Methylol group, reaction

These are water-soluble crystalline compounds sold as concentrated aqueous solutions. The methylol groups are highly reactive (118—122) and capable of being cured on the fabric by reaction with ammonia or amino compounds to form durable cross-linked finishes, probably having phosphine oxide stmctures after post-oxidizing. This finishing process, as developed by Albright Wilson, is known as the Proban process. 

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. 

In some resole appHcations, such as foam and foundry binders, a rapid cure of a Hquid resin is obtained at RT with strong acid. The reactions proceed in the same manner as those of novolak resin formation. Methylol groups react at ortho and para phenoHc hydrogen to give diphenyknethane units (41). 

These probably form the basis of the amorphous precipitates formed on cooling. The more soluble resins produced on continuation of the reaction probably contain pendant methylol groups formed by reactions of the NH groups with free formaldehyde Figure 24.3 I). 

Reaction of melamine with neutralised formaldehyde at about 80-100°C leads to the production of mixture of water-soluble methylolmelamines. These hydroxymethyl derivatives can possess up to six methylol groups per molecule and include trimethylolmelamine and hexamethylolmelamine (Figure 24.8) The methylol content of the mixture will depend on the melamine formaldehyde ratio and on the reaction conditions. 

The second phase in resole formation is reaction of the activated phenol with the aldehyde to form the phenol alcohol derivative. When the aldehyde is formaldehyde, the derivative is a hydroxymethyl phenol and the process is known as methylolation. Scheme 2 illustrates this reaction. Since resoles are usually made with excess aldehyde, more than one substitution may be made on the ring. When the reactants are phenol and formaldehyde, up to three methylol groups may be substituted. This reaction has been extensively studied and the rates of... 

In summary, it is clear that methylolation is a bimolecular, second-order reaction. As methylol groups are added to the ring, the ring undergoes general activation. Addition of o-methylol groups increases the acidity of the phenolic hydroxyl, which could increase reaction rates. However, all methylol groups ap-... 

The condensation reaction of resorcinol with formaldehyde, on an equal molar basis and under identical conditions, also proceeds at a rate which is approximately 10 to 15 times faster than that of the equivalent phenol-formaldehyde system [16-18,123]. The high reactivity of the resorcinol-formaldehyde system renders it impossible to have these adhesives in resol form. Therefore, only resorcinol-formaldehyde novolaks, i.e. resins not containing methylol groups can be produced. All the resorcinol nuclei are linked together through methylene... 

Phenolics or phenol-aldehydes include the important commercial phenolic resin bakelite based on phenol and formaldehyde. A one-step process produces resol resin from more than one molecule of formaldehyde per phenol molecule. A two-step process uses an excess of phenol to produce novolacs - resins that have no reactive methylol groups and must be mixed with an aldehyde o undergo further reaction. 

The Mannich reaction of polyacrylamide with formaldehyde and an amine may be used for the obtaining product that contains N-methylol groups (or ethers or ethers thereoO-... 

Chemical compounds that contain methylol groups (-CH2 OH) form stable, covalent bonds with cellulose fibers. Those compounds are well known and widely used in textile chemistry. Hydrogen bonds with cellulose can be formed in this reaction as well. The treatment of cellulose with methylolmelamine compounds before forming cellulose unsaturated polyesters (UP) composites decreases the moisture pickup and increases the wet strength of reinforced plastic [48,49]. 

The second step is the condensation reaction between the methylolphe-nols with the elimination of water and the formation of the polymer. Crosslinking occurs hy a reaction between the methylol groups and results in the formation of ether bridges. It occurs also by the reaction of the methylol groups and the aromatic ring, which forms methylene bridges. The formed polymer is a three-dimensional network thermoset ... 

The acid-catalyzed reaction occurs by an electrophilic substitution where formaldehyde is the electrophile. Condensation between the methylol groups and the benzene rings results in the formation of methylene bridges. Usually, the ratio of formaldehyde to phenol is kept less than unity to produce a linear fusible polymer in the first stage. Crosslinking of the formed polymer can occur by adding more formaldehyde and a small amount of hexamethylene tetramine (hexamine. 

Novolacs are prepared with an excess of phenol over formaldehyde under acidic conditions (Fig. 7.6). A methylene glycol is protonated by an acid from the reaction medium, which then releases water to form a hydroxymethylene cation (step 1 in Fig. 7.6). This ion hydroxyalkylates a phenol via electrophilic aromatic substitution. The rate-determining step of the sequence occurs in step 2 where a pair of electrons from the phenol ring attacks the electrophile forming a car-bocation intermediate. The methylol group of the hydroxymethylated phenol is unstable in the presence of acid and loses water readily to form a benzylic carbo-nium ion (step 3). This ion then reacts with another phenol to form a methylene bridge in another electrophilic aromatic substitution. This major process repeats until the formaldehyde is exhausted. 

Phenol, formaldehyde, and urea have been copolymerized to achieve resins and subsequent networks with improved flame retardance and lower cost relative to phenol-formaldehyde analogues. The condensation of a phenolic methylol group with urea (Fig. 7.32) is believed to be the primary reaction under the weakly acidic conditions normally used. 

The initial step of the polymerisation process is reaction of the amine groups with formaldehyde to generate methylol units, as illustrated in Reaction 1.9. Further heating of the polymer then leads to a variety of reactions. For example, the methylol groups can undergo self-condensation (Reaction 1.10). 

Alternatively the methylol groups can react with further amino groups, also evolving a molecule of water in what is another condensation reaction (Reaction 1.11). 

An amino-formaldehyde resin or acetone-formaldehyde resin has the capability to harden in alkaline media, in contact with a cement solution with a pH of 11 to 12. The presence of sintered CaO provides the required conditions for hardening of the methylol groups of the formaldehyde resin with Ca + ions and a further simultaneous reaction of the methylol groups that formed hydrate compounds, resulting in an improved dispersion and plastification of the solution. 

Methylene ether linkages by the reaction of two methylol groups (Figure 19.3b)... 

Methylene linkages by the reaction of methylol groups splitting out water and formaldehyde... 

Kinetic Studies. Self-condensation of A in pyridine solution was conducted at 100°C in 5 mm NMR tubes and the 60 MHz -NMR spectra of the reaction mixtures were recorded at room temperature after various reaction times. The relative concentrations of o- and p-methylol groups and of methylene ether linkages were determined from the relative intensities of the resonances observed at 6 5.25 (o-methylol), 5.0 (p-methylol), 4.8-4.9 (methylene ether) and 2.3 (CH3) ppm, respectively. Resonance areas were measured by cutting and weighing expanded spectra. Initial estimates of koo were obtained by considering the reaction to be a second order process... 

The simplest model to assume is based on the possibility that the reactivities of the o- and p-methylol groups are independent of the structure of the molecule to which they are attached. According to this model, three rate constants koo kop and kpp are sufficient to characterize the kinetic behavior of the reaction at a given temperature. (The subscripts associated with these rate constants define... 

Reactions between two reactants, each of which has one o-methylol and one p-methylol group require a factor of two because there are two ways the reaction can take place. 

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