Cure Terms Glossary

Resole

A resole is a B-stage phenol-formaldehyde condensation polymer obtained by thermally quenching the cure just short of the gel time. Phenol has a functionality of up to three (if all ortho and para groups are accessible) and formaldehyde has a functionality of two.

Thus, in principle, the ideal molar stoichiometry for full cure is 1.5:1 formaldehyde:phenol. In practice only a slight molar excess of formaldehyde is required to provide a crosslinking polymerisation and resoles typically use 1.1-1.5 moles of formaldehyde for each mole of phenol.

Phenol-formaldehyde condensations can operate under acid or basic catalysis; acids activating the formaldehyde and bases activating the phenol. The reaction passes through intermediate methyol groups which rapidly condense under acidic conditions but are more stable under basic conditions.

The initial reaction of phenol and formaldehyde gives a methylolphenol (ArCH2OH) and it is the subsequent attack of this on further aromatic species (e.g. ArCH2OH + ArX → ArCH2ArX + H2O) which gives rise to the condensation. The reaction of formaldehyde with aromatic rings is amenable to acid or basis catalysis, whilst that of methylol groups is particularly susceptible to acid catalysis. Resole resins are generally prepared under basic catalysis - which, to a degree, helps stabilise a methylol-rich intermediate cure state. In effect these are phenolic alcohols and resole resins are liquid. Their shelf life is usually short - perhaps no more than a couple of months.

As liquids, resoles are particularly useful for casting, bonding, lamination or for foam manufacture. Phenolic foams use physical blowing agents.