Note: Descriptions are shown in the official language in which they were submitted.
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Paper pigment, process for producing a paper product and paper product
Field of invention
This invention relates to new organic paper pigments, a process for
manufacturing
a paper product, the use of the paper pigments according to the invention in
the
manufacture of a paper product, and to new paper products.
Prior art technology
Fillers and pigments in paper manufacture
Paper pigments can be divided into filler pigments or fillers and precoating
and
surface coating pigments. Except for some special surface coating pigments,
paper pigments are inorganic mineral particles in structure. The pigments used
for
surface finishing are generally more finely divided than filler pigments, but
the
particle size of both is such that they contribute notably to the increase of
the
reflection surface. Paper pigments are light-scattering materials.
One of the most important characteristics strived for with pigments is thus
improvement of optical properties, such as opacity and gloss and even
porosity, of
the final product, i.e. paper or board. Through better surface properties, the
printing properties improve, the distribution of printing ink becomes more
uniform,
and the gloss can be further fine-adjusted by means of calendering. In
addition to
better characteristics, the use of paper pigments also influences the price of
the
final product. Replacing 1% of expensive fiber with a filler the price of the
final
product reduces 2.5 US$/ton, Baker, C. and Nazir, B., Practical ways for
achieving
higher filler content papers, Use of Minerals in Papermaking, Pira
International,
Surrey, U.K., (1988), pp. 83-92.
The availability of wood fiber has certain limits, and hence the use of paper
pigments also has significance in the environmental protection. Increased use
of
paper pigments has thus opened possibilities for increasing the manufacture of
paper products without a need to increase the use of wood.
The use of paper pigments of course has also its limits. When certain limits
are
exceeded, the tensile properties of paper decrease dramatically and the need
for
using expensive additional chemicals, for example, increases. In addition,
mineral
pigments are hard materials in structure, causing easily wear in paper
machines,
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finishing machines and printing machines. An excessive amount of mineral
particles also generates linting during printing.
Paper pigments are typically used in relatively great amounts in fine papers
and
magazine printing papers, but their use is increasing also in normal newsprint
as
well as in packing papers. Special products, such as laminated papers, bible
paper, tobacco paper, etc. easily contain 40% of paper pigments. In newsprint
the
amount of paper pigments is today between 0-10% (kaolin, talc, special
pigments), 20-30% (kaolin, talc) in uncoated magazines (SC), 0-25% in fine
papers (kaolin, talc, chalk, TiO2), and 0-10% in packing papers (kaolin, talc,
chalk,
TiO2).
Some of the properties required of the present paper pigments are:
= they must be chemically inert and insoluble in water
= high retention in paper machine
= high refractive index to achieve good opacity
= high light-scattering coefficient
= low density
= soft structure
= low price
An optimum particle size for a paper pigment would be 0.2-0.3 pm, i.e.
approximately half of the average light wavelength, which thus produces
maximum
opacity. For keeping the refining costs at an economically reasonable level, a
typical particle size of a paper pigment in the filler use, for example, is
approximately 0.4-5 pm.
The present mineral-based paper pigments can be classified as natural and
synthetic materials. The former are of course less expensive, the latter often
possess some of the above mentioned desired properties. Typical mineral-based
paper pigments include titanium dioxide, kaolin, calcined kaolin, talc,
gypsum,
chalk, ground calcium carbonate (GCC), precipitated calcium carbonate (PCC),
barium sulphate, Na Al silicate (Zeolex), Ca Al silicate, aluminium oxide,
kieseiguhr, zinc oxide, etc. In addition, a more advanced calcium oxalate
based
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paper pigment has been introduced to the market, which reduces somewhat the
ash content of the final product and has lower density.
Calcium carbonate is an alkaline paper pigment and hence it dissolves
completely
in water even at pH 6.5. Therefore its use is restricted to neutral and
alkaline
conditions only. For this reason the use of carbonates is strongly emphasized
in
the manufacture of wood-free papers.
