Novel sustainable synthesis of a formaldehyde-free thermosetting phenolic resin through solvent-free reactive extrusion†

This study proposes a new and elegant way to synthesize thermosetting phenolic resins through a green, solvent-free and versatile reactive extrusion route. In that frame, terephthalaldehyde (TPA), a non-toxic aromatic dialdehyde, has been selected to replace formaldehyde while resorcinol has been chosen as a replacement of phenol. The syntheses were performed without solvent at temperatures between 150 and 170 °C with a reaction time of around 3 minutes. The resins were synthesized at different TPA-to-resorcinol molar ratios (0.6 and 1.6). This study investigates the mechanism and chemical reactions occurring during the reactive extrusion by characterizing the resin composition through NMR and mass spectrometry (<1500 g mol⁻¹). In addition, differential scanning calorimetry (DSC) analyses were carried out to study the kinetics of the reactions and to estimate the activation energies (32–54 kJ mol⁻¹) through various calculation methods (Flynn–Wall–Ozawa, Friedman, and Vyazovkin methods). It was demonstrated that multiple and consecutive reactions (electrophilic aromatic substitution and condensation) occur during the reactive extrusion process. Additionally, the resins synthesized by reactive extrusion exhibited an exothermic post-reactivity signature in DSC, enabling the estimation of the conversion degrees of 0.63 and 0.59, respectively, for ratios of 0.6 and 1.6. Finally, the resins obtained through reactive extrusion demonstrate great thermal stability even prior to post-heating.