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 fill packaging 'beans', easily disposed of by dissolving in hot water once their useful life is over. In order to make useful molded products, the challenge is to produce foamable beads which can be easily molded (fused), and also to improve the durability of the molded products. A patent application filed in 1998 [2] from the Institute for Agrotechnical Research (ATO) at the University of Wageningen in The Netherlands describes a method for producing relatively thick molded parts in a single step which combines expansion and molding. Granules of TPS are conditioned to a water content around 15%, then coated with plasticizer which also acts as an adhesive. The granules are placed into a mold and then 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 molds.
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