Novolaks

Novolaks, or novolak resins, are a family of phenol-formaldehyde resins produced by acid-catalyzed condensation of phenol with formaldehyde, yielding relatively low molecular weight polymers. Unlike resol resins, which cure automatically under heat in basic conditions, novolaks are designed to cure only in the presence of a crosslinking agent, forming a hard, thermosetting network.

Manufacture typically uses a phenol-rich formulation under acidic conditions, which produces oligomers with terminal reactive groups. The resin is often handled as a liquid or low-melting solid and remains thermoplastic until cured. Curing is accomplished with external hardeners such as hexamethylenetetramine (HMTA) or other aldehyde or amine-type hardeners, with elevated temperatures facilitating crosslinking.

Novolaks can also be converted into epoxy novolac resins by reaction with epichlorohydrin, yielding epoxy-functional derivatives used in high-performance coatings and composites. This versatility makes novolaks a key binder in various applications.

Applications are widespread in wood products as binders for plywood, particleboard, and fiberboard, as well as in laminates and some paper coatings. They are valued for heat and chemical resistance and for high gloss, though cured products can be brittle and formaldehyde emissions may be a concern during processing and aging, requiring appropriate handling and ventilation.

Compared with resol resins, novolaks offer greater formulation flexibility because curing can be tailored with different hardeners, enabling a range of processing temperatures and end-use properties.