U.S. patent application number 12/738361 was filed with the patent office on 2010-08-19 for sheet-like article and method for making the same.
This patent application is currently assigned to TOKUSHU PAPER MFG. CO., LTD.. Invention is credited to Kousuke Akiyama, Masanori Imai.
Application Number | 20100206504 12/738361 |
Document ID | / |
Family ID | 40579519 |
Filed Date | 2010-08-19 |
United States Patent
Application |
20100206504 |
Kind Code |
A1 |
Akiyama; Kousuke ; et
al. |
August 19, 2010 |
SHEET-LIKE ARTICLE AND METHOD FOR MAKING THE SAME
Abstract
A sheet-like article including fibers and carbon nanotubes
and/or carbon nanohorns adhering to the surface of the fibers in a
uniformly dispersed state without agglomeration to form a network
structure on the fibers. The sheet-like article is preferably made
by converting the fibers and a dispersion of the carbon nanotubes
and/or carbon nanohorns by a wet papermaking process. To use
cellulose fibers as main fibers provides a good sheet-like
article.
Inventors: |
Akiyama; Kousuke; (Shizuoka,
JP) ; Imai; Masanori; (Shizuoka, JP) |
Correspondence
Address: |
YOUNG & THOMPSON
209 Madison Street, Suite 500
Alexandria
VA
22314
US
|
Assignee: |
TOKUSHU PAPER MFG. CO.,
LTD.
Shizuoka
JP
|
Family ID: |
40579519 |
Appl. No.: |
12/738361 |
Filed: |
October 22, 2008 |
PCT Filed: |
October 22, 2008 |
PCT NO: |
PCT/JP2008/069150 |
371 Date: |
April 16, 2010 |
Current U.S.
Class: |
162/181.9 ;
162/158; 977/734; 977/742 |
Current CPC
Class: |
D21H 15/12 20130101;
D21H 13/50 20130101; D21H 21/14 20130101 |
Class at
Publication: |
162/181.9 ;
162/158; 977/734; 977/742 |
International
Class: |
D21H 23/00 20060101
D21H023/00; D21H 11/00 20060101 D21H011/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 23, 2007 |
JP |
2007-274693 |
Claims
1-13. (canceled)
14. A sheet-like article comprising fibers and carbon nanotubes
and/or carbon nanohorns, the carbon nanotubes and/or carbon
nanohorns adhering to the surface of the fibers in a uniformly
dispersed state without agglomeration to form a network structure
on the fibers.
15. The sheet-like article according to claim 14, being obtained by
a wet papermaking process using the fibers and a dispersion of the
carbon nanotubes and/or carbon nanohorns.
16. The sheet-like article according to claim 14, being obtained by
impregnating a fiber aggregate free from carbon nanotubes and/or
carbon nanohorns with a dispersion of the carbon nanotubes and/or
carbon nanohorns.
17. The sheet-like article according to claim 16, wherein the fiber
aggregate is formed of fibers treated with a cationic organic
polymer.
18. The sheet-like article according to claim 14, wherein the
fibers mainly comprise cellulose fibers.
19. A method of making the sheet-like article according to claim
14, comprising the steps of mixing fibers and a dispersion of
carbon nanotubes and/or carbon nanohorns to prepare a mixed
dispersion and converting the mixed dispersion into a sheet form by
a wet papermaking process.
20. The method according to claim 19, wherein the dispersion of
carbon nanotubes and/or carbon nanohorns contains a surfactant.
21. The method according to claim 20, further comprising the step
of adsorbing a fixing agent for carbon nanotubes and/or carbon
nanohorns to the fibers before the step of mixing the fibers with
the dispersion of carbon nanotubes and/or carbon nanohorns.
22. The method according to claim 21, wherein the fixing agent has
a polarity opposite to that of the surfactant.
23. The method according to claim 22, wherein the surfactant is
anionic, and the fixing agent is cationic.
24. The method according to claim 23, wherein the fixing agent is
an organic polymer.
25. A sheet-like article obtained by the method according to claim
19.
26. The sheet-like article according to claim 15, wherein the
fibers mainly comprise cellulose fibers.
27. The sheet-like article according to claim 16, wherein the
fibers mainly comprise cellulose fibers.
28. The sheet-like article according to claim 17, wherein the
fibers mainly comprise cellulose fibers.
29. A sheet-like article obtained by the method according to claim
20.
30. A sheet-like article obtained by the method according to claim
21.
31. A sheet-like article obtained by the method according to claim
22.
32. A sheet-like article obtained by the method according to claim
23.
33. A sheet-like article obtained by the method according to claim
24.
Description
TECHNICAL FIELD
[0001] This invention relaters to a sheet-like article that is
fabricated of fibers having carbon nanotubes and/or carbon
nanohorns adhering thereto and is useful as an electrically
conductive material, an electromagnetic shielding material, an
electromagnetic absorbing material, a microwave heating element, an
electrode, an ultrafine filter, a sheet heating element, a catalyst
carrier, and so on. The invention also relates to a method for
producing the sheet-like article.
BACKGROUND ART
[0002] Carbon nanotubes (CNTs) and carbon nanohorns (CNHs) exhibit
very high electrical conductivity based on their unique structure
and are receiving attention for use as an electrically conductive
material, an electromagnetic shielding material, an electromagnetic
absorbing material, a microwave heating element, an electrode, an
ultrafine filter, a sheet heating element, and so on. Sheet
materials containing CNTs or CNHs have been studied for these
uses.
[0003] For example, patent document 1 (see below) proposes a sheet
composed of pulp and carbon fibers having a diameter of 0.01 to 4
.mu.m and an aspect ratio of 2 to 100,000. The sheet is produced by
a method including the step of mixing pulp and the carbon fiber.
