JP7616221B2 - Multi-stage copolymer for powder coating, its production method, and powder coating composition - Google Patents

Description

The present invention relates to a multistage copolymer for powder coatings and a method for producing the same, as well as a powder coating composition containing the multistage copolymer for powder coatings.
This application claims priority based on Japanese Patent Application No. 2020-111778, filed on June 29, 2020, the contents of which are incorporated herein by reference.

In recent years, powder coatings have come to be used in a wide range of fields because they do not generate organic solvents when the coating is baked, are organic solvent-free and therefore are excellent for the working environment, and are non-hazardous and resource-saving.

Compared to solvent-based paints, powder paints are characterized by being solvent-free and being able to produce thick films of 30 to 100 μm in a single coat. On the other hand, powder paints have issues such as reduced workability due to the thick coating film obtained by painting. Traditionally, polyester-based powder paints have been widely used for outdoor applications such as road materials and building materials. In recent years, polyester-based powder paints with even higher weather resistance have been studied from the perspective of maintenance-free use. There is an increasing demand to improve the toughness of the coating film obtained from these polyester-based powder paints with high weather resistance.

For example, Patent Document 1 discloses an example of applying a multi-layer polymer, in which a vinyl monomer is graft-polymerized onto a rubber-like polymer, to a powder coating. Patent Document 1 discloses that by dispersing a multi-stage copolymer having a polymer layer with a glass transition temperature of 20°C or less and a polymer layer with a glass transition temperature of 60°C or more in a powder coating, the processability and impact resistance of the coating film can be improved. However, there is a demand for the development of a powder coating that can produce a coating film with even improved toughness.

It has also been known that multi-layer polymer particles (multi-stage polymers) are blended with thermoplastic resins to improve impact resistance. Patent documents 2 to 4 disclose examples of using multi-layer polymer particles as impact resistance modifiers. In detail, Patent documents 2 to 4 disclose a technique for improving impact resistance, temperature dependency of haze, impact whitening resistance, etc., by adding a multi-stage polymer having a basic structure of three layers of hard-soft-hard to a hard resin such as a methacrylic resin. However, since the above technique is related to application to resin materials for molding parts, when the above technique is applied to powder coatings, the dispersibility of the multi-stage polymer in the thermoplastic resin is insufficient. In addition, powder coating compositions containing multi-stage polymers tend to increase in melt viscosity, so the appearance and toughness of the coating film obtained from the powder coating composition are insufficient.

JP 2000-1633 A Japanese Unexamined Patent Publication No. 48-55233 Japanese Patent Application Publication No. 10-338723 WO 2005/095480

The present invention has been made in consideration of the above circumstances, and aims to provide a multi-stage copolymer for powder paint that can give a coating film having good appearance and excellent toughness, a method for producing the same, and a powder paint composition containing the multi-stage copolymer for powder paint.

The present invention has the following aspects.
[1] A multi-stage copolymer for powder coating, comprising at least three polymer components: a first stage polymer (A), a second stage polymer (B), and a third stage polymer (C),
A powder coating composition comprising: a multi-stage copolymer for powder coating, the glass transition temperature of the polymer (A) being -15°C or higher, the glass transition temperature of the polymer (B) being -20°C or lower, and the glass transition temperature of the polymer (C) being 60°C or higher, the multi-stage copolymer for powder coating containing the polymer (A) in an amount of 1% by mass or more and 30% by mass or less when the total amount of the multi-stage copolymer for powder coating is taken as 100% by mass; and a thermoplastic resin which is a polyester .
[2] A multi-stage copolymer for powder coating, comprising at least an inner layer, an intermediate layer, and an outer layer,
the inner layer comprises a polymer (A) having a glass transition temperature of −15° C. or higher,
the intermediate layer contains a polymer (B) having a glass transition temperature of −20° C. or lower,
The outer layer contains a polymer (C) having a glass transition temperature of 60° C. or higher,
A powder coating composition comprising: a multi-stage copolymer for powder coating, the multi-stage copolymer for powder coating containing 2% by mass or more and 20% by mass or less of the polymer (A) when the total amount of the multi-stage copolymer for powder coating is taken as 100% by mass; and a thermoplastic resin which is a polyester .
[3] The powder coating composition according to [1] or [2], comprising: the polymer (B) obtained by polymerizing a monomer mixture in the presence of the polymer (A); and the polymer (C) obtained by polymerizing a monomer mixture in the presence of a polymer containing the polymer (A) and the polymer (B).
[4] A multi-stage copolymer for powder coating, comprising at least three polymer components: a polymer (A), a polymer (B), and a polymer (C),
3. A powder coating composition comprising the multi-stage copolymer for powder coating according to claim 1 or 2, wherein at least two peaks are present in a tan δ curve between -60°C and 140°C in a temperature dispersion measurement of dynamic viscoelasticity of the multi-stage copolymer for powder coating, and a thermoplastic resin which is a polyester .
[5] The powder coating composition according to any one of [1] to [4], wherein the polymer (A) contains 35% by mass or more and 99.5% by mass or less of methyl methacrylate units and 0.5% by mass or more and 5% by mass or less of a polyfunctional monomer, when the total amount of monomer units in the polymer (A) is taken as 100% by mass.
[6] The powder coating composition according to any one of [1] to [5], wherein the polymer (B) contains 70% by mass or more and 99.5% by mass or less of alkyl (meth)acrylate units having an alkyl group with 4 to 8 carbon atoms, and 0.5% by mass or more and 5% by mass or less of a polyfunctional monomer, when the total amount of monomer units in the polymer (B) is taken as 100% by mass.
[7] The powder coating composition according to any one of [1] to [6], wherein the polymer (C) contains 70 mass% or more and 100 mass% or less of methyl methacrylate units, when the total amount of monomer units in the polymer (C) is taken as 100 mass%.
[8] The powder coating composition according to any one of [1] to [7], wherein the volume average particle diameter of the primary particles is 0.1 μm or more and 10 μm or less.
[9] The powder coating composition according to any one of [1] to [8], wherein the volume average particle diameter of the secondary particles is 1 μm or more and 500 μm or less.
[10] The powder coating composition according to any one of [1] to [9], wherein the polymer (A) is contained in an amount of 1 mass% or more and 30 mass% or less, the polymer (B) is contained in an amount of 31 mass% or more and 94 mass% or less, and the polymer (C) is contained in an amount of 5 mass% or more and 39 mass% or less, when the total amount of the polymer (A), the polymer (B), and the polymer (C) is taken as 100 mass %.
[11] A method for producing a powder coating composition according to any one of [1] to [10],
a step (1) of polymerizing a first monomer mixture for constituting a polymer (A) to obtain a first dispersion containing the polymer (A), then dropping a second monomer mixture for constituting a polymer (B) into the first dispersion to polymerize the second monomer mixture to obtain a second dispersion containing the polymer (A) and the polymer (B), and further dropping a third monomer mixture for constituting a polymer (C) into the second dispersion to polymerize the third monomer mixture to obtain a third dispersion containing a multistage copolymer containing the polymer (A), the polymer (B), and the polymer (C);
and (2) spray-drying the third dispersion to obtain a powder containing the multistage copolymer .
[12] The powder coating composition according to any one of [ 1 ] to [ 10 ], wherein the content of the multistage copolymer for powder coating is 1% by mass or more and 20% by mass or less, when the total amount of the powder coating composition is 100% by mass.

