Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR MANUFACTURING PAPER AND BOARD
Field of the invention
The present invention relates to a fibre product that contains pulp or wood
fibre, and to the
manufacturing method of this product, wherein filler particles are attached
between the
pulp fibres or wood fibres as well as to fibrils, after which paper or board
is produced from
the fibre pulp.
Description of prior art
Typically, the fillers or pigments that are used in the manufacture of paper
and board have
an average particle size of less than 5 p.m and a light colour. The most
typical fillers
include kaolins, talcs, ground calcium carbonate, and precipitated calcium
carbonate. In
addition, there are more expensive, so-called special pigments, such as
precipitated
aluminium silicates, satin white, and titanium dioxide. Drawing an exact line
between
fillers and coating pigments is difficult; however, roughly speaking, fillers
have a larger
size than the pigments that are used in coating. From the point of view of a
maximum light
scattering, an optimal particle size for the most common fillers and coating
pigments
would be 0.4-0.5 pm. Typically, the average particle size of the coating
pigments is 0.5-1
p.m and that of the fillers 1.5-4 gm.
One problem with the fillers commonly used in the manufacture of paper and
board
products is that they weaken the final product. In particular, paper grades
that have large
filler contents, such as copying papers and specific magazine papers, would
generally need
improved stiffness compared to the present situation. The efforts to achieve
lighter basis
weights in the manufacturing of paper and cardboard also emphasize the need
for stiffness.
Generally, the stiffness of paper-weakens as the amount of filler in the paper
increases or if
the basis weight is reduced. This decrease in stiffness together with the
decrease in strength
constitutes the major problems with quality in the use of fillers in printing
papers.
Conventionally, this problem has either been solved by adding a separate
strengthening
additive or decreasing the amount of filler. On account of the price and
properties of pulp,
this is not profitable, however.
SUBSTITUTE SHEET (Rule 26)
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The most common clay mineral that is used as filler in the manufacture of
paper and board
is kaolin that mainly consists of kaolinite. The kaolin mineral has a two-
layer, laminar
structure that comprises a tetrahedral layer of silicon dioxide and an
octahedral layer of
aluminium oxide. The layers are interconnected by oxygen atoms. Because of the
silanol
groups of the surface of kaolin, the surfaces of the kaolin mineral are
negatively charged
and its edges are positively charged, when suspended in water. Kaolin can be
manufactured by dry or wet processes; however, in the wet process, it is
possible to have a
greater chemical effect on the final brightness of kaolin at the various
process stages. The
most common commercial kaolin grades include water-washed, delaminated,
calcined, and
chemically structured kaolins. The water-washed grade often contains several
kaolin sheets
that are attached to each other at their surfaces. In the delaminated grades,
kaolin surfaces
are separated by grinding them into smaller groups and single kaolin sheets.
The calcined
kaolins are made by allowing kaolin sheets, which have a conveniently small
particle size
distribution, to partly melt to each other at a temperature of about 1000 C.
At a
temperature of over 450 C, the structure of kaolin crystals begins to change.
Kaolin
crystals that are heated at a temperature of about 500-800 C are called
metakaolin. The
advantages of metakaolin and calcined kaolin are particularly visible as an
improvement in
brightness and opacity compared with water-washed and delaminated kaolins.
This is
based on the fact that, between the kaolin sheets in the calcined kaolin,
there are interfaces
of air and kaolin which are effective in scattering light. The pores formed in
the structure
of the structured kaolins further help in the setting of ink. The manufacture
of chemically
structured kaolins is based on the formation of the same interfaces, but in
this case, the
interfaces can be chemically bound to each other ¨ not by means of high
temperatures. The
light scattering efficiency of the chemically structured kaolins is normally
between that of
the delaminated and calcined kaolins.
The greatest disadvantage of using kaolin, as well as other fillers, is the
weakening of the
strength of the paper or board structure when replacing the mass with filler,
in particular.
This is because fillers prevent the formation of hydrogen bonds between the
fibres by
attaching themselves to the surfaces of the fibres.
