Note: Descriptions are shown in the official language in which they were submitted.
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Process for Preparing High Strength Paper
The present invention refers to polysilicate coated mineral particles, which
are suitable for
use as filler in paper, to an aqueous composition comprising these particles,
to a process for
the preparation of this aqueous composition, to paper filled with these
particles and to a
process for the preparation of filled paper.
Generally fillers, which are usually mineral materials, are included into
paper in order to
reduce the amount of fibres and thus costs. However, the paper strength
usually diminishes
as the level of filler is increased.
Thus, there is an ongoing search for paper having high filler content and at
the same time
acceptable paper strength.
US 6,623,555 describes a method of making a composite pigment of precipitated
calcium
carbonate and a silicon compound, for use, for example, as a filler for paper
making or as
coating pigment. The method includes the steps of introducing a soluble
silicate compound
into an aqueous medium containing a precipitate of calcium carbonate and
precipitating an
insoluble silicon compound upon the preceipitated calcium carbonate by
carbonation of the
reaction mixture. The temperature of the reaction mixture during deposition of
the silicon
compound upon the precipitated calcium carbonate is at least 50 C. The
obtained aqueous
composition containing the formed composite pigment is of low viscosity. The
disadvantage
of the composite pigment of US 6,623,555 is that it yields filled paper of low
strength when
used as filler.
WO 00/26305 describes a method for preparing an aqueous composition comprising
partly
silica coated calcium carbonate particles suitable as filler for paper. The
method includes
mixing calcium carbonate with about 3.5% by weight soluble silicate based on
the weight of
calcium carbonate and allowing the silicate slowly to react at room
temperature without pH
adjustment to form an insoluble silica coating. The disadvantage of this
method is that the
reaction time is very long, for example 50 hours, thus rendering the process
not technically
feasible from an industrial point of view. In addition, the amount of silica
in the silica coated
filler is very low, and the silica coated filler yields paper of a similar
strength to paper filled
with standard calcium carbonates.
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WO 95/03251 describes a method for preparing a mixed pigment of calcium
carbonate and
silica, useful as filler for paper. The method involves admixing lime milk and
an aqeous
sodium silicate solution and raising the temperature of the mixture to 55 to
170'C and
thereafter supplying carbon dioxide to the mixture until the pH of the mixture
falls to 7 or
below, thus precipitating a mixed pigment comprising calcium carbonate and
silica. The ratio
of silica/CaO is 3.6/1. Thus, the ratio of silica/CaCO3 in the obtained mixed
pigment is 2/1 (or
66/33) and thus very high.
EP 356 406 Al describes a process for the preparation of calcium carbonate
particles with
an acid-resistant coating, in which process a slurry of calcium carbonate
particles is mixed
simultaneously with the solution of a zinc compound and a solution of a silica-
containing
substance at a temperature of 70 to 95 C at pH 8 to 11.
US 5'164'006 describes a method for preparing an acid resistant calcium
carbonate pigment,
which method includes mixing a sodium silicate solution with a calcium
carbonarte slurry at
75 to 80 C, adjusting the pH to 10.2 to 10.7 by adding carbon dioxide, cooling
the reaction
mixture and adding zinc chloride.
The disadvantage of the methods of EP 356 406 Al and US 5'164'006 is that the
methods
involve the additional step of adding a zinc compound.
It is an object of the present invention to provide filled paper or paper
board of improved dry
strength, in particular of improved tensile strength and internal bond
strength, with respect to
the amount of filler.
This object is solved by the process of claim 1, the composition of claim 9,
the polysilicate
coated mineral particles of claim 15, the paper of claim 16 and the process of
claim 17.
Part of the invention is a process for the preparation of an aqueous
composition comprising
polysilicate coated mineral particles, wherein the process comprises the step
of subjecting an
aqueous composition comprising gelled polysilicate and mineral to shear of at
least
5'000 rpm for at least 30 seconds.
