Degradation of unstabilized medium‐density polyethylene pipes in hot‐water applications

Pipes of an unstabilized medium‐density polyethylene have been pressure tested with internal stagnant water and moderately circulating air as the external medium at temperatures ranging from 70 to 105°C and changes in molecular structure and crystallinity have been studied. The stage III (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 and size exclusion chromatography showed an earlier and more extensive increase in the quantities of oxidation end‐products and a more pronounced decrease in molar mass of the outer‐wall‐material than of the inner‐wall‐material of the pipe. Mass crystallinity, measured by differential scanning calorimetry, increased on an average by a quantity corresponding to 45 methylene groups per chain scission event. The life of the unstabilized pipe was divided into an induction period during which no detectable thermal oxidation occurred and a subsequent polymer degradation period. The induction period exhibited an Arrhenius‐temperature‐dependence with an activation energy of 75 kJ mol −1 .