Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
This invention relates to iron oxide pigments and
is directed particularly to a process for the preparation of
transparent iron oxide pigments having improved dispersibility.
Typically iron oxide pigments are produced by pre-
cipitation from solutions of water soluble salts, oxidation to
the ferric state followed by filtration washing and finally
drying at certain temperature ranges. During the drying process
pigments undergo crystalline aggregatiorand agglomeration.
This results in an aggregated and agglomerated pigment which
is subsequently difficult to disperse in the organic binder
solvent conventionally used in present-day paint systems due to
the fact that the organic binder and solvent is unable to wet
and separate the primary particles of the aggregate and the
agglomerate. Furthermore, when heating these iron oxide pig-
ments under normal conditions, the average particle size, because
of aggregation and agglomeration, becomes greater than 0.1
microns which is the maximum particle size desirable for good
transparency. In order for transparent pigments to be useful,
the particle size needs to be smaller than the wave length of
visible light which is about 0.1 microns. If aggregatlon
and agglomeration occurs, a greate~_-- ~~-~~~
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effort is required to disperse the pigment into particle
sizes of less than 0.1 micron. Transparent pigments are
presently in great demand. This is due to the fact that
there is an increased'use of these pigments in paints such
as in the automotive field wherein these pigments impart a
glossy finish to the metallic paint. The transparency of
the pigment enables the formulator to achieve a desired
; color to the top coat in addition to giving a metallic
effect. The iron oxide has the further advantage of
imparting durability and stability to the coating through
ultraviolet absorbing characteristics,
A typical manufacturing process for transparent
iron oxide pigment is taught in U. S. Patent 2,558,302.
Other teachings in the prior art as taught in
U. S. Patent 2,33$,760 and U. S. Patènt 2J384~579 employed
a process of kneading the pigrnent cake containing water
with large quantities of water-immiscible drying oils, alkyl
resins, nitrocellulose lacquers or other vehicles to expel
all the water. These processes require a great amount of
energy in order to expel sufficient quantities of water
from the pigment cake. In view of the prior art and the
need for easily dispersible iron oxide pigment having high
transparency, it is the object of this invention to provide
a process for producing such a transparen~ iron oxide
pigment,
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In accordance with this invention there is provided
transparent iron oxide pigments having improved dispersibility
and an improved process for their preparation.
In accordance with the invention, this improved
process for the preparation of such transparent iron oxide
pigment is of the type wherein an aqueous mixture of iron oxide
particles is formed. More particularly, this improved process
comprises:
(a) filtering said iron oxide particles from said
mixture and forming an iron oxide filter cake,
(b) washing said iron oxide filter cake with water,
(c) further washing with a solvent miscible with
water, immiscible and non-reactive with iron
oxide, having a boiling point below about 200C,
and having a surface tension of less than 60 dynes
per centimeter, and
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(d) removing a substantial portion of said
solvent from said iron oxide filter cake.
As mentioned supra, during the process of pre-
paring transparent iron oxide pigments the removal of
water results in an aggregation and agglomeration of the ~ -
particles of pigment. Due to this aggregation and agglom-
eration, the pigment is subsequently very difficult!to
disperse in use It is known that pigment particles which
are needle shaped such as in the transparent iron oxide
pigments J particle to particle contact occu~s on the long
.
axis Due to the great number of points of contact
pigment dispersion in a given binder solvent system is
most difficult It has been discovered that whçn the
water which envelops the primary particles is replaced by
a solvent of much lower surface tension, prior to drying,
the resultant pigment does not aggregate and agglomerate
to the same extent that it does upon drying from an aqueous
slurry Apparently the replacement of water involves
miscible displacement ~n the small capillaries The sol-
vent requirements are chemical miscibility with water,
immiscibility and non-reactivity with the iron oxide pig-
ment, having a surface tension of less than 60 dynes per
centimeter and a boiling point below about 200 Centigrade.
