JPH0236310B2 - - Google Patents

Description

[Detailed description of the invention]

TECHNICAL FIELD The present invention relates to a method for forming a multilayer coating film using a two-coat one-bake method, which is used, for example, for painting the surface of an automobile body. Conventional technology and problems Conventionally, for example, in the top coating of automobile bodies,
A widely used method is to form a two-coat, one-bake multilayer coating in which a base coat containing metallic pigments and/or colored pigments is applied, followed by a clear topcoat using a wet-on-wet method, and then the entire coating is cured simultaneously. It is being said. This method provides a high-quality coating film with excellent finished appearance, high durability, high solvent resistance, high chemical resistance, and high durability. Conventionally, in this two-coat one-bake painting system, various proposals have been made to improve the finished appearance, and these have been implemented on the production line. These are methods for preventing mixing of the base coat and top clear coat when applied wet-on-wet. Specifically, in order to reduce the compatibility or mutual solubility between the two, cellulose acetate butyl is added to the base coat. These methods include mixing a base coat with a base coat, preheating the base coat to a nonvolatile content above a certain level, and then applying a clear top coat, and increasing the solubility parameter between the base coat and the top coat above a certain range. However, until now, no study has been conducted on the basic characteristics that the multilayer coating film should have in order to obtain an excellent finished appearance. That is, it is easy to understand that if two types of coating films exist as a multilayer coating film, they will have different curing reactions during baking unless they are mutually dissolved and made uniform. Therefore, the curing start time, curing reaction rate, viscosity increase rate during baking, solvent evaporation rate, strain state due to curing shrinkage, etc. are naturally different between the two, and as a result, the finished appearance of the entire multilayer coating film is affected. That is clear. In particular, efforts have recently been made to improve the finished appearance by thickening the clear top coat, but a system that does not take into account the differences in the curing process between the base coat and the top clear coat cannot expect a significant improvement in the finished appearance. Will. Solution to the Invention The present invention involves coating the surface of an object to be coated with a thermosetting resin paint containing metallic pigments and/or colored pigments as a base coat, and then applying a thermosetting resin onto the base coat using a wet-on-wet method. This invention relates to a two-coat, one-bake method for forming a composite coating film, which comprises applying a clear coating as a top coat and then curing the entire coating film by heating. A feature of the present invention is that the curing start time TIb of the base coat alone and the curing start time TIc of the top coat alone are determined in advance under actual heat curing temperature conditions, and during that time, TIc-TIb≧0.3~ The purpose is to adjust the curing start times of the base coat and the top coat in advance so that a relationship of 5 minutes is established.
As a result, when formed into a multilayer coating, a high-finish appearance with smooth skin can be obtained. On the other hand, when TIc-TIb<0.3 minutes, the base coat has not yet reacted sufficiently and is dragged by the shrinkage strain of the top coat, resulting in poor smoothness of the multilayer coating as a whole. Detailed Description The curing start time can be adjusted by using curing agents and additives. Curing agents include polyvalent isocyanates, aminoplasts, and the like. Aminoplasts include condensates of urea, thiourea, melamine, benzoguanamine, etc. and formaldehyde, and lower alkyl (C ₁ -C ₄ ) condensates of these condensates.
