Method for granulating azo compounds, and resulting granules

Abstract

The present invention relates to a method for granulating an azo compound, comprising the following steps: a) a step of granulating by agitating an aqueous suspension of said azo compound, in the presence of an organic binder; b) an optional step of recovering the granules obtained in step a), preferably by filtration; and c) an optional step of drying the granules recovered in step b). The invention also relates to the granules obtainable using said method, and to the uses thereof.

Description

[0001]DESCRIPTION TITLE: PROCESS FOR GRANULATING AZO COMPOUNDS AND GRANULES OBTAINED The present invention relates to a process for granulating azo compounds, as well as the granules that can be obtained by this process and their uses. Azo compounds, and more particularly 2,2'-azobis(isobutyronitrile) called AZDN or AIBN, are well-known products. They are used as swelling agents, synthesis intermediates or initiators of polymerization reactions involving free radicals. These reactions can be bulk, solution, suspension or emulsion polymerization reactions and can use a wide variety of monomers, for example (meth)acrylic or vinyl monomers, such as acrylamide, acrylonitrile, alkyl (meth)acrylate, styrene, acetate and vinyl chloride or vinylidene chloride.The fields of application are therefore very diverse and concern in particular (but not exclusively) acrylic sheets or fibers, flocculants, paints, coating resins, grafted polyols, polystyrene, PVC, PVA, or PMMA. These azo compounds are conventionally obtained by oxidizing the corresponding hydrazo derivatives or amino-nitriles as described for example in documents US 2,515,628, WO 03 / 002521, US 3,390,146, CN 1309705 or EP 2821 393. After oxidation, the resulting suspension is generally drained, then possibly dried, to give a solid in powder form. However, azo compounds in powder form pose numerous problems. In particular, they generate dust that may present a risk of explosion and / or industrial hygiene.These dusts can in fact come into contact with the upper respiratory tracts of the people who handle them after being accidentally suspended in the air. Such contact can occur with operators present on the industrial installation, for example when manually loading the powder into the reactors or with operators who carry out the sampling necessary to control the manufacturing process. It is desirable to limit to a minimum, or even completely avoid any contact of this type, in order to ensure the safety of the operators and guarantee the hygiene of the manufacturing premises. In addition, this type of powder is not easily handled by operators. The powder is generally not sufficiently flowable to lend itself to easy transport and loading into the reactors.Azo compounds in powder form have poor flowability, often leading to caking problems during storage and loading. There is therefore a need for improved shaping of azo compounds, in particular one that generates little or no dust and is easy to handle by operators. Thus, one objective of the present invention is to provide a granulation process that is easy to implement, and preferably more environmentally friendly. Another objective of the present invention is to provide granules of azo compounds that avoid or reduce dust and / or that can be easily transported and handled. One objective of the present invention is to provide granules of azo compounds that are solid (i.e. resistant), in particular that retain their shaping during storage and handling.Another objective of the present invention is to provide granules of azo compounds having properties similar to those of powders. The present invention meets all or part of the above objectives. The inventors have discovered that the granulation of azo compounds can be carried out using a process using an aqueous suspension of these compounds, with stirring and in the presence of an organic binder. This process makes it possible in particular to avoid using organic solvents in large quantities, the suspension being aqueous. It can also make it possible to obtain granules without using surfactants or dispersants. For example, a mechanical type of granulation is known from document WO 00 / 24706. This involves compressing and then extruding powders of azo compounds to make granules. However, it is essential for this technique to control the shaping temperature. However, such control is not easy at the industrial level.This type of process involves risks of heating by friction or compression and consequently risks of violent thermal decomposition of the azo compounds during the formation of the granules. In addition, the granules obtained by mechanical granulation are not very resistant to crushing and disintegrate easily. The process according to the invention avoids these risks of heating and decomposition by using mild granulation conditions, which makes it possible to obtain granules of good purity. The application properties of the compounds, particularly in polymerization, are thus preserved. In addition, and surprisingly, the granules according to the invention are solid (resistant). More particularly, they have a crushing resistance significantly higher than other known shapings of azo compounds.The term "crushing strength" is understood to mean the maximum weight per unit of a surface area composed of granules that these granules can support