS). The second is that the pentane diffuses out of the beads readily during storage and transportation, so that the beads must be refrigerated to below 4 °C until they are pre-expanded. One countervailing benefit is that once pre-expanded the beads do not age as quickly as EPS, so can be stored for long periods before moulding without loss of mouldability, although shrinkage during moulding is noticeably greater than for EPS, as there is little pentane present to help expand the beads against the pressures of moulding.

Other manufacturers have found methods to produce blends of polystyrene and polyolefins for improved resilience (e.g., Dow [68], using their interpolymer technology), but at present the market only supports the two suppliers. The performance of these foams is markedly better than EPS on repeated drops [32] (see also Nova’s technical literature) and products made from them are much tougher so that repeated reuse and multiple impacts are far better tolerated. As the ‘first drop’ cushioning is almost as good as for standard EPS (not the case for expanded polypropylene; see Chapter 3), these materials find application where this performance is important, despite their high cost (up to four times that of EPS).

A recent US patent application on behalf of Nova Chemicals [69] contains a good description of the process of making these polyolefin–polystyrene interpolymer beads. This application is actually aimed at interpolymers which can be used in other processes such as injection



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moulding, sheet extrusion and foamed sheet processes, where the crosslinking of the polyolefin that occurs during polymerisation of the styrene is a handicap. By eliminating the crosslinking (a side effect of the dicumyl peroxide initiator for the styrene polymerisation), materials of lower melt viscosity and better processability can be made. This is not a particular benefit for expandable beads (and may actually reduce their resilience) but the application does contain a deal of useful information about the patent background as well as the production and use of styrene–olefin interpolymers.

In recent years several manufacturers have sought ways to achieve improved resilience at lower cost, and products such as Sunpor’s TM-B, Synbra’s SHOCK and Huntsman’s R-MERII have been marketed. These appear to be a half-way house between EPS and Arcel , employing the more conventional technology of high-impact polystyrene (droplets of styrene–butadiene copolymer dispersed within a polystyrene matrix) and made by the extrusion route (see Section 2.6.1). These grades can be processed just like EPS, and do have improved multi-drop cushioning, but are still relatively brittle, so have not achieved much market penetration to date.

2.5 Reduced Levels of EPS Blowing Agents and Alternatives

2.5.1 The Need to Reduce Volatile Organic Compound (VOC) Emissions

Pentane and other blowing agents commonly used in EPS are very effective but they are nowadays seen to have an environmental downside. When EPS beads are produced, stored, pre-expanded, moulded and post-aged, some blowing agent is vented to the atmosphere at each stage (particularly during pre-expansion and moulding). Typically beads are produced with some 5–6% of blowing agent, so that the 5 million tonnes of EPS produced in 2007 released 250,000–300,000 tonnes of volatile organic compounds (VOC) to the


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atmosphere (and as each pentane molecule will quickly be oxidised to five carbon dioxide molecules this adds almost 1.5 million tonnes of greenhouse gas, additional to that arising from the processing, transport and final disposal of the EPS). There is also a question mark over links between pentane emissions and ozone formation [70]. Increasingly, emissions are being regulated by national and regional governments, and the EPS industry is not immune. Although it is often possible to capture and destroy (usually by burning, which releases the carbon dioxide more quickly) the VOC emitted from polymerisation, pre-expansion and even moulding processes, it is never possible to capture all of the blowing agent – in thick-section products it may take weeks for the last of the pentane to escape. There is therefore a demand for grades which contain and emit smaller quantities of VOC, and most EPS manufacturers now offer ‘low-pentane’ grades (typically 3.0–4.5% pentane content, although Nova Chemicals offers an ‘ultralow-pentane’ grade with 2.5% pentane [71]).

2.5.2 ‘Low-pentane’ EPS Developments

The challenge to be met in developing an EPS with reduced blowing agent content is to obtain a material which can be expanded comparably quickly and to a similarly low density as standard grades. As the lower pentane level means that the starting Tg of the polymer is higher and there is also a lower internal gas pressure to expand the beads, some means must be found to weaken or plasticise the polymer. This enables the cells to expand more quickly and thus expansion can compete more effectively with evaporation of the blowing agent from the bead surface.

Various methods exist and have been tried for plasticising the polystyrene, and development work continues. There are probably rather more proprietary formulations than are to be found in the patent literature, as not all manufacturers are willing to publish their findings in this way – or they use obscure wording so that their patents are hard for competitors to find. A number of patents have been found and are discussed next.



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The first patents in this area were filed by BASF, which has a family of US patents dating to the early 1990s. One from 1992 [72] is a typical example, and claims a bead with a pentane level of 2.0–4.4%. The key to its improved expandability is stated to be the precise control of molecular weight distribution and limited branching. The molecular weight should be between 180,000 and 300,000, with polydispersity below 2.5 and less than 5% branching. Even so, a series of expansion stages, with the beads allowed to mature between them, is required in order to reach commercially useful (20 kg/m3 or less) product densities. More recent patents appear to contradict this prescription, but that was the state of development at that time.

The plasticisation approach was developed around the same time, and a variety of additives have been covered in patents from a number of companies. A Czech patent granted to Kaucuk [73] claims an ‘internal additive’ comprising a mixture of partial esters of unsaturated higher fatty acids with pentaerythritol (and optionally in combination with similar esters with glycerol) added at up to 0.8% A patent [74] granted to Kanegafuchi of Japan in 1999 describes the use of a ‘foaming assisting agent’ such as a mixture of hexane and cyclohexane, added to the beads at 0.3–2.0%. Another [75] granted to Cheil Industries Inc., of Korea in 2001 has a more complicated recipe for a fire-retarded EPS, with less than 4% pentane, 0.5–1.0% glycerol tristearate, 0.05–0.5% of an EVA/ethylene vinyl alcohol polymer, 0.1–1.0% HBCD and 0.05–0.5% dicumyl peroxide in a styrene-based polymer. All three of these patents were filed only in the home countries of the applicant companies (and in their own languages) and it is not clear how far these products have been successfully commercialised.

Within a very recent patent (see next [76]) is a summary of the technology of another Japanese patent [77] granted to Mitsubishi Kagaku in 2002, quoted as an example of prior and less satisfactory art. The bead formulation contains up to 5% of a plasticiser such as a liquid paraffin or an ester with a boiling point above 200 °C, as well as more than 0.1% residual styrene and less than 0.2% of total of aromatic solvents such as ethylbenzene, toluene, xylenes, etc.


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These are said to expand well, but to produce foamed beads that are hard and tough.

BP Chemicals were awarded a patent in 2005 [76] which claims improved expandability using a petroleum wax (alkanes from C18 to C80) and a nucleant (a Fischer–Tropsch or polyolefin wax). The description of the prior art discusses the (known) use of waxes to improve moulding by migrating to the bead surfaces and improving their adhesion to one another (although this also leads to clumping during pre-expansion) and also accelerated cooling. The petroleum wax of this patent is uniformly distributed within the beads – addition levels from 0.1 to 1.0%, although for low pentane contents 0.3–0.6% is best. There is a list with a number of trade names of potentially useful waxes.

A recent patent using the plasticisation approach to improve expandability was granted to Dow Global in 2008 [78], and claims advantage for a combination of three additives which together make up no more than 3.5% of the bead composition, and a pentane level of 3–5%. The examples use isobutyl stearate (up to 1.5%), a mixture of C20–C50 saturated aliphatic hydrocarbons (also up to 1.5%) and isopropylbenzene (0.1%) – the patent purports to show that all three additives are needed, otherwise the expansion is inadequate (densities 20 kg/m3) or the foamed beads are ‘hard and tough’. It is notable that the base polystyrene has a molecular weight in the region of 200,000–220,000 and polydispersity around 2.75 – a broader distribution than in a patent mentioned previously [72].

BASF has taken a slightly different path with another 2008 patent [79, 80] which demonstrates development since the early 1990s. Now a broad, even multimodal molecular weight distribution is seen as essential, and the ‘best’ examples are blends using a low-molecular weight terpolymer (Joncryl ADF1300) of styrene, acrylic acid and

-methylstyrene at up to 10% addition to polystyrene with molecular weight of 280,000 and polydispersity of 2.8. Another example has a 1:1 blend of two polystyrenes with molecular weight values of 195,000 and 280,000 and no low-molecular weight polymer –



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this also has improved expandability, although not as good at low pentane levels as the terpolymer blend. Such a low-molecular weight (between 2000 and 10,000) terpolymer is effectively a plasticiser, with Tg around 56 °C – the patent claims that it is most efficient when added at 3–8%. These blends can only be produced using an extrusion and micropelletising process (see Section 2.6.1) rather than via polymerisation (it is possible to obtain bimodal molecular weight distributions in suspension polymerisation, but not easy to incorporate the low-molecular weight terpolymer).

A particular type of low-pentane EPS is described in a family of BASF patents from the early 2000s [81-83]. These cover ‘expandable styrene polymers containing carbon particles’ – a Neopor type of product (see Section 2.3) which incorporates graphite particles to block infrared radiation in the finished foam insulation block, but also uses the graphite as a carrier for water as an auxiliary blowing agent. The description of the prior art mentions the known use to improve expansion of plasticisers such as the higher hydrocarbons mentioned in patents described previously, or the use of dimeric

-methylstyrene as a ‘regulator’, and also the advantage of a specific molecular weight distribution in avoiding the need for such remedies. The preferred embodiment contains 3–4% pentane, 3.5–6.0% water and around 5% finely powdered graphite (average particle size of 30 m) and can be expanded to around 15 kg/m3 without difficulty.

2.5.3 Alternative Blowing Agents

As has already become clear, pentane and similar organic solvents make ideal blowing agents for EPS – they dissolve in polystyrene and are therefore well retained in storage, they plasticise the polymer, helping to start and stabilise the foaming process and they volatilise at temperatures that lie conveniently between typical ambient values, Tg of the polymer and the temperature of the steam used for expansion. This does not guarantee that their use will continue to be encouraged or even permitted, as their flammability alone brings significant risks,


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and they also present health and environmental concerns. There is increasing pressure on the industry to reduce or eliminate as many of such hazards as possible, and it is therefore no surprise that EPS producers have been exploring ways to reduce levels and possible substitutes for the pentane they have been using for many years. The challenge remains formidable, however, and it is likely that complete replacement will only come about if legislative or customer pressure demands it, as there will inevitably be consequences for the production, expansion and moulding processes which are well developed and efficient. The resulting costs and compromises may affect the competitive position of EPS against other materials or systems in each of its applications.

One ‘obvious’ alternative approach is to use water dispersed within the beads as the sole blowing agent, and Nova Chemicals has pursued this route, in conjunction with workers at the Catholic University of Leuven and the University of Eindhoven, over a number of years [84-86]. The key challenges are firstly to incorporate water into the beads, secondly to retain it there and thirdly to be able to foam the beads in a conventional pre-expander and obtain mouldable expanded beads. Polystyrene and water do not mix well, and so a carrier is useful to hold the water within the beads until it is needed for expansion – microcrystalline starch was the chosen carrier in the Leuven studies. This has its own difficulties, as starch is incompatible with polystyrene – the first two papers described work done to improve the interaction, by grafting or compatibilisation, in order to be able to achieve a fine dispersion of starch in the polystyrene beads. A 2003 conference paper [87] describes these and the downstream processing challenges – in the prefoaming process the steam temperature must be above 100 °C in order to boil the water, and this is also above Tg of the polystyrene. As the temperature resistance of the polystyrene does not increase as expansion progresses (as occurs with pentane) there is a risk of collapsing the beads. Finally the moulding process requires more care, as there is no residual plasticisation to assist in fusing the beads together to form a strong product, and again higher steam temperatures are needed.



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A later paper [86], the final one in a series of four from Eindhoven University of Technology, takes a different approach. The polystyrene is first prepolymerised in a bulk polymerisation process, and the water is dispersed in it as a water-in-oil emulsion during this stage. Sodium chloride in the water is used to control its droplet size and dispersion and the polymer viscosity is sufficient to preserve the emulsion through the second stage. The prepolymer with its dispersed water is then finished in a normal free radical suspension polymerisation. Beads with up to 4% of finely dispersed water can be achieved in this way. In order to increase the melt strength and temperature resistance of the polystyrene so that it is more robust during the expansion and moulding processes, a proportion of PPE/PPO is added to the reaction mixture. The optimum recipe yields beads with up to 8% water (the PPE also increases the amount of water which can be held in the beads) and Tg around 108 °C. This paper does not report or discuss the expansion and moulding processes for these beads, simply stating that molecular weight is important, and that the PPE is expected to improve the processing window.

Although the foregoing suggests that water-expandable polystyrene (WEPS) has been developed to a workable level, such a product has not yet been commercialised. Presumably the economics are still adverse while conventional pentane-expanded grades are permitted and available (including low-pentane types).

A recent patent application from Nova Chemicals [88, 89] is very wide in its scope and may not be granted in its entirety. It describes a process which eliminates the maturing step for pre-expanded beads. Instead of using a blowing agent dissolved in the beads to generate the foaming, they are impregnated with air or carbon dioxide (or a mixture) at high pressure until they are saturated. This pressure may be 0.5–8 MPa, and the impregnation time for polystyrene beads may be 1–8 hours, at up to 90 °C. The beads can then be pre-expanded using heat and/or steam, before a drying step using a flow of hot air. These dry beads are stable, with a residual internal gas pressure. They are then transported directly to the moulding machine (or may be stored for a short time until the moulder is ready) and have


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sufficient expansion power to fuse well in the mould. The process may be applied to solid beads (which can be polystyrene, polyethylene, polypropylene or even other polymers) and allows the elimination of the volatile blowing agents normally used, as well as the large storage bags needed for the maturing step and the emissions that arise during it. It is, however, advantageous to ‘pre-nucleate’ the beads by carrying out a mild pre-expansion (e.g., from a bulk density of more than 600 kg/m3 to 400 kg/m3 or so) which creates the structure of foam nuclei for the later expansion and for which a (low) pentane content is very useful. This process may allow the elimination of the storage bags for maturing, but it does instead require equipment for the impregnation of the beads under high gas pressures. As this does not seem to be a great advance on the well-known PAT system for expanded polypropylene/polyethylene (see Chapter 3), in which prepressurisation of the prepuff beads gives them enough internal pressure to expand during moulding, it might not appear to be very inventive, and it is possible that the application will be revised and narrowed before it is granted.

Other possibilities have also been considered as a means to eliminate VOC, though this is easier in extruded polystyrene (XPS) sheet processes, where the material is allowed to expand immediately at the die of an extruder, having had the blowing agent injected into the melt shortly before the die. Two such XPS developments have been reported recently, and although it is difficult to see how they could be transferred directly to expandable bead production, as we will see later (Section 4.3), circumstances may dictate that some kind of alternative process may become necessary. These later methods are, therefore, included as signposts to possible future developments rather than existing technology for mouldable expandable foams.

The first of these papers [90] describes how an XPS process using carbon dioxide alone is difficult to control because of its poor solubility in the polymer (maximum around 4 wt%), but the addition of a similar level of ethanol (boiling point of 78 °C, so potentially useful in steam expansion) gives better plasticisation and expandability. The paper concludes that combining the two blowing



35

agents yields advantages which cannot be obtained by using one or the other alone.

The second paper [91] describes the use of water and n-butane together to produce ‘bi-cellular’ foam – a sheet with two cell size populations which reportedly improves the insulation performance of the sheet. The polystyrene also included a low percentage of precipitated silica which, by adsorbing water onto its hydrophilic surface, helped the dispersion of the water and also acted as a nucleation agent for the foam. With 5% butane, 2% water and 3% silica, the expansion ratio reached 35:1 (corresponding to a foam density around 30 kg/m3) with a well-defined two-peak distribution of cell sizes. Clearly this technique still relies on a volatile hydrocarbon, but exemplifies some advantages of combining two blowing agents.

