TECHNICAL FIELD

The present invention relates to a method of producing nanocellulose films or fibers, as well as nanocellulose films or fibers produced by such a method.

BACKGROUND ART

Nanocellulose is a renewable material typically manufactured from wood pulp. Free-standing nanocellulose films have demonstrated promise in a large variety of applications, such as use as barrier layers or as flexible substrates in electronic devices. Functional materials may also be added to the nanocellulose during production of such films in order to provide functional nanocellulose composite films.

A significant hinder to the commercial production of nanocellulose films is the difficulty in removing water during the production of such films in an energy-efficient manner, leading to excessive production costs.

Document W02020/095254 Al describes a method for dewatering a web comprising microfibril lated cellulose. The method comprises the steps of: providing a suspension comprising between 50 weight-% to 100 weight-% of m icrofi bril lated cellulose based on total dry weight, forming a fibrous web of said suspension on a support wherein said web has a dry content of 1-25% by weight, applying a dewatering felt into direct contact with the fibrous web, conducting said fibrous web, arranged between said dewatering felt and said support, through a pressing equipment. In this manner, it was found that it is possible to dewater a web comprising high amounts of microfibril lated cellulose.

There remains a need for an improved means of producing films comprising nanocellulose. SUMMARY OF THE INVENTION

The inventors of the present invention have identified a number of shortcomings with prior art means of producing nanocellulose films or fibers.

Due to the high aspect ratio of nanocellulose, it is necessary to have low concentrations of nanocellulose in a dispersion in order to obtain a uniform dispersion. If the nanocellulose concentration is too high, flocs are formed in the dispersion and the properties of the resulting film are impacted deleteriously. This is especially pertinent when producing composite films or fibers, where it is desirable to have a further material uniformly dispersed with the nanocellulose.

In the laboratory, films or fibers produced from dilute dispersions of nanocellulose are typically dried by slow evaporation, which is extremely time-consuming. Heating the nanocellulose film or fiber is conceivable, but is extremely energy-intensive for films or fibers produced from dilute dispersion, and therefore not commercially viable as a sole means of removing water. Even films or fibers formed from more concentrated dispersions typically form impermeable "filter cakes" when subjected to typical dewatering methods and are laborious to dewater.

It would be advantageous to achieve a method overcoming, or at least alleviating, at least some of the above-mentioned shortcomings. In particular, it would be desirable to obtain a method of effectively removing water from films or fibers produced from relatively dilute nanocellulose dispersion. In order to better address one or more of these concerns, a method for producing nanocellulose films or fibers having the features defined in the independent claim is provided.

The method comprises the steps: a) providing a gellable nanocellulose dispersion comprising a nanocellulose and water, wherein the gellable nanocellulose dispersion comprises from about 0.1 weight% to about 5 weight% total dry matter relative to the total weight of the gellable nanocellulose dispersion, and wherein the nanocellulose constitutes from about 10 weight% to about 100 weight% of the total dry matter; b) extruding the gellable nanocellulose dispersion through an extrusion die into a coagulation liquid to provide a strip of nanocellulose gel; and c) removing water from the strip of nanocellulose gel to provide a nanocellulose film or fiber.

By gellable nanocellulose dispersion it is meant a nanocellulose-comprising dispersion that is capable of being gelled, i.e. transformed from flowing to non-flowing. The nanocellulose in itself may provide suitably gellability, and no further gelling agents may be necessary, in particular if the nanocellulose is a charged nanocellulose. However, depending on the nature of the nanocellulose and amount of nanocellulose in the dispersion, it may be necessary to use further gelling agents in order to provide suitable gellablility.

By using a gellable dispersion of nanocellulose, extruding such a dispersion and flocculating/coagulating the extruded dispersion (extrudate) with a coagulation liquid, a strip of nanocellulose gel is formed. By the term "strip" it is meant that the gel is thin (< 20 mm) in the z-direction and its dimension is considerably less in the cross-direction (millimetre to meter scale) as compared to in the machine direction (which may be essentially continuous). The strip in practice may be fiber-like, filament-like, ribbon-like, or sheet-like, depending on the cross-direction dimension.

The intact strip (e.g. fiber, filament, ribbon or sheet) of nanocellulose gel can be dewatered, even at high water (low dry matter) content, to provide nanocellulose films or fibers. Thus, high-quality, uniform nanocellulose films or fibers may be produced in a relatively rapid and energy-effective manner. Without wishing to be bound by theory, it is thought that "locking" of the nanocellulose gel network by coagulation/flocculation increases the strength of the gel. This enables the intact gel having a high-water content to be successfully dewatered using initial dewatering treatments that could not successfully be used with conventional nanocellulose film/fiber production means. Thus, rapid, effective dewatering of the films or fibers may be achieved using relatively cost-effective dewatering means, such as dewatering means more typically utilized in the papermaking industry.

The method is amenable to the continuous production of nanocellulose film or fiber.

