Polylactic acid recycling with environmentally benign fluids

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2022-2
Bozcuoğlu, Çağla
In the past few decades, petroleum-based polymers have been replaced by biodegradable polymers since they bring about environmental problems. Polylactic acid (PLA) is a good candidate for this replacement due to its biocompatible and biodegradable nature and favorable mechanical and thermal properties. Due to the increase in usage and demand, there will be a PLA waste problem in the near future. Although PLA is classified as a biodegradable polymer, only a few microorganisms, which are not present in all soil types, can degrade PLA. In addition, relatively high temperature and humidity conditions should be provided for PLA biodegradation. The existing techniques for PLA degradation have not been fully developed yet, and degradation techniques are still being researched and developed. Hence, the aim of this study is to present an effective and environmentally benign solution for the PLA waste problem. In this study, the PLA degradation reaction was carried out in a deep eutectic solvent (DES) medium in two different systems with batch and semi-batch reactors. All degradation experiments were performed at 180°C for 8 hours. Zinc chlorideethylene glycol DES was synthesized, characterized, and used as a reaction medium in the degradation reaction of PLA. The complexation between zinc chloride and ethylene glycol was verified by shifting of the peak at 3390 cm-1 in the ethylene glycol FTIR spectrum. As a catalyst, aluminum-loaded silica aerogel was synthesized, characterized, and used in PLA degradation reaction. The support material was synthesized using the sol-gel method, and aluminum was loaded via the wet impregnation technique. Aluminum was loaded into silica aerogel supports in 15 and 25 weight percentages (SAUAl15 and SAUAl25, respectively). Both of them decreased the activation energy of the thermal degradation reaction of PLA, but SAUAl15 performed better in decreasing activation energy and degradation temperature. SAUAl15 decreased the activation energy from 262 kJ/mol to 193 kJ/mol, whereas SAUAl25 decreased the activation energy from 262 kJ/mol to 238 kJ/mol. The product distribution of the PLA degradation reaction in zinc chloride-ethylene glycol DES in the batch reactor was investigated with GC-MS analyses. Degradation of DES in the same reaction conditions as that of PLA degradation reaction was also investigated, and its degradation products were distinguished from PLA degradation products. Dioxane, decane and undecane were the main products formed by the degradation of DES. The main products formed in the PLA degradation reaction were diethylene glycol, triethylene glycol, and acetic acid hydrazino ethyl ester. In addition to these products, ethyl lactate and 3-methyl-2-hexanone were formed. The effect of SAUAl15 on product distribution and its interaction with zinc chlorideethylene glycol DES was investigated. Ethyl lactate amount increased by two-fold with the usage of catalyst the product distribution of PLA degradation reaction in zinc chloride-ethylene glycol DES carried out in the semi-batch reactor was also investigated with GC-MS analyses. 2-pentanol acetate, 3(2H)-furanone dihydro-2-methyl, and ethyl lactate were the main products formed by PLA degradation, while 1,3-dioxolane, 2,2- dimethyl, and dioxane were the main products formed by DES degradation. The effect of SAUAl15 on product distribution was investigated. With the usage of catalyst, 1,3-dioxolane, 2,2,4,5-tetramethyl was newly formed, and amounts of dioxane and 3(2H)-furanone dihydro-2-methyl increased. Analyses of gas products were carried out with GC analyses. In all experiments, hydrogen and carbon dioxide were mainly detected as gas products. In the semi-batch system, for DES degradation reaction, conversion of DES was 35.4%. In PLA degradation reaction, conversion of PLA was calculated as 97.3%. Yields of gas and liquid products and solid part in the reactor resulting from PLA were found to be 56.8%, 40.6%, and 2.6%, respectively. In PLA catalytic degradation reaction, conversion of PLA was 97%. Yields of gas and liquid products and solid part resulting from PLA catalytic degradation were calculated as 59.50 %, 37.4%, and 3.1%, respectively. Using zinc chloride-ethylene glycol DES as reaction medium, PLA was degraded, and value-added products were obtained. Aluminum loaded silica aerogel catalysts showed promising activity in the PLA degradation reaction.

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Citation Formats
Ç. Bozcuoğlu, “Polylactic acid recycling with environmentally benign fluids,” M.S. - Master of Science, Middle East Technical University, 2022.