What are the primary differences between mechanical recycling and chemical recycling of plastics?

Mechanical recycling and chemical recycling are two distinct approaches to plastic recycling, differing significantly in their processes and the quality of the resulting recycled material. Mechanical recycling involves physically processing plastic waste without significantly altering its chemical structure. The process typically includes sorting, cleaning, grinding, melting, and re-pelletizing the plastic to create new products. The primary advantage of mechanical recycling is its relative simplicity and lower energy consumption compared to chemical recycling. However, mechanical recycling is limited by the quality of the input waste; it works best with relatively clean, single-type plastics. Repeated mechanical recycling can also degrade the polymer chains, reducing the mechanical properties of the recycled plastic. Chemical recycling, on the other hand, involves breaking down the polymer chains into smaller molecules, such as monomers or other chemical building blocks, through chemical processes like pyrolysis, gasification, or depolymerization. These smaller molecules can then be used to synthesize new plastics or other chemicals. Chemical recycling can handle a wider range of plastic waste, including mixed plastics and contaminated plastics, which are often unsuitable for mechanical recycling. While chemical recycling can produce virgin-quality plastics, it generally requires higher energy input and can involve the use of chemicals, raising environmental concerns. For example, polyethylene terephthalate (PET) bottles can be mechanically recycled into fibers for carpets, but chemical recycling can break down PET into its original monomers, which can then be used to make new PET bottles with the same quality as virgin PET.