Plastic Pipes Conference Association # 2006 Washington DC
Nishimura, Kawaguchi, Igawa
This paper describes test methods for crosslinked polyethylene (PEX) pipes to evaluate long-term performance such as the resistance to chemical degradation due to residual chlorine at high temperature by the formation of crosslinked molecular structure and the addition of suitable antioxidants. As the methods of manufacturing PEX, the silane crosslinking process and the peroxide crosslinking process are used to produce PEX pipes for hot water supply and space and bath heating at Osaka Gas. The Oxidation Induction Test (OIT) and the gear oven test are accelerated tests and useful to evaluate relative thermal stability. However, these tests are not directly correlated to actual life. The immersion test is also useful to clarify the relation between immersion time and time to oxidation in a high concentration of aqueous solution of residual chlorine is obtained. The long-term performance of pipes can be generally evaluated by the stress rupture test. The stress rupture diagram is the most important data to indicate the actual creep strength of PEX pipes. The hoop stress corresponding to a operating pressure of 0.5 MPa even at 90 degrees C is 2 MPa, and the service life under continuous use is 3_104 hr or more, which is almost equivalent to 30 years under intermittent use at the rate of 3 hours a day even when a safety factor of 2 is assumed as the regression line for the experimental data. The stress rupture test by circulating a 3-ppm aqueous solution of chlorine at elevated temperature is a suitable test method to evaluate the actual life of PEX pipes.
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