Patent Application
20050261420
The invention relates to crosslinkable coatings based on polymers comprising hydroxyl groups and on crosslinking agents based on polyisocyanates and amine-formaldehyde resins.
Polyurethane coatings obtained by crosslinking between the hydroxyl functional groups of the polyol and the isocyanate functional groups of a polyisocyanate oligomer are widely used for their hardness, durability, chemical resistance and impact strength properties.
Coating compositions are also known in which the crosslinking agent is formed by amine-formaldehyde resins that react with the hydroxyl functional groups of the polyol in order to form a network. However, the mechanical properties of the coatings obtained are often unsatisfactory. In addition, these coatings have a tendency to be sensitive to the curing temperature, and this is not without application problems.
Moreover, FR 2 322 911 teaches compositions for curable varnishes comprising a blend of polymers containing hydroxyl groups, of amine-formaldehyde-type resins and of polyisocyanates, in which compositions the polyisocyanate may include compounds having a biuret group. However, the varnishes obtained have an unsatisfactory hardness.
The object of the invention is to provide coating compositions, especially paints or varnishes, that have a high hardness.
Another object of the invention is to provide coatings that have a high flexibility.
Another object of the invention is to provide coatings that have a high impact strength.
Another object is to provide coatings that have higher resistance to chemicals; more precisely, coatings that have a resistance with a rating of 3 or better (i.e. a rating of 3, 2, 1 or 0), for each of the attacks below:
Another object is to provide coatings and precursor binder compositions that have a resistance to butanone attack with a rating of 2 or preferably better (a resistance with a rating of better than 2 corresponding to a rating of less than 2).
Yet another object is to provide coating compositions that have a long pot life and can be processed in one-component systems called “1K” systems, i.e. in which all of the coating components, especially the crosslinkable compounds, are present in one and the same system.
The subject of the invention is a method for producing a coating having a high hardness, a high impact strength and a high resistance to chemical attack, characterized in that it comprises the application, to a substrate, of a blend, for simultaneous or successive addition, comprising:
The term “high hardness” is understood within the invention to mean a hardness of greater than 200 s (Persoz hardness), as measured one hour after the coating has been cured (minimum cure: 30 minutes at 140° C.).
The term “high impact strength” is understood to mean a strength of greater than 8 (Erichsen cupping).
Surprisingly, the inventors have furthermore discovered that coatings having the aforementioned properties could be obtained when, as crosslinking agent, a polyisocyanate composition is used in which at least some of the isocyanate groups are blocked by a blocking agent, the blocking agent, upon its release from the isocyanate group, reacting completely or partly with the amine-formaldehyde resin and thus participating in the formation of the network without causing defects in the coating film after the latter has been cured, and while limiting the amount of VOCs (volatile organic compounds).
It is advantageous for at least 10 mol %, advantageously 20 mol %, preferably at least 50 mol % and up to 100 mol % of the isocyanate functional groups to be blocked by a blocking agent or a mixture of blocking agents, these preferably being monofunctional.
Component (A) is advantageously a polyol having a hydroxyl content of between 1 and 5 g/l 00 g, advantageously between 3.5 and 4.5 g/100 g, expressed with respect to the solids content.
For this purpose, it is possible to use polyacrylates containing hydroxyl groups, polyesters or alkyds, or mixtures thereof. Most particularly preferred are polyacrylates containing hydroxyl groups, the molecular weight M, of which ranging from 3 000 to 50 000, advantageously from 5 000 to 30 000. It is also preferable for the molecular weight M, to range from 2 000 to 20 000, preferably from 3 000 to 10 000.
The molecular weight (Mw) is measured by gas chromatography (GC), polystyrene being used as reference. This method makes it possible at the same time to obtain Mw (weight-average molecular weight) and Mn (number-average molecular weight). The elution solvent is tetrahydrofuran (THF).
These polyols are as described on pages 40 to 49 of “Waterborne & Solvent based surface coating resins and their application”, Vol. III; John Wiley & Sons, (1998).
