Although the phenolic tannins of vegetable origin consist of both monohydridic and polyhidric molecular structure, it is more appropriate to discuss them in this section than under resorcinol formaldehyde "R.F." adhesive. The later and also resorcinol/phenol adhesive are discussed in the next section.
Condensation products of phenol and formaldehyde can be either potentially thermosetting and known as "Resols," or thermoplastic and known as "Novolaks."
A resol is the type of product formed when formaldehyde is used in molar excess under (normally) alkaline condition, while a novolak is the type of product formed when phenol is used in molar excess under (normally) acid conditions. By reacting with sufficient additional formaldehyde under alkaline conditions, it is possible to convert a novolak to a resol. The two-stage resol prepared in this way differ in certain physical properties, such as intrinsic viscosity, from a resol made by direct reaction of phenol and formaldehyde under alkaline conditions. The Novolak and Resol reactions in relation to the preparation of phenolic resin glues
The reaction of phenol and formaldehyde
The product of the first reaction between phenol and formaldehyde is either ortho- or para-monomethylol phenol (I), which can subsequently react with more formaldehyde to yield di- and tri-methylol phenol. In the ensuing reaction, a methylol group of one molecule may react either with the nucleus of a second phenolic molecule splitting of water to form a dihydroxydiphenylmethane type of compound (II and III) or it may react with a methylol group attached to another phenolic molecule to form a dihydroxydibenzyl ether (IV), water again being eliminated. Under both acid and alkaline condition, this ether in turn splits off formaldehyde to form a methylene linked compound, the formaldehyde becoming available for further reaction.
CH-R---CH2OH or OH-R-CH2OH (I)
OH-R---CH2OH---CH2R-OH (II)
OH-R---CH2OH---CH2-OH-R-CH2OH (III)
OH-R—CH2OCH2-R-OH (IV)
OH-R—CH2-R-OH)n-R-OH (V)
The extent to which these reactions take place depends on the ratio of phenol to formaldehyde, the temperature, the pH, and the catalyst.