MXPA96003914A - Metal pretracted with an aqueous solution quecontains a silicate or aluminate inorganicodisuelto, an orange functional silanum and a non-functional silanum for a resistance to corrosionmejor - Google Patents

Abstract

The present invention relates to a method for pretreating a metal to improve the corrosion resistance, characterized in that it comprises the steps of: providing an alkaline solution containing at least one of a dissolved inorganic silicate and a dissolved inorganic aluminate, a silane organofunctional and a crosslinking agent that includes two or more trialkoxyl or triacetoxysilyl groups, rinsing a metal sheet with the alkaline solution and drying the sheet to form a relatively insoluble composite layer containing siloxa

Description

METAL PRETRACTED WITH AN AQUEOUS SOLUTION CONTAINING A SILICATE OR DISSOLVED INORGANIC ALUMINATE, ONE YEAR ORGANOFUNCTIONAL AND A NON-FUNCTIONAL SILAO FOR A RESISTANCE TO IMPROVED CORROSION BACKGROUND OF THE INVENTION This invention relates to the pretreatment of a metal as a composite siloxane-containing layer to form an adherent covalent bond between an outer paint layer and the metal substrate. More particularly, the invention relates to a one-stage process for pretreating the metal with an alkaline solution containing at least one of a dissolved inorganic silicate and a dissolved inorganic aluminate, an organofunctional silane and a non-silane crosslinking agent. functional. It is known to improve the corrosion resistance of cold-rolled steels and metal coatings by passivating the surface with a chromate coating. Due to the toxic nature of hexavalent chromium, rinses containing chromate ions are undesirable for industrial use. It is also known to treat cold-rolled and metal-coated steels with a phosphate conversion coating to improve the adhesion of the paint. To improve the corrosion resistance, however, these phosphate steels generally require a final phosphate rinse. It has been proposed to improve the corrosion resistance and adhesion of the paint on cold-rolled and galvanized steel by coating with an inorganic silicate and then treating the silicate coating with an organofunctional silane. U.S. Patent No. 5,108,793 describes the formation of the silica coating by rinsing the steel with an alkaline solution containing dissolved silicate and metal salt. The steel is dried to form a silicate coating having a thickness of at least 20 Á. After this, the silica-coated steel is rinsed with an aqueous solution containing organofunctional silane at 0.5-5% by volume. The silane forms a relatively adherent covalent bond between the silica coating and an outer layer of paint. There have been numerous other proposals to improve the resistance to corrosion and the adhesion of the paint on cold rolled and galvanized steel. The pretreatment of the steel with a chromate solution containing colloidal silicate and / or aluminate and silane has been proposed. It has also been proposed to rinse the steel with a chromate solution and then rinse the chromed steel with a solution containing colloidal silicate and aluminate and silane. The rinsing of the steel with a solution containing polymer resin, colloidal silicate and silane has also been proposed. As is evident from the effort of the prior art, there has been a felt need for a long time to develop a process to improve the corrosion resistance of and adhesion of paint to a metal with the use of safe coating solutions from the environmental point of view that can be discarded in a non-expensive way. The process must be low cost, use non-toxic materials that can be easily discarded, provide long-term resistance in a humid environment and that do not require complex multi-stage processing or chrome plating. BRIEF DESCRIPTION OF THE INVENTION This invention relates to a pretreated metal in a one-step process, with a composite siloxane-containing layer to form an adherent covalent bond between the paint and the metal substrate. The invention includes rinsing the metal with an alkaline solution containing at least one of a dissolved inorganic silicate and a dissolved inorganic aluminate, an organofunctional silane and a crosslinking agent containing two or more trialkoxysilyl groups. The metal is then dried to completely cure the functional silane to form an insoluble composite layer strongly adhered to the metal substrate. Another feature of the invention includes the above alkaline solution, which contains (a concentration) 0.005 M of the silicate, aluminate or mixtures thereof. Another feature of the invention includes the aforementioned alkaline solution, which contains at least 0.1% by volume of each of the organofunctional silane and the crosslinking agent. Another feature of the invention includes the ratio of the organofunctional silane mentioned above to the crosslinking agent in the range of 2: 1 to 10: 1. Another feature of the invention includes the additional step of coating the metal with a phosphate layer before rinsing with the alkaline solution. A principal object of the invention is to improve the corrosion resistance and adhesion to the paint of a metal. Additional objects include improving the corrosion resistance and adhesion of the paint to the metal without using toxic materials such as chromates that produce toxic waste and having the possibility of producing a painted metal having high durability in a humid environment.

