AU736228B2 - Isocyanates masked using hydroxyaromatic compounds - Google Patents

Description

-la- ISOCYANATES MASKED USING HYDROXYAROMATIC COMPOUNDS The subject of the present invention is a new family of masked isocyanates. The invention relates more particularly to isocyanates masked using hydroxyaromatic derivatives, and to their use in the techniques for coating by means of powders.

For reasons associated with environmental protection and safety at work, it is increasingly sought to eliminate the use of solvents in coating techniques, and especially in paint techniques.

In this context, coating techniques using powders are being increasingly developed.

Masked isocyanates are starting to be used, but their use is limited by the few compounds satisfying the chemical requirements of the powders.

A first difficulty lies in the difficulty in finding masked isocyanates or mixtures of

V.

isocyanates which remain in powder form under the usual storage conditions which may vary greatly from one place to another. This would involve these compounds having a 15 relatively high melting point and/or glass transition temperature (Tg).

The derivatives which form the subject of the present study do not always have a Ssharp melting point, and in this case an apparent melting point is therefore determined, either with a koffler block or using a method of the so-called capillary type (for example the so-called "Buchi" melting point). A glass transition temperature may be measured by differential thermal analysis (DTA) techniques.

These compounds should also have glass transition temperatures and melting points which are sufficiently low to allow them to react in the conditions under which powders are used.

22570-0O.DOC/S -2- In addition, the compounds derived from crosslinking reactions should not be harmful either to the health of humans or animals, or to the environment.

It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

According to a first aspect, the invention provides a masked polyisocyanate whose melting point, or apparent melting point in the case of compounds which do not have a sharp melting point measured with a Koffler machine or using a method of capillary type, is at least equal to 30 0 C, and whose glass transition temperature is at least equal to 10 0 C, and wherein said masked polyisocyanate is obtainable by the condensation, with a 0.5 to 2% stoichiometric excess, of an aromatic derivative hydroxylated on the ring and bearing an electronwithdrawing group chosen from carbonyl and nitrile functions, with the condition •i that there is a direct bond between the ring and the electron-withdrawing function, unless there is already an electron-withdrawing group on the ring, or that this ring 15 is a six-membered heterocyclic ring, and with the condition that when the carbonyl function is an ester function, its alkyl portion contains not more than 6 carbon atoms and is branched when its carbon number exceeds 2; with a polyisocyanate containing more than two isocyanate functions, derived from a precondensation or prepolymerization of at least one elemental diisocyanate, this elemental diisocyanate containing at least one aliphatic isocyanate function; and in that it comprises not more than 5% of residual free isocyanate function, measured O RA by assay with dibutylamine.

The invention also provides compositions including a masked isocyanate of the rr 0" first aspect, and processes for the synthesis of a masked isocyanate of the first aspect.

22570-OO.DOC/W 2a Among the members of this family, those for which it is possible to determine an apparent melting point should be chosen, this measurement being made at room temperature (20 0 This melting point should be at least equal to 30 0 C (one significant figure) and advantageously to It is desirable for the products not to lump together; thus, compounds which, when ground and stored at room temperature, have a similar particle size after a 24-hour interval, are chosen.

*ooo* o o*o o o *o o oooo *o *o ••go* 22570-00.DOC -3- The lumping nature is generally more or less associated with the glass transition temperature (Tg) hence, the preferred compounds are those which have a glass transition temperature (Tg) at least equal to 10 0 C (two significant figures), advantageously to 20 0

C

(one, preferably two, significant figures) and preferably to 30 0 C (two significant figures).

The choice of alkyls may be important, especially for the alkyl hydroxybensoates, more specifically for the para-hydroxybenzoate. Thus, the esters whose alkyl portion is linear and contains more than two carbons either have an insufficiently high melting point or are syrupy and crystallize only after a long period ranging from one week to several months, which makes them difficult to use, and they are thus not preferred.

Hence, the n-propyl, n-butyl and more generally n-alkyl esters are difficult to use. In addition, long chains should also be avoided for similar reasons, especially those in which the number of carbons is greater than 6.

