Abstract:
A process is provided for preparation of carbon fibers based from fibers of poly(α(1→3) glucan). The method comprises three thermal exposures at progressively higher temperatures to drive off volatiles, thermally stabilize the glucan fiber, and carbonize the thermally stabilized fiber. The carbon fibers prepared according to the process hereof are strong, stiff, tough, and easily handled.

Description:
FIELD OF THE INVENTION 
     The invention relates to carbon fibers, processes of preparing the carbon fibers and the use of the carbon fibers in various applications. 
     BACKGROUND OF THE INVENTION 
     Carbon fibers are generally defined as a fiber containing at least about 92 wt-% of carbon. Carbon fibers containing 99 wt-% or more of carbon are often referred to as graphite fibers. Carbon fibers (CFs) are used in various applications owing to their excellent tensile properties, thermal and chemical stabilities (in absence of oxidizing agents) and thermal and electrical conductivities. The conventional applications of CFs include aircraft frames, turbine blades, automobile panels, sporting goods and industrial components. 
     Currently, the carbon fiber market is dominated by carbon fiber derived from polyacrylonitrile (PAN), with the balance being made up of fibers from pitch and rayon. CFs with distinct properties result from the processing of different precursor fibers. In a typical process in the art for converting organic polymer fibers into carbon fibers, the organic polymer fiber is first heat-stabilized in air in an oxidation process conducted at a temperature of 200 to 400° C. The thus stabilized precursor fibers then undergo controlled pyrolysis, i.e., a carbonization step, comprising heat-treating in an inert atmosphere such as nitrogen to a temperature of from about 300° C. to about 3000° C., which removes non-carbon elements such as hydrogen, oxygen and nitrogen from the oxidized fiber. It is known in the art that heating at the higher end of the temperature spectrum, e.g., between about 1000° C. and about 3000° C. may achieve higher carbon content, thereby producing CFs with higher Young&#39;s modulus values. 
     For automotive applications, desired mechanical properties for carbon fibers include tensile strength of &gt;1.72 GPa, tensile modulus of &gt;172 GPa and elongation at break of about 1%. 
     In addition to the limited mechanical properties of conventional CFs, the currently used methods of preparing CFs can be costly. For example, the cost of the precursor fiber amounts to approximately 40% to 50% of the total cost of preparing the carbon fiber. Therefore, there is a need in the art for lower cost precursor fibers that yield carbon fibers of excellent quality would significantly reduce the cost of CFs. An additional benefit would be to enable the expansion of CF applications to industries and markets such as those related to the automotive industry. 
     Furthermore, it is desirable to provide a source of carbon fibers that derives from a renewable source that does not contribute to global warming. 
     Polysaccharides have been known since the dawn of civilization, primarily in the form of cellulose, a polymer formed from glucose by natural processes via β(1→4) glycoside linkages; see, for example,  Applied Fibre Science , F. Happey, Ed., Chapter 8, E. Atkins, Academic Press, New York, 1979. Numerous other polysaccharide polymers are also disclosed therein. 
     Only cellulose among the many known polysaccharides has achieved commercial prominence as a fiber. In particular, cotton, a highly pure form of naturally occurring cellulose, is well-known for its beneficial attributes in textile applications. 
     It is further known that cellulose exhibits sufficient chain extension and backbone rigidity in solution to form liquid crystalline solutions; see, for example O&#39;Brien, U.S. Pat. No. 4,501,886. The teachings of the art suggest that sufficient polysaccharide chain extension could be achieved only in β(1→4) linked polysaccharides and that any significant deviation from that backbone geometry would lower the molecular aspect ratio below that required for the formation of an ordered phase. 
     More recently, glucan polymer, characterized by α(1→3) glycoside linkages, has been isolated by contacting an aqueous solution of sucrose with GtfJ glucosyltransferase isolated from  Streptococcus salivarius , Simpson et al., Microbiology, vol 141, pp. 1451-1460 (1995). Highly crystalline, highly oriented, low molecular weight films of α(1→3)-D-glucan have been fabricated for the purposes of x-ray diffraction analysis, Ogawa et al., Fiber Diffraction Methods, 47, pp. 353-362 (1980). In Ogawa, the insoluble glucan polymer is acetylated, the acetylated glucan dissolved to form a 5% solution in chloroform and the solution cast into a film. The film is then subjected to stretching in glycerine at 150° C. which orients the film and stretches it to a length 6.5 times the original length of the solution cast film. After stretching, the film is deacetylated and crystallized by annealing in superheated water at 140° C. in a pressure vessel. It is well-known in the art that exposure of polysaccharides to such a hot aqueous environment results in chain cleavage and loss of molecular weight, with concomitant degradation of mechanical properties. 
     Polysaccharides based on glucose and glucose itself are particularly important because of their prominent role in photosynthesis and metabolic processes. Cellulose and starch, both based on molecular chains of polyanhydroglucose are the most abundant polymers on earth and are of great commercial importance. Such polymers offer materials that are environmentally benign throughout their entire life cycle and are constructed from renewable energy and raw materials sources. 
     The term “glucan” is a term of art that refers to a polysaccharide comprising beta-D-glucose monomer units that are linked in eight possible ways, Cellulose is a glucan. 
     Within a glucan polymer, the repeating monomeric units can be linked in a variety of configurations following an enchainment pattern. The nature of the enchainment pattern depends, in part, on how the ring closes when an aldohexose ring closes to form a hemiacetal. The open chain form of glucose (an aldohexose) has four asymmetric centers (see below). Hence there are 2 4  or 16 possible open chain forms of which D and L glucose are two. When the ring is closed, a new asymmetric center is created at C1 thus making 5 asymmetric carbons. Depending on how the ring closes, for glucose, α(1→4)-linked polymer, e.g. starch, or β(1→4)-linked polymer, e.g. cellulose, can be formed upon further condensation to polymer. The configuration at C1 in the polymer determines whether it is an alpha or beta linked polymer, and the numbers in parenthesis following alpha or beta refer to the carbon atoms through which enchainment takes place. 
     
