WO1993016813A1

WO1993016813A1 - Method for improving an autodeposition type coating

Info

Publication number: WO1993016813A1
Application number: PCT/US1993/001184
Authority: WO
Inventors: Motoo Nakano; Takaomi Nakayama; Takumi Honda
Original assignee: Henkel Corporation
Priority date: 1992-02-24
Filing date: 1993-02-24
Publication date: 1993-09-02
Also published as: MX9300985A; JP3164874B2; JPH05237457A; AU3661693A

Abstract

The secondary adhesion of autodeposited resin film is improved by using hydrogen peroxide as the primary oxidant in a concentration not greater than 2.0 g/L, as measured by an oxidation-reduction titration of one or more samples of the autodeposition composition.

Description

Description METHOD FOR IMPROVING AN AUTODEPOSITION TYPE COATING

Technical Field

This invention relates to a method for improving the secondary adhesion of a deposited resin film obtained by immersing a substrate metal in an autodeposition type coating composition, so that a resin film is formed on the metal surface from the dispersed resin particles contained in the autodeposition composition. Background Art

Compositions which can form a resinous coating on a substrate metal surface by immersion of the substrate metal surface in the composition are known as autodeposition type coating compositions. Patents, such as JP Provisional Pub¬ lication No. Sho 47-17630 and 52-21006, have described coat¬ ing compositions which contain resin dispersed in water, oxidants such as H₂θ₂, and acids such as HF, wherein the pH level is less than about 3.8. The characteristic feature of an autodepositing composition is that when a metal sur¬ face is submerged in the composition, a coating film is formed due to the chemical reaction of the composition with the metal surface (in other words, resin particles associ¬ ate with the metal ions which eluted from the metal surface due to the etching action and deposit on the metal sur¬ face) . Thus, without applying electricity from an outside source such as needed for electrodeposition, a resin film is formed evenly on the metal surface.

The deposited resin film obtained by the above men¬ tioned autodeposition type coating composition has to be adherent to the metal surface. In this case, the require¬ ment for the adhesion between the deposited resin film and the metal surface is not only the adhesion with the metal surface immediately to shortly after the resin is dried (hereafter called the primary adhesion) , but also the adhe¬ sion achieved after a long period of time in an environment where such coated substrates are practically used, or in a laboratory simulation of such an environment, (hereafter called the secondary adhesion) . The secondary adhesion of the coating film to the metal surface is an important prop¬ erty that determines the quality of the processed material. Disclosure of the Invention

Problems to Be Solved by the Invention

The invention aims to solve the problems in the above mentioned conventional technology and to improve the secon¬ dary adhesion of the deposited film formed by the autodepo- sition type composition. Summary of the Invention

The method according to the invention uses H₂0₂ as an oxidant and its concentratio is not more than 2.0 grams per liter (hereinafter usually abbreviated "g/L") , and the secondary adhesion is improved by controlling the H₂0₂ con¬ centration with an oxidation-reduction titration. In one embodiment, prior to performing the oxidation-reduction ti¬ tration, the autodeposition type coating composition is fil¬ tered. Brief Description of the Drawings

Figure 1 is a correlation diagram between oxidation- reduction potential method values and hydrogen peroxide concentration values. Description of Preferred Embodiments The method according to the invention is characterized in that commonly known materials are used for the autodepo¬ sition type coating composition, but the concentration of oxidants, especially the concentration of H₂0₂, is kept with¬ in a fixed range. The range of the H₂0₂ concentration according to the invention is equal to or less than 2.0 g/L. It preferable if the range exceeds 0.04 g/L, and more preferable if it is not greater than 1.0 g/L. When the H₂0₂ concentration is equal to or less than 0.04 g/L, the quality, especially the secondary adhesion, of the film formed often shows incon¬ sistent results. When the concentration exceeds 2.0 g/L, the film obtained becomes too thin and has an undesirably rough surface. Thus the value of the product is signifi¬ cantly decreased.

The reason why the secondary adhesion is significantly improved when the H₂0₂ concentration is kept within the range characteristic of the invention has not been fully explained theoretically. It was an unexpected result.

