JP5120952B2 - Coating method and coated body obtained thereby - Google Patents

Description

  The present invention relates to a coating method for laminating two or more kinds of paints wet-on-wet and subjecting them to thermosetting treatment, 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 coating method in which thermosetting treatment is performed, all layers that make up the laminated coating film after all of the coatings have been conventionally laminated A method of selecting a thermosetting coating material for forming each layer so as to cure at the same heating temperature and curing the entire laminated coating film has been used. However, the conventional coating method has a problem that the skin and gloss of the laminated coating film are inferior compared to the case where the lower layer is baked and then the coating for 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 Laid-Open No. 2002-35679 (Patent Document 1) discloses a coating in which an intermediate coating, a base coating, and a clear coating are sequentially applied on an electrodeposited material, and three layers are baked and cured at a time. In the film forming method, the minimum viscosity with respect to the temperature at a non-volatile content of 90% by mass satisfies the conditions of intermediate coating paint ≧ base coating ≧ clear coating, and the curing start temperature satisfies the conditions of intermediate coating ≦ base coating ≦ clear coating. It is disclosed that an excellent finished appearance can be obtained by using a paint.

  Japanese Patent Application Laid-Open No. 2005-177680 (Patent Document 2) uses a difference in curing speed when applying an intermediate coating, a top coating base coating, and a top clear coating in a wet-on-wet manner and baking them simultaneously. Thus, a coating method in which the intermediate coating film is cured prior to the top coating film is disclosed, and it is also disclosed that the sharpness can be reliably ensured by this coating method.

However, in the appearance quality of a typical automobile as a coating industry product, for example, a Wa value (wavelength <0.3 mm) by wave scan is required to be 15 or less. However, in the conventional coating method, a Wa value of 20 or less is required. Although it was possible to make it to the extent, it was difficult to make it 15 or less.
JP 2002-35679 A JP 2005-177680 A

  The present invention has been made in view of the above-described problems of the prior art, and even when at least the uppermost layer is cured to ensure high durability by laminating two or more types of paints by wet-on-wet, 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, 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 determined that at least one of the lowermost layers of the laminated coating film has a predetermined loss elastic modulus at the start of gelation of the uppermost layer coating material. By forming using a lower layer coating that is less than or equal to the value, the fluidity of the lower layer formed using the lower layer coating is ensured even at the start of gelation of the uppermost layer, and the uppermost layer is cured. It is possible to minimize the formation of irregularities due to shrinkage of the laminated coating after the fluidity is significantly reduced, and the appearance quality is improved even when two or more types of paints are laminated on a wet-on-wet and subjected to thermosetting treatment. The inventors have found that an excellent laminated coating film (for example, having a Wa value of 15 or less) can be obtained, 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,
A thermosetting paint is prepared as the uppermost layer paint for forming the uppermost layer, and the gelation of the uppermost layer paint is started as at least one of the lower layer paints for forming the lower layer. Preparing a paint having a loss modulus of elasticity of 1 MPa or less at the time;
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;
Subjecting the uncured laminated coating film to a thermosetting treatment to cure at least the uppermost layer coating;
It is the method characterized by including.

  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. In the coating method of the present invention, it is preferable to use a thermosetting paint having a weight reduction rate of 0.5% by mass or less at the curing temperature of the uppermost layer paint as at least one of the lower layer paints. .

  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 substrate and an uppermost layer formed on the lower layer, and obtained by the coating method of the present invention. It is what was characterized.

  Even when two or more kinds of paints are laminated by wet-on-wet by the coating method of the present invention and the thermosetting treatment is performed, the reason why the unevenness of the surface of the laminated coating film is not necessarily clear is not certain. Guess as follows. That is, in the conventional multilayer coating formed by wet-on-wet, thermosetting paint is used in all layers including the uppermost layer, and each layer is cured at the same heating temperature, or curing is started in order from the lower layer. Because it is designed, when the thermosetting paint forming the uppermost layer is cured by heat treatment, the thermosetting paint is already cured in the lower layer, and the fluidity has already been lost. . In each layer of the multilayer coating film, the thermosetting paint is cured by an addition reaction after the condensation reaction or deblocking reaction of the curing agent, so that the volatile products generated by this condensation reaction or deblocking reaction together with the remaining solvent It volatilizes, 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 if the fluidity of the top layer is significantly reduced by curing, the lower layer also cures and becomes fluid Therefore, it is presumed that the unevenness on the surface of the substrate and the interface of each layer is 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, at least one of the lower layers is formed by using the lower layer paint whose loss elastic modulus at the start of gelation of the upper layer paint is equal to or less than a predetermined value. In the lower layer formed using the lower layer paint even when the film is cured, fluidity is ensured, and even when the laminated coating film contracts and irregularities are formed on the coating film surface, the lower layer flows. It is assumed that the unevenness is relaxed, and the manifestation of the unevenness on the surface of the coating film is suppressed.

  According to the present invention, even if at least the uppermost layer is cured by laminating two or more kinds of paints on a wet-on-wet basis to ensure high durability, etc., a laminated coating film with less irregularities on the surface of the uppermost layer is obtained. Can do. 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 thermosetting paint is prepared as the uppermost layer paint for forming the uppermost layer, and the gelation of the uppermost layer paint is started as at least one of the lower layer paints for forming the lower layer. Preparing a paint having a loss modulus of elasticity of 1 MPa or less at the time;
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;
Subjecting the uncured laminated coating film to a thermosetting treatment to cure at least the uppermost layer coating;
It is the method 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 on the uncured lower layer, and if necessary, the solvent or the like is evaporated by drying or the like to form an uncured uppermost layer. Thereafter, the obtained uncured laminated coating film is subjected to a heat curing treatment 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, a thermosetting paint is used as the top layer paint. As such a thermosetting paint for the uppermost layer, for example, a thermosetting resin capable of forming a coating film and a curing agent (for example, a compound having two or more functional groups capable of reacting with the functional group of the thermosetting resin) Or a resin), and examples thereof include thermosetting paints (for example, clear paints described in JP-A No. 2004-275966) used as the uppermost layer paint for ordinary baking coating. The form 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, and urethane resins, but are not limited thereto. . 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. These resins and curing agents may be used alone or in combination of two or more.

The curing temperature T _T of the uppermost layer-coating material for use in the present invention is not particularly limited but is usually 40 ° C. or higher 200 ° C. or less, preferably 160 ° C. 60 ° C. inclusive. The uppermost layer-coating material is preferably a thermosetting coating material that does not substantially generate a volatile product in the curing reaction. 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 a paint having such 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 the heat curing treatment and the fluidity is remarkably lowered. . From such a point of view, a thermosetting paint (a weight reduction rate of 0% by mass) that does not generate a volatile product is most preferable.

