EP0881954A1

EP0881954A1 - Multi-coat painting process

Info

Publication number: EP0881954A1
Application number: EP97903326A
Authority: EP
Inventors: Detlef Schlaak
Original assignee: Herberts GmbH
Current assignee: EIDP Inc
Priority date: 1996-02-23
Filing date: 1997-02-19
Publication date: 1998-12-09
Anticipated expiration: 2017-02-19
Also published as: AU1792497A; EP0881954B1; DE59700330D1; ES2140962T3; BR9707608A; US5976343A; JP2000505352A; WO1997030795A1; DE19606716C1; JP4096069B2

Abstract

A process for the multi-coat painting of a substrate with a first coat of a stoved dipping paint. Said process involves the application of a second coat with a dry thickness of 10 to 30 mu m of a first aqueous base coating agent containing polyurethane resin and the wet-on-wet application of a third coat with a thinner dry thickness of 7 to 15 mu m of a second aqueous base coating agent, the further application of a coat of clear lacquer without prior stoving, followed by stoving all together, where the first base coating agent has a higher polyurethane resin than the second.

Description

Process for multi-layer painting

The invention relates to a method for producing a multilayer coating, in particular metallic substrates, which is particularly suitable for automotive coating.

Today's automotive OEM coatings mostly consist of one

Basecoat / clearcoat topcoat that is applied to an electrophoretically primed body with a filler layer. Basecoat and clearcoat are preferably applied wet-on-wet, i.e. after a flash-off time, if necessary with heating, and after subsequent application of a clear coat, the basecoat is baked together with the latter. EP-A-0238 037 has disclosed a method for reducing stoving layers, in which a filler and an aqueous metallic basecoat are applied by the wet-on-wet method. The example section shows that the filler layer is applied in a dry film thickness of 35 μm that is customary for filler coatings.

From EP-A-0265363 it is known to provide a previously burned-in cataphoretically applied primer with a two-layer basecoat / clearcoat topcoat applied by the wet-on-wet method. The filler layer is saved. It is problematic, however, that stone chip damage to motor vehicles painted using this method becomes particularly apparent, since the primer layer is exposed in the event of stone chip by de-coating. Corrosion protection is then still guaranteed, but the visual impression cannot be tolerated.

For reasons of rationalization and the saving of materials, there is a desire for automotive paint systems to reduce the thickness of the paint structure, but without allowing significant losses in its overall level of properties.

The object of the invention is to provide a method for producing multi-layer coatings, in particular automotive coatings compared to the prior art to provide a comparable overall level of properties, but a reduced layer thickness of the overall paint structure, which can be carried out with as few baking steps as possible.

Surprisingly, the object is achieved by the method for multi-layer coating of a substrate provided with a baked first electrocoat layer, which is characterized in that on this first electrocoat layer a second coating layer corresponding to a dry layer thickness of 10 to less than 30 microns from a first aqueous , color and / or effect basecoat coating composition containing one or more polyurethane resins is applied, whereupon wet-on-wet a third coating layer corresponding to a dry layer thickness of 7 to 15 microns is applied from a second aqueous, color and / or effect basecoat coating composition and a fourth coating layer of a clear lacquer coating agent is applied to the third coating layer without prior baking and the second, third and fourth coating layers are baked together, a first aqueous basecoat is used berzugsmittel that a higher content of

Polyurethane resin has as the second basecoat based on the total weight of the respective aqueous basecoat, and the solid resin content in the first basecoat does not deviate by more than 20% from the absolute value of the weight content of solid resin in the second basecoat.

The process according to the invention makes it possible to apply an aqueous basecoat coating composition directly to a baked-on first coating layer created by electrodeposition, and this layer together with the subsequently applied second coating layer

Burn in the water-based lacquer layer and clear lacquer layer, eliminating the usual filler layers or other intermediate layers.

All known anodic or cathodic electrodeposition coatings (ETL), which are not subject to any particular limitation, can be used as the electrophoretically depositable coating agent for producing the first coating layer, namely a primer become.

This is e.g. to aqueous coating compositions with a solids content of up to 50% by weight, for example 10 to 20% by weight. The solid is formed from binders customary for electrocoating which carry ionic groups or groups which can be converted into ionic groups and, if appropriate, groups capable of chemical crosslinking, and any crosslinking agents which are present.

