JP3031153B2 - Painting method - 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 method, and more particularly, to an optimum coating method for a high solid paint.

[0002]

2. Description of the Related Art Conventionally, acrylic paints having excellent weather resistance and appearance have been widely used as cross-linking-curable paints used for painting automobiles and household electric appliances. The acrylic paint includes an alkyl (meth) acrylate such as butyl acrylate, methyl methacrylate or butyl methacrylate, and a functional group-containing monomer such as 2-hydroxyethyl methacrylate or 2-hydroxypropyl methacrylate. Acrylic resin obtained by radical copolymerization of is generally used. And a paint obtained by adding a heating type crosslinking agent such as a melamine resin or a polyvalent isocyanate to an organic solvent solution of such an acrylic resin is baked after painting to form a strong coating film having a three-dimensional network structure. Form.

[0003] One of the expected effects of the organic solvent in the paint for spray coating is the effect of adjusting the viscosity of the paint from the spray coating to the film formation process. That is,
A typical conventional paint has a mass ratio of about 40% with the paint before being sprayed from a spray gun, that is, in an initial state.
It contains a large amount of organic solvent of up to 60 wt% and is adjusted to low viscosity. Thereby, at the time of spray coating in which the paint is sprayed from the spray gun, the liquid paint can be smoothly atomized into mist. In addition, if the viscosity of the paint in the initial state is too high and the paint is not smoothly atomized at the time of spray coating, the resulting coating film may have poor appearance such as lack of luster.

[0004] Then, the paint atomized by spraying from the spray gun evaporates a part of the organic solvent having a low boiling point while flying to the object to be coated, and the viscosity increases. Due to this thickening effect, dripping when the paint is applied to an object to be coated is suppressed. On the other hand, when the amount of the organic solvent in the paint applied to the object to be coated is extremely small and the viscosity is extremely increased, the fluidity of the paint decreases, and the coated surface of the coating film becomes uneven. The appearance becomes poor. As described above, the organic solvent in the paint is used to properly adjust the viscosity of the paint in the process of film formation from spray coating, and to smoothly atomize the paint during spray coating,
It contributes to prevention of sagging during coating and formation of a smooth coating surface.

[0005] However, on the other hand, the problem of pollution caused by evaporation of the organic solvent expected to have the above-mentioned effects has been raised, and it has become necessary to reduce the amount of organic solvent used for resource saving reasons. I have. Therefore, development of water-based paints and high-solid paints has been actively conducted. However, water-based paints have a problem that water, which is a solvent, has a lower evaporation rate than an organic solvent, so that drying is slow and time until a film is formed is long. In addition, the amount of volatilization between spraying and application to the substrate is small,
There is also a problem that the rise in viscosity after coating is small and dripping easily occurs.

[0006] Therefore, Japanese Patent Laid-Open No. 3-262568, a wavelength in the coating particle is 2 to 10 ³ [mu] m in flight discharged from the coating gun
A coating method of irradiating the above-mentioned electromagnetic waves and evaporating the solvent in the coated particles by heating with the electromagnetic waves is disclosed. According to this coating method, the solvent in the coated particles is instantaneously heated and volatilized by the electromagnetic waves, so that the viscosity of the coated particles is considerably increased at the time of application to the object. Therefore, even if it is a water-based paint, the increase in viscosity after application is large, and dripping can be prevented.

In the case of high solid paints, there is a limit in the method of simply reducing the amount of the organic solvent because of the above-mentioned useful function of the organic solvent. In other words, when the amount of the organic solvent is reduced and the resin concentration is simply increased by that amount, the viscosity of the paint in the initial state becomes too high, so that smooth atomization is not performed at the time of spray coating and appearance defects such as glosslessness are caused. Or occur. Therefore, it is necessary to lower the viscosity of the coating film forming element itself such as a resin and to reduce the amount of the organic solvent so that spray coating is possible.

[0008]

In the case of painting on a production line, the paint is constantly circulated in order to keep the painting conditions constant. Therefore, the viscosity of the paint needs to be adjusted to a low viscosity of 100 to 300 cP in order to efficiently circulate and maintain the suitability of the paint at the time of spraying.

In order to further lower the viscosity of the high solid paint, it is desirable to further lower the viscosity of the coating film forming element itself, which serves as a matrix. However, as the organic solvent decreases and the content of the film-forming element increases, the difference in viscosity between the time of discharge from the spray gun and the time of application decreases, and the lower the viscosity of the film-forming element itself, the more There is a fundamental problem that sagging easily occurs.

