EP3350368A1

EP3350368A1 - Method for producing nanofibrillar cellulose

Info

Publication number: EP3350368A1
Application number: EP16779156.5A
Authority: EP
Inventors: Isko Kajanto; Mika Kosonen; Markus Nuopponen
Original assignee: UPM Kymmene Oy
Current assignee: UPM Kymmene Oy
Priority date: 2015-09-16
Filing date: 2016-09-16
Publication date: 2018-07-25
Anticipated expiration: 2036-09-16
Also published as: EP3350368B1; FI128901B; FI20155666A7; WO2017046455A1

Abstract

In a method for producing nanofibrillar cellulose starting from cellulose pulp (P), where cellulose is in native form, the cellulose pulp is prebeaten by circulating it in form of aqueous suspension of fibres through a refining gap (G) formed by opposite arrays of refining bars which perform relative movement in the gap. The cellulose pulp is circulated until a beating degree of below 55 ml CSF, preferably 50 ml CSF or less, more preferably 20 ml CSF or less is attained. The prebeaten pulp is forwarded to a disintegration process (2), where the prebeaten pulp is disintegrated to nanofibrillar cellulose (NFC).

Description

METHOD FOR PRODUCING NANOFIBRILLAR CELLULOSE Field of the application

The invention relates to a method for producing nanofibrillar cellulose starting from cellulose pulp.

Background

In the refining of lignocellulose-containing fibres by, for example, a disc refiner or a conical refiner at a low pulp consistency of about 3 to 4%, the structure of the fibre wall is loosened, and fibrils or so-called fines are detached from the surface of the fibre. The formed fines and flexible fibres have an advantageous effect on the properties of most paper grades.

Lignocellulose-containing fibres can also be disintegrated into smaller parts by detaching fibrils which act as structural components in the fibre walls, wherein the particles obtained become significantly smaller in size. The properties of so-called nanofibrillar cellulose thus obtained differ significantly from the properties of normal cellulose pulp. It is also possible to use nanofibrillar cellulose as an additive in papermaking and to increase the internal bond strength (interlaminar strength) and tensile strength of the paper or paperboard product, as well as to decrease the porosity of the paper or paperboard. Nanofibrillar cellulose also differs from pulp in its appearance, because it is gel-like material in which the fibrils are present in a water dispersion, and it is characterized by specific rheological properties in the form of gel. Because of the properties of nanofibrillar cellulose, it has become a desired raw material, and products containing it would have several uses in industry, for example as an additive in various compositions.

Nanofibrillar cellulose can be isolated as such directly from the fermentation process of some bacteria (including Acetobacter xylinus). However, in view of large-scale production of nanofibrillar cellulose, the most promising potential raw material is raw material derived from plants and containing cellulose fibres, particularly wood and fibrous cellulose pulp made from it. The production of nanofibrillar cellulose from cellulose pulp requires the decomposition of the fibres further to the scale of fibrils. Before the disintegration, pulp can be chemically treated to modify the molecular structure of the cellulose chemically. Nanofibrillar cellulose made from chemically treated pulp will in this case contain derivatized cellulose, whereas pulp where the cellulose is left unmodified will produce nanofibrillar cellulose in native form.

Thus, nanofibrillar cellulose is typically obtained by mechanical disintegration of cellulose pulp, carried out with suitable disintegration equipment. Mechanical disintegration is an energy consuming operation where the production capacity is limited. Thus several measures have been proposed for improving the disintegration process, such as modification of pulp prior to the disintegration. Said modification may comprise chemical modification of the cellulose to increase the susceptibility of the cellulose fibres to disintegration. Said chemical modification may be based for example on carboxymethylation, oxidation, esterification, or etherification of cellulose molecules. However, said chemical modification methods may result in grades of nanofibrillar cellulose, which are not desirable for all applications and thus also alternative methods have been studied, such as enzymatic treatment.

Paakko M. et al. in "Enzymatic hydrolysis combined with mechanical shearing and high-pressure homogenization for nanoscale cellulose fibrils and strong gels" Biomacromolecules 8 (2007) 1934-1941 disclose a method of producing cellulose fibrils using a combination of enzymatic hydrolysis and mechanical shearing. They report that previous attempts to prepare nanofibrillar cellulose only by extensive mechanical shearing resulted in that the homogenizer became blocked and the resulting material was non- homogeneous. However, the enzymatic treatment leaves traces of enzymes in the end product and an additional enzyme removal or inactivation step may be required before downstream applications. Additionally, the enzymes have a significant effect on the morphology of the cellulose nanofibrils: enzymatical pre-treament leads to decreased degree of polymerization, decreased length and decreased networking of the cellulose nanofibrils, and may lead to rod-shaped cellulose crystals or whiskers. Cellulose in its pure, original form, i.e. "native" cellulose is difficult to disintegrate from the level of fibers to the level of nanofibrillar cellulose, compared to derivatized cellulose where the original cellulose polymer chains are altered by chemical modification by adding functional groups. Especially oxidized and carboxymethylated celluloses are more prone to disintegration to fibrils, because the carboxyl and carboxymethyl groups labilize the supermolecular structure of cellulose. However, for some applications, there is need for nanofibrillar cellulose that is made up of cellulose in its native form.

Nanofibrillar cellulose is conventionally made from fibers in a fluidizer or homogenizer, commonly designated as "high pressure mechanical disintegration device". These types of devices contain narrow gaps for the passage of the pulp. If nanofibrillar cellulose is made from cellulose fibers made of native cellulose, the fibers tend to clog this type of equipment. Therefore, the native cellulose pulp is subjected to pre-beating to weaken the fibers mechanically so that they will be more flexible and will not block the equipment is not blocked by the fibers.

