Note: Descriptions are shown in the official language in which they were submitted.
The treatment o~ pigmentary titanium dioxide (TiO2)
in the rutile or ~natase ~orm wlth hydrous oxides, especially
silica and alumina, is known in the art to provide reæis-
tance to chalking and discoloration in paints, good opacity
in paper and good hiding power in ~lat paints. Generally,
the silica i~ applled to the pigment by precipitation from
sodium silicate with acid in an aqueous slurry o~ the
pigment. me resulting pigments, having a dense, i.e., non-
porous silica coating, have good dur~bility but do not pro-
vide as hlgh a degree of hiding power as is desirable inmany paint compositions. Applying a loose, i.e., porous,
alumina to den~e silica-coated TiO2 improves the di~pers-
ibility of dense silica-coated TiO2 without sacri~icing
durability, but does not signiflcantly enhance the hiding
power of the dense silica-coated TiO2. Applying a porous
coating consleting essentially o~ alumina and silica to
pigmentary titanium dioxide provides a coated pigment having !,'
generally better hiding power than the denæely coated pig-
ment, but lower durability than iæ desired ~or many -
appl~cat~ons.
This invention provides for a pigmentary TiO2
coated in such a way as to combine the durabilit~ o~ dense ;
silica coatings with the hiding power of loose hydrous
coatings.
For purpoæes of this application pigmentary means
that the material i8 pulv~rulent and of good ~rhitenes~ with
an average particle diameter o~ ~rom about 0.15 to about
0.3 micron.
mis in~ention provides for a coated TiO2 pigment
consisting essentially o~ a core o~ pigmentary TiO2, a
i -2
~ .,
,; ~, , ;
3g~
dense inner coating of from 1% to 10% by weight o~ dense
amorphous silica, calculated as SiO2, and based on the un-
coated pigmentary TiO2, and from 0~ to 5% by wsight o~
. ~ ~
alumina, calcul~ted as A1203, and based on the uncoated ;
pigmentary TiO2, and a porous outer coating o~ porous
silica and alumina. The porous silica in the porous outer
coating iB present in an amount ~rom 0.5~ to 15% by weight,
calculated as SiO~, and based on the uncoated pigmentary TiO2,
and the alumina in the porous outer coating is present in
an amount from 0.5% ~ 11% by weight, calculated as A1203,
and based on the uncoated pigmentary T102. The preferred
amount o~ dense amorphous silica in the dense inner coating
is ~rom 3.5~ to 6~ by weight. When alumina is present in
the dense Lnner coating the preferred amount is ~rom 1% to
3% by weight. In the porous outer coating it is pre~erre~
that from 5~ to 10~ by weight o~ porous silica and ~rom 5%
to 10% by weight of alu~ina be present.
In accordance with the present invention there is
provided a process for preparing coated TiO2 pigment by the
steps o~ (i) adJusting the pH o~ an aqueous slurr~ o~
pigmentary TiO2 to ~rom about 9 to 10, (ii) adding to said
slurry an aqueous solution of soluble silicate in an amount
calculated ~o provide from 1~ to 10~ by weight of silica,
calculated as SiO~, and based on said pigmentary TiO~,
(iii) slowly adding dilute aqueous acid to said slurry
until the pH o~ said slurry is ~rom 6 to 7.7, (iv) adding
to said slurry~ maintained at a pH ~rom 5 to 9, an aqueous
solution o~ a soluble silicate in an amount calculated
to provide ~rom 0.5~ to 15~ by weight of silica~ calculated
as SiO2, and based on æaid pigmentary TiO2 (v) adding to
-3- ~
' :'
~ .
