Wood-glue joint

Figure 1. Chain-link diagram of well-formed (proper) wood-glue joint. The materials and conditions that were used have led to a joint with good bulk wood strength, a strong interphase with good adhesion, and a strong glue film. (Reproduced with permission from ref. 1. Copyright 1950 American Furniture Manufacturers Association.)...

The formation of a proper wood-glue joint is the net result of several fluid actions. Marra (i, 2) outlined them as follows. 

Figure 11, Chain-link diagram of a joint involving weakened wood. The ultimate strength of even a well-formed wood-glue joint is usually limited by the strength of the wood. In the case of wood that has been weakened by decay or insect attack, joint strength is likely to be severely limited.

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. 

Integrity of Glue Joint in Structural Laminated Wood Products for Exterior Use, Test for D 1101... 

ASTM D-1101, Standard Test Method for Integrity of Glue Joints in Structural Laminated Wood Products for Exterior Use. Two methods are outlined in this standard for using an autoclave vessel to expose the joint alternately to water at... 

D-1101 Integrity of glue joints in structural laminated wood products for exterior use. D-1183 Resistance of adhesives to cyclic laboratory aging conditions. 

Emulsion Adhesives. The most widely used emulsion-based adhesive is that based upon poly(vinyl acetate)—poly(vinyl alcohol) copolymers formed by free-radical polymerization in an emulsion system. Poly(vinyl alcohol) is typically formed by hydrolysis of the poly(vinyl acetate). The properties of the emulsion are derived from the polymer employed in the polymerization as weU as from the system used to emulsify the polymer in water. The emulsion is stabilized by a combination of a surfactant plus a coUoid protection system. The protective coUoids are similar to those used paint (qv) to stabilize latex. For poly(vinyl acetate), the protective coUoids are isolated from natural gums and ceUulosic resins (carboxymethylceUulose or hydroxyethjdceUulose). The hydroHzed polymer may also be used. The physical properties of the poly(vinyl acetate) polymer can be modified by changing the co-monomer used in the polymerization. Any material which is free-radically active and participates in an emulsion polymerization can be employed. Plasticizers (qv), tackifiers, viscosity modifiers, solvents (added to coalesce the emulsion particles), fillers, humectants, and other materials are often added to the adhesive to meet specifications for the intended appHcation. Because the presence of foam in the bond line could decrease performance of the adhesion joint, agents that control the amount of air entrapped in an adhesive bond must be added. Biocides are also necessary many of the materials that are used to stabilize poly(vinyl acetate) emulsions are natural products. Poly(vinyl acetate) adhesives known as "white glue" or "carpenter s glue" are available under a number of different trade names. AppHcations are found mosdy in the area of adhesion to paper and wood (see Vinyl polymers). 

Glue was also used to join the numerous small plywood and wood details necessary to fabricate the structural frameworks typical of early aircraft. Fig. 1 shows a monoplane wing circa 1930 fabricated primarily from wood that was joined via more than one thousand glued joints [2]. Note the many small stringers and gussets bonded together to form the ribs and spars of the wing. 

In contrast, compare with the picture of a joint made with the same glue and wood but after 60 minutes open time and 0 minutes closed, Figure 4. The very thick glue line and almost total lack of solvent absorption are quite apparent. The wood failure in testing this joint was very low, while that of the previous figure was very high. 

General purpose epoxy adhesives are well suited for bonding common construction materials such as wood, stone, brick, metals, glass, rubber, plastic, ceramics, etc. They are particularly suitable for the repair of damaged items with closely fitting joint surfaces which require a fine glue... 

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