TECHNICAL FIELD
[0001] 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
[0002] 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., indium oxide/tin oxide as widely used for formation of ITO film, onto
surfaces as by a sputtering technique.
[0003] 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.
[0004] 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.
[0005] Another method has been proposed which involves loading into a resin the electrically
conductive powder prepared by coating amorphous silica or mica particles, 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).
[0006] 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 problem of reducing
transparency of the resin.
DISCLOSURE OF THE INVENTION
[0007] 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.
[0008] 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.
[0009] A second aspect of the present invention is characterized in that amorphous silica
fibers having a fiber length of 1 - 500 µm 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.
[0010] Such amorphous silica fibers can be obtained by treating calcium silicate fibers
with acid, as analogous to the first aspect.
[0011] Accordingly, the amorphous silica fibers for use in the second aspect may be those
obtained by treating calcium silicate fibers with acid.
[0012] 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.
[0013] 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.
[0014] 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
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.
[0015] 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.
[0016] 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
condition. Alternatively, hydrolyzing may be achieved after the addition of the solution
to the aqueous dispersion.
[0017] 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.
[0018] 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.
[0019] 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 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.
[0020] 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.
[0021] 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 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.
[0022] 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 µm and an aspect ratio of 5 - 5,000, more preferably
by a fiber length of 10 - 50 µm 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.
[0023] 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.
[0024] 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 mixed with the binder.
[0025] 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.
[0026] 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.
[0027] 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 10
10 Ω·cm and a total luminous transmittance of 30 - 100 %.
[0028] 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, the volume resistivity
of not exceeding 10
10 Ω·cm is mostly required. Also, the total luminous transmittance is preferably not
below 30 %, more preferably not below 50 %.
[0029] 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.
[0030] In accordance with the present invention, transparent conductive compositions can
be provided which have excellent transparency and electrical conductivity.
[0031] 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.
[0032] 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.
[0033] Also, the electrically conductive powder of the present invention, when formulated
in a coating composition, not 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
[0034] The present invention will be now described with reference to specific examples.
(PREPARATION OF ELECTRICALLY CONDUCTIVE POWDERS)
Preparation Example (Preparation of amorphous silica fibers)
[0035] 200 g of calcium silicate fibers (product name "BISTAL", manufactured by Otsuka Chemical
Co. Ltd., an average fiber length of 25 µm, an average fiber diameter of 0.77 µm)
was dispersed in 4 liter deionized water, and subjected to a treatment for 20 hours
which allowed a CO
2 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 µm, an average fiber diameter of 0.77 µm, an average aspect ratio of
30 and a refractive index of 1.5.
EXAMPLE 1
[0036] 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
[0037] 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 SiO
2, an average particle size of 16 µm, a refractive index of 1.48) was used a base material,
to prepare an electrically conductive powder.
COMPARATIVE EXAMPLE 2
[0038] 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 µm, 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
[0039] A transparent conductive sheet incorporating the electrically conductive powder of
Example 1 was prepared according to the following procedure.
[0040] 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 µm. A blending proportion of the electrically conductive powder
in the sheet was 30 % by weight.
COMPARATIVE EXAMPLE 3
[0041] 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
[0042] 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.
COMPARATIVE EXAMPLE 5
[0043] 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 µm, a refractive index of 2.90) was used as the base material,
to prepare an electrically conductive powder.
[0044] The procedure of the above Example 2 was followed with the use of this electrically
conductive powder to prepare a sheet.
[0045] 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 determining volume resistivity
[0046] 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 Ω·cm ) was determined
from this measured value.
Procedure for determining total luminous transmittance
[0047] Each sheet obtained was cut to an 8 cm by 8 cm square sample piece (thickness of
not exceeding 1 mm). A total luminous transmittance (%) of the sample was measured
utilizing a Haze computer (model number HGM-2DP: manufactured by Suga Shikenki Co.,
Ltd.).
Table 1
|
Electrically Conductive Powder |
Volume Resistivity (Ω·cm) |
Total Luminous Transmittance (%) |
Example 2 |
Example 1 |
1 X 105 |
85.3 |
Comp. Example 3 |
Comp. Example 1 |
2 X 107 |
69.0 |
Comp. Example 4 |
Comp. Example 2 |
3 X 106 |
81.2 |
Comp. Example 5 |
Titanium Oxide Fiber |
2 X 106 |
19.7 |
(PREPARATION OF TRANSPARENT CONDUCTIVE COATING FILMS)
EXAMPLE 3
[0048] The electrically conductive powder of the above Example 1 was loaded in a solvent-containing
solution-type urethane resin in the concentration of 40 % 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 µm.
COMPARATIVE EXAMPLE 6
[0049] The procedure of the above Example 3 was followed, except that the electrically conductive
powder of Comparative Example 1 was used, to form a coating film.
COMPARATIVE EXAMPLE 7
[0050] 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
[0051] 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.
[0052] 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
|
Electrically Conductive Powder |
Volume Resistivity (Ω·cm) |
Total Luminous Transmittance (%) |
Example 3 |
Example 1 |
4 X 104 |
91.2 |
Comp. Example 6 |
Comp. Example 1 |
3 X 106 |
77.6 |
Comp. Example 7 |
Comp. Example 2 |
5 X 105 |
83.5 |
Comp. Example 8 |
Titanium Oxide Fiber |
4 X 104 |
27.6 |
[0053] 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
[0054] 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.
1. An electrically conductive powder characterized in that amorphous silica fibers, obtainable
by treating calcium silicate fibers with an acid, are coated with one or more metal
oxides selected from tin oxide, indium oxide and zinc oxide.
2. The electrically conductive powder as recited in claim 1, characterized in that a
refractive index of said amorphous silica fibers is 1.4 - 1.8.
3. The electrically conductive powder as recited in claim 1, characterized in that the
loading of said metal oxide is 10 - 100 parts by weight, based on 100 parts by weight
of the amorphous silica fibers.
4. An electrically conductive powder characterized in that amorphous silica fibers having
a fiber length of 1 - 500 µm and an aspect ratio of 5 - 5,000 are coated with one
or more metal oxides selected from tin oxide, indium oxide and zinc oxide.
5. The electrically conductive powder as recited in claim 4, characterized in that said
amorphous silica fibers are those obtained by treating calcium silicate fibers with
an acid.
6. The electrically conductive powder as recited in claim 4, characterized in that a
refractive index of said amorphous silica fibers is 1.4 - 1.8.
7. The electrically conductive powder as recited in claim 4, characterized in that the
loading of said metal oxide is 10 - 100 parts by weight, based on 100 parts by weight
of the amorphous silica fibers.
8. A transparent conductive composition characterized in that an electrically conductive
powder, prepared by coating amorphous silica fibers having a fiber length of 1 - 500
µm and an aspect ratio of 5 - 5,000 with one or more metal oxides selected from tin
oxide, indium oxide and zinc oxide, is included in a binder.
9. The transparent conductive composition as recited in claim 8, characterized as containing
3 - 80 % by weight of the electrically conductive powder and having a volume resistivity
of not exceeding 1010 Ω·cm and a total luminous transmittance of 30 - 100 %.
10. The transparent conductive composition as recited in claim 8, characterized in that
said binder is a polymer.