Method for producing a multicoat coating

Abstract

A method for producing a multicoat coating (M) on a substrate (S) that includes:(!) producing a basecoat (B) on the substrate by applying an aqueous basecoat material (b) to the substrate (S), the basecoat material being a two-component coating composition, and (II) producing a clearcoat (K) directly on the basecoat (B) by applying an aqueous clearcoat material (k) directly to the basecoat (B), the clearcoat material being a two-component coating composition.

Claims

1. A method for producing a multicoat coating (M) on a substrate (S), the method comprising: (I) applying an aqueous basecoat material (b) to the substrate (S) to obtain a basecoat (B) on the substrate (S), the aqueous basecoat material (b) being a two-component coating composition comprising (b.1) a base component comprising (1) at least one polyurethane resin (1) having a hydroxyl number of 15 to 100 mg KOH/g and an acid number of 10 to 50 mg KOH/g, (2) at least one aqueous dispersion comprising water and a polyurethane resin fraction consisting of at least one polyurethane resin, the polyurethane resin fraction having a gel fraction of at least 50%, having a glass transition at a temperature of less than 20 C. and having a melting transition at a temperature of less than 100 C., (3) at least one color pigment, effect pigment, or both, and (b.2) a curing component comprising (4) at least one hydrophilically modified polyisocyanate (4) having an isocyanate content of 8% to 18%; and (II) applying an aqueous clearcoat material (k) directly to the basecoat (B) to obtain a clearcoat (K) directly on the basecoat (B), the aqueous clearcoat material (k) being a two-component coating composition comprising (k.1) a paint base component comprising (2) at least one aqueous dispersion comprising water and a polyurethane resin fraction consisting of at least one polyurethane resin, the polyurethane resin fraction having a gel fraction of at least 50%, having a glass transition at a temperature of less than 20 C. and having a melting transition at a temperature of less than 100 C., and (k.2) a curing component comprising (4) at least one hydrophilically modified polyisocyanate (4) having an isocyanate content of 8% to 18%.

2. The method as claimed in claim 1, wherein the at least one hydrophilically modified polyisocyanate (4) is selected from the group consisting of a polyether-modified polyisocyanate, a polyester-modified polyisocyanate, and mixtures thereof.

3. The method as claimed in claim 2, wherein the at least one hydrophilically modified polyisocyanate (4) is selected from the group consisting of a polyoxyethylene-modified polyisocyanate, a polyoxypropylene-modified polyisocyanate, a mixed polyoxyethylene-polyoxypropylene-modified polyisocyanate, and mixtures thereof.

4. The method as claimed in claim 3, wherein the polyoxyethylene-modified polyisocyanate, the polyoxypropylene-modified polyisocyanate, the mixed polyoxyethylene-polyoxypropylene-modified polyisocyanate, and the mixtures thereof are modified isocyanurates.

5. The method as claimed in claim 1, wherein the at least one polyurethane resin (1), based on a total amount of starting compounds in preparing the polyurethane resin (1), is prepared from between 30 and 80 wt % of at least one polyester diol prepared from at least one dimer fatty acid.

6. The method as claimed in claim 5, wherein, in preparing the at least one polyester diol, at least 50 wt % of dicarboxylic acids are the at least one dimer fatty acid.

7. The method as claimed in claim 1, wherein at least one color pigment (3) and the at least one polyurethane resin (1) in the base component (b.1) are comprised in a pigment paste.

8. The method as claimed in claim 7, wherein the pigment paste comprises: 1 to 60 wt % based on a total amount of the pigment paste of the at least one color pigment (3), 10 to 60 wt % based on a total amount of the pigment paste of the at least one polyurethane resin (1), and 30 to 80 wt % based on a total amount of the pigment paste of at least one organic solvent, wherein the at least one color pigment (3), the at least one polyurethane resin (1) and the at least one organic solvent comprise at least 80 wt % of a total weight of the paste.

9. The method as claimed in claim 1, wherein the polyurethane resin fraction of the at least one aqueous dispersion (2) has a glass transition at a temperature ranging from 100 C. to less than 20 C., has a melting transition at a temperature ranging from 20 C. to less than 90 C., and comprises particles having a particle size of greater than 1 micrometer.

