U.S. patent application number 16/623479 was filed with the patent office on 2020-06-18 for carbon fiber bundle and method of manufacturing same.
The applicant listed for this patent is Toray Industries, Inc.. Invention is credited to Naohiro Matsumoto, Haruki Okuda, Fumihiko Tanaka.
Application Number | 20200190705 16/623479 |
Document ID | / |
Family ID | 65002559 |
Filed Date | 2020-06-18 |
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United States Patent
Application |
20200190705 |
Kind Code |
A1 |
Matsumoto; Naohiro ; et
al. |
June 18, 2020 |
CARBON FIBER BUNDLE AND METHOD OF MANUFACTURING SAME
Abstract
A carbon fiber bundle is obtained by filtering a spinning dope
solution in which a polyacrylonitrile copolymer is dissolved in a
solvent, at a predetermined filtration speed, using a filter medium
having a predetermined particle retention and a filter basis
weight, then spinning the filtered spinning dope solution to obtain
a precursor fiber bundle for carbon fiber, and heat-treating the
obtained precursor fiber bundle for carbon fiber at an appropriate
temperature profile in an oxidizing atmosphere until reaching a
predetermined density to obtain an oxidized fiber bundle, and then
heat-treating the oxidized fiber bundle at a predetermined
temperature in an inert atmosphere.
Inventors: |
Matsumoto; Naohiro;
(lyo-gun, JP) ; Okuda; Haruki; (lyo-gun, JP)
; Tanaka; Fumihiko; (lyo-gun, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Toray Industries, Inc. |
Tokyo |
|
JP |
|
|
Family ID: |
65002559 |
Appl. No.: |
16/623479 |
Filed: |
June 28, 2018 |
PCT Filed: |
June 28, 2018 |
PCT NO: |
PCT/JP2018/024513 |
371 Date: |
December 17, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
D01F 6/18 20130101; D01F
9/22 20130101 |
International
Class: |
D01F 9/22 20060101
D01F009/22; D01F 6/18 20060101 D01F006/18 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 10, 2017 |
JP |
2017-134359 |
Claims
1-7. (canceled)
8. A method of manufacturing a carbon fiber bundle comprising:
filtering a spinning dope solution in which a polyacrylonitrile
copolymer is dissolved in a solvent with a filter medium having a
particle retention B (.mu.m) and a filter basis weight D
(g/m.sup.2), under conditions where a filtration speed A (cm/hour)
satisfies equations (1) to (3), spinning the filtered spinning dope
solution to obtain a precursor fiber bundle for carbon fiber,
D-600/(.alpha..times..beta.).gtoreq.0 (1) .alpha.=1-1/(1+exp(7-A))
(2) .beta.=1-1/(1+exp(-0.23.times.B)) (3) heat-treating the
obtained precursor fiber bundle for carbon fiber in an oxidizing
atmosphere until a density reaches 1.32 to 1.35 g/cm.sup.3,
heat-treating at 275.degree. C. or more and 295.degree. C. or less
in an oxidizing atmosphere until a density reaches 1.46 to 1.50
g/cm.sup.3 to obtain an oxidized fiber bundle, and heat-treating
the oxidized fiber bundle at 1200 to 1800.degree. C. in an inert
atmosphere.
9. The method according to claim 8, wherein a tension of the
oxidized fiber bundle when heat-treated at 275.degree. C. or more
and 295.degree. C. or less in an oxidizing atmosphere until the
density reaches 1.46 to 1.50 g/cm.sup.3 is 1.6 to 4.0 mN/dtex.
10. The method according to claim 8, further comprising
heat-treating the precursor fiber bundle for carbon fiber at
210.degree. C. or more and less than 245.degree. C. in an oxidizing
atmosphere until the density reaches 1.22 to 1.24 g/cm.sup.3, and
subjecting to the heat treatment process in an oxidizing atmosphere
until the density reaches 1.32 to 1.35 g/cm.sup.3, wherein the heat
treatment process performed until the density reaches 1.32 to 1.35
g/cm.sup.3 is performed at a temperature of 245.degree. C. or more
and less than 275.degree. C.
11. A carbon fiber bundle having an elastic modulus of strands of
240 to 280 GPa, a tensile strength of strands of 5.8 GPa or more, a
knot strength K [MPa] of -88d+1390.ltoreq.K (d: average single
fiber diameter [.mu.m]), and an average single fiber diameter of
6.5 to 8.0 .mu.m, wherein a probability that a flaw with a size of
50 nm or more exists on a fracture surface, which is collected when
a single fiber tensile test is performed with a gauge length of 10
mm, is 35% or less.
12. The carbon fiber bundle according to claim 11, having a knot
strength K of 770 MPa or more.
13. The carbon fiber bundle according to claim 11, having a mean
surface roughness Ra of 1.0 to 1.8 nm.
14. The carbon fiber bundle according to claim 11, wherein a skin
layer ratio of the single fibers of the carbon fibers is 90% or
more by area.
Description
TECHNICAL FIELD
[0001] This disclosure relates to a carbon fiber bundle and a
method of manufacturing the same.
BACKGROUND
[0002] Carbon fiber bundles have been widely used as reinforcing
fibers for composite materials, and there is a strong demand for
more performance. In particular, to reduce the weight of members
such as pressure vessels, it is required to improve mechanical
properties such as the tensile strength of resin-impregnated
strands and elastic modulus of resin-impregnated strands of the
carbon fiber bundle (hereinafter tensile strength of strands and
elastic modulus of strands) in a well-balanced manner. At the same
time, it is necessary to reduce the environmental load in the
manufacture of carbon fiber bundles. Generally, a
polyacrylonitrile-based carbon fiber bundle is obtained through a
process in which a precursor fiber bundle for carbon fiber is
heat-treated in an oxidizing atmosphere of 200 to 300.degree. C.
(oxidation process) and then heat-treated in an inert atmosphere of
1000.degree. C. or more (carbonization process). At that time,
since carbon, nitrogen and hydrogen atoms contained in
polyacrylonitrile are desorbed by thermal degradation, the yield of
the carbon fiber bundles (hereinafter carbonization yield) is about
half. It is necessary to increase the yield of carbon fiber bundles
with the same manufacturing energy from the viewpoint of reducing
the manufacturing energy per production amount, that is,
environmental load.
[0003] For this reason, so far, many techniques have been proposed
for the purpose of improving the tensile strength of strands or
carbonization yield of carbon fiber bundles by optimizing the
oxidizing conditions (Japanese Patent Laid-open Publication Nos.
2012-82541, S58-163729, H6-294020, 2013-23778 and 2014-74242).
[0004] In JP '541, studies have been made to improve the tensile
strength of strands of the carbon fiber bundle by minimizing the
amount of heat (Jh/g) given by high-temperature treatment in the
oxidation process. In JP '729, it was proposed to set the oxidation
temperature to a high temperature according to the amount of oxygen
added in the middle of the oxidation process and, in JP '020, it
was proposed to oxidize at a temperature as high as possible by
repeating heating and cooling so that the precursor fiber bundle
for carbon fiber does not thermally run away to shorten the
oxidation process. Moreover, JP '778 and JP '242 proposed attempts
to increase the carbonization yield by increasing the density of
oxidized fiber bundle in a short time by heating the precursor
fiber bundle for carbon fiber in an oxidizing atmosphere in an
initial stage of oxidation, and then bringing it into contact with
a high-temperature heating roller at 250 to 300.degree. C.
[0005] WO 2013/157613 A and Japanese Patent Laid-open Publication
No. 2015-096664 have proposed carbon fiber bundles with high knot
strength that reflect mechanical properties in a direction other
than a fiber axis direction and exhibit sufficient mechanical
properties in pseudo-isotropic materials.
[0006] Japanese Patent Laid-open Publication No. 2017-66580 has
proposed a carbon fiber bundle exhibiting a high carbonization
yield, excellent tensile strength of strands and elastic modulus of
strands in a well-balanced manner, and further satisfies excellent
knot strength at the same time since an oxidized fiber bundle
having a specific density can be obtained by performing
high-temperature heat treatment in the latter half at an
appropriate temperature profile in the oxidation process when
obtaining an oxidized fiber bundle having a specific density to
satisfy a high carbonization yield.
