JPS6227859B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a dip coating method and apparatus for forming a smooth coating surface on the surface of an article. More specifically, it is an object of the present invention to provide a method and apparatus for obtaining an extremely beautiful coating film with no irregularities on the object to be coated by applying ultrasonic waves to the coating solution and forming a uniform surface flow in the coating solution tank. In particular, when the present invention is applied to the surface coating treatment of advanced optical products such as mirrors and lenses, it brings about the remarkable effect that it is possible to obtain product quality of an excellent level that could not previously be achieved. BACKGROUND ART Conventionally, a method of forming a coating film on the surface of an object to be coated by immersing it in a coating liquid tank and pulling it up at a low speed has been widely used as a dip coating method. This method has the disadvantage that the shape of the object to be coated is limited to simple shapes with no liquid pools, and it is not possible to coat any part of the surface, but on the other hand, the resulting coating film has excellent smoothness. It has the advantage of being able to coat the entire surface of the object at once, and is used for surface coating such as undercoating and topcoating of flat or simply curved metals and synthetic resins. When the coated product is used as an optical product such as a mirror, lens, or glazing, or when it is subjected to secondary processing such as printing or plating, even better coating film smoothness with no irregularities is required. These uneven defects are known as "pinholes" in the industry.
"Butsu", "Hajiki", "Gas pin" and "Crater"
etc., and their removal is considered an important issue for ensuring quality. These coating film defects can be removed by cleaning the object to be coated with a detergent or the like in advance, removing foreign substances such as dust in the coating solution, and cleaning the entire atmosphere in which the coating is dried and cured by dip coating. However, the smoothness of the coating film was still insufficient. Furthermore, attempts have been made to ensure the smoothness of the coating film by causing the coating solution to flow very quietly in the coating tank, but this has not been satisfactory. As a result of extensive studies, the inventors of the present invention found that when applying ultrasonic waves to the coating liquid during dip coating of articles, the cavitation bubbles generated by the ultrasonic waves are successively flushed away from the surface of the coating liquid. It has been found that a strong uniform flow can be formed on the liquid surface of the liquid tank, and most of the defects in the coating film can be removed. Ultrasonic irradiation drives out dissolved air in the coating solution,
Furthermore, it is thought that it is necessary to destroy the air layer that is brought in while adhering to the surface of the object to be coated when the object is immersed in the coating solution. If air bubbles generated by cavitation remain around the object to be coated for a long time, they may cause defects such as "bumps" or "pinholes," or they may adhere to the surface of the object and be pulled up. This is thought to be necessary to prevent this from forming as traces of bubbles. Next, the present invention will be explained based on the drawings. FIG. 1 is a flow sheet for carrying out the present invention, in which the coating liquid in the coating liquid tank 1 overflows from the weir 4, passes through the overflow pocket 5, and reaches the intermediate tank 6. A funnel 9 is attached to the intermediate tank 6 for charging liquid to be added. The liquid in the intermediate tank 6 is pressurized by the circulation pump 7, and when it passes through the filter 8, foreign substances in the liquid are removed. The liquid coming out of the filter 8 is returned to the coating liquid tank 1 and the liquid is circulated. In addition, 2 in the figure is a water tank in which the ultrasonic transducer 3 is placed, and by immersing the coating liquid tank 1 in the water tank 2 and placing it near the ultrasonic transducer 3, the ultrasonic transducer 3 generates a liquid. The ultrasonic waves pass through the wall of the coating liquid tank 1 through the water in the water tank 2 and generate ultrasonic pressure waves in the coating liquid. Note that since the water in the water tank 2 serves as a medium for propagating ultrasonic waves and for removing heat generated from the ultrasonic vibrator 3, it is desirable to introduce new water from outside. FIG. 2 is a perspective view of the ultrasonic vibrator 3 installed on the bottom of the coating liquid tank, and the nozzles 10 and 11 in the figure
are the water supply nozzle and the water discharge nozzle, respectively. The temperature of the coating liquid tends to rise due to the ultrasonic vibration of the coating liquid, but if this is not desirable for the coating liquid, a cooler for the coating liquid can be added to the circulation line. In the method of the present invention, when a large number of objects to be coated are immersed in the width direction of the tank, it is effective to irradiate ultrasonic waves from the bottom of the tank as shown in Figure 2. When there are only one or two objects to be coated across the tank, the objects to be coated at the center in the width direction will not be in the shadow of the objects to be coated at the sides in the width direction, so ultrasonic waves should be irradiated from the side of the tank. You can also do it. In addition, here the second
As shown, the width of the tank is perpendicular to the way the liquid flows toward the overflow pocket, and the length of the tank is parallel thereto. The frequency of the ultrasonic waves irradiated to the coating liquid is 1KHz~
100KHz, preferably 5-50KHz.
If the frequency is less than 1 KHz, cavitation is less likely to occur, and if the frequency exceeds 100 KHz, it takes time for the cavitation bubbles to expand and contract in the liquid, so the amplitude becomes small and it does not work effectively. Also, the intensity of ultrasonic waves is 0.2w/cm ² to 5w/cm ²
and is preferably in the range of 0.5w/cm ² to 2w/cm ² . If the intensity of the ultrasonic wave is less than 0.2w/ ^cm2 , the cavitation pressure wave is weak and its intensity is
When the amount is more than 5w/cm ² , the cavitation group that occurs obstructs the propagation of ultrasonic waves. Furthermore, the intensity of ultrasonic waves relative to liquid volume is 10w/~
