Phenol-resorcinol wood glue

As polymer chemistry advanced in the 1930s and 1940s, stronger and more durable synthetic adhesives such as early phenol, resorcinol and urea formaldehydes began to supplant natural glues in wood aircraft manufacture. Around this time however, metal began to replace wood as the dominant material for aircraft manufacture. Aerospace adhesives research and development moved on to focus on metals, primarily aluminum, as the substrates of interest. 

Fig. 2.3.7 Lower GARField profiles showing a UF (urea formaldehyde) glue line acting as a barrier to water transport for up to 24 h. The glue line is at 800 pm on the scale. Wood is above and below this. The water reservoir is beyond 1300 pm. The profiles shown were recorded after 20 (thin line), 100 and 1400 (thick line) min of exposure to water. Upper plots of the magnetization signal intensity in the lower and upper wood layers as a function of time for three glues urea formaldehyde (squares), phenolic resorcinol formaldehyde (triangles), and poly (vinyl acetate) (diamonds).

Resorcinolic adhesives are either straight resorcinolic resins, or phenol-resorcinol copolymers. When military requirements dominated the field, during and after World War II, most of the beams were produced from oak, and the straight resorcinol resins were preferred. By the time of the Korean War, excellent phenol-resorcinol types had been developed. These glues were used in the production of the all-wood minesweepers used in naval operations. They demonstrated a high degree of durability in their many years of exposure to all the rigors of the Seven Seas. 

The major synthetic adhesives used for bonding wood include urea, phenol, and melamine formaldehyde resorcinol formaldehyde, phenol resorcinol, and polyvinyl acetate emulsions. More recently one-component, moisture cured polyurethane adhesives have become popular for bonding wood. Natural adhesives such as casein and animal glues are also often used for general-purpose wood bonding. Epoxies have been used for certain specialized wood joining applications such as when wood is bonded to metal substrates. 

Resorcinol resin glues set without the assistance of heat under mildly alkaline conditions. Therefore, no damage is caused to the wood by acid hardeners, and there are no restrictions on the moisture content of the wood up to 18 %. The resins can be cured with paraformaldehyde. Wood-to-wood glue joints with resorcinol resins are among the strongest and are weatherproof and resistant to solvents, oils, and fats. For reasons of economy, mixed condensates of resorcinol and phenol are used instead of pure resorcinol. 

Developments in glued laminated structures and panel products such as plywood and chipboard raises the question of the durability of adhesives as well as wood. Urea-formaldehyde adhesives are most commonly used for indoor components. For exterior use, resorcinol adhesives are used for assembly work, whilst phenolic, tannin and melamine/urea adhesives are used for manufactured wood products. Urea and casein adhesives can give good outdoor service if protected with well-maintained surface finishes. Assembly failures of adhesives caused by exudates from some timber species can be avoided by freshly sanding the surfaces before glue application. 

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