JPH0454709B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a solvent-based baking paint composition that does not generate pinholes.

Solvent-based baking paints tend to harden on the outside while the interior contains solvent, and thus tend to form pinholes when the internal solvent evaporates past the hardened or semi-hardened outside surface. There is. This tendency is particularly likely to occur when the film thickness is increased. Various methods have been tried in the past to prevent the occurrence of pinholes. For example, (1) a method that uses a slow-curing crosslinking agent (usually melamine resin) to prevent the surface from curing before the solvent inside the coating resin evaporates; (3) Using a material with a slow evaporation rate to prevent the painted surface from drying quickly, and (3) increasing the setting time after painting or lowering the baking temperature to prevent the internal solvent from evaporating. Measures have been taken to prevent the painted surface from hardening. For automotive top coat paints that require a beautiful finish, the occurrence of pinholes must be avoided as much as possible. For this reason, various prevention methods as mentioned above are used, but in reality there are various restrictions.

That is, using a crosslinking agent that cures slowly slows down the crosslinking reaction even if it is effective in preventing pinholes, and sufficient coating film performance cannot be obtained under the normally used drying oven conditions. Furthermore, when a solvent with a slow evaporation rate is used, sagging is likely to occur when painting a complex-shaped object such as an automobile. Furthermore, when the setting time is increased, the line equipment becomes too long and cannot be accommodated in reality.

The present invention has been made to solve the above problems, and provides a coating material with excellent pinhole resistance by combining a resin having a specific value range with benzoin or its alkyl ether. It is in.

Benzoin used in the present invention is represented by the following chemical formula.

Particularly preferable results are obtained when the alkyl ether is an ether compound such as benzoin methyl ether, ethyl ether, isopropyl ether, butyl ether, or isobutyl ether. For example, the structural formula of the methyl etherified product is as follows.

Of course, the phenyl group of benzoin may have a substituent such as methyl, ethyl, halogen, methoxyl, etc. These benzoin or its alkyl ether is used in an amount of 1.0 to 5%, preferably 1.5 to 4% by weight, based on the resin solid content in the paint. If benzoin was used in a smaller amount than 1.0%,
Although no particular effect was observed, it was found that when a larger amount was used, especially more than 1.5%, a remarkable pinhole prevention effect was exhibited. It is not clear why pinholes can be prevented by using benzoin in solvent-based paints, but we speculate that
It is presumed that the interaction between benzoin and the following resin system maintains the wettability of the surface of the object to be coated during baking, which is effective in preventing pinholes. In the case of benzoin, gasification and evaporation are less likely to occur than carbitol acetate, which has been tried to prevent pinholes in the past, making it possible to eliminate secondary causes of pinholes. Furthermore, the sagging that tends to occur when using high boiling point solvents is eliminated, and it becomes possible to obtain a uniform and beautiful finish as a paint.

Benzoin exhibits remarkable effects when combined with the following resin systems. That is, weight average molecular weight 8000 to 25000 (preferably 10000 to 20000),
Acid value 3-15 (preferably 5-10), OH value 70-140
(preferably 80 to 130) and an alkoxymethylol melamine system, and a weight average molecular weight of 8000 to 25000 (preferably 10000 to 20000),
Acid value 5-20 (preferably 7-15), OH value 60-100
(preferably 70 to 90) acrylic resin and alkoxymethylolmelamine.

Most pinholes occur in the early stages of baking, and whether or not the solvent that evaporates from inside the paint film during baking causes pinholes depends on the viscosity state at that time, and the appropriate viscosity is maintained. In order to do so, the above resin characteristic value range is required. In particular, it is essential that the weight average molecular weight falls within the above range. Acid value and OH value are regulated from the viewpoint of viscosity and coating performance. In terms of coating film performance, if the acid value is below the lower limit, the reaction with crosslinked melamine will be insufficient and sufficient coating hardness will not be obtained. When the acid value is above the upper limit, unreacted functional groups remain in the coating film, and coating film performance such as water resistance deteriorates. The OH value has the same effect as the acid value.

Examples of alkyd resins include polybasic acids,
(For example, phthalic anhydride, isophthalic acid, terephthalic acid, succinic anhydride, adipic acid, azelaic acid, sebacic acid, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, tetrabromo phthalic anhydride, tetrachlorophthalic anhydride, hectic anhydride, one or more acids selected from the group consisting of hymic anhydride, maleic anhydride, fumaric acid, itaconic acid, trimellitic anhydride, methylcyclohexentric anhydride, pyromellitic anhydride, etc.), and a polyhydric alcohol. , for example, ethylene glycol, propylene glycol, 1,
3-butylene glycol, 1,6-hexanediol, diethylene glycol, dipropylene glycol, neopentyl glycol, triethylene glycol, hydrogenated bisphenol A, bisphenol hydroxypropyl ether, glycerin, trimethylolethane, tolmethylolpropane, trishydroxy one or more alcohols selected from the group consisting of methylaminomethane, pentaerythritol, dipentaerythritol, etc.), semi-drying oil such as soybean oil, safflower oil, tall oil, and/or coconut oil , using non-drying oil such as castor oil, oil length is 40% or less (preferably
30% or less).

