Method for the mechanical and chemical treatment of cellulose fibres in a mixer

Abstract

The present invention relates to a method for the mechanical and chemical treatment of cellulose fibers in aqueous suspension in a mixer (1) comprising a body (10) defining a mixing chamber (11) whose wall forms a fixed compression surface (12), and at least one mixing axis (20) positioned in the mixing chamber (11) and comprising a rotatable shaft (21), and a plurality of mixing elements (22) fixed on the shaft (21), projecting radially from the shaft, and each provided with a rotatable compression surface (23). In said method, the compression surfaces (23) of the mixing elements (22) face the compression surface (12) of the mixing chamber (11), so as to define between them compression zones (ZC) of the fiber suspension (2) circulating between the body (10) and the mixing axis (20).The compression surfaces (23) of the mixing elements (22) have a compression profile whose radius relative to the shaft (21) varies according to the angular position of said shaft, so as to compress the suspension of fibers (2) in said compression zones (ZC) during the rotation of said mixing axis (20).

Description

Field of invention

[0001] The present invention relates to a method for the mechanical and chemical treatment of cellulose fibers in aqueous suspension, in particular to increase the binding potential of the fibers used in the composition of paper, the generic term "paper" covering paper, paperboard, cardboard and molded cellulose. Prior art

[0002] Fiber refining is one of the most important processes in the pulp industry. This operation is the key process for adjusting the strength properties as well as the quality of the paper, such as the mechanical, optical, and printability properties of the paper. Depending on the level of refining, the range of paper types varies from blotting paper (without refining) to tracing papers requiring intense refining. It also allows, when the pulp is enriched with mineral fillers, to improve the retention rate of these fillers in the paper, and to compensate for the drops in mechanical properties of the paper induced by the fillers, such as its tensile strength and / or tear strength. Refining is therefore an essential operation.

[0003] Paper pulp, also called "pulp," is a suspension of cellulose fibers dispersed in water. It is generally introduced into the air gap of a rotating refiner-type machine. The air gap is the space between two facing grooved surfaces arranged on discs or cones in relative motion. The speed differential at the periphery between the rotor (rotating part) and the stator (stationary part) is generally of the order of 20 to 25 m / s, and generates a shearing phenomenon. When the pulp passes through the air gap, which is approximately 100 µm to 300 µm (compared to the fiber dimensions of around 1 to 3 mm by 15 to 40 µm in diameter), the fibers are subjected to a succession of compression and shear forces, which induce morphological modifications of their ultrastructure.Three primary effects are generally associated with these morphological transformations: hydration, fibrillation, and cutting.

[0004] Compressive forces will compress the fibers, having the technical effect of hydrating the fibers, delaminating their walls, softening them, and making them more flexible. Increasing the flexibility of the fibers has the advantage of significantly increasing the bonding surface between the fibers, which is subsequently favorable for the formation of the paper sheet and its mechanical properties.

[0005] Shear forces will generate surface fibrillation by tearing off pieces on the outer part of the fiber wall. They will also generate fiber cutting, but also the tearing of surface fibrils, which will increase the fine element content of the paper pulp.

[0006] However, the presence of fine cellulosic elements in the paper pulp is detrimental to the subsequent paper sheet formation process. They will cause clogging of the grids and alter the "drainability" of the paper pulp, leading to an increase in the energy required to create the vacuum under the wire, a limitation of the speed of the paper production machines, therefore a reduction in productivity, and an increase in steam consumption during drying, therefore a significant increase in the energy consumption required for drying the paper. In addition, the fine elements, recovered in the process water, when not recycled, generate non-recoverable waste.

[0007] US 4,614,304, DE 102 56 856 and DE 102 36 962 describe methods for mechanical treatment of cellulosic fibers.

[0008] US 4,908,101 and US 5,466,334 describe a method for mixing chemical components and fibers.

[0009] Document EP 0 276 608 describes the ozone treatment of cellulosic fibers.

[0010] US 1,991,499 describes a process for preparing paper.

[0011] Document US 2009 / 0230223 describes a method for mixing a polymer composition.

[0012] The chemical modification of cellulose fibers also represents a challenge, particularly in the field of paper manufacturing, as this modification can improve the properties of the paper. Statement of the invention

[0013] The present invention aims to overcome these drawbacks (clogging of the fabrics, alteration of drainability, increase in the energy associated with the generation of the vacuum, limitation of the speed of the production machines, reduction in yield, increase in the consumption of steam and therefore of energy necessary for drying the paper, non-recycled fine elements, non-recoverable waste) by proposing an alternative method for treating the fibers (advantageously cellulosic), which does not generate shear, or at least so little that it becomes negligible, therefore reduces or even eliminates the generation of fine elements and cut fibers. The invention also aims to provide such a method which is less energy-intensive, while making it possible to improve the properties of the fibers and therefore intrinsically the properties of the paper, or even to develop new papers.The method according to the invention makes it possible to increase the binding potential of the fibers, but also the mechanical properties of a sheet of paper.

[0014] More precisely, for a given mechanical resistance of a sheet of paper, the present invention makes it possible to refine the cellulose fiber, while limiting, or even avoiding, the alteration of the drainability properties (°SR).

[0015] For this purpose, the invention relates to a method for mechanical and chemical treatment of cellulosic fibers by mechanical compression and chemical modification of cellulosic fibers, the method comprising the following steps: a) in a mixer, (1) mechanical compression and chemical modification of the cellulose fibers (2), in aqueous suspension having a concentration of between 5 and 20% by weight of cellulose fibers, in the presence of a chemical reagent to produce an aqueous composition of cellulose fibers having chemically modified functional groups, the mixer comprising: a body defining a mixing chamber delimited by a wall forming a fixed compression surface capable of coming into contact with the fiber suspension, at least one mixing axis positioned in the mixing chamber and comprising: ▪ a rotating shaft, ▪ a plurality of mixing elements (or paddles) fixed on the shaft, projecting radially from the shaft, and each provided with a rotating compression surface capable of coming into contact with the fiber suspension, the mixing elements having a helical or eccentric compression profile,method in which the compression surfaces of the mixing elements face the compression surface of the mixing enclosure, so as to define between said respective compression surfaces of the mixing elements and of the mixing enclosure compression zones of the suspension of fibers circulating between the body and the mixing axis, and in which the compression surfaces of the mixing elements have a compression profile whose radius relative to the shaft varies according to the angular position of said shaft, so as to compress the suspension of fibers in said compression zones during the rotation of said mixing axis (in other words, the compression profile, at a defined point of the mixing enclosure, varies during each complete rotation of the mixing element), the method having a speed differential between the at least one mixing axis and the mixing enclosure of less than or equal to 15 m / s,the mixer having a minimum distance separating the at least one mixing axis from the mixing enclosure of between 0.1 mm and 1 mm, the aqueous suspension of cellulose fibers has a residence time in the mixer of between 15 seconds and 15 minutes, b) optionally, at the mixer outlet, aging of the suspension of chemically modified cellulose fibers.

[0016] As already mentioned, the process allows to reduce or even eliminate the generation of fine elements and cut fibers. Thus, the chemical modification taking place in the mixer is optimized to the extent that the chemical reagent is not consumed by the fine elements.

