Phenolic Resins

Abstract Phenolic resins are a family of polymers and oligomers composed of a wide range of structures based on the various reaction products of phenols with formaldehyde. Applications vary from commodity construction materials to high technology applications in electronics and aerospace. Phenolic resins are prepared using acidic or basic catalysts. Weak Lewis acids, such as zinc acetate, are used for specialty resins. Phenolic novolaks are thermoplastic resins prepared from acid catalysts and a molar excess of phenol. They have a molecular weight of 500–5000 and a glass‐transition temperature, T g , of 45–70°C. Novolak resins are typically cured with 5–15% hexamethylenetetramine as the cross‐linking agent. Resole‐type phenolic resins are produced with a molar ratio of formaldehyde to phenol of 1.2:1 to 3.0:1 and alkaline catalysts such as NaOH, Ca(OH) 2 , and Ba(OH) 2 . A typical resin has an initial molecular weight of 150 to perhaps 1500. For systems of unsubstituted phenols, the final cross‐link density is 150–300 atomic mass units per cross‐link. In 1993, worldwide consumption of phenolic resins exceeded \documentclass{article}\usepackage{amssymb}\pagestyle{empty}\begin{document}${3\times 10^{6}{\hskip0.167em}{\hskip0.167em}{\rm{t}}}$\end{document} ; slightly less than half of the total volume was produced in the United States, where the largest‐volume application is in plywood adhesives and accounts for ca 49% of U.S. consumption. As a mature industry, U.S. production of phenolic resins has paralleled the growth in the GNP. Applications for phenolic resins include coatings, adhesives, carbonless copy paper, molding compounds, bonded and coated abrasives, friction materials, foundry resins, laminates, air and oil filters, wood bonding, fiber bonding, composites, spherical fillers, and fibers. Factors contributing to the health and safety of phenolic resin are related primarily to the presence of unreacted phenol and formaldehyde. Unreacted phenol in a resin can range from \documentclass{article}\usepackage{amssymb}\pagestyle{empty}\begin{document}${>{\hskip-0.167em}{\hskip-0.167em}10{\%}}$\end{document} for liquid resoles used in impregnation processes to \documentclass{article}\usepackage{amssymb}\pagestyle{empty}\begin{document}${<{\hskip-0.167em}{\hskip-0.167em}1{\%}}$\end{document} for novolaks intended for use as epoxy hardeners. Free formaldehyde can be 2–4% in liquid adhesives.