Process For The Production Of Carbon And Graphite Fibers

Abstract

Carbon fibers are produced by treating synthetic polymeric fibers with a solution of a condensation agent at a temperature of 180.degree. to 230.degree.C to effect cross-linking and/or cyclization within the polymeric fibers and thereafter heating the treated fibers to a temperature between 200.degree.-300.degree.C in an atmosphere containing an oxidizing gas and, subsequently, in an inert or reducing atmosphere, to a temperature of at least about 1,000.degree.C. Titanium tetrachloride, lead tetrachloride, tin tetrachloride, and tin tetrabromide are used as condensation agents, preferably in the form of complexes in a solvent such as high-boiling ethers and esters. Dimethylformamide is used as a complex former. o-nitroanisole, d-n-butylterephthalate, and iso-octadecylbenzoate are used as both solvent and complex former.

Description

My invention relates to a method of improving the suitability of synthetic polymer fibers for carbonization.

The production of carbon and graphite fibers by carbonization followed, if desired, by graphitization of synthetic polymer fibers such as regenerated cellulose, polyacrylonitrile or polyvinylchloride is already known. The synthetic polymer fibers are heated in an inert and/or reducing atmosphere to a temperature up to about 1,000.degree.C, for the purpose of graphitization, the carbon fibers so produced are then further heated in an inert atmosphere up to a temperature in the region of 2,500.degree.C. It has been found that the synthetic polymer fibers are partially depolymerized during their conversion to carbon fibers, particularly at temperatures within the range of from 200.degree. to 500.degree.C which the carbon content and, more important, the strength of the fibers decrease.

A number of methods of overcoming these problems have been described. For example, U.S. Pat. No. 2,697,028, describes methods of increasing the carbon yield, and thus also the strength of the fibers, by heating the synthetic polymer fibers in air at temperatures between 200.degree. and 300.degree.C, or by the addition of depolymerization inhibitors, such as ammonia, sulfur dioxide or methylamine. However, the improved strength produced is insufficient for many uses of the carbon fibers. High strength values are particularly required for those carbon and graphite fibers, which are to be used for the reinforcement of synthetic plastics, metals and ceramic materials.

It has also been proposed to improve the strength and rigidity of carbonized fibers by stretching the fibers at high temperatures in a direction parallel to the fibers axis. For example, in British Pat. No. 1,148,872, a method is described in which polyacrylonitrile is heated in a non-oxidizing atmosphere to at least 1,000.degree.C while simultaneously being held under tension so that the final length of the fibers is greater than the initial length thereof. U.S. Pat. No. 3,454,362 describes a method in which carbon or graphite fibers are subjected to a longitudinal tensile stress at temperatures above the flow temperature of the carbon fibers so that the fibers are stretched by at least 1 percent. Very high temperatures will be required for such a method.

The stretching of partially or completely carbonized fibers at temperatures greater than 200.degree.C causes a substantial alignment of the molecules in the direction of the fiber axis and results in a considerable increase in the strength and elasticity modulus values of the fibers. However, a disadvantage of such procedures is that complicated apparatus is necessary for stretching the fibers at relatively high temperatures and any breakages which occur in the fibers result in relatively long stoppages in production.

It is an object of my present invention to overcome or at least mitigate the aforesaid disadvantages and to provide carbon and graphite fibers having high strength values.

According to the present invention there is provided a method improving the suitability for carbonization of a non-cellulosic synthetic polymer fiber, in which the fiber is contacted with one or more condensation agents in order to effect cross-linking between and/or cyclization within polymer molecules, the synthetic polymer fiber then being oxidized by heating in the presence of a dehydrating agent at a temperature or temperatures within the range of from 175.degree. to 350.degree.C while free of any substantial tension.

