Process and seal coat for improving paint adhesion
Aqueous compositions useful as pretreatments prior to painting and to reduce the formation of rust in the uncoated condition consist essentially of water, an organo-functional silane, a compound of a Group IV-B element, and optionally a polyvinyl alcohol, stabilizing agents, wetting agents, thickeners, and biocides, and have an alkaline pH>7. A process for treating a metal surface includes contacting the surface with such an aqueous composition. The compositions and processes were found to have further benefits in comparison to the zinc phosphate metal pretreatment thought to be the standard in the industry. Preferably, the pH of the compositions is alkaline, which has been found to improve the efficacy of a silane as a surface treatment to improve uniformity of paint adhesion. By operating in the alkaline pH range, it was found that flash rusting and/or blush rust were significantly reduced.
1. An aqueous pretreatment composition for treating a metal surface, said composition comprising water, an organo-functional silane, a compound of a Group IV-B element, a polyvinyl alcohol in a concentration ranging from 1.0×10 −4 to 1.0×10 −3 wt. %, and optionally stabilizing agents, wetting agents, thickeners, and biocides, wherein the composition has an alkaline pH>7.
2. The aqueous pretreatment composition of claim 1 , wherein the compound of a group IV-B element is an acid selected from the group consisting of fluorozirconic acid, fluorotitanic acid, and fluorohafnic acid.
3. The aqueous pretreatment composition of claim 2 , wherein the acid is fluorozirconic acid.
4. The aqueous pretreatment composition of claim 1 , wherein the organo-functional silane is selected from the group consisting of an aminopropyltriethoxy silane, a mercapto silane, and an epoxy silane.
5. The aqueous pretreatment composition of claim 4 , wherein the organo-functional silane is an aminopropyltriethoxy silane.
6. The aqueous pretreatment composition of claim 1 , wherein the organo-functional silane is an aminopropyltriethoxy silane and the compound of a group IV-B element is fluorozirconic acid.
7. The aqueous pretreatment composition of claim 1 , wherein the composition has a pH of from >7 to about 11.
8. The aqueous pretreatment composition of claim 1 , wherein the composition has a pH of from >7 to about 8.5.
9. A process for treating a metal surface, said process comprising the step of: contacting the metal surface with an aqueous pretreatment composition comprising water, an organo-functional silane, a compound of a Group IV-B element, and a polyvinyl alcohol in a concentration ranging from 1.0×10 −4 to 1.0×10 −3 wt. %, and optionally stabilizing agents, wetting agents, thickeners, and biocides, wherein the composition has an alkaline pH>7.
10. The process of claim 9 further comprising, before the contacting step:
contacting the metal surface with an aqueous composition comprising fluorozirconic acid and colloidal silica, followed by a water rinse.
11. The process of claim 10 further comprising, after contacting the rinsed metal surface with the aqueous pretreatment composition:
applying a coating of paint to the metal surface.
12. The process of claim 10 further comprising, after contacting the rinsed metal surface with the aqueous pretreatment composition:
rinsing the metal surface with water; and
then applying a coating of paint to the metal surface.
13. The process of claim 12 , wherein the applying step comprises applying a coating of paint to the metal surface by electrocoating.
14. The process of claim 9 further comprising, before the contacting step:
cleaning the metal surface with an aqueous silicated cleaner and rinsing;
then contacting the rinsed metal surface with an aqueous composition comprising fluorozirconic acid and colloidal silica, followed by a water rinse.
15. The process of claim 14 further comprising, after contacting the rinsed metal surface with the aqueous pretreatment composition:
applying a coating of paint to the metal surface.
16. The process of claim 14 further comprising, after contacting the rinsed metal surface with an aqueous pretreatment composition:
rinsing the metal surface with water; and
then applying a coating of paint to the metal surface.
17. The process of claim 16 , wherein the applying step comprises applying a coating of paint to the metal surface by electrocoating.
18. The process of claim 9 , wherein the compound of a group IV-B element is an acid selected from the group consisting of fluorozirconic acid, fluorotitanic acid, and fluorohafnic acid.
19. The process of claim 18 , wherein the acid is fluorozirconic acid.
20. The process of claim 10 , wherein the organo-functional silane is selected from the group consisting of an aminopropyltriethoxy silane, a mercapto silane, and an epoxy silane.
21. The process of claim 20 , wherein the organo-functional silane is the aminopropyltriethoxy silane.
22. The process of claim 9 , wherein the organo-functional silane is an aminopropyltriethoxy silane and the compound of a group IV-B element is fluorozirconic acid.
23. The process of claim 9 , wherein the composition has a pH of from >7 to about 11.
24. The process of claim 9 , wherein the composition has a pH of from >7 to about 8.5.