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The resistance to slow crack growth and to internal over-pressure of pipes based on a commercial polyethylene gas pipe grade (BPE; 0.6 mol% butyl branches and ) blended with both an injection molding grade linear polyethylene (LPE; [Mathematical Expression Omitted]) and 0.5 ± 0.1 mm glass spheres was studied. The blends and the pure polymer components were characterized by density measurements, differential scanning calorimetry, small-angle X-ray scattering, and transmission electron microscopy. The fracture properties of extruded pipes were measured using hydrostatic pressure testing and notched uniaxial testing on samples cut from the pipe wall. The polymer components were macroscopically uniformly blended and differential scanning calorimetry indicated the occurrence of partial co-crystallization between the branched and linear components. Transmission electron microscopy showed molar mass segregation on a 100 nm level. Notched uniaxial testing showed that the slow crack growth resistance of pure BPE was considerably higher than that of the LPE/BPE blend with 30% LPE. Fractography indicated that the fracture-initiating particles were larger in pipes failing after shorter period of time in the hydrostatic pressure testing. The lifetimes of hydrostatic pressure tested pipes based on the BPE grade containing glass spheres were similar to those of pipes based on the LPE/BPE blend with 30% LPE., INTRODUCTION Failures by slow crack growth, as observed in polyethylene (PE) pipes at relatively low stresses, is believed to occur through the disentanglement of tie chains (1, 2). The average [...]

The microstructure and resistance to slow crack growth of two commercial polyethylene pipe materials were studied. Differential scanning calorimetry, small-angle X-ray scattering, and transmission electron microscopy were used to reveal the crystallite thickness and width distributions and the size of the lamellar stacks. The resistance to slow crack growth was assessed by uniaxial constant loading of notched specimens and by hydrostatic pressure testing of notched and unnotched pipes. The high-density material contained roof-lamellae, suggestive of a segregation of low molar mass species. Notched uniaxial testing revealed large differences in slow crack growth-resistance between the two PEs despite the fact that the average tie-chain concentration was similar. Hence, low-molar mass segregation, which was found to be higher for the high-density material, definitely decreases the resistance to slow crack growth. Notched uniaxial testing was a sensitive method for ranking these PEs according to their resistance to slow crack growth, and 15 times faster than that achieved in conventional unnotched pressure testing. Failure time extrapolations from higher temperatures to 20 ° C were made, using a multiple linear regression method (SEM-Q1), the Arrhenius equation, and universal shift-functions to investigate their applicability. The extrapolations resulted in longer life times compared with experimental data, regardless of the method used. The SEM-Q1 method (lower-confidence-limit data) gave the best fit to the 20 ° C experimental data followed by the Arrhenius equation., INTRODUCTION The lifetime of plastic pipes is controlled by material, environment, and loading factors. The creep rupture curve of polyethylene (PE) pipes may be divided into three parts, hereafter denoted [...]

An extrusion-grade medium-density polyethylene was fractionated by size exclusion chromatography and selective solvent extraction. The fractions were studied by infrared spectroscopy, differential scanning calorimetry, and transmission electron microscopy. Infrared spectroscopy showed that chain branching was concentrated to the low molar mass material. Transmission electron microscopy showed the presence of roof-shaped crystals in the low molar mass samples and curved S- or C-shaped lamellae in the high molar mass samples. These features could be used for identification of different molar mass species in the unfractionated sample., INTRODUCTION The development of polyethylenes of greater fracture toughness has occurred stepwise or through different 'generations.' The first generation of high-density polyethylenes (HDPE) used in gas pipes installed before 1975 [...]