PLA can be commercially produced from sugar cane, sugar beet or corn. Once milled, the raw sugar or corn are fermented in to produce lactic acid. Following concentration, this is then produced to lactide, the dimer of lactic acid, and then PLA. Chemical recycling converts PLA back into lactic acid. Alternatively, PLA can be physically recycled or composted.
PLA PRODUCTION PROCESS
Large-scale production of lactic acid (2-hydroxy propionic acid) is via microbial fermentation, which is high-yielding. Although lactic acid exists as two isomers, this route favours L-(+) lactic acid. Furthermore, this optical purity drives PLA’s favourable properties. Following the fermentation process, lactic acid is separated from the residue and purified by distillation to form an aqueous solution.
Lactide is a key precursor to PLA. It is formed via a two-stage process from lactic acid; polycondensation and depolymerisation. Polycondensation of results in short-chain oligomers in an equilibrium reaction that is promoted by the removal of water (azeotropic distillation). Depolymerisation results in the formation of lactide, the cyclic dimer of lactic acid, and is strongly dependent on temperature. Lactide is purified by distillation in preparation for the final stage of the process. This is carried out in the bulk, in the absence of solvents.
PLA is produced from lactide via controlled ring-opening polymerisation in the presence of a tin catalyst, Tin Octoate. In this vital step, the control of the catalyst to lactide ratio can be used to fine-tune the properties of the polymer. This process is sensitive to water and the catalyst employed has a good combination of activity and tolerance of impurities. Moreover, tin octoate has been approved by the American Food and Drug Administration for use as a catalyst. The reaction is also carried out in the absence of solvents. The polymer is purified by distillation where lactic acid, the by-product, is recycled. After drying, the polymer is converted into pellets ready for further processing and end-use.