Note: Descriptions are shown in the official language in which they were submitted.
CA 02611272 2012-12-14
Method of producing a fibrous web comprising a filler
The present invention relates to a method of manufacturing a fibrous web
including a filler,
comprising the steps of bringing the filler into contact with a dispersant to
improve the
light-scattering properties of the fibrous web, mixing the filler into a fibre
slush, forming
the fibrous web from the filler-bearing fibrous slush and drying the fibrous
web at a paper
or cardboard machine.
In a method such as this, a filler is added to a slush comprising vegetable
fibres, after
which the fibre slush forms a fibre web, which is dried at the paper or
cardboard machine.
The present invention also relates to use of a dispersant to improve the light-
scattering
property of a fibrous web, which comprises a filler, which in turn comprises
cellulose or
lignocellulose fibrils and light-scattering mineral particles which have been
precipitated
onto the fibrils, wherein the majority of the dispersant is attached to the
filler.
Fl Patent Specification 100729 describes a filler which is used in paper-
making and which
comprises porous aggregates formed of calcium carbonate particles which have
been
precipitated onto the surface of the fines. According to the patent
specification,
characteristic of this new filler is that the calcium carbonate has been
precipitated onto
fines fibrils which are prepared by a process of refining from cellulose
fibres and/or
mechanical pulp fibre. The size distribution of the fines fraction is in most
cases equivalent
to the fraction P100 of the wire screen. In the following, this filler is also
called by its
product name "SuperFill".
On the basis of the said patent specification, the calcium carbonate
percentage of the paper
can be increased by using the filler, in which case the grammage of the paper
can be
decreased. In product groups which have a relatively high fresh filler
percentage, the
presented product gives a considerable advantage.
In the context of this invention "Graphic papers" mean types of paper that are
used for
instance for posters, book-covers, advertisements, maps, illustrated books,
and as archival
papers. In other words, they are used for purposes where the smoothness, the
gloss and the
quality of the colour are important properties.
CA 02611272 2012-12-14
= 2
In coated graphic papers, the amount of the pigments in the broke is large,
which limits the
amount of the fresh filler, among others composite fillers such as SuperFill,
too.
Consequently, in these cases it is not possible to fully utilise the ability
of, for instance,
SuperFill to improve the light-scattering properties of the paper.
The purpose of the present invention is to eliminate the disadvantages
associated with the
known art and to generate a completely new way of producing paper and similar
fibre
products. In particular, the purpose of the present invention is to generate a
method which
can be used to produce fibre products possessing good mechanical and graphical
properties. Another purpose of the present invention is to generate a solution
for increasing
the light-scattering coefficient of paper at low fresh filler percentages.
The present invention is based on the idea that the uniformity of the filler
distribution is
increased by dosing dispersant into the composite filler, in which case the
agglomeration of
the calcium carbonate crystals is reduced and thus the light-scattering is
increased.
In filler applications, dispersants have earlier been used mainly to improve
the pumpability
of the filler slurries. The literature mentions that dispersant can be used to
increase the
uniformity of the filler distribution (Weigl, J. and Ritter, E., "Die
Bedeutung der
Teilchengrosse und ¨form von Fiillerstoffen sowie deren Verteilungen bei der
SC-
Papierherstellung, Wochenblatt für Papierfabrikation, 1995, no 17, pages 739-
747.)
The properties of the composite fillers, in turn, have been modified by using
several
pigments. Accordingly, EP Published Patent Application 0 892 019 describes how
CaCO3,
or another white pigment which turns insoluble, is precipitated onto the fine
fibres (fibrils).
After that, a further pigment is added in order that the precipitated pigment
will bind the
second pigment and the fine fibre together, with the result that a composite
pigment is
generated, one which binds better to paper than does the expensive pigment
(Ti02) by
itself.
When dispersants are used together with conventional fillers, much of the
retentive
property of the filler is lost. Unexpectedly, it has now been found that this
is not the case
when acting according to the present invention. The reason is that a composite
filler, such
--s CA 02611272 2013-10-09
3
as SuperFill, can be retained on the paper (or corresponding fibrous web) also
when it has
been dispersed. In this respect, too, it seems to clearly differ from
conventional fillers.
