Novel method of manufacturing carbon fiber, or precursor fiber therefor, using preliminary stretch

Abstract: Process of carbon fiber manufacture wherein the polyacrylonitrile precursor is stretched prior to oxidation in limited temperature range. Process permits greater throughput in precursor manufacture and reduces flaws in the precursor because significant stretching is at lower line speeds typically used for oxidation and carbonization in making carbon fiber.

Description: This invention relates to improvements in the manufacture of carbon fiber from polyacrylonitrile polymers. This invention, more particularly, relates to producing high quality and performance carbon fiber at significant cost savings through enhancing throughputs of precursor materials used in making such carbon fiber. This invention, still more particularly, relates to the discovery and exploitation of a limited temperature range wherein a multitude of filaments of the polyacrylonitrile polymer, such multitude often called "carbon fiber (polyacrylonitrile) precursor" or "precursor" for short, may be safely stretched up to four or more times its original length after formation but prior to the oxidation used in making the carbon fiber. PAN-based carbon fiber (for purposes of describing this invention hereafter called "carbon fiber") is a well-known material that is over 85% by weight elemental carbon. Such carbon fiber is notable for enabling manufacture of light weight composite materials and other articles of manufacture of exceptional tensile strength and modulus. Increased useage coupled with incremental improvements in the processes which manufacture carbon fiber have dramatically reduced its cost compared to when it was first introduced several years ago. Moreover, the attractiveness of light weight structures comprising carbon fiber has increased significantly due to recent marked improvements in the material properties of the carbon fiber and resin materials used in making the structures. Of late, however, despite even wider potential useage of carbon fiber through advances in material properties, continued significant decreases in its price, at one time dramatic and routine, have not proceeded as expected. One reason is that reliable manufacture of high quality carbon fiber remains an art; skillful control of process conditions used in making it and repeated testing for assuring quality remain a manufacturing requisite. Furthermore, recent advances in material property performance have been achieved through use of finer denier precursor, i.e., small diameter filaments, and such finer denier precursor requires the same or greater amount of precursor process control in making it but at generally lower throughputs. Accordingly, while the processing quality and performance characteristics of carbon fiber have been maintained and even improved over the years, the cost of making such high quality carbon fiber has not been reduced to the degree possibly anticipated, particularly with respect to newer carbon fibers made with finer denier precursor. It is evident that increases in the line speeds of the precursor and carbon fiber manufacturing processes reduce cost per weight of the final carbon fiber. However, further increases in line speeds, especially in precursor manufacturing speeds, increase the risk of lowering the quality of the resultant carbon fiber, particularly since precursor line speeds have generally approached their maximum limits with conventional equipment. The cost of making carbon fiber under current procedures can be divided into two categories. First, there is the cost of making the precursor, e.g. material costs of acrylonitrile, other monomers and solvent used in making the polyacrylonitrile polymer, costs associated with spinning the precursor, costs associated with treating the spun filaments such as stretching in cold and hot water and steam, and costs associated with process control and testing. Other costs in making carbon fiber include the costs of oxidizing and then carbonizing the precursor, costs associated with the process control of such oxidation and carbonization and testing costs. Since only about half by weight of the precursor becomes carbon fiber, it can be seen that reduction per pound of precursor manufacturing costs results in about twice that reduction per pound in carbon fiber. Exemplary description of making precursors useful for producing carbon fiber appear in UK 1,324,772, U.S. Patents 4,100,004; 4,001,382; 4,009,248; 4,113,847; 4,397,831; 4,448,740; 4,452,860 and Japanese Applications 53-24427 and 59-272341. These publications describe making precursors wherein polyacrylonitrile filaments are stretched in hot water, steam and air prior to oxidation. For example, in the "Example of