Definition of the conditions for obtaining lactic acid oligomers

The given work shows studying the process of obtaining oligomers of lactic acid (LA) from PURAC (Spain) which is used for the synthesis of lactide with subsequent polymerization for the manufacture of medical devices. Lactide synthesis includes the following steps: concentration of lactic acid solution; LA polycondensation in the presence of a catalyst ZnO; raw lactide synthesis; lactide purification. Concentration of 30 ml of LA and oligomers obtaining were combined and carried on a rotary vacuum evaporator Hei-dolph-VAP. The process was carried out at a temperature of 140...180 °C, the speed of the flask rotation on a vacuum rotary evaporator was 40... 100 revolutions per minute (depending on the viscosity of the reaction mass), at a negative pressure of 30... 50 mbar. The process time was from 4.0 to 5.0 hours. The molecular weight of the LA oligomer was determined in chloroform solution using gel-permeation chromatography (AGILENT TECHN. 1,200 calibrated against standard polystyrene samples). Synthesis of raw lactide was carried on a laboratory setup for vacuum distillation using an electromagnetic stirrer IKA C-MAG HS 7, at a temperature of 180... 210 °C and a pressure of 4...16 mbar. Raw lactide obtained from lactic acid in different yields was purified three times by re-crystallisation from dried ethyl acetate in the ratio 1:3 (lactide:ethyl acetate) and dried at 55 °C using a shallow vacuum of 300... 400 mbar in a vacuum drying oven. Analysis of raw materials and product identification was carried out using differential thermal analysis (2TG/DSK/DTA), infrared spectroscopy (IR-FT spectrometer Nicolet 5700) and melting temperature definition (MeltingPointM-560). Infrared spectra were recorded in the range from 400 to 4000 cm in KBr tablets. The data showed that the process of oli-gomerization of lactic acid in the presence of zinc oxide is significantly under effect of temperature and vacuum. Under these reaction conditions the molecular weight of the oligomer is increased to 1500 g/mol. Softening temperature of the LA oligomers acid is in the range of 40... 70 °C and the decomposition temperature is 280...295 °C. The optimum temperature of the LA oligomerization (150... 160 °C) and appropriate LA oligomers molecular weight (~1200 g/mol), in which there is a high yield of raw lactide, were identified. The speed of the reaction flask rotation has minimal impact influence on the LA oligomerization and lactide yield, as under the flask rotation speed of the 40 revolutions per minute acceptable conditions for mass and heat transfer in the reaction medium is achieved, compared to the conventional laboratory set for distillation.

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