JP2976857B2 - Paint baking method using a roll coater - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a coating baking method using a roll coater, and more particularly, to a coating baking method using a roll coater suitable for forming a smooth coating by suppressing the generation of streak patterns. About.

[0002]

2. Description of the Related Art Painted steel sheets are used in a wide range of fields such as home appliances, building materials, and automobiles, and require a coating film to have a required thickness. In addition, depending on the application, since it is used as it is on a visible part or is used after painting, there is a strict requirement for good appearance such as a smooth painted surface.

In such a coated steel sheet, as a method of controlling a coating thickness in a longitudinal direction to a required thickness, for example, Japanese Patent Application Laid-Open No. 55-134672 discloses a method of manufacturing a coated steel sheet continuously. A method of controlling the coating film thickness by comparing the coating film thickness on the output side with the coating film thickness set value, adjusting the steel sheet temperature based on the comparison result, and controlling the viscosity of the coating material is disclosed. And Japanese Patent Laid-Open No. 5-2
Japanese Patent No. 20441 discloses a method in which a gap between an applicator roll and a steel sheet is determined by applying an elastic fluid theory, and the influence of the swelling of the rubber lining of the applicator roll is corrected to control the coating film thickness.

[0004] As a method of adjusting the coating thickness of the steel strip,
In JP-A-63-7870, the surface temperature and the film thickness (and the color of the film surface) of the steel strip on which the film is baked are detected, and the temperature in the furnace of the baking furnace is determined based on the detected results. Also, a method for controlling a coating film of a coated steel sheet in which the coating amount is adjusted is disclosed.

Further, as a method for controlling the film thickness in the width direction of the steel strip, Japanese Patent Laid-Open Publication No. 3-196873 discloses a method of measuring the thickness of a coated strip in the width direction and calculating the width based on the measured value. By controlling the inter-roll pressure at both ends of the roll between the pickup roll and the applicator roll in accordance with the coating thickness pattern at both ends and the center in the direction, the coating thickness in the width direction of the strip is equalized. A method for controlling the coating thickness in the width direction is disclosed.

[0006] The method of applying a coating material to these strips is a coating method generally known as a "roll coating method", and is one of the general methods of applying various coating solutions to steel plates, plastics, papers and the like. is there.

[0007]

However, in the roll coating method shown in the above prior art, it is possible to control the film thickness in the longitudinal direction and the width direction. The thickness becomes uneven in a minute range in the width direction,
The generation of so-called streak patterns (also called rolls, striations, rovings, and wavings) cannot be suppressed, and remains as streak patterns on the product surface after drying and baking, resulting in poor appearance of the product.

In general, when a coating material is applied to a material to be coated, a natural coating method in which the applicator roll 13 and the backup roll 14 are rotated in the same direction as shown by a solid line in FIG. There is a reverse coating method in which the applicator roll 13 is rotated in a direction opposite to the rotation direction of the backup roll 14 as indicated by a broken line, and a streak pattern is generated in each case.

The cause of the streak pattern in the natural coating is that the material to be coated M and the applicator roll 1
It is said that the paint adhered to the material to be coated is torn by the applicator roll 13 when the material 3 is separated.

On the other hand, in the case of the reverse coating, the traveling direction of the workpiece M and the rotating direction of the applicator roll 13 are opposite to each other, and the tearing phenomenon unlike natural coating does not occur. There is no streak pattern. However, since the rotation directions of the pickup roll 12 and the applicator roll 13 are the same, a tear phenomenon occurs between the pickup roll 12 and the applicator roll 13 to generate a streak pattern, and this streak pattern is transferred to the material to be coated. Eventually, streaks appeared on the material to be coated.

[0011] This streak pattern, within the thickness adjustment range,
It cannot be adjusted by the rotation speed of the rolls or the pressure between the rolls, and it is necessary to reduce the line speed itself.

In order to solve this problem, the present applicant disclosed in Japanese Patent Application Laid-Open No. 4-330964 that when coating a plate material continuously with a roll coater, at least one of the coating rolls was used as a surface heating roll. Discloses a roll coating method in which coating is performed while increasing the temperature of the paint adhered to the surface.

