Coating Composition Comprising a Polyisocyanate and a Polyol

Abstract

The invention relates to a non-aqueous coating composition comprising a. a polyisocyanate, b. a polyol having an average functionality of more than 3 hydroxyl groups per molecule, c. a metal based curing catalyst for the addition reaction of isocyanate groups and hydroxyl groups, and d. a mercapto carboxylic acid,
wherein the coating composition does not contain a carboxylic acid wherein the carbonyl group of the carboxylic acid is in conjugation with a π-electron system.

Claims

1. A non-aqueous coating composition comprising a) a polyisocyanate, b) a polyol having an average functionality of more than 3 hydroxyl groups per molecule, and c) the reaction product of i) a metal based curing catalyst for the addition reaction of isocyanate groups and hydroxyl groups and ii) a mercapto carboxylic acid, wherein the coating composition does not contain a carboxylic acid wherein the carbonyl group of the carboxylic acid is in conjugation with a TT-electron system.

2. The coating composition according to claim 1, wherein the catalyst for the addition reaction of isocyanate groups and hydroxyl groups is based on a metal selected from the group consisting of tin, bismuth, zirconium, and mixtures thereof.

3. The coating composition according to claim 1, wherein the catalyst for the addition reaction of isocyanate groups and hydroxyl groups is based on tin.

4. The coating composition according to claim 1, wherein the amount of metal based curing catalyst is in the range of 0.001 to 10 weight-%, calculated on the non-volatile matter of the composition.

5. The coating composition according to claim 1, wherein the mercapto carboxylic acid is selected from the group consisting of 2-mercapto-carboxylic acids, 3-mercapto-carboxylic acids, and mixtures thereof.

6. The coating composition according to claim 1, wherein the amount of mercapto carboxylic acid is in the range of 0.1 to 18 weight-%, calculated on the non-volatile matter of the composition.

7. The coating composition according to claim 1, wherein component b) comprises a polyester polyol and a polyacrylate polyol.

8. The coating composition according to claim 1, wherein the composition further comprises a volatile organic solvent.

9. The coating composition according to claim 7, wherein the amount of volatile organic solvent does not exceed 420 g/l of coating composition.

10. A kit of parts for preparing a coating composition, comprising a) a binder module comprising a polyol having an average functionality of more than 3 hydroxyl groups per molecule, and b) a crosslinker module comprising a polyisocyanate, wherein a metal based catalyst for the addition reaction of isocyanate groups and hydroxyl groups and a mercapto carboxylic acid are distributed, individually or in combination, over one or more of the modules.

11. The kit of parts according to claim 10 wherein the metal based catalyst for the addition reaction of isocyanate groups and hydroxyl groups and the mercapto carboxylic acid are distributed, individually or in combination, only over one or more of modules a) and c).

12. The kit of parts according to claim 10, further comprising c) a diluent module comprising a volatile organic solvent.

13. A method for forming a coating layer, the method comprising applying the coating composition according to claim 1 to an automobile or a transportation vehicle.

14. The method according to claim 13, wherein the formed coating layer is a layer in a multi-layer lacquer coating.

15. The method according to claim 13, wherein the formed coating layer is a top coat layer in a multi-layer lacquer coating.

16. A process of coating a substrate comprising i) applying the coating composition according to claim 1 to a substrate, and ii) curing the coating composition at a temperature in the range of 40 to 80° C.

17. The process of coating a substrate according to claim 16, wherein component b) comprises a polyester polyol and a polyacrylate polyol and the amount of volatile organic solvent does not exceed 420 g/l of coating composition.

Description

EXAMPLES

[0043] Unless stated otherwise, the properties of the liquid coating compositions or the resulting coating films were determined as described below.

[0044] The DINC4 viscosity was measured in a DIN Flow cup (number 4). The viscosity is given in seconds.

[0045] The volatile organic content (VOC) was calculated theoretically from the solvent content of the coating compositions.