Although the properties of inorganic paper pigments, such as the crystal
structure,
have been developed to allow decreasing the density, for example, they are
still
relatively heavy as "stone materials". In addition, inorganic paper pigments
notably
reduce the calorific value of paper products and prevent or significantly
reduce
their utilization possibilities after use for example as a source of energy by
burning.
Despite of deinking, it is estimated that 24 millions of tons of paper
products are
taken to the dump areas every year in Europe only. The energy content of such
an
amount of paper products is approximately 8 Mtoe (Million tons oil
equivalent).
Disaccharides
Disaccharides can be defined as any sugars that are composed of two
monosaccharide units. They are crystalline carbohydrates, dissolving extremely
well in water due to their hydroxyl functions, of which a part is natural and
a part is
synthetic. The most common disaccharides are natural saccharose (cane sugar
and beet root sugar) and lactose (milk sugar) as well as maltose (a hydrolysis
product of starch) and cellobiose (a hydrolysis product of cellulose). The
greatest
production amounts are found with saccharose, which is produced approximately
134.1 millions of tons per year (in 2002). Due to its great production
amounts,
particularly saccharose but also lactose, which is generated in the waste flow
of
the milk processing industry, are inexpensive products that are available
worldwide and all year round.
Disaccharides are optically active compounds, i.e. they contain chiral carbon
atoms. For example, saccharose consists of two optically active
monosaccharides,
namely D-glucose and D-fructose. Lactose, in turn, is composed of D-galactose
and D-glucose. These two monosaccharides are joined together via a glucose
link.
This link may have further an a- or (3-configuration and the disaccharide may
be a
mixture of a disaccharide comprising these two different link configurations.
Maltose and cellobiose differ from each other in structure only with respect
to this
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glucose link; both have two glucose units, which are combined with an a-
glucoside
link in maltose and with a(3-glucoside link in cellobiose.
Among disaccharides, saccharose is used as such as a food product and as an
ingredient of food products. It is also used, like lactose, for example as
additive
components for pharmaceutical products due to its good preservability and
harmlessness.
These have also been used as starting materials in pharmaceutical synthetics,
in
which case it has been possible to protect the hydroxyl groups of
disaccharides for
a later selective disassembly for example as esters, typically as acetates due
to
their preferredness. The chemical and physical properties of such products are
greatly influenced by the type of the disaccharide and its stereochemistry.
For
example, the melting points of saccharose, lactose, maltose and cellobiose
octaacetates change owing to minor structural differences in the following
way: a-
saccharose octaacetate: about 82-83 C, P-lactose octaacetate: about 136-137 C,
P-maltose octaacetate: about 159-160 C, and a-cellobiose octaacetate about
225 C. Since the acetate esters of disaccharides, for example, are inert
molecules
as such, no other noteworthy use has been found for them. Therefore, the
production of disaccharide derivatives comprising longer alkyl or alkyl aryl
ester
chains has been insignificant.
Summary of invention
An objective of this invention is an organic paper pigment, poorly soluble in
water,
consisting of ester derivatives of disaccharide, the particle size of which is
0.15-50
pm.
Further, an objective of this invention is a process for producing a paper
product,
in which process an ester derivative of disaccharide, essentially insoluble in
water,
is used as a paper pigment.
Another objective of the invention is the use of an essentially insoluble
disaccharide as a paper pigment in the manufacture of a paper product.
Further, an objective of this invention is a paper product, for which paper
product
an ester derivative of disaccharide, poorly soluble in water, with a particle
size of
0.15-50 pm, has been used as a paper pigment.
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The invention now discovered is based on the surprising observation that
particles
composed of ester derivatives of disaccharides can be used as paper pigments.
This type of paper pigment is poorly soluble in water and has a high calorific
value
being an organic material. In addition, it is very lightweight and has good
5 brightness and a sufficiently high melting point to be used as a paper
pigment. Its
properties can be adjusted by the selection of the disaccharide and the
esterification substrate, for example, as well as by the selection of the
esterification degree. In addition, the stereochemical structure of the paper
pigment and thus the properties can be adjusted by the selection of the
esterification catalyst.