According to this method, the generation of streak patterns on the material to be coated can be reduced, but the generation of streak patterns cannot be completely eliminated. I had to do it.

In any case, according to the current technology, when applying the paint from the applicator roll to the material to be coated, if the line speed is not reduced, the paint is applied from the applicator roll to the material to be coated. It is inevitable that streaks are generated on the surface of the material to be coated.

Therefore, an object of the present invention is to perform coating without causing a decrease in productivity due to a decrease in line speed, leaving no streak pattern in the coated film after baking, and thereby, a smooth coating film surface. An object of the present invention is to provide a coated metal plate having an excellent appearance.

[0016]

According to the present invention, which solves the above-mentioned problems, there is provided a coating and baking method in which a paint is continuously applied to a conveyed metal plate using a roll coater and then baking is performed. After applying the coating material, the yield point temperature ± 1 at which the viscosity is minimized in the correlation between the temperature and the viscosity of the coating material
A coating and baking method using a roll coater, wherein drying and baking are started after holding a time of 8.0 seconds or more at a temperature within 0 ° C.

The present invention is not intended to suppress the generation of streak patterns by changing the conditions at the time of applying the paint from the applicator roll to the metal plate to be coated, as in the prior art described above. The purpose of the present invention is to smooth the surface of the material to be coated by utilizing the leveling action of the paint after application on the metal plate.

The leveling action refers to the smoothing action of the coating film due to gravity or the surface tension of the paint. This leveling action mainly depends on the dilution ratio, viscosity and temperature of the paint, and differs depending on the type of paint. Nevertheless, the leveling effect is generally increased by increasing the dilution ratio or decreasing the viscosity.

Therefore, if the dilution ratio is increased to increase the leveling action, the coating thickness after drying and baking is adversely affected, so that the paint cannot be excessively diluted. Further, in order to dilute the paint, a large amount of solvent is consumed, which is not practical.

Therefore, in the present invention, attention was paid not to changing the paint itself, but to using the paint itself in a low viscosity state so that the leveling action is effectively exhibited. The viscosity of the paint depends on the temperature when the dilution ratio is constant, and there is a correlation between the temperature and the viscosity in that case, for example, as shown in FIG. As can be seen from FIG. 5, there is a temperature at which the viscosity becomes minimum under the condition that the dilution ratio is constant.

The temperature at which the viscosity becomes minimum in the correlation between the temperature of the paint and the viscosity is defined as the "yield point temperature". This yield point temperature differs depending on the type of paint.

In order to effectively exert the leveling action, it is ideal to maintain the paint at the yield point temperature. However, as can be seen from FIG. 5, the viscosity of the paint falls within the temperature range near the yield point temperature. The difference is relatively small, typically ± 10%.

Further studies based on this finding further showed that the amplitude of the streak pattern (uneven thickness portion) at t seconds after the coating of the steel strip was Z ₁ , amplitude Z ₁ of was found to be given by the following equation (1).

[0024]

(Equation 1)

According to the equation (1), it can be seen that the lower the viscosity μ of the coating material, the smaller the coating thickness amplitude Z ₁ after the lapse of the same time. Therefore, the lower the viscosity of the paint, the smoother the surface of the material to be coated.

From the equation (1), it can be seen that the leveling action is further enhanced by increasing the time t to be maintained at the yield point temperature during the time required for bake hardening.
Therefore, in order to smooth the surface of the coating film layer, it is important that the temperature of the coating film after application is set to the yield point temperature of the paint and that the time for maintaining the yield point temperature is long.

More specifically, as shown in the examples described below, in order to obtain good smoothness of the coating film, drying and drying are performed from the time of application.
The temperature is maintained at a yield point temperature of ± 10 ° C. until the start of baking, and baking is started after a lapse of 8.0 seconds or more.

[0028]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be specifically described below with reference to the drawings. FIG. 1 is a schematic diagram of a coating apparatus used in the coating method according to the present invention. In such an apparatus, the paint in the paint pan 1 is picked up by the pickup roll 2 and transferred to the applicator roll 3. The applicator roll 3 applies a paint to a metal plate to be coated, for example, a steel strip M, which is continuously conveyed while being guided by the backup roll 4.

