The effects of recycling on the properties of carbon nanotube-filled polypropylene composites and worker exposures

Abstract

As nanocomposite materials enter the marketplace, it is critical to consider the entire life cycle of those products, including end of life and reuse of scrap. While recycling of polymer based materials is widely accepted for manufacturing and consumer scrap, the influence of the nanoscale filler on the recycling process has received little attention and may pose some unique challenges, such as the potential for human exposure to carbon nanotubes (CNTs) during the recycling process. In this work, the impact of recycling on CNT-filled polypropylene (PP) properties and exposures was studied by repeated injection molding and granulation up to twenty cycles, while simultaneously monitoring exposures. Characterization of chemical structure, melt rheology, mechanical properties, and morphology were performed on recycled materials. Both the neat and filled materials showed a reduction in viscosity with recycling, but the changes were greater for the neat materials. The mechanical properties (such as modulus, strength, toughness) were also affected by recycling. The Young's modulus, yield strain and stress for both neat PP and CNT-filled PP were found to be little affected by recycling. Strain and stress at break for neat PP decreased with recycling, but only slight changes were found for the CNT-filled PP. The CNT-filled PP showed an increase in toughness with recycling due to changes in the crystallization behaviour. This offers potential for addition of CNTs for the purpose of improving recycling resistance. However, recycling should be performed under proper exposure controls because grinding generated high exposures to nanoparticles and CNT-containing respirable fibers.