JP5260255B2 - Coating method and coated body obtained thereby - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a coating method which can obtain a laminated coated film with little unevenness in the uppermost layer surface even if two kinds of paints are laminated by a wet-on-wet method and baked, and each layer is hardened to secure high durability, etc. <P>SOLUTION: The coating method includes forming the laminated coated film provided with a lower layer formed on a substrate and the uppermost layer formed on the lower layer. The coating method also includes a process of preparing a thermoset paint containing a base substance resin of 5&deg;C or less in the glass transition temperature as a paint for a lower layer to form the lower layer and of preparing a thermoset paint in which the weight reduction rate in the curing temperature is 0.5 mass% or less as a paint for the uppermost layer to form the uppermost layer, a process of forming an uncured laminated coated film by laminating the paint for the lower layer and the paint for the uppermost layer by a wet-on-wet method on the substrate, and a process of applying a heat treatment to the uncured laminated coated film to cure the paint for the lower layer and the paint for the uppermost layer. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

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

  When two layers of paints are laminated wet-on-wet and then a laminated coating film is formed by a baking method, all layers constituting the laminated coating film have been heated at the same heating temperature after all the coating materials have been laminated. A method of selecting a thermosetting paint for forming each layer so as to be cured 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, in Japanese Patent Application Laid-Open No. 2007-283271 (Patent Document 1), a water-based base coating material and a water-based clear coating material using an isocyanate compound as a curing agent are applied wet-on-wet, and then both coating films are heated and cured together. In the multilayer coating film forming method, the solid content concentration of the base coating film is 85% by mass or more and the water absorption rate of the base coating film at 20 ° C. is 10% by mass or less at the time of application of the water-based clear coating material. It has been disclosed that diffusion and penetration of water from the clear layer to the base layer after coating can be suppressed, and an increase in viscosity of the clear coating film due to water absorption of the base coating film can be suppressed. Furthermore, it is disclosed that it is possible to improve the finished appearance of the multilayer coating film.

However, a clear coating using an isocyanate compound as a curing agent cures quickly during baking, and the clear coating loses its fluidity before the base coating is cured. In on-wet coating, after the clear coating is cured, the unevenness formed by the curing shrinkage that accompanies the progress of curing of the base coating is transferred to the surface of the clear coating without being relaxed. It has been difficult to improve the skin and gloss to the level required for automotive exterior quality.
JP 2007-283271 A

  The present invention has been made in view of the above-mentioned problems of the prior art. Even if each layer is cured in order to ensure high durability by laminating and baking two types of paints in a wet-on-wet manner, It is an object of the present invention to provide a coating method capable of obtaining a laminated coating film with less irregularities on the surface of the upper 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 the weight reduction rate at the curing temperature is predetermined when two types of thermosetting paints are laminated by wet-on-wet and baked. The uppermost layer of the laminated coating film is formed using a thermosetting paint in the range of 5 and the lower layer of the uppermost layer is formed using a thermosetting paint containing a base resin having a low glass transition temperature (Tg). As a result, the fluidity of the lower layer is ensured even after the uppermost layer is cured and the fluidity is remarkably lowered, and even in the subsequent cured state, high relaxation of the lower layer (high molecular mobility) is also obtained. As a result, the formation of unevenness due to shrinkage of the laminated coating can be minimized, and a laminated coating excellent in appearance quality can be obtained even if baking is performed after laminating two types of paints wet-on-wet see And it has led to the completion of the present invention.

That is, the coating method of the present invention is a coating method for forming a laminated coating film comprising a lower layer formed on a substrate and an uppermost layer formed on the lower layer,
Before Symbol weight loss percentage at the curing temperature as the uppermost layer-coating material for forming the uppermost layer is prepared 0.5 wt% or less of the thermosetting coating material, a glass transition as a lower layer-coating material for forming the lower layer Preparing a thermosetting paint containing a base resin having a temperature of 5 ° C. or less and having a loss elastic modulus of 1 MPa or less at the start of gelation of the uppermost layer 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;
Heat-treating the uncured laminated coating to cure the lower layer coating and the uppermost layer coating;
It is the method characterized by including.

