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 2001, worldwide consumption of phenolic resins exceeded 4 × 10 6 t; 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 concerns of phenolic resins are related primarily to the presence of unreacted phenol and formaldehyde. Unreacted phenol in a resin can range from >10% for liquid resoles used in impregnation processes to <1% for novolaks intended for use as epoxy hardeners. Free formaldehyde can be 2–4% in liquid adhesives.

Structure of phenol-formaldehyde polycondensates

“…This phenomenon can be substantiated with the strong tendency of oxymethylene derivatives to exist in equilibrium with quinone methides [4]. For unsymmetrical dihydroxydibenzyl ethers one possibility is depicted in Scheme 1.…”

“…It means that the resins contain about 2.25 moles of methylene containing functional groups to three aromatic positions. This is not surprising as there are many different reaction possibilities for the very active F and HM groups [4]. The role of side reactions (e.g.…”

“…Quite different resin structures can be obtained depending on the molar ratio of phenol to formaldehyde (P/F) and catalyst [4]. The reason is the two-step mechanism where polycondensation in participation of hydroxymethylphenols (HMP) should be regarded as * Corresponding author, jane@vkg.ee reaction with a new monomer different from initial F. The base-catalysed P/F reaction is more thoroughly studied because of practical importance.…”

“…It is clear that phenoxide ions both from P and HMP catalyse reactions [6]. Their quinoid transition states are shown as intermediate species for proton transfer but on 13 C NMR spectra only hydroxymethyl and methylene groups can be assigned [4].…”

Characterisation of a phenolic resin and sugar cane pulp composite

“…Phenolic resins are some of the principal thermofixed synthetic polymers, the third most important polymeric matrix for composites (Lubin, 1969), and are also known for their high temperature resistance (Knop and Pilato, 1985;Kopf and Little, 1991). Pure phenolic resin can be obtained through the condensation reaction between phenol (C 6 H 5 OH) and formaldehyde (CH 2 O), producing methylene bridges between the phenol molecules ( Figure 1).…”