Characteristics of the steam degradation of poly(lactic acid) and poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)

Abstract

The introduction of biodegradable plastics is considered a practical approach to reducing plastic waste accumulation in the environment. Regardless of their biodegradability, plastics should be recycled to effectively utilize and circulate carbon as a resource. Herein, the use of pyrolysis was examined as a method for recycling two common biobased/biodegradable plastics: PLA and PHBH. The pyrolysis of PLA produced lactides (10.7 wt% at 400 °C), but the yield was decreased when the pyrolysis temperature was increased. The presence of steam promoted the hydrolysis of PLA: a steam concentration of 25 vol % increased, the production of lactides at 400 °C to 17.4 wt%. The pyrolysis of PHBH primarily yielded crotonic acid (30.1 wt% at 400 °C), and the yield increased with increasing pyrolysis temperature (71.8 wt% at 800 °C). Steam injection increased the hydrolysis of oligomers, resulting in a 76.1 wt% yield of crotonic acid at 600 °C with a steam concentration of 25 vol %. Thus, we determined that hydrolysis and pyrolysis progress simultaneously under a steam atmosphere, increasing the chemical feedstock recovery from PLA and PHBH. These findings may lead to the proposal of effective degradation methods for treating biobased/biodegradable plastic wastes and ways to maximize the conversion efficiency and target product yields.

Solvent and Catalyst-Free Depolymerization of Polylactic Acid Waste to Lactide Enabled by Dry Steam

“…Dry steam is predominantly composed of monomeric water molecules with fewer intermolecular hydrogen bonding, which should be beneficial to the formation of hydrogen bonding between PLA and water molecules, thereby promoting depolymerization of high molecular weight PLA. 16,31 Meanwhile, the limited water content of dry steam may contribute to LT stabilization as LT easily decomposes in an aqueous environment. Furthermore, dry steam represents a green and sustainable technology that can improve energy efficiency, minimize wastewater production, and mitigate environmental impact.…”