Note: Descriptions are shown in the official language in which they were submitted.
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RUTILE TITANIUM DIOXIDE COATED
MICACEOUS PIGMENTS FORMED WITHOUT TIN
Nacreous or pearlescent pigments which are
titanium dioxide coatings on mica substrates are well
known. The pigments exhibit pearl-like and/or iridescent
effects from their reflection and transmission of light.
The titanium dioxide coating is actually transparent to
light. However, because the coatings are extremely
smooth and have a high index of refraction, they follow
the laws of thin films. Part of the light which strikes
each platelet is reflected and part transmitted to lower
platelets where multiple reflections can occur. These
multiple reflections from lower layers give a sense of
depth or sheen which simulates the real pearl. Also, if
the thickness of the titanium dioxide layer is
controlled, interference of light occurs and the
platelets act as optical filters separating light into
two components. A color is seen by reflection and a
complementary color by transmission.
Pearlescent pigments are used extensively in a
variety of applications including plastic incorporation,
automotive coatings and in cosmetics. The pearlescent
pigments which are titanium dioxide coated on a mica
substrate have a high index of refraction. The pigments
are normally dispersed in mediums such as paint films, or
nail enamel films which, when fully cured, have an index
of refraction of about 1.5. The index of refraction of
the pearlescent pigment must therefore be considerably
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higher than 1.5 if reflectivity of light is to occur.
This high index of refraction is provided by the titanium
dioxide layer whose index can vary between 2.3 and 2.5.
The mica substrate on which the titanium dioxide is
coated has an index about 1.5 and therefore, does not
take part in any reflectivity when incorporated in a
film. The rutile form of titanium dioxide has a higher
index than the anatase form and as a result, the rutile
modification will have greater reflectivity than the
anatase form. Therefore, the rutile modification of
titanium dioxide in a pearlescent pigment is more
desirable than the anatase modification.
There are many other reasons for preferring the
rutile modification. The rutile modification is more
stable in outdoor weathering than is the anatase
modification. The rutile modification of a titanium
dioxide coated mica results in a product which has better
luster and ref lectivity, better color and color
homogeneity and also contains fewer small particles. In
the processing stage during the formation of the titanium
dioxide on the mica, particles which are not attached to
the mica may form. These small particles, which resemble
pigmentary Ti02, cause light scattering. If too many
small particles are present, the pearlescent appearance
may be lost or diminished. The process for coating mica
in the rutile crystalline form results in very few small
particles compared to the anatase form.
If mica is coated with a layer of hydrous
titanium dioxide and then subjected to the normal
processing methods which include washing drying and
calcining usually from 750°C to 900°C, the titanium
dioxide which is formed is in the anatase form. The
presence of the mica causes the Ti02 to orient in the
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anatase crystalline form. Such pigments have been
described for example in Quinn et al. U.S. Patent No.
3,437,515 issued April 8, 1969 and Rieger et al U.S.
Patent 3,418,146 issued December 24, 1968 and Linton US
Patent 3,087,828.
If a rutile crystalline form is desired, a
layer of hydrous tin oxide is first precipitated on the
surface of the mica followed by a layer of hydrous
titanium dioxide. When this layered combination is
processed and calcined, the titanium dioxide is oriented
in the rutile form. This is described in detail in US
Patent 4,038,099 and also US Patent 4,086,100. Other
methods of forming rutile Ti02 on mica substrates using
tin oxide are also described.
Although many additives can aid in the
fonaation of rutile Tio2 per se, the formation of rutile
Tio2 on mica requires a very special additive. The
coating of Ti02 on the mica must be smooth and uniform.
If an irregular surface is formed, light scattering takes
place and the pigment no longer functions as a
pearlescent pigment. The coating of Ti02 must also
adhere strongly to the mica or else the coating of Ti02
will be separated from the mica during processing,
resulting in considerable breakage and loss of luster.
It is also necessary that the luster, color, and color
homogeneity be maintained. Small particle formation must
be suppressed. Otherwise, the small particles will
scatter light and diminish the pearlescent luster as was
mentioned previously. An additive which is used must
therefore perform many functions besides being a rutile
crystalline director. It has been difficult to find an
additive (other than tin) which can orient the TiOz to
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the rutile modification while still maintaining quality
and all of the other desirable characteristics.
Presently, additives other than tin for forming
the rutile modification of titanium dioxide on mica while
still maintaining all other desirable characteristics do
not exist. The use of tin oxide is the method of choice
- and is used universally in all commercial rutile titanium
dioxide coated micas.
