JPH09324140A - Powder coating and its coating - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain the subject coating containing a graft polymer having a polystyrene main chain and specific graft chains and having small particle diameters, suitable for thin film coatings, excellent in a positive charge property and useful for tribo-charge type spray guns, etc. SOLUTION: This powder coating contains a graft polymer having a polystyrene main chain and graft chains each having a skeleton of the formula [R1 is a 11-17C hydrocarbon; R2 , R3 are each H, a 1-8C hydrocarbon; R4 is a 1-12C alkylene which may contain an ether bond in the group; X<-> is an anion; (n) is 2-100] and having a volume-average particle diameter of 7-20μm. The content of the graft polymer is 0.5-10 pts.wt. per 100 pts.wt. of a binding resin. The graft polymer is obtained e.g. by introducing the graft chains to the functional group parts of a functional group-having styrenic resin (e.g. styrene/ chloromethylene styrene copolymer).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a powder coating having a small particle size, which is suitable for thin film coating and has excellent positive chargeability.

[0002]

2. Description of the Related Art It is well known that powder coatings have less volatile components and odors than solvent coatings and are very useful in terms of pollution control and environmental regulations. The powder coating materials that have hitherto been put on the market have a volume average particle diameter of 30 to 40 μm and have not been subjected to strict classification, so that the particle diameter distribution is very broad. Therefore, it has not satisfied the market needs such as improvement of the smoothness of the coating film, improvement of working efficiency by thinning the coating film, and total cost reduction. With powder coatings, in order to form a coating film having good smoothness, it is necessary to coat the coating so that the film thickness is 2 to 3 times the average particle diameter or more. In coating with the paint, only a thick coating film of about 60 μm could be formed.

Regarding the electrostatic coating machine, there is a corona charging type spray gun as a coating method of powder coating material which has been generally used conventionally. In this method, the powder paint negatively charged by corona ions generated from the corona electrode provided at the tip of the spray gun causes the electric field and the air flow formed between the object to be coated, which is a conductor, and the electrode. It flies along and adheres to the object to be coated. It has been found that such a corona charging method has two major technical problems. One is called the Faraday cage effect. Since the electric field (lines of electric force) is not formed in the concave part of the object to be coated, the powder coating adheres only a small amount to the concave part, and conversely the edge where electric lines of force concentrate. It is a phenomenon that a large amount adheres to the part.
The other is the phenomenon called reverse ionization, which is a phenomenon in which the accumulated charge of the powder coating material and free corona ions deposited on the object becomes too large and spark discharge occurs, causing crater-like defects on the coated surface. Is.

In order to solve these problems, a tribo-charging type spray gun has been used in recent years. In this system, the powder coating material carried by the air flow is positively charged by friction with the fluororesin portion existing on the inner wall of the spray gun, and is only flying and adheres to the object to be coated by the air flow. In this method, since no electric field is formed, the powder coating material adheres well to the recesses, and free ions are not generated, so that the reverse ionization phenomenon is unlikely to occur. However, since the powder paint is charged only by friction, the absolute charge amount is lower than that of the corona charging method, and the discharge amount of the powder paint from the spray gun is increased.
It has been clarified that when used in high humidity, a sufficient amount of charge cannot be obtained, resulting in insufficient coating of the powder coating material on the article to be coated. For these reasons, there is a high market need for a powder coating of small particle size, which has good coating properties with a tribo-charging type spray gun and is capable of thin film coating. However, in the tribo-charging type spray gun, the amount of charge of each powder particle is reduced due to the reduction of the surface area due to the reduction of the particle diameter of the powder coating material, which causes a problem that the coating property is deteriorated.

[0005]

DISCLOSURE OF THE INVENTION The present invention is applied when a powder coating of small particle size having a volume average particle size of 7 to 20 μm capable of thin film coating is applied using a tribo-charging type spray gun. The purpose is to improve poor adhesion.

[0006]

The present invention contains a graft polymer having a polystyrene main chain and a graft chain having a skeleton represented by the following general formula (1), and has a volume average particle size of 7: It is a powder coating material characterized by having a thickness of up to 20 μm.

