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Description  |
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BACKGROUND OF THE INVENTION
The invention pertains to a non-aqueous coating composition based on an oxidatively drying alkyd resin and a photo-initiator.
Such a coating composition has been proposed before in EP-A-234 641. The composition described in the document comprises an oxidatively drying alkyd resin of comparatively low-molecular weight and an allyl ether group covalently bonded thereto,
a siccative such as a cobalt salt and/or zirconium salt and, optionally, a photo-initiator.
A drawback to the coating compositions described in the document is that curing at temperatures of 10.degree. C. or lower, in particular of thick coats, is not satisfactory. A further drawback to the known coating compositions is that on curing
acrolein is released.
SUMMARY OF THE INVENTION
The invention now provides a coating composition which can be cured without any problems also at low temperatures even after it has been applied as a somewhat thicker coat.
The invention incorporates an acid or latent acid and one or more compounds belonging to the group of vinyl ethers, acetals, and alkoxysilanes which are reactive in the presence of an acid into a coating composition of the known type mentioned in
the opening paragraph.
It should be noted that EP-A-234 641 mentions in passing the possibility to modify the employed alkyd resins with compounds preferably having at least two reactive groups, such as polyisocyanates or polyalkoxysilanes. However, the reactivity of
these groups is intended for their reaction with the functional groups present in the alkyd resin during the preparation of the resin.
DETAILED DESCRIPTION OF THE INVENTION
At least part of the alkyd resin composition optionally comprising several alkyd resins in the non-aqueous coating compositions according to the invention is oxidatively drying as a result of incorporating a large number of unsaturated, aliphatic
compounds, at least a portion of which is poly-unsaturated. The unsaturated aliphatic compounds preferably are unsaturated aliphatic monocarboxylic acids, more particularly poly-unsaturated aliphatic monocarboxylic acids. Examples of mono-unsaturated
fatty acids are myristoleic acid, palmitoleic acid, oleic acid, gadoleic acid, erucic acid, and ricinoleic acid. Preferably use is made of fatty acids containing conjugated double bonds, such as dehydrated ricinus oil fatty acid and/or wood oil fatty
acid. Other monocarboxylic acids suitable for use include tetrahydrobenzoic acid and hydrogenated or non-hydrogenated abietic acid or its isomer. If so desired, the monocarboxylic acids in question may be used wholly or in part as triglyceride, e.g.,
as vegetable oil, in the preparation of the alkyd resin. If so desired, mixtures of two or more of such monocarboxylic acids or triglycerides may be employed, optionally in the presence of one or more saturated, (cyclo)aliphatic or aromatic
monocarboxylic acids, e.g., pivalic acid, 2-ethylhexanoic acid, lauric acid, palmitic acid, stearic acid, 4-tert.butyl-benzoic acid, cyclopentane carboxylic acid, naphthenic acid, cyclohexane carboxylic acid, 2,4-dimethyl benzoic acid, 2-methyl benzoic
acid, and benzoic acid.
If so desired, also polycarboxylic acids may be incorporated into the alkyd resin, such as phthalic acid, isophthalic acid, terephthalic acid, 5-tert.butyl isophthalic acid, trimellitic acid, pyromellitic acid, succinic acid, adipic acid,
2,2,4-trimethyl adipic acid, azelaic acid, sebacic acid, dimerised fatty acids, cyclopentane-1,2-dicarboxylic acid, cyclohexane-1,2-dicarboxylic acid, 4-methylcyclohexane-1,2-dicarboxylic acid, tetrahydrophthalic acid,
endomethylene-cyclohexane-1,2-dicarboxylic acid, butane-1,2,3,4-tetracarboxylic acid, endoisopropylidene-cyclohexane-1,2-dicarboxylic acid, cyclohexane-1,2,4,5-tetracarboxylic acid, and butane-1,2,3,4-tetracarboxylic acid. If so desired, the carboxylic
acids in question may be used as anhydrides or in the form of an ester, e.g., an ester of an alcohol having 1-4 carbon atoms.
