CN113242889A - Clear coating composition and method for forming clear coating film - Google Patents

Clear coating composition and method for forming clear coating film Download PDF

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Publication number
CN113242889A
CN113242889A CN201980085924.7A CN201980085924A CN113242889A CN 113242889 A CN113242889 A CN 113242889A CN 201980085924 A CN201980085924 A CN 201980085924A CN 113242889 A CN113242889 A CN 113242889A
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China
Prior art keywords
coating film
coating composition
clear coating
resin
acrylic resin
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Inventor
奥田英明
河野晋之介
水口克美
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Nippon Paint Automotive Coatings Co Ltd
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Nippon Paint Automotive Coatings Co Ltd
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Publication of CN113242889A publication Critical patent/CN113242889A/en
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    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09D—COATING COMPOSITIONS, e.g. PAINTS, VARNISHES OR LACQUERS; FILLING PASTES; CHEMICAL PAINT OR INK REMOVERS; INKS; CORRECTING FLUIDS; WOODSTAINS; PASTES OR SOLIDS FOR COLOURING OR PRINTING; USE OF MATERIALS THEREFOR
    • C09D133/00—Coating compositions based on homopolymers or copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and at least one being terminated by only one carboxyl radical, or of salts, anhydrides, esters, amides, imides, or nitriles thereof; Coating compositions based on derivatives of such polymers
    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09D—COATING COMPOSITIONS, e.g. PAINTS, VARNISHES OR LACQUERS; FILLING PASTES; CHEMICAL PAINT OR INK REMOVERS; INKS; CORRECTING FLUIDS; WOODSTAINS; PASTES OR SOLIDS FOR COLOURING OR PRINTING; USE OF MATERIALS THEREFOR
    • C09D175/00—Coating compositions based on polyureas or polyurethanes; Coating compositions based on derivatives of such polymers
    • C09D175/04—Polyurethanes
    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09D—COATING COMPOSITIONS, e.g. PAINTS, VARNISHES OR LACQUERS; FILLING PASTES; CHEMICAL PAINT OR INK REMOVERS; INKS; CORRECTING FLUIDS; WOODSTAINS; PASTES OR SOLIDS FOR COLOURING OR PRINTING; USE OF MATERIALS THEREFOR
    • C09D7/00—Features of coating compositions, not provided for in group C09D5/00; Processes for incorporating ingredients in coating compositions
    • C09D7/40—Additives
    • C09D7/65—Additives macromolecular

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  • Chemical & Material Sciences (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Wood Science & Technology (AREA)
  • Organic Chemistry (AREA)
  • Paints Or Removers (AREA)
  • Application Of Or Painting With Fluid Materials (AREA)

Abstract

The invention provides a clear coating composition which can form a clear coating film having excellent impact resistance even in cold regions, and can reduce the baking temperature and the baking time in the baking process of the clear coating composition. The clear coating composition of the present invention is a dried coating film of a clear coating composition comprising the following components (c-1) to (c-4) as a coating film-forming resin: a hydroxyl group-containing acrylic resin (c-1), a non-aqueous dispersion type acrylic resin (c-2), a diol resin (c-3), and a polyisocyanate compound (c-4), wherein the resins (c-1) to (c-4) of the clear coating composition have predetermined physical properties, and the diol resin (c-3) contains at least one selected from the group consisting of a polycarbonate diol resin, a polyether diol resin, a polyester diol resin, and a polycaprolactone diol resin.

Description

Clear coating composition and method for forming clear coating film
Technical Field
The present invention relates to a clear coating composition for multilayer coating films and the like and a method for forming clear coating films.
Background
In recent years, in the technical field of automobiles and the like, it has been required to shorten the coating process from the viewpoint of energy saving and reduction in carbon dioxide emission. For example, a lower baking temperature and a shorter baking time are required in the baking step of the coating composition.
Further, the vehicle body of an automobile or the like, and the automobile parts made of plastic or metal, and the like, need to be applied to various usage environments, and need to be applied even under extremely severe environments. For example, use in cold regions of the order of-20 ℃ is also envisaged.
In recent years, from the viewpoint of design, function, weight reduction, and the like, metal materials and plastic materials have been used in many cases for bodies of automobiles and the like, and coating films are also required to exhibit good adhesion to various objects to be coated (objects to be coated).
Patent document 1 discloses resin particles for a coating composition having a young's modulus, an elongation, and a breaking strength within specific ranges, and discloses that a coating film having excellent chipping resistance (chipping resistance) is formed by using a coating composition containing the resin particles.
Patent document 2 discloses a method for forming a multilayer coating film, in which a coating material having specific physical properties in terms of young's modulus and fracture energy is used as a first base coating material.
Patent document 3 discloses a method of applying a primer, a primer coat, and a topcoat to a metal plate.
Documents of the prior art
Patent document
Patent document 1: japanese patent laid-open publication No. 2007-270014;
patent document 2: japanese patent laid-open publication No. 2003-181368;
patent document 3: japanese patent laid-open No. 2000-204483.
Disclosure of Invention
Problems to be solved by the invention
However, in any of the inventions of patent documents 1 to 3, the obtained coating film does not exhibit sufficient impact resistance even at a low temperature (for example, -20 ℃). Therefore, neither of the inventions is applicable to automobile bodies and automobile parts which require impact resistance at low temperatures.
Further, the coating film is required to have a performance of following the deformation of the substrate without damaging the coating film (followability of the coating film), and sufficient performance cannot be obtained.
Further, there is still a demand for a lower baking temperature and a shorter baking time in the baking step of the coating composition.
In order to solve the above-mentioned problems, an object of the present invention is to provide a clear coating composition which has a coating film conformability, can form a clear coating film having excellent impact resistance even in cold regions, and can reduce the baking temperature and the baking time in the baking step of the clear coating composition.
It is another object of the present invention to provide a coating composition set for forming a multilayer coating film, which contains a clear coating composition exhibiting the above-described effects and can form a multilayer coating film having an excellent appearance.
In addition, the present invention provides a method for forming a multilayer coating film. According to this forming method, by using the clear coating composition of the present invention, the curing time of the coating composition can be greatly shortened, and the curing can be carried out at a temperature lower than the curing temperature.
Means for solving the problems
In order to solve the above problems, the present invention provides the following aspects.
[1] The clear coating composition of the present disclosure is the following clear coating composition: comprising a hydroxyl group-containing acrylic resin (c-1), a non-aqueous dispersion type acrylic resin (c-2), a diol resin (c-3), and a polyisocyanate compound (c-4) as a coating film-forming resin,
the hydroxyl value of the hydroxyl-containing acrylic resin (c-1) is 80mgKOH/g to 220mgKOH/g, the weight-average molecular weight is 4,000 to 10,000, and the glass transition temperature is 30 ℃ to 60 ℃,
the non-aqueous dispersion acrylic resin (c-2) has a core-shell structure, the hydroxyl value of the core part is 100mgKOH/g to 200mgKOH/g, and the glass transition temperature of the core part is 40 ℃ to 80 ℃,
the hydroxyl value of the diol resin (c-3) is 80mgKOH/g or more and 200mgKOH/g or less, and the diol resin (c-3) comprises at least one selected from the group consisting of a polycarbonate diol resin, a polyether diol resin, a polyester diol resin and a polycaprolactone diol resin.
[2] In a specific embodiment, the clear coating film formed from the clear coating composition according to the present disclosure has a DuPont impact strength of 4.9J or more at-20 ℃.
[3] In a specific embodiment, a multilayer coating film having a primer coating film formed from the primer coating composition according to the present disclosure, a base coating film formed from the base coating composition, and a clear coating film formed from the clear coating composition according to claim 1 or 2 has a dupont impact strength of 4.9J or more at-20 ℃.
[4] In embodiments, the clearcoat compositions to which the present disclosure relates further comprise a cellulose derivative.
[5] In a specific embodiment, the diol resin (c-3) is a polyester diol resin.
[6] In a specific embodiment, the equivalent ratio (NCO/OH) of the isocyanate group contained in the isocyanate compound (c-4) to the hydroxyl group contained in the hydroxyl group-containing acrylic resin (c-1) and the non-aqueous dispersion acrylic resin (c-2) is 0.7 or more and 2.0 or less.
[7] In a specific embodiment, the mass ratio of the hydroxyl-containing acrylic resin (c-1) to the non-aqueous dispersion acrylic resin (c-2) is (c-1)/(c-2)) 90/10-50/50.
[8] Specifically, the mass ratio of the total mass of the hydroxyl-containing acrylic resin (c-1) and the nonaqueous dispersion-type acrylic resin (c-2) to the glycol resin (c-3) is ((c-1) + (c-2))/(c-3) — (100/1 to 100/15.
[9] In a specific embodiment, the core portion of the nonaqueous dispersion type acrylic resin (c-2) contains a hydroxyl group-containing acrylate monomer having an ester moiety with 4 to 12 carbon atoms.
[10]In a specific embodiment, the core part of the non-aqueous dispersion type acrylic resin (c-2) contains a carboxyl group-containing compoundα,βEthylenically unsaturated monomers and monomers having hydroxyl groupsα,β-ethylenically unsaturated monomers.
[11] In a specific embodiment, the acid value of the core portion of the nonaqueous dispersion type acrylic resin (c-2) is 60mgKOH/g or less.
[12] In a specific embodiment, the hydroxyl group value of the shell portion of the non-aqueous dispersion type acrylic resin (c-2) is 50mgKOH/g or more and 160mgKOH/g or less, and the glass transition temperature is 0 ℃ or more and 80 ℃ or less.
[13] In a specific embodiment, the acid value of the shell portion of the nonaqueous dispersion type acrylic resin (c-2) is 30mgKOH/g or less.
[14] In another aspect, the present disclosure provides a method for forming a transparent coating film, comprising the steps of: a step of applying the above clear coating composition to an object to be coated to form an uncured clear coating film; and a step of forming a transparent coating film by heat-curing the uncured transparent coating film.
Effects of the invention
The clear coating composition of the present invention can form a clear coating film having excellent impact resistance even in cold regions (e.g., -20 ℃). More specifically, the clear coating composition of the present invention can form a coating film having the following properties of the coating film, excellent appearance, hardness, initial adhesion, moisture resistance, gasohol resistance, and impact resistance (e.g., DuPont impact strength at-20 ℃).
Further, the clear coating composition of the present invention can realize a reduction in baking temperature and baking time in the baking step, and can save energy and reduce cost.
Detailed Description
First, a process of achieving the present invention will be described. Vehicle bodies of automobiles and the like, and automobile parts made of plastic or metal, and the like are generally covered with a clear coating film or the like. For example, such a coating to be formed comprises a multilayer coating film comprising a transparent coating film. Examples of the multilayer coating film include: a multilayer coating film having a primer coating film formed from the primer coating composition, a base coating film formed from the base coating composition, and a clear coating film formed from the clear coating composition.