Further, to retain the maximum gel-forming ability of the nanofibrillar cellulose made up of native cellulose, it is desirable that there is minimum structural damage to the cellulose despite the high energy demand in disintegrating native cellulose fibers. During this process, cutting action is to be avoided. Native cellulose pulp has been conventionally pre-beaten in a PFI mill, which is a laboratory-scale equipment. PFI mill comprises a beater housing (stator) and in the housing a roll (rotor) equipped with bars and rotating excentrically with respect to the housing. The mill comprises also a loading device to enable a beating force per unit bar length. The batch of pulp to be beaten exists in the housing during the whole beating process. To ensure a desired beating degree (expressed as CSF), the PFI mill is run long enough, for example 9000 revolutions. The PFI mill is suitable for producing pre-beaten pulp of high purity, because the equipment, containing no pumps and pipes, is easy to clean. The major drawback is the poor capacity of the equipment, which is a bottleneck in large-scale manufacture of nanofibrillar cellulose from native cellulose fibers. Further, the mill works at medium consistencies, and the fibres typically suffer damages such as crimping under these conditions.

Summary

Accordingly, there exists a need to provide a method for production of nanofibrillar cellulose of high purity and with improved productivity.

Further, there exists a need to provide a method for effectively producing nanofibrillar cellulose where the cellulose is in native form. Native means that the original structure of the cellulose polymer is preserved in course of processing of the natural cellulosic raw material to cellulose pulp and subsequently to nanofibrillar cellulose, it being understood that some ionic species not being present originally in the natural cellulose may be bound to the native cellulose through ionic bonds. In other words, the cellulose is chemically underivatized cellulose. Having its origin in polymer found widely in nature, especially as structural component of plants, native cellulose has many advantages in various applications. Thus, cellulose pulp made from wide natural plant resources by retaining the cellulose in its underivatized, original chemical form is a preferable starting material for making the nanofibrillar cellulose by beating and disintegrating.

Further there exists a need to provide a method for production of nanofibrillar cellulose that has the cellulose in native form, is sterile and can be used in many applications in medicine and biology.

These needs are satisfied by a method where the cellulose starting material of natural origin and having the cellulose in native form is, prior to disintegrating it to nanofibrillar cellulose, subjected to beating to attain a beating degree that facilitates the subsequent disintegrating of the starting material to nanofibrillar cellulose. Prior to disintegration to nanofibrillar cellulose, the native cellulose pulp is beaten (pre-beating) to a beating degree that is below 55 ml CSF, preferably 50 ml CSF or less, more preferably 20 ml or less, to reduce or eliminate the risk of clogging in the disintegration process where the nanofibrillar cellulose is produced. The present application provides a method for producing nanofibrillar cellulose starting from hardwood cellulose pulp, where cellulose is in native form, comprising

- preheating the cellulose pulp by circulating the cellulose pulp in form of aqueous suspension of fibres through a refining gap formed by opposite arrays of refining bars which perform relative movement in the gap,

- circulating the cellulose pulp until a beating degree of below 55 ml CSF, preferably 50 ml CSF or less, more preferably 20 ml CSF or less is attained,

- forwarding the prebeaten pulp to a disintegration process, and

- disintegrating the prebeaten pulp in a disintegration process to nanofibrillar cellulose, wherein the preheating comprises

- preheating cellulose pulp in a conical refiner or disc refiner with a specific edge load of below 0.5 J/m.

The beating is performed in mild conditions so that no fiber cutting or fiber shortening occurs, contrary to conventional grinders such as "Masuko" grinders. The beating is performed in a refiner by continuously circulating the cellulose pulp through the refiner and by at the same time applying low specific edge load (SEL) in the refining gap, not higher than 0.5 J/m for hardwood.

Alternatively, the beating is performed in mild conditions in a hollander beater by continuously circulating the cellulose pulp through the hollander beater.

The beating is performed in clean conditions by arranging closed circulation for the starting material suspension (pulp) through the refiner or hollander beater. Closed circulation means that the pulp to be beaten flows along he circulation route isolated from the surrounding atmosphere to minimize the contaminations.

Performing the beating in clean conditions means conditions where the entry of micro-organisms is prevented or decreased. Measures for achieving these conditions will be described in further detail later.

The beating and subsequent processing to achieve nanofibrillar cellulose is preferably carried out under cleanroom standards meeting the cleanroom classification. In the context of this specification, conditions of ISO 8 of ISO 14644-1 cleanroom standards or stricter may refer to conditions assigned to and reproducibly meeting a cleanroom classification (ISO 14644-1 cleanroom standards) of at least Class ISO 8, or at least ISO 7, or at least ISO 6, or at least ISO 5, or at least ISO 4, or at least ISO 3, or at least ISO 2, or ISO 1 . The conditions stricter than conditions of ISO 8 of ISO 14644-1 cleanroom standards may thus refer to conditions of at least ISO 7 of ISO 14644-1 cleanroom standards, such as conditions of ISO 7, ISO 6, ISO 5, ISO 4, ISO 3, ISO 2 or ISO 1 . Such conditions are aimed at minimizing or preventing the introduction or contamination of viable microorganisms into the process, in contact with the bleached cellulose pulp fibers or the suspension containing said bleached cellulose pulp fibers and in contact with the nanofibrillar cellulose hydrogel.

