MULTI-LAYER PAINT STRUCTURE WITH IMPROVED LAYER ADHESION

Abstract

The present invention relates to a layer composite comprising a lower base paint layer and a cover layer arranged thereabove on a substrate, wherein the base paint is greater than or equal to 50 wt. % and less than 100 wt. % of polymers selected from the group consisting of polyacrylates, polyurethanes, polyether polyols, polycarbonate polyols, polyester polyols, melamine resins, alkyd resins or mixtures thereof; the cover layer is greater than or equal to 40 wt. % and less than or equal to 100 wt. % of prepolymers containing silane groups and/or the crosslinking products thereof; and the base paint is greater than or equal to 0.5 wt. % and less than or equal to 15 wt. % of prepolymers containing silane groups and/or the crosslinking products thereof, and wherein the prepolymers containing silane groups and/or the crosslinking products thereof have at least one thiourethane and/or urethane unit in the molecule. The invention further relates to a method for producing a claimed layer composite and to a vehicle or a vehicle body part having such a layer composite.

Claims

1. A composite coating comprising a lower basecoat arranged over a topcoat on a substrate, wherein the basecoat material comprises greater than or equal to 50 wt. % and less than 100 wt. % of polymers selected from the group consisting of polyacrylates, polyurethanes, polyether polyols, polycarbonate polyols, polyester polyols, melamine resins, alkyd resins, and mixtures thereof; the topcoat comprises greater than or equal to 40 wt. % and less than or equal to 100 wt. % of a compound selected from the group consisting of silane group-containing prepolymers and/or crosslinking products thereof; and the basecoat material comprises greater than or equal to 0.5 wt. % and less than or equal to 15 wt. % of a compound selected from the group consisting of silane group-containing prepolymers and/or crosslinking products thereof; and wherein the silane group-containing prepolymers and crosslinking products thereof have at least one of a thiourethane unit and a urethane unit in the molecule.

2. The composite coating as claimed in claim 1, wherein the silane group-containing prepolymers or their crosslinking products are selected from the group consisting of polyurethanes, polymeric polyisocyanates, reaction products of polymeric polyols with silane-containing compounds, polyether polyols, polyester polyols, polycarbonate polyols, polyacrylate polyols, polymethacrylate polyols, polyurea, polyurethane polyols, and mixtures thereof, and wherein individual prepolymers of this group each have at least one alkoxysilane group.

3. The composite coating as claimed in claim 1, wherein the silane group-containing prepolymers or crosslinking products thereof are selected from the group consisting of low molecular alcohols, polyacrylate polyols, polyester polyols, polysiloxane polyols, and mixtures thereof.

4. The composite coating as claimed claim 1, wherein the topcoat comprises a catalyst selected from the group consisting of phosphoric acid, dibutyl phosphate, bis(ethylhexyl phosphate), diphenyl phosphate, dimethyl phosphate, methyl phosphate, trimethyl phosphate, phenylphosphonic acid, phenylphosphinic acid, and mixtures thereof in a concentration of greater than or equal to 0.25 wt. % and less than or equal to 2.5 wt. %, based on the weight of the topcoat.

5. The composite coating as claimed in claim 1, wherein the upper topcoat at a layer thickness of 50 μm on a white basecoat has a delta-Lab value in relation to white basecoat of ΔL greater than or equal to 0.2 and less than or equal to 20, of Δa greater than or equal to −0.01 and less than or equal to −20, and Δb greater than or equal to −0.01 and less than or equal to −13, determined in accordance with DIN EN ISO 1166-4:2012-06.

6. The composite coating as claimed in claim 1, wherein the composite coating has a pendulum hardness, measured to DIN EN ISO 1522:2000-09, of greater than or equal to 60 s and less than or equal to 180 s.

7. The composite coating as claimed in claim 1, wherein the basecoat has a silicon fraction of greater than or equal to 1 wt. % and less than or equal to 10 wt. %.

8. A method for producing an at least 2-coat paint system comprising a lower basecoat and arranged an upper topcoat on a substrate, wherein the method comprises at least the following steps: a) applying a basecoat comprising polymers selected from the group consisting of polyacrylates, polyurethanes, polyether polyols, polycarbonate polyols, polyester polyols, melamine resins, alkyd resins, and mixtures thereof on a substrate; b) at least partly curing the basecoat; c) applying a topcoat material to the basecoat at least partly cured in step b), the topcoat material comprising, as structuring component, at least one of silane group-containing prepolymers and crosslinking products thereof, the cured topcoat material having an Si content of greater than or equal to 2.0 and less than or equal to 9.0 wt. % and a catalyst content of greater than or equal to 0.01 wt. % and less than or equal to 5 wt. %, and the catalyst being selected from the group consisting of protic acids, Lewis acids, and mixtures thereof; and d) at least partly curing the topcoat.