In addition, the alkyd resin can be composed of di- or polyvalent hydroxyl compounds. Examples of suitable divalent hydroxyl compounds are ethylene glycol, 1,3-propane diol, 1,6-hexane diol, 1,12-dodecane diol, 3-methyl-1,5-pentane diol,
2,2,4-trimethyl-1,6-hexane diol, 2,2-dimethyl-1,3-propane diol, and 2-methyl-2-cyclohexyl-1,3-propane diol. Examples of suitable triols are glycerol, trimethylol ethane, and trimethylol propane. Suitable polyols having more than 3 hydroxyl groups are
pentaerythritol, sorbitol, and etherification products of the compounds in question, such as ditrimethylol propane and di-, tri-, and tetrapentaerythritol. Preferably, use is made of compounds having 3-12 carbon atoms, e.g., glycerol, pentaerythritol
and/or dipentaerythritol.
The alkyd resins can be obtained by direct esterification of the constituent components, with the option of a portion of these components having been converted already into ester diols or polyester diols. Alternatively, the unsaturated fatty
acids can be added in the form of a drying oil, such as linseed oil, tuna fish oil, dehydrated castor oil, coconut oil, and dehydrated coconut oil. Transesterification with the other added acids and diols will then give the final alkyd resin. This
transesterification generally takes place at a temperature in the range of 115 to 250.degree. C., optionally with solvents such as toluene and/or xylene also present. The reaction generally is carried out in the presence of a catalytic amount of a
transesterification catalyst. Examples of transesterification catalysts suitable for use include acids such as p-toluene sulphonic acid, a basic compound such as an amine, or compounds such as calcium oxide, zinc oxide, tetraisopropyl orthotitanate,
dibutyl tin oxide, and triphenyl benzyl phosphonium chloride. The number average molecular weight of the alkyd resin thus prepared preferably is at least 1000 and not more than 2800; favourable results can also be achieved at higher molecular weights,
but this will be at the expense of the solids content in the final coating composition.
The vinyl ether, acetal and/or alkoxysilane compounds used according to the invention preferably contain at least two vinyl ether, acetal and/or alkoxysilane groups and have a molecular weight of 150 or higher. Since most commercially available
vinyl ether, acetal and/or alkoxysilane compounds contain only one vinyl ether, acetal and/or alkoxysilane group and in addition at most one functional amino, epoxy, thiol, isocyanate, acrylic, hydride or hydroxyl group, first an adduct is formed of such
a compound to a compound having at least two groups capable of reacting with an amino, epoxy, thiol, isocyanate, acrylic, hydride or hydroxyl group. As examples may be mentioned compounds having an epoxy, isocyanate, hydroxyl and/or ester group or
compounds with an ethylenically or ethynylenically unsaturated group incorporated therein.
Examples of at least difunctional, solid or liquid epoxy compounds suitable for use in the adduct in question include the di- or polyglycidyl ethers of (cyclo)aliphatic or aromatic hydroxy compounds such as ethylene glycol, glycerol, cyclohexane
diol, mononuclear di- or polyvalent phenols, bisphenols such as Bisphenol-A and Bisphenol-F, and polynuclear phenols; glycidyl ethers of fatty acids having, say, 6-24 carbon atoms; glycidyl(meth)acrylate; an isocyanurate group-containing epoxy compounds,
an epoxydated olybutadiene; hydantoin-epoxy resins; epoxy resins obtained by epoxydation of aliphatic and/or cycloaliphatic alkenes, such as dipentene dioxide, dicyclopentadiene dioxide, and vinyl cyclohexene dioxide, and glycidyl groups-containing
resins such as polyesters or polyurethanes containing one or more glycidyl groups per molecule, or mixtures of the epoxy resins in question. The epoxy group in these compounds is suitable for reaction with the amino-functional and thiol-functional vinyl
ether, acetal, and alkoxysilane compounds.
