Abstract:
The invention relates to a method for manufacturing nanostructured paper or board and a novel paper or board. The method comprises providing a liquid suspension of nanocellulose-containing material, forming a web from the suspension and drying the web in order to form paper or board. According to the invention, the water content of the suspension at the time of beginning of the drying is 50% or less by weight of liquids so as to form a paper or board having an average pore size between 200 and 400 nm. By means of the invention, very opaque paper, for example for printing applications, can be manufactured with low energy consumption.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to paper making. In particular, the invention relates to novel paper or board structures and their manufacturing methods. Generally, the present structures include a nanocellulose-based web. In the method, a web is formed from a nanocellulose-containing suspension, and the web is dried in order to form paper or board. 
       BACKGROUND OF THE INVENTION 
       [0002]    For more than 200 years the conventional papermaking process is based on a filtration process of aqueous suspensions of woodfibers. Due to the large flocculation tendency, which can cause optical inhomogenities in the final paper structure, typically low consistencies of about 0.5-2% (by weight) woodfibers are used in paper furnishes. A large part of the production energy is consumed by the drying process, as water forms typically about 50% (by weight) of the wet web structure after filtration and pressing, and has to be evaporated in the drying section of the process. 
         [0003]    Paper-like products have also been manufactured from non-cellulosic raw materials (e.g. ViaStone or FiberStone). Such products may consist of 80% calcium carbonate and 20% synthetic polymer resin, for example. By such materials, water consumption can be reduced or even avoided. 
         [0004]    In certain applications, woodfibers have been replaced with nanocellulose as the raw material. This enables opportunities for new products, and new papermaking processes. 
         [0005]    Henriksson et al,  Cellulose Nanopaper Structures of High Toughness, Biomacromolecules,  2008, 9 (6), 1579-1585 discloses a porous paper comprising a network of cellulose nanofibrils. The preparation of the paper starts from nanofibril-water suspension, where the water is removed so that a cellulose nanofibril network is formed. First, a 0.2% (by weight) stirred water suspension is vacuum filtrated in a filter funnel. The wet films obtained is dried under heat and pressure. Porosity of the product was increased by exchanging the water as a solvent for methanol, ethanol or acetone before drying. 
         [0006]    US 2007/0207692 discloses a nonwoven transparent or semitransparent highly porous fabric containing microfibrillated cellulose. The fabric can be obtained by a similar process as in the abovementioned article of Henriksson et al. by forming a web from aqueous suspension of microfibrillated cellulose, exchanging the water solvent for organic solvent and drying. According to the examples, the consistency of the aqueous suspension is 0.1% (by weight) before web-forming. Both the abovementioned methods utilize nanocellulose fibers that are smaller in size than the cellulose fibers (wood fibers) used in conventional paper making. Sheets manufactured from nanocellulose fibers are reported to have high toughness and strength. However, due to their transparency and/or exceptionally high porosity they are not very suitable as such for printing purposes, for example. 
         [0007]    In addition, there is a need for more efficient methods of manufacturing paper, paperboard or the like products from nanocellulose. 
       SUMMARY OF THE INVENTION 
       [0008]    It is an aim of the invention to produce a novel method for manufacturing opaque nanocellulose-containing products and a novel nanocellulose-containing paper, board or paper- or board-like product (for simplicity, hereinafter referred to as “paper or board”). A particular aim of the invention is to achieve an opaque paper or board which can be manufactured with reduced water consumption and a method reducing the energy consumption of paper making. 
         [0009]    According to a first aspect of the invention, there is provided a method where paper is manufactured from a suspension comprising nanocellulose fibers, the water content of the suspension at the time of beginning of the drying being 50% or less by weight of liquids so as to form a paper or board having an average pore size between 200 and 400 nm. 
         [0010]    It has been found that when the paper or board is dried from non-aqueous suspension, a product having an opacity of 85% or more, in particular 90% or more, and even 95% or more can be produced even without any opacifying additives. In other words, the web is dried from non-aqueous mass which is rich in nanocellulose fibers. The suspension typically comprises at least 50%, in particular at least 75%, preferably 95% (by weight) organic solvent, such as alcohol. The inventors have found that such suspensions significantly contribute to achieving high opacity, the screening of fiber-fiber interactions takes place and capillary forces are considerably reduced during the drying process. Thus, pore structures in the range of 200-400 nm can be achieved, the range being about half of the wavelength of the visible light (400-800 nm). While pores below 100 nm and above 800 nm do not scatter light efficiently, the light scattering is optimal exactly in this pore size range of half of the wavelength of visible light. In contrast, water-based nanocellulose papers are dense and therefore are not opaque but transparent, as will be shown later by experimental data. On the other hand, known nanocellulosic sheets are too porous and transparent to be used as a substitute for paper, e.g. in printing applications. 
         [0011]    According to a preferred embodiment at least 30% of the volume of the pores of the paper or board is contained in pores having a size between 200 and 400 nm. This ensures that high opacity is achieved at all wavelengths of visible light. 
         [0012]    According to a particular embodiment, the paper or board comprises
       10-90% by weight of solids nanocellulose fibers,   10-75% by weight of solids reinforcing macrofibers and/or filler, and   0-10% by weight of solids other additives,
 
