JP2009034668A - Coating method, and coated object prepared by the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a coating method by which a laminated coating film can be obtained with its uppermost layer having a surface of slight unevenness even if the uppermost layer and one or more of lower layer(s) are cured by baking after laminating two or more species of paint by a wet-on-wet method. <P>SOLUTION: The coating method for forming a laminated coating film comprising at least a lower layer and an uppermost layer formed on a substrate includes: a step of preparing, as paint for forming the uppermost layer, thermosetting paint that has a curing temperature T<SB>T</SB>of ≥40°C and ≤200°C and does not generate a volatile matter upon being subjected to a heat treatment, and further preparing thermosetting paint for the lower layer, which has a curing temperature T<SB>U</SB>satisfying the following equation (1): T<SB>U</SB>≤T<SB>T</SB>-30 (wherein T<SB>U</SB>is a curing temperature [°C] of the paint for the lower layer, and T<SB>T</SB>is a curing temperature [°C] of the paint for the uppermost layer) as at least one of the paint for forming the lower layer; a step of forming an uncured laminated coating film by laminating the paint for the lower layer and the paint for the uppermost layer on the substrate using a wet-on-wet method; a step of setting T<SB>L</SB>(a temperature [°C] for lower-temperature heating) and T<SB>H</SB>(a temperature [°C] for higher-temperature heating) so that T<SB>L</SB>and T<SB>H</SB>are each within respective specific temperature ranges; and a step of subjecting the uncured laminated film to a heat treatment at the temperature T<SB>L</SB>and at least curing the paint for the lower layer to form a laminated coating film where the paint for the uppermost layer is uncured, and further subjecting the resulting uncured laminated coating film where the uppermost layer is uncured to a heat treatment at the temperature T<SB>H</SB>to cure the paint for the uppermost layer. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a coating method in which two or more kinds of paints are laminated and baked by wet-on-wet, and a coated body obtained thereby.

  When two or more types of paints are laminated by wet-on-wet and then a laminated coating film is formed by a baking method, conventionally, all layers constituting the laminated coating film have the same heating temperature after all the paints are laminated. The thermosetting paint for forming each layer was selected so as to be cured at, and the entire laminated coating film was cured. In this case, there is a problem that the skin and gloss of the laminated coating film are inferior to the case where the lower layer is baked and then the coating material forming the uppermost layer is laminated and baked. For this reason, various methods have been proposed to improve the skin and gloss of the laminated coating film.

  For example, Japanese Patent Application Laid-Open No. 10-277478 (Patent Document 1) discloses a coating film in which a color base paint, a glittering material-containing base paint, and a clear paint are sequentially applied by a wet-on-wet method and then baked to cure each layer. A forming method is disclosed. In this method, the viscosity increase start time of each paint is adjusted to be longer in the order of the color base paint, the glitter-containing base paint, and the clear paint, and before the clear paint forming the uppermost layer increases in viscosity as it hardens. Curing of the color base paint and glittering material-containing base paint forming the lower layer is started.

  Japanese Patent Application Laid-Open No. 2005-193107 (Patent Document 2) discloses a method of coating the skin and gloss of a laminated coating film in a coating method in which an intermediate coating and a top coating are applied wet-on-wet and then cured simultaneously. The reason for the inferiority is thought to be the decrease in smoothness of the top coat film due to the mixed phase of the intermediate coat layer and the top coat layer. It is disclosed that a curing catalyst that accelerates curing of the intermediate coating layer is applied to the surface of the film layer.

  On the other hand, it is known that the cause of lowering the skin and gloss of the multilayer coating film is the residual solvent in the multilayer coating film before baking and curing. In particular, when the solvent in the laminated coating film is rapidly evaporated during the curing reaction, the surface of the laminated coating film becomes rough. In order to prevent this, the following method has been proposed. For example, Japanese Patent Laid-Open No. 2000-84463 (Patent Document 3) discloses a method for forming a coating film by two-stage heating including a step of applying a solution-type thermosetting paint, a low-temperature heating step, and a high-temperature heating step. Yes. Japanese Patent Application Laid-Open No. 2004-275966 (Patent Document 4) discloses a two-step heat treatment process, a process of sequentially applying an intermediate coating, a base paint, and a clear paint by wet-on-wet, a low-temperature heating stage, and a high-temperature heating stage. A method of forming a coating film containing the above is disclosed. In these methods, the solvent in the paint is gently evaporated without curing the paint in the low-temperature heating stage, and then the thermosetting resin contained in each layer of paint is cured in the high-temperature heating stage.

As described above, various methods have been proposed in the past to improve the skin and gloss of the laminated coating film. For example, for steel sheets for automobiles, a coated body having a better appearance quality has been demanded. Further improvement of the on-wet coating method is desired.
Japanese Patent Laid-Open No. 10-277478 JP-A-2005-193107 JP 2000-84463 A JP 2004-275966 A

  The present invention has been made in view of the above-mentioned problems of the prior art, in which two or more kinds of paints are laminated in a wet-on-wet manner, and the uppermost layer is cured to ensure high durability by baking, and further, It is an object of the present invention to provide a coating method capable of obtaining a laminated coating film with less unevenness on the surface of the uppermost layer even when one or more lower layers are cured, and a coated body excellent in appearance quality obtained thereby.

  As a result of intensive studies to achieve the above-mentioned object, the present inventors have established a thermosetting type that does not generate a volatile product in a curing reaction by heat treatment as a coating for the uppermost layer for forming the uppermost layer of a laminated coating film. Use at least one kind of lower layer paint to form a lower layer using a paint, a thermosetting paint having a curing temperature lower than the curing temperature of the uppermost layer paint, and wet-on-wet these paints First, the lower layer thermosetting paint is cured by heat treatment, and then the uppermost layer thermosetting paint is cured by heat treatment at a higher temperature, so that the uppermost layer is cured and the fluidity is significantly reduced. It is possible to minimize the shrinkage of the laminated coating after it has been applied, and even if two or more types of paints are laminated wet-on-wet and then baked, a laminated coating with excellent appearance quality is obtained. Heading to be, and have completed the present invention.

That is, the coating method of the present invention is a coating method for forming a laminated coating film comprising at least one lower layer formed on a substrate and an uppermost layer formed on the lower layer,
Preparing a thermosetting coating material having a curing temperature T _T of 40 ° C. or more and 200 ° C. or less and forming no volatile product in a curing reaction by heat treatment as the uppermost layer coating material for forming the uppermost layer; the curing temperature T _U is the following formula as at least one of the lower layer coating material for forming the lower layer (1)
T _U ≦ T _T -30 (1)
(In the formula (1), T _U represents a curing temperature [℃] of the lower layer for thermosetting paint, T _T denotes the curing temperature of the uppermost layer-thermosetting coating material [℃].)
A step of preparing a thermosetting paint for the lower layer satisfying
A step of laminating the lower layer coating material and the uppermost layer coating material on the substrate by wet-on-wet to form an uncured laminated coating film;
Following formula (2)
T _U −20 ≦ T _L ≦ T _T −30 (2)
And the following formula (3)
T _T -20 ≦ T _H ≦ T _T +40 (3)
(In the formula (2) and (3), _{T L} represents the low heating temperature [℃], _{T H} represents a high-temperature heating temperature [° C.], _{T U} and _{T T} is _{T U} and in the formula (1) it is synonymous with the _T T.)
Setting the heating temperatures T _L and T _H to satisfy
The uncured laminated coating film is subjected to a heat treatment at the temperature _TL to cure at least the thermosetting paint for the lower layer to form an uncured laminated coating film, and then the uppermost layer is uncured. curing the uppermost layer-thermosetting coating material is subjected to heat treatment at the temperature T _H in the multilayer coating film,
It is characterized by including.

The thermosetting paint for the uppermost layer is preferably a paint having a weight reduction rate of 0.5% by mass or less at the temperature T _T.

  When the lower layer is two or more layers, it is preferable that all of the lower layer coating materials for forming the lower layer are the lower layer thermosetting coating materials.

The coating method of the present invention, after the uppermost layer is reduced volatiles concentration of the multilayer coating film of the uncured 4 wt% or less, the top layer is a heat treatment at the temperature T _H in the multilayer coating film of uncured subjected It is preferable to cure the uppermost layer paint.

