EP1218432A1

EP1218432A1 - Thin dirt-repelling coatings

Info

Publication number: EP1218432A1
Application number: EP00960663A
Authority: EP
Inventors: Dirk Kasper; Stefan Küpper; Brigitte Calaminus; Andreas Kunz
Original assignee: Henkel AG and Co KGaA
Current assignee: Henkel AG and Co KGaA
Priority date: 1999-09-24
Filing date: 2000-09-15
Publication date: 2002-07-03
Also published as: CZ2002985A3; WO2001023452A1; BR0014226A; CA2388581A1; AU7287600A; PL353929A1

Abstract

The invention relates to aqueous compositions which contain at least one dispersed polyurethane prepolymer with at least partially blocked isocyanate groups as well as at least one additional cross-linkable polymer dispersion or polymer solution and optionally surface-active and dispersing agents and flow-control agents. The inventive compositions are useful for producing thin, dirt-repelling functional coatings for stainless steel surfaces. Stainless steel surfaces coated according to the invention are especially useful in households, in sanitary fields, in clinical fields as well as in the food-processing industry and pharmaceutical industry because said stainless steel surfaces are no longer prone to become dirty by virtue of food, cleaning agents or finger marks.

Description

"Thin dirt-repellent coatings"

The present invention relates to compositions for producing dirt-repellent functional coatings for stainless steel surfaces and to a method for coating stainless steel surfaces.

A large number of stainless steel appliances, devices and machines are used in the household, in the sanitary area, in the clinic area as well as in the food processing and pharmaceutical industries. Because of its high resistance to corrosion, the stainless steel is polished or brushed on the surface and used without further coatings. In this form, the stainless steel is very susceptible to soiling from food and cleaning agents or fingerprints. A number of surface treatments of the stainless steel have been proposed to improve the scratch resistance of the surface or to avoid soiling. These are both Painting processes of solvent-based and aqueous systems as well as aqueous electrophoretically separable immersion paints or around powder coatings.These coatings are intended to improve the soiling behavior and the scratch resistance of the surface.However, these coatings change the original appearance of the substrate, since they are very strong compared to untreated stainless steel shine or show color changes.

JP-A-07/258599 (1995) describes paints based on an acrylic polyol resin, at least one binder based on isocyanato compounds or melamines, at least one sol of metal oxides and catalysts and antioxidants. It is disclosed that these compositions are suitable for use with paints, which are weather resistant and have a better surface hardness than conventional acrylic resin-based paints and have good scratch resistance and resistance to soiling. They should be suitable for painting wagons, building materials and, among other things, stainless steel.

JP-A-07/171496 (1995) describes coatings made of a polyester resin and a topcoat made of a silicate resin and its use for coating stainless steel for outdoor applications.

JP-A-04150976 (1992) describes scratch-resistant and contamination-resistant coated stainless steel materials, the coating agent of which consists of a linear polyester resin and a melamine resin.

JP-A-01/151409 describes stainless steel surfaces with a surface roughness of 1 to 2.5 μm which have been coated with a polysiloxane, polyurethane or acrylic resin. It is said that the stainless steel sheets coated in this way are suitable for external cladding of railway carriages, elevator doors and the like.

JP-A-02/228372 (1990) describes a weather-resistant, high-gloss hard powder coating based on an acrylate polyester and a blocked polyisocyanate, as well as pigments for coating stainless steel.

Despite the extensive state of the art, there is still a need for improved coating compositions and processes for coating stainless steel surfaces; in particular, these coating systems should, if possible, be based on aqueous binder systems. The good soiling behavior and the scratch resistance of the coated surface should be given, and the natural look, especially of brushed stainless steel surfaces, should be preserved. Be about the latter property no statements made in the prior art. It was therefore the task of providing a thin, single-layer coating system which, in addition to the aforementioned criteria, also has a high resistance to the strongly alkaline and acidic cleaners, such as, for. B. used in kitchen appliances or devices in the food processing industry. Furthermore, the binder should be able to be applied using all conventional application methods.

The solution to the problem according to the invention can be found in the patent claims. It essentially consists in providing an aqueous composition which contains at least one dispersed polyurethane prepolymer, the isocyanate groups preferably being in blocked form, and furthermore the binder contains one or more crosslinkable polymer dispersions or dissolved polymers and, if appropriate, wetting and dispersing auxiliaries and leveling aids and, if appropriate further auxiliaries and additives.

