Melamine is an organic compound that is often combined with formaldehyde to produce melamine resin, a synthetic polymer which is fire resistant and heat tolerant. Melamine resin is a very versatile material with a highly stable structure. Uses for melamine include whiteboards, floor tiles, kitchenware, fire retardant fabrics, and commercial filters. Melamine can be easily molded while warm, but will set into a fixed form. This property makes it ideally suited to certain industrial applications.
Melamine resin is manufactured by mixing urea with formaldehyde under heat and pressure. The substances begin to polymerize and are forced into a mold which will create the desired shape. Under pressure, melamine releases water, which could make the plastic unstable if it is not removed. The materials finish polymerizing and create a finished product, melamine resin.
Melamine resin is known as a thermoset plastic, because the plastic is fixed after molding. If exposed to enough heat, melamine will melt. For this reason, melamine dishware should not be exposed to high temperatures like those in the oven and microwave. However, the plastic is able to withstand higher temperatures than other plastics. Because it is a thermoset plastic, melamine resin is difficult to recycle.
Melamine can be made into a foam product. Melamine foam has a distinctive structure composed of stacked bubble shapes, which are extremely hard and therefore can easily clean a wide variety of substances. Melamine foam is marketed under a variety of commercial names including Magic Eraser, a cleaning tool well known for removing scuffs and dirt from a wide range of surfaces.
Melamine resin is used in Formica and similar construction products made from composite materials. Formica is made using melamine resin, which is used to coat the fibers in the upper layer of the construction product. The melamine resin makes the end result heat resistant, so that hot objects can be set on the counter without concern. The surface of the material is designed to be easily wiped and cleaned, creating a long lived household product.
Melamine also plays a role in a wide range of flame resistant materials. These include textiles used in upholstery and the uniforms worn by firemen. Thermal liners, heat resistant gloves, and aprons to protect from splashback of hot substances are made using melamine. Melamine will protect a wearer from heat hazards, and will help to resist the spread of fire in aircraft and buses by providing a fire blocker.
Melamine is also used in the manufacture of some filters. The material is porous and will admit substances to pass through, but can be used to filter out particles of a particular size. Melamine filters are capable of handling a high capacity and can be used in hot environments due to the heat resistance of melamine. Melamine filters are also extremely efficient.
Aside from common commercial uses, melamine became a topic of much discussion in early 2007, when veterinary scientists determined it to be the cause of hundreds of pet deaths, because of pet food contamination. Prior to these reports, melamine had been regarded as non-toxic or minimally toxic. However, because of the unexplained presence of melamine in wheat gluten added to mass-produced dog and cat foods, it is the most likely cause. Pet owners report symptoms that are commonly associated with renal failure, which could be explained by the ammonia that may result from the digestion of the melamine.
In September 2008, several companies were implicated in a scandal involving milk and infant formula which had been adulterated with melamine, leading to kidney stones and other renal failure, especially among young children. By 22 September, nearly 53,000 people had become ill, with more than 12,800 hospitalizations and four infant deaths.
Melamine may have been added to fool government protein content tests after water was added to fraudulently dilute the milk. Because of melamine's high nitrogen content, it can cause the protein content of food to appear higher than the true value.[41][42] Officials estimate that about 20 percent of the dairy companies tested in China sell products tainted with melamine.
2008 Detection of melamine in "White Rabbit" candy
In September 2008, tests by the Connecticut Agriculture Experiment Station Laboratory determined that "White Rabbit Creamy Candy", imported from China, contains melamine.Detection of melamine in food
Until the 2007 pet food recalls, melamine had not routinely been monitored in food, except in the context of plastic safety or insecticide residue. This could be due to the previously assumed low toxicity of melamine, and the relatively expensive methods of detection.
Because melamine resin is often used in food packaging and tableware, melamine at ppm level (1 part per million) in food and beverage has been reported due to migration from melamine-containing resins. Small amounts of melamine have also been reported in foodstuff as a metabolite product of cyromazine, an insecticide used on animals and crops.
Synthesis
Melamine was first synthesized by the German chemist Justus von Liebig in 1834. In early production, first calcium cyanamide is converted into dicyandiamide, then heated above its melting temperature to produce melamine. However, today most industrial manufacturers use urea in the following reaction to produce melamine:
- 6 (NH2)2CO → C3H6N6 + 6 NH3 + 3 CO2
It can be understood as two steps.
First, urea decomposes into cyanic acid and ammonia in an endothermic reaction:
- 6 (NH2)2CO → 6 HCNO + 6 NH3
Then, cyanic acid polymerizes to form melamine and carbon dioxide:
- 6 HCNO → C3H6N6 + 3 CO2
The second reaction is exothermic but the overall process is endothermic.
The above reaction can be carried out by either of two methods: catalyzed gas-phase production or high pressure liquid-phase production. In one method, molten urea is introduced onto a fluidized bed with catalyst for reaction. Hot ammonia gas is also present to fluidize the bed and inhibit deammonization. The effluent then is cooled. Ammonia and carbon dioxide in the off-gas are separated from the melamine-containing slurry. The slurry is further concentrated and crystallized to yield melamine.Major manufacturers and licensors such as DSM, BASF and Eurotecnica have developed some proprietary methods.
The off-gas contains large amounts of ammonia. Therefore melamine production is often integrated into urea production which uses ammonia as feedstock.
Crystallization and washing of melamine generates a considerable amount of waste water, which is a pollutant if discharged directly into the environment. The waste water may be concentrated into a solid (1.5-5% of the weight) for easier disposal. The solid may contain approximately 70% melamine, 23% oxytriazines (ammeline, ammelide and cyanuric acid), 0.7% polycondensates (melem, melam and melon).
Recent production of melamine in mainland China
Between the late 1990s and early 2000s, both consumption and production of melamine grew considerably in mainland China. In the United States Geological Survey 2004 Minerals Survey Yearbook, in a report on worldwide nitrogen production, the author stated that "(mainland) China continued to plan and construct new ammonia and urea plants using coal gasification technology."
By early 2006, melamine production in mainland China is reported to be in "serious surplus". In April 2007, DSM's melamine industry update painted a grave global picture. Between 2002 and 2007, while the global melamine price remained stable, a steep increase in the price of urea (feedstock for melamine) has reduced the profitability of melamine manufacturing. Currently, China is the world's largest exporter of melamine, while its domestic consumption still grows by 10% per year. However, reduced profit has already caused other joint melamine ventures to be postponed there.
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