CA1046721A - Activated pitch based carbon fibers - Google Patents

Abstract

Abstract of the Disclosure High surface area activated carbon fibers, wherein the carbon has a surface area of at least about 300 m2/gm (square meters per gram) are produced by heating infusible cured modified pitch fibers in air from room temperature to about 250-450°C at a rate of temperature increase of 50-200 centigrade degrees per hour, and further heating the fibers in a non-oxidizing atmosphere to about 700-900°C at a rate of temperature increase of about 50-200 centigrade degrees per hour. The surface area of the carbon in such fibers may be increased by heating the fibers in steam at about 800-900°C to produce activated carbon fibers wherein the carbon has a surface area of at least about 1000 m2/gm.

Description

~04672~
ACTIVATED PITCH BASED CARBON FIBERS
Background of the Invention Because of the widespread availability, cheapness and attractive chemical properties of various pyrogenous residues such as pitch, they have found wide usage as starting materials for the fabrication of binders and coating materials. Pitches have been used as part of the component mixtures in the fabrication of carbonaceous articles of various kinds and in the formulation of carbon-aceous fibers. One of the uses for carbonaceous fibersstems from the treatment of the fiber by appropriate means to increase their surface area and absorptive properties.
Activated fibers of this type are useful in applications involving the purification of fluids. At the present time, fibers of this nature are derived by the carbonization and activation of relatively expensive organic resins. While pitch is inexpensive, it can be formed into stable fibers only by oxidative processes which are difficult and time consuming. No process is presently known for activating such fibers to make them absorbent. It is an object of the present invention therefore, to describe pitch fibers, modified by small amounts of added resins, which can be successfully made into highly absorbent activated carbon fibers and to further describe a process for making the fibers.
The invention pertains to a flexible absorbent carbon fiber in which the fiber is made from a mixture comprising a pitch and a phenol-formaldehyde novolac, the fiber having a surface area in the range of about 300 to about 2500 square meters per gram. The pitch-novolac mixture may comprise -1- ~

