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Title:
PROCESS FOR THE PREPARATION OF A PIGMENT -FIBRE COMPOSITE
Document Type and Number:
WIPO Patent Application WO/2011/098672
Kind Code:
A1
Abstract:
The invention relates to a process for the preparation of a pigment -cellulosic fibre composite, wherein the pigment comprises an inorganic pigment which comprises local cationic charges, said process comprises contacting an aqueous cellulosic fibre suspension and inorganic pigment particles such that crystals of said pigment are attached to the surface of the fibre, said crystals having a size of at most 5 μm. The produced composite product can be used as a filler pigment or coating pigment in the production of paper.

Inventors:
TURKKI, Tarja (Kivihaantie 9 C 25, Helsinki, FI-00310, FI)
HEISKA, Perttu (Kyyhkysmäki 9 A 5, Espoo, FI-02650, FI)
Application Number:
FI2011/050127
Publication Date:
August 18, 2011
Filing Date:
February 10, 2011
Export Citation:
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Assignee:
KEMIRA OYJ (Porkkalankatu 3, Helsinki, FI-00180, FI)
TURKKI, Tarja (Kivihaantie 9 C 25, Helsinki, FI-00310, FI)
HEISKA, Perttu (Kyyhkysmäki 9 A 5, Espoo, FI-02650, FI)
International Classes:
C01F11/46; C09C1/02; D21H17/66; D21H17/69; D21H17/70; D21H19/38; D21H19/42
Attorney, Agent or Firm:
BERGGREN OY AB (P.O. Box 16, Helsinki, FI-00101, FI)
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Claims:
Claims

1 . A process for the preparation of a pigment -cellulosic fibre composite, wherein the pigment comprises an inorganic pigment which comprises local cati- onic charges, said process comprises contacting an aqueous cellulosic fibre sus- pension and inorganic pigment particles such that crystals of said pigment are attached to the surface of the fibre, said crystals having a size of at most 5 μιτι.

2. The process according to claim 1 , wherein the inorganic pigment particles comprise gypsum, calcium carbonate, kaolin, titanium dioxide, talc, silica or silicate, preferably gypsum, calcium carbonate or kaolin. 3. The process according to claim 2 wherein the inorganic pigment particles additionally comprise calcium sulphate hemihydrate or calcium sulphate anhydrite which are converted to gypsum crystals which are attached to the surface of the fibre, said crystals having a size of at most 5 μιτι.

4. The process according to any of claims 1 to 3, wherein the net charge of the inorganic pigment is anionic.

5. The process according to any of claims 1 to 4, wherein the net charge of the cellulosic fibre is anionic.

6. The process according to any of claims 1 to 5, wherein the pigment crystals attached to the surface of the cellulosic fibre have a size between 0.1 and 5.0 μιτι. 7. The process according to any of claims 1 to 6, wherein the pigment particles are provided in the form of a powder or as a slurry.

8. The process according to any of claims 1 to 7, wherein the aqueous cellulosic fibre suspension and the inorganic pigment particles are contacted under high shear forces. 9. The process according to any of claims 1 to 8, wherein the weight ratio of pigment to cellulosic fibre on dry basis in the produced composite is in the range from 70:30 to 10:90, preferably from 50:50 to 10:90, more preferably from 40:60 to 20:80, and most preferably from 40:60 to 30:70.

10. The process according to any of claims 1 to 9, wherein the cellulosic fibre content on dry basis in the contacting stage is from 0.1 to 15 % by weight, preferably from 0.2 to 10 % by weight, more preferably from 0.3 to 8 % by weight.

1 1 . The process according to any of claims 1 to 10, wherein the content of pigment on dry basis in the contacting stage is from 0.3 to 20 % by weight, preferably from 0.5 to 20 % by weight, more preferably from 1 to 15 % by weight, and most preferably from 2 to 10 % by weight. 12. The process according to any of claims 1 to 1 1 , wherein the weight ratio of pigment to cellulosic fibre on dry basis in the contacting stage is from 95:5 to 20:80, preferably from 90:10 to 30:70.

13. The process according to any of claims 1 to 12, wherein the weigh ratio of pigment to water in the contacting stage is in the range from 0.005 to 0.6:1 , pref- erably from 0.01 to 0.6:1 , more preferably from 0.05 to 0.5:1 .

14. The process according to any of claims 1 to 13, wherein the cellulosic fibre comprises a chemical pulp fibre, such as a kraft pulp fibre, or a mechanical pulp fibre including chemi-mechanical pulp fibre, or a deinked pulp fibre.

15. The process according to claim 14, wherein the cellulosic fibre is refined cel- lulosic fibre, preferably having a length of at most 5 mm.

16. The process according to any of claims 1 to 15, wherein the pigment - cellulosic fibre composite is prepared in a pulp production unit.

17. Use of a pigment -cellulosic fibre composite produced according to any of claims 1 to 16 as a filler pigment or coating pigment in the production of paper. 18. The use according to claim 17, wherein the pigment -cellulosic fibre composite is added into the long circulation of the paper machine.

19. A paper product comprising a pigment -cellulosic fibre composite produced according to any of claims 1 to 16 as a filler pigment or coating pigment.

