The invention is on the field of bioprocessing technology

State of art and objectives

The objective of this invention is a method for production of concentrates of nanocellulose and microfibrillated cellulose, and possibilities for their use for technological purposes.

In publications and patents, tens of potential applications have been presented. Principal types of products marketed are nanofibrillated cellulose (CNF) produced by high intensity wet milling or pressure treatment, and crystalline nanocellulose (CNC) produced by acid hydrolysis. Starting material most often used has been wood based, usually also bleached cellulosic fibres. In addition, smaller amounts of nanocellulose from bacterial cultivations (BNC) is produced. High capital and production costs have been a barrier for applications in large scale. This has limited the use of nanocellulose principally to products with high added value.

In applications of nanocellulose, a criterion of quality is often mechanical properties of the final product, to improve these properties in the use, and often also to reduce amount of material needed for the purpose, and also to reduce costs thereby. Another common target is surface properties of the final product, especially permeability properties. For improving these properties, use of purified nanocellulose material is not always needed. Objective of this invention is preparation of concentrates which can be produced to more advantageous costs, and containing, besides nanometer-scale components, also other components contributing to mechanical or properties,

Nanocellulose and products containing it can be produced, in addition from wood-based materials, also from non-wood plants or parts of these. This enables exploiting biomass which is wild-grown or is by-product of agricultural production. When burned on the field it generates remarkable air pollution and health risks. When not collected and removed from the field it generates a corresponding amount of greenhouse gases.

In the Finnish patent application 20140067 (Nanorefix Oy) and the corresponding United States patent 1,273,632 B2, a method is presented for production of nanocellulose and its precursors from lignocellulosic feedstock which is in dry or air-dry state or in liquid organic media, using especially parts or by-products of non-wood plants, or products or by-products of industry processing cellulose. The objective of this invention is a method wherein separation operations are performed for their main part in water suspensions.

A great potential field of application is production of paper and cardboard. Above mentioned wood based microfibrillated products have been tested in several studies for improving mechanical properties. In the Finnish patent 127284 (Kemira Oy) an additive for this purpose is presented. It is prepared from non-wood plants and contains cellulosic microfibrils and clusters of microfibrils. Production steps consist of extraction in alkaline solution and homogenization.

Another great field for application is production of composites. Factors often mentioned limiting marketing for this purpose are the price of nanocellulose, its limited availability in large scale, and technical difficulties such as agglomeration, uneven distribution and weak contacting with hydrophobic matrices. Separated nanocellulose fibres and fibrils are known to have high tensile properties. Properties reached in composites have been weaker by orders of magnitude. Evident reasons for this are the short length of individual fibrils, tendency for agglomeration leading to uneven distribution, and hydrophilic properties of nanocellulose leading to weak contact to binding matrix.

The third potentially large application is reinforcing of concrete. The material mainly used in earlier studies has been crystalline nanocellulose (CNC), and the amounts used 3% of the cement or other binding ingredients. In recent studies (Hisseine et ak, J. Mater. Civ. Eng. 2018, 30(6):04018109) cellulosic filaments having diameter of 30 to 500 nm and ratio of length to diameter 200-5000 (U.S. Patent No 9051684 B2, 2015) have been tested. When the content of filaments has been 0.1-0.2 % of the amount of cement and fly ash, the level of mechanical properties reached has been by 25% higher than in the present concrete qualities. The method of the present invention enables to produce from by-product level lignocellulosic materials an additive of similar quality, to be used for concrete.

The fourth potentially important field of applications is using as components or starting material in electronic industries. Examples of possible applications are basic materials, separating membranes, materials for electrodes in batteries and supercapacitors, or production of electricity conducting components.

Brief description of the drawings

Figure 1. Separation of elementary fibrils from cellosin fibres, formation of micro and nanofibrils. Scale: unit equal to 1.2 pm.

Figure 2. Accumulation of nanofibrils on straw cellulose fibres. Scale: unit equal to 12 pm.

Figure 3. Fine structure of mixture of straw cellulose and nanocellulose. Scale: unit equal to 1.2 pm.

Figure 4. Composite of maize cellulose-nanocellulose-polyester. Distribution after impregnation of polyester matrix. Scale: unit equal tol2 pm.

Figure 5. Fine structure of composite after curing. Scale: unit equal to 1.2 pm.

Method

The method is based on separation of nanocellulose existing in parts of non-wood plants, or which can be economically produced from such materials, and application of it for improving mechanical or other functional properties of various materials.

An advantageous material for the purpose found so far is harvesting residues of maize. This feedstock is plentily available and can be collected to reasonable costs in maize cultivating countries. Cellulose of maize is already as such advantageous for improving tensile properties, based on the length fibres, low diameter, 10-15 pm, of fibre, and knowingly high crystallinity of it. In this study it has been found, that the crystallinity is high also in elementary fibrils and nanofibrils separated from different parts of maize. This appears as long and linear structures, which often are also oriented with each other. Another advantageous material for feedstock is straws of cereal plants. Possible raw materials are also other cultivated or wild-grown plants and various parts of them which have a tubular structure, such as stalks, leaves, grass, bagass and bamboo. Feedstock can also be cellulosic fibre from by-products of wood material processing industry. From fibrils of it elementary fibrils are separated in this process.

