U.S. patent number RE38,301 [Application Number 09/711,828] was granted by the patent office on 2003-11-11 for paper coating pigments, their production and use.
This patent grant is currently assigned to Imerys Minerals Limited. Invention is credited to Ian Stuart Bleakley, Philip Martin McGenity, Christopher Nutbeem.
United States Patent |
RE38,301 |
Bleakley , et al. |
November 11, 2003 |
Paper coating pigments, their production and use
Abstract
There is disclosed a method for the preparation of a
precipitated calcium carbonate (PCC) for use as a pigment in paper
coating compositions, the method comprising the steps of (a)
carbonating an aqueous lime-containing medium to produce an aqueous
suspension of a PCC predominantly in a selected crystal form, (b)
at least partially dewatering the PCC-containing suspension; and
(c) subjecting the PCC-containing suspension to comminution by high
shear attrition grinding with an attrition grinding medium. Steps
(b) and (c) may be applied in either order, ie. (b) followed by (c)
or alternatively (c) followed by (b). The dewatering step (b) is
preferably carried out using a pressure filter device operating at
a pressure of at least 5 MPa, preferably at least 10 MPa. A pH
reduction step may be applied after steps (b) and (c). Also
disclosed are pigments produced by the method and pigment and paper
coating compositions including such pigments.
Inventors: |
Bleakley; Ian Stuart (St.
Austell, GB), McGenity; Philip Martin (Holmfirth,
GB), Nutbeem; Christopher (St. Austell,
GB) |
Assignee: |
Imerys Minerals Limited
(GB)
|
Family
ID: |
10782072 |
Appl.
No.: |
09/711,828 |
Filed: |
November 13, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
728518 |
Oct 9, 1996 |
05833747 |
Nov 10, 1998 |
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Foreign Application Priority Data
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Oct 10, 1995 [GB] |
|
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9520703 |
|
Current U.S.
Class: |
106/464; 106/465;
423/432 |
Current CPC
Class: |
C09C
1/021 (20130101); D21H 19/385 (20130101); C09C
1/022 (20130101); C01P 2002/84 (20130101); C01P
2006/80 (20130101); D21H 19/40 (20130101); D21H
21/52 (20130101); C01P 2006/60 (20130101); C01P
2002/30 (20130101); C01P 2004/61 (20130101); C01P
2004/54 (20130101); C01P 2004/62 (20130101); C01P
2004/32 (20130101); C01P 2006/22 (20130101); C01P
2004/51 (20130101) |
Current International
Class: |
C09C
1/02 (20060101); D21H 19/00 (20060101); D21H
19/38 (20060101); D21H 21/52 (20060101); D21H
21/00 (20060101); D21H 19/40 (20060101); C09C
001/02 () |
Field of
Search: |
;106/464,465
;423/430,431,432 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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05 58 275 |
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Sep 1993 |
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EP |
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1 240 465 |
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Jul 1971 |
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GB |
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2 275 876 |
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Sep 1994 |
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GB |
|
6048732 |
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Feb 1994 |
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JP |
|
Other References
Norwegian Search Report, dated Feb. 6, 2002. .
WPI Abstract Accession No. 95-299360/39 JP7196317 A (Ind. Sci &
Tech) Dec. 28, 1993. .
WPI Abstract Accession No. 92-395143/48--Mar. 22, 1991 JP 4 295010
A (Okutama). .
WPI Acc. No. 89-351298/48 JP1261225A (Maruo Calcium KK) Apr. 12,
1998. .
WPI Abstract Accession No. 88-075176/11 JP 630303 16A (Ind. Sci
& Tech) Jul. 23, 1986. .
J.N. Ishley, E.J. Osterhuber & N. Roman, 1992 TAPPI Coating
Conference Proceedings, 335-348 (1992)--No Month. .
D.B. Crawshaw, C.H. Kahn-Schneider & P.C. Clark, 1982 TAPPI
Coating Conference Proceedings, 143-164 (1982)--No month. .
G. Engstrom & M. Rigdahl, Nordic Pulp & Paper Research
Journal. 90-101 (1992)--No Month. .
G. E. Lauterbauch, Wm. F. Parker, M.S. Crill and D.L. Breen, Energy
Conservation In Dispersing Calcium Carbonate Pigments, TAPPI
Conference, 1977--No Month..
|
Primary Examiner: Marcheschi; Michael
Attorney, Agent or Firm: Finnegan, Henderson, Farabow,
Garrett & Dunner L.L.P.
Claims
We claim:
1. A method for the preparation of a precipitated calcium carbonate
(PCC) for use as a pigment in paper coating compositions, the
method comprising the steps of (a) carbonating an aqueous
lime-containing medium to produce an aqueous suspension containing
a PCC in predominantly scalenohedral, rhombohedral, or aragonite
crystal form, (b) at least partially dewatering the PCC-containing
suspension; and (c) subjecting the PCC-containing suspension in wet
form to comminution by high shear attrition grinding with an
attrition grinding .[.mediums.]. .Iadd.medium .Iaddend.wherein the
comminution step is carried out such as to dissipate in the aqueous
suspension containing the PCC at least 100 kilowatt hours of energy
per dry tonne of PCC and wherein the PCC product produced following
steps (b) and (c) comprises particles having a particle size
distribution such that at least 70% .[.be.]. .Iadd.by
.Iaddend.weight of the particles have an equivalent spherical
diameter as measured by sedimentation of less than 1 .mu.m and at
least 50% by weight of the particles have an equivalent spherical
diameter of less than 0.5 .mu.m.
2. A method as claimed in claim 1 and wherein the dewatering step
(b) is carried out using a pressure filter device operating at a
pressure of at least 5 MPa.
3. A method as claimed in claim 2 and wherein the pressure filter
device comprises a tube press wherein material is pressure filtered
between two co-axially disposed tubular bodies.
4. A method as claimed in claim 1 wherein step (b) precedes step
(c) and a dispersing agent for the PCC is added to the aqueous
PCC-containing suspension prior to step (c).
5. A method as claimed in claim 1 and wherein the grinding medium
employed in step (c) comprises silica sand having a median particle
diameter in the range 0.1 mm to 4 mm.
6. A method as claimed in claim 1 and wherein an additional step
(d) is applied after steps (b) and (c) to reduce the pH of the
PCC-containing suspension.
