Abstract:
This invention relates to a method of carbonizing polyacrylonitrile fibers (PAN fibers) by exposing the fibers at an elevated temperature to an oxidizing atmosphere, then exposing the oxidized fibers to an atmosphere of an inert gas such as nitrogen containing a carbonaceous material such as acetylene. The fibers are preferably treated with an organic compound, for example benzoic acid, before the exposure to an oxidizing atmosphere. The invention also relates to the resulting fibers. The treated fibers have enhanced tensile strength.

Description:
ORIGIN OF THE INVENTION 
     The invention described herein was made by an employee of the United States Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     This invention relates to the carbonization of polyacrylonitrile (PAN) fibers. 
     BACKGROUND ART 
     Carbonized PAN fibers find use as reinforcement in various materials of construction, principally plastic materials. It is desirable that the fibers have a high tensile strength and for certain purposes that they also have a high electrical resistance. 
     Such fibers intended as reinforcement are oxidized by air at a relatively low temperature, typically about 260° C., and are then subjected to carbonization in an inert atmosphere at an elevated temperature. The carbonization is typically carried out in an atmosphere of nitrogen and it may be carried out at a relatively low temperature, for example 1200° to 1500° C. or at a higher temperature, for example about 2500° to 3000° C. The carbonized fibers resulting from the lower temperature carbonization have higher tensile strength than those prepared at the high temperature. 
     OBJECT OF THE INVENTION 
     It is an object of the present invention to improve upon the methods of producing carbonized PAN fibers and to produce PAN fibers of higher tensile strength, and higher modulus than heretofore and/or of higher electrical resistance. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention PAN fibers, after the oxidation step described above, are carbonized, preferably at an even lower temperature than indicated above, in the presence of a gaseous active carbon substance, such as, for example, acetylene and/or others mentioned hereinbelow. As will be apparent from the examples and data presented hereinbelow, tensile strength is greatly improved. Preferably, prior to the oxidation step, the PAN fibers are subjected to contact at a relatively low temperature, typically at about 140° to 200° C., with molten benzoic acid or other similar liquid or gaseous conditioning material. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As active carbon sources any one or a mixture of the following may be used: Methane, acetylene, ethylene, fuel gas and aliphatic hydrocarbons generally, provided they can be used in gaseous form; benzene and other aromatic hydrocarbons; also nonhydrocarbon organic materials such as carbon disulfide. In general any carbonaceous material that is gaseous at the temperature involved and which decomposes at temperatures below 1000° C. within a reasonable period of time, of the order of minutes, may be used for this purpose provided it does not react destructively with the PAN fibers. 
     In place of benzoic acid other aromatic carboxylic acids may be used such as phthalic acid, terephthalic acid and naphthoic acid. Hydroquinone may also be used. They may be used in liquid, e.g. molten state or in the gaseous state. 
     The examples below will serve further to illustrate the practice and advantages of the invention. In carrying out the experimental work, a standard procedure and a modified procedure were employed as follows: 
     STANDARD PROCEDURE 
     A tow of 96 filaments of 2.3 denier PAN was oxidized at 260° C. over a contact time of about 3 hours under a load of 15 g in a tubular reactor of about 2.45 cm diameter to produce a fiber with less than 5% shrinkage or extension. The sample was then heated at a rate of 20° C./min to temperatures within a range of 450°-1100° C. in a flow of 20 cc/min nitrogen atmosphere under a load of about 50 g in a tubular reactor of about 1.9 cm diameter. 
     MODIFIED PROCEDURE 
     In Examples 2, 4, 5 and 6 the same PAN tow was first passed through a molten benzoic acid bath at 175° with 1 g loading over a contact time of about 3 hours to produce fibers with less than 10% shrinkage or extension. In each of Examples 2 to 6 (after treatment with benzoic acid in Examples 2, 4, 5 and 6) the samples were oxidized at the control conditions under a load of 50 g to produce fibers with less than 5 percent shrinkage or extension. The oxidized fibers were then carbonized at the control conditions, except for the presence, as indicated, of a mixture of acetylene-nitrogen as the atmosphere (5.34% acetylene, 94.66% nitrogen). 
     The results are set forth in Table 1 below. It will be seen that substantial improvement in tensile strength resulted from carbonizing (Example 3) in an atmosphere of nitrogen and acetylene and that the best results were obtained by pre-treatment with benzoic acid followed by oxidation and carbonization in an atmosphere of nitrogen and acetylene (Example 4). Example 5 shows that treatment with benzoic acid, carbonizing in an atmosphere of nitrogen, cooling and then carbonizing in an atmosphere of nitrogen and acetylene substantially improves tensile strength but not as much as in Example 4 where the initial (and only) carbonization is carried out in an atmosphere of nitrogen and acetylene. 
     
