Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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S P E C I F I C A T I O N
ELECTRICALLY CONDUCTIVE POWDER AND TRANSPARENT
CONDUCTIVE COMPOSITION
TECHNICAL FIELD
The present invention relates to an electrically
conductive powder excellent in transparency and electrical
conductivity and also to a transparent conductive
composition incorporating the electrically conductive
powder.
BACKGROUND ART
There often is a need for shaped products, such as a
coating film, film, sheet and the like, which have imparted
thereto appropriate transparency and electrical conductivity
for antistatic purpose or for utility as electrodes, heating
elements or the like. Several methods have been proposed
whereby such transparency and electrical conductivity can be
imparted, including (1) subdividing antimony-doped tin oxide
particles to sizes of below the wavelength range of visible
light, mixing the subdivided particles in a resin to prepare
a film-forming or coating composition which is subsequently
processed to form a film or coating film, and (2)
depositing, in the form of a thin film, a compound, e.g.,
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indium oxide/tin oxide as widely used for formation of ITO
film, onto surfaces as by a sputtering technique.
However, the powder particles used in the method (1),
because of their fine sizes, exhibit high cohesive strength
that facilitates the formation of agglomerates as secondary
particles. The problem of poor dispersion arises when they
are formulated into a coating composition. Also, the amount
of those particles must be increased if they are to be
effective in forming electrically conductive paths.
The method (2), while effective to provide thin metal
films having excellent levels of electrical conductivity and
transparency, needs expensive film-forming materials and a
large-scale film-forming apparatus, which are problems.
Another method has been proposed which involves loading
into a resin the electrically conductive powder prepared by
coating amorphous silica or mica particle-s, known as being
capable of imparting relatively high transparency to a
coating film or regular film when incorporated therein, with
electrically conductive metal oxide such as tin oxide
(Japanese Patent Laid Open Nos. Hei 2-218768 and Hei 5-
116930) .
However, if the resin is to obtain a desired level of
electrical conductivity, the electrically conductive powder
must be incorporated therein in a large amount. The higher
loading of the electrically conductive powder has created a
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problem of reducing transparency of the resin.
DISCLOSURE OF THE INVENTION
It is an object of the present invention to provide an
electrically conductive powder having improved transparency
and electrical conductivity compared to the above-described
conventional electrically conductive powders, as well as a
transparent conductive composition incorporating such an
electrically conductive powder.
A first aspect of the present invention is
characterized in that amorphous silica fibers, obtainable by
treating calcium silicate fibers with acid, are coated with
one or more metal oxides selected from tin oxide, antimony
oxide, indium oxide and zinc oxide.
A second aspect of the present invention is
characterized in that amorphous silica fibers having a fiber
length of 1 - 500 um and an aspect ratio of 5 - 5,000 are
coated with one or more metal oxides selected from tin
oxide, antimony oxide, indium oxide and zinc oxide.
Such amorphous silica fibers can be obtained by
treating calcium silicate fibers with acid, as analogous to
the first aspect.
Accordingly, the amorphous silica fibers for use in the
second aspect may be those obtained by treating calcium
25, silicate fibers with acid.
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In the first and second aspects, the amorphous silica
fibers preferably have a refractive index in the range of
1.4 - 1.8. Maintaining their refractive index within the
above-specified range allows a resin or polymer including
such amorphous silica fibers to obtain the further improved
transparency.
In the first and second aspects, the amount of the
coated metal oxide is preferably in the range of 10 - 100
parts by weight, based on 100 parts by weight of the
amorphous silica fibers. If it is excessively small, the
sufficient electrical conductivity may not be obtained for
the electrically conductive powder. On the other hand, if
it is excessively large, an economical disadvantage becomes
significant while not showing the proportional improvement
in electrtical conductivity. Also, the transparency of the
conductive powder shows a decreasing trend.