Therefore, the carbon fibers form agglomerates even though they are
subjected to a treatment for improving dispersibility, such as
oxidation, coating, or grafting and fails to form a carbon fiber
network on the cellulose fibers. Because the connections between
carbon fibers are reduced due to the formation of carbon fiber
agglomerates, it has been difficult to achieve the best performance
characteristics of the carbon fibers, such as electrical
conductivity, electromagnetic shielding characteristics, and
microwave absorbing characteristics.
[0004] Patent document 2 (see below) discloses paper containing
carbon fibril agglomerates, in which carbon fibrils are intertwined
with cellulose fibers. However, since the carbon fibrils exist in
the form of agglomerates, they do not form a sufficient network in
the sheet, which makes it difficult to obtain the full performance
of carbon fibrils, as is the case with the sheet of patent document
1.
[0005] Patent document 3 (see below) proposes a method for covering
natural fibers with CNTs by immersing natural fibers in a mixture
of CNTs, a surfactant, and distilled water. This method, however,
fails to achieve sufficient covering of the cellulose fibers with
CNTs. Moreover, natural fibers containing a surfactant are not
easily entangled with each other so that, while usable as yarn for
woven fabric, they are difficult to be formed into a sheet by a wet
papermaking technique. Because the CNTs are coated with the
surfactant and are therefore inhibited from coming into contact
with each other, electron motion is interfered with so that
sufficient electrical conductivity is not provided.
[0006] Patent document 4 (see below) teaches a method for covering
natural fibers with CNTs by treating natural fibers with a
dispersion of CNTs in a chemical plating bath. According to the
method, natural fibers are covered with CNTs via a metal, such as
nickel. As a result, the performance characteristics essentially
possessed by CNTs, such as electromagnetic shielding
characteristics, microwave absorbing characteristics, and planarly
heat generating properties, are not fully exhibited. Although
natural fibers treated with a chemical plating bath are usable as
yarn for woven fabric as stated above, it is impossible to form
them into a sheet by a wet papermaking technique.
[0007] Patent documents 5 and 6 (see below) disclose a method for
dispersing CNTs and a sheet made by the method. The sheet is made
solely of CNTs and is therefore very expensive and unable to retain
the strength as a sheet. In addition, the CNTs have a surfactant
adhering thereto. The network formed of such CNTs does not
sufficiently exhibit the performance, such as electrical
conductivity, because the surfactant interferes with electrical
conduction.
[0008] Patent document 7 (see below) proposes paper containing CNTs
and a method for producing the paper. However, the CNT fixing
technique disclosed therein allows CNTs to agglomerate in the step
of preparing a pulp slurry, resulting in a failure to form a
satisfactory network of CNTs in the sheet. Therefore, it is
difficult to make the full advantage of the electrical conductivity
of CNTs, and it is necessary to add a large quantity of CNTs to
increase the conductivity of the sheet.
[0009] Thus, it has been difficult with conventional technology to
form a CNT- and/or CNH-containing sheet having very effective
electrical characteristics suited for use as an electrically
conductive material, an electromagnetic shield, a microwave heating
element, an electrode, and so forth.
Patent document 1: JP 63-288298A Patent document 2: JP 7-97789A
Patent document 3: JP 2005-256221A Patent document 4: JP
2005-256222A Patent document 5: JP 2007-39623A Patent document 6:
JP 2007-63107A Patent document 7: WO08/069,287
DISCLOSURE OF THE INVENTION
[0010] An object of the invention is to provide a sheet-like
article having excellent electrical characteristics and heat
generating characteristics and suitable for use as an electrically
conductive (hereinafter simply referred to as "conductive")
material, an electromagnetic shielding material, an electromagnetic
absorbing material, a microwave heating element, an electrode, and
so on.
[0011] The invention provides a sheet-like article including fibers
and a large number of CNTs and/or CNHs adhering to the fibers. The
CNTs and/or CNHs adhere to the surface of the fibers in an
uniformly dispersed state without agglomeration thereby to form a
network structure on the fibers.
[0012] The invention provides an embodiment of the sheet-like
article, which is obtained by a wet papermaking process using a
dispersion of the CNTs and/or CNHs and the fibers.
[0013] The invention provides an embodiment of the sheet-like
article, which is obtained by impregnating a fiber aggregate free
from the CNTs and/or CNHs with a dispersion of the CNTs and/or
CNHs.
[0014] The invention provides an embodiment of the sheet-like
article, in which the fiber aggregate is formed of fibers treated
with a cationic organic polymer.
[0015] The invention provides an embodiment of the sheet-like
article, in which main fibers forming the sheet-like article are
cellulose fibers.
[0016] The invention also provides a method of making the
sheet-like article. The method includes the steps of mixing fibers
and a dispersion of CNTs and/or CNHs to prepare a mixed dispersion
and converting the mixed dispersion into a sheet by a wet
papermaking process.
[0017] The invention provides an embodiment of the method, in which
the dispersion of CNTs and/or CNHs contains a surfactant.
[0018] The invention provides an embodiment of the method, which
further includes the step of adsorbing a CNT and/or CNH fixing
agent to the fibers before the step of mixing the fibers with the
dispersion of CNTs and/or CNHs.
[0019] The invention provides an embodiment of the method, in which
the fixing agent has a polarity opposite to that of the
surfactant.
[0020] The invention provides an embodiment of the method, in which
the surfactant is anionic, and the fixing agent is cationic.
[0021] The invention provides an embodiment of the method, in which
the fixing agent is an organic polymer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a schematic cross-section of a fiber forming the
sheet-like article of the invention, taken along the longitudinal
direction of the fiber.
[0023] FIG. 2 is an electron micrograph (.times.3000) of the
sheet-like article obtained in Example 1.
