Note: Descriptions are shown in the official language in which they were submitted.
WO 2010/122221 PCT/F12010/050316
1
PAPER PRODUCT
Field of the invention
The invention relates to a supercalendered paper product including a gypsum-
fibre
composite product as a coating pigment or a filler pigment. The invention also
re-
lates to a process for producing a supercalendered paper product and to the
use
of gypsum -fibre composite product as a coating pigment or a filler pigment in
the
production of a supercalendered paper product.
Background of the invention
A papermaking process starts with stock preparation where cellulosic fibers
are
mixed with water and mineral filler (usually clay or calcium carbonate or also
gyp-
sum). The obtained slurry is delivered by means of a head box on a forming
fabric
or press fabric or wire to form a fibrous web of cellulosic fibers at the
forming sec-
tion of the paper machine. Then water is drained in the draining section and
the
formed web is conducted to the press section including a series of roll
presses
where additional water is removed. The web is then conducted to the drying sec-
tion of the paper machine where most of the remaining water is evaporated typi-
cally by means of steam-heated dryer drums. Post drying operations include cal-
endering where the dry paper product passes between rolls under pressure,
thereby improving the surface smoothness and gloss and making the cali-
per/thickness profile more uniform. There are various calenders such as
machine
calenders where the rolls usually are steel rolls and include a heated roll
(thermo
roll), and supercalenders that use alternate hard and soft, heated rolls.
A supercalender is a stack of alternating hard and soft rolls through which
paper is
passed to increase its density, smoothness and gloss.
Gypsum or calcium sulphate dihydrate CaSO4-2H2O is suitable as material for
both coating pigment and filler, especially in paper products. Especially good
coat-
ing pigment and filler is obtained if the particular gypsum has high
brightness,
gloss and opacity. The gloss is high when the particles are sufficiently
small, flat
and broad (platy). The opacity is high when the particles are refractive,
small and
of equal size (narrow particle size distribution).
WO 2010/122221 PCT/F12010/050316
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The morphology of the gypsum product particles can be established by examining
scanning electron micrographs. Useful micrographs are obtained e.g. with a
scan-
ning electron microscope of the type Philips FEI XL 30 FEG.
The size of the gypsum product particles is expressed as the weight average di-
ameter D50 of the particles contained therein. More precisely, D50 is the
diameter of
the presumably round particle, smaller than which particles constitute 50% of
the
total particle weight. D50 can be measured with appropriate devices such as by
mi-
croscopy.
The flatness of a crystal means that it is thin. The form of flat crystals is
suitably
expressed by means of the shape ratio SR. The SR is the ratio of the crystal
length (the longest measure) to the crystal thickness (the shortest transverse
measure). By the SR of the gypsum product is meant the average SR of its indi-
vidual crystals.
The platyness of a crystal means that it is broad. Platyness is suitable
expressed
by means of the aspect ratio AR. The AR is the ratio between the crystal
length
(the longest measure) and the crystal width (the longest transverse measure).
By
the AR of the gypsum product is meant the average AR of its individual
crystals.
Both the SR and the AR of the gypsum product can be estimated by examining its
scanning electron micrographs. A suitable scanning electron microscope is the
above mentioned Philips FEI XL 30 FEG.
Equal crystal particle size means that the crystal particle size distribution
is nar-
row. The width is expressed as the gravimetric weight distribution WPSD and it
is
expressed as (D75-D25)/D50 wherein D75, D25 and D50 are the diameters of the
pre-
sumably round particles, smaller than which particles constitute 75, 25 and
50%,
respectively, of the total weight of the particles. The width of the particle
distribu-
tion is obtained with a suitable particle size analyzer such as the above
mentioned
type Sedigraph 5100.
Gypsum occurs as a natural mineral or it is formed as a by-product of chemical
processes, e.g. as phosphogypsum or flue gas gypsum. In order to refine the
gyp-
sum further by crystallising it into coating pigment or filler, it must first
be calcined
into calcium sulphate hemihydrate (CaSO4'1/2H2O), after which it may be
hydrated
back by dissolving the hemihydrate in water and precipitating to give pure
gypsum.
Calcium sulphate may also occur in the form of anhydrite lacking crystalline
water
(CaS04).
WO 2010/122221 PCT/F12010/050316
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Depending on the calcination conditions of the gypsum raw material, the
calcium
sulphate hemihydrate may occur in two forms; as a- and R-hemihydrate. The R-
form is obtained by heat-treating the gypsum raw material at atmospheric
pressure
while the a-form is obtained by treating the gypsum raw material at a steam
pres-
sure which is higher than atmospheric pressure or by means of chemical wet cal-
cination from salt or acid solutions at e.g. about 45 C.