                                           TABLE 1__________________________________________________________________________Effect of Processes on Tensile Strength of Final Carbon Fiber__________________________________________________________________________Product           Benzoic Acid,                   Air Oxidation at                            100% N.sub.2, at                                   5.34% C.sub.2 H.sub.2 /N.sub.2Sample Treatment           175° C., hr                   260° C., hr                            °C.                                   at °C.                                           % Elongation__________________________________________________________________________  Oxidized and carbonized in           --      3        700    --      0.76  N.sub.2 (standard process)  Treated with benzoic acid,           3       3        700    --      0.77  oxidized and carbonized  In N.sub.2  Oxidized and carbonized in           --      3        --     700     0.90  C.sub.2 H.sub.2  Treated with benzoic acid,           3       3        --     700     1.22  oxidized and carbonized  In C.sub.2 H.sub.2  Treated with benzoic acid,           3       3        700    700     --  oxidized and carbonized  in N.sub.2, cooled and  carbonized in C.sub.2 H.sub.2  Treated in benzoic acid           3       --       --     700     --  and carbonized in C.sub.2 H.sub.2__________________________________________________________________________                                    Tensile                                         % Change in                       Modulus                              Resistance                                    Strength                                         Tensile Strength       Sample Treatment                       (psi × 10.sup.-6)                              (Ω cm)                                    (psi)                                         From (A)__________________________________________________________________________       1.         Oxidized and carbonized in                       11.9   4.18   90596                                         0.00         N.sub.2 (standard process)       2.         Treated with benzoic acid,                       12.4   9.10   95160                                         +5.04         oxidized and carbonized         In N.sub.2       3.         Oxidized and carbonized in                       13.8   1.32  124400                                         +37.30         C.sub.2 H.sub.2       4.         Treated with benzoic acid,                       12.6   3.18  154170                                         +70.20         oxidized and carbonized         In C.sub.2 H.sub.2       5.         Treated with benzoic acid,                       --     --    129980                                         +43.50         oxidized and carbonized         in N.sub.2, cooled and         carbonized in C.sub.2 H.sub.2       6.         Treated in benzoic acid                       --     --    103100                                         +13.70         and carbonized in C.sub.2 H.sub.2__________________________________________________________________________ Note: Figures in Columns 1 and 2 are approximate. 
    
     In Table II the effects of acetylene concentration and temperature on tensile strength are set forth. The modified procedure was used in all cases. 
     
                                           TABLE II__________________________________________________________________________Effect of Acetylene Concentration and Temperature on Tensile Strengthduring Carbonizationof a PAN Sample Treated in Benzoic Acid at 175° C. for 3 hr andAir-Oxidized at 260° C. for 3 hr.Acetylene         Tensile Strength,                           Resistance,  ModulusSampleConcentration, %         Temperature, °C.                  (psi)    ohm-cm                                 % Elongation                                        (psi × 10.sup.-6)__________________________________________________________________________1    5.34     647      122,730  44.1  n.a.   n.a.2    5.34     705      154,170  3.18  1.22   12.63    5.34     800      191,180  .058  0.74   25.84    5.34     845      169,550  .019  0.70   24.21    11.3     610       73,590  254.7 --     --2    11.3     705      124,450  1.54  --     --3    11.3     750      136,320  .212  --     --4    11.3     805      116,860  .054  --     --1    14.02    500       68,370  18300 --     --2    14.02    590      116,930  657.6 --     --3    14.02    655      144,900  9.52  --     --4    14.02    703      133,280  n.a.  --     --Standardprocess0.00     705       90,590  4.18  0.76   11.9__________________________________________________________________________ 
    
     Table III illustrates the effect of varying the procedure. In the first case there was a straight heating in an atmosphere of acetylene and nitrogen. In the other cases the fiber was heated to 500° C. in 20 cc/min flow of pure N 2 , held for 10 minutes while the acetylene mixture replaced the N 2 , heated to the set temperature, the gas flow decreased to 0.5-1.0 cc/min during the 5-minute hold period, and the system purged with 90 cc/min pure N 2  on cooldown. 
     
                                           TABLE III__________________________________________________________________________The Effect of Variation of Carbonization Procedureon Tensile Strength            Acetylene                     Tensile Strength,                              Resistance   ModulusProcedure   Temperature, °C.            Concentration, %                     psi      ohm-cm                                    % Elongation                                           (psi × 10.sup.-6)__________________________________________________________________________  Straight   845      5.34     171,500  0.0191                                    0.71   24.2  heat-up at  20° C./min  in C.sub.2 H.sub.2  Modified   840      5.34     235,200   .0199                                    0.93   25.3  &#34;              2.0      256,433  --    1.01   25.4  &#34;              1.0      225,400  --    1.00   22.5  &#34;              0.0      104,920  --    0.61   17.2__________________________________________________________________________