Melamine from urea

Figure 14.11 shows the flow diagram for the manufacture of melamine from urea. 

Melting vessel 2 Gas scrubber 3 Huidized-bed reactor 4 Desublimation chamber Figure 14.11 Flow diagram for the production of melamine from urea... 

There are some well known examples of large scale industrial processes of this type, such as the steam cracking of hydrocarbons, the cracking of ethylene dichloiide to vinyl chloride, the c ytic leforming.of methane to synthesis gas, and the synthesis of melamine from urea These processes are often carried out in tubular reactors, the latter two with solid catalysts. A well known version of the melamine process is carried out in a fluidized bed reactpr. 

Melamine. Melamine (cyanurotriamide, 2,4,6-ttiainino-j -triazine) C H N, is a white crystalline soHd, melting at approximately 350°C with vaporization, only slightly soluble in water. The commercial product, recrystallized grade, is at least 99% pure. Melamine was synthesized eady in the development of organic chemistry, but it remained of theoretical interest until it was found to be a usehil constituent of amino resins. Melamine was first made commercially from dicyandiamide [461-58-5] (see Cyanamides), but is now made from urea, a much cheaper starting material (9—12) (see also... 

Since melamine resins are derived from urea, they are more cosdy and are therefore restricted to appHcations requiring superior performance. Essentially ad of the melamine produced is used for making amino resins and plastics. 

Miscellaneous Resins. Much less important than the melamine—formaldehyde and urea—formaldehyde resins are the methylo1 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). 

Table 24.1 Properties of mouldings prepared from urea-formaldehyde and melamine-formaldehyde mouldings compositions (Testing according to BS 2782)...

Today melamine is usually prepared from urea. The detailed mechanism of this reaction is believed to be as follows (Figure 24.7). 

Only one melamine molecule is formed from six urea molecules, whilst three molecules of ammonia carbamate are formed. Whilst this can be recycled to urea the conversion from urea to melamine per cycle is at most 35%. Both the main route and the recycling operation involve high pressures and the low process efficiency offsets some of the apparent economic attractions of the route compared to those from dicy . 

The MUF resin formulation is built up from combination of certain amount of formalin, melamine and urea (in initial and post refluxing stages) and also sorbitol. Variation on the formulation gives different resin properties. The optimum resin properties give the optimum MUF resin formulation. From the properties analysis data, the optimum formulation is determined by using Mixture Experimental Design D-optimal criterion. The selective criteria... 

The crosslinkers examined in this study were aminoplast resins 1-4 selected from melamine-formaldehyde, urea-formaldehyde, benzoguanamine-formaldehyde, and glycoluril-formaldehyde resins, all of which undergo the crosslinking sequence shown in Scheme 1. The response of these crosslinkers to acid catalysis in thin films is compared on a relative basis to the well studied methylated melamine, 1 19-11). 

Melamine can be produced from urea, dicyandiamide (a derivative of calcium cyanide) or hydrogen cyanide. In 2001 all world production was based on urea. The dicyandiamide process was the original melamine process from the 1930 s, but it was phased out in the 1980 s. Production of melamine from hydrogen cyanide was never commercialized114. 

Melamine is produced from urea by either a high- or a low-pressure process, and either process can consist of one or two stages. The net reaction in each process is114 ... 

Melamine can be produced from urea, dicyan-diamide, or hydrogen cyanide, but in 2001 all T nh2 ... 

Formaldehyde condensation products from urea and/or melamine are shown in Fig. 13.2. They are comparable to the corresponding products for permanent press and easy-care finishes (Chapter 5). They have the same advantages and disadvantages and they often improve the perspiration more than the washing fastness. Therefore they are mostly used for lining fabrics, where their price advantage is especially important. 

Researchers at the Eastern Forest Products Laboratory in Canada have evaluated the urea and melamine amino-resin systems (9, 57, 99-110). Their work demonstrates that both systems show good leach resistance and reduced flame spread. The stability of these resins is controlled by the rate of methylolation of the urea, melamine, and dicyandiamide. The optimum mole ratio for stability of these solutions is 1 3 12 4 for urea or melamine, dicyandiamide, formaldehyde, and orthophosphoric acid. However, even at the optimum mole ratios, the pot life of the melamine system is less than that of the urea system. In both systems the nitrogen is fixed to a greater degree than the phosphorus. However, the degree of fixation of the phosphorus is greater with the melamine than with the urea. The melamine structure may promote formation of compounds with phosphoric acid that are less soluble than those from urea and dicyandiamide. 

It was not until 1939 that melamine-formaldehyde resins were prepared and the commercial importance of melamine was realized. This led to extensive exploration of the commercial synthesis of melamine. Today, melamine is almost exclusively produced on an industrial scale from urea and dicyanodiamide (vide infra). 

Heating dicyanodiamide (1) above its melting point leads to melamine (3),389 but today melamine is manufactured from urea by a catalytic process390 (see Section 2.3.1.1.1.1.). Some substituted melamines can be obtained starting from substituted dicyanodiamides.391,392... 

The simple methylol compounds and the low molecular weight polymers obtained from urea and melamine are soluble in water. They are quite suitable for the manufacture of adhesives, molding compounds, and some kinds of textile treating resins. However, amino resins for coating applications require compatibility with the film-forming polymer resins with which they must react. Furthermore, even where compatible, the free methylol compounds are often too reactive and too unstable for use in a coating formulation that may have to be stored for some time before use. Reacting the... 

Application The low-pressure melamine process is used to produce melamine powder from urea. 

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