Further, a vehicle body and a part having such a clear coating film or a multilayer coating film including a clear coating film formed thereon are also expected to be used in cold regions, and a coating film such as a clear coating film covering them is also required to have physical properties suitable for use in cold regions.
For example, a clear coating film, preferably a multilayer coating film comprising a clear coating film, is required to have excellent impact resistance even in cold regions. Further, a clear coating film is also required to satisfy excellent appearance.
In addition, from the viewpoint of energy saving and cost reduction, it is also necessary to consider shortening the time required for forming a coating film. For example, various parts and various forms used for vehicle bodies and parts are required to form a coating film exhibiting excellent design properties and various physical properties in a shorter time.
Accordingly, the present inventors have focused on a clear coating composition which can form a clear coating film having excellent impact resistance even in cold districts and can shorten the time required for heat curing of the coating composition.
However, if the heat curing time of the conventional coating composition is shortened, the impact resistance of the obtained clear coating film may be deteriorated. Moreover, the viscosity, the polishing property, and the like of the clear coating film are also deteriorated, and the clear coating film is inferior not only in the physical properties but also in the appearance of the coating film.
As another countermeasure, a curing catalyst may be selected to shorten the drying time of the coating composition. However, in this case, there is a problem that the pot life becomes short and the coating efficiency becomes poor.
In order to solve the above problems, the present inventors have conducted extensive studies and, as a result, have found that: the specific clear coating composition of the present invention can form a clear coating film having low-temperature impact resistance (e.g., DuPont impact strength at-20 ℃) which can be sufficiently used even in cold regions. In addition to the above features, the clear coating composition of the present invention can form a coating film having excellent appearance, hardness, initial adhesion, moisture resistance, solvent resistance (e.g., alcohol vapor oil resistance), and good film-following properties. Further, the clear coating composition of the present invention has an appropriate pot life, is excellent in workability in a wide range of coating, and can be reduced in the time required for heat curing and at a low temperature.
For example, such effects can be exhibited also in a multilayer coating film having a primer coating film, a base coating film, and a clear coating film formed from the clear coating composition of the present invention.
Further, a coating composition set for forming a multilayer coating film, which is another embodiment of the present invention and comprises the specific clear coating composition according to the present invention, can form a multilayer coating film having impact resistance at low temperatures (e.g., dupont impact strength at-20 ℃) which can be sufficiently used even in cold regions, and can form a coating film having excellent appearance, hardness, initial adhesion, moisture resistance, solvent resistance (e.g., alcohol gasoline resistance) and good film-following properties.
The coating composition set for forming a multilayer coating film of the present invention has an appropriate pot life, is excellent in workability in a wide range of coating, can shorten the time required for heat curing and can reduce the temperature, and can reduce the working time and working temperature of the entire multilayer coating film.
Further, according to the method for forming a multilayer coating film according to another aspect of the present invention, by forming a multilayer coating film using the specific clear coating composition according to the present invention, the heat curing time of the coating film forming composition can be greatly shortened, and the production time of the multilayer coating film can be greatly shortened. The obtained multilayer coating film may have the above properties. As a result, energy saving and cost reduction can be achieved in the formation of a multilayer coating film, and a multilayer coating film having low-temperature impact resistance (e.g., dupont impact strength at-20 ℃) which can be sufficiently used even in cold regions can be formed.
The clear coating composition of the present invention having such an effect is a clear coating composition comprising a hydroxyl group-containing acrylic resin (c-1), a non-aqueous dispersion type acrylic resin (c-2), a diol resin (c-3), and a polyisocyanate compound (c-4) as coating film-forming resins,
the hydroxyl value of the hydroxyl-containing acrylic resin (c-1) is 80mgKOH/g to 220mgKOH/g, the weight-average molecular weight is 4,000 to 10,000, and the glass transition temperature is 30 ℃ to 60 ℃,
the non-aqueous dispersion acrylic resin (c-2) has a core-shell structure, the hydroxyl value of the core part is 100mgKOH/g to 200mgKOH/g, and the glass transition temperature of the core part is 40 ℃ to 80 ℃,
the hydroxyl value of the diol resin (c-3) is 80mgKOH/g or more and 200mgKOH/g or less, and the diol resin (c-3) comprises at least one selected from the group consisting of a polycarbonate diol resin, a polyether diol resin, a polyester diol resin and a polycaprolactone diol resin.
The clear coating film formed from the clear coating composition of the present disclosure has a dupont impact strength of 4.9J or more at-20 ℃.
In another embodiment, a multilayer coating film having a primer coating film formed from the primer coating composition, a base coating film formed from the base coating composition, and a clear coating film formed from the clear coating composition according to the present disclosure has a dupont impact strength of 4.9J or more at-20 ℃.
The clear coating composition of the present disclosure having such characteristics greatly reduces breakage of a vehicle body, an automobile part, and the like even when colliding with a snow bank (snowbank) in a cold region such as europe and china.
In addition, in the multilayer coating film having a clear coating film formed from the clear coating composition of the present invention, by setting the low-temperature impact resistance within the above range, good low-temperature impact resistance can be imparted even to a substrate having a thin portion.
Here, since the coating film forming resin according to the present invention contains a combination of specific resins having specific parameters, particularly the diol resin (c-3) according to the present invention, the coating film can be cured even by baking for a short time. Furthermore, the obtained coating film has excellent hardness even when baked and cured in a short time, and has excellent impact resistance at low temperatures.
Further, since the coating film-forming resin according to the present invention has the nonaqueous dispersion type acrylic resin (c-2) as the coating film-forming resin, the coating film can be cured even by baking for a short time, for example, due to a synergistic effect with the diol resin (c-3) according to the present invention. Furthermore, the obtained coating film has excellent hardness even when baked and cured in a short time, and has excellent impact resistance at low temperatures.
In addition, the multilayer coating film having a clear coating film formed from the clear coating composition of the present invention can exhibit such various effects.
The DuPont impact strength at-20 ℃ as described above can be measured by the method described in detail in the examples described later. Further, one skilled in the art can measure the DuPont impact strength at-20 ℃ for the clear coating film and the DuPont impact strength at-20 ℃ for the multilayer coating film based on the description of the present specification and examples.
In another aspect of the present invention, a coating composition set for forming a multilayer coating film and a method for forming a multilayer coating film include: a transparent coating film having the above characteristics.
The present invention will be described in detail below.
[ clear coating composition ]
The clear coating composition of the present disclosure comprises a hydroxyl group-containing acrylic resin (c-1), a non-aqueous dispersion type acrylic resin (c-2), a diol resin (c-3), and a polyisocyanate compound (c-4) as a coating film-forming resin.
Acrylic resin containing hydroxyl group (c-1)
The hydroxyl group-containing acrylic resin (c-1) has a hydroxyl value of 80mgKOH/g to 220mgKOH/g, a weight-average molecular weight of 4,000 to 10,000, and a glass transition temperature of 30 ℃ to 60 ℃.
In the present invention, the "acrylic resin" refers to a polymer obtained by polymerizing a monomer composition containing at least one monomer selected from acrylic acid and esters thereof, methacrylic acid and esters thereof.
The hydroxyl group-containing acrylic resin (c-1) has a hydroxyl value (OHV) of 80mgKOH/g or more and 220mgKOH/g or less, specifically 80mgKOH/g or more and 210mgKOH/g or less, for example 80mgKOH/g or more and 205mgKOH/g or less. In another embodiment, the hydroxyl value is 110mgKOH/g or more and 180mgKOH/g or less.
When the hydroxyl value is in such a range, a good crosslinking density can be imparted to the transparent coating film. Can impart excellent alcohol gasoline resistance, weather resistance and the like to a clear coating film and a multilayer coating film. Furthermore, hydrophilization of the coating film can be suppressed, and the transparent coating film and the multilayer coating film can be made excellent in water resistance and moisture resistance.
The hydroxyl value can be determined by a neutralization titration method using an aqueous potassium hydroxide solution described in JIS K0070.
The weight average molecular weight of the hydroxyl group-containing acrylic resin (c-1) may be 4,000 or more and 10,000 or less, in a specific embodiment 4,000 or more and 8,000 or less, in another embodiment 4,000 or more and 6,000 or less, for example, 4,000 or more and 5,500 or less.
When the weight average molecular weight of the hydroxyl group-containing acrylic resin (c-1) is in such a range, for example, when the clear coating film-forming composition is applied to a base coating film or an uncured base coating film, the occurrence of phase mixing of the clear coating film-forming composition and the base coating film can be suppressed, and a good appearance of the clear coating film (finished appearance) can be obtained. In addition, the quick-drying property of the clear coating film-forming composition can be improved.
Further, the clear coating composition can have an appropriate viscosity, and the use of a solvent that reduces the viscosity at the time of coating of the clear coating film-forming composition can be reduced, and the increase in Volatile Organic Compounds (VOC) can be suppressed. Furthermore, the resulting coating film can have excellent alcohol gasoline resistance, weather resistance and appearance.
The weight average molecular weight in the present specification is a value measured by gel permeation chromatography using HLC-8200 (manufactured by Tosoh corporation). The measurement conditions are as follows.
Column TSgel Super Multipore HZ-M3
Tetrahydrofuran as developing solvent
Column injection port oven 40 deg.C
Flow rate 0.35ml
Detector RI
PS oligomer kit manufactured by Tosoh Corp of Standard polystyrene
The hydroxyl group-containing acrylic resin (c-1) has a glass transition temperature of 30 ℃ or higher and 60 ℃ or lower, in a specific embodiment 30 ℃ or higher and 55 ℃ or lower, for example 30 ℃ or higher and 50 ℃ or lower. When the glass transition temperature is in such a range, the transparent coating film can have excellent stain resistance, scratch resistance and hardness, and the quick-drying property of the transparent coating film-forming composition can be improved. And may have an excellent appearance.
The glass transition temperature in the present specification is a value measured by the following procedure using a Differential Scanning Calorimeter (DSC) (thermal analysis apparatus SSC5200 (fine electronic products)). Specifically, in the step (step 1) of raising the temperature from 20 ℃ to 150 ℃ at a temperature raising rate of 10 ℃/min, the step (step 2) of lowering the temperature from 150 ℃ to-50 ℃ at a temperature lowering rate of 10 ℃/min, and the step (step 3) of raising the temperature from-50 ℃ to 150 ℃ at a temperature raising rate of 10 ℃/min, the value obtained from the graph (chart) at the time of raising the temperature in the step 3 is taken as the glass transition temperature.
In a specific embodiment, the solid Acid Value (AV) of the hydroxyl group-containing acrylic resin (c-1) is 2mgKOH/g or more and 30mgKOH/g or less, for example, 5mgKOH/g or more and 20mgKOH/g or less. In another embodiment, the ratio is 5mgKOH/g or less and 15mgKOH/g or less. When the acid value of the hydroxyl group-containing acrylic resin (c-1) is in such a range, a transparent coating film having an excellent appearance can be formed.