Further, even if the method disclosed in Japanese Patent Application Laid-Open No. 3-262568 is used, the effect of increasing the viscosity by volatilization of the diluent during flight is not expected because high solid paints originally have a small amount of diluent. Can not. Furthermore, JP-A-3-2625
In the method disclosed in Japanese Patent Publication No. 68, since the sprayed paint particles fly while spreading, it is necessary to increase the irradiation range when attempting to irradiate the entirety of the paint particles in flight with electromagnetic waves, and the device is large-scale. Become. Further, the flight time of the coating particles is almost constant and short, and it is difficult to change the irradiation time of the electromagnetic wave. Therefore, it is also difficult to change the viscosity at the time of coating by changing the irradiation time.

The present invention has been made in view of the above-mentioned circumstances, and uses a high solid paint in which the amount of an organic solvent to be used and the amount of an evaporating organic solvent are reduced as much as possible while maintaining a low viscosity required in an initial state. It is an object of the present invention to provide a coating method which is excellent in coating property and can contribute to solving pollution and resource saving problems.

[0012]

According to the present invention, there is provided a coating method comprising the steps of: supplying a liquid coating composition containing a polymerizable component which can be polymerized by irradiation of an electromagnetic wave to a spraying device in a pipe line; The method is characterized in that the object is irradiated with electromagnetic waves to polymerize the polymerization component, and the coating composition having an increased viscosity is discharged from a spray device to perform coating.

The above-mentioned coating composition containing a polymerization component includes:
For example, a macromonomer having a double bond such as a methacryloyl group at one end can be used. Further, a coating composition containing an acrylic resin, a heat-type cross-linking agent and an organic solvent, in which a part of the organic solvent is replaced by a polymerizable component capable of being polymerized by electromagnetic waves, may be used. The acrylic resin is not particularly limited, and is an alkyl (meth) acrylate such as butyl acrylate, methyl methacrylate or butyl methacrylate, and a functional group-containing monomer such as 2-hydroxyethyl methacrylate or 2-hydroxypropyl methacrylate. A general acrylic resin obtained by radical copolymerization with a body can be used.

The heating type crosslinking agent is not particularly limited, and a general heating type crosslinking agent such as melamine and polyisocyanate can be used corresponding to the acrylic resin. The organic solvent is not particularly limited, and common organic solvents such as toluene, xylene, and butanol can be used. However, due to the thickening effect of the polymerization component, it is possible to effectively prevent the occurrence of sagging at the time of application, so that it evaporates between the time when it is sprayed from the spray gun and when it is applied to the object to be coated. It is not necessary to use an organic solvent having a low boiling point. For this reason, it is preferable to use an organic solvent having as high a boiling point as possible, which can secure the fluidity of the coating film and contribute to the formation of a smooth coated surface after being applied to the substrate.

[0015] When the viscosity of the polymerization component increases due to polymerization and the suitability for spraying decreases, the coating composition is subjected to high shear stress by a spray device or heated to forcibly lower the viscosity by spraying. It is desirable to discharge from the device.

[0016]

According to the coating method of the present invention, the viscosity of the paint under normal conditions can be about 100 to 300 cP (10 to 30 cPa · s), and the paint can be smoothly circulated. Then, the coating composition is irradiated with electromagnetic waves in the middle of the pipeline before being discharged from the spray. Then, a polymerization reaction occurs in the polymerization components contained in the composition, and the viscosity of the coating composition increases.

When the suitability for spray coating is reduced due to an increase in viscosity, hot spraying or the like is used. In other words, by heating the thickened coating composition, the viscosity of the coating composition is forcibly reduced, and the viscosity becomes such that spray coating is possible, and smooth atomization is achieved. The coating particles discharged from the spray are finely divided, so that they have excellent smoothness of a coated surface when applied to a substrate. The coating particles recover to the same viscosity as in the original pipe mainly due to cooling by the atmosphere, and at the time when they are applied to the object to be coated, the viscosity has recovered to, for example, about 1000 cP (100 cPa · s). Therefore, dripping is prevented.

In addition, since electromagnetic waves are generally applied to small-diameter pipes, equipment for irradiating the electromagnetic waves may be small.
Furthermore, since the length of the conduit is generally not limited, the viscosity of the coating composition can be controlled by changing the length of the conduit for irradiating the electromagnetic wave, and the viscosity can be easily controlled. Furthermore, although the polymerization reaction is generally hindered by the presence of oxygen, the coating composition in the pipe is irradiated with electromagnetic waves to be polymerized, so that the hindrance of oxygen can be avoided and the polymerization reaction can be performed efficiently. Can be.

With the above configuration,
The polymerization of the polymerization component can be completely performed. Therefore, double bonds are prevented from remaining in the coating film, and various defects due to the remaining double bonds can be avoided.

[0020]

The present invention will be specifically described below with reference to examples. (Embodiment 1) FIG. 1 is an explanatory view of the structure of a coating apparatus used in this embodiment. The coating apparatus includes a paint tank 1, a supply pipe 11 for supplying paint from the paint tank 1 to a spray gun 2 by a pump 10, and a return pipe for returning paint to the paint tank 1 from before the spray gun 2 during non-painting. An ultraviolet lamp 3 is provided in a part of the supply pipe 11 before the spray gun 2. The spray gun 2 is provided with a heating device (not shown) so that hot spraying can be performed.

