Yellow pigments stable at high temperatures

A yellow pigment capable of withstanding elevated temperatures such as 200.degree. C. is provided by the formation of a spinel of iron and other metal, such as Mg, Zn, and Sn. The pigment is made by a method which does not involve a step of calcining at high temperature, such as 600.degree. to 1100.degree. C., but rather involves precipitation from aqueous solution and subsequent exposure to elemental oxygen, as by aeration, to form submicron-sized particles of desired spinel.

FIELD OF THE INVENTION 
The present invention relates to inorganic pigments and to a method for 
making them. More particularly, the present invention concerns yellow 
pigments derived from inorganic materials. Even more particularly, the 
present invention concerns yellow pigments stable at high temperatures. 
PRIOR ART 
As is known to those skilled in the art to which the present invention 
pertains, yellow inorganic pigments, except nickel titanate, are not 
suitable for processing into plastics because of their thermal instability 
at temperatures above 175.degree. C. Thus, yellow pigments such as iron 
oxide, lead chromate and zinc chromate are not well chosen for the yellow 
coloring of plastics such as polyethylene, polypropylene, polyvinyl 
chloride, polycarbonates, polyamide and the like. Furthermore, organic 
pigments show color degradation at temperatures of about 175.degree. C., 
thereby precluding their use in the processing of plastics. Thus, the need 
for yellow pigments stable at high temperatures is readily apparent. 
As will subsequently be detailed, the present invention provides such 
stable yellow pigments. 
STATEMENT OF RELEVANT PATENTS 
To the best of applicants' knowledge, the following patents are the ones 
most relevant to a determination of patentability: 
U.S. Pat. Nos. 2,904,395, 4,097,392, 3,822,210, 3,887,479, 4,075,029, 
3,832,455. 
Perhaps the most pertinent of the patents mentioned above is Iwase et al. 
U.S. Pat. No. 3,822,210. Although this patent teaches the making of zinc 
ferrite (spinel) pigments, it does not teach or suggest the present 
invention because it uses a method which is different from that of our 
invention, and its different method produces a different product. The 
products made by Iwase et al. are isotropic ferrites. They are made under 
conditions of temperature and mole ratio of alkali to metal salts 
different from those taught in accordance with the present invention. 
Iwase et al. teach that their product is formed only if the conditions of 
temperature and mole ratio of alkali to metal salts fall above the dashed 
curve in FIGS. 2A to 2C of the patent. Moreover, their method involves 
heating the precipitate while oxidation is occuring. 
The products made by the present invention are acicular (needle-like) 
ferrites. The product is formed under conditions of temperature and mole 
ratio of alkali to metal salts which are different from those disclosed in 
Iwase et al. Moreover, our method provides that heating of the precipitate 
takes place after oxidation has occurred. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, yellow pigments are provided by a 
spinel of iron and a metal selected from the group consisting of Mg, Zn, 
and Sn, as well as mixtures thereof. 
The spinels hereof may be produced by reacting a ferrous sulfate and a 
metal nitrate with a basic solution. The reaction proceeds at 5.degree. to 
50.degree. C., preferably room temperature. The precipitate is then 
aerated and reheated to obtain the spinels hereof. 
The spinels are temperature-stable up to about 900.degree. C. 
For a more complete understanding of the present invention, reference is 
made to the following detailed description and accompanying examples.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention provides yellow pigments which are temperature-stable 
up to about 900.degree. C. 
As is known to the skilled artisan, yellow iron oxide pigments are called 
"goethite". These pigments are considered to be hydrated oxides with a 
crystalline composition of alpha-FeOOH. It is the transformation of yellow 
alpha-FeOOH to a red alpha-Fe.sub.2 O.sub.3 which causes color 
instability. The transformation of alpha-FeOOH to alpha-Fe.sub.2 O.sub.3 
occurs over a wide temperature range, beginning at 175.degree. C., and is 
dependent upon the nature of the pigment and the particle size thereof. 
The transformation is a function of temperature and length of time of 
exposure to such temperature. Generally, those skilled in the art consider 
the transformation to involve the dehydration of alpha-FeOOH to 
alpha-Fe.sub.2 O.sub.3. A study of this phase transformation using 
differential thermal analysis shows that the transformation is complete at 
265.degree. C. to 277.degree. C., although the transformation commences at 
much lower temperatures. 
The present invention, as will subsequently be detailed, is based upon the 
fact that the phase transformation is related not solely to dehydration 
but also to crystalline structure. For example, all four hydrated iron 
oxides with a composition of FeOOH differ in color by virtue of their 
crystalline structure, only alpha-FeOOH being yellow. 
The present invention provides spinels of iron with various metals to 
produce stable yellow pigments. The various metals which are amenable 
hereto are selected from the group consisting of Mg, Zn, Sn, as well as 
mixtures thereof. 