According to the present invention, it is possible to provide a multi-stage copolymer for powder paint that can produce a coating film having good appearance and excellent toughness, a method for producing the same, and a powder paint composition containing the multi-stage copolymer for powder paint.

FIG. 2 is a graph showing a tan δ curve between −60° C. and 140° C. calculated by measuring the temperature dispersion of dynamic viscoelasticity of a multistage copolymer in Example 1.

The following describes embodiments of the multi-stage copolymer for powder paint and its manufacturing method according to the present invention, as well as a powder paint composition containing the multi-stage copolymer for powder paint.

[Multi-stage copolymer for powder coating]
The multi-stage copolymer for powder coating according to an embodiment of the present invention (hereinafter referred to as "the present embodiment") comprises a first stage polymer (A), a second stage polymer (B), and a third stage polymer (C). The polymer (A) has a glass transition temperature of -15°C or higher, and the polymer (B) has a glass transition temperature of -20°C or lower. The glass transition temperature of the polymer (C) is 60° C. or higher, and the polymer (A) is contained in an amount of 1% by mass to 30% by mass when the total amount of the multi-stage copolymer for powder coating is taken as 100% by mass. Includes the following:

The glass transition temperature (hereinafter also referred to as "Tg") is a value calculated from the following FOX formula (Formula (1)). In this specification, the unit of Tg is "°C". Specifically, in the case of a polymer (homopolymer) consisting of only a single monomer, the standard analytical value described in the "Polymer Data Handbook" compiled by the Society of Polymer Science and other publications can be adopted, and in the case of a copolymer obtained by polymerizing n (n is a natural number of 2 or more) types of monomers, the Tg can be calculated from the Tg of the homopolymer of each monomer using the following formula (1). Table 1 shows the literature values of Tg for representative homopolymers.

1/(273+Tg)=Σ(Wn/(273+Tgn)) (1)
In the formula, Wn represents the mass fraction of monomer n, and Tgn represents the glass transition temperature (°C) of a homopolymer of monomer n. Here, the mass fraction is the total amount of monomers charged. is the ratio of the amount of monomer n to the total amount of monomer n.

The abbreviations in Table 1 represent the following monomers.
・"MMA": methyl methacrylate・"EMA": ethyl methacrylate・"n-BMA": n-butyl methacrylate・"i-BMA": i-butyl methacrylate・"t-BMA": t-butyl methacrylate・"n-HMA": n-hexyl methacrylate・"2-EHMA": 2-ethylhexyl methacrylate・"MA": methyl acrylate・"EA": ethyl acrylate・"n-BA": n-butyl acrylate・"n-HA": n-hexyl acrylate・"2-EHA": 2-ethylhexyl acrylate・"MAA": methacrylic acid・"2-HEMA": 2-hydroxyethyl methacrylate

For homopolymers whose glass transition temperatures are not described in the literature, the glass transition temperature can be determined by measurement. In such cases, the Tg of the homopolymer may be measured using known methods such as differential scanning calorimetry, thermomechanical analysis, and dynamic viscoelasticity measurement. For multistage copolymers or resin compositions containing multistage copolymers, even if the Tg cannot be calculated using the above formula (1), the Tg may be measured by the above method.

The Tg of polymer (A) is -15°C or higher, and preferably 0°C or higher. If the Tg of polymer (A) is -15°C or higher, it is possible to obtain a multistage copolymer that can provide a powder coating composition capable of forming a coating film having excellent toughness and appearance.

The Tg of polymer (B) is -20°C or lower, and preferably -40°C or lower. If the Tg of polymer (B) is -20°C or lower, a multistage copolymer can be obtained that can provide a powder coating composition capable of forming a coating film with excellent toughness.

The Tg of polymer (C) is 60°C or higher, and preferably 80°C or higher. If the Tg of polymer (C) is 60°C or higher, the multistage copolymer is well dispersed in the thermoplastic resin, and a multistage copolymer can be obtained that can provide a powder coating composition capable of forming a coating film with excellent appearance.

Another multistage copolymer for powder coating of the present invention is composed of at least three polymer components, polymer (A), polymer (B), and polymer (C), and in a temperature dispersion measurement of dynamic viscoelasticity of the multistage copolymer for powder coating, there are at least two peaks in the tan δ curve between -60°C and 140°C. When there are at least two peaks in the tan δ curve, it is possible to obtain a multistage copolymer for powder coating that can provide a powder coating composition capable of forming a coating film excellent in toughness and appearance.

Of the at least two peaks present in the tan δ curve, the low-temperature peak is preferably present at -60°C to 15°C, more preferably at -60°C to 5°C, and even more preferably at -60°C to -5°C. The high-temperature peak is preferably present at 60°C to 140°C, more preferably at 70°C to 140°C, and even more preferably at 80°C to 140°C. When at least two peaks present in the tan δ curve are within the above ranges, a multi-stage copolymer for powder coating can be obtained that can provide a powder coating composition capable of forming a coating film having excellent toughness and appearance.

The dynamic viscoelasticity of the multi-stage copolymer for powder coating can be measured by a known method. In this embodiment, the dynamic viscoelasticity was measured using a viscoelasticity measuring device DMA6100 (manufactured by Hitachi High-Tech Science Corporation) in a bending mode at a frequency of 1 Hz and in the range of -100°C to 150°C for a film-like test piece of the multi-stage copolymer for powder coating prepared by press molding for 10 minutes under conditions of 180°C and 3 MPa, to calculate a tan δ curve.

"Composition of polymer (A), polymer (B) and polymer (C)"
The compositions of polymer (A), polymer (B) and polymer (C) constituting the multistage copolymer for powder coating of this embodiment are as follows.

When the total amount of monomer units constituting polymer (A) is taken as 100 mass%, polymer (A) preferably contains 35 mass% or more and 99.5 mass% or less of methyl methacrylate units and 0.5 mass% or more and 5 mass% or less of polyfunctional monomer units, more preferably contains 40 mass% or more and 99.5 mass% or less of methyl methacrylate units and 0.5 mass% or more and 5 mass% or less of polyfunctional monomer units, and even more preferably contains 45 mass% or more and 99.5 mass% or less of methyl methacrylate units and 0.5 mass% or more and 5 mass% or less of polyfunctional monomer units.
When the polymer (A) contains 35% by mass or more and 99.5% by mass or less of methyl methacrylate units, it is possible to obtain a multistage copolymer that can provide a powder coating composition capable of forming a coating film excellent in toughness and appearance.

When polymer (A) contains 0.5% by mass or more of multifunctional monomer units, the dispersion of the multistage copolymer in the thermoplastic resin is good, and a multistage copolymer that can provide a powder coating composition capable of forming a coating film with excellent appearance can be obtained. When polymer (A) contains 5% by mass or less of multifunctional monomer units, a multistage copolymer that can provide a powder coating composition capable of forming a coating film with excellent toughness can be obtained.