[0024] FIG. 3 is an electron micrograph (.times.10000) of the
sheet-like article obtained in Example 1.
[0025] FIG. 4 is an electron micrograph (.times.3000) of the
sheet-like article obtained in Comparative Example 3.
[0026] FIG. 5 is an electron micrograph (.times.10000) of the
sheet-like article obtained in Comparative Example 3.
[0027] FIG. 6 is a figure equivalent to FIG. 1 of a sheet-like
article out of the scope of the invention.
BEST MODE FOR CARRYING OUT THE INVENTION
[0028] One of the essential characteristics of the sheet-like
article of the invention is that CNTs and/or CNHs not only adhere
to the surface of main fibers forming the sheet-like article but
also form a specific network structure covering the surface of the
fibers. As used herein, the expression "main fibers (forming the
sheet-like article)" is intended to mean the fibers that form at
least 51% by mass of the sheet-like article. The electrical
conductivity (hereinafter simply referred to as "conductivity") of
a CNT or CNH is almost the same as that of copper. A CNT and a CNH
are physically very tough. Therefore, a sheet-like article
fabricated of fibers covered with a specific network of CNTs and/or
CNHs not only exhibits excellent electrical characteristics but
have strength and yet flexibility providing ease in handling, being
backed by the main fibers. The invention has thus succeeded in
settling the problems with conventional carbon black-containing
sheeting, such as fall-off of paper dust or carbon black particles.
The CNTs and/or CNHs existing on the fibers in the form of a
network structure allow efficient conversion of electron kinetic
energy, electromagnetic waves, and microwaves to thermal energy,
thereby to provide an excellent electromagnetic shielding sheet,
microwave heating element, and sheet heating element.
[0029] It is preferred that the specific network structure of CNTs
and/or CNHs preferentially cover the main fibers of the sheet-like
article. It is preferred that at least 5%, more preferably 20% or
more, of the surface area of the main fibers be covered with CNTs
and/or CNHs. When the surface area of the main fibers covered with
CNTs and/or CNHs is less than 5% based on the total surface area of
the main fibers, it is difficult for CNTs and/or CNHs to form a
network structure on the fibers and to provide good electrical
characteristics. That is, it is preferred for obtaining good
electrical characteristics that at least 5%, more preferably 20% or
more, of the surface area of the main fibers be covered with CNTs
and/or CNHs.
[0030] The specific network structure of CNTs and/or CNHs on the
fibers is the structure of a large number of CNTs and/or CNHs
adhering to the surface of a fiber A (the main fiber) in a
uniformly dispersed state without agglomeration, as schematically
illustrated in FIG. 1. A large number of CNTs and/or CNHs form a
continuous carbon layer (microscopic network structure) B with a
uniform thickness over substantially the whole length of the fiber
A. A great number of fibers A each having such a microscopic
network structure (i.e., carbon layer B) of CNTs and/or CNHs formed
thereon are three dimensionally entangled with each other to form a
sheet. Accordingly, the sheet-like article of the invention has, as
a whole, a macroscopic network of CNTs and/or CNHs to exhibit
excellent electrical characteristics and heat generating
characteristics.
[0031] As used herein, the term "network structure of CNTs and/or
CNHs" refers not only to the microscopic network structure
continuously formed by mutual connection of CNTs and/or CNHs on the
individual main fibers but also to the macroscopic network
structure formed by three dimensional entanglement of the fibers
covered with CNTs and/or CNHs. The CNTs and/or CNHs are thus in
continuous contact with each other throughout the sheet-like
article to provide passageways for electrons, which is considered
to provide the above discussed very advantageous electrical
characteristics. However, it is not always necessary for CNTs
and/or CNHs to form a carbon layer B with a uniform thickness on a
fiber A as illustrated in FIG. 1. The carbon layer B may have a
somewhat varying thickness or may have a surface roughness.
[0032] As stated, it is necessary for CNTs and/or CNHs to adhere to
the surface of individual main fibers forming the sheet-like
article of the invention in a uniformly dispersed state without
agglomeration, as schematically illustrated in FIG. 1, thereby to
achieve uniform conductivity through the sheet-like article. In the
case of, in contrast, a sheet-like article containing fibers having
CNTs and/or CNHs adhering thereto in which a large number of CNTs
and/or CNHs form a number of agglomerates C which continuously
connect to each other on the surface of a main fiber A, i.e., the
CNTs and/or CNHs are not uniformly dispersed as schematically
illustrated in FIG. 6, although electrons are allowed to pass
through the agglomerates C, the sheet-like article hardly provides
reliable electrical conduction as a whole. In this case, since no
improvement in conductivity is expected, CNTs and/or CNHs should be
used in an increased amount to obtain improved conductivity, which
will cause an increase in cost. Consequently, the sheet-like
article is not preferred. Moreover, the CNTs and/or CNHs added in a
largely increased amount will not only cover the fiber surface but
fill the fiber-fiber interstices. As a result, drainage of a
wet-laid fiber layer deteriorates, making sheet formation
difficult. Since the CNTs and/or CNHs are not uniformly distributed
in the sheet-like article, as schematically shown in FIG. 6,
conductivity inside the sheet-like article is not uniform, which
may lead to such a problem that electrification can cause a fire at
a portion having high resistance.
[0033] Where CNTs or CNHs are discontinuous on the fiber surface,
i.e., where a large number of CNTs and/or CNHs do not form a
network structure on the fiber, the CNTs and/or CNHs fail to
efficiently convert electromagnetic or microwave energy to thermal
energy and to exhibit sufficient effects when, in particular, the
sheet-like article is used as an electromagnetic shield or a
microwave heating element.
