TECHNICAL FIELD

The present invention relates to methods for the production of precursor fibers for the production of carbon fiber. The present invention further relates to precursor fibers obtained by such a method as well as carbon fibers obtained therefrom.

BACKGROUND ART

Increasing legislative requirements for improved fuel economy in vehicles is leading to an increased demand for light-weight materials for mass-market vehicle manufacture. Carbon fiber composites could address this demand provided that abundant, lower cost carbon fiber precursors were available.

Carbon fibers are however typically manufactured from polyacrylonitrile (PAN), a precursor material that is both expensive and derived from non-renewable petrochemical sources. The precursor material typically accounts for approximately half of the total cost of carbon fiber, and therefore, due to the high cost of PAN, applications of carbon fibers have typically been limited to speciality applications such as within the aerospace industry and high-end vehicles.

Lignin is an organic polymer present in the support issues of vascular plants. During paper pulping operations, a lignin fraction may be isolated from the pulping process. Lignin is thus an abundant and renewable feedstock. Attempts have been made to utilize lignin as a feedstock in the manufacture of carbon fibers. An overview of such attempts is provided in Baker, D. A., and Rials, T. G. "Recent advances in low-cost carbon fiber manufacture from lignin.", Journal of Applied Polymer Science, 130(2), 2013, pp. 713-728.

Attempts to produce lignin precursor fibres have most commonly utilized melt spinning techniques; see for example WO 2012/112108 Al. However, this places high demands on the melt properties of the lignin and thus often requires lignin that has undergone extensive purification or derivatisation in order to be spinnable.

Attempts have also been made to wet spin lignin blends. EP 2889401 B1 discloses a method of wet spinning regenerated cellulose fibers comprising lignin. The method utilises the viscose process which requires the in-situ formation of cellulose xanthate using carbon disulphide.

WO 2012/156441 discloses a method for wet spinning lignin-containing precursor fibers. The fibers are spun from a solution comprising lignin and cellulose or a cellulose derivative in at least one solvent. The solvent is selected from tertiary amine oxides, ionic liquids, polar aprotic solvents, dimethylformamide and/or dimethylacetamide. There remains a need for an improved method of producing precursor fibres from an abundant source.

SUMMARY OF THE INVENTION

The inventors of the present invention have identified a number of shortcomings with prior art methods of producing precursor fibers for carbonization. The most commonly applied method, melt spinning of PAN, requires a precursor material that is expensive and from non renewable sources. The production of precursor fibers from lignin commonly utilizes melt spinning, but this however requires lignins that have been subject to extensive purification or derivatisation, thus increasing the cost of the precursor material. The methods that exist for wet spinning lignin blends require either the use of toxic carbon disulphide in the case of the viscose method, or expensive solvents such as ionic liquids in the case of other regenerated celluloses. This increases the environmental impact of the fiber production, as well as the production costs due to the need for complex process equipment.

It is an object of the present invention to provide a method of producing a lignin-containing precursor fiber for the production of carbon fibers that overcomes or at least alleviates one or more of the above shortcomings.

These objects are achieved by a method for the production of precursor fiber according to the appended claims. The method comprises the steps: a) providing a spinning dope comprising a lignin and a gelling hydrocolloid dispersed in aqueous alkaline solution; b) extruding the spinning dope through a spinning nozzle to provide an initial fiber; and c) passing the initial fiber through a coagulation liquid to provide the precursor fiber; wherein the coagulation liquid is arranged to effect gelling of the gelling hydrocolloid by regulation of pH and/or ionicity.

The method of the invention allows a simple and robust means of spinning of a precursor fiber having significant lignin content using cheap, readily available materials of low toxicity. The method of the invention overcomes or alleviates several of the shortcomings identified in the prior art, and therefore achieves the object of the invention.

The gelling hydrocolloid may be a polysaccharide, such as a natural or synthetic

polysaccharide. The polysaccharide may be chosen from alginate, carrageenan, pectin, or combinations thereof, preferably alginate. These polysaccharides are readily available, non toxic as demonstrated by their widespread use in foodstuffs, and easily gelled under well- known conditions.

