Thermoplastic material, e.g. polystyrene, capable of being moulded to produce porous products, is formed by subjecting expanded pellets to pressure until their bulk density is between 5 and 20 lbs. per cubic foot, the pressure being insufficient to burst any substantial quantity of the pellets or of the intercellular walls, if any, within the pellets. The expanded pellets are produced by impregnating discrete thermoplastic particles with a low boiling hydrocarbon and then heat treating to cause expansion. The compressed pellets may be expanded in a mould by the technique described in Specification 807,237; it is preferable to expand them partially and age them in the air before the moulding operation.ALSO:Thermoplastic material e.g. polystyrene, capable of being moulded to produce porous products, is formed by subjecting expanded pellets to pressure until their bulk density is between 5 and 20 lbs. per cubic foot, the pressure being insufficient to burst any substantial quantity of the pellets or of the inter cellular walls, if any, within the pellets. The expanded pellets are produced by impregnating discrete thermoplastic particles with a low boiling hydrocarbon and then heat treating to cause expansion. The compressed pellets may be expanded in a mould by the technique described in Specification 807,237; it is preferable to expand them partially and age them in the air before the moulding operation.
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