Influence of branching density on the cross-linkability of ethylene-octene copolymers

Ethylene-octene copolymers (EOCs) with two different octene contents (20 and 35 wt%) and the same melt flow index (3 g per 10 min) were cross-linked using various levels (0.3, 0.5 and 0.7 wt%) of dicumyl peroxide at different temperatures. Cross-linking and degradation were analyzed by rubber process analyzer (RPA) within a temperature range of 150–200 °C. The highest s′max (maximum elastic torque) and the lowest tan(delta) were found for EOC-20 with low-octene content at all cross-linking temperatures. Lower peroxide efficiency was observed in the case of the high-octene copolymer. Increased degradation was observed with increasing cross-linking temperature. High-octene EOC was found to be more vulnerable to degradation. According to dynamic mechanical analysis, the storage modulus (M′) and the glass transition temperature (Tg) obtained from the tan(delta) peaks were found to decrease with increasing octene content. The differential scanning calorimetry (DSC) results show that the octene content has an inverse effect on the crystallinity (X) and melting point (Tm)—due to the reduction in the average number of consecutive ethylene units. Creep testing at 150 °C confirmed the cross-linkability results obtained by RPA and the gel content analyses. Increased β-scission due to high numbers of tertiary carbon atoms present in the chain has resulted in the poorer cross-linking and inferior properties of high-octene EOC.