JPS58157976A - Surface treatment before painting on zinc surface - Google Patents

Abstract

PURPOSE:To improve the paint adhesion on a treated surface and to reduce treating cost by treating a defatted and washed zinc surface successively in multi-stage alkali bath group having a specified concn. gradient, and multi-stage acid bath group wherein the liquid in the tank of the final stage is pure water. CONSTITUTION:The defatted and washed zinc surface is immersed successively in multi-stage alkali bath group which have a specified alkali concn. gradient from high to low concns. and controlled to a prescribed value in the alkali concn. of the liquid in the tank of the final stage, whereby said surface is etched. In succession, said surface is immersed successively in multi-stage acid bath group which have a specified acid concn. gradient from high to low concns. and in which the liquid in the tank of the final stage is pure water, whereby the unstable zinc hydroxide layer having some alkali component is neutralized. Then, the zinc hydroxide layer is removed and hardly soluble zinc salt film is formed. The zinc surface formed thereon with such chemically converted film is subjected immediately to a painting stage in a wet state, whereby the paint film having good adhesion is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a pre-painting surface treatment method for a zinc surface, and more specifically, a method for consistently performing surface treatment and painting on a zinc surface to obtain a coating with good adhesion. The present invention relates to a pretreatment method.

Zinc has been considered an extremely difficult metal to paint with old rice.

In addition, zinc easily undergoes chemical reactions with oxygen, carbon dioxide gas, moisture, etc. in the air, producing oxides including basic zinc carbonate, zinc hydroxide, zinc oxide, etc., but paints are applied to these oxides. It is also recognized that the paint film is extremely easy to peel off when . Therefore, various types of zinc products are commercially available, such as alloying with various compositions and chemical conversion coating treatment, which also serves as corrosion protection. However, even in the case of phosphate chemical conversion treatment, which is said to be the most suitable for painting, it does not significantly improve the adhesion of the paint, and it can only maintain the initial effect of painting for a relatively long period of time. Very little success has been achieved in improving the adhesion of paints from the surface treatment technology perspective. In addition, the types of zinc plates provided by material manufacturers are extremely diverse, and paints that have relatively good adhesion to certain materials may be ineffective against other materials. There's a wall.

Therefore, it is conceivable that the purchaser himself, for example, etches the surface of the material to return it to the original surface characteristics of zinc, and then performs the optimal pre-painting surface treatment under controlled conditions, but the optimal pre-painting surface treatment itself In reality, it remains largely untouched because it is not fully understood.

In the case of painting zinc surfaces, there is currently nothing that can be done from the standpoint of material manufacturers, paint manufacturers, or processing companies, and although various proposals are being made, there is no decisive technology. For example, in the case of galvanized steel sheets, very passive measures are taken such as removing the surface to be painted by mechanical polishing to expose the steel surface and painting on top of it. It can be said that there is no coating method that can obtain a coating film that is as good as it is.

The present inventors have conducted research into methods for forming coatings with good adhesion on various materials with zinc and zinc alloy surfaces, including those that have been subjected to conversion coating, using ordinary coating methods. The fact that alkaline etching is particularly useful for etching various materials with different surface characteristics to restore the original properties of zinc, that the water washing process for washing away the alkaline etching solution plays an important role in oxidizing the zinc surface, and that hydroxide etching Zinc itself does not cause many problems with adhesion to paints, but zinc oxide can cause the paint film to peel off very easily. If the coating is easy to create, and if the ground preparation and painting process of the zinc surface are to be carried out in succession, it is sufficient that the film that should exist at the interface between the zinc surface and the paint be a dense, homogeneous, extremely thin film. and
We discovered a number of basic and important facts, such as that this is preferable from the point of view of paint film adhesion, and that it is also possible to directly apply the coating process to a water-wet surface treatment material. , we have completed the present invention. That is, the present invention provides a multi-stage alkaline bath group in which the zinc surface to be degreased and cleaned has a constant alkaline concentration gradient from high concentration to low concentration, and the alkaline concentration of the final stage bath liquid is controlled to a predetermined value; The surface of a zinc surface before painting is characterized by being sequentially treated with a group of multi-stage acid baths arranged in series, each having a constant acid concentration gradient from to a low concentration, and the final stage bath liquid being pure water. The gist of this paper is ``Treatment Methods''.

