Patent Publication Number: US-2020283932-A1

Title: Method of making polyacrylonitrile based carbon fibers and polyacrylonitrile based carbon fiber fabric

Description:
RELATED APPLICATION 
     This application claims priority to U.S. Provisional Patent Application Ser. No. 62/813,359 filed on Mar. 4, 2019 which is hereby incorporated by reference in its entirety. 
    
    
     GOVERNMENT SUPPORT 
     This invention was made with government support under Grant No. W31P4Q-17-C-0009 awarded by the US Army AMRDEC and Materials Sciences Corporation. The government has certain rights in the invention. 
    
    
     TECHNICAL FIELD 
     This document relates generally to the field of carbon fibers and, more particularly to a new and improved method for making polyacrylonitrile-based carbon fibers with low thermal conductivity and a lobed profile for ablative composite and other applications. 
     BACKGROUND 
     Rayon-based carbon fibers have long been used for ablative composite applications such as for heat shields and the like required for space vehicles and other technologically advanced vehicles and applications. Environmental concerns related to the production of rayon have led to the cessation of the domestic production of rayon. As a result, the United States has had to rely on old stock and foreign production for the rayon used to produce carbon fibers having the desired characteristics for ablative composite applications. This is a strategic concern for which, to date, there has been no clear solution. 
     This document relates to the production of polyacrylonitrile or PAN-based carbon fibers having a unique combination of properties for PAN-based carbon fibers that approach those characteristic of rayon-based carbon fibers. More particularly, the PAN-based carbon fibers of the present method are characterized by (a) a lobed cross section that provides for enhanced mechanical interlocking between fibers, (b) a relatively low thermal conductivity characteristic of rayon-based carbon fibers, and (c) tensile properties similar to rayon-based carbon fibers. 
     SUMMARY 
     In accordance with the purposes and benefits described herein, a new and improved method is provided for the production of PAN-based carbon fibers. That method comprises the steps of: (a) extruding a PAN dope through a spinneret to produce a tow of PAN fibers having a lobed cross section, (b) washing, drying and winding the tow of PAN fibers without subjecting the PAN fibers to hot drawing and (c) carbonizing the tow of PAN fibers at a temperature of less than 1,000° C. to produce PAN-based carbon fibers. 
     The method may include using a spinneret with a +-shaped opening to produce PAN fiber filaments having a cross section with a lobed shape. 
     The method may further include the step of stabilizing the tow of PAN fibers prior to carbonization. That stabilizing may include heating the PAN fibers to 250°−280° C. 
     In some embodiments, during carbonizing, the highest fiber temperature reached may be between 850° C. and 950° C. In some embodiments, during carbonizing, the highest fiber temperature reached may be between 885° C. and 915° C. 
     PAN-based carbon fibers made by this method are also claimed. Those PAN-based carbon fibers may be characterized by the lobes of the PAN fiber filaments having a characteristic width greater than 10% of a width of the PAN fiber filaments at the largest dimension of the PAN fiber filaments. 
     In accordance with an additional aspect, a method is provided for producing a polyacrylonotrile-based/(PAN)-based carbon fiber fabric, comprising: (a) extruding a PAN dope through a spinneret to produce a tow of PAN fibers having a lobed cross section, (b) washing, drying and winding the tow of PAN fibers without subjecting the PAN fibers to hot drawing, (c) weaving the tow of carbon fibers into a woven PAN fiber fabric and (d) carbonizing the woven PAN fiber fabric at a temperature of less than 1,000° C. to produce a woven PAN-based carbon fiber fabric. 
     The method may include using a spinneret with a +-shaped opening to produce PAN fiber filaments having a cross section with a lobed shape. 
     The method may further include the step of stabilizing the tow of PAN fibers prior to carbonizing. That stabilizing may include heating the PAN fibers to 250°−280° C. 
     In some embodiments, during carbonizing, the highest fiber temperature reached may be between 850° C. and 950° C. In some embodiments, during carbonizing, the highest fiber temperature reached may be between 885° C. and 915° C. 
     PAN-based carbon fiber fabrics made by this method are also claimed. Those PAN-based carbon fiber fabrics may be characterized by the lobes of the PAN fiber filaments having a characteristic width greater than 10% of a width of the PAN fiber filaments at the largest dimension of the PAN fiber filaments. 
     In the following description, there are shown and described several embodiments of the methods as well as the PAN-based carbon fiber and PAN-based carbon fiber fabric products made from the methods. As it should be realized, the methods and the products of those methods are capable of other, different embodiments and their several details are capable of modification in various, obvious aspects all without departing from the methods and products as set forth and described in the following claims. Accordingly, the drawings and descriptions should be regarded as illustrative in nature and not as restrictive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       The accompanying drawing figures incorporated herein and forming a part of the specification illustrates several aspects of the methods and products of those methods and together with the description serves to explain certain principles thereof. 
         FIG. 1  is a schematic representation of the carbon fiber production process. 
         FIG. 2  illustrates the geometry of a +-shaped spinneret opening of the type that may be utilized to produce the lobed PAN fibers used in the production of the PAN-based carbon fibers. 
         FIG. 3  is a schematic representation of the carbon fiber fabric production method. 
         FIG. 4  is a cross section illustrating multiple fibers produced as a result of the lobed shaped cross section of those fibers. 
         FIG. 5  illustrates the lobe characteristics of the PAN-based carbon fibers. 
     