WO 88/05423 discloses a process for the preparation of gypsum by hydrating cal-
cium sulphate hemihydrate in an aqueous slurry thereof, the dry matter content
of
which is between 20 and 25% by weight. Gypsum is obtained, the largest measure
of which is from 100 to 450 pm and the second largest measure of which is from
10 to 40 pm.
AU 620857 (EP 0334292 Al) discloses a process for the preparation of gypsum
from a slurry containing not more than 33,33% by weight of ground hemihydrate,
thereby yielding needle-like crystals having an average size of between 2 and
200
pm and an aspect ratio between 5 and 50. See page 15, lines 5 to 11, and the
ex-
amples of this document.
US 2004/0241082 describes a process for the preparation of small needle-like
gypsum crystals (length from 5 to 35 pm, width from 1 to 5 pm) from an aqueous
slurry of hemihydrate having a dry matter content of between 5 and 25% by
weight. The idea in this US document is to reduce the water solubility of the
gyp-
sum by means of an additive in order to prevent the crystals from dissolving
during
paper manufacture.
DE 32 23 178 C1 discloses a process for producing organic fibres coated with
one
or more mineral substances. One embodiment comprises mixing cellulose fibres,
gypsum and water. The mixture is compacted to give a plastic mass which subse-
quently is dried and mechanically comminuted to give fine particles. The
obtained
product can be used as an additive or filler e.g. in bitumen masses or
putties.
WO 2008/092990 discloses a gypsum product consisting of intact crystals having
a size from 0.1 to 2.0 pm. The crystals have a shape ratio SR of at least 2.0,
pref-
erably between 2.0 and 50, and an aspect ratio AR between 1.0 and 10,
preferably
between 1.0 and below 5Ø
WO 2008/092991 discloses a process for the preparation of a gypsum product
wherein calcium sulphate hemihydrate and/or calcium sulphate anhydrite and wa-
ter are contacted so that the calcium sulphate hemihydrate and/or calcium sul-
WO 2010/122221 PCT/F12010/050316
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phate anhydrite and the water react with each other and form a crystalline
gypsum
product. The formed reaction mixture has a dry matter content of between 34
and
84% by weight.
WO 2007/003697 discloses a method for coating cellulose particles, produced
from dissolved cellulose by precipitation, by contacting the cellulose
particles with
a light scattering material, followed by the attachment of the light
scattering mate-
rial on the surface of the cellulose particles. The size of the cellulose
particles is
between 0.05 and 10 pm. The light scattering material comprises silica,
silicate,
PCC, gypsum, calcium oxalate, titanium dioxide, aluminium hydroxide, barium
sul-
phate or zinc oxide. The coated cellulose particles may be used as a filler or
coat-
ing pigment of paper or board.
Description of the invention
The aim of the invention is to provide a supercalendered paper product having
im-
proved properties, such as high brightness, high whiteness, low yellowness,
high
light scattering, high opacity, low roughness, high gloss and high density.
According to the present invention it was found that a particular gypsum -
fibre
composite product wherein the gypsum is crystallized on the surface of the
fibre
and attached fairly strongly to the fibre can be used as a filler pigment or
coating
pigment in the production of a supercalendered paper product resulting in unex-
pected improvements in respect of paper properties, such as high brightness,
high
whiteness, low yellowness, high light scattering, high opacity, low roughness,
high
gloss and high density. In the production of supercalendered paper also
improved
retention of the filler pigment and homogenous filler distribution can be
obtained.
Also higher filler load can be obtained.
Thus, according to a first aspect of the invention there is provided a
supercalen-
dered paper product comprising first cellulosic fibres and a gypsum -fibre
compos-
ite product as a filler pigment or coating pigment, wherein the gypsum in the
gyp-
sum -fibre composite product appears as crystals on the surface of the fibre,
and
wherein the gypsum crystals are obtained by contacting calcium sulphate hemi-
hydrate and/or calcium sulphate anhydrite and an aqueous second fibre suspen-
sion.
The gypsum -fibre composite product may be similar to the one described in Fin-
nish patent application FI 20085767 filed on 11 August 2008.
WO 2010/122221 PCT/F12010/050316
The gypsum is attached to the fibre and consequently the gypsum -fibre
composite
is shown by most measurement methods as a single piece. The shape and size of
the gypsum can roughly be estimated by means of microscopic images. The gyp-
sum crystals attached to the fibre can have the shapes and sizes described in
WO
5 2008/092990 and WO 2008/092991. However, according to the invention the crys-
tallized gypsum can also be needle-like.
The size of the gypsum crystals formed on the surface of the fibre is
preferably
from 0.1 to 5.0 pm, more preferably from 0.1 to 4.0 pm, and most preferably
from
0.2 to 4.0 pm However, in the finished supercalendered paper product the size
of
the gypsum crystals may be increased.