Step c) of removing water from the strip of nanocellulose gel may comprise a step (cl) of capturing the strip of nanocellulose gel on at least one foraminous support and transporting the strip out of the coagulation liquid. Simply by removing the gel from the coagulation liquid and allowing it to freely drain, the dry matter content of the gel may be increased significantly as compared to the dry matter content of the original dispersion. Without wishing to be bound by theory, it is hypothesised that the change from repulsion to attraction between the fibrils and/or osmotic pressure may be the cause of the concentration increase in this step.

Step c) of removing water from the strip of nanocellulose gel may comprise a step (c2a) of applying a porous material in contact with at least one surface of the nanocellulose gel to remove water by capillary action. It has been found that the formation of an intact nanocellulose gel by coagulation/flocculation of the nanocellulose dispersion enables effective dewatering using capillary action, despite the initially low dry matter content of the gel. Capillary action is a relatively rapid and cost-effective means of removing water from the nanocellulose gel.

Alternatively, or in addition, step c) of removing water from the strip of nanocellulose gel may comprise a step (c2b) of removing water by osmotic dehydration.

Step c) of removing water from the strip of nanocellulose gel may comprise a step (c3) of passing the nanocellulose gel through a press, to press water out of the nanocellulose gel. Such a step is particularly applicable once the nanocellulose gel has reached a suitable dry matter content, such as greater than about 4 weight%, by prior dewatering. A nanocellulose gel produced according to the disclosed method and having such a dry matter content has sufficient strength to be able to withstand pressing. Pressing is a rapid and cost-effective means of dewatering the nanocellulose gel to a relatively high dry matter content.

Step c) of removing water from the strip of nanocellulose gel may comprise a step (c4) of drying the nanocellulose gel by application of heat. Such a step is preferably performed only once the nanocellulose gel has already been dewatered to a relatively high dry matter content by other means, in order to increase the economy of the process.

Steps cl); c2a) and/or c2b); c3); and c4) may be performed sequentially. Step (c2b), if performed in addition to step (c2a) may be performed prior to step (c2a) or after step (c2a).

The nanocellulose may be a charged nanocellulose. The charged nanocellulose may preferably be an anionic nanocellulose, such as an anionic nanocellulose selected from the group consisting of carboxymethylated nanocellulose, TEMPO-mediated oxidised nanocellulose and phosphorylated nanocellulose. The use of a charged nanocellulose may provide the dispersion with suitable gelling properties without the need for a further gelling agent. Moreover, the coagulating/flocculating of such charged celluloses may be readily regulated by control of pH and/or salt concentration.

The coagulation liquid may comprise an aqueous acid and/or salt solution. For example, the coagulation liquid may suitably comprise an aqueous solution of hydrochloric acid. The gellable nanocellulose dispersions are readily coagulated/flocculated using such coagulation liquids.

The gellable nanocellulose dispersion may further comprise a gelling agent. The gelling agent may be selected from the list consisting of alginates, pectins, agars, carrageenans and guar gum. Inclusion of a gelling agent may render the nanocellulose film with altered or improved properties.

The gellable nanocellulose dispersion may further comprise a conductive additive, a photocatalytic additive, a magnetic additive, a photoconductive additive, a heat-transfer additive, a thermoplastic additive, and/or a nanoparticle. The conductive additive may be a conductive polymer. The conductive additive may be PEDOT:PSS, graphene and/or carbon nanotubes. Thus, composite nanocellulose films may be manufactured for a variety of applications.

According to a further aspect of the invention, the objects of the invention are attained by a nanocellulose film or fiber according to the appended independent claim.

The nanocellulose film or fiber may be obtained by the method as disclosed herein and as defined in the appended independent method claim.

The nanocellulose film or fiber may comprise from about 10 weight% to about 100 weight% nanocellulose by dry weight.

The nanocellulose film or fiber may further comprise a conductive additive, a photocatalytic additive, a magnetic additive, a photoconductive additive, a heat-transfer additive, a thermoplastic additive, and/or a nanoparticle. The conductive additive may be a conductive polymer. The conductive additive may be PEDOT:PSS, graphene and/or carbon nanotubes. In this manner, composite nanocellulose films or fibers having properties suitable for e.g. energy storage may be produced.