7. A method as claimed in claim 1 wherein step (c) precedes step
(b) and wherein the aqueous suspension formed in step (a) and
treated by comminution in step (c) has a solids concentration of
from 10% to 25% by weight and wherein the aqueous suspension after
application of step (b) has a solids concentration of at least 50%
by weight.
8. A pigment for paper coating which comprises a PCC produced by
the method claimed in claim 1.Iadd., wherein the particle size
distribution is such that the amount of particles having an
equivalent spherical diameter of less than 0.25 .mu.m ranges from
10% to 45% by weight.Iaddend..
9. A pigment .[.as claimed in claim 8.]. .Iadd.for paper coating
which comprises a PCC produced by the method claimed in claim 1
.Iaddend.and wherein the predominate form of the PCC comprises at
least 50% by weight aragonitic or scalenohedral crystals.
10. A dispersed aqueous suspension of the pigment claimed in claim
.[.8.]. .Iadd.9 .Iaddend.and which includes a dispersing agent for
the PCC.
11. A pigment composition which comprises a pigment as claimed in
claim .[.8.]. .Iadd.9 .Iaddend.mixed together with one or more
other pigments.
12. A composition as claimed in claim 11 and wherein the PCC is
mixed together with a platey kaolin clay.
13. A paper coating composition comprising an aqueous suspension
comprising an adhesive and a suspension as claimed in claim
10..Iadd.
14. A method as claimed in claim 1 and wherein the PCC produced
according to step (a) is in predominantly scalenohedral or
aragonite crystal form..Iaddend..Iadd.
15. A method as claimed in claim 14 and wherein the PCC produced
according to step (a) is in predominantly aragonite crystal
form..Iaddend..Iadd.
16. A method as claimed in claim 14 or 15 and wherein the
dewatering step (b) is carried out using a pressure filter device
operating at a pressure of at least 5 MPa..Iaddend..Iadd.
17. A method as claimed in claim 16 and wherein the pressure filter
device comprises a tube press wherein material is pressure filtered
between two co-axially disposed tubular bodies..Iaddend..Iadd.
18. A method as claimed in claim 14 or 15 wherein step (b) precedes
step (c) and a dispersing agent for the PCC is added to the aqueous
PCC-containing suspension prior to step (c)..Iaddend..Iadd.
19. A method as claimed in claim 14 or 15 and wherein the grinding
medium employed in step (c) comprises silica sand having a median
particle diameter in the range 0.1 mm to 4 mm..Iaddend..Iadd.
20. A method as claimed in claim 14 or 15 and wherein an additional
step (d) is applied after steps (b) and (c) to reduce the pH of the
PCC-containing suspension..Iaddend..Iadd.
21. A method as claimed in claim 14 or 15 wherein step (c) precedes
step (b) and wherein the aqueous suspension formed in step (a) and
treated by comminution in step (c) has a solids concentration of at
least 50% by weight..Iaddend..Iadd.
22. A pigment for paper coating which comprises a PCC produced by
the method claimed in claim 14 or 15..Iaddend..Iadd.
23. A dispersed aqueous suspension of the pigment claimed in claim
22 and which includes a dispersing agent for the
PCC..Iaddend..Iadd.
24. A pigment composition which comprises a pigment as claimed in
claim 22 mixed together with one or more other
pigments..Iaddend..Iadd.
25. A composition as claimed in claim 24 and wherein the PCC is
mixed together with a platey kaolin clay..Iaddend..Iadd.
26. A paper coating composition comprising an aqueous suspension
comprising an adhesive and a suspension as claimed in claim
23..Iaddend..Iadd.
27. A method for the preparation of a precipitated calcium
carbonate (PCC) for use as a pigment in paper coating compositions,
the method comprising: carbonating an aqueous lime-containing
medium to produce an aqueous suspension containing a PCC in
predominately scalenohedral or aragonite crystal form; partially
dewatering the PCC-containing suspension; and subjecting the
PCC-containing suspension in wet form to comminution by high shear
attrition grinding with an attrition grinding medium wherein the
comminution is carried out such as to dissipate in the aqueous
suspension containing the PCC at least 100 kilowatt hours of energy
per dry ton of PCC; and wherein the PCC product produced comprises
particles having a particle size distribution such that at least
70% by weight of the particles have an equivalent spherical
diameter as measured by sedimentation of less than 1 .mu.m and at
least 50% by weight of the particles have an equivalent spherical
diameter of less than 0.5 .mu.m..Iaddend..Iadd.
28. A method for the preparation of a precipitated calcium
carbonate (PCC) for use as a pigment in paper coating compositions,
the method comprising: carbonating an aqueous lime-containing
medium to produce an aqueous suspension containing a PCC in
predominantly aragonite crystal form; partially dewatering the
PCC-containing suspension; and subjecting the PCC-containing
suspension in wet form to comminution by high shear attrition
grinding with an attrition grinding medium wherein the comminution
is carried out such as to dissipate in the aqueous suspension
containing the PCC at least 100 kilowatt hours of energy per dry
ton of PCC; and wherein the PCC product produced comprises
particles having a particle size distribution such that at least
70% by weight of the particles have an equivalent spherical
diameter as measured by sedimentation of less than 1 .mu.m and at
least 50% by weight of the particles have an equivalent spherical
diameter of less than 0.5 .mu.m..Iaddend..Iadd.
29. A method as claimed in claim 27 or 28 and wherein the
dewatering is carried out using a pressure filter device operating
at a pressure of at least 5 MPa..Iaddend..Iadd.
30. A method as claimed in claim 29 and wherein the pressure filter
device comprises a tube press wherein material is pressure filtered
between two co-axially disposed tubular bodies..Iaddend..Iadd.
31. A method as claimed in claim 27 or 28 wherein the
PCC-containing suspension is dewatered prior to comminution and a
dispersing agent for the PCC is added to the aqueous PCC-containing
suspension prior to comminution..Iaddend..Iadd.
32. A method as claimed in claim 27 or 28 and wherein the grinding
medium employed during comminution comprises silica sand having a
median particle diameter in the range 0.1 mm to 4
mm..Iaddend..Iadd.
33. A method as claimed in claim 27 or 28 and wherein the pH of the
PCC-containing suspension is reduced after dewatering and
comminution..Iaddend..Iadd.