The invention further relates to a method for coating stainless steel with scratch-resistant, dirt-repellent and hydrolysis-resistant coatings, which comprises the following essential process steps: a) if necessary, the surface of the stainless steel is cleaned and degreased, b) if necessary, the surface is subsequently rinsed c) if necessary, the surface is treated with an adhesion promoter, d) the coating composition is applied in such an amount that, after curing, a coating with a mass per unit area of 0.1 to 10 g / m ² , preferably 0.5 to 5 g / m ^{2 is} obtained, e) if appropriate, the volatile constituents, in particular the water, are evaporated off, f) the coating is then cured at temperatures between 100 ° C. and 250 ° C. peak metal temperature (PMT) for one Period between 0.5 seconds and 40 minutes. The coating agent is preferably applied to the workpiece or metal strip surface by flooding / squeezing, spraying / squeezing, suitable scrapers / or roller applications or by brush application.

If the stainless steel strip is not to be coated directly in the coil coating process, but shaped stainless steel parts are to be coated, all spray systems commonly used in painting technology, such as e.g. airiess, air-assisted or electrostratically assisted spray processes.

The preferred uses of the stainless steel coated according to the invention are the production of machines, devices and objects for the household area, the sanitary area, the clinic area and for the food processing or pharmaceutical industry.

An essential component of the binder system to be used according to the invention is a preparation of a polyurethane prepolymer which essentially no longer contains any free isocyanate groups. This polyurethane prepolymer is constructed in a manner known per se from hydroxy-functional polyesters, hydroxy-functional acrylates or epoxides and aliphatic or cycloaliphatic polyisocyanates. The aliphatic or cycloaliphatic polyisocyanates to be used for this are preferably selected from the group 4,4'-dicyclohexylmethane diisocyanate (H ₁₂ MDI), 1-isocyanatomethyl-3-isocyanato-1, 5,5-trimethylcyclohexane (isophorone diisocyanate, IPDI), Cyclohexane-1,4-diisocyanate, hydrogenated xylylene diisocyanate (H ₆ XDI), 1-methyl-2,4-diisocyanato-cyclohexane, m- or p-tetramethylxylene diisocyanate (m-TMXDI, p-TMXDI), dimer fatty acid diisocyanate, Tetramethoxybutane-1,4-diisocyanate, butane-1,4-diisocyanate, hexane-1,6-diisocyanate (HDI), 1,6-diisocyanato-2,2,4-trimethylhexane, 1,6-diisocyanato-2,4 , 4-trimethylhexane, butane-1, 4-diisocyanate and 1, 12-dodecane diisocyanate (C ₁₂ DI). Possibly. the corresponding trimeric biuretization or isocyanuration products of the aforementioned diisocyanates can also be used. Suitable polyols are polyesters with molecular weights between 400 and 10,000, preferably between 400 and 5000, which are obtained by condensation of di- or tricarboxylic acids, such as adipic acid, sebacic acid, glutaric acid, azelaic acid, suberic acid, 3,3-dimethylglutaric acid, terephthalic acid, isophthalic acid, Hexahydrophthalic acid or dimer fatty acid with low molecular weight diols or triols such as ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, dipropylene glycol, 1, 4-butanediol, 1, 6-hexanediol, 1, 10-decanediol, dimer fatty alcohol, glycerol or trimethylolpropane can be prepared. Another group of the polyols to be used according to the invention are the polyesters based on ε-caprolactone, also called "polycaprolactones". Hydroxy-functional polycarbonates can also be used as the polyol component for the polyurethane preparation to be used according to the invention. However, polyester polyols of oleochemical origin can also be used. Such polyester polyols can, for example, by completely ring opening epoxidized triglycerides of an at least partially olefinically unsaturated fatty acid-containing fat mixture with one or more alcohols with 1 to 12 C atoms and then partial transesterification of the triglyceride derivatives to alkyl ester polyols with 1 to 12 C atoms in the alkyl radical getting produced. Other suitable polyols are polycarbonate polyols and dimer diols (from Henkel) and castor oil and its derivatives. The polytetramethylene glycols, which are produced, for example, by the acidic polymerization of tetrahydrofuran, are also at least co-usable as polyols. The molecular weight range of the polytetramethylene glycols is between 200 and 6000, preferably in the range from 400 to 4000.

The hydroxy-functional acrylates or methacrylates known per se can likewise be used as polyol, their molecular weight ranges between about 400 and 10,000.

