A method of production of nanofibrillated cellulose

Abstract

A method of production of nanofibrillated cellulose according to the present invention, comprises the following steps: adding 0.01 to 10 wt.% of cellulose, based on the total weight of the SiO2 and AOH dry mixture, to an aqueous solution of AOH having a molarity of 0.5 M to 20 M where A is selected from the group consisting of Li NA, K Rb or Cs, preferably a 5 M to 15 M AOH solution, most preferably 10 M NaOH solution, and stirring for at least 5 minutes; adding amorphous SiO2 powder or silica glass to the mixture in a quantity to ensure a silicate modulus n from 0,5 to 2, preferably from 1 to 2, heating it to the temperature of 50°C to 150°C, and stirring it at that temperature until complete dissolution of the SiO2 powder; evaporating water from the suspension at a temperature from 40°C to 150°C until reaching the dynamic viscosity at least 1 000 Pa.s. measured at 25 °C and a shear rate of 1 s-1, and the suspension becomes a paste; and applying high-speed mechanical stress with the minimal energy of 300 kJ per kilogram of the paste, at a temperature range of 1°C to 150°C, to obtain a paste containing nanofibrillated cellulose.

Description

[0001]A method of production of nanofibrillated cellulose Technical Field The invention relates to the field of nanomaterials, especially it relates to the techniques for synthesis, production, processing, and storage of nanofibrillated cellulose. In particular, it relates to mechanical techniques for nanocellulose production. Background Art Cellulose is a hierarchical material, in which each macrofibre of cellulose is composed of smaller fibres, or microfibres. These, in turn, are composed of i.e. smaller entities, defined as elementary nanofibres, having nanometric size (CNFs). Each CNF is in turn a stack of parallel anhydroglucose-based chains. CNFs is an extremely appealing material due to combination of several outstanding properties such as its renewability and sustainability; biodegradability; high strength and stiffness; lightweight; high surface area; transparency (nanocellulose films can be transparent); versatility for different applications; good barrier properties against gases; compatibility with other materials. However, the separation and isolation of the single nanofibres from regular macro- cellulose is a non-trivial task, due to the strong forces standing between each fibre which keep them stacked against one another. Depending on the method of nanocellulose production, the types of nanocellulose can be cellulose nanofibres (CNFs) or cellulose nanocrystals (CNCs). CNCs are commonly produced using acid hydrolysis of cellulosic materials dispersed in water. In general, concentrated sulfuric acid is used, which dissolves the amorphous regions of cellulose and the crystalline regions are left alone. The length and diameter of CNCs commonly vary from a length of 200–500 nm to a diameter of 3–35 nm (Habibi et al. 2010), depending of the hydrolysis parameters such as duration of the reaction, acid concentration, temperature, etc. CNFs contain both amorphous as well as crystalline cellulose domains within the single fibres. Typically, CNFs have a diameter of 5–50 nm and a length of a few micrometers. CNF extraction from cellulosic fibres can be obtained by three types of processes: (1) mechanical treatments (e.g. homogenization, grinding, and milling); (2) chemical treatments (e.g. TEMPO oxidation); and (3) a combination of the previous two (Sofiah et al. 2023). Among all, the mechanical route is any type of nanocellulose processing and synthesis that is based on the resorting of mechanical techniques. The set of techniques are convenient for being simple and scalable making them suitable for large-scale production, and versatile as they are capable of producing both CNCs and CNFs depending on the specific mechanical treatment. However, the main drawback is energy consumption: some mechanical methods may require significant energy input, especially high-pressure homogenization. The known methods for the mechanical disintegration of cellulose into micro- and nano-fibres cellulose are the following: - High-pressure homogenization (HPH), or fibre clogging. Cellulose fibres is thus forced to passed a narrow zig-zag orifice at high pressure in order to generate high shear stresses capable of initiating the defibrillation (Wang et al. 2019). This method requires high energy consumption if not preceded by chemical or enzymatic pre-treatments. - Microfluidization. Microfluidizers work similar to HPH in the production of nanocellulose fibre. Microfluidizers use an intensifier pump to enhance pressure, while the interaction chamber is used for shear and impact forces against colliding streams to defibrillate the fibres. Due to high pressure and the requirements of several passages under these conditions, high energy consumption is usually associated to this method to obtain a finely processed nanofibre cellulose product. The process of production is also lengthy, difficult to scale-up and a high-pressure equipment is required. - Ball milling. Ball milling is a mechanical process in which cellulose suspension is placed in a hollow cylindrical container for the production of CNF. This hollow cylindrical container is partially filled with balls made up of ceramic, zirconia, or metal; the container rotates and breaks cellulose cell walls through the high- energy collision between the balls. The