WO2009121455A2

WO2009121455A2 - Method for producing finish painted deformable sheet metal and sheet metal painted according to the method

Info

Publication number: WO2009121455A2
Application number: PCT/EP2009/001476
Authority: WO
Inventors: Alexandra Huber; Lothar Jandel; Dorothe Koberg; Bernhard Strauss
Original assignee: Voestalpine Stahl Gmbh; Basf Coatings Ag
Priority date: 2008-04-04
Filing date: 2009-03-02
Publication date: 2009-10-08
Also published as: DE102008017146B3; WO2009121455A3; EP2257389A2; EP2257389B1

Abstract

The invention relates to a method for producing painted deformable sheet metal, wherein a metal sheet is coated with a multilayer paint coating, wherein the paint coating comprises at least two outer clear paint coats disposed over each other, wherein the outer clear coat paint has a higher softening point in the cross-linked state than the intermediate clear coat paint thereunder, and the two top clear coat paints are adjusted with regard to the ratio of strength and elasticity so that a ratio of the tan δ_max-values of the top clear coat paint to the intermediate clear coat paint of ≤ 0.8 is maintained for sheet metal having high demands with regard to foldability, and a ratio of the tan δ_max-values of the top clear coat paint to the intermediate clear coat paint of ≥ 0.6 is maintained for sheet metal having high demands with regard to deep drawing capability.

Description

Process for the production of finished lacquered formable sheets and sheet metal painted by the process

The invention relates to a method for the manufacture of ready-coated formable sheets for the automotive sector and for domestic appliance and construction applications and the like.

Painting is a very investment-intensive and cost-intensive process. In the automotive production z. B. is carried out in a conventional manner, the painting after the body is assembled, usually several layers are applied and in particular after phosphating first a corrosion protection primer (KTL) is applied, then a single or multi-layer, color-giving paint with a final clearcoat. Even with household appliances sheet metal parts are painted in many cases only after forming and optionally joining.

Conventional painting lines are also problematic from the environmental point of view. The application efficiency of spray painting as well as volatile organic content (VOC) emissions in general are technological weak points which will require further significant improvement in the future as well. fen. Although more and more waterborne paints are being used, organic dilutions can not be completely eliminated from this range.

In addition, so-called tape coating or coil coating processes are known. These are processes in which paint is applied by roller application with an efficiency of nearly 100% on metal strips, the penetration takes place in largely closed systems with thermal utilization of the solvents used. VOC emissions are kept at very low levels with this technology.

The prerequisite for the use of the so-called coil coating process or the resulting painted metal strip is that special highly flexible coating systems are used, which join all forming steps crack-free and without surface modification.

Especially in applications requiring a scratch and chemical resistant glossy clearcoat surface, such as e.g. Automotive paint finishes, but also in other decorative applications in the construction and household appliance sector, these methods are not always applicable because the quality of the reshaped paint surface often does not meet the high requirements.

One major reason for this is the fact that pre-painted material after forming shows significant surface changes as a result of microscopic deformation processes in the metal substrate and the organic coating. The deformation processes in the polymeric coating are due, inter alia, to the formation of flow lines, which are partially reversible, but cause a gloss drop of the coating in reshaped areas. In order to counteract these effects, attempts have been made to further develop the paints, although all known paint systems still have a measurable loss of gloss depending on the degree of deformation, even if this could be reduced. However, the areas with the reduced loss of gloss by changing the paint systems also have a surface that does not meet the requirements of an automobile outer skin. In particular, it has been found that all previously known paint structures after stretching, in addition to gloss, also show a strong susceptibility to cracking as a result of heat treatment.

DE 100 47 982 A1 discloses a multicoat color and / or effect paint system with at least two superimposed clearcoats of different hardness and flexibility, this paint coating serving as a rockfall protection layer. Here, a flexible powder clearcoat or a flexible powder slurry clearcoat is present, to which a second hard scratch-resistant powder clearcoat and / or powder slurry clearcoat is applied.

