Refined cellulose fiber composition

Abstract

The present invention relates to a purified cellulose fiber composition useful as a strength enhancer for paper and paperboard, the purified cellulose fiber composition having a Schopper-Riegler (SR) number in the range of 80 to 98 as determined by standard ISO 5267-1, and a content of at least 12 million fibers having a length >0.2 mm per gram on a dry weight basis. The present invention also relates to a method for preparing the purified cellulose fiber composition, and to pulp paper and paperboard comprising the purified cellulose fiber composition.

Description

[Technical field]

[0001] Technical Field The present disclosure relates to strength enhancers for improving the strength properties of paper or paperboard, particularly Z strength and / or tensile strength. The disclosure further relates to the manufacture of such strength enhancers and to paper or paperboard containing such strength enhancers. [Background technology]

[0002] background Paperboard is composed of several layers (also called plies) of pulp and optional additives. These layers are selected and arranged to achieve the desired properties of the paperboard. The essential property of paperboard is its bending stiffness. The bending stiffness of paperboard is usually established by outer plies with high tensile stiffness and one or several lofty plies in between, so that the outer plies are placed at a desired distance from each other. The lofty ply / plies are often interlayer / interlayers.

[0003] The middle layer of the paperboard may be composed of mechanical pulps such as thermomechanical pulp (TMP) or chemi-thermomechanical pulp (CTMP), which generally have a higher bulk and therefore can be used to construct paperboard with the desired high stiffness at a lower gram weight compared to, for example, chemical pulps.

[0004] In the CTMP process, wood chips are impregnated with a lignin softening agent before pressure refining. This softens the lignin and causes fiber breakage during refining to be concentrated in the lignin-rich middle lamellae. This results in stiffer fibers and fewer fines and shives for a given energy input compared to TMP. A high content of long fibers is important for all products where bulk is required. Therefore, CTMP has an advantage over TMP for paperboard.

[0005] Paper strength is measured in three dimensions: along the grain (X-direction), along the cross grain (Y-direction) and perpendicular to the surface of the paper (Z-direction). The force required to tear a paper sample is recorded as the internal bond strength, or Z-direction tensile strength. High interlayer Z-strength is desired to avoid delamination of the paperboard interlayer and therefore delamination of the paperboard. However, such Z-strength must be achieved without compromising bending stiffness, i.e., without increasing the density of the paper web.

[0006] The Z-strength and density of a board layer are usually optimized by changing raw materials, choosing different operating conditions on the stock preparation and board making machine, and adding papermaking chemicals. Like many other strength properties, Z-strength increases with increasing density, an effect that is attributed to an increase in the bond area between the fibers. The relationship between density and out-of-plane strength may vary with pulp type and densification method. Refining increases strength more than wet pressing. The main purpose of refining is to improve fiber bonding. Changes that improve fiber bonding are internal and external fibrillation and fines generation. These three changes improve the pulp's water retention, density, strength properties such as tensile strength, stiffness, bursting strength, and compressive strength, as well as Z-strength.

[0007] While CTMP is bulky, its strength in the Z direction is relatively low.

[0008] It is not just paperboard that requires high peel resistance and bending stiffness: these properties are important in printing, converting and end-use applications, for example: making paper or board with high strength in the Z direction is crucial for many paper products.

[0009] Strengthening of textile and board products can be achieved by increasing fiber-to-fiber contact, such as surface fibrillation, the use of modified fibers, or the use of natural or synthetic strength enhancers such as polysaccharides. A recent development trend is the use of nanocellulose as a strength enhancer. Although nanocellulose is very useful as a strength enhancer, the manufacturing concept of nanocellulose is usually very energy demanding. To reduce energy costs, chemical pretreatments such as enzymatic pretreatment or derivatization, or fiber swelling have been proposed. However, biological, chemical and physicochemical treatments all require separate processing steps and require additional process solutions and investments if adopted for pulp mill integration / environment.

[0010] Therefore, improvements are needed to improve the tensile strength and Z strength of paper and paperboard. Summary of the Invention [Means for solving the problem]

[0011] Contents of the invention A specific object of the present invention is to provide a new type of strength enhancer which imparts excellent strength properties to paper and paperboard, particularly excellent tensile strength and excellent Z strength.

[0012] It is a further object of the present invention to provide a cellulosic strength improver that can be produced using less energy than previous cellulosic strength improvers.