Examples of at least difunctional isocyanate compounds suitable for use in the adduct in question include aliphatic, cycloaliphatic or aromatic di-, tri- or tetraisocyanates which may be ethylenically unsaturated or not, such as: 1,2-propylene
diisocyanate, trimethylene diisocyanate, tetramethylene diisocyanate, 2,3-butylene diisocyanate, hexamethylene diisocyanate, octamethylene diisocyanate, 2,2,4-trimethyl hexamethylene diisocyanate, 2,4,4-trimethyl hexamethylene diisocyanate,
dodecamethylene diisocyanate, .omega.,.omega.'-dipropyl ether diisocyainate, 1,3-cyclopentane diisocyanate, 1,2-cyclohexane diisocyanate, 1,4-cyclohexane diisocyanate, isophorone diisocyanate, 4-methyl-1,3-diisocyanato-cyclohexane, trans-vinylidene
diisocyanate, dicyclohexyl methane-4,4'-diisocyanate, 3,3'-dimethyldicyclohexyl-methane-4,4'-diisocyanate, a toluene diisocyanate, 1,3-bis(isocyanatomethyl)benzene, a xylene diisocyanate, 1,5-dimethyl-2,4-bis(isocyanatomethyl)benzene,
1,5-dimethyl-2,4-bis(2-isocyanatoethyl)-benzene, 1,3,5-tri-ethyl-2,4-bis(isocyanatomethyl)-benzene, 4,4'-diisocyanatodiphenyl, 3,3'-dichloro-4,4'-diisocyanatodiphenyl, 3,3'-diphenyl-4,4'-diisocyanatodiphenyl, 3,3'-dimethoxy-4,4'-diisocyanatodiphenyl,
4,4'-diisocyanatodiphenyl methane, 3,3'-dimethyl-4,4'-diisocyanatodiphenyl methane, a diisocyanatonaphthalene, the adduct of 2 molecules of a diisocyanate, e.g., hexamethylene diisocyanate or isophorone diisocyanate, to a diol such as ethylene glycol,
the adduct of 3 molecules of hexamethylene diisocyanate to 1 molecule of water (available under the trademark Desmodur N ex Bayer), the adduct of 1 molecule of trimethylol propane to 3 molecules of toluene diisocyanate (available under the trademark
Desmodur L ex Bayer), the adduct of 1 molecule of trimethylol propane to 3 molecules of isophorone diisocyanate, compounds such as 1,3,5-triisocyanatobenzene and 2,4,6-triisocyanatotoluene, and the adduct of 1 molecule of pentaerythritol to 4 molecules
of toluene diisocyanate. Preferably, an aliphatic or cycloaliphatic di- or triisocyanate having 8-36 carbon atoms is employed.
The isocyanate-functional compounds are suitable for reaction with the amino-functional, thiol-functional, and hydroxyl-functional vinyl ether, acetal, and alkoxysilane compounds.
As suitable di-, tri-, or polyvalent hydroxyl compounds may be mentioned ethylene glycol, propylene glycol, diethylene glycol, tetramethylene diol, neopentyl glycol, hexamethylene diol, cyclohexane diol, bis-(4-hydroxycyclohexyl)methane,
glycerol, trimethylol ethane, trimethylol propane, tris(2-hydroxyethyl)isocyanurate, and pentaerythritol. Polyols and other suitable hydroxy-functional compounds such as polyester diols and polyols and polyether diols and polyols have been described,
int. al., in H. Wagner and H. F. Sarx, Lackkunstharze, 5th edition, 1971. (Carl Hanser Verlag, Munchen).
The polyols are suitable for reaction with isocyanate-functional vinyl ether, acetal, and alkoxysilane compounds.
Suitable ester compounds are esters of polycarboxylic acids and low-boiling alcohols. Examples are methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, and tert-butyl esters of di-, tri- or tetracarboxylic acids, such as malonic acid, adipic
acid, direric fatty acids, maleic acid, fumaric acid, cyclohexane-1,2-dicarboxylic acid, phthalic acid, isophthalic acid, cyclohexane-1,3-dicarboxylic acid, cyclohexane-1,4-dicarboxylic acid, thiophene-1,5-dicarboxylic acid, trimellitic acid, ethylene
tetracarboxylic acid, acetylene dicarboxylic acid, and propane-1,1,2,3-tetracarboxylic acid. These compounds can enter into a reaction with hydroxy-functional and amino-functional vinyl ether, acetal or alkoxysilane compounds.