In addition, for example, in the case where the base coating film is formed adjacent to the clear coating film, the clear coating composition according to the present disclosure can be prevented from being mixed with the base coating composition (base coating film), and a clear coating film having excellent design properties can be formed.
Examples of suitable monomer compositions that can constitute the hydroxyl group-containing acrylic resin (c-1) according to the present invention and satisfy the above conditions include: hydroxyl-containing hydroxy acrylates including 2-hydroxyethyl acrylate, 4-hydroxybutyl acrylate, and the like; at least one kind of composition of hydroxyl methacrylate such as 2-hydroxyethyl methacrylate and 4-hydroxybutyl methacrylate containing hydroxyl group, and further, if necessary, contains acrylic acid; acrylic esters such as methyl acrylate, butyl acrylate, isobutyl acrylate, t-butyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate, isobornyl acrylate and the like; methacrylic acid; methacrylates such as methyl methacrylate, butyl methacrylate, isobutyl methacrylate, tert-butyl methacrylate, 2-ethylhexyl methacrylate, lauryl methacrylate, and isobornyl methacrylate; and at least one of ethylenically unsaturated monomers having an aromatic ring such as styrene. The composition of the monomer composition can be suitably adjusted depending on various physical properties required for the hydroxyl group-containing acrylic resin.
The monomer composition can be polymerized using a solvent such as butyl acetate. The kind of the solvent, the concentration of the monomer composition during polymerization, the kind and amount of the polymerization initiator, the polymerization temperature, the polymerization time and other polymerization conditions can be appropriately adjusted according to the various physical properties required for the hydroxyl group-containing acrylic resin (c-1). Therefore, the method for producing the hydroxyl group-containing acrylic resin (c-1) is not particularly limited, and a commercially available hydroxyl group-containing acrylic resin (c-1) can be used.
The amount of the hydroxyl group-containing acrylic resin (c-1) is more preferably 50 to 90 mass%, and still more preferably 60 to 90 mass%, based on 100 parts by mass of the total of the resin solid components of the clear coating composition comprising the hydroxyl group-containing acrylic resin (c-1), the non-aqueous dispersion type acrylic resin (c-2), and the diol resin (c-3). By using such an amount, the smoothness of the transparent coating film, for example, the transparent coating film formed on the base coating film (b) can be favorably maintained. Further, the clear coating composition is excellent in drying property, and can provide good workability to the clear coating composition.
(non-Water Dispersion type acrylic resin (c-2))
The non-aqueous dispersion acrylic resin (c-2) has a core-shell structure, wherein the hydroxyl value of the core part is 100mgKOH/g or more and 200mgKOH/g or less, and the glass transition temperature of the core part is 40 ℃ or more and 80 ℃ or less.
Further, the acrylic resin (c-2) was a Non-Aqueous Dispersion (Non Aqueous Dispersion).
The non-aqueous dispersion type acrylic resin (c-2) is considered to be present in the form of particles in the clear coating composition, and by using the non-aqueous dispersion type acrylic resin (c-2), a coating composition having a low viscosity can be obtained despite its high molecular weight. The non-aqueous dispersion acrylic resin (c-2) becomes a linear polymer in the step of heat curing (baking), and the drying property is improved. Further, since the non-aqueous dispersion type acrylic resin (c-2) is considered to be in the form of particles in the clear coating composition, the functional groups (carboxyl group and hydroxyl group) and the polyisocyanate compound can be separated in the clear coating composition until the heat curing (baking) step is performed. The functional group and the polyisocyanate compound react during the baking step. Therefore, the pot life of the coating composition can be extended and the reactivity can be improved.
Further, by containing the nonaqueous dispersion type acrylic resin (c-2), curing shrinkage can be suppressed, and a coating film having excellent appearance of the coating film can be formed.
In addition, the baking temperature in the baking step (heat curing step) can be lowered, and the baking time can be shortened. Moreover, the coating composition is excellent in maintainability, and can ensure excellent coating workability and production technology such as assembly and workability.
The core part has a hydroxyl value of 100mgKOH/g or more and 200mgKOH/g or less, specifically 110mgKOH/g or more and 200mgKOH/g or less, for example, 120mgKOH/g or more and 200mgKOH/g or less.
When the hydroxyl value of the core portion is in the above range, crosslinking of the coating film forming resin contained in the clear coating film is sufficiently advanced, and thus excellent alcohol gasoline resistance and weather resistance can be imparted to a coating film formed by applying the clear coating composition to, for example, a base coating film or an uncured base coating film and heat-curing (baking). In addition, hydrophilization of the coating film can be suppressed, and deterioration in water resistance can be prevented. Further, by using the non-aqueous dispersion having the above-mentioned constitution, the drying property immediately after heat curing (baking) can be improved, and the pot life of the coating composition can be improved.
The glass transition temperature of the core portion is 40 ℃ or more and 80 ℃ or less, in a specific embodiment 45 ℃ or more and 80 ℃ or less, for example 50 ℃ or more and 80 ℃ or less. By setting the glass transition temperature of the core portion within such a range, the clear coating composition can maintain good drying properties. For example, a coating film formed by applying a clear coating composition to a base coating film or an uncured base coating film and curing (baking) the coating film by heating can be improved in drying property. Further, firing required for forming a clear coating film can be performed in a shorter time than in the conventional art, and the required hardness and appearance of the coating film can be ensured. In addition, the transparent coating film obtained can have excellent smoothness and flexibility.
In a specific embodiment, the Acid Value (AV) of the core portion is 60mgKOH/g or less, for example, 50mgKOH/g or less, and preferably 40mgKOH/g or less. The lower limit value is not less than 0 mgKOH/g. When the acid value of the core portion is in the above range, a multilayer coating film formed by coating and baking the clear coating composition on the base coating film can have good water resistance.
The molecular weight of the core portion is high, such as about one hundred thousand, and the particle structure is collapsed by volatilization of the solvent during the baking step, and the core portion can be a linear polymer. In addition, the core portion may have a crosslinked structure.
The nonaqueous dispersion type acrylic resin (c-2) preferably contains, for example, a hydroxyl group-containing acrylate monomer having a highly reactive hydroxyl group and an ester moiety having 4 to 12 carbon atoms in the core portion (i). By including such an ester monomer, low-temperature short-time firing can be performed more efficiently.
In another embodiment, the non-aqueous dispersion type acrylic resin (c-2) may contain a carboxyl group-containing compound in the core part (ii)α,βEthylenically unsaturated monomers and monomers having hydroxyl groupsα,β-ethylenically unsaturated monomers.
(i) Of a core part comprising a monomer
The hydroxyl group-containing acrylate monomer (hereinafter, sometimes referred to as a long-chain hydroxyl group-containing monomer) contained in the core portion may be an acrylate monomer having a hydroxyl group, preferably a hydroxyl group at the end of an ester portion, and having 4 to 12 carbon atoms in the ester portion. The number of carbon atoms in the ester moiety is preferably 4 to 8.
Examples thereof include: acrylic acid 4-hydroxybutyl ester (4HBA), CH2＝C(R)COO(CH2)2O[CO(CH2)mO]nH (wherein R is hydrogen or C6 or lower alkyl, and m and n are ″ (CH)2)2O[CO(CH2)mO]n"a natural number having 4 or more and 12 or less carbon atoms at the site), and the like.
Further, as the hydroxyl group-containing acrylate monomer, commercially available ones can be used.
In the case where the core portion contains a hydroxyl group-containing acrylate monomer, the core portion may further contain a monomer having a carboxyl groupα,βEthylenically unsaturated monomers and monomers having hydroxyl groupsα,βEthylenically unsaturated monomers (hereinafter, sometimes referred to as short-chain hydroxyl group-containing monomers).
In a specific embodiment, the core may further include, as necessary: acrylic acid; acrylic esters such as methyl acrylate; methacrylic acid; methacrylic acid esters such as methyl methacrylate; monomers having 1 or 2 vinyl groups; a monomer having an isocyanate group; a monomer having an allyl group; a monomer having an epoxy group; anhydrides having 1 or 2 vinyl groups.
Having carboxyl group as contained in core partα,βExamples of the ethylenically unsaturated monomer (hereinafter, may be referred to as a COOH-containing monomer) include: acrylic acid, methacrylic acid, and the like.
As a core partHaving a hydroxyl group contained thereinα,βEthylenically unsaturated monomers, mention may be made of: an acrylate monomer having a hydroxyl group and an ester moiety having 1 to 3 or 13 or more carbon atoms, and among them, an acrylate monomer having an ester moiety having 1 to 3 carbon atoms is more preferable, and specific examples thereof include: hydroxyl-containing hydroxy acrylates such as 2-hydroxyethyl acrylate; hydroxyl group-containing hydroxy methacrylate such as 2-hydroxyethyl methacrylate.
(ii) Of a core part comprising a monomer
Having carboxyl group as contained in core partα,βSpecific examples of the ethylenically unsaturated monomer include: acrylic acid, methacrylic acid, and the like.
Having a hydroxyl group as contained in the coreα,βEthylenically unsaturated monomers, mention may be made of: an acrylate monomer having a hydroxyl group and an ester moiety having 1 to 3 or 13 or more carbon atoms, and among them, an acrylate monomer having an ester moiety having 1 to 3 carbon atoms is more preferable, and specific examples thereof include: hydroxyl-containing hydroxy acrylates such as 2-hydroxyethyl acrylate; hydroxyl group-containing hydroxy methacrylate such as 2-hydroxyethyl methacrylate.
Further, the core portion may include, as necessary: acrylic acid; acrylic esters such as methyl acrylate; methacrylic acid; methacrylic acid esters such as methyl methacrylate; monomers having 1 or 2 vinyl groups; a monomer having an isocyanate group; a monomer having an allyl group; a monomer having an epoxy group; anhydrides having 1 or 2 vinyl groups.
Examples of the core portion of the nonaqueous dispersion type acrylic resin (c-2) according to the present disclosure include: the case where the hydroxyl group-containing acrylate monomer having an ester moiety with 4 to 12 carbon atoms is contained (the core portion of (i)) and the case where the hydroxyl group-containing acrylate monomer is not contained (the core portion of (ii)).
In the case where the hydroxyl group-containing acrylate monomer having 4 to 12 carbon atoms in the ester moiety is contained, the hydroxyl group-containing acrylate monomer having a carboxyl group contained in the core moiety of the above (ii) may be further containedα,βEthylenically unsaturated monomers and having hydroxyl groupsOf the groupα,β-ethylenically unsaturated monomers. That is, the following 3 monomer compositions may be present in the core portion in the present invention: (I) the case of containing the above hydroxyl group-containing acrylate monomer; (II) containing the above hydroxyl group-containing acrylate monomer and having a carboxyl groupα,βEthylenically unsaturated monomers and monomers having hydroxyl groupsα,β-in the case of ethylenically unsaturated monomers; (III) a carboxyl group-containing acrylate monomer not containing the above hydroxyl group-containing acrylate monomerα,βEthylenically unsaturated monomers and monomers having hydroxyl groupsα,βThe case of ethylenically unsaturated monomers.