output is affected by various factors such as ball size, ball-to-cellulose weight ratio, grinding time, moisture content, and carboxylic charge. This method involves high yield of nanocellulose, but suffer from low homogeneity and poor morphological control over the produced CNF. Besides, high level of contamination is involved. - Grinding. In this process, pulp passes through a couple of stones, where one stone is fixed while the other stone rotates. This mechanism provides shear forces to break down the hydrogen bond and cell wall structure of fibres and convert pulp into nanoscale fibres. Friction of stones generates heat due to the fibrillation process, which helps to evaporate water content and raise solid content. This method suffers from the same problems as ball milling. - Cryocrushing. Cryocrushing is another mechanical method used to break the cellulose wall into nanosize fibres. In this method, fibres are kept in water and cellulose absorbs water in its cavity. Water-soaked cellulose is immersed in liquid nitrogen, which solidifies the water content, and is subsequently crushed by impact. Applying high-impact force on frozen cellulosic fibres leads to rupture due to applied pressure through ice crystals resulting in conversion to nanocellulose (Salimi et al. 2019). This method is very effective but costly as it requires liquid nitrogen for processing of the nanocellulose. A chemical treatment allows an easy production of high pure nano-fibres and a better control over the properties of the nanosized cellulose. Usually alkaline–acid pretreatment is the most common method used for lignin, hemicellulose, and pectin solubilization before mechanical isolation of NFC (Kim et al. 2016). This method could include the following steps: 1. Sodium hydroxides (NaOH). Soaking fibres in 12– 17.5wt% solution for 2 hours. This raises the surface area of cellulosic fibres and eases the hydrolysis. 2. Hydrochloric acid (HCL). Soaking fibres in 1M solution at 60– 80°C. This solubilizes the hemicelluloses. 3. Sodium hydroxides (NaOH). Treating with 2wt% solution for 2 hours at 60–80°C. This disrupts the lignin structure, and breaks the linkages between carbohydrate and lignin. Alkaline pretreatment is an effective method that can improve cellulose yield from 43% to 84%. However, this method usually resorts to harsh chemicals, leading to environmental concerns and the need for proper waste management. Another method for the nanocellulose production is the enzymatic treatment. This method is better than the previous in that it is eco-friendlier and provides specificity in cellulose degradation, resulting in well-defined nanocellulose. However, the main disadvantage consists of the process duration: enzymatic processes may take longer compared to some chemical or mechanical methods. Further, since cellulosic fibres contain many different organic compounds as a composite structure, a single specific enzyme cannot degrade the fibre and a set of enzymes are required to decay extra cellulose compound. The aim of this invention is to provide an inexpensive and eco-friendly method of production of nanofibrillated cellulose. Said method solve shortcomings of current techniques, including cost of production, energy consumption, eco-sustainability, efficiency and scaling up of the process, very important for industrial production. Moreover, the method also provides an efficient, low-cost and eco-friendly solution for the long-term storage of nanofibrillated cellulose. Summary of the Invention Technical problem solved: The method according to the present invention produces nanofibrillated cellulose through a mechanical disintegration of cellulose (micro or macro) from any source (pure cellulose, regenerated cellulose, cotton, paper, recycled paper, etc.). The method makes use of a semisolid aqueous-based medium composed of alkali metal silicate, preferably sodium silicate, in which the cellulose precursors are dispersed, and machines (such as hammers, punchers, rapid hydraulic presses, etc.) capable of imparting a powerful high-speed deformation, to massively breaks the cellulose chains and chop the fibres down to nanometric size. The sodium-silicate medium is low-cost, non-toxic, and easily removable, besides being a highly efficient storage medium, which keeps the nanosized fibres defibrillated, prevents any post-processing stacking of the nanofibrillated cellulose and protects the nanofibres from degradation (for instance bacterial and fungi degradation). The method of production of nanofibrillated cellulose according to the present invention comprises (and as a preferred embodiment it can also consist of) the following steps: i. Adding 0.001 to 10 wt.% of cellulose, based on the total weight of the SiO2 and AOH dry mixture, to an aqueous solution of AOH having a molarity of 0.5 M to 20 M, where A is selected from the group consisting of Li, Na, K, Rb or Cs, preferably Na, preferably a 5 M to 15 M AOH solution, the most preferably a 10 M NaOH solution and stirring for at least 5 minutes; ii. Adding amorphous SiO2 powder or silica glass to the mixture in a quantity to ensure a silicate modulus n (SiO2 / A2O ratio) from 0,5 to 2, preferably from 1 to 2, heating it to the temperature from 50°C to 150°C, and stirring it at that temperature until complete dissolution of the SiO2 powder or silica glass; iii. Evaporating water from the suspension at a temperature from 40°C to 150°C until reaching the dynamic viscosity at least 1000 mPa·s or more, measured at 25 °C and a shear rate of 1 s-1, and the suspension becomes a paste; iv. Applying high-speed mechanical stress with the minimal energy of 300 kJ per kilogram of the paste, at a temperature range of 1°C to 150°C, to obtain a paste containing nanofibrillated cellulose. Where the preferred method of production is as follows: i. Adding 0.001 to 10 wt.