Chemical Recycling of Poly(ethylene terephthalate) to Functional Glycolysates: Overcoming Phase Instability and Secondary Crystallization

Poly(ethylene terephthalate) (PET) waste management faces challenges as mechanical recycling limitations become apparent under strict EU regulations. This review critically evaluates PET glycolysis as a vital chemical recycling method, focusing on overcoming barriers to industrial implementation. While systematizing knowledge on reaction mechanisms and parameters, a significant research gap is identified: the necessity for utilizing a high initial mass fraction of waste PET in the reaction feed. Specifically, exceeding a critical concentration of PET-derived oligomers in the resulting glycolysis reaction mixture (typically when the initial waste PET input is above 40% by mass) inevitably triggers phase instability and secondary crystallization during storage. This instability at high concentrations is fundamentally driven by the altered oligomer molecular-weight distribution and the thermodynamic supersaturation of rigid aromatic segments upon cooling. Traditional laboratory approaches using a high excess of glycolyzing agent fail to meet industrial stability demands for subsequent polyester polyol synthesis. Currently, preventing crystallization relies on costly branched glycols or modifiers to disrupt molecular symmetry. This article highlights the urgent need for alternative, additive-free methods to achieve phase stability, such as the elimination of released ethylene glycol from the reaction environment. By addressing shortcomings in glycolysate shelf-life studies, this review charts innovative directions for developing technologies that convert high concentrations of waste PET into phase-stable glycolysates.