EP1458929A1

EP1458929A1 - Filler for the manufacture of base paper and method for the manufacture of base paper

Info

Publication number: EP1458929A1
Application number: EP02804592A
Authority: EP
Inventors: Markku Leskelä; Stina Nygard; Gudrun Pauler-Johansson; Maija Pitkänen; Eeva ANÄS
Original assignee: M Real Oyj
Current assignee: Metsa Board Oyj
Priority date: 2001-11-28
Filing date: 2002-11-21
Publication date: 2004-09-22
Also published as: AU2002366533B2; FI116573B; AU2002366533A1; WO2003050355A1; NZ533042A; FI20012328A; JP2005511916A; CN1596327A; US20040256067A1; CN1309904C; CA2466899A1; FI20012328A0

Abstract

The invention relates to the use of precipitated calcium carbonates (PCC) as fillers for the manufacture of base paper together with mechanical hardwood pulp and chemical softwood pulp, and to a method for the manufacture of base paper, in which method pulp produced from hard woods, in particular from the wood of the genus Populus, and chemical softwood pulp are used together with a PCC filler. The use of precipitated calcium carbonate (PCC) for the manufacture of base paper together with mechanical hardwood pulp and chemical softwood pulp is characterized in that >= 20 wt-% of the fibres of the mechanical hardwood pulp is included in the fibre size fraction of < 200 mesh and 10 - 40 wt-% of them is included in the fibre size fraction of 28/48 mesh, the brightness of the mechanical hardwood pulp is >= 75, the particle size distribution of the precipitated calcium carbonate is 90 % <= 9 microm, 50 % <= 5 microm and 20 % <= 1.5 microm, and the basis weight of the base paper is 25 - 150 g/m2.

Description

Filler for the manufacture of base paper and method for the manufacture of base paper

The present invention relates to the use of precipitated calcium carbonates (PCC) as fillers for the manufacture of base paper together with mechanical hardwood pulp and chemical softwood pulp, and to a method for the manufacture of base paper, in which method pulp produced from hard woods, in particular from the wood of the genus Populus, and chemical softwood pulp are used together with a PCC filler.

Prior art

In base papers, in particular in thin base papers, kaolins and ground calcium carbonates (GCC) are used as fillers. Thin base papers are traditionally manufactured in acidic conditions, wherein it is not possible to use calcium carbonates but other compounds, and thus PCC and GCC are used mainly in fine papers.

In coated base papers, the paper must be dense in order that coating colour shall stay on the surface of the paper forming a well-covering layer and shall not be able to penetrate into the base paper during the coating process. In order that coated paper should have good printability, it is desired that coating layers cover the base paper completely. Thin coating layers shall provide good coverage and the gloss of printed paper shall be high without disturbing variation. Moreover, the bonding strength of the paper, which is measured, when needed, as a Scott bond value, must be sufficiently high in order that the paper shall not crack during printing. Papers are printed typically with a heatset offset printing press, in which connection the surface of finished coated paper may bubble if the strength of the base paper in the z-direction is not sufficient. This phenomenon is particularly critical in the case of double-coated paper when the surface of the paper is dense and the vapours produced from the paper's own moisture in the dryer section of the heat offset printing press cannot escape through the surfaces of the paper. In view of the quality of base paper, the filler should additionally have a high opacity (light scattering) and brightness, but at the same time it is also very important that the filler does not reduce the bonding strength of the paper nor, on the other hand, increase the porosity of the base paper.

In sheet-fed offset printing, the strength in the z-direction is also critical even though printing ink is not dried in a separate dryer. In sheet-fed offset, the requirement for strength is caused by the fact that printing inks are very viscous and, at the outlet side of the printing nip, the paper is subjected to a force that tends to crack the paper.

As known, precipitated calcium carbonates (PCC) are good fillers in particular in uncoated fine papers. They provide high brightness and opacity and, as compared with other mineral fillers, bulk is also better. The advantages attained are basically based on the fact that in the process of manufacturing PCC the shape and size of particles can be controlled by varying process control parameters. High brightness is based on the fact that, since the process is synthetic, by means of pure raw materials it is possible to obtain an end result that is better than that of ground calcium carbonates. The manufacture of PCC is also economically competitive in large plants as compared with other fillers.

