JPS5952240B2 - Ultrafiltration method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention utilizes a selective filtration membrane such as an ultrafiltration membrane to
When obtaining a filtrate containing unnecessary components such as dissolved low molecular weight polymers, organic solvents that aid in dissolution, neutralizing agents, and contaminant ions from cationic electrodeposition paints.

and without reducing the furnace liquid level of the ultrafiltration membrane.
It's about how to maintain it at a high level. With the development of electrodeposition technology, control of electrodeposition solution composition has become an important issue.

In other words, an electrodeposition solution usually contains a basic or acidic synthetic resin in an aqueous medium in a state of being solubilized with a water-soluble acid or base, respectively. Since it is electrodeposited as a coating film, the acid or base used for solubilization remains in the form of ions in the electrodeposition solution. Also carbon dioxide (from the atmosphere) as it is used.
Unwanted materials such as organic solvents (eg, from supplies), salts (eg, from pretreatment liquids), and decomposition products (eg, from synthetic resins) will accumulate in the electrodeposition solution. As a result, the finish of the electrodeposited coating film deteriorates, and various drawbacks such as deterioration of electrical efficiency occur. Therefore, removing the above-mentioned unnecessary substances from the electrodeposition solution and maintaining its composition in a state that is not significantly different from the one at the time of use is a major problem in electrodeposition coating, and there are currently many treatments. A method has been proposed.

Membrane separation methods such as ultrafiltration are attracting attention as the most efficient method for recovering active ingredients from these insects. By using the membrane separation method, (1) the resin particles can be almost completely recovered, so the loss of paint carried away by the object to be coated can be significantly reduced. (2) Since not only water but also low-molecular impurities can be removed, the quality of the paint can be maintained at a high level even when the recovered resin particles are returned to the electrodeposition bath. Furthermore, (3) since resin particles are almost completely removed from the ultrafiltration filtrate, it can be used as washing water for objects to be coated. As a result of the above (4), the quality of electrodeposited products is improved, paint is saved, and washing water is saved, which results in cost and labor savings and washing water savings, which further reduces costs and labor. Various industrially advantageous effects, such as prevention of pollution, have come to be recognized. However, in the method of recovering electrodeposited active ingredients using membrane separation method,
Due to reasons such as the stability of the electrodeposition paint, changes in the concentration of the electrodeposition paint in the bath, and the contamination of impurities, the biggest problem is how to stabilize the amount of ultrafiltration liquid at a high level. In the past, it was extremely difficult to produce stable water over a long period of time because the level of water in the water frequently decreased for unknown reasons.

Conventionally, the method of keeping the amount of ultrafiltration furnace fluid at a high level and stable has been to increase the pressure of the electrocoating paint on the undiluted solution side applied to the membrane; The more the film was applied, the more it clogged, creating a problem in maintaining the membrane's performance at a stable and high level over the long term. The method of the present invention is completely different from conventional methods in that it fundamentally solves the cause of clogging, so the amount of liquid can be stabilized at a high level over a long period of time.

The present invention will be explained in more detail below.

The process of cationic electrodeposition coating is shown in Figure 1.

The cationic electrodeposition coating process consists of four steps: a degreasing process, a chemical conversion treatment process, an electrodeposition process, and a water washing process. Of these, the limit P
The supermodule 1 is used in a step connecting the electrodeposition step 2, washing step, and step 3. That is, the ultralacrimal membrane removes water, contaminant ions, and other low-molecular components from the paint 4 contaminated in the electrodeposition tank. The concentrated paint 5 is returned to the electrodeposition bath, and the drained liquid 6 containing water, contaminant ions, and other low-molecular resins is returned to the washing process as a washing liquid.
The above degreasing process involves cleaning the oil stains adhering to the surface of the electrodeposited material using an alkaline solution.
This is a process for efficiently electrodepositing an electrodeposition paint onto the surface of an electrodeposited object in an electrodeposition bath. The chemical conversion treatment step is carried out as a preliminary treatment for electrodeposition coating in order to improve paint adhesion and corrosion resistance, and phosphoric acid film treatment is usually performed.

The cationic electrodeposition process is a process in which a material to be electrodeposited is made into a negative electrode and a paint having a positive charge is deposited on the surface of the material to be electrodeposited.

