Methylol group ureas

Free formaldehyde, whieh is in steady state equilibrium with the remaining methylol groups and the post-added urea... 

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. 

Other spherical fillers include carbon black. This has several roles particularly in combination with elastomers, e.g., black pigment, anti-oxidant and UV stabiliser, reinforcing filler, and an electrical conductor when used at 60% concentration. Wood flour is particularly effective in phenol/formaldehyde and melamine or urea/formaldehyde thermoset resins because the phenolic lignin component in the wood reacts with the methylol groups (-CH2OH) in the growing polymer. 

First, it should be noted that the ZV-methylol group is activated by the carbonyl group. This reactive group is present in almost all ZV-methylol systems. Second, the reaction is an equilibrium reaction so that both forward and reverse reactions can occur. Third, the agent is not simply a dimethylol agent, but is predominantly a mixture of mono- and di-substituted ureas. 

The first synthetic plastics were the phenol-formaldehyde resins introduced by Baekeland in 1907 [1], Melamine and urea also react with formaldehyde to form intermediate methylol compounds which condense to cross-linked polymers much like phenol-formaldehyde resins. Paper, cotton fabric, wood flour or other forms of cellulose have long been used to reinforce these methylol-functional polymers. Methylol groups react with hydroxyl groups of cellulose to form stable ether linkages to bond filler to polymers. Cellulose is so compatible with these resins that no one thought of an interface between them, and the term reinforced composites was not even used to describe these reinforced systems. 

The methylol groups are very reactive, condensing with each other, with the N-H groups in urea, and with the -OH groups in cellulose. 

The possibility of crosslinking natural macromoleculcs, such as cellulose and proteic materials, via the Mannich reaction has also been inve.stigatcd. Cellulose derivatives, however, are mostly subjected to the analogous amidomcthylation reaction, " usually employing bis-methylol amides (urea, oxalyl, adipoyl derivativas, etc.) capable of reacting with the cellulose hydroxy groups. 

IR-spectroscopy was used in [27] to study the process of hardening of the urea-formaldehyde oligomers. It was found that the content of methylol groups versus time was of a nonmonotonous character. A possible mechanism of the appearance of fluctuations in the manifestation of this effect has been proposed, associated with the suggestion about simultaneous progress of the reaction of condensation and hydrolysis. 

These intermediates are water-soluble and may be used to impregnate cellulosic fillers prior to the subsequent crosslinking reaction. The crosslinking reaction (Scheme 1.17) involves firstly the reaction of methylol groups and urea to give a methylene bridge (1). 

The reaction is carried out under slightly alkaline conditions and will yield crystalline derivatives of dimethylol urea or methylol melamines containing up to six methylol groups. 

The rates of introduction into the urea molecule of one, two, and three methylol groups have been estimated to have the ratio 9 3 1. The formation of A, N -dimethylolurea from monomethylolurea is three times that of monomethylolurea from urea. 

The methylol group gets its name because it consists of a methyl group in which one hydrogen atom is replaced by an alcoho/ic hydroxyl. Note particularly that acid conditions give a condensation reaction, while basic conditions give an addition reaction. It is the latter route that interests us at present, because the water-soluble dimethylol urea could at this stage be converted to a water-soluble resin, by a switch to acid conditions ... 

The primary amino groups of melamine react with formaldehyde as do those in urea, but with a difference. Both hydrogen atoms may be converted to methylol groups, making a maximum of six for the... 

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