In the specification Fl 20020566, it has been proven that by using structured
filler
agglomerates of an average particle size of over 5 micrometres, improved
strength of the
fibre network is achieved with the same filler content than with normal
fillers of less than 5
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micrometres. The specification Fl 20085227 further shows that, by improving
the ability of
the surfaces of the filler agglomerates of over 5 micrometres to form hydrogen
bonds,
further improvement in the strength and stiffness of the fibre network is
achieved.
In this way, filler particles are obtained, which have a suitable size
category to fill the
openings between the pulp fibres. The required strength is thus achieved for
the fibre
product, and at the same time, part of the pulp can be replaced. Generally, in
prior art,
achieving the strength has been sufficient. Another purpose of fillers and
additives,
however, is the retention which is very difficult to improve in connection
with improving
the strength of the product.
In the manufacture of paper or board, it is known to form the paper or board
product by
dewatering solid matter pulp. Of all the raw materials, the amount of water is
clearly the
greatest and the idea is to remove it as quickly as possible from the end
product by means
of the wire, press and dryer sections. Dewatering is one of the most important
factors that
influence the economy of papermaking, and the idea is to influence it in a
chemical manner
through various flocculants and coagulants, among others. Concerning
mechanical
dewatering, the purpose is to influence it by means of the wire, press, and
dryer sections.
The more effective dewatering also leads to a reduction in the need for drying
energy in the
dryer section.
The various fillers also bind less water than fibres do, contributing to the
financial
advantage of their use due to the accelerated dewatering. In the wire, press,
and dryer
sections, the lower water retention capacity is visible as a quicker
dewatering and, thus,
lower energy costs in drying, when the filler is used.
Generally, the fillers and fibres have an anionic charge. Therefore, to
improve the filler
retention, a cationic charge in the form of ions or polymers should generally
be supplied to
the pulp to bind the filler to the fibre network. In printing papers, the
filler contents are
about 30%, calculated from dry fibre. The retention can be divided into a
mechanical and
chemical one, of which the chemical retention definitely has more
significance. The
mechanical retention has more meaning for boards of a high basis weight.
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The decrease in strength and stiffness of the paper or board product when
replacing the
fibre with filler is mainly caused by the fact that the fillers weaken the
formation of
hydrogen bonds between the fibres, since the surfaces of the fillers do not
form hydrogen
bonds. At present, the filler is added directly to the pulp. In the wire
section, only part of
the added filler is attached to the finished paper or board web. The rest of
the filler travels
through the white water system to finally constitute part of the finished
paper or board
structure, but then the risks of various runnability problems have increased
mainly due to
the attachment of various hydrophobic substances to the fillers of the white
water system.
In the paper or board machine, the runnability problems caused by this usually
appear as
contamination, breaks, of the wires and felts, for example. Part of the filler
of the white
water system also finally burdens the sewage treatment plant, because it is
never entirely
carried out of the process along with the finished paper or board.
Consequently, there is a need for a fibre product, wherein the filler would
attach itself to
the fibre more effectively and, at the same time, would give the product
advantageous
strength properties that would preferably be further improved compared to the
known
solutions.
Brief description of the invention
An object of the present invention is to provide a new paper or board product
that has good
strength and high opacity.
An object of the present invention, in particular, is to provide a new paper
or board product
that contains, as filler, granules that attach between the fibres and
carbonates that attach to
the fibrils.
The object of the present invention is to utilize an aqueous, carbonate-
bearing salt
composition to suspend structured kaolin in the manufacture of the paper and
board
products. In this way, as quick as possible dewatering and high wire retention
in the wire
section and the improvement of specific quality properties of paper and board
are ensured.
To incorporate the kaolin sheets, among others, latexes can be used as
binders, by means of
which the kaolin sheets and stacks can be attached to each other to form
granules or
agglomerates from the kaolin slurry by means of a mixing technique or spray
drying. The
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purpose is to provide structured kaolin with an average particle size of over
5 micrometres.
