Pigmented Paper and Coatings and Substrates for Pigmented Paper
Field of the Invention
[0001] The present invention relates to paper manufacturing, and in particular to application of
pigmented coatings to a paper substrate, for manufacture of paper suitable for digital printing.
Background
[0002] Digital printing refers to methods of printing from a digital based image direcdy on to a
media substrate. It is a widely used printing technique, which does not rely on printing plates
pressing arrangements - and finds application in ink jet printers, laser printers, colour printers and
copying machines.
[0003] In recent years, digital printing paper has emerged as a valuable product line, owing to
widespread availability of digital printers. Digital printing paper has traditionally been
manufactured by applying a pigmented coating to a hardwood pulp substrate. Use of hardwood
pulp as a substrate has however found to be expensive. Additionally, procurement and use of
hardwood pulp for substrate manufacture is known to contribute to deforestation. There is
accordingly a need for price effective and environment friendly methods for manufacture of high
quality digital printing paper.
[0004] Additionally, in adopting alternative or environment friendly materials for manufacturing
of printing paper substrates, it is necessary to develop pigmented coating that can be effectively
applied to such paper substrates to provide a paper product that satisfies aesthetic and quality
requirements.
Summary
[0005] In various embodiments of the present invention, the invention provides a pigmented
paper product comprising (i) a paper substrate having a first surface and a second surface, said
paper substrate comprising a first substrate component and a second substrate component,
wherein the first substrate component comprises wheat straw pulp, said wheat straw pulp
comprising between 45% and 65% by weight of the paper substrate and (ii) a pigment coating
applied to at least one of the first and second surfaces of the paper substrate, the pigment coating
comprising a pigment component, and a binding component comprising at least a first binder
component and a second binder component for binding the pigment component with the first
substrate component and the second substrate component.
[0006] The coat weight of the pigment coating applied to the paper substrate may comprise
between 4 gm/m2 and 8 gm/m2
•
[0007] The pigment component comprises 100 parts by weight of particles of wet ground calcium
carbonate, wherein at least 90% of the particles of wet ground calcium carbonate have a particle
size of 2 microns or less.
2
[0008] The first binder component may comprise between 40 and 120 parts by weight of oxidised
starch. The second binder component may comprises between 3 and 17 parts by weight of a latex
binder or a copolymer binder.
[0009] The binding component may comprise a third binder component, said third binder
component comprising between 4 and 17 parts by weight of a plastic binder.
[0010) The pigmented paper product may additionally comprise one or more of (i} between 1 and
7 parts by weight of a cross-linking agent, (ii) between 0.3 and 0. 7 parts by weight of a lubricant,
(iii) between 0.2 and 6.0 parts by weight of an optical brightening agent (iv) upto 0.5 parts by
weight of a thickener and (v) a buffering agent.
[00-11) The second substrate component may comprise a hardwood pulp and said second
substrate component may comprise between 35% and 55% by weight of the paper substrate.
[00l2) In an embodiment of the present invention, the invention provides a pigment coating for
a paper substrate comprising a first substrate component and a second substrate component,
wherein the first substrate component comprises wheat straw pulp, said wheat straw pulp
comprising between 45% and 65% by weight of the paper substrate, and the second substrate
component comprises hardwood pulp, said second substrate component comprising between 35%
and 55% by weight of the paper substrate. The pigment coating comprises (i) a pigment
component applied to the pape1' substrate, the pigment coating comprising 100 parts by weight,
wherein at least 90% of the pigment component particles have a particle size of 2 microns or less,
(ii} a binding component for binding the pigment component with the flrst substrate component
and the second substrate component, the binding component comprising between 40 and 120
parts of a first binder component and between 3 and 17 parts of a second binder component, and
optionally, one or more of (iii) between 4 and 17 parts of a third binder component, (iv) between
1 and 7 parts by weight of a cross-linking agent, (v) between 0.3 and 0.7 parts by weight of a
lubricant, (vi) between 0.2 and 6.0 parts by weight of an optical brightening agent (vii) uptd 0.5
parts by weight of a thickener, and (viii) a buffering agent.
[0013] In various embodiments of the present invention, the invention provides for a method for
manufacturing a pigmented paper product, comprising the steps of (i) preparing a paper substrate
comprising a first substrate component and a second substrate component, wherein (a) the first
substrate component comprises wheat straw pulp, said wheat straw pulp comprising between 45%
and 65% by weight of the paper substrate, and (b) the second substrate component comprises
hardwood pulp, said second substrate component comprising between 35% and 55% by weight
of the paper substrate. The method thereafter comprises (ii) applying the pigment coating to the
flrst surface and the second surface of the paper substrate, such that the coat weight of the pigment
coating applied to the paper substrate comprises between 4 gm/m2 and 8 gm/m2
•
[0014] The method further comprises preparing a pigment coating using a blend of the pigment
component with a binding component. The pigment component comprising 100 parts by weight,
wherein at least 90% of the pigment component particles have a particle size of 2 microns or less,
and the binding component comprising between 40 and 120 parts of a first binder component and
between } and 17 parts of a second binder component and optionally, one or more of (e) between
3
4 and 17 parts of a third binder component, (f) between 1 and 7 parts by weight of a cross-linking
agent, (g) between 0.3 and 0.7 parts by weight of a lubricant, (h) between 0.2 and 6.0 parts by
weight of an optical brightening agent, (i) upto 0.5 parts by weight of a thickener, and G) a
buffering.
