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Description  |
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FIELD OF THE INVENTION
This invention relates to a transparent conductive multi layer film
excellent in antistatic effect, electromagnetic wave shielding,
antireflection, mechanical strength and anti-smudge. More particularly it
relates to a transparent conductive film useful for antireflection of the
face panel (i.e., the front panel) of cathode-ray tubes, plasma displays,
etc.
BACKGROUND OF THE INVENTION
A transparent antistatic and electromagnetic wave shielding material has
been demanded for various electronic equipment. For example, a cathode-ray
tube or a plasma display used in TV sets or computer displays easily
attracts dust by static electrification of the front panel to reduce
visibility or radiates electromagnetic waves to give adverse influences to
the surroundings. An antireflection function has also become necessary to
cope with the tendency of the cathode-ray tube flattening. Additionally
the front panel is liable to receive scratches by the touch with fingers
or on cleaning.
For the purpose of statiec prevention, electromagnetic wave shielding and
antireflection, it has been proposed to form a conductive layer comprising
a metal such as silver or a conductive metal oxide such as indium-tin
oxide (ITO) directly on the front panel by vapor deposition or sputtering
technique. However, such vapor deposition and sputtering techniques
involve a vacuum treatment or a high temperature treatment, which
increases the production cost, or have poor productivity.
Thin conductive film formation techniques based on a sol-gel process have
also been proposed [see Habu et al., National Technical Report, Vol. 40,
No. 1, p. 90 (1994) and Y. Ono et al., SID 92DIGEST, 511 (1992)]. However,
these techniques also require a high temperature treatment. Besides, film
formation on a transparent substrate, i.e., a plastic film or a hard coat,
tends to induce denaturation of the substrate, which has limited the
choice of the substrate material.
Transparent conductive coatings having dispersed therein conductive oxide
particles or colloidal particles have also been proposed [see
JP-A-6-344489 and JP-A-7-268251 (The term "JP-A" as used herein means an
"unexamined published Japanese patent application") and H. Murakami et
al., SID 89 DIGEST, 270 and SID 93 DIGEST, 209, (1998)]; but the resulting
transparent conductive layer has insufficient conductivity.
In order to improve conductivity of the conductive films, JP-A-9-55175
proposes a transparent conductive film obtained by applying a coating
composition comprising metal particles. JP-A-10-142401 discloses a
low-reflection transparent conductive film obtained by applying an
antireflective coating composition comprising tetraethoxysilane, etc. on a
transparent conductive film. However, the film formed by merely coating a
transparent substrate with metal particles has insufficient mechanical
strength, and the antireflective coating such as tetraethoxysilane should
be treated in high temperature for a long time. Since formation of an
antireflective layer by a sol-gel process limits the material of the
transparent substrate, the above-mentioned low-reflection transparent
conductive film cannot be carried out except by directly applying the
coating composition to the glass face panel.
Instead of the method comprising direct application of a coating
composition to a face panel which entails high initial cost and requires a
high temperature treatment, methods of attaching a thin film formed on a
separate substrate to the face panel have been developed (see Taki et al.,
National Technical Report, Vol. 42, No. 3, pp. 264-268 (1996)). These
methods rely on thin film deposition techniques in a vacuum system, such
as vacuum evaporation, sputtering and the like for forming a conductive
metal oxide film (e.g., ITO), which are very costly and less productive as
previously mentioned.
On the other hand, it is necessary to ground a conductive layer for
electromagnetic wave shielding as described in JP-A-10-3868. Where the
conductive layer has a protective layer, it is difficult to lead a
grounding wire from the protective layer. In order to be grounded, the
conductive layer should be provided with some grounding terminals or be
partially exposed by some means. For example, grounding of the conductive
layer has been carried out by adhering a conductive tape to the conductive
layer before formation of a low-refractive layer thereon, or partly
peeling the surface protective layer, or piercing the protective layer, or
ultrasonic welding. Such a processing step for grounding is very likely to
be accompanied by damages such as scratches to the conductive layer, the
protective layer or any other functional layers, resulting in
deterioration of weather resistance and impairment of the commercial
value. These extra steps also contribute to an increase of cost and a
reduction of productivity.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a low-reflection
conductive multi layer film which is excellent in productivity as well as
antistatic properties, electromagnetic wave shielding performance,
reflection preventive properties, mechanical characteristics and
anti-smudge properties and which can be stuck to a face panel (i.e., a
front panel).
