JPH03188298A - Formation of coating film by electrodeposition - Google Patents

Abstract

PURPOSE:To improve the stability of an electrodeposition bath by specifying the concn. of solid matter and that of an org. acid in the bath, carrying out coating by cationic electrodeposition with the bath and spraying washing water on the coated surface in the form of mist. CONSTITUTION:At the time of coating by electrodeposition, a body to be coated is used as the cathode, the concn. of solid matter in an electrodeposition bath is regulated to 2-10wt.% and that of an org. acid to 30-60mmeq. When the body is coated by cationic electrodeposition with the bath, satisfactory dispersibility of a pigment is obtd., such trouble as the sedimentation of the pigment is not caused and a coating film having high surface smoothness is formed. Excess paint of the bath sticking to the coated surface can be satisfactorily washed off by spraying washing water on the coated surface in the form of mist because the electrodeposition bath used has the low concn. of solid matter.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention is aimed at removing a small amount of electrodeposited bath paint that has adhered to the object by electrodeposition when forming an electrodeposition coating film on the object. The present invention relates to a method for forming an electrodeposited coating film that can be washed and removed with washing water and that can maintain a stable electrodeposition bath.

(Prior art and its problems) Conventionally, after electrodeposition coating is performed using the object to be coated as a cathode, excess bath paint adhering to the object is washed with water using a dipping method or a shower method.
It had been removed.

However, in the immersion method, in order to perform sufficient water washing, it is necessary to provide a plurality of immersion washing baths, which requires a large amount of equipment investment and running costs.

In addition, when rinsing with a shower, ultrafiltrate is generally used as the rinsing water, but a large amount of rinsing liquid is required, and a large amount of electricity is required to obtain a large amount of rinsing water (ultrafiltrate). It is necessary to filter the deposited paint, but as the acid, which is a neutralizing agent, also flows out into the filtrate at the same time, the neutralization balance of the electrodeposition bath paint is disrupted and the stability of the electrodeposition bath is impaired. The problem was that it was easier.

Therefore, reducing the amount of ultrafiltrate used as washing water can contribute to maintaining the stability of the electrodeposition paint and downsizing the ultrafiltration device, which is advantageous from the viewpoint of bath management and equipment.

Furthermore, in recent years, a two-step electrodeposition coating method in which two layers of electrodeposition coating are formed has been proposed as a method for improving the anticorrosion properties of coating films.

In this method, after the first electrodeposition coating, the coating film is washed with water, and after the second electrodeposition coating is applied without curing,
It is common to bake to form a multilayer cured coating.

In this two-step electrodeposition coating method, when the shape of the object to be coated is complex, especially when it has a bag structure, the inside of the bag structure cannot be washed sufficiently with water after the first electrodeposition coating.
Since the second electrodeposition coating bath is considerably contaminated over time by the first electrodeposition paint, there is a problem in that the resulting composite cured coating gradually becomes unable to exhibit its original function.

Therefore, there has been a need for a method for forming an electrodeposition coating film that can reduce the amount of washing water and that can reduce contamination of the second electrodeposition coating bath even in a two-step electrodeposition coating method.

(Means for Solving the Problems) Therefore, the present inventors have devised a method that allows the excess paint adhered to the object to be coated after electrodeposition coating to be washed away with even a small amount of washing water, and which allows the object to be coated with a bag structure such as an automobile body. Even when a multi-layer electrodeposited coating is formed by electrodepositing the coating twice, the second electrodeposition coating bath is less contaminated (as a result of intensive research to find a method of electrodeposition coating that can The inventors have discovered that the above object can be achieved by spraying washing water as a fine mist after electrocoating using a bath, and have thus arrived at the present invention.

That is, the present invention uses the object to be coated as a cathode and the bath solid content concentration is 2.
Cationic electrodeposition coating is performed in an electrodeposition bath with a concentration of ~10% by weight and an organic acid concentration of 30 to 60 meq, and then the excess electrodeposition bath paint on the surface to be coated is removed by atomization mist of washing water on the surface to be coated. The purpose of the present invention is to provide a method for forming an electrodeposited coating film, which is characterized in that the electrodeposited coating film is removed by spraying with water and then washing with water.