Brief Description of the Accompanying Drawings
[0015] Figure 1 illustrates an exemplary pigmented paper product in accordance with the present
invention.
[0016] Figure 2 illustrates a facility for manufacture of pigmented coatings in accordance with the
present invention.
[0017] Figure 3 illustrates a facility for manufacture of pigmented paper products in accordance
with the present invention.
Detailed Description
[0018] The present invention provides novel, inventive, cost effective and environment friendly
pigmented papers products, paper substrates, coatings for paper substrates, and methods of
manufacturing pigmented paper products.
[0019] The novel and inventive pigmented paper product of the present invention comprises a
pigmented paper substrate. Manufacture of the pigmented paper substrate in accordance with the
present invention involves methods for manufacturing a paper substrate, and for applying a
pigmentation coat to one or both sides of the paper substrate.
[0020] Figure 1 illustrates a pigmented paper product 10 in accordance with the present invention.
Pigmented paper product 10 comprises a paper substrate 12 having a first surface 14 and a second
surface 16. In the illustrated embodiment, a first pigment coating layer 18 is applied to first surface
14, and a second pigment coating layer 20 is applied to second surface 16. ·
[0021] In accordance with the invention, paper substrate 12 may be a sheet substrate comprising
a first substrate component and a second substrate component. In the present invention, the flrst
substrate component comprises upto 60% by weight (w/w) of the total paper substrate weight,
while the second substrate component comprises the remainder of the total substrate weight.
[0022] The paper substrate may comprise a sheet substrate selected such that, after application of
pigment coating to the sheet substrate, the resulting pigmented paper product weighs between 65
gm/m2 (grams/ square meter) and 160 gm/m2
, preferably between 65 gm/m2 and 120 gm/m2
,
and in a preferred embodiment weighs 100 gm/m2
•
[0023] In an embodiment of the invention, the first substrate component comprises agro residue
pulp, which fust substrate component comprises between 40% and 75%, preferably between 45%
and 65%, and most preferably between 55% and 60% of the total substrate weight. The second
substrate component may comprise one or more of hardwood pulp, softwood pulp, bleach chemi
thermo mechanical pulp (BCTMP}, and one or more additives or flllers. In an embodiment of the
4
.·~ .
if:.
invention, the second substrate component comprises between 25% and 60%, preferably between
35% and 50%, and most preferably between 40% and 45% of the total substrate weight. In a
particular embodiment, the second substrate component comprises between 40% and 45% of the
total substrate weight, and is composed ofhardwood pulp or softwood pulp, bleach chemi thermo
mechanical pulp or a combination thereof.
[0024] In an embodiment of the invention, the agro residue pulp for the first substrate component
may comprise one or more of wheat straw pulp, rice straw pulp, corn stalks pulp, soy stalks pulp,
flax shives pulp, or hemp hurds pulp. In a preferred embodiment, the first substrate component
comprises wheat straw pulp.
[0025] It has been discovered that in manufacturing pigmented paper products comprising agro
residue pulps, achieving optimal paper qualities for digital printing requires a specific pigment
coating composition to be applied to surfaces of the underlying paper substrate. Additionally, it
has been discovered that application of said pigment coatings to the underlying paper substrate,
itself requires to be specifically controlled for achieving paper qualities that are desirable for digital
printing.
[0026] For achieving the qualities necessary for high definition digital printing, it has been
discovered that application of the pigment coating to one of, or each of the surfaces of the
underlying paper substrate requires to be carefully controlled. In particular, it has been discovered
that for optimal results, the coat weight of pigment coating applied to a single surface (either one
of the first and second surfaces) of the paper substrate comprises between 2% and 4.5% of the
weight of the underlying paper substrate, while coat weight of pigment coating applied to the first
and second surfaces of the underlying paper substrate comprises between 4% and 8.5% of weight
of the underlying paper substrate.
[0027] It has additionally been discovered that for optimal results, coat weight of pigment coating
applied to the first and second surfaces of the underlying paper substrate comprises between 4%
and 8% of total weight of the resulting coated paper product.
