Amidines as initiators for converting acrylic fibers into carbon fibers

Abstract

A method of preparing PANOX fibers useful in the production of superior carbon fiber material; the method based on the concept that the true initiators in the preparation of superior carbon fiber are amidines.

Description

The present non-provisional application is based on provisional application no. 60/636887 filed on Dec. 20, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is directed to the production of superior carbon fiber material by a novel process that allows the producer to prepare a starting material in such a manner as to allow formation of the superior material. The starting material in the invention is a homopolymer or co-polymer of acrylonitrile that is in the form of a spun fiber. The fiber can be wet-spun, dry-spun, or wet-dry-spun. If the fiber is prepared from a co-polymer, the co-polymer contains pendant groups that are carboxylic acid groups, carboxylic acid anhydride groups or salts of carboxylic acid groups. In a preferred embodiment, the co-polymer is an acrylonitrile-itaconic acid co-polymer.

Amidines are formed when nitrile units are reacted with primary amines, secondary amines or ammonia. If amidines were not formed, then acrylic fibers could not be converted to fine quality carbon fibers. The amidine functionality is comprised of a single carbon atom covalently bonded to two different nitrogen atoms. The other valency of the carbon atom is filled by a single covalent bond to another carbon atom which is, in the case of acrylic polymers, part of a carbon backbone. The single carbon atom, which is bonded to two nitrogen atoms, is singly bonded to one of the atoms and doubly bonded to the other.

When an amidine moiety is in proximity to a nitrile group (a carbon atom is triple bonded to a single nitrogen atom), the amidine moiety can react with the nitrile group to give a cyclic structure such as a naphthyridine ring system. This is essentially a self-polymerization reaction wherein the nitrile structures zipper themselves up via an original amidine formation to give a long polycyclic chain of naphthyridine rings. The nitrogen atom of each six-membered naphthyridine ring is singly bonded to one carbon atom and doubly bonded to another carbon atom of the six-membered ring.

When an amidine moiety is in proximity to a nitrile group bonded to a different carbon backbone, the amidine moiety can further react with the nitrile group to form a crosslink between two different polymer chains. Cross-linking of the polymer chains causes the fusion point of the fibers to be raised above 500° C. This is the key step for the formation of quality carbon fibers. This cross-linking must take place in the absence of “hot spots” and void formation. The method of heating the acrylic fibers to form the cross-links is critical.

Although formation of the cross-links between polymer chains, and thus also between fibers, is really the critical step in the preparation of precursors to carbon fiber; it is not possible to form only cross-links to the exclusion of naphthyridine ring formation. It is also not possible to control the amount of formation of one or the other. We do know that, with the formation of amidine moieties, some moieties will form cross-links and others will form ring structures. It is only necessary that the amount of cross-linking that occurs upon heating is enough to raise the fusion point of the fibers to 500° C. The density of the fibers is thus increased about 15-20%. That density is about 1.4 g/ml, which indicates that the fusion point of the fiber is high enough that it will not disintegrate when heated to about 2000° C.

When acrylonitrile polymeric fiber is heated in air at a temperature of about 250° C., ammonia comes off from the fiber. This ammonia can then react with the nitrile groups in the polymer fiber to initially form amidine groups. The fiber changes from the color white to the color yellow. When the amidine group further reacts with nitrite groups to form cross-links and internal cyclization, the softening point of the fiber is raised to approximately 500° C. The density of the fiber is also increased with increased cross-linking.

U.S. Pat. No. 2,758,003 to H. Kleiner et al. discloses a process for treating a polyacrylonitrile fiber and resulting product using an aliphatic mono- or polyamine, or ammonia at any stage in the process but preferably in a heating step.

U.S. Pat. No. 4,024,227 to Kishimoto et al. discloses a process for making carbon fiber of high tensile strength and high modulus of elasticity by short-time firing of an acrylonitrile fiber impregnated with a primary amine and/or quaternary ammonium salt so that the fiber is partly insoluble in a concentrated aqueous solution of sodium thiocyanate.

U.S. Pat. No. 4,336,022 to Lynch et al. discloses an acrylic fiber precursor having 93-99.4 mol percent acrylonitrile, 0.6-4.0 mol percent ammonia or amine (pKb 5), and a sulfonate co-monomer, preferably 0.8-2.0 mol percent sulfonic acid.

U.S. Pat. No. 4,364,916 to Kalnin et al. discloses a process for the thermal stabilization of acrylic fibers which involves contacting the fibers with hydroxylamine (pH 6-8) at 95° C. to 130° C., an oxidizing agent at 80° C. to 120° C. and heating the fibers in the presence of oxygen until the fibers are capable of undergoing carbonization.

U.S. Pat. No. 4,698,413 to Lynch et al. discloses an acrylic fiber of an acrylic polymer containing 93-99.4 mol percent acrylonitrile units, 0.6-4.0 mol percent ammonia or amine having a pKb 5. The fibers are heated for 4-20 minutes at 250° C.-260° C. The process is continued until the precursor fibers have a density of at least 1.40 g/cm³.

U.S. Statutory Invention No. H1052 to Peebles, Jr. et al. discloses a method for stabilizing polyacrylonitrile based fibers by heating them to a temperature between 150° C. to 350° C. in the presence of an oxidizer, preferably oxygen, and ammonia.

None of the above-referenced patents, whether taken individually or in combination, anticipate the present invention as disclosed and claimed.

SUMMARY OF THE INVENTION

The novel process includes the step of trapping a primary amine, a secondary amine, or ammonia in the polymeric fiber. A variety of methods can be employed to trap the nitrogen-containing compound in the fiber. The preferred method is to treat an acrylonitrile-itaconic acid co-polymer with a liquid solution of the nitrogen-containing compound at an elevated temperature. The fiber can then be dried and treated in further steps of the inventive process. The primary or secondary amine or ammonia is considered to be the pseudo-catalyst in the preparation of carbon fiber.

A second step in the process is obtaining an infrared spectrum (IR) of the starting material that has been chemically treated with the amine or ammonia. The spectrum is to be employed for comparison with a second IR scan in a later step of the process.