A third paper describing an XPS process [92] starts with a suspension polymerisation to produce beads of water-containing polystyrene, using activated carbon as the carrier for the water. These beads are then fed to an extruder and carbon dioxide injected into the melt to act as the primary blowing agent – the activated carbon holds the water so that it is not driven off in the extruder, and can contribute to the expansion of the sheet so that lower densities can be reached than are possible with carbon dioxide alone. The addition of around 3% of (wet – water content is not reported) activated carbon colours the sheet grey, and also possibly helps the improved insulation performance as described in Section 2.3.

Another approach that may have potential for the future is the use of Expancel expandable microspheres [93]. These have a diameter in the range 10–40 m and have a polymeric shell enclosing a volatile fluid such as a blowing agent. Once incorporated in a polymer matrix and heated above the softening point of the shell, the microspheres release their volatile contents to expand the matrix (although clearly this must also have been softened by the heat) and expansion ratios of up to 60 times are claimed. Although the microspheres are expensive, this approach may have potential in allowing the use of blowing agents that are not soluble in polystyrene.


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2.6 Developments in Making or Processing EPS Beads

2.6.1 Extrusion Processes for Bead Making

As mentioned in Section 2.1, the traditional process for producing expandable beads is via suspension polymerisation, which results in spherical beads of a range of sizes. Advances in process control, special additives and general ‘know-how’, usually proprietary to each manufacturer and jealously protected, can reduce the spread of sizes, but it is always a commercial challenge to find outlets for all the size ranges in the proportions that are generated. This is particularly true of the more specialised products such as coloured products, ‘high-temperature’ beads incorporating polyphenylene oxide and resilient grades such as Arcel , where there may simply not be a commercial outlet for the oversize or undersize beads. In recent years extrusion processes have been developed which allow beads to be made in the desired size range with little waste [94, 95]. There are a number of methods for generating a homogeneous melt of polymer and impregnating it with the blowing agent (closely following the development of XPS processes), but the key step is the micropelletisation. Desired bead sizes are usually in the range 0.5–2 mm – although the upper end of this range is close to what can be achieved by conventional die-face pelletisers, the very small sizes are much more difficult, especially where the melt already contains pentane. The first method for obtaining fine beads was developed in the 1980s [96], using an approach where thin extruded strands were drawn down before cooling and cutting. This resulted in ‘beads’ that were cylindrical rather than spherical, but small sizes (1 mm or so) were achievable. More recently, underwater microgranulators, with the water pressurised to keep the blowing agent in the beads until they are cool enough not to expand, have been developed. One major US/European manufacturer of such systems is Gala, but other manufacturers also produce such pelletisers.

Polystyrene is melted in an extruder (usually a twin-screw type, but the melt can be supplied direct from a reactive polymerisation,



37

via static mixers and melt pumps) and any desired additives compounded into it. Blowing agent can be pumped into the (sealed) extruder barrel and mixed into the polymer in a mixing zone, then the melt fed (sometimes via a ‘cooling extruder’ which can be a single-screw type, or directly via a melt pump – either ensures a steady flow of melt at a consistent pressure) to the inlet of the micropelletiser. This has a die with a large number of very fine holes arranged in clusters round an annular face, and fed through a number of ‘pots’ which pass through the heated body of the die. A multi-knife cutting head rotates against the die face at high speed under a flow of pressurised cooling water. To illustrate the challenges, a bead of 1 mm diameter has a mass of around 0.5 mg, so that a process running 300 kg/h must cut that throughput into some 10 million beads every minute. Die design is a critical part of a reliable process, and although there is much commercial secrecy, some approaches have been described [97-99]. The beads that result are egg-shaped rather than spherical, but close enough to be able to be processed in the same way as beads made by conventional suspension polymerisation.

Extrusion allows incorporation of additives such as colorants, fire retardants, property modifiers, etc., which could interfere with a suspension polymerisation, so it is likely to grow in popularity for making specialised grades of EPS. It is also now practicable to construct a continuous process line to start from styrene monomer, going through a polymerisation reaction step, thence to a compounding stage to add the blowing agent, nucleators and other modifiers, followed by micropelletisation. Sulzer AG took out a patent for a ‘monomer to beads’ production line in 1994 [100], and there are now increasing numbers of such lines in operation. An application by BASF [102] also points up the advantages of the extrusion route when seeking to blend polymers – it claims to be able to produce foamable beads from blends of polystyrene with, for example, styrene–butadiene block copolymers (for resilience), styrene acrylonitrile (for solvent and weathering resistance) and even polypropylene (presumably some kind of compatibiliser is needed here).


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2.6.2 Foam Nucleation

Nucleating additives act to nucleate the foaming process by forming the starting points for the myriad of cells in an expanded bead, and there are a wide variety of such agents used in EPS manufacture. The predominant mechanism employed is heterogeneous nucleation, and the list of typical agents includes finely divided particulate solids such as talc or chalk, and polyolefin waxes (which, because they are incompatible with polystyrene, form small droplets inside the beads). At the start of the pre-expansion process, these finely dispersed inhomogeneities nucleate bubbles of the vaporised blowing agent which then expand to create the fine cell structure of the EPS. The technology of nucleators is fairly well established, and each manufacturer has a preferred approach which suits its particular process, so there has not been much published activity in this field. A recent ANTEC paper [103] describes a theoretical study of nucleation in the polystyrene/carbon dioxide system and concludes that an ideal heterogeneous nucleant should have particles with numerous crevices of small semi-conical angles (a very rugged surface). This is in line with the qualitative model in which the interfaces between nucleant and polymer have small voids, into which blowing agent has diffused – when these voids feel the heating of the pre-expansion process, they are able to expand quickly and form the nuclei of foam cells. Subsequently, more blowing agent diffuses into the growing cells and they can expand further. Some cells coalesce, and blowing agent is lost from the bead surfaces by evaporation. The foam formation process is a competition between these mechanisms, and the resulting cell structure reflects the balance between cell formation and expansion, coalescence and collapse, and diffusion of blowing agent out of the bead – this is affected by pre-expansion conditions as well as by the nucleator and its internal distribution within the beads.

2.6.3 Moulding of EPS

Processing of EPS continues to develop, and moulding machines have been developed to mould EPS (and expanded polypropylene)



39

into or over metal parts [104]. Techniques have also been devised (or in one case, borrowed from the brick-making industry [104]) to increase production rates by use of multiple moulds which can be the same or different from one another, allowing several parts to be produced in parallel. Automation of pre-expanders and moulding machines is likely to become ever more sophisticated, as demand for process efficiency and consistent quality continues to grow. An interesting study into the relationships between EPS molecular weight and density and the quality of ‘lost foam’ castings [105] shows that demand for efficiency applies to all EPS processes – it was found that the molecular weight of the EPS controlled the rate of volatilisation, but that mould filling time also depended on the density of the foam.

2.6.4 Recycling of EPS

Recycling of EPS is well established and the industry has long met and exceeded its recycling targets under EC packaging directives. The most common method of recycling is to grind waste foam back to its constituent expanded beads, then add a proportion of them into virgin pre-expanded beads to mould new products – if the beads are clean they often have very little effect on the product properties. Waste foam can also be densified by crushing, allowing relatively economic transportation (a standard 12 m freight container holds less than 1 tonne of scrap EPS, whereas, if the foam can be densified to around 300 kg/m3, the same container will hold 12 tonnes or more). It can then be melted and reprocessed to a general-purpose polystyrene or modified to have greater added value. The economics of melt reprocessing are very sensitive to the price of oil, which not only controls the price of prime polymer (hence a ceiling on the price of recyclates) but also the cost of transportation of the scrap material from where it is generated to where it is recycled – an operation that is profitable today can become unprofitable tomorrow irrespective of the best efforts of the recycler.


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A comprehensive review of densification options and their relative costs has been published by the Environment and Plastics Industry Council of Canada [106] – it mentions the challenges from contamination (which lowers the value of the recovered EPS) and thermal degradation as well as the costs of densification equipment and the various disposal routes that may be used – recycling is frequently not the lowest cost option.

The experimental process using a solvent derived from citrus fruits which was being developed in the earlier years of the century and is described elsewhere [107] has not so far come to commercial fruition. This may partly be as a result of these fluctuating economics, but also because of the advantages of the regrinding/remoulding route described above.

Two papers from China report an unusual approach to the reuse of reground beads which was aimed at improving the fusion of these beads to one another or with virgin expanded beads. There is no residual blowing agent present in used beads, so they have little expansion capacity during moulding. The work explored the use of adhesives to improve the bead-to-bead fusion during the moulding process – it was found that product properties could be significantly improved, although the amount of adhesive needed was quite large (up to 10% of the EPS weight). A powdered hot melt adhesive worked better than a sprayed solvent-borne system [108], while measurements on 100% recycled EPS showed comparable properties to mouldings made from virgin beads [109]. It appears, however, that this approach is little used in practice, as the addition of a proportion of recycled beads to virgin beads is now well established and controlled.



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3 Expanded Polyolefin Moulded Foams (EPE and EPP) – Materials with Growing Application

3.1 Key Features of Polyolefin Moulded Foams, Compared with EPS

As the polyolefin polymers (polyethylene, polypropylene and their copolymers, in many different variants) together make up the largest volume production of the synthetic polymers, it is not unexpected that foaming of these polymers has been widely practised and studied. It is therefore surprising that the volume of mouldable beads produced in polyolefins is very much smaller than that in polystyrene, especially as foamed polyolefins have some significant property advantages. This disparity is due to the much greater complexity and cost of producing moulded foam products from polyolefins, compared with the particular suitability of polystyrene for the process.

The features of the EPS process described in Chapter 2 are not easily reproduced in other polymers, for reasons which will be explored in this and the following chapter. Polyolefins are commodity polymers that are produced in larger volumes than polystyrene, yet expandable bead production is very much smaller. In contrast to polystyrene, extrusion of expanded sheet from polyolefins is much more widely practised than manufacture of mouldable beads, which remain niche products for particular applications. This is a direct result of the differences between these polymers and polystyrene in relation to foaming and these differences must be considered in order to understand the reasons for the way moulded polyolefin foam technology has developed.


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The first key difference from EPS is that there are no blowing agents that can be impregnated into the beads and held there at ambient pressures for later expansion, in the way that pentane functions in EPS. Instead, once impregnated with blowing agent, the beads must be expanded immediately, or held under high pressure. Expanded beads have a high bulk, so that transporting them (and the finished products) is costly. Clearly, producing the beads on the same site as moulding the finished products would eliminate that cost, but those finished products could then require transporting over long distances to the customer – small-scale production units for expandable polyolefin beads are not economically attractive.

Secondly, the melt rheology (particularly in extension) of common polyolefin grades differs from that of polystyrene (which thickens on extension, helping to stabilise the foam cells) so that the foam cell structure is also more difficult to control. There have been developments in this area which will be discussed in Section 3.2.

The third area of difference arises when the beads are expanded and moulded. Polystyrene is amorphous with a Tg value around 100 °C, while the polyolefins are semi-crystalline – their Tg values lie below room temperature, but their crystalline melting points lie in the range 110–140 °C (polyethylenes) or 150–170 °C (polypropylenes). Because of these higher melting/softening points, high steam temperatures and pressures are needed for polyolefins, and the process window is very narrow – a few degrees wide, just below the crystalline melting point. Further, because of the slow crystallisation of polypropylene, expanded polypropylene (EPP) products often require post-ageing in an autoclave before they are fully stable. This all adds cost and complexity to the process, and further handicaps these polyolefin bead foams in the marketplace.

On the positive side, polyolefins are far more ductile than polystyrene, so their foams are also much tougher and more resilient. This is particularly useful for protection against repeated impacts, so the automotive industry has long been a major area of application as well as transport packaging.


Expanded Polyolefin Moulded Foams (EPE and EPP) – Materials with Growing Application

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Polyethylene bead foam processes were developed by BASF in the 1970s [1] and those for polypropylene by Japan Styrene Paper (JSP) in the 1980s [2], and a description of these processes can be found in the literature [3]. These processes differ in several key aspects from those for EPS – these differences add significant cost and have restricted the market penetration of polyolefin bead foams.

There are essentially three main production approaches for making mouldable expandable polyethylene (EPE) beads, all of which begin with an extrusion process (suspension polymerisation is not possible as it is for polystyrene) and a die with a multiplicity of very small holes (as in Section 2.6.1). The blowing agent can be injected into the melt before the die, so that the strands expand immediately and are cut to size at the die face – polyethylene beads may then be crosslinked, for example by electron beam irradiation, to stabilise their structure. Alternatively, polyethylene beads may be extruded and pelletised into microbeads which are impregnated with a crosslinking agent such as dicumyl peroxide in a suspension in water, then crosslinked by heating. Finally the blowing agent is impregnated into the beads before they are cooled, then expanded by steam. More recently, the JSP process [2], which can also be applied to suitable grades of polypropylene, was developed. Here, extruded microbeads are produced and suspended in water in an autoclave, where a blowing agent is impregnated into them under conditions of elevated temperature (close to the melting point) and pressure. The beads are cooled before depressurisation, and can retain the blowing agent for an hour or more. They must, however, be expanded by heat (usually steam) within a short time, then allowed to cool and the pressures inside and outside the beads to equilibrate. This prepuff can then be transported to the moulding site and may be stored almost indefinitely, as the blowing agent has been almost entirely lost.

The blowing agents used can be organic solvents such as butane, or hydrochlorofluorocarbon (HCFC) (formerly CFC) – pentane is less commonly employed. The critical factor is that these blowing agents diffuse into, through and out of polyolefins much more quickly than pentane does through polystyrene. A 2007 conference paper


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[4] discusses the relative diffusion rates of n-pentane in polystyrene and polypropylene – although the maximum solubility in the two polymers is not very much different (around 25 wt% in polystyrene and 17 wt% in polypropylene), n-pentane diffuses through polypropylene roughly two orders of magnitude faster. This means that polypropylene takes up pentane quickly, but also loses it very quickly, and storage of pentane-impregnated beads of polyolefins is not practicable. The same pattern of diffusion applies to other volatile blowing agents. This diffusivity issue also affects foaming behaviour – when a spherical bead containing a volatile blowing agent is heated, necessarily from the outer surface, two processes compete. The blowing agent can diffuse out of the bead surface or it can vaporise at internal nucleation sites, creating the cells that form the foam structure. As heating and vaporisation continue, the cells will grow and can coalesce as their walls become thinner, but the loss of blowing agent from the bead surface continues – this can lead to a ‘skin–core’ structure where the cells at the bead surface are relatively small compared with those in the interior, depending on the relative rates of cell formation, growth and coalescence, and blowing agent loss. Generally speaking, polyolefin foams tend to have coarser cell structures than EPS foams, because of their different rheological properties, in particular in extensional flow. Prefoaming times are shorter than for EPS, again because of the higher diffusivity, so that uniform expansion is harder to achieve.

As mentioned previously, and as described in more detail in Section 3.4, moulded polyolefin foams do have some advantageous properties, which lead to their use in a range of applications. What is also important here is the difference in utility between extruded foam sheet products and moulded bead products. The bulk of foamed polyolefin products are in the form of extruded sheet, for which the processes are relatively straightforward, but the foam structure tends to be less uniform than in bead foams and more likely to have open or connected cells. In contrast, sheets moulded from beads are more isotropic and have greater cushioning and resilience than extruded sheet, and can easily be made in significant thicknesses. Naturally, moulded bead products can also be made to specific shapes without



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waste, and this is of benefit in packaging applications. Although the market data for 1997 [5] do not separate bead products from extruded sheet and ‘plank’ polyolefin foams, given that the entire polyolefin foams market in that year was well below 10% of that for EPS, it is clear that polyolefin bead foams (EPO) were, although growing faster than EPS, still restricted to niche applications. A private source has indicated that in 2005 in Europe, some 1.5 million tonnes of EPS were produced against only around 26,000 tonnes of EPP and EPE (beads) combined. Data presented by Borouge (the joint venture that manufactures Borealis’ range of polyolefins) in its publicity for the Daploy foamable polypropylene grades suggest that low-density extruded polypropylene sheet was growing at 20% per year but was only at around 30,000 tonnes in 2005. Whatever the precise figures, it is clear that EPO command much smaller sales volumes than EPS.

