The relationship between the melt index

the tensile strength and the stress-crack resistance

and that between the end-group structure and thermal stability of copo-lymer of tetrafluoroethylene and hexafluoropropylene have been studied. It was shown that: (1) Within a definite copolymer composition and molecular weight distribution

the relationship between the melt index and the tensile strength of the copolymer can be expressed by the empirical equation: F= 350-18.3 (MI); (2) The copolymer with melt index larger than 4 is Poor in thermal stress-cracking properties;(3) When the end-group was converted into -CF=CF2 by heat treatment at 380℃

the copolymer was shown to be stable thermally during extrusion processing.