US3479150A - Carbonization method for cellulosic fibers - Google Patents

Description

United States Patent 3,479,150 CARBONIZATION METHOD FOR CELLULOSIC FIBERS Carlos L. Gutzeit, Long Beach, Calif., assignor to Hitco, Gardena, Calif. No Drawing. Filed July 14, 1965, Ser. No. 472,043 Int. Cl. C01b 31/07 US. Cl. 23209.1 25 Claims ABSTRACT OF THE DISCLOSURE Fibrous cellulosic materials are carbonized in the presence of an acidic oxidizing gas at a temperature between I about 300 F. and below a temperature which results in substantial impairment of the integrity of the fibrous materials until the exothermic reaction between the oxidizing gas and materials is substantially completed. The resulting material is thereafter heated at a higher temperature in the essential absence of oxygen and preferably in the presence of halogen or an alkaline reducing gas until carbonization is substantially completed.

The present invention generally relates to carbonization and more particularly relates to an improved method of carbonizing fibrous cellulosic materials.

Amorphous and graphitic carbon products are becoming increasingly important in various applications, for example in the space industry, where thermally insulative, ablation resistant materials are needed. Carbon products heretofore have been prepared by a wide variety of techniques in a number of forms, including monolithic forms and fibrous forms. The fibrous forms are newer and present special production problems. They include amorphous carbon and crystalline carbon (graphite) textiles, rovings, extended filaments, cord, tape and the like.

Whereas strong monolithic forms can be'fabricated by special rapid binding and hot pressing techniques, fibrous forms do not lend themselves to such fabrication techniques. Instead, time consuming carbonization procedures must be utilized to preserve, as much as possible, the strength and flexibility of the fibrous starting materials. However, present commercial techniques still result in substantial'decreases in strength, flexibility and durability of the fibers. In many instances, fiber integrity is seriously endangered. Moreover, commercial fibrous carbon products usually vary considerably from lot to lot in the indicated characteristics. In addition, considerable difficulties have been encountered in producing carbonized fibrous materials in a uniformly high yield. Commercial methods usually result in a recovery of only about 18-20 wt. percent of the product as opposed to a theoretical yield of about 44.4 wt. percent of carbon in the case of rayon and the like.

Inasmuch as the usual commercial carbonizing method ordinarily is relatively complicated, time consuming (of the order of 5-10 days) and expensive, it is obvious that it is of considerable importance that as high a yield as possible of uniformly high quality product be obtained. A further difliculty has been encountered in that existing commercial methods are not well adapted to vary selected characteristics of the carbon products. Thus, certain applications require fibrous carbon products of reduced surface area. Other applications desire high surface area products. Such products have been difficult or impossible to achieve.

It would be very desirable to provide an improved method of carbonization of fibrous cellulosic materials, which method should be capable of resulting in improved yields of uniformly high quality (high strength, flexible and durable) carbon products whether amorphous, or crystal- 3,479,150 Patented Nov. 18, 1969 line or any mixture thereof. It will be understood that, unless otherwise specified, reference herein to fibrous carbon products is intended to encompass fibrous amorphous carbon products and fibrous carbon products which contain up to about percent graphite.

Accordingly, it is a principal object of the present invention to provide an improved carbonization method.

It is also an object of the present invention to provide a simple relatively rapid and inexpensive method of carbonizing fibrous cellulosic materials to fibrous carbon products in high yield.

It is further object of the present invention to provide an improved method of carbonizing fibrous cellulose materials to fibrous carbon products of controlled characteristics, for example, relatively high tensile strength, flexibility, and durability, and controlled surface area and porosity.

It is a still further object of the present invention to provide an improved carbonization method capable of producing fibrous carbon products of uniformly high quality.

These and other objects are accomplished in accordance with the present invention by providing an improved carbonization method. The method comprises partial carbonization of fibrous cellulosic materials at relatively low temperatures in the presence of selected acidic oxidizing gas. The method is believed to operate by effecting rapid and controlled dehydration of the fibrous cellulosic material, thus initiating controlled breakdown of the cellulosic material so as to utimately result in a substantial improvement in yield and strength, flexibility and durability of the final fibrous carbon product. Such dehydration can be accomplished substantially completely and without substantial deterioration of the fiber integrity in the final product. Moreover, it can be accomplished at a lower temperature than possible in conventional carbonization techniques.

