The adhesion to metal of unmondified phenolic resins can increased by adding an epoxy resin, in particular a bisphenol a resin, and furthermore, since this addition reduces brittleness, band strength is increased. It, however, hot strength is nor to be seriously reduced, the amount of bisphenolic, it follows therefore that the cross linking of the epoxy resin (assuming no other curing agent is added) is provided for by the large excess of phenolic resin.
Interest in phenolic epoxy adhesives first came about through the demand for adhesives with improved hot strength capable of withstanding the dynamic heat generated by fast flying aircraft. The early reports published on this type of adhesive quoted hot strengths that were clearly an advance on values attainable at that time with other adhesive systems. But the excellent result obtained on stainless steel was good, the strength on prolonged heating deteriorated much more rapidly than on aluminum alloy. Some experienced conclude that the iron catalyzed the thermal decomposition of the adhesive, the decrease in joint strength is a consequence of this.
Some observation with phenolic epoxy adhesives, bonds made on titanium, brass, copper and low carbon steel also bad low resistance to heat ageing compared with aluminum. Some improvement in the behavior of stainless steel was brought about by specially treating the metal surface or by adding 1% manganese dioxide to the resin. An increase in the heat stability of stainless steel joints could be affected by adding certain metal chelates to the adhesive.
The ratio of phenolic to epoxy resin can be varied within wide limits, but the copositions that give the best hot strength (although probably not the best heat ageing) contain at least foru times as much phenolic as epoxy resin. In some formulations that have published, dicyandiamide is also added as well as a large amount of aluminum powder.