Cascading Crystallization in Mixed Polyolefins

Polyolefins constitute the largest share of plastics that enter the waste stream but have recycling rates less than 10 percent. The underlying difficulty is thermodynamic in nature. The post-consumer polyolefins – polyethylenes and polypropylenes - typically enter the recycling stream in mixed form and are processed together. Due to thermodynamic incompatibilities the various polyolefins mix and micro-separate into domain morphologies, and upon subsequent cooling, crystallize separately in a cascading fashion, frequently with poor interfacial adhesion - causing brittleness. Such cascading crystallization is critical to the final mechanical properties yet is poorly understood. Here we employ the rheo-Raman microscope, phase contrast microscopy, and differential scanning calorimetry to study the crystallization and the associated rheology in immiscible blends of high density polyethylene and isotactic polypropylene. We find that the crystallization kinetics and rheology are a strong function of domain structure and composition. In particular, we examine the compositional dependence of the continuity of the iPP domain and its impact on the crystallization kinetics. Our results indicate the importance of rheology and processing on the structure and properties of mixed waste-stream crystallizing polymers.