Pressurization and storage of thermoplastic resin foams prior to secondary expansion

Claim 1

In a process for decreasing the density of a thermoplastic olefin polymer resin foam material by secondary expansion, wherein a primarily foamed material of substantially closed-cell structure is impregnated with a gas, so that gas pressure within the cells of the foam is increased to an amount greater than atmospheric, and then said material is heated to a temperature above the melting point of the resin, thereby further expanding the primarily foamed material and producing a lower density foam product, the improvement, which allows retention of the superatmospheric internal foam pressure during storage prior to secondary expansion, comprising the steps of: (a) impregnating the primarily foamed resin material with a gas by subjecting said material to a gas at an impregnating pressure greater than atmospheric and a temperature below the melting point of the resin so that the pressure within the cells of the foamed material is increased; (b) cooling the gas impregnated foam material to a temperature below 0° C. while maintaining the external impregnating pressure at a pressure above atmospheric pressure; (c) releasing the external impregnating pressure while maintaining the gas impregnated foam material at a temperature below 0° C.; and (d) retaining a superatmospheric internal foam pressure during storage at an external pressure less than the impregnating pressure by maintaining the gas impregnated foam material at a temperature below 0° C.