Contains etherified products. Particularly preferred is a melamine/formaldehyde condensate in which part or all of the methylol groups are etherified with butanol. Adjustment of the curing start time using additives is performed by adding a curing accelerating catalyst or a curing retarder.
For example, in the case of a polyol and a lower alkyl etherified melamine/formaldehyde condensate, sulfonic acids such as p-toluenesulfonic acid, dodecylbenzenesulfonic acid, dinonylnaphthalenedisulfonic acid and mixtures thereof, and alkyl acid phosphates are used to accelerate curing. can be used. If these curing accelerators are used in too large a quantity, they will impair the water resistance of the coating film, so the amount should be 5 parts by weight or less per 100 parts by weight of the solid content of the paint. Further, as the retarder used in this system, amines that do not volatilize during coating but volatilize during baking hardening are preferable. Examples include diethylamine, triethylamine, n-butylamine, di-n-butylamine, tri-n-butylamine, isobutylamine, diisobutylamine, cyclohexylamine, monoethanolamine, diethylethanolamine, and the like. If the amount added is too large, the necessary curing will not be obtained, so the solid content of the paint
It should generally be less than 5 parts by weight per 100 parts by weight. As the polyol/polyvalent isocyanate-based curing accelerator, the above acids and/or amines can be used. Amine salts and ammonium salts are also good additives. The curing start time was measured by Naganuma et al.
29, 519 (1972) and Izumo et al. Shikizai, 54(10), 593−
598 (1981) DSA method (Dynamic Spring Ana-
This was done by dynamic viscoelasticity measurements using lysis.
That is, a spring with a diameter of approximately 2.0 mm and a number of turns of 20 was made from a copper wire with a diameter of 0.35 mm, and was used as a sample support. Approximately 0.01 g of the paint was uniformly applied to the support, dried under reduced pressure, and then subjected to measurement. The actual curing process of a paint film involves the presence of a solvent, but in that case, it is sufficient to use a sample of paint whose non-volatile content is the same as that of the paint film immediately before it is placed in a baking oven after painting. Here, a sample with a nonvolatile content of approximately 100% was prepared, and the curing start time was representative. For the dynamic viscoelasticity measurement, a forced stretching vibration type dynamic viscoelasticity measuring device (Hybron DDV-type; manufactured by Toyo Bald Winch Co., Ltd.) was used, and the frequency was 11 Hz and the temperature was constant. Quickly attach the sample to the
Assuming T ₀ , measure the relative elastic modulus over time, plot the relative elastic modulus on the vertical axis and time on the horizontal axis, and calculate the extension of the straight line part where the relative elastic modulus rises rapidly and increases linearly. The intersection of the line and the horizontal axis was determined as the curing start time TI. The accompanying drawing is a model of a graph representing the relative modulus change over time for a basecoat and a topcoat with the relationship TIc-TIb>0. The present invention includes a method of forming multilayer coatings using metallic coats and solid color coats as base coats. The paint for the base coat may be any commonly used paint, such as acrylic/melamine paint,
Preferred are acrylic/cellulose acetate butyrate/melamine paints, polyester/cellulose acetate butyrate/melamine paints, and acrylic/polyvalent isocyanate paints. Pigments contained in the base coat paint may include fillers and spreading agents such as kaolin and talc, in addition to ordinary inorganic and organic pigments. Metal flake pigments are also used for metallic finishes. The basecoat paint may optionally contain other commonly used additives. Examples include cross-linked polymer particles (microgels) that are insoluble in and stably dispersed in the paint system, viscosity modifiers such as organic montmorillonite, polyamides and polyethylene, silicones and organic polymers. These include surface conditioners, ultraviolet absorbers, hindered amines, and hindered phenols. As the clear top coat paint, any commonly used paint may be used, and acrylic/melamine-based and acrylic/polyisocyanate-based curable paints are preferred. The clear topcoat paint may also contain additives for the same purpose as the basecoat paint, if desired, except that it does not contain pigments. The curing start time TIc of the top coat alone is 0.8 minutes to 10 minutes, and the curing start time of the base coat alone