Preferably the first cellulosic fibres comprise conventional papermaking pulp
fibres
including chemical, mechanical, chemi-mechanical or deinked pulp fibres. Chemi-
cal pulps include kraft pulp and sulphite pulp. Mechanical pulps include stone
groundwood pulp (SGW), refiner mechanical pulp (RMP), pressure groundwood
(PGW), thermomechanical pulp (TMP), and also chemically treated high-yield
pulps such as chemithermomechanical pulp (CTMP). Deinked pulp can be made
using mixed office waste (MOW), newsprint (ONP), magazines (OMG) etc. Also
mixtures of different pulps can be used.
The average length of the first cellulosic fibre is preferably between 0.5 and
5 mm.
Cellulosic fibres derived from softwood typically have an average length of be-
tween 1 and 5 mm, preferably between 2 and 4 mm. Cellulosic fibres derived
from
hardwood typically have an average length of between 0.5 and 3 mm, preferably
between 1 and 2 mm.
Preferably the second fibre of the gypsum -fibre composite product comprises a
second cellulosic fibre such as a chemical, mechanical, chemi-mechanical or
deinked pulp fibre or a synthetic fibre, such as a polyolefine, e.g.
polypropene.
Chemical pulps include kraft pulp and sulphite pulp. Mechanical pulps include
stone groundwood pulp (SGW), refiner mechanical pulp (RMP), pressure ground-
wood (PGW), thermomechanical pulp (TMP), and also chemically treated high-
yield pulps such as chemithermomechanical pulp (CTMP). Deinked pulp can be
made using mixed office waste (MOW), newsprint (ONP), magazines (OMG) etc.
Also mixtures of different pulps can be used.
The average length of the second fibre is preferably between 0.5 and 5 mm.
Cellu-
losic fibres derived from softwood typically have an average length of between
1
WO 2010/122221 PCT/F12010/050316
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and 5 mm, preferably between 2 and 4 mm. Cellulosic fibres derived from hard-
wood typically have an average length of between 0.5 and 3 mm, preferably be-
tween 1 and 2 mm.
Said first cellulosic fibre and said second cellulosic fibre may be similar or
differ-
ent, preferably similar.
Preferably the weight ratio of gypsum to fibre in the gypsum -fibre composite
product on dry basis is in the range from 95:5 to 50:50, preferably from 75:25
to
50:50.
According to the invention the supercalendered paper product may additionally
comprise one or more of following substances: a natural or synthetic polymer
binder, an optical brightener, a rheology modifier and a sizing agent. These
sub-
stances or some of these substances may be introduced into the gypsum -fibre
composite product. The sizing agent may be a rosin size or a reactive size
such as
alkyl ketene dimer (AKD) or alkenyl succinic anhydride (ASA).
The amount of the gypsum -fibre composite product in the supercalendered paper
product is preferably from 10 to 60%, more preferably from 20 to 50% by weight
on dry basis. Correspondingly the amount of the first cellulosic fibres in the
paper
product is preferably from 40 to 90%, more preferably from 50 to 80% by weight
on dry basis.
When used as a coating pigment, the gypsum -fibre composite product comprises
gypsum crystals preferably having a size of between 0.1 and 1.0 pm, more pref-
erably between 0.5 and 1.0 pm. When used as a filler, the gypsum -fibre compos-
ite product comprises gypsum crystals preferably having a size of between 1.0
and
5.0 pm, more preferably between 1.0 and 4.0 pm. As was stated above the size
of
the gypsum crystals in the finished supercalendered paper product may be in-
creased.
Preferably the weigh ratio of calcium sulphate hemihydrate and/or calcium sul-
phate anhydrite to water in the crystallization is in the range from 0.03 to
0.6:1,
more preferably from 0.05 to 0.5:1.
Preferably the dry fiber content in the crystallization is from 3 to 30% by
weight.
Preferably the content of calcium sulphate hemihydrate and/or calcium sulphate
anhydrite in the crystallization is from 10 to 57% by weight.
WO 2010/122221 PCT/F12010/050316
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The obtained gypsum -fibre composite product may additionally be homogenized
to form a homogenized product or dried and comminuted to form a gypsum -fibre
composite product in the form of dry particles.
The second fibre may be comminuted before crystallizing the gypsum thereon.
However, it is more preferred to comminute the gypsum -fibre composite
product.
The gypsum -fibre composite product may be prepared at a pulp mill or in situ
at a
paper mill. In the latter case the gypsum -fibre composite product requires a
reten-
tion time of preferably at least 15 minutes.
A fixative can be introduced into the crystallization.
The fixative can be selected from the group consisting of poly aluminum
chloride,
poly diallyldimethylammonium chloride (poly DADMAC), anionic and cationic poly-
acrylates.
