Patent Description:
Aqueous coating compositions comprising a binder polymer dissolved or dispersed in an aqueous phase are well-known in the art. When hard coatings that are scratch-resistance and chemical resistant are required, crosslinking of the binder polymer is desired.

Binder polymers crosslinked with polyisocyanate are known to result in coatings with good hardness and good scratch-resistance and chemical resistance. Polyisocyanates are, however, undesirable from a health point of view.

Carbodiimides are known as crosslinkers for carboxyl functional resins. The reaction mechanism is explained in <NPL>. In <CIT> is disclosed the use of multifunctional polycarbodiimides as crosslinking agents in a coating composition that comprises a carboxylic acid functional polyurethane dispersed in water.

Carbodiimide crosslinked waterborne polyurethane coatings, however, cannot match the properties of isocyanate crosslinked waterborne polyurethane coatings, in particular in terms of chemical resistance and hardness.

Organo-functional alkoxysilanes are known as crosslinkers for various binder polymers, such as polyacrylates, polyesters, polyurethanes, or polyethers. In aqueous coating compositions, the amount of organo-functional alkoxysilane that can be used is limited in view of stability issues when the alkoxysilane is combined with an aqueous resin component.

In <CIT> is disclosed a wear-resistance coating composition, in particular for traffic marking, comprising a polycarbodiimide, an aqueous emulsion polymer reactive with carbodiimide moieties, and an alkoxysilane which is either reactive with the aqueous emulsion polymer, reactive with the polycarbodiimide, or attached to the polycarbodiimide. The amount of alkoxysilane used in the coating compositions of <CIT> is relatively low (between <NUM> and <NUM> wt% based on solid emulsion polymer).

There is a desire to provide a crosslinkable aqueous coating composition without the above-mentioned problems.

It has now been found that a two-component aqueous coating composition comprising a resin component and a hardener component can be provided that results in a coating with good hardness, scratch resistance, and chemical resistance without using polyisocyanate as crosslinker. By using a hardener component comprising a carbodiimide with non-ionic emulsifying groups and an alkoxysilane, in a weight ratio in the range of from <NUM>:<NUM> to <NUM>:<NUM>, a relatively high amount of alkoxysilane crosslinker can be used without stability issues upon mixing the hardener component with the aqueous resin component.

Accordingly, the invention provides in a first aspect a two-component aqueous coating composition comprising:.

wherein the weight ratio of the carbodiimide to the alkoxysilane in hardener component B is in the range of from <NUM>:<NUM> to <NUM>:<NUM>.

It is an advantage of the coating composition of the invention that it results in a coating with improved chemical resistance compared to a coating composition with carbodiimide as the only crosslinker whilst the hardness is similar or even improved. Without wishing to be bound to any theory, it is believed that due to the presence of the carbodiimide with non-ionic emulsifying groups, alkoxysilane can be brought in close proximity of the dispersed resin, provided the carbodiimide and alkoxysilane are present in the ratio specified. Moreover, a relatively high amount of alkoxysilane can be brought in close proximity of the dispersed resin. This results in improved crosslinking and chemical resistance. The coating composition shows very good initial hardness.

A further advantage is that hardener component B has a relatively low viscosity, so that components A and B can be combined relatively easy, even without organic solvent. If desired, the two components can be mixed by simple hand-stirring.

The coating composition provides a crosslinked coating with good hardness, scratch resistance, and chemical resistance and can therefore suitably be used as protective or decorative coating on any type of substrate, in particular metal, plastic, wooden, or ceramic substrates.

The coating composition according to the invention is a two-component aqueous coating composition. It comprises an aqueous resin component A comprising a carboxyl-functional resin dissolved or dispersed in an aqueous phase and a substantially water-free hardener component B. Components A and B are prepared and stored separately and are not combined until shortly before application of the coating composition.

Hardener component B comprises a carbodiimide and an alkoxysilane in a weight ratio of the carbodiimide to the alkoxysilane in the range of from <NUM>:<NUM> to <NUM>:<NUM>.

