Note: Descriptions are shown in the official language in which they were submitted.
CA 02871710 2014-10-27
WO 2014/001628 PCT/F12013/050678
1
LIGHT AND SMOOTH COATING FOR PAPER OR BOARD OR PAINT
COATING FORMED FROM A COMPOSITE STRUCTURE
Field of the invention
The present invention relates to a coating that employs a composite structure,
the
body of which consists of polysaccharide, into which carbonate is
precipitated. In
particular, the invention relates to the use of such a carbonate
polysaccharide
structure in the coating layer of paper or board, in the coating of paints, or
in
materials similar to paints (for example in varnishes). Said structure
improves the
smoothness, thickness, mottling, strength, opacity, and brightness of the
coating
layer of the finished paper or board or the paint coating, as well as the
optical
density of the ink, and the stiffness and sizing ability of the coated paper
or board.
Description of prior art
The most common coating pigments, which are used in the manufacture of paper
and board, are kaolins, talcs, gypsum, ground calcium carbonate (GCC), and
precipitated calcium carbonate (PCC). Additionally, there are more expensive
special pigments, such as precipitated aluminium silicates, satin white, and
titanium dioxide. In the manufacture of paper and board, coating pigments are
preferably used, as they improve the optical properties of the paper and board
and
the setting of printing ink (the printability), and make it possible to reduce
the basis
weight of the paper body stock, board or coating layer; in particular, if the
coverage of the coating is good. A good coverage appears as the lack of
mottling
to the human eye.
Paints employ the same coating pigments mentioned above. Titanium is used in
considerably larger amounts than in the coating of paper or board, however.
Another great difference, compared with the coating of paper and board, is
that in
latex paints, in particular, latex is used considerably more frequently as a
binder
than pigments. Regarding paints, the purpose is to form, on the surface that
is to
be painted, a wear-resistant film coat that is as solid as possible and has a
good
CA 02871710 2014-10-27
WO 2014/001628 PCT/F12013/050678
2
opacity. Additionally, various additives that improve the formation of a film
are
used in paints, softening the latex particles and helping to form a solid
paint film.
In both coating applications, i.e., in the coating of paper and board and in
paints,
the purpose of the binder is to attach the pigments to the base, which is to
be
coated, and to the coating. In paints, a solid coating is formed, while in the
coating
of paper and board, the purpose of the use of binders is to ensure that the
pigments of the coating and the actual coating remain attached to the paper or
board during the printing, in particular. In the coatings of paper and board,
there is
no solid film surface, due to the smaller amount of binder. The purpose of a
porous
surface, among others, is to provide a suitable setting velocity of ink for
each
printing method (e.g., offset, gravure printing, flexographic printing,
inkjet).
The aim is to make the surface of the base paper or board as even as possible,
so
as to obtain as good a coverage as possible, by using as small an amount of
coating as possible. For that reason, the base paper or board is, typically,
first pre-
calendered before the coating. In a machine calender, there are metal rollers
on
both sides of the paper or board; this keeps the thickness standard, but the
density
varies. In a soft calender, the paper or board travels between a hard and
rubber-
coated roller. One advantage thereof is that the density variation is minor.
In both
types of calender, generally, one metal roller is heated. There are also
supercalenders and multi-roll calenders, which comprise more than ten rollers,
between which the paper or board travels. In the pre-calendering, there are
often
two rollers and, in the finishing calendering (super and multi-roll
calendering),
more than ten rollers. In the calendering, the upper and lower surfaces of the
paper or board are mechanically pressed against each other. The finishing
calendering is carried out, among others, to provide an improved pre-coating
or a
printable front coat. The greatest disadvantage of calendering is that it
decreases
the thickness of the coated paper or board, i.e., it increases the density of
the
product. Other disadvantages include the weakening of stiffness levels,
strength
properties, and optical properties.
CA 02871710 2014-10-27
WO 2014/001628 PCT/F12013/050678
3
The poor smoothness of the coated paper or board appears as poorer coverage
and a need of a larger amount of coating to fill the holes on the surface that
is to
be coated. When a coated surface is printed, the poor coverage of the coating
is
often seen as an uneven print quality, i.e., so-called mottling. The poor
smoothness of the coating or the resulting poor coverage can also cause =gloss
mottling or unevenness of the gloss in glossy coated paper and board grades.
This
is also referred to as the opacity and brightness mottling of the coating.
Regarding
the mottling of the print, among others, the following types of mottling can
be
distinguished: back trap, hydrophobic, single-colour, density, and dry
mottling. It is
believed that the mottling is caused by the differences in microporosity of
the
coating structure or an uneven distribution of binder in the coating. The
mottling of
the print is caused, when more or less ink is absorbed in the mottled area of
the
paper or board than in the surrounding area.