Another object of the invention is to provide a transparent conductive film
which is excellent in productivity in directly grounding the surface of
the conductive film.
The above objects are accomplished by a low-reflection transparent
conductive multi layer film comprising, in the order described, a
transparent substrate, a hard coat, a transparent conductive layer
containing particles comprising at least one of a metal and a metal oxide,
and at least one transparent protective layer for the conductive layer
which has anti-smudge properties and has a refractive index different from
that of the transparent conductive layer.
The low-reflection transparent conductive film of the invention is directly
attached to a cathode-ray tube or a plasma display panel used in a TV set
or a computer display to perform the desired functions with greatly
simplified equipment through greatly simplified steps as compared with the
conventional vapor deposition techniques such as PVD or CVD or the
conventional method comprising applying a conductive coating directly to
the face panel. The invention also allows the surface of the protective
layer to be grounded directly, which leads to simplification of the
production steps.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic view illustrating the structure of the low-reflection
transparent conductive multi layer film of the invention.
DETAILED DESCRIPTION OF THE INVENTION
In FIG. 1 is shown a preferred example of the low-reflection transparent
conductive multi layer film (hereinafter sometimes simply referred to as a
multi layer film) of the invention, in which the multi layer film is
composed of, in the order described, a substrate 1, a hard coat layer 2, a
transparent conductive layer 3 containing conductive particles, and a
protective layer 4 for the conductive layer 3 (hereinafter simply referred
to as a protective layer), and an anti-smudge layer 5 as a top layer
(i.e., an outermost layer). The protective layer 4 may combine the
function for anti-smudge, in which case the anti-smudge layer 5 may be
omitted.
Having the hard coat, the multi layer film of the invention is protected
from scratches. Having the transparent conductive layer containing
conductive metal or metal oxide particles, the multi layer film is
antistatic and effectively shields electromagnetic waves radiated from
cathode-ray tubes, etc. Having the protective layer whose refractive index
is different from that of the transparent conductive layer, the multi
layer film reduces reflection of incident light. Where the protective
layer comprises a resin having a dielectric power factor of 0.01 or higher
(at 50 Hz), it is possible to lead a ground wire directly from the
protective layer. Further, the multi layer film is protected against
smudge by the protective layer having anti-smudge properties or the
separately provided anti-smudge top layer.
The substrate which can be used in the invention is a resin film. Suitable
resins include polyesters, such as polyethylene terephthalate,
polyethylene naphthalate, and polyethylene terephthalate/polyethylene
naphthalate copolymers or mixtures; polycarbonate resins; norbornene
resins (cyclic olefin copolymers); celluloses or cellulose esters, such as
cellulose triacetate and cellulose diacetate; polyarylate resins;
polyacrylates, such as polymethylacrylate; and polymethyl methacrylate.
The substrate preferably has a thickness of 20 to 500 .mu.m, particularly
100 to 200 .mu.m. Too thin substrate is mechanically weak, and too thick
substrate is so stiff and is difficult to apply.
The above-described resins, being hydrophobic, are desirably subjected to
an appropriate surface activation treatment for making the surface
adhesive to the layer formed thereon, such as a chemical treatment, a
mechanical treatment, a corona discharge treatment, a flame treatment, an
ultraviolet (UV) treatment, a radiofrequency treatment, a glow discharge
treatment, an active plasma treatment, a laser treatment, a mixed acid
treatment, an ozone oxidation treatment, and the like. It is also a
preferred manipulation that a primer coat is provided on the
surface-treated substrate or directly on the untreated substrate. The
primer coat can be a single layer of a resin containing both a hydrophobic
group and a hydrophilic group or a double layer composed of a first layer
showing good adhesion to the substrate, which is formed in contact with
the substrate, and a second layer showing good adhesion to the constituent
layer provided thereon.