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Preferably, the aqueous composition comprising gelled polysilicate and mineral
is subjected
to shear of at least least 8'000 rpm, more preferably to shear in the range of
10'000 to
30'000 rpm. Preferably, the shear is applied for at least 1 minute. The shear
can be applied
for any time. For practical reasons, however, shear is usually applied for
maximum 1 hour,
preferably maximum 30 minutes, more preferably maximum 15 minutes.
Preferably, the aqueous composition comprising polysilicate coated mineral
particles is
characterized by a viscosity of at least 500 mPas (when measured at a
concentration of 12%
by dry weight polysilicate coated mineral particles based on the weight of the
composition,
using a Brookfield viscometer at 25 C and 100 rpm, spindle 4, and the result
is taken 30
seconds after the start of the test).
Preferably, the aqueous composition comprising gelled polysilicate and mineral
is obtainable
by polymerization of a silicate in the presence of a mineral.
Usually, the silicate is water soluble. Preferably the silicate is an alkaline
earth metal silicate
such as magnesium silicate or an alkali metal silicate such as lithium,
portassium or sodium
silicate. More preferably it is an alkali metal silicate; most preferably it
is sodium silicate.
Preferred sodium silicate has a weight ratio of Na20 to Si02 is in the range
of 2:1 to 1:4 more
preferably it is in the range of 1:2 to 1:4, most preferably in the range of
1:2.5 to 1:3.5.
Preferably, the silicate is employed in form of an aqueous solution.
Examples of minerals are titanium dioxide, aluminium trihydrate, mineral
silicates such as
talc, mica, zeolite, clay, e.g. kaolin, and calcinated clay, precipitated
silicates and calcium
carbonates such as ground calcium carbonate (GCC) and precipitated calcium
carbonate
(PCC). A preferred mineral is calcium carbonate. The mineral is usually
substantially not
water-soluble under the conditions of polymerization. The mineral can be
employed in solid
form or as aqueous slurry.
The mineral can be already present at the start of the polymerization of the
silicate or it can
be added during polymerization of the silicate.
During polymerization of the silicate gellation occurs. Preferably, the
polymerization of the
silicate is performed under stirring. Once polymerization and thus gellation
is substantially
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completed, the intensity of the stirring is preferably increased to achieve an
aqueous
composition comprising gelled polysilicate and mineral of relatively low
viscosity, for example
below 300 mPas (when measured at a concentration of 12% by dry weight gelled
polysilicate
and mineral, using a Brookfield viscometer at 259C and 100 rpm, spindle 4, and
the result is
taken 30 seconds after the start of the test). This aqueous composition
comprising gelled
polysilicate and mineral of relatively low viscosity can then be applied to
the step of high
shear (at least 5'000 rpm, for 30 seconds).
The polymerization of the silicate can be initiated by adjusting the pH to a
pH of below 10 by
addition of an acid. Examples of acids are carbon dioxide, mineral acids such
as hydrochloric
acid or sulfuric acid, organic acids such as acetic acid and alkali metal
borates, aluminates or
stannates. Preferably, carbon dioxide or mineral acids are used as acid.
In the first embodiment of the process, where the mineral is already present
at the start of the
polymerization, the polymerization of the silicate is preferably initiated
using carbon dioxide,
and the pH is preferably adjusted to a pH in the range of 5 to 9, more
preferably about 7.
In the second embodiment of the process, where the mineral is added during
polymerization
of the silicate, the polymerization of the silicate is preferably initiated
using a mineral acid,
and the pH is preferably adjusted to a pH in the range of 2 to 10.5, more
preferably of 7 to 9.
If in the second embodiment of the process a mineral is used, which can
dissolve at acidic
pH, for example calcium carbonate, it is preferred that the pH is adjusted to
a pH in the range
of 8.5 to 10.
Preferably, in both embodiments, the polymerization of the silicate is
performed at a
temperature of below 50 C. More preferably, it is performed at a temperature
in the range of
15 to 35 C. Most preferably, it is performed at a temperature in the range of
20 to 30 C.
Usually the polymerization of the silicate is substantially complete within 12
hours after start
of the polymerization. Preferably, it is complete within 5 hours, more
preferably within 2 hour
and most preferably within 1 hour.