34. A method as claimed in claim 27 or 28 wherein comminution
precedes dewatering and wherein the aqueous suspension has a solids
concentration of at least 50% by weight after
comminution..Iaddend..Iadd.
35. A pigment for paper coating which comprises a PCC produced by
the method claimed in claim 27 or 28..Iaddend..Iadd.
36. A dispersed aqueous suspension of the pigment claimed in claim
35 and which includes a dispersing agent for the
PCC..Iaddend..Iadd.
37. A pigment composition which comprises a pigment as claimed in
claim 35 mixed together with one or more other
pigments..Iaddend..Iadd.
38. A composition as claimed in claim 37 and wherein the PCC is
mixed together with a platey kaolin clay..Iaddend..Iadd.
39. A paper coating composition comprising an aqueous suspension
comprising an adhesive and a suspension as claimed in claim
36..Iaddend.
Description
.Iadd.We claim priority to GB 9,520,703 issued Oct. 10,
1995..Iaddend.
The present invention relates to paper coating pigments and their
production and use.
In particular, the invention concerns an improved precipitated
calcium carbonate product for use as a paper coating pigment, a
process for preparing the same and paper coating compositions
containing such pigment.
Coated paper and coated paperboard is used for a large range of
products including packaging, art paper, brochures, magazines,
catalogues and leaflets. Such coated paper and paperboard is
required to give a range of properties, including brightness,
opacity and sheet gloss, as well as printing performance.
In an effort to attain the required properties, many paper makers
use small proportions of calcined clay and/or titanium dioxide
(TiO.sub.2) in their coating formulations. Such additives have the
advantage that they strongly scatter light, and thus give good
opacity and brightness, but their drawback is their relatively high
cost.
The general principle of using a precipitated calcium carbonate
(PCC) to replace partly or wholly such expensive additives has been
recognised before [J. N. Ishley, E. J. Osterhuber & N. Roman,
1992 TAPPI Coating Conference Proceedings, 335-348 (1992)].
Calcium carbonate can be precipitated from aqueous solution in
three different principal 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 temperatures.
The aragonite form crystallises as long, thin needles having a
length:diameter ratio of about 10:1, but the calcite form exists in
several different shapes of which the most commonly found are the
rhombohedral shape in which the length and diameter of the crystals
are approximately equal, and the crystals may be either aggregated
or unaggregated; and the scalenohedral shape in which the crystals
are like double, two-pointed pyramids having a length:diameter
ratio of about 4:1, and which are generally aggregated. All these
forms of calcium carbonate can be prepared by carbonation of milk
of lime by suitable variation of the process conditions.
The work of Ishley et al. reported in the reference specified above
refers to the use of rhombohedral calcitic PCC. The use of
aragonitic PCC in paper coating has also been reported [D. B.
Crawshaw, C. H. Kahn-Schneider & P. C. Clark, 1982 TAPPI
Coating Conference Proceedings, 143-164 (1982); and G. Engstrom
& M. Rigdahl, Nordic Pulp and Paper Research Journal, 90-101
(1992)], although this work does not refer specifically to light
scattering performance.
One of the problems with aragonitic PCC, produced by the reaction
of carbon dioxide with slaked lime, is that the reaction product
consists of aggregates of needle shaped particles. The aggregated
structure results in poor .[.Theological.]. .Iadd.rheological
.Iaddend.behaviour and poor paper coating performance (e.g. sheet
and print gloss). A similar but less pronounced problem can occur
with scalenohedral PCC.
According to the present invention in a first aspect there is
provided a method for the preparation of a precipitated calcium
carbonate (PCC) for use as a pigment in paper coating compositions,
the method comprising the steps of (a) carbonating an aqueous
lime-containing medium to produce an aqueous suspension of a PCC
predominantly in a selected crystal form, (b) at least partially
dewatering the PCC-containing suspension; and (c) subjecting the
PCC-containing suspension to comminution by high shear attrition
grinding with an attrition grinding medium.
Steps (b) and (c) may be applied in either order, ie. (b) followed
by (c) or alternatively (c) followed by (b). Where step (b) is
applied before step (c) a dispersing agent (as described below) is
likely to be required prior to application of step (c).
The dewatering step (b) is preferably carried out using a pressure
filter device operating at a pressure of at least 5 MPa, preferably
at least 10 MPa. Such a device may conveniently be of the known
tube press type wherein a material is pressure filtered between two
co-axially disposed tubular bodies. Such devices are described for
example in GB 907,485 and in GB 1,240,465. In GB 907,485 for
example, the tube pressure filter essentially comprises an upright
annular chamber formed between two co-axially disposed tubular
bodies, which chamber is divided into inner and outer
non-intercommunicating compartments by an impermeable elastic
sleeve, the arrangement being such that, in use, a material to be
pressure filtered is introduced into the compartment formed between
one side of the elastic sleeve and one of the tubular bodies, the
one tubular body supporting a filter element, and a hydraulic fluid
is introduced into the compartment formed between the other side of
the elastic sleeve and the other tubular body so as to compress the
material to be pressure filtered against the filter element.
The comminution step (c) is preferably carried out such as to
dissipate in the suspension in which the PCC is formed at least 100
kilowatt hours of energy per dry tonne of PCC. The dissipated
energy may be .[.200 kwhr.]. .Iadd.200 kWhr .Iaddend.or more per
tonne.
The grinding medium employed in step (c) may comprise one of the
hard, inorganic materials well known in the grinding of particulate
materials. For example, silica sand having a median particle
diameter in the range from about 0.1 mm to 4 mm, eg. 0.2 mm to 2
mm, is a preferred grinding medium. The grinding medium could
alternatively be aluminium oxide, zirconium oxide, hard steel or a
mixture of any of these materials.
Preferably, step (a) is carried out in an known manner by
carbonating a lime containing aqueous medium. Carbonation is
desirably carried out using a carbon dioxide containing gas.
We have found unexpectedly that by use of the method according to
the first aspect of the present invention PCC products can be
formed which have improved optical properties when compared with
those prepared in the conventional manner. Such products are
therefore especially suitable for producing paper coatings with
improved performance. Examples of such improvements are given
hereinafter.