Application 1" of Japanese Application 59-272341, wet spun polyacrylonitrile filaments are said to have been stretched over five (5) times over a hot water bath, washed and dried and then drawn 1.3 times at 170°C using dry heat prior to oxidation. U.S. Patent 4,104,004, on the other hand, discloses stretching up to 75% between 100°C and 160°C, e.g. 130°C. Now, in accordance with this invention, a limited temperature range has been discovered whereby the precursor may be stretched to four or more times its original length in air, steam or other fluid medium. As a result, greater throughput of precursor can be achieved under the same process control procedures used at less than half the throughputs while at the same time, only marginally increasing the cost of producing carbon fiber through such stretching. Furthermore, since the stretching achieved in this limited temperature range can be quite high, certain stretching during manufacture of the precursor may be reduced or even eliminated thereby allowing still further increase in throughputs and reductions in precursor manufacturing costs. This invention provides a process for making carbon fiber in the form of a tow or bundle comprising a multitude of high strength carbon fiber filaments, said process comprising (a) forming a spinning solution comprising a polyacrylonitrile polymer in a solvent for such polymer, (b) extruding said polymer through a die to provide a multitude of continuous filaments, (c) drawing these filaments to a longer length in making a precursor tow, and (d) oxidizing and then carbonizing said precursor tow in a series of ovens and furnaces to provide said carbon fiber, which comprises: (i) selecting precursor tow that is capable of being stretched at a limited temperature range of 140°C to below the temperature causing oxidation of said precursor tow; (ii) heating said precursor tow to a temperature within a limited temperature range above 140°C and below the temperature causing oxidation of said precursor tow; and (iii) stretching said precursor tow heated to said temperature so that its length after stretching is at least twice its length prior to stretching. Preferably, the precursor is stretched in the limited temperature range in air. Moreover, after the polyacrylonitrile polymer is extruded through the die, stretching prior to oxidation preferably consists essentially of stretching in the limited temperature range preceded by stretching at temperatures below boiling water temperature. Also, such stretching prior to stretching in the limited temperature range preferably comprises stretching under circumstances that plasticize the filaments. Figure 1 illustrates the thermal response curve for a preferred precursor of this invention. Figure 2 illustrates diagramatically a device that is useful in practice of this invention. Figures 3, 4 and 5 illustrate properties of precursors heated to a temperature in a range useful for practice of this invention. The polyacrylonitrile precursor useful in making carbon fiber in accordance with this invention comprises a polymer made by addition polymerization, either in solution or otherwise, of ethenic monomers (i.e. monomers that are ethylenically unsaturated), at least about 80 mole percent of which comprise acrylonitrile. The preferred polyacrylonitrile precursor polymers are copolymers of acrylonitrile and one or more other monofunctional ethenic monomers. Available ethenic monomers are diverse and include, for example, acrylates and methacrylates; unsaturated ketones; and acrylic and methacrylic acid, maleic acid, itaconic acid and their salts. Preferred comonomers comprise acrylic or methacrylic acids or their salts, and the preferred molar amounts of the comonomer ranges between 1.5 and 3.5%. (See U.S. Patent 4,001,382 and U.S. Patent 4,397,831 .) The polyacrylonitrile precursor polymers suitable for making carbon fiber hereof are soluble in organic and/or inorganic solvents such as dimethylsulfoxide, dimethyl formamide, zinc chloride or sodium thiocyanate solutions. In a preferred practice of making a polyacrylonitrile precursor for use in making the carbon fibers hereof, a solution is formed from water, polyacrylonitrile polymer and sodium thiocyanate at exemplary respective weight ratios of about 60:10:30. This solution is concentrated through evaporation and filtered to provide a spinning solution. The spinning solution preferably comprises about 15% by weight of the polyacrylonitrile polymer. The spinning solution is passed through spinnerets using dry, dry/wet or wet spinning to form the polyacrylonitrile precursor. The preferred polyacrylonitrile precursor is made using a dry/wet spinning wherein a multitude of filaments are formed from the spinning solution and pass from the spinneret