The crystallization can be carried out in the absence of crystallization habit
modifi-
ers.
The crystallization can also be carried out in the presence of a
crystallization habit
modifier.
The crystallization habit modifier can be added to water or aqueous fibre
suspen-
sion before the calcium sulphate hemihydrate and/or calcium sulphate
anhydrite.
The temperature of the water in the reaction mixture can be anything between 0
and 100 C. Preferably, the temperature is between 0 and 80 C, more
preferably
between 0 and 50 C, even more preferably between 0 and 40 C, most preferably
between 0 and 25 C.
The crystallization habit modifier may be an inorganic acid, oxide, base or
salt.
Examples of useful inorganic oxides, bases and salts are AIF3, AI2(S04)3,
CaCl2,
Ca(OH)2, H3B04, NaCl, Na2SO4, NaOH, NH4OH, (NH4)2SO4, MgCl2, MgSO4 and
MgO.
The crystallization habit modifier may also be an organic compound, which is
an
alcohol, an acid or a salt. Suitable alcohols are methanol, ethanol, 1-
butanol, 2-
butanol, 1-hexanol, 2-octanol, glycerol, i-propanol and alkyl polyglucoside
based
C8-Clo-fatty alcohols.
WO 2010/122221 PCT/F12010/050316
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The crystallization habit modifier is preferably a compound having in its
molecule
one or several carboxylic or sulphonic acidic groups, or a salt of such a
compound.
Among the organic acids may be mentioned carboxylic acids such as acetic acid,
propionic acid, succinic acid, citric acid, tartaric acid, ethylene diamine
succinic
acid (EDDS), iminodisuccinic acid (ISA), ethylene diamine tetraacetic acid
(EDTA),
diethylene triamine pentaacetic acid (DTPA), nitrilotriacetic acid (NTA), N-
bis-(2-
(1,2-dicarboxyethoxy)ethyl aspartic acid (AES), and sulphonic acids such as
amino-1-naphthol-3,6-disulphonic acid, 8-amino-1-naphthol-3,6-disulphonic
acid,
2-aminophenol-4-sulphonic acid, anthrachinone-2,6-disulphonic acid, 2-
mercaptoethanesulphonic acid, poly(styrene sulphonic acid),
poly(vinylsulphonic
acid), as well as the di-, tetra- and hexa-aminostilbenesulfonic acids.
Among the organic salt may be mentioned the salts of carboxylic acids such as
Mg
formiate, Na- and NH4 -acetate, Nat-maleate, NH4-citrate, Nat-succinate, K-
oleate, K-stearate, Nat-ethylenediamine tetraacetic acid (Na2-EDTA), Na6-
aspartamic acid ethoxy succinate (Na6-AES) and Na6-aminotriethoxy succinate
(Na6-TCA).
Also the salt of sulphonic acids are useful, such as Na-n-(C1o-C13)-
alkylbenzene
sulphonate, C10-C16-alkylbenzene sulphonate, Na-1-octyl sulphonate, Na-1-
dodecane sulphonate, Na-1-hexadecane sulphonate, the K-fatty acid sulphonates,
the Na-C14-C16-olefin sulphonate, the Na-alkyl naphthalene sulphonates with
ani-
onic or non-ionic surfactants, di-K-oleic acid sulphonates, as well as the
salts of di-
, tetra-, and hexaaminostilbene sulphonic acids. Among organic salts
containing
sulphur should also be mentioned the sulphates such as the C12-C14-fatty
alcohol
ether sulphates, Na-2-ethyl hexyl sulphate, Na-n-dodecyl sulphate and Na-
lauryl
sulphate, and the sulphosuccinates such as the monoalkyl polyglycol ether of
Na-
sulphosuccinate, Na-dioctyl sulphosuccinate and Na-dialkyl sulphosuccinate.
Phosphates may also be used, such as the Na-nonylphenyl- and Na-dinonyl
phenylethoxylated phosphate esters, the K-aryl ether phosphates, as well as
the
triethanolamine salts of polyaryl polyetherphosphate.
As crystallization habit modifier may also be used cationic surfactants such
as oc-
tyl amine, triethanol amine, di(hydrogenated animal fat alkyl) dimethyl
ammonium
chloride, and non-ionic surfactants such as a variety of modified fatty
alcohol eth-
oxylates. Among useful polymeric acids, salts, amides and alcohols may be men-
tioned the polyacrylic acids and polyacrylates, the acrylate-maleate
copolymers,
polyacrylamide, poly(2-ethyl-2-oxazoline), polyvinyl phosphonic acid, the
copoly-
WO 2010/122221 PCT/F12010/050316
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mer of acrylic acid and allylhydroxypropyl sulphonate (AA-AHPS), poly-a-
hydroxyacrylic acid (PHAS), polyvinyl alcohol, and poly(methyl vinyl ether -
alt.-
maleic acid).
