Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
-1-
TITLE
IMPROVED ELECTROCONDUCTIVE COMPOSITION
AND PROCESS OF PREPARATION
BACKGROUND OF T~iE INVENTION
The present invention relates to an improved
electroconductive composition which comprises antimony-
containing tin oxide in which the tin oxide is predomi-
nately crystalline and the composition exists in a
l0 unique association with silica or a silica-containing
material, e.g., a silicate. More particularly, the
present invention relates to an improved
electroconductive powder composition comprising tens of
microns to sub-micron size particles having a thin
15 surface layer of amorphous silica or silica-containing
material, said material having a thin surface coating
layer which comprises a network of antimony-containing
tin oxide crystallites and to a process for preparing
the composition.
20 U. S. Patents 4,373,013 and 4,452,830
describe the preparation of an electroconductive powder
having a structure comprising titanium oxide particles
as nuclei with a coating of antimony-containing tin
oxide on the surface of the titanium oxide particles.
25 The powder is prepared by mixing an aqueous dispersion
of titanium oxide particles with a solution containing
a hydrolyzable tin salt and a hydrolyzable antimony
salt. The titanium oxide particles become coated with
antimony-containing tin oxide and can then be recovered
30 by filtration.
"Journal of Materials Science"', 21 (1986),
pp. 2731-2734, describes the preparation of
antimony-doped Sn02 films by thermal decomposition of
tin 2-ethylhexanoate on glass substrates. Reagent
35 grade tin 2-ethylhexanoate and antimony tributoxide
were used as the sources of tin and antimony,
134~67~
-2-
respectively, and application of the film onto the
substrate was accomplished by dipping the substrate
into an alcoholic solution containing the
organometallic compounds and then drying the applied
5 solution. The~substrate used was coda-lime glass which
was previously coated with about a 30 nm layer of Ti02,
. Si02 or Sn02 (with 8 wt t Sb) by thermal decomposition
of organometallic compounds. The resistivity of the
resulting film in which the substrate had a precoating
of Si02 was one-thirtieth of the resistivity of the
antimony-doped tin oxide film on the uncoated glass
substrate. For the range of films prepared, however,
electrical properties were noted as being more or less
poor compared with films obtained by other methods,
15 such as, spraying or chemical vapor deposition.
Japanese Patent No. SHO 63[1988] 20342
describes a method of manufacturing fine electrocon-
ductive mica particles by coating them with a tin
oxide/antimony oxide mixture. This coating is accom-
20 plished by treating the mica with tin tetrachloride,
antimony trichloride, and a hydroxyl-containing,
low-molecular-weight fatty acid.
Compositions which are capable of imparting
electroconductive properties to thin films, such as, in
25 polymer films, magnetic recording tapes, work surfaces
and in paints, are not always economically attractive
or reliable for a given application. Electroconductive
compositions, e.g., powders, which are currently
available for use as conductive pigments in paint, for
30 example, suffer a variety of deficiencies. Carbon
black may be used to impart conductivity, but this can
limit the color of the paint to black, dark gray and
closely related shades. Titanium dioxide powders,
coated with antimony-doped tin oxide by methods of the
35 prior art, normally require high pigment/binder ratios,
e.g., 200/100, in order to achieve minimum acceptable
~~~os~o
surface conductivity. Such a high pigment loading is
expensive and can limit the color range and
transparency of the resulting paint to very light
shades and pastels. A simple powder of antimony-doped
tin oxide may be used, but cost and color limitations
can be unfavorable.
Mica powders can be made conductive by
coating the particles directly with antimony-doped tin
oxide, but the preparation of such powders can be
10 expensive and difficult because of the poor affinity of
tin and antimony intermediates for the surface of the
mica. Organic complexing agents and/or organic
solvents are typically used to facilitate the reaction
of tin and antimony intermediates with the mica
15 surface. Even with these additives, a significant
portion of the tin and antimony remain in solution or
as free particles. This reduces the effective
conductivity of the powder and increases the cost,
since a significant amount of the tin and antimony
20 values are lost when the coated particles are recovered
from the reaction medium. In addition, the tin and
antimony values remaining in solution must be removed
before the waste solution which remains is discharged.
Finally, the antimony-doped tin oxide layer has been
25 found to bond poorly to the mica and may delaminate
during subsequent processing, such as during milling or
during incorporation into a polymer vehicle, e.g., a
paint formulation or polyester film.
3 0 SUI~9~'iARY OF TIDE INVENTION
The present invention is an electroconductive
composition which comprises a two-dimensional network
of crystallites of antimony-containing tin oxide which
exists in a unique association with amorphous silica or
35 a silica-containing material. The antimony-containing
tin oxide forms a two-dimensional network of densely
~34os7o
-4-
packed crystallites on the surface of the silica or
silica-containing material. The silica or
silica-containing material is a substrate, and the
network comprises a generally uniform layer of
5 crystallites in which the crystallites form an
electrically conducting pathway to adjacent
crystallites. The layer of tin oxide crystallites is
typically about 5 to 20 nm in thickness but covers the
surface of a particle with major dimensions that are
l0 typically ten to ten thousand times as large as the
thickness of the tin oxide layer. The crystallites
are, thus, part of a continuous conducting layer in two
dimensions.
The silica substrate can be practically any
15 shape. In the form of flakes or hollow shells,
satisfactory results may be achieved when the
two-dimensional network is formed on only one side of
the silica substrate. In general, however, best .
results are obtained when practically all of the
20 exposed surface of the silica substrate is coated with
the crystallite layer.
According to one aspect of the invention, the
composition is a powder comprising shaped particles of
amorphous silica which are coated with a
25 two-dimensional network of antimony-containing tin
oxide [Sn02(Sb)~ crystallites. The finished particles,
typically, are tens of microns to sub-micron in size,
and they, in turn, are capable of forming an
electroconductive network within the matrix of a thin
30 film, such as within a paint film. The shaped
particles of amorphous silica may be in the form of
needles, platelets, spheres, dendritic structures or
irregular particles. These provide an extended surface
for the deposition of the antimony-containing tin
35 oxide.
- 5 -
In a preferred embodiment, the amorphous
silica powder comprises thin shells or platelets less
than about 20 nm in thickness. The powder, when
dispersed in a vehicle, is generally transparent, and
its presence as a component of pigment in paint has
little impact on color and related properties.
In another embodiment of the invention, the
composition is a powder comprising shaped particles,
each of which has a structure comprising an inert core
material having a surface coating layer of amorphous
silica, which, in turn, is coated with a two-
dimensional network of antimony-containing tin oxide
crystallites. These powders are particularly useful
for incorporation into plastics and elastomers where
the shear stresses involved in molding useful articles
might degrade otherwise conductive powders which
comprise hollow shell or thin flakes.
The present invention also includes a process
for preparing the electroconductive composition which
comprises:
(a) providing a substrate of amorphous
hydroxylated silica or active silica-containing
material,
(b) applying a coating layer to the
substrate surface consisting essentially of hydrous
oxides of antimony and tin, and
(c) calcining the coated substrate at a
temperature in the range of 400° to 900°C in an oxygen-
containing atmosphere.
The coating layer of hydrous oxides of
antimony and tin is preferably applied to the hydroxyl-
ated substrate surface by adding aqueous solutions of
hydrolyzable Sn and Sb salts to a slurry containing the
silica at a pH in the range of abut 1.5 to about 3.5,
preferably at a pH of 2Ø Calcining the coated silica
substrate perfects the crystalline phase of the
a
13406'0
-6_
Sn02(Sb) coating layer which imparts the desired
electroconductive properties to the individual
particles of the composition.
According to one aspect of the process, the
substrate of amorphous hydroxylated silica or active
silica-containing material is prepared by coating a
finely divided solid core material with active silica
and then removing the core material without unduly
disturbing the silica coating. The substrate thus
10 produced comprises hollow silica particles which are
substantially translucent and which have the general
shape of the core material. It will be appreciated
that the silica coating should be sufficiently thin,
for this purpose, so as not to reflect light. This
15 will normally mean a thickness of less~than about 250
nm. For most applications, thicknesses in the range of
about 5 to 20 nm are preferred.
Active silica is conveniently prepared by
gradually neutralizing an aqueous solution of sodium
20 silicate or potassium silicate with a mineral acid,
such as, for example, sulfuric acid or hydrochloric
acid.
