Abstract
There was a crucial need to eliminate formaldehyde (F) from high-performance thermoset formulations. The catalyzed reaction of innovative phenol (P)-terephthalaldehyde (TPA) resoles (basic conditions) was studied. For aerospace applications, alkali- or alkaline-earth-based catalysts should be avoided. To address this issue, we successfully employed DBU to catalyze the resole formation (with NaOH as reference system). The pre-polymerization (addition of phenol onto the first aldehyde of TPA) was performed in solution under mild conditions and followed by NMR and IR. Sodium hydroxide catalyzed rather effectively the reaction of ortho phenolic positions (1:1 ortho:para) whereas DBU catalyzed more para positions (ca. 1:2 ortho:para), because of the absence of chelating behavior of Na+. Based on non-isothermal DSC of the thermosets curing, we proposed a two-step mechanism pathway, directly arising from the multiple reactions occurring with TPA. The first reaction was assigned to the condensation reaction as its reactivity was enhanced by the unreacted aldehyde in para position (invariant aldehyde content measured in IR and weight loss recorded in TGA). The second was assigned to the addition of the phenol onto the second aldehyde (decrease of aldehyde and no weight loss). An in-depth non-isothermal thermo-kinetics study was efficient to compare both NaOH and DBU catalyzed reactions (isoconversional computational method of Vyazovkin). The isoconversional analysis allowed to discriminate the activation energies of the addition and condensation reactions (NaOH: 78 and 72 kJ·mol−1; DBU: 63 and 49 kJ·mol−1, respectively). Finally, TGA results showed that these new resoles presented increased thermal performances as compared to a commercial PF (Td10% ≥ 470 °C and char yield ≥ 64 wt%).