Solid-state shear pulverization of plastics: A green recycling process

A novel process called solid-state sheer pulverization ((SP)-P-3) has been developed at Northwestern University to recycle single or commingled postco nsumer or preconsumer polymeric waste without sorting by type or color. Thi s continuous, one-step process converts shredded plastic or rubber waste in to controlled-particle-size powder ranging from coarse (10 and 20 mesh) to fine (80 mesh) or ultrafine (200 mesh). As a result, the pulverization prod uct is usable in applications ranging from direct injection molding without prior pelletization, to rotational molding, to use in protective and decor ative powder coatings, as well as to blending with virgin resins and compou nding with additives. Scanning electron microscopy reveals that the fine pa rticles have a unique elongated shape that is attributed to the high shear conditions occurring during the pulverization process. Injection-molded par ts made from the powder product of the (SP)-P-3 process have mechanical and physical properties comparable to or better than the properties resulting from direct conventional processing of recycled single or commingled plasti cs. In addition, the part made from the powder product of the (SP)-P-3 proc ess are uniform in color, whereas parts injection-molded from multicolored recycled feedstock without prior pulverization via the S3P process are stre aked, reducing their commercial applicability. The improved mixing achieved via the (SP)-P-3 process is often accompanied by scission of the carbon-ch ain backbone of the polymers involved, as revealed by the generation of fre e radicals during S3P processing, associated mechanochemistry, and modifica tion of the melt flow rate of the polymers by the S3P process. The implicat ions of this chain scission process for in situ compatibilization of commin gled plastic waste via S3P processing are discussed.