In the present invention, the “coating temperature of the paint” refers to a curing time or the like for coating the target paint on the base material and applying a heat treatment to harden the coating film and fix it on the base material. It refers to the temperature at which curing can be performed most efficiently in relation to the curing conditions, and generally refers to 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 was coated on an aluminum foil as a target film thickness of a film thickness of the multilayer coating film after the heat treatment the coating of interest, the curing temperature T _T of the obtained aluminum foil sample the uppermost layer-coating material Gas chromatograph / mass spectrometer equipped with a thermal desorption introduction device (for example, Thermal Desorption System manufactured by GERSTEL) after drying for 90 minutes under a vacuum condition of 40 ° C. lower temperature [T _T −40 ° C.] and 10 ⁻² Torr or less. for example, Agilent Corp. 6890GC / 5975MSD) was heated for 30 minutes at a curing temperature _{T T} of the uppermost layer-coating material by the volatile amount of product using a (Rc (unit: g)) and to quantify the residual solvent amount, equation (1 ) To calculate the weight loss rate. 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 the present invention, as a preferable combination of the thermosetting resin and the curing agent of the uppermost layer coating material, 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 And / or a combination with a carboxyl group-containing resin. When a thermosetting paint composed of such a combination is used, it tends to be difficult to generate a volatile product in the heat treatment.

  The uppermost layer coating material used in 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 coating method of the present invention, at least one lower layer is formed on the substrate. At least one of the lower layers has a loss elastic modulus of 1 MPa or less (preferably at the start of gelation of the uppermost layer paint) Is 0.7 MPa or less, more preferably 0.5 MPa or less) (hereinafter referred to as “low-loss elastic modulus paint”). Specifically, when the lower layer is one layer, the lower layer is formed using the low-loss elastic modulus paint, and when the lower layer is two or more layers, at least one of them is formed as the low-loss elastic modulus. Form with paint. When there are two or more lower layers, it is preferable that the lower layer close to the uppermost layer is formed using the low-loss elastic modulus coating material in that the unevenness on the surface of the laminated coating film can be further relaxed. The low-loss elastic modulus paint is a paint having a loss elastic modulus of 1 MPa or less (preferably 0.7 MPa or less, more preferably 0.5 MPa or less) at the start of gelation of the uppermost layer coating material. A thermosetting type that forms a cross-linked structure or a non-cured type that does not cause a curing reaction and does not form a cross-linked structure may be used. On the other hand, it is preferable to use a non-curable coating material in terms of minimizing the shrinkage of the laminated coating film, and these can be appropriately used depending on the application.

  In the present invention, the “loss elastic modulus at the start of gelation of the uppermost layer coating material” is defined by the 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 heat 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 raised from room temperature (25 ° C.) to the curing treatment temperature T ₁ (the temperature at which the desired laminated coating film is baked and generally the curing temperature of the paint for the uppermost layer is adopted). The temperature is increased at a rate of 20 ± 4 ° C./min, and then the curing treatment temperature T ₁ is maintained. The measurement is carried out until 15 minutes or more have passed since the following inflection point.

When the measured value of the relative storage elastic modulus (E _r ′) obtained is plotted with respect to time, as shown in FIG. This change time is called “inflection point”. For the portion of 15 minutes after this inflection point, the following formula (2):
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 paint, the coating film is formed by adjusting the coating film thickness so that the film thickness after the heat treatment becomes a predetermined value as in the case of the uppermost layer paint, The relative loss elastic modulus (E _r ″) is measured under the same conditions as in the case of the uppermost layer coating material. The measurement is carried out until the target heat-treatment time (baking time) t _{1 of} the laminated coating film, and the time t _d and the relative loss elastic modulus (E _r ″ (t _d ) and E _r ″ (t ₁ )) of the coating film made of the lower layer coating material at the thermosetting time t ₁ are obtained.

Next, the target lower layer coating material is applied to a base material having a smooth surface from which a coating film after heat treatment such as a glass plate or a polypropylene plate can be peeled, and the film thickness after heat treatment becomes a predetermined value. The coating is applied while adjusting the coating thickness, and a predetermined preliminary drying treatment is performed as necessary, and the coating film is subjected to a thermosetting treatment under the target thermosetting condition of the laminated coating film. Then, the obtained cured film is peeled from the substrate. This cured film is cut into a predetermined shape, and using a dynamic viscoelasticity measuring apparatus (for example, DVA-220 model manufactured by IT Measurement Control Co., Ltd.), a tensile mode, a frequency of 1 Hz, a temperature rising rate of 5 ° C. / The loss elastic modulus (E ″) of the cured film is measured under the conditions of minutes, and the loss elastic modulus (E ″ (T ₁ )) of the cured film made of the lower layer coating material at the curing treatment temperature T ₁ is obtained.

The relative loss elastic modulus E _r ″ (t ₁ ) of the coating film made of the lower layer coating material at the thermosetting treatment time t ₁ can be regarded as the relative loss elastic modulus of the cured film made of the lower layer coating material. From the relative loss elastic moduli E _r ″ (t _d ) and E _r ″ (t ₁ ) of the coating film made of the obtained lower layer coating material and the loss elastic modulus E ″ (T ₁ ) of the cured film made of the lower layer coating material, Following formula (3):
E ″ (t _d ) = E _r ″ (t _d ) × E ″ (T ₁ ) / E _r ″ (t ₁ ) (3)
The loss elastic modulus (E ″ (t _d )) of the coating film made of the lower layer paint at the time t _d is obtained by the above-described method. In the present invention, this is expressed as “the lower layer paint at the start of gelation of the uppermost layer paint”. Loss elastic modulus ".

In the coating method of the present invention, at least one of the lower layer paint, the weight reduction rate in curing temperature T _T of the uppermost layer-coating material is preferably at most 0.5 mass% for use, It is more preferably 0.3% by mass or less, and particularly preferably 0.1% by mass or less. When at least one kind of lower layer coating material having such a small weight reduction rate is used, the shrinkage of the coating film tends to be minimized after the uppermost layer is cured by heat curing and the fluidity is remarkably lowered. From this point of view, the lower layer paint (weight reduction rate is 0% by mass) that does not generate a volatile product when the uppermost layer is thermally cured is most preferable.