The paper pigments according to the invention can surprisingly be used as
fillers,
precoating pigments and coating pigments. The paper pigment can be refined to
a
suitable size for each purpose.
The paper product, in which the paper pigments according to the invention have
been used, surprisingly showed excellent optical properties. Using the paper
pigment according to the invention, better opacity and light-scattering
coefficient
were achieved than for example with a high-opacity mineral pigment developed
for
a corresponding purpose. In addition, the tensile properties of the paper
product
were surprisingly at the same level.
As the paper pigment according to the invention is organic, the calorific
value of
the paper products to be manufactured is notably higher than in corresponding
paper products containing mineral-based paper pigments. This opens a
possibility
for a later use of the paper product or the slurry separated from it through
deinking
as bioenergy in combustion plants, for example.
The paper products manufactured according to the invention are also
lightweight.
Grammages can be reduced without impairing the working characteristics of the
final product. This reduces notably the transport costs, for example.
As the paper pigments according to the invention are also soft, they do not
cause
wear in the paper, board, finishing and printing machines in the same way as
the
currently used hard mineral pigments.
Figures
Figure 1 is a SEM image of refined lactose octaacetate (LOA), used in sheet
tests, having an average particle size of less than 2 pm,
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Figure 2 is a SEM image of a high-opacity PCC filler (Albacar HO) used as the
reference filler, having an average particle size of less than 2 pm,
Figure 3 shows the tensile index (Nm/g) as a function of the filler content
(%)
for test sheets, in which lactose octaacetate (LOA) or PCC has been
used as filler,
Figure 4 shows opacity (%) as a function of the filler content (%) for test
sheets,
in which lactose octaacetate (LOA) or PCC has been used as filler,
Figure 5 shows gloss (%) as a function of the filler content (%) for test
sheets,
in which lactose octaacetate (LOA) or PCC has been used as filler,
Figure 6 shows the light-scattering coefficient (m2/kg) as a function of the
filler
content (%) for test sheets, in which lactose octaacetate (LOA) or
PCC has been used as filler.
Detailed description of invention
The invention relates to a paper pigment, which is an ester derivative of a
disaccharide, essentially insoluble in water, having a particle size of 0.15-
50 pm.
Preferably the particle size of the paper pigment according to the invention
is
0.15-8 pm, and more preferably between 0.5-2 pm.
The ester derivative of the disaccharide can be regarded here as essentially
insoluble in water, of which a maximum of 2.5% by weight dissolves in water at
the
room temperature, preferably less than 1.5% by weight; and more preferably
less
than 1 % by weight.
As the carboxylate chain of the ester derivative of the disaccharide according
to
the invention is short (C2 i.e. acetate), a high, preferably a quantitative
degree of
convertion to esters of hydroxyl functions of the disaccharides is required to
achieve poor solubility in water for the paper pigment according to the
invention.
Preferred C2 carboxylate derivatives of disaccharides include lactose,
saccharose,
maltose and cellobiose octaacetates, for example.
When the number of carbon atoms in the carboxylate chain increases, it may not
be necessary any more to convert all hydroxyl functions of the disaccharides
to
esters for achieving the poor solubility in water according to the invention.
The
paper pigment according to the invention may be C2-C18 carboxylate of
disaccharide, preferably it is C2-C8 carboxylate and more preferably
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C2-C6 carboxylate, with either a quantitative or a partial esterification
degree of the
hydroxyl functions of the disaccharides. According to the invention the carbon
atoms of C2-C18 carboxylate can form straight or branched alkyl chains and
saturated or unsaturated ring structures. Further, the ester functions of the
disaccharide can be C2-C1$ carboxylates that are either similar with each
other or
alternatively different from each other.
The melting point of the paper pigment according to the invention should be
over
75 C. Preferably it is over 85 C and more preferably the melting point of this
ester
derivative of the disaccharide is over 100 C.
