FI123289B - Process for the preparation of nanofibrillated cellulosic pulp and its use in papermaking or nanofibrillated cellulose composites - Google Patents

Description

METHOD FOR PREPARING NANOFIBRILLED PULP PULP AND USE OF PULP IN PAPER MAKING OR NANOFIBRILLED PULP COMPOSITES

Field of the Invention

The invention relates to a process for the production of nanofibrillated cellulose pulp. The invention further relates to the use of pulp in papermaking or nanofibrillated cellulose composites.

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Background of the Invention

Many stages of mass production, especially refining, require a lot of energy. In particular, the production of nanofibrillated cellulose consumes a large amount of energy due to the multiple fibrillation steps required to obtain the nanoscale material. Because the energy consumption of the pulp produced is greatly increased by the presence of nanofibrillated cellulose in the pulp produced, an efficiency problem may arise when the pulp produced is at least partially composed of nanofibrillated cellulose. Another problem in the production of 20 nanofibrillated cellulosic pulp is sometimes the poor dewatering due to several strong bonds between the cellulose fibers and the water to be removed.

Due to the above-mentioned problems, it is preferable to add at least one compound capable of substantially inhibiting hydrogen bonding of the fibrils in the cellulose, particularly in the process of preparing nanofibrillated cellulose. this

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For this purpose, certain polyhydroxy compounds, such as sucrose, are used in the art. However, these known compounds have some problems, for example, known additives have only been used in grinding steps

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30, which causes additional additive costs. In that case, it would be to my advantage to use additives such as those later in the process

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§ would in any case be added for some other purpose, instead of known additives used exclusively for the above purpose.

35 2 Thus, a new solution is required to increase the efficiency of the production of nanofibrillated cellulose pulp. Thus, an additive is required which could provide lower energy consumption and improved dewatering capacity in the pulping processes during the pre-milling and / or fibrillation steps. Preferably, the new additive would have the properties of reducing fiber-to-fiber bonds, thereby improving the efficiency of refining due to the lower power consumption of the refining step. In addition, the additive would preferably have features that reduce fiber-water bonding and fiber-to-bond bonding that occurs during drying and thickening. The additive would also preferably be one of those additives that are often added for some other purpose to later stages of the process.

SUMMARY OF THE INVENTION The present invention solves at least some of the above problems by providing a pulping process wherein the pulp produced is at least partially composed of nanofibrillated cellulose. The method comprises the step of dispensing at least one type of optical brightening agent (OBA) before and / or after at least one pre-refining and / or fibrillation step. The invention also discloses the use of the prepared pulp in nanofibrillated cellulose composites or in the manufacture of paper or board, including the steps of making and finishing base paper, such as use in paper or board coatings.

Aspects of the invention are characterized by what is set forth in the independent claims 1, 10 and 11. The various embodiments of the invention are set forth in the 9 independent claims.

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The inventors of the present invention have surprisingly found that optical

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30 brighteners can improve the efficiency of the production of nanofibrillated cellulosic pulp if the additives are administered prior to the pre-milling step and / or

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§ during or during the fibrillation phase. Optical brighteners have been found to be able to form bonds with cellulose such that optical brighteners can act as substituents in the inter-fiber bonds and thus inhibit hydrogen bonding of fibrils in cellulose. The higher production efficiency is mainly due to the lower energy consumption of the fibrillation phase due to the substituent effect. Optical brighteners are also capable of forming bonds with water and thus improving the efficiency of drying and thickening processes.

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For the above reasons, optical brighteners are capable of redispersing nano-fibrillated cellulose. Due to its dispersing effect, the optical brightener can be used as a dispersant in the thickening and / or redispersing process of nanofibrillated cellulose and thus to promote 10 processes. In addition, due to the dispersing action, the quality of the nanofibrillated cellulosic pulp can be improved.

According to an embodiment of the present invention, at least one type of optical brightener is added before the pulp pre-refining step. According to another embodiment, at least one type of optical brightener is added before the pulp fibrillation step. According to another embodiment, at least one type of optical brightener is applied to the pulp during the pre-refining step. According to another embodiment, at least one type of optical brightener is administered to the pulp during the fibrillation step.

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According to one embodiment, the amount of nanofibrillated cellulose in the pulp produced is more than 30% by weight, preferably more than 40% by weight, 50% by weight, 60% by weight or 70% by weight, and may be up to 100% by weight.

