Latent curing agents, such as dicyandiamide, are commonly used in producing solid epoxy adhesives. Being latent, these curing agents react with epoxies only on heating. They are relatively insoluble in the epoxy resin at room temperature, yet melt and become soluble at an activation temperature. Once activated, they cure relatively quickly at elevated temperatures. Dicyandiamide provides excellent high temperature properties.
RESULTING PROPERTIES
In admixtures with DGEBA, dicyandiamide has demonstrated a room temperature storage life in excess of 4 years. Dicyandiamide is usually added to the solid epoxy resin in concentrations of about 3-6 phr. It melts at about 150°C. Cures can be conducted in the range of 120-175°C but are very slow at the lower temperatures. As a result, it is common practice to add accelerators such as BDMA and mono- or dichlorophenyl substituted ureas to these systems. Cure schedule for the formulations shown in the table above is 1 hr at 175°C and 120 psi.