before crushing or disintegrating (i.e. losing their shape): Crushing strength = weight of the load at the breaking point / surface area of ​​the granules To measure the crushing strength, the compression load necessary to cause the granule to break is generally measured. The crushing strength of the granules according to the invention is in particular greater than or equal to 25 g / cm. 2 , preferably still greater than or equal to 40 g / cm 2 They preferably have a maximum crushing resistance of 90 g / cm 2 , preferably 95 g / cm 2 , more preferably 100 g / cm 2 or even 500 g / cm 2 , including terminals. For example, their maximum crushing resistance is between 50 and 200 g / cm 2 , preferably between 55 and 95 g / cm2. They can therefore be transported and handled without breaking or disintegrating. Being preferably substantially spherical, they can flow easily from storage drums or bags and loading the reactors is facilitated. Thus, the granules according to the invention are preferably substantially spherical, or even spherical, and can have a diameter of between 0.5 mm and 5 mm, preferably between 2 and 3 mm. They are therefore generally larger than dust and in particular much larger than inhalable dust. The term "dust" is understood to mean in particular particles smaller than 100 µm. In particular, inhalable dust is smaller than 20 µm, preferably smaller than 5 µm. Also surprisingly, the granules obtained have properties similar to those of powders and in particular a similar dissolution time.The granules according to the invention are therefore entirely suitable for industrial use, like usual powders. BRIEF DESCRIPTION OF THE INVENTION The present invention relates to a process for granulating an azo compound of the following general formula (I): [Chem 1]. in which: ^ the radicals R 1 , R 2 , R 3 and R 4, identical or different, are chosen independently of each other from: - a linear or branched alkyl group, preferably a (C1-C6)alkyl group, optionally substituted by a hydroxyl, alkoxy group or by a halogen atom; - a cycloalkyl group, preferably a (C3-C6)cycloalkyl group, optionally substituted by a hydroxyl, alkoxy group or by a halogen atom; - an aryl group, preferably phenyl or naphthyl, optionally substituted by a hydroxyl, alkyl, alkoxy group or by a halogen atom; - an aralkyl group, preferably benzyl or phenethyl, optionally substituted by one or more alkyl, alkoxy, hydroxy group(s) or by one or more halogen atoms; or - at least one of the combinations of R 1 with R 2 and / or R 3 with R 4forms, with the carbon atom to which it is (or they are) linked, a cycloalkyl radical or a C(O) group; ^ the radicals R 5 and R 6, identical or different, are chosen independently of one another from the CN or NH2 groups; said process comprising the following steps: a) a step of granulation by stirring an aqueous suspension of said azo compound, in the presence of an organic binder; b) an optional step of recovering the granules obtained in step a), preferably by filtration; and c) an optional step of drying the granules recovered in step b). The present invention also relates to granules capable of being obtained or obtained or directly obtained by the process as according to the invention. The present invention also relates to granules comprising an azo compound of general formula (I) and an organic binder, as defined below. The present invention also relates to granules having a crushing strength greater than or equal to 25 g / cm 2 , preferably still greater than or equal to 40 g / cm 2. The present invention relates to the use of such granules as swelling agents, initiators of polymerization reactions involving free radicals, or as synthesis intermediates, in particular in the preparation of pharmaceutical or agrochemical compounds. DETAILED DESCRIPTION OF THE INVENTION The azo compounds have the following general formula (I): [Chem 1] in which: ^ the radicals R 1 , R 2 , R 3 and R 4, identical or different, are chosen independently of each other from: - a linear or branched alkyl group, preferably a (C1-C6)alkyl group, optionally substituted by a hydroxyl, alkoxy group or by a halogen atom; - a cycloalkyl group, preferably a (C3-C6)cycloalkyl group, optionally substituted by a hydroxyl, alkoxy group or by a halogen atom; - an aryl group, preferably phenyl or naphthyl, optionally substituted by a hydroxyl, alkyl, alkoxy group or by a halogen atom; - an aralkyl group, preferably benzyl or phenethyl, optionally substituted by one or more alkyl, alkoxy, hydroxy group(s) or by one or more halogen atoms; or - at least one of the combinations of R 1 with R 2 and / or R 3 with R 4forms, with the carbon atom to which it is (or they are) linked, a cycloalkyl radical (preferably a (C3-C6)cycloalkyl) or a C(O) group; ^ the radicals R 5 and R 6 , identical or different, are chosen independently of one another from CN (nitrile) or NH2 groups. Preferably, R 5 and R 6 are identical. In particular, when R 5 and R 6 are identical and represent an NH2 group, then R 1 with R 2 , and R 3 with R 4 each form with the carbon atom to which they are linked, a C(O) group or; when R 5 and R 6 are identical and represent a CN group, the radicals R 1 , R 2 , R 3 and R 4, identical or different, are chosen independently of each other from: - a linear or branched alkyl group, preferably a (C1-C6)alkyl group, optionally substituted by a hydroxyl, alkoxy group or by a halogen