Лако-красочные материалы — производство

Phenolic resins of both types — novolacs and resoles — are frequently chemically modified to improve their mechanical properties and to make them more suitable for coating applications. Rosin modi­fied phenolics have been widely used in oleoresinous varnishes and printing inks. They are prepared by reacting resole type phe­nolics (~10 to 20 % by weight) with rosin at high temperature (up to 270 °C). The phenolic and alcoholic hydroxyl groups of resoles react with unsaturated double bonds in rosin by what is known as chro- man ring formation. The product is then esterified with polyhyd — ric alcohols, such as glycerol or pentaerythritol, to reduce the acid number and build MW. Similarly, resoles from alkylated phenols are reacted to olefinic sites of drying oils or alkyds based on drying oils, and such products are useful in formulating spar varnishes and insulating varnishes.

Another important group of phenolic resins are those derived from cardanol, a substituted phenol obtained from a natural material, cashew nut shell liquid (CNSL). CNSL has a polyunsaturated C15 alkyl chain substituted at the m-position of phenol. The phenolic resins derived from cashew nut shell liquid have oxidative curing properties. Despite their dark color, they are very popular binders for anticorrosive air-drying primers.

In phenolic resins, the acidic phenol groups remain chemically unchanged even after chemical cross-linking, making their coa­tings sensitive to alkali. To obtain alkali-stable products, chemical modifications involving blocking of the phenolic hydroxyl, such as with acetyl chloride or allyl chloride, are used. Coatings produced from such resins have even greater chemical resistance and much reduced alkali sensitivity.

Diphenolic acids, a product from levulinic acid and phenol, are used to make water-dispersible phenolic resins with good storage sta­bility after neutralization of their carboxylic acid groups with a fugitive base.