Abstract:
The instant disclosure relates to a process for manufacturing a fibrillated cellulose powder suitable for being dispersed in an aqueous medium, comprising a step of adding a monovalent salt to a fibrillated cellulose suspension, followed by a lyophilisation step.

Description:
BACKGROUND 
       [0001]    The present invention relates to a method of manufacturing a fibrillated cellulose powder capable of being dispersed in an aqueous medium. More particularly, the present invention relates to such a method enabling to form a nanofibrillated or microfibrillated cellulose powder. 
       DISCUSSION OF THE RELATED ART 
       [0002]    Cellulose is a polymer which can be found in large quantity in the biomass, and particularly in the walls of plant cells. It is formed of linearly-connected glucose chains (β-1,4 bond) to form macromolecules which naturally organize in microfibrils. Many uses and applications of cellulose are known. In addition to conventional applications such as the manufacturing of paper, textile, or wrappings, the form of cellulose has been provided to be modified for other applications. 
         [0003]    It has in particular been provided to transform cellulose into fibrillated cellulose. When fibers have nanometer-range diameters, the fibrillated cellulose is known under acronym NFC, for NanoFibrillated Cellulose, or under acronym MFC, for MicroFibrillated Cellulose. Cellulose may also be transformed into cellulose crystals, better known as NCC (or “whiskers”), for NanoCristalline Cellulose. 
         [0004]    The fibers contains in fibrillated cellulose typically have a length in the range from 0.5 to 2 μm and a diameter in the range from 5 to 70 nm, and are formed of a group of glucose chains. Fibrillated cellulose is formed of crystal regions and of amorphous regions. Cellulose nanocrystals (NCC) have a length in the range from 50 to 500 nm and a diameter in the range from 5 to 10 nm. 
         [0005]    It should be noted that, in the following description, term “fibrillated cellulose” will be indifferently used for nanofibrillated or microfibrillated cellulose. 
         [0006]    To obtain fibrillated cellulose (NFC or MFC), a mechanical treatment step is carried out on a mixture of suspended cellulose fibers (pulped), for example, from wood. This step is a mechanical disintegration of the cellulose fibers, for example, by friction of the fibers, generally carried out in a homogenizing or friction machine. Pretreatments may be carried out before or after this friction step, for example, enzyme or chemical treatments such as a carboxymethylation, according to the final desired application of the fibrillated cellulose. 
         [0007]    The cellulose crystals (NCC) are obtained by means of chemical treatments, for example, by hydrolysis of the cellulose with a sulphuric acid treatment. 
         [0008]    Cellulose crystals and fibrillated cellulose have quite different properties associated with their different morphologies, dimensions, and crystallinities. Particularly, cellulose crystals may be used in varnish manufacturing or in anti-counterfeiting due to their organization in liquid crystals, which is not the case for MFCs. 
         [0009]    Fibrillated cellulose appears in suspension in the form of a thick gel. An application of this material comprises spreading this gel and then evaporating the water remaining in the gel, which provides transparent or translucent films having advantageous mechanical and/or physico-chemical properties. Particularly, such films have good properties in terms of mechanical resistance. Other properties of these films may be obtained by modifying the fibrillated cellulose manufacturing method (for example, by pretreatments). 
         [0010]    However, a major disadvantage of fibrillated cellulose suspended in water is that gels containing such fibrillated cellulose contain but a low proportion thereof, typically from 1 to 3% by weight. This makes the transport of fibrillated cellulose particularly expensive. 
         [0011]    Further, once fibrillated cellulose films have been formed, it is no longer possible to return to the initial gel form. Indeed, in the forming of films, strong hydrogen bonds are created between the amorphous portions of cellulose. The re-dispersion of such films in an aqueous medium is not directly possible, even by strongly stirring the produced suspension. 
         [0012]    Thus, a method enabling to obtain fibrillated cellulose under an easily transportable form and at a decreased cost is needed. 
