U.S. patent application number 10/523511 was filed with the patent office on 2005-12-08 for carbon fiber strand.
This patent application is currently assigned to Toho Tenax Co., Ltd.. Invention is credited to Matsuki, Toshitsugu, Muto, Shinichi, Nishimura, Isao, Saeki, Takao, Sakajiri, Kouichi.
Application Number | 20050271874 10/523511 |
Document ID | / |
Family ID | 32211795 |
Filed Date | 2005-12-08 |
United States Patent
Application |
20050271874 |
Kind Code |
A1 |
Sakajiri, Kouichi ; et
al. |
December 8, 2005 |
Carbon fiber strand
Abstract
A sizing agent for carbon fiber consisting of a reaction
products containing an unsaturated urethane compound as the
principal component obtained by reacting an unsaturated alcohol
with an isocyanate compound is disclosed in claim 1.
Inventors: |
Sakajiri, Kouichi;
(Shizuoka, JP) ; Saeki, Takao; (Shizuoka, JP)
; Muto, Shinichi; (Shizuoka, JP) ; Nishimura,
Isao; (Shizuoka, JP) ; Matsuki, Toshitsugu;
(Shizuoka, JP) |
Correspondence
Address: |
NORRIS, MCLAUGHLIN & MARCUS, P.A.
875 THIRD AVE
18TH FLOOR
NEW YORK
NY
10022
US
|
Assignee: |
Toho Tenax Co., Ltd.
38-16, Hongo 2-chome Bunkyo-ku
Tokyo
JP
113-0033
|
Family ID: |
32211795 |
Appl. No.: |
10/523511 |
Filed: |
February 4, 2005 |
PCT Filed: |
October 24, 2003 |
PCT NO: |
PCT/JP03/13639 |
Current U.S.
Class: |
428/364 |
Current CPC
Class: |
D06M 15/55 20130101;
Y10T 428/2913 20150115; D06M 2101/40 20130101; D06M 2200/50
20130101; D06M 2200/40 20130101; B29K 2063/00 20130101 |
Class at
Publication: |
428/364 |
International
Class: |
D02G 003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 31, 2002 |
JP |
2002-319003 |
Claims
1. A carbon fiber strand obtained by impregnating a carbon fiber
with a sizing agent composition containing a sizing agent
comprising at least two kinds of epoxy resins, wherein the sizing
agent composition is such that, when it is mixed with a given
curing agent at proportions of 100 parts by mass (the sizing agent
composition) and 30 parts by mass (the curing agent) to make a
composition for estimation, the composition for estimation is
heat-treated at 130.degree. C. for 2 hours, and the resulting cured
material for estimation is measured for dynamic viscoelasticity to
obtain its tan 6 of a relaxation peak and its tan .delta. of .beta.
relaxation peak, their product .alpha..sub.tan .delta.
.beta..sub.tan .delta. is 0.07 to 0.2.
2. A carbon fiber strand according to claim 1, wherein the sizing
agent composition has a Viscosity of 100 to 10,000 poises at
30.degree. C.
3. A carbon fiber strand according to claim 1, wherein the sizing
agent contained in the sizing agent composition contains a PO/EO
block copolymer in an amount of less than 30% by mass relative to
the epoxy resins.
4. A carbon fiber strand according to claim 1, wherein the content
of the sizing agent composition is 0.3 to 5.0% by mass.
5. A carbon fiber strand according to claim 1, which is constituted
by 1,000 to 50,000 single fibers.
6. A carbon fiber strand according to claim 1, wherein the carbon
fibers constituting the carbon fiber strand show a surface oxygen
concentration ratio O/C of 0.05 to 0.3 when measured by X-ray
photoelectron spectroscopy.
Description
TECHNICAL FIELD
[0001] The present invention relates to a carbon fiber strand
suitably used in applications such as resin reinforcing agent and
the like. More particularly, the present invention relates to a
carbon fiber strand which, when used for production of a composite
material (e.g. a carbon fiber-reinforced resin), can give a
composite material having sufficient adhesion between the carbon
fiber and the matrix resin and being superior in interlaminar shear
strength (ILSS) between the matrix resin and the carbon fiber.
