Chemical Recycling Of PLLA Based On The Efficient Pyrolysis For Giving L-lactide Under The Catalysis Of Complex Alkali

Currently,Polylactic acid?PLA?is regarded as one of the hottest materials to substitute traditional petroleum-based plastics.To develop the full chemical recycling technology of PLA is not only reducing the waste problems,but also regenerating the waste PLA.In the chemical life-cycle process of PLA,the catalytic synthesis of lactide by pyrolysis of PLA is the key technology to the whole cyclic procedure.However,some shortages have been realized when producing monomer lactide by low-molecule-weight PLA catalytic pyrolysis including low yield and serious racemization of lactide.Likewise,these are the similar issues occurring when synthesizing the lactide by high-molecule-weight PLA catalytic pyrolysis,besides a large dosage requirement of catalyst and the low catalytic activity problem.In order to solve the aforementioned issues,this dissertation developed a new full chemical recycling technology to effectively synthesize lactide by using easy-obtaining environmentally friendly base catalytic compounds?zinc lactate and sodium bicarbonate?to catalyze PLA pyrolysis.?1?Zn?La?₂ and NaHCO₃ were used to catalyze the low-molecular-weight poly L-lactic acid?Mw=1500Da?pyrolysis to synthesize low racemized L-lactide.After 3.25h,the yield of lactide was above 95.63%with a purity up to 97.86%at 195?and 3 torr.Later,the reaction conditions such as the catalyst dosage,temperature and pressure were studied to be optimized by GC,GPC,¹³C-NMR analysis.The reaction mechanism of highly efficient catalytic cracking of the compound base,and the racemization mechanism of the product lactide were analyzed through the study of the configuration characterization of the substrate and the product.?2?Zn?La?₂ and NaHCO₃ were used to catalyze the high-molecular-weight waste poly L-lactic acid?Mw>190k Da?pyrolysis to regenerate the monomer L-lactide.Two plausible degradation technical routes were investigated.The first approach is a combination of direct decompression and pyrolysis after mixing the catalyst in the mixer.The optimal monomer conversion rate and the purity was up to 89.93%and84.17%,respectively,within 3.5 hours at 210?and 3 torr.The second is a combination of ordinary pressure and decompression pyrolysis.After 3h reaction at 190?,the pressure was then decompressed to 3torr to continue reacting for 1 more hour.The optimal monomer conversion rate and the purity was up to 90.23%and 84.79%,respectively.Afterwards,the reaction conditions such as the catalyst dosages and the temperature were optimized.The reaction mechanism of the waste PLA pyrolysis catalyzed by the compound base was studied through the configuration characterization of the product components and the lactide.