A suitable range is about 100w/. In order to supply the coating liquid to the coating liquid tank on which ultrasonic waves are applied, it is necessary to form a uniform surface flow toward the overflow pocket on the surface of the coating liquid tank. This is to allow the cavitation bubbles generated by ultrasonic waves to act on the object to be coated and to smoothly wash them away from the surface of the coating liquid tank. If this flow is not carried out smoothly, the generated bubbles will Furthermore, dissolved air in the coating liquid or the destroyed air layer on the surface of the object to be coated stays in these bubbles, causing defects such as "bumps", "repellents", and "pinholes" on the coating film, and bubbles. This will leave traces and the purpose of the present invention will not be achieved. FIG. 3 shows a method of forming a uniform surface flow achieved by the present inventors based on this viewpoint, in which the liquid inlet 13 for supplying the coating liquid is located on the side of the overflow pocket 5 at the bottom of the coating liquid tank. It is connected to a blowing nozzle 14 having a large number of holes placed in the air. As shown in the figure, the liquid blown out from the blowout nozzle 14 is degassed by ultrasonic waves irradiated from the bottom or side of the coating liquid tank, rises against the wall of the tank facing the blowout nozzle 14, and is reversed at the liquid surface. The resulting surface flow crosses the weir 4 and reaches the overflow pocket 5. While the coating liquid flows down to the liquid level of the coating liquid tank and the overflow pocket 5, it comes into contact with air and increases the amount of dissolved air again, but it is circulated and flows again to the blowing nozzle 14.
The level of dissolved air is reduced by being blown out from the tank and subjected to ultrasonic irradiation at the bottom of the tank. This flow formed on the surface of the coating liquid tank also has the effect of quickly flushing dirt and dust floating on the surface of the coating liquid tank from the air or attached to the object to be coated into the overflow pocket 5. ing. Furthermore, since a strong circulating flow is created in the coating liquid tank 1 by this blow-off nozzle 14, there is no dead space everywhere in the tank. The above-mentioned blow-off nozzle 14 may have many holes or may have a slit-like hole, but it is preferable that the blow-off nozzle has a uniform blowing velocity over each location to ensure a uniform surface flow. It is important to obtain. The flow rate of the coating liquid in the coating liquid tank is not particularly limited as long as the flow rate can achieve the purpose of the present invention, but the flow rate is between 2 and 2.
100 cm/sec, preferably 10-50 cm/sec.
If the flow rate of the coating liquid is less than 2cm/sec, it will be "glue".
In terms of the occurrence of ``cissing'', the flow velocity is 100cm/
If it exceeds sec, it is unfavorable in terms of "occurrence of flow pattern". In addition to the coating liquid tank having the structure as shown in Fig. 3, there is also a method of blowing the inflowing liquid directly onto the tank wall on the opposite side of the overflow pocket as shown in Fig. 4, which is a coating liquid tank that obtains a uniform surface flow. There are disadvantages in that the inflowing liquid has little opportunity to receive ultrasonic irradiation from the bottom of the tank, and the overflow pocket side of the bottom of the tank tends to become a dead space. Further, in the case as shown in FIG. 5, the blown liquid flow hits the tank wall on the overflow pocket side and causes a backflow, so that no substantial surface flow can be obtained. Furthermore, although the method of uniformly blowing out the coating liquid from the bottom of the tank as shown in Figure 6 provides an extremely quiet surface flow, the partition wall that makes up the liquid outlet at the bottom of the tank obstructs the ultrasonic action and A fundamental problem arises in that the ultrasonic waves do not reach the coating. As is clear from the above description, the present invention irradiates the coating liquid with ultrasonic waves, and at the bottom of the coating liquid tank, on the side where the coating liquid overflows, the coating liquid is uniformly applied from the overflowing side to the opposite side, and This method is characterized by supplying the coating liquid by blowing it out parallel to the bottom of the tank, and the apparatus for realizing this method makes it possible to obtain a coating film with extremely excellent surface smoothness. The usefulness of the present invention will become clear from the following working examples. In addition, parts in the examples represent parts by weight, and the measurements and evaluations in the examples were performed in the following manner. (1) Measurement of surface smoothness a. Visual method 〇: The surface smoothness of the coating is good. ×: The surface is disordered and the smoothness is poor. B Frequency Measured using a lens meter (JIS B 7183-1975). The smaller the numerical value (absolute value), the better the surface smoothness. C. Astigmatism Measured using a lens meter (JIS B 7183-1975). The smaller the numerical value (absolute value), the better the surface smoothness. NBS from a distance of 5m using a telescope with 10x resolution
The image on the Resolution Test Chart is resolved through a sample placed in front of the telescope, and the numerical value of the resolved image is displayed. The larger the value, the better the resolution of the sample. (2) Judgment of surface defects (bumps or pinholes) 〇: Bumps are not noticeable ×: Bumps are noticeable Example 1, Comparative Examples 1 to 5 Trimethylolethane trimethacrylate 30
parts, 1.6-hexanediol diacrylate 10
parts, n-propyl alcohol 30 parts, toluene 30 parts
A dip coating device (Example 1) consisting of the process shown in FIG. 1 and the coating liquid tank shown in FIG. 3, and the process shown in FIG. 1 and the coating shown in FIG. A dip coating apparatus (Comparative Example 1) consisting of the process shown in FIG. 1 and a coating liquid tank shown in FIG. 5 (Comparative Example 2); The sample was placed in a dip coating apparatus (Comparative Example 3) consisting of the coating liquid tank shown in FIG. 6, and the apparatus was operated while operating the ultrasonic oscillator. The size of the tank for each device is 700 mm (length) x 250
mm (width) x 400 mm (depth), and the flow rate of the coating liquid in the tank was 20 cm/sec. Furthermore, the frequency and intensity of the ultrasonic oscillator were 28 KHz and 1.0 W/cm ² , respectively. A uniform methacrylic resin plate with a thickness of 3 mm is immersed in these dip coating devices, pulled up into clean air at a speed of 0.5 cm/sec, and immediately exposed to 5 % of ultraviolet rays using a 2 KW high-pressure mercury lamp in a clean nitrogen stream. Irradiated for seconds. The appearance of the obtained coated board was as shown in Table 1. Comparative Examples 4 and 5 are examples in which only the ultrasonic oscillation device was stopped while the liquid circulation remained in Example 1, and the ultrasonic oscillation device was kept as it was and only the liquid circulation was stopped. The obtained evaluation results are shown in Table 1.