Ethyl is an intermediate case and gives results which are acceptable (but only when 15 its content of starting material is low, below 2 and preferably below 1 in total (mass)) but not excellent. Isopropyl and especially methyl are preferred.

The nitrile and preferably the carbonyl functions may be attached to the ring either by a single bond or via a chain member which may be a chalcogen, nitrogen, or phosphorus bearing a hydrogen or a substituent, or optionally substituted methylene.

Given that the electron-withdrawing effect decreases or disappears on insertion of a chain member between the electron-withdrawing group and the ring, direct bonding between the ring and the electron-withdrawing group is preferred, provided that there is not already an electron-withdrawing group on the ring, or that this group is not naturally electron-poor (six-membered heterocycle for example).

22570-OO.DOC/S -4- Chalcogen chain members or chain members only bearing hydrogen or, and to a lesser extent, methyl are preferred; chain members based on element(s) from the second row (the row containing oxygen) of the Periodic Table of the Elements are preferred.

According to the present invention, the masked isocyanate, pure or as a mixture, is derived from a polyisocynate, that is to say one possessing at least two isocynate functions, advantageously more than two (possibility) of fractional values since these are generally mixtures of more or less condensed oligomers), which is itself usually derived from a precondensation or from a prepolymerization of unitary diisocyanate [or elemental diisocyanate, i.e. the isocyanates functions it bears were not subjected to condensation(s) with another isocyanate function (case of the biuret) or polymerisation(s) (case of the dimere or trimers, especially the one presenting the :isocyanuric cycle)]. As elemental isocyanates, one may cite those that are made of hydrocarbon skeletons wearing at least two isocyanate functions. Said skeleton is often an arylene radical, an alkylene radical (including aralkylene) such as the poly- 15 methylenes (generally and preferably hexamethylene), or the necessary to form the IPDI, one may mention that the skeletons may be alkyl for some extremities et arylic for the others. La atomic weight of these elemental isocyanates is advantageously at most 300 (one significant figure), preferably at most 200 (one significant figure).

i In general, the average molecular weights of these prepolymers or of these precondensates is not more than 2000 (one significant figure), more commonly than 1000 (one significant figure, preferably two).

Thus, among the polyisocyanates used for the invention, there may be mentioned those of the biuret type and those for which the di- or trimerization reaction has led to four-, five- or six-membered rings. Among the six-membered rings, there may be 22570-00.DOC/S mentioned the isocyanuric rings derived from a homo- or hetero- trimerization of various diisocyanates alone, with other isocyanate(s) [mono-, di- or polyisocyanate(s)] or with carbon dioxide, and in this case a nitrogen of the isocyanuric ring is replcaed by an oxygen.

The preferred polyisocyanates are those which have at least one aliphatic isocyanate function, in other words at least one isocyanate function masked according to the invention is attached to the skeleton via an sp 3 -type carbon advantageously bearing a hydrogen atom, preferably two.

The aromatic derivative hydroxylated on the ring, which serves to mask the isocyanate function, is advantageously chosen from those of formula Ar(r)n(Y-Z)m(OH)p

(I)

where Ar is an aromatic residue on which are grafted n substituent R, m polar functions S"Z chosen from nitriles and carbonyl groups, and p hydroxyl functions.

The values ofn, m and p are positive integers or zeros and are such that the sum n 15 m p is not more than the number of substitutable ring members, p is advantageously not more than 2, and is preferably equal to 1.

m is advantageously not more than two, and is preferably equal to 1.

n is advantageously not more than 3, preferably chosen from zero, one and two, and more preferably equal to zero.

R represents substituents which are immaterial with regard to the masking reaction and generally corresponds to hydrocarbon chains, usually alkyl chains in the etymological sense of the term, namely an alcohol whose hydroxyl function has been removed.

Two vicinal substituents R may be attached together to form a ring, which may, for example, be aromatic.

