The Influence of Carbon Nanotubes and Reprocessing on the Morphology and Properties of High-Density Polyethylene/Carbon Nanotube Composites

This study investigates virgin and recycled high-density polyethylene/multi-walled carbon nanotube (HDPE/MWCNT) composites using thermophysical and mechanical characterization techniques to generate knowledge and understand the recyclability of these composites. First, virgin samples with 0.1–5 wt% of MWCNTs were prepared by injection molding. Then, the HDPE/MWCNT composite waste, selectively collected based on the nanotube wt%, was mechanically recycled and successively reprocessed by injection molding. The experimental results show that the degradation process of the end-user properties (mechanical and electrical properties) depends on the MWCNT loading. The higher the carbon nanotube loading, the higher the degradation of the end-user properties. The HDPE/MWCNT composites were found to be resistant to degradation at carbon nanotube loadings below the percolation threshold (located around 3 wt%). In contrast, the recycled HDPE/MWCNT composites with 5 wt% showed a reduction in viscosity, mechanical and electrical properties with recycling. After four reprocessing cycles, degradation in the Young modulus (−35%), tensile strength (−25%), elongation at break (−60%), and electrical conductivity (−2 orders of magnitude) of the HDPE/MWCNT composite with 5 wt% was observed as compared with the virgin composite. From an industrial perspective, it is feasible to recycle HDPE/MWCNT composite waste by mechanical recycling and reuse it to manufacture products with favorable mechanical properties, covering insulating, antistatic and semiconducting ranges depending on the MWCNT loading, owing to the protective effect of carbon nanotubes against thermomechanical degradation.