BACKGROUND ART
[0002] Carbon fiber has features of being strong, high in elastic
modulus and light as compared with other fibers; therefore, it is
in wide use as a reinforcing material for a composite material
using a thermoplastic resin or a thermosetting resin as the matrix
resin. A composite material reinforced with carbon fiber is light
and highly strong and, therefore, is in wide use in various
industries including aerospace industry.
[0003] The process for producing a composite material using a
thermosetting resin includes a process of producing a prepreg of
desired shape and molding it. Such a composite material can also be
produced by a pultrusion process using a carbon fiber strand and a
thermosetting resin, a resin transfer molding (RTM) process, a
filament winding (FW) process, a sheet molding compound (SMC)
process, a bulk molding compound (BMC) process, a hand lay-up
process, etc.
[0004] As the thermosetting matrix resin used in production of a
thermosetting resin-based composite material, there can be
mentioned, for example, epoxy resin, unsaturated polyester resin,
vinyl ester resin, and phenolic resin. Of these, epoxy resin, in
particular, is preferred because the epoxy resin can produce a
composite material well-balanced in heat resistance, physical
properties, etc.
[0005] As the epoxy resin, there can be mentioned bisphenol A type
epoxy resin, polyfunctional epoxy resin produced from a raw
material such as tetraglycidylamine, triglycidylamine or the like,
novolac type epoxy resin, etc. Bisphenol A type epoxy resin, in
particular, is in wide use as a matrix resin because it is superior
in adhesivity, physical properties, etc. and is
general-purpose.
[0006] In producing a prepreg, a carbon fiber strand is processed
according to a given procedure. In this processing, the carbon
fiber strand is fretted by a guide, etc., which tends to cause
fluffing and accordingly makes difficult later handling. In order
to avoid this problem, a sizing agent is ordinarily added to the
carbon fiber strand. Covering the surface of the carbon fiber
strand with the sizing agent can improve the bundle ability and
accordingly improve fretting resistance and handleability of the
strand.
[0007] In general, adhesivity with matrix resin is considered in
selection of a sizing agent and, when the matrix resin is an epoxy
resin, an epoxy resin is used as the sizing agent (JP-B-62-56266
and JP-A-7-197381).
[0008] It was proposed to use a vinyl ester resin as a sizing agent
When the matrix resin is an unsaturated matrix resin. In these
proposals, however, since no consideration was made as to the
adhesivity between sizing agent and carbon fiber, the adhesivity
between sizing agent and carbon fiber is insufficient.
[0009] Meanwhile, in order to improve the adhesivity between sizing
agent and carbon fiber, there were proposed sizing agents having a
polar group which may react with a functional group present on the
surface of the carbon fiber (JP-A-56-167715, JP-A-63-50573,
JP-A-11-93078 and JP-A-2001-20181). A composite material produced
using a carbon fiber strand containing such a sizing agent, when
measured for its interlaminar shear strength (ILSS), shows a
superior value. Incidentally, the interlaminar shear strength
(ILSS) of a composite material is a yardstick showing the
adhesivity between the carbon fiber and matrix resin constituting
the composite matrix. However, in a composite material produced
using a carbon fiber containing the above-mentioned sizing agent,
when a tension is added thereto in its fiber axis direction, the
tension is not dissipated into the whole portion of the carbon
fiber and instead is concentrated in a relatively small number of
carbon fibers. Consequently, this small number of carbon fibers are
broken, it is repeated, and finally the whole composite material
comes to be broken. Therefore, the strength of the composite
material in the fiber axis direction is not large as expected and
its interlaminar shear strength is low currently.
DISCLOSURE OF THE INVENTION
[0010] In order to solve the above problems, the present inventor
made a study on a sizing composition to be added to a carbon fiber.