Consequently, according to the present invention, a composite filler is used
as a filler in
papers and cardboards in order to improve the light-scattering properties of
the fibrous
web. This filler is brought into contact with the dispersant before it is
mixed with the fibre
slush. More specifically, wherein the filler is a composite material,
comprising cellulose or
lignocellulose fibrils, onto which light-scattering material particles have
been precipitated,
wherein the dispersant used is anionic polyacrylate or polyphosphate and
wherein the
amount of dispersant is 0.05-1.0 weight % of the dry weight of the fibrous
web.
The use according to the present invention is, in turn, characterized in that
anionic
polyacrylate or polyphosphate is used as the dispersant and the amount of the
dispersant is
0.05-1.0 weight % of the dry weight of the fibrous web.
Considerable advantages are achieved with the invention. Thus, the present
invention can
considerably improve the light-scattering properties of paper and cardboard,
without
decreasing the retentive properties of the filler. According to the present
invention, it is
possible to have the filler uniformly distributed in all three directions,
whereas using the
dispersant in the solution described above (where use of filler in particulate
form was
discussed) has mainly led to improvement of the distribution in the z-
direction.
The solution presented here gives improved light-scattering properties
particularly in those
qualities of papers in which the percentage of the filler, particularly the
fresh filler, is
relatively small. Products of this kind are, among others, the above-mentioned
graphic
papers: poster papers, book covers and paper and cardboard used in
advertisements, map
paper, paper for illustrated books and archival paper.
In the following, the invention will be examined in more detail with the help
of the
accompanying drawing and a detailed explanation.
Figure 1 shows a bar chart of the light-scattering coefficient of papers which
comprise
three different fillers.
CA 02611272 2012-12-14
4
In the method according to the present invention, a fibrous web comprising
filler is
prepared at for instance a paper or cardboard machine using a method which is
known per
se. The production is described in more detail below.
The filler in the fibrous web is a composite material, one which comprises
cellulose or
lignocellulose fibrils, onto which light-scattering material particles have
been precipitated.
In order to improve the light-scattering properties of the fibrous web, the
filler is brought
into contact with the dispersant before it is mixed into the fibre slush. At
least most of the
dispersant is attached to the material particles of the filler.
The amount of the dispersant varies depending on the dispersant and the
material particles.
In order to determine the upper limit of the addition of the dispersant, in
practice it is
possible to proceed as follows: a water dispersion of mineral particles
(without fibril
matrices) is formed, after which dispersant is added to the dispersion. In
this way, the
upper limit of the addition of the dispersant is the same amount as that which
produces the
minimum viscosity of the slurry. Generally, the graph of the viscosity as a
function of the
dispersant amount is U-shaped, in which case, in the present invention, the
amount of
dispersant to be added is chosen as an amount which corresponds to the bottom
of the U.
The dispersant used can be conventional anionic polymers, such as anionic
polyacrylate ¨
for example anionic sodium polyacrylate, or polyphosphate. Sodium
polyacrylates are
marketed under the product names FennodispoTM A 40 and A 41 (supplier Kemira
Oyj), in
which A stands for acrylate and 40 and 41, respectively, indicate the
percentage of its
aqueous solution. Another corresponding product is DispexTM N40, which is a
sodium
polyacrylate, too, and the supplier of which is Ciba Specialty Chemicals.
Aminoalcohols
can be used as well.
For the above materials, the amounts to be added are approximately 0.05-1
weight % of the
dry weight of the fibrous web. Generally, the suitable amount to be added is
approximately
0.08-0.5 weight %, especially approximately 0.1-0.35 weight %, most suitably
approximately 0.1-0.3 weight %. These limits relate especially to anionic
polyacrylate,
especially anionic sodium polyacrylate.