The polyol polymer is generally dissolved in an organic solvent. As solvent, mention may be made in particular of esters, aromatic hydrocarbons, ethers, ether esters or amides. It is also possible to use aqueous solutions, emulsions or dispersions of polyols or aqueous-organic formulations.
According to one advantageous embodiment of the present invention, the polyol may be a polyol with a high solids content (SC), the SC of which is between 60 and 100%.
Constituent A is advantageously present in the compositions according to the invention with a solids content of between 10 and 60%, advantageously 20 and 40%, with respect to the total weight of components (A), (B), (C) and (D).
Constituent B is a resin of the amine-formaldehyde type, preferably an at least partially etherified precondensate of formaldehyde and of melamine or of urea. Solvent-free liquid products may be used for the purpose of applying varnishes or paints with a high solids content. However, it is preferred to use the standard amine-formaldehyde resins that contain solvents or aqueous or aqueous-organic solutions, dispersions or emulsions. As amines, it is possible to use triazines, triazoles, diazines, guanidines or guanamines, for example N,N′-dimethylurea, acetylenediurea, dicyanamide, benzoguanamine or alkyl-substituted melamines.
At least some, preferably all, of all the hydroxyl functional groups of the amine-formaldehyde precondensates are etherified. These mixed ether functional groups therefore carry, on one side, the chain of the precondensate and, on the other side, a hydrocarbon chain. This hydrocarbon chain is advantageously connected to said ether functional group via a carbon of sp3 hybridization; advantageously, they have at most 10 carbon atoms, preferably at most 5 carbon atoms. As hydrocarbon chains, mention may especially be made of methyl, ethyl, propyls, butyls (especially n-butyl) or benzyl.
The amine-formaldehyde resin is prepared in a known manner by acid-catalyzed condensation, aqueous formaldehyde preferably being used. Details may be found, for example, in the work by Houben-Weyl, “Methoden der Organischen Chemie” [Methods of Organic Chemistry], (1963), Volume 14/2, pages 319 et seq.
It is preferable for said amine-formaldehyde precondensates to be produced with an excess (generally a slight excess) of formaldehyde, thereby making it possible in particular to maximize the hydroxyl functional end groups. In general, the residual formaldehyde after condensation lies within the range from 1 per thousand to 2 percent by mass of the precondensate.
Thus, one particularly preferred melamine-formaldehyde resin is a resin having a functionality of at least 6, a dynamic viscosity (DIN 53177, 23° C.) of 500 to 2 500 mPa·s, a compatibility with n-heptane of between 2.0 and 5.0 ml/g, a free formaldehyde content of less than 1% and a density of about 1 g/mL.
Mention may be most particularly made of the resins sold by Vianova Resins, now Solutia, under the name MAPRENAL®.
Component (B) is advantageously used with a solids content of between 1 and 40%, advantageously 3 and 20%, by weight with respect to the total weight of components (A), (B), (C) and (D).
Component (B) is generally dissolved in an organic solvent, especially an alcohol.
Component (C) is a polyisocyanate composition comprising at least 30%, advantageously at least 50% by weight, preferably at least 60% by weight, of polyisocyanates that include at least one biuret group.
Advantageously, the polyisocyanate composition is obtained from polycondensation, advantageously catalytic double condensation or triple condensation, of initial isocyanate monomers in the presence of water, an alcohol or an amine.
The polyisocyanate composition may contain polyisocyanates the number of isocyanate functional groups (NCO) per molecule of which is greater than or equal to 2, such as 4-isocyanatomethyl-1,8-diisocyanato-octamethylene or the isocyanatoethyl ester of lysine diisocyanate.
