Note: Descriptions are shown in the official language in which they were submitted.
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HIGH GLOSS CALCIUM CARBONATE COATING COMPOSITIONS AND
COATED PAPER AND PAPER BOARD MANUFACTURED FROM SAME
FIELD OF THE INVENTION
The present invention relates to mixtures of calcium carbonate particles that
are useful in the
production of high quality coated papers having high sheet gloss.
BACKGROUND OF THE INVENTION
In paper manufacture, calcium carbonate, because of its excellent whiteness
properties, has been
used in coating applications to improve various properties such as the
brightness of the paper.
Both natural and synthetic calcium carbonates are used in the paper industry.
Natural calcium
carbonate, or limestone, is ground to a small particle size prior to its use
in paper, while synthetic
calcium carbonate is manufactured by a precipitation reaction and is called
precipitated calcium
carbonate (PCC). Precipitated calcium carbonates are generally preferred over
ground calcium
carbonates in paper production in that the morphology, the size. and the size
distribution of the
particles, as well as the purity of the as-produced calcium carbonate, can be
controlled.
When used as an additive for the paper industry, precipitated calcium
carbonates are commonly
prepared by the carbonation, with carbon dioxide gas, of an aqueous slurry of
calcium hydroxide
("milk of lime"). The precipitated calcium carbonate pigments are then applied
to the paper by
coating the paper with an aqueous slurry containing the precipitated calcium
carbonate and an
adhesive.
SUBSTITUTE Sl~EET (RULE 26)
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Calcium carbonate can be precipitated from an aqueous solution in three
different crystal forms:
the vaterite form which is thermodynamically unstable, the calcite form which
is the most stable
and the most abundant in nature, and the aragonite form which is metastable
under normal
ambient conditions of temperature and pressure, but converts to calcite at
elevated temperature.
The aragonite form has an orthorhombic shape that crystallizes as long, thin
needles that may be
either aggregated or unaggregated. The calcite form exists in several
different shapes of which
the most commonly found are the rhombohedral shape having crystals that may be
either
aggregated or unaggregated and the scalenohedral shape having crystals that
are generally
unaggregated. All these forms of calcium carbonate can be prepared by
carbonation of milk of
lime by suitable variation of the process conditions as is known in the art.
Although excellent in respect of the whiteness and absorptivity of printing
inks when used as a
pigment for paper coating as compared with kaolin clay consisting of platelet
particles,
conventional calcium carbonate pigments suffer a deficiency in that paper
coated using the same
I 5 is generally poor in sheet gloss. To date, the use of high levels of
precipitated calcium carbonate
in coating formulations, particularly single-coated applications, has
historically resulted in gloss
deficiencies compared to kaolin-based formulations. Thus, precipitated calcium
carbonate
particles have been used as pigments in kaolin-based compositions in lower
amounts, i.e., in
amounts of 25 weight percent or less.
RELATED ART
U.S. Pat. No. 5,861,209 teaches aragonitic precipitated calcium carbonate
pigments for coating
rotogravure printing papers, a method for the preparation of the pigment, a
paper coated with the
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coating pigment, and a method for preparing such a paper. The precipitated
calcium carbonate
particles have an aspect ratio of from about 3: I to about 15:1, preferably
from about 4:1 to about
7:1, and a multimodal particle size distribution, which is preferably bimodal
or trimodal.
Preferably, the aragonitic precipitated calcium carbonate is present in an
amount from about 20
percent to about 100 percent by weight of the coating pigment. The pigment may
also be used
with titanium dioxide, talc, calcined clay, satin white, plastic pigments,
aluminum trihydrate,
mica, or mixtures thereof.
The paper, "In Search of Synergy: Engineering Coatings for Maximum
Performance: Optimizing
Pigment Blends for Maximum Performance," by J. Drechsel (I999 Coating
Conference, pp. 4I3-
432), teaches the use of fine particle size kaolins and fine ground carbonates
in coatings to
improve the print gloss of coated papers.
The paper "Structure of the Coating Layer and Optical Properties of Coated
Paper," by L.
Jamstrom et al., Wochenblatt ~ Papierfabrikation 17, 736-74I, (1996), teaches
higher opacity
papers and positive synergistic effects achieved when a precipitated calcium
carbonate pigment
is mixed with a platelike kaolin for coating compositions.