10. The method as claimed in claim 1, wherein the aqueous basecoat material (b) and the aqueous clearcoat material (k) further comprise at least one further aqueous dispersion (5), a polyurethane resin fraction of the further aqueous dispersion (5) having a gel fraction of at least 50% and being present in the form of dispersed particles having a volume average particle size of 20 to 500 nanometers.

11. The method as claimed in claim 1, wherein the substrate (S) comprises a flexible foam substrate.

12. The method as claimed claim 1, wherein the basecoat (B) is produced directly on the substrate (S).

13. The method as claimed in claim 1, wherein the basecoat (B) and the clearcoat (K) are cured jointly at temperatures between 40 and 120 C., and no further coat is produced.

14. A multicoat coating produced by the method of claim 1.

15. A method for increasing stability of a substrate to external mechanical influences, the method comprising coating the substrate with the multicoat coating of claim 14.

Description

EXAMPLES

(1) Measurement Methods:

(2) Gel Fraction:

(3) The gel fraction of the polyurethane resin fractions of corresponding aqueous dispersions is determined gravimetrically in the context of the present invention. Here, first of all, the polymer present was isolated from a sample of an aqueous dispersion (initial mass 1.0 g) by freeze-drying. Following determination of the solidification temperaturethe temperature above which the electrical resistance of the sample shows no further change when the temperature is lowered furtherthe fully frozen sample underwent its main drying, customarily in the drying vacuum pressure range between 5 mbar and 0.05 mbar, at a drying temperature lower by 10 C. than the solidification temperature. By graduated increase in the temperature of the heated surfaces beneath the polymer to 25 C., rapid freeze-drying of the polymers was achieved; after a drying time of typically 12 hours, the amount of isolated polymer (solid fraction, determined by the freeze-drying) was constant and no longer underwent any change even on prolonged freeze-drying. Subsequent drying at a temperature of 30 C. of the surface beneath the polymer, with the ambient pressure reduced to the maximum degree (typically between 0.05 and 0.03 mbar), produced optimum drying of the polymer.

(4) The isolated polymer was subsequently sintered in a forced air oven at 130 C. for one minute and thereafter extracted for 24 hours at 25 C. in an excess of tetrahydrofuran (ratio of tetrahydrofuran to solid fraction=300:1). The insoluble fraction of the isolated polymer (gel fraction) was then separated off on a suitable frit, dried in a forced air oven at 50 C. for 4 hours, and subsequently reweighed.

(5) It was further ascertained that at the sintering temperature of 130 C., with variation in the sintering times between one minute and twenty minutes, the gel fraction found for the microgel particles is independent of the sintering time. It can therefore be ruled out that crosslinking reactions subsequent to the isolation of the polymeric solid increased the gel fraction further.

(6) Given that some of the aqueous, commercially acquired polymer dispersions used additionally include inorganic components such as silicates, and the fraction of these inorganic components is of course captured as well in the determination of the gel fraction, all of the dispersions were incinerated (800 C.) and any remaining ash content was then subtracted from the gel fraction found.

(7) Glass Transition and Melting Transition:

(8) The glass transition is determined on the basis of the glass transition temperature. The glass transition temperature in the context of the invention is determined experimentally in a method based on DIN 51005 Thermal Analysis (TA)Terms and DIN 53765 Thermal AnalysisDifferential Scanning calorimetry (DSC). A sample of the binder is applied with a wet film thickness of 100 m, using a doctor blade, to a glass plate, dried initially at 40 C. for 1 hour and then dried at 110 C. for 1 hour. For the measurement, a section of the film thus dried is removed from the glass plate and inserted into the measuring sleeve. This sleeve is then inserted into a DSC instrument. It is cooled to the start temperature, after which 1.sup.st and 2.sup.nd measurement runs are carried out at a heating rate of 10 K/min with inert gas flushing (N.sub.2) of 50 ml/min, with cooling again to the start temperature between the measurement runs.

(9) A glass transition can be seen in the DSC diagram as a section of the measurement curve that in terms of amount possesses a slope much greater in relation to the baselines before and after the transition. The slope greater in terms of amount is known to be attributable to the higher quantity of energy needed in the region of the phase transition in order to increase the temperature. A measurement curve section of this kind then, of course, possesses a point of inflection in the region with the slope that is greater in terms of amount. For the purposes of the present invention, the glass transition temperature of a glass transition is taken to be the temperature of this point of inflection in the 2.sup.nd measurement run.

