Melamine cyanurate is the reaction product of melamine (2, 4, 6-triamino-l, 3, 5-triazine.) and cyanuric acid (2,4,6- trihydroxy-1, 3, 5-triazine, or its tautomeric keto form). Hereinafter, the terms "cyanuric acid" and " (iso) cyanuric acid" will be used interchangeably for designating both the enol and keto tautomers.

Melamine cyanurate is mainly used, inter alia, as a flame retardant additive for various thermoplastic compositions, and specifically for polyamides, and also has other uses such as a lubricant and a plant nutrient.

In its most general form, the synthetic procedure for preparing melamine cyanurate involves the reaction of melamine and cyanuric acid in an aqueous medium. It has been proposed in the art to modify this general reaction scheme by adjusting the pH of the reaction mixture through the addition of either acidic or alkaline agents thereto.

A general teaching in this regard may be found in US 4,256,591. According to the description of the patent, melamine and (iso) cyanuric acid are dispersed in water, and are allowed to react, preferably at a temperature in the range of 5O ⁰ C to 100 ⁰ C, in the presence of either a mineral or an organic acid, or, alternatively, in the presence of an alkaline agent selected from the group consisting of caustic alkali, amine or ammonia, followed by neutralization at the end of the reaction.

EP 507677 discloses a process for preparing melamine cyanurate comprising reacting melamine and cyanuric acid in an aqueous medium in the presence of a strong acid, at a pH below 1.

JP 56-032470 discloses a process for preparing melamine cyanurate by reacting melamine with cyanuric acid in an aqueous solvent at a pH above 7.0. According to the examples included in this publication, melamine and cyanuric acid are reacted in water in the presence of sodium carbonate. Having completed the reaction, hydrochloric acid was added into the reaction mixture to neutralize the same, and the solid product was isolated by filtration. The resulting melamine cyanurate was finally dried and ground.

In a more recent publication, CN 1506356, it has been suggested to react melamine and cyanuric acid in water containing aqueous ammonia in a concentration of 2 to 14% (wt %), at temperature in the range of 100 ⁰ C to 200 ⁰ C, wherein said cyanuric acid is present in excess. The product is separated from the liquid phase by filtration, and is subsequently dried to afford a product having particle size of about 5 to 55μm.

The aforementioned processes afford melamine cyanurate in a powdered form. Though melamine cyanurate is commonly applied as a powdered material when used as a flame retardant additive for polyamides, attempts have been made in the art to provide granules of melamine cyanurate, since, as may be readily appreciated, granular material may be advantageous over a powdered product with respect to handling and transportation, provided, of course, that the polymer that

is made flame retarded by means of said granular material will ultimately satisfy the necessary requirements (e.g., acceptable flammability tests, mechanical properties, visual appearance) .

One of the approaches reported in the art for obtaining granular melamine cyanurate involves the use of various additives and binding agents. For example, EP 666259 discloses a process in which the melamine cyanurate is prepared in the presence of metal oxide nano-particles hydrosols. A more recent publication, WO 2003/35736, arrives at granular melamine cyanurate which comprises polyvinyl alcohol .

DE 4040276 and, similarly EP 487974, describe solid fire- retardant agglomerates with mean particle size of 0.1-5 mm, which agglomerates are prepared from primary particles having mean size in the range of 0.1-10 microns by means of a wet granulation procedure, and are subsequently dried.

JP 07-149739 discloses granules having an average grain diameter in the range of 100-2000μm, which are described as being composed of agglomerates of fine particles having average particle diameter in the range of 0.1-1 μm. It appears that the formation of the granules according to this publication involves the use of water.

A method for preparing a binder free granular melamine cyanurate, and the product obtained thereby, was also described in the art. EP 601542 discloses a process for preparing melamine cyanurate which comprises heating

melamine powder and cyanuric acid powder at 250 to 500 °C, substantially in the absence of any liquid medium. The melamine cyanurate can be obtained in a granular form when the heating is conducted subsequent to granulation of a mixture of said powders .

