Aging of polyethylene/polypropylene (PE/PP) dual layer pressure pipe by outdoor exposure

This project investigated the effect of PP skin on the PE core pipe in a PP/PE dual layer pipe during production and outdoor exposure under various radiation dosages by comparing the results with the corresponding uncoated pipe using different characterization techniques. It was found for unaged samples that after extrusion the adhesion reducer present in the PP skin migrated to the PE core pipe outer surface but had little effect on the electrofusion (EF) joint quality. The PP skin prevents the PE core pipe from quenching therefore more perfect PE crystal is formed as shown by a higher crystallinity and the residual stress is reduced as shown by a slit ring method. Due to the reduced residual stress, the skinned pipe had higher long term hydrostatic strength (LTHS) than the uncoated pipe. After outdoor weathering, photo-oxidation products were evident at the solar irradiated PP outer surface after 3 GJ/m2 weathering and the whole PP outer surface was oxidized after 10 GJ/m2 weathering. By deconvoluting the IR peaks, ketones, carboxylic acid and esters were found the main products. Although only slight photo-oxidation was identified after 10 GJ/m2 aging on the uncoated PE pipe outer surface, oxidation induction time (OIT) results indicated that the solar irradiated side of the surface lost most of its antioxidants after only 1 GJ/m2 weathering which led to production of weak layer in the EF joint. In the middle pipe wall and at the inner surface, a more gradual decrease of antioxidant was found. The skinned pipe showed better resistance to antioxidant loss than the uncoated pipe and still had adequate antioxidant for EF. The thermal effect of solar irradiation was thought to cause secondary crystallization of the uncoated pipe at the irradiated side and release of residual stress of both uncoated and skinned pipes after aging. The residual stress release rate was found to decline with weathering. As the pipe with thicker skin always had a lower residual stress, it can be inferred that the skinned pipe still had a higher LTHS value than the uncoated pipe even after aging.