[0007] On the other hand, carbon fiber is a brittle material, and
since a slight surface flaw and an internal flaw cause a decrease
in tensile strength of strands, delicate attention has been paid to
the generation of flaws. For example, Japanese Examined Patent
Publication No. H8-6210 has proposed to reduce flaws on the surface
of the carbon fiber to obtain a carbon fiber bundle having a high
tensile strength of strands by densification of the precursor fiber
bundle for carbon fiber, reduction of dust during the manufacturing
process, and removal of flaw by electrochemical treatment.
[0008] However, in JP '541, an attempt has been made to reduce the
integrated value of the amount of heat given in the oxidation
process, which is not sufficient to achieve both the tensile
strength of strands and the carbonization yield. In addition, in JP
'729 and JP '020, since the oxidation temperature has been
increased and the oxidation time has been shortened, the oxidation
temperature control that can satisfy the required tensile strength
of strands is not performed, and suppression of stress
concentration on a difference between skin-core structure has been
a problem. Moreover, in JP '778 and JP '242, while heat treatment
has been performed at a high temperature using a heating roller
with high heat transfer efficiency to perform heat treatment at a
high temperature in a short time in the latter half of the
oxidation process, sufficient tensile strength of strands has not
been obtained due to too short heat treatment time at a high
temperature, and generation of flaws due to adhesion between single
fibers when passing through the roller. Although WO '613 states
that the knot strength is increased by adjusting the oxidation
process even when the single fiber diameter is large, the effect is
limited due to the structure distribution in the single fiber at
the time of oxidation, and the level of knot strength has been
insufficient. Although JP '664 states that the knot strength is
increased by mainly adjusting the surface treatment of the carbon
fiber bundle and the sizing agent, it is limited to those having a
low single fiber diameter, and in having a low single fiber
diameter, the fracture tension of the single fiber is lowered
during the manufacturing process so that there is a problem that
the quality of the manufacturing process is lowered due to fiber
fracture. In JP '580, the tensile strength of strands and knot
strength have been increased by high-temperature heat treatment in
the latter half at an appropriate temperature profile in the
oxidation process, but the control of flaws affecting these
characteristics has been not sufficient, and there has been room
for improvement. In JP '210, although the flaws on the carbon fiber
surface can be effectively removed by electrochemical treatment,
strong electrochemical treatment is required to remove the flaws,
and a long electrochemical treatment bath is required so that there
has been a problem that it is difficult to implement industrially.
In addition, there has been also a problem that a brittle layer
that may lead to deterioration of mechanical properties of a
composite is formed on the surface of the carbon fiber by strong
electrochemical treatment. Furthermore, as a flaw, characteristics
of flaws in the fracture surface collected when the single fiber
tensile test was performed with a gauge length of 50 mm are
defined. However, the gauge length that affects the tensile
strength of strands and the tensile strength of the composite
material is shorter than 10 mm so that there has been also an
essential problem that the carbon fiber bundles that increase the
tensile strength of the composite material are not necessarily
obtained simply by defining the characteristics of flaws observed
at a gauge length of 50 mm.
[0009] It could therefore be helpful to provide a method of
manufacturing a carbon fiber bundle that exhibits the tensile
strength of strands and the elastic modulus of strands in a
well-balanced manner and has excellent knot strength without
impairing productivity.
SUMMARY
[0010] We thus provide a method of manufacturing a carbon fiber
bundle including:
[0011] filtering a spinning dope solution in which a
polyacrylonitrile copolymer is dissolved in a solvent, using a
filter medium having a particle retention B (.mu.m) and a filter
basis weight D (g/m.sup.2), under conditions where a filtration
speed A (cm/hour) satisfies equations (1) to (3),
[0012] spinning the filtered spinning dope solution to obtain a
precursor fiber bundle for carbon fiber,
D-600/(.alpha..times..beta.).gtoreq.0 (1)
.alpha.=1-1/(1+exp(7-A)) (2)
.beta.=1-1/(1+exp(-0.23.times.B)) (3)
[0013] heat-treating the obtained precursor fiber bundle for carbon
fiber in an oxidizing atmosphere until the density reaches 1.32 to
1.35 g/cm.sup.3,
[0014] heat-treating at 275.degree. C. or more and 295.degree. C.
or less in an oxidizing atmosphere until the density reaches 1.46
to 1.50 g/cm.sup.3 to obtain an oxidized fiber bundle, and
[0015] heat-treating the oxidized fiber bundle at 1200 to
1800.degree. C. in an inert atmosphere.
[0016] Also, the carbon fiber bundle has an elastic modulus of
strands of 240 to 280 GPa, a tensile strength of strands of 5.8 GPa
or more, a knot strength K [MPa] of -88d+1390.ltoreq.K (d: average
single fiber diameter [.mu.m]), and an average single fiber
diameter of 6.5 to 8.0 .mu.m, wherein a probability that a flaw
with a size of 50 nm or more exists on a fracture surface, which is
collected when a single fiber tensile test is performed with a
gauge length of 10 mm, is 35% or less.
[0017] When obtaining an oxidized fiber bundle, an oxidized fiber
bundle having a specific density can be obtained by heat-treating
at an appropriate temperature profile in the oxidation process,
whereby flaws governing the tensile strength of strands and the
knot strength are controlled to be very small, and thus a carbon
fiber bundle exhibiting the tensile strength of strands and the
elastic modulus of strands in a well-balanced manner and has
excellent knot strength can be manufactured without impairing
productivity. Moreover, the carbon fiber bundle satisfies
productivity at the time of manufacturing a composite material.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a scanning electron microscope (SEM) image of a
fracture surface of a carbon fiber. Radial streaks converging to
one point are confirmed.
[0019] FIG. 2 is an enlarged image of the vicinity of a fracture
origin in FIG. 1. Flaws like attached substances are confirmed.
[0020] FIG. 3 is an enlarged image of the vicinity of a fracture
origin of another fracture surface. Flaws like dents are
confirmed.
[0021] FIG. 4 is an enlarged image of the vicinity of a fracture
origin of another fracture surface. No noticeable morphological
features of 50 nm or more are confirmed.
DESCRIPTION OF REFERENCE SIGNS
(i): Fracture Origin
DETAILED DESCRIPTION
[0022] The carbon fiber bundle has a tensile strength of strands of
5.8 GPa or more, and preferably 6.0 GPa or more. In a tensile
strength of strands of 5.8 GPa or more, when a composite material
is manufactured using the carbon fiber bundle, the composite
material exhibits good tensile strength. The higher tensile
strength of strands of the carbon fiber bundle is better. However,
even when the tensile strength of strands is 7.0 GPa or less, a
sufficient tensile strength of the composite material can be
obtained. The tensile strength of strands can be determined by a
method described in the strand tensile test of the carbon fiber
bundle described later. In addition, this tensile strength of
strands can be controlled by using the method of manufacturing a
carbon fiber bundle described below.
[0023] The carbon fiber bundle has an elastic modulus of strands of
240 to 280 GPa, preferably 245 to 275 GPa, and more preferably 250
to 270 GPa. An elastic modulus of strands of 240 to 280 GPa is
preferable because of excellent balance between the elastic modulus
of strands and the tensile strength of strands. In particular, by
controlling the elastic modulus of strands to 250 to 270 GPa, a
carbon fiber bundle having excellent tensile strength of strands is
easily obtained. The elastic modulus of strands can be determined
by a method described in the strand tensile test of the carbon
fiber bundle described below. At this time, the strain range is set
to 0.1 to 0.6%. The elastic modulus of strands of the carbon fiber
bundle can be controlled mainly by applying tension to the fiber
bundle in any of the heat treatment processes in the manufacturing
process of the carbon fiber bundle, improving the difference
between skin-core structure that is the structure distribution
within the single fiber, or changing the carbonization
temperature.