In a specific embodiment, the total amount of the carboxyl group contained in the core part and the hydroxyl group of the long-chain hydroxyl group-containing monomer is 0.1 to 1.5mmol/g, for example, 0.2 to 1.4 mmol/g. When the total amount of the carboxyl group contained in the core part and the hydroxyl group of the long-chain hydroxyl group-containing monomer is within such a range, the clear coating composition has good drying properties when applied to, for example, a base coating film or an uncured base coating film, and exhibits excellent workability. Furthermore, the crosslinking of the coating film becomes sufficient, and the transparent coating film and the multilayer coating film can be made to have good alcohol gasoline resistance and weather resistance. In addition, in the heat curing step (baking step) of baking the clear coating composition, the reaction rate with the polyisocyanate compound (c-4) can be maintained in an appropriate range, and the curing shrinkage of the formed clear coating film and multilayer coating film can be suppressed, and the appearance (finished appearance) of the clear coating film (multilayer coating film) can be made good.
Shell part
The shell portion (dispersion resin) in the non-aqueous dispersion type acrylic resin (c-2) is preferably formed of a hydroxyl group-containing acrylic resin, and more preferably satisfies the conditions of a hydroxyl value of 50mgKOH/g or more and 160mgKOH/g or less, a glass transition temperature of 0 ℃ or more and 80 ℃ or less, and an acid value of 30mgKOH/g or less (lower limit value is 0 mgKOH/g).
Specifically, the hydroxyl value of the shell is, in a specific embodiment, 50mgKOH/g or more and 160mgKOH/g or less, for example, 60mgKOH/g or more and 150mgKOH/g or less, and in one embodiment, 70mgKOH/g or more and 140mgKOH/g or less. When the hydroxyl value of the shell portion is in such a range, crosslinking of the coating film-forming resin contained in the clear coating film becomes sufficient, and a coating film formed by applying the clear coating composition to, for example, a base coating film or an uncured base coating film and heat-curing (baking) the coating film becomes a coating film excellent in alcohol gasoline resistance and weather resistance. In addition, the transparent coating film can be inhibited from being hydrophilized and from decreasing in water resistance.
In a specific embodiment, the shell has a glass transition temperature of 0 ℃ or more and 80 ℃ or less, for example, 10 ℃ or more and 80 ℃ or less, and in one embodiment 20 ℃ or more and 70 ℃ or less. When the glass transition temperature of the shell portion is in such a range, the transparent coating composition can be coated on, for example, a base coating film or an uncured base coating film, and can be cured by heating with good drying properties. Further, the hardness of the formed transparent coating film can be made excellent. Further, the viscosity (viscosity at the time of coating) when the clear coating composition is coated on, for example, a base coating film or an uncured base coating film can be suppressed from increasing, and the appearance (finished appearance) of the clear coating film can be made good. In addition, in order to reduce the viscosity at the time of coating, the amount of the solvent used can be reduced, and the Volatile Organic Compound (VOC) in the step of heat curing (baking) the clear coating composition can be reduced.
In a specific embodiment, the acid value of the shell portion in the nonaqueous dispersion type acrylic resin (c-2) is 30mgKOH/g or less, for example, 20mgKOH/g or less, and in another embodiment, 10mgKOH/g or less. When the acid value of the shell portion is in such a range, hydrophilization and reduction in water resistance of a clear coating film formed by baking a clear coating composition applied to, for example, a base coating film or an uncured base coating film can be suppressed.
Examples of the monomer composition suitable for producing the shell portion of the nonaqueous dispersion type acrylic resin (c-2) include: hydroxyl-containing hydroxy acrylates including 2-hydroxyethyl acrylate, 4-hydroxybutyl acrylate, and the like; at least one kind of composition of hydroxyl methacrylate such as 2-hydroxyethyl methacrylate and 4-hydroxybutyl methacrylate containing hydroxyl group, and further, if necessary, contains acrylic acid; acrylic esters such as methyl acrylate, butyl acrylate, isobutyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate, isobornyl acrylate, and the like; methacrylic acid; methacrylic acid esters such as methyl methacrylate, butyl methacrylate, isobutyl methacrylate, n-butyl methacrylate, 2-ethylhexyl methacrylate, lauryl methacrylate and isobornyl methacrylate; and at least one of ethylenically unsaturated monomers having an aromatic ring such as styrene. The composition of the monomer composition can be appropriately adjusted according to various physical properties required for the shell portion.
Further, the monomer composition may contain, as required: monomers having 1 or 2 vinyl groups; a monomer having an isocyanate group; a monomer having an allyl group; a monomer having an epoxy group; anhydrides having 1 or 2 vinyl groups.
The monomer composition for forming the shell portion may be polymerized using a solvent such as butyl acetate.
The polymerization conditions such as the kind of the solvent, the concentration of the monomer composition at the time of polymerization, the kind and amount of the polymerization initiator, the polymerization temperature, and the polymerization time can be appropriately adjusted according to various physical properties required for the shell portion. Therefore, the method for producing the shell portion is not particularly limited, and commercially available materials can be used.
(production of non-aqueous Dispersion type acrylic resin (c-2) having core-Shell Structure)
For example, by reacting the shell portion with a monomer of the hydroxyl group-containing acrylate forming the core portion and/orα,βThe non-aqueous dispersion type acrylic resin (c-2) can be produced by polymerizing a monomer such as an ethylenically unsaturated monomer in a solvent in which the monomer forming the core part is dissolved but the polymer is not dissolved.
The nonaqueous dispersion type acrylic resin (c-2) is stably present in the solvent by the interaction of the hydroxyl group, ester group, carboxyl group, etc. present in the core portion and the shell portion. For example, by blending a long-chain hydroxyl group-containing monomer in the core portion, it is expected that the stability of the nonaqueous dispersion type acrylic resin (c-2) is further improved. The reason is considered to be that: since the ester portion of the long-chain hydroxyl group has a flexible structure in which the number of carbon atoms is 4 or more and is long, the efficiency of interaction with the hydroxyl group, the ester group, the carboxyl group, or the like of the shell portion is improved.
In addition to blending a long-chain hydroxyl group-containing monomer to the core part, in order to further improve the stability of the nonaqueous dispersion type acrylic resin (c-2), the core part may be grafted to the shell part, or the core part may be crosslinked, and these may be blended with a long-chain hydroxyl group-containing monomer to the core part.
In the graft polymerization, a graft structure or a crosslinked structure may be formed between an acryl group or a methacryl group, and a vinyl group or an isocyanate group, an allyl group, an epoxy group, or an acid anhydride group of a monomer contained in the shell portion or the core portion as necessary. By adjusting the amount of these functional groups, the degree of crosslinking can be adjusted.
In the graft polymerization, a solvent such as butyl acetate may be used. The polymerization conditions such as the kind of the solvent, the concentrations of the shell portion and the core portion at the time of polymerization, the kind and amount of the polymerization initiator, the polymerization temperature, and the polymerization time may be appropriately adjusted according to various physical properties required for the nonaqueous dispersion type acrylic resin (c-2), and are not particularly limited.
In a specific embodiment, the mass ratio of the hydroxyl group-containing acrylic resin (c-1) to the non-aqueous dispersion acrylic resin (c-2) is ((c-1)/(c-2)) -90/10-50/50, preferably ((c-1)/(c-2)) -85/15-60/40, for example ((c-1)/(c-2)) -80/20-65/35.
When the mass ratio of the hydroxyl group-containing acrylic resin (c-1) according to the present invention to the non-aqueous dispersion acrylic resin (c-2) according to the present invention is in such a range, the transparent coating film can maintain the hardness and good appearance after firing the transparent coating composition. Further, the composition can be fired at a lower temperature for a shorter time than a conventionally used clear coating composition to ensure the hardness and appearance of a clear coating film, and can reduce the load on the environment.
The amount of the nonaqueous dispersion type acrylic resin (c-2) contained in the clear coating composition is, for example, 10 to 50 mass%, specifically 10 to 40 mass%, based on 100 parts by mass of the total of the resin solids of the clear coating composition containing the hydroxyl group-containing acrylic resin (c-1), the nonaqueous dispersion type acrylic resin (c-2), and the diol resin (c-3) as the clear coating film-forming resin. When the amount of the non-aqueous dispersion type acrylic resin (c-2) is in such a range, the drying property of the clear coating composition becomes good, and a composition excellent in handling property can be obtained. Further, the smoothness of the formed transparent coating film can be improved.
(diol resin (c-3))
The clear coating composition according to the present invention contains a diol resin (c-3) as a coating film-forming resin. The hydroxyl value of the diol resin (c-3) is not less than 80mgKOH/g and not more than 200 mgKOH/g. Further, the diol resin (c-3) comprises at least one selected from the group consisting of a polycarbonate diol resin, a polyether diol resin, a polyester diol resin and a polycaprolactone diol resin. The diol resin is preferably liquid at room temperature of 20 ℃.
By providing the diol resin (c-3) with such characteristics, the cured coating film (clear coating film) after firing of the clear coating composition can secure excellent film properties and dupont impact strength. For example, the steel sheet can have more excellent dupont impact strength at low temperatures.
Further, since the viscosity is low, fluidity and appearance can be improved.
In a specific embodiment, the diol resin (c-3) has a hydroxyl value of 80mgKOH/g or more and 180mgKOH/g or less, for example, a hydroxyl value of 80mgKOH/g or more and 160mgKOH/g or less. In another embodiment, the hydroxyl value of the diol resin (c-3) is 90mgKOH/g or more and 180mgKOH/g or less, for example, the hydroxyl value of the diol resin (c-3) is 90mgKOH/g or more and 170mgKOH/g or less.
When the hydroxyl value of the diol resin (c-3) is in such a range, a cured coating film (clear coating film) after firing of the clear coating composition can secure more excellent coating film performance and dupont impact strength. For example, the steel sheet can have more excellent dupont impact strength at low temperatures.
In a specific embodiment, the glass transition temperature of the diol resin (c-3) is 0 ℃ or lower, for example, -20 ℃ or lower. In a specific embodiment, the glass transition temperature of the diol resin (c-3) is-50 ℃ or lower.
By setting the glass transition temperature of the diol resin (c-3) in such a range, the DuPont impact strength at low temperatures, for example, -20 ℃ can be made better.