(10) The temperature at which the polyurethane resin fraction has its glass transition is defined as follows in the context of the present invention: it is the temperature at the point of inflection of the curve section assignable to the glass transition (glass transition temperature). It is possible for there to be a number of curve sections present to which glass transitions are assigned. The system in question in that case has a number of glass transitions and glass transition temperatures. In such an event, the temperature at which the polyurethane resin fraction has its glass transition is the temperature at the point of inflection of the curve section in the highest temperature range. In such an event, indeed, it is only after the last glass transition that there is no longer any glasslike structure at all in the corresponding polyurethane resin fraction. The expression its glass transition can therefore be equated with the wording its complete glass transition or its glass transition with the highest temperature.

(11) The melting transition is likewise determined from the DSC diagram measured as described above. A melting transition is seen as a region which deviates from the baseline in the DSC diagram. Within this region, in fact, the system must be supplied with a greater quantity of energy, owing to the phase conversion of the crystallites, in order to bring about an increase in temperature. These regions, as is known, are seen as peaks with different widths.

(12) The temperature at which the polyurethane resin fraction has its melting transition is defined as follows in the context of the present invention: it is the temperature at the extreme point of the peak assignable to the melting transition. Where there are a number of peaks present to which melting transitions are assignable, the extreme point in question is that of the peak in the highest temperature range. In such an event, indeed, the system has a number of melting transitions. Accordingly, there is no longer any crystalline structure present in the corresponding polyurethane resin fraction only after the last melting transition. The expression its melting transition can therefore be equated with the wording its complete melting transition or its melting transition with the highest temperature.

(13) 1 Preparation of Basecoat and Clearcoat Materials and Production of Multicoat Coatings

(14) The paint base components of different coating materials were acquired directly as commercial product or prepared by successively combining the respective constituents and intimately mixing them in a dissolver (table 1a). Basecoat materials (b) and clearcoat materials (k) for inventive use carry the code I, comparative systems the code C. The curing components of the individual coating materials are listed in table 1b. The figures reported are in each case the parts by weight of the components that were used.

(15) TABLE-US-00001 TABLE 1a Paint base components Item Component b I1 k I1 k C1 k C2 k C3 1 Base varnish of a 100 commercial k 2 Base varnish of a 100 further commercial k 3 Aqueous dispersion (2) 53.31 72.041 72.041 4 First aqueous 17.45 23.585 23.585 dispersion (5) 5 Second aqueous 3.23 4.374 4.374 dispersion (5) 6 Pigment paste yellow 1 10.31 7 Pigment paste white 1 6.49 8 Pigment paste yellow 2 1.48 9 Pigment paste black 1 0.12 10 Pigment paste red 1 0.11 11 Water DI 7.5

(16) TABLE-US-00002 TABLE 1b Curing components Item Component b I1 k I1 k C1 k C2 k C3 12 Polyether-modified HDI 8 8 isocyanurate (4) having an isocyanate content of 12%, 70% strength in organic solvents 13 HDI isocyanurate 33 33 33 having an isocyanate content of 23% (Desmodur 3600), 56% strength in organic solvents

(17) Aqueous dispersion (2): The commercial dispersion comprises a polyurethane resin fraction with a gel fraction of 61.3%. The polyurethane resin fraction has its glass transition at a temperature of 47 C. and its melting transition at a temperature of 50 C. The polyurethane fraction comprises particles having a particle size of 1 to 100 micrometers. The solids content of the dispersion is 26.5% including 1.8% of inorganic components (silicates) (determined via incineration, 800 C.)

(18) Aqueous dispersions (5): The commercial dispersions comprise a polyurethane resin fraction having a gel fraction of 91% (first dispersion) or 95% (second dispersion). The average particle size (volume average) is 244 nanometers (first dispersion) or 56 nanometers (second dispersion). The polyurethane resin fractions have their glass transition at 48 C. (first dispersion) or 60 C. (second dispersion). Melting transitions at below 100 C. are not observed. The solids contents of the dispersions are 39.0% (first dispersion) and 37.0% (second dispersion).

(19) Pigment pastes: The pigment pastes each comprise a polyurethane resin (1) as pasting resin. The polyurethane resins used possess a hydroxyl number in the range between 20 and 40 mg KOH/g and an acid number in the range from 20 to 30 mg KOH/g. The fraction of the resins in the pastes is in the range from 10 to 30 wt %. Pigments included are commercial color pigments (fraction in the range from 5 to 60 wt %, according to type of pigment in each case). Likewise included are organic solvents (fraction in the range from 30 to 80 wt %).