In the course of the study, the present inventor found that when a
sizing agent composition is mixed with a curing agent and the
mixture is heat-treated to prepare a cured material of the sizing
agent composition, the cured material is used as a test sample and
measured for dynamic viscoelasticity to obtain a tan .delta. of a
relaxation peak (.alpha..sub.tan .delta.) and a tan .delta. of
.beta. relaxation peak (.beta..sub.tan .delta.), and there is
calculated a product of these two values, i.e. an .alpha..sub.tan
.delta. .beta..sub.tan .delta., this product has a large influence
on the strength of a composite material produced using a resin and
a carbon fiber to which the above-sizing agent has been added, in
its normal-to-fiber-axis direction. A further in-depth study by the
present inventor confirmed that any of the .alpha..sub.tan .delta.
and the .beta..sub.tan .delta. has no correlation with the strength
of the composite material in its normal-to-fiber-axis
direction.
[0011] The present invention has been completed based on the above
findings and aims at providing a carbon fiber strand capable of
giving a carbon fiber-reinforced resin composite material superior
in interlaminar shear strength greatly connected with its breaking
strength in its normal-to-fiber-axis direction.
[0012] The present invention which achieves the above aim, is as
follows.
[0013] [1] A carbon fiber strand obtained by impregnating a carbon
fiber with a sizing agent composition containing a sizing agent
comprising at least two kinds of epoxy resins, wherein the sizing
agent composition is such that, when it is mixed with a given
curing agent at proportions of 100 parts by mass (the sizing agent
composition) and 30 parts by mass (the curing agent) to make a
composition for estimation, the composition for estimation is
heat-treated at 130.degree. C. for 2 hours, and the resulting cured
material for estimation is measured for dynamic viscoelasticity to
obtain its tan .delta. of .alpha. relaxation peak and its tan
.delta. of .beta. relaxation peak, their product .alpha..sub.tan
.delta. .beta..sub.tan .delta. is 0.07 to 0.2.
[0014] [2] A carbon fiber strand set forth in [1], wherein the
sizing agent composition has a Viscosity of 100 to 10,000 poises at
30.degree. C.
[0015] [3] A carbon fiber strand set forth in [1], wherein the
sizing agent contained in the sizing agent composition contains a
PO/EO block copolymer in an amount of less than 30% by mass
relative to the epoxy resins.
[0016] [4] A carbon fiber strand set forth in [1], wherein the
content of the sizing agent composition is 0.3 to 5.0% by mass.
[0017] [5] A carbon fiber strand set forth in [1], which is
constituted by 1,000 to 50,000 single fibers.
[0018] [6] A carbon fiber strand set forth in [1], wherein the
carbon fibers constituting the carbon fiber strand show a surface
oxygen concentration ratio O/C of 0.05 to 0.3 when measured by
X-ray photoelectron spectroscopy.
[0019] The carbon fiber strand of the present invention is
constituted by impregnating a carbon fiber strand with a sizing
composition which, when cured under particular conditions, shows an
.alpha..sub.tan .delta. .beta..sub.tan .delta. of 0.07 to 0.2;
therefore, by using this carbon fiber strand as a reinforcing
material, there can be produced a carbon fiber-reinforced composite
material superior in interlaminar shear strength.
BRIEF DESCRIPTION OF THE DRAWING
[0020] FIG. 1 is a chart showing an example of the dynamic
viscoelasticity curve of a cured material for estimation obtained
by curing a sizing composition.
BEST MODE FOR CARRYING OUT THE INVENTION
[0021] The carbon fiber strand of the present invention is obtained
by impregnating a carbon fiber with a sizing agent composition.
[0022] The carbon fiber strand is a bundle of a large number of
carbon single fibers (filaments). It consists of preferably 1,000
to 50,000 carbon single fibers. Each carbon single fiber preferably
has 30 to 5,000 dtex.
[0023] Specific examples of the carbon single fiber constituting
the carbon fiber strand are a polyacrylonitrile (PAN)-based carbon
fiber, a pitch-based carbon fiber and a rayon-based carbon fiber.
