Base-Catalyzed Phenol-Formaldehyde Condensation Resols

Base-Catalyzed Phenol-Formaldehyde Condensation (Resols) 

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Resoles are usually mixtures of a number of methylol phenols, with small amounts of higher condensation products involving methylene and benzylic ether linkages. The kinetics of base-catalyzed phenol-formaldehyde reactions have been extensively researched. At high dilution and at a pH not exceeding 10, the rate equation can be expressed by Eq. (13), where [PhO ] denotes the concentration of phenoxide anion and [F] the concentration of unreacted formaldehyde, determined titrimetrically. " ... 

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-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). 

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. 

Formaldehyde reacts with phenol by electrophilic substitution at the 2-, 4- and 6-positions of phenol and will subsequently condense, forming a densely cross-linked network. This reaction can be catalyzed by acid as well as base catalysts. The nature of the product obtained is largely dependent on the type of phenol, the molar ratio of formaldehyde to phenol (f p) and the catalyst used. Phenolic resins are mainly divided into two broad classes resoles and novolacs. 

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