Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD FOR PREPARING COLOURED PIGMENTS
WITH TITANIUM DIOXIDE BASE
The invention concerns a process for the preparation of titanium dioxide based
colored pigments. The invention also concerns the colored pigments obtained
through this
process.
Rutile phase colored pigments, i.e., composed primarily of rutile titanium
dioxide
are commonly used as coloring substances in plastic materials, paints and
varnishes and
also in the cosmetics and galenic fields.
These prior art pigments are generally prepared by (i) mixing anatase titanium
dioxide with one or a plurality of pigmentation additives, (ii) grinding then
(iii)
calcination.
These colored pigments must meet a certain number of criteria and, in
particular,
must have good stability to temperature, light, and also to decomposition by
acid or basic
chemical agents. DE 36 04 317 proposes colored pigments having improved
thermostability containing from 0.1 to 2 wt.% magnesium in addifion to a
ternary system
composed of titanium, antimony and chromium.
2 0 It is, in fact, essential that the colors of the pigments do not shade
off, for instance,
when they are added to plastic materials or paints, or when they are subj
ected to
particularly severe climatic conditions. Along the same line of thought, EP
318 783
recommends incorporation of lithium at a rate of 0.0l up to 0.25 wt.%, to
limit the
tendency toward discoloration of rutile phase pigments composed of chromium
and
antimony subjected to a quenching treatment.
However, the colorations from rutile phase pigments used in industry to date
are
not always satisfactory from the point of view of intensity of the shades and
color
saturation.
The invention proposes to resolve the problem of intensifying the shades by
providing an improved process for the preparation of titanium dioxide based
colored
pigments. The colored pigments obtained through this process offer a palette
of colors
that are more intense than those currently available on the market.
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To that end, the invention concerns a process for the preparation of titanium
dioxide based colored pigments including the calcination of anatase titanium
dioxide until
complete transformation of the anatase titanium dioxide into rutile titanium
dioxide, in
which an anatase titanium dioxide with a rutilization temperature of at least
1000 ~ C,
preferably at least 1060 ~ C" and a sulfate level, expressed as S03, of at
most 1 wt.%,
preferably less than 0.8%,is used.
According to a first essential characteristic of the invention, the titanium
dioxide
used must have a sulfate level, expressed as S03, of at most 1 wt.%,
preferably less than
0.8%. It was noted that starting with an anatase titanium dioxide with an
excessively high
sulfate level yields a pigment shade which is not sufficiently intense. The
lowest possible
sulfate level is more advantageous.
The sulfate level is easily determined by the person skilled in the art using
any one
of the methods of analysis known in the art, for example, by x-ray
fluorescence. -
This sulfate content in the anatase titanium dioxide may be controlled during
the
manufacture of the anatase titanium dioxide.
The starting anatase titanium dioxides that may be used in the process
according
to the invention are prepared according to methods known to the person skilled
in the art.
In this regard, reference may be made to the works of Unman Band (vol. 18, p.
574) and
Kirk Othmer (vol. 23, p. 146). Preferably, the anatase titanium dioxide is
obtained in
three steps through ( 1 ) hydrolysis of a titanium sulfate solution, (2)
filtration of the
hydrolysate, and (3) washing of the filtrate with water.
The sulfate content of the titanium dioxide may be controlled, in particular,
during
the step (3) of washing of the filtrate.
According to a second essential characteristic of the invention, the titanium
dioxide used must have a rutilization temperature of at least 1000 ~ C,
preferably at least
1060~C.
Control of the rutilization temperature of a titanium dioxide is generally
performed during the synthesis of the titanium dioxide. The degree of purity
of the
anatase phase titanium dioxide may be a controlling element of this
temperature: the
closer the level of anatase titanium dioxide is to l00%, the higher the
rutilization
temperature.
This level of anatase may, in particular, be adjusted in the step (1) by
incorporation of seeds into the
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hydrolysis medium. These serve to direct the nature of the hydrolysate formed:
Depending on the nature of the titanium dioxide seeds introduced, the
resulting
hydrolysate, primarily made up of anatase titanium dioxide, will include a
higher or lower
percentage of rutile titanium dioxide, with the objective being to reduce the
percentage.
Anatase titanium dioxides that have the necessary characteristics to be used
in the
process according to the invention are marketed; for example, by the Rhone
Poulenc
Company. The titanium dioxide sold by the Rhone Poulenc Company under the
reference
G5 is a powder; the Rhone Poulenc Company also supplies a titanium dioxide as
a sol
under the reference SS-300.
