Influence of Branching Density in Ethylene-Octene Copolymers on Electron Beam Crosslinkability

Petr Svoboda

doi:10.3390/polym7121530

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.

Highlights

The development of catalysts has led to a new class of polyolefin copolymers [1]
The new type of copolymer is produced by constrained geometry catalyst technology (CGCT), which leads to copolymers with a narrow molecular weight distribution (MWD) and a very uniform comonomer distribution; in other words, a uniform short chain branching (SCB)
The main objective of this paper is to investigate of the influence of electron irradiation on the crosslinking behaviour of ethylene-octene copolymers with the same initial melt flow index but with a different octene content

Summary

Introduction

The older type of polyolefins produced by Ziegler-Natta catalysts had a broad molecular weight distribution (MWD) and a broad distribution of the comonomer. The new type of copolymer is produced by constrained geometry catalyst technology (CGCT), which leads to copolymers with a narrow MWD and a very uniform comonomer distribution; in other words, a uniform short chain branching (SCB). Processability (shear thinning) is enhanced by a controlled level of long chain branching (LCB). These copolymers provide an excellent opportunity to study structure-property relationships as a function of only one variable while all other variables are kept constant. This is a necessary condition for obtaining meaningful results. Bensason et al studied the influence of comonomer content [3], and Wood-Adams et al focused on the degree of LCB [4]

Objectives

Results

Conclusion