FR2730251A1 - Prodn. of micro=fibrillar cellulose used e.g. as thickener in paints - Google Patents

Abstract

A microfibrillated cellulose contains at least 80% of prim. walls, and is charged with carboxylic acids.

Description

Microfibrillated cellulose and its process for obtaining
from sugar beet pulp "
The present invention relates to a novel parenchymal cellulose and its method of production. More particularly, the present invention relates to a novel microfibrillated cellulose and its process for obtaining from sugar beet pulp after extraction of sucrose.

Cellulose is a product of great industrial importance which finds many applications. These include:
- food applications, as a thickener, for the stabilization of dispersions, emulsions and suspensions, for low caloric foods, for foods with a low proportion of fat or cholesterol, etc .;
- industrial applications, in paints, paper, textiles, agriculture, cosmetics, etc .;
pharmaceutical applications, as a drug excipient, release control agent, ointment or cream support, intestinal transit agent, etc.

 Celluloses known to date all have disadvantages.

 Weyerhaeuser WO 93/11182 discloses a bacterial cellulose having a crosslinked structure. In addition to being very expensive, such bacterial cellulose can cause contamination problems in food applications.

 FR-A-2,472,628 of ITT INDUSTRIES INCORPORATED describes a microfibrillated cellulose, consisting essentially of secondary walls, obtained from wood pulp. Such cellulose can not be easily resuspended once dehydrated. This causes significant storage and transportation problems due to the fact that the suspensions have a maximum cellulose content of about 4%.

 Weibel EP-A-0 102 829 discloses a method for simultaneously isolating cellulosic and hemicellulose constituents from sugar beet pulp. But, as for the above-mentioned FR-A-2,472,628, the parenchymal cellulose obtained can not, once dehydrated, be easily resuspended, causing the same problems of storage and transport.

 In addition, the valuation of sugar beet pulp is an important industrial necessity.

 An object of the present invention is to provide a microfibrillated cellulose resuspendable after being dehydrated.

 Another object of the present invention is to provide a process for obtaining cellulose derived from the valorization of sugar beet pulp.

 The present invention responds to these two objects.

 Other objects and advantages of the invention will appear on reading the description below.

 In general, the native cellulose is always in a microfibrillar form, these microfibrils being more or less associated in fibers, walls and membranes. Each cellulosic microfibrilla consists of a rigorous assembly of parallel cellulose chains, this assembly resulting from the mode of biosynthesis of cellulose. It is generally considered that cellulose microfibrils have few defects along their axis. Their mechanical properties are close to the theoretical mechanical properties of cellulose: toughness of the order of 130 GPa and breaking stress of the order of 13 GPa. Cellulosic microfibrils are therefore an interesting material if we can dissociate and restore them.

 Cellulose microfibrils are usually strongly associated with one another in walls or fibers. The secondary walls in which the microfibrils are organized in the form of highly oriented layers and the primary walls in which the microfibrils are deposited in a disorganized manner are distinguished. Parenchyma is a typical example of primary wall tissue. While it is very difficult, if not impossible, to separate the microfibrils from secondary walls without damaging them, it is easy to dissociate the microfibrils from primary walls, because of not only their looser organization but also that the Interstitial polysaccharides, often charged, constitute a significant percentage of these walls.

 The primary wall cellulose is therefore a material with interesting potential. The beet pulps consist mainly of parenchyma and therefore of primary walled cells.

The weight composition of the sugar beet pulp in the solid state may vary according to the origin of the pulp and the culture conditions. Pulps usually contain
- 15 to 30% of cellulose,
- 12 to 30% pectin,
- 12 to 30% of hemicelluloses,
2-6% protein,
- 2 to 6% of mineral matter,
2-6% of lignin, tannins, polyphenols and ferric ester.

It has already been proposed to treat sugar beet pulp to isolate cellulose from parenchymal cells. The aforementioned EP-A-0 102 829 relates to such a method and describes
- suspend beet pulp in acidic (pH <4.5) or basic (pH> 10.0) aqueous medium;
- Heat the suspension ion at a temperature above 1250C (0.5 MPa);
- maintain the suspension at a temperature above 1250C; during a period between 15 and 360 seconds;
subjecting the heated suspension to mechanical shear in a tubular reactor, followed by rapid expansion through small orifices in an atmospheric pressure zone;
filtering the suspension and recovering on the one hand the insoluble fraction which contains the parenchymal cellulose, on the other hand the soluble fraction (the filtrate) which contains the hemicelluloses;
treating the cellulosic fraction by bleaching with sodium hypochlorite and mechanical defibrillation to give a parenchyma cellulose paste consisting of cell wall fragments.

 As mentioned above, the cellulose obtained according to the process of EP-A-0 102 829 can not, once dehydrated, be easily resuspended.