Of the above-described surface treatments, preferred are a UV treatment, a
flame treatment, a corona discharge treatment, and a glow discharge
treatment. The UV treatment is preferably carried out in accordance with
the procedures taught in JP-B-43-2603, JP-B-43-2604, and JP-B-45-3828 [The
term "JP-B" as used herein means an "examined Japanese patent
publication]. A high-pressure mercury lamp emitting UV rays having
wavelengths of 180 to 320 nm is used preferably. The corona discharge
treatment can be conducted in a conventional manner. For example, the
methods disclosed in JP-B-48-5043, JP-B-47-51905, JP-A-47-28067,
JP-A-49-83767, JP-A-51-41770, and JP-A-51-131576 can be used. A suitable
discharge frequency is from 50 Hz to 5000 kHz, preferably 5 kHz to several
hundreds of kilohertz, still preferably 10 to 30 kHz. The flame treatment
can be effected with natural gas, liquefied petroleum gas (LPG), etc. A
gas/air ratio is of importance. A preferred LPG/air ratio is 1/14 to 1/22,
particularly 1/16 to 1/19, by volume, and a preferred natural gas/air
ratio is 1/6 to 1/10, particularly 1/7 to 1/9, by volume. The flame energy
to be applied is preferably 1 to 50 kcal/m.sup.2, particularly 3 to 20
kcal/m.sup.2. The glow discharge treatment, which is particularly
effective, is performed by any techniques known conventionally. Reference
can be made, e.g., in JP-B-35-7578, JP-B-36-10336, JP-B-45-220004,
JP-B-45-22005, JP-B-45-24040, JP-B-46-43480, U.S. Pat. Nos. 3,057,792,
3,057,795, 3,179,482, 3,288,638, 3,309,299, 3,424,735, 3,462,335,
3,475,307, and 3,761,299, British Patent 997,093, and JP-A-53-129262.
The primer which can be applied to the substrate includes various kinds of
polymers, such as those comprising monomers selected from vinyl chloride,
vinylidene chloride, styrene, butadiene, methacrylic acid (or esters) ,
acrylic acid (or esters), itaconic acid (or esters), maleic anhydride, and
so forth; polyethyleneimine, epoxy resins, grafted gelatin, and
nitrocellulose.
Hydrophilic polymers are also applicable as a primer, such as water-soluble
polymers, cellulose esters, latex polymers, and water-soluble polyesters.
Examples of the water-soluble polymers include gelatin, gelatin
derivatives, casein, agar, sodium alginate, starch, polyvinyl alcohol,
polyacrylic acid copolymers, and maleic anhydride copolymers. Examples of
the cellulose esters include carboxymethyl cellulose and hydroxyethyl
cellulose. Examples of the latex polymers include vinyl chloride
copolymers, vinylidene chloride copolymers, acrylic ester copolymers,
vinyl acetate copolymers, and butadiene copolymers. The primer composition
can contain a curing agent, such as chromium salts (e.g., chromium alum),
aldehydes (e.g., formaldehyde or glutaraldehyde), isocyanate compounds,
active halogen compounds (e.g., 2,4-dichloro-6-hydroxy-s-triazine), and
epichlorohydrin resins. The primer composition is applied by well-known
techniques, such as dip coating, air knife coating, curtain coating,
roller coating, wire bar coating, gravure coating, and the like. The
extrusion coating using a hopper described in U.S. Pat. No. 2,681,294 is
effective as well.
The hard coat layer which may be used in the present invention can be of
any known curing resins, including thermosetting resins and active energy
ray-curing resins. Examples of the thermosetting resins include those
curable on crosslinking of prepolymers, such as melamine resins, urethane
resins, andepoxyresins. Examples of the active energy ray-curing resins
include polyfunctional curing monomers, such as polyfunctional
(meth)acrylates, e.g., pentaerythritol tetra(meth)acrylate,
dipentaerythritol hexa(meth)acrylate, and trimethylolpropane
tri(meth)acrylate. Examples of the active energy rays include UV rays,
electron beams, and .gamma.-rays, with UV rays being preferred. In the
case of UV curing, a polymerization initiator is preferably added, if
necessary, to the curing monomers. Preferred active energy-curing resins
are active energy ray-curing compounds such as pentaerythritol
tetra(meth)acrylate and dipentaerythritol hexa(meth)acrylate.
The hard coat layer can contain, as a filler, fine particles or colloidal
particles of metal oxides such as silica, alumina, zirconia or titania to
have increased hardness. Harder particles produce better results.