[0017] Typically, the chemical modification of the cellulosic fibers results from a chemical reaction on the cellulosic fibers within the mixer during step a). This chemical reaction is advantageously chosen from oxidation, bleaching, carboxymethylation and reduction.

[0018] The helical vanes are advantageously oriented at 45° to each other.

[0019] According to a particular embodiment, the mixing elements are configured so as to provide an increasing compression profile in the mixer.

[0020] Advantageously, the mixer comprises at least two mixing axes positioned in the mixing enclosure, parallel to each other, the compression surfaces of the mixing elements of said mixing axes facing each other, so as to define between them additional compression zones of the suspension of fibers circulating between the mixing axes.

[0021] The mixer can be easily integrated into a continuous process, for example between the pulper and the paper production machine, after a thickening step (or thickening) of the pulp (aqueous suspension of cellulose fibers). The presence of a thickening step is particularly suitable for processes using recycled cellulose fiber pulp; in this case, a thickener can 1) separate the water from the paper pulp preparation circuit from the water from the paper machine and 2) feed the mixer.

[0022] Advantageously, the mixing enclosure comprises at least one inlet zone, one mixing zone and one outlet zone, the fiber suspension being introduced into the inlet zone and being extracted from the outlet zone, each mixing axis comprising at least one worm screw fixed on the shaft, arranged in the inlet zone and arranged to move the fiber suspension from the inlet zone to the outlet zone through the mixing zone, said plurality of mixing elements being arranged in the mixing zone and the outlet zone being or not integrated into the mixing zone.

[0023] Advantageously, the mixing zone comprises several successive mixing sectors, the mixing elements of each mixing axis being divided into series, one series corresponding to one mixing sector, the mixing elements of the same series being identical, and the mixing elements from one series to another being different in their shape and / or their thickness and / or their radius and / or their compression profile (the compression profile including the orientation of the mixing element relative to the axis).

[0024] Advantageously, the mixing elements consist of flat or complex parts, which have a section chosen from the group comprising an ovoid section comprising one lobe, an oblong section comprising two lobes, a triangular section comprising three lobes, a polygonal section comprising a number of lobes corresponding to the number of angles, and a helical or eccentric compression profile. The lobe(s) may have a flattened end.

[0025] Even if the conditions of use of the mixer make it possible to limit, or even eliminate, the shearing effects of the cellulose fibers, the helical profile and the eccentric profile are still less shearing than the flat profile and the notched profile.

[0026] Advantageously, the minimum distance separating the kneading axis(es) from the kneading enclosure is between 0.1 mm and 0.6 mm, preferably between 0.15 mm and 0.6 mm, in particular between 0.3 mm and 0.6 mm or between 0.3 mm and 0.5 mm. This minimum distance may in particular be between 0.15 mm and 0.4 mm. Beyond 1 mm, the kneader does not allow the cellulose fibers to be made flexible, although this is the desired effect in order to improve the properties of the cellulose fibers.

[0027] Advantageously, the minimum distance separating two mixing axes is identical to the minimum distance separating the mixing axis(es) from the mixing enclosure.

[0028] Advantageously, when the mixer comprises at least two mixing axes positioned in the mixing enclosure, the mixing axes are co-rotating, in that they rotate in the same direction of rotation.

[0029] Advantageously, when the mixer comprises at least two mixing axes positioned in the mixing enclosure, the mixing elements of two mixing axes whose compression surfaces face each other have an identical compression profile.

[0030] The speed differential between the periphery of the paddles (mixing elements) and the mixing enclosure is less than or equal to 15 m / s. This differential may be at least 0.1 m / s, for example at least 2 m / s, in particular between 0.1 and 15 m / s. It may in particular be between 0.5 and 10 m / s, in particular between 0.5 and 5 m / s, advantageously between 0.5 and 1.5 m / s. Beyond 15 m / s, the cellulose fibers undergo significant shear and are therefore shortened.

[0031] The residence time of the fiber suspension in the mixer is between 15 seconds and 15 minutes, for example between 15 seconds and 10 minutes, preferably between 15 seconds and 6 minutes, preferably between 15 seconds and 5 minutes, and more between 15 seconds and 3 minutes.

[0032] Advantageously, the residence time of the fiber suspension in the mixer and the speed differential between the mixing axis(es) and the mixing enclosure allow a number of compressions of at least 500, advantageously between 500 and 5000, more advantageously between 3000 and 5000.

[0033] Advantageously, the aqueous suspension of cellulosic fibers comprises a mixture of cellulosic fibers and water, and optionally mineral fillers. During the mechanical and chemical treatment, the proportion of dry cellulosic fibers is between 5 and 20% by weight, advantageously between 5 and 15% by weight, preferably between 8 and 15% by weight of the aqueous suspension. During the mechanical and chemical treatment, a concentration greater than 20% by weight, for example between 25 and 45% by weight, damages the cellulosic fibers and does not improve the mechanical properties, for example the flexibility allowing the bonding surface (binding effect) to be increased, of the cellulosic fibers. Indeed, above 20% by weight, the cellulosic fibers tend to be shortened and to be "kinked" and curled.Below 5% by weight, the aqueous suspension of cellulose fibers undergoes a phase separation phenomenon in the mixer, which prevents homogeneous treatment in particular.

[0034] Advantageously, the method according to the invention further comprises, prior to the mechanical and chemical treatment of the aqueous suspension of cellulosic fibers, a step of thickening said aqueous suspension of cellulosic fibers to form a paste. When necessary, this step makes it possible to increase the concentration of the aqueous suspension of cellulosic fibers, between 8 and 45% by dry weight of cellulosic fibers to feed the mixer, with a dilution between 8 and 15% at the inlet of the mixer if the paste is very concentrated.

[0035] In addition to the significant reduction of fine elements in the suspension during treatment, the treatment method of the invention has the advantage of minimizing waste, reducing production costs, improving the life cycle of the paper, and entering into a virtuous circle favorable to the environment.

[0036] Furthermore, since the fibers are compressed by kneading, and not refined by shearing, the long softwood fibers are cut less thanks to the process of the invention. Thus, to adjust the tear resistance in particular, it becomes possible to reduce the proportion of softwood in the hardwood / softwood mixture intended to form the sheet of paper. This results in savings on the cost of raw materials.

[0037] Another advantage of the process of the invention is that it leads to an improvement in the recycling of paper, insofar as the latter contains fewer fine elements, generally eliminated by the recycling process (flotation, washing in particular), further promoting the life cycle of the paper. Figures

[0038] The present invention and its advantages will appear better in the following description of several embodiments given as non-limiting examples, with reference to the figures. There Figure 1 is a plan view showing a mixer interposed between a pulper and a papermaking machine. Figure 2 is a radial section of the mixer of the Figure 1 . There Figure 3 is a perspective view of two mixing axes of the mixer of the Figure 2 , equipped with flat shear profile pallets. The Figure 4is a perspective view of another embodiment of the mixing axes of the Figure 3 , equipped with flat shear profile pallets. The Figure 5 illustrates two mixing axes having a helical profile, minimizing shear. The Figure 6 illustrates two mixing axes having a 90° flat paddle profile maximizing shear. The Figure 7 illustrates two mixing axes having an eccentric paddle profile mounted at 45°, reducing shear. The figure 8 illustrates two mixing axes having an eccentric paddle profile mounted at 90°, reducing shear. The Figure 9 illustrates two pallets having a helical profile. The Figure 10 illustrates two pallets having a flat profile. The Figure 11 illustrates a palette with an eccentric profile. The Figure 12shows the improvement in the mechanical properties of a sheet of paper (burst index) as a function of the additive used and the density of the paper.