  The thermosetting low-loss elastic modulus paint used in the present invention has a loss elastic modulus of 1 MPa or less (preferably 0.7 MPa or less, more preferably 0.5 MPa or less) at the start of gelation of the uppermost layer coating material. If it is a thing, it is possible to use suitably the thermosetting coating material used for normal baking coating, for example, the thermosetting resin and hardening agent (for example, the said thermosetting resin which can form a coating film) The loss elastic modulus at the start of gelation of the uppermost layer-coating material is 1 MPa or less (preferably 0.7 MPa or less, more preferably 0.7 MPa or less). 5 MPa or less). The form of the thermosetting low-loss elastic modulus paint may be any of solvent type, water-based, and powder, but is preferably water-based or powder from the viewpoint that the discharge amount of volatile organic compounds can be reduced.

Also, the thermosetting type that does not substantially generate volatile products when curing the top layer in that it can reduce the shrinkage of the coating after the top layer is cured and the fluidity is significantly reduced by the thermosetting process. It is preferable to use a low-loss elastic modulus paint. As such a coating material, the weight reduction rate at the curing temperature T _T of the 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, and 0.1% by mass. % Or less is particularly preferred. When such a thermosetting low-loss elastic modulus coating material with a small weight reduction rate is used as the coating material for the lower layer, the shrinkage of the coating film can be minimized after the uppermost layer is cured by the thermosetting treatment and the fluidity is remarkably lowered. There is a tendency. From such a viewpoint, a thermosetting low-loss elastic modulus paint (weight reduction rate of 0% by mass) that does not generate a volatile product when the uppermost layer is cured is most preferable.

  Examples of the thermosetting resin capable of forming a coating film contained in such a thermosetting low-loss elastic modulus coating material include acrylic resins, polyester resins, alkyd resins, epoxy resins, and urethane resins. Moreover, as a hardening | curing agent contained in the said thermosetting type low loss elastic modulus coating material, an isocyanate compound, an isocyanate resin, an amine compound, an amino resin, etc. are mentioned. Each of these resins and curing agents may be used alone or in combination of two or more. In the present invention, by adjusting the composition and blending ratio of the thermosetting resin and the curing agent, or by blending additives, for example, by lowering the glass transition temperature, the crosslinking density, etc., the thermosetting type The loss elastic modulus at the start of gelation of the uppermost layer-coating material for the low-loss elastic modulus coating material can be 1 MPa or less (preferably 0.7 MPa or less, more preferably 0.5 MPa or less).

  On the other hand, the non-curing low-loss elastic modulus coating material used in the present invention has a loss elastic modulus of 1 MPa or less (preferably 0.7 MPa or less, more preferably 0.5 MPa or less) at the start of gelation of the uppermost layer coating material. ), It is possible to use a non-curing coating material that does not substantially cause a curing reaction when the uppermost layer is cured, for example, it contains a resin capable of forming a coating film and does not contain a curing agent, and Examples include those having a loss elastic modulus of 1 MPa or less (preferably 0.7 MPa or less, more preferably 0.5 MPa or less) at the start of gelation of the uppermost layer coating material. The form of the non-curable low-loss elastic modulus paint may be any of solvent type, water-based, and powder, but is preferably water-based or powder from the viewpoint that the discharge amount of volatile organic compounds can be reduced.

In addition, it is possible to reduce the shrinkage of the coating after the uppermost layer is cured and the fluidity is remarkably lowered by the heat curing treatment. It is preferable to use a low-loss elastic modulus paint. As such a coating material, the weight reduction rate at the curing temperature T _T of the 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, and 0.1% by mass. % Or less is particularly preferred. When such a non-curing low-loss elastic modulus coating material having a small weight reduction rate is used, the shrinkage of the laminated coating film after the uppermost layer is cured by heat curing treatment and the fluidity is remarkably lowered tends to be reduced. From such a viewpoint, a non-curing low-loss elastic modulus paint (weight reduction rate of 0% by mass) that does not generate a volatile product when the uppermost layer is cured is most preferable.

  As a resin capable of forming a coating film contained in such a non-curable low-loss elastic modulus paint, a resin that does not cause a curing reaction by itself is preferable. For example, an intermediate coating paint described in JP-A-2004-275966 And resin components from which the curing agent is removed from the base paint and the like, specifically, an acrylic resin, a polyester resin, an alkyd resin, an epoxy resin, a urethane resin, and the like, but are not limited thereto. Two or more of these resins that do not cause a curing reaction may be selected and used in combination. In the present invention, by adjusting the glass transition temperature, molecular weight, etc. of the resin capable of forming a coating film, the loss elastic modulus at the start of gelation of the uppermost layer coating material for the non-curing low loss elastic modulus coating material is 1 MPa. Or less (preferably 0.7 MPa or less, more preferably 0.5 MPa or less).

  In the present invention, it is preferable to use a thermosetting low-loss elastic modulus paint in terms of ensuring the strength of the laminated coating film among the thermosetting low-loss elastic modulus paint and the non-curable low-loss elastic modulus paint, On the other hand, it is preferable to use a non-curing low-loss elastic modulus paint in terms of minimizing the shrinkage of the laminated coating film, and these can be appropriately used depending on the application.

  In the present invention, when the lower layer has two or more layers, if at least one of them is a layer formed using the low-loss elastic modulus coating material, the remaining layers are the top layer coating material. You may form using the coating material (henceforth "high loss elastic modulus coating material") whose loss elastic modulus at the time of a gelation start exceeds 1 Mpa. As such a high loss elastic modulus coating material, if the loss elastic modulus at the start of gelation of the uppermost layer coating material exceeds 1 MPa, even a thermosetting type that forms a cross-linked structure by heat treatment can be cured. However, it is preferable to use a thermosetting paint from the viewpoint of ensuring the strength of the laminated coating film, while minimizing the shrinkage of the laminated coating film. It is preferable to use a non-curable paint from the viewpoint of being able to be used, and these can be properly used depending on the application.

  As the thermosetting type high loss elastic modulus coating material used in the present invention, the thermosetting coating material used for ordinary baking coating is applicable as long as the loss elastic modulus at the start of gelation of the uppermost layer coating material exceeds 1 MPa. For example, 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). And the loss elastic modulus at the start of gelation of the coating for the uppermost layer exceeds 1 MPa. The form of the thermosetting high-loss elastic modulus coating material may be any of solvent type, aqueous type, and powder, but is preferably aqueous or powder from the viewpoint that the discharge amount of volatile organic compounds can be reduced.

