Study On Co-Pyrolysis Behavior And Mechanism Of Polylactic Acid And Forest-Derived Biomass

Petroleum-based plastics have caused irreversible harm to the environment.People have gradually paid attention to the use of biodegradable plastics to replace petroleum-based plastics in making disposable products,because biodegradable plastics can be completely degraded in nature.Polylactic acid(PLA)has become the most popular biodegradable plastic in the market due to its mechanical properties comparable to polypropylene,good biocompatibility,and recyclability of raw materials.Although PLA can be completely degraded,it is a good carbon source material with a carbon content of more than 50%.Direct degradation of PLA is not in line with the concept of resource recycling.Pyrolysis is a suitable method for the chemical upcycling of PLA,which can convert PLA into small molecular substances for reuse as chemical raw materials or fuels.With the deepening of research,people have gradually found that compared with the separate pyrolysis of waste,co-pyrolysis can produce better results in terms of energy and product quality,especially the pyrolysis of agricultural and forestry residues and waste plastics.Our country is rich in agricultural and forestry residues,but lack of rational utilization.At present,the feasibility of co-pyrolysis of agroforestry residues and PLA has not been studied.Therefore,in this study,the synergistic effects of different forest-derived biomass and PLA on the co-pyrolysis behavior and product distribution were investigated,and the mechanism of the synergistic effect was also investigated.The specific research content is as follows:(1)Poplar fiber was used as a representative of agroforestry residues to study the influence of its co-pyrolysis with PLA on the pyrolysis behavior and the distribution of pyrolysis products.Thermogravimetric analysis showed that the synergistic effect of wood fiber and PLA in the co-pyrolysis process was strong,which significantly reduced the temperature required for the co-pyrolysis of wood fiber and PLA.Subsequently,a series of thermodynamic and kinetic parameters were analyzed to quantify the synergistic effect on the co-pyrolysis of wood fiber and PLA.In addition,the production of carbon monoxide and acetaldehyde in the co-pyrolysis process of wood fiber and PLA was greatly reduced,thereby reducing the release of volatile organic products.In the co-pyrolysis process,the synergistic effect reduces the activation energy required for the free radical reaction of PLA pyrolysis,and the yield of Lactide in the co-pyrolysis product increases greatly.Lactide can be used as the raw material of PLA to synthesize PLA.In addition,the yield of small molecules of aromatic substances in the co-pyrolysis process was also improved,indicating that the synergistic effect of the co-pyrolysis of wood fiber and PLA promoted the co-pyrolysis of them respectively.(2)In order to explore the synergistic mechanism of co-pyrolysis of wood fiber and PLA,the co-pyrolysis behaviors of three main wood fiber components,cellulose,hemicellulose and lignin,and PLA were studied.Thermogravimetric analysis showed that the synergistic effect was mainly caused by the co-pyrolysis of lignin and PLA,while cellulose or hemicellulose had little or no effect on the co-pyrolysis.Cellulose and hemicellulose did not produce significant difference between the co-pyrolysis carbon and the single pyrolysis carbon,but the copyrolysis of lignin and PLA could reduce the re-polymerization of small molecular substances in the pyrolysis carbon.Pyrolysis kinetic analysis showed that the activation energy required for co-pyrolysis due to the synergistic effect was lower than that required for the pyrolysis of PLA or lignin alone.The principle of master diagram analysis showed that the synergistic effect changed the mechanism of co-pyrolysis of lignin and PLA from a single volume shrinkage model or reaction sequence model to a simultaneous reaction with multiple parallel pyrolysis mechanisms.At the same time,it was also clear that the synergistic effect of lignin and PLA pyrolysis changed the type and yield of co-pyrolysis products,promoted the free radical reaction of PLA pyrolysis,and made the order of pyrolysis products changed to esters→ hydrocarbons → carbon dioxide.The synergistic effect led to a substantial increase in the yield of Lactide.The co-pyrolysis of biomass and PLA can be optimized to the co-pyrolysis of lignin and PLA,which will produce better chemical recovery.(3)The effect of the ratio of lignin to polylactic acid on the strength of the synergistic effect of co-pyrolysis and on the pyrolysis products was investigated.Thermogravimetric analysis showed that no matter what ratio of lignin and PLA were co-pyrolysis,there was a strong synergistic effect between them,and the synergistic effect reduced the temperature required for the co-pyrolysis of lignin and PLA.Three artificial neural network(ANN)models with different algorithms were selected to simulate the pyrolysis behavior of co-pyrolysis of lignin and PLA at different ratios.The results showed that the RBF neural network model could not only distinguish the boundary of single and co-pyrolysis of lignin and PLA,but also better simulate the synergistic effect on the mass loss of co-pyrolysis of lignin and PLA.Copyrolysis of different proportions of lignin and PLA can improve the pyrolysis oil yield by inhibiting the production of pyrolysis gas.When the content of PLA is higher than that of lignin,the co-pyrolysis effect is better.The pyrolysis oil produced by the co-pyrolysis of lignin and PLA contains a large number of light-yellow solids,which can be separated from the liquid pyrolysis oil.The main component of these pyrolysis solids is lactide by NMR analysis.The solid yield in the pyrolysis oil produced by the co-pyrolysis of lignin and PLA was significantly higher than that produced by the pyrolysis of PLA alone,which also confirmed that the synergistic effect of lignin and PLA in the co-pyrolysis process promoted the production of a large amount of lactide.Finally,the synergistic mechanism was predicted by Py-GC/MS analysis.The free radicals of lignin in the pyrolysis process may be captured by PLA,thus promoting the free radical reaction of PLA,reducing the energy required for the reaction and increasing the yield of lactide.(4)The effect of degradation of PLA on its pyrolysis and co-pyrolysis is not clear,and different degrees of hydrolysis of PLA have been used to study this issue.A series of analytical methods were first used to characterize the molecular weight and ester groups of PLA before and after hydrolysis.The behavior of the pyrolysis of PLA with different degrees of hydrolysis alone or co-pyrolysis with lignin was subsequently analyzed.The results show that the main pyrolysis interval of PLA with different degrees of hydrolysis will be stable in a certain range.However,the synergistic effect produced by the co-pyrolysis of lignin and PLA will strongly promote the co-pyrolysis,and the synergistic effect will promote the pyrolysis reaction of PLA with different hydrolysis degrees in the lowest pyrolysis temperature range.When the PLA is pyrolyzed alone,the less ester bond content in the molecular chain,the more prone to free radical reaction of the PLA.The synergistic effect on the co-pyrolysis mechanism is much stronger than the influence of molecular weight and ester bond in PLA molecule on the copyrolysis mechanism.The synergistic effect makes the degraded PLA can undergo free radical reaction even at low pyrolysis temperature,thus promoting the increase of lactide production.Finally,the solubility of different substances in absolute ethanol can be used to separate Llactide and pyrolysis oil by the method of repeated dissolution and filtration in absolute ethanol,which is helpful to obtain pure lactide for the re-synthesis of PLA.