JPS585278B2 - How to manage an electrodeposition coating bath - Google Patents

Description

Detailed Description of the Invention The present invention is capable of reducing pollution by replenishing a specific paint composition during the electrodeposition coating process in which an electrodeposition paint containing an amino resin as the main crosslinking agent is continuously applied. The present invention relates to a method for managing an electrodeposition coating bath that enables continuous coating of a uniform coating film.

Generally, the main components of electrodeposition paints are a resin component and pigment as film-forming components, an alkali compound as a water solubilizer, an organic solvent as a water solubilizer or paint viscosity reducing agent, and water. It is publicly known.

Most of the components that are applied to the object to be coated in such electrodeposition paints are resin components and pigments, so if the electrodeposition is applied continuously, alkaline compounds and organic solvents will accumulate in the electrodeposition bath, which will affect the electrodeposition efficiency and finish. This results in a decrease in feelings.

It is well known that various methods are used in actual painting processes to prevent such problems.

In other words, methods for removing accumulated alkaline compounds include, for example, the diaphragm method using a cation exchange membrane at the coating cathode, the ion exchange method using an ion exchange resin, and furthermore, the method of removing alkaline compounds substantially from the electrodeposition coating bath. There is an acidic paint replenishment method that replenishes during the painting process a paint containing less alkali than the amount of alkaline in the bath.

However, in the diaphragm method and the ion exchange method, alkaline compounds are discharged from the system in proportion to the amount of coating, and especially in the ion exchange method, the used ion exchange resin is regenerated with a large amount of acid, so the amount of alkaline compounds generated in both methods is A large amount of expense is required to treat the waste liquid.

On the other hand, in the acidic paint replenishment method, substantially no alkaline compounds are discharged.

From this point of view, the acidic paint replenishment method is an excellent method for reducing the discharge of alkaline compounds during electrodeposition coating.

However, when an electrodeposition paint containing an amino resin as the main crosslinking agent is used in the acidic replenishment method, a reaction occurs between the amine resin and the film-forming resin from the time the replenishment paint is manufactured until it is used, causing the paint viscosity to accumulate. rise or even become a water-insoluble gel.

Generally, when a coating composition containing an amino resin is stored in a concentrated state for a long period of time, the following reaction occurs, resulting in polymerization or gelation.

(Here, R is a hydroxyl group-containing resin residue such as an alkyd resin or an acrylic resin, R', R'' is an amino resin residue, and R'' is hydrogen or an alkyl group.) Such a reaction is caused by the presence of hydrogen ions. While it is accelerated, it is inhibited in the presence of a base.

Furthermore, this reaction is inhibited by the presence of lower alcohols such as methyl alcohol, ethyl alcohol, butyl alcohol, and the like.

Usually, the aqueous resin used for electrodeposition coating is a carboxylic acid type resin and contains at least five times as many carboxyl groups as the resin used for organic solvent dilution type paint.

Therefore, when mixed with amine resin and no alkaline compound is added, carboxylic acid type resin acts catalytically and accelerates the above reaction, resulting in a significant increase in coating voltage and loss of smoothness of the painted surface. Otherwise, the resin becomes water-insoluble and cannot be painted.

In order to prevent this, it was customary to neutralize and stabilize most of the carboxyl groups contained in a basic substance, but when such a paint is used as a replenishment paint, a basic substance is released into the electrodeposition bath. This is not desirable because it accumulates.

On the other hand, in order to produce a stable concentrated paint without neutralization,
It has been found that at least 80 parts by weight of lower alcohol is required per 100 parts by weight of resin in the paint.

However, when using such a paint, the alcohol accumulates in the electrocoating bath as electrodeposition coating continues, and as the coating voltage decreases, the throwing power decreases, making it impossible to obtain a uniform film thickness. There is a flaw.

As a result of intensive research to eliminate such defects, the present inventor has found that
The invention has been completed.