[0046] The Enamel Hold Out (EHO) was determined as the total visual appearance. Each sample was rated for visual appearance on a scale of 1 to 10 (1=very bad, 10=excellent). The determination takes into account gloss, wrinkling, flow, and image clarity/distinctness of image. The average number will give the EHO.

[0047] The pinhole sensitivity was determined visually. Each sample was rated on a scale of 1 to 10 at comparable layer thicknesses of the coating (1=very bad, 10=excellent).

[0048] The warm tackiness was determined manually after 5 minutes at the end of the 60-65° C. cure cycle. Each sample was rated on a scale of 1 to 10 at comparable layer thicknesses of the coating (1=very bad, 10=excellent).

[0049] The open times were determined with a BK drying recorder. The drying recorder measurements were done to determine the drying rate and the open time of freshly applied coatings at the indicated temperature. With the drying recorder four different phases can be distinguished:

Phase 1: The needle track reflows to a closed film.
Phase 2: The needle track is regular and does not reflow to a closed film. [0050] The coating is damaged down to the substrate.
Phase 3: The needle track is irregular, with incidental damage down to the substrate.
Phase 4: The needle track is regular again, but the coating shows only superficial damage. [0051] At the end of stage 4 no damage is observed at all.

[0052] The coating layers used for the drying recorder experiments were applied with a K-Controle coating apparatus.

[0053] The Persoz hardness was measured according to ISO 1522.

[0054] The polyester polyol with a functionality of 2.8 which was used in the examples was prepared as described in international patent publication WO 2007/0020269, p. 22, Example E2.

[0055] The polyacrylate polyols used in the Examples were in an analogous way as described in international patent publication WO 2007/0020269, p. 19, 1. 16-p. 21, Examples A1-A4.

Abbreviations

[0056] 2-MPA 2-Mercaptopropionic acid [0057] 3-MPA 3-Mercaptopropionic acid [0058] TGA Thioglycolic acid [0059] E2-MPA Ethyl-2-mercaptopropionate [0060] E3-MPA Ethyl-3-mercaptopropionate [0061] DBTL Dibutyl tin dilaurate [0062] NVC Non-volatile content [0063] Tolonate HDT LV Aliphatic polyisocyanate HDI trimer ex Rhodia ppmc [0064] Desmodur XP 2511 Aliphatic polyisocyanate HDI trimer ex Bayer Material Science Ag [0065] BYK 331 Polydimethyl siloxane polyether (modified) additive, ex BYK Chemie GmbH [0066] BYK 355 Polyacrylate solution, ex BYK Chemie GmbH [0067] Vestanat T1890 E Cycloaliphatic polyisocyanate, ex Evonik Degussa GmbH [0068] BYK 392 Acrylate copolymer additive, ex BYK Chemie GmbH [0069] Exxsol D30 Naphta (petroleum), ex Exxon Mobil Chemical Company [0070] Butylcellosolve® acetate 2-Butoxyethyl acetate, ex DOW Chemical Company Ltd. [0071] PTMP Pentaerythritol tetrakis (3-mercaptopropionate) [0072] Tolonate HDT LV 2 Aliphatic polyisocyanate HDI, ex Rhodia ppmc [0073] SBP 140/165 Naphta (petroleum), ex Shell Chemicals Europe B.V. [0074] Byk 320 Polymethyl-alkylsiloxane, polyether (modified) additive, ex BYK Chemie GmbH [0075] Tinuvin 1130 Benzotriazole derivative mixture, ex Ciba Specialty Chemicals [0076] Tinuvin 292 Hindered amine light stabilizer, ex Ciba Specialty Chemicals [0077] T.sub.g Glass transition temperature (° C.) [0078] f(OH) Average OH functionality

