First Steps in the Formation of Phenolic Resins

The first step in the formation of phenolic resins is the addition of formaldehyde to phenol, giving p- or o-methylol phenol. This reaction 

The intermediate formation of o-methylol phenol can be shown to occur by using an excess of formaldehyde, in which case the products of the phenol alcohol that can be isolated are the acetal. 

With reference to the addition reaction and the subsequent condensation reaction, phenol is trifunctional (two ortho and one para positions) and formaldehyde is bifunctional. Therefore, to obtain non-cross-linked products, the molecular weight has to be kept low, which can be achieved by using less than the theoretical amount of formaldehyde. It has been found empirically that a formaldehyde/phenol molar ratio of 0.75 1 must not be exceeded in order to avoid the formation of a cross-linked product. This molar ratio corresponds to an equivalent ratio of the functional groups of r = (2 X 0.75) (3 x 1) = 0.5. Of course, theoretically, the equivalent ratio should be 0.5 in the absence of intramolecular reactions and with quantitative reaction at all the phenol reactive sites. Yet this agreement is fortuitous, since theory cannot correspond exactly with experiment because of the occurrence of intramolecular reactions, the nonequivalence of o- and p-phenol sites in these irreversible reactions, and the formation of open-chain formal structures. Correspondingly, the so-called pseudonovolaks, the o- and p-alkyl phenols, are only bifunctional. They do not give a cross-linked product with an excess of formaldehyde and therefore cannot be cured. 

The acid-catalyzed reaction is exothermic to a considerable degree, with an overall reaction enthalpy of A// = —98.5 kJ/mol. The addition reaction (addition of formaldehyde) contributes —20 kJ/mol and the condensation reaction (formation of methylene cross-links) contributes -78.5 kJ/mol. 

In the phenol/formaldehyde reaction, the rates for the addition and condensation reactions are in the ratio of 1 42. The overall activation energy is 84-100 kJ/mol. The p site in this acid-catalyzed reaction is about 2.4 times more reactive than the o site of the phenol. In general, then, p-methylol phenols are produced, but these are not as attractive commercially as the o,o -methylol-rich novolaks. The curing of novolak should, of course, occur as quickly as possible, which assumes that the reactive p sites are available for reaction (see below). 

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