  The coated body of the present invention is a coated body having a laminated coating film comprising at least one lower layer formed on a base material and an uppermost layer formed on the lower layer, and according to the coating method of the present invention. This is a coated product that is excellent in appearance quality such as skin and gloss.

  In addition, even when two or more kinds of paints are laminated by wet-on-wet and baked by the coating method of the present invention, the reason why the unevenness of the surface of the laminated coating film is not necessarily clear is not certain, but the present inventors I guess as follows. In other words, in a conventional multilayer coating film formed by wet-on-wet, a thermosetting paint for forming each layer is selected so that a curing reaction occurs at the same heating temperature in all layers including the uppermost layer. When the thermosetting paint forming the upper layer is cured by heat treatment, the thermosetting paint is cured also in the lower layer. At this time, in each layer of the laminated coating film, the thermosetting paint is cured by an addition reaction after the condensation reaction or the deblocking reaction of the curing agent. Therefore, a volatile product is generated and remains by this condensation reaction or deblocking reaction. It volatilizes with the solvent to be formed, and the laminated coating film contracts to form irregularities on the coating film surface. The unevenness on the surface of the coating film is alleviated by the flow while each layer has sufficient fluidity, but the unevenness is not alleviated if the fluidity of each layer, particularly the uppermost layer, is significantly reduced by curing. Concavities and convexities at the surface of the material and the interface of each layer are transferred to the surface of the uppermost layer, and the skin and gloss of the laminated coating film deteriorate.

On the other hand, in the coating method of the present invention, first, the thermosetting paint that forms the lower layer is cured by applying heat treatment to the lower layer at a temperature at which the thermosetting paint that forms the uppermost layer is not substantially cured, and then at a higher temperature. The thermosetting paint that forms the top layer is cured by heat treatment. In this method, when the uppermost layer is cured, the curing of the lower layer has already progressed considerably, so that the volatile products generated in the lower layer are reduced and the shrinkage of the lower layer is suppressed. Furthermore, since a thermosetting coating that does not substantially generate a volatile product in the curing reaction is used as the thermosetting coating for the uppermost layer, the volatile product is substantially volatilized during the curing of the uppermost layer. Therefore, it is presumed that the contraction due to the volatilization of the volatile product which affects the surface shape of the laminated coating film does not occur. In the present invention, “the volatile product is not substantially generated” and “the volatile product is not substantially volatilized” include that the contraction of the coating film due to the volatilization of the volatile product is the surface of the coating film. The case where a volatile product is generated and volatilized to such an extent that the shape is not affected is included. Specifically, if the weight reduction rate of the coating film is 0.5% by mass or less even when the paint is subjected to a heat treatment at a temperature T _T to produce a volatile product and volatilize, the paint It shall produce virtually no volatile product and shall not volatilize.

  According to the present invention, even if two or more kinds of paints are laminated on a wet-on-wet basis and are baked to ensure high durability, the uppermost layer is cured, and even if one or more lower layers are cured, the uppermost layer A laminated coating film with less surface irregularities can be obtained. Thereby, the coating body excellent in appearance quality, such as skin (surface smoothness) and gloss, can be obtained.

  Hereinafter, the present invention will be described in detail with reference to preferred embodiments thereof.

The coating method of the present invention is a coating method for forming a laminated coating film comprising at least one lower layer formed on a substrate and an uppermost layer formed on the lower layer,
Preparing a thermosetting coating material having a curing temperature T _T of 40 ° C. or more and 200 ° C. or less and forming no volatile product in a curing reaction by heat treatment as the uppermost layer coating material for forming the uppermost layer; the curing temperature T _U is the following formula as at least one of the lower layer coating material for forming the lower layer (1)
T _U ≦ T _T -30 (1)
(In the formula (1), T _U represents a curing temperature [℃] of the lower layer for thermosetting paint, T _T denotes the curing temperature of the uppermost layer-thermosetting coating material [℃].)
A step of preparing a thermosetting paint for the lower layer satisfying
A step of laminating the lower layer coating material and the uppermost layer coating material on the substrate by wet-on-wet to form an uncured laminated coating film;
Following formula (2)
T _U −20 ≦ T _L ≦ T _T −30 (2)
And the following formula (3)
T _T -20 ≦ T _H ≦ T _T +40 (3)
(In the formula (2) and (3), _{T L} represents the low heating temperature [℃], _{T H} represents a high-temperature heating temperature [° C.], _{T U} and _{T T} is _{T U} and in the formula (1) it is synonymous with the _T T.)
Setting the heating temperatures T _L and T _H to satisfy
The uncured laminated coating film is subjected to a heat treatment at the temperature _TL (hereinafter also referred to as “low-temperature heat treatment”) to cure at least the thermosetting paint for the lower layer to form an uncured laminated coating film. and, then, a step of the top layer to cure the temperature T _H in the heat treatment (hereinafter referred to as "high-temperature heat treatment".) the applied with thermosetting coating material for the uppermost layer to the multilayer coating film of the uncured,
It is characterized by including.

  In the coating method of the present invention, one or more lower layer thermosetting paints are applied on a substrate, and a solvent or the like is evaporated by drying or the like as necessary to form an uncured lower layer. Next, a thermosetting paint for the uppermost layer is applied on the uncured lower layer, and a solvent or the like is evaporated by drying or the like as necessary to form an uncured uppermost layer. Thereafter, the obtained uncured laminated coating film is subjected to a low-temperature heat treatment to cure at least the lower layer thermosetting paint, and then subjected to a high-temperature heat treatment to cure the uppermost layer thermosetting paint.

  The substrate is not particularly limited, and examples thereof include metals (iron, copper, aluminum, tin, zinc and alloys of these metals), steel plates, plastics, foams, paper, wood, cloth, glass, and the like. . Especially, this invention is applied suitably for the steel plate for motor vehicles with the required characteristic with respect to external appearance quality. These substrate surfaces may be subjected to a treatment such as electrodeposition coating in advance.

  In the coating method of the present invention, at least one lower layer is formed on the substrate, and at least one of the lower layers is formed using a thermosetting coating material having a curing temperature satisfying the formula (1). . Specifically, when the lower layer is a single layer, this lower layer is formed using a thermosetting paint, and when the lower layer is two or more layers, at least one of them is formed using a thermosetting paint. Form.

The lower layer heat-curable coating material curing temperature T _U is the following formula (1)
T _U ≦ T _T -30 (1)
(In the formula (1), T _U represents a curing temperature [℃] of the lower layer for thermosetting paint, T _T denotes the curing temperature of the uppermost layer-thermosetting coating material [℃].)
It is a thermosetting paint that satisfies the above. The curing temperature as the lower layer heat-curable coating material (1) can be expanded sufficiently the difference between the heating temperature T _L and the heating temperature T _H using a thermosetting coating material satisfying, as a result, the lower layer of the cured And the curing of the top layer can proceed independently. Further, the curing temperature _{T U} and _{T T} From this point of view the following formula (1a)
_{T U ≦ T T -40 (1a} )
(Formula (1a) in, _{T U} and _{T T} are as defined _{T U} and _{T T} in the formula (1).)
It is preferable to satisfy.

  In the present invention, the “curing temperature of the thermosetting paint” refers to a curing time for coating the target paint on the substrate, applying a heat treatment to cure the coating film, and fixing it on the substrate. The temperature at which curing can be performed most efficiently in relation to the curing conditions, and generally the baking temperature set (designed) for each paint. In the present invention, a catalog value can be adopted as the curing temperature (baking temperature).

  As the thermosetting paint for the lower layer, any thermosetting paint used in ordinary baking coating can be used as long as it satisfies the formula (1), preferably the formula (1a). Thermosetting resin and curing agent capable of forming a coating film such as intermediate coating and base coating described in Japanese Patent Application Publication No. 2004-275966 (for example, having two or more functional groups capable of reacting with the functional group of the thermosetting resin) Compound and resin). The form of the thermosetting paint for the lower layer may be either solvent-based or water-based, but water-based is preferable in that the amount of volatile organic compounds discharged can be reduced.

