Preparation of ultra high molecular weight polyethylene with MgCl2/TiCl4 catalyst: Effect of hydrogen and cocatalyst on molecular weight and molecular weight distribution

Abstract

The ultra high molecular weight polyethylene (UHMWPE) of a special class of HDPE was a linear polyethylene (PE) with a molecular weight numbering in the millions and was also known as high modulus PE or high performance PE. UHMWPE was a unique polymer with outstanding physical and mechanical properties. Most notable were its chemical inertness, lubricity, impact resistance and abrasion resistance. These properties of UHMWPE have been exploited in a wide range of industrial applications, including pickers for textile machinery, lining for coal chutes and dump trucks, runners for bottling production lines, as well as bumpers and siding for ships and harbors. Ethylene polymerization by typical MgCl2-supported Tibased catalysts, and trialkylaluminiums was the most commonly used cocatalyst. On the other hand, H2 is known to be the most efficient industrial regulator of the molecular weight of the olefin polymerization as a chain transfer agent. The use of H2 as a molecular weight regulator was based on the hydrogenolysis of metal to polymer bonds, and the rate of chain transfer to H2 was substantially higher than others. As reported by Hindryckx, the catalyst activity and the molecular weight drastically decreased with the introduction of H2 for ethylene polymerization, while the PDI increased with the increasing H2 pressure. It is well known that molecular weight distribution has a significant effect on the processability and end-use properties of olefin polymers. Therefore, it is necessary to control the molecular weight distribution of polyolefin to produce various grades suitable for the many end-use applications of polyolefin. However, the effects of H2 and cocatalyst on PDI value of UHMWPE have not been reported until now. Therefore, the aim of the present work was to study the effect of H2 feed ratio, H2 feeding time (tH) and cocatalyst on activity, thermal property, molecular weight and molecular weight distribution of UHMWPE produced by using MgCl2-supported Ti catalyst systems. The molecular weight distribution of UHMWPE was analyzed through both GPC and rheometry measurements in this study.