This invention relates to the preparation of cellulose nano-fibres from stalks of annual crops.

From the patent application US 20080146701 a method is known of preparing cellulose nano-fibers from a fibrous pulp derived from natural fibers, root crops, fibre crops which contain hemicellulose, lignin, pectin and mineral substance. The method comprises several consecutive steps of preparing the nano-fibres: hydrolysis of the fibrous pulp by means of a diluted acid at temperature of 70 - 90°C, extraction in alkaline medium, cryogenic disintegration, and defibrillation in a homogenizer at high pressure. The obtained nano- fibers may be used as reinforcement in resin composites prepared by film casting, forming or extrusion for application in medical devices and packaging.

From the patent description EP 2 415 821 known is a method of preparing a cellulose gel. The method consists in that the cellulose material is first treated with an oxidizing agent generating carboxylic groups on the cellulose surface. The suspension of the cellulose material is then homogenized and the obtained suspension of nano-fibers is treated with an organic-or inorganic acid to cause the cellulose nano-fibers aggregate into a gel. In order to solidify the gel, it is subjected to UV irradiation or to the action of a cellulase preparation or ozone and peroxides. The liquid gel is again mechanically processed. The outcome is a gel which contains cellulose nano-fibres holding free carboxylic groups on its surface. The prepared cellulose gel reveals low water absorption and swelling due to that the unreacted carboxylic groups are not as prone to dissociation as those reacted to salts.

Patent US 8546558 describes a method of preparing micro-fibril cellulose from a hemicellulose-containing sulfite pulp derived preferably from coniferous wood. The method involves mechanical grinding, action of hemicellulase or cellulase or mixture of the two and a further mechanical homogenization. Prospective uses of the micro- fibril cellulose are in food, cosmetics, pharmacy, paper, composites, coatings and thickeners.

A method to make cellulose nano-fibrils is described in the international patent application WO 2010092239. The method provides preparation of a suspension of cellulose fibers upon which adsorbed is a cellulose derivative (e.g. carboxymethylcellulose) or a polysaccharide or its derivative. The fibers are then disintegrated to nano-fibrils. Chemical-, mechanical-, thermo-mechanical or chemical- thermo-mechanical cellulose pulp was used as starting material. It was derived from wood, non-woody materials or recycled cellulose fibers. The nano-fibrils sized below 1 micrometer find their uses in food, concrete, drilling fluids, coatings, cosmetics, drugs and paper.

Known from the international patent application WO 2013188657 is a method for the manufacture of nano-fibres from a cellulosic material by processing of an aqueous suspension with a cellulose depolymerizing agent such as ozone, cellulase or a combination of the two, and a parallel or subsequent mechanical disintegration which delivers nano-fibers from the cellulosic material. Low energy consumption is an asset of the method in comparison to processes employing only mechanical disintegration. Nano fibers from the method find their use in paper making.

The method of preparing cellulose nano-fibers from stalks of annual plants according to the invention consist in that the stalks cut into pieces 10-70 mm long are processed in following steps:

a) action of water vapor at 1 ,5-3 bar for 5-60 minutes

b) mechanical defibrillation in a disk mill for 3 - 10 minutes

c) digestion in a digesting liquor containing 5-15wt% of NaOH on dry mass of the stalks; liquor module from 3:1 to 8:1, time 60 to 120 minutes, temperature 110- 160°C

d) delignification with oxygen in a medium containing 2-4 wt% of NaOH and 0,2-0,5wt% of MgS0 ₄ on dry mass of the stalks. Conditions: oxygen pressure at start-0,2 - 0,5 MPa, pulp concentration 8-15wt.%, time 60 - 120 minutes, temperature 110 - 120°C e) bleaching at least twice with NaC10 ₂ at concentration of 1 - l,5wt.% and H ₂ S0 ₄ concentration of 0,05 - 0,lwt.% at 60 - 70°C for 60-120 minutes, followed by alkaline extraction in aqueous NaOH with concentration of 0,5 - 1 wt% at 70 - 80°C, for 60- 120 minutes