         [0013]    It has been provided to transform fibrillated cellulose into a powder by methods of chemical grafting or of encapsulation of the fibrillated cellulose. This comprises, in the second case, drying a fibrillated cellulose gel after having introduced an encapsulating product, soluble in water, into the suspension. In the first case, such chemical surface grafting methods may be esterification or carbanilation methods. However, such methods are particularly complex and expensive to implement, and the obtained fibrillated cellulose once dispersed has different characteristics from the initial fibrillated cellulose. 
         [0014]    It has also been provided spray dry a fibrillated cellulose gel to form dry fibrillated cellulose clusters. Such clusters are more easily transportable than the fibrillated cellulose gel, but have the disadvantage of only containing few fibrillated celluloses agglomerated together. Further, a good dispersion of the obtained powder in an aqueous medium has not been demonstrated to date. 
       SUMMARY 
       [0015]    An object of an embodiment is to provide a method of manufacturing a fibrillated cellulose powder capable of being dispersed in an aqueous medium. 
         [0016]    An object of an embodiment is to provide such a method enabling to form a powder which, once dispersed in an aqueous medium, has properties identical to those of the initial product. 
         [0017]    An object of an embodiment is to provide an inexpensive method. 
         [0018]    Another object of an embodiment is to provide a method of dispersing a powder obtained by this manufacturing method. 
         [0019]    Thus, an embodiment of the present invention provides a method of manufacturing a fibrillated cellulose powder capable of being dispersed in an aqueous medium, comprising a step of adding a monovalent salt to a suspension of fibrillated cellulose, followed by a freeze drying step. 
         [0020]    According to an embodiment of the present invention, the monovalent salt is added at a concentration in the range from 5 to 20 mmol/L. 
         [0021]    According to an embodiment of the present invention, the monovalent salt is selected from the group comprising sodium chloride, potassium chloride, and lithium chloride. 
         [0022]    According to an embodiment of the present invention, the fibrillated cellulose suspension comprises, by weight, from 1 to 3% of fibrillated cellulose. 
         [0023]    According to an embodiment of the present invention, the salt addition step is preceded by a step of pretreating the suspended fibrillated cellulose. 
         [0024]    According to an embodiment of the present invention, the pretreatment of the fibrillated cellulose is an enzyme or chemical pretreatment, for example, a carboxymethylation. 
         [0025]    An embodiment of the present invention further provides a method of dispersing a fibrillated cellulose powder obtained by the above manufacturing method, comprising a step of incorporating the powder in an aqueous medium, followed by a step of stirring the obtained suspension. 
         [0026]    According to an embodiment of the present invention, the dispersion method further comprises a subsequent dialysis step. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, among which: 
           [0028]      FIG. 1  is a block diagram illustrating steps of a fibrillated cellulose powder manufacturing method according to an embodiment; 
           [0029]      FIG. 2  is a block diagram illustrating steps of a method of dispersing a powder formed by the method of  FIG. 1 ; 
           [0030]      FIG. 3  illustrates different results obtained, particularly by means of the manufacturing method of  FIG. 1 , according to the pH of the initial fibrillated cellulose suspension; 
           [0031]      FIG. 4  is an enlarged view of the result of an attempt to disperse a fibrillated cellulose film in an aqueous medium; 
           [0032]      FIGS. 5A to 5D  and  6 A to  6 D are enlargements of fibrillated cellulose films formed from fibrillated cellulose powder dispersed in an aqueous medium; and 
           [0033]      FIGS. 7A to 7C  and  8  are curves of the viscosity versus the shear rate of different samples obtained after dispersion of a powder formed by the method of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0034]    A method of manufacturing a fibrillated cellulose capable of being redispersed in an aqueous medium is provided herein, the powder form ensuring an easy and inexpensive transport. 
         [0035]      FIG. 1  is a block diagram illustrating a fibrillated cellulose powder manufacturing method according to an embodiment. 
         [0036]    At a first step  10 , it is started from a fibrillated cellulose suspension (NFC or MFC) in the form of a gel. A step  12  (ADD SALT) comprises adding, into the fibrillated cellulose suspension, a monovalent salt, for example, sodium chloride NaCl, potassium chloride KCl, or also lithium chloride LiCl. The salt is added by a quantity in the range from 5 to 20 mmol/L, for example, 10 mmol/L. 