9. The method as claimed in claim 8, wherein the silane group-containing prepolymers or the crosslinking products thereof have at least one of a thiourethane unit and/or a urethane unit in the molecule.

10. The method as claimed in claim 8, wherein the basecoat material is at least partly dried at a temperature of greater than or equal to 10° C. and less than or equal to 80° C.

11. The method as claimed in claim 8, wherein the catalyst of the topcoat has a pKa of greater than or equal to −14.0 and less than or equal to 6.

12. The method as claimed in claim 8, wherein the silane group-containing prepolymers have a number-average molecular weight, measured according to DIN EN ISO 55672-1:2016-03, of greater than or equal to 250 g/mol and less than or equal to 40 000 g/mol.

13. The method as claimed in claim 8, wherein the at least partial curing in step d) takes place in a temperature range of greater than or equal to 10° C. and less than or equal to 90° C.

14. A method of bonding, sealing or coating a substrate, wherein the method includes the composite coating as claimed in claim 1.

15. A vehicle or vehicle bodywork part having a composite coating as claimed in claim 1.

Description

EXAMPLES

[0200] All reported percentages are based on weight unless otherwise stated.

[0201] All experiments were conducted unless otherwise indicated at 23° C. and 50% relative humidity. NCO contents were determined titrimetrically in accordance with DIN EN ISO 11909:2007-05.

[0202] OH numbers were determined by titrimetry according to DIN 53240-2: 2007-11, acid numbers according to DIN EN ISO 2114:2002-06. The OH contents reported were calculated from the OH numbers determined by analysis. The reported values in each case relate to the total weight of the respective composition including any solvent also used.

[0203] The residual monomer contents were measured according to DIN EN ISO 10283:2007-11 by gas chromatography with an internal standard.

[0204] The solids content was determined according to DIN EN ISO 3251:2008-06.

[0205] The viscosity was ascertained at 23° C. according to DIN EN ISO 3219/A:1994-10.

[0206] König pendulum damping was determined in accordance with DIN EN ISO 1522:2007-04 on glass plates. The STP films described were drawn down onto the glass plates using a bar coater. The dry film thickness was 35-40 μm for all films.

[0207] Solvent and water resistances were ascertained to DIN EN ISO 4628-1:2016-07. The solvent resistances test was carried out using the solvents xylene (also abbreviated hereinafter to “Xy”), methoxypropyl acetate (also abbreviated hereinafter to “MPA”), ethyl acetate (also abbreviated hereinafter to “EA”), and acetone (also abbreviated hereinafter to “Ac”). The contact time was 5 min in each case. For the measurement of the water resistances, the contact time was 24 h in each case.

[0208] Rating took place in line with the recited standard. The test surface is assessed visually and by scratching, using the following classification: 0=no change apparent; 1=swelling ring, hard surface, only visible change; 2=swelling ring, slight softening; 3=distinct softening (possibly slight blistering); 4=significant softening (possibly severe blistering), can be scratched through to the substrate; 5=coating completely destroyed without outside influence.

[0209] The interface between the STP coat and the basecoat was determined using SEM/EDX in accordance with DIN EN ISO/IEC 17025:2018-03.

[0210] Materials used

[0211] Vestanat EP-M 95 was obtained from Evonik AG, Essen, and used without further purification or modification.

[0212] The diisocyanates used are products of Covestro Deutschland AG, Leverkusen, Germany. Dibutyltin dilaurate (DBTL) was obtained from TIB Chemicals, Mannheim, Germany.

[0213] Stabaxol I was used from Lanxess AG, Rhein Chemie, Mannheim, Germany.

[0214] Basecoat material (black) Spies Hecker Permahyd Basecoat 280 super tief schwarz. Dilution with DI water (95%). Baking conditions: 80° C., 10 min or around 30 min air drying.

[0215] Basecoat (white) Spies Hecker, Mischlack 280 WB 801, weiB. This basecoat can be used for determining the Delta-Lab values.

[0216] All other commercially available chemicals were obtained from Sigma-Aldrich, Taufkirchen, Germany.