The treated product of the described step of the present method can then be subjected to relatively rapid controlled higher temperature essentially complete carbonizing and, if desired, subsequent graphitization to provide the final desired product.

In one embodiment of the present method, a selected alkaline reducing gas contacts the acidic oxidizing gastreated product during such higher temperature carbonizing. This contacting has the unexpected result of substantially decreasing the total surface area and porosity of the ultimate fibrous carbon product.

In another embodiment, contacting with selected acidic oxidizing gas is continued throughout higher temperature carbonizing to assure a very high purity product.

Accordingly, the present method is relatively simple, inexpensive and rapid and provides a final fibrous carbon product of uniformly high quality, including high tensile strength, flexibility and durability, and of controlled surface area. Of considerable importance, the product is obtained rapidly and in an improved yield. The method is adapted for batch, semi-continuous or continuous operation. Further advantages of the present invention will be apparent from a study of the following detailed description.

Now referring more particularly to the present method, an initially low temperature heat treatment of fibrous cellulosic material in the presence of a selected acidic oxidizing gas is carried out in order to effectively initiate carbonization. It is believed that the acidic oxidizing gas, dehydrates the fibrous cellulosic material and speeds the carbonization process. The acidic oxidizing gas-contacting step constitutes the first major step of the method.

The fibrous cellulosic material may be any fibrous cellulosic material, natural or artificial, e.g., reconstituted,

for example, cotton, rayon, or the like, in woven or unwoven fibrous form. Thus, the starting material can be in carded or uncarded form, chopped fiber form, felted, etc. Woven textiles are used extensively because of their inherently great utility. It is preferred that the starting material be substantially devoid of foreign matter, including finishes, coatings, impregnants and the like. Accordingly, various conventional washing operations, solvent treatment steps and the like can be performed to bring the starting material to the desired state of purity. Such steps are not required, however, so long as the starting material is substantially free of foreign matter which would interfere with the desired carbonization and/ or reduce the desired purity of the final product.

, As an optional, although preferred, preliminary step, prior to contacting the starting material with the acidic oxidizing gas, the starting material can be heated in an inert gas, for example, nitrogen (of suflicient purity), helium, argon or the like, to any suitable temperature up to about 350 F. to remove surface moisture from the starting material. Temperatures above about 350 F. are not recommended, because of difficulties in preventing partial degradation of the cellulosic material. Such preliminary heating can occur over, for example, -15 minutes or the like, preferably while sweeping the surface of the fibrous cellulosic material with the inert gas. Such treatment can be terminated when moisture is no longer detected in the exiting inert gas. Such treatment in some instances slightly increases the ultimate yield of fibrous carbon product.

After the described optional moisture removal step, the fibrous cellulosic material is subjected to the required low temperature treatment (first major step) in the presence of the acidic oxidizing gas. The temperature at which the acid oxidizing gas treatment is carried out is one which is sufiicient to facilitate the relatively rapid dehydration of the fibrous cellulosic material in the presence of the acidic oxidizing gas. For most purposes, temperatures below about 300 F. are too low to permit a reaction rate sufiiciently rapid and complete for commercial preparation of the carbon fiber products. Temperatures within the range of from about 300 F. to above 350 F. are preferred. It is within this range that the desired reaction occurs relatively rapidly but at a readily controllable rate. Temperatures higher than about 350 F. can be used, for example, up to about 450 F. or so. However, no advantage is obtained by using the same. Moreover, difficulties may occur in controlling the rate of the reaction so as to prevent undesired changes in the fiber integrity or structure of the material under treatment. Accordingly, for most purposes, the reaction is carried out at temperatures of between about 300 F. and about 350 F. It will be noted that such temperature is substantially below conventional carbonization temperatures.

