A NEW RHEOLOGICAL METHOD TO EVALUATE THERMO-OXIDATIVE DEGRADATION OF ACOATING GRADE POLYETHYLENE

摘要

The conditions for melt processing of polymers, namely high temperature, the presence of oxygen andmechanical stresses, can cause chemical reactions to occur. Degradation is often a complex process involvingcombinations 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 tostudy the degradation of polyethylene is by passing five times the polymer through a grader at severe conditionsof temperature. The modification of the molecular structure can be easily evaluated through the change of themelt flow rate with the number of passes. Anyway this method is far from the conditions undergone by thepolymer in an extruder. A more complete rheological study, through the variation of the melt viscosity η* withthe time, can be obtained by using a parallel plate rotational rheometer. The results were given in terms of rateof increment of viscosity with respect to time. The rate of increment of viscosity increases with temperature atfixed frequency and strain level, while decreases with frequency and strain level at fixed temperature. Tosupport some of the results obtained, the analysis of carbonyl groups via changes in FTIR spectrum wasperformed.