[0039] THE Figures 1 to 11 use the following nomenclature: 1: Mixer 2: Fiber suspension 3: Pulper 4: Papermaking machine 5: Paper sheet 10: Body 11: Mixing chamber 12: Compression surface (fixed) 13: Inlet orifice 14: Outlet orifice 15 and 16: Inlet nozzles for injecting products, for example chemical reagents 20: Mixing shaft 20': Mixing shaft ( Figure 4 ) 21: Shaft 22: Mixing elements 22': Mixing elements ( Figure 4 ) 23: Compression surface (mobile) 24: Lobes 24': Lobes ( Figure 4 ) 25: Endless screw X: Axis of rotation ZC: Compression zones DM: Minimum distance ZE: Inlet zone ZM: Mixing zone ZS: Outlet zone

[0040] Even though the figures 1 to 8 show two mixing axes, the description of these figures can be generalized to mixers having a single mixing axis or more than two. Detailed description of the invention Mechanical compression and chemical modification applied to cellulose fibers

[0041] The method according to the invention relates to a method for mechanical treatment and chemical modification of cellulose fibers by mechanical compression and chemical modification of an aqueous suspension of cellulose fibers. The invention corresponds to an intensification of the method, the chemical modification being carried out jointly with the mechanical compression, while controlling the shear thanks to the specific profile of the mixing elements. Step a)

[0042] This process comprises the mechanical compression and chemical modification, in a mixer, of cellulosic fibers, in aqueous suspension having a concentration of between 5 and 20% by weight of cellulosic fibers, in the presence of a chemical reagent to produce an aqueous composition of chemically modified cellulosic fibers.

[0043] In particular embodiments, the aqueous suspension of cellulosic fibers is free of hemicellulose.

[0044] Advantageously, the chemical modification of the cellulose fibers results from a chemical reaction on the cellulose fibers within the mixer during step a). This chemical reaction is advantageously chosen from oxidation, bleaching, carboxymethylation and reduction. Preferably it is an oxidation reaction.

[0045] Examples of chemical reagent include, but are not limited to, periodic acid or a salt thereof, metaperiodic acid or a salt thereof, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), azaadamantane-N-oxyl (AZADO), hydrogen peroxide, ozone, chloroacetic acid, a hypohalite, or a hydrosulfite salt (sodium hydrosulfite, sodium bisulfite).

[0046] When the chemical reagent is TEMPO, the chemical reaction is advantageously carried out in the presence of sodium hypochlorite (NaClO) and sodium bromide (NaBr) under alkaline conditions or sodium hypochlorite (NaClO) and sodium chlorite (NaClO 2 ) under neutral conditions.

[0047] Preferably, the chemical reaction of step a) is an oxidation reaction, in particular a selective oxidation of the primary and secondary alcohol groups of the anhydroglucose units (AGU) of the cellulose.

[0048] In this case (oxidation), the reagent is advantageously periodic acid or one of its salts, in particular sodium periodate. In these embodiments, the oxidized cellulose obtained has aldehyde functions.

[0049] During step a), the chemical reagent / cellulose fiber molar ratio is advantageously between 0.1 and 1, more advantageously between 0.3 and 0.8, in particular when the chemical reagent is periodic acid or one of its salts.

[0050] With a ratio of 1, the number of moles of cellulose fibers corresponds to the number of moles of anhydroglucose.

[0051] Thus, preferably, the oxidant / AGU molar ratio varies from 0.1 to 1, advantageously from 0.3 to 0.8. A ratio equal to 1 means one mole of reagent for 1 mole of AGU.

[0052] The chemical reaction is carried out in the mixer at a temperature advantageously between 15 and 100°C, preferably 20 to 80°C and even more preferably 40 to 70°C, more advantageously 50 to 60°C.

[0053] The chemical reagent from step a) can be added pure (in solid, liquid or gaseous form), or in solution (diluted in a solvent, preferably in water).

[0054] When the chemical reagent is added in the form of a solution, the solvent used is typically one chosen to solubilize the reagent. A person skilled in the art, based on their knowledge, will be able to select a suitable solvent. This is advantageously water.

[0055] The chemical reagent from step a), pure or in solution, and the suspension of cellulose fibers can be introduced into the mixer separately or premixed. Preferably, they are added separately.

[0056] Advantageously, the suspension of cellulose fibers is diluted with the reagent solution.

[0057] When the reagent is an oxidant, particularly a periodate salt, it is advantageously used in solution, preferably in concentrated solution.

[0058] When the chemical reagent is used in solution, it is advantageously diluted in water, preferably so as to obtain a concentrated solution of chemical reagent, preferably at the limit of the solubility threshold. In this case, the chemical reagent solution can be heated so as to dissolve the chemical reagent, for example up to 100°C, preferably between 20 and 80°C, and even more preferably from 40 to 70°C, more advantageously from 50 to 60°C. Those skilled in the art will know how to adapt the temperature according to the nature of the chemical reagent, for example up to 70°C for periodate salts.

[0059] By "concentrated solution" we mean a solution at the limit of the solubility threshold of the chemical reagent in the solvent used, advantageously water.

[0060] According to a preferred embodiment, the chemical reagent is diluted in water, then mixed with the cellulose fibers when they are introduced into the mixer.

[0061] The use of a chemical reagent solution in water makes it possible to dilute the suspension of cellulosic fibers within the mixer. However, after dilution with the concentrated periodate solution, the concentration of cellulosic fibers is, as already indicated, between 5 and 20% by weight in the mixer. For example, an aqueous suspension comprising 30 to 50% by weight, advantageously 35 to 40% by weight, of cellulosic fibers can be introduced into the mixer and diluted with a chemical reagent solution to achieve a concentration between 5 and 20% by weight which corresponds to the concentration during mechanical and chemical treatment.

[0062] In embodiments, the reagent is added continuously.

[0063] According to a preferred embodiment, the reagent is introduced into the mixer continuously just after or at the same time as the cellulose fibers. A person skilled in the art will know how to adapt the addition of the chemical reagent according to the residence time of the cellulose fibers in the mixer.

[0064] Advantageously, the chemical reagent of step a) has a residence time in the mixer of between 15 seconds and 15 minutes. It is generally identical to the residence time of the aqueous suspension of cellulose fibers.

[0065] The fiber / reagent mixture is left in the mixer to react for a period of time ranging from 15 seconds to 15 minutes, more preferably from 15 seconds to 6 minutes, even more preferably from 15 seconds to 2 minutes. Step b)

[0066] Optionally, once removed from the mixer, the cellulose fibers are left in an aging tank, advantageously for up to 72 hours. This aging time can potentially improve the reaction yield of step a).

[0067] The aging after step a) is advantageously carried out at a temperature ranging from 15 to 60°C, more advantageously from 20 to 45°C.

[0068] During step a) and / or aging, cellulose fibers can swell by trapping water molecules.

[0069] After aging step b), the chemical reagent that has not reacted with the cellulose fibers is advantageously separated from the reaction medium and recovered for possible reuse in the treatment process. For example, when the chemical reagent is a periodate salt, it can be recycled according to the process described in document NL 2030671.