Also, the thermosetting type that does not substantially generate volatile products when curing the top layer in that it can reduce the shrinkage of the coating after the top layer is cured and the fluidity is significantly reduced by the thermosetting process. It is preferable to use a high-loss elastic modulus paint. As such a coating material, the weight reduction rate at the curing temperature T _T of the 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, and 0.1% by mass. % Or less is particularly preferred. When such a thermosetting high-loss elastic modulus coating material with a small weight loss rate is used as the coating for the lower layer, the shrinkage of the coating film can be minimized after the uppermost layer is cured by the thermosetting process and the fluidity is remarkably lowered. There is a tendency. From such a viewpoint, a thermosetting high-loss elastic modulus paint (weight reduction rate of 0% by mass) that does not generate a volatile product when the uppermost layer is cured is most preferable.

  Examples of the thermosetting resin capable of forming a coating film contained in such a thermosetting high-loss elastic modulus coating material include acrylic resins, polyester resins, alkyd resins, epoxy resins, and urethane resins. Moreover, as a hardening | curing agent contained in the said thermosetting type high loss elastic modulus coating material, an isocyanate compound, an isocyanate resin, an amine compound, an amino resin, etc. are mentioned. Each of these resins and curing agents may be used alone or in combination of two or more. In the present invention, by adjusting the composition and blending ratio of the thermosetting resin and the curing agent, for example, by increasing the glass transition temperature, the crosslinking density, etc., the uppermost layer for the thermosetting high-loss elastic modulus paint The loss elastic modulus at the start of gelation of the coating material can be over 1 MPa.

  On the other hand, as the non-curable high-loss elastic modulus coating material used in the present invention, it is possible to use a non-curable coating material as long as the loss elastic modulus at the start of gelation of the uppermost layer coating material exceeds 1 MPa. For example, a resin that can form a coating film, does not contain a curing agent, and has a loss elastic modulus of more than 1 MPa at the start of gelation of the uppermost layer-coating material. The form of the non-curing type high-loss elastic modulus coating material may be any of a solvent type, an aqueous type, and a powder, but an aqueous type or a powder is preferable from the viewpoint that the discharge amount of the volatile organic compound can be reduced.

In addition, it is possible to reduce the shrinkage of the coating after the uppermost layer is cured and the fluidity is remarkably lowered by the heat curing treatment. It is preferable to use a high-loss elastic modulus paint. As such a coating material, the weight reduction rate at the curing temperature T _T of the 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, and 0.1% by mass. % Or less is particularly preferred. When such a non-curing type high-loss elastic modulus coating material with a small weight reduction rate is used as the coating material for the lower layer, the shrinkage of the coating film can be minimized after the uppermost layer is cured by heat curing and the fluidity is remarkably lowered. There is a tendency. From such a viewpoint, a non-curing type high-loss elastic modulus paint (weight reduction rate of 0% by mass) that does not generate a volatile product when the uppermost layer is cured is most preferable.

  As a resin capable of forming a coating film contained in such a non-curing type high-loss elastic modulus coating material, a resin that does not cause a curing reaction by itself is preferable. For example, an intermediate coating material described in JP-A-2004-275966 And resin components from which the curing agent is removed from the base paint and the like, specifically, an acrylic resin, a polyester resin, an alkyd resin, an epoxy resin, a urethane resin, and the like, but are not limited thereto. Two or more of these resins that do not cause a curing reaction may be selected and used in combination. In the present invention, by adjusting the glass transition temperature, molecular weight, etc. of the resin capable of forming the coating film, the loss elastic modulus at the start of gelation of the uppermost layer coating material for the non-curing type high loss elastic modulus coating material is 1 MPa. Can be exceeded.

  In the present invention, the top layer having the same curing temperature is used when the thermosetting paint is used in both the top layer paint and the bottom layer paint (low loss elastic modulus paint and / or high loss elastic modulus paint). It is also possible to use a paint for a lower layer and a paint for a lower layer, and it is also possible to use a paint for an uppermost layer and a paint for a lower layer having different curing temperatures. The former case is suitable when the heat treatment described later is performed in a single stage, while the latter case is suitable when two or more stages of heat treatment are performed.

In the latter case, the curing temperature T _U of the curing temperature T _T and the lower layer-coating material of the uppermost layer-coating material, the following formula (4):
T _U ≦ T _T -30 (4)
It is preferable that the condition represented by If a coating material that satisfies the condition expressed by the above formula (4) is used as the lower layer coating, it is possible to perform a two-stage heat treatment of a low-temperature heat treatment and a high-temperature heat treatment, and a sufficient difference between these heating temperatures. As a result, the top layer and the bottom layer can be cured separately and independently. Further, the curing temperature _{T U} and _{T T} From this point of view the following formula (4a)
_{T U ≦ T T -40 (4a} )
(Formula (4a) in, _{T U} and _{T T} are as defined _{T U} and _{T T} in the formula (4).)
It is more preferable to satisfy the condition expressed in the conditions when the lower layer of two or more layers, which for any lower layer paint curing temperature T _U represented by the formula (4) or (4a) It is particularly preferable to satisfy it.

  The lower layer coating material (low-loss elastic modulus coating material and / or high-loss elastic modulus coating material) used in the present invention includes conventionally known color pigments, glitter pigments, and the like within a conventionally known range, if necessary. It may be. 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 low-loss elastic modulus paint. When there are two or more lower layers, at least one of them may be formed using the low-loss elastic modulus paint, and the remaining layers may be formed using the high-loss elastic modulus paint, The lower layer may be formed using the low-loss elastic modulus paint.

  When applying the coating for the lower layer, any known method such as air spray coating, air electrostatic spray coating, or rotary atomizing electrostatic coating can be used regardless of whether a thermosetting coating or non-curing coating is used. Can be applied.

  Although the film thickness of each lower layer can be appropriately set depending on the desired application, for example, the film thickness after the heat treatment is preferably 5 to 50 μm, and 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. Examples of a method for applying the uppermost layer coating material include conventionally known methods such as air spray coating, air electrostatic spray coating, and rotary atomizing electrostatic coating.

  The film thickness of the uppermost layer can be appropriately set depending on the desired application. For example, the film thickness after the 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 way, 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.