Nanofibrillated cellulose pulp, which can be prepared according to the invention o ™ and thus contains one or more optical brighteners, can be used o in various end product applications. The cellulosic pulp can be used, for example, in nanofibrillated cellulose composites and / or in papermaking, for example base paper and / or in the finishing of the paper produced.

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30 stages. The finishing steps of the paper produced include, for example, top-c3 finishing steps.

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BRIEF DESCRIPTION OF THE DRAWINGS In the following, the invention is illustrated by the drawings in which 4 figures 1a-d are microscopic images of cellulosic pulp, powdered cellulose pulp and nanofibrillated cellulose pulps; Figures 2a-c show the viscosity results obtained in the experiments as well as optical microscope images taken during the experiments on samples ground in a Masuko refiner; Figures 3a-b are optical microscope images of fluidizer samples taken during the experiment; Figures 4a-b are scanning electron microscope (SEM) images of samples also shown in Figures 3a-3b; and Fig. 5 shows turbidity and centrifugation test results for fluidized samples.

DETAILED DESCRIPTION OF THE INVENTION In this application, the term "cellulosic feedstock" refers to any source of cellulosic feedstock that may be used in the production of cellulose pulp, powdered pulp or microfibrillated cellulose. The cellulose raw material may be based on any plant cellulose containing cellulosic material, for example wood. The wood raw material may be derived from conifers, such as spruce, pine, fir, larch, Douglas or 9 Canadian hemlock, or hardwoods such as birch, aspen, poplar, oak, eucalyptus or acacia, or from the coniferous and deciduous tree. Non-wood based raw materials may be agricultural waste,

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30 such as straw, leaves, bark, seeds, pods, flowers,

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oj nats or fruits obtained from cotton, maize, wheat, oats,

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§ rye, barley, rice, flax, hemp, manila hemp, sisal hemp, jute, ramie, kenaf hemp, bagasse, bamboo or cane.

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The term "cellulosic pulp" as used herein means cellulosic fibers isolated from any cellulosic feedstock by chemical, mechanical, thermomechanical, or chemothermomechanical pulping processes. Fibers generally have a diameter of 15 to 25 µm and a length of more than 5 500 µm, but the present invention is not intended to be limited to these parameters.

As used in this application, the term "papermaking" refers to the process of manufacturing any paper-like material, such as cartons, papers and / or paper-composites.

According to an exemplary embodiment of the invention, at least a portion of the lignin contained in the cellulosic feedstock is preferably removed from the cellulose feedstock when processed into the cellulose pulp used to make the nano-15 fibrillated cellulose. Thus, chemical pulp can be used more advantageously in the production of nanofibrillated cellulose than mechanical pulp. According to an exemplary embodiment of the invention, the process yield when the cellulosic raw material is processed into the cellulose pulp used to make the nanofibrillated pulp has been at least 50%, at least 60%, at least 70% or at least 80%.

According to one exemplary embodiment, the cellulosic pulp used in the preparation of nanofibrillated cellulose may preferably be an unbleached or bleached chemical or chemical pulp, more preferably an unbleached or bleached chemical pulp, and most preferably the unbleached chemical pulp may be

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o Compared to other methods, it is most preferred that the cellulosic pulp used is 0 chemically produced unbleached pulp.

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30 The term "ground pulp" means ground cellulosic pulp. The pulping of my cellulose pulp is performed by suitable means such as a grinder, a grinder,

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A homogenizer, a colloidal mixer, a friction refiner, a fluidizer, such as a microfluidizer, a macrofluidizer, or a fluidizer-type homogenizer or ultrasonic ionizer. In general, not all cellulose-35 fibers are fully fibrillated; in addition to the ground cellulosic material, there are 6 large fractions of cellulosic fibers which have not changed in size. Large fibers in the pulp may have a fibrillated surface. The finest fraction of cellulose-based material in the 'pulp pulp' consists of nanofibrillated cellulose, i.e. cellulose microfibrils and microfibrillary bundles of less than 200 nm in diameter.

The term "nanofibrillated cellulose (NFC)" refers to a set of isolated cellulose microfibrils or microfibrils derived from a cellulosic feedstock. Microfibrils generally have a high aspect ratio: may be more than one 10 micrometers long, while the average number diameter is generally less than 200 The microfibrillary bundles may also be larger in diameter, but generally less than 1 µm. The smallest microfibrils are similar to so-called elemental fibrils, usually 2 to 12 nm in diameter. The dimensions of the fibrils or fibrils depend on the raw material and the method of digestion. contains hemicellulose, the amount may depend on the plant source.

