Phenol-formaldehyde resins base-catalyzed

Phenol-formaldehyde resins are the oldest thermosetting polymers. They are produced by a condensation reaction between phenol and formaldehyde. Although many attempts were made to use the product and control the conditions for the acid-catalyzed reaction described by Bayer in 1872, there was no commercial production of the resin until the exhaustive work by Baekeland was published in 1909. In this paper, he describes the product as far superior to amber for pipe stem and similar articles, less flexible but more durable than celluloid, odorless, and fire-resistant. ° The reaction between phenol and formaldehyde is either base or acid catalyzed, and the polymers are termed resols (for the base catalyzed) and novalacs (for the acid catalyzed). 

Phenol-formaldehyde reactions, 399, 380 base-catalyzed, 400-402 Phenol-formaldehyde resins, modified, 410-411... 

A Friedel-Crafts-type reaction of phenols under basic conditions is also possible. Aqueous alkaline phenol-aldehyde condensation is the reaction for generating phenol-formaldehyde resin.34 The condensation of phenol with glyoxylic acid in alkaline solution by using aqueous glyoxylic acid generates 4-hydroxyphenylacetic acid. The use of tetraalkylammonium hydroxide instead of sodium hydroxide increases the para-selectivity of the condensation.35 Base-catalyzed formation of benzo[b]furano[60]- and -[70]fullerenes occurred via the reaction of C60CI6 with phenol in the presence of aqueous KOH and under nitrogen.36... 

Condensations with carbonyl compounds phenol-formaldehyde resins. Acid or base catalyzes electrophilic substitution of carbonyl compounds in ortho and para positions of phenols to form phenol alcohols (Lederer-Manasse reaction). 

Phenolic polymers and phenol-formaldehyde resins are of great commercial interest for a number of electronic and industrial applications (7). However, there have been serious concerns regarding their use due to various toxic effects of formaldehyde and harsh synthesis environments (2). Peroxidase-catalyzed oxidative polymerization of phenol and substituted phenols provides an alternate route for the synthesis of phenolic polymers (3,4), The increased interest in this type of enzyme-based polymerization is mostly due to its environmental compatibility and potential for producing industrial polymers in high yield (5). 

The particleboard copolymer resin was applied at an 8% rate based on oven dry weight as the binder for flakeboards. A commercial phenol formaldehyde resin, catalyzed with resorcinol resin and extra sodium hydroxide was similarly used as the control resin binder for flakeboard evaluation. The density was 48 pounds per cubic foot for oak flakeboards and 43 pounds per cubic foot for pine. The oak flakeboards were pressed at a platen temperature of 350°F, however, the temperature had to be dropped to 335 F for the pine flakeboards due to a deficiency of steam pressure at the time they were to be made. The press times were ranging from 60 seconds to 120 seconds (push button close to push button open). 

Furan no-bake resins are two-component, r.t.-cur-ing, acid-catalyzed systems of furfiiryl alcohol prepolymer with additional monomer. Hot and warm box resins are mainly based on urea-modified fur-ftiryl alcohol-formaldehyde condensates that are cured at 100-170 °C. Also phenol-formaldehyde resins, modified with furfiiryl alcohol, are used. In cold box binder systems, the sand is mixed with a low-viscosity fiiran resin and a peroxide. The core is formed, and SO2 is blown into or generated in situ in the sand to cure the resin rapidly. 

Stopping the polymer at this point requires the ratio of formaldehyde to phenol to be less than unity. Both methylene and ether bridges are known to be present. The reaction is either acid or base catalyzed, and branching is uncommon at this stage. The products are variously known as A stage resins, novolacs, or resole prepolymers. 

This reaction is carried out under base-catalyzed conditions and with a formaldehyde/phenol ratio greater than unity. The resulting product is called a C state resin or resite. 

Phenolic resins were discovered by Baeyer in 1872 through acid-catalyzed reactions of phenols and acetaldehyde. Kleeberg found in 1891 that resinous products could also be formed by reacting phenol with formaldehyde. But it was Baekeland who was granted patents in 1909 describing both base-catalyzed resoles (known as Bakelite resins) and acid-catalyzed novolac products.2... 

The high-molecular-weight products formed by the condensation of phenols with carbonyl compounds (especially with formaldehyde) are known as phenolic resins. They are mixtures of structurally nonuniform compounds that are initially soluble and fusible but which can become crosslinked (cured) by subsequent reactions. One distinguishes between acid- and base-catalyzed condensations, since they lead to different end products the properties of the condensation polymer are also affected by the mole ratio of phenol to formaldehyde. 

