Thermal scission and cross-linking during polyethylene melt processing

We used temperature and time-dependent changes in vinyl and vinylidene concentration to develop a kinetic model that allows estimation of whether cross-linking or thermal scission will dominate during melt processing of major classes of polyethylenes (PE). Torque rheometer processed high-density, linear low-density-, and low-density PE exhibited losses of unsaturated groups when cross-linking was dominant and formation of unsaturated groups when thermal scission was dominant. Cross-linking was attributed to addition of alkyl radicals to olefin groups, and the activation energy for addition to vinyl groups determined was approximately 18 kJ/mol. The activation energy for beta-cleavage of secondary alkyl radicals to form vinyl groups was approximately 91 kJ/mol. Reliable estimates of activation energies for reactions involving vinylidene groups could not be determined but similar reactions were observed for vinylidenes as for vinyls. trans-Vinylene-group formation could not account for the vinyl concentration reductions observed.