Читать книгу Poly(lactic acid) - Группа авторов - Страница 76

It was earlier mentioned that the reversible lactide formation from polycondensated LA was initially explored by Carothers. He furthermore observed that temperature and pressure could be manipulated for pushing the equilibrium toward the lactide product. This was utilized later for the preparation of lactide, but the presence of other species (e.g., LA, water, lactoyllactic acid, lactoyllactoyllactic acid, and higher oligomers) necessitates further purification of the crude lactide to make it useful for polymerization purposes.

Various technologies for lactide manufacturing are found in the literature. Batch‐wise or continuous manufacturing processes have been described, as well as the use of different catalysts [90, 91]. A typical manufacturing process on an industrial scale involves heating PLA to 130–230°C at reduced pressure in the presence of 0.05–1.0 wt% of tin dust, or an organic tin compound derived from a carboxylic acid having up to 20 carbon atoms, in such a way that the produced lactide is distilled off and the PLA is continually or batch‐wise replenished [92]. In some processes, a fluid is used to make the separation of lactide more efficient. This can, for instance, be done by stripping off and recovering lactide from a gaseous nonreactive feed containing LA polycondensate [93]. It has been shown that keeping the crude lactide at elevated temperatures above the melting point prior to distillation for several hours will result in an increased LA oligomer content which enables a more efficient purification process [94]. The crude lactide will in most cases contain different impurities that will make the monomer mixture unsuitable for direct ring‐opening polymerization as such. The optical purity, acid number, and yield of the lactide will accordingly affect the economy of the manufacturing to a large extent. There are mainly three purification approaches suggested in the literature: solvent‐assisted purification, crystallization from the melt, and purification in the gas phase.