Chemical recycling of PCL
In the chemical recycling and reuse of chemically synthesized biodegradable plastic PCL, based on its excellent depolymerization, it can be easily converted into cyclic oligomers. The thermal decomposition reaction of PCL depends on the type of metal contained and the decomposition temperature, especially the residual metal catalyst which has catalytic function in the alcoholysis reaction, which has a great influence on the recycling process. Persenaire et al. proposed that the decomposition process can be divided into the following two stages: the first stage, β elimination at 317 °C; the second stage, the formation of rings at 338 °C.
In the report, it was pointed out that under the catalysis of zinc, PCL can undergo UNZIPPING depolymerization reaction below 300 °C, and β elimination and alcoholysis reactions above 300 °C. The thermal decomposition of some PCL is only carried out by the mechanism of UNZIPPING depolymerization, and some are subjected to UNZIPPING depolymerization under bulk conditions, and random hydrolysis is carried out in solution. This different result occurs, of course, the metal contained in it, and the end group structure of the polymer also has an influence.
PCL can be rapidly converted into cyclic oligomers under the action of lipase in organic solvents. Cyclic oligomers can easily undergo ring-opening polymerization under the action of lipase under concentrated solution or bulk conditions to regenerate high-molecular-weight PCL. Under lean conditions, PCL selectively generates cyclic dimer DCL through the action of lipase. For example, PCL with a molecular weight of 110,000 is depolymerized under the action of lipase, and converted into oligomers formed by a cyclic structure with a molecular weight of several hundred. The cyclic oligomers are polymerized again under the action of lipase to generate a molecular weight of about 80,000. PCL. In this decomposition reaction and polymerization reaction, if supercritical carbon dioxide is used instead of the organic solvent, the reaction will proceed faster.
The aliphatic polyester solution is continuously passed through the immobilized enzyme packed column, and cyclic oligomers can be quantitatively generated. The mobile phase can use not only organic solvents, but also environmental solvents, supercritical carbon dioxide. Moreover, compared with the use of hydrophobic organic solvents such as toluene alone, adding supercritical carbon dioxide to the toluene solution of PCL improves the decomposability of the polymer, and has decomposing activity at 40°C.
Chemical recycling of PBS
PBS is a biodegradable plastic derived from petrochemical resources, but it can also be obtained by polycondensation of succinic acid and 1,4-butanediol, so it is also a possible bio-based polymer in the future. PBS has been used in various fields of fermentation. It is the synthesis and recycling-type chemical recycling of PBS. PBS can be chemically recycled and reused, and can be decomposed into dicarboxylic acids and diols (used as raw materials again) by adding water, but acid and alkali catalysts must be used in the process, and they must be refined after decomposition.
Others use supercritical methanol decomposition to methylsuccinic acid and butanediol.
PBS can also be recycled and reused by enzymatic method. The polymer is dissolved in an appropriate organic solvent, lipase is added, and the mixture is stirred at about 70°C to generate a cyclic oligomer that can be repolymerized into PBS. This cyclic oligomer can easily undergo ring-opening polymerization under the action of lipase to regenerate a high molecular weight polymer. When the concentration of the polymer in the toluene solution during decomposition is low, the main component of the cyclic polymer is a cyclic dimer. Under the action of lipase, the cyclic dimer undergoes ring-opening polymerization to generate high molecular weight PBS (Mw=130000). The polymerizability varies according to the degree of polymerization of the cyclic oligomers, while the dimers are converted to high molecular weight PBS within minutes.
Another method is to convert PBS to monomeric RECYCLE of 1,4-butanediol and succinic acid using Aspergillus. Aspergillus has always been cultivated in solid cultures such as rice and soybeans, so consider whether solid plastics such as PBS can also be directly decomposed.