U.S. patent application number 16/972068 was filed with the patent office on 2021-04-22 for carbon fiber and method of producing same.
The applicant listed for this patent is Toray Industries, Inc.. Invention is credited to Naohiro Matsumoto, Haruki Okuda, Fumihiko Tanaka, Jun Watanabe.
Application Number | 20210115597 16/972068 |
Document ID | / |
Family ID | 1000005324345 |
Filed Date | 2021-04-22 |
United States Patent
Application |
20210115597 |
Kind Code |
A1 |
Okuda; Haruki ; et
al. |
April 22, 2021 |
CARBON FIBER AND METHOD OF PRODUCING SAME
Abstract
A carbon fiber having a strand elastic modulus of 360 GPa or
more, a strand strength of 3.5 GPa or more, and a single-fiber
diameter of 6.0 .mu.m or more, and having a residual twist count of
2 turns/m or more in a test in which one end is fixed end and
another end is free end which is capable of rotation about the axis
of a fiber bundle.
Inventors: |
Okuda; Haruki; (Iyo-gun,
JP) ; Watanabe; Jun; (Iyo-gun, JP) ;
Matsumoto; Naohiro; (Iyo-gun, JP) ; Tanaka;
Fumihiko; (Iyo-gun, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Toray Industries, Inc. |
Tokyo |
|
JP |
|
|
Family ID: |
1000005324345 |
Appl. No.: |
16/972068 |
Filed: |
June 17, 2019 |
PCT Filed: |
June 17, 2019 |
PCT NO: |
PCT/JP2019/023851 |
371 Date: |
December 4, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
D01F 9/22 20130101; D01F
6/18 20130101 |
International
Class: |
D01F 9/22 20060101
D01F009/22; D01F 6/18 20060101 D01F006/18 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 18, 2018 |
JP |
2018-115112 |
Jun 18, 2018 |
JP |
2018-115113 |
Aug 30, 2018 |
JP |
2018-161056 |
Claims
1-22. (canceled)
23. A carbon fiber having a strand elastic modulus of 360 GPa or
more, a strand strength of 3.5 GPa or more, and a single-fiber
diameter of 6.0 .mu.m or more, and having a residual twist count of
2 turns/m or more in a test in which one end is fixed end and
another end is free end which is capable of rotation about the axis
of a fiber bundle.
24. The carbon fiber as set forth in claim 23, meeting the
relationship represented by formula (1), wherein Es (GPa) is the
single-fiber elastic modulus and A (N) is the loop fracture load:
A.gtoreq.-0.0017.times.Es+1.02 (1).
25. The carbon fiber as set forth in claim 23, having a
single-fiber diameter of 6.0 .mu.m or more, satisfying the
relationship represented by formula (2) wherein E (GPa) is the
strand elastic modulus and B (MPa) is the knot strength determined
under conditions where the heat loss rate is 0.15% or less at
450.degree. C., and having a twist count of 20 to 80 turns/m:
B.gtoreq.6.7.times.10.sup.9.times.E.sup.-2.85 (2).
26. The carbon fiber as set forth in claim 23, wherein the total
fineness is 850 g/km.
27. The carbon fiber as set forth in claim 23, wherein the strand
elastic modulus is 440 GPa or more.
28. The carbon fiber as set forth in claim 23, wherein the twist
angle of the carbon fiber bundle surface layer is 2.0.degree. to
30.5.degree..
29. The carbon fiber as set forth in claim 28, wherein the twist
angle of the carbon fiber bundle surface layer is 4.8.degree. to
10.0.degree..
30. The carbon fiber as set forth in claim 23, wherein the
single-fiber diameter is 6.5 .mu.m or more.
31. The carbon fiber as set forth in claim 23, wherein single-fiber
diameter is 7.4 .mu.m or less.
32. The carbon fiber as set forth in claim 23, wherein the
crystallite size Lc (nm) and the orientation parameter of
crystallites .pi..sub.002(%) satisfy the relationship represented
by formula (3): .pi..sub.002.gtoreq.4.0.times.Lc+73.2 (3).
33. The carbon fiber as set forth in claim 23, wherein the
crystallite size Lc is 2.2 to 3.5 nm.
34. The carbon fiber as set forth in claim 23, wherein the strand
elastic modulus E (GPa) and the crystallite size Lc (nm) satisfy
the relationship represented by formula (4):
E.times.Lc.sup.-0.5.gtoreq.200 (GPa/nm.sup.0.5) (4).
35. The carbon fiber as set forth in claim 23, wherein the surface
oxygen concentration O/C is 0.05 to 0.50.
36. The carbon fiber bundle as set forth in claim 23, wherein the
filament number is 10,000 or more.
37. A carbon fiber meeting the relationship represented by formula
(1) wherein Es (GPa) is the single-fiber elastic modulus and A (N)
is the loop fracture load: A.gtoreq.-0.0017.times.Es+1.02 (1).
38. A carbon fiber having a single-fiber diameter of 6.0 .mu.m or
more, satisfying the relationship represented by formula (2)
wherein E (GPa) is the strand elastic modulus and B (MPa) is the
knot strength determined under conditions where the heat loss rate
is 0.15% or less at 450.degree. C., and having a twist count of 5
to 80 turns/m: B.gtoreq.6.7.times.10.sup.9.times.E.sup.-2.81
(2).
39. The carbon fiber as set forth in claim 37, wherein either the
single-fiber elastic modulus or the strand elastic modulus is 360
GPa or more.
40. A method of producing a carbon fiber comprising: a step in
which a precursor fiber bundle for carbon fiber is subjected to
stabilization (oxidation) treatment in an air atmosphere in the
temperature range of 200.degree. C. to 300.degree. C.; a step of
pre-carbonization in which the resulting stabilized fiber (oxidized
fiber) bundle is heat-treated in an inert atmosphere at or below a
maximum temperature of 500.degree. C. to 1,000.degree. C. until the
density reaches 1.5 to 1.8 g/cm.sup.3; and a step of carbonization
in which the resulting pre-carbonized fiber bundle is heat-treated
in an inert atmosphere, the precursor fiber bundle for carbon fiber
having a single-fiber fineness of 0.9 dtex or more, having a
tension controlled at 5 mN/dtex or more during the carbonization
treatment, meeting either (c) or (d), and having a strand elastic
modulus of 360 GPa or more: (c) the fiber bundle to be subjected to
the carbonization treatment has a twist count of 2 turns/m or more,
and (d) the total fineness, which is the product of the single
fiber fineness (g/km) and the filament number (number), of the
resulting carbon fiber is 740 g/km or more.
41. The method as set forth in claim 40, wherein the fiber bundle
to be subjected to carbonization treatment has a twist count of 16
turns/m or more.
42. The method as set forth in claim 40, wherein the maximum
temperature in the carbonization treatment step is 1,500.degree. C.
or more.
43. The method as set forth in claim 42, wherein the maximum
temperature in the carbonization treatment step is 2,300.degree. C.
or more.
44. The method as set forth in claim 40, wherein electrolytic
surface treatment with an amount of current of 2 to 100 c/g is
performed after the carbonization treatment step.
45. A carbon fiber having a strand elastic modulus of 360 GPa or
more, a strand strength of 3.5 GPa or more, a single-fiber diameter
of 6.0 .mu.m or more, and a total fineness, which is a product of
the single-fiber fineness (g/km) and the filament number (number)
of the carbon fiber, of 740 g/km or more, and meeting the
relationship represented by formula (1), wherein Es (Gpa) is the
single-fiber elastic modulus and A (N) is the loop fracture load:
A.gtoreq.-0.0017.times.Es+1.02 (1).
46. A carbon fiber having a strand elastic modulus of 360 GPa or
more, a strand strength of 3.5 GPa or more, a single-fiber diameter
of 6.0 .mu.m or more, and a total fineness, which is a product of
the single-fiber fineness (g/km) and the filament number (number)
of the carbon fiber, of 740 g/km or more, satisfying the
relationship represented by formula (2) wherein E (GPa) is the
strand elastic modulus and B (MPa) is the knot strength determined
under conditions where the heat loss rate is 0.15% or less at
450.degree. C., and having a twist count of 20 to 80 turns/m:
B.gtoreq.6.7.times.10.sup.9.times.E.sup.-2.85 (2).
47. A carbon fiber having a strand elastic modulus of 360 GPa or
more, a strand strength of 3.5 GPa or more, a single-fiber diameter
of 6.0 .mu.m or more, and a total fineness, which is a product of
the single-fiber fineness (g/km) and the filament number (number)
of the carbon fiber, of 740 g/km or more, wherein the twist angle
of the carbon fiber bundle surface layer is 2.0.degree. to
30.5.degree..
48. A carbon fiber having a strand elastic modulus of 360 GPa or
more, a strand strength of 3.5 GPa or more, a single-fiber diameter
of 6.0 .mu.m or more, and a total fineness, which is a product of
the single-fiber fineness (g/km) and the filament number (number)
of the carbon fiber, of 740 g/km or more, wherein the twist angle
of the carbon fiber bundle surface layer is 4.8.degree. to
10.0.degree..
49. A carbon fiber having a strand elastic modulus of 360 GPa or
more, a strand strength of 3.5 GPa or more, a single-fiber diameter
of 6.0 .mu.m or more, and a total fineness, which is a product of
the single-fiber fineness (g/km) and the filament number (number)
of the carbon fiber, of 740 g/km or more, wherein the crystallite
size Lc (nm) and the orientation parameter of crystallites
.pi..sub.002(%) satisfy the relationship represented by formula
(3): .pi..sub.002.gtoreq.4.0.times.Lc+73.2 (3).
50. A carbon fiber having a strand elastic modulus of 360 GPa or
more, a strand strength of 3.5 GPa or more, a single-fiber diameter
of 6.0 .mu.m or more, and a total fineness, which is a product of
the single-fiber fineness (g/km) and the filament number (number)
of the carbon fiber, of 740 g/km or more, wherein the crystallite
size Lc is 2.2 to 3.5 nm.
51. A carbon fiber having a strand elastic modulus of 360 GPa or
more, a strand strength of 3.5 GPa or more, a single-fiber diameter
of 6.0 .mu.m or more, and a total fineness, which is a product of
the single-fiber fineness (g/km) and the filament number (number)
of the carbon fiber, of 740 g/km or more, wherein the strand
elastic modulus E (GPa) and the crystallite size Lc (nm) satisfy
the relationship represented by formula (4):
E.times.Lc.sup.-0.5.gtoreq.200 (GPa/nm.sup.0.5) (4).
Description
TECHNICAL FIELD
[0001] This disclosure relates to carbon fiber and a method for the
production thereof.
BACKGROUND
[0002] Carbon fibers have high specific strength and specific
modulus and can serve as reinforcing fibers for carbon fiber
reinforced composites to realize large weight reduction in
producing members and, accordingly, they are used in a wide range
of fields as essential material in constructing a highly
energy-efficient society. Recently, they have been in wider use in
such fields as automobiles and electronic equipment housing where
there is a strong call for cost reduction, and manufacturers are
now strongly required to realize a reduction in cost for final
member production including the molding cost.
[0003] To effectively reduce the cost of final members, it is
important not only to achieve a cost reduction for the carbon fiber
itself, but also to take a comprehensive approach including
creation of carbon fibers showing enhanced performance to ensure a
decreased consumption and development of molding methods with
increased efficiency, thereby realizing a reduction in the overall
molding cost.
[0004] However, when trying to decrease the consumption of carbon
fiber while maintaining a required stiffness as one of the
important characteristics of the final member, for example, simple
application of an existing high modulus carbon fiber often fails in
realizing a reduction in the cost of the final member production.
This is because existing high modulus carbon fibers are low in
productivity and tend to be expensive and also because they are low
in moldability and require large total processing costs to provide
final members. The moldability of the carbon fiber depends on
features related to handleability and processability in various
steps performed to provide the final member including, for example,
the handleability of fiber bundles and degree of fuzz generation,
as well as the easiness of piecing yarns for the replacement of
carbon fiber bobbins in continuous production of a carbon fiber
reinforced composite.
[0005] In recent years, furthermore, carbon fibers are in wider use
in the form of discontinuous fibers, particularly when an emphasis
is placed on reducing costs. In general, when carbon fibers are
used as discontinuous fibers, the lengths of the carbon fibers tend
to be shortened as they are sheared and folded in the molding
process. This tendency is noticeable in existing high modulus
carbon fibers and adoption of carbon fibers with high tensile
modulus does not always serve effectively to provide high-stiffness
final members.