The rutilization temperature, which is not always indicated by the
manufacturer, is
easily determined by the person skilled in the art by implementing the
following operating
protocol,
The rutilization temperature is generally defined as being that at which 50
wt.%
of the anatase phase of an anatase titanium dioxide has been transformed into
the rutile
phase through calcination. According to the invention, the rutilization
temperature is
determined by high temperature x=ray diffractometry.
To avoid the influence of extrinsic parameters (such as the exact conditions
of the
rise in temperature), this determination takes place under very precise
operating
conditions: the anatase titanium dioxide sample is placed on a flat platinum
substrate and
kept in a confined atmosphere (without air currents) during calcination, for
example, in a
closed chamber. The temperature of the chamber is increased at the rate of 5 ~
C/min. up to
1250~C.
Diffractometry measurements are taken between 650~C and 1250~C. At regular
intervals (for example, every 5 0 ~ C), a dif&actogram is drawn, the
acquisition time being
fixed at 10 seconds over an angular range from 10 ~ to 60 ~ scanned by steps
of 0.032 ~ .
Preliminary experiments are performed in this manner with reference samples of
100% of rutile phase and 100% of anatase phase, respectively.
On the reference diffractograms obtained, the bands corresponding to the
anatase
and rutile titanium dioxides, respectively, are located.
The experiment is then performed under the same conditions with the titanium
dioxide sample to be analyzed.
By comparing the surface area of the most intense band of the two phases with
those marked on the diffractograms drawn from the reference samples, a
calculation is
made at each temperature of the proportion of rutile phase formed.
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The temperature at which an equal percentage of rutile phase and anatase phase
is
observed is the rutilization temperature.
According to the invention, the rutilization temperature must be at least 1000
~ C,
preferably at least 1060 ~ C. A temperature of less than 1000 ~ C is
insufficient from the
point of view of intensification of shades and of color saturation.
According to a preferred embodiment of the invention, the process includes the
following steps:
a) mixing the anatase titanium dioxide with at least one pigmentation
additive;
b) grinding the resulting mixture, and
c) calcination in an oxidizing atmosphere.
The mixing of anatase titanium dioxide with one or more pigmentation additives
may be performed dry or starting from an aqueous suspension.
To facilitate the mixing operation, selection of an anatase titanium dioxide
with a
BET specific surface of at least 250 g/m2 [sic m2/gJ is preferred. .
BET specific surface is understood to mean the specific surface measured by
the
BET method, i.e., by nitrogen adsorption, in accordance with ASTM standard
03663-78
established on the basis of the BRUNALJER-EMMETT-TELLER method described in
the periodical, "The Journal of the American Society", ,~Q, 309 (1938). When
the titanium
dioxide is in the form of an aqueous suspension (for example, a sol), the
aqueous
suspension is first atomized prior to measuring its specific surface.
When the mixing operation a) is performed dry, the anatase titanium dioxide is
in
the form of a powder composed of agglomerates of particles, with the average
size of
these agglomerates between l and 2 ~,m and the average size of the particles
between 40
and 60 nm.
Such a powder may be obtained by a prior art means, by implementing the steps
of
- hydrolysis of a titanium sulfate solution,
- filtration of the hydrolysate,
- washing of the filtrate with water,
- resuspension of the filtrate in water, and
- atomization of the resulting suspension.
According to another embodiment, the mixing a) may be performed in solution
starting from anatase titanium dioxide particles suspended in an aqueous
medium, with
the average size of the particles ranging between 40 and 60 nm. Prior to
grinding and
calcination, the water is, in this case, eliminated from the aqueous
suspension. This
second embodiment is even more advantageous insofar as it results
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in pigments that have even more intense colors, in comparison to the first
embodiment
according to which the mixing operation a) is performed dry starting from the
corresponding anatase titanium dioxide powder.
In all cases, a reduction in size of the particles facilitates the mixing
operation.
The anatase titanium dioxide based aqueous suspension may be obtained by
implementing the following steps:
- hydrolysis of a titanium sulfate solution,
- filtration of the hydrolysate,
- washing of the filtrate with water,
- redispersion of the filtrate in an acidified medium, for example, a nitric
acid or
hydrochloric acid solution. The use of sulfuric acid will be avoided at this
stage so as not
to increase the. sulfate level.
