Thermal stabilization of oxymethylene

Claim 1

METHOD FOR STABILIZING AN OXYMETHYLENE COPOLYMER HAVING A MELTING POINT OF AT LEAST 150*C. AND HAVING AT LEAST ONE CHAIN CONTAINING (1) AT LEAST 85 MOL PERCENT OF OXYMETHYLENE (-OCH2-) UNITS INTERSPERSED WITH (2) -O-R- UNITS WHEREIN R IS A DIVALENT RADICAL CONTAINING A LEAST TWO CARBON ATOMS DIRECTLY LINKED TO EACH OTHER AND POSITIONED IN SAID CHAIN BETWEEN THE TWO VALENCES, ANY SUBSTITUENTS IN SAID R RADICAL BEING INERT, SAID METHOD COMPRISING (1) HEATING SAID COPOLYMER, IN ADMIXTURE WITH A CHEMICAL STABILIZER, TO A TEMPERATURE ABOVE THE MELTING POINT OF SAID COPOLYMER BUT NOT HIGHER THAN ABOUT 60*C. ABOVE SAID MELTING POINT AND (2) MAINTAINING SAID COPOLYMER-STABILIZER MIXTURE WITHIN SAID TEMPERATURE RANGE WHILE MECHANICALLY WORKING AND CONTINUOUSLY EXPOSING FRESH SUFACES OF SAID MIXTURE TO PERMIT THE ESCAPE OF VOLATILE PRODUCTS UNTIL SAID MIXTURE HAS AN INITIAL THERMAL DEGRADATION RATE WHICH IS (A) LESS THAN 0.1 WEIGHT PERCENT PER MINUTE AND (B) LESS THAN 60% OF THE INITAIL DEGRADATION RANGE OF AN UNTREATED COOLYMER STABILIZER MIXTURE SAID DEGRADATION RATE BEING STATED IN TERMS OF WEIGHT LOST PER MINUTE WHEN SAID MIXTURE IS MAINTAINED IN AN OPEN VESSEL IN A CIRCULATING AIR OVEN AT 222*C., THE MIXTURE RESULTING FROM SAID TREATING BEING CAPABLY OF BEING INJECTION MOLDED.