JPH0580204A - Antistatic film, low reflection film having low refractive index and low reflection antistatic film - Google Patents

Abstract

PURPOSE:To obtain a low reflection antistatic film having satisfactory antistatic performance, low reflecting performance and high durability. CONSTITUTION:A liq. contg. dispersed Sb-doped SnO2 particles is mixed with a soln. contg. Ti(C5H7O2)n(OR)m (where n is 1-4, m is 0-3, n+m=4 and R is 1-4C alkyl) and a substrate is coated with the resulting mixture and heated to form an antistatic film. A film having a low refractive index is then formed on the antistatic film by applying a soln. contg. MgF2 and a compd. of Si, Zr, Ti, Al, Sn, etc., and by heating to obtain a low reflection antistatic film.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an antistatic film and a low reflection antistatic film applied to the surface of a substrate such as a cathode ray tube panel.

[0002]

2. Description of the Related Art Needless to say, the coating method of a low-reflection film has been used in conventional optical equipment as well as consumer equipment, especially T
V. Many studies have been conducted on cathode ray tubes (CRTs) for computer terminals. A conventional method is disclosed in, for example, JP-A-61-11893.
As described in No. 1, methods such as attaching a SnO ₂ layer having fine irregularities to the surface of the cathode ray tube to provide an antiglare effect, or etching the surface with hydrofluoric acid to provide irregularities have been performed.

However, these methods are called non-glare processing for scattering external light and are not methods for providing a low reflection layer by nature, so that there is a limit to the reduction of reflectance and the resolution is reduced in a cathode ray tube. It was also the cause.

Further, many studies have been conducted on the provision of an antistatic film. For example, as described in JP-A-63-76247, the surface of a cathode ray tube panel is heated to about 350 ° C. for CVD.
A method of providing a conductive oxide layer such as tin oxide and indium by the method has been adopted.

However, in this method, there is a problem in that in addition to the cost of the apparatus, the heating of the cathode ray tube at a high temperature causes the fluorescent substance in the cathode ray tube to fall off and the dimensional accuracy to decrease. Further, tin oxide is the most common material used for the conductive layer, but in this case, it is difficult to obtain a high performance film by low temperature treatment.

Further, JP-A-62-230617 describes that tin oxide particles are heat-treated in an acid or alkaline aqueous solution to produce a sol. However, even with this method, agglomeration of the particles is inevitable and a high-performance sol. There was a drawback that it was difficult to obtain.

Further, when low reflectivity and antistatic property are simultaneously provided, for example, in a two-layer film structure, it is necessary to dispose a low refractive index layer on the air side and a high refractive index layer on the substrate side, and the antistatic property is It may be applied to any layer. However, no attempt has hitherto been made to impart a high refractive index and an antistatic property to the layer on the substrate side.

[0008]

SUMMARY OF THE INVENTION The present invention is intended to solve the above-mentioned drawbacks of the prior art, and provides a high-performance antistatic film and a film having a high refractive index and a high electrical conductivity on the substrate side. It is an object of the present invention to newly provide a low-reflection antistatic film capable of exhibiting excellent low-reflection antistatic property by arranging a film having a low refractive index on the air side.

[0009]

Means for Solving the Problems That is, the present invention has been made to solve the above-mentioned problems, and Ti (C ₅ H ₇ O ₂ )
_n (OR) _m (however, n + m = 4, m = 0-3, n = 1-4, R
= C ₁ -C ₄ alkyl group), and a solution containing SnO ₂ particles doped with Sb, and
An antistatic film obtained by heating, and

MgF ₂ , Si compound, Zr compound, Ti compound, Al
A low refractive index and low reflective film obtained by applying a solution containing at least one of a compound and a Sn compound on a substrate and then heating the solution, and

A multi-layered low reflection antistatic film formed on a substrate, at least one of which is MgF ₂ , Si.
A low-refractive-index, low-reflection film obtained by applying a solution containing at least one of a compound, a Zr compound, a Ti compound, an Al compound, and a Sn compound, and then heating the solution, and at least one of multiple layers. Of Ti (C ₅ H ₇ O ₂ ) _n (OR) _m (n = 1 to 4, m = 0 to 3, n) formed on the substrate side of the low refractive index layer.
+ M = 4, R is an alkyl _{group), Si (OR) n R} m (n = 1~
4, m = 0 to 3, n + m = 4, R is an alkyl group), or at least one of these partial hydrolysates, and a solution containing Sb-doped SnO ₂ particles is applied and then heated. A low-reflection antistatic film, which is the obtained antistatic film.

