Thermal degradation of polyethylene in a nitrogen atmosphere of low oxygen content. IV. Structural changes occurring in different types of high‐density polyethylene

Abstract

AbstractThe thermal and thermo‐oxidative degradation of reprecipitated and “as received” samples of different types of high‐density polyethylene (HDPE) have been studied. Two of the samples were of conventional Phillips type, being essentially linear and having one vinyl group per molecule. The other two HDPE's were also produced by chromium catalysts, but according to Union Carbide's fluidized bed process. In one of these resins the molecules had vinyl end groups while the other resin was fully saturated.Contrary to earlier statements concerning the thermal degradation of polyethylene it was found that HDPE is not degraded according to a random scission type of mechanism. Instead both molecular diminishing and enlargement are found to occur simultaneously. This is, however, in accordance with our recent findings on low‐density polyethylene (LDPE).The samples were heated at temperatures between 284 and 355°C under high purity grade nitrogen during thermal degradation and under nitrogen containing up to 1.14% oxygen during thermo‐oxidative degradation. Changes in molecular weight distribution and degree of long chain branching (LCB) were followed by gel chromatography and viscosity measurements. Other structural changes were investigated by infrared spectroscopy and differential scanning calorimetry. Both reprecipitated and “as‐received” samples gave identical results.For all the HDPE's an increase in heating time and temperature and oxygen content was found to increase the formation of low molecular weight material, long chain branches, volatile products, carbonyl‐, ether‐, and olefinic groups, discoloration, and insoluble material. Although there was much in common between LDPE and HDPE there were also certain noticeable differences, even within the HDPE group. These differences are suggested to be due to different catalyst residues in the HDPE's causing, e.g., the fully saturated material to degrade at 100°C lower temperature than expected.