Classifications

• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08B—POLYSACCHARIDES; DERIVATIVES THEREOF
  • C08B15/00—Preparation of other cellulose derivatives or modified cellulose, e.g. complexes
  • C08B15/08—Fractionation of cellulose, e.g. separation of cellulose crystallites
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08B—POLYSACCHARIDES; DERIVATIVES THEREOF
  • C08B15/00—Preparation of other cellulose derivatives or modified cellulose, e.g. complexes
  • C08B15/02—Oxycellulose; Hydrocellulose; Cellulosehydrate, e.g. microcrystalline cellulose
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08B—POLYSACCHARIDES; DERIVATIVES THEREOF
  • C08B15/00—Preparation of other cellulose derivatives or modified cellulose, e.g. complexes
  • C08B15/02—Oxycellulose; Hydrocellulose; Cellulosehydrate, e.g. microcrystalline cellulose
  • C08B15/04—Carboxycellulose, e.g. prepared by oxidation with nitrogen dioxide
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08B—POLYSACCHARIDES; DERIVATIVES THEREOF
  • C08B3/00—Preparation of cellulose esters of organic acids
  • C08B3/08—Preparation of cellulose esters of organic acids of monobasic organic acids with three or more carbon atoms, e.g. propionate or butyrate
  • C08B3/10—Preparation of cellulose esters of organic acids of monobasic organic acids with three or more carbon atoms, e.g. propionate or butyrate with five or more carbon-atoms, e.g. valerate
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08B—POLYSACCHARIDES; DERIVATIVES THEREOF
  • C08B3/00—Preparation of cellulose esters of organic acids
  • C08B3/14—Preparation of cellulose esters of organic acids in which the organic acid residue contains substituents, e.g. NH2, Cl
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
  • C08L1/00—Compositions of cellulose, modified cellulose or cellulose derivatives
  • C08L1/02—Cellulose; Modified cellulose
  • C08L1/04—Oxycellulose; Hydrocellulose, e.g. microcrystalline cellulose
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
  • C08L1/00—Compositions of cellulose, modified cellulose or cellulose derivatives
  • C08L1/08—Cellulose derivatives
  • C08L1/10—Esters of organic acids, i.e. acylates
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
  • C08L1/00—Compositions of cellulose, modified cellulose or cellulose derivatives
  • C08L1/08—Cellulose derivatives
  • C08L1/26—Cellulose ethers
  • C08L1/28—Alkyl ethers
  • C08L1/286—Alkyl ethers substituted with acid radicals, e.g. carboxymethyl cellulose [CMC]

Definitions

• the invention relates to a process for preparing micro- and/or nanocrystalline cellulose material in the presence of an acid in the gas phase.
• the invention also relates to a cellulose product prepared by the said process, as well as the use thereof.
• microfibrils In plant cell walls cellulose is organized into microfibrils, in which most of the cellulose is in crystalline form. The microfibrils also contain less-organized or amorphous parts. The amorphous parts can be removed by a chemical reaction, i.e. selective acid hydrolysis, after which only the crystalline parts of the cellulose remain. The result is either microcrystalline or nanocrystalline cellulose, depending on the conditions. Both are actively used in modern material science applications.
• cellulose hydrolysis Due to the effect of the acid, the cellulose is hydrolyzed, meaning that it breaks at the glycosidic oxygen bridge, and at the same time one water molecule becomes attached to it. Cellulose hydrolysis therefore requires the presence of a water molecule.
• Organic or inorganic acids can be used in cellulose hydrolysis. In acid hydrolysis processes, high temperatures; (between about 100°C and about 200°C) or large acid concentrations, for example, 65% H 2 S0 4 (aq), have been used in the manufacture of nanocrystals.
• Patent publication RU 2281993 discloses a process for preparing microcrystalline cellulose by using gaseous hydrochloric acid (HCI).
• HCI gaseous hydrochloric acid
• the HCI gas is prepared separately by a reaction between concentrated liquid HCI and calcium chloride, after which the released gaseous hydrochloric acid is mixed with air and the mixture is transferred onto the cellulose substrate that is to be hydrolyzed.
• Patent publications EP 0248252 and FI 872378 disclose a process for preparing microcrystalline cellulose.