The compounds with an ethylenically or ethynylenically unsaturated group can be distinguished into compounds having electron-rich groups and compounds having electron-poor groups. The electron-rich groups can be divided up into two categories:
those capable of reacting with hydroxyl-, amino-, and silylhydride-functional vinyl ether, acetal, or alkoxysilane compounds, and those capable of reacting only with silylhydride-functional vinyl ether, acetal or alkoxysilane compounds. Examples of
electron-rich alkene or alkyn compounds reactive with hydroxyl, amino or silylhydride groups are: divinyl ethers, vinyl esters of polycarboxylic acids, polyenamines, poly(-1-alkenesulphides), dialkyl ethers, dialkyl thiolethers, alkyl esters of
polycarboxylic acids.
Examples of electron-rich alkene or alkyne compounds also capable of reacting with silylhydride-functional compounds are: unsaturated fatty acids and their esters or amides, other unsaturated carboxylic acids, except for
.alpha.,.beta.-unsaturated carboxylic acids, and their esters or amides. Examples of compounds having electron-poor ethylenic or ethynylenic groups are .alpha.,.beta.-unsaturated esters, amides, ketones, and other Michael acceptors known from the
literature (such as described, e.g., in J. March, Advanced Organic Chemistry, Reaction, Mechanism and Structure, 4th Ed. (Wiley Interscience: 1992), p. 795 and the references cited therein).
Representative examples of compounds having at least 2 acryloyl or methacryloyl groups include the (meth)acrylic esters of di-, tri- or polyvalent polyols, including polyester polyols and polyether polyols; adducts of, on the one hand, a hydroxyl
group-containing (meth)acrylic ester of a polyol to, on the other, an at least difunctional isocyanate compound; and adducts of (meth)acrylic acid to an at least difunctional epoxy compound.
According to the invention, preference is given to a coating composition in which the vinyl ether, acetal and/or alkoxysilane compounds are covalently bonded to the alkyd resin by addition via reactive group such as an amino, hydroxyl, thiol,
hydride, epoxy and/or isocyanate group. To this end these compounds have to possess at least one group capable of forming an adduct with the reactive groups present in the alkyd resin. In one embodiment, the alkyd resin has substantially no unsaturated
groups in the backbone.
To incorporate vinyl ether groups into the alkyd resin use is made of a vinyloxyalkyl compound the alkyl group of which is substituted with a reactive group, such as a hydroxyl, amino, epoxy or isocyanate group, which is capable of forming an
adduct with one or more reactive groups present in the alkyd resin.
Examples of vinyl ether compounds capable of being covalently bonded to the alkyd resin by addition are ethylene glycol monovinyl ether, butane diol monovinyl ether, hexane diol monovinyl ether, triethylene glycol monovinyl ether, cyclohexane
dimethanol monovinyl ether, 2-ethylhexane diol monovinyl ether, polytetrahydrofuran monovinyl ether, tetraethylene glycol monovinyl ether, trimethylol propane divinyl ether, and aminopropyl vinyl ether.
Adducts can be formed, e.g. by reacting the vinyl ether compound containing a hydroxyl group or amino group with an excess of a diisocyanate, followed by the reaction of this free isocyanate groups-containing adduct with the free hydroxyl groups
of the alkyd resin. Preferably, a process is employed in which first the free hydroxyl groups of the alkyd resin are reacted with an excess of a polyisocyanate, after which the free isocyanate groups are reacted with an amino group- or hydroxyl
group-containing vinyl ether compound. Instead of a diisocyanate, a diester may be employed. Transesterification of the hydroxyl groups present in the alkyd resin with an excess of ester groups of the diester, followed by transesterification or
transamidation of the remaining ester groups with hydroxyl-functional vinyl ether compounds and amino-functional vinyl ether compounds, respectively, results in vinyl ether-functional alkyd resins. Instead of using the process discussed above in which
an adduct is formed by reacting isocyanate, groups or ester groups with hydroxyl groups or amino groups, it is possible to incorporate (meth)acrylate groups into the alkyd resin during its preparation by carrying out the alkyd resin preparation in the
presence of a hydroxy-functional (meth)acrylate ester, such as hydroxyethyl methacrylate (HEMA), and then converting the thus functionalised alkyd resin by means of a Michael reaction with a compound containing a vinyl ether group and a primary amino
group, followed by reaction with, e.g., an isocyanate compound in order to obtain a non-basic nitrogen atom.