As already mentioned, in the present invention, the term "zinc surface" refers not only to surfaces made only of zinc, such as zinc plates and galvanized plates, but also to zinc surfaces containing various zinc alloys and subjected to chemical conversion coating. It is. Therefore, the term zinc material refers to a material containing such zinc. Furthermore, the present invention is characterized by subjecting the zinc material to a series of surface treatments prior to the painting process to control its surface properties, and the common treatments are clearly defined as etching, water washing, acid treatment, chemical conversion treatment, and water washing. However, it should be noted that these terms are used in the following description for the convenience of comparison with the prior art.

Now, in the present invention, first, a zinc surface to be degreased and cleaned is used to be subjected to the method of the invention. This degreasing removes oils and fats etc. adhering to the metal surface and satisfies the surface treatment of the method of the present invention. This method can be carried out using commonly used physical or chemical methods, and then the zinc material may be prepared using conventional methods such as washing with water or pickling to remove chemicals, dirt, etc. adhering to the metal surface. Processing is performed. Zinc materials come in forms with a wide variety of surface characteristics, such as galvanized surfaces or galvanized surfaces, and are not homogeneous due to a certain percentage of aluminum or rust. Etching is required to unify the surface with characteristics. Mechanical polishing and electrolytic polishing are available as methods for etching metal surfaces, but chemical immersion polishing is one that can be applied to relatively complex structures, and acid etching is usually used for chemical polishing of metals. is said to be effective. However, in the case of zinc, it is possible to etch it with both acid and alkali, and during acid etching, the reaction proceeds as follows, for example, Zn + 2HCI → ZnC1z + 2H2↑, and the ionization and solubility of the salt produced are large. The reaction rate is too high and difficult to control, and the acid is consumed, so its recovery is difficult and this replenishment is necessary.
In alkaline etching, for example, high temperature: Zn + 2HzO + NaOH- + NaZn
(OH)3+Hz↑Low temperature: NaZn(OH)3-+
ZnO + 820 + NaOH and other formed salts dissolve at an appropriate rate, and when the solution is cooled, zinc oxide precipitates as a sludge and the alkali is regenerated.Essentially, etching can be carried out without replenishment of alkali, so it is easier to use than an acid. However, etching with alkali is extremely advantageous.

Therefore, in the present invention, it was decided to selectively use an alkali for the purpose of etching the zinc surface. Any alkali can be used for this purpose, but inexpensive ones such as caustic soda, caustic potash, soda carbonate, and potassium carbonate are preferable.The alkali concentration depends on the quality of the surface of the material to be etched, the amount of etching, the processing temperature, time, etc. Generally, the purpose can be sufficiently achieved by immersion of 5 to 20% at a temperature of about 70 DEG C. in a liquid chamber for a short time of 1 to 10 minutes. Although the use of such an alkaline etching method for the purpose of pre-painting surface treatment is not new in itself, the alkaline bath treatment of the present invention is essentially different from simple alkaline etching in the following points. That is, in the past, it has been thought that after performing the usual etching process, sufficient rinsing with a large amount of running water is necessary to remove the treatment liquid from the metal surface. Therefore, the etching operation requires a large amount of running water, which poses problems in water quality control, and it is not allowed to discharge the wastewater as it is, so its treatment is essential. Furthermore, the inventors of the present invention have found serious doubts not only regarding the securing of water, purification means, and management techniques, but also regarding the washing itself after etching. In other words, in the case of a zinc surface, washing with water after etching not only removes water, soluble curved salt, and alkaline etching agent, but also removes Zn(OH)3−→Zn(OB )z A chemical change occurs from a water-soluble complex ion to water-insoluble zinc hydroxide, and zinc itself also reacts with water to form colloidal, insoluble water, as shown in Zn + 2H20→Zn(OH)z + 82↑. Zinc oxide will form on its surface. Moreover, the zinc hydroxide formed in this way not only cannot be removed by washing with water, but also changes to zinc oxide, which is more inert than Zn(OH)z 4 Zn0, as washing continues, and this change cannot be removed by washing with water. It was discovered that when the zinc material was taken out into the atmosphere and dried, the effect was particularly remarkable.