[0024] Also, in the carbon fiber bundle, a knot strength K obtained
by forming a knot part at the midpoint portion of the carbon fiber
bundle and performing a fiber bundle tensile test is preferably 700
MPa or more, more preferably 740 MPa or more, and further
preferably 770 MPa or more. The knot strength can be obtained by a
method described in the knot strength of the carbon fiber bundle
described below. The knot strength is an index that reflects
mechanical properties of the fiber bundle in a direction other than
a fiber axis direction. When manufacturing a composite material, a
force in a bending direction is loaded on the carbon fiber bundle.
When the number of filaments is increased to manufacture the
composite material efficiently, it is difficult to increase the
running speed of the fiber bundle during manufacture of the
composite material due to generation of fuzz. However, when having
a knot strength of 700 MPa or more, even in conditions where the
running speed of the fiber bundle is high, a composite material
with high quality can be obtained. To increase the knot strength of
the carbon fiber bundle, in the method of manufacturing a carbon
fiber bundle described below, it is particularly preferable to
control so that structural parameters in an oxidation process and a
precarbonization process are within preferable ranges. Further, the
knot strength can be also increased by reducing flaws on the
surface of the carbon fiber.
[0025] The carbon fiber bundle preferably has the number of
filaments of 10,000 to 60,000. When the number of filaments is
10,000 or more, a composite material can be manufactured with high
productivity. When the number of filaments is 60,000 or less, the
generation of fuzz at the time of manufacturing a composite
material can be suppressed, and the running speed of the fiber
bundle is increased so that the productivity is easily
increased.
[0026] Moreover, the carbon fiber bundle has a knot strength K
[MPa] (=N/mm.sup.2) of -88d+1390.ltoreq.K (where d is an average
single fiber diameter [.mu.m]). It is preferable that the carbon
fiber bundle satisfies a relational expression -88d+1410.ltoreq.K.
Such a relational expression indicates that the knot strength is
high for the average single fiber diameter. When the knot strength
K satisfies -88d+1390.ltoreq.K, during a filament winding molding
process, even in a carbon fiber bundle with a large average single
fiber diameter which is prone to fuzz due to abrasion with guide
parts or rollers, it is possible to suppress the generation of fuzz
and mold by increasing the running speed of the fiber bundle. To
satisfy this relational expression, it is preferable to
appropriately set the oxidizing conditions according to the average
single fiber diameter by the manufacturing method described
below.
[0027] In the carbon fiber bundle, the probability that a flaw with
a size of 50 nm or more exists on the fracture surface, which is
collected when a single fiber tensile test is performed with a
gauge length of 10 mm, is preferably 35% or less, more preferably
30% or less, and further preferably 25% or less. It is known that
the tensile fracture of carbon fiber starts from flaws. It is known
that there are various types of flaws to be fracture origins of
carbon fibers such as voids, damage on the fiber surface, dents,
attached substances, or adhesion marks that remain after single
fibers adhere to each other by the heat of heat treatment. However,
morphological features that can be observed by scanning electron
microscope (SEM) observation are collectively referred to as
"flaws" without particularly distinguishing all of them. We found
that the tensile strength of strands of the carbon fiber bundle is
greatly increased when the probability that a flaw with a size of
50 nm or more exists on the fracture surface, which is collected
when a single fiber tensile test is performed with a gauge length
of 10 mm, is set to 35% or less. What is important is that the
gauge length is set to 10 mm. When a single fiber tensile test was
performed with a longer gauge length, for example, a gauge length
of 50 mm, even if the probability that a flaw of a certain size or
larger exists was examined as described above, we found that the
probability is not necessarily correlated with the tensile strength
of strands and the tensile strength of the composite material. The
reason why it is effective to set the gauge length to 10 mm is
considered that the gauge length that governs the tensile strength
of strands and the tensile strength of the composite material
(generally referred to as effective gauge length) is shorter than
10 mm. The probability that a flaw with a size of 50 nm or more
exists on the fracture surface, which is collected when a single
fiber tensile test is performed with a gauge length of 10 mm, is
set to 35% or less, whereby flaws affecting the tensile strength of
strands of the carbon fiber bundle and the tensile strength of the
composite material are effectively reduced, as a result, the
tensile strength of strands and the tensile strength of the
composite material reach high levels. "The probability that a flaw
with a size of 50 nm or more exists on the fracture surface which
is collected when a single fiber tensile test is performed with a
gauge length of 10 mm" is reduced by controlling filtration
conditions of a spinning dope solution, that are filtration speed,
particle retention, and filter basis weight, according to the
methods described below, and effectively removing foreign
substances in the spinning dope solution.
[0028] In the carbon fiber bundle, the average single fiber
diameter is 6.5 to 8.0 .mu.m, preferably 6.7 to 8.0 .mu.m, more
preferably 7.0 to 8.0 .mu.m, further preferably 7.3 to 8.0 .mu.m,
and most preferably 7.5 to 8.0 .mu.m. As the average single fiber
diameter is smaller, the difference between skin-core structure
tends to decrease. However, when a composite material is prepared,
it may cause insufficient impregnation due to a high matrix resin
viscosity, which may lower the tensile strength of the composite
material. An average single fiber diameter of 6.5 to 8.0 .mu.m is
preferred because insufficient impregnation of a matrix resin is
unlikely to occur and a high carbonization yield and tensile
strength of strands are stably exhibited. The average single fiber
diameter can be calculated from the mass and density per unit
length of the carbon fiber bundle and the number of filaments. The
average single fiber diameter of the carbon fiber bundle is
increased by increasing the average single fiber diameter of the
precursor fiber bundle for carbon fiber, increasing the
carbonization yield in the carbonization process by controlling the
oxidizing conditions, and lowering the pre-carbonization stretch
ratio.
[0029] The carbon fiber bundle preferably has a mean surface
roughness Ra of a single fiber surface measured by an atomic force
microscope (AFM) of 1.8 nm or less. Details of the measurement
method will be described later. The mean surface roughness of the
precursor fiber bundle for carbon fiber is substantially maintained
even in the carbon fiber bundle. The mean surface roughness is
preferably 1.0 to 1.8 nm, and further preferably 1.6 nm or less.
When the mean surface roughness exceeds 1.8 nm, it tends to be a
stress concentration point during tension, and the tensile strength
of strands may decrease. The lower mean surface roughness is
better. However, when the mean surface roughness is less than 1.0
nm, the effect is often almost saturated. The mean surface
roughness of the carbon fiber bundle can be controlled by
appropriately controlling spinning conditions of the precursor
fiber bundle for carbon fiber (spinning method and coagulation bath
conditions) and reducing the surface flaws of the carbon fiber
bundle.
[0030] The carbon fiber bundle has an area ratio (hereinafter the
skin layer ratio) in a cross section of a blackened thickness of an
outer peripheral portion of the cross section perpendicular to the
fiber axis direction of the single fiber of the carbon fiber of
preferably 90% by area or more, more preferably 90 to 95% by area,
and further preferably 90 to 93% by area. The skin layer ratio is
an area ratio (%) obtained by dividing an area occupied by the
blackened thickness seen in the outer peripheral portion when
observing a cross section perpendicular to the fiber axis direction
of the single fiber of the carbon fiber with an optical microscope,
by an entire cross-sectional area. Since the degree of orientation
of the crystal part is low and the elastic modulus of strands is
low in the inside of the blackened thickness of the single fiber of
the carbon fiber, the higher the skin layer ratio, the more the
surface layer stress concentration can be suppressed so that high
tensile strength of strands can be exhibited. When the skin layer
ratio is low, a high carbonization yield and a high tensile
strength of strands are hardly exhibited. When the skin layer ratio
is 90% by area or more, the ratio of stress-bearing part on the
outer peripheral portion is sufficiently large so that stress
concentration on the surface layer is suppressed. When the skin
layer ratio exceeds 95% by area, the effect of suppressing stress
concentration on the surface layer is saturated, on the other hand,
the tensile strength of strands may decrease due to deviation of
the oxidization temperature from optimum temperature. The blackened
thickness can be measured by embedding a carbon fiber bundle in a
resin, polishing a cross section perpendicular to the fiber axis
direction, and observing the cross section with an optical
microscope. Details will be described later.