[0006] In polyacrylonitrile based carbon fiber, which is the most
widely used carbon fiber, the industrial production process
includes a stabilization step of converting precursor fibers for
carbon fiber into stabilized fibers in an oxidizing atmosphere at
200.degree. C. to 300.degree. C. and a carbonization step of
carbonizing them in an inert atmosphere of 300.degree. C. to
2,000.degree. C. In polyacrylonitrile based high modulus carbon
fiber, the industrial production process includes a graphitization
step of further graphitizing them in an inert atmosphere at or
below a maximum temperature of 3,000.degree. C. Such a
graphitization step can effectively increase the tensile modulus of
the carbon fibers, but on the other hand, it tends to require
equipment that is resistant to high temperatures or lead to final
carbon fibers having poor tensile strength or compressive strength
as a result of accelerated crystal growth in the carbon fibers.
Thus, when such a high modulus carbon fiber material is employed,
the productivity of carbon fibers in the aforementioned forms will
be low and the moldability in producing carbon fiber reinforced
composites will be also low. And their fiber lengths will be
decreased when they are used in the form of discontinuous
fibers.
[0007] Several methods have also been proposed to increase the
tensile modulus of carbon fibers by techniques other than
graphitization. As one of them, a method of developing a high
tension in a carbon fiber production process has been proposed.
[0008] International Publication WO 2008/047745 and Japanese
Unexamined Patent Publication (Kokai) No. 2009-256833 propose
techniques in which the molecular weight of a polyacrylonitrile
copolymer is controlled so that fuzz generation is suppressed even
if a high tension is developed in the carbonization step.
[0009] International Publication WO 2008/063886 proposes a
technique in which the strand elastic modulus is increased by
performing high-degree stretching in the stabilization step and the
pre-carbonization step.
[0010] Furthermore, precursor fiber bundles for carbon fiber are
entangled in the techniques proposed in Japanese Unexamined Patent
Publication (Kokai) No. 2001-49536, Japanese Unexamined Patent
Publication (Kokai) No. HEI-10-195718, Japanese Unexamined Patent
Publication (Kokai) No. 2000-160436 and Japanese Unexamined Patent
(Kokai) No. SHO-47-026964 or twisted in those proposed in Japanese
Unexamined Patent Publication (Kokai) No. SHO-56-091015 and
Japanese Unexamined Patent Publication (Kokai) No. 2002-001725 to
realize an increased processability in the carbonization step.
[0011] Japanese Unexamined Patent Publication (Kokai) No.
2014-141761 proposes a technique in which the gauge length
dependency of pre-carbonized fiber bundles is controlled by
entanglement or twisting during carbonization under high tension so
that deterioration in the adhesion between carbon fibers and matrix
is prevented while ensuring an increased strand elastic modulus of
the carbon fiber to be obtained.
[0012] International Publication WO 2013/157613 proposes a high
moldability carbon fiber having a high knot strength in spite of a
large single-fiber fineness, which is produced by controlling the
copolymerization composition of precursor fiber bundles for carbon
fiber.
[0013] In addition, International Publication WO 2013/157612
proposes a carbon fiber that suffers little deterioration in
mechanical properties in spite of a large single fiber diameter,
which is produced in a similar way.
[0014] Such conventional techniques, however, have problems as
described below.
[0015] In WO '745 and JP '833, the molecular weight of the
polyacrylonitrile copolymer is controlled, but does not
significantly improve the critical orientation tension in the
carbonization step, and a large increase in the strand elastic
modulus cannot be expected.
[0016] In WO '886, although the stretching ratio is high in and
before the pre-carbonization step, the stretching ratio is low in
the carbonization step, which should serve for easy improvement in
the strand elastic modulus of carbon fiber, and accordingly a large
increase in the strand elastic modulus cannot be expected.
[0017] JP '536, JP '718, JP '436, JP '964, JP '015 and JP '725 pay
no attention to increasing the stretching ratio in the
carbonization step and describe no idea of focusing on it.
[0018] JP '761 discloses that the strand elastic modulus, adhesion
to the matrix, and strand strength can be simultaneously maintained
at high levels, and that a high processability is achieved in the
carbonization step. However, no attention is paid to the
moldability in producing a carbon fiber reinforced composite or the
fiber breakage that may occur during use in the form of
discontinuous fibers, and there is no description about the idea of
focusing on them.
[0019] WO '613 and WO '612 pay no particular attention to the
stretching ratio in the carbonization step and, in some examples,
the strand elastic modulus is increased to 343 GPa maximum by
raising the carbonization temperature. Although not described, in
the conventional approach of raising the carbonization temperature,
the moldability in producing carbon fiber reinforced composites
tends to be low as in commercial carbon fibers of high modulus
grades. Furthermore, no attention is paid to the fiber breakage
that may occur during use in the form of discontinuous fibers, and
there is no description about the idea of focusing on it.
[0020] In summary, the conventional techniques contain no
description of a method by which the tensile modulus and
moldability of carbon fibers and, in addition, the ease of
maintaining required fiber lengths during use as discontinuous
fibers are simultaneously maintained at high levels, but
simultaneous realization of them at high levels has been an
important issue in achieving a reduction in the total cost of final
member production.
SUMMARY
[0021] We thus provide:
1) A carbon fiber having a strand elastic modulus of 360 GPa or
more, a strand strength of 3.5 GPa or more, and a single-fiber
diameter of 6.0 .mu.m or more, and further satisfying either (a) or
(b): (a) when one end is fixed end and the other end being free end
which is capable of rotation about the axis of the fiber bundle,
the residual twist count is 2 turns/m or more, and (b) the total
fineness, which is the product of the single-fiber fineness (g/km)
and the filament number (number) of the carbon fiber, is 740 g/km
or more. 2) A carbon fiber meeting formula (1), wherein Es (GPa) is
the single-fiber elastic modulus and A (N) is the loop fracture
load:
A.gtoreq.-0.0017.times.Es+1.02 (1).
3) A carbon fiber having a single-fiber diameter of 6.0 .mu.m or
more, satisfying formula (2) wherein E (GPa) is the strand elastic
modulus and B (MPa) is the knot strength determined under
conditions where the heat loss rate is 0.15% or less at 450.degree.
C., and having a twist count of 5 to 80 turns/m:
B.gtoreq.6.7.times.10.sup.9.times.E.sup.-2.85 (2).
4) The method of producing the carbon fiber, furthermore, includes
a step in which a precursor fiber bundle for carbon fiber is
subjected to stabilization treatment in an air atmosphere in the
temperature range of 200.degree. C. to 300.degree. C., a step for
pre-carbonization in which the resulting stabilized fiber bundle is
heat-treated in an inert atmosphere at or below a maximum
temperature of 500.degree. C. to 1,000.degree. C. until the density
reaches 1.5 to 1.8 g/cm 3, and a step for carbonization in which
the resulting pre-carbonized fiber bundle is heat-treated in an
inert atmosphere, the precursor fiber bundle for carbon fiber
having a single-fiber fineness of 0.9 dtex or more, having a
tension controlled at 5 mN/dtex or more during the carbonization
treatment, and meeting either (c) or (d): (c) the fiber bundle to
be subjected to the carbonization treatment has a twist count of 2
turns/m or more, and (d) the total fineness, which is the product
of the single-fiber fineness (g/km) and the filament number
(number), of the resulting carbon fiber is 740 g/km or more.
[0022] The carbon fiber is a carbon fiber that has both a high
tensile modulus and a high moldability in composite production and
easily maintains a required fiber length even when used in the form
of discontinuous fibers. The carbon fiber effectively reduces the
required consumption of carbon fibers, increasing the productivity
in composite production, and producing a composite having improved
mechanical properties.
DETAILED DESCRIPTION
[0023] Both a single fiber and an aggregate thereof of carbon fiber
are simply referred to as carbon fiber. Examples of such an
aggregate of single fibers of carbon fiber include bundles, webs,
composites thereof, and others in various forms. A method of
producing the carbon fiber will be described later.
[0024] The term "tensile modulus" collectively refers to the
single-fiber elastic modulus determined by single-fiber tensile
test of carbon fiber and the strand elastic modulus determined by
the method described later. The relation between the single-fiber
elastic modulus and the strand elastic modulus will be described
later.
[0025] The first example of the carbon fiber provides a carbon
fiber having a strand elastic modulus of 360 GPa or more, a strand
strength of 3.5 GPa or more, and a single-fiber diameter of 6.0
.mu.m or more, and further satisfying either (a) or (b). It is
preferable to satisfy both (a) and (b).
(a) When one end is fixed end and the other end being free end
which is capable of rotation about the axis of the fiber bundle,
the residual twist count is 2 turns/m or more. (b) The total
fineness, which is the product of the single-fiber fineness (g/km)
and the filament number (number) of the carbon fiber, is 740 g/km
or more. Each will be described below.
[0026] The strand elastic modulus is 360 GPa or more. The strand
elastic modulus is preferably 370 GPa or more, more preferably 380
GPa or more, still more preferably 400 GPa or more, and still more
preferably 440 GPa or more. A higher strand elastic modulus allows
the carbon fiber to more effectively increase the stiffness of the
resulting carbon fiber-reinforced composite, making it easier to
obtain a high-stiffness carbon fiber-reinforced composite. If the
strand elastic modulus is 360 GPa or more, it has a large
industrial value because it produces a carbon fiber-reinforced
composite having a greatly increased stiffness. From the viewpoint
of producing a high-stiffness carbon fiber-reinforced composite,
the strand elastic modulus of carbon fiber is preferably as high as
possible, but in conventional carbon fibers, an excessively high
strand elastic modulus tends to lead to a decreased moldability in
producing carbon fiber-reinforced composites or a decreased fiber
length during use in the form of discontinuous fibers. The strand
elastic modulus can be determined according to the tensile test of
resin-impregnated strands specified in JIS R7608 (2004). An
evaluation method for the strand elastic modulus will be described
later. There are various known methods that control the strand
elastic modulus, but it is preferable to control it by changing the
tension in the carbonization treatment step.
[0027] The strand strength is 3.5 GPa or more. The strand strength
is preferably of 3.7 GPa or more, more preferably 3.9 GPa or more,
and still more preferably 4.3 GPa or more. In general, a higher
strand strength tends to serve for producing a carbon
fiber-reinforced composite having a higher tensile strength, thus
leading to a high-performance fiber reinforced composite. A carbon
fiber having an extremely low strand strength may lead to a
decrease in moldability in producing a carbon fiber-reinforced
composite, but many times, serious problems will not occur if it is
3.5 GPa or more. The strand strength can be determined according to
the tensile test of resin-impregnated strands specified in JIS
R7608 (2004). An evaluation method for the strand strength will be
described later. There are various known methods that control the
strand strength, but when the conventional method of raising the
carbonization temperature is used, the strand strength often
decreases with an increasing strand elastic modulus. A carbon fiber
having a strand strength of 3.5 GPA or more in spite of a high
strand elastic modulus can be produced by our method of producing
the carbon fiber, which will be described later.
[0028] The single-fiber diameter is 6.0 .mu.m or more. The
single-fiber diameter is preferably 6.5 .mu.m or more, and more
preferably 6.9 .mu.m or more. In general, a larger single-fiber
diameter makes it more difficult to maintain both the strand
elastic modulus and the strand strength at high levels, but
according to the first example of the carbon fiber, both can be
maintained at high levels as described above even when the
single-fiber diameter is 6.0 .mu.m or more. Furthermore, a larger
single-fiber diameter ensures a higher moldability because it
prevents fuzz generation and fuzz accumulation on guide parts such
as rollers from being caused by the friction between carbon fibers
being unwound from the bobbin or their friction with the guide
parts in the step for producing a carbon fiber-reinforced
composite. There is no particular limitation on the upper limit of
the single-fiber diameter, but if it is too large, the strand
strength and the strand elastic modulus will easily decrease, and
accordingly, it is considered that the upper limit is practically
about 15 .mu.m. In addition, it is also preferable that the
single-fiber diameter is 7.4 .mu.m or less from the viewpoint of
easily maintaining both the strand elastic modulus and the strand
strength at high levels. Although a method of determining the
single-fiber diameter will be described later, it may be calculated
from the specific gravity, the metsuke, and the filament number of
the fiber bundle or may be determined by scanning electron
microscopy. If the measurement device to be used is correctly
calibrated, substantially the same results can be obtained by any
method. If the cross-sectional shape of the single fiber being
observed by scanning electron microscopy is not a perfect circle,
the equivalent circle diameter is used instead. The equivalent
circle diameter is the diameter of a perfect circle having a
cross-sectional area equal to the measured cross-sectional area.
