Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
S :~ 2
1 BACKGROUND OF THE INV~NTION
This invention relates to novel chalking-resistant
pigments, to their manufacture and to -thelr use as opacifying
extenders in exterior grade paint formulat:ions. More specif-
ically the invention relates to novel calcined clay pigments, thepigmen-ts being characterized by possessing a unique combination
of coarse particle si~e and low oil absorption value~
The paint industry supplies consumer-oriented products
of the solvent and emulsion types. Solvent paints are reLatively
simple systems, easy to formulate kut difficult for the consumer
to use. Solvent paints contain a binder (oil or resin), a sol-
vent (thinner), drying agents and pigments. Emulsion or so-
called "latex" paints are complex mixtures containing surfac-
tants, protective colloids, biocides, freeze-thaw stabilizers,
emulsifiers and water in addition to the one or more types of
pigment which may be used. Following their introduction after
World War II, latex paints have ga~ned substantially in market
acceptance. They now account for a majority of interior and
exterior paint trade sales.
Interior and exterior latex paints have generally
similar formulations. An important distinction however is that
exterior grade paints contain relatively more binder and prime
pigment but less extender pigment than interior paints. This is
because paint fiIm integrity and overall durability are more
critical in exterior paints than in interior grades
An important parameter in paint formulation is the
pigment volume concentration, or PVC. PVC is a control factor in
the design of paint Eormulations, because paint properties are
governed by volume rather than weight effects~ The following
equation defines the PVC as a percentage of volume of dried paint
2 ~
1 film:
PVC = volume of pigments _ _
(volume of pigments -~ volume of binder)
The crltlcal pigment volume concentration, or CPVC, is defined
as that PVC at which air interfaces are generated in the dry paint fil~
due to deficiency of binder with respect to pigment. It is ~ell known
that many paint volume properties change drastically at CPVC. The
relationship between PVC and CPVC is nonlinear. There is authorlty for
the view that different paints are properly compared on the basis of
lQ equal reduced pigment volume concentration, or RPVC. The RPVC is defined
by the following:
RPVC = PVC/CPVC
Generally exterior grade paints have an RPVC less than 1 and
interior paints have an RPVC greater than 1. There is considerable
controversy over whether in latex paint the CPVC is a characteristic of
the plgment or a characteristic of both pigment and binder. There is
published authority for both points of view.
CPVC is related inversely to the amount of binder that the
pigment particles 'iabæorb". A conventionally used technique eo determine
this property of a pigment or extender is the amount of linseed oi:L
needed to form a paste containing a given weight of pigment~ This is
referred to in the art as oil absorption. As used herein the term "oil
absorption" refers to the procedure described in ASTM D-281. Substltu-
tion of an equal amount of high oil absorption extender pigmeot for one
of low oil absorption results in a reduction of the CPVC of that paint.
This in turn restricts the range of PVC that can be utilized in exteriorformulations and the amount of extender pigments which can be employed.
,
1 Exterior grade latex paints contain a mixture of prime
and extender pigments with titanium dioxide most generally used
as the prime pigment because of its outstanding optical proper-
-ties. Zinc oxide is employed to a sMaller extent. The mos-t
commonly used e~tender pigments for exterior grade latex paints
are calcium carbonate and talc. Kaolin clays are rarely used in
exterior grade paint ~ormulations, and when they are employed
only small amounts are used for reasons which will be discussed
below.
The binder in emulsion paints consists of globules (0.1
to 1.0 micron diameter) of film-forminy polymer of 10,000 to
1,000,000 molecular weight. The latex particle size and
composition are varied to effect changes in such properties as
durability, gloss, glass transition temperature and the like~ At
present acrylic and vinyl-acrylic resins account for the majority
of binders used in latex paints.
The weatherability of exterior grade coatings is
determined by the ability of the coatings to resist chalking,
fading and brittlement, gloss reductlon, frosting and bleeding.
