Air Entrapment in the Use of Structural Adhesive Films

Abstract

Air entrapment during the preparation of bonds with structural adhesive films was examined by a microscopy study of glass/adhesive/glass specimens. It was observed that initially a thin film of air is trapped between the adhesive and adherend and covers over 50 % of the interfacial area. As the specimen is heat cured this air draws up into bubbles that are eventually displaced into the adhesive layer. Incomplete displacement occurs if the resin does not fully wet the adherend surface (contact angle greater than zero) or the resin does not become sufficiently fluid during the heat cure. The trapped air, whether displaced into the resin or held at the interface, could be eliminated from the bond by starting the cure in vacuum (5 mm Hg) and subsequently releasing the vacuum at the temperature at which the resin is in its most fluid condition. Specimens of bonded aluminum panels were tested in a T-peel configuration. An increase in bond strength of as much as 30% could be realized by complete void removal using the vacuum release technique. It was also found that the nylon support cloth used in the adhesive film tends to stabilize crack propagation in the peel tests but that this stabilization is not fully realized unless the bond is void free.