Example 1

[0079] Six clearcoat compositions were prepared by mixing the components in the weight proportions as indicated in Table 1. The molar ratio of isocyanate groups to hydroxyl groups was adjusted to 1.25:1. The clear coat compositions of Table 1 all contained the following additives:

0.6 parts by weight Tinuvin 1130
1.1 parts by weight Tinuvin 292
0.1 part by weight BYK 331
0.1 part by weight BYK 355

TABLE-US-00001 TABLE 1 Clear coat composition E1.1 E1.2 E1.3 E1.4 E1.5 E1.6 Acrylic polyol (f(OH) = 6.2; NVC = 55.6 55.6 55.6 55.6 55.6 55.6 65%, T.sub.g = 29° C.) Polyester polyol (f(OH) = 2.8; NVC = 19.9 19.9 19.9 19.9 19.9 19.9 100%; T.sub.g = −59° C.) n-butylacetate 16.1 16.1 16.1 16.1 16.1 16.1 Ethyl-ethoxy-propionate 0.9 0.9 0.9 0.9 0.9 0.9 SBP 140/165 4.5 4.5 4.5 4.5 4.5 4.5 Tolonate HDT LV/Desmodur XP 2511 44.9 44.9 44.9 44.9 44.9 44.9 Ethyl-ethoxy-propionate 9.6 9.6 9.6 9.6 9.6 9.6 n-butylacetate 9.6 9.6 9.6 9.6 9.6 9.6 DBTL 0.22 0.22 0.22 0.22 0.22 0.22 Methyl-amylketone 6.2 5.3 6.6 5.9 6.7 6.0 SBP 140/165 8.9 8.9 8.9 8.9 8.9 8.9 BYK-392 0.49 0.49 0.49 0.49 0.49 0.49 n-butylacetate — 0.87 — 0.87 — 0.87 2-Mercaptopropionic acid — — 0.24 0.24 0.12 0.12 PTMP 0.28 0.28 — — — — Benzoic Acid 0.28 0.28 — — — — DBTL — 0.1 — 0.1 — 0.1 VOC (g/l) ≦420 ≦420 ≦420 ≦420 ≦420 ≦420 Acrylic polyol (f(OH) = 6.2; NVC = 55.6 55.6 55.6 55.6 55.6 55.6 65%, T.sub.g = 29° C.) Polyester polyol (f(OH) = 2.8; NVC = 19.9 19.9 19.9 19.9 19.9 19.9 100%; T.sub.g = −59° C.) n-butylacetate 16.1 16.1 16.1 16.1 16.1 16.1 Ethyl-ethoxy-propionate 0.9 0.9 0.9 0.9 0.9 0.9 SBP 140/165 4.5 4.5 4.5 4.5 4.5 4.5 Tolonate HDT LV/Desmodur XP 2511 44.9 44.9 44.9 44.9 44.9 44.9 Ethyl-ethoxy-propionate 9.6 9.6 9.6 9.6 9.6 9.6 n-butylacetate 9.6 9.6 9.6 9.6 9.6 9.6 DBTL 0.22 0.22 0.22 0.22 0.22 0.22 Methyl-amylketone 6.2 5.3 6.6 5.9 6.7 6.0 SBP 140/165 8.9 8.9 8.9 8.9 8.9 8.9 BYK-392 0.49 0.49 0.49 0.49 0.49 0.49 n-butylacetate — 0.87 — 0.87 — 0.87 2-Mercaptopropionic acid — — 0.24 0.24 0.12 0.12 PTMP 0.28 0.28 — — — — Benzoic Acid 0.28 0.28 — — — — DBTL — 0.1 — 0.1 — 0.1 VOC (g/l) ≦420 ≦420 ≦420 ≦420 ≦420 ≦420

[0080] The clear coat compositions were based on two different DBTL blocking systems, namely PTMP/benzoic acid (E1.1, E1.2) and 2-MPA (E1.3 to E1.6). In samples E1.5 and E1.6 the amount of 2-MPA was half that in samples E1.3 and E1.4. In samples E1.2, E1.4 and E1.6 the amount of DBTL was increased. 2-MPA was used in equal molar quantities compared to benzoic acid. PTMP was used in equal molar SH quantities.

