| 摘要 |
Articles of foamable or pre-foamed thermoplastic resinous material may be formed by expanding the material in the presence of water e.g. 10-30% by weight, using dielectric heating, in a resiliently enlargeable or heat shrinkable mould. Suitable materials for use in this process include polymers and co-polymers of styrene, alpha-methyl styrene, vinyl toluenes, aryl mono-and di-chlorostyrenes, and aryl-dimethyl styrenes, methyl methacrylate, ethyl acrylate, vinyl acetate and butyral, ethylene, and cellulose acetate and nitrate. The material is preferably expanded in a rubber-like resilient mould which exerts a constant restraint on the material. The material may be wetted before or after filling the mould, which may also be wetted, and to aid the wetting the water may contain a surfactant e.g. a detergent. The temperature of the process may be increased above the boiling point of water by addition of alcohols or glycols, or by the use of vents in the mould which allow a certain pressure to build up before allowing escape of gas. Where non-expanded material is used a blowing agent, e.g. dichloro-, difluoro-methane, alkali metal carbonate or pentane, must be mixed with the material before wetting, and in cases where granular, pre-expanded material is used the moulding is best carried out in a mould which contracts on heating e.g. an envelope of polymerized vinylidene chloride. |