CA 02611272 2012-12-14
4a
The present invention is particularly suitable for fibrous webs which comprise
small or
average amounts of filler. Typically, the total percentage of the filler is at
maximum
approximately 25 weight %, especially at maximum approximately 20 weight %,
preferably at maximum approximately 15 weight %, of the dry matter of the web.
The
minimum percentage is approximately 0.1 weight %. The amount of fresh feed of
the filler
comprising composite filler is most suitably approximately 0.1-15, preferably
approximately 0.5-12 weight %, of the dry matter of the web to be prepared.
CA 02611272 2007-12-06
WO 2006/136651 PCT/F12006/000222
The filler used in the present invention is derived especially from the
fibrils obtained from
the chemical pulp. Here, chemical pulp means a pulp which has been treated
with cooking
chemicals in order to delignify the cellulose fibres. According to one
preferable
embodiment, the fibrils have been obtained by refining a pulp which has been
prepared
5 using the sulphate process or another alkaline method. The present
invention is suitable not
only for modifying chemical pulps but also for modifying fibrils obtained from
chemi-
mechanical and mechanical pulps.
Typically, the average thickness of the cellulose or lignocellulose fibrils is
less than 5 gm,
generally less than 1 gm. One or both of the following are characteristic of
the fibrils:
a. they correspond to a fraction that penetrates a 50 (or preferably 100)
mesh screen; and
b. their average thickness is 0.01-10 gm (preferably at maximum 5 p,m,
more preferably at maximum 1 grn) and their average length is 10-1500
gm.
The initial material of the fibrils, i.e. cellulose or other fibre-based
fines, is fibrillated by
refining it using a pulp refiner. When needed, the desired fraction can be
separated by
using a screen, but it is not always necessary to grade the fines. The
fractions P50-P400 are
suitable fibril fractions. Preferably, refmers having grooved blades are used.
The light-scattering particles of the filler are inorganic or organic salts,
which can be
formed from their initial materials by precipitation in an aqueous
intermediate agent. Such
compounds are calcium carbonate, calcium oxalate, calcium sulphate, barium
sulphate and
mixtures of them. The material particles have been precipitated onto the
fibres. The
amount of the inorganic salt compound relative to the amount of the fibrils is
approximately 0.0001-95 weight %, preferably approximately 0.1-90 weight %,
most
suitably approximately 60-80 weight %, calculated from the amount of filler
and
approximately 0.1-80 weight %, preferably approximately 0.5-50 weight %
calculated
from the paper.
In the following, the present invention is described especially on the basis
of the product
according to Fl Patent Specification 100729. However, it is obvious that the
present
CA 02611272 2007-12-06
WO 2006/136651 PCT/F12006/000222
6
invention can be applied to other above-mentioned products by making the
necessary
changes to the initial materials of the light-scattering pigment.
The filler is produced by precipitating the mineral pigment onto the surface
of the fines
fibrils which are prepared from cellulose fibre and/or mechanical pulp fibre.
Precipitation
of for instance calcium carbonate can be carried out in such a way that an
aqueous solution
of calcium hydroxide, which may comprise solid calcium hydroxide, and a
compound
which comprises carbonate ions and is at least partially dissolved in water,
are fed into the
water slush of fibrils. It is also possible to introduce carbon dioxide gas
into the aqueous
phase. In the presence of calcium hydroxide this carbon dioxide generates
calcium
carbonate. Beaded calcium carbonate crystal aggregates are formed, which are
kept
together by the fibrils, i.e. fine filaments, and in which the calcium
carbonate particles are
precipitated and are attached to the filaments of the fines. Together with the
calcium
carbonate the fines filaments form beaded filaments, which mainly resemble
pearl
necklaces in a pile. In the water (the slush), the ratio between the effective
volume and the
mass of the aggregates is very large compared with the corresponding ratio of
calcium
carbonate that is used as a conventional filler. The term effective volume
here means the
volume which the pigment requires.