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The amount of mineral is preferably in the range of 5 to 20%, more preferably
of 8 to 12%, by
weight based on the weight of aqueous composition comprising gelled
polysilicate and
mineral.
The amount of silicate is preferably in the range of 0.01 to 10%, more
preferably of 0.5 to
4%, more preferably of 1 to 3% by weight, based on the weight of the aqueous
composition
comprising gelled polysilicate and mineral.
Preferably, the amount of gelled polysilicate and mineral is in the range of 1
to 30%, more
preferably 5 to 15%, most preferably 10 to 14%, by dry weight based on the
weight of the
aqueous composition comprising the gelled polysilicate and mineral.
Also part of the invention is an aqueous composition comprising polysilicate
coated mineral
particles, which is obtainable by the process of the present invention.
Preferably, the aqueous composition comprising polysilicate coated mineral
particles is
characterized by a viscosity of at least 500 mPas (when measured at a
concentration of 12%
by dry weight polysilicate coated mineral particles based on the weight of the
composition,
using a Brookfield viscometer at 25 C and 100 rpm, spindle 4, and the result
is taken
30 seconds after the start of the test).
Preferably, the polysilicate coated mineral particles are not completely
coated with the
polysilicate. For example, a polysilicate coated calcium carbonate particle
dissolves at a pH
of below 6.
The amount of mineral can be between 40 and 99% by weight based on the weight
of the
polysilicate coated mineral particle. Preferably it is between 60 and 95% by
weight, more
preferably it is between 80 and 90% by weight, most preferably it is between
78 and 88% by
weight.
The amount of polysilicate can be between 1 and 60% by weight based on the
weight of the
mineral; preferably it is between 5 and 40% by weight, more preferably it is
between 15 and
25% by weight.
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The size of the polysilicate coated mineral particles can be between 1 and
1000 m,
preferably it is between 8 and 30 m, more preferably, it is between 10 to 27
m.
Preferably, the amount of polysilicate coated mineral particles is between 1
and 30% by
weight based on the weight of the aqueous composition comprising the
polysilicate coated
mineral particles. More preferably, it is between 10 and 15% by weight.
Also part of the present invention is a polysilicate coated mineral particle
obtainable from the
aqueous composition of the present invention by removal of water. Removal of
water can be
performed by any suitable method such as filtration, decantation or
distillation or any suitable
combination of methods.
Another part of the present invention is paper or paper board filled with the
polysilicate
coated mineral particles of the present invention.
Preferably, the weight ratio of fibre/polysilicate coated mineral particles in
the filled paper or
paper board is in the range of 90/10 to 30/70, preferably it is in the range
of 80/20 to 60/40,
and more preferably it is in the range of 75/25 to 65/35.
Also part of the invention is a process for preparing the filled paper or
paper board of the
present invention, which process comprises the step of adding the polysilicate
coated
mineral particles or the aquepous composition comprising the polysilicate
coated mineral
particles to a cellulosic suspension prior to drainage of the cellulosic
suspension on a wire,
where a web is formed, which is subsequently dried.
Preferably, filled paper is prepared.
The polysilicate coated mineral particles of the present invention can be used
in conjunction
with standard wet end additives such as cationic coagulants, dry strength
agents, retention
agents, sizing agents and optical brighteners. It is also possible that it is
used together with
other fillers such as calcium carbonate, although this is not particulary
preferred.
The polysilicate coated mineral particles can be added to the thick stock, the
thin stick or to
the white water.
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Also part of the invention is a method for improving the tensile strength and
internal bond
strength of filled paper or paper board with respect to the basis weight of
the filled paper or
paper board, which method involves adding the polysilicate coated mineral
particles or the
aqueous composition comprising the polysilicate coated mineral particles of
the present
invention to a cellulosic suspension prior to drainage of the cellulosic
suspension on a wire,
where a web is formed, which is subsequently dried.
The polysilicate coated mineral particles of the present invention have the
advantage that
when used as filler for paper, the ratio of tensile strength (breaking length)
and Internal
bonding strength (Scott bond strength)/ash content of filled paper is
improved. In other
words, the use of polysilicate coated mineral as filler leads either to filled
paper of increased
tensile strength and internal bonding strength for a given ash content, or to
paper of lower
ash content for a given tensile strength or Internal bonding strength. At the
same time, the
paper shows good formation and good opacity.