[0070] The suspension from step b) comprises water and the modified cellulose fibers. It is advantageously washed. It can also be concentrated, advantageously after being washed. Washing facilitates the recycling of chemical reagents that have not been consumed. The chemical reagents thus recycled can be reintroduced into the mixer, at the stage of step a). The present invention therefore makes it possible to optimize the different chemical compounds used.

[0071] Conventionally, the washing step is typically carried out in a medium containing a solvent and / or water. In particular, the solvent is preferably water. When the cellulose fibers are selectively oxidized, for example with a periodate salt, the modified functional groups are aldehyde functions (-CHO). Remarkably compared to the prior art, the method according to the invention makes it possible to obtain a concentration of aldehyde functions (-CHO) of the oxidized cellulose of between 0.5 and 5 mmol / g of fibers, preferably between 1 and 4 mmol / g and even more preferably between 1.5 and 4 mmol / g.

[0072] These functions can be used for grafting molecules of interest, thus making it possible to functionalize cellulose.

[0073] Furthermore, these aldehyde functions can typically be converted into carboxylic acid functions by post-reaction in a second oxidation reaction.

[0074] Thus, the process may comprise, after aging step b), a washing step and an optional chemical post-reaction step.

[0075] Advantageously, the chemical post-reaction is chosen from oxidation, reduction, sulfonation and amination.

[0076] In embodiments, after the chemical post-reaction step, in particular oxidation or sulfonation, the aqueous suspension of cellulosic fibers may be in the form of a gel.

[0077] Advantageously, in the case of an oxidation post-reaction, the oxidant used is chosen from hydrogen peroxide, ozone, a hypohalite such as a hypochlorite (ClO -< ), a chlorite (ClO 2 -< ) or a hyposulfite salt.

[0078] These carboxylic acid functions can also be functionalized, for example to generate ester or amide functions.

[0079] Carboxylic acid functions can be present in acid form (-COOH) or in salt form (-COOX with X = alkali or alkaline earth metal or ammonium).

[0080] In some embodiments, the cellulose is oxidized to directly obtain carboxylic acid functions in a single step in the mixer.

[0081] In this case, the alcohol groups of cellulose are converted directly into carboxylic acid groups, i.e. without intermediate transformation of the alcohol groups into aldehydes.

[0082] In these particular cases, the reagent used is a conventional oxidant known to the person skilled in the art such as hydrogen peroxide, ozone, a hypohalite such as a hypochlorite or a hyposulfite salt.

[0083] Advantageously, when the post-reaction is a sulfonation reaction, the reagent used is typically a bisulfite salt making it possible to prepare a modified cellulose having anionic sulfonate functions. Uses

[0084] The invention also relates to the use of mechanically and chemically modified fibers for preparing a paper pulp.

[0085] The resulting pulp can be used alone or mixed with paper pulp to form a sheet of paper.

[0086] The mechanical and chemical modification of fibers alters their properties. Thus, modified fibers have an electrostatic charge that can vary from 0.5 to 5 meq / g, preferably from 1.0 to 4 meq / g.

[0087] These high electrostatic charge values ​​result from the intensification of the process (combination of the intimate mechanical mixing of the mixer and the chemical reaction with very efficient mixing of the products, particularly in the fiber wall) of the mechanical and chemical modification process which allows the increase of the charge density on the cellulose fibers.

[0088] The invention also relates to the use of a modified cellulose (for example anionic) as an additive in the preparation of paper. In this case, preferably 5 to 25% by weight of modified cellulose is added to an aqueous suspension of cellulose fibers (thick pulp and / or thin pulp), advantageously 10 to 25% by weight relative to the weight of the aqueous suspension of cellulose fibers. The amount of modified cellulose can be adjusted depending on the desired improvement (gain) in mechanical properties. Mixer

[0089] In the illustrated embodiments, identical elements or parts bear the same reference numbers. In addition, terms that have a relative meaning, such as vertical, horizontal, right, left, front, rear, above, below, etc., must be interpreted under normal conditions of use of the invention, and as shown in the figures. The X, Y and Z axes are defined by an orthonormal reference frame illustrated in Figure 1. Furthermore, the geometric positions indicated in the description and the claims, such as "perpendicular", "parallel", "symmetrical" are not limited to the strict sense defined in geometry, but extend to geometric positions which are close, that is to say which accept a certain tolerance in the technical field considered, without influence on the result obtained. This tolerance is notably introduced by the adverb "substantially", without this term necessarily being repeated before each adjective.

[0090] The mechanical treatment method according to the invention applies to any aqueous suspension of fibers, preferably cellulosic fibers for the paper industry, covering paper, paperboard, cardboard and molded cellulose. The suspension may comprise a mixture of cellulosic fibers and water, and optionally mineral fillers, depending on the specifications. The proportion of dry cellulosic fibers may be significant and for example between 5 and 20% by weight, and preferably between 8 and 15% by weight of the aqueous suspension, without these values ​​being limiting. The ratio between the fibers and the chemical reagents is specified above.

[0091] As explained below, the process of the invention has the advantage of making the cellulosic fibers flexible by compression-kneading, that is to say of making them supple, by generating few or no fine elements and cut fibers, while modifying them chemically. The resulting improvement in mechanical properties makes it possible to reduce the grammage of the paper according to the objectives sought in terms of the quality / price ratio of the paper.

[0092] In addition, the method of the invention also makes it possible to improve certain properties of cellulosic fibers. This advantage makes it possible to consider mixtures of fibers from, for example, hardwoods, softwoods and others, and to vary the ratios from 0 to 100%, depending on the objectives sought in terms of the quality / price ratio of the paper. Fibers from softwoods having a cost price much higher than that of fibers from hardwoods can thus represent a lower share than that of the other fibers, without harming the properties of the paper, and in particular tearing.

[0093] Since the process according to the invention generates little or no shear, it is possible to use fibers from annual plants, even though these are generally sensitive to shear.

[0094] In reference to the Figure 1, the mechanical treatment method according to the invention consists of a step of compressing the aqueous suspension of cellulose fibers in a mixer, instead of the conventional compression and shearing step between two rotating blade surfaces. The chemical modification is carried out at the same time as the mechanical treatment by compression.

[0095] The mixer used in the said process is known in fields very different from papermaking. It is mainly used to homogenize viscous materials, such as polymers, compositions, composites or similar, used in cosmetics, adhesives, plastics, chemistry, etc. It may in particular be a UCP type mixer from the company HASLER.

[0096] In the present invention, the mixer 1 as shown in Figure 1is fed with an aqueous suspension of cellulose fibers 2 from a pulper 3 or any other equivalent machine. It feeds a papermaking machine 4 at the output to form a sheet or web of paper 5. The mixer 1 can be easily integrated into a continuous process, between the pulper and the machine after a pulp thickening step, allowing a gain in productivity and a reduced production cost.

[0097] Mixer 1 mainly includes: a hermetically sealed body 10, elongated along an axis X, defining a mixing chamber 11, delimited by a peripheral wall forming a fixed compression surface 12, said body 10 being provided with at least one inlet orifice 13 upstream, for the admission of the suspension of fibers to be treated, and an outlet orifice 14 downstream of the mixing chamber 11, for the reception of at least a fraction of the treated suspension, and at least one mixing axis 20, elongated along the axis X, positioned in the mixing chamber 10.

