JP2009034667A - Coating method, and coated object prepared by above method - 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 at least the uppermost layer is cured by baking for the purpose of obtaining high durability thereof 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 thermally uncurable paint as at least a species of paint for forming the lower layer(s), and further preparing thermosetting paint for forming the uppermost 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; and a step of subjecting the uncured laminated coating film to a heat treatment at least 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 the layers constituting the laminated coating film are cured by baking after all the coatings are laminated. Thermosetting paint was used for all layers so that the reaction occurred and the entire laminated coating 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) includes a two-step heating process, a process in which an intermediate paint, a base paint, and a clear paint are sequentially applied by wet-on-wet, a low-temperature heating stage, and a high-temperature heating stage. A coating film forming method 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-described problems of the prior art, and at least the uppermost layer is cured to ensure high durability by laminating and baking two or more kinds of paints by wet-on-wet. Another object of the present invention is to provide a coating method capable of obtaining a laminated coating film with less irregularities on the surface of the uppermost layer, and a coated body excellent in appearance quality obtained thereby.

  As a result of intensive studies to achieve the above object, the present inventors have formed at least one of the lowermost layers of the laminated coating film using a non-curable coating material that does not cause a curing reaction by heat treatment. By doing so, the shrinkage of the laminated coating after the uppermost layer is cured and the fluidity is remarkably reduced can be minimized, and baking is performed after laminating two or more kinds of paints by wet-on-wet. In addition, the present inventors have found that a laminated coating film having excellent appearance quality can be obtained and 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,
A non-curing paint that does not cause a curing reaction by heat treatment is prepared as at least one of the lower-layer paints for forming the lower layer, and a thermosetting type is used as the upper-layer paint for forming the uppermost layer Preparing a paint; and
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;
Applying a heat treatment to the uncured laminated coating to cure at least the uppermost layer coating;
It is characterized by including.

  The non-curable coating material is preferably a coating material having a weight reduction rate of 0.5% by mass or less at the curing temperature of the uppermost layer coating material, and more preferably a coating material that does not contain a curing agent.

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

  The uppermost layer-coating material is preferably a coating material having a weight reduction rate of 0.5% by mass or less at the curing temperature, and more preferably a coating material that does not generate a volatile product in a curing reaction by heat treatment. .

  In the coating method of the present invention, the volatile content concentration of the uncured laminated coating film is reduced to 3.5% by mass or less, and then heat-treated at a temperature of [the curing temperature of the top layer coating material—20 ° C.] or higher. It is preferable to cure the uppermost layer coating material. Further, the uncured laminated coating film is subjected to a heat treatment at a temperature lower than [the curing temperature of the uppermost layer paint −20 ° C.], and then subjected to a heat treatment at a temperature equal to or higher than the curing temperature of the uppermost layer paint −20 ° C. It is also preferable to apply.

  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. That is, in the conventional multilayer coating film formed by wet-on-wet, thermosetting paint is used in all layers including the uppermost layer. Therefore, when the thermosetting paint forming the uppermost 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, the laminated coating film shrinks, and irregularities are formed 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, since at least one of the lower layers is formed using a non-curable coating material, even if the uppermost layer is formed using a thermosetting coating material, the thermosetting coating material is subjected to heat treatment. In the lower layer formed by using a non-curing paint when cured, a curing reaction does not substantially occur and a volatile product is not substantially generated. As a result, the volatilization of the volatile product that affects the shrinkage of the conventional multi-layer coating film does not occur, and the shrinkage of the multi-layer coating film is suppressed to the minimum due to only the volatilization of the remaining solvent. The present inventors speculate that this is possible. In the present invention, “substantially does not generate a volatile product” and “volatilization of a volatile product does not substantially occur” includes shrinkage of a coating film due to volatilization of a volatile product. The case where the volatile product is generated and volatilized to such an extent that the surface smoothness of the coating film is not affected is included. Specifically, if the weight reduction rate of the coating film is 0.5% by mass or less even if a volatile product is generated and volatilized by heat-treating the paint, the paint is substantially volatile. It shall not produce product and will not volatilize.