Pigments, fillers and / or additives common in paint. The ionic or convertible groups can be anionic or convertible groups, e.g. acidic groups such as COOH groups or cationic or convertible groups, e.g. basic groups such as amino, ammonium, e.g. quaternaries

Ammonium groups, phosphonium and / or sulfonium groups. Binders with basic groups are preferred. Nitrogen-containing basic groups are particularly preferred. These groups can be quaternized or they are at least partially treated with a common neutralizing agent, an acid, e.g. an organic monocarboxylic acid, e.g. Formic acid or acetic acid, converted into ionic groups.

To produce the first coating layer, the electrodeposition coating layer, it is possible, for example, to use the usual anodically depositable electrodeposition coating binders and lacquers (ATL) containing anionic groups. These are, for example, binders based on polyesters, epoxy resins, poly (meth) acrylates, maleate oils or polybutadiene oils with a weight average molecular weight of, for example, 300 to 10,000 and an acid number of 35 to 300 mg KOH / g. The binders carry, for example, -COOH, -S0 ₃ H and / or -P0 ₃ H ₂ groups. After neutralization of at least some of the acidic groups, the resins can be converted into the water phase. The binders can be self-crosslinking or externally crosslinking. The lacquers can therefore also contain customary crosslinking agents, for example

Triazine resins, crosslinking agents containing groups capable of transesterification or blocked polyisocyanates. Specific examples of such in Anodically depositable electrodeposition paint binders and paints (ATL) containing anionic groups which can be used according to the invention are described, for example, in DE-A-2824418.

However, cathodically depositable electrocoat materials (KTL) based on cationic or basic binders are preferably used to produce the first coating layer. Such basic resins are, for example, resins containing primary, secondary or tertiary amino groups, the amine numbers of which are e.g. are 20 to 250 mg KOH / g. The weight average molar mass (Mw) of the base resins is preferably 300 to 10,000. Examples of such base resins are amino (meth) acrylate resins, aminoepoxy resins, aminoepoxy resins with terminal double bonds, aminoepoxide resins with primary OH groups, aminopolyurethane resins, amino group-containing polybutadiene resins or modified polybutadiene resins or amino group-containing polybutadiene resins Amine reaction products. This

Base resins can be self-crosslinking or they are used in a mixture with known crosslinking agents. Examples of such crosslinkers are aminoplast resins, blocked polyisocyanates, crosslinkers with terminal double bonds, polyepoxide compounds or crosslinkers which contain groups capable of transesterification. Specific examples of base resins and crosslinking agents which can be used in cathodic dip coating baths are e.g. in EP-A-0082 291, EP-A-0234 395, EP-A-0 227 975, EP-A-0 178531, EP-A-0333327, EP-A-0310971, EP-A-0456 270, US 3922 253, EP-A-0 261 385, EP-A-0 245786, DE-A-3324211, EP-A-0414 199, EP-A-0476514.

Such resins can be used alone or in a mixture. So-called "non-yellowing" CTL systems are preferably used, which prevent yellowing or discoloration of the multi-layer coating when baked. For example, these are KTL systems that crosslink using specially selected blocked polyisocyanates, as described, for example, in EP-A-0265 363.

In addition to the base resins and any crosslinking agents present, the electrocoat material (ETL) coating agent can contain pigments, fillers and / or additives customary in coating. The usual inorganic and / or organic pigments and come as pigments and fillers Fillers in question. Examples are carbon black, titanium dioxide, iron oxide pigments, phthalocyanine pigments, quinacridone pigments, kaolin, talc or silicon dioxide.

The pigments can be dispersed into pigment pastes, e.g. using known paste resins. Resins of this type are familiar to the person skilled in the art. Examples of paste resins that can be used in KTL baths are described in EP-A-0 183 025 and in EP-A-0469497.

The usual additives, as are known in particular for ETL coating agents, are possible as additives. Examples of these are wetting agents, neutralizing agents, leveling agents, catalysts, corrosion inhibitors, antifoams, solvents, but in particular light stabilizers, if appropriate in combination with antioxidants.

In the process according to the invention, the usual aqueous, color and / or effect basecoat coating compositions familiar to the person skilled in the art are used for the production of the second and third coating layers, as are used for the production of basecoat / clearcoat two-coat coatings and described in large numbers, for example, in the patent literature become.

The waterborne basecoats can be physically drying or can be crosslinked to form covalent bonds. With the under

The formation of water-based lacquers that crosslink covalent bonds can be self- or externally crosslinking systems. It can be one- or two-component systems, one-component systems are preferred.

The waterborne basecoats which can be used in the process according to the invention contain water-dilutable binders. These contain non-ionic and / or ionic groups which impart hydrophilicity to ensure their water dilutability.

