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).

The Application and Properties of Resorcinol Glue

In their application, the resorcinol and resorcinol/phenol glues are considered to be slightly more sensitive to ambient conditions than many other adhesives; the reason is the previously mentioned tendency to rapid drying-out. In conveniently rapid drying-out of an adhesive creates difficulties in spreading and reduces assembly time.

Cured resorcinol and resorcinol/phenol adhesives are known to withstand exposure that are in fact less detrimental to the glue than to most timbers. As assembly glues for wood they comply with the highest requirements of all specifications, including resistance to boiling water. They have acquired a reputation for making laminated beams and other load bearing structures used in buildings.



In dealing with performance of resorcinol adhesives it is usual to consider glued joints that have been cured at ambient air temperatures. With this in mind the following extract from a report is of interest; 'the durability of well-made phenol/resorcinol, and resorcinol resin glue joints, based on about 16 years experience, appears to be eventually equal to that of hot pressed phenol resin glue joint.

In order to obtain maximum durability in a wood glue component, the use of a preservative treated timber is essential and question of glueability therefore arises. In spite of the different nature of the substances used to impregnate timber, it may be conclude that, with certain qualifications, preservative treated timber can be glued satisfactorily with a resorcinol glue.

The use of resorcinol adhesive is not confined to wood. There is, however, little published information on their behavior with other adherents. Although they have been used for gluing metal, they can't in any way be considered as important metal adhesives. In metal to wood construction they have generally been successful for a number of years a resorcinol glue was used for bonding aluminum alloy sheet to ash framework for the roofs of London buses.

Addition Filler to Resorcinol Resins

There are no filler that are specifically suitable for use with resorcinol resins, so the remarks made in last article broadly apply here.

To incorporation of limited amounts of filler is often desirable in helping to reduce shrinkage stresses, retain the glue in the joint, and improve gap-filling properties. Moreover, a satisfactory gluing viscosity, for example 40-100 poises at working temperatures, demands a relatively high resin content if no filler are added. Commercial resorcinol adhesive therefore frequently contains some filler, either in the resin or in the hardener. In some applications it may be permissible and indeed advantageous for the user to add a further quantity of filler, perhaps also with some solvent. The incorporation of a small amount of filler does not lower the adhesive strength, indeed it frequently increases it, and moreover, also reduces the scatter of strength values which is a common features in any set of glued joint, whatever the adhesive. The use of pulverized fuel ash as a filler is interesting and has recently been favorably reported. The addition of white or pale colored filler or pigment reduces the dark color of a resorcinol resin to only a limited extent.

Curing at Elevated Temperature

Although most of the bonding that is done with resorcinol adhesives is carried out at ambient air temperature, elevated temperatures naturally accelerate the rate of curing. It is often asserted that it is inadvisable to cure a resorcinol resin above a temperature of about 70o-80o C. There would appear to be no fundamental reasons for such a temperature limit except that at high temperatures, the depolymerization of the para formaldehyde hardener is rapid, and the evolution of gaseous formaldehyde may result in foaming taking place in the glued joint. It is therefore desirable to liberate the formaldehyde more slowly; this may be accomplished by using as a source of formaldehyde methylol compounds such as methylol melamine of urea, or low molecular weight condensates derived from these which are relatively unstable and at elevated temperature split off formaldehyde. And at a temperature of 130oC or even higher, a source of formaldehyde and afford satisfactory and excellent adhesive strength.

Curing temperature is the temperature when the adhesive or resins is changed to their stabile structure. After over this temperature, the resin or adhesive form their own stabile condition and can't change or reprocess again. If the resin then formed a plastic material, then this plastic will have irreversible properties, the plastic itself called thermoplastic.

Next to Filler Adhesive.