The present invention provides a process for obtaining microfibrillated cellulose from sugar beet pulp after sucrose extraction comprising the steps of
(a) suspending the dehydrated pulp by acidic or basic impregnation;
(b) heating the suspension of step (a) to effect hydrolysis and solubilization of pectins and hemicelluloses;
(c) recovering the filtration residue from the suspension from step (b);
(d) extracting the residue of cellulosic material from step (c);
(e) recovering the residue of cellulosic material by filtering the slurry from step (d);
(f) bleaching the cellulosic material from step (e);
(g) recovering the cellulosic material by filtering the slurry from step (f); ;
(h) homogenizing the cell suspension from step (g); characterized in that
(i) the heating of step (b) is carried out at a temperature between about 60 ° C and 950 ° C, preferably between about 700 ° C and 850 ° C, more preferably about 80 ° C;
(ii) the extraction of step (d) is an alkaline extraction with a base, preferably chosen from sodium hydroxide and potassium hydroxide, whose concentration is less than about 9% by weight, preferably between about 1% and 6% by weight;
(iii) the homogenization of step (h) is carried out by mixing followed by passage of the cell suspension through a small diameter orifice, subjecting the suspension to a pressure drop of at least 20 MPa and to a high speed shear action followed by high velocity deceleration impact.

 Provisions (i), (ii), and (iii) are new and it will be shown later that they allow to obtain a new primary wall cellulose which has unique structural, morphological, chemical and rheological properties.

 The impregnation step (a) can be carried out in an acidic medium or in a basic medium.

 In the acidic impregnation, the pulp is mixed in a minimal amount of deionized water for a few minutes so as to homogenize the suspension. The suspension is then acidified to a pH of between 1 and 3, preferably between 1.5 and 2.5, with a concentrated solution of an acid such as hydrochloric acid and sulfuric acid. The pulp thus homogenized is kept impregnated for about 15 minutes before the heating step (b).

 In the basic impregnation, the pulp is mixed with a minimum amount of deionized water which is then brought to a pH of between 10 and 13 by adding, for example, an alkaline solution of sodium hydroxide or potassium hydroxide with a concentration of less than 9 % by weight, preferably less than 6% by weight. A small amount of a water-soluble antioxidant, such as sodium sulfite Na 2 SO 3, may be added to limit the oxidation reactions of the cellulose.

 The suspension obtained during the impregnation is then heated. According to the invention, this heating step (b) is carried out by heating the suspension at a "moderate" temperature of between about 60 ° C. and 950 ° C., preferably between about 700 ° C. and 850 ° C., more preferably equal to about 800 ° C. This contrasts with the very high temperatures (> 1250 ° C.) used in the prior art The duration of the heating step (b) is between about 1 hour and about 4 hours. (b) according to the invention, there is a partial hydrolysis with release and solubilization of pectins and hemicelluloses while preserving the molecular weight of the cellulose.

 In step (c), the suspension from the heating step (b) is filtered, the residue is washed with water and again filtered to recover the filter residue.

According to the invention, the residue of cellulosic material from step (c) undergoes, in step (d), an alkaline extraction with a base, preferably chosen from soda and potassium hydroxide, whose concentration is less than about 9% by weight, preferably between about 2% and about 6% by weight.

The Applicant has found that by performing this step (d) extraction according to the provisions of the invention is avoided irreversible transformation:
cellulose I -------> cellulose II
Such a transformation would destroy the microfibrillar structure that is necessary to obtain the specific properties of the product of the invention.

 The alkaline extraction step (d) according to the invention will consist of one or more successive extractions according to the desired purity of the cellulosic residue. The duration of the alkaline extraction step (d) is from about 1 to about 4 hours, preferably about 2 hours.

 The slurry from the alkaline extraction step of step (d) is then, in step (e), filtered, and the residue is washed extensively with water to recover the residue of cellulosic material.

 According to the invention, a certain percentage of noncellulosic acidic polysaccharides (pectins, hemicelluloses) are kept on the surface of the cellulose microfibrils, which has the particular effect of preventing them from associating with each other. This percentage of acid polysaccharides will generally be less than about 30% by weight, preferably less than 5% by weight. Too much acid polysaccharide would require too long homogenization times, however, according to the invention, this percentage should not reach 0.

 The cellulosic material of step (e) is then bleached, for example with sodium chlorite, sodium hypochlorite, etc., in a manner known per se. In the case of sodium chlorite bleaching, different concentrations can be used at temperatures between about 180 ° C. and 800 ° C., preferably between about 50 ° C. and 700 ° C. The duration of step (f) is between about 1 ° C. and about 700 ° C. hour and about 4 hours, preferably between about 2 and about 3 hours, to obtain a cellulosic material containing between 85 and 95% by weight of cellulose.

 The bleached suspension is then sent to the homogenization step which is carried out, according to the invention, by mixing followed by passage of the cell suspension through a small diameter orifice, subjecting the suspension to a pressure drop. at least 20 MPa and high speed shear action followed by high velocity deceleration impact.