In view of papermaking, the shape and size of the filler particles can be adjusted within fairly large limits. This has led, among other things, to the fact that PCC pigments have also become competitive as coating pigments. Calcium carbonate pigments can generally be used only in papermaking processes that have a neutral or alkaline pH. Carbonates dissolve under acidic conditions. It has been possible to avoid this limitation as well by developing PCC pigments that can be used in slightly acidic conditions.

PCC pigments have achieved a strong position as fillers of uncoated fine papers, the particles being typically complex in shape and fairly large in size, and also in coatings of fine papers, the particles being typically simpler and smaller in size.

Conventionally, PCC fillers are used in uncoated office papers in proportions of over 15 wt-%, the aim being to increase brightness and opacity and to reduce costs. The particle morphology of PCC has been designed so that bulk is also preserved as well as possible. As a result of this, the particles of PCC are "sea urchin"-like in morphology, which is again very weak from the viewpoint of the density and bonding strength of the web. Synthetic precipitated PCC is either aragonite or calcite depending on the manufacturing conditions. Typically, aragonite is needle-like in morphology, which is suitable for the coating of paper. Calcite precipitates as scalenohedral, i.e. mainly grain-shaped, or light-scattering rhombohedral, i.e. cubic agglomerates, which increase the bulk of paper, or as prismatic, i.e. semolina-shaped, spherical or roundish particles, which are often complex in shape, relatively large in size and represent, from the viewpoint of paper, a compromise in respect of their properties. The appended figure 1 illustrates the particles by means of SEM pictures. The particles used in the coatings of fine papers are simpler in type and smaller in size.

However, the PCC pigments have not achieved any particular success as fillers of thin base papers. In these products, it is desired that, in respect of quality, the filler shall have high brightness and opacity, i.e. light-scattering, but at the same time it is very important that the fillers do not lower the bonding strength of the paper and, on the other hand, do not increase the porosity of the base paper. In the development of paper, attempts are generally made to reduce the basis weights of paper, whereby opacity decreases, which, in turn, must be compensated for by increasing the light-scattering coefficient. In addition, density naturally deteriorates, i.e. decreases when the basis weight is reduced. The continuously increasing running speeds of paper machines and coating machines further emphasize the criticality of density. A problem with known PCC fillers is that they do not meet simultaneously all the requirements associated with the manufacture of thin base paper. Typically, the brightness and opacity of the base paper may be high but, at the same time, the bonding strength of the base paper has decreased to an alarming extent and its density has deteriorated such that the coating colour has penetrated into the base paper.

FI patent 100 729 describes a filler used in paper manufacture and mainly consisting of calcium carbonate, and a method for its manufacture. In the method, calcium carbonate is precipitated onto the surface of noil fibrils produced from cellulose fibre and/or mechanical pulp fibre by refining. The noil fibrils, onto whose surface calcium carbonate particles have precipitated, form fibres that resemble pearl necklaces, and the thus produced calcium carbonate aggregates resemble mainly clusters of pearl necklaces. This filler that is based on cellulose fibre or mechanical pulp fibre and on calcium carbonate imparts good optical properties and good strength to the paper.

FI patent 103 417 discloses a method for manufacturing a base paper suitable for the manufacture of coated fine paper by combining groundwood pulp made from hardwood and chemical softwood pulp. In the method, mechanical pulp manufactured from aspen or from wood of the genus Populus and chemical softwood pulp are used as a combination, thereby producing a pulp suitable for the manufacture of base paper in respect of its strength properties. The advantages of aspen pulp include high brightness and brightness stability as compared with spruce groundwood. This is due in particular to the low lignin content of aspen groundwood pulp or equivalent mechanical pulp and to its low concentration of carbonyl groups. By this means it is possible to manufacture fine paper with high brightness with a lower basis weight than usual.

Based on the foregoing it may be noted that there exists an obvious need for a PCC type filler which is suitable in particular for the manufacture of thin base paper together with mechanical hardwood pulp and chemical softwood pulp and which meets simultaneously all the requirements placed on the filler of base paper, as well as for a method for the manufacture of a base paper in which mechanical pulp based on hard woods, such as aspen or trees of the genus Populus, and chemical softwood pulp are used together with a PCC filler.

Object of the invention

An object of the invention is to provide a method for the manufacture of base paper, in particular for the manufacture of thin base paper, by means of which a base paper is obtained that has a low basis weight, a good internal strength and density, as well as improved light-scattering and opacity.