It has now been found that the presence of volatile bases, and specifically, ammonia/ammonium hydroxide in the slurry formed following the reaction of melamine and cyanuric acid in water, facilitates an easy separation of the melamine cyanurate product from the liquid phase by means of spray drying said slurry, to afford melamine cyanurate in a powdered form having advantageous properties. For the sake of brevity, throughout this description, the term "ammonium hydroxide" collectively refers to the species present in equilibrium in a solution of ammonia in water.

It has also been found that the powdered melamine cyanurate obtained following the process described hereinabove may be subsequently dry granulated, preferably by means of roller compaction, to give free flowing granules of melamine cyanurate, which granules are readily dispersible in polyamides, and hence are particularly suitable for the preparation of flame-retarded thermoplastic polyamide compositions .

Thus, in a first aspect, the present invention provides a process which comprises reacting melamine and cyanuric acid in an aqueous medium to form melamine cyanurate, removing the product slurry from the reaction vessel in the presence of a volatile base, which is most preferably ammonium

hydroxide, spray drying said product slurry and collecting melamine cyanurate in the form of a dry powder.

The term "volatile base" refers to an alkaline agent having a boiling point that is below the temperature employed in the spray drying stage, in order to allow the removal of said base from the melamine cyanurate product. The preferred volatile base according to the invention, ammonium hydroxide, may be introduced into the reaction vessel either before the commencement of the reaction, during the progress of the reaction or upon completion of the same, with the first and second options being particularly preferred. Other possible volatile bases that may be used according to the present invention are organic amines having a boiling point below 200 ⁰ C, and preferably below 100 ⁰ C, such as methyl amine or ethyl amine.

The starting materials used according to the present invention, melamine and cyanuric acid, are both commercially available or may be prepared by methods well known in the art.

Compositionally, the reaction medium according to the present invention most preferably comprises water and ammonium hydroxide dissolved therein, wherein the melamine and the cyanuric acid starting materials are suspended in said aqueous medium, such that the weight concentration of said solid reactants with respect to the total weight of the reaction mixture is preferably in the range of 5 to 40%, and more preferably in the range of 10 to 25%, and most preferably in the range of 15% to 20%. The weight

concentration of the ammonium hydroxide dissolved in the reaction mixture (calculated as ammonia) is preferably in the range of 0.05 to 1%, and more preferably in the range of 0.1 to 0.4%, relative to the water. Most conveniently, the ammonium hydroxide is provided in the form of an aqueous solution having a typical concentration of about 25 to 28% (w/w) NH ₃ dissolved therein, such that suitable quantities of this solution may be poured into the water serving as the reaction medium according to the invention. Alternatively, gaseous ammonia may be directly bubbled into the aqueous reaction medium.

It should be noted that the molar ratio between the total amounts of melamine and cyanuric acid which are allowed to react according to the present invention is preferably in the range of 0.98:1 to 1.02:1, and more preferably in the range 0.995:1 to 1.005:1. However, it has been observed that it is particularly advantageous to allow the reaction to proceed in the presence of an excess amount of melamine, and even to allow the reaction to reach completion such that the residual amount of melamine in the aqueous medium is up to 0.2% (molar percent) above the residual amount of the cyanuric acid present therein. Under these conditions, the slurry formed has relatively low viscosity. Accordingly, in view of the above, although water, ammonium hydroxide and the two solid reactants may be introduced into the reaction zone according to any desired order, it is preferred to carry out the process by introducing into the reaction zone suitable amounts of water, melamine powder and a solution of ammonium hydroxide, and subsequently gradually adding the cyanuric acid thereto, thus assuring that the reaction proceeds under an excess amount of melamine. The gradual

addition of the cyanuric acid into the reaction zone may be accomplished either continuously, or more preferably, in a portion-wise manner, such that approximately equal quantities of said cyanuric acid are sequentially charged into the reaction mixture over a period of about 30 to 120 minutes, at intervals of about 5 to 30 minutes.