The monomer that forms the polyfunctional monomer unit is a monomer having at least two polymerizable groups in the molecule, and examples thereof include unsaturated carboxylic acid allyl esters such as allyl acrylate, allyl methacrylate, diallyl maleate, diallyl fumarate, and diallyl itaconate, ethylene glycol dimethacrylate, propylene glycol dimethacrylate, 1,3-butylene glycol dimethacrylate, 1,4-butylene glycol dimethacrylate, and divinylbenzene. These monomers may be used alone or in combination of two or more.

When the total amount of monomer units constituting polymer (B) is taken as 100 mass%, polymer (B) preferably contains 70 mass% or more and 99.5 mass% or less of alkyl (meth)acrylate units having an alkyl group with 4 to 8 carbon atoms and 0.5 mass% or more and 5 mass% or less of polyfunctional monomer units, and more preferably contains 85 mass% or more and 99.5 mass% or less of alkyl (meth)acrylate units having an alkyl group with 4 to 8 carbon atoms and 0.5 mass% or more and 5 mass% or less of polyfunctional monomer units.
When polymer (B) contains 70% by mass or more and 99.5% by mass or less of alkyl (meth)acrylate units in which the alkyl group has 4 to 8 carbon atoms, a multistage copolymer can be obtained that can provide a powder coating composition capable of forming a coating film with excellent toughness.

When polymer (B) contains 0.5% by mass or more of multifunctional monomer units, the dispersion of the multistage copolymer in the thermoplastic resin is good, and a multistage copolymer that can provide a powder coating composition capable of forming a coating film with excellent appearance can be obtained. When polymer (B) contains 5% by mass or less of multifunctional monomer units, a multistage copolymer that can provide a powder coating composition capable of forming a coating film with excellent toughness can be obtained.

Examples of monomers forming alkyl (meth)acrylate units in which the alkyl group has 4 to 8 carbon atoms include (meth)acrylic acid esters such as n-butyl (meth)acrylate, s-butyl (meth)acrylate, i-butyl (meth)acrylate, t-butyl (meth)acrylate, hexyl (meth)acrylate, cyclohexyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate. These (meth)acrylic acid esters may be used alone or in combination of two or more.
Incidentally, "(meth)acrylate" is a general term for acrylate and methacrylate, and "(meth)acrylic acid" is a general term for acrylic acid and methacrylic acid.

In polymer (B), examples of monomers that form multifunctional monomer units include the monomers that form multifunctional monomer units in polymer (A) above.

Polymer (C) preferably contains 70% by mass or more and 100% by mass or less of methyl methacrylate units, and more preferably contains 70% by mass or more and 99.5% by mass or less, when the total amount of monomer units constituting polymer (C) is taken as 100% by mass.
When polymer (C) contains 70% by mass or more and 100% by mass or less of methyl methacrylate units, the dispersion of the multistage copolymer in the thermoplastic resin is good, so that a multistage copolymer can be obtained that can provide a powder coating composition capable of forming a coating film with excellent appearance.

It is preferable that polymer (C) contains a multi-functional monomer unit, since this provides a multi-stage copolymer that can provide a powder coating composition capable of forming a coating film with excellent appearance due to the good dispersion of the multi-stage copolymer in the thermoplastic resin. Examples of the multi-functional monomer unit include monomers that form the multi-functional monomer units in the above polymer (A).

The polymer (C) may contain a reactive group-containing monomer unit. The monomer forming the reactive group-containing monomer unit is a polymerizable monomer having a functional group in its molecule that can react with the thermoplastic resin or curing agent that constitutes the powder coating composition. Examples of the functional group include a hydroxyl group, a carboxyl group, and an epoxy group. Examples of polymerizable monomers having a hydroxyl group include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, and 2-hydroxybutyl (meth)acrylate. Examples of polymerizable monomers having a carboxyl group include methacrylic acid and acrylic acid. Examples of polymerizable monomers having an epoxy group include glycidyl (meth)acrylate. When the polymer (C) contains a reactive group-containing monomer unit, the dispersion of the multistage copolymer in the thermoplastic resin is improved, and a multistage copolymer that can provide a powder coating composition that can form a coating film with excellent appearance can be obtained. In addition, since the polymer (C) contains a reactive group-containing monomer unit, it can react with the thermoplastic resin or curing agent constituting the powder coating composition, and therefore a multi-stage copolymer can be obtained that can provide a powder coating composition capable of forming a coating film with excellent toughness. The reactive group-containing monomer unit is preferably a monomer unit formed from a polymerizable monomer having at least one of a hydroxyl group, a carboxyl group, and an epoxy group.

When the total amount of monomer units constituting polymer (C) is taken as 100 mass%, polymer (C) preferably contains 70 mass% or more and 100 mass% or less of methyl methacrylate units, and more preferably contains 70 mass% or more and 99.5 mass% or less of methyl methacrylate units and 0.5 mass% or more and 5 mass% or less of polyfunctional monomer units.
When the polymer (C) contains 0.5% by mass or more of the polyfunctional monomer unit, the multistage copolymer is well dispersed in the thermoplastic resin, and therefore a multistage copolymer capable of providing a powder coating composition capable of forming a coating film having excellent appearance can be obtained.When the polymer (C) contains 5% by mass or less of the polyfunctional monomer unit, a multistage copolymer capable of providing a powder coating composition capable of forming a coating film having excellent toughness can be obtained.

Polymer (A), polymer (B) and polymer (C) may contain monomer units other than the monomer units. The monomers forming the monomer units other than the monomer units (hereinafter also referred to as "other monomers") are not particularly limited as long as they are radically polymerizable, and examples thereof include (meth)acrylate compounds other than methyl methacrylate and the monomers forming the reactive group-containing monomer units, aromatic vinyl compounds, vinyl cyanide compounds, etc. Examples of aromatic vinyl compounds include styrene, α-methylstyrene, p-methylstyrene, etc. Examples of vinyl cyanide compounds include acrylonitrile, methacrylonitrile, etc. These other monomers may be used alone or in combination of two or more.

"Volume average particle size of primary particles"
The multistage copolymer for powder coating of this embodiment preferably has a volume average particle size of its primary particles of 0.1 μm or more and 10 μm or less, more preferably 0.3 μm or more and 3 μm or less, and even more preferably 0.4 μm or more and 1 μm or less.
Here, the primary particle refers to the smallest unit polymer particle constituting the powder containing the multistage copolymer recovered as a powder. If the volume average particle diameter of the primary particles is 0.1 μm or more, it is possible to obtain a multistage copolymer that can provide a powder coating composition capable of suppressing the increase in viscosity during melting of the powder coating composition and forming a coating film with excellent appearance. If the volume average particle diameter of the primary particles is 10 μm or less, it is possible to obtain a multistage copolymer that can provide a powder coating composition capable of forming a coating film with excellent toughness.

