Recycling technology of polylactic acid PLA
With the introduction of the concept of carbon peaking and carbon neutrality in various countries, around low-carbon environmental protection, renewable bio-based materials and plastic recycling have become market hotspots. A number of policy documents issued by the European Union and the United States propose that in the future, the use of virgin petroleum plastics will be eliminated and replaced with recycled materials and bio-based materials.
The polylactic acid PLA is derived from crop starch, which is a bio-based degradable material, and it is a hot spot in the market. The current range of applications is mostly single-use plastics. For example, in the Winter Olympics held in 2022, a large number of disposable degradable plastic products made of polylactic acid are used. Polylactic acid PLA has received attention, mainly in two points:
1. Bio-based sources have huge advantages in carbon emission reduction.
2. It can be degraded and can be composted, and it is taking advantage of the concept of "biodegradation".
1. Chemical cycle: polylactic acid-lactide-polylactic acid: Polylactic acid is currently the largest synthetic bio-based polymer material, and its raw materials are derived from renewable biomass resources. The carbon emission from raw materials to polymer production is about one-third that of general-purpose plastic polyethylene, and polylactic acid is used. Also biodegradable. However, due to the high TC temperature of polylactic acid, it is difficult to depolymerize and recover its polymerized monomer lactide with high recovery rate, and because lactide also has optical isomers, depolymerization and recovery of high optical purity lactide monomer body has not been resolved internationally.
After long-term basic research, the team of Academician Wang Yuzhong has made breakthroughs, realizing that the recovery rate and optical purity of polylactic acid depolymerization and recovery of lactide are far higher than the highest level of similar technologies in the world, so that polylactic acid can collect bio-based, Recyclable, easy to recycle and degradable have become a reality and become a typical low-carbon and environmentally friendly polymer material.
2. Chemical cycle: PLA-PLA, polymer to polymer:
Although polylactic acid can be degraded in nature, the process usually requires a long time and specific degradation conditions, and its degradation products are carbon dioxide and water, which cannot be directly and quickly recycled. The recycling of polylactic acid is realized by chemical recycling, which provides an effective solution to the post-treatment of waste polylactic acid. Most of the current research is to convert waste polylactic acid into alkyl lactate. However, to obtain high molecular weight polylactic acid material through this process, it is necessary to hydrolyze the alkyl lactate into lactic acid, prepolymerize it into oligomer, and dimerize it into lactide. , and then polymerized to obtain polylactic acid, which is expensive and inefficient (Figure 1). Therefore, it has research value and application prospect to realize the direct conversion of waste polylactic acid materials into new polylactic acid materials.
Recently, the Catalytic Polymerization and Engineering Research Group led by Wang Qinggang, a researcher at the Qingdao Institute of Energy, Chinese Academy of Sciences, has developed a new chemical cycle strategy for polymer degradation and repolymerization (“DE-RE polymerization” strategy). The recycling process of polylactic acid waste to new polylactic acid material is realized (Figure 2). The strategy has mild reaction conditions and few side reactions, which reduces the consumption of raw materials for completely reproducing polylactic acid, and improves the recycling efficiency of polylactic acid. During the repolymerization process, final materials with different properties can be obtained by adding different types of monomers. This achievement provides a new solution for the polylactic acid cycle. This strategy is effective in chemical recycling and holds promise in the modification and synthesis of polymers.
3. Chemical Cycle: Commercialized PLA Recycled Material
In October 2021, Total Corbion PLA (“TCP”) launched the world’s first commercialized chemically recycled bioplastic product.
Luminy® RPU grades share the same properties, characteristics and regulatory requirements as virgin Luminy® PLA, but are partially made from processed and post-consumer PLA waste. Total Corbion PLA is already receiving and depolymerizing reprocessed PLA waste, then purifying and polymerizing it into commercial Luminy® rPLA.
4. High-value recovery: efficient preparation of alanine
The Martin/Wang Meng research group of Peking University reported for the first time in the world a new process of catalytically converting polylactic acid plastics to alanine. Using Ru/TiO2 catalyst, under the condition of no added hydrogen, the high-efficiency preparation of alanine from polylactic acid plastic can be realized by simple heating treatment of ammonia water (77% yield, reaction temperature 140 degrees).
Studies have shown that PLA is first aminolyzed to form lactamide in ammonia water, and then lactamide is hydrolyzed to form ammonium lactate, which is further aminated on the catalyst surface to form alanine.
Isotope tracing experiments showed that the activation of ammonium lactate α-H was an important step in the reaction, and the reaction followed a dehydrogenation-amination-rehydrogenation route. The metal catalyst plays a key role in the activation of α-H and subsequent amination. The yield of alanine can be further improved by separation-recycling, the overall selectivity of alanine can reach 94%, and the purity is more than 95%.
The efficiency of this PLA-to-alanine process was evaluated using a commercial PLA pipette (containing about 83% PLA). A 5.0g PLA pipette was catalytically converted to obtain 3.0g of pure alanine, and the whole process only required adding ammonia and heating. The conversion of PLA can be achieved without the need for additional hydrogen.
This new method of converting PLA into high value-added chemicals under the guidance of the idea of "carbon circulation" has greater advantages than the natural degradation path, and will inspire other types of waste plastic recycling.
