US3607672A - Method for producing febrous carbon structures - Google Patents

Abstract

Carbonized fibrous material of improved resiliency and tensile strength is produced by impregnating rayon fibers with a solution of polyethylene or polypropylene dissolved in a volatile hydrocarbon solvent such as xylene, driving off the solvent, and thereafter heating the impregnated fibers to a temperature adequate to carbonize the rayon. After carbonization, the polyethylene-treated fibers showed a marked improvement in resiliency as well as an average tensile strength increase of about 55 percent over untreated fibers of the same type.

Description

United States Patent 6 Claims, No Drawings US. Cl 23 2094, 8/115.6, 8/116, 23/2092, 23/2094, 117/26, 117/46, 117/145, 264/29 Int. Cl C0lb 31/07 Field of Search 23/2091, 209.2, 209.4; 117/26, 46,145;8/115.5,1l5.6,116

References Cited UNITED STATES PATENTS 3,116,975 1/1964 Cross et a1. 23/2091 inventor Charles R. Schmitt Oak Ridge, Tenn.

Appl. No. 8,695

Filed Feb. 4, 1970 Patented Sept. 2 1, 1971 Assignee The United States of America as represented by the United States Atomic Energy Commission METHOD FOR PRODUCING FIBROUS CARBON STRUCTURES 3,121,698 2/1964 Orsinoetal. l17/l45X 3,281,261 10/1966 Lynch l17/46 3,294,489 12/1966 Millington et a1... 23/2094 3,305,315 2/1967 Bacon et a1. 23/2091 3,395,970 8/1968 Machell 8/1 16.2

Primary Examiner-Edward J. Meros AuomeyRoland A. Anderson ABSTRACT: Carbonized fibrous material of improved resiliency and tensile strength is produced by impregnating rayon fibers with a solution of polyethylene or polypropylene dissolved in a volatile hydrocarbon solvent such as xylene,

driving off the solvent, and thereafter heating the impregnated METHOD FOR PRODUCING FIBROUS CARBON STRUCTURES The present invention relates generally to the preparation of fibrous carbon products and more particularly to a method of treating rayon fibers with an organic polymer of polyethylene or polypropylene to improve the resiliency and tensile strength of the fibers when the latter are converted to a carbonaceous state. This invention was made in the course of, or under, a contract with the U. S. Atomic Energy Commission.

Textile materials such as provided by the weaving or knitting of cellulosic fibers with yarn, fabrics, braids, felts, or the like, are suitable structural materials because of their high tensile strength and flexibility. Conversion of these cellulosic materials to a carbonaceous state provides some additional features in that the well known chemical and physical properties of carbon can be used advantageously in many structural applications. However, while gaining such properties as attributed by the carbon, the carbonized material suffers considerable losses in the areas of tensile strength and resiliency so as to significantly detract from its usage in many applications where such physical characteristics are desired.

Accordingly, it is the aim and principal object of the present invention to overcome or substantially minimize the above and other shortcomings or drawbacks suffered by the carbonized cellulosic material as prepared by practicing previously known techniques. This goal is achieved by employing a method of preparing carbonized cellulosic material whereby the tensile strength and resiliency are significantly improved over those previously provided.

Broadly, the method of the present invention comprises the steps of impregnating or contacting fibers of regenerated cellulose, i.e., rayon, with an organic polymer of polyethylene or polypropylene dissolved in a suitable hydrocarbon solvent therefor, forming the impregnated fibers into the desired configuration, driving the solvent from the fibers such as by heating to a temperature below the melting point of the organic polymer but above the boiling point of the solvent, and thereafter heating the impregnated fibers to a temperature sufficient to convert the regenerated cellulose to carbon. The presence of the organic polymer during the pyrolysis or carbonization step apparently alters the surface of the fibers as well as the carbonized microstructure of the latter so as to provide the desirable tensile strength improvement which may be as high as about 55 percent over untreated carbonized fibers of the same type. In addition to the significant increase in tensile strength, the fibers also show unexpected resiliency which greatly exceeds that provided by untreated carbonized rayon fibers.

