Slow crack growth resistance of non-virgin polymers

Abstract

For economic and ecological reasons, the use of non-virgin polymeric materials for structural applications becomes more and more important. The dominating polymers for manufacturing of piping systems are polyethylene (PE), polypropylene (PP) and polyvinylchloride (PVC). Especially in the field of non-pressure applications, during the past years the international piping industry has put additional attention to an in-creased use of non-virgin reprocessed or recycled materials. For non-pressure pipes a minimum lifetime of 100 years will be required in future. Resistance against slow crack growth (SCG) is known as the most critical failure mechanism for long term pressure pipes. Compared to virgin materials, in non-virgin materials different effects are responsible for reduced SCG resistance such as impurities, polymer inhomogeneity or material degradation. Hence, a detailed knowledge about the SCG resistance of the used material is of essential importance for lifetime assessment. The current paper investigates the SCG resistance of different non-virgin PE , PP and PV C grades with the Cracked Round Bar (CRB) Test according ISO 18489. It is demonstrated that the SCG resistance is clearly lower than compared to virgin materials. These results are accompanied by optical fracture surface analysis in order to identify polymeric and inorganic impurities as two of the main reasons for decreased SCG resistance. To investigate effects of polymeric impurities in more detail, in a second step laboratory material were designed by a welldefined mixing of a PE-HD and a PP pipe grade in nine different ratios. The data show that already a very low amount of PE in PP and vice versa reduces the SCG resistance significantly. The current paper demonstrates that the CRB Test is a quick and sensitive method for the characterization of the SCG resistance not only for PE but also for PP and PVC. Moreover, it shows that already a small amount of a polymeric impurity has a big impact on the long-term relevant resistance against SCG.