Phenol resins are used as binding agents for various compound materials where high thermal stability is necessary in the final product. They are obtained by polymerization of phenolic compounds bridged by methylene or methyl ether groups.
The phenolformaldehyde resins are used as binders for various composite materials where is requested a high thermal stability of the final product. They are made by polymerizing phenolic compounds bonded through either methylene bridges or methyl ethers. There are two types of phenolformaldehyde resins: resols and novolacs. Resols have hydroxymethyl groups and cross-links without adding cross-linking agents by forming water so that are also more unstable than novolacs, while novolacs need a cross-linking agent which is thermally activated so that in powder form are especially suitable to dry mixing. Hexamethylene tetramine (HEXA) is usually utilized as cross-linking agent to novolacs.