Chemical recycling of poly(ethylene terephthalate) and investigatıon of conditions for effective resynthesis

The increase in plastic production leads to formation of more and more plastic waste, which causes the destruction of the natural environment system. Poly(ethylene terephthalate) (PET) has become one of the most investigated polymers because of its high consumption as bottles, textile products etc. The high consumption of PET-based products means huge quantities of PET waste, which causes significant environmental pollution. Therefore, recycling of PET waste is an important issue to be addressed.The main objective of the thesis was the production of high-quality PET from PET waste. Accordingly, the PET materials were chemically recycled in the first part of the study. Different types of PET materials; transparent PET beverage bottles, coloured PET beverage bottles, PET yarns without finish and PET yarns with finish were depolymerized by glycolysis method. (Zn(OAc)2), 1,3-Dimethylurea/Zn(OAc)2 deep eutectic solvent (1,3-DMU/Zn(OAc)2 DES) and Butyl-3-methylimidazolium bromine ([Bmim]Br) were used as catalysts in depolymerization reactions. The obtained Bis(2-hydroxyethyl) terephthalate (BHET) was characterized with regards to its chemical structure and thermal properties and compared to commercially available BHET. Additionally, the yields of different applied glycolysis processes were evaluated. It was found that the type of the input PET material does not cause any significant difference in the properties of the BHET obtained. Moreover, it was found that using the different catalyst systems did not lead to significant difference in the quality of BHETs. When BHET yield was considered, Zn(OAc)2 was the most efficient catalyst. On the other hand, 1,3-DMU/Zn(OAc)2 DES resulted in the shortest reaction time to reach complete glycolysis.In the second part of the thesis, experiments were conducted to resynthesize PET from recycled BHET and increase its molecular weight by solid state polymerization (SSP). The effects of different reaction conditions were investigated for an optimum synthesis reaction. The influence of the reaction time of the repolymerization and the applied vacuum amount and time on the chemical structure, physical and thermal properties of the PET product was investigated. Moreover, EG in various concentrations and phosphoric acid were added to the repolymerization reactions and their effects on the synthesized PET’s properties were evaluated. The catalyst concentrations were changed to determine the best catalyst concentration that leads to the formation of PET with the desired properties. Finally, a polymerization reaction was carried out with untreated BHET to evaluate the differences in the properties of PET obtained from recycled BHET compared to PET synthesized from untreated BHET. It was found that the use of different catalyst concentrations and the application of different vacuum times significantly affected the properties of PET polymer obtained. During the solid-state polymerization that was performed to increase the molecular weight of the polymer, different reaction conditions such as different reaction times and reaction temperatures were applied to the synthesized PET products. Besides, the effect of BHET particle size on the PET properties was investigated. The obtained recycled PET was characterized with regards to its chemical structure as well as its physical and thermal properties. It was found that the application of SSP resulted in an increase in the intrinsic viscosity (IV) of the PE