EP1399497A1

EP1399497A1 - Production of masked isocyanates, particularly masked polyisocyanates

Info

Publication number: EP1399497A1
Application number: EP02758520A
Authority: EP
Inventors: Jean-Marie Bernard
Original assignee: Rhodia Chimie SAS
Current assignee: Rhodia Chimie SAS
Priority date: 2001-06-25
Filing date: 2002-06-25
Publication date: 2004-03-24
Also published as: JP2004533471A; US20040242833A1; KR20040014580A; FR2826366B1; FR2826366A1; WO2003000763A1

Abstract

The invention relates to a method for the production of masked (poly)isocyanates using a masking agent with a C=N bond and a labile hydrogen atom on the α atom relative to the nitrogen of the C=N group, characterised in comprising the following steps: a) preparation of a masking agent with a C=N bond and a labile hydrogen atom on the α atom relative to the nitrogen of the C=N group which may be obtained by condensation of an aldehyde or ketone precursor with a nitrogen-containing precursor which has a functional group reactive towards the carbonyl moiety of the aldehyde or ketone, in an aqueous, organic or hydroorganic medium with liberation of water of condensation; b) in situ reaction of the masking agent with a (poly)isocyanate component for masking to give said (poly)isocyanates which are totally or partially masked.

Description

Preparation of masked isocyanates, especially masked polyisocyanates

The invention relates to the preparation of masked isocyanates, in particular masked polyisocyanates.

The invention more particularly relates to a process in which are directly chained the preparation of the masking agent and the masking of (poly) isocyanates, strictly speaking, without isolation of the intermediate reaction product. Polyisocyanates masked by means of an oxime-type masking agent are described in SU 727,637, SU 78-2665978, US 4,868,298 and DE 2342775.

In addition, SU 414259 and EP 159 117 describe polyisocyanates whose isocyanate functions are masked by a pyrazole.

Many other methods of masking isocyanates are described in the literature; all these methods comprise two stages: a first stage of preparation and isolation of the masking agent and a second stage in which the masking agent is reacted with the isocyanate functions present in the isocyanate composition to be masked.

The object of the present invention is to prepare a masked (poly) isocyanate composition in a controlled manner. The object of the present invention is also to provide a masked (poly) isocyanate composition, in part or in whole in organic, hydro-organic or aqueous medium in the form of a solution, suspension or emulsion, in which the content of masking groups can be determined in advance, which is obtained directly using the precursor compounds of the masking agents without loss of intermediate compounds and without intermediate isolation of the masking agent.

The work of the inventors at the origin of the present invention has made it possible to discover that for a type of masking agents In particular, it was possible to prepare masked polyisocyanates by a one-step process from precursors of the masking agents, without the absence of isolation of the masking agent interfering with the masking reaction. The invention relates to a process for the preparation of

(poly) isocyanates masked with a masking agent having a C = N bond and having a mobile hydrogen atom carried on the atom in α relative to the nitrogen atom of the group C = N , characterized in that it comprises the following stages: a) preparation of a masking agent having a bond

C = N and having a mobile hydrogen atom carried on the atom in α with respect to the nitrogen atom of the group C = N capable of being obtained by condensation of a precursor consisting of an aldehyde or a ketone with a nitrogenous precursor comprising a reactive functional group with respect to the carbonyl function of the aldehyde or of the ketone, in aqueous, organic or hydro-organic medium, with release of water of condensation; b) in situ reaction of the masking agent with a (poly) isocyanate composition to be masked in order to obtain said totally or partially masked (poly) isocyanates. Advantageously, said hydrogen atom mobile in α relative to the nitrogen atom of group C = N is carried by an oxygen or nitrogen atom.

By "capable of being obtained by condensation of a precursor consisting of an aldehyde or a ketone with a nitrogenous precursor comprising a functional group reactive with respect to the carbonyl function of the aldehyde or of the ketone" , it is understood that the masking agent can be obtained by this route, but is not necessarily so in the context of the process of the invention.

Condensation reactions between precursors of the type mentioned above are balanced reactions, the condensation of carbonyl precursors being in equilibrium with the hydrolysis reaction of the masking agent (Textbook of Practical Organic Chemistry; Vogel's; 5 ^th edition ; John Wiley &Sons; p. 1228). In addition, ketone derivatives often have a mobile hydrogen atom which is capable of inducing parasitic reactions during condensation with a nitrogen derivative.

The method of the invention therefore has a surprising character in that the skilled person expected that the unreacted starting materials and the annoying by-products present in the medium reaction at the end of step a) interfere, on the one hand, with the masking reaction and, on the other hand, have negative effects on the qualities of coatings obtained during the application of the (poly) isocyanate compositions masked of the invention, and would therefore have considered as a compulsory step the careful separation of the masking agent from the reaction medium of its preparation before masking itself. Among the by-products present in the reaction medium at the end of step a) and which could be considered to be troublesome in the light of the prior art, mention may, for example, be made of the water formed during the reaction of preparation of the masking agent.

Masking agents suitable for the process of the invention are preferably oximes, pyrazoles and hydrazones, the latter including semi-carbazones.

In general, the masking agents of the invention correspond to the following general formulas I to III:

in which :

Ri and R ₂ are chosen from an alkyl, perhaloalkyl, cycloalkyl, aryl, alkoxyl, acyl, acyloxyl, aryloxyl, alkoxycarbonyl and aryloxycarbonyl group optionally substituted, and / or interrupted by one or more heteroatoms chosen from S, O and N, preferably O;

R ₃ , R ₄ and R ₅ are chosen from a hydrogen atom, and an alkyl, perhaloalkyl, cycloalkyl, aryl, alkoxyl, aryloxyl, acyl, acyloxyl, alkoxycarbonyl and aryloxycarbonyl group, optionally substituted, and / or interrupted by one or several heteroatoms chosen from

S, O and N, preferably O;

Rβ represents a hydrogen atom, an alkyl, cycloalkyl or aryl group, as defined above or carbamoyie of formula CONR R ₈ , R ₇ and R _{8 which} can be chosen independently of one another from an alkyl, cycloalkyl group , or optionally substituted aryl.

In formulas I, II and III above, said hydrogen atom mobile at α of the nitrogen atom of the group C = N is in bold underlined characters.

Preferably, only one of Ri and R ₂ or R ₃ , R and R ₅ represents an alkoxyl, aryloxyl, acyloxyl, alkoxycarbonyl or aryloxycarbonyl group. When in formulas I, II and III, the groups Ri, R ₂ , R ₃ , R ₄ and R ₅ represent a hydrocarbon chain interrupted by a heteratome, there are preferably less than two single bonds CO, CS and / or CN on a single carbon atom.

Groups Ri to RQ advantageously have at most 15 carbon atoms, preferably at most 10 carbon atoms. Step a) of synthesis of the masking agent implements reactions well known to those skilled in the art.