  Examples of the thermosetting resin capable of forming a coating film included in the lower layer thermosetting paint include, but are not limited to, an acrylic resin, a polyester resin, an alkyd resin, an epoxy resin, and a urethane resin. . Moreover, as a hardening | curing agent contained in the said thermosetting coating material for lower layers, an amine compound, an amino resin, an isocyanate compound, an isocyanate resin, etc. are mentioned, However, It is not limited to these. These resins and curing agents may be used alone or in combination of two or more.

In the present invention, among the thermosetting paints for the lower layer, when the uppermost layer is cured, the uppermost layer is substantially cured at the time of the high-temperature heat treatment in that the shrinkage of the coating film after the upper layer is cured and the fluidity is significantly reduced can be reduced. In particular, it is preferable that the coating material does not generate a volatile product. As such a coating material, the weight reduction rate at the curing temperature T _T of the thermosetting coating material for the uppermost layer to be used is preferably 0.5% by mass or less, more preferably 0.3% by mass or less, Particularly preferred is 1% by mass or less. When such a thermosetting paint with a small weight reduction rate is used as the thermosetting paint for the lower layer, the shrinkage of the coating film tends to be minimized after the uppermost layer is cured by heat treatment and the fluidity is remarkably lowered. . From this point of view, a paint (a weight reduction rate of 0% by mass) that does not generate a volatile product during the high-temperature heat treatment is most preferable.

In the present invention, “weight reduction rate of paint” is a value measured by the following method. That is, the target paint is coated on the aluminum foil so that the film thickness after the heat treatment becomes the target film thickness in the laminated coating film, and the obtained aluminum foil sample is set to the curing temperature T of the thermosetting paint for the uppermost layer. Gas chromatograph / mass spectrometry with a heat desorption introducing device (for example, Thermal Destruction System manufactured by GERSTEL) after drying for 90 minutes under a vacuum condition of 40 ° C. lower than _T [T _T −40 ° C.] and 10 ⁻² Torr or less. Using an apparatus (for example, 6890GC / 5975MSD manufactured by Agilent), the amount of volatile products (Rc (unit: g)) and the amount of residual solvent were quantified by heating at the temperature T _T for 30 minutes, and the weight was calculated according to the formula (4). Calculate the reduction rate.

Weight reduction rate = 100 × Rc / W × 100 / (100-P) (4)
In formula (4), W represents the mass (unit: g) of the coating film obtained in the vacuum drying step, and P represents the mass (unit: g) of the pigment contained in 100 g of the coating film. In addition, the value (catalog value etc.) of the recipe of a coating material can be employ | adopted for the mass of a pigment. The weight reduction rate calculated in this way is the ratio of the volatile product amount to the total binder amount in the coating film.

  Examples of the thermosetting coating material that does not substantially generate a volatile product during the high-temperature heat treatment include a coating material in which a thermosetting resin capable of forming a coating film and a curing agent are subjected to an addition reaction by heat treatment to be cured. Specifically, the thermosetting paint includes a combination of a hydroxyl group-containing acrylic resin and an isocyanate compound and / or an isocyanate resin, a combination of an epoxy group-containing acrylic resin and a polyvalent carboxylic acid compound and / or a carboxyl group-containing resin, and the like. Can be mentioned.

Further, as the lower layer thermosetting paint used in the present invention, a paint having a relative loss elastic modulus of 1 s ⁻² or less at the start of gelation of the uppermost layer paint to be used tends to be preferable. The “relative loss elastic modulus at the start of gelation of the uppermost layer coating material” is defined by the relative loss elastic modulus measured by the following method. That is, first, the uppermost layer coating material is applied to a 40 mm × 50 mm stainless steel plate (thickness 0.5 mm) so that the film thickness after the thermosetting treatment is 35 ± 5 μm. Specifically, the stainless steel plate is placed on a horizontal base, an adhesive tape with a thickness of 70 μm is applied to each of the areas of about 5 mm from the two opposite edges of the stainless steel plate, and a knife with a straight edge is provided. The uppermost layer coating material is applied to the gap between the stainless steel plate and the blade edge of the knife by sliding on the tape.

In this way, the relative storage elastic modulus (E _r ′) of the coating film is measured 7 ± 1 minutes after the coating film made of the uppermost layer coating material is formed. The measurement is carried out using a rigid pendulum type physical property tester (RPT-5000, manufactured by A & D Co., Ltd.) equipped with an annular pendulum with a diameter of 74 mm to which a knife edge with a blade angle of 40 ° is attached. The temperature program at the time of measurement is set so that the temperature is increased from room temperature (25 ° C.) to the curing temperature of the uppermost layer coating material at a rate of temperature increase of 20 ± 4 ° C./min, and then the curing temperature is maintained.

When the measured value of the relative storage elastic modulus (E _r ′) obtained is plotted with respect to time, as shown in FIG. 1, the curve changes from a downward convex curve to an upward convex curve as time elapses (hereinafter referred to as “protruding curve”). This change time is called “inflection point”. The following equation (5) for the 15 minute portion from this inflection point:
E _r '= A [1-exp {k (t−t _d }] (5)
(In formula (5), A and k are constants, and t represents time.)
And the time axis intercept t _d is obtained by a nonlinear least square method. The t _d represents the time from the start of measurement to the uppermost layer-coating material starts to gel.

Next, for the lower layer coating material, a coating film is formed in the same manner as in the uppermost layer coating material, and the relative loss modulus (E _r ″) is measured under the same conditions as in the uppermost layer coating material. From this measurement result, the relative loss elastic modulus (E _r ″) at the time t _d is obtained, and this is defined as “relative loss elastic modulus at the start of gelation of the uppermost layer coating material”.

The relative storage elastic modulus (E _r ′) and the relative loss elastic modulus (E _r ″) are respectively the general storage elastic modulus (E ′) and the relative loss elastic modulus (E ″) and the following formula:
E _r '= BE'
E _r "= BE"
Can be associated with. Here, B is a value determined by measurement conditions, and the following formula:
B = (bh ² cos φ) / (I sin ³ θ)
(In the formula, b represents the length [unit: m] where the coating film and the knife edge are in contact, h represents the film thickness [unit: m] of the coating film, and φ represents the knife edge surface and the base of the stationary pendulum. Indicates the angle formed by the surface of the material (stainless steel plate in the above case), and I indicates the rotational moment of inertia [unit: kg · m ² ] around the pendulum blade edge)
It is represented by Therefore, if measurement conditions are fixed, B becomes a constant value.

In the present invention, by adjusting the composition and blending ratio of the thermosetting resin and the curing agent, blending additives, etc., for example, by lowering the glass transition temperature, the crosslinking density, etc., the lower layer heat The relative loss elastic modulus at the start of gelation of the uppermost layer-coating material for the curable coating material can be 1 s ⁻² or less.

  The thermosetting paint for a lower layer according to the present invention may contain conventionally known color pigments, glitter pigments, and the like within a conventionally known range, if necessary. In order to adjust various physical properties, various additives such as a viscosity control agent, a surface conditioner, a thickener, an antioxidant, an ultraviolet ray inhibitor and an antifoaming agent may be blended within a conventionally known range.

  Further, in the coating method of the present invention, when there are two or more lower layers, at least one of them is formed using the lower layer thermosetting paint, and the remaining layers are not subjected to a curing reaction by heat treatment. It can be formed using a curable paint.

Non-curable coating material for forming the lower layer, as long as it does not cause substantial curing reaction by the heat treatment, the weight reduction rate in curing temperature T _T of the uppermost layer-thermosetting coating material is 0.5 The thing of the mass% or less is preferable, the thing of 0.3 mass% or less is more preferable, and the thing of 0.1 mass% or less is especially preferable. When such a non-curing paint having a small weight reduction rate is used, the shrinkage of the coating film after the uppermost layer is cured by heat treatment and the fluidity is remarkably lowered tends to be reduced. Further, from such a viewpoint, a paint containing a resin capable of forming a coating film and containing no curing agent is most preferable. The form of the non-curable coating material may be either solvent-based or water-based, but water-based is preferable in that the amount of volatile organic compounds discharged can be reduced.

  The resin capable of forming a coating film contained in the non-curable coating material may be a resin that does not cause a curing reaction by heat treatment alone. For example, an intermediate coating material or a base described in JP-A-2004-275966 Examples of the resin component excluding the curing agent from the paint and the like, specifically, an acrylic resin, a polyester resin, an alkyd resin, an epoxy resin, a urethane resin, and the like, are not limited thereto. Two or more of these resins that do not cause a curing reaction by heat treatment may be selected and used in combination.

