JPH0826282B2 - Aqueous coating composition for dip coating - Google Patents

Description

TECHNICAL FIELD The present invention relates to automobiles, electric appliances, electronic parts, building materials, household products, and the like, and a high-thickness aqueous coating composition used for those parts.

[Prior Art] Conventionally, a method of increasing the viscosity of a coating material is known for forming a uniform high-thickness coating film, but this method is not preferable because of a problem in the coating technology. Further, there is a disclosure that a coating composition is mixed with a mountain bark (magnesium-containing clay mineral) so as to have thixotropic property to prevent sagging of a coating film and prevent sedimentation of other pigments (Japanese Patent Laid-Open No. 61-211373). ).

However, with the above coating composition air spray, dip coating,
Although general coating methods such as flow coating are possible, it has been said that particularly useful dip coating methods cannot provide a useful high-thickness coating film.

[Problems to be Solved by the Invention] In the present invention, a high film thickness can be obtained by a dip coating method,
Another object of the present invention is to obtain an aqueous coating composition capable of obtaining a coating film having a small difference in thickness between the top and bottom of the object to be coated.

[Means for Solving Problems] In a high-film-thickness aqueous coating composition containing a water-soluble coating resin, water and a pigment as main components of the present invention, the thixotropic index is 4.0 to 6.0 The nonvolatile content of
It is less than 40-60% by weight.

The thixotropic index is measured as follows. Use a B type viscometer to measure the viscosity (cps / 20 ° C) when the rotor speed is 2 rpm and the viscosity (cps / 2 ° C) when the rotor speed is 20 rpm.
It is expressed as the value divided by (0 ° C).

If the above thixotropic index exceeds 6.0, the coating film becomes too thick and the appearance of the coating film becomes poor. 4.0
If it is less than 1, the difference in film thickness becomes large, which is not preferable. The nonvolatile content is measured by heating 2.0 g of the coating material at 130 ° C for 1 hour, and when the nonvolatile content is less than 60% by weight of the solid content, the specific gravity of the coating material becomes large and the surface of the coating film becomes rough. If it is less than%, the coating film is thin and the difference between the top and bottom becomes large.

By setting the thixotropic index of the coating material within the above range, shearing force is applied to the coating material during coating, the viscosity of the coating material is lowered, and leveling is facilitated. Since the shearing force disappears immediately after coating, the viscosity of the coating increases and it is difficult for the coating to sag, resulting in a coating having a high film thickness with a small difference in the film thickness between the top and bottom of the coated object. Further, by setting the non-volatile content of the coating material within the above range, the specific gravity of the coating material is appropriately increased, the sag resistance is improved, and the appearance is improved.

At least one resin component such as water-soluble alkyd and water-soluble acrylic resin is used as a composition for obtaining a coating material having the above-mentioned properties, and it is preferable to use organic bentonite as a thixotropic agent.

In addition, known materials such as extender pigments, coloring pigments and additives can be added.

The organic bentonite has a thixotropic property by virtue of hydrogen bonding between the flakes.

As the solvent, in addition to water, butyl cellosolve, ethyl cellosolve, isopropyl alcohol, butyl carbitol and the like can be added.

As the pigment, inorganic pigments such as carbon black, red iron oxide, and titanium white, organic pigments such as phthalocyanine and quinacridone, and extender pigments such as magnesium carbonate and barium sulfate can be used. Further, as the additive, a drier, a defoaming agent, a leveling agent, etc. can be appropriately used.

[Operation and effect] Other than the dipping method, there are air spray and air spray as means for obtaining a high film thickness with a small film thickness difference between the upper and lower parts of the coating film, but these methods require modification of the coating equipment. In addition, when it is used twice or in combination with another coating method, the productivity is lowered and the coating cost is increased, which is not preferable.

In the case of the dipping method, when the coating composition of the present invention is used in the existing dip coating equipment, a high film thickness with a small film thickness difference between the top and bottom of the coating film is obtained and a good appearance is obtained by improving the sagging resistance.
Further, due to the shielding effect by increasing the film thickness, the water resistance and the corrosion resistance are improved, and the covering property of the edge portion of the article having a complicated shape is improved.