    
    
     DETAILED DESCRIPTION 
     Reference is now made to  FIG. 1  which schematically illustrates the new and improved method  10  for producing polyacrylonitrile-based carbon fibers/PAN-based carbon fibers. For purposes of this document, the terminology “PAN-based carbon fibers” includes carbon fibers made from polymer precursor fibers with a chemical composition consisting greater than 80% of the acrylonitrile monomer repeat units. 
     That method includes the step  12  of extruding a PAN dope of a type known in the art through a spinneret to produce a tow of PAN fibers wherein the filaments thereof have a lobed cross section. The spinneret  14  may include a plurality of openings  16  through which the PAN dope is extruded into a water coagulation bath of a type known in the art to be useful in the spinning of PAN fibers. More particularly, as illustrated in  FIG. 2 , each spinneret opening or hole  16  may have a +-shape adapted to produce fibers having the appearance of crenulated fibers. For example, each leg  18  may have a width W of 0.03 mm with the opposed legs having a total length L of 0.09 mm. 
     Following extruding, the method includes the washing, drying and winding  20  of the tow of PAN fibers without subjecting the PAN fibers to hot drawing. The absence of hot drawing is an important factor in producing PAN-based carbon fibers having the desired physical characteristics for use in ablative composite applications. 
     Following winding, the method includes stabilizing  22  the tow of PAN fibers by heating to a stabilizing temperature of, for example, 250°−280° C. in air and then cooling back to room temperature. In one particularly useful embodiment, the tow of PAN fibers is heated to 265° C. 
     Following stabilizing, the tow of PAN fibers is subjected to carbonizing  24  in an inert atmosphere at a temperature of less than 1,000° C. to produce PAN-based carbon fibers from the PAN fiber tow. In one possible embodiment of the method, the highest fiber temperature reached during carbonization is between 850° C. and 950° C. In another possible embodiment of the method, the carbonization temperature is between 885° C. and 915° C. In still another, the carbonization temperature is 900° C. In one possible embodiment, the fibers are maintained at peak carbonization temperature for between one and ten minutes. 
     Reference is now made to  FIG. 3  which illustrates the new and improved method  30  of producing a polyacrylonitrile-based carbon fiber fabric/PAN-based carbon fiber fabric. That method includes the step  32  of extruding a PAN dope of a type known in the art through a spinneret to produce a tow of PAN fibers having a lobed cross section. 
     As noted above, the spinneret  14  may include a plurality of openings  16  through which the PAN dope is extruded into a water coagulation bath of a type known in the art to be useful in the spinning of PAN fibers. More particularly, as illustrated in  FIG. 2 , each spinneret opening or hole  16  may have a +-shape adapted to produce fibers having the appearance of crenulated fibers. For example, each leg  18  may have a width W of 0.03 mm with the opposed legs having a total length L of 0.09 mm. 
     Following extruding, the method includes the washing, drying and winding  34  of the tow of PAN fibers without subjecting the PAN fibers to hot drawing. The absence of hot drawing is an important factor in producing PAN-based carbon fibers having the desired physical characteristics for use in ablative composite applications. 
     Following winding, the method includes the step  36  of weaving the tow of PAN fibers into a woven PAN fiber fabric. Durable weave patterns are those with more drape, such as  8  harness satin. 
     Following weaving, the method includes stabilizing  38  the tow of PAN fibers by heating in air to a desired stabilizing temperature of, for example, 250-280° C. In one particularly useful embodiment, the tow of PAN fibers is heated to 265° C. 