[0013] It is a further object of the present invention to provide a method for producing a paper or paperboard strength enhancer that requires less energy than the production of conventional cellulosic strength enhancers.

[0014] Moreover, a further object of the present invention is to provide a method for producing strength enhancers for paper and paperboard that can be integrated into a pulp mill without requiring investment in special equipment such as fluidizers or homogenizers.

[0015] The above objectives, as well as other objectives which will occur to those skilled in the art in light of the present disclosure, are achieved by various aspects of the present disclosure.

[0016] According to a first aspect exemplified herein, there is provided a purified cellulose fiber composition, the purified cellulose fiber composition having a Shopper-Riegler number (SR value) in the range of 80-98 as determined by standard ISO 5267-1, and the purified cellulose fiber composition having a content of at least 12 million fibers having a length >0.2 mm per gram on a dry weight basis. The content of fibers having a length >0.2 mm can be measured, for example, using an L&W Fiber tester Plus instrument (L&W / ABB).

[0017] The refined cellulose fiber composition of the present invention, which has a high SR value and a high content of fibers with lengths >0.2 mm, has been shown to be highly effective as a strength enhancer when incorporated into CTMP sheets, and has the potential to become a sustainable alternative as a raw material reducer in paper and board production.

[0018] The composition of the present invention is a refined cellulose fiber composition. The term "cellulose fiber" as used herein refers to natural cellulose fibers, i.e., not regenerated or manufactured fibers such as lyocell and viscose fibers. Natural cellulose fibers are still recognizable as being from a part of the original plant, as they have been processed as necessary to clean the fibers for use. Refining or beating cellulose pulp is the mechanical treatment of cellulose fibers to give them desired properties.

[0019] In some embodiments, the purified cellulose fiber composition has a Shopper-Riegler (SR) number (SR value), as determined by standard ISO 5267-1, in the range of 85 to 98, preferably in the range of 90 to 98. In some embodiments, the purified cellulose fiber composition has a Shopper-Riegler (SR) number, as determined by standard ISO 5267-1, in the range of 92 to 98, preferably in the range of 94 to 98.

[0020] In some embodiments, the purified cellulose fiber composition has a content of fibers having a length greater than 0.2 mm of at least 15 million fibers per gram on a dry weight basis, preferably at least 17 million fibers per gram on a dry weight basis, and more preferably at least 20 million fibers per gram on a dry weight basis.

[0021] A refined cellulose fiber composition typically contains no more than 50 million fibers per gram on a dry weight basis, preferably no more than 40 million fibers per gram on a dry weight basis, and more preferably no more than 30 million fibers per gram on a dry weight basis, having a length >0.2 mm.

[0022] In some embodiments, the purified cellulose fiber composition has a content of fibers having a length >0.2 mm in the range of 15-50 million fibers per gram on a dry weight basis, preferably in the range of 17-40 million fibers per gram on a dry weight basis, and more preferably in the range of 20-30 million fibers per gram on a dry weight basis. It has been found that a particular range of the content of fibers having a length >0.2 mm provides a favorable balance between performance and process economics.

[0023] The content of fibres 0.2mm or longer can be measured using the Fiber Tester Plus.

[0024] In some embodiments, the refined cellulose fiber composition has a krill value of at least 1.7, preferably at least 1.8, and more preferably at least 1.9. The krill value of the refined cellulose fiber composition will typically be less than 2.5. The krill value is measured using a Fiber Tester Plus.

[0025] In some embodiments, the purified cellulose fiber composition has an average fibril area of ​​at least 17%, preferably at least 20%, more preferably at least 22% of the fibers having a length >0.2 mm. The purified cellulose fiber composition typically has an average fibril area of ​​less than 50%, such as less than 40% or less than 30% of the fibers having a length >0.2 mm. As used herein, the term "average fibril area" refers to the length-weighted average fibril area. The average fibril area is determined using a Fiber Tester Plus instrument.

[0026] The refined cellulose fiber composition according to the invention can be produced from different raw materials, for example softwood pulp or hardwood pulp. The term "cellulose fiber" as used herein refers to natural cellulose fibers, i.e. not regenerated or manufactured fibers such as lyocell or viscose.

[0027] The selection of a softwood kraft pulp fraction offers the advantage of reduced energy demand, and therefore in some embodiments the cellulose pulp is a softwood pulp.

