Dicyandiamid

It is manufactured by heating dicyandiamide, H2N C(NH) NH CN, either alone or in the presence of ammonia or other alkalis, in various organic solvents. Melamine is an important material in the plastics industry. Condensed with melhanal and other substances it gives thermosetting resins that are remarkably stable to heat and light. U.S. production 1980 80 000 tonnes. 

Melamine - formaldehyde polymers. Melamine (2 4 6-triamino-1 3 5-triazine), obtained by heating dicyandiamide under pressure, condenses with formalin to give melamine - formaldehyde polymers (Beetle - Melamine), which have similar uses, but better stability to heat... 

Dicyandiamide reacts with the chlorhydrate of 2-aminothiazole compound 132 is produced (Scheme 87) (295). 

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. 

In the commonly used Welland process, calcium cyanamide, made from calcium carbonate, is converted to cyanamide by reaction with carbon dioxide and water. Dicyandiamide is fused with ammonium nitrate to form guanidine nitrate. Dehydration with 96% sulfuric acid gives nitroguanidine which is precipitated by dilution. In the aqueous fusion process, calcium cyanamide is fused with ammonium nitrate ia the presence of some water. The calcium nitrate produced is removed by precipitation with ammonium carbonate or carbon dioxide. The filtrate contains the guanidine nitrate that is recovered by vacuum evaporation and converted to nitroguanidine. Both operations can be mn on a continuous basis (see Cyanamides). In the Marquerol and Loriette process, nitroguanidine is obtained directly ia about 90% yield from dicyandiamide by reaction with sulfuric acid to form guanidine sulfate followed by direct nitration with nitric acid (169—172). 

Phosphoric Acid-Based Systems for Cellulosics. Semidurable flame-retardant treatments for cotton (qv) or wood (qv) can be attained by phosphorylation of cellulose, preferably in the presence of a nitrogenous compound. Commercial leach-resistant flame-retardant treatments for wood have been developed based on a reaction product of phosphoric acid with urea—formaldehyde and dicyandiamide resins (59,60). 

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... 

Other amino resias used ia the textile iadustry for rather specific properties have iacluded the methylo1 derivatives of acrylamide (46), hydantoia [461-79-3] (47), and dicyandiamide (48). 

In North America, calcium cyanamide is no longer used as fertiliser, but it has limited use in special agricultural appHcations for defoHants, fungicides, herbicides, and as a weed killer. The primary industrial use is as a chemical intermediate for the manufacture of calcium cyanide, hydrogen cyanamide solution, and dicyandiamide. Calcium cyanamide is also used to add nitrogen to steel. 

Reactions. Reactions of cyanamide are either additions to the nitrile group or substitutions at the amino group. Both are involved in the dimerization to dicyandiamide. 

Dimerization involves addition of the cyanamide anion to the nitnle group of an undissociated molecule to give the anion of cyanoguanidine, or dicyandiamide. This reaction takes place most readily at pH 8—10 where the reactants are present in favorable proportion. The product is a weaker acid than cyanamide and is protonated at once with generation of a new cyanamide anion. 

Sinulatly, nucleophilic reagents are suitable for addition reactions only if they are not so strongly basic as to produce the cyanamide anion in large amounts. In such cases, dicyandiamide is produced or a cyanamide salt is obtained. Ai,Ai-Disubstituted cyanamides do not ionize, of course, and react easily with strongly basic nucleophiles. 

For the manufacture of calcium cyanamide, cmde calcium carbide (ca 3.36 x 1.68 mm or 6 x 12 mesh) can be used, whereas for cyanamide and dicyandiamide, a 74 p.m (200 mesh) anhydrous carbide is used. 

The calcium cyanamide feed is weU mixed with the recycled slurry and filtrate ia a feed vessel. The calcium cyanamide is added at a rate to maintain a pH of 6.0—6.5 ia the cooling tank. The carbonation step can be conducted ia a turbiae absorber with a residence time of 1—2 min. After the carbonation step, the slurry is held at 30—40°C to complete the formation of calcium carbonate, after which the slurry is cooled and filtered. AH equipment for the process is preferably of stainless steel. The resulting solution is used directiy for conversion to dicyandiamide. 

The filtered cake produced from the manufacture of dicyandiamide contains about 86% calcium carbonate. American Cyanamid Co. blends the dried waste for the manufacture of calcium carbide-based desulfurized reagents as a gas releasiag agent. 

For production of commercial 50% solution and for recovery of crystalline cyanamide, this process is modified to improve purity and concentration. Calcium and iron may be removed by ion-exchange treatment. The commercial 50% solution is stabilized at pH 4.5—5.0 with 2% monosodium phosphate and contains less than 1.5% dicyandiamide and 0.2% urea. Such solutions are expected to show less than 1% change ia cyanamide content per month of storage below 10°C. It is advisable, however, to adjust the pH periodically duriag extended storage. Organic esters may be used iastead for improved stabihty (23). 

Specifications and Analysis. Cyanamide is sold as anhydrous, aqueous 50%, and calcium cyanamide. Aqueous 50% cyanamide solutions contain a buffer additive, usually 2% NaH2P04, to stabilize the pH and prevent formation of dicyandiamide and urea. Calcium cyanamide is stable under dry conditions. Table 2 gives a typical analysis of the three commercial forms. 

Handling and Storage. Cyanamide solution dimerizes to dicyandiamide and urea with the evolution of heat and a gradual increase in alkalinity accelerating the reaction. Storage above 30°C without pH stabilizer leads to excessive dimerization and can result in violent exothermic polymerization. Cyanamide should be stored under refrigeration and the pH tested periodically. Stabilized cyanamide can be kept at ambient temperature for a few weeks. 

Industrial uses make up most of the market for cyanamide. Calcium cyanamide is used directly for steel nitridation (34) and to some extent for desulfurization (36) (see Steel). Cyanamide is used to produce cationic starch (36) and calcium cyanide. Cyanamide is, of course, the raw material for dicyandiamide and melamine. New uses include intermediates for pesticides, detergents (37), medicines such as antihistamines, hypertension, sedatives, contraceptives, etc (38), the photography industry (39), as an additive for fuels and lubricants, as a paper preservative, and as a cement additive. 

Properties. Dicyandiamide (2) (cyanoguanidine [461-58-5]) is the dimer of cyanamide and crystallizes ia colorless monoclinic prisms. It is amphoteric, and generally soluble in polar solvents and insoluble in nonpolar solvents. Its properties are Hsted in Table 3. 

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). 

---