Resole Ammonia

Caustic soda is commonly used as the catalyst for the manufacture of resols for mechanical and decorative laminates. However, it is not used in electrical laminates because it adversely affects the electrical insulation properties. For electrical-grade resols ammonia is the usual catalyst, and the resins are usually dissolved in industrial methylated spirits. The use of cresylic acid (m-cresol content 50-55%) in place of phenol yields laminating resins of better electrical properties. 

In practice, ammonia is most frequendy used. With hexa, the initial reaction steps differ, but the final resole resins are identical, provided they contain the same number of nitrogen and CH2 groups. Most nitrogen from ammonia or hexa is incorporated as diben2ylamine with primary, tertiary, and cycHc amine stmctures as minor products. 

Health and Safety. Eree phenols may be present in phenoHc novolacs and resoles. Phenol [108-95-2] is poisonous and caustic, irritating the skin and mucous membranes. Eormaldehyde and ammonia [7764-41-7] are often emitted during the cure of novolacs and must be properly vented. Eormaldehyde is Hsted as a human carcinogen worker exposure and emissions are controUed by OSHA and the EPA. 

Two classes of resol are generally distinguished, water-soluble resins prepared using caustic soda as catalyst, and spirit-soluble resins which are catalysed by addition of ammonia. The water-soluble resins are usually only partially dehydrated during manufacture to give an aqueous resin solution with a solids content of about 70%. The solution viscosity can critically affect the success in a given application. Water-soluble resols are used mainly for mechanical grade paper and cloth laminates and in decorative laminates. 

Used for crosslinking novolacs or catalyzing resole syntheses, HMTA is prepared by reacting formaldehyde with ammonia (Fig. 7.5). The reaction is reversible at high temperatures, especially above 250°C. HMTA can also be hydrolyzed in the presence of water. 

The reaction mixture is dehydrated at temperatures as high as 160°C (higher temperatures can be tolerated than with resoles). The prepolymer is cooled, crushed, blended with 5-15% hexamethylenetetramine, (CH2)6N4, and sold to the fabricator. Hexamethylenetetramine, referred to as hexa, is the product of the reaction of 6 mol of formaldehyde and 4 mol of ammonia. Curring occurs rapidly on heating with the formation of both methylene and ben-zylamine crosslinking bridges between benzene rings. The crosslinked network is pictured as... 

Tsuruta, S., Historical note on the chemistry of synthetic resins. XXXVII. An introduction to ammonia resols, Net.su Koka.sey Juski, 10, 25, 1989 Chem. Abstr.. 111, 1.35219, 1989. 

Phenolic resol was synthesized by the usual procedure (See Scheme 1.) 1 mol of phenol, 1.2 mol of formaldehyde, and 0.04 mol of ammonia were heated at 70 C with vigorous stirring for 3 hours, then dehydration was carried out under vacuum. 

The route to crosslinked phenol-formaldehyde resins via resoles corre.sponds to that used by Baekeland in his original commercial technique, They now tend to be used for adhesives, binders, and laminates. The resole prepolymers are made typically in batch processes, using a trace of ammonia (about 2% on phenol) as the alkaline catalyst. Care has to be taken with this process since, despite the molar excess of formaldehyde, there is sufficient of each component present in the prepolymer to permit the formation of a highly crosslinked product. Indeed, such a product will form if the resole is heated excessively, but the problem can be avoided by careful attention to the conditions of reaction and by ensuring that polymerisation is not allowed to proceed for too long. 

Resols are produced by reacting a phenol with a molar excess of formaldehyde (commonly about 1 1.5—2) by using a basic catalyst (ammonia or sodium hydroxide). This procedure corresponds to Baekeland s original technique. Typically, reaction is carried out batchwise in a resin kettle equipped with stirrer and jacketed for heating and cooling. The resin kettle is also fitted with a condenser such that either reflux or distillation may take place as required. 

The resols are usually prepared in 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 one hour. Formaldehyde is often added in the form of formalin. The quantity of the added caustic or ammonia might comprise 1% 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 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. 