In addition to the calendering, efforts have been made to respond to the
unevenness of the rough base paper or board by adding plastic pigments to the
slip. The purpose of the use thereof is, among others, to increase the ability
of the
coating to react to the irregularities of* the base, in order to maintain the
light
structure of the coated paper or board by smaller calender loads in the
calendering. Generally, the light structure is also visible as an improved
opacity,
brightness, and optical density of the print. Ideally, after the calendering,
the
coating would have a good coverage, i.e., as little mottling as possible, it
would be
light, i.e., its thickness would be high, its smoothness good, and the other
qualities
of paper or board, such as gloss, would be at the target level and the
mottling
would be minor. The nip pressure of the calendering can be decreased, if some
coating provides the desired level of smoothness easier than another. In that
case,
a lighter, stiffer, and stronger coating is obtained, which still has the
desired level
of smoothness, and we would be closer to the ideal that is mentioned above.
With paints, there is no calendering, but also in this coating application, an
improved coverage of the surface that is to be painted is obtained by means of
a
smoother coat of paint. This means that a sufficient coverage can be obtained
with
a smaller amount of paint, whereby expensive titanium can be saved, in
particular.
CA 02871710 2014-10-27
WO 2014/001628 PCT/F12013/050678
4
The picking of the coating of paper and board, i.e., the peeling of the
fibres,
pigments, or the surface of the coating itself is a very harmful defect in
quality, in
the printing, in particular. In the printing machines, the material that has
peeled off
the coating accumulates, on the printing surfaces and in the dampening fluid
of the
first printing unit (in multi-colour printing), in particular. In printed
paper or board,.
this is visible as mottles, missing spots, the peeling of the coating, and
other
quality problems. Most generally, the picking of the coating surface causes
the
peeling of single fibres and fibre bundles; at its worst, the delamination of
part of
the coating can take place. The picking in the printing is caused by that, for
the
splitting of the ink in the printing nip, the force that is perpendicular to
the coating
surface exceeds the local strength of the coating surface at the outlet of the
printing nip. Binders are used in an attempt to prevent these quality
problems. An
example of these comprises starch.
The ability of starch to form the bonding strength of the coating is caused by
its
large number of hydroxyl groups, which increase its ability to form hydrogen
bonds. In plants, starch is found in small (2 ¨ 150 ptm) granules that are
separated
during the manufacturing process. All plants contain starch. Starch is
commercially
produced from potato, tapioca, barley, wheat, rice, and corn, however, and to
some extent, from tuberous roots, leguminous plants, fruit, and hybrids.
Generally, starch is not soluble in cold water. This is due to the fact that
starch
polymers are in a well-organized form, bound by hydrogen bonds in the starch
granules. When an aqueous starch solution is heated, the starch granules first
swell, whereafter single starch polymers are released from each other.
Starch is often treated to make the starch cationic, the stability of the
solution is
increased and/or its rheological properties are improved at higher dry matter
contents. Being a polyol, the chemical forming products of starch are,
generally,
ethers or esters. These treatment alternatives include hydroxy-alkylation,
cation ization, carboxymethylation, acetylation, thermo-mechanical treatment,
enzyme treatment, hydrogen peroxide treatment, sodium hypochlorite treatment,
CA 02871710 2014-10-27
WO 2014/001628 PCT/F12013/050678
and acid treatment. Starches that dissolve in cold water, i.e., so-called cold-
soluble
starches, are also provided by treating the starch.
Native (untreated) starch is anionic; therefore, its attachment to the fibre,
during
5 the paper and board manufacturing process, without a cationic treatment
is poor.
At present, cationic starches are ethers that are manufactured using an epoxy
chemical that contains a 'quaternary ammonium group. Cationized starch is the
treated starch that is used the most.
Typically, the starch polymers inside the starch granules should be released
either
by adding to hot water or cooking the aqueous starch solution. The cold-
soluble
starches mentioned above constitute an exception to this. Typically, the
purpose of
the coating slip that is used in the coating of paper or board is to provide
as high a
dry matter content as possible. Generally, an improved water retention,
runnability,
and quality properties are then achieved. When starch is used, therefore,
efforts
are made to minimize the amount of water needed in the manufacturing process
of
starch.
In addition to starch, other strength-improving additives have also been used.