       
                 
         
             
             
         
      
     
     The properties exhibited by a glucan polymer are determined by the enchainment pattern. For example, the very different properties of cellulose and starch are determined by the respective nature of their enchainment patterns. Starch or amylose consists of α(1→4) linked glucose and does not form fibers among other things because it is swollen or dissolved by water. On the other hand, cellulose consists of β(1→4) linked glucose, and makes an excellent structural material being both crystalline and hydrophobic, and is commonly used for textile applications as cotton fiber, as well as for structures in the form of wood. 
     Like other natural fibers, cotton has evolved under constraints wherein the polysaccharide structure and physical properties have not been optimized for textile uses. In particular, cotton fiber is of short fiber length, limited variation in cross section and fiber fineness and is produced in a highly labor and land intensive process. 
     O&#39;Brien, U.S. Pat. No. 7,000,000 discloses a process for preparing fiber from liquid crystalline solutions of acetylated poly(α(1→3) glucan). The thus prepared fiber was then de-acetylated resulting in a fiber of poly(α(1→3) glucan). 
     The inventive method described herein, results in carbon fibers meeting these desired mechanical benchmarks and would further reduce the costs making CFs available to additional industrial sectors. 
     SUMMARY OF THE INVENTION 
     A process comprising
         subjecting one or more filaments of poly(α(1→3) glucan) to a tension below the breaking strength of the one or more filaments at 350° C.;   subjecting the thus tensioned one or more filaments to a first thermal exposure by heating said one or more filaments to a temperature in the range of 160 to 200° C. in air for a duration in the range of 5 to 15 minutes;   subjecting the thus heated one or more filaments to a second thermal exposure by further heating said one or more filaments at a heating rate, still under tension, from a first temperature in the range of 200 to 250° C. to a second temperature in the range of 300 to 350° C., said heating rate being in the range of 0.1 to 1° C. per minute, thereby preparing one or more thermally stabilized filaments;   subjecting said one or more stabilized filaments in a zero tension state to a third thermal exposure by heating said one or stabilized filaments to a temperature in the range of 700 to 1500° C. in an inert atmosphere for a duration in the range of 0.5 to 5 minutes, thereby preparing one or more carbonized filaments.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a side view of the fiber spinning apparatus employed in the specific embodiments hereof. 
         FIG. 2  depicts a side view of the tube furnace arrangement used in the thermal stabilization step of the process hereof as executed in the specific embodiments thereof. 
         FIG. 3  depicts a side view of the carbonization apparatus used in the specific embodiments hereof. 
         FIG. 4A  depicts a top view, and  FIG. 4B  depicts a front view of the winding frame used to prepare the filament skeins employed in the specific embodiments hereof. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     When a range of values is provided herein, it is intended to encompass the end-points of the range unless specifically stated otherwise. Numerical values used herein have the precision of the number of significant figures provided, following the standard protocol in chemistry for significant figures as outlined in ASTM E29-08 Section 6. For example, the number 40 encompasses a range from 35.0 to 44.9, whereas the number 40.0 encompasses a range from 39.50 to 40.49. 
     As used herein, the term “filament” encompasses a thread-shaped compact unit comprising one or more molecules of a polymer comprising poly(α(1→3) glucan). The filament can further comprise additional polymers added, for example, order to control the morphology of the carbon fiber produced according to the process hereof. Such additives as are commonly employed in the art of carbon fiber production to enhance the properties or processing of organic polymers undergoing solution spinning and subsequent carbonization can also be included. 
     In the present invention, the term “fiber” and the term “filament” are used interchangeably. The present invention is directed to the preparation of high strength, high modulus carbon fibers from a fiber precursor comprising poly(α(1→3) glucan). Suitable poly(α(1→3) glucan) fibers are in the form of continuous filaments. Staple fibers are not well suited for the practice of the present invention. 
     According to the present invention a process is provided for the preparation of carbon fiber from a precursor fiber comprising poly(α(1→3) glucan), the process comprising
         subjecting one or more filaments comprising poly(α(1→3) glucan) to a tension below the breaking strength of the one or more filaments at 350° C.;   subjecting the thus tensioned one or more filaments to a first thermal exposure by heating said one or more filaments to a temperature in the range of 160 to 200° C. in air for a duration in the range of 5 to 15 minutes;   subjecting the thus heated one or more filaments to a second thermal exposure by further heating said one or more filaments at a heating rate, still under tension, from a first temperature in the range of 200 to 250° C. to a second temperature in the range of 300 to 350° C., said heating rate being in the range of 0.1 to 1° C. per minute, thereby preparing one or more thermally stabilized filaments;   subjecting said one or more stabilized filaments in a zero tension state to a third thermal exposure by heating said one or stabilized filaments to a temperature in the range of 700 to 1500° C. in an inert atmosphere for a duration in the range of 0.5 to 5 minutes, thereby preparing one or more carbonized filaments.       

