Mechanical Properties of High Density Polyethylene Sheets Exposed to Chlorinated Environment: Experimental Methods and Results

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Mechanical Properties of High Density Polyethylene Sheets Exposed to Chlorinated Environment: Experimental Methods and Results

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2020-08

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High density polyethylene is commonly used for plastic pipes because of its corrosion resistance and low cost compared with metals. In piping applications, disinfectants added to potable water supplies can lead to oxidation of the pipe materials. Longevity is critical; lifetimes in excess of 100 years are desirable. The interaction between stresses and environment, stress cracking, must be considered in predicting component lifetime. Homopolymer HDPE was extruded from pellets into 70-100 micron thick sheet and heat treated to achieve two initial crystallinities. These samples were exposed for up to 1250 hours in a chlorinated water environment, at 5ppm @ 70 C with a pH of 6.5. The Oxidation Reduction Potential of the system was maintained at 825 mV. Changes in the morphology, specifically crystallinity and molecular weight, were evaluated as a function of exposure time. Crystallinity was evaluated through differential scanning calorimetry and molecular weight was evaluated through gel permeation chromatography. Similarly, changes in the mechanical performance, specifically tensile and fracture properties were also evaluated as a function of exposure time. Tensile properties were evaluated with ASTM 1708 dog-bone specimens at strain rates ranging between 0.1 and 0.002 sec-1. Fracture properties were evaluated following the essential work of fracture method as applied to specimens at each exposure time. The same trends in morphological and mechanical properties were observed regardless of the initial crystallinity. As-extruded sheets had an initial molecular weight of 230 kg/mol and crystallinities of either 75% or 83%. Molecular weight decreased to less than 30% of the original after 1250 hours of exposure. The highest rate of decrease occurred between 250 and 750 hours exposure. At any exposure time, variation in molecular weight correlated with sample thickness, indicating correlation with a Fickian diffusion model. Crystallinities steadily increased with exposure time by approximately 13% over 1250 hours of exposure. Strain at break showed the most significant change of mechanical properties. Initial strain at break ranged between 4-10 mm/mm. Samples with higher initial crystallinities were completely embrittled after 750 hours, while those with lower initial crystallinities were embrittled after 1250 hours, both corresponding with approximately 90% crystallinity. The essential work of fracture required for the initiation and propagation of cracks during failure also indicated total embrittlement after 1000 hours exposure. The combination of decrease in molecular weight, increase in crystallinity, and embrittlement with exposure provide insight into the mechanisms behind degradation and loss of mechanical performance. Given that the ductility of the material can be attributed to the amorphous region, the data support the hypothesis that chain scission occurs within the amorphous region and that these shortened chains may either migrate into crystalline regions or undergo chemicrystallization.

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University of Minnesota M.S.M.E. thesis.August 2020. Major: Mechanical Engineering. Advisors: Susan Mantell, Mrinal Bhattacharya. 1 computer file (PDF); xvi, 157 pages.

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Cosgriff, Evan. (2020). Mechanical Properties of High Density Polyethylene Sheets Exposed to Chlorinated Environment: Experimental Methods and Results. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/217125.

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