The diol resin (c-3) may be, for example, at least one resin selected from the group consisting of a polycarbonate diol resin, a polyether diol resin, a polyester diol resin and a polycaprolactone diol resin. For example, the diol resin (c-3) is a polyester diol resin. The use of a polyester diol resin as the diol resin (c-3) provides a more favorable dupont impact strength at low temperatures.
The polyester diol resin can be produced by a general polymerization method. For example, the polyester diol resin can be produced by subjecting an alcohol compound to a condensation reaction with an acid. Specifically, the compound can be produced by condensation reaction of an alcohol compound such as methyl propylene glycol or diethylene glycol with an acid such as adipic acid or isophthalic acid.
The amount of the diol resin (c-3) contained in the clear coating composition (in which the solid content of the solvent is removed) is, for example, 1 to 15 parts by mass, and in a specific embodiment, 1 to 10 parts by mass, based on 100 parts by mass of the total of the resin solid contents of the hydroxyl group-containing acrylic resin (c-1) and the nonaqueous dispersion type acrylic resin (c-2).
When the amount of the diol resin (c-3) is in such a range, the resulting coating film has excellent low-temperature dupont impact strength after baking and can have a well-balanced coating film hardness required for a transparent coating film.
Further, by containing the diol resin (c-3) according to the present invention in the above range, the clear coating composition according to the present disclosure having a combination of the specific hydroxyl group-containing acrylic resin (c-1) according to the present disclosure, the specific non-aqueous dispersion type acrylic resin (c-2) according to the present disclosure, and the polyisocyanate compound (c-4) can be baked in a shorter time.
(polyisocyanate Compound (c-4))
The transparent coating film (c) according to the present invention contains a polyisocyanate compound (c-4) as a coating film-forming resin. By using the polyisocyanate compound (c-4), the curing reaction speed of the clear coating composition can be improved.
For example, when the core of the nonaqueous dispersion type acrylic resin (c-2) contains a hydroxyl group-containing acrylate monomer, the use of the polyisocyanate compound (c-4) can further increase the curing reaction rate of the clear coating composition.
The clear coating composition according to the present invention comprises the polyisocyanate compound (c-4) as a coating film-forming resin, and when the clear coating composition is applied, for example, a part of the polyisocyanate compound (c-4) can permeate into a base coating film which can be a lower layer or an uncured base coating film, thereby improving the crosslinkability between the base coating film (b) and the clear coating film (c).
The polyisocyanate compound (c-4) is not particularly limited as long as it is a compound having 2 or more isocyanate groups, and examples thereof include: aromatic diisocyanates such as tolylene diisocyanate, 4' -diphenylmethane diisocyanate, xylylene diisocyanate, and m-xylylene diisocyanate; aliphatic diisocyanates such as hexamethylene diisocyanate; alicyclic diisocyanates such as isophorone diisocyanate; monomers thereof, and polymers thereof such as biuret type, urethane type, adduct type and the like. For example, it further preferably has an isocyanurate and uretdione structure.
Commercially available products of polyisocyanate compounds include: duranate 24A-90PX (NCO: 23.6%, trade name, manufactured by Asahi Kasei Co., Ltd.), スミジュール N-3200-90M (trade name, manufactured by Kasei Bayer Urethane Co., Ltd.), タケネ - ト D165N-90X (trade name, manufactured by Mitsui Wutian Chemical Co., Ltd.), スミジュール N-3300, スミジュール N-3500 (trade name, manufactured by Kasei Bayer Urethane Co., Ltd.), Duranate THA-100 (trade name, manufactured by Asahi Kasei Co., Ltd.), and the like. Further, blocked isocyanates obtained by blocking these may be used as needed.
In a specific embodiment, the equivalent ratio (NCO/OH) of the isocyanate group contained in the polyisocyanate compound (c-4) to the hydroxyl group contained in the hydroxyl group-containing acrylic resin (c-1), the non-aqueous dispersion type acrylic resin (c-2) and the diol resin (c-3) is 0.7 or more and 2.0 or less, preferably 0.7 or more and 1.8 or less (NCO/OH), for example, 0.8 or more and 1.5 or less (NCO/OH), and in a specific embodiment, 0.8 or more and less than 1.0. When the equivalent ratio (NCO/OH) is in such a range, a transparent coating film having a sufficient crosslinking density and excellent strength can be obtained. Furthermore, a coating film having excellent weather resistance and hardness can be formed. It is preferable that the polyisocyanate compound (c-4) is contained in the clear coating composition in an amount having an equivalent ratio as described above between the hydroxyl group-containing acrylic resin (c-1) and the non-water dispersible acrylic resin (c-2).
In the present specification, unless otherwise specified, the term "equivalent ratio" (NCO/OH) refers to the equivalent ratio of isocyanate groups contained in the polyisocyanate compound (c-4) to hydroxyl groups contained in the hydroxyl group-containing acrylic resin (c-1), the non-aqueous dispersion type acrylic resin (c-2) and the diol resin (c-3).
(cellulose derivative)
The clear coating composition of the present disclosure may further comprise a cellulose derivative. By containing the cellulose derivative, the clear coating composition of the present disclosure can be fired at a lower temperature and in a shorter time than in the prior art, and can secure the hardness and appearance of the coating film required for the clear coating film even when fired at a low temperature and in a short time.
The cellulose derivative is not particularly limited. Examples thereof include: at least one cellulose derivative selected from the group consisting of cellulose acetate butyrate, nitrocellulose, cellulose acetate, and cellulose acetate propionate. Cellulose acetate butyrate is more preferable from the viewpoint of solubility with the coating film forming resin, development of tackiness, and the like.
For example, the degree of acetylation is 1 to 34% by mass and the degree of butyrylation is 16 to 60% by mass as measured by the method described in ASTM-D-817, and the viscosity is particularly preferably in the range of 0.005 to 20 seconds as measured by the method described in ASTM-D-1343.
Examples of cellulose acetate butyrate include: eastman Chemical Products corporation CAB-551-0.01 (viscosity 0.01 seconds, butyryl content 53%), CAB-551-0.2 (viscosity 0.20 seconds, butyryl content 52%), CAB-531-1 (viscosity 1.90 seconds, butyryl content 50%), CAB-500-1 (viscosity 1.00 seconds, butyryl content 51%), CAB-500-5 (viscosity 5.00 seconds, butyryl content 51%), CAB-553-0.4 (viscosity 0.30 seconds, butyryl content 46%), CAB-381-0.1 (viscosity 0.10 seconds, butyryl content 38 seconds), CAB-381-0.5 (viscosity 0.50 seconds, butyryl content 38 seconds), CAB-381-2.10 seconds, butyryl content 38 seconds), CAB-381-0.5 (viscosity 0.50 seconds, butyryl content 38 seconds, butyryl content 381-2.38 seconds, 0.321-0.321 seconds), and the like, Butyryl content 31.2%), CAB-171-15S (viscosity 15.00 sec, butyryl content 17%), and the like.
The cellulose acetate butyrate may be used singly or in combination of two or more.
In the embodiment where the clear coating composition contains a cellulose derivative, the amount of the cellulose derivative is 0.1 part by mass or more and 10 parts by mass or less, for example, 0.2 part by mass or more and 7 parts by mass or less, and in a specific embodiment, 0.5 part by mass or more and 5 parts by mass or less, based on 100 parts by mass of the total of the resin solid components of the clear coating composition containing the hydroxyl group-containing acrylic resin (c-1), the non-aqueous dispersion type acrylic resin (c-2), and the diol resin (c-3). When the cellulose derivative is contained in such a range, the coating workability of the clear coating composition can be further improved.
(other additives)
The clear coating composition for forming a clear coating film may further contain, in addition to the above-mentioned components, additives which are generally blended in the coating field. For example, a base color pigment, a metallic pigment may be contained within a range in which transparency is not suppressed. Further, an ultraviolet absorber, a hindered amine light stabilizer, an antioxidant, crosslinked resin particles, a surface conditioner, and the like may be blended.
The clear coating composition may contain a solvent such as butyl acetate which can be used in the production thereof. The concentration of the clear coating composition may be a concentration that is excellent in workability, and may be diluted with a solvent such as butyl acetate as needed.
The clear coating composition may contain known catalysts. For example, a tin catalyst may be included. The amount of the tin catalyst to be blended in the clear coating composition is 0 to 0.05 parts by mass, more specifically 0.01 to 0.02 parts by mass, based on 100 parts by mass of the total of the resin solid content of the hydroxyl group-containing acrylic resin (c-1) and the nonaqueous dispersion type acrylic resin (c-2) blended in the clear coating composition components. By blending the amount of the tin catalyst within such a range, the reaction rate with the coating film forming resin contained in the clear coating composition can be set within an appropriate range in the step of baking the clear coating composition, and the drying property of the coating composition can be improved. In addition, the obtained transparent coating film can obtain more sufficient hardness, coating film appearance and the like. Further, the pot life of the clear coating composition can be adjusted by appropriately selecting the amount of the catalyst to be blended, and therefore, the composition can exhibit good workability also for vehicle bodies, large-sized automobile parts, for example, bumper parts, and the like.
(formation of clear coating film)
The clear coating film is a dried coating film of a clear coating composition comprising a hydroxyl group-containing acrylic resin (c-1), a non-aqueous dispersion type acrylic resin (c-2), a diol resin (c-3), and a polyisocyanate compound (c-4) as a coating film-forming resin.
The thickness of the transparent coating film is preferably 15 to 50μAnd m is selected. If less than 15μm may cause a problem that unevenness of the substrate cannot be hidden, and if m exceeds 50, the thickness of the substrate may not be coveredμm may cause problems such as blistering and sagging (ワキ seed タレ) during coating. Preferably 20μm is 45 or moreμm is less than or equal to m. The dry film thickness can be measured by using SDM-miniR manufactured by SANKO.
(coating composition set for Forming multilayer coating film)
In one aspect, the present disclosure provides a coating composition set for forming a multilayer coating film.
The coating composition set for forming a multilayer coating film according to the present disclosure is a coating composition set for forming a multilayer coating film comprising a primer coating composition for forming a primer coating film, a base coating composition for forming a base coating film, and a clear coating composition (c) according to the present disclosure for forming a clear coating film,
the tensile elongation of the dried coating film of the primer coating composition at-20 ℃ is 5 to 35 percent,
the base coating composition includes a colorant.
By providing the specific coating composition according to the present disclosure to the multilayer coating film-forming coating composition set, the multilayer coating film formed using the multilayer coating film-forming coating composition set has excellent impact resistance even in cold regions (e.g., -20 ℃).
Furthermore, the coating composition set for forming a multilayer coating film according to the present invention can provide a multilayer coating film having excellent appearance, hardness, initial adhesion, alcohol gasoline resistance, moisture resistance, alcohol gasoline resistance, and impact resistance (e.g., DuPont impact strength at-20 ℃). Moreover, the coating film has good followability.