The coagulation liquid may be an aqueous solution of an inorganic acid or salt thereof, preferably an aqueous solution of a mineral acid or a magnesium, calcium, strontium, barium, lithium, sodium, potassium, caesium or ammonium salt thereof, even more preferably calcium chloride or hydrochloric acid. The low pH and/or metal salt content of the coagulation liquid effects gelling of the gelling hydrocolloid and provides a cohesive fiber structure. The acids and salts listed are readily available and are known to effect gelling in a variety of

hydrocolloids. The lignin may be chosen from kraft lignin, LignoBoost lignin, sulphite lignin, soda lignin, organosolv lignin, lignin from cellulosic ethanol production, or mixtures thereof, preferably LignoBoost lignin. Since the melt properties of the lignin are non-essential, the lignin may be chosen from a wide variety of commercially available lignins, without requiring further purification. The lignin may for example be a softwood lignin, which typically is difficult to use in the production of precursor fibers due to unsuitable melt properties.

The spinning dope may comprise at least 4 percent by weight of lignin relative to the total weight of the spinning dope, preferably at least 7 percent by weight. The spinning dope may comprise at least 4 percent by weight of gelling hydrocolloid relative to the total weight of the spinning dope, preferably at least 7 percent by weight. Such concentrations provide a spinning dope with suitable spinning properties. The upper concentration limits of the lignin and gelling hydrocolloid depend on the nature of the lignin, gelling hydrocolloid and spinning method used and may readily be determined by a skilled person. Such upper concentration limits may for example be up to about 25 percent by weight lignin and up to about 25 percent by weight polysaccharide.

The ratio of gelling hydrocolloid to lignin in the spinning dope may be from about 1:4 to about 4:1 by weight, such as from about 1:3 to about 3:1, from about 1:2 to about 2:1, or from about 1:1 to about 1:1. The ratio of gelling hydrocolloid to lignin may preferably be from about 1:3 to about 4:1 by weight, even more preferably from about 1:3 to about 1:1 by weight. This provides a precursor fiber having sufficient mechanical properties as well as a suitable lignin loading for carbonization. Moreover, a suitable ratio of gelling hydrocolloid to lignin is a prerequisite for wet spinning, since in order to be wet spun the spinning dope must have a sufficiently high amount of gelling hydrocolloid relative to lignin. The spinning dope may further comprises an alum, preferably potassium aluminium sulfate. This may assist in improving the spinnability of the spinning dope, for example by increasing viscosity, as well as improving the wet strength of the resulting precursor fibers.

The aqueous alkaline solution of the spinning dope may be from about 0.1 M to about 1 M sodium hydroxide solution, preferably from about 0.2 M to about 0.7 M sodium hydroxide solution. Such concentrations are suitable for dispersing the lignin in the dope.

The method may comprise a further step d) of passing the precursor fiber through a second coagulation liquid; wherein the second coagulation liquid may be the same or different from the first coagulation liquid. This may facilitate the spinning process, for example by preventing clogging of the spinning nozzle by the spinning dope. The method may comprise one or more subsequent steps ei) - e _x ) of washing the precursor fiber in water and/or an organic water-miscible solvent such as ethanol. This may facilitate drying of the precursor fiber, improve the purity of the obtained precursor fibers, and thus increase the quality of carbon fibers derived from the precursor fibers. The method may comprise one or more subsequent steps fi) - f _x ) of drying the precursor fiber, preferably at a temperature of 50 °C or higher, such as from about 50 °C to about 150 °C. Drying at these temperatures allows the removal of solvents, including water, without initiating changes at the molecular level of the fiber, such as crosslinking of the lignin.

According to a further aspect of the present invention, the objects of the invention are achieved by a precursor fiber according to the appended claims.

The precursor fiber may be produced by the method for the production of precursor fiber as herein described.