Furthermore, the present invention applies electrodeposition coating to the object to be coated as described above, and then sprays washing water on the surface to be coated in the form of an atomized mist to remove excess electrodeposited bath paint from the surface to be coated. After removal, electrodeposition coating is further performed to form a multilayer uncured electrodeposition coating, and then the multilayer electrodeposition coating is simultaneously baked and cured. The present invention provides a method.

Thus, according to the present invention, during electrodeposition coating, the solid content concentration of the electrodeposition bath is significantly lowered from the generally practiced solid content concentration of about 20% by weight, and is 2 to 10% by weight, preferably 4 to 4% by weight. 8% by weight, and organic acid concentration (paint solid content 10
0 g, 30 to 60 meq (milligram equivalents) of the organic acid used for neutralization, preferably 35 to 50 meq.
It is adjusted to meq. As a result, good pigment dispersibility is obtained in the electrodeposition bath, there is no problem such as pigment sedimentation (and a coating film with good coating surface smoothness is obtained.) When the solid content concentration is less than 2% by weight, pigment Poor dispersion stability
When stored over time, the amount of filtration residue due to 400-mesh wire mesh filtration increases, which is presumed to be due to pigment aggregation.

On the other hand, if it exceeds 10% by weight, when the washing water is sprayed in the form of a fine mist, the excess paint cannot be sufficiently washed and removed with a small amount of washing water (or the object to be coated may have a bag structure). If the electrocoating material has a large amount of paint, the amount of solid content of the electrocoating material that accumulates in the bag structure and is taken out increases, so a large amount of replenishment paint is required (and, in the case of two electrocoating coatings, the second electrocoating coating is necessary). Problems arise in contaminating the bath and increasing compositional changes in the second mold coating bath.

Further, if the organic acid concentration is less than 30 meq, the pigment dispersion stability will be poor, while if it exceeds 60 meq, there will be a problem that the surface smoothness of the resulting coating film will be poor.

The method of the present invention involves first performing cationic electrodeposition coating in an electrodeposition bath with a bath solid content concentration and an organic acid concentration within the above ranges, and then washing the excess electrodeposition bath paint on the surface to be coated with water. A one-time method for forming an electrodeposited film by spraying water in the form of an atomized mist, washing with water, removing it, and then baking it. The present invention includes a method for forming a multilayer electrodeposited coating film, in which a multilayer electrodeposition coating film is formed by further performing electrodeposition coating, and then baked. In addition, the term "wrapped objects" includes both those with and without a bag structure.

In the method of the present invention, when a workpiece having a bag structure is electrocoated twice, the first electrocoating paint (I) that has entered the bag during the first coating is generally Similar to the spray washing performed in the shower, the particles were not removed even by washing with atomized mist spray, and were carried into the second coating bath and mixed with the second coating paint (II). I will do it.

When coating is carried out successively, the proportion of the first cationic electrocoating paint (I) mixed into the second electrocoating paint bath inevitably increases gradually.

In terms of solid content, the proportion of the cationic electrocoating paint (I) in the first cationic electrocoating paint brought into the second electrocoating bath is
The solid content mA part and the cationic electrodeposition paint (II) that is applied to the object to be coated by the second electrodeposition coating and taken out of the system.
During the second electrodeposition coating, the cationic electrodeposition paint (II) enters the bag and is carried out of the system without being removed by the commonly used spray washing. ) is determined by the solid content C part. In other words, when coating is carried out continuously, the solid content of the second overcoat coating bath (II) is the sum of the solid content B part and 0 part taken out from the electrodeposition bath (II), and the solid content brought in. The solid content of the cationic electrocoating paint that is replenished to the second coating bath to keep the solid content constant is (B + C - A). ) section. As a result, the solid content that enters the second electrodeposition coating bath from the outside through replenishment is the cationic electrodeposition paint (I) part A and the replenishment paint (B+C-A) part. The ratio of solids entering from the outside is cationic electrodeposition paint (■) / replenishment paint = A /
(B+C-A).