Further objects, advantages and novel features of the present invention will become apparent to one skilled in the art from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the present invention and further objects and advantages of it, the detailed description set out below should be read together with the accompanying drawings, in which the same reference notations denote similar items in the various diagrams, and in which:

Fig. 1 schematically illustrates the use of an apparatus for producing nanocellulose films according to an exemplifying embodiment of the disclosure;

Fig. 2 is a flow chart illustrating an exemplifying embodiment of a method for producing nanocellulose films;

Fig. 3 is a chart illustrating the effect of capillary dewatering using various porous materials;

Fig. 4 shows images of nanocellulose film samples between crossed polarizers, and without polarizers;

Fig. 5 shows an image of a nanocellulose PEDOT:PSS composite film sample between crossed polarizers;

Fig. 6 is a chart showing conductivity measurements performed on model and extruded nanocellulose PEDOT:PSS composite film samples; and

Fig. 7 is a chart showing wide angle X-ray scattering (WAXS) measurements performed on model nanocellulose PEDOT:PSS composite film samples. DETAILED DESCRIPTION

Dispersion

The disclosed method utilizes a dilute gellable nanocellulose dispersion as starting material. By gellable nanocellulose dispersion it is meant a nanocellulose-comprising dispersion that is capable of being gelled, i.e. transformed from flowing to non-flowing. The nanocellulose in itself may provide suitably gellability, and no further gelling agents may be necessary, in particular if the nanocellulose is a charged nanocellulose. However, depending on the nature of the nanocellulose and amount of nanocellulose in the dispersion, it may be necessary to use further gelling agents in order to provide suitable gellablility.

Nanocellulose is a collective term used to describe a variety of nanocellulose products. The term nanocellulose encompasses nanofibrillated cellulose (NFC), nanocrystalline cellulose (NCC/CNC), and bacterial nanocellulose amongst other products. The nanocellulose is preferably nanofibrillated cellulose, which may also be referred to as cellulose nanofibrils (CNF) or microfi bri Hated cellulose (MFC). For example, the nanocellulose may preferably be cellulosic material that is produced through an at least partly mechanical nanofibrillation process, whereby the cellulosic material is disintegrated into a major fraction of individualized elementary nanofibrils and their aggregates. Nanofibrils have diameters of roughly 3-100 nm and can have lengths up to several micrometers.

Included among the mechanical treatments that can be used to obtain nanocellulose are high- pressure homogenization, ultrasonic homogenization, supergrinding/refiner-type treatments, combinations of beating, rubbing, and homogenization, high-shear refining and cryocrushing in various configurations, microfluidization, extrusion and ball-milling. As well as top-down methods of obtaining nanocellulose by disintegration of cellulosic fibers, nanocellulose may also be produced by bottom-up synthetic methods.

Cellulosic fibres may be obtained from any cellulose containing source, but especially wood pulp. Suitable wood pulps include, but are not limited to, kraft, soda, sulfite, mechanical, a thermomechanical (TMP), a semi-chemical, or a chemi-thermomechanical (CTMP) pulp. A raw material for the pulps can be based on softwood, hardwood, recycled fibres or non-wood fibres. The softwood tree species can be for example, but are not limited to: spruce, pine, fir, larch, cedar, and hemlock. Examples of hardwood species from which pulp useful as a starting material in the present invention can be derived include, but are not limited to: birch, oak, poplar, beech, eucalyptus, acacia, maple, alder, aspen, gum trees and gmelina. The raw material may comprise a mixture of different softwoods, e.g. pine and spruce. The raw material may also comprise a non-wood raw material, such as bamboo, sugar beet pulp, wheat straw, soy hulls, bagasse, kelp and seaweeds, such as cladophora. The raw material may also be a mixture of at least two of softwood, hardwood and/or non-wood.

The nanocellulose may be charged. Charged nanocellulose typically has superior gelling properties as compared to non-charged nanocellulose. Using charged nanocellulose therefore may assist in avoiding the need to include further gelling agents in the dispersion. By charged, it is meant that the nanocellulose is modified to comprise charged groups, e.g. by chemical or enzymatic modification. Preferably, the nanocellulose is anionic, i.e. comprises negatively charged groups. Such anionic nanocelluloses include, but are not limited to, carboxylated nanocellulose (e.g. carboxymethylated nanocellulose such as carboxymethylated CNF, or TEMPO-mediated oxidised nanocellulose), and phosphorylated nanocellulose. Typically, the charge is introduced to the cellulose prior to mechanical treatment, since the presence of charge may facilitate disintegration of the cellulose fiber to nanocellulose. The cellulose may be carboxyalkylated using carboxylic acids such as monochloroacetic acid (MCA) or 2- chloropropionic acid (CPA) to provide carboxyl groups. Alternatively, oxidation of the cellulose using for example the TEMPO process may be used to introduce carboxylate groups into the cellulose. If necessary, carboxylic acid moieties may be converted to their anionic form by treatment with a mild base such as sodium bicarbonate.

The gellable nanocellulose dispersion may comprise further components in order to alter the properties of the obtained nanocellulose films or fibers. Due to the high dilution of the gellable nanocellulose dispersion, the further components are able to be well dispersed in the dispersion and uniform composite films or fibers are ultimately obtained. Suitable further components for inclusion in the dispersion include, but are not limited to, conductive polymers, such as PEDOT :PSS; other conductive additives, such as carbon nanotubes, graphite or graphene; nanoparticles, such as titanium dioxide; photocatalytic additives; magnetic additives; photoconductive additives; heat-transfer additives; and/or thermoplastic additives. The further components may constitute from about 0 weight% to about 90 weight% of the total dry matter of the gellable nanocellulose dispersion, such as from about 10 weight% to about 70 weight%, such as from about 20 weight% to about 50 weight%.