The single-fiber diameter can be controlled by changing the rate of
discharge from the spinneret during spinning of the precursor fiber
bundle for carbon fiber or the stretching ratio in appropriate
steps.
[0029] The first example of the carbon fiber provides a carbon
fiber having a strand elastic modulus, a strand strength, and a
single-fiber diameter as specified above and satisfying one or more
of (a) and (b):
(a) when one end is fixed end and the other end being free end
which is capable of rotation about the axis of the fiber bundle,
the residual twist count is 2 turns/m or more, and (b) the total
fineness, which is the product of the single-fiber fineness (g/km)
and the filament number (number) of the carbon fiber, is 740 g/km
or more. Satisfying either or both of (a) or (b) enables prevention
of deterioration in moldability even when the strand elastic
modulus is high, thus realizing high industrial values.
[0030] The residual twist count is preferably 2 turns/m or more,
more preferably 5 turns/m or more, still more preferably 10 turns/m
or more, still more preferably 16 turns/m or more, still more
preferably 20 turns/m or more, still more preferably 30 turns/m or
more, and still more preferably 46 turns/m or more.
[0031] A fixed end means an appropriate portion of a fiber bundle
that is fixed so that it cannot rotate about the length direction
of the fiber bundle, which is assumed to be the axis, and can be
realized by restraining the rotation of the fiber bundle using
adhesive tape or the like. A free end means the end that comes out
when a continuous fiber bundle is cut in the cross-sectional
direction, which is perpendicular to the length direction, and is
not fixed to anything to allow the fiber bundle to rotate about its
length direction, which is assumed to be the axis. The residual
twist means the permanent twist of a carbon fiber bundle left with
one end fixed and the other free, and is represented in the number
of turns per meter. A semi-permanent twist means a twist that will
not unravel naturally without the application of an external force.
If a twisted specimen remains unraveled after being left for 5
minutes in a specific arrangement as described in the relevant
Examples with one end fixed and the other free, such a twist is
defined as a semi-permanent twist, or a residual twist. If the
residual twist count is 2 turns/m or more, it will be easy to
maintain a high moldability even when the strand elastic modulus is
high. The mechanism of this has not been clarified quantitatively,
but qualitatively, it is considered to be as follows. Specifically,
in a carbon fiber having a residual twist count of 2 turns/m or
more, the twist can work to fix the relative positions of single
fibers in the fiber bundle and, therefore, the single fibers in the
fiber bundle tend to remain undamaged by the friction between fiber
bundles and their friction with guide parts or the like. If the
residual twist count is 5 turns/m or more, furthermore, fuzz
generation is suppressed and, accordingly, a high tension can be
developed in the carbonization step that effectively increases the
strand elastic modulus. In addition, if the residual twist count is
20 turns/m or more, fuzz generation is suppressed strongly to
enable the control of the alignment of fiber bundles and, as a
result, smooth stress transfer can occur between fiber bundles,
easily causing enhancement in the knot strength, which will be
described later. For a fiber bundle mounted with one end fixed and
the other free, the residual twist count can be controlled by a
known method. Specifically, the residual twist count can be
controlled by adjusting the twist count of the fiber bundle in the
carbonization step.
[0032] As described previously, for the first example of the carbon
fiber, the total fineness is preferably 740 g/km or more, more
preferably 850 g/km or more, still more preferably 1,300 g/km or
more, still more preferably 1,600 g/km or more, and still more
preferably 2,000 g/km or more. A total fineness of 740 g/km or more
makes it easy to maintain a high moldability even when the strand
elastic modulus is high. The mechanism of this has not been
clarified quantitatively, but qualitatively, it is considered to be
as follows. Specifically, in a carbon fiber having a total fineness
of 740 g/km or more, it is considered that the single fibers
present in the outermost layer of a fiber bundle that are
susceptible to damage by friction as described above account for a
smaller proportion of the total number of single fibers in the
fiber bundle, leading to a smaller degree of damage by friction
suffered by the entire fiber bundle. The total fineness is the
product of the single-fiber fineness (g/km) and the filament number
(number) and therefore, it can be controlled by changing the
single-fiber fineness or the filament number.
[0033] The second example of the carbon fiber provides a carbon
fiber meeting the relationship represented by formula (1), wherein
Es (GPa) is the single-fiber elastic modulus and A (N) is the loop
fracture load:
A.gtoreq.-0.0017.times.Es+1.02 (1).
The constant in formula (1) is preferably 1.04 more, more
preferably 1.06 more, still more preferably 1.08 more, and
particularly preferably 1.10 more. The loop fracture load means the
weight working on a single fiber at the time it fractures as it is
gradually bent in a loop-like shape, and it is determined by the
method described later. Furthermore, the single-fiber elastic
modulus means the tensile modulus of a single fiber in a carbon
fiber material and has a correlation with the strand elastic
modulus described previously. As described in detail later, the
single-fiber elastic modulus can be determined by carrying out
single-fiber tensile test for a plurality of gauge lengths,
calculating the slope of the stress strain curve measured for each
of the gauge lengths, and removing the influence of the compliance
of the measuring system in consideration of gauge length
dependency. In general, an increase in the single-fiber elastic
modulus often leads to a decrease in the loop fracture load. If the
loop fracture load is small, the carbon fibers will be broken
easily in a molding step in which discontinuous fibers receive a
force in the bending direction, and the shortening of the fiber
length deteriorates the advantage of producing a fiber reinforced
composite having an increased stiffness. As the loop fracture load
increases, the single fibers become more resistant to the force in
the bending direction and accordingly, the fiber length will not be
shortened significantly in a molding step in which discontinuous
fibers receive a large force in the bending direction, thus
effectively producing a fiber reinforced composite having an
increased stiffness. If the loop fracture load A and the
single-fiber elastic modulus Es satisfy the relation represented by
formula (1), the carbon fiber is resistant to a force in the
bending direction even when the high single-fiber elastic modulus
is high, and the carbon fiber, used in the form of discontinuous
fibers, efficiently produces a carbon fiber reinforced composite
having an increased stiffness. A carbon fiber that satisfies the
relationship represented by formula (1) can be produced by the
method of producing the carbon fiber, which will be described
later. Furthermore, it is preferable that a carbon fiber according
to the first example also satisfies the requirements of the second
example. Even if the strand elastic modulus is high, such a carbon
fiber can not only effectively prevent a decrease in moldability,
but also maintain the fiber length when used in the form of
discontinuous fibers, thus enabling easy production of a
high-performance carbon fiber-reinforced composite.
[0034] The single-fiber elastic modulus is preferably 360 GPa or
more, more preferably 370 GPa or more, still more preferably 380
GPa or more, still more preferably 400 GPa or more, and still more
preferably 440 GPa or more. In conventional carbon fibers, a higher
sin-gle-fiber elastic modulus leads to a decrease in the loop
fracture load, and the fiber length are easily shortened during
their molding in the form of discontinuous fibers, whereas in the
second example of the carbon fiber, the loop fracture load is large
relative to the single-fiber elastic modulus and accordingly, a
carbon fiber reinforced composite having largely increased
stiffness can be produced in spite of a high single-fiber elastic
modulus. The single-fiber elastic modulus can be increased in the
same way as for the strand elastic modulus.
[0035] The third example of the carbon fiber provides a carbon
fiber having a single-fiber diameter of 6.0 .mu.m or more,
satisfying the relationship represented by formula (2) wherein E
(GPa) is the strand elastic modulus and B (MPa) is the knot
strength determined under conditions where the heat loss rate is
0.15% or less at 450.degree. C., and having a twist count of 5 to
80 turns/m:
B.gtoreq.6.7.times.10.sup.9.times.E.sup.-2.85 (2).
[0036] In the third example of the carbon fiber, the single-fiber
diameter is 6.0 .mu.m or more. The single-fiber diameter is
preferably 6.5 .mu.m or more, and more preferably 6.9 .mu.m or
more. In general, a larger single-fiber diameter makes it more
difficult to maintain both the strand elastic modulus and the knot
strength at high levels, but according to the third example of the
carbon fiber, both can be maintained at high levels even when the
single-fiber diameter is 6.0 .mu.m or more. Furthermore, a larger
single-fiber diameter ensures a higher moldability because it
prevents fuzz generation from being caused by the friction between
carbon fibers being unwound from the bobbin or their friction with
the guide parts such as rollers. For the third example of the
carbon fiber, there is no particular limitation on the upper limit
of the single-fiber diameter, but if it is too large, the knot
strength and the strand elastic modulus will easily decrease, and
accordingly, it is considered that the upper limit is practically
about 15 .mu.m. In addition, it is also preferable that the
single-fiber diameter is 7.4 .mu.m or less from the viewpoint of
easily maintaining both the strand elastic modulus and the knot
strength at high levels.
[0037] In the third example of the carbon fiber, the tensile
modulus E (GPa) of resin-impregnated strands and the knot strength
B (MPa) determined under conditions where the heat loss rate is
0.15% or less at 450.degree. C. satisfy the relationship
represented by formula (2):
B.gtoreq.6.7.times.10.sup.9.times.E.sup.-2.85 (2).
The heat loss rate at 450.degree. C., which will be described in
detail later, is calculated from the dif-ference in the mass of a
carbon fiber specimen between before and after heating it for 15
minutes in an oven filled with nitrogen at a temperature of
450.degree. C. The knot strength is an indicator that reflects the
mechanical properties of a fiber bundle in directions other than
the fiber axial direction. When producing a composite, a carbon
fiber bundle receives bending stress in directions other than the
fiber axis direction, and the knot strength affects the fuzz
generation, which represents fiber fractures that occur during the
composite production process. Fuzz is commonly generated if the
running speed of the fiber bundle is increased during the
production of a composite in an attempt to realize efficient
production of a composite, but an increase in the knot strength
serves to produce a high quality composite even when the running
speed of the fiber bundle is high. The knot strength tends to be
improved by applying a sizing agent to the carbon fiber bundle. On
the other hand, when the molding temperature of the matrix is so
high that the sizing agent may undergo thermal degradation to cause
a decrease in the adhesion strength between the carbon fiber and
the matrix, it may be desirable to avoid application of a sizing
agent from the viewpoint of ensuring an improved adhesion strength.
Therefore, the knot strength of a carbon fiber bundle in an unsized
state is used as an evaluation indicator. Specifically, the term
"determination under conditions where the heat loss rate is 0.15%
or less at 450.degree. C." means that an unsized fiber bundle is
used for evaluation or the sizing agent is removed before
evaluation when it is sized and has a heating loss rate of more
than 0.15% at 450.degree. C. Removal of a sizing agent may be
carried out by a generally known method such as, for example, using
a solvent that dissolves the sizing agent. If the knot strength is
low, fuzz tends to be generated during the molding step to produce
a carbon fiber-reinforced composite, possibly leading to a decrease
in the moldability. In general, an increase in the strand elastic
modulus tends to cause a decrease in the knot strength. If the
strand elastic modulus and the knot strength satisfy the
relationship represented by formula (2), both the strand elastic
modulus and the knot strength can be balanced at high levels. The
proportionality constant in formula (2) is preferably
6.9.times.10.sup.9, and more preferably 7.2.times.10.sup.9. A
carbon fiber that satisfies formula (2) that represents the
relationship between the strand elastic modulus and the knot
strength can be produced by the method of producing the carbon
fiber, which will be described later.
[0038] Furthermore, it is preferable that a carbon fiber according
to the first example also satisfies the requirements of the third
example and/or the second example. Even if the strand elastic
modulus is high, such a carbon fiber can effectively prevent a
decrease in moldability. In particular, when yarn piecing is
required in the molding step, it is advantageous for continuous
production because the yarn-pieced portions will not be fractured
easily.
[0039] For the third example of the carbon fiber, the twist count
is 5 and 80 turns/m. If the twist count is in the above range, fuzz
generation is suppressed strongly to enable the control of the
alignment of fiber bundles and, as a result, smooth stress transfer
can occur between fiber bundles, easily causing enhancement in the
knot strength. From the viewpoint of enhancing the handleability in
the molding step, it is preferable for the twist count in the third
example to be 20 to 80 turns/m.