3 turnovers for the second coating bath (1 turnover means that the amount of paint consumed becomes equal to the initial amount of electrodeposition coating while maintaining the electrodeposition coating bath by external replenishment etc.) 3 turnover means that the amount of paint consumed is equal to three times the initial amount charged in the electrodeposition bath.The rate of replacement of the electrodeposition bath by paint entering from the outside is:
Approximately 64% with 1 turnover, approx. 87 with 2 turnovers
%, approximately 95% after 3 coats. ), more than 95% of the composition of the second coating bath is from the paint coming in from the outside, that is, [cationic electrodeposition paint (I)].
)/Replenishment paint=A/(B+C-A)E Most of the paints have this composition.

In this formula, in order to prevent deterioration and contamination of the second coating, it is sufficient to reduce the solid content of the cationic electrodeposition paint (I), that is, to lower the solid content concentration of the cationic electrodeposition paint (I).

In the present invention, the electrodeposition paint used in one electrodeposition film forming method and the cationic electrodeposition paints (1) and (II) used in two electrodeposition coatings each have a resin binder component. Resin binders commonly used in the field of cationic electrodeposition coatings, such as polyamine resins such as amine-added epoxy resins, such as (i) polyepoxides and 1
(ii) polyepoxides and ketiminated primary amino groups; A secondary mono-
and adducts with polyamines (e.g. U.S. Patent No. 4,01
7,438): (iii) Reactant obtained by etherification of a polyepoxide with a ketiminated hydroxy compound having a primary amino group (for example, JP-A No. 5
9-43013)).

In addition, when good weather resistance is required for electrodeposited coatings,
As a resin binder, an amino group-containing or nonionic acrylic resin with excellent weather resistance may be used in combination with the amine-added epoxy resin.

Further, the resin binder may be an amino group-containing acrylic resin alone.

The resin binder may contain a crosslinking agent such as a polyisocyanate compound blocked with a blocking agent such as an alcohol, an oxime, or a phenol, or a melamine resin, in addition to a resin such as the above-mentioned amine-added epoxy resin.

Furthermore, as the resin binder, an amine-added epoxy resin that can be cured without using a crosslinking agent such as a blocked isocyanate compound can also be used, such as a resin in which a β-hydroxyalkyl carbamate group is introduced into a polyepoxide material. (For example, JP-A-59-1554
70); vine type resins that are cured by transesterification (see, for example, Japanese Unexamined Patent Publication No. 55-80436).

The preparation of a cationic aqueous dispersion using the resin binder described above is usually carried out by neutralizing the resin binder with a water-soluble organic acid such as formic acid, acetic acid, or lactic acid. The cationic electrodeposition paint containing the aqueous dispersion obtained in this manner as a main component may further contain a pigment, and the composition of the cationic electrodeposition paint may be appropriately selected depending on the desired coating performance and the like.

When performing two electrodeposition coatings, the compositions of the first and second electrodeposition coatings can be different; for example, a coating with emphasis on anti-corrosion properties may be used as the first coating; You may use a paint with an emphasis on weather resistance as the second paint.

The washing water used in the method of the present invention includes ultrafiltrate (Ultrafiltration filtrate), reverse osmosis filtrate (
Reverse osmosis filtrate), tap water, underground water, deionized water, etc. can be used, but among these, ultrafiltrate is preferred from the viewpoint of reusability.

In the present invention, the methods of spraying the washing water in the form of an atomized mist include: (1) supplying the washing water onto a high-speed rotating disk and scattering and atomizing it by centrifugal force, (2) pressurizing the washing water and spraying it from the orifice of the nozzle. Examples include a method of jet atomization, and a method of spraying washing water at low pressure using the principle of atomization using a jet of compressed gas such as compressed air, steam, or inert gas. In method (2), the diameter of the high-speed rotating disk is usually IC) ~ 35 cm, and the rotation speed is 3000 ~ 15000 rpm.
m, and the rewiring hole diameter in method (■) is 0.3 m.
-5 open, the pressure is about 20-700 Kg/cm2, and in method (■), the pressure of compressed gas is 1-8 Kg/cm2.
cm2, and the pressure of the washing water is about 1-5 Kg/Cm2. The particle size range of the spray in the present invention is 2 to 500 μm
, preferably 100 to 250 μm. In addition, in the case of the conventional shower method, the size of the droplets is usually about 1000 μm.
That's all.