When performing the heat treatment in the coating method of the present invention, one step of heat treatment may be performed, or two or more steps of heat treatment may be performed, but at least at a temperature above the temperature at which the uppermost layer is cured. process (hereinafter, referred to as "high-temperature heat treatment", the heating temperature at this time is "high temperature heating temperature T _H"), for example [hardening of the uppermost layer-coating material temperature T _T -20 ° C.] temperature above T _H It is preferable to perform the heat treatment at This high-temperature heating temperature _TH is expressed by the following formula (5) in either case of one-stage heat treatment or two-stage or more heat treatment:
T _T -20 ≦ T _H ≦ T _T +40 (5)
It is more preferable to satisfy the condition represented by the following formula (5a):
T _T ≦ T _H ≦ T _T +20 (5a)
It is particularly preferable to satisfy the condition represented by the following formula (5b):
T _T = T _H (5b)
It is most preferable that the condition represented by Incidentally, the formula (5), _{T T} in (5a) and (5b) has the same meaning as _{T T} in the formula (4). When the high temperature heating temperature is less than the lower limit, the uppermost layer tends to be not sufficiently cured, and when the upper limit is exceeded, the uppermost layer is excessively cured and tends to be broken or yellowed.

  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 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 or more and 45 minutes or less, and more preferably 18 minutes or more and 30 minutes or less. 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 lower temperature than the high temperature heating temperature T _H in front of the high-temperature heat treatment (hereinafter, referred to as "low temperature heating temperature T _L") heat treatment (hereinafter, referred to as "low temperature heating treatment" ) May be performed in two or more stages. It tends to be possible to reduce the concentration of volatile components in the laminated coating film before the uppermost layer is cured by this low-temperature heat treatment. Furthermore, in view of being able to prevent the top layer of the curing at a low temperature heat treatment, the low heating temperature T _L is the temperature of less than [the curing temperature T _T -20 ° C. of the uppermost layer-coating material] is preferably, [T _T A temperature of less than -30 ° C is more preferred, and a temperature of less than [T _T -40 ° C] is particularly preferred.

Furthermore, when using a thermosetting paint as the lower layer paint, the low temperature heating temperature _TL is expressed by the following formula (6) in that the lower layer can be sufficiently cured before the uppermost layer is cured.
T _U −20 ≦ T _L ≦ T _T −30 (6)
It is more preferable to satisfy the condition represented by the following formula (6a):
T _U ≦ T _L ≦ T _T −30 (6a)
It is particularly preferable to satisfy the condition represented by the following formula (6b):
T _U + 10 ≦ T _L ≦ T _T −40 (6b)
It is most preferable that the condition represented by Note that equation (6), (6a) is _{T U} and _{T T} in and (6b) is synonymous with _{T U} and _{T T} in the formula (4).

  Regarding the low-temperature heating time, when using a non-curable coating as the lower layer coating, the volatile content concentration of the laminated coating can be reduced without substantially curing the uppermost layer. It is preferably 10% or more and less than 50% of the curing time of the upper layer coating material, and more preferably 20% or more and 40% or less. Further, when a thermosetting paint is used as the lower layer paint, it is preferably 10% or more and 100% or less, more preferably 20% or more and 80% or less of the curing time of the lower layer paint. 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.

  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 before the heat treatment, particularly before the low temperature heat treatment. . The setting time is usually set to 1 to 20 minutes.

  Further, in the present invention, in order to obtain a higher-grade appearance, a coating is further applied on the uppermost layer of the coated body obtained by the coating method, and a thermosetting treatment is performed to form a surface layer. It is preferable. 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 loss elastic modulus of the lower layer coating material and the weight reduction rate due to the heat treatment of the coating material at the start of gelation of the uppermost layer coating material were measured by the following methods.

<Loss elastic modulus of the lower layer coating material at the start of gelation of the uppermost layer coating material>
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 the 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 is set so that the temperature is raised from room temperature (25 ° C.) to the curing temperature T _{T of the} paint for the uppermost layer at a rate of temperature increase of 20 ± 4 ° C./min, and then the curing temperature T _T is maintained. did.

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 paint, the coating film is formed by adjusting the coating film thickness so that the film thickness after the heat treatment becomes a predetermined value as in the case of the uppermost layer paint, The relative loss elastic modulus (E _r ″) is measured up to the baking time t ₁ of the target laminated coating film under the same conditions as in the case of the uppermost layer coating material, and the lower layer coating material at the time t _d and the baking time t ₁ . The relative loss elastic moduli (E _r ″ (t _d ) and E _r ″ (t ₁ ), respectively) of the coating film made of

Next, the target lower layer coating material is applied to a glass plate or polypropylene plate with a smooth surface while adjusting the coating film thickness so that the film thickness after the heat treatment becomes a predetermined value, and a preliminary drying treatment is performed. It was baked under the baking conditions of the target laminated coating film to obtain a cured film. Thereafter, the cured film was peeled from the substrate and cut into a predetermined shape. Using a dynamic viscoelasticity measuring apparatus (for example, DVA-220 type manufactured by IT Measurement Control Co., Ltd.), the loss elastic modulus of the cured film under the conditions of a tensile mode, a frequency of 1 Hz, and a heating rate of 5 ° C./min. (E ″) was measured, and the loss elastic modulus (E ″ (T _T )) of the cured film composed of the lower layer coating material at the curing temperature T _T was determined.

The relative loss elastic modulus E _r ″ (t ₁ ) of the coating film made of the lower layer coating material at the baking time t ₁ is regarded as the relative loss elastic modulus of the cured film made of the lower layer coating material, and the lower layer coating material obtained by the above measurement is used. From the relative loss elastic moduli E _r ″ (t _d ) and E _r ″ (t ₁ ) of the coating film made of and the loss elastic modulus E ″ (T _T ) of the cured film made of the coating material for the lower layer, the following formula (3):
E ″ (t _d ) = E _r ″ (t _d ) × E ″ (T _T ) / E _r ″ (t ₁ ) (3)
The loss elastic modulus (E ″ (t _d )) of the coating film made of the lower layer paint at the time t _d is obtained by the above, and this is referred to as “loss elastic modulus of the lower layer paint at the start of gelation of the uppermost layer paint”. did.

