Process for making low density chlorinated polyvinyl chloride foam

The process for making a low density chlorinated polyvinyl chloride polymer foam said polymer having a chlorine content of from 60% to 73% by weight having an essentially closed-cell cellular structure comprised predominantly of cells having a cell size of less than 500 microns, said process comprising:

• (a) dispersing a nucleating system comprised 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 within a chlorinated polyvinyl chloride polymer composition,
• (b) forming cell nuclei of hydrogen gas within said chlorinated polyvinyl chloride polymer composition by heating said nucleating system to cause said proton donor activator to produce protons which react with said alkali metal borohydride to form hydrogen gas, and
• (c) expanding said chlorinated polyvinyl chloride polymer composition into said foam product utilizing a primary blowing agent, said nucleating system being activated to form said cell nuclei at a temperature above 25.degree. C. but below the highest processing temperature which the CPVC polymer composition experiences before the CPVC polymer composition expands.