Further, the preheating takes place preferably at low consistencies of pulp, at 1 .0-5.0%, preferably at 1 .5-4.0%. By low consistency, crimping or curling of fibers is avoided. After the preheating, the pulp can be diluted to match the operating conditions of the high pressure disintegration device, or it can be disintegrated in the preheating consistency.

Brief description of the drawings

Fig. 1 is an overall flowchart of the method,

Fig. 2 is a schematic view of the apparatus according to the first embodiment, Fig. 3 is a schematic view of the apparatus according to the second embodiment,

Fig. 4 is a microscopic image from a sample obtained by the method, and Figs. 5 and 6 are microscopic images from samples obtained by a method of prior art.

Detailed description

As used herein, the term "nanofibrillar cellulose" refers to cellulose fibrils or fibril bundles separated from cellulose-based fiber raw material. These fibrils are characterized by a high aspect ratio (length/diameter): their length may exceed 1 μιτι, whereas the diameter typically remains smaller than 200 nm. The smallest fibrils are in the scale of so-called elementary fibrils, the diameter being typically in the range of 2-12 nm. The dimensions and size distribution of the fibrils depend on the refining method and efficiency. Nanofibrillar cellulose may be characterized as a cellulose-based material, in which the median length of particles (fibrils or fibril bundles) is not greater than 50 μιτι, for example in the range of 1-50 μιτι, and the particle diameter is smaller than 1 μιτι, suitably in the range of 2-500 nm. In case of native nanofibrillar cellulose, in one embodiment the average diameter of a fibril is in the range of 5-100 nm, for example in the range of 10-50 nm. Nanofibrillar cellulose is characterized by a large specific surface area and a strong ability to form hydrogen bonds. In water dispersion, the nanofibrillar cellulose typically appears as either light or turbid gel-like material. Depending on the fiber raw material, nanofibrillar cellulose may also contain small amounts of other wood components, such as hemicellulose or lignin. The amount is dependent on the plant source. Often used parallel names for nanofibrillar cellulose include nanofibrillated cellulose (NFC) and nanocellulose.

Different grades of nanofibrillar cellulose may be categorized based on three main properties: (i) size distribution, length and diameter (ii) chemical composition, and (iii) rheological properties. To fully describe a grade, the properties may be used in parallel. Examples of different grades include native (or non-modified) NFC, oxidized NFC (high viscosity), oxidized NFC (low viscosity), carboxymethylated NFC and cationized NFC. Within these main grades, also sub-grades exist, for example: extremely well fibrillated vs. moderately fibrillated, high degree of substitution vs. low, low viscosity vs. high viscosity etc. The fibrillation technique and the chemical pre-modification have an influence on the fibril size distribution. Typically, non-ionic grades have wider fibril diameter (for example in the range of 10-100 nm, or 10-50 nm) while the chemically modified grades are a lot thinner (for example in the range of 2-20 nm). Distribution is also narrower for the modified grades. Certain modifications, especially TEMPO-oxidation, yield shorter fibrils.

Depending on the raw material source, e.g. hardwood (HW) vs. softwood (SW) pulp, different polysaccharide composition exists in the final nanofibrillar cellulose product. Commonly, the non-ionic grades are prepared from bleached birch pulp, which yields high xylene content (25% by weight). Modified grades are prepared either from HW or SW pulps. In those modified grades, the hemicelluloses are also modified together with the cellulose domain. Most probably, the modification is not homogeneous, i.e. some parts are more modified than others. Thus, detailed chemical analysis is not possible - the modified products are always complicated mixtures of different polysaccharide structures.

In an aqueous environment, a dispersion of cellulose nanofibers forms a viscoelastic hydrogel network. The gel is formed at relatively low concentrations of, for example, 0.05-0.2% (w/w) by dispersed and hydrated entangled fibrils. The viscoelasticity of the NFC hydrogel may be characterized, for example, with dynamic oscillatory rheological measurements.

Nanofibrillar cellulose may be characterized with rheological properties. The nanofibrillar cellulose hydrogels are shear-thinning materials, which means that their viscosity depends on the speed (or force) by which the material is deformed. When measuring the viscosity in a rotational rheometer, the shear- thinning behavior is seen as a decrease in viscosity with increasing shear rate. The hydrogels show plastic behavior, which means that a certain shear stress (force) is required before the material starts to flow readily. This critical shear stress is often called the yield stress. The yield stress can be determined from a steady state flow curve measured with a stress controlled rheometer. When the viscosity is plotted as function of applied shear stress, a dramatic decrease in viscosity is seen after exceeding the critical shear stress. The zero shear viscosity and the yield stress are the most important rheological parameters to describe the suspending power of the materials. These two parameters separate the different grades quite clearly and thus enable classification of the grades. The dimensions of the fibrils or fibril bundles are dependent on the raw material and the disintegration method.

One way to characterize the nanofibrillar cellulose is to use the viscosity of an aqueous solution containing said nanofibrillar cellulose. The viscosity may be, for example, Brookfield viscosity or zero shear viscosity. In one example the apparent viscosity of the nanofibrillar cellulose is measured with a Brookfield viscometer (Brookfield viscosity) or another corresponding apparatus. Suitably a vane spindle (number 73) is used. There are several commercial Brookfield viscometers available for measuring apparent viscosity, which all are based on the same principle. Suitably RVDV spring (Brookfield RVDV-III) is used in the apparatus. A sample of the nanofibrillar cellulose is diluted to a concentration of 1 .5% by weight in water and mixed for 10 min. The diluted sample mass is added to a 250 ml beaker and the temperature is adjusted to 20°C±1 °C, heated if necessary and mixed. A low rotational speed 10 rpm is used.