  According to the present invention, even when at least the uppermost layer is cured for the purpose of ensuring high durability by laminating and baking two or more types of paints on a wet-on-wet basis, a laminated coating film with less irregularities on the surface of the uppermost layer is obtained. Obtainable. 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,
A non-curing paint that does not cause a curing reaction by heat treatment is prepared as at least one of the lower-layer paints for forming the lower layer, and a thermosetting type is used as the upper-layer paint for forming the uppermost layer Preparing the paint,
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;
Applying a heat treatment to the uncured laminated coating to cure at least the uppermost layer coating;
It is characterized by including.

  In the coating method of the present invention, one or more types of lower layer coating materials 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, the uppermost layer coating material is applied onto the uncured lower layer, and if necessary, the solvent is evaporated by drying or the like to form an uncured uppermost layer. A heat treatment is applied to cure at least the uppermost layer coating material.

  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, but at least one of the lower layers is formed using a non-curable coating that does not cause a curing reaction by heat treatment as the lower layer coating. The Specifically, when the lower layer is a single layer, this lower layer is formed using a non-curable paint, and when the lower layer is two or more layers, at least one of them is formed using a non-curable paint. Form. When there are two or more lower layers, it is preferable that all lower layers are formed using a non-curable coating material in that the shrinkage of the laminated coating film can be reduced.

  The non-curing coating material that does not cause a curing reaction by heat treatment may be any material that does not substantially cause a curing reaction by heat treatment, and the weight reduction rate at the curing temperature of the coating material for the uppermost layer to be used is 0.5% by mass or less. Are preferred, those with a mass of 0.3% or less are more preferred, and those with a mass of 0.1% or less are particularly preferred. When a non-curing coating material having a small weight reduction rate due to such heat treatment is used, the shrinkage of the laminated coating film after the fluidity of the uppermost layer is remarkably lowered by curing 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.

In the present invention, the “coating temperature of the paint” refers to a curing condition such as a curing time for coating the target paint on the base material and applying a heat treatment to cure the coating film and fix it on the base material. The temperature at which curing can be performed most efficiently is generally referred to, 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). The “weight reduction rate of the paint” is a value measured by the following method. That is, the target paint was coated on the aluminum foil so that the film thickness after heat treatment was the target film thickness in the laminated coating film, and the obtained aluminum foil sample was 40 ° C. higher than the curing temperature of the top layer paint. After drying at a low temperature and a vacuum condition of 10 ⁻² Torr or less for 90 minutes, a gas chromatograph / mass spectrometer (for example, 6890GC / 5975MSD manufactured by Agilent) equipped with a thermal desorption introduction device (for example, Thermal Desorption System manufactured by GERSTEL) is used. The volatile product amount (Rc (unit: g)) and the residual solvent amount are quantified by heating at the curing temperature of the uppermost layer coating material for 30 minutes, and the weight reduction rate is calculated by equation (1). 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) (1)
In formula (1), W is the mass (unit: g) of the coating film obtained in the vacuum drying step, and P is 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.

In addition, as the non-curable 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 (2) for a portion of 15 minutes from this inflection point:
E _r '= A [1-exp {k (t−t _d }] (2)
(In formula (2), 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.

  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 paint 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.

  In the present invention, when there are two or more lower layers, as long as at least one layer is a layer formed using the non-curable paint, the remaining layers may be formed using a thermosetting paint.

  As the thermosetting paint for the lower layer, a thermosetting paint used in ordinary baking coating can be used, and examples thereof include an intermediate paint and a base paint described in JP-A-2004-275966. The form of the thermosetting paint for the lower layer may be either a solvent type or an aqueous type, but an aqueous type is preferred in that the amount of volatile organic compounds discharged can be reduced. In addition, the thermosetting paint for the lower layer reduces the weight at the curing temperature of the uppermost layer paint used from the viewpoint of minimizing the shrinkage of the coating after the uppermost layer is cured by heat treatment and the fluidity is significantly reduced. A smaller rate is preferred.

  Specific examples of the thermosetting paint for the lower layer include thermosetting resins such as acrylic resins, polyester resins, alkyd resins, epoxy resins and urethane resins, and curing agents such as amine compounds, amino resins, isocyanate compounds and isocyanate resins. However, the present invention is not limited to these. Moreover, the thermosetting resin and the curing agent may be used alone or in combination of two or more.

  The thermosetting paint for the lower layer 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 present invention, a thermosetting paint is used as the paint for the uppermost layer. 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 reacting with the functional group of the thermosetting resin). Any thermosetting paint (for example, clear paint described in JP-A-2004-275966) used as an uppermost-layer paint for ordinary baking coating may be used. The form may be any of solvent type, aqueous type and powder.

