U.S. patent application number 10/577399 was filed with the patent office on 2007-04-19 for composite papyraceous material.
Invention is credited to Mikio Furukawa, Akira Ito, Norihiko Miki, Katsuyuki Toma, Yoshinao Yamada.
Application Number | 20070084575 10/577399 |
Document ID | / |
Family ID | 34544051 |
Filed Date | 2007-04-19 |
United States Patent
Application |
20070084575 |
Kind Code |
A1 |
Furukawa; Mikio ; et
al. |
April 19, 2007 |
Composite papyraceous material
Abstract
A composite papyraceous material comprising a fibrous
polytetrafluoroethylene (in particular, fibrous powder thereof) and
fibrous polyimide.
Inventors: |
Furukawa; Mikio; (Uji-shi,
JP) ; Toma; Katsuyuki; (Uji-shi, JP) ; Yamada;
Yoshinao; (Uji-shi, JP) ; Ito; Akira;
(Uji-shi, JP) ; Miki; Norihiko; (Settsu-shi,
JP) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Family ID: |
34544051 |
Appl. No.: |
10/577399 |
Filed: |
October 26, 2004 |
PCT Filed: |
October 26, 2004 |
PCT NO: |
PCT/JP04/15841 |
371 Date: |
April 27, 2006 |
Current U.S.
Class: |
162/157.1 ;
162/157.4; 162/205 |
Current CPC
Class: |
H05K 1/0366 20130101;
D21H 13/12 20130101; H05K 2201/0154 20130101; D21H 13/22 20130101;
H05K 2201/0278 20130101; D21H 13/14 20130101; H05K 2201/015
20130101; D21H 13/26 20130101 |
Class at
Publication: |
162/157.1 ;
162/157.4; 162/205 |
International
Class: |
D21H 13/26 20060101
D21H013/26; D21H 13/22 20060101 D21H013/22 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 31, 2003 |
JP |
2003-372813 |
Claims
1. A composite papyraceous material, comprising a fibrous
polytetrafluoroethylene and a fibrous polyimide.
2. The composite papyraceous material according to claim 1, wherein
the fibrous polyimide is a crystalline fiber.
3. The composite papyraceous material according to claim 1, wherein
the polyimide is thermoplastic.
4. The composite papyraceous material according to claim 1, wherein
the polyimide is a polyimide represented by the following Chemical
Formula (1): ##STR3## wherein R represents a quadrivalent aromatic
residue selected from monocyclic aromatic groups, condensed
polycyclic aromatic groups, and uncondensed polycyclic aromatic
groups in which the aromatic rings are bound to each other directly
or via a crosslinking group; X represents a direct bond or a
bivalent residue selected from hydrocarbon groups, a carbonyl
group, an ether group, a thio group or a sulfonyl group; and
Y.sub.1 to Y.sub.4 respectively represent a hydrogen or halogen
atom or a monovalent residue selected from an alkyl group or an
alkoxyl group.
5. The composite papyraceous material according to claim 1, wherein
the polyimide is a polyimide represented by the following Chemical
Formula (2): ##STR4##
6. The composite papyraceous material according to claim 1, wherein
the fibrous polyimide is a short fiber having an average fiber
diameter of 3 to 30 .mu.m and an average fiber length of 1 to 15
mm.
7. The composite papyraceous material according to claim 1, wherein
the fibrous polytetrafluoroethylene is a fibrous powder that has an
average fiber length of 100 to 5,000 .mu.m and an average shape
factor of 5 or more.
8. The composite papyraceous material according to claim 1, wherein
the polytetrafluoroethylene has a low temperature-sided peak area
ratio of 88.5% or more with respect to the total peak area, as
determined from the melting endothermic curve obtained at a heating
rate of 5.degree. C. per minute in differential scanning
calorimetry analysis.
9. The composite papyraceous material according to claim 1, wherein
the fibrous polytetrafluoroethylene is thermally fused and bonded
to the fibrous polyimide.
10. A method for producing the composite papyraceous material
according to claim 1, wherein the composite papyraceous material is
prepared from the fibrous polytetrafluoroethylene and the polyimide
by a papermaking method.
11. The method for producing the composite papyraceous material
according to claim 10, wherein the papermaking method is a wet
papermaking method.
12. The method for producing the composite papyraceous material
according to claim 11, wherein the papermaking method include a
dispersion step of dispersing a fibrous polytetrafluoroethylene, a
mixing step of mixing a fibrous polyimide, a papermaking step, a
pressurization step, and a drying step.
13. The method for producing the composite papyraceous material
according to claim 12, additionally including a heat-pressurizing
step after the drying step.