There are, however, two major disadvantages to
the use of tin to make rutile Ti02 coated mica. The
first is that tin oxide is not permitted in polymer
compositions which are to be used in contact with food.
Thus, any high quality pearlescent or interference
pigment which contains tin oxide cannot be used to color
the polymer film. The second is that in some countries,
notably Japan, the presence of tin oxide is not permitted
in cosmetic products. Cosmetic manufacturers are
therefore faced with a choice of either formulating
cosmetic products destined for Japan with anatase only
products and having a second line of the same products
for the rest of the world formulated with rutile products
or having a single anatase product line for the entire
world. The result is that polymer fonaulations in
contact with food and cosmetic lines to be used worldwide
use anatase products even though the rutile Ti02-coated
products have better color, color homogeneity and luster.
It is therefore the object of this invention to
provide a pearlescent pigment of titanium dioxide coated
mica in which the titanium dioxide is in the rutile
crystalline form and in which tin has not been used to
promote rutilization. A further object of this invention
is to provide a rutile Ti02-coated mica which has the
same advantages and characteristics of the tin containing
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product which includes luster, color, color homogeneity
and few small particle formation during manufacture.
These and other objects of the invention will become
apparent to one of ordinary skill in this art from the
following detailed description.
This invention relates to high quality TiOz-
coated micaceous pearlescent pigments. If a small
concentration of Fe and one or more of Zn, Ca and Mg
ions are introduced into the coating prior to the start
of the precipitation of hydrous titanium dioxide on mica,
the precipitation proceeds as if a layer of hydrous tin
oxide had been added. Complete rutile formation is
achieved. Both the pearl color and also the interference
colors are formed that have the same quality and
characteristics as the tin oxide-containing counterparts.
In accordance with the present invention, high
quality Ti02-coated micaceous pearlescent pigments are
formed in which the Ti02 is in the rutile form as a
result of the use of iron with zinc, calcium and/or
magnesium in the absence of tin.
It was found that the presence of Fe is as
effective as the Sn treatment in causing the Ti02 coating
to be in the rutile crystalline form. The presence of
Zn, Ca or Mg in most cases actually represses the
formation of rutile Ti02. The function of the zinc,
calcium and magnesium therefore is to aid in the
development of the critical and other essential features
necessary for formation of pearlescent pigments. The
combination of ions are necessary for the formation of a
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high quality rutile TiOz-coated mica similar in every
respect to the tin-containing pigment. The iron is used
as a rutile modifier and the zinc, calcium or magnesium
is used as a growth modifier. Together, they form a high
quality rutile Ti02-coated mica which does not contain
added tin.
If a large concentration of iron is added,
complete rutilization of the Ti02 layer can be achieved.
However, the resulting pigment acquires a tan or iron
color which is objectionable in many applications
especially at the white reflecting pearl color. The
concentration of Fe ions must therefore be reduced so
that only a trace of the tan color is found and is no
longer objectionable. If this is done, however, it is
found that the other desirable characteristics of the
pigment suffer. Luster, color and small particle
formation become unacceptable and the product no longer
resembles the product made with tin.
It was found that if a small quantity of Zn, Ca
and/or Mg ions are added to the coating bath together
with the small quantity of Fe ions, complete rutilization
can be achieved and quality and the other desirable
characteristics can be maintained. The tan color
imparted by Fe is no longer objectionable and the product
in all respects resembles the product made with tin.
The presence of iron, zinc, etc. in titanium
dioxide and titanium dioxide-coated mica has been noted
in the literature. Thus, Lipton in US Patent 3,087,828
teaches that light sensitivity of the titanium dioxide
coated on mica can be improved by employing an additional
metal oxide which is either deposited on or intermingled
with the titanium dioxide. One metal oxide mentioned is
iron oxide. Lipton does not indicate that iron is rutile
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directing. The Linton patent also states, with respect
to zinc, that "when a combined layer of hydrous Ti02 and
Zn0 is calcined, the resulting layer exhibits the X-ray
pattern of rutile Ti02 whereas the single layers of Ti02
show anatase Ti02".