Examples of suitable non-ionically stabilized binders are those binders whose water-dilutability is achieved by incorporating polyether segments into the resin molecule. Specific Examples of such stabilized polyurethane or polyurethane acrylate resins are described, for example, in EP-A-0354261, EP-A-0422 357 and EP-A-0424705.

The water-dilutable, ionic group-containing binders can be cationic or anionic group-containing binders.

Examples of the cationically stabilized binders which can be used according to the invention are completely or partially neutralized cationic (meth) acrylic copolymer, polyester,

Polyurethane and / or polyurethane urea resins, especially with one

Number average molecular weight (Mn) from 500 to 500000, an OH number from 0 to 450, an amine number from 20 to 200 and a glass transition temperature from -50 to + 150 ° C. Specific examples of these binders known to the person skilled in the art are described in DE-A-40 11 633.

The waterborne basecoats preferably contain binders which are stabilized via anionic groups. These are one or more film-forming resins as are customary in aqueous coating compositions, in particular in aqueous basecoats. The film-forming resins can be, for example, polyester, (meth) acrylic copolymer or preferably polyurethane resins. They can be self-curing, cross-linking or physically drying. Specific examples of suitable water-dilutable (meth) acrylic copolymers are e.g. in EP-A-0399427 and EP-A-0287 144.

Specific examples of suitable water-dilutable polyester resins are e.g. in DE-A-29 26854, DE-A-3832 142 and EP-A-0301 300.

It is also possible to use mixtures of binders. Particularly suitable binders are those in which (meth) acrylic copolymer and polycondensation resin are present covalently or in the form of interpenetrating resin molecules. Examples of such

The combination is (meth) acrylic copolymer and polyester resin. Specific examples of such a usable combination are, for example in EP-A-0226 171.

Anionically stabilized polyurethane resins are particularly preferably used. Such polyurethane resins are known to the person skilled in the art and are described in great variety in the literature. These are aqueous polyurethane dispersions or solutions or binders in which (meth) acrylic copolymer and polyurethane resin are present in a covalent manner or in the form of interpenetrating resin molecules. Examples of suitable polyurethane dispersions are stable, aqueous dispersions with a solids content of 20 to 50% by weight. The

Weight average molecular weight (Mw) of the resins can vary widely, e.g. from 1000 to 500000. The polyurethane resins can contain functional groups such as e.g. Contain hydroxyl groups or blocked isocyanate groups.

Specific examples of usable polyurethane dispersions are those which can be prepared by chain extension of isocyanate-functional prepolymers with polyamine and / or polyol. They are described for example in EP-A-0089497, EP-A-0228003, DE-A-3628 124 and EP-A-0512 524.

Further examples are polyurethane dispersions which can be prepared by chain-extending isocyanate-functional prepolymers with water, such as e.g. described in DE-A-39 15459 and DE-A-4224617.

Polyurethane dispersions can also be used which are produced by chain extension of polyurethane prepolymers containing polyisocyanates reactive with active hydrogen and containing polyisocyanates, such as e.g. in DE-A-3903804 and DE-A-4001 841.

Further examples of anionically stabilized polyurethane (PU) dispersions which can be used are those which can be obtained by chain-extending polyurethane resins containing at least one CH-acidic group in the molecule with compounds which can react with at least two CH-acidic groups. These are special examples polyurethane resin dispersions described in DE-A-42 28510.

PU dispersions based on polyurethane resins that are chain-extended via siloxane bridges can also be used. Such are e.g. known from DE-A-44 13 562.

Specific examples of anionically stabilized polyurethane-based binders in which (meth) acrylic copolymer and polyurethane resin are present covalently or in the form of interpenetrating resin molecules are e.g. in EP-A-0 353797, EP-A-0297 576, DE-A-41 22265, DE-A-41 22 266 and WO 95/16004.

The waterborne basecoats can contain a single aqueous binder, but several aqueous binders can also be present in combination. Aqueous binders based on anionically stabilized polyurethanes are preferably present. It may be appropriate if a part, e.g. up to 50 wt .-%, the polyurethane binder is replaced by resins based on a combination of (meth) acrylic copolymer and polyester resin.

Water-dilutable binders based on cellulose may also be present.

To prepare the waterborne basecoats, in particular if the binders used are not self-crosslinking, the customary crosslinkers known to those skilled in the art, such as, for example, formaldehyde condensation resins, such as phenol-formaldehyde condensation resins and amine-formaldehyde condensation resins, and free or blocked polyisocyanates can be used. The crosslinkers can be used individually or in a mixture. The mixing ratio of crosslinker to binder resin is preferably 10:90 to 40:60, particularly preferably 20:80 to 30:70, in each case based on the solids weight.