It has been found that the presence of small amounts of ammonium hydroxide within the reaction mixture, as provided by the preferred embodiment of the present invention, allows the reaction to proceed effectively even at temperatures below 75°C. Thus, according to a preferred embodiment of the invention, the reaction is carried out at a temperature in the range of 20 to 75 ⁰ C, and more preferably in the range of 35 to 55°C It should be noted that the reaction may be conducted at two or more distinct sub-ranges within the aforementioned temperature range. Thus, according to a particularly preferred embodiment of the invention, the reaction mixture comprising water, ammonium hydroxide and melamine is heated to a first temperature in the range of 38 to 42°C, prior to the introduction of the cyanuric acid thereto. During the- gradual addition of the cyanuric acid, the temperature of the system is maintained within the aforementioned sub-range. Having completed the addition of the cyanuric acid to the reaction mixture, the reaction slurry may be further heated to a second temperature in the sub-range of 48 to 52°C, thus assuring a more rapid completion of the reaction. Under the aforementioned temperature control, the duration of the reaction is in the range of 1 to 4 hours.

The presence of ammonium hydroxide in the reaction zone significantly reduces the viscosity of the reaction slurry, such that the reaction proceeds effectively even at relatively high concentrations of solid reactants (e.g., about 20% by weight) in a standard reactor equipped with turbine agitator. The viscosity of the slurry is preferably in the range of 50 to 35OcP at a shear rate of about 170 s ^"1 (for the concentration of solids indicated above) , wherein the temperature is not higher that 75 ⁰ C. More preferably, the viscosity of the product slurry is less than 150 cP under the aforementioned conditions.

Although it is preferred to carry out the reaction of melamine and cyanuric acid in the presence of ammonium hydroxide, it is also possible, according to an alternative embodiment of the invention, to react said starting materials in the absence of ammonium hydroxide, and then to add said reagent into the product slurry towards the end of the reaction, or upon substantially completing the same. It has been observed that the fluidity of the product slurry is markedly increased upon addition of ammonium hydroxide, such that the product becomes easily removable from the reaction vessel .

The product slurry may be sampled upon completion of the reaction, such that residual amounts of melamine or cyanuric acid may be neutralized by means of the counter reactant, thereby assuring that the melamine cyanurate product meets the necessary purity specifications.

Having completed the reaction, the resulting melamine cyanurate slurry is removed from the reaction vessel and is spray dried. The spray drying may be suitably carried out using a NIRO spray drier, following which the product is collected from the dryer using standard techniques. Typical spray drying parameters are illustrated in the examples below. In general, the inlet temperature is in the range of 200 to 600°C, and preferably in the range of 300 to 450 ⁰ C. The outlet temperature is in the range of 100 to 160 ⁰ C, and more preferably in the range of 130 to 150 ⁰ C.

Throughout the present description, the term "powdered melamine cyanurate" and the like refer to melamine cyanurate in the form of particulates the size of which is less than lOOμm, and preferably in the range of 1 to 50 μm, said particulates being preferably composed of particles having average size in the range of 0.5 to 6μm. The bulk density of the powdered melamine cyanurate is typically in the range of 0.15 to 0.35 g/ml.

The dry product collected following the process described hereinabove is an example of a powdered melamine cyanurate. This product is typically in the form of fairly spherical aggregates, having a diameter in the range between 5 and 30μm and a bulk density in the range of 0.20-0.35 g/ml, said aggregates being composed of particles having average size in the range of 0.5 to 4μm, and more preferably in the range of 1 to 3 μm, and most preferably about 2 μm, as determined by laser diffraction. It should be noted that the particles are held together to form the aggregates defined hereinabove without any auxiliary binder.

It has now been surprisingly found that the application of pressure on powdered rαelamine cyanurate promotes the formation of interparticle bonds to form a granule, and that the granule thus formed is characterized by good dispersibility, as determined by laser diffraction analysis (which is described in more detail hereinbelow) . Thus, in another aspect, there is provided a process which comprises dry granulating powdered melamine cyanurate to form granules of melamine cyanurate. The term "dry granulation", as used herein, indicates a compression process in which pressure is used to accomplish the formation of the granules.