In fractionating lignocellulosic material from non-wood sources using low temperature alkaline extraction it has now been observed surprisingly, that thin cellulosic filaments, nanocellulose, elementary fibrils and small-sized particles are separating during the extraction and subsequent homogenization. Separation now started is long-lasting. These particles bind cellulosic fibres or fibrils by bonding transversely (Figure 1). The main part of nano-sized particles is transferred in the black liquor, another part is adsorbed on cellulosic fibres.

After the alkaline extraction, refining and defibration stages the cellulose - nanocellulose fraction obtained is separated using methods known as such, such as screening or filtration, and pressed to consistency having 15 to 20% of dry matter. The filter cake obtained is stored at a temperature below 5°C until further treatments. This partial drying is sufficient to retard and inhibit formation of great aggregates, and the product is easily hydrated to be used for various applications.

Within the fibre network hydrogel and network of nanocellulose is now formed, and it follows the fibre network also at washing and refining stages. Cellulose network supports the nanocellulose network and hydrogel. Formation of microfibrils from elementary fibrils remaining in the suspension and their connecting to macrofibres continues, resulting to sheets of microfibril networks connected to fibres. (Figures 2 and 3). At the filtering stage macro- scaled fibres and particles fixed on them are most rapidly precipitating. They form a tight layer restraining the flow of nanosized particles. By interrupting the filtration or by retarding it, a hydrogel is formed from slowly precipitating particles. At further removal of water, a part of the hydrogel remains in the half-dry cellulose fraction, another part forms in the filter cake an aerosol consisting of nanocellulose supported by macro scale fibres.

In applications performed under aqueous conditions such as production of paper, cardboard or concrete, pressing cake is dispersed in water to a concentration used in the application, for example in the range of 0.2% in production of paper and cardboard. The dispersion obtained can be used as an additive for improving mechanical properties of paper, cardboard or concrete, or it can be screened or filtered and dried as such to paper or cardboard or to porous thin films or membranes having a surface containing nanocellulose. There is also a potential demand for such films or membranes in various applications in electronic industries, and in applications based on a high adsorption and absorption properties due to a large specific surface area.

In composite applications where the matrix is hydrophobic and no addition of water is included, an advantageous method is lamination. An applicable way for performing is converting the fibre mixture to a paper-like foil where nanocellulose is distributed as evenly as possible. The material is dispersed in water to a suspension containing 0.2 - 1 % of it, homogenized by blending with cutting action during 2 - 10 minutes and continuing the blending with no cutting action at least 2 minutes. The suspension is screened through sieves with pores of 0.05 - 0.30 mm. When needed, the screening can be performed with intermittent delays or retarding the flow to improve the yield of nanocellulose. Water separated can be removed using air pressure or vacuum filtration, water remaining in the fibres by microwave treatment or by solvent exchange. The network of macro scale fibres supports the hydrogel by preventing or limiting its collapsing. This support continues in the aerogel formed after removal of water or solvent.

Before impregnating the hydrophobic matrix, water has to be removed from the fibre material as completely as possible. Using conventional methods, removal of bound water is not possible. It can however be removed by microwave heating or by absorbing in a solvent. This drying leads also to diminution of volume, whereby the material in the foil shrinks, the form following the shape of the mould. A layer of nanocellulose is formed on the surface and protects from absorption of moisture under usual living room conditions.

To the now dried foil, selected matrix is now impregnated, and the desired lamination is performed. Impregnation shrinks pores of the foil, and thus an even impregnation is possible only into thin foils or fibres, depending on pressure differences and viscosity of the matrix. Unless porous composite is desired, removal of air in the pores has to be ensured using methods such as pressure, vacuum or high temperature, Curing of the composite is performed following the conditions provided for the matrix used. The method is thus applicable to be performed as manual operations, pultrusion or comparable lamination operations. Applicable stages of these operations can be uses also in other composite production methods known as such.

Example 1. Thin paper from maize and soya

From cellulose of maize stalk prepared by extraction with 3% sodium hydroxide at 102°C, a suspension in water containing 0.6% dry matter was prepared. To this suspension, 0.2% of finely milled soya hull flour was added. The mixture was homogenized with domestic blendor during 5 minutes, and casted for drying at room temperature. A membrane with a thickness of 0,1 mm and area weight of 14.9 g/m ² , resistant for handling was obtained. In microscopic observations, besides cellulosic fibres a three dimensional microfibril network containing also linear microfibrils with a length of over 5 cm was found.

