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Friday, January 30, 2009

Metal Adhesives

In many applications, such as in gluing metal to wood, a metal forms only one adherend; one extremely important example is the bonding of brake and clutch linings to metal. This application was one of the first to be investigated when epoxy adhesives became available, and although they are perfectly satisfactory, they are not used on anything like as large as scale as the two polymer adhesives based on phenolic/polyvinyl acetal systems.

Some specifications dealing with metal adhesives for aircraft structures demand resistance to variety of chemicals, and to salt solutions. The chemical resistance of bisphenol A adhesives depend on the curing agent and the curing cycle; it is therefore possible only to generalize by saying that epoxy resin have good resistance to acids and alkalis, to all alcohols and most hydrocarbons, but are somewhat sensitive to the lower ketenes. In many adhesive applications resistance to water is the most important requirement, and this is affected by the thickness of the glue line and nature of the adherents, as well as by the composition of the adhesive system. In a study of the chemical resistance (not in adhesive applications) of three types of hardener, anhydride, amine and boron trifluoride complex, it was shown that dimanodiphenyl sulphone gave the best chemical resistance and the best resistance to boiling water.

Fatigue, creep and impact resistance are important factors in aircraft adhesives, and these properties are obviously dependent on the hardener used and on the incorporation of additives especially flexibilisers. Specific information is available relating to the fatigue behavior or this adhesive generally. The creep resistance of glued metal joints has been included in a comprehensive paper by Hans.

The strength of epoxy adhesives shown extent to which this property can be improved by the addition of the polyamide Versamid or polysulphide Thiokol. The effect of asbestor on impact strength has been referred to under fillers.

Here it is appropriate to emphasize again the fact that unless accompanied by complete data such as type of metal, type of joint, curing temperature etc., the "bond strength" of an adhesive has only limited meaning. The result of detailed test can, however, be obtained from a number of sources, apart from some already noted.

Epoxy adhesives in film, on a carrier such as glass fiber, are specially suitable for bonding the skins to the honeycomb core in the making of sandwhich constructions. One advantage is that an adhesives can be formulated that has just the right amount of flow to enable good filleting to be achieved on the vertical edges of the honeycomb. This is an application where an asymmetric film is especially useful.