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The phenolic resins most widely used are low molecular weight butylated resols, which contain phenolic hydroxyl groups and etherified and unetherified methylol groups. The epoxy resins used have a molecular weight of 3000-4000 and therefore contain secondary hydroxyl groups.

Phenol-formaldehyde resin is prepared as follows: C6H5OH+H2C=O ---> [-C6H2(OH)CH2-]n One-Stage Resins. The ratio of formaldehyde to phenol is high enough to allow the thermosetting process to take place without the addition of other sources of cross-links. Two-Stage Resins. The ratio of formaldehyde to phenol is low enough to prevent the thermosetting reaction from occurring during manufacture of the resin. At this point the resin is termed novolac resin. Subsequently, hexamethylenetetramine is incorporated into the material to act as a source of chemical cross-links during the molding operation (and conversion to the thermoset or cured state).