Therefore, even if the zinc surface is etched with great effort, the treated surface will eventually be ruined by the water washing treatment, making quality control of the treated surface extremely difficult and insufficient.

Therefore, the present inventors integrated alkaline etching and water washing treatment as a means to homogenize and control the surface characteristics of the zinc surface, and decided to replace it with the following operating means with the condition that acid treatment was to follow. . That is,
The degreased and cleaned zinc surface is sequentially treated in a multistage alkaline bath group having a constant alkaline concentration gradient from high concentration to low concentration. In this case, the alkaline concentration of the initial alkaline bath depends on the composition of the zinc surface of the material to be treated, the amount to be etched, etc., but it can be set to an alkalinity of about 5 to 20 ml, which is normally used for alkaline etching. Further, baths having lower alkali concentrations are arranged one after another, and the alkali concentration of the final bath is maintained at a predetermined value between 1.times.10@-6 and 10@-1N. Conveniently, this multi-stage alkaline treatment bath treatment is carried out by sequentially immersing the zinc surface in these baths.

Although it is possible to spray the bath liquid if desired, this is not very preferable in order to maintain the alkaline concentration of the six baths constant. In the case of the immersion method, the alkaline concentration of the six baths is maintained, for example, as follows. That is, a part of the high concentration bath liquid is reverse osmosis filtered between any two tanks, and the F solution obtained is supplied to the low concentration bath, and a part of the low concentration bath liquid is transferred to the high concentration bath. It is something. If necessary, pure water is preferably replenished into the lowest concentration bath, and such a rosead system itself is well known to those skilled in the art. With this method, the bath solution adheres to the zinc surface and is carried to subsequent baths, decreasing the alkaline concentration in one bath and increasing the alkaline concentration in the other bath, changing the alkaline concentration gradient of the entire multi-stage alkaline bath. Moreover, since the system is designed to be a circulation system, consumption of alkali and water can be effectively prevented.

In the above treatment of the present invention, the zinc surface is etched in a highly concentrated alkaline bath to produce a water-soluble alkali zinc complex salt, but as the bath moves to a bath with a lower alkaline concentration, the amount of attached alkali decreases, and the Zn(OH)a −*Zn(O
H) Becomes water-insoluble zinc hydroxide with z. However, the alkaline bath in the final stage is not completely pure water, and is 1×10-6
Since the alkali concentration is controlled to a predetermined value between ~IQ-1N, the reaction of Zn(OH)z → ZnO due to unnecessary water washing as in the conventional method is prevented, and a zinc hydroxide layer is formed on the zinc surface. remains in a predetermined controlled state. However, in this case, the alkali content has not been completely removed from the final treated zinc surface, and the zinc hydroxide layer is in an unstable state and will easily oxidize if exposed to air11. It is.

Therefore, in the present invention, for the purpose of artificially stabilizing the unstable zinc hydroxide layer that still contains alkaline content and removing the alkali, acid treatment using a phosphoric acid and/or chromic acid-based chemical solution is performed. , which has a constant acid concentration gradient from high concentration to low concentration and is carried out using a multi-stage acid bath group leading to the final pure water. In this case, the acid treatment is sufficient to neutralize the unstable zinc hydroxide layer which has some alkali content generated in the treatment of the preceding alkaline bath group, and etching of zinc is unnecessary.