14. The composite papyraceous material according to claim 1, for
used as any one selected from the group consisting of seamless
belt, circuit board, stamping mold, filter, guard tube,
flame-resistant paper material, solder pattern paper, papyraceous
material for polishing, electrolyte film, lubricant member, sealing
member and cushioning material.
Description
TECHNICAL FIELD
[0001] The present invention relates to a composite papyraceous
material comprising a fibrous polyimide and a fibrous
polytetrafluoroethylene (PTFE).
BACKGROUND ART
[0002] Various papyraceous materials containing a high-performance
engineering plastic as a fiber component were proposed and expected
to be used widely, for example, as members in electronic devices
such as board material and heat-resistant structural materials.
Among them, the one containing a fluoroplastic fiber as a
constituent, which is advantageous in dielectric and friction
properties etc. has been studied intensively.
[0003] For example, Patent Document 1 proposes a papyraceous
material made of a fluoroplastic fiber and a heat-resistant
engineering plastic fiber.
[0004] Patent Document 2 proposes a sheet insulator made of a base
fiber material, mixture of an insulation fiber and a fluoroplastic
fiber, impregnated with an impregnating agent.
[0005] Patent Document 3 proposes a low-dielectric
printed-wiring-board material in combination of a mixed nonwoven
fabric of a fluoroplastic fiber and a heat- resistant engineering
plastic fiber with a cyanate resin.
[0006] Patent Document 4 proposes a fluorine resin composition in
combination of a polytetrafluoroethylene resin for sliding purpose
and a polyimide fiber.
[0007] As for papyraceous materials containing a polyimide fiber as
a constituent, for example, Patent Document 5 proposes a base
nonwoven fabric for laminated boards, in which the polyimide fibers
are bonded with a thermosetting binder.
[0008] However, the fluoroplastic fibers used in Patent Documents 1
to 3 have the shape of chopped strand substantially, and the fiber
diameter thereof is significantly larger than that of pulp-shaped
fibers if compared by the same weight, causing some concern about
uniformity when the fibers are used in combination with other
materials. In addition, the fluorine fibers used are sufficiently
burned in the manufacturing process of the fluoroplastic fiber, and
thus, do not function sufficiently as a binder for binding fibers
with each other. It is inevitable to blend a binder component to
produce a sheet product in a wet process. In many cases, the binder
component added causes deterioration of the properties of the
papyraceous material obtained.
[0009] Although the shape etc. of the polytetrafluoroethylene resin
in Patent Document 4 are not specified and are still unknown, it
seems practically impossible to prepare a thin paper-shaped
composite product, considering the manufacturing process.
[0010] In addition, a thermosetting resin is used as a binder of
the polyimide fiber in Patent Document 5. As described above, it is
only possible to prepare a papyraceous material having the
properties inherent to polyimide fiber impaired by such a
thermosetting resin. [0011] Patent Document 1: Japanese Patent
Laid-Open No. 10-212686 [0012] Patent Document 2: Japanese Patent
Laid-Open No. 11-144529 [0013] Patent Document 3: Japanese Patent
No. 2762544 [0014] Patent Document 4: Japanese Patent No. 2983900
[0015] Patent Document 5: Japanese Patent Laid-Open No.
11-200210
DISCLOSURE OF INVENTION
[0015] Technical Problems to be Solved
[0016] An object of the present invention, which was made in the
background of the conventional technology, to provide a papyraceous
material made of an engineering plastic fiber that is superior in
strength, thermal dimensional stability, chemical resistance, and
abrasion resistance and has unprecedented small values of water
absorption and dielectric property.
[0017] Specifically, the present invention relates to a composite
papyraceous material comprising a fibrous polyimide and a fibrous
polytetrafluoroethylene.
[0018] The fibrous polyimide is not particularly limited, but
preferably a fiber prepared, for example, by melt spinning of a
thermoplastic polyimide resin and cut to a particular length,
preferably a short fiber; particularly preferably a crystalline
fiber.
[0019] The thermoplastic polyimide preferably has a glass
transition temperature of 230.degree. C. or higher and a melting
point of 400.degree. C. or lower. A glass transition point of lower
than 230.degree. C. is not favorable, as it makes the fiber less
heat resistant. On the other hand, a melting point of higher than
400.degree. C. is unfavorable, as it makes thermal processing more
difficult.
[0020] Considering uniformity of a resulting composite papyraceous
material, the fibrous polyimide is preferably a short fiber, and
the average fiber length thereof is 1 to 15 mm, preferably 2 to 8
mm, and the average fiber diameter thereof is 3 to 30 .mu.m,
preferably 4 to 20 .mu.m.