     One benefit of the present invention over the known art is that the carbon fiber resulting from the process hereof is a “green” product—that is, it is biologically sourced because the poly(α(1→3) glucan) upon which it is based is produced by fermentation, and not from petroleum. 
     If the first thermal exposure is conducted at a temperature below 160° C., it may be ineffective. If the first thermal exposure is conducted at a temperature above 200° C., it can cause water molecules trapped within fiber pores to evaporate too quickly and rupture the fiber, causing points of weakness where the fiber can break. The duration of exposure less than 5 minutes is not highly effective. An exposure of greater than 15 minutes is not deleterious, but is unnecessary. In one embodiment of the process hereof, the first thermal exposure is effected at a temperature in the range of 175 to 185° C. for a duration of 7.5 to 12.5 minutes. 
     Thermal stabilization of the poly(α(1→3) glucan) fiber is effected in a second thermal exposure, which involves heating from a first temperature in the range 200 to 250° C., preferably 230 to 250° C., to a second temperature in the range of 300 to 350° C., preferably 310 to 330° C. At a temperature below 200° C., thermal stabilization does not occur or occurs at a rate that is impractically slow. At a temperature above 350° C., the fiber can melt and break. 
     In one embodiment of the process hereof, said second thermal exposure is effected in a series of well-defined steps between the first temperature and the second temperature, with a hold period between steps, and a heating rate from step to step in excess of 10° C. per minute. 
     The first and second thermal exposures are conducted in air or an oxygen containing atmosphere. If the first and second thermal exposures are conducted in an oxygen containing atmosphere other than air, the same sequence of steps will still be operative, but will be modified in detail to accommodate the atmosphere in question. 
     The third thermal exposure, the carbonization step, is effected in an inert environment. Any inert environment is satisfactory. A heavy nitrogen purge, as described in the specific embodiments infra, has been found to be satisfactory. The third thermal exposure is conducted in the temperature range of 700 to 1500° C., preferably 800 to 1000° C. At a temperature below 700° C., the necessary level of pyrolysis and carbonization does not occur. At temperatures above 1500° C., the resulting carbon fiber can undergo such deleterious changes as loss of integrity, melting and others. 
     When the third thermal exposure is conducted for a period of time less than 0.5 minutes, insufficient carbonization takes place. For a period of time more than 5 minutes, the resulting carbon fiber may undergo deleterious changes, particularly in the higher range of carbonization temperatures. In one embodiment, the third thermal exposure is effected in the temperature range of 800 to 1000° C. for a period of time of 1 to 2 minutes. 
     The resulting carbon fiber is strong, very stiff, and tough. 
     The invention is further described in, but not limited by, the following specific embodiments. 
     Examples 
     Materials 
     
       
         
               
               
               
             
           
               
                   
               
               
                 MATERIAL 
                 Description 
                 Vendor 
               
               
                   
               
             
             
               
                 Dialysis 
                 Spectrapor 25225-226, 12000 
                 VWR (Radnor, PA). 
               
