Effects of terminal vinyl groups on peroxide crosslinking of high density polyethylenes

AIREGUAMEN I. AIGBODION; JORGE A. RESSIA; ANDRES E. CIOLINO; MARCELO D. FAILLA; ENRIQUE M. VALLÉS

San Sebastián, España

Congreso; Fourth International Conference on Polymer Modification, Degradation and Stabilization. MODEST 2006; 2006

Polyethylene has become the largest commercially produced polymer because of its versatility. It is used in a wide range of applications both as a commodity resin, and in several cases, as a specialty polymer. However, in some of these applications, improved properties of the polyethylene are required thereby compelling further modification of its properties to meet the desired quality specification. Crosslinking is often used to achieve this objective. Radical crosslinking with organic peroxide is one major method frequently used in the industry to bring about crosslinking of polyethylene. This process is generally believed to involve peroxide homolysis yielding the peroxy radicals which abstract hydrogen atom from polyethylene thereby forming macroradicals. Crosslinking occurs through the combination of these macroradicals. It has been reported that abstraction of hydrogen atom by peroxy radical is significantly influenced by the presence of terminal vinyl groups in the parent polymer. On this premise, we conceived that hydrogenation of the terminal vinyl groups in one polymer type prior to crosslinking will give further insight and afford better understanding of the significance of vinyl groups in the modification process. This constitute the objective of the present work; i.e. to determine exclusively, the effects of terminal vinyl groups on peroxide crosslinking and rheological properties of HDPE using one polymer type instead of polymers from different sources having different vinyl concentrations. This was achieved by hydrogenation of the terminal vinyl groups of a commercial HDPE and comparing its level of peroxide crosslinking with that of the unhydrogenated polymer. Hydrogenation was performed on the solution of the polymer in xylene using (PPh3)3RhCl as catalyst. The hydrogenation was carried out  under hydrogen pressure of 700psi at 120 oC. A 2L pressure reactor was employed and reaction time was 24hr. The hydrogenated polymer powder was precipitated from the xylene solution with methanol. Larger amount of catalyst than specified for other unsaturated polyolefins is required for complete hydrogenation of the terminal vinyl groups present. SEC analyses of the samples show no evidence of chain scission upon hydrogenation.Complete hydrogenation of the terminal vinyl groups was verified by FTIR following the elimination of the strong absorption band at 908 cm-1 which is characteristic of terminal vinyl group.  Hydrogenated and unhydrogenated polymer samples were subsequently modified at 170 oC with different amounts of 2,5-dimethyl-2,5-di (tert- butylperoxy) hexane. Peroxide concentration used varied from 250ppm to 10,000ppm. Changes in the molecular structure were determined by FTIR, SEC and rheological measurements. Hydrogenation of the terminal groups significantly reduced the rate of modification or crosslinking. Concentration of terminal vinyl groups of the original polymer decreases rapidly with increase in the amount of peroxide used. Dynamic viscosity and elasticity increase with the level of peroxide modification in both polymers but  unhydrogenated samples showing more rapid increase in viscosity and elastic modulus. Indeed, hydrogenated sample requires 500% of the amount of peroxide needed for the unhydrogenated sample to attain similar structural changes. All these evidence support the important involvement of terminal vinyl groups in the crosslinking reactions.