EP 1 348 001 B1 discloses a clearcoat film which is part of a multicoat system comprising a top clearcoat layer which is suitable for the production of precoated metal strips, from which parts for automobiles can be produced by appropriate shaping processes (deep drawing). The intermediate clear coat layer and the top clearcoat layer differ in that the intermediate coat layer has a higher flexibility than the topcoat layer.

The object of the invention is to provide a method for producing finished pre-painted formable sheets with a decorative and protective clearcoat surface, which also after forming painted coated sheets gives a high surface quality.

The object is achieved by a method having the features of claim 1. Advantageous developments are characterized in the subclaims.

According to the invention, a paint system is formed in a multi-layered manner with at least three layers, the outer two layers being formed by a two-layer clearcoat. Preferably, the two-layer clearcoat builds on a fluorine-free binder chemistry, such as. B. PUR-crosslinked acrylate and polyester clearcoats.

Possible compositions of the clearcoats and their components based on acrylates and polyesters are described in DE 103 35 491 A1.

For the acrylate clearcoat materials used according to the invention, it was possible to use methacrylate copolymers for which acrylic acid, butanediol monoacrylate, butyl acrylate, butyl methacrylate, cyclohexyl methacrylate, hydroxyethyl methylacrylate, hydroxypropyl methacrylate styrene are suitable as building blocks. The preferred choice of these building blocks and their combination for the particular methacrylate copolymer depends on whether the clearcoats should be more flexible or rather hard.

Typical inventive layer thicknesses for the two-layer clearcoat have, for the first clearcoat layer, ie the clearcoat layer, which is applied to a coloring lacquer layer (intermediate clear coat) 3 to 30 μm and preferably 5 to 20 μm. The second clearcoat (covercoat) which is the outwardly facing clearcoat sits a thickness of 5 to 30 microns and preferably 10 to 25 microns.

According to the invention, the thermomechanical properties of the two clearcoat films are adjusted to one another in the crosslinked state, wherein the softening points and the tan δ _max values are set according to the invention. The thermomechanical properties are determined on free paint films by means of dynamic mechanical analysis (DMA) at 1 Hz and a heating rate of 2 K / min. The Tg values are determined according to the "Practice of Thermal Analysis of Plastics", 2nd edition, Hanser Verlag, Munich 2003 from the temperature situation at the loss modulus Maximum tan δ _{max of} the DMA measurements.

The top clear coat have a Tg ≥ 80 ⁰ C and the intermediate clearcoat in the crosslinked state in the crosslinked state has a Tg <60 ⁰ C.

In addition, the two cover clearcoats are adjusted according to the invention in terms of their ratio of strength and elasticity so that they meet different stress profiles.

For components with a high stress with regard to the foldability, a ratio of the tan δ _max values of the topcoat clearcoat to the intermediate clearcoat finish ≤ 0.8 is maintained.

According to the invention, a ratio of the tan δ _max values of topcoat lacquer to intermediate clearcoat lacquer ≥ 0.6 and preferably ≥ 0.9 is maintained for components with a high degree of stress in terms of deep drawability.

By the invention painted panels can be achieved, which are characterized in that the formed parts also after a heat treatment step at temperatures> Tg remain free of cracks (eg 15 minutes at 120 ⁰ C.), and that the paint system that is optimized for the folding, deformation strains of <2 T in accordance with DIN EN 13523-7: 2001 and the thermoformable optimized structure Dehngrade of> 20% crack-free and thermally stable can be produced.

According painted sheets according to the invention are characterized in that the surface changes that are induced by deformation and which lead to an impairment of gloss and sharpness of image can be eliminated by the invention following processing steps.

According to the invention, the production of a defined surface structure with respect to gloss and waviness at forming degrees <2%, preferably by heat treatment at temperatures> Tg. At forming degrees> 2%, the production of the defined surface structure is preferably carried out by a combination of local mechanical processing, in particular polishing and heat treatment. The need for mechanical processing increases with increasing degree of deformation and is additionally dependent on the component geometry.

Grinding and / or polishing processes can be combined with each other, whereby this targeted removal of a previous transformation is taken into account.

