Rheological determination of the sol–gel transition during the aqueous synthesis of resorcinol-formaldehyde resins

The sol–gel transition of organic gels prepared by polycondensation of resorcinol (R) with formaldehyde (F) in presence of sodium carbonate (C) was monitored by small amplitude oscillatory measurements, at a single frequency. The gelation time (tg) was determined from the evolution of the storage (G′) and loss (G″) moduli versus time. The influence of two synthesis variables, the resorcinol/sodium carbonate ratio (R/C) and the synthesis temperature (T), was investigated. The gelation time was found to decrease with increasing synthesis temperature and decreasing R/C ratio. The apparent activation energy of the reactional system leading to the gelation, calculated from the gelation times obtained between 50 and 90 °C, was found to be close to 80 kJ mol−1 and independent of the R/C ratio.