Of these carbon fibers, the PAN-based carbon fiber is particularly
preferred for the handleability, passability through production
steps, etc. Here, as the PAN-based carbon fiber, there is generally
used one obtained by making into a carbon fiber a copolymer which
contains a acrylonitrile structural unit as the main component and
a vinyl monomer unit (e.g. itaconic acid, acrylic acid or acrylic
ester) in an amount of 10 mole % or less.
[0024] When a composite material is produced using the carbon fiber
strand and a matrix resin, each carbon fiber constituting the
carbon fiber strand preferably has an oxygen concentration ratio
O/C at carbon fiber surface, of 0.05 to 0.3 when measured by X-ray
photoelectron spectroscopy, in order to increase the adhesivity
between the carbon fiber and the matrix resin. A carbon fiber
having a surface oxygen concentration ratio of less than 0.05 is
inferior in its adhesivity with matrix resin; therefore, a
composite material produced using such a carbon fiber is low in
physical properties. Meanwhile, a carbon fiber having a surface
oxygen concentration ratio of more than 0.3 is per se low in
strength and therefore is not preferred.
[0025] Control of the surface oxygen concentration ratio of the
carbon fiber in the above range can be achieved by, in the steps of
production of the carbon fiber, carbonizing a raw material fiber to
obtain a carbon fiber and then subjecting it to a surface
treatment.
[0026] The surface treatment includes a liquid phase treatment, a
gas phase treatment, etc. A liquid phase surface treatment by
electrolysis is preferred from the standpoints of productivity,
uniform treatment, stability, etc. These surface treatments per se
are known.
[0027] As the yardstick employed for controlling the degree of
surface treatment of carbon fiber, there is preferred a surface
oxygen concentration ratio O/C of carbon fiber which is measured by
X-ray photoelectron spectroscopy (XPS).
[0028] In the present invention, the O/C is determined according to
the following procedure using an X-ray photoelectron spectrometer,
ESCA JPS-9000 MX, produced by Japan Electron Optical Laboratory
Co., Ltd. That is, a sizing agent-removed carbon fiber is placed in
a measurement chamber of ESCA maintained at a low pressure of 10 to
6 Pa; then, an X-ray generated under the conditions of 10 kV
(electron beam acceleration voltage) and 10 mA is applied to the
carbon fiber with Mg used as a counter pole, and the spectra of the
photoelectrons emitted, by the application, from the carbon atom
and oxygen atom of carbon fiber surface are recorded; and an O/C is
calculated from the ratio of the areas of the individual
spectra.
[0029] The proportions of the photoelectrons emitted differ
depending upon the individual elements. In the case of using the
X-ray photoelectron spectrometer, ESCA JPS-9000 MX, produced by
Japan Electron Optical Laboratory Co., Ltd., the conversion factor
determined based on the characteristics of the apparatus is
2.69.
[0030] The carbon fiber as necessary subjected to a surface
treatment as above is preferably washed thoroughly to remove the
electrolyte which adhered during the surface treatment.
[0031] In the present invention, the sizing agent composition to be
impregnated into the carbon fiber contains, as the main component,
an epoxy type sizing agent mixture comprising at least two kinds,
preferably two to five kinds of epoxy resins. Since epoxy resin is
in wide use as a matrix resin of carbon fiber-reinforced resin,
there is used, in the sizing agent composition, a mixture of at
least two kinds of epoxy resins in an amount of at least 50% by
mass, preferably at least 60% by mass, particularly preferably at
least 70% by mass.
[0032] As to the epoxy type sizing agent, there is no particular
restriction and a commercial epoxy type sizing agent can be used.
It can be exemplified by bisphenol A type epoxy resin, bisphenol F
type epoxy-resin, dimer acid type epoxy resin, glycidyl ester type
epoxy resin, polyfunctional epoxy resins (e.g. amino epoxy and
novolac type epoxy), and modified epoxy resins [e.g. elastomer
(styrene-butadiene elastomer or the like)-modified epoxy resin and
urethane-modified epoxy resin]. The epoxy resin is preferably
liquid at room temperature.
[0033] Examples of the chemical structural formulas of commercial
epoxy type sizing agents are shown below. 1
[0034] The sizing agent composition may contain a known sizing
agent other than the epoxy type sizing agent, and various additives
and auxiliary agents.