The selected acidic oxidizing gas is any suitable acidic oxidizing gas which does not leave an appreciable residue in the carbonized product. For most purposes halogens, namely chlorine, bromine and iodine are preferred. However, halogen-yielding mixtures of hydrogen halides and oxygen (e.g. air) can be used as substitutes for the halogens, as by a mechanism believed to be analogous to the well-known Deacon process for chlorine generation. The oxygen concentration of such a mixture should be no greater than that necessary to convert all of the hydrogen of such halide to water i.e. no more than a stoichiometric amount. However, an about stoichiometric amount of oxygen should be used so that the halogen-yielding reaction is carried to substantial completion. Suitable mixtures of the described acidic oxidizing agents can be provided. It will be noted that uncombined oxygen, as for example, in air cannot be successfully used by itself, in the absence of the acidic gases above-described. Oxygen is not sufficiently reactive for the described dehydration at the indicated temperatures. Moreover, oxygen in any substantial concentration presents a certain hazard in that combustion of the fibrous cellulosic material can occur if the temperature of the material is inadvertently allowed to rise to above about 490 F. Accordingly, for the purposes of the present invention, the oxidizing agent is limited to acidic oxidizing gas, as described. Preferably, the acidic oxidizing gas is diluted with inert gas, e.g., pure nitrogen, helium, argon, etc. or mixtures thereof, to a suitable concentration, usually between about 1 and about 10-15 vol. percent, depending upon the reactivity of the acidic oxidizing gas, the reaction temperature selected, the surface area of the material, and other factors. Other suitable concentrations of the acidic oxidizing gas can be utilized.

The exothermic reaction which takes place when the acidic oxidizing gas is exposed to the fibrous cellulosic material at the initial reaction temperature within the above-described range, should be sufficiently controlled so as not to unduly stress the fibrous cellulosic material and so as to avoid preparation of a brittle product. Thus, it is generally desirable, when carrying out such contacting, to first stabilize the temperature of the fibrous cellulosic material within the described reaction temperature range in an inert gas containing no appreciable concentration of the acidic oxidizing gas. Such acidic oxidizing gas can then be added in controlled concentration insufiicient to cause the exothermic reaction to proceed so rapidly as to raise the temperature of the cellulosic material to more than the desired maximum temperature, e.g., about 350 F.

Accordingly, it is preferred to initially establish a temperature of about 300 F. in the cellulosic material in the contact zone in an acidic oxidizing gas-free inert gas environment, and then bleed the acidic oxidizing gas into the contact zone at a rate which assures no more than about a 50 F. rise in temperature in the cellulosic material during such treatment. Normally, the concentration of the acidic oxidizing gas is increased from a few vol. percent to an ultimate concentration within the desired range. Such acidic oxidizing gas is consumed during the reaction and must be replenished until the exothermic reaction is complete, usually within about 5-10 minutes, although longer and shorter periods of time are contemtemplated.

When the desired dehydration of the cellulosic material is completed at the reaction temperature, further utilization of the acidic oxidizing gas substantially stops. Further addition of acidic oxidizing gas to the contact zone, or further maintenance of the contacting thereof with the cellulosic material has no further effect on ultimate yield of the carbon product. The exothermic reaction can be carried out with or without sweeping of the contact zone with the acidic oxidizing gas. However, sweeping is preferred.

It will be understood that, if desired, the cellulosic material can be heated to reaction temperature in the presence of inert gas which already contains a controlled concentration of the acidic oxidizing gas. However, adequate temperature control of the cellulosic material in the contact zone is more easily achieved by following the previously described or a similar procedure.

Although the exact mechanism of the reaction occurring in the described low temperature acidic oxidizing gascontacting step (first major step) is not fully understood, it is believed to involve the described dehydration of the cellulose, which makes easier the complete carbonization of the cellulose with a lowered loss of carbon and with retention of fiber integrity, flexibility and tensile strength. Whatever the actual mechanism involved, the improved results are reproducible.

Once the dehydration step is carried out, further loss of volatiles from the cellulosic material proceeds easily and rapidly at higher temperatures until high carbon content fibrous residues which maintain the original shape of the cellulosic starting material are obtained. As a result of the described acidic oxidizing gas treating step, fibrous carbon product yields of up to 27 wt. percent or more with respect to the starting material, can be obtained, in contrast to conventional fibrous carbon yields of about 18-20 percent.

Once the low temperature acidic oxidizing gas treat ment step has been completed, further volatiles are removed from the fibrous material in a second major step by increasing the temperature of the fibrous material over a suitable period of time and in the absence of oxygen. The second major step ordinarily is required, since the product of the first major step usually is not in a commercially utilizable form. The environment for the second major step may be an inert or reducing gas, or a suitable concentration of selected acidic oxidizing gas, defined hereinafter alone or in inert gas, or an alkaline reducing gas, which may be ammonia, or an organic mono or dialkylamine such as methylamine, ethylamine, propylamine, dimethylamine, diethylamine, methylethylamine or the like, undiluted or diluted with inert gas.

