Poly(lactic acid). Группа авторов
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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.
3.2.1 Direct Condensation
The preparation of PLA from lactic acid by direct condensation can be divided into three principal stages: (a) removal of the free water content; (b) oligomer polycondensation; and (c) melt polycondensation of high‐molecular‐weight PLA:
1 Besides lactic acid, the feedstock also contains the so‐called free water. Due to the equilibrium of lactic acid and water, some low amount of oligomers of lactic acid (linear dimer, linear trimer, etc.) can already be formed in this stage. To convert lactic acid to PLA, first the free water has to be removed. The evaporation of the free water requires a system having good heat transfer and can be carried out in commonly used evaporators, such as falling film evaporators. Flash evaporation can also be used to remove the free water in lactic acid feedstock.
2 In the second stage, the lactic acid is converted into low‐molecular‐weight PLA or oligo(lactic acid). In this step, the removal of water is not critical because of the low viscosity of the reaction mixture. The rate‐determining step in this stage is usually the chemical reaction, which is significantly affected by the catalyst used [22]. Traditional polycondensation catalysts are strong acids, and organometallic compounds are also commonly used catalysts. The low‐molecular‐weight PLA polycondensation can also be carried out in an evaporator or alternatively in a stirred reactor having an agitator that generates good radial and axial mixing. The loss of lactic acid due to entrainment can be overcome by using a reflux condenser, a demister package, or a rectification column. Preferably, this stage is carried out in a system having a narrow residence time distribution (plug‐flow behavior) to obtain a prepolymer of lactic acid of narrow molecular weight distribution (small dispersion).
3 The third stage is the melt‐polycondensation in which the removal of water becomes critical. To enhance the polycondensation reaction, and not the transesterification reactions, the water formed in the reaction mixture should be removed efficiently. The rate‐determining step in this phase is the mass transfer of water. To enhance both mass and heat transfer, the melt‐polycondensation reaction should be applied in an apparatus having an efficient renewal of phase boundary layers. The apparatus should have intensive mixing and kneading in order to homogenize the reaction mixture. The removal of water from the viscous PLA mass can be further enhanced by carrying out the reaction under vacuum conditions in an inert atmosphere. A mathematical model for the polycondensation of lactic acid accounting for water removal by diffusion has been developed [23]. The increasing molecular weight of the PLA requires a system that can handle high‐viscosity mass. Such an apparatus could be a rotating disk type of reactor, generating a good surface renewal to enhance the mass transfer of the water formed. Such an apparatus should also have very good heat transfer to have a homogeneous temperature profile in the reaction mixture. Especially the mechanical heat formed due to mixing and kneading of the highly viscous PLA should be controlled. In this stage also a plug‐flow behavior is preferred to obtain a narrow molecular weight distribution.
Only a few studies have dealt with the influence of the catalyst when preparing PLA of high molecular weight through the direct bulk condensation reaction. In most studies with regard to catalysts, the polycondensations were carried out only to obtain low‐molecular‐weight polymers with an M w of a few thousands, before they were stopped. PLA having a molecular weight of as high as 130,000 g/mol (gel permeation chromatography (GPC) relative to polystyrene (PS) standards) was synthesized by direct bulk condensation polymerization at 180°C using titanium(IV) butoxide as catalyst [24]. In another study, several metal catalysts based on Ge, Sb, Zn, Fe, Al, Ti, and Sn were employed in the melt‐polycondensation reaction [25]. The most efficient catalyst was SnO with regard to molecular weight of the PLA, but the yield was below 40% when using this catalyst at 180°C (20 h). However, when using p‐toluenesulfonic acid as a co‐catalyst with SnCl2, the efficiency was drastically improved and molecular weights above 100,000 g/mol (GPC relative to PS standards in chloroform, 35°C) were achieved within 15 h of polycondensation. Sodium carbonate, calcium carbonate, and lanthanum oxide have also been used as catalysts when preparing PLA of high molecular weight [26]. Weight‐average molecular weights ranging from 63,000 to 79,000 g/mol (GPC relative to PS standards in chloroform at 40°C) were obtained by melt‐polycondensation but with poor yields (33–52%).
To achieve an increased molecular weight of the PLA, comonomers with functionality higher than two have been used. A process for making a star‐shaped PLA was described, where