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

  In the coating method of the present invention, the uncured laminated coating film is subjected to a heat treatment at a temperature lower than [the curing temperature of the uppermost layer-coating material—20 ° C.], and then [the curing temperature of the uppermost layer-coating material− It is preferable to perform heat treatment at a temperature of 20 ° C. or higher.

  The coated body of the present invention is a coated body having a laminated coating film comprising a 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 characterized by having a laminated coating film with excellent appearance quality such as skin and gloss.

  The reason why the unevenness of the surface of the laminated coating film is reduced is not always clear even when two types of paints are laminated by wet-on-wet and baked by the coating method of the present invention. I guess so. 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 at the same time, or curing is started in order from the lower layer. Therefore, when the thermosetting paint forming the uppermost layer is cured by heat treatment (baking treatment), the thermosetting paint has been cured in the lower layer and has already lost its fluidity. It is in a state. In each layer of such a laminated coating film, the thermosetting paint is cured by an addition reaction after a condensation reaction or a deblocking reaction of a curing agent, so that volatile products generated by this condensation reaction or deblocking reaction remain. 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 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.

  In addition, when a thermosetting paint containing an isocyanate compound or isocyanate resin as a curing agent is used as the uppermost layer paint, the uppermost layer is coated before the lower layer is cured because the uppermost layer paint is fast. Often loses liquidity. In this case, curing of the lower layer proceeds after the uppermost layer is cured, but the lower layer coating material used in conventional wet-on-wet coating has poor relaxation in the cured state, and due to shrinkage accompanying the progress of curing of the lower layer. The formed irregularities are not sufficiently relaxed, and the irregularities at the surface of the base material and the interface of each layer are transferred to the uppermost layer surface, and it is assumed that the skin and gloss of the laminated coating film deteriorate.

  On the other hand, in the coating method of the present invention, since the lower layer is formed using a thermosetting paint containing a base resin having a low Tg, high fluidity is secured in the lower layer even when the uppermost layer is cured. In addition, high relaxation (high molecular mobility) is obtained even in the subsequent cured state, so even if the laminated coating contracts and irregularities are formed on the coating surface, the irregularities are relaxed by the fluidity of this lower layer. It is presumed that unevenness on the surface of the coating film is suppressed.

  According to the present invention, it is possible to obtain a multilayer coating film with less unevenness on the surface of the uppermost layer even if each layer is cured by laminating and baking two types of paints wet-on-wet to ensure high durability. It becomes. 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 a lower layer formed on a substrate and an uppermost layer formed on the lower layer,
A thermosetting paint containing a base resin having a glass transition temperature of 5 ° C. or lower is prepared as a lower layer coating material for forming the lower layer, and a weight at the curing temperature as the uppermost layer coating material for forming the uppermost layer. A step of preparing a thermosetting paint having a reduction rate of 0.5% by mass or less;
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;
Heat-treating the uncured laminated coating to cure the lower layer coating and the uppermost layer coating;
It is the method characterized by including.

  In the coating method of the present invention, first, a coating material for a lower layer is applied on a substrate, and if necessary, a solvent or the like is evaporated by drying or the like to form an uncured lower layer. At this time, a thermosetting paint containing a base resin having a glass transition temperature of 5 ° C. or lower (hereinafter referred to as “low Tg thermosetting paint”) is used as the lower layer paint. 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 heat treatment (baking treatment) to cure each layer.

  The substrate used in the present invention 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, and glass. Etc. Especially, this invention is applied suitably for the steel plate for motor vehicles with the required characteristic with respect to external appearance quality. The surface of these base materials may be previously subjected to treatment such as electrodeposition coating or electrodeposition coating and intermediate coating.

