Regensburg 2004 – wissenschaftliches Programm
CPP 13: SYMPOSIUM: Understanding and Controlling Complex Structures: From Synthetic Polymers to Biomaterials I
CPP 13.6: Vortrag
Dienstag, 9. März 2004, 11:30–11:45, H 37
The Mobility of the Amorphous Phase in Polyethylene as a Determining Factor for Slow Crack Growth — •Yongfeng Men1, Jens Rieger1, Hans-Friedrich Enderle2, and Dieter Lilge2 — 1BASF Aktiengesellschaft, Polymer Physics, 67056 Ludwigshafen, Germany — 2Basell Polyolefine GmbH, R&D, 65926 Frankfurt, Germany
Polyethylene (PE) pipes generally exhibit a limited life-time, which is considerably shorter than their chemical degradation period. Slow crack growth failure occurs when pipes are used in long distance water or gas distribution though being exposed to a pressure lower than the corresponding yield stress. This slow crack growth failure is characterized by localized craze growth and craze fibril rupture. In literature, the life-time of PE pipes is often considered as being determined by the density of tie chains connecting adjacent crystalline lamellae. But this consideration cannot explain the excellent durability of the recent bimodal grade PE for pipe application. We show the importance of the craze fibril length as determining factor for the pipe life-time. The conclusions are drawn from stress analysis. It is found that longer craze fibrils sustain lower stress and are deformed to a lesser degree. The mobility of the amorphous phase is found to control the amount of material that can be sucked in by the craze fibrils and thus the length of the craze fibrils. The mobility of the amorphous phase can be monitored by dynamic mechanical analysis measurements. Excellent agreement between the mobility thus derived and life-times of PE materials as derived from FNCT-tests (full notch creep test) is given, thus providing an effective means to estimate the life-time of PE pipes by considering well defined physical properties.