 The mixing is, for example, carried out by mixing for a period ranging from a few minutes to about an hour, in a waring blender equipped with a four blade propeller under the following conditions: the concentration of dried pulp is between 0.2 and 10% by weight, preferably between 0.5 and 5% by weight. During mixing, the suspension warms up. The container is preferably equipped with a deflecting rib system whereby the liquid is returned to the propeller blades located in the center of the container.

 The actual homogenization will advantageously be carried out in a homogenizer of the MANTON GAULIN type in which the suspension is subjected to shearing action at high speed and pressure in a narrow passage and against a shock ring. The homogenization conditions are as follows: the concentration of dry pulp of the suspension to be homogenized is, after mixing, between 0.2 and 10% by weight, preferably between 0.5 and 5% by weight. The suspension is introduced into the homogenizer preferably after preheating at a temperature between 40 and 1000C, preferably between 50 and 800C. The temperature of the homogenization operation is maintained between 50 and 1000C, preferably between 70 and 950C. The suspension is subjected in the homogenizer at pressures of between 20 and 100 MPa, and preferably greater than 50 MPa.

 The homogenization of the cellulosic suspension is obtained by a number of passages which can vary between 1 and 50, preferably between 5 and 10, until a stable suspension is obtained.

 The Applicant has found that the treatment is more effective than the concentration of dry matter of the suspension to be homogenized is higher.

That is, as the cellulose concentration increases, the number of passes required decreases.

However, it will be limited by the viscosity of the suspension being treated which directly depends on the concentration of the suspension being treated. Indeed, the device is not, by design, adapted to work with suspensions too viscous.

 The Applicant has found that there are special valves for cell grinding that reduce the number of passages required. Moreover, the number of passages will be reduced if water-soluble dispersing agents, suspensoids or thickeners such as carboxymethylcellulose, cellulose ethers, gelling polysaccharides (guar, carob, alginates, etc.) are added to the suspension to homogenize. carrageenan, xanthan and their derivatives) in a cellulose / additive weight ratio in the range of about 1/1 to 10/1.

 In addition, beet pulps contain from 4 to 6% of water insoluble inorganic compounds.

 Among the mineral compounds present in beet pulps, there are soil residues, pebbles of relatively large size (> 1 mm). Among these insoluble mineral compounds, the applicant has demonstrated the presence of silica crystals and calcium oxalate monohydrate. The latter is found inside cells that are generally located in the libero-ligneous bundles, near the vessels. These calcium oxalate crystals are contained in certain cells and correspond to a form of storage of calcium by the plant. The nature, amounts, and proportions of these minerals may vary with the growing soil of the plant, the variety of beet, the climate during growth, and so on.

 The presence of such crystals of calcium oxalate is a problem during the homogenization step because they are very abrasive and it is preferable to eliminate them or at least greatly reduce their content. Such an elimination can be carried out by a treatment in an acidic medium, for example hydrochloric acid, such as an acidic impregnation treatment according to step (a), which makes it possible to convert calcium oxalate monohydrate into oxalic acid and into calcium chloride soluble in water.

 The removal of calcium oxalate can also be carried out by mechanical grinding and sieving. The Applicant has thus found that the concentration of mineral residues can be reduced by proceeding, before the impregnation step (a), to a grinding and sieving of the dehydrated pulp to retain only the fraction having a particle size of between about 20 Am and 1000 ssm, preferably between 75 um and about 600 um.

In the case where the cellulosic residue contains, after basic extraction or after bleaching, a significant amount of free calcium oxalate crystals or inside the cells, it is possible to carry out a moderate wet grinding, for example in a blender. Waring type
Blendor, allowing the cells to burst, followed by filtration or sieving on a suitable sieve.

The opening of the sieve will be easily determined by those skilled in the art, for example between 20 and 75 mm, such as an opening of 75, 60, 40 or 20 ssm, depending on the importance of mixing, that is to say ie, depending on the size of the cell fragments obtained, and the industrial feasibility.

 Another way to eliminate the troublesome calcium oxalate is to carry out an oxidation treatment, for example with ozone or hydrogen peroxide, associated with the bleaching treatment of step (f).

 In order to remove the calcium oxalate crystals, the bleaching of step (f) can also be carried out with ozone or hydrogen peroxide.

 All these means of removing calcium oxalate crystals, or at least reducing the content of such crystals, can be combined with each other or used alone, as will be readily determined by those skilled in the art in each particular case.

 The microfibrillated cellulose obtained by the process of the present invention consists of cellulose I.

 It is characterized in that it contains at least about 80%, more usually at least about 85%, primary walls, and is loaded with carboxylic acids. These carboxylic acids are generally uronic acids, such as galacturonic acid and glucuronic acid.

 The microfibrillated cellulose of the present invention is remarkable in that it is capable, after dehydration, of being resuspended.

 The microfibrillated cellulose of the invention has a crystallinity of 20 to 35%.