The paper "Optimized Binder Systems for Natural Calcium Carbonate Pigments
with Narrow
Particle Size Distribution," by R. Knappich et al., PTS Coating Symposium
(1999), pp. 13E to
13E-16, teaches the use of natural ground calcium carbonate pigments with
narrow particle size
distributions as providing a combination of high brightness, high opacity, and
excellent coverage
for coated paper and board.
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The paper, "Factors Governing Print Performance in Offset Printing of Matt
Papers," by P.G.
Drage et al., 1998 TAPPI Coating/Papermakers Conference, pp. 413-433, teaches
the production
of matt pigments for matt and Iow gloss papers using bimodal blends of coarse
and ultraline
components with GCC being focused on as the coarse fraction.
Japanese Pat. App. No. 10232253 teaches a multilayer coated paper for web
rotary offset having
dry strength, white paper gloss, multicolor printing gloss and blistering
resistance. The
multilayer paper includes coating layers having hollow or hemispheric polymer
particles.
1'0 Japanese Pat. App. No. 10-340790 teaches a coated paper for offset
printing having properties of
white paper glossiness prepared using an undercoating liquid of a pigment
component
comprising a wet pulverized needlelike or pillar-shaped precipitated calcium
carbonate in an
amount of 40 weight percent - 100 weight percent of the pigment component.
Japanese Pat. App. No. 11-065703 teaches a coated paper for offset printing
having printing
glossiness provided by a coating layer mainly of pigment containing a 60
weight percent - 90
weight percent fusiform wet ground causticized precipitated calcium carbonate
and a copolymer
latex having a 50nm - 80nm average particle diameter and 30 weight percent -
50 weight percent
gel content as the adhesive.
Japanese Pat. App. No. 11-008162 teaches a matt-coated paper for gravure
printing having
extremely low white paper glossiness. At least one side of the base paper is
coated with a
composition having 75 weight percent - 85 weight percent agglutinative spindle-
shaped
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precipitated calcium carbonate having average particle diameters of 3.O~m -
S.0 ~m in a
secondary particle shape and 15 weight percent - 25 weight percent of kaolin
having average
particle diameters of 1.Opm - 2.0 p,m.
Japanese Pat. App. No. 11-069426 teaches lightweight-coated paper for offset
printing having
blank paper glossiness and print glossiness. The paper has two coated layers
both continuing a
pigment and an adhesive, the top coat layer having 50 parts by weight - 85
parts by weight of
calcium carbonate having an average particle diameter not smaller than 0.2 p,m
and smaller than
0.5 um as the pigment and 8 parts by weight - 15 parts by weight copolymer
latex having SOnm -
70nm average particle diameter and 50 percent - 70 percent gel content as the
adhesive both
based on 100 parts by weight pigment.
Thus, there still remains a need for improved coating grade calcium carbonate
pigments for
producing high sheet gloss papers.
SUMMARY OF THE INVENTION
The present invention relates to a paper coating pigment which comprises a
blend of first and
second discrete aragonitic precipitated calcium carbonate (PCC) particles. The
first particle of the blended pigment has an average particle size (APS) of
about 0.4 microns
and the second particle of the blended pigment has an average particle size
(APS) of about 0.5
microns. The pigment preferably comprises about a 50:50 to about a 80:20
weight ratio of the
first 0.4 micron particle to the second 0.5 micron particle, with about a
60:40 weight ratio being
most preferred.
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The present invention also relates to a method for preparing the coated paper,
which comprises
preparing the blended aragonitic precipitated calcium carbonate pigment,
adding a clay, and
applying the pigment to the paper basestock in a slurry containing a binder
and other additives.
Preferably, the blended aragonitic precipitated calcium carbonate is present
in an amount of
from about 30 weight percent to about 85 weight percent of the mixture with
the about 70 weight
percent to about 15 weight percent balance being clay prior to preparing the
slurry.
DETAILED DESCRIPTION OF THE INVENTION
Unless otherwise specified, all reference to parts or percent herein refer to
percent by weight.