For example, the multilayer coating film can be obtained by applying a primer coating composition (a) for forming a primer coating film, a base coating composition (b) for forming a base coating film, and a clear coating composition (c) for forming a clear coating film to an object to be coated in this order and heat-curing (baking) the coating film.
The respective coating compositions (a) and (b) and the multilayer coating film will be described below.
The clear coating composition (c) according to the present disclosure may be referred to the above-mentioned clear coating composition.
In a specific embodiment, the multi-layer coating film having a primer coating film, a base coating film, and a clear coating film has a DuPont impact strength of 4.9J or more at-20 ℃. Thereby, even in cold regions (for example, -20 ℃), the impact resistance can be further improved.
(primer coating composition (a))
The primer coating composition (a) according to the present invention preferably has a tensile elongation at-20 ℃ of a dried coating film of the primer coating composition (a) of 5 to 35%. For example, the tensile elongation may be 10 to 30%.
In the present specification, the tensile elongation is a value measured by the method shown below.
(method of measuring tensile elongation)
(i) Coating film is strippableThe separated coating plate was coated to a dry film thickness of 30μm, and drying at 80 ℃ for 25 minutes after coating to form a coating film.
(ii) The test piece was made to have a length of 10mm × 50mm, masking tapes were stuck to both ends, and the remaining half of the masking tape was folded back.
(iii) The measurement was carried out at a tensile rate of 5 mm/min under an atmosphere of-20 ℃ by Shimadzu Autograph (AG-IS).
(iv) 5 samples were measured and the average was calculated.
When the tensile elongation of the primer coating film (a) is in such a range, the impact strength at a low temperature (for example, -20 ℃) is excellent. In addition, the coating film has good followability.
The primer coating composition is not particularly limited as long as the above tensile elongation is satisfied.
In particular, a conductive primer that can impart conductivity to a plastic substrate as an object to be coated is preferable. Among them, the water-based conductive primer is preferable, and examples thereof include: the primer resin and the conductive agent (carbon black, antimony-doped tin oxide-treated titanium oxide, etc.) may further contain a white pigment and other raw materials as required.
The blending ratio of water in the water-based conductive primer is preferably 45 to 90% by mass, and more preferably 50 to 80% by mass, based on the entire conductive primer. When the blending ratio of water is less than 45 mass%, the viscosity increases, and the storage stability and coating workability deteriorate. On the other hand, if the blending ratio of water exceeds 90 mass%, the ratio of the nonvolatile amount decreases, the coating efficiency deteriorates, and appearance abnormalities such as sagging and bubbles are likely to occur. The aqueous conductive primer may further contain an organic solvent, and the blending ratio thereof is usually 40% by mass or less with respect to the water contained therein.
As the resin component for the primer of the above-mentioned aqueous conductive primer, acid-modified polypropylene, acid-modified chlorinated polyolefin, epoxy resin, polyurethane resin; pigment dispersion resins such as water-based alkyd resins and water-soluble acrylic resins. These may also be all included.
The primer coating composition (a) for forming a primer coating film can be applied by a method such as spray coating or Bell coating. The substrate may be washed or degreased as necessary.
The primer coating film is preferably 5 to 30 in terms of dry film thicknessμAnd m is selected. If less than 5μm, the concealing property becomes insufficient, and if it exceeds 30μm, bubbles and sagging are likely to occur. Preferably 10 to 20μAnd m is selected. The dry film thickness can be measured by using SDM-miniR manufactured by SANKO.
The elongation of the coating film formed from the primer coating composition can be adjusted by adjusting the composition contained in the primer coating composition and the composition of the resin used by a known method. Further, the coating composition can be adjusted by using a softening agent such as alkyd resin for coating, polyester resin for coating, polybutylene glycol, polypropylene glycol, polyethylene glycol, or a mixture thereof.
(base coating composition (b))
The base coating composition of the present invention comprises a colorant. Such a base coating composition is not particularly limited, and may be a known coating composition. The colorant may be a known inorganic pigment, organic pigment, or the like. The blending amount thereof is also not particularly limited. The base coating composition may be either aqueous or solvent-based. Additionally, a variety of coating compositions forming two layers, a pigmented base layer and a mica base layer, may be included. The thickness of the base coating film formed from the base coating composition is preferably 10μm is more than and equal to 30μm is less than or equal to m. If less than 10μm, there is a possibility that the hiding property is insufficient, and if m exceeds 30μm, there is a possibility that defects such as sagging and air bubbles may occur. Preferably 15μm is more than or equal to 20μm is less than or equal to m. The dry film thickness can be measured by using SDM-miniR manufactured by SANKO.
The base coating composition is preferably a coating film formed from a solvent-based one-pack type base coating composition, a solvent-based two-pack type base coating composition, or an aqueous one-pack type base coating composition. Any of these forms may be suitable for the purposes of the present invention.
The base coating film can be formed from the base coating composition (b) by a known method.
In a specific embodiment, the coating composition set for forming a multilayer coating film of the present disclosure can form a multilayer coating film having a primer coating film formed from the primer coating composition, a base coating film formed from the base coating composition, and a clear coating film formed from the clear coating composition of the present disclosure, and the obtained multilayer coating film has a dupont impact strength of 4.9J or more at-20 ℃.
The coating composition set for forming a multilayer coating film of the present disclosure, which can form such a specific multilayer coating film, can form a multilayer coating film having excellent impact resistance even in cold regions and can form a multilayer coating film having excellent appearance. In addition, for example, the baking temperature in the baking step of the clear coating composition can be lowered, and the baking time can be shortened.
The DuPont impact strength at-20 ℃ of the multilayer coating film can be measured as described in examples below.
(method of Forming multilayer coating film)
In another aspect, the present disclosure discloses a method of forming a multilayer coating film as follows.
The disclosed method for forming a multilayer coating film comprises the following steps:
a step of forming an uncured clear coating film by applying the clear coating composition according to the present disclosure; and
and heating and curing the uncured transparent coating film to form a transparent coating film.
In the method for forming a multilayer coating film of the present disclosure, since a specific clear coating composition is used, for example, the curing time of the clear coating composition can be greatly shortened, and the curing time of the multilayer coating film can be greatly shortened. In addition, the curing temperature can be greatly reduced.
In addition, the multilayer coating film obtained by the formation method of the present disclosure can form a multilayer coating film having excellent impact resistance even in cold regions, and can form a multilayer coating film having excellent appearance.
The method for forming a multilayer coating film of the present disclosure includes: and heating and curing the uncured coating film to form a multilayer coating film. The uncured coating film can be cured by heating, for example, by baking.
In a specific embodiment, the conditions for heat curing are 65 ℃ to 90 ℃ inclusive, and 5 minutes to 45 minutes inclusive. The conditions for heat curing may be, for example, 70 ℃ to 80 ℃.
For example, by setting the heat curing in such a range, it is possible to achieve both rapid curing of the coating film and prevention of deformation of the plastic molded article. Furthermore, a coating film excellent in moisture resistance and low-temperature impact resistance (e.g., DuPont impact strength at-20 ℃ C.) can be formed.
Here, according to the production method of the present invention, for example, the baking time, which has conventionally been required to be about 20 minutes at 80 ℃, can be reduced to about half even at the same baking temperature. Further, a multilayer coating film can be formed on a large object to be coated, for example, a vehicle, a bumper, or the like in a shorter time than in the past. This is considered to be: the method of the present invention is directed to a clear coating composition comprising a combination of specific resins.
Even with such a heat curing (baking) time, the coating film can be sufficiently cured, and a multilayer coating film having excellent water resistance and alcohol gasoline resistance can be formed. Further, the heat curing time can be shortened as compared with the conventional one, and a multilayer coating film (cured coating film) excellent in tackiness and polishing properties can be formed.
Note that the heat curing (baking) time refers to a time during which the surface of the substrate is actually kept at the target baking temperature, and more specifically, does not take into consideration a time until the target baking temperature is reached, but refers to a time during which the temperature is kept after the target temperature is reached.
Examples of the heating device for simultaneously baking the uncured film of the coating composition include: a drying furnace using a heat source such as hot air, electricity, gas, or infrared rays. Further, it is preferable to use a drying furnace using two or more of these heat sources in combination because the drying time is shortened.
In a specific embodiment, the method for forming a multilayer coating film of the present disclosure includes the steps of:
a step of applying a primer coating composition to an object to be coated to form an uncured primer coating film;
applying a base coating composition to the uncured base coating film to form an uncured base coating film;
a step of applying the clear coating composition according to the present disclosure to an uncured base coating film to form an uncured clear coating film; and
and a step of heat-curing the uncured coating film to form a multilayer coating film.
In a specific embodiment, the multi-layer coating film obtained by the multi-layer coating film forming method according to the present disclosure has a dupont impact strength of 4.9J or more at-20 ℃.
The multi-layer coating film obtained by the multi-layer coating film forming method according to the present disclosure has, for example, a dupont impact strength of 4.9J or more at-20 ℃. This is considered to be: the method of the present invention is directed to a clear coating composition comprising a combination of specific resins having specific parameters.
The primer coating composition, the base coating composition and the clear coating composition which can be used in the method for forming a multilayer coating film of the present disclosure can be cited with respect to the respective compositions described above.
For example, a general coating method can be appropriately used as a method for forming a multilayer coating film within a range not departing from the scope of the present invention.
(object to be coated)
The object to be coated may be appropriately selected depending on the use, function, and the like.
In a specific embodiment, as the object to be coated, various objects to be coated that can be energized may be used. Examples of the substance to be coated that can be used include: cold-rolled steel sheet, hot-rolled steel sheet, stainless steel, electrogalvanized steel sheet, hot-dip galvanized steel sheet, zinc-aluminum alloy-based plated steel sheet, zinc-iron alloy-based plated steel sheet, zinc-magnesium alloy-based plated steel sheet, zinc-aluminum-magnesium alloy-based plated steel sheet, aluminum-silicon alloy-based plated steel sheet, tin-based plated steel sheet, and the like.
These objects to be coated may be objects to be coated which are subjected to known chemical conversion treatment or the like.
In a specific embodiment, a plastic substrate can be used as the object to be coated. For example, it may be a plastic substrate for automobile parts. For example, the plastic substrate comprises polypropylene or a polyolefin or an elastomer-modified polypropylene resin. The elastomer-modified polypropylene resin is not particularly limited, and known commercially available products can be used. Further, additives may be added as needed in addition to the resin. In addition, an object to be coated made of an ABS (acrylonitrile-butadiene-styrene) resin may also be used.
The dupont impact strength at-20 ℃ is measured by the method described in detail in the examples below.
Examples
The present invention will be described below with reference to examples. In the examples, the% in the blending ratio means mass% unless otherwise specified. The present invention is not limited to the examples described below.