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from about 5 to about 40 percent novolac, with the carbon fiber having a diameter ranging from about 0.1 to about 300 microns. The fiber is made by fiberizing a molten mixture of pitch and novolac, curing the fiber to make it infusible, heating the fiber in air to a temperature ranging from about 250C to about 450C, and subsequently heating the fiber to a temperature ranging from about 700C to about 900C under an inert atmosphere to form the adsorbent carbon fiber. The surface area of the fiber may be further increased by heating in steam at a temperature ranging from about 800C to about 900C.
The starting pyrogenous residues that can be used in the process of the present invention include a variety of pitches such as coal tar pitches, pitches obtained by distillation of oils, petroleum pitches, pyrogenous asphalts, and a variety of pitch-like substances produced as by-products of various industrial processes, such as distillation residues.
Preferably, the starting pyrogenous residue has a softening point of from about 80C to about 200C, more preferably from about 100C to about 150C. Preferably, the pyrogenous residue has a carbon to hydrogen ratio based on weight per-cent from about 18 to about 25. The content of aromatic and unsaturated components varies, depending upon the source of the raw material pyrogenous residue.
Preferably, pyrogenous residues or pitches used as starting materials have a beta-resin content greater than about 5 percent and preferably greater than lO percent.
The beta-resin is the benzene insoluble content of the pyro-genous residue minus the quinoline insoluble content. In making the determination, there are other solvents, such as ~046721 toluene, which can be substituted for benzene, and pyridine which can be substituted for quinoline. The beta-resin ; p~rtion of the pyrogenous residue appears to enhance the binding and adhesive qualitites thereof. It is believed " that a suitable amount of beta~resin contributes to rendering the fusible fiber infusible by a short curing process. The ,~ upper limit of the percent of beta-resin in the starting pyrogenous residue is not critical but is generally limited , by the type of pitch used and process conditions. Most ,~ 10 commercially available pitches have a beta-resin content of less than about 30 percent, but pitches with,a beta-resin content higher than 45 percent can be used in the present invention.
Generally, commercially available coal tar pitch has a benzene insoluble content of about 20 to about 50 percent and a quinoline insoluble content of about 10 to about 20 percent, with a resulting beta-resin content in the range of about 10 to 30 percent. These pitches are suited for use as a starting material in the process of the present invention without further modification.
Petroleum pitches and pyrogenous asphalts often have beta-resin contents less than about 5 percent. This is generally due to a low percentage of benzene insolubles, generally less than about 10 percent. In such a case, while the fusible fiber of pyrogenous residue and novolac can be rendered infusible by reacting with formaldehyde, the curing process is comparatively slow. Alternatively, it is possible to upgrade the pitch by increasing the beta-resin content.
Such upgrading can be done by reacting the pitch or asphalt with an aldehyde and phenolic compound in the presence of 104ti~21 an acid catalyst at a temperature sufficiently high to effect condensation between the pitch or asphalt, aldehyde and phenolic compound. Such a method is described in British specification No. 1,080,866 and u. S. Patent No. 3,301,803.
The amount of aldehyde and phenolic compound that is employed can vary widely depending on the degree of upgrading necessary. The reaction is carried out at a temperature from about 150F to about 600F for a suitable period of time.
The amount of quinoline insoluble in the starting pyrogenous residue should be less than about 20 percent and preferably less than about 10 percent. As the percentage of quinoline insoluble in the starting pyrogenous residue is decreased, the ease of fiberization of the melt is increased and the uniformity of the fibers is enhanced. The most preferred starting pyrogenous residue contains zero or a very low, percentage of quinoline insoluble. The quino-line insolubles represent material which is not soluble in the pyrogenous residue at the spinning temperature and which form an undesirable second phase. Removal of the quinoline insolubles can be accomplished by diluting the pitch in an appropriate solvent and filtering or centrifuging to remove the insolubles. Such a method is described in U. S. Patent No. 3,595,946.
A wide variety of novolac resins may be used as starting materials in the process of the present invention.
The term "novolac" refers to a condensation product of a -phenolic compound with formaldehyde, the condensation being carried out in the presence of a catalyst to form a novolac resin, wherein there are virtually no methylol groups, such 1~)46721 as present in resoles, and wherein the molecules of the phenolic compounds are linked together by a methylene group. The phenolic compound may be phenol, or phenol wherein one or more of the non-hydroxylic hydrogens are replaced by any of various substituents attached to the benzene ring, a few examples of which are the cresols, phenyl phenols, 3, 5-dialkylphenols, chlorophenols, resorcinol, hydroquinone, chloroglucinol and the like.
The phenolic compound may instead be naphthyl or hydroxy-phenanthrene or another hydroxyl derivative of a compoundhaving a condensed ring system.
For purposes of the present invention, any fusible novolac which is capable of further polymerization with a suitable aldehyde may be employed for the production of fibers. Stated another way, the novolac molecules must have two or more available sites for further polymerization.
Apart from this limitation, any novolac might be employed, including modified novolacs, i.e., those in which a non-phenolic compound is also included in the molecule, such as the diphenyl o~ide or bis phenol-A modified phenol-formalde-hyde novolac. Mixtures of novolacs may be employed or novolacs containing more than one species of phenolic com-pounds may be employed.
Novolacs generally have a number-average molecular weight in the range from about 500 to about 1200, although an exceptional case in which the molecular weight may be as low as 300 or as high as 2000 or more may occur. Unmodified phenol-formaldehyde novolacs usually have a number-average weight in the range from about 500 to about 900, most of the commercially available materials falling within this range.