The dry granulation of the powdered melamine cyanurate may be accomplished using various conventional compression methods, including, but not limited to, roller compaction. Roller compaction is a dry granulation method in which powders are densified by passing them between two rotating rollers. The powder is typically roller compacted at an applied force of about 1 to 10 ton/cm, and more preferably at about 3 to 7 ton/cm. The linear speed of the compactor roll is generally in the range of 3 to 50 cm/s and more preferably about 7 cm/s.

The resulting compacted material is subsequently grinded and classified by sieving in order to isolate a fraction of granules having suitable dimensions. In order to increase the total crop of the desired fraction, granules having sizes falling outside the selected fraction may be combined with fresh powdered melamine cyanurate and returned to the compaction device.

Thus, according to a particularly preferred embodiment of the present invention, the powdered melamine cyanurate obtained following the spray-drying step is dry granulated by means of roller compaction, followed by grinding and optionally sieving.

In addition to roller compaction, the dry granulation process of the present invention may be carried out using other compression apparatuses, such as hydraulic press. The pressure applied onto the powdered material in order to form the dense compact may vary in the range of 40 to 400 kg/cm ² and preferably 70 to 300 kg/cm ² . The resulting compacted material, in the form of tablets or pellets, is subsequently grinded and sieved to recover a desired fraction of granules .

It should be noted that the starting material for the dry- granulation process may be either powdered melamine cyanurate or a mixture thereof with one or more solid additives, and especially flame retardants additives, wherein the weight concentration of melamine cyanurate in said solid mixture is preferably not less than 60% (w/w) , and more preferably not less than 90%. Throughout this description, the terms "granules of melamine cyanurate", "granular melamine cyanurate" and the like refer to the melamine cyanurate product obtained following the dry granulation (using roller compaction or hydraulic press) of melamine cyanurate, or mixtures comprising the same.

In another aspect, the present invention provides dry granulated melamine cyanurate, and more preferably, binder- free, roller-compacted granules of melamine cyanurate,

preferably with a size in the range between 0.1 to 10 mm, and more preferably in .--the range of 0.2 to 2 mm. The invention also provides binder-free melamine cyanurate granules which are composed of particles having average diameter in the range of 0.3 to 12μm, and preferably in the range of 0.5 to 4μm, and more preferably in the range of 1 to 3 μm, and most preferably about 2 μm as determined by laser diffraction. The particle size distribution parameters are dso<5μm and dg ₈ <100μm, and more preferably d ₅ o<3 μm and dg8<25μm, ^' wherein d ₅ o is the median size and dgs is the particle size that is exceeded by only 2% of the particles population (by volume) . The bulk density of the binder free granules of the present invention is preferably not less than 0.60 g/ml, and more preferably not less than 0.63 g/ml.

In view of the above, it may be appreciated that the powdered and granular melamine cyanurate provided by the present invention are capable of disintegrating into particles having essentially the same average size, which is most preferably the range of 1 to 3μm, and most preferably about 2μm, as measured by laser diffraction.

The powdered and granular melamine cyanurate provided by the present invention may be suitably used, inter alia, as flame retardant agents for various polymers, including polyamides, polyesters and polypropylenes, with polyamide 6 or polyamide 6,6 being especially preferred. It has been surprisingly found that polymers compounded with the granular melamine cyanurate of the invention satisfy the desired mechanical properties and the requirements of the UL-94 flammability test. It is believed that the melamine cyanurate granules

according to the present invention are capable of disintegrating into sufficiently small particles during the compounding stage with the polymer, whereby said particles become essentially evenly distributed throughout the polymer. Thus, in another aspect, the present invention provides a process, which comprises adding the powdered or the granular melamine cyanurate of the present invention to a polymer. Methods for introducing melamine cyanurate into nylon are well known (e.g., US 4,314,927, EP 418210 and US 4,298,518). The resulting flame retarded polymer composition, and molded parts obtainable therefrom, constitute additional aspects of the invention.

In the drawings :

Figures Ia - Ib are Scanning Electron Microscopy photographs of the melamine cyanurate powder of the invention.

Examples

Viscosity measurements

The slurry viscosity was determined by a FANN viscosimeter, (from Baroid) at a temperature of 40° C. The device constitutes of two centric cylinders, a rotor and a stator. The speed of the rotor was in the range of 100 to 600 rpm, which is equivalent to a shear rate of 170 to 1000 sec ^"1 .

