JPH0645916B2 - Foaming prevention method in recovery water washing process of electrodeposition coating - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for preventing foaming in a washing step which is carried out immediately after an electrodeposition step in which cationic electrodeposition coating is carried out. Bath liquid UF
When performing a recovery water washing step in which the washing water containing the carry-out bath solution after washing with water is again returned to the electrodeposition bath using the solution, by adding polypropylene glycol to the UF solution for water washing, foaming in the water washing step is performed. In addition, the carry-out bath liquid can be collected under reflux without adversely affecting the electrodeposition bath.

(Prior Art) Electro-deposition coating on conductive substrates is widely practiced, and in particular, the cationic electro-deposition coating method is excellent in rust prevention and finish, and is therefore the mainstream of the electro-deposition coating method. There is.

This cationic electrodeposition coating is prepared by neutralizing a basic resin such as a polyamino resin with an acid such as acetic acid and adding a pigment to a vehicle in which a surfactant or the like is added and dispersed in water.

Then, this coating material is diluted with water to form an electrodeposition bath, and in the bath, a DC voltage is applied with the object to be coated as a cathode and a carbon electrode plate or the like as an anode to apply the cation particles in the bath to the cathode. Electrodeposition process that deposits a water-insoluble coating film on the surface of the coating, water-washing process that removes the applied coating liquid from the surface of the coating object pulled up from the electrodeposition bath, and baking to cure the coating film Electrodeposition coating is performed by sequentially and sequentially performing the steps.

Since the above-mentioned cationic electrodeposition coating uses an acid for paint neutralization, there is a problem of equipment corrosion.To avoid and reduce this, the amount of acid used is reduced to a low neutralization type resin. It is recommended to use an emulsion type electrodeposition bath by adding a surface active agent and the like, and by doing so, electrodeposition coating with extremely good throwing power can be achieved.

On the other hand, in the water washing step, the composition of the electrodeposition bath,
Using the effluent obtained by the UF liquid for controlling the concentration as washing water, a recovery washing step of returning the washing water obtained by washing out the carry-out bath liquid adhering to the surface of the coating object to the electrodeposition bath, It is done preferably.

(Problems to be Solved by the Invention) As described above, in the recovery water washing step after the cation electrodeposition step,
When the UF liquid of the electrodeposition bath is used as washing water, the UF liquid contains an excessive amount of the surfactant for emulsification added to the bath. Foaming on the surface, and further, the process in which the washing water containing the carry-out bath liquid flows down, and the foaming layer remarkably foams on the liquid surface of the water tank into which the water flows, and the foamed layer rises up to the transfer route of the coated object. It adheres to the coated surface or overflows out of the water tank, causing insufficient cleaning on the surface of the coated object and causing unevenness in the finish, and the amount of overflowed foam is lost.
There are problems that the composition of the electrodeposition bath is unbalanced, the operating environment is lowered, and the wastewater treatment load is increased.

It was considered to add an antifoaming agent to the washing water in order to prevent foaming in this washing zone, but in the case of silicone with a large defoaming property, it can be used because it adversely affects the electrodeposition conditions and causes the coating to repel. In the case of an alcohol-based compound that does not adversely affect the electrodeposition conditions, sufficient defoaming cannot be performed and a practical defoaming agent has not been found.

Therefore, the present situation is to avoid the problem by sufficiently performing the final washing with clean water in order to remove the insufficient washing portion due to the adhesion of bubbles, but this is not preferable because it increases the consumption of clean water. .

(Means for Solving Problems) In the present invention, in cationic electrodeposition coating, polypropylene glycol is added as a defoaming agent to the problem of foaming in the recovery washing step using the UF liquid of the electrodeposition bath as washing water. This has succeeded in solving the problem without causing any adverse effect on the coating system.

That is, according to the present invention, when the object to be coated pulled up from the cationic electrodeposition bath is subjected to a recovering and washing step with the UF solution of the electrodeposition bath solution, the washing UF solution has a molecular weight of 500 to 1500. 5 to polypropylene glycol
This is a method for preventing foaming, which is characterized in that it is added so as to be 10,000 PPM.

Polypropylene glycol decreases its surface tension and solubility in water with the increase of its molecular weight, and increases the defoaming property for an electrodeposition bath UF solution containing a surfactant, but on the contrary, repels the coating film and the coating film. The physical properties of the coating film tend to be deteriorated such as strength reduction, and the practical molecular weight range is 500 to 1500, and the preferable range is 900 to 1100.
Is. Further, the addition amount to the UF liquid is preferably the minimum amount at which a defoaming effect can be obtained, but it is adjusted depending on the composition of the UF liquid and is usually experimentally determined appropriately in the range of 5 to 10000 PPM.

The foaming prevention and defoaming effects of this polypropylene glycol could be confirmed by the following experiments.

Experimental conditions (1) Electrodeposition bath composition Epoxy-based polyamino resin electrodeposition paint (Electron No. 9400, gray manufactured by Kansai Paint Co., Ltd.) was diluted with deionized water, and a nonionic surfactant of HLB15 as a dispersant was added thereto. 2000PPM was added and mixed to form a bath.

(2) UF liquid UF device (UF system KCL-101 manufactured by Asahi Kasei Corporation)
0), the electrodeposition bath solution is treated at an excess ratio of 20%,
The liquid was used as a sample. The surfactant in the sample was 1500 PPM.