The antistatic film having a high refractive index used in the present invention is obtained by dispersing Ti (C ₅ H ₇ O ₂₎ in a liquid in which Sb-doped SnO ₂ particles are dispersed.
₂ ) _n (OR) _m (n + m = 4, m = 0 to 3, n = 1 to 4, R:
It can be obtained by using a solution obtained by mixing a solution containing at least one of alkyl groups). As SnO ₂ particles doped with Sb, those obtained by various known methods can be preferably used.

The dispersion medium and dispersion method of these particles are not particularly limited, and various types can be used. Preferably, SnO ₂ particles are added to an organic solvent such as water or alcohol,
Adjust pH by adding acid or alkali, colloid mill,
It can be obtained by dispersing with a commercially available pulverizer such as a ball mill, a sand mill and a homomixer. In this case SnO _{2 in} dispersion
The average diameter of the particles is preferably 300 nm or less.

The dispersion may be diluted with alcohol, water or the like. Further, instead of the above liquid in which Sb-SnO ₂ is dispersed, SnO ₂ , F-doped SnO ₂ ,
Alternatively, it is also possible to prepare and use a colloidal liquid in which a conductive oxide fine powder such as ITO is dispersed.

Further, the Ti salt is preferably Ti (OR) ₄
The alkoxide represented by (R: alkyl group) is dissolved in an organic solvent such as alcohol and acetylacetone is added,
After stirring and chelating, it is added to the SnO ₂ sol, or after chelating, water is added and stirred to be partially hydrolyzed before use. Also in this case, if chelation is not carried out, the stability of the liquid is poor, which is not preferable.

In this case, in order to improve the strength of the film, a silicon compound is preferably Si (OR) _m R _n (m = 1 to 4, n
= 0 to 3, R: alkyl group) or a partial hydrolyzate can also be used.

A preferable composition ratio is Sn in terms of oxide.
The weight ratio range of O ₂ : TiO ₂ = 100: 1 to 1:20 can be mentioned, and the total solid content is preferably 0.1 to 30 wt% with respect to the solvent.

As the low refractive index and low reflection layer used in the present invention, a solution containing MgF ₂ sol or a solution containing at least one selected from Si compound, Zr compound, Ti compound, Al compound and Sn compound. Obtained by using. In terms of refractive index, MgF ₂ is the lowest of the materials, and MgF ₂
It is preferable to use a solution containing a sol.

The solution containing MgF ₂ used in the present invention can be obtained by various synthetic methods, but MgX ₂ (X =
BF ₃ alkyl ether complex salt, BF ₃ phenol complex salt, BF that acts as a fluorinating agent with halogen elements other than fluorine)
A liquid containing MgF ₂ sol produced by the following reaction with at least one selected from ₃ alcohol complex salt and BF ₃ water-soluble complex salt is used.

This MgF ₂ sol has a BF of Mg salt (MgX ₂ ).
₃ Complex salt acts as a fluorinating agent, that is, Mg salt as Mg salt
When Cl ₂ is used, the reaction of the following formula proceeds and is formed. 3MgCl ₂ ＋ 2BF ₃ → 3MgF ₂ ＋ 2BCl ₃ ↑ However, some BCl _{3 on} the right side remains in the liquid in the form of 1 part boron oxide, but it has almost no effect on the characteristics of the low reflection film finally obtained. Absent. In addition, some of the left-hand side ones are unreacted and some remain, but there is almost no effect.

The solvent of this liquid is not particularly limited, but water, aqueous solution, alcohol, ester, ether and the like can be preferably used. The starting material preferably has a molar ratio of MgX ₂ and BF ₃ complex salt in the range of 1: 2 to 4: 1, particularly preferably 1: 1 to 2: 1, and the starting material is 1 to 30 wt% with respect to the solvent.
% Is preferably contained.