[0031] Our method of manufacturing a carbon fiber bundle is based
on the fact that we found that a carbon fiber bundle in which the
number of flaws governing the tensile strength of strands and the
knot strength is controlled to be extremely small, and a high
carbonization yield and excellent tensile strength of strands and
knot strength are exhibited is obtained by performing
high-temperature heat treatment in the latter half at an
appropriate temperature profile in the oxidation process so that
the oxidized fiber bundle has a specific density. A preferred
example will be described in detail below.
[0032] The precursor fiber bundle for carbon fiber can be obtained
by spinning a spinning dope solution in which a polyacrylonitrile
copolymer is dissolved in a solvent. At this time, the spinning
dope solution is filtered under specific conditions to effectively
remove foreign substances in the spinning dope solution, and then
the filtered spinning dope solution is spun to obtain a precursor
fiber bundle for carbon fiber. The obtained precursor fiber bundle
for carbon fiber is subjected to at least an oxidation process, a
pre-carbonization process and a carbonization process so that a
carbon fiber bundle having a high tensile strength of strands with
few flaws can be obtained. As the polyacrylonitrile copolymer, it
is preferable to use a polyacrylonitrile copolymer containing other
monomers in addition to acrylonitrile as a main component.
Specifically, the polyacrylonitrile copolymer preferably contains
90 to 100% by mass of acrylonitrile and less than 10% by mass of a
copolymerizable monomer.
[0033] The polyacrylonitrile copolymer preferably contains a
copolymer component such as itaconic acid, acrylamide and
methacrylic acid, from the viewpoint of improving the stability of
the spinning process, the viewpoint of efficiently performing the
oxidation treatment, and the like.
[0034] The method of manufacturing the polyacrylonitrile copolymer
can be selected from known polymerization methods. In the
manufacture of a precursor fiber bundle for carbon fiber, the
spinning dope solution is a solution prepared by dissolving the
polyacrylonitrile copolymer in a solvent in which polyacrylonitrile
such as dimethyl sulfoxide, dimethylformamide, dimethylacetamide or
nitric acid/zinc chloride/aqueous sodium rhodanide solution is
soluble.
[0035] Prior to spinning the spinning dope solution as described
above, it is preferable to pass the spinning dope solution through
a filter device to remove impurities mixed in the polymer raw
material and each process. The filter device means a facility that
filters and removes foreign substances present in the spinning dope
solution, composed of an inflow path that introduces the spinning
dope solution to be subjected to filtration into the filter device,
a filter medium that filters the spinning dope solution, an outflow
path that guides the filtered spinning dope solution to outside the
filter device, and a storage container. The filter medium is a
means for filtering a spinning dope solution stored in the filter
device.
[0036] As the form of the filter medium, a leaf disc type filter, a
candle type filter, a pleated candle type filter or the like is
used. The filter medium of the candle type filter or pleated candle
type filter has constant curvature, whereas the leaf disc type
filter can use the filter medium in a substantially planar form so
that this is preferable because it has an advantage that the pore
diameter distribution hardly spreads and the cleaning property is
easily maintained.
[0037] The filter medium is a member that plays a direct role in
removing foreign substances present in the spinning dope solution.
The filter medium is required to hold the determined opening
diameters with narrow variations and, additionally, chemical
stability to a substance to be treated, heat resistance and
pressure resistance are required. As such a filter medium, a wire
gauze prepared by weaving metal fibers, glass nonwoven fabric,
filter medium made of a sintered metal fiber tissue and the like
are preferably used. The material of the filter medium is not
particularly limited as long as it is inert to the spinning dope
solution and contains no elutable component into the solvent, but
metals are more preferable from the viewpoint of durability and
cost. As the specific metals, in addition to stainless steel
(SUS304, SUS304L, SUS316, SUS316L and the like), "INCONEL"
(registered trademark) and "HASTELLOY" (registered trademark),
various alloys based on nickel, titanium and cobalt are selected.
Methods of manufacturing metal fibers include so-called bundle
drawing in which a large number of wires are collected as a bundle
and the diameter is reduced by drawing, then individual wires are
separated to reduce the diameter, coiled sheet shaving, chatter
vibration shaving and the like. When the filter medium is wire
gauze, because the metal fibers are necessary to be of not fiber
bundles but single fibers, it is manufactured by a method including
repeating wire drawing and heat treatment or the like.
[0038] In filtration of the spinning dope solution, as the opening
of the filter medium is smaller, foreign substances in the spinning
dope solution are easily removed, but clogging of the filter medium
more frequently occurs. As the removal performance for foreign
substances, a "particle retention" is used. The particle retention
(.mu.m) is a particle diameter (diameter) of spherical particles,
95% or more of which are collected when the particles pass through
the filter medium. The particle retention can be measured according
to a method of JIS standard (JIS-B8356-8: 2002). The fact that the
particle retention is low and the fact that the particle retention
is excellent are synonymous. In addition, as the filter thickness
becomes thicker, foreign substances in the spinning dope solution
are easily removed, but the pressure loss in the filter medium
increases and the stability of the manufacturing process decreases.
Although the tendency described above has been known so far,
optimum filtration conditions are different for filter media, and
thus any generalizable knowledge has not been obtained for
filtration of the spinning dope solution. Accordingly, at the time
of changing the filter medium, it has taken a great deal of time
and cost to control the filtration condition.
[0039] In the method of manufacturing a carbon fiber bundle, when
the particle retention of the filter medium used to filter the
spinning dope solution is B (.mu.m) and the filter basis weight is
D (g/m.sup.2), the spinning dope solution is filtered under
conditions where a relationship of the filtration speed A
(cm/hour), a particle retention B (.mu.m), a filter medium basis
weight D (g/m.sup.2) satisfies equations (1) to (3), then the
filtered spinning dope solution is spun to obtain a precursor fiber
bundle for carbon fiber:
D-600/(.alpha..times..beta.).gtoreq.0 (1)
.alpha.=1-1/(1+exp(7-A)) (2)
.beta.=1-1/(1+exp(-0.23.times.B)) (3).
[0040] The filter basis weight D (g/m.sup.2) is a total basis
weight of a filter medium main body excluding a mesh layer that may
be laminated for the purpose of protecting the filter medium main
body. The filter basis weight D can be calculated by measuring the
mass of the filter medium cut out into an arbitrary area and
dividing this mass by the area.
[0041] As the filter basis weight D is larger, the trapping rate of
foreign substances is higher. Conversely, as the filter basis
weight D is smaller, foreign substances cannot be easily trapped
but tend to slip through. Therefore, when the effect of the filter
basis weight D on improvement of the quality of the precursor fiber
bundle for carbon fiber and suppression of clogging of the filter
was measured while changing the filtration speed A and particle
retention B, we confirmed that there was a minimum filter basis
weight that could achieve both improvement of the quality of the
precursor fiber bundle for carbon fiber and suppression of clogging
of the filter at an arbitrary filtration speed and particle
retention (hereinafter "minimum filter basis weight"). According to
the results of this experiment, the minimum filter basis weight can
be expressed using .alpha..times..beta., a product of mutually
independent parameters .alpha. and .beta., as shown in the second
term on the left side of equation (1). .alpha. is defined as a
function of the filtration speed A shown in equation (2), and
.beta. is defined as a function of the particle retention B shown
in equation (3). As the .alpha..times..beta. is larger, the minimum
filter basis weight is smaller, and as the .alpha..times..beta. is
smaller, the minimum filter basis weight is larger. As effects of
movement of the individual parameters, as the filtration speed A is
larger, .alpha. is smaller and the minimum filter basis weight is
larger. As the filtration speed A is smaller, .alpha. is larger and
the minimum filter basis weight is smaller. Similarly, as the
particle retention B is larger, .beta. is smaller and the minimum
filter basis weight is larger. As the particle retention B is
smaller, .beta. is larger and the minimum filter basis weight is
smaller. Both improvement of the quality of the precursor fiber
bundle for carbon fiber and suppression of clogging of the filter
can be achieved by performing filtration under conditions
satisfying equations (1) to (3). Although this mechanism is not
necessarily clarified, it is considered as follows. As the particle
retention is smaller, foreign substances are likely to be caught by
a flow path through the filter medium so that foreign substances
can be effectively trapped, whereas the filter is likely to clog.