  In addition, the non-curable coating material may contain conventionally known color pigments, glitter pigments, and the like within a conventionally known range, if necessary. In order to adjust various physical properties, various additives such as a viscosity control agent, a surface conditioner, a thickener, an antioxidant, an ultraviolet ray inhibitor and an antifoaming agent may be blended within a conventionally known range.

The coating method of the present invention, the curing temperature T _T as the uppermost layer-coating material is 40 ° C. or higher 200 ° C. or less, preferably used 60 ° C. or higher 160 ° C. or less of the thermosetting coating material. The uppermost layer thermosetting paint does not generate a volatile product in a curing reaction by heat treatment. In the present invention, the coating material that does not generate a volatile product may be any material that does not substantially generate a volatile product. Such a coating material preferably has a weight reduction rate of 0.5% by mass or less at the curing temperature T _T , more preferably 0.3% by mass or less, and particularly preferably 0.1% by mass or less. When such a thermosetting paint with a small weight reduction rate is used as the paint for the uppermost layer, the shrinkage of the coating film tends to be minimized after the uppermost layer is cured by heat treatment and the fluidity is significantly lowered. is there.

  As the thermosetting paint for the uppermost layer, a thermosetting resin capable of forming a coating film and a curing agent (for example, a compound or resin having two or more functional groups capable of undergoing addition reaction with the functional group of the thermosetting resin). And a thermosetting paint (for example, a clear paint described in JP-A No. 2004-275966) used as the uppermost layer paint for ordinary baking coating. The form of the thermosetting paint for the uppermost layer may be any of solvent type, aqueous type and powder.

  Examples of the thermosetting resin capable of forming a coating film included in the thermosetting paint for the uppermost layer include acrylic resins, polyester resins, alkyd resins, epoxy resins, urethane resins, and the like. Absent. Examples of the curing agent contained in the uppermost layer thermosetting paint include, but are not limited to, amine compounds, amino resins, isocyanate compounds, and isocyanate resins. However, it is necessary to select an appropriate combination of the thermosetting resin and the curing agent so that a volatile product is not generated during the curing reaction. These resins and curing agents may be used alone or in combination of two or more.

  Preferred combinations of the thermosetting resin and the curing agent include a combination of a hydroxyl group-containing acrylic resin and an isocyanate compound and / or an isocyanate resin, an epoxy group-containing acrylic resin and a polyvalent carboxylic acid compound and / or a carboxyl group-containing resin. The combination with is mentioned.

  Further, the uppermost layer coating material may contain conventionally known color pigments, glitter pigments, and the like within a conventionally known range, if necessary. In order to adjust various physical properties, various additives such as a viscosity control agent, a surface conditioner, a thickener, an antioxidant, an ultraviolet ray inhibitor and an antifoaming agent may be blended within a conventionally known range.

  In the coating method of the present invention, first, the lower layer coating material is applied onto the substrate, and if necessary, the solvent is evaporated by drying or the like to form an uncured lower layer. At this time, when the lower layer is one layer, the lower layer is formed using the lower layer thermosetting paint. When there are two or more lower layers, at least one of them can be formed using the lower layer thermosetting paint, and the remaining layers can be formed using the lower layer non-curable paint. In view of improving the strength of the laminated coating film, it is preferable to form all the layers using the lower layer thermosetting paint.

  When applying the coating for the lower layer, it is possible to apply a conventionally known method such as air electrostatic spray coating or rotary atomizing electrostatic coating when using either a thermosetting paint or a non-curing paint. it can.

  The film thickness of each lower layer can be appropriately set depending on the desired application. For example, the film thickness after heat treatment is preferably 5 to 50 μm, more preferably 10 to 40 μm. If the film thickness of each lower layer is less than the lower limit, it tends to be difficult to obtain a uniform lower layer coating film.On the other hand, if it exceeds the upper limit, it tends to absorb a large amount of the solvent contained in the uppermost layer coating film. Volatilization of the solvent contained in the layer itself is also suppressed, and the appearance quality of the laminated coating film tends to deteriorate.

  Next, the uppermost layer-coating material is applied on the uncured lower layer, and if necessary, the solvent is evaporated by drying or the like to form an uncured uppermost layer. Conventionally known methods such as air electrostatic spray coating and rotary atomizing electrostatic coating can be used as the coating method for the uppermost layer coating material.

  The film thickness of the uppermost layer can be appropriately set depending on the desired application. For example, the film thickness after heat treatment is preferably 15 to 60 μm, and more preferably 20 to 50 μm. When the film thickness of the uppermost layer is less than the lower limit, the fluidity is insufficient and the appearance quality of the laminated coating film tends to deteriorate. On the other hand, when the upper limit is exceeded, the fluidity becomes excessively large and the coating is applied in the vertical direction. There is a tendency for defects such as sagging to occur.

Next, the uncured laminated coating film formed by laminating the lower layer coating material and the uppermost layer coating material as described above is first subjected to a heat treatment at a temperature _TL , and then the lower layer thermosetting type. The paint is cured to form an uncured laminated coating. Thereafter, the uppermost layer is curing the curable coating top layer is subjected to heat treatment at a temperature T _H in the multilayer coating film uncured.

In the coating method of the present invention, the heating temperatures _TL and _TH are represented by the following formula (2):
T _U −20 ≦ T _L ≦ T _T −30 (2)
And the following formula (3)
T _T -20 ≦ T _H ≦ T _T +40 (3)
(In the formula (2) and (3), _{T L} represents the low heating temperature [℃], _{T H} represents a high-temperature heating temperature [° C.], _{T U} and _{T T} is _{T U} in the formula (1) And T _T. )
It is set to satisfy. By setting the heating temperatures T _L and T _H so as to satisfy the above formula, the lower layer can be cured first without curing the uppermost layer, and then the uppermost layer can be cured. From such a viewpoint, the heating temperature _TL is expressed by the following formula (2a)
T _U ≦ T _L ≦ T _T- 30 (2a)
It is preferable to satisfy the following formula (2b)
T _U + 10 ≦ T _L ≦ T _T −40 (2b)
It is more preferable to satisfy. The heating temperature _TH is expressed by the following formula (3a)
T _T ≦ T _H ≦ T _T +20 (3a)
It is preferable to satisfy the following formula (3b)
T _T = T _H (3b)
It is more preferable to satisfy.

Formula (2a), (2b), (3a) and (3b) _in, T _L, T H, _{T U} and _{T T} is _T L in formula (2) and _(3), T H, _{T U} and T Synonymous with _T.

  The low-temperature heating time is preferably 10% or more and 100% or less, and more preferably 20% or more and 80% or less of the curing time of the thermosetting paint for the lower layer. Specifically, when the curing time of the lower layer thermosetting paint is 30 minutes, the low-temperature heating time is preferably 3 minutes or longer and 30 minutes or shorter, and more preferably 6 minutes or longer and 24 minutes or shorter. If the low-temperature heating time is less than the lower limit, the lower layer tends not to be cured sufficiently. On the other hand, if the upper limit is exceeded, the entire heating time increases and productivity tends to be lowered.

  The high temperature heating time is preferably 50% or more and 150% or less, and more preferably 60% or more and 100% or less of the curing time of the thermosetting paint for the uppermost layer. Specifically, when the curing time of the thermosetting paint for the uppermost layer is 30 minutes, the high temperature heating time is preferably 15 minutes or longer and 45 minutes or shorter, and more preferably 18 minutes or longer and 30 minutes or shorter. When the high-temperature heating time is less than the lower limit, the uppermost layer tends to be not sufficiently cured, whereas when the upper limit is exceeded, the uppermost layer tends to be excessively cured and easily cracked or yellowed.

  In the coating method of the present invention, the volatile content concentration of the uncured laminated coating film before the high-temperature heat treatment is preferably 4% by mass or less, more preferably 3% by mass or less, and particularly preferably 2% by mass. Reduce to: This tends to minimize the shrinkage of the coating film after the uppermost layer is cured by heat treatment and the fluidity is remarkably lowered. In the present invention, the “volatile content of the laminated coating film” is a value calculated from the equation (6).