3.2 Developments in Materials for EPP and EPE Manufacture

The polyolefin family comprises a range of polymers and copolymers with widely varying properties, and a number of them have been exploited as foams and foamable beads. This runs from the softest ethylene-vinyl acetate (EVA) to the stiffest (polypropylene), although for foamable bead production, polypropylene and various polyethylenes predominate. Foams based on low-density polyethylene (LDPE) or linear low-density polyethylene (LLDPE) are soft and have relatively poor creep and compression set performance, while EPP is harder and stronger, but presents some specific processing challenges.

When a polymer is foamed, the primary mode of deformation is extensional – once cells are nucleated and begin to grow, their walls are stretched (and simultaneously thinned). Polystyrene is an ‘extension thickening’ material – as it is stretched the melt becomes more resistant to further stretching, so that the process is to some extent self-stabilising. LDPE can also behave in this way, because of


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its branched chains (polystyrene is not usually branched, but steric hindrance from the bulky side groups creates the same rheological effect in the rubbery phase) and can be foamed reasonably well. As the chains become more linear (and crystallinity increases), so the melt rheology of polyolefins tends more to the ‘extension thinning’ pattern, and once growing, cells tend to grow and quickly coalesce as their walls have less (melt) strength to stabilise them. The crystallinity of polypropylene is also a challenge to foaming – the material must be expanded at a temperature close enough to the crystalline melting point to allow the chains to move over one another, but not so close that the melt has no tensile strength – so it was clear that special grades or blends were required for a successful EPP process. The early EPP manufacturers used random copolymers of propylene and ethylene which have limited crystallinity (and therefore limited mechanical properties), and were also limited in the minimum density that could be achieved. There is also an approach, for which Kanegafuchi (Kaneka) obtained a US patent [6] in 1998, in which a proportion of the crystallinity is present as the (lower melting point) -form, promoted by the use of specific nucleating agents. This was claimed to reduce the temperature necessary for fusion of the beads and to widen the process temperature window. There were also applications filed in Europe and at the World Patent Office, but these were abandoned before their examination was completed.

Over time polymer manufacturers have seen the value of higher melt strength (also very useful in thermoforming) and have developed special grades to meet this need. The logical approach was to make polypropylene with a degree of branching, and a paper presented at ANTEC in 2001 [7] by workers from the University of Toronto and Borealis GmbH reported significant improvements in foamability by blending so-called ‘high melt strength’ (HMS) branched polypropylene into linear polypropylene. Not only did the cell density increase (showing less cell-wall breakage) with the proportion of HMS polypropylene, but also the minimum foam density and the temperature at which it was reached fell, although it was reported that the crystallisation temperature did not change. This work was



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done in a sheet extrusion process rather than beads, but the same principles were clearly applicable to bead making.

A paper presented at ANTEC in 2005 by a representative of JSP [8] discusses the impact of the newer metallocene types of catalysts developed for polyolefin synthesis on the foamability of the resulting polymers. The original Ziegler–Natta catalysts used for polyolefin synthesis produce essentially linear chains without branches, while metallocenes can yield chains of tailorable and controlled stereoregularity with narrow molecular weight distributions for predictable and reproducible material properties. One focus of this paper is on the opportunities arising from this ability to tailor the properties of EPP, especially in automotive applications. As the foam expands, so the polymer chains in the cell walls become biaxially oriented and also tend to form oriented crystalline structures, so that their strength and stiffness are enhanced by comparison with the random copolymers which can only crystallise to a limited extent. The final section of the paper outlines the innovative ‘porous EPP’ (PEPP) product in which the individual beads are in the form of hollow cylinders, or pipes. When moulded, the material retains a high level of porosity because of the connections between the spaces within these cylinders. This porosity is particularly useful for sound absorption, while the closed-cell nature of the foam provides good mechanical properties and cushioning.

Papers presented at conferences in 2005 [9] and 2006 [10] reported more on the HMS grades from Borealis GmbH and developed the themes of high melt strength and controlled crystallisation kinetics in foaming behaviour. The first [9] described two Borealis grades, a homopolymer WB130HMS and a newer copolymer WB260HMS. These were extrusion foamed either alone or in blends with linear polymers, showing that the controlled long-chain branching in the HMS grades improved elongational viscosity and foamability. The copolymer grade was claimed to give softer foams than the homopolymer, but without sacrificing processability. Blends with up to 50% of linear polypropylene, selected to obtain the desired mechanical properties, were shown to be easily expanded to densities down to 80 kg/m3.


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In the paper presented the following year at the next conference in the series [10], four different types of polypropylene were extrusion foamed using butane, with talc as the foam nucleator, and the influence of chain branching, degree of copolymerisation and butane level on foaming (maximum expansion, cell structure and mechanical properties) was explored. It was found that the HMS grades from Borealis were able to reach much lower foam densities (down to 20–25 kg/m3) over a wider process temperature and butane content range than two linear grades, a copolymer and a homopolymer. Copolymers yield a softer foam but with a wider processing window than homopolymers. A second paper at this conference [11] looked at the effect of repeated recycling on the foamability of LDPE expanded with a chemical blowing agent, azodicarbonamide, and concluded that elongational viscosity, particularly the reduction in elongational yield strength arising from damage to the polymer from repeated extrusion, was key to the foaming performance.

By 2008, another polymer manufacturer, SABIC, had entered the fray, and its work on high melt strength in polyethylenes has been described [12]. It was pointed out that too high a melt strength can be a disadvantage, as it limits the degree of expansion which can be achieved – the pressure of the blowing agent must be able to stretch the cell walls and make the cells grow. The control of molecular weight and chain morphology made possible by developments in catalysis allows polyethylene grades to be optimised for foaming to lower densities and with more uniform cell structures and better insulation performance.

Another approach, claimed in a recent Chinese patent [13], is to blend up to 10% or so of both an LDPE and a homopolymer polypropylene into a random polypropylene copolymer and to impregnate the resulting beads with pentane. Reportedly, the beads may be readily expanded to foam densities of 50 kg/m3 (many applications now call for much lower densities, in the range 10–30 k/gm3) but the process is claimed to work at low pressures and with low-cost equipment. Only an abstract is available in English, so to study the details of the



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description of the prior art and the claims would require a translation from Chinese.

A final comment on materials development is that as the polyolefin beads are produced by extrusion, a wide range of additives can be incorporated into them to obtain specific properties. These can include fire retardants, colorants, antistatic or conductive additives, nucleating agents (almost always included), etc. – the formulator is not limited, as he or she often is for EPS, to additives that do not interfere in the suspension polymerisation process. Fire-retarded EPO foams are able to meet the stringent requirements of the aerospace industries, among others [14].

3.3 Developments in Processing of EPP and EPE

The processes for impregnating, expanding and moulding polyolefin beads have been the subject of considerable development effort, not least because of their cost and complexity in comparison with EPS. For example, in the early 1990s, BASF developed the ‘pressure and temperature’ (PAT) system in order to reduce the density increase during moulding [14] (see later for further discussion).

Impregnation of beads with blowing agent was originally (and can still be) carried out in a water-suspended slurry in a reactor at elevated temperature – for EPP this is typically at 130–160 °C – and with elevated pressure of the blowing agent, since for polyolefins the amount of blowing agent taken up is proportional to the applied pressure (Henry’s law). BASF patented a continuous impregnation and expansion process in 1998 [15], in which the beads were suspended in water as a slurry and the slurry was then pumped into a pressurised system of pipes into which the blowing agent such as butane was injected at high pressure ( 20 MPa). The slurry and gas were heated (typically to around 115 °C, approaching the softening point of the beads) so that impregnation was accelerated. The volume of the pipework system and the slurry flow rate were designed to allow sufficient time for the beads to be fully impregnated. Finally,


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the slurry was pumped through an orifice into an expansion vessel where the blowing agent flashed into vapour and the heat-softened beads expanded. The blowing agent was recovered for re-injection or to be burnt to raise steam for the heating process. The patent does highlight the importance of keeping oxygen out of the system by blanketing the mixer where the slurry is first made with an inert gas, as the blowing agent is highly inflammable and at high pressures and temperatures there is a significant risk of explosion.

As micropelletisation methods were developed, so it became possible to inject blowing agent into the extruder, and underwater pelletisation under pressure can even permit the beads to be prevented from expanding at the die, and cooled fast enough to retain the blowing agent for a time (although never for long, as discussed in Section 3.2). Alternatively, the beads may be allowed to expand immediately after cutting, although this does demand good process stability and large-volume pipework to convey the expanding and expanded beads away from the extruder and die. BASF has obtained patents on the process of taking microbeads of polypropylene (and blends with other polypropylene types, or other polymers) and impregnating them in a reactor at high temperature (150 °C is stated to be best) and pressure for several hours, before cooling the reactor (under pressure) to below 50 °C and discharging the cooled beads. The patents [16, 17] claim that the beads are then expandable for a period of at least an hour – steam at 130–160 °C (0.2–0.45 MPa) is used for the expansion step. Although this is hardly comparable with the months-long shelf life of EPS beads, it does permit some flexibility in the layout and operation of the processes. The blowing agents claimed include pentane, hexane, heptane, 3,3-dimethyl-2-butanone and 4-methyl-2-pentanone, and the nucleation of the foam is expedited by a wax and/or talc. A later patent [18] claims the use of a coating on the beads to prevent caking during expansion, although the makeup of that coating is much as used in the EPS industry – a metal stearate (calcium in this case), glycerol tristearate and an antistat. This patent mentions the use of a pressurised underwater pelletiser to produce impregnated beads as an alternative to the reactor process for later impregnation of microbeads.



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One feature of EPP materials is that an unexpected melting peak sometimes appears during differential scanning calorimetry scans, at around 150 °C. This is explained [19] in terms of a terpolymer of propylene-co-ethylene-co-1-butene recrystallising at the expansion temperature (around 150 °C), forming more perfect crystals than usual. On cooling after complete melting, these do not reappear until the material has been annealed for more than 24 hours. The formation of these crystals is ascribed to self-nucleation during the expansion, which is much faster than homogeneous nucleation. It is clear that bead expansion, with a combination of heating, stretching and desorption of the blowing agent (changing the degree of plasticisation), is a highly dynamic process which can generate unexpected morphology in semi-crystalline polymers – this will be seen later in the discussion of expanded polylactic acid (Section 4.2).

Expansion of a foam comprises three main phases – nucleation, growth of the bubbles once they have reached a critical size and finally coalescence or stabilisation of the cells as the internal pressure of blowing agent is no longer sufficient to continue stretching the material of the walls (perhaps because the walls have become stiffer through orientation or cooling, or there is no longer sufficient fresh blowing agent diffusing into the growing cells). This process has been much studied, and a comparison has been done [20] of modelling and experiment in polypropylene/carbon dioxide expansion, with a focus on the effect of the rate of the pressure release on the number of cells and their growth. It was shown that there is a maximum in the nucleation rate with pressure release rate as the growing bubbles compete to consume the gas liberated, but that the bubble growth rate increases with pressure release rate.

Numerical simulation has also been used to explore the moulding process in which the expanded beads are filled into a mould, and then steam is applied to fuse the bead surfaces and create a monolithic moulding. Subsequent ageing may be required, especially for EPP as the crystallisation of polypropylene is slow, in order to obtain dimensionally stable mouldings. Work has been reported by a team at Tokyo University [21, 22] simulating the processes of steaming,


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depressurising, cooling and ageing which make up the moulding cycle, and thereby calculating the stresses on the mould.

As mentioned previously, as the expanded beads contain no residual blowing agent, they have no expansion power and mouldings tend to have a higher density than the prepuff because the steam compresses them to some extent. The PAT system [14] overcomes this to some extent by prepressurising the prepuff before moulding, using air at 0.3–0.4 MPa and 70–80 °C for several hours. This creates an internal pressure in the beads which resists compression by the steam, and may even permit some expansion during moulding so that low density can be maintained. This can be done in a batch process or made semi-continuous by using a large pressurising vessel supplying a smaller dosing tank which feeds the moulder. The process of Nova Chemicals [23, 24] referred to in Section 2.5.3 appears to be quite similar in approach and can in principle be applied to beads of several different polymers.

An article from 2005 [25], although oriented mostly to EPS, describes how moulding of particle foam articles has been developed to include the ability to overmould metal parts, or to mould foams into them. See Section 2.6.3 for more details.

3.4 Development of Applications for Expanded Polyolefin Bead Products

The major application area for EPO is in packaging, where their ability to absorb shock and retain their cushioning properties over multiple drops is far superior to that of EPS (see the discussion in Section 2.4). The cost of a package is often far smaller than the value of the product it is protecting, and when sustained rough treatment is anticipated in transit, EPO foams have unmatched performance. The author knows of an example where products such as plasma screens were shipped to the UK by sea from the Far East in EPP packaging, then unpacked and repacked in EPS for distribution within Europe, there being a saving of space/unit and therefore transport cost by using EPS on the less rigorous last lap.



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Another and growing area of application especially in Europe is in the automotive field. As the issues around disposal of ‘end-of-life’ vehicles have become more important, European car manufacturers in particular have striven to reduce the number of different polymers used in their vehicles, so that the polymeric part of the scrapped vehicle is less mixed and therefore more easily recycled. Polypropylene is often the preferred solution for internal, under-the-bonnet and bumper components, where a solid polypropylene skin may cover an energy-absorbing EPP foam layer (usually of fairly high density for optimum stiffness and strength) all carried on a steel armature. EPE foam finds application in sound-deadening within assemblies such as doors, and for soft cushioning tasks such as carpet underlays. ‘Porous EPP’ [8] is also intended for such applications, and other variants of Arpro (the trade name for JSP’s EPP) are tailored for specific roles in automotive interiors [26].

A niche but potentially large future market area for EPP foam is as an underlayer in synthetic sports surfaces [27] – a sheet of EPP provides the necessary level of cushioning between the hard substrate and the synthetic turf, while having sufficient toughness and resistance to compression set to have an adequate service life.

Although many applications of EPO are relatively small in volume [27, 28], there are many niches where the special properties of these foams fulfil the demands of the application better than other foams or alternative design solutions.

Because EPO are often used in roles where their protective performance is critical (because of their relatively high cost, they tend to be employed where their resilience and ductility are important), there has been effort devoted to testing and modelling of cushioning performance. Impact absorption is a compressive process at high strains and strain rates – this region can be hard to access using common testing equipment. Even drop testers (see Section 2.4) cannot easily explore the strain rates found in, for example, automotive accidents. A Canadian paper [29] reports compression testing carried out at rates up to 2500/s and compares the performance of EPS,


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EPP and polyurethane foams. The results are also compared against a common constitutive foam model and show that, above around 1000/s, strain rate dependence becomes very marked. Another study [30] explores the effect of multiple compression cycles on EPS and EPE, and compares their performance against simulations using finite element analysis. EPE (and EPP) is significantly more resilient to multiple loadings than EPS – the deformation of EPS quickly leads to breakage of the cell walls of the foam, as the matrix polymer is quite brittle, while polyolefins have much greater ductility and ‘absorb’ deformation by flexing and yielding without breaking – the cell structure therefore remains more or less intact and the cushioning power of the air inside the cells is retained.