For the preparation of acetal-functionalised alkyd resins generally use is made of a dialkyl acetal functionalised with an amino group. Examples of suitable acetal compounds are 4-aminobutyraldehyde dimethyl acetal and 4-aminobutyraldehyde
diethyl acetal. The alkyd resin is modified by adding the aminoacetal monomer to an alkyd resin functionalised with isocyanate groups, ester groups of a low-boiling alcohol, or (meth)acrylate groups. The thus obtained dialkyl acetal-modified alkyd
resin can be incorporated into a coating composition having a high solids content and a low viscosity. Alternatively, the preparation of acetal-functionalised alkyd resins can take the form of reacting a hydroxyacetal with the carboxyl groups of the
alkyd resin or by reacting a diisocyanate or diester compound with the hydroxyl groups of the alkyd resin.
For the incorporation of alkoxysilane groups into the alkyd resin use is made of a siloxane compound having one or more reactive groups which are subsequently reacted with one or more of the constituents making up the alkyd resin. In this
process favourable results have been achieved using an alkoxysilane satisfying the formula:
R.sub.1 has the meaning of an alkoxy or oxyalkylene alkoxy group or, if X stands for a hydrogen atom, of a halogen atom, R.sub.2 has the meaning of an aliphatic, cycloaliphatic or aromatic group, and X stands for a hydrogen atom or an alkyl group
substituted with an amino, isocyanate, mercapto or epoxy group, wherein a=1 through 3, b=1 through 3, c=0 through 2, and a+b+c=4.
R.sub.1 preferably is a lower alkoxy group having 1 to 4 carbon atoms in the alkoxy group and R.sub.2 preferably is a group having not more than 18 carbon atoms.
Examples of suitable siloxane compounds are 3-aminopropyl-triethoxysilane, polyglycol ether-modified aminosilane, 3-aminopropyl-trimethoxysilane, 3-aminopropyltris-methoxy-ethoxyethoxysilane, 3-aminopropyl-methyl-diethoxy silane,
N-2-aminoethyl-3-aminopropyl-trimethoxy-silane, N-2-aminothyl-3-aminopropyl-methyldimethoxy-silane, N-methyl-3-aminopropyl-trimethoxysilane, 3-ureidopropyl-triethoxysilane, 3,4,5-dihydroimidazol-1-yl-propyltriethoxysilane,
3-methacryloxypropyl-trimethoxysilane, 3-glycidyloxypropyl-trimethoxysilane, 3-mercaptopropyl-trimethoxysilane, and 3-mercaptopropyl-methyl-dimethoxysilane, triethoxy-silane, diethoxymethyl silane, dimethoxymethyl silane, trimethoxysilane,
trichlorosilane, triiodosilane, tribromosilane, dichloromethyl silane, and dibromomethyl silane.
The alkyd resin can be modified, e.g., by adding an amino group-modified alkoxysilane to an alkyd resin functionalised with a polyisocyanate or a polyester of a low-boiling alcohol. Hydride-functional alkoxysilanes can be bonded to the alkyd
directly, i.e., without modification with a coupling molecule such as a diisocyanate or diester, by adding a compound containing a silylhydride group to an ethylenically unsaturated group in the alkyd resin. This addition is catalysed by a transition
metal. In this process it is preferred to make use of a halogenated silylhydride and, on conclusion of the addition reaction, convert it into an alkoxysilane compound with a low-boiling alcohol. The addition reaction proceeds most favourably in the
absence of sterically hindering groups and is optimal when the ethylenically unsaturated groups are terminal groups, as is the case with esters of 10-undecenecarboxylic acid. The thus obtained alkoxysilane-modified alkyd resin can be incorporated into,
a coating composition having a high solids content and a low viscosity.
According to the invention, preference is given to a coating composition in which the ratio of the number of oxidatively drying groups present in the alkyd resin to the number of groups reactive in the presence of an acid is in the range of 1/10
to 15/1, with preference being given to a ratio in the range of 1/3 to 5/1. Instead of a single modified alkyd resin several alkyd resins may be employed, with one alkyd resin being highly modified and the others being less so or not at all. All that
is of importance is the ratio of the number of oxidatively drying groups to the number groups reactive in the presence of an acid.
