WO2014016469A1

WO2014016469A1 - A method and a system for preventing calender blackening

Info

Publication number: WO2014016469A1
Application number: PCT/FI2013/050767
Authority: WO
Inventors: Päivi SOLISMAA; Janne Varvemaa; Eero HAARLA; Kimmo FINNILÄ; Mikko Vuori
Original assignee: Upm-Kymmene Corporation
Priority date: 2012-07-27
Filing date: 2013-07-18
Publication date: 2014-01-30
Also published as: EP2877634A1; EP2877634B1; FI20125818A

Abstract

There is disclosed a method for preventing calender blackening of a printing paper product, a system for preventing calender blackening of a printing paper product, and a printing paper product obtainable by the method. The method comprises feeding cellulose based material comprising mechanical pulp to a system, feeding and admixing calcium hydroxide and carbon dioxide to the cellulose based fibres for precipitating calcium carbonate in a rhombohedral crystal form, and manufacturing a printing paper product comprising at least said cellulose based material and precipitated calcium carbonate in a rhombohedral crystal form. The amount of the precipitated calcium carbonate is at least 5 dry w-% calculated of the total amount of inorganic fillers in the printing paper product.

Description

A METHOD AND A SYSTEM FOR PREVENTING CALENDER BLACKENING

Field of the invention

The invention relates to a method and a system for preventing calender blackening. The invention also relates to a printing paper product.

Background of the invention

Printing paper is paper intended to be printed by printing machines and manufactured, for example, for magazines and newspapers. The printing paper may be coated, such as LWC paper (Light Weight Coated), or uncoated, such as SC paper (Super Calendered) or newsprint. The main raw material for the printing paper is usually cellulose based natural fibre, for example mechanically pulped natural fibre, chemically pulped natural fibre, and/or recycled fibre. In prior art, there are several established printing paper grades with their own established requirements for the properties of the paper.

Brief summary of the invention

The invention relates to a novel solution for preventing calender blackening during manufacturing process of a printing paper product. This invention further relates to a printing paper product. The printing paper product according to the present invention comprises continuously precipitated calcium carbonate (CPCC) in a rhombohedral crystal form.

The method according to the invention for preventing calender blackening is primarily characterized in what will be presented in claim 1 . The system according to the invention for preventing calender blackening is primarily characterized in what will be presented in claim 9. The printing paper product according to the invention is primarily characterized in what will be presented in claim 10. The printing paper product according to the present invention is always calendered, preferably supercalendered with a supercalender. It may be coated or uncoated, and it may comprise mechanically pulped cellulose based material (i.e. mechanical pulp) and/or chemically pulped cellulose based material (i.e. chemical pulp). Advantageously, the printing paper product comprises the mechanical pulp. Most advantageously, the printing paper product is uncoated and comprises the mechanical pulp.

The printing paper product may be, for example, fine paper (i.e. paper that does not comprise mechanical pulp), or LWC paper (i.e. Light Weight Coated paper, comprising mechanical pulp). However, most advantageously, the printing paper product according to the present invention is supercalendered paper (i.e. SC paper). SC paper is uncoated and calendered printing paper product comprising mechanical pulp. Its manufacturing process involves heavy calendering to obtain properties needed for printing papers without pigment coating.

Heavy calendering at thick web locations of the printing paper product may increase light penetration, hence, those locations seem distinctly darker than the surrounding area when observed at other angles than a gloss reflection angles. This phenomenon is called as calender blackening. The heavy calendering is used, for example, in the manufacture of an uncoated printing paper product to obtain sufficient smoothness and, thereby, suitable printing surface properties required of printing paper. Due to the calender blackening, a picture printed on the paper may change unwantedly. The problem may be, for example, in LWC papers and in some fine papers. However, most often this problem is in SC papers. Some factors, such as heavy loading, high moisture content, high temperature and uneven formation, may increase the risk for the calender blackening. In addition, as the filler content increases, the calender blackening typically increases at the same conditions. However, due to economic reasons, the filler content of the papers should typically be as high as possible.