 It consists of microfibrils having a section of between about 2 and about 4 nm.

 The microfibrillated cellulose of the present invention forms stable liquid crystal suspensions consisting of nematic domains.

The microfibrillated cellulose of the present invention has a group of interesting properties
- unique rheological properties which allow to obtain stable suspensions, having the appearance of a gel with concentrations higher than 1%.

 - unique physical and chemical properties in that the cellulose consists mainly of cellulose to which is associated a rate of pectins or residual hicellululoses which result in particular physical and chemical properties. The cellulose according to the invention consists of more or less individualized microfibrils, which are of the native type or cellulose I.

- very high chemical reactivity, very large accessible surface;
- excellent water retention capacity;
- high suspensive power;
- thickening power.

 The cellulose of the invention can, once obtained by the above process, be concentrated by precipitation, for example in an alcohol, such as ethanol, isopropanol or any other similar alcohol, by a freeze-thawing process. by passing on a filter press, by dialysis against a hygroscopic solution whose size of the molecules is greater than the pore size of the membrane used or by any other method known to those skilled in the art for concentrating such suspensions.

 The cellulose resulting from the process according to the invention or after a concentration step can be dried by evaporation, dehydration, spray-drying, roll drying, freeze-drying or critical point method, or any other method which makes it possible to obtain the product at the same time. dry state.

 The cellulose of the present invention further exhibits valuable film-forming properties.

 Indeed, when applying a suspension of microfibrils according to the invention on a surface, for example a metal surface, glass, ceramic, etc., and let it dry, the microfibrils form a film at the area.

 It is also possible to treat wet paper sheets during their manufacture with a suspension of cellulose according to the invention, thus improving their physical properties, in particular their tensile strength.

 The cellulose of the invention can also be deposited on the surface of a paper sheet, which improves the opacity and uniformity of the paper surface.

 The cellulose of the invention can be applied alone or with other compounds such as pigments and fillers, usually used in paper milling.

The present invention will be further illustrated in the following non-limiting Examples with reference to
Attached figures in which
Figure 1 is an optical microscope photograph of individualized parenchyma cells;
- Figure 2 is a photograph of native cellulose (cellulose I) taken under a transmission electron microscope;
FIG. 3 is a photograph of cellulose II taken by transmission electron microscope;
FIG. 4 is a photograph taken with a cell wall transmission electron microscope before homogenization in a Gaulin homogenizer;
- Figure 5 is a photograph taken under an electron microscope transmission of individual microfibrils after homogenization in a Gaulin homogenizer (6 passages).

 - Figure 6 is a photograph taken under an electron microscope transmission of individualized microfibrils after homogenization in a Gaulin homogenizer (10 passes).

EXAMPLE 1 Purification of beet pulp
The dehydrated pulp corresponding to beets harvested in the region of Nassandres, France, are resuspended in deionized water. In order to obtain better hydration, a Waring Blendor mixer equipped with a four-bladed propeller is used, and intermittently is mixed for 45 minutes. The suspension is acidified by adding a solution of
H2SO4 up to pH = 2. This suspension is kept at room temperature (250C) for 15 minutes and then brought to 800C for 2 hours with constant mechanical stirring.

This suspension is then filtered through a metal sieve and washed thoroughly with water. The solid residue after washing is extracted with an alkaline solution. It is resuspended in a soda solution of appropriate concentration so as to obtain a final concentration of sodium hydroxide of 2% by weight and a percentage of dry matter of 2.5% by weight relative to the total liquid. About 0.1% by weight of sodium bisulfite (Na 2 SO 3) is added to the total liquid. This suspension is brought to 800C for 2 hours with constant mechanical stirring. After this treatment, a sieve filtration of 0.6 mm is carried out. The solid residue is washed with water until a neutral filtrate is obtained.

 After this washing, the solid residue is resuspended at 2.5% in a solution of sodium chlorite (NaClO.sub.2) at 3.4 g / l, buffered with a mixture of sodium hydroxide and acetic acid at a pH = 4. 9. This suspension is brought to 700C for 3 hours with constant mechanical stirring.

Subsequently, the suspension is filtered through stainless steel sieves and then rinsed with water, until a colorless filtrate is obtained. A light gray cellulosic residue of 3 to 5% by dry weight can be obtained by filtration under reduced pressure using a büchner funnel.

The neutral sugar composition of the solid residue is obtained by a chemical analysis based on the gas chromatographic characterization of the alditol acetates obtained after acid hydrolysis of the polysaccharides, reduction and acetylation of the monomeric sugars. The identification of the alditol acetates is carried out in CPG and the determination of the sugars is carried out using the myo-inositol as internal standard, taking into account the response factors specific to each of the alditols. The chromatograph used is a Hewlett-Packard 5890 with a flame ionization detector connected to a Hewlett-Packard 3395 integrator. A column is used.
SP 2380 (0.53 mm x 25 m), and nitrogen U as carrier gas.