A third step in the process is oxidizing the chemically treated fiber in a relatively mild reaction to obtain a PANOX fiber (polyacrylonitrile, oxidized). The PANOX fiber contains an amount of amidine units and pseudo-amidine units in its structure. Some of the amidine units can form intramolecular cross-links between polymer chains. These cross-linked structures are the key to the formation of superior carbon fibers.

A fourth step in the novel process is obtaining an infrared spectrum (IR) of the PANOX fiber. In a preferred embodiment, the attenuated total reflectance method is employed.

A fifth step in the process is comparing the IR's obtained in steps two and four. This step is critical in that it allows the producer of carbon fiber to make a decision as to whether or not he wishes to proceed with the process of preparing the fiber. A superior fiber can be obtained only if the ratio of amidine groups to acrylonitrile groups in the PANOX fiber falls within a specified optimal range.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings are illustrative of embodiments of the invention and are not meant to limit the scope of the invention as encompassed by the claims.

FIG. 1 is a graph of the approximate locations of various group vibrations in the IR spectrum.

FIG. 2 is a tabular illustration of an analysis of four separate fiber samples, each being accorded a single letter designation (A, B, C, D). After the fibers have been heated, each is accorded a simple two-letter designation (AH, BH, CH, DH).

FIG. 3 is a graph of an infrared absorption spectrum of textile fiber A (polyacrylonitrile, untreated) prior to a heating step.

FIG. 4 is a graph of an infrared absorption spectrum of textile fiber AH (heated sample A).

FIG. 5 is a graph of an infrared absorption spectrum of textile fiber B (polyacrylonitrile co-polymer with itaconic acid; and chemically treated to contain 4060 ppm ammonia).

FIG. 6 is a graph of an infrared absorption spectrum of textile fiber BH (heated sample B).

FIG. 7 is a graph of an infrared absorption spectrum of textile fiber C (polyacrylonitrile co-polymer with itaconic acid; and chemically treated to contain 260 ppm ammonia).

FIG. 8 is a graph of an infrared absorption spectrum of textile fiber CH (heated sample C).

FIG. 9 is a graph of an infrared absorption spectrum of textile fiber D (acrylonitrile co-polymer with itaconic acid; and chemically treated to contain 4790 ppm ammonia).

FIG. 10 is a graph of an infrared absorption spectrum of textile fiber DH (heated sample D).

DETAILED DESCRIPTION OF THE INVENTION

An aspect of the present invention is the preparation of superior carbon fibers from acrylonitrile fibers that are easily obtainable. In a pre-carbonization phase, the acrylonitrile fibers are specially treated to obtain PANOX fibers having a specified amount of amidine groups.

The heating of the fiber is an exothermic step. The heat must be controlled so that it does not jump up too soon. The heat must be allowed to get out from between the fibers. A controlled amount of ammonia is critical for the preparation of a precursor fiber that has the properties for preparing high quality carbon fiber. With too much ammonia present during the step of formation of amidines, it is much more difficult for cross-links between polymer chains to form. Too little ammonia may prevent formation of the amidine group, although theoretically only one amidine group need be formed. This single amidine group can then react with pendant nitrile groups to obtain cyclic structures.

It is preferable that about 1-10 mole % of amidine functionality is formed, based on total amount (in moles) of functional groups which include amidine groups and nitrile groups, by the heating of acrylonitrile fibers. The amount of amidine formation can be determined by the use of infrared spectroscopy. An IR spectrum of the original acrylonitrile fiber shows no carbon-nitrogen double bonds and many carbon-nitrogen triple bonds. An IR spectrum of the preheated acrylonitrile fiber shows a decrease of carbon-nitrogen triple bonds and the presence of carbon-nitrogen double bonds. The presence of carbon-nitrogen double bonds proves the formation of amidine moieties. These amidine moieties are the true initiators in the formation of carbon fibers. The amidine moiety is the first formed structure and the necessary structure for the formation of carbon fibers. However, the amidine structure is present neither in the starting material nor in the final product. Thus controlling the formation of amidine moieties is the key to the preparation of high quality carbon fiber. Optimization of the yield of high quality carbon fiber is the object of the present invention. This optimization is obtained by a method of determining which acrylic fiber starting material produces useful levels of amidines, and thus will ultimately produce high quality carbon fiber.

Any fiber that is to be employed as starting material for the preparation of carbon fiber can be heated in the air for about 5 minutes at a temperature of about 210° C. to 250° C. to obtain a precursor fiber. This preheated precursor fiber can then be analyzed by infrared spectroscopy, employing the attenuated total reflection method, to calculate the ratio of carbon-nitrogen triple bonds to carbon-nitrogen double bonds. A fiber that is suitable for preparing carbon fiber must, after heating, have a reduction in nitrile absorbance of about 15% to about 25%.

When the preheated precursor fiber that contains amidine groups is heated below the melting point of the fiber, the amidine groups begin to react with cyano groups to obtain cross linked fibers and intramolecular cyclic structures. This is the first step in the formation of carbon fiber. It is also a critical step. The fusion point of the fiber must be raised to about 400° C. in order to move to the next step. Various degrees of crystallization are associated with improvement of modulus and tensile strength in the final carbon fiber. This first step governs the physical properties of the final carbon fiber product.

As an alternative to heating a starting material fiber in air, a fiber prepared from an acrylonitrile co-polymer can be heated in an inert atmosphere such as nitrogen. The co-polymer contains a co-monomer that can bind an amino group or ammonium ion. Such a co-monomer can be a carboxylic acid monomer or an anhydride monomer. In a preferred embodiment, the co-monomer is itaconic acid. The acrylontrile co-polymer is first treated with a primary amine, a secondary amine or ammonia to obtain a chemically modified acrylonitrile co-polymer. The chemically-modified co-polymer is then heated in an inert atmosphere for a time of about 5 minutes and at a temperature of about 210° C. to 250° C. to obtain a preheated precursor fiber. Because air causes the degradation of the precursor fiber, it is preferable to follow this alternative process.

