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

Abstract

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 °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.

Designing Schiff-Based Hyperbranched Polysiloxane for Simultaneously Enhancing Epoxy Resin with Mechanical Properties, Thermal Stability, and Recyclability

“…DMA is a critical tool for the characterization of the viscoelastic properties of polymeric materials, including the determination of dynamic energy storage modulus ( E ′) and loss tangent (tan δ). These measurements are particularly valuable for accurately assessing the glass transition temperatures ( T g ) of resin-based materials . In this study, DMA was employed to examine the T g values of both pure EP and 6-STHPSi/EP.…”

A Comprehensive Review on the Thermal Stability Assessment of Polymers and Composites for Aeronautics and Space Applications

“…In general, a higher degree of crosslinking and a lower mobility result in a higher T g . Therefore, epoxy composites must use a resin and a curing agent that can form a highly crosslinked and rigid network [ 128 , 131 , 132 ]. Optimizing the curing conditions: the curing conditions, such as the temperature, time, and pressure, affect the degree of conversion and the quality of the cross-linked network.…”

“…A higher degree of conversion and a smaller number of defects or voids lead to a higher T g . Therefore, epoxy composites must be solidified under the optimal conditions that can ensure a complete and uniform reaction [ 128 , 131 , 132 ]. Adding modifiers or fillers: modifiers or fillers can be added to the epoxy system to modulate its properties, such as its toughness, thermal conductivity, flame retardancy, etc.…”

“…In general, a higher degree of crosslinking and a lower mobility result in a higher T g . Therefore, epoxy composites must use a resin and a curing agent that can form a highly crosslinked and rigid network [ 128 , 131 , 132 ].…”

Carbon Black: A Thermally Conductive Reinforcement for Epoxy Based Composite

“…Glass transition temperature is one of the desirable properties of polymer composites. It is the temperature at which the material is expected to change its state from hard glassy to soft rubbery (Ruíz de Azúa et al, 2023). On the other hand, the coefficient of thermal expansion (CTE) is essential to study the structural changes of composite as a function of temperature.…”