The gellable nanocellulose dispersion may comprise further gelling agents. Depending on the nature and amount of nanocellulose in the dispersion, one or more further gelling agent may for example be required in order to obtain suitable gelling properties. One or more gelling agents may also be used to alter the mechanical properties of the obtained film or fiber. Suitable gelling agents include, but are not limited to, alginates (e.g. sodium alginate), pectins, agars (e.g. agar-agar), carrageenans and guar gum. The further gelling agents may in total constitute from about 0 weight% to about 90 weight% of the total dry matter of the gellable nanocellulose dispersion, such as from about 10 weight% to about 70 weight%, such as from about 20 weight% to about 50 weight%.

The gellable nanocellulose dispersion may contain appropriate further additives as conventional in the art. These may include, but are not limited to, stabilizers, antioxidants, pigments and plasticizers.

The various components of the gellable nanocellulose dispersion are dispersed in water by means conventional in the art. The gellable nanocellulose dispersion comprises from about 0.1 weight% to about 5 weight% total dry matter relative to the total weight of the gellable nanocellulose dispersion, preferably from about 0.2 weight% to about less than 1 weight% total dry matter, even more preferably from about 0.3 weight% to about 0.7 weight% total dry matter. The nanocellulose constitutes from about 10 weight% to about 100 weight% of the total dry matter, such as from about 30 weight% to about 100 weight%, preferably from about 45 weight% to about 100 weight% of the total dry matter, even more preferably from about 60 weight% to about 100 weight% of the total dry matter. In determining a suitable nanocellulose content for the dispersion, it is desirable that the content is sufficiently low in order to provide uniform dispersion and avoid premature floc formation. The exact content of nanocellulose that may be included without inducing flocculation depends on a number of factors, such as the nature and charge of the nanocellulose. The balance of dry matter may be composed of further components, further gelling agents, and/or further conventional additives as described above. Extrusion, coagulation and flocculation

The gellable nanocellulose dispersion is extruded into a coagulation liquid in a method similar to that utilized for the production of cellophane. Extrusion is performed through a spinneret or extrusion die, such as a slot extrusion die, having the desired dimensions. Using the terminology of papermaking, sheet-like films may be produced using an extrusion die having a large cross-direction dimension, e.g. several meters in magnitude, whereas filament-like films or fibers may be produced using a spinneret where the cross-direction dimension is similar or essentially the same as the z-direction dimension, e.g. a circular or square die outlet having a diameter in the millimetre scale. Naturally, all intervening cross-direction dimensions are also obtainable, and the final film or fiber may have a cross-direction dimension of from about 1 mm to about 10 m, such as from about 10 mm to about 1 m. Films may have a cross-direction dimension that is at least twice the z-direction dimension, such as at least ten times greater than the z-direction dimension. Fibers may have across-direction dimension that is similar or essentially the same as the z-direction dimension. The dimension in the z-direction is preferably sufficient to yield a film or fiber having the desired thickness after dewatering and drying, and the extrusion die may for example have a z-direction dimension of from about 0.5 mm to about 20 mm, such as about 1.5 mm to about 10 mm, preferably from about 2 mm to about 6 mm. If the method is performed continuously, the dimension in the machine direction is essentially continuous, and any desired length of film or fiber may be produced.

The outlet of the extrusion die or spinneret is preferably immersed in the coagulation liquid bath, i.e. the extrusion is a "wet" extrusion method. However, the outlet of the extrusion die or spinneret may be positioned above the coagulation liquid bath, allowing the extrudate to fall into the coagulation liquid. This may be termed a "dry-wet" or "airgap" method, in analogy with corresponding spinning methods.

The coagulation liquid may be any liquid suitable for inducing coagulation and/or flocculation in the gellable nanocellulose dispersion. When the nanocellulose is anionic, this may be achieved using an acidic coagulation liquid to convert the charged carboxylate groups on the nanocellulose to neural carboxylic acid groups. For example, an aqueous acid solution having a pH less than about 3, preferably a pH of about 2 or less, such as a pH of about 1 is suitable. The acid may be any suitable acid, such as a mineral acid, such as dilute hydrochloric acid or dilute sulphuric acid. The coagulation liquid may be buffered.