  The curing temperature of the uppermost layer thermosetting paint is not particularly limited, but is usually 40 to 200 ° C, preferably 60 to 160 ° C.

  Examples of the thermosetting resin capable of forming a coating film contained in the uppermost layer-coating material include acrylic resins, polyester resins, alkyd resins, epoxy resins, and urethane resins, but are not limited thereto. Preferred curing agents include, but are not limited to, amine compounds, amino resins, isocyanate compounds, and isocyanate resins. These resins and curing agents may be used alone or in combination of two or more.

  In the coating method of the present invention, the uppermost layer-coating material is preferably a coating material that does not substantially generate a volatile product in a curing reaction by heat treatment. Such a coating material preferably has a weight reduction rate of 0.5% by mass or less at the curing temperature, more preferably 0.3% by mass or less, and particularly preferably 0.1% by mass or less. When such a thermosetting paint having a small weight reduction rate is used as the uppermost layer paint, shrinkage of the coating film due to heat treatment tends to be minimized. From this point of view, a paint that does not generate a volatile product (weight reduction rate of 0% by mass) is most preferable.

  The combination of the thermosetting resin that does not generate a volatile product in the curing reaction by heat treatment and the curing agent includes 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 combination. Examples thereof include a combination with a polyvalent carboxylic acid compound and / or a carboxyl group-containing resin.

  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 a single layer, the lower layer is formed using the non-curable paint. When there are two or more lower layers, at least one layer can be formed using the non-curable paint, and the remaining layers can be formed using the lower layer thermosetting paint. It is preferable to form all the lower layers using the non-curable coating material in that it can be minimized. Moreover, when forming two or more lower layers, you may form the layer using a non-hardening-type paint in any position.

  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 regardless of whether a non-curable coating or a thermosetting coating is used. 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.

  In this manner, at least the uppermost layer coating material is cured by applying heat treatment to the uncured laminated coating film formed by laminating the lower layer coating material and the uppermost layer coating material by wet-on-wet. In the coating method of the present invention, the heat treatment is a heat treatment (hereinafter referred to as “high temperature heat treatment”) at a temperature higher than at least the temperature at which the uppermost layer is cured, for example, [the curing temperature of the paint for the uppermost layer−20 ° C.]. ) Is preferably included.

  Further, the high temperature heating temperature is preferably in a range of [the curing temperature of the top layer coating material ± 20 ° C.]. Specifically, when the curing temperature of the uppermost layer-coating material is 140 ° C., the high temperature heating temperature is preferably 120 ° C. or higher, and preferably 120 ° C. or higher and 160 ° C. or lower. The high temperature heating time is preferably 50% or more and 150% or less, and preferably 60% or more and 100% or less, of the curing time of the uppermost layer coating material. Specifically, when the curing time of the uppermost layer-coating material is 30 minutes, the high-temperature heating time is preferably 15 minutes to 45 minutes, and more preferably 18 minutes to 30 minutes.

  Further, in the coating method of the present invention, the volatile content concentration of the laminated coating film is preferably 3.5% by mass or less, more preferably 3% by mass or less, particularly without curing the uppermost layer before performing the high temperature heat treatment. Preferably, it is reduced to 2% by mass or less. This tends to minimize the shrinkage of the laminated coating after the uppermost layer is cured by heat treatment and the fluidity is significantly reduced. In the present invention, the “volatile content of the laminated coating film” is a value calculated from the formula (3).

V = (Wt−We) / Wt × 100 (3)
In formula (3), V is the volatile content concentration (unit: mass%) of the laminated coating film, Wt is the mass (unit: g) of the laminated coating film at an arbitrary heat treatment time t, and We is the laminated coating film at the end of the heat treatment. Mass (unit: g).

