melamine ullmann

c 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim10.1002/14356007.a16 171.pub2

Melamine and Guanamines 1

Melamine and GuanaminesGeorge M. Crews, Melamine Chemicals, Inc., Donaldsonville, Louisiana, United States (Chap. 1, 2, 3, 4, 5,6, 7, 8 and 9)Willi Ripperger, BASF Aktiengesellschaft, Ludwigshafen, Germany (Chap. 1, 2, 3, 4, 5, 6, 7, 8 and 9)Dietrich Burkhard Kersebohm, BASF Aktiengesellschaft, Ludwigshafen, Germany (Chap. 10)Thomas Guthner, Degussa AG, Trostberg, Germany (Chap. 11)Bernd Mertschenk, Degussa AG, Trostberg, Germany (Chap. 11)

1. Introduction . . . . . . . . . . . . . . . 12. Physical Properties . . . . . . . . . . . 23. Chemical Properties . . . . . . . . . . 23.1. Thermal Behavior . . . . . . . . . . . 23.2. Hydrolysis . . . . . . . . . . . . . . . . . 33.3. Salt Formation . . . . . . . . . . . . . . 33.4. Reaction with Aldehydes . . . . . . . 34. Production . . . . . . . . . . . . . . . . 44.1. Low-Pressure Processes . . . . . . . . 44.1.1. BASF Process . . . . . . . . . . . . . . . 54.1.2. Chemie Linz Process . . . . . . . . . . 54.1.3. Stamicarbon Process . . . . . . . . . . . 64.2. High-Pressure Processes . . . . . . . 74.2.1. Melamine Chemicals Process . . . . . 8

4.2.2. Montedison (Ausind) Process . . . . . 84.2.3. Nissan Process . . . . . . . . . . . . . . 95. Quality Specications . . . . . . . . . 106. Chemical Analysis . . . . . . . . . . . 107. Storage and Transportation . . . . . 108. Uses . . . . . . . . . . . . . . . . . . . . . 119. Economic Aspects . . . . . . . . . . . . 1110. Toxicology . . . . . . . . . . . . . . . . . 1111. Guanamines . . . . . . . . . . . . . . . 1211.1. Production . . . . . . . . . . . . . . . . 1311.2. Uses . . . . . . . . . . . . . . . . . . . . . 1411.3. Toxicology . . . . . . . . . . . . . . . . . 1512. References . . . . . . . . . . . . . . . . . 15

Melamines (2,4,6-triamino-1,3,5-triazines)are produced fromurea. They are used in the fab-rication of melamine formaldehyde resins forlaminating and adhesive applications. Melanineis used as cross-linker in heat-cured and high-solids paint systems.Guanamines (2,4-diami-no-1,3,5-triazines) are produced from dicyan-diamide and the corresponding nitrile by base-catalyzed cyclocondensation. They are used insimilar polymer applications as for melamine.They give a lower cross-link density and higherexibility because only two amino functionali-ties are present.

1. Introduction

Melamine was rst prepared and describedin 1834 by Liebig, who obtained it from fu-sion of potassium thiocyanate with ammoniumchloride. In 1885, A. W. von Hoffmann pub-lished its molecular structure. Melamine [108-

78-1] (2,4,6-triamino-1,3,5-triazine), C3N6H6,Mr 126.13, exists mainly in the amino form:

Not until 100 years later did melaminend industrial application in the productionof melamine formaldehyde resins ( AminoResins). The rst commercial plants came onstream in the late 1930s. Since that timemelamine has become an increasingly importantchemical commodity. In 1970, world capacitywas estimated at 200 000 t. Production in 1994is 610 000 t/a.Most of themelamine produced isstill used in the fabrication of melamine form-aldehyde resins.

Until about 1960, melamine was preparedexclusively from dicyandiamide [461-58-5] (Cyanamides, Chap. 3). This conversionwas car-

2 Melamine and Guanamines

ried out in autoclaves at 10 MPa and 400 C inthe presence of ammonia, according to the equa-tion

3 H2NC (NH)NHCN2 C3N6H6In the early 1940s, Mackay discovered that

melamine could also be synthesized from urea[57-13-6] at 400 Cwith or without catalyst [6].Today,melamine is produced industrially almostexclusively from urea. Most processes using di-cyandiamide as raw material were discontinuedor replaced at the end of the 1960s.

2. Physical Properties

Melamine is manufactured and sold as ne,white, powdered crystals. The most importantphysical data for melamine are summarized inthe following list [7, 8]:mp 350 C (subl.)Density 1.573 g/cm3Dissociation constants

Kb1 (25 C) 1.1109Kb2 (25 C) 1.01014

Heat of formationH0f (25 C) 71.72 kJ/molHeat of combustion (25 C) 1967 kJ/molHeat of sublimation (25 C) 121 kJ/molMolar heat capacity (25 C) 155 J K1 mol1Specic heat capacity cp ,J kg1 K1at 273 353 K 1470at 300 450 K 1630at 300 550 K 1720

Entropy (25 C) 149 J K1 mol1Entropy of formationS0f (25 C) 835 J K1 mol1Free energy of formationG0f (25 C) 177 kJ/mol

Solubility (30 C), g/100 mL, inEthanol 0.06Acetone 0.03Dimethylformamide 0.01Ethyl cellosolve 1.12Water 0.5

The temperature dependence of the vaporpressure (in 105 Pa) in the range 417 615 Kis described by the following equation [9]:

log p = 9.7334 6484.9/TMelamine solubility in water (in grams per

100 g of H2O) over the range 20 100 C coin-cides closely with the relationship [10]:

logL = 5.101 1642/TReasonably reliable values can be obtained

by means of this equation down to 0 C.Crystal Data. Melamine forms monoclinic

crystals, space group P21/a with a = 1.0537, b= 0.7477, c = 0.7275 nm, = 1129, and Z = 4[11].