22570-OO.DOC/S Z is advantageously chosen from groups having a carbonyl function. Among these functions, there should be mentioned alkoxycarbonyl functions (or in other words ester functions), the amide function, the ketone function with the preferred condition that there is no acidic hydrogen (in other words, the function advantageously does not bear hydrogen or, if it does, the corresponding pKa is a least equal to about 20 (one significant figure, preferably two) and is more preferably at least equal to about 25) in a position ac to the carbonyl function (ester, ketone or amide), thus, the preferred amides (including lactam or even urea) are advantageously substituted, preferably sufficiently for there to be no hydrogen on the nitrogen of the amide function or so that there is no reactive hydrogen, where Y is chosen from divalent groups, advantageously

NR'-,

-CR'R

n with R' and R n chosen from hydrocarbon radicals which are advantageously alkyl, of 1 to 6 carbon atoms, advantageously of 1 to 4, preferably methyl, more preferably hydrogen, and Y preferably represents a single bond.

It is preferable for the polar function or functions Z (in general chosen from the nitrile function and/or the carbonyl functions) not to be vicinal to the group Z as, for example, in salicylic acid.

The aromatic residue Ar consists of one or more rings, which are advantageously condensed hetero- or homocyclic rings. It is preferably for Ar not to contain more than two rings, and preferably not more than one ring.

The aromatic residue Ar may consist of one or more hetero- or homocyclic rings, usually homocyclic rings on account of their ease of access. However, the advantage of 22570-O.DOC/S -7- 6-membered heterocycles, which have a very much lower release temperature than that of the corresponding homocycles, should be emphasized.

It is desirable for the total number of carbons in the aromatic derivative hydroxylated on the ring to be not more than 20, preferably than 10 (one significant figure).

This ring advantageously contains 6 members, the ring members consisting of carbon or nitrogen with the necessary number of substituents for the valency of these atoms.

Among the acids whose derivatives give the most satisfactory results, acids grafted onto a benzene ring should be mentioned. Thus, meta-hydroxy- and parahydroxybenzoic acids, and especially the esters thereof, give good results.

As has already been mentioned, according to the present invention, it is preferable for the melting point of the compound or of the compound mixture obtained to have an apparent melting point at least equal to 30 0 C, preferably 50 0

C.

15 It is also preferable for the glass transition temperature to be at least equal to 20 0

C,

advantageously to 40 0

C.

It is preferable to choose compounds according to the present invention such that they react completely with a primary alcohol at 250 0 C in less than half an hour. The •°reaction is considered to be complete if it is achieved to 90 or more. The isocyanates for which the invention is most advantageous are those in which the nitrogen atom is attached to an sp3-hybridized carbon and more particularly to aliphatic isocyanates, and especially to polymethylene diisocyanates and the various condensation derivatives thereof (biuret, etc.) and di- and trimerization derivates thereof.

22570-OO.DOC/S -8- According to the present invention, it is preferable and sometimes necessary for the percentage of residual free isocyanate function to be not more than advantageously than 3 preferably than 1 The highest melting points or glass transition temperatures are obtained with percentages not exceeding 0.5 The contents of aromatic derivative hydroxylated on the ring are also advantageously low, that is to say not more than 5 advantageously than 3 preferably than 1 As has been mentioned at the start of the present description, the present invention is also directed towards powder compositions which contain a masked polyisocyanate or a mixture of masked polyisocyanate, according to the present invention.

In the present description, the particle size characteristics often make reference to notations of the type d, where n is a number from 1 to 99; this notation is well known in 4* many technical fields, but slightly rarer in chemistry, and it may thus be useful to recall 4 the meaning thereof. This notation represents the particle size such that n (in weight, 4 or more exactly in mass, since weight is not an amount of material but a force) of the 15 particles is less than or equal to the said size.

In the powder compositions according to this implementation, the masked isocyanates according to the present invention advantageously constitute a population (which is advantageously distinct from that of the co-reactants) of particles whose d 90 is not more than 200 microns, advantageously than 100 microns, preferably than microns; this particle population has a d 1 o at least equal to 1 micron, advantageously to microns, preferably to 10 microns.