Especially preferable crystallization habit modifiers are ethylene diamine
succinic
acid (EDDS), iminodisuccinic acid (ISA), ethylene diamine tetraacetic acid
(EDTA),
diethylene triamine pentaacetic acid (DTPA), nitrilotriacetic acid (NTA), N-
bis-(2-
(1,2-dicarboxyethoxy)ethyl aspartic acid (AES), the di-, tetra- and hexa-
aminostilbenesulfonic acids and their salts such as Na-aminotriethoxy
succinate
(Na6-TCA), as well as the alkylbenzenesulphonates.
The crystallization habit modifier can be used in an amount of 0.01 to 5.0%,
most
preferably 0.02-1.78%, based on the weight of the calcium sulphate hemihydrate
and/or calcium sulphate anhydrite.
In the crystallization R-calcium sulphate hemihydrate is typically used. It
may be
prepared by heating gypsum raw-material to a temperature of between 140 and
300 C, preferably from 150 to 200 C. At lower temperatures, the gypsum raw-
material is not sufficiently dehydrated and at higher temperatures it is over-
dehydrated into anhydrite. Calcinated calcium sulphate hemihydrate usually con-
tains impurities in the form of small amounts of calcium sulphate dihydrate
and/or
calcium sulphate anhydrite. It is preferable to use R-calcium sulphate
hemihydrate
obtained by flash calcination, e.g. by fluid bed calcination, whereby the
gypsum
raw-material is heated to the required temperature as fast as possible.
However, it
is also possible to use a-calcium sulphate hemihydrate in the crystallization.
It is also possible to use calcium sulphate anhydrite as starting. The
anhydrite is
obtained by calcination of gypsum raw material. There are three forms of anhy-
drite; the first one, the so called Anhydrite I, is unable to form gypsum by
reaction
with water like the insoluble Anhydrites II-u and II-E. The other forms, the
so called
Anhydrite III, also known as soluble anhydrite has three forms: R-anhydrite
III, R-
anhydrite III', and a-anhydrite III and Anhydrite II-s form pure gypsum upon
con-
tact with water.
After the calcium sulphate hemihydrate and/or calcium sulphate anhydrite, aque-
ous fibre suspension and optionally crystallization habit modifier have been
con-
tacted, they are allowed to react into calcium sulphate dihydrate i.e. gypsum.
The
reaction takes e.g. place by mixing, preferably by mixing strongly, said
substances
together for a sufficient period of time, which can easily be determined
experimen-
WO 2010/122221 PCT/F12010/050316
tally. At high dry matter contents strong mixing is necessary because, the
slurry is
thick and the reagents do not easily come into contact with each other.
Preferably
the hemihydrate and/or anhydrite, the aqueous fibre suspension and optionally
the
crystallization habit modifier are mixed at the above mentioned temperature
given
5 for the water. The initial pH is typically acidic, preferably between 3 and
7, more
preferably between 3 and 6. If necessary, the pH is regulated by means of an
aqueous solution of NaOH and/or H2SO4, typically a 10% solution of NaOH and/or
H2SO4.
Because gypsum has a lower solubility in water than hemihydrate and soluble an-
10 hydrite, the gypsum formed by the reaction of hemihydrate and/or anhydrite
with
water immediately tends to crystallize from the water medium on the second
fibre.
The crystallization can be regulated by means of the above mentioned
crystalliza-
tion habit modifier so that the useful gypsum -fibre composite product is
obtained.
The gypsum -fibre composite product can also be treated with other additives.
A
typical additive is a biocide which prevents the activity of microorganisms
when
storing and using the product.
According to a second aspect of the invention there is provided a process for
pro-
ducing a supercalendered paper product as defined above comprising providing a
stock where first cellulosic fibers are mixed with water and the gypsum -fibre
com-
posite product, the obtained slurry is delivered to the forming section of a
paper
machine, then water is drained in the draining section and the formed web is
con-
ducted to the press section where additional water is removed, then the web is
conducted to the drying section where most of the remaining water is
evaporated,
and finally supercalendering the paper to improve the surface smoothness and
gloss of the paper.
The gypsum -fibre composite product may be introduced in the form of an aque-
ous product or in a dried form, optionally comminuted into the form of
particles.
The supercalendering of the paper may be carried out in an on-line calender or
in
an off-line calender. In the latter case the calender is separate from the
actual pa-
per machine. The rolls of the supercalenders may be heated thermo rolls.
