Molecular structure and morphology of crosslinked polyethylene in an aged hot‐water pipe

Internally pressurized crosslinked polyethylene (XLPE) pipes fail according to one of the three following mechanisms: (a) stage I fracture occurs at the highest stresses and is ductile when large defects are absent; (b) stage II fracture is brittle and occurs at intermediate stress levels; (c) stage III fracture occurs at the lowest stress levels and is brittle. It has been assumed that the material in a pipe which fails according to the last mechanism is chemically degraded. This paper presents data obtained by thermal analysis, X‐ray diffraction, infrared spectroscopy, and gel permeation chromatography on samples taken at different radial positions from a pipe of XLPE (crosslinked by peroxide) which failed according to the stage III mechanism after 17,136 h when subjected to 2.62 MPa hoop stress at 110°C (internal water/external air). These data are compared with data from samples of an unexposed reference pipe. Highly degraded brown spots, referred to as “oxidation spots”, are visible in the aged pipe. The puncture fracture occurred in one of these oxidation spots. The increase in melting point and crystallinity, the decrease in fold surface free energy, the almost invariant crystal unit cell, the decrease in gel content, the decrease in molecular weight of the soluble fraction and the formation of carbonyl arid hydroxyl groups at the inner wall in the aged pipe compared with the properties of the unexposed pipe are consistent with an oxidative degradation of the amorphous chain segments including scission of entangled chains and interlamellar tie chains. The latter is the main reason for the major reduction in strength of the aged pipe leading to stage III failure. The thickness of the inner wall layer of highly oxidized material was about 5 mm in the oxidation spots and only 0.5 mm elsewhere in the aged 10 mm thick pipe.