Film-forming composition and process for its production

Details Film-forming composition and process for its production Film-forming composition and process for its production

2000-05-17

2002-05-07

Koslow, C. Melissa (Department: 1755)

Compositions: coating or plastic

Coating or plastic compositions

Coating repellent

C106S286200, C106S287170, C106S287340

Reexamination Certificate

active

06383270

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a film-forming composition and to a process for its production, and particularly relates to a composition for formation of hydrophilic films.
BACKGROUND ART
Coating solutions have conventionally been prepared to form various different coatings for protection of surfaces of a variety of articles. The coatings produced by these coating solutions include hydrophilic coatings and water-repellent coatings. Both hydrophilic coatings and water-repellent (hydrophobic) coatings are effective for surface protection and antifouling effects. Water-repellent resin coating films and the like have been widely used in the prior art. However, hydrophilic coatings are effective in avoiding blockage of visibility on window glass or mirrors, or in preventing adhesion of oily contaminants that are abundant in metropolitan regions. Formation of hydrophilic coatings has therefore become more desired in recent years.
Some of these coatings include photocatalysts such as titanium oxides. When irradiated with light, the photocatalysts in photocatalyst-containing coatings decompose organic substances. Such coatings, therefore, are often used for coating of a variety of familiar articles for the purpose of providing the articles with antibacterial properties, preventing adhesion of organic substances on air conditioner evaporators, and the like. A photocatalyst-containing coating of this type will normally exhibit hydrophobic properties when not irradiated with light, due to the photocatalyst itself or to alkyl groups present in the coating with the photocatalyst. When the coating is irradiated with light, however, the photocatalyst effect causes adsorption of hydroxyl groups onto the surface from moisture in the air, causing the coating to exhibit hydrophilic properties.
Furthermore, such hydrophilic coatings have low adhesive force on substrate surfaces and low surface hardness. Consequently, rubbing causes the coatings to easily peel from the substrate surfaces on which they are formed. Attempts to increase the peeling strength of the coatings usually require burning and other such steps, and therefore it has been difficult to form coatings that can be coated with simple procedures such as spraying and drying, and that exhibit strong adhesive force and high hardness.
Some photocatalyst-containing coatings exhibit hydrophilic properties when irradiated with light. However, as time passes in the absence of light irradiation, the surface hydrophilic groups are lost and hydrophobic properties are exhibited that result in adhesion of water droplets during nighttime, thereby promoting adhesion of dirt and making it impossible to adequately exhibit the antifouling effect of the photocatalyst, once such dirt has adhered.
Even coatings with hydrophilic groups on the surface usually have contact angles of about 50° for water and therefore cannot be said to have sufficient wettability, and since water droplets readily form when water adheres to the surface, a problem has existed in that it has not been possible to exhibit an adequate antifouling effect by hydrophilic groups.
It is an object of the present invention to overcome these problems by providing a film-forming composition that can form strong coatings that exhibit hydrophilic properties. It is another object to provide coatings with sufficient hydrophilic properties or electronic properties to prevent adhesion of dirt or dust.
SUMMARY OF THE INVENTION
In order to solve the aforementioned problem, the film-forming composition of the invention comprises a liquid containing a binder and a hydrotalcite-type compound represented by [M
2+
1−x
M
3+
x
(OH)
2
]
x+[A
n−
x

mH
2
O]
x−
(where M
2+
is a bivalent metal ion, M
3+
is a trivalent metal ion, A
n−
is an anion of valency n and 0<x≦0.33). The composition may be coated and dried to form a coating that exhibits hydrophilic properties. The coating is resistant to smearing, and can be provided with sufficient hardness to withstand a normal living environment.
The composition preferably contains transition metal oxide particles, preferably titanium oxide particles. Inclusion of transition metal oxide particles makes amount of adherent dirt or dust to coatings decrease extremely and hardness of coatings increase. Furthermore, inclusion of titanium oxide particles has also been confirmed to improve the wettability of formed coatings, impart antibacterial properties and cause oxidation reaction on SO
x
and NO
x
.
The mean particle size of the hydrotalcite-type compound is 3-500 &mgr;m, and it also preferably includes fine particles of a stable inorganic compound, having a mean particle size of 1 &mgr;m or smaller. This will increase the smoothness of the coating surface and reduce the contact angle for water and the like, thus providing improved wettability and reducing the degree of fouling of the coating surface. The mean particle size of the inorganic compound is preferably 0.001-0.5 &mgr;m, and even more preferably 0.01-0.1 &mgr;m. If the mean particle size exceeds 1 &mgr;m, the effect of improved wettability will be reduced, while the catalyst performance per unit weight is also thought to be inferior. On the other hand, it is very difficult to form fine particles with a mean particle size of smaller than 0.001 &mgr;m, and this will increase production cost and reduce the effect of improved smoothness of the coating surface.
The inorganic compound is preferably a transition metal oxide. Transition metal oxides can be easily formed into fine particles of 1 &mgr;m or smaller by chemical synthesis or the like. And dirt and dust hardly adheres to the coatings by inclusion of transition metal oxide particles to the film-forming composition (coating liquid or solution) while also increasing the coating hardness; by using substances with catalytic functions that decompose organic matter (such as titanium oxides) as the transition metal oxide it is possible to improve the antifouling property of the coating, while using water-absorbing substances (such as magnesium oxides and aluminum oxides) can improve the hydrophilic property and wettability.
Transition metal oxide particles in the coating film are covered with the hydrotalcite-type compound, therefore surface region of the coating film becomes N-type semiconductor and it becomes hardly charged with electricity. Then, anti-pollution property is obtained, i.e. it is preventable from adhesion of dirt or dust to the coating film. The coating films formed by using the film-forming composition (coating liquid or solution) of this invention have electronic properties of the semiconductor which has surface electric resistivity (specific resistance) of 10
8-
10
13
&OHgr; cm. For example, when dust charged with electricity is applied on the coating film, dust become unbalance in electricity and easily drop out of the surface of the coating film without adhering to the surface thereof.
Here, it is preferred for the transition metal oxide to be a substance that is stable in normal environments, and typical examples that may be used include titanium oxides (TiO, Ti
2
O
3
, TiO
2
), manganese oxides (MnO, Mn
2
O
3
, MnO
2
, Mn
3
O
4
, Mn
2
O
7
) andiron oxides (FeO, Fe
2
O
3
, Fe
3
O
4
). Other stable transition metal oxides that exist include oxides of Sc, V, Cr, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, Ef, Ta, W, Re, Os, Ir, Pt, Au and Hg. These can provide the coating itself with antibacterial properties and absorption or catalytic action on NO
x
and SO
x
.
It is preferred for at least a portion of the transition metal oxide to be titanium oxide. The titanium oxide particles may have any desired shape. The mean particle size of the titanium oxide is 1 &mgr;m or smaller, preferably 0.001-0.5 &mgr;m, and more preferably 0.01-0.1 &mgr;m.
The hydrotalcite-type compound preferably contains Mg
2+
as M
2+
, Al
3+
as M
3+
and CO
3
3−
as A
n−
. Magnesium-aluminum-hydroxide-carbonate hydrates are substan

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