The precursor fiber comprises a gelling hydrocolloid and a lignin. Features of the method herein described, such as the nature and relative concentrations of the gelling hydrocolloid and lignin, may equally well be applied to the resulting precursor fiber, where appropriate. So, for example, the gelling hydrocolloid may be a polysaccharide chosen from alginate, carrageenan, pectin, or combinations thereof, preferably alginate. The lignin may be chosen from kraft lignin, LignoBoost lignin, sulphite lignin, soda lignin, organosolv lignin, lignin from cellulosic ethanol production, or mixtures thereof, preferably LignoBoost lignin. The ratio of gelling hydrocolloid to lignin may be from about 1:4 to about 4:1 by weight, such as from about 1:3 to about 3:1, from about 1:2 to about 2:1, or from about 1:1 to about 1:1. The ratio of gelling hydrocolloid to lignin may preferably be from about 1:3 to about 4:1 by weight, even more preferably from about 1:3 to about 1:1 by weight. A suitable ratio of gelling hydrocolloid to lignin is a prerequisite for obtaining a wet spun fiber, since in order to be wet spun the spinning dope must have a sufficiently high amount of gelling hydrocolloid relative to lignin.

The precursor fiber may comprise from about 20 - 80 % by dry weight of the gelling hydrocolloid and/or about 20 - 80 % by dry weight of the lignin, preferably about 30 - 50 % by dry weight of the gelling hydrocolloid and/or about 50 - 70 % by dry weight of the lignin, relative to the total dry weight of the precursor fiber. The precursor fiber may have a fiber diameter of from about 1 pm to about 100 pm, preferably from about 5 pm to about 50 pm.

According to another aspect of the present invention, the objects of the invention are achieved by a method for the production of carbon fiber according to the appended claims. The method comprises the steps: a) producing a precursor fiber by the method for the production of precursor fiber as detailed herein; b) optionally stabilizing the precursor fiber by heating in an oxidizing atmosphere to a temperature of from about 200 °C to about 300 °C; and c) carbonizing the precursor fiber by heating in an inert atmosphere to a temperature of about 900 °C or higher.

The method thus allows for the production of carbon fibers from a precursor fiber that is simple and relatively environmentally benign to produce, from precursor materials that are cheap, renewable, abundant and non-toxic.

According to yet a further aspect of the present invention, the objects of the invention are achieved by a carbon fiber produced by the method for the production of carbon fiber described herein. The produced carbon fiber may be a structural carbon fiber, such as a discontinuous or continuous structural carbon fiber.

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. la schematically illustrates an apparatus for wet spinning of a precursor fiber Fig. lb schematically illustrates an apparatus for dry jet wet spinning of a precursor fiber

Fig. 2 is a flow chart schematically illustrating a method for the production of a

precursor fiber

Fig. 3 is a chart displaying the thermogravimetric analysis of precursor fibers during stabilization and carbonization

Fig. 4a is a SEM microscopy image showing carbon fibers obtained by the method herein

Fig. 4b is a SEM microscopy image showing a cross-section of a carbon fiber obtained by the method herein

DETAILED DESCRIPTION

The present invention provides a simple, robust and non-toxic method of producing precursor fiber for the production of carbon fiber from renewable sources. By renewable it is meant a material derived from a natural resource that, after exploitation, can return to its previous stock levels by natural processes of growth or replenishment. The method comprises the following steps: a) providing a spinning dope comprising a lignin and a gelling hydrocolloid dispersed in aqueous alkaline solution; b) extruding the spinning dope through a spinning nozzle to provide an initial fiber; and c) passing the initial fiber through a coagulation liquid to provide the precursor fiber. The coagulation liquid is arranged to effect gelling of the gelling hydrocolloid by regulation of pH and/or iconicity. In this manner, the initial fiber from the spinneret, which is relatively non- cohesive, is converted to a cohesive gel having mechanical properties suitable for spinning. The cohesive fibrous gel may then be treated in further process operations such as stretching, washing and drying to provide precursor fiber. Materials Spinning dope

The spinning dope comprises a lignin and a gelling hydrocolloid dispersed in aqueous alkaline solution. By dispersed, it is meant that the lignin and gelling hydrocolloid may each independently be dissolved in the aqueous alkali solution or form a colloidal suspension (sol) with the aqueous alkaline solution.