In practicing the present invention the fibers are formed of regenerated cellulose or rayon. Viscose rayon is the preferred type of rayon because of its ready availability and desirable physical properties, but other types of rayon such as cuprammonium rayon and saponified cellulose ester rayon may be satisfactorily employed as the fiber material. Natural high-cellulose-content fibers such as cotton and other flosses are not suitable for the purpose of this invention due to their weak carbonized strength. As briefly mentioned above, the regenerated cellulose fibers may be in any desired form such as long or chopped monofilaments, yarn, braids, fabrics, or the like, prepared by any well-known knitting or weaving procedure.

In order to provide the regenerated cellulose fibers with the improved resiliency and tensile strength characteristics upon carbonization, they are treated, i.e., impregnated, with polyethylene or polypropylene which has been dissolved in a suitable hydrocarbon solvent, as will be discussed below. Polyethylene is the preferred organic polymer used in the treatment of the fibers since improvements in resiliency and tensile strength are somewhat greater than provided by polypropylene. However, polypropylene, which is a linear hydrocarbon polymer containing little or no unsaturation and has properties similar in many respects to polyethylene, provides carbonized rayon fibers with tensile strengths greater than provided by untreated carbonized rayon and a resiliency factor greater than provided by treating the fibers with other organic materials, as will be discussed in detail below.

Polyethylene is basically a polymer of ethylene produced by the reaction N(C H (C I-I )n where N in the plastic grade is in the range of about 600 to 4,000. Polyethylene is a highly crystalline oriented material the crystallization of which is improved by stretching and further improved by annealing. The type polyethylene preferred in the subject application is the so-called low-density polyethylene, that is, about 0.91 or 0.92 gram per cubic centimeter (g./cc.) at 25 C. The polyethylenes characteristic of this group are preferred due to their relatively low softening points, ease in handling, dissolution by solvents, and by the fact that the polyethylenes in this range do not solidify upon volatilization of the solvent at a temperature which would be detrimental to the pyrolysis of the rayon. The polyethylenes in this range normally have a melting point of about l00l 10 C., which is satisfactory for this application. Normally, these polyethylenes have a molecular weight of about 2,000 to 20,000. Preferably, the polyethylene has a softening point of C., a density of approximately 0.91 g./cc. at 25 C., and a molecular weight of about 7,000.

Without being held to a specific theory as to why the presence of polyethylene and, to some extent, polypropylene have significant and positive effects on the quality of a carbon derived from rayon, the following postulations are offered. It is believed that the polyethylene-impregnated microstructure, after carbonizing, may contain oriented particles of carbon deposited in the spinneret holes in the fibers. The surface of the carbon yarn is apparently altered by the presence of polyethylene during pyrolysis, but merely providing a surface coating of carbon would not tend to have the profound effects on strength and flexibility that were observed for the polyethylene-treated products. It was also shown from experimental data that the polyethylene was probably present as a fluid during pyrolysis of the rayon and was later lost through volatilization. The influence of the polyethylene was apparent, not only from the increased quality of the product, but also from changes in the composition of pyrolysis products and changes in the surface of the resulting carbon yarn. For example, a comparative study of the rate of gas release from polyethylene-impregnated and unimpregnated rayon during pyrolysis has shown that a high release rate of carbon monoxide and carbon dioxide occurs for unimpregnated rayon at 300 C. The corresponding gas release rates obtained for the polyethylene-impregnated rayon were considerably lower, suggesting that polyethylene can react with rayon to inhibit the release of carbon monoxide and carbon dioxide during partial pyrolysis at 300 C. It is believed that the pyrolysis of the rayon yarn in the presence of the polyethylene resulted in the formation of a carbonaceous intermediate consisting of chains of four carbons each. Thus, the similarity in the basic structure of this intermediate and polyethylene could result in interaction and incorporation of polyethylene into the structure. Another alternative to the particular mechanism possibly causing the increased resiliency and tensile strength of the carbonized rayon is believed to be that the polyethylene actually altered the pyrolysis mechanism by acting as a source of radicals and therefore contributed little mass product. No significant mass effects have been observed for the treated rayon, and polyethylene alone leaves no char. Polypropylene is believed to function in a manner somewhat similar to the polyethylene, but the appearance of the carbonized yarns is different in that the parallel lines which are formed along the rayon fibers are visible with the polypropylene-treated yarns after carbonization, whereas the lines are substantially unobservable with the polyethylene-treated fibers. Even so, the lines along the fibers of the polypropylene-treated yarns are somewhat less evident than with the untreated yarns after carbonization.

