Titanium dioxide pigment and process for making same

The present invention provides an improved process for coating titanium dioxide with precipitated alumina (aluminum oxide) for use as a pigment in plastics wherein a first coating of amorphous alumina is precipitated onto the titanium dioxide and then a second coating of crystalline alumina is precipitated over the first coating. In operation, the pH of an aqueous slurry of titanium dioxide pigment is adjusted to be within a first pH range as the first alumina coating is effected by adding an amount of sodium aluminate to the aqueous slurry and then the pH of the slurry is adjusted to be within a second pH range while a second amount of sodium aluminate is added to form the second coating over the first coating. The present invention also contemplates an improved titanium dioxide pigment having a first coating of amorphous alumina and a second coating of boehmite alumina.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to improved titanium dioxide 
pigments and, more particularly, but not by way of limitation, to an 
improved titanium dioxide pigment having a first coating of amorphous 
alumina and a second coating of boehmite alumina. 
2. Description of the Prior Art 
Titanium dioxide has found extensive use as a pigment material in a wide 
range of applications. Certain characteristics of the pigment have been 
enhanced by coating the pigment with a hydrous metal oxide. For example, 
the weathering character of the pigment has been improved by coating the 
pigment with aluminum oxide. 
Various types of coatings have been applied to titanium dioxide pigments 
and various processes have been developed for applying such coatings. For 
example, U.S. Pat. No. 3,086,877, issued to Sheehan et al, disclosed a 
process wherein a strong acid was added to an aqueous slurry of titanium 
dioxide to adjust the pH to below about 4.0, and a sufficient amount of 
sodium aluminate was then added to the slurry to bring the pH of the 
slurry mixture up to a value of between 6.0 and 8.0. According to the 
Sheehan patent, this particular process produced an alumina coated 
titanium dioxide pigment having superior weathering properties. 
In some applications, multiple coatings of hydrous metallic oxides have 
been applied to titanium dioxide. The Rechmann patent, U.S. Pat. No. 
3,203,818, disclosed a process for applying two oxide coatings by 
subjecting the pigment to a first coating treatment, a calcination 
treatment, and then a second coating treatment, which was applied in a 
manner similar to the first coating treatment. The Allan patent, U.S. Pat. 
No. 3,383,231, disclosed the coating of titanium dioxide pigments with the 
oxides of at least two metals. The Allen patent, U.S. Pat. No. 3,897,261, 
disclosed the coating of titanium dioxide pigments with successive 
coatings of silica and alumina until a total of four oxide coatings were 
applied to the pigment. 
In general, many prior art processes involved the addition of a water 
soluble metal salt to an aqueous slurry of titanium dioxide, and the 
addition of a pH adjusting agent to neutralize the slurry and to form 
insoluble hydrous oxides on the pigment. In addition to those patents 
specifically mentioned above, the following patents disclosed various 
processes for coating titanium dioxide pigments: U.S. Pat. No. 2,284,772 
issued to Seidel; U.S. Pat. No. 2,357,101 issued to Geddes; U.S. Pat. No. 
3,251,705 issued to Rieck et al; U.S. Pat. No. 3,409,454 issued to Andrew 
et al.; U.S. Pat. No. 3,418,147 issued to Fields; U.S. Pat. No. 3,459,575 
issued to Andrew et al.; U.S. Pat. No. 3,510,344 issued to Goodspeed; U.S. 
Pat. No. 3,515,566 issued to Moody et al.; U.S. Pat. No. 3,545,994 issued 
to Lott, Jr. et al.; U.S. Pat. No. 2,671,031 issued to Whateley; U.S. Pat. 
No. 3,567,478 issued to Dietz et al.; U.S. Pat. No. 3,591,398 issued to 
Angerman; U.S. Pat. No. 3,595,822 issued to Swank; U.S. Pat. No. 3,660,129 
issued to Luginsland; U.S. Pat. No. 3,770,470 issued to Swank; and U.S. 
Pat. No. 3,859,109 issued to Wiseman et al. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Various problems have been encountered in processing titanium dioxide 
pigments which are used in plastics and which have been coated via prior 
processes, such as the various processes specifically referred to before, 
for example. More particularly, various problems have been encountered in 
the filtration of such coated, plastic grade pigments and in the 
micronizing or milling of such coated, plastic grade pigments. Referring 
particularly to the milling of such plastic grade, coated titanium dioxide 
pigments, the pigment is sticky and tends to stick to the milling 
apparatus and the material handling apparatus utilized in conjunction with 
the milling operation. The sticking of the pigment to the various 
apparatus makes it extremely difficult to load the pigment into the 
milling apparatus and to remove the pigment from the milling apparatus 
after the completion of the milling operation. Further, the sticking of 
the pigment tends to plug or foul the milling apparatus which increases 
the maintenance expenses and the maintenance time required to maintain the 
milling equipment operational. 
