FIELD OF TECHNOLOGY

The present invention relates to a microfibrillated cellulose structure for increased toughness of paper and a method of producing the paper.

BACKGROUND

The term "microfibrillated cellulose" refers to nanofibrils obtained from plant fibres (plant cells) by mechanical or chemical means, often a combination of chemical pre- treatment and mechanical disintegration, or from bacterial produced fibres. Such "microfibrillated cellulose" has a diameter typically less than 40 nm and is several micrometers in length and is termed nanofibrils in the following. Cellulose is the main reinforcing constituent in plant cell walls. It is present in the form of aligned β( l ,4)-D-glucan molecules in extended chain conformation assembled into nanofibrils of high modulus and tensile strength. Often, cellulose materials are based on plant cells, for instance in the form of wood pulp, but it can also be derived from bacteria. Despite good inherent properties of cellulose, the use of materials and products from cellulose tends to be motivated by low cost. Even in light of the recent interest in biocomposite materials, cellulose tends to be viewed as "filler" and it usually embrittles the polymer matrix. This disadvantage is balanced by its availability as a low cost constituent obtainable from renewable resources. However, in order to fully realize the potential of cellulose, it is promising to utilize it as a nanostructured high-performance constituent in the form of nanofibrils.

The importance of cellulose nanofibril network formation was first demonstrated in polymer nanocomposites. Nanocomposites were prepared from a water suspension of cellulose nanofibrils (tunicate whiskers) and a water-based thermoplastic latex. Favier et al. (Poly. Adv. Tech., 1995, 6, 351 ; Macromolecules, 1995, 28, 6365) demonstrated that the addition of as little as 6% tunicate whiskers is sufficient to form a network that will strongly increase the storage modulus above the glass transition temperature. Tunicate whiskers have high modulus, and form strong interfibrillar bonds between cellulose surfaces so that the network provides substantial stiffening to the rubbery matrix. Several reviews have been published on the subject of cellulose nanocomposites. The first studies on cellulose nanocomposites of high cellulose content, are published by Nakagaito and Yano (Appl. Phys. A: Mater. Sci. Process. 2005, 80, 155). A porous network of microfibrillated cellulose from wood pulp is impregnated by liquid low molar mass poly-phenol formaldehyde (PF) precursors which are subsequently polymerized. The materials show high modulus and strength, but are quite brittle. Materials based on melamine-formaldehyde show similar brittleness. However, Nakagaito and Yano illustrate that cellulose nanofibril networks have potential as high-performance materials and not only as low cost biocomposites. This observation is strengthened by the use of nanostructured cellulose networks in biomedical applications and in transparent materials for high-technology applications.

Lateral dimension in the nanometer scale and lengths in the micrometer range are key geometrical parameters that make cellulose fibrils potential excellent building blocks for construction of strong and tough materials. The reason for this could be their small diameter, high axial ratio (length /diameter), their semi-crystalline structure of extended chains causing intrinsically high mechanical properties. Such long entangled individual cellulose fibrils can be obtained through various disintegration processes, such as chemical modification and mechanical shearing, enzymatic treatment and homogenization using high-pressure homogenizers, steam explosion, ultra-fine friction grinder (supermasscolloider ^® ), and counter collision.

Compared to cellulose nanofibrils from wood, ribbons that consist of aggregates of bacterial cellulose fibrils can be modified during biosynthesis by the simple addition of water-soluble polymers in the culture medium of the bacterium. However, the effect of nanostructure change on the mechanical properties of bacterial cellulose /polysaccharide nanocomposite has not been investigated in great detail so far.

SUMMARY OF THE INVENTION

The object of the present invention is to present a paper comprising microfibrillated cellulose structures and a method of producing the same that overcome the drawbacks of the prior art. This object is achieved in a first aspect by the method as claimed in claim 1 wherein the method comprises the following steps: providing modified nanofibrils of cellulose wherein the modification comprises one of the treatments coating, formation of charge groups, mechanical beating and enzymatic degradation; providing a suspension of said modified nanofibrils at a concentration of less than 0.5 weight%; said nanofibrils being well dispersed in the suspension; filtering, dewatering and drying the nanofibrils.