V = (Wt−We) / Wt × 100 (6)
In formula (6), V represents the volatile content concentration (unit: mass%) of the laminated coating film, Wt represents the mass (unit: g) of the laminated coating film at an arbitrary heat treatment time t, and We is finally The mass (unit: g) of the obtained multilayer coating film is shown.

As a preferred method for reducing the volatile content concentration of the laminated coating film whose uppermost layer is uncured before the high temperature heat treatment, a method of performing a heat treatment at the temperature _TL can be mentioned.

  Furthermore, in the coating method of the present invention, in order to stabilize the uncured laminated coating film laminated by wet-on-wet, it is preferable to leave it at room temperature (setting) before the low-temperature heat treatment. The setting time is usually set to 1 to 20 minutes.

  In the present invention, in order to obtain a coated body having a higher-grade appearance, one or more kinds of paints are further applied on the uppermost layer of the coated body obtained by the coating method and subjected to a curing treatment. It is preferable to form a surface layer. As the coating material, those exemplified as the top layer coating material can be used. Examples of the method for applying the paint include conventionally known methods such as air spray coating, air electrostatic spray coating, and rotary atomizing electrostatic coating.

  The coated body of the present invention is manufactured by the coating method of the present invention, and has fewer irregularities on the surface of the multilayer coating film than the conventional multilayer coating film manufactured by wet-on-wet, and is excellent in appearance quality. Such a coated body is particularly useful as a vehicle body for automobiles such as passenger cars, trucks, buses, motorcycles, and parts thereof.

  EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example and a comparative example, this invention is not limited to a following example. The relative loss elastic modulus of the lower layer paint and the weight reduction rate due to heat treatment of the lower layer paint at the start of gelation of the uppermost layer paint were measured by the following methods.

(Relative loss elastic modulus of the lower layer paint at the start of gelation of the uppermost layer paint)
First, the uppermost layer-coating material was applied to a 40 mm × 50 mm stainless steel plate (thickness 0.5 mm) so that the film thickness after heat treatment was 35 ± 5 μm. Specifically, the stainless steel plate is placed on a horizontal base, an adhesive tape with a thickness of 70 μm is applied to each of the areas of about 5 mm from the two opposite edges of the stainless steel plate, and a knife with a straight edge is provided. The top layer paint was applied to the gap between the stainless steel plate and the blade edge of the knife by sliding on the tape.

In this way, the relative storage elastic modulus (E _r ′) of the coating film was measured 7 ± 1 minutes after the coating film made of the uppermost layer coating material was formed. The measurement was carried out using a rigid pendulum type physical property tester (RPT-5000, manufactured by A & D Co., Ltd.) equipped with a circular pendulum with a diameter of 74 mm to which a knife edge with a blade angle of 40 ° was attached. The temperature program at the time of measurement was set so that the temperature was raised from room temperature (25 ° C.) to the curing temperature of the uppermost layer coating material at a rate of temperature increase of 20 ± 4 ° C./min, and then the curing temperature was maintained.

The measured value of the relative storage elastic modulus (E _r ′) obtained was plotted against time, and the following equation (5) for the portion of 15 minutes from the above inflection point:
E _r '= A [1-exp {k (t−t _d }] (5)
(In formula (5), A and k are constants, and t represents time.)
And the time axis intercept, that is, the time t _d from the start of measurement to the start of gelation of the uppermost layer coating was obtained by the non-linear least square method.

Next, for the lower layer coating material, a coating film is formed in the same manner as in the uppermost layer coating material, and the relative loss modulus (E _r ″) is measured under the same conditions as in the uppermost layer coating material. From this measurement result, the relative loss elastic modulus (E _r ″) at the time t _d was obtained, and this was defined as “relative loss elastic modulus at the start of gelation of the uppermost layer coating material”.

(Measurement of weight loss rate)
Painted on an aluminum foil so that the film thickness after heat treatment of the coating to the subject is the target thickness of the multilayer coating film, less 40 ° C. than the curing temperature T _T of the obtained aluminum foil sample coating top layer The film was dried for 90 minutes at a temperature [T _T −40 ° C.] and a vacuum condition of 10 ⁻² Torr or less. The obtained aluminum foil sample provided with the uncured coating film was subjected to the above-described process using a gas chromatograph / mass spectrometer (for example, 6890GC / 5975MSD manufactured by Agilent) with a heat desorption introducing device (for example, Thermal Deposition System manufactured by GERSTEL). The amount of volatile products (Rc (unit: g)) and the amount of residual solvent were quantified by heating at temperature T _T for 30 minutes, and the weight reduction rate was calculated by the formula (4). This weight reduction rate is a ratio of the volatile product amount to the total binder amount in the coating film.

Weight reduction rate = 100 × Rc / W × 100 / (100-P) (4)
In formula (4), W represents the mass (unit: g) of the uncured coating film obtained in the vacuum drying step, and P represents the mass of the pigment contained in 100 g of the coating film. In addition, the mass of the pigment used the value of the coating composition table.

(Synthesis Example 1) Synthesis of Acrylic Resin A 4.5 parts by mass of methacrylic acid, 26.0 parts by mass of ethyl acrylate, a hydroxyl group-containing monomer (manufactured by Daicel Chemical Industries, Ltd., trade name “Placcel FM-1”) 64.5 parts by mass Part of the mixture, 5.0 parts by mass of methylstyrene dimer (trade name “MSD-100”, manufactured by Mitsui Toatsu Chemical Co., Ltd.) and 13.0 parts by mass of azoisobutyronitrile were prepared.

  A reaction vessel equipped with a stirrer, a temperature controller and a reflux condenser was charged with 82.0 parts by mass of xylene, and then 20.0 parts by mass of the mixed solution A was added and heated while stirring to raise the temperature. It was. Thereafter, the remaining 93.0 parts by mass of the mixed solution A was dropped over 3 hours while refluxing, and then a solution consisting of 1.0 part by mass of azoisobutyronitrile and 12.0 parts by mass of xylene was added over 30 minutes. The reaction was carried out dropwise. The obtained reaction solution was further refluxed with stirring for 1 hour to obtain a resin solution A containing an acrylic resin A having a number average molecular weight of 2000. The resin solution A was desolvated with an evaporator until the solid content concentration became 75% by mass to obtain an acrylic resin varnish A.

(Synthesis Example 2) Synthesis of Acrylic Resin B 5.0 parts by mass of acrylic acid, 17.0 parts by mass of 2-hydroxyethyl acrylate, 66.0 parts by mass of n-butyl methacrylate, 12.0 parts by mass of stearyl acrylate and A mixed solution B was prepared by mixing 0.8 part by mass of azobisisobutyronitrile.

  A reaction vessel equipped with a stirrer, a temperature controller and a reflux condenser was charged with 82.0 parts by mass of isopropyl alcohol, purged with nitrogen, and then heated to a temperature of 80 ° C. Next, after the mixed solution B (100.8 parts by mass) was dropped over 5 hours, stirring was continued for 1 hour to obtain a resin solution B containing an acrylic resin B having a number average molecular weight of 15000. After this resin solution B was desolvated with an evaporator until the solid content concentration reached 80% by mass, 6.0 parts by mass of dimethylethanolamine and 36.0 parts by mass of ion-exchanged water were added to obtain a solid content concentration of 60% by mass. An acrylic resin varnish B was obtained.

(Preparation example 1) Preparation of melamine curable aqueous intermediate coating material a In a reaction vessel, 337 parts by mass of acrylic resin varnish A having a solid content concentration of 75% by mass prepared in Synthesis Example 1 and titanium oxide (made by Ishihara Sangyo Co., Ltd., trade name) "CR-93") 1000 parts by mass and carbon black (manufactured by Degussa, trade name "FW-200P") 10 parts by mass were charged, and then 163 parts by mass of butyl acetate and 84 parts by mass of xylene were added. Thereafter, glass beads (particle diameter 1.6 mm) having the same mass as the total mass charged were charged and dispersed for 3 hours with a desktop SG mill. The particle size at the end of dispersion by a grind gauge was 5 μm or less. Thereafter, 84 parts by mass of xylene was added, and then the glass beads were separated by filtration to prepare a pigment paste. To this pigment paste, the acrylic resin varnish A and melamine resin (trade name “Cymel 254”, manufactured by Cytec Co., Ltd.) are coated so that the solid mass ratio of the acrylic resin and melamine resin is 10: 3. It was added so that the pigment concentration in the coating film was 50.0% by mass and diluted with ion-exchanged water to prepare a melamine curable aqueous intermediate coating material a having a solid content concentration of 50% by mass. The curing temperature of this melamine curable aqueous intermediate coating material a is 140 ° C.