[0098] The mixing shaft 20 comprises: a shaft 21 rotatable about said axis X, and a plurality of mixing elements 22, also called "paddles", fixed on the shaft 21, projecting from the shaft in a radial direction, each delimited by a peripheral wall forming a compression surface 23 integral in rotation with the shaft. The compression surface 23 is distant from the axis of rotation X by a non-constant radius, which varies between a maximum radius and a minimum radius depending on the point considered on said wall. As will be described in more detail in the remainder of this text, the irregular profile of the compression surface of the mixing elements relative to the axis of the shaft makes it possible to ensure, in cooperation with another corresponding compression surface, optimal compression of the fibrous suspension, thus leading to satisfactory mixing.

[0099] The mixing elements 22 arranged along the shaft 21 are preferably alternated (as opposed to eclipsed, along the X axis) in that they are oriented relative to each other in different angular positions to create a variable compression profile along the mixing axis 20. Thus, as shown in figures 5 , 7 and 8 , adjacent mixing elements can be offset, for example by 45° or 90°.

[0100] According to said method, the compression surfaces 23 of the mixing elements 22 which face the compression surface 12 of the mixing enclosure 11 on the one hand, and said compression surface 12 of the mixing enclosure 11 on the other hand, define between them compression zones ZC of the suspension of fibers 2 circulating between the body 10 and the mixing axis 20. The compression zones ZC thus correspond to the volume remaining between the mixing axis 20 and the mixing enclosure 11, which contains the suspension of fibers 2. They are identified very roughly by ovals at the Figure 2. Due to the variable compression profile of the mixing elements 22, the shape, location and volume of the compression zones ZC are not constant and vary continuously with the angular position of the shaft 21, having the effect of generating a large number of compression forces on the fiber suspension 2 during the rotation of the mixing axis 20 during the mixing operation. Thus, the cellulose fibers are kneaded, crushed, compressed, compressed, mixed, kneaded, but are not or very little cut or sheared because they do not undergo or very little shearing forces (the rotation speed of the paddles is low, typically of the order of 20 to 600, in particular between 20 and 350 revolutions per minute for example - rotation per minute or the acronym "rpm" in English - for a paddle diameter of approximately 25 mm, and therefore the shear at the wall is low, i.e. < 1 m / s for a laboratory mixer.For an industrial mixer, the diameter of the blades can for example be around 300 mm, for a wall shear of around 6 m / s, which remains low compared to the size of the mixer and the quantity of suspension treated of around 2 tonnes per hour). In other words, unlike conventional refining, the process of the invention induces a shear of the fibres which is negligible compared to their compression.

[0101] The mixer 1 may comprise more than one mixing axis 20, and for example two mixing axes 20 or more than two mixing axes, positioned in the mixing enclosure 11, parallel to each other. Thus, the compression surfaces 23 of the mixing elements 22 which face each other respectively define between them additional compression zones ZC for the suspension of fibers 2 circulating between the mixing axes 20. In this case, mixing axes 20 of identical or almost identical structure will preferably be chosen, positioned between them in such a way that the compression profile of one of the mixing axes matches the compression profile of the other mixing axis, and rotating at the same speed. In this preferred embodiment, it will be advantageous to ensure that two mixing elements 22 which face each other have the same shape. The mixing axes 20 can be co-rotating and thus rotate in the same direction of rotation, as shown in Figure 2 by the arrows in the counterclockwise direction, or counter-rotating and thus turn in opposite directions of rotation.

[0102] There Figure 2illustrates a radial section of a mixer 1 comprising two mixing shafts 20 parallel to each other in a mixing chamber 11. The mixing shafts 20 each comprise mixing elements 22 of oblong geometry. The mixing shafts 20 are angularly offset relative to each other, so that the two corresponding mixing elements 22 are positioned perpendicular to each other, without this relative position being obligatory and identical for the other mixing elements 22 not shown. This means, for example, that for a given position of the mixing shafts, a first pair of mixing elements facing each other can be offset by 90° relative to each other, while a second pair of mixing elements facing each other can be offset by 45° relative to each other. The mixing axes 20 rotate in the same direction of rotation and at the same speed.The center distance between the two shafts 21 of the mixing axes 20, as well as the profile of the compression surface 12 of the mixing enclosure 11 are defined as a function of the geometry of the mixing elements 22, to introduce a respective minimum distance DM between the mixing axes 20 and the mixing enclosure 11, and between the mixing axes 20 themselves.

[0103] The minimum distance DM, which is represented in the Figure 2by circles, is preferably between 0.1 mm and 0.6 mm, preferably between 0.15 mm and 0.6 mm, in particular between 0.3 mm and 0.6 mm or between 0.3 mm and 0.5 mm. This minimum distance may in particular be between 0.15 mm and 0.4 mm. This minimum distance DM allows in particular the rotation of the mixing axes 20 in the mixing enclosure 11 without conflict, while authorizing in a very limited manner the circulation of the suspension of fibers 2 in these restricted spaces. The limit zones located between the mixing axes 20 on the one hand, and between each mixing axis 20 and the mixing enclosure 11 on the other hand, which are defined by the minimum distance DM, therefore correspond to zones of high mechanical stress for the fibrous composition.When the mixing axes 20 rotate, these limit zones move, so as to alternately constrain the composition in the volume of the enclosure, which ensures optimal compression of the fibrous composition in the compression zones ZC.

[0104] The mixing elements 22 may have different geometries, thicknesses and compression profiles depending on the type of treatment and the degree of flexibility to be achieved, the nature and composition of the fiber suspension to be treated, and the specifications of the papermaker. The mixing elements 22 illustrated in Figure 2 are flat, oblong-shaped pieces, symmetrical with respect to two perpendicular planes passing through the axis of rotation X, and comprise two diametrically opposed lobes 24.

[0105] This example is of course not limiting. The mixing elements 22 may have an ovoid shape, symmetrical with respect to a plane passing through the axis of rotation X and comprising a single lobe (not shown). They may have a triangular shape, symmetrical with respect to a plane passing through the axis of rotation X and one of the vertices, and comprising three lobes (shown in Figure 4 ). They can also have a polygonal section, symmetrical or not with respect to a plane passing through the axis of rotation X, and whose number of lobes corresponds to the number of angles of the polygon (not shown). Finally, they can have a complex shape, symmetrical or not with respect to a plane passing through the axis of rotation X (not shown).

[0106] Similarly, the mixing elements 22 may comprise parts whose peripheral wall, i.e. the compression surface, is flat or not, and whose compression profile in the thickness of the part is variable. Thus, a succession along the shaft of parts whose thickness has a helical profile ( figures 5 And 9 ) leads to a helical mixing shaft forming a complete helix, each piece constituting a fraction of the helix. The Figure 11 illustrates a palette with an eccentric profile. The Figure 10 illustrates a palette (2 lobes) with a flat shear profile.

[0107] There Figure 3 illustrates two parallel, identical mixing axes 20, offset from each other by an angle of 90°, each provided with mixing elements 22 that are flat, oblong with two lobes 24, with a straight compression profile in the thickness, substantially identical to those shown in the Figure 2The mixing elements 22 of the same axis are angularly offset from each other by 90°, and the mixing elements 22 of the two mixing axes 20 are axially alternated to allow their radial interlocking. The mixing elements 22 of the Figure 3 And 6 have a flat profile ( Figure 10 ), that is, shearing.