As stated earlier, the metal oxide to be coated is
selected from tin oxide, antimony oxide, indium oxide and
zinc oxide. Preferably used are tin oxide containing
antimony oxide, a metal oxide consisting of indium oxide/tin
oxide, generally called ITO, and zinc oxide. For the
purpose of imparting high electrical conductivity by the
uniform coverage of amorphous silica fiber surfaces, the use
of tin oxide containing antimony oxide :is preferred. The
amount of antimony oxide incorporated is preferably in the
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range of 1 - 50 parts by weight, more preferably in the
range of 1 - 20 parts by weight, based on 100 parts by
weight of tin oxide. Tin oxide, when doped with antimony,
exhibits the enhanced level of electrical conductivity.
In the present invention, the amorphous silica fibers
may be coated with the metal oxide, for example, according
to the following procedure. A selected metal oxide, such as
in the form of its hydroxide, is deposited onto amorphous
silica fiber surfaces in an aqueous solution. Subsequent
dewatering, drying and heat treatment provides a coating
film of electrically conductive metal oxide on surfaces of
the amorphous silica fibers. The hydroxide or the like of
the metal oxide can be prepared by hydrolyzing a metallic
compound. The preferred metallic compounds are those which
can be solubilized into water or water-soluble organic
solvent, including those soluble in water in an acidic or
base condition, such as halides and oxides, and those
soluble in water-soluble organic solvents, such as metal
alcoholates and metal acetyl acetonates.
A specific manufacturing method involves adding a
solution of the aforementioned matallic compound to an
aqueous dispersion of amorphous silica fibers. Subsequent
hydrolyzing results in deposition of insolubles thereof onto
fiber surfaces. The metallic compound solution may be added
to the aqueous dispersion of fibers under a hydrolyzing
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condition. Alternatively, hydrolyzing may be achieved after
the addition of the solution to the aqueous dispersion.
A first hydrolyzing method utilizes an organic
compound, such as alcoholate or acetyl acetonate, for the
metallic compound. The organic compound, after dissolved in
a water-soluble organic solvent, is added to the above-
described aqueous dispersion of amorphous silica fibers,
whereby the metallic compound is hydrorized to deposit onto
fiber surfaces. Such a hydrolysis reaction may be effected
under the application of heat or in the presence of an
alkaline substance. Useful alkaline substances include
hydroxides and carbonates of alkaline metals, and ammonium
compounds, for example.
A second hydrolyzing method utilizes a halide for the
metallic compound. A halide solution in alcohol is added to
the aqueous dispersion of amorphous silica fibers. In this
method, the hydrolysis reaction may be effected under the
application of heat or in the presence of an alkaline
substance. A useful alkaline substance can be chosen from
those described in the aforementioned first method.
A third hydrolyzing method involves adding an aqueous
solution of the metallic compound to the aforementioned
aqueous dispersion of amorphous silica fibers. This method,
because of its exclusion of organic solvents, is forvored
from the following points; working atmosphere, environmetal
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pollution, prevention of disasters, and economics. Also in
this method, the hydrolysis reaction can be effected under
the application of heat or in the presence of an alkaline
substance.
In the first aspect, the amorphous silica fibers used
are those prepared by treating calcium silicate fibers with
an acid. Useful calcium silicate fibers may be those
comprised of wollastonite, xonotlite and the like, for
example.
The acid treatment of calcium silicate fibers results
in the removal of calcium therefrom, so that the amorphous
silica fibers are obtained. The acid treatment is not
particularly specified, so long as it can convert the
calcium silicate fibers to amorphous silica fibers while
maintaining the original fibrous form of the calcium
silicate fibers. However, the acid treatment using a weak
acid is generally preferred. For example, the preferred
acid treatment employs a carbonic acid. Preferably, a
carbonic acid gas is blown into the aqueous dispersion of
calcium silicate fibers. With such a treatment, calcium in
calcium silicate can be removed in the form of calcium
carbonate, resulting in the provision of amorphous silica
fibers which maintain the original fibrous form of the
calcium silicate. After the acid treatment using the
carbonic acid gas, the produced calcium carbonate may in
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some cases remain on the amorphous silica fibers. In such
cases, calcium carbonate can be decomposed for removal
therefrom by adding thereto an acid, such as oxalic acid,
nitric acid or the like.