               
                 tubing 
                 molecular weight cut-off 
                   
               
               
                 Sucrose 
                 15 wt-% solids aqueous solution 
                 VWR. 
               
               
                   
                 (#BDH8029) 
                   
               
               
                 Dextran 
                 T-10 (#D9260) 
                 Sigma Aldrich. 
               
               
                 Ethanol 
                 Undenatured (#459844) 
                 Sigma Aldrich 
               
               
                 Antifoam 
                 Suppressor 7153 
                 Cognis Corporation 
               
               
                   
                   
                 (Cincinnati, OH). 
               
               
                   
               
             
          
         
       
     
     All other chemicals were obtained from commonly used suppliers of such chemicals. 
     Preparation of Glucosyltransferase (gtfJ) Enzyme 
     Seed Medium 
     The seed medium, used to grow the starter cultures for the fermenters, contained: yeast extract (Amberex 695, 5.0 grams per liter, g/L), K 2 HPO 4  (10.0 g/L), KH 2 PO 4  (7.0 g/L), sodium citrate dihydrate (1.0 g/L), (NH 4 ) 2 50 4  (4.0 g/L), MgSO 4  heptahydrate (1.0 g/L) and ferric ammonium citrate (0.10 g/L). The pH of the medium was adjusted to 6.8 using either 5N NaOH or H 2 SO 4  and the medium was sterilized in the flask. Post sterilization additions included glucose (20 mL/L of a 50% w/w solution) and ampicillin (4 mL/L of a 25 mg/mL stock solution). 
     Fermenter Medium 
     The growth medium used in the fermenter contained: KH 2 PO 4  (3.50 g/L), FeSO 4  heptahydrate (0.05 g/L), MgSO 4  heptahydrate (2.0 g/L), sodium citrate dihydrate (1.90 g/L), yeast extract (Amberex 695, 5.0 g/L), Suppressor 7153 antifoam (0.25 milliliters per liter, mL/L), NaCl (1.0 g/L), CaCl 2  dihydrate (10 g/L), and NIT trace elements solution (10 mL/L). The NIT trace elements solution contained citric acid monohydrate (10 g/L), MnSO 4  hydrate (2 g/L), NaCl (2 g/L), FeSO 4  heptahydrate (0.5 g/L), ZnSO 4  heptahydrate (0.2 g/L), CuSO 4  pentahydrate (0.02 g/L) and NaMoO 4  dihydrate (0.02 g/L). Post sterilization additions included glucose (12.5 g/L of a 50% w/w solution) and ampicillin (4 mL/L of a 25 mg/mL stock solution). 
     Construction of Glucosyltransferase (gtfJ) Enzyme Expression Strain 
     A gene encoding the mature glucosyltransferase enzyme (gtfJ; EC 2.4.1.5; GENBANK® AAA26896.1, SEQ ID NO: 3) from  Streptococcus salivarius  (ATCC 25975) was synthesized using codons optimized for expression in  E. coli  (DNA 2.0, Menlo Park Calif.). The nucleic acid product (SEQ ID NO: 1) was subcloned into pJexpress404® (DNA 2.0, Menlo Park Calif.) to generate the plasmid identified as pMP52 (SEQ ID NO: 2). The plasmid pMP52 was used to transform  E. coli  MG1655 (ATCC47076™) to generate the strain identified as MG1655/pMP52. All procedures used for construction of the glucosyltransferase enzyme expression strain are well known in the art and can be performed by individuals skilled in the relevant art without undue experimentation. 
     Production of Recombinant gtfJ in Fermentation 
     Production of the recombinant gtfJ enzyme in a fermenter was initiated by preparing a pre-seed culture of the  E. coli  strain MG1655/pMP52, expressing the gtfJ enzyme, constructed as described infra. A 10 mL aliquot of the seed medium was added into a 125 mL disposable baffled flask and was inoculated with a 1.0 mL culture of  E. coli  MG1655/pMP52 in 20% glycerol. This culture was allowed to grow at 37° C. while shaking at 300 revolutions per minute (rpm) for 3 hours. 
     A seed culture, for starting the fermenter, was prepared by charging a 2 L shake flask with 0.5 L of the seed medium. 1.0 mL of the pre-seed culture was aseptically transferred into 0.5 L seed medium in the flask and cultivated at 37° C. and 300 rpm for 5 hours. The seed culture was transferred at optical density 550 nm (OD 550 )&gt;2 to a 14 L fermenter (Braun, Perth Amboy, N.J.) containing 8 L of the fermenter medium described above at 37° C. 
