Thursday, May 29, 2008

Resorcinol Formaldehyde Adhesive

Resorcinol Adhesive have a greater importance than the phenolic (resol) glue that cures on addition of a strong acid, is the resorcinol (novolak) glue that cures on addition of formaldehyde. Like the less expensive acid-setting phenolic, the resorcinol glue also sets without heat, but unlike the phenolic glue it cures under neutral or near neutral conditions. This behavior is not appreciably affected if a substantial part of the recorcinol is replaced by ordinary phenol.

The reaction in relation to the preparation of resorcinol formaldehyde glues.
Resorcinol, like ordinary phenol is trifunctional in its reactions with formaldehyde, but having a second hydoxyl group in the meta position it is more powerfully directing in the para- and ortho- positions. The reactivity is such that equimolar proportions of resorcinol and formaldehyde will react by them selves at room temperature to produce a gel. Such one stage resins are, however, of small practical worth as adhesives and so commercial resorcinol glues are two stages resins; the first stage is carried out by the manufacturer in forming the novolak, and the second stage is carried out by the user in converting the novolak to reside by adding formaldehyde.

In the preparation of a novolak having good stability, the ratio of formaldehyde must be kept well below the point at which there is no likehood of gelation occuring, for example, 1.0-1.5 moles formaldehyde to 2.0 moles resorcinol. The reaction (a condensation reaction in which water is split off) is carried out with heating, and under acid or alkaline conditions; strongly acid conditions may be used even as low as pH 1, but strongly alkaline conditions should be avoided, the limit being about pH 9. Whether condensed under acid or alkaline conditions the resulting novolak resins are completely soluble in water. Acid condensed novolaks, especially those made under strongly acid conditions, generally have superior film forming properties. Provided the ratio of reactants and the pH of the system are within the limits needed to produce novolak, and equally important, provided all the factor together allow reaction to be satisfactorily controlled, then a fairly wide tolerance is permissible.

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