Guar gum is a vegetable gum that is used most frequently. Guar gum and locust
bean gum are seed gums and consist of galactomannan. Karaya gum has a more
complex structure; it is a branched polysaccharide. The use of vegetable gums
has been limited by their price that is higher than starch. They have also
been
more difficult to treat than starch.
Cellulose and nanocellulose also belong to polysaccharides. Nanocellulose or
microfibrillated cellulose can be manufactured from all materials that contain
cellulose, such as wood. The structure of nanocellulose is considerably
smaller
than that of a normal cellulose polymer; therefore, it contains a considerably
larger
number of hydroxyl groups that form hydrogen bonds Nanocellulose is also
called:
microfibrillated cellulose, nanofibrillated cellulose, nanofibres, and
microfibres.
CA 02871710 2014-10-27
WO 2014/001628 PCT/F12013/050678
6
Accordingly, there is a need for a lighter coating of paper or board, or a
paint
coating, which provides a good coverage and improves the essential quality
properties. The light, smooth coating gives the manufacturer of paper or board
a
possibility to achieve a smooth finish that provides high-quality printing
results and
reduces the required coating material costs. A coating that has a good
coverage
can also decrease the basis weight of the base paper or board, to some extent.
Additionally, in the pigmenting of paper and board, in particular, an improved
pre-
coating can be achieved, compared to the known solution.
= Brief description of the invention
An object of the present invention is to provide a new coating pigment that is
suitable for paper and board products and paints, improving their smoothness,
strength, mottling, coverage, opacity, brightness, the optical density of the
print,
and their sizing ability.
The purpose of the invention, in particular, is to provide a coating pigment
that
contains a carbonate polysaccharide composite in its coating structure.
The purpose of the coating pigment in question is to provide an improved
smoothness and thickness, while keeping the brightness and opacity of the
paper
or board product and the paint, in particular, on a good level. The increase
in
thickness provides a lighter coating structure.
Thus, the present invention relates to a coating composition for the coating,
pigmenting, or surface sizing of the paper or board product, containing the
said
composite structure. This composition is suitable to be used, among others, as
the
coating pigment of the applications mentioned above.
To be more precise, the coating composition, according to the present
invention, is
characterized by what is presented in the characterizing part of claim 1; its
use as
a paint is characterized by what is presented in claim 9; and the application
of the
CA 02871710 2014-10-27
WO 2014/001628 PCT/F12013/050678
7
said paint on the surface to be painted is characterized by what is presented
in
claim 19.
Correspondingly, the coated paper or board product, according to the
invention, is
characterized by what is presented in the characterizing part of claim 10, and
the
method of manufacturing the same is characterized by what is presented in the
characterizing part of claim 14.
The present invention is multifunctional and improves various properties. The
composite structure, according to the invention, provides, among others, an
improvement in the smoothness properties of the products of various
applications
and a decrease in the mottling thereof, as well as an improvement in the
evenness
of the coating.
In the case of fibre products, their important properties, such as brightness,
opacity, and printability can also be kept on a good level, while part of the
raw
materials of the coating can .be replaced with the coating pigment, according
to the
invention, due to its lighter structure. The light structure and smoothness
that are
caused by the use of the composite, according to the invention, enable a
decrease
in the calender pressures in the nip, whereby the compaction of the paper or
board
can be reduced.
Brief description of the drawings
Fig. 1 shows SEM images of the coatings, wherein the carbonate starch
composite, according to the invention, is applied on the surface of LWC base
paper (Fig. 1A), and a reference coating that uses GCC (Fig. 1B).
Detailed description of the preferred embodiments of the invention
The present invention relates to a coating composition that comprises a
composite
structure, the body of which consists of polysaccharide, into which carbonate
is
precipitated.
CA 02871710 2014-10-27
WO 2014/001628 PCT/F12013/050678
8
The amount of polysaccharide in relation to the carbonate in the composite
structure can vary broadly, among others based on its different end uses and
the
ways of using it. According to a typical example, the amount is, however,
within
the range 1/20-2/1, preferably 1/15-1/1, and most suitably 1/15-1/5.
Also the size of the composite structure can vary broadly, and is generally
5.700pm.
The invention also relates to a coated paper or board product, on one or both
surfaces of which, the said composition is spread into one or more layers.
Another application, according to the invention, comprises paints, which are
certain kinds of coatings and, correspondingly, can consist of the coating
composition, according to the invention, or contain the composition.
The polysaccharide is preferably selected from starch, carboxy-methyl
cellulose,
guar gum, pectin, citosane, dextrins, galactomannan, alginates or
nanocellulose or
a mixture of any of the above, more preferably from starch or nanocellulose,
most
suitably from starch or cold-soluble starch.