The modification of the nanofibrils can be performed simultaneously as the nanofibrils are provided but it can also be done as a separate step.

In a preferred embodiment of the present invention, the modified nanofibrils are provided through one or more treatments of cellulose, preferably more than one treatment. These treatments may be, but are not limited to, enzymatic degradation and/or mechanical beating of the cellulose and coating and/or formation of charged groups on the nanofibrils.

In one embodiment of the present invention is the cellulose derived from plants, for example trees and in another embodiment it is derived from bacteria, for example Acetobacter xylinus or Acetobacter aceti.

One embodiment of the method according to the present invention comprises the formation of charged groups on the nanofibrils. These charged groups may be anionic, cationic or zwitterionic groups and they may be found on the surface at all time or they may be activated prior to or during the production of paper.

Another embodiment comprises the treatment of the nanofibrils with radicals to form the charged groups. This can be done for example by using 2,2,6,6- tetramethyl- 1 -piperidinyloxy (TEMPO) radicals or any other suitable radical containing or radical forming substance. Another embodiment comprises the use of glycidyltrimethylammonium chloride or any other suitable cationic agent at a suitable pH. Yet another embodiment involves the treatment with halogen acids to create charged groups. These halogen acids can for example be acetic chloride.

In yet another embodiment the method of the present invention comprises coating or grafting at least one polymer onto the nanofibrils from the bacterial cellulose.

In yet another embodiment the method of the present invention comprises using microfibrillated cellulose with a lateral dimension of approximately 15 nm or less.

Another embodiment of the method according to the invention comprises using microfibrillated cellulose with a lateral dimension of a narrow distribution, i.e. in a range of 15-25 nm.

Another embodiment of the present invention comprises nanofibrils of cellulose with an average degree of polymerisation of approximately at least 800.

One embodiment of the method according to the present invention involves further formation of porous structure in the paper. This may be accomplished via for example phase inversion, salt and/ or sugar leaching, freeze drying or any kind of phase separation suitable for the purpose.

In a further aspect of the invention a paper as claimed in claim 12 is provided, namely comprising a structure of modified cellulose nanofibrils wherein the nanofibrils are well dispersed.

In one embodiment is the cellulose derived from plants and in another from bacteria.

Another embodiment comprises the modification of the cellulose nanofibrils charged groups on said fibrils and yet another embodiment comprises the modification and coating of at least one polymer. These polymers may be of water soluble polysaccharide type or any other suitable type of polymer of variable length.

The well dispersed nanofibrils leads to an extremely strong paper and the thickness of the paper in one embodiment is 40 μm or less. Another embodiment is that the paper exhibits a tensile strength of at least 250 MPa. The paper may contain a porosity of at least 15%.

DESCRIPTION OF DRAWINGS

Fig. 1: FE-SEM micrographs of freeze-dried BC.

Fig. 2: Weight average molecular mass distribution curves.

Fig. 3: FE-SEM micrographs of the surfaces of cellulose nanopaper films.

Fig. 4: Tensile stress-strain curves of the bacterial cellulose (BC) nanopaper films.

DETAILED DESCRIPTION OF THE INVENTION

In the present application the term "lateral dimension" is defined as the largest cross-sectional width of a fibril. For a cylindrically shaped fibril, the lateral dimension would be its diameter.

In the present application the term "modified" is defined as a material treated or changed via one or more treatments to obtain chemical and /or structural modifications. These treatments can be chemical treatments, such as hydrolysis, degradation, grafting, side group attachment or coating; enzymatic treatments, such as enzymatic degradation; or physical treatments, such as mechanical beating.

