STRUCTURE AND CRACK-GROWTH IN GAS PIPES OF MEDIUM-DENSITY AND HIGH-DENSITY POLYETHYLENE

The microstructure and resistance to slow crack growth of two commerci al polyethylene pipe materials were studied. Differential scanning cal orimetry, small-angle X-ray scattering, and transmission electron micr oscopy were used to reveal the crystallite thickness and width distrib utions and the size of the lamellar stacks. The resistance to slow cra ck growth was assessed by uniaxial constant loading of notched specime ns and by hydrostatic pressure testing of notched and unnotched pipes. The high-density material contained roof-lamellae, suggestive of a se gregation 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 ran king 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 d egrees 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 S EM-Q1 method (lower-confidence-limit data) gave the best fit to the 20 degrees C experimental data followed by the Arrhenius equation.