[0028] In an embodiment of the invention, the resulting coated paper product weighs 100 gm/ m2
,
the total coat weight of the pigment coating applied to the first and second surfaces of the
underlying paper substrate is between 4 gm/m2 and 8 gm/m2
, and the coat weight of pigment
coating applied to a single surface of the underlying paper substrate is between 2 gm/m2 and 4
gm/mz.
[0029] Maintaining the above coat weight parameters have been found to provide optimal digital
print quality on the resulting pigment coated paper products. On the other hand, it has been found
that venturing outside of the above parameters results in poor application of print ink or print
toner during printing, with associated loss of print quality and print gloss.
[0030] Pigment coatings in accordance with teachings of the present invention comprise an
aqueous suspension which includes a pigment component, a binding component, and water. The
aqueous suspension may optionally comprise one or more of an alkali, a cross-linking agent, a
lubricant, an optical brightening agent (OBA) and a thickener.
5
[0031] In an embodiment of the invention, at the time of application to the underlying paper
substrate, the pigment component may exhibit solids levels of between 25% and 40%.
[0032] In an embodiment of the invention, the pigment component comprises calcium carbonate
(CaC03) particles. The calcium carbonate particles may be obtained in any number of different
ways- including by way of example by way of wet grinding. For the purposes of the present
invention, at least 90% (and preferably 95%) of particles of the calcium carbonate pigment
component have particle size of 2 microns or less.
[0033] In an embodiment of the invention, not more than 5% of particles of the pigment
. component have particle size greater than 2 microns. In a more particular embodiment, at least
95% of particles of the pigment component have particle size of 2 microns or less, and I or at
least 98.9% of particles of the pigment composition have particle size of 3 microns or less, and I
or at least 99.8% of particles of the pigment composition have particle size of 4 microns or less,
and I or 100% of particles of the pigment component (or substantially 100% of particles of the
pigment component) have particle size of 7 microns or less.
[0034] In an embodiment, the pigment component may comprise wet ground calcium carbonate
in accordance with any of the above embodiments, exhibiting solids levels of between 70% and
75% prior to being combined with other constituents of the pigment coating.
[0035] In an embodiment of the invention, the pigment coating comprises 100 parts (by weight)
of the pigment component.
[0036] The binding component within the pigment coating of the present invention may comprise
at least a first binder component and a second binder component.
[0037] In an embodiment of the invention, the first binder component comprises starch -which
starch may comprise either a native starch or a modified starch. In a preferred embodiment of the
invention, the first binder component is a cooked starch or an oxidised starch (such as a cooked
or oxidised starch derived from the starch compound marketed under the trade name Amyloplast
10TM). In an embodiment of the invention, the first binder component is a non-ionic cooked or
oxidised starch.
[0038] The first binder component enables binding of the pigment component with the
underlying paper substrate. The first binder component may exhibit solids levels of between 20%
and 40% (and preferably between 20% and 30%) prior to being combined with other constituents
of the pigment coating.
[0039] In an embodiment of the invention, the pigment coating comprises between 40 and 120
parts (by weight} of the first binder component. In a specific embodiment, the pigment coating
comprises 100 parts (by weight} of oxidised starch by way of a first binder component.
[0040] The second binder component may comprise a latex or copolymer binder. In an
embodiment of the invention, the second binder component may be an anionic aqueous
copolymer of styrene and butadiene (such as the copolymer product marketed under the trade
6
name Styronal DD718apTM) having a pH of between 6.0 and 7.0. In a particular embodiment, the
second binder component may comprise an anionic aqueous copolymer of styrene and butadiene.
[0041] The second binder component improves binding between the entire pigment coating
chemistry and the underlying paper substrate. The second binder component may exhibit solids
levels in the region of 45% to 55% (and preferably 50.5%) prior to being combined with other
constituents of the pigment coating.
[0042] In an embodiment of the invention, the pigment coating comprises between 3 and 17 parts
(by weight) of the second binder component. In a specific embodiment, the pigment coating
comprises 7 parts (by weight) of an anionic aqueous copolymer of styrene and butadiene having a
pH of between 6.0 and 7.0, by way of the second binder component.
[0043] In an embodiment of the invention, the binding component within the pigment coating of
the present invention may additionally comprise a third binder component. The third binder
component may comprise a plastic binder, such as for example an aqueous anionic dispersion
comprising primarily of styrene and having a pH of between 5.5 and 7.0 (marketed under the trade
name Styronal PR8758ap.TM).
[0044] It has been discovered that inclusion of the third binder component in the binding
component improves print gloss of the resulting pigmented paper product. The third binder
component may exhibit solids levels in the region of 45% to 50% prior to being combined with
other constituents of the pigment coating.
[0045) In an embodiment of the invention, the pigment coating comprises betWeen 4 and 17 parts
(by weight) of the third binder component. In a specific embodiment, the pigment coating
comprises 6 parts (by weight) of an aqueous anionic dispersion comprising primarily of styrene
and having a pH of between 5.5 and 7.0, by way of the third binder component.