The paint system can consist of a primer layer, a color-giving basecoat and the two final clearcoat films which are applied in the coil coating process.

Primer and base coat are modified commercial coil coating paints. Preferably, the primer has a thickness of 10 to 25 microns and the basecoat applied thereon also has a thickness of 10 to 25 microns.

The total paint layer thickness is about 60 to 80 microns and thus differs greatly from conventional automotive coatings, which have a typical thickness of 90 to 130 microns. In contrast to conventional automotive coatings, the layer structure here provides a two-layer clearcoat, while in conventional automotive finishes the clearcoat is single-layered.

According to the invention, the thermomechanical and mechanical properties of the coating can be adjusted via the chemistry or curing reaction of the two-layer clearcoat.

In addition, after being fully cured and formed, such a varnish may be processed with one or more sanding or polishing steps, the upper topcoat serving as a wear layer. With the grinding or polishing step, the roughness or waviness of the lacquer is locally changed so that the specified characteristic values - which usually correspond to those on the flat metal sheet - are also achieved in shaped areas

It should be noted that it must of course be known in advance in this inventive method, how high the degrees of deformation in individual areas of the sheet metal strip, so that targeted and depending on the degree of deformation, the corresponding mechanical processing of the paint can take place. It is sufficient to use empirically determined data in computer models for simulation purposes and to perform the mechanical processing in the corresponding intensity in the corresponding areas on the basis of the simulation. A conventional automotive paint has a corrosion protection primer (KTL), an overlying filler, then a single-coat coloring paint and a single-coat clearcoat. The thickness is typically 90 to 130 μm. Such paints are usually applied by dip coating (KTL) and spray painting by painting robots.

The paint system according to the invention has a significantly lower layer thickness of 50 to 90 microns compared to conventional paint structures and is usually constructed four-layered. The first layer facing a sheet substrate is a conventional primer with a thickness of 5 to 25 microns, on which a coloring paint is applied with 10 to 25 microns.

According to the invention, a two-layer clearcoat is applied in which the first clearcoat layer has a thickness of from 3 to 30 .mu.m and preferably from 5 to 20 .mu.m and the outercoat layer has a thickness of from 5 to 30 .mu.m and preferably from 15 to 25 .mu.m.

The two clearcoat films differ in their mechanical properties.

Decisive for the invention are the coordinated thermomechanical properties of the two clearcoats in the crosslinked state. These thermomechanical properties are firstly characterized by the Tg values, ie the softening values at temperature. According to the invention, these values are different, wherein the intermediate clearcoat in the crosslinked state has a Tg <60 ⁰ C and the cover clearcoat in the crosslinked state has a Tg ≥ 80 ⁰ C. In addition, according to the invention, the two cover clearcoats are adjusted in terms of their ratio of strength and elasticity such that for components with high stress with regard to the foldability (ie a deformation in which the sheet is strongly bent along one edge) a ratio of the tan δ _max values ( Deckklarlack to intermediate clearcoat) of ≤ 0.8.

For components with a high stress in terms of deep drawability, a ratio of the tan δ _max values (cover clearcoat to intermediate clearcoat) of ≥ 0.6 is maintained. Particularly preferred for components with high stress in terms of thermoformability is a ratio of tan δ _max values of> 0.9.

In particular, it follows that the outer varnish is designed to be thicker and, in particular, harder as a mechanical processing reserve than the underlying clear varnish.

The inventive setting of the thermo-mechanical properties of the two upper lacquer layers to each other, in particular the setting of the Tg values and the adjustment of tan δ _max values and optionally providing a polishing reserve, a defined surface structure with respect to the gloss and waviness of the paint after Reshaping takes place by carrying out a heat treatment at temperatures> Tg at degrees of deformation <2%.

With degrees of deformation of more than 2%, it is possible to produce a defined surface structure of the paint with regard to gloss and waviness by polishing and heat treatment. Preferably, at least the outer coating is such that possibly present polishing marks can be even further compensated by a final optional heat treatment. Here, the thermal stability of such a paint is particularly advantageous.