The inventors of the present invention have surprisingly found that precipitated calcium carbonate (PCC) in a rhombohedral crystal form decreases the calender blackening phenomenon. Advantageously, the method for preventing calender blackening during manufacturing process of a printing paper product comprises:

- feeding cellulose based material comprising mechanical pulp to a system,

- feeding and admixing calcium hydroxide and carbon dioxide to the cellulose based fibres for precipitating calcium carbonate in a rhombohedral crystal form, and

- manufacturing a printing paper product comprising at least said cellulose based material and precipitated calcium carbonate in a rhombohedral crystal form, the amount of the precipitated calcium carbonate being at least 5 dry w-%, preferably at least 10 dry w-%, calculated of the total amount of inorganic fillers in the printing paper product.

Advantageously, the method further comprises:

- feeding clay to the system in such a way that the amount of the clay is between 10 and 70 dry w-% calculated of the total amount of inorganic fillers in the printing paper product.

Alternatively or in addition, the method further comprises:

- feeding aragonite PCC to the system in such a way that the amount of the aragonite PCC is between 10 and 50 dry w-% calculated of the total amount of inorganic fillers in the printing paper product.

Advantageously, the total amount of the PCC(s) calculated of the total amount of inorganic fillers in the printing paper product is at least 10 w-%, or at least 20 w-%, more preferably at least 40 w-% or at least 60 w-%, and most preferably at least 70 w-%, or at least 80 w-%, and 100 w-% at the most. In addition or alternatively, the method comprises

Introducing inorganic pigment into the system,

admixing calcium hydroxide and carbon dioxide to said inorganic pigment, for precipitating calcium carbonate in said inorganic pigment, and manufacturing a printing paper product comprising said inorganic pigment with precipitated calcium carbonate.

In addition or alternatively, the method comprises

- calendering the printing paper product to be manufactured with at least a gradient calender, a supercalender, or a soft calender, preferably at least with the supercalender.

Advantageously, the amount of the mechanical pulp is at least 50 dry w-% calculated of the total amount of the cellulose based materials.

Advantageously, the amount of the precipitated calcium carbonate is between 15 and 100 dry w-%, more preferably between 20 and 50 dry w-%, calculated of the total amount of inorganic fillers in the printing paper product.

Advantageously, the system for preventing calender blackening comprises:

- a paper machine,

- a supplying device for feeding cellulose based material comprising fibres to the system,

- means for adding calcium hydroxide and carbon dioxide and for precipitating calcium carbonate in a rhombohedral crystal form, and

- means for supplying the cellulose based material and precipitated calcium carbonate in the rhombohedral crystal form to the paper machine, for manufacturing a printing paper product.

Advantageously, the calender blackening of the product is 52 or 51 .5 at the most, more preferably 51 or 50.5 at the most, and most preferably 50 at the most. In an example, the method according to the invention comprises one or more of the following steps.

Feeding mechanic pulp into the system.

Feeding chemical pulp into the system.

Feeding at least one inorganic pigment into the system.

- Dividing at least part of the used raw materials into at least two separated accepted fractions. Admixing milk of lime and carbon dioxide for precipitating calcium carbonate onto the surfaces of the cellulose based material.

Admixing milk of lime and carbon dioxide to the inorganic pigment 2c, for precipitating calcium carbonate onto the surfaces of the inorganic pigment 2c.

Admixing at least one additional chemical into at least one formed fibre fraction.

Supplying the raw materials of the printing paper product either as a uniform pulp suspension or as separate material flows to a paper machine.

Forming a printing paper product in the paper machine.

Calendering the printing paper product with a calender.

The printing paper product according to the invention has preferably one, two, three, four, five, six, seven or all of the properties listed below:

- The printing paper product is SC paper.

- The printing paper product is uncoated.

- The printing paper product is calendered.

- The density of the calendered printing paper product is between 1000 and 1200 kg/m³, preferably between 1050 and 1 150 kg/m³, most preferably between 1 100 and 1 150 kg/m³.

- The gloss is over 35 %, preferably over 40 % and most preferably over 45 %.

- The porosity (Bendtsen) of the calendered printing paper product is below 30 ml/min, preferably below 25 ml/min and most preferably below 20 ml/min.

- The smoothness (PPS10) is below 1 .5 μιτι, preferably below 1 .35 μιτι and most preferably below 1 ,25 μιτι.

- The strength of the calendered printing paper is at a level sufficient for runnability at a printing works.

Thanks to the novel solution, the calendering blackening in the printing paper product may be avoided or at least decreased. Therefore, it is possible to manufacture a printing paper with otherwise similar properties, wherein the amount of the calender blackening is decreased. Description of the drawings

In the following, the invention will be described in more detail with reference to the appended drawings, in which

Figs 1 a-b show SEM images of printing paper products,

Figs 2a-g show reduced schematic views of elements in a system according to some embodiments of the invention for manufacturing a printing paper product, and shows an example of filler content of a printing paper product according to the invention.