[0046] It has been found that wheat straw and other agro residues are comprised of relatively
short fibres, in comparison with hardwood and softwood derived pulps that have been previously
used for manufacture of print quality paper and which are known to have relatively longer fibres.
In experimenting with shorter fibres derived from agro residues, it was observed that paper
products manufactured using short fibre agro residues suffer from "dusting" problems- wherein
interaction between the paper product and printer moving parts (such as rollers) cause the short
fibres to be mechanically separated from the paper substrate, resulting in loss of print quality,
impermanence of print, as well as dust and fiber accumulation within printer machinery. A
previously known solution for this problem was to increase the coat thickness and coat weight of
coating applied to the underlying paper substrate.
[0047] The present invention however relies on a significandy improved pigment coating, and an
improved binding component (which in turn comprises at least first and second binder
components, and optionally a third binder component) within said pigment coating to ensure that
dusting problems do not arise despite use of the relatively short wheat straw fibres in the
underlying paper substrate and despite use of significandy lower coat weights for the pigment
coating applied to the underlying paper substrate.
7
[0048] In developing the pigment coating and the binding component, it was observed that the
specific ratios of the first~ second and optional third binder components within the binding
component are significant to achieve the desired qualities of digital print paper. It has been
discovered that use of starch as the first binding component requires to be supplemented by use
of a latex or copolymer binder by way of a second binder component, to ensure that the applied
pigment coating prevents dusting problems associated with short fiber agro residue pulps.
Simultaneously, it has been discovered that using the latex or copolymer binder by way of a second
binder component in quantities outside of the ranges prescribed in the specification, results in
undesired coloring or loss of brightness and poor visual appeal of the resulting pigment coated
paper product. Likewise, it has been discovered that addition of a plastic binder by way of a third
binder component, and within the specific quantities or ranges prescribed above results in
significant improvements to print gloss of the resulting digital printing paper and sharpness of
print on the resulting digital printing paper.
[0049] In a specific embodiment of the invention, the binding component within the pigment
coating comprises between 4 7 and 154 parts (by weight) of the pigment coating. In more particular
embodiment, (i) the first binder component comprises between 40 and 120 parts (by weight) of
the pigment coating (ii) the second binder component comprises between 3 and 17 parts (by
weight) of the pigment coating and / or (iii) the third binder component comprises between 4 and
17 parts (by weight) of the pigment coating.
[0050] The pigment coating may additionally comprise an alkali (such as for example, caustic soda
or ammonia) as a buffering agent to maintain a target pH level. The alkali may be present within
the pigment coating aqueous suspension in a quantity sufficient to ensure a target pH of between
6.0 and 9.5, or between 8.5 to 9, and in certain embodiments approximately 7.5 pH.
[0051] The pigment coating may include a cross-linking agent. The cross-linking agent is a
hydrophobic chemical which helps to provide water resistance properties to natural binders within
the binding component (e.g. the starch component within the binding component) and improves
water resistance properties of the underlying paper substrate. In a preferred embodiment, the
cross-linking agent is a reactive poly-hydroxyls based reagent, ·such as for example the
commercially available cross-linking agent distributed under the trade name Cartabond FI
Liquid™.
[0052] The cross linking agent may in an embodiment comprise dry content of between 40% and
45% (and preferably 40%) prior to being combined with other constituents of the pigment coating.
[0053] In an embodiment of the invention, the pigment coating comprises between 1 and 7 parts
(by weight) of the cross-linking agent. In a specific embodiment, the pigment coating comprises 3
parts (by weight) of a reactive poly-hydroxyls based reagent, by way of the cross-linking agent.
[0054] The pigment coating may further include a lubricant. The lubricant is a softener which
controls viscosity and prevents the pigment coating from becoming britde and cracking during the
process of application to the underlying paper substrate (e.g. during calendaring). In an
embodiment, the lubricant may achieve the necessary softening effect by controlling viscosity of
the pigment coating. The lubricant may in an embodiment comprise a calcium stearate (such as
the lubricant marketed under the trade name Sapcote C104TM).
8
[0055] The lubricant may exhibit solids levels of between 45% and 50% prior to being combined
with other constituents of the pigment coating.
[0056] In an embodiment of the invention, the pigment coating comprises between 0.3 and o~ 7
parts (by weight) of the lubricant. In a specific embodiment, the pigment coating comprises 0.7
parts (by weight) of a calcium stearate, by way of the lubricant.
[0057] The pigment coating may include an optical brightening agent - which improves the
brightness and overall visual appeal of the pigmented paper product. The optical brightening agent
may in an embodiment comprise a urea free hexa sulphonated stilbene optical brightener (such as
the optical brightening agent marketed under the trade name Deepwhite AS Liquid CI3S7TM).
