Wood and Metal Combinations

Improved methods of bonding wood to metal have been developed through research. Rigid bonds are now possible, an improvement over the past methods which were sufficient only when elevated temperatures and long-term-aging effects were of little importance. Conventional thermoplastic materials can be used by spreading on both the wood and metal surfaces, allowing solvents to escape, and then bonding together by means of pressure or heat and pressure. Bonds of this nature are limited in performance especially in regard to durability at elevated temperatures due to the nature of the resins applied. Thermosetting resins are not yet available, and the absence of adequate nondestructive testing procedures results in a hesitancy to use these adhesives in the aircraft industry. They are, however, used in bonding wood-to-metal such as in railroad passenger coaches, and bonding metal to plywood such as in truck bodies and railroad cars. Resorcinol resins have excellent resistance to temperature and chemicals but for bonding wood-to-metal it is always necessary to prime the metal surface with a cohesive type of primer in order to effect a bond of reasonable strength. Since these primers are thermoplastic, many of the advantages of resorcinol are lost. Casein-rubber latex adhesives cure at room temperature but the bond has poor water resistance and relatively tow strength. Thermosetting vinyls and thermosetting epoxy-amine emulsions appear to give excellent results in bonding metal to wood. Both require formulation before use, but have exceptionally long pot lives and cure at room or elevated temperatures. Preparation of the wood surface is rarely any different than conventional laminating techniques. Preparation of the metal surface is the most critical factor in satisfactory metal to wood bonding. Vapor degreasing or acid bath etching or both is required. Metals such as aluminum and copper usually require electrolytic etching prior to bonding so the surface will be adequately wetted by the adhesives. Mastics, thermoplastics, and rubber cements require only contact pressure for bonding. Epoxy resins bond best at low pressure. Fifty to 125 psi bonding pressure must be employed with resorcinol, catalyzed polyvinyl acetates, and epoxy-amine emulsions. Adhesives which cure under low temperatures and pressures are more likely free from printing or telegraphing of core irregularities. Due to the high coefficient of thermal expansion of metal, prolonged heating of adhesives requiring heat to cure may result in dishing or cupping upon cooling. Cold bonding is most desirable where a high degree of flatness is necessary. In cold pressing of metal to wood, the amine emulsion types exhibit advantages of long pot life in liquid form and relatively short room temperature cure cycle.

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