TABLE-US-00002 TABLE 2 Viscosity development of samples E1.1 to E1.6 at 20° C. Sample T.sub.0 min T.sub.15 min T.sub.30 min T.sub.45 min E1.1 16.1 17.3 23.4 38.1 E1.2 16.1 17.6 25.9 46.5 E1.3 16.1 16.1 16.3 16.7 E1.4 16.0 16.0 16.5 17.3 E1.5 15.9 15.9 16.6 18.5 E1.6 15.8 15.8 17.1 20.0

[0081] From Table 2 it can be inferred that the viscosity increase of samples E1.1 and E1.2 is the fastest in time. Samples E.1.3 and E1.4 have the slowest viscosity increase in time, followed by samples E.1.5 and 1.6.

[0082] The blocking behaviour of 2-MPA is better than the PTMP/benzoic acid catalyst blocking system. Even with higher amounts of DBTL and half the amount of 2-MPA (sample E1.6) the viscosity increase is slower than in sample E1.1. A slower viscosity increase indicates a longer pot life. In addition, the use of 2-MPA over PTMP/benzoic acid is preferred from an economic point of view.

[0083] Drying recorder experiments at 60° C. and 20° C. are in line with the viscosity increase experiments of Table 2. Sample E1.3 has the longest open time (the slowest viscosity increase).

[0084] A long open time is advantageous for air and solvent escape from the coating layer. Samples E1.1, E1.3, and E1.5 are directly comparable.

TABLE-US-00003 TABLE 3 Drying recorder results at 60° C. Sample Phase 1 (cm) Phase 2 (cm) Phase 3 (cm) E1.1 1.3 0.2 1.7 E1.3 3.4 0.5 2.3 E1.5 3.1 0.4 1.8

TABLE-US-00004 TABLE 4 Drying recorder results at 20° C. Sample Phase 1 (cm) Phase 2 (cm) Phase 3 (cm) E1.1 1.8 0.4 1.7 E1.3 4.7 2.5 5.7 E1.5 3.1 2.7 5.7

Example 2

[0085] A clear coat composition was prepared by mixing the components in the weight proportions as indicated in Table 5. The molar ratio of isocyanate groups:hydroxyl groups was adjusted to 1.03:1. The clearcoat compositions of Table 5 below contained the following additives: [0086] 0.6 parts by weight Tinuvin 1130 [0087] 1.0 part by weight Tinuvin 292 [0088] 0.1 part by weight BYK 331 [0089] 1.0 part by weight BYK 392

TABLE-US-00005 TABLE 5 Clear coat compositions E2.1 E2.2 E2.3 E2.4 Acrylic polyol (f(OH) = 52.3 52.3 52.3 52.3 6.2; NVC = 75%, T.sub.g = −12° C.) Polyester polyol 19.9 19.9 19.9 19.9 (f(OH) = 2.8; NVC = 100%, T.sub.g = −59° C.) n-butylacetate 25 25 25 25 Ethyl-ethoxy-propionate 0.9 0.9 0.9 0.9 DBTL 0.2 0.2 0.2 0.2 Tolonate HDT LV 2 33.6 33.6 33.6 33.6 Vestanat T1890 E 12.8 12.8 12.8 12.8 Methyl-isoamyl-ketone 3.5 3.5 3.5 3.5 n-butylacetate 3.5 3.5 3.5 3.5 n-butylacetate 14.3 14.1 13.7 14.4 Xylene 5.2 5.2 5.2 5.2 Butylcellosolve ® 5.2 5.2 5.2 5.2 acetate 2-Mercapto-ethanol 0.29 2-MPA 0.39 Benzoic acid 0.45 PTMP 0.45 0.45 VOC gr/L ≦420 ≦420 ≦420 ≦420

[0090] The clear coat compositions were based on 4 different DBTL blocking components, namely 2-MPA (E2.1, according to the invention), PTMP (E2.2, comparative), PTMP/benzoic acid (E2.3, comparative), and 2-mercapto-ethanol (E2.4, comparative). The catalyst blocking agents were used in equal molar quantities or, in the case of PTMP, replaced by equal molar SH quantities.