In addition to the usual physically drying and / or chemically crosslinking binders, the waterborne basecoats used in the process according to the invention contain pigments. The expression used here "Pigments" include inorganic and / or organic colored pigments and / or effect pigments and any fillers present. All common pigments can be used; Examples are titanium dioxide, iron oxide pigments, carbon black, azo pigments, phthalocyanine pigments, metal pigments, for example made of titanium, aluminum or copper, interference pigments, such as, for example, titanium dioxide-coated aluminum, coated mica, graphite effect pigments, platelet-shaped iron oxide, platelet-shaped copper phthalocyanine pigments, calcium oxide, barium sulfate, barium sulfate.

It is preferred if the first and the second basecoat coating compositions differ in their pigment / volume concentration (PVC) by no more than 30% of the absolute value of the pigment / volume concentration in the second basecoat coating composition. The pigment / volume concentration of the first basecoat coating composition is preferably below that of the second

Basecoat coating agent. The pigment / volume concentration in a coating agent represents the ratio of pigment volume to pigment volume + binder volume and is calculated using the following equation:

Volume pigment

PVK = • 100%

Volume of pigment + volume of binder

The same pigments are preferably used in both basecoat coating compositions, particularly preferably the same pigmentations. Pigmentation is understood to mean the relative composition of a pigment mixture, i.e. H. the type of pigments and their respective proportions in the pigment mixture. It is also preferred to use pigmentations with a similar average specific weight in the two basecoat coating compositions (this results from the relative proportions of the individual pigments). The average specifics preferably differ

Weights of the pigmentations in the two basecoat coating compositions by no more than 20%, based on the average specific weight of the pigmentation in the second waterborne basecoat. The coloring absorption pigments (colored pigments) and optionally fillers are generally rubbed into part of the water-dilutable binders. The rubbing can preferably also take place in a special water-thinnable paste resin. The paste resins known to the person skilled in the art and suitable for aqueous systems can be used. A specific example of a paste resin which can preferably be used and which is based on an anionically stabilized polyurethane resin can be found in DE-A-4000889. The grinding is done in conventional units known to those skilled in the art. Then complete with the remaining portion of the aqueous binder or the aqueous paste resin for finished pigment pigment grinding. If paste resins are present in the waterborne basecoat, they add up to binders plus any crosslinker present. Both binders, crosslinkers and paste resins together result in the solid resin content of the waterborne basecoats.

The waterborne basecoats may also contain customary auxiliaries, e.g. Catalysts, leveling agents, anti-foaming agents, anti-cratering agents or in particular light stabilizers, if appropriate in combination with antioxidants.

The waterborne basecoats, for example, have a solids content of 10 to 50% by weight, for effect basecoats it is preferably 15-30% by weight, for plain-colored basecoats it is preferably higher, for example 20-45% by weight. The ratio of pigment to binder plus optionally crosslinking agent plus optionally paste resin in the waterborne basecoat is, for example, between 0.03: 1 to 3: 1, for effect basecoats, for example, it is preferably 0.06: 1 to 0.6: 1, for monochrome

Basecoats are preferably higher, for example 0.06: 1 to 2.5: 1, in each case based on the solids weight. The first water-based lacquer used to produce the second coating layer preferably has a lower pigment / binder ratio than the second used to produce the third coating layer

Water-based paint. The term binder includes the binder or binders as such plus any crosslinking agent plus any paste resins present, the sum of these components representing the solid resin content of the water-based paints.

It is essential to the invention that the first water-based lacquer used to produce the second coating layer is distinguished from the second water-based lacquer used to produce the third coating layer by a higher proportion by weight of polyurethane resins. The absolute value of this excess of polyurethane resins in the first waterborne basecoat preferably corresponds to between 5 and 50%, particularly preferably between 10 and 35% of the absolute value of the weight content of solid resin in the second coating agent. Accordingly, if the second basecoat coating agent has a solid resin content of 20% by weight, the preferred absolute value for the polyurethane content in the first basecoat coating agent is calculated as the sum of the polyurethane content X (% by weight) of the second coating agent plus 5 to 50% of 20 % By weight, ie the polyurethane content of the first coating agent is then preferred: X + (1 to 10)% by weight. However, the absolute value of the weight content of solid resin in the first basecoat coating composition does not differ by more than 20% from the absolute value of the weight content of solid resin in the second basecoat coating composition; Thus, in the example above, if the solid resin content of the second basecoat coating composition is 20% by weight, the total solid resin content of the first basecoat coating composition can accordingly be 16 to 24% by weight. The solid resin content of the first coating agent is particularly preferably equal to or higher than that of the second basecoat coating agent. For example, according to a preferred procedure, the first basecoat coating composition can be produced from the second basecoat coating composition by admixing a corresponding amount of polyurethane resin.

