Phenolic resin bonds, in particular phenol-formaldehyde resin bonds, are the most common resin bonds; tools made of this bonding type represent the largest market segment for conventional wheels after vitrified tools [KREB06, KLOC05a, p. 65, MARI07, p. 119]. Originally, this bond type was known as bakelite and for this
Furfural
Fig. 3.3 Structures of phenol, relevant aldehydes, and phenolic resin [GARZ00, p. 8, 14, 21, HARR89] reason retains the letter “B” in many wheel specifications [MARI04, p. 413, GARZ00, p. 318]. In comparison to other resins, phenolic resin is less expensive and easier to mold [ASAM10, p. 310].
Phenolic resins are obtained by the reaction of phenol and aldehyde [GARZ00, p. 3, 111]. Phenols are aromatic compounds with the hydroxyl group bonded to the aromatic nucleus (Fig. 3.3) [GARZ00, p. 3]. The phenol synthesis is commonly done by the cumene process, an oxidation process of cumene (isopropylbenzene) and air to cumene hydroperoxide, which is cleaved to phenol and acetone [GARZ00, p. 5, 8]. Safety is a critical aspect for plant design and operation, because the oxidation takes place close the flammability limit and cumene hydroperoxide is an unstable material [GARZ00, p. 8].
Relevant aldehydes for bond production of abrasive tools are formaldehyde, furfural, and hexamethylenetetramine. Formaldehyde is a hazardous chemical with potential eye, nose and throat irritation above a certain concentration (Fig. 3.3) [GARZ00, p. 14]. Hexamethylenetetramine, also called hexamine, is a common hardener in phenolic resin bonds [KREB06, HARR89] (Fig. 3.3). Basic oxides such as calcium oxide or magnesium oxide are curing accelerators in phenolic resins [GARZ00, p. 321].
The mode of catalysis and molar ratio of phenol and aldehyde result in a resin that is either of resole type or novolak type [GARZ00, p. 24]. Resoles are easily cured by acid, base, or thermal conditions; novolaks are cured with formaldehyde from hexamethylenetetramine, solid resoles, or other methods [GARZ00, p. 61]. Resoles occur either as solid resole, resole solution or aqueous resole; novolaks appear as solid resins, novolak solutions, aquenous novolak dispersions, and powder resins with hexamethylenetetramine [GARZ00, p. 122]. For the manufacturing of abrasive tools, aquenous resole and powder novolak are the most important forms [GARZ00, p. 122].
The phenolic resin bond for grinding wheels contains liquid (resole) and powdered (novolak), straight and modified phenolic resins, powdered resins with wetting agents, or low melting phenolic resin combined with powdered phenolic
resin [GARZ00, p. 323]. Various modifications with epoxy resins, rubber, polyvinyl butyral, etc. are possible [GARZ00, p. 325]. Furthermore, phenolic resin bonds for superabrasives are enhanced by SiC grits and solid lubricants [METZ86, p. 54].
Phenolic resins are cured at around 150-200 °C through polycondensation [KREB06, MARI04, p. 413]. Gardziella et al. give detailed compositions of the single liquid and powdered resins used for the production of abrasive tools [GARZ00, p. 324 and 326].