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PLA Recycling PLA

PLA Recycling PLA recycling via thermal depolymerization by Ramani Narayan Xiangke Shi Daniel Graiver Biobased Materials Research Group Michigan State University Sample # Lactide monomer % Note 1 93.49 PLA Resin 2 93.49 PLA Resin 3 91.79 NatureWorks Ingeo cups Table 1. Recovery of lactide by catalytic thermal depolymerization PBAT content Lactide recovered –no catalyst Table 2. Recovery of lactide monomer from PLA-PBAT blends Figure 3. Laboratory scale recovery of lactide from PLA polymers and blends Lactide recovered 0.1% SnO 2 0% 99.7% 99% 10% 98.7% 99.6% 25% 96.5% 81.6% 50% 80.0% 68.2% Background PLA, poly(lactic acid) is a commercial 100% biobased thermoplastic polymer that has found wide spread industrial applications. It derives its value proposition from having a zero material carbon footprint arising from the short (in balance) sustainable biological carbon cycle. This is different from the process carbon footprint (the carbon and environmental footprint arising from converting the feedstock to product, use, and ultimate disposal, typically covered by LCA methodology [1, 2]. Many issues of bioplastics MAGAZINE have showcased the commercial applications of PLA, and it is the biobased plastic of choice in the market today. The typical end of life option for the PLA product is in industrial composting systems, where it is readily and completely assimilated by the microorganisms present in the compost environment as “food” (completely biodegraded in industrial composting environment) releasing energy that it utilizes for its life processes. A viable end-of-life option for PLA is chemical recycling back to monomer – a virtual cycle of monomer to polymer and back to monomer – a circular biobased economy. PLA can be manufactured by the direct condensation polymerization of lactic acid with concomitant removal of water. However, it is difficult to obtain the high molecular weights necessary for plastics applications because of the low equilibrium constant of lactic acid esterification and the difficulty of water by-product removal in the increasingly viscous reaction mixture. 42 bioplastics MAGAZINE [03/13] Vol. 8

PLA Recycling O O OH OH HO HO (R,R)-lactide (R) or D-lactic acid (S) or L-lactic acid H O O 3 C H O O 3 C H O O 3 C O O CH 3 O O (R,S) or meso-lactide CH O O 3 CH 3 (S,S)-lactide O O Hydrolysis HO HO ( OH O n OH Lactic Acid Purification Condensation O O O Lactide O Polymerization Depolymerization O Poly (latic acid) ( Figure 1. Stereochemistry of the lactide monomers Figure 2 Chemistry of the interconversion between lactic acid, lactide, and poly(lactic acid), Today’s industrial processes are based on the ring opening polymerization of the lactide monomer. First lactic acid is heated under vacuum in a high surface area-to-volume process to obtain PLA oligomers with degree of polymerization between 2 and 25. A metal catalyst is added and the resultant lactide removed by distillation. The PLA system has a rich stereochemical architecture which controls physical and performance properties of the resultant product – the stereoisomers of lactic acid and lactide monomers are shown in Figure 1. The percent meso or D lactide in the L-lactide monomer would affect rate and percent crystallinity and the eventual polymer properties of the polymer product. Reversible Kinetics model approach for PLA recycling Current approaches to PLA recycle is to hydrolyze it to lactic acid, purify it and then reform into lactide which can then enter into the polymerization step. However, the authors and their workgroup have shown that the polymerization of lactide to PLA follows a reversible kinetic model [3]. “They have used this reversible polymerization to recycle PLA to lactide monomer using catalytic thermal depolymerization with success [4]. The chemistry scheme is shown in Figure 2. Proof of concept was established in a laboratory scale setup with 10-50 gram samples of commercial PLA resin from NatureWorks. Tin(II) 2-ethylhexanoate catalyst was used. Melting occurred at 180-185 °C and the depolymerization reaction started at 185°C.The reaction was carried out under vacuum in a distillation set-up. The lactide distilled over driving the reaction forward. The melting point of lactide is 92-94°C and electric heating tape was used to keep the lactide in the liquid phase after vapor condensed. A trap was used to prevent lactide vapor from clogging the vacuum line. The reaction temperature was kept between 185-210°C by using an oil bath. Figure 3 shows the laboratory scale distillation set up with the pale yellow lactide clearly visible in the receiver flask. As can be seen from Table 1 the total yield of lactide was around 94% on a mass basis. Commercial Ingeo thermoformed cups were obtained from the marketplace, cut into small squares and added to the reactor vessel, 92% of lactide was recovered on a mass basis. The composition and optical purity of the lactide was established by 1H NMR (Proton Nuclear Magnetic Resonance) (Figure 4). The resonance peak at 1.7 ppm corresponded to meso lactide and the resonance peak at 1.65 ppm corresponded to either LL or SS lactide (Figure 4). Due to the isomeric nature of LL and SS lactide, their resonance peaks are identical in the 1H NMR spectrum. According this analysis, the meso content of this product is 9.5%, very close to the previously reported14 meso content of PLA 3051D (8%). Since PLA is also blended with other polyesters to incorporate biobased content, it was important to establish lactide recovery from such blends. One such blend is a PLA-PBAT (polybutylene adipate-co-terephthalate) reactive blend marketed (e.g.) under the BASF trademark of Ecovio ® . Experiments were run with and without tin oxide catalyst. Table 2 shows lactide recovery from blends with varying amount of PBAT resin content. The SnO 2 catalyst in the resin did not aid in depolymerization but was processed with the resin. If was found that higher amounts of Ecoflex ® (PBAT) in the blended samples prevented recovery of lactide from PLA. PLA samples containing 50 wt% of Ecoflex yielded 80% recovery of available lactide in the samples without resin catalyst and 68% recovery in the samples with resin catalyst. However, the bioplastics MAGAZINE [03/13] Vol. 8 43

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