Resol, preparation

Two classes of resins are generally distinguished. Resols prepared with ammonia as catalysts are spirit-soluble resins having good electrical insulation properties. Water-soluble resols are prepared with caustic soda as catalyst. In aqueous solutions (with a solids content of about 70%) these are used mainly for mechanical grade paper and cloth laminates and in decorative laminates. 

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. ... 

Several types of alkaline catalysts may be used in the resole preparation stage. These are LiOH, NaOH, KOH, Ca(OH)2, Ba(OH)2, MgO, ZnO, NH2OH, and organic alkyl or alkanol amines. The rate of the reaction varies with each catalyst, and the resin properties may be somewhat different [46]. 

With a bulk process, resole resins, in neat or concentrated form, must be produced in small batches (ca 2—9.5 m ) in order to maintain control of the reaction and obtain a uniform product. On the other hand, if the product contains a large amount of water, such as Hquid plywood adhesives, large reactors (19 m ) can be used. Melt-stable products such as novolaks can be prepared in large batches (19—38 m ) if the exotherms can be controlled. 

Resoles. The advancement and cure of resole resins foUow reaction steps similar to those used for resin preparation the pH is 9 or higher and reaction temperature should not exceed 180°C. Methylol groups condense with other methylols to give dibenzyl ethers and react at the ortho and para positions on the phenol to give diphenyknethylenes. In addition, dibenzyl ethers eliminate formaldehyde to give diphenyknethanes. 

Neoprene—phenohc contact adhesives, known for thein high green strength and peel values, contain a resole-type resin prepared from 4-/-butylphenol. The alkyl group increases compatibiHty and reduces cross-linking. This resin reacts or complexes with the metal oxide, eg, MgO, contained in the formulation, and increases the cohesive strength of the adhesive. In fact, the reactivity with MgO is frequently measured to determine the effectiveness of heat-reactive phenoHcs in the formulation. 

Both novolaks and resols are prepared in similar equipment, shown dia-grammatically in Figure 23.16. The resin kettle may be constructed from copper, nickel or stainless steel where novolaks are being manufactured. Stainless steel may also be used for resols but where colour formation is unimportant the cheaper mild steel may be used. 

In the manufacture of resols a molar excess of formaldehyde (1.5-2.0 1) is reacted with the phenol in alkaline conditions. In these conditions the formation of the phenol alcohols is quite rapid and the condensation to a resol may take less than an hour. A typical charge for a laboratory-scale preparation would be ... 

In the manufacture of pure resorcinol resins, the reaction can be violently exothermic unless controlled by the addition of alcohols. Because the alcohols perform other useful functions in the glue mix, they are left in the liquid adhesive. PRF adhesives are generally prepared firstly by reaction of phenol with formaldehyde to form a PF resol polymer, that has been proved to be in the greatest percentage, and often completely, linear [95], In the reaction step that follows the resorcinol chemical is added in excess to the PF-resol to react it with the PF-resin -CH2OH groups to form PRF polymers in which the resorcinol groups can be resorcinol chemical or any type of resorcinol-formaldehyde polymer. 

Phenolic oligomers are prepared by reacting phenol or substituted phenols with formaldehyde or other aldehydes. Depending on the reaction conditions (e.g., pH) and the ratio of phenol to formaldehyde, two types of phenolic resins are obtained. Novolacs are derived from an excess of phenol under neutral to acidic conditions, while reactions under basic conditions using an excess of formaldehyde result in resoles. 

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. 

In another exemplary study, optical microscopy revealed that the void content of resole networks ranged from 0.13 to 0.21.82 Resole networks prepared from different F/P molar ratios showed comparable void size distributions. A bimodal distribution was observed for all networks, which was attributed to... 

Phenol-formaldehyde resole resins, preparations of, 429 Phenolic-based networks, 376 Phenolic compounds, 62 Phenolic cyanate resins, 418... 

The crosslinking of resoles is slightly more straightforward than that of novolaks, if only because there are fewer possible chemical structures involved in the setting reaction or finished structure. Since resoles are prepared under alkaline conditions, crosslinking is generally preceded by neutralisation. This enhances the ease with which network structures can form when the resin is simply heated. 

Phenol-formaldehyde resins using prepolymers such as novolaks and resols are widely used in industrial fields. These resins show excellent toughness and thermal-resistant properties, but the general concern over the toxicity of formaldehyde has resulted in limitations on their preparation and use. Therefore, an alternative process for the synthesis of phenolic polymers avoiding the use of formaldehyde is strongly desired. 

Acid Curing. Urea-formaldehyde resins and resol-phenol-formaldehyde resins can be acid-cured by wastes from the production of maleic anhydride [1902]. The waste from the production of maleic anhydride contains up to 50% maleic anhydride, in addition to phthalic anhydride, citraconic anhydride, benzoic acid, o-tolulic acid, and phthalide. The plugging solution is prepared by mixing a urea-formaldehyde resin with a phenol-formaldehyde resin, adding the waste from production of maleic anhydride, and mixing thoroughly. 

Various attempts have been made to prepare adhesives from lignin. The preparation of resol resin adhesives has been studied especially extensively. The introduction of phenols into the a or /2-position of the sidechain of the phenylpropane unit (phenolation of lignin) has been considered a key reaction for the formulation of these types of adhesives with adequate glu-ability. 

There have been many attempts to prepare resol resin adhesives from kraft lignin (23). Phenolation of lignin can, in these cases, also be considered a key reaction. That is, effective modification of lignin with phenol can enhance its reactivity, resulting in good gluability. 

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