(Preparation Example 2) Preparation of Melamine Curing Type Aqueous Base Paint “a” Melamine resin (product name “Cymel 325” manufactured by Cytec Co., Ltd.) and acrylic resin was added to acrylic resin varnish B having a solid content concentration of 60% by mass produced in Synthesis Example 2. Add so that the mass ratio of the solid content with the melamine resin is 10: 2, and add the aluminum paste for water-based paint so that the pigment concentration in the base coating film is 17.7% by mass, Dilution was performed to prepare a melamine-curable water-based base paint a having a solid content concentration of 20% by mass. The curing temperature of this melamine curable aqueous base coating material a is 140 ° C.

Preparation Example 3 Preparation of Isocyanate Curing Clear Paint A A polyol, an additive and a solvent were mixed in the ratio shown in Table 1 to prepare a main component of a two-component isocyanate curing clear paint. Moreover, the isocyanate hardening agent shown in Table 1 was used as a hardening | curing agent of the said isocyanate hardening type clear coating material. In Examples 1 to 5 and Comparative Examples 1 and 3 below, a mixture (solid content concentration 55% by mass) of the main agent and a curing agent mixed at a ratio shown in Table 1 was used as the isocyanate curable clear coating material A. The curing temperature of this isocyanate curable clear coating material A was 140 ° C., and the weight reduction rate at 140 ° C. was 0% by mass.

(Preparation Example 4) Preparation of Isocyanate Curing Clear Paint B A polyol, an additive and a solvent were mixed at the ratio shown in Table 1 to prepare a main component of a two-component isocyanate curing clear paint. Moreover, what added 21.5 mass parts 3,5- dimethyl pyrazole as a blocking agent with respect to solid content 100 mass parts of isocyanate hardening | curing agent (The product made by Bayer, brand name "Desmodur N3390 Ba / SN"). Used as a curing agent for the thermosetting clear paint. In the comparative example 2, what mixed this main ingredient and hardening | curing agent in the ratio shown in Table 1 was used as the block isocyanate hardening type clear coating material B. FIG. The curing temperature of this blocked isocyanate curable clear coating B was 140 ° C., and the weight reduction rate at 140 ° C. was 6.1% by mass.

Example 1
The isocyanate-curable clear coating A (curing temperature = 140 ° C., weight loss rate (140 ° C.) = 0% by mass) prepared in Preparation Example 3 was used as the uppermost layer coating, and the curing temperature T was used as the thermosetting coating for the lower layer. A block isocyanate-curing solvent-based base paint A (trade name “SFX800” manufactured by Kansai Paint Co., Ltd.) having a _U of 80 ° C. was used. In addition, since the said isocyanate curable clear coating material A hardens | cures by the addition reaction of an isocyanate compound, a volatile product is not produced | generated substantially. The blocked isocyanate-curable base coating material A is cured by an addition reaction of an isocyanate compound generated by the deblocking reaction of the blocked isocyanate, and a volatile blocking agent is generated in the deblocking reaction. The relative loss elastic modulus of the blocked isocyanate curable base coating material A at the start of gelation of the isocyanate curable clear coating material A was 0.60 s ⁻² .

  The block isocyanate curable base coating A is applied to the surface of an electrodeposition coating plate (made by Shinto Herberts Co., Ltd., trade name “Sucsade 80V Gray”) so that the film thickness after heat treatment is 25 μm, and at 60 ° C. for 10 minutes. The organic solvent was volatilized by heating. Next, the isocyanate curable clear coating A (curing temperature = 140 ° C., weight reduction rate (140 ° C.) = 0% by mass) is formed on the layer of the blocked isocyanate curable base coating A to a film thickness after heat treatment of 35 μm. Thus, an uncured laminated coating film obtained by laminating the blocked isocyanate curable base coating material A and the isocyanate curable clear coating material A by wet-on-wet was obtained. The uncured laminated coating film was allowed to stand at room temperature for 10 minutes, then subjected to low-temperature heat treatment at 90 ° C. for 10 minutes to cure the blocked isocyanate curable base coating material A, and then at a high temperature of 140 ° C. for 30 minutes. The isocyanate-curable clear paint A was cured by heat treatment. During this time, the mass Wt (unit: g) of the multilayer coating film was measured at a predetermined timing, and the volatile content concentration V (unit: mass%) of the multilayer coating film was calculated from the equation (6).

V = (Wt−We) / Wt × 100 (6)
In formula (6), We is the mass (unit: g) of the obtained laminated coating film.

  In addition, wave scan values [Wa (wavelength <0.3 mm), Wb (wavelength 0.3 to 1 mm), Wc (wavelength 1 to 1 mm) using wave scan (Wave-Scan Dual manufactured by BYK-Gardner) together with the mass measurement]. 3 mm), Wd (wavelength 3 to 10 mm)]. These wave scan values mean that the smaller the Wa, the better the gloss, and the smaller the Wd, the better the skin.

  Table 2 shows Wa to Wd of the obtained multilayer coating film. Further, FIG. 2 shows the relationship between V and Wa during the heat treatment.

(Example 2)
The curing temperature T _U is 80 ° C. isocyanate curing type instead of blocked isocyanate-curable solvent-based coating material A as the lower layer heat-curable coating solvent type (2-liquid type) base paint B (tradename, Kansai Paint Co. A laminated coating film was prepared in the same manner as in Example 1 except that “Letane PG60 Kai” was used, and V and Wa to Wd were measured. In addition, since the said isocyanate curable base coating material B hardens | cures by the addition reaction of an isocyanate compound, a volatile product is not produced | generated substantially. The relative loss elastic modulus of the isocyanate curable base coating B at the start of gelation of the isocyanate curable clear coating A was 0.30 s ⁻² .

  Table 2 shows Wa to Wd of the obtained multilayer coating film. Further, FIG. 2 shows the relationship between V and Wa during the heat treatment.

(Comparative Example 1)
The melamine curable aqueous base paint a (curing temperature = 140 ° C.) prepared in Preparation Example 2 was used in place of the blocked isocyanate curable solvent-based base paint A (curing temperature = 80 ° C.) as the lower layer thermosetting paint. Except for the above, two steps of heat treatment (90 ° C. and 140 ° C.) were performed in the same manner as in Example 1 to prepare a laminated coating film, and V and Wa to Wd were measured. In the melamine curable base coating material a, the hydroxyl group of the acrylic resin and the melamine resin undergo a condensation reaction to cure, and volatile alcohol and water are generated in the condensation reaction. The relative loss elastic modulus of the melamine curable base coating material a at the start of gelation of the isocyanate curable clear coating material A was 1.1 s- ² .

  Table 2 shows Wa to Wd of the obtained multilayer coating film. Further, FIG. 2 shows the relationship between V and Wa during the heat treatment.

(Comparative Example 2)
Instead of the isocyanate curable clear paint A, the blocked isocyanate curable clear paint B (curing temperature = 140 ° C., weight reduction rate (140 ° C.) = 6.1% by mass) prepared in Preparation Example 4 was used as the clear paint. A laminated coating film was produced in the same manner as in Example 1 except that V and Wa to Wd were measured. In addition, although the said block isocyanate hardening type clear coating material B hardens | cures by the addition reaction of the isocyanate compound produced | generated by deblocking reaction of blocked isocyanate, a volatile blocking agent produces | generates in this deblocking reaction. The relative loss elastic modulus of the blocked isocyanate curable base coating material A at the start of gelation of the blocked isocyanate curable clear coating material B was 0.95 s ⁻² .

  Table 2 shows Wa to Wd of the laminated coating film after the heat treatment. Further, FIG. 2 shows the relationship between V and Wa during the heat treatment.