In the case of starch, its particles can be treated before using it in the
said
composite structure, so that the particle is expanded, or even fully or partly
broken.
Generally, this is carried out by heating which, in the case of the present
invention,
can be carried out, for example, in a solution that contains carbonate ions
and
other states of carbonate in the prevailing pH (i.e., in a so-called carbonate
ion
solution).
Calcium and/or magnesium carbonate is precipitated into cold-soluble starch,
nanocellulose or a mixture thereof from the carbonate ion solution by means of
low
pressure, heating, mixing, the dosing of an additive that contains alkaline,
or a
mixture of any of the above.
CA 02871710 2014-10-27
WO 2014/001628 PCT/F12013/050678
9
The carbonate that is included in the composite structure and precipitated
into the
polysaccharide is preferably calcium or magnesium carbonate or a mixture
thereof, whereby the carbonate ion solution also contains calcium or magnesium
ions of a mixture thereof.
Regarding their crystal structure, precipitated calcium carbonates are
typically
calcite, aragonite, or vaterite. Typically, calcite is found as flaky and
cubic crystal
forms. Scalenohedral, rhombohedral, acicular, spherical, and prismatic crystal
forms are also possible to obtain by changing the precipitation conditions,
such as
the concentration of slaked lime and the reaction temperature.
The manufacture of the composite structure according to the invention is
described in the patent application 20125569, and the invention of this
application
relates to the use of the composite structure according to the above-mentioned
application in the coating of paper or board, or in paints.
"Carbonate ion solution" refers to a carbonate system that can contain
carbonate
ions (C032"), bicarbonate ions (HCO3-), carbonic acid (H2CO3), and even carbon
dioxide (CO2), but this content and the mutual ratios of these states vary
according
to the pH of the solution.
The carbonate system, thus, refers to the change of different carbonate
states,
according to the pH. The main states are as follows:
H2CO3 HCO3- C032-
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 up to a pH of about 10. In a highly alkaline range (pH > 10),
carbonate
(C032") is the main state. The alkalinity of carbonates refers to the content
of
strong acid, with which the aqueous solution can be titrated to the end point
of
phenolphthalein. At that time, all of the C032- has been converted into the
ion state
CA 02871710 2014-10-27
WO 2014/001628 PCT/F12013/050678
HCO3-. This takes place at a pH of about 8.3. In the most important pH range
of
the paper and board manufacturing process, the pH of 6-8, bicarbonate (HCO3")
is
the most predominant state. Furthermore, reasonably well dissolved carbon
dioxide and some carbonic acid and colloidal carbonate have come to this pH
5 range from the acidic range of the pH range. The solution in the acidic
pH range is
herein called the acidic carbonate ion solution.
By increasing the pH or temperature, the bicarbonate ions can be made change
into carbonate particles from the carbonate ion solution. Correspondingly,
when
10 the temperature is raised, carbon dioxide is released and the
bicarbonate reacts
with free calcium or magnesium ions, according to the following reaction
equation:
Ca2+ + 2HCO3" ¨>CaC031+ CO21+ H201.
The reaction above also works, when a sufficiently warm carbonate ion solution
is
subjected to low pressure or centrifugal force, so that the carbon dioxide is
released from the solution in the air.
When the pH is increased with an alkali, for example with NaOH or Ca(OH)2, the
carbonate particles can be precipitated according to the following reaction
equations:
Ca2+ + 2HCO3" + 2NaOH --CaC031+ Na2CO3 + 2H20.
Ca2+ + 2HCO3- + Ca(OH)2 ¨>2CaCO3I+ 20H-.
The bicarbonate in the acidic carbonate ion solution is assumed to help the
generated carbonate filler to attach to the polysaccharide polymer. The
bicarbonate ion, as such, includes a possibility for the formation of hydrogen
bonds, which is important for the attachment of polysaccharides to the fibre,
among others. It could be considered that bicarbonate controls the attachment
to
the hydroxyl groups of polysaccharides before precipitating into carbonate.
CA 02871710 2014-10-27
WO 2014/001628 PCT/F12013/050678
11
When a polysaccharide that is treated with cationization is used, in the
invention, it
is probable that the cationic groups help the generated carbonate
polysaccharide
structure attach to the other components of the coating. Due to these reasons,
the
composite pigment is capable of enhancing the strength bonds of the binders of
the coating to the pigments that are used.
As mentioned above, for example, starch, carboxy-methyl cellulose, guar gum or
nanocellulose or a mixture of any of the above, more preferably starch or
nanocellulose, most suitably starch can be used as the polysaccharide.