The advantages of the invention include forming a composite layer which is insoluble, has excellent affinity for painting on cold rolled steel and with metal coating, including phosphatized and metallic cold rolled steel, and has good corrosion resistance. The process of the invention does not use or create hazardous substances from the point of view of the environment, is inexpensive and has applicability to a variety of paints. DETAILED DESCRIPTION OF THE PREFERRED MODALITIES An important aspect of the invention is to pretreat a sheet of metal to be painted with a composite layer containing at least one of an inorganic silicate or an inorganic aluminate and siloxane. The siloxane stabilizes the composite layer, thereby increasing the corrosion resistance and forming a tenacious covalent bond between an outer layer of paint and other polymers and the metal substrate. Unlike an uncured silane, the siloxane has a "Si-O-Si-hydrolytically stable structure impermeable to water and is believed to form a better adhesion because the siloxane interdiffs throughout the inner composite layer and the external paint. That is, siloxane and paint become an impenetrable network. The siloxane also improves the wettability of the paint to the composite layer and ensures a continuous film of moisture impervious paint. To form a continuous adherent composite layer containing siloxane, an alkaline solution is prepared which contains at least one of a dissolved inorganic silicate, a dissolved inorganic aluminate or a mixture thereof, an organofunctional silane and a silane crosslinking agent which it has no organic functionality other than two or more trialkoxysilyl groups. The organofunctional silane has the general formula R1-R2-Si (0X3) 3 wherein R- ^ is an organofunctional group, R2 is an aliphatic or aromatic hydrocarbon group and X is an alkyl group. For example, R ^^ can be a group -NH2, R2 can be a propyl group and X preferably is CH3 or C2H5. Alternative groups for R2 include any chain (CH2)? where x is preferably the integer 3. A preferred organofunctional silane found to work very well in the invention was ei? -aminopropyltriethoxy silane (APS). Examples of other silanes that may be used include? -glycidoxypropyltrimethoxy (GPS), -y-methacryloxypropyltrimethoxy (MPS), N- [2- (vinylbenzylamino) ethyl] -3-aminopropyltrimethoxy (SAAPS), mercaptopropyltriacetoxy, diaminosilanes such as NH2-CH2 -NH-CH-CH2-CH2-Si (OX) 3 and vinylpropyltrimethoxy silane. An alkaline solution means an aqueous solution having a pH greater than 7 and preferably at least 12, it is important that the rinse solution be alkaline because the organofunctional silanes work much better. It is also important that the solution does not contain an organic solvent due to environmental concern since the pre-treatment solution is generally contained in a tank open to the atmosphere. The non-functional silane or crosslinking agent includes two or more trialkoxysilyl groups having the general structure R3- (SiOY3) n wherein R3 is an aliphatic or aromatic hydrocarbon, and can be a methyl, ethyl or acetoxy group and n is an equal whole or greater than 2. A preferred silane crosslinking agent is 1,2 bis trimethoxysilyl ethane (TMSE), for example (C2H50) 3 Si-CH2-CH2-Si (C2H50) 3. Other possible crosslinking agents include (CH30) 3SiCH2CH2CH2Si (OCH3) 3 ,. (CH30) 3YES (CH2) 6Si (OCH3) 3, Si (OC2H5) 4, (CH30) 3SÍCH2CH2Si (CH30) 2-0 -YES (CH3) 2CH2CH2Si (OCH3) 3, OR The concentration of the non-functional silane crosslinking agent in the alkaline rinse solution should be at least 0.02% by volume, at least 0.2% by volume which is the preferred concentration. The concentration should be at least 0.02% by volume because the reactivity of the alkaline solution would be too slow at lower concentrations. The concentration of organofunctional silane in the alkaline rinse solution should be at least 0.1% by volume, with at least 0.8% by volume being preferred to ensure that a continuous film is formed. The ratio of the organofunctional silane concentration to the concentration of the silane crosslinking agent should preferably be at least 2: 1 but not exceed about 10: 1. If the concentration of the organofunctional silane is less than twice that of the crosslinking agent, the amount of the crosslinking agent present is excessive and is wasted and the number of functional groups is too low to ensure good adhesion of the paint to the composite layer. On the other hand, if the concentration of the organofunctional silane is more than about ten times that of the crosslinking agent, the amount of the crosslinking agent present may be insufficient to completely react all the organofunctional silane and convert it to siloxane. A preferred ratio of functional silane to the crosslinking agent is 4: 1. Neither