Processing stability of high density polyethylene:: effect of adsorbed and dissolved oxygen

The amount of oxygen adsorbed on the surface of a Phillips type HDPE polymer was determined by polarography. Oxygen content was decreased by dt gassing the samples several times, while it was increased by introducing oxygen into the hopper of the extruder. Polymer samples containing various amounts of oxygen were extruded without stabilizer acid in the presence of a hindered phenolic antioxidant. The MFI and the color of the processed samples were measured by standard methods, while their chemical structure was characterized by FTIR spectroscopy. The amount of oxygen adsorbed on the samples is in the range of 20-25 ppm proving that processing takes place in an oxygen poor environment. The polymer powder adsorbs more oxygen than the granules due to its larger specific surface area and porous structure. Permeability measurements indicate that about the same amount or more oxygen is dissolved in the polymer than is adsorbed on the surface. Its estimated amount is around 40-70 ppm. In the oxygen poor environment of processing, the viscosity of the polymer decreases, while its color increases with the increasing oxygen content. The discoloration of stabilized samples is especially strong. Stability also decreases considerably with the increasing oxygen content. (C) 2000 Elsevier Science Ltd. All rights reserved.