[0028] In some embodiments, the refined cellulose fiber composition is substantially free of lignin, and preferably the cellulose pulp has a lignin content of 10% by weight or less, based on the total dry weight of the pulp.

[0029] In some embodiments, the purified cellulose fiber composition is obtained by subjecting a fine fiber fraction obtained by fractionating cellulose pulp to purification at a total refining energy in the range of 100 to 1500 kW / t, preferably in the range of 500 to 1500 kW / t, and more preferably in the range of 750 to 1250 kW / t.

[0030] The present invention further relates to a method for producing the purified cellulose fiber composition according to the first aspect with reduced energy content.

[0031] Thus, according to a second aspect exemplified herein, there is provided a method of producing a purified cellulose fiber composition for use as a strength enhancer for paper or paperboard, the method comprising: a) providing a fine fiber fraction obtained by fractionating cellulose pulp; b) subjecting said fine fiber fraction to purification to a consistency in the range of 0.5-30% by weight and a Shopper-Riegler (SR) number in the range of 80-98 as determined by standard ISO 5267-1, to obtain a purified cellulose fiber composition.

[0032] The fine fiber fraction is obtained by classifying cellulose pulp into a fine fiber fraction and a crude fiber fraction.

[0033] The problem with refining non-fractionated pulp is the high energy consumption. One way to reduce energy consumption is to derivatize the pulp or to use enzymes to promote fibrillation, as described in International Patent Application WO2007091942A1. However, the use of chemicals and additives has limitations, especially in integrated pulping processes. Chemicals increase costs and can interfere with other chemicals.

[0034] The refining process is very important to control, since it is an energy-intensive process and has a significant impact on the properties of many paper products. Currently, the refining process is mainly controlled by online measurements of the drainage resistance of the raw material (e.g. the Schopper-Riegler value) and sometimes the geometric dimensions of the fibers. A very important variable remains the possibility of the fibers bonding together. The fibrils that are partially or completely loosened from the fibers are called krill. The krill that arises during the refining process significantly improves the bonding strength of the fibers together.

[0035] Experiments performed by the inventors show that unfractionated pulp can indeed be refined to a high SR by increasing the refining energy, which is expected and in line with the prior art. However, the results also show that the content and krill value of fibres with a length >0.2 mm remain relatively low. When 5 wt% refined undefibrated pulp is used in the CTMP sheets, both the Z strength and tensile index increase, but the increase is low to moderate.

[0036] On the other hand, if the pulp is fractionated into fine and coarse fibre fractions and refining is carried out on the fine fraction, experiments have shown that high SR values ​​can be reached, regardless of the coniferous wood mixture, but with a high content of fibres over 0.2 mm in length and a high krill value, even though refining is carried out at a lower consistency.

[0037] The refined cellulose fiber composition of the present invention, which has a high SR value and a high content of fibers with lengths >0.2 mm, has been shown to be highly effective as a strength enhancer when incorporated into CTMP sheets, and has the potential to become a sustainable alternative as a raw material reducer in paper and board production.

[0038] The fines fraction used as starting material in the process of the invention is obtained by size fractionation of a cellulose pulp starting material into a fines fraction and a coarse fiber fraction. The fines fraction has a higher content of short, thin fibers compared to the starting material. The fines fraction can be obtained, for example, by separating the cellulose pulp starting material with a pressure screen to obtain a fraction having shorter, thinner fibers. The dry weight of the fines fraction can comprise, for example, less than 75%, less than 50%, less than 25% by weight of the total dry weight of the unfractionated cellulose pulp starting material.

[0039] The fine fiber fraction typically has an average fiber length (determined according to ISO 16065-2) of fibers having a length >0.2 mm of 1.7 mm or less, and a content of at least 5 million fibers having a length >0.2 mm per gram on a dry weight basis. The content of fibers having a length >0.2 mm in the fine fiber fraction is typically less than 10 million fibers per gram on a dry weight basis.

[0040] The coarse fibre fraction thus obtained can be used, for example, for the production of tissues, since it has a reduced amount of fine powder and fine fibres.

[0041] In some embodiments, the cellulose pulp in step a) is a softwood pulp. In some embodiments, the cellulose pulp in step a) is a pulp that has never been dried. In some embodiments, the cellulose pulp in step a) is an unbeaten pulp.

