Coating processes – With post-treatment of coating or coating material – Heating or drying
Reexamination Certificate
2002-11-13
2003-10-21
Cameron, Erma (Department: 1762)
Coating processes
With post-treatment of coating or coating material
Heating or drying
Reexamination Certificate
active
06635315
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for improving the smoothness of a film formed from a thermosetting liquid coating composition.
2. Description of Related Art
The smoothness of a film formed from a thermosetting liquid coating composition greatly influences the finished appearance of the coated article. Therefore, improving the smoothness of films of thermosetting liquid coating compositions is an important issue in the paint industry.
Generally, the heat curing process of a thermosetting liquid coating composition applied to a substrate comprises: the start of volatilization of the solvent from the wet film immediately after application; fluidization of the film by heat; the start of a curing reaction; and substantially complete volatilization of the solvent and curing of the film. In this process, the fluidity of the film caused by heat before the start of the curing reaction is presumed to be a significant factor in determining the smoothness of the resulting film.
Usually, a coating composition that forms a film with low fluidity before the start of the curing reaction results in a cured film with low smoothness.
On the contrary, a coating composition that forms a film with high fluidity before the start of the curing reaction produces a cured film with high smoothness. However, such a coating composition causes the problem of sagging when applied to a substrate having vertical planes. For example, when coating a substrate having horizontal and vertical planes, such as an automobile, a coating composition with high fluidity forms a film that has excellent smoothness on the horizontal planes, but has reduced smoothness on the vertical planes because of sagging of the coating composition.
Therefore, it is necessary to control the thermal fluidity of a film formed from a thermosetting liquid coating composition before the start of the curing reaction, thereby preventing the reduction in film smoothness owing to sagging on vertical planes of the substrate, and achieving satisfactory film smoothness on horizontal planes.
It is usually difficult to control the thermal fluidity of a film before the start of the curing reaction only by selecting and combining the resins, pigments, organic solvents, and other basic constituents of the coating composition. Therefore, a flow modifier, leveling agent, organic solvent, and other additives are added to the coating composition to control the thermal fluidity. Specifically stated, a suitable flow modifier, leveling agent, or organic solvent is found and formulated for each of the coating compositions that differ in their resin or pigment component. Further, the effect of the formulation of such additives is evaluated by testing the smoothness of the heat-cured film.
However, the action of a flow modifier and other additives varies depending on the resin, pigment, organic solvent, or other components of the coating composition. Accordingly, there has been a problem in that the optimum formula needs to be found for each of the coating compositions that differ in their resin component, pigment component, etc., to achieve good film smoothness.
Thus, when a thermosetting liquid coating composition is applied to a substrate, it is desired that the thermal fluidity of the film before the start of the curing reaction in the heat curing process be easily controllable so that good film smoothness is obtained on both the horizontal and vertical planes of the substrate.
BRIEF SUMMARY OF THE INVENTION
An object of the present invention is to provide a method for improving the smoothness of films formed from various thermosetting liquid coating compositions that vary in their resin component, pigment component, etc., not only on the horizontal planes but also on the vertical planes of a substrate.
The present inventors carried out extensive research on the relationship between the viscosity/elasticity of a coating composition and the smoothness of the resulting film. The inventors found that, in the application and heat curing of a coating composition onto a substrate, when adjustments are made in such a manner that, at a specific temperature at which the film is fluidized by heat before the start of the curing reaction, the film has a specific storage modulus, loss modulus, and ratio of these moduli within predetermined ranges, the cured film has improved smoothness on both the horizontal and vertical planes of the substrate. The present invention has been accomplished based on these findings.
The present invention provides the following methods for improving the smoothness of a film formed from a thermosetting liquid coating composition.
1. A method for improving the smoothness of a film formed from a thermosetting liquid coating composition, comprising making adjustments in the application and heat curing of the thermosetting liquid coating composition onto a substrate, in such a manner that, at a temperature at which the thermal fluidity of the film reaches a maximum before the start of the curing reaction, the film has a storage modulus G′ of about 0.5 to about 20 Pa at a stress of 0.5 Pa and a frequency of 0.1 Hz, a loss modulus G″ of about 1.0 to about 20 Pa at a stress of 0.5 Pa and a frequency of 0.1 Hz, and a storage modulus/loss modulus (G′/G″) ratio of about 0.3 to about 1.0.
2. A method according to item 1, wherein the temperature at which the thermal fluidity of the film before the start of the curing reaction reaches a maximum is about 25 to about 90° C.
3. A method according to item 1, wherein the thermosetting liquid coating composition is a clear coating composition, and the adjustments are made in such a manner that the film has the storage modulus G′ of about 0.5 to about 2.0 Pa, and the loss modulus G″ of about 1.0 to about 2.5 Pa.
4. A method according to item 1, wherein the thermosetting liquid coating composition is a colored coating composition, and the adjustments are made in such a manner that the film has the storage modulus G′ of about 1.0 to about 20 Pa and the loss modulus G″ of about 2.0 to about 20 Pa.
5. A method according to item 1, wherein the adjustments are made in such a manner that the film has a storage modulus/loss modulus (G′/G″) ratio of about 0.4 to about 0.9.
6. A method according to item 1, wherein the adjustments are made by modification of the thermosetting liquid coating composition before application.
7. A method according to item 6, wherein the modification of the thermosetting liquid coating composition before application is carried out by addition of a flow modifier and/or addition of a solvent.
8. A method according to item 7, wherein the flow modifier is at least one member selected from the group consisting of fine silica powders, fine barium sulfate powders, fine particulate organic resins, clay-containing flow modifiers, polyamide-containing flow modifiers, urea-containing flow modifiers, urethane-containing flow modifiers, high acid value acrylic emulsion-containing flow modifiers, polycarboxylic acid salt-containing flow modifiers, and cellulose-containing flow modifiers.
DETAILED DESCRIPTION OF THE INVENTION
There is no limitation on the substrate used in the method of the present invention. Examples of substrates include metal substrates, such as steel sheets or plates (e.g., cold rolled steel plates, galvanized steel plates, zinc alloy plated steel plates, stainless steel plates, tinned steel plates, etc.), aluminum sheets or plates, aluminum alloy sheets or plates, and magnesium alloy sheets or plates; the above-mentioned metal substrates surface-treated with phosphate, chromate, composite oxide, or the like; plastic substrates; inorganic ceramic substrates, such as glass, cement, slate, mortar, concrete, and tile; paper; coated substrates prepared by coating the above-mentioned substrates; and processed articles of these substrates. Examples of zinc alloy plated steel sheets or plates are steel sheets or plates coated with a zinc alloy, such as iron/zinc, nickel/zinc
Fujitani Toshihide
Furusawa Satoru
Yabe Masami
Armstrong Westerman & Hattori, LLP.
Cameron Erma
Kansai Paint Co. Ltd.
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