Particle Sizing and dispersibility test

The particle sizing of the spray dried melamine cyanurate powder and the granular product was carried out using a laser diffraction system [Malvern (UK) Model Mastersizer 2000] according to the procedure of the Mastersizer 2000 Operation Guide. The dispersion medium was isopropanol. The pretreatment was performed in a 1 L

volume ultrasound bath [Cole Partner (USA) Model 8890; 2L volume, output 70 watts at 42 kHz] . The methodology was as follows :

1 g of product was added to 10 mL of isopropanol in a vial. The mixture was put in the ultrasound bath for 1 min. A sample of the mixture was then added to the measurement cell filled with isopropanol. The amount of sample added was as required to obtain obscuration in the range of 10 to 20%. Internal ultrasound was applied for 5 min at 60% of the maximal intensity (the maximal intensity was 27 W at a nominal frequency of 40 kHz) . The data were retrieved according to the following model parameters:

Product refractive index: 1.52; Absorption 0.1; isopropanol refractive index 1.39. Analysis model: general purpose; Particle shape: irregular (default).

The average particle size was defined as the volume weighted mean D [4, 3]. d ₅₀ is the volume median diameter and dgs is the top cut, defined as that particle diameter that is only exceeded by 2 % of the particle population by volume .

Residual water

Residual water in the product was determined by Karl Fisher titration method.

Residual excess of melamine and cyanuric acid 3 g of a dry product was mixed with 22 mL of distilled water for one hour. The sample was then filtered. The filtrate was analyzed for residual excess of melamine or cyanuric acid by HPLC method using a column REGIS NH ₂ , wherein the eluent was acetonitrile and buffer phosphate.

Process Completion

The completion of the reaction was determined by thermal gravimetric analysis of the dry product using an analyzer from TA Instrument Model 2950 (heating rate: 10°C/min) . The reaction completion was characterized by a weight loss of less than 1% at 290°C.

Flammability test (UL-94 V) :

Flammability vertical burning test according to Underwriters

Laboratories Inc.

Limiting oxygen index (LOI) :

According to ASTM D 2863-77 using a Stanton Redcroft FTA

Flammability Unit.

Notched Izod Impact:

According to ASTM D256 using a Zwick 5102 pendulum impact tester.

Tensile properties of plastics:

According to ASTM D638, Type I Dumbell, with a speed of test of 5 mm/min., using a Zwick 1435 material testing machine.

Example 1

A three liter jacketed reactor was loaded with water (2530 g) ; ammonium hydroxide solution containing 25 % NH ₃ (10.5 g of solution) and melamine (327.2 g, 2.59 mole) . The reactor content was agitated mechanically. The reaction mixture was

/ heated to 40 ⁰ C and cyanuric acid 97.5% (343.4 g, 2.59 mole) was added in one portion to the reactor. The reaction temperature was maintained in the range of 40 to/45 ⁰ C for an

hour. The viscosity of the product slurry was 13 cP and 60 cP for a shear rate of 1000 s ^"1 and 170 s ^"1 , respectively. The product was dried in a bench-scale spray-dryer Minor Unit from NIRO (Denmark) . Inlet temperature: 400 ⁰ C and outlet temperature: 120°C. The residual water in the product was of 0.25% (w/w) .

The quality of the final product (completion of the reaction) was confirmed by a weight loss of 0.6% at 29O ⁰ C using thermal gravimetric analysis. The product was composed of particles having average size of 2.5 μm as determined by laser diffraction and the particle size distribution of the product was as follows: d ₅₀ =2. lμm and d ₉₈ =17μm.

Example 2

The procedure of example 1 was repeated at a temperature of about 70-75 ⁰ C. The viscosity of the slurry was 80 cP and 320 cP for a shear rate of 1000 s ^"1 and 170 respectively.