Aqueous alkaline solution

By aqueous alkaline solution it is meant a solution of any base that forms hydroxide ions upon dissolution in water, including basic salts of alkali metals and alkali earth metals, as well as ammonia. The aqueous alkaline solution is preferably a sodium hydroxide solution. The aqueous alkaline solution may have any concentration suitable for dispersing the lignin and polysaccharide. Such a concentration may be for example from about 0.1 M to about 1 M, or from about 0.2 M to about 0.7 M.

Lignin

Lignin is an amorphous polyphenolic material created through the enzymatic polymerisation of coniferyl-, sinapyl- and p-coumaryl-alcohols in lignocellulosic materials such as wood. The lignin for use in the present invention may be obtained from any lignocellulosic source material. These include wood, annual crops and agricultural waste.

Suitable woods may include softwoods and hardwoods. 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 lignin suitable as a starting material in the present invention may be derived include, but are not limited to: birch, oak, poplar, beech, eucalyptus, acacia, maple, alder, aspen, gum trees and gmelina. The raw material for lignin production 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, corn stover, bagasse and grasses such as switchgrass and elephant grass.

Since the lignin can be produced from various green resources, such as wood, agricultural residues and annual crops, it is thus abundant, renewable and biodegradable. The lignin may be isolated as a by-product of a pulping process for the manufacture of paper or board. Common pulping processes are the kraft (sulphate) process, sulphite process, soda process and organosolv processes that may utilize a variety of solvents including but not limited to ethanol, methanol, butanol, ethylene glycol, acetic acid, formic acid, acetone and mixtures thereof. The lignin may be obtained from a LignoBoost process whereby high-quality lignin is obtained by at least partially neutralising kraft black liquor using carbon dioxide in order to precipitate the lignin. The LignoBoost process is further described in: Tomani, Per; The Lignoboost Process; Cellulose Chem Technol., 44(1-3), 53-58 (2010).

The lignin may be isolated as a by-product of cellulosic ethanol production. When fermenting a lignocellulosic biomass feedstock to produce ethanol, typically 15 to 30 percent of the biomass remains unconverted after fermentation. This residual biomass comprises primarily lignin.

The lignin used in the present invention is preferably non derivatised lignin. By non-derivatised lignin it is meant lignin that is not subject to any extensive derivatisation either during isolation or through post-isolation modification. Non-derivatised lignins may be subject to some degree of hydrolysis or oxidation during isolation, depending on the process used for isolating the lignin, but this is an unintentional consequence of the isolation process and the primary lignin structure remains substantially intact and unmodified. For example, lignins isolated by the kraft and soda pulping processes are considered to be non-derivatised. Lignosulfonates isolated as a by product of the sulphite pulping process are not considered to be a non-derivatised due to the abundance of sulfonate groups formed on the lignin primary structure. Ogranosolv lignins may or may not be considered non-derivatised depending on the extent of derivatisation (e.g. acetylation) occurring during isolation.

The lignins used may be fractionated by any means known in the art, e.g. ultrafiltration or precipitation, in order to provide a purer lignin or a lignin with reduced dispersity. The lignin is preferably provided in pulverized form for use in the methods of the invention.

Gelling hydrocolloid Hydrocolloids are a heterogeneous group of long chain polymers (both polysaccharides and proteins) characterised by their property of forming viscous dispersions and/or gels when dispersed in water. A subset of hydrocolloids can form hydrogels under specific conditions, herein termed gelling hydrocolloids. Hydrocolloids that are gelled via crosslinking of hydrocolloid chains by ions are termed ionotropic gelling hydrocolloids. The gelling hydrocolloid may be native, naturally-derived, semi-synthetic or synthetic. The gelling hydrocolloid may be a polysaccharide gelling hydrocolloid. Such hydrocolloids include but are not limited to alginates such as sodium alginate, pectins, in particular low ester pectins, and carrageenans, such as /oto-carrageenan and kappa-carrageenan. Ionotropic gelation may be controlled by control of ionicity and/or pH. For further information regarding gelling hydrocolloids see PHILLIPS, Glyn O.; WILLIAMS, Peter A. (ed.). Handbook of hydrocolloids. Elsevier, 2009. In particular, Table 1.5 of the aforementioned reference text lists gelling hydrocolloids and their gelling mechanisms, and is herein incorporated by reference. Alginates such as sodium alginate form non-thermoreversible gels upon exposure to divalent cations such as calcium, barium or strontium cations. Alginic acid gels may also be formed from alginates by lowering the pH of the alginate solution to approximately pH 4 or less. Alginates are commercially available as sodium, potassium or ammonium salts in a variety of mesh sizes and viscosity levels. Precursor fibers comprising alginates may be easily spun, as demonstrated by the examples herein.

