JPS6025573A - Powder coating method - Google Patents

Abstract

PURPOSE:To prevent bubbles from being incorporated in a coated film and enhance appearance and mechanical properties, by performing powder coating by heating in two stages, namely, heating to such an extent as to melt a particulate coating material and heating to such an extent as to harden the melted coating material. CONSTITUTION:A steel pipe 4 as an object to be coated is passed sequentially through the first heating furnace 2, a coating booth 1 and the second heating furnace 3 while being supported by skew rollers 5. In the first heating furnace 2, the pipe 4 is heated to a temperature of 120-200% of the substantial melting temperature of a thermosetting particulate resin such as an epoxy resin. In the coating booth 1, a predetermined amount of the particulate coating material is applied to the steel pipe 4 thus heated, and is melted to form a coated film, which starts to be hardened, and is fed into the second heating surface 3 together with the pipe 4. In the second heating furnace 3 the temperature in which is higher than the coated film forming temperature, the coated film is sufficiently cured while being supplied with an amount of heat required for not only hardening but also obtaining various characteristics.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a powder coating method, and more particularly to a powder coating method using a thermosetting powder resin such as an epoxy resin.

Traditionally, epoxy powder coating involves heating the object to be coated to a high temperature and coating the heated object with epoxy powder, or coating the object to be coated at room temperature with epoxy powder. This was done by heating it to a high temperature.

However, in any case, in conventional epoxy powder coating, the heating of the epoxy powder is done temporarily, and as the epoxy powder undergoes a rapid curing reaction at the same time as it melts, it forms on the coated object. The air contained in the epoxy powder coating when it was powdered and the gas generated during the curing reaction are trapped in the coating film without being sufficiently removed, and as a result, the coating film formed on the object is contains many bubbles, and the presence of such bubbles has been an obstacle to further improving mechanical properties such as impact resistance and bending resistance. Furthermore, since the epoxy powder melts and hardens at the same time, the coating film on the object to be coated is not smooth and uniform, and there is a drawback that roughness tends to occur.

This invention has been made in view of the above-mentioned conventional circumstances, and is a powder that can prevent the inclusion of air bubbles in a powder coating film and provide a coating film with good mechanical properties and a good appearance. The purpose is to provide a body painting method.

That is, in the powder coating method of the present invention, a thermosetting powder resin is coated on an object to be coated at a temperature of 120% above the substantial melting temperature of the powder resin.
It is characterized by melting at a temperature of ~200°C, thereby forming a molten coating film of powder resin on the object to be coated, and then raising the temperature of the bath-melted coating film to sufficiently key the coating film. It is.

This invention will be explained in more detail below.

FIG. 1 shows a painting line used in one embodiment of the present invention. In the painting line shown in the figure, a first heating furnace 2 is provided in front of a coating booth 1, and a second heating furnace 3 is provided behind the coating booth 1. A steel pipe 4, which is an object to be coated, is supported by skew rollers 5 and passes through a first heating furnace 2, a coating booth 1, and a second heating furnace 3 in that order.

In this invention, the steel pipe 4 is coated in the following manner on the coating line shown in FIG. First, the steel α4 is heated to 120°C, which is the substantial melting temperature of the thermosetting powder resin, in the first heating furnace 2.
Melt at a temperature of ~200 degrees.

Here, the effective bath melting temperature rtL of the thermosetting powder resin refers to the temperature at which, when the thermosetting powder resin is heated, the powder state collapses and melting occurs in appearance. This point will be explained in more detail by taking as an example a commercially available epoxy resin powder coating used in Examples described later. FIG. 2 shows the melting temperature of the epoxy resin powder coating measured by a differential calorimeter. As shown in FIG. 2, it can be seen that a rapid endothermic reaction occurs at 63° C., and microscopic melting begins at that temperature. It can be seen that when the temperature is further increased, the endothermic amount and the calorific value are balanced in the range of 80 to 90'C, and when the temperature exceeds 90°C, the exothermic reaction becomes stronger and the curing progresses. The above is the microscopic process of melting and curing of paint.
Macroscopically, when we actually applied the above paint to a thickness of 350μ on a steel plate and observed the melting and hardening process, we found that:
At 63°C, the paint does not appear to be melting at all;
The powder state deteriorated in the range of 0 to 90°C, and melting occurred only when the temperature reached 90°C, and a coating film began to be formed. Therefore, the melting temperature of this paint is 63°C, but when handling this paint, the actual melting temperature at which the powder paint coating the object to be coated melts in the bath and forms the coating is a problem. In that sense, the substantial melting temperature of the above-mentioned coating material is 90°C.

Moreover, as mentioned above, the reason why the heating temperature in the first heating furnace 2 is set to 120 to 200% of the substantial melting temperature of the powder coating is as follows.

The powder coating adheres to the object to be coated, in this case the steel pipe 4, and then melts in the bath, and heat is transferred to the object that adheres later through the molten layer of the powder coating, which has a poor calorific value 4! ll Melting progresses.

Therefore, if the heating temperature for the object to be coated is set to the substantial melting temperature, the amount of heat added to the powder coating material that is subsequently attached will be insufficient. Therefore, in order to melt the entire powder coating adhered to the steel pipe 4, it is necessary to set the heating temperature of the steel pipe 4 to about 20 degrees higher than the actual melting temperature.

That is, when the heating temperature is less than 120% of the melting temperature of the powder coating, it becomes impossible to completely melt the entire coating material attached to the steel pipe.