The solution for forming the low refractive index and low reflection layer of the present invention includes MgF ₂ film forming compound or a solution or sol containing MgF ₂ , as well as the adhesion strength of the low refractive index film. and to improve the hardness, or the solution or sol of liquid stability binders or fillers in order to improve, or Si (oR) as an additive ₄ (R is an alkyl group) and Si (oR) _x · R
It is also possible to add _4-x (x = 3, 4, R is an alkyl group) or a partial hydrolyzate thereof to precipitate SiO ₂ .

Similarly, as a binder or in addition to the silicon compound, Zr (OR) ₄ (where R is an alkyl group), Ti (OR) ₄ (where R is an alkyl group), Al (OR) ₃ (Where R is an alkyl group), Zr (C ₅ H ₇ O ₂ ) _n · (OR) _m (where n = 1 to 4, m = 0 to 3, n + m = 4, R is an alkyl group), Ti (C ₅ H ₇ O ₂ ) _n · (OR) _m (where n = 1 to 4, m = 0
~ 3, n + m = 4, R is an alkyl group), Al (C ₅ H ₇ O ₂ ) _n · (O
R) _m (where n = 1 to 3, m = 0 to 2, n + m = 3, R
Is an alkyl group), Zr (OR) _x · R _4-x (x = 1 to 3, R is an alkyl group), Ti (OR) _x · R _4-x (x = 1 to 3, R is an alkyl group) ), Al (OR) _x · R _{3 -x} (x = 1 or 2, R is an alkyl group)
Or by adding these partial hydrolysates, ZrO ₂ , TiO _2, Al
₂ O ₃ or a mixture of at least two of these,
Alternatively, the composite may be precipitated simultaneously with MgF ₂ , MgF ₂ and SiO ₂ .

Alternatively, various surfactants may be added to improve the wettability with the substrate. For example, as the surfactant to be added, linear sodium alkylbenzene sulfonate, alkyl ether sulfate, etc. may be mentioned.

For the purpose of imparting conductivity, a metal oxide having conductivity (for example, In ₂ O ₃ (ITO) containing SnO ₂ , Sn) is used.
Etc.) metal (eg, Sn, In, etc.) acetylacetonate, alkoxide, etc. (eg, Sn (C ₅ H ₇ O ₂ ) _n
(OR) _m ((n = 0 to 2, m = 0 to 2, n + m = 2, R is an alkyl group) or (n = 0 to 4, m = 0 to 4, n + m =
4, R is an alkyl group))), a metal salt such as an organic metal salt, a halide, an acetate salt, a nitrate salt or a chelate compound, and a solution containing an Mg salt and a BF ₃ complex salt used in the present invention,
Or which further added to the liquid containing _{SiO 2, ZrO 2, TiO 2} , Al 2 O at least one to form a compound of _3, In ₂ O ₃ conductive oxide such as (ITO) comprising SnO ₂ or Sn the MgF _2, or it is also possible to simultaneously deposit and MgF ₂ and other oxides.

Separately prepared Sb-SnO ₂ (Sb-doped
It is also possible to prepare and use a colloidal solution in which fine powder of conductive oxide such as SnO ₂ doped with SnO ₂ ), SnO ₂ or F, or ITO is dispersed.

Although the above various substances can be added, it is preferable to use a solution containing a silicon compound from the viewpoints of hardness and scratch resistance of the film.

As the liquid containing such a silicon compound, various liquids can be adopted, and particularly preferable is a liquid containing silicon alkoxide or a partial hydrolyzate of silicon alkoxide. Silicon alkoxide
As (Si (OR) ₄ : R; alkyl group), various ones can be used, but monomers or polymers of silicon ethoxide, silicon methoxide, silicon isopropoxide, and silicon butoxide are preferably used. Is.

Silicon alkoxide can be used by dissolving it in alcohol, ester, ether or the like, or can be used by hydrolyzing it by adding hydrochloric acid, nitric acid, acetic acid or aqueous ammonia solution to the above solution.

The starting material is 1 to 30 wt% with respect to the solvent.
% Is preferably contained.

The low refractive index and low reflection layer used in the present invention may be added with a solution containing the above-mentioned various Zr, Ti, Al and Sn compounds, but especially Zr acetylacetone alkoxide Zr (C ₅ H ₇ The use of O ₂ ) _n (OR) _m is preferable because the stability of the coating solution is improved.