 The alditol acetates are eluted with retention times characteristic of the column. Studies have been done to determine the relative response factor for each alditol acetate. Knowing the area and the amount of starting inositol, it is possible from the surface of the peaks of each of the alditol acetates, to deduce therefrom the amount of the corresponding monosaccharides, and to calculate the percentages by weight of each neutral sugar monomer obtained. relative to the total mass of neutral sugars of the sample studied.

Glucose comes almost entirely from the hydrolysis of cellulose; the percentage of glucose thus gives an indication of the purity of the cellulose of the sample.

The other neutral sugars are mainly xylose, galactose, mannose, arabinose and rhamnose, they give an estimate of the quantities of pectins and residual hemicelluloses.

 The chemical dosage of the cellulosic residue obtained indicates a percentage of 85% glucose.

EXAMPLE 2 Purification of Beet Pulp
The entire series of treatments of Example 1 was repeated adding after the treatment with sodium chlorite and the corresponding rinses, a second sodium chlorite treatment identical to the first. A whitish cellulosic residue is thus obtained whose chemical composition in neutral sugar has changed little during the second whitening. The chemical dosage indicates a percentage of 86% glucose of the cellulosic residue obtained.

EXAMPLE 3 Purification of beet pulp
The dehydrated pulps are resuspended in deionized water and then subjected to acid hydrolysis according to the method described in Example 1. Filter and wash with water to remove solubilized pectins and hemicelluloses. The solid residue is then extracted with an alkaline solution according to the method described in Example 1. This alkaline treatment is repeated a second time. The solid residue is washed with water until a neutral filtrate is obtained, before carrying out two successive bleachings with sodium chlorite according to the process described in Example 1. The chemical determination indicates a percentage of 89% of glucose. .

 Examples 1, 2 and 3 show that the greater the number of extractions, the more pure the cellulose of the residue.

EXAMPLE 4 Purification of beet pulp
The entire series of treatments of Example 1 was repeated substituting the sulfuric acid solution with hydrochloric acid solution to bring the pH of the suspension to pH = 2.

 A percentage of 90% cellulosic residue glucose, similar to that obtained in Example 3, is obtained.

EXAMPLE 5 Purification of beet pulp
Dehydrated pulps are resuspended in deionized water. In order to obtain better hydration, a Waring Blendor mixer equipped with a four-bladed propeller is used, and intermittently is mixed for 45 minutes. The suspension is then made alkaline by adding a sodium hydroxide solution of appropriate concentration so as to obtain a final sodium hydroxide concentration of 2% by weight and a dry matter percentage of 2.5% by weight relative to the total liquid. About 0.1% by weight of sodium bisulfite (Na 2 SO 3) is added to the total liquid.

This suspension is brought to 800C for 2 hours with constant mechanical stirring. After this treatment, a sieve filtration of 0.6 mm is carried out. The solid residue is washed with water until a neutral filtrate is obtained. This alkaline treatment is repeated a second time. The solid residue is washed with water until a neutral filtrate is obtained, before carrying out two successive bleachings with sodium chlorite according to the process described in Example 1.

 The chemical dosage indicates a percentage of 87% glucose.

EXAMPLE 6 Purification of beet pulp
The entire series of treatments of Example 5 are carried out with three successive alkaline treatments with a sodium hydroxide solution of appropriate concentration so as to obtain a final sodium hydroxide concentration of 2% by weight, instead of two treatments as in Example 5. The solid residue is then washed with water until a neutral filtrate is obtained, before carrying out two successive bleaches with sodium chlorite according to the process described in Example 1.

 The chemical dosage indicates a percentage of 92% glucose.

EXAMPLE 7 Purification of beet pulp
The entire series of treatments of Example 5 are carried out, replacing the two alkaline treatments with sodium hydroxide by two successive treatments with a potassium hydroxide solution of appropriate concentration so as to obtain a final potash concentration of 2%. The solid residue is then washed with water until a neutral filtrate is obtained, before carrying out two successive blanchings with sodium chlorite according to the process described in Example 1. Using potash, residues are obtained. cellulosic purity similar to that obtained with soda.

EXAMPLE 8 Influence of the Sodium Concentration
The dehydrated pulp was resuspended in deionized water by mixing identical to that described in Example 1. The suspension thus obtained was refluxed for 20 minutes and then filtered through a 0.6 mm sieve.

The solid residue is then resuspended in a soda solution of appropriate concentration so as to obtain a final sodium hydroxide concentration of 2% or 8% by weight and a dry matter percentage of 2.5% by weight relative to the liquid. total. This suspension is kept under magnetic stirring for three hours at 200C. After this treatment, a sieve filtration of 0.6 mm is carried out and a washing with water until a neutral filtrate is obtained.