On the other hand, if the heating temperature is set to a temperature that exceeds the melting temperature of the powder coating by 200 degrees, the powder coating will melt and harden at the same time, causing problems such as bubbles being included in the formed coating film.

Next, a predetermined amount of powder coating is applied to the heated steel σ4 in the coating booth 1, and the adhered powder coating melts to form a coating film and begins to harden. The steel pipes 4 and the steel pipes 4 are then transported into the second heating furnace 3. This second
In the heating furnace 3, the coating film is not only cured, but also sufficiently heated by being given the amount of heat necessary to ensure the performance of various coating films. Then, the coating film hardens and the ski-low 2
After reaching a hardness that does not cause scratches on the surface when it comes into contact with the steel 4, the steel 4 is supported by ski-rollers 5 and conveyed, and cooled in a water cooling zone 6 to a temperature that does not interfere with post-processing such as inspection. cooled down. The lower limit of the amount of heat applied in the second heating furnace 3 is set as a temperature exceeding the coating film formation temperature, but the upper limit depends on the curing speed of the thermosetting resin used and the coating speed. It is determined. In other words, if the coating speed is to be increased to meet the curing speed, it is necessary to raise the temperature significantly in the second heating furnace 3; conversely, if the coating speed is to be slowed to meet the curing speed, The temperature increase in the heating furnace 3 may be relatively small.

Next, examples of this invention will be shown.

Example 1 Using the commercially available epoxy resin shown in FIG.
The outer diameter is 1,4:20 mm at the painting line shown in the figure.

Thickness: 16 +n+a&i4%ft, f&loading speed: 1.5
#) '1nin, coating thickness was 350μ. The temperature conditions for coating were set as follows.

That is, for a paint's substantial melting temperature of 90'C,
The heating temperature in the first heating furnace 2 is set to 120°C so that it can be coated at 110"C, which is 20% higher than that, and the steel pipe 4 heated to 120°C in the first heating furnace 2 is placed in a coating booth. 1 and was coated with paint in the coating booth 1, heated to 230° C. in the second heating/furnace 3, cured for about 4 minutes, and then cooled with water.

Example 2 Other conditions were the same as in Example 1, and painting was performed in the following manner. That is, the actual melting temperature of the paint is 90°C.
The heating temperature in the first heating furnace 2 was set to 190°C, and the heating temperature in the second heating furnace 3 was set to 230°C so that the paint layer could be coated at 180°C, which is twice as high.

Example 3 Powder coating was carried out using the coating line shown in FIG. 3 under the same conditions as in Example 1 except for the other conditions. That is, the pipe 4 at room temperature is coated with paint in the coating booth 7, the steel f4 is heated to 160°C in the first heating furnace 8, and heated in the heating furnace 9 in 2 below for about 1 minute.
The temperature was raised to 30°C, cured for about 4 minutes, and then cooled with water.

Comparative Example The same steel pipe as in each of the above Examples was coated with the same paint by a conventional method. The heating temperature for the steel pipe was 230°C, and the temperature of the copper pipe when spraying paint in the paint booth was maintained at 220°C, so that the paint film would harden in about 5 minutes before cooling with water and be sufficiently quenched. did.

Table 1 shows the coating temperature conditions in each of the above examples and comparative examples.

Table 1: Coating temperature conditions The surface and cross-sectional appearance of the coating films obtained in each Example and Comparative Example were inspected, and (mechanical strength tests!) were conducted. show.

Table 2 Paint film performance As shown in Table 2, the surface of the paint film obtained in the comparative example had small irregularities on the skin, and paint powder floating in the paint booth (=J On the other hand, the surface of the first coating obtained in each example was 1f!, and was in good condition. In contrast to the comparative examples, where air bubbles were observed to be scattered, no air bubbles were present in the coating films obtained in each of the examples.

In this way, each embodiment
is dense, so as shown in Table 2, −4
In the bending test and the impact test at 0° C., the coating films obtained in each example gave better results than the coating films of the comparative examples.

As described above, according to the present invention, powder coating is performed by heating in two stages: heating to melt the powder paint and heating to harden the melted paint. It is possible to prevent the coating film from containing the air that the coating material had when it was powdered and the gas generated when the molten coating material is cured, and it is possible to obtain a coating film with high visual strength. Moreover, a coating film with a smooth appearance and no rough skin can be obtained.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram of a coating line used in an embodiment of the present invention, and Fig. 2 is a diagram showing the results of measuring the bath melting temperature of a commercially available epoxy resin powder coating used in an embodiment of the invention. . 1...Painting booth, 2...First heating furnace, 3...
Second heating furnace, 4-.../J$4 tube, 5... Skew roller.

Claims (3)

[Claims] (1) Melting the thermosetting powder resin on the object to be coated at a temperature of 120 to 200% of the substantial melting temperature of the powder resin,
As a result, a molten coating film of powder resin is formed on the object to be coated,
A powder coating method characterized in that the temperature of the molten coating film is then raised to sufficiently stiffen the coating film. (2) The step of forming a molten coating film of thermosetting powder resin on the object to be coated is performed by heating the object to be coated in advance to a temperature of 120 to 200 degrees higher than the melting temperature of the powder resin. The powder according to claim 1 is coated with the powder resin, and then coats the object to be painted with the powder resin, and melts and applies the powder resin to the painted surface of the object. Painting method. (3) The step of forming a molten coating film of thermosetting powder resin on the object to be coated is performed after coating the object to be coated at room temperature with the powder resin. 2. The powder coating method according to claim 1, wherein the powder coating method is carried out by heating to a temperature of 120 to 200 degrees higher than the melting temperature of the powder resin.