In the method of the present invention, the washing water is sprayed in the form of an atomized mist as described above, but since an electrodeposition bath with a low solid content concentration is used, the paint adhering to the object to be coated is sprayed in the washing water. (It is also possible to sufficiently wash by spraying the washing water in the form of an atomized mist.) When the washing water is sprayed in the form of a mist, the amount of washing water per unit time is lower than that of shower-like washing. The amount used can be significantly reduced, and in the method of the present invention, since the water is easily washed away by the mist, a long washing time is not required, so the total amount of washing water can be reduced.

The washing time for spraying in the form of an atomized mist is not particularly limited, and may be about the same time as in the shower method, but is usually about 0.5 to 10 minutes. Further, the water washing step may be performed in one stage or in multiple stages.

In addition, in the method of the present invention, when the object to be coated has a bag structure, after electrodeposition coating, the object to be coated may be tilted or rotated to remove paint accumulated inside the bag structure. is preferred.

(Effects of the Invention) According to the method of the present invention, washing with water can be carried out as well as the conventional shower method, and the amount of washing water used can be significantly reduced, so that the stability of the electrodeposition paint bath can be improved. Further, when the object to be coated has a bag structure and is subjected to two electrodeposition coatings, contamination of the second mold coating bath can be reduced.

EXAMPLES The present invention will be specifically explained below with reference to Examples and Comparative Examples. "Parts" and "%" mean "parts by weight" and "% by weight," respectively.

Example 1 The following cationic electrodeposition paint (paint A) using a water-soluble epoxy polyamino resin equivalent to Nireclone No. 9000 manufactured by Kansai Paint Co., Ltd. was adjusted to a solid content of 7.7% with deionized water, and The organic acid concentration was adjusted to 36.2 meq and electrodeposition coating was performed under the conditions shown in Table 2.

Then, the obtained electrodeposition coating film was washed with water by spraying the ultrafiltrate as washing water for 1 minute, and then washed with water for about 18 hours.
Baking for 30 minutes in an electric dryer at 0°C formed a cured coating. Further, this electrodeposition coating was carried out until the electrodeposition paint was turned over three times, and the condition of the coated surface of the baked electrodeposition paint film and the filtration residue of the electrodeposition paint at this time were measured. The test results are shown in Table 3 below.

Paint'LΔ Resin: Pigment = 10 Total oil absorption 8,7 *l Resin base value 8 Add fat to acetic acid 0: 49 5 0 epoxy polyamino tree Dispersed in water with an equivalent amount of 0.50 For 7 hard It turned into...

*2 Manufactured by Asahi Carbon Co., Ltd. Product name: Carbon ASMJ.

*3 Water-containing amorphous silicon dioxide pigment (oil absorption 300) manufactured by Fuji Devison Co., Ltd., product name.

In addition, in Example 1, when the washing conditions and time were changed, the state of washing of the electrodeposition paint excessively attached to the object to be coated was as shown in Table 1 below.

Table 1 Example 24 The same procedure as in Example 1 was carried out except that the solid content of the cationic electrodeposition paint was adjusted to 6% and the organic acid concentration to 40.5 meq.

Comparative Example 1 The same procedure as in Example 1 was carried out except that washing with water was performed using an ultrafiltrate shower.

Comparative Examples 2 to 4 Table 3 shows the solid content and organic acid concentration of cationic electrodeposition paints.
Example 1 was carried out in the same manner as in Example 1 except as shown in .

The results of Example 2 and Comparative Examples 1 to 4 are shown in Table 3.

In this table, evaluation was performed in accordance with the following.

○: Good ■2 A slight amount of excess paint remains. △: A considerable amount of excess paint remains.

Notes in Table 3 are as follows.

*4 Washing method: Using a Tilden M-1 pump manufactured by Japan Machinery Co., Ltd., a jet of compressed air is used to spray the ultrafiltrate, which is the washing water, into a mist based on the atomizing principle.
Or it was ejected in the form of a shower. The mist and shower conditions were as shown in the table below.

