Читать книгу Poly(lactic acid) - Группа авторов - Страница 62

From a chemistry point of view, lactic acid can form PLA by means of the reaction of the hydroxyl and carboxylic acid groups of lactic acid. By removing the water formed during this condensation reaction, the reaction proceeds toward the product side, PLA:

where n and m ≥ 1.

The removal of water becomes more difficult and can be rate determining when producing a higher‐molecular‐weight PLA due to the increased viscosity of the reaction mixture. Through applying vacuum and high temperature, the water removal can be enhanced. However, during the polycondensation of lactic acid, other side reactions also occur, such as transesterification, resulting in the formation of ring structures of different sizes [19]. These side reactions have a negative influence on the properties of the polymer. The formation of ring structures, such as lactide, lowers the overall molecular weight and the removal of the lactide formed reduces the first‐pass yield of the process. The formation of lactide cannot be excluded, but to suppress the lactide formation and increase the first‐pass yield of the polycondensation reaction of lactic acid, the lactide can be returned back to the reaction mixture. A partial condenser (reflux condenser) or a rectification column placed on top of the polycondensation reaction can be used to recycle lactide back to the reaction mixture. The addition of stabilizers, such as antioxidants or phosphorous compounds, can reduce the coloration [20].

FIGURE 3.2 Theoretical relation between concentration of chain terminator and number‐average molecular weight (M _n) at different conversions of the functional groups.

The lactide formation becomes substantial at high reaction temperatures (>200°C) [21]. To suppress the lactide formation, the polycondensation reaction should thus be carried out at temperatures below 200°C. Conducting the polycondensation at low temperatures again has a negative effect on the removal of water due to the relatively high viscosity of the reaction mixture in addition to a lowered reaction rate. Since polycondensation should be the main reaction, the removal of water should be as high as possible without allowing the reaction mixture to undergo hydrolysis and transesterification reactions.

Besides the removal of reaction water from the viscous reaction mixture, the quality of the monomer (lactic acid) with respect to chain terminators such as monocarboxylic acids (formic acid, acetic acid, propionic acid, etc.) or monohydroxy alcohols (methanol, ethanol, propanol, etc.) is important. To obtain a desired high molecular weight, the amount of end terminators must be limited. In Figure 3.2, the theoretical relationship between the concentration of chain terminator in mol% and the corresponding number‐average molecular weight (M _n) at different conversions of the functional groups is shown. With a chain terminator content of 0.1 mol%, a maximum M _n of 72,000 g/mol can be obtained at 100% conversion of the functional groups. This clearly demonstrates the importance of using high‐purity lactic acid (often also called polymer‐grade lactic acid) during the polycondensation reaction.