The present invention discloses a method of determining superior acrylic fiber for the preparation of carbon fiber. It also includes a method of preparing high quality carbon fiber including the steps of: (1) obtaining an acrylic fiber starting material selected from the group consisting of polyacrylonitrile homopolymer fiber and chemically-modified polyacrylonitrile co-polymer fiber, said co-polymer fiber prepared from acrylonitrile monomer and a co-monomer which is a member selected from the group consisting of unsaturated carboxylic acid and unsaturated carboxylic acid anhydride and which chemically-modified polymer is obtained by treating the copolymer with a nitrogen-containing compound which is a member selected from the group consisting of primary amines, secondary amines and ammonia; (2) heating the acrylic fiber for a time of about 5-10 minutes at a temperature of about 210° C. to 250° C. to obtain a preheated precursor fiber that contains amidine groups and cyano groups; (3) analyzing the heated fiber by means of an infrared spectrophotometer; (4) analyzing the unheated fiber by means of an infrared spectrophotometer; (5) calculating the amount of reduction of nitrile absorbance based on the IR scans of the heated fiber and the unheated fiber; (6) selecting the fiber for preparing carbon fiber when the nitrile absorbance is reduced by about 15% to about 25%; (7) heating the selected precursor fiber in an oxidation zone at a temperature below the fusion temperature of the precursor for a time sufficient to initiate cross-linking reactions between the amidine groups and the pendant cyano groups of the (co)polymer; (8) increasing the heating in subsequent stages, as the fusion temperature of the precursor increases, to a temperature of about 400° C. for a time sufficient to increase the fiber density to about 1.40 grams per cubic centimeter; (9) withdrawing an oxidized precursor from the oxidation zone; (10) passing the oxidized precursor to a carbonization zone; (11) carbonizing the oxidized precursor at a temperature of about 1000° C. to about 2000° C. in an inert atmosphere for a time of about 1 to about 5 minutes; and (12) withdrawing a carbon fiber.

In the alternative embodiment of heating an acrylic polymer in an inert atmosphere, the starting material cannot have over 20% amine. If too much amine or ammonia is employed, the excess amine or ammonia can be harmful to the fiber. The amine or ammonia can be captured in the fiber in any number of ways. The fiber can be treated in a boiling solution of aqueous amine as in dyeing. The fiber can be heated in the presence of an amine, and optionally another liquid. The fiber can be prepared by directly spinning the acrylic composition and the amine in the presence of a common solvent. Some examples of suitable amines are: hydroxyl amines, diethylene triamine and polyethylene imine. The fiber can be impregnated with the amine or ammonia by forming the nitrogen salts of pendant carboxylic acid groups found in the acrylic copolymer, e.g. an acrylonitrile co-polymer of vinyl carboxylic acid or vinyl sulfonic acid. The useful range of amidine initiator in the polymer is about 1.0 to about 10 mole %. The total amount of amidine is calculated. This includes both the amidine formed from the chemical reaction of the primary or secondary amine (or ammonia) with the pendant cyano group of the polyacrylonitrile, and the amidine formed as a result of the thermal degradation of the polyacrylonitrile in air or oxygen. Amines that are useful for preparing the amidine moiety by reaction with the pendant cyano group are as follows: all amines that have at least one hydrogen directly attached to the nitrogen atom. These amines are primary amines, secondary amines, polyamines or polymeric amines.

The amount of amidine present in the fiber must be controlled. One method is to heat the fiber in air for about 10 minutes at a temperature of about 260° C. The second step of the method is to take an IR of the heated fiber. The infrared analysis is conducted using the Attenuated Total Reflection method. The third step of the method is to calculate the ratio of carbon-nitrogen double bonds to carbon-nitrogen triple bonds. The carbon-nitrogen double bonds represent the amidine structure. The carbon-nitrogen triple bonds represent the cyano structure. Fibers that are useful for making superior carbon fiber have a ratio of carbon-nitrogen double bond to carbon-nitrogen triple bond of about 0.1 to about 1.0, as calculated in the IR spectrum.

Prior to the instant invention, all manufacturers of carbon fiber failed to calculate the amount of amidine functionality in the fiber precursor. However, the amount of amidine is one of the most important features in the preparation of high quality carbon fiber. Failure to recognize this parameter can result in the manufacture of poor quality carbon fiber. The present invention prevents this possibility.

In the present process, the precursor fiber is formed by heating an amount of fiber in air or oxygen for a short period of time and at reduced temperature to obtain a fiber containing polymeric chains that contain an amount of amidine functionality. This precursor fiber is then further heated in two independent heating steps. In a first heating step, the precursor fiber is heated in air or in an inert atmosphere at a temperature below the melting point of the fiber. This heating is continued until the fiber density is increased. The final density of the fiber after the heating is about 1.35 g/ml to about 1.40 g/ml. When this density is reached, the fusion point of the fiber is high enough to advance to the next heating step. In this final heating step, the fiber is heated in an inert atmosphere at a temperature of about 500° C. to about 2000° C. for a time of about 10 minutes.

The amidine structure is easily detectable in the early stages of forming the carbon fiber, but it is not present in either the starting material or the final product. However, if one obtains samples of starting materials (the acrylonitrile polymer or copolymer), then the starting material can be heated for a short period of time to obtain a precursor fiber that contains an amount of amidine structure. By placing a small amount of precursor fiber between potassium bromide pellets, and then running an IR, one can calculate the ratio of carbon/nitrogen double bonds to carbon/nitrogen triple bonds. If the ratio falls between 0.1:1.0 and 1:1, then the precursor fiber can make excellent carbon fiber. Within this ratio, the amount of pendant nitrile groups in the starting material is reduced between 25 and 50%.

The infrared spectrum that is obtained from conducting an IR on a precursor fiber can be readily analyzed for the presence of certain functional groups. Frequencies that are characteristic of functional or structural groups are known as group frequencies

Amidine content of a precursor fiber can be readily calculated by the following process: Obtain a starting material polyacrylonitrile fiber (as made) and conduct an IR scan on a small sample of the fiber to obtain a first infrared absorption spectrum. Obtain a precursor fiber by heating an amount of the starting material polyacrylonitrile fiber in air for about 3 minutes at a temperature of about 220° C. Conducting an IR scan on a small sample of the precursor fiber that contains amidine groups to obtain a second infrared spectrum of the precursor fiber. The first IR spectrum contains no absorbance for amidine functional groups, but does contain absorbance for nitrile (or cyano) groups. The second IR spectrum contains absorbance both for amidine functional groups and cyano functional groups. However, the second IR spectrum cannot distinguish between amidine present in cyclic naphthyridine rings on a polymer chain and amidine present as a cross-linker between polymer chains. The infrared spectroscopy is conducted from 700 to 4000 cm.⁻¹ (wave numbers).