Active silica-containing materials may
conveniently be applied as coatings for a selected core
25 material by including other components along with the
active silica in the reacting solution. For example,
by adding sodium borate along with the sodium or
potassium silicate, a silica-boria coating may be
obtained. Such coatings are effective as a substrate
30 in practicing this invention so long as the surface of
the coating contains hydroxylated silica
functionalities. If the other component or components
present in the silica-containing substrate inhibit the
retention of hydroxyl groups on the substrate surface,
35 then the subsequent Sn02(Sb) coating may not adhere
completely and may, thus, be less effective.
_,_ :l3~Ofi70
According to another aspect of the invention,
the core material may remain encapsulated within the
amorphous silica coating so long as its presence does
not adversely affect the proposed end-use of the
5 finished composition and so long'ns it remains stable
during subsequent processing.
In a preferred embodiment,. the core is a mica
platelet with a thickness of less than 250 nm.
Platelets of this type are nearly transparent when
10 dispersed in a suitable vehicle, yet they provide
conductivity at low loadings in the vehicle. Muscovite
is a preferred form of mica for use in the invention.
In yet another aspect of the process, the
coating layer of hydrous oxides of antimony and tin is
15 applied to the hydroxylated silica substrate surface in
the presence of a grain refiner, or a mixture of grain
refiners, selected from soluble compounds of alkali
metals, alkaline earth metals, transition metals, and
rare earth elements. Alkaline earth chlorides and zinc
20 chloride are preferred. In this regard, the present
invention includes electroconductive powders which are
prepared by applying, i.e., depositing, a coating layer
of hydrous oxides of antimony and tin to the surface of
a substrate other than amorphous hydroxylated silica
25 where the deposition is accomplished in the presence of
from about 500 parts per million up to about 3 molar of
a grain refiner as defined above. The finished
composition can contain up to about 10% by weight of
the grain refiner, although a concentration of from 110
30 ppm to 1% by weight is preferred.
The composition of this invention in a
preferred embodiment comprises a powder which is
particularly useful as a pigment in paint formulations
for automotive paint systems. The finished powder of
35 this invention comprises particles capable of forming a
generally transparent conductive network within the
~34~J67p
_8_
paint film at a pigment/binder loading ratio as low as
15/100 or even lower, such that the transfer efficiency
can be improved when a subsequent coat, e.g., the top
coat, is applied electrostatically. According to one
5 aspect of the invention, the particles are shaped and
preferably needle-like which results in a generally low
pigment volume concentration within the paint vehicle.
BRIEF DESCRIPTION OF THE DRAWINGS
10 Fig. 1 is an electron micrograph which shows
a group of electroconductive particles, in the form of
shells, which have been prepared according to the
process of the invention.
Fig. 2 is an electron micrograph, at higher
15 magnification, of a fragment of a shell which is coated
with a conducting layer of antimony-containing tin
oxide crystallites according to the invention.
Fig. 3 is a sectional view of a device used
to measure dry powder resistivities of individual
20 samples of compositions which were prepared as dry
powders according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is a composition which
25 comprises a two-dimensional network of
antimony-containing tin oxide crystallites which exist
in a unique association with amorphous silica or with a
silica-containing material. The composition, when in
the form of particles, is uniquely capable of forming
30 an interconnecting conductive network when incorporated
as a component within a carrier matrix or a solution
which is applied and dried on a surface as a thin film.
The carrier matrix may take any of a variety of forms,
including paint film, fiber, or other shaped article.
35 The particles represent an association of the
two-dimensional network of antimony-containing tin
13~~670
-g-
oxide crystallites with amorphous silica or a
silica-containing material which is accomplished by the
process of this invention and comprises the steps of:
(a) providing a substrate of amorphous
hydroxylated silica or active silica-containing
material,
(b) applying an outer conductive coating
layer to the substrate surface consisting essentially
of hydrous oxide of antimony and tin, and
l0 (c) calcining the coated substrate at a
temperature in the range of 400' to 900'C. in an
oxygen-containing atmosphere.
The term silica-containing material' as used
herein means a material, i.e., a composition, such as a
metal silicate, amorphous silica-containing materials,
or, in general, a material having an extensive covalent
network involving Si04 tetrahedra. Such compositions
offer the potential for surface hydroxyl formation, a
feature believed to be important in the chemical
interaction between the silica-containing solid and the
aqueous solution of tin and antimony salts in forming
the compositions of this invention.
The term active silica-containing material
as used herein means a silica-containing composition
that has been activated by the creation of surface
hydroxyl groups. This is most conveniently achieved by
direct precipitation, from aqueous solution, of
amorphous silica, alkaline earth silicates, or
transition metal silicates such as zinc silicate onto
the surface of the core particles. Active borosilicate
compositions may also be prepared in this manner. In
general, silicate surfaces which have been dried or
heated extensively will no longer contain effective
concentrations of surface hydroxyl groups and will be
inactive. Such surfaces may, however, be reactivated
by extended treatment with reactive aqueous solutions,
~34os7o
-10_
such as hot caustic. Mica surfaces may, for example,
be activated in this manner, but this type of activated
surface typically is not as reactive with tin/antimony
intermediates as is a freshly precipitated active
5 silica coating. In general, active silica-containing
material which is prepared by direct precipitation from
aqueous solution is preferred.
Generally speaking, maximum utility for the
composition of this invention ie realized when the
l0 substrate comprises a powder, i.e., finely divided
particles which are tens of microns to sub-micron in
size. The powder particles are composed of amorphous
silica or a silica-containing material, or they are
composed of an inert core material having an amorphous
15 silica coating or a coating of a silica-containing
material.
According to one aspect of the invention, the
powder particles are shaped particles which are
somewhat elongated rather than spherical or equiaxial
20 and have an aspect ratio of from at least about 2.0 up
to about 50. An important criterion for the silica, or
silica-containing, particles is that, as a finished dry
powder, they are capable of forming an interconnecting
electroconductive network within a thin film, such as a
25 paint film, or when used as a filler in a bulk
polymeric material.
Particle shapes which are capable of forming
such an effective interconnecting network and which are
contemplated for use in this invention are selected
30 from rods, whiskers, platelets, fibers, needles, shells
and shell parts, and the like. Particles of this
invention which are equiaxial in shape may also be
used, and they may even be preferred in applications
where very high electrical conductivity is needed and
35 higher pigment/binder ratios can be tolerated.
-11-
In one aspect of the invention, the powder
particles have the shape of platelets. This shape
facilitates the particles forming an interconnecting
electroconductive network within a thin film. In a
5 preferred embodiment of the invention, the particles
are platelets of mica, with a thickness of less than
250 nm. These particles, when dispersed in selected
binders, are practically transparent, yet they provide
electrical conductivity at relatively low powder
loadings.
Polymeric materials may be conveniently
rendered conductive by filling the polymer composition
with a powder of equiaxial, i.e., generally spherical,
particles of this invention. It will be appreciated
15 that the preferred particle shape for any specific
application will depend on many factors. While
acicular particles are generally preferred for use in
paint films, and equiaxial shaped particles are
generally preferred for use in filled plastics, other
20 factors may lead to a different preference in a
specific application.
In a preferred embodiment of this invention
the substrate of amorphous silica is a hollow shell
which is prepared by coating a finely divided core
25 material with active silica and then removing the core
material which leaves behind a silica shell as the
substrate for receiving the antimony-containing tin
oxide surface coating layer. A primary function of the
core material is merely to provide a shaped particle on
30 which the amorphous silica substrate can be deposited.
The core material must, of course, largely maintain its
physical stability during the silica coating process.
Formation of the silica substrate.can be
accomplished by first suspending the core material in
35 water and then adding active silica while maintaining
the pH of the suspension at a value in the range of 8
13~p6~~
- 12 -
to 11. This procedure is described in greater detail
in U.S. Patent 2,885,366 (Iler). In general, active
silica is very low molecular weight silica, such as
silicic acid or polysilicic acid or metal silicates,
which may be added as such to the suspension, or formed
in situ as by the reaction of an acid with a silicate.
Suitable core materials are carbonates such
as, for example, BaC03 and CaC03. Other materials may
also be used provided that they will readily accept an
adherent skin of amorphous hydroxylated silica, they
have low solubility at the coating conditions, they can
be easily removed from the silica shell by a variety of
techniques including extraction, reaction and
oxidation, and/or their chemical composition will not
interfere with application of the tin oxide coating
layer. The use of BaC03, CaC03 and SrC03 as the core
material is particularly advantageous because each can
provide an in situ source of grain refiner, the
importance of which is discussed in more detail
hereinafter.