However, it is considered that when the filtration speed is low,
deformation and spreading of foreign substances in the filter
medium due to pressure drop are suppressed so that the flow path in
the filter medium hardly clogs.
[0042] In addition, as an example of the method of manufacturing a
carbon fiber bundle, a filter medium with the particle retention B
(.mu.m) satisfying equation (4) is preferably used:
B.gtoreq.3 (4).
[0043] When the particle retention B is 3 or more, suppression of
clogging of the filter can be made more effective. Although the
reason for this phenomenon is not necessarily clarified, it is
considered that, as the value of particle retention B is larger,
the filtration pressure tends to decrease, and thus the degree of
deformation of foreign substances is so smaller that a filter
clogging suppressing effect tends to appear.
[0044] Next, a method of manufacturing a precursor fiber bundle for
carbon fiber suitable for obtaining a carbon fiber bundle will be
described. In manufacturing a precursor fiber bundle for carbon
fiber, it is preferable to obtain a precursor fiber for carbon
fiber with a small mean surface roughness on the surface of a
single fiber by using a dry-jet wet spinning method. A method of
manufacturing a precursor fiber bundle for carbon fiber includes a
spinning process that extrudes a spinning dope solution from a
spinneret into a coagulation bath by a dry-jet wet spinning method
and spinning fibers, a water washing process that cleans the fibers
obtained in the spinning process in a water bath, a water bath
stretching process that stretches the fibers obtained in the water
washing process in a water bath, and a dry-heat treatment process
that dries and heat treats the fibers obtained in the water bath
stretching process, and may include a steam stretching process of
steam stretching the fibers obtained in the dry-heat treatment
process may be included, as necessary.
[0045] In the manufacture of a precursor fiber bundle for carbon
fiber, the coagulation bath preferably contains a coagulant and a
solvent used as a solvent for the spinning dope solution. As the
coagulant, a component that does not dissolve a polyacrylonitrile
copolymer and is compatible with the solvent used in the spinning
dope solution can be used. Specifically, it is preferable to use
water as the coagulant.
[0046] In the manufacture of a precursor fiber bundle for carbon
fiber, the water bath temperature in the water washing process is
preferably 30 to 98.degree. C., and is preferably washed using a
water washing bath having a plurality of stages.
[0047] In addition, the stretch ratio in the water bath stretching
process is preferably 2 to 6 times.
[0048] After the water bath stretching process, it is preferable to
apply an oil agent made of silicone or the like to the fiber bundle
for the purpose of preventing adhesion between single fibers. As
such a silicone oil agent, it is preferable to use a modified
silicone, and it is preferable to use one containing an
amino-modified silicone having high heat resistance.
[0049] A known method can be used for the dry-heat treatment
process. For example, the drying temperature is 100 to 200.degree.
C.
[0050] A precursor fiber bundle for carbon fiber more suitably used
for the manufacture of a carbon fiber bundle is obtained by further
performing the steam stretching process, after the water washing
process, water bath stretching process, oil agent application
process, and dry-heat treatment process described above. As the
steam stretching process, it is preferable to stretch 2 to 6 times
in pressurized steam.
[0051] The mean fineness of single fibers contained in the
precursor fiber bundle for carbon fiber thus obtained is preferably
0.7 to 1.5 dtex, and more preferably 0.9 to 1.2 dtex. By setting
the single-fiber fineness to 0.7 dtex or more, the occurrence of
fiber bundle fracture due to the accumulation of single fiber
fracture due to contact with rollers and guide parts is suppressed,
and the process stability of each of the spinning process,
oxidation process, precarbonization process and carbonization
process can be maintained. Also, by setting the single-fiber
fineness to 1.5 dtex or less, the skin layer ratio in each single
fiber after the oxidation process is reduced, the process stability
in the subsequent carbonization process and the tensile strength of
strands and elastic modulus of strands of the resulting carbon
fiber bundle can be improved. To adjust the single-fiber fineness
of the resulting precursor fiber bundle for carbon fiber, it is
only necessary to adjust the extrusion amount of the spinning dope
solution in the spinning process for extruding the spinning dope
solution from the spinneret and spinning fibers.
[0052] The resulting precursor fiber bundle for carbon fiber is
usually continuous fibers. Also, the number of filaments per one
fiber bundle is preferably 10,000 to 60,000.
[0053] The method of manufacturing a carbon fiber bundle is such
that the precursor fiber bundle for carbon fiber is heat-treated in
an oxidizing atmosphere until the density reaches 1.32 to 1.35
g/cm.sup.3, and then heat-treated at 275.degree. C. or more and
295.degree. C. or less in an oxidizing atmosphere until the density
reaches 1.46 to 1.50 g/cm.sup.3. That is, the precursor fiber
bundle for carbon fiber is heat-treated until reaching a
predetermined density in the former half of the oxidation process,
and then heat-treated at a high temperature of 275.degree. C. or
more and 295.degree. C. or less in the latter half of the oxidation
process.
[0054] The oxidizing atmosphere is an atmosphere containing 10% by
mass or more of a known oxidizing substance such as oxygen and
nitrogen dioxide, and an air atmosphere is preferable from the
viewpoint of simplicity.
[0055] The density of the oxidized fiber bundle is generally used
as an index indicating the progress of the oxidation reaction. When
the density is 1.32 g/cm.sup.3 or more, the oxidized fiber bundle
has a high heat resistant structure so that it is difficult to be
decomposed when heat-treated at a high temperature, and the tensile
strength of strands of the resulting carbon fiber bundle is
improved. In addition, when the density is 1.35 g/cm.sup.3 or less,
a long heat treatment time at a high temperature can be secured in
the subsequent process so that the tensile strength of strands of
the carbon fiber bundle can be improved. In the oxidation process,
to enable the process temperature to be switched as described above
at the density specified by the oxidized fiber bundle, it is only
necessary to collect the fiber bundles during the former half and
the latter half of the oxidation process and measure their
densities. A method of measuring the density will be described
below. For example, when the measured density of the oxidized fiber
bundle is lower than specified, the density of the oxidized fiber
bundle can be adjusted by raising the temperature or prolonging the
oxidation time in the former half of the oxidation process.
[0056] In the oxidation process, first, the precursor fiber bundle
for carbon fiber is heat-treated in an oxidizing atmosphere, at
preferably 210.degree. C. or more and less than 245.degree. C.,
more preferably 220.degree. C. or more and less than 245.degree.
C., and further preferably 225.degree. C. or more and less than
240.degree. C., thereby obtaining an oxidized fiber bundle with a
density of preferably 1.22 to 1.24 g/cm.sup.3, and more preferably
a density of 1.23 to 1.24 g/cm.sup.3. When the density of the
oxidized fiber bundle is 1.22 g/cm.sup.3 or more, the chemical
structure of the single fiber in the oxidation process is
stabilized by heat treatment, and the difference between skin-core
structure of the single fiber does not deteriorate even when the
subsequent heat treatment is performed at a high temperature so
that the tensile strength of strands is often improved. Further,
when the density is 1.24 g/cm.sup.3 or less, the total amount and
time of heat treatment including the subsequent heat treatment is
reduced, which is often superior in terms of tensile strength of
strands and productivity. Regarding the temperature, a temperature
of 210.degree. C. or more is preferable because the difference
between skin-core structure can be sufficiently suppressed. At a
temperature of less than 245.degree. C. is preferable because it is
an oxidation initial temperature sufficiently low to suppress the
difference between skin-core structure regarding the single fiber
diameter of the precursor fiber bundle for carbon fiber, the
tensile strength of strands is often increased.