[0058) The optical brightening agent may exhibit solids levels of between 17% and 25% prior to
being combined with other constituents of the pigment coating.
[0059) In an embodiment of the invention, the pigment coating comprises between 0.2 and 6.0
parts (by weight) of the optical brightening agent. In a specific embodiment, the pigment coating
comprises 2.8 parts (by weight) of a urea free hexa sulphonated stilbene optical brightener, by way
of the optical brightening agent.
[0060) The pigment coating may additionally comprise a thickener for controlling the viscosity of
the pigment coating. It has been discovered that for optimal application of the pigment coatings
of the present invention to an underlying wheat straw composite paper substrate, viscosity of the
pigment coating is required to be maintained within the range 20 cP to 1000 cP. Thickener may
accordingly be added to the pigment coating in quantities sufficient to achieve the desired viscosity
of the pigment coating.
[0061) The thickener may in an embodiment comprise an aqueous anionic dispersion of a
copolymer of ethyl acrylate and carboxylic acids (such as the thickener marketed under the trade
name Sterocol FDTM). The thickener may exhibit solids levels of between 20% and 30% (and in a
preferred embodiment 25.6%), prior to being combined with other constituents of the pigment
coating.
[0062} In an embodiment of the invention, the pigment coating comprises between 0.01 and 0.5
parts (by weight) of the thickener. In another embodiment of the invention, the thickener may be
entirely omitted from the pigment coating.
[0063} Tables 1 to 5 summarise working examples of pigment coatings developed in accordance
with the teachings of the present invention.
[0064] Table 1 presents data corresponding to an exemplary pigment coating developed using wet
ground calcium carbonate (WGCC), wherein 90% of the WGCC particles have size of 2 microns
or less, the WGCC having a solids level of 70.5%. The working example was carried out with a
view to develop a pigment coating having a target solids level of 25% without addition of thickener
to the pigment coating.
9
Table 1
Pigment 100
component
(Wet ground
calcium
carbonate
having between
Example 90% and 95% 25 21.6 7.8
1 particles of
particle size less
than2
First 100 20-30
component
(Oxidised
Starch derived
from
Second binder 7 50.5
component
(Styronal
DD71
Third 6 45-50
component
(Styronal
PR875
Alkali (caustic As necessary
soda at to achieve
concentrations target pH (in
of between 1 00 Example 1,
and 300 target pH
gm/litre) was between
8.0 and 9.5
Cross-linking 3 41.3
agent
(Carta bond FI
Lubricant 0.7 45-50
(Sapcote
C1
Optical 2.8 17-25
brightening
agent
AS w
10
Liquid
CI357™)
Thickener 0
(Sterocol FDTM)
Water As necessary
to achieve
target solids
levels (in
Example 1,
target solids
level was
25%)
[0065) Table 2 presents data corresponding to an exemplary pigment coating developed using wet
ground calcium carbonate (WGCC) wherein 90% of the WGCC particles have size of 2 microns
or less, said WGCC having a solids level of 70.5%. The working example was carried out with a
view to develop a pigment coating having a target solids level of 25% with addition of thickener
to the pigment coating.
Table 2
Pigment 100 70-75
component
(Wet ground
calcium
carbonate
having between
Example 90% and 95% 26 31.2 7.8
2 particles of
particle size less
than2
First binder 100 20-30
component
(Oxidised
Starch derived
from
Second binder 7 50~5
component
(Styronal
DD71
Third binder 6 45-50
11
(Styronal
PR8758apTM)
Alkali (caustic
soda at
concentrations
. of between 100
and 300
gm/litre)
Cross-linking
agent
(Cartabond FI
L!guidTM)
Lubricant
(Sapcote
C104TM)
Optical
brightening
agent
(Deepwhite AS
Liquid
CI357™)
Thickener
(Sterocol FDTM)
Water
As necessary
to· achieve
target pH (in
Example 1,
target pH
was between
8.0and 9.5
pH)
3
0.7
2.8
0.3
As necessary
to achieve
target solids
levels (in
Example 2,
target solids
level was
25%)
41.3
45-50
17-25
25.6
[0066] Table 3 presents data corresponding to an exemplary pigment coating developed using wet
ground calcium carbonate (WGCC) wherein 95% of the WGCC particles have size of 2 microns
or less, and said WGCC has a solids level of 74.7%. The working example was carried out with a
view to develop a pigment coating having a target solids level of 25% with addition of thickener
to the pigment coating.