Chalking, which is manifested by the formation of a powder on a
painted surface, is one of the most undesirable performance
characteristics of a paint. It involves the chemical degradation
of the paint binder by atmospheric and meteorological attack,
from which loose, removable powder (the pigment) is evolved from
the paint film at or just beneath the surface. Two distinct
mechanisms are believed to be responsible for chalking. One
involves direct ultraviolet degradation of the binder. It is
related to the ultraviolet stabilizer of the binder. With
present-day use of ultraviolet screens in paints this is no
longer much of a problem. The second mechanism occurs when a
~ ~S5~2
1 pigment acts as a catalyst for chemical oxidation o~ the binder.
Thus it is evident -that chalking is a characteristic of a paint
film and the terms "chalkiny pigments" and "chalking-resistant
pigments" as used herein will refer to chalking of the paint film
containing pigment. Chalking is a problem still considered sub-
stantially unsolved by those in the paint industry, and methods
for reducing chalking are continually sought by the paint indus-
try and its suppliers.
Extender pigments profoundly influence the properties
of latex paints. They control texture, optical and flow pro-
perties. Extenders involve a large group of materials with
diverse chemical properties. In most published studies of ex-
-tenders, the pigments have been evaluated in terms of PVC rather
than RPVC. This makes quantitative comparisons of extenders dif-
ficult. Kaolin extender pigments are widely used in interiorformulations, whereas the undesirable weathering properties of
kaolin clay pigments have severely limited their use in exterior
grade paint formulations. Several hypotheses have been suggested
to describe chalking by TiO2, but little or no work has been
done with extender pigments.
Commercially availabler pigment-grade hydrous kaolins
have oil absorption generally in the range 25-40 grams oil per
100 grams clay. Hydrous kaolins having the lower values in this
range are desirable in exterior grade latex formulations but
their chalking properties preclude such use. The high oil
absorption values resulting ~rom conventional calcination of
kaolin clay pigments, which is typically in ~he range of 45-60
grams oil per 100 grams clay, preclude their use as prime
extenders in latex paints formulated belo~ CPVC.
An extensive study of simple extender pig~ent in vinyl
5~2
1 polymer at 50% PVC is described by F. Liberti, Official Digest,
vol. 33, March 196l, page 390. Liberti Eound that four particle
si~es oE talcr coarse (ASP~ ~00) and fine (ASP 10~ hydrous
kaolin, calcined kaolin (nonspecified particle sizes) and fine
calcium carbonate all chalked at essentially the same rate, that
is, these pigments all rated "fair" on a scale of good-Eair-poor.
Liberti also found that coarse calcium carbona-te, rated as
"good", chalked less than the fine calcium carbonate, but the
RPVC was not controlled.
Kaolin clay pigments are supplied as pigments and ex-
tenders in uncalcined (hydrous) grades and calcined grades, the
latter being favored where opacification (hiding power) is an
important criterion. The hydrous grades include products com-
posed predominantly of relatively fine and relatively coarse
particles and are frequently supplied as blenas of fractions of
different particle sizes. Hydrous grades that contain an ap-
preciable content of particles larger than 2 microns as deter-
nlined by sedimentation include a significant quantity of natur-
ally occurring stacks or booklets as well as the individual
platelets known to characterize particles of kaolin that are
finer than about 2 microns. The finer grades are composed
predominantly of such individual platelets. Delaminated hydrous
kaolins are produced by mechanically altering the naturally
occurring stacks or booklets in whole ~unfractionated) crude
clays or coarse particle size fractions thereof. The delaminated
grades generally have higher oil absorption values than naturally
occurrin~ clay of similar particle size distribution as deter-
mined by sedimentation. Most commercially available grades of
calcined kaolin pigments have average particle size~ in the range
of about 1.0 to 3 microns, and oil absorption values above 40
~ ~ 6S5 :~ 2
g./100 g. Generally oil absorption increases inversely with
average particle size. Ultrafine grades oE calcined clay having
an average particle size below 1 micron are used as extenders and
coating pigments by the paper industry. Oil absorption values
5 exceed 80 g. oil/100 ~. clay. Coarse particle size calcined clay
pigments have been supplied by the industry as products having
average particle sizes in the range of 4 to 7 microns with oil
absorption values above 45 g./100 g. A process for preparing such
products involving stage-wlse fractionation of a dispersed pulp
10 of crude clay to selectively reject undersized and oversized
particles is described in U. S. ~,928,751 to Lyons. The calcined
coarse kaolin pigment, as disclosed in this patent, is
contemplated for use as an ingredient in the manufacture of
ceramics.