Correlation PH and Mole of Formaldehyde

The relationship between pH and gelation time is important because, although the position and shape of the curve may differ with different resins, once the curve is established the information can be used to predict the change in pot-life that can be affected by altering the pH. The important part of the curve is between pH 6 and pH 9, and within these limits a pot-life variation of several hours can generally be obtained. The relationship, with regard to a pure resorcinol novolak and a resorcinol/phenol novolak, each with a para-formaldehyde hardener, is shown the graphic below.

Formaldehyde may be used in any form in which it can be made available to the resin under the curing conditions. In practice the polymeric solid form of formaldehyde known as para-formaldehyde is most frequently used as the hardening or curing agent. Aqueous solution of formaldehyde in an immediately available form, thereby coverting the novolak to resite and effecting complete cure of the resin at a lower temperature than is possible when using para-fromaldehyde. The rate of depolymerisation temperatures, and is further accelerated by acids and alkalis. The curing reaction may be exothermic depending on the novolak and the form in which the formaldehyde is added.

Formaldehyde denote as phenolic resin can be used to cure resorcinol glue, and moreover such a hardener can be used by separate application. There are certain advantages in using a phenolic resin a hardener, such as a reduction in the rate of drying out and the ability to cure at temperatures as low as 5 oC. Unfortunately we impossible make an inherently liquid resorcinol resin.

The Hardening of Resorcinol Adhesives.

The novolak resin is converted to a thermosetting resin through the addition of formaldehyde by the user, and herein lies the most important feature of a resorcinol adhesive and its difference from the ordinary phenol resin. Theoretically, sufficient formaldehyde is needed to bring the molar ratio of formaldehyde to resorcinol to the region 1:1. A final ratio of slightly less than 1:1 is adequate for curing a resin that is neutral or weakly alkaline, but in practice it is usual to have formaldehyde in slight molar excess, the actual amount desirable for cross linking depending on the characteristics of the novolaks.

Although it is commonly supposed that resorcinol formaldehyde adhesives are neutral, this is not always the case unless we interpret "neutral" rather widely. The limit for commercial products are usually between about pH 6 and pH 9, a range of values that can be harmful to few adherents.

The pot-life of resorcinol adhesives is influenced by pH, the relationship between pH and gelatine time (in this context the terms gelation times and pot-life are considered to have the same meaning) for more or less strandar resorcinol formaldehyde resin. The longest gelation time (minimum reactivity) to occur between pH 3 to 4 and the shortest gelation time to occur around pH 7 – 8.

The relation between pH and gelation time is important because, although the position and shape of the curve may differ with the different resins, once the curve is established the information can be used to predict the change in pot-life that can be effected by altering the pH. The important part of the curve is between pH 6 and pH 9, and within these limits a pot-life variation to a pure resorcinol novolak and a resorcinol/phenol novolak, each with a paraformaldehyde hardener.

Resorcinol Phenol Formaldehyde Adhesive

Resorcinol alone was used for some years as the phenolic substance. Later it was realized that the important properties of a resorcinol formaldehyde adhesive could be retained, and the cost reduced, by replacing part of the resorcinol with ordinary phenol. Cresol is less suitable but limited amounts of vegetable tannins may be used. All the earlier remarks relating to "pure" resorcinol, apply broadly to resorcinol/phenol adhesives, at least in the amounts in which phenol is normally used to replace resorcinol. The usual replacement is up to one half the molecular proportion of total phenolic constituents, but as much as 80% has been suggested. Without sacrificing too many desirable properties it is certainly possible to replace considerably more than one half, but the process of room temperature hardening after gelatin becomes progressively slower as the proportion of phenol is increased. Another disadvantage of using too much phenol is that the characteristic phenolic smell (completely absent with resorcinol) may be pronounced, even in the fully hardened glued joint.

There are two principal methods of preparing resorcinol/phenol resins; formaldehyde may be reacted with a mixture of resorcinol and phenol, or the resorcinol and phenol may be reacted with formaldehyde in separate stage. Both methods can lead to similar products; properties of the product depend much more on the molecular ratio and pH of the reaction than on anything else.