[0021] It is possible to enhance orientation in the crystalline
region of the filament obtained, for example, by melt spinning of a
crystalline thermoplastic polyimide resin, by heat stretching under
a suitable condition. The fibrous polyimide highly oriented in the
crystalline region, which shows a smaller dimensional change during
heating or cooling, improves the thermal dimensional stability of
the resulting papyraceous material, has smaller water absorption,
and thus, is extremely resistant to the change in dimension and
electrical characteristic by water absorption. The "crystalline
fiber" is defined by the crystallinity of fibrous polyimide, and
preferably has a crystallinity of 15% or more, preferably 20% or
more, particularly preferably 25% or more, as determined by X-ray
diffractometry. The crystallinity of less than 15% is unfavorable
as it leads to increase in thermal dimensional change and water
absorption.
[0022] For satisfying the properties above, the polyimide is a
polyimide having the chemical structure represented by the
following General Formula (1) as its repeating unit: ##STR1##
(wherein R represents a quadrivalent aromatic residue selected from
monocyclic aromatic groups, condensed polycyclic aromatic groups,
and uncondensed polycyclic aromatic groups in which the aromatic
rings are bound to each other directly or via a crosslinking group;
X represents a direct bond or a bivalent residue selected from
hydrocarbon groups, a carbonyl group, an ether group, a thio group
or a sulfonyl group; and Y.sub.1 to Y.sub.4 respectively represent
a hydrogen or halogen atom or a monovalent residue selected from an
alkyl group or an alkoxyl group).
[0023] Among the polyimides, the polyimide represented by the
General Formula (1) wherein R is a monocyclic aromatic group is
more preferably; X is a direct bond is more preferable; and each of
Y.sub.1 to Y.sub.4 is a hydrogen atom is more preferable.
[0024] Favorable typical examples of the polyimides include
polyimides having the following Chemical Formula (2) as the
repeating unit: ##STR2## (wherein n is preferably 5 to 200).
[0025] Such a polyimide is commercially available, for example, as
Aurum (trade name, manufactured by Mitsui Chemicals Inc.).
[0026] During preparation of polyimide fibers, another polyimide
having a different chemical structure may be blended; and another
polymer such as polyester, polyolefin, polyamide, polyphenylene
sulfide, polyether imide, and polyether ether ketone, or
fluororesin, as well as an inorganic filler such as titanium oxide,
zinc oxide, magnesium oxide, alumina, silica, aluminum nitride,
silicon nitride, boron nitride, silicon carbide, carbon black,
graphite, or mica may be blended so far as the properties required
for the polyimide and the papyraceous material used in the present
invention are not impaired.
[0027] The fibrous polytetrafluoroethylene for use in the present
invention is fibrous powder in shape. The fibrous powder of
polytetrafluoroethylene is preferably prepared, for example, by
beating polytetrafluoroethylene powder, and such a fibrous powder
has ununiform whisker-like branches and yet a fibrous shape as a
whole, but shows a phenomenon as powder at the visual level. The
fibrous powder preferably has an average fiber length of 5 to 2,000
.mu.m and an average shape factor of 5 or more. The powder having
these values lower than the above ranges leads to decrease in water
filtering property and productivity in the paper making step and
only gives a paper lower in air permeability. The fibrous powder
having these values higher than the above ranges leads to
deterioration in the paper surface state and makes it difficult to
produce thin uniform paper. In addition the fibrous powder
preferably has a specific surface area, as determined by nitrogen
absorption method, of 4.0 m.sup.2/g or more. The fibrous powder
having a lower specific surface area is inferior in binding
efficiency, and thus, the papyraceous material obtained has a lower
strength. The average shape factor is a value obtained by dividing
fiber length by fiber width.
[0028] The polytetrafluoroethylene may be a tetrafluoroethylene
homopolymer or a copolymer of tetrafluoroethylene with a trace
amount of monomer other than tetrafluoroethylene that is
non-processable by melting (hereinafter, referred to as modified
polytetrafluoroethylene).
[0029] Examples of the trace amount monomers include
perfluoroolefins, perfluoro(alkylvinylether)s, cyclic fluorinated
monomers, perfluoroalkylethylenes, and the like.
[0030] Examples of the perfluoroolefins include hexafluoropropylene
and the like; examples of the perfluoro(alkylvinylether)s include
perfluoro(methylvinylether), perfluoro(propylvinylether), and the
like; examples of the cyclic fluorinated monomers include
fluorodioxol, and the like; and examples of the
perfluoroalkylethylenes include perfluoromethylethylene and the
like.
[0031] The fibrous polytetrafluoroethylene is preferably a
partially baked polytetrafluoroethylene having a low
temperature-sided peak area ratio of 88.5% or more with respect to
the total peak area in the melting endothermic curve, as determined
by differential scanning calorimetry (DSC) analysis at a programmed
heating rate of 5.degree. C. per minute. Such a fibrous
polytetrafluoroethylene gives a papyraceous material having
smoother surface and superior in air permeability. The upper limit
is preferably 99.0%. A low temperature-sided peak area ratio lower
than the range above makes it difficult to prepare a surface-smooth
paper, while that higher than the range above gives a papyraceous
material unfavorably structured and drastically lower in
usability.