through an air gap or other gap between the spinneret and a coagulant preferably comprising aqueous sodium thiocyanate. After exiting from the coagulant bath, the spun filaments are washed and then stretched again to several times their original length in hot water and steam. (See U.S. Patent 4,452,860 and Japanese Application 53-24427 [1978].) In addition, the polyacrylonitrile precursor is treated with sizing agents such as silane compounds (see U.S. Patent 4,009,248). The polyacrylonitrile precursor (preferably silane sized) is in the form of tows in bundles comprising a multitude of filaments (e.g. 1,000, 10,000 or more). The tows or bundles may be a combination of two or more tows or bundles, each formed in a separate spinning operation. A thermal response curve in air of a preferred polyacrylonitrile precursor suitable for use in making the carbon fibers of this invention is shown in Figure 1. This precursor is made from a monomer composition comprising about ninety eight mole percent (98%) acrylonitrile and about two mole percent (2%) methacrylic acid. The tex (denier) per filament of the polyacrylonitrile precursors desirably ranges between 0.07 (0.6) and 0.7 (6.0) or higher. The particular denier of the polyacrylonitrile precursor chosen influences subsequent processing of the precursor into carbon fiber hereof. For example, larger tex (denier) precursor, e.g. 0.148 tex (1.33 denier) per filament or above precursor is preferably stretched at temperatures below 200°C (e.g. about 150 - 160°C) to reduce its tex (denier) to less than 0.09 (0.8) prior to significant oxidation. Greater stretching in the limited temperature range reduces the need for such stretching in later, e.g., during oxidation, manufacturing steps in making the carbon fiber. Through stretching at temperatures between 145 and 165°C, the resultant precursor is up to 4.0 times or more its original length; and due to the minimal reaction at temperatures within this range may be in amounts selectively calculated in advance to provide the denier desired for subsequent oxidation and stabilization. For example, a 0.24 tex (2.2 denier) per filament precursor may be stretched to twice its original length to yield a 0.12 tex (1.1 denier) per filament material by a Stretch Ratio (S.R.) of 2 according to the following formula: d n = ds S.R. where S.R. equals Lo Li where L o is length out, L i is length in (i.e. original length), d S is original tex (denier) and d N is new tex (denier). Desired stretch ratio (S.R.) may be achieved by drawing the Desired stretch ratio (S.R.) may be achieved by drawing the precursor faster through the desired heated zone (e.g. temperature between 150°C and 160°C) that it is permitted to enter this zone. Figure 2 illustrates in schematic form the design of stretching device 20 useful in practice of this invention. Device 20 of Figure 2 comprises supports 22 which carry platform 24 on which inlet roll stack 26, steam heated roll stack 28, steam heated tube 30 and outlet roll stacks 32 and 34 are mounted. Steam heated tube 30 is heated by steam passing through a jacket surrounding the tube walls. In addition, gas inlet tube 36 is mounted to the downstream end of steam heated tube 30 and provides for entry of heated air or other gas when countercurrent gas heating is used in heat-up of the precursor traveling between about inlet and outlet roll stacks 26, 34. Control 38 monitors inlet and outlet counter-rolls 40, 42 which can also measure tension and line speed as read out in control 38. Access port 44 permits stringing precursor through steam heated roll stack 28. Steam heated roll stack 28 is optionally used in providing preliminary heat transfer to the precursor as it travels around the steam heated rolls 28. This preliminary heat transfer permits shorter heated lengths of steam heated tube 30, if desired. Features of the Device 20 of Figure 2 such as length of steam heated tube 30, use or non-use of steam heated roll stack 28 and gas flow velocity, direction and placement of inlet tube 36 as well as temperatures used for heating thereof are selected as a matter of design choice so as to provide rapid heat-up of the precursor as well as compatibility with the oxidation and carbonization line speeds in the carbon fiber process. Initial tow spacing that ranges preferably between 2 and 4.5 tows per 2.54 centimeter for 12000 filament tows are preferred for flow of heat transfer medium, such spacing advantageously inherently increasing as stretching proceeds. The polyacrylonitrile precursor after stretching in the limited temperature range according to this invention, is oxidized in one or more ovens preferably maintained at temperatures between 200°C and 300°C. A variety of oven geometries are known to provide appropriate