Melamine processes

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. 

Figure 13.1. DSM High-Pressure Melamine Process (Reproduced by permission of DSM)...

Melamine process technologies can be licensed from DSM, BASF, Agrolinz, AUied-Eurotecnica, Nissan, Montedison, and Melamine Chemicals114. 

Melamine Process Block Diagram, Eurotecnica - Melamine, www.eurotecnica.it/melamine.htm, Milan, Italy, 2003. 

The major melamine process technologies and the suppliers of these technologies are listed in Reference 114. 

Fig. 22.23. Eurotecnica high-pressure melamine process. (Reproduced by permission of Eurotecnica.)...

The cyanuric acid needs to be minimized or pyrolyzed to isocyanic acid for reaction with ammonia to form urea. It might also be fed into a urea to melamine process. 

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

Description The melamine process is a catalytic vapor-phase process operated at pressures below 10 bar. 

Table 2. Observations concerning modelling in the development of melamine process.

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. 

Uses. The principal use of adiponitrile is for hydrogenation to hexamethylene diamine leading to nylon-6,6. However, as a result of BASE s new adiponitrile-to-caprolactam process, a significant fraction of ADN produced may find its way into nylon-6 production. Adipoquanamine, which is prepared by the reaction of adiponitrile with dicyandiamide [461-58-5] (cyanoguanidine), may have uses in melamine—urea amino resins (qv) (see "Benzonitrile, Uses"). Its typical Hquid nitrile properties suggest its use as an extractant for aromatic hydrocarbons. 

A small amount of particleboard is made with a fire-retardant treatment for use in locations where codes require this material, as in some offices and elevators. Particleboards receive overlay and finishing treatments with ease. Wood veneers, melamine overlays, printed paper overlays, vinyl overlays, foils, and direct grain printing can all be done quite simply. A small amount of particleboard is also made in the form of shaped, molded articles such as furniture parts, paper roU plugs, bmsh bases, and even toilet seats. There is another small increment of particleboard made by the extmsion process. These products are made in small captive operations owned by furniture manufacturers which consume all of this production in their furniture. The extmsion process differs from conventional flat-pressed particleboard in that the wood furnish is forced between two stationary heated surfaces. The mats are formed from one edge and this edge is alternately formed and pushed between the heated platens, which are maintained at a distance equal to the thickness of board produced. This is an old, slow, small-scale process, but is stiU in use in at least one location. 

THPC—Amide Process. The THPC—amide process is the first practical process based on THPC. It consists of a combination of THPC, TMM, and urea. In this process, there is the potential of polymer formation by THPC, melamine, and urea. There may also be some limited cross-linking between cellulose and the TMM system. The formulation also includes triethanolamine [102-71-6J, an acid scavenger, which slows polymerization at room temperature. Urea and triethanolamine react with the hydrochloric acid produced in the polymerization reaction, thus preventing acid damage to the fabric. This finish with suitable add-on passes the standard vertical flame test after repeated laundering (80). 

The commonly used resins in the manufacture of decorative and industrial laminates ate thermosetting materials. Thermosets ate polymers that form cross-linked networks during processing. These three-dimensional molecules ate of essentially infinite size. Theoretically, the entire cured piece could be one giant molecule. The types of thermosets commonly used in laminates ate phenoHcs, amino resins (melamines), polyesters, and epoxies. 

Figure 4d represents in situ encapsulation processes (17,18), an example of which is presented in more detail in Figure 6 (18). The first step is to disperse a water-immiscible Hquid or soHd core material in an aqueous phase that contains urea, melamine, water-soluble urea—formaldehyde condensate, or water-soluble urea—melamine condensate. In many cases, the aqueous phase also contains a system modifier that enhances deposition of the aminoplast capsule sheU (18). This is an anionic polymer or copolymer (Fig. 6). SheU formation occurs once formaldehyde is added and the aqueous phase acidified, eg, pH 2—4.5. The system is heated for several hours at 40—60°C.

Fig. 6. Flow diagram of microencapsulation process that utilises acid-cataly2ed in situ polymerisation of melamine or urea with formaldehyde to form a...

