TECHNICAL FIELD
[0001] The present invention relates to a carbon fiber bundle and a method for manufacturing
the same.
BACKGROUND ART
[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, in order to reduce
the weight of members such as pressure vessels, it is required to improve mechanical
properties such as tensile strength of resin-impregnated strands and elastic modulus
of resin-impregnated strands of the carbon fiber bundle (hereinafter simply referred
to as tensile strength of strands and elastic modulus of strands) in a well-balanced
manner. At the same time, it is necessary to reduce 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°C (oxidation process) and then
heat-treated in an inert atmosphere of 1000°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
also referred to as 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 (Patent Documents 1 to 5).
[0004] In Patent Document 1, studies have been made to improve the tensile strength of strands
of the carbon fiber bundle by minimizing the amount of heat (J · h/g) given by high-temperature
treatment in the oxidation process. In Patent Document 2, 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 Patent Document 3, 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, in order
to shorten the oxidation process. Moreover, Patent Documents 4 and 5 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°C.
[0005] Patent Documents 6 and 7 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] Patent Document 8 has proposed a carbon fiber bundle that exhibits 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 in order
to satisfy a high carbonization yield.
[0007] On the other hand, the 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 flaw. For example, Patent Document 9
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.
PRIOR ART DOCUMENTS
PATENT DOCUMENTS
[0008]
Patent Document 1: Japanese Patent Laid-open Publication No. 2012-82541
Patent Document 2: Japanese Patent Laid-open Publication No. S58-163729
Patent Document 3: Japanese Patent Laid-open Publication No. H6-294020
Patent Document 4: Japanese Patent Laid-open Publication No. 2013-23778
Patent Document 5: Japanese Patent Laid-open Publication No. 2014-74242
Patent Document 6: WO 2013/157613 A
Patent Document 7: Japanese Patent Laid-open Publication No. 2015-096664
Patent Document 8: Japanese Patent Laid-open Publication No. 2017-66580
Patent Document 9: Japanese Examined Patent Publication No. H8-6210
SUMMARY OF THE INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION
[0009] However, in the proposal of Patent Document 1, an attempt has been made to reduce
the integrated value of the amount of heat given in the oxidation process, which is
not sufficient for achieving both the tensile strength of strands and the carbonization
yield. In addition, in the proposals of Patent Documents 2 and 3, 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 the proposals of Patent Documents 4 and
5, while heat treatment has been performed at a high temperature using a heating roller
with high heat transfer efficiency in order 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 flaw due to adhesion between single fibers when passing through
the roller. Although the proposal of Patent Document 6 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
the proposal of Patent Document 7 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 the case of 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 the proposal of Patent Document 8, 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 Patent Document 9, 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.
[0010] An object of the present invention is to provide a method for 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 in order to solve the above-described problems in the prior art.
SOLUTIONS TO THE PROBLEMS
[0011] In order to achieve the above object, the method for manufacturing a carbon fiber
bundle of the present invention is a method including filtering a spinning dope solution
in which a polyacrylonitrile copolymer is dissolved in a solvent, using a filter medium
having a particle retention B (µm) and a filter basis weight D (g/m
2), under conditions where a filtration speed A (cm/hour) satisfies following equations
(1) to (3), then spinning the filtered spinning dope solution to obtain a precursor
fiber bundle for carbon fiber,
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
3, then heat-treating at 275°C or more and 295°C or less in an oxidizing atmosphere
until a density reaches 1.46 to 1.50 g/cm
3 to obtain an oxidized fiber bundle, and then heat-treating the oxidized fiber bundle
at 1200 to 1800°C in an inert atmosphere.
[0012] Also, the carbon fiber bundle of the present invention is 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 ≤ K (d: average single fiber
diameter [µm]), and an average single fiber diameter in the range of 6.5 to 8.0 µm,
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.
EFFECTS OF THE INVENTION
[0013] According to the method of the present invention, 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 that exhibits 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, according to the carbon
fiber bundle of the present invention, it becomes a carbon fiber bundle which satisfies
the productivity at the time of manufacturing a composite material.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014]
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.
Fig. 2 is an enlarged image of the vicinity of a fracture origin in Fig. 1. Flaws
like attached substances are confirmed.
Fig. 3 is an enlarged image of the vicinity of a fracture origin of another fracture
surface. Flaws like dent are confirmed.
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.
EMBODIMENTS OF THE INVENTION
[0015] The carbon fiber bundle of the present invention 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 for
manufacturing a carbon fiber bundle of the present invention described later.
[0016] The carbon fiber bundle of the present invention 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 later. 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.
[0017] Also, in the carbon fiber bundle of the present invention, 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 later.