An actual example of the auto deposition type coating composition that can be used for this invention is, for ex¬ ample, a composition consisting of dispersed resin, acids (for example HF) , ferric fluoride, and pigment as needed (for example, carbon black), with a pH level between 1.6 and 5.0. (Cf. JP Patent Publication {Kokoku} No. Sho 52- 35692) . The content of each constituent is: from about 5 to 550 g/L of dispersed resin solids; from about 0.4 to 5 g/L of HF; from about 1 to 50 g/L of ferric fluoride (tri- hydrate salt) ; from 0.04 to 2.0 g/L of the oxidant H₂0₂; and the appropriate amount of a pigment of preferred color.

There is no particular restriction on the dispersion resin.

One important part of this invention is that an oxi- dation-reduction titration is used as the method to measure the H₂0₂ concentration in the composition. Any known oxida¬ tion-reduction titration can be used, and there is no re¬ striction on these methods and titration solutions.

When the autodeposition type coating composition is used continuously, especially when a metal being coated contains iron, a excess amount of ferrous ions is accumu¬ lated in the coating composition. As a result, the film eventually becomes thin and the composition itself becomes unstable. JP Patent Publication {Kokoku} Nos. Sho 52-35692 and 53-28461 propose a continuous stabilization method (hereinafter called continuous use) in which ferric ions (from FeF3) are added to the above mentioned composition from the beginning. Also, as a method to control these problems, JP Patent Publication {Kokoku} No. Sho 59-275 can be referred to as the prior technology. This reference proposes a control method in which a known amount of H₂0₂ is added to the autodeposition type coating composition con- taining known amounts of organic resin particles, hydro¬ fluoric acid, and ferric fluoride (FeF₃) , and the concen¬ tration of the ferrous ions in the composition is monitored by the oxidation-reduction electric potential between a reference electrode and a platinum electrode immersed- in the autodeposition composition.

However, it has been found that the oxidation-reduc¬ tion electric potential measurement control method is not advantageous for improving the secondary adhesion of the deposited resin film obtained by the auto deposition type coating composition according to this invention, because of the reasons given below, and it is better to measure the H₂0₂ concentration by oxidation-reduction titration of a sample of the autodeposition composition. 1. The oxidation-reduction electric potential (ORP) method, in a conventional autodeposition composition as described above, cannot measure the concentration of indi¬ vidual oxidants directly. For example, if the metal object to be coated is iron, so that the composition after even a few minutes of use contains both ferric and ferrous ions, the ORP is expressed by the following equation (the Nernst equation) :

ORP value = E₀ + 0.059 log ([Fe³⁺]/[Fe²⁺]) In this case, E_Q, the Standard ORP for this particular oxidation-reduction couple, is 0.4 volt versus the normal hydrogen electrode.

The effect of the oxidant is expressed by the chemical equation: H₂0₂ + 2Fe²⁺ + 2H⁺ → 2H₂0 + 2Fe³⁺. Thus, controlling by ORP does not measure the concentration of the oxidant H₂0₂ directly, but controls the balance of Fe³⁺ and Fe²⁺. Therefore, it cannot measure the concentration of the oxidant alone.

2. With the ORP method, as the concentration of the oxidant increases, the change of the ORP values becomes smaller, thus, the ORP method is not always sensitive enough to measure the oxidant concentration as precisely as has been found to be desirable as part of the present in- vent ion .

In order to establish the above mentioned fact, the. concentration of the oxidant H₂0₂ is measured by the ox¬ idation-reduction titration method, with the H₂0₂ concen- tration calculated as stoichiometrically equivalent to the amount of KMn04 consumed in the titration, and the ORP val¬ ues according to the prior art ORP method are measured for each H₂0₂ concentration, so that the correlation is ob¬ tained. The results are shown in Figure 1. As seen in Figure l, beyond the area where the H₂0₂ concentration exceeds 0.1 g/L, the ORP value shows less and less change, and at the high concentration range, the curve becomes horizontal and approaches to a fixed value. That is, at the high concentration range, correlation between the ORP values and the H₂0₂ concentrations almost disap¬ pears.