     Cells of  E. coli  MG1655/pMP52 were allowed to grow in the fermenter and glucose feed (50% w/w glucose solution containing 1% w/w MgSO 4 .7H 2 O) was initiated when glucose concentration in the medium decreased to 0.5 g/L. The feed was started at 0.36 grams feed per minute (g feed/min) and increased progressively each hour to 0.42, 0.49, 0.57, 0.66, 0.77, 0.90, 1.04, 1.21, 1.41 1.63, 1.92, 2.2 g feed/min respectively. The rate was held constant afterwards by decreasing or temporarily stopping the glucose feed when glucose concentration exceeded 0.1 g/L. Glucose concentration in the medium was monitored using a YSI glucose analyzer (YSI, Yellow Springs, Ohio). 
     Induction of glucosyltransferase enzyme activity was initiated, when cells reached an OD 550  of 70, with the addition of 9 mL of 0.5 M IPTG (isopropyl β-D-1-thiogalacto-pyranoside). The dissolved oxygen (DO) concentration was controlled at 25% of air saturation. The DO was controlled first by impeller agitation rate (400 to 1200 rpm) and later by aeration rate (2 to 10 standard liters per minute, slpm). The pH was controlled at 6.8. NH 4 OH (14.5% weight/volume, w/v) and H 2 SO 4  (20% w/v) were used for pH control. The back pressure was maintained at 0.5 bars. At various intervals (20, 25 and 30 hours), 5 mL of Suppressor 7153 antifoam was added into the fermenter to suppress foaming. Cells were harvested by centrifugation 8 hours post IPTG addition and were stored at −80° C. as a cell paste. 
     Preparation of gtfJ Crude Enzyme Extract from Cell Paste 
     The cell paste obtained above was suspended at 150 g/L in 50 mM potassium phosphate buffer pH 7.2 to prepare a slurry. The slurry was homogenized at 12,000 psi (Rannie-type machine, APV-1000 or APV 16.56) and the homogenate chilled to 4° C. With moderately vigorous stirring, 50 g of a floc solution (Aldrich no. 409138, 5% in 50 mM sodium phosphate buffer pH 7.0) was added per liter of cell homogenate. Agitation was reduced to light stirring for 15 minutes. The cell homogenate was then clarified by centrifugation at 4500 rpm for 3 hours at 5-10° C. Supernatant, containing crude gtfJ enzyme extract, was concentrated (approximately 5×) with a 30 kilo Dalton (kDa) cut-off membrane. The concentration of protein in the gftJ enzyme solution was determined by the bicinchoninic acid (BCA) protein assay (Sigma Aldrich) to be 4-8 g/L. 
     Enzymatic Synthesis of Poly(α(1→3) Glucan) 
     Several batches of poly(α(1→3) glucan) polymer were prepared by combining the materials listed in Table 1 in the amounts shown. The pH was adjusted to pH 6.8-7.0 by addition of 10% KOH. De-ionized water was then added to bring the volume up to level specified in Table 1. The buffer concentration in the thus prepared solution was 50 mM. 
     The thus prepared pH-adjusted solution was then charged with the enzyme extract prepared supra in an amount sufficient to bring the enzyme concentration to 0.30% by weight in each batch. Each thus prepared reaction mixture was then allowed to stand at ambient temperature for 144 hours. The resulting poly(α(1→3) glucan) solids were collected on a Buchner funnel using a 325 mesh screen over 40 micron filter paper. The filter cake was re-suspended in deionized water and filtered twice more as above to remove sucrose, fructose and other low molecular weight, soluble by-products. Finally two additional washes with methanol were carried out, the filter cake was pressed out thoroughly on the funnel and dried in vacuum at room temperature, yielding a white flaky solid in the amounts shown in Table 1. 
                                                                           TABLE 1                   Batch           KH2PO4               Batch   size   Sucrose   Dextran   Buffer   Ethanol       Number   (L)   (g)   T-10 (g)   (mL)   (mL)   Yield                                1   20   1000   4.0   1000   0   120.0       2   20   1000   4.0   1000   0   114.5       3   20   1000   4.0   1000   0   113.0       4   20   1000   4.0   1000   0   86.0       5   3   450   2.4   150   150   47.3       6   3   450   3.0   150   300   32.1       7   3   450   6.0   150   300   49.0       8   3   450   9.0   150   300   56.6                    
Preparation of 1,3 Alpha Glucan Triacetate
 