The polyisocyanate composition advantageously contains at least 15% by weight of a compound of formula (I):
in which R1, R2 and R3, which are identical or different, represent a hydrogen atom, a linear, branched or cyclic C1-C20 hydrocarbon chain that is optionally substituted, preferably substituted with one or two isocyanate functional groups and/or optionally with a group derived from an isocyanate functional group, namely a carbamate, allophanate, isocyanurate, uretidione, iminooxadiazinedione or oxadiazinetrione group, with the proviso that at most only one of R1, R2 and R3 represents a hydrogen atom.
The composition according to the invention advantageously comprises at least 15%, advantageously at least 30% and preferably at least 50% by weight of compounds of formula (I), in which R1, R2 and R3, which are identical or different, are linear, branched or cyclic C1-C20 hydrocarbon chains that advantageously include an isocyanate functional group. These compounds are also called “true biurets”.
The polyisocyanate composition of the invention also advantageously contains from 10 to 70%, preferably 15 to 60%, by weight of compounds that include more than one biuret group, namely compounds of general formula (I) in which at least one of R1, R2 and R3 comprises a biuret group of formula:
The polyisocyanate composition of the invention furthermore advantageously comprises from 0 to 80%, preferably from 0 to 50%, by weight of isocyanurate compounds that include one or more isocyanurate rings.
Advantageously, the polyisocyanate composition contains from 0 to 50%, preferably from 0 to 30%, by weight of monoisocyanurate compounds.
The polyisocyanate composition furthermore contains:
The content of isocyanate monomers depends on the volatility of the latter. It is preferable to work with a low monomer content (advantageously at most 1% by mass) when these monomers are volatile.
When it is desired not to work with low isocyanate monomer contents, monomers of very low volatility may therefore be chosen, particularly triisocyanate monomers.
The polyisocyanate compositions of the invention may be obtained by a method comprising the following steps:
Reference may also be made to the methods described in EP-A-0 259 233 or U.S. Pat. No. 5,103,045.
The tricondensation catalyst is advantageously a catalyst for the formation of a biuret functional group and, in particular, moderately strong acids having a pKa of at least 1, advantageously at least 2 (for example, phosphoric or phosphonic acids) and preferably at least 3, or else a carboxylic acid as described in FR 86/12524.
To obtain a blocked polyisocyanate composition, a blocking agent or mixture of blocking agents, these preferably being monofunctional, is added to the reaction mixture, before or during step a), after step b) or after step c), in an amount corresponding to the proportion of isocyanate functional groups that it is desired to block, and the reactants are left to react under the reaction conditions for reaction between the isocyanate functional group and the reactive hydrogen of the blocking agent according to the scheme:
Am—H+ISO—N═C=O→Am—CO—NH—ISO,
ISO being as defined above and Am representing the residue of the blocking agent after removal of the reactive hydrogen.
The most commonly used blocking agents are those cited by M. Wicks in his article “Blocked isocyanates”, Progress in Organic Coatings (1975), Vol. 3, p. 73, their deblocking temperatures advantageously being above 50° C., preferably of about 90° C.
The blocking agents may be divided into three groups:
Among those whose mobile hydrogen is carried by a chalcogen (preferably a light chalcogen, namely sulfur and oxygen), those in which the chalcogen is an oxygen are most particularly used. Among the latter, mention may in particular be made of:
Mention may also be made of the compounds disclosed in application EP-A-0 661 278.
Among those whose mobile hydrogen is carried by a nitrogen, mention may in particular be made of:
Mention may also be made of the compounds disclosed in application EP-A-0 661 278.
Among those whose mobile hydrogen is carried by a carbon, mention may essentially be made of compounds of malonic nature, that is to say an RCH< radical carrying two electron-withdrawing groups (such as carbonyl, nitrile, Rf or perfluoroalkyl).
In this regard, mention may especially be made of the following pairs of blocking agents: methyl amyl ketoxime/2-hydroxypyridine and dimethylpyrazole/2-hydroxypyridine.