<Measurement of weight loss rate>
Coating a coating of interest on an aluminum foil so that the film thickness after heat treatment becomes a target thickness of the multilayer coating film, the resulting 40 ° C. than the curing temperature T _T of the aluminum foil sample the uppermost layer-coating material A gas chromatograph / mass spectrometer (for example, Agilent) equipped with a thermal desorption introduction device (for example, Thermal Destruction System manufactured by GERSTEL) after drying at low temperature [40-T _T ° C.] and a vacuum condition of 10 ⁻² Torr or less for 90 minutes. GMBH made 6890GC / 5975MSD) was heated for 30 minutes at a curing temperature _{T T} of the uppermost layer-coating material by using a volatile amount of product (Rc (unit: g)) and to quantify the residual solvent amount, by weight by the formula (1) The reduction rate was calculated. 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 Emulsion R-1 The following monomers were mixed to prepare a monomer mixture.
<Monomer mixture composition>
Methyl methacrylate 69.3 parts by weight Butyl acrylate 94.5 parts by weight 2-hydroxyethyl acrylate 63.0 parts by weight Styrene 78.8 parts by weight Acrylic acid 9.5 parts by weight This monomer mixture 315 parts by weight, n-dodecyl mercaptan 4 Part by mass, 105 parts by mass of water and 14 parts by mass of an anionic surfactant (“New Coal 707-SN” manufactured by Nippon Emulsifier Co., Ltd.) were mixed and stirred using a mixer to emulsify to prepare a monomer pre-emulsion. .

  Next, 280 parts by mass of water and an anionic surfactant (manufactured by Nippon Emulsifier Co., Ltd.) were added to a reaction vessel for producing an acrylic resin emulsion equipped with a stirrer, a thermometer, a dropping funnel, a reflux condenser and a nitrogen introduction tube. 5.6 parts by mass of Newcol 707-SN ") and 0.7 parts by mass of an aqueous ammonium persulfate solution (ammonium persulfate (manufactured by Aldrich)) and 13.3 parts by mass of water as a polymerization initiator ) 20 parts by mass were charged and heated to 80 ° C. with stirring. To this solution, 5% by mass of the monomer pre-emulsion was added and held at 80 ° C. for 10 minutes. Thereafter, the remaining monomer pre-emulsion was added dropwise with stirring over 4 hours. After completion of the dropwise addition, stirring was further continued at 80 ° C. for 1 hour for reaction. Thereafter, 56 parts by mass of water was added and cooled to room temperature. After cooling, the pH of the reaction solution was adjusted to 7.2 using a 10% by mass dimethylethanolamine aqueous solution to obtain an acrylic emulsion R-1 having a nonvolatile content of 38.1% by mass.

(Synthesis example 2) Synthesis | combination of acrylic emulsion R-2 The following monomer was mixed and the monomer liquid mixture was prepared.
<Monomer mixture composition>
Methyl methacrylate 10.7 parts by mass Butyl acrylate 203.2 parts by mass 2-hydroxyethyl methacrylate 50.4 parts by mass Styrene 42.5 parts by mass Acrylic acid 8.2 parts by mass In place of the monomer mixture described in Synthesis Example 1, Acrylic emulsion R-2 having a non-volatile content of 38.1% by mass and a glass transition temperature of −20 ° C. in the same manner as in Synthesis Example 1 except that 315 parts by mass of the monomer mixture was used and the pH of the reaction solution was adjusted to 7.4. Got.

(Synthesis example 3) Synthesis | combination of acrylic emulsion R-3 The following monomer was mixed and the monomer liquid mixture was prepared.
<Monomer mixture composition>
Methyl methacrylate 48.8 parts by weight Butyl acrylate 133.6 parts by weight 2-Hydroxyethyl methacrylate 50.4 parts by weight Styrene 74.0 parts by weight Acrylic acid 8.2 parts by weight In place of the monomer mixture described in Synthesis Example 1, An acrylic emulsion R-3 having a non-volatile content of 38.1% by mass and a glass transition temperature of 10 ° C. was prepared in the same manner as in Synthesis Example 1 except that 315 parts by mass of the monomer mixture was used and the pH of the reaction solution was adjusted to 7.4. Obtained.

(Synthesis Example 4) 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, methylstyrene dimer (trade name “MSD-100”, manufactured by Mitsui Toatsu Chemical Co., Ltd.) 5.0 parts by weight and azoisobutyronitrile 13.0 parts by weight were mixed to prepare a mixed solution.

  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 was added and heated while stirring to raise the temperature. . Thereafter, the remaining 93.0 parts by mass of the mixture 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 performed 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 5) 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 was prepared by mixing 0.8 parts 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 liquid (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 Colored Pigment Paste In a SUS container, 123 parts by weight of water, 30 parts by weight of urethane dispersion (“Hydran WLS-202” manufactured by DIC Corporation), a wetting dispersant (“Disperbyk 181” manufactured by BYK Chemie) ) 1.5 parts by mass, defoaming agent (“SN deformer 1340” manufactured by San Nopco) and rutile titanium oxide (“CR-90-2” manufactured by Ishihara Sangyo Co., Ltd.) 323.4 After charging the mass part and premixing for 3 minutes, glass beads (particle size: 1.6 mm) having the same volume as the total charged volume were charged and dispersed for 1 hour in a desktop sand mill. The particle size at the end of dispersion as measured with a grind gauge was 5 μm or less.

(Preparation Example 2) Preparation of thermosetting (melamine curable) waterborne intermediate coating P-1 In a container, 244.9 parts by mass of the color pigment paste obtained in Preparation Example 1 was charged, and this was a synthesis example while stirring. 170.6 parts by mass of acrylic emulsion R-1 obtained in 1 and 40.3 parts by mass of methylated melamine resin (“Cymel 325” manufactured by Nippon Cytec Industries, Ltd.) were added and stirred for 5 minutes. Thereafter, 20 parts by mass of water, 8 parts by mass of butyl diglycol and 16 parts by mass of butyl glycol were added and stirred for 5 minutes. Further, an alkali thickener (“Viscalex HV30” manufactured by Ciba Specialty Chemicals Co., Ltd.), dimethylethanolamine and water are added in an appropriate amount, and a melamine curable aqueous intermediate coating P- having a non-volatile content of 48.3% by mass and a pH of 8.4 1 was obtained. The curing temperature of this aqueous intermediate coating P-1 was 140 ° C. Further, the total pigment content mass (%) (hereinafter referred to as “PWC”) with respect to the total solid content mass in the formulation of the aqueous intermediate coating material P-1 was 42.

(Preparation Example 3) Preparation of thermosetting (melamine curable) aqueous intermediate coating P-2 337 parts by mass of acrylic resin varnish A having a solid content concentration of 75% by mass prepared in Synthesis Example 4 and titanium oxide ( Ishihara Sangyo Co., Ltd., trade name “CR-93”) 1000 parts by mass and carbon black (Degussa, trade name “FW-200P”) 10 parts by mass were charged, followed by butyl acetate 163 parts by mass and xylene 84 parts by mass. And were added. Thereafter, glass beads (particle size 1.6 mm) having the same weight as the total weight 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 added so that the solids weight 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 melamine curable aqueous intermediate coating material P-2 having a solid content concentration of 50% by mass. The curing temperature of this aqueous intermediate coating P-2 was 140 ° C.

