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Saturday, June 28, 2008

The Gluing of Oak

Before concluding this section on resorcinol formaldehyde adhesives, it may be of interest to mention that some users hold the opinion that the product of one manufacturer may give better results on certain kinds of timber than that of another. The author has heard this opinion expressed with particular reference to the room temperature gluing of oak. No satisfactory explanation for the behavior has been given, but the following experience suggests one, or at least throws some light on the problem.

Test pieces cut from a specimen of English oak (species not known) repeatedly give low strength when glued with resorcinol or resorcinol/phenol glue, but gave excellent strength with urea glue. The acidity of the oak was suspected to be a contributory factor; an aqueous extract was found to have a pH value of 3.7 (determined by extracting 10 g of oak in 100 ml of distilled water). It was recognized that acidity of this order might seriously prolong the gelation and setting, at least at room temperature, of a relatively unbuffered resorcinol glue. Therefore the surface of the wood was washed with a dilute solution of sodium acetate, and after this treatment adhesion with both resorcinol and resorcinol/phenol glue was excellent.

The inference to be drawn is that some resorcinol adhesives are either more alkaline than others or have a buffer capacity that enables them to resist a marked change in pH. If the pH value of the adhesive at the glue line is considerably reduced, the setting will be protected and result in so little cross linking that glued joints, even if they exhibit satisfactory adhesion initially, will have poor cold-water-resistance and poor resistance to weathering. With 'difficult' timber a higher strength can always be obtained by curing a elevated temperatures, (a corollary to which is that a boiling water test may improve the strength of a glued joint rather than reveal its weakness).

With further reference to this subject, it is interesting to note that is has been suggested that in gluing oak with a 'pure' resorcinol resin a curing cycle of 10 hours at 60 oC be used in order to give high resistance to exterior exposure (compared with 10 hours at 27 oC for Douglas fir and yellow pine).