1~)467Zl Preferably, novolacs with a molecular weight from about 500 to about 1200 are employed in the method of the present invention. The temperature at which low molecular weight novolacs soften and become tacky is usually comparatively low. Therefore, it is necessary to cure the fiberized novolac at a very low temperature to avoid adherence and/or deformation of the fiber. It is usually undesirable to employ such curing temperatures since the curing rate increases dramatically with the increase in temperature, and a low curing temperature entails the practical disadvantages of a prolonged curing cycle. It is generally preferred to employ a novolac having a moderately high molecular weight to permit curing in a reasonable time without adherence and/
or deformation, but to avoid the extreme upper end of the molecular weight range to minimize problems in fiberizing due to gelling.
A mixture of pyrogenous residue, or pitch, and novolac may be formed by any convenient technique such as dry blending or melting the pyrogenous residue and novolac by heating together to form a homogenous mixture. Mixtures containing from about 5 to about 40 percent novolac can be used for preparing the fibers of the present invention. Since the pyrogenous residue is the most economically available component of the mixture it is preferred to employ less than about 35 percent novolac. It is preferable that the novolac content be at least about 10 percent and more preferably that it be at least about 25 percent in the mixture so that the spinability of the fiber is enhanced and the curing time can be sufficiently reduced. Preferably, the mixture consists essentially of the pyrogenous residue and novolac.

1~;)467Zl The fiberization can be performed by any convenient method, such as drawing a continuous filament downwardly from an orifice in the bottom of a vessel containing a molten mixture of pitch and novolac. The filament is wound and collected on a revolving take-up spool mounted below the orifice. The take-up spool also serves to attenuate the filament as it is drawn from the orifice before it cools and solidifies upon contacting the atmosphere bet-ween the orifice and the spool. The melt can also be formed into short staple fibers by methods known in the prior art, such as by blowing the melt through a fiberizing nozzle and collecting the cooled fibers, or by blowing a thin stream of melt into the path of a hot blast of gas. These methods produce a staple consisting of a multiplicity of fusible uncured pitch-novolac fibers of variable length and diameter.
The diameter of such fibers can vary from 0.1 micron to about 300 microns.
When producing a continuous filament having a uniform diameter by melt spinning, the fibers preferably have diameters from about 10 to about 30 microns. The filament diameter depends primarily upon two factors; the drawing rate and the flow rate of the melt through the orifice.
The fiber diameter decreases as the drawing rate is increased, and increases as the flow rate of the melt is increased.
The flow rate of the melt depends primarily upon the diameter and length of the orifice, and the viscosity of the melt, increasing as the orifice diameter is increased, decreasing as the length of the orifice is increased, and increasing as the viscosity of the melt is decreased. An increase of flow rate may also be effected, if desired, by applying pressure 10467Zl to the melt to force it through the orifice. Alternatively, fibers of the modified pitch material can be produced by pouring a mixture of pitch and novolac resin in a vessel which is connected to a nozzle. The nozzle in turn is connected to a source of air pressure for forcing the mixture through the nozzle. In this manner short staple or blown fibers of appropriate diameter can be collected. The fibers produced either by drawing or by blowing are fusible and may be treated further to cause the fibers to cure or crosslink at least to the point of infusibility.
The curing is generally effected by heating the fibers in the presence of a source of methylene groups, such as formaldehyde, and preferably also in the presence of a suitable catalyst, such as an acid. Blowing produces a staple comprising fibers of varying lengths and diameters, diameters as small as about 0.1 micron or less being attain-able, as well as considerably thicker fibers. Melt spinning may be employed to produce fibers in the form continuous filaments if desired, having diameters as small as about 4 microns, up to as large as about 300 microns or more. After curing, the fibers may be processed by various techniques to produce rovings and yarns, paper felt, woven or knitted fabrics, and various other textile forms. A particularly desirable method for the preparation of infusible cured phenol-formaldehyde novolac fibers is set forth in detail in U. S. Patent 3,650,102, issued to James Economy et al which is assigned to The Carborundum Company. In accordance with the present invention, the infusible modified pitch fiber is carbonized by heating it in air from room temperature (about 25C) up to an intermediate temperature in the range 1~46721 fr~m about 250c to about 450c, the temperature being continually increased at a rate which may vary from about 50C per hour to about 200C per hour. Heating is then continued in a non-oxidizing atmosphere such as nltrogen or a similar inert gas, from said intermediate temperature to a final ~emperature in the range from about 700~C to about 900C, the temperature being continually increased at a rate of from about 50C per hour to about 200C per hour.
While the precise nature of the conversion effected thereby has not been ascertained, it appears that the heating in air results in enhanced crosslinking, partial pyrolysis and carbonization of the starting fiber to produce a partially crosslinked and carbonized fiber which is further carbonized during the heating in the non-oxidizing atmosphere.
The method results in the production of a carbon fiber wherein the carbon generally has a surface area of at least 300 m2/gm to about 800 m2/gm. It has further been found that, if the starting cured modified pitch fiber is swelled by immersing it in a highly polar sovent before carbonizing it by the method just described, the resulting carbon fiber generally has a somewhat greater surface area of at least about 400 m2/gm and usually within the range from about 400 m2/gm to about 1000 m2/gm. A particularly desirable feature of the method described is that carbon fibers may be produced thereby which, in addition to having a carbon surface area in the range from about 300 m2/gm to about 1000 m2/gm, are relatively strong and very flexible.
Another interesting feature of this invention is that if the modified pitch fiber, after curing in the presence of formaldehyde, is heated in air at a heating rate of 10C