In the present application the term "well dispersed" is defined as when cellulose nanofibrils remain in a suspension for at least 6 months after centrifuging at 800 £ for 5 minutes.

The present invention provides a procedure to produce a paper with highly improved mechanical properties. Without wishing to be bound by any theory, it is believed that incorporation of charged groups or formation of coatings onto the nanofibrils of cellulose will lead to well dispersed nanofibrils. The incorporation of charged groups and the formation of coatings are proposed to form loose bundles of nanofibrils which in turn results in for example increased tensile strength.

The charged groups can be formed via radicals or via reaction with halogen acids. A preferred radical is the 2,2,6,6-tetramethyl- l-piperidinyloxy radical, but other suitable radicals or radical forming substances may also be used such as azo- compounds (for example azobis-isobutyro nitrile), peroxides or persulphates. Suitable radical containing or radical forming substances are known to a person skilled in the art. Glycidyltrimethylammonium chloride at basic pH is one example on how to create cationic groups. The formation of these charged groups is preferably performed initially in the paper forming production.

The fibrils of the present invention have also a relatively small lateral dimension and a relatively narrow size distribution of the lateral dimension. The average lateral dimension of a fibril may be as low as 15 nm or less and the distribution range can be as low as 15-25 nm. This also contributes to the high dispersion of the fibrils.

The molecular weight of the cellulose is also important to the mechanical properties of the final paper. High molecular weight causes increased entanglement which in turn increases for example strain at failure and yield stress. The degree of polymerisation, i.e. the number of β( l ,4)-D-glucos repeating units, especially for the plant derived cellulose nanofibrils is preferably above 400, more preferably above 800 and most preferred above 1000. The molecular weight and degree of polymerisation can be measured using for example Size Exclusion Chromatography (SEC).

A preferred strategy to provide modified nanofibrils of cellulose is to treat pulp or cellulose with enzymatic degradation and/or mechanical beating and/or formation of charged groups.

A preferred treatment of the nanofibrils is the enzymatic degradation and/or the mechanical beating. The enzymatic degradation can be performed using a variety of enzymes known to a person skilled in the art but most preferred is to use endoglucanase. The mechanical beating can be carried out by using a laboratory beater or any other suitable instrument or tool.

The preparations of the porous structures are environmentally friendly routes starting from nanofibril -water suspensions. During one step the water is removed so that a cellulose nanofibril network is formed. Cellulose nanofibrils of different average molar mass may be used, and other solvents than water may be introduced so that the porosity can be varied in the films. Other solvents could for example be different alcohols or other highly or partly water mixable solvents. Another way of creating a porous structure is through phase inversion where the nanofibril -water suspension is placed in a solvent that does not dissolve the nanofibrils but causes the fibrils to precipitate. Phase separation can also be accomplished via temperature or pressure or a combination thereof. A further example of a phase separation technique is freeze drying. The mentioned water suspension may be replaced with any mixture that keeps the cellulose nanofibrils in suspension.

One preferred method of producing the paper comprises the following steps: providing modified nanofibrils of cellulose and optionally modifying them; providing a suspension of said modified nanofibrils at a concentration of less than 0.5 weight%; filtering, dewatering and drying the microfibrillated cellulose. Preferably the concentration should be between 0.1 and 0.3 weight%. These steps may be performed in a variety of manners. When the nanofibrils are modified by formation of charged groups it can be performed using TEMPO radicals. The treatment is executed in a water suspension together with sodium bromide, or any other suitable salt. It is preferred that the water suspension is kept basic during the reaction. Optionally, the cellulose may be further oxidised using NaClO or any other suitable oxidising salt, preferably at an acidic pH. The TEMPO -mediated oxidation of the cellulose may be performed according to Saito et al. (Biomacromolecules, 2006, 7(6), 1687- 1691). During the treatment the nanofibrils become well dispersed in the suspension.