~aid slurry, main*ained at a pH from 4 to g~ an aqueous
solution o~ soluble alumin~te in an a~ount calculated to
i provide ~rom 0-5% to 11% by weight of alumina~ calculated
- as A1203, and ba~ed on said pigmentary TiO2 (vi) ad~ust-
ing the pH o~ said slurry to ~rom 7 to 9, and (vii) separat-
ing the resulting coated TiO2 plgment ~rom said ~lurry. To
incorporate al~mina ln the inner coating, the above-
de~cribed process comprlses the additional ~tep (liia)
of adding to said ~lurr~, a~ter step (iii) and prior to
10 ~tep (iv), an aqueous solution of soluble a~umanate in an .
amount calculated to provide ~rom 0.5~ to 5% by weight o~
alumina, calculated as A1203, and based on said pigmentar~
TiO2, while the pH o~ said slurry is ma~n~ained from 6 to
9. ~ .
The order of process ~teps (iv) and (v~ is not
particularly critical ln that a ~uitably coated pigment
re~ults when the order o~ these steps i~ reversed or per-
formed simultaneou31y.
Although curing the slurry at variou3 ~tages in ~ -
the proce~s iB not essential to the attainment of good
pigment properties, in applications where curing i~ deemed
de~irable the ~lurry can b~ cured, for ex~mple, at tempera-
tures of at least 40C~, and pre~erably at least 70C.y
for at least 15 mlnutes, and pre~erably at lea~t 30 minutes,
a~ter a~y or all of proce~s steps (i), (iii), (i~), and (v).
m~ base titanium dioxide pigment may be pre-
pared by the high temperature vapor phase oxidation o~
TiCl~, the ~apor pha~e hydrolysis o~ TiC14, or the
~ydrolysis o~ colloidall;y seeded sul~uric acid ~olutio~
o~ titani~erou~ raw materials such as ilmenite. me
-4-
'~'
: , . . .
S3~
.. ~ :~. .
hydrolysate o~ the sul~uric acid process must be washed
and calcined to develop the cxystalline characteristic -~
and particle size needed ~or the good light æcattering
characteristics o~ pigments. The temperature o~ the
slurry being treatod may vary from room temperature to
above 90C., but it i~ pre~erred that the temperature be
from 80C. to 90C. me slurry should be agitated through-
out the precipitation o~ both the silica and the alumina to
in~ure general uniformity o~ the pH conditions within the `~
specified ranges.
Any soluble silica may be u~ed in the process
including sodium or potassium silicate. Commercially
availa~le water soluble sodlum silicates wi~h SiO2/Na20
weight ratioæ from about 1,6 to about 3.75 and varying
~rom 32% to 54% by weight o~ solids, with or without
~urther dilution are the most practic~l. To apply dense
amorphous silica to the pigmentary TiO2 the slurry must be
basic during the addition o~ the ef~ective portion o~ the
æoluble silicate, then slowly acidi~ied over a period of -
pre~erably at least 45 minutes until a pH from about 6 to
7.7 is reached. To apply a porous silica to the coated
pigment the slurry should pre~erably be acid during the
addition o~ the e~ective portion o~ the soluble silicate.
m e acid used may be any acid, such as HClj H2S04, ENC
or H3P04 having a diæsociation constant suf~iciently high `;
to precipltate silica and used in an amount su~icient to
maint&in an acid oondition in the slurry. Compounds such
as TiOS04 and TiCl~ whlch hydrol~ze ~o form acid may al~o be
used. Alternative to adding all the acid ~ir~t, the soluble ~ ~:
~ilicate and the acid may be added simultaneou~ly ~o long
".''.~ :
"', ' '' '
~ .
as the acidity of the slurry is pre~erably maintained at
a p~ of below about 7.~
me alumina o~ the inner coatlng or outer coating
must be precipitated in an environment having a pH ~rom about
4 to about g. To avoid sub~ecting the treated pigment to un-
duly acid or alkaline conditions, a pH ~rom about 6 to 8 is
pre~erred. For purpo~es o~ this invention it i5 pre~erable
to add about 75~ o~ the total desired alumina a~ sodium alum-
inate, while maintaining the slurry p~ betwee~ about 6 and 8.
The remainder o~ the sodium ~luminate can be added, allowing
the slurry pH to rlse to between about 8 and 9.
