New Reheological Method to Evaluate Thermo-Oxidative Degradation of a Coating Grade Polyethylene, 2001 European PLC Conference Proceedings

P.Mariani, G.Carianni, A.Monteverdi--The conditions for melt processing of polymers, namely high temperature, the presence of oxygen and mechanical stresses, can cause chemical reactions to occur. Degradation is often a complex process involving combinations of different mechanisms 1,2 . The rates of oxidation of various polyethylenes such as HDPE, LLDPE and LDPE depend on details of their chemical structure. Until now the main rheological method to study the degradation of polyethylene is by passing five times the polymer through a grader at severe conditions of temperature. The modification of the molecular structure can be easily evaluated through the change of the melt flow rate with the number of passes. Anyway this method is far from the conditions undergone by the polymer in an extruder. A more complete rheological study, through the variation of the melt viscosity n* with the time, can be obtained by using a parallel plate rotational rheometer. The results were given in terms of rate of increment of viscosity with respect to time. The rate of increment of viscosity increases with temperature at fixed frequency and strain level, while decreases with frequency and strain level at fixed temperature. To support some of the results obtained, the analysis of carbonyl groups via changes in FTIR spectrum was performed.