(20) Subsequently, using the basecoat and clearcoat materials, multicoat coatings were produced on foam substrates. In this case the respective paint base component was not mixed homogeneously with the curing component until immediately prior to application. Application was made in a two-coat system (pneumatic manual coating), first basecoat then clearcoat. To start with the basecoat material was applied directly to the substrate, flashed at 25 C. for 10 minutes, and then the clearcoat material was applied directly to the basecoat and the multicoat coating was then cured in a forced air oven at 80 C. for 30 minutes. The film thicknesses (cured) are in each case 20-25 m for both basecoat and clearcoat. Substrates used were thermoplastic polyurethane bead foams in the form of sheets with a thickness of about 1 cm. The polyurethane bead foams were produced beforehand by expanding and fusing corresponding polyurethane pellets in corresponding molds by means of steam.

(21) The following multicoat coatings (M) were produced (table 2):

(22) TABLE-US-00003 TABLE 2 Multicoat coatings Multicoat MI1 MC1 MC2 MC3 coating System b I1 b I1 b I1 b I1 k I1 k C1 k C2 k C3
2 Investigations on the Multicoat Coatings

(23) Investigations were carried out into the flexibility and elasticity and also the stonechip resistance of the multicoat coatings disposed on the film substrates. Tests conducted were as follows: (a) The coated substrate was bent over the edge of a steel panel 2 mm thick. A determination was made of the bending angle at which cracking/breaking was seen in the coating system. (b) The coated substrate was folded (180 bend) centrally with the coated side outward, and was grasped at both ends. The loading trial was carried out by moving the hands in opposite directions (twisting movement). The coated substrates were subsequently inspected. (c) The coated substrate was tested for its impact capacity using a 5 euro cent coin. This was done by placing the coin upright on the film surface by its edge and pressing it into the substrate at constant pressure. The maximum depth of impression was 5 mm. (d) Stonechip test according to DIN EN ISO 20567-1.

(24) Results found were as follows:

(25) The inventive multicoat coating MI1 showed no cracking/damage at all at a maximum possible bend of 180 over the metal edge (test (a)). Nor was it possible to determine the changes after bending with the coated side outward (180). The twisting movement as well showed no effect at all on the appearance of the coating (test (b)). In the case of the 5 euro cent impact test, coating system MI1 likewise showed no damage at all to the surface (test (c)). No damage at all was found in the stonechip test.

(26) In the case of the variants MC2 and also MC3, breaking/cracking of the coating system was observed/heard after 45 and 30 bending, respectively, in test (a). Furthermore, the unprotected substrate was visible. During the 180 folding (b), moreover, the samples undergo defined breaking along the bending edge; as a result of the twisting movement, the coating system tears and breaks without coordination in all directions. In test (c), the coatings broke with an audible crack at a depth of penetration of 2 mm. In both systems, the stonechip test led to a large number of cracks in the coating, which were readily visible using commercial magnifying glasses.

(27) The flexibility characteristics of system MC1 were similar to those of inventive example MI1. After a 180 bend (film surface on the inside), however, about the metal edge, creasing was found along the bending edge, and was still visible even after the substrate had been removed from tension (significantly delayed fold reformation). The stonechip test did not lead to any damage at all.

(28) The investigations show that the inventive multicoat coatings have advantages in terms of elasticity, flexibility, and stonechip resistance.

(29) 3 Production of Further Coatings and Investigation of these Coatings

(30) As described above, further coatings MI1 were produced. In addition, coatings were produced for which no clearcoat material was applied. The coatings therefore contain only one film of the basecoat material b I1, disposed directly on the foam substrate.

(31) Investigations were carried out into chemical resistances and abrasion resistance: (a) For the determination of the chemical resistances, the procedure of DIN EN ISO 2812-3 was carried out, using a pad. The evaluation scale runs from 0 (no traces) to 5 (detachment). (b) The abrasion resistance of the coated surfaces was conducted according to GS97034-5 (BMW) using a crockmeter (method B, in accordance with DIN EN ISO 105-X12) (10 double rubs, using cotton rub fabrics wetted with different media). The samples are inspected both for visible traces of abrasion on the coated substrate and for perceptible color rub-off on the cotton rubbing fabrics used. The evaluation scale runs from 0 (complete abrasion) to 5 (no abrasion at all).