[0035] The additives and auxiliary agents include a known
dispersing agent, a surfactant, an emulsifier, a lubricant, a
stabilizer, etc.
[0036] The sizing agent other than the epoxy type sizing agent can
be exemplified by resins such as polyurethane, polyester, polyamide
and the like.
[0037] The sizing agent other than the epoxy type sizing agent can
be exemplified by known sizing agents such as polyalkyl ether,
unsaturated polyester, poly(vinyl ester), acrylic resin and the
like.
[0038] A particularly preferred sizing agent other than the epoxy
type sizing agent is a propylene oxide (PO)/ethylene oxide (EO)
block copolymer. As the PO/EO block copolymer, there can be used
one which is known as an emulsifier. Specifically, there is
preferred a PO/EO block copolymer having a propylene oxide and
ethylene oxide molar ratio of 2.about.8:8.about.2 and a viscosity
at 25.degree. C. of 5,000 to 30,000 mPa.multidot.s.
[0039] The sizing agent composition contains the propylene oxide
(PO)/ethylene oxide (EO) block copolymer in an amount of preferably
less than 30% by mass, more preferably 5% by mass to less than 30%
by mass.
[0040] When a sizing agent composition containing the propylene
oxide (PO)/ethylene oxide (EO) block copolymer is added to a carbon
fiber strand, the resulting carbon fiber strand is used to produce
a composite material, and the composite material undergoes an
external stress, the external stress is not concentrated in a small
region of the composite material and is dissipated uniformly
therein. As a result, the composite material shows a high
interlaminar shear strength.
[0041] The lubricant which is liquid at room temperature can be
exemplified by a higher aliphatic ether type polyoxyethylene
adduct, a higher aliphatic polyoxyethylene adduct, a higher fatty
acid ester of polyhydric alcohol, and a polyoxyethylene adduct of
higher fatty acid ester of polyhydric alcohol. These are known
lubricants.
[0042] The surfactant may be any of nonionic type, cationic type
and anionic type. By adding auxiliary agents such as lubricant,
surfactant and the like to the sizing agent composition, the carbon
fiber impregnated with the composition is improved in
handleability, fretting resistance, fluffing resistance, and
impregnability with sizing agent.
[0043] The sizing agent composition used in the present invention
contains the above-mentioned sizing agent and, as necessary,
additives and auxiliary agents. As described below, the sizing
agent composition shows an intended dynamic viscoelasticity when it
is mixed with a particular curing agent to prepare a composition
for estimation and heat-treated to prepare a cured material for
estimation.
[0044] That is, the cured material for estimation prepared using
the sizing agent composition used in the present invention gives a
product (.alpha..sub.tan .delta. .beta..sub.tan .delta.) of 0.07 to
0.2, preferably 0.07 to 0.15, wherein the .alpha..sub.tan .delta.
.beta..sub.tan .delta. is a product of a tan .delta. of a
relaxation peak (.alpha..sub.tan .delta.) and a tan .delta. of
.beta. relaxation peak (.beta..sub.tan .delta.), both obtained from
a measured dynamic viscoelasticity curve shown in FIG. 1.
[0045] The present inventor thought that a sizing agent composition
capable of giving a cured material for estimation showing a
.alpha..sub.tan .delta. .beta..sub.tan .delta. of 0.07 to 0.2 is
preferred, based on the following reason. That is, the tan .delta.
values obtained from the dynamic viscoelasticity measurement of the
cured material for estimation are yardsticks for evaluating the
dissipation of thermal energy relative to external stress and, from
these values, the toughness of a material can be estimated.
Specifically explaining, high toughness can be expected with a
material of high tan .delta. values. When the sizing agent is an
epoxy resin, there exist mainly an .alpha. relaxation attributed to
the molecular movement of polymer main chain and a .beta.
relaxation attributed to local movement. These two relaxations are
thought to be main factors governing the toughness of a material.