     Next, the woven PAN fiber fabric is subjected to carbonizing  40  in an inert atmosphere at a temperature of less than 1,000° C. to produce a PAN-based carbon fiber fabric from the PAN woven PAN fiber fabric. In one possible embodiment of the method, the highest fiber temperature reached during carbonization is between 850° C. and 950° C. In another possible embodiment of the method, the carbonization temperature is between 885° C. and 915° C. In still another, the carbonization temperature is 900° C. 
     As should be appreciated, the crenulated or lobed cross section of the PAN fiber filaments allows the fibers F to better mechanically interlock into the matrix material of the final composite. 
     The PAN-based carbon fiber filaments produced by the method disclosed herein include lobes or legs having a characteristic width W 1  greater than 10% of a width W f  of the fiber filaments at the largest dimension of the fiber filaments: that is W 1 /W f &gt;=0.1 (see  FIG. 5 ). 
     The PAN-based carbon fibers are characterized by a unique combination of physical characteristics that make them particularly useful for ablative composite applications. The PAN-based carbon fibers have a thermal diffusivity of less than 2 mm 2 /sec along the fiber axis. More specifically, with the low-fire carbonization temperature of 900° C. an axial thermal diffusivity of as low as 1.503 mm 2 /sec has been achieved. The incumbent rayon based carbon fiber has an axial thermal diffusivity of 1.944 mm 2 /sec. 
     The PAN-based carbon fibers have a modulus of less than 100 GPa along the fiber axis in tension. More specifically, under the no hot drawing condition, a modulus of as low as 60.06 GPa has been achieved. The rayon based carbon fiber is approximately 39.7 GPa. 
     The PAN-based carbon fibers have a break stress of less than 1 GPa along the fiber axis in tension. More specifically, under the no hot drawing condition, a break stress of 0.506 GPa has been achieved. The rayon based carbon fiber is approximately 0.454 GPa. 
     Finally, the PAN-based carbon fibers have a lobe width W 1 /max fiber diameter W f  of greater than 0.1. More specifically, under the indicated processing conditions, a W 1  of 6 microns and a W f  of 12 microns for a W 1 /W f  of 0.5 has been achieved. 
     Experimental Section: 
     Dope Preparation: 
     14 wt % of polyacrylonitrile (PAN) polymer powder (relative to the solvent, dimethylsulfoxide, DMSO plus the PAN masses) was mixed and prepared into a spinning solution or dope. For example, 196 g of PAN was mixed with 1204 g of DMSO. It is recommended that the PAN powder be dried under vacuum for 1 hour at 50° C. to remove moisture. After hand mixing to break up any large polymer powder agglomerates, the dope was poured into a purpose-built dope mixer and heated to 55° C. for overnight mixing. The dope was then extracted from the mixer with a 15 psi overpressure of nitrogen, directly into the spinning pump system. Next, the dope in the pump system was placed under vacuum to remove any entrained air bubbles in the dope. This degassing step is important for good fiber spinning. 
     Spin Preparation: 
     First the spinneret was inspected for cleanliness by optical microscopy, to ensure that no capillaries were clogged. Inspection of individual capillaries is best done under the microscope. If any are clogged, the spinneret is placed into a container of clean dimethylacetamide (DMAc) and sonicated using a wand sonicator at an intensity of 20% for 15 minutes. Similarly, after a spin run, this cleaning process was repeated and the spinneret was allowed to sonicate for one hour. 
     A new 3 micron filter element was placed in the filter housing. Then the spinning pump system, the filter housing, and the spinneret die head were heated to 50° C. On back to back spin runs, the same filter element is typically used. Down the line, the heated godet dryer rollers were heated to 90° C. The pump and heated godet were given an hour to fully heat up. The adapter hardware for the wet spinning spinneret was also placed in an oven at 50° C. 
     A 0.65 wt % silicone oil based emulsion spin finish was prepared by mixing 3.25 g of a proprietary spin finish concentrate, with 196.75 g of deionized water (1.625 wt %). As well a 0.5 wt % polyvinyl alcohol (PVA) spin/warp finish was prepared by mixing 11 g of a warp sizing, with 189 g of deionized water (5.5 wt %). 