Extrusion into the coagulation liquid yields a nanocellulose gel strip having a dimension in the cross-direction and z-direction determined by the dimension of the extrusion die or spinneret. In a continuous process, the strip is essentially continuous in the machine direction. By the term "strip" it is meant that the gel is thin (< 20 mm) in the z-direction and its dimension is considerably less in the cross-direction (millimetre to meter scale) as compared to in the machine direction (which may be essentially continuous). The strip in practice may be filament-like, ribbon-like, or sheet-like, depending on the cross-direction dimension. The coagulation liquid bath is dimensioned to provide sufficient residence time in the bath for complete formation of the nanocellulose gel. The coagulation bath may have an essentially laminar flow in the machine direction to assist extrusion, but is not stirred or turbulent, meaning that the nanocellulose gel strip remains intact once formed, and the network structure of the gel is "locked" prior to subsequent processing. Without wishing to be bound by theory, it is thought that, when a charged nanocellulose is used, neutralization of the charged groups on the nanocellulose decreases the repulsion between nanocellulose fibers and allows the fibers to form a physically bonded gel network. It is thought that the formed fibril network provides the nanocellulose gel with the strength to be able to withstand mild dewatering means such as capillary or osmotic dewatering

Removal of water

After the step of coagulation/flocculation is completed, the nanocellulose gel may be picked up by, or transferred to, a foraminous support. This support allows excess water to drain from the gel. For example, the support may be a continuous belt that picks up the nanocellulose gel in the coagulation bath and transports it out of the bath to initiate dewatering. By foraminous, it is meant that the support is supplied with a multiplicity of holes capable of allowing water to drain from the gel, i.e. it is water-permeable. The support may for example be a wire (mesh conveyor belt) as used in a conventional papermaking process. A single support under the nanocellulose gel strip may be used, or the gel strip may be sandwiched between two supports. The residency time on the support is preferably sufficient to allow as much water as possible to passively drain from the nanocellulose gel prior to initiating active dewatering means. The draining of excess water is however typically relatively fast, and residence on the support prior to further dewatering steps may be in the order of 1 minute or less. Depending on the nature of the nanocellulose used, this passive dewatering may be sufficient to bring the gel to a dry matter content of about 1 weight% to about 2 weight%.

The strength and cohesion of the nanocellulose gel increases with each successive dewatering step, and the ability to successfully perform each step may therefore be contingent on a previous, milder dewatering step. For example, after dewatering by draining under gravity, the nanocellulose gel may be subjected to a subsequent mild dewatering step. Osmotic dehydration, e.g. by immersing the nanocellulose gel in an osmotic solution, could be used at this stage in order to further decrease the amount of water in the gel. However, it has been found that the gel after draining may withstand capillary dewatering, and that such dewatering may be effective in dewatering the nanocellulose gel to a dry matter content in the range of from about 3 weight% to about 12 weight%, preferably from about 4 weight% to about 12 weight%. Since capillary action is cheap and relatively rapid to perform, it is the preferred means of dewatering after having performed initial passive dewatering (draining). Capillary dewatering is performed by applying a porous material in contact with at least one surface of the nanocellulose gel. By in contact, it is meant either directly in contact, and/or indirectly in contact, i.e. via a foraminous support when the porous material is capable of absorbing water from the gel via this route. A porous material may be applied to one or both faces of the nanocellulose gel, optionally via a foraminous support at one or both faces. The porous material may be any substrate capable of absorbing water from the gel, such as blotting paper, textile (e.g. cotton and/or regenerated cellulose), sponge, or press felt. Preferably, the porous material is capable of itself being dewatered by pressing so that it can be regenerated and used in a continuous dewatering process. The residence time in the capillary dewatering section should be sufficient to ensure that the nanocellulose gel has reached sufficient dry matter content to be able to withstand more mechanically demanding dewatering means, and this may be in the range of from about 10 seconds to about 20 minutes, typically in the range of from about 1 minute to about 20 minutes.

After mild dewatering, the nanocellulose gel is strong enough to withstand more demanding dewatering methods. The preferred means of dewatering at this stage is by pressing. Pressing is a well-established and very efficient technique, but requires that the nanocellulose gel network is capable of handling the loads applied. Therefore, pressing may be performed in a number of subsequent pressing steps, such as from 1 up to 5 sequential pressing steps, where the load applied is increased in each step. Suitable pressing means are known in the art and are widely used, for example, in papermaking. By pressing, the dry matter content of the nanocellulose gel may be increased considerably, to a range of from about 20 weight% to about 35 weight%. Residence time in the press section is typically shorter than in the capillary dewatering section, and may be typically be in the range of from about 1 second to about 2 minutes, preferably from about 10 seconds to about 1 minute.

Not all water can be pressed from the nanocellulose gel, and therefore in a final step for removing water, the nanocellulose gel may be brought to the desired dry matter content by heating. Heating may provide a film or fiber having any desired dry matter content, such as from about 50 weight% to about 100 weight%. Since only about 2-4 kilograms of water per kilogram dry matter remain after pressing, the heating section is not required to evaporate large amounts of water, and the step is therefore relatively rapid and commercially viable. Heating may be performed at any desired temperature, such as a temperature in the range of from about 50 °C to about 180 °C, such as about 90 °C to about 150 °C, preferably from about 120 °C to about 150 °C. Suitable means for drying films or fibers by heating are known in the art, such as IR lamps or heated dryer rolls.

After final drying, the nanocellulose film or fiber may be wound on a reel.