[0217] Polymeric polyols A)

[0218] Polymeric polyol A1) 70% solution in butyl acetate of polyacrylate polyol prepared from 6.3% ethyl acrylate, 0.7% acrylic acid, 17.6% isobornyl acrylate, 21.1% hydroxyethyl methacrylate, 7% methyl methacrylate and 14.3% styrene. OH content: 2.4-2.7%; acid number: 7±1 mg; viscosity (23° C.): 1200±200 mPas; solid content: 70.0%±2.0%.

Example 1

Preparation of isocyanatosilane 1

[0219] 1340 g (8 mol) of HDI were admixed under dry nitrogen at a temperature of 70° C. with 1240 g (1 mol) (3-mercaptopropyl)triethoxysilane and, after addition of 0.25 g (2.04 mmol) of DABCO, the mixture was stirred for 2 h until an NCO content of 39.3%, corresponding to full conversion, had been attained. Subsequently, the unconverted monomeric HDI was removed on a thin-film evaporator at a temperature of 140° C. and a pressure of 0.1 mbar. This gave a virtually colorless, clear isocyanatosilane having the following characteristics: NCO content: 9.8%; viscosity (23° C.): 60 mPas.

Example 2

Isocyanatosilane 2

[0220] 2220 g (10 mol) of isophorone diisocyanate (IPDI) were admixed under dry nitrogen at a temperature of 80° C. with 196 g (1.0 mol) of mercaptopropyltrimethoxysilane and, after addition of 0.06 g (25 ppm) of dibutyltin dilaurate (DBTL), the mixture was stirred for 3 hours until an NCO content of 33.0%, corresponding to a full conversion, had been attained. Subsequently, the unconverted monomeric IPDI was removed on a thin-film evaporator at a temperature of 150° C. and a pressure of 0.1 mbar. This gave a virtually colorless, clear isocyanatosilane having the following characteristics: NCO content=9.7%; solids fraction=100%; viscosity (23° C.)=5400 mPas.

Example 3

Preparation of STP-1

[0221] 765 g of the isocyanatosilane prepared in example 1 and 8.9 g (0.06 mol) of tetraethyl orthoformate (TEOF) were admixed under dry nitrogen at 80° C. with 128 g (0.9 mol) of 2-ethyl-1,3-hexanediol and 0.02 g (0.03 mmol) of dibutyltin(IV) dilaurate (DBTL) and the reaction was conducted until a residual NCO of 0.3% was attained. The crude product was admixed with 100 g of butylacetate (BuAc). This gave a virtually colorless, clear STP-1 having a number-average molecular weight Mn of 942 g/mol and the following characteristics: Residual NCO content: 0.3%; viscosity (23° C.) 280 mPas; solids content: 80%.

Example 4

Preparation of STP-2

[0222] 506 g of the isocyanatosilane prepared according to example 1, 15 g (0.1 mol) of TEOF and 10 drops of DBTL were charged to a reactor and heated to 80° C. Then a mixture of 728 g of polyol Al and 36.9 g (0.1 mol) of Stabaxol I was added. The reaction mixture was stirred at 80° C. for 5 h. The product was lastly admixed with 214 g of BuAc and used without further purification. This gave a virtually colorless, clear STP-3 having a number-average molecular weight Mn of 1740 g/mol and the following characteristics: Residual NCO content: <0.24%; viscosity (23° C.): 490 mPas; solids content: 70%.

Example 5

Preparation of STP-3

[0223] 540.1 g of example 2, 4.59 g of tetraethyl orthoformate (TEOF) and one drop of DBTL are admixed under dry nitrogen at 80° C. with 89.2 g of 2-ethyl-1,3-hexanediol. After 5 h, a further drop of DBTL is added and the reaction is stirred until a residual NCO of <0.3% is attained. The crude product is admixed with 127.0 g of butyl acetate (BuAc). This gives a virtually colorless silane having the following characteristics: Residual NCO content=<0.3%; viscosity (23° C.)=9000 mPas; solids content=80%.

[0224] Basecoat Formulation

[0225] In order to detect the migration of silane-terminated prepolymers into the basecoat material, an aqueous basecoat, black, based on a secondary acrylate (OH-containing) was prepared. For this purpose, the components were weighed out successively, mixed and, as specified in the formulation, dispersed with a dissolver having a dispersing disk.