A special type of EPP is manufactured by Brock (USA), in which the beads are not fused by steam but glued together with a polyurethane adhesive. This creates a very open texture, through which water can be poured – the porosity is of the order of 25%. This product is particularly aimed at the application discussed above in synthetic sports surfaces, where good drainage can be very important. Work has been reported [31] that aimed to compare these foams with conventional EPP for application in protective cycle helmets (where the impermeability of EPS presents ventilation issues for the wearer). Sadly for the investigators, the conclusion of their study was that the permeability of the bonded foam was insufficient to provide good ventilation and the compressive and impact-absorbing properties were a little poorer than those of a conventional EPP.

In conclusion, it is likely that EPO will continue to find new applications where their resilience and ductility add value, and although these may begin as small-volume niches, at some point they may grow to be much more significant. The application as an underlayer for synthetic sports surfaces is a good example, or growing displacement of other materials such as polyurethane or polyvinyl chloride foams in the automotive market. As world trade grows, high-performance transport packaging for delicate products can only increase in importance (and many countries have far worse road surfaces than those to which Europeans are accustomed).



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EPS will continue to hold its leading position as a packaging foam, although EPO and biodegradable foams (see Chapter 4) are likely to gain increasing shares of this very important market in the coming years.

References

1. F. Statsny, R. Gaeth and G. Treischmann, inventors; BASF, assignee; US3616365, 1971.

2. K. Hirosawa and S. Shimida, inventors; Japan Styrene Paper Corporation, assignee; US4379859, 1983.

3. Polymeric Foams and Foam Technology, 2nd Edition, Eds., D. Klempner and V. Sendijarevic, Carl Hanser Publishers, Munich, Germany, 2004, Chapter 8.

4. X. Zhang, Z. Zhu, C.B. Park, E.K. Lee, N. Chen and H.E. Naguib in Proceedings of SPE ANTEC 2007, Cincinnati, OH, USA, 2007, p.3057.

5. D. Eaves, Polymer Foams, Trends in Use and Technology, Rapra Technology, Shawbury, UK, 2001.

6. Y. Munakata and K. Senda, inventors; Kanegafuchi Kagaku Kogyo Kabushiki Kaisha, assignee; US5716998, 1998.

7. H.E. Naguib, J.X. Xu, C.B. Park, A. Hesse, U. Panzer and N. Reichelt in Proceedings of SPE ANTEC 2001, Dallas, TX, USA, 2001, p.1623.

8. S.R. Sopher in Proceedings of SPE ANTEC 2005, Boston, MA, USA, 2005, p.2577.

9. M. Stadlbauer, R. Folland and P. de Mink in Proceedings a Rapra Technology Conference - Blowing Agents and Foaming Processes, Stuttgart, Germany, 2005, Paper No.10.


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10. J. Wang, C.B. Park, M. Stadlbauer, R. Folland and W. Pirgov in Proceedings of a Rapra Technology Conference - Blowing Agents and Foaming Processes, Munich, Germany, 2006, Paper No.19.

11. E.T. Kabamba and D. Rodrigue in Proceedings of a Rapra Technology Conference - Blowing Agents and Foaming Processes, Munich, Germany, 2006, Paper 20.

12. J. Krist and P. Sengupta in Proceedings of a Rapra Technology Conference - Blowing Agents and Foaming Processes, Berlin, Germany, 2008, Paper No.25.

13. W. Zhang, S. Zhang, T. Liu, H. Yin, J. Shao, M. Lu, L. Zhang, S. Duan, Y. Lu, X. Wang, K. Li, H. Zhang and L. Chen, inventors; China Petroleum and Chemical, assignee; CN101104716(A), 2008.


15. T. Hall and Y. Trivedi, inventors; BASF Corporation, assignee; US5753157, 1998.

16. C. Maletzko, K. Hahn, I. de Grave, G. Ehrmann and F-J. Dietzen, inventors; BASF AG, assignee; US6448300, 2002.

17. C. Maletzko, K. Hahn, I. de Grave, G. Ehrmann and F-J. Dietzen, inventors; BASF AG, assignee; US6476089, 2002.

18. C. Maletzko, V. Keppeler, K. Hahn and D. de Grave, inventors; BASF AG, assignee; US6864298, 2005.

19. J.B. Choi, M.J. Chung and J.S. Yoon, Industrial and Engineering Chemistry Research, 2005, 44, 8, 2776.

20. K. Taki, Chemical Engineering Science, 2008, 63, 14, 3643.



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21. S. Nakai, K. Taki, I. Tsujimura and M. Ohshima in Proceedings of SPE ANTEC 2006, Charlotte, NC, USA, 2006, p.2726.

22. S. Nakai, K. Taki, I. Tsujimura and M. Ohshima, Polymer Engineering and Science, 2008, 48, 1, 107.

23. M.F.J. Berghamns, K.C. Bleijenberg, J. Teubert and A.C.G. Metsaars, inventors; Nova Chemicals, assignee; EP1654304A2, 2005.

24. M.F.J. Berghmans, K.C. Bleijenberg, J. Teubert and A.C.G. Metsaars, inventors; Nova Chemicals, assignee; WO2005019310/A2, 2007.

25. J.H. Schut, Plastics Technology, 2005, 51, 4, 68.

26. High Performance Plastics, 2008, August, 5.

27. Plastics Engineering, 2002, 58, 52.

28. Advanced Materials and Processes, 2006, 164, 4.

29. S. Ouellet, D. Cronin and M. Worswick, Polymer Testing, 2006, 25, 6, 731.

30. U.E. Ozturk and G. Anlas, Journal of Polymer Engineering, 2007, 27, 8, 607.

31. N.J. Mills and A. Gilchrist, Journal of Materials Science, 2007, 42, 3177.


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4 ‘Sustainable’ Polymers – The Future?

4.1 Why Sustainable Polymers?

As the twenty-first century unfolds, many of the assumptions of our twentieth-century lives are being challenged by changing circumstances and by new thinking. The finite nature of fossil fuel supplies has come into sharper focus, leading to a search for alternative raw materials to produce many of the materials we need – polymers in particular. There has also been an increasing awareness of the potential environmental consequences of the growth in lightweight plastics packaging – from one hand have come demands for such packaging to biodegrade rapidly once discarded, from another for easier and more efficient recycling, and there is not as yet any sign of consensus on the optimum way forward for the packaging industry. It is, however, clear that there is a significant interest in using polymers that can be made from renewable raw materials (biomass of various kinds, from algae to wood), whether the packaging is recyclable or biodegradable once it has fulfilled its primary purpose. The use of such polymers can be seen as reasonably ‘carbon-neutral’ over their lifetimes, though there is still energy used to create the polymers, process them and transform them into useful packaging products. Life cycle analysis (LCA) is often called in to promote the ‘green’ credentials of particular materials – this remains a contentious (and expensive) arena in which to participate.

To date, no mouldable particle foam products from renewable sources have been commercialised in Europe (though see Section 4.2), and the potential size of the market is very difficult to estimate. However, if even 5% of the expandable polystyrene (EPS) used for packaging


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was replaced by foam from renewable sources over the next 10 years, volumes would become significant and require large investments in polymer production capacity.

Polystyrene can, in principle, be manufactured from renewable sources, as styrene may be obtained from several trees and shrubs, most notably Liquidambar orientalis [1]. It is, however, extremely unlikely that there could be sufficient monomer available from these plants to supply even the EPS demand, let alone the rather more solid polystyrene manufactured every year. In order to be realistically ‘sustainable’, economics and scale require that such a polymer must be made from widely grown plants, preferably not required for food use (because the supply of food crops is under increasing pressure from a growing world population), and that the yield of polymer is reasonably high and efficient in the energy used to manufacture and process it.

Over the past few years, a wide range of sustainable polymers has been promoted as potential solutions to these issues, and it remains unclear which of them will be among the winners and losers – some may find specific niches while some may succeed on a larger scale and others fall by the wayside [2]. Long-established materials such as starch and cellulosic polymers have come under fresh consideration, together with polyhydroxyalkanoates (PHA), polylactic acid (PLA) and other more exotic polymers (Nylon 12, for example, is made from castor oil). There has also been an announcement [3] that Braschem SA of Brazil is to produce a fully renewable linear polyethylene derived from sugar cane, although this is intended for packaging film production at present. Producers and users of mouldable particle foams are also keenly interested in this developing area – if, for example, governments legislate (or large customers decree) to require packaging to be sustainable or degradable then cost constraints that presently apply may become much less important and the ability to deliver a product that works much more critical.

The issue of how post-use foam packaging can best be handled in an environmentally acceptable manner is still a subject of much


‘Sustainable’ Polymers – The Future?

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debate [4], and it is possible that legislation will dictate (therefore potentially differently in different countries) how the material must behave. For example, a foam material that begins to degrade as soon as it is exposed to ultraviolet light and moisture (e.g., when it is discarded as litter) may quickly cease to be a visual affront, but as well as the potential for degradation before it is desirable (while it is still in use as packaging), it is then lost to any potential recycling. There are already masterbatches available which can trigger so-called bio-oxo-degradation (e.g., see product literature for d2w , Total Degradable Plastics Additives or Biobatch ), although these are intended and marketed as a means to make synthetic polymers (often polyolefin films and bags) ‘disappear’ once discarded. There is no reason why similar technology should not be used to accelerate the degradation of bio-based polymers – by their nature these tend to biodegrade, although sometimes very slowly – and it may be that there will be a market for such pro-degradants even in the sustainable polymers field.

Biodegradability was earlier seen as a positive benefit of bio-based materials, and remains so in some niche applications such as the medical field, but has actually proved to be a handicap in many markets. As long ago as 1997, it was concluded [5] that the growth prospects for biodegradable polymers would most likely depend on legislation to restrict the disposal of organic materials into landfill – this would affect both bio-based and synthetic polymers and foams. Increasingly, ‘sustainability’ is seen as more important than biodegradability for packaging, as collection for re-use, recycling or incineration becomes better established – the idea of using a package once and then allowing it to decompose in the environment is viewed as less environmentally responsible. Composting may be an acceptable way to use biodegradability in a beneficial way – at least the raw materials and energy put into making a package are recovered to some extent.

Compostability is still therefore seen as desirable, as collection and recycling costs for low-density and often contaminated items such as post-use foam packaging can be prohibitive, and there are now


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standards which lay down the requirements for materials to be acceptable into the wide variety of composting processes already in operation around the world. Because composting conditions vary, from heap systems (self-heating when they are large enough) to pressurised retorts, and the timescale of the process can vary with the operator, some polymers may be regarded as compostable in some systems but not in others. There are now standards such as EN 13432 [6], ISO 14855 [7] and ASTM D5338 [8] which have established test specifications and criteria for the assessment of compostability and biodegradability. These standard test methods are oriented towards industrial-scale composting plants where temperatures are relatively high (55–60 °C) – some authorities such as Vincotte (Belgium) also offer certification for compostability in much cooler domestic composting.

Recycling is an alternative to disposal by composting, and can, as with EPS (see Section 2.6.4), take several routes. Waste foam may be disintegrated to the individual expanded beads, then remoulded, or may be remelted and reprocessed to beads or other products. The viability of each route will depend on the degree to which the polymer chain length has already been degraded in service and in the recovery process (although polyesters such as PLA and PHA can have their chain length rebuilt by using suitable chain extenders in a melt process).

In the following sections, four of the candidate polymer classes will be reviewed and recent developments discussed. As already mentioned, it is still far from clear how the market will develop, or even what constraints will drive it, but we may be sure that cost will remain a major competitive issue – there are materials solutions other than mouldable foams to all packaging issues, and their relative costs will also help to winnow the field.

The important issues surrounding any candidate for mouldable foam production include:

• The cost of the polymer and its processing,



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• Selection of blowing agent(s) and their interaction with the polymer,

• Temperatures for processing and in service, and

• Whether the polymer is recyclable or degradable or compostable after use.

Each of the polymer types will be considered against these criteria and to what extent recent reported work allows us to judge their potential for finding suitable applications in the near future.

The cost of a packaging product moulded in particle foam depends to a large extent on the base cost of the polymer – the subsequent processes do add cost, and a material which is expensive to process will carry a handicap, but a rule of thumb in the plastics industry is that the product cost is usually between 1.5 times and twice the polymer cost. Accordingly, a material that is expensive to synthesise (e.g., more than twice the cost of polystyrene) will tend to be regarded with disfavour, and alternative packaging solutions are likely to become competitive against it. One of the factors driving the interest in sustainable polymers is, however, the volatility in the price of crude oil (which determines the cost of synthetic polymers) and the certainty that the long-term trend is to higher oil prices as oil fields dry up and world production declines. There are many contending voices in the debate over how soon the so-called ‘peak oil’ (maximum output) will occur, though few doubt that it will be sometime in the near to medium term – some would argue that the world is already at that point. As oil prices increase, so the cost of producing bio-based materials will also be affected, as energy is needed for their synthesis and processing, but it is anticipated that such increases will be smaller than the increases in the price of synthetic polymers. Over the next few years, therefore, we can expect the competitive position of sustainable polymers to improve, although not without excursions – the world recession of 2009 lowered oil prices to a third of the peak reached in 2008. Such instability is not helpful to companies considering investments in sustainable polymer capacity,


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and it will not be a surprise if the number of players in this field suffers significant attrition in the next few years. What is more important when considering the options for foams made from sustainable polymers is the comparative costs of the candidates – here PLA and starch-based materials have a clear advantage over polyalkanoates and cellulosics. As will be seen below, PLA-based materials have seen by far the greatest amount of effort in recent years.

Blowing agents are, as we have seen in previous chapters, a vital component of a mouldable particle foam system. The ideal combination is, as with EPS, a volatile plasticiser for the polymer, which is well retained in the beads at ambient temperatures. After pre-expansion, if some of the blowing agent remains in the beads for later use in the moulding process, to expand and fuse the beads together, that is even more desirable. This is, however, only likely to work when the polymer has a Tg value well above ambient temperature, and a good affinity for the blowing agent. As suspension polymerisation is not available as a route to the production of suitably sized (0.5–1.5 mm) beads, all of these polymers would most probably need to be extruded as microbeads and impregnated with a blowing agent (in the extruder or subsequently) before pre-expansion and later moulding.

Processes for EPS and EPO rely on steam as the medium for heat transfer to the beads – it has a high heat capacity and can be controlled in temperature over quite a wide range, by varying the pressure or diluting with air. Existing equipment for expansion and moulding has been developed and tuned for use with these types of beads – if a new bead material requires a significant redesign of the process equipment then that will add to its costs and make it less likely to be adopted.

As all of the candidate polymers are to some degree crystalline (there is so far no bio-based analogue of polystyrene with its amorphous character and conveniently located Tg), process temperatures for foaming and moulding tend to be quite critical, and typically lie just below the melting point of the polymer. The (usually slow) rate



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of crystallisation during and after the steps of pre-expansion and moulding will also affect these process steps, and may require control by the use of nucleants or co-monomers.

The issue of recyclability or compostability or degradability has been discussed above – this will be a constraint that may vary between different applications and/or territories, as legislation and customer preference are likely to be the decisive factors.

The development of particle foams in these polymers is not as yet very advanced, but production of extruded expanded sheet has made much more progress. Technologically this is a less demanding application – chemical blowing agents can be used, or a physical blowing agent can be injected into the melt in the extruder just before the die and need not be retained in the melt since expansion occurs immediately it emerges from the die and is depressurised. The discussion of the literature which follows will, therefore, necessarily include reference to such sheet products and consider how the techniques described may, or may not, be applicable to mouldable beads.

4.2 Polylactic Acid (PLA)

4.2.1 Production and Properties of PLA Polymers

In recent years interest in polymers of lactic acid (2-hydroxypropanoic acid) has been increasing quite markedly, especially as the cost of the material now appears to be competitive with some of the oil-based polymers. Initially PLA was seen as a low-volume, high-value polymer mostly for medical applications (such as fibres for degrading sutures, or temporary scaffolds that disappear over time in the body). The position changed in 2002 when Natureworks LLC (initially a joint venture between Dow and Cargill) commissioned a large production facility (nameplate capacity of 140,000 tpy) for lower cost PLA in the USA, and there are now smaller volume suppliers in Asia. In 2008, two European investments in PLA production


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facilities were announced, including one for Synbra Technology BV targeted specifically at foamable bead production [9]. The material cost is indicated as being comparable with polyethylene terephthalate (PET) or somewhat higher – still not comparable with EPS costs, but low enough to make PLA the most popular sustainable polymer for investigation, based on its cost and balance of properties. As more suppliers come on stream, so the cost is likely to fall, and as more experience is gained with it, so will the processing costs and hence the cost of products made from the material.