Further additives

Further additives may be added to the spinning dope, for example in order to improve the spinnability of the dope and/or improve the properties of the resulting fiber. For example, it has been found that addition of an alum such as potassium aluminium sulfate to the dope increases the viscosity and spinnability of the dope, as well as the wet strength of the resulting fiber.

Coagulation liquid

The coagulation liquid is a liquid capable of effecting gelling of the gelling hydrocolloid. The exact nature of the coagulation liquid depends on the gelling hydrocolloid used, and a suitable coagulation liquid for each gelling hydrocolloid may be determined by the skilled person. Divalent salts are capable of gelling a variety of hydrocolloids, and therefore the coagulation liquid may commonly be an aqueous solution of a divalent salt such as calcium chloride. The coagulation liquid may also effect gelling by control of the pH of the gelling hydrocolloid. The coagulation liquid may therefore be a dilute aqueous solution of a mineral acid, such as a dilute hydrochloric acid solution or dilute sulfuric acid solution. The coagulation liquid may comprise either salts or acids, or may comprise both salts and acids in order to effect gelation by a regulation of both ionicity and pH. The concentration of the coagulation liquid should be adapted to provide suitable properties of the gelled fiber. A coagulation solution comprising of from about 1 percent to about 10 weight percent divalent salt relative to the total weight of the coagulation liquid maybe suitable. Likewise, for acidic coagulation solutions a suitable mineral acid concentration may be from about 0.1 M to about 1M.

Gelling may be effected in several stages by passing the fiber through a series of coagulation liquids, such as a first coagulation liquid and a second coagulation liquid. This may for example assist in preventing clogging of the spinning nozzle. In such a case, the series of coagulation liquids may be the same, or may be different. For example, the series of coagulation liquids may utilize the same salt, e.g. calcium chloride, but in different concentrations. For example, the second coagulation bath may have a higher acid and/or salt concentration than the first coagulation bath. The series of coagulation liquids may also differ, such that for example the first coagulation bath utilizes a salt and the second coagulation bath utilizes an acid. Method

Preparation of the spinning dope

The spinning dope is prepared by dispersing/dissolving the lignin, the gelling hydrocolloid, and any further spinning dope components such as alum, in aqueous alkaline solution. This may be done by any means known in the art, such as by adding the components as powders to an aqueous alkaline solution of the desired molarity. The lignin and gelling hydrocolloid may be dispersed/dissolved as separate solutions and then combined to form a spinning dope. For example, the lignin may be dispersed in a relatively concentrated alkaline solution and the gelling hydrocolloid dispersed in water. Mixing the lignin and gelling hydrocolloid solutions in the desired proportions then provides a spinning dope having the desired alkaline molarity and lignin/ gelling hydrocolloid content.

The spinning dope may be filtered after preparation or before use in order to remove any undispersed matter.

Spinning An apparatus for wet spinning of the precursor fiber from the spinning dope is schematically illustrated in Figure la, and an apparatus for dry jet wet spinning of the precursor fiber is illustrated in Figure lb. A flow chart depicting the process for producing the precursor fiber is shown in Figure 2.

