Phenolic Resin Varnish

any of the varnishes consisting of phenolic resins (chiefly, phenol-formaldehyde resins) and various products of the resins’ modification.

The solvents for phenolic resin varnishes include ethanol and certain hydrocarbons. Phenolic resin spirit varnishes are made with resol and novolac resins. Resol varnishes, referred to as varnishes of the Bakelite type, form coatings characterized by high hardness, good electrical insulation properties, and resistance to water, acids, salts, oils, and organic solvents. Among the disadvantages are poor adhesion to metals, dark color, low resistance to alkalies and oxidizing agents, and brittleness. The last of these is reduced by plasticization with, for example, phthalates. Spirit varnishes based on novolacs are preferred to shellac varnishes in the finishing of wood products. The use of these varnishes is limited because the coatings darken upon exposure to light.

The products of the reaction of phenol-formaldehyde resins with vegetable oils, the latter serving to plasticize the resins, have found broad use as film-forming substances. The solvents for these varnishes are solvent naphtha, petroleum spirits, and turpentine. The resins that are compatible with vegetable oils are obtained by modification of the resol resins with rosin, followed by etherification, for example, with glycerol (the products here being known as artificial copals). They are also obtained by the synthesis of resins from alkylphenols (butylphenols, amylphe-nols) and the etherification of resol resins by butyl alcohol (resin butanolization). Artificial copals and butanolized resins form the basis of preservative varnishes. Alkylphenol-formaldehyde resins are used in making primers for surfaces exposed to tropical conditions.

Water-soluble paint and varnish materials obtained by, for example, the cocondensation of phenol-formaldehyde resins with oils or with alkyd or polyacrylic film-forming substances constitute a special type of phenolic resin varnish. These materials, which form corrosion-resistant coatings, are used when the application is effected through electrodeposition.