Acid-Catalyzed Phenol-Formaldehyde Condensation Novolaks

Acid-Catalyzed Phenol-Formaldehyde Condensation (Novolaks) 

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Formation of novolak involves an acid-catalyzed reaction of formaldehyde with excess phenol (i.e., formaldehyde-to-phenol mole ratio less than 1). The initial methylol phenols condense with the excess phenol to form dihydroxydiphenyl methane, which undergoes further condensation yielding low-molecular-weight prepolymer or novolak. Unlike resoles, novolaks do not contain residual methylol groups. They are fusible and insoluble. 

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). 

Resols are solid materials which are obtained from the primary oily condensation products of phenols and alkylphenols with formaldehyde (catalyzed by alkali) by heating to about 120° C. Further heating leads to Resitols , the product of final curing is called Resit. Novolaks are the primary condensation products if acids are used as catalyst. 

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