If the second water-based lacquer contains polyurethane resin, the higher proportion of polyurethane resin can be determined by the same or different polyurethane resins. Examples of polyurethane resins that can make up the higher proportion in the first water-based lacquer are the polyurethane resins described above as being suitable as binders for water-based lacquers. In addition to the distinguishing feature of the invention which is higher than that of the second water-based lacquer in the first water-based lacquer, the proportion of polyurethane resin used for the second and third coating layers produced by the process according to the invention can also be different from other water-based lacquers, but preferably similar water-based lacquers. Particularly preferably, the two water-based paints differ only in the above-mentioned distinguishing feature essential to the invention. This is explained in more detail below.

It is particularly preferred if the first and the second water-based lacquer, apart from the excess polyurethane of the first water-based lacquer, have the same solid resin composition, ie the same binders and optionally crosslinking agents and optionally paste resins are present. It is preferred if, taking into account the specifications according to the invention, the quantity of the respective components of the solid resin composition, apart from the excess polyurethane, only have a fluctuation range of less than 30%, particularly preferably less than 20% and particularly preferably less than 15%. In particular, it is preferred if the two waterborne basecoats do not differ in terms of their qualitative or quantitative solid resin composition, with the exception of the distinguishing criterion essential to the invention. In addition, it is preferred if the waterborne basecoat used to produce the second coating layer has a color tone which is close to or particularly preferably identical to that of the waterborne basecoat used to produce the third coating layer. In the context of the present invention, it is preferable to understand the closest color shades in the context of the present invention that the color difference, composed of the difference in brightness, hue and difference in color, between the hues of the second and third coating layers determined in each case with opaque coating and a measurement geometry of (45/0) -fold ΔE * (CIELAB) - value does not exceed. The ΔE * (CIELAB) reference value is the value which results in the CIE-x, y diagram (chromaticity diagram) familiar to the person skilled in the art based on DIN 6175 for the color of the third coating layer, and the following relationship applies:

n <90 in the area of the CIE-x, y diagram marked with ΔE * = 0.3, n <50 in the area of the CIE-x, y diagram marked with ΔE * = 0.5, n <40 in the area with ΔE * = 0.7 marked area of the CIE-xy diagram, n <30 in the area marked with ΔE * = 0.9 of the CIE-x, y diagram.

In principle, all customary clear lacquers or transparent colored or colorless pigmented coating compositions are suitable as clear coating compositions for the production of the fourth and possibly further coating layers. These can be single-component or multi-component clear lacquer coating compositions. They can be solvent-free (liquid or as a powder clearcoat), or they can be systems based on solvents, or they can be water-dilutable clearcoats, the binder systems of which are suitable, e.g. anionic, cationic or non-ionic, are stabilized. The water-dilutable clear lacquer systems can be water-soluble or water-dispersed systems, for example emulsion systems or powder slurry systems. The clear lacquer coating agents harden when stoved to form covalent bonds as a result of chemical crosslinking.

The clearcoats which can be used in the process according to the invention are the customary clearcoat coating compositions which are known to the person skilled in the art and which contain one or more customary base resins as film-forming binders. If the base resins are not self-crosslinking, they may also contain crosslinking agents. Both the base resin component and the crosslinker component are not subject to any limitation. Polyester, polyurethane and / or (meth) acrylic copolymer resins, for example, can be used as film-forming binders (base resins).

In addition to the chemically crosslinking binders and, if appropriate, crosslinking agents, the clear lacquers which can be used in the process according to the invention can contain conventional lacquer additives, such as, for example, catalysts, leveling agents, dyes, but especially rheology control agents, such as microgels, NAD (= non-aqueous dispersions), disubstituted ureas ("sagging control agents "), and light stabilizers, if appropriate in combination with antioxidants.

Specific examples of one (1K) and two-component (2K) non-aqueous clearcoat systems which can be used as clearcoat in the process according to the invention can be found in DE-A-3826693, DE-A-40 17075, DE-A-41 24 167, DE-A-41 33704, DE-A-4204518, DE-A-4204 611, EP-A-0257 513, EP-A-0408858, EP-A-0523267, EP-A-0557822, WO-92 11 327.