% of cellulose, calculated in respect of the total weight of the SiO2+NaOH dry mix, into a 10M NaOH water solution and stirring for at least 5 minutes; ii. Adding amorphous SiO2 powder or silica glass to the mixture in a quantity to ensure a sodium silicate modulus n (SiO2 / Na2O ratio) from 1 to 2, heating it to the temperature from 50°C to 150°C, and stirring it at that temperature until complete dissolution of the SiO2powder or silica glass; iii. Evaporating water from the suspension at a temperature from 40°C to 150°C until reaching the dynamic viscosity at least 1000 mPa·s or more (measured at 25 °C and a shear rate of 1 s-1) and the suspension becomes a paste; iv. Applying high-speed mechanical stress with the minimal energy of 300 kJ per kilogram of the paste, at a temperature range of 1°C to 150°C, to obtain a paste containing nanofibrillated cellulose. Wherein the input materials comprise cellulose, amorphous SiO2 powder or silica glass, AOH, where A is alkali metal selected from Li, Na, K, Rb or Cs, preferably Na and water. For simplicity, the following description refers to (10M) NaOH as a representative example; however, the same applies mutatis mutandis to other AOH compounds. In the context of this invention the term cellulose means cellulose microfibres or cellulose macrofibres or mixture thereof. In the context of this invention the term nanofibrillated cellulose means mixture of cellulose microfibres and nanofibres with the proportion of nanofibres at least 20%, preferably at least 50%, more preferably at least 80%, the most preferably over 90%. In the context of this invention the term pure nanofibrillated cellulose means cellulose where the proportion of nanofibers is substantially (is approaching) 100%. Where the term cellulose macrofibres includes cellulose fibres having length above 0.5 mm and possibly thickness above 0.050 mm. The term cellulose microfibres includes cellulose fibres having length in the range from 2 to 500 μm and possibly thickness in the range from 0.5 to 50 μm. The term cellulose nanofibers include cellulose fibres having length below 2000 nm and possibly thickness below 500 nm. The measurement of the dynamic viscosity can be performed through any suitable standardized measuring methods such as “cone-and plate method”, “concentric cylinder method”, “parallel plates method” or “Mooney / Ewart method”. These methods guarantee the calculation of the rheological parameters in the form of absolute units independent of the individual measuring system. Cellulose macro-fibres can be obtained from any source, for example a pure cellulose, regenerated cellulose, cotton, paper, recycled paper, etc. The quality of the source material can affect the quality of the final product. In the step i. 0.001 to 10 wt.% of cellulose has to be added into preferably a 10M NaOH water solution. At higher concentrations of cellulose (over 10 wt.%) the final paste gradually loses its plastic properties. Preferably, 0.01 to 5 wt.% of cellulose should be added to the mixture, the most preferably 0.01 to 1. The solution is stirred for at least 5 minutes, preferably from 30 to 60 minutes. In this step the cellulose is functionalized by Na intercalation of the cellulose molecular chains and a crystalline transition from Cell I to a Na-Cell II occurs. In the step ii. amorphous SiO2 powder or silica glass is added to the mixture in a quantity to ensure a sodium silicate modulus n (SiO2 / Na2O ratio) from 0,5 to 2, preferably from 1 to 2. This range of sodium silicate modulus guarantees an appropriate plasticity of the paste. Above this range, the sodium silicate becomes to brittle, and below this range the excessive hygroscopic behaviour of the paste renders the evaporation stage (step iii) hard to properly accomplish. However, for high concentration of cellulose (5 wt. % and 10 wt. %), the chemical modulus should preferably be at the lower end of said range, i.e. (from 0,5 to 1.5) to achieve the proper desired viscosity and elastic properties. The mixture is heated to the temperature from 50°C to max. 150°C, as at the temperature above 150°C the cellulose would tarnish, which is undesirable. Preferably, the mixture is heated to the temperature from 65 to 75°C, the most preferably at 70°C. The mixture is stirred at that temperature until complete dissolution of the SiO2 powder or silica glass. In this step the sodium silicate network (waterglass) intercalates the nanofibres of cellulose composing the cellulose macrofibres. This range of temperature would limit excessive cellulose thermal degradation. In the step iii. the water is evaporated from the suspension at a temperature from 40°C to 150°C, preferably at a temperature from 60°C to 100°C, more preferably at the temperature of 70°C until reaching the dynamic viscosity at least 1000 mPa.s, preferably at least 30000 mPa.s, more preferably at least 100000 mPa.s, (measured at 25 °C and at a shear rate of 1 s-1), the most preferably until the evaporation is no longer possible due to the hygroscopic properties of the Cellulose / Sodium silicate system, and the solution becomes a semisolid paste. The elimination of water decreases the distance between the suspended colloidal species (increase of silicate- silicate and fibre-silicate interactions), resulting in higher viscosity. 