  As a method of reducing the volatile content concentration of the laminated coating film without curing the uppermost layer, heat treatment (hereinafter referred to as “low temperature heat treatment”) at a temperature lower than [the curing temperature of the uppermost layer coating material—20 ° C.] The method of applying is preferred. Further, the low-temperature heating temperature is preferably less than [the curing temperature of the uppermost layer paint −30 ° C.], particularly preferably less than [the curing temperature of the uppermost layer paint −40 ° C.]. Specifically, when the curing temperature of the paint for the uppermost layer is 140 ° C., the low temperature heating temperature is preferably less than 120 ° C., more preferably less than 110 ° C., and particularly preferably less than 100 ° C. . The low temperature heating time is preferably 10% or more and less than 50%, and preferably 20% or more and 40% or less of the curing time of the uppermost layer coating material. Specifically, when the curing time of the uppermost layer-coating material is 30 minutes, the low-temperature heating time is preferably 3 minutes to 15 minutes, and more preferably 6 minutes to 12 minutes. When the uncured laminated coating film is heat-treated in the range of the low temperature heating temperature and the low temperature heating time, the volatile content concentration of the laminated coating film tends to be reduced without substantially curing the uppermost layer.

  Furthermore, in the coating method of the present invention, in order to stabilize the uncured coating film laminated by wet-on-wet, it is preferable to leave it at room temperature (setting) before the 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 (2) for the portion of 15 minutes from the above inflection point:
E _r '= A [1-exp {k (t−t _d }] (2)
(In formula (2), 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)
The target paint is coated on the aluminum foil so that the film thickness after the heat treatment becomes the target film thickness of the laminated coating film, and the obtained aluminum foil sample is at a temperature 40 ° C. lower than the curing temperature of the uppermost layer paint. After drying for 90 minutes under a vacuum condition of 10 ⁻² Torr or less, using a gas chromatograph / mass spectrometer (for example, 6890GC / 5975MSD manufactured by Agilent) equipped with a thermal desorption introducing device (for example, Thermal Destruction System manufactured by GERSTEL) The amount of volatile products (Rc (unit: g)) and the amount of residual solvent were quantified by heating at the curing temperature of the upper layer coating material for 30 minutes, and the weight reduction rate was calculated by equation (1). 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) (1)
In formula (1), W is the mass (unit: g) of the coating film obtained in the vacuum drying step, and P is the mass (unit: g) 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 thermosetting aqueous intermediate coating material A 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. In this pigment paste, the acrylic resin varnish A and the melamine resin (product name “Cymel 254”, manufactured by Cytec Co., Ltd.) are added so that the solid content mass ratio of the acrylic resin to the melamine resin is 10: 1.5, and It was added so that the pigment concentration in the intermediate coating film was 50.0% by mass and diluted with ion-exchanged water to prepare a thermosetting aqueous intermediate coating material A having a solid content concentration of 50% by mass. The curing temperature of the thermosetting aqueous intermediate coating material A was 140 ° C., and the weight reduction rate at 140 ° C. was 1.6% by mass (calculated as P = 50.0).

(Preparation Example 2) Preparation of thermosetting water-based intermediate coating material B The acrylic resin varnish A and melamine resin (made by Cytec Co., Ltd., trade name “Cymel 254”) have a solid mass ratio of 10 to acrylic resin and melamine resin. : A thermosetting aqueous intermediate coating material B having a solid content concentration of 50% by mass was prepared in the same manner as in Preparation Example 1 except that the composition was used so as to be 3. The curing temperature of the thermosetting aqueous intermediate coating material B was 140 ° C., and the weight reduction rate at 140 ° C. was 3.3% by mass (calculated as P = 50.0).

(Preparation example 3) Preparation of non-curable water-based base coating material A To acrylic resin varnish B having a solid content concentration of 60% by mass prepared in Synthesis Example 2, 1-t-butoxy-2-propanol having the same mass as the solid content was added. Further, an aluminum paste for water-based paint is added so that the pigment concentration in the base coating film is 17.7% by mass, diluted with ion-exchanged water, and the solid content concentration is 20% by mass. Paint A was prepared. The weight loss rate of this non-curable water-based base coating material A at 140 ° C. was 0% by mass.

(Preparation Example 4) Preparation of thermosetting water-based base coating material B 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 mass% prepared 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, A thermosetting water-based base coating material B having a solid content concentration of 20% by mass was prepared by dilution. The curing temperature of this thermosetting water-based base coating material B was 140 ° C., and the weight reduction rate at 140 ° C. was 1.6% by mass (calculated as P = 17.7).

(Preparation Example 5) Preparation of thermosetting water-based base coating material C The acrylic resin varnish B and melamine resin (trade name “Cymel 325”, manufactured by Cytec Co., Ltd.) have a solid mass ratio of 10: A thermosetting water-based base coating material C having a solid content concentration of 20% by mass was prepared in the same manner as in Preparation Example 4 except that 1 was used. The curing temperature of this thermosetting water-based base coating material C was 140 ° C., and the weight reduction rate at 140 ° C. was 0.8% by mass (calculated as P = 17.7).