Thus, the reaction between a 1, 3-diketone (or β-diketone) with hydrazine in basic medium to lead to a substituted pyrazole is described in particular in "Vogel's Textbook of Practical Organic Chemistry" (BS Furniss, AJ Hannaford, PWG Smith, AR Tatchell), ^5th edition.

In addition, the reaction between hydroxylamine with a ketone is a well-known reaction described in particular in Advanced Organic Chemistry by Jerry March, 3 ^rd edition, J. Wiley & Sons, pp 534, 729, 805 and 1170.

The preparation of hydrazones from hydrazine or one of its derivatives and a ketone or a ketoalcohol is also well known to those skilled in the art and described in particular in (Textbook of

Practical Organic Chemistry; Vogel's; 5th edition; John Wiley &Sons; p. 1245).

By "alkyl" group within the meaning of the invention, generally means a saturated hydrocarbon group having only hydrogen and carbon atoms, linear or branched and generally having from 1 to 20, preferably from 1 to 10, more preferably from 1 to 6 carbon atoms.

By "cycloalkyl" means an alkyl group as defined above comprising a unicycle and advantageously having 3 to 12 carbon atoms, for example a cyclopropyl or cyclopropylmethyl group.

By "aryl" group is meant a monocyclic or bicyclic aromatic hydrocarbon group comprising from 6 to 10 carbon atoms.

By "alkoxyl" within the meaning of the invention means a group -O-alkyl, alkyl being as defined above, including in particular cycloalkyl and aralkyl.

By "aryloxyl" within the meaning of the present invention means a group-O-aryl, aryl being as defined above. By "acyl" within the meaning of the invention, is meant a group -C (O) -alkyl or -C (O) -aryl, alkyl and aryl being as defined above, and alkyl including cycloalkyl and aralkyl.

By "acyloxyl", within the meaning of the invention, is meant a group -OC (O) -alkyl or -OC (O) -aryl, alkyl and aryl being as defined above, and alkyl including cycloalkyl and aralkyl .

By "alkoxycarbonyl" within the meaning of the invention means a group -C (O) -O-alkyl, alkyl being as defined above, and including cycloalkyl and aralkyl. By "aryloxycarbonyl" within the meaning of the present invention means a group -C (O) -O-aryl, aryl being as defined above.

The substituents of the alkyl groups may be aryl groups, ORg, SRg, NRgRio, PO RgRιo or polyoxyethylene, Rg and Rio identical or different representing an alkyl, cycloalkyl or aryl group as defined above.

The substituents of the aryl groups can be alkyl, aryl, ORg, SRg, NR ₉ R ₁₀ , PO RgRι ₀ or polyoxyethylene groups in which Rg and Rio, which are identical or different, represent an alkyl, cycloalkyl or aryl group as defined above. . The preferred Ri and R ₂ groups are alkyl groups, in particular methyl and ethyl, or one of Ri and R ₂ is an alkoxy carbonyl group, in particular methyloxycarbonyl or aryloxycarbonyl.

The preferred R ₃ , R and R _{5 groups} are a hydrogen atom or an alkyl group, in particular methyl or aryl, in particular phenyl. Preferably at least one group of R ₃ , R ₄ and R ₅ represent a hydrogen atom.

Preferred compounds of general formula I are methyl ethyl ketoxime (MEKO), benzophenone oxime, alkyl pyruvate oximes, in particular methyl oxime pyruvate (POME) or ethyl oxime pyruvate and cyclohexanonoxime.

Oximes derived from α-diketones are not preferred.

Preferred compounds of general formula II are pyrazole, 3-methylpyrazole and 3,5-dimethylpyrazole. Preferred compounds of general formula III are acetaldehyde hydrazone, methylhydrazone, acetone, cyclopentanone and methyl ethyl ketone.

According to a first embodiment of the invention, reacting in step a) the hydroxylamine with a ketone, an α-ketoester, or even a β-ketoester to obtain an oxime.

The ketones in this case include the hydroxycarbyloxy-carbonyl compounds, in particular the alkyloxy-aryloxy-carbonyl or ketoesters and preferably the α-ketoesters, in particular those described in WO 97/24386, the ketonitriles, the diaikoyiamides, the aldols and ketols, including sugars and their derivatives and cyclic esters, in particular lactones as well as keto alcohols, in which the alcohol function is preferably secondary or tertiary.

According to a second embodiment of the invention, reacting in step a) hydrazine with a β-diketone to obtain a pyrazole compound.

According to a third embodiment of the invention, reacting in step a) hydrazine or a hydrazine derivative with a ketone or an ε-keto alcohol preferably secondary or tertiary or with a ketolactone to get a hydrazone.

More specifically, the oximes corresponding to general formula I can be obtained by condensing one mole of hydroxylamine with one mole of ketone of general formula la:

in which Ri and R ₂ are as defined above, with the elimination of one mole of water. The reaction generally takes place between pH 3 and 10, advantageously between 4 and 6, preferably at pH 5 ± 0.5.

The pyrazoles of general formula II are obtained by reaction of a mole of hydrazine: NH ₂ -NH ₂ with a mole of β-diketone of general formula lia with elimination of two moles of water:

R ₃ , R ₄ and R ₅ being as defined above.

The hydrazones of general formula III are obtained by reaction of a mole of hydrazine of general formula IIIa: NH ₂ -NHR ₆ (IIIa) where R ₆ is as defined above with one mole of ketone of formula general la) as defined above, with elimination of a water molecule.

The first step of the method of the invention is carried out in a manner known per se in organic, hydro-organic or aqueous medium. The choice of solvent is guided by that of the final formulation.

If one seeks to obtain a (poly) isocyanate composition in an organic medium, one will use a conventional solvent of formulations of this type, in particular an ester (n-butyl acetate), an ether or a hydrocarbon, preferably an aromatic hydrocarbon, for example SOLVESSO®.

If one seeks to obtain a composition in hydro-organic medium, one will choose an organic solvent miscible with water. An ether, an alcohol or an amide, such as NMP, will be preferred. If one seeks to obtain an aqueous composition, the reaction will be carried out entirely in water.

It has been surprisingly shown in the context of the present invention that the reaction could be carried out in a biphasic manner without interfering with the condensation of the carbonylated products with the nitrogenous products.

The reaction medium must be understood as being able to be homogeneous or heterogeneous, without interfering with the condensation of carbonylated products with nitrogenous products and without interfering with the subsequent masking reaction of the (poly) isocyanate composition. w In each of these cases, it should be understood that the presence of the water formed during the reaction for the preparation of the masking agent does not constitute a hindering element for the masking reaction itself. It can also be envisaged to add water to the reaction medium. The water content of the reaction medium will thus be

Advantageously equal to once, preferably 1, 5 times, preferably still 2 times the amount of water formed during the preparation of the masking agent

The reaction temperature is generally between 20 and 100 ° C, and is preferably around 50 ° C. The reaction time is generally of the order of 30 minutes to 8 hours, advantageously of the order of 30 minutes to 4 hours.