Zr (C ₅ H ₇ O ₂ ) _n (OR) _m (where n + m =
4, n = 1 to 3, m = 1 to 3, R = C _{1 to} C ₄ alkyl group) may be added as they are to the MgF ₂ sol and the silicon alkoxide solution, or an alcohol, an ether, It can also be used by dissolving it in ester, aromatic hydrocarbon or the like. When the MgF ₂ sol, the silicon alkoxide solution, and the solution containing the Zr salt are mixed, the final total solid content is preferably in the range of 0.1 to 5.0 wt% in terms of oxide. Also Mg
The weight ratio of F ₂ and SiO ₂ can be arbitrarily changed, but the addition amount of ZrO ₂ is preferably in the range of 1 to 50 wt% with respect to SiO ₂ .

The heating temperature must be 50 ° C. or higher, but the upper limit is usually determined by the softening point of the glass, plastic, etc. used for the substrate. Considering this point, the preferable temperature range is 100 to 400 ° C. Various methods such as a spin coating method, a dipping method, a spraying method, a roll coater method, and a meniscus coater method can be considered as the method of attaching to the film, and the spin coater method is particularly preferable because it is excellent in mass productivity and reproducibility. By such a method, a low reflection film having a thickness of 100 Å to 1 μm can be formed.

In the present invention, the substrate for forming the antistatic film or the low reflection film is not particularly limited, and soda lime silicate glass, aluminosilicate glass, borosilicate glass, lithium aluminosilicate glass is used according to the purpose. Glass such as quartz glass, single crystals such as steel balls, translucent ceramics such as magnesia and sialon, and plastics such as polycarbonate can be used.

The method for producing an antistatic film and a low reflection antistatic film of the present invention can be applied to the production of a multilayer low reflection film. As the structure of the multilayer low-reflection film having the antireflection property, the wavelength to be antireflection is set to λ, and the high refractive index layer-low refractive index layer is formed with an optical thickness of λ / 2-λ / 4 from the substrate side. Low-reflecting film of three layers, a medium-refractive index layer-high refractive index layer-low refractive index layer formed from the substrate side with an optical thickness of λ / 4-λ / 2-λ / 4. A typical example is a four-layer low reflection film in which a refractive index layer, a medium refractive index layer, and a high refractive index layer are formed with an optical thickness of λ / 4-λ / 4-λ / 2-λ / 4. There is.

In the present invention, it is known as a low refractive index layer. In the present invention, the low reflection film of the present invention is used as the low refractive index layer and the multilayer antistatic film used in the present invention as the high refractive index layer. It is also possible to produce an antistatic film using.

That is, since the antistatic film in the present invention contains a Ti salt, it has a high refractive index and, for example, has a two-layer structure of λ.
When applied to a / 2-λ / 4 or λ / 4-λ / 4 film, a preferable combination is a substrate / SnO ₂ -TiO ₂ / MgF ₂ -SiO ₂ , a substrate / Sn
O ₂ -TiO ₂ / SiO _2, the substrate _{/ SnO 2 - TiO 2 -SiO 2} / SiO 2 and the like.

[0038]

In the antistatic film of the present invention, a Ti salt can be added to SnO ₂ sol to stabilize and increase the refractive index. Although the mechanism of stabilization is not always clear,
Believed to be due to aggregation of the SnO ₂ particles is prevented by the interaction of SnO ₂ particles, also because of using a Ti salt, after heating TiO ₂ is believed to increase the refractive index of precipitated. Further, since the Ti salt used in the present invention is chelated with acetylacetone,
The stability of the coating solution is improved as compared with the solution using alkoxide represented by Ti (OR) ₄ (R: alkyl group).

Further, since a high refractive index film can be obtained, it is considered that when a low refractive index liquid containing MgF ₂ or the like is used as the upper layer, low reflectivity is exhibited due to the interference of light.