Removing water from the nanocellulose gel may preferably be performed in the sequence: draining-capillary dewatering-pressing-heating. Some of the steps may be performed recurrently, such as multiple pressing operations. Osmotic dehydration may be used as a complement or alternative to capillary dewatering. If osmotic dehydration is used to complement capillary dewatering, it may be performed prior to capillary dewatering. For example, it may be performed after the coagulation/flocculation step and prior to draining on a foraminous support. Alternatively, osmotic dehydration may be performed after the passive draining, since this may allow the use of greater osmotic pressures without risking the integrity of the nanocellulose gel.

Figure 1 schematically illustrates the use of an apparatus for producing nanocellulose films according to an exemplifying embodiment of the disclosure. A gellable nanocellulose dispersion is extruded from extruder die 105 into coagulation bath 107, where it coagulates to form a nanocellulose gel strip 109. The strip 109 is picked up by the foraminous support 103 (e.g. wire) and transported out of the coagulation liquid 107 to a draining section 110. In draining section 110, water may freely drain from the nanocellulose gel, through the foraminous support 103. After passing through the draining section 110, the nanocellulose gel strip 109 proceeds to the capillary dewatering section 111, where a porous support 113 (e.g. dewatering felt) in contact with the nanocellulose gel strip 109 absorbs water from the gel. After the capillary dewatering section 111, the nanocellulose gel strip 109 proceeds to a press section 115 where it is pressed between two press rolls 116 to further remove water. After the press section, the nanocellulose gel strip 109 proceeds to drying section 115 where IR heaters 119 evaporate water from the nanocellulose gel strip 109, ultimately providing dried nanocellulose film 121. The nanocellulose film 121 is wound on a reel 123.

Figure 2 is a flow chart illustrating an exemplifying embodiment of a method for producing nanocellulose films. Step s200 denotes the start of the method. In step s201 a gellable nanocellulose dispersion comprising a nanocellulose and water is provided. In step s203 the nanocellulose dispersion is extruded through an extrusion die into a coagulation liquid to provide a strip of nanocellulose gel. In step s205 the strip of nanocellulose gel is captured on at least one foraminous support and transported out of the coagulation liquid. In step s207 a porous material is applied in contact with at least one surface of the nanocellulose gel to remove water by capillary action. In step s209 the nanocellulose gel is passed through a press to press water out of the nanocellulose gel. In step s211 the nanocellulose gel is dried by application of heat. Step s213 denotes the end of the method.

The apparatus exemplified in Figure 1 and method exemplified in Figure 2 may readily be adapted for production of nanocellulose fiber. This may be done e.g. by using an extrusion die where the cross-direction dimension is similar or essentially the same as the z-direction dimension, such as circular or square die outlet having a diameter in the millimetre scale. In order to avoid deformation (flattening) of the fiber, the pressing step s209 may also be avoided. Nanocellulose film or fiber

The nanocellulose films or fibers obtained by the method described herein have a dry matter composition that correspond essentially to the dry matter composition of the gellable nanocellulose dispersion. That is to say, they may comprise nanocellulose in a range of from about 10 weight% to about 100 weight% by dry weight, such as from about 30 weight% to about 100 weight%, preferably from about 45 weight% to about 100 weight%, even more preferably from about 60 weight% to about 100 weight%. The balance of the dry weight of the film or fiber may be composed of further components, further gelling agents, and/or further conventional additives as described herein. The films are typically dewatered and dried to a dry matter content of from about 50 weight% to about 100 weight%, such as from about 70 weight% to about 90 weight%.

Due to the method of production from dilute dispersions, the obtained films are very homogenous and have properties that may differ from films or fibers produced by conventional methods. For example, fiber orientation may be greater in films or fibers produced by the method disclosed herein, and anisotropic films or fibers may be produced. Such differences in properties are especially apparent in nanocellulose composite films or fibers produced according to the method herein. For example, a nanocellulose PEDOT:PSS composite film or fiber produced according to the method described herein may have a lower conductivity than corresponding films or fibers produced by prior art methods.

The films produced by the method described herein may be filament-like, ribbon-like or sheetlike. The film may have a cross-direction dimension of from about 1 mm to about 10 m, such as from about 10 mm to about 1 m. The thickness (z-direction dimension) of the final film is significantly less than the corresponding cross-direction dimension in the nanocellulose gel.

Nanocellulose films may be produced even by extrusion from dies where the z-direction is the same as the cross-direction dimension, since the nanocellulose gel strip may be flattened during dewatering by e.g. pressing. Nanocellulose fibers, i.e. products having a z-direction dimension similar or essentially the same as the cross-direction dimension may be produced using extrusion dies where the cross-direction dimension is similar or essentially the same as the z-direction dimension, e.g. a circular or square die outlet. In such a case, in order to obtain fibers, case should be taken in the selection of subsequent dewatering steps in order to avoid deformation of the nanocellulose strip resulting from extrusion. For example, it may be desirable to avoid pressing.

