Of the miscellaneous compounds that have been proposed as curing agents for epoxy resins, many are probably never tested in adhesive systems unless they arouse the interest of the more experiment-minded user.
A compound that has established itself as an important hardener for adhesives, especially for metal bonding; is dicyandiamide. This substance melts at about 200oC, and being non-reacting at room temperature is convenient for use in the form of powder on rod. On heating, dicyandiamide. If finely divided, the dicyandiamide will remain in suspension in a liquid resin provided a substance such as Aerosil (a finely divided silicon doxide) is also added to create thixotropy. Dicyanidimide is difficulty soluble substance, but limited amounts can be dissolved in certain polar organic solvents, in particular methoxymethanol (methyl “Cellosolve”) and added as a solution.
An example of the strength/temperature relationship of a bisphenol A epoxy adhesive cured with discyadiamide is shown in the figure.
The hot water (90oC) resistance of the same adhesive system has been shown to fall rapidly during the first moth but to retain at least one-half of its original strength during six months. In a test of this sort, the adherent is important because of the effect of the water at the adhesive/metal interface, and consequently the surface treatment of the adherent is also
Strength/temperature relationship of bisphenol A resin cured with discyandiamide