[0040]

EXAMPLES The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

[Example 1] SnO ₂ doped with Sb in an amount of 10 mol%
15 g of powder (average particle size 0.5 μm) was added to water, kept at 30 ° C. and stirred for 1 hour with a homomixer to prepare a sol.
(Liquid A) Ethanol solution of Ti (OC ₄ H ₉ ) ₄ (TiO ₂ equivalent solid content 20 wt%)
Acetylacetone was added to Ti (OC ₄ H ₉ ) _{4 in} an amount of 1.0 mol, and the mixture was stirred for 1 hour. H ₂ O was added to Ti (OC ₄ H ₉ ) ₄ in 2 mol.
The mixture was added in a specific amount and further stirred for 1 hour. (B liquid)

Liquids A and B were diluted with ethanol so that each of them would be 3 wt% in terms of oxide, and then mixed so that the ratio of liquid A: liquid B was 2: 1 and the rotation speed of 1200 rpm was applied to the surface of the cathode ray tube panel. Coating for 5 seconds and then heating at 200 ° C. for 30 minutes to obtain a film having a thickness of about 100 nm.

The evaluation method of the coating film obtained in the examples is as follows. 1) Conductivity evaluation Measure the surface resistance value of the film surface with a Hiresta resistance meter (made by Mitsubishi Yuka)

2) Scratch resistance After the film surface was reciprocated 100 times with an eraser under a load of 1 kg, the scratches on the surface were visually judged and the evaluation criteria were as follows. ◯: No scratches were given. Δ: Some scratches are formed. X: Many scratches were formed or the film was peeled off.

Example 2 The starting material of the liquid B was Ti (OC ₃ H ₇ ) ₄
The same procedure as in Example 1 was carried out except that

[Example 3] The same procedure as in Example 1 was carried out except that SnO ₂ : TiO ₂ was mixed at a weight ratio of 1: 1.

[Comparative Example 1] Solution A was solidified with ethanol to give a solid content of 3 parts.
A 100 nm film was obtained by diluting it so that it would be wt% and using it as a coating solution. The evaluation was performed in the same manner as in Example 1. Examples 1-3,
The results of Comparative Example 1 are shown in Table 1.

[0048]

[Table 1]

Example 4 3 g of H ₂ O was added to 100 g of ethanol, and 0.05 mol of MgCl _{2 and} 0.033 mol of BF ₃ .C ₂ H ₅ OH were added.
Was added, and the completely mixed and dissolved solution was placed in a flask equipped with a reflux condenser and reacted at 85 ° C. for 1 hour to obtain MgF ₂ sol.

An ethanol solution of silicon ethoxide was added to this solution in an amount of 3 wt% in terms of total solid content in terms of oxides and fluorides.
MgF ₂ : SiO ₂ = 4: 6 weight ratio was mixed. Further Zr in the solution _{(C 5 H 7 O 2)} 2 (OC 4 H 9) ₂ ethanol solution ZrO ₂
The mixture was added and mixed so as to be 10 wt% of SiO ₂ in conversion. (D liquid)

On the film prepared in the same manner as in Example 1, 20 parts of D liquid were applied.
The coating was carried out at a rotation speed of 00 rpm for 5 seconds and then heated at 200 ° C. for 30 minutes to obtain a 100 nm film. It evaluated similarly to Examples 1-3.

[Example 5] The same procedure as in Example 4 was carried out except that after applying the liquid D in Example 4, heating was carried out at 450 ° C for 30 minutes.

[Embodiment 6] A solution in which MgF ₂ : SiO ₂ = 5: 5 was used at the time of preparation of the solution D in Example 4, and this solution was applied.
The same procedure as in Example 4 was carried out except that heating was carried out at 450 ° C. for 30 minutes.

[Embodiment 7] Instead of the liquid D in Embodiment 4, a partial hydrolyzate of ethyl silicate (solid content of SiO _{2 of} 3 wt.
%) Was performed in the same manner as in Example 4.

Comparative Example 2 The procedure of Example 4 was repeated except that the partial hydrolyzate of ethyl silicate was coated on the film obtained in Comparative Example 1 as in Example 7.

In addition, in Examples 4 to 7 and Comparative Example 2, evaluation of low reflectivity was also performed. The evaluation method is as follows. The single-sided luminous reflectance of the film was measured with a GAMMA spectroscopic reflection spectrophotometer, and the results are shown in Table 2.