  As is apparent from the results shown in Table 2, a lower layer is formed using a thermosetting paint that cures at a lower temperature than the uppermost layer as in the present invention, and the lower layer is cured by heat treatment at a low temperature. Next, Wa to Wd of the laminated coating films (Examples 1 and 2) obtained by performing the heat treatment at a high temperature to cure the uppermost layer are all formed using the thermosetting paint having the same curing temperature. The lower and upper layers and lower layers are simultaneously heat-treated and cured to lower the thickness of the multilayer coating film (Comparative Example 1). Both were confirmed to be improved over the laminated coating film of Comparative Example 1.

  In addition, all of Wa to Wd of the laminated coating film (Examples 1 and 2) using an isocyanate curable clear coating that does not shrink in a curing reaction by heat treatment (Examples 1 and 2) undergo a curing reaction accompanied by a deblocking reaction by heat treatment, Compared to the multilayer coating film (Comparative Example 2) using a shrinkable thermosetting clear paint as the uppermost layer, the multilayer coating film of Examples 1 and 2 are both glossy and skin better than the multilayer coating film of Comparative Example 2. It was confirmed that there was an improvement.

  As shown in FIG. 2, the laminated coating films of Examples 1 and 2 were subjected to low-temperature heat treatment, and the volatile component concentrations V were reduced to about 4% by mass and about 3% by mass, respectively. The coating film decreased only to about 5% by mass. As a result, in the high-temperature heat treatment, the volatile component concentration V was reduced by about 5% by mass in Comparative Examples 1 and 2, whereas in Examples 1 and 2, the decrease in the volatile component concentration V was about 4% by mass and about It was suppressed to 3% by mass. The volatile matter concentration V in Comparative Example 2 includes the volatile matter produced by the deblocking reaction by heat treatment at 140 ° C.

  As is clear from the results shown in FIG. 2 and Table 2, the laminated coating films (Examples 1 and 2) having a small decrease in the volatile component concentration V during the heat treatment at 140 ° C. were subjected to the heat treatment at 140 ° C. Compared with the laminated coating film (Comparative Examples 1 and 2) in which the amount of decrease in the volatile component concentration V was large, it was confirmed that Wa to Wd were all small and excellent in gloss and skin.

  In the laminated coating films of Examples 1 and 2 and Comparative Examples 1 and 2, the clear paint starts to cure by heat treatment at 140 ° C. (In the case of Comparative Example 2 as well, the deblocking reaction occurs quickly and immediately. The same curing reaction as in Example 1 occurs.) However, after the fluidity of the thermosetting clear coating layer is significantly reduced by curing, the amount corresponding to the decrease in the volatile concentration V by this high-temperature heat treatment Shrinkage of the laminated coating occurs. Therefore, the laminated coating film of Examples 1-2 in which the decrease amount of the volatile component concentration V during the high-temperature heat treatment is small is higher than the laminated coating film of Comparative Examples 1-2 in which the decrease amount of the volatile component concentration V is large. Shrinkage due to heat treatment is surely suppressed, and as a result, formation (prominence) of unevenness on the surface of the laminated coating film is suppressed, Wa to Wd are all reduced, and gloss and skin are laminated coatings of Comparative Examples 1 and 2 It was confirmed that the film was improved over the film.

(Example 3)
The isocyanate curable clear coating A (curing temperature = 140 ° C., weight reduction rate (140 ° C.) = 0% by mass) prepared in Preparation Example 3 was used as the uppermost layer coating, and the curing temperature T was used as the thermosetting intermediate coating. An isocyanate curable (two-component) solvent-type intermediate coating material A (trade name “SFX5333” manufactured by Kansai Paint Co., Ltd.) having an _U of 80 ° C. was used. Further, the blocked isocyanate curable base coating A (curing temperature = 80 ° C.) used in Example 1 was used as the thermosetting base coating. The isocyanate-curable clear coating material A and the isocyanate-curable intermediate coating material A are cured by an addition reaction of an isocyanate compound, so that a volatile product is not substantially generated. The blocked isocyanate-curable base coating material A is cured by an addition reaction of an isocyanate compound generated by the deblocking reaction of the blocked isocyanate, and a volatile blocking agent is generated in the deblocking reaction. The relative loss elastic modulus of the isocyanate-curable clear coating painted in the isocyanate-curable at gelation initiation of A paint A and the blocked isocyanate-curable base coating material A was respectively 17s ^-2 and 0.60S ^-2 .

  The isocyanate-curing intermediate coating A is applied to the surface of an electrodeposition coating plate (made by Shinto Herberts Co., Ltd., trade name “Sucsade 80V Gray”) as a first lower layer so that the film thickness after heat treatment is 20 μm. The organic solvent is volatilized by heating at 60 ° C. for 10 minutes, and the blocked isocyanate curable base coating material A (curing temperature = 80 ° C.) is heat-treated as a second lower layer on the isocyanate curable intermediate coating material A layer. After coating so that the film thickness becomes 15 μm, the isocyanate-curable clear coating A (curing temperature = 140 ° C., weight reduction rate (140 ° C.) = 0 mass on the block isocyanate-curable base coating A layer. %) Was applied in the same manner as in Example 1 except that the film thickness was 35 μm after the heat treatment, and V and Wa to Wd were measured.

  Table 3 shows Wa to Wd of the obtained multilayer coating film. Further, FIG. 3 shows the relationship between V and Wa during the heat treatment.

Example 4
Instead of the isocyanate curable intermediate coating material A, the main component of the isocyanate curable intermediate coating material A (two-component type (curing agent: isocyanate compound)) used in Example 3 was used as a non-curable solvent-based intermediate coating material. A laminated coating film was produced in the same manner as in Example 3 except that V and Wa to Wd were measured. Note that the non-curing intermediate coating does not undergo a curing reaction, and therefore does not generate a volatile product due to the curing reaction. The relative loss elastic modulus of the non-curing intermediate coating at the start of gelation of the isocyanate-curable clear coating A was 12.5 s ⁻² .

  Table 3 shows Wa to Wd of the obtained multilayer coating film. Further, FIG. 3 shows the relationship between V and Wa during the heat treatment.

(Example 5)
The isocyanate curable clear coating A (curing temperature = 140 ° C., weight reduction rate (140 ° C.) = 0% by mass) prepared in Preparation Example 3 was used as the uppermost layer coating, and the curing temperature T was used as the thermosetting intermediate coating. A blocked isocyanate-curing solvent-type intermediate coating material B (trade name “SFX3300CD” manufactured by Kansai Paint Co., Ltd.) having a _U of 90 ° C. was used. Further, solvent-based blocked isocyanate curing type curing temperature T _U as thermosetting base paint 90 ° C. base paint C was used (tradename, Kansai Paint Co. "SFX420"). In addition, since the said isocyanate curable clear coating material A hardens | cures by the addition reaction of an isocyanate compound, a volatile product is not produced | generated substantially. The blocked isocyanate curable intermediate coating material B and the blocked isocyanate curable base coating material C are cured by an addition reaction of an isocyanate compound generated by the deblocking reaction of the blocked isocyanate. In this deblocking reaction, a volatile blocking agent is used. Generate. The relative loss elastic moduli of the blocked isocyanate curable intermediate coating B and the blocked isocyanate curable base coating C at the start of gelation of the isocyanate curable clear coating A are 0.70 s ⁻² and 0.09 s ⁻² , respectively. Met.

  The block isocyanate curable intermediate coating B is used in place of the isocyanate curable intermediate coating A (curing temperature = 80 ° C.) as the first lower layer thermosetting coating, and the second lower layer thermosetting coating is used as the lower layer thermosetting coating. Example 3 except that the blocked isocyanate curable base coating material C was used in place of the blocked isocyanate curable base coating material A (curing temperature = 80 ° C.), and the first heating condition was changed to 100 ° C. for 10 minutes. Similarly, a laminated coating film was prepared, and V and Wa to Wd were measured.

  Table 3 shows Wa to Wd of the obtained multilayer coating film. Further, FIG. 3 shows the relationship between V and Wa during the heat treatment.