FIG. 1 relates to a graph of approximate locations of various group vibrations in the IR spectrum. Both the group frequency region and the fingerprint region are included in the group. The following absorbencies (% transmission) are noted on each IR spectrum: (1) absorbance at 1380 cm⁻¹ for C—H stretching (used to compensate for sample thickness); (2) absorbance at 1650 cm⁻¹ for the amidine moiety; and (3) absorbance at 2245 cm⁻¹ for the nitrile moiety. A crude analysis clearly shows an increased absorbance at 1650 cm⁻¹ when one goes from the first IR spectrum (sample as made) to the second IR spectrum (precursor fiber that has been heated). Similarly, a crude analysis clearly shows a decrease in absorbance at 2245 cm⁻¹ when one goes from the first IR spectrum to the second IR spectrum.

Four sets of graphs relating to infrared absorption spectra of four different polyacrylonitrile fibers are disclosed. In each set, one absorption spectrum relates to a textile fiber as made; i.e., a fiber that can be used as starting material for preparing carbon fiber. This fiber is not heated. A second absorption spectrum relates to a textile fiber that has been heated in air for a specified amount of time.

FIGS. 3 and 4, designated as A and AH, display IR spectra for an unheated fiber that is 1.2 dpf (denier per fiber) and a heated fiber that has the same dpf. The heated fiber is prepared by placing the fiber in an oven at 220° C. for a time of 3 minutes. FIGS. 5 and 6 include graphs that are designated as B and BH. The IR spectrum designated as B relates to an unheated fiber prepared from acrylonitrile and itaconic acid and pretreated with ammonia. Ammonia, which is retained in the fiber by the itaconic acid, is present in the fiber in an amount of about 4060 ppm. The amount of acid in the co-polymer is 5% by weight. The IR spectrum designated as BH refers to a heated fiber prepared from acrylonitrile and itaconic acid. Itaconic acid is present in the co-polymer in an amount of 5% by weight. Ammonia, which is retained in the fiber by the itaconic acid, is present in the fiber in an amount of about 4060 ppm. The fiber is heated at 220° C. for a time of 3 minutes.

FIGS. 7 and 8 are designated as C and CH. The IR spectrum designated as C refers to an unheated fiber prepared from acrylonitrile and itaconic acid. Itaconic acid is present in the polymer in an amount of 5% by weight. Ammonia, which is retained in the fiber by itaconic acid, is present in the fiber in an amount of 260 ppm. The IR spectrum designated as CH relates to the heated fiber of graph C. Thus the heated fiber was prepared from the monomers acrylonitrile and itaconic acid. The fibers are treated with ammonia to provide a polymer containing 260 ppm ammonia.

FIGS. 9 and 10 contain graphs that are designated as D and DH. The IR spectrum designated as D refers to an unheated fiber prepared from acrylonitrile and itaconic acid, the itaconic acid being present in an amount of 5% by weight. The fiber is pretreated with ammonia so that 4790 ppm ammonia is retained in the fiber. The unheated fiber has a thickness of 5 dpf (denier per fiber). The IR spectrum designated as DH refers to a heated fiber prepared from acrylonitrile and itaconic acid. The itaconic acid is present in the amount of 5% by weight. Ammonia, which is retained in the fiber by itaconic acid, is present in the fiber in an amount of 4790 ppm.

FIG. 2 relates to an analysis of the IR spectra for the four fiber samples, both heated and unheated. In the table, fiber samples are grouped as A and AH, B and BH, C and CH, and D and DH (where H stands for “heated”). In each case the % transmission is calculated for C—H stretching at 1380 cm⁻¹ (used to compensate for sample thickness), N—H stretching at 1650 cm⁻¹ (representing the amount of amidine present in the fiber), and —CN bending at 2245 cm⁻¹ (which represents the amount of cyano groups present in the fiber). In a second column, the ratios of amidine groups to CH groups and cyano groups to CH groups are calculated. The difference of the ratios between heated fiber and unheated fiber is also calculated. In comparing an unheated fiber to a heated fiber, the amount of amidine functionality increases upon heating and the amount of nitrile groups decreases upon heating. Amine content of the fiber appears not to be critical to amidine formation. Relatively small amounts of amine, for example, 260 ppm ammonia, form a large amount of amidine. In theory, only one molecule of ammonia (or primary amine, etc.) can cause complete consumption of all pendant nitrile groups in the polymer. The amount of amidine formation is critical to the formation of superior quality carbon fiber. Although it is impossible to calculate the amount of amidine that forms cross-links between polymer chains, without these cross-links a carbon fiber cannot be formed.

The present invention is based on the discovery that the true initiators in the formation of carbon fibers from polyacrylonitrile starting material are amidine moieties. These amidine moieties, which are both pendant from a carbon backbone and also are a crosslink between two different carbon backbones, are formed in various processes. One process is air oxidation of the polyacrylonitrile fiber under suitable conditions of temperature and pressure to begin degradation of the fiber, whereby vaporous amines are generated. These vaporous amines can then penetrate the fiber and react with nitrile groups to obtain amidines. The process can be conducted at atmospheric pressure and at a temperature of about 150° C. and about 250° C. Oxygen can be employed rather than air.

A second process for preparing a polymer containing amidine moieties is heating the polyacrylonitrile starting material with an amine. The process can be performed neat or with a solvent. A single amine or a mixture of amines can be employed. The amine is a member selected from the group consisting of a primary amine and a secondary amine. Ammonia can be employed in place of the amine. The heating can be conducted at atmospheric pressure and at a temperature of about 150° C. to about 250° C.

A third process for preparing the polymer containing amidine moieties comprises the step of boiling an aqueous suspension of polyacrylonitrile fibers and an amine. This process is similar to a dyeing process. Mixtures of amines can also be employed. The amine is a member selected from the group consisting of a primary amine and a secondary amine. Ammonia can be used in place of the amine.