[0040] When the carbon fiber is in the form of a carbon fiber
bundle, it is preferable that the twist angle of the carbon fiber
bundle surface layer is 2.0.degree. to 30.5.degree.. The twist
angle of the carbon fiber bundle surface layer means the angle
between the fiber axial direction of the single-fibers present in
the outermost layer of the carbon fiber bundle and the long axis
direction of the whole carbon fiber bundle, and it may be directly
observed, but can be calculated more precisely from the twist
count, the filament number, and the single-fiber diameter as
described later. If the twist angle is controlled in the above
range, fuzz generation is suppressed and, therefore, a high tension
can be applied in the carbonization step to ensure an effective
increase in the strand elastic modulus. The twist angle of the
carbon fiber bundle surface layer is preferably 4.8.degree. to
30.5.degree., more preferably 4.8.degree. to 24.0.degree., still
more preferably 4.8.degree. to 12.5.degree., and still more
preferably 4.8.degree. to 10.0.degree.. A carbon fiber bundle
having a twist angle in the above range can be produced by the
method of producing our carbon fiber as described later.
Specifically, the twist angle of the carbon fiber bundle surface
layer can be controlled by adjusting the twist count of the fiber
bundle and, in addition, adjusting the filament number and the
single-fiber diameter in the carbonization step. As the filament
number and single-fiber diameter of a carbon fiber bundle are
increased, a larger twist angle can be maintained if the twist
count of the fiber bundle is constant, thereby enhancing the effect
of twisting.
[0041] It is preferable that the crystallite size Lc (nm) and the
orientation parameter of crystallites .pi..sub.002(%) satisfy the
relationship represented by formula (3):
.pi..sub.002.gtoreq.4.0.times.Lc+73.2 (3).
The crystallite size Lc is an indicator representing the thickness
in the c-axis direction of the crystallites present in the carbon
fiber. In general, it is determined by observing the fiber bundle
by wide-angle x-ray diffraction, but it may also be determined by
separately observing each of three single fibers by microbeam
wide-angle x-ray diffraction and averaging the measurements to give
the average crystallite size Lc (s). To determine the average
crystallite size Lc (s) when the size of the microbeam is equal to
or smaller than the single-fiber diameter, measurements taken at a
plurality of points aligned in the diameter direction of a single
fiber are averaged to give an evaluation value to represent that
single fiber, and such values obtained in the same manner from a
total of three single fibers are averaged and adopted. The
evaluation technique will be described in detail later. The
wide-angle x-ray diffraction data of a single fiber is essentially
the same as the generally known wide-angle x-ray diffraction data
of a fiber bundle, and the average crystallite size Lc (s) is
nearly the same as the crystallite size Lc. Our studies have shown
that the orientation parameter of crystallites .pi..sub.002 tends
to increase with an increasing crystallite size Lc, and formula (3)
empirically shows an upper limit assumed on the basis of existing
data on carbon fibers. In general, as the crystallite size Lc
increases, the strand elastic modulus also increases, but the
strand strength, knot strength, loop fracture load, and moldability
in producing a carbon fiber-reinforced composite often tend to
decrease. In addition, the orientation parameter of crystallites
.pi..sub.002 strongly affects the strand elastic modulus, and the
strand elastic modulus increases with an increasing orientation
parameter of crystallites. The fact that the orientation parameter
of crystallites .pi..sub.002 satisfies the relationship represented
by formula (3) means that the orientation parameter of crystallites
.pi..sub.002 is large relative to the crystallite size Lc, and even
when the strand elastic modulus is high, it will be possible to
effectively prevent a decrease in the strand strength, knot
strength, loop fracture load, and moldability, thereby realizing
high industrial values. The constant in formula (3) is preferably
73.5, and still more preferably 74.0. A carbon fiber that satisfies
the relationship represented by formula (3) can be produced by
increasing the orientation tension in the carbonization step.
[0042] The crystallite size Lc is preferably 2.2 to 3.5 nm, more
preferably 2.4 to 3.3 nm or more, still more preferably 2.6 to 3.1
nm or more, and particularly preferably 2.8 to 3.1 nm. A
crystallite size Lc of 2.2 nm or more ensures effective stress
bearing inside the carbon fiber to allow the single-fiber elastic
modulus to be increased easily, whereas a crystallite size Lc of
3.5 nm or less prevents stress concentration to ensure high levels
of the strand strength, knot strength, loop fracture load, and
moldability. The crystallite size Lc can be controlled by changing
the treatment time and maximum temperature mainly in the
carbonization step.
[0043] The orientation parameter of crystallites .pi..sub.002 is
preferably 80.0% to 95.0%, more preferably 80.0% to 90.0%, and
still more preferably 82.0% to 90.0%. The orientation parameter of
crystallites .pi..sub.002 is an indicator representing the
orientation angle with respect to the fiber axis of crystallites
present in carbon fiber. As in the crystallite size, it may also be
determined by separately observing each of three single fibers by
microbeam wide-angle x-ray diffraction and averaging the
measurements to give the average orientation parameter of
crystallites .pi..sub.002 (s). To determine the average orientation
parameter of crystallites .pi..sub.002 (s) when the size of the
microbeam is equal to or smaller than the single-fiber diameter,
measurements taken at a plurality of points aligned in the diameter
direction of a single fiber are averaged to give an evaluation
value to represent that single fiber, and such values obtained in
the same manner from a total of three single fibers are averaged
and adopted. The evaluation technique will be described in detail
below. The wide-angle x-ray diffraction data of a single fiber is
essentially the same as the generally known wide-angle x-ray
diffraction data of a fiber bundle, and the average orientation
parameter of crystallites .pi..sub.002 (s) is nearly the same as
the orientation parameter of crystallites .pi..sub.002. A
orientation parameter of crystallites of 80.0% or more makes it is
easy to maintain a high strand elastic modulus. The orientation
parameter of crystallites .pi..sub.002 (s) can be controlled by
changing the orientation tension in addition to the temperature and
time of the carbonization step.
[0044] It is preferable that the strand elastic modulus E (GPa) and
the crystallite size Lc (nm) satisfy the relationship represented
by formula (4):
E.times.Lc.sup.-0.5.gtoreq.200 (GPa/nm.sup.0.5 (4).
[0045] We found that both the strand elastic modulus and the
moldability are easily maintained at particularly high levels when
carbon fiber satisfies formula (4). The reason for the fact that
the strand elastic modulus and the moldability are easily
maintained at high levels when formula (4) is satisfied has not
been clarified yet, but it is speculated as follows. Specifically,
as can be seen in the Hall-Petch equation, which is widely used in
the field of polycrystalline materials, if the -0.5th power of the
crystallite size Lc is assumed to be an indicator of some kind of
toughness of the material, it can be interpreted that a larger
Lc.sup.-0.5 indicates a tougher material whereas a smaller
Lc.sup.-0.5 indicates a brittler material. Therefore, when formula
(4) is satisfied, it means that the product of the strand elastic
modulus and the toughness of the material is equal to or larger
than a certain value and suggests that the strand elastic modulus
and the toughness of the material are maintained at high levels.
Such a carbon fiber that satisfies formula (4) can be produced by
increasing the orientation tension in the carbonization step.
[0046] It is preferable that the surface oxygen concentration O/C
is 0.05 to 0.50. The surface oxygen concentration is an indicator
representing the number of oxygen-containing functional groups
introduced in the surface of a carbon fiber and can be determined
by photoelectron spectroscopy, which will be described later. A
higher surface oxygen concentration achieves a larger increase in
the adhesion between the carbon fiber and the matrix and production
of a carbon fiber-reinforced composite having better mechanical
properties. It is more preferable for the surface oxygen
concentration O/C to be 0.07 to 0.30. If the surface oxygen
concentration O/C is 0.05 or more, a sufficiently strong adhesion
to the matrix is achieved, whereas if it is 0.50 or less, it
prevents peeling of the surface of the carbon fiber due to
excessive oxidation, thereby leading to a carbon fiber-reinforced
composite having improved mechanical properties. Methods of
controlling the surface oxygen concentration O/C in the above range
will be described later.
[0047] When the carbon fiber is in the form of a carbon fiber
bundle, it is preferable that the filament number is preferably
10,000 or more. The filament number is more preferably 15,000 or
more, and still more preferably 20,000 or more. If the twist count
is constant, a larger filament number ensures a larger distance
between the central axis of the twist and the outer periphery of
the fiber bundle. Accordingly, the twist is stabilized easily, and
fuzz generation and fracture are suppressed easily even when a high
tension is applied in the carbonization step, thereby effectively
realizing an increase in the strand elastic modulus and an increase
in the moldability.
[0048] A method of producing the carbon fiber is described
below.
[0049] A precursor fiber bundle for carbon fiber serving as a
material for producing the carbon fiber can be prepared by spinning
a spinning dope solution of a polyacrylonitrile copolymer.
[0050] Examples of the polyacrylonitrile copolymer include not only
homopolymer prepared from acrylonitrile alone, but also copolymers
produced from other monomers in addition to acrylonitriles as main
components. Specifically, it is preferable that a polyacrylonitrile
copolymer contains 90 to 100% by mass of acrylonitrile and less
than 10% by mass of a copolymerizable monomer.
[0051] Useful monomers that are copolymerizable with acrylonitrile
include, for example, acrylic acid, methacrylic acid, itaconic
acid, alkali metal salts thereof, ammonium salts, lower alkyl
esters, acrylamide, derivatives thereof, allyl sulfonic acid,
methallylsulfonic acid, and salts or alkyl esters thereof.
[0052] A spinning dope solution is prepared by dissolving a
polyacrylonitrile copolymer as described above in a solvent such as
dimethyl sulfoxide, dimethyl formamide, and dimethyl acetamide,
nitric acid, aqueous solutions of zinc chloride, and aqueous
solutions of sodium rhodanide, that can dissolve the
polyacrylonitrile copolymer. When the solution polymerization
technique is used for preparing a polyacrylonitrile copolymer, it
is preferable to use the same solvent for both polymerization and
spinning because it eliminates the necessity of steps for
separating the resulting polyacrylonitrile copolymer and
re-dissolving it in a spinning solvent.
[0053] A precursor fiber bundle for carbon fibers can be produced
by spinning a spinning dope solution prepared as described above by
a wet or dry-jet wet spinning technique.
[0054] The spinning dope solution is coagulated by introducing it
into a coagulation bath, and the resulting coagulated fiber bundle
is passed through a water washing step, an underwater stretching
step, an oil agent treatment step, and a drying step, thereby
providing a precursor fiber bundle for carbon fiber. The water
washing step may be omitted to send the coagulated fiber bundle
directly to the underwater stretching step, or the water washing
step may be carried out to remove the solvent before the underwater
stretch step. In general, it is preferable for the underwater
stretching step to be carried out in a single or multiple
stretching baths that are controlled at temperatures of 30.degree.
C. to 98.degree. C. Furthermore, a dry heat stretching step or a
steam stretching step may be added to the above steps.
[0055] It is preferable for the precursor fiber bundle for carbon
fiber to have a single-fiber fineness of 0.9 dtex or more, more
preferably 1.0 dtex or more, and still more preferably 1.1 dtex or
more. A precursor fiber bundle for carbon fiber having a higher
single-fiber fineness ensures less frequent fracture generation in
the fiber bundle due to contact with the rollers and guide parts
and stabler implementation of the steps for spinning, carbon fiber
stabilization, pre-carbonization, and carbonization. If the
single-fiber fineness of the precursor fiber bundle for carbon
fiber is 0.9 dtex or more, it allows the process stability to be
maintained easy. If the single-fiber fineness of the precursor
fiber bundle for carbon fiber is too high, it will be difficult
sometimes to perform uniform treatment in the stabilization step
and it will cause a decrease in the stability of the production
process, possibly leading to a carbon fiber bundle and a carbon
fiber having deteriorated mechanical properties. The single-fiber
fineness of the precursor fiber bundle for carbon fiber can be
controlled by known techniques such as changing the rate of
spinning dope solution discharge from the spinneret, the stretching
ratio and the like.
[0056] The resulting precursor fiber bundle for carbon fiber is
normally in the form of continuous fibers. The filament number per
fiber is preferably 1,000 to 80,000. A plurality of precursor fiber
bundles for carbon fiber may be combined as required to adjust the
filament number per fiber of the carbon fiber to be produced.
[0057] The carbon fiber can be produced by subjecting the
aforementioned precursor fiber bundle for carbon fiber to
stabilization treatment, pre-carbonization treatment, and
carbonization treatment in this order.
