Process for making low density chlorinated polyvinyl chloride foam

A low density foam product comprising expanded chlorinated polyvinyl chloride polymer said polymer having a chlorine content of from 60% to 73% by weight having an essentially closed-cell cellular structure composed predominantly of cells having a cell size less than 500 microns, said cells containing a gas derived from a primary blowing agent and hydrogen gas derived from the activation of a nucleating system composed of an alkali metal borohydride and a proton donor activator compatible with said polymer selected from the group consisting of hydrated salts and organic acids, said foam having a density less than 20 lbs/ft.sup.3.