Blends of post-consumer recycled polypropylene (PP)/polyethylene (PE): rheology, morphology and mechanics

Abstract

The commingled plastic waste of polypropylene (PP) and polyethylene (PE) can be mostly found in plastic packaging waste. The similar density of PP and PE is an obstacle to separate into individual plastic type by using density separation in water during mechanical recycling. Hence, the commingled packaging waste (PP and PE) can be recycled by blending at the end phase of mechanical recycling process. The obtained blend from mechanical recycling of post-consumer packaging waste is known as regranulate. The unknown composition and poor impact resistance of regranulates are two hurdles to overcome. In this work, compatibilization of high-density polyethylene (PE-HD) and PP virgin model blends (PE-HD rich PE-HD/PP) for mechanical recycling simulation of post-consumer detergent bottle was first investigated as a preliminary study. Based on results from contact angle measurements, adhesion tests, tensile as well as tensile impact strength tests, it was found that ethylene-based olefin block copolymer was the most effective compatibilizer due to the encapsulated morphology, best adhesion, and thereof improved impact resistance of virgin model blend. After preliminary study with model virgin blend, the compatibilization of recycled post-consumer detergent bottle waste with ethylene-based olefin block copolymer and their rheological, morphological, thermal, and mechanical properties were studied. The results indicated that the addition of ethylene-based olefin block copolymer into 15 wt-% PP contaminated PE-HD (worst-case scenario) improved significantly impact resistance at room temperature as well as at -20 oC even in the presence of pigments and detergent residues. The morphological analysis with scanning electron microscopy (SEM) indicated that the addition of ethylene-based olefin block copolymer as a compatibilizer improved adhesion between rPE-HD and rPP from recycled detergent bottle waste. The rheological analysis proved interfacial interaction between ethylene-based olefin block copolymer, rPE-HD and rPP.Other hurdle to overcome is the determination of unknown composition in regranulates from mechanical recycling. The model mixtures from PE-LD/PP as well as PE-HD/PP were prepared, and the calibration curves were plotted using FTIR and DSC methods. With the help of calibration curves, the unknown composition of regranulates were determined. In addition to compositional analysis of regranulates, the impact resistance of regranulates was improved with the addition of 5 wt-% ethylene-based olefin block copolymer as a compatibilizer.