Melam Melamine

Melamine and its salts are widely used in formulations of fire retardant additives, particularly of the intumescent type (4-71. The role played by melamine structures in these additives is however not yet understood. The thermal behaviour is of paramount importance in studies of the fire retardance mechanism. It is known that melamine undergoes progressive condensation on heating with elimination of ammonia and formation of polymeric products named "melam", "melem", "melon" (8.91. The following schematic reaction is reported in the literature (10-121 ... 

While there is clear evidence that melam Is the dimer of melamine in which two s-triazine rings are connected by a NH bridge (di 6,[2,4 diamino-1,3,5-triazine]amine) (10) ... 

Hydrobromide. A complex process takes place in the first step of weight loss of melamine hydrobromide (250-400 C) as shown by the DTG curve of Figure 6. The products evolved in this step were shown to be a mixture of melamine hydrobromide (IR) and ammonium bromide (IR and X-ray). The IR spectrum of the residue (55% at 400 0 which is compared with the original salt in Figure 7, is identical with that of reference melam hydrobromide. Furthermore, the IR of the residue... 

The by-product ammonium bromide is shown to volatilise in similar conditions in TG. This is a different behaviour altogether from that of melamine in whose thermal condensation melam can be isolated with difficulty only in carefully controlled experimental conditions (11.17). Indeed melem is the condensation product most easily isolated and this led some authors to doubt that melam could be an intermediate product of the condensation of melamine to melem and melon (19.22). 

Quantitative condensation of melamine to melam on heating at 290 C in the presence of zinc chloride and acids was previously reported in the literature (20.23). The reason for limitation of the condensation process to the dimer has however not been discussed. 

Melam hydrobromide undergoes thermal degradation between 450-550 0 (2nd step, Figure 6) with complete elimination of bromine either through evolution of ammonium bromide, which is the major product, or melamine hydrobromide as shown by IR. Also a small amount of free ammonia is evolved in this step in which melon is formed as shown by the IR of the residue at 550 0 (ca. 35% of original melamine hydrobromide). 

Similarly to melamine hydrobromide, in the second step of weight loss (370-450 0, Figure 8), IR data show that melam nitrate evolves further ammonium nit rate,ammonia and melamine nitrate giving melon which decomposes completely to volatile products above 500 C. The degradation scheme is then ... 

Figure 7. IR of melamine hydrobromide (A) and of residue at 400 C (B) melam hydrobromide (by comparison with reference compound).

Ammonia is evolved here in the condensation process of melamine to melam whereas the respective ammonium salts are evolved in the case of the hydrobromide and nitrate. Reactions 9 and 9b show that ammonia should be neutralised by pyrosulphuric acid groups. In TG we have found that diammonium pyrosulphate eliminates ammonia above 350 C, with however complete decomposition ... 

Moreover, simultaneous evolution of water implies that condensation of melamine is combined with that of polyphosphoric structures. Assuming the reaction scheme proposed above, melam polyphosphate and ammonium polyphosphate groups should be formed from melamine polyphosphate ... 

Owing to overlapping of different processes in this range of temperature, quantitative considerations based on data obtained in programmed temperature TG cannot be made. However, it can roughly be estimated that up to 410 C, that is to the end of the process characterised by the narrow DTG peak 3 in Figure 12, about one mole of ammonia 1s evolved per mole of melamine. This would support the formation of melam by Reaction 13, followed by elimination of ammonia (reaction 14). 

The triazine compound melamine (2,4,6-triamino-1,3,5-triazine) was first isolated, and named, in 1834 by Justus Liebig, who obtained it by reacting alkali with melam. A. Claus in 1875 improved on Liebig s method. S. Cloez and S. Cannizzaro in 1851, and E. Dreschel in 1875 and 1876, demonstrated that melamine was formed when cyanamide or dicyandiamide were heated at elevated temperatures (Scheme 32). In 1885, A. W. Hofmann made melamine from thiocyanuric esters by heating them in sealed tubes with concentrated aqueous ammonia101. 

Melamine [108-78-1]. This inexpensive solid melts with vaporization at about 350°C and, if evaporation is constrained, decomposition also occurs to ammonia and nonvolatile condensation products— melam, melem, and eventually melone. It is occasionally used by itself as a flame retardant but more often used as a blowing agent and flame-retardant adjuvant in intumescent coatings, elastomers, and plastic formulations (138) as well as in flexible polyurethane foams (139,140). 

Upon heating, to whatever extent it does not sublime away, melamine can undergo progressive condensation with evolution of ammonia and formation of thermally stable condensates, known as melam, melem, and melon. This reaction competes with melamine volatilization and is more pronounced if the volatilization of melamine is impeded for example, by trapping in the charred polymer. 

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