• Higgins et at. 1982 Higgins and Ho, Hydrolysis of cellulose using hydrogen chloride: a comparison between liquid phase and gaseous phase processes.
• Agricultural Wastes 4(2) :97-116, 1982 discloses the hydrolyzation of cellulose obtained from newsprint, paperboard, and wheat straws by using both liquid 41.7% hydrochloric acid and gaseous hydrochloric acid.
• Patent publication WO 1996/025553 discloses a method and equipment for hydrolyzing lignocellulose material under pressurized conditions at a temperature between 160°C and 230°C.
• the object of the invention is to solve the above-mentioned problems. More specifically, the object of the invention is to provide a process for chemically modifying cellulose material, by means of which the micro- or nanocrystalline cellulose material can be dispersed without separate dispersing agents.
• the object of the invention is reached by a process for preparing micro- and/or nanocrystalline cellulose material in the presence of an acid in the gas phase, the process comprising steps, in which the cellulose material is hydrolyzed in the presence of at least one acid in the gas phase, the moisture content of the cellulose being between 1% and 80%, and the hydrolyzed cellulose material is mechanically treated in order to obtain micro- and/or nanocrystalline cellulose material.
• the surface of the micro- and nanocrystals is modified, so that the crystals would form a homogeneous dispersion in water.
• nanocrystals to be dispersed in water can be obtained.
• New functional groups can be introduced through chemical reactions onto the surface of micro- and nanocrystals either by pre- and/or post-treatments.
• the cellulose material can be modified before and/or after the hydrolysis.
• a good dispersion of nanocrystals in aqueous solution requires electrostatic repulsion between the individual cellulose nanocrystals.
• the surface of the nanocrystals is modified to provide hydrophobicity.
• an acid can be used having such a vapour pressure that it gasifies into the gas phase and adsorbs onto the cellulose surface at room temperature.
• one of the following acids can be used: hydrochloric acid (HCI), nitric acid (HN0 3 ) and/or trifluoroacetic acid.
• HCI hydrochloric acid
• N0 3 nitric acid
• trifluoroacetic acid trifluoroacetic acid.
• the concentration of the acid used can be at least 1% by volume.
• the hydrolyzed cellulose can be dispersed in water or in a suitable solvent, such as formic acid or ethyl acetate. Hydrolyzed cellulose can be mechanically broken down, which can comprise for example mechanical stirring and/or ultrasound treatment.
• a further object of the invention is a product that contains micro- and/or nanocrystalline cellulose material.
• the invention also relates to a cellulose product prepared by the said process of the invention and the use thereof in food applications and in optical, cosmetic and medical applications.
• hydrolysis speed is relatively high at room temperature and normal atmospheric pressure.
• a gaseous acid such as hydrochloric acid or nitric acid, breaks the cellulose down into micro- and/or nanocrystals. The hydrolysis speed of hydrochloric acid is higher than that of nitric acid due to a greater gas pressure.
• Fig. 1 shows an experimental arrangement for hydrolyzing cellulose in the presence of gaseous hydrochloric acid (HCI).
• HCI gaseous hydrochloric acid
• Liquid HCI is placed on the bottom of the desiccator.
• the cellulose material here a filter paper made of cotton, is hydrolyzed in the desiccator in the presence of gaseous hydrochloric acid.
• Fig. 3 shows the change of cellulose's degree of polymerization (DP) as a function of time with different hydrochloric acid concentrations.
• Fig. 4A shows the development of cellulose's degree of polymerization (DP) in a sample of cotton cellulose (Whatman 1 filter paper) in the presence of gaseous HCI.
• Fig. 4B shows an atomic force microscopy (AFM) 5 ⁇ 5 ⁇ 2 image of cellulose nanocrystals made from the filter paper hydrolyzed with HCI gas and dispersed in formic acid.
• AFM atomic force microscopy
• Fig. 5 shows the development of cellulose's degree of polymerization (DP) in a sample of cotton cellulose (Whatman 1 filter paper) in the presence of gaseous HN0 3 .
• the acid concentrations of the liquid phase are 7.7 and 15.4 mol/l.
• Fig. 6 shows the adding of sulphate groups onto the surface of cellulose (55% H 2 S0 4 , 60°C, 2h), when the substrate (filter paper) is first (A) hydrolyzed for 3 hours with 35% HCI vapour or (B) left untreated.