After this extraction, the glucose percentages obtained for treatment with 2% or 8% sodium hydroxide are reported in Table I.
TABLE I
Effect of soda concentration on the purity of
cellulosic residue

<tb><SEP> Concentration <SEP> of <SEP> soda <SEP>% <SEP> of <SEP> glucose
<tb> in <SEP> in <SEP> weight <SEP> molar <SEP> in <SEP> weight
<tb><SEP> 2 <SEP> 2 <SEP>% <SEP> 0.5 <SE> M <SEP> 48
<tb><SEP> 8 <SEP> 8 <SEP>% <SEP> 2 <SEP> M <SEP> 58
<Tb>
It is thus seen that during the first alkaline extraction, the use of a more concentrated soda results in a cellulose of better purity.

EXAMPLE 9 Influence of the Sodium Concentration
The dehydrated pulp was treated according to Example 6.

The cellulosic residue thus purified gives, after filtration under reduced pressure using a buchner funnel, a 4% paste which is treated simultaneously and in a similar manner in eight separate experiments. 0.6 ml of this sample is added to 50 ml of a 2%, 7%, 9%, 9.5%, 10%, 12%, 14% and 17% by weight sodium hydroxide solution. The treatment is carried out at the temperature T = 200 ° C. with constant magnetic stirring for two hours. After neutralization with hydrochloric acid and dialysis of these suspensions against distilled water, the residue is dried in an oven at 500C in small containers, so as to obtain thin films of the cellulosic residues of each experiment.

 In other tests, a drop of suspension after dialysis is deposited on an electron microscope grid and then dried before being observed.

 X-ray studies show that the films resulting from cellulose treated with soda solutions of 2 to 9% by weight diffract identically to the starting cellulose. They are identified as cellulose I (interference at 0.54 nm, 0.4 nm and 0.258 nm) having a degree of crystallinity of the order of 35%. On the other hand, for cellulose films treated with sodium hydroxide at concentrations of 9.5% and above, a characteristic spectrum of cellulose II is obtained. It is characterized in particular by interference at 0.7 nm, 0.44 nm, 0.4 nm and 0.258 nm.

 It can be observed by observation under an electron microscope that the samples which have been treated in sodium hydroxide by 2 to 9% are in the form of an assembly of smooth and entangled cellulose microfibrils capable of sliding against each other (FIG. ). On the other hand, after passing through the sodium hydroxide at 9.5% and above, the sample agglomerated in the form of grains of microgels consisting of elements welded together (FIG. 3). This transformed cellulose no longer has the characteristic properties of the cellulose of the invention.

 To preserve the particular properties of the cellulose of the invention, it will be necessary to preserve the crystalline structure of the native cellulose; therefore, if a sodium hydroxide solution is used during the alkaline extraction, never exceed a concentration of 9%.

EXAMPLE 10 Elimination of the mineral matter
The dried pulp, before being rehydrated, was passed through a grinder equipped with a 1 mm grid for ten minutes. Sieving on sieves of 600 and 75 ssm at the mill outlet makes it possible to recover a fraction of particles less than 75 μm in size and another fraction that is very predominant between 75 and 600 μm. After calcination at 5600C for 8 hours, the mass of ash and reported to the initial mass of the sample introduced. The ash content is thus obtained for each fraction: between 75 and 600 ssm, a fraction is isolated which comprises 5% of mineral matter and below 75 ssm, a fraction with 12% of mineral matter.

 It can be seen that this grinding followed by sieving makes it possible to obtain a depleted fraction of mineral matter.

EXAMPLE 11: Elimination of the mineral matter
The residues resulting from the purifications described in
Examples 1 to 7 were mixed in a Waring Blendor at the highest speed for three minutes after the bleaching step, and then filtered through a 25 μm sieve.

The effectiveness of such a treatment is observable under an optical microscope because the calcium oxalate crystals have the particularity of being very birefringent when they are observed in polarized light. Before the treatment, numerous crystals are observed between the cells on the bottom of the observation plate as well as crystals inside certain cells. At the end of this treatment, there are no more observable crystals between the cells on the bottom of the plates.

 This example shows that depending on the size of the mix and the abundance of sieves on sieves of appropriate porosity, we can eliminate these crystals.

EXAMPLE 12 Effect of Homogenization
The suspensions resulting from the treatments described in Examples 1 to 7 are suspensions of purified cells, consisting mainly of cellulose. An observation under the microscope shows that they are more or less individualized cells. These suspensions are passed through a Gaulin homogenizer at 40 MPa fifteen consecutive times at a concentration of 2% after preheating for one hour at 600 ° C. The temperature rises rapidly to 80 to 1000C.

 In the homogenizer, the purified suspension is pushed into a conduit by a piston at high speed and then passes through a small diameter orifice in which the suspension is subjected to a significant pressure drop and is then projected against a shock ring. .