The diameter of the calcium carbonate particles in the aggregates is
approximately 0.1-5
gm, typically approximately 0.2-3 pim. The sizes of fibrils correspond
especially to the
fractions P50 (or P100) ¨ P400 of the wire screen. In the filler, at least 80
%, preferably
even 90 %, of the precipitated light-scattering pigment particles are attached
to the fibrils.
According to the present invention, the volumetric efficiency of the pigment
particles is at
least 60 weight % (of the filler weight), preferably 70 weight % or more, but
less than 85
weight %. Within this range, a good dewatering at the paper or cardboard
machine is
achieved and a constant air permeance in the fibrous web, too.
The composite filler is modified by means of a dispersant before it is fed to
the paper
machine or cardboard machine. The modification can be done simply by mixing a
predetermined amount of dispersant with the filler in a mixing tank, wherein
the dispersant
is allowed to be adsorbed or otherwise be attached during mixing to at least
the mineral
particles of the filler. The mixing time is especially approximately 1 minute
¨24 hours,
CA 02611272 2007-12-06
WO 2006/136651 PCT/F12006/000222
7
most suitably approximately 5 minutes ¨ 10 hours. The mixing is carried out
especially at a
relatively high dry matter percentage, which can even be increased due to
addition of the
dispersant. Typically, the dry matter percentage is approximately 40-80 weight
%.
Besides the mixing tank, continuous mixers can be used, too, in which the
mixing of the
filler slurry and the dispersant is carried out in pipelines, possibly aided
by static mixers
inside them.
The bulk of the dispersant that is fed into the filler dispersion, typically
at least 80 weight
%, preferably at least 90 weight %, is attached to the filler sufficiently
well that it will not
be essentially detached from it in the conditions of the headbox. In the
headbox, any free
dispersant remaining would easily become attached to, for instance, particle-
like fillers
which might be circulating in the short or the long circulation water coming
from the paper
or the cardboard machine to the headbox. If a conventional filler is
dispersed, the retention
of such a filler decreases significantly.
Consequently, any (part of the) dispersant that remains unattached is
preferably circulated
back to the elutriation phase of the composite filler in order to attach it as
efficiently as
possible to the very same filler.
The dispersed composite filler is fed and mixed into the fibre slush. In this
case, the fibre
raw material of the paper or cardboard pulp is first slushed to a suitable
consistency in a
way which is known per se (typically to a solids percentage of approximately
0.1-1 %).
The above-mentioned filler, generally approximately 1-25 weight % of the
weight of the
fibres of the fibre pulp, is added into the fibre slush most suitably in the
headbox of the
paper or cardboard machine. In practice, at least when coated paper and
cardboard qualities
are being produced, the amount of pigments in the broke is so large that the
amount of the
fresh feed must be limited to less than 15 weight % of the dry weight of the
fibres fed into
the headbox.
The percentage of the composite filler in the total amount of filler of the
web is at least 20
weight %, preferably at least approximately 30 weight %, most suitably at
least 40 weight
% and especially 50 weight %. More preferably, 75-100 weight % of the filler
is composite
filler.
CA 02611272 2007-12-06
WO 2006/136651 PCT/F12006/000222
8
At the paper or cardboard machine, the fibre pulp is webbed to form the paper
or cardboard
web. After that, the web is led from the press section to the drying section,
where it is dried
in a way which is known per se. The fibrous web is dried and may be coated and
optionally
after-treated, for instance by calendering.
It is also possible to produce a multi-layer product which comprises the
present filler
especially in the surface layers of the product. A multi-layer webbing
technique can be
used to produce such products. Suitable pulp feeding architectures have been
described for
instance in FT Patent Specification 105 118 and EP Published Patent
Application 824 157.
The multi-layer headbox is used most suitably together with a gap former. In a
device such
as this, a slice jet formed by the headbox is fed in between two wires and the
water is
removed from the pulp through the wires in two different directions. Using a
gap former, it
is possible to make the fines gather on the surfaces of the layer thereby
producing an arc-
shaped filler distribution ¨ like the shape of a smile. When the multilayer
headbox is used
together with the gap former, a desired multilayer structure is generated
simply by feeding
the paper or cardboard pulp in layers in between the wires by the means
described above.