_g_ 104~;7Zl to 50C/hr. up to 250C, before heating rapidly to higher temperatures, at rates of 50-200C/hr., the resulting fibers, in addition to being absorbent and active are also found to possess improved flexibility. Another particularly important feature of the method described is that the carbon surface area of the carbon fiber produced thereby may subsequently be increased, if desired, from an initial sur-face area of about 300-1000 m2/gm to as much as about 2500 m /gm by heating the fiber in steam at a temperature in the range from about 800C to about 900C. This steam treatment is in addition to, and subsequent to, the second heating stage in nitrogen or similar inert gas. This is quite important, since the adsorptive capacity of the carbon, and the flexibility of the fiber, generally increase with increasing surface area.
It should be noted that infusible fibers with pitch contents approaching 95 percent may be successfully made by the process of the invention and heat treated to give fibers of greatly increased surface area. This behavior is entirely unexpected in view of the behavior shown by fibers consisting wholly of a pyrogenous residue or pitch. These latter fibers are thermoplastic and pass through a fluid state when heated to carbonization temperatures of 800C. When such fibers are subjected to an activation treatment such as that described for the activation of modified pitch fibers, products with surface areas of less than 1-5 m2/gm are obtained. Con-sidering this behavior, the marked change brought about by the addition of as lit~le as 5 percent of resin to the modified fiber is completely unexpected. While the exact reasons are not completely ascertained, it is believed that 1l)467Zl crosslinking of the modified pitch material during the treatment of curing and activation may be one of the factors in producing the superior fibers of the invention.
The invention will be further described partly with reference to the following examples, which are intended to illustrate and not to limit the scope of the invention.
Example 1 A starting coal tar pitch (Allied Chemicals Company) had a softening point of 125C, a beta-resin content of 22 percent, and a quinoline insoluble content of 13.6 percent.
The pitch was mixed with a novolac resin (Varcum) having a molecular weight of about 800 to 1000, in the proportion of 70 percent of pitch to 30 percent of resin in the final mixture. The novolac and pitch were heated together to 190C to form a homogeneous mixture and the resulting mixture was poured into a fiberization vessel. The vessel was a cylinder having an orifice at the bottom and a plunger at the top for forcing liquid through the orifice. The vessel was mounted onto the fiberization equipment, which included a spool attached to the shaft of a variable speed electric motor mounted beneath the vessel for gathering the fibers.
The vessel was surrounded by an electrical heating coil connected to an adjustable source of electricity, whereby a controlled amount of heat was imparted to the vessel and its contents. The fibers were spun through an orifice of about 1.5 mm in length, having an internal diameter of about 0.3 mm The vessel containing the mixture of resin was maintained at a temperature of about 120C while the bottom portion with the orifice was maintained at about 150C.
The mixture of pitch and novolac was driven through the 1;;)467Zl orifice by a ram at a pressure of about 110 p.s.i. The resulting mixed pitch-novolac filament had an average dia-meter of about 15 to 25 microns, and was taken up on a graphite cylindrical cone at the rate of 500 r.p.m. The ~iber bundle thus obtained was cured by hanging the graphite cone containing the fiber on a graphite support and immersing in a curing solution. The solution was prepared by mixing equal portions of an aqueous solution containing about an 18 percent concentration of hydrochloric acid and the same concentration of paraformaldehyde. The curing solution with the graphite cone containing the fiber immersed therein was heated from room temperature to 100C by increasing the temperature from 25C to 50C over a period of 1 hour, increasing the temperature from 50C to 100C over a period of 1/2 hour and maintaining the temperature at 100C for 4 hours for a total residence time of about 5-1/2 to 6 hours.
The cured fibers were removed, washed with water, and dried in air at about 60C. The cured fibers were infusible and did not soften when heated in an oven or in a flame.
Example 2 A mixture of pitch and novolac resin prepared according to the procedure of Example 1 was poured into a fiberization vessel equipped with a nozzle. The nozzle was connected to a source of air pressure for forcing the mixture of pitch-novolac in air through the nozzle. In this manner, short staple fibers or blown fibers were collected on a plate after being cooled by falling through the air. The nozzle was heated at about 250C and the air pressure used was about 20 p.s.i. The fibers collected in this manner were cured by placing in a graphite container to form a mat and lQ467Zl curing in a liquid state in a manner similar to Example 1.
The average diameter of the cured fiber was about 2 microns.
These fibers, as in Example 1, were infusible.
Example 3 A tow of infusible modified pitch filaments comprising coal tar pitch and phenol-formaldehyde novolac was prepared in substantial accordance with the procedure of Example 1, the filaments having diameters of about 12-18 mi~rons. The tow was cut into six-inch (a~out 15 cm) segments, and 5 gm.