That is, the present invention provides an electrodeposition coating process in which an electrodeposition paint containing an alkyl etherified amine resin as a crosslinking agent is continuously applied, in which the amount of alkali compound contained in the paint is smaller than that required during electrodeposition coating. Contains the minimum amount of alkali compound necessary to emulsify the film-forming components in water, and also contains at least 20% of the total amount of organic solvent in the paint.
By replenishing an organic solvent in which % or more is an alcohol having 3 or more carbon atoms and a water-dispersed paint containing 30% or more of water based on the total amount of organic solvents and using an alkyl etherified amino resin as a crosslinking agent, electricity can be The concentration of the electrodeposition coating bath is corrected, and if necessary, the PH and specific conductivity of the electrodeposition coating bath, which vary minutely, can be corrected using either the ion exchange method, the diaphragm method using a part of the cathode, or the ultrafiltration method. The present invention relates to a method for controlling an electrodeposition coating bath, which is characterized by controlling the electrocoating bath by using the above-described method or a combination thereof.

According to this method, even if electrodeposition coating is performed continuously, the increase in the amount of alkaline compounds and organic solvent in the electrodeposition bath is suppressed, so compared to the conventional electrodeposition coating process containing amine resin. As a result, the amount of alkali compound waste is reduced to at least ⅓ or less, or becomes zero.

Furthermore, since the accumulation of organic solvents is almost negligible, electrodeposition coating is possible under constant coating conditions.

When using an electrodeposition paint made by replenishing such a water-dispersed paint, there is almost no need to adjust the pH and specific conductivity of the electrodeposition paint bath, but if you want to make more precise adjustments, you can usually It is sufficient to carry out ion exchange using an H-type cation exchange resin, or to perform filtration using a diaphragm ultrafiltration device, and a portion of the filtrate may be discarded.

The film-forming resin used in the electrodeposition paint of the present invention is a carboxyl-containing resin, and acrylic resins and polyester resins used in ordinary electrodeposition coatings are suitable, and the alkyl etherified amino resin is alkyl etherified melamine resin. Preferred are resins, benzoguanamine resins, acetoguanamine resins, urea resins, and the like.

Although the alkyl etherified amine resin used may be water-soluble, it is preferable that at least a partially water-soluble amine resin or a water-insoluble amine resin contains at least 20% or more of the water-soluble amino resin.

Furthermore, hydrous acid group solvents are effective as organic solvents, but
It is necessary to contain at least 20% or more of the total organic solvents, which are at least good solvents for carboxyl group-containing resins and have at least 3 or more carbon atoms in the molecule.

Examples of such alcoholic solvents include aliphatic alcohols such as n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, iso-butyl alcohol, 5ec-butyl alcohol, amyl alcohol, and octyl alcohol, ethylene glycol monomethyl ether, and ethylene. These include ethylene gelol monoethers such as glycol monoethyl ether and ethylene glycol monobutyl ether, and benzyl alcohol.

Among them, aliphatic alcohols are preferred.

Furthermore, the amount of water added to the coating bath for dispersing the paint must be at least 30% organic solvent.

The alkali compound for emulsifying the paint may be the same substance as the alkali compound used in the electrodeposition coating bath,
One or more alkaline compounds used in ordinary electrodeposition coatings can be used.

That is, the alkali compounds in the present invention include, for example, alkali metal hydroxides; alkylamines such as monomethylamine, dimethylamine, trimethylamine, isopropylamine, and diethylamine; monoethanolamine, jetanolamine,
Alkanolamines such as triethanolamine and diisopropanolamine; Alkylamines such as dimethylaminoethanol; morpholine, ammonia, and the like.

In the replenishment water-dispersed paint of the present invention, when an alcohol with a carbon number of 2 or less is used, the evaporation rate of the alcohol is fast, so it is easily volatilized from the paint, and the reaction between the amino resin and the film-forming resin occurs. This is not desirable as it may occur.

Furthermore, if the content of alcohols having 3 or more carbon atoms is less than 20% in the total organic solvent, the reaction between the amine resin and the film-forming resin cannot be prevented.

Furthermore, if the water content is less than 30% of the total amount of organic solvents, it is not preferable because a uniform, low-viscosity water-dispersed paint cannot be obtained.