It has been found among the solvents that are useful are
aliphatic alcohols containing 1 to 4 carbon atoms, glycol
ethers, alkanolamines, ketonesJ dimethylsulfoxide, di- ~,
me~hylform~mLde, meth~l pyrrolLdone, nitropropane, and
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dioxane. Among the alcohols which may be employed are
methanol, ethanol, propyl, isopropyl, normal, secondary
and tertiary butyl alcohols. Those glycol ethers which
are contemplated are the mono- and dimethyl and ethyl
ethers of ethylene glycol, diethylene glycol, propylene
glycol, and dipropylene glycol. The solvents selected
from the alkanolamines may be ethanolamine, diethanolamine, ;
isopropanolamine and diisopropanolamine. Among the ketones,
solvents such as acetone, methylethylketone, and methyl-
isobutylketone may be employed. Mixtures of any of the
above may also be employed. It has also been found pos-
sible to employ aromatic solvents in admixture with the
above solvents Accordingly, solvents such as benzene,
toluene and xylene may be employed in conjunction with the
alcohols, ketones and the like, depending upon mutual com-
patibility with one another and limited only by miscibility
of the mixture with water. For example, it is possible to
employ a mixture of 90% butanol and 10~ xylene in the
practice of this invention.
The temperatures which may be employed for the
washing step may range from 20 Centigrade to 100 Centi- , :
grade, depending upon the boiling point and the volatility
of the sol~ent employed. The amounts which are employed
for the washing will depend upon the amount of moisture
;~ which is present in the pigment slurry. It is contemplated
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that the normal range of solvent employed is from 1000 to 1500
milliliters for every lS0 to 300 grams of iron oxide filter
cake. Upon sufficient washing, the iron pigment filter cake may
be dried at temperatures ranging from 25 to 110 Centigrade.
In some instances the removal of the solvent may be hastened by
drying the iron oxide filter cake under a vacuum of about 1 or
2 millimeters of mercury.
The yellow pigment obtained by this process can be
converted to easily dispersible transparent red iron oxide
pigment by calcination at temperatures rangi~g from 150C to
400C.
The invention and its advantages will be better
understood with reference to the following description taken in
connection with the accompanying drawing which illustrates
a typical curve obtalned for Brightness values versus dispersion
times employing a Red Devil paint shaker. As can be seen,
this drawing illustrates that solvent washing results in pigment
having greater dispersibility compared to pigment prepared by
prior art procedures as indicated by higher Brightness values.
In order to teat the iron oxide pigment, obtained in
accordance with the invention a dispersion procedure is employed
wherein one and a half grams of the dried pigment are added to
a four-ounce bottle containing 100 grams of 2 mm glass beads and
4.5 grams of a Dispersion Vehicle which consists of 80 weight
percent acrylic resin, 4 weight percent normal butanol and 16
weight percent xylene. The bottle is stoppered and placed on
a paint shaker for various lengths of time, upon which the bottle
is cooled and 24 grams of Let-down Vehicle consisting of about
; 45 weight percent acrylic resin, 20 percent melamine resin, 7
percent butanol and 28 percent xylene. The mixture is then re-
shaken, the glass beads filtered out, and the filtrate then
tested for Brightness ~Y values.
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The dispersibility of the pigment is conveniently
determined ~y tristimulus measurements of the Y values and
is the difference between the Y values of the specularly
reflected light and the diffusely reflected light, This
difference is expressed as the Brightness ~Y value. These
values are deter~ined employing such suitable measuring
e~uipment as the Carey Model 14 Spectrophotometer equipped
with an integrating sphere and a 45 light source. The
Brightness,~Y values are an indication of the ability of
the pigment to reflect the incident lights with a minimum
of scattering caused by aggregation and agglomeration of
the particles. The higher the4 Y value obtained, the finer
the particle size distribution. Higher~ Y values indicate
that most of the light is reflected and very little is
diffused, Lower~ Y values indicate less reflected light
and more light is scattered due to the larger particle
size distribution,
The procedure employed for determining the
Brightness ~Y is to form a spot of dispersed pigment of
about 20 millimeters in diameter onto an alumin~lm backed
.card or a clear mylar or glass sheet. The control pigment
prepared in a similar manner without solvent washing is
spotted next to it. A wire wound film applicator is used
to draw down the two pools of pigment to the bottom of the
card, following which the draw down is dried and cured.
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Brightness ~Y measurements are then made using a spectro-
photometer. The curve indicated in the drawing is plot
of Brightness QY values versus various dispersion times
on a Red Devil paint shaker. It is seen that the solvent
washed pigment has a higher Brightness ~Y value compared
to the control pigment. This indicates that the solvent
washed pigment has a higher dispersibility and smaller
particle size. The following examples are illustrative
of the present invention and are not intended in any way
as a limitation upon the scope thereof. Parts and percents
are by weight unless otherwise indicated, These examples
show that sol~ent washing of iron oxide pigments results
in smaller particle sizes, increased dispersibility and
higher transparency of the resulting iron oxide pigment.
