Structures and impact strength variation of chemically crosslinked high-density polyethylene: effect of crosslinking density

Impact strength of high-density polyethylene (HDPE), especially at low temperature, is crucial for its applications outdoors because of its poor impact strength. In order to improve the impact strength of HDPE, crosslinked HDPE was prepared by the addition of a peroxide crosslink agent, bis(tert-butyldioxyisopropyl)benzenehexane, and the effect of the crosslinking density on the microstructures and mechanical properties, especially impact strength between −60 °C and 23 °C, were investigated. The results show that the crosslinking density is controlled by varying the content of the crosslinking agent. It is found that, at room temperature, with increase in the content of crosslink agent from 0% to 0.5–0.7%, the impact strength increases from 4 kJ m(−2) to about 80 kJ m(−2) and the elongation at break increases from 20% to about 550%. With further increase in the content of crosslink agent to 1.5%, the impact strength and the elongation at break reduce to 64 kJ m(−2) and 360% respectively. With increase in crosslink agent, the flexural modulus, yield strength, crystallinity, mean lamellar thickness, crystal size and spherulitic size and the brittle–ductile transition temperature (BDTT) decrease, and the gel content, impact strength of the HDPE at low temperature, intensity of β transition increase significantly. In considering both the room temperature mechanical properties and low temperature impact strength, the optimized content of the crosslink agent is about 0.7%. Overall, crosslinking significantly improves the toughness and impact strength of HDPE and extends its application, especially at low temperature.