An object of the invention is also the use of precipitated calcium carbonate (PCC) as a filler for the manufacture of base paper together with mechanical hardwood pulp and chemical softwood pulp, which filler meets in particular the requirements placed above on the filler of base paper.

An object of the invention is further to provide a method for the manufacture of base paper, in which method mechanical pulp based on hard woods, such as aspen or trees of the genus Populus, and chemical softwood pulp are used together with a PCC filler, whereby all the desired properties of the base paper can be achieved simultaneously. Features characteristic of the invention

The features characteristic of the use of a PCC filler in the manufacture of base paper together with mechanical hardwood pulp and chemical softwood pulp, as well as of the method for the manufacture of base paper according to the invention are set forth in the claims.

Description of the invention

It has been found that when mechanical hardwood pulp and chemical softwood pulp are used together with a PCC filler for the manufacture of base paper, unexpectedly all the characteristics required of base paper can be achieved simultaneously. In the method in accordance with the invention, mechanical hardwood pulp, in particular pressure groundwood (Pressure Ground Wood, PGW) or chemimechanical pulp (CTMP), made from aspen or from wood species of the genus Populus, is advantageously combined with chemical softwood pulp and a PCC filler is used which has a particle size distribution of 90 % < 9 μm, 50 % < 5 μm and 20 % < 1.5 μm and which is semolina-shaped, grain-shaped or spherical in its morphology.

For the base paper it is possible to use mechanical hardwood pulp which is advantageously produced from aspen or from hard woods of the genus Populus, i.e. wood species of the genus Populus, which are preferably selected from the group of the following species: P. tremula, P. tremuloides, P. balsamea, P. balsamifera, P. trichocarpa and P. heterophylla, or aspen species crossbred from different mother aspens, such as hybrid aspen species, and species produced by gene technology, or poplar, or from a blend of mechanical pulps produced from the above-mentioned species. Particularly preferable wood species are native aspen P. tremula, Canadian aspen P. tremuloides, poplar and hybrid aspen. Mechanical hardwood pulp may optionally contain 70 wt-% of spruce or pine at the maximum. Instead of aspen it is also possible to use other hardwood, such as, birch, eucalyptus or acacia. Such mechanical hardwood pulps can be prepared as blends that contain, for example, two hard woods and then, for example, spruce, as softwood. The proportion of spruce may be 70 % at the maximum, preferably 50 % at the maximum, and particularly preferably below 30 %.

Mechanical hardwood pulps have shorter fibres than chemical birch pulp or mechanical spruce pulps. Therefore the same basis weight of mechanical hardwood pulp comprises more fibres than chemical birch pulp or mechanical spruce pulp. This results in a higher light-scattering ability, a good formation, i.e. a lower variation of basis weight in a small scale, in a low surface roughness, and bulk is also good.

Mechanical hardwood pulp and in particular chemimechanical pulp (CMTP) and pressure groundwood (PGW) made from aspen or from other wood species of the genus Populus contain a large quantity of short fibres, which impart bulk and light-scattering to the pulp. By combining mechanical pulp made from aspen or from wood species of the genus Populus with chemical cellulose pulp made from softwood using a PCC filler it is possible to produce a base paper that has excellent light-scattering and opacity, a high brightness and an even surface, a good strength and density.

20 - 70 wt-% of mechanical hardwood pulp, preferably aspen-CTMP or aspen- PGW or a combination of them, and 80 - 30 wt-% of bleached chemical softwood pulp, calculating from the dry solids of stock, are used, and at least 20 % of the fibres of the mechanical hardwood pulp are included in the fibre size fraction of < 200 mesh and preferably 10 - 40 wt-% of them are included in the fibre size fraction of 28/48 mesh. Preferably, at least 30 wt-% and particularly preferably at least 50 wt-% of the fibres of the hardwood pulp originate from aspen, hybrid aspen and/or poplar. Hardwood raw material is chipped, after which mechanical pulp, refiner mechanical pulp (TMP) or chemimechanical pulp (CTMP) is manufactured from the chips in a manner known in itself. When a groundwood method is used (GW or PGW), raw material is fed into the process as logs. A stock is prepared from the mechanical pulp together with chemical pulp, and a PCC filler having a particle size distribution of 90 % < 9 μm, 50 % < 5 μm and 20 % < 1.5 μm is used as a filler. In addition, the stock may contain additives, such as different starches, starch derivatives and retention agents. The dry solids content of the stock is 0.1 - 5 wt-%. As the water phase of the stock it is possible to use, for example, the circulation water of the paper machine. Bleached chemical softwood pulp is preferably used as the chemical pulp. The amount of the mechanical pulp is then 20 - 80 wt-%, preferably 30 - 70 wt-% and the amount of the bleached chemical softwood pulp is 80 - 20 wt-%, preferably 70 - 30 wt-%, calculated from the dry solids of the stock.