In another aspect of the invention, the core
material remains encapsulated with the shell of
amorphous silica or silica-containing material, i.e.,
it is not removed. Examples of suitable core materials
for this embodiment include Ti0" mica, Kaoline, talc,
and BaS04. In either case, the silica coating is
coherent and is bound upon the core material forming a
coating layer which is substantially uniform in
thickness from about 5 to 20 nm. In applications
where transparency is a desirable feature of the
polymer matrix or where flexibility in coloring the
polymer matrix is important, then the core material
for the electroconductive powder should have an index
of refraction no higher than that of mica.
C
~~~0070
-13-
In practice, an aqueous suspension, i.e.,
dispersion, of the desired core material is prepared,
and the dispersion is brought to a pH of 10 by adding
an appropriate amount of an alkali, such as NaOH, KOH,
5 or NH40H. The particles comprising the core material
should generally have a specific surface area (BET
method N2 adsorption) in the range of 0.1 to 50
m2/g, but for best results a specific surface area of 2
to 8 m2/g is preferred. In general, the preferred
1o surface area will be in the lower part of the above
range for high density materials and in the higher part
of the above range for low density materials.
The concentration of the core material in the
dispersion is not especially critical. It can range
15 from 100 to 400 g/liter, but for best iesults the
dispersion should be uniform. Having prepared a
dispersion of the core material, a soluble silicate,
such as sodium silicate or potassium silicate, is added
to facilitate the formation of the silica coating. A
20 convenient form of sodium silicate is a clear aqueous
solution with a Si02/Na20 molar ratio of 3.25/1 which
has been filtered to remove all insoluble residue. A
range of 2 to 50% by weight of silica based on the
amount of core material in the dispersion can be added,
25 but 6 to 25% by weight of silica is preferred. To
promote the reaction rate, the dispersion, i.e.,
slurry, is heated to a temperature in the range of
about 60' to 100'C.
The alkali component of the sodium silicate
30 or potassium silicate is next neutralized by adding a
dilute acid slowly to the slurry over a predetermined
period of time which is dictated by the amount of
silica present so as to avoid the formation of "'free"'
silica, i.e., silica particles which are not attached
35 to the core material. Mineral acids selected from
H2S04, HC1, HN03 and the like are suitable for the
:134Q670
-14-
neutralization. Acidic metal salts, such as calcium
chloride, may also be used. In this procedure, some
calcium becomes incorporated into the silica coating
and later becomes available as a grain refiner in the
5 tin oxide coating step. The larger the amount of
silica present, the longer will be the time required
for neutralization: however, a silica deposition rate
of 3% of the weight of the base powder per hour is
normally satisfactory to insure formation of the silica
10 coating layer. The important consideration is to keep
the addition rate slow enough to avoid precipitating
free silica. The slurry is then held at temperature
for at least one-half hour after neutralization to
ensure a complete reaction of the hydroxylated silica
15 coating layer. The silica coated particles can then be
isolated, washed, and dried prior to beginning the next
step of the process, or they can be retained as a
slurry, and the process continued.
Alternatively, the amorphous hydroxylated
20 silica may be prepared by simultaneously adding the
alkali silicate solution and the acid solution to a
heel, i.e., a quantity already present in the reactor,
of alkaline water containing the powder to be coated.
With this technique, the pH can be kept constant
25 throughout most of the reaction. Under certain circum-
stances, this can facilitate the uniform coating of the
silica onto the substrate.
Iiyd~oxylated silica is silica which has
hydroxyl groups on the surface. This may be obtained
30 by precipitating the silica from aqueous solution.under
alkaline conditions. Preferred amorphous hydroxylated
silicas are obtained by carrying out the precipitation
slowly (over 1-3 hours) and at elevated temperatures,
such as around 90'C. Under these processing
35 conditions, the silica is coherent, i.e., the silica
adheres to the substrate and takes the general shape of
~.34p67p
-15-
the substrate particle. Typically, particles coated
with a coherent silica coating will have a surface
area, by nitrogen adsorption, which is approximately
the same as, or slightly lower than, the area of the
5 uncoated powder. Particles with a non-coherent, e.g.,
porous, silica coating will have much higher surface
areas, as much as 10 to 100 times higher. While
coherent coatings are preferred in practicing the
invention, a moderate degree of porosity in the coating
10 is not particularly harmful. In particular, when
hollow shells are desired, a small amount of porosity
is beneficial in facilitating the extraction of the
core material.
As noted above, the formation of the
15 amorphous hydroxylated silica is preferably carried out
at a temperature of 60' to 90'C to facilitate
densification of the silica. However, lower
temperatures in the range of 45' to 75'C can be used if
a densification aid, such as, for example, B203, is
20 present in the reaction mixture.
When the process is continued from previously
dried silica coated particles, they are first
re-dispersed in water, and the resulting slurry is
heated to a temperature in the range of about 40' to
25 100'C. Next, the core material may be dissolved and
extracted by treating, for example, with an acid. This
may be accomplished by heating an aqueous slurry of the
silica coated particles to 40' to 100'C, adding
hydrochloric acid while stirring until the pH reaches a
30 value in the range or 1.5 to 3.5, but preferably the pH
should be 2.0 for best results. The core material
dissolves, leaving hollow shaped particles of amorphous
silica which are the substrates onto which the
antimony-doped tin oxide coating is applied.
~~~os7o
-16-
The core material can be extracted by other
means, such as, for example, by oxidation during
calcining where the core material is a graphite powder.
Other core materials contemplated for use according to
5 this invention include finely divided metal powders,
such as aluminum and copper, and metal oxides such as
iron oxide.
Where BaC03~is the core material, an
appropriate solvent is HCl, which dissolves the BaC03
10 liberating C02 and Ba++ ions in solution. The choice
of solvent is critical to the extent that a solvent
which will react with the core material to form an
insoluble reaction product should not be used.
As previously mentioned, according to one
15 aspect of the invention, the core material may remain
encapsulated throughout final processing. The presence
or absence of a core material in practicing the
invention may enhance certain optical or other
properties and is for the convenience of the operator.
20 In a preferred embodiment of this invention, the use of
a removable core material, especially BaC03 or CaCOg,
facilitates the formation of a shaped amorphous silica
substrate. Alternatively, any convenient source of
amorphous hydroxylated silica or hydroxylated
25 silica-containing material, preferably hydroxylated
silica, can be used as a substrate in practicing this
invention.
The outer conductive coating layer can be
applied to the amorphous hydroxylated silica substrate
30 by preparing separate aqueous solutions of hydrolyzable
tin and antimony'salts and adding them simultaneously
to the substrate slurry along with an appropriate
amount of a strong base to maintain.the pH of the
slurry in the desired range. While it is generally
35 preferred to add the tin and antimony solutions
simultaneously, and indeed they may conveniently be
~J~~~70
-17-
first mixed together and then added as one solution, it
is also possible to add the solutions sequentially.
Solvents for preparing the individual tin and antimony
salt solutions can be any solvent which dissolves the
salt without adverse reaction. However, water or
acidic aqueous solutions are preferred solvents. The
tin salt solution may conveniently be prepared by
dissolving SnC14.5H20 in water. The antimony salt
solution may conveniently be prepared by dissolving
SbCl3 in a nominal 37t aqueous solution of HC1. Sn and
Sb chlorides are the preferred salts, but other salts,
such as, for example, sulfates, nitrates, oxalates, and
acetates can be used. In general, tetravalent tin
salts and trivalent antimony salts are preferred as
starting materials. Although the concentration of the
salts in solution is not critical, it is preferred that
the concentrations are kept within the practical ranges
of 50 to 500 g of tin oxide/liter and 0.5 to 250 g
Sb/liter to facilitate uniform coating while avoiding
unnecessary dilution. According to one aspect of the
invention, the individual Sn and Sb solutions can be
combined into a single solution which is then added to
the slurry slowly over a predetermined period of time
based on the percent Sn02(Sb) being added. Typically,
a rate of 25% of the total Sn02 and Sb can be added per
hour. Rapid addition of the Sn02(Sb) solution will
result in nonuniform coating of the Sn02(Sb) onto the
silica substrate while very slow addition of the
Sno2(Sb) solution will unnecessarily prolong the
operation. The temperature of the slurry during
deposition of the antimony-doped tin oxide coating
layer is maintained in the range of 25' to 100'C under
continuous agitation.