3. Chemical Properties

The chemical properties of melamine are sum-marized in detail in [1] and [2]. The s-triazinering is very stable and cleaves only under drasticconditions (e.g., heating above 600 C or fusionwith alkali compounds).

By X-ray diffraction studies, crystallinemelamine has been shown to exist only in thesymmetrical triamino structure; the same is truefor the vapor phase and for both neutral and al-kaline solutions. Although reactions are in somecases observed at the ring nitrogen atoms (theproducts being substituted isomelamines), themost important commercial reactions involveonly the NH2 groups, which behave chemi-cally as amido rather than amino functions.

3.1. Thermal Behavior

When melamine is heated above 300 C in theabsence of ammonia or at low ammonia par-tial pressure, deammoniation and condensationlead to compounds with higher molecular mass.Degradation starts with the release of ammoniaand the formation of melem [1502-47-2] (2,5,8-triamino-1,3,4,6,7,9,9b-heptaazaphenalene):

Further heating to ca. 600 Cyieldsmore am-monia and melone [32518-77-7] [12]:


Various sources disagree on hydrogen anal-ysis and other analytical data for melone. Thematerial may represent a mixture of substancessuch as 1 and 2.

Melam [(N-4,6-diamino-1,3,5-triazin-2-yl)-1,3,5-triazine-2,4,6-triamine], [3576-88-3]seems not to be an intermediate in the thermaldegradation of melamine although the evidenceis not entirely clear [12 14]. This substancecan be prepared by heating melamine salts be-low 315 C, and it arises as a byproduct ofmelamine synthesis.

The three deammoniation productsmelam,melem and meloneare formed reversibly; ad-dition of ammonia at high pressure and temper-ature regenerates melamine. Indeed, processesfor melamine production invariably rely uponexcess ammonia to suppress formation of thesebyproducts. If melamine is heated to 600 Cor higher it is partially cracked, leading tocyanamide alongwith other products. Hydrogencyanide may also appear in the crack products,especially in the absence of oxygen [15].

3.2. Hydrolysis

Melamine is hydrolyzed bymineral acid or inor-ganic alkali. Hydrolysis proceeds stepwise, withloss of one, two, or all three amino groups:

The product spectrum varies with tempera-ture, pH, and concentration; the end product iscyanuric acid [108-80-5]. Even small amountsof the oxotriazines (especially cyanuric acid andammelide) markedly effect the condensation ofmelamine with formaldehyde by increasing therate of condensation [16].

3.3. Salt Formation

Melamine is a weak base, forming well-denedsalts with both organic and inorganic acids. Themelamine ion is assigned the following struc-ture:

The water solubility of organic and inorganicsalts of melamine is no higher than that of freemelamine (see Table 1). Melamine cyanurate,melamine picrate, andmelamine perchlorate arevery insoluble in water, and are useful in thequantitative determination of melamine.

3.4. Reaction with Aldehydes

Melamine reacts with aliphatic and aromaticaldehydes to give a variety of products. Most

4 Melamine and GuanaminesTable 1. Industrially important melamine salts

Molecular formula Solubility (20 C), g per 100 gH2O

Mr mp, C

C3N6H6 H3PO4 0.43 224.12 223C3N6H6 H2SO4 0.19 350.33 380 *C3N6H6 HNO3 0.68 189.14 298 *C3N6H6 C17H35COOH 0.17 410.42 154C3N6H6 HCOOH 1.56 172.16 250

(decomp.)* Sublimes at 316 C. Sublimes at 186 C.

important is the resinous material obtained fromthe reaction of formaldehyde with melamine:

Me(NH2)3+6CH2OMe[N(CH2OH)2]3where Me represents that part of the melaminemolecule that is not involved in the reaction.

All hydrogen atoms on the melaminemolecule can be replaced by methylol groups,and products ranging from the monomethylolto the hexamethylol derivatives have been ob-served. The methylolmelamines are sparinglysoluble in most solvents and are very unsta-ble due to further condensation or resinication,e.g.,

MeNHCH2OH+H2NMe

MeNHCH2NHMe+H2O

2MeNHCH2OH

MeNHCH2OCH2NHMe+H2O

Melamine formaldehyde condensationproducts are characterized by good heat re-sistance and superior water-resisting properties.They are used, usually in combination with urea formaldehyde resins, as glues in the wood-working industry, as impregnating resins fordecorative laminates, and as a binder in mold-ing materials containing a ller (e.g., celluloseor sawdust). Methylolmelamines can be ether-ied by heating with alcohol in the presenceof an acid catalyst. Industrially most importantare the products formed with methanol, n-bu-tanol, and isobutanol. They are used as curingagents for surface coatings and as auxiliaries inthe paper and textile industries. For additionalinformation see Amino Resins [17].

4. Production

Melamine can be synthesized from urea at 390 410 C:6 H2NCONH2

C3N3(NH2)3+6NH3+3CO2The overall reaction is endothermic, requir-

ing 649 kJ per mole of melamine starting withmolten urea at 135 C.