Set TIb to 0.5 minutes to 5 minutes, and set the time difference between the two to 0.3.
Preferably, the time is between 5 minutes and 5 minutes. In the process of the present invention, a basecoat composition is applied to the surface of a previously primed or conventionally treated substrate. As for the painting method, normal air spray painting or electrostatic painting is preferable. The substrates are metals such as iron, aluminum, and copper commonly used in automobile manufacturing, but also materials such as ceramics and plastics, as long as they can withstand the temperatures at which these multilayer coatings undergo final curing. , can be used. After applying the base coat, it may be left at room temperature or heated if necessary, then a top clear coat is applied by wet-on-wet, and after setting or preheating as necessary, it can be cured at room temperature or at the required temperature. By baking and curing the base coat and clear top coat at the same time, a multilayer film with a high finished appearance can be formed. Also, the thickness of the base coat film is 10~
30μ, top clear coat film thickness is 20μ
~70μ (all dry film thicknesses) is preferred. Examples of the present invention are shown below, and "parts" and "%" in the examples are based on weight unless otherwise specified. Resin Production Example Production Example-1 80 parts of xylene and 10 parts of methyl isobutyl ketone were charged into a container equipped with a stirrer, a temperature control device, and a reflux condenser. Next, add 20 parts of a solution with the following composition: methacrylic acid 1.8 parts, methyl methacrylate 39.4, ethyl acrylate 43.6, isobutyl methacrylate 3.2, 2-hydroxyethyl acrylate 12.0, azobisisobutyronitrile 1.5, and heat with stirring. , the temperature was increased. The remainder of the above mixed solution while refluxing
81.5 parts were added dropwise over 3 hours, and then a solution consisting of 0.3 parts of asobisisobutyronitrile and 10 parts of xylene was added dropwise over 30 minutes. After stirring and refluxing the reaction solution for another 2 hours to increase the conversion rate to resin, the reaction was terminated and the nonvolatile content was 50% and the number average molecular weight was
Obtained 18000 acrylic resin varnish. Production Example-2 Using the same equipment as Production Example-1, xylene
55 parts, methyl isobutyl ketone 7 parts, and then a solution with the following composition: Styrene 5.0 parts Ethyl acrylate 8.1〃 Isobutyl methacrylate 61.0〃 Ethylhexyl acrylate 8.4〃 2-Hydroxyethyl methacrylate 15.0〃 Methacrylic acid 2.5〃 Azobisisobutyronitrile 20 parts of 4.0〃 was added to the container and heated while stirring to raise the temperature. While refluxing, the remaining 84 parts of the mixed solution was added dropwise over 2 hours, and then a solution consisting of 0.3 parts of asobisisobutyronitrile, 35 parts of xylene, and 3 parts of methyl isobutyl ketone was added dropwise over 30 minutes. The reaction solution was further stirred and refluxed for 2 hours to increase the conversion rate to resin, and then the reaction was terminated.
An acrylic resin varnish with a nonvolatile content of 50% and a number average molecular weight of 3500 was obtained. Production example -3 Using the same equipment as production example -1, xylene 70
20 parts of n-butanol, and then a solution with the following composition: Methacrylic acid 1.2 parts Styrene 26.4〃 Methyl methacrylate 26.4〃 N-Butyl acrylate 36.0〃 2-Hydroxyethyl methacrylate 10.0〃 Azobisisobutyronitrile 1.0〃 20 parts of the mixture were added and heated while stirring to raise the temperature. The remainder of the above mixed solution while refluxing
