How impurities affect the lifetime of plastic products - A circularity case study on polymer pipes

The transition to a circular economy has significantly increased the demand for recycled plastics in durable applications. However, the long-term performance of post-consumer recyclates (PCR) remains poor due to the complex influence of polymeric and non-polymeric impurities on product lifetime. This study systematically investigates the impact of impurities on both material properties and product-level performance, on the example of plastic pipes. The Cracked Round Bar (CRB) test was used to evaluate the slow crack growth (SCG) resistance of a material. However, to assess the influence of inhomogeneities on the structural performance, the hydrostatic pressure tests (HPT) was applied. It could be demonstrated that incorporating post-consumer recycled polypropylene (PP) into virgin, extrusion grade PP substantially reduces pipe lifetimes under internal pressure as the recyclate content increases. If not carefully selected, even a 10 % addition of an unsuitable recyclate, containing large non-polymeric inhomogeneities, can reduce ultimate failure time by a factor of more than 30. This drastic decrease occurs because rigid inorganic particles act as stress concentrators, accelerating crack initiation and propagation. Results highlight the critical need for enhanced sorting, purification during recycling, and stricter quality control before manufacturing. This work aims to raise awareness within the polymer industry about the often-overlooked issue of polymeric and particularly non-polymeric impurities by demonstrating a clear correlation between content and size of impurities and performance.