Читать книгу Poly(lactic acid) - Группа авторов - Страница 40

A lactide synthesis reactor invariably produces a crude lactide stream that contains lactic acid, lactic acid oligomers, water, meso‐lactide, and further impurities. The specifications for lactide are stringent mainly for free acid content, water, and stereochemical purity. Basically, two main separation methods, distillation and crystallization, are currently employed for lactide purification:

 Distillation. Splitting the multicomponent mixture consisting of lactide, water, lactic acid, and its oligomers into pure fractions requires considerable know‐how on kinetics and operation of vacuum equipment. Distillates and bottoms may be recycled, but the accumulation of impurities from the feed or the production of meso‐lactide during the process requires careful fine‐tuning of temperatures and residence times. Distillation is well described in the patent by Gruber et al. [68]. The crude lactide from the synthesis is distilled in the first column to remove the acids and water, and then meso‐lactide is separated from lactide in the second column. As the boiling points of all compounds are in the range of 200–300°C, low pressures are used. Since the difference in boiling temperature of lactide and meso‐lactide is quite small, this distillation requires a lot of theoretical stages (>30). The NatureWorks distillation uses a series of distillation columns and is performed continuously [4]. Part of the distillation can also be integrated with the reaction [79].

 Solvent Crystallization. A commonly used laboratory method for lactide purification is recrystallization from mixtures of toluene and ethyl acetate [4]. Lactide of extremely high purity can be obtained by repeated crystallization with different toluene/ethyl acetate ratios. Several patents also mention the use of solvents for the crystallization of lactide, but for large scale, melt crystallization without the use of solvents is preferred.

 Melt Crystallization. Lactide crystallizes easily, and several patents describe how crystallization can yield lactide with required specifications regarding lactic acid content, oligomers, meso‐lactide, and water. An early patent describes such a crystallization method and includes some information on the thermodynamic equilibria (eutectica) of the lactide/lactic and the lactide–meso‐lactide system, which define the maximum yield as a function of these impurities in the feed [80]. In patents, the use of different types of equipment is mentioned: static equipment, falling film crystallizers, vertical column with scraper to remove crystal mass from the cooled wall, and scraped heat exchanger coupled to a wash column [70, 80, 81]. For large scale, it is a challenge to design and scale‐up the crystallization equipment with respect to the needed heat transfer areas and hydrodynamics, and the possible increase of viscosity of mother liquor by oligomerization of lactide and residual acid.

The choice between distillation, crystallization, or novel separation methods such as absorption or membrane separation is determined by the desired stereochemical purity of the product. Crystallization yields highly pure lactide, suitable, for example, for high‐melting PLLA homopolymer of high molecular weight. Affordable distillation equipment does not fully remove all meso‐lactide, and consequently, a lactide monomer mixture for PLA copolymers with other thermal properties is obtained upon ring‐opening polymerization.

The design of the separation system relies on detailed knowledge of the thermodynamic properties of the compounds and the kinetics of the reactive system. Obtaining such know‐how requires sophisticated analytical methods for lactic acid and its oligomers, lactides, and residues. Impurities can also be formed in lactide synthesis, similar to PLA degradation reactions, and gas chromatography (GC) methods are needed to identify these compounds and determine their fate in the process.