  In the coating method of the present invention, a lower layer is formed on the substrate using a thermosetting paint, and this lower layer is a low Tg thermosetting paint containing a base resin having a glass transition temperature of 5 ° C. or lower. It is formed using. Thus, even when the lowermost layer is formed using the low-Tg thermosetting paint and the uppermost layer is cured and the fluidity is remarkably lowered, the fluidity of the lower layer is sufficiently secured, and the lower layer is cured. Since sufficient relaxation is ensured even in the state, it is possible to relieve surface irregularities due to shrinkage of the laminated coating film, and to obtain a laminated coating film having excellent appearance quality.

  The low Tg thermosetting paint used in the present invention includes a base resin having a Tg of 5 ° C. or lower, preferably a base resin having a Tg of −5 ° C. or lower, and a base having a Tg of −15 ° C. or lower. A resin containing resin is more preferable. When the Tg of the base resin exceeds the upper limit, when the fluidity of the uppermost layer is remarkably lowered by curing, the lower layer is also cured and the fluidity cannot be sufficiently secured, and the relaxation property when the lower layer is in a cured state However, the unevenness of the coating film surface is not relaxed, and the skin and gloss of the laminated coating film tend to deteriorate.

In the present invention, the “base resin” means a main component of the resin contained in the paint. For the glass transition temperature (Tg (unit: K)) of such a base resin, refer to the following Fox formula (Bulletin of the American Physical Society, 13, p123 (1956)):
1 / Tg = w ₁ / Tg ₁ +... + W _i / Tg _i +... + W _n / Tg _n
Where w _i represents the mass fraction of monomer i (i = integer from 1 to n) and T g _i is the glass transition temperature (unit: K) of the homopolymer of monomer i (i = 1 to n) )
Can be used to calculate. The Tg of the homopolymer is described in J. Org. Jpn. Soc. Color Mater. , 64, p594-p595 (1991) can be applied, and the Tg of homopolymers not described in this document is “POLYMER HANDBOOK (FOURTH
EDITION) ”, J. BRANDRUP, EH IMMERGUT and EA GRULKE, JOHN WILEY & SONS, INC., And the values described above can be applied. A base resin of Tg can be prepared.

  Examples of the base resin used in such a low Tg thermosetting paint include, but are not limited to, acrylic resin, polyester resin, alkyd resin, epoxy resin, and urethane resin. These resins may be used alone or in combination of two or more. The Tg of such a base resin can be calculated by the Fox equation, and a base resin having a predetermined Tg can be obtained by adjusting the monomer composition.

  Examples of the curing agent contained in the low Tg thermosetting paint include isocyanate compounds, isocyanate resins, amine compounds, and amino resins. These curing agents may be used alone or in combination of two or more.

  In the present invention, the form of the low Tg thermosetting paint may be either a solvent type or an aqueous type, but an aqueous type is preferable in that the amount of volatile organic compounds discharged can be reduced. The low Tg thermosetting paint may contain conventionally known color pigments, glitter pigments, and the like within a conventionally known range, if necessary. Furthermore, in order to adjust various physical properties, various additives such as a viscosity control agent, a surface conditioner, a thickener, an antioxidant, an ultraviolet absorber, and an antifoaming agent may be blended within a conventionally known range.

  In the coating method of the present invention, the lower layer paint preferably has a weight reduction rate at the curing temperature of the uppermost layer paint to be used of 0.5% by mass or less, more preferably 0.3% by mass or less. Particularly preferred is 0.1% by mass or less. When the 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 treatment and the fluidity is remarkably lowered. Further, from this point of view, a lower layer coating material (a weight reduction rate of 0% by mass) that does not generate a volatile product when the uppermost layer is cured is most preferable.

