Preparation, Acid Curing, and Thermal Stability of Various Formulated Phenolic Resole Resins: Unfilled and Nanocomposites

Abstract

Several phenol-formaldehyde resole resins were prepared with variety in monomers ratio, catalyst type, and content, having different nanoclay levels; then they were cured using various acids in the wide range of concentration. The acid-curing process was studied, considering gel time (tG) and cure time (tC). In addition, the thermal stability was investigated before and after cure for all samples. With increasing F/P ratio, tG, tC and tG-tC decreased and also CC (critical concentration). By using more catalyst in the synthesizing step, the curing was done more rapidly in the order of NaOH, Ba(OH)2 and NH3. Stronger acids having smaller pKa made a more realizable cure. However, the weak boric acid had no curing effect. Nanocomposites had shorter tG and tC, mainly at lower acid concentration. The structure peak of nanoclay shifted to the lower angles in nanocomposites, especially in the cured state. Crosslinked samples had higher degradation temperature (TD) and lower weight decrease (Δw) related to the primary resoles. For uncured resins, thermal stability increased with decreasing of F/P ratio, and the inverse effect was found for the cured resins. Resins cured with HCL had higher TD and lower Δw. However, at 30% concentration the sample cured by H2SO4 was more stable. With increasing catalyst amount and reactivity, TD increased and Δw decreased. In all acid concentrations, at the presence of nanoclay the better thermal resistance was observed. TD increased and Δw decreased as the nanoclay level increased.