20. The paper product according to claim 19, wherein the amount of the pigment -cellulosic fibre composite is from 10 to 80 %, preferably from 20 to 60% by weight on dry basis.

Description:
PROCESS FOR THE PREPARATION OF A PIGMENT -FIBRE COMPOSITE Technical field

The invention relates to a process for the preparation of a pigment -cellulosic fibre composite product. The invention also relates to the use of the pigment -cellulosic fibre composite product as a coating pigment or a filler pigment in the production of paper.

Background of the invention

A papermaking process starts with stock preparation where cellulosic fibres 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 fibres at the forming section 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 typically by means of steam-heated dryer drums. Post drying operations include calendering 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).

The mineral filler is usually introduced in the form of a dispersed filler. Useful dispersing agents are the following: lignosulphonates such as Na lignosulphonate, condensation products of aromatic suplhonic acids with formaldehyde such as the condensed naphthalene sulphonates, dispersing anionic polymers, and copolymers made from anionic monomers or made anionic after polymerization, polymers containing repeating units having anionic charge such as carboxylic and sul- phonic acids, their salts and combinations thereof. Also phosphates, non-ionic and cationic polymers, polysaccharides and surfactants may be used. The amount of dispersing agent typically used is from 0.01 to 5.0 %, such as from 0.05 to 3.0 %, based on the weight of the mineral filler.

Gypsum or calcium sulphate dihydrate CaSO 4 -2H 2 O is suitable as material for both coating pigment and filler, especially in paper products. Especially good coating 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).

In the following the dimensions of gypsum particles will be discussed. However, what is said in respect of gypsum applies as well to other pigments, such as calcium carbonate and kaolin.

The morphology of the gypsum product particles can be established by examining scanning electron micrographs. Useful micrographs are obtained e.g. with a scanning electron microscope of the type Philips FEI XL 30 FEG. The size of the gypsum product particles is expressed as the weight average diameter D 50 of the particles contained therein. More precisely, D 50 is the diameter of the presumably round particle, smaller than which particles constitute 50 % of the total particle weight. D 50 can be measured with an appropriate particle size analyzer, such as Sedigraph 5100. 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 claimed gypsum product is meant the average SR of its individual 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 claimed 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 narrow. The width is expressed as the gravimetric weight distribution WPSD and it is expressed as (D 7 5-D25) D 5 o wherein D 75 , D 2 5 and D 5 o are the diameters of the presumably 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 (CaSO 4 ½H 2 O), 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 (CaSO 4 ). Depending on the calcination conditions of the gypsum raw material, the calcium sulphate hemihydrate may occur in two forms; as a- and β-hemihydrate. The β- 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 pressure 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 calcium 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 μιτι and the second largest measure of which is from 10 to 40 pm.

AU 620857 (EP 0334292 A1 ) 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 μιτι and an aspect ratio between 5 and 50. See page 15, lines 5 to 1 1 , 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 μιτι, width from 1 to 5 μιτι) 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.

US 5 320 677 discloses a composite material produced by mixing gypsum and host particles, especially wood fibres, and sufficient water to make a dilute slurry, followed by heating the slurry under pressure to convert the gypsum to calcium sulphate alpha hemihydrate, and then separating a major portion of the water, followed by rehydrating the hemihydrate back to gypsum. The composite material is useful for making building and plaster products, especially fire-resistant wall- boards. 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, calcined gypsum and water. The mixture is compacted to give a plastic mass which subsequently 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 essentially intact crystals having a size from 0.1 to 2.0 μιτι. The crystals preferably have a shape ratio SR of at least 2.0, more preferably between 2.0 and 50, and an aspect ratio AR between 1 .0 and 10, more preferably between 1 .0 and below 5.0. The gypsum product can be used as a coating pigment or as a filler pigment in the production of paper.

WO 2008/092991 discloses a process for the preparation of a gypsum product wherein calcium sulphate hemihydrate and/or calcium sulphate anhydrite and water are contacted so that the calcium sulphate hemihydrate and/or calcium sul- 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, preferably between 50 and 84 % by weight. The gypsum product can be used as a coating pigment or as a filler pigment in the production of paper.

The aim of the invention is to provide a pigment -cellulosic fibre composite wherein the pigment is attached fairly strongly to surface the fibre, which composite can be used as a coating pigment or a filler pigment in the production of paper.

Summary of the invention

According to the present invention it was surprisingly found that a filler for use in the manufacturing of paper can be produced by contacting an aqueous cellulosic fibre suspension and undispersed or partially dispersed gypsum particles or some other undispersed or partially dispersed pigment particles under such conditions that the pigment which comprises local cationic charges is attached to the surface of the fibre to form a pigment -cellulosic fibre composite. Brief description of the drawings

Fig. 1 shows scanning electron microscope (SEM) micrograph of fibres from prior art gypsum filler in filler trial,

Figs. 2-13 show SEM micrographs of gypsum/ kraft pulp composites of the present invention at various initial pulp concentrations,

Fig. 14 shows SEM micrograph of gypsum/ mechanical pulp composite of the present invention,

Figs. 15-18 show SEM micrographs of PCC/ kraft pulp composites of the present invention at various initial pulp concentrations, Fig. 19 shows SEM micrograph of kaolin/ kraft pulp composite of the present invention,

Fig. 20 shows SEM micrograph of gypsum+calcium sulfate hemihydrate/ kraft pulp composite of the present invention, and

Figs. 21 -22 show SEM micrograph of titanium dioxide/ kraft pulp composites of the present invention.