[0032] The peak area obtained by differential scanning calorimetry
is proportional to the quantity of heat and generally to the number
of the molecules in sample in an allowable range. Thus, the ratio
of disentangled polytetrafluoroethylene molecules can be evaluated
with a peak ratio of the area of low temperature-sided peaks to the
total peak area obtained by differential scanning calorimetry. A
double peak or a single peak having a distinct shoulder may be
considered mathematically as a master curve of three or more
multiple normal-distribution curves, but it is sufficiently
reasonable to separate the peak having two vertexes into two
normal-distribution or similar curves, and such an assumption gave
adequate results also in evaluation of the present invention.
Accordingly, it is possible to assume that partially disentangled
molecules are also included in the normal distribution curve of
undisentangled molecules in evaluation, because the quantity of
heat needed for disentanglement is smaller.
[0033] The combined absorption peak can be separated normally by
approximation with using Gaussian-Lorentian curve. The
Gaussian-Lorentian curve characteristically has a smaller deviation
than either Gaussian curve or Lorentian curve, and most of
calculation software installed in commercial analyzers employs this
method. In the present invention, basic peak positions are
determined, by supplying two apparent vertexes of the raw
polytetrafluoroethylene powder as the initial values and making
approximation without restricting. The basic peak positions thus
obtained, for example, of the fibrous polytetrafluoroethylene used
in Example 1 of the present invention are 339.14.degree. C. and
343.01.degree. C.; and a combined curve was separated into two
curves and the peak areas thereof were obtained by performing
approximation using these values as standards while restricting
only the peak temperatures to temperatures lower by 0.6 to
0.7.degree. C. than the initial values but not restricting the
linear half-value widths. In the evaluation above, information on
the raw powder was used for shortening the period needed for value
convergence, but the values may also be determined directly from
the melting curve of fibrous powder.
[0034] The composite papyraceous material according to the present
invention is prepared as a paper-shaped composite in combination of
the fibrous polyimide and the polytetrafluoroethylene described
above. The composite-forming method is not particularly limited,
but the composite can be prepared easily in a paper-making process,
as the raw materials are blended. Among many paper-making methods,
application of a wet paper-making method is preferable, for
production of a thin composite papyraceous material having a
uniform formation.
[0035] The wet paper-making method include a dispersion step of
dispersing at least fibrous polytetrafluoroethylene in water, a
mixing step of mixing fibrous polyimide, a paper-making, and a
drying step.
[0036] In the dispersion step, a slurry of fibrous
polytetrafluoroethylene is obtained by adding the fibrous
polytetrafluoroethylene in water and defibrating it in a pulper,
refiner, or the like equipped with agitator. Normally,
polytetrafluoroethylene has a great contact angle with water and it
is hard to disperse it in water uniformly, and thus, it is
preferable to add a dispersant previously to the fiber or fibrous
powder or to blend it in the water for dispersion. The dispersant
for use is not particularly limited, but preferably a nonionic
polyoxyethylene alkylethers, because it is effective for dispersion
even when added in a small amount.
[0037] In the mixing step of mixing the fibrous polyimide, a mixed
slurry is prepared by adding fibrous polyimide to the slurry of
fibrous polytetrafluoroethylene obtained in the dispersion step and
processing in a manner similar to the dispersion step. The fibrous
polyimide is normally dispersible in the slurry uniformly without
use of a dispersant.
[0038] The blending ratio of fibrous polyimide to
polytetrafluoroethylene may be determined properly according to
applications, but the lower limit value in blending ratio of the
fibrous polyimide is 5 percent by mass, preferably 10 percent by
mass. The upper limit ratio is 90 percent by mass, more preferably
80 percent by mass. A fibrous polyimide blending ratio of less than
5 percent by mass may lead to insufficient thermal dimensional
stability and strength, while a ratio of more than 90 percent by
mass to insufficient handling efficiency in the production process,
and insufficient strength and dielectric property of composite
papyraceous material.