(Preparation Example 6) Preparation of Thermosetting Clear Paint A 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 thermosetting clear paint. Moreover, the isocyanate hardening agent shown in Table 1 was used as a hardening | curing agent of the said thermosetting type clear coating material. In Examples 1, 3 to 4 and Comparative Examples 1 to 4, 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 thermosetting clear coating material A. The curing temperature of this thermosetting clear paint A was 140 ° C., and the weight reduction rate at 140 ° C. was 0% by mass.

(Preparation Example 7) Preparation of thermosetting clear paint B 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 thermosetting clear paint. Moreover, what added 4.3 mass part 3, 5- dimethyl pyrazole as a blocking agent with respect to solid content of 100 mass parts of isocyanate hardening | curing agent (The Bayer company make, brand name "Desmodur N3390 Ba / SN"). Used as a curing agent for the thermosetting clear paint. In Example 2, a mixture obtained by mixing the main agent and the curing agent at a ratio shown in Table 1 was used as the thermosetting clear coating material B. The curing temperature of this thermosetting clear paint B was 140 ° C., and the weight reduction rate at 140 ° C. was 1.5% by mass.

Example 1
The thermosetting clear coating material A (curing temperature = 140 ° C., weight loss rate (140 ° C.) = 0% by mass) prepared in Preparation Example 6 was used as the uppermost layer coating material, and the lower layer coating material was prepared in Preparation Example 3. Non-curable water-based base paint A (weight reduction rate (140 ° C.) = 0% by mass) was used. The relative loss elastic modulus of the non-curable aqueous base paint A at the start of gelation of the thermosetting clear paint A was 0.29 s ⁻² .

  On the surface of the electrodeposition coating plate (made by Shinto Herberts, trade name “Sucsade 80V Gray”), the non-curable water-based base paint A prepared in Preparation Example 3 is applied so that the film thickness after heat treatment is 20 μm. Heating at 80 ° C. for 3 minutes volatilized water and organic solvent. Next, the thermosetting clear coating material A prepared in Preparation Example 6 was applied onto the layer of the non-curable water-based base coating material A so that the film thickness after heat treatment was 35 μm. An uncured laminated coating film obtained by laminating the thermosetting clear paint A with wet on wet was obtained. The uncured laminated coating film was allowed to stand (setting) at room temperature for 10 minutes, and then subjected to a heat treatment at 90 ° C. for 10 minutes and a heat treatment at 140 ° C. for 30 minutes to cure the thermosetting clear coating material A. . 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 formula (3).

V = (Wt−We) / Wt × 100 (3)
In formula (3), We is the mass (unit: g) of the laminated coating film at the end of the heat treatment.

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

(Example 2)
Except for using the thermosetting clear paint B (curing temperature = 140 ° C., weight reduction rate (140 ° C.) = 1.5 mass%) prepared in Preparation Example 7 instead of the thermosetting clear paint A as the clear paint. Prepared a laminated coating film in the same manner as in Example 1, and measured V and Wa to Wd. The relative loss elastic modulus of the non-curable water-based base paint A at the start of gelation of the thermosetting clear paint B was 0.35 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.

(Comparative Example 1)
Instead of the non-curable water-based base paint A, the heat-curable water-based base paint B prepared in Preparation Example 4 (curing temperature = 140 ° C., weight reduction rate (140 ° C.) = 1.6% by mass) is used as the base paint. A laminated coating film was prepared in the same manner as in Example 1 except that V and Wa to Wd were measured. The relative loss elastic modulus of the thermosetting water-based base coating material B at the start of gelation of the thermosetting clear coating material A was 1.1 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 clear from the results shown in Table 2, a non-curable water-based base paint is used for the lower layer of the uppermost layer, and a thermosetting clear paint that shrinks by heat treatment with a deblocking reaction is used for the uppermost layer. As for Wa to Wd of the laminated coating film of the present invention (Example 2), a thermosetting water-based base coating material is used for the lower layer of the uppermost layer, and a thermosetting clear coating material that does not shrink in the curing reaction by heat treatment is the uppermost layer. It was confirmed that the laminated coating film of Example 2 was superior to the laminated coating film of Comparative Example 1 both in terms of gloss and skin. In addition, Wa to Wd of the laminated coating film (Example 1) in which a non-curable water-based base paint is used for the lower layer of the uppermost layer and a thermosetting clear paint that does not shrink in the curing reaction by heat treatment is used for the uppermost layer. It is about 1/2 of the laminated coating film of Example 2 and 1/2 or less of the laminated coating film of Comparative Example 1, and the laminated coating film of Example 1 is very excellent in both gloss and skin. confirmed.

