Textile wastes to polyols: study of PET aminolysis pathway

This research investigated the chemical recycling of polyethylene terephthalate (PET) from textile waste via aminolysis for the production of polyols to be employed in the synthesis of rigid polyurethane (PUR) foams. Primary and secondary alkanolamines were utilized as bifunctional cleaving agents; their chemical structure presented both the hydroxyl terminal, required for the subsequent polyurethane production, and the amine end-group, which were primarily responsible for converting the polyester fibers into oligomeric polyols. Initial experimental trials were conducted using neat mono-alkanolamines (ethanolamine, 3-amino-1-propanol, and 2-amino-1-butanol) to isolate and characterize the resulting aromatic terephthalamide monomers, which were recovered as solid crystalline products. Subsequently, the addition of glycols (polyethylene glycol 300, diethylene glycol, and dipropylene glycol) as solvents was studied to recover single-phase polyols with manageable processability. The introduction of these glycols, however, resulted in final polyols characterized by unfavorable rheological behavior, attributed to extensive intermolecular hydrogen bonding, and relatively high hydroxyl values (HV). In contrast, diethanolamine (DEA), a disubstituted alkanolamine, was successfully employed to selectively depolymerize the polyester while aiming to increase the functionality of the recovered products. Although depolymerization by DEA alone produced a homogeneous matrix, the resulting product was highly viscous; therefore, the incorporation of glycols was required to modulate the viscosity. The control of the moisture content within the reaction mixture emerged as a critical variable in limiting the extent of secondary reactions. Residual water in the system promoted hydrolysis, leading to the formation of carboxylic acid terminals; these groups subsequently underwent esterification due to the high concentration of hydroxyl end-groups provided by both the glycol and the free DEA, further accelerating water evolution. As a result, although the final product presented a suitable viscosity and an HV within the desired range, the Acid Value (AV) of 30-35 mg KOH/g remained inadequate for subsequent foam synthesis. This challenge was addressed by implementing a two-step solvolysis strategy, consisting of a sequential diethylene glycol-mediated glycolysis followed by a targeted DEA aminolysis. The initial stage was conducted at 220°C for one hour, after which the temperature was reduced to 160°C to introduce the DEA and facilitate its reactivity. The recovered polyester-amide (PEA) polyols targeted a dynamic viscosity of 120–150 Poise at 25°C, a hydroxyl value of 400–500 mg KOH/g, and a significantly reduced AV of 5–8 mg KOH/g. Subsequently, rigid polyurethane foams were synthesized by progressively substituting virgin, petrochemical-based polyols with the recycled products. Characterizations of the obtained PUR foams confirmed that an increase in recycled polyol content resulted in an overall improvement in both compressive strength and thermal conductivity.