Degradation of Unstabilized Medium Density Polyethylene Hot-Water Pipes [PE]

The life of a stabilized polyolefin hot-water pipe may be divided into three phases denoted as Regimes A, B and C. Regimes A and B are characterized by different antioxidant consumption mechanisms. Regime C is defined as the time period from the stan of the thermooxidative attack on the polymer itself to the failure of the pipe. The scope of this work was to study Regime C. Pipes of an unstabilized medium density polyethylene (MDPE) have been pressure tested at different temperatures and changes in molecular structure and crystallinity have been assessed. The stage 111 (fracture induced by thermal oxidation) life of the unstabilized polyethylene pipes was less than 12% of the life of the corresponding stabilized polyethylene pipes. Infrared spectroscopy, size exclusion chromatography and differential scanning calorimetry were used to determine the increase in the concentration of oxidation end products, decrease in molar mass and increase in mass crystallinity. A computer program was developed which simulates the changes in the molar mass distribution during degradation as determined by size exclusion chromatography. The program was applied to molar mass distributions of samples taken from the unstabilized MDPE, a stabilized MDPE and a stabilized polybutylene-1 pipe grade.