the concentration of the crosslinking agent nor that of the organofunctional silane should exceed about 5.0% by volume in the alkaline solution due to the excessive cost and that the thickness of the composite layer can be excessive to cause the composite layer to be brittle. The alkaline solution also contains at least one dissolved inorganic silicate, a dissolved inorganic aluminate or a mixture of the silicate and the aluminate. It is important that the composite layer formed from the alkaline solution contains silicate and / or aluminate to provide excellent corrosion protection for a painted metal sheet. The silicate and / or aluminate composite layer preferably has a thickness of at least 10 A, more preferably at least 20 A and more preferably a thickness of 50 A. The composite layer should have a thickness of at least 10 A to ensure a continuous layer tightly bonded to the metal substrate and impervious to moisture. It was determined that a minimum concentration of the silicate and / or aluminate in the solution of about 0.005 M ensures that a continuous composite layer is formed. At concentrations greater than about 0.05 M, the corrosion resistance is not improved, the costs become excessive and the thickness of the composite layer can become excessive. The composite layer should not have a thickness in excess of about 100 A, because a thick coating is brittle or brittle and tends to crack and flake off when the coated metal is manufactured. Examples of silicates that can be used include Na (SiO3) ?, for example, liquid sodium silicate, sodium metasilicate or sodium polysilicate. Examples of the aluminates that may be used include Al (OH) 3 dissolved in NaOH or A1203 dissolved in NaOH. When inorganic silicate is used, the alkaline solution preferably includes a metal salt such as an alkaline earth metal salt. Any of the alkaline earth salts of Ba (N03) 02, Ca (N03) 2 or Sr (N03) 2 are acceptable for this purpose. After being formed on a steel sheet, the silicate containing a siloxane and / or aluminate layer should not be dissolved during subsequent processing or should not be dissolved by the corrosive environment within which the painted sheet is placed. The function of the metal salt is to make the composite silicate layer insoluble. Since the metal salt in the alkaline solution reacts in direct proportion to the dissolved silicate, the concentration of the salt must be at least equal to the concentration of the dissolved silicate. Therefore, a minimum acceptable concentration of the metal salt is approximately 0.005 M as well. The composite layer of the invention can be applied to metal sheets such as hot-rolled and pickled steel, cold-rolled steel, hot-dip or electro-plated metal-coated steel, chromium alloy steel and stainless steel. A composite layer of aluminate of the invention has particular use for pre-treating non-ferrous metals such as aluminum or aluminum alloy or steel coated with aluminum or aluminum alloy. Aluminum coatings can be included in metal coatings, aluminum alloy, zinc, zinc alloy, lead, lead alloy and the like. The meaning of sheet includes a strip or continuous sheet or cut sections. The present invention has particular utility for providing good adhesion of the paint to the phosphatic steels to be painted. The steel sheets to be painted, particularly cold rolled steel, can first be coated with a phosphate conversion layer before applying the siloxane-containing composite layer of the invention. The composite layer improves the protection against corrosion and reinforces the bond between the paint and the phosphate substrate. An advantage of the invention is that it is capable of rapidly pre-processing a sheet of metal in a short period of time. Coating times in excess of 20 seconds in general do not lend themselves to industrial applicability. It was determined that a phosphatized steel, pretreated with the composite layer of the invention can be formed in short rinse times of less than 30 seconds, preferably less than 10 seconds. Another advantage is that a high rinse temperature is not required for the alkaline solution when the composite layer is formed. The ambient temperature, for example 25 ° C, and rinsing times as fast as 2-5 seconds can be used with the invention. Example 1 By way of example, galvanized steel test panels were pretreated by hot dip with an alkaline solution of the invention. After these test panels were painted, their corrosion resistance was compared to conventionally pre-treated hot-dip galvanized steel test panels. The pretreatment coatings formed on several comparison panels were formed by rinsing with standard solutions including a phosphate conversion solution, a chromate solution and an alkaline solution containing dissolved silicate. These standard pretreatment coatings