[0058] The stabilization treatment of the precursor fiber bundle
for carbon fiber is preferably carried out in an air atmosphere at
a temperature of 200.degree. C. to 300.degree. C. The precursor
fiber bundle for carbon fiber is subjected to stabilization
treatment to provide a stabilized fiber bundle.
[0059] The aforementioned stabilization treatment is followed by
pre-carbonization of the stabilized fiber bundle. In the
pre-carbonization step, it is preferable for the stabilized fiber
bundle resulting from the stabilization treatment step to be
subjected to heat treatment in an inactive atmosphere at or below a
maximum temperature of 500.degree. C. to 1,000.degree. C. until the
density reaches 1.5 to 1.8 g/cm.sup.3. The stabilized fiber bundle
is then subjected to the pre-carbonization treatment step to
provide a pre-carbonized fiber bundle.
[0060] In addition, the aforementioned pre-carbonization step is
followed by carbonization of the pre-carbonized fiber bundle. In
the carbonization step, the pre-carbonized fiber bundle resulting
from the pre-carbonization treatment step is subjected to the
carbonization treatment step in an inactive atmosphere. The maximum
temperature in the carbonization treatment step is preferably
1,500.degree. C. or more, and more preferably 2,300.degree. C. or
more. The maximum temperature in the carbonization treatment step
is preferably as high as possible from the viewpoint of providing a
carbon fiber having a high strand elastic modulus and a high
single-fiber elastic modulus, but if the maximum temperature is at
least 1,500.degree. C. or more, it ensures production of a carbon
fiber that is high not only in the strand elastic modulus and
single-fiber elastic modulus, but also in the knot strength and
loop fracture load. If the carbonization temperature is too high,
on the other hand, the knot strength and loop fracture load tend to
decrease, and for the carbonization step, therefore, it is
recommended that an appropriate maximum temperature is adopted to
allow the required strand elastic modulus and single-fiber elastic
modulus to be in good balance with the knot strength and loop
fracture load. The carbon fiber can easily realize these physical
properties in good balance even when the maximum temperature in the
carbonization step is 2,300.degree. C.
[0061] Furthermore, the tension in the carbonization step is 5
mN/dtex or more, preferably 5 to 18 mN/dtex, more preferably 7 to
18 mN/dtex, and particularly preferably 9 to 18 mN/dtex. The
tension in the carbonization step is calculated by dividing the
tension (mN) measured at the outlet of the carbonization funace by
the total fineness (dtex), which is the product of the single-fiber
fineness (dtex) and filament number of the precursor fiber bundle
for carbon fiber used. Adjusting the tension in the above range
allows the orientation parameter of crystallites .pi..sub.002 to be
controlled appropriately without significantly affecting the
crystallite size Lc of the resulting carbon fiber, and enables the
production of a carbon fiber satisfying the relationship
represented by formula (1) and/or formula (2). From the viewpoint
of increasing the strand elastic modulus and the single-fiber
elastic modulus of the carbon fiber, the tension is preferably as
high as possible, but if it is too high, it may cause a decrease in
the processability in the carbonization step and deterioration in
the quality of the resulting carbon fiber, and therefore, it is
preferable that an appropriate tension is adopted in light of both
of them.
[0062] It is more preferable that the method of producing the
carbon fiber further satisfies either of (c) and (d). It is still
more preferable that both (c) and (d) are satisfied.
(c) The fiber bundle to be subjected to the carbonization treatment
has a twist count of 2 turns/m or more. (d) The total fineness,
which is the product of the single-fiber fineness (g/km) and the
filament number (number), of the resulting carbon fiber is 740 g/km
or more. Satisfying either of (c) or (d) enables production of a
carbon fiber having high moldability in spite of a high strand
elastic modulus.
[0063] The fiber bundle has a twist number of 2 turns/m or more in
the carbonization treatment step. The twist number is preferably 5
turns/m or more, more preferably 10 turns/m or more, still more
preferably 16 turns/m or more, still more preferably 30 turns/m or
more, and still more preferably 46 turns/m or more. Although there
is no particular limitation on the upper limit of the twist count,
it is effective to control it about 60 turns/m or less to increase
the productivity and the stretching limit in the carbonization
step. If the twist number is controlled in the above range, fuzz
generation is suppressed in the carbon fiber production process to
realize application of a high tension, thus enabling easy
production of a carbon fiber having a high strand elastic modulus
and a high single-fiber elastic modulus. The twist number of the
fiber bundle in the carbonization treatment step means the twist
count pos-sessed by the fiber bundle being treated for
carbonization. If the tension in the carbonization step is
increased without giving a twist, there may occur fractures of
single fibers and increased fuzz generation to cause a decrease in
the processability in the carbonization step or fractures of the
whole fiber bundle, possibly making it impossible to maintain a
required tension. The twist count can be controlled by a method in
which a precursor fiber bundle for carbon fiber, stabilized fiber
bundle, or pre-carbonized fiber bundle is once wound up on a bobbin
and then a bobbin is rotated in the plane perpendicular to the
direction of unwinding the fiber bundle, or a method in which,
instead of winding up on a bobbin, rotating rollers, belts and the
like, are brought into contact with the traveling fiber bundle to
give a twist.
[0064] The filament number of the fiber bundle in the carbonization
treatment step is preferably 10,000 or more, more preferably 15,000
or more, and still more preferably 20,000 or more. If the twist
count of the fiber bundle in the carbonization treatment step is
constant, a larger filament number ensures a larger distance
between the central axis of the twist and the outer periphery of
the fiber bundle to allow the twist to more easily suppress fuzz
generation, thereby more effectively producing a carbon fiber
having a higher single-fiber elastic modulus. There is no
particular limitation on the upper limit of the filament number,
and an appropriate upper limit may be adopted in light of the
intended use.
[0065] Examples of the inert gas used for the inert atmosphere
include nitrogen, argon, and xenon, of which nitrogen is preferable
from an economical point of view.
[0066] The carbon fiber bundle produced by the above production
method may be further subjected to additional graphitization
treatment in an inert atmosphere at or lower than 3,000.degree. C.
to appropriately adjust the single-fiber elastic modulus in light
of the intended use.
[0067] It is preferable that the carbon fiber bundle thus produced
is subjected to surface treatment after the carbonization treatment
step to introduce oxygen-containing functional groups to increase
the strength of adhesion between the carbon fiber and the matrix.
Useful surface treatment methods include gas phase oxidization,
liquid phase oxidization, and liquid phase electrolytic
oxidization, of which liquid phase electrolytic oxidization has
been preferred from the viewpoint of high productivity and uniform
treatment. There is no specific limitation on the technique to be
used for liquid phase electrolytic oxidation and a generally known
one may be selected appropriately. For the electrolytic surface
treatment to perform the liquid phase electrolytic oxidation, the
amount of current is preferably 2 to 100 c/g, and more preferably 2
to 80 c/g. If the amount of current during the electrolytic surface
treatment is 2 c/g or more, a sufficient number of
oxygen-containing functional groups are introduced to the surface
of the carbon fiber to easily realize a required adhesiveness to
the resin and prevent a decrease in the elastic modulus of the
composite, whereas if it is 100 c/g or less, it prevents the
electrolytic surface treatment from causing flaw formation on the
surface of the carbon fiber and prevent a decrease in the loop
fracture load.
[0068] Performing surface treatment such as electrolytic surface
treatment serves to introduce oxygen-containing functional groups
into the carbon fiber bundle and control the surface oxygen
concentration O/C of the carbon fiber bundle. To control the
surface oxygen concentration O/C in the preferable range, the
amount of current and the treatment time for the surface treatment
may be adjusted by known methods.
[0069] After the electrolytic treatment, a sizing agent may also be
attached to further enhance the handleability and high-order
workability of the resulting carbon fiber bundle or increase the
strength of adhesion between the carbon fiber and the matrix. An
appropriate sizing agent may be adopted to suit the type of the
matrix used in the carbon fiber-reinforced composite. In addition,
the amount of agent and the like may be finely adjusted from the
viewpoint of handleability and high-order processability.
Furthermore, when the molding temperature of the matrix is so high
that the sizing agent may undergo thermal degradation to cause a
decrease in the strength of adhesion between the carbon fiber and
the matrix, it may be desirable to minimize the amount of the
sizing agent to be attached or avoid the implementation of sizing
treatment.
[0070] The methods used to measure the various physical properties
described herein are described below. The evaluations were made
based on one measurement (n=1) unless otherwise specified.
Strand Strength and Strand Elastic Modulus of Carbon Fiber
[0071] The strand strength and strand elastic modulus of a carbon
fiber are determined by the following procedure according to JIS
R7608 (2004) "Resin-impregnated strand test method." When the fiber
bundle of carbon fiber has a twist, evaluations are made after
unraveling the twist by twisting it in the reverse rotation
direction by the same number of turns as its original twist count.
The resin mixture used consisted of Celloxide (registered
trademark) 2021 P (manufactured by Daicel Chemical Industries,
Ltd.), boron trifluoride monoethyl-amine (manufactured by Tokyo
Chemical Industry Co., Ltd.), and acetone at a ratio of 100/3/4
(parts by mass), and the curing conditions used included
atmospheric pressure, a temperature of 125.degree. C., and test
time of 30 minutes. Ten strands formed of carbon fiber bundles are
examined and the measurements taken are averaged to represent the
strand strength and the strand elastic modulus. The strain should
be 0.1% to 0.6% when determining the strand elastic modulus.
Average of Single Fiber Diameter of Carbon Fiber
[0072] A cross section of a single fiber of a carbon fiber sample
is observed by scanning electron microscopy to examine its
cross-sectional features. The diameter of a perfect circle that has
the same cross-sectional area as the observed one is calculated and
adopted as the diameter of the single fiber. A total of 50 single
fibers (N=50) are examined and their diameters are averaged and
adopted. The acceleration voltage is set to 5 keV.
[0073] The scanning electron microscope used for the Examples given
herein was an S-4800 scanning electron microscope (SEM)
manufactured by Hitachi High-Technologies Corporation.
Residual Twist Count Measured with One End Fixed and the Other
Free
[0074] A guide bar is installed at a height of 60 cm from a
horizontal surface, and an appropriate portion of a carbon fiber
bundle is taped to the guide bar to be a fixed end, followed by
cutting the carbon fiber bundle at a position 50 cm away from the
fixed end to create a free end. The free end is enclosed in a
folded piece of tape so that the fiber bundle will not unraveled
into single fibers. To eliminate that part of a twist which is not
semi-permanent, that is, temporal or releasable over time, the
specimen is left in this state for 5 minutes and the free end is
rotated while counting the number of turns, followed by recording
the number of turns n (turns) required for complete untwisting. The
residual twist count is calculated by the following formula. The
above measuring procedure is performed three times repeatedly and
the results are averaged to represent the residual twist count:
Residual twist count (turns/m)=n (turns)/0.5 (m).
Single-Fiber Elastic Modulus of Carbon Fiber
[0075] The single-fiber elastic modulus of a carbon fiber is
determined as described below according to JIS R7606 (2000). First,
a carbon fiber bundle of about 20 cm is divided into approximately
four equal portions, and single fibers are sampled from the four
bundles one by one in turn so that specimens are collected as
evenly as possible from the entire bundle. The single fibers
sampled are secured to pasteboard sheets having holes of 10, 25, or
50 mm. For the securing, an epoxy based adhesive (Araldite
(registered trademark), fast curing type, manufactured by Nichiban
Co., Ltd.) is used and, after application, it is allowed to stand
at room temperature for 24 hours to ensure sufficient curing. Each
pasteboard sheet with a secured single fiber is mounted on a
tensile test machine, and tensile test was performed on 15
specimens under the conditions of a gauge length of 10, 25, or 50
mm and a strain rate of 40%/min. In the stress (MPa)-strain (%)
curve of each single fiber, the slope (MPa/%) is determined in the
strain range of 0.3% to 0.7%, and the apparent single-fiber elastic
modulus is calculated by the formula below:
Apparent single-fiber elastic modulus (GPa)=slope (MPa/%)
in the strain range of 0.3%-0.7%/10.