Polymers. AH nitro alcohols are sources of formaldehyde for cross-linking in polymers of urea, melamine, phenols, resorcinol, etc (see Amino RESINS AND PLASTICS). Nitrodiols and 2-hydroxymethyl-2-nitro-l,3-propanediol can be used as polyols to form polyester or polyurethane products (see Polyesters Urethane polymers). 2-Methyl-2-nitro-l-propanol is used in tires to promote the adhesion of mbber to tire cord (qv). Nitro alcohols are used as hardening agents in photographic processes, and 2-hydroxymethyl-2-nitro-l,3-propanediol is a cross-linking agent for starch adhesives, polyamides, urea resins, or wool, and in tanning operations (17—25). Wrinkle-resistant fabric with reduced free formaldehyde content is obtained by treatment with... 

Titanium alkoxides are used for the hardening and cross-linking of epoxy, siUcon, urea, melamine, and terephthalate resins in the manufacture of noncorrodable, high temperature lacquers in the sol-gel process as water repellents and adhesive agents (especially with foils) to improve glass surfaces as catalyst in olefin polymeri2ation, and for condensation and esterification. 

Ammonia is used in the fibers and plastic industry as the source of nitrogen for the production of caprolactam, the monomer for nylon 6. Oxidation of propylene with ammonia gives acrylonitrile (qv), used for the manufacture of acryHc fibers, resins, and elastomers. Hexamethylenetetramine (HMTA), produced from ammonia and formaldehyde, is used in the manufacture of phenoHc thermosetting resins (see Phenolic resins). Toluene 2,4-cHisocyanate (TDI), employed in the production of polyurethane foam, indirectly consumes ammonia because nitric acid is a raw material in the TDI manufacturing process (see Amines Isocyanates). Urea, which is produced from ammonia, is used in the manufacture of urea—formaldehyde synthetic resins (see Amino resins). Melamine is produced by polymerization of dicyanodiamine and high pressure, high temperature pyrolysis of urea, both in the presence of ammonia (see Cyanamides). 

The decorative plastic laminates widely used for countertops and cabinets are based on melamine—formaldehyde resin (see Laminates). Several layers of phenohc-saturated kraft paper are placed in a press and a sheet of a-ceUulose paper printed with the desired design and impregnated with melamine—formaldehyde resin is placed over them. Then a clear a-ceUulose sheet, similarly impregnated with the resin, is placed on top to form a clear, protective surface over the decorative sheet. The assembly is cured under heat and pressure up to 138°C and 10 MPa (1450 psi). A similar process is used to make wall paneling, but because the surfaces need not be as resistant to abrasion and wear, laminates for wall panels are cured under lower pressure, about 2 MPa (290 psi). 

The thermoplastic or thermoset nature of the resin in the colorant—resin matrix is also important. For thermoplastics, the polymerisation reaction is completed, the materials are processed at or close to their melting points, and scrap may be reground and remolded, eg, polyethylene, propjiene, poly(vinyl chloride), acetal resins (qv), acryhcs, ABS, nylons, ceUulosics, and polystyrene (see Olefin polymers Vinyl polymers Acrylic ester polymers Polyamides Cellulose ESTERS Styrene polymers). In the case of thermoset resins, the chemical reaction is only partially complete when the colorants are added and is concluded when the resin is molded. The result is a nonmeltable cross-linked resin that caimot be reworked, eg, epoxy resins (qv), urea—formaldehyde, melamine—formaldehyde, phenoHcs, and thermoset polyesters (qv) (see Amino resins and plastics Phenolic resins). 

Reactions. The reactions of dicyandiamide resemble those of cyanamide. However, cycUzations take place easily and the nitrile group is less reactive. Under pressure and ia the presence of ammonia, dicyandiamide cyclizes to melamine. Considerable toimages of melamine have been made ia this manner however, melamine is produced chiefly by the urea process (43). 

Additional catalytic processes. Nitrobenzene is hydrogenated to aniline (U.S. Patent 2,891,094). Melamine and isophthalouitrile are produced in catalytic fluidized-bed readers. Badger has announced a... 

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 . 

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. 

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