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 in order 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. In order to increase
the knot strength of the carbon fiber bundle, in the method for manufacturing a carbon
fiber bundle of the present invention described later, it is particularly preferable
to control so that structural parameters in an oxidation process and a pre-carbonization
process are within preferable ranges. Further, the knot strength can be also increased
by reducing flaws on the surface of the carbon fiber.
[0018] 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.
[0019] Moreover, the carbon fiber bundle has a knot strength K [MPa] (= N/mm
2) of -88d + 1390 ≤ K (where d is an average single fiber diameter [µm]). It is preferable
that the carbon fiber bundle satisfies a relational expression -88d + 1410 ≤ 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 ≤ 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. In order 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 of the present invention described later.
[0020] 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 a flaw. It is known that there are various types of flaws
to be fracture origins of carbon fibers, such as voids, damages 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, in the present invention, morphological
features that can be observed by scanning electron microscope (SEM) observation are
collectively referred to as "flaws" without particularly distinguishing all of them.
As a result of studies by the present inventors, it has been 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 here 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, it has been found that the
probability is not necessarily correlated with the tensile strength of strands and
the tensile strength of the composite material as a result of the studies by the present
inventors. 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
later, and effectively removing foreign substances in the spinning dope solution.
[0021] In the carbon fiber bundle of the present invention, the average single fiber diameter
is 6.5 to 8.0 µm, preferably 6.7 to 8.0 µm, more preferably 7.0 to 8.0 µm, further
preferably 7.3 to 8.0 µm, and most preferably 7.5 to 8.0 µm. 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 µm 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.
[0022] 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.
[0023] The carbon fiber bundle has an area ratio (hereinafter referred to as 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. Here, 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.
[0024] In order to solve the problems of the present invention, the method for manufacturing
a carbon fiber bundle of the present invention is based on the fact that it has been
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 embodiment for carrying out the present
invention will be described in detail below.
[0025] 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.
[0026] 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.
[0027] The method for 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.
[0028] 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. Here, the filter device means a facility
for filtering and removing foreign substances present in the spinning dope solution,
composed of an inflow path for introducing the spinning dope solution to be subjected
to filtration into the filter device, a filter medium for filtering the spinning dope
solution, an outflow path for guiding the filtered spinning dope solution to outside
the filter device, and a container for storing these. Here, the filter medium is a
means for filtering a spinning dope solution stored in the filter device.
[0029] 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.
[0030] The filter medium is a member that plays a direct role for 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,
etc.), "INCONEL" (registered trademark) and "HASTELLOY" (registered trademark), various
alloys based on nickel, titanium and cobalt are selected. Methods for 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. In a case where 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.
[0031] 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. In the present invention, as
the removal performance for foreign substances, a "particle retention" is used. Here,
the particle retention (µm) 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 optimize the filtration condition.
[0032] In the method for manufacturing a carbon fiber bundle of the present invention, when
the particle retention of the filter medium used for filtering the spinning dope solution
is B (µm) and the filter basis weight is D (g/m
2), the spinning dope solution is filtered under conditions where a relationship of
the filtration speed A (cm/hour), a particle retention B (µm), a filter medium basis
weight D (g/m
2) satisfies the following equations (1) to (3), then the filtered spinning dope solution
is spun to obtain a precursor fiber bundle for carbon fiber.
[0033] Here, the filter basis weight D (g/m
2) is a total basis weight of a filter medium main body excluding a mesh layer which
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.
[0034] 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, it was 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 described as "minimum filter
basis weight"). According to the results of this experiment, the minimum filter basis
weight can be expressed using α × β, a product of mutually independent parameters
α and β, as shown in the second term on the left side of equation (1). Here, α is
defined as a function of the filtration speed A shown in equation (2), and β is defined
as a function of the particle retention B shown in equation (3). As the α × β is larger,
the minimum filter basis weight is smaller, and as the α × β is smaller, the minimum
filter basis weight is larger. As effects of movement of the individual parameters,
as the filtration speed A is larger, α is smaller and the minimum filter basis weight
is larger. As the filtration speed A is smaller, α is larger and the minimum filter
basis weight is smaller. Similarly, as the particle retention B is larger, β is smaller
and the minimum filter basis weight is larger. As the particle retention B is smaller,
β 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. In other words, 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.
[0035] In addition, as an example of the method for manufacturing a carbon fiber bundle
of the present invention, a filter medium with the particle retention B (µm) satisfying
the following equation (4) is preferably used.
[0036] 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.