the total amount of said components amounting to 100% by weight of solids. The macrofibers and filler contribute to achieving a product which has mechanical and/or optical properties comparable to those of conventional printing papers, increase the bulk of the product and help to reduce nanocellulose consumption.
       
 
         [0016]    In addition to high opacity, by means of the invention, considerable energy savings are achieved because the heat of vaporization of non-aqueous solvents is typically lower than that of water. Moreover, it has been found by the inventors, that owing to the small particle size, flocculation of the nanofibers is about negligible for the optical homogeneity of the final web structure. This enables the use of suspensions with higher consistencies for drying and, if desired, even for high consistency web forming. The consistency of the suspension can be 0.5-90% (by weight). A relatively high consistency at this range further assists in achieving the desired pore size distribution and high opacity. According to a particular embodiment, the consistency is 1-50% (by weight), preferably at least 3% (by weight). Thus, the amount of liquids is initially significantly lower than in conventional papermaking. No special equipment is needed for nanocellulose-based high-consistency web forming. 
         [0017]    Another advantage of the use of nanocelluloses compared to conventional woodfibers is the immense increase of contact points of the formed fiber web, which enables the use of non-aqueous suspensions during drying. Due to the reduced fiber-fiber interaction, woodfibers do not form any comparable, mechanically stable paper structures from typical non-aqueous (e.g. alcoholic) suspensions. In contrast, mechanically stable, porous and highly opaque paper-like web structures can be formed from alcoholic suspensions of cellulose nanofibers. Owing to a lower evaporation energy, the drying of nanocellulose webstructures from alcoholic suspensions is much more energy efficient compared to water-based web formation processes. Due to the much higher number of binding sites, also higher porosities and mechanical stabilities can be achieved using the same amount of nanocellulose compared to woodfibers, which allows reduction in raw materials use and higher contents of filler particles. 
         [0018]    It has also been found by the inventors that cellulose particles with a high specific surface area form mechanically stable sheet-like structures (like paper) also from non-aqueous systems (e.g. ethanolic suspensions). This is a great improvement as compared with conventional sheets made from non-aqueous suspensions using wood-fibers, which do not hold together very well due to the much lower surface area of the much larger wood-fibers and the resulting much lower contact area. 
         [0019]    The potential of the described new papermaking process compared to the conventional papermaking process is about 100% water savings, 60% energy savings, and 30-50% raw materials savings. 
         [0020]    According to another aspect of the invention, there is provided a novel paper comprising a network of nanocellulose fibers and reinforcing macrofibers and inorganic filler as additives. 
         [0021]    According to one embodiment, the high-consistency non-aqueous suspension or the paper formed contains 10-90% (by weight of solids), in particular 25-75% additives such as macrofibers (in contrast to nanofibers) and/or filler. The macrofibers are preferably organic macrofibers, such as wood fibers used in conventional paper making. Macrofibers have been found to have a significant reinforcing effect on the paper. The filler is preferably organic (e.g. cellulosic) or inorganic filler such as pigment, in particular mineral pigment having an additional opacifying, whitening, brightening or coloring effect on the paper. 
         [0022]    According to one embodiment, the amount of organic macrofibers is 1-30% (by weight of solids), in particular 1-10%. By this embodiment, mechanically more stable products can be manufactured. 
         [0023]    According to one embodiment, the amount of filler is 10-75% (by weight of solids), in particular 25-75%. By this embodiment, the specific volume (bulk) or visual appearance, such as whiteness, brightness, color or opacity can be increased, depending on the type of filler. According to one embodiment, the suspension contains hydrophobization agent, such as sizing agent. The content of such agent can be, for example, 0.1-5% by weight. For example, alkenyl-succinic anhydride (ASA), can be used as the hydrophobization agent, in particular in the amount of 1-3 wt-%. One purpose of the hydrophobization agent is shielding of fiber-fiber interactions by hydrogen bonding and adjusting the porosity and/or bulk of the end product. Another purpose of the hydrophobization agent is to adjust the hydrophobic/lipophilic interactions for improved wettability, which is of importance in printing applications. 
         [0024]    Organic solvent-based suspensions are compatible also with most other conventional additives used in papermaking. 
         [0025]    According to a preferred embodiment, the porosity of the product is in the range of 10-50%, which is considerably smaller than achieved in US 2007/0207692 and allows the product to be used in printing applications, for example. 
         [0026]    According to one embodiment, the paper of board is manufactured, i.e. formed and dried, directly from non-aqueous suspension. Such method comprises the following steps:
       non-aqueous suspension is conveyed from suspension container to means for forming a web from the non-aqueous suspension,   the formed web is conveyed to drying zone for solvent removal,   the dried web is guided out of the drying zone for storage, and   optionally, solvent is collected (e.g. condensed) at the drying zone and recovered or circulated back to the process.       
 