A paper web is formed for a paper machine from the stock of mechanical hardwood pulp and chemical pulp in accordance with the prior art, for example, by using a gap former.

In the laboratory tests carried out it has been found that, when using mechanical hardwood pulp, preferably aspen-based pulp, having a low freeness, CFS < 150 ml and a high brightness > 75, preferably > 77, and chemical softwood pulp together with a PCC filler having a particle size distribution of 90 % < 9 μm, 50 % < 5 μm and 20 % < 1.5 μm, preferably 90 % < 6.3 μm, 50 % < 2.7 μm and 20 % < 0.8 μm, and particularly preferably 90 % < 4.3 μm, 50 % < 1.8 μm and 20 % < 0.5 μm and which is grain-shaped, semolina-shaped or spherical in its morphology, it is possible to manufacture a thin base paper which, in respect of its quality, meets the requirements placed on base paper and the properties of which are stated below: • basis weight 25 - 150 g/m², preferably 25 - 80 g/m²,

• light-scattering coefficient > 45, preferably 50 and very preferably > 55,

• bonding strength > 200, preferably > 250 and very preferably > 300, • porosity of paper < 500 ml/min, preferably < 300 ml/min and very preferably < 250 ml/min (in basis weight of 50 g/m²),

• bulk of paper 1.2 - 1.8 cm³/g

• brightness > 75 %.

The fibre composition of the base paper in accordance with the invention comprises 20 - 70 wt-% of mechanical hardwood pulp, preferably aspen-based pulp, most preferably aspen groundwood or aspen-CTMP pulp and very preferably aspen-CTMP pulp, and 80 - 30 wt-% of chemical softwood pulp, preferably bleached chemical pine pulp.

The base paper can be coated by any suitable method known in the art, whereby a coating layer having a basis weight of 5 - 50 g/m², preferably 5 - 30 g/m², is formed on at least one surface, preferably on both surfaces of the paper web.

High-quality fine paper is obtained from the base paper in accordance with the invention by coating it with a suitable pigment-containing coating colour. The coating colour can be applied to the material web in a manner known in itself. The coating of the paper can be carried out on-line or off-line by means of a conventional coating device, i.e. by blade coating, or by means of film coating or surface spraying.

The base paper manufactured by the method in accordance with the invention and the coated paper manufactured further from it can be calendered by any calendering method known in the prior art. Preferably, calendering is accomplished on-line as soft calendering, thereby obtaining smooth and glossy or matte-surfaced products whose bulk, opacity and stiffness meet the requirements. Examples

The invention is illustrated with the help of the following examples. However, it is clear to a person skilled in the art that the invention is not meant to be limited to the embodiments of the examples only.

Example 1

Laboratory tests with different fillers

In a series of tests, laboratory sheets were produced on a Formette Dynamique sheet mould. The fibre pulps, broke and different fillers described below were used at test points. A standard amount of retention agents and starch was also used.

The following fibre pulps were used in the tests:

- Nordic bleached chemical softwood pulp, which was refined to an SR number of 24 on an Escher Wyss laboratory refiner; - bleached CTMP containing 85 % aspen and 15 % spruce, which was refined to a CSF value of 48 ml on a Voith Sulzer laboratory refiner (consistency 4.2 %, specific edge load 0.3 J/m, specific energy consumption 90 kWh/t). The fibre distribution of the pulp and its most important paper technical values measured from laboratory sheet were:

- length-weighted fibre length: 0.69 mm

- McNett classification +28 mesh: 3.3 %

- McNett classification 28/48 mesh: 39.4 %

- McNett classification 48/100 mesh: 22.4 % - McNett classification 100/200 mesh: 11.0 %

- McNett classification -200 mesh: 23.9 % - tensile index 37.8 Nm/g

- tear index 3.4 mNm²/g - bulk 1.79 cm³/g - Scott bond 152 J/m² - scattering coefficient 47.4 m²/kg

- brightness 80.9

There was in total 12 % of filler at all test points. In a paper mill manufacturing coated paper, in practical conditions, the filler of base paper is composed both of so-called fresh filler and of the filler coming from coated paper broke (the filler containing the mineral pigments of coating layer). For this reason, the filler was dosed to some of the test points as coated broke from a paper mill, in which broke the mineral main component was the ground carbonate contained in the coating. At these points, fresh filler constituted 6 percentage units of the filler and the filler coming from broke constituted 6 percentage units thereof.

