Cure Terms Glossary

Novolak (Novolac)

A novolak is a phenol-formaldehyde condensation prepolymer obtained by using a stoichometric deficiency of formaldehyde. 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. To avoid full cure, novolaks are produced with a molar excess of phenol - typically in the region of 25% excess (e.g. 0.8:1 formaldehyde to phenol.

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. Novolak resins are prepared under acid catalysis to ensure complete conversion of methylol groups (and a storage-stable product). The resins are virtually devoid of methylol groups and are effectively methylene-bridged polyphenols. Novolak resins are solid.

Novolaks require the action of a curative (hardener in thermoset terminology) to generate the final product. If hexamethylenetetramine is the curative, then no additional catalyst is required, although an accelerator may be used. Hexamethylenetetramine is the usual curative - its action, in part, may be considered as the supply of formaldehyde (to restore the deficiency of the prepolymer step. Simplistically, the action of hexamethylenetetramine may be considered as the reverse of its manufacture (from formaldehyde and ammonia).

Such a simplistic scheme belies chemistry of some complexity, and it is quite likely that ammonia-formaldehyde adducts such as HN(CH2OH)2 are involved, and that some —CH2NHCH2— linkages are present in the crosslinks. The presence of ammonia and the organic bases provides for in situ catalysis.

Novolak-based compounds provide phenolic moulding powders.

Casting, bonding or laminating resins are prepared in a different way - these compounds are based on (liquid) resole phenolic resins.