Embedded image (Wherein, R ₁ is a hydrocarbon group of C ₁₁ ~C _17, R ₂ and R
₃ is hydrogen or a C ₁ -C ₈ hydrocarbon group, or R ₂ and R ₃ may be connected to each other to form an aromatic ring. R ₄ is C ₁ which may contain an ether bond in the group.
To C ₁₂ alkylene group, X ⁻ is an anion, and n is 2 to
In addition, the coating method of the present invention is characterized in that the powder coating material is positively charged by friction with a member containing a fluorine atom installed in the coating material transporting portion, and then sprayed onto an object to be coated. And

Hereinafter, the present invention will be described in detail. The greatest feature of the present invention is that it contains a graft polymer having a polystyrene main chain and a graft chain having the skeleton represented by the general formula (1) as a constituent of the powder coating material. By using this graft polymer, the rise time of triboelectrification is shortened, and the triboelectrification type spray gun is easily charged to a sufficient charge amount. Further, it is possible to obtain a practically sufficient charge amount even in a high humidity environment. Further, the above-mentioned graft polymers are often colorless or light-colored and do not hinder the color matching of powder coatings, and are therefore suitable for color powder coatings.

The graft polymer used in the present invention has the general formula (1) at the functional group portion of the styrene resin having a functional group.
The graft chain represented by is introduced. Therefore, the polystyrene-based main chain of the present invention is obtained by removing the functional group portion of the styrene-based resin having a functional group.

Examples of the styrene resin having a functional group include a styrene resin having a halomethyl group and a styrene resin having an amino group. Examples of the styrene-based resin having a halomethyl group include a styrene monomer, a halomethylated styrene monomer and, if necessary, a copolymer of other monomers. Chloromethylstyrene as the halomethylated styrene monomer,
Promomethyl styrene etc. are mentioned. Preferred among these is chloromethylstyrene. On the other hand, examples of the styrene-based resin having an amino group include a styrene monomer, an amino group-containing monomer and, if necessary, a copolymer of other monomers. Examples of the amino group-containing monomer include amino group-containing styrene (1-styrylmethylimidazole, etc.), amino group-containing (meth) acrylate {(dimethylaminoethyl (meth) acrylate, 1- [2- (meth) acryloyloxyethyl] imidazole. Etc.) and amino group-containing (meth) acrylamide {dimethylaminopropyl (meth) acrylamide, 1- (meth) acryloylimidazole, etc.} and the like. Among these, preferred are amino group-containing (meth) acrylates and amino group-containing (meth) acrylates.
It is acrylamide. In the styrene-based resin having a halomethyl group and the styrene-based resin having an amino group, as the other monomer used as a copolymerization component, if necessary, a styrene substitution product, a (meth) acrylic-based monomer and other vinyl-based monomers are used. Can be mentioned.

Examples of the substituted styrene include alkyl-substituted styrene (α-methylstyrene, p-methylstyrene, p-cumylstyrene, etc.), halogen-substituted styrene [(poly) chlorostyrene, (poly) promostyrene, etc.], acetoxy. Examples thereof include styrene and hydroxystyrene. Further, (meth) alkyl (meth) acrylate [(methyl (meth) acrylate as the acrylic monomer (meth) acrylate {C ₁ ~C _{1 8,} ethyl (meth) acrylate, butyl (meth) acrylate, 2-ethylhexyl (Meth) acrylate, lauryl (meth) acrylate, stearyl (meth) acrylate and the like], hydroxyl group-containing (meth) acrylate [hydroxyethyl (meth) acrylate and the like]}, (meth) acrylamide [(meth) acrylamide and the like], Examples thereof include nitrile group-containing (meth) acrylic monomers [(meth) acrylonitrile, etc.].
Other vinyl-based monomers include vinyl esters (vinyl acetate, etc.), aliphatic vinyl hydrocarbons (butadiene, isoprene, etc.), halogenated olefins (vinyl chloride, vinyl bromide, etc.), unsaturated mono- or polycarboxylic acids [( (Meth) acrylic acid, crotonic acid, maleic acid,
Itaconic acid, cinnamic acid, etc.], and their anhydrides (maleic anhydride, etc.) and the like.