Therefore, as for the initial acid concentration, the conventional method does not require any high concentration, but depending on the acid and treatment time, it is necessary to create an acid gradient from IN or a much lower concentration of about 0.5N. Can be done. Furthermore, if a high concentration bath is used, the treatment can be carried out in a short time and the effect of shortening the process is enormous. The final bath must be set to pure water, for example, highly demineralized water with a specific conductivity of 10 PS/work or less, so that acid content does not remain on the zinc surface. Note that the acid concentration gradient is created using the same means used to maintain the concentration gradient in the alkaline bath group, i.e., a part of the high concentration bath liquid is subjected to reverse osmosis filtration, and the F solution obtained is supplied to the low concentration bath. It is advantageous to have the bath liquid flow sequentially from the water tank towards the highest concentration bath and to keep it constant. However, the acid is consumed by neutralizing the alkali content and reacting with zinc hydroxide, and it is difficult to recover the acid by itself, so it is necessary to replenish the highest concentration bath with acid according to the amount consumed and maintain the acid concentration constant. Needless to say, there is a need for updating as appropriate. In addition, deionized water with a specific conductivity of 10 μS/cyx or less is replenished into the final stage water tank as required from evaporation or removal of water by the treatment material.

This acid treatment neutralizes the alkali and converts 3Zn(OH)z + 2H3PO4→Zn5(po4
)z + 6H20 The zinc hydroxide layer is removed and a sparingly soluble zinc salt coating is formed.

As the acid in the acid bath, phosphoric acid-based or chromic acid-based chemical conversion treatment agents can be used, but even in that case, the acid for etching is unnecessary, so a chemical conversion treatment solution with a relatively low concentration can be used. , it is possible to neutralize zinc hydroxide and form a very thin zinc conversion coating.

3Me(H2PO4)2 , :' 4H3PO4+ M
e3(PO4)2 The inventors of the present invention have found that a zinc surface on which a chemical conversion film has been formed by the above treatment can be immediately subjected to the painting process while still wet, and a coating film with extremely good adhesion can be obtained. I found out that it can be done. This was completely unexpected considering the conventional wisdom that it was necessary to dry the metal surface before painting.Moreover, in the case of the zinc surface of the present invention, which has an extremely thin coating, drying after preparing the base is completely unexpected. On the contrary, it promotes the formation of oxides and is even harmful, and one of the characteristics of the present invention is that the preparation of the base and the painting are carried out consistently. Coating can be carried out by conventional snow blowing methods, but it has also been found that electrocoating methods are particularly useful for producing high quality coated surfaces. The zinc chemical conversion coating obtained by the method of the present invention is much thinner than that obtained by conventional chemical conversion coating formation methods, for example, about 0.1 to 0.3p, and the adhesion of the coating is extremely low. In good condition. This is due to the fact that in ordinary chemical conversion coatings, the crystal alignment is relatively coarse, but even though they are obtained in a laminated state and have excellent overall corrosion resistance, when painted, chlorine ions, etc. freely pass between these crystals, causing the coating film to On the other hand, in the chemical conversion coating formed by the method of the present invention, the crystals are arranged in a densely thin layer,
It is thought that this is to effectively prevent passage of chlorine ions, etc.

In the method of the present invention, water washing following alkaline etching is stopped at the stage of washing with a solution with slightly alkaline concentration,
To prepare a zinc surface with a very thin protective film that contains neither alkali nor acid by sequentially carrying out neutralization with acid and washing with water, and immediately subjecting it to the painting process to provide a coating with excellent adhesion to the zinc surface. Moreover, it can minimize the amount of running water required for washing, greatly reducing the amount of alkali and acid consumption, and can be applied to a wide variety of zinc surfaces with different surface characteristics. This is possible and constitutes an extremely useful invention industrially.

The present invention will be explained below with reference to Examples and Comparative Examples. All "parts" and "%" in the example sentences are by weight unless otherwise specified.

