Study of the curing kinetics of a benzimidazole/phthalonitrile resin system

• For the first time, rheology and calorimetry were introduced to describe the curing kinetics of benzimidazole-promoted phthalonitrile resins.
• Microphase separation is responsible for the extremely mild curing process and apparent self-acceleration model of this curing system.
• A new diffusion factor equation is established.
• This equation follows variable activation energy concept.
• This equation provides direct insight into diffusion controlled process.

The isothermal curing behaviour of a benzimidazole/phthalonitrile (PNBI/PNTI) resin system was studied by rheology, quasi-isothermal modulated differential scanning calorimetry (QMDSC) and polarised optical microscopy (POM) in combination with theoretical model analysis. The rheological curing kinetics are consistent with an apparent self-acceleration model dα/dt = kαm(1 − α)n below conversion a = 80% with a total reaction order m + n of approximately 2.3. Viscosity analysis and POM show that microphase separation has already occurred in the initial cure stages, which is responsible for the extremely mild curing process and apparent self-acceleration model. Changes in the effective activation energy of the curing system with conversion α were introduced to include the effects of diffusion control. A new equation for diffusion factor (DF) based on the law of addition of kinetic resistance and the equation of effective activation energy has been induced, indicating that the values of DF are primarily determined by the effective activation energy.

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