Particles obtained in a PCC produced as in step (a) in the method
according to the first aspect of the present invention will
comprise aggregates as described hereinbefore. We have found
unexpectedly that substantial breaking down of such aggregates
occurs in both steps (b) and (c) in the method according to the
first aspect of the present invention. The contribution to
breakdown of the aggregates by step (b) is greater when step (b)
precedes step (c) and this is one of the factors which may lead an
operator to choose to apply step (b) before step (c).
We have found that when the particle aggregates are broken down in
steps (b) and (c) the pH of the aqueous suspension being treated
rises. We believe that the reason for this is that when PCC is
formed as in step (a) unconverted lime becomes entrapped in the PCC
crystal aggregates. When the aggregates are broken down this free
lime is released and dissolves in the host aqueous medium. The PCC
produced as in the prior art or in step (a) may for example contain
by weight up to 5% free lime, eg. 0.2% to 2% free lime. The pH may
rise to pH11 or more after the application of the first of step (b)
and step (c). Such a pH level is undesirable in the paper coating
applications in which the PCC may be employed, as described
hereinafter, because it is potentially harmful to machinery and to
operators who have to process the suspension.
Desirably, an additional step (d) to reduce the pH of the aqueous
PCC-containing suspension is applied preferably after both steps
(b) and (c) have been applied although it could be applied after
the first of these two steps. The additional step (d) may be
applied until the pH falls to a suitable level, eg. below pH 9
preferably to or below pH 7.5. The additional step (d) may comprise
further carbonation of the PCC-containing suspension.
Alternatively, or in addition, a material known for use in a
reducing the pH of a mineral suspension may be added. Such a
material may, for example, comprise a mild mineral acid such as
phosphoric acid.
In the method according to the first aspect of the present
invention the aqueous suspension formed in step (a) may have a (dry
weight) solids concentration of from 10% to 25%. After application
of step (c) the PCC-containing suspension may have a solids
concentration by weight of at least 50%, eg. greater than 65%.
Desirably, the suspension formed after step (c) is suitable for use
in the formation of a paper coating composition without further
dewatering.
For example, we have found that for a given predominantly
aragonitic PCC produced in a manner to give a solids level of
approximately 18% by weight, the properties given in Table 1 as
follows can be obtained by a method embodying the first aspect of
the present invention wherein step (b) follows step (a) and step
(c) follows step (b).
TABLE 1 Particle Particle Particle Particle PCC Size Size Size Size
Product Parameter Parameter Parameter Parameter Gloss Stage X1 X2
X3 X4 (%) Product of 90 75 10 <5 Step (a) Product of 90 75 50 25
55 Step (b) Product of 98 93 67 32 59 Step (c)
In the method employed to obtain the PCC product whose properties
are shown in Table 1 step (b) was carried out using a tube press
providing a pressure of >7 MPa and step (c) was carried out
using silica sand grinding using a grinding energy expenditure of
100 kwhr per dry tonne of product.
In Table 1, the particle size parameters X1 to X4 are the
percentages by weight of particles in the product at the given
product stage having an esd less than respectively 2 .mu.m, 1
.mu.m, 0.5 .mu.m and 0.25 .mu.m.
Thus, it can be seen from Table 1 that the combination of steps (b)
and (c) applied after step (a) unexpectedly and beneficially
improves the particle size distribution of the particles of the PCC
product and this provides a consequential improvement in optical
properties.
A dispersing agent, e.g. one of the agents specified below, may be
employed during the grinding step (c). This may conveniently be
applied before step (c) is begun.
The product of step (b) or step (c) (or step (d) if employed), may
be formed into a dispersed aqueous suspension, by adding a
dispersing agent for the PCC, e.g. in an amount of from 0.01% to
2%, e.g. 0.02% to 1%, by weight based on the dry weight of the
pigment, the suspension containing at least 60%, preferably at
least 70%, by weight of dry calcium carbonate and having a
viscosity of not more than 500 mPa.multidot.s as measured by means
of a Brookfield Viscometer at a spindle speed of 100 rpm. This
dispersed suspension may then be incorporated into a paper coating
composition together with an adhesive. The adhesive may be one of
the adhesives known in the art and may form from 4% to 30%, eg.
less than 20% by weight, of the composition, based on the dry
weight of the calcium carbonate. For example the adhesive for the
pigment may generally be chosen from the known materials for use in
paper coating compositions, eg. the group consisting of starches,
proteinaceous adhesives such as casein, and latices of, for
example, styrene butadiene rubber and acrylic polymers.
According to a second aspect of the present invention there is
provided, therefore, a pigment for paper coating which comprises a
PCC produced by the method of the first aspect.
A preferred form of PCC which may be the PCC according to the
second aspect of the present invention has a particle size
distribution such that at least 70 percent by weight and desirably
at least 90 percent by weight of the PCC particles have an
equivalent spherical diameter (as measured by sedimentation) of
less than .Iadd.1 .Iaddend.micrometer. Desirably, at least 50% by
weight (based on the dry PCC weight) have an equivalent spherical
diameter of less than 0.5 micrometers. A preferred product particle
size distribution for the preferred PCC is one in which the
particle size distribution is such that the percentage by weight of
particles .[.have.]. .Iadd.having .Iaddend.an equivalent spherical
diameter (measured by sedimentation) smaller than 1 .mu.m, 0.5
.mu.m and 0.25 .mu.m, respectively is as follows: 96 to 99 wt
%<1 .mu.m 50 to 80 wt %<0.5 .mu.m 10 to 45 wt %<0.25
.mu.m.Iadd...Iaddend.
Such a distribution has not been achieved for PCC products in the
prior art. The usefulness of such a distribution is demonstrated
hereinafter.
The selected crystal form of the PCC according to the second aspect
is preferably a form which is predominantly aragonite although a
form which is predominantly calcite of the scalenohedral habit or
shape is also acceptable. Desirably, the length to diameter ratio
of the crystals of the selected form averages at least 3:1. The
process conditions during the precipitation process required
generally to achieve either principally aragonitic or scalenohedral
PCC are known to those skilled in the art.
A preferred form of a method according to the first aspect to
produce predominantly an aragonitic PCC comprises the following
steps prior to steps (b) and (c) as described above: (i) mixing
quicklime with water at a temperature not exceeding 60 C. to give
an aqueous suspension containing from 0.5 to 3.0 moles of calcium
hydroxide per liter of suspension under conditions such that the
temperature of the suspension increases by not more than 80 Celsius
degrees; (ii) cooling the suspension of slaked lime prepared in
step (a) to a temperature in the range from 30 C. to 50 C. (iii)
passing a carbon dioxide-containing gas through the cooled
suspension at a rate such that not more than 0.02 moles of carbon
dioxide are supplied per minute per mole of calcium hydroxide to
precipitate calcium carbonate in the suspension while the
temperature thereof is maintained within the range from 30 C. to 50
C. until the pH has fallen to a value within the range from 7.0 to
7.5.