U. S. Patent No. 3,519,453 to Morris et al discloses
blending calcined, delaminated clay with hydrous coating clays
for coating applications in paper. Calcination is shown to
increase oil absorption and coarsen particle size, mostly by
agglomeration of fine material. U. S. 3,403,041 discloses the
use of a chemically modified calcined mechanically delaminated
clay in a paint formulated for interior use. U. S. 3,171,718
discloses tcolumn 11) alkyd paint formulations containing cal-
cined delaminated clay at 50% and 60% PVC. These ~aints are
also formulated for interior use.
While hydrous and calcined clays have enjoyed wide-
spread use as extenders for interior paints, to the best of my
knowledge known Eorms of kaolin pigments or extenders, both
hydrous and calcined grades, have not been used extensively by
the paint industry as the principal extender in the formulation
30 of exterior grade paints. Furthermore, to the best of my know-
~ ~5512
1 ledge known Eorms of kaolin clay do not possess the combination
of properties required for such use. These properties include
adequate resistance to chalking, ability to be Eormulated at the
high PVC necessary in exterior grade latex paint formulation and
acceptable opacification properties.
It is known in the art that wet or dry milling of
pulverized calcined clay pigments serve to breakup agglomerates
in the calcined clay. This reduces oil absorption but has been
associated with a significant loss of opacifying ability when the
calcined clay pigments were evaluated in their intended end uses,
namely in paper and in interior grade paints. Thus, post-milling
has been a step avoided by suppliers of calcined clay pigments
designed or prior uses of such clays.
THE INVENTION
lS The present invention results from a series of Eindings
relative to those properties of clay pigments that have hereto-
fore placed a constraint upon their utility in exterior grade
paint formulation. I have discovered that contrary to the prior
art knowledge of the adverse effect of milling on the opacifying
properties of calcined clay pigments for most uses, milling of
calcined clay particles, in particular coarse particles of
calcined clayl to an extent sufficient to reduce susbstantially
the oil absorption thereof, can be practiced to permit adequate
loading of the calcined clay without substantial impairment of
the opacifying properties of the clay when the calcined and then
milled clay is formulated below CPVC in exterior grade paint
formulations. I have also discovered that such calcined and
milled clay can be used as an extender pigment, preferably the
principal extender in exterior grade paint formulations, to
produce paint films having chalk resistance comparable to
~ ~ ~5~
1 heretofore used talc and calcium carbonate extenders and superior
hiding power.
In its broadest aspect novel calcined cl~y pigments of
the present invention have an average particle size of 3 microns
or above, preferably in the range 3 to lO microns, and an oil
absorption value more characteristic of hydrous kaolin, for
example about 30 g. oil/100 g. clay or below, than typical
calcined kaolin. The pigment may be produced by calclning a
coarse particle size ~rac-tion of hydrous kaolin clay crude and
subjecting the calcined material to controlled milling.
In a presently preferred embodiment of the invention
calcined kaolin clay e~tender pigments of further reduced oil ab-
sorption value are produced by blending a minor amount of a fine
fraction of hydrous kaolin clay of average particle size below
about l micron with a coarse fraction of hydrous kaolin clay of
average particle size 3 to lO microns, the major component of the
blend being the coarse fraction, calcining the blend and milling
the calcined blend, whereby mechanical problems associated with
milling fine calcined clay are minimized. The particle sizes of
coarse and fine fractions of kaolin particles and the proportions
thereof are such that the blended, calcined and milled product
has an average particle size of about 3 microns or above, prefer-
ably in the range 3 to lO microns. It will be recognized that
the scope of the invention also includes calcining coarse ancl
fine fractions separatelyl then blending and milling, or calcin-
ing and milling separately then blending.