The present invention is related to a paper coating composition containing a
blend of calcium
carbonate particles, a method of using the composition to improve sheet gloss,
and a process of
producing. a paper having high sheet gloss and the paper made from the
process. The calcium
carbonate is preferably a precipitated aragonite (i.e., orthorhombic
crystalline form). When used
in pigment formulations, the calcium carbonate pigment blend of the present
invention provides
improvement in the sheet gloss, when compared to typical prior art coating
grade carbonates, and
is particularly advantageous in the production of high-gloss papers including
paper board.
The improvement in sheet gloss of coated papers using the blended pigment is
unexpected and is
attributed to a synergy created by using particles having particle size
distributions with
mean values that are from about 0.1 microns to about 0.2 microns in difference
in the blended
pigment. In addition, the calcium carbonate pigment of the present invention
has other clear
advantages over other calcium carbonate pigments including its ease of
calendering a paper sheet
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and the resultant sheet gloss and print gloss of coated papers using the
pigment blend.
The calcium carbonate component particles useful in the blended calcium
carbonate pigment
blend of the invention are preferably synthesized (i.e., precipitated) by the
carbonation with
carbon dioxide gas of an aqueous slurry of calcium hydroxide ("milk of time")
to produce
discrete aragonitic particles. In a preferred embodiment, preparation of the
blended calcium
carbonates of the invention is accomplished by mixing component particles
having mean particle
sizes of 0.40 microns and 0.50 microns, which are~commercially available from
Minerals
Technologies Inc., New York, New York, as OPACARB~ A40 PCC and OPACARB~ A50
PCC, respectively.
More specifically, OPACARB'~A40 PCC and OPACARB~A50 PCC are aragonitic
precipitated
calcium carbonate particles with average particle sizes of about 0.4 microns
and 0.5 microns,
respectively, having narrow particle size distributions of ~ 0.02 microns
about the mean.
The average particle size of the particles, according to the present
invention, were determined by
using a Micromeritics Sedigraph 5100 Analyzer, an instrument for measuring
particle size
distributions using Stokes law (see CRC Handbook of Chemistry and Physics,
69'" Edition 1988-
1989, pale F-105), which gives the rate of fall of a small sphere in a viscous
fluid. From this,
particle size distributions on a mass (weight) basis and average particle size
are determined.
Improved coating results are obtained with the aragonitic PCC pigment of the
present invention,
either alone or in blends with any other conventional coating pigment. The PCC
content of the
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pigment when mixed with clay can range from about 30 weight percent to about
85 weight
percent of the coating formulation, with 60 weight percent being 'preferred.
The pigment mixture
of the present invention is particularly advantageous for use in high-gloss
printing papers, and
may be mixed with one or more conventional binders, thickeners and/or
lubricants as is known in
the art. The coating can also contain dilution water in an amount needed to
bring the final solids
content of the coatings to a range of from about 50 weight percent to about 70
weight percent.
Embodiments of the present invention will now be described by way of example
only with
reference to the following Examples. The following non-limiting examples are
merely
illustrative of the present invention, and are not to be construed as limiting
the invention, the
scope of which is defined by the appended claims.
In each experimental formulation, 40,total parts of the precipitated calcium
carbonate pigment
was used and mixed with a coating grade clay having 72 percent solids using a
conventional flat-
blade Cowles-type mixer. The clay used for all examples below was ALPHAGLOSS~
clay
available from Huber Corporation. OPACARB~A40 PCC; OPACARB~A50 PCC; and
ALBAGLOS~ S PCC particles were provided alone and in combination to evaluate
the effect of
various calcium carbonate particles.
Specifically, Coating Mixture Nos. l and 7 were provided to evaluate the
effect of
OPACARB~A40 PCC aragonitic particles and OPACARB~A50 PCC aragonitic particles,
respectively, when used alone with ALPHAGLOSS~ clay. Coating Mixture Nos. 2,
3, and 4
were provided to evaluate the effect of OPACARB~A40 PCC aragonitic particles
when used in
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varying weight ratios ranging from about 40:60 to about 60:40 with ALBAGLOS~ S
PCC calcite
particles traditionally used with clay-based paper coating compositions.
Coating Mixture Nos. 6
and 5 were provided to evaluate blended aragonitic PCC pigments having
OPACARB°A40 PCC
aragonitic particles and OPACARB° A50 PCC aragonitic particles in
weight ratios according to
the present invention.
