resols

Resols, or resole resins, are a class of phenol–formaldehyde resins produced under alkaline conditions with excess formaldehyde. They consist of phenol–formaldehyde condensates that contain hydroxymethyl groups and ether or methylene bridges between aromatic rings. The high formaldehyde content and basic catalyst promote further condensation, yielding resins that are liquid or low-molecular-weight solids at room temperature and that crosslink irreversibly upon curing.

Preparation typically involves reacting phenol with formaldehyde in an alkaline medium, using catalysts such as sodium hydroxide at moderate temperatures. The reaction forms methylol phenols that rapidly condense to higher-molecular-weight polymers. Because resols retain reactive methylol groups and excess formaldehyde, they are thermosetting and can cure by heating alone or with mild acid to drive condensation and water loss, forming a rigid network.

Applications for resol resins are wide-ranging. They are widely used as binders in foundry sands for molds and cores, providing good green strength and hot strength. They also serve as adhesives and binders in wood-based panels and plywood, as well as coatings, impregnants, and components in ceramic or refractory binders. In many formulations, resols are paired with curing agents or additives to tailor curing temperature and performance.

Resols are distinct from novolac resins, which have lower formaldehyde content and require an external curing agent (such as hexamethylenetetramine) to crosslink. The curing of resols tends to occur with heat and the presence of residual methylol groups. Health and safety concerns relate to formaldehyde emission during processing and early-stage curing, necessitating adequate ventilation and controls.