Detailed description of the invention

In the following disclosure, all percentages are by dry weight, if not indicated otherwise. In this application, the measured values for density, gloss, grammage, porosity, and smoothness refer to measurements according to the following standards (valid in 201 1 ):

for density: ISO 534,

for gloss: ISO 8254-1 ,

for grammage: ISO 536,

for porosity: ISO 5636-3, and

for smoothness: ISO 8791 -4.

Calender blackening, i.e. Calender Blackening Index, CBI, was measured according to Dr. Hermann Praast theory using an apparatus of KP333, Emco Image Analyzer, Dr Praast together with a computer having lAS-analysis program. The apparatus KP333 was checked according to the manual of the apparatus before it was used for the measurements. The checking was implemented according to the manual of the apparatus by using MN033 checking normal. In addition, if needed, the apparatus was calibrated according to the manual. Also illumination was checked according to the manual. The lamp of the apparatus was checked regularly and, if needed, changed. A4-standard sized sheets were used. Every measured sample was clean and undamaged, as well as the apparatus used. Every sheet was measured 10 times / side. Five of the measurements were measured from the cross direction (CD) of the sheet, and five of the measurements were measured from the machine direction (MD) of the sheet.

Typically, if the measured value is < 49, the calender blackening is very weak or cannot be seen, therefore, it is almost always at an acceptable level. If the measured value is between 50 and 52, the calender blackening is weak but typically acceptable. If the measured value is between 53 and 55, the calender blackening can be seen, but it may, in some cases, be at acceptable level. If the measured value is between 56 and 60, the calender blackening is too strong and at unacceptable level. If the measured value is > 60, the calender blackening is very strong and cannot be acceptable.

The term "gsm" refers to g/m².

The term "cellulose based material" refers to various fibres and particles of natural origin. The cellulose based material may comprise, for example, mechanically pulped fibre, dissolving cellulose fibre, sulphite cellulose fibre, sulphate cellulose fibre, or viscose fibre and/or particles made of the above mentioned source(s). Advantageously, the cellulose based material is selected from the group of wood-based fibres, fibres of plant origin, and their derivatives and mixtures. The cellulose based material may be selected, for example, from the wood, sisal, jute, hemp, flax, straw, and other annual plants, as well as mixtures of these. Preferably, the printing paper product according to the invention comprises wood fibres as the main raw material. The content of cellulose based material in the printing paper product, calculated as dry content, is advantageously not higher than 80 w-% and more advantageously not higher than 70 w-%, but preferably at least 50 w-%. Advantageously, at least 30 wt.% or at least 40 wt.%, more preferably at least 50 wt.% or at least 60 wt. %, and most preferably at least 80 wt. % or at least 90 wt.% of the cellulose based material is wood based. The printing paper product according to the present invention preferably comprises mechanical pulp, i.e. mechanically pulped cellulose based particles and/or fibres. The mechanical pulp is preferably selected from thermomechanical pulp (TMP), chemithermomechanical pulp (CTMP), bleached chemithermomechanical pulp (BCTMP), pressurized ground wood (PGW), ground wood (GW), refiner mechanical pulp (RMP), recovered fibre (RCF), or mixtures of these. The main raw material is preferably softwood, such as pine, Southern pine, and/or spruce. Advantageously, the main raw material of natural fibre is spruce. Advantageously, the amount of the mechanical pulp is at least 30 wt.% or at least 40 wt.%, more preferably at least 50 wt.% or at least 60 wt. %, and most preferably at least 70 wt. % or at least 80 wt.% of the cellulose based material.

Alternatively or in addition to the mechanical pulp, the printing paper product may comprise chemical pulp, i.e. chemically pulped cellulose based material. In an example, the chemical pulp comprises pine, spruce, birch, eucalyptus, and/or acacia.

In this application, reference is made to Figs. 1 to 3, in which the following reference numerals are used:

1 printing paper product,

2 raw material(s) for the printing paper product,

2a mechanical pulp,

2b chemical pulp,

2c inorganic pigment,

3 separator,

4 means for precipitating calcium carbonate in a rhombohedral crystal form,

5 means for chemical processing of natural fibres, and

7 papermaking machine.