may also have been rinsed with another solution containing a silane. A silicate solution was prepared by dissolving 0.015 M liquid sodium silicate and 0.015 M Ca (N03) 2 in water. An organofunctional silane solution was prepared by dissolving 2.4% by volume APS in water. A non-functional silane solution was prepared by dissolving the 0.6% by volume TMSE crosslinking agent in water. To form an embodiment of an alkaline solution of the invention, equal volumes of the three solutions were mixed together immediately after being hydrolyzed in the ratio of 1: 1: 1 with the pH adjusted to 12 with the use of NaOH. The alkaline solution of the invention contains 0.005 M silicate, 0.005 M salt, 0.8% by volume APS and 0.2% by volume TMSE. After being cleaned of the solvents, the test panels were given several pretreatments. The phosphate conversion process included using zinc phosphate sold under the tradename Chemfil 952. The test panels of the invention were rinsed with the alkaline solution for 10 seconds to form a composite layer containing organofunctional silicate and silane. The organofunctional silane was cured in the presence of air by the siloxane crosslinking agent which diffuses throughout the composite layer. The composite layer had an average thickness of approximately 15A on each side of the test panels. Then all test panels were coated with a standard internal automotive E coating plus a standard external automotive acrylic-melamine surface coating. The thickness of the coating E and the acrylic surface coating was approximately 100 μm. After painting, the test panels were scratched by the paint and the composite layer and on the steel base metal. Then the striped panels were exposed for eight weeks to the standard Cyclic General Motors crust corrosion test. After the completion of the test, the panels were washed in water, dried and the loose paint was removed by brushing. The test panels were visually observed for slippage, that is, the propagation of corrosion under the paint from the scratch mark. The results are summarized in table 1. Table 1 Pre-treatment Bleed (mm) Fos only 1.40 Fos + chromate 1.13 Fos + silicate 0.93 Fos + APS silane 1.26 Fos + silicate + APS silane 0.90 Of the invention (Fos + silicate + APS silane + TMSE crosslinker) 0.75 The results show that a conventional phosphate pretreatment followed by a chromate rinse (the generally accepted industrial standard) would be better than conventional phosphate pretreatment alone. Additional improvements can be obtained with the use of a conventional silicate pretreatment. The addition of a final rinse with silane to the panels pretreated with the conventional pretreatments with phosphate or chromate, however, adds little resistance to further corrosion, for example the shift is reduced from 0.93 mm to 0.90 mm. A significant improvement in corrosion resistance, for example, shifting reduced to 0.75 mm, was obtained when the phosphate test panels were pretreated with an alkaline solution of the invention containing a non-functional silane crosslinking agent. Example 2 In another example, galvanized steel test panels were evaluated by hot dip as to corrosion, as well as paint adhesion, similar to those described in example 1, except that none of the test panels of Comparison were pretreated with a phosphate conversion coating after cleaning. In addition to being evaluated using the GM crust test, the test panels were given an NMPRT paint adhesion test * as well. The results are summarized in table 2. Table 2 Pretreatment Shifting (mm) NMPRT (min.) * None 2.2 1.5 APS silane only 1.8 2.0 Silicate only 1.7 2.3 Silicate + APS silane 1.4 9.5 Of the invention (silicate + APS + crosslinking agent TMSE *) 1.1 30 * NMPRT is a measure of the adhesion of the paint to the substrate, N-methyl pyrrolidone is used as a dilation solvent to separate the paint as it is measured in minutes. This test is described in a document where the applicant is a co-author and published in the Journal of Adhesion Science and Technology, 7, 897 (1993), incorporated herein by reference. Again the results clearly show that by using the one-step alkaline solution of the invention, which contains a non-functional silane crosslinking agent, it can be expected to provide the best performance against corrosion and especially paint adhesion. The results of the NMPRT test suggest that the adhesion of the paint to the test panels of the invention was approximately three times better than the comparative test panels rinsed with a conventional alkaline solution containing organofunctional silicate and silane but did not contain a reticulation. These results illustrate that the composite coating of the invention provided improved corrosion resistance and improved paint adhesion for bare, ie non-phosphatized, metals.