Detailed description of the invention

According to one aspect of the invention there is provided a process for the preparation of a pigment -cellulosic fibre composite, wherein the pigment comprises an inorganic pigment which comprises local cationic charges, said process comprises contacting an aqueous cellulosic fibre suspension and inorganic pigment particles such that crystals of said pigment are attached to the surface of the fibre, said crystals having a size of at most 5 μιτι.

The pigment is attached to the fibre and consequently the pigment -cellulosic fibre composite is shown by most measurement methods as a single piece. The shape and size of the pigment can roughly be estimated by means of microscopic images.

An important feature the present invention is that the pigment comprises at least some sites that have a cationic charge. These cationically charged sites form together with the anionic fibres a bond between the pigment and fibres. The net charge of the pigment does not necessarily have to be cationic. The net charge can even be anionic.

In a preferred embodiment the net charge of the inorganic pigment is anionic.

Preferred inorganic pigment particles comprise gypsum, calcium carbonate, espe- cially precipitated calcium carbonate (PCC), kaolin, titanium dioxide, talc, silica or silicate. Especially preferred pigment particles are gypsum, calcium carbonate or kaolin. It is also possible to use a mixture of two or more of these pigments. Preferably the pigments are introduced in the form of crystals and the pigments also appear as crystals attached to the fibres. The inorganic pigment particles may additionally comprise calcium sulphate hemi- hydrate or calcium sulphate anhydrite which are converted to gypsum crystals which are attached to the surface of the fibre, said crystals having a size of at most 5 μιτι. When the calcium sulphate hemihydrate or calcium sulphate anhydrite comes into contact with the aqueous cellulosic fibre suspension it will react with water to form gypsum crystals on the surface of the fibres. It is believed that calcium sulphate hemihydrate or calcium sulphate anhydrite acts as a binder and, thus, improves the adherence of the other pigment to the fibres.

In one embodiment gypsum crystals are attached to the fibre. The gypsum crystals used in the production can have the shapes and sizes described in WO 2008/092990 and WO 2008/092991 . However, according to the invention the gypsum crystals can also have other shapes, such as being needle-like.

The size of the pigment particles attached to the surface of the cellulosic fibre is preferably from 0.1 to 5.0 μιτι, more preferably from 0.1 to 4.0 μιτι, and most preferably from 0.2 to 4.0 μιτι. The size of the pigment particles may also be from 0.1 to 2.0 μηη or from 0.2 to 2.0 μηη.

The pigment particles may be provided in the form of a powder or as a slurry, such as aqueous slurry.

Preferably the cellulosic fibre of the pigment -cellulosic fibre composite product comprises a chemical pulp, mechanical pulp including chemimechanical pulp or deinked pulp fibre. Chemical 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 net charge of the cellulosic fibre is preferably anionic.

Preferably the weight ratio of pigment to cellulosic fibre on dry basis in the pro- duced composite is in the range from 70:30 to 10:90, more preferably from 50:50 to 10:90, even more preferably from 40:60 to 20:80, and most preferably from 40:60 to 30:70.

Preferably the cellulosic fibre content on dry basis in the contacting stage is from 0.1 to 15 % by weight, more preferably from 0.2 to 10 % by weight, and most pref- erably from preferably from 0.3 to 8 % by weight.

Preferably the content of pigment on dry basis in the contacting stage is from 0.3 to 20 % by weight, more preferably from 0.5 to 20 % by weight, even more preferably from 1 to 15 % by weight, and most preferably 2 to 10 % by weight.

Preferably the weight ratio of pigment to cellulosic fibre on dry basis in the contact- ing stage is from 95:5 to 20:80, more preferably from 90:10 to 30:70.

Preferably the weigh ratio of pigment to water in the contacting stage is in the range from 0.005 to 0.6:1 , more preferably from 0.01 to 0.6:1 , and most preferably from 0.05 to 0.5:1 .

According to the invention the cellulosic fibre may be unrefined or refined cellulosic fibre. The refined cellulosic fibre preferably has a length of at most 5 mm.

The process of the invention may additionally comprise the step of comminuting the obtained product to form a comminuted pigment -cellulosic fibre composite product.

Furthermore, the process of the invention may additionally comprise the steps of drying and comminuting the obtained product to form a pigment -cellulosic fibre composite product in the form of particles.

According to the invention the pigment -cellulosic fibre composite product may be produced in a pulp production unit and shipped as such or in a dewatered form or as a dry product to the paper manufacturing unit where the product, if necessary, can be diluted to desired consistency. Preferably the cellulosic fibre used for producing the pigment -cellulosic fibre composite product, is produced at this same pulp production unit. The pigment may be provided in the form of an undispersed pigment or in the form of a predispersed pigment.

According to the invention it is also possible to add a retention chemical into the process for the preparation of the pigment -cellulosic fibre composite. Suitable re- tention chemicals include cationic polyacryl amide and microfibrillated cellulose. Other suitable retention chemicals are well known to the man skilled in the art.