[0091] The clearcoat compositions were sprayed by a coating robot, 30 minutes after mixing, at 25° C. and 75-80% relative humidity. Aluminium panels were used as a substrate. The substrates were pre-coated successively with a chromate-free etch primer, a non-sanding filler, and a water borne metallic base coat. Once the coating had been applied, the panels were immediately transferred to an oven kept at 60° C. without intermediate flash-off time.

TABLE-US-00006 TABLE 6 Visual/physical characteristics of the clear coat compositions Persoz hardness Sample EHO Flow Warm tackiness (after 20 hours) E2.1 8 8 7 38 E2.2 6-7 7 8 37 E2.3 7 7 8 35 E2.4 2 2 8 35

[0092] Sample E2.1 according to the invention was judged to be the best among the four samples. The Enamel Hold Out and the flow were better than in the case of the comparative Examples. The warm tackiness of sample E2.1 was rated to be 7. The warm tackiness of samples E2.2-E2.4 was rated to be 8.

[0093] The formulation E2.1, containing 2-MPA, has the longest open time, at both 20° and 60° C. A long open time is advantageous for the flow of a coating and for air and solvent escape from the coating layer. A proper solvent and air escape is advantageous for the EHO.

TABLE-US-00007 TABLE 7 Open time at 20° C. Phase 1 (cm) Phase 2 (cm) Phase 3 (cm) E2.1 5.4 0.9 2.2 E2.2 3.0 0.4 1.0 E2.3 3.4 0.4 1.9 E2.4 2.9 0.4 0.9

TABLE-US-00008 TABLE 8 Open time at 60° C. Phase 1 (cm) Phase 2 (cm) Phase 3 (cm) E2.1 2.3 0.3 2.0 E2.2 1.6 0.3 1.5 E2.3 1.8 0.3 2.0 E2.4 1.3 0.3 1.3

Example 3

[0094] Five clear coat compositions were prepared by mixing the components in the weight proportions as indicated in Table 9.

[0095] The clear coat compositions of Table 9 below contained the following additives: [0096] 0.6 parts by weight Tinuvin 1130 [0097] 1.1 parts by weight Tinuvin 292 [0098] 0.1 part by weight BYK 331 [0099] 0.1 part by weight BYK 355

TABLE-US-00009 TABLE 9 Clear coat composition E3.1 E3.2 E3.3 E3.4 E3.5 Acrylic polyol (f(OH) = 6.2; NVC = 55.6 55.6 55.6 55.6 55.6 65%, T.sub.g = 29° C.) Polyester polyol (f(OH) = 2.8; NVC = 19.9 19.9 19.9 19.9 19.9 100%; T.sub.g = −59° C.) n-butylacetate 16.1 16.1 16.1 16.1 16.1 Ethyl-ethoxy-propionate 0.9 0.9 0.9 0.9 0.9 SBP 140/165 4.5 4.5 4.5 4.5 4.5 Tolonate HDT LV/Desmodur XP 2511 44.9 44.9 44.9 44.9 44.9 Ethyl-ethoxy-propionate 9.6 9.6 9.6 9.6 9.6 n-butylacetate 9.6 9.6 9.6 9.6 9.6 DBTL 0.22 0.22 0.22 0.22 0.22 2-MPA 0.24 E2-MPA 0.31 3-MPA 0.24 E3-MPA 0.31 TGA 0.21 SBP 100-140 9 9 9 9 9 Methyl amyl ketone 6 6 6 6 6 VOC g/l ≦420 ≦420 ≦420 ≦420 ≦420

[0100] The clear coat compositions were based on 5 different DBTL blocking components, namely 2-MPA (E3.1, according to the invention), E2-MPA (E3.2, comparative), 3-MPA (E3.3, according to the invention), E3-MPA (E3.4, comparative), and TGA (E3.5, according to the invention). All blocking agents were used in equal molar quantities, and are therefore directly comparable.