(Synthesis example C-1-1)
Production of hydroxyl-containing acrylic resin Ac (1)
444.27g of butyl acetate was charged into a 2L separable flask equipped with a temperature regulator, stirring blades (stirring blades), a reflux tube, and a nitrogen inlet, and after the inside of the flask was placed under a nitrogen atmosphere, the temperature was raised to 130 ℃ and kept constant. On the other hand, a mixture of 255g of Styrene (ST), 8.5g of methacrylic acid (MAA), 394.4g of 2-hydroxyethyl methacrylate (HEMA), 117.47g of 2-ethylhexyl acrylate (EHA), 74.72g of isobutyl methacrylate (iBMA) and 102g of Kayaester O was charged into a dropping funnel and added dropwise over 3 hours.
After the reaction was continued for 1 hour, a mixture of 204g of butyl acetate and 20.4g of Kayaester O was added dropwise over 30 minutes as a post-initiator, and the reaction was continued for 1 hour to obtain a hydroxyl group-containing acrylic resin Ac (1) having a solid content of 60%. The blending amount and physical properties of the hydroxyl group-containing acrylic resin Ac (1) are shown in table 1.
(Synthesis examples C-1-2 to C-1-9)
Synthesis of hydroxyl-containing acrylic resins Ac (2) -Ac (9)
The hydroxyl group-containing acrylic resins Ac (2) to Ac (9) shown in Table 1 were obtained by performing the same synthesis procedures and operations as those of Synthesis example (C-1-1) except that the same apparatus as used in Synthesis example (C-1-1) was used and the amounts of the solvent, monomer, initiator and polymerization temperature were changed as shown in Table 1. The physical properties are also shown in Table 1.
(hydroxyl value (OHV))
The hydroxyl value was determined by a neutralization titration method using an aqueous potassium hydroxide solution described in JIS K0070.
(glass transition temperature)
The glass transition temperature (Tg) of the hydroxyl group-containing acrylic resins Ac (1) to Ac (9) was measured by the following method. That is, after removing the solvent from the hydroxyl group-containing acrylic resin obtained by polymerization by distillation under reduced pressure, the following steps 1 to 3 are carried out using a Differential Scanning Calorimeter (DSC) (product of Seiko electronic industries, Ltd.; thermal analysis device SSC/5200H):
step 1: 20 ℃→ 100 ℃ (heating rate 10 ℃/min)
And a 2 nd step: 100 ℃→ -50 ℃ (cooling rate 10 ℃/min)
And a 3 rd step: -50 ℃→ 100 ℃ (ramp rate 10 ℃/min)
The glass transition temperature of the hydroxyl group-containing acrylic resin is measured by the method described in Japanese patent laid-open No. 2015-168693 from the time of temperature rise in the 3 rd step.
(weight average molecular weight)
The weight average molecular weight (Mw) of the hydroxyl group-containing acrylic resins Ac (1) to Ac (9) is a value measured by GPC (gel permeation chromatography) and is a weight average molecular weight in terms of polystyrene.
(measurement of solid Acid Value (AV))
The acid value was measured in accordance with JIS.K. 5601-2-1.
[ Table 1]
Figure 377529DEST_PATH_IMAGE001
(Synthesis example C-2-1)
Production of non-aqueous Dispersion acrylic resin (NAD-1)
Synthesis of Shell portion of non-Water Dispersion acrylic resin (non-Water Dispersion resin Shell Ac (1))
376.8g of butyl acetate was charged into a 1L separable flask equipped with a temperature regulator, a stirring blade, a reflux tube and a nitrogen inlet, and after the inside of the flask was placed under a nitrogen atmosphere, the temperature was raised to 110 ℃ and kept constant. On the other hand, a mixture of 9.36g of 2-hydroxyethyl methacrylate, 91g of 2-hydroxyethyl acrylate, 100g of lauryl methacrylate, 55g of methyl methacrylate, 4g of methacrylic acid, 140.64g of isobornyl methacrylate and 12g of Kayaester O was charged into a dropping funnel and added dropwise over 3 hours.
After the reaction was continued for 1 hour, a mixture of 40g of butyl acetate and 4.8g of Kayaester O was added dropwise over 30 minutes as a post-initiator, and the reaction was continued for 1 hour to obtain a non-dispersed resin shell Ac (1) having a solid content of 50%. The weight average molecular weight of the resulting resin was 20,000.
Synthesis of non-aqueous Dispersion type acrylic resin (NAD (1))
133.33g of the nonaqueous dispersion resin shell Ac (1) prepared as described above and 174.98g of butyl acetate were placed in a 500mL separable flask equipped with a temperature controller, a stirring blade, a reflux tube and a nitrogen inlet, and after the inside of the flask was put under a nitrogen atmosphere, the temperature was raised to 110 ℃ and kept constant.
Next, a mixture of 39.44g of 2-hydroxyethyl methacrylate (HEMA), 2.83g of Styrene (ST), 52.5g of Methyl Methacrylate (MMA), 4.59g of 2-ethylhexyl acrylate (EHA), 0.64g of Acrylic Acid (AA) and 1.0g of Kayaester O was charged into a dropping funnel and added dropwise over 3 hours.
Then, the reaction was continued for 1 hour, and then a mixture of 10g of butyl acetate and 0.1g of Kayaester O was added dropwise thereto as a post-initiator over 30 minutes, followed by further continued reaction for 1 hour to obtain a non-aqueous dispersion type acrylic resin (NAD (1)) having a solid content of 50%. No aggregates were observed in the resulting non-aqueous dispersion acrylic resin (NAD-1). The blending amount and physical properties of the nonaqueous dispersion type acrylic resin (NAD (1)) are shown in Table 2.
(Synthesis examples C-2-2 to C-2-9)
The procedures and operations similar to those of Synthesis example (C-2-1) were carried out using the same apparatus as used in Synthesis example (C-2-1) except that the amounts of the dispersion resin, solvent, monomer, initiator and polymerization temperature were changed to the amounts described in Table 2, to obtain non-aqueous dispersion acrylic resins (NAD (2)) -non-aqueous dispersion acrylic resins (NAD (9)) described in Table 2. The physical properties of the nonaqueous dispersion type acrylic resin (NAD (2)) to the nonaqueous dispersion type acrylic resin (NAD (9)) are shown in table 2.
In Table 2, 4HBA means 4-hydroxybutyl acrylate. Placcel FM1 is a caprolactone-modified hydroxy (meth) acrylate (product name, manufactured by Daicel chemical Co., Ltd.). These all represent hydroxyl-containing acrylate monomers having a hydroxyl group and an ester moiety with 4 to 12 carbon atoms.
The glass transition temperature, hydroxyl value and acid value of the non-aqueous dispersion acrylic resin (NAD (1)) to the non-aqueous dispersion acrylic resin (NAD (9)) were measured in the same manner as for the hydroxyl group-containing acrylic resin.
[ Table 2]
Figure 481620DEST_PATH_IMAGE003
(example 1)
(formation of clear coating film)
The clear coating composition diluted with butyl acetate was applied to a polypropylene substrate so as to have a dry film thickness of 35 ℃ by a spray gun (manufactured by Anest Seisakusho K.K.; W-101-μm, placing the test piece in a coating environment with the temperature of 20 +/-5 ℃ and the relative humidity of less than 78% for 10 minutes.
The diol resin (c-3) used in example 1 was BECKOLITE WHF348 (a polyester diol resin available from DIC), and had a hydroxyl value of 110mg KOH/g and a glass transition temperature (Tg) of not more than room temperature (20 ℃ C.).
As the polyisocyanate compound, N3300 (product of Covestro, NCO% ═ 22) was used.
Subsequently, the raw material was heated to 80 ℃ by a dryer, and then dried and cured by heating at 80 ℃ for 10 minutes to obtain a test piece having a base material and a transparent coating film.
The blending, various physical properties, and evaluation results of the obtained transparent coating film are shown in table 3.
(examples 2 to 11, comparative examples 1 to 13)
The details of each component used in examples and comparative examples, except for the components used in example 1, are as follows.
Synthesis example (C-3-1) 3 production of functional polyol
36.8g of trimethylolpropane and 563.2g of trimethylolpropane were placed in a 1L separable flask equipped with a temperature controller, a stirring blade, a reflux tube and a nitrogen inletεCaprolactone, 0.3g of dibutyltin laurate, and the reaction was continued at 160 ℃ for 6 hours. The resulting resin had a branch number of 3, a hydroxyl equivalent of 730 and was liquid at room temperature and 20 ℃.
Diol component
Ziziphali polyol P-1010 (manufactured by Kuraray Co., Ltd.) (P-1010)
A polyester diol having a molecular weight of 1000, a hydroxyl value of 110mg KOH/g, a melting point of-71 DEG C
As a seed, Coloray polyol P-2010 (manufactured by Kuraray Co., Ltd.) (P-2010)
A polyester diol having a molecular weight of 2000, a hydroxyl value of 55mg KOH/g, a melting point of-67 DEG C
As a seed, Placcel 210 (manufactured by Daicel corporation, Ltd.) (PCL-210)
Polycaprolactone diol having a molecular weight of 1000, a hydroxyl value of 110mg KOH/g and a glass transition temperature (Tg) of 20 ℃ or lower
As a seed, Duranol T5651 (manufactured by Asahi chemical Co., Ltd.) (T-5651)
Polycaprolactone diol with number average molecular weight of 1000, hydroxyl value of 110mg KOH/g, and melting point below-5 DEG C
Cellulose derivatives
As a seed, Eastman CAB 551-0.01 (Eastman Chemical Co., Ltd.)
Butylated 53% cellulose derivative with molecular weight 16000
(evaluation of physical Properties of coated article)
Clear coating films were formed in the same manner as in example 1 except that the clear coating compositions of examples 2 to 11 were prepared in the blending ratios shown in Table 3.
Clear coating films were formed in the same manner as in example 1 except that the coating compositions of comparative examples 1 to 13 were prepared in the blending ratios shown in Table 4.
Various physical properties of the transparent coating films of examples and comparative examples were evaluated. Evaluation methods and evaluation criteria for various physical properties are as follows.
(appearance)
The 60 ℃ gloss of the test piece having a clear coating film was measured by a specular gloss meter in accordance with JIS K-5600-4-7, and evaluated according to the following criteria.
". o": the 60 ℃ gloss was 85 or more, and no surface abnormality of the coating film such as bulge, crack, pinhole, orange peel or the like was observed.
"×": the 60 DEG gloss is less than 85, or at least one of swelling, cracks, pinholes, orange peel and other coating surface abnormalities is observed.
(hardness)
The hardness of the test piece having a clear coating film was measured in accordance with JIS K-5600-5-4. The measurement value was evaluated as "B" or more and "2B" or less as "X".
