Typical Commercial Preparations

The resols are usually prepared in typical reaction kettles, using 1.5–2.0 moles of formaldehyde per mole of the phenol. The reactions are rapid and the condensations to resoles might be accomplished in 1 h. Formaldehyde is often added in the form of formalin. The quantity of the added caustic or ammonia might comprise one percent of the phenol in the reaction mixture. These reactions are carried out at water reflux for a specified time. The pH is then lowered to neutral and the water is distilled off, usually at reduced pressure. The progress of the condensation is followed by measuring the melting point, the gel time (time required for the material to become thermoset at a specified temperature), solubility, or free phenol content. Better quality novolacs and resoles are prepared in stainless steel resin kettles. For novolacs, a typical recipe might call for mole of phenol to 0.8 moles of formaldehyde (usually added as formalin, a 37%solution in water). Acid catalysts, like oxalic, hydrochloric, or others, are added in amounts of 1–2% by weight of the phenol. Oxalic acid is favored over hydrochloric, sulfuric, or phosphoric due to corrosion problems. In addition, vapors of hydrochloric acid tend to react with vapors of formaldehyde and form a carcinogenic compound, 1,10-dichlorodimethyl ether. The reactions are conducted at the reflux temperature of water for 2–4 h. Maleic acid is sometimes used to form high melting novolacs. In a typical preparation of novolacs, molten phenol, usually kept at 65C, is introduced into the reaction kettle and heated to 95C. The catalyst is then added. This is followed by addition of the formaldehyde solution to the kettle with stirring, at a rate that allows a gentle reflux. After addition, heating and stirring are continued until almost all the formaldehyde is used up. At that point, the resins separate from the aqueous phase. Water is distilled off and the temperature is raised in the process to about 160C. The unreacted phenol is removed by vacuum distillation. The end of the reaction may be determined by the melting point of the product or by its melt viscosity. Cresols are also often used in preparations of phenolic resins. These may be individual isomers or mixtures of all three. Cresilic acids, mixtures of all three isomers, rich in m-cresol and low in o-cresol are preferred. Xylenols (all six isomers) are now also in common use to form alkali-resistant grades of phenolic resins. High 3,5-xylenol mixtures are preferred. Also, resorcinol, which forms very reactive phenolic resins, is used in preparations of cold-setting adhesives. Higher homologues of phenol, like Bisphenol A, are used to prepare special phenol-formaldehyde condensates.

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مواضيع ذات صلة

Silicone Elastomers

Polysiloxanes

Melamine-Formaldehyde Resins

Urea-Formaldehyde Resins

Ammonia-Catalyzed Phenolic Resins

Novolacs

Resols

Cycloaliphatic Epoxides

The Cross-linking Reactions

Preparation of Commercial Epoxy Resins

Polyurethane Foams

Polyurethane Elastomers