Then, for each of the gauge lengths 10, 25 and 50 mm, the average
single-fiber elastic modulus E.sub.app (GPa) is calculated, and its
reciprocal, i.e., 1/E.sub.app (GPa.sup.-1), on the vertical axis
(Y-axis) is plotted against the reciprocal of the gauge length
L.sub.0 (mm), i.e., 1/L.sub.0 (mm.sup.-1, on the horizontal axis
(X-axis). The reciprocal of the intercept of the curve on the
y-axis shows the single-fiber elastic modulus corrected for
compliance, and this value is adopted as the single-fiber elastic
modulus.
[0076] The tensile tester used for the Examples given herein was a
Tensilon RTF-1210 tensile tester manufactured by A&D Company,
Limited.
Loop Fracture Load
[0077] A single fiber with a length of about 10 cm is placed on a
glass slide, and 1 to 2 drops of glycerin are put onto the central
portion. Ten, both ends of the single fiber is twisted slightly in
the fiber's circumferential direction to form a loop in the central
portion of the single fiber, and a cover glass is put on the loop.
This is mounted on the stage of a microscope and recorded on video
under the conditions of a total magnification of 100 times and a
frame rate of 15 frames/sec. With the stage occasionally adjusted
to maintain the loop within the field of view, both ends of the
looped fiber are pulled in the opposite directions at a constant
rate while pressing them against the glass slide to strain the
single fiber until it is fractured. The video is viewed by
frame-by-frame playback to identify the frame just before loop
fracture, and the width W of the loop just before loop fracture is
determined by image analysis. The single-fiber diameter d is
divided by W to calculate the d/W ratio. The test was performed on
20 specimens (n=20), and the average d/W ratio is multiplied by the
single-fiber elastic modulus Es to calculate the loop strength as
Es.times.d/W. In addition, it is multiplied by the cross-sectional
area .pi.d.sup.2/4 determined from the single-fiber diameter and
.pi.Es.times.d.sup.3/4 W is adopted as the loop fracture load.
Heating Loss Rate at 450.degree. C. of Carbon Fiber Bundle
[0078] A carbon fiber bundle being examined is cut to a mass of 2.5
g and wound up on a reel having a diameter of about 3 cm, and its
mass w.sub.0 (g) before the heat treatment is determined. Then, it
is heated in a nitrogen atmosphere in an oven at a temperature of
450.degree. C. for 15 minutes, and allowed to cool to room
temperature in a desiccator, followed by determining its mass
w.sub.1 (g) after heating. The heating loss rate at 450.degree. C.
is calculated by the following formula. Tree measurements are taken
and their average is adopted.
Heating loss rate (%) at 450.degree.
C.=(w.sub.0-w.sub.1)/w.sub.0.times.100(%)
Knot Strength of Carbon Fiber Bundle
[0079] Carbon fiber bundles with a heating loss rate at 450.degree.
C. of 0.15% or less were used for the measurement of the knot
strength. When evaluating a carbon fiber bundle treated with a
sizing agent, the carbon fiber bundle was cleaned in acetone to
remove the sizing agent and used after drying. The dried carbon
fiber bundle is subjected to measurement of its heating loss rate
at 450.degree. C. and cleaning is repeated until it becomes 0.15%
or less.
[0080] When the carbon fiber bundle has a twist, evaluations are
made after unraveling the twist by twisting it in the reverse
rotation direction by the same number of turns as its original
twist count. A carbon fiber bundle as described above having a
length of 150 mm is divided or combined with others to form a
carbon fiber bundle having a total fineness of 7,000 to 8,500 dtex,
and the resulting carbon fiber bundle is used for measurement. It
is noted that the total fineness of a carbon fiber bundle means the
product of the average fineness (dtex) of the single fibers in the
carbon fiber bundle and the filament number. A grip with a length
of 25 mm is attached to each end of this carbon fiber bundle to
prepare a test specimen, and during this test specimen preparation,
a load of 0.1.times.10.sup.-3 N/denier is applied to parallel the
carbon fibers. One knot is formed at the center of the test
specimen, and fiber bundle tensile test is performed with a
crosshead speed of 100 mm/min maintained while it is pulled.
Measurements are taken from a total of 12 fiber bundles, and after
eliminating the largest and smallest ones, the remaining the 10
measurements are averaged and used to represent them. Then the
standard deviation of the 10 values is calculated and used as the
standard deviation of knot strength. To calculate the knot strength
to be used, the largest load obtained in the tensile test is
divided by the average cross-sectional area of the carbon fiber
bundles. Twist angle of carbon fiber bundle surface layer
[0081] The overall diameter (.mu.m) of a carbon fiber bundle is
calculated first by the following formula from the single-fiber
diameter (.mu.m) and the filament number described above, and the
twist angle (.degree.) of the carbon fiber bundle surface layer is
calculated by the following formula using the twist count
(turn/m):
Overall diameter of carbon fiber bundle (.mu.m)={(the single-fiber
diameter).sup.2.times.filament number}.sup.0.5
Twist angle (.degree.) of carbon fiber bundle surface layer=a
tan
(overall diameter of fiber
bundle.times.10.sup.-6.times..pi..times.twist count).
Crystallite Size Lc and Orientation Parameter of Crystallites
.pi..sub.002 of Carbon Fiber Bundle
[0082] The fibers in a carbon fiber bundle being examined are
paralleled and bound using a collodion-alcohol solution to prepare
a test specimen of a square prism having a length of 4 cm and a
base of 1 mm.times.1 mm. The test specimen prepared is examined by
wide angle x-ray diffraction under the following conditions.
1. Measurement of Crystallite Size Lc
[0083] X-ray source: CuK.alpha. beam (tube voltage 40 kV, tube
current 30 mA)
Detector: goniometer+monochromator+scintillation counter
Scanning range: 2.theta.=10.degree.-40.degree.
Scan mode: Step scan, unit step 0.02.degree., counting time 2
sec
[0084] In the diffraction pattern obtained, the half-width of the
peak appearing in the vicinity of 2.theta.=25.degree. to 26.degree.
is measured, and the crystallite size is calculated from this value
by the Scherrer equation:
Crystallite size (nm)=K.lamda./.beta..sub.0 cos .theta..sub.B
[0085] wherein [0086] K=1.0, .lamda.=0.15418 nm (wavelenghth of
X-ray)
[0086] .beta..sub.0=(.beta..sub.E.sup.2-.beta..sub.1.sup.2).sup.1/2
[0087] .beta..sub.E: apparent half-width (measured) (rad),
.beta..sub.1: 1.046.times.10.sup.-2 (rad) [0088] .theta..sub.B:
Bragg diffraction angle.
2. Measurement of Orientation Parameter of Crystallites
.pi..sub.002
[0089] The crystal peak described above is scanned in the
circumferential direction to determine the intensity distribution
and calculation is performed from its half width by the formula
below:
.pi..sub.002=(180-H)/180 [0090] wherein [0091] H: apparent
half-width (deg). The above measuring procedure is performed three
times repeatedly and the measurements taken are arithmetically
averaged to give the crystallite size and the orientation parameter
of crystallites of the carbon fiber bundle.
[0092] In the Examples and Comparative Examples below, XRD-6100
manufactured by Shimadzu Corporation was used as the wide-angle
x-ray diffraction apparatus.
Average crystallite size Lc (s) and average orientation parameter
of crystallites .pi..sub.002 (s) of single fiber of carbon
fiber
[0093] Single fibers are randomly sampled from a carbon fiber
bundle and examined by wide angle x-ray diffraction using an
instrument that can generate an X-ray .mu.-beam. For the
measurement, a microbeam with a wavelength of 0.1305 nm that is
shaped to 3 .mu.m in the fiber axis direction and 1 .mu.m in the
fiber diameter direction is used to scan the single fiber in 1
.mu.m steps in the fiber diameter direction. The irradiation time
is 2 seconds in each step. The camera length, which is the distance
between the detector and the specimen, is set within the range of
40-200 mm. The camera length and the coordinate position of the
beam center are determined from measurements taken by using cerium
oxide as standard sample. A two dimensional diffraction pattern
recorded after removing the specimen is subtracted from the
observed two dimensional diffraction pattern to remove the dark
noise attributed to the detector and the scattering noise
attributed to air to provide a corrected two-dimensional
diffraction pattern. The corrected two-dimensional diffraction
patterns obtained from different positions in the fiber diameter
direction of the single fiber are added together to provide an
averaged two dimensional diffraction pattern in the fiber diameter
direction of the single fiber. In this averaged two dimensional
diffraction pattern, sector integration is performed over the angle
range of .+-.5.degree. around a direction perpendicular to the
fiber axis to provide a diffraction intensity profile in the
2.theta. direction. The diffraction intensity profile in the
2.theta. direction is least-squares fitted using two Gaussian
functions to calculate the angle 2.theta..sub.m (.degree.), which
is the 2.theta. value where the diffraction intensity is at a
maximum, and the full width at half maximum FWHM (.degree.) of the
composite function of the two Gaussian functions. In addition,
circumferential integration is performed over the range of
5.degree. around the angle 2.theta..sub.m (.degree.) where the
diffraction intensity profile in the 2.theta. direction is at a
maximum to obtain a diffraction intensity profile in the
circumferential direction. The diffraction intensity profile in the
circumferential direction is least-squares fitted using a Gaussian
function to calculate the full width at half maximum
FWHM.sub..beta. (.degree.). The crystallite size Lc (s) and the
orientation parameter of crystallites .pi..sub.002 (s) of the
single fiber is determined by the formula below, and the results
from three single fibers are averaged to calculate the average
crystallite size Lc (s) and the average orientation parameter of
crystallites .pi..sub.002 (s):
Lc (s)(nm)=K.lamda./FWHM cos(2.theta..sub.m/2).
Scherrer factor K is 1.0 and the X-ray wavelength .lamda. is 0.1305
nm. The full width at half maximum FWHM and 2.theta..sub.m, both in
degrees (.degree.), are used after conversion into values in radian
(rad):
.pi..sub.002 (s) (%)=(180-FWHM.sub..beta.)/180.times.100(%).
[0094] For the Examples herein, the second hatch of SPring-8
Beamline BL03XU (FSBL) was used as the facility that can generate
X-ray .mu.-beam and a C9827DK-10 flat panel detector (pixel size 50
.mu.m.times.50 .mu.m) manufactured by Hamamatsu Photonics K.K. was
used as the detector.
Surface Oxygen Concentration O/C of Carbon Fiber
[0095] The surface oxygen concentration O/C of a carbon fiber
bundle can be determined by X-ray photoelectron spectroscopy
according to the procedure described below. First, dirt and the
like attached on the surface of a carbon fiber bundle is removed
with a solvent, and it is cut to about 20 mm and spread on a
specimen support table made of copper. Then, the specimen support
table is placed in a specimen chamber, and the pressure in the
specimen chamber is maintained at 1.times.10.sub.-8 Torr.
Subsequently, measurements are taken using AlK.alpha..sub.1,2 as
X-ray source and assuming a photoelectron takeoff angle of
90.degree.. To perform peak correction for electrification that
occurs during measurement, the value of binding energy of the
C.sub.1s primary peak (peak top) is adjusted to 286.1 eV, and the
C.sub.1s peak area can be determined by drawing a straight baseline
in the range of 282 to 296 eV. Furthermore, the O.sub.1s peak area
can be determined by drawing a straight baseline in the range of
528 to 540 eV. The surface oxygen concentration is calculated from
the ratio between the 01, peak area and the C.sub.1s peak area,
which represents the ratio in the number of atoms, using an
inherent sensitivity correction value of the equipment. The x-ray
photoelectron spectroscopy equipment used for the Examples given
herein was ESCA-1600, manufactured by Ulvac-Phi, Incorporated, and
the sensitivity correction value inherent in this equipment was
2.33.
Running Stability
[0096] For model evaluation for moldability, the running stability
is determined as follows. A running stability evaluation unit is
constructed on which five V-groove rollers with a diameter of 50
mm, a groove width of 10 mm, and a groove depth of 10 mm are fixed
along a straight line at intervals of 300 mm. A carbon fiber bundle
being evaluated, which is free of a sizing agent, is set in a
zigzag pattern so that its top surface, bottom surface, top
surface, bottom surface, and top surface are in contact with the
V-groove rollers of the running stability evaluation unit, and
moved for 30 minutes at a linear speed of 10 m/min while applying a
tension of 1 kg using a dancer weight. Then, the carbon fiber
bundle is removed and the condition of the five V-groove rollers is
visually inspected and ranked as follows:
A: The rollers are free of carbon fibers adhered thereon. In
addition, a test piece ranked as A is further subjected to the
running test for additional 150 minutes and ranked as AA if the
rollers are still free of carbon fibers adhered thereon. B: A few
carbon fibers are found attached on the rollers (attached on one or
two of the five rollers). C: Carbon fibers are found attached on
the rollers (attached on three or four of the five rollers). D:
Many carbon fibers are found attached on the rollers (attached on
all of the five rollers).
