Physical and chemical characteristics
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Melamine is pure white monoclinic crystal, tasteless, and its density is 1.573g/cm3 (16℃). Normal pressure melting point 354℃ (decomposition); Rapid heating sublimation, sublimation temperature is 300℃. Soluble in hot water, slightly soluble in cold water, slightly soluble in hot ethanol, insoluble in ether, benzene and carbon tetrachloride, soluble in methanol, formaldehyde, acetic acid, hot ethylene glycol, glycerol, pyridine, etc. Low toxicity. It is generally stable, but it may release cyanide at high temperature.
It is weakly alkaline (pKb=8) and can form melamine salt with hydrochloric acid, sulfuric acid, nitric acid, acetic acid and oxalic acid. Under neutral or weak alkaline conditions, all kinds of hydroxymethyl melamine are condensed with formaldehyde, but under weak acidic conditions (pH 5.5 ~ 6.5), they are condensed with hydroxymethyl derivatives to produce resin products. When hydrolyzed with strong acid or strong alkali aqueous solution, amino groups are gradually replaced by hydroxyl groups and become cyanuric acid diamide, which is further hydrolyzed to form cyanuric acid monoamide, and finally cyanuric acid is generated.
Main uses Melamine is a basic organic chemical intermediate product, which has a wide range of uses. Its main use is as a raw material for producing melamine formaldehyde resin (MF). Melamine can also be used as flame retardant, water reducer and formaldehyde cleaner. The hardness of this resin is higher than that of urea-formaldehyde resin, and it is nonflammable, water-resistant, heat-resistant, aging-resistant, arc-resistant and chemical-resistant. It has good insulation performance, luster and mechanical strength, and is widely used in wood, plastics, coatings, paper-making, textile, leather, electric power, medicine and other industries. Its main uses are as follows:
(1) Decorative board: it can be made into fire-proof, earthquake-resistant and heat-resistant laminated board, bright-colored, high-strength and heat-resistant decorative board, used as veneer for airplanes, ships and furniture, as well as fire-proof, earthquake-resistant and heat-resistant building decorative materials.
(2) Coatings: After etherification with butanol and methanol, it can be used as the base material of advanced thermosetting coatings and solid powder coatings, and can be used to make metal coatings and advanced amino resin decorative paints for automobiles and electrical appliances.
(3) Molding powder: Melamine plastic can be made by mixing, granulating and other processes. It is not over standard, resistant to contamination, and can still maintain good electrical properties when wet. It can be made into white, fall-proof household utensils, sanitary ware, porcelain-like tableware, electrical equipment and other advanced insulating materials.
(4) Paper: It can be used as a paper treatment agent after etherification with ether to produce high-grade paper such as anti-wrinkle, shrink-proof and non-perishable banknotes and military maps.
(5) Melamine formaldehyde resin can be mixed with other raw materials to produce fabric finishing agent, leather tanning agent, polishing agent and waterproof agent, rubber adhesive, combustion improver, high-efficiency cement water reducer, steel desalter, etc.
Biological toxicity
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At present, melamine is considered to be slightly toxic, and the lethal dose to rats is above 3 g/kg body weight. According to an experimental report of 1945, rats, rabbits and dogs were fed with large doses of melamine, and no obvious poisoning phenomenon was found. Long-term intake of melamine in animals will cause damage to reproductive and urinary systems, bladder and kidney calculi, and further induce bladder cancer. 1994 Volume III of the International Handbook of Chemical Safety and the International Chemical Safety Card jointly edited by the International Chemical Safety Agency and the European Commission only show that long-term or repeated intake of melamine may affect the kidneys and bladder, leading to stones. However, the preliminary investigation of pet food pollution in the United States in 2007 showed that wheat protein powder mixed with melamine ≤6.6% was the cause of pet food poisoning, which put a question mark on the above conclusion of slight toxicity. However, for safety reasons, melamine tableware will be marked "Do not use in the microwave oven".
Pseudoprotein principle
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Melamine is often used as a food additive by unscrupulous businessmen to improve the protein content index in food testing due to the defects in the detection methods of protein content in food and feed industries, so melamine is also called "protein essence".
Protein is mainly composed of amino acids, and its nitrogen content generally does not exceed 30%, while the molecular formula of melamine contains about 66% nitrogen. The common protein test method "Kjeldahl method" estimates the content of protein by measuring the content of nitrogen. Therefore, adding melamine will make the protein content of food high, so that inferior food can pass the inspection of food inspection agencies. It is estimated that the content of test protein in plant protein powder and feed will increase by one percentage point, and the cost of using melamine is only 1/5 of that of real protein raw materials. Melamine, as a white crystalline powder, has no smell and taste, and it is not easy to be found after doping.
Milk powder incident: the protein content in all brands of milk powder is 15-20% (marked as 10-20% on supermarket packaging at night), and the average nitrogen content in protein is 16%. If the protein content of a qualified milk is 2.8% and the nitrogen content is 0.44%, the protein content of a qualified milk powder is 18% and the nitrogen content is 2.88%. The nitrogen content of melamine is 66.6%, which is 15 1 times that of milk and 23 times that of milk powder. Adding 0. 1g melamine to every 100g milk can increase protein by 0.4%.
Slightly soluble means that the solute 1g(ml) can be dissolved in the solvent 100 ~ less than 1000ml, and melamine is slightly soluble in water. For the solubility in oil-in-water emulsion of milk, no experimental data have been found yet, but I think it is better than water and needs to be verified.
synthetic technique
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Melamine was first synthesized by Justus von Liebig in 1834. In the early stage, dicyandiamide method was adopted: calcium carbide (CaC2) was hydrolyzed and dimerized to produce dicyandiamide, and then heated and decomposed to produce melamine, so as to prepare calcium cyanamide (CaCN2). At present, due to the high cost of calcium carbide, dicyandiamide method has been eliminated. Compared with this method, urea method has low cost and is widely used at present. Urea is boiled at 380-400℃ with ammonia as the carrier and silica gel as the catalyst, which is decomposed to produce cyanic acid and then condensed to produce melamine.
6 (NH2)2CO → C3H6N6 + 6 NH3 + 3 CO2
Cooling and trapping the generated melamine gas to obtain a crude product, then dissolving, removing impurities and recrystallizing to obtain a finished product. The production of melamine by urea method consumes about 3800 kilograms of urea and 500 kilograms of liquid ammonia per ton of product.
According to different reaction conditions, the synthesis process of melamine can be divided into high-pressure method (7- 10MPa, 370-450℃, liquid phase), low-pressure method (0.5- 1MPa, 380-440℃, liquid phase) and atmospheric method (
Most foreign melamine production processes are named after technology development companies, such as BASF process in Germany, Linz process in Austria, Lurgi process, United signal chemistry in the United States, Nissan process in Japan, DSM process in the Netherlands and so on. According to the different synthesis pressure, these production processes can be basically divided into three processes: high pressure process, low pressure process and atmospheric pressure process. At present, Japanese Nissan process, Italian Allied-Eurotechnica high-pressure process, Dutch DSM low-pressure process and German BASF atmospheric process are advanced and competitive processes in the world.
Melamine production enterprises in China mostly adopt semi-dry atmospheric pressure process. Under 0. 1MPa, urea is used as raw material and silica gel is used as catalyst to synthesize melamine at about 390℃. The melamine is crystallized in the condenser, and the crude product is dissolved, filtered and crystallized into the finished product.