It has been found that the plastic grade titanium dioxide pigment of the 
present invention has less of a tendency to stick to the milling apparatus 
and the various devices associated with the milling process and, in 
general, the titanium dioxide pigment of the present invention has 
substantially improved material handling characteristics with respect to 
one particular process, it was found that the process of the present 
invention substantially reduced the manufacturing cost and the production 
rate was essentially doubled while producing a plastic grade titanium 
dioxide pigment with equal or superior overall properties as compared to 
plastic grade titanium dioxide pigment produced via prior processes. 
The structure of the alumina coating on titanium dioxide pigment varies 
with the pH range in which the hydrous oxide is precipitated. It has been 
found that alumina precipitated in an acidic range is of an amorphous 
form, while alumina precipitated in a basic pH range is of a crystalline 
form or, more specifically, of a boehmite form or type. The boehmite form 
of alumina coating provides the desired characteristics for the titanium 
dioxide pigment, while the amorphous form of alumina coating improves the 
handling characteristics associated with the manufacturing processes 
involved in producing the pigment. 
It has been found that titanium dioxide coated with about 1% alumina 
possesses superior handling properties when approximately half of the 
alumina coating is precipitated from an acidic medium (providing an 
amorphous alumina) and the remaining portion of the alumina coating is 
precipitated in a basic pH range (producing a boehmite alumina). Thus the 
present invention contemplates an improved process for adding or applying 
multiple layers of alumina in a single process treatment. 
In accordance with the present invention, an aqueous slurry of titanium 
dioxide is prepared (a floccing agent may be added if desired in a 
particular application). A first coating of alumina is formed on the 
pigment by adjusting the pH of the slurry to a level within a first pH 
range, the pH of the slurry preferably being below about 2.0, and then 
adding a sufficient amount of basic aluminum salt to produce the first 
alumina coating and to adjust the pH to a level between about 8.0 and 9.0. 
Following the formation of the first coating, a second coating of alumina 
is precipitated onto the pigment by adding additional basic aluminum salt 
while maintaining the pH of the slurry within a second pH range, the 
second pH range preferably being at the level between about 8.0 and 9.0. 
After the final addition of basic aluminum salt, the pigment may be 
digested if desired. In one particular process, the digestion is 
accomplished by subjecting the slurry to a temperature of between about 
120.degree. F. and 140.degree. F. for approximately 15 minutes. 
The improvements in the handling characteristics of titanium dioxide 
pigment coated with the dual amorphous and boehmite alumina coatings of 
the present invention are demonstrated by the results appearing in TABLE I 
below: 
TABLE I 
______________________________________ 
A. B. C. D. E. 
%Al.sub.2 O.sub.3 
pH range Cake (mm) Milling 
Plastic 
______________________________________ 
Test 1 0.5 8.0-9.0 5.0 Poor Good 
Test 2 0.5 8.0-9.0 3.5 Fair Marginal 
Test 3 0.5 2.0-7.5 14.0 Bad -- 
Test 4 1.0 8.0-9.0 6.0 Fair Acceptable 
Test 5 1.0 8.0-9.0 5.0 Good Marginal 
Test 6 1.0 2.0-9.0 13.0 Good Good 
______________________________________ 
Each of the tests reported in the TABLE I involved a pigment coated by a 
process involving the addition of sodium aluminate to an aqueous slurry of 
titanium dioxide at a controlled pH range as shown in column B of TABLE I. 
The use of sodium aluminate is convenient because of its capacity to 
increase the pH level of the aqueous slurry. The pH level of the slurry 
was adjusted by adding an appropriate amount of acid. It is also possible, 
and within the contemplation of the invention, to use an acid aluminum 
salt and then add a basic solution for adjusting the pH level. 
Column A of TABLE I indicates the quantity of hydrous oxide coating, 
expressed as aluminum oxide, that was added to the titanium dioxide for 
the tests recorded in TABLE I. 
The test results recorded in TABLE I generally indicate that better results 
are obtained when 1% of oxide coating (expressed as aluminum oxide) is 
provided. That is, the handling characteristics of tests 4 through 6 (1% 
Al.sub.2 O.sub.3) were better than those conducted with lesser amounts of 
aluminum oxide (0.5% Al.sub.2 O.sub.3). 
Column B of TABLE I provides the pH range of the slurry in which the 
alumina coating was precipitated. Tests 1 and 4; tests 2 and 5; and tests 
3 and 6 represent comparable results at the 0.5% and the 1.0% weight level 
of alumina coating added, respectively. Tests 1 and 4 were conducted by 
maintaining the pH of the slurry between about 8.0 9.0. A flocculant 
consisting of magnesium sulfate was added to the aqueous slurry of tests 2 
and 5, and the pH of the slurry was maintained at a level between about 
8.0 and 9.0 during the precipitation of the alumina in the manner of tests 
1 and 4 (it should be noted the flocculant may consist of any of the 
soluble alkaline earth metal sulfates, as may be desired in a particular 
application). Test 3 was conducted in an acidic range of from about 2.0 to 
about 7.5. 