The process according to the present invention makes it possible to manufacture a printing paper product in such a way that the amount of calender blackening may be lower than in the solutions of prior art. The printing paper product according to the present invention always comprises precipitated calcium carbonate (PCC) in the rhombohedral crystal form. Figures 1 a to 1 b show SEM images of printing paper products according to the present invention comprising precipitated calcium carbonate (PCC) in the rhombohedral crystal form.

In the solution according to the present invention, the final product is a printing paper product, such as, for example, magazine paper or newsprint, most advantageously supercalendered (SC) paper, which contains precipitated calcium carbonate in the rhombohedral crystal form as a filler and cellulose based material in fibre form as the main fibre raw material. In addition, the printing paper product preferably contains aragonite PCC and/or clay. Advantageously, at least part of said cellulose based fibres and/or particles are at least partly coated with the precipitated rhombohedral crystal calcium carbonate. The PCC in the rhombohedral crystal form increases porosity of the printing paper product in such a way that the paper is still suitable for the printing purpose.

By the method according to the present invention, it is possible to manufacture, for example, SC paper in such a way that the product contains less calender blackenings than conventionally but still obtains the printing properties required for SC paper.

The grammage of the printing paper product is preferably between 27 and 80 gsm, more preferably between 35 and 70 gsm and most preferably between 45 and 60 gsm.

Advantageously the printing paper product comprises mechanical pulp. The amount of the mechanical pulp is preferably between 50 and 100 dry w-%, more preferably between 60 and 98 dry w-% or between 70 and 95 dry w-%,, and most preferably at least 80 or at least 90 w-% calculated of the total amount (dry weigh) of the cellulose based material in the printing paper product. According to an advantageous example, the mechanical pulp comprises or consists of TMP. Advantageously, the total inorganic filler content of the printing paper product is between 15 and 40 dry w-% of the printing paper product, more preferably between 20 and 36 dry w-% of the printing paper product and most preferably between 25 and 33 dry w-% of the printing paper product. The total inorganic filler content of the printing paper product typically depends on the type, grade and grammage of the final product.

In the solution according to the invention, the filler in the printing paper product comprises precipitated calcium carbonate in a rhombohedral crystal form. The amount of the PCC in the rhombohedral crystal form is preferably between 10 and 100 dry w-%, more advantageously between 15 and 50 dry w-% and most advantageously between 20 and 40 dry w-% calculated of the total amount of inorganic fillers in the printing paper product. If the printing paper product is the SC paper, the amount of the PCC in the rhombohedral crystal form is preferably between 10 and 50 dry w-%, more advantageously between 15 and 40 dry w-% and most advantageously between 20 and 30 dry w-% calculated of the total amount of inorganic fillers in the printing paper product.

Advantageously, the paper product also comprises clay and/or PCC in aragonite form (i.e. aragonite PCC), more advantageously the product comprises both clay and aragonite PCC. The amount of the clay is preferably between 10 and 50 dry w-%, more advantageously between 15 and 40 dry w-% and most advantageously between 20 and 30 dry w-% calculated of the total amount of inorganic fillers in the printing paper product. The amount of the PCC in aragonite form is preferably between 10 and 50 dry w-%, more advantageously between 15 and 40 dry w-% and most advantageously between 20 and 30 dry w-% calculated of the total amount of inorganic fillers in the printing paper product. PCC in the fine rhombohedral crystal form gives rather good pigment distribution, especially together with clay and aragonite PCC. The rhombohedral crystal form may give alone very high ink absorption. However, rhomobohedral PCC together with clay and aragonite PCC gives good ink absorption properties for the printing paper product.

Figures 2a to 2g show some examples of some steps of the system according to the invention in reduced schematic charts. Naturally, the system according to the invention may also comprise other partial steps or less partial steps of printing paper manufacture than those shown in Figures 2a-g. The system according to the present invention typically comprises a paper machine 7. The paper machine 7 typically comprises at least a forming section, a press section, and a drying section. In addition, the system typically comprises a short circulation, which typically comprises, among other things, a wire pit, deaeration means, as well as means for removing impurities. Calcium carbonate in the rhombohedral crystal form is precipitated 4 to the pulp suspension before the particles are led to the paper machine 7. The mechanical pulp comprising the PCC in the rhombohedral crystal form is then led to a paper machine 7.

