Page/Link:Page URL:HTML link:The Free Library. Retrieved Apr 28 2020 fromINTRODUCTIONThe influence of chemicals on the environmental stress crackingbehavior of polyethylene is of major significance, specifically withregard to the creep strength of molded pipes and for forecastinglong-term properties (1-3). Whereas the environmental stress crackingbehavior of various type of polyethylene has been investigated,including the influence of molecular parameters such as molecularweight, degree and length of branching, or concentration of the chemicalmedium (4-9), studies of blends of different polyethylenes are few. EXPERIMENTALMaterials and Blend PreparationA copolymer made in a gas phase process using 1-butene was used asthe standard high-density polyethylene for mixing with LLDPE or HBPE,respectively, with the properties as shown in Table 1, and the molecularweight averages determined through SEC at 135 degrees C in 1, 2,4-trichlorobenzene (HDPE 5). The properties of the high-densitypolyethylenes involved (HDPE 1-3H), the polyethylene of very highmolecular weight (HDPE 4), of the LLDPE with octene as a comonomer (LLDPE) and of the slightly long-chain branched LLDPEs, also usingoctene as a comonomer, (HBPE 1-4), are also shown in Table 1.Table 1. Figure 1 ILLUSTRATION OMITTEDA 1.6-mm-deep peripheral notch was cut in the middle of thespecimen by a razor blade. Number 8 test specimens were made for onetest each.
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The clamping length was 70 mm. The tests were made on aspecial-purpose creep test equipment (Fig. 2).Figure 2 ILLUSTRATION OMITTEDThe tests were carried out using ethylene glycol as a test medium,at a temperature of 80 degrees C. The applied weight was 5 kg as thepreferred condition, and 3 kg. The test stresses were 5.8 to 6.2 MPa and3.8 to 4.1 MPa, respectively.The median of the time-to-failure t.sub.f up to breakage of thetest piece was established as the parameter for environmental stresscracking resistance.Calculation of the Tie-Molecule DensityThe density of the tie-molecules was calculated according to Brownet al.
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Environmental Stress Cracking as a Tool for Evaluating Polyethylene Piping Materials. The relative resistance to sluw crack growth in polyethylene gas pipe. Corrugated high-density polyethylene (HDPE) pipe 36 in. (900 mm) in diameter was evaluated for its stress crack resistance with the notched constant ligament stress test (ASTM F 2136).
(18, 19) using the Gaussian probability distribution ofend-to-end distances for molecular-uniform polyethylene chains and themelting temperatures determined by means of DSC and the lamellar thicknesses of the crystalline areas established therefrom.
STRESS CRACKING BEHAVIOR OF HDPE GEOMEMBRANES AND ITS PREVENTIONContact
https://www.epa.gov/aboutepa/about-national-risk-management-research-laboratory-nrmrl
Citation:
Koerner, R. M., Y. Hsuan, AND A. R. Lord. STRESS CRACKING BEHAVIOR OF HDPE GEOMEMBRANES AND ITS PREVENTION. U.S. Environmental Protection Agency, Washington, D.C., EPA/600/R-93/092 (NTIS 93-196616), 1993.
Description:
Geomembranes made from high density polyethylene (HOPE) have a high percent crystallinity and are therefore of concern with regard to stress cracking. A review of the literature plus our field exhuming of various sites-of-opportunity gave rise to twenty-five (25) situations where stress cracking of HDPE geomembranes has occurred since the introduction of this liner material in 1980. The stress cracks varied from very small lengths of a few centimeters to large shattering types of failure. All failures have occurred on exposed geomembranes where ultraviolet light exposure, high temperature oxidation and low temperature contraction are continuously ongoing processes. The currentiy used ASTM D1693 bent strip test method for quantifying HDPE geomembranes contains some generally acknowledged limitations. Therefore an alternative (and more challenging) test method was investigated. The result of this search was the development of a notched constant tensile load (NCTL) test. In this test dumbbell shaped test specimens are centrally notched and stressed to a prescribed percentage of their yield stress. They are then incubated in a wetting agent at 50°C constant temperature and their failure times recorded. Evaluation of a series of test specimens at different applied stresses produces a ductile-to-brittle response curve where the onset of brittle behavior is termed the transition point. The test method and procedure was also verified for reproducibility and was evaluated in a series of interlaboratory tests with satisfactory results. Utilization of the above described NCTL test on 18 commercially available sheet materials and 7 field exhumed geomembranes led to the recommendation that for an acceptable stress crack resistant HDPE geomembrane the test results should indicate a transition time of greater than 100 hours. The above recommendations have been shown to be clearly more challenging than those resulting from the bent strip test. It is recommended to discontinue sole reliance on the bent strip test for qualification of HDPE geomembranes in favor of the proposed NCTL test. In consideration of the length of testing time required to completely develop the full NCTL test response, a single point (SP) test has also been developed. In the SP-NCTL test, five notched test specimens are stressed at 30% of then- yield stress and evaluated in a similar procedure as the NCTL test The recommendation is that none of the test specimens should fail within 200 hours. Still further, a seam constant tensile load (SCTL) test has been developed whereby five notched seam specimens are stressed at 30% of their yield stress and, again, are evaluated in a similar procedure as the NCTL test The recommendation is that none of these test specimens should fail within 200 hours.This report was submitted in fulfillment of Cooperative Agreement CR 815692 by the Geosynthetic Research Institute of Drexel University under the partial sponsorship of the US Environmental Protection Agency. This report covers a period from May 11,1989 to May 10,1992 and was completed as of July 31,1992.
URLs/Downloads:STRESS CRACKING BEHAVIOR OF HDPE GEOMEMBRANES AND ITS PREVENTIONRecord Details:Record Type: DOCUMENT (PUBLISHED REPORT/REPORT)
Record Last Revised: 09/11/2008
Record ID: 126388
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