THERMAL STABILITY AND LONG-TERM STRENGTH OF PLASTIC PIPES REQUIRED FOR HOT WATER SUPPLY AND SPACE HEATING SYSTEMS

摘要

Plastic pipes such as PEX and PB pipes have been widely used for water and hot water supply, and for central heating for residential use in Japan. The qualities of PEX and PB pipes and joints have been maintained at high levels as products and as jointing techniques for 20 years or more in Japan. There are few leakages and failures in actual use. The PEX and PB pipe systems have been evaluated for their high quality. The polyethylene for raised temperature (PERT) pipes has recently been introduced also for central heating and floor heating for residential use as they have a possibility to reduce the pipe cost. There are three modes of failure for plastic pipes, which are ductile failure(l), brittle failure(ll), and chemical degradation failure(Ⅲ) as well known. The amount of remaining antioxidants gradually decreases with time. To prevent brittle failure and chemical degradation failure and to obtain a high stress rupture strength, a high molecular weight resin should be selected for plastic pipes. To delay the occurrence of chemical degradation failure, it is important for antioxidants to be integrated, considering thermal stability and minimizing degradations and to be sufficiently added in a polymer. To maintain long-term resistance of plastic pipes to thermal and chemical degradations due to hot water and residual chlorine at elevated temperature for hot water supply and also for central heating and floor heating, suitable antioxidants are added in general. The Oxidation Induction Time (OIT) test is also a useful test to evaluate relative thermal stability. A value of 100 minutes or more by OIT is required for thermal stability at 210 degrees C in an oxygen atmosphere. The long-term strength of plastic pipes can generally be evaluated by the stress rupture test.A 30-years-or-more service life of the system for central heating should be guaranteed at 90 degrees C under an inner pressure of 0.25 Mpa. No failure at 12,000 hrs or more should occur at 110 degrees C by the accelerated stress rupture test specified in ISO1167. The full-notch tensile creep test at elevated temperature is also useful to evaluate long-term strength as an accelerated test method.