Thermoplastic foam and process for the preparation thereof

A method for the preparation of synthetic resinous thermoplastic foam wherein a synthetic resinous thermoplastic resin in admixture with a blowing agent, is heat plastified under pressure and subsequently extruded to a zone of lower pressure wherein the blowing agent provides gas pressure within the gel forming a plurality of closed cells within the gel and subsequently cooling the foamed mobile gel to a temperature at which it is below its heat plastification temperature, the improvement which comprises employing a major portion of the heat plastifiable resin, an ethylenic addition polymer of mer units having the general formula ##STR4## wherein A is at least one tetravalent organic radical having a first pair of valences or vicinal carbon atoms attached to the carbonyl groups and a second pair of valences or vicinal carbon atoms forming part of the polymer chain and arising by addition polymerization of a C.dbd.C ethylenic double bond in the monomeric form of A, i.e. an ethylenically unsaturated dicarboxylic acid cyclic anhydride, B is the polymerized mer unit corresponding to at least one ethylenically unsaturated addition polymerizable monomer, and x and y are numbers such that the ratio thereof represents the average proportion on a mole basis of the respective mer units in the polymer and the sum thereof represents the average degree of polymerization in the polymer, and as blowing agent a member selected from the group consisting of ammonium carbonate, ammonium bicarbonate, ammonium oxalate and mixtures thereof.