  As shown in FIG. 2, the laminated coating film of Example 1 has a volatile concentration V of up to about 2 mass% by heat treatment at 90 ° C., and the laminated coating film of Example 2 has a volatile concentration V of about 2.5 mass. %, While the laminated coating film of Comparative Example 1 was reduced only to about 4% by mass. As a result, in the heat treatment at 140 ° C., in Comparative Example 1, the volatile concentration V decreased by about 4% by mass, whereas in Example 1, the decrease in the volatile concentration V was suppressed to about 2% by mass. In Example 2, the decrease in the volatile concentration V was suppressed to about 2.5% by mass. In addition, the volatile matter concentration V in Example 2 includes the volatile matter generated by the deblocking reaction by heat treatment at 140 ° C.

  As is clear from the results shown in FIG. 2 and Table 2, the smaller the amount of decrease in the volatile component concentration V during the heat treatment at 140 ° C. (Example 1 <Example 2 <Comparative Example 1), the more Wa to Wd are. Example 1 <Example 2 <Comparative Example 1) With respect to gloss and skin, the laminated coating film of Example 1 is the best, followed by the laminated coating film of Example 2, and the laminated coating film of Comparative Example 1 It was confirmed that the film was inferior in gloss and skin.

  In each of the laminated coating films of Examples 1 and 2 and Comparative Example 1, the thermosetting clear paint starts to be cured by heat treatment at 140 ° C. (In the case of Example 2 as well, the deblocking reaction occurs quickly and immediately. However, after the fluidity of the thermosetting clear coating layer is significantly reduced by curing, the amount of decrease in the volatile component concentration V during the heat treatment at 140 ° C. Corresponding to the shrinkage of the laminated coating film occurs. Therefore, the smaller the amount of decrease in the volatile content concentration V during the heat treatment at 140 ° C., the more reliably the shrinkage after the fluidity of the thermosetting clear coating layer is significantly lowered by curing is suppressed. It was confirmed that the formation (prominence) of irregularities on the surface of the laminated coating film was further suppressed, Wa to Wd were further reduced, and gloss and skin were further improved.

(Example 3)
Instead of the non-curable water-based base paint A, a polyurethane resin-based thermosetting paint (manufactured by Kansai Paint Co., Ltd., trade name “Retan PG60 Kai”, two-component type (curing agent: isocyanate compound)) is used as a non-curable type. A laminated coating film was prepared in the same manner as in Example 1 except that this non-curable solvent-based base paint was applied so that the film thickness after heat treatment was 25 μm. Wd was measured. The weight reduction rate at 140 ° C. of the non-curable solvent-based base paint (the main component of the polyurethane resin thermosetting paint) was 0% by mass. Further, the relative loss elastic modulus of the non-curable solvent-based base paint at the start of gelation of the thermosetting clear paint A was 0.16 s ⁻² .

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

(Comparative Example 2)
A laminated coating film was prepared in the same manner as in Comparative Example 1 except that the thermosetting water-based base coating material B was applied so that the film thickness after heat treatment was 25 μm, and V and Wa to Wd were measured.

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

  As is apparent from the results shown in Table 3, Wa to Wd of the laminated coating film (Example 3) by wet-on-wet using a non-curing solvent-based base coating as the lowermost layer as in the present invention is as follows. Both are smaller than the multilayer coating film (Comparative Example 2) using a thermosetting water-based base coating as the lowermost layer, and the multilayer coating film of Example 3 is both glossy and skin better than the multilayer coating film of Comparative Example 2. It was also confirmed that it was improved.