         [0037]    A subsequent step  14  comprises freeze drying (FREEZE DRYING) the obtained gel, that is, freezing the obtained suspension and performing a drying causing the sublimation of the water contained in the gel. This provides a powder at a step  16  (POWDER). 
         [0038]    Advantageously, adding a salt into the suspension enables to limit the forming of hydrogen bonds between the glucose chains forming the fibrillated cellulose, as well as to limit the forming of strong hydrogen bonds from the carboxyl groups (COOH) which are irregularly distributed on the glucose chains (hemicellulose impurity in the form of traces). Indeed, the hydrogen atom of carboxyl groups is replaced with the cation present in the salt, which limits the forming of bonds during the subsequent freeze drying step. For example, if sodium chloride is used as a salt, Na +  ions dissolve in the suspension of fibrillated cellulose and associate with carboxylate ions to form —COO − /Na +  groups. 
         [0039]    Advantageously, carrying out a method such as described in  FIG. 1  ensures the forming of a fibrillated cellulose powder capable of being dispersed in an aqueous medium for the forming of a gel having properties identical to those of the initial gel used to form the powder. This will be described in further detail hereafter. 
         [0040]    More particularly, the method of  FIG. 1  may be carried out as follows: based on a fibrillated cellulose gel, this gel may be diluted by addition of distilled water to obtain a more fluid suspension. After a control of the pH, the salt is then added. The freeze drying step may be carried out, for example, at a −81° C. temperature. 
         [0041]      FIG. 2  is a block diagram illustrating steps of a method of dispersing in an aqueous medium a powder formed by the method of  FIG. 1 . 
         [0042]    At a step  20  (POWDER+WATER), a powder obtained by the method of  FIG. 1  is integrated into an aqueous medium, for example, water. It should be noted that the obtained mixture may have a ratio, by weight, of  1 % of fibrillated cellulose. A subsequent step  22  (MIX) comprises strongly stirring the obtained suspension, which provides, at a step  24  (NFC/MFC), a fibrillated cellulose gel of same consistency as the initial gel. 
         [0043]    Advantageously, the salt added in the suspension during the forming of the powder dissolves in the aqueous medium, which provides the fibrillated cellulose gel. It should be noted that, if a conventional drying is performed to form a fibrillated cellulose film, the salt molecules do not disturb the forming of hydrogen bonds and of bonds between carboxyl groups for the obtaining of the film. 
         [0044]    An alternative embodiment comprises, between steps  22  and  24 , carrying out a step  26  of dialysis (DIALYSIS) of the obtained suspension, after stirring step  22 , to remove the residual salt from the suspension after dispersion. As an example, the dialysis may be carried out for 24 hours. 
         [0045]      FIG. 3  illustrates different results obtained, particularly by means of the manufacturing method of  FIG. 1 , according to the pH of the initial fibrillated cellulose suspension. To modify the pH of the initial suspension, sodium hydroxide may for example be added into this suspension. 
         [0046]    More particularly,  FIG. 3  illustrates the result obtained when a fibrillated cellulose film is desired to be dispersed in an aqueous medium and when a fibrillated cellulose powder obtained by one of the methods described in relation with  FIG. 2  is desired to be dispersed in an aqueous medium. 
         [0047]    In these examples, the fibrillated cellulose is obtained from whitened wood pulp comprising, by weight, 60% of broadleaf trees and 40% of resinous trees. 
         [0048]    In  FIG. 3 , a first sample  10  comprises a fibrillated cellulose gel (initial gel). As can be seen in this drawing, gel  10  is particularly viscous, although it comprises a low percentage of fibrillated cellulose (smaller than 3%). 
         [0049]    In the example of  FIG. 3 , four powder samples  30  are disclosed, obtained by freeze drying of a gel  10  respectively having a pH of 4, 6, 8, and 10 (with no addition of salt). Reference  32  illustrates the result of an attempt of dispersion of the four powder samples  30  in an aqueous solution, after stirring. As can be seen, the samples  32  obtained at various pHs do not disperse in the aqueous solution: 30 seconds after the stirring, a sedimentation effect appears. 
