Synthesis And Properties Of PBT-based Random Copolyesters Modified By Lactic Acid

As one of the greatest inventions of the 20th century,plastic greatly facilitated people’s lives.However,general-purpose plastics can stay in the environment for hundreds to thousands of years without disappearing.The excessive use of such environmentally persistent plastics,especially the use of a large number of plastic films in the field of disposable packaging in recent years,has caused the earth’s environment to be increasingly severely polluted by plastics,whether on land or in the ocean.In the long run,the development and use of degradable plastic products to replace general-purpose non-degradable plastic products is a fundamental and effective way to prevent this problem from continuing to develop.From the point of view of material selection,aliphatic polyester ester bonds are more susceptible to molecular chain breaks under the action of water,oxygen,and microorganisms.After being placed in compost or soil for several months,microbial enzymatic degradation can occur,and they will be completely decomposed into CO₂ and water.However,currently only a few products such as PLA,PBAT,PBS,etc.are generally high in price,poor in heat resistance and mechanical properties,and their degradation rate has been proved to be severely slowed in marine and other water environments,so their application expansion is restricted.Aromatic polyesters using terephthalic acid as raw materials such as PBT and PET,although not biodegradable,have excellent thermal and mechanical properties,and are easy to process,and the raw materials are relatively cheap.Synthetic modification of the existing polyester materials can improve the hydrophilicity and crystallinity of the plastic,increase the probability of the interaction of the plastic with water and microorganisms,thereby improving its degradation performance in water.For example,a certain proportion of PBA is introduced into the PBT segment to form a biodegradable semi-aromatic copolyester polybutylene adipate terephthalate（PBAT）.In order to further improve the hydrolysis performance of PBT polyesters in water and obtain a class of all-natural degradable polyester materials,this paper designed polylactic acid as an easily hydrolyzable segment into the PBT polyester segment,and melted and polycondensed by one-step method.Three types of PBT-based random copolyesters were prepared by copolymerizing lactic acid,terephthalic acid and butylene glycol,and their basic thermal,mechanical properties and degradation properties were investigated.The specific work is as follows:（1）Synthesis and performance study of lactic acid modified PBT random copolyester（PBTL）Through one-step melt polycondensation,different proportions of lactic acid（LA）,terephthalic acid（PTA）and 1,4-butanediol（BDO）were copolymerized to synthesize a series of random copolyesters with higher molecular weight―Poly（terephthalate-1,4-butanediol-lactic acid）ester（PBTL）.The results showed that the introduction of LA units disturbed the regularity of the PBT main chain,thereby reducing the crystallization ability of the copolyester.As the LA content increases,the melting point of the copolyester decreases,but the thermal performance remains at a relatively high level.The melting point is between 155℃ and 220℃,and it is thermally stable below 374℃.It is the decline in crystalline properties that makes PBTL increase the elongation at break from 182%to 564%while sacrificing part of the strength,and the toughness is greatly improved,and the Young’s modulus is generally above 1000 MPa.In addition,the PBTL series of copolyesters showed compost degradability in the composting experiment,accompanied by obvious changes in disintegration morphology,and the greater the LA content,the faster the degradation rate.After three months,the final mineralization The rate is as high as 48%.In the hydrolysis experiment,the PBTL spline immersed in the buffer solution for 400 days showed breakage and hydrolysis porosity,and the lactic acid content and the average segment length of BT were significantly reduced.After 90 days of soaking in natural seawater,PBTL with high lactic acid content has shown preliminary signs of degradation.Inserting polylactic acid fragments into the main chain of PBT as easily hydrolyzable units indeed endows this refractory aromatic polyester with good degradation performance while losing part of its thermal and mechanical properties.In short,the PBTL obtained in this study is a kind of compostable and hydrolyzable random copolyester with good heat resistance,strong mechanical strength and toughness.（2）Preparation and properties of PBTL random copolyester（PBA_mTL）toughened and modified by adipic acidIn view of the insufficient toughness and slow degradation of the low lactic acid component in the PBTL series of copolyesters,the LA was controlled to be 20%of the total moles of dibasic acid,and the fatty diacid adipic acid was introduced into the PBTL main chain through one-step melt polycondensation.（AA）was modified as the fourth monomer to obtain a high molecular weight series of random copolyester―Poly（terephthalate-1,4-butylene-adipate-lactic acid）ester（PBA_mTL）.The results showed that the introduction of AA further weakened the crystallization ability of the copolyester,and the crystalline structure changed when the lactic acid content was high.Compared with the low lactic acid content PBTL,the PBAm TL copolyester was significantly tougher,and the elongation at break changed from 247%increased to 1467%,but the melting point and strength decreased rapidly.（3）Preparation and performance study of lactic acid modified PBA_mTL random copolyesterIn view of the rapid decrease in the melting point and strength of the material in the PBA_mTL copolyester,in this work,we further adjusted the lactic acid content in the copolyester PBA_mTL,and synthesized a series of random copolyesters―Poly（terephthalate-1,4-butanediol-adipate-lactic acid）ester（PBATL_m）.The results showed that when the molar feed ratio（LA:T:AA）increased from 1:4:1 to 3:4:1,the elongation at break of PBATL_m copolyester increased from395%to 618%,and the strength（34.0 MPa-35.5 MPa）and the melting point（160℃-181℃）only slightly changed.Consistent with our modified expectations.In addition,the introduction of lactic acid,compared with the equimolar amount of long carbon chain adipic acid,gives the PBT backbone more easily hydrolyzable ester bond sites,which is beneficial to promote the early hydrolysis process of the copolyester biodegradation,thereby accelerating the overall biology degradation rate.