Chemical disintegration of nanofibrillated cellulose from cellulosic feedstock, cellulose pulp, or pulp is accomplished by suitable means such as a refiner, a grinder, a homogenizer, a colloid mixer, a friction grinder, an ultrasonic ionizer, a fluidiser, a microfluidizer, such as a microfluidizer. Nanofibrillated cellulose may also be derived from any chemically or physically modified cellulose microfibrils or microfibrils. Chemical conversion may be based, for example, on a carboxymethylation, oxidation, esterification or etherification reaction of cellulose molecules. The conversion could also be accomplished by physical anionic, cationic or nonionic agents, or any combination thereof on the cellulose surface. The described conversion may be carried out before, during or after the preparation of nanofibrillated cellulose g.

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30

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£ 3 There are several widely used synonyms for nanofibrillated cellulose.

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g For example: nanocellulose, microfibrillated cellulose, nanofibrillated cellulose, cm cellulose nano fiber, nano-grade fibrillated cellulose, microfibrillated cellulose (MFC) or cellulose microfibrils. The nano-35 fibrillated cellulose described in this application is not the same material as the so-called. cellulose whiskers (7), also referred to as cellulose nano-whiskers, cellulose nanocrystals, cellulose nano-sticks, rod-like cellulose microcrystals or cellulose nano-wires. In some cases, similar terminology is used for both materials, for example in Kuthcarlapati et al.

5 (Metals Materials and Processes 20 (3): 307-314, 2008), where the material studied was referred to as '' cellulosic nano fiber '', although these were clearly cellulose nano-whiskers. Generally, these materials do not have amorphous segments in the fibril structure as in microfibrillated cellulose, which results in a stiffer structure. Cellulose whiskers are also shorter than microfibrillated cellulose; the length is generally less than 1 pm.

Conventional pulping generally aims to obtain long, intact, fibrillated fibers, whereas nanofibrillated cellulose is intended to crush the fibers into smaller pieces. Nanofibrillated cellulose and plain cellulose are generally produced by different refiners, because conventional refinement refiners cannot be used, at least not effectively, in the manufacture of nanofibrillated cellulose. However, refiners used in conventional pulping can be used as pre-refiners in the production of nanofibrillated cellulose. As used herein, the term '' fibrillation step 'refers to a step which produces more fibrillated cellulose, and the term' pre-refining step 'refers to a step which may advantageously be used for pre-refining prior to the fibrillation step in the preparation of nanofibrillated cellulose.

The characteristics of nanofibrillated cellulosic pulp differ greatly from conventional cellulosic pulp due to the

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o of a plurality of nanoparticle particles, and thus nanofibrillated o cellulose cannot be considered as the same material as conventional cellulose. Nanofibrillated cellulose is, for example, a gel-like material

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30 even at low consistency, and its drainage rate is generally low. Paper sheets containing a lot of nanofibrillated cellulose have special

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§ properties compared to ° sheets made from normal cellulosic pulp: for example, they generally have good strength properties, very low porosity, and the sheets are generally (at least partially) transparent.

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The differences between cellulosic pulp, powdered cellulosic pulp and nanofibrillated cellulosic pulp are illustrated in the optical microscopic images of Figures 1a-1d. The magnification is the same in Figures 1a-1c. Figure 1a is an optical microscope image of a typical cellulose pulp. Figure 5a-1b shows an image of a typical pulp pulp taken with an optical microscope. Figures 1c and 1d show microscopic images of a typical nanofibrillated cellulose pulp. As can be seen in Figure 1c, the large cellulosic fibers shown in Figures 1a and 1b are no longer clearly visible. Fig. 1d shows the same situation as in Fig. 1c but with a smoother magnification of butter-10, whereby individual microfibril and microfibril bundles with a diameter less than 100 nm can be detected.

According to an exemplary embodiment, the present invention provides a process for preparing a nanofibrillated cellulosic pulp by pre-milling a club and / or fibrillating the pulp in the presence of at least one optical brightener. Furthermore, according to another exemplary embodiment, the present invention provides the use of the pulp produced in papermaking or nano-fibrillated cellulose composites.