[0034] The sheet-like article of the invention exhibits
conductivity in its thickness direction, too, because of the
network structure of CNTs and/or CNHs extending in not only planar
directions (two dimensional directions) but also in the thickness
direction (three dimensional directions) of the sheet-like article.
Therefore, the sheet-like article is suited for use as, for
example, a current collector of a battery cell. The network
structure of CNTs and/or CNHs extending in the thickness direction
also brings about great improvements in electromagnetic shielding
characteristics and the like. In the cases where the main fibers
are partly covered with CNTs and/or CNHs, the ratio of the surface
area of the fibers covered with CNTs and/or CNHs to the total
surface area of the main fibers is of importance. In such cases, a
single fiber may have CNTs and/or CNHs adhering to a divided
part(s) of its surface. The electromagnetic wave shielding
characteristics or the characteristics as a microwave heating
element are secured as long as the ratio of the surface area of the
main fibers covered with CNTs and/or CNHs to the total surface area
of the main fibers (hereinafter referred to as "covered surface
area ratio") is 5% or more. With a covered surface area ratio of
20% or more, the performance as a sheet heating element is markedly
improved. With a covered surface area ratio of 50% or more,
conductivity is remarkably improved, which will allow application
to new fields where conductivity is of concern. The covered surface
area ratio, a measure of the degree to which the surface of the
main fibers is covered with CNTs and/or CNHs, is a significant
parameter; for it relates to the degree of the formation of the
network by CNTs and/or CNHs. The degree of the formation of the
network by CNTs and/or CNHs is easily demonstrable by direct
observation under an electron microscope
[0035] The sheet-like article having the above discussed network
structure of CNTs and/or CNHs is preferably fabricated by (1) a wet
papermaking process using fibers and a dispersion of the CNTs
and/or CNHs (hereinafter also referred to simply as "CNT/CNH
dispersion") or (2) impregnating a fiber aggregate formed without
using CNTs and/or CNHs with a CNT/CNH dispersion. In either method,
to use a CNT/CNH dispersion is a key point for forming the network
structure of CNTs and/or CNHs to make the sheet-like article.
[0036] The method (1) is sheet formation by a wet papermaking
process. A wet papermaking process is a technique commonly used to
make paper or nonwoven fabric, which includes dispersing fibers in
water, adding necessary chemicals to the resulting slurry as
appropriate, and converting the slurry into a sheet form using wire
cloth. CNTs and CNHs are usually composed solely of surface atoms
so that they are attracted to themselves by van der Waals force and
eventually exist as agglomerates. If CNTs and CNHs are used to make
a sheet in the form of powder without being dispersed in a liquid
medium, such as water, they would exists in the resulting sheet in
the form of agglomerates as schematically illustrated in FIG. 6 and
fail to exhibit their intrinsic performance discussed above. It is
therefore desirable in the invention that the sheet-like article be
fabricated using a CNT/CNH dispersion. The dispersion preferably
contains a surfactant helping CNTs and/or CNHs to be dispersed in a
liquid medium.
[0037] The manner of dispersing CNTs and/or CNHs, i.e., the method
of preparing a CNH/CNHs dispersion is not particularly limited.
Examples of useful dispersions include, but are not limited to, a
dispersion prepared by dispersing CNTs and/or CNHs using a
surfactant by ultrasonication, bead-milling, or like means; a
dispersion prepared by dispersing using an organic solvent by a
physical treatment, such as ultrasonication or bead-milling; a
dispersion prepared by making use of a repulsive force between
molecules of the same polarity; a dispersion obtained by dispersing
CNTs and/or CNHs having a magnetic substance adhering thereto; a
dispersion prepared by dispersing CNTs and/or CNHs with their
surface modified; and a dispersion prepared by a combination of
these techniques. A CNT/CNH dispersion in water is particularly
advantageous to form a network structure of CNTs and/or CNHs on the
surface of fibers having a hydrophilic group, such as cellulose
fibers.
[0038] Production of the sheet-like article of the invention by a
papermaking process (the method (1)) will be described taking, for
instance, the case of using cellulose fibers as the main fibers
constituting the sheet-like article. The method includes the steps
of mixing fibers and a CNT/CNH dispersion to prepare a mixed
dispersion and converting the mixed dispersion into a sheet form.
In order to form the above described specific network structure, it
is significant to select a combination of the manner of dispersing
CNTs and/or CNHs and a fixing agent. The fixing agent preferably
has a polarity opposite to that of the surfactant. Cellulose fibers
are known to be negatively charged when suspended in water. For
example, when CNTs and/or CNHs are dispersed using an anionic
surfactant, a cationic fixing agent is used to fix the CNTs and/or
CNHs, whereby the CNTs and/or CNHs show good adhesion to the
cellulose fibers while forming their network structure probably
through the following mechanism. On being added to the cellulose
fibers, the cationic fixing agent adheres to the surface of the
cellulose fibers to change the surface charges of the cellulose
fibers to positive. Thus, the CNTs and/or CNHs dispersed with the
anionic surfactant are successfully adsorbed to the surface of the
cellulose fibers by an electrical affinity.