The volume average particle size of the primary particles of the multistage copolymer can be measured by a known method, for example, a laser diffraction method (laser diffraction/scattering method). In this embodiment, the volume average particle size of the primary particles is the average value obtained by measuring the primary particle sizes of the multistage polymer (dispersed particles) in a dispersion liquid containing the multistage copolymer using a laser diffraction/scattering type particle size distribution measuring device (product name: LA-960, manufactured by Horiba, Ltd.).

"Volume average particle size of secondary particles"
The multistage copolymer for powder coating of this embodiment preferably has a volume average particle size of its secondary particles of 1 μm or more and 500 μm or less, more preferably 1 μm or more and 300 μm or less, and even more preferably 1 μm or more and 100 μm or less.
Here, the secondary particles refer to a powder containing the multistage copolymer recovered as a powder. If the volume average particle diameter of the secondary particles is 1 μm or more, dusting and the like can be suppressed, and the powder can be easily handled. If the volume average particle diameter of the secondary particles is 500 μm or less, the dispersibility with the components constituting the powder coating is good, and a multistage copolymer that can provide a powder coating composition capable of forming a coating film with excellent appearance can be obtained.

The volume average particle size of the secondary particles of the multistage copolymer can be measured by a known method, such as a laser diffraction method (laser diffraction/scattering method), in the same manner as the volume average particle size of the primary particles. In this embodiment, the volume average particle size of the secondary particles is the average value obtained by measuring the secondary particle sizes of the multistage polymer (dispersed particles) in a dispersion liquid containing the multistage copolymer using a laser diffraction/scattering type particle size distribution measuring device (product name: LA-960, manufactured by Horiba, Ltd.).

"Multi-stage copolymer for powder coatings"
The multi-stage copolymer for powder coating of the present embodiment is a polymer (B) obtained by polymerizing a monomer mixture in the presence of a polymer (A), and a mixture of the polymer (A) and the polymer (B). It is preferable that the powder coating composition contains a polymer (C) obtained by polymerizing a monomer mixture in the presence of a polymer containing the monomer (C). This makes it possible to form a coating film having excellent appearance and toughness. It is possible to obtain a multi-stage copolymer which can provide the above.
As the monomers which become the monomer units constituting each of the polymer (A), the polymer (B) and the polymer (C), those mentioned above can be used.

The multistage copolymer for powder coating of this embodiment preferably contains 1 mass % or more and 30 mass % or less of polymer (A), 31 mass % or more and 94 mass % or less of polymer (B), and 5 mass % or more and 39 mass % or less of polymer (C), when the total of polymer (A), polymer (B), and polymer (C) is taken as 100 mass %.
When the content of polymer (A) is in the range of 1% by mass or more and 30% by mass or less, a multistage copolymer can be obtained that can provide a powder coating composition capable of forming a coating film excellent in toughness and appearance.
When the content of polymer (B) is 31% by mass or more and 94% by mass or less, it is possible to obtain a multistage copolymer which can provide a powder coating composition capable of forming a coating film having excellent toughness.
If the content of polymer (C) is 5% by mass or more, the dispersion of the multistage copolymer in the thermoplastic resin is good, so that a multistage copolymer capable of providing a powder coating composition capable of forming a coating film having excellent appearance can be obtained.If the content of polymer (C) is 39% by mass or less, a multistage copolymer capable of providing a powder coating composition capable of forming a coating film having excellent toughness can be obtained.
From the viewpoint of achieving both toughness and appearance of the coating film, the multistage copolymer for powder coating of this embodiment preferably contains 2% by mass or more and 20% by mass or less of polymer (A), 45% by mass or more and 93% by mass or less of polymer (B), and 5% by mass or more and 35% by mass or less of polymer (C), and even more preferably contains 3% by mass or more and 15% by mass or less of polymer (A), 50% by mass or more and 93% by mass or less of polymer (B), and 5% by mass or more and 35% by mass or less of polymer (C).

Another embodiment of the multistage copolymer for powder coating of the present invention is a multistage copolymer for powder coating comprising at least an inner layer, an intermediate layer and an outer layer, the inner layer containing a polymer (A) having a glass transition temperature of -15°C or higher, the intermediate layer containing a polymer (B) having a glass transition temperature of -20°C or lower, and the outer layer containing a polymer (C) having a glass transition temperature of 60°C or higher. The intermediate layer is present outside the inner layer, and the outer layer is present outside the intermediate layer. The polymer constituting the multistage copolymer may contain a polymer other than the polymer (A), the polymer (B) and the polymer (C) (hereinafter also referred to as "other polymer"). In this case, the other polymer may be present inside the polymer (A), may be present inside the polymer (B), may be present outside the polymer (B), or may be present outside the polymer (C).
Other polymers can be used as long as they do not impair the function of the multistage copolymer for powder coating of this embodiment. When the total amount of polymers constituting the multistage copolymer for powder coating of this embodiment is taken as 100 mass%, the content of other polymers is preferably 20 mass% or less.

According to the multi-stage copolymer for powder coating of this embodiment, the multi-stage copolymer is composed of at least three polymer components, a first-stage polymer (A), a second-stage polymer (B), and a third-stage polymer (C), and the glass transition temperature of the polymer (A) is -15°C or higher, the glass transition temperature of the polymer (B) is -20°C or lower, and the glass transition temperature of the polymer (C) is 60°C or higher. When the total amount of the multi-stage copolymer for powder coating is taken as 100% by mass, the multi-stage copolymer contains 1% by mass to 30% by mass of the polymer (A), and therefore can provide a multi-stage copolymer that can provide a powder coating composition capable of forming a coating film having good appearance and excellent toughness. Note that, as long as the order of the first, second, and third stages is maintained, other polymer components may be composed before or after the first stage, after the second stage, or after the third stage.

The description in this specification of the powder coating composition of the present embodiment is also applicable to the powder coating compositions of the other embodiments of the present invention.

[Method for producing multi-stage copolymer for powder coating]
The method for producing a multistage copolymer for powder paint of this embodiment includes the steps of: polymerizing a first monomer mixture for constituting polymer (A) to obtain a first dispersion containing polymer (A); then adding a second monomer mixture for constituting polymer (B) dropwise to the first dispersion to polymerize the second monomer mixture to obtain a second dispersion containing polymer (A) and polymer (B); and further adding a third monomer mixture for constituting polymer (C) dropwise to the second dispersion to polymerize the third monomer mixture to obtain a third dispersion containing a multistage copolymer containing polymer (A), polymer (B) and polymer (C); and spray-drying the third dispersion to obtain a powder containing the multistage copolymer.

The multi-stage copolymer for powder paint of this embodiment can be produced, for example, by a known emulsion polymerization method. A suitable example of a method for producing the multi-stage copolymer for powder paint of this embodiment is shown below, but the method for producing the multi-stage copolymer for powder paint of this embodiment is not limited to this. Here, an example in which emulsion polymerization is used in step (1) is shown.

Step (1) includes a step (1-1) of polymerizing a first monomer mixture for constituting polymer (A) to obtain a first dispersion containing polymer (A), a step (1-2) of dropping a second monomer mixture for constituting polymer (B) into the first dispersion to polymerize the second monomer mixture and obtain a second dispersion containing polymer (A) and polymer (B), and a step (1-3) of dropping a third monomer mixture for constituting polymer (C) into the second dispersion to polymerize the third monomer mixture and obtain a third dispersion containing a multistage copolymer containing polymer (A), polymer (B), and polymer (C).