In the second major step, whatever the treating gas, it is essentially free of oxygen, so as to prevent combustion of the fibers. The temperature of the fibrous material is increased at any suitable rate in increments or continuously from the acidic oxidizing gas treatment step (first major step) temperature previously described to a temperature which assures removal of most of the remaining volatiles from the carbon product. In the case of an inert gas environment, this temperature level is usually about 1100-1200 F. As an example, the temperature of the carbon fibers can be increased from 350 F. to about 1200 F. in about one to four hours, i.e., much more rapidly than in conventional processes.

In the event that the second major step is carried out in inert gas or the usual reducing gas such as hydrogen gas, the contact zone is swept free of acidic oxidizing gas from the first major step by utilizing inert gas or reducing gas as a purge. During the heating from about 300-350 F. to, for example, about 1200 F., the heat treating zone is preferably swept with the inert gas or reducing gas.

In one embodiment of the present invention, the carbonaceous fiber product of the first major step is treated in the second major step by increasing the temperature thereof to a suitable level, e.g., about 1100-1200 F. while maintaining contact of the product with selected acidic oxidizing gas. Such gas is undiluted or diluted to any suitable concentration, e.g. 1-10 vol. percent, with inert gas. Such acidic oxidizing gas constitutes one or a mixture of halogens. It will be understood that the use of hydrogen halides with oxygen as acidic oxidizing gas is excluded. So also are nitrous oxide and nitrogen pentoxide. This is because the elevated temperatures of the second major step preclude the presence of oxygen in contact with the carbon fiber, otherwise combustion of the fiber would occur. Such higher temperature treatment can be carried out in the previously described manner for the second major step, i.e., over any suitable period of time, for example, with a sufficiently slow temperature rise per hour to assure retention of fiber integrity, strength and flexibility and with or without sweeping of the gas through the contact zone.

The selected acidic oxidizing gas in the second major step acts to assure essentially complete removal of inorganic impurities from the carbon fibers so that a very high purity product is produced.

The contacting with the selected acidic oxidizing gas in the first major step causes the product to have a somewhat greater total surface area than if such first major step had been carried out only in inert gas. For example, the usual surface area encountered in carbon cloth prepared from rayon textile samples in accordance with a conventional process involving initial heat treating with inert gas at from about 450 F. to about 1100 F., followed by firing at 1800 F. is about 50-100 m. /g., whereas when the same type of samples are treated with acidic oxidizing gas in the first major step (followed by treatment with inert gas and/or acidic oxidizing gas in the second major step) the products usually have surface areas of up to -500 m. g. Accordingly, the combination of acidic oxidizing gas in the first major step of the present method with acidic oxidizing gas and/ or inert gas in the second major step affords a way of controllably increasing the surface area of carbon fiber products.

The substantial increases in yield of carbon fiber products afforded by the first major step of the present method are retained, along with the improved tensile strength and flexibility, whether the second major step is carried out only in an inert or usual reducing gas, or in the acidic oxidizing gas (undiluted or diluted with inert gas) or in the described alkaline reducing gas (undiluted or diluted with inert gas). However, the alkaline reducing gas imparts additional properties to the carbon fibers. Thus, such alkaline reducing gas reduce the surface area of the carbon fibers to a very low level, for example, to about 10-50 m. /gm., regardless of the surface area of the material prior to said contacting. By suitably controlling the concentration and type of alkaline reducing gas and the temperature and time of contacting in the second major step, control of the surface area of the carbon fiber product can be accomplished. Dilution of alkaline reducing gas can be accomplished with inert gas such as that referred to in connection with the first major step. The time and temperature utilized in the second major step employing the diluted or undiluted alkaline reducing gas can be as previously described for the embodiments utilizing acidic oxidizing gas and/ or inert gas in the second major step or any other suitable time and temperature. Preferably, the temperature is increased to about 1200-1400 F.