This technique can also be used to manufacture products in which the
thicknesses of the
layers are smaller than when a regular multilayer technique is used.
The paper or cardboard product generated according to the present invention
can be coated
or it can be delivered uncoated.
The coating can be carried out as either a single coating or as a double
coating, thus the
coating pastes can be used as single coating pastes and as pre-coating and
surface coating
pastes. Triple coatings are possible, too. Generally, the coating colour
according to the
present invention comprises 10-100 parts by weight of at least one pigment or
a mixture of
pigments, 0.1-30 parts by weight of at least one binder and 1-10 parts by
weight of other
additives known per se. Examples of such pigments are precipitated calcium
carbonate,
ground calcium carbonate, calcium sulphate, calcium oxalate, aluminium
silicate, kaolin
(hydrous aluminium silicate), aluminium hydroxide, magnesium silicate, talc
(hydrous
magnesium silicate), titanium dioxide and barium sulphate, and mixtures of
them. It is
possible to use synthetic pigments, too. Of the pigments mentioned above, the
main
pigments are kaolin, calcium carbonate, precipitated calcium carbonate and
gypsum, which
CA 02611272 2007-12-06
WO 2006/136651 PCT/F12006/000222
9
in general constitute over 50 % of the dry solids in the coating mix. Calcined
kaolin,
titanium dioxide, satin white, aluminium hydroxide, sodium silicoaluminate and
plastic
pigments are additional pigments, and their amounts are in general less than
25 % of the
dry solids in the mix. Of the special pigments, special-quality kaolins and
calcium
carbonates, together with barium sulphate and zinc oxide, should be mentioned.
The coating mix can be applied to the material web in a manner known per se.
The method
according to the present invention for coating paper and/or cardboard can be
carried out
with a conventional coating apparatus, i.e. by blade coating, or film coating
or surface
spraying (JET application).
When at least one surface of the paper web is coated, preferably both
surfaces, a coating
layer is formed, the grammage of which is 5-30 g/m2. The uncoated side can be
treated for
instance with surface sizing.
Furthermore, after that the products can be calendered off-line. Naturally,
the calendering
can be carried out already at the paper machine (on-line).
By means of the present invention it is possible to generate cellulose and
lignocellulose-
bearing material webs which have excellent printability properties, good
smoothness and
high opacity and brightness. Here, "cellulose-bearing" material means, in
general, paper or
cardboard or a corresponding cellulose-bearing material, which is sourced from
lignocellulose-bearing raw-material, especially wood or annual or perennial
plants. The
material in question can be wood-containing or wood-free and it can be
prepared from
mechanical, cherni-mechanical or chemical pulp. The pulp and the mechanical
pulp can be
bleached or unbleached. The material can also comprise recycled fibres,
especially
recycled paper or recycled cardboard. The grammage of the web formed of the
material
may typically vary between 35 and 500 g/m2, especially between approximately
50 and
450 g/m2.
Generally, the grammage of the base paper is 20-250 g/m2, preferably 30-80
g/m2. By
coating a base paper of this type, which has a grammage of approximately 50-70
g/m2,
with a 2-20 g coating/m2/side and by calendering the paper, a product is
generated having a
grammage of 50-110 g/m2, a brightness of at least 90 % and an opacity of at
least 90 %.
CA 02611272 2007-12-06
WO 2006/136651 PCT/F12006/000222
The following non-restrictive examples illustrate the present invention. The
results of
measurements of the paper properties in the examples are determined using the
following
standard methods:
5
Surface texture: SCAN-P76:95
Air resistance: SCAN-M8, P19
Example 1
The way in which dispersed fillers work to improve the light-scattering
coefficients is
measured by using a raw-material composition in which the ratio between the
hardwood
pulp and the softwood pulp is 70/30 (Table 1) and the beating degrees are 22.2
SR
(hardwood) and 25.3 SR (softwood).