In practice of the invention milling is terminated
before the particles of calcined clay are so reduced in particle
size as to create a fine particle size calcined clay having an
average particle size appreciably below 3 microns; for example 2
I ~ 6~2
1 - 2 1/2 microns average particle size. Such pigments will lack
chalk resistance achievable when milled coarser calcined clay
particles are utillzed.
Pigments of the invention may be used to extend a
titania or other primary pigment in latex or solvent paint
formulations. The pigments of the present invention are capable
of being incorporated in exterior grade paints at higher loadings
than previously attainab]e.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Crude kaolin clays useful in the production of pigments
of the invention are preferably those that contain a sub-
stantial proportion of particles (clay booklets) larger than 2
microns e.s.d. ~he coarse clay particles are concentrated from
such crude clay by blunging the crude in water, degritting the
resulting pulp, centrifuging the degritted clay to concentrate
one or more coarse size clay fractions as the centrifuge under-
flow and, if necessary, repeating the centrifugation one or more
times to remove fine particles as an overflow and oversize, e.g.,
plus 20 micron particles. The recovered coarse size fraction of
hydrous clay should have an average particle size in the range of
about 3 to 10 microns, preferably in the range of about 4 to 8
microns. Particle size of the hydrous clay calciner feed is
reflected in the particle size of the calcined and pulverized
clay Since coarse calcined clay has greater chalk resistance
than finer calcined clay, it is desirable to produce from the
crude a fraction that has maximum average particle size. However,
calcined clay particles that are too large, e.g., larger than 10
microns generally lack the tinting power of finer particles.
Clay fractions having an average particle size in the range of
about 4 to 8 microns, e.s.d., strike a balance between these
11 3 ~;~5~2
1 considerations. Generally, hydrous clay particles larger than
about 20 microns should be avoided or at least should be present
in minimal amount.
Those knowledgeable about the nature of crude clays and
clay fractions will recognize that naturally occurring clays are
polydisperse. In other words, they are composed of particles of
varying sizes. The same is true of coarse size fractions of
clays recovered from clay crude in conventional centrifuges. For
example a coarse size fraction of kaolin clay having an average
particle size o 4.5 microns may contain 90% by weight of parti-
cles finer than 14 microns, 70% by weight of particles finer than
7 microns and 25% by weight iner than 2 microns.
All particle sizes used herein refer to values obtained
by conventional wet sedimentation techniques using 2.58 as the
value for particle density in the case of hydrous clays and 2.63
as the value for particle density for calcined clays. These val-
ues are conventionally expressed as "equivalent spherical diame-
ter" or "e.s.d.". Average particle size, as used herein, refers
to that value of particle size at which 50% by weight of the ma-
terial is finer than and 50% is coarser than the designatedvalue.
The coarse size fraction of the kaolin clay crude
should not be mechanically delaminated prior to calcination.
Among other effects, delamination will result in undesirable
increase in oil absorption and decrease in particle size.
The cOarsQ size fraction of clay recovered as the
underflow pulp from the centrifuges may be flocculated by
addition of acid and/or alum prior to bleaching when using any
conventional filtration apparatus such as a rotary vacuum filter.
Al~ernatively the pulp can be dewatered without flocculation in
~S~l~
1 an electrically augmented vacuum filter. The filter cake may be
washed and then dried in conventional drying equipment such as a
rotary dryer. ~lternatively the filter cake can be dispersed by
addition of a suitable dispersant such as a condensed phosphate
salt, sodium salt~ an organic dispersant or ammonia, and then
spray dried. After drying by any of these techniques the dry
clay should be pulverized before the calcination stepO
Calcination occurs under conditions of time and temperature such
that the kaolin particles are substantially dehydrated by passing
through the characteristic endotherm - about 550C. Milling,
understood herein to mean reduction of particle size and grind~
ing, of the calcined "metakaolin" product follows calcination.
If desired, calcination may also be carried out such that the
kaolin passes through the characteristic exotherm at about 980C.
Kaolin pigments so treated are generally brighter but also more
abrasive.
It is especially preferred, however, to use acid kaolin
as the calciner feed. The salt content in the clay should be
kept low, as the presence oE significant quantities will cause
sintering during calcination and undesirably large increases in
oil absorption.