The processes themselves may be subdividedinto two categories:

1) noncatalytic, high-pressure ( 8 MPa) pro-cesses, and

2) catalytic, low-pressure processes (ca. 1 MPa).Each type includes three stages:synthesismelamine recovery and puricationoff-gas treatment

4.1. Low-Pressure Processes

Typical low-pressure processes utilize a u-idized catalyst bed at pressures from atmo-spheric to ca. 1 MPa and temperatures of 390 410 C. The uidizing gas is either pure am-monia or the ammonia carbon dioxidemixtureformed during the course of the reaction. Cata-lysts include alumina and materials of the silica alumina type. Melamine leaves the reactor ingaseous form together with the uidizing gas; itis separated from ammonia and carbon dioxideby quenching the gas stream either with water(followed by crystallization) or with cold reac-tion gas (desublimation).

In the catalytic processes the rst reactionstep is decomposition of urea to isocyanic acid


and ammonia, after which the isocyanic acid istransformed into melamine:

The overall reaction mechanism is not yetfully understood, but isocyanic acid from thedecomposition of urea is believed to be catalyti-cally disproportionated into carbon dioxide andcyanamide or carbodiimide, which then trimer-izes to melamine [18 20]:

The melamine yield is ca. 90 95% basedon urea. Byproducts includemelam,melem, andmelone, as well as oxotriazines such as amme-line, ammelide, and cyanuric acid. Ureidotri-azine is also observed as a product of reactionbetween melamine and isocyanic acid.

Some byproducts are formed in the reactorduring synthesis; others are not generated untilthe melamine recovery section, where deammo-niation or hydrolysis occurs [20 29].

Worldwide, three low-pressure processes arein commercial operation: the BASF process, theChemie Linz process, and the Stamicarbon pro-cess.

4.1.1. BASF Process

The BASF process (see Fig. 1) is a one-stage, low-pressure, catalytic vapor-phase pro-cess. Molten urea is fed to the uidized catalyticbed reactor (a) at 395 400 C and atmosphericpressure. Alumina is used as a catalyst, and u-idization is accomplished with an NH3 CO2mixture (the process off-gas).

The reactor temperature is held at ca. 395 Cby molten salt circulated through internal heat-ing coils (b). The uidizing gas is also preheatedto 400 C. To secure an ammonia-rich atmo-sphere in the reaction zone, make-up ammoniais added to both the uidizing gas and the ureanozzles.

Gas leaving the reactor is a mixture ofgaseous melamine, traces of melem, and un-reacted urea (in the form of its decompositionproducts isocyanic acid and ammonia), as wellas ammonia and carbon dioxide (part newlyformed, part uidizing gas). In addition, thegas mixture contains entrained catalyst nes;coarser catalyst particles are retained by cycloneseparators inside the reactor.

The gas mixture leaving the reactor is cooledin the gas cooler (d) to a temperature at whichonly the byproduct melem crystallizes. Precipi-tated melem, in the form of a ne powder, is re-moved together with the entrained catalyst nesin adjacent gas lters (e).

The ltered gas mixture enters the top of thecrystallizer (f) where it is blended countercur-rently with recycled off-gas (140 C). The tem-perature in the crystallizer is thereby reducedto 190 200 C, and more than 98% of themelamine crystallizes as ne crystals.Melamineis recovered from the gas in a cyclone (g), afterwhich it is cooled and stored. It can be usedwith-out further treatment and has a minimum purityof 99.9%.

The nearly melamine-free gas stream fromthe cyclone is fed to the urea washing tower (i)where it is scrubbed with molten urea (135 C),which provides both cooling andwashing. Cleangas leaving the urea scrubber (after passingthrough droplet separators) is partially recycledto the reactor as uidizing gas and partially re-cycled to the crystallizer as quenching gas. Thesurplus is fed to an off-gas treatment unit.

A single-stage reactor has the advantage ofconverting the corrosive intermediate isocyanicacid immediately to melamine; also, the heat ofthis exothermic reaction is used directly for theendothermic decomposition of urea, therst stepin melamine synthesis.

4.1.2. Chemie Linz Process

The Chemie Linz process (see Fig. 2) is a two-stage process. In the rst step, molten urea is de-composed in a uidized sand-bed reactor (b) toammonia and isocyanic acid at ca. 350 C and0.35 MPa. Ammonia is used as the uidizinggas. Heat required for the decomposition is sup-plied to the reactor by hot molten salt circulatedthrough internal heating coils. The gas stream


Figure 1. BASF processa) Reactor; b) Heating coils; c) Fluidizing gas preheater; d) Gas cooler; e) Gas lter; f) Crystallizer; g) Cyclone; h) Blower;i) Urea washing tower; j) Heat exchanger; k) Urea tank; l) Pump; m) Droplet separator; n) Compressor

Figure 2. Chemie Linz processa) Heat exchanger; b) Urea decomposer; c) Converter; d) Quencher; e) Heat exchanger; f) Suspension tank; g) Heat exchanger;h) Centrifuge; i) Mother-liquor vessel; j) Disk dryer; k) Elevator; l) Delumper; m) CO2 absorption column; n) Compressor;o) Heat exchanger

is then fed to the xed-bed catalytic reactor (c)where isocyanic acid is converted to melamineat ca. 450 C and near-atmospheric pressure.