atom; - a cycloalkyl group, preferably a (C3-C6)cycloalkyl group, optionally substituted by a hydroxyl or a halogen atom; - an aryl group, preferably phenyl or naphthyl, optionally substituted by a hydroxyl, alkyl, alkoxy group or by a halogen atom; - an aralkyl group, preferably benzyl or phenethyl, optionally substituted by one or more alkyl, alkoxy, hydroxy group(s) or by one or more halogen atoms; or - at least one of the combinations of R 1 with R 2 and / or R 3 with R 4forms, with the carbon atom to which it is (or they are) linked, a cycloalkyl radical. Most preferably, R 5 and R 6 represent a CN (nitrile) group. In particular, R 1 and R 3 are identical and / or R 2 and R 4 are identical. Preferably, the radicals R 1 , R 2 , R 3 and R 4 are chosen from methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, cyclopropyl, phenyl. In particular, the radicals R 1 , R 2 , R 3 and R 4are chosen from alkyl groups as defined above. In particular, when the substituent(s) are alkyls and / or alkoxys (also referred to as alkoxys), they comprise between 1 and 6 carbon atoms. Among the halogens, mention may be made in particular of fluorine, chlorine, bromine and iodine. Preferably, the compounds of general formula (I) are symmetrical. Examples of azo compounds of general formula (I) include: 2,2'-azobis(isobutyronitrile), 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(2-methylhexylonitrile), 2,2'-azobis(2-cyclopropylpropionitrile), 2,2'-azobis(2-phenylpropionitrile), 2,2'-azobis(2-methylbutyronitrile), 1,1'-azobis(1-cyclohexanecarbonitrile), and azodicarbonamide. The corresponding structures are given in the table below: [Table 1] Structure Name 2,2'-azobis(2,4-dimethyl-valeronitrile) CH3CH3CH3CH3 The preferred compound according to the invention is 2,2-azobis(isobutyronitrile), generally called AZDN or AIBN. It corresponds to the compound for which R 1 , R 2 , R 3 and R 4 are a methyl group and R 5 and R 6are a CN group (CAS No. 78-67-1). Before granulation, the azo compound of formula (I) is generally in the solid state, most often in powder form. Thus, said azo compound is preferably in the form of a powder comprising particles having a size between 10 µm and 200 µm, preferably between 40 µm and 150 µm. In particular, said powder has a particle size distribution (Dv50) between 10 µm and 200 µm, preferably between 40 µm and 150 µm, more preferably between 90 µm and 130 µm. The particle size of a powder is generally defined as the statistical distribution of the particles that make up the powder as a function of their dimensions (size and shape of the elementary particles). The particle size distribution or particle size distribution (Dv50) is a known parameter.It corresponds to the particle diameter (µm) below which 50% of the particle volume is located on the distribution curve expressed in cumulative frequency. For example, if Dv50 = 100 µm, 50% of the sample particle volume has a diameter less than 100 µm and 50% of the particle volume has a diameter greater than 100 µm. The two other characteristic diameters usually used to describe the particle size distribution of powders are Dv10 and Dv90: - Dv10: it corresponds to the diameter (µm) below which 10% of the particle volume is located; and - Dv90: it corresponds to the diameter (µm) below which 90% of the particle volume is located. In particular, the AZDN (especially dry) is in the form of a powder with a Dv50 of between 80 µm and 200 µm, preferably between 90 µm and 110 µm. In particular, the AZDN (especially wet) is in the form of a powder with a Dv50 of between 80 µm and 200 µm, preferably between 110 µm and 130 µm.Said azo compound powder may thus comprise between 0.1% and 10%, preferably between 5% and 10% by weight of water, relative to the total weight of the powder. Preferably, said compound of formula (I) has a solubility in the organic binder of at least 10 g / L, preferably between 10 g / L and 100 g / L and more preferably between 20 g / L and 70 g / L. Said azo compound is therefore preferentially soluble in said organic binder. This solubility can be determined according to conventional methods. It can also be determined as follows: 1.5 g of compound of formula (I) are added to 10 g of organic binder at 20°C. The bottle is shaken for a period of between 1 h and 10 h, for example 6 h. After decantation, the said binder is analyzed to determine its content of compound of formula (I): this content corresponds to the solubility of the azo compound in the organic binder in g / L. The analysis is carried out by gas chromatography.A standard range is produced between 0 and 150g / l by diluting the compound of formula (I) in acetone. The azo compounds of formula (I) according to the invention are known and are commercially available, generally in the form of a powder. Examples include AZDN marketed by Arkema, the products V-40, V-59, AIBN, V-65 marketed by Fujifilm Wako or the products Vazo®52, Vazo®64, Vazo®67 and Vazo®88 marketed by Chemours. An azo compound recovered directly after its production can also be used. For example, the azo compound can be used, drained, washed and obtained in the form of a wet powder after oxidation of the corresponding hydrazo or amino-nitrile compound. In this case, the powder can comprise between 5% and 10% by weight of water, relative to the total weight of the powder. We can use this same azo compound wrung out, washed and then dried.In this case, the