The calcined product has a much higher oil absorption
than the hydrous kaolin feed material. ~illing reduces the oil
absorption and may be continued until oil absorption of the
calcined kaolin has been lowered enough to provide an improved
pigment, which when formulated below CPVC in exterior paints has
low-chalking properties without significantly reduced opacifying
power. Generally it is preferred to have the oil absorption
below about 35 g. oil/lO0 g. clay, or in the range 10-30 g.
oil/lO0 g. clay, in which most extender pigments fall. Milling
12
g ~
1 equipment in which compression, densifying or comp~cting forces
predominate is preferred, since the compressive-type forces pro-
duced in such a mill appear to assist in obtaining optimum pack-
ing and low oil absorption. Ring-roller and muller are examples
of preferred types of milling equipment.
Achieving low oil absorption with pigments of the pre-
sent invention is a function of type of mill, time of milling and
choice of particle size in the blend. On commercial scale equip-
ment achievement of very low oil absorption through milling can
become prohibitively expensive. In the laboratory values as low
as about 20 g. oil/100 g. clay have been achieved wi-th some pig-
ments of the present invention, and thus it appears obtaining oil
absorp-tion values below 20 g./g. may be difficult to achieve
commercially with pigments of the present invention.
Fine particle size calcined clay may be obtained by
fractionating the same or a different crude clay used as a source
of the coarse clay. Known centrifugation techniques are used to
recover from a dispersed aqueous pulp of degritted hydrous kaolin
clay -- a fine particle fraction that is at least 70% by weight,
~0 and preferably at least 80~ by weight, and most preferably at
least 90% by weight riner than 2 microns. A typical fine parti-
cle size cut is 100% finer than l0 microns, 90% finer than 2 mi-
crons, 70~ finer than l micron, and 50~ finer than 0.6 microns.
Therefore particle size frac-tions can be blended as an aqueous
~5 pulp with the coarse fraction before filtration and drying or the
blending can take place after calcination and before milling or
after milling. Blending with fines, when desired, may be carried
out by techniques conventional in the art. The hydrous clay, for
exampler may be blended in slurry form as various cuts exit from
fractionating devices. Fine material may be blended with coarse
13
~ ~ ~55 ~ 2
1 material in ratios up to 50% by weight of fine material. The
preferred ratios will depend to a certain degree on the average
particle si~es of the coarse and fine cuts. For coarse cuts of
average particle size 6-8 microns, the preferred blendiny ratio
is in the ran~e of 20-30% fine fraction by weight, the fine
fraction having an average particle size between 0.3 and 0.9
microns, and the remainder coarse fraction. When blending, es-
pecially good results are achieved when the amount of material in
the coarse cut of particle size smaller than 2 microns is kept
below about 10% by weight of the coarse cut.
Pigments of the invention may be used in latex or sol-
vent paints without departing from conventional formulations or
formulation techniques. The pigment may be used as an extender
in conjunction with titania or other primary pigment. A signi-
Eicant advantage of the pigment of the present invention is that,relative to other common extenders, it may be used to replace
more of the very expensive titania primary pigment in common
formulations without decreasing chalking resistance or opacityO
Chalking performance of pigments is commonly measured
by direct exposure to atmospheric conditions on so-called "test~
fences" situated in various locations. These tests take several
years to produce results. In order to produce results in a
shorter time, accelerated weathering tests have been devised.
panel, coated with the paint formulation containing the pigment
to be tested, may be exposed to ultraviolet radiation, high hu-
midity and high temperature in a UVCON~ test chamber, available
from the Atlas Electric ~orporation~ Testing procedures and
conditions follow ASTM G53-77, in which the instrument is also
described. After periods of exposure a chalking rating may be
determined qualitatively by visual comparison with standard
1~
~ ~;55~2
1 samples. A ra~ing of 1 is poorest relative to chalking and a
rating of 10 is best (lowest chalking), the rating procedure
following ASTM D659-74. Weight loss methods may also be used.