[0039] Then, a papyraceous material (paper-made product) is
prepared through a paper-making step of sheet-making the mixed
slurry in a known wet papermaking machine such as cylinder wet
paper machine, short-screen wet paper machine, inclined
short-screen wet paper machine or inclined fourdrinier wet paper
machine, and the obtained paper-made product is put into a drying
step of drying it in a hot-air, contact or radiant dryer installed
close to the paper machine. Thus, a composite papyraceous material
is obtained. It is possible to bond the fibrous
polytetrafluoroethylene to the fibrous polyimide under compression
and give a composite papyraceous material superior in shape
stability, when a pressurization step of applying a pressure onto
the paper-made product in the thickness direction by nip rolls or a
pair of metal rolls, is provided in the paper making process. The
pressurization may be performed at room temperature; the pressure
is normally, approximately 1 to 10 kgf/cm; but of course, a
pressure may be applied under heat if possible. The pressurization
step may be placed between the paper-making step and the drying
step or after the drying step.
[0040] The paper-making process is controlled to make the
papyraceous material after drying have a basis weight of 50 to
1,500 g/m.sup.2, preferably 100 to 1,200 g/m.sup.2. The basis
weight of less than that leads to deterioration in easiness in
handling in papermaking process, making it difficult to make paper
reliably. The greater basis weight may lead to notable problems in
production such as decrease in filtered water in the paper-making
step and insufficient drying.
[0041] Then, the composite papyraceous material thus prepared is
pressurized under heat in the thickness direction in a
heat-pressurizing apparatus such as a roll-pressing machine having
a pair of metal rolls such as calendering rolls or a double
belt-pressing machine having a pair of facing metal belts etc.; and
the fibrous powder of polytetrafluoroethylene is fused on the
fibrous polyimide, drastically increasing the strength of the
composite papyraceous material. That is, a composite papyraceous
material having fibrous powder of polytetrafluoroethylene fused on
fibrous polyimide, i.e., a densified composite papyraceous
material, is obtained by the processing in the step of preparing a
composite papyraceous material by the wet paper making method and
additionally in the step of heat-pressurizing the papyraceous
material obtained. Needless to say, the heat-pressurization step
may be performed continuously after the paper-making step or
separately in another line.
[0042] The heating temperature during the heat pressurization is
preferably not lower than the melting point of
polytetrafluoroethylene. A final heating temperature of lower than
the melting point may result in insufficient improvement in
mechanical strength property. It also means that it is possible to
keep the binding force in a particular range until the final
heating step if the papyraceous material is heat-treated at a
temperature of lower than the melting point and to obtain processed
goods such as laminate by using such an intermediate. The heating
temperature is preferably not higher than the melting point of the
polyimide for the fibrous polyimide, and if the polyimide is a
crystalline polyimide, the heating temperature is preferably lower
than the melting point of the polyimide and more preferably lower
by 10.degree. C. or more than the melting point. A heating
temperature of higher than the melting point of the polyimide is
undesirable, because it leads to significant fusion of the fibrous
polyimide and insufficient blending effects by the fibrous
polyimide. The heating temperature is normally, approximately 340
to 380.degree. C. The melting point is a value obtained by using a
differential scanning calorimeter (Pyrisl DSC, manufactured by
PerkinElmer Co., Ltd.).
[0043] The pressure during heat pressurization is not particularly
limited, but a higher applied pressure leads to a lower porosity
and densification of the composite papyraceous material obtained,
and thus, for example, a pressure of approximately 0.05 to 10 MPa
may be applied.
[0044] Pressurization under heat of two or more sheets of the
composite papyraceous material according to the present invention
in the step above gives a densified composite papyraceous material
without interlayers, and thus, it is possible to prepare a
densified papyraceous material having a desired thickness by
changing the number of the sheets used. It is also possible then to
prepare a composite papyraceous material in which the composition
of polyimide and polytetrafluoroethylene varies in the thickness
direction, by using the composite papyraceous materials according
to the present invention different in the composition in respective
layers.
[0045] The thickness of the papyraceous material thus obtained is
determined according to the basis weight in the paper-making step
and the number of sheets used in lamination and the degree of
densification during heat pressurization, but preferably 20 to
2,000 .mu.m. more preferably 25 to 800 .mu.m, from the points of
application, productivity, and uniformity in physical properties.
The thickness of smaller than the lower limit leads to
deterioration in the easiness of handling and lower yield in the
production process, while the thickness of greater than the upper
limit tends to give only papyraceous materials lower in dimensional
uniformity in the heat-pressurization step. The apparent density
thereof is preferably 0.3 to 2.1 g/cm.sup.3, and the density of
lower than the range may lead to deterioration in strength, while
the density higher than that to practical difficulty of
production.
[0046] The composite papyraceous material according to the present
invention may contain various additives in the range that does not
impair the advantages of the present invention. Examples of the
additives include short fibers or pulps of other organic or
inorganic material, i.e., short fibers or pulps such as of aramide,
polyester, polyether imide, polyether ether ketone, polysulfone,
polyphenylene sulfide, polyketone, carbon, glass, and alumina; and
various particulate matters (fillers) such as titanium oxide, zinc
oxide, magnesium oxide, alumina, silica, aluminum nitride, silicon
nitride, boron nitride, silicon carbide, carbon black, graphite,
mica, and molybdenum disulfide; and the like.