In the present invention, the term "curing temperature of the coating material", is one general refers to baking temperature which is set for each paint (design). 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 is applied on the aluminum foil so that the film thickness after the heat treatment becomes the target film thickness in the laminated coating film, and the obtained aluminum foil sample is 40 times higher than the curing temperature of the uppermost layer paint. 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) after drying at a low temperature of 90 ° C. and a vacuum condition of 10 ⁻² Torr or less. The amount of volatile products (Rc (unit: g)) and the amount of residual solvent are quantified by heating at the curing temperature of the coating material for the uppermost layer 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 the coating method of the present invention, the lower layer paint preferably has a loss elastic modulus of 1 MPa or less, more preferably 0.7 MPa or less, at the start of gelation of the uppermost layer paint used. Particularly preferred is one having a viscosity of 5 MPa or less (hereinafter referred to as “low-loss elastic modulus paint”). By using such a low-loss elastic modulus coating as a coating for the lower layer, the fluidity is secured in the lower layer even if the uppermost layer is cured and the fluidity is remarkably lowered. Therefore, the unevenness of the surface due to shrinkage of the laminated coating film can be further relaxed, and a laminated coating film having excellent appearance quality can be obtained.

  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. 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”. 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 ".

  Further, in the present invention, by adjusting the composition and blending ratio of the thermosetting resin and the curing agent or blending additives, for example, by adjusting the glass transition temperature, the crosslinking density, etc. The loss elastic modulus of the lower layer coating material at the start of gelation of the upper layer coating material can be controlled.

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

  Examples of the base resin contained in the uppermost layer coating material include, but are not limited to, an acrylic resin, a polyester resin, an alkyd resin, an epoxy resin, and a urethane resin. Preferred curing agents include, but are not limited to, isocyanate compounds and isocyanate resins. These resins and curing agents may be used alone or in combination of two or more. In the present invention, the coating material for the uppermost layer is prepared by combining the base resin and the curing agent so that the weight reduction rate falls within the above range. Examples of the combination of the thermosetting resin that does not substantially generate a volatile product in the curing reaction by heat treatment and the curing agent include a combination of a hydroxyl group-containing acrylic resin and an isocyanate compound and / or an isocyanate resin. .

  The form of the uppermost layer-coating material may be either solvent-based or water-based. Moreover, although the curing temperature of the coating material for uppermost layers is not specifically limited, Usually, it is 40-200 degreeC, and it is preferable that it is 60-160 degreeC.

  Among such uppermost layer coating materials, thermosetting coating materials containing a base resin having a Tg of more than 5 ° C. are preferred. If a thermosetting paint containing a base resin having a Tg of not more than the lower limit is used as the uppermost layer paint, the mechanical properties and durability of the laminated coating film tend to be lowered. The Tg of the base resin used in the uppermost layer coating material can be calculated by the Fox equation, and a base resin having a predetermined Tg can be obtained by adjusting the monomer composition.

  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 absorber, and an antifoaming agent may be blended within a conventionally known range.

  In the coating method of the present invention, first, the low Tg thermosetting paint is applied on the substrate, and the solvent is evaporated by drying or the like as necessary to form an uncured lower layer. When applying the lower layer coating material, conventionally known methods such as air spray coating, air electrostatic spray coating, and rotary atomizing electrostatic coating can be applied.

  Although the film thickness of a lower layer can be suitably set with a desired use, For example, it is preferable that it is 5-50 micrometers by the film thickness after heat processing, and it is more preferable that it is 10-40 micrometers. If the film thickness of the 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 the upper layer exceeds the upper limit, the lower layer itself tends to absorb much solvent contained in the uppermost coating film. Volatilization of the solvent contained therein 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 the method for applying the uppermost layer coating material include conventionally known methods such as air spray coating, air electrostatic spray coating, rotary atomizing electrostatic 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, the uncured laminated coating film formed by laminating the lower layer coating material and the uppermost layer coating material by wet-on-wet is subjected to heat treatment (baking treatment) to cure each layer. In the coating method of the present invention, the heat treatment is a heat treatment at a temperature at least equal to or higher than the temperature at which the uppermost layer is cured, for example, [the curing temperature of the uppermost layer paint −20 ° C.]. It is preferable to contain.