The disaccharide of the ester derivative of the disaccharide according to the
invention can be for example any stereoform of lactose, saccharose, maltose or
cellobiose, or a mixture of these stereoforms. The ester derivative of the
disaccharide according to the invention can also be a mixture of the above
mentioned disaccharides and further of their various stereoforms. Neither does
the
invention exclude the use of other disaccharides and their various stereoforms
as
the disaccharide of the ester derivative.
The esterification of disaccharides can be performed, for example, by using
natural carboxyl acids or their derivatives, such as anhydrides or acid
chlorides.
Especially preferred substrates are anhydrides, such as acetic anhydride. The
stereochemistry of the product created can be adjusted by the selection of the
catalyst, for example. It is also possible to use synthetic esterification
substrates.
A specially preferred paper pigment according to the invention is lactose
octaacetate. Another preferred paper pigment according to the invention is
saccharose octaacetate.
The objective of the invention is also a process for producing a paper
product, in
which process an ester derivative of a disaccharide, essentially insoluble in
water,
is used as a paper pigment. The paper pigment according to the invention is
defined as above.
In one embodiment of the process according to the invention, the paper pigment
particle formed by the ester derivative of the disaccharide essentially
insoluble in
water can be a filler, the particle size of which is 0.15 pm - 50 pm,
preferably 0.5
pm - 8 pm; and most preferably 1pm - 2 pm.
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In another preferred embodiment of the process according to the invention, the
paper pigment particle formed by the ester derivative of the disaccharide
essentially insoluble in water can be a precoating pigment, the particle size
of
which is 0.15 pm - 2 pm, preferably 0.5 pm - 1.5 pm.
Further, in another preferred embodiment of the invention, the paper pigment
particle formed by the ester derivative of disaccharide essentially insoluble
in
water can be a surface coating pigment, the particle size of which is 0.15 pm -
2 pm, preferably 0.5 pm -1.0 pm.
The invention also relates to the use of an ester derivative of a disaccharide
essentially insoluble in water in the manufacture of a paper product, in which
this
ester derivative of the disaccharide is used as a paper pigment, the particle
size of
which is 0.15 pm - 50 pm. The paper pigment according to the invention is
defined
as above.
The 'paper product' is used here to refer to paper or board of any type or of
any
weight, white or colored.
The paper pigment according to the invention can be used as a filler in the
manufacture of the paper product. In this case the particle size of ester
derivatives
of the disaccharide essentially insoluble in water is 0.15 pm - 50 pm,
preferably
0.5 pm - 8 pm; and most preferably 1 pm - 2 pm.
Further, the paper pigment according to the invention can be used as a
precoating
pigment in the manufacture of a paper product. In this case the particle size
of
ester derivatives of the disaccharide essentially insoluble in water is 0.15
pm - 2
pm, preferably 0.5 pm - 1.5 pm.
In yet another preferred embodiment of the invention, the paper pigment
according
to the invention can be used as a surface coating pigment in the manufacture
of
the paper product. In this case the particle size of ester derivatives of the
disaccharide essentially insoluble in water is 0.15 pm - 2 pm, preferably 0.5
pm -
1.0 pm.
Preferably the paper pigment according to the invention has good brightness;
however, pigments with lower brightness can also be used to provide opacity in
paper products for which brightness is not an important criterium.
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The invention also relates to a paper product, in which an ester derivative of
a
disaccharide, poorly soluble in water, the particle size of which is 0.15 pm -
50
pm, has been used as a paper pigment. The paper pigment according to the
invention and the particle sizes providing various preferred properties to the
paper
product are defined as above.
In the paper product according to the invention, the portion of the ester
derivative
of the disaccharide, poorly soluble in water, from the total mass of the final
product
can vary between 1-55 % by weight, preferably 3-40 % by weight and most
preferably 5-35 % by weight.