powder may comprise between 0.01% and 0.2% by weight of water, relative to the total weight of the powder. The granulation step a) of the azo compound is carried out in the presence of an organic binder. Said organic binder is preferably liquid (under the operating conditions of the process according to the invention). In particular, said organic binder is sparingly soluble in water. By "sparingly soluble in water" is meant in particular a binder which has a solubility in water of less than or equal to 25 g / L, preferably between 0.1 g / L and 20 g / L, more preferably between 5 g / L and 15 g / L. To measure the solubility of the organic binder in water, conventional methods may be used. It is also possible to mix 100 g of water and 20 g of binder with stirring at 20°C for 1 hour. The aqueous phase is decanted and then analyzed by gas chromatography to determine the binder content in g / L, corresponding to its solubility.Preferably, the organic binder is chosen from aliphatic acetates, aliphatic carbonates, non-halogenated aromatic hydrocarbons, aliphatic ketones and aliphatic ethers, or mixtures thereof. Among the aliphatic acetates, alkyl acetates are preferred, said alkyl possibly comprising at least 4 carbon atoms, preferably between 4 and 6 carbon atoms. They are in particular of the following formula: CH3-C(O)OR. a , R a being an alkyl comprising at least 4 carbon atoms, preferably between 4 and 6 carbon atoms. Among the aliphatic carbonates, dialkylcarbonates are preferred, said alkyls being able to comprise at least 2 carbon atoms, preferably between 2 and 3 carbon atoms. They are in particular of the following formula: R b -OC(O)-OR c , R b and R cbeing independently of each other an alkyl comprising at least 2 carbon atoms, preferably between 2 and 3 carbon atoms. Preferably R b and R care identical. By non-halogenated aromatic hydrocarbons is meant in particular alkylbenzenes and preferably toluene, xylene (o-, m- and p-xylenes) and cumene. By alkylbenzene is meant in particular a benzene substituted by at least one alkyl comprising at least 1 carbon atom, preferably between 1 and 5 carbon atoms, more preferably methyl. Among the aliphatic ketones, dialkylketones are preferred, which may comprise at least 6 carbon atoms, preferably between 6 and 8 carbon atoms. They are in particular of the following formula: CH3-C(O)-Rd, Rd being an alkyl comprising at least 4 carbon atoms, preferably between 4 and 6 carbon atoms. Among the aliphatic ethers, dialkyl ethers are preferred, which may comprise at least 6 carbon atoms, preferably between 6 and 8 carbon atoms.They are in particular of the following formula: CH3-O-Rf, Rf being an alkyl comprising at least 5 carbon atoms, preferably between 5 and 7 carbon atoms, and preferably cyclic. It is understood that said alkyls mentioned above may be linear, branched or cyclic. Preferably, the organic binder is chosen from the group consisting of: toluene, m-xylene, p-xylene, o-xylene, n-butyl acetate, isobutyl acetate, cyclopentylmethyl ether (CPME or methoxycyclopentane), diethyl carbonate, methylisobutyl ketone and methylpentyl ketone. Preferably, the organic binder is selected from the group consisting of: toluene, m-xylene, n-butyl acetate, isobutyl acetate, cyclopentylmethyl ether (CPME or methoxycyclopentane) and methylisobutyl ketone. Preferred organic binders are isobutyl acetate and cyclopentylmethyl ether, more preferably isobutyl acetate.Before step a) of granulation, the process may comprise a step of preparing the aqueous suspension of the azo compound (i.e. the heterogeneous mixture in which the liquid phase is water and the solid dispersed phase is the azo compound). For example, the azo compound in the form of a powder, preferably as defined above, is mixed with water, and preferably with stirring. A person skilled in the art may prepare this suspension by any conventional method. In the context of the present invention, it is also possible to use the acidic aqueous suspension of azo compound obtained directly after the step of oxidation of the hydrazo compound or of the corresponding amino-nitrile (for example, after the step of chlorination of hydrazo-bis-isobutyronitrile, in the case of AZDN). Such a suspension may comprise between 2% and 15% by weight of HCl, relative to the total weight of the suspension.In said suspension, the azo compound / water mass ratio may be between 1 / 99 and 40 / 60, preferably between 10 / 90 and 25 / 75; and more preferably between 15 / 85 and 25 / 75. The process according to the invention comprises a step a) of granulation of an azo compound of general formula (I) as defined above, in suspension in water, by stirring, and in the presence of an organic binder. Preferably, the organic binder is added to the aqueous suspension. It is possible to add the organic binder to the azo compound, then add the water, or to put the three components at the same time in the reactor, even if these embodiments are not preferred. The addition of the organic binder may be done by any means known to those skilled in the art. It may be punctual or continuous, preferably punctual.Indeed, it is not necessary to gradually add the organic binder: the total quantity of the latter can be added at one time to the aqueous suspension of azo compound. It is thus preferred to carry out a rapid addition of the organic binder, for example for a duration of between 1 min and 30 