Fig. 1 shows the correlation between breaking length and sheet ash content of
handsheets
containing Calopak F, Calopak F and polysilicate, and polysilicate coated
Calopak F.
Fig. 2 shows the correlation between breaking length and sheet ash content of
handsheets
containing Albaca@)HO, Albaca@)LO, Syncarb F0474, respectively, Miconapaque HB
and the
respective polysilicate coated calcium carbonates.
Fig. 3 shows the correlation between Scott bond strength and sheet ash content
of
handsheets containing Calopak F, Calopak F and polysilicate, and polysilicate
coated
Calopak F.
Fig. 4 shows the correlation between Scott bond strength and sheet ash content
of
handsheets containing Albaca@)HO, Albaca@)LO, Syncarb F0474, respectively,
Miconapaque
HB and the respective polysilicate coated calcium carbonates.
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Examples 1 to 5
Preparation of aqueous compositions comprising polysilicate coated calcium
carbonate
particles using carbon dioxide as acid
100 g oven dry weight of calcium carbonate (see table 1) is taken and diluted
to 930 g using
deionised water. To this is added 70 g of sodium silicate (28.5% (w/v) Si02,
8.9% (w/v)
Na20). Thus, the amount of calcium carbonate is 10% by dry weight based on the
volume of
the reaction mixture, and the amount of sodium silicate is 2% by dry weight
based on the
volume of the reaction mixture. The two aqueous materials are mixed thoroughly
at room
temperature using a magnetic stirrer and carbon dioxide is added using a gas
diffusion tube
to achieve a pH of 7.0 at room temperature. In some instances the pH of 7 is
not achieved
prior to gelling of the polysilicate. The reaction mixture is allowed to gel
at room temperature.
Minutes after gellation is substantially complete the gel is broken down by
the action of
the magnetic stirrer and a slurry containing visible particles (>2 mm) is
obtained. This slurry
15 is then sheared using an ultra thurrax homogeniser for 1 minute at 13'500
rpm. The aqueous
composition obtained contains 12% by dry weight polysilicate coated calcium
carbonate
based on the volume of the composition.
The calcium carbonates used are listed in table 1 along with their particle
size and the
20 particle size of the polysilicate coated calcium carbonate particles
obtained. The particle
sizes are analyzed using a Malvern MasterSizer 2000 Particle Size Analyser.
Trade name of Mean particle size of Mean particle size of
calcium carbonate calcium carbonate before polysilicate coated
used coating [xn] calcium carbonate [xn]
example 1 Calopak F 3.191 17.987
example 2 Albacar HO 2.673 13.917
example 3 Albacar LO 3.419 10.676
example 4 Syncarb F0474-GO 2.425 26.757
example 5 Miconapaque HB 2.649 18.988
Table 1.
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Calopak(OF is a dry precipitated calcium carbonate, Albaca@)HO, Albaca@)LO and
Syncarb
F0474-GO are dispersed precipitated calcium carbonates and Miconapaque HB is a
dispersed ground calcium carbonate. Calopak(OF, Albaca@)HO and Albaca@)LO are
sold by
Mineral Technologies and Miconapaque HB is sold by Columbia River Carbonates.
The viscosities of the aqueous compositions comprising the polysilicate coated
calcium
carbonates of examples 1, 4 and 5 are measured using a Brookfield DVII-Pro
viscometer and
the results are taken 30 seconds after the start of the test.
Trade name of Brookfield viscosity
calcium carbonate (spindle 4, 100 rpm, 25 C)
used
[mPas]
Before the shear step After the shear step
example 1 Calopak F 250 596
example 4 Syncarb F0474-GO 166 960
example 5 Miconapaque HB 198 620
Table 2.
When trying to acidify the aqueous compositions comprising polysilicate coated
calcium
carbonates of examples 1 to 5 to a pH of about 1.5 using hydrochloric acid,
the calcium
carbonate rapidly dissolves before the pH reaches 6 indicating that the silica
surface is not a
complete coating providing a barrier to the acid.