The PCC form achieved in practice is unlikely to be 100% of the
selected form. It is quite usual for one PCC crystal form even when
predominant to be mixed with other forms. Such mixed forms will
give suitably improved product properties. We prefer that at least
50% by weight, desirably at least 80% by weight of the crystals in
the PCC product produced in step (a) are of the selected form.
As noted above, the PCC product of the second aspect may be
dispersed in an aqueous medium using a dispersing agent to form a
dispersed aqueous suspension of the PCC. According to the present
invention in a third aspect, therefore, there is provided a
dispersed aqueous suspension of the PCC product of the second
aspect which incorporates a dispersing agent. The dispersed aqueous
suspension formed preferably contains at least 60% preferably at
least 70% by weight of calcium carbonate based on the dry weight of
calcium carbonate present and has a viscosity of not more than 500
mPa.multidot.s as measured by a Brookfield Viscometer at a spindle
speed of 100 revolutions per minute. The dispersing agent may be
present in an amount of from 0.01 percent to 2.0 percent, e.g. 0.02
percent to 1.5 percent by weight based upon the dry weight of PCC
present.
The dispersing agent may be selected from the dispersing agents
known in the art for the dispersion of calcium carbonate. The
dispersing agent may.Iadd., .Iaddend.for example.Iadd.,
.Iaddend.comprise a polycarboxylate which may be a homopolymer or
copolymer which contains a monomer unit comprising a vinyl or
olefinic group which is substituted with at least one carboxylic
acid group, or a water soluble salt thereof. Examples of suitable
monomers are acrylic acid, methacrylic acid, itaconic acid,
crotonic acid, fumaric acid, maleic acid, maleic anhydride,
isocrotonic acid, undecylenic .[.acic.]. .Iadd.acid.Iaddend.,
angelic acid and hydroxyacrylic acid. The number average molecular
weight of the polycarboxylate dispersing agent should be not
greater that 20,000, and preferably in the range from 700 to
10,000, as measured by the method of gel permeation chromatography
using a low angle laser light scattering detector.
According to the present invention in a fourth aspect there is
provided a pigment composition for use in paper coating which
comprises a mixture of pigments one of which comprises a PCC
according to the second aspect or a dispersed aqueous suspension
according to the third aspect. The said PCC may comprise
predominantly an aragonitic PCC or predominantly a scalenohedral
PCC, the PCC having a particle size distribution such that at least
70 percent of the particles have an equivalent spherical diameter
of less than 1 micrometer and at least 50 percent of the particles
have an equivalent spherical diameter of less than 0.5
micrometers.
The other pigment or pigments incorporated in the mixture according
to the fourth aspect may for example be selected from titanium
dioxide, calcined clay, talc, calcium sulphate, kaolin clay,
calcined kaolin and precipitated or ground calcium carbonate. The
pigment mixture desirably includes a mixture of the product of the
second aspect and a kaolin clay. Such a pigment mixture may
comprise from 5 percent to 99 percent, especially 40 percent to 70
percent, by weight of the PCC product. Platey kaolin clay is
especially preferred to form the pigment mixture with the PCC
product optionally together with other pigment ingredients for the
reasons explained hereinafter. By "platey" kaolin clay is meant a
kaolin clay having an aspect ratio of at least 20:1, preferably at
least 30:1.
The pigment mixture may be formed by mixing aqueous suspensions of
each of the required pigments to form an aqueous suspension
incorporating the mixture of pigments. Such an aqueous suspension
may be a dispersed aqueous suspension and the individual aqueous
suspension of pigments employed to form the mixture may each
incorporate a dispersing agent. The dispersing agents employed to
disperse the pigments in the individual aqueous suspensions mixed
together, and the concentrations of such suspensions, may be the
same or different.
According to the present invention in a fifth aspect there is
provided a paper coating composition which comprises an aqueous
suspension of a PCC product according to the second aspect mixed
together with an adhesive. The PCC product employed in the
composition may be mixed with one or more pigments as described
above. The adhesive may form from 4 percent to 30 percent by weight
based on the total dry weight of pigment or pigments present. The
adhesive may be one of the known paper coating adhesives employed
in the art, e.g. chosen from the group consisting of starches,
proteinaceous adhesives such as casein and latices of, for example,
styrene butadiene rubbers and acrylic polymers.
The paper coating composition according to the fifth aspect may
also include one or more optional additives conventionally used in
paper coating compositions, eg. a thickener, e.g. in an amount of
up to two percent by weight based upon the total dry weight of
pigment or pigments present. The thickener may comprise one or more
substances employed as thickeners in the prior art, e.g. sodium
carboxymethyl cellulose or synthetic acrylic thickeners.
The paper coating composition according to the fifth aspect may be
formed by mixing together an aqueous dispersed suspension according
to the third aspect, optionally with one or more further aqueous
dispersed suspensions containing other pigments, with the adhesive
and any other optional constituents e.g. thickener, in a manner
familiar to those skilled in the art.
We have found that paper coating compositions according to the
fifth aspect when applied to woodfree paper or board, especially as
a topcoat on a precoated base substrate, gives excellent sheet
gloss, print gloss and brightness. The performance of the material
when coated onto a wood containing base, especially in a light
weight coating, has been surprisingly good in two respects: (i) The
sheet opacity and brightness attained have been such that, in a
paper coating composition comprising a mixture of white pigments
comprising 85 parts by weight kaolin, 10 parts by weight metakaolin
(calcined kaolin), and 5 parts by weight TiO.sub.2, up to 10 parts
of calcined kaolin plus 5 parts of TiO.sub.2 and 25 parts of kaolin
can be replaced with 40 parts of the dry PCC product according to
the second aspect with no deleterious effect on sheet properties.
