Oxidation and stabilisation chemistry of metallocene-based polyolefins during melt processing

Abstract

Metallocene catalyzed linear low density polyethylene (m-LLDPE) is a new generation of olefin copolymer. Based on the more recently developed metallocene-type catalysts, m-LLDPE can be synthesized with exactly controlled short chain branches and stereo-regular microstructure. The unique properties of these polymers have led to their applications in many areas. As a result, it is important to have a good understanding of the oxidation mechanism of m-LLDPE during melt processing in order to develop more effective stabilisation systems and continue to increase the performance of the material. The primary objectives of this work were, firstly, to investigate the oxidative degradation mechanisms of m-LLDPE polymers having different comonomer (I-octene) content during melt processing. Secondly, to examine the effectiveness of some commercial antioxidants on the stabilisation of m-LLDPE melt. A Ziegler-polymerized LLDPE (z-LLDPE) based on the same comonomer was chosen and processed under the same conditions for comparison with the metallocene polymers. The LLDPE polymers were processed using an internal mixer (torque rheometer, TR) and a co-rotating twin-screw extruder (TSE). The effects of processing variables (time, temperature) on the rheological (MI, MWD, rheometry) and molecular (unsaturation type and content, carbonyl compounds, chain branching) characteristics of the processed polymers were examined. It was found that the catalyst type (metallocene or Ziegler) and comonomer content of the polymers have great impact on their oxidative degradation behavior (crosslinking or chain scission) during melt processing. The metallocene polymers mainly underwent chain scission at lower temperature (<220°C) but crosslinking became predominant at higher temperature for both TR and TSE processed polymers. Generally, the more comonomers the m-LLDPE contains, a larger extent of chain scission can be expected. In contrast, crosslinking reactions were shown to be always dominant in the case of the Ziegler LLDPE. Furthermore, it is clear that the molecular weight distribution (MWD) of all LLDPE became broader after processing and tended generally to be broader at elevated temperatures and more extrusion passes. So, it can be concluded that crosslinking and chain scission are temperature dependent and occur simultaneously as competing reactions during melt processing. Vinyl is considered to be the most important unsaturated group leading to polymer crosslinking as its concentration in all the LLDPE decreased after processing. Carbonyl compounds were produced during LLDPE melt processing and ketones were shown to be the most imp0l1ant carbonyl-containing products in all processed polymers. The carbonyl concentration generally increased with temperature and extrusion passes, and the higher carbonyl content fonned in processed z-LLDPE and m-LLDPE polymers having higher comonomer content indicates their higher susceptibility of oxidative degradation. Hindered phenol and lactone antioxidants were shown to be effective in the stabilization of m-LLDPE melt when they were singly used in TSE extrusion. The combination of hindered phenol and phosphite has synergistic effect on m-LLDPE stabilization and the phenol-phosphite-Iactone mixture imparted the polymers with good stability during extrusion, especially for m-LLDPE with higher comonomer content.