Environmental stress crack growth in medium‐density polyethylene pipe

Abstract

Environmental stress cracking (ESC) has been investigated as a tool for evaluating the relative quality of piping materials used for distributing fuel gas. Heretofore unexplained differences in failure times of pipe extruded by different manufacturers from the same medium-density polyethylene resin have been observed. Lot-to-lot variations have also been noted for which no plausible cause could be found. Scanning electron microscopy has revealed that surface features of the pipe can contribute to premature failure in polyethylene piping material. EXPERIMENTALTwo lots of 1-in.-diam (2.54 cm) polyethylene pipe corresponding to ASTM designation P E 2306,' both made from the same material but processed by different extruders, were tested in the compressed-ring constant strain test. The test was first described by Rader2 and is being balloted by ASTM for possible adoption as a recommended procedure. In effect, the test approximates the stress configuration in the more common bent strip test developed by Bell Laboratorie~.~The compressed-ring method (for 1-in. pipe) consists of cutting a 12.7-mm-wide ring out of the pipe and pressing a razor blade 0.64 mm deep into the outside wall a t the center of the ring, parallel to the edge. The notch is 19 mm long and is consistently located in the region of minimum wall thickness.Eight of these specimens are mounted between two compression plates with the notched area positioned parallel to the direction of compression. The specimens are compressed so that the plates are 9.14 mm apart and are then placed in a 25% solution of the stress cracking agent a t 50OC. The times for crack initiation are recorded. A picture of the compressed ring fixture with specimens prior t o immersion in Igepal are shown in Figure 1.The stress cracking agent used in this test, Igepal CO-630 (GAF Corp.), is a highly polar, nonionic surfactant with the following structure:

Environmental stress cracking resistance of polyethylene: The use of CRYSTAF and SEC to establish structure-property relationships

“…These external variables can generally be related to changes in crystal morphology and surface defects. 11,12 For instance, ESCR increases by quenching polyethylene from the melt. Because smaller lamella are formed during the rapid cooling of a polymer melt, the percentage of chains functioning as tie molecules is likely to increase.…”

Improvement of the Test Procedure of an Environmental Stress Cracking Resistance Test of High-Density Polyethylene

“…So, ES crack initiated from the notch root and propagated slowly in a radial direction around the notch until the crack reaches to the surface. Lustiger 12 and Lustiger et al 14 also described very similar behavior of subsurface crack initiation under ESCR test, but they did not observe the actual fracture surface. The shape of ES crack is gradually changed into a half‐elliptical shape due to the variation of crack driving forces, which are related to the applied bending stress and geometry.…”

“…The loading condition of the ESCR test is bending, so the highest bending stress is at the surface. However, due to the surface notch that is fabricated parallel with the longitudinal direction of the specimen, the stress concentration is happening at the notch root (Lustiger et al 14 ). So, ES crack initiated from the notch root and propagated slowly in a radial direction around the notch until the crack reaches to the surface.…”

The effect of molecular composition and heterogeneity on the environmental stress cracking resistance (ESCR) of propylene impact copolymers

“…Like most plastic materials, this group of polymers suffers from environmental stress cracking (ESC). The ESC of polyolefins, especially the polyethylenes, has been studied by many groups, over several decades [2][3][4][5][6][7][8][9][10][11][12]. It is reported [13] that PP can undergo ESC but does not suffer as much as PE.…”