[0057] The oxidized fiber bundle is heat-treated until the density
reaches 1.22 to 1.24 g/cm.sup.3 and then heat-treated in an
oxidizing atmosphere to obtain an oxidized fiber bundle with a
density of 1.32 to 1.35 g/cm.sup.3, and more preferably 1.33 to
1.34 g/cm.sup.3. This heat treatment process is performed in an
oxidizing atmosphere at preferably 245.degree. C. or more and less
than 275.degree. C., and more preferably 250.degree. C. or more and
less than 270.degree. C. When the density is 1.32 g/cm.sup.3 or
more, the chemical structure of the single fiber in the oxidation
process is further stabilized by heat treatment, and the difference
between skin-core structure does not deteriorate even when the
subsequent heat treatment is performed at a higher temperature so
that the tensile strength of strands is often improved. Further,
when the density is 1.35 g/cm.sup.3 or less, the total amount and
time of heat treatment including the subsequent heat treatment are
reduced, and the tensile strength of strands and productivity are
superior. When the heat treatment temperature is 245.degree. C. or
more, the total amount and time of heat treatment are reduced, and
the tensile strength of strands and productivity are often
superior. When the heat treatment temperature is less than
275.degree. C., even when heat-treating an oxidized fiber bundle
with a density of 1.22 to 1.24 g/cm.sup.3, the difference between
skin-core structure can be suppressed, and high tensile strength of
strands is often exhibited.
[0058] Subsequently, the obtained oxidized fiber bundle is
heat-treated in an oxidizing atmosphere at a temperature of
275.degree. C. or more to 295.degree. C. or less, and preferably
280.degree. C. or more to 290.degree. C. or less to obtain an
oxidized fiber bundle with a density of 1.46 to 1.50 g/cm.sup.3.
When the heat treatment temperature is 275.degree. C. or more, the
amount of heat applied when increasing the density can be reduced,
whereby the tensile strength of strands is improved. When the heat
treatment temperature is 295.degree. C. or less, it is possible to
proceed the oxidation reaction without decomposing the structure of
the single fiber, and maintain the tensile strength of strands. To
measure the heat treatment temperature, it is only necessary to
insert a thermometer such as a thermocouple into a heat treatment
oven in the oxidation process to measure oven temperature. When
there are temperature unevenness and temperature distribution when
measuring the oven temperature in several points, the simple
average temperature is calculated.
[0059] The final density of the oxidized fiber bundle is 1.46 to
1.50 g/cm.sup.3, preferably 1.46 to 1.49 g/cm.sup.3, and further
preferably 1.47 to 1.49 g/cm.sup.3. Since the density of the
oxidized fiber bundle correlates with the carbonization yield, the
higher density is better, from the viewpoint of reducing
manufacturing energy. When the density is 1.46 g/cm.sup.3 or more,
the carbonization yield can be sufficiently increased. When the
density is 1.50 g/cm.sup.3 or less, the effect of increasing the
carbonization yield is not saturated, which is effective from the
viewpoint of productivity. To complete the heat treatment at the
specified density, it is only necessary to adjust the oxidation
temperature and time.
[0060] In the process of heat treatment at 275.degree. C. or more
and 295.degree. C. or less in an oxidizing atmosphere until the
density of the oxidized fiber bundle reached 1.46 to 1.50
g/cm.sup.3, the tension applied to the oxidized fiber bundle
(oxidation tension) is preferably 1.6 to 4.0 mN/dtex, more
preferably 2.5 to 4.0 mN/dtex, and further preferably 3.0 to 4.0
mN/dtex. The oxidation tension is represented by a value obtained
by dividing the tension (mN) measured on an exit side of the
oxidation oven by the fineness (dtex) of the precursor fiber bundle
for carbon fiber in complete dryness. When the tension is 1.6
mN/dtex or more, the orientation of the carbon fiber bundle is
sufficiently increased, and the tensile strength of strands is
often improved. When the tension is 4.0 mN/dtex or less, quality
deterioration due to fuzz tends to be small.
[0061] Generally, when the density of the oxidized fiber bundle is
increased to obtain a high carbonization yield, the tensile
strength of strands of the carbon fiber bundle tends to decrease.
In the method of manufacturing a carbon fiber bundle, even when the
density of the oxidized fiber bundle is increased by performing
high-temperature heat treatment in the latter half at an
appropriate temperature profile in the oxidation process, the
difference between skin-core structure of the single fiber is
greatly suppressed, and the structure is stabilized so that both
high carbonization yield and high tensile strength of strands can
be achieved.
[0062] Except for the oxidation process, a known method of
manufacturing a carbon fiber bundle may be basically followed.
However, in our method of manufacturing a carbon fiber bundle, it
is preferable to perform a pre-carbonization process, following the
spinning process and the oxidation process. In the
pre-carbonization process, it is preferable to obtain a carbonized
fiber bundle by heat-treating the oxidized fiber obtained in the
oxidation process in an inert atmosphere at a maximum temperature
of 500 to 1000.degree. C. until the density reaches 1.5 to 1.8
g/cm.sup.3.
[0063] A carbonization process is performed, following
pre-carbonization. In the carbonization process, it is preferable
to obtain a carbon fiber bundle by heat-treating the pre-carbonized
fiber bundle in an inert atmosphere at a maximum temperature of
1200 to 1800.degree. C., and preferably 1200 to 1600.degree. C.
When the maximum temperature is 1200.degree. C. or more, the
nitrogen content in the carbon fiber bundle is reduced, and the
tensile strength of strands is stably exhibited. When the maximum
temperature is 1800.degree. C. or less, a satisfactory
carbonization yield can be obtained.
[0064] The carbon fiber bundle obtained as described above is
preferably subjected to an oxidation treatment so that an oxygen
containing functional group is introduced to improve adhesion to a
matrix resin. As the oxidation treatment method, gas phase
oxidation, liquid phase oxidation, liquid phase electrolytic
oxidation and the like are used. From the viewpoint that high
productivity and uniform treatment can be achieved, liquid phase
electrolytic oxidation is preferably used. The method of liquid
phase electrolytic oxidation is not particularly specified, and may
be performed by a known method.
[0065] After such an electrochemical treatment, a sizing treatment
can also be performed to impart convergency to the obtained carbon
fiber bundle. As the sizing agent, a sizing agent having good
compatibility with the matrix resin can be appropriately selected
according to the type of the matrix resin used for a composite
material.
[0066] The measuring methods of various physical property values
described in this specification are as follows.
Tensile Strength of Strands and Elastic Modulus of Strands of
Carbon Fiber Bundle
[0067] The tensile strength of strands and elastic modulus of
strands of the carbon fiber bundle are determined in accordance
with a resin-impregnated strand test method of JIS-R-7608 (2004),
according to the following procedure. Ten resin-impregnated strands
of the carbon fiber bundle are measured, and the average value
thereof is defined as the tensile strength of strands. Strain is
evaluated using an extensometer. The strain was evaluated at a
strain range of 0.1 to 0.6%. As a resin formulation, "CELLOXIDE
(registered trademark)" 2021P (manufactured by Daicel Chemical
Industries, Ltd.)/boron trifluoride monoethylamine (manufactured by
Tokyo Chemical Industry Co., Ltd.)/acetone=100/3/4 (parts by mass)
are used. As curing conditions, atmospheric pressure, a temperature
of 125.degree. C., and a time of 30 minutes are used.
Density Measurement
[0068] 1.0 to 3.0 g of the oxidized fiber bundle is collected and
completely dried at 120.degree. C. for 2 hours. Next, after
measuring an absolute dry mass A (g), the oxidized fiber bundle is
impregnated with ethanol and sufficiently defoamed, then a fiber
mass B (g) in the ethanol solvent bath is measured, and a density
is determined by density=(A.times..phi./(A-B). .rho. is a specific
gravity of ethanol at the measurement temperature.
Skin Layer Ratio of Single Fiber of Carbon Fiber
[0069] A carbon fiber bundle to be measured is embedded in a resin,
a cross section perpendicular to the fiber axis direction is
polished, and the cross section is observed using a 100 times
objective lens of an optical microscope at a total magnification of
1000. The blackened thickness of the outer peripheral portion is
measured from the cross-sectional microscopic image of the polished
surface. Analysis is performed using image analysis software Image
J. First, in a single fiber cross-sectional image, black and white
area division is performed by binarization. For the luminance
distribution in the single fiber cross section, the average value
of the distribution is set as a threshold value, and binarization
is performed. In the obtained binarized image, the shortest
distance from a point on the surface layer to a lined region from
black to white is measured in the fiber diameter direction. This is
measured for five points in the circumference of the same single
fiber, and the average value is calculated as the blackened
thickness at that level. Further, the skin layer ratio is
calculated from the area ratio (%) of the blackened thickness
portion with respect to the entire cross section perpendicular to
the fiber axis direction of the single fiber of the carbon fiber.