f) enzymatic treatment of the prepared cellulose pulp by cellulase at activity of endo-l,4-beta-glucanase of 10-1000 J/g on dry mass ; pH of 4-7, temperature 40 - 60°C, time 30-480 minutes, agitation-50 - 500 followed by washing of the cellulose pulp with demi water, and vacuum filtration, and deactivation of the residual enzyme in the pulp at 90 - 125°C, for 10 - 30minutes

g) homogenizing of the aqueous cellulose suspension with concentration of 0,5 - 3 wt%, at 5000 - 20000 rpm for 1 - 10 minutes. During the agitation, the suspension is quenched with refrigeration brine at about -15°C. Next, the suspension is centrifuged at 4000 - 15000 rpm for 5-30 minutes to obtain a supernatant containing cellulose nano-fibres.

The digesting in step c) can, according to the invention, be run in a liquor with a content on dry mass of 5 - 15 wt.% of NaOH and 0,1 - 0,5 wt.% of anthraquinone or a liquor with 5 - 15 wt% of NaOH, 3 - 6 wt% of H ₂ 0 ₂ , 0.1 - 0,3 wt% of ethylenediaminetetraacetic acid (EDTA), 0,2 - 0,5 wt% of MgS0 ₄ and 2 - 5 wt% of water glass.

The cellulose pulp is, according to the invention, in addition to the bleaching in step e) preferably put to de-lignification by the action of 1 - 2 wt% of peracetic acid and 0,2 - 0,5 wt% of MgS0 _4> calculated on dry mass. Conditions: temperature of 80 - 90°C for 60-120 minutes.

The cellulose pulp holding about 50 - 70 wt% of water is, after the bleaching in step e) and prior to the enzymatic treatment in step f), preferably put for 20 - 90 minutes to mechanical defibrillation followed by homogenizing in water suspension.

According to the invention, the sediment formed after homogenization and centrifugation in step g) can be recirculated to step f) and put to repeated enzymatic treatment with a solution of cellulase. The multi-stage de-lignification combined with bleaching of the lingo-cellulose raw material, according to the invention, offers the chance of removing the non-cellulosic substance without a far-going degradation of cellulose thus providing a high output of cellulose pulp with excellent purity. Moreover, the multi-stage mechanical treatment of the pulp combined with enzymatic hydrolysis of cellulose makes it possible to prepare cellulose nano-fibres having a high proportion of length to thickness. The method according to the invention enables the utilization of agriculture residues such as linen and hemp waste straw remaining after the separation of seeds for oil expression.

The nano-fibres made according to the invention can be used as reinforcement in the production of composite materials, as thickener or constituent of a variety of coatings

The invention is illustrated with following embodiments which should not be construed as limiting the invention in any manner.

Example 1

Hemp stalks cut to pieces 40 - 60 long were loaded into a 15 dm swing digester and put for 10 minutes to the action of 2 bar steam and then three times defibrillated in the disk mill of Sprout- Waldron type with a consecutive decrease of the distance between the defibrillating elements (0,5, 0,2 and 0,1 mm). The slackened plant raw material was for 60 minutes digested at 110°C in a liquor containing: 5 wt.% of NaOH, 5,5 wt.% of H ₂ 0 ₂ , 0,3 wt. of EDTA, 0,5 wt.% of MgS0 ₄ and 5% water glass calculated on dry mass at fluid module of 4: 1.