We generally work with an equivalent ratio

functions / nitrogen functions of the precursor, at most equal to 0.9, advantageously 1, preferably with an excess of equivalents of

25 nitrogenous functions of the precursor compared to the equivalent functions up to 20%, preferably 10%. We can also work with a

slight excess of function ^{== 0} . However, this embodiment is not preferred since the products tend to yellow, which could prove troublesome for certain uses of the masked (poly) isocyanate compositions of the present invention. At the end of the first step, a reaction product of general formula I, II or III is obtained, water resulting from the condensation of the carbonyl reagent with the nitrogen reagent and, where appropriate, an organic solvent .

In a first embodiment of the invention, step a) takes place in a hydro-organic organic medium. The water produced by condensation of the compounds of general formula I, II or III with the corresponding carbonyl compound can form an aqueous phase which can optionally be removed, for example by decantation or any other method known to those skilled in the art, and the reaction product of general formula I, II or III recovered in the organic phase which is not necessarily dry, either in solution or in suspension. This does not imply complete elimination of the water.

In a second embodiment of the invention, the water produced in step a) is not removed and the reaction product is obtained in solution or suspension in an aqueous or hydro-organic medium depending on whether the medium initial reaction or not contained an organic solvent.

In a third embodiment of the invention, the reaction takes place entirely in an aqueous medium. In this case, the equilibrium should be shifted, for example by precipitation of the compound formed. In general, this embodiment is not preferred in view of the fact that the balance is shifted towards the starting compounds.

The masking step b) is carried out directly on the reaction medium obtained at the end of step a) without subsequent treatment thereof, if not, if necessary, a decantation step, but in all cases without isolation of the intermediate masking product. The reaction of the isocyanate with the masking agent being exothermic, the reaction temperature increases as a function of the nature of the masking agent, of the isocyanate compound and of the concentration of reagents in the medium.

According to a first operating mode, the isocyanate which it is sought to mask the NCO functions is introduced into the reaction medium, containing the masking agent, preferably gradually and continuously, so as to maintain the temperature of the medium lower reaction at the release temperature of the masking group, preferably less than 100 ° C and better still at 80 ° C.

The introduction of the isocyanate generally takes place at a temperature between 20 and 120 ° C, preferably around 50 ° C. It is however also possible to heat the reaction medium if one seeks to accelerate the reaction.

According to a second procedure, the masking agent from step a) is added to the (poly) isocyanate composition to be masked.

In the case where the (poly) isocyanate is added to the masking agent, all the isocyanate functions are masked insofar as the functional group of the masking agent reacting with the isocyanate / isocyanate functions is> 1.

Otherwise, one can stop the addition of the masking agent to (poly) isocyanate and obtain partially masked (poly) isocyanates, the masking agent / isocyanate ratio being less than 1 and the distribution of masking agents being statistical, that is to say that practically all the masking agents block all or part of the isocyanate functions of each species (poly) isocyanate present in the initial (poly) isocyanate composition. It is therefore very possible to introduce a second masking agent or a chain extender (diol, prepolymer, etc.).

In both cases, there may be the formation of urea and possibly biuret by reaction of the isocyanate with H ₂ O but according to the present invention this reaction is clearly in the minority / blocking reaction unless it is voluntary ( addition of chain extender or catalyst). The determination of the free NCO functions is carried out by the dibutylamine method, or monitored by infrared at 2250 cm ⁻¹ (NCO band).

The masking reaction can be promoted by the addition of a catalyst, but this addition is not necessary.

According to a particular embodiment of the invention, one can use for step a) two different carbonyl compounds which will lead to two different oximes, for example MEKO and POME, or two different pyrazoles, for example pyrazole and 3-methylpyrazole, and carry out the subsequent masking with the mixed masking agents obtained.

One can also choose the pairs of compounds la and / or lla / hydrazine or hydrazine derivative so as to obtain a mixture of a masking agent of a type (for example, oxime) and an agent masking of different type (for example, pyrazole) which is subsequently reacted with the (poly) isocyanate composition to be masked in order to obtain mixed masked (poly) isocyanates.

The isocyanate which is reacted with the masking agent can be a monomeric isocyanate, in particular a diisocyanate or a triisocyanate. Preferred are the monomers in which at least one of the isocyanate functions is aliphatic, that is to say carried by a hybridization carbon (sp ³ ) which also advantageously carries at least one, preferably two atoms 'hydrogen. Mention may in particular be made of the following isocyanate monomers:

- 1,6-hexamethylene diisocyanate,

- 1,12-dodecane diisocyanate,

- cyclobutane-1, 3-diisocyanate, - cyclohexane-1, 3 and / or 1, 4-diisocyanate,

- 1-isocyanato-3,3,5-trimethyl-5-diisocyanato-methylcyclohexane (isophorone diisocyanate, or even IPDI),

- 2,4- and / or 2,6-hexahydrotoluylene diisocyanate,

- hexahydro-1, 3- and / or -1, 4-phenylene diisocyanate, - perhydro-2,4'- and / or -4,4'-diphenylmethane diisocyanate,

- 1, 3- and / or 1, 4-phenylene diisocyanate,

- 2,4- and / or 2,6-toluylene diisocyanate,

- diphenylmethane-2,4'- and / or -4,4'-diisocyanate,

- isocyanato- (4) -methyloctylene diisocyanate (LTI or NTI), - triphenylmethane-4,4 ', 4 "-triisocyanate,

- 1, 3-bisisocyanatomethylcyclohexane,

- bis-isocyanatomethylnorbornane (NBDI), - 2-methylpentamethylene diisocyanate.

The isocyanate can also be a product of homo- or hetero-condensation of alkylene diisocyanates, comprising in particular products of the "BIURET" type and of the "TRIMERES" type, or even "PREPOLYMERS" with isocyanate functions, comprising in particular urea, urethane, allophanate, ester, amide, or mixtures of the aforementioned products.

The isocyanate can also be a precondensation product with a compound having at least one mobile hydrogen function, in particular a polyol or a polyamine.