The pigmentary additives are those generally used in the art..These are
chosen, for
example, from among the compounds of the transition elements Sb, Cr, Ni, Co,
Zn, Cu,
Mn and W, and are either in the oxide form (particularly, Sb205, Cr203, NiO,
CuO, Mn0
and W03), of in the form of compounds oxidizable in an oxidizing atmosphere,
such as
that used during the calcination stage.
Preferably, a mixture of titanium dioxide and compounds of chromium and
antimony, in which the molar ratio of antimony to chromium is close to 1, for
example,
between 0.8 and 1.2, will be selected.
For informational purposes, it should be noted that calcination may take place
in
an oxygen or air atmosphere, with the calcination temperature advantageously
between
800 and l400~C.
Prior to grinding and calcination of the titanium dioxide based mixture, it is
possible to add mineral additives to it (called "mineralizers" in the art),
which are, for
example, sodium fluoride, sodium chloride, potassium chloride, calcium
fluoride or
calcium chloride. It is also possible to envisage the addition of lithium
compounds such
as those described in EP 3l8 783 (carbonate, fluoride, chloride, oxide,
hydroxide, sulfate,
nitrate, phosphate, lithium antimonate or lithium titanate) or magnesium
compounds,
such as are recommended in DE 36 04 317 (magnesium carbonate and oxide).
The invention also concerns the colored pigments obtained through the process
described above. These may be used in plastic materials, paints and varnishes,
and also in
the cosmetics and galenic fields.
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And finally, the invention concerns the use of an anatase titanium dioxide
with a
rutilization temperature of at least 1000 ~ C, preferably at least 1060 ~ C,
and a sulfate level,
expressed as S03, of at most 1 wt.%, preferably less than 0.8%, for the
preparation of
titanium dioxide based colored pigments.
The following examples are for the purpose of facilitating comprehension of
the
invention without restricting it.
- Determination of the rutilization temperatures of anatase titanium
dioxides.
The samples used in the examples have the following characteristics:
- sample 1: GS powder marketed by the Rhone Poulenc Company. This titanium
dioxide has a BET specific surface of 300 m2/g and a sulfate level, expressed
as S03, of
less than 0.8 wt.%; the powder is in the form of an agglomerate of particles,
with the
average size of the agglomerates close to 1.5 p,m.
- sample 2: comparative powder having a BET specific surface of over 250 m2/g
and a sulfate level, expressed as 503, of 5 wt.%; it is in the form of
agglomerates of
particles, with the average size of the agglomerates close to 1.5 Vim.
The rutilization temperatures of these samples are measured as follows.
First, the reference samples powders are studied by high temperature x-ray
di~action. As a reference for the location of the bands corresponding to the
anatase and
rutile phases, 100% anatase and 100% rutile powders are used. Each reference
powder to
be tested is placed on a flat platinum substrate situated in a closed chamber.
The
temperature of the chamber is progressively increased at a heating speed of 5
~ C/min.
Dif&actograms are drawn at the temperatures of 650, 700, 750, 800, 850, 900,
950, 1000,
1050, 1100, 1150, 1200 and 1250~C, respectively, under the following
conditions:
- acquisition time: 10 s
- angular range 10 ~ to 60 ~ in steps of 0.032 ~ .
The bands corresponding to the rutile and anatase phases are located.
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The same measurements are made on samples 1 and 2. The dif&actograms
obtained enable the evaluation of the percentage of the rutile and anatase
phases by
comparing the surface area of the most intense bands of the two phases with
those marked
on the dif&actograms drawn from the reference samples.
The results are then visualized in the form of curves representing the
variation in
the percentage of the rutile or anatase phase as a function of temperature.
These curves,
proposed in figures l and 2, enable an easy graphic determination of the
rutilization
temperature.
Figure 1 corresponds to sample 1, which shows a rutilization temperature of
1080~C.
Figure 2 corresponds to sample 2, which shows a rutilization temperature of
850~C.
Only the powder from sample 1 may be used in the process according to the
invention since its rutilization temperature is greater than 1000 ~ C.
EXAMPLE 2 - Preparation of the anatase titanium dioxide based colored pigments
with a rutilization temperature of 1080~C.