The filtering and dewatering can be performed using a selection of filters, with different pore sizes, and techniques, all known to a person skilled in the art. The final paper can be transparent and exhibits very low thermal expansion, see M Bergenstrahle, LA Berglund, K Mazeau, J Phys Chem B (2007), 1 1 1 , 9138 for details. The thermal expansion may be as low as 0.5-7 nm/K* 10 ⁵ .

One way of coating or grafting a nanofibril is by producing bacterial cellulose (BC) in the presence of an appropriate polymer. The modified cellulose can be a hydroxylaliphaticcellulose such as hydroxylethylcellulose (HEC), hydroxylpropylcellulose, hydroxylbutylcellulose and so on. The structure and formation of the bacterial cellulose network can be affected by spontaneous interference of polymers added with cellulose assembly. Addition of carboxymethylcellulose, methylcellulose, glucomannan, pectin, arabinoxylan or xylan in the culture medium of Acetobacter xylinus has been shown to influence the properties of the nascent BC, in particular its crystallite dimension, crystallinity and water content. Xyloglucan/BC composite hydrogel has been prepared and used as a model to study the effect of plant cell wall enzymes on its mechanical properties. Figure 1 shows FE-SEM micrographs of freeze-dried BC produced in the presence of 2 % w/v HEC in the culture medium (a) and the control BC (b). Transmission electron micrographs of a loose bundle of aggregated BC fibrils produced in the presence of 2 % w/v HEC in the culture medium (c) and ribbons of the control BC (d). 0.2 % w/v water suspensions of c and d observed at rest between crossed polarizers were shown in e and f, respectively. Figure 3 shows FE- SEM micrographs of the surfaces of cellulose nanopaper films prepared from water suspensions of BC microfibrils, a, control BC; b, BC produced in the presence of 2 % w/v HEC. c, drawing illustrate the structure of a ribbon of cellulose fibril aggregates from a, and compartmentalized bacterial cellulose fibril aggregates with soft matrix (HEC) nanocoating from b mimicking tendon ultrastructure.

Purified and freeze-dried control BC fleeces (~ 150 mg) were obtained as described in Example 3. About 25% of the D-glucose present in the culture medium was utilized by the bacterium and incorporated into cellulose after 7 days of culture. The bacteria can be cultured under both static and dynamic conditions.

Interestingly, the yield of the BC fleeces obtained by growing the bacterium in the presence of HEC (BCHEC) increased with the amount of HEC present in the culture medium. Typically, relative yields of 128%, 138%, 155% and 190% with respect to the control BC cultures ( 100%) were obtained in the presence of 0.5, 1.0, 2.0 and 4.0% (w/v) HEC, respectively. The weight average molecular mass (M _w ) of BC produced in the presence of HEC in the culture medium was comparable to that of the control BC (M _w of 2.1 x 10 ⁶ ), with a polydispersity index (M _w /M _n , where M _n is the number-average molecular mass) of 1.9. Lower molecular weight fractions of HEC (Mw of 5.9 x 10 ⁴ , Mw/Mn of 1.6) were incorporated into BC as shown by size exclusion chromatography, Figure 2. From the ratios between the peak areas of the chromatograms, it can be estimated that the amount of incorporated HEC was of 18 % and 19 % for the fleeces prepared in the presence of 2 % and 4 % (w/v) HEC in the culture medium, respectively.

Without wishing to be bound by any theory it can be proposed that HEC self- assembles with the cellulose fibrils, which co-aggregate into larger fibril aggregates during biosynthesis, i.e. the BC fibrils are coated with HEC, thereby altering the structure of the cellulose crystals. As a consequence, the formation of BC ribbons is hindered and loose bundles of nanofibril aggregates are compartmentalized. The same reasoning

The cellulose nanopaper films prepared from the water suspension of the well dispersed cellulose nanofibrils show dramatically increased tensile strength and work to fracture compared to the control BC, and compared to previous studies on BC sheets and wood-based cellulose nanopapers. The key might be the novel biomimetic nanostructural composites concept of nanofibrils compartmentalized by thin coatings of a polysaccharide (HEC). As individual nanofibrils fracture during the latter part of the strain-hardening region, catastrophic fracture is delayed by the crack-deflecting function of the thin nanofibril coating. This preparation approach for uniquely structured nanocomposites represents a low-energy and cost-effective process method for building high-strength cellulose-based nanocomposite materials.