This proceæs may be applied to pigments which
ha~e been pre~iously treated with oxides such as titania~
but their presence is not essential to the benefi~s hereini
obtained~ After treatment according to this process the
pigment is recovered by known procedures including
neutralization of the ~lurry i~ neceæ~ary, flltration,
washing, ~rying ~nd frequentl~ a dry grinding step such as
micronizing. Drying is not necessary, however, as ~ thick
slurry of the product can be used directly in preparing
emul~ion paints where water is the liquid ~hase. The
process provide~ a method rOr obtaining good hiding power
and good durability in titanium dioxide pigmente ~or
both indoor and outdoor applicationsO
In the examples which ~ollo~, the test results
are obtained by the procedures describe~ below.
Acid solubilit~ o~ the coated TiO2 pigments of
this ~vention is measuxed by digesting a predetermined
quantit~ of eoated pigment in sulfuric acid and spectro~
photometricall~ comparing it to a standard TiO2 sample
~ ~ - . t
.
similarly treated~ The standard spectrophotometric curve
i~ prepared as follows: a standard ~olution i~ prepared
b~ di~solving 15 g. o~ ammonium sulfate and 1c0118 gO of
TiO2, containing 98.83% by weight TiO2J in 20 ml. of 66
Bé. sulfuric acid and diluting to 800 ml. with waterO
Elghty milliliters o~ the sul~uric acid solution is t,hen
added to this solution. A~ter allowing the resulting
solution to cool to room temperature, it is diluted to
one liter ~ith water. m e solution contains 0.001 g. o~
TiO2 per;ml. and ~hould be allowed to ~tand ~or one week
before using.
Samples 2, 4~ 6 and 8 ml. o~ the above- ;
pre~ared standard solution are each mixed with 10 ml. o~
30% hydrogen peroxide and diluted to 100 ml. with 10% sul-
furic acid. A~ter standing one hour, the abæorbance
o~ these solutions is read using a Beckman Model DK, DU or
B Spectrophotometer against a reference solution, prepared
by diluting 10 ml, of 30% hydrogen peroxide to 100 ml. with
. . .
10% sulfuxic acid at 400 m~ using 10 mm. cells. A plot o~ - -
titanium peroxide concentration in ~g./l. versus optical
densit~ i5 made for these samples.
To determine acid solubility o~ the coated TiO2
pigment, 0.2000 g. of coated TiO2 pigment is added with
~tirring to 10 mlc 66 B~. sul~uric acid at 175C. The
pigment is digested at 175C. ~or one hour. A~ter digestion ;~
the s~mple is quenched by pouring into crushed ice, m~ e
from distilled water. The sample is diluted to 100 ml.
with water and ~iltered. Ten milliliters o~ the filtrate
is mixed with 2 ~1. of 30% hydrogen peroxide and diluted
3 to 25 ml. wi-th 10% sul~uric acid. ~ter one hour the
' -7~
~ 3~
.
ab~orbence of the sample 1~ read against a reference solution,
prepared by dlluting 2 ml. of 30~ hydrogen peroxide to
25 ml. with 10% sulfurlc acld.
The concentr~tion of soluble TiO2 i8 determlned
from the above-prepared ~t~ndard spectrophotometric curve
from the mea~ured optical den~ity and the percent soluble
T102, i~e,, the acld solublllty, i8 calculated by dividlng
the concentr~tlon o~ soluble T102 by 8
The tinting ~trength test compares ~he green
re~lectance o~ paints ~hich contaln the sa~e ratio of TiO2
pigment ~o tinting gel. Although any color tint could be
u~ed for thi~ te~t, green or blue tints are preferred
becau~e the eye i8 more sen~itive to changes in ~tr~ngth :
wlth the~e colors. The tlnting gel i8 . prepa~ed by mlxlng
the followlng lngredients:
rt~ by wei~
~18 tilled water 77.6
A co~merciAlly avallable organic 20.0
pigment ~old by the Du Pont Company
20 under ~he Trade ~ark Mona~tral ~:~
Green B
Hydroxyethyl cellulose 2.4
Ten grams of the above-described tlntlng gel i9
mixed with 100 g. of paint. The paint i~ a conventlonal
lndoor water-bAsed emul~lon paint containing 2,5 lbs./gal.