[0042] In some embodiments, the cellulose pulp is substantially free of lignin, and preferably the cellulose pulp has a lignin content of 10% by weight or less, based on the total dry weight of the pulp.

[0043] In some embodiments, the cellulose pulp has a hemicellulose content in the range of 10-30% by weight, based on the total dry weight of the pulp.

[0044] The fine fiber fraction may optionally be treated by oxidation or by the addition of a supplemental strength enhancing agent prior to refining in step b), hi some embodiments, the fine fiber fraction is treated with an oxidizing agent prior to refining.

[0045] In some embodiments, a supplemental strength enhancing agent is added to the fine fiber fraction prior to refining.

[0046] In some embodiments, the fine fiber fraction of step a) has a Shopper-Riegler (SR) number of less than 70, preferably less than 50, measured by standard ISO 5267-1.

[0047] In some embodiments, the fine fiber fraction of step a) has a content of fibers having a length greater than 0.2 mm in the range of 1 to 10 million fibers / gram, preferably in the range of 5 to 10 million fibers / gram.

[0048] In some embodiments, the fine fiber fraction of step a) has an average fiber length in the range of 1 to 2, preferably in the range of 1 to 1.7.

[0049] In some embodiments, the fine fiber fraction is subjected to purification at a consistency in the range of 1-10% by weight.

[0050] In some embodiments, the fine fiber fraction is subjected to refining with a total refining energy in the range of 100-1500 kW / t, preferably in the range of 500-1500 kW / t, more preferably in the range of 750-1250 kW / t.

[0051] In some embodiments, the purified cellulose fiber composition of step b) has a Shopper-Riegler (SR) number, as determined by standard ISO 5267-1, in the range of 85 to 98, preferably in the range of 90 to 98. In some embodiments, the purified cellulose fiber composition has a Shopper-Riegler (SR) number, as determined by standard ISO 5267-1, in the range of 92 to 98, preferably in the range of 94 to 98.

[0052] By purifying the fine fiber fraction, a purified cellulose fiber composition is obtained that contains a large amount of highly fibrillated fibers having a length of more than 0.2 mm and a high crill value.

[0053] In some embodiments, the purified cellulose fiber composition has a content of fibers having a length greater than 0.2 mm of at least 12 million fibers per gram on a dry weight basis, preferably at least 15 million fibers per gram on a dry weight basis, more preferably at least 17 million fibers per gram on a dry weight basis, and even more preferably at least 20 million fibers per gram on a dry weight basis.

[0054] In some embodiments, the refined cellulose fiber composition has a krill value of at least 1.7, preferably at least 1.8, and more preferably at least 1.9. The krill value of the refined cellulose fiber composition will typically be 2.5 or less. The krill value is measured using a Fiber Tester Plus.

[0055] In some embodiments, the purified cellulose fiber composition has an average fibril area of ​​at least 17%, preferably at least 20%, more preferably at least 22% of the fibers having a length >0.2 mm. The purified cellulose fiber composition typically has an average fibril area of ​​less than 50%, such as less than 40% or less than 30% of the fibers having a length >0.2 mm. As used herein, the term "average fibril area" refers to the length-weighted average fibril area. The average fibril area is determined using a Fiber Tester Plus instrument.

[0056] The average length of fibers with a length >0.2 mm, the fibril area of ​​fibers with a length >0.2 mm, and the crill value were measured using an L&W Fiber Tester Plus (L&W / ABB) instrument (also referred to herein as "Fiber Tester Plus" or "FT+") according to the ISO 16065-2 standard, defining fiber as a fibrous particle longer than 0.2 mm.

[0057] A known sample weight of 0.100 g was used for each sample and the content of fibres with length >0.2 mm (million fibres per gram) was calculated using the following formula: Million fibres / gram = (number of fibres in sample) / (sample weight) / 1 000 000 = (property ID 3141) / property ID 3136) / 1 000 000.

[0058] The krill measurement method makes use of the ability of particles to absorb and emit different wavelengths of light depending on their diameter. By passing a pulp suspension through sources of UV and IR light and placing a detector on the other side, it is possible to detect the presence of small particles in the solution. The more particles there are, the more light is emitted or absorbed. Small particles like krill scatter and absorb the light from the UV light source, while fibres affect the light from the IR light source. The krill content is obtained as a quota when UV / IR is detected.