(Preparation Example 4) Preparation of thermosetting (melamine curable) water-based base paint B-1 A container is charged with 183.7 parts by mass of the acrylic emulsion R-2 obtained in Synthesis Example 2, and this is stirred with methyl. 40 parts by mass of melamine resin (“Cymel 325” manufactured by Nippon Cytec Industries, Ltd.), 150 parts by mass of water and 20 parts by mass of butyl glycol were added and stirred for 5 minutes. Further, an appropriate amount of an alkali thickener (“Viscalex HV30” manufactured by Ciba Specialty Chemicals), dimethylethanolamine and water was added to obtain an aqueous resin solution having a nonvolatile content of 23 mass% and a pH of 8.5.

  In another container, 53 parts by mass of butyl glycol and 5 parts by mass of a phosphoric ester compound (“Lublizol 2062” manufactured by Nippon Lubrizol Co., Ltd.) were charged and stirred for 5 minutes. To this solution, 20.0 parts by mass of two types of aluminum paste (“Hydrolan 2154” manufactured by ECKART GmbH and “Hydrolan 2156” manufactured by ECKART GmbH) were added, respectively, and then stirred for 1 hour to obtain an aluminum paste solution.

  Next, 101.6 parts by mass of this aluminum paste solution was added to 478 parts by mass of the aqueous resin solution while stirring, and the mixture was further stirred for 1 hour to have a nonvolatile content of 24.7% by mass and pH 8.0, a melamine curable aqueous base. A paint B-1 was obtained. The weight reduction rate of this water-based base coating material B-1 at 140 ° C. was 3.6% by mass (calculated as P = 22.4).

(Preparation Example 5) Preparation of Thermosetting (Melamine Curing Type) Water-Based Paint B-2 Except for using 183.7 parts by mass of the acrylic emulsion R-3 obtained in Synthesis Example 3 instead of the acrylic emulsion R-2 In the same manner as in Preparation Example 4, a melamine curable aqueous base paint B-2 having a nonvolatile content of 24.7% by mass and a pH of 8.0 was obtained. The weight reduction rate of this water-based base coating material B-2 at 140 ° C. was 3.8% by mass (calculated as P = 22.4).

(Preparation Example 6) Preparation of non-curable water-based base coating material B-3 The acrylic resin varnish B having a solid content concentration of 60% by mass prepared in Synthesis Example 5 was added to 1-t-butoxy-2- with the same weight as the solid content. Propanol is added, and 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 solid content concentration is 20% by mass. An aqueous base paint B-3 was prepared. The weight loss rate of this non-curable water-based base coating material B-3 at 140 ° C. was 0% by mass.

(Preparation Example 7) Preparation of thermosetting (isocyanate curable) clear paint A Main component of two-component thermosetting (isocyanate curable) clear paint by mixing polyol and additives in the proportions shown in Table 1. 51 parts by weight were prepared. 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 the following Examples and Comparative Examples, a mixture (solid content concentration 55% by mass) of this main ingredient and a curing agent mixed at a ratio shown in Table 1 was used as the thermosetting (isocyanate curing type) clear paint 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. The time _{t d} to gelation starts was 10.2 minutes.

Example 1
The thermosetting clear coating material A (curing temperature = 140 ° C., weight loss rate (140 ° C.) = 0% by mass) prepared in Preparation Example 7 was used as the uppermost layer coating material, and the lower layer coating material was prepared in Preparation Example 2. Use melamine curable aqueous intermediate coating P-1 (curing temperature = 140 ° C.) and melamine curable aqueous base coating B-1 prepared in Preparation Example 4 (weight reduction rate (140 ° C.) = 3.6% by mass) did. The loss elastic moduli of the aqueous intermediate coating material P-1 and the aqueous base coating material B-1 at the start of gelation of the clear coating material A were 2.4 MPa and 0.37 MPa, respectively.

  The aqueous intermediate coating P-1 is applied to the surface of an electrodeposition coating plate (trade name “Sucsade 80V Gray” manufactured by Shinto Herberts Co., Ltd.) so that the film thickness after heat treatment is 30 μm. Water and an organic solvent are volatilized by heating for a minute, and the aqueous base coating material B-1 is applied on the layer of the aqueous intermediate coating material P-1 so that the film thickness after the heat treatment becomes 20 μm. Heat at 3 ° C. for 3 minutes to volatilize water and organic solvent, and apply the clear paint A on the layer of the aqueous base paint B-1 so that the film thickness after the heat treatment is 35 μm, and cure melamine An uncured laminated coating film obtained by laminating a mold-type waterborne intermediate coating P-1, a melamine curable aqueous base paint B-1, and a thermosetting clear paint A by wet-on-wet was obtained. The uncured multilayer coating film was allowed to stand (setting) at room temperature for 10 minutes, and then subjected to a heat treatment at 140 ° C. for 30 minutes to obtain a multilayer coating film.

  About the obtained laminated coating film, the wave scan value [Wa (wavelength <0.3 mm), Wb (wavelength 0.3 to 1 mm), Wc (wavelength) using wave scan (Wave-Scan Dual manufactured by BYK-Gardner)] 1-3 mm), Wd (wavelength 3-10 mm)]. The results are shown in Table 2. These wave scan values mean that the smaller the Wa, the better the gloss, and the smaller the Wd, the better the skin.

(Example 2)
The thermosetting clear coating material A (curing temperature = 140 ° C., weight loss rate (140 ° C.) = 0% by mass) prepared in Preparation Example 7 is used as the uppermost layer coating material, and the blocked isocyanate curable solvent is used as the lower layer coating material. In-mold coating P-3 (trade name “SFX3300CD”, curing temperature = 90 ° C., manufactured by Kansai Paint Co., Ltd.) and solvent type base coating B-4 of block isocyanate curing type (trade name “SFX420”, manufactured by Kansai Paint Co., Ltd.) Curing temperature = 90 ° C.) was used. The loss elastic moduli of the solvent-based intermediate coating P-3 and the solvent-based base coating B-4 at the start of gelation of the clear coating A were 3.2 MPa and 0.40 MPa, respectively.