     Spinning Procedure: 
     The spinning pump was connected to the filter manifold and die head by flexible pressure hoses. 40-50 ml of dope was purged through all components to eliminate contaminates and air bubbles. On back to back spin runs only 20 ml of dope was purged through the system. The spinneret was attached to the preheated adapter hardware and then attached to the die head. At an initial flow rate of 3.0 ml/min the spinneret (500 holes) was submerged into a 78 wt % DMSO/DI water coagulation bath and the nascent 500 filament tow is formed. The spinneret used was a special shaped hole spinneret as to generate fibers with cross sectional shapes that emulated crenulated fiber—(as observed in lobed fibers). Out of coagulation the tow was pulled on to the Y0 (initial) godet set to 0.9 m/min. Upon stable spinning, the flow rate was incrementally stepped down from approximately 4 ml/min (for start-up) to 1.38 ml/min. After coagulation, the fiber tow was then stretched 2.1× through a 50 wt % DMSO/DI water bath and pulled onto the Y1 (second) godet at 1.9 m/min. The fiber tow then travelled down the line through six deionized water wash baths at 1.9 m/min speeding up slightly to 2-2.2 m/min to keep the tow taught. No hot drawing was applied to the tow (typically precursor PAN fiber is significantly drawn). The fiber tow was then passed through the silicone emulsion spin finish and onto the heated godet (90° C.). After drying on the first four rollers, the fiber tow passed through the PVA spin finish bath at the fifth roller. The fiber was then dried on the remaining three rollers of the heated godet and taken onto cardboard spools at 2 m/min. The tow was traverse wound on cardboard cores and accumulated for subsequent weaving processing. 
     Weaving 
     The spun tow, in continuous form, was woven into a satin-harness fabric. An approximately 6 inch wide by 10s of feet long fabric was woven from 180 warp ends of the low k PAN precursor fiber. 
     Thermal Conversion: 
     Thermal conversion can be done on the spun PAN fiber, or on the woven fabric. For the purposes of ablative carbon composites, the woven is preferred. 
     Stabilization 
     The woven fabric was placed in an air convection oven, unconstrained, and heated at a rate of 10 C/min to 175° C., then 0.5° C./min to 265° C. Then it was allowed to cool to room temperature. 
     Carbonization 
     The stabilized fabric was carbonized under an inert atmosphere by ramping to 900° C. at 5° C./min. We consider this a ‘low-fired’ carbonization temperature. Higher carbonization temperatures increased the resulting carbon fiber thermal diffusivity (and thermal conductivity) beyond values of interest, see Table 1. The HTT needs to be &lt;1000 C. 
     The primary goal was to demonstrate a PAN-based carbon fiber with as similar as possible thermal, and mechanical properties to the incumbent rayon-derived carbon fiber—shown in Table 1 as Rayon derived carbon fiber. First, the data suggested that a highest heat treatment temperature (HTT) during carbonization of 900 C resulted in the PAN-based carbon fiber with thermal diffusivity most similar to the Rayon derived carbon fiber. Second, the 900 C HTT resulted in the modulus of the PAN-based carbon fiber most similar to the Rayon derived carbon fiber. Third, the no-draw condition of the PAN precursor, coupled with the low fire HTT of 900 C resulted in the PAN-based carbon fiber most similar to the Rayon derived carbon fiber. Lastly, the lobed (or crenulated) shape of the PAN precursor fiber, rendered a PAN-based carbon fiber similar to the Rayon carbon fiber. 
     Together these three issues; low fire carbonization temperature, no precursor draw, and lobed shape, result in a unique low k PAN-based carbon fiber, similar thermally, mechanically, and in cross sectional shape, to the Rayon based carbon fiber. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Thermal and mechanical properties of low-k PAN derived carbon fibers 
               
