Improving Glass Transition Temperature and Toughness of Epoxy Adhesives by a Complex Room-Temperature Curing System by Changing the Stoichiometry

The glass transition temperature (Tg) of room-temperature curing epoxy adhesives is limited by the temperature used during curing. It is already known that the excess of epoxy groups can undergo a homopolymerization reaction initiated by tertiary amines at elevated temperatures, resulting in an increase in Tg. However, there is no evidence of this reaction occurring at room temperature. In the present work, the influence of formulation stoichiometry on Tg and mechanical properties was investigated. Dynamomechanical, rheological and mechanical properties of epoxy adhesives were determined by DSC, DMA, rheometer and tensile and shear strength testing. It has been probed that an excess of epoxy resin combined with a complex curing system composed of a primary amine, a polymercaptan and a tertiary amine leads to an increase in Tg up to 70 degrees C due to the homopolymerization reaction that takes place at room temperature. However, as the excess of epoxy resin is increased, gel time becomes slower. Regarding mechanical properties, it has been proven that an excess of epoxy resin provides a tighter and tougher material but maintains flexibility of the stoichiometric formulation, which is meant to enhance the resistance to impact-type forces, thermal shock and thermal cycling.

Automated summary

This study investigated the influence of formulation stoichiometry on the glass transition temperature (Tg) and mechanical properties of epoxy adhesives through DSC, DMA, rheometer, and tensile and shear strength testing. The results showed that an excess of epoxy resin combined with a complex curing system can undergo homopolymerization reaction at room temperature, resulting in an increase in Tg up to 70 degrees C. However, as the excess of epoxy resin is increased, the gel time becomes slower. In terms of mechanical properties, an excess of epoxy resin provides a tighter and tougher material while maintaining the flexibility of the stoichiometric formulation to enhance resistance to impact, thermal shock, and thermal cycling.