Additionally the invention relates to the use of a gypsum -fibre composite
product
wherein the gypsum appears as crystals on the surface of the fibre and wherein
the gypsum crystals are obtained by contacting calcium sulphate hemihydrate
WO 2010/122221 PCT/F12010/050316
11
and/or calcium sulphate anhydrite and an aqueous fibre suspension, as a filler
pigment or coating pigment in the production of a supercalendered paper
product.
Preferably the amount of the gypsum -fibre composite product in the supercalen-
dered paper product is from 10 to 60%, preferably from 20 to 50% by weight on
dry basis. Correspondingly the amount of said cellulosic fibres in the paper
product
is preferably from 40 to 90%, more preferably from 50 to 80% by weight on dry
ba-
sis.
Short description of the drawings
Figures 1-8 show electron microscope micrographs of calcium sulfate dihydrate -
fiber composite products of examples 1-8, and Figures 9-17 show various proper-
ties of supercalendered (SC) paper samples wherein the filler is a calcium
sulfate
dihydrate -fiber composite product, precipitated calcium sulphate (PCS) or
kaolin
clay.
Figure 1 a shows SEM micrograph of calcium sulfate dihydrate/ TMP composite at
hemihydrate solids content of 18% (HH/(HH+water)),
Figure 1 b shows SEM micrograph of the same composite as in Figure 1 a washed
in saturated calcium sulfate solution,
Figure 2a shows SEM micrograph of calcium sulfate dihydrate/ TMP composite at
hemihydrate solids content of 42% (HH/(HH+water)),
Figure 2b shows SEM micrograph of the same composite as in Figure 2a washed
in saturated calcium sulfate solution,
Figure 3 shows SEM micrograph of calcium sulfate dihydrate/ eucalyptus kraft
pulp composite at hemihydrate solids content of 6.25% (HH/(HH+water)),
Figure 4 shows SEM micrograph of the fibers from calcium sulfate dihydrate/ eu-
calyptus kraft pulp composite at hemihydrate solids content of 7.5%
(HH/(HH+water)),
Figure 5a shows SEM micrograph of calcium sulfate dihydrate/ pine kraft pulp
composite using poly aluminum chloride as fixative, composite being washed
with
saturated calcium sulfate solution,
WO 2010/122221 PCT/F12010/050316
12
Figure 5b shows SEM micrograph of the same composite as in Figure 5a being
stirred with Heidolph laboratory mixer at 350 rpm for a couple of minutes,
Figure 6a shows SEM micrograph of calcium sulfate dihydrate/ pine kraft pulp
composite using poly-DADMAC as fixative, composite being washed with satu-
rated calcium sulfate solution,
Figure 6b shows SEM micrograph of the same composite as in Figure 6a being
stirred with Heidolph laboratory mixer at 350 rpm for a couple of minutes,
Figure 7 shows SEM micrograph of calcium sulfate dihydrate/ birch kraft pulp
composite washed with saturated calcium sulfate solution,
Figure 8 shows SEM micrograph of calcium sulfate dihydrate/ plastic fiber
compo-
site washed with saturated calcium sulfate solution,
Figure 9 shows ISO Brightness of SC paper samples,
Figure 10 shows CIE Whiteness of SC paper samples,
Figure 11 shows Yellowness of SC paper samples,
Figure 12 shows Light scattering of SC paper samples,
Figure 13 shows Opacity of SC paper samples,
Figure 14 shows PPS Roughness of SC paper samples,
Figure 15 shows Gloss of SC paper samples,
Figure 16 shows Gloss vs. paper bulk of SC paper samples, and
Figure 17 shows Air permeability (Bendtsen porosity) of SC paper samples.
EXAMPLES
In the following the invention will be illustrated in more detail by means of
exam-
ples. The purpose of the examples is not to restrict the scope of the claims.
In this
specification the percentages refer to% by weight unless otherwise specified.
WO 2010/122221 PCT/F12010/050316
13
First, general information about the syntheses and product analyses is
disclosed.
Then, data about each example is presented.
Synthesis
General information is first presented. A method optimization for the paper
pig-
ments was carried out. The parameters were:
HH (initial hemihydrate, w-%) 5-57
Fiber concentration (w-%) 3-30
Additive concentration (w-% of DH(dihydrate)) 0.100-1
The reaction was carried out at system pH. The amount of habit modifier
chemical
is calculated as per cent of the precipitated calcium sulfate dihydrate (w-%
of DH)
The experiments were performed with the following equipment.
The reactor was of Hobart type N50CE. The hemihydrate and the chemicals are
added batchwise to the aqueous fiber suspension phase and a hemihydrate slurry
with an initial solids of 5 - 57 w-% is obtained. Mixing speed is about 250 -
500
rpm. Reaction is carried out at system pH.
Analysis
Morphology of calcium sulfate dihydrate was studied by using FEI XL 30 FEG
scanning electron microscope. Conversion of hemihydrate to dihydrate was ana-
lyzed using Mettler Toledo TGA/SDTA85 1 /11 00-thermogravimetric analyzer
(TG).