The carbodiimide comprises in the range of from <NUM> to <NUM> carbodiimide moieties, preferably in the range of from <NUM> to <NUM> carbodiimide moieties, even more preferably of from <NUM> to <NUM>. Reference herein to the number of carbodiimide moieties is to the average number of carbodiimide moieties. The -NCN- equivalent weight, i.e. the molecular weight for <NUM> mole of carbodiimide moiety, is preferably in the range of from <NUM> to <NUM>,<NUM>/mole. The carbodiimide preferably has a number average molecular weight in the range of from <NUM> to <NUM>,<NUM>/ mole, more preferably of from <NUM>,<NUM> to <NUM>,<NUM>/mole. The carbodiimide preferably has a weight average molecular weight in the range of from <NUM>,<NUM> to <NUM>,<NUM>/ mole, more preferably of from <NUM>,<NUM> to <NUM>,<NUM>/mole. The number average and weight average molecular weight can be determined by means of gel permeation chromatography using polystyrene standards.

Carbodiimides are known in the art and are suitably prepared by reacting polyisocyanates and optionally monoisocyanates in the presence of a catalyst at a temperature in the range of rom <NUM> to <NUM> to form a carbodiimide prepolymer with terminal isocyanate groups. Chain extenders and chain stoppers may be used to control the length of the polymer chain and the molecular weight. Functional groups other than the carbodiimide moieties may be built-in via such chain extenders and/or chain stoppers or by using polyisocyanates and/or monoisocyanates with such functional groups. In <CIT> is for example described carbodiimides with alkoxysilyl or methacryloyl functional groups prepared by using alkoxysilyl- or methacryloyl-functional monoisocyanates in the carbodiimide forming reaction.

The carbodiimide in component B may be an aliphatic or an aromatic carbodiimide.

The carbodiimide in component B comprises terminal and/or pendant non-ionic emulsifying groups, such as for example polyether groups. Terminal polyether groups are suitably built into the carbodiimide by reacting the isocyanate-functional and carbodiimide-functional prepolymer with a polyether mono-alcohol or a polyether monoamine as chain stopper. Pendant polyether groups are suitably built into the carbodiimide by reacting the isocyanate-functional and carbodiimide-functional prepolymer with a diol or a diamine with a pendant polyether group.

Preferably, the terminal and/or pendant non-ionic emulsifying groups are selected from the group consisting of poly(ethyleneglycol)alkyl ether groups, poly(propyleneglycol)alkyl ether groups, and poly(ethyleneglycol/ propyleneglycol)alkyl ether groups with a molecular weight in the range of from <NUM> and <NUM>,<NUM> Dalton. Poly(ethyleneglycol)methyl ether groups, poly (propyleneglycol)methyl ether groups, and poly(ethyleneglycol/propylene glycol)methyl ether groups with a molecular weight in the range of from <NUM> and <NUM>,<NUM> Dalton are particularly preferred.

The carbodiimide may have any suitable content of non-ionic emulsifying groups. Preferably, the content of the non-ionic emulsifying groups is in the range of from <NUM> to <NUM> wt% of the total weight of the carbodiimide, more preferably in the range of from <NUM> to <NUM> wt%, even more preferably in the range of from <NUM> to <NUM> wt%. Reference herein to the content of the non-ionic emulsifying groups is to the wt% of monomers with non-ionic emulsifying moieties used in the polymerization of the carbodiimide, based on the total weight of monomers used.

The carbodiimide in component B may comprise further functional groups with reactivity towards the carboxyl-functional resin in component A or with a capability of self-condensation or of condensation with the alkoxysilane, directly or after hydrolysis. Examples of suitable further functional groups are mentioned at page <NUM> of <CIT>, incorporated herein by reference. The carbodiimide preferably comprises alkoxysilyl or alkylalkoxysilyl functional groups as further functional groups, more preferably alkoxysilyl or alkylalkoxysilyl functional groups with one or two carbon atoms in any one of its alkyl and alkoxy moieties, such as for example trimethoxysilyl, triethoxysilyl, methyldimethoxysilyl, ethyldimethoxysilyl, or dimethyl ethoxysilyl groups. Particularly preferred further functional groups are trimethoxysilyl and triethoxysilyl groups.