At least the ultraviolet lamp 3 in the supply line 11
Is formed of a transparent glass tube and is capable of transmitting ultraviolet light. The ultraviolet lamp 3 is composed of a high-pressure mercury lamp (6 kW × 2), is provided at a distance of 300 mm from the supply pipe 11, and is configured to be able to irradiate ultraviolet rays toward the supply pipe 11. Next, the high solid paint used in this example is composed of 80% by weight of a macromonomer having a weight average molecular weight (Mw) of 2600 and having a methacryloyl group at one end, and 20% by weight of xylene. To 100 parts by weight of the solid paint, 4 parts by weight of a photopolymerization initiator ("Darocur 1173" manufactured by Ciba Geigy) was added to obtain a coating composition used in this example.

This paint composition is placed in a paint tank 1,
The pump 10 is driven to circulate the paint tank 1 → the pump 10 → the supply pipe 11 → the return pipe 12 → the paint tank 1. At this time, the temperature of the coating composition was 25 ° C., and the viscosity was 200 cP (20 cPa · s). During this circulation, the ultraviolet lamp 3 is turned off. Therefore, the coating composition does not increase in viscosity during circulation and has low viscosity, so that the composition is smoothly circulated.

The object 4 was placed at a position 300 mm away from the spray gun 2 and the coating composition was discharged from the spray gun 2 to perform hot spray coating.
At this time, ultraviolet rays were irradiated from the ultraviolet lamp 3 to the supply conduit 11. The dose of ultraviolet rays, 22 mJ / cm ² by changing the irradiation width of the ultraviolet, 44 mJ / cm ² and 3 of 66 mJ / cm ²
The hot spray coating was performed for each level. The coating composition is heated to 60 ° C. by a heating device (not shown), increases in viscosity by irradiation with ultraviolet light, and is reduced in viscosity by heating, and is discharged from the spray gun 2 at a viscosity of 200 cP (20 cPa · s). Accordingly, the discharged coating particles have a low viscosity and are smoothly atomized.

The object 4 to be coated when coated in this way
The viscosity of the coating composition at the time of application was measured, and the appearance of the coating (the degree of sagging) was visually evaluated. The results are shown in Table 1. As a comparison, coating was performed in the same manner except that ultraviolet irradiation was not performed, and the evaluation results are shown in Table 1.

[0025]

[Table 1] From Table 1, it is clear that the viscosity after application increases in proportion to the amount of ultraviolet irradiation, and that dripping is thereby prevented. (Example 2) 38 parts by weight of a heat-crosslinkable clear paint ("O-150" (manufactured by Nippon Paint Co., Ltd.) composed of an acrylic resin and a melamine resin, and 98.5% by weight of neopentyl glycol diacrylate hydroxypivalate (HPNDA)) Then, 32 parts by weight of a polymerization component comprising 1.5% by weight of the same photopolymerization initiator as in Example 1 and 30 parts by weight of an organic solvent were mixed to obtain a coating composition used in this example.

This coating composition was supplied to the same coating apparatus as in Example 1, and coating was performed in the same manner as in Example 1. The amount of ultraviolet irradiation is 66 mJ / cm ² . As a result, the viscosity during circulation was 200 cP (20 cPa · s), and the viscosity after application was 1230 cP (123 cPa · s). The result was the same as that of Example 1 without sagging. Although a photopolymerization initiator was added in the above-described embodiment, such a polymerization initiator may not be required depending on the type of the polymerization component and the coating conditions.

As the electromagnetic wave, a microwave, an electron beam, a far-infrared ray and the like can be used other than the ultraviolet ray.

[0028]

That is, according to the coating method of the present invention, dripping of the high solid paint can be effectively prevented. Therefore, the concentration of the coating film forming element in the high solid can be increased, so that it is possible to further save resources and prevent environmental pollution.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a configuration of a coating apparatus used in one embodiment of the present invention.

[Explanation of symbols]

1: paint tank 2: spray gun 3: ultraviolet lamp 4: substrate

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) B05D 1/02 B05D 3/06 B05D 7/24

Claims (2)

(57) [Claims] 1. A method for supplying a liquid coating composition containing a polymerizable component which can be polymerized by irradiation with an electromagnetic wave to a spraying device in the middle of a pipe for supplying the coating composition with the electromagnetic wave to polymerize the polymerized component. A coating method, wherein the coating composition having an increased viscosity is discharged from the spray device to perform coating. 2. The coating method according to claim 1, wherein the spray device forcibly lowers the viscosity of the coating composition and discharges the coating composition.