Thermal Study of Gels Obtained From Resol Type Phenolic Resins Modified With Lignin

Gels were obtained from resol type phenolic resins, with resorcinol, formaldehyde and lignin as precursors. Thermal response was evaluated by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and curing kinetics were determined from the SDTA curves using Flynn–Wall–Ozawa and Friedman iso-conversional models. Additionally, it was used Fourier transform infrared spectroscopy (FTIR) to evaluate the progress of the curing reaction. These analyzes were performed for two gels, one of them prepared with lignin (lignin resin), and a reference (resorcinol resin). Lignin resins tend to be more thermally stable than resorcinol resins. Average values found for activation energy of the curing processes are 55.07 kJ mol⁻¹ and 60.60 kJ mol⁻¹ for resorcinol resin and lignin resins respectively using Flynn–Wall–Ozawa method; values of 51.94 kJ mol⁻¹ and 53.14 kJ mol⁻¹ were obtained using Friedman method. It was possible to conclude that the lignin affects positively the thermal stability of the resin; the curing reaction takes place in the first six hours and the Flynn–Wall–Ozawa model fits better.