[0101] Below, the viscosity development at 20° C. of samples E3.1 to E3.5 measured by DIN cup 4 is summarized

TABLE-US-00010 TABLE 10 Viscosity increase (pot life) DC 4, 20° C. Sample T.sub.0 min T.sub.15 min T.sub.30 min T.sub.45 min T.sub.60 min T.sub.75 min E3.1 15.6 16.0 16.4 16.7 17.1 17.6 E3.2 15.6 16.0 16.8 18.5 22.5 32.9 E3.3 15.5 16.0 16.3 17.3 18.2 20.9 E3.4 15.5 16.0 17.0 19.3 25.3 44.3 E3.5 15.7 16.0 16.3 16.7 17.3 18.0

[0102] From Table 10 it is evident that sample E3.1 has the lowest viscosity after 75 minutes. The compositions according to the invention containing 2-mercapto-carboxylic acid (E3.1 and E3.5) have a lower viscosity after 75 minutes compared to the comparative formulations containing the ethyl ester of 2-mercapto-carboxylic acid functionality (E3.2). The same trend of viscosity development is observed between samples E3.3 (slowest) and E3.4 (fastest). The samples E3.1, E3.3, and E3.5 demonstrate that a thiol and carboxylic acid functionality in close proximity is beneficial for good catalyst blocking behaviour. This phenomenon can most likely be explained by bidentate-coordination of the thiol-carboxylic acid ligand towards the catalyst.

[0103] Drying recorder experiments at 60° C. show the same observations as in the case of the pot life experiments; the formulation containing 2-MPA has the slowest viscosity development.

TABLE-US-00011 TABLE 11 Drying recorder at 60° C.: Sample Phase 1 (cm) Phase 2 (cm) Phase 3 (cm) E3.1 3.1 0.6 1.4 E3.2 1.6 0.3 1.2 E3.3 1.8 0.2 2.1 E3.4 1.3 0.3 0.9 E3.5 2.6 0.5 1.1

Example 4

[0104] Using the Lesonal™ HS 420 top coat line two car body resprays were performed according to the technical data sheet of the product. The Lesonal HS 420 top coat line is a ≦420 gr/l solvent borne polyester/polyacrylate polyol-isocyanate top coat system, especially designed for the car refinish market. Polyester polyol: f(OH)=4.4; NVC=80%; t.sub.g=−2° C. Polyacrylate polyol: f(OH)=4.1; NVC=74%, t.sub.g=20° C. The red colour RAL 3020 was selected as A-component and prepared from the Lesonal™ HS 420 top coat toner assortment. Organic pigmented (e.g. red/blue) A-components are known to have high pinhole sensitivity. The Lesonal™ HS420 hardener was used as the hardener for both samples. One top coat ready-to-spray mixture was finished with sample E4.1 (comparative), the other sample was finished with sample E4.2 (according to the invention)

TABLE-US-00012 TABLE 12 Sample E4.1 E4.2 2-Mercaptopropionic acid 1.5 BYK-392 4.25 Methyl-isoamyl-ketone 24 22 Methyl-amyl-ketone 24 10.25 Ethyl-ethoxy-propionate 14.25 Exxsol D30 37.75 Butylcellosolve acetate ® 10 Acetyl acetone 50 Byk 320 2.0

[0105] The two car body resprays were performed in a spray booth at 35° C. and 25% humidity. The car bodies used were similar to a normal sized 5-door passenger car. The two samples were sprayed 15 minutes after the preparation of the ready-to-spray compositions. The top coat was applied in two layers, with 5-10 minutes flash-off between the layers. After the top coat had been applied, there was no flash-off time and the applied coating was cured at 60-65° C. for 35 minutes.