min, more preferably between 1 min and 20 min, in particular between 1 min and 5 min. The granulation step a) is carried out with stirring. This stirring can be carried out by any known stirring means or rotor, for example any type of blade (straight or inclined) or helical ribbon. The reactor may comprise one or more stages of stirring rotor(s). More particularly, the stirring speed (corresponding for example to the rotation speed of the stirring rotor) must be sufficient to obtain a homogeneous suspension of the azo compound in water and a homogeneous dispersion of the organic binder, without however causing the formation of an emulsion.A speed of 500 to 900 rpm is generally used when operating in a reactor of 1 to 10 liters or a speed of 50 to 300 rpm can be used when operating in a reactor of the order of 100 liters. For example, the stirring speed can be increased or decreased throughout step a), but it is preferred to decrease it gradually. Generally, the formation of the granules occurs rapidly, i.e. a few minutes after stirring in the presence of said organic binder. In particular, the granules are formed over a period of time ranging from 1 min to 30 min, for example between 2 min and 10 min. Spherical (or substantially spherical) granules are generally obtained having a maximum diameter of between approximately 0.5 and 5 mm, for example between 1 and 5 mm, preferably between 2 and 3 mm. Continued agitation after their formation can serve to consolidate the resulting granules and / or reduce their dispersion.Step a) can be carried out for a period of between 1 minute and 10 hours, preferably between 30 minutes and 5 hours, more preferably between 1 hour and 5 hours. Granulation step a) can comprise or consist of the following two steps: A1) adding the organic binder to the aqueous suspension of azo compound to obtain a granulation medium, with stirring; then A2) maintaining stirring of the granulation medium. Preferably, step a) is carried out in the absence of surfactants and / or dispersing agents. Preferably, step a) is carried out without adding surfactants and / or dispersing agents. For example, step a) is carried out in the absence or without adding sodium dioctylsulfosuccinate. Granulation step a) is in particular carried out at a temperature that does not cause degradation of the azo compound. It can be between 5°C and 45°C, preferably between 10°C and 40°C, for example between 10°C and 20°C.It is generally carried out at atmospheric pressure. The azo compound / water mass ratio may be between 1 / 99 and 40 / 60, preferably between 10 / 90 and 25 / 75; and more preferably between 15 / 85 and 25 / 75. The organic binder / azo compound mass ratio may be between 0.2 and 0.5, preferably between 0.3 and 0.5, more preferably between 0.3 and 0.4. The process according to the invention optionally comprises subsequent steps of recovering and drying the granules obtained in step a). These steps may be carried out in a conventional manner. For example, the granules may be recovered by filtration. They may then be dried. In particular, they may be dried at a temperature between 10°C and 45°C, preferably between 20°C and 40°C. They can be dried under reduced pressure or more preferably under a sweep of air, depleted air or an inert gas such as nitrogen.Drying can last a few hours, for example between 1 and 10 hours, preferably between 3 and 5 hours. The term "granules" is understood to mean in particular solid and cohesive agglomerates of constituent particles, said particles possibly having a size of between 10 µm and 200 µm, preferably between 90 µm and 110 µm. Thus, the present invention relates to granules capable of being obtained (or obtained or directly obtained) by the process according to the invention. Such granules are novel. The present invention also relates to granules comprising an azo compound of general formula (I) as defined above and an organic binder as defined above. In particular, in the granules according to the invention, said binder is present in trace amounts; in particular when they are dried after recovery, which results in evaporation of the organic binder.The granules according to the invention may thus comprise between 20 ppm and 3000 ppm, for example between 20 ppm and 1000 ppm of organic binder. More particularly, they comprise between 20 ppm and 500 ppm of organic binder, more preferably between 50 ppm and 300 ppm of organic binder, more preferably between 50 ppm and 200 ppm of organic binder, in particular after drying. In particular, the granules according to the invention have a crushing strength greater than or equal to 25 g / cm. 2 , preferably still greater than or equal to 40 g / cm 2 They advantageously have a maximum crushing resistance of 90 g / cm 2 , preferably 95 g / cm 2 , more preferably 100 g / cm 2 or even 500 g / cm 2 , including terminals. For example, their maximum crushing resistance is between 50 and 100 g / cm 2 , preferably between 55 and 95 g / cm 2The granules according to the invention are generally of substantially spherical or spherical shape. They may have a diameter of between 0.5 and 5 mm and more preferably between 1 and 5 mm, for example between 2 and 3 mm. The present invention also relates to granules comprising an azo compound of general formula (I) as defined above and having a crushing strength greater than or equal to 25 g / cm 2 , preferably still greater than or equal to 40 g / cm 2. Such granules may comprise one or more of the characteristics mentioned above. One of the advantages of the granules such as according to the invention is their solidity, which makes them very easy to transport and handle. They are also of good purity, with a low content of residual organic binder. The present invention relates to their use as swelling agents, initiators of polymerization reactions involving free radicals, or as synthesis intermediates, in particular in the preparation of pharmaceutical or agrochemical compounds. The polymerization reactions may be bulk, solution, suspension or emulsion polymerization reactions and may use a wide variety of monomers, for example (meth)acrylic or vinyl monomers, such as acrylamide, acrylonitrile, alkyl (meth)acrylate, styrene, vinyl acetate and chloride or vinylidene chloride.The fields of application relate in particular (but not exclusively) to acrylic sheets or fibers, flocculants, paints, coating resins, grafted polyols, polystyrene, PVC (polyvinyl chloride), PVA (polyvinyl acetate), or PMMA (polymethyl methacrylate). In particular, the granules according to the invention can be used in the preparation of polyols grafted with a mixture of styrene and acrylonitrile or in the preparation of polyacrylonitriles precursors of carbon fibers. EXAMPLES The particle size of the dry AZDN used in the examples is 52.5 / 101 / 180 microns respectively dv(10), dv(50) and dv(90). That of the wet AZDN is 61 / 119 / 203 microns respectively dv(10), dv(50) and dV(90). This particle size measurement is carried out using a Masterziser® S device. The measurement is carried out using water and a drop of Igepal® surfactant (ethoxylated nonylphenol) as dispersant.The particle size measurement is carried out after 10 minutes of circulation in the measuring cell. EXAMPLE 1: Granulation in a 500 ml reactor 1- Operating methods: Granulation: The reactor is a 500 ml double-walled glass reactor, maintained at 15 ° C by circulating cold water. It is equipped with mechanical stirring. 22.4 g of dry AZDN are weighed into a beaker and then 100 ml of water are added. After homogenizing the mixture using a spatula, the aqueous suspension is transferred into the reactor. 100 ml of water are then added to recover the AZDN remaining in the beaker. The suspension, in the reactor, is stirred at 1000 -1100 rpm, so that the AZDN remaining on the surface is carried away by the stirring. After a few minutes, the organic binder is quickly added to the reactor. The stirring speed is then reduced to 850 rpm. After approximately three hours of stirring, the formation or absence of granules is noted.The reactor is drained onto a filter. Optionally, the aqueous filtration solution, saturated with binder, is used to finish rinsing the reactor. The filtered granules are washed and left to dry in the open air under ventilation for approximately 24 hours. The fragility of the formed and dried granules is noted by their resistance to manual crushing. Measurement of solubility of the organic binder in water: The measurements were carried out by bringing 100g of water and 20g of organic binder into contact with each other under stirring at 20°C for 1 hour. The aqueous phase is decanted and then analyzed by gas chromatography to determine the concentration of organic binder. Measurement of the solubility of AZDN in the organic binder: The measurements were carried out by adding 1.5g of AZDN to 10g of organic binder at 20°C. The flask is stirred for approximately 6 hours. After decantation, the organic binder is analyzed to determine its AZDN concentration.The analysis is carried out by gas chromatography with an injector temperature set at 220°C (under these conditions, AZDN is essentially transformed into tetramethylsuccinonitrile in the injector). A standard range is produced between 0 and 150g / l by diluting AZDN in acetone. Gas chromatographic analyses: The chromatographic column is an OV1701 macrobore column (diameter = 0.25mm, length = 30m, film thickness = 0.25 microns), the chromatographic apparatus is a Hewlett Packard HP 6890 equipped with an FID (flame ionization) type detector. 2- Results obtained: The results obtained are presented in the following table: [Table 2] *. - positive: the dried granule does not crush under light pressure from the spatula; - negative: the dried granule crushes under light pressure from the spatula and gives powder. ** Test B carried out with a 5% aqueous HCl solution instead of water. Following granulation as according to the invention, granules of size between 1 and 3 mm are obtained, which are sufficiently solid to be recovered, dried and handled. EXAMPLE 2: Granulation in a 2L reactor 1- Granulation and drying: Granulation: Apparatus similar to that of Example 1 is used but with a two-liter reactor. The quantities used are given in the table below. The agitator is an agitator with inclined blades and the initial agitation is set at 800 rpm then lowered to 600 rpm after introduction of the organic binder as in the previous example.Drying of the granules: A 6 cm diameter porous glass filter is used, equipped with a double jacket allowing the filter walls to be heated by circulating hot water. 100g of undried filtered granules are introduced into the filter. A constant flow of nitrogen (2L / min) is then injected from the bottom of the filter at different temperatures. The granules obtained are given in the table: [Table 3]. 