[0037] Next, a method for 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 for manufacturing a precursor fiber bundle
for carbon fiber includes a spinning process for extruding 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 for cleaning the fibers obtained in the spinning process
in a water bath, a water bath stretching process for stretching the fibers obtained
in the water washing process in a water bath, and a dry-heat treatment process for
drying and heat treating the fibers obtained in the water bath stretching process,
and may include a steam stretching process for steam stretching the fibers obtained
in the dry-heat treatment process may be included, as necessary.
[0038] 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.
[0039] 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°C, and is preferably washed using
a water washing bath having a plurality of stages.
[0040] In addition, the stretch ratio in the water bath stretching process is preferably
2 to 6 times.
[0041] 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.
[0042] A known method can be used for the dry-heat treatment process. For example, the drying
temperature is 100 to 200°C.
[0043] A precursor fiber bundle for carbon fiber that is 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.
[0044] 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. In order 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.
[0045] 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.
[0046] The method for manufacturing a carbon fiber bundle of the present invention 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
3, and then heat-treated at 275°C or more and 295°C or less in an oxidizing atmosphere
until the density reaches 1.46 to 1.50 g/cm
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°C or more and 295°C or less in the latter half of the
oxidation process.
[0047] Here, 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.
[0048] 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
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
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, in order 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 for measuring
the density will be described later. 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.
[0049] In the oxidation process, first, the precursor fiber bundle for carbon fiber is heat-treated
in an oxidizing atmosphere, at preferably 210°C or more and less than 245°C, more
preferably 220°C or more and less than 245°C, and further preferably 225°C or more
and less than 240°C, thereby obtaining an oxidized fiber bundle with a density of
preferably 1.22 to 1.24 g/cm
3, and more preferably a density of 1.23 to 1.24 g/cm
3. When the density of the oxidized fiber bundle is 1.22 g/cm
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
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°C or more is preferable
because the difference between skin-core structure can be sufficiently suppressed.
At a temperature of less than 245°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.
[0050] The oxidized fiber bundle is heat-treated until the density reaches 1.22 to 1.24
g/cm
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
3, and more preferably 1.33 to 1.34 g/cm
3. This heat treatment process is performed in an oxidizing atmosphere at preferably
245°C or more and less than 275°C, and more preferably 250°C or more and less than
270°C. When the density is 1.32 g/cm
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
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°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°C, even when
heat-treating an oxidized fiber bundle with a density of 1.22 to 1.24 g/cm
3, the difference between skin-core structure can be suppressed, and high tensile strength
of strands is often exhibited.
[0051] Subsequently, the obtained oxidized fiber bundle is heat-treated in an oxidizing
atmosphere at a temperature of 275°C or more to 295°C or less, and preferably 280°C
or more to 290°C or less to obtain an oxidized fiber bundle with a density of 1.46
to 1.50 g/cm
3. When the heat treatment temperature is 275°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°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. In order 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. In
a case where there are temperature unevenness and temperature distribution when measuring
the oven temperature in several points, the simple average temperature is calculated.
[0052] In the present invention, the final density of the oxidized fiber bundle is 1.46
to 1.50 g/cm
3, preferably 1.46 to 1.49 g/cm
3, and further preferably 1.47 to 1.49 g/cm
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
3 or more, the carbonization yield can be sufficiently increased. When the density
is 1.50 g/cm
3 or less, the effect of increasing the carbonization yield is not saturated, which
is effective from the viewpoint of productivity. In order to complete the heat treatment
at the specified density, it is only necessary to adjust the oxidation temperature
and time.
[0053] In the process of heat treatment at 275°C or more and 295°C or less in an oxidizing
atmosphere until the density of the oxidized fiber bundle reached 1.46 to 1.50 g/cm
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
[0054] Generally, when the density of the oxidized fiber bundle is increased in order to
obtain a high carbonization yield, the tensile strength of strands of the carbon fiber
bundle tends to decrease. In the method for manufacturing a carbon fiber bundle of
the present invention, 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.
[0055] Except for the oxidation process, a known method for manufacturing a carbon fiber
bundle may be basically followed. However, in the method for manufacturing a carbon
fiber bundle of the present invention, 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°C until the density reaches 1.5 to 1.8 g/cm
3.
[0056] A carbonization process is performed, following the 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°C, and
preferably 1200 to 1600°C. When the maximum temperature is 1200°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°C or less, a satisfactory
carbonization yield can be obtained.
[0057] The carbon fiber bundle obtained as described above is preferably subjected to an
oxidation treatment so that an oxygen containing functional group is introduced, in
order 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.
[0058] 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.
[0059] 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>
[0060] 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°C, and a time of 30 minutes are used.
<Density Measurement>
[0061] 1.0 to 3.0 g of the oxidized fiber bundle is collected and completely dried at 120°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 × ρ)/(A - B). ρ is a specific gravity of ethanol at the measurement temperature.
