Chemical recycling is the process of chemically treating plastics into useful low-molecular-weight products for reuse. There are petrochemical energy recovery and monomer recovery and reuse of recycled plastic raw materials. In the latter sense, there are more cases of utilization.
From the perspective of reaction types, chemical recycling and reuse are divided into two categories: thermal decomposition and chemical depolymerization. As a method of recycling plastics as chemical raw materials and fuels (thermal decomposition technology), the common ones include thermal decomposition in isolation of air and thermal decomposition under hydrogen. I tried to put the initial thermal decomposition in a vacuum to reduce to monomers, but during high-temperature thermal decomposition, it is easy to produce side reactions in addition to the target main reaction, which is very difficult to control.
However, recently, by adding a catalyst to the thermal decomposition reaction, the reaction can be gradually controlled, and a precise depolymerization technology corresponding to the precise polymerization technology has been developed. The method of chemical depolymerization to generate monomers (depolymerization technology) can be divided into hydrolysis, alcoholization, and glucose decomposition according to different catalysts and solvents. In addition, biochemical methods that use enzymes and microorganisms as renewable natural catalysts have also received attention.
The chemical structure of biodegradation and depolymerization of biodegradable plastics is basically the same. It started with PLA and PHA. Later, many biodegradable plastics such as PCL, aliphatic polycarbonate, and polyamino acid were found to have chemical recycling and reuse functions. Biodegradable plastics are mixed according to physical requirements, so it is more effective to separate chemically recovered through depolymerization than thermal decomposition. Moreover, with the development of new technologies that utilize super (sub)critical carbon dioxide, water and methanol, the advantages of chemical recycling and reuse will become greater.
The basic reaction in monomer reduction chemical recovery and reuse is a reaction that utilizes the equilibrium state of polymerization and depolymerization. In the ring-opening polymerization of cyclic monomers, the energy of ring-opening is the driving force for polymerization. On the other hand, the energy consumed by the polymerization causes the balance to proceed in the direction of the cyclic monomers, thereby forming a polymerization-depolymerization equilibrium state.
Ring-opening polymerization is reversible as the reaction conditions change, and equilibrium is established. Therefore, in order to promote the depolymerization of the monomers, methods such as decompression and other methods are used to vaporize the generated monomers and isolate them from the system. However, the thermal decomposition reaction generally carried out at high temperature is affected by side reactions and the structure of the terminal group of the polymer bond, so it is more difficult to control than precision depolymerization and precision polymerization.