How do the processing parameters in mechanical recycling affect the final properties of the recycled plastic?

The processing parameters in mechanical recycling significantly affect the final properties of the recycled plastic. First, the sorting and cleaning processes influence the purity of the recycled material. Inadequate sorting can lead to contamination with different types of plastics, while insufficient cleaning can leave residual contaminants that degrade the final product's properties. Second, the grinding process affects the particle size and shape of the recycled plastic. Excessive grinding can generate fine particles that are difficult to process and can reduce the mechanical strength of the final product. Third, the melting temperature and residence time during melt processing affect the degree of polymer degradation. High temperatures and long residence times can cause chain scission and oxidation, reducing the molecular weight and mechanical properties of the recycled plastic. Fourth, the cooling rate during solidification influences the crystallinity and morphology of the recycled plastic. Slow cooling promotes higher crystallinity, which can increase the stiffness and strength of the material. Fast cooling, on the other hand, can result in a more amorphous structure with improved flexibility but reduced strength. Fifth, the addition of additives, such as stabilizers, antioxidants, and compatibilizers, can improve the properties and processability of the recycled plastic. For example, adding antioxidants can prevent thermal degradation during melt processing, while adding compatibilizers can improve the mixing of different types of recycled plastics. Sixth, the number of recycling cycles also affects the final properties. Each time a plastic is mechanically recycled, it undergoes thermal and mechanical stress, leading to further degradation and a reduction in its properties. For example, repeatedly recycling PET bottles can lead to a decrease in their intrinsic viscosity and tensile strength.