(ii) Even more surprisingly, it has been found that a blend of
material embodying the fifth aspect with a "platey" kaolin clay,
i.e. a kaolin clay of high particle aspect ratio (ratio of diameter
of a circular platelet of equivalent area to average platelet
thickness) of at least 20, can, in some cases, give a superior
gloss to either pigment alone, and the sheet brightness attained
with the blend is markedly greater than would be expected by
interpolating from the brightnesses of the sole pigments.
Embodiments of the present invention will now be described by way
of example only with reference to the following Examples.
EXAMPLE 1
Predominantly aragonitic PCC for use as a paper coating pigment as
prepared by the following method embodying the present
invention.
A sample of quick lime was slaked in water at a temperature of 47
C. to give a 2 molar suspension of hydrated lime, i.e. 148 g of
Ca(OH).sub.2 per liter of suspension. During this slaking the
temperature of the suspension rose to a temperature 72 C. The
slaked lime suspension was then cooled to a temperature of 40 C.
before carbonating.
25 m.sup.3 batches of this suspension were carbonated at a constant
temperature of 40 C. Carbon dioxide was applied at a rate of
introduction of 0.0026 moles of carbon dioxide per minute per mole
of a calcium hydroxide. Carbonation was continued until the pH
began to drop, and then for another 30 minutes thereafter, given a
final pH of 7.5. This gave a slurry of 18 wt % solids, which was
diluted to 16 wt % after rinsings from the carbonator were added At
this point the precipitated calcium carbonate typically had a
particle size distribution such that 80% by weight of the particles
had an equivalent spherical diameter smaller than 2 .mu.m and 25%
by weight of the particles had an equivalent spherical diameter
smaller than 1 .mu.m.
The precipitated calcium carbonate was then comminuted to break up
aggregates in an attrition grinding mill fitted with a 250
horsepower (186 kW) motor and containing as the grinding medium
silica sand consisting of grains having sizes in the range from 0.5
mm to 1.0 mm. Flow and density sensors were coupled to a
kilowatt-hour meter to give control of grinding energy. Typically
about 100 to 200 kilowatt hours per tonne were needed to attain the
desired particle size of about 95% to 100% by weight of the
particles having an equivalent spherical diameter smaller than 1
.mu.m. After grinding, the product was passed through a 370 mesh
(nominal aperture 40 .mu.m) screen.
The suspension of ground precipitated calcium carbonate was then
partially dewatered in a tube pressure filter of the type described
in British Patent Specification No. 1240465. This gave a cake
solids of about 71 wt % to 72 wt %.
The pH of the suspension was adjusted in one of the ways described
above to pH7.5.
In preparation for paper coating experiments, partially dewatered
calcium carbonate prepared in the method embodiment described above
was redispersed in water containing 0.8 wt % of a sodium
polyacrylate dispersing agent in a high shear mixer, to give a
fluid suspension containing about 70% to 71% by weight of dry
calcium carbonate, and having a viscosity as measured by means of a
Brookfield Viscometer at a spindle speed of 100 r.p.m. of about 200
mPas.
EXAMPLE 2
Five paper coating compositions were prepared according to the
recipes set out in Table 2 below:
TABLE 2 Composition A B C D E Ingredient % by weight Clay 1 85 90
60 60 60 Calcined Clay 10 10 -- -- -- Titanium dioxide 5 -- -- --
-- PCC 1 -- -- 40 -- -- PCC 2 -- -- -- 40 -- PCC 3 -- -- -- -- 40
Styrene-butadiene 8 8 8 8 8 latex adhesive Corn Starch adhesive 8 8
8 8 8 Sodium hydroxide to 8.5 8.5 n.a n.a n.a give a pH of: Water
to give a solids 56.0 56.5 57.5 56.8 57.3 concentration of: (% by
weight)
Note: In this Table, and in subsequent Tables, "n.a." means "not
applicable". In these cases no sodium hydroxide was added.
Clay 1 was a fine hydrous kaolin such that 95% by weight consisted
of particles having an equivalent spherical diameter (e.s.d.) of
less than 2 .mu.m and 89% by weight consisted of particles having
an e.s.d. of less than 1 .mu.m.
The calcined clay was metakaolin such that 91% by weight consisted
of particles having an e.s.d. of less than 2 .mu.m.
The titanium dioxide was of the rutile type and was marketed by Du
Pont de Nemours Int.S.A. under the registered trade mark
"TI-PURE".
PCC 1 was of the predominantly aragonitic type (prepared as in
Example 1) and was ground such that 96% by weight consisted of
particles having an e.s.d. of less than 1 .mu.m. Its powder
brightness, or percentage reflectance to light of 457 nm
wavelength, was 94.3.
PCC 2 was of the predominantly aragonitic type (prepared as in
Example 1) and was ground such that 75% by weight consisted of
particles having an e.s.d. of less than 1 .mu.m.
PCC 3 was of the predominantly aragonitic type (prepared as in
Example 1) and was ground such that 93% by weight consisted of
particles having an e.s.d. of less than 1 .mu.m.
Each of PCC 1, PCC 2 and PCC 3 had at least 50 percent of particles
having an esd less than 0.5 .mu.m. The differentiation in particle
size between PCC1, PCC2 and PCC3 was achieved by controlling the
work input during the attrition grinding step employed in
production. It should be noted that in compositions C, D and E no
sodium hydroxide to give pH adjustment was required at the
composition forming stage.
Each of the coating compositions A to E was formed in a known way
by mixing together dispersed aqueous suspensions of the relevant
pigments together with the other ingredients incorporated into the
composition. The suspensions of pigment comprising clay, calcined
clay or TiO.sub.2 each contained 0.3 percent or less of a sodium
polyacrylate dispersing agent. The dispersed suspension of PCC was
produced by the above method embodiment.
Compositions A to E were each separately applied to a 39 gsm LWC
(light weight coated) offset basepaper, using a Valmet pilot coater
with a short dwell head at a coating speed of 1200 m/min and with a
blade holder angle of 45. Coat weights of approximately 6, 8 and 10
gsm were obtained by adjusting the pressure applied to the blade.
Samples of the papers so coated with compositions A to E were
conditioned for 24 hours at 23 C., 50% relative humidity, and were
calendered by passing them 6 times through a Perkins laboratory
supercalender at a temperature of 65 C., a pressure of 45 bar and a
speed of 36 m/min.
The papers so calendered were then subjected to the paper and
offset printing tests described below.