Table 1
Main raw material Pulp Percentage
Short stock Mixed hardwood pulp 70 %
Long stock Softwood pulp 30 %
The dispersant dose (as a percentage of the pigments) is determined by
measuring the
viscosity of the PCC filler slurry as a function of the dose (Table 2). A
suitable dose range
was obtained from the minimum point of the viscosity. The levels used were
0.10, 0.15 and
0.20 parts of the calcium carbonate.
Because the slurry contains very little solid material, the viscosity response
does not
necessarily show up in this kind of measurements. Therefore, measurements of
the
concentrated commercial PCC, which has the same crystal structure as
SuperFill, were also
carried out. It is probable that, using an appropriate accuracy, the best
working dose is the
same regardless of the composition of the slurry. The materials are mainly
adsorbed onto
the calcium carbonate, not onto the fines of the paper pulp.
The viscosity measurements were carried out using spindle no. 2, by spinning
at 100 rpm
for 15 seconds, in order to prevent the SuperFill from sedimenting.
CA 02611272 2007-12-06
WO 2006/136651 PCT/F12006/000222
11
Table 2
Temperature
Levels C BF (m Pas)
Superfill without dispersant 20 - 130
SuperFill with A 40 0.10 20 -124
0.15 20 --117
0.20 20 --110
Albacar LO without dispersant 20 - 131
Albacar with A 40 0.10 20 - 39
0.15 20 -26
0.20 20 - 27
The dose selected was 0.2 parts (of calcium carbonate) of dispersant (Kemira's
Fennodispo
A40), which is added to the SuperFill filler.
Example 2
When measuring the light-scattering coefficient (Table 3), an adequate
dispersant dose was
used. PAC and an anionic polyacrylamide for PCC were used as retention agents.
The
amounts of retention agents used were always kept to the minimum.
Table 3
Test no. 44 Sheet1 2 3 4 5 6 7
8
no.
DOSE chemical/sheet
Starch of the
C*B0ND 35845 % 0.50 0.50 0.50 0.50 0.50 0.50 0.50
0.50
raw-material
Filler 1 ALBACAR LO " 10.0 20.0
Filler 2 SUPERFILL Ref. /I 10.0 20.0
Filler 3 SUPERFILL with A40 " 10.0 20.0
PAC 18 "
0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25
Retention agent PERCOL HC 4000 " 0.04 0.04 0.02 0.02 0.02 0.02 0.02
0.02
(anionic)
The following adjustments were used during the measuring (Table 4).
CA 02611272 2007-12-06
WO 2006/136651 PCT/F12006/000222
12
Table 4
ADJUST- results Sheet
no. 1 2 3 4 5 6 7 8
MENT
Formette drum speed r/min
1250 I
Raw-material
bar 3.20
pump
Cylinder
speed m/min 1.50
drier 3
pressure bar 1.50 -
temperature C 100
drying time min 5.0
Grammage
pc or ondi ruoclt
g/m2 79.8 81.5 79.8 79.4 79.5 81.1 78.9 77.9
Filler contents % 10.7 20.1 10.1 20.2 10.5 20.7 10.4
19.7
The advantages achieved with the present invention are shown in the
accompanying
drawing. In Figure 1, the light-scattering coefficients of paper are
interpolated to the same
tensile index level (39 Nm/g) for dispersed SuperFill, for the same SuperFill
without
dispersion, and for the commercial PCC (Albacar LO), which has the
corresponding
particle size distribution.
The filler level was approximately 11 %, and the ratio between the hardwood
pulp and the
softwood pulp is, as mentioned above, 70/30, and the beating degrees SR 22.2
(hardwood)
and 25.3 (softwood).
As the figure shows, the dispersion of SuperFill substantially improves the
light-scattering
coefficient compared with the same SuperFill filler which is not dispersed.
The effect of
the dispersion is significant, especially at low or middle range percentages
of filler.
Further analyses were carried out on the products and it was found that the
other properties
of the papers (density, strength, opacity and brightness) had remained good,
regardless of
the changed conditions.