[0039] CNTs and CNHs are known to have a very strong tendency to
agglomerate due to van der Waals attraction. In order for CNTs
and/or CNHs to be selectively fixed onto the cellulose fibers to
form a good network structure, it is necessary to prevent them from
agglomerating before they are fixed on the cellulose fibers. Among
the manipulations that may be used to achieve this is addition of a
cationic fixing agent to an aqueous dispersion of cellulose fibers
(i.e., a pulp slurry) to fix the cationic fixing agent onto the
cellulose fibers followed by addition of a dispersion of CNTs
having the surface charged anionically that is prepared by, for
example, using an anionic surfactant as a dispersant. It is
preferred that addition of the cationic fixing agent into the pulp
slurry be followed by stirring thoroughly. That is to say, it is
preferred that the fixing agent for CNTs and/or CNHs be adsorbed
onto the fibers before the fibers are mixed with the CNT/CNH
dispersion. Thus, the cationic fixing agent is uniformly adsorbed
on the surface of the cellulose fibers, followed by adsorbing the
anionically charged CNTs to the adsorbed cationic fixing agent. As
a result, the CNTs and/or CNHs successfully form a network
structure on the surface of the cellulose fibers while being
prevented from agglomerating. In the case when the cationic fixing
agent is added after the addition of the dispersion into the pulp
slurry, not only adsorption on the cellulose fibers but also
considerable agglomeration of the CNTs and/or CNHs occur.
[0040] For use as a fixing agent (cationic fixing agent) organic
polymers are preferable to inorganic fixing agents generally used
in papermaking, such as aluminum sulfate, probably for the
following reason. It is known that an organic polymeric fixing
agent is adsorbed on a fiber surface in a train-loop-tail
configuration. "Train" segments are directly adsorbed on the
surface of a fiber and "loop" or "tail" segments spread out into
the solvent. When a dispersion of anionically charged CNTs and/or
CNHs is added to the fiber slurry in which the polymeric fixing
agent is adsorbed on the fiber in such a configuration, the loop
and tail segments of the cationic organic polymeric fixing agent
are adsorbed on the negatively charged surface of CNTs to form
bridges. This is believed to accelerate adsorption of the CNTs
and/or CNHs to the cellulose fiber surface. Accordingly, in order
to obtain the fixing effect to the maximum extent, it is preferred
that adsorption of the fixing agent on the cellulose fibers, which
may be achieved by adding the fixing agent to an aqueous dispersion
of the cellulose fibers, be followed by adsorption of CNTs and/or
CNHs on the cellulose fibers, which may be achieved by adding a
dispersion of the CNTs and/or CNHs to the aqueous dispersion of the
cellulose fibers.
[0041] An inorganic fixing agent, like aluminum sulfate, which is
generally used in the field of papermaking, is considered to be
fixed by a mechanism such that it is adsorbed on the surface of a
particle to neutralize the charges and fixed by van der Waals
force. In this case, although fixation of the CNTs and/or CNHs to
the cellulose fibers and formation of a network structure by the
CNTs and/or CNHs are observed, the CNTs and/or CNHs undergo
agglomeration due to the very strong van der Waals force between
themselves. As a result, the resulting sheet-like article is
inferior in performance to the sheet-like article obtained using an
organic polymeric fixing agent.
[0042] Formation of a satisfactory network by CNTs and/or CNHs on
the surface of cellulose fibers is also achieved when the CNTs
and/or CNHs are dispersed with a cationic surfactant. In this case,
the pulp slurry may previously be made anion-rich by adding to the
pulp slurry an anionic polymer, microfibrillated cellulose, or
carboxymethyl cellulose, to thereby accelerate fixation of the CNTs
and/or CNHs on the cellulose fibers.
[0043] The method of making the sheet-like article by the
papermaking process is advantageous in that the surfactant used to
disperse the CNTs and/or CNHs is removed simultaneously with the
sheet formation. When a process other than the wet papermaking
process, such as a dry process, is followed, the residual
surfactant tends to act on the fiber surface to interfere with the
formation of hydrogen bonds between cellulose fibers. Furthermore,
the surfactant present on the surface of the CNTs and/or CNHs tends
to inhibit the CNTs and/or CNHs from coming into direct contact
with each other during the formation of the network structure. By
using a wet papermaking process, the CNTs and/or CNHs are allowed
to form a network structure in the wet-laid fiber sheet, which is
then dewatered to be freed of the surfactant present on the surface
of the CNTs and/or CNHs. The above discussed problems are thus
settled.
[0044] The sheet-like article of the invention is also obtained by
the method (2) described above. The method (2) is known as an
impregnation method. More specifically, the method (2) includes the
steps of making a CNT- or CNH-free sheet or fiber aggregate of the
main fibers and soaking the fiber aggregate in a CNT/CNH dispersion
to infiltrate the dispersion deep into the inside of the fiber
aggregate, whereby the CNTs and/or CNHs cover the surface of the
individual fibers to form a microscopic network while forming a
three dimensional, macroscopic network throughout the sheet. The
manner of covering the individual main fibers with the CNT and/or
CNH network structure is not particularly restricted. For example,
the sheet or fiber aggregate may be formed of the main fibers
having been treated with a cationic organic polymer so as to
facilitate fixing the CNTs and/or CNHs, and the resulting fiber
aggregate is impregnated with a CNT/CNH dispersion. It is also
effective to use cationic pulp to make main fibers with a cationic
surface and convert the main fibers into a sheet, which is then
impregnated with a dispersion of CNTs and/or CNHs having been
treated to become anionic to form a network structure of the CNTs
and/or CNHs on the main fibers. Preferred examples of the CNT/CNH
dispersion include a dispersion prepared by dispersing using an
organic solvent by a physical treatment, such as ultrasonication or
bead-milling and a dispersion prepared by dispersing CNTs and/or
CNHs with their surface modified.
[0045] When in using a CNT/CNH dispersion containing a surfactant,
the resulting sheet-like article may be subjected to post treatment
to eliminate the aforementioned interference of the surfactant and
to obtain the effects of CNTs and/or CNHs to a sufficient extent.
For example, the sheet-like article may be washed by immersing in a
solvent which elutes the surfactant to have improved
conductivity.
