Recycling of Mixed Plastic Waste - Is Separation Worthwhile?

Abstract

The automated separation of plastic waste fractions intended for mechanical recycling is associated with substantial investments. It is therefore essential to evaluate to what degree separation really brings value to waste plastics as raw materials for new products. The possibility of reducing separation requirements and broadening the range of possible applications for recycled materials through the addition of elastomers, mineral fillers or other additives, should also taken into consideration. Material from a Swedish collection system for rigid (non-film) plastic packaging waste was studied. The non-film polyolefin fraction, which dominated the collected material, consisted of 55% polyethylene (PE) and 45% polypropylene (PP). Mechanical tests for injection-moulded blends of varying composition showed that complete separation of PE and PP is favourable for yield strength, impact strength, tensile energy to break and tensile modulus. Yield strength exhibited a minimum at 80% PE whereas fracture toughness was lowest for blends with 80% PP. The PE fraction, which was dominated by blow-moulded high density polyethylene (HDPE) containers, could be made more suitable for injection-moulding by commingling with the PP fraction. Nucleating agents present in the recycled material were found to influence the microstructure by causing PP to crystallise at a higher temperature than PE in PP-rich blends but not in PE-rich blends. Studies of sheet-extruded multi-component polyolefin mixtures, containing some film plastics, showed that fracture toughness was severely disfavoured if the PE-film component was dominated by low density polyethylene (LDPE) rather than linear low density polyethylene (LLDPE). This trend was reduced when the non-film component was dominated by bottle -grade HDPE. A modifier can be added if it is desired to increase fracture toughness or if there are substantial variations in the composition of the waste-stream. A very low density polyethylene (VLDPE) was found to be a more effective modifier than poly(ethylene-co-vinyl acetate) and poly(1-butene). The addition of 20% VLDPE to multi-component polyolefin mixtures increased the tensile strength and tear propagation resistance by 30% on average, while standard deviations for mechanical properties were reduced by 50%, which would allow product quality to be kept more consistent. ABS was found to be more sensitive to contamination by small amounts of talc-filled PP than viceversa. Contamination levels over 3% of talc -filled PP in ABS gave a very brittle material whereas talcfilled PP retained a ductile behaviour in blends with up to 9% ABS. Compatibility in blends of ABS, high-impact polystyrene and talc -filled PP was poorer at high deformation rates, as opposed to blends of PE and PP from rigid packaging waste where incompatibility was lower at fast deformation. This difference was explained by a higher degree of interfacial interaction through chain entanglements in PE/PP blends.