The pKa value of the acid under the influence of which the acid-reactive compounds are activated is <5, preferably <3. Examples of suitable acids are sulphonic acid derivatives, such as methane sulphonic acid, p-toluene sulphonic acid,
dodecyl benzene sulphonic acid, phosphoric acid derivatives such as phosphoric acid dibutyl ester and/or suitable substituted carboxylic acid derivatives such as trichloroacetic acid and trifluoroacetic acid. Up to now, optimum results have been
obtained using phosphoric acid dibutyl ester.
The photo-initiators suitable for use according to the invention generally are compounds activated by low UV-intensities and daylight. Suitable photo-initiators are acyl phosphine oxides, thioxanthone compounds, and mixtures thereof. Both
monoacylphosphine oxide photo-inititors, such as are disclosed, for example, in EP-A-0 007 508 and EP-0 413 657, and bisacylphosphine oxide photo-initiators, such as are disclosed, for example, in EP-A-0 184 095, EP-A-0 413 657, GB 2,259,704, and GB
2,292,740, can be used as photo-initiators in the coating composition of the present invention. An example of a monoacylphosphine oxide photo-initiator is (2,4,6-trimethylbenzoyl)-diphenyl-phosphine oxide (Lucirin.RTM. TPO). Examples of
bisacylphosphine oxide photo-initiators include bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentyl-phosphine oxide, bis(2,6-dimethoxybenzoyl)-2,2,4-trimethylpentyl-phosphine oxide, and bis(2,4,6-trimethylbenzoyl)-phenyl-phosphine oxide. Examples of
thioxanthone compounds include 2-isopropyl-thioxanthone, 1-chloro-4-propoxythioxanthone, 2,4-diethoxy thioxanthone, and 2-chlorothioxanthone. Very favourable results have been obtained up to now using
bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentyl-phosphine oxide as photoinitiator.
In general, favourable results are obtained when the photo-initiator is present in an amount of 0,01 to 5 wt. %, calculated on the overall amount of ethylenically unsaturated alkyd resin present. In this case preference is given to an amount in
the range of 0,01 to 2 wt. %, more particularly, in the range of 0,05 to 2 wt. %. When a sensitiser is employed, preference is given to an amount making up 10 to 200 wt. % of the photo-initiator.
While the ethylenically unsaturated alkyd resin is cured by oxidative drying under the influence of a photo-initiator, the vinyl ethers, acetals and/or alkoxysilanes are cured under the influence of an acid and, in the case of acetals and/or
alkoxysilanes, also in the presence of a small amount of moisture from the air. Thus, in order to enhance the storage stability of the coating compositions according to the invention, advantageous use is made of a photo-initiator which releases an acid
under the influence of electromagnetic radiation. Such photo-initiators have been disclosed, int. al., by G. Li Bassi et al. in "Photoinitiators for the simultaneous generation of free radicals and acid hardening catalysts," in a paper distributed
during a symposium of Chemspec Europe 87 BACS.
As an example may be mentioned the compound (MDTA), 2-methyl-1-[4-(methylthiophenyl]-2-[4-methylphenylsulphonyl]propan-1-one, which is commercialised by Fratelli Lamberti Spa, Varese, Italy.
Other latent acids are disclosed, for example, in EP-A-0 139 609, EP-A-0 164 314, EP-A-0 199 672, EP-A-0 571 330, EP-A-0 780 729, and GB 2,306,958.
Alternatively, use may be made of latent acids which are unblocked with moisture from the air. As an example may be mentioned the silyl esters of sulphonic acids, such as trimethylsilyl p-toluene sulphonic acid.
Generally, a small amount of organic solvents is incorporated into the coating compositions according to the invention. Preference is given in that case to solvents which have a flashpoint of at least 55.degree. C. and a boiling point in the
range of 140.degree. C. to 260.degree. C. As an example may be mentioned aliphatic, cycloaliphatic or aromatic hydrocarbons having on average 9-16 carbon atoms per molecule, alcohol ethers, and alcohol ether acetates or mixt | | |
