Methylol reaction, condensation

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 addition to the two main reactions, ie, methylolation and condensation, there are a number of other reactions important for the manufacture and uses of amino resins. For example, two methylo1 groups may combine to produce a dimethylene ether linkage and Hberate a molecule of water ... 

With the present state of knowledge it appears that in the first part of the second-stage methylol ureas condense with each other by reaction of an CH20H group of one molecule with an NH2 group of another (Figure 24.2). 

In Fig. 3 there appears to be only one reaction step for the novolac from beginning to end. This indicates that novolac methylolation and condensation reactions occur simultaneously at all temperatures. This, in turn, shows that the activation energy for the condensation reaction is similar to or less than that for the methylolation. In other words, the methylolation is the rate-determining step in the process. 

The final phase of resole manufacture is known as the condensation stage (Scheme 3). This is the actual process by which molecular weight is developed and involves the combination of the hydroxymethyl phenol intermediates to form oligomers. It can be reasonably well separated from the resole methylolation reaction in practice by maintaining reaction temperatures below about 70°C. The activation energy for condensation is higher than that for methylolation. This is not to say that condensation does not occur at temperatures below 70°C. It simply means that the methylolation is much faster than condensation at this temperature. 

The observation of condensation products at 30°C may seem to contradict statements made earlier regarding our ability to separate the methylolation and condensation reactions by holding reaction temperatures below 70°C. Flowever, there is no conflict. The differences in the situations are primarily matters of absolute rate. The relative rates are still similar for methylolation and condensation. No... 

Melamine, a white powder, was discovered and identified by Liebig in 1834 but commercial manufacture came only in 1939, by Cyanamid Company of America with dicyandiamide as raw material. Melamine is 2,4,6-triamino-l,3,5-triazine with a structure as shown in Figure 53. On reaction with formaldehyde in aqueous solution the melamine powder dissolves rapidly on heating to form various methylol melamines, as in Figure 54. After further heating and the elimination of water the methylol melamines condense to form resinous polymers. 

Unlike the methylolations reaction, the condensation (curing) reaction is catalyzed by acids only. Apparently the methylol group is protonated and a molecule of water is lost to give the intermediate carbonium-immonium ion. This then reacts with an amino group to form a methylene link. These reactions are illustrated in Equations 9-12. 

The urea-formaldehyde polymer is formed by a multi-step reaction process between urea and formaldehyde. The initial phase is a methylolation of the urea under slightly alkaline conditions with a formaldehyde-urea (F/U) molar ratio of 2.0 1 to 2.k ]. Condensation of the methylolureas from the methylolation reaction is at atmospheric reflux with a pH of to 6. This condensation polymerization continues to a pre-determined viscosity, at which time the pH is adjusted with a suitable base to 7.3 to 8.0. The adhesive is then concentrated to a total solids content of 50 to 60 percent by vacuum distillation. Additional urea is then normally added to produce a final F/U molar ratio of 1.6 1 to 1.8 1. 

Formation of novolak involves an acid-catalyzed reaction of formaldehyde with excess phenol (i.e., formaldehyde-to-phenol mole ratio less than 1). The initial methylol phenols condense with the excess phenol to form dihydroxydiphenyl methane, which undergoes further condensation yielding low-molecular-weight prepolymer or novolak. Unlike resoles, novolaks do not contain residual methylol groups. They are fusible and insoluble. 

In the condensation of phenols and formaldehyde using basic catalysts, the initial substitution reaction (i.e., the formaldehyde attack on the phenol) is faster than the subsequent condensation reaction. Consequently, phenolic alcohols are initially the predominant intermediate compounds. These phenolic alcohols, which contain reactive methylol groups, condense either with other methylol groups to form ether links, or more commonly, with reactive positions in the phenolic ring (ortho or para to the hydroxyl group) to form methylene bridges. In both cases water is eliminated. 

The reaction of urea and formaldehyde is basieally a two-step process, usually eon-sisting of an alkaline methylolation (hydroxymethylation) step and an acid condensation step. The methylolation reaction, which usually is performed at a high molar ratio (F/U = 1.8 to 2.5), is the addition of up to three (four in theory) molecules of bifunctional formaldehyde to one molecule of urea to give methylolureas the types and the proportions... 

The most commonly known phenolic composite group is phenol formaldehyde polymers (phenoplasts). They are produced by polycondensation of a phenol and a mixture of phenols (phenol and phenol derivatives like cresol-resorcinol or para tertiary butyl phenol) with an aldehyde, usually formaldehyde and hexamethylene tetramine. Reaction of formaldehyde with phenol (up to 3 moles of formaldehyde can react with one mole of phenol - phenol acts as a three functional monomer) yields methylol groups in the ortho and para positions of the phenol molecule. In a further reaction, the methylol groups condenses with another molecule of phenol to form a methylene bridge. In practice, a prepolymer (usually a powder) is prepared first which is then cured later to the shape of the article in the mould. 

Fig. 1. Reaction mechanism for the formation of formaldehyde-based amino resins. 1, methylolation 2, condensation.

In the applications of SRH additives for typical industrial molded or extruded elastomers, there has been considerable study of formulating variables which has thrown some light on the mechanism. Chemisorption or methylol reaction with the non-elastomer substrate is, of course, probably similar to that of RFL dips, assuming that a significant level of resorcinol-formaldehyde condensate can find its way into the interface. 

Melamine and formaldehyde react to yield methylol-melamines, the degree of methylolation depending on the melamine/formaldehyde ratio. The principal cross-linking reaction involves methylol-methylol condensation to —CH2— O—CH2—, although methylene links may be formed by the elimination of formaldehyde from the bridge or by methylol-amine condensation, the first two reactions being similar to those of UF prepolymer s cross-linking mechanisms. 

Polymerization takes place through a series of methylol and condensation reactions to form amino resins which are clear and colourless. This makes them easier to colour than the phenolic resins which are naturally dark coloured. 

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