Adhesion of metals to polymers. I. Study of interfaces modified by inductive heating

The objective of the work reported in this paper is to explain theoretically the observations made in an earlier study that castings of an epoxide resin reinforced by electrically conductive inserts, after having been postcured by inductive heating, had strength properties superior to those achieved by a purely thermal postcure. A search of the literature on adhesion suggested that among the theories presented, those dealing with electrical double layer formation, interdiffusion, and mixed polymer grafting on the insert–polymer interface deserve to be considered. Experiments were then planned in such a manner as to permit a choice between these theories. Epoxide resin specimens reinforced with various metals (pure silver, silver with an oxide layer, pure copper, copper with an oxide layer, aluminum, and stainless steel) were prepared. They were then either postcured in a heated oven, or postcured in an induction furnace for various lengths of time. Bonding strength was determined by a knife‐edge method, and the exposed insert and resin surfaces were tested for electrical potential and surface wettability (angle‐of‐contact). Most strikingly, the curves obtained with increasing induction heating times for bonding strength, electrical potential, and wettability were all sinusoidal in shape, and their maxima and minima generally were found to coincide. For oven‐postcured specimens, bonding between different metal inserts and an epoxide resin is explained by double electrical layer formation (insert positive, resin negative), with bonding strength increasing as the stability of the oxide forming the surface layer of the metal decreases. Metals with no oxide surface layer thus have the highest bonding strengths. For inductively postcured specimens, bonding is effected by both electrical double layer formation and mixed polymer grafting through the oxygen atoms in the metal surfaces, with the more stable metal oxide giving the stronger bonding in grafting. Where the insert does not carry an oxygen surface layer, bonding takes place through electrical forces only. Alternating build‐up and internal discharge of electrical double layers are the direct cause of the sinusoidal shape of the electrical potential curves and the indirect cause of the comparable shape of the wettability curves. Their combination results in the observed periodicity of the breaking load curves.