Example 2. Porous paper from straw cellulose and nanocellulose

2.7 g of rye straw was cut in pieces and suspended in 200 mL of 3% sodium hydroxide solution. The mixture was extracted at 102°C for two hours, and the cellulose fraction was separated by sieving through cloth with pores of 0.25 mm. The pulp was suspended in 300 mL of water, homogenized for 2 minutes with cutting domestic blender, and the solid material was separated by sieving. 25% of the homogenized wet pulp was suspended in 180 mL of water, and homogenized further with non-cutting blending for 2 minutes, and the solid material was separated by sieving at atmospheric pressure. The precipitate containing water not separated was left on the sieve for 15 minutes at atmospheric pressure, to reinforce the hydrogel formed in it. After this time, 25 mL of acetone was added to the mixture for removing water. The precipitate now solidified was separated, and removing water was continued by immersing it in 25 ml of acetone for 30 minutes. Supporting veil tissues were removed and the precipitate was left to dry at room temperature. After 6 hours, it had dried to a stiff layer with a thickness of 0.45 mm, surface weight of 53 g/m ² and volume weight of 0.122 g/cm ³ . A great part of the nanocellulose had been adsorbed on cellulosic fibres as sheet-formed networks and small agglomerations, another part was as thin networks or elementary fibrils and small agglomerations between the fibres (Figures 2 and 3). After two days microfibril networks had been expanded leading to more even distribution in the mixture. During storing at room temperature, the weight of the stiff layer sample had not altered, but the thickness had increased to 0.5 mm. In the fine structure, plenty of linear, mutually oriented microfibrils were observed. After 9 days of curing their lengths were between 40 - 150 pm.

Example 3. Preparation and testing of paper sheets

30 g of leaf material of maize post-harvest residue was cut in pieces, mixed in 300 mL of 3% sodium hydroxide and extracted at 102°C for 90 minutes. Black liquor was separated by screening, the solid material was diluted to 1000 mL and homogenized with cutting domestic blender for 2 minutes, The mixture was screened, the solid material diluted with water to 500 mL and homogenization repeated. After 5 minutes the mixture was screened, and separating water was pressed from the cake. The wet weight of the precipitate after these operations was 85 g, its dry matter content 15.6%. The cake was stored at +4°C.

37 g of nearly harvest ripe rye straw and 19 g of harvest ripe rye straw were cut in pieces and extracted in 600 mL of 3 % sodium hydroxide solution at 102°C for 120 minutes. Screening, washing and homogenizations were performed as above for maize material, In the first homogenization the liquid volume was 1000 mL and the duration 5 minutes, in the second homogenization the volume was 800 mL and duration 3 minutes. The wet weight after the treatments was 136 g and dry matter of the cake 19.5%. The cake was stored at +4°C.

As control sample and basic material for the mixtures, Kraft cellulose made of mixture of pine and spruce (Botnia PineRma) from Metsa Fibre Oy, Rauma, Finland, was used. It was ground in water suspension one day before its use in the experiments by a Valley grinder (ISO 5264). Grinding time was 20 minutes, and milling density 15.6 g/L. For testing mechanical properties, following hand sheets were prepared:

1. Control, pine-spruce cellulose

2. Straw cellulose and nanocellulose

3. Maize cellulose and nanocellulose

4. Mixture 80% pine-spruce and 20% straw-nanocellulose

5. Mixture 80% pine-spruce and 20% maize cellulose.

The sheets were dried at 90°C, and stored at 50% relative humidity conditions. To follow the developing mechanical properties, two sheets of each sample were tested after two weeks, three sheets after three weeks from preparing. Results are presented in Table 1. The results indicate that the mechanical properties of sheets of straw cellulose, as such as in mixture with softwood cellulose are on the same level as those of softwood cellulose. Sheets prepared from maize leaf fraction cellulose as such have mechanical properties which are weaker than those of softwood cellulose, probably due to lower proportion of long fibres, but in the 20% mixture with softwood cellulose maize cellulose has elevated the tensile strength by 12.3%, and tensile energy absorption by 33%.

Example 4. Preparation of composite

From the maize leaf fraction cellulose prepared according to Example 2, stored in refrigerated conditions, a dispersion of 0.2% of it in water was prepared. The suspension was filtered through a tissue having pores of 0.25 mm, initially at atmospheric pressure. For reinforcing the hydrogel formed it was left standing for 3 minutes, after which water was removed from it by two subsequent treatments with acetone, and the foil was left to dry at room temperature. A sample of 0.2 g was cut from it, and impregnated with 1.2 g of polyester (Biltema, Helsingborg, Sweden, Art. 3-075), added with 3% of hardening (2-butanoperoxide, Art. 36- 077). The sheet was cured for one day at room temperature under over-pressure of 0.1 bars for removing air, and after it at a temperature of 120°C without over-pressure for two hours. The content of fibre material in the final product was 16.7%. In microscopic study, a part of the nanocellulosic material was found to be bound to microfibres and fibrils, the remaining part was as a fine structure forming a homogenic network of nanofibrils (Figures 4 and 5). Separate agglomerates having diameters higher than 50pm were not observed. Micro and nanofibrils formed were often as tranverse binding of cellulose fibres reinforcing their network, A part of them had formed an aerogel within this network, from which during storage nano and microfibrils were further separated forming bonds in the structure.

Table 1. Mechanical properties of paper samples after storage.