Optical Brightening Agents (OBAs) are dye-like compounds that absorb short-wave light in the ultraviolet and violet regions of the electromagnetic spectrum, which are invisible to the human eye, and re-emit light in the wavy blue region. In other words, optical brighteners make the material, such as paper, look less yellow to the human eye, and thus the human eye interprets blue light as a stronger whiteness. In the prior art optical brighteners are used

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o to obtain better optical properties of the produced paper, for example the bleaching effect of the produced paper. Types of optical brightener commonly used in the pulp and paper industry are, for example, di-, tetra-, and

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30 hexanesulfonated silbene compounds. The amount of sulfonated groups affects the chemical properties of the c3 optical brightener and thus the optical properties used

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The type of brightener may affect the process of the exemplary embodiment of the invention. In general, the more sulfonated groups there are in the optical brightener, the greater the effect of the used optical brightener 35 on the process of the invention. Some of the other commercially available optical brighteners for the pulp and paper industry 9 are based on the chemical properties of coumarin and pyrazoline. The types of optical brightener mentioned are just a few examples, and other types of optical brighteners known in the art can be used in the present invention.

However, according to one embodiment of the invention, in practice, the chemical types mentioned in the application, namely stilbene, coumarin and pyrazoline, are the most advantageous for the use of this invention. Of these chemicals, anionic stilbene compounds may be most preferred for use in the invention.

Optical brighteners are very expensive additives, and therefore the solutions of the present invention are intended to be most effective if optical brighteners are used in the preparation of nanofibrillated pulp, which would otherwise be added to the pulp or the final product at a later stage. In papermaking, optical brighteners are typically added at the wet end of the papermaking process, including, for example, a mixing pump or machine container. The use of optical brighteners according to some exemplary embodiments of the invention does not necessarily increase the cost of the additive but, conversely, the retention of the optical brightener in the nanofibrillated mass can be improved if the optical brightener is added before or during the pre-refining step. the total cost of the brightener can be reduced. In other words, it is possible that, according to an exemplary embodiment of the invention, the total amount of optical brightener required may be reduced if the optical brightener is added in accordance with some exemplary embodiments of the invention due to multiple interactions between fiber and optical brightener.

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the bonds that may be formed by nanofibrillated cellulose pre-milling

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o and / or during the fibrillation steps.

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Thus, the efficiency of the production of nanofibrillated cellulose can be further improved by adding the optical brightener to the pulp produced before or during the refining step, not only to reduce energy consumption.

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§ but also because of the lower cost of additives. In addition, the dosage of the optical brightener for the preparation of nanofibrillated cellulose according to some exemplary embodiments of this invention may enhance the ability of nanofibril-35 cellulose to carry the optical brightener, thereby reducing the need for other optical brightener carriers such as polyvinyl alcohol (PVOH). Thus, the invention can also increase the production efficiency in this way.

According to one embodiment of the invention, the method comprises the step of dispensing at least one type of optical brightener (OBA) as a refining additive to a pulp containing cellulose. In the method, dosing is performed prior to the pre-milling and / or fibrillation step. In addition, or alternatively, one type of optical brightener may be added to the pulp during the pre-refining or fibrillation step. It is then possible to add the optical brightener to the pre-refining step, and / or the optical brightener may be added to any or all of the following fibrillation steps. According to a preferred embodiment of the invention, the pulp is fibrillated in at least one fibrillation step after the addition of the additive, regardless of the step in which the additive is added to the process.

The anionic optical brightener is able to prevent the formation of hydrogen bonds between the cellulose fibrils in the cellulose and can therefore be used to provide a dispersing effect which can improve the quality of the nanofibrillated cellulose produced. Due to its dispersing effect, the optical brightener can be used as a dispersant in the thickening and / or redispersing process of nanofibrillated cellulose and thus to promote the process.

Nanofibrillated cellulose containing an optical brightener can improve the refining efficiency and quality of the pulp produced, but in one embodiment of the invention

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o according to the embodiment, also the strength and optical properties of the final mass product o. The improvements in the strength properties are due mainly to the properties of nanofibrillated cellulose, and the optical properties of

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These improvements are mainly due to the properties of the optical brightener.

A method according to an exemplary embodiment of the invention comprises at least the following: 35 - introducing into the system a raw material containing cellulose; 11 - dispensing at least one type of optical brightener as a refining additive; or in a fibrillation step to form a fibrillated cellulose material.

When compared to prior art pulp, the new invention can provide at least some of the following benefits: 10 - lower energy consumption to achieve the desired pulp fibrillation rate, dispersion properties, 15 - better strength properties compared to the same total energy consumption of refining, and - better optical properties compared to the same amount of optical brightener (due to better retention of optical brightener).