According to the invention it is also possible to add a fixative into the process for the preparation of the pigment -cellulosic fibre composite. Suitable fixatives are selected from the group consisting of poly aluminum chloride, poly diallyldimethyl- ammonium chloride (poly DADMAC), anionic and cationic polyacrylates.

The process of the present invention can be carried out at atmospheric pressure and a temperature of between 0 and 100 °C, preferably between 0 and 80 °C, more preferably between 10 and 50 °C.

The process of the present invention is preferably carried out by mixing, preferably by mixing strongly, the aqueous cellulosic fibre suspension and pigment particles together for a sufficient period of time, which can easily be determined experimentally. 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. The initial pH is typically between 3.5 and 9.0, most preferably between 4.0 and 7.5. It is pre- ferred that the initial pH is 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 H 2 SO 4 , typically a 10 % solution of NaOH and/or H 2 SO 4 .

Additional substances such as a natural or synthetic polymer binder and/or an optical brightener and/or a rheology modifier and/or sizing agents may be added to the produced pigment -cellulosic 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 produced pigment -cellulosic fibre composite product can also be treated with other additives. A typical additive is a biocide which prevents the activity of micro- organisms when storing and using the product.

The present invention also relates to the use of a pigment -cellulosic fibre composite produced according to the above described process of the present invention, as a filler pigment or coating pigment in the production of paper. According to the invention the pigment -cellulosic fibre composite may be added to the short circulation or long circulation of a paper machine.

The "short circulation" refers to the system wherein water is separated from the stock in web forming in the paper machine or drying section wire and used for dilu- tion of the stock to be fed into the head box. The water separated from the stock is called "white water".

The "long circulation" refers to the system in which excess white water from the short circulation and other waters are collected at the paper machine and drying machine and used for stock dilution or other purposes in stock preparation. The addition of the pigment into the pulp is typically carried out under mechanical share, especially by vigorous mixing. Vigorous mixing is needed to provide sufficient collosion frequency and energy to obtain good pigment/fiber interaction to increase pigment retention on the fibre as compared to the state of the art. Typical addition points are e.g. pulper, refiner, various continuous or batch type mixers, pumps or static mixers in e.g. pipes. Especially preferred mixers are high-speed mixers providing high shear forces, such as mixers of the Diaflo mixer type having a mixing speed of at least 500 rpm.

By using the pigment -cellulosic fibre composite in the production of paper, improved retention of the filler pigment and homogenous filler distribution can be ob- tained. This in turn makes it possible to use lower levels of retention agents and to obtain better printing behaviour. Also improved strength and improved optical properties of the paper can be obtained.

Additionally the present invention provides a paper product comprising a pigment -cellulosic fibre composite produced according to the above described process of the present invention as a filler pigment or coating pigment.

The paper product of the present invention preferably comprises in addition to the pigment -cellulosic fibre composite, cellulosic fibres.

Preferably the cellulosic fibres comprise conventional papermaking pulp fibres including chemical, mechanical pulp including chemi-mechanical pulp or deinked pulp fibres. Chemical 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.

Said cellulosic fibres can be similar to or different from the fibres in the pigment - cellulosic fibre composite product, and preferably the fibres are similar. For fine papers the cellulosic fibres are preferably kraft pulp fibres. Examples of fine papers are writing and printing grade papers including offset, bond, duplicating and photocopying papers.

Preferably the amount of the pigment -cellulosic fibre composite in the paper product is from 10 to 80 %, preferably from 10 to 60 %, more preferably from 20 to 60%, and most preferably from 20 to 50 % by weight on dry basis. Correspondingly the amount of said cellulosic fibres in the paper product may be from 20 to 90 %, preferably from 40 to 90 %, more preferably from 40 to 80 %, and most preferably from 50 to 80 % by weight on dry basis.

According to a preferred embodiment the pigment -cellulosic fibre composite con- stitutes the whole filler content of the paper product of the present invention meaning that no additional filler is added into the paper manufacturing process. It is also possible that the pigment -cellulosic fibre composite forms only a part of the total filler content of the paper product, meaning that an additional filler, such as gypsum, PCC or kaolin is introduced into the paper manufacturing process. Preferably the pigment -cellulosic fibre composite forms at least 50 %, more preferably at least 60 % by weight of the total filler content of the paper product.

According to one embodiment of the invention, the composite product is a coating pigment and comprises pigment crystals preferably having a size of between 0.1 and 1 .0 μιτι, more preferably between 0.5 and 1 .0 μιτι. According to another em- bodiment, it is a filler and comprises pigment crystals preferably having a size of between 1 .0 and 5.0 μιτι, more preferably between 1 .0 and 4.0 μιτι. The pigment crystals in the filler composite may also have a size of between 1 .0 and below 2.0 μιτι. A preferred pigment crystal comprises gypsum crystal.