Amphoteric starches function in a wider pH range than cationic starches.
Hence,
they are advantageous polysaccharides to be used in the present invention.
Other
forming alternatives of starch include hydroxy-alkylation, cationization,
carboxymethylation, acetylation, thermo-mechanical treatment, enzyme
treatment,
hydrogen peroxide treatment, sodium hypochlorite treatment, and acid
treatment.
Vegetable gums are more difficult to treat than starch. Correspondingly,
polyacrylamide (PAM), nanocellulose, and carboxy-methylcellulose (CMC) do not
need to be treated, as PAM is already found in either a cationic or anionic
form,
and the others already contain a sufficient number of hydroxyl groups that
form the
hydrogen bonds. Consequently, in their untreated form, they are well suited to
be
used as polysaccharides in the invention.
The applications of the above mentioned composite structure according to the
invention can include the coating of paper, coating of board, and paints.
Generally,
the composite structure then functions as a coating pigment in the end
product.
In paper or board coatings or paint coatings, the composite structure can be
used
together with other coating pigments or as a single coating pigment.
The composite structure is most suitably dosed into the paint or coating slip
in as
high a dry matter content as possible.
CA 02871710 2014-10-27
WO 2014/001628 PCT/F12013/050678
12
The fibre product to be manufactured can be coated, surface-sized or pigmented
printing paper, packing board, kraft paper or another paper grade that uses
mechanical pulp or chemical pulp fibre or both.
The patent application 20125569 shows that, by raising the pH of the carbonate
ion solution, which is at an acidic pH, to the neutral range, when the
polysaccharide is in the acidic carbonate ion Solution (pH>6.5), by means of
heating, the dosing of an additive that contains alkaline additives, low
pressure,
mixing or a combination of any or all of the above, the carbonate can be
precipitated on the polysaccharide. Additionally, it is possible to use any
known
additives, such as acids that influence the crystal form and structure of the
precipitated carbonate. The method of precipitating carbonate on
polysaccharide,
which is described in the said patent application and, particularly, its
claims, can
thus also be implemented in connection with the formation of the coating
composition of the present invention. In the manufacture of paper or board, in
addition to the opacity, brightness, and printability (an improvement in the
absorption properties of ink), this composite structure of precipitated
carbonate
and polysaccharide gives the surface-sized, pigmented or coated end product or
the latex paints a decrease in mottling and an improvement of smoothness,
above
all.
Typically, the carbonate polysaccharide composite, according to the invention,
is
used in the coating, surface sizing or pigmenting of paper or board, and in
paints,
according to the following alternatives (A¨D). The coating, surface sizing or
pigmenting can be carried out on one or both sides, one or more times,
alternatively, using one or more on-machine or off-machine coating units. When
the coating is carried out several times, drying can be carried out between
the
coating layers, or a new coating layer can be applied directly on top of the
wet
coating. The coating layer herein refers- to a coating layer that is provided
on the
base paper or board by coating, pigmenting or surface sizing. The base paper
or
board can be precalendered or uncalendered.
CA 02871710 2014-10-27
WO 2014/001628 PCT/F12013/050678
13
A) The composite according to the invention is mixed with a coating slip
together
with other pigments, binders, and other additives that are used in the slip,
so that
the dry matter content of the slip is at least 55%. The paper or board is
coated with
this slip at least once. Thereafter, the dried and coated paper or board can
be
calendered to a desired target smoothness and thickness. Other possible layers
of
coating or pigmenting do not necessarily contain the composite, according to
the
invention, in the slip. When the paper or board is coated with more than one
layers
of coating, the previous coating layer can be dried and/or calendered before
the
new coating layer is spread on top of the previous one. Alternatively, the new
coating layer can also be spread directly on top of the previous layer without
drying and/or calendering.
B) The composite according to the invention is mixed with a pigmenting paste
together with other pigments, binders, and other additives, which are used in
the
paste, so that the dry matter content of the paste is at least 20%. The paper
or
board is pigmented with this paste at least once. Thereafter, the dried and
pigmented paper or board can be calendered to a desired target smoothness and
thickness. The base paper or board, which is pigmented one or more times, can
be coated with one or more coating layers, according to alternative A, or with
one
or more coating layers, which do not contain the said composite. When the
paper
or board is coated with more than one layer of coating, the previous coating
layer
can be dried and/or calendered before a new coating layer is spread on top of
the
previous layer. Alternatively, the new coating layer can also be spread
directly on
top of the previous layer without drying and/or calendering.