These high strength papers can be used in a variety of areas. Besides paper and filter paper, they can be used in a wide range of biomedical applications due to their biocompatible structures. Implant material such as vascular graft, scaffold for tissue growth and/ or as a drug delivery vehicle are some areas where this material would be suitable. It has also potential as a membrane in speaker systems and in batteries, for example lithium ion batteries. Figure 4 shows tensile stress-strain curves of the BC nanopaper films prepared from microfibrils obtained from the BC produced with HEC in the culture medium (a), from the control BC (b) or from a blend of control BC and HEC (c). As seen the tensile strength is at least 250 MPa for the BC produced with HEC in the culture medium. Tensile tests were performed, if nothing else is stated, using a Universal Materials Testing Machine from Instron, USA, equipped with a 500 N load cell. The cross-head speed was set to 4 mm/min.

EXAMPLES

Example 1

Preparation of MFC

The different kinds of microfibrillated cellulose (MFC) used herein are termed DP-X where X corresponds to the average degree of polymerization (DP) of the specific MFC sample, estimated from viscosity data.

Oxidation and formation of charged groups on the cellulose fibrils may for example be conducted on cellulose (2 g) suspended in water (150 ml) containing TEMPO (2,2,6,6-tetramethyl-l-piperidinyloxy) (0.025 g) and sodium bromide (0.25 g). The pH was adjusted by adding NaClO and was then maintained at 10.5. To terminate the reaction the pH was lowered through addition of HCl to a pH of around 7. The whole procedure was performed at room temperature. The product was thoroughly washed with water.

The MFC was prepared from softwood dissolving pulp. The pulp was subjected to a pre-treatment step, in 4Og batches, followed by disintegration into MFC by a homogenization process with a Microfluidizer M-1 10EH, Microfluidics Inc., USA. There is no upper limit for the amount that can be processed in the Microfluidizer (flow speed is about 400 ml/min). In the pre-treatment step the pulp is exposed to a combination of enzymatic degradation and mechanical beating in a laboratory beater. The enzyme used is an endoglucanase, Novozym 476, manufactured by Novozymes A/ S, Denmark, believed to preferably degrade cellulose in disordered regions. This way to prepare MFC is based on the method explained in detail by Henriksson et al (Eur. Polym. J., 43, page 3434, 2007) with a few modifications. During the enzymatic treatment a phosphate buffer prepared from 1 1 mM NaH2PO4 and 9 mM Na ₂ HPO ₄ with pH 7 was used. The fibres were incubated at 50 ⁰ C for 2 h, washed and thereafter incubated at 90 ⁰ C in order to stop the enzyme activity. Different concentrations of enzymes used in the pre-treatment step correspond to different degrees of polymerization for the resulting MFC. The enzyme concentrations used per gram pulp fibres were 5 μl, 5 μl and 0.2 μl. This resulted in MFC with average DP of 410, 580, and 820, respectively. The reason why the same concentration, 5 μl per gram pulp fibres, resulted in two different average DP's is that for the DP 410 case, the enzyme activity was not stopped immediately after incubation. After pre-treatment, the pulp was passed 12 times through the Micro fluidizer in order to produce cellulose nanofibrils. During the first three passes, chambers with dimensions of 400 μm (first chamber) and 200 μm (second chamber) were used. The pressure was 950 bar. During the 9 last passes, chambers with dimensions of 200 μm (first) and 100 μm (second) were used. During these passes, the pressure was 1650 bar. The MFC's termed DP-410, DP-580, and DP-820 are prepared by this method.