Other patents and papers are concerned only
with the formation of pigmentary Ti02 and not Ti02 coated
on mica. Thus, Hoffman et al. in US 3,453,129 speaks of
a rutile titanium dioxide-calcium sulfate composite
pigment. This is made by heating a TiOS04-FeSOd solution
with an aqueous slurry of CaSO, anhydrate. Before
calcining, 0.2-2% ZnO is added and the mixture calcined
at 800-1000°C. The calcined product is ground to produce
a rutile Ti02-CaSO4 composite pigment. Also in the paper
"Kinetics and Mechanism of the Anatase/Rutile
Transformation, as Catalyzed by Ferric Oxide and Reducing
Conditions", Emerson F. Heald and Clair W Weiss, American
Mineralogist, Vol. 57, pp. 10-23 (1972), it was noted
that traces of ferric oxide have a strong catalytic
effect on both nucleation and growth phases of the
transformation of Ti02 from anatase to rutile.
In most reported cases, the primary concern is
the formation of the rutile structure since coatings are
not made on mica. Other considerations which are of
primary importance in the formation of high quality
micaceous pearlescent pigments which include the
maintenance of extremely smooth surfaces, uniformity of
thickness and color homogeneity are not pertinent. There
is no indication in these papers nor in the literature
that an extremely small concentration of both ions of Fe
and of Zn, Ca and/or Mg would accomplish all objectives.
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A pearlescent pigment which comprises particles
having an adherent layer of rutile titanium dioxide and
which does not contain tin as a rutile promoter is
achieved by precipitating the hydrous titanium dioxide on
the mica in the presence of both iron and zinc, calcium
and/or magnesium ions. The procedure is generally the
- same as that employed to form tin-containing rutile Ti02-
coated mica with the exception that no tin is employed
and the iron and zinc, calcium and/or magnesium ions are
substituted for the tin.
In the coating process mica is dispersed in
water, which is preferably distilled. Although muscovite
mica is the preferred mica because of its white color,
other micas can be used which include phlogopite,
lipidolite or synthetic micas.
The average particle size of the mica which is
used can vary from an average particle size of about 3
microns to an average particle size of about 100 microns.
The concentrations of mica in the water can vary from
about 5% to 25%. The generally preferred concentration
varies between about 10% to 20%.
After the mica is dispersed in the water and
placed in an appropriate vessel, both Fe and one or more
of Zn, Ca and Mg ions are introduced. The Fe ions are
introduced as chloride ions but other forms can be used
such as nitrates or sulfates etc. The zinc, calcium
and/or magnesium ions are also preferably introduced as
chlorides, but other forms can also be used which include
sulfates and acetates.
The quantity of iron which is employed is
important. There is a minimum below which the quality of
the product deteriorates as evidenced by a decrease in
luster even though complete rutile formation is achieved.
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The minimum amount can vary depending on the mica
fraction used, the interference color to be made and the
luster whiteness which is desired. The Fe content can
vary between about 0.125% Fe to about 1.0% Fe based on
the weight of mica. Most preferably, the amount of iron
is about 0.25% to 0.75% based on the weight of mica
A white pearl reflecting TiOz coated mica can
tolerate only a very small concentration of Fe before the
tan color iri the product is observable. Interference
colors, on the other hand, can tolerate larger
concentrations of Fe. In fact, higher concentrations of
Fe in interference pigments enhance the reflection color.
The amount of zinc, calcium and/or magnesium
employed can also vary over a larger range since it has
been found that regardless of the amount employed, only a
small amount is incorporated in the product. Generally
the amount is at least about 0.05% and preferably is
about 0.25 to 0.75%
It is most convenient to treat the micaceous
particles by adding the ions of iron followed by zinc,
calcium and/or magnesium ions to the coating bath. They
could also be added simultaneously. The pH is then
adjusted to about 3.0 and heated to a temperature of
75°C. When that temperature is reached, the pH is then
lowered to 1.6 by addition of a 1.1 HCl:water solution.
Other methods for the addition of the iron and zinc,
calcium and/or magnesium ions are possible. For example,
it is possible to precipitate the iron on the mica at
constant pH 3.5 by slow addition of the iron salt. Zinc,
calcium and/or magnesium ions are then added to the
coating bath and the precipitation is continued by
addition of the hydrous titanium dioxide. Alternately,
both the iron and zinc, calcium and/or magnesium can be
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precipitated on the mica. The iron can be precipitated
at constant pH 3.5 and the second metal at constant pH
5.8, followed by precipitation of the hydrous titanium
dioxide at constant pH 1.6.
The pH at which the hydrous titanium dioxide is
precipitated is important. Above about pH 1.9 complete
rutilization will not occur. Below that value, it is
dependent on the particular system although complete
rutilization is formed at about pH 1.6 and a pH below
about 1 should be avoided. Between pH 1.6 and pH 1.9,
mixtures of anatase and rutile titanium dioxide usually
form. Generally the pH should be at least about 1.4.