The gypsum crystals are preferably produced by a crystallization process. Pre- ferred gypsum crystals and processes for producing the same are described in WO 2008/092990 and WO 2008/092991 , the contents of which are herewith included by reference. The gypsum crystal starting material used in the present invention may be produced by the processes disclosed in these documents. According to WO 2008/092990 the gypsum crystals are produced by contacting calcium sulphate hemihydrate or calcium sulphate anhydrite, water and a crystallization habit modifier under such conditions that the dry matter content of the reaction mixture is between 50 and 84 %by weight.

According to WO 2008/092991 the gypsum crystals are produced by contacting calcium sulphate hemihydrates or calcium sulphate anhydrite, water and optionally a crystallization habit modifier under such conditions that the dry matter content of the reaction mixture is between 34 and 84 %by weight.

In the processes for producing gypsum crystals, calcium sulphate hemihydrates, preferably β-calcium sulphate hemihydrate is typically used. It may be prepared by heating gypsum raw-material (such as natural gypsum mineral or gypsum formed as a by-product of chemical processes, e.g. as phosphogypsum or flue gas gypsum) 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 contains impurities in the form of small amounts of calcium sulphate dihydrate and/or calcium sulphate anhydrite. It is preferable to use β-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 material for produc- ing gypsum crystals. The anhydrite is obtained by calcination of gypsum raw material. There are three forms of anhydrite; the first one, the so called Anhydrite I, is unable to form gypsum by reaction with water like the insoluble Anhydrites ll-u and ll-E. The other forms, the so called Anhydrite III, also known as soluble anhydrite has three forms: P-anhydrite III, P-anhydrite III', and a.-anhydrite III and Anhydrite ll-s form pure gypsum upon contact with water. EXAMPLES

In the following the invention will be illustrated in more detail by means of exam- pies. 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 and gypsum refers to calcium sulphate dihydrate. The experiments were carried out at system pH with following equipments. The reactor was of Hobart type N50CE or Diaf dissolver 20 VH (also called Diaflo mixer). Mixing speed is about 250 - 500 rpm for the Hobart N50CE mixer and about 500 - 1000 rpm for the Diaf dissolver 20 VH. After the mixing the samples were disintegrated using Lorentz & Wettre wet disintegrator according to ISO 5263 standard and 10000 cycles.

Analysis

Morphology of the pigments was studied by using FEI XL 30 FEG scanning electron microscope. The pigment content of the samples was determined as follows: When the mixing was stopped sample was disintegrated using Lorentz & Wettre wet disintegrator according to ISO 5263 standard using 10000 cycles. Fiber samples were picked up and dried in Memmert aging oven; gypsum samples at 45°C, calcium carbonate and kaolin at 75°C. Ash content was determined for the dried samples using Nabertherm C250 chamber furnace. Gypsum samples were heated to 850°C and kept there for three hours. Calcium carbonate and titanium dioxide samples were heated to 525°C for four hours and kaolin to 900°C for two hours. Pigment content of the sample was weighted and for gypsum correction factor 1 .265 was used to get the original calcium sulfate dihydrate content from the calcium sulfate anhydrite ash. For other pigments no correction was used. The refined pine pulp had shopper-riegler value of 29. Zeta potential measurements

Zeta potential (Z, mV) was measured using Malvern Zetasizer ZS. Samples were prepared by taking 1 ml sample and diluted with 250 ml 1 mM KCI solution. pH was measured and adjusted either with dilute KOH or HCI. The results are set forth in following table.

Kaolin 1 GCS Disp. Talc PCS Kaolin 2 Undisp. PCC

Talc

Z, mV PH Z, mV PH Z, mV PH Z, mV PH Z, mV PH Z, mV PH Z, mV PH

-18 3.5 -19 5.5 -43 3.8 -23 4.8 24 2.9 -19 3.2 -14 7.6

-21 4.3 -17 6.2 -63 7.3 -22 7.1 35 3.9 -27 3.9 -16 8.9

-66 5.5 -19 6.9 -68 8.1 -19 7.4 47 5.9 -48 6.3 -23 10

-55 6.1 -18 7.1 -64 9 -21 7.9 -58 6.2 -53 7

-75 8.4 -19 8 -20 8.9 -63 8.2 -53 8.5

-76 9.3 -17 9 -62 8.9 Additionally, the measured zeta potential was -44 mV at pH 6.7 for rutile titanium dioxide, and -17 mV at pH 6.7 for anatase titanium dioxide.

A negative zeta potential means anionicity. GCS is ground gypsum

PCS is precipitated gypsum

PCC is precipitated calcium carbonate

Kaolin 1 is filler kaolin

Kaolin 2 is coating kaolin. Reference example 1

By contacting a fiber suspension and dispersed gypsum normally used as filler in the production of paper, the gypsum particles are not adhered to the fibers to any notable extent. This is shown in figure 1 which shows scanning electron microscope (SEM) micrograph of fibers from prior art gypsum filler in filler trial. Accord- ing to figure 1 only a few gypsum particles are adhered to the fiber.

Example 1

Gypsum filler pigment was precipitated as described in WO 2008/092991 . Hobart N50CE mixer was used as reactor. Unrefined fiber suspension had solids content of eight per cent and it was placed in the reactor. 91 g of undispersed gypsum filler pigment powder was added into 800 g of pine kraft pulp suspension under stirring. Mixing was carried out for three minutes after the filler addition was completed. When the mixing was stopped sample was disintegrated using Lorentz & Wettre wet disintegrator according to ISO 5263 standard and 10000 cycles. Fiber samples were picked up and scanning electron photographs were taken. Pigment con- tent of the disintegrated sample was 25%. The obtained pigment -cellulosic fiber composite is shown in figure 2.