of the cut fibers were placed in a tube furnace. The fibers were heated from room temperature to an intermediate tempera-ture of 400C at a rate of temperature rise of 200C per hour while passing a slow current of air through the tube to remove the volatiles dissipated by the fibers and to maintain an air atmosphere. Upon reaching 400C, the air current was replaced by a slow current of nitrogen and heating was continued in the nitrogen atmosphere at a rate of tempera-ture rise of 200C per hour up to a final temperature of 900C, after which the resulting carbon fibers were allowed to cool to room temperature, nitrogen being employed to provide a non-oxidizing atmosphere during the cooling cycle to preclude oxidation of the carbon. A yield of 3 gm of flexible carbon fibers was obtained, the carbon having an average surface area of 720 m2/gm.
Example 4 Example 3 was repeated, but the fibers were slowly heated in air to an intermediate temperature of 200-250C, at a rate of 10-50C/hr., and then more rapidly at 100C/hr.
to 450C with the heating continued in nitrogen to a final temperature of 700C at a rate of 100C/hr. The resulting 10467Zl fibers were found to be very flexible. While the exact mechanism has not been ascertained, it is believed that the slow heating of the formaldehyde cured fibers to 250C
renders the fibers more completely cross-linked, which is believed to improve mechanical properties. These fibers had an average surface area of 705 m2/gm with individual fibers having carbon surface areas up to about 1000 m2/gm.
Example 5 The Gurface area of the carbon fibers produced according to the invention as illustrated in Examples 3-4 may be increased, if desired, by heating the carbon fibers in steam at a temperature in the ranges from about 800C to about 900C. It is thereby possible to increase the surface area from about 300-1000 m2/gm to as high as about 2500 m /gm. During the heating, the carbon fibers gradually increase in porosity and surface area and lose weight as carbon is burned off, and will be completely dissipated if heated too long. Therefore, while the heating should be carried out for a time sufficient to effect an increase in surface area, it should not be unduly prolonged, and the preferred time is generally that which gives the desired surface area with minimum weight loss. The higher the temperature within the range specified, the shorter the time required for a given increase in surface area. Thus, for example, at 800C approximately 90 minutes was required to increase the surface area to 2000 m2/gm whereas only about 20 minutes was required at 900C, the latter tempera-ture being preferred. Slightly longer periods are required to attain surface areas of about 2500 m2/gm.
Example 6 The carbon fibers produced in Example 4 were placed in 1t)46721 a tube furnace at 800C under a slow current of steam and held under these conditions for two hours, after which the fibers were cooled to room temperature in a non-oxidizing (nitrogen) atmosphere. The resulting carbon fibers had an average carbon surface area of 2400 m2/gm and exhibited good flexibility and mechanical properties.
The method of the invention may be carried out with the fibers in virtually any desired form including, for example tows, rovings and yarns, staple and batting, felt, paper, woven or knitted fabrics and the like, the form of choice depending primarily upon the intended use for the carbon fibers. Similarily, the fiber may be of any desired diameter, which is selected primarily with refexence to the intended use for the resulting carbon fibers. While the surface area does not appear to be dependent upon diameter, the tensile strength of the carbon fibers tends to increase with increasing diameter and their flexibility tends to increase with decreasing diameter. It will be apparent that while the examples illustrate the invention as carried out in a batchwise fashion, suitable apparatus for carrying out the steps in a continuous manner may be readily devised.
The carbon in fibers produced according to the invention is amorphous (glassy) and is of the type known as hard carbon, i.e., highly cross-linked carbon which is very difficult to graphitize. The high surface area carbon fibers, in various forms, have numerous application. For example, they are particularly useful as an adsorbent in gas masks and in adsorbent protective clothing and filter media.
Percentages referred to herein are by weight unless ~04672~
otherwise expressly stated or clearly indicated by the con-text. Surface areas as set forth herein are surface areas as determined with a Model 2200 Automatic Surface Area Analyzer (Micromeritics Instrumental Corp. Norcross, Georgia) in accordance with the BET method and eq1~ation of srunauer, Emmett and Teller (see J. Amer, Chem. Soc. 60; 309-316 (1938)), which involves a determination of the quantity of a gas such as nitrogen which is required to form a monomolecular layer adsorbed on the surface of the sample.
While the invention has been described herein with reference to certain examples and preferred embodiments, it is to be understood that various changes and modifications may be made by those skilled in the art without departing from the concept of the invention, the scope of which is to be determined by reference to the following claims.

