Melamine (pronounced
/ˈmɛ lə miːn/ melamine (
help·
info)) is an
organic base and a
trimer of
cyanamide, with a 1,3,5-
triazine skeleton. Like cyanamide, it contains 66%
nitrogen by mass and, if mixed with resins, has
fire retardant properties due to its release of nitrogen gas when burned or charred, and has several other industrial uses. Melamine is also a
metabolite of
cyromazine, a
pesticide. It is formed in the body of mammals who have ingested
cyromazine.
[2] It has been reported that cyromazine can also be converted to melamine in plants.
[3][4]Melamine combines with
cyanuric acid to form
melamine cyanurate, which has been implicated in the
Chinese protein export contaminations.The German word
melamin was coined by combining the names of 2 other chemical products:
Melam (a distillation derivative of ammonium thiocyanate) and
Amine.
[5] [6]Melamine is combined with
formaldehyde to produce
melamine resin, a very durable
thermosetting plastic used in
Formica, and
melamine foam, a
polymeric cleaning product. The end products include countertops, dry erase boards, fabrics,
glues, housewares and flame retardants. Melamine is one of the major components in Pigment Yellow 150, a colorant in inks and plastics.
Melamine also enters the fabrication of melamine poly-sulfonate used as
superplasticizer for making high-resistance
concrete. Sulfonated
melamine formaldehyde (SMF) is a polymer used as
cement admixture to reduce the water content in concrete while increasing the fluidity and the workability of the mix during its handling and pouring. It results in concrete with a lower porosity and a higher mechanical strength exhibiting an improved resistance to aggressive environments and a longer life-time.
The use of melamine as fertilizer for crops had been envisaged during the '50s and '60s because of its high nitrogen content (2/3).
[7] However, the hydrolysis reactions of melamine leading to the nitrogen mineralisation in soils are very slow, precluding a broad use of melamine as fertilizing agent.
Melamine derivatives of
arsenical drugs are potentially important in the treatment of African
trypanosomiasis.
[8]Melamine use as
non-protein nitrogen (NPN) for cattle was described in a 1958 patent.
[9] In 1978, however, a study concluded that melamine "may not be an acceptable non-protein N source for ruminants" because its
hydrolysis in cattle is slower and less complete than other nitrogen sources such as
cottonseed meal and urea.
[10]Melamine is sometimes illegally added to food products in order to increase the apparent protein content. Standard tests such as the
Kjeldahl and
Dumas tests estimate protein levels by measuring the nitrogen content, so they can be misled by adding nitrogen-rich compounds such as melamine.
[11]Melamine is described as being "Harmful if swallowed, inhaled or absorbed through the skin. Chronic exposure may cause cancer or reproductive damage. Eye, skin and respiratory irritant.” However, the toxic dose is on a par with common table salt with an
LD50 of more than 3 grams per kilogram of bodyweight.
[12] FDA scientists explained that when melamine and cyanuric acid are absorbed into the bloodstream, they concentrate and interact in the urine-filled renal microtubules, then crystallize and form large numbers of round, yellow crystals, which in turn block and damage the renal cells that line the tubes, causing the kidneys to malfunction.
[13]The European Union set a standard for acceptable human consumption of melamine at 0.5 milligrams per kg of body mass, Canada declared a limit of 0.35 mg and the US FDA’s limit was put at 0.63 mg, but was later reduced to 0.063 mg daily. The
World Health Organization’s food safety director estimated that the amount of melamine a person could stand per day without incurring a bigger health risk, the "tolerable daily intake" (TDI), was 0.2 mg per kg of body mass.
[14]Melamine is reported to have an oral LD50 of 3248
mg/kg based on rat data. It is also an irritant when inhaled or in contact with the skin or eyes. The reported dermal LD50 is >1000 mg/kg for rabbits.
[15] In a 1945 study. A study by
USSR researchers in the 1980s suggested that
melamine cyanurate, commonly used as a fire retardant,
[16] could be more toxic than either melamine or cyanuric acid alone.
[17] For rats and mice, the reported LD50 for melamine cyanurate was 4.1 g/kg (given inside the stomach) and 3.5 g/kg (via inhalation), compared to 6.0 and 4.3 g/kg for melamine and 7.7 and 3.4 g/kg for cyanuric acid, respectively.
A toxicology study conducted after recalls of contaminated pet food concluded that the combination of melamine and cyanuric acid in diet does lead to acute renal failure in cats.
[18]Ingestion of melamine may lead to
reproductive damage, or
bladder or
kidney stones, which can lead to
bladder cancer.
[15][19][20][21][22]A study in 1953 reported that dogs fed 3% melamine for a year had the following changes in their urine: (1) reduced
specific gravity, (2)
increased output, (3) melamine
crystalluria, and (4)
protein and
occult blood.
[23]A survey commissioned by the American Association of Veterinary Laboratory Diagnosticians suggested that crystals formed in the kidneys when melamine combined with cyanuric acid, "don't dissolve easily. They go away slowly, if at all, so there is the potential for chronic toxicity."
[24][25][26]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:
(NH2)2CO → HCNO + 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.
[27] 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).
[28]