Specific examples of one-component (1-component) or two-component (two-component) water-clear lacquer systems which can be used as clear lacquer in the process according to the invention can be found in DE-A-39 10829, DE-A-4009931, DE-A-4009932, DE-A -41 01 696, DE-A-41 32 430, DE-A-41 34 290, DE-A-42 03 510, EP-A-0365098, EP-A-0365775, EP-A-0496079, EP-A -0546640.

Specific examples of powder clearcoat systems which can be used as clearcoat in the process according to the invention can be found in EP-A-0 509392, EP-A-0509393, EP-A-0 522648, EP-A-0 544 206, EP-A- 0555 705, DE-A-42 22 194, DE-A-42 27 580.

The transparent coating can be applied in a single layer or in the form of several layers from the same or from several different transparent coating agents. However, the transparent coating layer is expediently applied as a fourth layer comprising only one clear lacquer coating agent.

Electrically conductive materials such as metals are suitable as the substrate for the method according to the invention. Particularly suitable are e.g. Automobile bodies or parts thereof, they can consist of pretreated or untreated metal, of metal provided with an electrically conductive layer or of electrically conductive or plastic provided with an electrically conductive layer. The first coating layer, in particular in the form of a

Corrosion protection primer, electrophoretically in the usual way and in a dry layer thickness of usual for ETL primers for example, 15 to 35 microns deposited and baked. In the special case of precoated substrates, multi-layer coatings are obtained with more than the four coating layers produced according to the invention, but within the scope of the present invention the electrocoat layer applied first in the method according to the invention is referred to as the first coating layer.

The second coating layer composed of the first coloring and / or effect-imparting water-based lacquer is preferably applied to the substrate provided with the baked ETL layer in one spray pass in one of the

Coverage of the color tone-dependent dry layer thickness of 10 to less than 30 μm, for example 10 to 29 μm, preferably 10 to 25 μm, particularly preferably 10 to 20 μm. Examples of suitable spray application methods are compressed air spraying, airless spraying or electrostatic (ESTA) high-rotation spraying, electrostatic spraying being the preferred application method for applying the first water-based lacquer. After a short flash-off time, if necessary at elevated temperatures of up to 80 ° C., but preferably at ambient temperature, the third coating layer becomes a color and / or effect-giving layer

Waterborne basecoat in one spray pass, preferably by compressed air spraying, preferably in a smaller dry layer thickness than the second coating layer, applied from only 7 to 15 μm and briefly vented, if appropriate at temperatures up to 80 ^° C., for example for 1 to 5 minutes, for example by the action of infrared radiation. After the application and the flashing off of the third coating layer, the clear lacquer is applied in a wet-on-wet process.

The fourth coating layer is overpainted from a customary liquid clear lacquer or powder clear lacquer (in this case it is a dry-in-wet application) in a dry layer thickness customary for clear lacquer layers of, for example, 30 to 80 μm, preferably 30 to 60 μm, and together baked with the second and third coating layers. For example, the stoving temperature is when the three are stoved together

Coating layers between 80 and 160 ^* C, preferably below 140 ^° C. If necessary, further clear lacquer layers of the same or of which various clear lacquer coating agents are applied before or after baking.

The method according to the invention permits the production of four- or multi-layer coatings, in particular automotive coatings with a reduced overall layer thickness and comparable overall property level compared to the prior art, which includes filler layers and / or other intermediate layers. Only two burn-in steps are necessary. It has been shown that excellent properties are achieved by the procedure according to the invention, although this makes it possible to dispense with conventional filler layers.

Example 1 (state of the art, painting with filler layer)

A water-based paint (blue metallic) with a dry layer thickness of 15 μm is applied by spraying to a customary phosphated body panel pre-coated by cathodic dip painting and with a 30 μm thick filler layer. After application, the product is briefly flashed off and then predried at 80 ° C. for 10 minutes. It is then overpainted with a commercially available 2K-HS automotive series clear coat (two-component high-solid clear coat based on acrylic resin / polyisocyanate) in a dry layer thickness of 40 μm and dried for 30 minutes at 130 ^° C (object temperature). The water-based paint (blue metallic) consisted of the following components:

17.00 parts of aqueous polymer dispersion from DE-A-3628124, preparation example 1

9.20 parts of aqueous polyurethane dispersion from DE-A-42 24617, preparation example 2

3.10 parts of aqueous polyurethane resin according to the example from DE-A-4000889

2.70 parts hexamethoxymethylmelamine

1.00 parts aluminum bronze

1.60 parts Cu phthalocyanine pigment 0.90 parts talc

0.50 parts of standard paint defoamer

0.50 parts of customary wetting agent 7.00 parts butoxyethanol

2.50 parts of 2-propanol

2.50 parts of 2-butanol

51.50 parts deionized water

The solid resin content (absolute value of the weight content of solid resin) of this water-based lacquer is 13.8% by weight.