Preferably, the carbodiimide comprises alkoxysilyl or alkylalkoxysilyl functional groups in such amount that the carbodiimide has a Si content in the range of from <NUM> to <NUM> wt%, more preferably of from <NUM> to <NUM> wt%, based on the weight of the carbodiimide. The Si content can suitably be determined by means of elemental analysis using X-ray fluorescence spectroscopy.

The alkoxysilane is selected from the group consisting of an alkoxysilane of general formula (I):.

R<NUM>[-(CH<NUM>)n-Si(OR<NUM>)<NUM>-m(R<NUM>)m]p     (I).

and a tetraalkylorthosilicate of general formula (II).

Preferably, the alkoxysilane is an alkoxysilane of general formula (I). R<NUM> may be directly covalently linked to the silicon atom of alkoxysilyl or alkylalkoxysilyl group Si(OR<NUM>)<NUM>-m(R<NUM>)m (n is <NUM>), or indirectly via a hydrocarbon linker group (CH<NUM>)n with <NUM> to <NUM> carbon atoms (n is in the range of from <NUM> to <NUM>). If R<NUM> is indirectly linked to the silicon atom of the (alkyl)alkoxysilyl group via a hydrocarbon linker group (CH<NUM>)n, then n is preferably <NUM> or <NUM>. Only if p is, R<NUM> may be a hydrogen atom. If R<NUM> is a hydrogen atom, then n is preferably in the range of from <NUM> to <NUM>, more preferably is <NUM> or <NUM>.

Preferably, R<NUM> comprises an epoxide group, a vinyl group, or a (meth)acryloyl group, more preferably an epoxide group or a vinyl group, even more preferably an epoxide group. If R<NUM> comprises an epoxide group, the alkoxysilane is an epoxysilane; if R<NUM> comprises a vinyl group, the alkoxysilane is a vinylsilane.

Particularly preferred alkoxysilanes are selected from the group consisting of <NUM>-glycidyloxypropyltrimethoxysilane, <NUM>-glycidyloxypropylmethyldiethoxy silane, <NUM>-glycidyloxypropylmethyldimethoxysilane, <NUM>-glycidyloxypropyltriethoxysilane, vinyltriethoxysilane, and vinyltrimethoxysilane.

The organic group R<NUM> does not comprise a carboxylic functional group, a mercapto-functional group, or an amine-functional group. Such groups are reactive with the carbodiimide moieties in the carbodiimide and would result in an unstable hardener component B.

If the alkoxysilane is a tetraalkoxyorthosilicate of general formula (II), R<NUM> preferably is an alkyl group with <NUM> or <NUM> carbon atoms. A particular suitably alkoxysilane of general formula (II) is tetraethylorthosilicate (TEOS).

The weight ratio of the carbodiimide to the alkoxysilane in component B is in the range of from <NUM>:<NUM> to <NUM>:<NUM>, preferably of from <NUM>:<NUM> to <NUM>:<NUM>.

Hardener component B is substantially water-free. Reference herein to substantially water-free is to a water content of less than <NUM> wt%, preferably less than <NUM> wt%, even more preferably less than <NUM> wt% of water, based on the total weight of component B. Water is preferably not explicitly added, e.g. to adjust the viscosity of component B, but merely present, if at all, in small amounts as part of any additives present in component B.

Component B may comprise additives such as antioxidants or surfactants in typical amounts. Preferably, the total amount of additives in component B is in the range of from <NUM> to <NUM> wt%, based on the total weight of component B.