Since the polyurethane preparations are aqueous systems, the polyurethane prepolymers generally do not have free isocyanate groups, but rather at least for the most part they are blocked with the aid of blocking agents. These blocking agents are selected from aldoximes, ketoximes, lactams, imidazole compounds, ß-dicarbonyl compounds such as dialkyl malonate or acetoacetic ester. Other blocking agents known per se can be alcohols, phenols, in particular alkylphenols, thioalcohols, thiophenols, secondary amines, amides, imides or hydroxamic acid esters. These blocking groups are then released during the curing or crosslinking of the binder system, possibly with the aid of catalysts, so that the isocyanate groups which are then released can react with other reactive groups such as hydroxyl groups, amino groups from other binder constituents and thus to increase the molecular weight or to crosslink the Binder system contribute.

The coating compositions to be used according to the invention contain, as further crosslinkable polymer dispersions or polymer solutions, binders based on (meth) acrylate copolymers, hydroxyl-functional polyurethane dispersions based on polyester polyols, polycarbonate polyols or polyether polyols. Further binder components can be epoxy resin dispersions - modified with free hydroxyl groups or with amines if necessary - or water-soluble or water-dispersible melamine-formaldehyde resins or urea

Be formaldehyde resins. Instead of the (meth) acrylate homo- and / or copolymer dispersions, butadiene copolymer dispersions or styrene copolymer dispersions can also be used, at least in part. The aforementioned copolymers based on olefinically unsaturated monomers can also contain crosslinkable comonomers which are known per se. Examples include acrylic acid, methacrylic acid, glycidyl (meth) acrylate, acrylamide, N-methylolacrylamide, N-methylol methacrylamide, N, N-bis-alkoxymethyl methacrylamide and similar thermally crosslinkable groups, where the alkoxy group can have 1 to 4 carbon atoms. Crosslinkers known per se can also be used in the form of epoxy resin dispersions or urea derivatives. Self-crosslinking or externally crosslinkable are particularly preferred (Meth) acrylate dispersions or emulsions, epoxy resins or methylolated melamine-formaldehyde resins such as methoxymethyl-methylol-melamine.

In addition to the abovementioned binders, the aqueous preparation according to the invention also contains additives which have a positive influence on the leveling properties and the film formation of the dispersions or solutions applied. Examples include small amounts of essentially volatile organic water-miscible solvents such as e.g. Butylglycol, 2-butanol and solutions or dispersions of mercaptoesters, fluorinated polysilicon glycols, silicone glycols, low molecular weight acrylate copolymers, polyether-polysiloxane copolymers and similar leveling agents. Furthermore, so-called “crosslinkers” in the form of polyfunctional crosslinkers containing aziridine groups can be used for polymers containing carboxyl groups or else polyepoxide dispersions. Colloidal silica solutions can also be used as auxiliaries. If the silica particles are in nanoscale form, they increase the scratch resistance of the coating without to influence the visual appearance.

The preparations according to the invention contain

0 to 25 wt.% Preferably 5 - 10 wt.% Polyurethane prepolymer (as

Aktivsubsstanz)

0.5 to 150 wt.%, Preferably 1 to 100 wt.% Crosslinking

Polymer component, based on the polyurethane active substance,

0.01 to 5, preferably 0.1 to 1% by weight (based on the total preparation) of leveling agent and

2 to 15, preferably 5 to 10% by weight (based on the total composition) of film-forming solvents.

If the compositions do not contain a blocked polyisocyanate polyurethane prepolymer, other crosslinking polymer components must be present, particularly Epoxy resin emulsions in combination with polyamine adducts (as a solution in water).

The term "active substance" excludes the dispersion medium such as water and volatile solvents, while these are included in the "overall composition". The pH of the preparations is between 7.0 and 9.0.

It may be advisable to use an adhesion promoter before the organic binder is applied. Examples of such adhesion promoters are Alodine NR 1453 (from Henkel) and similar adhesion promoters used in the coil coating process based on hexafluorotitanates, phosphoric acids or polyphenols, such as those e.g. WO 99/16827 or WO 98/47631 for the pretreatment of metallic substrates. Organophosphonic acids or organofunctional silanes can also be used as adhesion promoters.

In addition to the aforementioned leveling agents and wetting agents and film formers, pigments can only be used if their particle size is in the nanometer range, since the preparations according to the invention should be transparent. Conventional anti-aging agents and inorganic auxiliaries can also be used.