The copolymerization ratio of the styrene monomer and the halomethylated styrene monomer or the amino group-containing monomer and the other monomer used as the case requires is on a molar ratio basis.
Styrene monomer is 50 to 99.5%, halomethylated styrene monomer or amino group-containing monomer is 0.5 to
30%, other monomers are 0 to 40%, preferably styrene monomers are 60 to 99%, halomethylated styrene monomers or amino group-containing monomers are 1 to 20%,
Other monomers are 0 to 30%. It should be noted that the styrene-based resin having a functional group does not necessarily have to copolymerize a monomer having a functional group from the beginning. That is, the desired functional group may be introduced by a reaction after polymerizing a styrene resin having no functional group. For example, a styrene resin may be halomethylated to obtain a styrene resin having the same structure as a styrene resin having a halomethyl group.

Next, in the general formula (1), C of R ₁ is
Examples of the hydrocarbon group of _{11 to} C ₁₇ include an alkyl group (undecyl group, dodecyl group, tridecyl group, tetradecyl group, pentadecyl group, hexadecyl group, heptadecyl group, etc.), alkenyl group (undecenyl group, dodecenyl group,
Tridecenyl group, tetradecenyl group, pentadecenyl group, hexadecenyl group, heptadecenyl group, etc.), alkylphenyl group (pentylphenyl group, hexylphenyl group, heptylphenyl group, octylphenyl group, nonylphenyl group, decylphenyl group, undecylphenyl group, etc. ) And the like. Among these, preferred are C _{11 to} C ₁₇ alkyl groups and alkenyl groups. Particularly preferred are C ₁₁ -C ₁₇ alkyl groups. The hydrogen of R ₂ and R ₃ or the hydrocarbon group of C _{1 to} C ₈ includes hydrogen, an alkyl group (methyl group, ethyl group, propyl group, butyl group, hexyl group, octyl group, etc.), aryl group ( Phenyl group) and aralkyl group (benzyl group). Examples of the aromatic ring in which R ₂ and R ₃ are connected to each other include a benzo group. Among these, preferred are hydrogen, methyl group, ethyl group and benzo group. In addition, among R ₄ , C ₁ ~
As the C ₁₂ alkylene group, a methylene group, an ethylene group,
Examples thereof include trimethylene group, tetramethylene group, hexamethylene group, octamethylene group and dodecamethylene group. Further, among R ₄ , C _{1 to} C ₁₂ containing an ether bond
The alkylene group of is —CH ₂ CH ₂ OCH ₂ CH
_{2 -, - CH 2 CH 2} OCH 2 OCH 2 CH 2 -, - CH 2 C
_{H 2 OCH 2 CH 2 OCH 2} CH 2 -, and the like.
Among these, preferred are an ethylene group, trimethylene group, tetramethylene group, hexamethylene group, dodecamethylene _{group, -CH 2 CH 2 OCH 2 CH} 2 - and -CH ₂ C
H ₂ OCH ₂ OCH ₂ CH ₂ —, more preferred are trimethylene group, tetramethylene group, hexamethylene group, dodecamethylene group and —CH ₂ CH ₂ OCH ₂ CH ₂ —.
It is.

Further, X ^- as the anion a halide ion (chloride ion, bromide ion, iodide ion, etc.),
Sulfate ion, nitrate ion, phosphate ion, sulfonate ion (p-toluenesulfonate ion, methylsulfonate ion, hydroxynaphthosulfonate ion, etc.),
Borate ion (tetrafluoroborate ion, tetraphenylborate ion, metaborate ion, metaborate-
Glycerin complex ion etc.), oxo acid ion (molybdate ion, tungstate ion etc.) and carboxylate ion (acetate ion etc.) and the like. Among these, preferred are halogen ion, sulfonate ion, borate ion and oxo acid ion.

The molecular weight of the graft polymer in the present invention is not particularly limited, but usually the number average degree of polymerization of the polystyrene main chain, that is, the number average degree of polymerization of the styrene resin is 5 to 500, The number average degree of polymerization n of the chain is 2 to 100. The graft polymer does not need to have a structure in which only one end of all graft chains is bonded to a polystyrene-based main chain, but a structure in which both ends of some graft chains are bonded to a polystyrene-based polymer. May be.

