Melamine methylol derivatives

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

Most amino resins used commercially for finishing textile fabrics are methylolated derivatives of urea or melamine. Although these products are usually monomeric, they may contain some polymer by-product. 

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. 

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. 

Melamine (MF) resins (Cymel, Melmac, Resimene) are produced by the formylation of melamine (2,4,6-triamino-1,3,5-triazine). Melamine has six active hydrogen atoms and hence forms mono-, di , tri-, tetra-, penta, and hexamethylo melamines. The methylol derivatives may be etherified with alcohols such as 1-butanol. A hexamethyl ether (hexamethoxymethylmela-mine) is commercially available and may be used as an intermediate. Some of the reactions leading to resinification of melamine are shown in Figure 15.6. 

KAURIT M90 is an extensively etherified, highly methylolated derivative of melamine with approx. 90% active substance. 

Easy care/crease resistance/ wash-wear A -methylol derivatives of ureas, triazones, carbamates, melamine... 

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). 

Pyrovatex CP (Ciba Geigy), the A-methylol derivative of dimethylphosphonopropionamide reacted with a melamine/formaldehyde condensate used with an acid catalyst such as phosphoric acid ... 

Urea/formaldehyde (UF) or melamine/formaldehyde (MF) thermoset resins are also formed via the initial addition of HCHO to form methylol derivatives (dimethylol urea and hexamethylol melamine). Curing or cross-linking with acids yields thermoset polymers. 

Benzoguanamine closely resembles melamine in that reaction with formaldehyde gives methylol derivatives and then resinous condensates. Butylated... 

Thiourea closely resembles urea in that reaction with formaldehyde gives methylol derivatives and then resinous condensates which on continued heating yield network structures. Thiourea-formaldehyde resins are slower curing than urea-formaldehyde resins and the hardened products are more brittle and more water-resistant. At one time thiourea-formaldehyde resins were added to urea-formaldehyde resins to give mouldings and laminates with improved water-resistance. These mixed resins have now been largely superseded by melamine-formaldehyde resins which give products with better resistance to heat. 

MF resins are produced by first reacting melamine and formaldehyde in the presence of aqueous sodium carbonate. The degree of methylolation depends upon the molar ratio of formaldehyde to melamine. Although methylolation will result in a mixture of partially methylolated derivatives, the reaction is usually carried out to optimize the content of either hexa-methylolmelamine or symmetrieal trimethylolmelamine, as in Figure 18. 

Melamine, the cyclic trimer of cyanamide, reacts with almost neutral or mildly alkaline formaldehyde. solution, producing methylohnelamines From one to six mols of foimaldehvde react readilv with one mol of melamine, apparently yielding all the methylol derivatives theoretically possible. Hexamethylolmelamine, for example, is obtained by dissolving 0.1 mol of melamine in 75 part.s of neutral 32 per cent formaldehyde... 

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 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. 

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). 

For illustration we focus on the formation, the change of concentration per unit of time, of mono-methylol melamine (label c ) out of dissolved melamine (label b ) and formaldehyde. This production depends on ki, the concentrations of formaldehyde and dissolved melamine and the number of possibilities for the binding of formaldehyde to dissolved melamine. In this case there are six places where the formaldehyde can be bound. The reverse reaction depends on the concentration mono-methylol melamine and water. For this case we only have one possibility for the loosening. Following these rules for the reaction kinetics and denoting the formaldehyde concentrations with [jPM], the water concentration with [H2O] and the concentration of a methylol melamine by its corresponding label inside square brackets, we can derive the differential equations for all the species with the labels c to k , formaldehyde and water. As an example we will give the differential equation for mono-methylol melamine (label c ). 

The reaction mechanisms occurring with systems based on melamine (2,4,6-triamino-1,3,3,5-triazine), a monomer that is synthesized from urea, are not different from those affording UF resins. It is important to mention a higher reactivity of MF resins compared to that of UF resins and the real hexavalence of melamine, which gives rise to the hexamethylol derivative shown below and other methylols by reaction with formaldehyde. 

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