Gelation and isoconversional kinetic analysis of synthesis of lignin–resorcinol–glyoxal resin curing

Abstract

Two resins were prepared from Stipa tenacissima L. and their physical and chemical behaviors were evaluated. The resins were produced by modification of extracted lignin from the S. tenacissima L. using phenolation reaction (resorcinol). Glyoxal and formaldehyde as cross-linking agents were employed to produce lignin–glyoxal resin and lignin–resorcinol–formaldehyde resins (LRFR), respectively. The curing process of two lignin-based resins (lignin–resorcinol–glyoxal, LRGR, and lignin–resorcinol–formaldehyde, LRFR) was studied by rheological measurements and differential scanning calorimetry (DSC) techniques, which showed that the gel formation obeys an isoconversional principle. In fact, an analysis on curing kinetics was performed by processing its non-isothermal DSC data using Ozawa’s isoconversional methods and found that good fittings could be obtained. This method allowed the determination of activation energy and the pre-exponential factor dependencies on the degree of curing. In kinetics method, the variation in activation energy, evaluated at different curing degrees, revealed that the mechanism of phenolic resins is based on two reactions: methylolation followed by condensation. Finally, the gel points of these systems were found to be around 63 and 72 %, for LRFR and LRGR, respectively. These values were compared to those calculated from Flory and Stockmayer equation and found to be close to each other.