Innovative Process Boosts Emerging Bioplastics

Novel reaction technology makes polylactic acid with improved properties at lower cost.

By Philip Nising, Sulzer Chemtech

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The first plant pioneering a breakthrough process developed by Sulzer Chemtech for making polylactic acid (PLA) is operating in Etten-Leur, The Netherlands. Synbra Technology successfully put the 5,000-mt/yr plant (Figure 1) into service in 2011. Since then, Sulzer has started up a second plant (Figure 2) for its own use. The 1,000-mt/yr facility, located in Pfäffikon, Switzerland, is expected to produce high-quality PLA samples by the end of this year.


The patented process allows production of PLA with very high stereochemical purity, which has a high crystallinity, and, thus, a much better heat stability than other PLA products currently available in the market. In addition, Sulzer’s proprietary static reaction and devolatilization equipment help reduce energy consumption and optimize operational expenses. All in all, these advances enable PLA-based bioplastics to become even more interesting alternatives to petrochemical plastics in a broader range of areas.

The biopolymer resembles conventional high-volume petrochemical plastics like polystyrene or polyethylene terephthalate (PET) in many characteristics and can be processed using typical equipment. PLA and PLA blends usually come as granulates and currently are used in commercial applications such as packaging, one-way food services and disposable tableware.

PLA commonly is produced through the ring-opening polymerization of lactide monomers, which are based on lactic acid produced by the fermentation of sugar or starch. Selective mixing of lactides made from the natural L(+) isomer of lactic acid and lactides from the D(-) isomer allows producers to tune important characteristics of the polymer like the degree of crystallinity.

Researchers are working on new fermentation processes using cellulosic material from wood, grass, or agricultural wastes to replace sugar as a raw material, thus making PLA an even more attractive and interesting material for the future. This “next generation” process also will create synergies with other industrial sectors working with cellulose, such as the pulp and paper industry.

THE NEW PRODUCTION PROCESS
Major shortcomings of current bioplastics are their low heat resistance and high price — as well as the lack of sufficient amounts of high quality resin. Therefore, in 2008 Sulzer Chemtech and Purac, a company of the Dutch CSM group, began to jointly develop a cost-efficient polymerization process. It uses Purac’s high purity lactide monomers and relies upon Sulzer’s proven proprietary polymerization technology. Output consists of PLA grades with exceptionally high stereochemical purity and a wide range of molecular weights. The substantially better heat resistance enables the PLA grades to endure temperatures of up to 180°C, opening up potential applications in the automotive, electronics and textile industries.