The amount of nanofibrillated cellulose in the pulp according to the invention may be 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95 or 100% by weight, including all regions and sub-regions.

The pulping process of the invention has at least one fibrillation step, optionally at least 2, 3 or 4 fibrillation steps. According to the invention, at least one type of optical brightener is added o - prior to at least one of said fibrillation steps or before a pre-refining step, optionally before at least 2, 3, 4 or 5 said fibrillation steps, or before a pre-fibrillation step, 30 and / or

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£ 3 - in at least one of said fibrillation steps or pre-refining steps g, optionally at least 2, 3, 4 or 5 in said fibrillation step or pre-fibrillation step.

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In addition, according to the invention, the pulp is fibrillated after at least one dose of additive in at least one fibrillation step to form a nanofibrillated cellulosic material.

As stated above, the addition of the optical brightener before or during the milling reduces the binding of cellulose to the cellulose by hydroxyl groups by forming hydrogen bonds with the cellulose fibrils. At the same time, the addition of the optical brightener produces a dispersing effect on the pulp suspension due to the rejection forces between the anion groups.

10 Tests

Experimental tests were carried out in processes in which some amounts of optical brighteners were added as grinding additives. In addition, paper sheets were made from the pulps produced and tested later.

15 1. Used mass

The bleached birch pulp produced by the conventional chemical pulping process was used as the crude cellulose pulp.

2. Optical brighteners used ^ Two different types of optical brighteners were used as a refining additive: w 25 disulfone type optical brighteners and hexasulfone type optical brighteners. In addition, control samples to which no optical brightener was added were made.

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tr 3. Foreword

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g The pre-refining step was performed on a Voith refiner. The £ 3 increase in optical brightener used was always 2 wt%. The entire amount was metered into the pulp before the pre-milling step, after which the samples were milled with 200 kWh / t energy. The pre-milling was done at 4% consistency.

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The resulting pulps were diluted to 1.6% consistency for the fluidizer and 2% consistency for the Masuko refiner. Optical brighteners were added to the pre-milling process to improve their binding to the 5 fiber surface.

4. Sample Preparation 4a: Fluidizer 10 Some of the samples were fluidized with an M-700 Microfluidics Processor.

These samples were dispersed with a mixer at a 1.6% consistency for 30 minutes. After dispersion, the samples were passed three times through the fluidizer so that, for the first time, only one APM chamber of 500 µm diameter was used. On the second run, the fiber suspension was passed through two successive chambers of 500 µm and 200 µm in diameter. The third time was performed by passing the fiber suspension through successive chambers of 500 µm and 100 µm in diameter. The fluidiser condenser was turned off for all of these experiments as it was found to improve fibrillation in the first part of the experiments.

4b: Masuko ultra-fine friction grinder.

The milled samples were dispersed at 2% consistency with a mixer for 15 minutes prior to treatment with the Masuko. The dispersed ^ 25 samples were passed four times through a Masuko grinder such that the first time the gap between the refining stones was wider than the next three times. The flour was first and third washed afterwards.

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30 5. Characterization

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The gel-like fiber suspensions obtained from the Masuko refiner and fluidizer were characterized by measuring the viscosity (Brook field) and turbidity of the samples and examining the images taken with the optical microscope and 35 scanning electron microscope. In addition, centrifugation measurements measured the solids content of both the liquid and solid phases to determine the amount of nanoscale material in the sample.

5 6. Making paper sheets

The pulps produced were made into sheets of paper in the laboratory.

7. Test results 10

The milling result can be determined by measuring the viscosity level of the nanofibrillated cellulose sample, since the viscosity level of the pulp material varies with the proportion of nanofibrillated cellulose contained in the pulp. In general, the finer ground and thus the more fibril-15 nanofibrillated cellulose is a more gel-like material than the less nanofibrillated cellulose. Therefore, the more gel-like material means more nanofibrillated cellulose in the sample, and this higher proportion of nanofibrillated cellulose can be seen at a higher viscosity level of the sample. Samples with dose-20 saturated optical brighteners were clearly more viscous than control samples. The amount of non-fibrillated fibers can be determined from images taken under an optical microscope.

From both the viscosity results and the optical microscope images, the optical brightener dosage has a clear effect on the fibrillation. Some of these results can be seen in Figures 2a.