  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.

  Moreover, in the coating method of this invention, it is preferable to reduce the volatile matter density | concentration of a laminated coating film, without hardening the uppermost layer, before performing the said high temperature heat processing. 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.

  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 within 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 heat 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 glass transition temperature (Tg) of the base resin and the weight reduction rate due to the heat treatment of the paint were measured by the following methods.

<Calculation of glass transition temperature>
The following Fox formula:
1 / Tg = w ₁ / Tg ₁ +... + W _i / Tg _i +... + W _n / Tg _n
Where w _i represents the mass fraction of monomer i (i = integer from 1 to n) and T g _i is the glass transition temperature (unit: K) of the homopolymer of monomer i (i = 1 to n) )
It calculated using. In addition, Tg of the homopolymer of the monomer used by the Example and the comparative example is shown below.
Methyl methacrylate 105 ° C
Butyl acrylate -54 ° C
2-Hydroxyethyl methacrylate 55 ° C
Styrene 100 ° C
Acrylic acid 106 ℃

<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 40 ° C. lower than the curing temperature of the top layer paint. And 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 introduction apparatus (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 coating for the uppermost layer 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 Emulsion R-1 The following monomers were mixed to prepare a monomer mixture.
<Monomer mixture composition>
Methyl methacrylate 10.7 parts by mass Butyl acrylate 203.2 parts by mass 2-hydroxyethyl acrylate 50.4 parts by mass Styrene 42.5 parts by mass Acrylic acid 8.2 parts by mass This monomer mixture 315 parts by mass, 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, the reaction was further continued at 80 ° C. for 1 hour with stirring. 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 7.4 using a 10% by mass aqueous dimethylethanolamine solution to obtain an acrylic emulsion R-1 having a nonvolatile content of 38.1% by mass and Tg = −20 ° C.

(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 22.7 parts by mass Butyl acrylate 178.6 parts by mass 2-Hydroxyethyl methacrylate 50.4 parts by mass Styrene 55.1 parts by mass Acrylic acid 8.2 parts by mass In place of the monomer mixture described in Synthesis Example 1, An acrylic emulsion R-2 having a nonvolatile content of 38.1% by mass and Tg = −10 ° C. was obtained in the same manner as in Synthesis Example 1 except that 315 parts by mass of the monomer mixture was used.

(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 36.2 parts by mass Butyl acrylate 155.6 parts by mass 2-hydroxyethyl methacrylate 50.4 parts by mass Styrene 64.6 parts by mass Acrylic acid 8.2 parts by mass In place of the monomer mixture described in Synthesis Example 1, An acrylic emulsion R-3 having a nonvolatile content of 38.1% by mass and Tg = 0 ° C. was obtained in the same manner as in Synthesis Example 1 except that 315 parts by mass of the monomer mixture was used.

(Synthesis example 4) Synthesis | combination of acrylic emulsion R-4 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-4 having a nonvolatile content of 38.1% by mass and Tg = 10 ° C. was obtained in the same manner as in Synthesis Example 1 except that 315 parts by mass of the monomer mixture was used.

