Determination of thermal oxidation of high density polyethylene pipes using differential scanning calorimetry

Fifteen high density polyethylene pipes were investigated by differential scanning calorimetry. The internal surface layer of thermal oxidized pipes was found to have a lower melting temperature and lower crystallinity compared with the material in a similar position in nonoxidized pipes. Analysis of the radial gradient of melting temperature and crystallinity showed significant differences between oxidized and nonoxidized pipes. This is expected since thermal oxidation decreases the regularity of the chemical structure by bond scission, branching and cross-linking of molecules, and thereby reduces crystallinity and melting temperature.