Example 2

Gypsum filler pigment was precipitated as described in WO 2008/092991 . Hobart N50CE mixer was used as reactor. Unrefined fiber suspension had solids content of eight per cent and it was placed in the reactor. 45.5 g of undispersed gypsum filler pigment powder was added into 800 g of pine kraft pulp suspension under stirring. Mixing was carried out for three minutes after the filler addition was completed. When the mixing was stopped sample was disintegrated using Lorentz & Wettre wet disintegrator according to ISO 5263 standard and 10000 cycles. Fiber samples were picked up and scanning electron photographs were taken. Pigment content of the disintegrated sample was 18 %. The obtained pigment -cellulosic fiber composite is shown in figure 3. Example 3

Gypsum filler pigment was precipitated as described in WO 2008/092991 . Hobart N50CE mixer was used as reactor. Unrefined fiber suspension had solids content of six per cent and it was placed in the reactor. 91 g of undispersed gypsum filler pigment powder was added into 800 g of pine kraft pulp suspension under stirring. Mixing was carried out for three minutes after the filler addition was completed. When the mixing was stopped sample was disintegrated using Lorentz & Wettre wet disintegrator according to ISO 5263 standard and 10000 cycles. Fiber samples were picked up and scanning electron photographs were taken. Pigment content of the disintegrated sample was 34%. The obtained pigment -cellulosic fiber composite is shown in figure 4. Example 4

Gypsum filler pigment was precipitated as described in WO 2008/092991 . Hobart N50CE mixer was used as reactor. Unrefined fiber suspension had solids content of four per cent and it was placed in the reactor. 91 g of undispersed gypsum filler pigment powder was added into 800 g of pine kraft pulp suspension under stirring. Mixing was carried out for three minutes after the filler addition was completed. When the mixing was stopped sample was disintegrated using Lorentz & Wettre wet disintegrator according to ISO 5263 standard and 10000 cycles. Fiber samples were picked up and scanning electron photographs were taken. Pigment content of the disintegrated sample was 34%. The obtained pigment -cellulosic fiber composite is shown in figure 5. Example 5

Gypsum filler pigment was precipitated as described in WO 2008/092991 . Hobart N50CE mixer was used as reactor. Unrefined fiber suspension had solids content of two per cent and it was placed in the reactor. 91 g of undispersed gypsum filler pigment powder was added into 800 g of pine kraft pulp suspension under stirring. Mixing was carried out for three minutes after the filler addition was completed. When the mixing was stopped sample was disintegrated using Lorentz & Wettre wet disintegrator according to ISO 5263 standard and 10000 cycles. Fiber samples were picked up and scanning electron photographs were taken. Pigment content of the disintegrated sample was 36%. The obtained pigment -cellulosic fiber composite is shown in figure 6. Example 6

Gypsum filler pigment was precipitated as described in WO 2008/092991 . Hobart N50CE mixer was used as reactor. Unrefined fiber suspension had solids content of one per cent and it was placed in the reactor. 91 g of undispersed gypsum filler pigment pow- der was added into 800 g of pine kraft pulp suspension under stirring. Mixing was carried out for three minutes after the filler addition was completed. When the mixing was stopped sample was disintegrated using Lorentz & Wettre wet disintegrator according to ISO 5263 standard and 10000 cycles. Fiber samples were picked up and scanning electron photographs were taken. Pigment content of the disintegrated sample was 30%. The obtained pigment -cellulosic fiber composite is shown in figure 7.

Example 7

Gypsum filler pigment was precipitated as described in WO 2008/092991 . Hobart N50CE mixer was used as reactor. Unrefined fiber suspension had solids content of eight per cent and it was placed in the reactor. 91 g of undispersed gypsum filler pigment pow- der was added to small amount of water and mixed vigorously using Diaf dissolver 20 VH. Pigment slurry was then added to 800 g of pine kraft pulp suspension under stirring. Mixing was carried out for three minutes after the filler addition was completed. When the mixing was stopped sample was disintegrated using Lorentz & Wettre wet disintegrator according to ISO 5263 standard and 10000 cycles. Fiber samples were picked up and scanning electron photographs were taken. Pigment content of the disintegrated sample was 34%. The obtained pigment -cellulosic fiber composite is shown in figure 8.

