U.S. patent application number 13/798951 was filed with the patent office on 2014-01-02 for light and smooth coating for paper or board, or a paint coating, formed using a composite structure.
The applicant listed for this patent is Nordkalk OY AB. Invention is credited to Teemu Gronblom, Lars Gronroos, Sakari Saastamoinen.
Application Number | 20140004340 13/798951 |
Document ID | / |
Family ID | 49778458 |
Filed Date | 2014-01-02 |
United States Patent
Application |
20140004340 |
Kind Code |
A1 |
Saastamoinen; Sakari ; et
al. |
January 2, 2014 |
LIGHT AND SMOOTH COATING FOR PAPER OR BOARD, OR A PAINT COATING,
FORMED USING A COMPOSITE STRUCTURE
Abstract
The present invention relates to a coating composition that
comprises a composite structure with a body of polysaccharide, into
which carbonate has been precipitated. The invention also relates
to a coated paper or board product, onto which the said composition
has been spread on one or both sides into one or more layers; as
well as to paint that is formed of the coating composition
according to the invention, or that contains the said
composition.
Inventors: |
Saastamoinen; Sakari;
(Hameenlinna, FI) ; Gronblom; Teemu; (Pargas,
FI) ; Gronroos; Lars; (Pargas, FI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Nordkalk OY AB; |
|
|
US |
|
|
Family ID: |
49778458 |
Appl. No.: |
13/798951 |
Filed: |
March 13, 2013 |
Current U.S.
Class: |
428/342 ;
106/135.1; 106/15.05; 106/198.1; 106/217.01; 106/217.9; 156/230;
427/201; 427/209; 427/255.24; 427/358; 427/364; 427/394; 427/395;
427/419.1; 427/420; 427/427; 427/458; 427/471; 427/542; 428/537.5;
524/27 |
Current CPC
Class: |
D21H 17/70 20130101;
D21H 19/385 20130101; D21H 25/06 20130101; D21H 19/54 20130101;
D21H 23/56 20130101; D21H 19/826 20130101; D21H 21/16 20130101;
Y10T 428/31993 20150401; D21H 19/82 20130101; Y10T 428/277
20150115; D21H 19/14 20130101 |
Class at
Publication: |
428/342 ;
427/420; 427/209; 427/427; 427/471; 427/458; 427/201; 427/358;
427/364; 427/542; 427/394; 427/255.24; 427/395; 156/230; 428/537.5;
427/419.1; 106/217.9; 106/217.01; 106/198.1; 106/135.1; 106/15.05;
524/27 |
International
Class: |
D21H 19/14 20060101
D21H019/14; D21H 19/82 20060101 D21H019/82; D21H 25/06 20060101
D21H025/06 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 28, 2012 |
FI |
20125746 |
Claims
1. A coating composition for the coating, pigmenting, or surface
sizing of a paper or board product, comprising containing a
composite structure with a polysaccharide body, into which
carbonate has been precipitated.
2. The coating composition according to claim 1, wherein the
polysaccharide is starch, cold-soluble starch, carboxy-methyl
cellulose, guar gum or nanocellulose or a mixture of any of the
above, preferably starch or nanocellulose, most preferably
cold-soluble starch, whereby the shape of the polysaccharide body
is preferably swollen or partly or fully broken.
3. The coating composition according to claim 1, wherein the
precipitated carbonate is calcium or magnesium carbonate or a
combination thereof, and at least part of the carbonate is
preferably precipitated from the solution directly into the
polysaccharide body.
4. The coating composition according to claim 3, wherein part of
the carbonate is added carbonate, which is preferably calcium
carbonate, such as precipitated calcium carbonate (PCC) or ground
calcium carbonate (GCC) or a mixture thereof.
5. The coating composition according to claim 1, further comprising
other pigments, binders, and additives, such as latex, polyvinyl
alcohol, CMC, starch, kaolin, calcined kaolin, talc, titanium
dioxide, composite pigments, granule pigments, protein, casein,
dispersing agents, anti-foam agents, foam removers, pH regulators,
hardening agents, lubricants, regulators of water retention and
rheology, synthetic thickeners, lubricants, optical clarification
agents, colouring agents, and/or microbicides.