A fourth process for preparing a polymer containing amidine moieties comprises the steps of spinning a mixture of polyacrylonitrile, an amine and a solvent to obtain a fiber. The fiber is then heated in a heating zone at a temperature of about 150° C. to about 250° C.

A fifth process for preparing the polymer that contains amidine moieties comprises the steps of preparing a copolymer from acrylonitrile and a second monomer which is capable of retaining amines, forming a fiber from the copolymer, contacting the copolymer fiber with an amine or ammonia to obtain a copolymer fiber containing salt groups, and heating the fiber under suitable conditions of temperature and pressure to dissociate the salt into a free amine or ammonia whereby the free amine penetrates into the fiber and reacts with pendant nitrile groups to obtain amidine moieties. The heating step can be conducted under atmospheric pressure and at a temperature of about 150° C. to about 250° C. The second monomer is a member selected from the group consisting of vinyl carboxylic acids, allylic carboxylic acids, vinyl sulfonic acids and allylic sulfonic acids. In a preferred embodiment, the second monomer is itaconic acid. The second monomer can be a mixture of monomers, such as a mixture of itaconic acid and acrylic acid in any ratio.

The fifth process is a preferred process because the presence of carboxylic acid groups in the copolymer assists in the prevention of gelation during fiber formation. The presence of carboxylic acid groups in the fiber acts as a metering system during the heating step whereby amines are released in a controlled fashion by thermal dissociation of the salt groups.

In all of the above processes, the heating step is critical in that an exothermic reaction occurs. As in all exothermic reactions, there is a point where the reaction can become uncontrollable and a “runaway reaction” takes place. The heating step must be carefully monitored. This is usually done by incremental increases in temperature over a very long period of time. This preliminary heating step is often called the oxidation step. The fibers obtained after the oxidation step are often called PANOX fibers. PANOX fibers are precursors for making carbon fiber.

Carbon fibers prepared form acrylonitrile polymers and copolymers are produced in a process comprising three steps. A relatively low temperature heat treatment or oxidation step is followed by a carbonization step. The third step is an optional high temperature heat treatment called graphitization.

The first step of oxidative heat treatment that forms PANOX fibers causes a well-oriented ladder polymer structure to be developed under tension. This structure is formed when the initially formed amidines react further with a nitrile group in an intra-molecular reaction to obtain a cyclic structure that contains naphthyridine rings. Other mechanisms for formation of naphthyridine rings are intramolecular cyclization of nitrile groups and reactions of adjacent amidine groups.

The acrylonitrile polymers and copolymers are prepared by any of the known processes in the prior art. Such processes include solvent polymerization, mass polymerization, emulsion polymerization, suspension polymerization, precipitation polymerization and the like. Processes that employ solvents can use either organic-based systems or aqueous-based systems. Organic solvents that can be employed are: dimethylformamide, dimethylacetamide, dimethyl sulfoxide and the like. In a preferred embodiment, an aqueous system is employed. A preferred aqueous system is a mixture of water, nitric acid, zinc chloride and sodium thiocyanate.

The acrylonitrile polymer or copolymer is then spun into a fiber by any of the known spinning processes. Examples of spinning processes are wet spinning, dry-wet spinning and dry spinning. In dry-wet spinning, a polymer or copolymer solution is extruded through a spinning orifice and into an inert gas atmosphere. The extruded material is then added to an aqueous coagulating bath to form coagulated fibers. In a preferred embodiment, a water-swollen acrylonitrile polymer or copolymer fiber is wet spun from an aqueous suspension. The preferred acrylonitrile fiber is a copolymer prepared from acrylonitrile and one or more monomers. The one or more monomers can be selected from the group consisting of acrylic acid, methacrylic acid, itaconic acid and crotonic acid.

The oxidation step of the carbon fiber process is critical to the development of a high strength carbon fiber material. Prior to this step, the polyacrylonitrile fiber is frequently stretched by 100% to 500% at a temperature of about 100 degrees Centigrade. The stretching improves the alignment in the polymer structure and reduces the fiber diameter, as well as increasing the tensile strength and Young's modulus of the final carbon fiber.

In the past, the oxidation step has been conducted for a time of about 1 to about 5 hours. The step is slow and adds significant expense to the overall process. Process temperatures must be maintained below the fusion temperature of the fibers to prevent instantaneous temperature surges within the interior of the fibers. Temperature surges produce bubbles of gaseous products that ruin the physical properties of the carbon fiber. The oxidation step is conducted in an oxidizing atmosphere, usually air, at a temperature of about 150° C. to about 250° C. The reaction is an exothermic one, and a runaway reaction is always possible.

A major advance in conducting the oxidation step to prepare PANOX fibers from acrylonitrile fibers is recorded in U.S. Pat. Nos. 6,054,214 and 5,804,108, both patents issued by the United States Patent and Trademark Office to the present inventor, W. Kenneth Wilkinson. The two patents disclose and claim a process whereby the oxidation step is reduced from 30-90 minutes to about 8-15 minutes. The process comprises the steps of: (a) obtaining an extruded fiber comprising a substantially metal-free, substantially vinyl-sulfonic acid monomer-free polyacrylonitrile copolymer, wherein the copolymer is prepared from acrylonitrile monomer in an amount of about 95% to about 98% based on weight, a vinyl carboxylic acid monomer in an amount sufficient to retain in the copolymer ammonium ion or amine catalyst in an amount of about 1% to about 4% based on molar ratio, and optionally a vinyl carboxylic acid ester monomer in an amount up to about 2% based on weight; (b) adding to the fiber an oxidation catalyst which is a member selected from the group consisting of ammonia and low molecular weight amines; (c) washing, drying and stretching the fiber to form a precursor; (d) removing the precursor to an oxidation zone; (e) heating the precursor at a temperature below the fusion temperature of said precursor for a time sufficient to initiate cross-linking reactions between the ammonium ion or amine catalyst and pendant cyano groups of the copolymer; (f) increasing the heating in subsequent stages, as the fusion temperature of the precursor increases, to a temperature of about 400° C. for a time sufficient to increase the fiber density to about 1.40 grams/cc.; and (g) withdrawing the oxidized precursor from the oxidation zone. In a preferred embodiment, step (f) is conducted for a time of about 8 minutes to about 20 minutes.