PLA can be produced from a very wide variety of plant types, including corn starch, sugar beet and sugar cane, potatoes and other biomass (which need not necessarily be a food crop). The carbohydrates in a suitable feedstock are fermented to lactic acid by lactobacillus bacteria under conditions arranged to optimise the yield and purity of the product. Direct polymerisation of lactic acid to usefully high molecular weights is not practicable as the water eliminated as each monomer unit adds to the chain tends to break the chain back down by hydrolysis – only low-molecular weight oligomers are achieved. This problem is overcome by catalytically cracking the oligomers down to the cyclic dimer, lactide, and removing the water. The high-molecular weight polymer can then be produced in a ring-opening polymerisation reaction, often catalysed by tin octoate, although other catalysts are used. A fuller account of the technology as it stood in 2007 can be found elsewhere [10]. PLA polymers are also described as polylactides – there is as yet no firm consensus over nomenclature.

As lactic acid is a chiral molecule (see Figure 4.1), in which the methyl group can be either on one side of the chain backbone (‘looking’ from the double carbon–oxygen bond) or on the other, there are potentially many types of ‘PLA’ which can be produced. By controlling the mix of lactides (L–L, L–D, D–D) fed into the ring-opening polymerisation process, polymers can be made with all L-lactic acid units in their chains (PLLA) or with all D-lactic acid units (PDLA), or with the chains comprising a random mixture sometimes referred to as P(DL)LA. Most of the useful grades are strongly biased towards PLLA



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with limited proportions of the D-isomer – the higher the D-content the lower the crystallinity and thereby the heat resistance. Generally, polymers with D-contents below 5% crystallise well, whereas if it is above 10–12% the polymers are fully amorphous – a fuller account of the relationship between composition and properties of PLA polymers can be found elsewhere [11].

L-Lactic acid

D-Lactic acid

Figure 4.1 L- and D-Lactic Acids, showing the chirality of the molecules

The properties of PLA polymers are in some ways comparable with those of polystyrene or amorphous polyethylene terephthalate (A-PET), even though PLA is semi-crystalline. They are usually transparent and stiff, relatively brittle in unmodified form, with a high specific gravity (1.25–1.29). Tg is rather low, around 58 °C, while the crystallinity and crystalline melting points depend on the degree of stereoregularity – pure PLLA is reasonably crystalline and has a melting point around 170–180 °C, but as the content of D-lactic acid units in the polymer increases, so the crystallinity is reduced and the crystals melt at lower temperatures. The rule of thumb is that for each 1% of D-units the melting point falls by 4 °C. There is, however, a specific crystalline


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form, the so-called stereo-complex, where a mix of equal parts of L- and D-chains can co-crystallise in a specific crystal morphology with a high melting point at around 230 °C. In general, PLA polymers are slow to crystallise and often require nucleation to achieve tolerable crystallinity after processing. The low Tg together with this slow crystallisation has restricted the material’s application. Although it can be thermoformed quite easily, the resulting products have limited heat stability and service temperatures above 40–50 °C can cause shrinkage and distortion. Much effort has been expended on attempts to control crystallisation behaviour so that, for example, extruded sheets of PLA are amorphous enough to be easily thermoformed but can be persuaded to crystallise quickly to a level that gives the formed product adequate heat resistance. Typical nucleating agents for PLA crystallisation include talc (yielding translucent or opaque parts) or ethylene-bis-stearamide (EBS) wax for greater transparency.

4.2.2 Processing of PLA-based Beads – A Trend Towards Carbon Dioxide

As PLA is made from renewable raw materials, it has recently been seen as preferable that the blowing agents used to foam it should be also – this suggests alcohols or carbon dioxide (CO2) as the most obvious candidates. Other agents have been tried (see also Section 4.5 below) but the bulk of recent foam development work appears to have been focused on CO2. PLA, particularly when CO2 is used to expand it, presents some unusual demands in processing, as will be explained further below.

There have been many patents and patent applications filed in respect of foamed PLA in the past 15 to 20 years – a search carried out in 2007 found some 85 filings in Japan alone, with considerable overlaps – and it would be impractical to attempt to summarise them all. The following account is therefore an attempt to identify some key documents and the teaching contained within them about the challenges of working with PLA and how they have been addressed with more or less success.



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Commercial PLA foam mouldings have remained elusive, although there have been announcements of imminent launches. For example, in 2005, both Unitika and Kaneka announced in Japan that they were about to introduce mouldable PLA foam bead products [12, 13]. In 2008, the Biopolymer Network of New Zealand was awarded an International Bioplastics Award [14] for their 2006 development of mouldable PLA foam beads, said to have been the result of a collaboration between three Crown Research Institutes in New Zealand and an Australian packaging group, Alto. A patent application covering this work was filed in August 2008 and is discussed in Section 4.2.4. Further information on the current status of these three product introductions is difficult to find, but it is clear from these and from a recent announcement in Europe [9] that moulded PLA foam packaging will be in service in different parts of the world before very long.

Some of the earliest patents of interest were granted to DuPont in 1992, for foamed extruded sheet products [15]. The blowing agents referenced included CO2 and nitrogen, but hydrofluorocarbons (made by DuPont) were favoured. This patent claimed the benefits of using star-shaped polymers from a special polymerisation process, and extra benefit could be gained by crosslinking the polymer. Expansion was achieved by extrusion or by placing the sheet, containing the blowing agent, in a mould and heating so that it expanded to fill the mould. Mitsui Toatsu claimed a process for extrusion-foamed sheet in a 1995 patent [16] which introduced some of the concepts mentioned frequently thereafter. The process used a mixture of PLLA (crystalline) and P(DL)LA (amorphous) polymers, and additives such as chain extenders and plasticisers were compounded into the melt. A range of blowing agents such as butane is mentioned in the description of the invention, although only chemical blowing agents appear in the claims – interestingly, CO2 is not mentioned as a blowing agent. The examples show that foamability improved as the proportion of amorphous PLA was increased.

In 1994, BASF [17] patented a mouldable particle foam in which the expandable beads were made from an amorphous PLA, and


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expanded using a mix of methyl formate and pentane as the blowing agent. These expanded beads were then fused during moulding by means of finely powdered, unexpanded PLA mixed with them before moulding, to act as an adhesive, as the crystallinity developed during expansion inhibits bead fusion. A similar approach was described by Kanebo in a Japanese patent application of 2001 [18], where the preferred adhesives were polybutylene succinate (PBS) or polycaprolactone (PCL). Another member of Kanebo’s long list of 2001 patent applications [19] claims the use of blends ranging from crystalline (5% D-form) to amorphous (60% D-form) PLA. There is also a chain extender, plasticisers such as glycerol, erythritol or pentaerythritol, talc as foam nucleator and a volatile blowing agent such as isopentane or butane. The blowing agent is impregnated into the beads after they have been made by an extrusion process. Methanol or ethanol or acetone mixed with the blowing agent are claimed to act as foaming assistants. These patents highlight one of the difficulties when trying to mould pre-expanded beads of PLA – there is little expansion power in the beads and it is difficult to heat their surfaces sufficiently to fuse them without damage to the cell structure, or even collapsing the beads. In general, as with expanded polypropylene (EPP), moulding expanded PLA results in an increase in density compared with the pre-expanded beads, and ways to minimise this effect have been much sought after.

4.2.3 Processing of PLA-based Beads –Interactions Between PLA and Carbon Dioxide

As the emphasis has shifted from the biodegradability of PLA foams to their sustainability, CO2 has become the favoured blowing agent, and the majority of recent patents and applications focus on it. The use of CO2 brings its own difficulties – it is a gas at normal temperatures, freezes/sublimes at 78.5 °C (it does not liquefy except at high pressures) and has its critical point at 31 °C. Generally, therefore, it is stored as a liquid under pressure and must be either injected into the barrel of an extruder as a supercritical fluid or impregnated into pre-made beads close to room temperature but



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at moderately high pressures. It does have reasonable solubility in the amorphous regions of PLA, so that, given time, more than 10 wt% can be absorbed, depending on the applied pressure. The rate of diffusion is, however, quite high, so that absorbed gas can be lost quite quickly once the pressure is released and the beads exposed to atmosphere – they can lose sufficient gas to be unusable within an hour or so.

Sheet extrusion foaming of PLA by CO2 injected into the extruder upstream of the die is relatively straightforward, and there have been numerous patents filed to cover this type of process, recent examples being to Coopbox of Italy [20] and Natureworks [21]. The process of CO2 extrusion foaming of PLA, on its own and in blends with thermoplastic starch (TPS), has also been studied by a Canadian team [22] with some interesting findings. Using a laboratory-scale extruder they melted the polymer components using a crystalline (low D-content) PLA (which was grafted with maleic anhydride for blending with a well-dried TPS with glycerol as the plasticiser), compounded talc into it as a nucleator and injected CO2 upstream of the die. They found that foaming was very poor until the CO2 level in the melt reached 7%, and that above this level the foam density, at 20 kg/m3, remained constant as the gas level was raised as far as 10%. The foams had a significant proportion of open cells, suggesting that some of the gas escaped before it could be used to foam the polymer, and that the cell walls were prone to breaking. The other interesting finding was that the combination of plasticisation by the CO2 and orientation during expansion allowed the development of a reasonable degree of crystallisation in the PLA – this somewhat unexpected behaviour of PLA is discussed further next.

The effects of absorbed CO2 on PLA were explored in a paper presented at the ANTEC conference in 2005, by another group of Canadian researchers [23]. They moulded thin discs of a (potentially) crystalline PLA from Unitika with a D-content of less than 2%, but by quenching after moulding ensured that they were fully amorphous. These discs were impregnated with CO2 for two


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days at room temperature and different pressures, the uptake of gas was measured and finally the discs were allowed to foam by releasing the pressure and heating them to temperatures up to 160 °C for a short time – as little as five seconds and temperatures in the range 90–100 °C appeared to be optimal for good foam structure. The solubility of CO2 in PLA rises fairly linearly with applied gas pressure, while the diffusion coefficient rises exponentially. Interestingly, they found that above about 2.1 MPa impregnation pressure, the PLA began to crystallise at 25 °C, and that the level of crystallinity then rose with pressure to a plateau of 24% above 3.5 MPa. As a result, the foaming generated from lower impregnation pressures was softer and had better cell structure. At the higher pressures, the increased diffusivity and increased polymer stiffness arising from the crystallinity meant that the gas escaped more quickly or was used to nucleate foam cells, with the result that the foam structure was finer but less uniform.

Another interesting study was published in 2007 by a team from the University of Thessaloniki, Greece, reporting experiments on the foaming of PLA (this time with a D-content of 12%, so amorphous) impregnated with supercritical CO2 and comparing it with polystyrene [24]. This work was again carried out on discs of the two polymers, placed in a pressure vessel filled with supercritical CO2 (at up to 40 MPa) and held for four hours, followed by release of the pressure at a controlled rate. The temperatures used for the PLA lay between 35 and 55 °C, while for polystyrene they were higher, 80–100 °C. The impregnation pressures and pressure release rates were varied and the density and cell structure of the foaming that resulted were quantified. The faster the pressure was released, the more numerous but also the smaller the cells – the foam density increased with the rate of pressure drop, as more gas was used to create new cells than to grow the existing ones. The effect of higher temperature was, as would be expected, to reduce foam density – the cells grew faster because the gas was able to diffuse into them more quickly. Although the conditions used in this study do not appear to relate very closely to real process conditions, the work does cast some light on the dynamics of foaming.



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It is clear from the previous discussion that the interactions between PLA and CO2 make the use of CO2 as a blowing agent far from simple. The gas plasticises the polymer, reducing its already low Tg (results [22] suggest that Tg falls below 25 °C with around 8% CO2), but the gas diffuses through it quite quickly so is not well retained. The promotion of crystallisation by the combination of this plasticisation and the biaxial orientation which develops during expansion means that, after expansion, beads of crystallisable grades of PLA can have a significant crystallinity. Although this boosts heat resistance, which is desirable in the finished product, it also reduces the ability of the beads to expand and fuse together during the moulding process. Fully amorphous PLA has quite poor heat resistance, with a maximum service temperature around 40 °C, a little too low for even the majority of packaging applications. PLA with some crystallinity has better temperature stability, so that companies wishing to market foamed PLA products have to find a way to impregnate their beads with a controlled level of CO2 then expand and mould the beads to a product with good fusion and mechanical properties – how they have achieved that is discussed next.

4.2.4 Processing of PLA-based Beads – Review of Recent Patents and Patent Applications

The great bulk of patents and applications on the topic of foamable PLA beads come from Japanese companies, most of them filed only in Japan, so that only the abstracts are available in English. The exceptions are JSP Corporation, which has filed in Europe and the USA, and the Biopolymer Network of New Zealand, which has recently filed a world patent application – much can be learnt from the descriptions in those documents. Filings by other Japanese companies will also be reviewed in less detail – since 2003, Kanebo Ltd, Unitika Ltd, Sekisui Corporation, Kanegafuchi Chemical Industries (Kaneka) and Achilles Corporation have also made applications which appear relevant.

JSP’s first application dates back to 2003 [25] and claims an expanded bead containing at least 50 mol% of lactic acid units (allowing for


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copolymers or blends with other polymers, although none of the examples have any other polymers in their makeup). The principal focus of the description and claims is on the crystallinity of the beads, in the expanded state, where it should not be too high. As amorphous but potentially crystalline PLA is heated, it can undergo an exothermic ‘cold crystallisation’ process at 80–90 °C – as this coincides with typical prefoaming temperatures, it can limit the expansion and interfere with bead fusion in the later moulding step, but also brings improved heat resistance to the moulded product. The description emphasises that the conditions under which the beads are extruded, impregnated and then expanded are critical to the control of the crystallinity, especially the temperature at which the blowing agent is impregnated and the duration of the heating during expansion. Both crystallisation speed and maximum crystallinity of the material can also be controlled by blending crystalline (higher L-content) and amorphous (higher D-content) grades of PLA, and the examples cover a range of compositions from 100% of the crystalline grade to 50% – the best results appear to be found at 50%, but many other factors are varied including impregnation and expansion conditions and also the post-ageing of the mouldings. Talc is used as a nucleator for both crystallisation and foaming.

The blowing agents mentioned in this patent include a list of organic agents such as propane, butane, pentane, hexane and haloethanes, but it is stated that inorganics such as air, nitrogen, argon and CO2 are better from the cost and environmental standpoint, particularly CO2, and all the examples use it. Impregnation of the beads is carried out under pressure in an autoclave, either ‘dry’ or suspended in water, at a temperature below the final Tg of the impregnated polymer – there is a formula to determine the optimum temperature based on the desired CO2 content, which yields values below 40 °C. The impregnation pressure is in the range 0.5–10 MPa, applied for up to 12 hours, to charge the beads with up to 20% CO2 (the examples range from 6 to 14%). The expansion is best carried out using a mixture of steam and air, at a temperature around 60–70 °C. The expanded beads are then repressurised with air or CO2 before moulding using steam, as this improves the fusion. Finally, the mouldings are aged for up to



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24 hours at 60–70 °C to complete the crystallisation process and optimise heat resistance.

