Synthesis of PET-based urethane-modified alkyd resins from depolymerization intermediates of post-consumer PET bottles: coating properties and thermal behaviors

Urethane-modified alkyd resins were synthesized using the depolymerization intermediates obtained from simultaneous hydrolysis–glycolysis reactions of post-consumer poly(ethylene terephthalate) (PET) bottles with dipropylene glycol (DPG) and water. For this aim, first, the synthesis of a four-component reference alkyd resin having an oil content of 50% was synthesized by using tall oil fatty acid (TOFA), trimethylolpropane (TMP), phthalic anhydride (PA), and dipropylene glycol (DPG). The PET-based alkyd resins were also synthesized using different amounts and types of purified and fractionated depolymerization intermediates (water-insoluble fraction, WIF, and water-soluble and crystallizable fraction, WSCF) under the same reaction conditions. For the synthesis of PET-based alkyd resins, the depolymerization intermediates were used partially or completely instead of dibasic acid and/or diol components in alkyd resin formulations. In order to determine the optimum modification ratio for urethane-modified alkyd resin synthesis, the reference alkyd resin was reacted with toluene diisocyanate (TDI) at different molar ratios (NCO/OH: 1/1, 1/2, and 1/3), and the optimum NCO/OH ratio was determined as 1/1, according to the surface coating properties. And, then, all PET-based urethane-modified alkyd resins were prepared by the modification reactions of the PET-based alkyd resins with TDI at an optimum ratio. At the end of the study, it was observed that the use of waste PET intermediates (WIF or WSCF) did not cause any negative effects on physical/chemical surface coating and thermal properties; on the contrary, some properties were obtained as superior than that of the reference resin. In conclusion, it seems possible to use PET-based urethane-modified alkyd resin could be used as a cheap, environmentally friendly, and relatively economic binder component in synthetic paint formulations. In the synthesis of urethane-modified alkyd resin, using a certain amount of waste PET intermediate instead of dibasic acid and/or diol component will reduce raw material costs and also benefit the environment by re-evaluation of recycled PET. Thus, it could be both possible to recycle of post-consumer PET bottles, which are a valuable waste, and to provide cheap raw materials without compromising its features for paint formulations. These results are remarkable in terms of sustainability and waste management besides are also important for the economy and paint industry.