Step s200 denotes the start of the process. In a step s201 a spinning dope as described above is provided. The spinning dope is held in a dope tank 1. The dope is then extruded through the spinning nozzle 5 in a step s203 using a metering pump 3. The produced fiber is then in a step s205 submerged in at least one coagulation bath 7. In wet spinning (Fig. la) the spinneret is submerged in the coagulation bath such that the initial fiber is immediately contacted with the coagulation liquid upon extrusion from the spinneret, whereas in dry jet wet spinning (Fig. lb) the spinneret is non-submerged and the initially extruded fiber must first pass through a gap prior to submersion in the coagulation bath. After the step s205, the produced fiber may be washed in a step s207 in one or more washing baths 9, dried in a step s209 on a dryer roll 11, and collected in a step s211 on a collection reel 13. Step s213 denotes the end of the process.

In order to spin fiber the spinning dope is extruded through a spinning nozzle such as a spinneret. The spinneret may be of the monofilament type or the multifilament type, and may comprise orifices of any suitable diameter, such as about 5 pm to about 100 pm. Suitable process parameters such as flow rate may be determined by a person skilled in the art. After leaving the spinning nozzle the extruded fiber is passed through a coagulation bath comprising the coagulation liquid. Depending on the spinning technique utilized the spinning nozzle may extrude the fibers directly into the coagulation liquid, i.e. wet spinning, or the spinning nozzle may first extrude to an intermediate gaseous phase, such as air or an inert gas, prior to the fibers being submerged in the coagulation liquid. This is known as dry jet wet spinning and is also known as air-gap wet spinning. Any suitable apparatus known in the art may be used for spinning the precursor fibers.

Gelling of the initial fiber is typically effected by passing the fiber through a single coagulation liquid as described above. However, gelling of the initial fiber may also be effected in several stages by passing the fiber through a series of coagulation liquids, such as a first coagulation liquid and a second coagulation liquid. This may for example assist in preventing clogging of the spinning nozzle. In such a case, the series of coagulation liquids may be the same, or may be different. For example, the series of coagulation liquids may utilize the same salt, e.g. calcium chloride, but in different concentrations. For example, the second coagulation liquid may have a higher acid and/or salt concentration than the first coagulation bath. The series of coagulation liquids may also differ, such that for example the first coagulation bath utilizes a salt and the second coagulation bath utilizes an acid.

After gelling, the formed precursor fiber may be subjected to further operations known in the art, such as stretching, washing and drying. Stretching may for example be achieved by control of the extrusion rate relative to the rotation rate of the collection roller. Stretching may increase the alignment of the spun fibers and improve the mechanical strength and properties of the stretched fiber.

The precursor fiber may be washed by submersion in one or more baths. The washing solution may comprise or consist of water, or may comprise or consist of an organic solvent. Water may assist in removing residual salts from the fiber. Using organic solvents may assist in removing water from the fiber and/or precipitating the fiber. This in turn facilitates drying of the fiber and makes the drying step less energy-demanding. Suitable organic solvents may include relatively low-boiling water-miscible solvents such as acetone, methanol, ethanol and isopropanol. The washing liquid may be heated somewhat to a suitable temperature below the boiling point of the washing liquid in order to improve diffusion of the washing liquid into the fiber as well as increase the solubility of any impurities in the washing liquid. The fiber may be dried by any means known in the art, for example by drying around a heated roller. Suitable drying temperatures may be in excess of 50 ° C, such as from about 50 °C to about 150 °C. Excessive drying temperatures may inadvertently initiate crosslinking of the lignin in the fiber.

Precursor fiber The precursor fiber obtained from the method above comprises gelling hydrocolloid and lignin. The precursor fiber may comprise from about 20- 80 % by dry weight of the gelling hydrocolloid and/or about 20 - 80 % by dry weight of the lignin, preferably about 30 - 50 % by dry weight of the gelling hydrocolloid and/or about 50 - 70 % by dry weight of the lignin, relative to the total dry weight of the precursor fiber. The precursor fiber may consist of gelling hydrocolloid and lignin only, or may consist essentially of these components, i.e. comprise only residual impurities from the manufacturing process over and above the gelling hydrocolloid and lignin. Alternatively, the precursor fiber may comprise further additives in order to provide the precursor fiber with desired features. Such additives may be determined by a person skilled in the art.

The obtained precursor fibers may have a fiber diameter of from about 1 pm to about 100 pm, preferably from about 5 pm to about 50 pm.