U.S. Pat. Nos. 6,054,214 and 5,804,108 are hereby incorporated by reference in their entirety. After the oxidized precursor is withdrawn from the oxidation zone, it is added to a carbonization zone and carbonized at a temperature of about 1000° C. to about 2000° C. in an inert atmosphere for a time of about 1 to about 5 minutes. A high strength carbon fiber is then withdrawn from the carbonization zone.

Referring to the heating or oxidation step, amidines are initially formed and the fibers begin to cross-link. When the fusion point of the fibers is raised to a temperature of about 400° C. and above, the cross-linking of the fibers is adequate for carbonization treatment, which removes all atoms except backbone carbon. Depending on the heat history of the fibers, certain degrees of crystallization give improved modulus and tensile strength.

Rather than depending on the fusion point of the fibers, the oxidation step can be performed until the density of the fibers increases about 15-20%. Fiber density is directly related to the formation of cross-links between separate polymer molecules, the cross-links resulting from formation of amidine units in an intermolecular fashion. When the oxidation or heating step is performed in the presence of an amine or mixture of amines, the amine must have at least one reactive hydrogen atom. The amine can be a hydroxylamine, a polyamine or a polymeric amine.

Amidine formation is a critical and necessary step in the mechanism of formation of carbon fibers from polyacrylonitrile starting material. Amidines can be formed simply by heating acrylonitrile fibers in air. This is because amines are formed during the process of oxidative degradation. Amidine formation occurs both intra-molecularly and inter-molecularly. Intermolecular amidine formation is known as cross-linking, which cross-linking is critical for the carbonization step.

The present invention relates to a precursor PANOX fiber for preparing carbon fiber. The precursor fiber comprises a chemically treated solid acrylonitrile polymer wherein the polymer comprises nitrile and amidine pendant groups as well as cross-links comprising amidine functionality. The molar ratio of amidine groups to nitrile groups is from about 0.1:1 to about 1:1. The chemically treated solid acrylonitrile polymer is the reaction product of solid polyacrylonitrile and a nitrogen-containing compound. The nitrogen-containing compound is in the liquid or gaseous state. Examples of nitrogen-containing compounds are primary amines, secondary amines, ammonia and mixtures thereof. Examples of primary amines are methyl amine, ethyl amine, n-propyl amine, isopropyl amine, ethanol amine and mixtures thereof. Examples of secondary amines are dimethyl amine, diethyl amine, methylethyl amine, di(n-propyl) amine, diisopropylamine, diethanol amine, methyl (n-propyl) amine, ethyl (n-propyl) amine, methyl (isopropyl) amine, ethyl (isopropyl) amine, and mixtures thereof.

The solid acrylonitrile polymer can be a solid acrylonitrile homopolymer or a copolymer of acrylonitrile and a second monomer or monomers. The second monomer or monomers is preferably a monomer that can form an ionic complex with a nitrogen-containing compound such as a primary amine, a secondary amine or ammonia. Examples of this type of monomer are: itaconic acid, acrylic acid, crotonic acid, maleic anhydride, methacrylic acid and the like. Basically, salts of vinyl carboxylic acids are formed. Different rates of chain extension are obtained by employing different amines. For example, diethylamine is more reactive than ammonia. Thus different rates of cross-linking and cyclization are obtained.

The concentration of the nitrogen-containing compound cannot be chosen randomly when adding the nitrogen-containing compound to the acrylonitrile polymer. If the concentration of the amines (or ammonia) is too high, many short sequences of cyclic structures are formed. It is possible that with a high concentration of amine, no cyclic structures at all will be formed, but only the formation of the amidine moiety on every second carbon atom of the carbon backbone. When the amine concentration is too low, no appreciable cross-linking and densification will occur within a reasonable amount of time. Any cross-linking and cyclization that occurs will be too slow to be of economic value.

The present invention relates to a process for preparing a precursor fiber, said precursor fiber being useful in preparing carbon fiber. The process comprises the steps of: (a) preparing a suspension of a solid acrylonitrile polymer in a solvent; (b) adding to the suspension a liquid or gaseous chemical treating agent which is a member selected from the group consisting of a primary organic amine, a secondary organic amine, ammonia and mixtures thereof; (c) spinning the suspension to obtain a fiber; (d) removing the fiber to a heating zone; (e) heating the fiber to obtain a precursor which comprises a chemically treated solid acrylonitrile polymer wherein the treated polymer comprises cyano pendant groups and amidine pendant groups; and (f) withdrawing the precursor fiber from the heating zone to obtain a precursor fiber containing amidine groups in an amount of about 1 mole percent to about 10 mole percent, based on total amount of functional groups.

The solid polyacrylonitrile polymer can be a copolymer prepared from acrylonitrile monomer and a co-monomer selected from the group consisting of itaconic acid, acrylic acid, methacrylic acid, crotonic acid, maleic anhydride, p-vinyl benzoic acid, itaconic anhydride and mixtures thereof. In an alternative embodiment, the solid polyacrylonitrile copolymer can be a terpolymer wherein a third monomer is a member selected from the group consisting of alkyl acrylates having 1-4 carbon atoms in the alkyl group, alkyl methacrylates having 1-4 carbon atoms in the alkyl group, vinyl acetate, vinyl propionate, styrene, vinyl chloride, vinylidene chloride and mixtures thereof.

The chemically treated solid polyacrylonitrile copolymer contains nitrile pendant groups and amidine pendant groups. The amidine pendant groups are present in the copolymer in an amount of about 1-10 mole percent, based on total amount of functional groups in moles.

The present invention also relates to a process for making a precursor fiber for preparing carbon fiber. The process comprises the steps of: (a) placing in a heating zone a fiber comprising an acrylonitrile copolymer comprising at least 90 mole percent acrylonitrile units, based on total amount of functional groups in moles; (b) heating the fiber in air at a temperature of about 150 degrees Centigrade to about 250 degrees Centigrade for a time of about 5 minutes to about 15 minutes; and (c) withdrawing a precursor fiber wherein the precursor fiber contains amidine groups in an amount of about 1-10 mole percent, based on total amount of functional groups in moles.