Preferably, the total content of carbodiimide and alkoxysilane in component B is at least <NUM> wt%, more preferably at least <NUM> wt%, or even at least <NUM> wt%. The total content of carbodiimide and alkoxysilane in component B may be up to <NUM> wt%, preferably up to <NUM> wt%. A total content of carbodiimide and alkoxysilane in the range of from <NUM> to <NUM> wt% is particularly preferred.

Component B preferably comprises at most <NUM> wt% of organic solvent, more preferably at most <NUM> wt%, even more preferably is free of organic solvent. Reference herein to organic solvent is to any volatile organic compound (VOC) as specified in <NPL>.

Component B is typically obtained by mixing a carbodiimide (optionally provided as a mixture of the carbodiimide and a plasticizer for controlling the viscosity), the alkoxysilane, and optionally one or more additives. Reference herein to a plasticizer is to an organic compound having an initial boiling point at standard pressure above <NUM>. Such compound is non-volatile and therefore not considered an organic solvent.

Component B may thus comprise a plasticizer, preferably a plasticizer of a hydrophilic nature such that it can easily be dispersed in aqueous component A, when components A and B are combined. Examples of suitable plasticizers include tributoxyethyl phosphate, tetraethylene glycol dimethyl ether, triethylene glycol mono(<NUM>-hexylhexanoate), and non-volatile coalescing agents. The amount of plasticizer is preferably in the range of from <NUM> to <NUM> wt%, more preferably of from <NUM> to <NUM> wt%, based on the weight of the carbodiimide.

The viscosity of component B has to be sufficiently low for component B to be dispersible into aqueous component A. Preferably, component B has a viscosity of at most <NUM> mPa. s, determined using a rotary viscometer at a shear rate of <NUM>,<NUM>-<NUM> and a temperature of <NUM>. Preferably, the viscosity is at least <NUM> mPa. s, more preferably of least <NUM> mPa. s determined using a rotary viscometer at a shear rate of <NUM>-<NUM> and a temperature of <NUM>. The viscosity may be controlled by adjusting the weight ratio of the carbodiimide to the alkoxysilane, the size and functional groups of the carbodiimide, the type of alkoxysilane, and the optional use of a plasticizer or organic solvent.

Component A comprises a carboxyl-functional resin dissolved or dispersed in an aqueous phase, preferably dispersed in an aqueous phase. The carboxyl-functional resin is a film-forming polymer and may have any suitable polymer backbone, preferably a polyurethane, polyacrylate, polyester backbone or a hybrid of any thereof such as a polyester-urethane hybrid or a polyurethane-polyacrylate hybrid. Even more preferably, the carboxyl-functional resin is a carboxyl-functional polyurethane, a carboxyl-functional polyacrylate, or a carboxyl-functional polyurethane-polyacrylate hybrid. The carboxyl-functional resin may be a combination of two of more carboxyl-functional resins.

The carboxyl functionality of the resin is reactive with the carbodiimide moieties in the carbodiimide. The resin comprises at least two carboxyl groups per molecule of resin.

Preferably, the carboxyl-functional resin has an acid value in the range of from <NUM> to <NUM> KOH/g polymer, more preferably of from <NUM> to <NUM> KOH/g polymer. Acid value is determined by Method A in ISO <NUM>.

The carboxyl-functional resin may comprise functional groups other than the carboxyl groups, preferably crosslinkable functional groups such as silanol or alkoxysilyl groups.

Component A may comprise a further resin other than the carboxyl-functional resin. Such further resin may be any resin suitable for use in an aqueous coating composition, preferably a resin dispersed in water. Examples of such further resins include polyacrylate, alkyd, polyurethane, polyester, and polyamine resins, and combinations of two or more thereof.

Component A preferably comprises at most <NUM> wt% of a further resin, based on the total weight of resin solids in component A, more preferably at most <NUM> wt%, even more preferably at most <NUM> wt%. In one embodiment, component A is free of any further resin other than the carboxyl-functional resin.

Components A preferably comprises in the range of from <NUM> to <NUM> wt% resin solids, more preferably of from <NUM> to <NUM> wt%, based on the total weight of component A.

