The condensation products of phenol and formaldehyde can be either potentially thermosetting and known as Resols or thermoplastic and known as Novolaks. Resol is a type of product that is formed when formaldehyde used in molar excess under alkaline condition. By reacting with sufficient additional formaldehyde under alkaline conditions, it is possible to convert a novolak to a resol. The two resol prepared in this way differ in certain physical properties, such as intrinsic viscosity, from a resol made by direct reaction of phenol and formaldehyde under alkaline conditions.
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