The production process is preferably continuous, and therefore the film or fiber may be produced to essentially any desired length in the machine direction. The films or fibers are free-standing or self-supporting. That is to say they have ample mechanical integrity in themselves are not dependent on a support substrate to provide strength.

Potential applications for films produced by the method herein include use as barrier films. Composite nanocellulose films or fibers produced by the method as described herein may find application in organic electronics applications, for example for use in energy storage applications.

The invention will now be described in more detail with reference to certain exemplifying embodiments and the drawings. However, the invention is not limited to the exemplifying embodiments discussed herein and/or shown in the drawings, but may be varied within the scope of the appended claims. Furthermore, the drawings shall not be considered drawn to scale as some features may be exaggerated in order to more clearly illustrate certain features.

Examples

Production of nanocellulose dispersion

The starting pulp was a softwood sulphite dissolving pulp (Domsjb Dissolving plus) from Domsjb Fabriker AB, Sweden. The never-dried fibers were dispersed in water (approx. 1.5% (w/w)) at 10000 revolutions by using an ordinary laboratory pulper. The fibers were then exchanged to ethanol by washing the fibers in one liter of ethanol four times with a filtering step in between.

After the solvent exchange process, the fibers (30 grams, approx. 20% (w/w)) were impregnated for 30 minutes with a solution of 2.7 grams of monochloroacetic acid in 107 grams of 2-propanol. The fibers were then added in portions to solutions that had been heated just below their boiling temperature containing. The solutions consisted of 4.4 grams of NaOH, 108 grams methanol, and a total of 428 grams of 2-propanol. The carboxymethylation reaction was allowed to continue for one hour. Following the carboxymethylation step, the fibers were filtrated and washed in three steps. First, the fibers were washed with 20 liters of water. Thereafter, the fibers were washed with two liters of acetic acid (0.1 M) and finally with 10 liters of water. The fibers were then impregnated with two liters NaHCO3 solution (4% w/w solution) for 60 minutes in order to convert the carboxyl groups to their sodium counterion form. Finally, the fibers were washed with 15 liters of water, and drained on a Buchner funnel.

After the carboxymethylation, the pulp was homogenized (Microfluidizer M-110EH, Microfluidics Corp., USA) four times at 1700 bar at 1%, and thereafter four more times at 0.5%, by two serial reaction chambers with smallest dimension 200 micron and 100 micron, respectively. The produced dispersion was then diluted to 0.4% by magnetic stirring, and stored in the refrigerator until use.

Extrusion and coagulation of gellable nanocellulose dispersion

The CNF dispersion is removed from the refrigerator and stirred at 10000 rpm for 10 minutes using a laboratory homogenizer (Polytron PTT 3100D). Bubbles are then removed from the dispersion by centrifuging at 1000 g for 5 minutes. The CNF is then ready for use.

The following parameters are used when extruding the nanocellulose films. The extrusion die has a slot outlet 20 mm wide and 2 mm high. The coagulation bath is a dilute hydrochloric acid solution having a pH of approx. 0.7 to 0.8. The outlet of the extrusion die is submerged in the coagulation bath. The flow rate through the extrusion die is set to 50 ml/minute, and an adequate laminar flow is arranged in the coagulation bath to carry the extrudate along the bath. A section of nylon papermaking wire, 0.5 mm mesh, is placed at the bottom of the coagulation bath in order to pick up the nanocellulose gel after coagulation. The gel is allowed to reside in the bath for 2 minutes prior to being picked up on the wire and allowed briefly to drain.

Removing water from the nanocellulose gel

The wire retaining the nanocellulose gel on top is placed on top of a porous sheet. A further section of wire may optionally be placed above the nanocellulose gel, and a further porous sheet is placed as the top layer of the sandwich-like construction. This sandwich construction is weighted with a weight of c:a 5 kg on top in order to ensure good contact between the layers, and is left to dewater the nanocellulose gel for a predetermined period of capillary dewatering. The sandwich construction is dismantled and the partially dewatered nanocellulose gel is transferred to lie flat upon section of Teflon belt. A sheet of blotting paper is placed on top of the nanocellulose gel, and the gel is dewater by pressing the gel beneath the sheet of blotting paper using a flat substrate (e.g. flat metal sheet) and pressure applied by hand. The pressing operation is repeated three times, each time with a fresh sheet of blotting paper.

Finally, the nanocellulose gel on the Teflon belt was dried at 50 °C under a halogen IR lamp for 2 minutes to provide the final nanocellulose film.

Various porous sheets and periods of capillary dewatering were tested. As the porous sheets, various combinations of blotting paper, dry absorptive textile (Wettex® or VWR, mixture of regenerated cellulose and cotton fibers, SE140344 Cl) and wet absorptive textile (in order to mimic continuous pressing process) were tested. The capillary dewatering period was varied from 30 seconds up to 12 minutes (720 seconds).