In a preferred embodiment, and a critical
feature of the invention, simultaneously with the
addition of the salts to the slurry, the pH of the
13~0~70
-18-
system is kept constant at a value of from 1.5 to 3.5,
and most preferably at 2.0, by adding alkali, e.g.,
NaOH, KOH, or the like during the addition. In this pH
range the active, or hydroxylated, silica surface of
5 the substrate becomes very receptive to an association
with, i.e., the deposition of, hydrous oxides of tin
and antimony. Brief excursions of pH to levels above
or below the 1.5 to 3.5 range are generally not
harmful, but extensive processing substantially outside
10 this range will degrade the continuity of the
two-dimensional network of antimony-doped tin oxide
crystallites and, thus, will adversely affect the
conductive properties of the resulting powder. The tin
and antimony salts hydrolyze and deposit on the surface
15 of the silica and form a generally uniform layer
typically having a thickness in the range of about 5 to
20 nm, and more typically a thickness of about 10 nm.
After calcination, the Sn02(Sb) crystals are typically
about 10 nm in diameter, but individual crystals may be
20 as large as 20~nm in diameter or larger. It will be
appreciated that some crystallites may be significantly
larger than 20 nm, ranging up to 50 or 6o nm. The
limited quantity of these larger crystallites does not
affect the overall translucency of the powder. It has
25 been observed that as the quantity of
antimony-containing tin oxide in the outer coating
layer increases, the resistivity of the finished dry
powder will decrease, i.e., the conductivity will
increase. Generally, the antimony content of the tin
30 oxide layer can range from 1 to 30~ by weight, but best
results are achieved when the antimony content is about
l0~ by weight.
The coated particles obtained in this manner
are then isolated by any convenient solid-liquid
35 separation procedure, such as, for example, by
filtration, and then washed free of salts with water
1340670
-19-
and dried. Drying can be conveniently accomplished at
a temperature of up to about 120'C: however, drying is
optional if the particles are to be calcined
immediately following isolation and washing.
The isolated particles~are next calcined in
an oxygen-containing atmosphere at a temperature in the
range of from 400' to 900'C, preferably 600' to 750'C,
for a period of time sufficient to develop the
crystallinity of the tin oxide phase and establish the
conductivity. The time required will depend on the
temperature and on the geometry of the furnace and do
processing conditions. In a small batch furnace, for
example, the time required for calcination is typically
from 1 to 2 hours. Calcination is critical to the
process of the invention because it serves to perfect
the crystal phase of the antimony-containing tin oxide
outer coating layer which, in turn, imparts the
electroconductive properties to the particles.
In yet another aspect of the invention, the
conductive properties of the composition can be
enhanced by accomplishing the deposition of the
antimony-containing tin oxide outer coating layer in
the presence of a grain refiner, or a mixture of grain
refiners, selected from alkali metals, alkaline earth
metals, transition metals and rare earth elements which
enhance the uniformity of Sn02 deposition on the Si02
surface and minimize grain growth during subsequent
calcination. The exact function of the grain refiners
is not entirely understood, but concentrations of as
little as 50o parts per million or up to about 3 molar
or higher of a grain refiner, or mixture of grain
refiners, in the slurry during the deposition of the
tin oxide conducting phase results, after calcination,
in improved electroconductive properties of the
composition. Preferred grain refiners are soluble
salts of Ba, Ca, Mg, and Sr, although soluble salts of
~13~0670
-20-
alkali metals, rare earth metals, other alkaline earth
metals and certain transition metals, such as Fe and
Zn, are expected to produce satisfactory results.
When the coating layer of hydrous antimony
and tin oxides~is to be applied according to the
process of the invention in the presence of a grain
refiner as defined above, it has been found that
substrates other than amorphous hydroxylated silica,
such as a substrate selected from BaS04, 8rS04, CaS04,
10 graphite, carbon, and Ti02, can be used which yield
powders having unexpected electroconductive properties.
Preferred grain refiners for such substrates are
selected from Ca++, Ba++, and Sr++. Such non-silica
substrates are generally powders which have a low
15 solubility under the reaction conditions used to apply
the coating of hydrous antimony and tin oxides.
Suitable substrates are also inert and generally
unreactive with the antimony and tin oxides during
calcination. Electroconductive powders based on a
20 non-silica substrate will generally contain from about
100 parts per million, or more, of the grain refiner,
or mixture of grain refiners.
The electroconductive powders of this
invention are characterized by a high surface area, as
25 determined by nitrogen adsorption, relative to the
surface area that would be expected for the average
particle size.as observed by electron microscopy. As
previously noted, the electroconductive powder of this
invention is typically submicron to tens of microns in
30 particle size. As observed under an electron
microscope, the silica surface is seen to be densely
populated with fine crystallites of antimony-doped tin
oxide, each crystallite typically in the range of 5 to
20 nm. This crystallite size range is confirmed by
35 X-ray diffraction line broadening. The high surface
area results from the population of fine crystallites.
~3~os7o
-21-
The actual surface area, as measured by nitrogen
adsorption, is typically in the range of 30 to 60 m2/g.
Referring now to the Figures, Fig. 1 is an
electron micrograph which shows a group of
5 electroconductive particles, in the form of shells,
which have been prepared according to the invention.
Three lighter areas can be seen which are believed to
be holes in the shells~which were formed as. the core
material was being removed during processing. The
to surfaces of the shells, seen in a somewhat
cross-sectional view, are uniformly coated with a
two-dimensional network of antimony-doped tin oxide
crystallites. Fig. 2 is an electron micrograph, at
higher magnification, of a fragment of a shell which
15 has been prepared according to the invention. The
two-dimensional network of antimony-doped tin oxide
crystallites can be seen in this view. Some of the
crystallites appear very dark, while others appear as
various lighter shades of grey to near-white. This
20 variation is due to the random orientation of the
crystallites on the silica surface and does not
indicate a variation in composition.
Figs. 1 and 2 show closely packed
antimony-doped tin oxide crystallites on the surface of
25 the amorphous silica with the result that the
interstices, i:e., pores, between the crystallites are
very small. Thus, electrical resistance between
crystallites,- and between individual coated particles
which are in contact, is minimized. The equivalent
30 pore diameter, as measured by nitrogen
adsorption/desorption is below 20 nm, and preferably
below 10 nm.
The electroconductive powders of this
invention are further characterized by a low
35 isoelectric point, e.g., in the range of from 1.0 to
4.0, typically 1.5 to 3Ø By contrast, antimony-doped
1340670
-22-
tin oxide powders, prepared in the absence of silica,
will have an isoelectric point substantially below 1.0,
and typically below 0.5. The silica itself has an
isoelectric point of from 2 to 3.
Electroconductive powder samples which were
prepared according to this invention were evaluated by
comparing dry powder resistances. A relative
comparison of dry powder samples is made possible so
long as the particle size and shape do not vary
to substantially among the samples. Generally, the lower
the relative resistance in dry powder evaluation, the
lower the resistivity in an end-use system, although
many other factors, such as, for example, the ability
to form an interconnecting network in the end-use
carrier matrix or vehicle system, may also affect
end-use conductance.
In an end-use paint primer system, the
electroconductive powder of this invention can be
evaluated by measuring the surface conductivity of the
dry paint film in which the powder has been
incorporated as a component of the paint pigment. A
simple meter has been developed by the Ransburg
Corporation to measure the surface conductivity of
paint films. This meter, which fs known as the
Ransburg Sprayability Meter, is calibrated in Ransburg
Units (RU's) from a value of 65 to a value of 165. Any
paint film which demonstrates a surface conductivity of
more than 120 RU's is considered to have satisfactory
surface conductivity.
The dry powder technique which was used for
early evaluations of the conductive powder of this
invention utilizes a device as shown in partial section
in Fig. 3. The device comprises a hollow cylinder 10
of a non-conducting material, such as plastic, having a
copper piston 12 located at one end and held in place
by an end cap 14. A copper rod 16 of a predetermined
13~~~~~
-23-
length shorter than the cylinder is placed inside the
cylinder in contact with the piston as shown, and a
powder sample to be measured 18 is placed in the hollow
portion of the cylinder which remains. A second
end-cap 20 is placed over the end of the cylinder which
contains the powder sample, and copper leads are
attached to the ends of the cylinder for connection to
an ohm meter. In practice, the copper piston drives
the copper rod to compress the individual powder
samples to a given compaction, and resistivity is
measured by the ohm meter for each sample. In the
examples described below, the relative resistances were
measured by filling the cylindrical cavity (0.64 cm in
diameter by 1.72 cm long) with powder, and tightening
the end-caps manually to compress the powder.
The electroconductive composition of this
invention and its method of preparation are illustrated
in more detail in the following examples. For
convenience, the examples are summarized in Table d.