Example 3 In another example, the galvanized steel hot dip test panels were evaluated for corrosion and paint adhesion similar to that described in Examples 1 and 2. That is, some of the test panels were pretreated with a zinc phosphate conversion coating after cleaning, in a manner similar to that in Example 1 and others were not pretreated with the phosphate as in Example 2. After the pretreatments, the test panels are coated with A standard polyester powder paint. The powder paint is cured at 170 ° C for 30 minutes. The paint had a thickness of approximately 25 mm. The results of corrosion and adhesion of the paint are summarized in table 3. Table 3 Phosphating Pre-treatment ** Bleed (mm) None 1.2 Chromate 0.8 Silicate 1.0 Silicate + APS silane 0.6 Of the invention (Silicate + APS + TMSE crosslinking agent) 0.4 ** All test panels were phosphatized before receiving the indicated pretreatment. For example, the panel indicated by "none" was phosphatized only and the panel indicated by "Chromate" was phosphatized and then rinsed with chromate, etc. Non-Phosphating Pretreatment Shifting (mm) NMPRT (min.) None 1.6 3.0 APS silane only 1.3 > 45 Silicate only *** 0 Silicate + APS silane 0.8 > 45 Of the invention (silicate + APS silane + crosslinking agent TMSE *) 0.6 > Four. Five *** Total delamination The results demonstrate again that by using the one-step alkaline solution of the invention, which contains a non-functional silane crosslinking agent, it can be expected to provide the best performance against corrosion, with or without a pretreatment with phosphate. Example 4 In another example, the steel test panels were evaluated for corrosion in a manner similar to that described in example 1, except that the test panels were cold rolled steel without a zinc metal coating. In this example, the same concentrations were used in the alkaline solution of the invention, but the different organofunctional silanes were replaced by APS for some of the test panels. For all the test panels of the invention, the alkaline rinsing time was reduced to five seconds instead of ten seconds. These test panels were evaluated with the use of a standard Japanese cyclic corrosion test, this is CCT-4. In this test, the corrosion is less aggressive than that of the GM crust test and was exposed for a standard exposure time of three months. The results are summarized in table 4. Table 4 Pre-treatment Bleeding (mm) Fos only 0.93 Fos + chromate 0.75 Of the invention: Fos + silicate + GPS silane + crosslinking agent TMSE 1.32 Fos + silicate + MPS silane + crosslinking agent TMSE 1.07 Fos + silicate + SAAPS silane + crosslinking agent TMSE 0.71 Fos + silicate + APS silane + TMSE 0.52 crosslinking agent The results demonstrate that using the alkaline solution of the invention, containing APS or SAAPS silane and a non-functional eilane crosslinking agent, can be expected to provide improved corrosion efficiency for cold rolled, phosphating steel.

Example 5 In another example, the test panels were again evaluated for corrosion, in a manner similar to that described in example 1, except that the test panels were cold rolled steel, the test panels were phosphatized with iron phosphate instead of zinc phosphate and the pretreated panels were painted with a conventional solvent-based polyester paint. After painting, the test panels were scratched through the paint and the composite layer and to the base of steel metal. Then the scratched panels were exposed for a week to the GM crust corrosion test. After the completion of the test, the panels were washed in water, dried and the dried paint was removed using tape. The percentages of paint raised from the clogged surface area are summarized in Table 5. Table 5 Pre-treatment% Painting Lift Fos only 60-70 Fos + chromate 30-40 Of the invention (Fos + silicate + APS + crosslinking agent TMSE 0 The results with the use of the tape showed that the use of the alkaline solution of the invention, containing APS silane and a non-functional silane crosslinking agent can be expected to improve the adhesion of the paint to the steel rolled in cold phosphated, compared to cold rolled steel pretreated with conventional phosphate or phosphate plus chromate The painted steel sheet, pretreated with a composite silicate layer containing siloxane has excellent protection against long-term corrosion and paint adhesion The inorganic silicate forms the necessary base for a protective layer against corrosion impervious to moisture.The organofunctional silane establishes an strong covalent bond between the silicate and the steel substrate and between the silicate and the paint. The efficiency of organofunctional silane is improved when cured by a non-functional silane, in such a way that the silicate and / or aluminate is more stable. That is, a crosslinked silane forms a dense network that has improved adhesion to a metal substrate. The silicate provides a large number of silanol groups which are the reaction sites for the silane and the crosslinking agent. Thus, the network is denser and impermeable to water. It will be understood that various modifications can be made to the invention without deviating from the spirit and scope thereof. Accordingly, the limits of the invention should be determined from the appended claims.

