Designing catechol-poly(ethylene terephthalate) as a depolymerizable adhesive for PET packaging

Abstract

2,3-Dihydroxyterephthalate, a catechol-functionalized derivative of the conventional poly(ethylene terephthalate) monomer, was converted into a reactive monomer that polymerized into polyesters that exhibit adhesive properties. The monomer synthesis required two steps and proceeded with an overall yield of 95%, incorporating photoprotecting groups, ortho-nitrobenzyl (ONB), on the catechol to prevent side reactions. Polymerization with ethylene glycol produced a catechol-protected material, poly(ONB-2,3-dihydroxyterephthalate) (ONB-c-PET), and by varying the polymerization conditions, molecular weights ranged from 3000 to 8000 g mol⁻¹ with dispersity values ranging from 1.13 to 1.72. These polymers were deprotected using a UV lamp (370 nm), producing poly(2,3-dihydroxyterephthalate), c-PET. The glass transition temperatures (T_g) measured for ONB-c-PET and c-PET were 34 °C and 56 °C, respectively. Adhesion lap-shear testing of c-PET (ASTM D1002) showed adhesion strengths of 0.77 ± 0.09 MPa and 0.38 ± 0.11 MPa on a PET substrate. Degradation studies showed complete conversion to monomers via methanolysis over 16 hours. The resulting monomers, 2,3-dihydroxyterephthalic acid and 2,3-dihydroxyterephthalate, can be separated from ethylene terephthalate due to their water solubility. These results highlight the potential of this adhesive to facilitate the recycling of plastic materials contaminated with residual adhesives.