25
35
130670
-24-
TABLE 1
Ac id carce
S
l~wmpl. tillcateCory silica Cors Iausediate
po- Eauree 14t-rialrialDanosltion Isolation
5 1 la2Si0j EaC03 8ZS0~ SC1 liltar
s s2sio3 c.cos scl acl riltsr
3 ~SiOS CaC03 8C1 /C1 Decant
4 lsZSiOI iaCOy 1C1 1C1 Daoaat
5 L1S103 8a0D; xl 1C1 Tiltsr
1 0 6 laZSiO~ Ii02 1250 loos liltar
(v i
v/o
CaOD~)
7 BaZSi03 EaSO' 1250, ona liltar
8 LjSi03 !pt t1028C1 ions BOae
9 i25i03 Tpt fi02HC1 loos lone
1 5 10 EZS103 saC03 HC1 HC1 ~ liltar
v E203
ii Boas laSO' tons ion, loan
12 L1S103 Rica 8C1 ane 80~
l~ iZSi03 Lolinlte8C1 lane Hone
20
Example 1
(A) In an 18-liter, agitated polyethylene
beaker, 3 liters of water were brought to a pH of 10.0
with sodium hydroxide. A stock solution of sodium
25 silicate was prepared and filtered to remove insoluble
material. The stock solution has a Si02/Na20 molar
ratio of 3.25/1, and contained 398 g of Si02 per liter
of solution. 65 ml of this solution were added to the
18-liter beaker. Thereafter, 1350 g of BaC03, which
30 had been predispersed in one liter of water, was added
to form a slurry. The slurry was heated to 90'C in
one-half hour by the introduction of steam, after which
the pH was 9.7. Next, a sodium silicate solution and a
sulfuric acid solution were simultaneously added~over a
35 period of 3 hours, while stirring the slurry vigorously
and while maintaining the pH at 9Ø The sodium
~340~7~
-25-
silicate solution was prepared by diluting 342 ml of
the above sodium silicate stock solution to 600 ml with
water. The sulfuric acid solution was prepared by
diluting 69 g of 96% H2S04 to 600 ml with water. All
5 of the sodium silicate solution was added to the
slurry. Sufficient sulfuric acid was added to maintain
the pH at 9Ø After the simultaneous addition was
complete, the slurry was then digested at 90'C for
one-half hour, and the resulting silica-coated BaC03
10 particles were isolated by filtration, washed with
water to remove soluble salts, and dried overnight at a
temperature of 120'C. 1485 g of dry powder were
recovered.
(B) In a 3-liter, agitated glass flask, 250
15 g of the powder prepared in (A) above~were dispersed in
1 liter of water, and the resulting slurry was heated
to a temperature of 90'C. 164 ml of nominal 37% HC1
was then added slowly to the slurry which. lowered the
pH to a value of 2.0 and dissolved the BaC03 material.
20 Next, a SnCl4/SbCl3/HC1 stock solution was prepared by
dissolving SnC14.5H20 in water and dissolving SnCl3 in
nominal 37% HCl. These were combined in a ratio to
give the equivalent of 10 parts of Sn02 to 1 part of
Sb, and diluted with water to yield a solution
25 containing the equivalent of 0.215 g Sn02/ml and 0.0215
g Sb/ml. 256 ml of this Sn/Sb/HC1 solution was then
added to the slurry over a period of 2 hours
simultaneously with sufficient 10% NaOH to maintain the
pH of the slurry at 2Ø The slurry was digested for a
30 half-hour at pH = 2.0 and at a temperature of 90'C, and
then the resulting particles were filtered, washed to
remove soluble salts, and dried overnight at a
temperature of 120'C. The dried particles, which
comprised a powder, were then calcined in air at.750'
35 for 2 hours. 106 g of dry powder were recovered. The
finished powder product had a dry powder resistivity of
-26- ~~~0~~0
ohms. By X-ray fluorescence analysis, the powder was
found to contain 46% Sn (as Sn02), 22% Si (as Si02),
18% Ba (as Ba0), and 4% Sb (as Sb203). This powder,
when examined under the electron microscope, was found
5 to consist of hollow shells of silica with fins
crystallites of antimony-doped tin oxide forming a
uniform, two-dimensional network on~the surface of the
silica. The powder was formulated with a test paint
carrier at a pigment/binder loading of 25/100 and
10 applied to a test surface. The resulting dry paint
film exhibited a surface conductivity of 140 Ransburg
units.
Example 2
15 (A) In an 18-liter, agitated polyethylene
beaker, 3 liters of water were brought to a pH of 10.0
with NaoH. A stock solution of potassium silicate was
obtained having a Si02/K20 molar ratio of 3.29 and
containing 26.5% Si02 by weight. 100 g of this stock
20 solution were added to the solution in the 18-liter
beaker, and, thereafter, 1350 g of precipitated CaC03
powder, with a surface area of 4 m2/g, were added to
form a slurry. The slurry was heated to 90'C in
one-half hour by the introduction of steam, after which
25 the pH was 9.7. Next, 3875 g of the above potassium
silicate stock solution were diluted with 1000 ml of
water and added to the slurry over a period of 5 hours.
The pH was maintained at 9.0 during the addition by the
simultaneous addition of hydrochloric.acid. 262 g of
30 37% HCl, diluted to 1000 ml with water, were required
to maintain the pH at 9Ø The slurry was then
digested at 90'C for one-half hour, after which the pH
of the slurry was adjusted to a value of 7.0 by the
addition of hydrochloric acid, and the resulting
35 silica-coated particles were isolated by filtration,
1340670
-27-
washed to remove soluble salts, and dried at 120'C for
24 hours. 1607 g of powder were recovered.
(B) In a 3-liter, agitated glass flask, 250
g of powder prepared in (A) above were dispersed in 1
liter of water, and the resulting slurry was heated to
a temperature of 90'C. 355 ml of nominal 37% HCl were
then added to the slurry to adjust the pH to 2.0 and to
dissolve the core material. Next, an aqueous solution
of SnCl4, SbCl3 and HC1 was prepared by combining 158
10 ml of an aqueous SnCl4 solution containing the
equivalent of 0.286 g Sn02/ml, with 20 ml of an aqueous
HCl solution of SbCl3, containing the equivalent of
0.235 g Sb/ml. This solution was added to the slurry
over a period of 2 hours, simultaneously with
15 sufficient 10% NaOH to maintain the pH of the slurry at
2Ø The slurry was digested at a temperature of 90'C
and pH of 2.0 for one-half hour, and then the resulting
particles were filtered, washed with water to remove
soluble salts, and calcined at 750'C for 2 hours. The
20 finished powder product had a dry powder resistance of
18 ohms. When analyzed by X-ray fluorescence, the
powder was found to contain 48% Sn (as Sn02), 47% Si
(as Si02), 6% Sb (as Sb203)) and 0.3% Ca (as Ca0).
When examined under the electron microscope, the powder
25 was found to consist of hollow shells of silica and of
fragments of shells of silica, with fine crystallites
of antimony-doped tin oxide forming a uniform,
two-dimensional network on the surface of the silica.
By transmission electron microscope, the average
30 antimony-doped tin oxide crystallite size was found to
be 9 nm. By X-ray diffraction line broadening, the
crystallite size was 8 nm. The powder had a surface
area, by nitrogen adsorption, of 50 m2/g and an average
pore size of 7.7 nm. The powder had a specific gravity
35 of 3.83 g/cc and a bulk density of 0.317 g/cc.
_28_ 1340670
25.9 g of a high solids polyester/melamine/
castor oil resin and 12.3 g of the dry powder of this
example were added to a 4 oz. glass jar to form a mill
base. The jar was sealed and shaken for 5 minutes on a
5 paint shaker. 8.5 g of butanol/xylene/diisobutyl
ketone solvent and 160 g of 20-30 mesh zirconia beads
were added to the jar, and it was shaken for an addi-
tional 10 minutes. The zirconia beads were then
removed by screening, and 22.8 g of mill base were
10 recovered. A 9.7 g sample of this mill base was then
diluted with 7.6 g of resin to give a slurry having a
pigment (dry powder)/binder ratio of 15/100.
0.06 g of catalyst (Cycat 600, a
dodecylbenzenesulfonic acid catalyst in a dimethyl
15 oxazoladine solvent) were added and the slurry was
stirred. A slurry, i.e., paint, film was then cast on
a glass plate using a draw-down blade with a 0.015 mil
gap. The film was cured by heating to 163'C for
one-half hour. The resulting cured film had a Ransburg
20 reading of 158.
A repeat of the procedure using 10.8 g of the
mill base diluted with 4.4 g of binder to give a
pigment/binder ratio 20 was also done. 0.05 g of
catalyst were added, and a film was prepared as
25 described above. The resulting cured film had a
conductivity which exceeded the maximum Ransburg
reading of 165 units.