[0046] The CNTs and CNHs for use in the invention are not
particularly limited. There are various methods for making CNTs,
such as CVD, arc discharge, and laser ablation. The effects of the
invention do not depend on the method of making CNTs. There are two
types of CNTs: single-walled CNTs that can be conceptualized by
wrapping a single graphene sheet into a cylinder and multi-walled
CNTs composed of two or more concentric single-walled CNTs, either
of which is useful. The diameter of the CNT is preferably 1 to 75
nm, more preferably 1 to 50 nm. The length of the CNT is preferably
0.1 .mu.m or longer, more preferably 1 .mu.m or longer. The
diameter of the CNH is preferably 150 nm or less.
[0047] The main fibers (main fibers forming the sheet-like article)
that can be used in the invention are not particularly limited.
Examples of fibers useful as the main fibers include cellulose
fibers, such as wood pulp fibers, e.g., softwood bleached kraft
pulp (NBKP), hardwood bleached kraft pulp (LBKP), softwood bleached
sulfite pulp (NBSP), and thermomechanical pulp (TMP), bast fibers,
e.g., paper mulberry, oriental paperbush, and gampi (Diplomorpha
sikokiana), non-wood pulp fibers, e.g., straw, bamboo, kenaf, and
bagasse, microfibrillated cellulose fibers obtained by treating
cellulose fibers, biocellulose fibers, rayon fibers, other
surface-treated cellulose fibers, and carboxymethyl cellulose
fibers; synthetic pulp; synthetic fibers; semisynthetic fibers;
inorganic fibers; and metal fibers. These fibers may be used either
individually or as an appropriate combination thereof. If desired,
carbon fibers, activated carbon fibers, conductive fibers, and
metal fibers may be used as well.
[0048] To use cellulose fibers as the main fibers is preferred
because CNTs and/or CNHs are fixed to the fibers while efficiently
forming the network structure for some unknown reason. It is
believed that CNTs and/or CNHs are fixed onto the surface of the
fibers while forming a network structure through the above
discussed mechanism because cellulose fibers are negatively charged
due to their hydroxyl groups. Additionally, cellulose fibers have
good dispersibility in water owing to the hydrophilic hydroxyl
groups, which is advantageous when the sheet is formed by a wet
papermaking process.
[0049] To use microfibrillated cellulose obtained by disintegrating
cellulose fibers under high pressure and shearing force is
preferred as providing a denser three dimensional network
structure. Microfibrillated cellulose is fine fibers obtained by
treating cellulose fibers so as to have a number average length of
0.05 to 3 mm and a water retention of 200 ml or more. Using such
fine fibers to be covered with CNTs and/or CNHs provides a denser
and more uniform network structure. Microfibrillated cellulose is
very well fixable on cellulose fibers, such as pulp fibers.
Therefore, using microfibrillated cellulose or a mixture of
cellulose fibers and microfibrillated cellulose to be covered with
CNTs and/or CNHs is extremely effective in forming a network
structure of CNTs and/or CNHs.
[0050] In making the sheet-like article by a wet papermaking
process, commonly employed papermaking additives may be used within
ranges that do not impair the performance. Examples of such
additives include water soluble polymeric strength agents, such as
starch, guar gum, and polyvinyl alcohol; polymeric strength agents,
such as polyacrylamide and vinylamine polymers; wet strength
agents, such as melamine and polyamide polyamine epichlorohydrin;
loading materials, such as titanium oxide, calcium carbonate,
kaolin, and talc; sizes, such as a rosin size, an alkylketene
dimer, a sodium alkenyl succinic anhydride, and styrene-acrylic
monomer copolymers; and fixing agents, such as aluminum sulfate,
cationized polyacrylamide, and anionized polyacrylamide. If
desired, carbon black may be added.
[0051] The sheet-like article of the invention, especially the
sheet-like article containing CNTs is useful as an ultrafine
filter. This is considered to owe to the ultrafine voids formed by
CNTs in which fine particles are trapped.
[0052] The sheet-like article of the invention, particularly the
sheet-like article containing CNHs is usable as a catalyst carrier
with the CNH's property of taking in a substance being taken
advantage of.
EXAMPLES
[0053] CNTs were dispersed in accordance with the following
dispersing methods.
Dispersing Method 1:
[0054] CNTs were kneaded with sodium dodecylbenzenesulfonate as a
dispersant in a mortar for 20 minutes. The mixture was treated in
an ultrasonic bath operating at a frequency of 38 kHz and a power
of 120 W for 2 hours to provide a CNT dispersion of anionic
surfactant type.
Dispersing Method 2:
[0055] A CNT dispersion of cationic surfactant type was prepared in
the same manner as dispersing method 1, except for using
dododecyldimethylammonium bromide as a dispersant.
Dispersing Method 3:
[0056] CNTs were kneaded in a mortar for 20 minutes and then
dispersed in acetone to give a CNT solid content of 2 mass %. The
dispersion was processed with ultrasonic waves at 100 W for 20
minutes to give a CNT dispersion of physical treatment type.
Example 1
[0057] Fifty percent by mass of hardwood bleached kraft pulp and 50
mass % of softwood bleached kraft pulp were dispersed in water and
beaten in a double disc refiner to prepare a pulp slurry having a
CSF freeness of 350 ml. Cationic starch (Neotack L-1, from Nihon
Syokuhin Kako Co., Ltd.) was mixed therein in an amount of 2 mass %
relative to the pulp mass. The CNT dispersion of anionic surfactant
type prepared by the dispersing method 1 was then mixed therein to
give a slurry having a CNT solid content of 5 mass % relative to
the pulp mass. The slurry was converted into a sheet-like article
having a grammage of 100 g/m.sup.2 by a wet papermaking process
using a wire cloth.