The coatings according to the invention have good resistance and resistance to soiling against a large number of agents such as are found, for example, in the household sector or in the food industry: good resistance to black tea, black ink, condensed milk, fruit juices, vinegar, mustard, ketchup, mayonnaise, onion. The fingerprints left on uncoated stainless steel surfaces after touching are also avoided with stainless steel coated according to the invention. In particular, they are also resistant to acidic and strongly alkaline cleaning agents such as detergents, oven sprays, so-called "steel shine" or all-purpose cleaners stable at temperatures up to 60 ° C. The visual appearance of the brushed surface is also completely retained, even if absolute gloss measurements show deviations from the uncoated stainless steel.

The invention is to be explained in more detail with reference to the following exemplary embodiments, the selection of the examples not being a limitation of the scope of the subject matter of the invention. Unless otherwise stated, all amounts in the examples below are percentages by weight or parts by weight based on the total composition.

Examples

In analogy to JP-A-07/171496, coatings were produced and tested for their suitability as protective protective functional coatings for stainless steel surfaces. The coatings showed a strong yellowing and were not resistant to acids and alkalis as they occur in food.

Coating agents made of linear polyester resin and a melamine resin analogous to JP-A-04/150976 also gave thick yellowish layers and were not resistant to hydrolysis.

Coatings consisting of a polysiloxane and a polyurethane analogous to JP-A-01/151409 were not resistant to acids, alkali or food.

In the subsequent coatings according to the present invention, brushed stainless steel sheets were used, which were pretreated with a 2.5% solution of Ridoline C 72 (Henkel company) by spraying for 20 to 30 seconds at 60 °. An adhesion promoter was then applied by means of a paint spinner or in an immersion bath and dried. The drying method of the adhesion promoter was based on the application process: 2 minutes were used when using the paint spinner 70 ° C dried in the oven, using the immersion method, the coated substrate was dried after immersion by blowing with compressed air. Alternatively, 1 to 20% solutions of Alodine NR 1453 or Granodine 1455 (both from Henkel) were used as adhesion promoters. The dirt-repellent functional coatings according to the invention were then applied with the aid of a paint spinner or a laboratory spray gun, so that after baking at 170 ° C. for 20 minutes in a forced air oven, a coating with a mass per unit area of 1 to 3 g / m ² was obtained. For the coatings according to the invention, binders and auxiliaries were used as an aqueous dispersion or solution in organic solvents, so that the coating compositions could be prepared by simple mixing with stirring until homogeneous. The individual results are shown in Tables 1 and 2 below.

Table:

In the table above, DPM in line 3 means dipropylene glycol monomethyl ether. The blocked polyisocyanates according to lines 4 and 5 are auxiliary solvent-free in water-dispersed polyisocyanate derivatives blocked with 2-butanone oxime and having a latent NCO content (calculated based on solids) of 10 and 11%, respectively. The polyurethane resins (PUR) according to lines 5 and 6 are anionic dispersions of polyurethane resins with crosslinkable, blocked isocyanate groups with a functionality greater than three. The acrylate copolymer emulsion according to line 7 is a dispersion of (meth) acrylate copolymers from C1-C4 esters of (meth) acrylic acid with N- Alkylolacrylamide groups, viscosity 36 mPas / 25 ° C, MFT 15 ° C. The polyurethane according to line 9 is an anionic polyurethane dispersion based on an aliphatic polycarbonate, which contains N-methylpyrrolidone as a film-forming aid. The fluoropolymer emulsion according to line 12 is a fluoroethylene-vinyl ether copolymer and, like the polyether-modified polysiloxane according to line 15, serves as a surfactant to improve the substrate wetting of the aqueous coating composition. The melamine resin according to line 13 is a mixture of methoxymethylmethylolmelamine isomers.

The degree of gloss was measured with a Micro-TRI-gloss measuring device from Byck-Gardner. Since the surface of the stainless steel sheets was structured by the brushing, the measurement was measured once along the brushing direction and once across it and compared to the standard (unpainted material). In both cases, there is a significantly lower gloss level value across the direction of brushing than along the direction of brushing. It can be seen from the measured values that the optical impression of the brushing after coating with the compositions according to the invention was largely retained, which was also confirmed by the visual assessment. The colorimetric determination of the color differences was carried out using the CIELAB formula, only the b value being taken into account as an indication of yellowing of the colorless coating itself. As can be seen from the comparison to the uncoated standard in line 19, hardly any yellowness can be determined for all coatings, and this is also confirmed by visual assessment.

The results of the surface property and the resistance tests of the coatings according to the invention are summarized in Table 2 below. The surface properties, in particular the scratch resistance and the sensitivity to fingerprints, are at least equal to the untreated standard in all the coatings according to the invention, in most cases even significantly better than the untreated standard. The same applies to the resistance to common foods and mild cleaners. The compositions according to the invention can even be designed so that they are also resistant to very aggressive cleaners such as oven cleaners or ceramic cleaners.