The filtrate, obtained when the coated powder
was filtered from the reaction slurry, was analyzed for
30 Sn and Sb by inductively coupled plasma spectra and
found to contain less than 1 part per million (the
detection limit of the method) of each element.
18 g of the conductive powder, prepared
above, were mixed with 77.7 g of a commercial vinyl
35 acrylic latex paint and 6 g of water. The ingredients
were first mixed together manually and then mixed in a
134os7o
-29-
commercial paint shaker for 10 minutes, using 160 g of
20-30 mesh zirconia beads. The resulting paint was
drawn down on commercial corrugated cardboard at a
thickness of approximately 2 mils. After drying the
5 painted surface had a Randsburg reading of over 120
unites.
Example 3
(A) In an 18-liter, agitated polyethylene
beaker, 3 liters of water were brought to a pH of 10.0
with NaOH. 100 g of potassium silicate (26.5% Si02)
were added to form a solution. Thereafter, 1350 g of
CaCO3, which had previously been dispersed in 1 liter
of water, were added. The slurry was heated to 90'C in
15 one-half hour by the introduction of steam, after which
the pH was 9.9. Next, 1027 g of potassium silicate
solution (26.5% Si02), predispersed in 1 liter of
water, and 262 ml of nominal 37% HCl, diluted to 1
liter with Water, were added simultaneously to the
20 slurry over a period of 5 hours. The pH was maintained
at 9.0 during the addition of the two solutions. The
slurry was then digested at 90'C for one-half hour, the
pH was adjusted to 7.0 with hydrochloric acid, and,
after sedimentation, the supernatant was decanted and
25 the resulting mixture reheated to 90'C.
(B) Next, nominal 37% HC1 was added until
the pH reached 2Ø 1016 ml of an aqeuous SnCl4
solution containing the equivalent of 0.286 g Sn02/ml,
and 129 ml of an SbCl3/HC1 solution, containing the
3o equivalent of 0.235 g Sb/ml were combined and added to
the slurry over a period of 2 hours simultaneously with
sufficient 30% NaOH to maintain the pH of the slurry at
2Ø The slurry was digested at a temperature of 90'C
for one-half hour, and the resulting particles were
35 filtered, washed with water to remove soluble salts,
and then calcined at a temperature of 750'C for 2
-30- 13406'70
hours. The finished powder product had a dry powder
resistance of 3 ohms. By X-ray fluorescence analysis,
the powder was found to contain 46% Sn (as Sn02), 47%
Si (as Si02), 6% Sb (as Sb203) and 0.2% Ca (as Ca0).
'
Examgle 44
(A) In an 18-liter, polyethylene beaker, 3
liters of water Were brought to a pH of 10.0 with NaOH.
90 g of sodium silicate, in the form of the stock
solution of Example 1, were added to form a solution
and, thereafter, 1350 g of calcined BaC03, with a
surface area of 2.3 m2/g, were added. The slurry was
heated to 90'C in one-half hour, after which the pH was
9.7. Next, 343 ml of the sodium silicate stock solu-
tion of Example 1 were diluted to 600.m1 with water and
added to the slurry over a period of one-half hour.
Then 143 ml of nominal 37% HC1, diluted to 600 ml with
water, were added to the slurry over a period of 3
hours, until the pH reached 7Ø The slurry was then
digested at a temperature of 90'C for one-half hour at
a pH of 7Ø Next, after sedimentation, the
supernatant was decanted, and the remaining mixture was
reheated to 90'C.
(8) Nominal 37% HCl was then added until the
pH of the reaction mass reached 2Ø Next, 909 ml of
an SnCl4 solution were prepared which contained the
equivalent of 0.286 g Sn02/ml, and 111 ml of an SbCl3
solution were prepared which contained the equivalent
of 0.235 g Sb/ml, and these solutions were mixed
together and added to the slurry over a period of 2
hours, while simultaneously adding 30% NaOH to maintain
the pH at a value of 2Ø The slurry was digested for
one-half hour at a temperature of 90'C and a pH of 2Ø
The resulting particles were then filtered, washed with
water to remove soluble salts, and calcined at a
temperature of 750'C for 2 hours. The finished powder
1340670
-31-
had a dry powder resistance of 4 ohms. 480 g of powder
were recovered. By X-ray fluorescence analysis, the
powder was found to contain 54.2% Sn (as Sn02), 33.8%
Si (as Si02), 6.4% Sb (as Sb2O3), and 4.6% Ba (as Ba0).
Example 5
(A) In an 18-liter, polyethylene beaker, 3
liters of water were brought to a pH of 10.0 with
sodium hydroxide. 100 g of the potassium silicate
10 stock solution of Example 2 were added, followed by
1350 g of barium carbonate powder, with a surface area
of 2.3 m2/g. The slurry was heated to 90'C in one-half
hour, at which time the pH was 9Ø 515 g of the
potassium silicate stock solution were diluted to 600
15 ml with water and added to the agitated slurry over a
period of one-half hour. 139 ml of nominal 37% HC1
were diluted to 600 ml with water, and added to the
agitated slurry over a period of 3 hours,~at which time
the pH had dropped to 7. The slurry was held at 90'C
20 and a pH of 7 for one-half hour. The product was then
filtered, washed free of soluble salts, and dried at
120'C. 1498 g of powder were recovered.
(B) 250 g of the powder prepared in (A)
above was dispersed in 1 liter of water by mixing in a
25 high speed blender for 2 minutes. The slurry was
heated to 90'C and nominal 37% HC1 was added until the
pH had dropped to 2. 185 ml of the nominal 37% HC1
were required. A SnCl4/SbCl3/HC1 stock solution was
prepared as in Example 1, but containing the equivalent
30 of 0.254 g of Sn02/ml and 0.064 g Sb/ml of solution.
178 nl of this solution was added to the stirred slurry
over a period of 3 hours, along with sufficient 10%
NaOH to maintain the pH at 2. The slurry was then held
at 90'C and a pH of 2 for an additional one-half hour.
35 The product was filtered, washed free of soluble salts,
and dried at 120'C and calcined in air at 750'C for 2
1340670
-32-
hours. 79 g of powder were recovered. This powder had
a dry powder resistance of 22 ohms. It had a surface
area, by nitrogen adsorption, of 49.8 m2/g and an
average pore diameter of 9.4 nm. When examined under
5 the electron microscope, the product was found to
consist of hollow shells of silica with fine
crystallites of antimony-doped tin oxide forming a
uniform, two-dimensional network on the surface of the
silica. By transmission electron microscopy, the
l0 average crystallite size was 10 nm. By X-ray
diffraction line broadening, the average crystallite
size was 8 nm. By X-ray fluorescence analysis, the
powder contained 57% Sn (as Snot), 34% Si (as Si02), 7%
Sb (as Sb203), and 1.3% Ba (as Ba0). the powder had a
15 specific gravity of 4.31 g/cc and a tapped bulk density
of 0.333 g/cc. The powder had an isoelectric point of
2.3.
Example 6
20 (A) In an agitated, 18-liter polyethylene
beaker, 3000 g of 97% pure rutile titanic powder, with
a 6.8 m2/g surface area, were dispersed in 6 liters of
water. The pH was brought to 10.0 with NaOH. 454 ml
of the sodium silicate stock solution of Example 1 were
25 added to the agitated slurry. The slurry was heated to
90'C in one-half hour by the direct introduction of
steam. Then,. 10% sulfuric acid was added gradually
over a period of 2 hours, until a pH of 7 was reached.
The slurry was then held at 90'C and a pH of 7 for an
30 additional one half-hour, and the resulting
silica-coated titanic particles were isolated by
filtration, washed to remove soluble salts, and dried
overnight at a temperature of 120'C. 3108 g of powder
were recovered.
