melamine

melamine, a colourless crystalline substance belonging to the family of heterocyclic organic compounds, which are used principally as a starting material for the manufacture of synthetic resins. Melamine is rich in nitrogen, a property that is similar to protein.

Manufacture and applications of melamine

Melamine can be manufactured from dicyandiamide, hydrogen cyanide, or urea. Modern commercial production of melamine typically employs urea as a starting material. Urea is broken down to cyanuric acid, which then can be reacted to form melamine. Its most important reaction is that with formaldehyde, forming melamine-formaldehyde resins of high molecular weight. These compounds form under the influence of heat and then become fixed into an insoluble and infusible mold; this process is known as thermosetting. Melamine-based thermoset materials contain cross-linked polymers, which make the fixed molds strong and durable. Usually formulated with fillers and pigments, melamine resins can be molded into dishes, containers, utensils, handles, and the like or used as laminating agents or coating materials for wood, paper, and textiles. Formica and Melmac are well-known trade names for products based on melamine resins.

Melamine has also been incorporated into a variety of flame-retardant materials. This application is based on the compound’s high nitrogen content. When exposed to heat, melamine degrades and releases nitrogen. The freed nitrogen takes the place of oxygen in the surface air surrounding the material, which prevents the material from burning. Butylated melamine resins, made by incorporating butyl alcohol into the melamine–formaldehyde reaction mixture, are fluids used as ingredients of paints and varnishes. A copolymer containing melamine, formaldehyde, and sodium bisulfite produces a foam with sound-absorbing and flame-retardant properties. The foam has a notably hard microbubble structure, which gives it an abrasive quality that has been utilized in the development of cleaning products.

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major industrial polymers: Melamine-formaldehyde polymers

Melamine and food safety

The misuse of melamine, namely the adulteration of various food products with the chemical, raised significant public health and food safety concerns in the first decade of the 21st century. Beginning in the late 1950s, melamine was utilized as a nonprotein nitrogen (NPN) compound in ruminant feed, since microorganisms in the ruminant stomach are able to metabolize the chemical into protein. However, the addition of melamine to animal feeds was later discouraged, since the process of microbial protein generation from melamine was inefficient and resulted in the release of potentially toxic amounts of ammonia byproduct. Although many manufacturers switched to the use of more-efficiently metabolized NPNs such as urea, melamine use in animal feeds persisted, particularly in China, where manufacturers illegally added large amounts of melamine to a variety of products to pass industry tests, specifically the Dumas test and the Kjeldahl method. These tests are frequently used to detect protein levels in milk and other food products based on nitrogen content. Because melamine is abundant in nitrogen, these tests are unable to distinguish between melamine-contaminated and melamine-free products.

Evidence of misuse of the chemical in China emerged in 2007, when more than 4,000 dogs and cats in the United States died from kidney failure as a result of ingesting melamine-tainted pet food. The substance was found in wheat gluten and in rice protein concentrate that had been imported from China and used in the United States to make the foods. Some 100 brands of pet foods were recalled as a result of the incident. The following year, melamine contamination of infant formula in China led to the hospitalization of more than 50,000 infants as a result of renal dysfunction. Studies of melamine toxicity in humans and pets revealed that, although low-level exposure to the chemical does not usually cause adverse health affects, in high concentrations it crystallizes in the urine, leading to kidney stones and other renal disorders. An investigation into the contamination of infant formula conducted within China in 2008 traced the production of tainted products to more than 20 dairy manufacturers. Chinese dairy product exports, including certain chocolate and milk products, were also found to contain melamine and were subsequently recalled in Australia, New Zealand, Hong Kong, Singapore, and Thailand.

Melamine-based plastics sold as dinnerware and kitchenware have also raised safety concerns. Melamine plastics degrade and release chemical constituents upon heating to temperatures utilized for cooking in ovens and microwaves, as well as upon exposure to acidic foodstuffs. As a result, there is a risk of melamine and formaldehyde discharge into foods prepared or stored in melamine-containing dishes, bowls, and other products in the home. Although the extent of impact on human health from chemical degradation due to heating remains unclear, manufacturers of melamine plastics typically supply instructions on the safe use of their products in order to minimize risk. Advancements in heat-stability of melamine-based plastics have also helped reduce the risk of chemical leaching.