[0057]

[Table 2]

[0058]

According to the present invention, a strong and highly conductive antistatic film can be provided. Further, by providing a low reflection film made of MgF ₂ sol or the like on the antistatic film, excellent low reflection antistatic property can be imparted to the substrate.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Keisuke Abe, 1150, Hazawa-machi, Kanagawa-ku, Yokohama, Kanagawa Prefecture Central Research Institute, Asahi Glass Co., Ltd. (72) Satoshi Takemiya, 1150, Hazawa-machi, Kanagawa-ku, Yokohama, Asahi Glass Co., Ltd. Central Research Institute (72) Ken Kawari, 1150 Hazawa-machi, Kanagawa-ku, Yokohama, Kanagawa Prefecture Asahi Glass Co., Ltd. Central Research Institute (72) Inventor, Kazuya Hiratsuka, 1150, Hazawa-machi, Kanagawa-ku, Yokohama City Asahi Glass Co., Ltd. In-house

Claims (9)

[Claims] 1. Ti (C ₅ H ₇ O ₂ ) _n (OR) _m (where n + m = 4, m =
0 to 3, n = 1 to 4, R = C _{1 to} C ₄ alkyl group), and a solution containing SnO ₂ particles doped with Sb on a substrate, and then heating. Antistatic film obtained by. 2. An antistatic film comprising a single layer or a multi-layer film formed on a substrate, at least one of which is made of Ti.
(C ₅ H ₇ O ₂ ) _n (OR) _m (however, n + m = 4, m = 0-3, n = 1
~ 4, R = C ₁ -C ₄ alkyl group), at least one kind,
An antistatic film, which is a film obtained by applying a solution containing SnO ₂ particles doped with Sb and Sb on a substrate and then heating. 3. MgF ₂ , Si compound, Zr compound, Ti compound, Al
A low-refractive-index, low-reflection film obtained by applying a solution containing at least one of a compound and a Sn compound onto a substrate and then heating the solution. 4. A low-reflection antistatic film composed of multiple layers formed on a substrate, at least one layer of which is MgF ₂ , Si.
A low-refractive-index, low-reflection film obtained by applying a solution containing at least one of a compound, a Zr compound, a Ti compound, an Al compound, and a Sn compound, and then heating the solution, and at least one of multiple layers. Of Ti (C ₅ H ₇ O ₂ ) _n (OR) _m (n = 1 to 4, m = 0 to 3, n) formed on the substrate side of the low refractive index layer.
+ M = 4, R is an alkyl _{group), Si (OR) n R} m (n = 1~
4, m = 0 to 3, n + m = 4, R is an alkyl group), or at least one of these partial hydrolysates, and a solution containing Sb-doped SnO ₂ particles is applied and then heated. A low reflection antistatic film, which is the obtained antistatic film. 5. The antistatic film according to claim 1, wherein the solution coated on the substrate further contains a Si compound. 6. A solution for forming a low refractive index and low reflection film comprises Mg salt.
The low-refractive-index low-reflection film according to claim 3 or the low-reflection antistatic film according to claim 4, which is a solution containing an MgF ₂ sol produced by a reaction with a BF ₃ complex salt. 7. A solution for forming a low refractive index and low reflection film is Si (OR)
_n R _m (n = 1 to 4, m = 0 to 3, n + m = 4, R is an alkyl group), Zr (C ₅ H ₇ O ₂ ) _n (OR) _m (n = 0 to 4, m = 0 ~
4, n + m = 4, R is an alkyl group), Ti (C ₅ H ₇ O ₂ ) _n (OR) _m
(N = 0 to 4, m = 0 to 4, n + m = 4, R is an alkyl group), Al (C ₅ H ₇ O ₂ ) _n (OR) _m (n = 0 to 3, m = 0 to 3, n
+ M = 3, R is an alkyl group), Sb-doped SnO ₂ , Sn
(C ₅ H ₇ O ₂ ) _n (OR) _m ((n = 0 to 2, m = 0 to 2, n + m
= 2, R is an alkyl group) or (n = 0 to 4, m = 0 to
4, n + m = 4, R is at least one selected from the group consisting of alkyl groups)), and the low refractive index / low reflection film according to claim 3 or 6, or 4 or 6.
The low-reflection antistatic film described. 8. A glass article on which the antistatic film according to any one of claims 1 to 7, a low refractive index low reflection film, or a low reflection antistatic film is formed. 9. A cathode ray tube having the antistatic film according to any one of claims 1 to 7, a low refractive index and low reflection film, or a low reflection antistatic film formed on the surface thereof.