Particles having a Mohs'hardness of 6 or more are preferred. The
particlespreferably have a particle size of 1 to 100 nm. Particles greater
than 100 nm tend to cause haze, and particles finer than 1 nm are
difficult to disperse, failing to produce the effects as a filler. The
particles are preferably added in an amount of 5 to 50% by volume,
particularly 20 to 45% by volume, based on the curing resin. Addition of
more than 50% makes the film brittle, and addition of less than 5%
produces insubstantial effects. The metal oxide particles are preferably
subjected to a surface modification treatment to improve dispersibility
and compatibility with the resin. Suitable surface modification treatments
include treatment with a silane coupling agent containing a (meth)acryl
group or a (meth)acrylic acid derivative containing a polar group such as
a carboxyl group or a phosphoric acid group, and the like.
The hard coat layer preferably has a thickness of 2 to 30 .mu.m, still
preferably 4 to 10 .mu.m. If desired, the hard coat layer may contain an
anionic surfactant or a cationic surfactant or may be subjected to a
surface treatment such as a corona discharge treatment or a glow discharge
treatment, to improve the surface hydrophilicity or adhesion.
The conductive layer according to the present invention is a layer
containing at least one of a conductive metal and a conductive metal
oxide. The conductive layer has a surface resistivity of 10 k.OMEGA./sq.
(applied voltage: 90 V) or less, preferably 10 to 1000 .OMEGA./sq., still
preferably 10 to 700 .OMEGA./sq. The conductive layer could be formed by
carrying out vapor-deposition, sputtering or plating of a metal or a metal
oxide. Also, it is preferably formed by coating with conductive particles
of a metal or a metal oxide from the standpoint of productivity. Examples
of the conductive metal particles include gold, silver, copper, aluminum,
iron, nickel, palladium, platinum, and alloys thereof. Examples of the
conductive metal oxide particles include indium oxide, tin oxide, antimony
oxide, zinc oxide, aluminum oxide, silicon oxide, iron oxide, and
composite oxides thereof. Metal particles are preferred as conductive
particles. Silver particles or metal alloy particles mainly consisted of
silver are more preferred, and silver particles are particularly
preferred. From the standpoint of weatherability, a silver-palladium alloy
is preferred. The palladium content of the alloy is preferably 5 to 30% by
weight. Too small palladium content is ineffective on weatherability, and
too high palladium content reduces conductivity.
Methods of forming the metal particles include the ordinary low-vacuum
evaporation techniques and the method of preparing metal colloid
comprising reducing an aqueous solution of a metal salt.
The metal or metal oxide particles preferably have an average particle size
of 1 to 200 nm. Greater particles than 200 nm will absorb much light,
resulting in a reduced light transmission and an increased haze of the
conductive layer. Smaller particles than 1 nm are difficult to disperse.
Moreover, the conductive layer will have a drastically increased surface
resistivity, failing to provide a multi layer film having low resistance
enough to achieve the object of the invention. To secure high
conductivity, it is preferred for the transparent conductive layer to
consist substantially solely of conductive particles, not containing
non-conductive materials such as a binder resin.
The transparent conductive layer containing the metal or metal oxide
particles is formed by coating the hard coat with a dispersion of the
metal or metal oxide particles in a solvent mainly comprising water.
Solvents that can be mixed into water preferably are alcohols, such as
ethyl alcohol, n-propyl alcohol, isopropyl alcohol, 1-butyl alcohol,
2-butyl alcohol, t-butyl alcohol, methyl cellosolve, and butyl cellosolve.
The metal or metal oxide particles are preferably applied in an amount of
50 to 150 mg/m.sup.2. Too small amount of the conductive particles fails
to secure conductivity, and too large amount of the conductive particles
deteriorates transparency.
The transparent conductive layer should have a surface resistivity of 1000
.OMEGA./sq. or smaller in order to fulfill TCO (Tianstemanners Central
Organisation) Guidelines specified by Swedish Central Labor's Society. The
transparency is preferably 50% or more in terms of light transmittance. As
for transparency, the transparent conductive layer may be dark as far as a
display panel to which the multi layer film is attached is practically
visible but preferably has a visible light transmittance of 50% or higher,
still preferably 55% or higher, particularly preferably 60% or higher.