The present invention is not limited to the nature of the isocyanate monomers employed. Thus, the isocyanate monomers may be aliphatic, including cycloaliphatic, diisocyanates or triisocyanates, such as:
The preferred isocyanates according to the invention are those in which at least one, advantageously two and preferably three of the above conditions are fulfilled:
In general, the preferred initial isocyanates (monomers) are those having at least one polymethylene chain sequence (comprising from 2 to 6 methylene chain links).
Isocyanates, particularly aliphatic diisocyanates, especially C1-C10 alkyl isocyanates, are preferred in which the alkyl chain is linear or lightly branched. The term “lightly branched” is understood to mean the absence of any tertiary and/or neopentyl carbon.
HDI, IPDI, NBDI, H12MDI and MPDI are particularly preferred.
Component (C) is generally diluted in a solvent, so that the solids content of the composition is between 60 and 75% by weight.
Component (D) may be chosen from one of more of the following compounds: cellulose esters, spreading agents, plasticizers, silicone oils, thixotropic agents, wetting agents and mar-resistance agents, pigments or fillers, for example titanium dioxide, carbon black, organic or mineral colored pigments, talc or barium sulfate, crosslinking catalysts, UV stabilizers, etc.
Examples of suitable catalysts are, on the one hand, sulfonic acids, especially methanesulfonic acids or trifluoromethanesulfonic acids, for example para-toluenesulfonic acid, and, on the other hand, certain tin compounds, such as dibutyltin dilaurate or latent forms of these compounds, such as salts of para-toluenesulfonic acid, methanesulfonic acid or perfluoroalkanesulfonic acids.
It is also possible to use latent forms of these catalysts, such as the amine salts of these strong acids.
The crosslinking catalysts are present in a proportion ranging from 0 to 3%, preferably from 0 to 1%, by weight.
Component (D) may be contained in the proportions of 0 to 65%, preferably 0 to 50%, by weight.
Advantageously, the ratio of isocyanate functional groups to hydroxyl functional groups is less than 3, advantageously less than 2 and preferably between 0.3 and 1.5.
The subject of the invention is also a coating composition, characterized in that it comprises:
Components (A), (B), (C) and (D) are as defined above. Component (E) furthermore comprises the solvents described above for the formulation of components (A) and (B), optionally one or more other solvents for the formulation of the final coating composition, n-butyl acetate, ethylene glycol acetate, methyl ethyl ketone or xylene.
The examples below illustrate the invention without however limiting it.
The following ingredients were mixed in order and homogenized:
*polyol:SC = 75%; dry OH = 3.64%;
**melamine-formaldehyde resin.
Various varnish formulations were prepared by adding to compound 1 or to compound 2 a polyisocyanate composition 1 (PIC 1) containing 100% isocyanate functional groups blocked by 3,5-dimethylpyrazole (3,5-DMP) having the following composition before blocking:
At the same time, varnish formulations (comparative example) were prepared by adding, to compounds 1 and 2 above, a polyisocyanate composition PIC 2 having an identical composition except that the biuret was replaced with isocyanurate in the HDI biuret, the HDI bis(biuret) and the heavy fractions.
The above varnish compositions were applied to glass or steel substrates using a 100 μm spiral filmograph in the case of the steel substrate and in the case of the glass plates.
After 30 minutes of desolvation, the compositions were cured at 140° C. and 150° C. for 30 minutes.
After 7 days at room temperature, the following properties were measured:
impact strength.
*Values greater than or equal to “9” were not to be considered as the piston of the cup passed through the plate.
Ratings 0 to 5:
It is clearly apparent that the results obtained with composition 1 are superior to the results obtained with composition 2.
For compounds without the amine-formaldehyde resin, the isocyanate-based compositions (PIC 2) give better results, whereas when a melamine-formaldehyde resin is used the compositions containing biuret give, in all cases, results that are at least as good and are often better.
| Number | Date | Country | Kind |
|---|---|---|---|
| 01/02912 | Mar 2001 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR02/00775 | 3/4/2002 | WO | 1/5/2004 |
