Accelerating Effect of Acetylene on the γ-Radiation-Induced Cross-Linking of Polyethylene

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The γ-radiation-induced cross-linking of polyethylene was investigated in the presence of acetylene under an absolute pressure of 3 kg/cm2 at 30—200°C with a dose rate of 1.1×105 rad/hr. The weight of the polyethylene increased almost proportionally to the radiation dose, and the rate of the increase was larger at 30 and 90°C than at 150 and 200°C. The gel fraction increased with rising irradiation temperature, passed through a maximum at around 105°C, and thereafter decreased sharply to become almost constant above 150°C. At temperatures below the melting temperature of polyethylene, the gel fraction was much higher than in vacuo. As a characteristic structural change, the formation of terminal vinyl unsaturation was observed, and the rate of the formation increased with the temperature. The formation of hydrogen and trans-vinylene unsaturation was little affected by acetylene. On the basis of these results, the role of acetylene in the γ-radiation-induced cross-linking of polyethylene was discussed from the view-point of the reaction mechanism.