A 2006 filing [26, 27] demonstrates further refinement of the technology. Again, the description covers blends of crystalline and amorphous PLA grades, possibly also copolymers and blends with other polymers such as biodegradable polyesters similar to BASF’s Ecoflex (see also Section 4.3.2). The account of the prior art describes how the use of high-pressure steam for moulding, necessitated by the high crystallinity of the expanded beads, can damage the amorphous regions of the beads and cause shrinkage and an uneven surface. The solution to this problem includes the careful control of crystallinity as laid out at length [25] previously, but also the use of a polyolefin wax, preferably a polyethylene, of closely defined molecular weight and crystallinity, as a nucleating agent to promote a fine cell structure in the expanded beads. Carbon dioxide is again the preferred blowing agent, preferably impregnated into a suspension of the beads in warmed water as before, to a level between 5 and 15%. Another co-blowing agent may be used, such as isobutane, although this is not in the examples.

There is also a claimed benefit from incorporation of a plasticiser as well as a ‘fusion bonding improver’, flame retardants, antistatic agents, etc. There is a long list of useful plasticisers, although the most suitable is stated to be glycerin diacetomonocaprylate – the function of the plasticiser is to lower Tg of the polymer by a few degrees, thereby improving the secondary expandability and fusion during moulding. Interestingly, the plasticiser is optimally applied to the outer surface of the beads during the CO2 impregnation process. This ensures that the plasticiser penetrates the surfaces of the beads, and helps during expansion to create what is claimed to be an advantageous cell structure where a 300 m layer at the surfaces of the beads has larger cells than the core. The expansion, repressurisation to an internal pressure of 0.05–0.2 MPa and moulding processes are as in the previously discussed patent [25], but there is in this patent no need for a post-ageing of the mouldings.


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In the examples, where moulded densities down to 50 kg/m3 are reported, a range of polyethylene waxes are compared – such waxes are also used as nucleators in EPS. Most of the examples were made with 20% crystalline PLA and 80% amorphous PLA (both from Mitsui Chemicals), although later examples were at higher levels of the crystalline grade. In all cases the processing of the beads was the same, as the main purpose of the examples was to explore the effect of the different nucleators. The impregnation with CO2 was carried out at 30 °C at a pressure of 3.0 MPa, falling gradually to 2.5 MPa over three hours – this gave a concentration in the beads of around 5–6%, which proved sufficient to expand them down to 40–50 kg/m3. Despite the extensive description of their benefits, plasticisers were not used in the examples. The beads were expanded at 80 °C for one to two minutes, and after repressurisation with air at 0.2 MPa, moulded with steam at 0.02 MPa, yielding mouldings with densities from approximately 50 kg/m3 upwards.

The Biopolymer Network application [28] takes a very different approach, using liquid CO2 for impregnation, at a moderately low temperature (10 °C appears optimal) but high pressures (in order to maintain the CO2 in the liquid state). In this process, PLA beads (which are claimed not to require nucleation) are immersed in liquid CO2 at around 10 °C and a pressure of 6 MPa for a period of 30 minutes to 4 hours. Under these conditions, the beads can absorb 30% or more CO2 – the pressure is then released and the beads cooled to 20 °C and stored for a period (which can be as long as a month). During the first day or so of storage, the CO2 concentration falls by some two-thirds, then settles and falls only slowly thereafter – the gas is also claimed to redistribute itself within the beads, and this ageing process is said to be important for foamability.

Following the storage period, the beads contain 8–12% CO2 and can be expanded at a temperature in the range 50–70 °C – some of the examples were expanded in hot water. The pre-expansion yields a prepuff that can then be moulded immediately (no need for the maturing process which is typical of EPS) or stored (again in a freezer) for moulding at a later date. Moulding is carried out at temperatures



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up to 110 °C, although the claims cover both the case where the moulding is effected at or above the pre-expansion temperature and the case where it is done at or below the pre-expansion temperature, and the examples are similarly various. What is made clear is that the pre-expansion should not exhaust the expansion potential of the beads, and that enough CO2 should remain to drive further expansion during moulding.

The claims make no reference to whether amorphous or crystalline PLA is desirable, although they do cover the possibility of using blends with other polymers such as EVA or synthetic polyesters such as PBS, and also fillers such as talc, chalk and ground pine bark. The description and examples do explore the amorphous–crystalline PLA balance and although all the subsequent examples use amorphous PLA, the first reports successful expansion and moulding of blends with up to 50% of a crystalline grade. The description does, however, observe that the more crystalline the PLA, the less effective the process is usually found to be – presumably this is why the blends exemplified all had at least 50% of an amorphous grade, and the beads might be expected to have little or no crystallinity. This is unsurprising, in the light of the accelerating effect of absorbed CO2 on crystallisation discussed in Section 4.2.3. Unless the low temperatures prevent it, these high CO2 levels would accelerate any potential crystallisation, making the beads much harder to expand. If the beads do remain amorphous (and the application does not discuss the development of crystallinity during the processes), the heat resistance of foamed objects made by this process will not be high. Amorphous PLA has a long-term service temperature limit close to 40 °C, so foamed amorphous materials would also be very limited in terms of the ambient temperature range they could tolerate without undergoing softening, shrinkage or distortion.

Kanebo has three 2006 applications in Japan [29-31]. The summary of the first [29] indicates that the use of an ‘aprotic’, non-aqueous dispersion medium improves the gas impregnation process, and also refers to the expansion process being carried out at between 10 and 35 °C above Tg of the PLA polymer. Moulding is best carried out by


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preheating the expanded beads to 10 °C below Tg to 30 °C above Tg, then moulding them at between 30 and 60 °C above Tg. The English summary of the second application [30] is very general, claiming that use of 0.5–10% of additives including a plasticiser allows the volume expansion ratio to be controlled. The objective of the third application [31] is to provide antistatic mouldings at an expansion ratio of 30, by incorporating 1–3% of a secondary alkanesulfonic acid sodium salt into the PLA.

Unitika’s applications are mostly in Japanese and the English summaries are often so brief as to give little idea of the technical content of the application, but there are two applications available in English that provide more details [32, 33]. Both are oriented principally towards foamed sheet products of relatively high density, but reveal a preference for crosslinking the PLA using a peroxide and an ester of methacrylic acid. The more general application [32] covers a wide range of applications including blow mouldings and injection mouldings, but does have one example that describes foamable beads which can be expanded to a ratio of 45 using butane, impregnated into the beads at 10 MPa and 130 °C. The compositions include nucleating agents (which promote crystallisation as well as foaming) and ‘foaming aids’ such as calcium or magnesium stearate, or stearic acid. The purpose of the invention described in the other application [33] is to reduce the water vapour permeability of a (CO2) foamed PLA sheet which can be thermoformed into containers, by incorporating a wax (candelilla or paraffin wax) and low-density polyethylene (LDPE).

Sekisui Plastics’ recent intellectual property appears to take a different approach to the moulding of foamed articles, that of foaming the beads in the mould, to expand and fuse them at the same time. A 2007 Japanese application [34] claims a benefit from using water as the heating medium, at 60–100 °C. A world patent application filed in 2008 [35] describes a die-face pelletising system for cutting beads as they expand from the die – this produces roughly spherical expanded beads, which further expand isotropically during the moulding process and fuse together strongly. It is not clear from the



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abstract how much expansion is achieved at the die and how much more during moulding, nor what blowing agents or other additives are required.

Two very recent applications [36, 37] from Kanegafuchi/Kaneka also relate to an in-mould foaming process for PLA beads initially foamed using a hydrocarbon blowing agent. These expanded beads are then pressurised with CO2 for a short period (0.5–10 minutes at 0.1–2 MPa pressure) before moulding. This appears to be a step back from using CO2 as the only blowing agent, as others have done in recent times – perhaps the shelf life of beads between impregnation and expansion is considered too short.

Achilles Corporation claims to have extended shelf life (before expansion) by using cyclopentane as a foaming aid in beads impregnated with one of the common hydrocarbon blowing agents (isobutane, n-butane, isopentane, n-pentane) [38].

In conclusion, then, we can say that the various challenges faced in producing mouldable foam beads from PLA, especially when CO2 is used as blowing agent, have been tackled in different ways by a number of workers. That moulded articles from foamed PLA remain scarce is perhaps an indication that the costs of producing them are not yet competitive – perhaps that will change in the near future.

4.2.5 End-of-life Aspects of PLA Foam Products

PLA of a useful molecular weight is insoluble in water, and takes water up only slowly from its surroundings. Once water does penetrate the material, it can break down the polymer chains by hydrolysis of the ester linkages, principally in the amorphous phase – the broken chain ends include carboxylic acid groups which catalyse further degradation. In general, this process is rather slow, and although products made from PLA can decompose in industrial composting systems when mixed with other wastes, if there is too much PLA present the composting process can be excessively prolonged.


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There have been problems, for example when large numbers of PLA drinking cups were collected from outdoor events and proved slow to break down. Foamed PLA can be expected, by virtue of its lower density and open structure (once the cell walls begin to break down), to compost faster than solid items, but each type of product may need to be tested and shown to be compostable before it can be granted certification.

Recycling of PLA is entirely practicable – waste material may be remelted and reprocessed into new products. Predrying is important to reduce hydrolysis during processing, or chain extenders may be added to rebuild the molecular weight. It is likely that recycling of PLA packaging products will be preferred wherever possible, including where used foam packaging can be economically collected, just as for EPS.

Incineration of waste, including plastics, is now common practice in Europe, and where the energy is recovered by using the heat of combustion to generate electricity is a reasonably carbon-neutral means of disposal for the renewable polymers discussed in this chapter. It is certainly better than burial in landfill, where organic materials such as waste plastics tend to break down anaerobically and release methane, a short-lived but potent greenhouse gas.

There is in principle, no reason why foamed PLA (or PHA or cellulosics) cannot be used for long service life applications such as insulation boards, though it is hard to see how this could be achieved for TPS because of its high water absorption. With suitable flame retardants and antibacterial additives, or a coating of some kind to exclude water, it should be possible to retain integrity for many years, though this would require demonstration and certification to satisfy, for example, construction standards. It might appear counterintuitive to use an inherently biodegradable material for a long-life product, but as oil prices increase it may eventually be cost-advantageous to use sustainable polymers instead of EPS, and there are numerous other biodegradable materials such as wood widely used in construction. At the end of life such materials can then be recycled, incinerated or perhaps treated in some way to accelerate their biodegradation.



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4.3 Starch and Starch-based Foams

4.3.1 Production and Properties of Starch-based Polymers

Starch is an inexpensive and widely available plant-derived polymer, which is hygroscopic. Water plasticises the polymer, and in excess dissolves it – this renders it processable but places limitations on its usefulness in applications where durability is important (construction, and even much protective packaging). Improvement of the serviceability of TPS has therefore been a long-running research interest, although to date most applications have been where its rapid breakdown by water is an advantage, such as in disposable hygiene products, golf tees and fast-food packaging.

Starch is a polymer of D-glucose, with two types of repeat units, linear amylose and branched amylopectin (see Figure 4.2), the proportions affecting the properties of the polymer. In its raw form it consists of hydrophilic crystalline beads some 15–100 m in diameter, with a melting point above its decomposition point around 250 °C. The crystalline structure can be broken down in a gelatinising or destructuring process, using water and other plasticisers, heat and shear, so that a melt-processable form, TPS, results. It is essentially amorphous in nature and is readily plasticised by water – adjustment of the water content allows the processing behaviour to be controlled to suit the application [39]. In the main, useful TPS grades are at least 70% (linear) amylose.

The properties of TPS are quite poor – it softens at a temperature that depends on the content of water and other plasticisers, but is usually between 60 and 120 °C. Films made from pure TPS are hard and brittle and much of the development work done on starch-based polymers has had the objective of improving the mechanical, thermal and durability properties without losing the cost and biodegradability advantages. There are a number of commercially available blends such as Novon (Warner-Lambert and Chisso Corporation), Mater-Bi (Novamont) and Plantic


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(Plantic Technologies and Du Pont) with properties tailored to particular applications.

Amylose

Amylopectin

Figure 4.2 Structures of amylose and amylopectin, the component repeat units of starch

4.3.2 Processing of Starch-based Foamable Beads

Water contained within TPS beads can be used successfully as an environmentally friendly and cheap blowing agent. If the material is heated quickly, the water boils and foams the material rather than being driven off. A long-established and popular use is for loose



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fill packaging ‘beans’, easily disposed of by dissolving in hot water once their useful life is over [40]. In order to make useful moulded products, the challenge is to produce foamable beads which can be easily moulded (fused), and also to improve the durability of the moulded products, discussed in more detail in Section 4.3.3.

A patent application filed in 1998 [41] from the Institute for Agrotechnical Research (ATO) at the University of Wageningen in The Netherlands describes a method for producing relatively thick moulded parts in a single step which combines expansion and moulding. Granules of TPS are conditioned to a water content around 15%, then coated with a plasticiser which also acts as an adhesive. The granules are place into a mould and there heated using microwave energy. This heats the water to steam which expands the starch and, with the aid of the adhesive, fuses the beads into a single foamed piece. There is no record of this application proceeding to granted status – the method is clearly only practicable with a high-power microwave oven and non-metallic moulds.

Using water as the blowing agent for starch has the disadvantage that the concentration of water must be tightly controlled before expansion, as it affects the rheology of the material as well as providing the driving force for the expansion. If the water concentration is too high, the material may have insufficient melt strength to sustain a good cell structure, too low and it may not be able to expand sufficiently. This makes the processing of the material inherently unstable and unpredictable. Further, employing the conventional EPS processes, using steam to expand and mould foamed TPS risks damage to the foam structure (‘burning’ of the beads) as the temperature of the steam will usually be above the softening point of the material. BASF has addressed this issue by using more conventional blowing agents, in beads made from starch-based blends [42] which have more heat and moisture resistance. This patent claims blends of a biodegradable copolyester with starch or a cellulose-derived polymer, expanded using one of propane, butane, pentane, methanol, ethanol or propanol as the blowing agent. The copolyester is the condensation product of a C2–C12 alkanediol or a C5–C10 cycloalkanediol with a mixture of an


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aliphatic or cycloaliphatic C2–C20 dicarboxylic acid and an aromatic dicarboxylic acid. The blowing agent is injected into the barrel of an extruder before the blend is pelletised under water and under pressure (as in Section 2.6.1) to produce cooled expandable beads for later expansion and moulding in conventional EPS equipment. As there are no examples it is not entirely clear which copolyester is intended, although BASF offers such a product with the trade name Ecoflex , to be used alone or as a modifier for biodegradable polymers such as PLA. It is a synthetic polymer, not bio-based, but is certified as being biodegradable. The claimed compositions range from 1 part copolyester to 9 parts starch or cellulosic, to the converse – it is not stated in the text where the optimum composition (processing as well as service properties) may lie, as this will depend on the chosen application. Ecoflex is a soft and flexible polymer, so the properties of blends with TPS or cellulosic polymers depend on their proportions – more Ecoflex improves toughness and ductility and, in general, foamability.

A 2006 paper written by a team of collaborators from the University of Pisa, Italy, and the Agriculture Research Service of the US Department of Agriculture [43] reports work on making foamed sheets from a blend of unmodified potato starch, polyvinyl alcohol (PVOH, a synthetic degradable and water-soluble polymer) and corn fibres, a waste product from the fermentation of corn (maize) to make ethanol. These ingredients were mixed together with water at room temperature to make a batter. This was then injected into a steel tray mould and baked at 200 °C for two to three minutes. The batter foamed during the baking to yield plates whose mechanical properties were measured. The conclusions were that corn fibres did not improve mechanical properties, but did not interfere with foaming and improved the water resistance of the foamed material. PVOH also improved the water resistance and mechanical properties. Clearly this work was not aimed at making expandable beads, but it does show the types of starch blends being studied for foaming performance.

