The expansion of polystyrene using water as the blowing agent has been investigated. Water is ad- or absorbed by a hydrophilic additive that is introduced in styrene before the polymerization. The different additives used in this work are ordinary corn starch and high amylose starch, zeolite A and Y, and SiO_2. The influence of additive and water content, of the type of additive, and the polymerization conditions on the density and the morphology of the expanded product were investigated. The characteristics of the foam depend on the expansion procedure. The density obtained with high frequency electric field heating is lower that what can be realized with hot air.
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