The results from these tests are given in Table 3 below,
interpolated to a coat weight of 8 gsm (the exceptions to this are
the printing results which are obtained from measurements at one
coat weight, at or very close to 8 gsm).
TABLE 3 Dry Dry Gloss Brightness Opacity Print Print Composition %
(ISO) (ISO) Gloss Density A (reference) 57 72.7 91.2 67 1.48 B
(reference) 56 71.3 90.0 69 1.45 C (invention) 59 72.9 91.1 65 1.46
D (invention) 53 72.8 90.7 61 1.43 E (invention) 56 72.9 90.9 64
1.45
It can be seen that with Compositions C and E, which contain the
finer PCC samples, but no calcined clay or titanium dioxide, the
sheet gloss of the reference Compositions can be matched or
exceeded. With Compositions C, D and E, the sheet brightness and
opacity of reference Composition B (with 10 parts of calcined clay)
is exceeded. The brightness and opacity of reference Composition A,
with 10 parts calcined clay and 5 parts TiO.sub.2, is matched by
Composition C (with the finest PCC sample) and is approached by
Compositions D and E.
EXAMPLE 3
Three coating Compositions F, G and H were prepared according to
the recipes given in Table 4 below in the manner described with
reference to Example 2:
TABLE 4 Composition F G H Ingredient % by weight Clay 2 100 90 50
Calcinated clay -- 10 -- PCC 1 -- -- 50 Styrene-butadiene 12 12 12
latex adhesive Carboxy methyl 0.5 0.5 0.5 cellulose thickener
Sodium hydroxide to 8.5 8.5 n.a. give a pH of: Water to give a
solids 58.0 56.3 60.3 concentration of: (% by weight)
Clay 2 was a moderately platey paper coating kaolin clay (average
aspect ratio about 30) having a particle size distribution such
that 85% by weight consisted of particles having an e.s.d. smaller
than 2 .mu.m.
The calcined clay was as used in Example 1.
Compositions F, G and H were separately applied to a 39 gsm LWC
offset basepaper using a Valmet pilot coater with a short dwell
head with a coating speed of 1200 m/min, using a blade holder angle
of 45 degrees. Coat weights of between 5 and 10 gsm were obtained
by adjusting the pressure applied to the blade. Samples of the
papers so coated with Compositions F to H were conditioned for 24
hours at a temperature of 23 C. and 50% relative humidity, and were
calendered by passing them 10 times through a Perkins laboratory
supercalender at a temperature of 65 C., a pressure of 69 bar and a
speed of 36 m/min. The papers so calendered were then subjected to
the paper tests described below.
The results from these tests are given in Table 5 below,
interpolated to a coal weight of 7 gsm.
TABLE 5 Gloss Brightness Opacity Composition % (ISO) (ISO) F
(reference) 56 70.3 89.5 G (reference) 58 71.3 90.0 H (invention)
61 72.8 89.8
It can be seen that Composition H gives superior gloss to both of
the reference Compositions F and G. The brightness is superior and
the opacity similar to that given by Composition G, which contains
10 parts by weight of calcined clay.
EXAMPLE 4
Three coating compositions were prepared according to the recipes
given in Table 6 below:
TABLE 6 Composition J K L Ingredient % by weight Clay 3 100 -- 50
PCC 1 -- 100 50 Styrene-butadiene 12 12 12 latex adhesive Carboxy
methyl 0.5 0.5 0.5 cellulose thickener Sodium hydroxide to 8.5 n.a
n.a give a pH of: Water to give a solids 59.8 63.4 60.3
concentration of: (% by weight)
Clay 3 was a kaolin refined such that 94% by weight consisted of
particles having an e.s.d. of less than 2 .mu.m and 85% by weight
consisted of particles of e.s.d of less than 1 .mu.m.
Compositions J, K and L were applied separately to a 39 gsm LWC
offset basepaper using a Valmet pilot coater with a short dwell
head with a coating speed of 1200 m/min, using a blade holder angle
of 45 degrees. Coat weights of between 5 and 11 gsm were obtained
by adjusting the pressure applied to the blade. The papers so
coated with Compositions J, K and L were conditioned for 24 hours
at a temperature of 23 C., 50% relative humidity, and were
calendered using a Perkins laboratory supercalender under the same
conditions as those employed in Example 2. The papers so calendered
were then subjected to the paper tests described below.
The results from these tests are given in Table 7 below,
interpolated to a coat weight of 8 gsm.
TABLE 7 Gloss Brightness Opacity Composition % (ISO) (ISO) J
(reference) 61 69.9 89.6 K (invention) 62 74.3 89.6 L (invention)
64 73.2 89.9
It can be seen that: (i) Composition K and L both showed superior
gloss and brightness compared with reference Composition J; (ii)
Composition L (incorporating a blend of pigments) gives slightly
superior gloss to that of either of the sole pigments (compositions
J and K); (iii) Composition L has a brightness over a unit higher
than that which would be expected by interpolating from the
brightnesses given by the sole pigments.
EXAMPLE 5
Four coating compositions were prepared according to the recipes
given in Table 8 below in the manner described with reference to
Example 2 above.
TABLE 8 Composition M N P Q Ingredient % by weight Clay 3 30 30 --
-- Clay 4 -- -- 50 50 PCC 1 -- 70 -- 50 .[.PCC 1.]. .Iadd.GCC
1.Iaddend. 70 -- 50 -- Styrene-butadiene 11 11 12 12 latex adhesive
Carboxy methyl 0.5 0.5 0.5 0.5 cellulose thickener Sodium hydroxide
to 8.5 n.a. 8.5 n.a. give a pH of: Water to give a solids 66.3 63.7
63.1 61.7 concentration of: (% by weight)
Clay 3 was a kaolin refined such that 94% by weight consisted of
particles having an e.s.d. of less than 2 .mu.m and 85% by weight
consisted of particles of e.s.d. of less than 1 .mu.m.
GCC 1 was a ground marble with a powder brightness of 94.5 and a
particle size distribution such that 95% by weight consisted of
particles having an e.s.d. of less than 2 .mu.m.
Clay 4 was a kaolin having a particle size distribution such that
91% by weight consisted of particles having an e.s.d. of less than
2 .mu.m and 81% by weight consisted of particles having an e.s.d.
of less than 1 .mu.m.
