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The present invention relates to a method for forming a coating film on a plastic substrate. Coating of plastic substrates has heretofore been carried out by spray coating methods such as air spraying, airless spraying and the like.
However, in order to save energy and to reduce harmful wastes emitted into the environment, electrostatic coating, which achieves high coating efficiency, is coming into wide use. Therefore, electrostatic coating is usually carried out after the plastic substrate itself or surface thereof is made electrically conductive. For example, before a plastic substrate is electrostatically coated, an electrically conductive primer is applied to provide the substrate with electrical conductivity.
The electrically conductive primer to be used usually contains a resin component and an electrically conductive filler.
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The electrically conductive fillers usually used are particles of electrically conductive carbon, metal, electrically conductive metal oxides and the like. The particle form of the electrically conductive fillers is usually powdery, acicular, fibrous or spherical. Carbon powder or carbon fiber, which has a low specific gravity, is preferable as the electrically conductive filler in that the conductive effect can be obtained by the use of a small quantity thereof.
However, there are some problems that need to be solved.
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The whiteness, or the lightness, of the coating film is decreased, so that the color tone of any overcoat is affected. Although metal powders or metal zz have high electrical conductivity, the quantity thereof required is large, decreasing the whiteness of the coating film or the stability of the coating composition, because the particles need to be in contact with one another in order to form electrically conductive paths through the coating film.
Likewise, though acicular or fibrous conductive fillers are favorable for providing high electrical conductivity, they are scattered through the air when the coating composition is produced or the coated plastic substrate is recycled, gravely affecting the environment and the human body.
Problems also arise when a spherical conductive filler is used. The amount thereof required is large, resulting in an unstable coating composition or involving high cost. Japanese Unexamined Patent Publication No. This method has made it possible to shorten the coating process and to reduce cost. However, the undercoat containing a volatile conductive substance has problems in view of the recent regulation on volatile organic compounds VOCs.
The sulfonium salt compound, however, adversely affects the environment when coating films are baked jid when coated plastic products are recycled, and the whiteness of the coating films is not sufficient. The lightness of a multilayer coating film thus obtained is not sufficient. In addition, the lightness of the multilayer coating film thus produced is not satisfactory. An object of the present invention is to provide a method for forming a coating film on a plastic substrate, wherein the plastic substrate, after being coated with an aqueous white primer, can be easily electrostatically coated with a clear colored coating composition and a clear coating composition.
Another object of the invention is to provide a method for forming a coating film on a plastic substrate, wherein a light-colored multilayer coating film with a high degree of lightness can be obtained.
In consideration of the aforementioned related art, the present inventors conducted extensive research to develop a method for forming a coating film on a plastic substrate and thereby to achieve the objects stated 87211. Based on these novel findings, the present inventors carried out s research and accomplished the present invention. The present invention relates to a method for forming a coating film on a plastic substrate as follows. A method for forming a coating film on a plastic substrate which comprises the steps of:.
The method for forming a coating film according to mis above item 1, wherein the aqueous white primer A comprises a chlorinated polyolefin, white pigment and water.
The method for forming a coating film according to the above item 4, wherein the aqueous white primer A further comprises one or more modifier resins. A coated plastic article obtained by the method for forming a coating film according to the above item 1. A detailed description is given below of the method for forming a coating film of the present invention.
There are no restrictions on the plastic substrate to be used in the present invention. Examples thereof include various plastic materials used for the exterior parts of a car, such as bumpers, spoilers, grilles, fenders and the like, the outside parts of a household electrical appliance, and so on. Materials particularly useful as the plastic substrate are, for example, polyolefin obtained by polymerizing at least one kind of olefin having 2 to 10 carbon atoms, such as ethylene, propylene, butylene, hexene and the like.
However, the material is not limited to these. Polycarbonate, ABS resin, urethane resin, nylon and the like may also be used. If necessary, these plastic substrates can be subjected to pretreatments such as degreasing, washing with water, etc. Consequently, the coating film can be electrostatically coated in an easy manner with a thermosetting clear colored coating composition B and a thermosetting clear coating composition C.
Suitably used as the aqueous white primer A is one that comprises a chlorinated polyolefin, white pigment and water. A chlorinated polyolefin is a chlorinated product of a polyolefin, and examples of the base polyolefin encompass radical homopolymers or copolymers of at least one kind of olefin selected from ethylene, propylene, butene, methylbutene, isoprene, etc. The chlorinated polyolefin generally has a weight-average molecular weight of about 30, to about , and preferably about 50, to aboutEspecially preferred examples of the chlorinated polyolefin are chlorinated polyethylene, chlorinated polypropylene, chlorinated ethylene-propylene copolymer, chlorinated ethylene-vinyl acetate copolymer, and the like.
It is also possible to use graft polymers of chlorinated polyolefins with polymerizable monomers. Examples of the polymerizable monomers used for graft polymerization include meth acrylic acid alkyl esters, meth acrylic acid alkoxy alkyl esters, glycidyl meth acrylate, adducts of glycidyl meth acrylate and monocarboxylic acids, hydroxyalkyl meth acrylates, acrylic acid, methacrylic acid, and so on.