The amorphous silica fibers, as obtained in the manner
as stated above, retain the original fibrous form of the
starting material, i.e., calcium silicate fibers. The
amorphous silica fibers can thus be obtained which have the
fiber length and aspect ratio approximate in levels to those
of the starting calcium silicate fibers. Therefore, the
fiber length and aspect ratio of the amorphous silica fibers
can be adjusted by suitably selecting the fiber length and
aspect ratio of starting wollastonite fibers. The fibrous
configuration of the amorphous silica fibers is represented
preferably by a fiber length of 1 - 500 um and an aspect
ratio of 5 - 5,000, more preferably by a fiber length of 10
- 50 um and an aspect ratio of 10 - 100. In order to assure
transparency when the electrically conductive powder of the
present invention is loaded in a resin or the like, it is
preferred that a refractive index of the electrically
conductive powder loaded is as close to that of the resin or
the like as possible. In view thereof, the referactive
index of the electrically conductive powder is preferably in
the range of 1.4 - 1.8, more preferably in the range of 1.4
- 1.6.
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The tranparent coductive composition of the present
invention is characterized in that the aforemetnioned
electrically conductive fibers of the present invention is
included in a binder. Examples of binder materials include
synthetic polymer compounds such as thermoplastic and
thermosetting resins; natural resins and their derivatives;
metal-containing organic compounds; inorganic binder
materials; and emulsions of organic or inorganic compounds.
Specific examples of thermoplastic resins are engineering
plastics such as polyolefins, polyvinyl chloride, ABS resin,
polystyrene, acrylics, POM resin, PBT resin and PPS resin.
Specific examples of thermosetting resins are phenol resin,
epoxy resin and the like. Other synthetic polymer compounds
include polyphosphazene and the like. One or more of the
above-listed binder materials can be selectively employed
depending upon the particular purposes and uses sought.
In the case where the transparent conductive
composition of the present invention is provided in the form
of the conductive resin composition, an additive such as a
filler, reinforcer, pigment, anti-oxidant, antistatic agent,
lubricant, heat stabilizer, or flame retarder may suitably
be incorporated therein within the limits not to lose the
required transparency. Also, the electrically conductive
powder of the present invention may be subjected to surface
treatment with a coupling agent or the like before it is
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mixed with the binder.
Also in the preparation of the tranparent conductive
resin composition, conventional mixing operations may
suitably be employed including, for example, Banbury mixing,
internal mixing, extrusion pelletizing and the like.
Also, the transparent conductive composition of the
present invention, because of its excellent transparency,
becomes advantageous when used in the processed forms such
as a coating film, film, sheet and the like. It can be
formed into a coating film when used in the form of
conventional coating compositions such as solvent-based,
water-based and emulsion coating compositions. Also, it can
be rendered into a film or sheet by a conventional film- or
sheet-forming process.
Preferably, the transparent conductive composition of
the present invention contains 3 - 80 % by weight of the
electrically conductive powder and has a volume resistivity
of not exceeding 101° ~~cm and a total luminous
transmittance of 30 - 100 0.
The excessively low loading of the electrically
conductive powder may result in the failure to obtain the
desired electrical conductivity. On the other hand, the
excessively high loading of the electrically conductive
powder may result in the reduced transparency. For uses
which need electrical conductivity for antistatic purposes,
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the volume resistivity of not exceeding 101° ~~cm is mostly
required. Also, the total luminous transmittance is
preferably not below 30 %, more preferably not below 50 0.
In accordance with the present invention, electrically
conductive powders can be provided which, when loaded in a
resin or the like, are able to impart both transparency and
electrical conductivity thereto.
In accordance with the present invention, transparent
conductive compositions can be provided which have excellent
transparency and electrical conductivity.
The electrically conductive powder of the present
invention are also useful for electrostatic coating and
electrodepsition primer in the coating field of steel plates
and the like.