Specific examples of the graft polymer of the present invention are shown by the combination of the above-mentioned functional group-containing styrene resin and a graft chain. Styrenic resin having functional group: Styrene / chloromethylstyrene copolymer Graft chain: Graft chain represented by the following structural formula

Embedded image Styrenic resin having a functional group: styrene / butyl acrylate / chloromethylstyrene copolymer Graft chain: Graft chain represented by the following structural formula

Embedded image

Styrenic resin having a functional group: styrene / butyl acrylate / chloromethylstyrene copolymer Graft chain: Graft chain represented by the following structural formula

Embedded image Styrenic resin having a functional group: styrene / butyl acrylate / dimethylaminoethyl methacrylate copolymer Graft chain: Graft chain represented by the following structural formula

[Chemical 6]

Styrenic resin having a functional group: Styrene / butyl methacrylate / chloromethylstyrene copolymer Graft chain: Graft chain represented by the following structural formula

[Chemical 7] Styrenic resin having functional group: Styrene / butyl methacrylate / chloromethylstyrene copolymer Graft chain: Graft chain represented by the following structural formula

Embedded image

Styrenic resin having a functional group: styrene / butadiene acryloyloxyethyl imidazole copolymer Graft chain: Graft chain represented by the following structural formula

Embedded image The content of the graft polymer in the powder coating material is 0.5 to 10 parts by weight, preferably 1 to 5 parts by weight, based on 100 parts by weight of the binder resin. If the amount added is less than 0.5 part by weight, it is difficult to obtain the positive charge contribution effect. On the contrary, even if added in an amount of more than 10 parts by weight, the charge amount reaches a saturation value and becomes in an equilibrium state, so the improvement in coatability is less than that in the case of adding 0.5 to 10 parts by weight, and the cost is low. It is not good because it will be up.

As the binder resin used in the powder coating material of the present invention, polyester resin, epoxy resin, acrylic resin, phenol resin, xylene resin, urea resin, melamine resin and the like can be used. Further, isocyanate, amine, polyamide, acid anhydride, polysulfide, trifluoroboric acid, acid dihydrazide, imidazole, etc. can be used as a curing agent. Further, the powder particles include fillers such as barium sulfate, calcium carbonate, aluminum oxide and calcium silicate, leveling agents such as acrylic oligomer and silicone, coloring agents such as titanium oxide, chromium oxide, iron oxide and carbon black, A foaming inhibitor and the like can be added as appropriate.

The powder coating material of the present invention is obtained by dry-mixing the above-mentioned composition, hot-melt kneading, pulverizing and classifying the composition, or polymerizing the composition by a suspension polymerization method, an emulsion polymerization method or the like. May be obtained by

The particle diameter of the powder coating material of the present invention must be 50% in volume measured by Coulter Counter TAII type, that is, the volume average particle diameter in the present invention is in the range of 7 to 20 μm. Since powder particles having a volume 50% diameter of less than 7 μm have a large interparticle force due to van der Waals force and the like and have poor fluidity, they are likely to aggregate in a coating machine and easily form lumps (projections) on the coated surface. Conversely, the volume 50% diameter is 20
If it exceeds μm, the height difference between the peaks and valleys of the surface of the adhered particle layer becomes large, and the packing density of the powder particles decreases, so that a thin film coating cannot provide a sufficiently smooth coating film. .

Inorganic fine particles such as silica, alumina and titanium oxide may be attached to the surface of the powder coating material particles of the present invention, if necessary. In order to make the inorganic fine particles adhere to the surface of the powder particles, they may be dry-mixed with a high-speed mixer such as a Henschel mixer manufactured by Mitsui Miike Co., Ltd., or a super mixer manufactured by Kawada Seisakusho Co., Ltd. In the powder coating material of the present invention, both a thermosetting powder coating material and a thermoplastic powder coating material can be obtained by selecting a binder resin.