In order to impart water dispersibility to the chlorinated polyolefin, it is possible to carry out the graft polymerization of the chlorinated polyolefin by a known method with at least one kind of hydrophilic monomer such as polymerizable unsaturated dicarboxylic acids or anhydrides thereof.
The polymerizable unsaturated dicarboxylic acid or anhydride thereof is a compound having one polymerizable unsaturated bond and two or more carboxyl groups or the anhydrous group s thereof per molecule. Examples of the polymerizable unsaturated dicarboxylic acid or the anhydride thereof are maleic acid or the anhydride thereof, itaconic acid or the anhydride thereof, citraconic acid or the anhydride thereof, and so on.
The graft polymerization of the chlorinated polyolefin with the monomer can be carried out by a method known per se. In order to impart water solubility or water dispersibility to the chlorinated polyolefin subjected to graft polymerization with the polymerizable unsaturated dicarboxylic acid or anhydride thereof, it is preferable to neutralize some or all of the carboxyl groups contained in the molecules of the chlorinated polyolefin with an amine compound.
Examples of the amine compound encompass tertiary amines such as triethylamine, tributylamine, dimethylethanolamine, triethanolamine and the like; secondary amines such as dimethylamine, dibutylamine, diethanolamine and the like; and primary amines such as monoethanolamine and the like. These amine compounds can be used singly or in combination of two or more. To increase water solubility or water dispersibility, the amine compounds can be used in combination with surfactants.
In order to improve the adhesion of the coating film, a chlorinated polyolefin is needed as the resin component of the aqueous white primer A.
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In combination with the chlorinated polyolefin, one or more modifier resins can be used to adjust the flexibility or rigidity of the coating film or to achieve better film-forming properties. The usable modifier resin is acrylic resin, polyester resin, polyurethane resin or the like. The ratio of the modifier resin to be used is usually about 10 to about 50 parts by weight, relative to parts by weight of chlorinated polyolefin.
Acrylic resins that can be preferably used as the modifier resin are hydroxyl-containing acrylic resins. It is preferred that the acrylic resin contains carboxyl groups to enhance water solubility or mis dispersibility, crosslinking, and so on.
The hydroxyl-containing acrylic resin can be obtained by polymerizing a hydroxyl-containing monomer, meth acrylic acid alkyl ester monomer and optional other comonomer s by known polymerization methods such as solution polymerization and the like.
The hydroxyl-containing monomer is a compound containing a hydroxyl group and a polymerizable unsaturated group; examples thereof include monoesterified compounds of alkylene glycols having 2 to 10 carbon atoms and meth acrylic acid such as hydroxyethyl meth acrylate, hydroxypropyl meth acrylate, hydroxybutyl meth acrylate, etc. Examples of the meth acrylic acid alkyl ester monomer are monoesters of monoalcohols having 1 to 20 carbon atoms and meth acrylic acid such as methyl meth acrylate, ethyl meth acrylate, propyl meth acrylate, butyl meth acrylate, hexyl meth acrylate, 2-ethylhexyl meth acrylate, lauryl meth acrylate, stearyl meth acrylate, etc.
The optionally usable comonomers are compounds having a polymerizable unsaturated bond, other than the above hydroxyl-containing monomers and meth acrylic acid alkyl ester monomers. Examples of such monomers are carboxyl-containing monomers such as meth acrylic acid, maleic acid and the like; epoxy-containing monomers such as glycidyl meth acrylate and the like; meth acrylamide; acrylonitrile; styrene; vinyl acetate; vinyl chloride; and so on.
Polyester resins used as the modifier resin are typically obtained by the esterification reaction of a polybasic acid and a polyhydric alcohol. The polybasic acid is a compound having two or more carboxyl groups per molecule or anhydrides thereofand the polyhydric alcohol is a compound having two or more hydroxyl groups per molecule.
Usable as the polybasic acid and polyhydric alcohol are those used in this field.
In addition, the jus resin can be modified with monobasic acids, higher fatty acids, his components, etc. The polyester resin may contain hydroxyl groups, and the introduction thereof can be carried out by the combined use of alcohols having three or more hydroxyl groups per molecule with dihydric alcohols. The polyester resin may also contain carboxyl groups jsi addition to the hydroxyl groups, and generally has a weight-average molecular weight of about 1, to about , preferably about 1, to about 70, Polyurethane resins especially suitable as a modifier resin are those hydrophilic polyurethane resins that are water-soluble or water-dispersible.
Especially preferable is a water-dispersed self-emulsifying polyurethane resin a an average particle diameter of about 0.
Examples of the white pigment which is added to achieve high whiteness include titanium oxide such as rutile titanium oxide, anatase titanium oxide and the likewhite lead, zinc oxide, zinc sulfide, lithopone and the like. Of these, titanium oxide is preferable to enhance chemical resistance and whiteness. The most suitable white pigment to be used is rutile titanium oxide with an average particle diameter of about 0.
