Understanding the curing kinetics of boron-based hyperbranched polysiloxane reinforced and toughened epoxy resin by rheology

The curing kinetics of epoxy resin (EP) play an important role in optimizing the final properties of the resin, such as thermal stability, flame retardancy and mechanical properties, etc. However, full understanding of the curing kinetic remains a formidable challenge due to the progressively crosslinked structure and the multi-stage exothermic curing reactions. In this study, a hyperbranched polysiloxane (PBPS) with a Si-O-B backbone, containing reactive hydroxyl and epoxy end groups, is synthesized to facilitate the amine curing of EP. For the first time, the contribution of boron to the curing kinetics of epoxy resin is systematically revealed. The Kamal-Sourour model and the Arrhenius four-parameter model, having good fittings to rheology data, successfully describe the isothermal and non-isothermal curing kinetics of the epoxy-amine system loaded with PBPS, respectively. Acting as a Lewis acid, the boron in PBPS coordinates with epoxy groups, efficiently reducing the induction time of curing of the epoxy-amine system. The reaction rate of the non-catalytic stage is increased with reduced activation energy. Meanwhile, a gentle autocatalytic curing stage is achieved with PBPS. The effect of PBPS on the initial viscosity of EP leads to an increase in the activation energy of the viscous flow stage of the non-isothermal process. The curing behavior determines the final properties of EP. The combined effect of improved curing conditions, which endow EP with high crosslink density, and the flexible segments of PBPS exhibit excellent toughening effects. PBPS cavities can in situ form filament structures, thereby reinforcing and toughening the EP. This systematic study fills the knowledge gap regarding the mechanism of hyperbranched polysiloxane participating in the curing of epoxy resin and its influence on the final properties.