Depolymerization of PLA by Phase Transfer Catalysed Alkaline Hydrolysis in a Microwave Reactor

Abstract

Poly(lactic acid) (PLA) is one of the most important polyester bioplastics, produced from agricultural renewable resources and due to its excellent properties already has found applications in several industrial sectors, including packaging. Its amount, relative to PET, in the waste stream is continuously increasing. Although waste bioplastics are biodegradable, the process sometimes needs long degradation times. Therefore, sufficient recycling techniques should be developed in terms of sustainable chemistry. Hydrolysis of PLA under microwave irradiation in an alkaline solution was investigated aiming in the chemical recycling of this biodegradable polymer and the recovery of the monomer, lactic acid. Several process parameters were examined, including the presence or not of an alkali solution, its amount and concentration, the presence or not of methanol in the reaction medium, together with reaction temperature and time. Moreover, several phase transfer catalysts were employed at various relative amounts to PLA. Reaction temperature, pressure and microwave power did not change significantly during degradation. Optimum experimental conditions, leading to PLA degradation more than 90%, were achieved using hexadecyltrimethylammonium bromide as phase transfer catalyst in a 10% w/v NaOH medium at 100 °C for 10 min irradiation time. Using such low temperature and degradation times results in great environmental benefit since it does not consume significant amount of energy compared to other similar techniques proposed in literature and thus leads to the sustainability of the process. Therefore, it seems to be a very efficient method to be used in the recycling of large amounts of this polymer.