One-Component Adhesive Formulations

In one-component epoxy adhesives, the curing agent and resin are compounded together as a single product by the adhesive formulator. The curing agent system is chosen so that it does not react with the resin until the appropriate processing conditions are applied. 

A second method of achieving latency is by using a curative precursor, which is chemically inactive at room temperature but then converts to an active curative at the cure temperature. Examples of this type include Monuron and di-urea adduct of toluene diisocyanate and dimethylamine. Full cure in about 1 h at 125°C is achievable with these materials. 

Dicyandiamide (sometimes referred to as dicy ) or its derivatives are used in most commercial one-component epoxy adhesives. This curative is a white crystalline solid and is easy to incorporate into an epoxy formulation as a finely ground powder. When cured with epoxy resin, dicyandiamide provides an excellent set of performance properties. 

Dicyandiamide has an activation temperature of about 177°C. However, this can be reduced to about 120°C by the use of accelerators. When mixed into liquid DGEBA resins, dicyandiamide has very good shelf life due to its latency. The shelf life of unaccelerated systems is about 6 months at room temperature or longer if refrigerated. The shelf life is reduced significantly with the addition of accelerators. 

A typical dicyandiamide cured epoxy adhesive formulation is described in Table 12.7. These formulations are unaccelerated so that they have a shelf life of greater than 6 months at room temperatures. The adhesives will cure in 30 to 90 min at 149 to 177°C. Tensile shear strength on aluminum is in the range of 2500 to 3000 psi. 

---