• Fig. 7 shows AMF images of a cotton fibre hydrolyzed with HCI and dispersed in formic acid (A) 5x5 Mm 2 and (B) 2x2 ⁇ 2 .
• Fig. 8 shows AMF images of a cotton fibre hydrolyzed with HN0 3 and dispersed in formic acid (A) 5x5 pm 2 and (B) 2x2 pm 2 .
• Fig. 9 shows a transmission electron microscopy (TEM) image indicating that in formic acid is made to disperse nanocrystals prepared with acid vapour and having the same length (7 nm) as when prepared with liquid acid.
• TEM transmission electron microscopy
• Fig. 10 shows an AMF image of a cotton fibre hydrolyzed with HN0 3 and dispersed in ethyl acetate.
• the invention relates to a process for preparing micro- and nanocrystalline cellulose material in the presence of an acid in the gas phase.
• the invention also relates to a cellulose product prepared by said process and the use thereof in food and liquid crystal applications as well as in optical, cosmetic and medical applications.
• cellulose material refers to any cellulose raw material source. Almost any types of cellulose raw materials are suitable as a cellulose material for the method and process of the present invention, as described below.
• the cellulose material used in the invention can be obtained from wood-based or non-wood-based material.
• cellulose material that comprises pulp, such as chemical pulp, mechanical pulp, thermomechanical pulp or chemithermomechanical pulp.
• the cellulose material can be based on any plant material containing cellulose.
• the plant material can be wood-based or non-wood-based.
• the wood can be of softwood, such as spruce, pine, fir, larch, Douglas spruce or hemlock, or of hardwood, such as birch, aspen, poplar, alder, eucalyptus or wattle, or it can be a mixture of soft- and hardwoods.
• the non-wooden material can be from agricultural remnants, grasses or other plant materials, such as straws, leaves, bark, seeds, peels, flowers, vegetables or fruits of cotton, corn, wheat, oat, rye, barley, rice, flax, hemp, abaca, sisal, jute, ramie, kenaf, bagasse, bamboo or cane.
• the cellulose material can be of cellophane.
• the cellulose can also originate from prochordata (Urochordata or Tunica ta).
• Cellulose “nanocrystals” (whiskers) are nano-sized rods of crystalline cellulose. Nano- and microcrystalline cellulose differ from each other in their particle size. Nanocrystals are single cellulose crystals: their width corresponds to the width of the native cellulose microfibril, about 3-10 nm (e.g. 7 nm in cotton cellulose) and their length the length of the crystalline area in the microfibril, about 50-2000 nm (e.g. between 5 nm and 300 nm in cotton cellulose) depending on the source of cellulose. The diameter of microcrystalline cellulose particles is several micrometers or tens of micrometres.
• cellulose microcrystals consist of nanocrystalline particles, i.e. microcrystals are aggregates of nanocrystals. Microcrystals are easier to manufacture than nanocrystals, since with microcrystals there is no need to control or prevent the aggregation that easily occurs.
• cellulose nanocrystals are commonly referred to as cellulose nanocrystals, nanocrystalline cellulose, cellulose whiskers, cellulose nanowhiskers.
• nanocrystalline cellulose has also been referred to as microcrystalline cellulose.
• nanocrystalline cellulose has an exact reaction window. In case of microcrystalline cellulose, there is no desire to achieve a stable dispersion. Microcrystalline cellulose must be filtered. Nanocrystalline cellulose requires dialysis.
• nanocrystals are prepared by hydrolysis in about 64% by weight liquid sulphuric acid.
• the reaction is stopped with a 10-fold dilution, followed by centrifugation, dialysis, ion exchange and dispersion by ultrasound.
• the hydrolysis with H 2 S0 4 is not an environmentally friendly process.
• the process requires a great amount of water and when washing reaction products, a great amount of waste water is formed.
• Use of high liquid acid concentrations also involves a recycling problem: since the acid must be washed out of the micro- and nanocrystals, it is substantially diluted and cannot be completely recovered.
• the process is quite laborious and precautionary measures are needed, because the sulphuric acid is quite caustic.
• the surface of the nanocrystal can be charged or it can be to some extent neutral, depending on the hydrolizing acid.
