According to a review by Paul McKeown and Matthew Jones of the University of Bath: The article "Chemical Recycling of PLA" pointed out that the synthesis of polylactic acid (PLA) is generally based on starch extracted from some plants such as corn, wheat, and cassava as the initial raw material. Glucose is decomposed to obtain lactic acid after being fermented by lactic acid bacteria, and then high-purity polylactic acid can be obtained through chemical polymerization. The ester bond in the PLA molecule can undergo hydrolysis or alcoholysis, and can generate lactic acid in the organism or in the soil through the degradation of microorganisms. The final metabolite products are water and carbon dioxide.
The degradation of PLA is an extremely complex process, which mainly includes the intrusion of solvents, the breaking of ester bonds, the diffusion of soluble oligomers, and the decomposition of fragments. Industrialized composting refers to the process by which microorganisms degrade solid and semi-solid organic matter under controlled conditions to produce stable humus. The general cycle is 180 days. Industrial composting of PLA requires special sites and facilities. Therefore, in addition to composting, the recycling and reuse of PLA through certain chemical methods has also become a sustainable choice.
The chemical recovery of PLA mainly includes two forms of hydrolysis and alcoholysis (as shown in the figure above). In view of the solubility characteristics of PLA, the hydrolysis of PLA generally requires high temperature and/or strong acid/base conditions. According to the molecular weight and size of PLA, the temperature and pH value of the hydrolysis system, the hydrolysis of PLA can be divided into overall etching and surface etching. The overall etching of PLA can uniformly lose the quality and molecular weight of the sample, while the surface etching only involves Degradation to the surface molecular weight. The hydrolysate of PLA, lactic acid, can form a series of high-value by-products after oxidation, esterification, dehydration and dimerization (as shown in the figure below).
The most common form of alcoholysis of PLA is to use methanol for alcoholysis, and the product is methyl lactate; the product of alcoholysis using ethanol is ethyl lactate. During the alcoholysis of PLA, it is often necessary to add a small amount of catalyst, such as H2SO4, ZnCl2, 4-pyrrolidinepyridine, TBD, etc., and a certain temperature is required at the same time. Methyl lactate and ethyl lactate can be used as green solvents, with good biodegradability and low toxicity. At the same time, alkyl lactate can generate lactide through ring-closing reaction, so as to realize the recycling of PLA.