[0059] The purified cellulose fiber composition of the present invention can be preferably used as a strength-enhancing additive in cellulose pulp (e.g., CTMP) to increase the strength of paper or paperboard produced from the pulp. The purified cellulose fiber composition is typically added to the pulp to be reinforced at a concentration of at least 0.1% by weight, preferably at least 1% by weight. In some embodiments, the purified cellulose fiber composition is added to the pulp to be reinforced at a concentration in the range of 1 to 25% by weight, preferably in the range of 1 to 15% by weight, more preferably in the range of 1 to 10% by weight, and most preferably in the range of 2 to 7% by weight, based on the dry weight. In some embodiments, the purified cellulose fiber composition is added to the pulp to be reinforced at a concentration in the range of 2 to 5% by weight, based on the dry weight.

[0060] According to a third aspect exemplified herein there is provided a cellulose pulp for producing paper or paperboard comprising at least 0.1 wt. % of a purified cellulose fibre composition as described herein with reference to the previous aspect, preferably in the range 1 to 25 wt. %, more preferably in the range 1 to 10 wt. %, most preferably in the range 2 to 7 wt. %, based on dry weight.

[0061] In some embodiments, the cellulose pulp is a chemi-thermomechanical pulp (CTMP).

[0062] Paper and paperboard containing the purified cellulose fiber composition of the present invention exhibit significantly increased tensile strength and improved Z strength compared to corresponding papers and paperboard that do not contain the purified cellulose fiber composition of the present invention.

[0063] According to a fourth aspect presented herein there is provided a paper or paperboard comprising one or more plies, at least one ply comprising at least 0.1 wt. % of a purified cellulose fibre composition as described herein with reference to the previous aspect, preferably in the range 1-25 wt. %, more preferably in the range 1-10 wt. %, most preferably in the range 2-7 wt. %, based on dry weight.

[0064] Paper is generally a material made into thin sheets from fibrous substances including wood pulp and cellulose fibers, and is used for writing, painting, printing, and as packaging material.

[0065] Paperboard is a strong, thick paper or cardboard made from cellulose fibers used for packaging such as boxes. Paperboard can be bleached or unbleached, coated or uncoated, and produced in a variety of thicknesses depending on the end use.

[0066] According to a fifth aspect exemplified herein, there is provided a use of a purified cellulose fiber composition as described herein with reference to the previous aspects for improving the Z strength and / or tensile strength of paper or paperboard.

[0067] The strength enhancers in the third, fourth and fifth aspects may be further defined as described above with reference to the first and second aspects.

[0068] As used herein, the term "% by dry weight" (eg, in relation to a pulp composition or pulp fraction) means % by weight based on the total dry weight of the composition.

[0069] In relation to the present invention, the following is further disclosed: [1] 1. A purified cellulose fiber composition comprising: The refined cellulose fiber composition has a Shopper-Riegler (SR) number in the range of 80 to 98 as determined by standard ISO 5267-1; and The purified cellulose fiber composition has a content of at least 12 million fibers having a length >0.2 mm per gram on a dry weight basis. [2] [1] The purified cellulose fiber composition according to the present invention, characterized in that the purified cellulose fiber composition has a Shopper-Riegler (SR) number in the range of 85 to 98, preferably in the range of 90 to 98, as measured according to standard ISO 5267-1. [3] The purified cellulose fiber composition according to any one of [1] to [2], wherein the purified cellulose fiber composition has a content of fibers having a length greater than 0.2 mm of at least 15 million fibers per gram based on dry weight, preferably at least 20 million fibers per gram based on dry weight. [4] The purified cellulose fiber composition according to any one of [1] to [3] above, characterized in that the purified cellulose fiber composition has a cryl value of at least 1.7, preferably at least 1.8, and more preferably at least 1.9. [5] The purified cellulose fiber composition according to any one of [1] to [4], wherein the purified cellulose fiber composition has an average fibril area of ​​at least 17%, preferably at least 20%, and more preferably at least 22% of fibers having a length >0.2 mm. [6] The purified cellulose fiber composition according to any one of [1] to [5], wherein the purified cellulose fiber is a purified softwood cellulose fiber. [7] 1. A method for producing a purified cellulose fiber composition for use as a strength enhancer for paper or paperboard, comprising: a) providing a fine fiber fraction obtained by fractionating cellulose pulp; b) subjecting said fine fiber fraction to purification to a consistency in the range of 0.5-30% by weight and a Shopper-Riegler (SR) number in the range of 80-98 as determined by standard ISO 5267-1, to obtain a purified cellulose fiber composition. [8] The method according to [7], wherein the cellulose pulp is softwood pulp. [9] The method according to any one of [7] to [8], wherein the cellulose pulp is a pulp that has never been dried.

