Chemical recycling of poly(lactic acid) via controlled degradation with protic (macro)molecules

Abstract

Designing of a new method of poly(lactic acid) chemical recycling leading to potentially useful products is the purpose of this paper. Controlled chemical degradation processes of commercial poly(lactic acid) in the presence of small molecules, such as diols, dipentaerythriol, diamines and adipic acid or oligo(ethylene glycol) as well as polyesterodiols were studied. The processes were found to be efficient and in the presence of a catalytical amount of tin(II) octanoate lead to the formation of homo- or block diol type copolymers or oligomers of lactic acid. Diamines appear to be the most efficient, even without catalyst at quite low temperature, whereas adipic acid eventually at 200 °C reacts with moderate efficacy. It is demonstrated that Mw of the products is much lower with respect to that of poly(lactic acid). Degradation agents incorporate into the products' structures, however, in almost all cases some fraction of macro-α,ω-hydroxyacids is present in the samples, which seems to be unavoidable because of degradation mechanism and some residual moisture present in the polymers used. Products rich in ABA type triblock copolymers may be obtained by the degradation method presented utilizing macrodiols. The processes of poly(lactic acid) degradation with macrodiols can be carried out at temperatures of 120 °C or higher in solvent (e.g. xylene) or in bulk as well as an extruder. The reaction time depends on temperature and it is in the range from 30 min to 4 h. Aliphatic copolymers are homogeneous, whereas these containing aliphatic–aromatic macrodiols are of two-phases. The latter ones show different morphology based on composition. Triblock systems reveal multistage differential thermal analysis curves. Robust method of chemical recycling of poly(lactic acid) by controlled degradation with protic compounds is presented.