(o.36 kilo~raw~/liter) TiO2 pigment, about 1.4 lbs./gal,
(0.19 kilograms/llter) of extender plgmen~ and about
2.3 lb~./gal~ (0.20 kilograms/llter~ of vinyl RCe~a~e-
ethylene emulslon, The pigment volum~ concentr~tion o~
thls paint i8 about 50~, Th@ TiO2 plgment u~ed ln the
stand~rd paint i9 a commercial T102 pigment produeed by
the vapor phase oxldat~on of TlC14 and t~ ated with A1203
-8-
r ~ .
~S3~
; .
and SiO2 according to the procedure o~ Angerman U,S.
Patent 3 591 398, Thl~ TiO2 plgment, designa~ed a~
Control ID in Table I, conslst~ essentially of ~7.6% by
weight of Tio2~ 6.2% by weight of A12~3 and 6.2% by
welght o~ S102, ba~ed on the weight of the treated ,
pigment,
A ~ilm of prepared p~int 1~ drawn on white
lacquered paper using ~ draw-down blade with o. oo8 lnch i ~. .
(0.020 cm.) clearance, The film i~ drled ~or at le~st
two hour~. The reflectanc`e of the dried ~ilm i~ measured
u~ing a green ~ilter on Neot~c "Du-Color n~ Model 220 Re~lecto-
mcter. From the value of thi~ refleetance~ R ~, the value
of the ratio o~ ~he ab~orption coe~ficient, K, to the
scattering coe~icient, S, i8 ~ound using the
Xubelka-Munk Table (D. B. Judd and G. Wy~zeckl, "Color ln ,~
Buslness, Science, and Industry", John Wll~y, Dr, San~, 19633
Appendi~ D)~ V~lues beyond the range ~hown in the Table can
be calculated by 2
(l-R~ ) -
K/S =
Expre~slng the tlnting ~trength on a relatlve :
ba~is, a value of 100 i8~ a~signéd as the`tinting ~trength of . - .
the standard, The relative val~e ~or the sample i~ calcu~
lated a~ ~OllQW~
(K/Si Standard
Relative Tlnting Strength - ~ X 100
~ The indoor Aldin~ power is determined uæing a
conventional lndoor water-based e~ulsion paint containing ' :~
3 lbs/g~l. (0.36 kllogram/llter~ o~ TiO2 plgment, about
~ 1.6 lbs~/gal. (0.19 kilqgram/liter~ o~ slllcate ~xtender~ .
and about 1.7 lbs/gal. (0,20 kilogram/liter~ o~ vinyl
* denote~ trade mark
_9_ :
., ;
.
~6~53~37
acryllc resinous binder in an aqueou~ emulsion. The TiO2
pigment volume concentratlon of this paint i9 about 49~3
the remaining 51% by volume o~ solids consisting essentially
: of silicate extender~ and benders. The TiO2 pigmen~ used in
the standard lndoor paint i8 the commerclal Ti~2 pigment
used for the tinting strength test described above,
The outdoor hiding power ls determined using a
conw~ntion~l outdoor paint and a ccmmercial ~iO2 pigment
produced by the vapor phase oxidation of TiC14 and treated
with dense amorphou~ sillca and alumina according to the
: procedure o~ Werner U.~. 3 ~37 502, Thl3 outdoor ~tandard ~:
T102 plgment, designated a~ "Control OD" in ~able I,
con~ists essentially o~ 92.6~ by weight of TiO2, 5.5% by
weight of dense amorphous ~ilica, calculated as SiO2, and
l.9~ by wei~ht of alumina, calculated a~ Al203.
. .
The conventional outdoor palnt used i8 a water
basod emul~ion containing about 2.5 lbs./gal. (0.30
kilogr~m/liter) of TiO2 plgment, about 2~ 0 lbs . /gal. (O. 24
kilogram/liter) of magnesium silicate ttalc) extender, and
20 about 4, 6 lbs. /gal, (O. 55 kilograms/liter) of an acrylic
emulslon (50% by weight of sollds) The TiO2 pigment volume
~- concentratlon of this paint ls about 18.5~ and the total
pigment volume concentration l~ about 40~.