A subfamily of compounds of this type relates to polyisocyanates resulting from a prepolymerization with alcohols, in general triols or polyamines, in particular triamines, with a polyisocyanate, in general diisocyanate, the quantity of isocyanate functions being greater than that of the mobile hydrogen functions (such as amines and / or alcohols), so that at the end of the prepolymerization the number of residual isocyanate functions per molecule is on average greater than two, advantageously at least equal to 2.5; preferably at least three. By way of example of alcohol compounds, mention may be made of monoalcohols such as methanol, ethanol, propanol, butanol, polyols having from 2 to 30 carbon atoms, such as glycerol, propylene glycol , butanediol, trimethylolpropane, pentaerythritol, diglyme, phenols, such as phenol, cresols, xylenols, and nonylphenol. As isocyanate compound, in particular the

1, 6-hexamethylene (HDI) diisocyanate, the cyclotrimer of 1, 6-hexamethylene diisocyanate, that is to say the monoisocyanurate compound obtained by cyclotrimerization of 3 moles of HDI on itself, the cyclodimer of HDI , that is to say the compound uretidione obtained by cyclodimerization of 2 moles of HDI on itself, as well as the biuret derivatives, and allophanates of these compounds. When the reaction medium of step b) is an organic medium, that is to say that it has been optionally freed from the aqueous phase, in particular by decantation, the masked isocyanate is obtained in solution or suspension in the organic medium depending on the nature of the masking agent and / or the percentage of masking groups present in the composition.

When it is desired to obtain a masked isocyanate composition in aqueous emulsion, it is advantageous to carry out step a) exclusively in aqueous medium and to add to the reaction product obtained before, during or after the step b) a surfactant. The addition of a surfactant is however not necessary, the reaction possibly taking place in a heterogeneous medium.

The surfactant useful for the formation of the emulsion is chosen from standard surfactants known to those skilled in the art for their properties in forming emulsions (for example, polyethylene glycol monoalkylether).

The surfactant can either be foreign to the isocyanate, or be itself an isocyanate and result from the reaction of an isocyanate on a precursor having a reactive hydrogen function (said precursor either having significant hydrophilicity, or being it even already amphiphilic), or be a mixture of the two. This synthesis can either have been carried out prior to masking, or be carried out simultaneously.

The surfactants used can be nonionic with an HLB greater than 10, preferably of the order of approximately 10 to approximately 20, anionic, cationic, zwitterionic or amphoterically advantageous with HLB greater than approximately 10.

The nonionic surfactants may be chosen from alkoxylated fatty acids, polyalkoxylated alkylphenols, polyalkoxylated fatty alcohols, polyalkoxylated or polyglycerol fatty amides, alcohols and polyglycerol alphadiols, polyalkyleneglycols, oxide block polymers. ethylene propylene oxide, as well as alkylglucosides, alkylpolyglucosides, sucroethers, sucroesters, sucroglycerides, sorbitan esters, and the ethoxy compounds of these sugar derivatives advantageously having an HLB of at least about 10.

[0086] Anionic surfactants may be chosen from the alkylbenzenesulfonates, the monoalkyl sulfates, alcoyléthersulfates, the alcoylaryléthersulfates, the dialcoylsulfosuccinat.es, the alkyl phosphates, well differentiated ether phosphates such as those constituting alkali, ammonium, preferably quaternaries, advantageously having an HLB of at least about 10. Certain preferred surfactants will be detailed below. Among the cationic surfactants, there may be mentioned aliphatic or aromatic fatty amines, aliphatic fatty amides, quaternary ammonium derivatives advantageously having an HLB of at least about 10.

Among the zwitterionic or amphoteric surfactants, there may be mentioned betaines and their derivatives, sultaines and their derivatives, lecithins, imidazoline derivatives, glycinates and their derivatives, amidopropionates, fatty amine oxides advantageously having an HLB of at least about 10.

In general, when these surfactants are nonionic surfactants, they have hydrophilic groups such as, for example, ethylene oxide groups in sufficient number, generally greater than about 10, to allow easy emulsification of the polyisocyanates, masked or not. These surfactants also have a hydrophobic part which can be chosen from aromatic groups carrying aliphatic chains or simply from aliphatic chains with a number of carbons between 8 and 50. Other hydrophobic units such as silicone or fluorinated units can also be used for specific applications.

Mention may be made, by way of nonlimiting examples, of the derivatives of polyoxyalkylene esters of fatty acids, of alkyl phenol ethoxyls, of phosphate esters with a polyalkylene alkylene glycol chain (such as the polyoxy and / or propoxy ethylene glycol, for example), and polyethylene chain tristyryl phenols. Said surfactant can also consist of an agent (or a mixture of agents) which is neutral or has an anionic function. The latter are preferred.

Thus, advantageously, said surfactant (or a mixture of agents) contains a compound comprising an anionic function and a polyethylene glycol chain fragment of at least one, advantageously at least 5, preferably at least 7 alkyleneoxy units of formula:

- (CHR-CH ₂ -O) - with R = H or CH ₃ , advantageously ethylenoxyls. Said surfactant (or one of its constituents) is advantageously based on a compound which has an anionic function. Surfactants of this type are described for example in WO 99/10402 to which reference may be made for more details.

A preferred surfactant is one whose anion corresponds to the following formula:

with, when q is zero

• where p represents zero or an integer between 1 and 2 (closed intervals, that is to say including the limits);

• where m represents zero or an integer between 1 and 2 (closed intervals, that is to say including the limits);

• where the sum p + m + q is at most equal to three;

• where the sum 1 + p + 2m + q is equal to three or five;

• where X and X ', similar or different, represent an arm comprising at most two carbon links; • where n and s, similar or different, represent an integer chosen between 5 and 30 advantageously between 5 and 25, preferably between 9 and 20 (closed intervals, that is to say including the limits);

• where Ri and R ₂ , similar or different, represent a hydrocarbon radical, advantageously chosen from aryls and alkyls optionally substituted in particular by halogen atom, in particular fluorine. The phosphate and polyalkylene glycol esters are preferred.

Mention may in particular be made of mixtures of polyethoxylated phosphate mono- and di-esters having an average number of EO units from 3 to 20, advantageously from 8 to 15 and having an ethoxylated nonylphenol chain or an acid chain. ethoxylated fat, for example lauryl or a Cβ-C ₂ o hydrocarbon chain. Mention may in particular be made of the products sold under the name RHODAFAC®. The counter-cation is advantageously monovalent and is chosen from mineral cations and organic cations which are advantageously non-nucleophilic and consequently of a quaternary or tertiary nature (in particular oniums from column V such as phosphoniums, ammoniums, and even columnar VI such as sulfoniums) and their mixtures, most often ammoniums, generally derived from an amine, advantageously tertiary. Advantageously, it is avoided that the organic cation presents a hydrogen reactive with the isocyanate function, hence the preference with respect to tertiary amines.

The mineral cations can be sequestered by phase transfer agents such as crown ethers. The pKa of the cations (organic [ammonium, etc.] or minerals) is advantageously between 8 and 12.

When step a) is carried out in an aqueous medium, it is also possible to obtain the final masked (poly) isocyanate composition in hydro-organic emulsion by addition to the reaction medium before, during or after step b) of a water-dispersible solvent. Mention may in particular be made of methyl phosphate, an ether or alternatively preferably aromatic hydrocarbons of the SOLVESSO® type. It is advantageous in this case, although not necessary, to add to the hydro-organic medium a surfactant of the type described above in order to obtain a stable oil-in-water emulsion.

