Melamine, synthesis

Step I is endothermic, step 2 is exothermic the overall reaction is endothermic Because of the largo quantities of CO- and HN s generated, the process often is undertaker in connection with urea manufacture, which permits the off-gases to lx recycled usefully. The melamine synthesis maybe carried out at low or medium pressures with the assistance of a catalyst or at higher pressures without a catalyst. 

Back in 1966, Schmidt described a process to thermolyze urea into NH3 and HNCO gas on an industrial scale [35]. In this process, solid urea was blown into a fluidized bed of inert material, which was heated above 300 °C. The produced HNCO gas was used for melamine synthesis. 

The byproducts biuret and cyanuric acid are observed in the largest quantity at low and moderate temperatures up to about 300 °C [36] however, additional byproducts form in smaller amounts as shown in Fig. 16.2 [11, 36, 37]. Substitution of the OH groups of cyanuric acid by NH3 yields the more stable triazines ammelide, ammeline, and melamine [36]. Ammelide may also be formed by the reaction of biuret with HNCO, if water is eliminated instead of NH3 [11]. Melamine may also be produced by trimerization of cyanamide [36] since cyanamide is an intermediate in melamine synthesis [35]. Cyanamide was not observed in a... 

Fig. 28.22. Melamine synthesis-0.S.W. (Hydrocarbon Processing, 48, No. 11, 209, Nov. 1969. Copyright 1969 by Gulf Publishing and used by permission of the copyright owners.)...

Co-ordination chemistry is not the only possible approach to the successful preparation of supramolecular dendrimers. Other examples include hydrogen-bonded dendrimers, which have been prepared using tetracar-boxylic acid moieties as the linking species, and others have been prepared where the hydrogen bonds were developed using melamine and cyanuric acid at the focal point (i.e. the point towards which convergent synthesis is directed). 

Guanidine dearranges less readily than urea and yidds ammonia and cyanamide as would be expected from the familiar method for its synthesis. If it is heated with aniline, the cyanamide shows no tendency to combine with that substance but combines with itself, as it does in the absence of aniline, to form melamine. 

A recent new synthesis of biguanide, affording 50—60% yields of product, utilises the reaction between 0-adkylisoureas and equimolar proportions of guanidine and its hydrochloride in ethanol (612). Small amounts of melamine and cycuioguanidine are formed as by-products. 

Scheme 9 Synthesis of three generation melamine dendrimer on silica surfaces...

The major one-carbon feedstock is methane and it serves as the feedstock to a number of important monomers including hexamethylene tetramine and melamine, used in the synthesis of a number of cross-linked thermosets as well as vinyl acetate, ethylene, ethylene glycol, and methyl methacrylate (Table 17.1). 

The amino resins or plastics, closely related to the phenolics in both synthesis and applications, are obtained by the polymerization of formaldehyde with urea (XXXVII) (/ = 4) or melamine (XXXVIII) (f — 6). Synthesis of the amino plastics can be carried out either in alkaline or acidic conditions [Drumm and LeBlanc, 1972 Nair and Francis, 1983 Updegraff, 1985]. Control of the extent of reaction is achieved by pH and temperature control. The prepolymer can be made at various pH levels depending on the reaction temperature chosen. Polymerization is stopped by cooling and bringing the pH close to neutral. Curing of the prepolymer involves heating, usually in the presence of an added acid catalyst. 

There are many examples of such syntheses. The development of JV-cyanoamidines as a starting material is a major advance in the synthesis of 1,3,5-triazines bearing a different substituent on each carbon. Dicyandiamide continues to be a valuable starting material for the synthesis of 2,4-diamino-l,3,5-triazines, although it has been superseded by urea as the starting material in the industrial synthesis of melamine. 

Uses. IIic most important commercial use for benzonitiile is the synthesis of benzoguanamine, which is a derivative of melamine and is used in protective coatings and molding resins. See also Amino Resins and Cyanamides. 

Cyanoguanidine is used in fertilizers as a stabilizer of NC in org synthesis of melamine, barbituric acid guanidine salts in pharmaceutical products dyestuffs and in expls. Some examples of expls contg cyanoguanidine are as follows ... 

Uses. The most important commercial use for benzonitrile is the synthesis of benzoguanamine, which is a derivative of melamine and is used in protective coatings and molding resins (see Amino RESINS Cyanamides). Other uses for benzonitrile are as an additive in nickel-plating baths, for separating naphthalene and alkylnaphthalenes from nonaromatics by azeotropic distillation (qv), as a jet-fuel additive, in cotton bleaching baths, as a drying additive for acrylic fibers, and in the removal of titanium tetrachloride and vanadium oxychloride from silicon tetrachloride. 

We wanted to explore the limits of the self-assembly approach for the synthesis of well-defined structures with nanoscale dimensions by further increasing the size of the assemblies. Our group has shown that calix[4]arenes diametrically substituted with two melamine units at the upper rim are very good platforms for the formation of double rosette assemblies. 

The use of polyols such as pentaerythritol, mannitol, or sorbitol as classical char formers in intumescent formulations for thermoplastics is associated with migration and water solubility problems. Moreover, these additives are often not compatible with the polymeric matrix and the mechanical properties of the formulations are then very poor. Those problems can be solved (at least partially) by the synthesis of additives that concentrate the three intumescent FR elements in one material, as suggested by the pioneering work of Halpern.29 b-MAP (4) (melamine salt of 3,9-dihydroxy-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5,5]-undecane-3,9-dioxide) and Melabis (5) (melamine salt of bis(l-oxo-2,6,7-trioxa-l-phosphabicyclo[2.2.2]octan-4-ylmethanol)phosphate) were synthesized from pentaerythritol (2), melamine (3), and phosphoryl trichloride (1) (Figure 6.4). They were found to be more effective to fire retard PP than standard halogen-antimony FR. 

Liu, Y. and Wang, Q. 2006. Catalytic action of phospho-tungstic acid in the synthesis of melamine salts of pentaerythritol phosphate and their synergistic effects in flame retarded polypropylene. Polym. Deg. Stab. 91 2513-2519. 

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