Study On The Synthesis Of Polylactic Acid From Lactic Acid From Non-food Biomass Sources

As a typical biodegradable aliphatic polyester,PLA monomer lactic acid is derived from starch,polysaccharide and other biomass resources.After use,PLA can be completely degraded into carbon dioxide and water in nature.It is a green and environment-friendly material that comes from nature and returns to nature.PLA has excellent biodegradability,biocompatibility,easy processability,and good mechanical properties,are already widely used in industry,medicine,building,transportation,agricultural,electrical and electronic,automobile,environmental protection material,and other fields,in today’s"lent"environment,polylactic acid is becoming a hot spot of materials research and development.In this study,the research status of biodegradable materials,especially PLA,was reviewed.It was found that the main problems in the preparation of PLA were as follows:1)the effective removal of by-product water;2)Control of reaction kinetics—mass and heat transfer in bulk polymerization;3)Design of high activity and high selectivity catalytic system;4)Effectively inhibit the high temperature degradation of the target product polylactic acid.Compared with the synthesis methods,direct melt polycondensation is one of the most potential preparation methods.Firstly,we studied the influence of the polarity of the reaction system on the activity of the traditional Lewis acid catalyst（SnCl₂·2H₂O）,from the aspects of the way of adding catalyst,the amount of catalyst,reaction temperature,reaction time and the optimization of the reaction device.The process conditions for the preparation of polylactic acid by melt condensation under the traditional Lewis acid catalyst（SnCl₂·2H₂O）were improved.The PLA with a molecular weight of about 3.0×10⁴ was obtained by the preparation of a multi-stage pore supported Sn ZA molecular sieve catalyst with excellent mass transfer ability for the melting condensation of lactic acid.FTIR,SEM,TEM,XPS,BET,XRD and ICP-MS were used to characterize the prepared Sn-ZA molecular sieve.The final product of PLA was characterized by FTIR,GPC,¹H-NMR,¹³C-NMR and DSC.On the basis of the above,the best catalyst and other technological conditions for the preparation of PLA by melting polycondensation of lactic acid monomer from non-food biomass were studied.The study showed that the catalytic effect of the catalyst could be maximized when SnCl₂·2H₂O catalyst was added at one time in the final high temperature polymerization stage.On this basis,0.4 wt%catalyst was added and the polymer was polymerized at 165℃for 24 h.At this time,the molecular weight of PLA reached3.7×10⁴,the product was white yellowish powder,and the yield could reach 92%.The molecular weight distribution index is about 2,and the molecular weight distribution is wide.Sn-supported ZA molecular sieves have multiple pore structures,large specific surface area and more Sn active sites,which endow them with outstanding mass transfer ability and catalytic activity.It was found that ZA-Ⅲ-Sn²⁺(Sn⁴⁺/Sn²⁺=41.9/58.1)had the best catalytic effect,indicating that the main catalytic effect was divalent tin.The three-step treatment had a great influence on the pore structure of the molecular sieve,and its specific surface area,pore volume and pore diameter were significantly increased,exposing more reactive active sites.The multistage pore structure has better mass transfer ability.The optimum conditions for the synthesis of polylactic acid from ZA-Ⅲ-Sn²⁺were as follows:the dosage of catalyst was 0.5 wt%,the catalyst was added in three batches,and the final polymerization temperature was160℃for 26 h.The molecular weight distribution of the product is narrower than that of the product catalyzed by SnCl₂·2H₂O,the molecular weight is about 3.3×10⁴,the yield is about 80%,and the product is white and black powder.Finally,poly（lactic acid）was prepared by melt polycondensation of lactic acid monomer from non-grain biomass.The lactic acid monomer from non-grain biomass（glucose as substrate）contained nearly 1/1 D-lactic acid and L-lactic acid.The results show that both ZA-Ⅲ-Sn²⁺and SnCl₂·2H₂O catalysts have a certain catalytic effect.The molecular weight of PLA catalyzed by ZA-Ⅲ-Sn²⁺is about 2.0×10⁴,the yield is40%,and the product is yellow-colored powder;SnCl₂·2H₂O has a good catalytic effect on the direct melting condensation of lactic acid monomer from non-grain biomass,and the optimal process conditions are as follows:When 0.4 wt%SnCl₂·2H₂O was added in the high temperature section of 170℃for 26 h,the PLA obtained was amorphous,with vitrification transition at 45.33℃,no crystallization temperature and melting point,molecular weight index was only about 1.3,molecular weight distribution was narrow,molecular weight was about 3.3×10⁴,and the yield was about 50%,the product is white yellowish melt.