Other than the modifications noted above, the
procedure to form the tin free, rutile titanium dioxide-
coated micaceous pigment is conventional.
Various non-limiting examples are set forth
below to further illustrate the present invention. In
these, as well as throughout the balance of this
specification and claims, all parts and percentages are
by weight and all temperatures are in degrees Centigrade
unless other indicated.
EXAMPLES 1-25
A coating procedure was adopted in which 200
grams of muscovite mica having an average particle size
of about 18 microns (by laser light scattering) were
dispersed in 1 liter of water. Iron and zinc when
employed were introduced in the form of 7.5 grams of a
39% aqueous solution of ferric chloride further dissolved
in 50 ml of distilled water and 2.1 grams of zinc
chloride dissolved in 50 ml of water. In all procedures,
the pH of the slurry was adjusted to 3.0 using a 35%
aqueous sodium hydroxide solution and the slurry heated
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to a temperature of 75°. The pH was then lowered to 1.6
by addition of hydrochloric acid and a 40% aqueous
solution of titanium tetrachloride was added at a rate of
100 ml/hour while the pH was maintained at 1.6 by the
addition of 3.5% aqueous sodium hydroxide. The titanium
introduction was continued until either a white pearl or
interference colors which include gold, red, blue and
green and also second and higher colors had been reached.
When the desired end point was achieved, the slurry was
filtered on a Buchner Funnel and washed with additional
water. The coated platelets were then dried and calcined
at 900°C.
The luster quality of the pigments produced in
the foregoing fashion were determined by reflectance
measurements made on standard drawdowns on a hiding power
chart (Form 2-6 Opacity Charts of The Leneta Company),
half of which is black and half of which is white. A
coating on the black part of this chart displays the
reflection color when it is examined by specular
reflection while the coating on the white portion
displays the transmission color when it is viewed at an
angle which is not equal to the angle of incidence.
The standard drawdowns are prepared by
suspending 3% pigment in a nitrocellulose lacquer which
contains
Nitrocellulose RS type 15-20 sec. 2~9%
Nitrocellulose RS type 30-40 sec. 6.6
Isopropanol 5.1
Amylacetate 44.8
n-Butyl acetate 37.6
Mono-butoxydiethylene glycol 3.0
100.0%
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The two grades of nitrocellulose provide the desired
combination of solids content and a viscosity of
approximately 2000 centiposes at 25°C. The mono-butoxy-
diethylene glycol is used to prevent "blushing" or
clouding of the lacquer film by condensation of water
vapor from the atmosphere.
The drawdowns are made with a Bird film
applicator which produces a wet film of approximately
0.003 inch (about 0.008 cm) thickness on the hiding power
chart held firmly against a Bird vacuum plate. The
spectrophotometric curve of the sample is determined with
a Leres Trilac spectrophotometer using an angle of
incidence of viewing of 15° to the normal. The
reflectance is measured relative to a pressed cake of
barium sulfate. Reflectance at the maximum (Rm"~) and the
average reflectance are measurements of pearlescent or
nacreous luster. The wavelength at the maximum is an
indication of color, although the entire curve is
required to describe the color completely.
Besides the luster measurements, the pigments
were also analyzed for the percentage of rutile and
anatase that was present in each sample by x-ray
diffraction.
For comparison purposes, a number of different
pigments were prepared at various thicknesses. one was
made by the conventional Sn procedure. A second
experiment was made with both Fe and Zn added. Other
experiments included pigments which contained Fe but no
Zn, Zn but no Fe, and a control which contained neither
Sn, Fe, Zn. The results are shown in the following
tables.
In all cases, except at the blue reflection,
the Fe and Zn combinations produced products with quality
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equal to or exceeding the products which contained Sn.
All the products were 100% rutile with no anatase
present. It is also to be noted that with the exception
of the white pearl, the presence of Zn actually repressed
the formation of rutile. The presence of the Zn caused
the quality of the pigment to equal that of the Sn.
From this data, the function of the Fe was to
cause complete rutilization of the Ti02 and the function
of the Zn was to modify the growth so that equivalent
quality to the Sn containing product was achieved.