The fibrous raw material for preparing cellulose pulp for preheating and subsequent disintegration to nanofibrillar cellulose is obtained normally from cellulose raw material of plant origin. The raw material can be based on any plant material that contains cellulosic fibers, which in turn comprise microfibrils of cellulose. The fibers may also contain some hemicelluloses, the amount of which is dependent on the plant source. The plant material may be wood. Wood can be from softwood tree such as spruce, pine, fir, larch, douglas-fir or hemlock, or from hardwood tree such as birch, aspen, poplar, alder, eucalyptus or acacia, or from a mixture of softwoods and hardwoods. Non-wood material can be from agricultural residues, grasses or other plant substances such as straw, leaves, bark, seeds, hulls, flowers, vegetables or fruits from cotton, corn, wheat, oat, rye, barley, rice, flax, hemp, manila hemp, sisal hemp, jute, ramie, kenaf, bagasse, bamboo or reed.

One preferred alternative is fibers from non-parenchymal plant material where the fibrils of the fibers are in secondary cell walls. The fibrils originating in secondary cell walls are essentially crystalline with degree of crystallinity of at least 55 %. The source can be wood or non-wood plant material. For example wood fibres are one abundant fibrous raw material source. The raw material can be for example chemical pulp. The pulp can be for example softwood pulp or hardwood pulp or a mixture of these.

The common characteristics of all wood-derived or non-wood derived fibrous raw materials is that nanofibrillar cellulose is obtainable from them by disintegrating the fibers to the level of nanofibrils. The preheating is performed to fibrous raw material which exists as a suspension in a liquid, that is, pulp. Raw material means in this connection starting material of the process, which may be the result of separation process of the fibrous material from its original natural source. This separation process is commonly chemical pulping when the original natural source is plant material, especially wood.

Because the chemical pulping includes a bleaching stage, cellulose pulp for the preheating process can be collected after the bleaching stage. The pulp can be collected in clean form in the chemical pulp mill immediately after the bleaching stage where the conditions are favourable for prevention of the micro-organisms.

According to the first embodiment, the beating is performed in a conical or disc refiner by circulating an aqueous suspension of fibres through a refining gap. The aqueous suspension of fibres is preferably at a consistency of 1 .0- 5.0%, more preferably 1 .5-4.0%. The refining gap is formed between two refining surfaces (refining blades or fillings), a rotor and a stator, which both comprise bars. The operation of these kinds of refiners can be characterized by specific edge load (SEL) with J/m as unit (energy per unit length of bar crossings) and it is inversely proportional to cutting edge length (CEL). Specific edge load is based on a well-known theory developed for describing the intensity of refining of pulp. By selecting the SEL low enough, a gentle beating action on fibres is achieved, which is important for the final quality of the nanofibrillar cellulose, the end product.

According to the second embodiment, the beating is performed in a hollander beater, which is a device known for its gentle beating action on fibres. In a hollander beater, the aqueous suspension of fibres is forced through a converging narrow refining gap formed between a cylindrical rotor or beater roll revolving around a horizontal axis and a stationary bottom of a tub where the aqueous suspension of fibres circulates. The surface of the cylindrical rotor and the surface of the bottom beneath the rotor are equipped with bars. The aqueous suspension of fibres is preferably at a consistency of 1 .0-5.0%, more preferably 1 .5-4.0% also in this case. Consistency is calculated with known formula:

(oven-dry weight^* of pulp x 100) / (weight of pulp + weight of water)

^* oven-drying at 105°C

In both embodiments, the aqueous suspension of fibres is circulated until a beating degree of below 55 ml CSF, preferably 50 ml CSF or less, more preferably 20 ml or less is attained. CSF is a standard measure of freeness of pulp (Canadian Standard Freeness) and it decreases with increasing beating degree of pulp. With these values, the risk of clogging in the subsequent disintegration process can be reduced or eliminated. The Canadian Standard Freeness (CSF) number may be determined according to ISO 5267-2.

Further, in both preheating embodiments, the circulation of pulp takes place in a closed system. In a closed system, the pulp is isolated from the surrounding air to exclude all possible contaminations. In the first embodiment the pipes circulating the pulp through the refiner and the pump that brings the pulp in motion, as well as a possible intermediate tank create a closed circulation by being all isolated from the surrounding air. In the second preheating embodiment the tub of the hollander beater is covered to isolate it from the surrounding air. In both embodiments, the same batch of cellulose pulp is circulating through the refining gap.

To ensure a sufficient preheating result, total specific energy (SEC, specific energy consumption) of at least 150 kWh/t, preferably in the range of 150- 500 kWh/t, more preferably 150-400 kWh/t should be used for hardwood, and total specific energy of at least 400 kWh/t should be used for softwood. The energy consumption is calculated per metric ton (t) of dry pulp.

SEC [kWh/t] is determined according to the generally known formula Pnet/throughput, where P_net is effective refining power [kW] (Total applied power minus power consumed under no load), and throughput [t/h] is defined as amount of oven dry pulp [metric ton, t] per unit of time [hour, h]. To increase the cleanness of the preheating process, water having high purity is used for preparing the pulp. The water can be distilled, ion- exchanged or sterilized. Sterilization can be used as complementary measure after distillation or ion-exchange.