1000 mPa.s is the least viscosity which guarantees appropriate shear stress during the stage of mechanical deformation to defibrillate the micro and nanometric fibrils from the initial cellulose macrofibres. The higher the viscosity the more efficient the defibrillation process. In the step iv. the high-speed mechanical stress imparted in the step iv. should have minimal energy of 300 kJ per kilogram of the paste, preferably 1.2 to 18 MJ, the most preferably 4.5 MJ. The values of energy are mathematically correlated with the power and the velocity of deformation. The temperature range should be 1°C to 150°C. At the temperature below 1°C the cellulose would spoil and at the temperature above 150°C it would tarnish, as already mentioned. The high-speed mechanical stress in step iv. is preferably imparted by using machinery capable of applying to the paste a high-speed deformation such as hammers, punchers, rapid hydraulic presses, extruders, homogenizers, mixers and so on. In case of hammers, punchers, rapid hydraulic presses and extruders the number of beats (or cycles for extrusion) per 1 kg of the paster should preferably be 1 to 100, more preferably, 10 to 50, the most preferably 30. More than 100 beats are to be avoided for energy and time consumption. Accordingly, the puncher and the hammers should have punching force per kilogram of paste of 0.5 tons to 5 tons. Preferably, 2.5 tones. The upper limit should be avoided for safety reasons due to excessive weight of the components. Equivalently, the hydraulic press should have a range of pressure per kilogram of paste of 100 MPa to 1000 MPa, preferably 500 MPa. In case of mixers or homogenizers, the high-speed mechanical stress imparted in the step iv. should have the minimal power of 1000 W per kilogram of the paste, and it should be imparted for at least 5 minutes. 1000 W per kg of the paste is the minimal power that the machine should have to overcome the shear frictions of the viscose system and thus create defibrillation. For higher volumes of the paste, the power supply should increase proportionally. Preferably the high-speed mechanical stress should have a power 2000 W to 10000 W per kilogram of the paste, the most preferably 5000 W per kilogram of the paste. The mechanical work should be imparted for at least 5 minutes, preferably 15 to 60 min., more preferably 10 to 30 minutes, the most preferably 15 minutes. This time of mechanical deformation is appropriate to have enough formation of nanocellulose fibres and appropriate homogenization. In theory, the mechanical deformation does not have a maximal threshold of time but it should be limited due to issues related to energy and time consumption. The term high-speed means, in case of homogenizers, a rotational speed between 200 rpm and 3000 rpm, preferably 1000 rpm. In case of hammers, a load speed 100 to 500 mm / s, preferably 300 mm / s. In case of punchers, a load speed 100 to 500 mm / s, preferably 250 mm / s. In case of extruders, a load speed 5 to 100 mm / s, preferably 50 mm / s. In case of hydraulic presses 50 to 300 mm / s, preferably 200 mm / s. These ranges of speed are the most appropriate to guarantee appropriate homogenization and defibrillation, and in the same time to give enough energy for breaking cellulose molecular chains. The high-speed (high-intensity) mechanical stress overcomes the Van der Walls forces keeping the nano-fibres altogether and provides enough energy to break the anhydroglucose chains (cleavage), thus defibrillating the cellulose fibres and chopping them down to nano-size (max. length 2 μm and thickness between 50 and 100 nm). This task is accomplished through viscosity forces given by the waterglass medium which is capable of intercalating concurrently the single glucose-chains with Na cations and the fibres with the sodium silicate aqueous network (red medium in the figure below). Upon water removal, the sodium silicate network shrinks, packing increases and viscosity increases. The viscous system thus provides enough frictions between cellulose fibres to delaminate (or to defibrillate) small fibres packages (the sodium silicate intercalation facilitates the defibrillation). When high-speed mechanical stress is applied, the friction due to viscosity is enough to even part the glucose chains of the cellulose. The temperature and time vary according to the deformation methodologies of applying mechanical stress. This stress will be imparted through one of the following methods: hammering, punching, rapid hydraulic press, extrusion, rolling, high-power homogenizing and mixing. According to the preferred embodiment, the step iv. is followed by: 1. dissolving the paste in water, preferably 0.5 l to 1l of water per 1kg of paste, more preferably 2.5 l to 5l of water per 1kg of paste, to obtain a liquid suspension of nanofibrillated cellulose in a diluted alkali metal-silicate solution (sodium-silicate solution), 2. removing the alkali metal silicate (sodium-silicate), preferably by filtering it off or using centrifuge, and optionally 3. washing out the resulting mixture in water and diluted HCl solution (0.01-1M), to obtain nanofibrillated cellulose. According to another preferred embodiment, the step iv. is followed by: a. Mixing