Influence of branching density in ethylene-octene copolymers on electron beam crosslinkability

Abstract

Two ethylene-octene copolymers (EOC) with the same melt flow index (MFI = 3 g/10 min) but different octene contents, being 20 and 35 wt % (EOC-20 and EOC-35), were compared with regard to sensitivity to electron beam crosslinking. Dynamic mechanical analysis (DMA) revealed a large influence of the octene content on the storage modulus and the glass transition temperature (Tg) but a smaller influence of irradiation on the properties below melting point (Tm). Rheology at 150 °C pointed out large differences in samples crosslinked in the 0-60 kGy range and at lower frequencies (0.1-1 Hz). The loss factor tanδ confirmed that before irradiation the two copolymers were very similar, while after irradiation to 120 kGy, the EOC-35 had considerably lower tanδ than EOC-20, which corresponds to a better elasticity (or a higher level of crosslinking). A high-temperature creep test showed a considerably lower creep for EOC with a higher octene content. An analysis of the insoluble gel content exhibited higher values for EOC-35 confirming a higher level of crosslinking. Analysis according to the Charlesby-Pinner equation revealed increased crosslinking-to-scission ratio, G(X)/G(S), for EOC-35. While the G(X) value changed only slightly, a significant decrease in the G(S) value was discovered. © 2015 by the authors.