[0108] Advantageously, the mixing elements with the eccentric profile have a rounded compression surface 23. In other words, according to this embodiment, the edges of the blade are rounded (i.e. not protruding).

[0109] There Figure 4illustrates two parallel, identical 20' mixing axes, each provided with flat, triangular mixing elements 22' with three equidistant 24' lobes, with a straight compression profile in the thickness. The mixing elements 22' of the same axis are angularly offset from each other by 30°, and the mixing elements 22' of the two mixing axes 20' fit together radially. The mixing elements 22 of the Figure 4 have a flat profile, that is to say shearing.

[0110] There Figure 5 illustrates two parallel, identical mixing axes, without angular offset, each provided with helical mixing elements, with two equidistant lobes, with a helical compression profile in the thickness. The mixing elements ( Figure 5 ) of the same axis are angularly offset from each other by 45°, and the mixing elements of the two mixing axes fit radially. The mixing elements of the Figure 5have a helical profile, i.e. less shearing, or even non-shearing. The mixing axes of the Figure 5 include, in the area of ​​introduction of the cellulose fiber suspension, a worm screw.

[0111] There Figure 6 illustrates two parallel, identical mixing axes, without angular offset, each provided with flat, two-lobe equidistant mixing elements, with a flat compression profile in the thickness. The mixing elements ( Figure 6 ) of the same axis are angularly offset from each other by 90°, and the mixing elements of the two mixing axes fit radially. The mixing elements of the Figure 6 have a flat profile, i.e. shearing. The mixing axes of the Figure 6 include, in the area of ​​introduction of the cellulose fiber suspension, a worm screw.

[0112] There Figure 7illustrates two parallel, identical mixing axes, without angular offset, each provided with mixing elements with an eccentric compression profile in the thickness. The mixing elements ( Figure 7 ) of the same axis are angularly offset from each other by 45°, and the mixing elements of the two mixing axes fit radially. The mixing elements of the Figure 7 have an eccentric profile. The mixing axes of the Figure 7 include, in the introduction zone of the cellulose fiber suspension, a worm screw and, in the exit zone, a series of helical blades.

[0113] Generally speaking, positioning the mixing elements (whatever the profile) on the same axis by angularly offsetting them by 45° relative to each other reduces shear compared to an offset of 90°.

[0114] There figure 8illustrates two parallel, identical mixing axes, without angular offset, each provided with mixing elements with an eccentric compression profile in the thickness. The mixing elements ( figure 8 ) of the same axis are angularly offset from each other by 90°, and the mixing elements of the two mixing axes fit radially. The mixing elements of the figure 8 have an eccentric profile, i.e. non-shearing. The mixing axes of the figure 8 include, in the introduction zone of the cellulose fiber suspension, a worm screw and, in the exit zone, a series of helical blades.

[0115] The mechanical treatment method according to the invention offers a flexible, modular, adaptable, scalable compression kneading solution, making it possible to vary, modify, refine the properties of the fibers depending on the paper to be manufactured. This method is also part of a research and development approach for new papers. For this purpose, the structure of the kneader 1 as well as its operating parameters can be easily chosen, modified and / or combined.

[0116] As seen above, the structure of the mixer 1 is determined by the number of mixing axes 20 as well as by the arrangement and choice of geometry of the mixing elements 22, but not only. The mechanical processing method can also comprise several steps to achieve different degrees of flexibility. These different steps can be carried out in different ways: by a mixer 1 comprising a mixing chamber 11 and at least one mixing shaft 20 on which are mounted one or more series of mixing elements 22 of different and appropriate structures, in which the fiber suspension 2 is rotated in a loop; by an extended mixer 1 comprising a mixing chamber 11 and at least one mixing shaft 20 on which are mounted several series of mixing elements 22 of different and appropriate structures; by a mixer (not shown) comprising several successive mixing chambers, connected in series, each comprising one or more mixing shafts of different and appropriate structures; or by several successive mixers (not shown), connected in series, of different and appropriate structures.

[0117] In these different cases, the treated fiber suspension 2 can be extracted at each stage of the process and / or at the end of the process stages as required.

[0118] The operating parameters of the mixer 1 can also be selected, modified and / or combined to vary the mixing characteristics. Examples of the modifiable parameters include, but are not limited to, the following: the feed rate of the mixer 1 in fiber suspension 2, defining the mixing duration;the mixing temperature: the mixer operates at room temperature, but can also operate at a temperature above or below room temperature thanks to the body 10 of the mixer 1 which can be made up of a double jacket (not shown) allowing the circulation of a heat transfer fluid. In more detail, the mixer is kept at room temperature by cooling to compensate for the temperature rise due to mixing, in order to better control the viscosity of the paste. In practice, a temperature rise of approximately +10°C to +30°C is observed depending on the operating concentration, the rotation speed, and the residence time. Alternatively, it is possible to work hot, in particular to carry out oxidation reactions in conjunction with mixing (intensification of the process) or cold, for example in the presence of soda to dissolve the cellulose;the mixing pressure: preferably the mixing operation is carried out in a mixing chamber 11 at atmospheric pressure, but depending on the needs could be carried out at a pressure higher or lower than atmospheric pressure. For example, it may be interesting to work at a pressure higher than atmospheric pressure to adjust the filling speed of the feed screw (for example of the endless screw type as described in the remainder of this text); the rotation speed of the mixing axis(es) 20: this speed is preferably relatively low, for example between 20 and 600 rpm, preferably between 20 and 400 rpm or between 20 and 350 rpm; the speed differential between the mixing axes 20 (rotating part) and the mixing chamber 11 (fixed part) is less than or equal to 15 m / s thus producing little or no shearing phenomenon;the direction of rotation of the mixing axes 20: co-rotating or counter-rotating; the mixing power transmitted to the fiber suspension 2 by measuring the resistive torque of the mixing axes 20; the mixing time: the residence time of the fiber suspension 2 in the mixing enclosure 11 may be between 15 seconds and 15 minutes, preferably between 15 seconds and 10 minutes, and more preferably between 15 seconds and 5 minutes; the minimum distance DM between the mixing axes 20 and the mixing enclosure 11 by modifying the mixing elements 22; the thickness of the mixing elements 22 to increase or decrease the compression surfaces.

[0119] The kneading tests carried out with the method of the invention revealed an unexpected and counter-intuitive physical phenomenon for those skilled in the art of papermaking: the higher the rotation speed of the kneading shafts 20 (20 to 600 rpm), the shorter the duration of application of the mechanical compression action on the cellulose fibers, and the better the kneading result. A high speed makes it possible to limit the shearing and fibrillation of the cellulose fibers and therefore the formation of fine elements. These conditions favor brief compressions, but a high number of compressions, advantageously at least 500, more advantageously between 500 and 5000, and even more advantageously between 3000 and 5000.

[0120] In summary, the rotation speed of the mixing axes (20 to 600 rpm) and the mixing conditions (speed differential ≤ 20 m / s and residence time between 15 seconds and 15 minutes) allow for short and frequent mechanical compression, which has the effect of limiting the alteration of the cellulose fibers. Thus, the cellulose fibers are less damaged, or even preserved, compared to long and frequent or infrequent mechanical compressions.