(32) Tables 3 and 4 show the results.

(33) TABLE-US-00004 TABLE 3 Chemical resistance Methyl ethyl Ethyl Sample Ethanol ketone Acetone acetate B I1 0 2 1 1 MI1 0 1 0 0

(34) In these investigations it is apparent that the two-coat system (pigmented basecoat plus clearcoat, MI1) exhibits better resistance overall with respect to chemicals/solvents in relation to the one-coat system (pigmented basecoat material only).

(35) TABLE-US-00005 TABLE 4 Abrasion resistance Water Cockpit spray Ethanol Rub Rub Rub Sample Substrate fabric Substrate fabric Substrate fabric B I1 5 5 5 4-5 5 4 MI1 5 5 5 5 5 5

(36) The two-coat system (pigmented basecoat plus clearcoat, MI1) shows better resistance in its abrasion behavior than the one-coat system (pigmented basecoat material only).

(37) 4 Further Multicoat Coatings and their Investigation

(38) In analogy to the production process identified above, further comparative multicoat coatings were produced and compared with multicoat coatings MI1. Tables 5a and b and also 6 show all of the coating materials used (and/or the paint base components, tables 5a and b) and the multicoat coatings produced (table 6).

(39) TABLE-US-00006 TABLE 5a Paint base components Item Component b I1 b C1 b C2 k I1 k C4 k C5 1 Aqueous 53.31 72.041 dispersion (2) (see table 1a) 2 Further aqueous 42.73 67.4 dispersion (5).sup.1 3 Aqueous 42.73 67.4 dispersion used for comparison, containing polyurethane.sup.2 4 First aqueous 17.45 17.45 17.45 23.585 27.5 27.5 dispersion (5) (see table 1a) 5 Second aqueous 3.23 3.23 3.23 4.374 5.1 5.1 dispersion (5) (see table 1a) 6 Pigment paste 10.31 10.31 10.31 yellow 1 (table 1a) 7 Pigment paste 6.49 6.49 6.49 white 1 (table 1a) 8 Pigment paste 1.48 1.48 1.48 yellow 2 (table 1a) 9 Pigment paste 0.12 0.12 0.12 black 1 (table 1a) 10 Pigment paste 0.11 0.11 0.11 red 1 (table 1a) 11 Water DI 7.5 3.9 3.9 .sup.1This dispersion (5) has a solids content of 35%. The polyurethane fraction has a gel fraction of 60% and has its glass transition at a temperature of 45 C. Melt transitions at less than 100 C., however, are not present. The average particle size (volume average) is 43 nanometers. .sup.2The dispersion has a solids content of 35%. The polyurethane resin fraction has a gel fraction of only 1% and has its glass transition at a temperature of 42 C. Melting transitions at less than 100 C. are not present.

(40) TABLE-US-00007 TABLE 5b Curing components Item Component b I1 b C1 b C2 k I1 k C4 k C5 12 Polyether- 8 8 8 8 8 8 modified HDI isocyanurate having an isocyanate content of 12%, 70% strength in organic solvents

(41) TABLE-US-00008 TABLE 6 Multicoat coatings Multicoat coating MI1 MC4 MC5 System b I1 b C1 b C2 k I1 k C4 k C5

(42) Investigations were made of the gloss (using a BYK micro-TRI-gloss meter, 20) and also of the flexibility and elasticity (by means of the methods (a) to (c) described in section 2).

(43) Table 7 shows the corresponding results of gloss measurement. Low values correspond to low gloss.

(44) TABLE-US-00009 TABLE 7 Gloss measurement Multicoat coating MI1 MC4 MC5 Gloss (20) 1.1 28 55

(45) The inventive multicoat coating MI1 possesses an extremely low gloss/high matting effect and has an optically high-grade and refined appearance in comparison to the comparative systems.

(46) The investigation of flexibility and elasticity found that the multicoat coatings MC4 and MC5, while they did not suffer cracking, nevertheless showed greatly increased fold formation after bending and twisting movements, in comparison to MI1. Fold reformation was significantly delayed. Overall, the flexibility and elasticity of the comparative systems are much poorer than in the case of system MI1.