A process for preparing a foam article of expanded polystyrene with a continuous coating of thermoplastic resin as well as free-flowing molding material for use in the method.

摘要

A free-flowing moulding composition comprises expandable beads of a styrene polymer, from 0.33 to 3.0 parts by weight, per part by weight of expandable beads, of a non-expandable thermoplastic resin powder having a density greater than that of the expandable beads, and from 3 to 40% by weight, based on the weight of the expandable beads, of retained water in the form of a hydrate containing at least 25% by weight of chemically bound water. By "styrene polymer" is meant styrene homopolymers and copolymers containing at least 50% by weight of styrene. The preferred amount of thermoplastic resin powder is from 0.67 to 1.50 parts by weight per part beads. The composition may contain not more than 20% by weight, based on the weight of the beads, of an inert drying substance. Many thermoplastic resins are mentioned, polyethylene being the preferred one. Many hydrates are mentioned, sodium silicate and sodium tetraborate being the preferred ones. The composition may be used in a process where it is placed in a heated mould and moved in a source of heat so that the denser non-expandable thermoplastic resin moves to a position against the mould walls and fuses against them to form an external coating, the polystyrene beads foaming and fusing together and to the coating. The function of the hydrate is to provide water which is converted into steam and acts as a heat distributing agent to ensure uniform heating of the beads. In the examples ethylene-ethyl acrylate copolymer and ethylene-vinyl acetate copolymer are used as non-expandable resins.