The thixotropic index of the coating composition is
By increasing it to 4.0 to 6.0, the shearing force is applied to the paint during coating, the viscosity of the coating composition decreases and it evenly adheres to the object to be coated, and the shearing force disappears immediately after coating so that the viscosity of the paint increases and it does not easily sag. A high-thickness coating film with little difference in film thickness is formed.

[Examples] Hereinafter, the present invention will be described with reference to examples.

(Example 1) 50 parts of water-soluble alkyd resin ("Haridip U-1502" manufactured by Harima Kasei Co., Ltd., nonvolatile content: 60% by weight), calcium carbonate (35 parts of "calcium carbonate special grade" manufactured by Sankyo Seiki), carbon black (Mitsubishi Made by Kasei Kogyo "Carbon Black MA-10"
0 ") 10 parts, organic bentonite (NLIrd, Inc." Benton EW ") 3 parts, defoamer (San Nopco" Nopco DF-122-N "
S ”) and 50 parts of butyl cellosolve in a ball mill for about 2
Water-soluble alkyd resin (50 parts), metal drier (Harima Kasei Kogyo's "Cyclodex Co 6%"), and water (40 parts) were added to the paste obtained by spin-dispersion for 0 hours, and the mixture was stirred and mixed to obtain an aqueous coating composition. It was Table 2 shows the results of coating film evaluations on test panels obtained by dip-coating this coating composition on a degreased mild steel plate (300 × 70 × 0.8 mm).

(Examples 2 to 5) Table 1 shows the compounding ratio of the aqueous coating composition prepared in the same manner as in Example 1. However, the melamine curing agent used in Example 5 is "Nikalac MX-031" manufactured by Sanwa Chemical Co., Ltd. A test panel was also prepared in the same manner as in Example 1 to evaluate the coating film. The evaluation results are shown in Table 2.

(Comparative Examples 1 to 5) In the same manner as in Example 1, pigments, additives, solvents and resins were mixed in the proportions shown in Table 1 to prepare aqueous coating compositions. In addition, the production evaluation of the coating film was performed in the same manner as in Example 1.
The results are shown in Table 2.

The viscosity in Table 2 is measured using a No. 4 Fort Cup viscometer.
It was measured at 20 ° C.

The coating film forming conditions of Examples 1 to 4 and Comparative Examples 1 to 3 were set to 5 minutes after immersion and baked at 70 ° C. for 20 minutes. In Example 5 and Comparative Example 4, the baking temperature was 120 ° C. × 20 minutes.

Regarding the film thickness measurement site, the upper part and the lower part shown in FIG. 1 were measured.

As a result, in the example, the appearance of the coated surface was good, and the difference in the thickness of the film was as small as 6 μm or less, and a high-thickness coating film of 26 to 30 μ was formed. On the other hand, in Comparative Examples 1 and 5, since the thixotropic index is small, there is a large difference in the film thickness. In Comparative Example 2 in which the amount of non-volatile matter is small, the viscosity of the paint is low and the film thickness is thin, so that a high-thickness coating film cannot be formed. On the contrary, in Comparative Example 3 in which the amount of non-volatile components is large, the viscosity of the coating is high, the appearance is poor due to the skin roughness of the coating film, and the film thickness is high at 40 μm. Comparative Example 4 in which the thixotropic index is too large is not preferable because the appearance of the coated surface causes sagging defects. The composition having the characteristics of the present invention has a substantially uniform film thickness (25-30 μm) as shown in the examples, and forms an excellent coating film showing a good appearance.

[Brief description of drawings]

 FIG. 1 is a schematic diagram showing a coating film thickness measurement site.

Front Page Continuation (72) Inventor Keiro Asaka, 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation (56) References JP 61-106677 (JP, A) JP 58-152068 (JP) , A) JP-A-61-211373 (JP, A)

Claims (2)

[Claims] 1. A high-thickness water-based coating composition comprising a water-soluble coating resin, water and a pigment as main components to form a high-thickness coating film by a dip coating method, and having a thixotropic index of 4.0 to 6.0. And the nonvolatile content in the paint is 40-60% by weight
An aqueous coating composition for dip coating which is less than. 2. The aqueous coating composition according to claim 1, which contains organic bentonite as a thixotropic agent.