TABLE-US-00013 TABLE 13 Sample 4 E4.1 E4.2 Pinholes 5 9 EHO 7 8 Sprayability first layer 7 7 Sprayability second layer 6 7 Warm tackiness 8 7 Flow 7 8

[0106] The results in Table 13 demonstrate improvements in pinhole sensitivity, EHO, sprayability, and flow of sample E4.2 over E4.1.

Example 5

[0107] Application of 420 g/l top coats in an industrial manner (e.g. airless/electrostatic apparatus) can be problematic due too significant amounts of air entrapment in top coats. Air entrapment causes pinholes during curing. This example shows the advantage of thinner sample E5.2 according to the invention over comparative Example E5.1 based on known catalyst blocking compounds. These two samples were used together with the Sikkens™ Autocoat BT LV 351™ Topcoat and Hi Flo hardener. The Sikkens™ Autocoat BT LV 351™ is a solvent borne 420 g/l polyester/polyacrylate polyol-isocyanate top coat system. Polyester polyol: f(OH)=4.4; NVC=80%; t.sub.g=−2° C. Polyacrylate polyol: f(OH)=4.1; NVC=74%, t.sub.g=20° C. The ready-to-spray mixture was prepared according to the technical data sheet of the product. The red colour Ral 3020 was selected as the A-component and prepared from the top coat toner assortment of Sikkens™ Autocoat BT LV 351™. Organic pigmented top coats (e.g. red/blue) are known to have relatively high pinhole sensitivity. Application of the two ready-to-spray mixtures was performed on steel panels precoated with sanding filler. Subsequently the two samples were applied with a Graco airless spraying apparatus and immediately cured at 60° C. for 30 min. No flash off time was used.

TABLE-US-00014 TABLE 14 Sample E5.1 E5.2 2-Mercaptopropionic acid 1.5 BYK-392 2.0 4.5 Ethyl-ethoxy-propionate 45.5 36.5 Butylcellosolve ®-acetate 25 SBP 140/165 28.0 30 DBTL 0.3 0.3 n-Butylacetate 2.7 1.35 Xylene 1.35 PTMP 1.5 Acetylacetone 20.0

TABLE-US-00015 TABLE 15 Example E5.1 E5.2 Pinholes 4 9 Warm tackiness 8 7

[0108] The results in Table 15 clearly demonstrate the improvements in pinhole sensitivity of coating E5.2 according to the invention. The warm tackiness indicates the curing is almost on the same level as in comparative Example E5.1. Hence, it has been demonstrated that the balance of properties has been improved.

Example 6

[0109] Example 6 demonstrates the influence of the functionality of the polyol used in the coating composition. Example E6.1 is an example according to the invention, containing a acrylic polyol having an average functionality of 6.2 hydroxyl groups. Example E6.2 is a comparative Example wherein the acrylic polyol of Example E6.1 was replaced with an acrylic polyol having an average functionality of 3.0 hydroxyl groups. The other properties of the acrylic polyols used in these Examples, in particular glass transition temperature and molecular weight, were the same. The amount if acrylic polyols in Examples E6.1 and E6.2 was selected such that the total amount of hydroxyl groups was the same in both compositions. The amount of additives was selected such that their proportion on non-volatile content was the same in both compositions. Hence, any differences in the coating compositions and the coatings can be attributed to the difference in functionality. The components of the compositions are summarized, in parts by weight, in Table 16.

