The Optimization Of Experimental Process Conditions For Preparation Of Polylactic Acid (PLA) By Microbial Fermentation Of L-lactic Acid

Polylactic acid（PLA）is a new type of biodegradable material,which is obtained by chemical synthesis of lactic acid dimer（lactide）using microbial fermentation product lactic acid as monomer and polymerization.PLA has a wide range of applications and can be completely degraded by microorganisms.Therefore,PLA is a new way to solve the problem of environmental pollution caused by traditional plastics.However,the high production cost affects its market competitiveness.Currently,the most common manufacturing method of PLA is the ring-opening polymerization of lactide.The quality of lactide plays a decisive role for the quality of PLA,and the raw material of lactide,the lactic acid,also needs to be refined and purified.These factors have led to a long and complex technical process for PLA production,with many variables that can affect the final product.This article has conducted in-depth research on the experimental process of lactic acid-lactide-polylactic acid.In terms of lactide,high-purity L-lactic acid was obtained by screening high-temperature and high-permeability resistant lactobacillus strains and optimizing the fermentation and refining conditions.Then,high-purity L-lactide was synthesized using L-lactic acid vacuum distillation method,and the synthesis and purification conditions were optimized to prepare high-purity L-lactide.Finally,PLA was produced using the lactide ring-opening polymerization method,and the polymerization process was optimized to obtain high-quality PLA products.The main research achievements are as follows:（1）From different samples of self-made yogurt by herdsmen in Mulei area of Gongliu County,Xinjiang,20 strains of bacteria with high acid production were screened,and 5strains were selected through high-temperature and high-permeability resistance screening.16S r DNA sequence analysis was performed,and the highest optical purity of L-lactic acid was produced by Lactobacillus rhamnosus（Lr-3#）,with a product optical purity of 97.4%.（2）The optimal fermentation parameters of strain Lr-3#were determined by single-factor experiments.The carbon source was a mixture of glucose and molasses（1:1）,the nitrogen source was a mixture of yeast powder and corn steep liquor（1:1）,and the KH₂PO₄concentration 2 g/L.The optimal fermentation conditions were inoculation amount 4.5%,inoculation time 20h,fermentation temperature 45℃,and p H 6.0.The optimal molecular distillation conditions for the purification of L-lactic acid were scraper rotor speed 100 rpm,feed rate 70 m L/h,and evaporation temperature 75℃.Based on these results,the fermentation conditions were further optimized using response surface methodology（RSM）.The optimal process parameters were determined to be inoculation amount 4.598%,inoculation time 20.137 h,fermentation temperature 45.130℃,and p H6.094.Under these conditions,the theoretical production of L-lactic acid was 34.778 g/L.Meanwhile,it was found that the inoculation amount had the most significant effect on L-lactic acid production,followed by inoculation time,fermentation temperature,and p H value.After modifying the optimal process parameters for practical experimental operation,the L-lactic acid production from fermentation was increased to 35.5 g/L with an optical purity of 99.5%,which was 2.1%increase compared to the unoptimized condition.（3）The optimal parameters for the synthesis and purification of lactide were obtained by single-factor optimization method,which were catalyst addition amount 1.5%,polycondensation time 4.0 h,cracking temperature 245℃,and recrystallization 4 times.The optimal process successfully produced lactide with high purity and high melting point,and the structure was confirmed by infrared spectroscopy.The optical purity was determined to be 99.47%by high-performance liquid chromatography（HPLC）,and the melting point was determined to be 101.47℃by differential scanning calorimetry（DSC）.（4）The optimal conditions for the synthesis of polylactic acid（PLA）were determined by single-factor optimization method,which were catalyst addition amount 0.03%,polymerization temperature 140℃,and polymerization time 10 h.The optimal process was further determined using RSM,which was a catalyst addition amount 0.031%,polymerization temperature 139.148℃,and polymerization time 10.087 h.Under these conditions,the theoretical weight-average molecular weight（Mw）of PLA was 188370,and the results showed that the impact on Mw in decreasing order was catalyst addition amount>polymerization temperature>polymerization time.After modifying the optimal experimental process conditions,the following indicators of synthesized PLA were determined:weight-average molecular weight（Mw）189817,molecular weight distribution width（PD）1.5797,melting temperature（Tm）175.45℃,monomer content 0.28%,and average color b value 0.99.Compared with the basic process,the monomer content was reduced by 66.3%,and the particle color was significantly improved.In summary,this study developed an experimental process for the synthesis of polylactic acid（PLA）by ring-opening polymerization of lactide obtained from lactic acid which was produced by microbial fermentation.The obtained preparation method provides a feasible reference for industrial applications.