EXAMPLES
[0097] Our carbon fibers and methods will now be illustrated in
detail with reference to Examples, but it should be understood that
this disclosure is not construed as being limited thereto.
[0098] In the Examples 1 to 11 and Comparative Examples 1 to 16
given below, the procedure described in the following Comprehensive
Example is carried out under the conditions specified in Tables 1
or 2.
Comprehensive Example
[0099] A monomer compound consisting of acrylonitrile and itaconic
acid was polymerized by the solution polymerization method using
dimethyl sulfoxide as solvent to prepare a spinning dope solution
containing a polyacrylonitrile copolymer. A coagulated fiber was
produced through a dry-jet wet spinning process in which the
resulting spinning dope solution was first filtered, discharged
into air through a spinneret, and introduced into a coagulation
bath containing an aqueous solution of dimethyl sulfoxide. Then,
the coagulated fiber was rinsed, stretched in a hot water bath at
90.degree. C. to an underwater stretching ratio of 3, treated with
an oil agent, dried using a roller heated at a temperature of
160.degree. C., and subjected to pressurized steam stretching to a
stretching ratio of 4 to prepare a precursor fiber bundle for
carbon fiber having a single-fiber fineness of 1.1 dtex.
Subsequently, four of such precursor fiber bundles for carbon fiber
were combined to form a bundle containing 12,000 single fibers and
heat-treated at a stretching ratio of 1 in an air atmosphere in an
oven at 240.degree. C. to 280.degree. C., there by converting it
into a stabilized fiber bundle.
Example 1
[0100] After obtaining a stabilized fiber bundle by the procedure
described in the Comprehensive Example, the resulting stabilized
fiber bundle was twisted to 75 turns/m and subjected to
pre-carbonization treatment at a stretching ratio of 0.97 in a
nitrogen atmosphere at a temperature of 300.degree. C. to
800.degree. C. to provide a pre-carbonized fiber bundle. Then, thus
pre-carbonized fiber bundle was subjected to carbonization
treatment under the conditions shown in Table 1 and electrolytic
surface treatment with a quantity of electricity of 30 coulombs per
gram of the carbon fiber bundle using an aqueous sulfuric acid
solution as electrolyte to provide a carbon fiber bundle having a
surface oxygen concentration (0/C) of 0.09. The processability in
the carbonization step was high and the resulting carbon fiber
bundle had good quality. It was very highly rated as AA in
moldability. Evaluation results of the carbon fiber obtained are
described in Table 1.
Example 2
[0101] Except that the twist count was 50 turns/m and that the
tension in the carbonization treatment step was 5.2 mN/dtex, the
same procedure as in Example 1 was carried out to produce a carbon
fiber bundle. The processability in the carbonization step was high
and the resulting carbon fiber bundle had good quality. It was very
highly rated as AA in moldability. Evaluation results of the carbon
fiber obtained are described in Table 1.
Example 3
[0102] Except that the tension in the carbonization treatment step
was 10.2 mN/dtex, the same procedure as in Example 2 was carried
out to produce a carbon fiber bundle. The processability in the
carbonization step was high and the resulting carbon fiber bundle
had good quality. It was very highly rated as AA in moldability.
Evaluation results of the carbon fiber obtained are described in
Table 1.
Example 4
[0103] Except that the twist count was 20 turns/m and that the
tension in the carbonization treatment step was 10.3 mN/dtex, the
same procedure as in Example 1 was carried out to produce a carbon
fiber bundle. The processability in the carbonization step was high
and the resulting carbon fiber bundle had good quality. It was very
highly rated as AA in moldability. Evaluation results of the carbon
fiber obtained are described in Table 1.
Example 5
[0104] Except that, unlike the Comprehensive Example, eight
precursor fiber bundles were combined to integrate 24,000 single
fibers, the same procedure as in Example 3 was carried out to
produce a carbon fiber bundle. The processability in the
carbonization step was high and the resulting carbon fiber bundle
had good quality. It was very highly rated as AA in moldability.
Evaluation results of the carbon fiber obtained are described in
Table 1.
Example 6
[0105] Except for performing carbonization treatment at or below a
maximum temperature of 2,350.degree. C. under a tension of 6.5
mN/dtex in the carbonization treatment step, the same procedure as
in Example 2 was carried out to produce a carbon fiber bundle. The
processability in the carbonization step was high and the resulting
carbon fiber bundle had good quality. It was highly rated as A in
moldability. Evaluation results of the carbon fiber obtained are
described in Table 1.
Example 7
[0106] Except that the tension in the carbonization treatment step
was 9.1 mN/dtex, the same procedure as in Example 6 was carried out
to produce a carbon fiber bundle. The processability in the
carbonization step was high and the resulting carbon fiber bundle
had good quality. It was highly rated as A in moldability.
Evaluation results of the carbon fiber obtained are described in
Table 1.
Example 8
[0107] Except that the tension in the carbonization treatment step
was 11.6 mN/dtex, the same procedure as in Example 6 was carried
out to produce a carbon fiber bundle. The processability in the
carbonization step was high and the resulting carbon fiber bundle
had good quality. It was highly rated as A in moldability.
Evaluation results of the carbon fiber obtained are described in
Table 1.
Example 9
[0108] Except that the twist count was 20 turns/m and that the
tension in the carbonization treatment step was 11.0 mN/dtex, the
same procedure as in Example 5 was carried out to produce a carbon
fiber bundle. The processability in the carbonization step was high
and the resulting carbon fiber bundle had good quality. It was very
highly rated as AA in moldability. Evaluation results of the carbon
fiber obtained are described in Table 1.
Example 10
[0109] Except that the twist count was 5 turns/m, the same
procedure as in Example 9 was carried out to produce a carbon fiber
bundle. The processability in the carbonization step was high and
the resulting carbon fiber bundle had good quality. It was very
highly rated as AA in moldability. Evaluation results of the carbon
fiber obtained are described in Table 1.
Example 11
[0110] Except that, unlike the Comprehensive Example, two precursor
fiber bundles were combined to integrate 6,000 single fibers, the
same procedure as in Example 3 was carried out to produce a carbon
fiber bundle. The processability in the carbonization step was high
and the resulting carbon fiber bundle had good quality. It was
highly rated as A in moldability. Evaluation results of the carbon
fiber obtained are described in Table 1.
Comparative Example 1
[0111] Except that the twist count was 0 turn/m and that the
tension in the carbonization treatment step was 5.3 mN/dtex, the
same procedure as in Example 1 was carried out to produce a carbon
fiber bundle. The processability in the carbonization step was high
and the resulting carbon fiber bundle had good quality. Since the
residual twist count was not within our range, it was rated as B in
moldability, indicating a deterioration compared with Example 1.
Evaluation results of the carbon fiber obtained are described in
Table 2.
Comparative Example 2
[0112] Except that the twist count was 0 turn/m and that the
tension and the maximum temperature in the carbonization treatment
step were 5.3 mN/dtex and 1,400.degree. C., respectively, the same
procedure as in Example 3 was carried out to produce a carbon fiber
bundle. The processability in the carbonization step was high and
the resulting carbon fiber bundle had good quality. Since the
residual twist count was not within our range, it was rated as B in
moldability, indicating a deterioration compared with Example 1.
Evaluation results of the carbon fiber obtained are described in
Table 2.
Comparative Example 3
[0113] Except that the tension in the carbonization treatment step
was 1.0 mN/dtex, the same procedure as in Example 2 was carried out
to produce a carbon fiber bundle. The processability in the
carbonization step was high and the resulting carbon fiber bundle
had good quality. Although it was highly rated as A in moldability,
the tension in the carbonization treatment step was not within our
range and, as a result, the carbon fiber prepared had a lower
elastic modulus than in Example 1. Evaluation results of the carbon
fiber obtained are described in Table 2.
Comparative Example 4
[0114] Except that the precursor fiber bundle for carbon fiber used
had a single-fiber fineness of 0.8 dtex and that the tension and
the maximum temperature in the carbonization treatment step were
10.3 mN/dtex and 1,400.degree. C., respectively, the same procedure
as in Example 2 was carried out to produce a carbon fiber bundle.
The processability in the carbonization step was high and the
resulting carbon fiber bundle had good quality. Since the precursor
fiber bundle for carbon fiber used had a small single-fiber
fineness, it was rated as B in moldability, indicating a
deterioration compared to Example 2. Evaluation results of the
carbon fiber obtained are described in Table 2.
Comparative Example 5
[0115] Except that the tension in the carbonization treatment step
was 1.0 mN/dtex and that specimen was free of twist, the same
procedure as in Example 2 was carried out to produce a carbon fiber
bundle. The processability in the carbonization step was high and
the resulting carbon fiber bundle had good quality. It was rated as
B in moldability, indicating a small deterioration. Evaluation
results of the carbon fiber bundle obtained are described in Table
2.
Comparative Example 6
[0116] Except that the precursor fiber bundle for carbon fiber used
had a single-fiber fineness of 0.8 dtex and that the tension and
the maximum temperature in the carbonization treatment step were
10.3 mN/dtex and 1,900.degree. C., respectively, the same procedure
as in Example 2 was carried out to produce a carbon fiber bundle.
The processability in the carbonization step was high and the
resulting carbon fiber bundle had good quality. Since the residual
twist count was not within our range, it was rated as B in
moldability, indicating a deterioration compared to Example 2.
Evaluation results of the carbon fiber bundle obtained are
described in Table 2.
Comparative Example 7
[0117] Except that the tension in the carbonization treatment step
was 1.6 mN/dtex, the same procedure as in Example 6 was carried out
to produce a carbon fiber bundle. The processability in the
carbonization step was high and the resulting carbon fiber bundle
had good quality. It was rated as B in moldability, indicating a
small deterioration. Evaluation results of the carbon fiber
obtained are described in Table 2.
Comparative Example 8
[0118] Except that the twist count was 0 turns/m, the same
procedure as in Example 3 was carried out to perform carbonization.
The phenomenon of fracturing of the fiber being treated in the
carbonization occurred so frequently that it was difficult to
sample the carbon fiber bundle.
Comparative Example 9
[0119] Except that the twist count was 0 turn/m, the same procedure
as in Example 2 was carried out to produce a carbon fiber bundle. A
certain degree of fuzz generation was seen in the carbonization
step, but it was possible to sample the carbon fiber bundle. The
carbon fiber bundle prepared suffered fuzz generation and was low
in quality. Since the residual twist count was not within our
range, it was rated as B in moldability, indicating a deterioration
compared to Example 2. Evaluation results are described in Table
2.
Comparative Example 10
[0120] Except that the tension in the carbonization treatment step
was 3.4 mN/dtex, the same procedure as in Comparative Example 9 was
carried out to produce a carbon fiber bundle. The processability in
the carbonization step was high and the resulting carbon fiber
bundle had good quality. Since the tension in the carbonization
treatment step was not within our range, the carbon fiber prepared
had a lower elastic modulus than in Example 2. Furthermore, since
the residual twist count was not within our range, it was rated as
B in moldability, indicating a deterioration compared to Example 2.
Evaluation results are described in Table 2.
Comparative Example 11
[0121] Except that, unlike the Comprehensive Example, two precursor
fiber bundles were combined to integrate 6,000, that the twist
count was 0 turn/m, and that the tension in the carbonization
treatment step was 3.4 mN/dtex, the same procedure as in Example 2
was carried out to produce a carbon fiber bundle. The
processability in the carbonization step was high and the resulting
carbon fiber bundle had good quality. Since the tension in the
carbonization treatment step was not within our range, the carbon
fiber prepared had a lower elastic modulus than in Example 2. Since
the residual twist count and the total fineness were not within our
ranges, it was rated as C in moldability, indicating a
deterioration compared to Example 2. Evaluation results are
described in Table 2.
Comparative Example 12
[0122] Except that the twist count was 50 turn/m, the same
procedure as Comparative Example 11 was carried out to produce a
carbon fiber bundle. The processability in the carbonization step
was high and the resulting carbon fiber bundle had good quality.