[0121] This phenomenon, which cannot be detected or envisaged before implementing the method of the invention, makes it possible to further increase the mixing performance and the advantages of the method which result from it, such as better flexibilization of the fibers, improved fiber content, possible new paper pulp formulations, reduced power consumption allowing significant energy savings, very little wear of the moving parts allowing reduced maintenance of the mixer 1.

[0122] In the example of mixer 1 shown in Figure 1, the mixing enclosure 11 comprises an inlet zone ZE, a mixing zone ZM and an outlet zone ZS, which follow one another axially. The inlet zone ZE is coupled to the inlet orifice 13 and preferably comprises a worm screw 25 fixed on the corresponding shaft 21 of each mixing axis 20, to move the fiber suspension 2 axially towards the mixing zone ZM and then the outlet zone ZS. The mixing zone ZM comprises the mixing elements 20 described above. The outlet zone ZS can be confused with the end of the mixing zone ZM and also comprises mixing elements 20 which may or may not be different from those of the mixing zone ZM. The outlet zone ZS communicates with the outlet orifice 14 which can be gravity-driven or combined with any other extraction means.The mixer 1 may comprise several inlet taps 15, 16 which can be used to introduce additives, fillers and other materials into the fibre suspension 2 during mixing. It may also comprise several outlet taps (not shown) which can be used to extract all or part of the fibre suspension 2 during mixing, gases and other condensates generated during processing. This is particularly advantageous because the operator can recover several fractions of mixed fibres having different degrees of compression and therefore different properties depending on their respective outlet tap, and this during the same mixing operation.

[0123] The present invention is of course not limited to the embodiments described but extends to any modification and variant obvious to a person skilled in the art within the limits of the appended claims. Furthermore, the technical characteristics of the different embodiments and variants mentioned above may be, in whole or in part, combined with each other. Examples

[0124] In the examples, the percentages are percentages by weight. 1. Comparison between cellulose fibers oxidized according to the process of the invention and cellulose fibers oxidized in a conventional reactor Example INV-1: Protocol for the preparation of oxidized cellulosic fibers according to the process of the invention (INV-1)

[0125] The cellulose fiber suspension (eucalyptus) is introduced into a mixer comprising a mixing shaft and sodium periodate to produce an aqueous cellulose fiber suspension having aldehyde groups.

[0126] The process has a speed differential between the mixing axis and the mixing enclosure equal to 0.5 m / s.

[0127] The mixer has a minimum distance between the mixing axis and the mixing chamber of 0.3 mm.

[0128] At the outlet of the mixer, the suspension of chemically modified cellulose fibers is left to age.

[0129] The mixing profile is a helical profile, with the helical paddles offset by 45°, which helps to minimize shear.

[0130] The reaction conditions used are shown in Table 1 below. Counterexample CE-1: Protocol for the preparation of oxidized cellulosic fibers in a conventional reactor (CE-1)

[0131] Comparative example CE-1 was carried out in a conventional reactor, from fibers modified in a conventional reactor at low concentration (2% by weight).

[0132] The reaction conditions used are shown in Table 1 below. Table 1

[0133] Table 1: Reaction conditions of chemically modified cellulose fibers according to the process of the invention (INV-1) in comparison with fibers modified in a conventional reactor (CE-1) Example CE-1 (conventional reactor) INV-1 (mixer + aging) Fiber concentration 2% 10% in the mixer Terms 2 hours at 20°C in the reactor 2 minutes at 20°C in the mixer Reagent (quantity) Sodium periodate (ratio 1) Sodium periodate (ratio 0.6) Aging - 2 hours at 20°C Amount of aldehyde in mmol / g 0,5 2

[0134] In Table 1, the amount of reagent in step a) corresponds to the molar ratio between the reagent (oxidant = sodium periodate) and the anhydroglucose units (AGU) of the cellulose of the fibers. The amount of aldehyde resulting from the reaction in step a) is expressed in mmol per gram of cellulose fibers.

[0135] Example INV-1 shows that the amount of aldehyde functionalizing the cellulose fibers is 4 times greater than for the comparative example (2 vs 0.5 mmol / g).

[0136] On the other hand, the present invention makes it possible to treat suspensions having higher concentrations of cellulose fibers (10% vs. 2%).

[0137] Thus, the present invention makes it possible to optimize the reaction in the mixer (INV-1 = oxidation) and, consequently, the functionalization by formation of aldehyde functions. In summary, the process according to the invention makes it possible to obtain greater functionalization than conventional reactor processes, while reducing the necessary quantity of chemical reagent (oxidant in the example INV-1). 2. Effect of the modified cellulose fibers according to the invention on the mechanical properties of a sheet of paper

[0138] The mixer and conditions of Example INV-1 were used for Examples DCC, DSC, and INV-2 to INV-10. Table 2

[0139] Table 2: Reaction conditions for cellulose fibers chemically modified according to the process of the invention (sodium periodate in solution in water at 110 g / L) Example DCC DSC Reagent from step a) (quantity) Sodium periodate (ratio 0.8) 50°C Sodium periodate (ratio 0.8) 50°C Aging (stage b)) 72 hours at 20°C 72 hours at 20°C Post-reaction Sodium chlorite Sodium bisulfite

[0140] According to the DCC example, the aldehyde functions formed during step a) (NaIO 4 ) react with the oxidant sodium chlorite (NaClO 2 ) to form di-carboxyl cellulose during the post-reaction (post-reaction: 30 g of NaClO 2 per liter of aqueous suspension at 30 g / L of cellulose fibers for 12 hours at 20°C).

[0141] According to the DSC example, the aldehyde functions formed during step a) (NaIO 4 ) react with the sulfonating agent (NaHSO 3 ) to form cellulose di-sulfonate during the post-reaction (post-reaction: 30 g of NaHSO 3 per liter of aqueous suspension at 30 g / L of cellulose fibers for 12 hours at 20°C).

[0142] DCC and DSC modified celluloses were used as additives in the manufacture of a paper sheet from an aqueous suspension of eucalyptus fibers (Table 3) that had been refined, unrefined, or modified in a mixer. Table 3

[0143] Table 3: Burst index of paper sheets according to their density and composition Example Composition of the fiber suspension Density (kg / m 3 < ) Bursting index (kPa.m 2< / g) Fiber (% by weight of dry matter) Additive (% by weight of dry matter) CE-2 EUCA (100) N 529 0,83 CE-3 EUCA (M)< (100) N 700 3,41 EC-4 EUCA (shelf) < (100) 0 kWh / t N 502 0,63 EC-5 EUCA (shelf) < (100) 50 kWh / t N 590 2,03 CE-6 EUCA (shelf) < (100) 100 kWh / t N 635 2,91 what-7 EUCA (shelf) < (100) 200 kWh / t N 737 4,67 CE-8 EUCA (dense)< (100) N 588 1,1 EC-9 EUCA (dense)< (100) N 662 1,35 EC-10 EUCA (dense)< (100) N 699 1,4 CE-11 EUCA (dense)< (100) N 741 1,7 INV-2 EUCA (90) DCC (10) 565 2,53 INV-3 EUCA (75) DCC (25) 600 4,61 INV-4 EUCA (97.5) DSC (2.5) 557 1,34 INV-5 EUCA (95) DSC (5) 590 2,03 INV-6 EUCA (90) DSC (10) 575 2,7 INV-7 EUCA (75) DSC (25) 608 5,09 INV-8 EUCA (M)< (94.85) DSC (5) 566 1,71 DADMAC (0.15) INV-9 EUCA (M)< (90) DSC (10) 703 8,36 INV-10 EUCA (M)< (75) DSC (25) 696 8,4