     The several batches of poly(α(1→3) glucan) as shown in Table 1 were combined in the amounts shown, respectively, in Table 2 to make three 130 g blends for subsequent acetylation. 
     The blends were boiled for one hour in deionized water. Each thus boiled blend was then added to a mixture containing 890 mL of methylene chloride, 600 mL of acetic acid and 870 mL of acetic anhydride in a 4 L reaction kettle provided with a nitrogen blanket. Mixing was effected with an egg beater style mixing blade that covered the entire depth of the liquid. The resulting mixture was then cooled to approximately −5° C. Separately, a catalyst mixture was prepared by addition of 9 mL of 70% aqueous perchloric acid to 370 mL of chilled acetic anhydride. The catalyst mixture was then added dropwise to the rapidly stirred reaction mixture at −5° C. Subsequent to catalyst addition, the reaction kettle was immersed in a hot water bath contained in a 2 gallon plastic bucket, and heated to 30° C. When the temperature of the reactants was observed to exceed 32° C., the reaction kettle was removed from the hot water bath and suspended in the air until the reaction temperature was observed to reach 27° C. at which point the reaction kettle was again immersed in the hot water bath. This procedure was continued for a period of 2-4 hours until reaction was complete. The reaction was deemed to be complete when no particulate matter was observed by visual inspection of the translucent reaction mixture. 
     In small aliquots, the mixture was coagulated in methanol in a Waring blender, the resultant suspension was filtered, washed with methanol twice more, water washed until neutral pH was obtained, and then washed with methanol and dried under vacuum. Yield of the resulting triacetate is shown in Table 2 
                                                   TABLE 2               Blend   Polymer Batches   Wt. (g)   Triacetate Yield (g)                                1   1/2    30/100   190.4       2   3/4   43.7/86.3   204.6       3   5/6/7/8   25/20/40/45   207.94                    
Spinning Solution
 
     Spinning solutions A and B were prepared from the thus prepared acetylated poly(α(1→3) glucan). 100 parts by weight of trifluoroacetic acid were diluted with 8 parts by weight of water. The thus prepared solution was added to two 1-quart zip-lock bags, each containing 120 g of the respective acetylate poly(α(1→3) glucan) blends, as indicated in Table 3, in an amount sufficient to prepare a 37.5% solids solution in each case. Each bag was then sealed, and was subject to hand kneading to homogenize. The bag was allowed to stand at ambient conditions overnight. In order to dissolve the polymer therein, the mixture of polymer and solvent was first stirred by hand using a stainless steel spatula in order to homogenize the mixture. The homogenized mixture was then pumped back and forth through 13 cycles between two syringes connected by a short length of 3 mm ID stainless steel tubing. 
                                                                     TABLE 3                       Spinning   Glucan Triacetate Blend                    Solution   #   Weight (g)                        A   1   94               2   26           B   3   120                    
Fiber Spinning of Glucan Triacetate
 
     The thus prepared spinning solutions were solution-spun into continuous filaments using the spinning apparatus depicted in  FIG. 1 . The spinning solution was charged to the cell ( 13 ) that was provided with a piston ( 11 ) connected to ram ( 12 ) which pushed solution through a spin pack containing a screen pack ( 14 ) provided with stainless steel support screens including 100 mesh support screen and a 325 mesh filter screen, and a 20-hole spinneret ( 15 ). Each spinneret hole was characterized by a diameter of 0.005 in. and a length to diameter ratio of 6. The piston ( 11 ) was driven by a drive screw (not shown) that drove the ram at a metered rate. The filaments ( 16 ) emerging from the spinneret ( 15 ) were directed into a coagulation bath ( 17 ) consisting of 100% methanol. The fiber was passed around Teflon guide pins ( 18 ) within the coagulation bath and exiting the bath to a traverse ( 19 ) with a guide pin ( 110 ) distributing the fiber evenly across a width to a windup ( 111 ) where the fiber is collected on a bobbin. The bobbins so prepared were soaked overnight in methanol. Spinning conditions are provided in Table 4. The yarns so produced are herein designated GYA-1 and GYA-2. 
                                                                                       TABLE 4                       Spin-   Jet               Wind               ning   Veloc-   Bath   Bath   Air   up   Spin           Solu-   ity   Temp   length   Gap   speed   Stretch           tion   (fpm)   (° C.)   (ft)   (in)   (fpm)   Factor                                    GYA-1   A   17   −1   11.8   0.3   52   3.1       GYA-2   B   22   −19   11.8   0.75   60   2.7                    
Saponification
 