• the acid hydrolysis with a sulphuric acid introduces negatively charged sulphate groups (S0 4 ) onto the surface of the nanocrystals.
• S0 4 negatively charged sulphate groups
• the surface of the nanocrystals is neutral and the nanocrystals tend to form aggregates. Without surface modification, the cellulose nanocrystals have a tendency to agglomerate.
• the neutral cellulose nanocrystals can be dispersed homogeneously in formic acid using vigorous ultrasonication.
• Cellulose nanocrystals are mainly neutral, depending on the hydrolyzing acid to be used.
• the cellulose nanocrystals can be modified by esterifying, adding palmitate groups (performed with gaseous palmitoyl chloride) or acetylating with gaseous trifluoroacetic acid into hydrophobic.
• Gaseous hydrochloric acid causes the degree of polymerization to decrease at room temperature, which is required for the formation of micro- and nanocrystalline cellulose material.
• the decrease of the degree of polymerization can take place within a few hours, preferably in 30 minutes.
• Micro- and nanocrystals are obtained by mechanical breaking down following the hydrolysis, for example, by mechanical stirring and/or a following ultrasound treatment.
• a gaseous acid it is possible to avoid several of the environmentally harmful micro- and nanocrystalline cellulose material manufacturing steps, which may also be difficult to apply on industrial scale. Large amounts of water are not needed for rinsing the sample, recycling of the acid is easier and the dialysis used for purification can be omitted. Therefore recycling of the material and its processability are improved.
• the cellulose On the surface of the cellulose fibre, there is a thin water layer, meaning that a high local acid concentration is formed when the gaseous acid adsorbs onto the fibre surface.
• the cellulose is dry, but not entirely dry.
• the moisture content of the cellulose is between 1% and 80%.
• the moisture content of the cellulose is between 1% and 10%, more preferably between 2% and 9%, and even more preferably between 3% and 8%, between 4% and 7%, or between 5% and 6%.
• the absolute amount of water on the fibre surface is so small that the local H 3 0 + concentration is high. This leads to a surprisingly high breakdown rate.
• dry hydrolyzed surface-modified micro- and/or nanocrystalline cellulose substrate can be obtained, from which mono- and oligosaccharides, such as sugars, can be separated by using water, and the cellulose can be mechanically broken down.
• One embodiment of the invention provides a process for preparing micro- and/or nanocrystalline cellulose material in the presence of an acid in the gas phase, the process comprising steps in which the cellulose material is hydrolyzed in the presence of at least one acid in the gas phase, the moisture content of the cellulose material being between 1% and 80%, the cellulose material is surface-modified, and the hydrolyzed cellulose material is mechanically treated in order to obtain micro- and/or nanocrystalline cellulose material.
• the inventive process can comprise a step, in which water-soluble mono- and oligosaccharides can be separated from the hydrolyzed cellulose by extraction.
• Mono- and oligosaccharides can comprise, for example, glucose and its oligomers, arabinose, xylose, mannose and other disintegration products of hemicelluloses.
• Mono- and/or oligosaccharides for example, sugars, can be separated from the hydrolyzed cellulose by extraction. The TOC of the sugars is determined, they are analyzed by HPLC and the amount of eventual residual acid is measured. The separated sugars can be further fermented into ethanol.
• the surface modification of the cellulose material can be chemical or physical modification.
• the chemical modification can be based on, for example, acetylation, carboxymethylation / oxidation, esterification or etherification reactions of cellulose molecules.
• the modification can also be performed by a physical adsorption of anionic, cationic or non-ionic agents or any combination of thereof onto the surface of cellulose.
• the described modification can be performed before, after or during the acid hydrolysis of the cellulose material.
• the chemical modification can comprise, for example, a TEMPO oxidation, acetylation and/or carboxymethylation.
• the cellulose material can be comprised of labile chemically modified pulp or cellulose raw material.
• TEMPO 2,2,6,6-tetramethylpiperidine-l-oxyl
• CH 2 )3(CME 2 )2NO is a chemical compound of formula (CH 2 )3(CME 2 )2NO. It is a stable radical, and it is used as a catalyst in oxidation of primary alcohols into aldehydes.
• carboxyl groups are introduced onto the surface of a nanocrystal and thus it disperses homogeneously in water.