To increase the preheating effect, the conductivity of the pulp that is prebeaten is preferably low, which can be achieved by washing the fibres. The pulp has preferably conductivity below 20 S/cm when measured at 1 % consistency in water. Low conductivities promote the swelling of fibres, which has a positive effect on the efficiency of preheating.

Further, before the preheating, the pulp is preferably washed to Na⁺ form which causes uniform swelling of fibres when the pulp is at low conductivity. Preferably, in preparation of the washing water, water of high purity as explained above is used, that is, the water is distilled or ion-exchanged, and preferably also sterilized. In the practice the COOH groups which exist mainly in a hemicellulose portion of the fibres have been converted to salt form (COONa) by ion-exchange using a solution of sodium ions. If needed, after the ion-exchange the pulp is washed with the high-purity washing water to reach the above-mentioned low conductivity levels.

Finally, the pH of the pulp that is prebeaten is preferably 7.0-8.5. This is the preferable range of the final product, the nanofibrillar cellulose hydrogel, and when the preheating is conducted in this pH range, no pH adjustment is needed before the disintegration process. Further, the fibers are suitably swollen state at this pH.

The above measures and properties can be applied equally well to both above-mentioned embodiments (conical or disc definer and hollander beater).

After the preheating, the prebeaten pulp is either diluted and supplied to the disintegration process at a lower consistency, or supplied in the same consistency where it was prebeaten to the disintegration process. If dilution is used, the dilution water will also have high purity. The dilution water can be distilled, ion exchanged or sterilized water. The consistency of the pulp supplied to the disintegration process where it is disintegrated to nanofibrillar cellulose is 0.7-3.0%, preferably 1 .0-2.5%.

The disintegration process is a high pressure mechanical disintegration process which uses the combination of high pressure applied to the pulp and a narrow gap which cause mechanical disintegration of the prebeaten fibres. The term "high pressure mechanical disintegration" refers here to disintegration process using high pressure, typically 200 bar or more, such as 1000 bar or more, particularly suitably 1500 bar or more, resulting in liberation of cellulose fibrils. The upper limit for the above mentioned recommended pressure ranges is preferably 2000 bar. The high pressure mechanical disintegration is suitably carried out from 1 to 10 passes, particularly suitably from 1 to 5 passes. High pressure mechanical disintegration may be carried out for example using a pressure type homogenizer, preferably high pressure homogenizer or high pressure fluidizer, such as microfluidizer, macrofluidizer or fluid izer-type homogenizer.

The prebeaten cellulose is subjected to the high pressure mechanical disintegration until target rheological properties are obtained. The rheological properties of the nanofibrillar cellulose measured by the method to be explained later can be characterized by:

- a zero shear viscosity in the range of 100 to 8000 Pa s, preferably 200- 2000 Pa s measured at a consistency of 0.5% in water by rotational rheometer, and

- a yield stress in the range of 0.5 to 8 Pa, preferably 1-4 Pa, measured at a consistency of 0.5% by rotational rheometer.

Further, the prebeaten cellulose is preferably subjected to the high pressure mechanical disintegration until NTU of 200 or less, preferably 150 or less, more preferably 140 NTU or less, is achieved. The turbidity may be between 50 and 200 NTU, more preferably between 80 and 150 NTU, such as 80, 90, 100, 1 10, 120, 130, 140 or 150, most preferably between 100-140 NTU in water at concentration of 0.1 w%. In this way it can be ensured that fibril bundles are substantially disintegrated and uniform nanofibrillar cellulose is obtained. The nanofibrillar cellulose obtained as end product contains cellulose fibrils where the cellulose is in native form. It forms an aqueous gel at low concentrations, typically already at 0.5%. It has shear thinning behaviour characteristic of nanofibrillar cellulose.

To verify the success of fibrillation, rheological measurements of the samples in the form of nanofibrillar cellulose hydrogels were carried out with a stress controlled rotational rheometer (ARG2, TA instruments, UK) equipped with four-bladed vane geometry. Samples were diluted with deionized water (200 g) to a concentration of 0.5 w% and mixed with Waring Blender (LB20E^*, 0.5 I) 3 x 10 sec (20 000 rpm) with short break between the mixing. Rheometer measurement was carried out for the sample. The diameters of the cylindrical sample cup and the vane were 30 mm and 28 mm, respectively, and the length was 42 mm. The steady state viscosity of the hydrogels is measured using a gradually increasing shear stress of 0.001-1000 Pa. After loading the samples to the rheometer they are allowed to rest for 5 min before the measurement is started, room temperature. The steady state viscosity is measured with a gradually increasing shear stress (proportional to applied torque) and the shear rate (proportional to angular velocity) is measured. The reported viscosity (=shear stress/shear rate) at a certain shear stress is recorded after reaching a constant shear rate or after a maximum time of 2 min. The measurement is stopped when a shear rate of 1000 s^"1 is exceeded. The method is used for determining zero-shear viscosity.

On the other hand, turbidity may be measured quantitatively using optical turbidity measuring instruments. There are several commercial turbidometers available for measuring quantitatively turbidity. In the present case the method based on nephelometry is used. The units of turbidity from a calibrated nephelometer are called Nephelometric Turbidity Units (NTU). The measuring apparatus (turbidometer) is calibrated and controlled with standard calibration samples, followed by measuring of the turbidity of the diluted NFC sample.