Example 3

A 220 liter reactor was loaded with 181.6 kg of distilled water, 16.2 kg of melamine and 730 g of ammonium hydroxide solution (25% NH ₃ ) . The mixture was agitated and heated to 40 ⁰ C. Four portions of 4.25 kg cyanuric acid were added at intervals of 15 min, during which period the reaction mixture was maintained at 40 ⁰ C. The temperature was then raised to 50 ⁰ C, and the reaction mixture was maintained at this temperature for one hour. The viscosity of the product slurry was 25 cP and 100 cP for a shear rate of 1000 s ^"1 and

170 s ^"1 , respectively. The slurry was charged into a. spray- dryer (NIRO, Denmark) . The spray drying parameters are listed below: Inlet temperature: 400°C Outlet temperature: 130-150 ⁰ C Atomizer speed: 24,000 rpm Slurry feed rate: 20 L/hour Air flow rate: 400 kg/hour

The residual water in the product was of 0.15%. The quality of the final product (completion of the reaction) was checked by a weight loss of 0.5% at 290°C using thermal gravimetric analysis.

The dried powder collected consisted of fairly spherical aggregates having sizes in the range of 10 to 30 μm. The average diameter of the particles of which said aggregate was composed was 1.5 μm, as determined by laser diffraction, and the particle size distribution was as follows: d ₅ o=l .3μm and d ₉₈ =6μm. The bulk density of the powdered melamine cyanurate was 0.30 g/ml. Figure Ia is a Scanning Electron Microscopy photograph of the melamine cyanurate aggregates obtained by the present invention. Figure Ib is a Scanning Electron Microscopy photograph of said aggregate, showing the particles composing the same.

Example 4

A three liter jacketed reactor was loaded with water (2671 g) ; ethyl amine solution 70 % (10.5 g of solution) and melamine (233.3, 1.85 mole). The reactor content was agitated mechanically and heated to 4O ⁰ C. Cyanuric acid 97.5% (245 g, 1.85 mole) was added in four portions of 61.25

g to the reactor, each portion added at intervals of 15 min, during which period the reaction mixture was maintained at 40 ⁰ C. The temperature was then raised to 50 ⁰ C, and the reaction mixture was maintained at this temperature for one hour. The viscosity of the product slurry was 4 cP and 6 cP for a shear rate of 1000 s ^"1 and 170 s ^"1 , respectively. The product was dried in a bench-scale spray-dryer Minor Unit from NIRO (Denmark) . The residual water in the product was of 0.25% (w/w) .

The quality of the final product (completion of the reaction) was checked by a weight loss of 0.8% at 290°C using thermal gravimetric analysis. The product was composed of particles having average size of 1.5μm as determined by laser diffraction and the particle size distribution was as follows: d ₅ o=1.2μm and d ₉₈ =8μm.

Example 5 (comparative)

The procedure of Example 1 was repeated with the omission of NH ₄ OH solution and the temperature maintained at 50 ⁰ C for four hours. The reaction was not completed, according to X-ray diffraction analysis of the final product after drying .

Example 6 (comparative)

The procedure of Example 2 was repeated with the omission of NH ₄ OH solution. After one hour, the reaction was completed. The product slurry obtained was so viscous that it was not possible to discharge it through the bottom opening of the reaction vessel.

Example 7

The procedure according to Example 6 was repeated, following which 52.5 g of NH ₄ OH solution (25% NH ₃ ) were added to the product slurry,- which was then mixed for 30 min at 75°C. The fluidity of the slurry was improved, such that it was easily discharged from the bottom opening of the reaction vessel .

Example 8

Dry granulation of powdered melamine cyanurate using roller compaction

1000 g of the powdered product of Example 3 were compacted using a double-roll press (Hutt, Germany) . The diameter of the roll was 22 cm, and its length was 6 cm. The roll compactor produced pillow shaped bodies. The force applied was 6 ton/cm and the rotation speed was 6 rpm. The material was twice recycled in the compactor, in order to increase the strength of the compacts. The compacted material was then grinded and sieved through lmm and 0.5mm sieve. The fraction of granules with a size in the range of 0.5 to 1 mm was separated (550 g) . The coarse fraction (+lmm; 50 g) and the fines (-0.5mm, 350 g) were considered as a residual material .