Table 3
12
(Wet ground
calcium
carbonate
having between
Example 90% and 95% 24.7 72 7.9
3 particles of
particle size less
than 2 microns)
First binder 100 20-30
component
(Oxidised
Starch derived
from
Amyloplast 1 0)
Second· binder 7 50.5
component
(Styronal
DD718apTM)
Third binder 6 45-50
component
(Styronal
PR8758apTM)
Alkali (caustic As necessary
soda at to achieve
concentrations target pH (in
of between 1 OQ Example 1,
and 300 target pH
gm/litre) was between
8.0 and 9.5
pH)
Cross,. linking 3 41.3
agent
(Cartabond FI
LiquidTM)
Lubricant 0.7 45-50
(Sapcote
C104TM)
Optical 2.8 17-25
brightening
agent
(Deepwhite AS
Liquid
CI357TM)
Thickener 0:3 25.6
(Sterocol FDTM)
Water As necessary
to achieve
target solids
levels (in
Example 3,
target solids
13
f level
~5%)
was. t
[0067] Table 4 presents data corresponding to an exemplary pigment coating developed using wet
ground calcium carbonate (WGCC) wherein 95% of the WGCC particles have size of 2 microns
or less, said WGCC having a solids level of 74.7%. The working example was carried out with a
view to develop a pigment coating having a target solids level of 40% without addition of thickener
to the pigment coating.
Table 4
Pigment 70-75
component
(Wet ground
calcium
carbonate
having between
Example 90% and 95% 39.4 787 7.8
4 particles of
particle size less
than 2
First binder 100 20-30
component
(Oxidised
Starch derived
from
7 50.5
component
(Styronal
DD71
Third binder 6 45-50
component
(Styronal
PR87
Alkali (caustic As necessary
soda at to achieve
concentrations target pH (in
of between 100 Example 1,
and 300 target pH
gm/litre) was between
8.0and9.5
14
Cross-linking 3 41.3
agent
(Carta bond FI
Liquid™)
Lubricant 0.7 45-50
(Sapcote
C10:4TM)
Optical 2.8 17-25
brightening
agent
(Deepwhite AS
Liquid
CI357™)
Thickener 0
(Sterocol FDTM)
Water As necessary
to achieve
target solids
levels (in
Example 4,
target solids
level was
.40%)
[0068] Table 5 presents data corresponding to an exemplary pigment coating developed using wet
ground calcium-carbonate (WGCC} wherein 95% of the WGCC particles have size of 2 microns
or less, said WGCC having a solids level of 7 4. 7%. The working example was carried out with a
view to develop a pigment coating having a target solids level of 40% with addition of thickener
to the pigment coating.
Table 5
Pigment 100 70-75
component
(Wet ground
calcium
carbonate
having between
Example 90% and 95% 40.2 800 8.0
5 particles of
particle size less
than 2
First binder 100 20-30
t
15
/
(Oxidised
Starch derived
from
Amyloplast 1 0)-
Second binder
component
(Styronal
DD718ap™)
Third binder
component
(Styronal
PR8758lp™)
Alkali (caustic
soda at
concentrations
of between 1 00
and 300
gm/litre)
Cross-linking
agent
(Cartabond FI
LiquidTM)
Lubricant
(Sapcote
C104TM)
Optical
brightening
agent
(Deepwhite AS
Liquid
CI357TM)
Thickener
(Sterocol FDTM)
Water
7
6
As necessary
to achieve
target pH (in
Example 1,
target pH
was between
8.0 and 9.5
pH)
3
0.7
2.8
0.3
As necessary
to achieve
target solids
levels (in
Example 5,
target solids
level was
40%)
50.5
45-50
41.3
45-50
17-25
25.6
[0069] Figure 2 illustrates an exemplary embodiment of a facility used to develop pigment coatings
in accordance with the teachings of the present invention. In the illustrated embodiment, facility
200 comprises hopper 202, conveyer 204, cooker 206, screen 208 and storage tank 210.
16
[0070] Hopper 202 may comprise a hopper used to store starch or other embodiments of the first
binder component of the pigment coating. The starch or first binder component is transferred by
means of conveyer 204 to cooker 206-.
[0071] While cooker 206 may have any dimension or configuration suitable to the specific
pigment coating under manufacture, for the purposes of generating the data in Tables 1 to 5,
cooker 206 was a starch cooker having a 4 m3 capacity. Temperature of cooker 206 may be
modulated using steam or any other appropriate means for temperature regulation.
[0072] Cooker 206 serves as a receptacle into which various pigment coating constituents may be
added in appropriate quantities, at appropriate concentrations, at appropriate temperatures and /
or at appropriate solids levels to achieve the target pigment coatings.
[0073] In an embodiment of the invention, starch conveyed to cooker 206 may be oxidised by
cooking at .a temperature of 90 °C or above - which serves to modify the molecules to first swell
and burst into uniform molecules suitable for binding the pigment component. Cooking of the
starch may be accompanied by agitation or stirring (for example by one or more agitators within
cooker 206) for 20 minutes or more at temperatures of 90 oc or above.