The alkyd resin referred to in the present invention consists of one or more acid components, one or more alcohol components, and oil (semi-drying oil and/or non-drying oil), and has a weight average molecular weight of 8,000 to 8,000. 25000, acid value 3
~15, and the hydroxyl value was adjusted to a range of 70 to 140.

The acrylic resin suitable for the solvent-based baking paint of the present invention has a weight average molecular weight of 8,000 to 25,000 (preferably
10000-20000), acid value 5-20 (preferably 7-15),
It has an OH value of 60 to 100 (preferably 70 to 90).

Specific monomers constituting the acrylic copolymer resin include methyl acrylate, ethyl acrylate, isopropyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylate, methyl methacrylate, ethyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, n-hexyl methacrylate,
Lauryl methacrylate, acrylic acid, methacrylic acid, maleic acid, itaconic acid, methacrylic acid 2-
Hydroxyethyl, hydroxypropyl methacrylate, 2-hydroxyethyl acrylate, hydroxypropyl acrylate, acrylamide, N-
Examples include acrylic monomers such as methylol acrylamide, diacetone acrylamide, glycidyl methacrylate, and acrylonitrile.
Other monomers such as styrene, vinyl ketone, vinyl acetate, etc. may also be blended.

Using the above monomers, a polymer is obtained by a known polymerization reaction such as solution polymerization, emulsion polymerization, suspension polymerization, or bulk polymerization in the presence of a conventional polymerization catalyst. acid value,
Adjustment of OH number is known to those skilled in the art.

Examples of alkoxymethylolmelamines suitable for blending with the above-mentioned alkyd resins or acrylic resins suitable for the present invention include methylated melamine resins (for example, Cymel 303, manufactured by Mitsui Toatsu Co., Ltd.), n-butylated melamine resins (for example, Yuban 128), , Mitsui Toatsu Co., Ltd.)
Isobutylated melamine resin (e.g. Super Betsukamine G-821, Dainippon Ink Co., Ltd.) and methyl,
Butyl mixed melamine resin (e.g. Nikalatsuku MX)
-40, Sanwa Chemical Co., Ltd.), etc.

Alkyd resin and alkoxymethylol melamine are 9/1 to 6/4, particularly preferably 8/2 to 6/4.
Blend in paint at a ratio of 7/3. The blending ratio of acrylic resin and alkoxymelamine is 9/1 to 6/
4, especially 8/2 to 7/3.

A variety of solvents can be used in the present invention. for example,
Toluol, Xylol, Sorbetso 100 (Etsuso
Co., Ltd.), aromatic hydrocarbons such as Solbetsuso 150 (Etsuo Co., Ltd.), alcohols such as methanol, ethanol, isopropyl alcohol, n-butanol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, Ethylene glycol derivatives such as ethylene glycol monoethyl ether acetate, diethylene glycol derivatives such as diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, and diethylene glycol monobutyl ether, ketones such as acetone, methyl ethyl ketone, and methyl isobutyl ketone, and esters such as ethyl acetate and butyl acetate. However, the solvents that can synergistically act synergistically with benzoin or its derivatives to more effectively exhibit the pinhole prevention effect in the present invention are the ethylene glycol derivatives and diethylene glycol derivatives.

By using the solvent-based baking paint of the present invention, the critical film thickness at which pinholes occur can be increased, the setting schedule can be significantly shortened, and baking can be carried out at high temperatures and in a short time.

The present invention will be explained below with reference to Examples.

Example 1 Alkyd resin (oil length 30%, weight average molecular weight 10,000, acid value 8, OH value 130) and after the reaction, xylol/n-butanol=
The non-volatile content was adjusted to 65% using a 7/3 solvent.

20 parts of this alkyd resin, titanium oxide pigment
65 parts of RCR-3 (manufactured by Ishihara Sangyo Co., Ltd.) and 15 parts of xylene were thoroughly stirred with a disper and then dispersed with a sand ground mill. The particles were dispersed using a 50μ grind gauge until the coarse particles became 5μ or less.

Next, 50 parts of the above alkyd resin and 32.5 parts of n-butanol-modified butylated melamine resin Yuban 128 (manufactured by Mitsui Toatsu Co., Ltd.) were added to 100 parts of this dispersion base, and PWC (pigment weight concentration; pigment weight/pigment weight + Resin solid weight x
100) 50%, and the ratio (solid content) of alkyd resin and melamine resin (nonvolatile content 60%) was 70/30.

Next, 1.2 parts of benzoin (3.4% based on the resin solid) was added to 100 parts of this paint, and the mixture was mixed with a disper.
The benzoin was stirred for 15 minutes to uniformly dissolve the benzoin in the paint. SPCC-SD steel plate (JIS-G3141) was treated with zinc phosphate treatment solution (Nippon Paint Co., Ltd. "Granogin").
16N") for chemical conversion treatment, then washed with water,
Next, electrodeposition was performed using a cationic electrodeposition paint (Nippon Paint Co., Ltd.'s "Power Top U-30"), and after washing with water, baking was performed at 180℃ for 20 minutes to form an electrodeposition coating with a dry film thickness of 20μ. I let it happen.