The method of the invention also makes it possible to obtain a masked (poly) isocyanate composition in highly concentrated organic solution. In this case, step a) can be carried out in an aqueous medium and at the end of step b) a hydrophilic solvent is added to the reaction medium after which the organic and aqueous phases are left and the composition (poly) is recovered isocyanate masked in very concentrated organic solution. A preferred solvent for this purpose is n-butyl acetate.

The invention also relates to a partially or totally masked (poly) isocyanate composition obtained by the process according to the invention.

The (poly) isocyanates partially or totally masked according to the invention can be used as a basis for the preparation of polymers and / or reticulates and can be used in particular as one of the main constituents of coatings of all kinds, such as varnishes and paints . In such uses, the hardness qualities of the crosslinkable polymers are among those which are sought from a technical and functional point of view.

This polymer preparation process comprises the following steps:

- Putting a protected polyisocyanate according to the invention (I) in the presence of a co-reactant which contains derivatives having reactive hydrogens in the form of alcohol, phenol, thiol, certain amines including anilines; these derivatives can have aliphatic, alicyclic or aromatic hydrocarbon skeletons, preferably alkyls, including cycloalkyls and aralkyls, aryls, linear or branched, substituted or not; (these co-reactants, in general polyols are in themselves known)

- And heat the reaction medium thus formed either at a high temperature for a short time or at a low temperature for a long time. The first case corresponds to the technique often designated by the English term "coil coating" and corresponds to a duration of at most 15 minutes at 180 ° C and at most 5 minutes at 200 ° C, with in addition a good resistance to coloring (in general, yellowing) for cases where there is overcooking. It is generally preferred to work at a temperature of 150 ° C + 10 ° C.

In the other extreme, it has a softer cooking, in general, at a temperature of at most 160 ° C for a period of at most two hours, more often of at most one hour. Advantageously, the temperature is at most equal to 150 ° C, preferably between 80 ° C and 140 ° C and, more preferably still, between 110 ° C and 130 ° C for a period less than or equal to 3 pm, preferably at 10 am and, more preferably still, at 8 am.

One can plan to include an organic solvent in the reaction medium. It is also possible to provide a suspension in water.

This optional solvent is as defined above. Preferred are slightly polar solvents, that is to say those whose dielectric constant is hardly greater than or equal to 4 or more preferably to 5. The derivatives entering into the composition of the coreactive agent are generally di-, oligo-, or poly-functional. They can be monomers or derived from di-, oligo- or polymerization and are used for the preparation of optionally crosslinked polyurethanes. Their choice will be dictated by the functionalities expected for the polymer in the final application and by their reactivity.

It is preferred to avoid using derivatives having reactive hydrogens which catalyze the release of the masked isocyanate. Thus, among the amines, it is preferred to use only those which do not catalyze the decomposition or the transamidation of the masked isocyanate functions according to the present invention. These co-reactants are generally well known to those skilled in the art.

The invention therefore also relates to paint compositions comprising for successive or simultaneous addition: - a masked (poly) isocyanate according to the invention;

- a reactive hydrogen co-reactant as described above;

- Any catalysts known per se for oximes, pyrazoles and / or hydrazones, in particular those based on tin; - optionally at least one pigment;

- optionally titanium dioxide;

- optionally an aqueous phase;

- optionally a surfactant to maintain in emulsion or in suspension the constituent components of the mixture; - optionally an organic solvent;

- possibly a desiccant.

The invention also relates to the paints and varnishes obtained by the use of these compositions, according to the above method.

The invention is illustrated by the following examples:

In the following examples, for the characterization of the products, reference will be made to these IR bands.

Infrared bands of the characteristic functions of free or masked polvisocvanates:

The infra-red analysis of the finished products is carried out on a tablet of

KBr, after evaporation of the solvents and water at room temperature. The dry extract is then examined in film on a KBr slide.

Characteristic bands of HDT polyisocyanates with isocyanate functions masked by 3,5-dimethylpyrazole

- NCO: 2250 cm ^'1

- C = O isocyanurate: 1688 cm ^"1 and 1465 cm ^'1 - Strips blocked by 3,5-dimethylpyrazole:

-C = O blocking: 1720 cm ^"1 and -C (= O) -NH- blocking: 1518 cm ^{" 1}

- NH- at 3398cm ^"1

Characteristic bands of HDT polyisocyanates with isocyanate functions masked by methyl ethyl ketoxime

- NCO: 2250 cm ^"1

C = O Isocyanurate: 1688 cm ^"1 and 1465 cm ^{" 1} MEKO blocking strips:

C = O blocking: 1731 cm ^"1 and -C = O-NH- blocking: 1508 cm ^{" 1}

Characteristic bands of HDT polyisocyanates with isocyanate functions masked by the alkyl oxide pyruvate

- NCO: 2250 cm ^"1

C = O Isocyanurate: 1688 cm ^'1 and 1465 cm ^"1 Oxime blocking bands: C = O blocking: 1731 cm ^{" 1} and -C = O-NH- blocking: 1508 cm ^"1 Ester bands: 1732 cm ^{" 1}

Example 1:

Synthesis of an aqueous suspension of 3,5 dimethylpyrazole

135 g of hydrazine hydrate at 51% by weight in water, or 2, are successively introduced into a 1 liter, thermostatically controlled reactor equipped with a condenser, a thermometer and conventional stirring. , 15 moles of hydrazine. 47 g of water are still added. 200 g of acetylacetone (2 moles) are added to the reaction medium in 2 hours 45 minutes while maintaining the temperature around 50 ° C. At the end of the addition, the temperature of the reaction medium is brought to 70 ° C. for another 2 hours to ensure the completion of the reaction.

The aqueous DMP solution can be used as it is or diluted to obtain an aqueous suspension of DMP in water at 40%. The DMP formed precipitates during the cooling of the solution. The structure of the product obtained is confirmed by infrared and NMR analysis.

It is preferred to use the hydrazine hydrate aqueous solution as the source of hydrazine because it does not lead to the co-production of salts. Another route for the synthesis of 3,5-dimethylpyrazole, described in

Textbook of Practical Organic Chemistry VOGEL'S 5th edition John Wiley and Sons New York on page 1149, uses hydrazine sulfate and leads to the production of salts (sodium sulfate) which must be eliminated before or after blocking of the isocyanate functions.

In the following examples, an aqueous solution of DMP at 39% (suspension at room temperature) resulting from this example and therefore free of salts will be used.