81.0 parts were added dropwise over 2 hours, and then a solution consisting of 0.3 parts of asobisisobutyronitrile and 10 parts of xylene was added dropwise over 30 minutes. The reaction solution was further stirred and refluxed for 2 hours to complete the reaction, and an acrylic resin varnish with a nonvolatile content of 50% and a number average molecular weight of 8,000 was obtained. Production Example-4 Using the same equipment as Production Example-1, xylene 57
Styrene 30.0 parts Ethylhexyl methacrylate 45.2 parts Ethylhexyl acrylate 5.5 parts 2-Hydroxyethyl methacrylate 16.2 parts Methacrylic acid 3.1 parts Azobisisobutyronitrile 20 parts of the 4.0 parts was added and heated with stirring to raise the temperature. While refluxing, remove the remaining 84% of the above mixed solution.
1 part was added dropwise over 2 hours, and then 0.5 part of azobisisobutyronitrile, 23 parts of xylene, and n-butanol were added.
A solution consisting of 14 parts was added dropwise over a period of 20 minutes. The reaction solution was further stirred and refluxed for 2 hours to complete the reaction, and an acrylic resin varnish with a nonvolatile content of 50% and a number average molecular weight of 3,400 was obtained. Production Example-5 The following raw materials were charged into a container equipped with a stirrer, a temperature control device, and a decanter, and heated while stirring. Ethylene glycol 39 parts Neopentyl glycol 130〃 Azelaic acid 236〃 Phthalic anhydride 186〃 Xylene 30〃 Heating was continued until the acid value reached 150 while water produced as the reaction progressed was azeotropically removed with xylene. . Thereafter, the temperature was cooled to 140° C., 314 parts of Cardiura E-10 (epoxy resin, manufactured by Shell Co., Ltd.) was added, and stirring was continued for 2 hours to complete the reaction. The obtained resin has an acid value of 9, a hydroxy value of 90, and a number average molecular weight.
It was 1050. This resin was diluted with xylene to a non-volatile content of 60% to obtain a polyester resin varnish having a Gardner viscosity of Y. Production Example -6 Using the same equipment as Production Example -1, xylene 80
2-sulfoethyl methacrylate 2.0 parts methacrylic acid 0.8 parts Methyl methacrylate 39.0 parts Ethyl acrylate 43.0 parts Isobutyl methacrylate 3.2 parts 2-Hydroxyethyl acrylate 12.0 parts Azobis 30 parts out of 1.5 parts of isobutyronitrile was added and heated with stirring to raise the temperature. The remainder of the above mixed solution while refluxing
71.5 parts were added dropwise over 3 hours, and then a solution consisting of 0.8 parts of asobisisobutyronitrile and 10 parts of xylene was added dropwise over 30 minutes. The reaction solution was further stirred and refluxed for 2 hours to increase the rate of conversion to resin, and then the reaction was terminated, with a nonvolatile content of 50% and a number average molecular weight.
Obtained 16000 acrylic resin varnish. Manufacturing example of base coat paint-1 Acrylic metallic base paint Weigh the materials with the following composition in a stainless steel container,
The paint was prepared by stirring with a laboratory stirrer. Varnish of Production Example-1 100 parts Super Bethcamine 14-559 (melamine resin manufactured by Nippon Reichhold, non-volatile content = 60%) 21 parts Aluminum paste 1109MA (manufactured by Toyo Aluminum Co., Ltd.)
13 parts Modaflow (manufactured by Mitsubishi Monsanto) 0.3 parts-2 Acrylic metallic base paint A paint was prepared in the same manner as in Production Example-1. Varnish of Production Example-1 100 parts Yuban 125 (melamine resin manufactured by Mitsui Toatsu Co., Ltd., non-volatile content = 60%) 21 parts Aluminum paste 1109MA 13 parts Modaflow 0.3 parts -3 Metallic base paint Apply the paint in the same manner as in Production Example-1. It was adjusted. Varnish of Production Example-2 100 parts Yuban 20 SE-60 (melamine resin manufactured by Mitsui Toatsu Co., Ltd., non-volatile content = 60%) 21 parts Aluminum paste 1109MA 13 parts Sea soap #103 (manufactured by Shiraishi Calcium Co., Ltd.) 1 part -4 For base coat solid paint