The powder compositions advantageously contain at least one polyol (at least diol) or, in certain cases, polyamines. It is also possible to have polyfunctional compounds having at least two functions chosen from amine functions or -ols (phanols or preferably 22570-OO.DOC/S -9alcohols) and the above compounds may additionally have other functions (for example an acid function such as a carboxylic or sulphonic function) on condition that these functions do not prevent the condensation or the crosslinking.

These polyols or polyamines themselves also form powders and satisfy the same melting point and glass transition temperature constraints as those mentioned above.

It is preferable for the melting point of the compositions according to the present invention to have a melting point at least equal to 50C, and it is even desirable for the softening temperature to be such that there is not sintering of the powder at a temperature of at least It is also preferable for the glass transition temperature thereof to be at least equal to Advantageously, the powder compositions also contain at least one catalyst, o generally and preferably catalysts based on tin or zinc.

Where appropriate, they contain additives which are common in the art, such as fillers, pigments (TiO 2 etc.) and additives for enhancing the physical properties (surface tansion, resistance to ageing and light, ease of use, etc.).

se*. According to the present invention, one synthesis process consists in placing the -free, or partially free, isocyanate in contact -with the hydroxyaromatic compound, that is "to say the compound of phenolic type, in a solvent.

When the compounds according to the invention and the precursors thereof are stable under the conditions below, the synthesis may be performed without solvent but in the molten state. The final product is than cooled, for example by making flakes, which may be obtained by abrupt cooling effected by pouring the reaction mixture onto a cold wall. The flakes obtained may be ground. In order to obtain good (that is to 22570-OO.DOC/S say low) percentages of residual free isocyanate function, it is important to introduce the aromatic derivative hydroxylated on the ring in an amount very close to stoichiometry.

It is preferable to be in a slight stoichiometric excess (of 0.5 to 2 preferably not more than 1 It is also preferable to add a catalyst for the condensation of the isocyanates to -ol functions; these condensation catalysts are often based on tin or tertiary amine.

The temperature at the end of the condensation is advantageously not more than 100 0 C (one significant figure, preferably two), preferably than 80°C and advantageously at least equal to 50C, preferably to 60 0 C. Indeed, if overheated, there is a risk of the percentage of free isocyanate being too high.

When a solvent is present, it is preferably chosen so as to be sufficiently polar to 4* dissolve at least 50, preferably at least 100 and more preferably at least 200 grams per S litre of initial isocyanate.

Once the reaction is complete, the final product should be precipitated, according 15 to a standard crystallization technique and more preferably by addition of a precipitating compound which is sufficiently nonpolar to bring about the precipitation without there necessarily being crystallization.

0 The precipitating compound is, obviously, a compound of volatile type and usually compounds of light hydrocarbon mixture type of the petroleum ether type, or of the hexane or heptane type. It is also possible to use, alone or as a mixture, ethers of light alcohol (that is to say alcohols containing not more than six carbon atoms, preferably 4).

Compounds of the alkane or alkene type in which the number of carbons is less than 20 and greater than 4 are generally used.

The non-limiting examples which follow illustrate the invention.

22570-OO.DOCIS 11 Example 1 Synthesis of methyl p-hydroxvbenzoate blocked with tolonate HDT: The following are loaded into a 500 ml reactor: hexamethylene diisocyanate trimer sold under the trade name Tolonate HDT®- 54.2 g (NCO number 22.1 Solvesso 100® 25 g.

The following is then added in several portions, with stirring and at room temperature: methyl p-hydroxybenzoate 47.6 g (0.31 mol).

10 The reaction mass is heated to 60 0 C and maintained at this temperature until the o NCO functions have disappeared.

After cooling, the desired product (blocked polyisocyanate) precipitates. It is reduced to a powder and washed using n-hexane: n-hexane 41.2 g.

15 The reaction mass is filtered and the solid obtained is washed with several fractions of hexane and then ground and again dried.

mass obtained 95.7 g melting point 85 0

C.

3 peaks are observed on NMR: 7.8 ppm (hydrogen borne by the nitrogen of the carbamate) 7.9 ppm (aromatic hydrogen ortho to the carbamic ester function) 7.10 ppm (aromatic hydrogen ortho to the carbonyl function) Example 2 Glazed formulation of the above product: 22570-OO.DOCIS -12- The intermediate product obtained is formulated in the following way: g Desmophen 690 14.0 g OH 2 i.e. an NCO/OH ration 1.