C) In surface sizing, the composite according to the invention is used,
whereby the
picking problem of the surface can be kept under control, and a coating is
obtained, which improves the brightness, opacity, and the optical density of
the
print. When the composite according to the invention is used in surface
sizing,
other pigments, binders, and additives can possibly also be used in the
surface-
sizing paste. The surface sizing can be used as the first coating on the base
paper
or board before the pigmenting of alternative B and/or the coating of
alternative A,
or before the spreading of pigmenting or coating layers that contain no
composite.
CA 02871710 2014-10-27
WO 2014/001628 PCT/F12013/050678
14
When the paper or board is coated with more than one layer of coating, the
previous coating layer can be dried and/or calendered before the new coating
layer is spread on top of the previous one. Alternatively, the new coating
layer can
also be spread directly on top of the previous layer without drying and/or
calendering.
Consequently, one or more layers of pigmenting, or one or more layers of
coating,
or one or more layers of surface size contain the composite according to the
invention, whereas the other layers can either contain the said composite or
not.
D) The use of the composite according to the invention in paints;
particularly, to
reduce the required amount of paint, due to an improved coverage, without
weakening the picking strength, opacity or wear resistance of the paint layer.
The other additives mentioned above include other pigments, binders, and
additives, such as latexes, polyvinyl alcohol, CMC, starches, proteins,
caseins, or
dispersing agents, anti-foam agents, foam removers, pH regulators, hardening
agents, regulators of water retention and rheology, synthetic thickeners,
lubricants,
optical clarification agents, colouring agents, and microbicides.
Latex is a binder that is used the most frequently both in the paper or board
applications and paint applications. Typically, latexes are combinations of
styrene
butadiene, polyvinyl acetate, and acrylate latexes or the combinations of
their
different copolymers. In both of the applications above, efforts have been
made to
reduce the portion of latex in the coating slip or paint to decrease the
costs.
The amount of latex varies according to the printing method. In offset
printing, the
surface of the paper is moistened by water, whereby the picking strength (wet
strength, in particular) is of importance and, generally, 10-15 parts of latex
are
needed. In gravure printing, no dampening solution is used, but the smoothness
of
the paper is an important property. In that case, the amount of latex is
generally 4-
5 parts of latex.
CA 02871710 2014-10-27
WO 2014/001628 PCT/F12013/050678
The most common of the other binders are starch, carboxy-methyl cellulose
(CMC), polyvinyl alcohol, protein, and casein. Polyvinyl alcohol and CMC yield
the
best bonding strength, but being more expensive than latex, they are not used
to a
great extent.
5
Regarding the other additives that are used in coating slips include
dispersing
agents, anti-foam agents, foam removers, pH regulators, hardening agents,
regulators of water retention and rheology, synthetic thickeners, lubricants,
optical
clarification agents, colouring agents, and microbicides.
The base paper or board that is coated with the composite structure mentioned
above, particularly, contains chemical pulp fibre and/or wood fibre and/or
synthetic
fibre. In the present invention, the fibres can be any natural fibres and/or
synthetic
fibres. In particular, the fibres can comprise chemical pulp or mechanical
pulp or a
mixture thereof. For example, sulphate and sulphite cellulose fibres,
dissolving
pulp, nanocellulose, chemi-mechanical (CTMP), thermo-mechanical (TMP)
pressure groundwood (PWG), ground pulp, recycled fibre or the fibres of de-
inked
pulp, can comprise the solid matter. Typically, sulphate and sulphite
celluloses are
called chemical pulps, and thermo-mechanical pulp, pressure groundwood, and
groundwood pulp are called mechanical pulps.
In addition to the coated, surface-sized or pigmented printing papers and
packing
materials, different special paper grades (e.g., wall paper base and kraft
paper),
and different paints constitute good applications, since in these products,
the
smoothness and mottling of the coating layer are of great importance. In these
applications, particularly, the surface sizing and pigmenting are essential.
The present invention also relates to a method of manufacturing the coated
paper
or board product, wherein the coating composition mentioned above is spread on
one or both sides of an uncoated paper or board product to form one or more
layers.
CA 02871710 2014-10-27
WO 2014/001628 PCT/F12013/050678
16
The method of coating, i.e., the method of spreading the composition, can
employ
methods that contact the paper or board during the coating, or non-contacting
coating methods. The most important non-contacting methods are the curtain
coating and spray coating. Blade coating, rod coating, air brush coating and
film
transfer coating are the most important contact methods of coating. The blade
coating can be further divided into long dwell time, short dwell time, and jet
application. In these methods mentioned above, the coating slip is prepared in
an
aqueous solution, which is dried after the coating. In the coating of paper
and
board, the drying is, generally, carried out with hot air or infrared lamps.