Example 8

Gypsum filler pigment was precipitated as described in WO 2008/092991 . 91 g of undispersed gypsum filler pigment powder was added to small amount of water and mixed vigorously using Diaf dissolver 20 VH. Unrefined fiber suspension had solids content of eight per cent and it was placed in a beaker and mixed with Diaf dissolver as well. Pigment slurry was then added to 800 g of pine kraft pulp suspension under stirring. Mixing was carried out for three minutes after the filler addition was completed. When the mixing was stopped sample was disintegrated using Lorentz & Wettre wet disintegrator according to ISO 5263 standard and 10000 cycles. Fiber samples were picked up and scanning electron photographs were taken. Pigment content of the disintegrated sample was 30%. The obtained pigment -cellulosic fiber composite is shown in figure 9. Example 9

Gypsum filler pigment was precipitated as described in WO 2008/092991 . 91 g of un- dispersed gypsum filler pigment powder was added to small amount of water and mixed vigorously using Diaf dissolver 20 VH. Unrefined fiber suspension had solids content of four per cent and it was placed in a beaker and mixed with Diaf dissolver as well. Pigment slurry was then added to 800 g of refined pine kraft pulp suspension under stirring. Mixing was carried out for three minutes after the filler addition was completed. When the mixing was stopped sample was disintegrated using Lorentz & Wettre wet disintegrator according to ISO 5263 standard and 10000 cycles. Fiber samples were picked up and scanning electron photographs were taken. Pigment content of the disintegrated sample was 39%. The obtained pigment -cellulosic fiber composite is shown in figure 10.

Example 10

Gypsum filler pigment was precipitated as described in WO 2008/092991 . Hobart N50CE mixer was used as reactor. Unrefined fiber suspension had solids content of four per cent and it was placed in the reactor. Fennopol K3400R retention polymer (cationic polyacrylamide) was added to fiber suspension. 91 g of undispersed gypsum filler pigment powder was added into 800 g of pine kraft pulp suspension under stirring. Mixing was carried out for three minutes after the filler addition was completed. When the mixing was stopped sample was disintegrated using Lorentz & Wettre wet disintegrator according to ISO 5263 standard and 10000 cycles. Fiber samples were picked up and scanning electron photographs were taken. Pigment content of the disintegrated sample was 34%. The obtained pigment -cellulosic fiber composite is shown in figure 1 1 .Example 11

Gypsum filler pigment was precipitated as described in WO 2008/092991 . Hobart N50CE mixer was used as reactor. Unrefined fiber suspension had solids content of four per cent and it was placed in the reactor. Microfibrillated cellulose was added as retention aid to fiber suspension. 91 g of undispersed gypsum filler pigment powder was added into 800 g of pine kraft pulp suspension under stirring. Mixing was carried out for three minutes after the filler addition was completed. When the mixing was stopped sample was disintegrated using Lorentz & Wettre wet disintegrator according to ISO 5263 standard and 10000 cycles. Fiber samples were picked up and scanning electron photographs were taken. Pigment content of the disintegrated sample was 37%. The obtained pigment - cellulosic fiber composite is shown in figure 12. Example 12

Gypsum filler pigment was precipitated as described in WO 2008/092991 . Hobart N50CE mixer was used as reactor. Unrefined fiber suspension had solids content of six per cent and it was placed in the reactor. 73 g of undispersed platy gypsum filler pigment powder was added into 800 g of pine kraft pulp suspension under stirring. Mixing was carried out for three minutes after the filler addition was completed. When the mixing was stopped sample was disintegrated using Lorentz & Wettre wet disintegrator according to ISO 5263 standard and 10000 cycles. Fiber samples were picked up and scanning electron photographs were taken. Pigment content of the disintegrated sample was 26%. The obtained pigment -cellulosic fiber composite is shown in figure 13.

Example 13

Gypsum filler pigment was precipitated as described in WO 2008/092991 . Hobart N50CE mixer was used as reactor. Groundwood pulp suspension had solids content of six per cent and it was placed in the reactor. 90 g of undispersed platy gyp- sum filler pigment powder was added into 800 g of pine pulp suspension under stirring. Mixing was carried out for three minutes after the filler addition was completed. When the mixing was stopped sample was disintegrated using Lorentz & Wettre wet disintegrator according to ISO 5263 standard and 10000 cycles. Fiber samples were picked up and scanning electron photographs were taken. Pigment content of the disintegrated sample was 49%. The obtained pigment -cellulosic fiber composite is shown in figure 14.

Example 14

Precipitated Calcium carbonate filler pigment was taken. Hobart N50CE mixer was used as reactor. Unrefined fiber suspension had solids content of 6.5 % and it was placed in the reactor. 91 g of undispersed aragonite calcium carbonate filler pigment slurry was added into 732 g of pine kraft fiber suspension under stirring. Mixing was carried out for three minutes after the filler addition was completed. When the mixing was stopped sample was disintegrated using Lorentz & Wettre wet disintegrator according to ISO 5263 standard and 10000 cycles. Fiber samples were picked up and scanning electron photographs were taken. Pigment content of the disintegrated sample was 26%. The obtained pigment -cellulosic fiber composite is shown in figure 15. Example 15

Precipitated Calcium carbonate filler pigment was taken. Hobart N50CE mixer was used as reactor. Refined fiber suspension (SR (shopper riegler number) =29) had solids content of 4 % and it was placed in the reactor. 60 g of non-dispersed ara- gonite calcium carbonate filler pigment slurry was added into 800 g of pine kraft fiber suspension under stirring. Mixing was carried out for three minutes after the filler addition was completed. When the mixing was stopped sample was disintegrated using Lorentz & Wettre wet disintegrator according to ISO 5263 standard and 10000 cycles. Fiber samples were picked up and scanning electron photo- graphs were taken. Pigment content of the disintegrated sample was 34%. The obtained pigment -cellulosic fiber composite is shown in figure 16.