[0074] The pigment component (for example wet ground calcium carbonate) may be added to
cooker 206 and blended with the cooked starch. The first binder component may thereafter be
added to cooker 206 and blended. Likewise, the second binder component may subsequendy be
added into cooker 206 and blended.
[0075] One or more of optional constituents such as the third binder component, alkali, crosslinking
agent, lubricant, optical brightening agent, thickener and water may also be added and
agitated or blended to ensure that aqueous suspension-like properties are achieved.
[0076] Addition and blending of each constituent within cooker 206 may be accompanied by a
corresponding resting time of 4 to 8 minutes.
[0077] It would be understood that each of the constituents added to cooker 206 may be in
accordance with the quantities, concentrations and solid levels already described hereinabove.
[0078] The resulting aqueous suspension is passed through screen 208, which may in an
embodiment comprise a vibrating screen or particulate filter with pores sized to remove unwanted
particulate matter. The aqueous suspension is passed through screen 208 and into storage tank
210, where it is maintained at between 45 octo 55 oc and between 8.0 and 9.5 pH, for subsequent
application onto a paper substrate at a size press or other appropriate mechanism. Storage tank
210 may include one or more agitators to maintain a suspension state of the pigment coating.
[0079] While storage tank 210 may have any dimension or configuration suitable to the specific
pigment coating under manufacture, for the purposes of generating the data in Tables 1 to 5,
storage tank 210 was a tank having a 12m3 capacity.
[0080] Figure 3 illustrates an exemplary system 300 for manufacturing a pigment coated paper
product in accordance with the present invention.
17
.,-_
0..
[0081) Octopus central header 302 provides an inlet for paper stock slurry, by providing a set of
uniformly spaced inlet orifices for passing paper stock slurry into head box 304. It would be
understood that for the purposes of the invention, the paper stock slurry introduced through
octopus central header 302 may comprise a combination of agro residue and hardwood pulp,
softwood pulp and / or bleach chemi thermo mechanical pulp in accordance with the paper
substrate constituents discussed in detail hereinabove. In an embodiment of the invention, the
paper stock may comprise an agro residue derived first substrate component of between 40% and
75%, preferably between 45% and 65%, and most preferably between 55% and 60% of the total
paper stock weight. The second substrate component may comprise one or more of hardwood
pulp, softwood pulp, bleach chemi thermo mechanical pulp and one or more additives or fillers.
In an embodiment of the invention, the second substrate component comprises between 25% and
60%, preferably between 35% and 50%, and most preferably between 40% and 45% of the total
paper stock weight. In a particular embodiment, the second substrate component comprises
between 40% and 45% of the total paper stock weight, and is composed of hardwood pulp or
softwood pulp, bleach chemi thermo mechanical pulp, or a combination thereof.
[0082) Head box 304 pushes the paper stock slurry though a nozzle, whereafter wire part 306 and
top former 308 are used to dewater the paper stock slurry and to form a layer of paper substrate
(i.e. web). Nip press 310 (which may include one or more than one nips) thereafter receives and
further dewaters the web, whereinafter the dewatered web is passed through heated cylinders
within pre-dryer 312, resulting in a dried web or paper substrate. The dried paper substrate
thereafter moves through size press 314, at which stage pigment coating received from storage
tank 210 (of Figure 2) may be applied to one or both surfaces of the paper substrate.
[0083) In an embodiment of the invention the size press may be specifically configured to apply
the pigment coating of the present invention to the paper substrate of the present invention in a
manner that achieves the coat weight parameters described in detail hereinabove.
[0084] The pigment coated paper is thereafter passed through post-dryer 316 - which in an
embodiment results in a dried pigment coated paper having approximately between 4.4% and 6.5%
moisture content. The coated paper is passed through calendar 318 ·which smooths the paper,
through quality control scanner 320, and finally through pope reel 322, which is configured to
wind the pigment coated paper onto a paper roll.
[0085) In various working examples using the above described production facilities and
machinery, pigment coated paper generated in accordance with the teachings of the present
invention were found to exhibit (i) moisture content less than 6.5% (ii) smoothness levels less than
170 mL/min on each of the top former and wire sides respectively (iii) taber stiffness greater than
2.5 in machine direction, and taber stiffness greater than 1.5 in cross direction, (iv) brightness
greater than 89.0% ISO (v) CIE whiteness greater than 145 (vi) Opacity greater than 95%.
[0086] It would be understood that the examples and embodiments discussed in the present
specification are illustrative only. Those skilled in the art would immediately appreciate that various
modifications in form and detail may be made without departing from or offending the spirit and
scope of the invention as defined by the appended claims.