Component A may comprise further compounds such as extender pigments, color pigments, biocide, coalescing solvent, and any additives typically used in coating compositions such as for example one or more of a surfactant, wetting agent, corrosion inhibitor, defoaming agent, dispersing agent, leveling agent, UV stabilizer, or antioxidant. Component A may comprise any suitable amount of pigment, depending on the type of coating composition. Typically, component A comprises in the range of from <NUM> to <NUM> wt% extender pigments and/or color pigments.

Component A preferably comprises a base to control the reactivity of the carboxyl groups towards the carbodiimide groups in the carbodiimide in component B, more preferably a volatile base, even more preferably a volatile base selected from the group consisting of ammonia and organic amine compounds having up to <NUM> carbon atoms, such as methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine, dimethyl ethylamine, and triethylamine.

The coating composition, i.e. components A and B combined, preferably comprises in the range of from <NUM> to <NUM> wt% alkoxysilane, based on the solid weight of the carboxyl-functional resin, more preferably of from <NUM> to <NUM> wt%, even more preferably of from <NUM> to <NUM> wt%.

Reference herein to solids content with respect to an organic compound is to the non-volatile content as determined according to ISO <NUM>:<NUM>(E) by heating a sample of <NUM> gram for <NUM> minutes at <NUM>.

The coating composition may comprises any suitable weight ratio of component B to component A. Preferably, the coating composition comprises at least <NUM> wt%, more preferably in the range of from at least <NUM> to <NUM> wt%, even more preferably of from <NUM> to <NUM> wt% or from <NUM> to <NUM> wt% of hardener component B, based on the weight of resin component A.

The coating composition may comprise extender pigments, color pigments, biocide, and any additives typical for aqueous coating compositions. The pigments, biocide, coalescing solvent, and any additives may be present in resin component A, in hardener component B, or in both components. Preferably, any extender pigments and any color pigments are comprised in resin component A.

The coating composition may comprise a crosslinking catalyst, in particular a catalyst catalyzing crosslinking of alkoxysilyl or silanol groups formed upon hydrolysis of alkoxysilyl groups. Such catalysts are known in the art. Suitable crosslinking catalysts comprise strong acids or strong bases, optionally blocked, and organometal salts such as organotin salts. Examples of suitable catalysts include para-toluene sulfonic acid, methyl sulfonic acid, <NUM>,<NUM>-diazabicyclo[<NUM>. <NUM>]undec-<NUM>-ene (DBU), <NUM>,<NUM>-diazabicyclo[<NUM>. <NUM>]octane (DABCO), <NUM>,<NUM>,<NUM>,<NUM>-tetramethylguanidine (TMG), and dibutyltin dilaurate (DBTL).

The coating composition may suitably be applied to a substrate after components A and B have been combined. The coating composition can be cured at any suitable temperature, both at ambient temperatures, for example in the range of from <NUM> to <NUM>, and at elevated temperatures, for example in the range of from <NUM> to <NUM>. When applied to a substrate and allowed to cure, a crosslinked coating with good hardness, scratch resistance, and chemical resistance is formed.

The coating composition may be used as protective or decorative coating on any type of substrate, in particular metal, plastic (including composite substrates), wooden, or ceramic substrates (including concrete and glass). The coating composition may be used as a primer coating, i.e. directly applied upon a substrate, or may be used as a topcoat on a primed substrate. It may also be used as an intermediate layer, top layer, or clear coat in a multiple layer coating system.

The invention is further illustrated by means of the following non-limiting examples.

The ingredients used in the examples are shown in Table <NUM>.

For various hardener components, differing in weight ratio of carbodiimide and alkoxysilane, the ease of emulsification in an aqueous phase was determined as follows. Hardener components were prepared by mixing carbodiimide (CDI) and alkoxysilane (silane) in the ratios indicated in Table <NUM>. The hardener component was then mixed with water in a <NUM>:<NUM> weight ratio. Ease of emulsification was ranked from <NUM> (phase separation) to <NUM> (easy to emulsify by manual stirring) as follows:.