Although the applications do not wish to be bound by any theory, it appears 
that, by virtue of the distribution of iron and other atoms within the 
spinel structure, the mobility of the iron atom is greatly inhibited and 
restricted. This restriction is further fortified by the electrostatic 
interaction between the metals, iron, oxygen and hydroxyl groups in the 
pigment. This restricted mobility results in a higher requirement of 
thermal energy to bring about the yellow-to-red color transformation of 
the iron oxide. 
The spinels hereof may be produced by the reaction of hydrated ferrous 
sulfate and a metal nitrate hydrate or its equivalent in a solution, 
initially acidic, which has been subsequently brought to an alkaline pH. 
The reaction preferably proceeds at room temperature although those 
skilled in the art will appreciate that other temperatures such as 
5.degree. to 50.degree. C. also may be used, if desired. The precipitate 
so obtained is then oxidized to promote the oxidation of the ferrous iron. 
To state the present invention in its method aspect comprehensively, this 
invention may be viewed as comprising a method of making a pigment having 
good stability at high temperatures, said method comprising the steps of 
forming a first aqueous solution consisting essentially of water, a 
soluble ferrous salt, and at least one other salt, said salt being a 
soluble salt of a non-ferrous metal selected from the group consisting of 
magnesium, zinc, and tin, said ferrous salt and said non-ferrous salt 
being present in said solution in proportions such that the respective 
quantities of iron and other metal which are present will yield, when said 
solution is so adjusted in pH as to cause precipitation, the formation of 
a substantial quantity of a precipitate oxidizable to a spinel of the 
formula XFe.sub.2 O.sub.4, in which X is a metal selected from the group 
consisting of magnesium, zinc, and tin; mixing with said first aqueous 
solution a second aqueous solution to form a reaction mixture, said second 
solution containing a substantial proportion of a soluble alkalizing 
compound selected from the group consisting of the carbonates, 
bicarbonates, and hydroxides of the alkali metals, the said second 
solution being used in such quantity and the said first and second 
solutions being in such a state of dilution that there results upon said 
mixing the formation of a precipitate in the form of particles of 
sub-micron size, the liquid phase of said reaction mixture being as a 
result of such precipitation substantially completely depleted in its 
content of metal ions having a valence greater than one; oxidizing at 
15.degree. to 35.degree. C. the said precipitate to form a spinel in 
aqueous solution; then heating said aqueous solution having said spinel 
therein to a temperature of from about 75.degree. to 100.degree. C.; and 
recovering said spinel. 
Although we have worked particularly with hydrated metal nitrates, among 
which Mg(NO.sub.3).sub.2.6H.sub.2 O and Zn(NO.sub.3).sub.2.6H.sub.2 O may 
be specifically mentioned, those skilled in the art will appreciate that 
it may in certain instances be possible to achieve the desired results 
with other equivalent materials which will suggest themselves to those 
skilled in the art. The use of hydrated salts is in general to be 
preferred, because such salts usually can be dissolved in water somewhat 
more quickly, other things being equal, than their anhydrous counterparts. 
After an aqueous solution of a desired strength has been obtained, it 
makes no difference whether the salt was or was not originally in the 
hydrated form. 
Various other soluble anions may sometimes be used in place of nitrate, 
such as chloride or sulfate. As has been mentioned elsewhere, divalent tin 
may be used as a cation. 
Suitable bases or solutions thereof are alkali-metal bases such as the 
sodium, potassium, or lithium carbonates, hydroxides, bicarbonates and the 
like. A particularly preferred base is sodium carbonate. Generally, a 
stoichiometric equivalent or base is employed. To be more specific, this 
means the use of one mole of sodium carbonate (or its equivalent, such as 
two moles of sodium bicarbonate) for each mole of, for example, hydrated 
ferrous sulfate. Insofar as its effect upon, for example, ferrous sulfate 
is concerned, the sodium carbonate may be considered as taking part in the 
reaction 
EQU FeSO.sub.4 +H.sub.2 O+Na.sub.2 CO.sub.3 .fwdarw.Fe(OH).sub.2 +Na.sub.2 
SO.sub.4 +CO.sub.2 .uparw. 
Insofar as its effect upon, for example, magnesium nitrate is concerned, 
the sodium carbonate may be considered as taking part in the reaction 
EQU Mg(NO.sub.3).sub.2 +H.sub.2 O+Na.sub.2 CO.sub.3 .fwdarw.Mg(OH).sub.2 
+2NaNO.sub.3 +CO.sub.2 .uparw. 