oxidation in making carbon fiber and any of these ovens may be suitably employed in accordance with this invention. Preferably, however, a series of ovens (or series of passes through a single oven) are employed according to this invention with the precursor that is undergoing oxidation optionally stretched to a longer length than the length it has upon entering the oxidation oven. After exit from the oxidation oven or ovens, the oxidized (and stabilized) precursor is passed to one or more furnaces for carbonization in an inert atmosphere. Preferably, at least two furnaces are employed respectively at temperatures between 400°C and 800°C and between 1000°C and 1400°C. Still higher modulus carbon fiber is made through using still another furnace having temperatures above 1800°C, e.g. between 2000°C and 2800°C. The fiber undergoing carbonization is desirably stretched or at least not allowed to shrink in the temperature range between 400°C and 800°C and 2000°C and 2800°C. After exit from the high temperature furnace the carbonized fiber is surface treated. A variety of surface treatments are known in the art. Preferred surface treatment is an electrolytic surface treatment and the degree of surface treatment is ordinarily a function of the carbonization temperatures used in making the fiber. The preferred electrolytic surface treatment comprises passing the fiber through a bath containing an aqueous ammonium bicarbonate or sodium hydroxide solution (e.g. 0.5 - 3% by weight). The current is applied to the fiber at between about 0.1 and 5 coulumbs/2,54 cm (inch) of fiber per 12,000 filaments. The resulting surface treated fiber is then preferably sized with an epoxy compatible sizing agent such as Shell epoxy Epon 834. The following examples are intended to illustrate this invention and not to limit its broader scope as set forth in the appended claims. In these examples, all temperatures are in degrees Centrigade (°C) and all parts are parts by weight unless otherwise noted. Example 1 (reference example) Polyacrylonitrile precursors were made using an air gap wet spinning process. The polymer of the precursor had an intrinsic viscosity between 1.9 and 2.1 deciliters per gram using a concentrated sodium thiocyanate solution as the solvent. The spinning solution and coagulants comprised an aqueous solution of sodium thiocyanate. The polymer was made from a monomer composition that was about 98 mole % acrylonitrile and 2 mole % methacrylic acid. The precursors were stretched so that their length in tow form was about six times greater after steam stretching compared to their length after extrusion from the spinnerets. Table A shows the characteristics of the resulting precursor which were nominal 0.14 and 0.09 tpf (tex per filament) (1.3 and 0.8 dpf (denier per filament)). Precursor Properties TPF DPF (NOMINAL) 0.14 (1.3) 0.09 (0.8) Precursor Properties Tow Tex (g/1000m) 1.784 1.063 Tow Denier (g/9000m) (16.053) (9.570) Tow Tenacity cN/dtex (g/d) 4.3 (4.9) 4.9 (5.6) Tow Modulus cN/dtex (g/d) 85 (96) 90.1 (102) DHT cN/dtex (g/d) 0.125 (0.142) 0.147 (0.166) Boil-off Shrinkage (%) 5.5 5.7 US Content (%) 0.60 0.88 Sodium Content (ppm) 580 568 Residual Solvent (%) - 0.0073 Moisture Content (%) 0.92 0.60 Filament Diameter Cv (%) 4.1 4.4 Monster Filaments 0 0 The precursors were then stretched at a temperature of 158°C with a residence time of about six (6) minutes at that temperature. After stretching, the stretched precursor was oxidized in stages generally at 234°C and then 249°C. The oxidized fiber was then passed through a low temperature furnace with a non-oxidizing atmosphere (nitrogen) at a temperature between 600°C and 800°C (tar removal) and then passed to a high-temperature furnace having a temperature between 1200°C and 1400°C and a non-oxidizing atmosphere (nitrogen). During passage through the low and high temperature furnaces the fiber undergoing carbonization in these furnaces was either allowed to shrink or was stretched across the length of the furnaces. Tables B and C, set forth below, show the stretch of the fiber undergoing carbonization as it passes through the tar removal (TR) and high temperature (Cl) furnaces along with the calculated dpf of the filament based on the stretch imparted at 158°C. Under each level of stretch in these tables the tensile strength and modulus properties are listed for the fiber having the calculated tpf (dpf) and stretch during passage through the TR and Cl furnaces. (Table B shows results from using the 0.14 tpf (1.3 dpf) precursor of Table A; Table C shows the result of using the 0.09 tpf (0.8 dpf) precursor of Table A.) The first value in columns 4 through 9 of Table B and C is tensile strength of the resultant carbon fiber in Mega Pascal (MPa) times 1000 (psi