In the method, a nanofibrillar cellulose sample is diluted in water, to a concentration below the gel point of said nanofibrillar cellulose, and turbidity of the diluted sample is measured. Said concentration where the turbidity of the nanofibrillar cellulose samples is measured is 0.1 %. HACH P2100 Turbidometer with a 50 ml measuring vessel is used for turbidity measurements. The dry matter of the nanofibrillar cellulose sample is determined and 0.5 g of the sample, calculated as dry matter, is loaded in the measuring vessel, which is filled with tap water to 500 g and vigorously mixed by shaking for about 30 seconds. Without delay the aqueous mixture is divided into 5 measuring vessels, which are inserted in the turbidometer. Three measurements on each vessel are carried out. The mean value and standard deviation are calculated from the obtained results, and the final result is given as NTU units.

The novel nanofibrillar cellulose product has a typical turbidity below 200, preferably below 150 NTU in the above mentioned measurement conditions.

After the disintegration process, the nanofibrillar cellulose dispersion is preferably sterilized. Sterilization is carried out preferably by autoclaving or irradiating, for example using UV irradiation. If the nanofibrillar cellulose is to be transported, it is preferably sterilized in packages where it will be transported.

Figure 1 illustrates the whole process, starting from pulp and ending in nanofibrillar cellulose having the target properties.

The process comprises supplying pulp P of chemically underivatized cellulose preferably at a consistency of 1 to 5% to a beating step 1 , where the pulp is circulated through a circulation loop C1 so that it passes through a refining gap G a number of times N1 (N1 passes) until it has the desired CSF value as a result of the beating. The prebeaten pulp PB having the desired CSF value as mentioned above is taken from the beating step and passed to the disintegration step 2, where it is disintegrated to nanofibrillar cellulose NFC. The prebeaten pulp is circulated through a circulation loop C2 so that it passes mechanical disintegration means M a number of times N2 (N2 passes) until it has reached the target rheological properties as indicated above. As explained above, N2 can be 1-10, particularly suitably 1-5. The mechanical disintegration means M are configured to perform high pressure mechanical disintegration as described above. As a rule, they comprise a passage for the prebeaten pulp which is shaped so that when the pulp is supplied under high pressure to the passage, it will be disintegrated gradually to fibrils when flowing through the passage.

Broken lines illustrate some optional additional steps. The pulp may be chemical pulp collected after an operation in a chemical pulping process, for example after a bleaching stage B and supplied to the beating step 1 . Further, the nanofibrillar cellulose NFC taken out from the disintegration step 2 is preferably subjected to sterilization S. Between the beating step 1 and the disintegration step 2, the prebeaten pulp may be diluted with dilution water D to match the consistency of the pulp to the disintegration step, for example in a range of 0.7-3.0%, preferably 1 .0-2.5%. Water of high purity as described above is preferably used as the dilution water D.

The beating step 1 and the disintegration step 2 are typically batch processes where the material to be processed is circulated the required number of times N1 or N2. After the material (prebeaten pulp PB from the beating step 1 or nanofibrillar cellulose NFC from the disintegration step 2) is taken out from the process, the equipment used in the process is used for a new batch of material . Before the new batch of material is supplied to the process, the equipment used in the process is preferably cleaned or sterilized.

Further, the beating step 1 is performed under the above-mentioned cleanroom standards. This is achieved by arranging the circulation C1 to take place in a closed circulation, that is, the piping, the refining gap and possible intermediate tanks of the equipment form a contained space where provisions are made to reduce particulate contamination so that the required standard is achieved.

Preferably both the beating step 1 and the disintegration step 2 are performed under the above-mentioned cleanroom standards.

Figure 2 is a schematic representation of the equipment used in the beating step 1 according to a first embodiment. The refining gap through which the pulp to be beaten is passed is formed between the rotor R and the stator of a disc refiner 3. Although disc refiner is shown, a conical refiner can also be used. The opposite rotor and stator surfaces have refining bars which are arranged at such a density that the above-mentioned low specific edge loads (SEL) are obtainable with the rotation speeds (rpm) used.

The equipment further comprises piping which connects the disc refiner 3 (or conical refiner), a pulper 5 and a pump 4 so that the closed circulation C1 is formed. The pump 4 is preferably a pump operating on progessing cavity principle, so called "mono" or "mohno" pump. The equipment further comprises shut-off valves V in the supply pipe and in the discharge pipe to form the closed circulation C1 , as well as other valves V in washing and sterilizing lines, through which the cleaning and sterilizing of the equipment can be carried out. The valves V are preferably aseptic valves known in food, dairy and pharmaceutical industry. Aseptic valves isolate the valve passage totally from the surroundings in their closed and open position and they are usually diaphragm valves.

Fig. 3 shows a beating equipment according to another embodiment. The equipment includes a hollander beater 3, which has a barred rotor R or beater roll horizontally placed in a tub, and a barred bottom portion of the tub, so called "bedplate" for forming the refining gap G with the rotor. The closed circulation C1 of the pulp is formed in the tub along a horizontal loop which is formed by an intermediate wall W in the tub, dividing the loop in the beating section, which is on the side of the tub where the rotor R is located, and in the return section, which is on the other side of the tub. The circulation movement of the pulp is brought about by the rotation of the rotor R which also causes the beating action.

The hollander beater also has a lid L which seals the interior of the tub from the surroundings so that the required cleanroom standards are achieved.

Further, the supply pipe and discharge pipe (not shown) which are connected to the hollander beater are equipped with shutoff valves which are preferably aseptic. The supply pipe and the discharge pipe can be connected to the bottom of the tub. Example 1

In the following, a preheating trial is described, which shall not be construed as restricting the scope of the method.