The use of a stoichiometric quantity of sodium carbonate or the like is 
usually to be preferred, but satisfactory results can be obtained in some 
cases, whether the quantities used are in a stoichiometric relation or 
not. By "satisfactory results", we mean obtaining by reaction in an 
aqueous medium an appreciable yield of a fine-particled spinel having a 
desired composition and good high-temperature stability. If the departure 
from the quantities dictated by stoichiometry is not too great, an 
appreciable yield of the desired spinel can be obtained, even though the 
process is operated somewhat wastefully in respect to its use of carbonate 
or of metal salt. It is necessary to use at least enough of sodium 
carbonate to precipitate both the iron and a substantial proportion of the 
other metal, such as zinc. 
In the practice of the present invention, obtaining particles of the 
desired size is an important consideration. The particle size is in part 
dependent upon the degree of dilution of the reactant solutions employed; 
the use of solutions more dilute yields particles of finer size, other 
things being equal. 
It will be understood that the oxidation may be done conveniently, in most 
cases, by aeration at about room temperature, i.e., approximately 
15.degree. to 35.degree. C. 
Oxidation can be performed in any suitable manner. One way is aeration 
(spraying the suspension-containing solution into air from a perforated 
pipe). Another is bubbling air or a mixture of oxygen and inert gas 
through the suspension-containing solution. Another is shaking the 
suspension-containing solution in a vessel along with air or a mixture of 
oxygen and an inert or unreactive gas. Still another possibility is the 
use of a suitable chemical oxidizing agent, such as hydrogen peroxide or 
any of a number of chemical oxidants which can provide oxygen but will not 
(as potassium permanganate or sodium dichromate would) tend to impart any 
color to the solution. The oxidation action which is required is one that 
it is not practical to obtain by allowing the solution to stand while 
exposed to air or an atmosphere of oxygen. Oxidation can be monitored by 
titration to determine when it has been completed. 
The solution is then heated to a temperature of from about 75.degree. C. to 
about 100.degree. C. It is desirable to maintain the high temperature for 
a time long enough to "set the crystal", i.e., ensure the development of 
desired crystalline forms in the product. X-ray diffraction tests can be 
used to monitor the progress and ensure the completion of the development 
of the desired crystalline forms. 
The spinel is then recovered by conventional filtration and washing 
techniques. 
The spinel thus obtained has a very fine particle size. The spinel 
particles are substantially all of a size under one micron in maximum 
dimension, and in most cases, they are even finer, such as 0.1 micron in 
maximum dimension. The performance of pigments containing the spinel 
particles is dependent upon their having particles of the size indicated. 
In general, a fusion process for obtaining spinels results in particles 
substantially larger than those obtained with the present invention. Fine 
particles made according to the present invention give purer colors than 
larger particles, made by a fusion process, do. 
In further description of the temperature-stable spinel product made in 
accordance with the invention, it may be stated that a procedure of the 
kind detailed above gives particles which are of acicular (needle-like) 
shape and, as is revealed in the drawings, of a length which is on the 
order of 0.01 to 0.13 microns. FIG. 1 corresponds to the product of 
Example III, below, and FIG. 2 is a photomicrograph of a similar product, 
but one made by starting with zinc sulfate instead of zinc nitrate. FIG. 3 
shows the effect of calcination upon the product shown in FIG. 1; the 
particles are no longer acicular. 
The photomicrographs reveal that the process discussed above yields a 
product different from any encountered in the above-mentioned U.S. 
patents. The particles of U.S. Pat. No. 2,904,395 are said to be acicular, 
but with a length of 0.4 to 1.0 micron, as compared with about 0.01 to 
0.13 micron for the particles shown in FIGS. 1 and 2. 
That the materials shown in FIGS. 1 and 2 are spinels has been confirmed by 
X-ray diffraction tests. The same materials exhibit an absence of the 
characteristic endotherm in differential thermal analysis at a temperature 
around 260.degree. to 277.degree. C. Such endotherm is characteristic of 
phase transformation of yellow alpha-FeOOH to red Fe.sub.2 O.sub.3. This 
means that the materials in FIGS. 1 and 2 are spinels and are not 
alpha-FeOOH. 
U.S. Pat. No. 3,832,455 does not provide information concerning the size 
and shape of the particles produced by its teachings, but its method 
involves a necessary calcining step, and in view of FIG. 3, it appears 
unlikely that it yields a product containing acicular particles. U.S. Pat. 
No. 3,887,479 similarly involves use of high temperatures and does not 
contain information concerning the morphology of its product particles. 
The differences between the particles of U.S. Pat. No. 3,822,210 and those 
of the present invention are apparent from a consideration of FIGS. 1 to 3 
hereof and the photomicrographs in that patent, allowance being made for 
the difference in scales between them and FIGS. 1 to 3. 
After they have been obtained as indicated above, the fine-particled 
spinels made according to the invention may be incorporated in plastics as 
a pigment to produce a desired yellow, by using proportions and practices 
well known to a person of ordinary skill in the art. 