【table】

【table】 [Brief explanation of the drawing]

Fig. 1 is a flow sheet showing an embodiment of the present invention, Fig. 2 is a perspective view of an ultrasonic vibrator installed on the bottom of the coating liquid tank, and Fig. 3 is a coating liquid tank and a coating liquid according to the present invention. A perspective view showing the flow pattern is shown in the fourth
5 and 6 are perspective views showing other coating liquid tanks and flow patterns of the coating liquid, respectively.
is a coating liquid tank, 3 is an ultrasonic vibrator, 4 is a weir, 13
1 is a coating liquid inlet, and 14 is a blowing nozzle.

Claims (1)

[Scope of Claims] 1. A method in which an article is immersed in a coating liquid tank in which the coating liquid is externally circulated and then pulled up to form a coating film on the surface of the article, in which the coating liquid is irradiated with ultrasonic waves, and the coating liquid is irradiated with ultrasound at the bottom of the coating liquid tank. A dip coating method characterized by spraying and supplying a coating liquid uniformly from the side where the coating liquid overflows to the opposite side and parallel to the bottom of the tank. 2. In a device that immerses an article in a coating liquid tank in which the coating liquid circulates externally and then pulls it up to form a coating film on the surface of the article, an ultrasonic vibrator is installed on the bottom inside or outside the coating liquid tank, and At the bottom of the coating liquid tank on the side where the coating liquid overflows, a nozzle that can blow out and supply the coating liquid uniformly from the side where the coating liquid overflows to the opposite side and parallel to the bottom of the tank is installed. A dip coating device characterized by being provided with.