The coating method using the powder coating material of the present invention is
The powder coating composition is characterized in that the powder coating composition is charged to a positive polarity by friction with a member containing a fluorine atom formed in a powder coating composition transporting section inside the spray gun, and then sprayed onto an object to be coated. The member containing the fluorine atom is directly formed in the powder coating material conveying section inside the spray gun or is surface-treated. Fluorine atom-containing members have the advantage of less physical deterioration after long-term continuous use, and have a strong negative charge, so that the powder paint side is charged to positive polarity by friction between this member and the powder paint. can do.
As the member containing a fluorine atom, polytetrafluoroethylene, polytrifluorochloroethylene, polyvinyl fluoride, polyvinylidene fluoride, polydichlorodifluoroethylene or the like is used.

[0024]

EXAMPLES The present invention will be described below based on examples. <Example 1> The raw materials having the above blending ratios are mixed with a super mixer, melted and kneaded at 120 ° C. using a pressure kneader, pulverized with a jet mill, and then with a dry air stream classifier to have a volume average particle diameter of 13 μm. The powder was classified to obtain the powder coating material of the present invention. <Example 2> In Example 1, raw materials having the same mixing ratio as in Example 1, except that the amount of the graft polymer added was changed to 5 parts by weight, were mixed with a super mixer, and then mixed with a pressure kneader.
After hot-melt kneading at 0 ° C., the mixture was pulverized with a jet mill and then classified with a dry air stream classifier to have a volume average particle diameter of 13 μm to obtain a powder coating material of the present invention.

<Example 3> Raw materials having the same mixing ratio as in Example 2 were mixed in a supermixer, melted and kneaded at 120 ° C using a pressure kneader, pulverized in a jet mill, and then dried in a dry air stream classifier. The particles were classified so that the volume average particle diameter was 7 μm to obtain the powder coating material of the present invention. <Example 4> Raw materials having the same mixing ratio as in Example 2 were mixed in a super mixer, hot melt kneaded at 120 ° C using a pressure kneader, pulverized by a jet mill, and then volume averaged by a dry airflow classifier. The powder coating material of the present invention was obtained by classification so that the particle diameter was 20 μm.

<Example 5> The raw materials having the above blending ratios are mixed with a super mixer, melted and kneaded at 120 ° C. using a pressure kneader, pulverized with a jet mill, and then with a dry air stream classifier to have a volume average particle diameter of 13 μm. The powder was classified to obtain the powder coating material of the present invention. <Example 6> The raw materials having the above blending ratios are mixed with a super mixer, melted and kneaded at 120 ° C. using a pressure kneader, pulverized with a jet mill, and then with a dry air stream classifier to have a volume average particle diameter of 13 μm. The powder was classified to obtain the powder coating material of the present invention.

<Comparative Example 1> In Example 1, raw materials having the same mixing ratio as in Example 1 except that the graft polymer was not added were mixed in a supermixer, and a pressure kneader was used to prepare 1
After hot-melt kneading at 20 ° C., the mixture was pulverized with a jet mill and then classified with a dry airflow classifier to have a volume average particle size of 13 μm, to obtain a powder coating material for comparison. <Comparative Example 2> Raw materials having the same mixing ratio as in Example 2 were mixed in a supermixer, hot-melt kneaded at 120 ° C using a pressure kneader, pulverized by a jet mill, and then volume-averaged by a dry airflow classifier. The particles were classified so that the particle diameter was 5 μm to obtain a powder coating material for comparison.

<Comparative Example 3> Raw materials having the same mixing ratio as in Example 2 were mixed in a supermixer, melted and kneaded at 120 ° C. using a pressure kneader, pulverized in a jet mill, and then dried in a dry air stream classifier. Then, classification was performed so that the volume average particle diameter was 35 μm, and a powder coating material for comparison was obtained. <Comparative Example 4> In Example 5, raw materials having the same mixing ratio as in Example 5, except that the graft polymer was changed to non-added, were mixed in a super mixer, and hot melt kneaded at 120 ° C using a pressure kneader, followed by jet milling. Was pulverized, and then classified by a dry airflow classifier to have a volume average particle diameter of 13 μm to obtain a powder coating material for comparison.

<Comparative Example 5> In Example 6, raw materials having the same mixing ratio as in Example 6 except that the graft polymer was not added were mixed in a super mixer, and the mixture was mixed with a pressure kneader to prepare 1
After hot-melt kneading at 20 ° C., the mixture was pulverized with a jet mill and then classified with a dry airflow classifier to have a volume average particle size of 13 μm, to obtain a powder coating material for comparison.