times 1000) and the second value is modulus in MPa times 1 000 000 (psi times 1,000,000). Modulus and tensile strength measurements were made using strand and tow test (Impregnated Strand) procedures. Example 2 Polyacrylonitrile precursor was made generally according to the conditions previously described except that it had no steam stretching and its denier was 0.13 tpf (1.2 dpf). The 0.13 tpf (1.2 dpf) polyacrylonitrile precursor fiber was stretched to twice its original length (i.e. s. r. equals 2) at a temperature of 158°C and wound around a spool and stored. The precursor was then oxidized by passing it through air circulation ovens at temperatures for the times shown in the following Table D. Temperatures Time (minutes) 158°C 2.05 240°C 17.73 245°C 14.43 248°C 17.72 250°C 17.72 250°C 4.43 The oxidized precursor passed from the last oxidation oven through a low temperature (tar removal) furnace. Then the partially carbonized fiber passed through a first low temperature furnace held at 1425°C and then a second high temperature furnace held at 2500°C. The stretch in each of the low temperature, first high and second high temperature furnaces are shown below (values are %) for four distinct runs in Table E below. Run Overall TR C1 C2 135-1 0.1 4.5 -5.3 0.9 135-2 2.4 6.9 -5.1 0.9 135-3 4.9 9.3 -5.0 1.0 135-4 6.9 11.3 -4.1 0.2 Table F, below, shows the properties of carbon fiber made according to the procedures of this Example. Run Modulus Tensile Strength Density 135.1 0.401 (58.2) 4.18 (606) 135.2 0.414 (60.1) 4.24 (615) 135-3 0.424 (61.5) 4.33 (628) 135-4 0.423 (61.4) 3.85 (558) Example 3 In this Example, precursor was made under conditions generally described heretofore. The precursor had a 0.186 tpf (1.67 dpf) (12,000 filaments per tow) and had not been stretched under steam. The precursor was passed through a device like that shown in Figure 2 and stretched four (4) times its original length (i.e. S.R. equals 4) after a residence of 0.8 minutes at about 158°C. In similar studies, a precursor run at the same temperature but at a residence time of 0.25 minutes broke prior to achieving a stretch that was equivalent to making it 2.3 times longer than its original length. When the residence time was increased to 0.33 minutes at 158°C, the tows broke unacceptably after a stretch that made them 3.3 their original length. However, the precursor had reduced cosmetics at stretch ratios (S.R.) which equaled 2.0 and 2.3 at these respective 0.25 minutes and 0.33 minutes residences at 158°C. Example 4 In this Example the precursor used was made from a polyacrylonitrile polymer using sodium thiocyanate as solvent and coagulant and an air gap spinning process, as described heretofore. The fiber was only stretched in water and had a 0.297 tpf (2.67 dpf) and 12,000 filaments per tow. Methacrylic acid was used in making the polyacrylonitrile polymer. The 0.297 tpf (2.67 dpf) precursor was stretched in a prototype device like that shown in Figure 2, also described hereinbefore. Hot air at 158°C was circulated in counter-current flow in tube 30 around the tows which were spaced about 1.8 toss per centimeter.Steam at 0.49 MPa (71 psig) was passed into the jacket of tube 30 and into the rolls of roll stack 28. The line speed was gradually increased with the stretch ratio held at 3.9 (i.e. precursor was 3.9 times as long after stretching as compared to length prior to stretching). Tensions of the precursor were measured in the rolls stacks 26, 34. Results of this Example are shown in Table G, below. Effect of Increasing Speed on Break Stretch Ratio Stretch Ratio Line Speed No. of Tows Tension (gpd) at Pin Position 1 4 10 3.9X 25(x30.48cm (ft)/min.) 10 0.347 0.335 0.333 3.9X 25.8 10 0.347 0.335 0.333 3.9X 26.6 10 0.347 0.335 0.333 3.9X 27.5 10 0.347 0.335 0.333 3.9X 28.3 10 0.349 0.335 0.335 3.9X 29.1 10 0.349 0.335 0.335 3.9X 30.0 10 0.349 0.335 0.335 Fiber broke during 9.2 m (30 ft)/min. run. Run used 0.297 tpf (2.67 dpf)/12K precursor, hot air at 158°C, steam tube and rolls at 0.49 MPa (71 psig). 1 gpd = 0.883 cN/dtex Table H below shows the results of making a nominal 0.09 tpf (0.8 dpf) precursor from the aforedescribed 0.297 (2.67) precursor described above using the procedures of this invention. A standard steam stretched precursor stream (See Table A) having a 0.09 tpf (0.8 dpf) was used as a control in making carbon fiber and compared against carbon fiber made from a 0.09 tpf (0.8 dpf) (calculated) precursor which was made using the procedures of this invention. The results are shown in Table I below. As can be seen, nearly equivalent properties are obtained using the procedures of this invention even though the amount of stretching in oxidation is less. Example 5 The tension developed at increasing temperatures and 