EFFECT OF BETA-NUCLEATION ON AGING AND CRACK GROWTH RESISTANCE OF POLYPROPYLENE EXPOSED TO CHLORINATED WATER

Papers # 2018 Las-Vegas

This research addresses the effect of nucleation on the global aging behavior and the fatigue crack growth (FCG) resistance of commercial alpha- and beta-nucleated polypropylene random copolymer pipe grades (PP-R) when exposed to chlorinated water (5 mg/l free chlorine) at an elevated temperature of 60°C. Comparing the two PP-R grades, for the alpha-nucleated PP-R a significantly higher embrittlement due to exposure to hot chlorinated water was obtained in tensile tests with micro-sized specimens. In the FCG experiments with cracked round bar specimens, the beta-nucleated PP-R also outperforms the alpha-nucleated type in non-chlorinated water. Experiments with PP-beta in chlorinated water revealed a significantly reduced FCG resistance, most likely caused by enhanced local crack tip aging.

In many world regions, water is polluted with bacteria, viruses and parasites, and disinfection of contaminated water is required as a prevention of waterborne diseases. Chlorine is the most widely utilized and the most affordable water disinfectant, as it is easy to use and highly efficient against different kinds of waterborne pathogens. In this context, it is well known that polyolefins in general exhibit a significant degree of aging when exposed to water disinfectants. However, for the case of polypropylene (PP), little information is available on how the aging behavior and crack growth resistance in chlorinated water environments is affected by the material morphology. Hence, the aging behavior and crack growth resistance of two commercial PP random copolymer pipe grades was systematically investigated. One of the PP grades was of a conventional alpha-nucleated type (PP-alpha), whereas the other was specifically beta-nucleated (PPbeta). Material exposure and testing was performed at an elevated temperature of 60°C, and under environmental conditions of non-chlorinated and chlorinated water, the latter containing 5 mg/l chlorine. In addition to global aging and fatigue crack growth (FCG) experiments, tensile tests and DSC experiments were conducted. While for the microsized specimens of PP-alpha the exposure to chlorinated water at 60°C resulted in significant embrittlement after 1000 h, beta-nucleated PP-R exhibited a high resistance to chlorinated water exposure. In the FCG experiments, PP-beta turned out to be superior to PP-alpha under non-chlorinated water environment. Furthermore, in chlorinated water crack growth rates for PP-beta were found to be higher than in non-chlorinated water.

Related keywords : nucleation effects; aging; fatigue crack growth resistance; polypropylene pipe grades; chlorinated water.
Please note that the whole article content is available on PPCA website onlySource : 2018 Las-Vegas


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