The mixture of the two powders is ground until a perfectly homogenous mixture with a particle size of less than 50 ,n is obtained.

Some of this powder is applied as a layer 300 uan thick onto a steel plate and is heat-treated at various temperatures for 30 or 60 minutes.

CURING 30 minutes 60 minutes Test Solvent Hardness Test Solvent Hardness while hot while hot (1) 130 0 C D M D M 160 0 C D M D M 190°C D M I VG 200 0 C I VG I VG The film obtained is qualified by its hardness and its solvent-resistance: VG very good: M mediocre Deposition of a drop of methyl ethyl ketone and observation of the deterioration of the film, D the film is degraded by the action of solvent I the film is intact after the action of solvent Procedure: Example 3 The following products are introduced into a reactor: 22570-00.DOC/S 13 Tolonate HDT®: 100 g (0.529 mol NCO) The following is added thereto: Methyl p-hydroxybenzoate: 81.3 g (0.529 mol).

The mixture is heated and stirred until melting of the blocking agent is obtained, at about 85°C; at about 100 0 C the medium is totally clear and colourless.

It is heated to 120 0 C and maintained at this temperature for 1 hour.

After cooling, the product is in the form of a slightly sticky hard gum Tg 8 0

C

By assaying with dibutylamine, the content of free NCO functions is determined to 10 be about S Example 4 The following products are introduced into a reactor: Tolonate HDT": 100 g (0.529 mol NCO) The following is added thereto: 15 Methyl p-hydroxybenzoate: 81.3 g (0.529 mol) Triethylamine (TEA): 0.2 g.

The mixture is heated and stirred until melting of the blocking agent is obtained, at about 85 0 C; at about 100°C the medium is totally clear and colourless.

After maintaining at 100 0 C, the content of free NCO functions is measured by assaying using dibutylamine.

1 h at 100°C: free NCO 6.2 2h at 100C: NCO 5.6 h at 100°C NCO 5.6 22S70-OO.DOC/S -14- Example The following products are introduced into a reactor: Tolonate HDT®: 100 g (0.529 mol NCO) The following is added thereto: Methyl p-hydroxybenzoate: 81.3 g (0.529 mol) Triethylamine (TEA): 0.2 g.

The mixture is heated and stirred until melting of the blocking agent is obtained, at about 85 0 C; at about 100 0 C the medium is totally clear and colourless.

The reaction mass is then cooled gradually to 60 0 C and then maintained at this 10 temperature.

The content of free NCO functions is measured by assaying with dibutylamine.

30 min at 60°C: free NCO 3.1 1 h at 60°C: NCO 2.6 4 h at 60C: NCO 2.6% 15 Example 6 The following products are introduced into a reactor: Tolonate HDT®: 100 g (0.529 mol NCO) The following is added thereto: Methyl p-hydroxybenzoate: 81.3 g (0.529 mol) Triethylamine (TEA): 0.5 g.

The mixture is heated and stirred until melting of the blocking agent is obtained, at about 85 0 C; at about 100 0 C the medium is totally clear and colourless.

The reaction mass is then cooled gradually to 60 0 C and then maintained at this temperature.

22570-.DOC/S The content of free NCO functions is measured by assaying with dibutylamine.

3 h at 60 0 C: free NCO 1.2 A further 0.5 g of TEA is added to the reaction medium and, after again maintaining the temperature, the free NCO functions are then assayed: 3 h at 60C: NCO 0.2 After cooling, the product is in the form of a hard gum which may ground.

Tg 24 0

C

Example 7 The procedure is identical to that of Example 4 except for the use, in place of the 10 methyl p-hydroxybenzoate, of the following product; ethyl p-hydroxybenzoate: 88.9 g (0.529 mol).

After cooling, the product is in the form of a solid gum.

Tg- 18.8°C Example 8 15 The procedure is identical to that of Example 4 except for the use, in place of the methyl p-hydroxybenzoate, of the following product: butyl p-hydroxybenzoate: 77.8 g (0.529 mol).

