Polyethylene Cross-linking by Two-step Silane Method: A Review

Cross-linking of polyethylene is a subject of interest, having been emerged as a result of the need to meet application requirements which were not satisfied by the neat polyethylene itself. Although this review is aimed at silane method of cross-linking, the other cross-linking methods, namely: radiation, peroxide and azo methods have been presented briefly for a better understanding of the merits of perspectives of the more recent methods, especially the two-step silane method. Freeradical grafting of unsaturated hydrolyzable alkoxy-silanes onto polyethylene chains by a peroxide initiator followed by moisture cross-linking is the most versatile crosslinking method and may be successfully used for other thermoplastics as well. Different techniques of silane cross-linking, i.e., the “one step” Monosil® process and the “two-step” Sioplas® process have been discussed with more emphases on Sioplas® process, as it is less expensive and readily achievable. The grafting step which is performed by reactive processing is the major and key process in Sioplas® technique. The state-of-the-art of a two-step silane grafting and cross-linking has been presented. In this regard, the effects of various parameters, such as the type and quantity of silane, peroxide, stabilizing agent, catalyst, micro and macromolecular structures and physical form of polyethylene, additives, and reaction temperature and time have been described in relation to efficiency of grafting and cross-linking. These data were evaluated in turn by torque, MFI, FTIR, and gel content studies.