A Sustainable Option for Chemical Recycling of Polylactic Acid

Polylactic acid (PLA) is a plastic derived from lignocellulosic waste such as sugar cane and rice. It can be decomposed through compost. Compared with petroleum-based polymers, the energy required to produce PLA can be reduced to 55%, making it more sustainable. But PLA is much less degradable than other biodegradable polymers, and this slow decomposition hinders PLA's widespread application. Converting PLA into its monomer, lactic acid or better yet lactide, through chemical recycling is a sustainable way to make PLA more commercially attractive.

3D Printing Materials Product List for Research and Development

CAS	Product Name
124578-11-6	Polystyrene-b-poly(ethylene-ran-butylene)-b-polystyrene-g-maleic anhydride
134490-19-0	Poly(l-lactide-co-caprolactone-co-glyco&
26023-30-3	Polylactic acid
26100-51-6	Polylactic Acid, Mw ~60,000
26680-10-4	3,6-Dimethyl-1,4-dioxane-2,5-dione homopolymer
26780-50-7	Poly(d,l-lactide-co-glycolide)
9000-70-8	Gelatin

The researchers designed a sustainable depolymerization method with reference to the hydrolysis and photolysis of PLA into lactic acid and CO₂ in the natural environment. The team obtained the selective product lactide by embedding α-Fe₂O₃, a narrow-bandgap, low-cost, low-toxicity, high-yield photocatalyst, into a PLA matrix, then placing the composite material in water and exposing it to ultraviolet light. In addition, this photocatalytic hydrolysis technology can be extended to other plastics and other photocatalysts can be used.

The researchers first dissolved discarded strip-shaped PLA in dichloromethane (DCM), and then dispersed α-Fe₂O₃ particles in the PLA polymer matrix (10 wt%) to obtain the composite material α-Fe₂O₃/PLA (DCM).

The researchers confirmed that α-Fe₂O₃ is single-phase through powder X-ray diffraction (XRD) characterization. The band gap was determined to be 2.20 eV using UV-visible diffuse reflectance spectroscopy calculations. With the help of scanning electron microscope (SEM), it was observed that α- Fe₂O₃ nanoparticles were dispersed in PLA and affected the surface morphology of PLA.

The researchers used thermogravimetric analysis to compare the performance of α- Fe₂O₃/PLA, PLA and PLA (DCM). FTIR analysis showed that the chemical properties of PLA did not change before and after the reaction and before and after α-Fe₂O₃ embedding. Based on this, the researchers concluded that the composite would break down over time as photocatalytic hydrolysis occurs.

After PLA, PLA (DCM) and α-Fe₂O₃/PLA composites were irradiated for 90 h, the HPLC results showed that only α-Fe₂O₃/PLA containing α-Fe₂O₃ produced lactic acid and lactide. The researchers proposed the photocatalytic hydrolysis mechanism of PLA. The photocatalytic hydrolysis of PLA is divided into two stages. Polylactic acid is cracked to form lactic acid, and then a dehydration reaction occurs to produce a larger amount of lactide. For comparison, the researchers tested another Fe₂O₃ (γ-Fe₂O₃) embedded in PLA using the same procedure and found that this material was also active, but only promoted the polymer cleavage step rather than the subsequent dehydration reaction. , so lactic acid becomes the main product.

Therefore, the researchers concluded that the photocatalyst controls the formation of lactide through a dehydration step. The photocatalytic process is based on photogenerated electron and hole pairs in photocatalysts. Electron-hole pairs can generate hydroxyl radicals or guide electron transfer in the surrounding environment. After embedding the photocatalyst into PLA, compared with the hydrolysis and photolysis processes, the additional radical formation or electron transfer of the photocatalyst should be the main factor leading to the formation of lactide.

Reference

1. Photocatalytic Hydrolysis─ A Sustainable Option for the Chemical Upcycling of Polylactic Acid.
   ACS Environmental Au 3.6 (2023): 342-347.