Claims (11)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A flexible adsorbent carbon fiber made from a mixture comprising a pitch and a phenol formaldehyde novolac, having a surface area in the range from 1000 m2/gm to 2500 m2/gm.

2. A carbon fiber according to claim 1 in which the mixture comprises from about 5 to about 40 percent novolac.

3. A carbon fiber according to claim 1 in which the mixture comprises from about 10 to about 25 percent novolac.

4. A carbon fiber according to any of claims 1, 2 or 3 in which the fiber diameter ranges from about 0.1 to about 300 microns.

5. A carbon fiber according to any of claims 1, 2 or 3 in which the fiber diameter ranges from about 10 to about 30 microns.

6. A method for making a flexible adsorbent carbon fiber, comprising the steps of:
(a) fiberizing a molten mixture of pitch and novolac having at least two available sites for further polymerization;

(b) curing the fiber with acidic formaldehyde to make it infusible;
(c) heating the fiber in air to a temperature ranging from 250°C to 450°C; and subsequently (d) heating the fiber under an inert atmosphere to a temperature ranging from 700°C to 900°C to form the adsorbent carbon fiber.

7. A method according to claim 6 in which the mixture of pitch and novolac has a novolac content ranging from about 5 to about 40 percent.

8. A method according to claim 6 in which the mixture has a novolac content ranging from about 10 to about 25 percent.

9. A method according to any of claims 6, 7 or 8 in which the fiber diameter ranges from about 0.1 to about 300 microns.

10. A method according to any of claims 6, 7 or 8 in which the fiber diameter ranges from about 10 to about 30 microns.

11. A method according to any of claims 6, 7 or 8 comprising, in addition, the subsequent step of heating the carbon fiber in steam at a temperature in the range from about 800°C to about 900°C for a time sufficient to increase the surface area of the carbon fiber to at least 1000 m2/gm.