[10] The method according to any one of [7] to [9], wherein the cellulose pulp is unbeaten pulp.

[11] The method according to any one of [7] to

[10] , wherein the cellulose pulp is substantially free of lignin, and preferably the cellulose pulp has a lignin content of 10 wt. % or less based on the total dry weight of the pulp.

[12] The method according to any of [7] to

[11] , characterized in that the cellulose pulp has a hemicellulose content in the range of 10 to 30% by weight, based on the total dry weight of the pulp.

[13] The method according to any one of [7 to

[12] , characterized in that the fine fiber fraction is treated with an oxidizing agent before purification.

[14] The method according to any one of [7] to

[13] , characterized in that an auxiliary strength enhancer is added to the fine fiber fraction before purification.

[15] The method according to any of [7] to

[14] , wherein the fine fiber fraction in step a) has a Shopper-Riegler (SR) number measured according to standard ISO 5267-1 of less than 70, preferably less than 50.

[16] The method according to any one of [7] to

[15] , characterized in that the fine fiber fraction is subjected to purification at a consistency within the range of 1 to 10% by weight.

[17] The method according to any one of [7] to

[16] , characterized in that the fine fiber fraction is subjected to purification with a total purification energy in the range of 100 to 1500 kW / t, preferably in the range of 500 to 1500 kW / t, more preferably in the range of 750 to 1250 kW / t.

[18] The method according to any of the claims [7] to

[17] , wherein the purified cellulose fiber composition of step b) has a Shopper-Riegler (SR) number in the range of 85 to 98, preferably in the range of 90 to 98, as determined by standard ISO 5267-1.

[19] The method according to any one of [7] to

[18] , wherein the purified cellulose fiber composition has a content of fibers having a length greater than 0.2 mm of at least 12 million fibers per gram on a dry weight basis, preferably at least 15 million fibers per gram on a dry weight basis, more preferably at least 17 million fibers per gram on a dry weight basis, and even more preferably at least 20 million fibers per gram on a dry weight basis.

[20] 11. The method of any preceding claim, wherein the purified cellulose fiber composition has a cryl value of at least 1.7, preferably at least 1.8, and more preferably at least 1.9.

[21] 11. The method of any preceding claim, wherein the purified cellulose fiber composition has an average fibril area of ​​at least 17%, preferably at least 20%, and more preferably at least 22% of the fibers have a length value >0.2 mm.

[22] A cellulose pulp for producing paper or paperboard, comprising at least 0.1% by weight, based on the dry weight, of the purified cellulose fiber composition according to any one of [1] to [6] or the purified cellulose fiber composition obtained by the method according to any one of [7] to

[21] , preferably 1 to 25% by weight, more preferably 1 to 10% by weight, and most preferably 2 to 7% by weight.

[23] 2. The cellulose pulp for producing paper or paperboard according to claim 22, characterized in that the cellulose pulp is a chemical thermomechanical pulp (CTMP).

[24] A paper or paperboard comprising one or more plies, wherein at least one of the plies contains, based on the dry weight, at least 0.1% by weight, preferably 1 to 25% by weight, more preferably 1 to 10% by weight, and most preferably 2 to 7% by weight, of the purified cellulose fiber composition according to any one of [1] to [6] or the purified cellulose fiber composition obtained by the method according to any one of [7] to

[21] .

[25] Use of a purified cellulose fiber composition according to any one of [1] to [6] or obtained by a method according to any one of [7] to

[21] for improving the Z strength and / or tensile strength of paper or paperboard. While the invention has been described with reference to various exemplary embodiments, those skilled in the art will recognize that various modifications can be made and equivalents can be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications can be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but rather to include all embodiments falling within the scope of the appended claims.

[0070] example analysis The average length of fibers with a length >0.2 mm, the fibril area of ​​fibers with a length >0.2 mm, and the crill value were measured using an L&W Fiber Tester Plus (L&W / ABB) instrument (FT+) according to the ISO 16065-2 standard, where fibers are defined as fibrous particles longer than 0.2 mm.