Phenolic Resins. Phenolic resins [9003-354] (qv) are thermosets prepared by the reaction of phenol with formaldehyde, through either the base-catalyzed one-stage or the acid-catalyzed two-stage process. The liquid intermediate may be used as an adhesive and bonding resin for plywood, particle board, fiberboard, insulation, and cores for laminates. The physical properties for typical phenolic laminates made with wood are listed in Table 1. 

The functionality may vary with reaction conditions. For example, in base-catalyzed copolymerization of phenol and formaldehyde, both monomers are bifunctional at ambient temperature, but phenol becomes trifunctional if the temperature is raised sufficiently. Copolymerization at ambient temperature can produce essentially linear, liquid, resole-type "prepolymers" of low molecular weight. Upon acidification and heat-curing, methylene and ether crosslinks formed by the now trifunctional phenol units transform the polymer into an insoluble resin [7] (see next page). The original Bakelite was such a "thermosetting" product. 

Acid Catalyzed Condensation Polymerizations. The strong protonic acids produced by the photolysis of onium salts I-III can also be employed to catalyze the condensation of phenolic, melamine, and urea formaldehyde resins. Very durable photoresists based on these inexpensive and readily available resins can be made. Such resists generally require a postbake prior to development to complete the condensation and to enhance image formation. 

The phenolic resins are condensation products of phenol and formaldehyde [144-146, 148]. These materials were among the earliest commercial synthetic plastics. Two different methods [144-146] are used to prepare them. In the first one, the condensations are base catalyzed, while in the second one, they are acid-catalyzed. The products formed with basic catalysts are called resols and with acidic ones novolacs. Phenolic resins are used widely in coatings and laminates. The pure resins are too friable for use as structural materials by themselves. They become useful plastics, however, when filled with various fillers. 

Epoxy NovolaC Resins. Epoxy novolacs are multifunctional epoxies based on phenolic formaldehyde novolacs. Both epoxy phenol novolac resins (EPN) and epoxy cresol novolac resins (ECN) have attained commercial importance (53). The former is made by epoxidation of the phenol-formaldehyde condensates (novolacs) obtained from acid-catalyzed condensation of phenol and formaldehyde (see Phenolic Resins). This produces random ortho- and para-methylene bridges. 

There are two types of phenol-formaldehyde condensation polymers resoles and novolacs (117). Phenol-formaldehyde polymers prepared from the base-catalyzed condensation of phenol and excess formaldehyde are called resoles. In most phenolic resins commonly used with epoxies, the phenolic group is converted into an ether to give improved alkali resistance. At elevated temperatin-es (>150°C), resole resins react with the hydroxyl groups of the epoxy resins to provide highly cross-linked polymers. 

Phenol has unique chemical properties due to the presence of a hydroxyl group and an aromatic ring, which are complementary in that they facilitate both electrophilic and nucleophilic reactions. The aromatic ring of phenol is highly reactive towards electrophilic snbstitntion, which assists its acid-catalyzed reaction with formaldehyde. Phenol is a weak acid and easily forms sodium phenoxide (NaPh) in a base-catalyzed medinm. In the presence of sodium phenoxide, the nucleophilic addition of the phenolic aromatic ring to the carbonyl group of formaldehyde occurs. Thus, phenol can react with formaldehyde under acidic or basic conditions, leading to either novolac or resole resins (Weber and Weber, 2010). 

These aldehydes react on acid condensation with phenols to give novolac-type products. Base-catalyzed condensation is not practical with acetaldehyde since it undergoes rapid aldol condensation and self-resinification reactions. Acid condensations involving acetaldehyde or its trimer paraldehyde and phenol give soluble and permanently fusible resins, comparable to the novolacs. Aldehyde with no a hydrogens react in a manner similar to formaldehyde ... 

Phenolic resins are used in paper-, cloth-, and synthetic-backed products. These bond systems give the greatest strength and heat resistance compared to conventional adhesives. Today s high performance coated abrasive products would not be possible without the use of phenolic resins. The typical systems are base catalyzed and water miscible, having water tolerances in the range of 50-300%. Most phenolic make d size bonds are blends of two or more resins, usually a high (e.g., 1.8) and a low (e.g., 1.1) F/P ratio (formaldehyde-to-... 

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