As already mentioned, since the most suitable range of the H₂0₂ concentration for the improvement of the secondary adhesion includes the range where the ORP values show very little correlation with the H₂0₂ concentrations, therefore the ORP method is less than optimal to control the measure¬ ment of the H₂0₂ concentration in this invention.

3. The ORP method can have a destabilizing effect on the coating composition. When the autodeposition type coating composition is continuously used, the ORP values often drop rapidly as a result of the accumulation of fer¬ rous ions by dissolution from the large total area of sub¬ strates coated during the continuous use, and if one tries to maintain the ferrous ion at a lower level, by maintain- ing higher concentrations of H₂0₂, the accuracy of control drops rapidly. Thus control by ORP becomes a cause of de¬ stabilizing the coating composition, even when control by ORP was adequate at earlier stages of using the bath, when there was less accumulated iron and the ORP value was in its relatively high accuracy of control range.

In summary, controlling by ORP has the drawbacks of not being able to measure the oxidant concentration di- rectly, inability to control as accurately as desired when the oxidant concentration is relatively high, and destabi¬ lizing the coating composition. ^'

On the other hand, with the method of the invention, the oxidant H₂0₂ concentration can be directly measured, and it can also handle the high concentration area of H₂0₂, and its use does not destabilize the coating composition. With the method, the adhesion between the metal surface and the deposited resin film can be controlled, and as a result, the secondary adhesion of the deposited resin film is im¬ proved.

An important feature in one embodiment of the inven¬ tion is that, for the continuous use of coating composi¬ tion, a portion of the composition is filtered through a filter that excludes materials with a high molecular weight and/or large particle size, and the H₂0₂ concentration is measured by the oxidation-reduction titration using the filtrate. When measuring H₂0₂ with the oxidation-reduction titration, if the coating composition is directly titrated, it has been found to be difficult to judge the end point of the titration, because of the presence of resins and pig¬ ments in the composition. However, by filtering a portion of the composition through a high molecular film, resins and pigments, etc. are removed. Thus, the judgment of the end point becomes easier when the filtered solution is used for the oxidation reduction titration.

For the continuous use of coating composition, dis¬ persed resins (including pigments) and acid ingredients such as HF are added as needed. The dispersed resins are controlled by the periodic measurement of the solids con¬ tent in the composition. For HF, control by measurement with the Lineguard™ 101 meter proposed in US Patent 3,329, 585 can be used satisfactorily. Or, for HF or other acids, the acid concentration of the coating composition can be easily measured by a neutralization titration of the fil¬ tered composition. By adopting the method according to the invention, not only can the coating composition be maintained in a stable state, but also resin films with an excellent secondary adhesion can be obtained. The invention may be further appreciated by consider¬ ation of the following working examples and comparison ex¬ amples.

Examples General Conditions Applicable to All Examples and Comparison Examples

The base coating composition, i.e., the composition exclusive of the hydrogen peroxide content, used in these examples and comparison examples had the characteristics shown in Table 1.

Table 1

Constituent Concentration (g/L)

Daran® SL-143 latex (54 % solids) 90.17

Newcol® 261A emulsifier (45 % solids) 0.30 Aquablack® 255 (carbon black dispersion) 3.87

Ferric fluoride 3.00

Hydrofluoric acid 0.75

The balance of the composition was demineralized water.

Cold-rolled steel sheets 7 x 15 centimeters in size were used for the metal substrate coated. The sheets were cleaned by an alkali degreasing and washed with water be¬ fore coating. After being coated, they were washed with water, then dried in an oven at 110° C for 25 minutes be¬ fore tests were performed.

To determine the H₂0₂ concentration in the coating com¬ position, a portion of the coating composition was filtered through a high molecular film with a mean pore diameter of less than 1.0 micrometer ("μm") , and the filtered solution was used for an oxidation-reduction titration. The oxida¬ tion-reduction titration was performed by the following operation. A 10 milliliter ("mL") volume was collected from the filtered solution and 5 mL of 50 % sulfuric acid was added; this mixture was then titrated with 0.025 N KMn0₄ solution. When the collected filtered solution shows a light red color, it has reached the end point. From the volume of 0.025 N KMn0₄ solution, the H₂0₂ concentration in the coating composition can be calculated.