[0071] A known sample weight of 0.100 g was used for each sample and the content of fibres with length >0.2 mm (million fibres per gram) was calculated using the following formula: Million fibres / gram = (number of fibres in sample) / (sample weight) / 1 000 000 = (property ID 3141) / property ID 3136) / 1 000 000.

[0072] Drainage resistance (tap water) was measured based on the Schopper-Riegler method ISO 5267-1.

[0073] The properties of the sheets were measured according to the following criteria: Tensile Test ISO 1924-3:2005 Z-strength test ISO 15754:2009

[0074] Starting materials Softwood pulp 1 (SW1) is a softwood kraft pulp (spruce / pine mix) with an average fiber length (>0.2 mm) of 2.1 mm according to ISO 16065-2 and a content of 4.2 million fibers / g with a length >0.2 mm according to FT+.

[0075] Softwood pulp 2 (SW2) is softwood kraft pulp (pine) with an average fiber length (>0.2 mm) of 2.1 mm according to ISO 16065-2 and a fiber content of 3.6 million fibers per gram with a length of 0.2 mm or more according to FT+.

[0076] Softwood pulp 3 (SW3) is a softwood kraft pulp (spruce) with an average fiber length (>0.2 mm) of 2.6 mm according to ISO 16065-2 and a fiber content of 3.1 million fibers per gram with a length >0.2 mm according to FT+.

[0077] Softwood pulp 4 (SW4) is a softwood kraft pulp (spruce / pine mix) with an average fiber length (>0.2 mm) of 2.4 mm.

[0078] Fractionation The fine fiber fraction was achieved by separating the pulp through a pressure screen (equipped with a screen basket with 1.2 mm pore size) with the aim of obtaining a fraction rich in short fibers. A two-step procedure separated 4-7% of the pulp from the feed pulp stream. This two-step procedure reduced the fiber length and increased the number of fibers per gram. In addition to the average length measurements with the FT+, a Valmet Fiber Image Analyzer FS5 was used to measure the average length, with fibers defined as fibrous particles longer than 0.2 mm according to the ISO 16065-2 standard.

[0079] [Table 0]

[0080] purification Purification of the unfractionated and fine fibre fraction samples was carried out using a Voith Sulzer LR1 refiner (with 2mm disc refiner) at a consistency of ∼4% and a flow rate of 100 litres / min as follows.

[0081] Example 1 (unfractionated). The SW1 pulp was refined using a conical refiner filling with 3 mm bars, a cutting angle of 60 degrees, and an edge load of 1.0 Ws / m, resulting in a refining energy of 466 kWh / t.

[0082] Example 2 (unfractionated). The SW1 pulp was refined using a conical refiner filling with 3 mm bars, a cutting angle of 60 degrees, and an edge load of 1.0 Ws / m, resulting in a refining energy of 1032 kWh / t.

[0083] Example 3 (Unfractionated) . The SW3 pulp was refined using a disc refiner packed with 2 mm bars at a cut angle of 40 degrees and an edge load of 0.25 Ws / m, yielding a refining energy of 750 kWh / t.

[0084] Example 4 (unfractionated). SW2 pulp was refined using a conical refiner filling with 3 mm bars, a cutting angle of 60 degrees, and an edge load of 1.0 Ws / m, resulting in a refining energy of 950 kWh / t.

[0085] Example 5-9. Fine fiber fractions. The fine fiber fraction was refined using a disc refiner packed with 2 mm bars at a cutting angle of 40 degrees and an edge load of 0.25 Ws / m, resulting in a refining energy of 1039–1550 kWh / t.

[0086] Preparation of sheets Five percent by weight of the refined sample was added to CTMP (Spruce CTMP, Canadian standard freeness 600 ml CSF) and sheets for strength testing were made in a Formette dynamic sheet former. The pulp consistency was 3 g / litre and mixed at 1000 rpm for 1 min. After mixing, the sheets were pressurized with a nozzle 2514, pressure 2.5 bar, rotation speed 1050 rpm and basis weight 100 g / m2. 2 The following steps were performed: 1. Dewatering time 2 min., counting with blotter paper, 1st roll press at 1 bar between felts, 2nd roll press at 5 bar between felts with blotter replaced, 3rd roll press at 5 bar without felt with blotter replaced, drying in bow dryer at 95 °C, conditioning at 23 °C and 50% RH. For efficient mixing of the pulp raw materials, the Formette dynamic sheet former is equipped with a baffled mixing chest, which is a modified version of the Britt Dynamic Drainage Jar. The sheets were then analyzed for Z strength and tensile index.