TABLE I
White Pearl Ret7ecting
Ti0= Coated Mica
Treatment % Rutile % Anatase Rmax
Sn 100 0 86.0
FelZn 100 0 89.5
Fe - No Zn 100 0 82.5
Zn - No Fe 15 85 71.0
No Fe, Zn or Sn 10 90 68.5
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TABLE II
Gold Interference Reflecting
Ti0= Coated Mica
Treatment % Rutile % Anatase Rmax
Sn 100 0 68.5
FelZn 100 0 71.0
Fe - No Zn 100 0 68.5
Zn - No Fe 25 75 57.0
No Sn; Fe or Zn 40 60 I 51.5
TABLE IB
1 o Red Interference Reflecting
TiOI Coated Mica
Treatment % Rutile % Anatase R,o,
Sn 100 0 60.0
FelZn 100 0 60.5
Fe - No Zn 100 0 51.0
15 Zn - No Fe 22 78 26.0
No Sn, Fe or Zn 35 65 24.0
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TABLE IV
Blue Interference Reflecting
Ti0= Coated Mica
Treatment % Rutile % Aaatase Rmax
Sn 100 0 63.0
Fe/Zn 100 0 59.0
Fe - No Zn 100 0 56.0
Zn - No Fe 33 67 31.0
No Sn, Fe or Zn 50 50 28.0
TABLE V
1 o Green Interference Reflecting
Ti02 Coated Mica
Treatment % Rutile % Anatase Rmax
Sn 100 0 56.5
Fe/Zn 100 0 57.0
Fe alone 100 0 55.0
Zn alone 4~, 60 31.5
No Fe, Zn or Sn 55 45 ~ 31.0
EXAMPLES 26-40
The combined iron and zinc procedure of the
foregoing examples was repeated at three different
concentrations of iron based on the mica. The results
are set forth in the following table:
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TABLE VI
Quality
and Rutile
Formation
for Varying
Iron Content
Iron % Rutile Pearl Gold Red Blue Green
Content R~ R~ R,,o, R~ Rm,s
0.5 % 100 88 71 60.5 59.0 57.0
0.25 % 100 84 69 62.5 64.0 63.0
i
0.125 % 99-100 70 62 45.0 50.0 50.0
The results indicate that the iron content can
be reduced from 0.5% to 0.25% without a decrease in
quality while still maintaining 100% rutile. When the
iron content was decreased to 0.125%, however, the rutile
content was no longer 100% and the quality of the
products decreased. The iron content of 0.25% is the
lower limit if quality and 100% rutile are to be
maintained.
EXAMPLES 41-44
The combined iron/zinc procedure of Examples 1-
is repeated at four different concentrations of zinc.
They were 0.075, 0.1, 0.5 and 1.0% based on the weight of
20 the mica. Analysis of the supernate indicates that
essentially the same amount of zinc was incorporated into
the pearlescent pigment and the balance of the zinc
remained in solution.
EXAMPLES 45 - 49
25 Tin-free pearlescent pigments were prepared
following the combined iron and zinc procedure of
Examples 1-25, but substituting the corresponding sulfate
and acetate salts for the zinc chloride there used.
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EXAMPLES 50 - 55
The procedure of Examples 1- 25 were repeated
using 0.29% iron and either 0.5% A1, 0.7% al, 0.5% Zr,
0.5% Ca, 0.5% Zn or 0.7% Mg to produce pearl or green
refecting products. The results are shown in the
following Tables VII, VIII and IX:
TABLE VII
White Pearl Reflecting 02 Coated
Ti Mica
Treatment % Rutile % Anatase Rmax max
1o FelZr 83 17 62 420
Fe/Al (0.5 %) 33 67 70 420
FeJAl (0.7 %'o ) 65 35 420
FeJCa 95 5 87 530
Fe/Mg 100 0 88 420
FeJZn 100 0 90 420
TABLE VIII
Green Interference
Reflecting Ti02 Coated
Mica
Treatment % Rutile % Anatase Rmax max
FelZr 97 3 50.5 500
2o Fe/Al (0.7%) 95 5 53 500
FeICa ca. 100 trace 75 500
FelMg 100 0 75 500
Fe/Zn 100 0 77 500
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TABLE IX
COLOR PURITY RELATIVE TO
Fe/Zr
Fe/Ca GOOD
Fe/Mg GOOD
Fe/A1 (0.5%) POOR
Fe/A1 (0.7%) POOR
Fe/Zr POOR
Various changes and modifications can be made
in the products and process of the present invention
without departing from the spirit and scope thereof. The
various embodiments which have been disclosed herein were
set forth for the purpose of illustrating the invention
but were not intended to limit it.
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