The structured kaolin thus dried can then be converted into calcined kaolin or
metakaolin
by heating it in a furnace. In the manufacture of paper or board, an
improvement is
achieved by using the structured kaolin that is suspended in the aqueous
solution,
5 according to the invention, whereby quicker dewatering and higher
retention are utilized in
the wire section. From the kaolin slurry that is prepared in the aqueous salt
composition,
carbonate filler is precipitated to the fibre structure by means of an
increase in the pH,
pressure or temperature, preferably at the tail of the wire section and/or
after the press
section. The goal is to introduce a desired amount and distribution of
precipitated
carbonate filler into the lumen and fibrils of the fibre structure, whereby
the effect of
structured kaolin that weakens the stiffness can be prevented. At the same
time, the
carbonate filler thus created increases the brightness, opacity, and
printability that are
achieved by using structured kaolin alone. In an ideal paper or board
structure, the
carbonate filler thus created would be contained in the fibrils of the fibres,
increasing the
stiffness, among others, and the structured kaolin would be contained in the
holes of the
fibre network, whereby the weakening of the strength and stiffness of the
fibre network
caused by the prevention of the formation of hydrogen bonds between the fibres
of the
filler would be lesser than when using fillers or pigments with an average
particle size of
less than 5 micrometres.
The present invention thus relates to a fibre product that contains pulp fibre
or wood fibre,
the product preferably being paper or board, and to the manufacturing method
of this
product, wherein filler particles are attached between the pulp or wood fibres
and to the
fibrils, after which the said paper or board is produced from the pulp.
To be more precise, the fibre product according to the present invention is
characterized by
what is stated in the characterizing part of claim 1.
The manufacturing method of the fibre product according to the invention is,
in turn,
characterized by what is stated in the characterizing part of claim 10.
The present invention is multifunctional and it improves various properties:
the quality
properties of paper and board and the economic activity of the manufacturing
process. By
combining carbonates of a suitable size with particles of structured kaolin of
a suitable
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size, the invention makes it possible, among others, to improve the
brightness, opacity, and
printability; at the same time, the dewatering can be accelerated and the
retention improved
in the wire section. An additional advantage of the use of kaolin also
comprises the effect
of further improving the stiffness, opacity, brightness, and printability that
are given to the
fibre structure by the precipitated carbonate filler. At the same time, part
of the fibre of the
end product is now replaced with the filler without weakening the strength
properties.
Detailed description of the preferred embodiments of the invention
The present invention relates to a fibre product that contains pulp fibre or
wood fibre,
wherein filler particles are attached between the fibres and to the fibrils,
part of the
particles consisting of structured agglomerates or granules of kaolin,
metakaolin or
calcined kaolin, and part consisting of the salts or esters of carbonic acid
or a combination
thereof, preferably the various states of carbonate.
This invention proves that when the aqueous salt composition is used in the
manufacture of
structured kaolin slurry and when the pH of the paper or board pulp that is
diluted with this
composition is increased with an alkali and/or the temperature is increased,
possibly
simultaneously with increasing the solid matter content of the pulp, a
carbonate filler can
be precipitated to the paper or board structure. This precipitated carbonate
filler has a
positive effect on the brightness, opacity, printability (the absorption
properties of the ink),
thickness, and stiffness of the paper or board product.
The product can contain as much as 25% by weight of structured kaolin
particles from the
dry matter, at least 5% by weight from the fibre. They are essentially
spherical in shape
and have a size of > 5 pm, preferably 10-40 m, more preferably 20-40 pm. They
consist
of chemically structured kaolin agglomerates or granules, which are optionally
processed
so that a part thereof, preferably the surface, is calcined or changed into
metakaolin. The
kaolin of the filler is preferably delaminated, water-washed, dry-classified,
or treated by
means of two or more of the said treating methods.
The amount of carbonate in the product is at least 0.01% by weight from the
dry matter, for
example 0.01-5% by weight, particularly 0.01-3% by weight. At normal pressure,
the salts
of carbonic acid comprise carbonate or bicarbonate salts, preferably
bicarbonate and
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colloidal carbonate. They may have an average particle size of < 0.3 nm, most
suitably <
0.1 nm.
"Colloidal carbonate particles" in the present application refer to carbonates
that have a
small average particle size of less than 300 nm, preferably less than 100 nm.
