OBTAINING LACTIDE BY CATALYTIC DEPOLYMERIZATION OF POLYMER WASTES BASED ON POLYLACTIDE

Abstract

This paper presents a comparative assessment of the results of thermocatalytic recycling of polymer wastes based on polylactide, a biodegradable polymer, to a monomer (lactide), which can be used as a secondary raw material, along with primary plant resources, to produce polylactide. Off-grade polylactide, polylactide sheets obtained by hot pressing, a filament for 3D printing and polylactide items made by 3D printing were used as polymer wastes. Crude lactide was obtained with a yield of 72–83% by the destruction of polymer wastes at a temperature of 180–250 ℃ in the presence of zinc oxide as a catalyst. The yield of purified lactide after crude lactide recrystallization from ethanol and ethyl acetate exceeded 40%. By gas chromatography, the product was found to contain residual amounts of meso-lactide, lactic acid and lactic acid linear oligomers impurities. The influence of the type of the raw polymer on the yield and quality of the monomer was analyzed. The destruction of polylactide previously treated at a high temperature and under mechanical force (by the methods of extrusion during 3D printing and hot pressing) was characterized by a lower yield of crude lactide and obtaining of the monomer with a higher content of impurities capable of violating the stereoregularity and reducing the molecular weight of the polymer synthesized from lactide. In order to assess the possibility of using the monomer obtained from polymer waste for the synthesis of polylactide, lactide was subjected to polymerization in the presence of stannous octoate as a catalyst. A product with a number average and weight average molecular weight of about 9.7 kDa and 16.3 kDa, respectively, was obtained.