Example 2:

Synthesis of a masked polyisocyanate composition HDT 3,5-dimethylpyrazole

365 g of an aqueous suspension of 3,5-dimethylpyrazole (DMP) are introduced at ambient temperature into a 1 liter reactor, thermostatically controlled, equipped with a condenser, a thermometer and conventional stirring.

40% dry extract, or 146 g of 3,5-DMP, prepared as described in Example 1.

The reaction medium is heated to a temperature of 60 ° C., the temperature at which the DMP dissolves slowly.

380 g of a solution of polyisocyanate based on hexamethylene diisocyanate trimer (isocyanurate) (TOLONATE HDT) in n-butyl acetate (AcO) are added over 1 hour 45 minutes to the stirred reaction medium. n-Bu) (i.e. 253 g of HDT at 0.52 mole of NCO per 100 g dissolved in 127 g of AcO n-Bu).

The temperature of the reaction medium is brought to 80 ° C. for one hour 40 minutes until the infrared analysis of a sample of reaction mass indicates the absence of an isocyanate band at 2250 cm ⁻¹ .

100 g of water and 86 g of n-butyl acetate are added to the reaction medium, and the reaction medium is decanted. The organic phase is washed with 450 g of water and a new decantation is carried out.

The hydro-organic phase containing the HDT polyisocyanate masked by DMP (437 g) is then stored in a container without additional drying.

A dosage on this organic phase indicates the presence of 2% water by weight.

The level of n-butyl acetate assayed by ¹ H NMR is 36.3%. The content of DMP masked HDT in the final organic solution is 62%.

The infrared spectrum of the emulsion shows that the HDT-DMP product formed is practically identical to a solution of masked polyisocyanate HDT-DMP obtained according to a conventional process in organic phase from 3,5-dimethylpyrazole in dry powder. The DMP masked HDT polyisocyanate solution is used as it is in the application tests (cf. application examples).

If a higher dry extract is desired, it is possible to distill part of the solvent by evaporation under partial vacuum at a maximum temperature of 80 ° C. It is also shown that the addition of alcohol (1 g of ethanol) to

1.5 grams of hydro-organic solution of HDT masked DMP leads to a completely homogeneous and translucent solution.

Example 3 Synthesis of an HDT 3,5-dimethylpyrazole Masked Polyisocyanate Composition The procedure is as for Example 2 using 314 g of an aqueous suspension at 39% of DMP obtained according to Example 1 (122.5 g of DMP and 191.5 g of water).

The organic solution of HDT polyisocyanate (245.5 g in 122.7 g of n-butyl acetate) is added to the reaction medium over approximately 50 minutes at 60 ° C. After adding all of the HDT solution, the temperature of the reaction medium is brought to 80 ° C. After 1 hour 20 minutes of finishing at 80 ° C, the solution is decanted. 170.5 g of aqueous phase are withdrawn. The organic phase is adjusted so that the dry extract is around 69%.

Analysis of the solvents shows the following distribution: water 1.6% by weight and n-butyl acetate: 29.5% by weight.

The potential NCO titer is 10.03%. The infrared analysis on a KBr film after evaporation of the solvents is characteristic of the spectrum of HDT masked by 3,5-dimethylpyrazole and shows the absence of free isocyanate functions.

Example 4 Synthesis of an HDT 3,5-dimethylpyrazole Masked Polyisocyanate Composition

The procedure is as for Example 3 using 340 g of a 39% aqueous suspension of DMP obtained according to Example 1 (122.5 g of DMP and 191.5 g of water).

The HDT polyisocyanate organic solution (265.8 g in

132.8 g of triethylphosphate) is added to the reaction medium in approximately 15 minutes. The temperature of the reaction medium goes from 60 ° C at the start of addition to 75 ° C at the end of addition. The mixture is left stirring for 2 hours at 80 ° C. to finish the masking reaction.

The solution is then decanted and 233.5 g of phase are withdrawn. After addition of 233.5 g of triethylphosphate to the organic phase, a clear hydro-organic masked polyisocyanate solution containing 54% of dry extract in HDT-DMP is obtained, with a viscosity equal to 330 mPa.s at 25 ° C. Analysis of the solvents shows the following distribution: water 28.1% by weight and triethylphosphate: 18% by weight.

The potential NCO titer is 0.239 or 10%.

The infrared analysis on a KBr film after evaporation of the solvents is characteristic of the spectrum of HDT masked by 3,5-dimethylpyrazole and shows the absence of free isocyanate functions.

Example 5:

Synthesis of an HDT masked polyisocyanate composition

3,5-dimethylpyrazole

The procedure is as for Example 4 using 287 g of a 39% aqueous suspension of DMP obtained according to Example 1 (112 g of DMP and 175 g of water).

The HDT polyisocyanate (224.3 g) is added directly to the reaction medium over approximately 40 minutes. The temperature of the reaction medium is maintained at 60 ° C. At the end of the addition of HDT 21, 5 g of N-methylpyrolidone, ie 6% by weight, are poured into the reaction medium and the temperature rose to 75 ° C. The mixture is left stirring for 2 hours at 85 ° C. to finish the masking reaction. The 63% solids solution in HDT-DMP is then stored.

During storage, a decantation in two phases is observed, which indicates that N-methylpyrrolidone is not the best solvent for formulation of HDT-DMP.

Example 6:

Synthesis of a hydro-organic solution of HDD masked DMP To the reaction medium of Example 1, a solution of HDT in n-butyl acetate (382.4 g of HDT in 41.6 g of acetate) is added over 55 minutes and at approximately 70 ° C. n-butyl). The temperature of the reaction medium rises to 75 ° C.

The two-phase reaction medium is then decanted and after elimination of most of the water, the organic phase is drained into a storage container.

Infrared analysis indicates the absence of isocyanate bands at 2500 cm ⁻¹ and the presence of urea bands corresponding to the masking.

Example 7:

Synthesis of a hydro-organic solution of HDD masked DMP

The procedure is as in previous example 6, with the difference that the HDT solution is a solution in a Solvesso® 100 / n-butyl acetate mixture (50/50) by weight.

The presence of Solvesso® leads to a more difficult decantation and a whitish milky emulsion of HDD masked DMP is obtained.

Example 8:

Synthesis of a hydro-organic solution of HDD masked DMP

The procedure is as for Example 4 using 370 g of a 39% aqueous suspension of DMP obtained according to Example 1 (148 g of DMP (1.54 mole) and 222 g of water).

The HDT polyisocyanate solution (297 g of NCO title 0.519 mole of NCO per 100 g) is directly added to Solvesso® 100 (148.2 g) to the reaction medium over 1 hour. The temperature of the reaction medium is maintained at 35 ° C. At the end of the addition of HDT, the viscosity of the medium increases and the temperature of the reaction medium is brought to 75 ° C. to facilitate agitation.