     0.54 g of sodium methoxide were added to 100 mL of methanol. The bobbin of GYA-2 yarn was placed into the solution so formed for a period of 48 hours to regenerate glucan fiber from the glucan triacetate fiber. The so-treated bobbin was then rinsed with methanol, and soaked for an additional 24 hours in neat methanol, and allowed to air dry. The resulting regenerated glucan fiber yarn is herein designated GY-1. 
     Oxidation Treatment 
     Referring to  FIG. 2 , a tube furnace ( 21 ) having an entry port ( 22 ) and an exit port ( 22 ′) was equipped with an air supply fan ( 23 ) that flowed air, at rates stated in Table 5, infra, into the entry port ( 22 ) and through the furnace to the exit port ( 22 ′). A skein of fiber ( 24 ) was fed end-wise through the tube furnace. The skein was disposed to pass over a pulley ( 25 ) at each end of the tube furnace. Each end of the skein was formed into a loop ( 26 ), through which a hook ( 27 ) was passed. Affixed to the hook was a weight ( 28 ). The weight employed is stated in the examples, infra. The heated section of the tube inside the tube furnace was a 2 inch schedule 5 tube with an inner diameter of 57 mm and a length of 54 inches. Each specimen was subject to a temperature of 180° C. in air for 10 minutes. The temperature was then increased in a series of steps, as described in the thermal profile provided in the examples, infra. It took less than 1 minute to make the temperature changes between adjacent steps in the thermal profile. 
     Carbonization Treatment 
     Referring to  FIG. 3 , nitrogen was provided to the tube furnace ( 21 ) at six locations ( 33 ): one at the entry port ( 22 ) and one at the exit port ( 22 ′) of the tube furnace, two at the tubing before the entrance port and two at the tubing before the exit port ( 22 ′). The nitrogen was fed through six flow meters ( 34 ). The oxidized fiber skein ( 35 ) was attached to an Inconel® transport wire 0.9 mm in diameter ( 36 ) using metal crimps ( 37 ) in order to keep the fiber skein in a zero tension state. The Inconel® wire was wrapped around pulleys ( 25 ) located at the entry port ( 22 ) and exit port ( 22 ′) in order to move the fiber skein into and out of the furnace. The fiber skein thus disposed was then subject to heating according to the schedule disclosed in the specific embodiments infra. 
     Preparation of Filament Skeins. 
     Referring to  FIG. 4 , a skein of filaments having more than 20 ends was prepared by winding the skein around four posts ( 41 ) that were set at the corners of a square ( 42 ), 24 inches apart from each other. A fiber skein was wrapped around the posts until the skein contained the desired number of filaments. The skein was cut at one post, resulting in a length of 8 feet. 
     Example 1 
     Two 60-inch skeins, consisting each of 20 filaments of GY-1 were prepared for oxidation as described supra. To each skein, herein designated GY-1-A and GY-1-B, a 3.5-gram weight was affixed at each end as shown in  FIG. 2 . Under an air flow rate of 6 standard cubic feet per minute (scfm), each skein was individually heated to 230° C., held for 60 minutes, then heated to 250° C., held for 60 minutes, then heated to 270° C., held for 60 minutes, then heated to 290° C., held for 60 minutes, then heated to 310° C., held for 60 minutes. No breakage had occurred at the end of the five-hour thermal exposure process. The resulting oxidized skeins are herein designated GY-1-AO and GY-1-BO. 
     The GY-1-AO oxidized skein was prepared for carbonization as described supra. The skein was heated at 800° C. for 90 seconds under a nitrogen purge of 120 scfh. The skein, herein designated GY-1-AC, was removed from the oven and spooled. The skein was black in color, pliable enough to be spooled, but fragile. If the skein was wrapped tightly, filaments would break. 
     The GY-1-BO oxidized skein was prepared for carbonization as described supra. The skein was heated to 1000° C. for 90 seconds under a nitrogen purge of 120 scfh. The skein was black in color. The filaments seemed stronger than GY-1-AC, but upon removal from the oven, many filaments were caught on the equipment and broken. 
     Example 2 
     Referring to  FIG. 4 , a 440 filament skein was prepared by wrapping a 20-filament length of GY-1 around the posts 22 times. A second skein was prepared in the same manner. The skeins so prepared were cut at one post, resulting in two lengths of 8 feet each, designated GY-1-C and GY-1-D. 
     Each of GY-1-C and GY-1-D were prepared for oxidation as described, supra. Each was oxidized separately. To each skein a 50-gram weight was affixed at each end as shown in  FIG. 2 . Under an air flow rate of 10 scfm, each skein was heated to 250° C., held for 40 minutes, then heated to 270° C., held for 40 minutes, then heated to 290° C., held for 40 minutes, then heated to 310° C., held for 40 minutes, then heated to 330° C., held for 40 minutes. No breakage occurred at the end of the 200-minute temperature profile. The resulting oxidized skeins are herein designated GY-1-CO and GY-1-DO. 
     c. Carbonization 
     Oxidized skein GY-1-CO was prepared for carbonization as described supra. The skein was heated to 800° C. under a nitrogen flow rate of 120 standard scfh for 120 seconds. The thus heated skein, herein designed GY-1-CC, was removed from the furnace. The skein was black in color, pliable, and easy to spool. 
     Oxidized skein GY-1-DO was treated in a manner identical to that of GY-1-CO except that the temperature was 1000° C. The thus heated skein, herein designed GY-1-DC, was removed from the furnace. The skein was black in color, very pliable, and very easy to spool. 
     In the thus carbonized skeins fiber diameter was determined by scanning electron microscopy; denier, using a TexTechno Vibromat ME denier testerand (TexTechno H.Stein GMBH &amp; Co.); and, mechanical properties, using an Instron® Universal Testing Machine. Results are shown in Table 5. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
               