After cooling, the product is in the form of a vitreous liquid.

Tg 5.5 0

C

Example 9 The procedure is identical to that of Example 4 except for the use, in place of the methyl p-hydroxybenzoate, of the following product: isopropyl p-hydroxybenzoate: 73.1 g (0.529 mol).

22570-00.DOC/S -16- After cooling, the product is in the form of a solid gum.

Tg 23C Example Glazed formulation of the above product: The intermediate product obtained methyl p-hydroxybenzoate blocked with Tolonate HDT is formulated in the following way: I= 38.3 g Johnson 587 61.7 g OH 2.8 i.e. an NCO/OH ratio 1.1.

The mixture of the two powders is ground until a perfectly homogeneous mixture with a particle size of less than 50 anm is obtained.

Some of this powder is applied as a layer 200 /im thick onto a steel plate and is heat-treated at various temperatures for 30 minutes.

CURING 30 minutes Test Solvent Hardness while hot (1) 130 0 C D M 140 0 C D M 150 0 C D M 160°c I VG The film obtained is qualified by its hardness and its solvent-resistance: VG very good: M mediocre 22570-OO.DOC/S -17- Deposition of a drop of methyl ethyl ketone and observation of the deterioration of the film, D the film is degraded by the action of solvent I the film is intact after the action of solvent o *o ooooo *oo o *ooo/ oo 22570-OO.DOC/S

Claims (2)

18- THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: 1. Masked polyisocyanate whose melting point, or apparent melting point in the case of compounds which do not have a sharp melting point measured with a Koffler machine or using a method of capillary type, is at least equal to 30 0 C, and whose glass transition temperature is at least equal to 10 0 C, and wherein said masked polyisocyanate is obtainable by the condensation, with a 0.5 to 2% stoichiometric excess, of an aromatic derivative hydroxylated on the ring and bearing an electron- withdrawing group chosen from carbonyl and nitrile functions, with the condition that there is a direct bond between the ring and the electron-withdrawing function, S 10 unless there is already an electron-withdrawing group on the ring, or that this ring is a six-membered heterocyclic ring, and with the condition that when the carbonyl function is an ester function, its alkyl portion contains not more than 6 carbon atoms and is branched when its carbon number exceeds 2; with a polyisocyanate containing more than two isocyanate functions, derived from 15 a precondensation or prepolymerization of at least one elemental diisocyanate, this *i elemental diisocyanate containing at least one aliphatic isocyanate function; and in that it comprises not more than 5% of residual free isocyanate function, measured by assay with dibutylamine. 2. Masked polyisocyanate according to Claim 1, comprising at least one masked isocyanate function according to the invention, linked to the skeleton via a carbon of sp 3 type bearing at least one hydrogen atom.