Occasionally the accelerated tests do not correlate as accurately
as might be desired with actual test fence data, and paint manu-
facturers often use both tests. rhe accelerated test has the
great advantage of requiring only about two months, not two or
more years, for results.
In order to more fully illustrate the nature o the in-
vention, the following examples, not to be construed as limit-
ing, are presented:
EXAMPLE 1
A coarse-grade hydrous kaolin clay having a particle
size distribution of 100% finer than 44 microns, 90% finer than
14 microns, 70% finer than 7 microns, 50% finer than 4.1 microns,
33~ finer than 2 microns, 24~ finer than 1 micron and 10% finer
than 0.4 microns was used as the parent material for obtaining a
coarse-fraction hydrous kaolin clay for further processing. The
coarse-grade clay had been produced by blunging a Georgia crude
clay, degritting the crude and fractionating the degritted crude
by standard techniques in the clay art. This conventionally
processed clay, an acid kaolin, was found to have very minor
amounts of residual soluble salts, for example less than 0.1%
weight.
Two hundred and fifty grams (250g.) of this clay was
calcined in a fixed bed in a muffle furnace at 450C. for 90
minutes. After this calcination treatment the sample had an oil
absorption value of 38 g. oil/100 g. clay by ASTM. The sample
had lost 7~ of its crystalline water as measured by loss on igni-
tion before and after calcinatio~. The sample was then ~illed in
5 1 2
1 a mortar and pestle until an oil absorption value of 30 g./100 g.
was reached. It was then micropulverized through a 0.020-inch
screen to "flufE-up" the ~igment which alds in dispersing the
pigment in paint formulation processes. The calcined and milled
product had a particle size dis-tribution o 100~ finer than 30
microns, 90% finer than 1~ microns, 69~ finer than 7 microns and
50~ finer than 4.6 microns, 26% finer than 2 microns and 15%
finer than 1 micron.
This and subsequent samples were used as sole pigments
in latex paint formulations, described in another example follow-
ing, for the purpose of rating chalking characteristics of the
pigments. The oil absorption values obtained herein by following
the ASTM procedure were reproducible to within +2 units Eor oil
absorption numbers up to about 46 g. oil/100 g. clay.
EXAMPLE 2
Same as Example 1, except calcination was accomplished
at ~OOC. for 2 hours. Under these conditions the sample lost
92~ of its crystalline water as measured by L.O.I. beEore and
after calcination. The oil absorption value after calcination
20 was 41 g. oil/100 g. clay, and after milling it was 30 g./100 g.
Particle size distribution of the final product was the same as
in Example 1.
EXAMPLE 3
Same as Example lr except calcination was accomplished
at 900C. for 2 hours. Under these conditions the sample lost
98~ of its crystalline water as measured by L.O.I. before and
after calcination. The oil absorption value after calcination
was 46 g. Gil/100 g. clay, and after milling it was 30 g./100 g.
Particle size distribution of the final product was the same as
in Example 1.
16
~ ~ 6~512
l EXAMPLE 4
A sample of the coarse-grade hydrous kaolin of Example
l was acid flocced and washed with water to remove vir-tually all
traces of soluble sal-ts, and air dried prior to calcination in a
muffle furnace. Two~hundred and fifty grams (250 g~) was cal-
cined in a laboratory muffle furnace at 900C. for l hour. This
calcined clay had an oil absorption of 37 g. oil/lO0 g. clay and
was milled in a l,ancaster Mixer-Muller, Type P~, for l hour,
after which treatment of the oil absorption fell to 31 g~/lO0 g.
The sample was then micropulverized through a 0.020-inch screen.
Particle size distribution of the calcined and milled product was
the same as in Example l.
EXAMPLE 5
From the coarse-gradej hydrous kaolin clay of Example
}5 l, i.e. the "parent" clay having an average particle size of 4.1
microns, a coarser fraction of average particle size 6.4 microns
was obtained by acid-washing the coarse-grade clay to remove any
dispersant thereon, water ~ashing and then re-dispersing the clay
in water containing sodium hydroxide as the only dispersant. The
clay was then fractionated by sedimentation techniques familiar
to those in the art. The coarse fraction of the parent material
was then dried and stored for use. The particle size
distribution of the coarse fraction of the parent material was
found by sedimentation techniques (Sedigraph) to be 100% finer
2~ than 40 microns, 90% finer than 15 microns, 70% finer than 9~2
microns, 50~ finer than 6.4 microns, 30% finer than 4.~ microns,
10% finer than 3 microns and 5% finer than 2 microns.