               
                 as a function of highest heat treatment temperature, compared to Rayon derived carbon fiber. 
               
               
                 Standard derivations are shown in parenthesis. 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                   
                 Corrected 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Heat 
               
               
                   
                   
                 Diffusivity 
                 Stress at Break 
                 Modulus 
                 Strain at 
                   
                 Capacity at 
               
               
                   
                 HTT 
                 (mm 2 /s) 
                 (MPa) 
                 (GPa) 
                 Break (%) 
                 H/C Ratio 
                 25C (J/gC) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Rayon 
                 N/A 
                 1.944 
                 (0.052) 
                 454 
                 (144) 
                 39.7 
                 (2) 
                 1.14 
                 (0.35) 
                 0.01645 
                 0.8740 
               
               
                 derived 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 carbon fiber 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Baseline 
                  900 
                 1.549 
                 (0.079) 
                 981 
                 (381) 
                 135.3 
                 (15) 
                 0.71 
                 (0.24) 
                 0.02337 
                 1.0589 
               
               
                 Low-k PAN 
                 1000 
                 3.805 
                 (0.242) 
                 1786 
                 (404) 
                 204 
                 (13) 
                 0.87 
                 (0.19) 
                 0.00862 
                 0.6802 
               
               
                 (367) 
                 1200 
                 5.83 
                 (0.127) 
                 1889 
                 (411) 
                 206 
                 (14) 
                 0.92 
                 (0.19) 
                 0.00643 
                 0.6302 
               
               
                   
                 1400 
                 9.713 
                 (0.573) 
                 1774 
                 (693) 
                 231 
                 (13) 
                 0.76 
                 (0.28) 
                 0.00435 
                 0.6647 
               
               
                   
                 1600 
                 13.879 
                 (1.744) 
                 2010 
                 (570) 
                 248 
                 (23) 
                 0.81 
                 (0.23) 
                 0.01383 
                 0.6343 
               
               
                 Unstretched 
                  900 
                 1.503 
                 (0.108) 
                 1167 
                 (469) 
                 92 
                 (14) 
                 1.27 
                 (0.50) 
                 0.02432 
                 0.8310 
               
               
                 Low k PAN 
                 1000 
                 3.662 
                 (0.285) 
                 1678 
                 (443) 
                 175 
                 (9) 
                 0.95 
                 (0.23) 
                 0.00605 
                 0.6222 
               
               
                 (368) 
                 1200 
                 5.868 
                 (0.199) 
                 1863 
                 (441) 
                 191 
                 (12) 
                 0.98 
                 (0.22) 
                 0.00865 
                 0.7299 
               
               
                   
                  1400* 
                 7.964 
                 (0.226) 
                 2025 
                 (511) 
                 210 
                 (17) 
                 0.96 
                 (0.20) 
                 0.00402 
                 0.8092 
               
               
                   
                 1600 
                 13.706 
                 (0.465) 
                 1134 
                 (387) 
                 228 
                 (29) 
                 0.49 
                 (0.15) 
                 0.00581 
                 0.6167 
               
               
                 No Draw 
                  900 
                   
                   
                 506.31 
                 (168.8) 
                 60.06 
                 (3.43) 
                 0.833 
                 (0.237) 
                   
                   
               
               
                 (374) 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
               
            
           
         
       
     
     Typically the fibers were hot stretched in the spinning process in 2 stages, which involved a hot water stretch (stage 1) followed by a hot glycerol stretch (stage 2). Here “Unstretched”=stretched in hot water, but not stretch in hot glycerol, or no 2nd stage stretch. “No Draw”=not stretched in hot water or hot glycerol (no stage 1 or stage 2). However, in all cases, the fibers were drawn approximately 2× just after their initial coagulation in the first step of the washing process. 
     The foregoing has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the embodiments to the precise form disclosed. Obvious modifications and variations are possible in light of the above teachings. All such modifications and variations are within the scope of the appended claims when interpreted in accordance with the breadth to which they are fairly, legally and equitably entitled.