Durability of non-pressure polypropylene pipes
For plastics pipes used for underground drainage and sewerage as well as for other nonpressure pipe applications there are at present no internationally accepted methods for the evaluation of the durability of the material. In practice pipe material selection is based upon comparative operational experience of the different materials over a number of years. This means that there are no objective means of assessing new pipe designs and new pipe materials coming onto the market. Reasonably there will be a spread in material qualities with some materials having good properties and some being of lower quality (e.g.recycled materials or materials with very high filler content). Without a recognised testing method it is not possible to rank these alternatives or restrict their use to specific application areas without awaiting the outcome from their use in the field. It is today an established fact that plastics pipes are well suited for underground water and sewer systems due primarily to their flexibility to soil movement and their corrosion resistance. Pipe deflections of up to 10% of the diameter has been shown to be no problem for plastics pipes according to extensive field studies performed by the Teppfa organisation. However, it is also a fact, not so well known, that the stresses in the pipe wall can still be at a relatively high level after 10 or 50 years of use under such conditions. With new pipe structures and possibly lower quality pipe materials, the stresses and strains in the pipe may well exceed the limiting properties of the material and thereby put the reliable function of the pipe system at risk. One main objective of this study has been to look for limiting values of strain for the different materials, above which excessive deformation or failure can occur. The evaluation method presented is based on the use of a range of durability test methods on mainly polypropylene but also on one polyethylene material. The important characteristics studied include the stress relaxation/strainability properties, slow crack growth / notch resistance, thermo-oxidative degradation and environmental stress cracking. The basic properties of the materials are developed by tests on solid wall pipes. The relaxation behaviour of the materials was also modelled and compared using CAED methodology.