TABLE-US-00016 TABLE 16 Clearcoat composition E6.1 E6.2 Acrylic polyol (f(OH) = 6.2; NVC = 65%, 55.6 OH number = 140 Mg KOH/g Polyester polyol (f(OH) = 2.8; NVC = 100%; 19.9 19.9 T.sub.g = −59° C. OH number = 303 mg KOH/g Acrylic polyol f(OH) = 3.0 NVC = 63.8% 117.8 OH number = 67.3 n-butylacetate 16.1 21.9 Ethyl-ethoxy-propionate 0.9 0.9 SBP 140/165 4.5 4.5 Tolonate HDT LV/Desmodur XP 2511 44.9 44.9 Ethyl-ethoxy-propionate 9.6 9.6 n-butylacetate 9.6 9.6 DBTL 0.20 0.28 Methylamyl ketone 6.6 6.6 SBP 140/165 8.9 8.9 Tinuvin 292 1.1 1.4 BYK-355 0.1 0.14 BYK-331 0.1 0.14 BYK-392 0.5 0.7 2-Mercaptopropionic acid 0.24 0.33 VOC (g/L) ≦420 ≦420

[0110] In Table 17 the viscosity development at 20° C. of samples E6.1 and E6.5 measured by DIN cup 4 is summarized.

TABLE-US-00017 TABLE 17 Viscosity increase (pot life) DC 4, 20° C. Sample T.sub.0 min T.sub.15 min T.sub.30 min T.sub.45 min E6.1 15.3 15.8 16.0 16.4 E6.2 18.0 19.2 19.7 20.1

[0111] From Table 17 it can be inferred that sample E6.1 according to the invention has a lower starting viscosity than comparative sample E6.2. furthermore, the viscosity of sample E6.2 increases faster. In can be concluded that clearcoat sample E6.1 according to the invention can be longer processed can be longer processed that comparative sample E6.2.

[0112] The drying rate of applied coatings from samples E6.1 and E6.2 at 20° C. and at 60° C. was determined using a BK drying recorder. The results are summarized in Table 18.

TABLE-US-00018 TABLE 18 Sample Phase 1 (cm) Phase 2 (cm) Phase 3 (cm) Drying recorder results at 20° C. E6.1 7 7 5.1 E6.2 4.8 20.0 nd Drying recorder results at 60° C. E6.1 2.9 0.6 2.9 E6.2 3.0 3.9 3.8

[0113] The overall drying time of clearcoat sample E6.1 according to invention is shorter then E6.2, both at 20° C. and 60° C. degrees Celsius. This is particularly surprising, as the viscosity increase in the pot of sample E6.1 is slower, as demonstrated above.

[0114] The clearcoat compositions were sprayed by a coating robot, 20 minutes after mixing, at 22° C. and 45% humidity. Steel panels were used a substrate. The substrates were pre-coated successively with a sanding filler and a water borne, deep black, basecoat. Once the coating had been applied, the panels were left for five minutes flash off time before being transferred to an oven kept at 60° C. The clearcoat panels were cured for 40 minutes and left for 24 hours before being judged.

TABLE-US-00019 TABLE 19 Visual characteristics of the clear coats Sample EHO Sagging Pinholes E6.1 9-10 9-10 9-10 E6.2 8-9  8-9  6-7 

[0115] Form table 19 it can be inferred that clearcoat formulation E6.1 has better visual properties and less defects compared to sample E6.2. Sample E6.1 according to the invention was rated better for enamel hold out and sagging. A significant difference was observed in solvent pop (pinhole) sensitivity. Comparative clear coat E6.2 was judged on average 6-7 (on a scale of ten), while sample E6.1 was judged on average a 9-10.

[0116] From the results of Example 6 it can be concluded that sample E.6.1 according to the invention exhibits a slower viscosity increase, i.e. longer potlife, faster drying, and better visual properties than the comparative sample E6.2. The only difference between these samples is the functionality of the polyol used in both samples. Hence, the improved properties of sample E6.1 can be attributed to the use of a polyol having a average functionality of more than 3 hydroxyl groups per molecule.