Novolaks are normally made under acid conditions, when the formation of methylol compounds is slow, whereas the conversion of methylol compounds to diphenylmethanes is rapid. Thus, with a molar excess of phenol, methylol group do not accumulate in the reaction mixture and the resulting molecules are linear (II and V), giving a permanently fusible resin, a novolak. The ratio of formaldehyde to phenol is usually 0.6 - 1,0 and strong acids, for example para-toluene slphonic acid, oxalic and sulfuric acid are common catalysts.
In the preparation of the potentially thermosetting (resol) adhesives, a trifuctional phenol is required in order to produce breached polymers capable of forming a three dimensional cross linked network. Thus, phenol itself is most frequently used; mixed cresol (chiefly: meta and para isomers) and mixed xylenols may be used to replace part of or all of the more expensive phenol, with some loss of functionally. The narrowing of the price gap over recent years has made the use of mixed cresols less attractive.
In the production of resol adhesives the ratio of formaldehyde to phenol ranges from about 1:1 to 3:1. The commonest catalyst are strong alkalis such as sodium hydroxide, ammonium is also employed. Under alkaline conditions, the formation of methylol compound is rapid, but the condensation reaction to give diphenylmethanes is comparatively slow. Thus with a molar excess of formaldehyde, mono-, di-, and trimethylol compounds tend to accumulate in the reaction mixture, together with polynuclear compounds (eg. III and VI) resulting from further condensation.
At this point, the resol is described as being in in the "A" stage, being easily melted and soluble in a number of organic liquid, in particular the lower alcohols and ketones.