Читать книгу Poly(lactic acid) - Группа авторов - Страница 39

In 1992, Gruber et al. [68] described a continuous lactide synthesis in which prepolymer is fed continuously to a reactor, crude lactide is evaporated under vacuum, and residue is removed. Typical operating conditions for the reactor were residence time around 1 h, vacuum pressure 4 mbar, temperature 213°C, and catalyst amount 0.05 wt% tin(II) octoate on feed. The conversion per pass was around 70%, and the overall yield was increased by recycling the residue to the lactic acid section of the process, where the oligomers are hydrolyzed again.

Especially in the patent literature, several different reactor types are described for continuous lactide synthesis:

 Stirred tank reactor with different stirrer types [76]. On a bench scale, the reactor is jacketed for heating.

 Stirred reactor with a distillation section on top of the reactor to fractionate the product [50].

 Thin film evaporator with a typical conversion of 80% on pilot scale [70].

 Horizontal wiped film evaporator. In a patent by Kamikawa et al. [77], the use of horizontal wiped film is described. In the horizontal mode, the residence time of the reaction mixture can be controlled and a conical form is used in which wipers transport the viscous residue.

 Distillation column. In a patent by O'Brien et al. [75], a distillation column with perforated plates and optional use of packing material and heating on the stage are described. In an experiment with a single tray, a DP 10 feed was fed to the top, and N2 was used to strip the lactide from the liquid. At different residence times, the conversion on the tray could be as high as 93% at 210–215°C. In other patents, the use of N2 gas as a stripping agent is mentioned, but it is to be expected that in large‐scale equipment the processing of large amounts of inert gases will be less economical compared with the use of vacuum systems.

Reviewing the literature provides a list of process aspects that need consideration in the design of a solventless synthesis operated with vacuum equipment.

 Temperature. Intrinsic reaction rates increase with temperature. At higher temperature also, the vapor pressure of lactide above the reaction mixtures increases. The reaction rate of racemization will also increase with temperature. In Witzke's Ph.D. study, information on activation energies can be found [6].

 Pressure. Pressures of 10 mbar or less are used. At higher pressures, the driving force for lactide evaporation will be lower, and the overall reaction rate will be lower. Low pressures will require detailed considerations of equipment size, vacuum systems, condensers, and so on.

 Feed DP. The feed DP has two effects. First, a low DP feed will contain more monomer lactic acid that boils at a lower temperature than lactide, and this will contaminate the crude lactide distilled off from the reactor. Also, monomer lactic acid can be released from DP 3 with the catalyst, leading to more acidity in the crude lactide. Second, it is to be expected that at a higher feed DP the residue in the reactor will have a higher DP and viscosity with consequences for equipment design. The influence of prepolymer DP on the meso‐lactide level formed during lactide synthesis was discussed by Gruber et al. [69]. Increasing feed DP clearly resulted in a decrease in the lactic acid concentration in the crude lactide. A drawback is that the meso‐lactide concentration also increased significantly.

 Catalyst Concentration. More catalyst will increase the overall reaction rate. In practice, this effect may not be linear, since next to kinetics mass transfer in the equipment will play a role.

 Racemization. In the production of stereochemically pure lactide, formation of the other lactic acid enantiomer and meso‐lactide is unwanted. Higher temperatures, longer reaction times, and increased catalyst levels result in increased rates of racemization [4, 6, 69]. Since temperature and catalyst influence the rate of lactide formation as well, controlling the racemization rate can become quite complex.

 Impurities. Data in the literature on the role and fate of impurities from the feed in the synthesis are scarce. Some metal cations such as sodium and potassium in the feed increase racemization risk, while other metals (Al, Fe) are catalytically active in transesterification, resulting in competitive polylactide formation [68, 69]. Through corrosion, metals may be released in the residue and will build up there [6, 75]. Some patents discuss the presence of acid impurities in the process [6, 7, 67, 78]. Mono‐ and dicarboxylic fermentation acids are responsible for stoichiometric imbalance in the lactic acid polycondensation reaction. Consequently, the composition of the obtained lactic acid oligomer chains can differ from pure PLA, resulting in impeded and incomplete catalytic depolymerization of the oligomers into lactide. In PLA manufacture, degradation reactions play a role, mainly via intramolecular chain scission, and this may also affect lactide synthesis.

On the one hand, it can be concluded that the lactide synthesis is straightforward in the sense of making a prepolymer and releasing lactide by thermal catalytic depolymerization at low pressure. On the other hand, it can be concluded that the scale‐up from a lab‐scale process to an economical, large‐scale process with high yield and no compromises on stereochemical purity is a complex multifaceted task.