Crystal structure was determined with Philips X'pert x-ray powder
diffractometer
(XRD).
Example 1
1. 800 g of water is placed into the Hobart N50 CE laboratory mixer. Couple of
drops of biocide (Fennosan IT 21) is added.
2. 200 g of TMP (Thermomechanical pulp) with solids content of 36% is added to
the reactor.
3. Fluidized bed calcined a-calcium sulphate hemihydrate is evenly added to
the
reactor with the operation speed of the stirrer set to position 1. The total
amount of
WO 2010/122221 PCT/F12010/050316
14
hemihydrate added is 200 g (giving 18% by weight of HH/(HH+water)). After the
addition the operation speed of the stirrer is raised to position 2. Composite
is
stirred for five minutes.
4. Wait for the formation of calcium sulfate dihydrate for one hour.
The obtained pigment-fiber composite is shown in figure la, after washing with
calcium sulfate saturated water in figure 1 b.
Example 2
1. 430 g of water is placed into the Hobart N50 CE laboratory mixer. Couple of
drops of biocide (Fennosan IT 21) is added.
2. 570 g of TMP (Thermomechanical pulp) with solids content of 36% is added to
the reactor.
3. Fluidized bed calcined a-calcium sulphate hemihydrate is evenly added to
the
reactor with the operation speed of the stirrer set to position 1. The total
amount of
hemihydrate added is 570 g (giving 42% by weight of HH/(HH+water)). After the
addition the operation speed of the stirrer is raised to position 2. Composite
is
stirred for five minutes.
4. Wait for the formation of calcium sulfate dihydrate for one hour.
The obtained pigment-fiber composite is shown in figure 2a, after washing with
calcium sulfate saturated water in figure 2b.
Example 3
1. 456.5 g of eucalyptus kraft pulp with solids content of 17.7% is placed
into the
Hobart N50 CE laboratory mixer.
2. Fluidized bed calcined a-calcium sulphate hemihydrate is evenly added to
the
reactor with the operation speed of the stirrer set to position 1. The total
amount of
hemihydrate added is 25 g (giving 6.25% by weight of HH/(HH+water)). After the
addition the operation speed of the stirrer is raised to position 2. Composite
is
stirred for five minutes.
3. Wait for the formation of calcium sulfate dihydrate for one hour.
WO 2010/122221 PCT/F12010/050316
The obtained pigment-fiber composite after washing with calcium sulfate
saturated
water is shown in figure 3.
Example 4
1. 47 g of water is placed into the Hobart N50 CE laboratory mixer. Couple of
5 drops of biocide (Fennosan IT 21) is added.
2. 295.5 g of eucalyptus kraft pulp with solids content of 17.7% is added to
the
reactor.
3. Fluidized bed calcined a-calcium sulphate hemihydrate is evenly added to
the
reactor with the operation speed of the stirrer set to position 1. The total
amount of
10 hemihydrate added is 25 g (giving 7.5% by weight of HH/(HH+water)). After
the
addition the operation speed of the stirrer is raised to position 2. Composite
is
stirred for five minutes.
4. Wait for the formation of calcium sulfate dihydrate for one hour.
The obtained fiber product is shown in figure 4.
15 Example 5
1. 44.8 g of water is placed into the Hobart N50 CE laboratory mixer. 1.6 g of
poly
aluminum chloride and couple of drops of biocide (Fennosan IT 21) is added.
2. 640 g of pine kraft pulp with solids content of 7% is added to the reactor.
3. Fluidized bed calcined a-calcium sulphate hemihydrate is evenly added to
the
reactor with the operation speed of the stirrer set to position 1. The total
amount of
hemihydrate added is 160 g (giving 20% by weight of HH/(HH+water)). After the
addition the operation speed of the stirrer is raised to position 2. Composite
is
stirred for five minutes.
4. Wait for the formation of calcium sulfate dihydrate for one hour.
The obtained pigment-fiber composite after washing with calcium sulfate
saturated
water is shown in figure 5.
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Example 6
1. 15.8 g of water is placed into the Hobart N50 CE laboratory mixer. 0.6 g of
poly
DADMAC and couple of drops of biocide (Fennosan IT 21) is added.
2. 226 g of pine kraft pulp with solids content of 7% is added to the reactor.
3. Fluidized bed calcined a-calcium sulphate hemihydrate is evenly added to
the
reactor with the operation speed of the stirrer set to position 1. The total
amount of
hemihydrate added is 300 g (giving 57% by weight of HH/(HH+water)). After the
addition the operation speed of the stirrer is raised to position 2. Composite
is
stirred for five minutes.