The mixture is left stirring for approximately 2 hours at 75 ° C. for the completion of the masking reaction (control of the isocyanate functions by IR). The solution is drained hot to give cold a hydro-organic emulsion of HDT with isocyanate functions masked by DMP, the dry extract of which is 49%.

The amount of free dimethyl pyrazole is 0.03%. Analysis of the solvents gives the following distribution:

Solvesso® 100: 19.5% and water: 30.5% by weight.

During storage, a decantation in two phases is observed but the masked polyisocyanate HDT-DMP is on the other hand stable.

Example 9:

Process for the preparation of a composition based on the isocyanurate trimer of hexamethylene diisocyanate with isocyanate functions masked by the oxime of methyl pyruvate.

In a 1 L reactor, thermostatically controlled, equipped with a condenser, a thermometer and conventional stirring, 83 g of hydroxylamine sulfate at 99% purity, ie 0.5 mol, are successively introduced at room temperature. 240 g of water. The temperature of the reaction medium goes from 20.7 ° C to 15.5 ° C. An organic solution of methyl pyruvate is prepared by mixing 1 18.5 g of this compound with 120 g of n-butyl acetate.

This solution is added to the reaction medium over 1 hour. 10 minutes after the start of pouring (approximately 40 ml) of this organic solution, 250 g of an aqueous 4N sodium hydroxide solution are added in parallel, over approximately 1 hour. The temperature of the reaction medium goes from 15.5 ° C to 45 ° C at the end of the reaction.

After 3 hours at 40 ° C., the organic phase is decanted and the organic phase is washed twice with 2 times 50 g of water to remove the majority of the sodium sulfate formed. 534 g of aqueous phase and 102.5 g of aqueous washing phases are thus withdrawn.

On the hydro-organic phase of methyl pyruvate oxime (269 g) brought to 40 ° C with stirring, 193 g of polyisocyanate TOLONATE- Title HDT 0.519 in NCO functions per 100 g are added in 2 hours with a temperature gradient of 40 ° C.

The temperature profile adopted for this addition is as follows: 1 hour at 40 ° C followed by a rise to 85 ° C in 30 minutes and maintains the temperature for 1 hour.

The absence of free isocyanate functions is checked by infrared analysis.

437 g of a hydro-organic composition containing 74% of dry extract of polyisocyanate-Tolonate-HDT with isocyanate functions masked by methyl pyruvate oxime with a viscosity of 25 ° C. are thus obtained.

1380 mPa.s and a potential NCO titer equal to 16.14% (0.384 moles of NCO per 100 g).

The structure of the product obtained is confirmed by infrared and NMR analysis.

Example 10:

The procedure is as in Example 9, except that the aqueous phase laden with sodium sulphate is not decanted after the methyl pyruvate oxime has been formed and that the masking reaction is carried out on the biphasic non-decanted reaction medium by adding HDT to the reaction medium.

At the end of the masking reaction determined by infrared analysis, the aqueous saline phase is then decanted onto the organic phase containing the polyisocyanate-Tolonate-HDT with isocyanate functions masked by the oxime of methyl pyruvate. The organic phase is washed with twice 100 ml of distilled water to remove the residual traces of sodium sulfate. This gives a solution of polyisocyanate-Tolonate-HDT with isocyanate functions masked by methyl pyruvate oxime at 75% of dry extract with a viscosity equal to 1500 mPa.s at 25 ° C.

Conclusion: This example shows that it is possible to chain the synthesis of the masking agent and the masking of the isocyanate functions of a polyisocyanate in a biphasic medium without intermediate isolation of the masking agent. The elimination of the salts can be postponed after the masking reaction.

Example 11:

Synthesis of an aqueous suspension of 3,5-dimethylpyrazole in the

SOLVESSO® 100

The procedure is as in Example 1. A variant is added to the final treatment.

350 mL of trimethylbenzene (SOLVESSO® 100) are added to the aqueous DMP solution.

The water is then removed by azeotropic distillation from the reaction mixture.

After removing the water, the solution is allowed to cool. A suspension of DMP is thus obtained in Solvesso® 100 which is then used for masking the isocyanate functions of a polyisocyanate in the organic phase. The concentration of DMP in Solvesso® 100 is around 46%.

Example 12:

Preparation of an organic composition of HDT polyisocyanate with isocyanate functions masked by 3,5-dimethylpyrazole in SOLVESSO® 100

The suspension of DMP is used in Solvesso® 100 prepared as described in Example 11. In a 1 L reactor, thermostatically controlled, equipped with a condenser, a thermometer and conventional stirring, the suspension of DMP is introduced into Solvesso® 100 at room temperature, ie 152.5 g of DMP in 174 , 5 g of Solvesso® 100, ie 1.59 mole of DMP. In this suspension, 302 g of

Polyisocyanate-Tolonate-HDT having a rate of isocyanate functions of 0.519 moles per 100 g. The temperature of the reaction medium thus goes from 23 ° C to 52 ° C. At the end of the addition of the HDT polyisocyanate, the reaction medium is heated to 80 ° C. for a further 1 hour 15 minutes so that the masking reaction is complete.

625 g of a solution of HDT with isocyanate functions masked by DMP are thus obtained, the dry extract of which is 73.4%, the rate of potential isocyanate functions of 10.5%.

The dosage of free 3,5-dimethylpyrazole in the final solution is 0.07%.

The analysis of the solvents gives as distribution:

- 25.5% of Solvesso® 100;

- 1.1% in water.

The infrared spectrum is consistent with that of the expected infrared spectrum.

This example shows that the chained process (synthesis of masking agent - masking reaction of the isocyanate functions) makes it possible to obtain a solution with a high dry extract without intermediate isolation of the masking agent and therefore without loss of the latter.

Comparative Example 13:

Synthesis of an organic DMP masked HDT solution according to a conventional process

In a 1 L reactor, thermostatically controlled, equipped with a condenser, a thermometer and conventional stirring, 258 g of n-butyl acetate solvent and 242 g of 3,5-dimethylpyrazole (i.e. 2.52 moles). The temperature of the reaction medium is brought to 60 ° C. and 500 g of polyisocyanate HDT at 0.52 mole of NCO per 100 g are added over a period of one hour. The temperature of the reaction medium goes from 60 ° C to 80 ° C. After one hour of finishing at 80 ° C., the infrared spectrum shows an absence of band corresponding to the isocyanate band. 960 g of organic HDT polyisocyanate solution masked by DMP are packaged in a 500 ml bottle.

This composition has a dry extract of 74%, a potential NCO titer of 10.6% and a viscosity of 1210 mPa.s at 25 ° C. The dry extract is adjusted to a rate of 70% in order to be able to use this derivative in the application tests.

This composition is used to make comparative tests (example 14).