Next, apply oil-free baking type intermediate coating paint (Nippon Paint Co., Ltd.'s "Olga 810 Intermediate Coating Gray") to a dry film thickness of 30μ, and after setting for 10 minutes, bake at 140℃ for 30 minutes. I did it.
After baking, the intermediate coated surface was wet-sanded uniformly using No. 600 water-resistant abrasive paper to prepare a test plate.

The above-mentioned top coat was diluted with a No. 4 Ford cup viscometer at 20° C. for 22 seconds on this test plate, and then applied with an automatic coating machine to a dry film thickness of 30 μm. then
Coating was performed 10 times in total while sequentially shifting the test plate every 30 seconds so that the dry film thickness was 5 μm, so that a test board with a gradient of dry film thickness from 30 μm to 80 μm was obtained. After setting for 7 minutes, it was placed in a drying oven preset at 140°C and baked at 140°C for 20 minutes. This baked test plate was visually observed, and the film thickness just before a pinhole appeared was recorded as the pinhole limit film thickness (if it was unclear whether it was a pinhole, it was determined by observing it with a microscope at 20 to 100x magnification). ). The pinhole limit film thickness of this paint was 60μ.

Example 2 Isobutylated melamine resin G-821 (manufactured by Dainippon Ink Co., Ltd.) was used as the crosslinking agent in Example 1, and benzoin methyl ether was added at 1.5% based on the resin solid content.
Mixed. The pinhole limit was measured in the same manner as in Example 1 and was found to be 58μ.

Example 3 Using an ordinary polymerization kettle equipped with a reaction tank, a stirrer, a monomer dropping tank, and a heating/cooling device, xylol/
Polymerization of the following monomer composition was carried out in a 1/1 solution of Solbetsuso 100 (manufactured by Etsuso Co., Ltd.) to obtain an acrylic resin varnish with a nonvolatile content of 50%.

Styrene 21.5 parts Butyl methacrylate 40 parts Acrylic acid 1.5 parts 2-ethylhexyl methacrylate 5 parts Ethylhexyl acrylate 10 parts 2-hydroxyethyl methacrylate 22 parts 2,2'-azobisisobutyronitrile 1.3 parts The weight average molecular weight of this acrylic resin is 14,000. ,
The acid value was 12 and the OH value was 95.

26 parts of this acrylic resin, titanium oxide pigment CR95
After stirring well with a disper, 65 parts of Celolbu acetate and 9 parts of Celolbu acetate were dispersed in a sand grind mill until the coarse particles became 5μ or less.

Next, to 100 parts of this dispersion base, 85 parts of the above acrylic resin, n-butanol modified butylated melamine resin Yuban 20SE (manufactured by Mitsui Toatsu Co., Ltd., non-volatile content 60
%) was added, and the mixture was made such that PWC was 45% and the ratio of acrylic resin to melamine resin (solid content) was 70/30.

Next, benzoin ethyl ether was mixed into this paint in an amount of 4.5% based on the resin solid content.

The pinhole limit was measured in the same manner as in Example 1 and was found to be 58μ.

Comparative example 1 30 parts of coconut oil, 40 parts of phthalic anhydride, 20 parts of TMP,
Example 1 using 10 parts of neopentyl glycol
oil length by 30%, by extending the reaction time of
It was adjusted to have a weight average molecular weight of 14,000, an acid value of 4, and an OH value of 65. After the reaction was completed, the nonvolatile content was adjusted to 65% using a xylol/n-butanol=7/3 solvent.

A paint was prepared in the same manner as in Example 1, and 3.4% benzoin was added to the resin solid.

The pinhole limit of this material was 48μ.

Comparative Example 2 The pinhole limit in Example 1 without using benzoin was 45μ.

Comparative example 3 An acrylic resin with the following formulation was prepared.

Styrene 21.5 parts Butyl methacrylate 38 parts Acrylic acid 1.5 parts 2-ethylhexyl methacrylate 5 parts Ethylhexyl acrylate 10 parts 2-hydroxyethyl acrylate 24 parts 2,2'-Azobisisobutyronitrile 1.3 parts The weight average molecular weight of this acrylic resin is 15,000 parts. ,
The acid value was 12 and the OH value was 105.

When it was made into a paint in the same manner as in Example 3, the pinhole limit was 47μ.

Comparative Example 4 The pinhole limit of the acrylic resin paint of Example 3 without the addition of benzoin ethyl ether is
It was 46μ.

Claims (1)

[Claims] 1 Weight average molecular weight 8000-25000, acid value 3-15,
A solvent-based paint containing a resin selected from the group consisting of an alkyd resin having a hydroxyl number of 70 to 140 or an acrylic resin having a weight average molecular weight of 8,000 to 25,000, an acid number of 5 to 20, and a hydroxyl number of 60 to 100, an alkoxymethylol melamine, and a solvent. A solvent-based baking top coating containing 1 to 5% by weight of benzoin or its alkyl ether based on the resin solid content of the coating.