Methylolated melamine formaldehyde

Melamine reacts similarly to produce methylol derivatives, which form the familiar melamine—formaldehyde resins on heating (63) (see Aminoresins). 

The chemistry of melamines and phenoHcs is quite similar. In both cases formaldehyde [50-00-0] is added to the reactive sites on the patent ring to form methylol phenols (3) or methylol melamines (4) (see Phenolresins Aminoresins). There ate six reactive sites on the triazine ring of melamine [108-78-1] (1) so it is possible to form hexamethylolmelamine. However, the most common degree of methylolation is 1.5—2.0. The ortho and para positions of phenol ate active thus phenol can be trimethylolated (2). However, as with melamine, lower degrees of methylolation such as 1.2—2.5 ate... 

Amino Resins. Amino resins (qv) include both urea- and melamine—formaldehyde condensation products. They are thermosets prepared similarly by the reaction of the amino groups in urea [57-13-6] or melamine [108-78-1] with formaldehyde to form the corresponding methylol derivatives, which are soluble in water or ethanol. To form plywood, particle board, and other wood products for adhesive or bonding purposes, a Hquid resin is mixed with some acid catalyst and sprayed on the boards or granules, then cured and cross-linked under heat and pressure. 

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. 

In a typical process a jacketed still fitted with a stirrer and reflux condenser in charged with 240 parts 37% w/w (40% w/v) formalin and the pH adjusted to 8.0-8.5 using sodium carbonate solution with the aid of a pH meter. One hundred and twenty six parts of melamine (to give a melamine formaldehyde ratio of 1 3) are charged into the still and the temperature raised to 85°C. The melamine goes into solution and forms methylol derivatives. For treatment of fabrics, paper and leather this product may be diluted and cooled for immediate use. It may also be spray dried to give a more stable product. Cooling the solution would yield crystalline trimethylolmelamine, which may be air dried but which is less soluble in water than the spray-dried product. 

Figure 7.1 Reaction of melamine with formaldehyde and subsequent dimerization of the methylolated melamine monomers.

Pentaerythritol, monomer, dimer, trimer Phenol-formaldehyde resins Methylol melamine... 

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. 

Melamine chemistry begins with the addition of 2-3 mols of formaldehyde to form methylol melamines. 

Another category of durable hand builders are formaldehyde-containing thermosetting polymers. These products are usually supplied as dispersions or aqueous solutions of precondensates of urea or melamine with formaldehyde, for example di-methylol urea or di- to hexa-methylol melamine and their methyl ethers. The thermosetting polymers are comparatively inexpensive and provide fabrics with stiffness and resilience. However, they have a tendency to reduce abrasion resistance, yellow after exposure to heat, and release formaldehyde. Melamine-based hand builders are more highly crosslinked than urea-based products and are accordingly more durable. Butylated urea condensates are especially useful for rayon fabrics (see also Chapter 5 Easy-care and durable press finishes of cellulosics). 

Melamine will react with formaldehyde to form methylol melamines. 

Acrylic ester PVC Polymer 3 melamine formaldehyde N-methylol acrylamide Water-based coating Leeson and Ludwig V.S. Pat., 4,007,147, 1977 Goodrich... 

A melamine laminating resin used to saturate the print and overlay papers of a typical decorative laminate might contain two moles of formaldehyde for each mole of melamine. In order to inhibit crystallization of methylol melamines, the reaction is continued until about one-fourth of the reaction product has been converted to low molecular weight polymer. A simple determination of free formaldehyde may be used to foUow the first stage of the reaction, and the build-up of polymer in the reaction mixture may be followed by cloud-point dilution or viscosity tests. 

Miscellaneous Resins. Much less important than the melamine—formaldehyde and urea—formaldehyde resins are the methylol carbamates. They are urea derivatives since they are made from urea and an alcohol (R can vary from methyl to a monoalkyl ether of ethylene glycol). 

Production of melamine-formaldehyde polymers involves reactions essentially similar to those of UF resins, that is, initial production of methylol derivatives of melamine, which on subsequent condensation, ultimately form methylene bridges between melamine groups in a rigid network structure (15). 

On further heating, the methylol melamines undergo condensation reactions, and a point is reached where hydrophobic resin separates out. The rate of resinification depends on pH. The rate is minimum at about pH 10.0-10.5 and increases considerably both at lower and higher pH. The mechanism of resinification and cross-linking is similar to that observed for urea-formaldehyde (Figure 4.26) and involves methylol-amine and methylol-methylol condensations. 

Figure 1 Methylolation (hydroxymethylation) and subsequent condensation reactions to form melamine-formaldehyde adhesive systems.

---