Example 2 (comparison, painting without filler layer)

The waterborne basecoat (blue metallic) from Example 1 is applied to a customary phosphated and pre-coated bodywork sheet by cathodic dip coating in a dry layer thickness of 15 μm. After application, the product is flashed off briefly and then a second layer of the same waterborne basecoat is applied in a dry layer thickness of 10 μm, likewise by spraying. It is predried at 80 ^° C. for 10 minutes. Then the 2K-HS automotive series clearcoat from Example 1 is overpainted in a dry layer thickness of 40 μm and dried for 30 minutes at 130 ° C (object temperature).

Example 3 a-c (according to the invention)

Example 2 is repeated with the difference that the 15 μm thick lacquer layer applied to the KTL layer is not applied from the water-based lacquer (blue-metallic) from example 1, but from a modified water-based lacquer (blue-metallic), while the subsequently in 10 μm dry layer thickness applied lacquer layer as in example 2 is produced from the water-based lacquer (blue-metallic) from example 1. The modified waterborne basecoat (blue metallic) is produced by mixing 87 parts of the waterborne basecoat (blue metallic) from Example 1 with 13 parts of an aqueous polyurethane resin adjusted to a solids content of 34% by weight. The following are used as aqueous polyurethane resins:

a) aqueous polyurethane dispersion from DE-A-42 24617, preparation example 2 b) aqueous polyurethane resin according to the example from DE-A-4000889 c) aqueous polyurethane dispersion from WO 95/28 249, preparation example 1

The solid resin content (absolute value of the weight content of solid resin) of the water-based paints 3a), 3b) and 3c) is in each case 16.4% by weight. There is therefore an excess of (16.4-13.8) × 100 / 13.8 = 18.9% by weight of solid resin and thus also polyurethane resin, in comparison with the water-based lacquer from Example 1. Example 4 (state of the art, painting with filler layer)

The waterborne basecoat (black) described below is applied to a customary phosphated and pre-coated by cathodic dip painting and pre-coated with a 30 μm thick filler layer in a dry layer thickness of 20 μm. After application, the product is briefly flashed off and then predried for 10 minutes at βo'c.

Subsequently, the 2K-HS automotive series clearcoat from Example 1 is overpainted in a dry layer thickness of 40 μm and dried for 30 minutes at 130'C (object temperature). The water-based paint (black) consisted of the following components:

11.00 parts of the 60% by weight binder from WO 95/16004, point 2, example 4

19.90 parts of aqueous polyurethane dispersion from DE-A-42 24617,

Production Example 2

12.90 parts of aqueous polyurethane resin according to the example from DE-A-4000889

3.80 parts hexamethoxymethylmelamine 3.00 parts carbon black

2.00 parts of talc

0.70 parts of standard defoamer

1.20 parts of customary wetting agent

0.70 parts of adhesion promoter 5.70 parts of butoxyethanol

4.50 parts of diethylene glycol monobutyl ether

3.00 parts of N-methylpyrroldione 0.30 parts of dimethylethanolamine 31.30 parts of deionized water

This waterborne basecoat has a solid resin content of 21.9% by weight. Example 5 (comparison, painting without filler layer)

The water-based paint (black) from Example 4 is applied to a customary phosphated and pre-coated bodywork sheet by cathodic dip coating in a dry layer thickness of 15 μm. After application, the product is briefly flashed off and then a second layer of the same waterborne basecoat is applied in a dry layer thickness of 10 μm, likewise by spraying. It is predried at 80 ° C. for 10 minutes. Subsequently, the 2K-HS automotive series clearcoat from Example 1 is overpainted in a dry layer thickness of 40 μm and dried for 30 minutes at 13 ° C. (object temperature).