The results show that emulsification of the hardener component by simple manual stirring is poor (rankings <NUM> and <NUM>) if the weight ratio of carbodiimide to alkoxysilane is outside the range of from <NUM>:<NUM> to <NUM>:<NUM>.

Different coating compositions were prepared by combining a resin component with a water-free hardener component as indicated in Table <NUM>. The hardener components comprising carbodiimide and alkoxysilane were prepared by mixing the carbodiimide (CDI) and the alkoxysilane (silane) in the wt% indicated in Table <NUM>. Coating compositions <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM> are according to the invention; coatings compositions <NUM>-<NUM>, <NUM>, <NUM>-<NUM>, <NUM>-<NUM>, and <NUM>-<NUM> are comparison compositions.

The coating compositions were applied on tin panels using a draw-down bar at a wet film thickness of <NUM>µ and were dried for a week at <NUM> and <NUM>% relative humidity.

The chemical resistance of the dried coatings was determined according to Method A of ASTM D5402 using methyl ethyl ketone (MEK) as solvent.

The film hardness of the dried coatings was determined according to the pencil test of ISO <NUM>.

The results of the chemical resistance and hardness measurements are shown in Table <NUM>. The addition of a hardener component with a carbodiimide and an alkoxysilane improves the chemical resistance of the resulting coating in comparison with a coating composition without such hardener or with a hardener that only comprises a carbodiimide or an alkoxysilane. The pencil hardness of the coating is improved or is comparable to the pencil hardness of a coating from a composition with a hardener that only comprises a carbodiimide or an alkoxysilane.

Two different coating compositions were prepared by combining a resin component A (R5: same resin as in R1 but now with a small amount of n-butyl glycol as coalescing solvent) with a water-free hardener component B, as indicated in Table <NUM>. The hardener components comprising carbodiimide and alkoxysilane were prepared by mixing the carbodiimide and the alkoxysilane in the wt% indicated in Table <NUM>.

The coating compositions were applied on tin panels as described in EXAMPLE <NUM>. The chemical resistance and the film hardness of the dried coatings were determined as described in EXAMPLE <NUM>. The results are shown in Table <NUM>.

Two different coating compositions were prepared by combining a resin component A (R1) with a water-free hardener component B, as indicated in Table <NUM>. The hardener components comprising carbodiimide and alkoxysilane were prepared by mixing the carbodiimide and the alkoxysilane in the wt% indicated in Table <NUM>.

Claim 1:
A two-component aqueous coating composition comprising:
A an aqueous resin component comprising a carboxyl-functional resin; and
B a substantially water-free hardener component comprising:
- a carbodiimide comprising in the range of from <NUM> to <NUM> carbodiimide moieties and terminal and/or pendant non-ionic emulsifying groups; and
- an alkoxylsilane selected from the group consisting of an alkoxysilane of general formula (I)

        R<NUM>[-(CH<NUM>)n-Si(OR<NUM>)<NUM>-m(R<NUM>)m]p     (I)

and a tetraalkylorthosilicate of general formula (II)

        Si(OR<NUM>)<NUM>     (II)

wherein:
m is <NUM> or <NUM>;
n is an integer with a value in the range of from <NUM> to <NUM>;
p is <NUM> or <NUM>, preferably <NUM>;
R<NUM> is an organic group comprising an epoxide group, a vinyl group, or a (meth)acryloyl group, or is a hydrogen atom
and R<NUM> does not comprise a carboxylic functional group, a mercapto-functional group, or an amine-functional group;
R<NUM> is an alkyl group having <NUM> to <NUM> carbon atoms; and
R<NUM> is an alkyl group having <NUM> to <NUM> carbon atoms,
wherein the weight ratio of the carbodiimide to the alkoxysilane in hardener component B is in the range of from <NUM>:<NUM> to <NUM>:<NUM>.