(3) Experimental method 100C.C. of the above liquid was placed in a No. 4 forward cup, and 5
00C.C. From the height of 100 cm above the bottom of the graduated cylinder, the whole amount was poured down into the cylinder, and the amount of foam formed on the liquid surface and the disappearance time of the foam layer were measured by adding polypropylene glycol (molecular weight 1000). The rate was changed and measured.

(4) Experimental results From the above experimental results, polypropylene glycol was added to the UF liquid containing 1500 PPM of surfactant in an amount of 5%.
It was found that the addition of 00 to 5000 PPM was effective, but if the addition amount of polypropylene glycol was experimentally determined so as to obtain a desired defoaming effect, depending on the kind and the mixing amount of the surfactant in the UF liquid. Well, usually the range is 5 to 1000 PPM.

On the other hand, when a conventionally known defoaming agent such as butyl alcohol was added at 5000 PPM, the foaming amount was about the same as that of polypropylene glycol, but the defoaming time required 5 minutes, and the foaming preventing effect was obviously inferior. Was there.

(Function) Polypropylene glycol is especially U of the cationic electrodeposition bath solution.
Although the mechanism of action for obtaining the antifoaming effect superior to the conventionally known alcohol-based antifoaming agent to the liquid F cannot be clearly explained, it is probably due to its water solubility, low surface tension, molecular weight, etc. , It seems that the defoaming property occurs in a balanced manner.

Further, even if polypropylene glycol is mixed in the electrodeposition bath together with the washing water in the washing and recovery step, the electrodeposition performance and the electrodeposition operating conditions are not adversely affected up to the amount of about 1% by weight. On the other hand, in the case of a known alcohol-based antifoaming agent such as butyl alcohol, the inclusion of 0.5% by weight deteriorates the throwing power on electrodeposition, and the operating conditions must be changed.

(Example) An example in which the method of the present invention is applied to a specific cationic electrodeposition coating step will be described.

The drawing is an explanatory device layout view when the foaming prevention means of the present invention is applied to a conventionally used continuous cationic electrodeposition coating device.

In the drawings, 1 is an electrodeposition bath, 2 is an overflow bath,
Reference numeral 3 is a first cleaning tank, 4 is a second cleaning tank, and a conveyor rail 30 for transferring an object to be coated is provided above them.

A bath liquid recirculation pipe 6 is provided between the overflow bath 2 and the electrodeposition bath 1, and the UF filter 1 is branched from the pipe 6.
Piping 7 for returning the concentrated liquid to the overflow tank 2 via 0,
In addition, the liquid is passed through the liquid tank 11 to the second cleaning tank 4
The pipe 8 for supplying the

The pipe 8 has a discharge pipe 9 for discharging a part of the liquid to the outside of the system.
Is provided via a valve.

The first cleaning tank 3 is connected so as to receive the cleaning water overflowed from the second cleaning tank 4, and the respective cleaning tanks 3 and 4 are connected to the shower ring devices 13 and 14 arranged above the article transfer path by piping 15 , 16 connected.
A tray is provided so that all of the washing water of the showering device 14 flows down to the second washing tank 4, some of the washing water of the showering device 13 flows down to the overflow tank 2, and the other flows to the first washing tank 3. .

A pump P is provided in each of the pipes 6, 8, 15 and 16.

The above is no different from the conventional apparatus for performing the electrodeposition process and the water washing recovery process.

Reference numeral 20 denotes a polypropylene glycol tank provided for the method of the present invention, which supplies an appropriate amount of polypropylene glycol to the second cleaning tank via a pipe 21 or to the liquid tank 11 via a pipe 22 shown by a dotted line.

Using the above equipment, a cation electrodeposition bath (150 t) having the same composition as in the above experiment was constructed, and while the bath was managed by the same UF filter as in the experiment, a washing and recovery process with a UF liquid was carried out to carry out an automobile. The body was continuously electrodeposited.

When rinsing was performed while dripping and supplying polypropylene glycol to the water surface of the second cleaning tank 4 through the pipe 21 so as to be 500 PPM with respect to the UF liquid, the foam layer thickness on the water surface was always maintained at 10 cm or less, and the shower Due to the ring, the foaming on the surface of the object to be coated is very small and swiftly drained away, there is little foaming on the tray, and a very good cleaning state is obtained, and the foaming layer thickness on the water surface of the first cleaning tank 3 is also 15 cm. Never exceeded.

Also, add polypropylene glycol to the liquid tank 1.
The same anti-foaming effect as described above was obtained, except that the thickness of the foam layer on the water surface in the second washing tank 4 was about 15 cm.

(Effects of the Invention) As described above, according to the present invention, it is possible to completely eliminate the adverse effect of foaming when a UF liquid is used in the recovery water washing step in cationic electrodeposition coating, and further, to adversely affect the electrodeposition step. Therefore, an excellent coated product can be obtained.

[Brief description of drawings]

The drawing is an explanatory device layout view of a cationic electrodeposition coating device for carrying out the method of the present invention. 1 ... Electrodeposition bath, 2 ... Overflow bath, 3 ... First
Cleaning tank, 4 …… Second cleaning tank, 6,7,8,15,16,20,2
1 ... Piping, 10 ... UF filter, 11 ... Liquid tank, 13, 14 ... Shower ring device, 20 ... Polypropylene glycol tank, P ... Pump

Claims (1)

[Claims] 1. A UF solution for water washing having a molecular weight of from 500 to 1 in carrying out a water washing step of recovering an object to be coated pulled up from a cationic electrodeposition bath with the UF solution of the bath solution.
500 to 500,000 polypropylene glycol
A method for preventing foaming in a recovery water washing step of electrodeposition coating, which is characterized in that PPM is added.