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Extrusion and granulation process of waste plastics

by:TENGDA     2022-12-28
Waste plastics are different from new resins in performance because they have undergone the heat history and shear history of the molding process, and have experienced the effects of heat, oxygen, light, climate and various media during use. Therefore, the mechanical properties of recycled materials, including tensile strength and impact properties, are lower than those of virgin resins. Cracks cause changes in surface structure, and the appearance quality is not as good as before. The color is yellow and the transparency is reduced. The performance changes of various materials are different. The change of polyolefin material is relatively small. The reduction in relative molecular weight due to processing, especially multiple processing in plastic extruders, can be compensated by crosslinking reactions, so that the processability can be kept constant to a certain extent; the situation is different for styrene copolymers, Tensile properties are reduced with each processing pass. After about four processing processes, the toughness decreases very seriously. Moreover, the effectiveness of the rubber phase impact modifier is also reduced due to crosslinking. Although it is high-impact polystyrene, its impact toughness is not better than general-purpose polystyrene.
The performance of waste plastics can be improved by blending new materials or adding specific stabilizers and additives, such as adding antioxidants and heat stabilizers, which can reduce the adverse effects of heat and oxygen during the operation of the plastic extruder. In some mixed waste plastics, a compatibilizer can also be added appropriately, such as adding EPDM or EVA to mixed waste plastics of polyethylene and polypropylene. Filling modification can also be carried out in the plastic extruder, such as adding 10%~35% filler, 3%~6% lubricant, and 2%~4% color masterbatch to PP waste film at the same time. The recycled material made of CaCO3 filler is used for injection products, which can effectively shorten the molding cycle, improve the rigidity of the product, increase the heat distortion temperature, and reduce the shrinkage rate.
Lubricants improve the fluidity of the melt. In the recycling of some engineering plastics, filling, strengthening and alloying can also be carried out. For some hygroscopic materials, such as PA, PET, etc., moisture will cause degradation during processing, which will reduce the relative molecular weight, melt viscosity and physical properties. Before processing, the moisture in the waste plastic should be removed and fully dried to ensure the quality of the recycled material produced by the plastic extruder.

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