4. Wait for the formation of calcium sulfate dihydrate for one hour.
The obtained pigment-fiber composite after washing with calcium sulfate
saturated
water is shown in figure 6.
Example 7
1. 116 g of water is placed into the Hobart N50 CE laboratory mixer. Couple of
drops of biocide (Fennosan IT 21) is added.
2. 800 g of birch kraft pulp (solids content 14.5%) is added to the reactor.
3. Fluidized bed calcined a-calcium sulphate hemihydrate is evenly added to
the
reactor with the operation speed of the stirrer set to position 1. The total
amount of
hemihydrate added is 200 g (giving 20% by weight of HH/(HH+water)). After the
addition the operation speed of the stirrer is raised to position 2. Composite
is
stirred for five minutes.
4. Wait for the formation of calcium sulfate dihydrate for one hour.
The obtained pigment-fiber composite after washing with calcium sulfate
saturated
water is shown in figure 7.
Example 8
1. 600 g of water is placed into the Hobart N50 CE laboratory mixer. Couple of
drops of biocide (Fennosan IT 21) is added.
2. 10 g of synthetic polypropene fiber is added to the reactor.
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3. Fluidized bed calcined a-calcium sulphate hemihydrate is evenly added to
the
reactor with the operation speed of the stirrer set to position 1. The total
amount of
hemihydrate added is 300 g (giving 34% by weight of HH/(HH+water)). After the
addition the operation speed of the stirrer is raised to position 2. Composite
is
stirred for five minutes.
4. Wait for the formation of calcium sulfate dihydrate for one hour.
The obtained pigment-fiber composite after washing with calcium sulfate
saturated
water is shown in figure 8.
Example 9
Pigment-filler composite was prepared as described in Example 1 by using
thermo-mechanical pulp (TMP) at fiber solids content of 8%. Calcined a-calcium
sulphate hemihydrate was added in an amount giving 20% of HH/(HH+water).
Application test on uncoated supercalendered paper was carried as follows.
Composite samples were disintegrated using Hollander refiner. In sheet making
the composite was mixed with untreated fiber fraction and compared with tradi-
tional kaolin and PCS (precipitated calcium sulphate) fillers. The used filler
level
was 30% (for the composite the filler level refers to the gypsum level), and
the tar-
get basis weight was 60 2 g/m2. Filler level was adjusted by changing the
ratio
between composite and untreated fiber. Samples were calendered using 2+2 nips
at line loads of 100, 175 and 250 kN/m. Roll temperature was 80 C, relative hu-
midity 85%, and calendaring speed 30 m/min.
The ISO-Brightness of paper samples was measured at R457. The results are
shown in figure 9. The results show that the effect of PCS and the Composite
of
the present invention in paper brightness was similar, whereas the kaolin clay
gave a brightness of about 4% units lower than for PCS and Composite.
The CIE Whiteness of paper samples was measured. The results are shown in
figure 10. The results show that the filler-fiber composite gave significantly
higher
whiteness to the paper than PCS and kaolin clay.
The Yellowness (CIE yellow colour coordinate b*(C/20)) of paper samples was
measured. The results are shown in figure 11. The results show that the filler-
fiber
composite gave significantly lower yellowness values than PCS and kaolin clay.
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The Light scattering of paper samples was measured. The results are shown in
figure 12. The results show that the filler-fiber composite improved light
scattering
with about 5-10 units compared to PCS and kaolin clay.
The Opacity of paper samples was measured. The results are shown in figure 13.
From the results it can be concluded that improvement in light scattering had
also
positive impact on the opacity values. At lower line loads the filler-fiber
composite
showed about one unit higher opacity than kaolin and about 2 units higher than
PCS. The differences increased at higher line loads being about 2 and 3 units
higher than kaolin and PCS, respectively.
The PPS Roughness of paper samples was measured. The results are shown in
figure 14. The results show that kaolin and filler-fiber composite had similar
rough-
ness while PCS had about 0.1 pm higher roughness. The filler-fiber composite
had
also lower difference in roughness between top side (TS) and wire side (WS).
The Gloss 75 of paper samples was measured. The results are shown in figure
15. The results show that the filler-fiber composite had an about 4 units
higher cal-
endered gloss than PCS and one unit higher than kaolin.
In figure 16 the calendered gloss is shown against paper bulk. From the
results it
can be seen that the filler-fiber composite had about 0.1 units higher bulk
than
kaolin at the same gloss value.
The porosity of paper samples was measured by the Bendtsen method. This
method measures the rate at which air will pass through a sheet of paper at a
set
of pressure. A high porosity indicates the paper allows the air to travel
through
relatively easy. The results are shown in figure 17. The results show that the
filler-
fiber composite had a lower porosity, i.e. higher density than PCS. Kaolin
samples
had the highest density.