6. The coating composition according to claim 1, wherein a coating
slip is having a dry matter content of at least 55%.
7. The coating composition according to claim 1, wherein a
pigmenting paste is having a dry matter content of at least
20%.
8. The coating composition according to claim 1, wherein a surface
sizing paste is having a dry matter content of at least 2%.
9. Use of the coating composition according to claim 1 as
paint.
10. A coated paper or board product, wherein, on one or both
surfaces thereof, the composition according to claim 1 is spread
into one or more layers.
11. The coated paper or board product according to claim 10,
wherein also one or more layers of other coating, pigmenting or
surface sizing pastes are spread on one or both surfaces
thereof.
12. The coated paper or board product according to claim 10,
wherein the coating composition, which has been spread on its
surface, has formed after the drying one or more layers of coating,
pigmenting, or surface sizing, the basis weight of which is at
least 0.05 g/m.sup.2.
13. The coated paper or board product according to claim 10,
wherein it is a surface-sized or pigmented printing paper or
packing material, or a special paper grade, such as a wallpaper
base or kraft paper.
14. A method of manufacturing the coated paper or board product
according to claim 10, wherein a coating composition containing a
composite structure with a polysaccharide body, into which
carbonate has been precipitated, is spread on one or both sides of
the uncoated paper or board product into one or more layers.
15. The method according to claim 14, wherein a contacting or
non-contacting coating method is used for the spreading of the
coating composition, whereby the non-contacting methods are
selected from the group of curtain and spray coating, electric
field-assisted coating, and dry powder coating; and the contacting
methods are selected from a group of blade, rod, and air brush
coating, and film transfer coating.
16. The method according to claim 14, wherein the coating
composition is prepared in an aqueous solution, and after spreading
onto the paper or board product, excess water is removed by drying,
preferably with hot air or infrared lamps.
17. The method according to claim 14, wherein the coating is made
without a separate intermediary rolling.
18. The method according to claim 14, wherein the coating is made
at a separate coating station after intermediary rollings.
19. A method of spreading paint on a surface to be painted, wherein
the coating composition according claim 1 is mixed with optional
additives and binders, whereafter the composition is spread onto
the surface in wet form with a roller or by spraying or in dry form
by powder painting, whereafter the coat of paint is dried.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is entitled to the benefit of and
incorporates by reference essential subject matter disclosed in
Finnish Patent Application No. 20125746 filed Jun. 28, 2012.
TECHNICAL FIELD
[0002] 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.
BACKGROUND
[0003] 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.
[0004] 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 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.
[0005] 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).
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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 .mu.m) 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.
[0012] 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.
[0013] 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, cationization, carboxymethylation, acetylation,
thermo-mechanical treatment, enzyme treatment, hydrogen peroxide
treatment, sodium hypochlorite treatment, and acid treatment.
Starches that dissolve in cold water, i.e., so-called cold-soluble
starches, are also provided by treating the starch.
[0014] Native (untreated) starch is anionic; therefore, its
attachment to the fibre, during 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.
[0015] 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.
[0016] 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.
[0017] 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.
[0018] 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.
SUMMARY
[0019] 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.
[0020] The purpose of the invention, in particular, is to provide a
coating pigment that contains a carbonate polysaccharide composite
in its coating structure.
[0021] 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.
[0022] 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.
[0023] To be more precise, the coating composition, according to
the present invention, containing a composite structure with a
polysaccharide body, into which carbonate has been precipitated;
its use as a paint; and the application of the said paint wherein
on the surface to be painted is such that the coating composition
is mixed with optional additives and binders, whereafter the
composition is spread onto the surface in wet form with a roller or
by spraying or in dry form by powder painting, whereafter the coat
of paint is dried.
[0024] Correspondingly, the coated paper or board product,
according to the invention, is characterized by on one or both
surfaces thereof, is spread into one or more layers, and the method
of manufacturing the same is characterized by the coating
composition is spread on one or both sides of the uncoated paper or
board product into one or more layers.