  As shown in FIG. 3, the laminated coating film of Example 3 decreased to a volatile content concentration V of about 3 mass% by heat treatment at 90 ° C., whereas the laminated coating film of Comparative Example 2 decreased to about 4 mass%. Only a drop. As a result, in the heat treatment at 140 ° C., the volatile content concentration V decreased by about 4% by mass in Comparative Example 2, whereas the decrease in the volatile content V in Example 3 was suppressed to about 3% by mass. It was. The decrease in the volatile content concentration V during the heat treatment at 140 ° C. corresponds to the shrinkage of the laminated coating film. Accordingly, the multilayer coating film of Example 3 contracted after the fluidity of the thermosetting solvent-type clear coating layer was significantly reduced by curing during the heat treatment at 140 ° C. as compared with the multilayer coating film of Comparative Example 2. As a result, formation (prominence) of unevenness on the surface of the multilayer coating film is suppressed, Wa to Wd are all reduced, and gloss and skin are improved as compared with the multilayer coating film of Comparative Example 2. It was confirmed that

Example 4
The thermosetting clear coating material A (curing temperature = 140 ° C., weight loss rate (140 ° C.) = 0% by mass) prepared in Preparation Example 6 was used as the uppermost layer coating material, and the lower layer coating material was prepared in Preparation Example 1. Thermosetting water-based intermediate coating material A (curing temperature = 140 ° C., weight reduction rate (140 ° C.) = 1.6% by mass) and non-curable water-based base coating material A prepared in Preparation Example 3 (weight reduction rate (140 ° C. ) = 0% by mass). The relative loss elastic moduli of the thermosetting water-based intermediate coating material A and the non-curable water-based base coating material A at the start of gelation of the thermosetting clear coating material A are 4.7 s ^-2 and 0.29 s, respectively. ^-2 .

  The thermosetting water-based intermediate coating material A prepared in Preparation Example 1 is applied to the surface of an electrodeposition coating plate (made by Shinto Herberts, trade name “Sucsade 80V Gray”) so that the film thickness after heat treatment is 20 μm. , After heating at 100 ° C. for 3 minutes to volatilize water, organic solvent, etc., the non-curable aqueous base coating material A prepared in Preparation Example 3 is heat-treated on the thermosetting aqueous intermediate coating material A layer. Other than coating so that the thickness is 15 μm, and coating the thermosetting clear paint A prepared in Preparation Example 6 on the layer of the non-curable water-based base paint A so that the film thickness after the heat treatment is 35 μm. Prepared a laminated coating film in the same manner as in Example 1, and measured V and Wa to Wd.

  Table 4 shows Wa to Wd of the laminated coating film after the heat treatment. FIG. 4 shows the relationship between V and Wa during the heat treatment.

(Comparative Example 3)
Instead of the thermosetting water-based intermediate coating A, the thermosetting aqueous intermediate coating B prepared in Preparation Example 2 (curing temperature = 140 ° C., weight reduction rate (140 ° C.) = 3.3% by mass ) And the thermosetting water-based base paint B prepared in Preparation Example 4 instead of the non-curable water-based base paint A (curing temperature = 140 ° C., weight loss rate (140 ° C.) = 1.6 mass) %) Was used in the same manner as in Example 4 except that V and Wa to Wd were measured. The relative loss elastic modulus of the thermosetting water-based intermediate coating material B and the thermosetting water-based base coating material B at the start of gelation of the thermosetting clear coating material A is 7.5 s ⁻² and 1.1 s, respectively. ^-2 .

  Table 4 shows Wa to Wd of the laminated coating film after the heat treatment. FIG. 4 shows the relationship between V and Wa during the heat treatment.

(Comparative Example 4)
Instead of the non-curable water-based base paint A, the thermosetting water-based base paint C (curing temperature = 140 ° C., weight reduction rate (140 ° C.) = 0.8% by mass) prepared in Preparation Example 5 was used as the base paint. A laminated coating film was prepared in the same manner as in Example 4 except that V and Wa to Wd were measured. The relative loss elastic modulus of the thermosetting water-based base paint C at the start of gelation of the thermosetting clear paint A was 1.0 s ⁻² .

  Table 4 shows Wa to Wd of the laminated coating film after the heat treatment. FIG. 4 shows the relationship between V and Wa during the heat treatment.

  As is apparent from the results shown in Table 4, Wa to Wd of the laminated coating film (Example 4) by wet-on-wet using a non-curing type water-based base coating as the lowermost layer as in the present invention is any Is smaller than the multilayer coating film (Comparative Examples 3 to 4) using only the thermosetting water-based base coating as the lowermost layer, and the multilayer coating film of Example 4 is that of Comparative Examples 3 to 4 for both gloss and skin. It was confirmed that it was improved over the laminated coating film.

