Plastic waste management is a colossal problem for modern society, that has resulted into several legislations and technological solutions across the world. Waste plastic can be part of the circular economy by their catalytic conversion into monomers and useful products. This work presents development and design of technologies to solve plastic waste issues using depolymerization, re-polymerization and composting. In the first part, depolymerisation of polyethylene terephthalate (PET) and Nylon 6 waste was studied using 2-L high pressure autoclave reactor at molten state and autogenous pressure in the presence of excess of water under subcritical conditions for various time intervals. Operation parameter such as reaction temperature, time, and concentration for both catalysts were studied. The obtained monomers (terephthalic acid (TPA) and ethylene glycol) were characterised by qualitative and quantitative analysis. In comparison with zinc acetate (used before), PEG 400 was the best catalyst. Concentration profiles were developed for PET, oligomer and terephthalic acid (TPA) using HPLC. A new mechanism of solid (polymer)-liquid(melt)-liquid (water) phase transfer catalysis (PTC) for hydrolysis was proposed and validated. This work is published in the Journal of cleaner production[1]( doi.org/10.1016/j.jclepro.2023.138312).A similar approach was used for depolymerization of nylon 6 into 6-aminocaproic acid (ACA) in the first stage using PEG as the phase transfer catalyst and the aqueous phase containing ACA and PEG was subjected to dehydration and cyclization to caprolactam in the second stage. The hydrolytic depolymerization method was applied to nylon 6 by using pure water as depolymerization agent, reacting under subcritical water 230-250 °C and autogenous pressure, the reaction time was 60 min. PEG 400 was discovered as efficient phase transfer catalyst for hydrolysis of nylon 6. A new theory was developed to interpret dehydration of nylon 6 to 6-aminocproic acid (ACA) where the reaction takes place in subcritical water phase. The highest yield of caprolactam reached 96% in reaction time of 60 min. plastic. There is challenge of separation of plastic waste (i.e PET or Nylon) from organic waste. Our next approach was to prepare biobased and biodegradable/compostable polyester which can be used for preparation compostable bags. For this, the polymer must have a high molecular weight for processing conditions. So, this study aims to prepare high molecular weight biobased and biodegradable polyester. The polymerization synthesis methodology was developed to synthesize high molecular weight polymers (60-80 kg/mol) namely polybutylene adipate co-terephthalate (PBAT), polybutylene sebacate co-terephthalate (PBSeT), and polybutylene azelate co-terephthalate (PBAzT) were synthesized using a developed methodology. The polymers obtained were characterized by intrinsic viscosity, acid number, and molecular weight and compared with a commercial polymer. The extent of reaction was determined by monitoring acid group in the reaction mixture. Next step, the food waste and compostable bags were mixed with a composition of brown and green. The reactor feed composition was maintained the same for all experiments. Various runs were conducted at different temperatures. The percent loss of dry mass was calculated by ASTM-D2974 method. The compostable bags were observed by visual inspection and pictures were recorded for it. It was observed that within 8-10 days all bags disintegrated and disappeared from the mixture. Overall, this work contributes to the concepts of circular economy and developing sustainable technology. Study of depolymerization using solvolysis, repolymerization for redesigning polymer for end of life will help to solve the issues of menace by plastic waste.
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