Role of interfacial crystallization in designing polyolefin blends from mixed stream recycle feeds

Polyolefin production requires ~8% of global oil and natural gas production for monomer supply and the energy required for polymerization; often these polyolefins are used in short term applications such as packaging. While researchers work toward long term solutions involving sustainable polymers, the short term focus on how to better recycle polyolefins currently in the production/consumption cycle must be addressed. Given their chemical similarity and similar density, recycled polyolefins are difficult to separate from recycle streams often resulting in mixed stream recycle feeds. Previously we presented the role of residual oligomer after Ziegler-Natta polymerization of polyethylene (PE) and isotactic polypropylene (iPP) in preventing cross interfacial crystallization of immiscible PE-iPP bilayers which resulted in weak interfacial adhesion. We also presented strategies for promoting cross interfacial crystallization via processing (rapid interfacial quenching) and materials selection (thickened interfaces) in PE-iPP bilayers. Here we investigate the role of interfacial adhesive strength between three PE-iPP blends in the absence of applied shear during processing. With poor interfacial adhesion between PE/iPP, brittle failure of each blend was observed, as expected with immiscible polymer pairs. When interfacial adhesion strength exceeded that of the strength of component homopolymer, exciting synergism was observed between PE/iPP blends. Processing in the presence of applied shear flows (injection molding and film extrusion) will also be discussed. This finding highlights the importance of considering interfacial strength when designing mixed polyolefin recycle streams.