Example 2
[0058] A sheet-like article having a grammage of 100 g/m.sup.2 was
made in the same manner as in Example 1, except for replacing the
CNT dispersion of anionic surfactant type obtained by the
dispersing method 1 with the CNT dispersion of cationic surfactant
type obtained by the dispersing method 2 and using 3 mass % of
carboxymethyl cellulose in place of the cationic starch.
Example 3
[0059] A sheet-like article having a grammage of 100 g/m.sup.2 was
made in the same manner as in Example 1, except for replacing the
CNT dispersion of anionic surfactant type obtained by the
dispersing method 1 with the CNT dispersion of physical treatment
type obtained by the dispersing method 3.
Example 4
[0060] A sheet-like article having a grammage of 100 g/m.sup.2 was
made in the same manner as in Example 1, except for using 2 mass %,
relative to the pulp mass, of aluminum sulfate as a fixing
agent.
Example 5
[0061] Fifty percent by mass of hardwood bleached kraft pulp and 50
mass % of softwood bleached kraft pulp were dispersed in water and
beaten in a double disc refiner to prepare a pulp slurry having a
CSF freeness of 350 ml. A polyamide epichlorohydrin resin (WS4002,
from Seiko PMC Corp.) was mixed therein as a wet strength agent in
an amount of 0.5 mass % relative to the pulp mass. The resulting
pulp slurry was converted into a sheet having a grammage of 100
g/m.sup.2 by a wet papermaking process. The sheet was impregnated
with the CNT dispersion obtained by the dispersing method 3 and
dried to give a CNT-impregnated sheet-like article. The CNT content
of the sheet-like article was 1.5% as calculated from the amount of
the dispersion infiltrated into the sheet.
[0062] A CNT-impregnated sheet-like article was made in the same
manner as in Example 5, except that the unimpregnated sheet was
impregnated with a 1% solution of cationic starch as a fixing agent
and dried. The resulting impregnated sheet was further impregnated
with the CNT dispersion of anionic surfactant type obtained by the
dispersing method 1 to give a CNT-impregnated sheet-like
article.
Comparative Example 1
[0063] Fifty percent by mass of hardwood bleached kraft pulp and 50
mass % of softwood bleached kraft pulp were dispersed in water and
beaten in a double disc refiner to prepare a pulp slurry having a
CSF freeness of 350 ml. Two percent by mass of a polyamide
epichlorohydrin resin (WS4002, from Seiko PMC Corp.) was mixed
therein. The resulting pulp slurry was converted into a sheet
having a grammage of 100 g/m.sup.2 by a wet papermaking process
using a wire cloth.
Comparative Example 2
[0064] A sheet-like article was fabricated using cellulose fibers
and, as a binder, polyethylene fibers by a dry process. The article
was apparently found to have CNTs agglomerated.
Comparative Example 3
[0065] Fifty percent by mass of hardwood bleached kraft pulp and 50
mass % of softwood bleached kraft pulp were dispersed in water and
beaten in a double disc refiner to prepare a pulp slurry having a
CSF freeness of 350 ml. A CNT powder, which was not dispersed in a
liquid medium, was then mixed therein in an amount of 5 mass % on a
solid basis relative to the pulp mass, and 2 mass % of aluminum
sulfate was also added thereto. The resulting slurry was converted
into a sheet-like article having a grammage of 100 g/m.sup.2 by a
wet papermaking process using a wire cloth.
[0066] Each of the sheet-like articles obtained in Examples 1 to 6
and Comparative Examples 1 to 3 was evaluated for performance in
terms of CNT-covered surface area ratio, electromagnetic shielding
properties, microwave heating properties, planarly heat generating
properties, and conductivity in accordance with the methods
described below. The results of evaluation are shown in Table 1.
The sheet-like article of Comparative Example 2 was not evaluated
because of its incontestable inferiority due to the apparent
agglomeration of CNTs.
CNT-Covered Surface Area Ratio
[0067] The surface of a sample (sheet-like article) was
photographed using an electron microscope (JSM-6360LA from JOEL,
Ltd.) at a magnification of 3000 times. The ratio of the area
covered with CNTs as calculated on the image was ranked AA to C as
follows. Ranks AA, A and B are appraised as passable. The
evaluation was made on randomly chosen 10 points per sample.
AA: 20% or more A: 10% or more and less than 20% B: 5% or more and
less than 10% C: less than 5%
Electromagnetic Shielding Properties
[0068] Electromagnetic shielding properties of 100 MHz and of 3 GHz
were determined by placing a sample (sheet-like article) between a
micro loop antenna for radiating electromagnetic signals (loop
diameter: 5 mm; available from Keycom Corp.) and a micro loop
antenna for receiving electromagnetic signals (magnetic field probe
CP-25, from NEC Glass Corp.), each connected to a spectrum analyzer
(R3132, from Advantest Corp.). The electromagnetic shielding
properties at 100 MHz and 3 GHz were expressed in decibel (dB) and
ranked AA to C as follows. Samples ranked AA and A are
passable.
At 100 MHz:
[0069] AA: 15 dB or more A: 10 dB or more and less than 15 dB B: 5
dB or more and less than 10 dB C: less than 5 dB
At 3 GHz:
[0070] AA: 20 dB or more A: 10 dB or more and less than 20 dB B: 5
dB or more and less than 10 dB C: less than 5 dB
Microwave Heating Properties
[0071] A sample (sheet-like article) was microwaved in a domestic
microwave oven at 600 kW for 30 seconds, and the surface
temperature of the sample was measured with a non-contact
thermometer. A sample that heated to a temperature 10.degree. C. or
more higher than before being microwaved was ranked B. A sample
that heated to a temperature 20.degree. C. or more higher than
before being microwaved was ranked A. A sample that did not heat to
a temperature 10.degree. C. or more higher than before being
microwaved was ranked C. Ranks A and B are passable.