• N- oxyl-mediated oxidation for example with 2,2,6,6-tetramethyl-l-piperidine-N-oxide (TEMPO), can lead to a very labile cellulose material.
• TEMPO 2,2,6,6-tetramethyl-l-piperidine-N-oxide
• Acid hydrolysis of the carboxymethylated pulp with gaseous HCI introduces charges onto the surface of the nanocrystal and thus it disperses in water.
• the TEMPO- oxidated and carboxymethylated pulp is hydrolyzed in HCI vapour and thereafter dispersed in water or other solvents, many process steps can be avoided.
• Mechanical treatment refers to mechanical breaking down, such as grinding, crushing, dispersing, ultrasound treatment or other breaking down, but not limited to these.
• the mechanical treatment can be performed by any known method of breaking down.
• the mechanical treatment can be performed by a suitable apparatus, such as a refiner, grinding machine, homogenizer, crusher, friction grinding machine, fluidizer, such as a microfluidizer, macrofluidizer or fluidizer-type homogenizer or an ultrasound sonicator.
• the process according to the invention is preferably performed under atmospheric conditions, i.e. normal atmosphere and normal pressure.
• the process can be carried out at a temperature that is between 0°C and 100°C, preferably the temperature is between 10°C and 40°C, more preferably between 15°C and 35°C, most preferably between 18°C and 22°C.
• the temperature is, for example, room temperature.
• the process uses at least one acid in the gas phase.
• the vapour pressure of the gas used in the invention is such that it gasifies at room temperature, so that the acid adsorbs onto the cellulose surface.
• the acid hydrochloric acid, nitric acid and/or trifluoroacetic acid.
• the concentration of the acid can be at least 1% by volume.
• the concentration of the acid in the gas phase is between 2% and 99% by volume, for example, 15% by volume, 38% by volume, or 68% by volume.
• the natural vapour pressure of the acid/water mixture for example, a HCI/water mixture, which is sufficiently great to gasify the HCI into the vapour phase can be utilized in the inventive process, meaning that a separate reaction is not needed.
• HCI can also be transferred onto a cellulose substrate without separate mixing with air and without mechanical flow.
• the dispersion of nanocrystals in a hydrophobic solvent such as toluene or chloroform
• a hydrophobic solvent such as toluene or chloroform
• the cellulose material used as the starting material is modified and/or treated before hydrolysis.
• the hydrolyzed cellulose can be modified and/or treated after hydrolysis.
• Pre- and post- treatments can contribute to dispersion and/or enable the different applications of the nanocrystals. Examples of treatments before and after the hydrolysis include hydrofobization, TEMPO-mediated oxidation and carboxymethylation.
• the hydrolyzed cellulose can be dispersed using suitable dispersion processes known in the art. Mechanical dispersion processes can also be used in the process of this invention.
• solvents, in which the cellulose material can be dispersed include formic acid and ethyl acetate.
• the cellulose hydrolyzed with HCI can be dispersed in formic acid using vigorous ultrasonication after the cellulose nanocrystals are refined into powder.
• the cellulose material hydrolyzed with nitric acid can be dispersed in formic acid using vigorous ultrasonication after refining.
• the HCI in the gas phase hydrolyzes the TEMPO-oxidated pulp into cellulose nanocrystals. This is due to the fact that the TEMPO oxidation adds carboxyl groups onto the surface of the nanocrystal and thus is disperses homogeneously in water or other solvents.
• the HCI in the gas phase hydrolyzes the carboxymethylated pulp into cellulose nanocrystals, producing a charge onto the surface of the nanocrystal and contributing to the dispersion in water or other solvents.
• the inventive process can be carried out within a few hours, preferably, for example, in 30 minutes.
• the viscosity of the hydrolyzed and broken-down cellulose is determined. Also the following analysis tools can be used in the inventive process: Cuen viscosity, gel permeation chromatography, and the CCOA method.
• An object of the invention is also a product that contains micro- and/or nanocrystalline cellulose.
• the invention also relates to a cellulose product prepared by the said process and the use thereof in food and liquid crystal applications as well as in optical, cosmetic and medical applications.
• micro- and/or nanocrystalline cellulose can be used, for example, in the following applications: water-based varnishes in hardwood floorings, iridescent NCC films in security papers, architectonic applications and polymer reinforcements.