(Preparation example 1) Preparation of melamine curable aqueous base paint B-1 In a container, 183.7 parts by mass of the acrylic emulsion R-1 having a Tg of −20 ° C. obtained in Synthesis Example 1 was charged and stirred. 40 parts by mass of a methylated 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, 30 parts by mass of two types of aluminum pastes (“Hydrolan 2154” manufactured by Eckart GmbH and “Hydrolan 2156” manufactured by Eckart GmbH) were added, 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 457.7 parts by mass of the aqueous resin solution while stirring, and the mixture was further stirred for 1 hour to have a non-volatile content of 24.7% by mass and a pH 8.0 melamine curable type. An aqueous base 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 2) Preparation of Melamine Curing Water-Based Paint B-2 Except for using 183.7 parts by mass of acrylic emulsion R-2 having a Tg of −10 ° C. obtained in Synthesis Example 2 instead of acrylic emulsion R-1 In the same manner as in Preparation Example 1, a melamine curable aqueous base paint B-2 having a non-volatile content of 24.7% by mass and a pH of 8.0 was obtained. The weight loss rate of this water-based base coating material B-2 at 140 ° C. was 3.7% by mass (calculated as P = 22.4).

(Preparation Example 3) Preparation of Melamine Curing Water-Based Paint B-3 Except for using 183.7 parts by mass of acrylic emulsion R-3 with Tg = 0 ° C. obtained in Synthesis Example 3 instead of acrylic emulsion R-1. In the same manner as in Preparation Example 1, a melamine curable aqueous base paint B-3 having a nonvolatile content of 24.7% by mass and a pH of 8.0 was obtained. The weight loss rate of this water-based base coating material B-3 at 140 ° C. was 3.8% by mass (calculated as P = 22.4).

(Preparation Example 4) Preparation of Melamine Curing Water-Based Paint B-4 Except for using 183.7 parts by mass of acrylic emulsion R-4 with Tg = 10 ° C. obtained in Synthesis Example 4 instead of acrylic emulsion R-1. In the same manner as in Preparation Example 1, a melamine curable aqueous base paint B-4 having a non-volatile content of 24.7% by mass and a pH of 8.0 was obtained. The weight loss rate of this water-based base coating material B-4 at 140 ° C. was 3.8% by mass (calculated as P = 22.4).

(Preparation Example 5) Preparation of isocyanate-curable aqueous base paint B-5 The amount of acrylic emulsion R-1 charged with Tg = −20 ° C. was changed to 210 parts by mass, and water-dispersible polyisocyanate was used instead of methylated melamine resin. Except for using 25 parts by mass (“Bernock DNW5000” manufactured by DIC Corporation), an isocyanate-curable aqueous base paint B-5 having a non-volatile content of 24.7% by mass and a pH of 8.0 was obtained in the same manner as in Preparation Example 1. It was. The weight loss rate of this water-based base coating material B-5 at 140 ° C. was 0% by mass (calculated as P = 22.4).

(Preparation Example 6) Preparation of Isocyanate-Curable Water-Based Paint B-6 Prepared except that 210 parts by mass of acrylic emulsion R-2 with Tg = -10 ° C. obtained in Synthesis Example 2 was used instead of acrylic emulsion R-1. In the same manner as in Example 1, an isocyanate-curable aqueous base paint B-6 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-6 at 140 ° C. was 0% by mass (calculated as P = 22.4).

(Preparation example 7) Preparation of isocyanate curable aqueous base paint B-7 Preparation example except that 210 parts by mass of Tg = 0 ° C. acrylic emulsion R-3 obtained in Synthesis Example 3 was used instead of acrylic emulsion R-1 In the same manner as in Example 1, an isocyanate-curable aqueous base coating material B-7 having a non-volatile content of 24.7% by mass and a pH of 8.0 was obtained. The weight loss rate of this water-based base coating material B-7 at 140 ° C. was 0% by mass (calculated as P = 22.4).

(Preparation Example 8) Preparation of Isocyanate-Curable Water-Based Paint B-8 Preparation Example except that 210 parts by mass of Tg = 10 ° C. acrylic emulsion R-4 obtained in Synthesis Example 4 was used instead of acrylic emulsion R-1 In the same manner as in Example 1, an isocyanate-curable aqueous base coating material B-8 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-8 at 140 ° C. was 0% by mass (calculated as P = 22.4).