Planarly Heat Generating Properties
[0072] A sample (sheet-like article) was placed on a glass plate. A
voltage of 12 V was applied to the sample for 5 minutes using
electrodes spaced 10 cm. After the voltage application, the surface
temperature of the sample was measured with a non-contact
thermometer in a constant temperature room set at 23.degree. C. A
sample the surface temperature of which rose by 10.degree. C. or
more was ranked B. A sample the surface temperature of which rose
by 20.degree. C. or more was ranked A. A sample the surface
temperature of which did not rise by 10.degree. C. or more was
ranked C. Ranks A and B are passable.
Conductivity
[0073] A sample (sheet-like article) was conditioned under the
conditions described in JIS P8111 for one day. The volume
resistivity of the conditioned sample was measured by the
four-terminal method in accordance with JIS K7194 as a measure of
conductivity. The measurement was taken using Lorest MCP-HT450 from
Mitsubishi Chemical Analytech Co., Ltd. at 23.degree. C. and 50%
RH. The measured volume resistivity was ranked as follows. Ranks AA
to B are passable.
AA: less than 1.times.10.sup.1 .OMEGA.cm A: 1.times.10.sup.1
.OMEGA.cm or more and less than 1.times.10.sup.2 .OMEGA.cm B:
1.times.10.sup.2 .OMEGA.cm or more and less than 1.times.10.sup.3
.OMEGA.cm c: 1.times.10.sup.3 .OMEGA.cm or more
TABLE-US-00001 TABLE 1 Electromagnetic Covered Shielding Microwave
Planarly Heat Sheet Formation Surface Area Properties Heating
Generating Method Ratio 100 MHz 3 GHz Properties Properties
Conductivity Example 1 wet papermaking AA AA AA A A AA Example 2
wet papermaking AA A AA A A A Example 3 wet papermaking A A A B B B
Example 4 wet papermaking B A A A A A Example 5 impregnation B A A
B B B Example 6 impregnation A A A A A A Compara. wet papermaking C
C C C C C Example 1 Compara. dry process N.D. N.D. N.D. N.D. N.D.
N.D. Example 2 Compara. wet papermaking C C B C C C Example 3
[0074] FIGS. 2 and 3 are electron micrographs taken of the surface
of the sheet-like article obtained in Example 1. In FIG. 3, the
portion indicated by the arrow is a portion covered with CNTs
corresponding to the carbon layer B of FIG. 1. The micrographs of
FIGS. 2 and 3 reveal that CNTs adhere to the cellulose fibers, the
main fibers fabricating the sheet-like article, in a uniformly
dispersed state without agglomeration thereby covering the
cellulose fibers while forming a network structure. In particular,
FIG. 2 provides visual observation of the network structure
covering the cellulose fibers at a surface area ratio of 50% or
more.
[0075] The sheet-like article of Example 1, which was prepared
using a combination of an anionic surfactant for CNT dispersion and
a cationic fixing agent, had better performance than the product of
Example 2, which was prepared using a combination of a cationic
surfactant for CNT dispersion and an anionic fixing agent and that
of Example 3 prepared using a CNT dispersion of physical treatment
type. This is considered to be because the above described fixation
mechanism of the cationic polymer promotes covering of cellulose
fibers with CNTs.
[0076] Electron microscopic observation of the surface of the
sheet-like article of Example 4, which was prepared using an
inorganic fixing agent, revealed that the CNTs adhere to the
cellulose fibers in a uniformly dispersed state without
agglomeration to cover the surface of the cellulose fibers while
forming a network structure and that agglomerates of CNTs fill the
interstices of the cellulose fibers. The wet papermaking system of
Example 4 took a slightly longer time for dewatering as compared
with the systems of Examples 1 through 3. This is probably because
the CNT agglomerates filling the interstices of the cellulose
fibers interfere with drainage.
[0077] The sheet-like articles of Examples 5 and 6 are both made by
the impregnation method. The sheet-like article of Example 6
generally exhibits better performance than that of Example 5. This
is considered to be because of good fixation of CNTs to the
cellulose fibers, being accelerated by treating the cellulose
fibers with the cationic polymer.
[0078] Containing no CNTs, the sheet-like article of Comparative
Example 1 showed poor results in various performance
characteristics. While the sheet-like article of Comparative
Example 2 was not evaluated for performance, agglomeration of CNTs
was noticeable. In addition, many of the CNTs fell off the surfaces
of the fibers, apparently suggesting inferiority in
performance.
[0079] FIGS. 4 and 5 are electron micrographs taken of the surface
of the sheet-like article obtained in Comparative Example 3. In
FIGS. 4 and 5, the portions indicated by the arrows are
agglomerates of CNTs corresponding to the agglomerates C of FIG. 6.
FIGS. 4 and 5 reveal that the CNTs exist in the form of
agglomerates, failing to sufficiently cover the surface of the
cellulose fibers. The noticeable agglomeration of CNTs observed in
Comparative Example 3 is believed to be because the CNTs were added
to the dispersion of fibers in powder form (not in a dispersed
state as in Examples).
INDUSTRIAL APPLICABILITY
[0080] The sheet-like article of the invention is fabricated of
fibers having a large number of CNTs and/or CNHs adhering to the
constituent fibers thereof in a uniformly dispersed state without
agglomeration to build up a network structure on the fibers. Owing
to this structure, the sheet-like article exhibits excellent
conductivity, electromagnetic shielding properties, microwave
heating properties, and planarly heat generating properties and are
suited for use as a conductive sheet, an electromagnetic shielding
sheet, a microwave heating sheet for cooking, an anti-fog sheet for
mirror, an electrode, an ultrafine filter, and so forth.
* * * * *