[0047] For improvement in strength etc., a thermoplastic resin, for
example a polyimide resin or the precursor thereof, or a
thermosetting resin, for example an epoxy resin or the like, may be
added according to applications. In such a case, the polyimide
resin or the precursor thereof or the epoxy resin or the like is
normally used in the emulsion or solution state for coating,
spraying or impregnation.
[0048] One or more of these additives may be added in an amount
that does not impair the advantage of the present invention, the
total mass after drying preferably remains 30 percent by mass or
less with respect to the total mass of the composite paper
according to the present invention, for preserving various
favorable properties.
[0049] As described above, the composite papyraceous material
according to the present invention may be processed into the state
in which the fibrous powder of polytetrafluoroethylene is fused and
bonded to the fibrous polyimide in the heat-pressurization step,
and such a densified papyraceous material has a superior strength.
In addition, such a papyraceous material, even when laminated in
the heat-pressurization step, does not show a phenomenon of
breakage from the interlayer during use and retains a strength at a
substantially same level as that of a single-layer papyraceous
material. The strength thereof is determined by the blending ratio
of fibrous powder of polytetrafluoroethylene to fibrous polyimide,
the degree of densification, or the amount of the third component
blended, but, for example, the average breaking length, as
determined according to the test method specified by JIS-P8113, is
normally in the range of 0.5 to 7 km.
[0050] The composite papyraceous material according to the present
invention is resistant to thermal dimensional change and superior
in dimensional stability when used at high temperature. As for the
dimensional stability, both the average linear expansion
coefficients at 20 to 230.degree. C. in the production and width
directions of the composite papyraceous material are preferably in
the range of -20 to 30 .mu.m/m.degree. C. (as determined according
to JIS-K7197). The average linear expansion coefficient outside the
region of -20 to 30 .mu.m/m.degree. C. may lead to increase in the
dimensional stability when used at high temperature and make the
papyraceous material unsuitable for use.
[0051] In the present invention, it is possible to prepare a
papyraceous material superior in dimensional stability and its
directional balance, by preparing a papyraceous material wherein
the fibrous polyimide and the polytetrafluoroethylene described
above are dispersed randomly.
[0052] The composite papyraceous material preferably has lower
water absorption, and in a preferred embodiment of the present
invention, the water absorption, as calculated according to the
following formula when left in an environment at 25.degree. C. and
a relative humidity 60% for 24 hours, is preferably 0.5% or less.
Water .times. .times. absorption = W - Wo Wo 100 .times. ( % ) (
Formula .times. .times. I ) ##EQU1## (wherein W represents mass of
nonwoven fabric after moisture absorption, and W.sub.0 represents
mass of nonwoven fabric when absolutely dried).
[0053] Generally, polyimides are known to have a relatively higher
water absorption because of the strong polarity of the imide groups
therein, and the molded products thereof often had problems of the
change in dimension and electrical characteristic after absorption
of moisture. Papyraceous materials of conventional amorphous
polyimide fibers also had the problem in dimensional change by
water absorption; but in the present invention, it is possible to
prepare a high-performance composite papyraceous material lower in
water absorption than ever, by using a crystalline polyimide fiber
and a fibrous powder of polytetrafluoroethylene lower in water
absorption as the main components. In particular, the polyimide
represented by Chemical Formula (1) or (2) has crystallinity and a
low imide-group content in the chemical structure, and thus, has
especially lower water absorption among polyimides; and from these
viewpoints, use of the fibrous polyimide above is preferable.
[0054] Both polyimide and polytetrafluoroethylene have a lower
dielectric constant and are superior in insulating performance. The
papyraceous material according to the present invention does not
contain a third component as a binder, and thus, the composite
papyraceous material thereof can be given the same properties as
above. The electric properties can be evaluated at room temperature
and 2.45 GHz by using a cylindrical dielectric constant meter
(constituted of a resonator manufactured by Kanto Electronics
Application & Development Inc. and a network analyzer
manufactured by Agilent Technologies). The dielectric constant of
the composite papyraceous material according to the present
invention may vary according to the content ratio of fibrous
polyimide to fibrous powder of polytetrafluoroethylene and the
apparent density of composite papyraceous material, but the
dielectric constant of a densified composite papyraceous material
as determined by the test method above is normally in the range of
2.0 to 3.1. The dielectric loss, as determined similarly, is in the
range of 1.times.10.sup.-5 to 3.times.10.sup.-3.