(initial adhesion)
A single-edge cutting tool prescribed in JIS K-5600-5-6 was perpendicularly abutted on the coated surface of the test piece having a clear coating film, and parallel lines 1 were drawn in parallel so that 11 cuts could reach the texture (base material). Parallel lines 2 which perpendicularly intersect the parallel line 1 and in which 11 cuts reaching the texture were drawn at the same intervals as the parallel line 1 were formed, and a checkerboard portion of 100 squares surrounded by 4 straight lines was produced. The spacing between the parallel lines 1, 2 was set to 2 mm. The transparent pressure-sensitive adhesive tape specified in JIS K-5600-5-6 was adhered to the checkerboard portion so as not to contain air bubbles between the coated surface and the pressure-sensitive adhesive tape. Thereafter, the tape was peeled off at once for 0.5 to 1.0 second, and the state of peeling of the checkerboard portion was evaluated visually (checkerboard adhesion test). Then, the case where no peeling was observed was evaluated as "o", and the case where peeling was observed was evaluated as "x".
(moisture resistance)
Evaluation was made in accordance with JIS K-5600-7-2. Specifically, the test piece having the transparent coating film was left to stand in an atmosphere of 50. + -. 2 ℃ and 98. + -. 2% humidity for 240 hours, and then the surface of the coating film was observed and the checkerboard adhesion test was performed within 1 hour. The checkerboard adhesion test was performed in the same manner as the checkerboard adhesion test described in the above "initial adhesion". Then, the case where no abnormality was observed on the surface of the coating film and no peeling was observed was evaluated as "o", and the case where abnormality was observed or peeling was observed on the surface of the coating film was evaluated as "x".
(weather resistance)
The test piece having a clear coating film was subjected to a weather resistance test using a Sunshine weather meter (manufactured by Suga test machine Co., Ltd.), and the adhesion and appearance after 1200 hours were evaluated. The adhesion was evaluated by performing a checkerboard adhesion test similar to the checkerboard adhesion test described in the above "initial adhesion". The appearance was evaluated visually, with color difference, and with gloss value. Then, the case where no peeling was observed at all, no significant abnormality was observed by visual observation, the color difference (Δ E) from the initial state (before the weather resistance test) was 3.0 or less, and the gloss was 80 or more was evaluated as "o", the case where 1 of the above 4 items was not satisfied was evaluated as "Δ", and the case where 2 or more of the above 4 items was not satisfied was evaluated as "x".
(DuPont impact Strength)
Evaluation was carried out in accordance with JIS K5600-5-3 by holding the temperature of the coating film at-20 ℃ in a cylinder having an impact type radius of 6.35. + -. 0.03, a receiving table (pedestal) diameter of 4.8cm and a plate thickness of 2 mm.
Using a 500g falling weight, the limit value at which the polypropylene material as the object to be coated is not damaged was measured and evaluated according to the following criteria.
O: 4.9J or more
X: less than 4.9J
(alcohol resistance gasoline)
After a polypropylene resin base sheet (3 cm. times.3 cm) having a clear coating film formed thereon was immersed in alcohol gasoline (20 ℃ C.) prepared by adding 10 vol% ethanol (alcohol) to regular gasoline, a sheet having no peeling at 30 minutes was marked as "O" and a sheet having no peeling at less than the above was marked as "X".
[ Table 3]
Figure 199041DEST_PATH_IMAGE004
[ Table 4]
Figure DEST_PATH_IMAGE005
(formation of multilayer coating film)
(example 12)
Test pieces having a multilayer coating film were produced as follows. Test pieces were prepared in a coating environment at a temperature of 20. + -. 5 ℃ and a relative humidity of 78% or less.
BR-116CD, a primer coating composition for polypropylene resin, was applied to a degreased polypropylene resin substrate (150 mm. times.100 mm, 2mm thickness: Bumper grade) using a spray gun (W-101-μm, and then left to stand in the above coating atmosphere for 1 minute. In the state of being left for 1 minute, the coating film was not cured.
Next, R-301S Silver, a base coating composition containing a colorant, was applied by means of a spray gun (manufactured by Anest Seika corporation; W-101-μAnd m is selected. The resultant was left to stand in the above coating atmosphere for a further 1 minute. In the state of being left for 1 minute, the coating film was not cured.
The clear coating composition prepared in example 1 was applied to the above-mentioned transparent coating film by using the above-mentioned spray gunThe uncured base coating film was formed so that the dry film thickness was 35μm, and left to stand for 10 minutes in the above coating environment.
Subsequently, the raw material was heated to 80 ℃ by a dryer, and then dried and cured by heating at 80 ℃ for 10 minutes to obtain a test piece (automobile part) having a plastic base and a multilayer coating film.
The multilayer coating film included in the obtained automobile parts was tested in the same manner as the transparent coating film. As a result, all the evaluation results were "o" in terms of appearance, hardness, initial adhesion, moisture resistance, weather resistance, dupont impact strength, and alcohol gasoline resistance, and a multilayer coating film having excellent physical properties was obtained.
The clear coating composition of the present invention exhibits excellent results in all aspects of appearance, hardness, initial adhesion, moisture resistance, weather resistance, dupont impact strength (low temperature) and gasoline resistance.
Further, the clear coating composition according to the present invention has a pot life suitable for coating a large object to be coated, and thus the workability is improved. Further, the coating film has the following properties.
In addition, the clear coating composition of the present invention has particularly excellent effects in terms of moisture resistance and low-temperature impact resistance (e.g., DuPont impact strength at-20 ℃), although the curing time is very short as compared with that of a commonly used clear coating composition.
For example, the multilayer coating film in example 1 was examined for the suitability of tackiness, polishability, and polishing properties, and all the physical properties were good.
On the other hand, the non-aqueous dispersion acrylic resins of comparative examples 1 to 4 were outside the scope of the present invention. Thus, comparative example 1 is inferior in hardness, moisture resistance, weather resistance and alcohol gasoline resistance, comparative example 2 is inferior in dupont impact strength, comparative example 3 is inferior in moisture resistance, weather resistance and dupont impact strength, and comparative example 4 is inferior in moisture resistance, weather resistance and alcohol gasoline resistance.
The hydroxyl group-containing acrylic resins of comparative examples 5 to 10 were outside the scope of the present invention. Therefore, comparative example 5 is inferior in hardness, initial adhesion, moisture resistance and weather resistance, comparative example 6 is inferior in moisture resistance, weather resistance and gasoline resistance, comparative example 7 is inferior in appearance and dupont impact strength, comparative examples 8 and 9 are inferior in hardness, moisture resistance, weather resistance and gasoline resistance, and comparative example 10 is inferior in appearance and dupont impact strength.
Comparative examples 11 to 13 contained no diol resin, or the diol resin was outside the scope of the present invention. Comparative examples 11 to 13 were poor in all of the appearances and also insufficient in the DuPont impact strength.
Industrial applicability
The clear coating composition of the present invention can be suitably used for, for example, automobile vehicles, automobile parts.

Claims (14)

1. A clear coating composition comprising the following components (c-1) to (c-4) as a coating film-forming resin: a hydroxyl group-containing acrylic resin (c-1), a non-aqueous dispersion type acrylic resin (c-2), a diol resin (c-3), and a polyisocyanate compound (c-4),
the hydroxyl value of the hydroxyl group-containing acrylic resin (c-1) is 80mgKOH/g to 220mgKOH/g, the weight-average molecular weight is 4,000 to 10,000, and the glass transition temperature is 30 ℃ to 60 ℃,
the non-aqueous dispersion acrylic resin (c-2) has a core-shell structure, the hydroxyl value of the core part is 100mgKOH/g to 200mgKOH/g, and the glass transition temperature of the core part is 40 ℃ to 80 ℃,
the diol resin (c-3) has a hydroxyl value of 80mgKOH/g or more and 200mgKOH/g or less, and the diol resin (c-3) contains at least one selected from the group consisting of a polycarbonate diol resin, a polyether diol resin, a polyester diol resin and a polycaprolactone diol resin.
2. The clear coating composition according to claim 1, wherein the clear coating film formed from the clear coating composition according to claim 1 has a DuPont impact strength of 4.9J or more at-20 ℃.
3. The clear coating composition according to claim 1 or 2, wherein the multi-layer coating film having a primer coating film formed from the primer coating composition, a base coating film formed from the base coating composition, and a clear coating film formed from the clear coating composition according to claim 1 or 2 has a dupont impact strength of 4.9J or more at-20 ℃.
4. The clear coating composition according to any one of claims 1 to 3 further comprising a cellulose derivative.
5. The clear coating composition according to any one of claims 1 to 4, wherein the diol resin (c-3) is a polyester diol resin.
6. The clear coating composition according to any one of claims 1 to 5, wherein an equivalent ratio NCO/OH of the isocyanate group contained in the isocyanate compound (c-4) to the hydroxyl group contained in the hydroxyl group-containing acrylic resin (c-1) and the non-aqueous dispersion type acrylic resin (c-2) is 0.7 or more and 2.0 or less.
7. The clear coating composition according to any one of claims 1 to 6, wherein the mass ratio of the hydroxyl group-containing acrylic resin (c-1) to the non-aqueous dispersion acrylic resin (c-2) is (c-1)/(c-2) of 90/10 to 50/50.
8. The clear coating composition according to any one of claims 1 to 7, wherein the mass ratio of the total mass of the hydroxyl-containing acrylic resin (c-1) and the non-aqueous dispersion acrylic resin (c-2) to the glycol resin (c-3) is ((c-1) + (c-2))/(c-3) ═ 100/1 to 100/15.
9. The clear coating composition according to any one of claims 1 to 8, wherein the core of the non-aqueous dispersion acrylic resin (c-2) contains a hydroxyl group-containing acrylate monomer having an ester moiety with 4 to 12 carbon atoms.
10. The clear coating composition according to any one of claims 1 to 9, wherein the core of the non-aqueous dispersion acrylic resin (c-2) contains a carboxyl group-containing compoundα,βEthylenically unsaturated monomers and monomers having hydroxyl groupsα,β-ethylenically unsaturated monomers.
11. The clear coating composition according to any one of claims 1 to 10, wherein the acid value of the core part of the non-aqueous dispersion acrylic resin (c-2) is 60mgKOH/g or less.
12. The clear coating composition according to any one of claims 1 to 11, wherein the non-aqueous dispersion acrylic resin (c-2) has a hydroxyl group value of the shell portion of 50mgKOH/g or more and 160mgKOH/g or less and a glass transition temperature of 0 ℃ or more and 80 ℃ or less.
13. The clear coating composition according to any one of claims 1 to 12, wherein the acid value of the shell portion of the non-aqueous dispersion acrylic resin (c-2) is 30mgKOH/g or less.
14. A method for forming a clear coating film, comprising the steps of:
a step of applying the clear coating composition according to any one of claims 1 to 13 to an object to be coated to form an uncured clear coating film; and
and heating and curing the uncured transparent coating film to form a transparent coating film.
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