Degradation and stabilization of HDPE during processing [antikvár]

Chapter 1 Chapter 1 Introduction Polyethylene is the leading commodity polymer at the moment, which is produced in large quantities and used in flie most various fields of application. Fihns are extensively applied for packaging purposes, but hollow articles, gas and hot water pipes, technical articles, etc. are also prepared from this polymer. Polyethylene is produced by the most various technologies. Plants using the old gas phase process developed by ICI are still in operation and large amounts of the polymer is produced by Ziegler-Natta or Phillips type chromiiun catalysts. The latest technology utilizes the newest one active site metallocene catalysts, which produce polymers with pre-detemiined and well defined properties. The polymerization technology determines the chemical stracture and the molecular characteristics of the produced polymer. Each application field requires certain combination of properties, the window of property variation is extremely narrow in certain areas. The virgin polymer usually leaves the polymerization reactor in the form of powder, which is immediately melted and homogenized with various additives. During homogenization its properties change because degradation reactions take place in the polymer melt. The extent of property changes depends on many factors, on the inherent properties of the polymer, processing conditions, the amount of oxygen present in the extrader, additive package etc. The extent and direction of the modifications in polymer properties, however, must be controlled in order to obtain a product of constant quality. Before its use, the polymer undergoes at least one further processing operation, in which it obtains its final form. Naturally further, and not necessarily the same, reactions occur during this processing step as well. It is obvious from these considerations that the knowledge ofthe degradation reactions taking place during the processing of polyethylene is very important in practice both for the producer and the user. Polyethylene is produced also in Hungary by various technologies (ICI, Phillips, BASF) in the different plants of TVK. The Department of Plastics and Rubber Technology, Technical University of Budapest has a very good, long standing relationship with the company. During the years, the department was involved in numerous projects from recipe development to the solution of problems which occurred during the processing or application of various PE grades both at the different plants of TVK or at customers. A ?lorough analysis of these problems clearly showed that most of the problems are related to the degradation and stabilization of the polymer. The reactions taking place in the processing machine, which were mentioned above, affected considerably both the quality of the granides and the properties of the final product. During the years it became obvious that only a deep knowledge of the basic questions of polyethylene degradation and stabilization may lead to significant improvement in product quaUty and to the reduction of customer complaints.