         [0031]    This embodiment has the advantage that even higher consistency suspensions can be used for web-forming as organic solvents have a significant positive effect on the rheology of the suspension and broaden the usable consistency range. 
         [0032]    According to another embodiment, the web is formed from aqueous suspension, after which the aqueous solvent is exchanged with an organic solvent for drying. Such method comprises the following steps:
       an aqueous suspension is conveyed from suspension container to means for forming a web from the aqueous suspension,   the aqueous solvent is exchanged with organic solvent,   the formed web is conveyed to drying zone for solvent removal,   the dried web is guided out of the drying zone for storage, and   optionally, solvent is condensed at the drying zone and recovered or circulated back to the process.       
 
         [0038]    This embodiment has the advantage that aqueous suspensions, in which nanocellulose is typically produced, can be directly used for web-forming. In the solvent exchange step, at least 50%, typically at least 90% (by weight) of the aqueous solvent is replaced with non-aqueous solvent. 
         [0039]    The grammage of the resulting paper is preferably 30-160 g/m 2  and the grammage of the resulting board is preferably 120-500 g/m 2 . 
       DEFINITIONS 
       [0040]    The term “nanocellulose” in this document refers to any cellulose fibers with an average diameter (by weight) of 10 micrometer or less, preferably 1 micrometer or less, and most preferably 200 nm or less. The “cellulose fibers” can be any cellulosic entities having high aspect ratio (preferably 100 or more, in particular 1000 or more) and in the abovementioned size category. These include, for example, products that are frequently called fine cellulose fibers, microfibrillated cellulose (MFC) fibers and cellulose nanofibers (NFC). Common to such cellulose fibers is that they have a high specific surface area, resulting in high contact area between fibers in the end product. The term “nanocellulose-based” paper or board means that the paper or board comprises a continuous network of nanocellulose fibers bound to each other so as to form the backbone of the paper or board. 
         [0041]    The terms “macrofibers” (“woodfibers”) refer to conventional (wood-originating) cellulose fibers used in papermaking and falling outside the abovementioned diameter ranges of nanocellulose. 
         [0042]    The term “non-aqueous suspension” refers to content of water in the suspension of 0.01-50%, typically 0.01-20%, in particular 0.01-5%, by weight of the total liquid phase of the suspension. Thus, the majority of the liquid phase of the suspension is other liquid than water, for example alcohol. In practice, a minor amount of water is contained in all technical qualities of organic solvents, such as alcohols. This is, in fact, necessary, as a small amount of water is needed for the hydrogen bonding of the nanofibers. However, even a water content of significantly less than 1% (by weight) is sufficient. 
         [0043]    The term “high consistency” of suspension refers to a consistency significantly higher than the cellulose suspension of conventional paper making, in particular a consistency of 5% (by weight) or more. Although high consistency suspension is preferred due to the reduced need of liquid removal and increased runnability, it is to be noted that the invention can generally be applied to low-consistency suspensions too. The preferred consistency range is about 0.05%-90%, in particular about 1-50% (by weight). 
         [0044]    The term “filler” includes all non-fibrous raw materials which can be bound to the pores of a nanocellulose-containing web. In particular, such materials comprise pigments, such as mineral and/or polymer pigments, optical brighteners and binders. Examples of pigments are particles selected from the group consisting of gypsum, silicate, talc, plastic pigment particles, kaolin, mica, calcium carbonate, including ground and precipitated calcium carbonate, bentonite, alumina trihydrate, titanium dioxide, phyllosilicate, synthetic silica particles, organic pigment particles and mixtures thereof. 
         [0045]    Next, embodiments and advantages of the invention will be discussed in more detail with reference to the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0046]      FIG. 1  illustrates schematically manufacturing apparatus according one embodiment. 
           [0047]      FIG. 2  shows measured properties of exemplary ethanol suspension-based nanocellulose papers, conventional copy paper and aqueous suspension-based nanocellulose papers. 
           [0048]      FIGS. 3   a  and  3   b  show pore size distributions of paper sheets manufactured from non-aqueous and aqueous suspensions, respectively. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0049]    The invention describes water-free paper production processes based on nanocelluloses, and sheet-like products made by these processes. The term water-free refers to cellulose suspensions which are not water-based (e.g. including hydrocarbon solvent, such as bio-ethanol). Low amounts of water can be still present, as it is typically the case in technical qualities of alcohols. The water-content of the liquid phase of the cellulose suspension has to be lower than 50%, preferably below 5% (by weight). 
