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Foam Particle Foams from Thermoplastic Starch – Waiting for Technology? Article contributed by Robin Britton Consultant and Part-Time Lecturer at Loughborough University, UK TPS Loose Fill (iStockphoto) Particle foam , generic picture, no TPS (iStockphoto) Readers of bioplastics MAGAZINE will be familiar with thermoplastic starch (TPS) materials and the various methods which have been employed to render them more easily processed and water resistant, though for some applications, their sensitivity to water is an advantage. One such is the well-known loose fill packaging ‘beans’ or ‘chips’ and extruded foam profiles, which have very low density, good cushioning power and easy disposal, either by dissolution in water or by composting. In other applications, where more durability is required, a greater degree of water resistance is desirable. There is a much larger market (several million tonnes per year) for lowdensity moulded packaging ‘cushions’ which is currently dominated by expanded polystyrene (EPS) particle foams – these low density beads are easily moulded into quite complex shapes, but disposal after they have served their protective purpose is a significant problem. EPS is widely recycled, but collection and transport of used consumer packaging can be so costly as to be uneconomic. Synbra Technology bv, with its BioFoam ® development, is already addressing this issue (see pages 30ff), but could there be an opportunity here for TPS? EPS Protective Packaging The conventional processes for expanding and moulding particle foams rely on steam, a cheap and very controllable source of heat with a high energy density. (See, for example, [1] for more detail.) In EPS manufacture, millimetre-scale beads of polystyrene impregnated with a blowing agent (usually pentane) are expanded in stirred vessels fed with steam at controlled pressure and densities down to as low as 10 g/l can be achieved. Once matured to stabilise the internal pressure, the ’prepuff’ beads are fed into a mould and more steam piped in. This creates further expansion and fuses the bead surfaces together to produce a strong moulded part. Expanded polypropylene and polyethylene are expanded rather differently because they retain blowing agents much less well, but are moulded in a similar way to EPS. From the point of view of current moulders of protective packaging, an ideal ’green’ particle foam material would be a drop-in replacement for EPS. That is, it should be delivered in a dense form, be expandable in their existing steam expanders and moulded in their existing steam moulding machines. Any changes will be seen as barriers to innovation, as they are likely to add cost and require investment. Although the packaging industry is aware that such an ideal material is unlikely to exist, and that barriers are there to be surmounted, the smaller the adaptations required, the easier will be the process of introduction of a new mouldable packaging particle foam. 28 bioplastics MAGAZINE [01/11] Vol. 6

Foam Particle foams from TPS – where is the technology today? Water contained within thermoplastic starch beads is used successfully as an environmentally friendly and cheap blowing agent for packaging ’chips’ - when the material is heated quickly enough, the water boils and foams the material before it can be driven off. 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. The steam which is the heat source in EPS processing is the enemy here – it tends to degrade or ’burn’ the pre-puffed TPS rather than expanding and fusing the beads together. The challenge of making expandable TPS which can be moulded has been addressed in recent years, but so far without commercial success. In 1998, a group from the Institute for Agrotechnical Research at the University of Wageningen in the Netherlands applied for a patent using microwaves to expand and fuse starch beads in one step [2]. Their idea was to condition thermoplastic starch beads to a water content around 15%, and coat them with a plasticiser which could also act as an adhesive. The beads were then placed in a non-metallic mould and heated in a microwave oven – the water in the beads was thereby heated to produce steam which expanded the beads and fused them, with the help of the adhesive, to yield a moulded part. Although this approach is clearly practicable, there is no record of the patent being granted. With microwave heating technology now considerably more advanced, this method would appear worth revisiting – moulds must be non-metallic, and the oven must be large enough to contain the products to be made but neither issue should be an insuperable problem. More recently, BASF took a different approach in a US patent [3] applied for in 2003. Rather than using water as the blowing agent, their method uses more conventional hydrocarbons or alcohols as blowing agents (propane, butane, pentane, methanol, ethanol, propanol). The thermoplastic starch is also blended with a biodegradable copolyester (Ecoflex ® ) to give it more heat and moisture resistance. The blend components are compounded together in an extruder, the blowing agent injected into the barrel as a final step before the material is pelletised under pressurised water (to prevent expansion before the beads have cooled and solidified). These beads, ready impregnated with the blowing agent, can later be expanded and moulded in standard EPS equipment. The proportions of copolyester to starch claimed in the patent cover a wide range, from 1:9 to 9:1 – as the proportion of starch is increased, the material becomes less expensive but more water sensitive, less ductile and less easily processed – the copolyester is a soft, flexible, biodegradable (but not biobased) material. As with the Dutch microwave process of [2], this technology does not yet appear to have been successfully commercialized. Yet another approach to making TPS foamable and potentially mouldable was described by a group from the US Agricultural Research Service in a paper of 2007 [4]. Their blend formulations included, as well as water, sorbitol or glycerol and ethylene vinyl alcohol (EVOH) as a biodegradable thermoplastic binder. The blends were extruded as pellets or mixed together and milled to small particles, then expanded by heating for 20 seconds or more at 190-210°C. Higher water contents, up to 25%, meant lower expansion temperatures as the material was more plasticised. The purpose of this study was to assess how different types of starch and other additives affected the foam density, so moulding of the expanded beads was not attempted, but there seems no insuperable reason why it should not be possible, using microwave or even steam processes. So what stands in the way of TPS particle foams? The key issues are the formulation of the material (selection of the right balance of plasticisers, blowing agent and foam nucleating agents, plus possibly waterproofing additions), the optimization of the expansion process and development or adaptation of the moulding process. Finally, of course, the solutions found must also be economical for the purchasers of protective packaging – a package is no more than a temporary expedient to ensure that the more valuable product within it reaches the end user in good condition, and as such is seen as a cost to be minimized as far as possible. The need to reduce the water sensitivity of thermoplastic starch, in order to improve its processability and durability has been addressed by a number of different companies in recent years, though as yet no-one seems to have developed particle foams. There is a wide range of blends using starch and hydrocarbon-based polymers (for example the Mater- Bi materials from Novamont), whose water resistance and biodegradability can be tailored to fit both process and application. It can only be a matter of time before such blends are considered for use as particle foams, and practical solutions found? In conclusion, therefore, we can say that moulded foam products based on starch are likely to become technically feasible as development effort is applied. The protective packaging market is both very large and ripe for more sustainable alternatives to EPS, EPP and EPE, so we can expect ‘market pull’ to bring new products forward in the coming years - starch-based systems should be able to take their share. References: [1] Britton, R.N.; Update on Mouldable Particle Foam Technology; iSmithers 2009 [2] World Patent Application WO98/51466A1 [3] US Patent US657330308, 2003 [4] Journal of Agricultural and Food Chemistry, 2007, 55 (10), p3936 bioplastics MAGAZINE [01/11] Vol. 6 29

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