The protective layer which can be used in the present invention is
preferably formed of a resin having a dielectric power factor of 0.01 or
more (at a frequency of 50 Hz). The "dielectric power factor" is one of
the attributes of an electrical insulator. Resins having a higher
dielectric power factor easily cause insulation destruction and thereby
are preferred in the present invention. For the details of the "dielectric
power factor", refer to Kagaku Binran Kiso-hen II, pp. 1177-1179, Maruzen
(1975). Examples of the resins having a dielectric power factor of 0.01 or
more for use in the invention preferably include, but are not limited
thereto, polyisoprene (dielectric power factor (hereinafter the same):
about. 0.03 or more), chlorosulfonated polyethylene (about 0.03 or more),
polysulfide rubber (about 0.1), fluororubber (about 0.03), casein (about
0.06), phenolic resins (about 0.05 or more), polysulfide epoxy resins
(0.01 or more), urea resins (about 0.03 or more), melamine resins (about
0.03), nylon 6 (0.01 or more), nylon 66 (0.01 or more), polymethyl
methacrylate (about 0.05), ethyl acrylate-ethylene copolymers (0.01 or
more), polyvinyl chloride (0.01 or more), polyvinylidene chloride (about
0.03 or more), polyvinylidene fluoride (about 0.05), cellulose mono-, di-
or triacetate (about 0.02), and nitrocellulose (about 0.1).
Preferred among these resins are fluororubber, phenolic resins, urea
resins, melamine resins, nylon 6, nylon 66, polymethyl methacrylate,
polyvinyl chloride, polyvinylidene chloride, polyvinylidene fluoride,
polyvinyl formal, cellulose mono-, di -or triacetate, and nitrocellulose.
Active energy ray-cured (e.g., UV-, electron- or .gamma.-ray-cured) resins
are also preferred. Active energy ray-polymerized resins prepared from a
polyfunctional vinyl derivative of a polyol (e.g., polyfunctional
(meth)acrylic polyesters) are particularly preferred for their surface
hardness and mechanical strength. Preferred examples of the polyfunctional
vinyl derivatives include trimethylolpropane tri(meth)acrylate,
pentaerythritol tetra(meth)acrylate, and dipentaerythritol
hexa(meth)acrylate. If desired, a polymerization initiator can be added to
the active energy ray-curing resin precursor.
The thickness of the protective layer is not limited as long as
conductivity can be obtained from its surface but is preferably 10 to 2000
nm, still preferably 20 to 1000 nm, particularly preferably 20 to 500 nm,
especially preferably 10 to 500 nm, the most preferably 30 to 300 nm. The
surface resistivity of the protective layer is preferably 10 k.OMEGA./sq.
or less, still preferably 5 k.OMEGA./sq. or less, particularly preferably
2 k.OMEGA./sq. or less, especially preferably 1 k.OMEGA./sq. or less
(applied voltage: 90 V). The practical minimal surface resistivity is
preferably 10 .OMEGA./sq. Where the protective layer is 300 nm thick, a
surface resistivity of 1 k.OMEGA. or less can be obtained at an applied
voltage of 10 V. The ratio of the surface resistivity after formation of
the protective layer (i.e., the surface resistivity of the protective
layer) to that before formation of the protective layer (i.e., the surface
resistivity of the transparent conductive layer) preferably falls within a
range of from 0.5 to 1.5. By forming the protective layer, the metal, the
metal particles, the metal oxide or the metal oxide particles of the
conductive layer can be firmly fixed, and the surface resistivity is
reduced. As a result, the change in resistivity before and after the
formation of the protective layer is reduced. Essentially being
insulating, the protective layer increases its resistivity as its
thickness increases, and the ratio of the resistivity after the protective
layer formation to that before the formation increases as a result. The
protective layer can perform the function as an antireflective coat with
its refractive index controlled as hereinafter described.
If desired, the protective layer can contain a metal oxide. Suitable
examples of metal oxides include oxides of silica, alumina, zirconia, and
titania. These oxides are added for the purpose of improving the film
strength or varying the refractive index of the layer. It is also possible
to further provide an overcoat layer on the protective layer. Preferred
materials for the overcoat layer include well-known low-surface energy
compounds containing fluorine, such as silicone compounds containing a
fluorinated hydrocarbon group and polymers containing a fluorinated
hydrocarbon group. These compounds may be provided not only in the
overcoat layer but in the protective layer. Those compounds which are
orientated in the vicinities of the layer surface after being added
thereby serving for surface modification are preferred.