The salts or esters of carbonic acid are preferably made of a corresponding
oxide or
hydroxide and they consist of an inorganic or organic salt or a composite or
mixture of
several salts, most preferably calcium salt or magnesium salt or a mixture
thereof.
According to a preferred embodiment of the invention, the filler particles are
attached to
each other and to the fibres by a binder, which is preferably latex, silicon
dioxide, alum or
aldehyde or a mixture thereof, most preferably in an amount of 0.5-50% by
weight.
The product can further contain other retention agents or flocculating or
coagulating
microparticles or a mixture thereof, preferably at least microparticles, most
preferably
together with conventional retention agents.
Various synthetic and natural polymers function as retention agents in the
invention.
Natural polymers are generally called polysaccharides. Examples of these
include starch,
which is the most commonly used natural polymer in the manufacture of paper
and board,
if fibres are not taken into account. Regarding synthetic polymers,
polyacrylamides should
be mentioned. Polymer, in particular, is selected from a group of
polyacrylamide,
polyethyleneimine, starch, polydadmac, polyacrylamide, polyamine, starch-based
coagulant, any copolymer of the above or a mixture of two or more such
polymers or
copolymers. The polymer is most preferably polydadmac, polyamine,
polyacrylamide or
the copolymer of two or more of these.
Inorganic, so-called microparticles are preferably used together with these
polymeric
retention agents to improve the dewatering, retention, and formation. Of these
inorganic
microparticles, colloidal silicon dioxide (polysilicic acid, silicon dioxide
sol, microgel,
etc.) and bentonite are especially well-suited to this purpose. Other
alternatives include
other sols, gels, microgels, silicic acids, and polysilicic acids or mixtures
thereof that
contain bentonites or silicon dioxides.
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The product can also contain one or more chemicals known as such, which are
selected
from a group of aluminium compounds, stock sizes, surface sizes, antislime
agents,
colouring agents, starches, optical brighteners, dispersing agents, anti-foam
agents, plastic
pigments, and conventional fillers and coating materials.
The present invention also relates to the manufacturing method of such a fibre
product,
wherein
- oxide or hydroxide is added to the aqueous solution to form
hydroxide slurry, and
the pH of the solution is decreased to a range of 6.0-8.3 by conveying carbon
dioxide to the solution, so that the content of the salts of carbonic acid
formed from
the carbon dioxide and hydroxide slurry is at least 0.01%, calculated from the
entire
weight of the solid matter of the solution, whereby a salt composition is
formed;
- kaolin sheets or stacks or both are suspended in water together with
a binder,
whereby a kaolin composition is formed;
- kaolin drops are produced in a spray drier from the kaolin composition,
which is
formed from the kaolin sheets or stacks or both and which contains the binder,
whereby excess water is also evaporated, whereby structured kaolin particles
are
formed; or these structured kaolin particles are formed, by means of a mixing
technique, from the kaolin sheets or stacks or both that are in the form of
slurry
with the binder in the above-described salt composition or its dissolved
portion;
- optionally, the spray-dried structured kaolin particles are mixed with
the salt
composition described above or its dissolved portion;
- the structured kaolin particles thus formed are admixed, together with
the salt
composition or its dissolved portion, to the paper or board pulp, whereby a
fibre
dispersion is formed; and
- the carbonate from the salt composition is precipitated into particles in
the
dispersion, while the dispersion is filtered, pressed, and dried into paper or
board.
Briefly, in the method according to the present invention, the solid matter of
the paper or
board pulp is diluted with the kaolin composition that is suspended in the
salt composition,
according to the invention. The salt composition consists of the states of
carbonate, i.e.,
carbonates and bicarbonates, and calcium or magnesium ions or a mixture
thereof, which
are added to and produced in the aqueous solution at a pH that remains below
8.3
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throughout the production state. These states of carbonate can include, among
others,
colloidal-size carbonate partices, bicarbonate ions, carbonate ions and
carbonic acid, which
are formed in the aqueous solution, when the pH is below 8.3.
