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any of the oligomers produced by the poly-condensation of phenol, homologues of phenol (cresols, xyle-nols), or polyhydric phenols (such as resorcinol) with aldehydes (formaldehyde and furfural). Phenol-formaldehyde resins, which are obtained from phenols (mainly, phenol itself) and formaldehyde, have the greatest practical importance. Depending on the relative amounts of the reactants and the nature of the catalyst, the reaction produces either thermoplastic resins (novolacs) or thermosetting resins (resols). Thus, in the presence of acid catalysts (usually hydrochloric or oxalic acid) and an excess of phenol, novolac resins are obtained; in the presence of basic catalysts, such as NaOH, Ba(OH)2, and NH4OH, and an excess of formaldehyde, resols are formed.
Novolac resins are for the most part linear oligomers, the phenol rings being joined by methylene radicals (as in I below); they contain almost no methylol groups (—CH2OH). Resols are mixtures of linear and branched oligomers (II) containing a large number of methylol groups capable of undergoing further transformations.
Novolac resins are produced batchwise and in continuous processes, while resols are produced only in batches. Production includes stages of polycondensation, carried out at the boiling point of the mixture (90°–98°C), and drying, carried out at a residual pressure of 13.30–19.98 newtons/m2 (100–150 mm Hg). The temperature at the end of the drying stage is 120°–130°C for novolac resins and 90°–105°C for resols. Novolac resins are produced as solids (glassy lumps, flakes, and granules), and resols as solids and liquids.
Novolac resins and resols (molecular weights, 600–1,300 and 400–1,000, respectively) are readily soluble in alcohols and acetone; their color depends on the type of catalyst used in production and ranges from light yellow to reddish. Phenol-formaldehyde resins harden during processing upon heating; with novolac resins, however, a hardener is required (usually hexamethyl-enetetramine, in an amount equal to 6–14 percent of the weight of the resin). The hardening of resols involves an initial (A), an intermediate (B), and a final (C) stage. In the A stage, the physical properties of the resin (resol) are analogous to those of novolac resins since resol dissolves and melts. In the B stage, the resin (resitol) has the ability to soften upon heating and swell in solvents. In the C stage, the resin (resite) neither melts nor dissolves.
The hardened resins are characterized by high resistance to heat, water, and acids and good dielectric properties; combined with fillers, they have great strength. Hardened novolacs are inferior to resites in resistance to heat, water, and chemical action and in dielectric properties.
To effect desired changes in the properties of phenol-formaldehyde resins, components capable of reacting with phenol and formaldehyde are introduced during production. Thus, the dielectric properties and water resistance are improved upon the introduction of aniline, and photostability is improved with the addition of urea. Rosin and tert-butyl alcohol impart the capacity to dissolve in nonpolar solvents and combine with vegetable oils; resins of this type are commonly used in making phenolic resin varnishes. Phenol-formaldehyde resins are combined with other oligomers and polymers. Polyamides, for example, increase elasticity and resistance to heat and water; polyvinyl chloride improves water resistance and resistance to chemicals; natural rubbers increase impact strength; and polyvinyl butyral improves adhesion (resins of this type being used for butvar-phenolic adhesives). Phenol-formaldehyde resins are used in the hardening of epoxy resins in order to impart higher resistance to heat, acids, and alkalies. The former are the resins most commonly used in the production of many types of plastics. Novolacs are used to obtain molding powders, and resols to obtain molding powders and plastic laminates. Plastic foams and honeycomb plastics are made from novolacs and resols.
REFERENCEEntsiklopediiapolimerov, vol. 3. Moscow, 1977.
G. M. TSEITLIN