Solid-State Shear Pulverization of Plastics: A Green Recycling Process

Abstract

A novel process called solid-state shear pulverization (S3P) has been developed at Northwestern University to recycle single or commingled postconsumer or preconsumer polymeric waste without sorting by type or color. This continuous, one-step process converts shredded plastic or rubber waste into controlled-particle-size powder ranging from coarse (10 and 20 mesh) to fine (80 mesh) or ultrafine (200 mesh). As a result, the pulverization product is usable in applications ranging from direct injection molding without prior pelletization, to rotational molding, to use in protective and decorative powder coatings, as well as to blending with virgin resins and compounding with additives. Scanning electron microscopy reveals that the fine particles have a unique elongated shape that is attributed to the high shear conditions occurring during the pulverization process. Injection-molded parts made from the powder product of the S3P process have mechanical and physical properties comparable to or better than the properties resulting from direct conventional processing of recycled single or commingled plastics. In addition, the parts made from the powder product of the S3P process are uniform in color, whereas parts injection-molded from multicolored recycled feedstock without prior pulverization via the S3P process are streaked, re during their commercial applicability. The improved mixing achieved via the S3P process is often accompanied by scission of the carbon-chain backbone of the polymers involved, as revealed by the generation of free radicals during S3P processing, associated mechanochemistry, and modification of the melt flow rate of the polymers by the S3P process. The implications of this chain scission process for in situ compatibilization of commingled plastic waste via S3P processing are discussed.