TABLE-US-00001 Amount Basecoat 1 (g) I.)  Bayhydrol A 2542, as-supplied form. 34.81    Distilled water 25.25    Dimethylethanolamine, 10% in dist. 6.02    water (for pH 8-8.5)    2-Ethyl-1-hexanol 2.79    BYK 347, as-supplied form. 0.17    BYK 345, as-supplied form. 0.17    BYK 011, as-supplied form. 1.45    Byketol AQ, as-supplied form. 2.76    Solus 3050, 20% in butyl glycol/ 2.61    dist. water/DMEA (50.00/28.58/1.42)    Rheovis AS 1130, as-supplied form. 1.75    n-Butanol 0.14    -disperse at about 10.5 m/s for 5 min.- II.) Pigment paste, black, consisting of: 6.20    Setaqua B E 270, as-supplied form. 10.40    dist. water 41.60    Borchi Gen 0851, as-supplied form. 32.00    Colour Black FW 200 16.00    -disperse at about 10.5 m/s for 30 min.- III.)  Dist. water 15.88 Total weight 100.00

[0226] This produces a solids content at spray viscosity of 21.7%, a flow time in the DIN cup, (4 mm) of 30 s, and a pH of around 8.3.

[0227] Topcoat Formulations

[0228] The amount of the flow control agent added was calculated based on the solid resin content. The amount of catalyst was calculated based on the solid resin content. The topcoats were produced by initially introducing the binders and then adding, with stirring, the solvent and subsequently in mandated order the additives at room temperature with continued stirring. The solvent was butyl acetate. The amounts of solvent were chosen such that the solids contents were the same. The topcoats were produced freshly immediately prior to application.

TABLE-US-00002 Clearcoat 1 Clearcoat 2 Clearcoat 3 STP-1 100.00 Vestanat EP-M 95 100.00 STP-2 100.00 Butyl acetate (BA) 48.63 74.86 22.40 BYK 331, 10% in BA 0.85 1.00 0.70 Nacure 4000, 10% in BA 6.38 7.50 5.25 Total weight 155.86 183.36 128.35 Solids content (theo.), wt %. 55.0 55.0 55.0 Catalyst, solid based on 0.75 0.75 0.75 resin solids

[0229] Migration Experiments

[0230] For the migration experiments, the basecoat was drawn down onto a polypropylene (PP) plate by means of 50 μm spiral-wound coating bar, flashed off at room temperature for 5 minutes and then dried in an air circulation paint drying cabinet at 80° C. for 20 min Immediately after being cooled down (20 min RT), the clearcoat under test was then applied to the basecoat by means of a spiral-wound coating bar (formulation 1=100 μm, formulation 4=150 μm, formulation 6=110 μm), flashed off at room temperature for 5 minutes, and then baked in an air circulation paint drying cabinet at 100° C. for 30 min. The film thicknesses of basecoat (around 12 μm dry film) are identical in all of the experimental setups. The plates were then stored under standard conditions for 24 hours, and the paint system was peeled from the PP plate, and the basecoat was then analyzed on its underside by means of an FT-IR spectrometer (Tensor II with platinum ATR unit (diamond crystal) from Bruker). Single measurements were conducted. The spectra were evaluated by performing a Min-Max standardization in the 3900-3800 cm-1 range; no baseline correction was performed.

[0231] The signals indicated below were evaluated as characteristic bands for the silane-terminated prepolymers and also for the basecoat. These signals are employed in order to describe the diffusion of the STP topcoat into the basecoat:

TABLE-US-00003 3323/3304 cm.sup.−1 signal height 1685/1651 cm.sup.−1 signal height 1590-1485 cm.sup.−1 integration over absorption region

[0232] Evaluation of the Migration Experiments

[0233] The FT-IR spectra of the water-based basecoat, of the STP clearcoat and of the multicoat system based on these two components were compared. Surprisingly it was observed here that there is migration of the STP topcoat into the basecoat. This was found on the basis of the IR spectral evaluation and demonstrated by various experiments. In addition, the diffusion was characterized quantitatively by means of energy-dispersive X-ray spectroscopy (SEM/EDX) (see end of section). For this purpose, firstly, the signal heights of the absorption maxima of the STP clearcoat were compared with the corresponding absorption intensities of the water-based basecoat. Secondly, integration took place over an absorption region of the multicoat system−basecoat+STP clearcoat −and the area obtained was compared with that of an aqueous basecoat without clearcoat.