The present invention is applicable to the majority of non-cellulosic synthetic polymer fibers since most fibers, if not all fibers, of this type are capable of undergoing intra-molecular cylization within and/or a certain amount of cross-linking between polymer chains leading to formation of a kind of "ladder" polymer. The most suitable polymers for use in the method of the invention are polymers, and copolymers with suitable copolymerizable monomers, of acrylonitrile. However, polymers and copolymers of vinylchloride, vinylesters and vinylalcohol, and 1,2-cis-polybutadiene can also be used. The intramolecular and/or intra-molecular reactions which take place in the presence of the condensation agents result in the substantial retention of the molecular orientation during the subsequent heating of the fibers in an oxidizing atmosphere and later in an inert or reducing atmosphere.

The nature of the inter-molecular and/or intra-molecular reactions, which occur will of course depend upon the particular polymer used. In the case of polyacrylonitrile, the cyclizing and/or cross-linking pre-treatment takes place in a similar manner to the reaction which has been observed with monomeric acrylonitrile which can undergo cyclization to yield dihydropyridine derivatives. A reaction similar to cyclization of acrylonitrile occurs with polyacrylonitrile, leading to a cross-linked product which contains dihydropyridine rings.

The reaction in the presence of a condensing agent or catalyst is generally carried out at temperatures of from 180.degree. to 230.degree.C and preferably from 190.degree. to 200.degree.C. The cross-linking agents used are generally Lewis acids of a strength sufficient to saturate the free pairs of electrons of the polymer. While sulfur trioxide, for example, can be used as condensation catalyst, this is not a convenient material for working with on an industrial scale. The preferred catalysts for use in the method according to the present invention are halides of elements of the fourth main group and sub-group of the Periodic Table. Particularly preferred condensation agents are titanium tetrachloride, lead tetrachloride, tin tetrachloride and tin tetrabromide.

It has been found particularly advantageous to use the condensation agents in the form of complexes, more especially in the form of amine complex compounds, ester complex compounds or ether complex compounds. A particular advantage of the use of complexes is that the above-mentioned halide condensation agents are normally highly hydroscopic, but in the form of complex compounds, they possess a substantially grater water resistance. The complexes are used in solution in suitable solvents such as high-boiling ethers and/or esters. Those solvents which are at the same time complex formers are particularly advantageous. Examples of compounds which can be used as both solvent and complex former are o-nitroanisole, di-n-butylterephthal-ate and iso-octadecylbenzoate. Dimethylformamide can be used simply as complex former. When the synthetic polymer fibers are immersed in a solution of a condensation agent, the solution advantageously contains at least 0.8 percent by weight, and preferably from 1.0 to 1.5 percent by weight, of the condensation agent in the form of a complex.

The on-cellulosic synthetic polymer fibers used in the process of the present invention are advantageously subjected to stretching at low temperature, generally the ambient temperature, prior to being contacted with the condensation agent at an elevated temperature. The stretch ratio to which the fibers are subjected is advantageously at least 1:4 and the tensile strength of the stretched fibers should be at least 4 P/den. (pond per denier). This preliminary step, of course, is commonly used in the textile art and serves to align the polymer molecules.

Advantages of the use of the method of the present invention are as follows:

A. Carbon and graphite fibers of high strength can be obtained from fibers pretreated in the aforespecified manner;

B. the need to subject the fibers to stretching at elevated temperatures is obviated so that complicated equipment is no longer required and stoppages in production caused by breakages in the fibers are avoided; and

C. the pretreatment of synthetic polymer fibers prior to carbonization and graphitization can be carried out in a very short time. The fibers can be carbonized and, if desired, graphitized in the same oven as that used in the oxidation step.

During the oxidation step, the temperature of the fibers is advantageously gradually raised at a rate of from 40.degree. to 200.degree.C per hour. In a preferred mode of operation, oxidation is carried out at temperatures within the range of from 200.degree. to 300.degree.C, the temperature of the fibers being increased at a rate of 100.degree.C per hour during oxidation. The oxidation step is advantageously carried out at a temperature of from 180.degree. to 250.degree.C and can be completed in from 1 to 5 hours. The dehydrating agent used is advantageously air, although chlorine or liquid sulfur can be used for this purpose.