(Step (1-1))
In the step (1-1), deionized water and, if necessary, an emulsifier are added to a reaction vessel, and then a first monomer mixture containing monomers for constituting the polymer (A) is added to polymerize the first monomer mixture to obtain a first latex (first dispersion) containing first dispersed particles made of the polymer (A).

(Step (1-2))
In the step (1-2), a second monomer mixture containing a monomer for constituting the polymer (B) is dropped into the first latex (first dispersion) to polymerize the second monomer mixture, thereby obtaining a second latex (second dispersion) containing second dispersed particles consisting of the polymer (A) and the polymer (B). The second dispersed particles are polymers having a two-layer structure in which the first layer (inner layer) consists of the polymer (A) and the second layer (outer layer) consists of the polymer (B).

(Step (1-3))
In step (1-3), a third monomer mixture containing a monomer for constituting polymer (C) is dropped into the second latex (second dispersion) to polymerize the third monomer mixture, thereby obtaining a third latex (third dispersion) containing a multistage copolymer (third dispersed particles) containing polymer (A), polymer (B) and polymer (C). The multistage copolymer is a three-layer polymer in which the first stage (inner layer) is made of polymer (A), the second stage (middle layer) is made of polymer (B) and the third stage (outer layer) is made of polymer (C).

The emulsion polymerization uses a radical polymerization initiator and an emulsifier.
The radical polymerization initiator is not particularly limited, but examples thereof include peroxides, azo-based initiators, and redox-based initiators that combine an oxidizing agent and a reducing agent.
The emulsifier is not particularly limited, but is preferably, for example, various carboxylates such as sodium sulfonate, sodium sarcosinate, potassium fatty acid, sodium fatty acid, dipotassium alkenyl succinate, and rosin acid soap, because they have excellent stability of the latex during radical polymerization and can increase the polymerization rate.

The polymerization temperature in emulsion polymerization varies depending on the type and amount of radical polymerization initiator, but is preferably 40°C to 120°C, and more preferably 60°C to 95°C. The radical polymerization initiator is added to at least one of the aqueous phase and the monomer phase.

(Step (2))
The multi-stage copolymer obtained by emulsion polymerization is usually in the form of a latex, and therefore in step (2), the multi-stage copolymer is recovered from the latex (third latex) of the multi-stage copolymer obtained in step (1).
In step (2), examples of the method for recovering the multistage copolymer from the latex of the multistage copolymer include a wet method in which the latex of the multistage copolymer is coagulated into a slurry by introducing it into hot water in which a coagulant is dissolved, and a spray drying method in which the latex of the multistage copolymer is sprayed into a heated atmosphere to recover a powder containing the multistage copolymer by spray drying, etc. The aggregated particles obtained by the spray drying method are not strongly bonded to each other among the primary particles, so that it is difficult to form a higher-order particle structure, and the primary particles can be uniformly dispersed and directly recovered, and therefore the recovery method is preferably the spray drying method.

The method for producing a multi-stage copolymer for powder coating of this embodiment includes the above-mentioned steps (1) and (2), and therefore can provide a multi-stage copolymer that can provide a powder coating composition capable of forming a coating film having good appearance and excellent toughness.

[Powder coating composition]
The powder coating composition of the present embodiment contains the above-mentioned multi-stage copolymer for powder coating and a thermoplastic resin.

Examples of thermoplastic resins include polyester, epoxy resin, acrylic resin, etc. Among these, polyester is preferred from the viewpoints of toughness, appearance, light resistance, and cost of the powder coating composition containing the thermoplastic resin.

When the powder coating composition of this embodiment contains polyester as the thermoplastic resin, it becomes a polyester-based powder coating. When the powder coating composition of this embodiment contains polyester and epoxy resin as basic components, it becomes a polyester-epoxy hybrid powder coating. When the powder coating composition of this embodiment contains acrylic resin as a basic component, it becomes an acrylic powder coating. When the powder coating composition of this embodiment contains epoxy resin as a basic component, it becomes an epoxy-based powder coating.

There are no particular limitations on the polyester, so long as it is one that is used by those skilled in the art as a polyester-based powder coating. For example, polyester resins that have two or more hydroxyl or acid groups in one molecule and have a softening point in the range of 60°C to 150°C are commonly used as thermoplastic resins.

There are no particular limitations on the epoxy resin, so long as it is one that is used as an epoxy powder coating by those skilled in the art. Those that have an average of two or more epoxy groups per molecule are generally used, such as bisphenol A epoxy resins, bisphenol B epoxy resins, bisphenol F epoxy resins, novolac epoxy resins, brominated epoxy resins, and alicyclic epoxy resins.

There are no particular limitations on the acrylic resin, so long as it is one that is used as an acrylic powder coating by those skilled in the art. For example, a (meth)acrylic acid ester copolymer having a softening point in the range of 60°C to 150°C, synthesized by copolymerization of a (meth)acrylic acid ester monomer and a (meth)acrylic acid ester monomer having a glycidyl group in the molecule, is commonly used as a thermoplastic resin.

In the powder coating composition of this embodiment, when the total amount of the powder coating composition is taken as 100 mass%, the content of the multi-stage copolymer for powder coating is preferably 1 mass% or more and 20 mass% or less, more preferably 1 mass% or more and 15 mass% or less, and even more preferably 1 mass% or more and 10 mass% or less.
When the content of the multistage copolymer for powder coating is 1% by mass or more, a powder coating composition capable of forming a good coating film having excellent toughness can be provided.When the content of the multistage copolymer for powder coating is 20% by mass or less, an increase in the melt viscosity of the powder coating composition can be suppressed, and a powder coating composition capable of forming a smooth coating film can be provided.

The powder coating composition of this embodiment may contain, in addition to the above-mentioned multi-stage copolymer for powder coating and thermoplastic resin, a curing agent, a pigment, and various other additives.

The powder coating composition of this embodiment can be generally produced by a known method, i.e., by dry-mixing the multi-stage copolymer for powder coating, the thermoplastic resin, and optionally a curing agent, a pigment, and other additives, melt-kneading the mixture at a temperature equal to or higher than the softening point of the thermoplastic resin, and then pulverizing and classifying the mixture as required.
Examples of other additives include known additives used in ordinary powder coating compositions, such as surface conditioners, ultraviolet absorbers, antioxidants, and anti-popping agents.

In dry mixing of the powder coating composition, various mixers such as a Henschel mixer, a Banbury mixer, a high-speed mixer, or a Nauta mixer can be used.
Apparatuses used for melt-kneading the powder coating composition include, for example, a heated roller machine, a heated kneader machine, an extruder, and the like.
Examples of the pulverizer for pulverizing the powder coating composition after melt-kneading include impact pulverizers such as a hammer mill and a pin mill.
As a classifier for classifying the powder coating composition after pulverization, for example, a vibrating sieve or the like can be mentioned.