For blending purposes a fine-grade hydrous kaolin clay
having a particle size distribution of 100% finer than 5 microns,
30 90~ finer than 2 microns, 70~ finer than 0.55 microns/ 50% finer
5 ~ ~
1 than 0.27 microns, 30% finer than 0.16 microns and 10% finer than
0.10 microns was used. This clay was obtained from blunged, de
gritted, fractionatecl and bleached crude kaolin from Georgia.
The fine-grade clay was acid flocced by acidifying the slurry to
5 a pH of about 2.5-3.0 wi-th a 10% sulfuric acid solution to remo~e
dispersant. The suspension was then filtered and the solids
washed with water, air dried at 50C. overnight and micropul-
verized through an 0.020-inch screen prior to use.
The fine-grade clay of average particle size (e.s.d.)
of 0-27 microns was dry-blended with the coarse fraction clay of
average particle size (e.s.d.~ of 6.4 microns in the ratio 1 part
fine to ~ parts coarse by weight. The blend was calcined in a
laboratory muffle furnace at 925C. for 90 minutes then milled in
a Lancaster Mixer-Muller for 1 hour and subsequently micropulver-
ized using an 0.020 inch diameter round-hole screen. The parti-
cle size distribution of the resulting product was 100% finer
than 44 microns, 90% finer than 13.5 microns, 70% finer than 7.3
microns, 50~ finer than 4.8 microns, 30% finer than 2.8 microns,
20~ finer than 0.9 microns and 10% finer than 0.45 microns. The
20 oil absorption of the calcined and milled blend was 25 g. oil/100
g. clay by ASTM. By way of comparison, the oil absorption of the
blend prior to calcining was 38 g. oil/100 g. clay by ASTM.
The chalking rating was evaluated by using 100% blended
pigment in an acrylic latex formulation discussed in the example
following:
EXAMPLE 6
Single pigment paints for chalking tests were formu-
lated in a latex s~stem at 30% PVC. The CPVC's of the pigments
varied but were generally above 48%. The latex paint formula-
tions based on 100 gallons of paint ~ollow:
18
~ ~ fi~
1 30~ PVC LATEX PAINT FORMULATION
I~ G:r .i nd "
ComponentW ~ lhs.) Volume (gal.)
Water 25.0 3.0
5 Natrosol~250HR
cellulosic thickener 120.0 14.38
in 2g solution
Igepal C0630 wetting agent 3.0 0.34
Tamol~731 dispersant in
25~ solution 2.0 ().26
Pigment 190-210 9.0
Super Ad-it~ biocide 1.0 0.12
Colloid 581~ defoamer 1.5 0.22
Above ingredients were dispersed for 15 minutes at high speed on
a Premier Mill Model 200 Dispersator with a 2-1/2 inch Cowles
blade. The speed was then reduced and the following ingredients
were added and mixed 10-15 minutes (or until smooth appearance
achieved) at reduced speed:
"Let-Down"
ComponentWeight (lbs.) Volume (gal.)
Colloid 581B Defoamer 1.5 0.22
; UCAR~ 366 acrylic latex 370.0 40.88
Water and 2% Natrosol
250HR thickener286.4 32.16
-
- 100.0
The amount of Natrosol thickener added to the "Let-Down" portion
of the mix was just sufficient to give 85 Ku (Krebs units) on the
Stormer viscometer, an instrument for viscosity measurement
widely used by those in the paint art. For the purposes of
generating laboratory-size samples, the weight values given above
were scaled down to give a sample size of approximately one pint.