Refining intensity based on effective refiner load and bar edge length per second was first introduced by Wultsch and Flucher (Wultsch, F., Flucher, W., Das Papier 12(13):334 (1958)) and later defined as specific edge load by Brecht and Siewert (Brecht, W., Siewert, W.H., Das Papier 20(1 ):4 (1966)).

Specific edge load (SEL) [J/m] is calculated using the following formula:

SEL = Pnet / CEL, where P_net is effective refining power [W] and CEL is cutting edge length (cutting speed of bars) [mis].

The CEL in turn is calculated using the formula:

CEL = Z_R x Z_s x I x (n/60), where Z_R and Z_s are the number of rotor bars and number of stator bars, respectively, I is the bar effective length (common contact length of opposite bars) [m] and n is the number of revolutions of the refiner [rpm].

Equipment: Voith-Sulzer beater (conical refiner)

Refining gap: conical fillings, cutting edge length CEL 2.43 km/s at 3000 rpm

(48.6 m/revolution, obtained by dividing CEL by the value of revolutions/second)

Cellulose pulp: birch pulp

Pulp consistency: 3.2%

SEL: 0.5 J/m

Pulp was circulated through the beater until refining energy was SEC = 200 kWh/t. When this energy level was reached, the pulp had circulated through the refining zone 55 times in average.

The beating degree obtained by this energy level, as expressed by Schopper-Riegler was 83°SR, which is below 50 ml when expressed by CSF. The Schopper-Riegler (SR) number may be determined according to ISO 5267-1 .

Beating was continued until SEC = 500 kWh/t. At this stage refining level was 96°SR, which is below 20 ml and in fact close to zero as expressed in CSF. Microscopic images were taken from this sample (Figure 4).

From Figure 4 we can see that there is a lot of fines (foggy material) but also a lot of unrefined fibers. Comparison with Figure 5, PFI beating, a preheating method of prior art at high consistency using the PFI mill, tells that there is no substantial difference. Also in PFI beating many of the fibers appear unbeaten and fines are created. However, in PFI beating, there are more damages typical to high consistency beating, like crimps in the fibers (Figure 6).

The quality of the prebeaten pulp can be further improved by using specific edge loads below 0.5 J/m, preferably below 0.2 J/m, most preferably below 0.1 J/m. This is achievable with fillings where the bar pattern is more dense (with narrower bar widths and groove widths), thus increasing the cutting edge length (CEL) at a given rpm value.

Example 2

Washing the pulp to the Na+ form, washing the pulp to a low conductivity, preheating the pulp, and disintegrating the prebeaten pulp to nanofibrillar cellulose.

2000 g of wet native cellulose pulp obtained from bleached birch pulp was filtered and the solid mass was diluted with 0.01 M aqueous HCI and to obtain suspension having dry matter content of approx. 1 % by weight. The suspension was allowed to stand for approx. 15 min with occasional agitation. The suspension was then filtered, washed twice with deionized water and filtered. Then the solid mass was suspended in a 0.005 M aqueous NaHCO₃ solution to obtain suspension having dry matter content of approx. 1 % by weight, the pH of the obtained suspension was adjusted between 8 and 9 with 1 M aqueous NaOH solution and the obtained suspension was allowed to stand for 15 min with occasional agitation. The suspension was filtered and the solid mass was washed with deionized water until the conductivity of the filtrate was less than 20 S/cm.

Washed pulp was pre-grinded with Voith-Sulzer beater.

Refining gap: conical fillings, cutting edge length 2.43 km/s at 3000 rpm (48.6 m/revolution).

Cellulose pulp: birch pulp

Pulp consistency: 4%

SEL: 0.5 J/m

Pulp was circulated through the beater until refining energy was SEC = 250 kWh/t. The beating degree obtained by this energy level, as expressed by CSF was 41 ml.

The pre-refined pulp obtained was diluted to 1 .8 w% consistency and was subjected to high pressure mechanical disintegration where nanofibrillar cellulose hydrogel was obtained. Sample was fibrillated in Microfluidics Fluidizer (M100EH-30), 1500 bar, once trough ΑΡΜ+200μηη chambers and six times through APM+100 μιτι chambers until the turbidity was below the target level < 200 NTU. The final turbidity for the product, Sample 2, was 190 NTU and viscosity 30500 mPa s (Brookfield 10 rpm at 1 .5% concentration of NFC).

Applications of the nanofibrillar cellulose

The nanofibrillar cellulose obtained is useful in cell culture applications, such as in cell culture matrix or drug delivery composition. The native nanofibrillar cellulose derived from plant material in the above-described process may be used without any modifications as biomimetic human ECM for 3D cell culture. Nanofibrillar cellulose hydrogel is an optimal biomaterial for 3D cell scaffolds for advanced functional cell based high throughput screening assays in drug development, in drug toxicity testing and in regenerative medicine and further to drug and cell delivery in vivo. Due to its ECM-mimicking properties and non-toxicity, the nanofibrillar cellulose may be used in any kinds of applications involving cell or tissue contact, such as drug delivery, cell delivery, tissue engineering, wound treatment, or implants, or as a wound healing agent, an anti-inflammatory agent, or a hemostatic agent.

The matrix for cell culture or drug delivery composition may further comprise suitable additives selected from the group consisting of special extra cellular matrix components, serum, growth factors, and proteins.