Melamine is recovered from the reaction gasby quenchingwithwater andmother liquor fromthe centrifuges (h). The quencher (d) is speciallydesigned to work quickly, thereby preventingsignicant hydrolysis of melamine to ammelideand ammeline. The melamine suspension fromthe quencher is cooled further to complete themelamine crystallization process. After beingcentrifuged, the crystals are dried, milled, andstored. A separate recrystallization step is notrequired.

Exhaust gas from the quencher is fed to an ab-sorber (m) where carbon dioxide is removed asammonium carbamate by washing with a leancarbamate solution from the off-gas treatment

section. The wet ammonia gas is dried withmake-up ammonia. Part of it is compressed andrecycled to the urea decomposer, and part is ex-ported. Remaining ammonia and carbon dioxidein the liquid efuent are then recovered in theoff-gas treatment section.

4.1.3. Stamicarbon Process

Like the BASF process, the DSM Stamicarbonprocess (see Fig. 3) involves only a single cat-alytic stage. However, it differs from the formerin that it is operated at 0.7 MPa, the uidizinggas is pure ammonia, the catalyst is of the sil-ica alumina type, and melamine is recoveredfrom the reactor outlet gas by water quench andrecrystallization.


Figure 3. Stamicarbon processa) Urea tank; b) Reactor; c) Preheater; d) Heating coils; e) Internal cyclone; f) Quench cooler; g) Scrubber; h) Hydrocyclone;i) Desorption column; j) Heat exchanger; k) Heat exchanger; l) Mixing vessel; m) Heat exchanger; n) Dissolving vessel;o) Precoat lter; p) Vacuum crystallizer; q) Pump; r) Heat exchanger; s) Mother-liquor vessel; t) Hydrocyclone; u) Centrifuge;v) Pneumatic dryer; w) Hydrocyclone; x) Absorption column; y) Compressor

Ureamelt is fed into the lower part of the reac-tor (b). The silica alumina catalyst is uidizedby preheated (150 C) ammonia, which entersthe reactor at two points: at the bottom of the re-actor to uidize the catalyst bed, and at the ureanozzles to atomize the urea feed. The reaction ismaintained at 400 C by circulating molten saltthrough heating coils within the catalyst bed.

The melamine-containing reaction mixturefrom the reactor is quenched rst in a quenchcooler (f) and then in a scrubber (g) with recy-cled mother liquor from the crystallization sec-tion. The resulting melamine suspension is con-centrated to ca. 35 wt %melamine in a hydrocy-clone (h), afterwhich it is fed to a desorption col-umn (i) where part of the ammonia and carbondioxide dissolved in the suspension is strippedoff and returned to the scrubber. The precedingsteps are all carried out at reaction pressure; forthe following stages, the pressure is reduced.

The suspension leaving the bottom of thedesorber is diluted with recycled and preheatedmother liquor and water. Activated carbon andlter aids may also be added. The melamine dis-solves completely, although separate dissolvingvessels (n) are necessary to allow sufcient timefor dissolution. The resulting solution is lteredusing precoat-type lters (o). Crystallization ofmelamine is carried out in a vacuum crystallizer

(p), and crystals are separated from the motherliquor by hydrocyclone (t) and centrifuge (u).The crystals are dried in a pneumatic dryer andthen conveyed to product bins.

Surplus ammonia must be recovered as u-idizing gas from the wet ammonia carbondioxide mixture leaving the desorption columnand the scrubber. The hot gas mixture is partlycondensed by heat-exchange (k)with themotherliquor from melamine dissolution. The conden-sate and uncondensed gas are then passed at 0.7MPa to an absorption column (x). Liquid make-up ammonia is fed to the top of this column tocondense any carbon dioxide remaining in theammonia gas. The ammonia is then compressedand recycled as uidizing and urea-atomizationgas for the reactor.

4.2. High-Pressure Processes

High-pressure melamine synthesis systems dif-fer from low-pressure processes by producingmelamine in the liquid instead of the vaporphase. They have the advantage of providinghigh-pressure off-gas more suitable for use inthe urea synthesis facility. Liquid phase opera-tion also lends itself to smaller reaction vessels,but the highly corrosive nature of the system dic-


tates use of expensive, corrosion-resistant con-struction materials such as titanium.

High-pressure reactions occur without cat-alyst at > 7 MPa and > 370 C. In general,molten urea is injected at high pressure into amolten melamine urea mixture in the reactor,where it undergoes conversion tomelamine. Suf-cient residence time is provided in the reactor toensure complete reaction, leading to melaminewith a purity> 94%. Heat is supplied to the re-actor either by electric heater elements or by amolten salt heat-transfer system. Various typesof off-gas separation and melamine puricationfollow.

High-pressure synthesis of melamine fromurea proceeds via the intermediate cyanuric acid,which is subsequently converted to melamineunder high pressure in an ammonia environment[30, 31]:

The net reaction is the same as in the low-pressure process.

4.2.1. Melamine Chemicals Process

Melamine Chemicals uses a continuous high-pressure single-stage process that producesmelamine with a purity of ca. 96 99.5%.Molten urea is converted into melamine in a liq-uid-phase reactor. The off-gases (ammonia andcarbon dioxide) are separated in a gas-separatingvessel. Liquid melamine is then quenched in acooling unit, where liquid ammonia is used tosolidify the crystals.

Process Description. Incoming urea is pre-heated by using it to scrub the reactor off-gasstream. This scrubber performs various func-tions, including (1) driving off any water thatmay be present in the urea feed, (2) preheat-ing the molten urea, (3) removing melaminefrom the off-gases, and (4) recovering excessheat energy for subsequent use. The reactoris heated to ca. 370 425 Cwith a heating-coil

system and pressurized to about 11 15 MPa.Mixing is provided by heat convection and gen-eration of gaseous reaction products.