The main benefits of the invention now discovered include the following facts
and
characteristics:
= organic paper pigments, which have
o very low density
o excellent optical properties
o very poor water solubility
o high brightness
o high calorific value
o soft structure and easy workability to a suitable particle size
o resistance in even acid paper production processes
= production of paper pigments using disaccharides
o disaccharides are a feedstock available worldwide and all year round
o especially saccharose and lactose commodities
o esterification reagents are typically natural carboxyl acids or their
derivatives
o inexpensive feedstock and production processes
o the properties of the paper pigment are adjustable by changing the
feedstock saccharide and esterification function
= due to their softness the paper pigments cause less abrasion in paper
machines, finishing machines and printing machines
= new environmental-friendly paper products, which
o have a high calorific value and can thus be used as bioenergy
o are high-quality products with excellent optical properties
~ opacity
~ light-scaftering coefficient
~ gloss, etc.
o are lightweight
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~ reduce transport costs
~ are user-friendly
o have good strength properties.
5
Examples
The paper pigment according to the invention and its use for manufacturing a
paper product as well as the thus manufactured paper product are described
10 below without restricting the invention, however, to the examples set
forth.
Example 1
Lactose octaacetate
Lactose octaacetate (P-lactose octaacetate) to be used as a paper pigment was
produced in several batches with a standard procedure by esterification of a-D-
lactose (A.C.S reagent, Sigma-Aldrich) with an equimolar amount (8
equivalents)
of acetic anhydride using sodium acetate as catalyst, diagram 1(stereochemical
structures not considered). The progress of the reaction was monitored with
HPLC
and the conversions of the reaction were practically quantitative. After
carrying out
the reaction, the product was separated from the reaction mixture by
precipitating
with water. After this the product was flushed with water and dried in a
heating
chamber (105-115 C). The extremely high purity degree of the products was
still
verified with HPLC before using them in sheet tests.
OH OH OAc OAc
O O Ac20 O O
HO OH ' Ac0 OAc
NaOAc
HO OH HO OH AcO OAc OAc
Diagram 1.
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The water solubility of the produced P-lactose octaacetate was determined (DI-
water, pH 5.45) at the room temperature, and the water solubility was defined
to
be about 45 ppm. Thus the product prepared is extremely poorly soluble in
water.
The melting point of the prepared P-lactose octaacetate was measured and the
result was 135.5-137 C.
Before using the P-lactose octaacetate as a paper pigment, the product was
refined using a Dyno laboratory mill. The particle size distribution was
verified with
SEM and with a measuring device for the particle size distribution (Malvern).
Example 2
Several sheets with a grammage of 80 g/m2 were produced in the sheet tests
using the Ernst Haage sheet forming unit. In the sheet tests, a stock mixture
was
used, which included 70% of bleached softwood pulp and 30% of bleached
birchwood pulp (M-Real). The initial consistencies of the stocks were 3.9% for
hardwood pulp and 4.0% for birchwood pulp. The freeness degrees CSF
(Canadian Standard Freeness) of stock were about 500 for hardwood pulp and
about 450 for birchwood pulp. The retention aid used was Fennopol K3400R
(Kemira).
The previously prepared lactose octaacetate was used as filler in
consistencies of
about 5 and 9 percent. The reference filler used was a commercially available
high-opacity PCC filler (Albacar HO) in corresponding consistencies. The
technical
characteristics of the fillers are described in Table 1. LOA and PCC used in
the
sheet tests were scanned with SEM (Scanning Electron Microscopy), Figures 1
and 2. The average particle size of the fillers used is below 2 pm. It should
be
noted in the table that the density of lactose octaacetate is only 47% of the
density
of PCC used as the reference pigment.
Table 1
Particle size R457* Solubility Density Refractive
d50, index
pm
% ppm g/ml
LOA <2 92 45 1.27
PCC <2 96 - 2.7 1.6
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*R457 refers to ISO brightness. The ISO brightness or the diffuse blue
reflectance
factor is a reflectance factor, which is determined with a device whose
maximum
receptor sensitivity coincides with a wavelength of 457 nm.
When using lactose octaacetate as filler, water-based deforming agent was used
due to the low density and hydrophobicity of the filler (Aerotech 1630V,
Kemira; 2
ml/sheet mould).