In practicing the present invention, the spinels are preferably derived 
from iron and Mg, Zn, or mixtures thereof. Such spinels show temperature 
stability up to about 900.degree. C. Particularly good results have been 
obtained with the iron-zinc spinels, which are preferred. 
Those skilled in the art will know how to incorporate the spinel pigments 
into plastics or the like. The necessary proportions, procedures, and 
equipment do not require explanation. 
A comparison of weight loss versus temperature between a yellow iron oxide 
pigment of the prior art and an iron-zinc spinel of the present invention 
establishes that the spinel has less water, present as hydroxyl, than the 
pure iron oxide pigment. This is shown in Table I, below. 
TABLE I 
______________________________________ 
% Wt. Loss 
Temp., .degree.C. 
Iron-Zinc Spinel 
Iron Oxide 
______________________________________ 
100 2.7 1.9 
250 6.2 7.2 
350 7.7 13.7 
400 8.1 14.0 
______________________________________ 
The chemical compositions of the spinels hereof generally correspond to the 
structure XFe.sub.2 O.sub.4 where X is a metal as denoted above. 
As hereinbefore noted, mixtures of metals can be utilized. However, 
electroneutrality in the crystal must be maintained. 
Following are specific examples illustrating the principles of the present 
invention. These examples are to be construed as illustrating and not 
limiting the present invention. 
EXAMPLE I 
Into a suitable reaction vessel equipped with titration means, cooling 
means, aeration means and agitation means, there was added a three liter 
solution of 27.8 grams per liter (gpl) of FeSO.sub.4.7H.sub.2 O and 17 gpl 
Mg(NO.sub.3).sub.2.6H.sub.2 O. With stirring, 200 milliliters of 231 gpl 
Na.sub.2 CO.sub.3 solution was added thereto. The temperature in the 
vessel at the time of carbonate addition was 19.degree. C. 
With stirring, a precipitate is formed in the flask. 
After precipitation ceased, air was bubbled into the flask via the aeration 
means. Contemporaneous with aeration, the oxidation of the ferrous sulfate 
was monitored by titration with potassium permanganate. 
After aeration was completed, the precipitated solution was heated to 
90.degree. C. and maintained thereat for two hours, then cooled to room 
temperature. The precipitate was then recovered by filtration of the 
solution. The precipitate was then washed and dried. A temperature-stable 
yellow pigment was thus obtained. In other words, the pigment displayed, 
upon being subjected to differential thermal analysis, an absence of the 
transformation from goethite to hematite of the kind discussed herein 
above. The pigment also has been tested by being subjected to elevated 
temperatures, up to about 900.degree. C., showing no change of color. 
EXAMPLE II 
Example I was repeated using an iron-metal solution of 27.8 gpl 
FeSO.sub.4.7H.sub.2 O and 19.8 gpl Zn(NO.sub.3).sub.2.6H.sub.2 O, to which 
was added 46 gms of Na.sub.2 CO.sub.3 solution. A yellow pigment which was 
temperature-stable was obtained. 
EXAMPLE III 
A solution containing 67 lbs. (30.4 kg.) of ferrous sulfate heptahydrate 
and 47 lbs (21.3 kg.) of Zn(NO.sub.3).sub.2.6H.sub.2 O was prepared in 150 
gallons (567.8 liters) of water maintained at 20.degree. C. To this 
solution was added a solution of 37 lbs. (16.8 kg.) of Na.sub.2 CO.sub.3 
dissolved in 19.3 gallons (73 liters) of water. The reaction mixture was 
aerated at 3 cubic feet per minute (84.9 liters per minute) of air until 
maximum oxidation of the ferrous ion occurred, as determined by potassium 
permanganate titration. The reaction mixture was then heated to 90.degree. 
C., filtered, washed and dried. A temperature-stable yellow pigment was 
thus obtained. 
EXAMPLE IV 
Example III was repeated, except that 46 lbs. (20.9 kg.) of zinc sulphate 
heptahydrate were used in place of the indicated quantity of zinc nitrate 
hexahydrate. Once again, a temperature-stable yellow pigment was thus 
obtained. 
EXAMPLE V 
Samples of each of the pigments of Examples I-IV hereof were heated for 1/2 
hour at 280.degree. C. in an oven. Each sample remained yellow in color. 
When heated to 800.degree. C. for 1/2 hour, the samples still remained 
yellow. 
In the claims, the term "transparent pigment" is used to indicate a pigment 
which is capable of being incorporated in a vehicle to provide a coating 
which is substantially transparent to visible light. This implies that the 
particles of pigment have a size less than the wavelength of visible 
light. 
While we have shown and described herein certain embodiments of our 
invention, we intend to cover as well any change or modification therein 
which may be made without departing from its spirit and scope.