The powder coating composition of the present embodiment can be applied to an object to be coated by a common coating method such as electrostatic coating or fluidized bed coating, and then heated and cured to form a coating film.
The heating temperature (baking temperature) and time for forming the coating film can be appropriately set. For example, the heating temperature is usually set to be equal to or higher than the melting point of the thermoplastic resin.

Examples of substrates to which the powder coating composition of this embodiment can be applied include metals such as iron, zinc, tin, stainless steel, copper, and aluminum, inorganic materials such as glass, and substrates made of these metals or inorganic materials that have been subjected to a plaster treatment, primer coating, or undercoat coating as necessary.

The thickness of the coating film formed by the powder coating composition of this embodiment is not particularly limited, but it is preferable that it be 15 μm or more and 1 mm or less.

Before forming a coating film using the powder coating composition of this embodiment, a primer coating may be formed on the coating surface of the above-mentioned substrate (the surface to which the powder coating composition is applied) using a known primer coating.

According to the powder coating composition of this embodiment, since it contains the above-mentioned multi-stage copolymer for powder coating and a thermoplastic resin, it is possible to provide a powder coating composition capable of forming a coating film having good appearance and excellent toughness.

The present invention will be specifically described below with reference to examples. Note that the present invention is not limited to these examples. Note that "parts" hereinafter refer to "parts by mass."
The various measurement and evaluation methods are as follows.

[Measurement of Volume Average Particle Size of Primary Particles of Multistage Copolymer]
The volume average particle size of the primary particles of the multistage copolymer was measured by the following method.
The primary particle diameter of the dispersed particles in the dispersion liquid containing the multistage copolymer was measured using a laser diffraction/scattering type particle size distribution measuring device (product name: LA-960, manufactured by Horiba, Ltd.). The average value of the primary particle diameters was taken as the volume average particle diameter of the primary particles. The relative refractive index of the resin particles and the dispersion medium was all 1.12. Ion-exchanged water was used as the dispersion medium.

[Measurement of Volume Average Particle Size of Secondary Particles of Multistage Copolymer]
The volume average particle size of the secondary particles of the multistage copolymer was measured by the following method.
The secondary particle diameter of the dispersed particles in the dispersion liquid containing the multi-stage copolymer was measured using a laser diffraction/scattering type particle size distribution measuring device (product name: LA-960, manufactured by Horiba, Ltd.). The average value of the secondary particle diameters was taken as the volume average particle diameter of the secondary particles. The relative refractive index of the resin particles and the dispersion medium was all 1.12. Ion-exchanged water was used as the dispersion medium.

[Evaluation of Appearance of Coating Film]
The appearance of the coating film obtained by applying and curing the powder coating composition was evaluated based on the leveling property of the coating film. The leveling property of the coating film was evaluated by the following method.
The powder coating composition was sprayed onto a metal substrate (test piece; Q-panel QD46, thickness 0.5 mm) and cured at 200° C. for 10 minutes to form a coating film with a thickness of 100 μm.
Thereafter, the long wavelength (LW) of the reflected light of the light irradiated onto the coating film was measured using a Microwave Scan AW-4824 (manufactured by BYK Corporation).
From the obtained measured values, the leveling property of the coating film was judged according to the following criteria: If LW was less than 50, the leveling property was judged to be good.
LW50 or more: ×, LW40 or more to less than 50: 〇, LW less than 40: ◎

[Evaluation of coating toughness]
The toughness of the coating film formed by applying and curing the powder coating composition was evaluated by the cupping resistance of the coating film. The cupping resistance of the coating film was evaluated by the following method.
The powder coating composition was sprayed onto a metal substrate (test piece; Q-panel QD46, thickness 0.5 mm) and cured at 200° C. for 10 minutes to form a coating film with a thickness of 40 μm.
The indentation depth of the coating was then measured according to ISO 1520 using a PCE-CPT manual cupping tester.
From the obtained measured values, the cupping resistance was evaluated according to the following criteria: If the indentation depth was 2.5 mm or more, the cupping resistance was considered to be good.
Indentation depth less than 2.5 mm: ×, indentation depth 2.5 mm or more and less than 4.0 mm: ◯, indentation depth 4.0 mm or more: ◎

[Evaluation of impact resistance of coating film]
The toughness of the coating film formed by applying and curing the powder coating composition was evaluated by the impact resistance of the coating film. The impact resistance of the coating film was evaluated by the following method.
The powder coating composition was sprayed onto a metal substrate (test piece; Q-panel QD46, thickness 0.5 mm) and cured at 200° C. for 10 minutes to form a coating film with a thickness of 40 μm.
Thereafter, a ball drop test was carried out on the coating surface in accordance with ASTM D-2794, and the impact resistance of the coating was measured in inch-pounds (in-lbn).
From the obtained measured values, the impact resistance was judged according to the following criteria: If the inch-pound value was 100 or more, the impact resistance was deemed good.
Less than 100 in-lbn: ×, 100 in-lbn to less than 140 in-lbn: ◯, 140 in-lbn or more: ◎

Example 1
[Step (1-1)]
In a 5-liter separable flask equipped with a thermometer, a nitrogen gas inlet tube, a stirring rod, a dropping funnel and a cooling tube, 63.8 parts of deionized water was placed, and nitrogen gas was bubbled through for 30 minutes to replace the oxygen dissolved in the deionized water.
Then, the nitrogen gas flow was stopped, and the temperature was raised to 80° C. while stirring at 200 rpm.
Thereafter, when the temperature inside the separable flask reached 80°C, 2.96 parts of methyl methacrylate, 1.97 parts of n-butyl acrylate, 0.19 parts of allyl methacrylate, and 0.03 parts of 1,3-butylene glycol dimethacrylate were added all at once, and further 0.05 parts of potassium persulfate and 2.38 parts of deionized water were added, and the mixture was maintained for 60 minutes to obtain a latex containing polymer (A) particles.

[Step (1-2)]
To the latex obtained in the step (1-1), 0.04 parts of PELLEX OT-P, 0.05 parts of potassium persulfate, and 4 parts of deionized water were added, and 75.1 parts of n-butyl acrylate, 2.85 parts of allyl methacrylate, 1.52 parts of 1,3-butylene glycol dimethacrylate, 0.75 parts of PELLEX OT-P, and 51.6 parts of deionized water were added dropwise over 180 minutes.
Thereafter, the mixture was maintained for 1 hour to complete the polymerization, thereby obtaining a latex containing particles in which the polymer (B) was polymerized around the polymer (A).

[Step (1-3)]
To the latex obtained in the step (1-2), 17.7 parts of methyl methacrylate, 0.35 parts of ethyl acrylate, 1.9 parts of 2-hydroxyethyl methacrylate, 0.20 parts of 1,3-butylene glycol dimethacrylate, 0.20 parts of PELLEX OT-P, and 15 parts of deionized water were added dropwise over 140 minutes.
Thereafter, the mixture was maintained for 2 hours to complete the polymerization, thereby obtaining a latex containing a multi-stage copolymer in which the polymer (C) was polymerized around particles made of the polymer (A) and the polymer (B).