Hence, the present inventor thought that by taking a large value
for the product of the two relaxations (.alpha..sub.tan .delta. and
.beta..sub.tan .delta.), i.e. .alpha..sub.tan .delta.
.beta..sub.tan .delta., the properties of a carbon fiber-reinforced
resin can be improved.
[0046] A .alpha..sub.tan .delta. .alpha..sub.tan .delta. of less
than 0.07 is not preferred because the sizing agent composition in
composite material, when cured, is low in toughness and the carbon
fiber-reinforced resin (composite material) is inferior in
properties.
[0047] When the .alpha..sub.tan .delta. .alpha..sub.tan .delta. is
more than 0.2, the cured sizing agent composition in composite
material, is low in hardness and is unable to remain stable at the
interface between the matrix resin and the carbon fiber.
[0048] The curing agent added to the sizing agent composition for
preparation of a composition for evaluation has the following
chemical structure. This compound is a curing agent for epoxy
resin, well known to those skilled in the art as Kayahard MCD
(trade name) (a product of Hitachi Chemical Company, Ltd.). The use
amount of the curing agent is 30 parts by mass relative to 100
parts by mass of the sizing agent composition. The heat treatment
conditions are 2 hours at 130.degree. C. 2
[0049] The sizing agent composition used in the present invention
has a viscosity at 30.degree. C. of preferably 100 to 10,000
poises. When the viscosity is less than 100 poises, the carbon
fiber strand impregnated with the sizing agent composition is too
soft. In this case, when the strand comes in touch with a guide
roller in a prepreg production step, a filament winding step, etc.,
fluffing, for example, tends to occur. When the viscosity is more
than 10,000 poises, the carbon fiber strand obtained by
impregnating a carbon fiber with the sizing agent composition is
too hard and moreover the impregnatability of a resin into the
carbon fiber strand is inferior.
[0050] In the present invention, the sizing agent composition is
impregnated into the above-mentioned carbon fiber to obtain a
carbon fiber strand of the present invention.
[0051] The impregnation amount of the sizing agent composition in
the carbon fiber strand is preferably 0.3 to 5.0% by mass, more
preferably 1.0 to 4.0.
[0052] When the impregnation amount of the sizing agent composition
in the carbon fiber strand is less than 0.3% by mass, the bundling
of the-carbon fiber strand is inferior. Further, the composite
material produced using this carbon fiber strand is insufficient in
adhesivity between the carbon fiber and the matrix resin and is low
in shear strength.
[0053] When the content of the sizing agent composition is more
than 5.0% by mass, the carbon fiber strand is inferior in
spreadability. As a result, in production of a composite material,
the infiltrability of the matrix resin into the carbon fiber strand
is low, which is not preferred.
[0054] The method for impregnation of the sizing agent composition
into the carbon fiber can be conducted by any method selected from
spraying method, dipping method, transfer method, etc., all
well-known to those skilled in the art. The dipping method is
preferred because it is superior in general-purpose properties,
efficiency and uniform impregnation. When the carbon fiber is
dipped in the sizing agent composition in the dipping method, it is
preferred that the spreading and squeezing of the carbon fiber are
conducted repeatedly using a roller provided in the sizing agent
composition to infiltrate the sizing agent composition sufficiently
into the carbon fiber strand.
[0055] The impregnation method of the sizing agent composition
includes a solution method which comprises dipping a carbon fiber
in a solution of a sizing agent composition containing at least two
kinds of epoxy resins, etc., dissolved in a solvent such as acetone
or the like; and an emulsion method which comprises dipping a
carbon fiber in an aqueous emulsion obtained by emulsifying a
sizing agent composition in water using an emulsifier or the like.
The emulsion method is preferred from the standpoint of safety to
human body and prevention of pollution of natural environment.
