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

Abstract

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 ( t g) 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.