The same evaluation is performed on 30 single fibers in the carbon
fiber bundle, and the average value thereof is used.
Average Single Fiber Diameter of Carbon Fiber Bundle
[0070] A mass A.sub.f (g/m) and a density B.sub.f (g/cm.sup.3) per
unit length are determined for a carbon fiber bundle composed of a
large number of carbon filaments to be measured. The number of
filaments of the carbon fiber bundle to be measured is defined as
C.sub.f, and the average single fiber diameter (.mu.m) of the
carbon fibers is calculated by the equation below:
Average single fiber diameter of carbon fibers
(.mu.m)=((A.sub.f/B.sub.f/C.sub.f)/.pi.).sup.(1/2).times.2.times.10.sup.3-
.
Knot Strength of Carbon Fiber Bundle
[0071] A grip part having a length of 25 mm is attached to both
ends of a carbon fiber bundle with a length of 150 mm to prepare a
test specimen. In the preparation of the test specimen, a load of
9.0.times.10.sup.-5 N/dtex is applied to the carbon fiber bundle
for alignment. One knot is made at the midpoint of the test
specimen, and the test specimen is subjected to a fiber bundle
tensile test at a crosshead speed at tension of 100 mm/min. A total
of 12 fiber bundles are subjected to the measurement. The average
value of 10 fiber bundles excluding the maximum value and the
minimum value is used as the measured value. As the knot strength,
a value obtained by dividing the maximum load value obtained in the
fiber bundle tensile test by the average cross-sectional area value
of the carbon fiber bundles is used.
Probability that Flaw with Size of 50 nm or More Exists
[0072] A single fiber tensile test of the single fibers of the
carbon fibers is performed in accordance with JIS R7606 (2000), and
a sample of the single fibers of the carbon fibers after fracture
including a fracture surface (hereinafter "fracture surface") is
collected. The number of single fibers to be tested is one set of
50 fibers, and when it is not possible to collect 30 or more pairs
of fracture surfaces on both sides, another set of 50 fibers is
subjected to a single fiber tensile test to collect 30 or more
pairs of fracture surfaces on both sides. The strain rate during
the tensile test is set to 0.4 mm/min.
[0073] From the pairs of fracture surfaces collected as described
above, 30 pairs are randomly selected and observed with a scanning
electron microscope (SEM). Before the observation, a vapor
deposition treatment for applying conductivity is not performed,
and the observation is performed at an acceleration voltage of 1
keV and at a magnification of 25,000 to 50,000. In addition, to
make it easy to determine the presence or absence of minute flaws,
a stage is rotated such that the fracture origin faces the front
side, and the stage is tilted by 30.degree. to observe the fracture
origin from the oblique upside as shown in FIGS. 1 to 4.
[0074] Because traces of the fracture radially progressing from the
fracture origin (i) remained as radial streaks on the original
fracture surface caused by tensile fracture of the carbon fiber, a
portion on which the streaks present on an SEM observation image
converged to one point when traced is identified as a fracture
origin (i). When the streaks cannot be recognized or when the
streaks can be recognized, but stain is adhered near the fracture
origin (i) so that the streaks are hardly observed on at least one
side of the fracture surfaces on both sides, the pair of such
fracture surfaces is excluded from evaluation. The fracture surface
reduced by the exclusion is replenished as appropriate so that 30
pairs of fracture surfaces will eventually be observed.
[0075] Once the fracture origin (i) can be identified, it is
examined whether there are any morphological features. There are
various types of morphological features such as dents, attached
substances, traces of the fiber surface being partially peeled,
damages and adhesion marks. The morphological features to be
fracture origins that can be observed by SEM are collectively
referred to as "flaws." Lengths measured along the circumferential
direction of the fiber, that is, those with a size of 50 nm or
more, are uniformly classified as a "fracture surface in which a
flaw with a size of 50 nm or more exists" regardless of differences
in appearance. When it is performed on the fracture surfaces on
both sides and either one is classified as the "fracture surface in
which a flaw with a size of 50 nm or more exists," the pair is
taken as having the "fracture surface in which a flaw with a size
of 50 nm or more exists." This classification is performed on all
30 pairs of fracture surfaces observed with SEM, and the total
number of "fracture surfaces in which a flaw with a size of 50 nm
or more exists" is divided by 30 which is the total number of the
pairs of fracture surfaces observed by SEM and multiplied by 100 to
calculate a "probability (%) that a flaw with a size of 50 nm or
more exists."
[0076] The single fiber tensile test was performed by TENSILON
"RTC-1210A" manufactured by A&D Company, Limited, with a gauge
length of 10 mm, using a commercially available cyanoacrylate
instant adhesive to fix the carbon fiber to a test piece mount, and
use a special test jig designed to perform in water. Further, a
scanning electron microscope (SEM) "5-4800" manufactured by Hitachi
High-Technologies Corporation was used to observe the collected
fracture surfaces.
Mean Surface Roughness
[0077] Using ten single fibers of the carbon fibers to be evaluated
that are placed on a sample stage and fixed with an epoxy resin as
samples, evaluation is performed using an atomic force microscope
(in Examples, NanoScope V Dimension Icon, manufactured by Bruker
AXS). In the Examples, a three-dimensional surface shape image is
obtained under the following conditions: Probe: silicon cantilever
(OMCL-AC160TS-W2 manufactured by Olympus)
Measurement mode: tapping mode Scanning speed: 1.0 Hz Scanning
range: 600 nm.times.600 nm Resolution: 512 pixels.times.512 pixels
Measurement environment: room temperature, in air.
[0078] For a single fiber, a three-dimensional surface shape image
is measured under the above conditions, and the obtained
measurement image is subjected to image processing using, "flat
treatment" for removing undulation of data derived from the device
using the attached software (NanoScope Analysis), taking a
curvature of a fiber cross section into account, "median 8
treatment" which is a filter treatment that replaces a central
value of a matrix from a median value of Z data in the 3.times.3
matrix, and "three-dimensional tilt correction" for carrying out
fitting of a cubic curved surface by a least square method from all
image data and correcting in-plane tilt, then surface roughness
analysis is performed with the attached software to calculate mean
surface roughness. The mean surface roughness (Ra) is a
three-dimensional extension of a centerline roughness Ra defined in
JIS B0601 (2001) so that it can be applied to surface measurement
and is defined as a mean value of absolute values of deviations
from a reference surface to a designated surface. As to
measurement, 10 different single fibers are randomly sampled, and
the measurement is performed once for each single fiber, 10 times
in total, and the average value thereof is taken as the measured
value.
Number of Fuzzes of Carbon Fiber Bundle
[0079] The quality of the carbon fiber bundle affecting
productivity during manufacture of the composite material is
evaluated by a method of directly counting the number of fuzzes by
the following method. By visually observing the running carbon
fiber bundle at a running speed of 1.5 m/min and a stretch ratio of
1 time, the number of fractures single fibers protruding 5 mm or
more from the surface of the carbon fiber bundle is counted at a
length of the carbon fiber bundle of 20 m to evaluate the number of
fuzzes per 1 m (fuzzes/m).