(Comparative Example 3)
The melamine curable aqueous intermediate coating material a (curing temperature = 140 ° C.) prepared in Preparation Example 1 is used in place of the isocyanate curable intermediate coating material A (curing temperature = 80 ° C.) as the thermosetting paint for the lower layer of the first layer. The melamine curable aqueous base paint a (curing temperature = 140 ° C.) prepared in Preparation Example 2 instead of the blocked isocyanate curable base paint A (curing temperature = 80 ° C.) as the second layer thermosetting paint for the lower layer A two-stage heat treatment (90 ° C. and 140 ° C.) was carried out in the same manner as in Example 3 except that it was used to produce a laminated coating film, and V and Wa to Wd were measured. In the melamine curable base coating material a and the melamine curable intermediate coating material a, the hydroxyl group of the acrylic resin and the melamine resin undergo a condensation reaction, and the curing proceeds. In this condensation reaction, volatile alcohol or water is generated. . The relative loss elastic moduli of the melamine curable aqueous intermediate coating material a and the melamine curable aqueous base coating material a at the start of gelation of the isocyanate curable clear coating material A are 7.5 s ^-2 and 1.1, respectively. . s- ² .

  Table 3 shows Wa to Wd of the obtained multilayer coating film. Further, FIG. 3 shows the relationship between V and Wa during the heat treatment.

  As is apparent from the results shown in Table 3, a thermosetting paint that cures at a lower temperature than the uppermost layer is used as the lower layer thermosetting paint as in the present invention, and the lower layer is subjected to heat treatment at a low temperature. Each of Wa to Wd of the laminated coating films (Examples 3 to 5) obtained by curing and then heating at a high temperature to cure the uppermost layer is a thermosetting type having the same curing temperature for the uppermost layer and the lower layer. Compared to the multilayer coating film (Comparative Example 3) obtained by applying and curing the top layer and the lower layer at the same time using the paint, the multilayer coating films of Examples 3 to 5 are both glossy and skin. It was confirmed that the improvement was higher than the laminated coating film of Comparative Example 3.

  As shown in FIG. 3, the laminated coating films of Examples 3 to 4 were reduced to a volatile content concentration V of about 3% by mass and the laminated coating film of Example 5 was reduced to about 2% by mass by low-temperature heat treatment. Thus, the laminated coating film of Comparative Example 3 was reduced only to about 6% by mass. As a result, in the high-temperature heat treatment, the volatile component concentration V was reduced by about 6% by mass in Comparative Example 3, whereas the decrease in the volatile component concentration V was about 3% by mass in Examples 3 to 4 and in Example 5. It was suppressed to about 2% by mass. The decrease in the volatile content concentration V due to the high temperature heat treatment corresponds to the shrinkage of the laminated coating film. Therefore, the multilayer coating films of Examples 3 to 5 are surely suppressed from shrinkage due to high-temperature heat treatment as compared with the multilayer coating film of Comparative Example 3, and as a result, the formation of unevenness (prominence) on the surface of the multilayer coating film is suppressed. As a result, Wa to Wd were all reduced, and it was confirmed that gloss and skin were improved as compared with the laminated coating film of Comparative Example 3.

  In addition, it is guessed that the difference in the shrinkage | contraction amount of the laminated coating film in an Example and a comparative example arises as follows. In the laminated coating films of Comparative Examples 1 to 3 using a thermosetting paint that is cured by a condensation reaction at the same curing temperature for the uppermost layer and the lower layer, the fluidity of the paint for the uppermost layer is greatly reduced by a high-temperature heat treatment that significantly decreases due to the curing reaction. The upper layer and the lower layer are simultaneously cured, and during this curing, the hydroxyl group of the acrylic resin and the melamine resin are condensed to generate and volatilize volatile alcohol and water. On the other hand, in the laminated coating films of Examples 1 to 5 using a thermosetting paint having a lower curing temperature than the uppermost layer thermosetting paint as the lower layer thermosetting paint, the lower layer was cured by the low-temperature heat treatment. The uppermost layer is cured by heat treatment. At this time, as the thermosetting paint for the lower layer, a thermosetting paint that does not substantially generate a volatile product by heat treatment, or a volatile product such as a blocking agent or a condensation product is generated at the time of curing by low-temperature heat treatment. Because it uses a thermosetting paint that generates relatively little volatile product during the subsequent high-temperature heat treatment (especially when the fluidity of the paint for the top layer is significantly reduced by the curing reaction) The volatilization of volatile products during the curing of the top layer by high temperature heat treatment is substantially reduced. As a result, in Comparative Examples 1 to 3, the coating film shrinks due to the formation and volatilization of the volatile product when the uppermost layer is cured, whereas in Examples 1 to 5, the volatilization of the volatile product occurs when the uppermost layer is cured. It is presumed that the shrinkage of the coating film is reduced due to the small amount of the film.

  As described above, according to the present invention, even when at least the uppermost layer is cured by laminating and baking two or more kinds of paints on a wet-on-wet basis, it is possible to obtain a laminated coating film with less unevenness on the surface of the uppermost layer. it can. Thereby, the coating body excellent in appearance quality, such as skin (surface smoothness) and gloss, can be obtained.

  Therefore, the present invention is useful as a coating method capable of obtaining a coated body excellent in appearance quality even when two or more kinds of paints are laminated and baked by wet-on-wet, and particularly, passenger cars, trucks, buses, motorcycles. It is useful as a painting method for automobile bodies and parts thereof.

It is a graph which shows typically a time-dependent change of relative storage elastic modulus (Er '). It is a graph which shows the relationship between the volatile matter density | concentration V and the wave scan value Wa in the heat processing of the laminated coating film produced in Examples 1-2 and Comparative Examples 1-2. It is a graph which shows the relationship between the volatile matter density | concentration V and the wave scan value Wa in the heat processing of the laminated coating film produced in Examples 3-5 and Comparative Example 3. FIG.

Explanation of symbols

P ... inflection point, t ... time, t d _... time from the start of measurement of the relative storage elastic modulus for the uppermost layer-coating material until gelation starts.

Claims (5)

A coating method for forming a laminated coating film comprising at least one lower layer formed on a substrate and an uppermost layer formed on the lower layer,
Preparing a thermosetting coating material having a curing temperature T _T of 40 ° C. or more and 200 ° C. or less and forming no volatile product in a curing reaction by heat treatment as the uppermost layer coating material for forming the uppermost layer; the curing temperature T _U is the following formula as at least one of the lower layer coating material for forming the lower layer (1)
T _U ≦ T _T -30 (1)
(In the formula (1), T _U represents a curing temperature [℃] of the lower layer for thermosetting paint, T _T denotes the curing temperature of the uppermost layer-thermosetting coating material [℃].)
A step of preparing a thermosetting paint for the lower layer satisfying
A step of laminating the lower layer coating material and the uppermost layer coating material on the substrate by wet-on-wet to form an uncured laminated coating film;
Following formula (2)
T _U −20 ≦ T _L ≦ T _T −30 (2)
And the following formula (3)
T _T -20 ≦ T _H ≦ T _T +40 (3)
(In the formula (2) and (3), _{T L} represents the low heating temperature [℃], _{T H} represents a high-temperature heating temperature [° C.], _{T U} and _{T T} is _{T U} and in the formula (1) it is synonymous with the _T T.)
Setting the heating temperatures T _L and T _H to satisfy
The uncured laminated coating film is subjected to a heat treatment at the temperature _TL to cure at least the thermosetting paint for the lower layer to form an uncured laminated coating film, and then the uppermost layer is uncured. curing the uppermost layer-thermosetting coating material is subjected to heat treatment at the temperature T _H in the multilayer coating film,
The coating method characterized by including. The coating method according to claim 1, wherein the thermosetting paint for the uppermost layer is a paint having a weight reduction rate of 0.5 mass% or less at the temperature T _T.   The coating method according to claim 1 or 2, wherein the lower layer has two or more layers, and all of the lower layer paints for forming the lower layer are the lower layer thermosetting paints. After the top layer is reduced volatiles concentration of the multilayer coating film of the uncured 4 wt% or less, the uppermost layer-coating material by subjecting the top layer to heat treatment at the temperature T _H in the multilayer coating film of the uncured Curing is performed. The coating method according to any one of claims 1 to 3.   It is a coating body which has a laminated coating film provided with the at least 1 lower layer formed on the base material, and the uppermost layer formed on the said lower layer, Comprising: It is a coating body as described in any one of Claims 1-4. Painted body obtained by the coating method of

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