(Preparation Example 9) Preparation of thermosetting (isocyanate curable) clear paint C Main component of two-component thermosetting (isocyanate curable) clear paint by mixing polyol and additives in the ratio 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 of 55% by mass) of the main agent and the curing agent mixed at a ratio shown in Table 1 was used as the thermosetting (isocyanate curing type) clear paint C. The curing temperature of this thermosetting clear paint C was 140 ° C., and the weight reduction rate at 140 ° C. was 0% by mass (calculated as P = 0). The time _{t d} to gelation starts was 10.2 minutes.

Example 1
On the surface of the steel sheet (manufactured by Nippon Route Service Co., Ltd.) subjected to intermediate coating and electrodeposition coating, the melamine curable aqueous base paint B-1 obtained in Preparation Example 1 (Tg of the base resin = −20 ° C.). The film was baked to a film thickness of 15 μm and heated at 80 ° C. for 3 minutes to volatilize water and organic solvent. Next, the thermosetting clear paint C obtained in Preparation Example 9 was applied onto the layer of the aqueous base paint B-1 so that the film thickness after baking was 35 μm, and the melamine curable aqueous base paint B- 1 and the thermosetting clear coating material C were obtained by laminating wet and wet coatings. The loss elastic modulus of the water-based base paint B-1 at the start of gelation of the thermosetting clear paint C was 0.20 MPa.

  After leaving this uncured laminated coating film at room temperature for 10 minutes (setting), a heating process (baking process) at 90 ° C. for 10 minutes and a heating process (baking process) at 140 ° C. for 30 minutes were sequentially performed. Cured to obtain a laminated coating film.

  About the obtained laminated coating film, wave scan values (Wa (wavelength <0.3 mm), Wb (wavelength 0.3 to 1 mm), Wc, Wc using a wave scan ("Wave-Scan Dual" manufactured by BYK-Gardner)), Wc (Wavelength 1 to 3 mm), Wd (wavelength 3 to 10 mm)] were measured. 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)
Except for using the melamine curable water-based base paint B-2 obtained in Preparation Example 2 (Tg of the base resin = -10 ° C.) instead of the melamine curable water-based base paint B-1, the same as in Example 1, A laminated coating film was obtained. 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. The loss elastic modulus of the water-based base paint B-2 at the start of gelation of the thermosetting clear paint C was 0.45 MPa.

(Example 3)
Lamination was carried out in the same manner as in Example 1 except that the melamine curable aqueous base paint B-3 obtained in Preparation Example 3 (Tg of the base resin = 0 ° C.) was used instead of the melamine curable aqueous base paint B-1. A coating film was obtained. 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. The loss elastic modulus of the water-based base coating material B-3 at the start of gelation of the thermosetting clear coating material C was 0.72 MPa.

Example 4
In the same manner as in Example 1 except that the isocyanate curable aqueous base paint B-5 obtained in Preparation Example 5 (Tg of the base resin = −20 ° C.) was used instead of the melamine curable aqueous base paint B-1. A laminated coating film was obtained. 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. The loss elastic modulus of the water-based base paint B-5 at the start of gelation of the thermosetting clear paint C was 0.22 MPa.

(Example 5)
In the same manner as in Example 1 except that the isocyanate curable aqueous base paint B-6 obtained in Preparation Example 6 (Tg of the base resin = −10 ° C.) was used instead of the melamine curable aqueous base paint B-1. A laminated coating film was obtained. 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. The loss elastic modulus of the water-based base paint B-6 at the start of gelation of the thermosetting clear paint C was 0.48 MPa.

(Example 6)
Lamination was carried out in the same manner as in Example 1 except that the isocyanate curable water-based base paint B-7 (Tg of the base resin = 0 ° C.) obtained in Preparation Example 7 was used instead of the melamine curable water-based base paint B-1. A coating film was obtained. 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. The loss elastic modulus of the water-based base paint B-7 at the start of gelation of the thermosetting clear paint C was 0.78 MPa.