Example 6 a-c according to the invention

Example 5 is repeated with the difference that the 15 μm thick lacquer layer applied to the KTL layer is not applied from the water-based lacquer (black) from example 4, but from a modified water-based lacquer (black), while the one subsequently applied in a 10 μm dry layer thickness Lacquer layer as in Example 5 is produced from the water-based lacquer (black) from Example 4. The modified waterborne basecoat (black) is prepared by mixing 85 parts of the waterborne basecoat (black) from Example 4 with 15 parts of an aqueous polyurethane resin adjusted to a solids content of 34% by weight. The following are used as aqueous polyurethane resins:

a) aqueous polyurethane dispersion from DE-A-42 24617, preparation example 2 b) aqueous polyurethane resin according to example from DE-A-4000889 c) aqueous polyurethane dispersion from WO 95/28249,

Production Example 1 The water-based paints 6a), 6b) and 6c) each have a solid resin content of 23.7% by weight. The excess of solid resin and in the present case also of polyurethane resin for the water-based lacquer of Example 4 is therefore (23.7-21.9) x 100 / 21.9 = 8.3% by weight.

Both the multi-layer coating from Examples 1, 2 3a-c and from Examples 4, 5, 6a-c do not differ in either their adhesion or their resistance to condensation, the results today correspond to the usual technological requirements.

Table 1 summarizes the results of the stone chip tests carried out.

Table 1

Multi-layer painting

1 2 3a 3b 3c 4 5 6a 6b 6c

VDA rockfall * '1 3 1 1 ./. 1 3 1 1 • / •

Rockfall after 7 12 3 5 6 5 10 4 4 4 "Split method" ²⁵ 0- 1 4-5 0-1 0-1 1 0-1 4-5 0-1 0-1 1

^x> Testing with a stone chip tester according to VDA (Erichsen, model 508, at + 20 ° C with 2 x 500 g steel shot (angular, 4-5 mm), accelerated by compressed air of 1 bar. Evaluation: characteristic value 0 = best Value, characteristic value 10 = worst value

²⁾ Test using a stone chip simulation device in accordance with "Paint and varnish", 8/1984, pages 646 - 653, test temperature: -20 ° C, test specimen: sphere with a mass of 0.15 g and a diameter of 2 mm, impact angle: 5 degrees , Impact speed: 250 km / h.

./• - not examined.

Assessment: Specification of the circular damage area in mm * and the degree of rust determined on the damaged area by exposure to 1% copper sulfate solution for 10 minutes, degree of rust 0 = best value, degree of rust 5 = worst value.

Claims

Claims:

1. A process for the production of a multi-layer coating on a substrate provided with a baked-on first electrocoat layer, characterized in that on the first electrocoat layer a second coating layer corresponding to a dry layer thickness of 10 to less than 30 μm from a first, aqueous coloring and / or effect basecoat coating agent, which contains one or more polyurethane resins, is applied, whereupon wet-on-wet, a third coating layer corresponding to a lower dry layer thickness of 7 to 15 μm is applied from a second aqueous coloring and / or effect-imparting basecoat coating agent and a fourth is applied to the third coating layer without prior baking Coating layer applied from a clear lacquer coating agent and the second, third and fourth coating layer are baked together, using a first aqueous basecoat coating agent which has a higher content of polyurethane resin a ls the second basecoat coating composition, based on the total weight of the respective aqueous basecoat coating composition, and the solid resin content in the first basecoat coating composition does not differ by more than 20% from the absolute value of the weight content of solid resin in the second basecoat coating composition.

2. The method according to claim 1, characterized in that the excess of polyurethane resin in the first aqueous basecoat coating agent 5 to 50% of the absolute value of

Weight content of solid resin in the second basecoat is.

3. The method according to claim 1 or 2, characterized in that the first basecoat coating composition has a pigment volume concentration which differs by no more than 30% from the absolute value of the pigment / volume concentration of the second basecoat coating composition.

4. The method according to any one of claims 1 to 3, characterized in that the first and the second aqueous basecoat coating compositions contain the same or different polyurethane resins.

5. The method according to any one of claims 1 to 3, characterized in that the second aqueous basecoat coating agent is free of polyurethane resins.

6. The method according to any one of claims 1 to 5, characterized in that, apart from the excess polyurethane resin content of the second basecoat coating composition, the resins which form the solid resin content of the first and second basecoat coating composition have the same qualitative and quantitative composition.

7. The method according to any one of claims 1 to 6, characterized in that the first aqueous basecoat was prepared by admixing polyurethane resin to the second basecoat.

8. The method according to any one of claims 1 to 7, characterized in that the same pigmentations are present in both basecoat coating compositions or that the pigmentations of the two basecoat coating compositions have similar specific weights on average.

9. The method according to any one of claims 1 to 8, characterized in that the pigment content based on the solids content in the second basecoat is higher than in the first basecoat.

10. The method according to any one of claims 1 to 9, characterized in that it is carried out for painting motor vehicles or their parts.