In the replenishment paint of the present invention, the composition ratio of resin, total organic solvent, and water is within the range of 30 to 70% resin, 6 to 35% total organic solvent, and 5 to 60% water. It is preferable that the total amount of organic solvents is 100%, and that the total amount of organic solvents is 20 to 80% of the resin.

The water-dispersed paint having such a composition may be added to the electrodeposition coating bath as it is, or may be added after being mechanically premixed with the electrodeposition bath.

The preliminary mixing may be performed using a mixing tank or may be continuously replenished using a line mixer.

Control of the electrodeposition bath is usually not necessary when using such replenishment paints, but in painting processes such as aluminum sash painting where contaminant ions from the alumite process are often introduced from the outside, contaminant ions are usually removed by anion exchange. However, when the pH fluctuates, the pH may be controlled substantially less often than anion exchange.

A more effective method for removing contaminant ions is to use ultrafiltration.

In the method of the present invention, since there is little variation in the alkaline compounds in the bath, the electrodeposition coating bath can be controlled by simply discarding a small amount of the r-liquid discharged from the ultrafiltration device.

According to the method of the present invention, it is possible to reduce the BOD (biological oxygen demand) in waste water generated by electrodeposition coating of a paint containing an amine resin to at least 1/2 or less.

Hereinafter, the present invention will be explained in more detail with reference to production examples and examples.

In addition, parts and parts in the production examples and examples indicate parts by weight unless otherwise specified.

Production Example 1 15 parts of styrene and 35 parts of methyl methacrylate in a reaction vessel
parts, 2-hydroxyethyl methacrylate 15 parts, ethyl acrylate 15 parts, 2-ethylhexyl acrylate 10 parts
80 parts, 10 parts of acrylic acid, 20 parts of isopropyl alcohol and 1 part of azobisisobutyronitrile.
Polymerization was carried out at ℃ for 12 hours.

The resulting varnish had a solid content of 83%, an acid value of 78, and a peak molecular weight of 23,000.

This resin solution is called varnish A.

Production Example 2 35 parts of phthalic anhydride, 35 parts of trimethylolpropane and 24 parts of coconut oil fatty acid were placed in a reaction vessel and reacted at 170°C for 30 minutes while removing condensed water, then 7 parts of trimellitic anhydride was added. The reaction temperature was raised to 200° C., and the reaction was continued for 2.5 hours while removing condensed water.

The obtained resin had a 60% xylene solution viscosity of Z1 and a resin acid value of 50.2.

This resin is called resin B.

Examples 1-2 and Comparative Examples 1-2 Using Resin B, the following four types of replenishment paints were prepared based on the compositions shown in Table 1 in a conventional manner.

The prepared paints 1 to 3 exhibited a uniform emulsion state,
Paint 4 was a clear resin solution.

After putting this paint in a sealed container and leaving it at a temperature of 30°C for two weeks, the can was opened and the state of the paint was observed.The paint of Comparative Example 1 turned into a gel, and even if amine was added, it did not dissolve in water. It did not dissolve and could not be used as an electrodeposition paint.

The paint of Example 1 had a viscosity of 95 poise before storage and had increased to 110 poise after storage, but 4.5 parts of triethylamine was added to 224.5 parts of the paint and dispersed in water.
When a 10% electrodeposition bath was prepared and electrodeposition was performed on an aluminum alloy plate (6063S) which had been subjected to an alumite treatment using a known method, a coating film having the same coating surface condition as before storage was obtained.

The paint of Example 2 had a viscosity of 82.3 poise before storage, which remained almost unchanged at 85.6 poise after storage, indicating a good paint condition.

When this product was coated in the same manner as in Example 1 with an electrodeposition bath prepared, a coating film with a good coating surface was obtained.

The paint of Comparative Example 2 had significantly thickened after storage, and was made into a uniform dispersion by adding 4.5 parts of triethylamine to 204.5 parts of the paint and dispersing it in water to a solid content of 10%. However, as a result of coating in the same manner as in Example 1, the coated surface had many pinholes, resulting in a coating film with low gloss and no practical use.