In the invention, slurry of structured kaolin is added to a salt composition
similar to the one
described above, at the manufacturing stage of the paper or board product that
is before the
headbox of the paper machine, or the structured particles are formed in this
salt
composition.
and improving the retention in the wire section of the paper machine. For
example, in the
drying of paper or board, carbonate can be precipitated to the fibre structure
from the salt
composition, the carbonate enhancing the light scattering efficiency of
kaolin, whereby the
opacity and brightness increase, while the total amount of filler remains the
same. In
Said "aqueous solution" can be any watery solution.
According to a preferred embodiment of the invention, however, this aqueous
solution is
raw water, chemically or mechanically purified water, wire water, filtered
water that is
purified to various degrees of purity, or another kind of water used in the
paper factory, or
a mixture thereof, preferably filtered water or process water, from which the
solid matter is
According to another preferred embodiment of the invention, the said aqueous
solution
employed consists of chemical pulp (sulphate or sulphite pulp), mechanical or
chemi-
mechanical pulp, pulp manufactured by means of alkalis, recycled fibre,
deinked pulp
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(purified by washing or flotation), nano-cellulose, coated reject, uncoated
reject or a
mixture thereof.
Particularly according to this second preferred embodiment, paper pulp is
first
5 manufactured from the aqueous solution, the solid matter of the paper
pulp being mixed
with the solution in the paper pulp, whereafter the stages of the method
mentioned above
are carried out.
In addition to the said aqueous solution, the "salt composition" thus contains
salts of
10 carbonic acid. The salt composition preferably consists of the
carbonates or bicarbonates of
magnesium or calcium or a mixture thereof, and it is preferably manufactured
by adding
the slurry of oxide or hydroxide to the aqueous solution and by conveying
carbon dioxide
to the solution, so that the pH in the aqueous solution remains essentially
below 8.3
throughout this stage, whereby bicarbonate and colloidal carbonate are formed,
their
average particle size being < 0.3 nm, most preferably < 0.1 nm.
In paper and board machines, the aim is usually to keep the pH of the white
water system
within 6-8. For example, the chemistry of carbonate ions and the buffering of
pH provided
by the same are then utilized.
At an acidic pH, soluble carbon dioxide (CO2) and, to a minor extent, carbonic
acid
(H2CO3), are the main states of carbonate. In the neutral (on both sides of a
pH of 7) and
alkaline ranges, bicarbonate or hydrocarbonate (HCO3-) is the main state of
carbonate all
the way to a pH of 10. "Main" means that at least 50% by weight of the states
comprise
carbonate. In a very alkaline range (pH > 10), carbonate (C032) is the main
state. When
moving from the alkaline range towards the acidic one, essentially all of the
C032- has been
= converted into a form of HCO3- at a pH of about 8.3. In the most
important pH range of the
paper and board manufacture, the pH of 6-8, bicarbonate (HCO3) is thus the
prevailing
state.
"Structured kaolin particles" are produced from the kaolin sheets or stacks of
the kaolin
composition, which are possible further processed, so that at least part of
the kaolin,
preferably of its surface, is converted into metakaolin or calcined kaolin.
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The granules or agglomerates of structured kaolin are manufactured by spray-
drying the
kaolin slurry by means of a binder. For the spray-drying, a rotating atomizer,
discharge
nozzles, a double-fluid nozzle, ultrasound or a combination of the foregoing
can be used.
Metakaolin or calcined kaolin can be manufactured from these granules or
agglomerates by
heating, in a furnace, the spray-dried kaolin granules or agglomerates or
those that are
manufactured by means of the binder using the mixing technique. In the
manufacture of
"structured" kaolin, i.e., spray-drying, raw materials other than kaolin can
also be used.
Before spray-drying or using the mixing technique together with the binder,
for example,
calcium carbonate, titanium dioxide, talc or silicon dioxide or several
substances, among
others, can be added to the kaolin slurry that is manufactured in the aqueous
solution,
before manufacturing the granules or agglomerates.