Since the tension in the carbonization treatment step was not
within our range, the carbon fiber prepared had a lower elastic
modulus than in Example 2. Since the total fineness was not within
our range, it was rated as B in moldability, indicating a
deterioration compared to Example 2. Evaluation results are
described in Table 2.
Comparative Example 13
[0123] Except that, unlike the Comprehensive Example, the precursor
fiber bundle had a single-fiber fineness of 0.8 dtex and that the
tension in the carbonization treatment step was 3.4 mN/dtex, the
same procedure as Example 2 was carried out to produce a carbon
fiber bundle. The processability in the carbonization step was high
and the resulting carbon fiber bundle had good quality. Since the
tension in the carbonization treatment step was not within our
range, the carbon fiber prepared had a lower elastic modulus than
in Example 2. Since the precursor fiber bundle for carbon fiber
used had a small single-fiber fineness, it was rated as B in
moldability, indicating a deterioration compared to Example 2.
Evaluation results are described in Table 2.
Comparative Example 14
[0124] Except that the twist count was 0 turn/m, the same procedure
as Comparative Example 13 was carried out to produce a carbon fiber
bundle. The processability in the carbonization step was high and
the resulting carbon fiber bundle had good quality. Since the
tension in the carbonization treatment step was not within our
range, the carbon fiber prepared had a lower elastic modulus than
in Example 2. Since the precursor fiber bundle for carbon fiber
used had a small single-fiber fineness and the residual twist count
was not within our range, it was rated as D in moldability,
indicating a lower stability compared to Example 2. Evaluation
results are described in Table 2.
Comparative Example 15
[0125] Except that, unlike the Comprehensive Example, two precursor
fiber bundles were combined to integrate 6,000 single fibers, the
same procedure as Comparative Example 13 was carried out to produce
a carbon fiber bundle. The processability in the carbonization step
was high and the resulting carbon fiber bundle had good quality.
Since the tension in the carbonization treatment step was not
within our range, the carbon fiber prepared had a lower elastic
modulus than in Example 2. Since the precursor fiber bundle for
carbon fiber used had a small single-fiber fineness and the total
fineness was not within our range, it was rated as C in
moldability, indicating a deterioration compared to Example 2.
Evaluation results are described in Table 2.
Comparative Example 16
[0126] Except that the twist count was 0 turn/m, the same procedure
as Comparative Example 15 was carried out to produce a carbon fiber
bundle. The processability in the carbonization step was high and
the resulting carbon fiber bundle had good quality. Since the
tension in the carbonization treatment step was not within our
range, the carbon fiber prepared had a lower elastic modulus than
in Example 2. Since the precursor fiber bundle for carbon fiber
used had a small single-fiber fineness and the residual twist count
the total fineness was not within our range, it was rated as D in
moldability, indicating a lower stability compared to Example 2.
Evaluation results are described in Table 2.
Reference Example 1
[0127] Evaluation results of Torayca (registered trademark) T700S
(manufactured by Toray Industries, Inc.) are described in Table 2.
A sized specimen had a knot strength of 826 MPa. It was rated as B
in moldability, indicating a small deterioration.
Reference Example 2
[0128] Evaluation results of Torayca (registered trademark) M35J
(manufactured by Toray Industries, Inc.) are described in Table
2.
Reference Example 3
[0129] Evaluation results of Torayca (registered trademark) M40J
(manufactured by Toray Industries, Inc.) are described in Table
2.
Reference Example 4
[0130] Evaluation results of Torayca (registered trademark) M46J
(manufactured by Toray Industries, Inc.) are described in Table
2.
Reference Example 5
[0131] Evaluation results of Torayca (registered trademark) M40
(manufactured by Toray Industries, Inc.) are described in Table
2.
TABLE-US-00001 TABLE 1 precursor fiber twisting carbonization
single-fiber twist maximum filament number in fineness count
temperature tension carbonization step dtex turns/m .degree. C.
mN/dtex number Example 1 1.1 75 1,900 6.1 12,000 Example 2 1.1 50
1,900 5.2 12,000 Example 3 1.1 50 1,900 10.2 12,000 Example 4 1.1
20 1,900 10.3 12,000 Example 5 1.1 50 1,900 10.2 24,000 Example 6
1.1 50 2,350 6.5 12,000 Example 7 1.1 50 2,350 9.1 12,000 Example 8
1.1 50 2,350 11.6 12,000 Example 9 1.1 20 1,900 11.0 24,000 Example
10 1.1 5 1,900 11.0 24,000 Example 11 1.1 50 1,900 10.2 6,000
carbon fiber twist angle orientation average single- of parameter
average orientation fiber single- strand residual fiber of crystal-
parameter of elastic loop form- form- form- moldability fiber total
strand elastic twist surface crystallite crystallites lite size
crystallites knot formula mod- fracture ula ula ula running
diameter density fineness strength modulus count layer size L.sub.c
.pi..sub.002 L.sub.c(s) .pi..sub.002 (s) strength (2) ulus load (1)
(3) (4) stability .mu.m g/cm.sup.3 g/km GPa GPa turns/m .degree. nm
% nm % MPa yes/no GPa N yes/no yes/no yes/no -- Example 7.4 1.72
888 4.1 367 73 10.5 2.8 85.2 2.8 85.5 343 yes 350 0.49 yes yes yes
AA 1 Example 7.3 1.74 874 5.3 364 47 6.7 2.9 85.6 2.9 85.5 358 yes
345 0.52 yes yes yes AA 2 Example 7.2 1.74 850 4.3 392 47 6.6 2.9
87.2 2.9 87.1 343 yes 370 0.47 yes yes yes AA 3 Example 7.2 1.74
850 4.5 408 19 2.7 2.9 87.1 3.0 86.8 307 yes 400 0.47 yes yes yes
AA 4 Example 7.2 1.73 1690 4.1 385 47 9.4 2.9 87.2 2.9 86.8 375 yes
370 0.46 yes yes yes AA 5 Example 6.5 1.91 761 4.0 480 47 6.0 4.6
88.2 4.5 88.4 186 yes 464 0.31 yes no yes A 6 Example 6.5 1.91 761
4.4 499 47 6.0 4.5 90.2 4.5 90.0 219 yes 470 0.30 yes no yes A 7
Example 6.6 1.84 755 4.3 493 47 6.1 4.5 90.4 4.5 90.3 156 yes 473
0.30 yes no yes A 8 Example 7.2 1.74 1700 4.5 400 18 3.6 2.9 87.3
2.9 87.0 370 yes 388 0.47 yes yes yes AA 9 Example 7.2 1.74 1700
4.8 405 4 0.8 2.9 87.5 2.9 87.1 380 yes 392 0.46 yes yes yes AA 10
Example 7.2 1.74 425 4.4 394 47 4.7 2.9 87.2 2.9 87.1 343 yes 374
0.47 yes yes yes A 11
TABLE-US-00002 TABLE 2 precursor fiber twisting carbonization
single-fiber twist maximum filament number in fineness count
temperature tension carbonization step dtex turns/m .degree. C.
mN/dtex number Comparative 1.1 0 1,900 5.3 12,000 Example 1
Comparative 1.1 0 1,400 5.4 12,000 Example 2 Comparative 1.1 50
1,900 1.0 12,000 Example 3 Comparative 0.8 50 1,400 10.3 12,000
Example 4 Comparative 1.1 0 1,900 1.0 12,000 Example 5 Comparative
0.8 50 1,900 10.3 12,000 Example 6 Comparative 1.1 50 2,350 1.6
12,000 Example 7 Comparative 1.1 0 1,900 10.2 12,000 Example 8
Comparative 1.1 0 1,900 5.2 12,000 Example 9 Comparative 1.1 0
1,900 3.4 12,000 Example 10 Comparative 1.1 0 1,900 3.4 6,000
Example 11 Comparative 1.1 50 1,900 3.4 6,000 Example 12
Comparative 0.8 50 1,900 3.4 12,000 Example 13 Comparative 0.8 0
1,900 3.4 12,000 Example 14 Comparative 0.8 50 1,900 3.4 6,000
Example 15 Comparative 0.8 0 1,900 3.4 6,000 Example 16 Reference
-- -- -- -- 12,000 Example 1 Reference -- -- -- -- 12,000 Example 2
Reference -- -- -- -- 12,000 Example 3 Reference -- -- -- -- 12,000
Example 4 Reference -- -- -- -- 6,000 Example 5 carbon fiber orien-
average tation orien- twist para- tation angle meter para- single-
strand res- of of average meter of fiber loop mold- single- elastic
idual fiber crystal- crystal- crystal- crystal- form- elastic frac-
form- form- form- ability fiber total strand mod- twist surface
like lites lite lites knot ula mod- ture ula ula ula running
diameter density fineness strength ulus count layer size L.sub.c
.pi..sub.002 size L.sub.c(s) .pi..sub.002(s) strength (2) ulus load
(1) (3) (4) stability .mu.m g/cm.sup.3 g/km GPa GPa turns/m
.degree. nm % nm % MPa yes/no GPa N yes/no yes/no yes/no --
Comparative 7.4 1.72 888 4.3 348 0 0.0 2.8 85.3 2.8 85.6 340 no 335
0.45 no yes yes B Example 1 Comparative 7.4 1.79 924 4.6 314 0 0.0
2.0 82.6 2.0 82.3 402 no 301 0.50 no yes yes B Example 2
Comparative 7.5 1.70 901 4.3 294 47 6.9 2.8 82.5 2.8 82.7 360 no
287 0.46 no no no A Example 3 Comparative 5.3 1.81 479 5.3 361 47
4.9 2.1 85.1 2.1 85.5 391 yes 355 0.33 no yes yes B Example 4
Comparative 7.5 1.71 914 4.3 296 0 0.0 2.7 82.6 2.7 82.3 323 no 282
0.51 no no no B Example 5 Comparative 5.3 1.81 475 5.0 391 47 4.9
2.9 87.2 2.9 86.7 287 yes 378 0.28 no yes yes B Example 6
Comparative 6.8 1.90 828 3.3 412 47 6.3 4.6 86.9 4.5 86.3 219 no
393 0.32 no no no B Example 7 Comparative Sampling of carbon fiber
is difficult. Example 8 Comparative 7.4 1.74 898 4.6 350 0 0.0 2.8
85.2 2.8 85.4 343 no 340 0.44 no yes yes B Example 9 Comparative
7.4 1.74 898 4.7 340 0 0.0 2.9 84.0 2.9 83.9 355 no 325 0.46 no no
no B Example 10 Comparative 7.4 1.74 449 4.8 335 0 0.0 2.8 84.1 2.8
84.2 350 no 320 0.47 no no yes C Example 11 Comparative 7.4 1.74
449 4.5 339 47 4.8 2.9 84.2 2.9 83.9 352 no 322 0.47 no no no B
Example 12 Comparative 5.4 1.80 495 5.0 342 47 5.0 2.9 83.9 2.9
84.1 325 no 324 0.28 no no yes B Example 13 Comparative 5.4 1.80
495 5.0 340 0 0.0 2.9 84.1 2.9 84.2 321 no 325 0.28 no no no D
Example 14 Comparative 5.4 1.80 247 4.9 337 47 3.5 2.9 83.9 2.9
83.9 320 no 322 0.27 no no no C Example 15 Comparative 5.4 1.80 247
4.9 333 0 0.0 2.9 83.9 2.9 84.0 323 no 323 0.27 no no no D Example
16 Reference 7.0 1.80 831 4.9 230 0 -- -- -- 1.7 -- 262 no 232 0.43
no -- -- B Example 1 Reference 5.2 1.75 446 4.7 343 0 -- -- -- 3.3
-- 313 no 333 0.28 no -- -- C Example 2 Reference 5.2 1.75 446 4.4
377 0 -- -- -- 3.7 -- 127 no 350 0.24 no -- -- C Example 3
Reference 5.1 1.84 451 4.2 436 0 -- -- -- 4.9 -- 98 no 413 0.25 no
-- -- C Example 4 Reference 6.6 1.77 363 3.0 387 0 -- -- -- -- -- 8
no 375 0.18 no -- -- C Example 5
INDUSTRIAL APPLICABILITY
[0132] Our carbon fiber has both a high tensile modulus and a high
moldability in composite production and easily maintains a required
fiber length even when used in the form of discontinuous fibers,
also relates to a method for the production thereof. With such good
features, our carbon fiber bundles produced according to our method
can be used suitably for manufacturing members of aircraft,
automobiles, ships and the like, and general industrial articles
including sporting goods such as golf shafts and fishing rods.
* * * * *