It was found that blotting paper was most effective for capillary dewatering, followed by dry absorptive textile, and then wet absorptive textile. However, it was found that even the wet absorptive textile was capable of dewatering the nanocellulose gel from an initial dry matter content of about 2.0 weight% to a dry matter content of about 3.7 weight% after 12 minutes. The effect of drying with blotting paper (squares) or wet Wettex (circles) for various periods of time are shown in Figure 3. The effect of pressing the wet-Wettex-dewatered nanocellulose gel is also shown. The dry matter content is increased from about 3.7 weight% to about 32.8 weight% by pressing by hand (the triangle).

Characterisation of nanocellulose film

The obtained film has a thickness of 21.1 micron (standard deviation 0.9 micron) as measured using a Lorentzen and Wettre micrometer.

Fiber alignment was observed using crossed polarizers. Two crossed polarizers and a camera (ProgRes Jenoptik, Jena) were used to take images (1360 x 1024 pixels 24 bit RGB, TIFF) of film samples with a white diffuse light source. Images of the film samples between crossed polarizers (images c) and d)) and without polarizers (images a) and b)) are shown in Figure 4. The light and dark spots (inhomogenities in fibril alignment) correspond well with the transport wire geometry. Light lines are fibre fragments. It is concluded that the extruded film is homogeneous and the induced variations are due to the drying.

Synthesis and characterisation of Nanocellulose PEDOT:PSS composite films

Nanocellulose composite films comprising PEDOT:PSS were produced, dewatered and dried by a similar procedure as described above. PEDOT:PSS (PH 1000) was obtained from Heraeus GmbH & co. KG. The pH of PEDOT : PSS dispersions was adjusted to 7 by the addition of NaOH before mixing with the CNF dispersion. The CNF dispersion was diluted to 0.3% by magnetic stirring overnight and mixed with the pH-adjusted PEDOT:PSS suspension to result in mixtures with a range of PEDOT:PSS/CNF ratios. Each mixture was mixed with a Polytron PT 100D in 10000 rpm for three minutes and then kept in a vacuum desiccator overnight to remove air bubbles. Each mixture comprised a dry matter content of 0.25 weight% and the dry matter was composed of nanocellulose and PEDOT:PSS in a ratio ranging from 90:10 to 60:40.

In Figure 5 it can be seen by observing the obtained film between crossed polarizers that the PEDOT-containing nanocellulose film has a high degree of homogenity.

In order to investigate conductivity, a number of model nanocellulose PEDOT:PSS films having varying ratios of nanocellulose to PEDOT:PSS were produced as follows. A dispersion having the relevant ratio was poured into a petri dish in a thin layer. The dispersion was gelled by careful addition of 0 pH aqueous HCI (approx. 1 M) at an edge point of the petri dish without stirring to provide a final pH of 2 in the gelled dispersion. The gelled dispersion was left to dry in the petri dish for 7 days, was soaked in ethylene glycol overnight, and then the film was dried again under ambient conditions for 7 days. The film was then conditioned as defined below.

The thickness of each model film was determined using a Lorentzen and Wettre micrometer, and the electrical conductivity of the films was measured using a four-point probe and Keithley 2401 electrometer (Keithley Instruments GmbH, Germany) under controlled conditions of 23 °C and 50% RH.

The same measurement was performed using an extruded film produced by the method described herein and having 0.4 parts PEDOT:PSS. The results of the conductivity measurements are shown in Figure 6. It can be seen that the conductivity can be tuned from very low to high by controlling the amount of PEDOT:PSS in the film. It can also be seen that the extruded film has a lower conductivity than a corresponding model film. It is thought that this is due to the higher anisotropy of the extruded film.

A number of the model films were also investigated using wide angle x-ray scattering (WAXS). WAXS measurements were performed with the aid of an Anton Paar SAXSpoint 2.0 system (Anton Paar, Graz, Austria) equipped with a Microsource X-ray source (Cu K-alpha radiation having a wavelength of 0.15418 nm) and a Dectris 2D CMOS Eiger R IM detector with 75 pm by 75 pm pixel size. All measurements were performed with a beam size of approximately 500 pm in diameter, at ambient temperature with a beam path pressure at about 1-2 mbar. The samples were exposed to vacuum during the measurement. The result of the measurements is shown in Figure 7, and demonstrates that there is PEDOT:PSS in the network and that it has a crystalline structure.

The degree of anisotropy, fl, defined as fl = (180 -fwhm) / 180, where fwhm is the full width at half the maximum in degrees of the in-plane orientation distribution of cellulose chains, can be measured by for example WAXS. When fl = 0, the film is isotropic and when fl = 1, the cellulose is fully aligned. With this method it is not only possible to produce isotropic films, but it is also possible to produce anisotropic films. The anisotropic films can have fl = 0.05 - 0.9, preferably 0.2 - 0.7, or more preferably 0.4 - 0.6.

Other nanocellulose composite films

Composite nanocellulose films comprising carbon black have been produced having a mass ratio of carbon back to nanocellulose as high as 90:10.