Test 6 was conducted according to the present invention by adding to the 
aqueous slurry a sufficient amount of sulfuric acid to reduce the pH of 
the slurry to below about 1.0, then a first quantity of sodium aluminate 
to raise the pH of the slurry to between 8.0 and 9.0 (this step produces a 
predominantly amorphous type of alumina coating). Then, a second quantity 
of sodium aluminate was added to the slurry mixture while simultaneously 
maintaining the pH between about 8.0 and 9.0 by adding more of the 
sulfuric acid solution (this step produces an alumina coating which is 
predominantly boehmite). 
Column C of the TABLE I provides the cake thickness of the filtration of 
the tests which were made on a commercially available leaf filter of the 
type manufactured by EIMCO Corporation of Salt Lake City, Utah. 
Column D of the table indicates the milling characteristics of the alumina 
coated titanium dioxide. The milling step of the process was accomplished 
utilizing a fluid energy type of mill (a micronizer) of the type 
commercially available from such manufacturers as the Sturtevant Mill 
Company of Boston, Mass., for example. The notations of "Good", "Poor", 
"Fair", and "Bad" in column D relate to the tendency of the titanium 
dioxide pigment not to stick to the milling apparatus. 
Column E of TABLE I indicates the character of the pigment in a plastic 
matrix. As shown by the results in TABLE I, the pigment produced in 
accordance with the present invention in test 6 was the only test that 
gave good results in both the milling and plastics characteristics, 
thereby demonstrating the superior handling characteristics achieved by 
the invention while maintaining the required characteristics of the 
pigment for use in a plastic matrix.

By way of further illustration, the following example is provided. 
EXAMPLE 1 
An aqueous slurry of titanium dioxide was prepared by adding 1,145 grams of 
titanium dioxide (TiO.sub.2) to a sufficient amount of water to provide a 
total slurry volume of 3.2 liters. The slurry was heated to 60.degree. C., 
and 8 milliliters of concentrated sulfuric acid (H.sub.2 SO.sub.4) was 
added over a period of about 5 minutes to the slurry to adjust the pH 
level of the slurry to less than 2.0 and in a range between about 1.5 and 
about 2.0. 
Sodium aluminate solution served as the alumina coating agent, with 41 
milliliters of the solution containing an equivalent of 11.5 grams of 
Al.sub.2 O.sub.3. This was determined to be adequate to precipitate 1% 
Al.sub.2 O.sub.3 based on the weight of TiO.sub.2. A first portion 
(approximately 30 milliliters) of this sodium aluminum solution was added 
drop-wise to the slurry to raise the pH of the slurry to about 8.7. 
Following the above steps, the pH was maintained in a range of 8.4 to 8.7 
by adding H.sub.2 SO.sub.4, while a second portion (approximately 11 
milliliters) of the sodium aluminate solution was added drop-wise to the 
slurry mixture. 
After all of the sodium aluminate solution had been added to the slurry, 
the pH of the slurry was adjusted to 6.0. The slurry was then digested at 
about 120.degree. F. to about 140.degree. F. for 15 minutes, and the pH 
once again was adjusted to 6.0 by adding H.sub.2 SO.sub.4. 
The slurry mixture was filtered and the precipitate was washed with water. 
A leaf filter produced a 13 millimeter cake in a 20 second immersion and a 
filtration test was carried out under a vacuum of about 28 inches of 
mercury. The cake was removed and dried at about 110.degree. C., and the 
dried product was micronized or milled. 
The pigment produced by the above method milled easily and received a 
"good" rating in the plastic test. 
The above example discloses a process in which a basic aluminum salt and 
acid are added to the titanium dioxide slurry. While this technique is 
convenient, other constituents may also be used. For example, it is 
possible to add acid and aluminate solution, or an acid aluminum salt and 
a base, simultaneously or in any manner that produces an amorphous alumina 
precipitate in an amount from about 0.3 to 0.7% based on the weight of the 
pigment. This first coating then is followed by a second coating of 
hydrous alumina in an amount from about 0.3 to 0.7% based on the weight of 
the pigment, the second coating being precipitated under conditions which 
precipitate alumina predominantly in a boehmite form. 
It has been found that the application of the invention reduces the 
manufacturing cost of plastic grade titanium dioxide pigment and 
production rates have been essentially doubled through the use of the 
present invention, while providing a pigment having equal or superior 
properties to pigment produced by prior processes. 
Changes may be made in the process or in the steps of the process or in the 
sequence of the steps of the process of the present invention without 
departing from the spirit and scope of the invention as defined in the 
following claims.