The paint is applied at a temperature within a yield point temperature ± 10 ° C. at which the viscosity becomes minimum in the correlation between the temperature and the viscosity of the paint, and 8.0 seconds or more from the point of application to the point of start of drying and baking. After a lapse of time, printing is started.

In the example shown in the figure, a reverse coating system is used, but the present invention is also applied to a natural coating system in substantially the same manner.

[0031]

Embodiments of the present invention will be described below. (Example 1) With a size of 0.8 mmt x 1220 mm width,
For a steel sheet conveyed at a line speed of 100 mpm, a solvent-type epoxy resin-based paint (curing temperature: 160 ° C., yield point temperature: 50 ° C., dilution degree: 23%) is applied by a reverse coating method shown in FIG. , 50 ° C. After 20.7 seconds after the application, the maximum heating rate is 6.
Drying and baking oven with a capacity of 0 ° C / sec (furnace length 57m
/ 5 zone), and then dried and baked with a temperature gradient shown in FIG.

As a result, the smoothness of the coating film of the product was extremely excellent without streaks. FIG. 2 also shows a typical example of the related art. In this conventional example, a streak pattern slightly remains.

(Example 2) Since it was found from Example 1 that the holding time and the holding temperature affected the smoothness of the coating film, the holding time, the holding temperature and the line speed were variously changed to improve the smoothness. Upon examination, the results shown in Table 1 were obtained. The results are shown in FIG. Adjusting the holding time in relation to the line speed was performed by adjusting the temperature in the former stage of the baking / drying furnace.

[0034]

[Table 1]

From these results, in order to obtain excellent smoothness of the coating film surface, the temperature of the paint and the holding time from the start of coating to the start of drying and baking are factors. It can be seen that the best is obtained only within the range specified in the invention.

When the film is held at a temperature higher than the yield point temperature by more than 10 ° C., the viscosity is already low at the time of coating, and the evaporation speed of the solvent in the coating film is high during the holding process. It is considered that the leveling action is not sufficient due to the increase in viscosity due to the decrease in the dilution ratio.

When the film is held at a temperature lower than the yield point temperature by more than 10 ° C., the viscosity is high at the time of coating, and the leveling effect in the holding process cannot be expected. Even if the viscosity decreases, it is considered that the viscosity starts increasing again immediately with the progress of drying, and drying and baking are performed until leveling is completed.

The heating rate in the drying / baking furnace is determined mainly by the capability of the drying / baking furnace itself and the conditions that do not cause the armoring limit (avatar-like defect) of the coating film. Therefore, the upper limit of the holding time is naturally determined by the capacity and equipment length of the drying and baking furnace, the line length including the coater, and the passing speed of the steel sheet. Generally, the upper limit of the holding time is 35 seconds. The retention time is preferably 10
３０30 seconds.

[0039]

As is apparent from the above description, according to the present invention, coating can be carried out without causing a decrease in productivity due to a decrease in line speed, and a streak pattern is left on the coated film after baking. In addition, it is possible to provide a coated metal plate having an excellent appearance with a smooth coating film surface.

[Brief description of the drawings]

FIG. 1 is a schematic view of a coating apparatus used for coating according to the present invention.

FIG. 2 is a graph showing an example of the present invention in comparison with a conventional example.

FIG. 3 is a graph showing the results of Examples.

FIG. 4 is a schematic view of a conventional coating apparatus.

FIG. 5 is a graph showing the relationship between the temperature and the viscosity of a paint.

[Explanation of symbols]

1 ... Paint pan, 2 ... Pickup roll, 3 ... Applicator roll, 4 ... Backup roll, M ... Coating material.

Claims (1)

(57) [Claims] 1. A coating and baking method in which a paint is continuously applied to a conveyed metal plate using a roll coater, and then dried and baked. A baking method using a roll coater, wherein drying and baking are started after holding a time of 8.0 seconds or more at a temperature within a yield point temperature of ± 10 ° C. at which the viscosity is minimized in relation to the viscosity. .