Research On The Preparation Process Of PLA By Ring-opening Polymerization Of Lactide

Poly(lactic acid)(PLA), a kind of aliphatic polyesters, bears wonderful biologic compatibility and biodegradable properties. PLA that can finally biodegrade into CO2 and H₂O under the influence of microorganism, water, acid or alkali, is harmless and non-toxic to the environment. As a thermoplastic polymer with good processability and application performance, PLA will be used very widely. The research of PLA will benefit economy and society.In this paper, poly(lactic acid) was synthesized by indirect method. The synthesis process was chiefly divided into two steps---Preparing lactide and Ring-opening polymerization.L-lactide was prepared from lactic acid. Using the difference of the hydrolysis speed in the water between meso-lactide and other lactides, high pure L-lactide was obtained by water washing-four times recrystallization. Comparing to the only recrystallization method, the yield of lactide purified by water washing-four times recrystallization was increased, the mass of solvent was reduced, and the operating steps were easy.Polylactic acid was obtained by ring-opening polymerization with stannous caprylate (Sn(Oct)₂) as the catalyst. The polymer structure was identified with IR spectra and 1H-NMR, and the polymer molecular weight was measured with viscometer and GPC. Under certain vacuum, the relation of catalyst amount, reaction temperature, reaction time with the molecular weight of PLA was investigated respectively. Further more we investigated how the purity of lactide and the circumstance of vacuum affect the molecular weight of PLA. According to the research, high-molecular weight of PLA can be obtained by means of the ring-opening polymerization under controlling conditions. The optimal parameters are: I/M:1:4000, polymerization temperature: 150℃, polymerization time:35h. The highest viscosity- average molecular weight has exceeded 170000.The effect of several complex antioxidants on polymerization process and the moleculer weight of PLA was investigated. To a certain extent, moleculer weight was increased and the color stability of polymer was improved.In addition, complex antioxidant can favor the subsequent manufacture of PLA goods, and prolong the useful life of the goods through delaying the deterioration of high moleculer weight material.