Environmental stress cracking of polyethylene pipe: Changes in physical structures leading to failure

Abstract

The environmental stress cracking resistance (ESCR) was studied for pipe made of bimodal poly(ethylene-co-1-butene) copolymer. Ductile or brittle failures of the pipe were observed after pipe exposure to different temperatures and hydrostatic pressures. Changes in the phase composition, size of crystalline and amorphous domains and molecular mobility were studied in failed test pipes by DSC, SAXS, solid-state 13C NMR spectroscopy and time-domain 1H NMR T2 relaxometry. The ESCR test at 20 °C causes a decrease in crystallinity, which suggests lamellar fragmentation, whereas a slight crystallinity increase is observed at a test temperature of 70 °C. At both temperatures, molecular mobility in the amorphous phase decreases, suggesting stress-induced elongation of chain segments in the amorphous phase. ESCR tests at 95 and 110 °C led to crystallinity increases and lamellar thickening owing to partial melting and recrystallization during long-term pipe exposure to high temperatures. The test causes an increase in molecular mobility in the amorphous phase due to a change in partitioning of ethyl chain branches between the crystal-amorphous interface and the amorphous phase. The role of molecular mobility in the amorphous phase on the type of failure and on reorganization of semicrystalline morphology during the ESCR test is discussed.