[0056] After the sizing agent composition has been impregnated into
the carbon fiber, ordinarily the carbon fiber impregnated with the
sizing agent is sent to a drying step to remove the dispersing
agent (water or solvent) which adhered to the carbon fiber during
impregnation of the sizing agent composition. As the drying method
which can be employed in the drying step, there are, for example, a
method of passing the carbon fiber impregnated with the sizing
agent composition through a drying oven, and a method of contacting
the carbon fiber impregnated with the sizing agent composition,
with an overheated roller. The drying temperature is not
particularly restricted; however, it is set ordinarily at 80 to
200.degree. C. when there is employed an aqueous emulsion type
sizing agent composition of general use. It is possible that after
the drying step, a heat treatment step of 200.degree. C. or more is
conducted to adjust the viscosity of the sizing agent composition
in the carbon fiber strand.
[0057] The present invention is described more specifically below
by way of Examples.
EXAMPLES
[0058] Physical properties were measured according to the following
methods.
[0059] <Dynamic Viscoelasticity Measurement>
[0060] 100 parts by mass of a sizing agent and 30 parts by mass of
a curing agent (Kayahard MCD, a product of Hitachi Chemical
Company, Ltd.) were mixed to prepare a composition for estimation;
the composition was molded and cured at 130.degree. C. for 2 hours
in a mold to obtain a cured material for estimation. This cured
material for estimation was cut into a size of 30 mm
(length).times.6 mm (width).times.3 mm (thickness) to obtain a test
piece for measurement of dynamic viscoelasticity.
[0061] The test piece was measured for dynamic viscoelasticity
using a dynamic mechanical analyzer (Model Rhogel E-4000, a product
of UBM). The measurement conditions were temperature elevation
rate: 4.degree. C./min, frequency: 10 Hz; and measurement
temperature range: -100 to 200.degree. C.
[0062] From the dynamic viscoelasticity curve obtained (an example
of the curve is shown in FIG. 1), a tan .delta. of a relaxation
peak (.alpha..sub.tan .delta.) and a tan .delta. of .beta.
relaxation peak (.beta..sub.tan .delta.) were determined, and their
product .alpha..sub.tan .delta. .beta..sub.tan .delta. was
calculated.
[0063] <Interlaminar Shear Strength (ILSS)>
[0064] A resin composition was prepared by mixing 70 parts by mass
of EPN 1138 (trade name) (a phenolic novolac type epoxy resin
produced by Ciba Geigy Japan Limited), 12 parts by mass of Epikote
834 (trade name) (a bisphenol A type epoxy resin produced by Japan
Epoxy Resins Co., Ltd.), and 18 parts by weight of Epikote 1002
(trade name) (a bisphenol A type epoxy resin produced by the same
company.). To the resin composition were added 5 parts by mass of a
curing agent DICY (dicyandiamide produced by the same company.) and
10 parts by mass of a curing accelerator DUMU
(3-[3,4-dichlorophenyl]-1,1-dimethylurea produced by Hodogaya
Chemical Co., Ltd.) to prepare a resin composition for prepreg
production. This composition was coated on a release paper using a
film coater, in an amount of 44 g/m.sup.2, to obtain a resin film
consisting of a release paper and a resin composition layer formed
thereon.
[0065] On this resin film was placed a carbon fiber strand
impregnated by the sizing agent composition at equal intervals in a
stretched state, and they were heated to impregnate the resin of
the resin film into the carbon fiber strand to produce a
unidirectional (UD) prepreg having an area weight of 150 g/m.sup.2
and 37% by mass in impregnated resin content.
[0066] The UD prepreg was laminated in a plurality of layers so as
to give a thickness of 3 mm. The laminate was placed in a mold and
molded at 130.degree. C. for 2 hours at a pressure of 686 kPa (7
kg/cm.sup.2) to produce a unidirectional carbon fiber-reinforced
plastic plate (CFRP plate). This CFRP plate was measured for ILSS
at room temperature according to ASTM D 2344.