The compositions of the coating mixtures prepared are shown in Table 1 below
with the amounts
of the calcium carbonate particles and clay being present following weight
percentages.
TABLE 1
PIGMENT 1 2 3 4 5* 6* 7
MIXTURE #
ALBAGLOS ~ 25 r '20 15
S
PCC (wt. Percent)
OPACARB ~A40 40 15 20 25 25 20
PCC (wt. Percent)
OPACARB ~A50
15 20 40
PCC (wt. Percent)
ALPHAGLOSS~ 60 60 60 .60 60 60 60
CLAY (wt. Percent)
* Calcium carbonate pigment according to the present invention.
After mixing the coating formulations above, binder was added to each and
mixed again using a
conventional flat-blade Cowles-type mixer. Each calcium carbonate pigment
formulation
contained the same binder containing 11 parts GENFLO~ 5905 styrene/butadiene
latex available
from Gencorp Corporation (now Omnova Corporation), 3 parts hydroxyethylated
starch
available from Penford Starch Corporation as PENFORD~ 280 Gum, and 1 part
calcium stearate
lubricant. A standard paper thickener available from Hercules Corporation as
ADMIRAL° 3089
was added to each coating sample to achieve a target Brookfield 100
revolutions per minute
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(rpm) viscosity of 1200 centipoise (cps). Generally, the amount of thickener
used to achieve the
target viscosities increased as the carbonate level increased. '
Pigment coatings were formulated at approximately 60 percent solids, and
tested for percent
solids and water retention character as determined by the AA-GWR method
(Kaltec Scientific,
USA). Low shear viscosities in centipoise were measured at 10 rpm, 20 rpm, 50
rpm and 100
rpm using a Brookfield model RVT viscometer. High shear viscosity measurements
were made
using a Hercules high shear viscometer from Kaltec Scientif c, USA. The
Hercules viscosities
were run using the following conditions: E bob, 400,000 dyne-cm/cm spring
constant, 0 rpm -
4400 rpm, room temperature. The formulation data for the coatings tested are
provided in Table
2.
TABLE 2
COATING # 1 2 3 4 5 * 6 * 7
SOLIDS, percent60.4 60.2 60.5 60.3 60.2 60.1 60.2
AA-GWR, grams
per square 89 90 84 91 94 88 97
meter
(gsm)
HERCULES
mscosITY, 50.0 47.9 46.5 42.5 45.1 41.0 3 8.9
cps @4400
rpm
BROOKFIELD
SITY
V
ISCO
_
_ _ 1420 1100 1200 1060 1310 1310 1120
_
cps @100rpm
cps @ 50 rpm 2560 1800 1940 1720 2090 2240 1840
cps @ 20 rpm 5250 3650 3900 3500 4275 4700 3800
cps @ 10 rpm 9600 6600 7000 6400 7600 8600 7000
pH (adjusted 8.5 8.5 8.5 8.5 8.5 8.5 8.5
to 8.5-
8.9 with NaOH)
Addition Thickener
27 30 30 30 25 30 25
1 ~ * Aragomtic precipitated calcmm carbonate pigment according to the present
invention.
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After preparation, the coatings were applied to a 51.1 pounds per ream (70
grams per square
meter) paper basestock using a Cylindrical Laboratory Coater (CLC-6000). The
coat weight
target was 9 pounds per 3300 square feet. The coated papers were then
calendered at 150
degrees Fahrenheit on a laboratory supercalender having two sets of rollers
with nips providing
800 pounds per square inch (psi).
Standard testing of the coated sheets included paper sheet gloss, print gloss,
brightness and
opacity. The coated sheet test data are given in Table 3, with the print gloss
values being
determined using the Nancy Plowman Test Method (NPA) and all remaining data
being
determined using standard TAPPI test methods, which test methods will be
readily recognized by
those skilled in the art.
TABLE 3
IS
COATING # 1 2 3 4 5 * 6 * 7
SHEET GLOSS,
percent 73.6 72.6 72.1 72.7 75.7 74.6 73.3
NPA PRINT
GLOSS, percent90.6 92.8 92.I 91.2 91.8 91.6 92.4
Rapida SFO
Black
BRIGHTNESS g6.0 85.7 86.0 85.8 86.0 85.9 85.9
TAPPI, percent
OPACITY g9.6 90.2 89.7 89.9 90.1 90.0 90.0
TAPPI, percent
coLOR 92.4 92.4 92.4 92.4 92. 92.4 92.4
NTER L S
(-IU O O O O -O l -O
O O O O 1 -O 1
HUNTER A . . . . . . .