The fibrous pulp was subjected to delignification with oxygen in a medium containing 2,5 wt% of NaOH and 0,5 wt% of MgS0 ₄ on dry mass with oxygen pressure at start of 0,2 MPa, pulp concentration 8wt%, for 60 minutes, at 120°C. After the delignification, the cellulose pulp was bleached twice with sodium chlorite at NaC10 ₂ concentration of l,4wt% and H ₂ S0 ₄ concentration of 0,1 wt% at 70°C, for 120 minutes. After every chemical treatment, the cellulose pulp was washed with demi water and centrifuged. The prepared cellulose pulp was characterized by lignin content of 0,45% and alfa-cellulose content of 85,5 wt%, and average polymerization degree of 1266. Total output of the process was 65,9%. An aqueous 2,5 wt% suspension of the cellulose in a conical flask was placed on the platform of incubator with a rotation shaker Labfors (Infers) and put to enzymatic treatment with cellulase (Ecostone L900, AB Enzymes) having endo-l,4-beta-glucanase activity of 500 J/g at pH of 4.8 at 50°C, for 180 minutes at agitation rate of 150 rpm. The processing finished and cellulose filtered on a Biichner funnel with filtration paper, the residual enzyme in the cellulose pulp was deactivated in an autoclave at 121 °C, for 20 minutes. The 1.5wt% aqueous suspension was for 10 minutes homogenized in the homogenizer Ultra Turrax T50 (IKA) at 10000 rpm. During the homogenization, the suspension was quenched with refrigerating brine at -15°C. Homogenization finished, the suspension was for 20 minutes centrifuged on centrifuge otina 420 at 4500 rpm to separate the cellulose- - nano-fibers- containing supernatant.

The prepared fibers were characterized by thickness in the range of 30 to 120 nm and average molecular degree of 130.

Example 2

Stalks of fire type flax cut to pieces 30 - 50 mm long were steam-processed and defibrillated as in Example 1 and, then, for 60 minutes at 160°C digested in a liquor containing 15wt.% of NaOH and 0,1 wt% of anthraquinone at fluid module of 7:1. Delignification, bleaching with sodium chlorite and alkaline extraction were carried out like in Example 1. The cellulose pulp was again put for 120 minutes at 80°C to delignification in a solution containing peracetic acid at concentration of 2 wt% and MgS0 ₄ at concentration of 0,5wt%. The obtained material was washed with demi water and centrifuged. The prepared cellulose pulp was characterized by lignin content of 9 wt% and alfa-cellulose content of 80,3 wt%, and average polymerization degree of 1306. Total output of the process was 69,5%. The cellulose pulp was put to mechanical treatment for 60 minutes at 40 rpm in a laboratory shredder. The l,5wt% aqueous suspension of cellulose was put to enzymatic treatment like in Example 1 save amount of enzyme which was 300 J/g. The residual enzyme in the cellulose pulp was deactivated like in Example 1. The l,5wt.% aqueous suspension of cellulose was homogenized by means of the homogenizer Ultra Turrax T50 in 3 cycles with the G45F tool and 3 cycles with W65SK tool at 10 000 rpm for 3 minutes in each of the cycles. During the homogenization, the suspension was quenched with refrigerating brine at about -15°C. The cellulose suspension was centrifuged as in Example 1 and the cellulose- -nano-fibers- containing supernatant was separated. The prepared fibers were characterized by thickness in the range of 40 to 160 nm and average molecular degree of 155.

Example 3

Stalks of oil flax cut to 30 - 60 mm pieces were steam-treated, defibrillated, digested, put to delignification in oxygen medium, bleached with sodium chlorite and alkaline- extracted like in Example 1. The prepared cellulose pulp was characterized by lignin content of 14 wt% and alfa-cellulose content of 72,4 wt%, and average polymerization degree of 1151. Total output of the process was 92,3%. The cellulose pulp was enzyme-treated like in Example 1. A 2,5 wt% suspension of the cellulose in water was next prepared and put to homogenization in the homogenizer Ultra Turrax T50 at 10 000 rpm for 3 minutes in each of the 5 homogenization cycles. During the homogenization, the suspension was quenched with refrigerating brine at about -15°C. The cellulose suspension was thereafter for 20 minutes centrifuged at 4 500 rpm. After the separation of the supernatant, the deposit was again put to enzyme treatment like in Example 1, employing cellulase with endo-l,4-beta-glucanase activity of 200 J/g. From the remaining sediment prepared was a 1,5 wt% cellulose suspension in demi water which was for 3 minutes mechanically processed in each of the 3 cycles by means of the homogenizer Ultra Turrax T50 at 10 OOOrpm. The suspension was in the consecutive step for 20 minutes centrifuged at 4 500 rpm. Supernatants from both steps of the processing were combined thus improving the output of the process. The prepared fibers were characterized by thickness in the range of 45 to 200 nm and average molecular degree of 190.