The fillers used are shown in the following Table 1 and the appended Fig. 2 illustrates the particle size distributions graphically. The values have been measured on a Malvern Master Size device.

Table 1

The properties of the sheets are shown in the following Table 2. The tensile and tear indexes are geometric means from the machine and cross-direction results

Table 2

It is seen from the results, as expected, that brightness and the scattering coefficient become higher when PCC is used as compared with calcium carbonate. Unexpectedly, it is seen, however, that, in addition to superior optical properties, PCC1 and particularly PCC2 can additionally even improve the strength properties. This has occurred at the points comprising broke in respect of Scott bond bonding strength and tear strength. The porosity of papers has also remained at a sufficiently low level and also here attention is drawn to the surprisingly good results of PCC2.

Example 2

Manufacture of base paper on a pilot paper machine as well as its coating and calendaring

The following fibre pulps were used in tests:

- mill-ground Nordic bleached chemical softwood pulp,

- bleached CTMP containing 85 % aspen and 15 % spruce, which was after- refined on a pilot-scale refiner with a low specific edge load while the specific energy consumption was 140 kWh/t. Before the after-refining, the CSF of the pulp was 115 ml. After the after-refining, the most important properties of the pulp and laboratory sheets were:

- CSF 59 ml

- length- weighted fibre length 0.73 mm

- McNett classification +16 mesh: 0.0 % - McNett classification 16/30 mesh: 5.4 %

- McNett classification 30/50 mesh: 31.3 %

- McNett classification 50/200 mesh: 34.1 %

- McNett classification -200 mesh: 29.1 %

- brightness 78.6 - tensile index 46.7 Nm/g

- tear index 3.9 mNm /g - density 570 kg/m³ (bulk 1.75 cm³/g)

- Scott bond 329 J/m²

- scattering coefficient 51.5 m²/kg.

When these results are compared with the properties of the pulp after-refined in the laboratory in Example 1, it is seen that the pilot-scale refiner has yielded clearly more 200 mesh fraction, whereas there is clearly less 30/50 mesh fraction (corresponds to the fraction 28/48 mesh in Example 1) in the pilot test. The result is typical and it has been found in practice that the refiner used in the pilot test corresponds to the paper mill conditions very well. The refiner is equivalent to a mill refiner in its structure, however, so that the size of the pilot refiner is smaller.

Based paper was manufactured from the pulps on a pilot-scale paper machine. The paper machine had a gap former. The same fillers were added to the paper as in Example 1 as follows:

- PCC1, scalenohedral

- PCC2, rhombohedral.

A comparison with ground calcium carbonates was not made any more but two of the best PCCs were included in the test.

Standard amounts of starch and the same retention agents were added to the stock at all test points.

The base papers were made into rolls which were dried and then analyzed. The measurement results of the following Table 3 are means from the measurements of four rolls. The tensile and tear indexes are geometric means in the machine and cross direction. Table 3

It is seen from Table 3 that the properties of the base papers were relatively close to each other. The comparison is made more difficult by the higher content of PCC1. As in the laboratory test, better optical properties, i.e. brightness and scattering coefficient, were achieved with PCC2.

After that, the base papers were coated with the same coating colours and supercalendered. The target coating amount was 18 g/m² on both sides of paper. The lineal pressure of calendering was 280 kN/m.

The most important properties of the finished paper are shown in the following Table 4 (the properties are the means of the sides of the paper). Table 4

According to Table 4, the only clear difference after coating is brightness, which is better in the case of PCC2. The usual expectation is, however, that if brightness is as many as 1.3 units higher in finished paper, the opacity of the paper having higher brightness is expected to be clearly lower than it is in the paper having lower brightness. Thus, the optical properties of the PCC2-containing paper must be considered superior to those of the PCCl paper. There is no practical difference in the surface properties of the paper (smoothness, gloss).