[0025] 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.
[0026] 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
[0027] The present invention and its advantages will now be
described in further details with reference to the accompanying
drawings where:
[0028] FIGS. 1A and 1B show 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
[0029] The present invention relates to a coating composition that
comprises a composite structure, the body of which consists of
polysaccharide, on which carbonate is precipitated.
[0030] 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.
[0031] 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.
[0032] 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.
[0033] 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).
[0034] 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.
[0035] 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
[0036] 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.
[0037] The manufacture of the composite structure according to the
invention is described in Finnish patent application Serial No.
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.
[0038] "Carbonate ion solution" refers to a carbonate system that
can contain carbonate ions (CO.sub.3.sup.2-), bicarbonate ions
(HCO.sub.3.sup.-), carbonic acid (H.sub.2CO.sub.3), and even carbon
dioxide (CO.sub.2), but this content and the mutual ratios of these
states vary according to the pH of the solution.
[0039] The carbonate system, thus, refers to the change of
different carbonate states, according to the pH. The main states
are as follows: [0040] H.sub.2CO.sub.3
HCO.sub.3.sup.-CO.sub.3.sup.2- At an acidic pH, soluble carbon
dioxide (CO.sub.2) and, to a minor extent, carbonic acid
(H.sub.2CO.sub.3), are the main states of carbonate. In the neutral
(on both sides of a pH of 7) and alkaline ranges, bicarbonate or
hydrocarbonate (HCO.sub.3.sup.-) is the main state of carbonate up
to a pH of about 10. In a highly alkaline range (pH>10),
carbonate (CO.sub.3.sup.2-) 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 CO.sub.3.sup.2- has been
converted into the ion state HCO.sub.3--. 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 (HCO.sub.3.sup.-)
is the most predominant state. Furthermore, reasonably well
dissolved carbon dioxide and some carbonic acid and colloidal
carbonate have come to this pH range from the acidic range of the
pH range. The solution in the acidic pH range is herein called the
acidic carbonate ion solution.
[0041] By increasing the pH or temperature, the bicarbonate ions
can be made change into carbonate particles from the carbonate ion
solution. Correspondingly, when the temperature is raised, carbon
dioxide is released and the bicarbonate reacts with free calcium or
magnesium ions, according to the following reaction equation:
Ca.sup.2++2HCO.sub.3.sup.-.fwdarw.CaCO3.dwnarw.+CO2.uparw.H.sub.2O.uparw-
..
[0042] 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.
[0043] When the pH is increased with an alkali, for example with
NaOH or Ca(OH).sub.2, the carbonate particles can be precipitated
according to the following reaction equations:
Ca.sup.2++2HCO.sub.3.sup.-+2NaOH.fwdarw.CaCO3.dwnarw.+Na.sub.2CO.sub.3+2-
H.sub.2O.
Ca.sup.2++2HCO.sub.3.sup.-Ca(OH).sub.2.fwdarw.2CaCO3.dwnarw.+2OH.sup.-.
[0044] 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.
[0045] 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.
[0046] 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.
[0047] 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.
[0048] 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.
[0049] 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.
[0050] 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.
[0051] The composite structure is most suitably dosed into the
paint or coating slip in as high a dry matter content as
possible.
[0052] 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.
[0053] The Finnish patent application Serial No. 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.
[0054] 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.
[0055] 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.
[0056] 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.
[0057] 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. 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.
[0058] 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.
[0059] 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.
[0060] 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.
[0061] 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.
[0062] 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.
[0063] 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.
[0064] 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.
[0065] 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.
[0066] 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.
[0067] 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.
[0068] 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/m.sup.2/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.
[0069] 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.
[0070] 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/m.sup.2/side. A pond size press or sym-sizer film coater is
conventionally used in the surface sizing.
[0071] 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/m.sup.2/side, depending on the paper and board grade and the
purpose of use.
[0072] 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.
[0073] The following non-limiting examples illustrate the invention
and its advantages.