Using a filtered solution just as mentioned above, the HF concentration was measured by neutralization titration on a 10 mL sample from the filtered solution. Bromophenol blue indicator was added. Then, the sample was titrated with 0.1 N NaOH solution. When the collected filtered so¬ lution turns pink, it has reached the end point. From the volume of 0.1 N NaOH solution which was required to reach the end point, the HF concentration in the coating compo¬ sition is obtained.

The oxidation-reduction electric potential in the coat¬ ing composition was measured using a Pt electrode versus an Ag/AgCl (saturated) electrode at a temperature of 20° C. The external appearance of the film was judged by vis¬ ual observation after the substrate was coated and dried. In Table 2 below, "0" indicates no abnormality; "X" indi¬ cates pin holes and roughness were observed.

To measure film adhesion, a grid of scribes was cut through the coated film on each test substrate sheet to be evaluated for this purpose. The grid was placed so as to form 100 separated zones, each a square 1 mm on each side, on each coated and scribed test sheet before and after it was soaked for approximately 240 hours in water at 40° C, and the number of zones where the coating films peeled off with tape was counted. The value before soaking is called the primary adhesion and the value after soaking is called the secondary adhesion in Table 2 below. Example 1 A composition which consisted of 1 liter of the above- mentioned water soluble base coating composition plus 0.10 g of H₂0₂ was used for coating. The film thickness on each

substrate was measured at two different places on each coated sheet, designated arbitrarily as "Site 1" and "Site 2" in Table 2. The performance of the coated substrate sheet is shown in Table 2. The H 0₂ and HF concentrations in the composition were measured by the oxidation-reduction titration and the neutralization titration as described above respectively. At the same time, the oxidation reduc¬ tion electric potential was measured. The value when the composition was prepared is designated "A" and the value after coating a total area of 8.4 x 10^"2 square meters ("m²") is designated "B". The results are shown in Table

Example 2 to 4 and Comparison Examples 1 - 3

These were performed in the same manner as Example 1, except that the concentrations of H 0 were varied as fol¬ lows: 0.20, 0.50, and 1.00 g/L for Examples 2, 3, and 4 re¬ spectively and 0.02, 0.04, and 2.20 g/L for Comparison Ex¬ amples 1, 2, and 3 respectively. The results are shown in Tables 2 and 3. Benefits of the Invention

This invention can improve the adhesion of the depos¬ ited resin film to the metal surface, particularly the sec¬ ondary adhesion, and can stabilize the composition by keep¬ ing the concentration of the oxidant H₂0₂, as measured by an oxidation-reduction titration method, within a fixed range.

Claims

1. A method for coating an active metal substrate with an autodeposition composition including hydrogen peroxide as the primary oxidant, characterized in that the concentra- tion of H₂0₂, as measured by oxidation-reduction titration of a sample of the composition before, during, and/or after the coating, is maintained during the coating at a value not greater than 2.0 g/L.

2. A method according to Claim 1 wherein the concentra- tion of H₂0₂, as measured by oxidation-reduction titration of a sample of the composition before, during, and/or after the coating, is maintained during the coating at a value greater than 0.04 g/L.

3. A method according to claim 2 wherein the concentra- tion of H₂0₂, as measured by oxidation-reduction titration of a sample of the composition before, during, and/or after the coating, is maintained during the coating at a value not greater than 1.0 g/L.

4. A method according to any of claims 1 to 3, wherein the concentration of H₂0₂ is measured by oxidation-reduction titration of a sample of the composition that has been filtered before titration through a sufficiently fine filter to exclude substantially all of the dispersed resin and pigment content of the autodeposition composition.

5. A method according to claim 4, wherein the composi¬ tion has been filtered before titration through a filter with a pore size of about 1 micrometer.