Blending of recycled mixed polyolefins with recycled polypropylene: effect on physical and mechanical properties

The similar densities of polyethylene ( PE) and polypropylene ( PP), complicate the separation for these materials in the mechanical recycling of post-consumer plastics. Therefore, recycled mixed polyolefins (rMPO) are widely available as recycled (r) polymer material. These blends are commonly cheaper, but also inferior in properties to relatively pure waste streams of rPP or rPE. In order to improve these materials, blends of rMPO were made with more expensive, relatively high quality rPP. This approach was purposefully 'bottom up', exploring how to upcycle the lower grade rMPO instead of downcycling higher grade rPP with a measure of added (cheaper) rMPO. Blends were made with two types of rMPO, the one a HDPE-PP blends from hard plastic waste and the other a LDPE-HDPE-PP blends, based mostly on foil materials. Each of these materials had one type of mechanical property in common with the high quality rPP, these being Young's modulus and toughness respectively. Blends were made at different concentrations (0-50% in steps of 10%) and injection moulded into test samples. Density, MFI, mechanical strength, impact toughness and transition temperatures were experimentally determined for each blend. It was evaluated in how far the law-of-mixtures could apply to these blends and for those occurences where it did not, practical rules-of-thumb were composed.