Compositions M and N were separately applied to a 81 gsm precoated
woodfree basepaper with a ISO sheet brightness of 92.0 using a
Valmet pilot coater with a Roll Applicator with a coating speed of
800 m/min and blade holder angles of 47 and 48 degrees
respectively. Coat wights of 10, 12 and 14 gsm were obtained by
adjusting pressure applied to the blade. The papers so coated with
compositions M and N were calendered by passing them 11 times
through a Valmet Supercalender at a temperature of 100 C, a
pressure of 300 kN/m.sup.2 and a speed of 800 m/min.
Compositions P and Q were separately applied to a 39 gsm LWC wood
containing basepaper with an ISO sheet brightness of 67.5 using a
Valmet pilot coater with a short dwell head, with a coating speed
of 1200 m/min and a blade holder angle of 45 degrees. Coat weights
of 6, 8 and 10 gsm were obtained by adjusting the displacement of
the blade towards the paper surface. The papers so coated with
compositions M and Q were conditioned for 24 hours at a temperature
of 23 C., 50% relative humidity, and were calendered using a
Perkins laboratory supercalender under the same conditions as were
employed in Example 3.
The calendered papers coated with compositions M,N,P and Q were
subjected to the paper tests described in Example 2. The results
are shown in Table 9 below.
TABLE 9 Gloss Brightness Opacity Composition % (ISO) (ISO)
Precoated Woodfree Basepaper M 72 88.8 89.1 N 78 88.4 89.4 LWC
Mechanical Basepaper P 67 71.8 89.5 Q 72 73.0 90.8
EXAMPLE 6
Six paper coating compositions were prepared according to the
recipes set out in Table 10 below:
TABLE 10 Composition R S T U V W Ingredient % by weight GCC 1 --
100 -- 100 -- 100 GCC 2 50 -- 50 -- 50 -- PCC 4 50 -- -- -- -- 50
PCC 5 -- -- 50 -- -- -- PCC 6 -- -- -- -- 50 -- Styrene- 11 11 11
11 11 11 butadiene latex adhesive Carboxy 0.5 0.5 0.5 0.5 0.5 0.5
methyl cellulose thickener Water to give a 68.5 69.1 68.7 69.3 67.2
67.3 solids concen- tration of: (% by weight)
GCC 2 was a ground marble with a powder brightness of 94.5 and a
particle size distribution such that 90% by weight consisted of
particles having an e.s.d. of less than 2 .mu.m.
PCC 4 was of the aragonitic type produced as in Example 1 and was
ground such that 97% by weight consisted of particles having an
e.s.d. of less than 1 .mu.m.
PCC 5 was of the predominantly scalenohedral type produced by a
method embodying the invention and was ground such that 98% by
weight consisted of particles having an e.s.d. of less than 1
.mu.m.
PCC6 was a predominantly scalenohedral PCC produced without the
grinding step (c) of the method according to the first aspect,
having a particle size distribution such that 74% by weight
consisted of particles having an e.s.d. of less than 1 .mu.m.
Compositions R and S were applied separately to a precoated
woodfree base paper with a substance of 113 gsm using laboratory
coating machine supplied by Denver and a coating speed of 4000
m/min.
Compositions T and U were separately applied under identical
conditions to R and S, but in a separate exercise.
The papers so coated were calendered using the same conditions a
those described in Example 2 and were subjected to the paper tests
described in Table 10.
The results of these tests interpolated to a coatweight of 10 gsm
are also given in Table 11 below.
TABLE 11 Gloss Brightness Opacity Composition % (ISO) (ISO) R
(Invention) 80 85.0 94.2 S (Reference) 77 84.7 94.3 T (Invention)
81 87.8 91.5 U (Reference) 77 87.8 91.4 V (Reference) 72 85.6 94.4
W (Reference) 75 85.2 94.3
It can be seen that, i) Compositions R and T both show superior
gloss to the reference compositions S,U,V and W. ii) Compositions R
and T are similar with respect to their performance relative to
their respective references S and U, iii) Compositions R and T both
give superior gloss to Composition V which contains a precipitated
calcium carbonate which was not prepared in accordance with the
first aspect of the present invention.
EXAMPLE 7
Four paper coating compositions were prepared according to the
recipes set out in Table 12 below:
TABLE 12 Composition X Y Z Ingredient % by weight GCC 1 70 GCC 2 35
35 Clay 5 30 30 30 PCC 7 35 PCC 8 35 Styrene-butadiene 11 11 11
latex adhesive Carboxy methyl 0.5 0.5 0.5 cellulose thickener Water
to give a solids 65.5 65.7 65.2 concentration of: (% by weight)
Clay 5 was a fine hydrous kaolin, refined such that 92% by weight
consisted of particles having an e.s.d. of less than 2 .mu.m and
83% by weight consisted of particles having an e.s.d of less than 1
.mu.m.
PCC 7 was produced as in Example 1 and was of the predominantly
aragonitic type and was ground such that 94% by weight consisted of
particles having an e.s.d. of less than 1 .mu.m.
PCC 8 was produced by a method embodying the invention and was of
the predominantly scalenohedral type ground such that 96% by weight
consisted of particles having an e.s.d. of less than 1 .mu.m.
The above compositions were separately applied on to a 95 gsm
surface sized precoated woodfree base using a Valmet pilot coater
with a Roll Applicator and a coating speed of 1000 m/min. Coat
weights of 8, 10 and 12 gsm were obtained by adjusting the pressure
applied to the blade. The papers so coated with compositions X,Y
and Z were calendered by passing them 11 times through a Valmet
Supercalender at a temperature of 25.degree. C., a pressure of 185
kN/m.sup.2 and a speed of 400 m/min. The calendered papers were
subjected to the test described in Table 13 below. The results of
these tests interpolated to a coatweight of 10 gsm are also given
in Table 13.
TABLE 13 Gloss Brightness Opacity Composition % (ISO) (ISO) X
(Reference) 71 87.5 93.4 Y (Invention) 73 87.9 93.4 Z (Invention)
75 87.9 93.5
It can be seen that the compositions Y and Z, both give superior
gloss and brightness to the reference composition X.
PCCs produced above in a predominantly scalenohedral form were
prepared in a known way using conditions known to give the required
form. The lime molarity used was about 2M and the temperature was
about 25 C. to 30 C. rising during the carbonation reaction of
CO.sub.2 and Ca(OH).sub.2.
* * * * *