Process for the improved radiation cross-linking of tetrafluoroethylene-propylene copolymer

Amine compounds such as tetraethylene pentamine and hexamethylene diamine, and basic oxides such as zinc oxide and magnesium oxide promote the cross-linking of tetrafluoroethylene-propylene copolymer even with low dose of irradiation. The process comprises irradiating tetrafluoroethylene-propylene copolymer in the presence of fluorine capturing agents such as amine compounds and basic oxides to promote cross-linking of the copolymer. The ratio of gel content can be controlled between 10% and 50% or more by using a radiation dose between 5 and 6 Mrad. In practice, it is desirable for the rubber of tetrafluoroethylene-propylene copolymer to have a molecular weight of 20,000 or more. A fluorine capturing agent may be added the quantity of which is 0.1% to 20% by weight of the copolymer, with 0.2% to 10% preferable. The total absorption dose of ionizing radiations may be 103 to 108 rad at a dose rate of 102 to 109, preferably 103 to 108 roentgen per hour. In examples, some samples of tetrafluoroethylene-propylene copolymer of molecular weight 30,000 were mixed with the following promoters: 6% of calcium oxide, 3% of zinc oxide, 1% of magnesium oxide, 3.7% of polyethylene pentamine and other promoters respectively. They were hot rolled at 60 degrees to form a sheet 1 mm thick. The sheets were exposed to electron beams from an electron beam accelerator of energy level 1.5 MeV and a flux density of 0.48 μA/cm2. As a result of testing, the gel content was 26% to 46%, the breaking strength 50 to 80 kg/cm2 and the elongation 600 to 900%. (Iwakiri, K.)