Liquid melamine is separated from the off-gas in a gas separator, the product being col-lected at the bottom. The separator is held atabout the same temperature and pressure as thereactor. The gaseous phase (ammonia and car-bon dioxide, saturated with melamine vapor)is removed overhead to a urea scrubber. Themelamine stream leaving this separator is theninjected into the product cooling unit.

The product cooling unit employs liquid am-monia to both cool and solidify melamine. Thisis accomplished at a controlled temperature andpressure to minimize formation of such impuri-ties as melam and melem. Product is removedfrom the pressurized cooling unit through a se-ries of pressure-reducinghoppers.Dependingonthe required degree of purity, it may then be re-crystallized [32].

The off-gas stream represents high-pres-sure (> 10 MPa) ammonia and carbon dioxide,which can be used directly as feed to the ureafacility. Alternatively, this gas stream can betreated in a monoethanolamine (MEA) scrubberto recover ammonia and remove carbon dioxide.

4.2.2. Montedison (Ausind) Process

The Montedison process (see Fig. 4) operates at370 C and 7 MPa. The required temperature ismaintained by amolten-salt heating system con-sisting of concentric bayonet-type tubes.

Molten urea at 150 C is fed to the reactor (a)together with preheated ammonia. Average res-idence time of the mixture in the reactor is about20 min. As the reaction mixture leaves the reac-tor, the pressure is lowered to 2.5 MPa, and themixture is treated at 160 C in a quencher (b)with an aqueous solution of ammonia and car-bon dioxide to precipitate melamine. The water-saturated mixture of ammonia and carbon diox-ide leaving the top of the quencher can be recy-cled to a plant for urea or fertilizer production.

The aqueous melamine slurry remains in thequencher at 160 C for some time to decomposeunconverted urea and such byproducts as biuretand triuret to ammonia and carbon dioxide. Itis then fed to a steam stripper (c), where any


Figure 4.Montedison processa) Reactor; b) Quencher; c) Stripper; d) Absorption column; e) Heat exchanger; f) Filter; g) Vacuum crystallizer; h) Filter;i) Pneumatic dryer; j) Heat exchanger; k) Cyclone; l) Blower

remaining ammonia and carbon dioxide are re-moved. Off-gas from the stripper is dissolved inwater in an absorption column (d) and this solu-tion is recycled to the quencher.

The ammonia- and carbon dioxide-freemelamine slurry leaving the bottom of the strip-per is diluted with mother liquor to dissolvemelamine. Sodium hydroxide is also added, andthe solution is then claried with activated car-bon (f). Melamine is crystallized from the clar-ied solution in a crystallizer (g) operated adia-batically under vacuum. Melamine crystals areseparated from the mother liquor in a rotary l-ter (h), dried in a pneumatic conveyor dryer(i), and stored.

4.2.3. Nissan Process

The Nissan melamine process (see Fig. 5) op-erates at 10 MPa and 400 C. One characteris-tic feature is urea washing of the reactor off-gas. For this purpose, molten urea is also pres-surized to 10 MPa and passed through a high-pressure washing tower (c) where it absorbs anymelamine and unreacted urea present in off-gasleaving the reactor. The urea then ows into thereactor (a) by gravity. Ammonia is also fed tothe reactor.

In a so-called level tank (b), efuent fromthe reactor is separated into gaseous and liquidphases. The gaseous phase passes through thepreviously described urea washing tower to anoff-gas treatment facility. The liquid phase con-

sists mainly of molten melamine. This melt ismixed with hot gaseous ammonia and fed to acushion vessel (e) for aging (i.e., to allow by-products to be reconverted to melamine).

After aging, the melamine melt is quenched(f) under pressure with aqueous ammonia, inwhich it dissolves. The resulting 20 30 wt %melamine solution is retained in the quencherat 180 C until any remaining impurities havedecomposed.

Most of the added ammonia is next removedfrom the solution in an ammonia stripper (g) (op-erated at 1.5 MPa) and the solution is ltered.Recovered ammonia is recycled. Crystallizationtakes places in two crystallizers (k) operated inseries. Mother liquor and melamine crystals areseparated in centrifuges (l), after which the crys-tals are dried and crushed before storage.

Further treatment of the mother liquor startswith an ammonia stripper (n), in which oxoami-notriazines precipitate. The slurry from this am-monia recovery tower is therefore alkalinizedbefore being fed to a third crystallizer (o) operat-ing at reduced temperature and pressure. Addi-tional melamine crystallizes here, and after sep-aration from the liquid (p) it is returned to thesecond crystallizer.

Lowering the pH of the mother liquor causesoxoaminotriazines to precipitate; these are re-moved by decantation (q). The clear motherliquor is used to absorb ammonia leaving thecrystallizers and is subsequently recycled to the


Figure 5. Nissan processa) Reactor; b) Level tank; c) Off-gaswashing tower; d) Steam drum; e) Cushion vessel; f) Quencher; g) NH3-stripper; h) NH3-distillation column; i) Absorber; j) Filter; k) Crystallizers; l) Centrifuge; m) Pneumatic dryer; n) Ammonia recovery tower;o) Crystallizer; p) Separator; q) Decanter

quencher together with ammonia released in theammonia recovery tower.