• a filter paper sheet of cotton cellulose (type Whatman 1, solids content about 95%) was placed in an desiccator with a small amount of liquid hydrochloric acid (HCI) on the bottom.
• the test conditions comprised normal air pressure and room temperature.
• the acid used was 2.1%, 15%, 68%, and 99% HCI in the gas phase, corresponding to 20%, 25%, 30% and 37% liquid acid, respectively.
• Table 1 shows the concentrations of gaseous hydrochloric acid as compared to the corresponding liquid hydrochloric acid.
• Table 1 The concentration of hydrochloric acid in liquid and gas phase in normal air pressure and room temperature.
• the experimental arrangement is shown in Fig. 1.
• the natural vapour pressure of liquid HCI was great enough to gasify the HCI into the gas phase at room temperature and transfer the HCI onto the cellulose substrate, which was cotton cellulose.
• the results can be analyzed, for example, by the following methods:
• CCOA measurements indicated that no oxidation of cellulose occurred in the system during the hydrolysis of gaseous HCI.
• Fig. 2 shows the results of the acid hydrolysis when using 2.1%, 15%, 68%, and 99% gaseous hydrochloric acid, corresponding to 20%, 25%, 30%, and 37% liquid hydrochloric acid.
• concentration of the hydrochloric acid was increased at room temperature, the LODP point was reached more rapidly. Nanocrystalline cellulose was formed, when the LODP value exceeded 100.
• Table 2 shows the hydrolysis time, viscosity and DP V with different acid concentrations.
• Hydrolyses were performed in a vacuum desiccator at a normal air pressure and room temperature.
• the liquid hydrochloric acid, HCI (35%) or nitric acid, HN0 3 (65%) was placed on the bottom of the desiccator and allowed to evaporate.
• the replacement of the excess air with acid/vapour mixture was enabled by opening and closing the desiccator valve repeatedly over 6-12 hours.
• the hydrolysis time needed to achieve nanocrystals with 35 % HCI by weight was 3 hours. When 15.5 mol/l HN0 3 was used, nanocrystals were formed within 24 hours (Figs. 4 and 5).
• Adding of sulphate groups to the hydrolyzed filter paper was performed with 55% H 2 S0 4 at 60°C for 2 hours.
• cellulose nanocrystals were obtained by a HCI acid hydrolysis and gaseous nitric acid.
• the atomic force microscopy (AMF) image of Fig. 6A shows that stable dispersions of nanocrystals can be obtained, when sulphate groups are added to the cellulose substrate hydrolyzed with gaseous HCI (35%, 3h).
• Fig. 6B is demonstrated that, when the pre-hydrolysis is not performed with an acid in the gas phase, nanocrystals are observed but fewer and, moreover, their sulphation is inadequate.
• the hydrolysis of cotton fibres was performed as in Example 2. After HCI(g) and HN0 3 (g) hydrolysis, the filter paper was refined into powder in a Wiley mill. Thereafter, the powder was dispersed in 85% formic acid (1 g/l dispersion) by using ultrasonic bath (Figs. 7 and 8). In formic acid were made to disperse nanocrystals prepared with acid vapour and having the same length (7 nm) as when prepared with liquid acid (Fig. 9).
• the hydrolysis of cotton fibres was performed as in Example 2. After HN0 3 hydrolysis, the filter paper was refined into powder in a Wiley mill. Thereafter, the powder was dispersed in ethyl acetate (1 g/l dispersion) by using ultrasonic bath (Fig. 10).
• the hydrolysis of a TEMPO pulp was performed in a vacuum desiccator at a normal air pressure at room temperature.
• the liquid hydrochloric acid was placed on the bottom of the desiccator and allowed to evaporate.
• the replacement of the excess air with acid/vapour mixture was enabled by opening and closing the desiccator valve repeatedly over 6-12 hours.
• the TEMPO pulp was broken down into nanocrystals, which disperse in water by ultrasound bath.
• the following treatments are performed after the hydrolysis, i.e. they are performed, as a LOPD value is achieved by an acid in the gas phase.
• the reaction can be performed at room temperature in negative pressure, provided that e.g. a vacuum pump is attached to the vacuum desiccator.
• a reaction time of 24 hours is adequate in order to modify the surface of any cellulose substrate.

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