(533b) Enhancing the Sustainability, Recyclability and Properties of PLA, PET, and Polypropylene Via Novel Low-Temperature Processing of Homopolymers and Composites with Cellulose

We have successfully addressed several grand challenges in the field of ?green polymers? using a novel, industrially scalable, near-ambient-temperature process called solid-state shear pulverization (SSSP): (1) enhance the effective recyclability of synthetic polymers, specifically poly(ethylene terephthalate) or PET, for high-value applications; (2) overcome limitations in the processability and properties of renewable, biobased polymers, namely poly(lactic acid) or PLA, via addition of low levels of cellulose; (3) enhance the properties of a synthetic polymer, specifically polypropylene (PP), via addition of high levels of cellulose. These achievements have resulted from direct SSSP processing of homopolymer, virgin or recycled, or a mixture of homopolymer with microcrystalline cellulose (MCC). The SSSP apparatus is a modified twin-screw melt extruder in which the barrel is cooled rather than heated and employs trilobe and bilobe screw elements. Process variables are identical to those of twin-screw extrusion (feed rate, temperaturea, screw design and speed), and the powder or particulate output can be directly melt processed into final products by injection molding, extrusion, etc.

When SSSP is applied to PET, the product exhibits both a tunable increase in melt viscosity (factor of 1.2 to 2.0) and a major enhancement in crystallizability. This increase in melt viscosity originates from mechanochemistry during SSSP leading to lightly branched PET and addresses a long-standing problem regarding the high-value recycling of PET, i.e., melt processing PET at ~280 oC into high-value products results in a molecular weight reduction, thus making the viscosity too low for effective use of recycled PET in high-value applications. When a two-step process of SSSP followed by melt mixing is applied to PLA with 1 wt% MCC, a fine micro-/nano-dispersion of MCC results, leading to factor of 20 reductions in PLA crystallization half-times, crystallinity levels (~38%) in the as-processed state near the theoretical limit (40-45%) for PLA, and improvements in both Young's modulus and yield stress. When 20-30 wt% MCC is added to PP by SSSP, 100-115 % increases in Young's modulus are achieved relative to neat PP. These results indicate that SSSP may contribute in significant ways to making sustainable, green engineering of polymers synonymous with major property improvements.