  The above-mentioned solvent-type intermediate coating P-3 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 30 μm, and at 60 ° C. The organic solvent is volatilized by heating for 10 minutes, and the solvent-based base coating material B-4 is applied on the layer of the solvent-based intermediate coating material P-3 so that the film thickness after the heat treatment is 20 μm, The organic solvent is volatilized by heating at 60 ° C. for 10 minutes, and the clear paint A is applied on the layer of the solvent-type base paint B-4 so that the film thickness after the heat treatment is 35 μm. An uncured laminated coating film obtained by laminating the intermediate coating material P-3, the solvent-based base coating material B-4, and the clear coating material A by wet on wet was obtained. This uncured laminated coating film was allowed to stand (setting) at room temperature for 10 minutes, then subjected to low-temperature heat treatment at 100 ° C. for 10 minutes, and then subjected to high-temperature heat treatment at 140 ° C. for 30 minutes to obtain a laminated coating film. . For the obtained multilayer coating film, Wa to Wd were measured in the same manner as in Example 1. The results are shown in Table 2.

(Comparative Example 1)
Melamine curable water-based base coating B-2 (weight reduction rate) prepared in Preparation Example 5 instead of melamine curable water-based base coating B-1 (weight reduction rate (140 ° C.) = 3.6% by mass) as the lower layer coating A laminated coating film was prepared in the same manner as in Example 1 except that (140 ° C.) = 3.8% by mass), and Wa to Wd were measured. The results are shown in Table 2. The loss elastic modulus of the water-based base paint B-2 at the start of gelation of the clear paint A was 1.14 MPa.

(Comparative Example 2)
The thermosetting clear coating material A (curing temperature = 140 ° C., weight reduction rate (140 ° C.) = 0% by mass) prepared in Preparation Example 7 was used as the uppermost layer coating material, and the lower layer coating material was prepared in Preparation Example 3. Melamine curable aqueous intermediate coating material P-2 (curing temperature = 140 ° C.) and non-curable water-based base coating material B-3 prepared in Preparation Example 6 (weight reduction rate (140 ° C.) = 0 mass%) were used. The loss elastic moduli of the water-based intermediate coating P-2 and the non-curable water-based base coating B-3 at the start of gelation of the clear coating A were 17.0 MPa and 1.28 MPa, respectively.

  The water-based intermediate coating P-2 is applied to the surface of an electrodeposition coating plate (trade name “Sucsade 80V Gray” manufactured by Shinto Herberts Co., Ltd.) so that the film thickness after heat treatment is 20 μm. Water and organic solvent are volatilized by heating for a minute, and the non-curable water-based base paint B-3 is applied on the layer of the water-based intermediate coating P-2 so that the film thickness after the heat treatment is 15 μm. Then, it is heated at 80 ° C. for 3 minutes to volatilize water and organic solvent, and the film thickness after the heat treatment of the clear paint A on the layer of the non-curable water-based base paint B-3 is 35 μm. And an uncured laminated coating film obtained by laminating the melamine curable aqueous intermediate coating material P-2, the non-curable aqueous base coating material B-3, and the thermosetting clear coating material A by wet-on-wet was obtained. This uncured laminated coating film was allowed to stand (setting) at room temperature for 10 minutes, then subjected to low-temperature heat treatment at 90 ° C. for 10 minutes, and then subjected to high-temperature heat treatment at 140 ° C. for 30 minutes to obtain a laminated coating film. . For the obtained multilayer coating film, Wa to Wd were measured in the same manner as in Example 1. The results are shown in Table 2.

  As is clear from the results shown in Table 2, as in the present invention, wet using a paint having a loss elastic modulus of 1 MPa or less at the start of gelation of the paint for the uppermost layer as at least one of the lower layers of the uppermost layer. Wa to Wd of the on-wet laminated coatings (Examples 1 and 2) are all laminated using a paint whose loss elastic modulus at the start of gelation of the paint for the uppermost layer exceeds 1 MPa for all of the lower layers of the uppermost layer. Compared to the coating film (Comparative Examples 1 and 2), the Wa value of the laminated coating film of Examples 1 and 2 is 15 or less, whereas the Wa value of the laminated coating film of Comparative Examples 1 and 2 is 15 or less. It was confirmed that the laminated coating films of Examples 1 and 2 were superior to the laminated coating films of Comparative Examples 1 and 2 in both gloss and skin.

  In addition, it is guessed that the difference in the unevenness | corrugation of the coating-film surface in an Example and a comparative example arises as follows. In the laminated coating films of Comparative Examples 1 and 2, since all of the lower layers of the uppermost layer are paints having a loss elastic modulus exceeding 1 MPa at the start of gelation of the uppermost layer paint, at the start of gelation of the uppermost layer It is presumed that the fluidity of the lower layer is poor, and many irregularities formed on the surface of the coating when the uppermost layer is cured are not relaxed, and many irregularities remain on the surface of the coating. On the other hand, in the laminated coating films of Examples 1 and 2, at least one of the lower layer coatings of the uppermost layer is a coating material having a loss elastic modulus of 1 MPa or less at the start of gelation of the uppermost layer coating material. Therefore, the fluidity of the lower layer is ensured even at the start of gelation of the uppermost layer, and the unevenness slightly formed on the coating surface when the uppermost layer is cured is alleviated by the flow of the lower layer, and the unevenness of the coating surface is reduced. It is guessed that it was less.

  As described above, according to the present invention, even when two or more kinds of paints are laminated by wet-on-wet and subjected to heat treatment, a laminated coating film with less unevenness on the surface of the uppermost layer can be obtained. 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 having excellent appearance quality even when two or more kinds of paints are laminated by wet-on-wet and subjected to heat treatment, in particular, passenger cars, trucks, It is useful as a painting method for automobile bodies such as buses and motorcycles and parts thereof.

Is a graph showing changes with time in the relative storage elastic modulus (E _r ') and the relative loss elastic modulus (E r _").

Explanation of symbols

P: Inflection point, t _d : Time from the start of measurement of the relative storage elastic modulus to the start of gelation for the uppermost layer coating material.

Claims (4)

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 thermosetting paint is prepared as the uppermost layer paint for forming the uppermost layer, and the gelation of the uppermost layer paint is started as at least one of the lower layer paints for forming the lower layer. Preparing a paint having a loss modulus of elasticity of 1 MPa or less at the time;
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;
Subjecting the uncured laminated coating film to a thermosetting treatment to cure at least the uppermost layer coating;
The coating method characterized by including.   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.   3. The thermosetting paint according to claim 1, wherein at least one of the lower layer paints is a thermosetting paint having a weight reduction rate of 0.5% by mass or less at the curing temperature of the uppermost layer paint. Painting method.   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 1-3. Painted body obtained by the coating method of

Images