In addition to these granules and agglomerates, the "fibre dispersion"
contains a source of
fibre, binder, and the salt composition mentioned above. In the present
invention, the fibres
can consist of chemical pulp or mechanical pulp. For example, sulphate and
sulphite
cellulose fibres, dissolving pulp, nano-cellulose, chemi-mechanical pulp
(CTMP),
thermomechanical pulp (TMP), pressure groundwood (PGW), ground pulp, recycled
fibre,
or fibres of deinked pulp can function as fibres. The binder has settled on
the surfaces of
the formed kaolin particles, and it functions by binding the particles to each
other and,
particularly, by binding the particles to the fibres.
Flocculants, coagulants or microparticles or a mixture or a copolymer thereof
can be
added, as a retention agent, to the aqueous solution or the fibre dispersion,
in an amount of
at least 0.01%, particularly about 0.01-3%, calculated from the total weight
of the solid
matter of the solution or dispersion, preferably at least microparticles, most
preferably
together with conventional flocculants or coagulants.
One or more chemicals known as such can also be added to the fibre dispersion,
the
chemicals being selected from a group of aluminium compounds, stock sizes,
surface sizes,
colouring agents, starches, optical brighteners, plastic pigments, natural and
synthetic
polymers, and fillers and coating materials.
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At the drying stage of the paper or board manufacture or by increasing the pH,
the
bicarbonate ions contained in the salt composition or mixed with the fibre
dispersion can
be converted into carbonate particles. Correspondingly, when increasing the
temperature,
carbon dioxide is released and the bicarbonate reacts with free calcium or
magnesium ions
according to the following reaction equation:
Ca2+ + (HCO3)2 ¨> CaCO3,1, + CO2 + H201'.
When the pH is increased with an alkali, e.g, NaOH or Ca(OH)2, the carbonate
particles
can be precipitated according to the following reaction equations:
Ca2+ + (HCO3)2 + 2NaOH -4 CaCO3.1. + Na2CO3 + 2H20.
Ca2+ + (HCO3)2 + Ca(OH)2 ¨> 2CaC04 + 20H-.
The carbonate particles thus formed fit between the fibrils and fibre, keeping
the fibrils in
their outward-oriented positions and giving the structure of the paper or
board opacity,
brightness, stiffness, and thickness (bulkiness). The carbonates on the
surface of the paper
or board, in particular, improve the adsorption of the printing ink. Part of
the precipitated
carbonate is also inside the lumens and pores of the fibres. Structured
kaolin, in turn, fills
the holes that remain between the fibres of the fibre network, whereby the
strength and
stiffness are reduced less than when using, e.g., pure fillers with an average
particle size of
3 micrometres or less.
The dispersion formed from these ingredients is then mixed to form the pulp,
which at the
last stage of the method according to the invention is filtered, pressed, and
dried into the
paper or board product. At this stage, usually, openings of about 10
micrometers remain
between the fibres. The kaolin agglomerates of a size of about 10 micrometres,
which are
utilized in the present invention, however, drift from the pulp into these
openings when
water is removed, thus increasing the surface area in which the hydrogen bonds
can be
formed. With commercial fillers, the decrease in strength is more pronounced,
since there
is less surface area between the fibres for the formation of hydrogen bonds.
Therefore,
structured kaolin with an average particle size of about 15 m is used in the
present
invention.
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The following examples describe the specific preferred embodiments of the
present
invention. They are intended to illustrate the benefits and advantages
achieved by the
invention, and not to limit the scope of the invention.
Examples
The results below indicate that the best results for brightness, opacity,
printability, strength,
and stiffness are obtained by combining structured kaolin and the carbonate,
which is
precipitated in the dispersion and which originates in the salts of carbonic
acid.
Suspending the kaolin in acidic water and using it among the pulp accelerates
the
dewatering and improves the retention in the wire, press, and dryer sections.
Example 1. Manufacture of structured kaolin and acidic water
In this example, the salt composition, which hereinafter is called acidic
water (below, also
"AW"), was made in ion-exchanged water. First, 25 kg of ion-exchanged water
were
weighed into each one of closable plastic cans (of a volume of 30 litres). 170
g of burnt
lime (CaO) were added thereto, having been slaked in 600 g of ion-exchanged
water at 45
C before the addition. By adding carbon dioxide to the weak calcium hydroxide
slurry
thus formed, Ca(OH)2, the pH was dropped from about 12 to 6.3. This solution
was
allowed to sediment for 12 hours, after which the colloidal portion that had
not sedimented
was separated from the can. The precipitate that sedimented on the bottom was
not used in
the tests.
