Mechanical properties and molecular structures of virgin and recycled HDPE polymers used in gravity sewer systems

The widespread use of plastics in the conditioning, packaging and building material sectors generates an enormous amount of industrial waste which could be recycled for wastewater pipes and fittings. Nevertheless, current manufacturing standards in the piping industry recommend against the use of post-consumer recycled materials—a policy based on inadequate understanding of the properties and long-term mechanical performance of recycled materials. The present study compared the material characteristics of virgin and recycled high-density polyethylene (HDPE) plastics commonly found in the piping industry. Mechanical testing, oxidative induction time (OIT), melt flow index (MFI) and thermal analysis were used in conjunction with X-ray fluorescence (μ-XRF), size exclusion chromatography and ¹³C solid-state NMR to evaluate mechanical behavior and molecular structure as well as contaminant or filler contents. This study provides evidence for the degradation processes impact that can occur when post-industrial and post-consumer polymers are recycled. However, the study identified two measures to improve the material qualities of post-consumer recycled HDPE: 1) reducing the amount of contaminants or, alternatively, improving their compatibility with HDPE resins, and 2) improving current sorting and recycling processes to increase the amount of tie molecules in HDPE materials.