In an alternative embodiment, the present invention relates to an improvement in a process for preparing thermally stabilized precursor fiber for preparing carbon fiber. The process comprises the steps of treating a fiber comprising an acrylonitrile copolymer having pendant nitrile groups and pendant carboxylate groups, the carboxylate groups being ionically associated with a nitrogen-containing compound wherein the nitrogen-containing compound is a member selected from the group consisting of a primary amine, a secondary amine, ammonia and mixtures thereof; heating the fiber in a heating zone below its melting point to obtain a fiber containing amidine pendant groups; and withdrawing from the heating zone a thermally stabilized precursor fiber. The improvement in the process comprises controlling amidine formation to obtain a precursor fiber having about 1 mole percent to about 10 mole percent amidine groups, based on total amount of functional groups in moles. The amount of amidine groups in the fiber can be controlled by selecting a solid polyacrylonitrile copolymer that contains about 1 mole percent to about 10 mole percent of a moiety selected from the group consisting of carboxylic acids, salts of carboxylic acids and mixtures thereof. Even if this polyacrylonitrile copolymer is not chemically treated with a nitrogen-containing compound, it exhibits enhanced rate of density increase upon heating in air for about 3 minutes or so. Amines produced by oxidation of the fiber (degradation of the nitrile groups) are captured by the carboxylate groups. Amidine groups are then generated in situ during the heating process. Intramolecular cyclization and intermolecular cross-linking are obtained from originally formed amidine groups.

In a further embodiment, the present invention comprises a method of selecting a precursor fiber for preparing a carbon fiber. The method comprises the steps of: (a) obtaining a fiber comprising a chemically treated solid polyacrylonitrile homopolymer or copolymer wherein the chemically treated solid polyacrylonitrile homopolymer or copolymer is the reaction product of solid polyacrylonitrile and a nitrogen-containing compound, the nitrogen-containing compound being in the liquid or gaseous state; (b) removing the fiber to a heating zone; (c) heating the fiber in air at a temperature of about 150 degrees Centigrade to about 250 degrees Centigrade for a time of about 3-10 minutes; (d) withdrawing the heated fiber from the heating zone; (e) cooling the fiber; (f) analyzing the fiber by means of an infrared spectrophotometer, employing the attenuated total reflection method, to obtain an IR spectrum; (g) calculating the ratio of C(triple bond)N to C(double bond)N as represented in the IR spectrum; (h) comparing the ratio to a range of about 0.1:1.0 to about 1.0:1.0; and (i) selecting the fiber for preparing carbon fiber when the calculated ratio is within the range recited in step (h). The nitrogen-containing compound is a member selected from the group consisting of a primary amine, a secondary amine ammonia and mixtures thereof. The primary amine is a member selected from the group consisting of methyl amine, ethyl amine, n-propyl amine, isopropyl amine ethanol amine and mixtures thereof. The secondary amine is a member selected from the group consisting of dimethyl amine, diethyl amine, methylethylamine, di(n-propyl) amine, diisopropylamine, diethanol amine, methyl (n-propyl) amine, ethyl (n-propyl) amine, methyl (isopropyl) amine, ethyl (isopropyl) amine and mixtures thereof. When the polyacrylonitrile is a copolymer, it can be prepared with a co-monomer selected from the group consisting of itaconic acid, acrylic acid, methacrylic acid, crotonic acid, maleic anhydride, p-vinyl benzoic acid, itaconic anhydride and mixtures thereof. The polyacrylonitrile can be a terpolymer wherein a third monomer is a member selected from the group consisting of alkyl acrylates having 1-4 carbon atoms in the alkyl group, alkyl methacrylate groups having 1-4 carbon atoms in the alkyl group, vinyl acetate, vinyl propionate, styrene, vinyl chloride, vinylidene chloride and mixtures thereof.

EXAMPLE 1

A simple test on a sample from a batch of acrylic fibers indicates whether the fibers can be employed to prepare superior carbon fiber. The test is based on the formation and detection of the amidine moiety in the acrylic polymer.

A reference sample is prepared by obtaining an amount of finely chopped acrylic fiber taken from the batch of acrylic fibers, and forming a pellet for use in infrared spectroscopy. The pellet is prepared by obtaining 0.4 grams of potassium bromide and 0.1 grams of the finely chopped acrylic fiber, and forming a blend. The blend is then placed into a mold and pelletized under pressure. The pellet is about 2.5 to 5.0 millimeters thick. An IR spectrum is then obtained, using the pellet (Spectrum A).

A test sample is prepared by obtaining a small amount of acrylic fiber from the same batch of fibers from which the reference sample with circulating air, and heated for a time of about 3 to 5 minutes at a temperature of about 220° C. The sample is then removed from the oven, cooled and chopped. About 0.1 grams of the chopped sample is then blended with 0.4 grams of potassium bromide. The blend is placed into a mold and pelletized under pressure. The pellet is about 2.5 to 5.0 millimeters thick. An IR spectrum is then obtained, using the pellet (Spectrum B).

The spectra are compared by placing Spectrum B over Spectrum A on a light table. If Spectrum B contains a new absorption at or near 1600 cm⁻¹, then the amidine structure has been formed as by heating in the oven, and will act as the primary catalyst in the formation of carbon fiber. The amount of amidine formation can be calculated using standard techniques. The useful range of amidine initiator in the polymer is about 1.0 to about 10.0 mole %.

Numerous modifications and variations of the present invention are possible, and there is not intent to limit the scope of the invention in the description above, except as set forth in the appended claims.

Claims

1. A precursor fiber for preparing carbon fiber comprising a chemically treated solid acrylonitrile polymer wherein the polymer comprises cyano and amidine pendant groups and wherein the amidine groups are present in an amount of about 1-10 mole percent, based on total amount of functional groups.

2. A precursor fiber according to claim 1 wherein the chemically treated solid acrylonitrile polymer is the reaction product of solid polyacrylonitrile and a nitrogen-containing compound, the nitrogen-containing compound being in the liquid or gaseous state.