A fll~ ~ prepared paint i8 drawn on a Morest
Chart Form 09, a glo~sy paper chart having black and whlte
areas, uslng a draw-down blade with an 000025 inch (O. oo64
cm.~ clearance3 at 50~ R.H. and 72F, (22C.~. Nhen the
~ fllm is dried, the reflectances over the whlte and b~ ck
~ back~round~ are mea~ured u~ing a Neotec '!Du-Color" Model 220
3 Reflectometer using a green ~ilt~r, One readlng is taken
-10-
''~4` ~ '
~353~7
- ` .
Dn each of two ~white" and "black" sections o~ each of
~our charts.
The ~cattering power (SX) i~ determined, using the
: above-mea~ured refl~ctances, ~rom ~he appropriate Kubelka-
Munk opacity Chart (D.B, Judd, "Optical Specification of
Light Scattering M~terial~", Journal o~ ~esearch o~ the
National Bureau of Stan~ards~ Vol. 19, 1937, p. 2~7~.
- The relativ~ hiding power (H.P.~ of the qamples
19 calculated a~:
H P ~
~ ~ o s an ar
The titanium dioxide used in all the following
examples 1~ produced by the vapor phase oxidation of ~itanium
tetrachloride~ me percentage composition~ of alumina and
silica, re~erred to hereinunder and elsewhere in the specifl-
catlon, are by welght, based on the weight o~ the TiO2 pig-
ment, and are calculated as SiO2 and A1203, respectively,
unles~ other~i~e ~pecified.
The determination of chalk~fade index is in
accordance with the~procedure descrlbed by W.H. Daiger and
W~Ho Mad~on in Vol. 3~, July 1967, o~ the ~}~YI~LLJ~C E~L~
, pp, 39~-410. Specifically, the te~t~ u ed ~or
obtaining data in thlg applicatlon are made on paints o~ the
~ollowing formulation:
Gramæ
.
: TiO2 pigment 48.o
Blue organic pigment 6,o
(a commercial organic pigment sold by
: the Du Pont Company under the trade
mark "Rampo" Blue BP-366-D) :
3o Alkyd re~ln (a commercial resin ~old by 120~0
Celanese Coatings Company under the
trade mark "Syntex" 3533, 50% ~.V.~
. . ,, , , ' ", . ',,. ~ ', :.' ~,
., , : ,, ~ . .
Organic ~olv~ntg boiling range 160C.- 36.0
: 193C. (co~mereial solvent ~old by
~umble Oil and Re~ining Co. under the
trade mark "Varsol'~ #1)
~ylene 36.0
24~ Pb naphthen~te drier 0.75
6~ Co naphthenate drler 0~40
6g Mh naphthenate drier 0.20
The~e in~redients are ground in a rotating gla98
~ar contalnlng 400 g. of 0.5 inch (1~27 cm). glass balls ~or
abou~ 42 hours and the resulting pain~ sprayed, wl~hout
r~duction, onto aluminum panel~ for expo~ure te~ts. Control
panels prepared ~sing the standard outdoor pigment de~cribed
above are placed ln each 8et for expo ure.
