Phenolics

Phenol-formaldehyde resins or simply phenolics are prepared from the condensation of a mixture of phenol (i.e., carbolic acid) and cresols with formaldehyde as follows: n C6 H5 OH + n HCHO —> [—C6 H2 (OH)CH2 —]n + nH2 O Addition of formaldehyde ensures the formation of di- and trimethylolphenol, which later condense and polymerize rapidly.

Phenolics are cheap thermosets that exhibit good strength, heat stability, and impact resistance along with a good machinability. Chemically speaking, phenolics demonstrate high chemical resistance and moisture penetration, except towards strong alkalis.

Two processes are currently used: (i) In the one-stage process, formaldehyde, phenol and an alkaline catalyst are introduced into a stainless steel vessel and reacted together. The elevated ratio of formaldehyde to phenol allows the thermosetting process to take place without any addition of another cross-linking agent. After discharge, further heating terminates the polymerization yielding an insoluble and non fusible resin. (ii) In the two-stage process, formaldehyde, phenol and concentrated sulfuric acid are introduced in a stainless steel vessel with a low ratio of formaldehyde to phenol in order to prevent the thermosetting reaction from occurring during manufacture of the resin. After 4 hours at 150°C, separation of condensation water and resin occurs and overlying water is simply removed by vacuum pumping. At this point the resin is a viscous liquid termed novolac resin. Subsequently, an activator, hexamethylenetetramine, is incorporated into the material to complete the polymerization and yields the final thermoset in the cured state. Ground phenolic resin can be mixed with a plasticizer and fillers such as asbestos, graphite, or silica to give materials with desired properties.

Due to their chemical resistance, phenolics are widely used as linings and impregnating resins for chemical process equipment for handling strong acids. Other uses include brake linings, electrical components, laminates, glues, adhesives, molds and binders.