Table 2:

1) visual assessment, no change in appearance

Endurance test sought

2) determined with hardness test rod 318, Erichsen 3) optical inspection of fingerprints 4) soiled test trays are stored at 60 ° C for 1 h, washed off with water and visually inspected. Food: mustard, tea, mayonnaise,

Ketchup, lemon juice etc., mild cleaners: all-purpose cleaners, detergents etc., aggressive cleaners: oven cleaners, ceramic cleaners etc.

5) Test plates angled at 90 ° are treated with steam for 2 hours and visually inspected. 6) Test plates are left in the Uvasol 2 radiation system (Hönle) for 48 hours. Then the ΔE value is measured using a "micro-color" colorimeter

(From Lange)

7) Test plates are exposed to a temperature of 150 ° C for 1 h. The ΔE value is then determined using a "micro-color" color measuring device (from Lange)

test results

equal to the untreated standard better than untreated standard worse than untreated standard n.a. not available

Claims

claims

1.) Aqueous composition for coating stainless steel containing a) at least one dispersed polyurethane prepolymer with at least partially blocked isocyanate groups b) at least one further crosslinkable polymer dispersion or polymer solution c) optionally wetting and dispersing aids and leveling aids.

2.) Composition according to claim 1, characterized in that the dispersed blocked polyurethane prepolymer is composed of low molecular weight polyols and aliphatic diisocyanates.

3.) Composition according to claim 2, characterized in that the blocking agent is selected from aldoximes, ketoximes, lactams, imidazole compounds, β-dicarbonyl compounds, alcohols, phenols, thioalcohols, thiophenols, secondary amines, amides, imides or hydroxamic acid esters.

4.) Composition according to claim 2 or 3, characterized in that an aliphatic or cycloaliphatic diisocyanate is used as the polyisocyanate selected from the group

4,4'-dicyclohexylmethane diisocyanate (H ₁₂ MDI), 1-isocyanatomethyl-3-isocyanato-1, 5,5-trimethyl-cyclohexane (isophorone diisocyanate, IPDI), cyclohexane-1,4-diisocyanate, hydrogenated xylylene diisocyanate ( H ₆ XDI), 1-methyl-2,4-diisocyanatocyclohexane, m- or p-tetramethylxylene diisocyanate (m-TMXDI, p-TMXDI), dimer fatty acid diisocyanate, tetramethoxybutane-1, 4-diisocyanate, butane-1, 4 -diisocyanate, hexane-1,6-diisocyanate (HDI), 1,6-diisocyanato-2,2,4-trimethylhexane, 1,6-diisocyanato-2,4,4-trimethylhexane, butane-1,4-diisocyanate as well 1, 12-dodecane diisocyanate (C ₁₂ DI).

5.) Composition according to claim 1, characterized in that the further crosslinkable polymer component (b) is selected from reactive (meth) acrylate copolymers, polyurethane dispersions on polyesterol, Polycarbonate or polyether base, epoxy resin dispersions or water-soluble or water-dispersible melamine-formaldehyde resins.

6.) Use of the composition according to claim 1 to 5 for the production of thin, dirt-repellent hydrolysis-resistant and scratch-resistant

Coatings on stainless steel.

7.) Process for coating stainless steel with scratch-resistant, dirt-repellent thin layers characterized by the following essential steps

• if necessary, cleaning and degreasing the stainless steel,

If necessary rinsing the surface,

Optionally treating the metal surface with an adhesion promoter,

Coating the surface with a composition according to at least one of claims 1 to 5, so that after curing a coating with a mass per unit area of 0.1 to 10 g / m ^2, preferably 0.5 to 5 g / m ² , is obtained,

Evaporation of the volatile constituents, if necessary,

• Curing of the coating at temperatures between 100 ° and 250 ° C for a period of 0.5 seconds to 40 minutes.

8.) Method according to claim 7, characterized in that the composition according to claim 1 to 5 by flooding / squeezing, spraying / squeezing, suitable scraper or roller applications is applied to the belt surface.

9.) The method according to claim 7, characterized in that the composition according to claim 1 to 5 is applied by spray application or by brush application to the surface of the shaped workpiece.

10.) Method according to claim 9, characterized in that an airiess, air-assisted or electrostatically assisted spraying method is used as the spraying system.

11.) Use of the coated stainless steel according to claim 7 to 10 for the production of machines and devices for the household, sanitary and clinical area and for the food processing or pharmaceutical industry