The carbon fiber product of the second major step usually still retains a very low but detectable concentration of volatiles, e.g., less than 1 percent by wt. Remaining volatiles can be removed from the fibrous carbon product by carrying out an optional, although preferred, third major step i.e. firing at above 1200 F., preferably at at least about 1800 F. and more preferably at about 2000 2200 F. in an oxyegn-free inert gas environment over a suitable period of time, for example, a few seconds to a few minutes. Cooling of the fired carbon fiber product usually is carried out to ambient temperature in inert gas and at a sufficiently slow rate to assure retention of fiber integrity. Although a vacuum can be substituted for inert gas in the firing and cooling steps, commercial practicality usually dictates the use of inert gas.

It will be understood that the carbon fiber products provided through the described firing step are basically turbostratic carbon. They can easily be converted to high quality graphitic products by heating them in an inert atmosphere to graphitization temperature, for example, to about 4000 F. over a suitable interval of time e.g. one hour. The graphitization can be carried out as an extension of, in substitution for or in addition to the firing step previously described. The amorphous carbon fiber material is partially or essentially completely converted to crystalline carbon, that is, graphite, depending upon the particular graphitizing conditions. Although conventional graphite-containing fibrous products do not largely consist of graphite, they contain graphitic properties in varying degree, depending on their extent of graphitization. It is conventional to term those products which exhibit at least some graphitic properties as graphite products. It will be understood that the present invention extends to amorphous carbon fiber products, to essentially graphite fiber products and to fiber products consisting of mixtures of amorphous carbon and crystalline carbon.

The following examples further illustrate certain features of the invention:

EXAMPLE I Substantially pure rayon textile free of sizing and having. a denier of 1620-1690, and an average fiber diameter of about 00004 inch isp laced in 600 gm. amount in a contact zone comprising a mufile furnace and heated to 300 F. in helium over a period of about 20 minutes. It is maintained at 300 F. for 10 minutes while being swept with helium at the rate of about 0.1 ftfi/min. At the end of this time, the textile is free of surface moisture. Chlorine is then bled into the helium sweep gas entering the muffle furnace to provide an initial acidic oxidizing gas concentraton of about 1 vol. percent. The chlorine content in the inert sweep gas is built up to a final concentration of about 10 vol. percent over a 10 minute period, during which time the temperature in the furnace increases to about 330 F due to the exothermic reaction between the rayon and chlorine. The exothermic reaction terminates at the end of the 10 minute period, after which the chlorine input to the furnace is shut off and the furnace is swept free of chlorine through the use of helium purge gas.

The temperature of the furnace and textile is then slowly raised to 1200 F. (over a three hour period) while sweeping the furnace with helium at the rate of about 0.1 ft. min. At the end of this time, the textile is rapidly heated in helium to 2000 F. and held at that temperature for 30 seconds while continuing to purge the furnace with helium, after which the textile is cooled to ambient temperature over a 2 hour period under a helium blanket, and then removed from the furnace and tested.

The resulting textile is essentially pure (substantially less than 0.5 percent impurity) high quality turbostratic carbon fiber textile with a tensile strength of about 19,000 and is very flexible. It exhibits unimpaired fiber form, and weighs about 160 grams, representing about 27 wt. percent of the initial weight of the rayon, a substantial increase from the usual 18-20 percent wt. carbon yield of conventional processes. It has a surface area of about 300 m. gm.

Separate parallel test substituting bromine, iodine,

nitrous oxide and nitrogen pentoxide in the first major 1 step provide comparable results. Further parallel tests utilizing suflicient HCl and air of HBr and air or HI and air in the first major step to yield comparable concentrations of the respective halogens result in carbon yields of about 2425 wt. percent.

EXAMPLE II Rayon textile identical with that of Example I is subjected to a substantially identical treatment, except for the substitution of bromine for chlorine in the 300- 350 F. treating step, and except for the addition of bromine in a 10 volume percent concentration to the inert gas in heat treating to 1100 F. The bromine is then removed from the furnace, argon is substituted and the textile is fired at 2100 F. for seconds. The textile after firing and cooling is comparable to that of Example I in yield, tensile strength, flexibility and fiber integrity. Moreover, it has an essentially nil concentration of impurities. In addition, its surface area is about 300 m. gm.

Separate parallel tests utilizing chlorine and iodine as acidic oxidizing gases for both major steps provide comparable results. So also do tests utilizing nitrous oxide, nitrogen pentoxide and hydrogen halide and air mixtures in the first major step and one or more halogens in the second major step.