Generally,
the paints dry by air-drying. There are also waterless or powder coating
methods,
which are meant to make charged dry pigments attach to the surface to be
coated.
These methods may also employ the use of binders. The coating can be applied
directly on the surface of the manufactured paper or board without a separate
intermediary rolling of ink (on-machine coating) or after an intermediary
rolling of
ink at a separate coating head (off-machine coating). Both sides of the paper
or
board can be coated simultaneously or at different stages. The coating can be
carried out one or more times, whereby drying can be carried out between the
different runs of coating. Typically, the dry matter content of the coating
slips is
about 60% or more, and the amount of coating is 10-30 g/m2/side. In addition,
surface sizing and pigmenting are carried out, wherein the composition of the
pastes is simpler, and the dry matter contents and desired amounts of coat are
lower.
In the surface sizing, the picking strength and stiffness of the paper or
board is
enhanced, and any partly loose fibres and other components can be bound to the
surface. This also improves the smoothness, porosity, gloss, and the
absorption of
ink. The surface sizing can also constitute a pre-treatment for the paper and
board that is to be coated. Starch, as such, can be used in the surface sizing
pastes in a dry matter content of about 10%. In the surface sizing, the amount
of
surface size on the surface of the base paper or board is, generally, 0.5...3
g/m2/side. A pond size press or sym-sizer film coater is conventionally used
in the
surface sizing.
CA 02871710 2014-10-27
WO 2014/001628
PCT/F12013/050678
17
A light coating, i.e., pigmenting, improves the properties and printability of
the
surface of the paper/board. The paste often contains a pigment that is added
to
the starch solution, often calcium carbonate, and possible additives. The dry
matter contents in the pigmenting are about 20-55%. The amount of coating is
1...10 g/m2/side, depending on the paper and board grade and the purpose of
use.
Regarding the coverage of the coating, the blade coating provides a surface
with a
poorer coverage on a rough base paper or board than, for example, the air
brush
coating, film transfer coating, and the non-contacting coating methods. This
is due
to the incapability of the blade coating to follow the irregularities of the
surface that
is coated.
The following non-limiting examples illustrate the invention and its
advantages.
Examples
Example 1
A carbonate ion solution was prepared, so that 170 g of burnt lime (CaO) was
mixed with 1000 g of water at 55 C. The Ca(OH)2- slurry that was thus
generated
was allowed to react, in the mixture, with the carbon dioxide that was
conducted
thereto, so that the final pH was 6.2. This solution was allowed to sediment
for 48
hours, whereafter the clear solution was separated from the sedimented
ingredient. The dry matter content of this solution was 42%. The carbonate ion
= 25 solution that was thus prepared was used as raw .material in the
cooking of the
= coating starch in the tests described below.
Example 2
The carbonate ion solution that was prepared, according to the previous
example,
.was used as the cooking water of starch in this examplel. In test point A, 20
parts
of starch were cooked (95 C, 30 minutes, mixing) together with the carbonate
ion
solution of Example 1, so that the final dry matter content of the paste was
50%.
The starch that was used was potato starch (coating quality) from Chemigate
Oy.
CA 02871710 2014-10-27
WO 2014/001628 PCT/F12013/050678
18
In test point B, the same starch was used as in test point A. In test point B,
starch
was first cooked (95 C, 30 minutes, mixing), and ground calcium carbonate
(Hydrocarb 90, Omya) was added thereto by gently mixing, so that the
proportion
of starch is 20 parts and the final dry matter content is 50%.
Three different rods were used to coat an uncoated copying paper of 80g/m2 to
three different coat weights. The coating was carried out by a laboratory
coater
(RK K303 multicoater), using a velocity of 6. Thereafter, the sheets were
dried in a
conditioning room for 48 hours, at 23 C and a relative humidity of 50%.
Thereafter,
their coat weights were verified and they were soft-calendered in the
following
conditions:
= 50 C
= two nips
= pressure of 200kN/m
The following properties were determined from the calendered samples:
= ISO brightness (Minolta Spectrophotometer 3610d), ISO 2470
= Opacity (Minolta Spectrophotometer 3610d), ISO 2471
= Thickness (L&W Thickness tester SE51), ISO 534
= Picking strength of paper (Wax pick test), T 459 OM 93
= Smoothness (Parker print surface), ISO 8791-4
The results are shown in Table 1. All of the results are normalized to a coat
weight
of 8g/m2.
Table 1. The results that are normalized to a coat weight of 8 g/m2.