  As shown in FIG. 4, the laminated coating film of Example 4 decreased to a volatile content concentration V of about 2 mass% by heat treatment at 90 ° C., whereas the laminated coating films of Comparative Examples 3 and 4 each had about 5%. It decreased only to mass% and about 4 mass%. As a result, during the heat treatment at 140 ° C., in Comparative Examples 3 and 4, the volatile concentration V decreased by about 5 mass% and about 4 mass%, respectively, whereas in Example 4, the volatile concentration V decreased. It was suppressed to about 2% by mass. The decrease in the volatile content concentration V due to the heat treatment at 140 ° C. corresponds to the shrinkage of the laminated coating film. Therefore, after the fluidity of the layer of the thermosetting solvent-based clear coating material was significantly reduced by curing, the multilayer coating film of Example 4 was subjected to heat treatment at 140 ° C. as compared with the multilayer coating films of Comparative Examples 3 and 4. As a result, the formation (prominence) of unevenness on the surface of the multilayer coating film is suppressed, Wa to Wd are all reduced, and gloss and skin are more than those of the multilayer coating films of Comparative Examples 3 and 4. It was also confirmed that it was improved.

  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 4 using a thermosetting base paint for all of the lower layers, the lower layers are cured by heat treatment at 140 ° C., and at that time, the hydroxyl group of the acrylic resin and the melamine resin react to become volatile. Alcohol and water are produced. On the other hand, the fluidity of the layer of the thermosetting solvent-type clear paint is remarkably lowered by curing in the laminated coating films of Examples 1 to 4 in which the non-curable base paint is used for at least one of the lower layers. In the heat treatment at 140 ° C., the curing reaction does not substantially occur in the lower layer formed by the non-curing base paint, and no volatile product is generated. As a result, in Comparative Examples 1 to 4, the volatile product volatilizes and the coating film shrinks, whereas in Examples 1 to 4, the volatile product does not substantially form, so the coating film shrinks less. It is assumed that

  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 is particularly suitable for 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 in the heat processing of the laminated coating film produced in Examples 1-2 and the comparative example 1, and the wave scan value Wa. It is a graph which shows the relationship between the volatile matter density | concentration V in the heat processing of the laminated coating film produced in Example 3 and Comparative Example 2, and the wave scan value Wa. 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 Example 4 and Comparative Examples 3-4.

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 (9)

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,
A non-curing paint that does not cause a curing reaction by heat treatment is prepared as at least one of the lower-layer paints for forming the lower layer, and a thermosetting type is used as the upper-layer paint for forming the uppermost layer Preparing the paint,
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;
Applying a heat treatment to the uncured laminated coating to cure at least the uppermost layer coating;
The coating method characterized by including.   The coating method according to claim 1, wherein the non-curable coating material is a coating material having a weight reduction rate of 0.5% by mass or less at the curing temperature of the uppermost layer coating material.   The coating method according to claim 1, wherein the non-curable paint is a paint that does not contain a curing agent.   The said lower layer is two or more layers, and all the coating materials for lower layers for forming the said lower layer are non-curable coating materials, The coating method as described in any one of Claims 1-3 characterized by the above-mentioned. .   The coating method according to claim 1, wherein the uppermost layer-coating material is a coating material having a weight reduction rate of 0.5% by mass or less at the curing temperature.   The coating method according to any one of claims 1 to 5, wherein the uppermost layer-coating material is a coating material that does not generate a volatile product in a curing reaction by heat treatment.   After reducing the volatile content concentration of the uncured laminated coating film to 3.5% by mass or less, heat treatment is performed at a temperature of [the curing temperature of the uppermost layer coating material −20 ° C.] or higher to cure at least the uppermost layer coating material. The coating method according to claim 1, wherein the coating method is performed.   The uncured laminated coating film is subjected to a heat treatment at a temperature lower than [the curing temperature of the uppermost layer paint −20 ° C.], and then subjected to a heat treatment at a temperature equal to or higher than the [curing temperature of the uppermost layer paint −20 ° C.]. The coating method as described in any one of Claims 1-7 characterized by the above-mentioned.   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 1-8. Painted body obtained by the coating method of

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