0% stretch (i.e. constant length) for various precursors was measured. The tension measured versus the temperature to which each precursor was heated at this 0% stretch is shown in Figure 3 for four polyacrylonitrile materials. The monomer composition for the precursor of Curve A in Figure 3 included acrylonitrile, methacrylic acid and methylacrylate. The monomer composition used in making the precursor of Curve B in Figure 3 included acrylonitrile and methacrylic acid. The monomer composition used in making the precursor of Curve C included acrylonitrile, itaconic acid and methylacrylate. Precursors of various compositions were heated to various temperatures and the break stretch ratio (i.e. stretch ratio (SR) when the precursor filaments broke) determine for that temperature for each precursor composition. Curve A, Figure 4, shows a plot of break stretch ratio, or stretch ratio at the time the filaments of the precursor broke, for a precursor made from ingredients noted above in connection with Figure 3. Likewise, Curves B and C of Figure 4 show results for precursors having compositions indicated above with respect to Figure 3. Figure 5 shows the results of stretching different denier precursors until breaking at various temperatures. The monomer composition used in making each of the precursors in Figure 5 was 98 mole % acrylonitrile and 2 mole % methacrylic acid. Precursors D, E and F were not stretched in steam and have therefore somewhat less previous stretch imparted than precursors G and H (which were steam stretched). In Figure 3, the conversion 1 gdp = 0.883 cN/dte applies.

Claims: A process for making carbon fiber in the form of a tow or bundle comprising a multitude of high strength carbon fiber filaments, said process comprising: (a) forming a spinning solution comprising a polyacrylonitrile polymer in a solvent for such polymer; (b) extruding said polymer through a die to provide a multitude of continuous filaments; (c) drawing these filaments to a longer length in making a precursor tow; and (d) oxidizing and then carbonizing said precursor tow in a series of ovens and furnaces to provide said carbon fiber, which comprises: (i) selecting precursor tow that is capable of being stretched at a limited temperature range of 140°C to below the temperature causing oxidation of said precursor tow; (ii) heating said precursor tow to a temperature within a limited temperature range above 140°C and below the temperature causing oxidation of said precursor tow; and (iii) stretching said precursor tow heated to said temperature so that its length after stretching is at least twice its length prior to stretching. The process in accordance with Claim 1, wherein said heating of step (ii) comprises heating in a gaseous medium. The process in accordance with Claim 1, wherein said heating of step (ii) comprises heating through contact with heated rollers. The process in accordance with Claim 1, wherein said limited temperature range is between 150°C and 160°C. The process in accordance with Claim 1, wherein said polyacrylonitrile is made from a monomer composition comprising 98 mole % acrylonitrile and 2 mole % methacrylic acid. The process in accordance with Claim 1, wherein said precursor tow that is stretched in step (iii) is passed in a heated condition to an oxidation oven. The process in accordance with Claim 1, wherein said drawing of step c consists essentially of drawing at a temperature below 100°C. The process in accordance with Claim 1, wherein said polyacrylonitrile is heated to a temperature between 142°C and 180°C so as to permit said precursor tow to be stretched at least to a length that is four times its original length prior to said stretching and then stretching said precursor tow at said temperature so it would be four times longer after said stretching than prior to said stretching at said temperature. A method of making a precursor suitable for a process for manufacturing a high quality carbon fiber in accordance with Claim 1, said method comprising polymerizing acrylonitrile and one or more other monomers in a solvent for the resultant polymer, extruding said polymer through a spinneret comprising a multitude of apertures to provide a precursor tow comprising a multitude of filaments and stretching said precursor tow wherein said stretching consists essentially of first and second stretching operations wherein said first stretching operation is at a temperature below 100°C and said second stretching operation consists of stretching at a temperature between 142°C and a temperature wherein said filaments begin to oxidize.

Inventors: PAUL JAMES TONER JR, PAUL, JAMES TONER, JR.
Applicants: HEXCEL CORP, HEXCEL CORPORATION
Application year: 1989
Main classification: D01F9
IPC codes: D01F6, D01F9