Enhancing Toughness of Post-Consumer Recycled Polyolefins with Polybutadiene-Derived Block Copolymers

There is a rapidly expanding need for economically viable approaches for ameliorating the wasteful disposal of post-consumer plastics in landfills and the leakage of these materials into the environment. Here, we assess the efficacy of poly(ethylene)-block-poly(ethylene-ran-ethyl ethylene)-block-poly(ethylene) (EXE) triblock copolymers as compatibilizers in post-consumer recycled poly(ethylene) (rPE) and isotactic polypropylene (rPP) containing ca. 10–15% PP and PE impurities, respectively. E67X138E67 was prepared by anionic polymerization of butadiene followed by catalytic hydrogenation, where the subscripts indicate block molecular weight in kg/mol. This triblock copolymer was mixed with rPE and rPP, along with blends formed from virgin PE and PP, and the recycled and blended materials were characterized by differential scanning calorimetry (DSC), atomic force microscopy (AFM), and tensile testing. The presence of phase-separated impurities in the recycled plastics was confirmed by DSC and AFM, and shown to contribute to inferior mechanical properties, e.g., strain at break, εb < 20% for melt-molded specimens cooled at 38 °C/min, referred to as fast cooling. Addition of 1 wt % of E67X138E67 to rPE and rPP led to improvements in ductility, dependent on the rate of cooling of melt-molded specimens. Fast cooling produced marginal gains in ductility, εb ≈ 90 and 30% in rPE and rPP, respectively. However, industrially relevant very fast cooling (380 °C/min) dramatically improved the ductility, where εb ≥ 500% for both recycled plastics. Similar results were obtained with virgin PE/PP blends containing 1 wt % E67X138E67. These findings are compared with the results described in a previous report, where 1 wt % E65X88E65 was added to virgin PE/PP blends, indicating that the X block molecular weight plays an important role in the efficacy of compatibilization.