Research On The Reclaiming And Purifying Of Waste Liquid From Lactic Acid Polymerization

Polylactide (PLA) is a goog biomedical material. PLA is synthesized by ring opening polyaddition of lactide. The lactide which was synthesized by lactic acid had much impurity. In order to get a higher molecular weight PLA, the lactide must be purified. Usually, ethyl acetate was used as the recrystallization solvent of lactide. In the recrystallization,a lot of ethyl acetate was polluted. In the testing of the molecular weight of PLA, chloroform was also polluted. If the polluted solution was not reclaimed, not only the environment was polluted but also much resource was wasted.According to the components of the recrystallization solution, reduced pressure distillation was firstly used to reclaim the solution. The fraction acquired under different temperature was investigated by IR. Ethyl acetate can form azeotrope with water. In order to get higher ethyl acetate, salting was used to concentrate ethyl acetate. The salting effect of different salt was researched.It was found that the IR of the fraction acquired under 45±5℃ was the most nearly to the IR of pure ethyl acetate. GC showed that the concentration of the fraction acquired under 45±5℃ rose to 99.9 % when it was salted by sodium carbonate. GC-MS was used to test the fraction acquired under 45±5℃ and the ester phase acquired by salting. It was found that there were two obvious peaks in the TIC, and their retained time were the same. The two composition was separately ethyl acetate and a five-ring ester who had alcohol structure through MS analysis.When the recrystallization solution and the ester phase acquired by sodium carbonate salting were tested by coordinating titration, it was found that the two solution had a competent which could form coordinate complex salt with Ca²⁺. This competent was 3,4-dihydroxy-5-methyl-dihydrofuran-2-one by analyzing theory. The IR of raw lactide and recrystallization solution both had an obvious peak at about 1350cm^-1 .This was the tertiary alcohol absorption peak .But the IR of the lactide which was recrystallized 3 times had a weak peak at this site. This showed that the five-ring ester was rinsed out with ethyl acetate when the lactide was recrystallized. The melt point of lactide which acquired by recrystallization three times was agreement with the document reported. The impurity five-ring ester had no influenceon the purity of lactide.The purity of chloroform can be reached 99.55 % when it was distillated under 76±2°C.Reclaiming and purifying the polluted solvent had grate significance that it not only protects environment but also economizes resource.