Example 1 Galvanized steel sheet (zinc layer 30μ, 70x150x 0.
8w+) was immersed for 3 minutes at room temperature in a 10% aqueous degreasing solution containing commercially available orthosilicate powder and a surfactant as main components to degrease and clean, and then washed with tap water. This degreasing plate was washed with a sodium hydroxide solution (2N NaOH).
) for 3 minutes at 50°C to remove the natural oxide film and etch it, then immerse it in IN NaOH, 0, OIN N
The samples were sequentially immersed in alkali baths of aOH, 0.00 IN NaOH, and alkaline baths having a concentration gradient for 5 seconds each.

Subsequently, 0.1N H3PO4, 0.01N H3
It was immersed for 5 seconds each in six baths of acid baths of PO4 and 7 cm of deionized water with a specific conductivity of 10 μS. Both the alkaline bath and acid bath were maintained at room temperature. Next, apply the cationic electrodeposition paint Nippon Paint to the treated board while it is still wet.
3 at 30℃ and 200V using Power Top U-30 made by
Set the mold for 30 minutes, then bake at 170℃ for 30 minutes.
A coating film of μ was obtained.

For comparison, the same degreased and cleaned galvanized steel sheet as above was immersed in an 8% sodium hydroxide aqueous solution at 50°C for 3 minutes, then washed with tap water for 3 minutes, and then immersed in a bath of phosphoric acid-based chemical conversion treatment solution. After soaking at 50℃ for 3 minutes and washing with tap water for 3 minutes,
After drying with hot air, cationic electrodeposition coating was applied in the same manner as above. Both were subjected to Tsuruto spray test (JIS Z-2371
) (SST) and 5% NaCl solution (40°C, 5%
0° C.) for a certain number of days and subjected to a tape peeling test, it was found that the coating according to the present invention had extremely good coating adhesion as shown below.

Number of days leading to peeling and degree of peeling Invention 30 days 0 10 days Δ 10 days × Comparative example 10 days × 3 days Δ
1 day △O no peeling, Δ slight change, ×5- or more peeling Example 2 To effectively manage the treatment bath on the zinc surface after degreasing and cleaning, and to perform treatment operations efficiently, a circulation type was used. An example using a multi-stage treatment bath group will be shown.

As shown in Figure 1, seven treatment tanks (a-g) are prepared,
The first four (a to d) are used in an alkali bath and the alkali concentrations are adjusted to 2N, IN, 0.01N and 0.001NK.

A reverse osmosis device RO is installed at the bottom of the alkaline tank to remove permeation F.
The liquid is supplied into the subsequent bath d, and the baths s, c, d
The bath surface of the bathtub is lowered stepwise towards the front as shown in the figure, and the bathtub d
The bath liquid is caused to flow down in the direction from b to b sequentially due to overflow.

The last three (e+'+g) of the processing bath are 0 and 1, respectively.
A bath of N 83PO4,0,01N 83PO4 and deionized water with a specific conductivity of 10 pS/cm is provided, a reverse osmosis device RO is installed at the bottom of bath e, 1, the permeated liquid F is supplied to bath g, and the bath surface of bath e+f+g is is gradually lowered in the direction from g to e, so that the bath liquid flows down in order from g to e as an overflow.

In addition, bath e has H3PO4, and g has a specific conductivity of 10.
, us/m, respectively, as desired. By operating the reverse osmosis devices of the above device at the same time, bath liquid is circulated in the alkaline bath and the acid bath, respectively.Although not shown in the diagram, each reverse osmosis device or the phosphoric acid and deionized water supply lines are connected to each bathtub. The system monitors the drug concentration in the bathtub and automatically operates based on the results to maintain the drug concentration in the bathtub at a predetermined level. Further, if desired, a heating device is disposed in the alkaline bath a to adjust the bath temperature.

As in Example 1, the degreased and water-washed galvanized steel sheet was
In the treatment bath group shown in the figure, a...50℃ 3 minutes b... RT 15℃ 3 seconds C...
・・・ Same as above d・・・・・・・・・
Same as above e... Same as above f... Same as above g...
...Sequentially immersed at RT15°C for 3 seconds. Next, a water-wet cationic ED paint Power Top U-30 manufactured by Nippon Paint ■ was applied to the mold at 30°C for 3 minutes at 200V, and then baked at 170°C for 30 minutes to form a 20μ coating. The above operations were performed continuously, and the performance was tested by sampling as appropriate.