Degradation on expo~ure cau~e~ the blue to become
whlte due to chalklng, The degree of degradation i8 measured
by recordin~ the red re~le~tance at various lnterval~. Red
re~l~ctance incr~ase~ a~ the chalking progresse~, To deter-
mine the index numbers reported, the pa-nel~ are expo~ed out-
door~ until the instrument gi~e3 a flxed readin~, arbitrarily
. 20 ~et ~o indlcate a well-de~ined degree of chalklng, The
chalk~Rde index i8 the ratio of time units rQguired ~or the ~ t
test ~ample to reach thls re~lectanee d~vided b~ the tim~
units r~qulred by ~h~ control times 100. Th`e lnstrument
;,
u8e~ iS a sp~ctrophotometer ad~pted to r~ad ligh~ lntensity
r~lected l~rom the panel ~urface through a re~ ~ilt~r havin~; :
a p~ak transm~sion at ~bout 5760 A. -~
~t r~
Three thousand grams o~ piementary TiO~ is slurried
.l in ~u~icient water to achieve 300 gO T102 per liter~ The
30 ~lurry is heated to 90C. ~nd the pH i~ ad~usted to 9~,5 with
' -12-
,
-- ~S3~
a 50% by weight aqueous solution of sodium hydroxide. At
this ternperature a solution Or sodium silicate (weight ratio
of SiO2/Na20 is 3.25/1) at a concentration of 400 g. o~ SiO2
per liter is gradually added with agitation in an amount
suf'~icient to provide the percent of dense SiO~ ~hown ln
rrable I. Maintalnlng the temperature at about 90C., a 10%
by welght aqueous solution of H2$04 is added to the slurry
over a period of about one hour until the pH of the slur~y
drops to from 7.3 to 7.6. Then additional sodium silicate
solution (described above) ls added to the slurry in an
amount sufficient to provide the percent of porous SiO2
shown in Table I. During the addition of the sodium
: ..
silicate solution the ~H of the slurry is maintained between
5 and 7 by the slmultaneous addition of 96% H2S04. A~ter
the addition the ~lurry is allowed to stand for 30 mlnutes.
; Then 96~ sulfuric acid and a sodium alumi~ate solution con-
taining 383 g./l. of A1203 are simultaneously added at rates
which malntaln the pH of the slurry between 6 and ~. The
sodium aluminate solution is added in an amount sufficiPnt
to provide the percent of A1203 shown in Table I. The pH
of the slurry is then ad~usted to from 705 to 8Ø The pig-
.. . .
ment is recovered by filtering, washing, drying at about120C., and micronizing. The properties of the pigments
prepared and the Controls are shown in Table I.
B amples 19-26
Four thousand grams of pigm~ntary TiO2 is slurrled
in sufficient water to achieve 300 g. TiO2 per liter~ The
slurry is heated to 90C. and the pH is ad~usted to 9.5 with
a 50~ by weight aqueous so~ution of sodium hydroxide. At
this temperature a solution of sodium silicate~ described in
-13-
'~
. . . , . , , . ,
, . , , , , , . , ... , , . , ~
.
3 ~
Example lg at a concentration o~ 200 g. of SiO2 per liter is
gradually added with agitation in an amount su~icient to
provide the percent of dense silica shown in Table II~ Main-
taining the temperature at about 90C., a 5% by weight
aqu~ous solution of H2S04 is added to the slurry over a
perlod of about 2 hours until the pH of the slurry drops to
~rom 7.3 to 7.6. The slurry is maintained at about 90C. ~or
30 minutes. Then 50~ æulfuric acid and a sodium aluminate
~olution containing 370 g./l. Or A1203 are simultaneously
10 added at rates which maintain the pH o~ the slurry between
8 and 9~ The sodium aluminate solution ls added in an
amount to provide the percent of A1203 ~hown in the third
column of Table II. Then additional sodium silicate solu-
tion, described above, is added to the ~lurry in an amount
su~icient to provide the percent o~ porou~ SiO2 shown in t
Table II. During the addition o~ the sodium silicate ~olu-
tion the pH o~ t~e slurry is maintained between 8 and 9 by
the simultaneous addition of 50% H2S04. To the amount of
additional A1203 shown in the ~i~th column o~ Table II, 50%
20 sulfuric acid and the sodium aluminate solution~ described
above, are simultaneously added at rate~ which maintain the
pH o~ the slurry between 8 and 9~ Then the slurry ie cured
~or 30 minutes at a~out 90C. The p~ o~ the ~lurry is then
ad~us~ed to about 7. The pigment i~ recovered in the manner
described in Example 1~ All the pigments exhibit good .:~
hiding power with respect to the indoor and outdoor atan~
dardsO The ~cid solub~ility and chalk/fade index oP ~he
pigments so prepared are shown in Table II~ :~
-14-
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