Improved silane grafting of high-density polyethylene in the melt by using a binary initiator and the properties of silane-crosslinked products

This work aimed at finding optimum conditions for producing high-density polyethylene (HDPE) with improved silane grafting while maintaining the overall process efficiency. The silane-grafted product was then crosslinked utilizing water as the crosslinking agent. The properties of the samples before and after silane curing were analyzed using differential scanning calorimetry, thermogravimetric analysis, heat distortion temperature, and tensile testing. In the work, vinyltrimethoxysilane-grafted HDPE has been prepared in the melt by reactive extrusion. The effects of single and binary initiator systems, and of extrusion conditions, on silane grafting and undesirable PE self-crosslinking have been investigated. The use of binary initiators (i.e. benzoyl and dicumyl peroxide mixture) successfully led to an HDPE with improved level of grafting and diminished amount of gel from crosslinking side reaction. Results of the swelling factor, in combination with the gel content of the silane-cured products, showed that different crosslink network structures (loose or tight networks) formed during the progress of the crosslink reaction. A tight network was formed after the maximum swelling factor was reached. The structure of the crosslink network affected the tensile and heat aging properties of the materials greatly. The samples of tight networks were able to withstand the high heat of 132 °C which is close to the melting temperature of HDPE. Only 10 % reduction in tensile properties was observed for the crosslinked samples after heat aging.