Isothermal curing kinetics of a flame retardant epoxy resin containing DOPO investigated by DSC and rheology

• Curing at temperatures lower than Tg∞ was diffusion-controlled at high conversion α.
• Modified Kamal model with diffusion factor described dα/dt curves quantitatively.
• Curing degree θ from storage modulus revealed liquid to solid transition much later than α.
• α at the modulus transition time was the same for curing at different temperatures.

The isothermal curing kinetics of a flame-retardant epoxy resin containing DOPO (1.5 wt% of phosphorus) with hardener DICY was investigated using differential scanning calorimetry and oscillatory shear rheometry. The glass transition temperature Tg increased with conversion α measured by DSC. When the curing temperature is lower than Tg∞ of the completely cured epoxy, the curing kinetics at high α became diffusion-control due to that cured epoxy restricted the diffusion of the reactants to decelerate curing. The modified Kamal model including the diffusion factor described curing conversion rate curves quite well. The curing degree θ was established with relative storage modulus, which was measured by the cyclic frequency sweep, and revealed that the transition from liquid to solid during curing appeared later with a slower rate than that for the conversion change, because the network began to form after formation of branched polymer chains. At the modulus transition time, the epoxy samples cured at different temperatures had the same conversion of 0.409, indicating that above this conversion of reactants the effective network was formed to bear the stress.

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