Calcium carbonate is preferably precipitated from calcium hydroxide Ca(OH)₂ into the rhombohedral crystal form on the surfaces of the cellulose based material by first admixing carbon dioxide to the fibre fraction and then adding calcium hydroxide in an aqueous solution (i.e. so called milk of lime). Alternatively, carbon dioxide is precipitated from calcium hydroxide Ca(OH)₂ into the rhombohedral crystal form on the surfaces of cellulose based material by first admixing milk of lime to the fibre fraction and then precipitating the milk of lime with carbon dioxide to form calcium carbonate on the surfaces of the cellulose based material, such as fibres.

In continuous precipitation of calcium carbonate, calcium carbonate is precipitated in connection with a selected material in such a way that calcium carbonate adheres at least partly to the surfaces of the cellulose based particles. The solution according to the invention always comprises a step in which calcium carbonate in a rhombohedral crystal form is precipitated from calcium hydroxide Ca(OH)₂ in a continuous process (continuous precipitated calcium carbonate, CPCC), in such a way that calcium carbonate in a rhombohedral crystal form is precipitated onto the surfaces of mechanically pulped natural fibres and/or particles. The reactive mineral material used for said filler is calcium hydroxide, advantageously an aqueous solution of calcium hydroxide, which is also called milk of lime (MOL). The precipitating chemical used for said filler is advantageously carbon dioxide, preferably pure or almost pure carbon dioxide (degree of purity preferably at least 85 %, more preferably between 95 and 100%). In an example, the printing paper product comprises, in addition to the precipitated calcium carbonate in the rhombohedral crystal form, at least one inorganic pigment 2c other than said precipitated calcium carbonate in the rhombohedral crystal form. Advantageously, the other inorganic pigment comprises clay and/or aragonite PCC. Figure 2b shows a part of the system according to the present invention, wherein calcium carbonate is precipitated 4 before at least one kind of inorganic filler is further added to the system. Figure 2c shows a part of the system according to the present invention, wherein at least one kind of inorganic filler is further added to the system before calcium carbonate is precipitated 4. It is also possible to add at least one kind of inorganic filler at both stages, i.e. before the precipitation step and after said precipitation step.

Fig. 2d shows an example part of the system according to the present invention, in which the printing paper product comprises, in addition to precipitated calcium carbonate in the rhombohedral crystal form, at least one inorganic pigment 2c other than said precipitated calcium carbonate in the rhombohedral crystal form. In this case shown in Fig. 2d, milk of lime and carbon dioxide are supplied to an aqueous solution of the inorganic pigment 2c, for precipitating calcium carbonate onto the surfaces of said inorganic pigment. Thus, a new hybrid pigment is formed, in which said inorganic pigment is coated at least partly with precipitated calcium carbonate in the rhombohedral crystal form. According to an advantageous example, said inorganic pigment comprises titanium dioxide, kaolin, talc, milled calcium carbonate, chalk, feldspar, mica, and/or waste pigment flow from a deinking plant. Most advantageously, the said inorganic pigment comprises clay and/or aragonite PCC.

Figure 2f shows an example part of the system according to the present invention, wherein a separator 3 is used to divide pulp into at least two separated accepted fraction, which both are led to the paper machine 7.

Advantageously at least mechanical pulp is divided into at least two different fractions before calcium carbonate is precipitated 4 in order to obtain PCC in a rhombohedral crystal form in connection with at least one fraction of the mechanical pulp. Alternatively or in addition, chemical pulp can be divided into at least two different fractions before calcium carbonate is precipitated in order to obtain PCC in a rhombohedral crystal form in connection with at least one fraction of the chemical pulp. However, advantageously the precipitation of calcium carbonate is performed for at least one mechanical pulp fraction and/or to the whole pulp comprising cellulose based fibres. Advantageously, the mechanical pulp comprises or consists of bleached TMP which has been fractionated into at least two separate fibre fractions before precipitating 4 calcium carbonate in the rhombohedral crystal form onto the surface of at least one fibre fraction.

If the separator 3 is used, it may be, for example, a screen or a cyclone. According to an advantageous example, the separator 3 is a pressure screen.

