Conversion of poly(lactic acid) to lactide via microwave assisted pyrolysis

Microwave assisted pyrolysis (MAP) of poly(lactic acid) (PLA) was performed using different MW absorbers (tire, carbon, or Fe), apparatus set-up, and MW power, obtaining a liquid rich in 3,6-dimethyl-1,4-dioxan-2,5-dione (lactide) and other oxygenated organic compounds. Pure form of L-lactide was separated from the collected liquids while the meso-form of lactide was present in liquid and identified through GC-MS analysis. Up to 27.7 % of lactide was formed and might be recovered. Furthermore simple acids such as acetic acid and propionic acid (up to 17.1 %), carbonyl compounds, and fragments of PLA backbone randomly cleaved were present in liquids. The pyrolysis of PLA in the presence of other polymers, which might pyrolyze to aromatic compounds, such as those present in tires, gave cross reactions in the course of the pyrolysis affecting both yield and characteristics of liquid. Aromatic compounds formed from tire pyrolysis prevented the precipitation of lactide.