                   
                 GY-1-CC 
                 GY-1-CD 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Diameter (micrometers) 
                 17.0 ± 0.4 
                 19.6 ± 1.7  
               
               
                   
                 Denier 
                     3.581 ± 0.789 
                 3.076 ± 0.674 
               
               
                   
                 Tenacity (gpd) 
                  1.3 ± 0.5 
                 2.0 ± 1.0 
               
               
                   
                 Tensile Strength (MPa) 
                 189 ± 79 
                 203 ± 100 
               
               
                   
                 Tensile Modulus (GPa) 
                 28 ± 4 
                 27 ± 6  
               
               
                   
               
             
          
         
       
     
     Comparative Example A 
     One 60-inch skein consisting of 20 filaments of glucan triacetate GYA-1 was prepared for oxidation as described supra. A 4.5 g weight was affixed to each end of the skein as shown in  FIG. 2 . Under an air flow rate of 6 scfm, the bundle was heated to 230° C. After one minute, the skein broke. 
     Comparative Example B 
     Two 200 filament skeins were prepared by wrapping the 20-filament glucan triacetate GYA-1 ten times around the posts of the apparatus in  FIG. 4 . Each skein was cut at one post, resulting in two lengths of 8 feet. 
     A 60-inch skein was cut from each of the thus prepared 8 foot lengths, herein designated GYA-1-1 and GYA-1-2. Each 60-inch skein was prepared for oxidation as described supra. Each skein was oxidized separately. A 16 g weight was affixed to each end of the GYA-1-1 skein, and a 40 g weight was affixed to each end of GYA-1-2. The skeins were heated for 10 minutes at 180° C. under an air flow rate of 6 scfm. skeins broke after 10 minutes at 180° C. 
     Comparative Example C 
     PANOX® Thermally Stabilized Textile Fiber, an oxidized poly(acrylonitrile) fiber was obtained from The SGL Group, Ross-Shire, UK. Three PANOX fiber skeins, herein designated PANOX-1, PANOX-2, and PANOX-3, consisting of approximately 12,000 filaments per skein were prepared for carbonization as described supra. Three 60-inch length skeins were heated to 800° C. under a nitrogen atmosphere of 120 scfh. PANOX-1 was held for 60 seconds, PANOX-2 was held for 90 seconds, PANOX-3 was held for 120 seconds. PANOX-1 caught on the furnace during removal and was bunched up. No further testing was performed. PANOX-2 was frayed and could not be spooled. PANOX-3 was removed from the oven, herein designed PANOXC-3, and spooled. 
     A further 12,000 filament 60 inch skein of PANOX, herein designated PANOX-4, was heated to 1000° C. under a nitrogen atmosphere of 120 scfh for 120 seconds. PANOX-4 was removed from the oven, herein designated PANOXC-4 and spooled. 
     PANOXC-3 and PANOXC-4 were analyzed in the manner of the specimens in Example 2. Results are shown in Table 6. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 6 
               
               
                   
               
               
                   
                 PANOXC-3 
                 PANOXC-4 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Diameter (micrometers) 
                 8.0 ± 0.3 
                 9.9 ± 0.3 
               
               
                   
                 Denier 
                 0.779 ± 0.040 
                 1.111 ± 0.070 
               
               
                   
                 Tenacity (gpd) 
                 9.4 ± 2.1 
                 2.7 ± 1.7 
               
               
                   
                 Tensile Strength (MPa) 
                 1440 ± 317  
                 387 ± 247 
               
               
                   
                 Tensile Modulus (GPa) 
                 85 ± 6  
                 15 ± 8