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Materials M

Materials M Electrodialysis Feed Solution o E. Coli C o C C C C C o o o o H 2 O Anode - - - - - - - - + + + + + + + + Dilute - - - - - - - - + H 2 O + OH + + + + Cathode + + Concentrate O O O O Biomass Fermentation A Separation / Purification B Conversion C Resin Manufacturing D Schematic diagram of (A) fermentation, (B) Separation and Purification, (C) Lactide Conversion and (D) PLA polymerisation. Four-unit process technology for PLA manufacturing By Hong, Chae Hwan Kim, Si Hwan Soe, Ji Yeon Han, Do Suck CAE & Materials Research Team Hyundai·Kia Motors Gyeonggi-do Uiwang Samdong, South Korea Polylactide (PLA) is one of the most important biodegradable and biocompatible polyesters derived from annually renewable resources. The most efficient method for preparation of PLA is ring-opening polymerisation of the dimeric cyclic ester of lactic acid, i.e. lactide. Fermentative production of the PLA precursor, lactic acid, offers the great advantage of producing optically pure L-or D-lactic acid depending upon the strains selected for fermentation. The optical purity of lactic acid is crucial for the physical properties of PLA. Though L-lactic acid can be polymerised to give a crystalline product (PLLA) suited to commercial uses, its application is limited by its low melting point. Complexing PLLA with poly-D-lactic acid (PDLA), however, raises the melting point thus presenting an attractive solution to the heat sensitivity of PLA. However, fermentation of sugars to D-lactic acid has been studied very little and its microbial productivity is not well known. Therefore, Hyundai·Kia Motors investigated D-lactic acid fermentation with a view to obtaining improved strains capable of producing D-lactic acid with enhanced productivity, and finally a maximum lactic acid production of 60 g/l was achieved. A fermentation-based process requires maintenance of a near neutral pH for high productivity and this necessitates the addition of alkali in most of the cases. Alkali addition produces a salt of lactic acid instead of lactic acid itself. To overcome this salt problem, the processes based on electrodialysis that do not require then addition of acid or alkali to convert lactate salts into lactic acid was tested. Electrodialysis technology (see picture) is based on electromigration of ions through a stack of cation and anion exchange membranes. Basically, it involves two steps. The first step called conventional electrodialysis (CED) separates and concentrates lactate salts. The second step called bipolar electrodialysis (BED) converts lactate salts into lactic acid. These two processes were adopted and D-lactic acid was produced. Lactide is prepared in a two-stage process: first, the lactic acid is converted into oligo(lactic acid) by a polycondensation reaction; second, the oligo(lactic acid) is thermally depolymerised to form the cyclic lactide via an unzipping mechanism. Through catalyst screening test for polycondensation and unzipping depolymerisation reaction a new method was developed to shorten the whole reaction time to 50% of the conventional method. Poly(L-)lactide was obtained from the ring-opening polymerisation of L-lactide. Various catalysts and polymerisation conditions were investigated resulting in the best catalyst system and the scale-up technology. 50 bioplastics MAGAZINE [01/12] Vol. 7

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