         [0050]    With another portion of gel  10 , the method of  FIG. 1  is carried out on samples of gel  10  having respective pHs of 4, 6, 8, and 10. Reference  34  illustrates the result obtained after forming of the method of  FIG. 1 . A powder is obtained, whatever the pH of the initial fibrillated cellulose gel. Reference  36  illustrates the result of a dispersion of the four powder samples  34  in an aqueous solution, after stirring. The samples  36  obtained at various pHs seem at first sight to correctly disperse back in the aqueous solution, and no sedimentation effect appears. 
         [0051]    Reference  38  illustrates the result obtained from samples bearing reference  36  after dialysis step  26  has been carried out. It can be observed that the gel consistency is kept and no sedimentation can be observed. 
         [0052]      FIG. 4  is an enlarged microscope view of the result of an attempt to form a fibrillated cellulose film after redispersion in an aqueous medium of the fibrillated cellulose powder of samples  30  (attempt to form a film from samples  32 ). 
         [0053]    It should be noted that the results of  FIG. 4 , of  FIGS. 5A to 5D , and of  FIGS. 6A to 6D  are obtained by scanning electron microscopy (SEM) integrating a field emission gun. As concerns  FIG. 4 , the acquisition has been performed with a 3 kV acceleration voltage for a 6.4-mm working distance. A droplet of the diluted suspension has been deposited on a substrate covered with a carbon tape and has been covered, after drying, with a 2-nm layer of a gold and palladium alloy. To characterize the samples of  FIGS. 5A to 5D  and  6 A to  6 D, an energy dispersive X-ray spectrometry (EDX) is coupled with the microscope to identify the salt distribution at the sample surface. To perform the analyses, a 15-kV voltage combined with a low vacuum (5.6.10 −4  Torr) is used. A drop of each suspension has been deposited on a substrate covered with a carbon tape and has been dried to form a film. 
         [0054]    The enlargement of  FIG. 4  is performed on a film formed by evaporation of a suspension  32  having an initial pH equal to 8. It can be seen in this enlargement that the obtained film contains no fibrillated cellulose (no presence of fibers), but rather aggregations of carved film portions. This is due to the forming of hydrogen bonds, during the freeze drying enabling to form powders  30 , the bonds being maintained during the dispersion attempt. 
         [0055]      FIGS. 5A to 5D  are enlargement of fibrillated cellulose films obtained by evaporation of the dispersed fibrillated cellulose samples  36  of  FIG. 3 , according to the pH of initial suspension  10  and with no dialysis step. 
         [0056]    These drawings show the reappearing of the cellulose fibrils which are, for certain samples, intertwined with the salt crystals (reference numeral  40  in  FIGS. 5B and 5D ). 
         [0057]    The obtained fibrils have diameters in the order of 23 nm, to within more or less 8 nm, which effectively corresponds to the basic sample. No cluster forms, as in the case of  FIG. 4 , which proves the effect of salt as a hydrogen bond blocker facilitating redispersion. 
         [0058]    It should be noted that the form of the fibrillated cellulose film seems optimal for a pH around  8 . Indeed, when the pH of the initial sample is equal to or smaller than  4 , no carboxylate ion is present at the surface of the glucose chains. Thus, when added, the salt acts on hydrogen bonds, but the salt cation does not react with —COOH groups. With a pH equal to 6, a few carboxylate ions are present on the fibrillated cellulose, which implies a combination with the salt cations, and limits the forming of bonds between carboxyl groups. The optimal value of combination with the salt is obtained at a pH equal to 8, for which most carboxylate ions are available. 
         [0059]      FIGS. 6A to 6D  are enlargement of fibrillated cellulose films obtained by evaporation of the dispersed fibrillated cellulose samples  38  of  FIG. 3 , according to the pH of the initial suspension, after a dialysis step. 