Example 1
Into a 12000 ml reaction flask was added 161.4
grams of FeS04, 7H20 crystals and 6000 mls of water. To
this solution was added ~00 mls of an aqueous solution
containing 16,7 weight percent sodium carbonate. By means
of an air lance placed into the flask, air was bubbled
through the solution forming a slurry of ferric hydroxide
until a test with potassium permanganate showed that
oxidation to the ferric state was complete. The slurry
was then heated at 90C for one hour. The slurry was
filtered through Whatman No, 4 filter paper employing a
B~chner funnel. The pigment residue was then washed with
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8000 mls of water and was dried in a vacuum oven at 105C
for 24 hours. The resulting dry yellow pigment was ground
in a mortar with a pestle and sieved through a 200 mesh
screen, This was used as the control pigment,
Solvent Washing Procedure
Into a B~chner funnel was added 150 grams of
yellow ferric hydroxide pigment slurry containing a~out
60% water prepared as described above prior to filtration.
This slurry was filtered through Whatman No, 4 filter paper,
The residue of pigment was then washed with about 8000 mls
of water, This was fo~lowed by 1200 mls of dimethylform-
amide (DMF) at the rate of about 100 mls/minute, The DMF
water mixture was then recycled through the pigment until
the moisture content of the pigment was reduced to less than
2~ as determined by a Karl Fischer titration, The pigment
was then dried in a vacuum oven at 105C for 24 hours,
The resulting yellow dry pigment was ground in a mortar
with a pestle and sieved through a 200 mesh screen,
Dispersion Procedure
Into a four-ounce bottle was added 100 grams of
2 mm glass beads and 4,5 grams of a dispersion vehicle con-
sisting of 80 weight percent acrylic resin, 4 weight per-
cent n-butanol and 16 weight percent xylene and 1,5 grams
of the dry DMF washed pigment, The bottle was stoppered
with a cap and placed on a Red Devil paint shaker for 15
minutes, ~fter coollng the bottle, 24 grams o~ Let-down
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Vehicle consisting of 45.2 weigh~ percent acrylic resin,
19.4 weight percent melamine resin, 7.0 weight percent
n-butanol and 28.4 weight percent xylene was added to the
bottle and reshaken for ~ minutes. The glass beads were
filtered out and the filtrate was retained for Brightness
testing.
Brightness Test
A portion of the above filtrate forming a spot
of about 20 mm was poured onto an aluminum backed card. A
control pigment was spotted next to it A "60 micron"
spiral was used to "draw-down" the two pools of pigment to
the bottom of the card. The "draw-down" was allowed to
air dry for 10 minutes, then oven dried for 30 minutes at
120C. The Brightness ~ Y was then measured employing a -
Carey spectrophotome~er with an integrating sphere and a
45 light source. Additional samples were prepared on the
Red Devil shaker for various lengths of time with~Y
results as indicated below in Table I.
Table I
Shaker TLme, Minutes
Example
DMF washed, ~Y 42 50 60 7
Control, ~Y ~2 40 50 60
These data indicate tha~ solven~ washing inhibits
~ aggregation and agglomeration of the particles since the
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Y values are higher for the solvent washed pigment com-
pared to the control pigment, Similar differences were
- observed for a red pigment produced by calcining the yellow -:
pigment at 280C for 20 minutes prior to the dispersion
procedure.
Examples 2-5
The solvent washing procedure of Example 1 was
followed except that as indicated various solvents were
employed and the pigment was dried at 70C for 24 hours.
The Brightness aY values are tabulated in Table II below,
Table II
Brightness ~Y values
Shaker time, minutes
Example Solvent -I5 30 o 120
2 Ethanol 37 45 55 65
3 Dimethyl-
sulfoxide 45 55 65 75
4 90% Butanol ~'
10% ~ylene 35. 45 55 65
Acetone 35 47 53 65
Control 32 40 50 60
These data indicate that various solvents may be
employed for washing the pigment resulting in improved dis-
persibility of the pigment.
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