EXAMPLE II Rayon textile identical to that of Example I is subjected to substantially identical treatment as in Example I, except for the substitution of iodine for the chlorine and nitrogen for the helium in the 300350 F. treating step (first major step) and except for the substitution of pure ammonia (100 volume percent concentration) for the inert gas in heat treating (second major step) to about 1400 F. Ammonia is then swept out and helium is substituted, after which the textile is fired, as per Example I, at 1900 F. for 1 minute. It is then cooled over 2 hours in helium to ambient temperature and tested. The carbon fiber textile is obtained in about 27.5 wt. percent yield, has high purity and a surface area of 20 m. gm. and exhibits tensile strength and flexibility comparable to those of the carbon fiber textiles of Examples I and II. So also do carbon textiles provided in parallel tests substituting the acidic oxidizing gases chlorine, bromine, nitrous oxide and nitrogen pentoxide in the first major step. Further parallel tests substituting methylamine, ethylamine, dimethylamine, diethylamine, methylethylamine and propylamine for the ammonia in the second major step provide comparable results.

EXAMPLE IV The turbostratic carbon fiber textile products of Examples I, II, III are subjected to graphitization in a furnace under a helium blanket by heating to 4000 F. over a 1 hour period. The textiles are then allowed to cool under the inert gas blanket to ambient temperature, after which they are tested for extent of graphitization, tensile strength, fiber integrity and flexibility. High quality graphite textiles exhibiting high tensile strength, fiber integrity and relatively high flexibility are produced. The graphitization substntially reduces the surface area to low levels. for example, l-10 m. gm.

EXAMPLE V The tests performed in Examples I to IV, inclusive, are duplicated except for the substitution (in successive series of tests) of cotton cloth, cotton roving, cotton batt, cotton felt, rayon roving, rayon monofilaments, rayon batt and rayon felt (chopped fibers) for the rayon textiles of Examples I to IV. Comparable results are obtained in every respect.

The preceding examples clearly illustrate that the present method is simple, utilizes simple equipment, is inexpensice and is relatively rapid. Moreover, it provides fibrous carbon products of improved and controlled characteristics. Such products have flexibility and tensile strength at least equivalent to conventionally produced carbon fiber products and have surface areas which can be controlled so as to be substantially larger than, comparable to, or substantially smaller than those of conventionally prepared carbon fiber products. Yields of the carbon fiber products have been substantially increased, as per the present method, from the conventional level of about 18-20 weight percent, based on the weight of the initial material, to 25-27 weight percent or more.

In the present method, the heat treating of the fibers at above about 350 F. up to substantially complete removal of the volatiles at about 11001400 F. can be accomplished in a very short period of time. Whereas conventional procedures usually require days of treatment Within this temperature range, this step in the present method can be carried out in, for example, about one to about four hours. Firing operation can also be relatively rapid, as can be the graphitization, if any. The low temperature heat treatment at about 300350 F. also is very rapid, in that it usually can be carried out within about 5-15 minutes. Accordingly, carbon fiber products of amorphous, graphitic or mixed carbon form can be prepared by the present method, either batchwise, semicontinuously or continuously from fibrous cellulosic material within as little total treating time as about 2 hours. This is in contrast to conventional methods which require days to perform, for example, about 8-10 days.

Despite the rapidity of the present method, the integrity of the fibers, Whether in textile, chopped fiber or other form, is maintained. Thus, volatiles are easily and rapidly removed from the fibers without damaging such fibers, due to the use of the acidic oxidizing gas in the low temperature treating step. The influence of the acidic oxidizing gas is such that the subsequent higher temperature treating step or steps can then be greatly shortened, still without danger of damaging the fibers.

The present method is flexible in that it can be carried out utilizing inert or reducing gas, or alkaline reducing gas diluted or undiluted with inert gas or selected acidic oxidizing gas (halogen) diluted or undiluted with inert gas, all in the absence of oxygen, for heat treating at between about 350 F. and about 1100-1400 F. The alkaline reducing gas results in a controllable decrease in surface area in the product. The acidic oxidizing gas during this step results in a more purified final product. Accordingly, the present method reflects improvements in operational speed and efficiency, quality, yield, control of product characteristics and adaptability to various starting materials and various forms of fibrous carbon end products. Various other advantages are as set forth in the foregoing.

Various changes, modifications, alterations, and substiutions can be made in the present method, its steps and parameters and in the equipment for carrying out the stpes. All such changes, modifications, alterations and substitutions as are within the scope of the appended claims form a part of the present invention.