EXAMPLES
Example 1
[0080] A copolymer composed of 99% by mass of acrylonitrile and 1%
by mass of itaconic acid was polymerized by solution polymerization
using dimethyl sulfoxide as a solvent to produce a
polyacrylonitrile copolymer to obtain a spinning dope solution. The
spinning dope solution was allowed to flow into a filter device and
filtered. The filter medium used was a sintered metal filter having
a particle retention B of 1 .mu.m, a filter medium thickness C of
800 .mu.m, and a filter basis weight D of 2500 g/m.sup.2, and
filtration was performed under a filtration condition with a
filtration speed A of 3 cm/hour. Fibers were spun by a dry-jet wet
spinning method in which the filtered spinning dope solution was
once extruded through a spinneret into the air and introduced into
a coagulation bath composed of an aqueous solution of 35% dimethyl
sulfoxide controlled at 3.degree. C. The spun fiber bundle was
washed with water at 30 to 98.degree. C., and 3.5 times water bath
stretching was performed at that time. Subsequently, an
amino-modified silicone-based silicone oil agent was applied to the
fiber bundle after the water bath stretching in the water bath and
dried using a roller heated to a temperature of 160.degree. C. to
obtain a fiber bundle with a number of single fibers of 12,000. The
fiber bundle was stretched 3.7 times in pressurized steam to make
the total stretch ratio of the yarn 13 times. Then, the fiber
bundle was subjected to entangling treatment by air having a fluid
extrusion pressure of 0.35 MPa with a tension of 2 mN/dtex being
applied to the fiber bundle to obtain a precursor fiber bundle for
carbon fiber with a single-fiber fineness of 1.1 dtex and a number
of single fibers of 12,000. Next, using the oxidizing conditions
described in Condition 1 in Table 1, the precursor fiber bundle for
carbon fiber was heat-treated in an oven in an air atmosphere at a
stretch ratio of 1.0 to obtain an oxidized fiber bundle.
[0081] The obtained oxidized fiber bundle was subjected to a
pre-carbonization treatment at a stretch ratio of 0.95 in a
nitrogen atmosphere at a temperature of 300 to 800.degree. C. to
obtain a pre-carbonized fiber bundle. The obtained pre-carbonized
fiber bundle was subjected to a carbonization treatment at a
maximum temperature of 1350.degree. C. in a nitrogen atmosphere.
The obtained carbon fiber bundle was subjected to surface treatment
and sizing agent coating treatment to obtain a final carbon fiber
bundle. The number of fuzzes of the carbon fiber bundle at this
time was less than 0.1/m, and almost no fuzz was confirmed and the
quality was good.
[0082] Table 2 shows the tensile strength of strands, elastic
modulus of strands, skin layer ratio of the single fibers of the
carbon fibers, and average single fiber diameter of the obtained
carbon fiber bundle.
TABLE-US-00001 TABLE 1 Oxidation Oxidized fiber bundle density
Oxidation temperature tension First Second Final First Second Final
Final oven exit oven exit oven exit oven oven oven oven g/cm.sup.3
.degree. C. mN/dtex Condition 1 1.23 1.32 1.48 235 260 285 1.5
Condition 2 1.23 1.32 1.48 225 260 285 1.5 Condition 3 1.23 1.32
1.48 245 260 285 1.2
TABLE-US-00002 TABLE 2 Filtration conditions Filtration Particle
Filter medium Filter basis D - 600/ speed A retention B thickness C
weight D .alpha. .beta. (.alpha. .times. .beta.) cm/h .mu.m .mu.m
g/m.sup.3 -- -- -- Example 1 3 1 800 2500 0.98 0.44 1120 Example 2
3 9 3200 6400 0.98 0.11 947 Example 3 6 1 800 2500 0.73 0.44 646
Example 4 6 1 800 2500 0.73 0.44 646 Example 5 6 1 800 2500 0.73
0.44 646 Comparative 3 9 1600 3200 0.98 0.11 -2253 Example 1
Comparative 6 9 1600 3200 0.73 0.11 -4125 Example 2 Comparative 6 9
3200 6400 0.73 0.11 -925 Example 3 Comparative 8 1 800 2500 0.27
0.44 -2539 Example 4 Comparative 12 1 800 2500 0.01 0.44 -199979
Example 5 Example 6 3 1 800 2500 0.98 0.44 1120 Example 7 3 1 800
2500 0.98 0.44 1120 Carbon fiber bundle Probability that Tensile
Elastic flaw with size Skin layer ratio Average Knot strength of
modulus of of 50 nm or of single fiber single fiber strength Mean
surface strands strands more exists of carbon fiber diameter K -88d
+ 1390 roughness GPa GPa % % .mu.m MPa MPa nm Example 1 5.9 256 17
91 7.5 736 730 -- Example 2 5.8 257 22 91 7.5 752 730 -- Example 3
5.8 253 29 91 7.5 741 730 1.5 Example 4 6.0 250 29 91 7.3 772 748
-- Example 5 6.2 263 29 91 7.0 785 774 1.4 Comparative 5.7 252 52
91 7.5 722 730 -- Example 1 Comparative 5.6 257 57 91 7.4 778 739
2.1 Example 2 Comparative 5.6 247 38 91 7.5 698 730 -- Example 3
Comparative 5.4 261 37 91 7.5 838 730 -- Example 4 Comparative 5.2
262 81 91 7.5 867 730 -- Example 5 Example 6 5.8 261 17 97 7.5 731
730 -- Example 7 5.8 245 17 85 7.5 732 730 --
Example 2
[0083] A precursor fiber bundle for carbon fiber and a carbon fiber
bundle were obtained in the same manner as in Example 1, except
that the filter medium was changed to a sintered metal filter
having a particle retention B of 9 .mu.m, a filter medium thickness
C of 3200 .mu.m, and a filter basis weight D of 6400 g/m.sup.2.
Example 3
[0084] A precursor fiber bundle for carbon fiber and a carbon fiber
bundle were obtained in the same manner as in Example 1 except that
the filtration speed A was changed to 6 cm/hour under the
filtration conditions.
Examples 4 and 5
[0085] A precursor fiber for carbon fiber and a carbon fiber bundle
were obtained in the same manner as in Example 3, except that the
stretch ratio during pre-carbonization was 1.05 times in Example 4
and 1.10 times in Example 5.
Comparative Example 1
[0086] A precursor fiber bundle for carbon fiber and a carbon fiber
bundle were obtained in the same manner as in Example 2, except
that the filter medium was changed to a sintered metal filter
having a filter medium thickness C of 1600 .mu.m and a filter basis
weight D of 3200 g/m.sup.2. The number of fuzzes of the carbon
fiber bundle was 0.2/m, and the quality deteriorated.
Comparative Example 2
[0087] A precursor fiber bundle for carbon fiber and a carbon fiber
bundle were obtained in the same manner as in Comparative Example 1
except that the filtration speed A was changed to 6 cm/hour under
the filtration conditions.
Comparative Example 3
[0088] A precursor fiber bundle for carbon fiber and a carbon fiber
bundle were obtained in the same manner as in Example 2 except that
the filtration speed A was changed to 6 cm/hour under the
filtration conditions.
Comparative Example 4
[0089] A precursor fiber bundle for carbon fiber and a carbon fiber
bundle were obtained in the same manner as in Example 3 except that
the filtration speed A was changed to 8 cm/hour under the
filtration conditions.
Comparative Example 5
[0090] A precursor fiber bundle for carbon fiber and a carbon fiber
bundle were obtained in the same manner as in Example 3 except that
the filtration speed A was changed to 12 cm/hour under the
filtration conditions.
Example 6
[0091] A carbon fiber bundle was obtained in the same manner as in
Example 1 except that Condition 2 in Table 1 was used as the
oxidizing condition. The skin layer ratio of the carbon fibers was
97%, and the tensile strength of strands decreased as compared to
that in Example 1.
Example 7
[0092] A carbon fiber bundle was obtained in the same manner as in
Example 1 except that Condition 3 in Table 1 was used as the
oxidizing condition. The skin layer ratio of the carbon fibers was
85%, and the tensile strength of strands decreased as compared to
that in Example 1.
INDUSTRIAL APPLICABILITY
[0093] We can obtain an oxidized fiber bundle having a specific
density by heat-treating at an appropriate temperature profile in
the oxidation process, whereby flaws governing the tensile strength
of strands and the knot strength are controlled to be very small,
and thus can manufacture a carbon fiber bundle that exhibits the
tensile strength of strands and the elastic modulus of strands in a
well-balanced manner and also exhibits high knot strength without
impairing productivity. Moreover, the carbon fiber bundle satisfies
productivity at the time of manufacturing a composite material. The
carbon fiber bundle to be obtained is suitably used for general
industrial uses such as aircraft, automobile and ship members,
sports uses such as golf shafts and fishing rods, and pressure
vessels, taking advantage of such characteristics.
* * * * *