35 (B) 100 g of the powder prepared in (A)
above was dispersed in one liter of water, using a high
-33-
speed mixer. The slurry was transferred to an
agitated, 3-liter glass flask and 200 g of barium
carbonate powder were added. The slurry was then
heated to 90'C and the pH was adjusted to 2.0 by the
5 addition of hydrochloric acid. Then, 197 ml of a
SnCl4/SbCl3/HC1 solution were added to the slurry over
a period of 2 hours, t~hile maintaining the pH at 2.0 by
the simultaneous addition of a 10% NaOH solution. The
SnCl4/SbCl3/HCl solution contained the_equivalent of
10 0.254 g Sn02/ml, 0.0262 g Sb/ml and was prepared as in
Example 1. The slurry was held an additional one
half-hour at 90'C and pH 2.0, after completion of the
simultaneous additions. The resulting particles were
filtered, washed to remove soluble salts, and dried
15 overnight at a temperature of 120'C. The powder was
then calcined in air at 600'C for 2 hours. 155 g of
powder were recovered. The dry powder resistivity was
3 ohms. By X-ray fluorescence analysis, the powder
contained 32% Sn (as Sn02), 4% Si (as Si02), 4% Sb (as
20 Sb203), and 60% Ti (as Ti02). Examination of the
powder under the electron microscope revealed that the
titania particles were coated with silica, and that the
silica surface was coated with fine crystallites of tin
oxide. The crystallites of antimony-containing tin
25 oxide were uniformly dispersed as a two-dimensional
network on the silica surfaces. The isoelectric point
of this powder was determined to be 3.1. The surface
area, by nitrogen adsorption, was 15.4 m2/g and the
average pore diameter was 9 nm. By X-ray diffraction
30 line broadening, the tin oxide crystallite size was
determined to be 15 nm. By transmission electron
microscope, the average antimony-doped tin oxide
crystallite size was determined to be 9 nm. The
finished product had a dry powder resistance of 3.2
35 ohms.
13~0~70
- 34 -
30 grams of the calcined powder were then
incorporated into 70 grams of low density polyethylene
by blending and extruding through a Banbury mill. The
polyethylene resin had a melting point of 105°-107°C,
and the mixture was blended in the mill for 2 minutes
at 110°-120°C at 230 rpm. The mixture was extruded at
a ram pressure of 50 - 60 psi, and the extruded blend
was pressed into sheets of 10 mil thickness. The
sheets had a specific conductance of 0.68 ohm-cm.
Example 6 was repeated without the addition
of BaC03 in part B, and the dry powder resistance
increased to 166 ohms. Examination of the powder under
the electron microscope showed less complete
development of the two-dimensional network of tin oxide
crystallites on the silica surface. The isoelectric
point of this powder was 5.0, and the surface area was
20.4 mz/g. The Sn02 crystallite size, by X-ray line
broadening, was 11 nm.
Example 6 was repeated, but with both the
silica coating and the BaC03 eliminated from the
procedure. The dry powder resistance of the resulting
powder was 3000 ohms, and examination of the powder
under the electron microscope showed incomplete
development of the surface net-work of tin oxide
crystallites. Much of the tin oxide appeared to have
entered into a solid solution with the titania.
Example 7
(A) In an agitated, 18-liter polyethylene
beaker, 3000 g of barium sulfate (Blanc Fixer"') with a
surface area of 3.3 m'-/g, were dispersed in 6 liters of
water. The pH was adjusted to 10.0 with sodium
hydroxide, and 454 ml of the stock sodium silicate
solution from Example 1 were added. The slurry was
heated to 90°C in one-half hour by the introduction of
steam. Then, 10% sulfuric acid was added at the rate
c
1~~0670
-35-
of 100 ml/hr until the pH reached 7Ø The particles
were filtered, washed to remove soluble salts, and
dried overnight at 120'C. 3130 g of dry powder were
recovered.
5 (B) In an 18-liter, agitated polyethylene
beaker, 500 g of the powder prepared in (A) above and
500 g of CaC03 were dispersed in 5000 ml of water. The
slurry was heated to 90'C and the pH adjusted to 2.0
with hydrochloric acid. 325 ml of .a SnCl4/SbCl3/HC1
10 solution were then added to the slurry over a period of
2 hours, while maintaining the pH at 2.0 by the
simultaneous addition of a 10% solution of NaOH. The
temperature was maintained at 90'C throughout this
addition. The SnCl4/SbCl3/HC1 solution was prepared as
15 in Example 1 and contained the equivalent of 83 g Sn02
and 8.3 g of Sb. The slurry was held at 90'C and a pH
of 2.0 for an additional half-hour. The product was
then filtered, washed to remove soluble salts, dried
overnight at 120'C and calcined in air at 750'C for 2
20 hours. 557 g of product were recovered, having a~dry
powder resistance of 12 ohms. By X-ray fluorescence
analysis, the powder contained 14% Sn (as Sn02), 2% Sb
(as Sb203), 5% Si (as Si02) and 79% Ba (as BaS04).
The Example was repeated without the addition
25 of calcium carbonate, and the dry powder resistivity
was 1200 ohms. The Example was again repeated without
either the silica coating or the calcium carbonate
addition, and the dry powder resistance increased to
1400 ohms.
Example 8
2 liters of deionized water were placed in a
3-liter beaker and heated to 90'C. 25 g of CaCl2 were
added to the bath. Over a period of 2 hour, 400 g of
35 potassium silicate solution, with a Sf02/K20 molar
ratio of 3.29/1 and containing 24% Sio2 by weight, were
-36- 134070
added to the solution while maintaining the pH at 9.5
with nominal 37% HCl. Good agitation was maintained
during the silica precipitation. Following the
addition of the potassium silicate solution, the pH was
5 adjusted to 7.0 with HC1 and held 'for one-half hour.
The pH was then lowered to 2.0 with concentrated HC1.
A solution of SnCl4/SbCl3 was prepared as follows:
2000 g of SnC14.5H20 were dissolved in water and
adjusted to a total volume of 3000 ml. 250 g of SbCl3
10 were dissolved fn 500 ml of nominal 37% HCl. For the
stock solution, 600 ml of the SnCl4 6olution, along
with 73 ml of the SbCl3 solution, were mixed together.
The stock solution was added to the calcium modified
silica slurry over a 2 hour period, while maintaining
15 the slurry at a pH of 2.0 by the addition of 20% NaOH.
The temperature was maintained at 90'C. After a
half-hour cure, the product was isolated by filtering
and washed free of soluble salts. The product was then
dried for 12 hours at 120'C. The dried product was
20 then calcined i~n a silica dish at 750'C for 2 hours.
296 g of dry powder were recovered. The surface area
of the dried product was 80 m2/g, and the surface area
of the calcined product was 48 m2/g. The calcined
powder had a dry resistance of 6 ohms. The powder
25 composition, reported as oxides, was 55% Sn02, 7%
Sb203, 3?% Si02, and 0.3% CaO. When examined under the
electron microscope, the powder was found to consist of
particles of silica with fine tin oxide crystallites
dispersed in a continuous two-dimensional network on
30 the surface of the silica. The powder had an
isoelectric point of 2.3.
When the above Example was repeated without
the calcium chloride, the dry powder resistance was 8
ohms. The calcined powder had a surface area of X60
35 m2/g.
~.3~os~o
-37-
Example 9
2 liters of deionized water were placed in a
3-liter beaker and heated to 90'C. 15 g of Ba(OH)2.H20
were added to the heated water. Over a period of 2
5 hours, 400 g of the potassium silicate solution of
Example 8 were~added to the solution while maintaining
the pH at 9.5 with nominal 37% HCl.. Good agitation was
maintained during the silica precipitation. Following
the addition of the potassium silicate solution, the pH
10 was adjusted to 7.0 and held for one-half hour. The pH
was then lowered to 2.0 with nominal 37% HC1. A stock
solution of SnCl4/SbCl3 was prepared as follows: 2000
g of SnC14.5H20 were dissolved in water and adjusted to
a total volume of 3000 ml with deionized water. 250 g
15 of SbCl3 were dissolved in 500 ml of nominal 37% HC1.
600 ml of the SnCl4 solution and 73 ml of the SbCl3
solution were mixed together for the stock solution for
addition to the precipitated silica. The. stock
solution was added over a 2 hour period at a pH of 2
20 and 90'C, using a 20% NaOH solution to control the pH.
After a half-hour cure, the product was isolated by
filtering and washed free of soluble salts. The
product was dried for 12 hours at 120'C. The dried
product was calcined in air in a silica dish at 750'C
25 for 2 hours. 295 g of the dry powder were recovered.
The surface area of the dried product was 83 m2/g, and
the surface area of the calcined product was 39 m2/g.
The powder composition, reported as oxides, was 58%
Sn02, 7% Sb203, 35% Si02, and 0.4% BaO. The powder had
30 an isoelectric point of 2Ø
This Example was repeated without the
presence of silica or barium by simply adding the
SnCl4/SbCl3/HC1 stock solution to water at 90'C, while
maintaining the pH at 2.0 by the addition of NaOH. The
35 resulting dry powder had an isoelectric point of 0.5.
_.