Dry Covergloss (Kamin LLC) kaolin powder was suspended in ion-exchanged water
to a
dry matter content of 20%. In the elutriation, 0.2% of the Dispex N40 (BASF)
dispersing
agent and 8% of latex (Acronal S505, BASF) were used, calculated from the
weight of the
kaolin. After this, the suspension was spray-dried (Niro, mobile minor). The
feed rate of
the suspension was 50 ml/min, the rotation speed of the atomizer was about 25
000
rotations per minute, the temperature of the drying air was 250 C, and the
temperature of
the output air was 110 C. The dried, structured kaolin (below, "struc") was
cooled to room
temperature and suspended in acidic water into slurry of 20%. The average
particle size of
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the structured kaolin thus manufactured (struc+AW) was 15 micrometres
(Sedigraph 5120,
Micromeritics).
Example 2. Effect of the addition of structured kaolin sluriy on the
properties of the paper
made of the pulp.
In this test series, a Valley grinder was used to first grind a mixture of
bleached pine pulp
and bleached birch pulp to an SR number of 30. The amount of pine pulp from
the weight
of the pulp was 30% and that of birch pulp was 70%. The pulp was ground
according to
the standard method SCAN-C 25:76. This pulp was diluted with the ion-exchanged
water
according to the invention to a consistency of 0.2%, to which 0, 20 or 40%,
calculated
from dry pulp, of the 20% slurry of structured kaolin and acidic water
(struc+AW)
manufactured according to the previous example were added. In addition, for
comparison
of results, pulps diluted to 0.2% with pure ion-exchanged water were
manufactured, to
which 0, 20 or 40% of precipitated calcium carbonate, calculated from dry
pulp, (PCC,
Precarb FS-240, Shaefer Finland Oy), Covergloss (Kamin LLC) or Alphatex
(Imerys) were
added. Fom all of these, a 20% slurry was made in ion-exchanged water before
adding it to
the pulp. The test points are called struc+AW, struc, PCC, Covergloss, and
Alphatex.
Regarding the abbreviations:
"Struc" refers to structured kaolin;
"PCC" refers to precipitated calcium carbonate;
"Covergloss" is a filler that contains conventional, non-structured kaolin;
and
"Alphatex" is conventional, non-structured, calcined kaolin.
Slurries were manufactured from all of these either in acidic water (whereby
the test point
was also called by the abbreviation AW) or in pure ion-exchanged water, and
this slurry
was added to the paper slush, from which the pulp according to the test points
was formed.
From the pulps of consistencies of 0.2% thus manufactured, sheets of 80 g/m2
were
produced in a sheet mould without circulated water, according to the standards
SCAN-C
26:76 (SCAN-M 5:76). 10 sheets were made from each test point by using
cationic
polyacrylamide (Praestaret PK 435) as retention agents. 250 g/t of
polyacrylamide were
added by mixing without shearing forces. After this, the sheets were wet-
pressed and dried
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in a drum drier (120 C, 2 hours), as described in the publication of Pertti
Aaltonen from
1986 (Pertti Aaltonen: Kuituraaka-aineen ja paperin testausmenetelmia (Testing
methods
for fibre raw material and paper), Otakustantamo, 1986). All sheets thus
manufactured
were conveyed to be aerated for 48 hours at 23 C and a relative humidity of
50%. After
5 this, the basis weights of the sheets were verified and these properties
were determined:
= Filler content (525 C and 2 hours)
= ISO brightness (L&W Elrepho Spectrophotometer SE070), ISO 2470
= Opacity (L&W Elrepho Spectrophotometer SE070), ISO 2471
= Scott bond (Internal bond tester Huygen), Tappi-UM403
10 = Stiffness (L&W paper bending tester SE160), ISO 2493/SCAN-P 29:95
The basis weights of the sheets were at the target basis weight of 80 g/m2,
with an accuracy
of -0.6 g/m2. In this test, the assessment of the printing properties of the
sheets was made
by measuring the optical density. The sheets were printed by a Universial
Testprinter
15 (Testprint B.V.) using a Cold set black (Sun Chemical, viscosity 7.3
Pas) with 10
milligrammes of ink on the wire side of the sheet. The optical densities were
measured
using a densitometer (Macbeth) from aerated and dried samples after 24 hours
from the
printing. The Universial testprinter employed a pressure of 630 N and a
velocity of 1 m/s.