[Step (2)]
The latex obtained in the step (1-3) was spray-dried using a spray dryer (product name: L-8i type, manufactured by Okawara Kakoki Co., Ltd.) under the conditions of inlet temperature/outlet temperature=120° C./60° C. and disk rotation speed of 20,000 rpm, to obtain a multistage copolymer (P-1).
The volume average particle size of the primary particles of the multi-stage copolymer was 0.70 μm. The results are shown in Table 2.
FIG. 1 shows a tan δ curve between −60° C. and 140° C. calculated by measuring the temperature dispersion of the dynamic viscoelasticity of the multi-stage copolymer.
The Tg of each of the polymers (A), (B) and (C) was calculated using the formula (1) from the Tg of the homopolymer shown in Table 1 corresponding to the monomer unit constituting each polymer.

Example 2
A multi-stage copolymer (P-2) was obtained in the same manner as in Example 1, except that the respective components of the monomers constituting the monomer units constituting the polymer (A), the polymer (B), and the polymer (C) were in the mass ratio shown in Table 2. The results are shown in Table 2.

Example 3
A multi-stage copolymer (P-3) was obtained in the same manner as in Example 1, except that the monomer components constituting the monomer units constituting the polymer (A), the polymer (B), and the polymer (C) were in the mass ratio shown in Table 2. The results are shown in Table 2.

Example 4
A multistage copolymer (P-4) was obtained in the same manner as in Example 1, except that the monomer components constituting the monomer units constituting the polymer (A), polymer (B), and polymer (C) were in the mass ratio shown in Table 2. The results are shown in Table 2.

Example 5
A multi-stage copolymer (P-5) was obtained in the same manner as in Example 1, except that the monomer components constituting the monomer units constituting the polymer (A), polymer (B), and polymer (C) were in the mass ratio shown in Table 2. The results are shown in Table 2.

Example 6
A multistage copolymer (P-6) was obtained in the same manner as in Example 1, except that the monomer components constituting the monomer units constituting the polymer (A), the polymer (B), and the polymer (C) were in the mass ratio shown in Table 2. The results are shown in Table 2.

Example 7
[Step (1-1)]
In a 5-liter separable flask equipped with a thermometer, a nitrogen gas inlet tube, a stirring rod, a dropping funnel and a cooling tube, 63.8 parts of deionized water was placed, and nitrogen gas was bubbled through for 30 minutes to replace the oxygen dissolved in the deionized water.
Then, the nitrogen gas flow was stopped, and the temperature was raised to 80° C. while stirring at 200 rpm.
Thereafter, when the temperature inside the separable flask reached 80°C, half of the monomer mixture solution of 6.00 parts of methyl methacrylate, 4.00 parts of n-butyl acrylate, 0.39 parts of allyl methacrylate, and 0.05 parts of 1,3-butylene glycol dimethacrylate was added all at once, and further, 0.05 parts of potassium persulfate and 2.38 parts of deionized water were added, and the mixture was maintained for 60 minutes.
Next, the remaining half of the above monomer mixture solution was added dropwise over 30 minutes.
Thereafter, the mixture was maintained for 30 minutes to complete the polymerization, thereby obtaining a latex containing polymer (A) particles.

[Step (1-2)]
To the latex obtained in the step (1-1), 0.04 parts of PELLEX OT-P, 0.05 parts of potassium persulfate, and 4 parts of deionized water were added, and 70.0 parts of n-butyl acrylate, 2.66 parts of allyl methacrylate, 1.42 parts of 1,3-butylene glycol dimethacrylate, 0.70 parts of PELLEX OT-P, and 48.1 parts of deionized water were added dropwise over 180 minutes.
Thereafter, the mixture was maintained for 1 hour to complete the polymerization, thereby obtaining a latex containing particles in which the polymer (B) was polymerized around the polymer (A).

[Step (1-3)]
To the latex obtained in the step (1-2), 17.7 parts of methyl methacrylate, 0.35 parts of ethyl acrylate, 1.9 parts of 2-hydroxyethyl methacrylate, 0.20 parts of 1,3-butylene glycol dimethacrylate, 0.20 parts of PELLEX OT-P, and 15 parts of deionized water were added dropwise over 140 minutes.
Thereafter, the mixture was maintained for 2 hours to complete the polymerization, thereby obtaining a latex containing a multi-stage copolymer in which the polymer (C) was polymerized around particles made of the polymer (A) and the polymer (B).

[Step (2)]
The latex obtained in the step (1-3) was spray-dried to obtain a multi-stage copolymer (P-8) in the same manner as in Example 1. The results are shown in Table 2.

<Comparative Example 1>
A multistage copolymer (P-8) was obtained in the same manner as in Example 1, except that the respective components of the monomers constituting the monomer units constituting the polymer (A), the polymer (B), and the polymer (C) were in the mass ratio shown in Table 2. The results are shown in Table 2.

The compounds in Table 2 are as follows:
"MMA": Methyl methacrylate (manufactured by Mitsubishi Chemical Corporation)
"n-BA": n-butyl acrylate (manufactured by Mitsubishi Chemical Corporation)
"AMA": Allyl methacrylate (manufactured by Mitsubishi Chemical Corporation)
"BDMA": 1,3-butylene glycol dimethacrylate (manufactured by Mitsubishi Chemical Corporation)
"EA": Ethyl acrylate (manufactured by Mitsubishi Chemical Corporation)
"2-HEMA": 2-hydroxyethyl methacrylate (manufactured by Mitsubishi Chemical Corporation)
"MAA": Metallurgic acid (manufactured by Mitsubishi Chemical Corporation)

Example 8
The components of the powder coating composition are as follows:
Polyester resin: SP-6400 (manufactured by Sun Polymer International)
Curing agent: Triglycidyl isocyanurate (TGIC) (manufactured by Aalchem)
Leveling agent: Resiflow P-67 (manufactured by Estron Chemical)
Defoaming agent: Benzoin (manufactured by Estron Chemical Co.)
Pigment: TiPure R960 (Venator)
Toughness imparting agent: multi-stage copolymer (P-1)

61.4 parts of SP-6400, 4.6 parts of TGIC, 1 part of Resiflow P-67, 0.5 parts of Benzoin, 30 parts of TiPure R960, and 2.5 parts of the multi-stage copolymer (P-1) were pre-kneaded in a Vitamix for 8 seconds.
Thereafter, the pre-mixed mixture was melt-kneaded in a twin-screw extruder APV 19 mm (set at 100° C., screw speed 500 rpm, chill roll 18 rpm).
The melt-kneaded mixture was then pulverized in a strand mill and sieved through a 140 mesh sieve to obtain a powder coating composition.
The resulting powder coating composition was used to form a coating film, which was then evaluated for leveling property, cupping resistance and impact resistance. The results are shown in Table 3.

<Example 9>
A powder coating composition was obtained in the same manner as in Example 8, except that the multi-stage copolymer (P-2) was used instead of the multi-stage copolymer (P-1).
The resulting powder coating composition was used to form a coating film, which was then evaluated for leveling property, cupping resistance and impact resistance. The results are shown in Table 3.