In order to treat the fibers with the polyethylene or the polypropylene it is necessary that these organic polymers be in a solution which will facilitate the impregnation of the fibers.

The formation of a polyethylene or polypropylene solution is readily achieved by using a suitable hydrocarbon solvent such as xylene, toluene, or benzene. The dissolution of these organic polymers is normally achieved at an elevated temperature, e.g., 60 C. or higher, since the organic polymers are relatively insoluble at lower temperatures even in the particular solvents described.

it has been found that the quantity of the organic polymers in the solution adequate to provide the rayon fibers with the necessary quantity of impregnant is usually about 5-10 weight percent. Employment of more than about weight percent does not appear to be advantageous in that sufficient polyethylene or polypropylene is incorporated in the fibers using the weaker solutions. Further, the use of such weaker solutions assures that greater penetration into the fibers will be accomplished. Less than about 5 weight percent does not appear to be adequate since insufficient quantities of the polymer penetrate the fibers to accomplished the the necessary property changes during pyrolysis.

Rayon fibers in the form of fabrics, cloths, weaves, yarns, monofilaments, etc., are impregnated with the polymer by immersing the fibers into a bath consisting of the dissolved polymer. The impregnation is usually accomplished within a relatively short period of about 30 minutes. increased penetration of the fibers may be achieved by employing well-known pressure impregnation techniques. Upon completing the impregnation of the fibers they are removed from the bath and formed into the desired configuration. The fibers are then heated to a temperature below the melting point of the polymer to drive ofi the volatile solvent. This driving off of the volatile solvent is usually achieved at a temperature less than about l00 C. with the solvents listed above. Upon completion of the removal of the solvent the fibers are heated in an inert atmosphere, e.g., argon, to a temperature sufficient to convert the regenerated cellulose to a carbonaceous state. Usually, a temperature of about 900-l ,000" C. is adequate for pyrolysis. This temperature is preferably maintained for a duration of about 60 hours to assure complete conversion of the fibers to carbon. Exhausting the gases generated during the carbonization step is important since insufficient gas removal will result in inferior products. If desired, the carbonized rayon may be converted to graphite by heating the carbonized rayon in an inert atmosphere to a temperature in the range of about 2,500 to 3,000 C.

Regenerated cellulose fibers, when treated and carbonized in accordance with the teachings of the present invention, demonstrate a remarkable improvement in the resiliency over fibers which have not been treated or which have been treated with other chemically impregnating plastics. in establishing the improvement in the resiliency of the fibers produced by the impregnation with polyethylene or polypropylene, carbon cones were prepared by chemically impregnating rayon velvet with polypropylene, polyethylene, polystyrene diluted in xylene solvent, and formamide, acetic acid, tributylamine, and furfuryl alcohol in a partially polymerized state. After carbonizing the impregnated cones the resulting carbon cones had dimensions of 1% inches in height and 1.3 to 1.5 inches in diameter. These cones were subjected to a compressive load at the apex of the cone at a constant strain rate of 0.05 inch per minute. The data resulting from this testing of the cones impregnated by various polymers are shown in the table below. The data set forth for the cone impregnated with polypropylene are derived from the testing of a larger cone.

TABLE.COMPARATIVE RESILIENCY 0F CARBONIZED RAYON VELVET (AFTER VARIOUS CHEMICAL TREAT- MENTS PRIOR TO COKING) Specimen was very ..-.resi sn Table Continued The polyethylene-treated carbonized rayon velvet had by far the highest resiliency in that no fracture occurred, even after completely flattening the apex of the cone. Polypropylene, while not providing the resiliency afforded by the polyethylene, did show greater resiliency than the other carbonized specimens, as is evident upon viewing the data in the above table. The use of long-chained aliphatic hydrocarbons as impregnants had no effect on the carbon products with respect to improved resiliency or tensile strengths. The vapor pressure of these hydrocarbon materials indicated a probable volatilization prior to pyrolysis of the yarn. Resins that were found to be undesirable included indene, vinyl indene, divinylbenzene, and polystyrene. Polystyrene is reported in the above table. All the products produced by these last-mentioned resins were very weak and brittle.