Examples 1 to 8 and Comparative Examples 1 to 3
[0067] A sizing agent-free carbon fiber strand (Besfight produced
by Toho Tenax Co., Ltd., 24,000 filaments) having a surface oxygen
concentration ratio O/C of 0.2 as measured by X-ray photoelectron
spectroscopy, was continuously dipped in a sizing bath. Each sizing
agent composition aqueous emulsion used was an aqueous emulsion
obtained by emulsifying 100 parts by mass of each sizing agent
composition [whose formulation is shown in Table 1 and which is a
mixture of two or three components of different molecular weights
selected from bisphenol A type epoxy resins (Epikote 828 and
Epikote 1002, products of Japan Epoxy Resins Co., Ltd.), a
bisphenol F type epoxy resin (Epikote 807, a product of Japan Epoxy
Resins Co., Ltd.), a dimer acid type epoxy resin (Epikote 871, a
product of Japan Epoxy Resins Co., Ltd.), a tetrafunctional
aminoepoxy resin (Epikote 604, a product of Japan Epoxy Resins Co.,
Ltd.), a glycidyl ester type epoxy resin (Epikote 191P, a product
of Japan Epoxy Resins Co., Ltd.) and an elastomer-modified epoxy
resin (Epikote YX 310, a product of Japan Epoxy Resins Co., Ltd.)],
with 20 parts by mass of a PO/EO block copolymer [Leocon ED274R
(trade name) produced by Lion Corporation, viscosity: 6,900
mPa.multidot.s at 25.degree. C.].
[0068] The carbon fiber strand impregnated with each sizing agent
composition emulsion was dried (150.degree. C. for 3 minutes) to
remove the water of the strand to obtain each carbon fiber strand.
At that time, the bath concentration was adjusted to obtain carbon
fiber strands shown in Table 1. By using these carbon fiber
strands, ILSS were measured. The results are shown in Tables 1 and
2.
[0069] Meanwhile, each sizing agent composition mentioned above was
heat-treated and cured according to the method described in the
above <dynamic viscoelasticity measurement>, to produce each
test piece for dynamic viscoelasticity measurement. These test
pieces were measured for dynamic viscoelasticity to obtain
respective dynamic viscoelasticity curves and, from the curves, an
.alpha..sub.tan .delta. .beta..sub.tan .beta. of each sizing agent
composition was calculated. The results are shown in Tables 1 and
2.
[0070] As shown in Table 1, Examples 1 to 6 each showing an
.alpha..sub.tan .delta. .beta..sub.tan .delta. of 0.07 to 0.2 are
high in ILSS. However, as shown in Table 2, Comparative Examples 1
to 6 each showing an .alpha..sub.tan .delta. .beta..sub.tan .delta.
not falling in the above range are low in ILSS.
1 TABLE 1 Examples 1 2 3 4 5 6 7 8 Sizing agent composition
formulation, parts by mass EP828 -- -- 70 -- -- 60 35 10 EP191P 70
-- -- 70 -- -- -- -- EP807 -- -- -- -- 80 -- 35 40 EP871 -- 50 --
-- -- -- -- 50 EP604 -- -- -- -- -- 40 -- -- EP4007 -- 50 -- 30 20
-- -- -- EP1002 30 -- 30 -- -- -- 30 -- .alpha..sub.tan .delta.
.multidot. .beta..sub.tan .delta. 0.076 0.072 0.074 0.091 0.092
0.070 0.085 0.19 Sizing agent composition, 3.4 2.7 1.3 2.3 1.6 2.6
2.0 3.0 % by mass ILSS, MPa 100 98 99 105 102 97 100 99
[0071]
2 TABLE 2 Comparative Examples 1 2 3 4 5 6 7 Sizing agent
composition formulation, parts by mass EP828 40 -- 50 -- -- -- 10
EP191P -- -- -- -- 50 -- -- EP807 -- 40 -- 50 -- -- 20 EP871 -- --
-- -- -- 20 70 EP604 -- -- -- -- -- -- -- EP4007 -- -- 50 50 50 80
-- EP1002 60 60 -- -- -- -- -- .alpha..sub.tan .delta. .multidot.
.beta..sub.tan .delta. 0.049 0.061 0.044 0.054 0.062 0.063 0.23
Sizing agent composition, 3.3 2.5 2.0 2.5 1.3 1.8 1.3 % by mass
ILSS, MPa 91 93 90 89 91 88 85
* * * * *