We claim:
1. A pigmented paper product comprising:
a paper substrate having a first surface and a second surface, said paper substrate
comprising a first substrate component and a second substrate component, wherein the first
substrate component comprises wheat straw pulp, said wheat straw pulp comprising between 45%
and 65% by weight of the paper substrate; and
a pigment coating applied to at least one of the first and second surfaces of the paper
substrate, the pigment coating comprising:
a pigment component; and
a binding component, comprising at least a first binder component and a second
binder component, for binding the pigment component with the first substrate component
and the second substrate component.
2. The pigmented paper product as claimed in claim 1, wherein coat weight of the pigment
coating applied to the paper substrate comprises between 4 gm/m2 and 8 gm/m2
•
3. The pigmented paper product as claimed in claim 1, wherein the pigment component
comprises 100 parts by weight of particles of wet ground calcium carbonate.
4. The pigmented paper product as claimed in claim 3 , wherein at least 90% of the particles
of wet ground calcium carbonate have a particle size of 2 microns or less.
5. The pigmented paper product as claimed in claim 1, wherein the ftrst binder component
comprises between 40 and 120 parts by weight of oxidised starch.
6. The pigmented paper product as claimed in claim 1, wherein the second binder component
comprises between 3 and 17 parts by weight of a latex binder or a copolymer binder.
7. The pigmented paper product as claimed in claim 1, wherein the binding component
comprises a third binder component, said third binder component comprising between 4 and 17
parts by weight of a plastic binder.
8. The pigmented paper product as claimed in claim 1, wherein the pigment coating
comprises between 1 and 7 parts by weight of a cross-linking agent.
9. The pigmented paper product as claimed in claim 1, wherein the pigment coating
comprises between 0.3 and 0.7 parts by weight of a lubricant.
10. The pigmented paper product as claimed in claim 1, wherein the pigment coating
comprises between 0.2 and 6.0 parts by weight of an optical brightening agent.
19
11. The pigmented paper product as claimed in claim 1, wherein the pigment coating
comprises upto 0.5 parts by weight of a thickener and a buffering agent.
12. The pigmented paper product as claimed in claim 1, wherein the second substrate
component comprises hardwood pulp.
13. The pigmented paper product as claimed in claim 12, wherein the second substrate
component comprises between 35% and 55% by weight of the paper substrate.
14. A pigment coating for a paper substrate comprising a first substrate component and a
second substrate component, wherein the first substrate component comprises wheat straw pulp,
said wheat straw pulp comprising between 45% and 65% by weight of the paper substrate, and the
second substrate component comprises hardwood pulp, said second substrate component
comprising between 35% and 55% by weight of the paper substrate, the pigment coating
compnsmg:
a pigment component applied to the paper substrate , the pigment component comprising
100 parts by weight, wherein at least 90% of the pigment component particles have a particle size
of 2 microns or less; and
a binding component for binding the pigment component with the first substrate
component and the second substrate component, the binding component comprising between 40
and 120 parts of a first binder component and between 3 and 17 parts of a second binder
component; and
optionally, one or more of:
between 4 and 17 parts of a third binder component;
between l.and 7 parts by weight of a cross-linking agent;
between 0.3 and 0. 7 parts by weight of a lubricant;
between 0.2 and 6.0 parts by weight of an optical brightening agent;
upto 0.5 parts by weight of a thickener; and
a buffering agent.
15. A method for manufacturing a pigmented paper product, the method comprising the steps
of:
preparing a paper substrate comprising a first substrate component and a second substrate
component, the first substrate component comprising wheat straw pulp, said wheat straw pulp
comprising between 45% and 65% by weight of the paper substrate, the second substrate
20
component comprising hardwood pulp, said second substrate component comprising between
35% and 55% by weight of the paper substrate; and
applying a pigment coating to the first surface and the second surface of the paper
substrate, such that coat weight of the pigment coating applied to the paper substrate is between
4 gm/m2 and 8 gm/m2
•
16. The method as claimed in claim 15, comprising: preparing a pigment coating using a blend of
the pigment component with a binding component, the pigment component comprising 100 parts
by weight, wherein at least 90% of the pigment component particles have a particle size of 2
microns or less.
17. The method as claimed in claim 16, wherein the binding component comprises between 40
and 120 parts of a first binder component and between 3 and 17 parts of a second binder
component.
18. The method as claimed in claim 15, wherein the pigment coating comprises one or more
of:
between 4 and 1 7 parts of a third binder component;
between 1 and 7 parts by weight of a cross-linking agent;
between 0.3 and 0. 7 parts by weight of a lubricant;
between: 0.2 and 6.0 parts by weight of an optical brightening agent;
upto 0.5 parts by weight of a thickener; and
a buffering agent;
19. A pigmented paper product or a pigment coating substantially as described in the written
description and. accompanying drawings.
20. A method for manufacturing a pigmented paper product substantially as described in the
written description and accompanying drawings.