The obtained precursor fibers are continuous, i.e. they may be spun in long lengths without breakage, e.g. lengths in excess of 1 meter. Fibers that are flexible, non-tack and having good wet- and dry strength may be obtained by the method above.

Although termed herein as a "precursor fiber", the fiber obtained from the method above may have applications in other fields besides the manufacture of carbon fiber. For example, the fibers obtained may be woven and/or used in new composite materials, such as continuous fiber composites. Depending on the porosity of the fibers, they may be used in filtration or purification applications, either with or without subsequent carbonization.

Carbon fiber

The precursor fibers may be further converted to carbon fibers using techniques known in the art. Typically, this involves stabilizing the precursor fibers by heating in an oxidative atmosphere for a predetermined time, prior to carbonizing the stabilized fibers in an inert atmosphere at higher temperatures. For example, the stabilization may be performed in air using a temperature ramp of 0.1 - 0.1 °C/min from a temperature of about 100 °C up to a temperature of about 250 °C. This may be followed by carbonization under an inert atmosphere, such as under nitrogen or argon, using a temperature ramp of about 10 °C/min up to a temperature of about 1000 °C. The precursor fibers obtained by the method above may be converted to carbon fibers in this fashion. The carbon fibers obtained are structural carbon fibers, i.e carbon fibers suitable for load-bearing applications.

Examples Spinning dope

The following materials were used as received in spinning precursor fibers:

Lignin (softwood kraft lignin) from Backhammar Mill

Sodium alginate (medium viscosity) from Fluka Chemicals

Sodium hydroxide (ACS reagent > 97 %) from Sigma-Aldrich

Alum (aluminium potassium sulfate-12 hydrate)

Calcium chloride (technical grade) from VWR chemicals

Ethanol (99%)

Spinning dopes were prepared according to Table 1 below. Sample 1 was prepared by dissolving 9% alginate in deionised water using mechanical stirring, followed by addition of a solution of 21% lignin in 0.5M NaOH to give a final alginate/lignin ratio of 30/70. This dispersion was then filtered through a 10 pm filter using a syringe pump prior to use.

Samples 2, 3, and 4 were prepared by dispersing lignin in 0.5M NaOH and thereafter, whilst vigorously stirring the lignin dispersion, adding powered sodium alginate to give a final concentration of 7.5% by weight of each of lignin and alginate. These dispersions were then filtered through a 10 pm filter using a syringe pump prior to use.

A small quantity of alum (approx. 0.1 - 0.5 %) was added during the preparation of sample 3.

A defined quantity of alum (0.25 %) was added during the preparation of sample 4. Sample 5 was prepared in an analogous manner to sample 4, but was allowed to stand overnight with stirring prior to filtration.

Table 1 - Composition of spinning dopes

Sample no. Alginate Lignin NaOH Alum ϊ Ϊ5% 10.5% 0.25 M -

2 7.5% 7.5% 0.5 M 3 7.5% 7.5% 0.5 M c:a 0.1 - 0.5 %

4 7.5% 7.5% 0.5 M 0.25%

5 7.5% 7.5% 0.5 M 0.25%

Spinning of precursor fibers

The wet spinning of fibers was performed using a spinneret having 50 holes (75 pm diameter) submerged in a calcium chloride solution, followed by a wash bath of deionised water, drying over a heated dryer roll and finally collection on a collection reel. The spinning dope was fed at a rate of 1.0 - 1.5 ml/min and the fibers were thereafter drawn through the apparatus with a gradually increasing speed from 3.2 m/min closest to the spinneret to 3.8 m/min at collection.

For spinning of sample 1, a calcium chloride concentration of 10 % was ultimately chosen for the coagulation bath. A single washing bath containing deionised water was used, and the dryer roll was heated to 105 °C.

Sample 2 was spun in an analogous way to sample 1. The fibers resulting from sample 2 had a greater mechanical integrity than those from sample 1.

Sample 3 was spun using a coagulation bath containing 2% calcium chloride.