The aforementioned residual material (about 40Og) was mixed with 60Og of fresh powder, and the aforementioned compaction/grinding/sieving procedure was repeated several times, to obtain a granular product. The granular melamine cyanurate had a bulk density of 0.65g/ml. The granules were composed of particles having average size of 2μm, as

determined by laser diffraction, and the particle size distribution was as follows:

Example 9

Dry granulation of powdered melamine cyanurate using roller compaction

The granulation procedure of Example 8 was repeated using a commercially available melamine cyanurate starting material (Melapur MC25 from CIBA, Switzerland) , having an average particle size of 5.3 microns, as determined by laser diffraction, and the following particle size distribution: d ₅ o=3.6 microns, microns.

The granular product thus obtained had a bulk density of 0.73 g/ml. The granules were composed of particles having average size of 10 microns, as determined by laser diffraction, and the particle size distribution was as follows: d ₅ o=4.1 microns, d ₉₈ =80 microns.

Example 10

Dry granulation of powdered melamine cyanurate using hydraulic press

Powdered melamine cyanurate of Example 3 was compacted using a hydraulic press. 15 gr tablets were prepared in a tungsten carbide cylindrical mold of 2.5 cm in diameter. The pressure applied was in the range 40 to 350 kg/cm ² . The density of the compact was determined by measuring its volume and weight. Crushing strength was measured by standard compression test. The tablets were then grinded and sieved through lmm and 0.5mm sieve. The particle size of the

granules obtained were , analyzed by laser diffractometry , The results are presented in Table 1 below.

Table 1

Example 11 (comparative) Wet granulation of powdered melamine cyanurate

153 gr of a melamine cyanurate powder obtained in Example 3 was mixed with 47 g of water and pressed through a screen with an opening of 1.6 mm. The product obtained was sieved through a screen with an opening of 0.5 mm. The weight of the coarse fraction was 135 gr and the fines 60 g. The product of the coarse fraction was dried overnight in an oven at 110 ⁰ C. 105 g of granular product were recovered. The granules were composed of particles having average size of 19.5 μm, as determined by laser diffraction, and the particle size distribution was as follows: dso=2.3 μm, d ₉₈ =160 μm.

Example 12 (comparative) Wet granulation of powdered melamine cyanurate

The procedure below follows the working example according to JP 07-149739. Cyanuric acid 99% (310 g; 2.38 moles) with average particle size of 40 microns and melamine (300 g; 2.38 moles) with average particle size of 57 microns were put in a Henshel mixer. The mixture was agitated and the

jacket of the mixer was heated with heating medium at 100 ⁰ C. 100 g of water were then added slowly during 30 min using a peristaltic pump. The agitation was maintained under these conditions for a supplementary half an hour. • The heating medium was then heated to 120 °C and the agitation was pursued at this temperature for one hour.

Granules of melamine cyanurate were obtained. The average size of the granules was about 250 microns and the moisture content thereof was 0.2%. The completion of the reaction was checked by X-ray diffraction and the weight loss was about 0.9% at 290° C using thermal gravimetric analysis.

The average size of the particles, of which the granules were composed, as determined by laser diffraction was about 55μm and the particle size distribution was as follows: The size of the sub-particles as determined by scanning electron microscopy was in the range between 0.2 and 1 micron.

Example 13

88 parts of Nylon 6,6 (Zytel EFE 1032 NC 10 ex. Du Pont) were compounded with 12 parts of powdered melamine cyanurate (the product of Example 3) in a Berstorff ZE25 co-rotating twin screw extruder (D=25 mm, L/D =32) with open vent at temperature range of 260 - 27O ⁰ C. Gravimetric feeding system ex. K-Tron was used. The compounded strands were pelletized. Produced pellets were dried in a vacuum oven at 80 °C overnight.

The pellets obtained were used to mold test specimens having thickness of 0.8 and 1.6 mm for the UL-94 flammability test,

and test specimens having thickness of 3.2 mm for determining other properties of the melamine cyanurate containing polyamide. The results are presented in Table 2.

Example 14

The procedure according to Example 13 was repeated with the granular, roller-compacted, melamine cyanurate obtained in Example 8. The results are presented in Table 2.

Table 2