Electron beam crosslinking of ethylene-octene copolymers

Abstract

Seven ethylene-octene copolymers (EOC) with a wide range of octene content (17–45 wt. %) were crosslinked by e-beam radiation (in range 30–120 kGy). Testing methods included differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), high-temperature creep test, analysis of the gel content and rheology. It has been found that copolymers with higher initial molecular weight (or lower MFI) and higher branching density (or higher octene content) attain a higher level of crosslinking density. Increasing octene content brings about lower melting point (Tm), crystallization temperature (Tc), crystallinity (X), glass transition temperature (Tg) and storage modulus. Crosslinking influenced the properties below Tm only marginally. However, the properties above Tm were extremely influenced (high temperature creep). Above Tm (without the presence of crystals) only the chemical bonds hold the amorphous chains together which was manifested by creep gradually decreasing at 150 °C with an increasing irradiation level. Loss factor (tanδ at 0.1 Hz), which is a very important parameter in evaluation of molecular structure, was measured by rheology at 150 °C, and it revealed a decreasing trend (or increasing crosslinking level) with the increasing octene content, surprisingly regardless of the initial molecular weight (or MFI) at 120 kGy.