The electrically conductive powder of the present
invention, when added to a base layer made from a metallic
coating composition commonly used in the coating field of
steel plates and the like, can make the base layer directly
coatable without the interposition of a primer. A coating
composition for use in such a base layer may contain the
electrically conductive powder of the present invention, an
aluminum paste, a pigment and a polyester-melamine binder,
for example.
Also, the electrically conductive powder of the present
invention, when formulated in a coating composition, not
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only imparts the desired electrical conductivity but also
exhibits the excellent flatting effect, which makes it very
suitable for applications in the aforementioned coating
fields.
BEST MODES IN CARRYING OUT THE INVENTION
The present invention will be now described with
reference to specific examples.
(PREPARATION OF ELECTRICALLY CONDUCTIVE POWDERS)
Preparation Exam,~le (Preparation of amorphous silica fibers)
200 g of calcium silicate fibers (product name "BISTAL",
manufactured by Otsuka Chemical Co. Ltd., an average fiber
length of 25 um, an average fiber diameter of 0.77 um) was
dispersed in 4 liter deionized water, and subjected to a
treatment for 20 hours which allowed a COZ gas to bubble
through the dispersion at a flow rate of 100 ml/min, while
the dispersion was stirred and maintained at 70 °C. After
the treatment was terminated, the dispersion was cooled to
40 °C. 240 ml of a concentrated (67.5 ~;) nitric acid was
added dropwise to the dispersion, while stirred, over 10
minutes, followed by an additional 1 hour of stirring.
Thereafter, the product was suction filtered, washed with
water, dewatered and dried. The resulting product was
amorphous silica fibers having an average fiber length of 23
um, an average fiber diameter of 0.77 um, an average aspect
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ratio of 30 and a refractive index of 1.5.
EXAMPLE 1
250 g of the amorphous silica fibers obtained in
Preparation Example, as a base material, was dispersed in
2.5 liter of deionized water. A mixed solution of tin
chloride and antimony chloride in hydrochloric acid and an
aqueous solution of sodium hydroxide were concurrently but
separately added dropwise to the dispersion while stirred
and maintained at 70 °C. The dispersion was stirred for an
additional 2 hours, while its pH was kept at about 3. The
dispersion was then filtered, washed with water, dewatered,
dried at 110 °C for 10 hours, and then heat treated at 450 °C
to obtain a fibrous-form electrically conductive powder of
the present invention. Its tin oxide and antimony oxide
contents were 25 parts by weight and 5 parts by weight,
respectively, based on 100 parts by weight of the base
material.
COMPARATIVE EXAMPLE 1
The procedure of the above Example 1 was followed, with
the exception that white carbon (product name "NIPSIL",
manufactured by Nippon Silica Kogyo Co., Ltd., comprised
principally of Si02, an average particle size of 16 dun, a
refractive index of 1.48) was used a base material, to
prepare an electrically conductive powder.
COMPARATIVE EXAMPLE 2
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The procedure of the above Example 1 was followed,
except that muscovite (product name "Z-20", manufactured by
Hikawa Kogyo Co., Ltd., flake-form, an average particle size
of 50 um, a refractive index of 1.56) was used as a base
material, to prepare an electrically conductive powder.
(PREPARATION OF TRANSPARENT CONDUCTIVE SHEETS)
EXAMPLE 2
A transparent conductive sheet incorporating the
electrically conductive powder of Example 1 was prepared
according to the following procedure.
A pellet-form polypropylene resin was melt in a twin-
screw kneader, the electrically conducitive powder of
Example 1 was supplied from a side hopper, and the mixture
was kneaded and extruded into a sheet-form, i.e. an
electrically conductive sheet, which had a thickness of 30
um. A blending proportion of the electrically conductive
powder in the sheet was 30 % by weight.
COMPARATIVE EXAMPLE 3
The procedure of the above Example 2 was followed,
except that the electrically conductive powder of
Comparative Example 1 was used, to prepare a sheet.
COMPARATIVE EXAMPLE 4
The procedure of the above Example 2 was followed,
except that the electrically conductive powder of
Comparative Example 2 was used, to prepare a sheet.
