A foam of thermoplastic polymer which is stretched and then acquired hot

摘要

A method of preparing a heat-shrunk polymer comprises extruding a foamable thermoplastic polymer, chilling one surface of this extrudate to form a tough, non-porous skin, allowing the major portion of the polymer to expand to form a foamed layer thicker than, and integrally bound to the skin, hot-stretching the polymer in at least one direction, and finally shrinking the same by heat-treatment. The polymer may be any thermoplastic resin, and includes polyamides, polyesters, polyurethanes, india-rubber, and cellulose esters and ethers, but preferred are styrene polymers which contain at least 70% by weight of styrene in their structure. If other types of polymer are to be used, it is preferred that they should be rigid, and relatively non-elastic. Foaming is brought about by volatile organic liquids, which liquids are not reactive with, and have no more than a slight swelling action on the polymer, and which are adsorbed on a conventional, finely-divided absorbent, the whole being mixed with the polymer before extrusion. Chemical blowing agents in solid form may also be used. To ensure even bubble formation, nucleating agents are used, these being combinations of compounds which react to yield carbon dioxide and water. Wetting agents may also be used. Chilling is conveniently carried out by an air or air-water mist stream, flowing at 40-100 feet/second, and having a temperature between 0-100 DEG C. Stretching is performed by rollers. The shrinking process needs, in general, only a brief heating, as the process is rapid. A stream of steam or hot air is conveniently used for this purpose. The foams of the invention have particular application as form-fitting insulants for irregularly shaped objects, and also as a water-pipe insulant. Reference has been directed by the Comptroller to Specification 998,180.