Next, the following tests were conducted on the powder coating materials of the above Examples and Comparative Examples, and the results are shown in Table 1. <Coating efficiency> The coating efficiency of the powder coating materials of the above-mentioned examples and comparative examples was confirmed using a tribo-charging type spray gun in which a member containing a fluorine atom was formed in the coating conveyance section.
The confirmation method of the coating efficiency is as follows. Tribo electrification type spray gun manufactured by Matsuo Sangyo Co., Ltd. Product name: T-1a Object to be coated 1000 mm 300 mm fixed to the center of a square iron plate
× 300mm × 1mm iron plate Coating conditions ・ Reciprocating stroke: 1m up and down ・ Reciprocating speed: 20m / min ・ Specific coated object moving speed (conveyor speed): 4m / min ・ Gun paint discharge: 200g / min ・ Gun Distance from the object to be coated: 180 mm ・ Coating environment: 25 ° C / 50% RH Calculating method of coating efficiency Calculate the time (min) that the gun moved on the object to be coated 300 mm square and calculate from this time and the gun. The value obtained by multiplying the coating material discharge amount (g / min) was defined as the theoretical discharge amount. next,
The weight of the coating material adhered to the object to be coated was actually measured, and this weight was divided by the above theoretical discharge amount to obtain the coating efficiency (%).

<Coating state> The powder coating materials of the above-mentioned examples and comparative examples were applied to a tribo charging type spray gun (manufactured by Matsuo Sangyo Co., Ltd.).
Using the product name: T-1a), the film thickness is 40 on the SPCC plate (product name: PB-137M) manufactured by Nippon Test Panel Co., Ltd.
The test piece of each of Examples and Comparative Examples was obtained by applying the coating solution so as to have a thickness of μm and baking at 180 ° C. for 20 minutes.
Next, the surface smoothness and surface condition of the test piece were visually observed and evaluated. In addition, in the evaluation of the coating film state in Table 1, the test piece having a good surface condition without unevenness was evaluated as ◯, and the test piece having unevenness was evaluated as x.

[0032]

[Table 1]

As is clear from the results shown in Table 1, the powder coating material of the present invention has a coating efficiency of 81% or more, a good coating property of the powder coating material, and an excellent smoothness without unevenness in the surface condition. It was confirmed to have. On the other hand, the powder coating materials of Comparative Examples 1, 2, 4 and 5 have a practical problem because the coating efficiency is less than 80%, and the powder coating material of Comparative Example 2 has a convex portion due to agglomerated powder on the surface. It was confirmed that the powder coating composition of Comparative Example 3 had a rugged surface-like surface roughness, and the smoothness was poor.

[0034]

As described above, in the present invention, the volume average particle diameter of the powder coating material is 7 to 20 μm, so that the coating film can be made thin, and the graft polymer represented by the general formula (1) is obtained. The addition of the compound improves the positive chargeability when coated using a tribo spray gun. Therefore, according to the coating method of the present invention, the adhesion of the powder coating material to the object to be coated does not deteriorate due to the reduction in particle size, and the smoothness is improved and the work efficiency is improved by thinning the coating film. It is possible to obtain a good coated surface even after baking.

Claims (3)

[Claims] 1. A graft polymer having a polystyrene-based main chain and a graft chain having a skeleton represented by the following general formula (1), and having a volume average particle size of 7 to 20 μm.
m, a powder coating. Embedded image (Wherein, R ₁ is a hydrocarbon group of C ₁₁ ~C _17, R ₂ and R
₃ is hydrogen or a C ₁ -C ₈ hydrocarbon group, or R ₂ and R ₃ may be connected to each other to form an aromatic ring. R ₄ is C ₁ which may contain an ether bond in the group.
To C ₁₂ alkylene group, X ⁻ is an anion, and n is 2 to
Represents an integer of 100) 2. The content of the graft polymer is the binder resin 1
0.5 to 10 parts by weight with respect to 00 parts by weight.
Powder coating as described. 3. The powder coating composition according to claim 1, which is positively charged by friction with a member containing a fluorine atom formed in a coating conveyance section inside a spray gun, and then sprayed onto an object to be coated. How to apply powder paint.