It is also advantageous if the outer clearcoat is applied at a residual thickness of 10 to 25 microns with a thickness that ensures a corresponding polishing reserve.

In the invention, it is advantageous that a method is provided which allows the surface to be conditioned with a special paint system in such a way that, after the forming or, if appropriate, only one heat treatment step, a paint surface is obtained which also in the formed regions high quality requirements for a painted surface are sufficient.

The advantage here is that a significant cost reduction can be achieved because of the manufacturers on separate paint particularly advantageous.

In the invention, it is advantageous that a method is provided which allows the surface to be conditioned with a special paint system in such a way that, after the forming or, if appropriate, only one heat treatment step, a paint surface is obtained which also in the formed regions high quality requirements for a painted surface are sufficient. It is advantageous that a significant cost reduction can be achieved, since the manufacturers can be dispensed with separate Lackierstrassen. In addition, the constancy of the coating quality can be improved, as in the coil coating process on flat sheets easier and more accurate control of the painting process is possible as in the spray coating on the finished formed component. In particular, all sheet metal parts are uniformly and uniformly covered with varnish. In addition, this also ensures that optimum protection of the steel substrate is achieved in flange areas.

Claims

claims

Anspruch [en] A process for producing varnished, deformable sheets, wherein a metal sheet is coated with a multilayer varnish layer, wherein the varnish layer comprises at least two outer, clear varnish layers arranged on one another, wherein the outer clear varnish in the crosslinked state has a higher softening point than the intermediate varnish underneath and the two cover clearcoats are adjusted in terms of their ratio of strength and elasticity so that for sheets with high stress in terms of folding a ratio of tan δ _max values of topcoat to intermediate clearcoat ≤ 0.8 is maintained and for sheets with high stress in terms of deep drawability a ratio of tan δ _ma χ values of cover clearcoat to intermediate clearcoat of ≥ 0.6 is maintained.

2. The method according to claim 1, characterized in that in sheets with very high stress in terms of drawability a ratio of tan δ _ma χ values of ≥ 0.9 is maintained.

3. The method according to any one of the preceding claims, characterized in that the cover clearcoat is set in the crosslinked state to a Tg value ≥ 80 ⁰ C.

4. The method according to any one of the preceding claims, characterized in that the intermediate clear coat in the crosslinked state to a Tg value ≤ 60 ⁰ C is set.

5. The method according to any one of the preceding claims, characterized in that lacquers are used with a fluorine-free binder chemistry as clear coats.

6. The method according to claim 5, characterized in that PUR-crosslinked acrylate and polyester clearcoats are used.

7. The method according to any one of the preceding claims, characterized in that the intermediate clearcoat is used with layer thicknesses of 3 to 30 microns and preferably 5 to 20 microns.

8. The method according to any one of the preceding claims, characterized in that the cover clearcoat is used with layer thicknesses of 5 to 30 microns and preferably 10 to 25 microns.

9. The method according to any one of the preceding claims, characterized in that the coatings are applied in the coil coating process.

10. The method according to any one of the preceding claims, characterized in that for the production of a defined surface structure with respect to the gloss and the waviness after forming at degrees of deformation <2%, a heat treatment at temperatures> Tg is performed.

11. The method according to any one of the preceding claims, characterized in that the production of a defined surface structure with respect to the gloss and waviness after forming at forming degrees> 2% by a grinding step and / or polishing step and / or a heat treatment are performed.

12. The method according to any one of the preceding claims, characterized in that a total layer thickness of the overall paint buildup of at least 40 microns and at most 90 microns is formed.

13. The method according to claim 11, characterized in that in the polishing step, only the uppermost lacquer layer is polished or in a grinding step and polishing step, only the uppermost lacquer layer is ground and polished, wherein provided in the areas to be machined, the uppermost lacquer layer with a thick addition is that the removal is taken into account by the mechanical processing.

14. A coated deformable sheet metal for use in the automotive industry, the home appliance production, characterized in that the deformable sheet has a multilayer paint system with two outer clearcoat films, which is achieved by a method according to any one of the preceding claims.