What is claimed is:

1. An improved method of obtaining high yields of carbonized fiber, which method comprises maintaining cellulosic fiber in the presence of an acidic oxidizing gas selected from the group consisting of halogens, nitrous oxide, nitrogen pentoxide and a mixture of hydrogen halide and oxygen at a low temperature of at least about 300 F. and below about 450 F. until the exothermic reaction between said acidic oxidizing gas and said fiber is substantially completed, and thereafter in the essential absence of oxygen heat treating said fiber at a higher temperature in the presence of alkaline reducing gas selected from the group consisting of ammonia and an alkylamine until carbonization is substantially completed, whereby an improved yield of low surface area carbon fiber is obtained.

2. The method of claim 1 wherein said strongly acidic oxidizing gas comprises halogen.

3. The method of claim 1 wherein said acidic oxidizing gas comprises chlorine.

4. The method of claim 1 wherein said acidic oxidizing gas comprises bromine.

5. The method of claim 1 wherein said acidic oxidizing gas comprises iodine.

6. The method of claim 1 wherein said acidic oxidizing gas comprises nitrous oxide.

7. The method of claim 1 wherein said acidic oxidizing gas comprises nitrogen pentoxide.

8. The method of claim 1 wherein said acidic oxidizing gas comprises a mixture of hydrogen halide and oxygen, said oxygen being present in an amount approximating and not exceeding a stoichiometric amount necessary to convert all of said halide to halogen and water vapor.

9. The method of claim 8 wherein said hydrogen halide comprises hydrogen chloride.

10. The method of claim 8 wherein said hydrogen halide comprises hydrogen bromide.

11. The method of claim 8 wherein said hydrogen halide comprises hydrogen iodide.

12. The method of claim 1 wherein said low temperature treatment is carried out at between about 300 F. and about 350 F. and wherein said higher temperature treatment is carried out by progressively increasing the temperature of said fiber up to about 1400 F.

13. The method of claim 1 wherein said alkaline reducing gas comprises ammonia.

14. The method of claim 1 wherein said alkaline reducing gas comprises a monoalkylamine.

15. The method of claim 14 wherein said monoalkyl amine comprises methylamine.

16. The method of claim 14 wherein said monoalkylamine comprises ethylarnine.

17. The method of claim 14 wherein said monoalkylamine comprises propylamine.

18. The method of claim 1 wherein said alkaline reducing gas comprises dialkylamine.

19. The method of claim 18 wherein said dialkylamine comprises dimethylamine.

20. The method of claim 18 wherein said dialkylamine comprises diethylamine.

21. The method of claim 18 wherein said dialkylamine comprises methylethylamine.

22. The method of claim 1 wherein the product obtained after said higher heat treating is fired in an oxygenfree inert environment at a temperature in excess of about 1800 F. for a time sufficient to remove remaining volatiles from said product.

23. The method of claim 22 wherein graphitizing of said fibers in an oxygen-free inert environment is carried out.

24. The method of claim 1 wherein the concentration of acidic oxidizing gas is controlled such that the temperature increase of the cellulosic fiber caused by the exothermic reaction does not exceed about F.

25. An improved method of carbonizing cellulosic fiber, which method comprises maintaining the fiber in the presence of an acidic oxidizing gas selected from the group consisting of nitrous oxide, nitrogen pentoxide, and a mixture of hydrogen halide and oxygen, at a temperature of at least about 300 F. and below a temperature which results in substantial impairment of the integrity of the fiber until the exothermic reaction between the acidic oxidizing gas and the cellulosic fiber is substantially completed, and thereafter in the essential absence of oxygen heat treating the resulting product at a higher temperature in the presence of an alkaline reducing gas selected from the group consisting of ammonia and an alkylamine until carbonization is substantially completed.

References Cited UNITED STATES PATENTS 3,116,975 1/1964 Cross et al 23209.4 3,179,605 4/1965 Ohsol 23209.2 X 3,294,489 12/1966 Millington et al. 23-209.1 3,305,315 2/1967 Bacon et al. 23209.1 3,313,597 4/ 196 7 Cranch et al 23 209.1 X 3,333,926 8/1967 Moyer et al. 23209.1

EDWARD J. MEROS, Primary Examiner US. Cl. X.R.