The following Examples illustrate the invention:

EXAMPLE 1

Fibers of a copolymer of 95 percent by weight acrylonitrile and 5 percent by weight methyl methacrylate were stretched while wet to five times their original length. The tensile strength of the stretched fibers was 4 ponds/den. The fibers were then dried at a temperature of 120.degree.C and pulled over rollers while free of any tension other than that required to pull them over the rollers, through a tank containing a 1.5 percent solution of PbC1.sub.4 .sup.. 2 di-n-butyltere-phthalate in di-n-butylterephthalate. The bath temperature was 200.degree.C and the residence time of the fibers there was about 30 minutes. The fibers were then drawn through the nip of wringer rollers and led into a second tank containing acetone in which the di-n-butylterephthalate was washed off them. The fibers were then washed in a third tank containing water and dried in a tube dryer at about 120.degree.C.

The fibers were then subjected to an oxidation treatment. For this purpose, they were heated in a tube furnace in which the temperature control was such that a linear rise in temperature of 100.degree.C per hour was obtained in a temperature range of 200.degree. to 300.degree.C. During the residence of the fibers in the tube furnace, a stream of air was continuously passed through the furnace.

The oxidized fibers were then subjected to carbonization. For this purpose, they were pulled through a second furnace to which a stream of nitrogen was continuously passed, the temperature control in the second furnace being such that there was a linear rise in temperature of about 2,000.degree.C per hour in the temperature range between 300.degree. to 1,000.degree.C. The fibers were subsequently further heated in a nitrogen atmosphere to 1,700.degree.C, the temperature control being such that a linear rise in temperature of 5,000.degree.C per minute occurred.

After the carbonization, the fibers had the following mechanical properties:

E (Elasticity modulus) = 2.00 .times. 10.sup.6 kp/cm.sup.2

.delta. (Tensile strength) = 2.1 .times. 10.sup.4 kp/cm.sup.2

EXAMPLE 2

A copolymer of about 95 percent by weight acrylonitrile and about 5 percent by weight methylmethacrylate was dry stretched to a stretch ratio of 1:6. The stretched fibers, which had a tensile strength of 7 ponds/den. were immersed in iso-octadecylbenzoate containing dissolved therein 1 percent by weight of a complex of SnBr.sub.4 and iso-octadecylbenzoate. The temperature of the solution was 190.degree.C and the fibers were immersed therein for 60 minutes. The fibers were then washed, oxidized and carbonized in the manner set out in Example 1. The fibers obtained after carbonization had the following mechanical properties:

E = 2.55 .times. 10.sup.6 kp/cm.sup.2

.delta. = 2.6 .times. 10.sup.4 kp/cm.sup.2

Claims

I claim:

1. Method of producing a carbonaceous fiber from a thermoplastic polymeric fiber selected from polyacrylonitrile and polyacrylonitrile copolymers, comprising the steps of:

a. stretching or elongating the polymeric fiber to a stretching ratio of at least 1:4 until the tensile strength of the stretched fiber is at least 4 p/den;

b. heating the fiber in a solution of a solvent selected from o-nitroanisole, di-n-butylterephthalate or i-octadecylbenzoate containing a complex of said solvent and a halide selected from the group consisting of titanium tetrachloride, lead tetrachloride, tin tetrachloride and tin tetrabromide, to a temperature of 180 to 230.degree.C; and

c. heating the fiber without external tensile stress to a temperature between 200.degree. and 300.degree.C in an atmosphere containing oxidizing gases and, subsequently, in an inert or reducing atmosphere, to approximately 1,000.degree.C.

2. The method of claim 1, wherein di-n-butylterephthalate is used as both complexing agent and solvent for lead tetrachloride.

3. The method of claim 1, wherein iso-octadecylbenzoate is used as both complexing agent and solvent for tin tetrabromide.

4. The method of claim 1, wherein the solution contains at least 0.8 percent by weight of a complex of said halide.

5. The method of claim 4, wherein the solution contains from 1.0 to 1.5 percent by weight of a complex of said halide.

6. The method of claim 1, wherein the synthetic polymeric fiber is a copolymer of acrylonitrile and methylmethacrylate.

7. The method of claim 1, wherein the fiber in step (c) is heated to 2,500.degree.C.