The water washing step is a step q in which excess paint components adhering to the surface of the electrodeposited object are washed away with a washing liquid, and components effective for electrodeposition overflow into the electrodeposition bath.

The concentration of the electrodeposition solution in the electrodeposition tank can be kept constant by controlling the amount of the electrodeposition solution overflowing from the water washing step and the concentration of the electrodeposition solution using an ultrafiltration membrane or the like. Due to the action of the ultrafiltration membrane, water, low molecular weight ions, etc. in the electrodeposition tank are filtered out to maintain the balance of the paint concentration in the electrodeposition tank during continuous electrodeposition processing in the cationic electrodeposition process. It can be selectively removed as a bottom liquid, while the concentrated paint can be returned to the electrodeposition bath.

By this selective treatment, a desired coating concentration can be achieved without removing any coating material or resin that is an effective component for electrodeposition from the electrodeposition bath. The components extracted into the sap include water as well as anions, cations and nonionic substances such as acids, alkali metal ions, phosphates, chromates, sulfates, solvents and dissolved carbon dioxide. . However, when concentrating electrocoating paint using this ultrafiltration method is operated continuously for a long period of time, impurities are deposited on the membrane surface, crushing the pores on the surface that have a selected filtration function, and causing the so-called clogging phenomenon. As a result, the deep water level decreases,
Eventually, no more filtrate will come out.

Why does this phenomenon of lowering the level of deep water occur?
The cause was unknown. As a result of intensive research into the cause of this problem, the inventors found that by disassembling the module that caused the blockage,
It was discovered that large and small crystals were deposited on the filtration side of the membrane (the side from which the P solution exits), and these crystals clogged the pores. These filtrate drop phenomena are due to the fact that few modules have been operated for a long period of time, the lack of reproducibility in the filtrate drop module, and the presence of too many factors that cause clogging due to decomposition of the mixture. , which has not been elucidated until now. As a result of analyzing these crystals, the present inventors found that metal carbonate whose main component is carbonate was precipitated.

That is, when the electrodeposition liquid is used for a long period of time, carbon dioxide gas from the air is dissolved into the electrodeposition liquid as carbon dioxide gas.

However, the electrodeposition solution contains solubilized resins with quaternary ammonium groups, solvents used to improve the water dispersibility of these resins and to improve the appearance of the deposited resin film (for example, hydrocarbons). , alcohols, ethers, dalicols, etc.) pigments (iron oxide, lead oxide, strontium chromate,
Carbon black, coal dust, titanium dioxide, talc, barium sulfate, colored materials such as cadmium yellow, cadmium red, chromium yellow, etc.), other surfactants, reaction catalysts, and electrolytes for controlling the conductance of liquid media. Salts (Ka, AlCl3, K4Fe(CN
)6, BaC12, NaC1, A1(NO3)3, Mg
C12, CaC12, SnC14, NH4C1, LiC
1, H3P04, H2S04, etc.) are added. It is a mixed system of many substances, and these are successfully solubilized, stabilized as counterions, and the pH is controlled, so even if carbonic acid is dissolved, carbonates such as lead carbonate do not precipitate. .

However, on the permeation side of the membrane, the cationic electrodeposition resin, pigment, etc. cannot pass through the membrane, so the solution becomes an aqueous solution of low molecular weight substances. At this time, the metal acetate contained as a reaction catalyst, which had been ionized and stabilized by dissociation around the cationic resin, becomes unstable because it is separated from the cationic resin in the stream. Therefore, it is thought that lead acetate reacts with carbonic acid to become lead carbonate, which is deposited on the membrane surface.
As a result of examining this phenomenon from various aspects, the present inventors found that
By washing the film with acetic acid, which had begun to degrade the edge liquid, it was possible to suppress the growth of lead carbonate crystals without adding any components to the electrodeposition solution that would reduce the electrodeposition rate. In other words, the membrane begins to drop in the P liquid (a drop in the P liquid refers to the fact that the permeation capacity becomes 90% or less of what it was at the start of operation).