The tensile strength of the polyethylene-treated rayon is significantly greater before and after coking than that of untreated rayon. Random samplings of untreated rayon yarn 0.024 inch in diameter and polyethylene-impregnated yarn 0.020 inch in diameter provided average tensile strengths of 9,095 p.s.i. and 14,081 psi, respectively. Similar samplings of carbonized untreated rayon yarn 0.016 inch in diameter and carbonized yarn 0.014 inch in diameter which had been impregnated with polyethylene provided average tensile strengths of 263 psi, respectively. The above data demonstrate that the carbonized rayon yarns have an increased tensile strength of about 75 percent over the untreated rayon yarn, and the polyethylene-impregnated rayon yarn showed about a 55 percent improvement in tensile strength over the untreated rayon yarn upon carbonization. Comparative tensile strength data on polyethylene-impregnated monofilaments of rayon having a diameter of approximately 10 microns after carbonizing have shown average tensile strengths of approximately 70,000 psi. as compared to average tensile strengths of only approximately 50,000 psi. after carbonizing untreated rayon monofilaments of the same diameter.

Flexible fabrics, yarns, fibers, felts, and the like, of carbon presently have widespread industrial use and the added resiliency and tensile strength afforded to these carbonaceous materials will greatly improve their use in thermal and electrical applications, particularly where high temperatures are involved.

What is claimed is:

1. A method of producing a resilient carbonaceous fiber comprising the steps of contacting a fiber of regenerated cellulose with a solution consisting essentially of an organic polymer selected from the group consisting of polyethylene and polypropylene and dissolved in a solvent therefor volatile at a temperature less than the melting temperature of the organic polymer, removing the fiber from the solution, heating the fiber to a temperature sufficient volatilize the solvent, and thereafter further heating the fiber in an inert atmosphere to a temperature sufficient to convert the cellulose to carbon.

2. The method of producing a resilient carbonaceous fiber as claimed in claim 1, wherein the solution consists essentially of about 5 to weight percent of the organic polymer with the remainder being provided by the solvent.

3. The method of producing a resilient carbonaceous fiber as claimed in claim 1, wherein the fiber of regenerated cellulose is selected from the class consisting of viscose rayon, cupraammonium rayon, and saponified acetate rayon, and wherein the temperature sufficient to convert the fiber to carbon is in the range of about 900 to 1,000 C.

4. The method of producing a resilient carbonaceous fiber as claimed in claim 3, wherein the solvent is a hydrocarbon solvent selected from the group consisting of xylene, toluene, and benzene, and wherein the temperature sufficient to 20,000, and a density of about 0.91 gram per cubic centimeter.

Claims (5)

1. 2. The method of producing a resilient carbonaceous fiber as claimed in claim 1, wherein the solution consists essentially of about 5 to 10 weight percent of the organic polymer with the remainder being provided by the solvent.
2. 3. The method of producing a resilient carbonaceous fiber as claimed in claim 1, wherein the fiber of regenerated cellulose is selected from the class consisting of viscose rayon, cupraammonium rayon, and saponified acetate rayon, and wherein the temperature sufficient to convert the fiber to carbon is in the range of about 900* to 1,000* C.
3. 4. The method of producing a resilient carbonaceous fiber as claimed in claim 3, wherein the solvent is a hydrocarbon solvent selected from the group consisting of xylene, toluene, and benzene, and wherein the temperature sufficient to volatilize the solvent is less than about 100* C.
4. 5. The method of producing a resilient carbonaceous fiber as claimed in claim 3, including the additional step of heating the carbonized fiber to a temperature in the range of about 2,500* to 3,000* C. for converting the carbon to graphite.
5. 6. The method of producing a resilient carbonaceous fiber as claimed in claim 1, wherein the polymer is polyethylene having a melting temperature in the range of about 100* to 110* C., a molecular weight in the range of about 2,000 to 20,000, and a density of about 0.91 gram per cubic centimeter.