Changes in polymer structure and antioxidant concentration have been systematically studied as functions of temperature, hoop stress, exposure time, and location in pipe wall on pressure tested pipes of medium density polyethylene. The pressure tests have been performed with water as the internal medium and air as the external medium at temperatures in the range 80 to 105°C. Infrared spectroscopy shows that oxidation is initiated at the inner wall surface just prior to the onset of the so-called stage 111 fra…
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