         [0050]    According to one embodiment, the relative permittivity of the solvent is at least 10 (e.g. ethanol: 24). 
         [0051]    The process is characterized by the use of non-water based suspensions, which can be used at moderately high to high consistencies between 0.5% and 90%, preferably between 1 and 50%, typically 3-20% (by weight). High consistency of the suspension in the beginning of web-forming process minimizes the need of solvent removal/circulation and thus energy consumption. High-consistency organic solvent based forming thus has major positive economic and environmental effects. In conventional wood fiber-based paper making, high-consistency forming has required special high consistency formers, which have a different operating principle as in conventional low-consistency forming. Organic solvents have a significant effect on the rheology of the suspension and broaden the consistency range of conventional forming techniques at paper mills. 
         [0052]    The specific area of the nanocellulose used within the invention is preferably at least 15 m 2 /g, in particular at least 30 m 2 /g. The cellulose fibers may be prepared from any cellulose-containing raw material, such as wood and/or plants. In particular, the cellulose may originate from pine, spruce, birch, cotton, sugar beet, rice straw, sea weed or bamboo, only to mention some examples. In addition, nanocellulose produced partly or entirely by bacterial processes can also be used (bacterial cellulose). 
         [0053]    As concerns the manufacturing of nanocellulose, we refer to methods known per se, for example, as disclosed in US 2007/0207692, WO 2007/91942, JP 2004204380 and U.S. Pat. No. 7,381,294. The aqueous suspensions obtained by such method can be converted to non-aqueous suspensions within the meaning of the present invention by solvent exchange either before of after web-forming. However, it is also possible to produce directly alcoholic suspensions of nanocelluloses, e.g. by grinding ethanolic suspensions of dry pulp. 
         [0054]    The web formation process can be performed by filtration of the non-aqueous suspension, e.g. vacuum filtration on a porous support, or by drying of the wet web structure on a non-porous support, e.g. belt drying, or by combinations of these methods. 
         [0055]    The drying of the web can be performed by employing thermal energy, e.g. IR irradiation, or generating thermal energy in the wet web structure, e.g. microwave drying. Belt drying as the preferred drying process enables 100% retention of the raw material and of any additives to improve product performance or processability. Combinations or cascades of different drying techniques may also be employed. 
         [0056]    Further possible process steps can be included, such as condensation and circulation of the solvent, and calandering or wetting of preformed sheets e.g. for the formation of layered structures. 
         [0057]    As organic solvents are more expensive than water, recovery or circulation of the removed solvent is a preferred option. 
         [0058]      FIG. 1  shows schematically the manufacturing process according to one embodiment of the invention. In the process, aqueous or non-aqueous suspension is conveyed from suspension container  11  to a high-consistency (&gt;1%) web former  12 . If the suspension is aqueous, the formed web is subjected to a solvent exchange process. The formed non-aqueous web  13  is conveyed using a belt conveyer  14 , through drying zone  15  containing a drier  16  and solvent condenser  17 . Dried web is guided out of the drying zone for storage. From the solvent condenser  17 , the liquid solvent is circulated back to the suspension container  11  through a circulation conduit  18 . 
         [0059]    According to a preferred embodiment of the invention, there is provided as a starting material a nanocellulose-based furnish including inorganic filler particles as additives. The range of filler content is typically 1-90%, preferably 10-75% (by weight). As nanocellulose-based paper structures prepared from such furnishes have relatively low tensile stiffness compared to conventional paper (see Table 2,  FIG. 2 ), wood fibers can be used as an additional additive to improve both tensile stiffness and tear strength. The wood-fiber content ranges from 1 to 30%, preferably from 1 to 10% (by weight). 
         [0060]    The preparation from non-aqueous furnishes is compatible also with other additives used in papermaking, e.g. sizing agents which can be used for nanofiber hydrophobization (see Table 2 and  FIG. 2 ). Hydrophobized nanofibers can be used for adjusting the porosity, bulk and/or hydrophobic/lipophilic interactions. Thus, the formed paper or board can be designed suitable for high quality printing applications, in which the porosity and wettability, in particular, must be in a desired range. 
         [0061]    According to one advantageous embodiment, the present nanocellulose-based paper comprises
       25-75% (by weight) nanocellulose fibers,   1-30% (by weight) reinforcing macrofibers, and   0-75% (by weight) fillers,   0-10% (by weight) other additives,
 
