Fracture characterization of recycled HDPE/nanoclay composites

High density polyethylene (HDPE) is one of the most widely used engineering plastics in infrastructures, particularly in a pipeline system. Current HDPE pipes are made from pristine resin which directly relies on petroleum supply. Utilizing recycled HDPE to replace a portion of pristine resin certainly provides benefits in improving the sustainability of the HDPE pipes and saving raw materials. However, challenges still remain to apply the recycled material for engineering applications which specifically require the long-term service life performance. Recycled materials generally possess inferior material properties to the pristine counterpart, and they have a higher risk of failure caused by stress cracking within a desired lifetime. To suggest a solution to the concern, this research focuses on a new class of materials which are polymer/nanoclay composites (PNCs). The presence of a small amount of nanoclay significantly enhances the properties by increasing mechanical properties, flammability resistance and surface scratch resistance while decreasing gas permeability. However, the fracture resistance of the recycle-based PNCs, particularly stress crack resistance (SCR), has not been studied yet. This research therefore evaluates the effect of nanoclay on the short-term and long-term fracture resistance of pristine/recycled HDPE pipe material. The short-term fracture resistance was characterized using the essential work of fracture (EWF) concept test. The results revealed that nanoclay has not produced significant toughening effect and causes embrittlement of the materials. Adding nanoclay to pristine HDPE, a steady decrease in fracture toughness was measured until 4-wt% of nanoclay after which the change was insignificant. Conversely, nanoclay content more than 2-wt% in recycle-blends greatly decreased the fracture toughness value. The long-term SCR via slow crack growth (SCG) mechanism was evaluated by means of the Notched Constant Ligament Stress (NCTL) test. The result revealed that, unlike the short-term fracture resistance, the nanoclay enhanced SCR and extended the failure time. By integrating the short-term and long-term failure behavior employing the elastic-plastic fracture mechanics, a correlation between the fracture toughness and SCG was accomplished in this study.