[0234] The table below presents the absorption maxima for various coatings and coating systems. The migration of the STP clearcoat into the basecoat is described here, illustratively, on the basis of STP-1. In order to be able to detect migration, an absorption maximum of the clearcoat 1 (STP 1) was employed that exhibits an intense absorption band in comparison to the basecoat with the corresponding wavenumber; for clearcoat 1 (STP 1), 3304 cm.sup.−1 was chosen.

TABLE-US-00004 Absorption Absorption Difference Percent maximum maximum relative to (ratio: at 3304 at 3323 basecoat Clearcoat X/ No. System cm.sup.−1 (a.u.) cm.sup.−1 (a.u) (a.u.) basecoat 1) 1 Basecoat 1 46.4 48.2 . 2 Clearcoat 1.sup.[1] 217.5 — . (Topcoat) 3 Clearcoat 1.sup.[1] + 110.0 — 64 237% Basecoat 1 4 Clearcoat 2.sup.[2] — 259.6 — — (Topcoat) 5 Clearcoat 2.sup.[2] + — 201.6 153 418% Base coat 1 6 Clearcoat 3.sup.[3] — 178.8 — — (Topcoat) 7 Clearcoat 3.sup.[3] + — 49.0 1 102% Basecoat 1 .sup.[1]STP 1; .sup.[2]Vestanat EP-M 95; .sup.[3]STP-2.

[0235] The absorption maxima (FT-IR) of a water-based basecoat, of an STP topcoat and of a multicoat system consisting of basecoat and STP topcoat were investigated in the range between 3304 cm.sup.−1 and 3323 cm.sup.−1. For evaluation, to start with, determinations were made of the intensity (a.u) of the IR absorption band of STP 1 (No. 2) and of the basecoat (No. 1) at 3304 cm.sup.−1. Subsequently an IR spectrum of the multicoat system (No. 3)—Basecoat with STP 1—was recorded. In these IR investigations it was found that characteristic bands of the STP are repeated in the reverse-side IR spectrum of the basecoat. This will be discussed using, as an example, clearcoat 1, based on STP-1: for the basecoat, identified here as basecoat 1 (No. 1), a signal with an intensity of 46.4 a.u. was measured at a wavenumber of 3304 cm.sup.−1. The multicoat system (No. 3), in contrast, showed a much more intense signal, at 110.0 a.u.; this corresponds to an intensity difference of 237%. This intense absorption band was found beforehand with a stronger intensity for the pure clearcoat 1 (No. 2). On this basis, the diffusion of the clearcoat 1 into the basecoat is verified.

[0236] The table below shows the absorption maxima (FT-IR) of a water-based basecoat, of an STP topcoat and a multicoat system composed of basecoat and STP, in the region between 1651 cm.sup.−1 and 1685 cm.sup.−1.

TABLE-US-00005 Absorption Absorptions Difference maximum maximum relative to At 1651 at 1685 the basecoat No. System cm.sup.−1 (a.u.) cm.sup.−1 (a.u) (a.u.) Percent 1 Basecoat 1 75.9 167.0 — — 2 Clearcoat 1.sup.[1] 600.5 — — — (Topcoat) 3 Clearcoat 1.sup.[1] + 376.2 — 300 496% Basecoat 1 4 Clearcoat 2.sup.[2] — 911.2 — (Topcoat) 5 Clearcoat 2.sup.[2] + — 821.6 655 492% Basecoat 1 6 Clearcoat 3.sup.[3] 610.8 — — (Topcoat) 7 Clearcoat 3.sup.[3] + 158.0 — 82 208% Basecoat .sup.[1]STP 1; .sup.[2]Vestanat EP-M 95; .sup.[3]STP-2.

[0237] From the table it is apparent that analogous results were obtained for the absorption maxima of the clearcoats at 1651 cm.sup.−1 and 1685 cm.sup.−1. The results are discussed using, as an example, the case of STP 2. At 1651 cm.sup.−1, an intensity of 75.9 a.u. was observed for the basecoat (No. 1). For the clearcoat 3 (No. 6), an intensity of 610.8 a.u. was found. In the multicoat system (No. 7)—basecoat and clearcoat 3—an intensity of 158.0 (a.u.) was measured. The latter corresponds to an increase in the basecoat signal (No. 1) by 208%. The increased signal intensity at a wavenumber of 1651 cm.sup.−1 is attributed to clearcoat 3, since at the wavenumber stated, this clearcoat exhibits a very strong absorption band, which is reflected here partly in the clearcoat 1—basecoat system. These results demonstrate that there is diffusion of the clearcoat 3 into the basecoat.