The anatase titanium dioxide powder of sample 1 (cf. example 1 ) is mixed with
chromiurrl oxide (CR203) and antimony oxide (SbZOs). This mixture is formed
into
pellets.
Then these pellets are calcinated for 12 hrs. at 920 ~ C in an oxygen
atmosphere.
Next, the pellets are ground prior to calcinating them again, first for 12
hrs. at 980~C in
an oxygen atmosphere, then for 4 hrs. at 970~C in air. The resulting powder is
ground.
Two samples of orange-colored pigment are prepared,in this manner:
~ sample 3: this has as a total formula (Tio.9Cro.osSbo.os)02
~ sample 4: this has as a total formula (Tio.~Cro,oosSbo.oos)Ox~
The chemical formula of the final compound is adjusted using the respective
quantities of chromium oxide, antimony oxide and titanium dioxide.
The two pigments obtained have been tested from the point of view of color
saturation.
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The coloration is quantified here by the chromatic coordinates L*, a* and b*
of
the 1976 CIE system (L*, a*, b*) as defined by the International Commission on
Illumination and listed in the Compendium of French Standards (Recueil des
Normes
Fran~aises (AFNOR)), colorimetric color no. X08-12 (1983). They are determined
using
a DATACOLOR colorimeter marketed by the SCIENTIFIC PACIFIC Company.
L* gives a measurement of reflectance and gives information on the clarity of
the
color.
a* and b* express the colored tendencies:
a value of a* positive indicates red,
a value of a* negative indicates green,
a value of b* positive indicates yellow, and
a value of b* negative indicates blue.
Thus, schematically, L* represents the variation from white to black
(clarity), a*
the variation from green to red and b* the variation from blue to yellow.
The measurements taken from samples 3 and 4 yielded the results reported in
Table 1 below:
TABLE 1
Pi ent L* a* b*
Sam le 3 67.5 24 59
Sam le 4 84 6 3 8
EXAMPLE 3 (comparative) - Preparation of anatase titanium dioxide based
colored
pigments with a rutilization temperature of 850~C.
Two colored pigments (samples 5 and 6) corresponding respectively to total
formulas (Tio.9Cra.oSSbo.os)OZ ~d (Tio.~,Cro.~SSbo.oos)Oz ~'e prepared
starting from the
anatase titanium dioxide of sample 2 as described in example 1 in accordance
with the
operating mode of example 2.
The measured chromatic data. are reported in table 2 below:
TABLE 2
Pi ent L* a* b*
Sam le 5 66 20 57
Sample 6 -- J g5.g-_I _. 3-2 28.2
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It is then noted, by comparing the pigments with the same chemical formula
(samples 3 and 5, on the one hand, and samples 4 and 6, on the other) that the
pigments
according to the invention have higher values for a* and b* and thus, overall,
a more
intense orange-color.
EXAMPLE 4 - Preparation of anatase titanium dioxide based colored pigments
with
a rutilization temperature of 1080~C.
The anatase titanium dioxide used is in sol form. It is marketed by the Rhone
Poulenc Company under reference SS-300. The titanium dioxide in question is
characterized by a BET specific surface of 300 mz/g and a sulfate level,
expressed as S03,
of less than 0.8 wt.% and a rutilization temperature of l 080 ~ C. The dry
extract of the
SS-300 sol is 20 wt.%.
The titanium dioxide colored pigments are prepared by (i) mixing the SS-300
sol
with chromium nitrate and antimony nitrate, (ii) atomization of the resulting
aqueous
mixture, and (iii) calcination/grinding as in example 2.
Two orange-colored samples are thus prepared:
- sample 7 has as a total formula: (Tio.9Cro_osSbo.os)02
- sample 8 has as a total formula: (Tio.99Cro.oosSbo.oos)02~
The chemical formula of the pigments is adjusted using the respective
quantities
of antimony nitrate, chromium nitrate and SS-300 sol.
Samples 7 and 8 were analyzed from the point of view of color saturation. The
results obtained are reported in Table 3. ,
TABLE 3
Pi ent L* a* b*
Sam le 7 68 24 61.5
Sam le 8 85 8 41.2
A comparison of the values for a* and b* in the case of samples 3 and 7, on
the
one hand, and 4 and 8, on the other, reveals a more intense coloration of the
pigments of
samples 7 and 8.
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Thus, it is verified that pigments prepared from an anatase titanium dioxide
sol
according to the invention result in an intensification of shades.