The white pigment is used usually in an amount of jiis 50 to about parts by weight, and preferably about 70 to about parts by weight, per parts by weight of solids of the resin component containing chlorinated polyolefin and optional modifier resin. In order to improve coating film performance such as water resistance etc. Examples of the crosslinking agent are 8712 polyisocyanate compounds having unreacted isocyanate groups, blocked polyisocyanate compounds having the isocyanate groups blocked with a blocking agent, melamine resins, epoxy resins, carbodiimide resins, oxazoline compounds and so on.
The hydrophilization of the polyisocyanate compound can be carried out, for example, by introducing a hydrophilic group such as a carboxyl group, sulfonic group, tertiary amino group and so on into the compound and neutralizing the compound with a neutralizing agent such as dimethylol x acid or like hydroxycarboxylic acid, ammonia, tertiary amine and so on.
The polyisocyanate compound can also be mixed with a surfactant to convert the same into a self-emulsifying polyisocyanate compound.
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The blocked polyisocyanate compound is obtained by adding a blocking agent to the isocyanate groups of the polyisocyanate compound to be blocked.
Usable melamine resins are methylolated melamine resin produced by reacting melamine and formaldehyde; partially or fully etherified melamine resin obtained by reacting methylolated melamine resin and a monoalcohol having 1 to 10 carbon atoms; and the like.
These usable melamine resins may also have coexisitent imino groups. The melamine resins may be hydrophobic or hydrophilic; especially suitable are low-condensation hydrophilic melamine resins that are etherified with methanol and have a number average molecular weight of about 3, 872 less, preferably about to about 1, The epoxy resin has two or more epoxy groups per molecule, and is effective for crosslinking chlorinated carboxyl-containing polyolefins, carboxyl-containing acrylic resins, carboxyl-containing polyester resins, carboxyl-containing polyurethane resins, etc.
A typical example of the epoxy resin is a copolymer of a polymerizable epoxy-containing monomer and a polymerizable vinyl monomer. Examples of the polymerizable epoxy-containing monomer are glycidyl acrylate, glycidyl methacrylate, methyl glycidyl acrylate, methyl glycidyl methacrylate, and the like. Examples of the polymerizable vinyl monomers other than polymerizable epoxy-containing monomers are meth acrylic acid alkyl ester, acrylonitrile, styrene, vinyl acetate, vinyl chloride and the like.
The copolymerization of these monomers can be performed by known methods. The obtained polymer preferably has an epoxy equivalent of about 20 to about 2, particularly about 30 to about and a number average molecular weight of about 3, to about , particularly about 4, to about 50, Also usable as a crosslinking agent are epoxy resins of glycidyl etherified bisphenol, the hydrogenated product thereof, epoxy resins of glycidyl etherified aliphatic polyhydric alcohols, glycidyl ester based epoxy resins, alicyclic epoxy resins and the like.
Preferably, these epoxy resins have a molecular weight of about to about 20, particularly about to about 5, The oxazoline compounds are hydrophilic compounds which are effective for crosslinking chlorinated carboxyl-containing polyolefins, carboxyl-containing acrylic resins, carboxyl-containing polyester resins, carboxyl-containing polyurethane resins, etc.
These crosslinking agents are preferably used in an amount of 0 to about 50 parts by weight, particularly about 5 to about 40 parts by weight, per parts by weight of the total solids of the resin component containing chlorinated polyolefin. The aqueous white primer A is used to adjust the water content of the coating film within a specific range and thereby to provide the electrical conductivity required for electrostatically coating the primer film with a coating composition.
It is not necessary to use an electrically conductive filler, which may lower the whiteness level of a coating film; however, a small quantity of filler may be optionally added for enhancing electrical conductivity. Examples of the conductive fillers are particles of electrically conductive metal oxides such as tin oxide, antimony tin oxide ATOindium tin oxide ITOfluorine-doped tin oxide FTOphosphorus-doped tin oxide, zinc antimonate, indium-doped zinc oxide, ruthenium oxide, rhenium oxide, silver oxide, nickel oxide, copper oxide and the like.
The aqueous medium used in the production of the components may be used as is for the aqueous medium of the primer. To reduce volatile organic compounds VOCsit is preferable to minimize the use of organic solvent. Examples of the usable organic solvent are ketone solvents such as methyl ethyl ketone, methyl isobutyl ketone and the like; ester solvents such as ethyl acetate, butyl acetate and the like; ether solvents such as ethylene glycol monobutyl ether and the like; alcohol solvents such as isopropyl alcohol, n-butyl alcohol, isobutyl alcohol and the like; aliphatic hydrocarbon solvents such as n-heptane, n-hexane, isooctane and the like; aromatic hydrocarbon solvents such as toluene, xylene and the like; and other solvents such as N-methyl-pyrrolidone, etc.