- 38
Example l0
(A) 3000 ml of determined water was placed
in a paddle beaker equipped with a paddle stirrer. The
pH was adjusted to 10.5 with a 20% NaOH solution, and
the temperature of the mixture was raised to 75°C using
a hot plate. Separately, a stock coating solution was
prepared by mixing together 615 g of potassium silicate
solution (24~ Si02) with 200 g of NazB204.8Hz0. 150 g of
the stock coating solution were added to the stirred
solution in the 5-liter beaker over a period of 2
minutes. Immediately following the addition of the
stock coating solutions, 1350 g of BaC03 powder was
added over about a 2 minute period. The remainder of
the stock coating solution (665 g) was then added to
the slurry. Over a period of 3 hours, while
maintaining a temperature of 75°C, a total of 1660 ml
of 6N HC1 were added to the stirred slurry. When the
HC1 addition was completed, the slurry was held at pH 7
and 75°C for one-half hour. The SiO~/BZO~ coated BaC03
was isolated by filtering with a coarse sintered glass
filter. The product was washed with deionized water to
7000 micromhos and then dried 12 hours at 120°C. The
product coma fined 12 % S i02/ B20, .
(b) 250 g of the BaC03 powder, coated with
12% SiO.,/Bz03 as prepared in (A) above, were placed in a
Waring'''" blender with 500 ml of deionized water and
blended for 2 minutes. The material was added to 1300
ml of water in a 4-liter beaker equipped with a paddle
stirrer. The slurry was heated to 60°C and nominal 37%
HC1 was added dropwise to the stirred slurry to remove
the BaCO~ core. 187 ml of nominal 37% HC1 were
required. The pH stabilized at 2.0 when all the
available BaC03 had been removed. A stock solution of
SnCl4/SbCl3 was added to the slurry at pH 2.0, over a
period of 2 hours. The pH was maintained at 2.0 by
simultaneously adding a 20g solution of NaOH. The
C
1340670
-39-
product was then filtered and washed to 7000 micromhos.
The washed product was dried for 12 hours at 120'C, and
calcined in air for 2 hours at 75o'C. 84 g of dry
powder were recovered. In the dry powder cell, the
5 product had a resistance of 8 ohms. The product had a
surface area of 128.9 m2/g.
EXAMPLE 11
(A) 300 g of barium sulfate (Blanc Fix) were
dispersed in one liter of water in a 3-liter agitated
glass flask and heated to 90'C. Over a period of 2
hours, 197 ml of an SnCl4/SbCl3/HC1 solution, contain-
ing the equivalent of 50 g of Sn02 and 5.0 g of Sb, and
prepared according to the procedure of Example 1, was
15 added to the slurry. When the pH reached 2, 10% sodium
hydroxide was added along with the SnCl4/SbCl3/HC1
solution to maintain the pH at 2 for the remainder of
the addition. The slurry was then held an additional
one-half hour at a pH of 2 and at a temperature of
20 90'C. The product was filtered, washed free of soluble
salts, and calcined in air at 750'C for 2 hours. 354
g of dry product were recovered.
(B) Part (A) was repeated, except that 333
g of CaCl2 were dissolved in the one liter used to form
25 the BaS04 slurry. 354 g of dry product were recovered.
(C) 3000 g of BaS04 were dispersed in 6
liters of water in an 18-liter agitated polyethylene
beaker. The pH was adjusted to 10.0 by the addition of
10% NaOH. 628 g of a sodium silicate solution,
30 containing 28.7% Si02 and 8.9% Na20 were added, and the
slurry was then heated to 90'C in one-half hour. The
pH was then 10.15. A 25% H2S04 solution was then added
at a rate of 100 ml/hour until the pH reached 7Ø The
slurry was held at pH 7 and 90'C for one-half hour.
35 The resulting product was filtered, washed free of
1340670
-40-
soluble salts, and dried overnight at 120'C. 3088 g of
dry powder were recovered.
(D) In a 3-liter agitated glass flask, 300
g of the powder from step (C) above were dispersed in
5 one liter of water and then heated to 90'C. Over a
period of 2 hours, 197 ml of the SnCl4/SbCl3/HC1
solution of Part (A) were added to the slurry. When
the pH dropped to 2.0, sufficient 10~ caustic was added
along with the SnCl4/SbCl3/HC1 solution to maintain the
10 pH at 2, and the temperature was maintained at 90'C.
The resulting product was filtered, washed free of
soluble salts, and then calcined in air for 2 hours at
a temperature of 750'C. 356 g of dry powder were
recovered.
15 (E) Part (D) was repeated, except that 333
g of CaCl2 were dissolved in the one liter of water
used to form the slurry. 154 g of dry powder were
recovered.
Dry powder resistances, pore diameters and
20 surface areas for the powders produced in steps (A),
(B), (C), and (D) were measured, and the results are
shown in Table 2.
25 Pore Surface
Diameter Area,
Part ,g~Q~ ~aCl~ Resistance nm m2/a
(A) No No 200 ohms 12.0 8.4
(B) No Yes 60 ohms 9.9 7.6
30 (C) Yes No 75 ohms 11.5 11.4
(D) Yes Yes 2 ohms 7.5 9.2
134067Q
-41-
T ABLE 3
% Ba as % Sn as % Si as % Sb as % Ca
as
Part BaSOd SnOZ SiO~ Sb2_03- Sao
(A) 83 14 0 1.7 < 0.05
5 (B) 83 14 0 1.7 ~ 0.05
(C) 79 14 6.5 1.7 ~ 0.05
(D) 79 14 6.4 1. 6 ~ 0.05
EXAMPLE 12
10 (Aj 188 g of wet-ground Muscovite mica, with
a surface area of 8.7 m2/g, was dispersed with 0.8% of
triethanolamine in 2000 ml of distilled water. The
process temperature was raised to 90'C and held there
for the remainder of the aqueous processing. The pH
15 was adjusted to 10.0 with 20% NaOH, and 50 g of 3.29
ratio potassium silicate (25% (25% Si02) was added to
the stirred slurry over two minutes. 20% HCl was then
added to the slurry over a 2 hour period, bringing the
pH to 8Ø The pH was then further adjusted to 7.0
20 with 20% HC1, and the slurry was stirred for 30
minutes. The pH was then adjusted to 2.0 with 20% HC1,
and 220 g of CaCl2 were added to the bath over a
five minute period. 220 ml of a SnCl4 solution (0.445
g Sn02/mlj and 42 ml of a SbCl3 solution (0.235 g
25 Sb/ml) were mixed together and added to the slurry over
2 hours, maintaining the pH at 2.0 by the addition of
20% NaOH. The slurry was held at 90'C and a pH of 2
for 30 minutes. It was then filtered, washed free of
soluble salts and dried at 120'C for 12 hours. The
30 dried product was calcined at 75'C for 2 hours. By
X-ray fluorescence analysis, the powder was found to
contain 33.1% Sn (as Sn02), 4.0% Sb (as Sb203), 31.2%
Si (as Si02), 22.0% Al (as A1203), and 6.3% R (as R20).
By X-ray diffraction line broadening, the average Sn02
35 crystallite size was 7 nm. A polyester/melamine/castor
oil primer paint was prepared as in Example 2.
134067Q
-42-
(B) The procedure of Part (A) was repeated,
except that the silica coating was eliminated. After
dispersing the mica in water and triethanolamine, the
pH was lowered to 2.0 by the addition of 20% HCl. The
5 calcium chloride was added, and the Part (A) procedure
was followed from that point on.
(C) The procedure of Part (A) was repeated,
except that the calcium chloride solution was not used.
The composition and electroconductive
10 performance of the resulting powders were found to be
as follows. The compositions were determined by X-ray
fluorescence analysis, and the crystallite size was
determined by X-ray diffraction line broadening.
15 TABLE 4
% % $ $
ppm %
wder SnO~ Sb203, SiO~ A1~0~ ~Q j$~Q
A 33.1 4.0 31.2 22.0 100 6.3
B 33.8 , 4.2 28.9 22.6 100 6.3
20 C 33.5 3.6 33.6 23.3 100 6.3
Crystallite Size Performance in Paint
of Sn02-Sb Conductivity,
ow nm P/B Ransburc Units
25 A 7 48 over 165
25 145
B 8 48 140
25 75
C 8 48 90
30 25 75
EXAMPLE 13
Example 12, Part (A) was repeated, except
that 188 g of delaminated Kaolinite clay were
35 substituted for the 188 g of mica, the amount of SnCl4
solution was increased to 252 ml, and the amount of
134670
-43-
SbCl3 solution was increased to 48 ml. The Kaolinite
had a surface area of 12.7 m2/g. A sample of the
powder was incorporated into a
polyester/melamine/castor oil paint as in Example 2.
5 The resulting paint film had a conductivity of 135
Ransburg units at a P/B of 50.
15
25
35
