DIELECTRIC MONITORING OF PHASE-SEPARATION DURING CURE OF BLENDS OF EPOXY-RESIN WITH CARBOXYL-TERMINATED POLY(BUTADIENE-CO-ACRYLONITRILE)

Curing experiments were carried out at several temperatures on blends of diglycidyl ether of bisphenol A (DGEBA) epoxy resin with 34.4 phr of 4,4'-diamino-3,3'-dimethyldicyclohexylmethane (3DCM) hardener and 15 wt% carboxyl-terminated poly(butadiene-co-acrylonitrile) (CTBN) rubber. Simultaneous measurements were made of turbidity and of dielectric properties over the frequency range 0.1 to 20 kHz. Changes in viscosity, gel fraction and enthalpy were determined in separate experiments. The results were compared with those for the unmodified resin-hardener mixture. In CTBN blends, relative permittivity epsilon' at low frequencies rose sharply during phase separation, owing to an interfacial polarization process, which can be described by the Maxwell-Wagner-Sillars model: relaxation times tau are in the order of 1 ms. In the neat resin, epsilon' fell continuously throughout the cure over the whole frequency range, the fall being particularly steep as the resin gelled and began to vitrify. It is concluded that dielectric techniques provide an effective method for monitoring phase separation, gelation and vitrification in resin blends.