Recycling or reprocessing of "multi-layer" or "multi-material" plastic products is a particular problem for plastic compounders or recyders. Examples of multi-layer plastic products include painted or coated plastic items, such as automotive body panels, or multi-layer plastic packaging, such as that used in high barrier applications. In each of these cases, the coating or multi-layer structure is utilized to enhance either the product's overall performance or appearance. Unfortunately, the presence of other material layers will generally have a negative impact on the recyclability of the primary thermoplastic material as they often act as incompatible contamination. For example, most of the paints or coatings used to paint plastic parts are thermosetting in nature and act as solid paniculate inclusions in the recycled matrix material. Methods to remove coatings from coated plastic parts have been developed, but are not always cost effective. Similarly, separating multi-material or multi-layer structures may or may not be technically feasible depending on their specific construction. As an alternative, these complex material constructions can sometimes be effectively recycled if the contaminating layers can be reduced in particle size and effectively mixed into the continuous thermoplastic matrix material. A single screw mixing or "grating" section has been designed and evaluated experimentally in this study. Experimental trials were conducted using reground, painted thermoplastic olefin automobile bumpers. The reground bumper scrap was extruded using a single screw equipped with this grating sections, and several more conventional extrusion screw designs. The paint flake size distribution of the extrudate, and the physical properties of samples molded from the extrudate were evaluated for each screw design, both with and without melt filtration.
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