Sample 4 was spun using two separate coagulation baths: a first coagulation bath containing 1% calcium chloride solution, and a second coagulation bath containing 2% calcium chloride solution. The fiber was then washed in two separate wash baths containing deionised water prior to drying over the dryer roll at a temperature of 95°C. Additional drying subsequent to the dryer roll was provided using a heat gun.

Sample 5 was performed in a similar manner to sample 4, but the second wash bath was instead filled with ethanol. This meant that the temperature of the dryer roll could be reduced to 60°C. Carbonization of precursor fibers

A number of trials were made to carbonize precursor fibers derived from sample 1. Stabilization and carbonization was performed in a thermogravimetric analysis instrument using a temperature ramp as shown in Figure 3, wherein axis x denotes time in minutes, axis yl denotes relative weight % and axis y2 denotes temperature in °C. Line 301 denotes the temperature curve of trial 1, line 302 denotes the weight curve of trial 1. Line 303 denotes the temperature curve of trial 2 and line 304 denotes the weight curve of trial 2. Stabilization was performed in air with a ramp of 50 °C/min up to 105 °C followed by a ramp of 0.2 °C/min (trial 2) or 0.5 °C/min (trial 1) up to 250 °C. Thereafter, the gas flow was switched to nitrogen gas and the temperature was increased to 1000 °C using a ramp of 10 °C/min.

Results

All spinning dope samples could be spun to continuous fibers. It was however noted that addition of alum (samples 3-5) provided fiber with greater wet strength and therefore less tendency to break during spinning. The resulting dry fibers were also improved, had greater flexibility, and could be produced to a smaller diameter without breakage, down to at least as low as approximately 20 pm. The addition of a wash step in ethanol (sample 5) had a considerable effect on the drying of the produced fiber. Despite using a lower dryer roll temperature (60 °C) the fibers were nearly completely dry upon collection on the collection reel and showed no tendency to agglomerate or stick to each other.

The carbonization trials demonstrated that it is possible to carbonize the precursor fibers obtained by the method described herein.

Further example of spinning precursor fiber (PF) and conversion to carbon fiber (CF)

A softwood kraft lignin (38 mg/mL) was dissolved in 0.5 M NaOH overnight and mixed with alginate powder to form a lignin-alginate dope consisting of 11.3 wt% alginate, 3.8 wt% lignin and 0.5 M NaOH.

Prefibres (PFs) were wet-spun from the obtained dope, by pumping it through a spinneret (50 holes, 100 pm diameter) into a coagulation bath containing 1.5 % CaCI ₂ in deionized water at a rate of 1.25 mL/min. The tow of PFs were then pulled through a bath containing 1% KAI(S0 ₄ ) ₂ , followed by 3 consecutive washing baths with only deionized water, and finally a sizing bath containing 5 % fabric softener (Neutral ^R , Unilever, Copenhagen, Denmark). Drying was done by a hot air blower and winding the tow around a heated cylinder (90 °C) five times. The tow was pulled at a speed of 3.2 m/min after the coagulation bath and gradually increased to a final speed of 5.2 m/min during the winding onto a spool. Stabilisation of the PFs was performed in air (7 L/min) using a KSL-1200X muffle furnace (MTI Corporation, Richmond, CA, USA). The PFs were stabilised at 0.2 °C/min to 200 °C and then to 250 °C at 1.0 °C/min, where the temperature was kept for one hour. Carbonisation was performed in a Model ETF 70/18 tube furnace (Entech, Angelholm, Sweden) under nitrogen atmosphere (flow rate 200 mL/min) by heating at 1 °C/min to 600 °C and then at 3 °C/min to 1000 °C, before cooling to room temperature.

The obtained CF was easily separated reflecting a successful conversion, since fibre fusion commonly is otherwise commonly seen in lignin-based CF. Figures 4a and 4b are high resolution scanning electron microscopy (SEM) images of the obtained carbon fibers, both in plan view (Figure 4a) and in cross-section (Figure 4b). The outer surface of the produced CFs was smooth (Figure 4a) and the cross section was slightly bean shaped and solid without any pores (Figure 4b). This indicates a potential use as load-bearing fibres in composites, i.e. a structural carbon fiber was produced, as opposed to a porous carbon fiber.