[0144] In Table 3: EUCA: unrefined eucalyptus fibre sheet EUCA (raf)<: eucalyptus fibre sheet that has been refined in a conventional twin-disc refiner (kWh / t: energy in kW.h per tonne of cellulosic fibre; CE-4 at 0 kWh / t = no refining) EUCA (M)<: eucalyptus fibre sheet that has been mechanically modified in a kneader EUCA (dens)<: unrefined eucalyptus fibre sheet that has been densified by wet pressing at increasing pressure DCC: dicarboxyl cellulose DSC: disulfonate cellulose DADMAC: diallyldimethylammonium chloride polymer N: no additives

[0145] The addition of DCC and DSC modified celluloses (at a ratio of 2.5; 5; 10; 25%) to eucalyptus pulp (modified or not by compression in the mixer) made it possible to obtain very high mechanical strengths, similar to those obtained by conventional refining in a disc refiner. In addition, mechanical modification of the fibers by compression and the addition of mechanically and chemically modified fibers (10 to 25% DSC) made it possible to further increase the bursting strength.

[0146] The bonds created by DCC or DSC anionic fibers are electrostatic bonds that operate at greater distances than hydrogen bonds and are much stronger. Therefore, conventional refining in a disc refiner or mechanical modification by compression in a kneader expand the bonding areas to create hydrogen bonds while the presence of anionic cellulose (DSC or DCC) increases the bond strength, complementary effects as shown by the burst strength values.

[0147] Thus, the mechanically and chemically modified cellulose fibres according to the invention allow the formation of stronger bonds and, thus, multiply by 8 the bursting strength of a standard paper (INV-9 and INV-10 vs CE-10).

[0148] In Example INV-8, the paper sheet was prepared from an aqueous suspension containing, in terms of dry matter, 94.85% unrefined eucalyptus fibers and 5% fluidized DSC. Fluidized DSC is a solution of anionic cellulose polymer that is obtained by shearing a DSC solution in water at pH 9. This anionic cellulose polymer was added in the presence of a cationic polymer (0.15% DADMAC), the role of which is to retain the anionic dissolved cellulose.

[0149] Unfluidified DSC comprises long fibers (some reaching about 300 µm) that are highly hydrated / swollen due to their very high anionicity.

[0150] The tests in Table 3 show that: Mechanical densification by additional wet pressing of unrefined eucalyptus fibers (CE-8 to CE-11) increases density but increases mechanical strength little because the fibers have not been flexibilized and the bonding surface is small with only short-range hydrogen bonds in the contact areas. Conventional refining (between 2 discs) and kneading allow densification of the sheet without additional wet pressing and result in an increase in mechanical strength due to better fiber flexibilization and a larger contact surface to develop short-range hydrogen bonds. The addition of highly anionic DSC fibers allows the development of new long-range bonds in the contact areas, in addition to hydrogen bonds: electrostatic bonds that are much stronger than hydrogen bonds.the combination of mechanical modification (by kneading) and the addition of DSC fibers allows a synergistic effect: kneading develops the bonding surfaces while DSC develops electrostatic bonds, and therefore the intensity of the bonds.

Claims

1. A method for the mechanical and chemical treatment of cellulosic fibers by mechanical compression and chemical modification of cellulosic fibers, the method comprising the following steps: a) in a mixer (1), mechanical compression and chemical modification of cellulosic fibers (2), in aqueous suspension having a concentration of between 5 and 20% by weight of cellulosic fibers, in the presence of a chemical reagent to produce an aqueous composition of cellulosic fibers having chemically modified functional groups, the mixer (1) comprising: - a body (10) defining a mixing chamber (11) delimited by a wall forming a fixed compression surface (12) capable of coming into contact with the fiber suspension (2), - at least one mixing axis (20) positioned in the mixing chamber (11) and comprising: ▪ a shaft (21) movable in rotation, ▪ a plurality of mixing elements (22) fixed on the shaft (21),projecting radially from the shaft (21), and each provided with a rotationally movable compression surface (23) capable of coming into contact with the fiber suspension (2), the kneading elements (22) having a helical or eccentric compression profile, a method in which the compression surfaces (23) of the kneading elements (22) face the compression surface (12) of the kneading enclosure (11), so as to define between said respective compression surfaces (23, 12) of the kneading elements (22) and of the kneading enclosure (11) compression zones (ZC) of the fiber suspension (2) circulating between the body (10) and the kneading axis (20), and in which the compression surfaces (23) of the kneading elements (22) have a compression profile whose radius relative to the shaft (21) varies according to the angular position of said shaft,so as to compress the suspension of fibers (2) in said compression zones (ZC) during the rotation of said mixing axis (20), - the method having a speed differential between the at least one mixing axis (20) and the mixing enclosure (11) less than or equal to 15 m / s, - the mixer having a minimum distance (DM) separating the at least one mixing axis (20) from the mixing enclosure (11) of between 0.1 mm and 1 mm, - the aqueous suspension of cellulose fibers has a residence time in the mixer of between 15 seconds and 15 minutes, b) optionally, at the mixer outlet, aging of the suspension of chemically modified cellulose fibers., 2. Treatment method according to claim 1, wherein the chemical reagent is selected from the group consisting of periodic acid or one of its salts, metaperiodic acid or one of its salts, 2,2,6,6-tetramethylpiperidine-1-oxyl, azaadamantane-N-oxyl, hydrogen peroxide, ozone, chloroacetic acid, hypohalites or one of their salts, hydrosulfite salts, and mixtures thereof.

3. Treatment method according to one of the preceding claims, in which the chemical reagent / fiber molar ratio varies from 0.1 to 1 when the chemical reagent is periodic acid or one of its salts.

4. Treatment method according to one of the preceding claims, in which when the chemical reagent is TEMPO the chemical reaction is carried out in the presence of sodium hypochlorite NaClO and sodium bromide NaBr at alkaline pH or sodium hypochlorite NaClO and sodium chlorite NaClO2 at neutral pH.

5. Treatment method according to one of the preceding claims, in which an oxidation reaction is carried out at a temperature between 15 and 100°C, preferably 20 to 80°C and even more preferably 40 to 70°C.

6. Treatment method according to one of the preceding claims, in which the method comprises step b) of aging during which the suspension of modified cellulose fibers is left to age for up to 72 hours at a temperature of from 15 to 60°C, advantageously from 20 to 45°C.

7. Treatment method according to one of the preceding claims, in which the method comprises step b) of aging and, after step b) of aging, a washing step and a chemical post-reaction step.

8. Treatment method according to claim 7, wherein the chemical post-reaction is chosen from oxidation, sulfonation, reduction and amination.

9. Composition of modified cellulose fibers obtained according to the process of one of claims 1 to 8.

10. Use of a cellulosic fiber composition according to claim 9 as an additive in the preparation of paper or cardboard.

11. A method of preparing a sheet of paper or board, comprising adding a cellulosic fiber composition according to claim 9 to a suspension of cellulosic fibers, and forming a sheet of paper or board.

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