When the reaction of precipitating calcium carbonate is allocated to one or more partial fractions of the fibre raw material, it is possible to reduce the content of so-called interfering substances (such as resin, dissolved and colloidal detrimental elements), which have been developed in said fraction in connection with pulping and/or bleaching, in the aqueous phase. This reduction may be, for example, at least 10 percentage units or at least 20 percentage units compared with a situation without the method according to the invention for producing a filler. The contents of interfering substances can be detected, for example, by measuring them on component level, for example, as contents in ppm. According to an advantageous example, the required quantity of retention material after the precipitation reaction is about 30% lower compared with a situation without the method of manufacture according to the invention. Figure 2g shows an example part of the system according to the present invention.

Advantageously, at least one mechanically pulped and/or chemically pulped fraction is treated 5 with a chemical, such as starch, after the fractionation 3 in the process according to the invention. According to an advantageous example, said chemical is selected from starch, cationic starch, xylan, galactoglucomannan, calcium hydroxide, peracetic acid, or other reacting chemicals, as well as other polymers with a long-chained and/or branched molecular structure, as well as various modifications of the above mentioned chemicals. According to an example, at least one mechanically and/or chemically pulped fraction is treated with a cationic chemical after the fractionation.

In an example (not shown in Figures), said chemical treatment 5 as well as said precipitation 4 of calcium carbonate are performed for at least one and the same fibre fraction. Thus, in an advantageous example, the adherence of the filler formed onto the surfaces of the natural fibre raw material in the calcium carbonate precipitation reaction 4 (CPCC) is intensified by a chemical treatment by dosing one or more chemicals into one or more fibre fractions in which calcium carbonate is precipitated. According to an advanta- geous example, said chemical to enhance the adherence is selected from starch, cationic starch, xylan, galactoglucomannan, calcium hydroxide, peracetic acid, or other reacting chemicals, as well as other polymers with a long-chained and/or branched molecular structure, as well as various modifications of the above mentioned chemicals.

In an example, said chemical treatment 5 and the precipitation 4 of calcium carbonate are performed for different fractions (shown in Fig 2g). Thus, in an example, the chemically treated fraction is returned to a different step in the process compared with the fraction in which calcium carbonate has been precipitated. Thus, the retention time of said chemically treated fraction in the process may be substantially different from the retention time of at least one other fraction.

The mechanical pulp 2a used as the raw material may be bleached already before it is supplied into the process according to the invention, and/or it can be bleached in the process according to the invention. According to an example, the bleaching of the mechanical pulp 2a is done in a process in which lignin causing blackening of the fibre raw material is diluted with chemicals. Such a bleaching process may be, for example, peroxide bleach- ing. According to an advantageous example, mechanical pulp is divided into at least two fractions or fibre fractions before a separate bleaching, after which one or more fibre fractions are led into said bleaching process. According to another example, said bleaching process is carried out on homogeneous mechanical pulp. The bleaching is suitably performed before the step of precipitating calcium carbonate.

According to an advantageous example, the dry content of mechanical pulp 2a which has been bleached at least partly is increased after the bleaching step. According to an example, the dry content of one or more natural fibre fractions is increased to a fibre consistency of at least 8% and preferably higher than 20%. The dilution of each pulp fraction to a predetermined consistency after the precipitation is preferably performed by using the circulation waters of the paper machine as the dilution water.

In an example, the printing paper product 1 according to the invention is formed by mixing all raw materials to a pulp suspension upstream of the paper machine 7 and by leading this pulp suspension to the paper machine, for forming a paper product. According to another advantageous example, the printing paper product 1 according to the invention is formed by forming partial fractions and leading the fractions, either as such or partly mixed with each other, to the paper machine 7, to form a printing paper product with a layered structure. Thus, the desired partial fractions can be led to the middle layer of the printing paper product, and the desired partial fractions to the surface layers of said paper. In this way it is possible to optimize the structure of the paper. According to an advantageous example, the fractions which improve the bulk and/or the strength in the z-direction are led to the middle layer of the paper, and the fractions which improve printability are led to the surface layers.

Example 1

Continuous PCC trials with rhombohedral PCC were implemented, wherein the effect of the rhombohedral PCC on the calender blackening of the printing paper was examined. Printing paper product comprising rhombohedral PCC as filler (test sample) was compared with a similar printing paper product that did not have rhombohedral PCC but aragonite PCC (reference sample). As can be seen from Table 1 , calender blackening was clearly decreased because of rhombohedral PCC. Maximum value of the test sample was only 52.5, and maximum value of the reference sample was 56.1 . Thus, the maximum value of the test sample was decreased more than 6.4 % thanks to the rhombohedral PCC. In addition, the median value of the test sample was 50.6/50.7 while the median value of the reference sample was 51 .8/51 .9. Thus, the median value of the test sample was decreased more than 2.3 % thanks to the rhombohedral PCC. This is very important difference, i.e. the calender blackening of the test sample was always below 53, i.e. it was always weak, while the calender blackening of the test sample was sometimes even strong and, hence, unacceptable.