[0087] [Table 1]

Claims

1. 1. A purified cellulose fiber composition comprising: The refined cellulose fiber composition has a Shopper-Riegler (SR) number in the range of 80 to 98 as determined by standard ISO 5267-1; and The purified cellulose fiber composition has a content of at least 12 million fibers having a length of >0.2 mm per gram on a dry weight basis, The refined cellulose fiber composition has a krill value of at least 1.

7. The purified cellulose fiber composition has an average fibril area of ​​at least 17% of the fibers having a length >0.2 mm. A refined cellulose fiber composition, wherein the refined cellulose fibers are refined softwood cellulose fibers.

2. 2. The purified cellulose fiber composition of claim 1, wherein the purified cellulose fiber composition has a Shopper-Riegler (SR) number in the range of 85 to 98, as measured by standard ISO 5267-1.

3. The purified cellulose fiber composition of any one of claims 1 to 2, wherein the purified cellulose fiber composition has a content of at least 15 million fibers per gram on a dry weight basis having a length greater than 0.2 mm.

4. 1. A method for producing a purified cellulose fiber composition for use as a strength enhancer for paper or paperboard, comprising: a) providing a fine fiber fraction obtained by fractionating cellulose pulp; b) subjecting said fine fiber fraction to purification to a consistency ranging from 0.5 to 30% by weight and a Shopper-Riegler (SR) number ranging from 80 to 98 as determined by standard ISO 5267-1, to obtain a purified cellulose fiber composition; Moreover, the cellulose pulp is unbeaten pulp.

5. 5. The method of claim 4, wherein the cellulose pulp is softwood pulp.

6. The method according to any one of claims 4 to 5, characterized in that the cellulose pulp is a never-dried pulp.

7. The method of any one of claims 4 to 6, wherein the cellulose pulp is substantially free of lignin.

8. 8. The method according to any one of claims 4 to 7, characterized in that the cellulose pulp has a hemicellulose content in the range of 10 to 30% by weight, based on the total dry weight of the pulp.

9. A method according to any one of claims 4 to 8, characterized in that the fine fibre fraction is treated with an oxidizing agent before purification.

10. A method according to any one of claims 4 to 9, characterised in that a supplemental strength enhancer is added to the fine fibre fraction before refining.

11. 11. The method according to any one of claims 4 to 10, wherein the fine fibre fraction of step a) has a Shopper-Riegler (SR) number of less than 70, measured according to standard ISO 5267-1.

12. 12. The method according to any one of claims 4 to 11, characterized in that the fine fibre fraction is subjected to purification at a consistency in the range of 1 to 10% by weight.

13. 13. The method according to any one of claims 4 to 12, characterized in that the fine fiber fraction is subjected to refining with a total refining energy in the range of 100 to 1500 kW / t.

14. 14. The method according to any one of claims 4 to 13, wherein the purified cellulose fiber composition of step b) has a Shopper-Riegler (SR) number in the range of 85 to 98, as determined by standard ISO 5267-1.

15. 15. The method of any one of claims 4 to 14, wherein the purified cellulose fiber composition has a content of at least 12 million fibers per gram on a dry weight basis having a length greater than 0.2 mm.

16. The method of any one of claims 4 to 15, wherein the refined cellulose fiber composition has a krill value of at least 1.

7.

17. 17. The method of any one of claims 4 to 16, wherein the purified cellulose fiber composition has an average fibril area of ​​at least 17% of the fibers having a length value >0.2 mm.

18. A cellulose pulp for making paper or paperboard comprising at least 0.1% by weight, based on dry weight, of the purified cellulose fiber composition according to any one of claims 1 to 3.

19. 19. The cellulose pulp for making paper or paperboard according to claim 18, characterized in that the cellulose pulp is a chemi-thermomechanical pulp (CTMP).

20. 4. A paper or paperboard comprising one or more plies, at least one ply containing at least 0.1% by weight, on a dry weight basis, of the purified cellulose fiber composition of any one of claims 1 to 3.

21. Use of the purified cellulose fiber composition according to any one of claims 1 to 3 or obtained by the method according to any one of claims 4 to 17 for improving the Z strength and / or tensile strength of paper or paperboard.

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