[Table] was weighed into a dispersion container, mixed and dispersed using a paint conditioner, and then

【table】 Adjusted by adding. -5 Acrylic solid blue for base coat

[Table] -6 Silver metallic paint for base coat A paint was prepared in the same manner as in Production Example-1. Production Example - Varnish of 6 100 parts Yuban 128 (melamine resin manufactured by Mitsui Toatsu Co., Ltd.) 21 parts Aluminum paste 51-231 (manufactured by Toyo Aluminum Co., Ltd.) 13 parts Modaflow 0.6 parts Manufacturing of paint for top clear coat In a stainless steel container, the following composition was added. Weigh the materials,
The paints of each production example were prepared by stirring with a laboratory stirrer. -1 Varnish of Production Example-3 100 parts Yuban 125 36 parts Modaflow 0.1 part-2 Varnish of Production Example-3 100 parts Super Betsucomin 14-559 36 parts Modaflow 0.1 part-3 Varnish of Production Example-4 100 parts Yuban 20SE -60 36 parts Tributylamine 1 part -4

【table】 -5

[Table] Creation of samples for DSA and multi-layer coated plates for appearance evaluation Samples for measurement of curing start time were prepared by uniformly applying the paint of each production example to the spring and drying at room temperature under reduced pressure to adjust the non-volatile content to 100%. I used the one I made. Judgment of the finished appearance was made by creating a multilayer coating film in the following manner. First, among the manufacturing examples of each paint, the base coat paint was diluted with the following mixed solvent (hereinafter referred to as base thinner): Cellosolve Acetate 50, Butyl Acetate 30, Xylene 20, in a No. 4 food cup for 15 seconds at 20°C. . In addition, the clear paint of each production example was diluted with the following mixed solvent (hereinafter referred to as top clear thinner) xylene 50% Swasol 1000 (Maruzen Sekiyu) 50% using a No. 4 food cup for 25 seconds at 20°C. Then, hold the solvent-degreased tin plate horizontally and apply each base paint once with Wider 71 (air cup gun made by Iwata Yaiki Co., Ltd.) at room temperature (20℃) to form a dry film.
After coating to a thickness of 15 μm, the base film was left at room temperature for 3 minutes. Next, the diluted top clear paint was applied in combination with each example.
One coat of Wider 71 was applied over the base film by wet-on-wet. The clear film was dried and set to 35μ. Then at room temperature
After leaving it for 10 minutes, it was baked in a dryer at 140°C or 100°C for 30 minutes to obtain a multilayer film. Examples Comparative Example 1 Paint Production Example The curing start time of base-1 and clear-1 was measured at 140°C, and the results are shown in the table. Furthermore, as a result of creating a multilayer coating film, the reaction of the clear proceeded too quickly, resulting in shrinkage distortion, poor skin quality, and poor finished appearance. Example 1 Coating Production Example The curing start time at 140° C. of Base 2 and Clear 2 was measured, and multilayer coatings were prepared. The results are shown in the table. By setting the clear curing start time later than that of the base, the finished appearance was good. Example 2 The curing start time of Paint Production Example-3 (for base) and -3 (for clear) at 140°C was measured, and a multilayer coating film was prepared. The results are shown in the table. In order to obtain a good finished appearance even in a two-coat, one-bake high-solids coating, it is possible to achieve the objective by adjusting the curing start time between the base and clear coat. Comparative example 2 Manufacturing example -4-1 (for base) and -4-
1 (for clear), the curing start time was measured at 140°C, or a multilayer coating film was created, and the results are shown in the table.
The skin was poor as a solid color with 2 coats and 1 bake. Example 3 Production example-4-2 (for base) and -4-
Regarding No. 2 (for clear), the curing start time at 140°C was measured, and the results of creating a multilayer coating film are shown in the table. The finished appearance of 2-coat, 1-bake solid colors can be greatly improved by adjusting the curing start times of the base and clear. Comparative Example 3 The curing start time of base paint-5-1 and clear paint-5-1 was determined by DSA at 100°C. The results are shown in the table. The table also shows the appearance of the multilayer coating. The clear reaction was fast and a shrinking pattern appeared. Example 4 Base paint-5-2 and clear paint-
Determine the curing start time at 100℃ in 5-2,
In addition, a multilayer coating film was prepared and its appearance was evaluated, and the results are shown in the table. It had a better appearance than Comparative Example 3. Example 5 The curing start time of base paint-6 at 140°C was determined. Then, wet-on-wet coatings of -6 base and -2 clear were prepared, and the finished appearance was evaluated. The results are shown in the table. It had a good finished appearance.

【table】 [Brief explanation of drawings]

The drawing shows the curing start time TIb of the base coat alone.
This is a graph model of relative elastic modulus versus time when the relationship between TIc and curing start time TIc of the top coat alone is TIc-TIb>0.

Claims (1)

[Scope of Claims] 1. A thermosetting resin paint containing metallic pigments and/or colored pigments is applied as a base coat to the surface of the object to be painted, and then a thermosetting resin clear paint is applied as a top coat on the base coat by wet-on-wet coating. In a two-coat, one-bake method of forming a multilayer coating film, the coating is applied using a two-coat one-bake method, and the entire coating film is then heated and cured.
Determining in advance a curing start time TIb of the base coat alone and a curing start time TIc of the top coat alone under the heat curing temperature condition,
A method for forming a multilayer coating film, characterized in that the curing start times of the base coat and the top coat are adjusted in advance so that a relationship of TIc-TIb≧0.3 to 5 minutes is established during that time. 2 The above TIc is between 0.8 minutes and 10 minutes, and the above
The method of the first term, where TIb is between 0.5 and 5 minutes. 3. The method of item 1, in which the curing start time is adjusted using a curing agent, and the reactivity of the curing agent in the base coat is relatively faster than the reactivity of the curing agent in the top coat. 4. The method of item 1, in which the curing start time is adjusted using an acid catalyst and/or an amine.