2- Determination of the residual organic binder content: During the drying of the granules described above, approximately one gram of granules obtained with isobutyl acetate is taken over time and analyzed by gas chromatography to determine the residual binder content. The results obtained are presented in the following table: [Table 4] With CMPE, at 30 °C, drying is faster and after 3 hours a residual CPME concentration of 0.01% is observed which does not change significantly thereafter (5 hours drying). The preparation and drying of the granules can therefore be carried out under satisfactory industrial conditions. After a few hours of drying, granules are obtained with a residual mass content of organic binder between 0.01% and 0.02%, i.e. between 100 ppm and 200 ppm of residual organic binder (i.e. between 100 and 200 mg of organic binder per kg of granules). 3- Crushing test of the dried granules: 20g of AZDN granules obtained in example 2 are placed and distributed evenly in a 6cm diameter glass crystallizer, then a 5cm diameter flat-bottomed glass beaker (mass 200g empty) is placed on top.If no crushed or broken granules are observed, additional weights of 200g are gradually added above the beaker until the beginning of crushing of the granules is observed, visible by transparency in the bottom of the beaker or on the sides of the glass crystallizing dish. The surface area of ​​the beaker is S = π x (2.5). 2 = 19.625 cm 2.The crushing strength is: [total weight (beaker + additional weight(s)) at breaking point] / [S] Comparative test: Aggregates were prepared by extrusion according to the procedure described in application WO 00 / 24706 in examples 1 and 2. The AZDN powder is that used for the previous examples of the present application. The aggregates obtained have a diameter of 5 mm and a length of 1.5 cm on average (between 1 and 2 cm). Once dried, these aggregates appear very fragile when handled. They show a very low resistance to the crushing test, much lower than that obtained for the granules according to the invention. The results obtained are presented in the following table: [Table 5] * The granules according to the invention have significantly improved crushing resistance. They are easy to handle without disintegrating. EXAMPLE 3: Granulation in a 100 L reactor 1- Granulation and drying: A 100 liter AE100 enameled stainless steel reactor from DE DIETRICH is used. The internal diameter of the reactor tank is 508 mm, the useful height is 375 mm. Agitation is provided by a 300 mm diameter impeller type agitator with three straight blades providing radial agitation. The agitation speed can vary from 0 to 200 revolutions per minute. The reactor is equipped with a ball type bottom valve connected to a drying filter. The filter diameter is 55 cm. The filter is equipped with a 48 cm diameter scraper agitator which can be raised or lowered into the filter and, when lowered to the level of the filter, allows mechanical evacuation of the dried product through a side opening. The agitation speed can vary from 0 to 60 rpm.The filtration cloth is a 20 micron mesh cloth. The test is carried out at room temperature (19-20°C). The reactor is maintained under nitrogen by a gentle flush of 50 l / h. 43.1 kilograms of demineralized water are introduced. Stirring is set at 200 rpm, the temperature is 18°C. 11.3 kilograms of wet AZDN powder with 8% water content are introduced over 5 minutes. After 5 minutes, 4.1 kilograms of isobutyl acetate are added over two minutes. The formation of granules is visible from the first minutes. After 5 minutes, stirring is lowered to 100 rpm and these conditions are maintained for 5 hours. The reactor is then drained through the bottom valve into the filter. The granules are filtered by applying nitrogen pressure to evacuate and recover the aqueous juices. A 10 m nitrogen sweep. 3 / h is then placed for 24 hours at room temperature. The granules, once dried, are solid and can be recovered by starting the mechanical scraper agitator set at a speed of 6 revolutions per minute. 10.2 kilograms of AZDN are thus recovered in the form of granules approximately 2 to 3 millimeters in diameter. 2- Crushing resistance test: A crushing resistance test of the granules as described in the previous example was carried out and shows very good solidity of these granules with a resistance of up to 91.72 g / cm 2. 3- Dissolution test: A granule dissolution test is carried out at room temperature. A glass Erlenmeyer flask equipped with a magnetic stirring bar is placed on a magnetic stirrer. With stirring stopped, 13 g of AZDN are introduced into the Erlenmeyer flask. 100 ml of acetone are then quickly introduced and stirring is started (100 rpm). The time taken for non-solubilized AZDN crystals to no longer be seen visually is noted. Commercial recrystallized AZDN (130-257-438 microns / dv 10-50-90), AZDN granules prepared and dried as obtained above and the dry AZDN powder used in Example 1 are compared. [Table 6] It can be seen that the spherical AZDN granules obtained according to the invention dissolve as quickly as the initial AZDN in powder form. On the other hand, the recrystallized AZDN crystals, although of average diameter smaller than that of the spherical granules of the invention, take much longer to dissolve in acetone. It can be noted that there is no visible insoluble material for each of the samples.