The combination of these two phenomena, pressure drop and decelarization impact, produces a shearing action and the individualization of cellulose microfibrils. By passing the suspension through the orifice several times, a stable suspension of individualized cellulose microfibrils is obtained. This appears clearly by optical or electron microscopic observation. In photo 4, the entanglement structure of cellulose microfibrils constituting the primary wall of beet parenchyma cells clearly appears. In photo 5, we can see the cellulose microfibrils more or less separated from each other. This effect of individualization is directly consequent on the homogenization treatment in the Gaulin homogenizer.

 The samples thus treated are suspensions of microfibrils individualized and have the appearance of a gel.

 The resulting cellulose has at least 90% primary walls.

Its percentage of crystallinity, observed at
X-rays, is 33%.

 Observation under the electron microscope indicates that the average cross section of the microfibrils is 2-4 nm and that their length is greater than 7 ssm, and can reach a length of 15-20 Am.

EXAMPLE 13: Effect of homogenization (time)
The beet pulps were treated according to Example 3 and then mixed in a three-minute high-speed Waring Blendor, and separated into three parts. The first is kept as it is, the second is passed 6 times in the Gaulin homogenizer at 50 MPa, the third is subjected to 10 passages at 50 MPa. Figure 6 shows the individualized microfibrils after 10 passages in a Gaulin homogenizer.

These suspensions are studied using a rheometer
Carri-Med CSL50 with a cone-plane geometry. The flow threshold corresponds to the minimum stress to be applied to obtain a viscosity value, a value directly related to the strength of the gel. The suspension obtained is also characterized by the value of viscosity at the shear rate of 57.6 sl. The results obtained for the three suspensions ions studied at a concentration of 1% are collated in Table II.

TABLE II
Rheological characteristics of suspensions
in flow

<tb><SEP> Number <SEP> of <SEP> q <SEP> (MPa.s)
<tb> Sample <SEP> Passages <SEP> a0 <SEP> (Pa) <SEP> to <SEP> 57.6 <SEP> se <SEP>
<tb><SEP> 1 <SEP> 0 <SEP> 1.4 <SEP> 16
<tb><SEP> 2 <SEP> 6 <SEP> 4.3 <SEP> 186
<tb><SEP> 3 <SEP> 10 <SEP> 7.6 <SEP> 328
<Tb>
It is clear that the effect of homogenization which is an effect of individualization of cellulose microfibrils, causes a considerable improvement of the rheological characteristics. The characteristics of the microfibrils are similar to those of Example 12, except that the percentage of primary walls is 90%.

EXAMPLE 14: Stability of suspensions
An important characteristic of the suspensions obtained according to Example 12 is their ability to form stable suspensions.

 Such suspensions treated according to Example 12 were stored for several months at concentrations ranging from 0.1% to 7% without ever giving a decantation volume of less than 95%.

EXAMPLE 15: Suspension Stability
A suspension of cellulose microfibrils treated according to Example 12 was treated with 0.1 M trifluoroacetic acid solution at 200 ° C. for 2 hours.

 Analysis of the neutral sugars by the corresponding alditol acetates gives a percentage of cellulose of 95%. The suspension obtained is not stable.

 Trifluoroacetic acid has the characteristic of allowing the preferential hydrolysis of pectins and hemicelluloses. There is therefore a loss of stability correlated with the hydrolysis of pectins and hemicelluloses.

 It is clear from this example that the stability of these suspensions is due to the presence of pectins and hemicelluloses which were bound to cellulose microfibrils.

EXAMPLE 16: Resuspension
A sample as prepared according to Example 12 is taken and dried in an oven in a flat-bottomed polyethylene container. A dry cellulose film is obtained after 12 hours at 100 ° C. This film (0.2 g) is soaked in 10 ml of water at room temperature (250 ° C.) and triturated slightly with a stirring rod. At the end of 30 minutes a thick paste is obtained, by diluting this paste with water a suspension of cellulose microfibrils with properties identical to the initial suspension is obtained.

EXAMPLE 17: Resuspension
A sample as prepared according to Example 12 is taken and dried in an oven in a flat-bottomed polyethylene container. After 12 hours at 1000C, dry cellulose films are obtained. These films are cut into shreds and put in a Waring blender
Blendor with deionized water. After 15 minutes of stirring, these flaps are disintegrated and a suspension of microfibrils is obtained with properties similar to the starting suspension.

EXAMPLE 18 (Comparative): Resuspension
Samples as prepared according to Example 12 are taken and dried in an oven at 600C for 12 hours. Cellulose films which are treated with trifluoroacetic acid are obtained, as in Example 15. The films thus treated are resuspended in water.

 These samples are difficult to disperse and the initial properties of the cellulose are not found.

 This Example demonstrates that uncharged cellulose outside the scope of the present invention can not be practically resuspended after dehydration without significant loss of its rheological properties.