Example 16

Precipitated Calcium carbonate filler pigment was taken. Hobart N50CE mixer was used as reactor. Refined fiber suspension had solids content of 2 % and it was placed in the reactor. 30 g of non-dispersed aragonite calcium carbonate filler pigment slurry was added into 800 g of pine kraft fiber suspension under stirring. Mixing was carried out for three minutes after the filler addition was completed. When the mixing was stopped sample was disintegrated using Lorentz & Wettre wet disintegrator according to ISO 5263 standard and 10000 cycles. Fiber sam- pies were picked up and scanning electron photographs were taken. Pigment content of the disintegrated sample was 40%. The obtained pigment -cellulosic fiber composite is shown in figure 17. Example 17

Precipitated Calcium carbonate filler pigment was taken. Hobart N50CE mixer was used as reactor. Refined fiber suspension had solids content of 1 % and it was placed in the reactor. 15 g of non-dispersed aragonite calcium carbonate filler pigment slurry was added into 800 g of pine kraft fiber suspension under stirring. Mixing was carried out for three minutes after the filler addition was completed. When the mixing was stopped sample was disintegrated using Lorentz & Wettre wet disintegrator according to ISO 5263 standard and 10000 cycles. Fiber sam- pies were picked up and scanning electron photographs were taken. Pigment content of the disintegrated sample was 36%. The obtained pigment -cellulosic fiber composite is shown in figure 18. Example 18

Hobart N50CE mixer was used as reactor. Unrefined fiber suspension had solids content of six per cent and it was placed in the reactor. 100 g of predispersed platy kaolin filler pigment powder was added into 800 g of pine kraft fiber suspension under stirring. Mixing was carried out for three minutes after the filler addition was completed. When the mixing was stopped sample was disintegrated using Lorentz & Wettre wet disintegrator according to ISO 5263 standard and 10000 cycles. Fiber samples were picked up and scanning electron photographs were taken. Pigment content of the disintegrated sample was 26%. The obtained pigment - cellulosic fiber composite is shown in figure 19.

Example 19

Gypsum filler pigment was precipitated as described in WO 2008/092991 . Hobart N50CE mixer was used as reactor. Refined pine kraft pulp suspension had solids content of eight per cent and it was placed in the reactor. 50 g of undispersed gyp- sum filler pigment powder and 50 grams of calcium sulfate hemihydrate were added to the reactor. Pigment slurry was then added to 800 g of pine pulp suspension under stirring. Mixing was carried out for three minutes after the filler addition was completed. When the mixing was stopped sample was disintegrated using Lorentz & Wettre wet disintegrator according to ISO 5263 standard and 10 000 cy- cles. Fiber samples were picked up and scanning electron photographs were taken. Pigment content of the disintegrated sample was 34%. The obtained pigment -cellulosic fiber composite is shown in figure 20.

Example 20

Precipitated Calcium carbonate filler pigment was taken. Three experiments with fiber solids of 0.7, 0.5 and 0.3 % were carried out. 800 grams of refined pine kraft fiber suspension (SR = 29) was placed in a beaker and mixed with Diaf dissolver. 10.5, 7.5 and 4.5 g of non-dispersed aragonite calcium carbonate filler pigment slurry was added into 800 g of fiber suspension with the respective fiber solids under stirring. Mixing was carried out for three minutes after the filler addition was completed. When the mixing was stopped sample was disintegrated using Lorentz & Wettre wet disintegrator according to ISO 5263 standard and 10000 cycles. Pigment contents of the wet disintegrated samples are shown in following table 1 . Table 1

Example 21

Retention of rutile form titanium dioxide filler pigment was tested. Refined fiber suspension had solids content of 4 % and it was placed in Hobart N50CE reactor. 30 g of rutile pigment slurry was added into 800 g of refined pine kraft fiber suspension (SR = 29) under stirring. Mixing was carried out for three minutes after the filler addition was completed and then mixed for an additional three minutes with Diaf dissolver. When the mixing was stopped sample was disintegrated using Lor- entz & Wettre wet disintegrator according to ISO 5263 standard and 10000 cycles. Fiber samples were picked up and scanning electron photographs were taken. Ashing was carried out at 525 °C. Pigment content of the disintegrated sample was 49.4%. The obtained pigment -cellulosic fiber composite is shown in figure 21 . Example 22

Retention of anatase form titanium dioxide filler pigment was tested. Refined fiber suspension had solids content of 4 % and it was placed in Hobart N50CE reactor. 30 g of anatase pigment slurry (calculated as dry pigment) was added into 800 g of refined pine kraft fiber suspension (SR = 29) under stirring. Mixing was carried out for three minutes after the filler addition was completed and then mixed for an additional three minutes with Diaf dissolver. When the mixing was stopped sample was disintegrated using Lorentz & Wettre wet disintegrator according to ISO 5263 standard and 10000 cycles. Fiber samples were picked up and scanning electron photographs were taken. Ashing was carried out at 525 °C. Pigment content of the disintegrated sample was 50.7%. The obtained pigment -cellulosic fiber composite is shown in figure 22.