<Skin Layer Ratio of Single Fiber of Carbon Fiber>
[0062] 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>
[0063] A mass A
f (g/m) and a density B
f (g/cm
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
f, and the average single fiber diameter (µm) of the carbon fibers is calculated by
the following equation.
[0064] <Knot Strength of Carbon Fiber Bundle>
[0065] 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 × 10
-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>
[0066] 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 simply referred to
as "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.
[0067] 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, in order 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° to observe the fracture origin from the oblique
upside (see Figs. 1 to 4).
[0068] 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.
[0069] 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" in the present invention 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".
[0070] Here, 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 for fixing the carbon fiber to a test piece mount,
and using a special test jig designed to perform in water. Further, a scanning electron
microscope (SEM) "S-4800" manufactured by Hitachi High-Technologies Corporation was
used for observing the collected fracture surfaces.
<Mean Surface Roughness>
[0071] 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 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 × 600 nm
Resolution: 512 pixels × 512 pixels
Measurement environment: room temperature, in air.
[0072] 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 × 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. Here,
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>
[0073] The quality of the carbon fiber bundle, which affects 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)
[0074] 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 µm, a filter medium
thickness C of 800 µm, and a filter basis weight D of 2500 g/m
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°C. The spun fiber bundle was washed with water at 30 to 98°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°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.
[0075] 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°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°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.
[0076] 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 1]
|
Oxidized fiber bundle density |
Oxidation temperature |
Oxidation tension |
First oven exit |
Second oven exit |
Final oven exit |
First oven |
Second oven |
Final oven |
Final oven |
g/cm3 |
°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 2-1]
|
Filtration conditions |
Filtration speed A |
Particle retention B |
Filter medium thickness C |
Filter basis weight D |
α |
β |
D-600 / (α×β) |
cm/h |
µm |
µm |
g/m3 |
- |
- |
- |
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 Example 1 |
3 |
9 |
1600 |
3200 |
0.98 |
0.11 |
-2253 |
Comparative Example 2 |
6 |
9 |
1600 |
3200 |
0.73 |
0.11 |
-4125 |
Comparative Example 3 |
6 |
9 |
3200 |
6400 |
0.73 |
0.11 |
-925 |
Comparative Example 4 |
8 |
1 |
800 |
2500 |
0.27 |
0.44 |
-2539 |
Comparative Example 5 |
12 |
1 |
800 |
2500 |
0.01 |
0.44 |
-199979 |
Example 6 |
3 |
1 |
800 |
2500 |
0.98 |
0.44 |
1120 |
Example 7 |
3 |
1 |
800 |
2500 |
0.98 |
0.44 |
1120 |
[Table 2-2]
|
Carbon fiber bundle |
Tensile strength of strands |
Elastic modulus of strands |
Probability that flaw with size of 50 nm or more exists |
Skin layer ratio of single fiber of carbon fiber |
Average single fiber diameter d |
Knot strength K |
- 88d+1390 |
Mean surface roughness Ra |
GPa |
GPa |
% |
% |
µm |
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 Example 1 |
5.7 |
252 |
52 |
91 |
7.5 |
722 |
730 |
- |
Comparative Example 2 |
5.6 |
257 |
57 |
91 |
7.4 |
778 |
739 |
2.1 |
Comparative Example 3 |
5.6 |
247 |
38 |
91 |
7.5 |
698 |
730 |
- |
Comparative Example 4 |
5.4 |
261 |
37 |
91 |
7.5 |
838 |
730 |
- |
Comparative Example 5 |
5.2 |
262 |
81 |
91 |
7.5 |
867 |
730 |
- |
Example 6 |
5.8 |
261 |
17 |
97 |
7.5 |
731 |
730 |
- |
Example 7 |
5.8 |
245 |
17 |
85 |
7.5 |
732 |
730 |
- |
(Example 2)
[0077] 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 µm, a filter medium thickness
C of 3200 µm, and a filter basis weight D of 6400 g/m
2.
(Example 3)
[0078] 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)
[0079] 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)
[0080] 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 µm and a filter basis
weight D of 3200 g/m
2. The number of fuzzes of the carbon fiber bundle was 0.2/m, and the quality deteriorated.
(Comparative Example 2)
[0081] 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)
[0082] 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)
[0083] 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)
[0084] 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)
[0085] 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)
[0086] 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
[0087] The present invention 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,
according to the carbon fiber bundle of the present invention, it becomes a carbon
fiber bundle which satisfies the productivity at the time of manufacturing a composite
material. The carbon fiber bundle to be obtained by the present invention 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.
DESCRIPTION OF REFERENCE SIGNS
[0088] (i): Fracture origin