Research on a strengthening method and mechanism of expanding polypropylene pipe

Abstract The expansion of deformation‐induced crystallization of polypropylene (PP) pipes was studied. The crystal structure and melting characteristics of the samples after expanding at different expansion strains and expansion temperatures were analyzed. The results showed that the crystallinity was improved when the PP pipes were expanded, and the expansion effect made the crystal structure more perfect while the expanding temperature did not exceed the crystallization temperature. However, once expanding temperature exceeded crystallization temperature, some β lamellae crystal were stretched and transformed into α crystals structure. It showed that the expansion effect could cause the crystal phase transformation of the sample. In addition, when the expanding strain was less than 15%, the β crystal diffraction peak intensity of the sample decreased, but the α crystal diffraction peak intensity increased, and the original sparse distribution lamellae structure gradually became compact. With the expanding strain continued to increase, the oriented molecular chains underwent stress relaxation, which hindered the conversion of β crystals to α crystals. The oriented crystal structure formed by the crystal phase transformation made the PP pipe have excellent tensile and impact strength. As a result, the hoop tensile strength is increased by 13%, and the impact strengths are increased by 36%.