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o 2c, showing viscosity results and optical microscope images of samples ground by a Masuko refiner. Figure 2a shows the viscosity results for control sample 21, sample 22,

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30, which was dosed with a disulfonic optical brightener, and sample 23, which was dosed with a hexulfonic optical brightener. Figure 2b is

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The optical microscope image of the reference sample 21, and Figure 2c is the optical microscope image of the sample 23 dosed with hexanesulfonic optical brightener.

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Fluidized samples were also subjected to preliminary redispersion experiments after the samples were oven dried. Differences in the disintegration of the dried films in water were observed. Comparative samples did not disintegrate to any small extent, but samples modified with optical brightener clearly disintegrated. Figures 3a and 3b show optical microscope images of fluidizer samples showing a control sample (shown in Figure 3a) and a sample 32 having a hexanesulfonic optical brightener (shown in Figure 3b). The sample shown in Figure 3b to which the optical brightener was administered is clearly better fibrillated than the control sample shown in Figure 3a. Figures 4a and 4b show the same situation as scanning electron microscope images (magnification: 10,000 χ), showing a comparative sample (Figure 4a) and a sample supplemented with a hexanesulfonated optical brightener (Figure 4b). As can be seen in the pictures, a sample with hexanesulfonated optical brightener 15 looks much better than a control sample, since many smaller fibrils can be seen in the picture.

The quality of nanofibrillated cellulose can be analyzed using centrifugation of the nanoscale material and measuring the turbidity of the samples. The centrifugation results are expected to reflect the degree of fibrillation, so that a more nanoscale material will fibrillate more efficiently than a less nanoscale material. For turbidity results, the lower the turbidity value, the more nano-sized material should be present in the sample. Figure 5 shows turbidity and centrifugation results for fluidized samples. This figure shows a control sample 51, a sample 52 dosed with a disulfone-type optical scanner.

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o dew, and sample 53 supplemented with a hexane sulfone-type optical dew dew. As can be seen in the picture, the samples that were dosed with optical brightener were the most nanosized.

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Centrifugation results also showed that the amount of nanoscale material increases with the addition of optical brightener.

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O) o cm Retrospectively tested sheets of paper showed that the specific strength values were significantly higher for sheets according to the invention than for 35 control sheets without added optical brightener.

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It will be obvious to one skilled in the art that various embodiments of the invention may have applications in environments where it is desired to optimize the fibrillation of the nanofibrillated cellulosic pulp. Accordingly, it is to be understood that the present invention is not limited to the above embodiments, but may be modified within the scope of the appended claims.

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Claims (12)

A pulping process comprising producing at least 30% by weight of nanofibrillated cellulosic material measured on a dry basis, comprising: - introducing into the system a raw material containing cellulose; in the presence of an optical brightener in at least 10 pre-refining or fibrillation steps to form a fibrillated cellulose material. A method according to claim 1, characterized in that the milling is carried out in the presence of a dosed optical brightener in at least one fibrillation step to form a fibrillated cellulosic material. Process according to Claim 1 or 2, characterized in that the prepared pulp contains at least 40% by weight, at least 50% by weight, at least 60% by weight or at least 70% by weight of nanofibrillated cellulosic material, measured on dry weight basis. Method according to one of Claims 1 to 3, characterized in that at least part of the raw material is selected from: - unbleached chemical pulp, 25. bleached chemical pulp, O 4 - unbleached thermochemical pulp, and CO 0 - bleached thermochemical pulp. (Mo o A method according to any one of claims 1 to 4, characterized in that CL 30 comprises dispensing at least one type of optical brightener prior to the pre-grinding step. CD 0) 0 cm A method according to any one of claims 1 to 5, characterized in that at least one type of optical brightener is added to the pre-refining step. A method according to any one of claims 1 to 6, characterized in that at least one type of optical brightener is administered prior to the fibrillation step. 5 A method according to any one of claims 1 to 7, characterized in that at least one type of optical brightener is administered to the fibrillation step. Method according to one of the preceding claims 1 to 8, characterized in that the type of the at least one dispensed optical brightener is selected from the group consisting of stilbene, coumarin and pyrazoline compounds. Use of a pulp in nanofibrillated cellulose composites, wherein at least 15 parts of the pulp are made by a process according to any one of the preceding claims. Use of pulp in the production of paper or board, including - base paper production and 20. paper or board finishing steps, wherein at least a portion of the pulp is made by a process according to any one of claims 1-9. Use according to claim 11, wherein said finishing steps include coating paper or cardboard. (M i CO o {M O X en CL CO CO (M CD O) o o {M