the total amount of components amounting to 100%.
       
 
       Examples 
       [0066]    Table 1 shows examples of nanocellulose-based papers including additives (filler and wood-fibers). The filler used for the samples shown in Table 1 was ground calcium carbonate (GCC) (Hydrocarb HO, supplied by Omya, Finland). Reinforcing wood fibers were obtained from bleached birch Kraft pulp. All listed compositions have been found to be processable from non-aqueous suspensions and to the porosity range according to the invention. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Grammage 
                 Filler 
                 Enforcement 
               
               
                   
                 (g/m 2 ) 
                 amount 
                 fibres 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 NFC 100-5 + 
                 80 
                  0% 
                 — 
               
               
                   
                 filler 
                 80 
                 50% 
                 — 
               
               
                   
                   
                 80 
                 50% 
                 2% 
               
               
                   
                   
                 80 
                 50% 
                 5% 
               
               
                   
                   
                 80 
                 50% 
                 10%  
               
               
                   
                 NFC 100-5 + 
                 120 
                  0% 
                 — 
               
               
                   
                 filler 
                 120 
                 25% 
                 — 
               
               
                   
                   
                 120 
                 50% 
                 — 
               
               
                   
                   
                 120 
                 75% 
                 — 
               
               
                   
                   
               
             
          
         
       
     
         [0067]    Table 2 shows grammage examples of nanocellulose-based papers prepared from aqueous suspensions (ethanol), including the use of sizing agent (ASA). All listed paper grades have been found to be processable from non-aqueous suspensions and to the porosity range according to the invention. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Material 
                 grammage (g/m 2 ) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 NFC 100-5 
                 30 
               
               
                   
                   
                 60 
               
               
                   
                   
                 120 
               
               
                   
                 NFC (2%) ASA 
                 60 
               
               
                   
                   
               
             
          
         
       
     
         [0068]    Table 3 shows measurement data on mechanical and optical properties of papers according to the invention and comparative papers. The data is shown graphically in  FIG. 2 . NFC  5  and NFC  9  refer to the ‘water-free’ papermaking approach, compared also to other NFC sheet structures made from aqueous suspensions, like NFC  2  and NFC  8 . 
         [0069]    The NFC  2  and NFC  5  papers were composed of 100 wt-% plain nanofibrillated cellulose 100-5 (ground beech fibers) and the NFC  8  and  9  papers were composed of 100 wt-% ASA-treated nanofibrillated cellulose 100-5 (ground beech fibers) (amount of ASA 2 wt-%). The raw NFC 100-5 was obtained from Rettenmaier &amp; Sohne GmbH, Germany. No other additives, pigments, wood-fibers have been used for those NFC films were contained in the samples tested. 
         [0070]    For film formation suspensions of NFC and ASA-NFC, respectively, were prepared in water or ethanol with concentrations in the range of 0.2-1 wt %. The suspensions were homogenized by using a Waring 38-BL40 laboratory blender. Subsequently the sheets were formed in a Büchner funnel by filtration under reduced pressure. The obtained wet NFC sheets were dried at 50° C. between glass plates in a Memmert 400 drying oven. 
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 tensile 
                   