EXAMPLES
Example 1
[0074] A carbonate ion solution was prepared, so that 170 g of
burnt lime (CaO) was mixed with 1000 g of water at 55.degree. C.
The Ca(OH).sub.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 solution that was thus prepared
was used as raw material in the cooking of the coating starch in
the tests described below.
Example 2
[0075] The carbonate ion solution that was prepared, according to
the previous example, was used as the cooking water of starch in
this example. In test point A, 20 parts of starch were cooked
(95.degree. 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. In test point B, the same starch was
used as in test point A. In test point B, starch was first cooked
(95.degree. 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%.
[0076] Three different rods were used to coat an uncoated copying
paper of 80 g/m.sup.2 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.degree. C. and a relative
humidity of 50%. Thereafter, their coat weights were verified and
they were soft-calendered in the following conditions: [0077]
50.degree. C. [0078] two nips [0079] pressure of 200 kN/m
[0080] The following properties were determined from the calendered
samples: [0081] ISO brightness (Minolta Spectrophotometer 3610d),
ISO 2470 [0082] Opacity (Minolta Spectrophotometer 3610d), ISO 2471
[0083] Thickness (L&W Thickness tester SE51), ISO 534 [0084]
Picking strength of paper (Wax pick test), T 459 OM 93 [0085]
Smoothness (Parker print surface), ISO 8791-4
[0086] The results are shown in Table 1. All of the results are
normalized to a coat weight of 8g/m.sup.2.
TABLE-US-00001 TABLE 1 The results that are normalized to a coat
weight of 8 g/m.sup.2. Smoothness Thickness Picking PPS-10 Test
point Brightness % Opacity % .mu.m strength (.mu.m A 86.5 89.7 149
18 1.4 B 83.1 85.4 135 14 2.7
[0087] 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
[0088] 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).
[0089] In test point A, 35 parts of starch were cooked (95.degree.
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.degree. 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%.
[0090] Three different amounts of coating were run by the CLC
coating machine (CLC 6000) on an LWC base paper of 40 g/m.sup.2. A
blade coating unit and a velocity of 800 m/min were used in the
coating. The target amounts of coating were within 3-9 g/m.sup.2.
Thereafter, the sheets were conditioned for 48 hours at 23.degree.
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 from the calendered samples:
[0091] ISO brightness (Minolta Spectrophotometer 3610d), ISO 2470
[0092] Opacity (Minolta Spectrophotometer 3610d), ISO 2471 [0093]
Thickness (L&W Thickness tester SE51), ISO 534 [0094] IGT pick
(AIC2-5), 4 m/s, medium viscous oil, ISO 3783 [0095] Smoothness
(Parker print surface), ISO 8791-4
[0096] The mottling was assessed as follows. The samples were
printed at a four-colour Heatset-offset line frequency of 60 l/cm,
using round dots. The C70 and B70 frames on the "mottling" areas on
both sides of the samples were measured, so that some paper around
them was also included in the image. Herein, the clustering method
was used, wherein the mottles (0.5-10 mm) 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 system software was used. In the
measurement, the settings of the system software were according to
Table 2.
TABLE-US-00002 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
[0097] The results are shown in Table 3. All of the results are
normalized to a coat weight of 8 g/m.sup.2.
TABLE-US-00003 TABLE 3 The results that were normalized to a coat
weight of 8 g/m.sup.2. Bright- IGT Test ness, Opacity, Thickness,
pick, Smoothness Mottling point % % .mu.m m/s PPS-10 .mu.m index A
73.8 89.3 79 3.3 1.7 37 B 70.3 85.7 65 3.1 2.5 78
[0098] 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 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.
[0099] The selection of base paper has obviously influenced the
levels of brightness. Any other selection would also have provided
improved brightness results.
[0100] The SEM images of FIG. 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.
[0101] While the present invention has been illustrated and
described with respect to a particular embodiment thereof, it
should be appreciated by those of ordinary skill in the art that
various modifications to this invention may be made without
departing from the spirit and scope of the present.
* * * * *