A blend system that was aimed towards expandable beads is described in a paper from the US Agricultural Research Service [44]. The



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formulations of the blends included as plasticisers for the starch, sorbitol, glycerol, ethylene vinyl alcohol (EVOH) and water, extruded into pellets or ground into particles. On heating at 190–210 °C for more than 20 seconds at atmospheric pressure, the beads expanded to low densities. As the water concentration was raised from 10 to 25%, so the foaming temperature fell by more than 20 °C. All of the additives except the EVOH acted to reduce the foam density, and wheat or potato starches gave lower densities than corn starch. There is no mention of moulding the expanded beads, although it would be expected to be feasible as for the other systems described here.

4.3.3 End-of-life Aspects of Starch-based Foam Products

As mentioned previously, starch itself is hydrophilic and readily absorbs water from the air, so its biodegradation by bacterial enzymes tends to be rapid. This affects the serviceability of starch-based materials, which can even deteriorate in use, before their packaging task has been fulfilled. This is naturally undesirable, and shows that there is always a balance to be struck between degradability and durability, dependent on the application, and that great care needs to be taken in material selection and testing before ‘green’ claims can be made with safety.

In order to obtain more useful properties and greater durability, many blends have been made with other sustainable polymers, such as PHA (Section 4.4), cellulose acetates (Section 4.5) and PCL. In addition to the blends discussed above, starch has also been blended with oil-based polymers including polyolefins and even expandable polystyrene [45] in order to reduce their cost and accelerate their disintegration in the environment (not always with demonstrable success). If the starch domains are fully encapsulated by a more hydrophobic polymer, degradation can be significantly delayed [5] as the enzymes are unable to reach the starch. There was an enthusiasm for supermarket carrier bags made from starch-filled polyethylene in the early 1990s, but it was eventually shown that such bags could remain virtually intact for many years even after burial. Again, any


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product made from a starch blend must be designed and tested to show that it meets the requirements.

Recycling of starch-based polymers is unlikely to be a preferred option, not least because of the variety of plasticisers and blend additions present in the different grades – waste material could be very mixed in nature and challenging to process. As starch-based materials are essentially sold for their biodegradability, composting (or burning for energy recovery) will be the most attractive disposal route.

4.4 Polyhydroxyalkanoates (Including Polyhydroxybutyrate (PHB) and Copolymers)

4.4.1 Production and Properties of PHA Polymers

PHA polymers are true ‘biopolymers’, as they are synthesised within the bodies of bacteria grown in special digesters, and subsequently extracted from the biomass. The polymer can make up to 80% of the biomass, stored by the bacteria as an energy reserve. These polymers have been of interest (though sadly not sufficiently to make them profitable volume materials) for a number of years and ownership of the technology has changed hands several times since ICI first marketed Biopol in 1990. One current manufacturer is Metabolix (as Mirel) which has a 50,000 tonnes per year plant in Iowa, USA, while other companies are reportedly also investing in PHA capacity [2]. Cost remains a severe handicap [46], and has to date inhibited the introduction of these materials into the very cost-sensitive foam packaging market – Mirel prices were quoted in 2008 [47] at more than $4.4 per kg, far above the price of oil-derived polymers such as polystyrene or polyolefins. As for all potential applications of ‘sustainable’ polymers, legislation or customer diktat can change the economics to some extent, but it appears that unless the production costs of PHA reduce significantly with increased scale (for other applications), they are unlikely to become a major player in the near future.



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Figure 4.3 Structures of polyhydroxybutyrate (PHB), polyhydroxyvalerate (PHV) and the copolymer, PHBV

The composition of a PHA copolymer – polyhydroxybutyrate-co-hydroxyvalerate (see Figure 4.3), with 5–12% of valerate units randomly incorporated along the chains – is controlled by manipulation of the ‘food’ (mostly glucose) supplied to the bacteria. Tg of pure PHB is 5 °C and its melting point is 170–180 °C – the presence of the hydroxyvalerate (HV) units acts to lower the melting point, increase impact strength and flexibility and reduce tensile strength. The copolymers are crystalline, although as the HV content rises so the rate of crystallisation reduces, and nucleants are routinely added to accelerate crystallisation during processing. Plasticisers are also used to improve processability and flexibility.

The properties of PHA vary with the composition, but generally speaking the more common grades resemble polyolefins – at mid-range HV contents they are tough like polypropylenes, at higher levels they are softer and more like polyethylenes. This has led to applications in packaging such as films and bottles where their degradability brings added value sufficient to offset their high cost.


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4.4.2 Blowing Agents and Processing of PHA Foamable Beads

Kaneka Corporation, also known as Kanegafuchi, has been active in the technology of foaming biodegradable polymers, and has filed two patents in recent years on expandable PHA. In the first of these [48], solid beads of poly(3-hydroxyalkanoate) (Kaneka’s patent writers prefer this version of the chemical name for PHA copolymers) are formed, then suspended in a blowing agent in a sealed vessel, pressurised and heated to start the expansion. The blowing agent is preferably dimethyl ether, diethyl ether or methyl ethyl ether – all have low boiling points and will impregnate the polymer. Once the beads are sufficiently saturated with ether and heated to a temperature not far below their melting point, the vessel is opened and the beads complete their expansion. The patent goes into much detail about the necessity for controlling the valerate co-monomer level and having two crystalline melting points, and much less detail about the way the beads are first produced. Moulding of the expanded beads into shaped products is discussed without much detail – they are first prepressurised with air (as in EPP moulding), then fused with steam.

The second Kaneka patent [49] follows much the same lines, but additionally claims benefits from the addition of an isocyanate compounded into the polymer (as a chain extender). A wider range of blowing agents is also claimed, including isobutane and pentane. There are no details of the bead-making process, and again the moulding process is barely discussed. In the given, the temperature in the impregnation vessel is 120–130 °C, close to the polymer melting point, and the pressure is held at 2.5 MPa for an hour.

The importance of controlling crystallisation behaviour is emphasised by a joint 2008 patent application from Kanegafuchi Chemical (Kaneka) and the Tokyo Institute of Technology [50]. The text of this patent is in Japanese, but the English abstract reveals that it is claimed that there is a need to accelerate crystallisation in PHA in order to improve processability. This applies to all melt processing –



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injection moulding, blow moulding, fibre spinning, extrusion foam moulding and bead foam moulding are all quoted – and is achieved by the use of a sugar alcohol (galactitol) as a nucleating agent. Differential scanning calorimetry traces shown in the abstract have melting curves of PHB and PHBV (the valerate used as co-monomer) with no nucleant, with 2% hexagonal boron nitride and with 2% galactitol – clear differences are visible, with the galactitol increasing the size of the melting peaks but also their peak temperature.

4.4.3 End-of-life Aspects of PHA Foam Products

When PHBV articles are exposed to microbially active environments, enzymes begin to attack the surface of the polymer, eventually breaking it down into carbon dioxide and water in aerobic conditions, or carbon dioxide and methane in anaerobic conditions. In composting conditions it may take a reasonably long time for solid parts to decompose fully, but foamed material can be expected to degrade rather more quickly. As with all thermoplastics, PHA are potentially melt reprocessable, although their heat stability is limited, and temperatures above 180 °C should be avoided.

4.5 Cellulosic and Other Sustainable Polymers

Cellulose-based polymers were the first commercial thermoplastics, developed in the middle of the nineteenth century (cellulose acetate, cellulose acetate butyrate, cellulose nitrate) and still find a range of applications. They are obtained by acidic digestion of plant-derived cellulose, followed by further treatments to render the materials processable and reasonably durable. They tend to be expensive (more than three times the cost of polystyrene) which severely limits their market share.

The raw material, cellulose, is a polysaccharide which makes up a significant proportion of the cell walls of many plants. Cotton fibres can be as much as 90% (dry weight) cellulose, while wood is closer to 50%. Pure cellulose is believed to be essentially polyanhydroglucose


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(C6H10O5)n, with linear chains, extensively hydrogen bonded, which prevents it from being water soluble and gives it the ability to crystallise (see Figure 4.4).

Cellulose

Figure 4.4 Structure of cellulose (anhydroglucose)

Cellulose acetates are nowadays the most commonly used cellulosic polymers, produced by the reaction of acetic anhydride on wood or cotton linters (the short waste fibres remaining after textile-grade fibres have been removed) to yield materials with a range of degrees of substitution (DS; acetylation of the anhydroglucose base unit) typically between 1.7 and 2.5. Pure cellulose has DS = 0 and cellulose triacetate has DS = 3. In general they have a high crystalline melting point (around 200 °C) which must be reduced by plasticisation to improve processing. As the materials are hard, tough and transparent, they still find reasonably wide application in sheet and film form for packaging and adhesive tapes, and are also moulded into such decorative items as combs, toothbrush handles, etc. The rate of biodegradation depends on DS, faster at lower levels, and at DS = 2.5 the polymers are very slow to degrade, because of the slow rate of attack by deacetylase and cellulase enzymes [5]. Certainly, few commercially available cellulosic materials decompose fast enough to be certified compostable – one objective in their original development was to prevent rapid deterioration and render them serviceable for durable products. The reader is referred elsewhere for an example of a more comprehensive description of the technology of cellulosic polymers [1].



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An early Japanese patent to Teijin Ltd [51] describes a method of producing a biodegradable foam moulding using expanded cellulose acetate. Foamed beads are made by extruding a mixture of cellulose acetate and water through a fine-hole die at temperatures above 120 °C, then cutting the strand to form beads which expand immediately. These beads are then coated with 3–50 wt% of a low melting point (around 60 °C), biodegradable polymer, PCL, and the beads fused by heating them in a mould. This is a similar approach to that which can be taken with other polymers that are difficult to fuse once expanded (see Sections 2.6.4 and 3.4), but does add to the cost and density of the moulded product.

The BASF patent [42] discussed in Section 4.3.2 also claims the use, in expandable beads, of cellulosic polymers in a blend with a biodegradable copolyester. This does not fully meet the need for sustainability, as the copolyester is a synthetic polymer derived from oil, and even if the product is practicable and economic, it may not be easily accepted if full sustainability is mandated by governments or customers.

A wide-ranging presentation [52] was made by workers from the Fraunhofer Institute for Chemical Technology at a symposium in 2006, hosted by Gala GmbH, makers of underwater pelletising systems. Part of their paper described a project, supported by the state government of Baden-Württemburg, on foamable biodegradable materials. Successfully mouldable (fusible) foamed beads were produced from cellulose acetate using pentane as blowing agent – less success was met with PLA, as the foam nucleation was poor and the beads expanded with just one (large) cell in each. This demonstration study was not intended to develop commercial products, and it appears that the high cost of the cellulosic base material is likely to discourage any industrial development at present. The biodegradation of cellulosics tends to be very slow, unless the polymer has been ‘primed’ using a plasticiser which also promotes degradation – compounds such as Bioceta (currently produced by Mazzucchelli 1849 SpA) can meet compostability standards.


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Although there are a variety of other bio-based or biodegradable polymers now available, such as the PCL mentioned previously and PVOH (a synthetic but degradable polymer), there has been little interest in using them in foamable beads. Cost (principally), processability and control of their degradation behaviour have tended to inhibit their exploitation. As mouldable particle foams come a long way down the hierarchy of applications to be explored when new polymeric materials are introduced, it is unlikely that such materials will appear as foamable beads in the near future.

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5 Concluding Remarks – What Forces Will Drive Development in this Field?

It is always very dangerous to attempt prediction, certainly in any detail – as the history of the plastics industry shows, the unexpected should be expected at every turn. The course of recent events has only confirmed that volatility is increasing, and companies involved in the mouldable particle foam industry will need courage to plan their strategies through the coming years and make the most profitable investments in future capacity. There are, however, some general principles which will influence the way in which this industry (and the wider plastics industry) is able to develop and compete in the future. It must be borne in mind that moulded foam products are only one approach to the requirements for protective packaging, insulation, etc. – alternative technologies may be able to take advantage of developments to displace existing applications of moulded foams, or new opportunities may arise. This chapter has no references as there is such a wide variety of forward-looking material available, with very many different starting points, scenarios and messages – I leave it to the reader to review as many as possible, choose those which appear most credible and then to try to make the best decisions about the future.

The critical factors that are likely to influence foamable bead producers and processors are largely the same as they have always been, although the balance between them is likely to continue to change with events. Important drivers are the costs of raw materials, investment in production capability, transport of products to customers and disposal of products at the end of their service life – but the most important will be the performance of these products when compared in cost-effectiveness with the


108


alternatives. For example, if the price of crude oil rises sharply and remains high, biomass-derived sustainable polymers should, although not unaffected by the higher costs of energy for processing and transport, become more competitive even in the absence of legislation driving their adoption. If brominated fire retardants are proscribed, much of the construction insulation market may be lost to expandable polystyrene (EPS) unless acceptable alternatives can be found which are not excessively costly. Unless customers continue to select particle foams as their preferred solutions for packaging, protection, insulation and so on, the industry will wither away, however its technology develops.

The most likely driver of change over the medium term will be the increase in the price of crude oil as production peaks and begins to decline, although it is almost inevitable that political and economic instabilities will impose strong fluctuations on the rate of that increase. Those instabilities and fluctuations will also have an impact on the investment decisions made by the companies active in this industry – some will be bold and perhaps lose their shirts, some will be more cautious and maybe miss golden opportunities. It is, however, possible to see where technological development effort could be most usefully directed to give the industry the best options to face the coming challenges (not forgetting that some of these challenges will be new and unexpected).

The EPS industry faces the likelihood of increasing raw material and energy costs, increased restrictions on its emissions and increasing costs for transport both of its products and of the products for which EPS is used as packaging. It is therefore likely that development which enables further reduction of foam density (and therefore product weight), while retaining or improving performance, will be fruitful, as will reduction of volatile organic compounds (VOC) emissions during manufacture and processing of EPS. Lower densities will of course have an impact on the costs of recovery and recycling, activities which may increasingly be mandatory. As alluded to above, fire performance may become a critical issue for the insulation market – perhaps the Xire technology (see Section 2.2) or something comparable to it will


Concluding Remarks – What Forces Will Drive Development in This Field?

109

enable growth in a different direction, or a different solution will be found to the challenge of flame retardation.

The polyolefin particle foams, EPE and expanded polypropylene (EPP), are likely to continue to find new markets into which to grow, where their cushioning and durability can be exploited. Increasing transport costs for expanded beads (or if the beads are moulded in fewer locations, the transport to the customer) could drive development of small-scale impregnation and expansion equipment, so that relatively dense beads can be transported to a site close to the end-user, then expanded and moulded as needed. It is even possible that, as this technology seems likely to be needed for the processing of polylactic acid (PLA) beads (the front-running sustainable foam material), there will be some useful spin-off back to the polyolefin foam processors.

The ‘sustainable’ polymers present a very interesting prospect, but, as their history shows, there can be unexpected turns in the road. In the late 1990s, biodegradability was seen as a ‘unique selling point’ which would solve the perceived end-of-life issues for foam packaging products and enable biodegradable polymer foams to gain a market foothold from which to grow. As the trend away from the ‘throwaway society’ mentality has led to the establishment of better collection and recycling infrastructure for waste packaging, so the emphasis has shifted to a focus on raw material sources and their sustainability – renewability is the new unique selling point for these polymers. At present PLA polymers, alone or in combination with other polymers, appear to be leading the field, largely based on their relatively low cost and the wide variety of biomass that can be used to produce them. This situation may change – there is active research going on into all kinds of materials formerly considered ‘exotic’, oriented around the availability of biomass which is not in competition with food uses. Some feel that as the world’s population continues to grow, anything edible is more likely to be eaten rather than turned into polymers. Cellulose is an extremely common plant material, but not so far exploited to the full – perhaps new methods can be developed to process it to useful products. Whichever foam system


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using sustainable polymers is adopted, its process technology will be different from that of EPS, and can be expected to drive innovations which will open up new opportunities and ways of organising the industry to meet customers’ needs.