Example 14:

Comparative application tests

A coating obtained with an HDT polyisocyanate masked by DMP is compared according to the following claimed process: synthesis of masking agent - masking of the isocyanate functions of HDT (example 3) and a polyisocyanate of the same nature HDT with isocyanate functions masked by

DMP obtained according to a conventional process (example 13).

The results obtained with the hardener of Example 3, which is the subject of the invention, are comparable to those obtained with Comparative Example 13. This shows that the synthesis process implemented has no negative influence on the film properties. Composition of the formulations of Examples 3 and 13

% NCO ES

Polyisocyanate of Example 3 10.03 69 Polyisocyanate of Example 13 10.2 70

Claims

1. Process for the preparation of masked (poly) isocyanates using a masking agent having a C = N bond and having a mobile hydrogen atom carried on the atom in α relative to the atom nitrogen of group C = N, characterized in that it comprises the following stages: a) preparation of a masking agent having a C = N bond and having a mobile hydrogen atom carried on the atom in α relative to to the nitrogen atom of group C = N capable of being obtained by condensation of a precursor consisting of an aldehyde or a ketone with a nitrogenous precursor comprising a functional group reactive with respect to the carbonyl function of l aldehyde or ketone, in aqueous, organic or hydro-organic medium, with release of water of condensation; b) in situ reaction of the masking agent with a (poly) isocyanate composition to be masked in order to obtain said totally or partially masked (poly) isocyanates.

2. Method according to claim 1, characterized in that said hydrogen atom mobile in α relative to the nitrogen atom of group C = N is carried by an oxygen or nitrogen atom.

3. Method according to claim 1 or 2, characterized in that in step a), hydroxylamine is reacted with a ketone to obtain an oxime of general formula I:

in which R 1 and R ₂ are chosen from an alkyl, perhaloalkyl, cycloalkyl, aryl, alkoxyl, acyl, acyloxyl, aryloxyl, alkoxycarbonyl and aryloxycarbonyl group optionally substituted, and / or interrupted by one or more heteroatoms chosen from S, O and N, preferably O.

4. Method according to claim 1, characterized in that in step a), the hydrazine is reacted with a β-diketone to obtain a pyrazole compound of general formula II:

in which R ₃ , * and R ₅ are chosen from a hydrogen atom, and an alkyl, perhaloalkyl, cycloalkyl, aryl, w alkoxyl, aryloxyl, acyl, acyloxyl, alkoxycarbonyl and aryloxycarbonyl group, optionally substituted, and / or interrupted by one or more heteroatoms chosen from S, O and N, preferably O.

5. Method according to claim 1, characterized in that in step a), an optionally substituted hydrazine is reacted with a ketone to obtain a compound of general formula III:

in which :

R 1 and R ₂ are chosen from an alkyl, perhaloalkyl, cycloalkyl, aryl, alkoxyl, acyl, acyloxyl, aryloxyl, alkoxycarbonyl and aryloxycarbonyl group optionally substituted, and / or interrupted by one or more heteroatoms chosen from S, O and N, de preferably O; and

R ₆ represents a hydrogen atom, an alkyl, cycloalkyl or aryl group, as defined above or carbamoyie of formula 25 CONR R ₈ , R7 and Rβ can be chosen independently of one another from an alkyl, cycloalkyl group , or optionally substituted aryl.

6. Method according to claim 1 or claim 2, characterized in that the compound obtained at the end of step a) is methyl ethyl ketoxime (MEKO), benzophenone oxime, an alkyl pyruvate oxime, in particular methyl oxime pyruvate (POME) or ethyl oxime pyruvate or cyclohexanonoxime.

7. Method according to claim 4, characterized in that the compound obtained at the end of step a) is pyrazole, 3-methylpyrazole or 3,5-dimethylpyrazole.

8. Method according to claim 5, characterized in that the compound obtained at the end of step a) is acetaldehyde hydrazone, methylhydrazone acetone, cyclopentanone and methyl ethyl ketone.

9. Method according to any one of claims 1 to 8, characterized in that step a) is carried out in an organic or hydroorganic medium

10. Method according to any one of claims 1 to 8, characterized in that step a) is carried out in an aqueous medium.

11. Process for the preparation of a solution or suspension in an organic medium of a masked (poly) isocyanate using an agent for masking the NCO function, chosen from oximes, pyrazoles and hydrazones, according to one of claims 1 to 8, characterized in that step a) is carried out in an organic medium.

12. Process for the preparation of an aqueous emulsion of masked (poly) isocyanate using an agent for masking the NCO function, chosen from the oximes, pyrazoles and hydrazones according to any one of claims 1 to 8, characterized in that step a) is carried out in the middle aqueous and in that, before, during or after step b), a surfactant is added to the reaction medium.

13. Method according to claim 12, characterized in that s the surfactant is an anionic agent whose anion corresponds to the following formula:

• where p represents zero or an integer between 1 and 2 (closed intervals 0, that is to say including the limits);

• where the sum p + m + q is at most equal to three;

• where the sum 1 + p + 2m + q is equal to three or five; 5 • where X and X ', similar or different, represent an arm comprising at most two carbon links;

• where n and s, similar or different, represent an integer chosen between 5 and 30 advantageously between 5 and 25, preferably between 9 and 20 (closed intervals, that is to say including the limits); 0 • where R- | and R2, which are similar or different, represent a hydrocarbon radical, advantageously chosen from aryls and alkyls optionally substituted in particular by halogen atom, in particular fluorine.

14. Process for the preparation of a masked (poly) isocyanate in hydro-organic emulsion, characterized in that step a) is carried out in an aqueous medium and in that before, during or after step b), add a water-dispersible solvent to the reaction medium.

15. The method of claim 14, characterized in that 0 is also added to the reaction medium a surfactant.

16. Method according to any one of the preceding claims, characterized in that the polyisocyanate is an alkylene diisocyanate or a product of homo- or hetero-condensation of alkylene diisocyanates, comprising in particular products of the type "BIURET", "TRIMERES", or "PREPOLYMERS" with isocyanate functions, comprising in particular urea, urethane, allophanate, ester, amide functions, or mixtures of the aforementioned products.

17. The method of claim 16, characterized in that the w polyisocyanate is chosen from hexamethylene diisocyanate and its homo- or poly-condensation products of the biuret, trimer and prepolymer type.

18. A partially or totally masked (poly) isocyanate composition obtained by the process according to one of claims 1 to 17.

15

19. Composition for coating, characterized in that it comprises, for successive or simultaneous addition: a) a partially or completely masked (poly) isocyanate composition according to claim 18; B) a reactive hydrogen co-reactant.

20. Process for the preparation of polymers, characterized in that it comprises the following steps: a) placing a partially or totally masked (poly) isocyanate according to claim 18 in the presence of a co-reagent which contains derivatives having hydrogen reagents; and b) heating the reaction medium thus formed to a temperature allowing crosslinking of the (poly) isocyanate with the co-reagent.

30