Test Brightness Opacity Thickness Picking
Smoothness PPS-10
point cyo cyo lam strength m
A 86.5 89.7 149 18 1.4
- B 83.1 85.4 135 14 2.7
CA 02871710 2014-10-27
WO 2014/001628 PCT/F12013/050678
19
Table 1 shows that, when the structure of the coating can be kept light (high
thickness) and the surface of the coating smooth, excellent brightness and
opacity
properties are also achieved. If the coated samples of both of these test
points had
been calendered to the same level of smoothness, the properties of the coating
according to the invention (test point A) would even have been better, because
the
calendering could have been carried out at a lower nip pressure.
Example 3
The carbonate ion solutions that were prepared, according to Example 1, were
used as the cooking water of the cooking of starch in this example. The potato
starch that was used in the tests was from Chemigate Oy (Raisamyl).
In test point A, 35 parts of starch were cooked (95 C, 30 minutes, mixing)
together
with the carbonate ion solution of Example 1, so that the final dry matter
content of
the paste was 40%. In test point B, the same starch was used as in test point
A. In
test point B, starch was first cooked (95 C, 30 minutes, mixing), and ground
calcium carbonate (Hydrocarb 90, Omya) was added thereto by gently mixing, so
that the proportion of starch is 35 parts and the final dry matter content is
40%.
Three different amounts of coating were run by the CLC coating machine (CLC
6000) on an LWC base paper of 40g/m2. A blade coating unit and a velocity of
800m/min were used in the coating. The target amounts of coating were within 3-
9g/m2. Thereafter, the sheets were conditioned for 48 hours at 23 C and a
relative
humidity of 50%, before measurements. Thereafter, the amounts of coating
thereof were verified and they were calendered in the same conditions as in
the
previous Example 2. The following properties were determined frorn the
calendered samples:
= ISO brightness (Minolta Spectrophotometer 3610d), ISO 2470
= Opacity (Minolta Spectrophotometer 3610d), ISO 2471
= Thickness (L&W Thickness tester SE51), ISO 534
= !GT pick (AIC2-5), 4 m/s, medium viscous oil, ISO 3783
= Smoothness (Parker print surface), ISO 8791-4
CA 02871710 2014-10-27
WO 2014/001628 PCT/F12013/050678
The mottling was assessed as follows. The samples were printed at a four-
colour
Heatset-offset line frequency of 60 I/cm, using round dots. The C70 and B70
frames on the "mottling" areas on both sides of the samples were measured, so
5 that some paper around them was also included in the image. Herein, the
clustering method was used, wherein the mottles (0.5-10mm) are searched by the
threshold method, and grouped according to the size and contrast. As the
measuring instrument, the Hewlett-Packard ScanJet 7400C desktop scanner was
used, with which the version 3.02 of the Hewlett-Packard Precision Scan Pro
10 system software was used. In the measurement, the settings of the system
software were according to Table 2.
Table 2. Settings of the Precision Scan pro software for determining the
mottling.
Resolution 1200 dpi
Exposure time "Highlights" 235
Exposure time "Shadows" 0
Exposure time "Midtones" 2.2
Crispening None
Noise removal None
15 The results are shown in Table 3. All of the results are normalized to a
coat weight
of 8 g/m2.
Table 3. The results that were normalized to a coat weight of 8 g/m2.
Test Brightness, Opacity, Thickness, IGT Smoothness Mottling
point pm pick, PPS-10, index
m/s p,M
A 73.8 89.3 79 3.3 1.7 = 37
70.3 85.7 65 3.1 2.5 78
20 Table 3 shows that, when the structure of the coating can be kept light
(high
thickness) and the surface of the coating smooth, the mottling index is also
improved (a lower value). The mottling index is an index that emphasizes the
size
CA 02871710 2014-10-27
WO 2014/001628 PCT/F12013/050678
21
and contrast of the mottles by the number. If the coated samples of both of
these
test points had been calendered to the same level of smoothness, the
properties
of the coating according to the invention (test point A) would even have been
better, because the calendering could have been carried out at a lower nip
pressure.
The selection of base paper has obviously influenced the levels of brightness.
Any
other selection would also have provided improved brightness results.
The SEM images of Figure 1 show that the composite of the invention (test
point
A) provides a surface that consists of small granules (Fig. 1A). The coating
that
contains ground calcium carbonate (test point B) obviously constitutes a
denser
and coarser surface (Fig. 1B) than the test point A, according to the
invention (Fig.
1A). The enlargement of the figures is considerably smaller than the
resolution of
the human eye. When examined visually, the smaller granule provides an
improved coverage, due to its homogeneous and small granules.