5. Quality SpecicationsMany consumers of melamine are satised witha purity of 99.9%. Some, however, specify ad-ditional measurable criteria such as: content ofinorganic ash, moisture, and ammeline-relatedcompounds (alkali solubles); particle-size distri-bution; pH; resin reaction time; and resin color.Average particle sizes ranging from 15 to 100m are available for various applications. Therate of dissolution of melamine in formaldehydesolutions depends on particle size, an importantparameter that is generally also reported by pro-ducers.

6. Chemical Analysis

Melamine is difcult to characterize by tradi-tional chemical methods. Purity specicationsare usually based on differences obtained af-ter subtracting determined impurity levels formoisture, ash, and alkali solubles. Instrumentalanalysis is possible using liquid chromatogra-phy [33] and spectroscopic methods, although

melamine tends to form complexes with the pH-control buffers used in liquid chromatography,leading to variability in the UV absorption ob-served at different pHs.Melamine can be precip-itated for quantitative determination as the per-chlorate or picrate salt (see Section 3.3).

7. Storage and Transportation

Melamine is stable when stored under normalwarehouse conditions.Although not particularlyhygroscopic, powdered melamine must still beprotected from wetting because, like most pow-ders, it will pack and lump over extended storageperiods.

In the VDI guideline 2263 melamine is clas-sied as having a burning index of 2, i.e., dur-ing a re it ignites quickly but the ame israpidly extinguished. Flammability tests per-formed in accordance with the EEC guideline84/449 A 10 showed that a glowing platinumwire (>1000 C) was not able to produce conti-nous burning of melamine.

Shipping considerations are typical of thosefor other nonhazardous powders. Melamine isavailable in standard-weight paper bags andsemibulk bags. Bulk shipment is by truck andrailroad car.

Melamine and Guanamines 11Table 2.Melamine applications, in percent, by region

Application Europe United States Japan

Laminates 47 35 6Glue, adhesives 25 4 62Molding compounds 9 9 16Coatings 8 39 12Paper, textiles 11 5 3Other 8 1Total 100 100 100

8. Uses

Most melamine is reacted with formaldehyde toproduce resins for laminating and adhesive ap-plications [16, 17, 34]. One of the major usesof melamine is in the upper sheet of laminatedcounter- and tabletops.

Another important use of melamine is as theamino cross-linker in heat-cured paint systems.In this case the methylated methylolmelamineis used, with varying molar ratios of melamine,formaldehyde, and alcohol for different paintsystem applications. High-solids paint systemsfor automotive applications also constitute ama-jor market for melamine.

Other uses include the preparation ofwet-strength resins for paper, water clarify-ing resins, ion-exchange resins, plastic mold-ing compounds, adhesives, re retardants inpolyurethane foams, and intumescent paints.Important new applications are under develop-ment in the eld of re retardants for polymericmaterials, especially polyurethane foams. Ap-plications and uses of melamine differ widelyamong the main consumer countries or regions.Estimates are provided in Table 2.

9. Economic Aspects

About 18 melamine producers exist worldwide.Rated annual production capacity is ca. 550000 t (data for 1990/1991; see Table 3). In thelast decade the average annual growth rate formelamine consumption was ca. 2%.

10. ToxicologyAcute Toxicity. From the standpoint of acute

toxicity, melamine is not classied as a healthrisk. The oral LD50 for rats is > 5000 mg perkilogramof bodyweight [35].Melamine applied

to the skin and eyes of rabbits is a non-irritant[35]. Skin sensitization could not be provokedby patch tests on humans [36] or guinea pigs[37].

Table 3.Melamine production capacity worldwide

Country Company Capacity, t/a

Fed. Rep. Germany BASF 42 000Austria Chemie Linz 55 000Netherlands DSM 90 000Italy Ausind 28 000France Norsolor 15 000Western Europe 230 000

Poland Polimex Cekop 28 000Rumania Romchim 12 000Soviet Union Techmashimport 10 000Eastern Europe 50 000

United States American MelamineInd.

50 000

Melamine Chem. 47 000America 97 000

Japan MitsubishiPetrochemical

32 000

Mitsui Toatsu Chemical 38 000Nissan Chemical 42 000

Korea Korea Fertilizer 16 000Taiwan Taiwan Fertilizer 10 000Saudi Arabia Safco 20 000China Sichuan Chemical

Works12 000

India Gujarat State Fertilizer 5 000Middle and Far East 175 000Total 552 000

Genetic Toxicity. Investigations into the po-tential genetic effects ofmelaminehave includedthe following tests:

In vitro methods

1) Bacterial tests2) Ames Test [38]Escherichia coli plate test [39]

3) Test with eukaryotes4) Test with eukaryotes


Saccharomyces cerevisiae (gene conver-sion) [39]Rat hepatocytes (DNA repair) [40]Mouse lymphoma test (point mutation) [41]CHO cells (chromosomal aberrations) [42]

In vivo methods

Drosophila melanogaster (sex-related lethaltest) [43]Mouse micronucleus test (oral administrationof 1000 mg per kilogram of body weight)[44]In none of these studies could melamine-

induced mutagenicity or damage to genetic ma-terial be demonstrated.

Metabolism. Investigations into themetabolism and toxicokinetics of melamineshowed that a single oral dose of 0.38 mg of14C-labeled melamine administered to a rat waseliminated unchanged in the urine to the extentof 90% [45]. The plasma half-life was foundto be 2.7 h, with highest concentrations in thebladder and kidneys.

