Mechanism of thermal degradation of alternating ethylene-chlorotrifluoroethylene copolymers

Abstract

Thermal degradation of the 1:1 alternating ethylene-chlorotrifluoroethylene copolymer (ECTFE) starts above 400 °C (TGA: N 2, 10 °C/min) and is characterized by elimination of HCl and HF. Structural defects in the polymer chains lead to a decrease in stability and we have observed that, in copolymers whose polyethylene sequence content had been enhanced, a low rate of dehydrohalogenation starts at about 300 °C. At the beginning, the molar ratio of HCl evolved is about twice that of HF, although the Cl to F ratio is 1:3 in the original copolymer. The HCl/HF ratio tends to 1, however, during later stages of the degradation. Infrared and ultraviolet spectroscopy indicate that the dehydrohalogenation process leads first to isolated double bonds which develop into polyene sequences of conjugated double bonds of relatively short length (<about 5 double bonds). The melt viscosity of ECTFE decreases initially when dehydrohalogenation begins, but later increases. This shows that heating induces chain scission at first while crosslinking becomes predominant with increasing double bond content. The initial dehydrohalogenation step (in which Cl is eliminated in excess of F) is likely to involve structural defects of the polymer, whereas the main dehydrohalogenation process might occur through a chain radical mechanism propagated by Cl radicals. Once the first double bond is created, the next hydrogen abstraction is likely to involve the mobile allylic hydrogen atom of the same ethylene unit from which hydrogen abstraction has already occurred. When two conjugated double bonds are thus obtained, by sequential elimination of Cl and F, the following hydrogen abstraction can occur with the same probability either from the ethylene unit next to the unsaturated structure or from any other unit in the polymer.