(Comparative Example 1)
Lamination was carried out in the same manner as in Example 1 except that the melamine curable aqueous base paint B-4 obtained in Preparation Example 4 (Tg of substrate resin = 10 ° C.) was used instead of the melamine curable aqueous base paint B-1. A coating film was obtained. 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. The loss elastic modulus of the water-based base paint B-4 at the start of gelation of the thermosetting clear paint C was 1.12 MPa.

(Comparative Example 2)
Lamination was carried out in the same manner as in Example 1 except that the isocyanate curable aqueous base paint B-8 (Tg of the base resin = 10 ° C.) obtained in Preparation Example 8 was used instead of the melamine curable aqueous base paint B-1. A coating film was obtained. 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. The loss elastic modulus of the water-based base paint B-8 at the start of gelation of the thermosetting clear paint C was 1.15 MPa.

  As is apparent from the results shown in Table 2, as in the present invention, a thermosetting paint is used for each of the lower layer and the uppermost layer, and of these, a base resin having a glass transition temperature of 5 ° C. or lower as the lower layer paint Wa to Wd of laminated coating films (Examples 1 to 6) using wet-on-wet using a thermosetting paint containing a base material having a glass transition temperature exceeding 5 ° C. is a thermosetting paint containing a base resin. It was smaller than the conventional laminated coating film (Comparative Examples 1 and 2) used as the lower layer coating material, and was excellent in appearance quality.

  Further, when the laminated coating films of Examples 1 to 3 and Comparative Example 1 and the laminated coating films of Examples 4 to 6 and Comparative Example 2 are compared, the Tg of the base resin contained in the lower layer thermosetting paint is decreased. It was confirmed that Wa to Wd were reduced and the appearance quality was further improved. In particular, the laminated coating films of Examples 1, 2, 4, and 5 were confirmed to have very excellent appearance quality.

  Furthermore, when the laminated coating film of Examples 1-3 and the laminated coating film of Examples 4-6 are compared, when an isocyanate curable aqueous base paint is used (Examples 4-6), a melamine curable aqueous base is used. It was confirmed that Wa to Wd (particularly Wa) became smaller and the appearance quality was further improved as compared with the case where the paint was used (Examples 1 to 3). This is because the weight reduction rate at 140 ° C of the isocyanate-curable aqueous base paint is smaller than that of the melamine-curable aqueous base paint, and the use of the isocyanate-curable aqueous base paint suppresses the shrinkage of the coating film during heat treatment. It is guessed that this is because

  As described above, according to the present invention, it is possible to obtain a laminated coating film with less unevenness on the surface of the uppermost layer even when two types of paints are laminated by wet-on-wet and baked to cure each layer. Thereby, the coating body excellent in appearance quality, such as skin (surface smoothness) and glossiness, 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 types of paints are laminated and baked by wet-on-wet, and particularly for passenger cars, trucks, buses, motorcycles, etc. It is useful as a painting method for automobile bodies 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 a lower layer formed on a substrate and an uppermost layer formed on the lower layer,
Before Symbol weight loss percentage at the curing temperature as the uppermost layer-coating material for forming the uppermost layer is prepared 0.5 wt% or less of the thermosetting coating material, a glass transition as a lower layer-coating material for forming the lower layer Preparing a thermosetting paint containing a base resin having a temperature of 5 ° C. or less and having a loss elastic modulus of 1 MPa or less at the start of gelation of the uppermost layer 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;
Heat-treating the uncured laminated coating to cure the lower layer coating and the uppermost layer coating;
The coating method characterized by including.   The coating method according to claim 1, wherein the lower layer 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 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 method of coating according to claim 1 or 2, characterized by applying. It is a coating body which has a laminated coating film provided with the lower layer formed on the base material, and the uppermost layer formed on the said lower layer, Comprising: By the coating method as described in any one of Claims 1-3. A painted body characterized by being obtained.

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