Comparative example 1
Preparation of an aqueous composition comprising a polysilicate
An aqueous composition comprising solely polysilicate without calcium
carbonate is prepared
in analogy to examples 1 to 5, except that no calcium carbonate is added. The
amount of
sodium silicate is again 2% by weight based on the volume of the reaction
mixture. The
aqueous composition obtained contains 2% by dry weight polysilicate based on
the volume
of the composition
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Examples 6 to 10
Preparation of handsheets containing polysilicate coated calcium carbonates
Handsheets containing silica coated calcium carbonates are prepared as
follows:
A standard laboratory fine paper stock containing 0.5% by dry weight fibres
(70% bleached
birch fibres and 30% bleached pine fibres) based on the weight of the stock is
beaten to
47 SR. The stock is stirred at 1000 rpm for 5 seconds. An aqueous solution
containing 0.5%
(w/v) RaisamyD50021, a cationic potato starch, is added to obtain a
concentration of 5 kg/t
RaisamyD50021 based on the dry weight of the final paper and stirring at 1000
rpm is
continued for 30 seconds. The required amounts of the aqueous compositions of
examples
1 to 5 comprising the polysilicate coated calcium carbonate particles (see
table 2) are added
and stirring at 1000 rpm is continued for further 30 seconds. An aqueous
solution containing
0.1 %(w/v) Percol 175, a cationic polyacrylamide, is added to obtain a
concentration of
500 g/t Percol 175 based on the dry weight of the final paper and stirring at
1000 rpm is
continued for further 30 seconds. An aqueous solution containing 0.1% (w/v)
Hydrocol 2D6,
a water-swellable montmorrilonite clay, is added to obtain a concentration of
2000 g/t
Hydrocol 2D5 based on the dry weight of of the final paper and stirring is
continued for
15 seconds at 500 rpm. The treated stock is formed into sheets using a semi
automatic sheet
maker, pressed and dried at 95C.
The order of addition for the preparation of handsheets of examples 6 to 10 is
summarized in
the diagramm 1 below:
5s 30s 30s 30s 15s
Polysilicate-
Percol Hydrocol
Stock 44 Starch 44 coated calcium 44 44 44
175 2D6
carbonate
1000 1000 1000 1000 500
rpm rpm rpm rpm rpm
Diagramm 1.
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Comparative examples 2 to 8
Preparation of handsheets containing calcium carbonate, but no polysilicate
coated calcium
carbonate
Comparative handsheets are prepared as described for the handsheets of
examples 6 to 10,
but without adding the polysilicate coated calcium carbonates. Instead the
respective calcium
carbonate is added before the addition of starch. The order of addition for
the preparation of
handsheets of comparative examples 2 to 8 is summarized in the diagramm 2
below:
5s 5s 30s 30s 15s
Stock 4 4 Calcium 44 Starch 44 Percol 44 Hydrocol
44
carbonate 175 2D6
1000 1000 1000 1000 500
rpm rpm rpm rpm rpm
Diagramm 2.
Comparative example 9
Preparation of handsheets containing calcium carbonate and polysilicate, but
no polysilicate
coated calcium carbonate
Handsheet of comparative example 9 is prepared as described for handsheets of
examples 6
to 10, but the polysilicate of comparative example 2 is used instead of the
polysilicate coated
calcium carbonates. In addition, the respective calcium carbonate is added
before the
addition of starch. The order of addition for the preparation of handsheet of
comparative
example 9 is summarized in diagramm 3 below:
5s 5s 30s 30s 30s
Stock 4 4 Calcium 44 Starch 44 Poly- 44 Percol 44 Hydrocol
carbonate silicate 175 2D6
1000 1000 1000 1000 1000
rpm rpm rpm rpm rpm
Diagramm 3.