Claims (23)

1. CLAIMS 1. A method for pretreating a metal to improve corrosion resistance, characterized in that it comprises the steps of: providing an alkaline solution containing at least one of a dissolved inorganic silicate and a dissolved inorganic aluminate, an organofunctional silane and an crosslinking agent including two or more trialkoxyl or triacetoxysilyl groups, rinsing a metal sheet with the alkaline solution, and drying the sheet to form a relatively insoluble composite layer containing siloxane.
2. 2. The method according to claim 1, characterized in that it includes the additional step of painting the composite layer.
3. 3. The method according to claim 1, characterized in that the alkaline solution contains at least 0.005 M of the silicate.
4. 4. The method according to claim 1, characterized in that the alkaline solution includes at least 0.1% by volume of the crosslinking agent.
5. 5. The method according to claim 1, characterized in that the alkaline solution includes at least 0.1% by volume of the organofunctional silane.
6. 6. The method according to claim 4, characterized in that the alkaline solution includes 0.2 -5.0% by volume of the organofunctional eilane.
7. The method according to claim 1, characterized in that the ratio of the organofunctional silane to the crosslinking agent in the alkaline solution is in the range of 2: 1 to 10: 1.
8. The method according to claim 1, characterized in that the metal sheet is cold-rolled steel with a layer of zinc phosphate or iron phosphate before being rinsed with the alkaline solution.
9. 9. The method according to claim 1, characterized in that the alkaline solution has a pH > 12 and the organofunctional silane is APS.
10. 10. The method of compliance with the claim 3, characterized in that the alkaline solution includes at least 0.005 M of a metal salt.
11. 11. The method according to claim 1, characterized in that the crosslinking agent is TMSE.
12. 12. The method in accordance with the claim 1, characterized in that the metal sheet is made of aluminum or an aluminum alloy and the alkaline solution contains at least 0.005 M of the aluminate.
13. The method according to claim 1, characterized in that the metal sheet is made of steel 2b Coated with a metallic coating of aluminum or aluminum alloy and the alkaline solution contains at least 0.005 M of the aluminate.
14. A method for the pretreatment of steel, to improve the corrosion resistance and adhesion of the paint, characterized in that it comprises the steps of: providing an alkaline solution containing at least 0.005 M of one of a dissolved inorganic silicate and a dissolved inorganic aluminate, 0.1-5.0% of an organofunctional silane, at least 0.1% by volume of a crosslinking agent including two or more trialkoxyl or triacetoxylylyl groups, rinsing a steel sheet with the alkaline solution, removing the sheet for forming a relatively insoluble composite layer containing siloxane, and painting the composite layer, whereby the siloxane forms an adherent covalent bond between the paint and the steel alloy.
15. 15. An aqueous alkaline solution, characterized in that it contains at least one of a dissolved inorganic eilicate and a dissolved inorganic aluminate, an organofunctional silane and a crosslinking agent including two or more trialkoxyl or triacetoxy silyl groups.
16. 16. The composition according to claim 14, characterized in that it contains at least 0.005 M of the silicate. 2 G
17. 17. The solution according to claim 15 or claim 16, characterized in that it includes at least 0.1% by volume of the crosslinking agent.
18. 18. The solution according to any of claims 15 to 17, characterized in that it includes at least 0.1% by volume of the organofunctional silane, preferably 0.2-5.0% by volume of the organofunctional silane.
19. 19. The solution according to any of claims 15 to 18, characterized in that the ratio of the organofunctional eilane to the crosslinking agent is in the range of 2: 1 to 10: 1.
20. 20. The solution according to any of claims 15 to 19, characterized in that it has a pH of at least 12.
21. 21. The solution according to any of claims 15 to 20, characterized in that the organofunctional silane is APS.
22. 22. The solution according to any of claims 15 to 21, characterized in that it includes at least 0.005 M of a metal salt.
23. 23. The solution according to any of claims 15 to 22, characterized in that the crosslinking agent is TMSE.