Example 3 100 parts of resin B, 25 parts of diacetone alcohol methyl ether, 25 parts of n-butanol, 50 parts of hexakismethoxymethylmelamine, and 9 parts of triethylamine were thoroughly mixed, and deionized water was added to disperse the mixture to form a solid. minute 10%
An electrodeposition bath was prepared.

This coating bath 31 was placed in an electrodeposition tank, and a continuous coating experiment was conducted using an aluminum alloy plate that had been alumite-treated using a known method.

In order to compensate for the decrease in bath solid content due to continuous coating, the replenishment paint shown in Example 1 and the current coating bath composition were exactly the same, but the amount of deionized water was reduced by 1 to make the solid content 40%.
Table 2 shows the results when using the replenishment paint prepared as follows.

After replenishment, when the coating bath amount became less than 31, deionized water was added to correct the bath amount to 31.

When the refill paint of Example 1, which is the paint of the present invention, is used, a good electrodeposition coating condition is obtained even after 25 m2 of coating is applied, but with the conventional replenishment paint of the same composition as the bath composition, 5 m of paint is applied. It is necessary to previously control the pH of the bath by ion exchange, U/F (Ultrafiltration) or other methods.

In addition, the PH fluctuation when using the replenishment paint of Example 1 is as follows:
Although it does not affect the coating conditions and surface condition, if you want to control it more strictly, use the ion exchange method, U
/F etc. may be used to temporarily control a decrease in pH or a slight change in pH due to addition of an amine.

Examples 4-5 and Comparative Examples 3-4 Paints 5-4 based on the compositions shown in Table 3 using Varnish A
Paint stability and continuous coating experiments were conducted under the same conditions as Examples 1 to 3.

The solid content of the prepared varnishes was about 40% for all paints.

In a storage test at a temperature of 30° C., Paint 7 separated and precipitated after one day, the precipitate turned into a gel after four days, and did not dissolve even when dimethylaminoethanol was added.

Paint 5 and Paint 6 exhibited a white uniform dispersion state and were stable for 30 days or more in a storage test at 30°C.

Paint 8 was in the form of a transparent aqueous solution and was stable for 30 days or more in a storage test at 30°C.

The composition of paint 8 was diluted with deionized water to 10% solids,
A continuous coating experiment was conducted in the same manner as in Example 3.

As a result, when paint 5 is replenished, the coating area is 25 m.
Even after processing No. 2, there was no abnormality on the coated surface, which was good.

When paint 6 was replenished, the gloss decreased after a coating area of 15 m2, so an ultrafiltration device was used to filter the paint liquid.
% of the filtrate was removed, and by adding deionized water in an amount equivalent to the removed filtrate, a condition in which a good coated surface could be obtained was restored.

When Paint 8 was replenished, gloss reduction and pinholes were observed on the painted surface after 1 m2 of painted area, and with a total painted area of 4 m2,
It has deteriorated to the point where a continuous film cannot be obtained.

Comparative Example 5 Electrodeposition was carried out continuously in the same manner as in Example 3, except that n-butanol in the electrodeposition bath of Example 3 and the replenishment paint (paint 1) shown in Example 1 was replaced with ethanol.

The results are shown in Table 4.

Claims (1)

[Claims] 1. In the electrodeposition coating process in which an electrodeposition paint containing an alkyl etherified amine resin as a crosslinking agent is continuously applied, the amount of alkali compound is less than that required during electrodeposition coating,
It also contains the minimum amount of alkali compounds necessary to emulsify the film-forming components contained in the paint in water, and furthermore, at least 20% of the total amount of organic solvent in the paint is an alcohol having 3 or more carbon atoms. The concentration of the electrodeposition coating bath is corrected by replenishing an organic solvent and a water-dispersed paint containing 30% or more of water based on the total amount of organic solvent and using an alkyl etherified amine resin as a crosslinking agent. How to manage an electrodeposition coating bath.