         [0060]    It should be noted in these drawings that the dialysis has enabled to remove the different salt crystals. It should be noted that the fibrillated cellulose keeps dimensions close to those of the initial sample, that is, lengths in the range from 1 to 2 μm and a diameter in the order of 21 nm, to within more or less 9 nm. 
         [0061]      FIGS. 7A to 7C  and  8  are curves of the viscosity (in Pa.s) of a sample versus the shear rate applied to the sample (in s −1 ). Such measurements are obtained by using a calibrated rheometer, the sample of suspended fibrillated cellulose being placed in a closed container avoiding the evaporation of water, and thus avoiding the forming of fibrillated cellulose films. 
         [0062]    It is here intended to limit bonds (hydrogen and covalent bonds) during the freeze drying, but also to enable to regenerate these bonds once the powder fibrillated cellulose has been redispersed in an aqueous medium. Rheological analyses enable to verify this point: it is here desired to study the viscosity of the obtained gel, which is a direct image of the number of hydrogen bonds formed during the dispersion. The greater the aggregation between fibrils, due to the many hydrogen bonds, the lower the viscosity in suspension. 
         [0063]      FIGS. 7A to 7C  illustrate the viscosity of samples originating from fibrillated cellulose having pHs varying from 4 to 10, according to the shear rate applied to the sample. More particularly,  FIG. 7A  illustrates the viscosity for samples  32  (obtained after attempting to disperse powder  30 ),  FIG. 7B  illustrates the viscosity for non-dialyzed samples  36  (obtained after the dispersion of fibrillated cellulose powder formed by the method of  FIG. 1 ) and  FIG. 7C  illustrates the viscosity for dialyzed samples  38  (obtained after dispersion of fibrillated cellulose powder formed by the method of  FIG. 1 ). 
         [0064]    As shown in these drawings, the viscosity of samples  32  ( FIG. 7A ) is in the order of one tenth of the viscosity of samples  36  and  38  ( FIGS. 7B and 7C ). 
         [0065]      FIG. 8  shows another comparison between different samples, a first curve  50  illustrating the viscosity of initial sample  10 , before carrying out the method of  FIG. 1 , a curve  52  illustrating the viscosity of a fibrillated cellulose sample obtained after forming of a powder by the method of  FIG. 1  and dispersion of this powder according to the method of  FIG. 2  (sample  36  or  38 ), and a curve  54  illustrating the same curve for a sample  32 . 
         [0066]    In this drawing, it can be observed that the properties of samples  10  and  36 / 38  are almost identical, while the viscosity of sample  32  is much lower. The viscosity of the gel, and thus its capacity to form hydrogen bonds, are not affected by the passage through the powder state. 
         [0067]    X-ray diffraction also enables to determine that the method provided herein provides a fibrillated cellulose gel having diffraction properties identical to those of the initial gel, whatever the pH of the initial gel. Indeed, diffractometry shows that the crystallinity index is not modified by the presence of salt in the gel. Thus, the crystal structure is not modified and the quality of the crystals present in the fibrillated cellulose is identical according to samples. 
         [0068]    The method provided herein thus provides an easily-transportable powder having physico-chemical properties after dispersion identical to those of the base product. Further, a conventional drying of the suspension obtained after dispersion of the powder (obtained by the method provided herein) enables to obtain fibrillated cellulose films having the same properties as fibrillated cellulose films obtained by direct drying of the basic sample. 
         [0069]    Thus, the method provided herein does not modify the morphological and structural properties of fibrillated cellulose, as is the case for the other known methods. 
         [0070]    Specific embodiments of the present invention have been described. Various alterations and modifications will occur to those skilled in the art. In particular, during the freeze drying step, it may be provided to modify the structure of the fibrillated cellulose powder to incorporate therein elements providing, after dispersion in an aqueous medium, improved properties of the fibrillated cellulose films. 
         [0071]    Further, although fibrillated cellulose (NFC/MFC) has been considered herein, it should be noted that the method provided herein also applies to fibrillated cellulose having been submitted to pretreatments, during the method for obtaining it as a gel or after this method, for example, enzyme or chemical pretreatments such as a carboxymethylation.