The at least one protective layer whose refractive index is different from
that of the transparent conductive layer has a refractive index of 1.70 or
smaller. The protective layer may have such a layer structure that the
outermost layer thereof has an anti-smudge function. In this case, the
protective layer preferably has a refractive index of 1.30 to 1.70,
particularly 1.35 to 1.60. If the refractive index exceeds 1.70, the
antireflective effect is small. If it is less than 1.30, the reflection on
the interface with the transparent conductive layer becomes large.
Materials capable of forming a film having a refractive index between 1.30
and 1.70 include organic synthetic resins, such as polyester resins,
acrylic resins, epoxy resins, melamine resins, polyurethane resins,
polyvinyl butyral resins, and UV-curing resins (for more specific
examples, refer to Polymer Handbook, 4th Ed., VI-571, John Wiley & Son,
Inc. (1999)); hydrolysate of metal (e.g., silicon) alkoxides; and organic
or inorganic compounds, such as silicone monomers or silicon oligomers.
Preferred are active energy ray-curing resin (precursors), such as
pentaerythritol tetra(meth)acrylate or dipentaerythritol
hexa(meth)acrylate, which may contain fine particles of silica, alumina,
etc. to brings about increased surface hardness.
For securing antireflection performance, the thickness of the protective
layer is selected to produce effects on reflectance reduction, preferably
from a range of from 50 to 150 nm. It is preferred that the product of the
refractive index and the thickness (nm) of the protective layer falls
within a range of from 100 to 200.
The protective layer can contain a fluorine- and/or silicon-containing
compound to improve the anti-smudge properties. Such a compound preferably
includes well-known fluorine compounds or silicon compounds, compounds
having a block containing a fluorine- and silicon-containing moiety, and
compounds comprising a segment compatible with a resin or a metal oxide,
etc. and a segment containing fluorine or silicon. Addition of such a
compound to the protective layer whose refractive index is different from
that of the transparent conductive layer results in localization of
fluorine or silicon in the vicinity of the surface of the protective
layer.
Specific examples of the F- and/or Si-containing compounds include block or
graft copolymers comprising an F- or Si-containing monomer unit and a
hydrophilic or lipophilic monomer unit. Examples of the F-containing
monomer includes perfluoroalkyl group-containing (meth)acrylic esters,
such as hexafluoroisopropyl acrylate, heptadecafluorodecyl acrylate,
perfluoroalkylsulfonamide ethylacrylate, and perfluoroalkylamide
ethylacrylate. Examples of the Si-containing monomer includes the one
having a siloxane group obtained by the reaction between
polydimethylsiloxane and (meth)acrylic acid, etc. Examples of the
hydrophilic or lipophilic monomer includes (meth)acrylic esters (e.g.,
methyl acrylate), esters between a polyester having a hydroxyl group at
the terminal and (meth) acrylic acid, hydroxyethyl (meth) acrylate, and
polyethylene glycol (meth)acrylate. These F- and/or Si-containing
compounds are commercially available under trade names Defensa MCF-300,
312 and 323 (acrylic oligomers having a micro-domain structure of a
perfluoroalkyl chain), Megafac F-170, F-173 and F-175 (perfluoroalkyl
group/lipophilic group-containing oligomers), Megafac F-171
(perfluoroalkyl group/hydrophilic group-containing oligomers) (all these
products are available from Dai-Nippon Ink & Chemicals, Inc.); and Modiper
F-200, 220, 600 and 820 (fluoroalkyl type block polymers of a vinyl
monomer, comprising a segment showing excellent migration and a resin
compatible segment) and Modiper FS-700 and 710 (silicon type) (Modiper
series are available from Nippon Oil & Fats Co., Ltd.).
The F- and/or Si-containing compound can be added in such an amount that
the compound may localize to the surface of the protective layer to
increase the contact angle to 90.degree. or greater, preferably
100.degree. or greater. More specifically the compound is added in an
amount of 1 to 50%, preferably 5 to 30%, by weight based on the protective
layer. When the amount is too small, the anti-smudge effect is small. When
the amount exceeds 50% by wei | | |
