Polypropylene Dissolution Kinetics: Effects of Solvent, Temperature, and Particle Size.

Polypropylene (PP) is widely used and currently very little recycled. A promising method for recycling the PP present in plastic waste involves its selective dissolution and subsequent separation from undissolved compounds. We address here the fundamentals of PP dissolution. Specifically, we present a model that describes the different phenomena involved in the dissolution of semicrystalline PP and validate the model with the experimental results on the decrystallization and dissolution kinetics of PP pellets. The model provides detailed time-resolved and position-resolved information on composition (i.e., crystalline PP, amorphous PP, and solvent) and solvent diffusivity (which depends on composition) across the dissolving polymer particle, in different solvents and temperatures. Such information is unavailable experimentally or difficult to obtain. The key fitted parameters that capture decrystallization and polymer chain disentanglement decrease with increasing temperature following an Arrhenius relationship, with activation energies higher than that for crystallization and comparable to that for melt viscosity. Both decrystallization and dissolution times increase with particle size. For smaller particles, decrystallization and dissolution occur nearly simultaneously, while for larger particles, their interior remains solvent-poor and crystalline for longer times. This work offers insights into the interplay of decrystallization and polymer chain disentanglement during the time-course of PP dissolution. Further, this work facilitates the design and optimization of a dissolution-precipitation recycling process that can unlock value from the million tons of PP annually that is currently being landfilled or incinerated following its use.