[0055] Polyimide and polytetrafluoroethylene are known to be
superior both in chemical and oxidation resistances, and the
composite papyraceous material according to the present invention
is also superior in these properties. The chemical resistance can
be evaluated by determining the change in weight, color, and shape
after a composite papyraceous material is immersed in a test
chemical at room temperature for a week; for example, large change
in weight and dimension is observed when it corroded significantly
with the chemical, and some increase in weight is observed when it
swells therein; and the composite papyraceous material according to
the present invention is know to be resistant to many common polar
solvents such as alcohol solvents, ether solvents, ketone solvents,
ester solvents, and amide solvents, and nonpolar solvents such as
hydrocarbon solvents, and various oils and fuels. The oxidation
resistance can be evaluated by determining the deterioration of the
composite papyraceous material by oxidation when it is immersed in
the so-called Fenton reagent, i.e., a hydrogen peroxide solution
containing a small amount of an activation agent such as iron
sulfate (III); and the composite papyraceous material according to
the present invention showed almost no deterioration when immersed
therein, for example, at 70.degree. C. for 48 hours.
[0056] The composite papyraceous material according to the present
invention, which has a structure wherein polyimide superior in
abrasion resistance and polytetrafluoroethylene superior in
lubricity are dispersed uniformly, is superior in frictional
abration resistance. For example, it can be evaluated by using a
thrust frictional abrasion tester (EMFIII-E, manufactured by Toyo
Baldwin) at room temperature without use of a lubricant, and the
composite papyraceous material according to the present invention,
shows a favorable abrasion resistance when it is slid in contact
with a counter material of SUS material or aluminum material under
pressure at a constant sliding velocity for 48 hours and does not
damage the counter material.
[0057] The composite papyraceous material according to the present
invention, which has all the properties described above, is a raw
material most favorably used in products that demand strength, heat
resistance, and thermal dimensional stability such as seamless
belt, stamping mold, guard tube, flame-resistant papyraceous
material, valve sheet, solder pattern paper and cushioning
material. It is also superior in electrical characteristics and
thus may be applied to a circuit board. It is also superior in
chemical and abrasion resistances and thus, it is a raw material
favorable as filter, paper material for polishing, electrolyte
film, and sealing material.
[0058] Hereinafter, the present invention will be described with
reference to Examples.
Production Example of Fibrous Polyimide
[0059] The polyimide resin used was Aurum (trade name: manufactured
by Mitsui Chemicals Inc.). The polyimide resin was melted at
415.degree. C. and extruded through a nozzle of 0.2 mm in diameter
into a strand, which was drawn at a velocity of 700 m/min,
solidified by cooling, and wound around a paper jointing tube, to
give a fibrous polyimide having a fiber diameter of approximately
23 .mu.m. Then, the fibrous polyimide was drawn in a heater at
350.degree. C. approximately thrice, to give a fibrous polyimide
having a fiber diameter of approximately 12 .mu.m. The fibrous
polyimide obtained had a crystal orientation increased by
stretching. In this Example, short fiber prepared by cutting the
fibrous polyimide to a length of 5 mm was used. The crystallinity
of the short fiber was 26%, and the melting point was 389.degree.
C.
Production Example of Fibrous Polytetrafluoroethylene Powder
[0060] A polymer obtained by emulsion polymerization of 100 mol %
of tetrafluoroethylene was used as a raw polytetrafluoroethylene
powder (average diameter: 570 .mu.m). The raw
polytetrafluoroethylene powder obtained was fed into a hopper by a
feeder. Then, the polytetrafluoroethylene powder was fed, assisted
with dry air as needed, into a stretching tank equipped with a
rotating blade (inner tank diameter: 160 mm.phi.) and stretched
therein. Part of the bottom face of the stretching tank had a mesh
structure, and the powder having a diameter smaller than a
particular size was allowed to be discharged from the stretching
tank. The powder was processed with a standard classification
screen, while removing the powder having a diameter of 5 .mu.m or
less, to give a raw fibrous polytetrafluoroethylene powder.
[0061] The fibrous polytetrafluoroethylene powder obtained had:
[0062] an average fiber length of 1.5 .mu.m; [0063] an average
shape factor of 40; [0064] a specific surface area of 6.38
m.sup.2/g; and [0065] a low temperature-sided peak area ratio of
92.3%.
EXAMPLE 1
[0066] To an aqueous dispersion containing 0.1 part by weight of a
polyoxyethylene alkylether in 1,000 parts by mass of water, added
was 3.2 parts by mass of the fibrous polytetrafluoroethylene
powder; and the mixture was agitated in an agitating machine
(pulper), to give a uniformly dispersion.
[0067] Then, 0.8 part by mass of the polyimide short fiber cut to 5
mm in length was added to the dispersion, and the mixture was
agitated to give a slurry in which respective raw materials were
mixed and dispersed uniformly.