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The weight ratios of fibre, silica coated calcium carbonate, calcium
carbonate, respectively,
polysilicate used in the handsheets of examples 6 to 10 and comparative
examples 2 to 9 is
summarized in table 2:
Handsheets
Dry Dry weight Dry weight
Tradename of weight silica treated Calcium Dry weight
No. Calcium carbonate Fibre calcium carbonate Polysilicate
used carbonate [%]1
[%]1 [%]1 [%]1
comp. ex. 2 Calopake F 85 - 15 -
comp. ex. 3 Calopake F 65 - 35 -
comp. ex. 4 Calopake F 75 - 25 -
comp. ex. 9 Calopake F 70 - 25 5
6 Calopake F 70 302 - -
comp. ex. 5 Albacar HO 75 - 25 -
7 Albacar HO 70 302 - -
comp. ex. 6 Albacar LO 75 - 25 -
8 Albacar LO 70 302 - -
comp. ex. 7 Syncarb F0474-GO 75 - 25 -
9 Syncarb F0474-GO 70 302 - -
comp. ex. 8 Miconapaque HB 75 - 25 -
Miconapaque HB 70 302 - -
5
Table 2. 1Based on the total weight of fibre, silica treated cacium carbonate,
calcium
carbonate and polysilicate.2 Containing 25% by weight calcium carbonate and 5%
by weight
polysilicate.
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Evaluation of the handsheets of examples 6 to 10 and comparative examples 2 to
9
The handsheet samples of examples 6 to 10 and comparative examples 3 to 9 are
conditioned for at least 24 hours at 50% relative humidity and 23 C according
to the Tappi
test method T402.
Basis weight is a function of weight and handsheet area and is calculated from
the
handsheet weight. Ash content is determined using a CEM microwave furnace at a
temperature of 525 C. 2 x 15 mm strips are taken from each handsheet and are
tested for
tensile strength (Breaking Length) according to Tappi T494 using an EJA Single
column
tensile tester. Internal bond strength (Scott Bond Strength) is determined
using a Scott bond
Instrument in accordance with TAPPI Test Method T569
Handsheet
Basis weight Ash Content Breaking Length Scott Bond Strength
Example No.
Lgm-2l L%l Lml LJm-2l
comp. ex. 2 71.0 16.7 3869.6 409
comp. ex. 3 71.4 36.1 1760.4 120
comp. ex. 4 71.1 27.9 2537.5 245
comp. ex. 9 72.2 31.4 2680.7 187
6 72.2 26.9 3356.0 298
comp. ex. 5 67.4 32.5 2226.9 187
7 71.5 39.7 2522.9 178
comp. ex. 6 73.1 33.0 2523.9 254
8 71.5 35.5 2651.3 216
comp. ex. 7 64.6 22.1 3735.1 446
9 71.8 38.2 2878.5 241
comp. ex. 8 71.8 36.4 2999.3 320
10 72.2 45.3 3257.3 289
Table 3.
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The results for handsheets of comparative examples 2 to 4 show that the higher
the fibre
content and the lower the calcium carbonate content, the higher the Breaking
Length and
Scott Bond Strength. Replacement of part of the fibres with polysilicate while
keeping the
amount of calcium carbonate constant leads to an increase in Breaking Length
(see results
for handsheets of comparative examples 4 and 9).
The correlation of breaking length and ash content for the handsheets of
example 6 and
comparative examples 2 to 4 and 9 is shown in Fig. 1.
It can be assumed that the correlation of breaking length and ash content of
the handsheets
of example 6 and comparative example 9 behaves in analogy to the correlation
of breaking
length and ash content as shown for the handsheets of comparative examples 2
to 4. This
means that one can draw a virtual line through the dot for example 6,
respectively,
comparative example 9, which is parallel to the line formed by the dots for
comparative
examples 2 to 4. By doing so, it becomes obvious that at a given ash content,
a higher
breaking length can be achieved for handsheets containing polysilicate coated
Calopak F
(example 6) compared to handsheets containing only Calopak F (comparative
examples 2
to 4) and also compared to handsheets containing a mixture of Calopak F and
polysilicate
(comparative example 9). Or in other words: the same breaking length can be
achieved at
lower ash content for handsheets containing polysilicate coated Calopak F
compared to
handsheets containing only Calopak F or a mixture of Calopak F and
polysilicate.