ChronicToxicity andCarcinogenicity. Thetarget organ system for melamine toxicity afterprolonged administration to mice and rats is theurinary tract. Both species were administered, insome cases extremely high doses in the diet (750 30 000 ppm; ca. 62 2490 mgper kilogramofbody weight per day) over periods of 14 d, 90 d,and 2 a. All studies led to changes in the kid-ney (inammation, calcied concretions in theproximal tubuli) and bladder (inammation, ul-ceration, epithelial hyperplasia, bladder stones).Daily dosages up to 15 000 ppm for mice and5000 ppm for rats administered in the diet over14 d produced no changes. Female rats toler-ated doses of 9000 ppm/d for 13 weeks withoutsymptoms. The incidence of bladder carcinomaincreased only for male rats receiving melaminein the diet over a two-year period (dosage: 4500ppm). Of eight rats displaying tumors, sevenalso had bladder stones, suggesting that chronicmechanical irritation of the mucous membranesof the bladder may have been responsible forthe tumors. Dosages that do not result in blad-der stones are not expected to be carcinogenic.Female rats in this study displayed neither blad-der stones nor carcinoma [39, 46].

An initiation-promotion study on femalemouse skin failed to reveal any tumor-inducingeffect [47]. A single dermal application of 1mol of melamine followed by a 31-week treat-ment (twice a week) with 10 nmol of a pro-moter (12-O-tetradecanoylphorbol-13-acetate)resulted in no increased incidence of tumor-bearing animals.

Reproduction Toxicology. A study reportedby Thiersch on the reproduction toxicologyof melamine was published in 1957. Intraperi-toneal administration of a dose of 70 mg/kg topregnant rats on days 4 and 5, 7 and 8, or 11 and12 of pregnancy had no inuence on the ma-ternal or fetal development, nor was there anyteratogenic effect [48].

11. Guanamines

In the course of heating guanidine acetate,Nencki (1874) obtained a new compound towhich he assigned the name guanamine; laterhe changed it to acetoguanamine after the dis-covery of other homologues. Weith recognizedin 1876 that this substance was in fact 2,4-di-amino-6-methyl-1,3,5-triazine. The term gua-namine has since been applied generally to2,4-diamino-1,3,5-triazines substituted in the 6-position with alkyl, aryl, or alkaryl residues.Such substances are named on the basis of thecarboxylic acid that contains one more carbonatom than is present in the substituent on thetriazine ring:

Compounds of this type that have majorcommercial signicance are benzoguanamine[91-76-9], C9H9N5, Mr 187.20, and acetogua-namine [542-02-9], C4H7N5, Mr 125.14 [49].

The physical properties of aceto-, benzo-, andcaprinoguanamine [5921-65-3], C12H23N5,Mr237.35, are summarized in Table 4. Figure 6 il-lustrates the most important reactions of aceto-and benzoguanamine [50 53]. For informationregarding other guanamines and their reactions,see [54 56].

Melamine and Guanamines 13Table 4. Physical properties of the most important guanamines

Acetoguanamine Benzoguanamine Caprinoguanamine

mp, C 277 228 105 120aSolubility (at 20 C),g/L, inWater 11.2 0.3 insolubleAcetone 1.04 18.0 25.2Benzene 0.07 0.3 11.2Dimethylformamide 0.88 120.0 67.0

a Liquid crystalline intermediate phase

Figure 6. Reactions of guanamines

11.1. Production

2,4-Diamino-1,3,5-triazines can be preparedfrom several aliphatic C-N-compounds [5] in-cluding the historic synthesis from biguanideswith esters [57, 58]. Aromatic guanamines can

be obtained by Pd-catalyzed Suzuki couplingfrom 1,3,5-triazine precursors [59]. The onlymethod of industrial relevance, however, in-volves the reaction of dicyandiamide (cyanogua-nidine) with nitriles [60]:


Use of dinitriles leads to bisguanamines. Re-action occurs at 105 120 C in the presence ofalkaline catalysts (e.g., KOH) in polar solvents,usually alcohols, and high yields are obtained.The rate of reaction depends on the structure ofthe nitrile, the nature of the solvent, and the con-centration of alkali. Aliphatic nitriles generallyreact more slowly than aromatic nitriles. The re-action can also be performed under microwaveirradiation [61] or in ionic liquids [62].

11.2. UsesAcetoguanamine is used as a condensation

component in amino resins. Occasional use ismade of the pure resins that result from re-action of acetoguanamine with formaldehyde.Such resins display a high degree of water tol-erance and can be cured thermally. The rate ofthe condensation reaction is considerably lessdependent on pH than in the case of melamine.Condensationmaybe carried out inweakly basicor weakly acidic media. Compared to melamine formaldehyde resins, the curing of acetogua-namine resins occurs more slowly and at a moreacidic pH.

Acetoguanamine is normally used to mod-ify melamine formaldehyde resins, conferringimproved elasticity, higher gloss, and reducedresin shrinkage. Main application is in thedecorative layer of high-pressure laminates(HPL) used for ooring and, e.g., kitchenplates. Acetoguanamine-modied melamineresins provide high gloss, low stain receptivity,and postforming properties. Even low amountsof acetoguanamine (5-10% based on melamine)are sufcient to allow small bending radii with-out affecting surface hardness or thermal stabil-ity [63]. Newer developments aim at reducingthe formaldehyde content of melamine ace-toguanamine resins in order to avoid formalde-hyde emissions after production [64]. Due to therelatively high cost of acetoguanamine,mixtureswith other modifying agents, e.g., dicyandi-amide have been applied, especially for short cy-cle laminates on wood-based materials [65](seealso Wood). Partially alkoxylated acetogua-

namine formaldehyde resins have been pro-posed for high-temperature bers with goodexibility [66]. Acetoguanamine cyanurate, ei-ther alone or withmelamine cyanurate, serves asan effective ame retardant in polyamides [67,97].

