U.S. patent application number 11/311246 was filed with the patent office on 2006-06-22 for amidines as initiators for converting acrylic fibers into carbon fibers.
Invention is credited to Kenneth Wilkinson.
Application Number | 20060134413 11/311246 |
Document ID | / |
Family ID | 36596228 |
Filed Date | 2006-06-22 |
United States Patent
Application |
20060134413 |
Kind Code |
A1 |
Wilkinson; Kenneth |
June 22, 2006 |
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.
Inventors: |
Wilkinson; Kenneth;
(Waynesboro, VA) |
Correspondence
Address: |
Leander F. Aulisio
#705
320 23rd Street
Arlington
VA
22202
US
|
Family ID: |
36596228 |
Appl. No.: |
11/311246 |
Filed: |
December 20, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60636887 |
Dec 20, 2004 |
|
|
|
Current U.S.
Class: |
428/364 |
Current CPC
Class: |
D01F 6/18 20130101; D01F
6/38 20130101; D01D 10/02 20130101; Y10T 428/2913 20150115; D01F
6/54 20130101 |
Class at
Publication: |
428/364 |
International
Class: |
D02G 3/00 20060101
D02G003/00 |
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.degree. C. to about 250.degree. 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.degree. C. to about 275.degree. 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.
Description
[0001] The present non-provisional application is based on
provisional application no. 60/636887 filed on Dec. 20, 2004.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.degree. 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.
[0007] 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.degree. 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.degree. C.
[0008] When acrylonitrile polymeric fiber is heated in air at a
temperature of about 250.degree. 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.degree. C. The density of the fiber is also
increased with increased cross-linking.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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.degree. C.
to 130.degree. C., an oxidizing agent at 80.degree. C. to
120.degree. C. and heating the fibers in the presence of oxygen
until the fibers are capable of undergoing carbonization.
[0013] 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.degree.
C.-260.degree. C. The process is continued until the precursor
fibers have a density of at least 1.40 g/cm.sup.3.
[0014] 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.degree. C. to
350.degree. C. in the presence of an oxidizer, preferably oxygen,
and ammonia.
[0015] None of the above-referenced patents, whether taken
individually or in combination, anticipate the present invention as
disclosed and claimed.
SUMMARY OF THE INVENTION
[0016] 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.
[0017] 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.
[0018] 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.
[0019] 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.
[0020] 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
[0021] 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.
[0022] FIG. 1 is a graph of the approximate locations of various
group vibrations in the IR spectrum.
[0023] 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).
[0024] FIG. 3 is a graph of an infrared absorption spectrum of
textile fiber A (polyacrylonitrile, untreated) prior to a heating
step.
[0025] FIG. 4 is a graph of an infrared absorption spectrum of
textile fiber AH (heated sample A).
[0026] 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).
[0027] FIG. 6 is a graph of an infrared absorption spectrum of
textile fiber BH (heated sample B).
[0028] 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).
[0029] FIG. 8 is a graph of an infrared absorption spectrum of
textile fiber CH (heated sample C).
[0030] 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).
[0031] FIG. 10 is a graph of an infrared absorption spectrum of
textile fiber DH (heated sample D).
DETAILED DESCRIPTION OF THE INVENTION
[0032] 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.
[0033] 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.
[0034] 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.
[0035] 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.degree. C. to 250.degree.
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%.
[0036] 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.degree. 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.
[0037] 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.degree. C. to 250.degree. C. to
obtain a preheated precursor fiber. Because air causes the
degradation of the precursor fiber, it is preferable to follow this
alternative process.
[0038] 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.degree. C.
to 250.degree. 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.degree. 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.degree. C. to about 2000.degree. C. in an
inert atmosphere for a time of about 1 to about 5 minutes; and (12)
withdrawing a carbon fiber.
[0039] 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.
[0040] 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.degree. 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.
[0041] 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.
[0042] 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.degree. C. to about 2000.degree. C. for a time of about 10
minutes.
[0043] 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%.
[0044] 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
[0045] 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.degree. 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..sup.-1 (wave numbers).
[0046] 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.sup.-1 for C--H
stretching (used to compensate for sample thickness); (2)
absorbance at 1650 cm.sup.-1 for the amidine moiety; and (3)
absorbance at 2245 cm.sup.-1 for the nitrile moiety. A crude
analysis clearly shows an increased absorbance at 1650 cm.sup.-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.sup.-1 when one goes from the first IR spectrum to the
second IR spectrum.
[0047] 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.
[0048] 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.degree. 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.degree. C. for
a time of 3 minutes.
[0049] 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.
[0050] 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.
[0051] 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.sup.-1 (used to
compensate for sample thickness), N--H stretching at 1650 cm.sup.-1
(representing the amount of amidine present in the fiber), and --CN
bending at 2245 cm.sup.-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.
[0052] 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.degree. C. and about 250.degree. C. Oxygen can be
employed rather than air.
[0053] 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.degree. C. to about 250.degree. C.
[0054] 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.
[0055] 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.degree. C. to about 250.degree. C.
[0056] 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.degree. C. to about
250.degree. 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.
[0057] 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.
[0058] 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.
[0059] 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.
[0060] 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.
[0061] 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.
[0062] 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.
[0063] 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.
[0064] 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.degree. C. to about 250.degree. C. The
reaction is an exothermic one, and a runaway reaction is always
possible.
[0065] 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.degree. 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.
[0066] 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.degree. C. to about 2000.degree. 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.
[0067] 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.degree. 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.
[0068] 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.
[0069] 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.
[0070] 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.
[0071] 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.
[0072] 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.
[0073] 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.
[0074] 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.
[0075] 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.
[0076] 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.
[0077] 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.
[0078] 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
[0079] 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.
[0080] 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).
[0081] 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.degree. 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).
[0082] 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.sup.-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 %.
[0083] 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.
* * * * *