Study On Synthesis Of Poly(Lactic Acid) By One-step Method And Chain Entension And Crosslink Of PLA

Polylactic acid(PLA) is a biodegradable material, and the raw material of PLA isfrom renewable resources, so it s attract extensive attentions of scientists. Though thedirect polycondensation of PLA has low cost, PLA prepared by the method has poorperformance and narrow application. This paper studied the direct meltpolycondensation of lactic acid, PLA with ultraviolet(UV) activity was prepared by endgroup modification of PLA. PLA with ultraviolet(UV) activity can be crosslinked afterUV irradiation. It can be used in paint, printing ink, etc.Firstly, the melt polycondensation conditions for PLA had been discussed such asdosage of catalysts, the reaction time and temperature. Drawing on the experience of thefirst study, hydroxyl-terminated PLA(OHPLA) was prepared through adding polyol(ethanediol2s, glycerol3s, pentaerythritol4s) in the process of melt polycondensationof PLA. Then PLA with the double bond (prepolymer) was prepared by the solutionpolycondensation. The detection of acid value changes was used to reflect the degree ofreaction in this study. The influence of factors on the reaction degree were discussed andthe best preparation technology of prepolymer was obtained. In the preparation step ofhydroxyl-terminated PLA, acid value at reaction equilibrium became bigger with theincrease of arm number of polyols when the content of polyols were all5wt%. The timeacid value reach balance gradually extended with the decrease of content of polyols. Inthe preparation step of PLA terminated with acrylic acid(DPLA), the decrease of acidvalue became unobvious after5h.The infrared spectra (IR), nuclear magnetic resonance(NMR), gel permeation chromatography (GPC) and differential scanning calorimeter(DSC) were used to test and analysis the structures and performance of samples. Itindicated that molecular weight of DPLA became larger and the molecular structure ofprepolymer was more uneven when arm numbers of the polyols were more at the samequality. The prepolymer with more arm number had the lower glass transitiontemperature and melting temperature. Branched structure is beneficial to increaseacrylic end capping rate(Da), Da of4sDPLA(1%) was88%, more than2sDPLA(1%).The influence of light initiator, active diluents and prepolymer on curing rate andcoating performance were studied. The optimal formula of the UV curing system wasobtained. The coating performance of4sFPLA and3sFPLA were better than2sFPLA.The gel content of4sFPLA(35%) was75%, The film properties of4sFPLA(35%) were:pencil hardness2H, adhesion1degree. Molecular weight of the sol part of3sFPLA was3.4times than that before UV curing, it indicated that light irradiation played a role inexpanding chain.The thermal performance of cured films were tested by DSC and TGA.The results showed the thermal stability of the product was improved after crosslinking.According to the study on the thermal and high humidity degradation of product, thedegradation speed of cured film of higher gel content was slower.