                   
               
               
                   
                   
                   
                   
                 air 
                 bright- 
                   
                 tensile 
                 Tensile 
                   
                 energy 
                 tensile 
                 TEA 
               
               
                   
                 grammage 
                 thickness 
                 bulk 
                 permeance 
                 ness 
                 opacity 
                 strength 
                 index 
                 stretch 
                 absorption 
                 stiffness 
                 index 
               
               
                   
                 (g/m2) 
                 (microns) 
                 (cm3/g) 
                 (ml/min) 
                 (%) 
                 (%) 
                 (kN/m) 
                 (Nm/g) 
                 (%) 
                 (J/m2 
                 (kN/m) 
                 (J/g) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 copy paper MD 
                 82.2 
                 103 
                 1.25 
                 836 
                 97.5 
                 90.8 
                 4.8 
                 58.4 
                 1.1 
                 34 
                 712 
                 0.414 
               
               
                   
                 copy paper CD 
                 82.2 
                 103 
                 1.25 
                 836 
                 97.5 
                 90.8 
                 1.68 
                 20.4 
                 3.4 
                 45 
                 207 
                 0.547 
               
               
                 NFC 2 
                 NFC 100-5 
                 76.7 
                 75.8 
                 0.99 
                 1 
                 76.6 
                 35.9 
                 4.45 
                 58.0 
                 3.2 
                 110 
                 321 
                 1.434 
               
               
                 NFC 5 
                 NFC 100-5 
                 72.3 
                 139 
                 1.93 
                 6 
                 91.7 
                 93.6 
                 1.68 
                 23.2 
                 3.8 
                 47.6 
                 155 
                 0.658 
               
               
                   
                 (ethanol 
               
               
                 NFC 8 
                 NFC (2% ASA) 
                 55.4 
                 72.8 
                 1.31 
                 3 
                 86.8 
                 71.2 
                 1.83 
                 33.0 
                 1.9 
                 23.2 
                 166 
                 0.419 
               
               
                 NFC 9 
                 NFC-2% ASA 
                 72.4 
                 190 
                 2.62 
                 413 
                 93.2 
                 95.2 
                 0.437 
                 6.0 
                 2.4 
                 8.2 
                 39.6 
                 0.113 
               
               
                   
                 (ethanol) 
               
               
                   
               
             
          
         
       
     
         [0071]    As can be seen from Table 3, ethanol-based suspensions (NFC  5 , NFC  9 ) resulted in thicker, more bulky, brighter and more opaque papers than the comparison papers manufactured from water-based suspensions (NFC  2 , NFC  8 ). Also other properties measured indicate that such papers have the potential of being widely used in similar applications as conventional copy papers. 
         [0072]    The pore size distributions of NFC  5  and NFC  2  test papers were measured by mercury intrusion porosimetry (MIP). The method is based on the gradual intrusion of mercury into the pores of the formed NFC sheets. For this purpose a high pressure station, Pascal 440 (Thermo Scientific), was been employed. It allows measurements at high pressures up to 400 MPa and by this the intrusion of pores in the single nanometer range. The experimental data is obtained in form of dependence of filled pore volume upon the applied pressure. These data are converted into a pore size distribution histogram by applying the Washburn equation describing the relation between mercury pressure and pore radius. 
         [0073]    Results of the measurements are shown in  FIGS. 3   a  and  3   b , respectively. The relative pore volume is shown in percentages as vertical bars for a plurality of pore diameter ranges and the cumulative pore volume is shown in cubic centimeters per gram as a curve. As can be seen, the sheet dried from alcohol-based suspension (NFC  5 ,  FIG. 3   a ) contains almost two orders of magnitude smaller pore size than the sheet dried from aqueous suspension (NFC  2 ,  FIG. 3   b ). The average pore size of the former lies in the advantageous range of 200-400 nm, whereas average pore size of the latter is over 20 μm. The indicated dominant geometry of the pores of the NFC sheets is cylindrical. 
         [0074]    The embodiments and specific examples disclosed above and issuprated in the attached drawings are non-limiting. The invention is defined in the attached claims which are to be interpreted in their full scope taking equivalents into account.