19
l TABLE I
CHALKING RATING (ASTM D659-74) OF VARIOUS PIGMFNTS
1~ .G~ r~r ~r~ ul~rl~
AE`TER 1500 HO~RS EXPOSURE IN UVCON
Pigment Rating
Pigment o~ Example 1 (coarse calcined kaolin) 7
Pigment of Example 2 (coarse calcined kaolin) 9
Pigment of Example 3 (coarse calcined kaolin) 9
Pigment of Example 4 (coarse calcined kaolin) 9
10 Pigment of Example 5 (calcined kaolln blend) 9
Chem Carb~ 44 pigment (coarse calcium carbonate)
~SP~ 400 clay (coarse hydrous kaolin) 5
A rating of 10 indicates no chalking. Superior resistance to
chalking is e~hibited by the pigments of Examples 2~4r which ha~
been calcined to substantial dehydration, i7e~ kaolin clay that
had passed through the characteristic endotherm. Kaolin clay
which had been partially dehydrated (Pigment of Example l) showed
improved chalking resistance over hydrous kaolin clay. Kaolin
clay that had been substantially dehydrated showed the highest
resistance to chalking of the samples tested, indeed virtually
not chalking at all. Although no difference was found relevant
to chalking resistance between kaolin clay pigment that had been
calcined at 600C. and that which had been calcined at 900COI
the 900C. calcination temperature is preerred because it yields
a higher brightness pigment. Indeed, calcining at temperatures
below 800C. will generally lead to pigments of unsatisfactory
brightness~
EXAMPLE 7
In order to show that pigments of the present invention
possess high opacifying power and good optical properties,
5 ~ ~
1 several paint samples were formulated at 45% PVC and evaluated
for optical properties. The formulation used is given below:
~5~ PVC LATEX PAINT FORM~LATION
_
"Grind"
5 Component Weight~lbs.)
Water ~6.0 3.12
Natrosol HR2.50 cellulosic
thickener in 2% solution120.0 14.38
Igepal C0630 wetting agent 3.0 0.34
Tamol 731 dispersant in 25% solution 5.0 0~66
Tio2 Primary Pigment 225.0 6.76
Extender pigment 122-128 5.70
Super Ad-it biocide 1.0 0.12
Colloid 581B defoamer 1.5 0.22
Above ingredients were dispersed for 15 minutes at high speed on
a Premier Mill Model 2001 Dispersator with a 2-1/2 inch Cowles
blade. The speed was -then reduced and the following ingredients
were added and mixed 10-15 minutes (or until smooth appearance
achieved) at reduced speed:
"Let-Downl'
Component Weight(lbs.) Volume(gal.)
Colloid 581B de~oamer 1.5 0.22
UCAR 366 acrylic latex266.4 29.6
Water and 2~ solution of Natrosol 324.9 38.99_
100. 0
Extender pigments included calcined kaolin of the present
invention, hydrous kaolin, and calcium carbonateO The amount of
Natrosol thickener added to the "Let-Down" portion of the mix was
just sufficient to give 85 Ku (Krebs units) on the Stormer
Viscometer. For the purpose of generating laboratory-si~e
samples, the weight of each ingredient was scaled down to give a
21
s :~ ~
1 sample size of approximately one pint.
Samples of paint were so prepared using various
extender pigments to evaluate the optical properties. rrhese are
shown in Table II below:
TABLE II
OPTICAL PROPERTIES OF VARIOUS EXTENDER PIGMENTS IN FULL
FORMULATION, 45~ PVC LATEX PAINT
.. . .. _~
Hiding Power % Tinting Percent
Extender Pi~mentft._/gal.*Stren~th** 85 Sheen
Hydrous Kaolin 332 100 7.7
(ASP 400 pigment)
Pigment of Example 4 339 100 6.7
(coarse calcined kaolin)
Pigment of Example 5 326 100 5.7
(calcined kaolin blend)
Chem Carb 44 Pigment 269 93 6.7
(coarse calcium carbonate)
Gold Bond~ R Pigment 298 97 2.9
~ (Foarse silica)
* at 0.98 contrast ratio, determined by Kubelka-Munk analysis.
** relative to ASP 400 hydrous kaolin.
These results show that the desirable optical properties of
kaolin pigments are retained by the calcined kaolin pigments of
the present invention.