DP-800 was delivered from Innventia and is prepared by a similar method as above. The pulp used was bleached sulphite softwood (Domsjό ECO Bright) which has higher hemicellulose content than the dissolving pulp. DP-1 100 is prepared from the same kind of softwood dissolving pulp as above. The pulp is carboxymethylated in a chemical pre-treatment step and then run once through the Microfluidizer.

The DP- 1 100 sample has the highest average molar mass, but also shows some other differences compared with samples based on enzymatic pre-treatment. The degree of dispersion of nanofibrils is higher (higher suspension viscosity and more transparent suspension) and the cellulose surface contains carboxylic acid groups due to the chemical pre-treatment.

Example 2

Preparation of porous cellulose nanopaper

Cellulose nanopaper films were prepared by vacuum-filtration of a 0.2% (by weight) MFC suspension. Prior to filtration the suspension was stirred for 48 h to ensure well dispersed nanofibrils. All films, except DP-800, were filtrated on a glass filter funnel (1 1.5 cm in diameter) using Munktell filter paper, grade OOH, Munktell Filter AB, Sweden. Films prepared of DP-800 were filtrated on a glass filter funnel (7.2 cm in diameter) using filter membrane, 0.65 μm DVPP, Millipore, USA. After filtration, the wet films were stacked between filter papers and then dried at 55 ⁰ C for 48 h at about 10 kPa applied pressure. This resulted in MFC films with thicknesses in the range 60-80 μm.

Porous films are prepared by solvent exchange on the filtered film before drying. After filtration the wet film was immersed in methanol, ethanol or acetone for 2 h. The solvent was replaced by fresh solvent and the film was left for another 24 h. Then the film was dried in the same way as described above. This resulted in films of various porosities and thicknesses in the range of 40-90 μm.

Uniaxial tensile tests were performed to determine the mechanical properties of the produced papers.

Example 3

Preparation of cellulose nanopaper using bacterial nanofibrils

The Acetobacter aceti strain was pre-cultivated in the Hestrin-Schramm (HS) medium for 7 days at 27 ⁰ C, and 5 mL of this pre-culture was used to inoculate 30 mL of fresh HS medium. A series of BCHEC samples were prepared in the presence of 0.5, 1.0, 2.0, and 4.0% (w/v) HEC (Aldrich cat # 308633; average M _w 250,000) in the culture medium. The control BC was obtained by cultivating the bacterium in the absence of HEC in the medium. The control BC and BCHEC fleeces were harvested after 7 days of culture at 27 ⁰ C under static conditions. They were treated with 0. 1 M NaOH at 80 ⁰ C for 3 h and washed with de-ionized water. This process was repeated 3 times and the BC fleeces were finally washed with de-ionized water for several days until neutrality was reached. Aqueous suspensions of BC microfibrils with a solid content of 0.2% were obtained by homogenizing the control BC or the BCHEC fleeces with a Waring® blender. Typically, 400 mL of the water suspensions were vacuum filtered on a glass filter funnel (7.2 cm in diameter) using a 0.65-μm DVPP filter membrane from Millipore. After filtration, the wet films were stacked between filter papers and dried at 55 ⁰ C for 48 h under a 10-kPa applied pressure. This resulted in BC nanopaper films with thicknesses in the range 40-70 μm. Tensile tests for bacterial cellulose nanopaper films were performed at 50% relative humidity and 23 ⁰ C, using an Instron 5567 universal testing machine equipped with a load cell of 100 N. The films were cut in thin rectangular strips of 5 x 60 mm. The gauge length was 40 mm for all samples and the strain rate was of 10% min ¹ . Stress-strain curves were plotted and the Young's modulus was determined from the slope of the low strain region. The strength and strain-to-failure were also determined for each specimen. Toughness is defined as work to fracture and is calculated as the area under the stress-strain curve. Mechanical tensile data were averaged from at least three specimens.