Benzoguanamine is also used primarily inamino resins. Compared to acetoguanamine,benzoguanamine produces signicantly morehydrophobic resins that have lower lightfastnessdue to the aromatic ring.

Themost important application of benzogua-namine is in resins for industrial paints. Methy-lolated benzoguanamine resins that have beenetheried with butanols are compatible with hy-drocarbon solvents, oils, and various syntheticresins (e.g., alkyd, polyester, epoxy [68] andacrylic resins [69]).Upon curing, theseOHfunc-tional resins react with the butoxymethyl groupsof the benzoguanamine resin to form new cross-links, by splitting off butanol. In contrast to sim-ilar melamine resins, benzoguanamine resinsgive a lower cross-link density, and thus a higherexibility and better surface quality. Typical ap-plications are can coatings, coil coatings, and au-tomotive basecoats. Benzoguanamine-carbox-ylic acids have been proposed as cross-linkersfor water-borne coatings [70].

Due to its favorable physicochemical prop-erties, tetra(methoxymethyl)benzoguanamine[4588-69-6] has been proposed as a cross-linkerfor powder coatings based on hydroxy poly-esters [71, 72]. However, bubbles formed duringthe elimination of methanol have limited its useto date.

Fully condensed formaldehyde benzogua-namine resins that mainly consist of methylene-bridged structures are amorphous, insolublesolids with high thermal stability. Due to theirhigh refractive index, these resins are used aswhite pigments or colorants in plastics [73] or, inconjunction with uorescent dyes, as pigmentsfor daylight uorescent inks and paints [74, 75].Benzoguanamine is used as a exibilizing agentin phenolic resins to improve the punchability,water resistance, and ame-retardance of FR2-type printed circuit boards [76]. In the sameway, it can be used as additive to urea form-aldehyde foams [77] or bismaleimide copoly-mer resins [78]. Both, acetoguanamine and ben-zoguanamine have been proposed as epoxy cur-

Melamine and Guanamines 15Table 5. Typical storage lifetime of formaldehyde solutions stabilized with guanamines

Formaldehyde concentration Guanamine added Storage temperature Storage lifetime

40% 0.1% benzoguanamine 10 C 2 weeks40% 0.05% caprinoguanamine 10 C 4 weeks50% 0.05% caprinoguanamine 40 C 4 weeks

ing agents with improved properties for mi-croelectronic encapsulation [79, 80]. By reac-tionwith 4,4-diaminodiphenyl ether, benzogua-namine forms a bridged bisguanamine whichwas proposed for resin applications [81].

Benzoguanamine can be used instead of di-chlorophenyltriazine to produce Pigment Yel-low 184, an anthraquinone dye with a triazinesubunit [82].

Caprinoguanamine is oneof the best knownstabilizers for aqueous formaldehyde solutions.It prevents or retards the precipitation of poly-meric paraformaldehyde at low storage temper-atures and/or high formaldehyde concentrations[83]. Though less effective, benzoguanamine issometimes used for the same purpose. Typicalstability data of aqueous formaldehyde solutionsis given in Table 5.

Caprinoguanamine has been applied as ad-ditive for melamine formaldehyde impregna-tion resinswith improvedwetting characteristics[84]. It is also used in styrenic foams to improvethe foam structure and provide ame retardancy[85] (see Flame Retardants).

Other Guanamines. Guanamines with un-saturated groups in the substituent R canalso be converted to polymers and copoly-mers [86]. The compound CTU (bis-3,9-cyanoethyl-2,4,8,10-tetraoxaspiro[5.5]-undecan)-guanamine [22535-90-6] (formulashown below) is employed (like benzogua-namine) primarily in paints [87].

Guanamines with imidazole side chains,e.g., 2,4-diamino-6-(2-methylimidazol-1-yl)-ethyl-1,3,5-triazine are used as epoxy curingagents and accelerators as such or in form ofadducts with, e.g., cyanuric acid [88].

Guanamine structures have also found ap-plication in pharmacology as c-AMP specic

phosphodiesterase inhibitors [89], the mostprominent being Irsogladine maleate (2,5-Di-chlorobenzoguanamine, [84504-69-8]).

11.3. Toxicology

The acute oral LD50 for acetoguanaminein rats is 2740 mg/kg; for benzoguanamine,1470 mg/kg. The corresponding value forcaprinoguanamine is > 10 000 mg/kg, and itsdermal LD50 (rabbit) is> 2800 mg/kg [90]. Allthree compounds are not irritating to skin andeyes [91, 92]. Acetoguanamine and benzogua-namine did not reveal mutagenic activity in theAmes test [93]. Long term feeding studies withbenzoguanamine in male rats and mice failedto reveal carcinogenic potential [94]. Benzogua-naminewas adoptedby theScienticCommitteeon Food as a monomer for food contact mate-rials in List 3 with a restriction of 5 mg/kg ofbenzoguanamine in food [95]. The evaluation ofthis substance under theOECD/HighProductionVolume Program resulted in the conclusion, thatthere is low priority for further work (no furtherwork recommended) [96].

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