The lactid synthesis in the inert atmosphere

The process of L-lactide production was investigated. L-Lactide through the ring-opening polymerization gives polylactide used for processing of various biocompatible absorbable medical products and environmental packaging. Lactide synthesis includes the following stages: concentrating the lactic acid (LA); LA polycondensation in the presence of ZnO catalyst; raw- lactide synthesis; lactide purification, waste disposal. To increase the lactide yield the reaction of lactide obtaining is usually carried out in an inert gas atmosphere. Therefore, it was interesting to assess the impact of the inert medium and gas injection method on the lactide yield and purity. The stages of 30 ml LA PURAC concentration and LA oligomer obtain were combined and carried out on the rotary vacuum evaporator Heidolph Hei-VAP. The process was conducted at a temperature of 150°C, flask rotational speed on the rotary vacuum evaporator 60 rpm, and vacuum of 30-50 mbar. The process time was 240 minutes. The molecular weight of the LA oligomer was determined in a chloroform solution using gel - permeation chromatography (Agilent Technologies 1260 Infinity, calibrated by polystyrene standards). Raw-lactide synthesis was performed on a laboratory set for vacuum distillation under vacuum of 5-10 mbar. The process was carried out using an air condenser, electromagnetic stirrer IKAC-MAGHS 7 and vacuum station Vacuubrand PC 3001 VARIO. The fractions boiling at a vapor temperature of 100-180°C were samled. Depolymerization process lasted 1-2,5 hours. LA oligomer and raw-lactide obtaining was conducted under the following conditions: with no inert medium (air), in an inert atmosphere of nitrogen or argon, and also under reaction mixture bubling with an inert gas. Raw-lactide obtained from the LA was purified by recrystallization from dried ethyl acetate. Nitrogen was prepared by the nitrogen generator NG-200, which provides a volume fraction of nitrogen not less than 99.6% of volume. Impurities contain oxygen (0.4%), argon, neon and helium. Argon was used from cylinders (TU 6-21-12-94). Analysis of the feedstock and product identification was carried out by differential thermal analysis (2n/DSC/DTA), high performance liquid chromatography (Young Lin Clarity YL9100 chromatography), melting temperature (Melting Point M-560) and infrared-spectroscopy (IR-Fourier spectrometer Nicolet 5700). To solve the objected problem it LA oligomer was obtained by described method in a nitrogen atmosphere. Oligomer was divided into three equal parts, used for the raw-lactide synthesis (without an inert medium, in an inert atmosphere of nitrogen and while bubbling the reaction mass with nitrogen) under reduced pressure with the qualitative and quantitative evaluation of the obtained fractions. It was shown that the use of an inert environment (nitrogen, argon) increases the raw-lactide purity and raw-lactide yield on 5-8%. Bubbling the inert gas through the LA oligomer melt leads to a substantial increase in the raw-lactide yield and purity.

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