the paste with a polymeric powder, where the mass ratio of cellulose to polymeric powder is preferably from 1:0.1 to 1:10, more preferably from 1:1 to 1:5, at the temperature in the range of 1°C to 150°C, preferably at 40°C, most preferably at 70°C at 10 to 1000 revolutions for at least 25 minutes or until complete homogenization. The complete homogenization is achieved when no powder clumps are visible. b. Dissolving the mixture in water, preferably 2.5 l to 5l of water per 1kg of mixture, to obtain a liquid suspension of mixture of nanofibrillated cellulose with polymer in diluted alkali metal silicate (sodium silicate), c. Removing the alkali metal silicate (sodium silicate), preferably by filtering it off or using centrifuge, and optionally d. Washing out the resulting mixture in water and 0.01 to 1M HCl solution, to obtain a mixture of polymeric composite with homogenously distributed nanofibrillated cellulose, and optionally e. Processing the mixture according to the selected polymeric powder (i.e. thermoset or thermoplastic polymers), in order to form a bulk polymeric composite. In the step a) any polymeric powder which is stable in a sodium silicate basic environment can be used. Either synthetical polymer such as PVA, PVC, PVDF, PTFE, PE, PP, Nylon etc.., or natural polymer, such as sodium alginate, albumin, chitosan etc… or biodegradable polymers such as PHA, PHB, PLA etc… According to a further embodiment, the step iv. is followed by drying the paste at a temperature 100 to 150°C, preferably 120°C to remove chemically bound water, to obtain a nanocellulose composite. Alternatively, the step iv. is followed by treating the medium with 0.01-1M solutions of monoprotic strong acid preferably HCl or HF, to form water soluble alkali metal salts (sodium salts) and SiO2 washable by filtration and / or centrifuge, to obtain a nanocellulose composite. According to yet another embodiment, the step iv. is followed by diluting the medium in water, preferably 0.5l to 1l of water per 1kg of paste, and treating it with 5% solution of phosphoric or sulfuric acid in order to precipitate SiO2and alkali metal phosphate or alkali metal sulphate salts (sodium phosphate or sodium sulphate salts), to obtain highly ordered cellulose / salt nanostructures. According to yet another embodiment, the paste of step iv. can be dissolved in water to obtain a liquid suspension of nanofibrillated cellulose in a diluted alkali metal- silicate solution (sodium-silicate solution) and a drop is left to dry on a substrate (usually glass, but it can be any material which is water-compatible). The nano- material is then separated from the micro- and macro-sized fibres (unprocessed fibres) through Marangoni’s effect. The nanofibrillated cellulose is collected. Unprocessed micro- and macro-sized fractions can be stored or reused in further production of nanofibrillated cellulose. Marangoni flows were found to effectively separate nano fraction from the non- fibrillated macrosized one. After mechanical deformation, the paste was rediluted, a small amount is withdrawn and deposited on a heated substrate. It is possible to observe how upon drying, the nano-sized fibre fraction is dragged towards the drop’s outer edges (top right insert in Fig. 3B), while the rough non-fibrillated cumulates at the drop’s centre (bottom right insert in Fig. 3B). This selective flow pattern arises because smaller fibres typically have a higher surface-to-volume ratio compared to the larger ones, inducing different size- dependent flow velocities or trajectories. Smaller fibres, with their higher surface-to- volume ratio, experience stronger surface tension effects and are thus transported more effectively by the flow (see Figure 3A). The paste containing nanofibrillated cellulose of step iv. having the viscosity 30000 mPa.s or more, preferably 100000 mPa∙s, can be used as shock absorption material, to dissipate and absorb kinetic energy generated by impacts or sudden forces, thus reducing the transmitted force to the object or surface they're protecting. Compared to the similar materials on market (e.g. D3O®), it has similar shock absorption properties it is composed of an ecological polymeric material. The paste containing nanofibrillated cellulose of step iv having the viscosity 5000 mPa.s or more, preferably 30000 mPa∙s, can be used as a semisolid ionically conductive but electrically insulating material in electrochemical applications such as solid electrolytes, having ionic conductivity of approx. 10-6S / cm at room temperature, one order of magnitude higher than many solid fast-ionic conductors (NaSICONs). To further improve control over the size of the cellulose fibres (mixed micro- and nano-fibres are usually obtained, and sometimes macro-fibres are found), one or more of the following steps can be performed: a. Longer exposure of the paste to high-speed mechanical stress, 30 min. to 4 h., preferably 1 to 1.5 h. b. Ad-hoc tailoring of the mechanical stress (choice of the deformation mode, i.e. rolling, hammering, extrusion etc. and the parameters to it related e.g. speed of deformation, temperature, length of the treatment, geometry of the set-up, etc.) c. Separation of the nano-sized fibres from the micro-sized and macro-sized fibres through Marangoni’s flows and collection of the macro-sized fibres to be utilized again in the process of the present invention. The method according to the present invention has following advantages: ^ low-cost, as the synthesis of the fully recyclable waterglass is made up of inexpensive input materials; ^ easy to reproduce and it does not require expensive and sophisticated machines; ^ makes use of aqueous medium and therefore eco-friendly and non-toxic. ^ effective and efficient for the production of nanofibrillated cellulose and it does not require long time of processing and / or pre-treatments; ^ high quality and purity of the produced nanofibrillated cellulose, as the aqueous medium is easily washable; ^ scalable and suitable for mass-production, as up to a few kg of product can be produced in one day; ^ versatile and flexible, since the waterglass medium can be used for storage or further synthesis of nanocomposite; ^ the medium is also a storage medium for the preservation of defibrillated nanocellulose; ^ provide the same quality of the final product with less energy consumption and under atmosphere pressure. Further, sodium silicate can be produced in high quantity, by dissolving silica glass / gel into concentrated hydroxide solutions. This approach can be easily scalable being straightforward. The water or the sodium silicate can be further recycled for new batches, limiting the consumption of raw materials. The temperature involved are low (max to 150 °C) which can be more easily set for bigger volumes of materials. No toxic gases are generated during these reactions. The heat of dissolution of NaOH (strongly exothermic reaction) can be also stored and employed. The nanocellulose prepared by the method according to the present invention is suitable for further development of nanocomposites for miscellaneous applications, especially ceramics, metallic and polymeric nanocomposites. Said nanocellulose can be further dried and - mixed to a polymeric matrix to obtain a nanocellulose-reinforced composite, or - mixed to an inorganic powder to form a ceramic nanocomposite, or - dispersed in natural lubricant for storage and further exploitation in other applications. The paste prepared by the method according to the present invention can serve as a storage medium for the nanocellulose contained and dispersed within. The nanocellulose stored in such way is preserved in defibrillated state until further exploitation in other applications. The sodium silicate and nanocellulose mixture (paste) obtained after step iv or step 1 can be also employed as alkali-activator in the synthesis of a geopolymeric nanocomposite as cellulose chains have a high tensile strength (2-3 GPa) and high modules of elasticity (100-220 GPa), by addition of an aluminosilicate source (e. g. metakaolin, fly ash, etc.). The synthesised geopolymers has at room-temperature an average compressive strength of about 120 MPa, which is 3 times higher than the average compressive strength of a standard metakaolin-based geopolymer. The paste of step iii, owing to its high viscosity can be used as a shock- adsorbing / damper material for applications requiring fast dissipation of mechanical energy (e.g. car bumpers). Brief description of Drawings Figure 1a) is a SEM image of pristine macrocellulose fibres Figure 1b) is a SEM image of CNFs obtained after the defibrillation but before the final refining step. Figure 2. are SEM images of CNFs obtained after the final refining step. Insert on the left: EDS analysis showing the purity of the refined fibres. Figure 3a) is a scheme depicting Marangoni-driven particle sorting occurring during drying of a sessile drop Fig. 3b) is a Marangoni-driven fibre sorting obtained by drying a sessile drop of diluted paste: insert on top right shows the separated nano-sized fraction deposited at drop’s edges while the insert at the bottom right shows the non-fibrillated micrometric fraction which deposits at the drop’s center. Detailed Description Hereinafter, embodiments for carrying out the present invention will be described in detail. However, the present invention is not limited to the following embodiments. Example 1 Preparation of medium containing nanocellulose The waterglass was prepared so as to have a chemical modulus n= 1.8 (molar ratio SiO2 / Na2O). The weight percentage of cellulose was calculated with respect to the total weight of the SiO2+NaOH dry mix. The medium containing nanocellulose was produced as follows: 1 wt.% of cellulose fibre precursor of pristine cellulose fibres of macrometric and / or micrometric size was added into a 10M NaOH water solution (1000 mL H2O, 400 g NaOH) and stirred for 5 minutes; SiO2 powder was added to the mixture in a quantity to ensure a sodium silicate modulus n (SiO2 / Na2O ratio) 1.8, and kept under stirring at 70 °C until complete dissolution of the SiO2 powder; The suspension was then stirred at the temperature of 70°C, to evaporate water from the suspension until evaporation was no longer possible due to the hygroscopic properties of the Cellulose / Sodium silicate system, and the solution has become a paste. The suspension was weighed every 30 min. and after achieving three consecutive identical results (within the measurement error), evaporation phase was considered to be completed - the maximum viscosity of the suspension was reached at a given temperature. The 10 g of the paste has been obtained. The paste was then removed for the heat source and exposed to hammering deformation of 300 J and repeated for 30 times, at room temperature. A paste containing nanocellulose was obtained. Example 2 Preparation of nanocellulose The process of