The future will, therefore, present the particle foam technologist with new challenges and opportunities. These may be similar to those which have already been addressed, and for which the solutions have been described in this update, or they may be completely new. Of one thing we can be certain, there will always be a drive to fulfil existing needs in a cheaper and ‘smarter’ fashion, and to meet new needs as quickly and efficiently as possible. Without such progress, the industry will be left behind and wither away, so there will continue to be ample scope for the technologist who can invent, adapt and improve.


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Abbreviations for Mouldable Particle Foam

A-PET Amorphous polyethylene terephthalate

ASTM American Society for Testing and Materials

ATO Agrotechnological Research Institute

CFC Chlorofluorocarbon(s)

CO2 Carbon dioxide

DIN Deutsches Institut für Normung (German Institute for Standardisation)

DMPP Dimethylphenylphosphonate

DOPP 9,10-Dihydro-9-oxa-10-phosphaphenanthrene 10-oxide

DS Degree of substitution

EBS Ethylene-bis-stearamide

EPE Expanded polyethylene

EPO Expanded polyolefin

EPP Expanded polypropylene

EPS Expandable polystyrene

EVA Ethylene-vinyl acetate

EVOH Ethylene vinyl alcohol copolymer

HBCD Hexabromocyclododecane

HCFC Hydrochlorofluorocarbon(s)

HMS High melt strength

HV Hydroxyvalerate

ISO International Standards Organisation

JSP Japan Styrene Paper


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LCA Life cycle analysis

LDPE Low-density polyethylene

LLDPE Linear low-density polyethylene

NAFTA North American Free Trade Agreement

P(DL)LA Poly (D,L) lactic acid

PAT Pressure and temperature

PBS Polybutylene succinate

PCL Polycaprolactone

PDLA Poly (D) lactic acid

PEPP Porous expanded polypropylene

PET Polyethylene terephthalate

PHA Polyhydroxyalkanoates

PHB Polyhydroxybutyrate

PHBV Polyhydroxybutyrate-valerate

PHV Polyhydroxyvalerate

PLA Polylactic acid

PLLA Poly (L) lactic acid

PPE Polyphenylene ether

PPO Polyphenylene oxide

PVOH Polyvinyl alcohol

RDP Resorcinol diphosphate

REACH Registration, Evaluation, Authorisation and Restriction of Chemicals

SMA Styrene-maleic anhydride

Tg Glass transition temperature

TPP Triphenyl phosphate

TPS Thermoplastic starch

TPY Tonnes Per Year

VOC Volatile organic compounds

WEPS Water-expandable polystyrene

XPS Extruded polystyrene (foam)


113

Author Index

A

Abe, T 43Ajioka, M 103Allen, R B 49Allmendinger, M 42, 45, 47, 48, 49Andreas, W 49Angelopoulou, A G 104Anlas, G 67Aoyama, T 106Araya, A 43Arduini-Schuster, M 44Aw, H D 50Aw, H S 50

B

Barry, R P 47Bastioli, C 103Batscheider, K-H 44Berghmans, H 48Berghmans, M 43, 48, 67Berrios, J 105Biglione, G 49Bleijenberg, K 43, 48Bleijenberg, K C 48, 67Boukami, S 42Braun, F 43, 44, 105Brydson, J A 102Buijk, C M G 43


114


C

Cai, Z 48Carlier, C 47Casalini, A 45, 46, 49Champagne, M F 48Chaudhary, B I 47Chen, L 66Chen, N 65Chiellini, E 105Chiou, B-S 105Chiou, N-R 48Choi, J B 66Chung, M J 66Cigna, G 49Cinelli, P 105Cink, K 103Cronin, D 67Culkova, A 47

D

Daniel, T 48Datko, A 45, 49Daum, M 46Day, M 104de Grave, I 66de Mink, P 65de Swart, H J 43Dehnert, P 47Delaviz, Y 48Der Hovanesian, M 105Dietzen, F-J 42, 43, 45, 47, 48, 49, 66Douay, D 47Duan, S 66


Author Index

115

E

Eaves, D 5, 41, 50, 65, 66Eberstellar, R 44Ehrmann, G 42, 45, 47, 48, 49, 66Enamoto, K 103Erbay, E 46Ezdesir, A 46

F

Favis, B D 103Feil, H 105Felisari, R 45, 46Fiordelisi, F 103Fischer, J 43, 44Folland, R 65, 66Ford, T M 103Francis, T 43, 46Frigiero, G 45Fuminobu, H 106

G

Gaeth, R 65Galewski, J-M 47Gan, J 43Gendron, R 48Ghidoni, D 45, 46Gilchrist, A 67Gilead, D 105Glenn, G M 105Glenz, W 5Gluck, G 42, 43, 44, 48, 105Gougeon, B 41Grave, D de 66Green, J R 49Guo, Z 48Gupta, A P 103


116


H

Hahn, K 42, 43, 44, 45, 47, 48, 49, 66Hall, T 66Haraguchi, K 104Harclerode, W H 47Hayashi, T 47Henn, R 43, 44Hesse, A 65Hirokado, N 104Hirosawa, K 65Hirose, K 106Hiroshi, N 103Hiroyuki, M 106Hohwiller, F 43, 44Holoch, J 45, 47, 48, 49Holtman, K M 105Hood, L S 47Hu, X 104Huang, Z 105Huneault, M A 103Husemann, W 44

I

Imam, S H 105Inoue, Y 106Irving, D W 105Isao, K 46

J

Jager, J 45Jönsson, L 49Jung, D C 47

K

Kabamba, E T 66Kaempfer, K 44


Author Index

117

Kannah, K 47Katsunori, N 104Keisuke, O 105Keitaro, S 105Keller, A 43Kelusky, E C 46Kemperman, W P T 43Keppeler, V 66Keulen, G 43Kim, M C 47Kim, S G 48Kim, S R 47King, B 43Kitahara, Y 103Klamczynski, A K 105Klement, E 5Klempner, D 65Klodt, R-D 41Knutsen, J C 47Kono, K 47Kovarik, J 47Krist, J 66Krupinski, S M 47Kuhn, J 44Kumar, V 103Kurcharikova, I 47

L

Lamprecht, J 43, 44Landa, A 43Lanfredi, R 45Lanzani, F 103Laun, M 45, 49Lauzon, M 102Lawton, J W 105Lee, E K 48, 65Lee, K-M 48


118


Lee, L J 48Lee, S G 50Lemstra, P 48Leung, S N 49Levchik, S V 41Li, H 103Li, K 66Lia, X 104Liu, T 66Liu, Y 105Lopez, P 46Lu, M 66Lu, Y 66Lye, S W 50

M

Makoto, Y 106Maletzko, C 66Masamichi, K 46Mat, J Z 47Matsumoto, T 104Matsuoka, F 104Metsaars, A C G 67Michimiro, H 104Mihai, M 103Mills, N J 67Miyagawa, T 106Moore, G F102Moreira, A 46Movilli, W 103Mronga, N 44Muhlbach, K 49Munakata, Y 65

N

Naegele, D 43, 44Naguib, H E 48, 65, 104


Author Index

119

Naioki, N 105Nakai, S 67Namikawa, H 47Nangeroni, J 103Nawaby, A V 104Nehls, B 43Nelissen, L 48Noordegraaf, J 43, 45

O

Ohara, H 47Ohshima, M 67Ohta, H 104Oogi, Y 104Orts, W J 105Ouellet, S 67Ozturk, U E 67

P

Pallay, J 48Panayiotou, C 104Panzer, U 65Paquet, A N 41Park, C B 48, 49, 65, 66Pavlicek, J 47Pekich, B J 47Pinkert, R 47Pirgov, W 66Platt, D K 105Polasky, M E 48Ponticiello, A 45, 46Priddy, D B 41

R

Randall, J R 103Rathbun, M 43


120


Rego, J M 43Reichelt, N 65Reiger, J 46Rensen, P F M 43Riethues, M 5, 43, 49Rinaldi, R 49Rodrigue, D 66Roesch, J 44Ruch, J 42, 45, 47, 48, 49

S

Sandler, J 43Sands, M 50Sanford, F L 49Sasaki, H 104Saunders, S M 102Schellenberg, J 47Schennink, G G J 105Scherzer, D 42, 43, 44Schiers, J 41Schmeid, B 42, 43, 45, 47, 48, 49Schouren, P W M 43Schueneman, H H 46Schuler, P 46Schut, J 103Schut, J H 49, 67Scott, G 105Sei, Y 103Senda, K 65, 106Sendijarevic, V 65Sengupta, P 66Shah, S 104Shao, J 66Shey, J 105Shi, X 105Shimida, S 65Shinichi, F 104


Author Index

121

Shinko, Y 103Shinohara, M 104Shiv Kumar, S 50Simonelli, A 45, 46Smeets, E P W 43Smith, J C 103Snijders, E 48Song, G I 47Sopher, S R 65Spinu, M 103Stadlbauer, M 65, 66Statsny, F 65Sterzel, H-J 103Suh, K W 41Sunal, G 46Suzuki, K 103

T

Takaaki, H 104Takayoshi, K 103Taki, K 66, 67Taskiran, I 46Tetsuo, K 104Tetsuya, O 104Teubert, J 48, 67Tokiwa, T 104Toshiyuki, I 104Treischmann, G 65Trivedi, Y 66Trn, N J 47Troitzsch, J 42Tsujimura, I 67Tsunahiro, N 103Tzivintzelis, I 104


122


U

Ueda, K 104Uyanik, N 46

V

Van Leuven, R A H 105Vannini, C 103Vink, D 103Vo, C V 42Volz, W E 47Voss, J C 47

W

Wagner, F 45Walter, B 106Wang, J 66Wang, X 66Warzelhan, V 43Wassmer, K-H 43, 44Wassner, E 43, 46Watanabe, T 103Weil, E D 41Wiman, J V 47Witt, M R J 104Wong, A 49Wood, D 105Worswick, M 67

X

Xu, F 105Xu, J X 65

Y

Yamaguchi, A 103Yang, J 48


Author Index

123

Yeh, S-K 48Yin, H 66Yoon, J S 66Yoshihito, Y 104Youngson, C G 43Yu, S 105Yukikage, M 106

Z

Zach, J 47Zamperlin, L 45Zavodska, V 47Zhang, H 66Zhang, L 66Zhang, S 66Zhang, W 66Zhang, X 65Zhu, Z 65


124



125

Subject Index

A

Ageing 9, 62Agglomeration 7Amylopectin, structure of 92Amylose, structure of 92Antistatic mouldings 88

B

Bead foam moulding 99Bead-making process 36, 98Biodegradability 71Blow moulding 88, 99Blowing agents 31

C

Cellulose 109Cellulosic polymers 99, 100, 101Chlorofluorocarbons 11Coating, hydrophobicising 18Copolymerisation 22, 58Crystallisation 61, 78, 83, 84, 85, 97, 98

Cold 84Cushioning performance 23, 24

D

Dewatering 7Differential scanning calorimetry 61, 99


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Dimethylphenylphosphonate 13Dihydro-oxa-phosphaphenanthrene oxide 14, 15

E

Ethylene vinyl alcohol 95Ethylene-bis-stearamide wax 78Ethylene-vinyl acetate 55 Expanded polyethylene 55, 64, 109

Mouldable beads 53Expanded polyolefin 62, 63, 65, 74

Bead products 62Moulded foams 51

Expanded polypropylene 52, 55, 56, 61, 63, 80, 98, 109Expanded polystyrene 2-4, 7, 10-17, 19, 21, 23, 25-28, 31, 32,

38, 40, 52, 54, 55, 59, 60, 62, 64, 65, 69, 74, 86, 90, 93, 94, 108, 110

Beads 7, 23, 36Blowing agents 27Industry 28, 60, 108Low-pentane developments 28Moulding 38, 39Packaging 23Unexpanded beads 7

Extruded foamed sheet 15, 34Extruded polystyrene process 35, 36Extrusion 8, 13, 17-21, 31, 37, 51, 58, 99

F

Fibre spinning 99Fire retardation 12Foam moulding 9, 101Foam nucleation 38Foam products 10Foaming 1, 31

Assisting agent 29In-mould 89


Subject Index

127

G

Gas impregnation process 87Gelatinising 91Ground stabilisation 1

H

Hexabromocyclododecane 11, 13, 14, 17High melt strength 56Hydrochlorofluorocarbon 11, 53Hydrolysis 76, 90Hygroscopic 91

I

Impregnation 34, 59, 84, 85, 86, 98Injection moulding 9, 26, 88, 99

J

Japan Styrene Paper Process 53

K

Knudsen Effect 20

L

Life cycle analysis 69Linear low-density polyethylene 55Lost wax process 3Low-density polyethylene 55, 58

M

Macroinitiator 23Melt process 72, 98Melt rheology 52Metal casting 3Micropelletisation 31, 37, 60Mouldable bead foams 3


128


Mouldable particle foam industry 1, 107Moulded expandable foams 3Moulded polylactic acid foam packaging 79Moulding 9, 22, 26-28, 30, 32, 33, 34, 40, 53, 59, 62, 74, 75, 79,

83, 84, 86, 87, 89, 94, 95, 98,

O

Olefinic oligomer 19Olefinic polymer 26

P

Particle foam industry 1Pelletiser 36, 60Polylactic acid 76, 81, 83

Recycling 90Plasticisation 29, 30, 32, 34, 83Plasticisers 80Plastics foam industry 1Plastics packaging 69Plastics processing 9Polybutylene succinate 80Polycaprolactone 80, 102Polydimethylsiloxane prepolymer 22, 23Polyethylene bead foam processes 53Polyethylene terephthalate 76

Amorphous 77Polyhydroxyalkanoates 70, 96Polyhydroxybutyrate 97Polyhydroxybutyrate-co-hydroxyvalerate 97Polyhydroxyvalerate 97Polylactic acid 70, 109Polymeric matrix 20Polymers, sustainable 96, 109Polymerisation 7, 8, 10, 18, 21, 25, 27, 31, 33, 36, 37, 76

Suspension 7, 8, 18, 19, 31, 35, 36, 37


Subject Index

129

Polyolefins 20, 25, 27, 51, 53-57, 59, 97Foams 3, 55Moulded foams 51Particle foams 109Polymers 51Wax 30, 38

Polystyrene 10-12, 20-22, 25, 28, 30-33, 35, 36, 51-53, 55, 56, 70, 73, 82

Water-expandable 33Polyvinyl alcohol 102Prefoaming 9Prepuff 62

Beads 9

R

Recycling 40, 69, 71, 96Resorcinol 14Resorcinol diphosphate 15

S

Self-nucleation 61Sheet extrusion foaming 57, 81Styrene-maleic anhydride 23

T

Thermal insulation 1Thermoforming 56Thermoplastic polymers 1Thermoplastic starch 81, 90Triphenyl phosphate 13, 14

V

Volatile organic compounds 27, 28, 108


130


W

Waste packaging 109

Z

Ziegler-natta catalysts 57


Moulded particle (bead) foam products are ubiquitous, in packaging and

construction, from drinking cups to motorway foundations. The industry

which started with expanded polystyrene (EPS) has grown spectacularly and

now also includes expanded polypropylene and expanded polyethylene, and

its technology and machinery has become so specialised that it is seen as an

almost isolated branch of the plastics industry.

This update summarises the present status of particle foam technology and

how specific challenges have already driven its development. This includes

the potential threats to the fire retardants and blowing agents which have

been used for many years.

The potential for particle foams made from ‘renewable’ polymers such as

starch, polylactic acid, polyhydroxyalkanoates and cellulosics is also covered.

For each of these, existing technology is reviewed, together with the issues

for research and development.

This update is written by a plastics technologist who works in the EPS

industry. It will be of interest to both relative newcomers and those who

already have long experience but wish to know more technical detail about a

fascinating branch of plastics technology.

Shawbury, Shrewsbury, Shropshire, SY4 4NR, UK Telephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118 Web: www.rapra.net

Published by iSmithers, 2009

Documents

Update on Mouldable Particle Foam Technology