EXAMPLE 19: Reactivity
The cellulose of the invention prepared according to Example 12 was incubated with an enzyme mixture of Trichoderma reesei CL-847. In a 25 ml Erlenmeyer flask, 810 mg of cellulose of the invention is resuspended in 30 ml of distilled water. The mixture is stirred in such a way as to obtain a homogeneous suspension of cellulose microfibrils and the temperature is equilibrated for 15 minutes at 500.degree. The enzyme solution is prepared by dissolving 31.10 mg of enzyme (corresponding to 25 FPU / g of cellulose) in 15 ml of sodium citrate buffer at pH = 4.8. The solution is added to the reaction medium which is left to incubate at 500C with horizontal agitation of 50 round trips per minute. After 4 hours, 8 hours and 24 hours of reaction, 3 ml of well homogeneous reaction medium (so as not to change the concentration) is taken and taken in a reflux eppendorf cone at 1000 ° C. for 20 minutes to denature the enzymes and therefore stop enzymatic reactions. It is centrifuged for 10 minutes at 10,000 g and then filtered through a micropore cellulosic membrane with a porosity of 0.45 ssm. The assay is performed by HPLC using glucose and cellobiose standards. It is found that the cellulose of the invention is rapidly hydrolysed by the enzymatic mixture to give 0.45 mg of reducing sugars (glucose and cellobiose) per mg of starting cellulose after 4 hours of hydrolysis, 0.58 mg per mg of starting cellulose after 8 hours of hydrolysis and 0.85 mg per mg of starting cellulose after 24 hours of hydrolysis.

Claims (16)

 1. Microfibrillated cellulose characterized in that it contains at least about 80% of primary walls and in that it is loaded with carboxylic acids.  2. Microfibrillated cellulose according to claim 1, characterized in that it contains at least about 85% of primary walls.  3. Microfibrillated cellulose according to any one of claims 1 and 2, characterized in that the carboxylic acids are uronic acids.  4. Microfibrillated cellulose according to claim 3, characterized in that the uronic acids are selected from the group consisting of galacturonic acid and glucuronic acid.  5. Microfibrillated cellulose according to any one of claims 1 to 4, characterized in that it has a crystallinity of 20 to 35%.  6. Microfibrillated cellulose according to any one of claims 1 to 5, characterized in that it comprises microfibrils having a section between about 2 and about 4 nm.  A process for preparing the microfibrillated cellulose of claim 1 from sugar beet pulp after sucrose extraction, containing cellulose, pectins, hemicelluloses, proteins and minerals, comprising the steps of  (a) suspending the dehydrated pulp by acidic or basic impregnation;  (b) heating the suspension of step (a) to effect hydrolysis and solubilization of pectins and hemicelluloses;  (c) recovering the filtration residue from the suspension from step (b);  (d) extracting the residue of cellulosic material from step (c);  (e) recovering the residue of cellulosic material by filtering the slurry from step (d);  (f) bleaching the cellulosic material from step (e);  (g) recovering the cellulosic material by filtering the slurry from step (f);  (h) homogenizing the cell suspension from step (g); characterized in that  (i) the heating of step (b) is carried out at a temperature between about 60 ° C and 950 ° C;  (ii) the extraction of step (d) is an alkaline extraction with a base having a concentration of less than about 9% by weight  (iii) the homogenization of step (h) is carried out by mixing followed by passage of the cell suspension through a small diameter orifice, subjecting the suspension to a pressure drop of at least 20 MPa and to a high speed shear action followed by high velocity deceleration impact.  8. The method of claim 7, characterized in that the heating of step (b) is performed at a temperature of between about 70 and 85 ° C.  9. The method of claim 8, characterized in that the heating of step (b) is performed at a temperature of about 80 ° C.  10. Process according to any one of claims 7 to 9, characterized in that the alkaline extraction step (d) is carried out with a base selected from sodium hydroxide and potassium hydroxide.  11. A process according to any one of claims 7 to 10, characterized in that the concentration of the base used in step (d) is in the range of about 2% to about 6% by weight.  12. Method according to any one of claims 7 and 11, characterized in that is added to the suspension to be homogenized in step (h) an additive selected from the group consisting of dispersing agents, suspensoids and thickening agents soluble in water, in a cellulose / additive weight ratio of between approximately 1/1 and 10/1.  13. Method according to any one of claims 7 and 12, characterized in that it is carried out prior to the impregnation of step (a) a grinding and sieving of the dehydrated pulp to maintain the fraction having a particle size between about 20 μm and about 1000 μm, preferably between about 75 μm and 600 μm.  14. Process according to any one of claims 7 to 13, characterized in that the bleaching treatment of step (f) is combined with an oxidizing treatment with ozone or with hydrogen peroxide.  15. Method according to any one of claims 7 to 13 characterized in that the bleaching treatment of step (f) is carried out using ozone or hydrogen peroxide.  16. Method according to any one of claims 7 to 15, characterized in that after the bleaching step (f) is carried out a moderate wet grinding of the cellulosic suspension followed by a filtration with a sieve aperture of from about 20 to about 75 ssm.