), the membrane should be brought into contact with the solution mixed with acetic acid, and the amount of acetic acid added at this time is between 500 and 100.
00 ppm, and if more than this is added, P in the electrodeposition solution will increase.
H is significantly reduced and has a negative effect. Moreover, if the amount is less than this, the carbonate deposited on the membrane surface cannot be completely dissolved, and the amount of P liquid will not be recovered. There are several ways to bring the membrane into contact with the liquid containing acetic acid, but the most typical method is to fill the liquid containing acetic acid into the liquid side when periodically stopping the module. This is a method (3) in which backwashing is performed with a filtrate obtained by adding acetic acid to the finished liquid during regular backwashing times.

In the case of this method, the contact time between the acetic acid-containing furnace solution and the membrane is preferably about one day and night.

In addition, if acetic acid is to be encapsulated periodically, it is recommended to do this about once every month. In the case of method (B), the backwashing time with the acetic acid-containing bottom liquid is 30 to 60 minutes depending on the continuous filtration time.
It is best to backwash for 0.5 to 1 minute after continuous filtration.

Acetic acid is most suitable because the metal catalyst in the electrodeposition paint is contained in the form of acetate, and the degree of dissociation is not so large that it does not reduce the electrodeposition efficiency. In addition to acetic acid, aliphatic and aromatic mono- or polyhydric acids such as formic acid, hydroxyacetic acid, glycolic acid, lactic acid, malic acid, etc. can be used in this method.

Acids other than these are not preferred because they reduce the electrodeposition efficiency of the electrodeposition paint. In principle, the membrane used in this method is
Any membrane may be used as long as it can separate the active ingredients and low molecular weight substances in the electrodeposition paint, but a membrane with an average pore diameter of 1 μm or less is preferred.

If the pore diameter is larger than this, paint components, resins, application materials, etc. that are effective for electrodeposition will pass through. Preferably, the average pore size is 0.
Ultrafiltration membranes, reverse osmosis membranes, and microfilters with a molecular weight cut-off of 5 μm or less or a molecular weight cut-off of 5 million or less are preferred, and selective permeation membranes with a skin layer on the surface that blocks large molecules and allows only small molecules to pass through. A membrane with a so-called asymmetric membrane structure, which has pores with high pores and a supporting layer below which the pores become continuously larger, is preferable because it shows good results in both selectivity and flow rate.

Example 1 Kansai Paint Co., Ltd. cationic electrodeposition paint Elecron #900
Regarding 0 (G), Asahi Kasei UF module KCV
-4010 was used for ultrafiltration, and the used KCV-
4010 was sealed with various acid solutions and the recovery of the amount of the solution was investigated.

Table 1 shows the history of the modules tested.

Conditions for acid encapsulation (all acids were added to the liquid).

) The acid solution with the above composition was added to the module (KCV-40) shown in Table 1.
10), and after standing for 24 hours, the sealed liquid was removed, and the sample was briefly washed with pure water, and subjected to ultrafiltration using an electrodeposition coating solution.

The results are shown in Table 2. Example 2 Table 3 shows the test results of UF module KCV-4010 manufactured by Asahi Kasei Kogyo Co., Ltd. on the line of the K-Mori S factory using cationic electrodeposition paint Elekron #9000 clay manufactured by Kansai Paint Co., Ltd.

The conditions for filtration and backwashing were as follows. (Test No. 2, 4, and 6 are without backwashing)

[Brief explanation of the drawing]

FIG. 1 is a process diagram of cationic electrodeposition coating. 1... Ultrafiltration module, 2... Electrodeposition process, 3.
... Water washing process, 4... Contaminated paint, 5... Concentrated paint, 6... Finished liquid.

Claims (1)

[Claims] 1. In the process of recovering cationic electrodeposition paint by ultrafiltration, the filtrate side of the ultrafiltration membrane is temporarily or continuously soaked in an aliphatic and aromatic monohydric or polyhydric acid solution. An ultrafiltration method characterized by contacting with. 2. The ultrafiltration method according to claim 1, wherein in the step of recovering the cationic electrodeposition paint by ultrafiltration, the membrane is backwashed using a solution obtained by adding an acid to the filtrate. 3. The method according to claim 1, characterized in that in the step of recovering the cationic electrodeposition paint by ultrafiltration, a solution obtained by adding an acid to the filtrate is sealed in the filtrate side of the ultrafiltration module. Ultrafiltration method.