Claims

CLAIMS 1. Process for granulating an azo compound of the following general formula (I): [Chem 1] in which: * the radicals R 1 , R 2 , R 3 and R 4 , identical or different, are chosen independently of each other from: - a linear or branched alkyl group, optionally substituted by a hydroxy, alkoxy group or by a halogen atom; - a cycloalkyl group, optionally substituted by a hydroxy, alkoxy group or by a halogen atom; - an aryl group, optionally substituted by a hydroxy, alkyl, alkoxy group or by a halogen atom; - an aralkyl group, optionally substituted by one or more alkyl, alkoxy, hydroxy group(s) or by one or more halogen atoms; or - at least one of the combinations of R 1 with R 2 , and / or R 3 with R 4forms, with the carbon atom to which it is (or they are) linked, a cycloalkyl radical or a C(O) group; * the radicals R 5 and R 6 , identical or different, are chosen independently of one another from the CN or NH2 groups; said process comprising the following steps: a) a step of granulation by stirring an aqueous suspension of said azo compound, in the presence of an organic binder; b) an optional step of recovering the granules obtained in step a), preferably by filtration; and c) an optional step of drying the granules recovered in step b).

2. Granulation process according to claim 1, in which said organic binder is added to said aqueous suspension.

3. A granulation process according to claim 1 or 2, wherein the azo compound of general formula (I) is selected from the group consisting of: 2,2'-azobis(isobutyronitrile), 2,2'-azobis(2,4-dimethyl-valeronitrile), 2,2'-azobis(2-methylhexylonitrile), 2,2'-azobis(2-cyclopropylpropionitrile), 2,2'-azobis(2-phenylpropionitrile), 2,2'-azobis(2-methylbutyronitrile), 1,1'-azobis(1-cyclohexanecarbonitrile) and azodicarbonamide; preferably 2,2'-azobis(isobutyronitrile).

4. A granulation process according to any one of the preceding claims, wherein the organic binder is selected from aliphatic acetates, aliphatic carbonates, non-halogenated aromatic hydrocarbons, aliphatic ketones and aliphatic ethers. 5.Granulation process according to any one of the preceding claims, in which the azo compound / water mass ratio may be between 1 / 99 and 40 / 60, preferably between 10 / 90 and 25 / 75; and more preferably between 15 / 85 to 25 / 75.

6. Granulation process according to any one of the preceding claims, in which the organic binder / azo compound mass ratio may be between 0.2 and 0.5, preferably between 0.3 and 0.5, more preferably between 0.3 and 0.

4.

7. Granules comprising an azo compound of general formula (I) as defined in any one of claims 1 or 3 and an organic binder as defined in any one of claims 1 or 4.

8. Granules according to claim 7, characterized in that they are of substantially spherical shape, preferably with a diameter of between 0.5 and 5 mm.

9. Granules capable of being obtained by the process as according to claims 1 to 6. 10.Use of the granules according to any one of claims 7 to 9, as swelling agents, initiators of polymerization reactions using free radicals, or as synthesis intermediates, in particular in the preparation of pharmaceutical or agrochemical compounds.

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