3. A precursor fiber according to claim 2 wherein the nitrogen-containing compound is a member selected from the group consisting of a primary amine, a secondary amine, ammonia, and mixtures thereof.

4. A precursor fiber according to claim 3 wherein the primary amine is a member selected from the group consisting of methyl amine, ethyl amine, n-propyl amine, isopropyl amine, ethanol amine, and mixtures thereof.

5. A precursor fiber according to claim 3 wherein the secondary amine is a member selected from the group consisting of dimethyl amine, diethyl amine, methylethyl amine, di(n-propyl) amine, diisopropylamine, diethanol amine, methyl (n-propyl) amine, ethyl (n-propyl) amine, methyl (isopropyl) amine, ethyl (isopropyl) amine, and mixtures thereof.

6. A precursor fiber according to claim 2 wherein the solid acrylonitrile polymer is a member selected from the group consisting of a solid polyacrylonitrile homopolymer and a solid polyacrylonitrile copolymer.

7. A precursor fiber according to claim 6 wherein the solid polyacrylonitrile copolymer is prepared from acrylonitrile monomer and a co-monomer selected from the group consisting of itaconic acid, acrylic acid, methacrylic acid, crotonic acid, maleic anhydride, p-vinyl benzoic acid, itaconic anhydride, and mixtures thereof.

8. A precursor fiber according to claim 7 wherein the solid polyacrylonitrile copolymer is a terpolymer where a third monomer is a member selected from the group consisting of alkyl acrylates having 1-4 carbon atoms in the alkyl group, alkyl methacrylates having 1-4 carbon atoms in the alkyl group, vinyl acetate, vinyl propinate, styrene, vinyl chloride, vinylidene chloride, and mixtures thereof.

9. A process of making a precursor fiber for preparing carbon fiber comprising:

(a) preparing a suspension of a solid acrylonitrile polymer in a solvent;

(b) adding to the suspension a liquid or gaseous chemical treating agent which is a member selected from the group consisting of a primary organic amine, a secondary organic amine, ammonia and mixtures thereof;

(c) spinning the suspension to obtain a fiber;

(d) heating the fiber to obtain a precursor which comprises a chemically treated solid acrylonitrile polymer wherein the treated polymer comprises cyano and amidine pendant groups; and

(e) separating the precursor fiber from the suspension whereby the precursor fiber contains amidine groups in an amount of about 1-10 mole percent, based on total amount of functional groups.

10. A process according to claim 9 wherein the solid acrylonitrile polymer is a member selected from the group consisting of an acrylonitrile homopolymer and an acrylonitrile copolymer.

11. A process according to claim 11 wherein the acrylonitrile copolymer is prepared

from acrylonitrile monomer and a co-monomer selected from the group consisting of itaconic acid, acrylic acid, methacrylic acid, crotonic acid, maleic anhydride, p-vinyl benzoic acid, itaconic anhydride, and mixtures thereof.

12. A process for making a precursor fiber for preparing carbon fiber comprising:

(a) placing in a heating zone a fiber comprising an acrylonitrile copolymer comprising at least 90 mole percent acrylonitrile units, and about 1 mole percent to about 10 mole percent nitrogen-containing compound as neutralizing cation for carboxylate groups, said compound incorporated into the polymer by copolymerization of one or more copolymerizable, carboxylate containing co-monomers;

(b) heating the fiber in air at a temperature of about 200° C. to about 250° C. for a time of about 3 minutes to about 15 minutes; and

(c) withdrawing a precursor fiber wherein the precursor fiber contains amidine groups in an amount of about 1-10 mole percent, based on total amount of functional groups.

13. A method of selecting a precursor fiber for preparing a carbon fiber, the method comprising the steps of:

(a) obtaining a fiber comprising a chemically treated solid acrylonitrile polymer which is the reaction product of solid polyacrylonitrile and a nitrogen-containing compound, the nitrogen-containing compound being in the liquid or gaseous state,

(b) analyzing the fiber by means of an infrared spectrophotometer;

(c) removing the fiber to a heating zone;

(d) heating the fiber in air at a temperature of about 250° C. to about 275° C. for a time of about 3-10 minutes;

(e) withdrawing the fiber from the heating zone;

(f) cooling the fiber;

(g) analyzing the cooled fiber by means of an infrared spectrophotometer;

(h) calculating the amount of reduction of nitrile absorbance based on the IR scans of the original unheated fiber and the heated fiber; and

(i) selecting the fiber for preparing carbon fiber when the nitrile absorbance is reduced by about 15% to about 25%.

14. A method according to claim 13 wherein the nitrogen-containing compound is a member selected from the group consisting of a primary amine, a secondary amine, ammonia and mixtures thereof.

15. A method according to claim 14 wherein the primary amine is a member selected from the group consisting of methyl amine, ethyl amine, n-propyl amine, isopropyl amine, ethanol amine, and mixtures thereof.

16. A method according to claim 13 wherein the secondary amine is a member selected from the group consisting of dimethyl amine, diethyl amine, methylethyl amine, di(n-propyl) amine, diisopropylamine, diethanol amine, methyl (n-propyl) amine, ethyl (n-propyl) amine, methyl (isopropyl) amine, ethyl (isopropyl) amine, and mixtures thereof.

17. A method according to claim 13 wherein the solid acrylonitrile polymer is a member selected from the group consisting of a solid polyacrylonitrile homopolymer and a solid polyacrylonitrile copolymer.

18. A method according to claim 17 wherein the polyacrylonitrile copolymer is prepared from acrylonitrile monomer and a co-monomer selected from the group consisting of itaconic acid, acrylic acid, methacrylic acid, crotonic acid, maleic anhydride, p-vinyl benzoic acid, itaconic anhydride, and mixtures thereof.

19. A method according to claim 18 wherein the polyacrylonitrile copolymer is a terpolymer where a third monomer is a member selected from the group consisting of alkyl acrylates having 1-4 carbon atoms in the alkyl group, alkyl methacrylates having 1-4 carbon atoms in the alkyl group, vinyl acetate, vinyl propinate, styrene, vinyl chloride, vinylidene chloride, and mixtures thereof.