Silica containing blowing agent compositions as plate-out preventives in foamed plastics processes

A thermoplastic composition which comprises a thermoplastic polymeric material selected from polyolefins and olefin copolymers, polystyrenes and styrene copolymers, and polyvinyl chlorides and vinyl chloride copolymers;

• 0.01 to 10% azodicarbonamide;
• and at least one finely divided colloidal or fumed silica having a BET surface area of 200 to 400 m.sup.2 /g and an available water content of 2 to 20% by weight, the weight ratio of silica to azodicarbonamide being from 1:5 to 5:1 and such as to provide 1.5 to 20 parts by weight available water per 100 parts by weight azodicarbonamide.

A process for preparing an article of a foamed plastics material which comprises heating a thermoplastic polymeric composition comprising a polymeric material selected from polyolefins and olefin copolymers, polystyrenes and styrene copolymers, and polyvinyl chloride and vinyl chloride copolymers;

• 0.01 to 10% azodicarbonamide;
• and at least one finely divided colloidal or fumed silica having a BET surface area of 200 to 400 m.sup.2 /g and an available water content of 2 to 20% by weight, the weight ratio of silica to azodicarbonamide being from 1:5 to 5:1 and such as to provide 1.5 to 20 parts by weight available water per 100 parts by weight azodicarbonamide, to a temperature which is below the decomposition temperature of the polymeric material and above the decomposition temperature of the azodicarbonamide, to cause part of the azodicarbonamide to decompose, and subsequently cooling the foamed composition.

In a process for extruding or molding an article of foamed plastics material which comprises heating the thermoplastic polymeric composition with azodicarbonamide as a blowing agent and extruding or molding said thermoplastic polymeric composition in an extrusion or molding apparatus and wherein the deposition of decomposition products of the azodicarbonamide on said apparatus occurs, the improvement which comprises minimizing said deposition by incorporating into said thermoplastic polymeric composition at least one colloidal or fumed silica having a BET surface area of 200 to 400 m.sup.2 /g and an available water content of 2 to 20% by weight, the weight ratio of silica to azodicarbonamide being from 1:5 to 5:1 and such as to provide 1.5 to 20 parts by weight available water per 100 parts by weight azodicarbonamide.