Process for the production of a heat expansible thermoplastic resin

Claim 1

A PROCESS FOR THE PRODUCTION OF A HEAT EXPANSIBLE THERMOPLASTIC SYNTHETIC RESIN IN GRANULAR FORM, WHICH COMPRISES STIRRING A DISPERSION OF THERMOPLASTIC SYNTHETIC RESIN GRAINS IN AN AQUEOUS EMULSION OF TWO MUTUALLY MISCIBLE, LIQUID BLOWING AGENTS WHICH HAVE BOILING POINTS BELOW THE SOFTENING TEMPERATURE OF SAID SYNTHETIC RESIN, ONE OF SAID BLOWING AGENTS BEING SOLUBLE IN SAID THERMOPLASTIC SYNTHETIC RESIN AND THE OTHER ONE INSOLUBLE IN THE SAME, STIRRING BEING EFFECTED UNDER SUBSTANTIALLY ATMOSPHERIC PRESSURE AT A TEMPERATURE BELOW THE BOILING POINTS OF SAID BLOWING AGENTS UNTIL SAID GRAINS HAVE RETAINED FROM 5 TO 15 PERCENT BY WEIGHT THEREOF OF SAID INSOLUBLE BLOWING AGENT AND A PROPORTION BY WEIGHT OF SOLUBLE BLOWING AGENT BETWEEN 2 TO 15 PERCENT; THEN SEPARATING SAID GRAINS; AND EXTRACTING SAID GRAINS WITH A LIQUID SOLVENT FOR SAID SOLUBLE BLOWING AGENT, WHICH IS A NON-SOLVENT FOR SAID INSOLUBLE BLOWING AGENT, TO LOWER THE PROPORTION OF SAID SOLUBLE BLOWING AGENT IN SAID THERMOPLASTIC SYNTHETIC RESIN GRAINS TO BELOW 3 PERCENT BY WEIGHT.