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Issue 05/2017

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Production Reducing PLA

Production Reducing PLA production cost Earlier this year at a bioplastics conference in Bangkok, “Jem’s Law” about the growth of the PLA market was presented. Jem’s Law basically says that PLA volumes doubled every 3 to 4 years in the past and therefore will continue to do so in the future. With some knowledge of the actual production capacities one can calculate that the PLA market will be around 600,000 t/a in 2022 / 2023. All in all, this would mean that there is a need for 5 additional PLA plants with a capacity of 75,000 t/a until 2022. This is a promising perspective, not only for an engineering company like Uhde Inventa-Fischer, a subsidiary of thyssenkrupp Industrial Solutions. Even though all forecasts have to be treated with the necessary caution, Jem’s Law can be considered fairly realistic compared with earlier ones about the markets for bioplastics. PLA economics: size, price, efficiency If PLA plants are to be built in the future, economics will of course play a crucial role. Besides the well-known factors of plant size (the bigger the better) and feedstock prices (the lower the better), raw material conversion – which determines specific feedstock demand – must not be neglected. What factors influence the conversion of lactic acid to PLA? One is the formation of side-products. In the case of PLA, provided one uses the right catalyst, this is comparatively low. In practice more than 95 % of what is theoretically possible can be converted into lactide and polylactide. Unwanted meso-lactide increases production cost But lactic acid is an optical active substance with a L(+)- and a D(-)-configuration, and three different types (enantiomers) of lactides: L-lactide, D-lactide and mesolactide. Each one results in different PLAs in terms of properties and processing behavior. The repartition of the enantiomers in the lactide feedstock determines PLA properties like crystallinity/crystallization time to a major extent and consequently also heat distortion temperature and hydrolysis resistance. What’s more, the lactide composition cannot be adjusted to the desired level without separation of meso-lactide, the lactide enantiomer with a L(+)- and a D(-)-configuration. Using optically pure L(+)-lactic acid is not sufficient to obtain an optically pure lactide. Racemization of L-lactide (or D-lactide), mainly during depolymerization of lactic acid polycondensate to lactide, leads to the formation of mesolactide. In many applications a small percentage of mesolactide is advantageous. But there are also applications where meso-lactide should be as low as possible. And it appears that their share is growing, for example in durables and most fibers, or if high heat is required. In general more meso-lactide is produced than is needed. This raises the question of what to do with the surplus meso-lactide. To write it off as a loss is not an option as this would increase production cost severely. Fig. 1 shows production cost as a function of raw material conversion. A loss of 10 % due to racemization leads to a decrease in conversion from 96 % to 83 % which in turn increases production cost by more than 12 % (all calculations based on Uhde Inventa-Fischer’s PLAneo ® technology for an industrial scale plant on a European price basis). Selling or downgrading back to lactic acid have drawbacks. A better option is to hydrolyze meso-lactide back to lactic acid. Technically this is not a challenge. But due to its racemic nature the quality of the lactic acid is lower than the feedstock lactic acid. It goes without saying that the conversion of a high grade lactic acid into a low grade one is economically unfavorable. Besides bad economics a producer of PLA has the drawback of having to deal with two completely different markets – selling PLA on the one hand 120% Fig 1: Production cost as a function of raw material conversion VAC Fig 2: Process flow diagram of thyssenkrupp’s PLAneo process Increase in Production Cost 115% 110% 105% 100% Crude Lactide Meso-Lactide Purrification PLAneo ® 95% 75% 80% 85% 90% 95% 100% Lactic Acid Conversion: Percentage of theoretical maximum L-Lactide Purrification Ring Opening Polymerisation Demonomerisation 24 bioplastics MAGAZINE [05/17] Vol. 12

Production By: Udo Mühlbauer thyssenkrupp Industrial Solutions Uhde Inventa-Fischer GmbH Berlin, Germany and lactic acid for example to the cosmetics industry on the other (unless he is already a lactic acid producer). An even better option would be to sell meso-lactide as a chemical intermediate or monomer for different applications and to different markets – with the aim of achieving higher prices. As meso-lactide has not existed as a commercial product before, there is no established market. New applications have to be developed and markets have to be found. Whether these markets will develop and to what size remains to be seen. Using polymerized meso-lactide to form a single product: PLAneo The solution that Uhde Inventa-Fischer has developed initially appears obvious: like L-lactide, meso-lactide is purified and polymerized. This is easier said than done. Beside the fact that meso-lactide is much more sensitive to sidereactions than usual polymer-grade lactide, the molecular weight of poly-meso-lactide has to be comparatively high in order to obtain good mechanical properties. Both facts add up to stringent requirements for the purity of polymer-grade meso-lactide. The second step of the PLAneo technology is not as obvious. Instead of producing a second polymer, which would have limited possible application due to its amorphous nature, polymeso-lactide is blended with the main crystallizable PLA-melt, both polymerized continuously in parallel lines, to give one product. Optimized yield, same product properties The resulting polymer maintains all relevant mechanical, optical and physical properties: tensile strength, E-modulus, crystallization behavior and melting point do not change. Only the b*-value of the PLA pellets is slightly increased. This holds true irrespective of whether distillation or crystallization is used to purify the main lactide stream. Processing of PLAneo PLA is just as straightforward as standard PLA. Applying separate polymerization of meso-lactide and L-Lactide and blending it afterwards means no meso-lactide has to be discarded or used in a less economical way. The specific demand of lactic acid converges to its theoretical minimum of 1.25 kg per kg of PLA. Nobody knows exactly how the PLA market will develop. We will see whether Jem’s law will continue to prove true in the future and how many new plants will come on stream. But the ones using technology that maximizes raw material yield will definitely have an advantage. 201 200 Fig 3: Comparison of PLA properties with and without amorphous PLA contingent D- Content [%] Intrinsic Viscosity [dl/g] Residual Monomer [%] Tm [°C] b*- Value [-] Haze [%] 159 160 4069 3881 cPLA PLAneo ® bioplastics MAGAZINE [05/17] Vol. 12 25 Fig 4: Comparison of BOPLA film properties with and without amorphous PLA contingent 4201 4265 96 113 85 100 PLA 2 1.87 < 0.02 162 4 0.38 PLAneo 5 1.87 < 0.02 162 6 0.35 MD TD MD TD MD TD Tensile Strength [N/mm²] E-Modulus [N/mm²] Elongation at Break[%]

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