Paint adhesion was measured using Salt Spray Test (SST) and 5
% salt water immersion test: 35°C SST 5% salt water immersion W15% salt water immersion 40°C
50℃ 1st 30th 0 10th to 10th X
100th 30th 0 10th △ 10th X 200th 30th 0 10th 10th Ta. The painted surface was also highly resistant to mechanical deterioration. After 200 treatments, the sludge and contamination of the alkaline solution in tank a became significant, so it was discarded.

The acid solution in tank e was also discarded at the same time because it was noticeably contaminated.

The disposal of these two tanks is extremely advantageous in that the wastewater treatment is extremely easy and inexpensive because the wastewater is not diluted with a large amount of flowing water as in the conventional method, and pollution prevention measures can be completely implemented. There are no restrictions on the use of chromic acid-based chemical conversion treatment solutions, and a completely closed system has been achieved. In the next operation, the bath from tank B is used in tank A to construct an alkaline bath, and the subsequent bath liquids can be used sequentially, which has a great resource saving effect.

Conventional chemical conversion treatment required a large amount of water, but this method saves water resources to a large extent, and the cost burden of water quality management such as deionized water of 10 μS 7 cm or less is hardly a problem, and compared to conventional methods. We were able to achieve significant cost reductions.

[Brief explanation of the drawing]

FIG. 1 shows a preferred multistage treatment bath used in carrying out the method of the present invention, in which a is an etching bath, b, c, and d are alkaline baths, e and f are acid baths, g is a pure water bath, 1 is a heater, and 2 is a reverse osmosis filtration device, 3 is a feed pump

Claims (7)

[Claims] (1) The zinc surface to be degreased and cleaned is divided into a multi-stage alkaline bath group in which there is a constant alkaline concentration gradient from high concentration to low concentration and the alkaline concentration of the final stage bath liquid is controlled to a predetermined value, and A pre-painting surface treatment of a zinc surface characterized by sequential treatment using a treatment bath group arranged in series with a multi-stage acid bath group having a constant acid concentration gradient and the final stage bath liquid being pure water. Law. (2) The alkaline bath group has a constant alkali concentration gradient from 5N to 1X10N, and consists of a multi-stage alkali bath group in which the alkaline concentration of the final stage tank liquid is controlled to a predetermined value between 1x10 and 1X10N. A method according to claim 1. (3) The acid bath group consists of a common knowledge having a constant acid concentration gradient from IN to I x 10'-3N and a final stage deionized water bath having a specific conductivity of 1 OPS/B or less.
The method described in any of Section 2. (4) The method according to claim 3, wherein the acid solution in the acid bath group is a phosphoric acid or chromic acid-based chemical solution. (5) The alkaline concentration gradient in the alkaline bath group is such that the F solution obtained by reverse osmosis filtration of a part of the high concentration bath liquid is supplied to the low concentration bath, and the bath liquid flows from the lowest concentration bath to the highest concentration bath. The specific conductivity is increased to 70μS as necessary
2. The method according to claim 1, wherein the concentration is kept constant by replenishing the lowest concentration bath with deionized water of less than / cm. (6) The acid concentration gradient in the acid bath group is such that the F solution obtained by reverse osmosis filtration of a part of the high concentration bath liquid is supplied to the low concentration bath, and the bath liquid is flowed from the pure water tank toward the highest concentration bath. 70- parts in sequence,
Claim 1: A constant level is maintained at all times by replenishing the highest concentration bath with acid according to the amount consumed and, as necessary, replenishing deionized water with a specific conductivity of 10 μS/c1n or less into the pure water tank. The method described in section. (7) The method according to claim 1, wherein the material carrying the zinc surface to be treated is sequentially immersed in a multi-stage alkaline bath group and a multi-stage acid bath group.