In the use, the cell culture or drug delivery matrix comprises living cells. The cell culture or drug delivery composition forms a hydrogel and wherein the cells are present in the matrix in a three-dimensional or two-dimensional arrangement.

Cells can be any cells. Any eukaryotic cell, such as animal cells, plant cells and fungal cells are within the scope of the present invention as well as prokaryotic cells. Prokaryotic cells comprise micro-organisms such as aerobic or anaerobic bacteria, viruses, or fungi such as yeast and molds. Even stem cells, such as non-human stem cells may be grown using the matrix comprising nanofibrillar cellulose. Depending on the cell line, the experiments are carried out on 2D or 3D, i.e. the cells are cultivated on the CNF (cellulose nanofibril) membranes or gels or the cells are dispersed homogeneously in the CNF hydrogels or CNF membranes. Cells are growing in the 3D matrix or on the matrix. The matrix could be injectable hydrogel or sheet-like membrane optionally with appropriate surface topology. The composition comprising cellulose nanofibers or derivatives thereof can be used for immobilizing cells or enzymes.

The properties of CNF are close to optimal for cell and tissue culturing, maintenance, transporting and delivery: transparent, non-toxic, highly viscous, high suspending power, high water retention, good mechanical adhesion, non-animal based, resembles ECM dimensions, insensitive to salts, temperature or pH, not degradable, no autofluorescence. CNF has negligible fluorescence background due to the chemical structure of the material. Furthermore, CNF gel is not toxic to the cells. It is known that strong interactions are formed between adjacent nanofibrils due to the surface hydroxyl groups, and this in combination with the high stiffness results in a rigid network that improves the stiffness and strength of polymer based nanocomposites also. In addition to improved mechanical properties, the advantages with nanofibrillar cellulose as reinforcement in composites are increased thermal stability, decreased thermal expansion, and increased thermal conductivity. If a transparent composite matrix is used, it is possible to maintain most of the transparency due to the fine scale of the nanofibrils. Further, high degree of crystallinity and DP are physical properties that are useful to the elaboration of strong nanofibrillar cellulose composites.

Further, the rheological properties, transparency, non-toxicity, and insensitivity to salts, temperature or pH render the nanofibrillar cellulose desired in cosmetics, personal care compositions, flocculant or water- treatment systems, composites, as a bulking agent, a thickener, a rheology- modifier, a food additive, a paint additive, a paper, board or pulp additive. Compared to chemically modified grades, such as TEMPO-oxidized grade, nanofibrillar cellulose of native cellulose is insensitive to salts, temperature or pH which may be beneficial in many end-uses.

Thereby improved pharmaceuticals, cosmetics, food, agrochemicals, paints, coatings, paper, board, pulp, filters, composite products, adhesives, displays, personal care compositions, tooth paste, or cell or tissue culture matrixes, or cell or tissue delivery matrixes may be obtained from the nanofibrillar cellulose prepared in the process.

Claims

Claims:

1 . A method for producing nanofibrillar cellulose starting from hardwood cellulose pulp, where cellulose is in native form, comprising

- forwarding the prebeaten pulp to a disintegration process, and

2. The method according to claim 1 , characterized in that it comprises

- pretreating the cellulose pulp by washing until it has conductivity below 20 S/cm when measured at 1 % consistency, and

- after the pretreating, beating the cellulose pulp.

3. The method according to claim 1 or 2, characterized in that it comprises

- ion-exchanging the cellulose pulp to Na⁺ form

- preheating the cellulose pulp in Na⁺ form.

4. The method according to claim 1 , 2 or 3, characterized in that it comprises preheating the cellulose pulp at pH 7.0-8.5.

5. The method according to any of the preceding claims, characterized in that during the preheating, the cellulose pulp is circulated in a closed circulation isolated from the surrounding air.

6. The method according to any of the preceding claims, characterized in that it comprises

- preheating the cellulose pulp in a conical refiner or disc refiner, or

- preheating the cellulose pulp in a hollander beater.

7. The method according to claim 6, characterized in that it comprises

- preheating hardwood cellulose pulp in a conical refiner or disc refiner with a specific edge load of below 0.2 J/m, most preferably below 0.1 J/m.

8. The method according to any of the preceding claims, characterized in that it comprises

- preheating hardwood cellulose pulp to said beating degree using total specific energy of at least 150 kWh/t, preferably at least 150 kWh/t and 500 kWh/t at the most, more preferably 150-400 kWh/t or

- beating softwood cellulose pulp to said beating degree using total specific energy of at least 400 kWh/t.

9. The method according to any of the preceding claims, characterized in that it comprises

- preheating the cellulose pulp in a consistency of 1 .0-5.0%, preferably 1 .5- 4.0%.

10. The method according to any of the preceding claims, characterized in that it comprises

- prior to preheating, collecting cellulose pulp after a bleaching stage of cellulose pulp and forwarding the cellulose pulp to beating.

1 1 . The method according to any of the preceding claims, characterized in that it comprises

- disintegrating the cellulose pulp in a high pressure disintegration process, such as by means of a fluidizer or homogenizer.

12. The method according to any of the preceding claims, characterized in that the nanofibrillar cellulose obtained in the disintegration process has

13. The method according to any of the preceding claims, characterized in that the preheating is configured to operate under conditions of ISO 8 of ISO 14644-1 cleanroom standards or stricter.

14. The method according to claim 13, characterized in that the preheating and subsequent process including the disintegration process to nanofibrillar cellulose is configured to operate under conditions of ISO 8 of ISO 14644-1 cleanroom standards or stricter.