HUNTER B 2.3 2.3 2.3 2.3 2.4 2.4 2.4
Gurley Porosityg3 93 86 91 94 91 95
(sec @ l
Occ)
PPS-to Roughness1.32 1.41 1.45 1.36 1.34 1.37 1.42
IGT Pick, 91 91 91 76 82 82 76
(cm @ 3.0
m/s)
SOLIDS, percent60.4 60.2 60.5 60.3 60.2 60.1 60.2
Lalctum carbonate pigment accoramg to the present tnventton.
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These data demonstrate the improved sheet gloss that is provided by the
coating pigment of the
present invention and also demonstrates that the calcium carbonate coating
pigment can also be
used in combination with clay in high quantities without degrading print gloss
properties. When
comparing the sheet gloss of Coating Nos. 6 and 5, a marked improvement is
seen when using
blended calcium carbonate pigments according to the present invention having
OPACARB~A40
PCC aragonitic particles and OPACARB~A50 PCC aragonitic particles in weight
ratios of from
about 50:50 to about 80:20, and, preferably, about 60:40.
Moreover, these particle mixtures provide for improved sheet gloss without the
attendant
decrease in print gloss normally associated with the use of higher carbonate
contents.
Specifically, the sheet and print gloss ranges of 74.6 percent - 75.7 percent
and 9I .6 percent
Rapida SFO Black - 91.8 percent Rapida SFO Black achieved using OPACARB~A40
PCC
particles and OPACARB~A50 PCC particles in weight ratios according to the
present invention
are higher than the respective sheet and print glosses achieved when using
OPACARB~A40
PCC alone in Coating No. 1 (73.6 percent, 90.6 percent Rapida SFO Black) and
is higher than
the sheet gloss and comparable to the print gloss achieved when using
OPACARB~A50 PCC
alone in Coating No. 7 (73.3 percent, 92.4 percent Rapida SFO Black).
Moreover, a synergistic
effect is observed upon using higher ratios of OPACARB~A40 PCC to OPACARB~A50
PCC
which is contrary to the much lower print gloss (90.6 percent Rapida SFO
Black) achieved when
using OPACARB~A40 PCC alone with clay (Coating No. 1 ) and the print gloss
(92.4 percent
Rapida SFO Black) achieved when using OPACARB~A50 PCC alone with clay (Coating
No. 7).
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In comparing the use of the aragonitic blended pigment of the present
invention with aragonite
mixtures containing calcite particles, improved sheet glosses are also
observed. Specifically,
when using OPACARB°A40 PCC particles and OPACARB°A50 PCC
particles in weight ratios
according to the present invention (Coating Nos. 6 and 5), sheet gloss values
ranging 74.6
percent -75.7 percent were markedly improved over the sheet gloss values of
72.1 percent -72.7
percent obtained using OPACARB'~A40 PCC aragonitic particles in weight ratios
ranging from
40:60 to 60:40 with ALBAGLOS~ S PCC calcite particles (Coating Nos. 2-4).
Moreover, this
marked improvement in sheet gloss for the blended aragonitic pigment was
obtained with
comparable print gloss properties to those containing the aragonite/calcite
pigment mix.
It will be observed that the particles utilized in the blends according to the
present invention
exhibit particle size distributions with mean-values that are from about 0.1
microns to about 0.2
microns in difference. It is envisioned that other blends having similar
particle size distribution
differences would exhibit similar synergistic effects on coated sheet
properties. Moreover, it is
expected that in addition to the aragonite/aragonite blends, other like-kind
mixtures of calcium
carbonate morphologies (eg., calcite/calcite) meeting the above particle size
distribution criteria
would exhibit similar synergistic effects on coated sheet properties.
While embodiments and applications of this invention have been shown and
described, it will be
appreciated by those skilled in the art that modifications and embodiments are
possible without
departing from the inventive concepts herein described. Therefore, it is
intended that the
appended claims cover all such modifications and embodiments that fall within
the true spirit and
scope of the present invention.
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