Example 10
A powder coating composition was obtained in the same manner as in Example 8, except that the multi-stage copolymer (P-3) was used instead of the multi-stage copolymer (P-1).
The resulting powder coating composition was used to form a coating film, which was then evaluated for leveling property, cupping resistance and impact resistance. The results are shown in Table 3.

Example 11
A powder coating composition was obtained in the same manner as in Example 8, except that the multi-stage copolymer (P-4) was used instead of the multi-stage copolymer (P-1).
The resulting powder coating composition was used to form a coating film, which was then evaluated for leveling property, cupping resistance and impact resistance. The results are shown in Table 3.

Example 12
A powder coating composition was obtained in the same manner as in Example 8, except that the multi-stage copolymer (P-5) was used instead of the multi-stage copolymer (P-1).
The resulting powder coating composition was used to form a coating film, which was then evaluated for leveling property, cupping resistance and impact resistance. The results are shown in Table 3.

<Example 13>
A powder coating composition was obtained in the same manner as in Example 8, except that the multi-stage copolymer (P-6) was used instead of the multi-stage copolymer (P-1).
The resulting powder coating composition was used to form a coating film, which was then evaluated for leveling property, cupping resistance and impact resistance. The results are shown in Table 3.

<Example 14>
A powder coating composition was obtained in the same manner as in Example 8, except that the multi-stage copolymer (P-7) was used instead of the multi-stage copolymer (P-1).
The resulting powder coating composition was used to form a coating film, which was then evaluated for leveling property, cupping resistance and impact resistance. The results are shown in Table 3.

<Comparative Example 2>
A powder coating composition was obtained in the same manner as in Example 2, except that the multi-stage copolymer (P-8) was used instead of the multi-stage copolymer (P-1).
The resulting powder coating composition was used to form a coating film, which was then evaluated for leveling property, cupping resistance and impact resistance. The results are shown in Table 3.

<Comparative Example 3>
63.7 parts of SP-6400, 4.8 parts of TGIC, 1 part of Resiflow P-67, 0.5 parts of Benzoin, and 30 parts of TiPure R960 were premixed in a Vitamix for 8 seconds.
Thereafter, the pre-mixed mixture was melt-kneaded in a twin-screw extruder APV 19 mm (set at 100° C., screw speed 500 rpm, chill roll 18 rpm).
The melt-kneaded mixture was then pulverized in a strand mill and sieved through a 140 mesh sieve to obtain a powder coating composition.
The resulting powder coating composition was used to form a coating film, which was then evaluated for leveling property, cupping resistance and impact resistance. The results are shown in Table 3.

The multi-stage copolymer for powder coatings of the present invention can provide a powder coating composition capable of forming a coating film having good appearance and excellent toughness, and therefore can be suitably used for outdoor applications such as road materials and building materials.

Claims (12)

A multi-stage copolymer for powder coating, comprising at least three polymer components: a first stage polymer (A), a second stage polymer (B), and a third stage polymer (C),
A powder coating composition comprising: a multi-stage copolymer for powder coating, the glass transition temperature of the polymer (A) being -15°C or higher, the glass transition temperature of the polymer (B) being -20°C or lower, and the glass transition temperature of the polymer (C) being 60°C or higher, the multi-stage copolymer for powder coating containing the polymer (A) in an amount of 1% by mass or more and 30% by mass or less when the total amount of the multi-stage copolymer for powder coating is taken as 100% by mass; and a thermoplastic resin which is a polyester. A multi-stage copolymer for powder coating comprising at least an inner layer, an intermediate layer and an outer layer,
the inner layer comprises a polymer (A) having a glass transition temperature of −15° C. or higher,
the intermediate layer contains a polymer (B) having a glass transition temperature of −20° C. or lower,
The outer layer contains a polymer (C) having a glass transition temperature of 60° C. or higher,
A powder coating composition comprising: a multi-stage copolymer for powder coating, the multi-stage copolymer for powder coating containing 2% by mass or more and 20% by mass or less of the polymer (A) when the total amount of the multi-stage copolymer for powder coating is taken as 100% by mass; and a thermoplastic resin which is a polyester. The powder coating composition according to claim 1 or 2, comprising the polymer (B) obtained by polymerizing a monomer mixture in the presence of the polymer (A), and the polymer (C) obtained by polymerizing a monomer mixture in the presence of a polymer containing the polymer (A) and the polymer (B). A multi-stage copolymer for powder coating, comprising at least three polymer components: a polymer (A), a polymer (B), and a polymer (C),
3. A powder coating composition comprising the multi-stage copolymer for powder coating according to claim 1 or 2, wherein at least two peaks are present in a tan δ curve between -60°C and 140°C in a temperature dispersion measurement of dynamic viscoelasticity of the multi-stage copolymer for powder coating, and a thermoplastic resin which is a polyester. The powder coating composition according to any one of claims 1 to 4, wherein the polymer (A) contains 35% by mass or more and 99.5% by mass or less of methyl methacrylate units and 0.5% by mass or more and 5% by mass or less of a polyfunctional monomer, when the total amount of monomer units in the polymer (A) is taken as 100% by mass. The powder coating composition according to any one of claims 1 to 5, wherein the polymer (B) contains 70% by mass or more and 99.5% by mass or less of alkyl (meth)acrylate units having an alkyl group with 4 to 8 carbon atoms, and 0.5% by mass or more and 5% by mass or less of a polyfunctional monomer, when the total amount of monomer units in the polymer (B) is taken as 100% by mass. The powder coating composition according to any one of claims 1 to 6, wherein the polymer (C) contains 70% by mass or more and 100% by mass or less of methyl methacrylate units, when the total amount of monomer units in the polymer (C) is taken as 100% by mass. 8. The powder coating composition according to claim 1, wherein the volume average particle size of primary particles of the multistage copolymer for powder coating is 0.1 μm or more and 10 μm or less. 9. The powder coating composition according to claim 1, wherein the volume average particle size of secondary particles of the multistage copolymer for powder coating is 1 μm or more and 500 μm or less. The powder coating composition according to any one of claims 1 to 9, comprising 1% by mass or more and 30% by mass or less of the polymer (A), 31% by mass or more and 94% by mass or less of the polymer (B), and 5% by mass or more and 39% by mass or less of the polymer (C), when the total of the polymer (A), the polymer (B), and the polymer (C) is taken as 100% by mass. A method for producing the powder coating composition according to any one of claims 1 to 10, comprising the steps of:
a step (1) of polymerizing a first monomer mixture for constituting a polymer (A) to obtain a first dispersion containing the polymer (A), then dropping a second monomer mixture for constituting a polymer (B) into the first dispersion to polymerize the second monomer mixture to obtain a second dispersion containing the polymer (A) and the polymer (B), and further dropping a third monomer mixture for constituting a polymer (C) into the second dispersion to polymerize the third monomer mixture to obtain a third dispersion containing a multistage copolymer containing the polymer (A), the polymer (B), and the polymer (C);
and (2) spray-drying the third dispersion to obtain a powder containing the multistage copolymer. The powder coating composition according to any one of claims 1 to 10, wherein the content of the multi-stage copolymer for powder coating is 1% by mass or more and 20% by mass or less, when the total amount of the powder coating composition is 100% by mass.

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