harvesting the nanocellulose, i.e. separation of nanocellulose from the paste prepared in the Example 1, was accomplished as follows: The paste was dissolved in water to obtain a liquid suspension of nanofibrillated cellulose in a diluted sodium-silicate solution. The sodium silicate was filtered off and the resulting mixture was washed out in water and diluted HCl solution (1M), to obtain nanofibrillated cellulose. Example 3 The initial pristine cellulose fibres as well as the nanofibrillated cellulose prepared in the Example 1 and 2 were evaluated through scanning electron microscopy and transmission electron microscopy. Results: As can be seen pristine cellulose fibres with micrometric size (fig. 1a)) was effectively disintegrated down to nano-sizes (fig. 1b)), and kept defibrillated by the sodium silicate medium. The precursor cellulose fibres are depicted in figure 1a). The size of these fibres is macrometric, with length up to 5 mm and thickness always higher than 10 μm. After treatment of the Example 1 the precursor fibres were chopped down to nanometric size, as depicted in the figure 1b). Range 500 nm – 1000 nm, and thickness within the range 50 nm-100 nm. The morphology depicted in the picture can be thus considered a stack of cellulose nanofibrils. Therefore, this method allows a massive and efficient production of nanocellulose, with the sodium silicate medium acting as an effective fibres’ separator preventing them from restacking. Figure 2a) shows the chemical analysis through EDS of the SEM observation of the nanocellulose produced in the Example 2. The analysis confirmed a highly pure nanocellulose throughput. Moreover, Figure 2a) shows a magnification of the same SEM observation, evidencing that the removal of the sodium silicate medium induces a restacking of the nanocellulose, but the fine and disordered morphology of the cellulose confirms that the pristine cellulose is efficiently processed into nanocellulose after the application of the method according to the invention. Example 4 The paste prepared in the Example 1 was dissolved in water (1L of water each kg of paste) and a liquid suspension of nanofibrillated cellulose in a diluted sodium-silicate solution were obtained. A drop of the suspension was left to dry on a glass substrate. The nano- and micro-sized material was separated from the macro-sized fibres (unprocessed fibres) though Marangoni’s effect. The nanometric product, on the border of the drop, is collected using a spatula. Example 5 The paste prepared in the Example 1 was dried at a temperature 120°C to remove chemically bound water, to obtain a nanocellulose nanocomposite. Example 6 The paste prepared in the Example 1 was treated with diluted solutions 0.1M of HCl to form water soluble sodium salts and SiO2. The SiO2 is further removed with centrifuge to obtain a cellulose nanocomposite. Example 7 The paste prepared in the Example 1 was diluted in water and treated with 5% solution of phosphoric acid in order to precipitate SiO2 and sodium phosphate salts, to obtain highly ordered cellulose / salt nanostructures. Example 8 In a similar manner, samples 1 to 7 were prepared under the preparation conditions listed in Table 1. Tab. 1 Preparation conditions Results Dynamic viscosity Nanofiber (mPa.s) Energy cellulose Type of portion of Item Modulus measured per kg (wt. %) deformation cellulose at 25 °C (MJ) (%) and shear rate 1 s-1p 76 0.3 m 72 3000 891.5pm 83 1 0.01 2.0 p 89 1.2 m 86 30 000 p 95 4.5 m 90 p820.3 m 77 5000 p901.5 m 83 2 0.1 1.8 p 87 1.2 m 81 50 000 p954.5 m 92 p 73 1.2 m 65 10 000 p 82 3 1 1.6 4.5 m 79 p8580 000 4.5 m 81 p 87 15 m 86 321.2pm 21 30 000 p 55 4.5 m 43 4 10 1.3 p 31 4.5 m 10000022p 39 18 m 32 p stands for punch deformation; m stands for mixing (homogenizers). Results: The analysis of the different compositions (Tab. 1) was accomplished through image editing on SEM observations of the dried drops on a carbon conductive tape substrate, similarly to that showed in the Fig. 3b). This method was found to be the most reliable for the evaluate of the nanometric population. For higher concentration of cellulose (such as 1 wt. % to 10 wt. %), the method gradually loses effectiveness due to sodium silicate saturation. As expected, irrespective of the composition, the higher viscosity and higher imparted energy increases the effectiveness of the method. It was also found that punchers and hammering seem to be slightly more efficient than homogenizers. For high concentration of cellulose (5 wt. % to 10 wt. %), the chemical modulus should be lowered (below 1.5) in order to achieve the proper desired viscosity and elastic properties. In the same table, the temperature of mechanical deformation is not reported as the temperature was adapted to achieve the desired viscosity (the viscosity is inversely proportional to the temperature). Measurement of the dynamic viscosity: The measurement of the dynamic viscosity of the suspension in the step iii) was operated through a “cone-and-plate” measuring system using a universal Rheometer Anton Paar Physica MCR 501 (Czechia). This method is a standard system of viscosity measurements according to the ISO 3219 (1993) and DIN 53019-1 (1976). The conical rotating plate was kept to a distance of 0.049 mm from the fixed plate and approx. 2 ml of suspension was squeezed in this gap for viscosity measurements. A sample of the suspension was taken regularly every 30 minutes until the desired viscosity was reached.