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COMPARATIVE EXAMPLE 5
The procedure of the above Example 1 was followed,
except that titanium oxide fibers (product name "FTL-200",
manufactured by Ishihara Sangyo Co., Ltd., rutile-form, an
average fiber length of 5 um, a refractive index of 2.90)
was used as the base material, to prepare an electrically
conductive powder.
The procedure of the above Example 2 was followed with
the use of this electrically conductive powder to prepare a
sheet.
The sheets obtained in Example 2 and Comparative
Examples 3 - 5 were respectively measured for volume
resistivity and total luminous transmittance in accordance
with the following procedures. The results are shown in
Table 1.
Procedure for determinina volume resistivit~
A sample piece was placed between two electrodes. A DC
voltage was applied across the sample piece, and a value of
the current flowing though the sample piece was measured. A
volume resistivity (numerical value given by dividing the
voltage by the current flowing through a unit volume of the
sample piece: a unit of ~~cm ) was determined from this
measured value.
Procedure for determining total luminous transmittance
Each sheet obtained was cut to an 8 cm by 8 cm square
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sample piece (thickness of not exceeding 1 mm). A total
luminous transmittance (o) of the sample was measured
utilizing a Haze computer (model number HGM-2DP:
manufactured by Suga Shikenki Co., Ltd.).
Table 1
Electrically Volume Total
Luminous
Conductive Resistivity
Transmittance
Powder (S2 cm)
( o )
Example 2 Example 1 1 X 105 ~~ 85.3
Comp. Example Comp. Example 2 X 10' 69.0
3 1
Comp. Example Comp. Example 3 X 106 81.2
4 2
Comp. Example Titanium Oxide 2 X 106 19.7
5
Fiber
(PREPARATION OF TRANSPARENT CONDUCTIVE COATING FILMS)
EXAMPLE 3
The electrically conductive powder of the above Example
1 was loaded in a solvent-containing solution-type urethane
resin in the concentration of 40 o by weight, based on a
total solids weight, followed by mixing thereof with
sufficient stirring. The mixture was then applied onto a
PET film and cured by heat drying to obtain a coating film
having a dry film thickness of 10 um.
COMPARATIVE EXAMPLE 6
The procedure of the above Example 3 was followed,
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except that the electrically conductive powder of
Comparative Example 1 was used, to form a coating film.
COMPARATIVE EXAMPLE 7
The procedure of the above Example 3 was followed,
except that the electrically conductive powder of
Comparative Example 2 was used, to form a coating film.
COMPARATIVE EXAMPLE 8
The procedure of the above Example 3 was followed,
except that the same electrically conductive powder as used
in Comparative Example 5, i.e., the electrically conductive
powder prepared from titanium oxide fibers as the base
material was used, to form a coating film.
The coating films obtained in Example 3 and Comparative
Examples 6 - 8 were measured, respectively, for volume
resistivity and total luminous transmittance according to
the same procedures as described above. The results are
shown in Table 2.
Table 2
Total
Electrically Volume
Luminous
Conductive Resistivity
Transmittance
Powder (~2 cm)
Example 3 Example 1 4 X 104 91.2
~
Comp. Example Comp. Example 3 X 106 77.6
6 1
Comp. Example Comp. Example 5 X 105 83.5
7 2
Comp. Example Titanium Oxide 4 X 104 27.6
g
Fiber
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As can be clearly seen from Tables 1 and 2, the sheet
of Example 2 and the coating film of Example 3, respectively
prepared by using the electrically conductive powder of
Example 1 in accordance with the present invention, exhibit
the reduced levels of volume resistivity, markedly increased
levels of total luminous transmittance and improved degrees
of electrical conductivity and transparency, compared to
those of Comparative Examples 3 - 8.
AVAILABILITY IN INDUSTRY
The present invention is applicable to members for
which transparency and electrical conductivity have been
conventionally sought, such as display device parts,
antistatic films or packages, transparent electrodes and
transparent heating units.
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