The correlation of breaking length and ash content for the handsheets of
example 7 to 10
and comparative examples 5 to 8 is shown in Fig. 2.
Fig. 2 shows that at a given ash content, a higher breaking length can be
achieved for
handsheets containing polysilicate coated AlbacaCHO (example 7), Albaca@)LO
(example 8),
Syncarb F0474 (example 9), respectively, Miconapaque HB (example 10) compared
to
handsheets containing the respective calcium carbonate without polysilicate
coating
(comparative examples 5 to 8). Or in other words: the same breaking length can
be achieved
at lower ash content for handsheets containing polysilicate coated Albaca@)HO,
Albaca@)LO,
Syncarb F0474, respectively, Miconapaque HB compared to handsheets containing
the
respective calcium carbonate without polysilicate coating.
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The correlation between Scott bond strength and ash content for the handsheets
of
example 6 and comparative examples 2 to 4 and 9 is shown in Fig. 3, and for
the handsheets
of example 7 to 10 and comparative examples 5 to 8 is shown in Fig. 4. Here it
can also be
assumed that the correlation of Scott bond strength and ash content of the
handsheets of
examples 6 to 10 and comparative examples 5 to 9 behaves in analogy to the
correlation of
Scott bond strength and ash content as shown for the handsheets of comparative
examples
2 to 4. This again means that one can draw a virtual line through the dot for
the respective
example, which is parallel to the line formed by the dots for comparative
examples 2 to 4.
When comparing the parallel lines, it can be seen that the correlation between
Scott bond
strength and ash content is almost identical for handsheets containing only
Calopak F
(comparative examples 2 to 4) or a mixture of Calopak F and polysilicate
(comparative
example 9), but that at a given ash content, a higher Scott bond strength can
be achieved for
handsheets containing polysilicate coated Calopak(OF (example 6), Albaca@)HO
(example 7),
Albaca@)LO (example 8), Syncarb F0474 (example 9), respectively, Miconapaque
HB
(example 10) compared to handsheets containing the respective calcium
carbonate without
polysilicate coating (comparative examples 5 to 8). Or in other words: the
same Scott bond
strength can be achieved at lower ash content for handsheets containing
polysilicate coated
calcium carbonate compared to handsheets containing only calcium carbonate or
a mixture
of calcium carbonate and polysilicate.
In summary: The use of polysilicate coated calcium carbonate leads either to
paper of
increased breaking length and Scott bond strength at the same ash content or
to paper of
lower ash content, i.e. containing a reduced amount of fibres and/or calcium
carbonate at the
same breaking length or Scott bond strength. Thus the ratio of breaking
length/ash content of
filled paper, and the ratio Scott bond strength/ash content of filled paper
are improved.
Example 11
Preparation of compositions comprising aqueous polysilicate coated calcium
carbonate using
sulfuric acid as acid
To 200 L water (standard town water) 21 L of sodium silicate (P sodium
silicate from PQ
corporation, 28.7% (w/v) Si02, 8.9% (w/v) Na20, density at 68 F: 1.38 g/cm3)
is added. After
stirring to achieve a homogenous mixture sufficient concentrated sulfuric acid
is added to
reduce the pH of the sodium silicate from 11.3 to 9.5 at room temperature. The
sodium
CA 02667420 2009-04-21
WO 2008/049750 PCT/EP2007/060949
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silicate is allowed to react for 3 minutes prior to the addition of 50 L of
dispersed ground
calcium carbonate (GCC) slurry (Hyrocarl090 sold by Omya, 60% by weight GCC).
The
calcium carbonate is mixed into the slurry well, as well as a further 29 L of
water before the
silicate starts to form a solid 3 dimensional matrix. The solid gel is broken
down by the action
of the mixer and the mixture is stirred for further 2 hours. After this time a
grainy slurry is
produced with visible particle in the range of 0.5 to 2 mm. 10 L samples of
this material is
taken and sheared for 10 minutes at 14,000 rpm using a Polytron Emulsifier to
give a uniform
viscous aqueous based polysilicate coated calcium carbonate. The polysilicate
coated
calcium carbonate is then ready to use.
