Energy-efficient recycling of post-consumer polyethylene terephthalate (PET) is a persisting challenge in the field of plastic circular economy. Today, the major method for PET recycling is mechanical recycling, in which the quality of the recycled PET decreases with each cycle. To address this, chemical recycling methods have been developed where PET is depolymerized to its parent monomers that can be repolymerized to yield virgin PET. This Ph.D. thesis is primarily focused on an energy-efficient chemical recycling method to convert discarded PET into its parent monomers for remanufacturing virgin PET. For this purpose, the impact of different catalysts, diols, and melt-pretreatment on depolymerization rates of PET has been investigated. Initially, pretreatment was effective in eliminating the crystalline regions that hinder the depolymerization process. Furthermore, the addition of catalyst and diol during the melt-pretreatment process could reduce the chain length of the polymer while active sites were created to accelerate the rate of depolymerization within the chunk of polymer. PET samples were subjected to methanolysis at temperatures ranging from 140 °C to 200 °C, and results revealed that the time for full depolymerization for pretreated and control (without pretreatment) samples were significantly different. For the optimized melt-pretreatment process and reaction condition, in the case of methanolysis, at 200°C, the time of full depolymerization shortened from 166 min to 7 min yielding >99% dimethyl terephthalate (DMT), while a minimum of 8-fold decrease in the energy demand for the depolymerization of melt-pretreated PET in comparison to the untreated PET was achieved. In the case of PET glycolysis, the optimal pretreatment could reduce the depolymerization time from 181 min to 9 min (under the same optimal reaction conditions) at 180 °C and yielded ~85% monomer. The scope of the research was further expanded and two organic catalysts, were employed as alternatives for zinc-based catalysts. The addition of 0.5 mol% of catalyst and diol during melt-pretreatment confirmed the striking effect of this extrusion-quench pretreatment on the organocatalytic depolymerization at 190 ◦C enabling full conversion of PET within 30 to 32 minutes in the presence of either organic catalyst, while conserving at least 38.5% of the required energy. With the growing production and consumption of PET, this project can help to convert billion tons/year of waste PET bottles into valuable materials and save resources. In a separate study, a technoeconomic analysis (TEA) was performed for a novel ionic polybutylene adipate-co-terephthalate (CPBAT) as a paper coating material with excellent water-in-oil resistance. The TEA determined the total capital investment for a production capacity of 1 ton of CPBAT per day. The minimum selling prices of CPBAT coated on Kraft paper (CPBAT-K) and CPBAT coated on starch-coated paper (CPBAT-S) are estimated to be $1.327/m2 and $1.864/m2, respectively. Additionally, the results of a sensitivity analysis show that the production of CPBAT-K and CPBAT-S is highly sensitive to the plant production capacity, raw material costs, the energy efficiency of the coating process, and reaction energy, as well as reaction yield. Additionally, recovery of the ionization solvent only marginally increases the selling prices of CPBAT-K and CPBAT-S, hence it is highly suggested. In the base case scenario, the price of CPBAT-K is ~40%, and CPBAT-S is ~96% more than that of commercial polyethylene-coated paper (PE Paper). With increased production capacity, lower cost of raw material, use of more energy-efficient coating machines, and partial recovery of the energy produced from the reactions, the MSPs will reduce to 0.588 and $0.914/m2, for CPBAT-K and CPBAT-S respectively. Conclusively, with comparable mechanical and barrier properties to PE paper and the added benefit of biodegradability and recyclability, the CPBAT offers an economically feasible and sustainable alternative to current coated paper packaging.
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