Table 1 . Calender blackening of the reference sample vs. the test sample

Rhombohedral No rhombohedral

PCC PCC

Side 1 Side 2 Side 1 Side 2

median 50,8 50,6 51 ,9 51 ,8

mm 49,8 49,4 49,9 50

max 52,4 52,5 56,1 54,8

As can be seen, even the median value and maximum value measured from both sides was lower with rhombohedral PCC, there were no changes in other values. Therefore, the printing paper product comprising rhombohedral PCC as filler was as good as the test sample when compared other properties, even with the significantly decreased calender blackening values. This can be seen in Tables 2-3. Table 2. Properties of the test sample.

Rhombohedral PCC

PPS

Grammage Density Gloss S10 Porosity g/m² kg/m3 % μηη ml/min

Side 1 Side 2 Side 1 Side 2 median 50,9 1119 43,4 43,6 1,25 1,49 26 min 50,4 1098 42,1 41,4 1,2 1,39 23 max 51,2 1137 46,7 45,6 1,3 1,8 30

Table 3. Properties of the reference sample

No rhombohedral PCC

Gramm PPS

age Density Gloss S10 Porosity g/m² kg/m3 % μηη ml/min

Side 1 Side 2 Side 1 Side 2

median 50,9 1102 43,9 44,4 1,32 1,29 25 min 50,7 1064 40,3 40,6 1,21 1,18 19 max 53,5 1142 46,9 48 2,47 3,05 30

It should be noted that the invention is not limited solely to the examples presented in Figs.1 to 3 and in the above description, but the invention is characterized in what will be presented in the following claims.

Claims

Claims:

1. A method for preventing calender blackening of a printing paper product, wherein the method comprises:

- feeding cellulose based material comprising mechanical pulp to a system,

- manufacturing a printing paper product comprising at least said cellulose based material and precipitated calcium carbonate in a rhombohedral crystal form, the amount of the precipitated calcium carbonate being at least 5 dry w-% calculated of the total amount of inorganic fillers in the printing paper product.

2. The method according to claim 1 , wherein the method further comprises: - feeding clay to the system in such a way that the amount of the clay is between 10 and 50 dry w-% calculated of the total amount of inorganic fillers in the printing paper product.

3. The method according to any of the preceding claims, wherein the method further comprises:

feeding aragonite PCC to the system in such a way that the amount of the aragonite PCC is between 10 and 50 dry w-% calculated of the total amount of inorganic fillers in the printing paper product.

4. The method according to any of the preceding claims, wherein the amount of the mechanical pulp is at least 50 dry w-% calculated of the total amount of the cellulose based materials.

5. The method according to any of the preceding claims, wherein the method comprises

Introducing inorganic pigment (2c) into the system,

admixing calcium hydroxide and carbon dioxide to said inorganic pigment (2c), for precipitating calcium carbonate in said inorganic pigment, manufacturing a printing paper product comprising said inorganic pigment with precipitated calcium carbonate.

6. The method according to any of the preceding claims, wherein the method comprises

calendering the printing paper product to be manufactured with a supercalender.

7. The method according to any of the preceding claims, wherein the cellulose based material is supplied in the form of at least two separate fibre flows to the paper machine (7), for manufacturing a printing paper product having a layered structure.

8. The method according to any of the preceding claims, wherein the amount of the precipitated calcium carbonate is between 15 and 50 dry w-% calculated of the total amount of inorganic fillers in the printing paper product.

9. A system for preventing calender blackening of a printing paper product, the system comprising

- a paper machine (7),

- means (4) for adding calcium hydroxide and carbon dioxide and for precipitating calcium carbonate in a rhombohedral crystal form, means for supplying the cellulose based material and precipitated calcium carbonate in the rhombohedral crystal form to the paper machine (7), for manufacturing a printing paper product.

10. A printing paper product obtainable by a method according to any of the claims 1 to 8, wherein the printing paper product comprises precipitated calcium carbonate in a rhombohedral crystal form, and calender blackening of the product is < 52.