22570-00.DOC/W -19- 3. Masked polyisocyanate according to Claim 2 wherein the carbon of sp 3 type bears two hydrogen atoms. 4. Masked polyisocyanate according to any one of Claims 1 to 3, wherein said elemental diisocyanate is a polymethylene diisocyanate. 5. Masked polyisocyanate according to any one of Claims 1 to 4, wherein said polyisocyanate is a condensation and/or di- and trimerization derivative of one or more aliphatic elemental diisocyanates. 6. Masked polyisocyanate according to any one of Claims 1 to 5, wherein said polyisocyanate is a compound of biuret type or a trimer of one or more aliphatic .10 diisocyanates. 7. Masked polyisocyanate according to any one of Claims 1 to 6, wherein said polyisocyanate is a trimerization derivative of a polymethylene diisocyanate. 8. Masked polyisocyanate according to any one of Claims 1 to 7, wherein said aliphatic elemental diisocyanate is hexamethylene diisocyanate. -o 15 9. Masked polyisocyanate according to any one of Claims 1 to 8, wherein said hydroxylated aromatic derivative corresponds to the formula (I) SAr(R)n(Y-Z)m(OH) (I) where Ar is an aromatic residue; where R represents a hydrocarbon substituent, generally an alkyl substituent; where Z is chosen from nitrile functions and carbonyl groups; where Y is chosen from divalent groups, advantageously a single bond, NR'-, CR'R"- with R' and R" chosen from hydrogen or hydrocarbon radicals; 22570-00.DOCW and where n m p is not more than the number of substitutable ring members of Ar. Masked polyisocyanate according to Claim 9, wherein R' and R" are alkyl radicals of 1 to 6 carbon atoms. 11. Masked polyisocyanate according to Claim 9 or 10, wherein R' and R" are alkyl radicals of 1 to 4 carbon atoms. 12. Masked polyisocyanate according to Claim 11, wherein R' and R" are methyl. 13. Masked polyisocyanate according to Claim 9, wherein R' and R" are hydrogen. 14. Masked polyisocyanate according to any one of Claims 9 to 13, wherein p is equal to 1 and that m is not more than 2. S" "10 15. Masked polyisocyanate according to any one of Claims 9 to 14, wherein Z is chosen from the groups: alkoxycarbonyl; amide; alkylcarbonyl; S* °15 on condition that Z does not bear an acidic hydrogen. 16. Masked polyisocyanate according to any one of Claims 9 to 15, wherein the hydroxyl function is not vicinal to a or to the function Z. 17. Masked polyisocyanate according to any one of Claims 9 to 16, wherein Ar is chosen from carbon- or nitrogen-based six-membered aromatic rings. 18. Masked polyisocyanate according to any one of Claims 1 to 17, wherein the hydroxylated aromatic derivative is an hydroxybenzoic acid. 22570-OO.DOC/W -21 19. Masked polyisocyanate according to any one of Claims 1 to 18, wherein said hydroxylated aromatic derivative is a derivative of one of the para- or meta- hydroxybenzoic acids. Masked polyisocyanate according to any one of Claims 1 to 18, wherein said hydroxylated aromatic derivative is para- or meta-hydroxybenzoic acid. 21. Masked polyisocyanate according to any one of Claims 1 to 19, wherein the aromatic hydroxylated derivative is hydroxy benzonitrile. 22. Masked polyisocyanate according to any one of Claims 17 to 21, wherein the said aromatic hydroxylated derivative is para-hydroxybenzonitrile. i 10 23. Masked polyisocyanate according to any one of Claims 1 to 22, wherein its apparent melting point is at least equal to 50 0 C. 24. Powder composition, especially for coating, including at least one of the masked polyisocyanates according to one of Claims 1 to 23, the said masked polyisocyanate being in the form of a powder. 15 25. Composition according to Claim 24, additionally including a catalyst based on zinc or tin. 26. Composition according to Claim 24 or Claim 25, additionally containing a polyol powder. 27. Composition according to any one of Claims 17 to 19, additionally including a polyamide in powder form. 28. Process for the synthesis of a masked isocyanate according to any one of Claims 1 to 23, including the placing in contact of the said hydroxylated aromatic derivative with R A. the said isocyanate at a temperature of not more than 100 0 C. 22570-00.DOCdW -22- 29. Process according to Claim 28, additionally including the step of precipitation of the masked isocyanate by means of the addition of an apolar solvent of alkane or alkene type. Process according to Claim 29, wherein the apolar solvent of the precipitation step is a C 4 to C 2 0 alkane. 31. Process according to any one of Claims 29 to 30, wherein said solvent is chosen such that the isocyanate is soluble to an extent of at least 50 grams per litre. 32. Process according to any one of claims 29 to 31 wherein said solvent is chosen such that the isocyanate is soluble to an extent of at least 100 grams per litre. S" 10 33. Process according to any one of claims 29 to 32 wherein said solvent is chosen such that the isocyanate is soluble to an extent of at least 200 grams per litre. 34. A masked isocyanate, substantially as herein described with reference to any one of the accompanying Examples. 35. A process for the synthesis of a masked isocyanate, substantially as herein o 15 described with reference to any one of the accompanying Examples. DATED this 1st Day of May 2001 RHONE POULENC CHIMIE o 22570-00.DOC/CmW