Method for imparting hydrophilicity to substrate

Details Method for imparting hydrophilicity to substrate Method for imparting hydrophilicity to substrate

2002-04-12

2003-12-23

Cameron, Erma (Department: 1762)

Coating processes

With post-treatment of coating or coating material

Chemical agent applied to treat coating

C427S344000, C427S354000, C427S419400

Reexamination Certificate

active

06667075

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a method for imparting hydrophilicity to a substrate whereby high hydrophilic properties and water-holding properties can be maintained for a long period of time.
BACKGROUND TECHNIQUE
Various measures such as the use of defogging mirrors or eyeglasses have been taken to prevent fog due to vapor or rain. For example, there is a method for applying a surface active agent to the surface of a glass or lens (hereinafter referred to as “substrate”), a method for applying a water absorbing agent to the surface of the substrate, a method for applying a water repellent agent to the surface of the substrate, or a method for coating the surface of the substrate with inorganic hydrophilic substance by a wet method.
In the method for applying the surface active agent to the surface of the substrate, by positioning a hydrophilic group on the surface of the substrate, a water screen is uniformly leveled without water adhering to produce a defogging effect.
The method for applying the water absorbing agent to the surface of the substrate is to produce a defogging effect by causing water adhering to the surface of the material to be absorbed by the water absorbing agent.
In the method for applying the water repellent agent to the surface of the substrate, by providing a coating such as a fluoroethylene resin whose surface tension is less on the substrate to increase the contact angle of water droplets, water adhering to the surface is repelled to produce a defogging effect.
In the method for coating the surface of the substrate with an inorganic hydrophilic substance, by providing a hydrophilic thin film with a small contact angle for the water droplets on the surface of the substrate, the water screen is made uniformly level without water adhering to produce a defogging effect, wherein a wet method such as a sol-gel method or a spin coat method is adopted as a coating process.
DISCLOSURE OF THE INVENTION
In the method for applying the surface active agent, there is a problem that the surface active agent cannot be used for a long time because it is easily swept away by water.
In the method for applying the water absorbing agent, there is a problem that the water absorbing agent cannot withstand long term use because it will soon be saturated if there is a lot of water or if an extremely thick water absorbing agent coating is required to allow the water absorbing agent to exhibit a water absorbing effect to some extent and as a result, the reflected image is distorted, or the water absorbing agent exhibits inferior flaw resistance because of the presence of an organic material.
In the method for applying the water repellent agent, there is a problem that though the water repellent agent repels water, it does not have sufficient defogging properties to repel even minute water droplet and it is difficult to maintain the defogging property for a long time.
Further, in the method for coating with an inorganic hydrophilic substance, there is a problem that uniform coating is not possible over a large area such as a mirror because it is difficult to control the coating thickness in the order of nm. It is complicated because there is a plurality of treating processes such as spraying of coating solution and burning, and it is difficult to maintain the defogging properties for a long time because the mechanical strength on the coating surface is insufficient.
To solve the above-mentioned problems, according to the present invention, an SiO
2
film is formed directly or through an undercoat layer on a substrate under a reduced pressure of 100 Pa or less and immediately after the SiO
2
film is formed, the SiO
2
film is treated with water to selectively form silanol groups (SiOH) on the surface of the SiO
2
film. The silanol groups (SiOH) formed on the SiO
2
film exhibit hydrophilic properties.
In the present invention, the term “SiO
2
film” is used to clearly express a silicon dioxide film, but this also includes a film in which the ratio of an Si atom to an O atom is not always 1:2 stoichiometrically. This also applies to an SnO
2
film.
A TiO
2
film, an Al
2
O
3
film, a Nb
2
O
5
film, a laminated film prepared by laminating the TiO
2
film on the Nb
2
O
5
film, or a low emissivity film is desirable as the undercoat layer. In this case, it is desirable that another SiO2 film be formed between the undercoat layer and the substrate. It is also desirable that the SnO
2
film be selected as a structure for the low emissivity film. If the SnO
2
film which is formed through a laminated film prepared by laminating the SnO
2
film and the SiO
2
film on the substrate in turn by a CVD method is selected, it is possible not only to decrease an interference reflected color of a thin film, but also to prevent seepage of alkalis (Na) from a glass substrate.
An SiN film is also effective as the undercoat layer provided to prevent seepage of the alkalis (Na). A thickness of 1~20 nm is desirable for the SiN film.
As described above, the SiO
2
film is formed under a reduced pressure of 100 Pa or less. However, there is a method for forming the SiO
2
film by a plasma CVD under an atmosphere of 100~1 Pa of which the main components are silane gas and oxidizing gas, a method for forming the SiO
2
film by sputtering under a reduced pressure of 10~0.1 Pa, or a method for forming the SiO
2
film by vacuum deposition under a reduced pressure of 1 Pa or less, or the like. Above all, the method for forming the SiO
2
film by sputtering is most excellent as a method for treating a larger area. When the SiO
2
film is formed by sputtering an Si target under an oxygen atmosphere, if 1~400% (preferably 1~200%, more preferably 60~80%) of argon or nitrogen is mixed relative to the oxygen in an oxygen atmospheric gas, it is possible to selectively form a silanol group by providing a large oxygen deficiency on the surface of the SiO
2
film.
In the case where a glass substrate is selected for the substrate, it is also possible to obtain a hydrophilic treating surface by burning, after the water treatment to form silanol groups, the glass substrate at a temperature of 300~500° C. under an atmospheric pressure in which 0~400% of nitrogen is provided relative to air. In this manner, it is possible to remove any organic substance adhering to the surface which would become water repellent component and to form a highly hydrophilic film.
It is also desirable that 0.1~20% by weight of Al component be used to dope an Si component in the SiO
2
film. In this manner, adsorption of water on the silanol group is stabilized and a film with high water-holding properties can be formed.
Further, it is desirable that the substrate have transparency or mirror reflection and the transparency or the mirror reflection be maintained even after the hydrophilic treatment.
PREFERRED EMBODIMENTS FOR WORKING THE INVENTION
Preferred embodiments of the present invention will now be explained hereunder.
In a method for imparting hydrophilic properties to a substrate according to the present invention, an SiO
2
film is formed on the substrate by a film-forming method under a reduced pressure and immediately after formation thereof, the SiO
2
film is treated with water. There is no specific limit to the material for the substrate used in the present invention as far as required to provide the substrate with a defogging function, hydrophilic function or water-holding function. For example, a suitable material includes glass, a mirror, a lens (including a plastic lens), tile or an aluminum panel.
As methods for forming an SiO
2
film on the substrate under a reduced pressure of 100 Pa or less, there are a plasma CVD method, a sputtering method, a vacuum deposition method and the like. The reason why a silanol group is formed by performing water treatment immediately after an SiO
2
film is formed under a highly reduced pressure is because water reacts before chemical adsorption of an organic substance onto the surface of chemically unstable SiO
2
film formed by a film forming method under reduced pressur

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