Tertiary amines [70], some inorganic salts [71], ammonia [70], and l,3,5,7-tetraazatricyclo[3.3.3.P ]decane (hexamethylenetetramine) [72] can catalyze the formation of resoles. The basic pH in the reaction mixture produees a phenolate anion. This anion possesses a higher electron density at the ortho and para positions. Therefore, the electrophilic attack of the carbon end of the methanal occurs more efficiently in these positions. Furthermore, the dehydration of the hydroxymethylol group oecurs slower because the cation of the basic catalyst stabilizes the hydroxymethylated phenols. A typical example for such a stabilized structure is given in Seheme 3. 

Another alkaline catalyst applied for the preparation of resoles is ammonia It is well known that ammonia can dissociate in aqueous solution to form ammonium hydroxide which can also function as a catalyst. Furthermore, the anunonium ion has a comparable ionic radius to the potassium ion. However, ammonia also catalyses the formation of Mannich-bases, Schiffsch-bases, and other structures that are represented as by-products in the mixture of resoles formed. These by-products contribute to a coloring ofthe... 

The pH value used for prepolymer synthesis is an important parameter in order to obtain novolacs or resoles. Novolacs are formed if the reaction between phenol and methanai occurs in an acidic medium. Typical catalysts used are oxalic acid, sulfuric acid, and 4-methylbenzene sulfonic acid. The use of slight acidic conditions and bivalent metal salts (e.g. Ca, Zn, Mg, Pb, Co, Cu, Ni) to catalyze the phenol methanai reaction results in ortho, ortho -novolacs. Furthermore, no catalyst is necessary for the reaction of a substituted phenol with paraformaldehyde at temperatures between 150-200 C for the manufacture of novolacs. Resoles are formed when phenol reacts with an excess of methanai atpH values larger than 5. This process is catalyzed by inorganic salts, ammonia, and 1,3,5,7-tetraazatricyclo-... 

Usually, NaOH alkali is used as catalyst, in an amount up to one mole per mole phenol (molar ratio NaOH/P), which corresponds to a portion of alkali in the liquid resin of approximately 10 mass%. The pH of commercial phenolic resin resols is in the range 10-13. Besides NaOH, other basic catalysts also can be used, like Ba(OH)2, LiOH, Na2C03, and ammonia. The type of catalyst significantly determines the properties of the resins. ... 

This increase in Tg is higher than that expected if only phenol and formaldehyde were used, and is a result of the hydrogen-bonding interaction between the backbone amine units and the phenolic hydroxyls. Taking advantage of this effect, hexa and ammonia have been frequently used to produce solid, grindable, and water-insoluble resoles for molding compoimds. 

Phenolic resins such as resol-type resins generate water and formaldehyde during curing as by-products of condensation reactions. Hexamethylenetetramine (HMTA) can be used as a source of formaldehyde for curing novolac-type resins, producing formaldehyde and ammonia as by-products. During curing. 

Typically, reaction is carried out batch-wise in a stirred reactor, jacketed for heating and cooling. The reactor is also fitted with a condenser such that either reflux or distillation may take place, as required. A mixture of phenol, formalin and ammonia (about 1-3% on the weight of phenol) is heated under reflux at about 100°C for 0.25-1 hour and then water is removed by distillation under reduced pressure (to limit further reaction). Distillation is continued until a cooled sample of the residual resin has a melting point in the range 45—50°C. The resin is then quickly discharged and cooled to give a hard, brittle solid. In an alternative procedure, the removal of water is not taken to completion but is halted when the resol content of the residual aqueous solution reaches about 70%. The solution is then used directly, mainly in the preparation of paper laminates. In the preparation of such aqueous solutions, the preferred catalyst is... 

As indicated previously, resols prepared with ammonia as catalyst are different in some respects from those produced with metal hydroxides. The ammonia probably enters into the resin structure since various nitrogen-containing compounds have been obtained by treating methylolphenols with ammonia. Included in the compounds which have been obtained in this way are bis- and tris(hydroxybenzyl) amines, e.g. ... 

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