Results of the sheet tests are shown in Table 2 and in Figures 3-6. Figure 3
shows the tensile index (Nm/g) as a function of the filler content (%), Figure
4
shows opacity (%), Figure 5 shows gloss (%) and Figure 6 shows the light-
scattering coefficient (m2/kg) as a function of the filler content (%).
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Table 2
Test No. 1 2 5 6
Filler type Ref Ref LOA LOA
PCC PCC
Filler content [%] 5.3 9.5 9 5.2
Grammage, g/mz 79.4 80.1 78.8 80.7
Total thickness, pm 164.7 170.7 167.6 167.2
Total density, kg/m3 4818 469.4 469.9 482.4
Tensile strength, kN/m 3.1 2.7 2.5 2.9
Tensile index, Nm/g 39.3 34.1 31.1 36.1
Opacity*, % 85.5 87.9 90.6 88.6
Gloss, % 86.1 86.9 83.9 84.6
Light-scattering coefficient**, 46.7 55.6 55.9 49.9
m2/kg
L' 96.3 96.6 94.8 95.2
a' -0.4 -0.3 -0.3 -0.4
b' 3.8 3.6 2.8 3.0
* opacity is a measure of untransparency of a paper product. The opacity of a
completely untransparent material is 100% and that of a completely transparent
material is 0%.
** the light-scattering coefficient is a measure indicating how well an
indefinitely
thin material layer scatters light.
As shown in Table 2 and Figures 3-6, lactose octaacetate (LOA) used as paper
pigment surprisingly provides the currently prepared paper sheets both a
higher
light-scattering coefficient and opacity than a corresponding amount of PCC
used
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as reference. These excellent optical properties were achieved without notable
impairing of the tensile index or tensile strength. The results are
surprising,
because LOA is an organic pigment, while PCC used as the reference pigment is
specifically of the high opacity quality. In this way it was possible to
increase the
amount of a low-density organic material in the paper product not only by
maintaining but also by simultaneously improving the optical properties of the
final
product without impairing notably the strengths of the final products.
Example 3
In the sheet tests, commercially available saccharose octaacetate (Aldrich)
with a
particle size of 13 pm was used as paper pigment. In the reference tests,
precipitated calcium carbonate (PCC) with a particle size of 0.7 pm was used
as
filler. The test arrangements were as in Example 2. Results of the sheet tests
are
shown in Table 3.
Table 3
Test No. 1 2 3 4 5
Filler type - PCC PCC SOA SOA
Filler content (% 0 10 20 10 20
Gramma e/m2 85.6 84.9 86.1 84.7 81.4
Total thickness, pm 124 124 120 142 137
Total density k/m 693 687 717 595 593
Tensile strength kN/m 6.58 4.51 3.17 2.96 3.67
Tensile index Nm/ 76.9 53.1 36.8 34.9 45.1
Opacity (%) 78.8 87.3 90.8 86.2 82.0
When using saccharose octaacetate as filler, it was observed that the total
density
of the sheets decreased clearly compared to the sheet, in which a pigment was
not used. The decrease of the total density is even more clearly visible when
comparing the results to the results obtained with an inorganic pigment.
Regardless of the total density decrease of the sheets, which is an preferred
aspect when producing paper, the tensile strength of the sheets treated with
SOA
did not deviate notably compared to the sheet treated with an inorganic
pigment.
Similarly, differences were not found in opacity compared to the sheets
treated
with an inorganic pigment, especially in the filler content of 10% (tests 4
and 2). It
should be noted that the particle size of the filler used is unusually large
compared
to the particle size of fillers that are normally used. The particle size of
calcium
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carbonate used as the reference material, in turn, was optimum for filler
applications.
Regardless of the dispreferred particle size, the optical properties achieved
in the
sheet tests 4 and 5 are comparable to the results obtained when using an
5 inorganic filler.
It is to be assumed that when a saccharose octaacetate with a smaller particle
size is used as filler, the properties of the obtained paper improve
essentially
compared to those gained in this test.