[0238] As well as the signal height, i.e., the intensity, it is also possible to employ the area under the absorption bands for characterizing the diffusion of the STP clearcoats through a basecoat. One such is discussed, illustratively, by using the example of the STP Vestanat EP-M 95 (table, see below). This is a chemically modified STP which has only one urethane group and no supplementary thiourethane group. In the absorption region from 1590 cm.sup.−1 to 1485 cm.sup.−1 (see above), the area under the absorption band was determined at 8394 (a.u.) for basecoat 1 (No. 1). The area of the STP (No. 6) is 39 908 (a.u.). For the multicoat system (No. 7), the area is found to be 33 013 (a.u.). This corresponds to an increase of 393%. This figure is close to the figure for the integral area of the STP clearcoat (No. 6).

TABLE-US-00006 Integrated Difference area in the relative to Absorption region absorption the % age No. (1590-1485 cm.sup.−1) region (a.u.) basecoat difference 1 Basecoat 1 8394 — — 2 STP 1 (clearcoat) 33745 — — 3 STP 1 + Basecoat 1 20890 12496 248% 4 STP 2 (clearcoat) 33561 — 5 STP 2 + basecoat 1 9603 1209 114% 6 Vestanat EP-M 95 39908 — — (clearcoat) 7 Vestanat EP-M 95 + 33013 24619 393% Basecoat 1

[0239] These experiments show that there is diffusion through the basecoat irrespective of the structure and molecular weight of the STP. For this, both the signal intensity and the area of the signal were used.

[0240] This diffusion can be correlated very well with the mechanical properties of the composite coatings. Without being tied to the theory, preferred mechanical properties and, in particular, especially good adhesion of the composite coating result from the diffusion of the STP into the basecoat.

[0241] Measurements by means of energy-dispersive X-ray spectroscopy (SEM/EDX) confirmed the surprising findings with phosphorus-containing catalysts. This method can also be used to determine the extent of the diffusion by means of the characteristic elements from the coating materials. In this case, the coatings not only are studied with the surprisingly suitable phosphorus-containing acid catalysts, but also are compared with sulfur-containing acid catalyst systems. The latter likewise exhibit diffusion into the clearcoat, this being desirable to the skilled person for known technical reasons.

[0242] The following examples use a physical mixture of different STPs in order to describe the diffusion of the clearcoat into the basecoat. The measurements show that there is diffusion of the clearcoat into the basecoat of at least 5 gm. This surprising finding shows that in combination with the stated catalyst systems, the STP coating promotes adhesion between the coats and the crosslinking of the basecoat.

TABLE-US-00007 Clearcoat 4 Clearcoat 5 Component (g) (g) STP-1 56.00 56.00 STP-2 21.00 21.00 STP-3 8.00 8.00 Dibutyl phosphate/diphenyl 0.99/3.30 — phosphate (10% in BA) H.sub.2SO.sub.4 (10% in MeOH) — 3.30 BYK 315N (10% in MPA) 1.98 1.98 MPA (41.53) (41.53)

[0243] For the sulfur content and silicon content, which each originate from the clearcoat, 7.5% and 6%, respectively, were found in the EDX measurements. From the boundary layer up to 15 μm within the basecoat, a decrease in the sulfur content from 6.5% to 1.5% is found; for silicon, a decrease from 4.5% to 1.0% is found (clearcoat 4).

TABLE-US-00008 No. Coating system Clearcoat 4 Clearcoat 5 Sulfur content 7.5% 7.0% (Clearcoat) Sulfur content 6.5%/4.0%/2.5%/1.5% 6.0%/2.0%/0%/0% (basecoat, black) (0 μm*/5 μm/10 μm/15 μm) Silicon content 6% 6% (clearcoat) Silicon content 4.5%/3.0%/2.0%/1.0% 5.0%/1.5%/0%/0% (basecoat, black) (0 μm*/5 μm/10 μm/15 μm) Carbon content 65% 64% (clearcoat) Carbon content 70%/74%/77%/79% 66%/75%/78%/78% (basecoat, black) (0 μm*/5 μm/10 μm/15 μm) *Interface between clearcoat and basecoat (black)

[0244] With sulfur-containing catalysts such as sulfuric acid, a lower diffusion is observed for the STP coating (clearcoat 5); the diffusion limit thereof is reached above 6 μm.