*5 Condition of water washing of adhered paint: Evaluation was performed according to the same evaluation criteria as in Table-1.

*6 Filtration residue: The amount of filtration residue obtained when the electrodeposition bath after 3 turnovers was filtered through a 400-mesh wire mesh.

Example 3 A rectangular parallelepiped (with an internal volume of 15 x 15 x 200 mm) made of cold-rolled mild steel plate treated with Parbond #3004 (manufactured by Nippon Parr Ising Co., Ltd.) with a thickness of 0.5 m/m was created. A total of two holes with a diameter of 6 mm were drilled on one of the four sides at a distance of 1 am and 10 cm from the edge as the object to be coated. The first cationic electrodeposition coating was performed by supporting the substrate so that its longitudinal direction was substantially vertical and completely immersing it in the first electrodeposition bath.

For the first electrodeposition bath, the coating material 1-A was adjusted to a solid content of 7.7% by deionization, and the organic acid concentration was adjusted to 36%.
The amount adjusted to 2 meq was used.

The conditions for electrodeposition were as follows: a voltage of 200 V, a current application time of 2 minutes, a bath temperature of 28° C., and a film thickness of 20 μm, using the object to be coated as a cathode. After electrodeposition coating, it was pulled up and set for 30 seconds while maintaining the vertical position. As a result, the amount of paint remaining in the bag of the object to be coated is approximately 21
．． It became 8cc.

The object to be coated was washed with water using a mist of ultrafiltrate while maintaining the object in a vertical position. The water washing conditions are: filtrate usage: 0.1
2/min for 1 minute.

Next, this object to be coated is suspended almost vertically in the reverse direction in a second electrodeposition bath with a capacity of 4ρ, and after mixing the remaining paint in the bag, the object to be coated is completely submerged and electroplated. I did it.

As the second electrodeposition bath, a cationic electrodeposition paint (Paint B) using an epoxy resin containing an acrylic resin, which is equivalent to Nireclone No. 9600 manufactured by Kansai Paint Co., Ltd., was used with a solid content of 19.2%. , the organic acid concentration was adjusted to 24.2 meq.

After electrodeposition coating, it was washed with water and baked in an electric dryer at about 180° C. for 30 minutes to form a cured coating film. This two-time electroplating was carried out 20 times, and when the appearance of the coated surface of the outer surface of the coated object was tested after the 20th time, no abnormality was observed. In addition, when the electrodeposition bath paint after the test was filtered through a 400-mesh wire mesh, the amount of filtration residue was 1.
It was 5 mg/β, which was good.

Comparative Example 6 In Example 3, the paint A adjusted to have a solid content of 19.2% and an organic acid concentration of 24.2 meq was used as the first electrodeposition bath, and a shower of ultrafiltrate (filtrate) was used. When the same procedure as in Example 3 was carried out except for washing with water at a usage rate of 5 I2/min for 1 minute, some spots were observed on the coated surface of the outer surface of the object to be coated for the 20th time. In addition, the electrodeposition bath paint after the test was
The amount of filtration residue when filtered through mesh wire mesh is llomg/
β and was poor.

Claims (3)

[Claims] (1) Using the object to be coated as a cathode, perform cationic electrodeposition coating in an electrodeposition bath with a bath solid content concentration of 2 to 10% by weight and an organic acid concentration of 30 to 60 meq, and then remove excess electrodeposition from the surface to be coated. A method for forming an electrodeposited coating film, which comprises washing and removing the bath paint by spraying washing water onto the surface to be coated in the form of an atomized mist. (2) Using the object to be coated as a cathode, perform cationic electrodeposition coating in an electrodeposition bath with a bath solid concentration of 2 to 10% by weight and an organic acid concentration of 30 to 60 meq, and then remove excess electrodeposition from the surface to be coated. After washing and removing the bath paint by spraying washing water in the form of a fine mist onto the surface to be coated, electrodeposition is further performed to form a multilayer uncured electrodeposition coating. A method for forming an electrodeposition coating film, which is characterized by simultaneously baking and curing multiple layers of electrodeposition coating films. (3) The method for forming an electrodeposition coating film according to claim 1 or 2, wherein the object to be coated has a bag structure.