Claims

Claims 1. A method of production of nanofibrillated cellulose, characterised in that it comprises the following steps: i. Adding 0.001 to 10 wt.% of cellulose, based on the total weight of the SiO2 and AOH dry mixture, to an aqueous solution of AOH having a molarity of 0.5 M to 20 M where A is alkali metal selected from the group consisting of Li, Na, K, Rb or Cs, preferably a 5 M to 15 M AOH solution, most preferably 10 M NaOH solution, and stirring for at least 5 minutes; ii. Adding amorphous SiO2powder or silica glass to the mixture in a quantity to ensure an alkali metal-silicate modulus n from 0,5 to 2, preferably from 1 to 2, heating it to the temperature of 50°C to 150°C, and stirring it at that temperature until complete dissolution of the SiO2 powder; iii. Evaporating water from the suspension at a temperature from 40°C to 150°C until reaching the dynamic viscosity at least 1 000 mPa·s, measured at 25 °C and a shear rate of 1 s-1, and the suspension becomes a paste; iv. Applying high-speed mechanical stress with the minimal energy of 300 kJ per kilogram on the paste, at a temperature range of 1°C to 150°C, to obtain a paste containing nanofibrillated cellulose.

2. The method according to claim 1, wherein the high-speed mechanical stress in the step iv. is imparted by hammering, punching, rapid hydraulic press, extrusion, rolling, high-power homogenizing or mixing.

3. The method according to any of the preceding claims, wherein the energy applied in the step iv. is 1.2 to 18 MJ per kilogram of the paste, the most preferably 4.5 MJ.

4. The method according to any of the preceding claims, wherein the further steps are:

1. Dissolving the paste in water, preferably 2.5 l to 5l of water per 1kg of paste, to obtain a liquid suspension of nanofibrillated cellulose in diluted alkali metal silicate, 2. Removing the alkali metal silicate, preferably by filtering it off or using centrifuge, and optionally 3. Washing out the resulting mixture in water and 0.01 to 1M HCl solution, to obtain nanofibrillated cellulose.

5. The method according to any of the claims 1, 2 or 3, wherein the further steps are: a. Mixing the paste with a polymeric powder, where the ratio of cellulose content to polymeric powder is from 1:0.1 to 1:10, at the temperature in the range of 1°C to 150°C, at 10 to 1000 revolutions per min. until complete homogenisation, b. Dissolving the mixture in water, preferably 2.5 l to 5l of water per 1kg of mixture, to obtain a liquid suspension of mixture of nanofibrillated cellulose with polymer in diluted alkali metal silicate, c. Removing the sodium silicate, preferably by filtering it off or using centrifuge, and optionally d. Washing out the resulting mixture in water and 0.01 to 1M HCl solution, to obtain a mixture of polymeric composite with homogenously distributed nanofibrillated cellulose, and optionally e. Processing the mixture to form a bulk polymeric composite.

6. The method according to any of the claims 1, 2 or 3, wherein the further step is drying the paste at a temperature 100 to 150°C, preferably 120°C to remove chemically bound water, to obtain a nanofibrillated cellulose nanocomposite.

7. The method according to any of the claims 1, 2 or 3, wherein the further step is treating the paste with 0.01-1M solutions of monoprotic strong acid preferably HCl or HF, to form water soluble alkali metal salts and SiO2, and removing the SiO2,preferably by filtering it off or using centrifuge, to obtain a nanofibrillated cellulose nanocomposite.

8. The method according to any of the claims 1, 2 or 3, wherein the further step is diluting paste in water, preferably 0.5l to 1l of water per 1kg of paste, and treating it with 5% solution of phosphoric or sulfuric acid in order to precipitate SiO2 and alkali metal phosphate or alkali metal sulphate salts, to obtain highly ordered cellulose / salt nanostructures.

9. The method according to any of the claims 1, 2 or 3, wherein the high-speed mechanical stress in step iv is applied by extruder, rapid hydraulic press, puncher, hammer or homogenizer.

10. The method according to any of the claims 1, 2 or 3, wherein the paste is: - dissolved in water, preferably 0.5l to 1l of water per 1kg of paste, to obtain a liquid suspension of nanofibrillated cellulose in a diluted alkali metal-silicate solution, and - drops of the suspension is left to dry on a substrate to separate the nano- product from the micro-sized fraction though Marangoni’s effect, - collecting the pure nanofibrillated cellulose, and preferably storing or reusing the unprocessed micro- and macro-sized fractions in further production of nanofibrillated cellulose.

11. Use of the paste of any of the claims 1 to 3 as a storage medium for preservation of nanofibrillated cellulose contained within.

12. Use of the paste of any of the claims 1 to 3 as activator for the production of geopolymers.

13. Use of the paste of any of the claims 1 to 3 as a shock-adsorbing / damper material.

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