According to the filler content determined from the sheets (525 C and 2
hours), the results
are linearly normalized to the same filler content of 10%. A reliability of
95% means a
confidence interval of 95%.
Table 1. A sheet of 80 g/m2, normalized to a filler content of 10%.
Test point Opacity, % Brightness, Scott Bond, Stiffness, Optical
J/m2 1.1Nm density, 10 g
Covergloss 85.5 82.2 242 78 1.30
Alphatex 87.5 83.4 260 65 1.42
PCC 87.1 84.5 267 68 1.44
Stntc 88.4 83.0 312 85 1.52
Struc+AW 89.2 85.3 315 138 1.56
Reliability of -0.4 -0.2 12 6 -0.06
95%
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16
As indicated by the results, adding the structured kaolin that was suspended
in acidic water
(struc+AW) to the pulp and combining the structured kaolin with the carbonate
thus
precipitated improve the brightness, opacity, strength, stiffness, and
printability (optical
density) more than the use of structured kaolin alone (struc) or the use of
PCC alone
(PCC). The strength (Scott bond) also increases, while the round structured
kaolin of a size
of about 15 micrometres weakens the formation of hydrogen bonds between the
fibres less
than when using the fillers of a smaller size (Covergloss, Alphatex, and PCC).
Example 3. The effect of conventional kaolin suspended in acidic water on the
properties
of paper
In this example, dry Intrafil C (Imerys), which is conventional kaolin, is
suspended in
acidic water or ion-exchanged water to a dry matter content of 20%. The acidic
water used
is made as in example 1.
The Valley grinder was used to first grind a mixture of bleached pine pulp and
bleached
birch pulp to an SR number of 35. The amount of pine pulp from the weight of
the pulp
was 30% and that of birch pulp was 70%. The pulp was ground according to the
standard
method SCAN-C 25:76. This pulp was diluted to a consistency of 0.2%, to which
Intrafil C
(Intra) that was suspended in 0, 20 or 40% of ion-exchanged water or Intrafil
C
(Intra+AW) that was suspended in acidic water was added.
From the pulps of consistencies of 0.2% thus manufactured, sheets of 80 g/m2
were
produced in a sheet mould without circulated water, according to the standards
SCAN-C
26:76 (SCAN-M 5:76). 10 sheets were made from each test point using cationic
polyacrylamide (Praestaret PK 435) as retention agents. 250 g/t of
polyacrylamide were
added by mixing without shearing forces. Thereafter, the sheets were wet-
pressed and
dried in a drum drier (120 C, 2 hours).
All sheets thus made were taken to be aerated for 48 hours at 23 C and a
relative humidity
of 50%. After this, the basis weights of the sheets were verified and these
properties were
determined:
= Filler content (525 C and 2 hours)
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17
= ISO brightness (L&W Elrepho Spectrophotometer SE070), ISO 2470
= Opacity (L&W Elrepho Spectrophotometer SE070), ISO 2471
= Stiffness (L&W paper bending tester SE160), ISO 2493/SCAN-P 29:95
= Thickness (L&W Thickness tester SE51), ISO 534
Table 2. A sheet of 80 g/m2, normalized to a filler content of 10%.
Test point Opacity, % Brightness, % Thickness, p.m Stiffness,
.t.Nm
Intra 84.3 82.4 132 74
Intra+AW 86.5 84.6 173 109
Reliability of 0.4 -0.2 2 8
95%
As the results indicate, the opacity and brightness, in particular, do not
improve as much
with conventional kaolin as with structured kaolin.