Claims

1. The use of precipitated calcium carbonate (PCC) for the manufacture of base paper together with mechanical hardwood pulp and chemical softwood pulp, characterized in that > 20 wt-% of the fibres of the mechanical hardwood pulp is included in the fibre size fraction of < 200 mesh and 10 - 40 wt-% of them is included in the fibre size fraction of 28/48 mesh, the brightness of the mechanical hardwood pulp is > 75, the particle size distribution of the precipitated calcium carbonate is 90 % < 9 μm, 50 % < 5 μm and 20 % < 1.5 μm, and the basis weight of the base paper is 25 - 150 g/m².

2. The use as claimed in claim 1, characterized in that the brightness of the mechanical hardwood pulp is > 77, the particle size distribution of the precipitated calcium carbonate is 90 % < 6.3 μm, 50 % < 2.7 μm and 20 % < 0.8 μm, and the basis weight of the base paper is 25 - 80 g/m².

3. The use as claimed in claim 1 or 2, characterized in that the mechanical hardwood pulp has been manufactured from aspen and/or from wood species of the genus Populus.

4. The use as claimed in any one of claims 1 to 3, characterized in that the mechanical hardwood pulp contains CTMP pulp or pressure groundwood (PGW) or a combination of them, and the hardwood pulp has been manufactured from wood species of the genus Populus selected from the group of the following species: P. tremula, P. tremuloides, P. balsamea, P. balsamifera, P. trichocarpa, P. heterophylla, aspen species crossbred from different mother aspens, such as hybrid aspen species, species produced by gene technology, and poplar.

5. The use as claimed in any one of claims 1 to 4, characterized in that at least 30 % of the fibres of the mechanical hardwood pulp originates from aspen and/or from wood species of the genus Populus.

6. The use as claimed in any one of claims 1 to 5, characterized in that the mechanical hardwood pulp contains at least 50 % fibres that originate from aspen and/or wood species of the genus Populus.

1. The use as claimed in any one of claims 1 to 6, characterized in that the mechanical hardwood pulp contains 70 - 100 % fibres that originate from aspen and/or trees of the genus Populus and 0 - 30 % fibres that originate from soft woods and/or other hard woods.

8. A method for the manufacture of base paper, in which method a paper web is formed from a fibre raw material for a paper machine, characterized in that the fibre raw material is formed by combining precipitated calcium carbonate (PCC) with mechanical hardwood pulp and chemical softwood pulp, and > 20 wt-% of the fibres of the mechanical hardwood pulp is included in the fibre size fraction of < 200 mesh and 10 - 40 wt-% of them is included in the fibre size fraction of 28/48 mesh, the brightness of the mechanical hardwood pulp is > 75, the particle size distribution of the precipitated calcium carbonate is 90 % < 9 μm, 50 % < 5 μm and 20 % < 1.5 μm, and the basis weight of the base paper is 25 - 150 g/m².

9. A method as claimed in claim 8, characterized in that the brightness of the mechanical hardwood pulp is > 77, the particle size distribution of the precipitated calcium carbonate is 90 % < 6.3 μm, 50 % < 2.7 μm and 20 % < 0.8 μm, and the basis weight of the base paper is 25 - 80 g/m .

10. A method as claimed in claim 8 or 9, characterized in that the mechanical hardwood pulp has been manufactured from aspen and/or from wood species of the genus Populus.

11. A method as claimed in any one of claims 8 to 10, characterized in that the mechanical hardwood pulp contains CTMP pulp or pressure groundwood (PGW) or a combination of them, and the hardwood pulp has been manufactured from wood species of the genus Populus selected from the group of the following species: P. tremula, P. tremuloides, P. balsamea, P. balsamifera, P. trichocarpa, P. heterophylla, aspen species crossbred from different mother aspens, such as hybrid aspen species, species produced by gene technology, and poplar.

12. A method as claimed in any one of claims 8 to 11, characterized in that at least 30 % of the fibres of the mechanical hardwood pulp originates from aspen and/or wood species of the genus Populus.

13. A method as claimed in any one of claims 8 to 12, characterized in that the mechanical hardwood pulp contains at least 50 % fibres that originate from aspen and/or wood species of the genus Populus.

14. A method as claimed in any one of claims 8 to 13, characterized in that the mechanical hardwood pulp contains 70 - 100 % fibres that originate from aspen and/or wood species of the genus Populus and 0 - 30 % fibres that originate from soft woods and/or other hard woods.