[0068] The slurry was diluted with water additionally to be a
slurry concentration of 0.02 percent by mass, and fed into an
inclined continuous short-screen paper machine, to give a
paper-made product. The paper-made product obtained had a water
content of approximately 30 percent by mass.
[0069] Then, the fibrous polytetrafluoroethylene powder and the
polyimide short fiber were adhered to each other under a linear
pressure of 0.1 N/mm by using nip rolls equipped with a pair of
stainless steel rolls at room temperature.
[0070] The resultant was fed into and dried in a conveyor-type hot
air dryer attached to the paper machine, to give a composite
papyraceous material having a water content of approximately 0
percent by mass.
[0071] The composite papyraceous material obtained had a favorable
strength and shape stability because the fibrous
polytetrafluoroethylene powder was bonded under pressure to the
fibrous polyimide; and the fibrous polyimide and the fibrous
polytetrafluoroethylene powder were dispersed and blended
uniformly. The papyraceous material obtained through the above
processes was designated as a composite papyraceous material A.
[0072] The composite papyraceous material A obtained by the wet
paper making method above was preheated at 240.degree. C. for 2
minutes under no pressure in a continuous belt-pressing machine
(manufactured by Sandvik), heated at 350.degree. C. for 5 minutes
under a pressure of 25 N/mm, and then, cooled rapidly to 50.degree.
C. under the same pressure, to give a densified composite
papyraceous material continuously. The composite papyraceous
material obtained, in which the fibrous polytetrafluoroethylene
powder was melted and bonded to the polyimide short fiber surface,
had a smooth surface and a densified structure. The densified
papyraceous material obtained through the above processes was
designated as a composite papyraceous material B.
[0073] The composite papyraceous material B obtained had a basis
weight of 256 g/m.sup.2 and a thickness of 154 .mu.m. The average
linear expansion coefficient, breaking length, water absorption,
and dielectric constant thereof were also determined. The results
are summarized in Table 1.
[0074] The thickness was determined as an average of the 50
thicknesses measured at an interval of one point in 100 cm.sup.2
(10.times.10 cm) by using a digital thickness gage.
[0075] The breaking length was determined according to JIS-P8113 by
using a universal tester (manufactured by Intesco).
[0076] The average linear expansion coefficient was determined
according to JIS-K7197 by using a TMA measuring apparatus (TMA2940,
manufactured by TA Instruments).
[0077] The water absorption was determined according to the General
Formula I.
[0078] The dielectric constant was determined according to the
method of using the cylindrical dielectric constant meter described
above. The test sample used was a rod-shaped product of 1.4 mm
square prepared by densifying the laminated composite papyraceous
materials A to a thickness of 1.4 mm in a similar manner to the
composite papyraceous material B and cutting the densified product
into pieces of 1.4 mm width.
EXAMPLES 2 TO 5
[0079] A composite papyraceous materials B were prepared in a
manner similar to Example 1, except that the blending ratio of the
fibrous polyimide to the fibrous polytetrafluoroethylene powder was
changed to the value shown in Table 1. Various physical properties
of the composite papyraceous materials obtained are summarized in
Table 1.
COMPARATIVE EXAMPLE 1
[0080] A composite papyraceous material A of the polyimide short
fiber was prepared in a manner similar to Example 1, except that no
fibrous polytetrafluoroethylene powder was used and the polyimide
short fiber was added in an amount of 4 parts by mass.
[0081] In the papyraceous material obtained in Comparative Example
1, which contains no fibrous polytetrafluoroethylene powder, the
fibers are not bonded under pressure to each other and thus, the
product did not have paper-like shape, and it was not possible to
obtain a favorable papyraceous material A.
COMPARATIVE EXAMPLE 2
[0082] A papyraceous material B of the fibrous
polytetrafluoroethylene powder was prepared in a manner similar to
Example 1, except that no polyimide short fiber was used and the
fibrous polytetrafluoroethylene powder added in an amount of 4
parts by mass. Various physical properties of the composite
papyraceous material B obtained are summarized in Table 1.
COMPARATIVE EXAMPLE 3
[0083] A composite papyraceous material A was prepared in a manner
similar to Example 1, except that a polytetrafluoroethylene short
fiber having an average fiber length of 5 mm (trade name: Toyofron,
manufactured by Toray Fine Chemicals Co. Ltd.) was used instead of
the fibrous polytetrafluoroethylene powder.
[0084] The polytetrafluoroethylene short fiber used in the
Comparative Example had: [0085] an average shape factor of 250
[0086] a specific surface area of 0.1 m.sup.2/g, and [0087] a low
temperature-sided peak area ratio of 100%.
[0088] In this Comparative Example, the fibers are not bound under
pressure to each other probably because the polytetrafluoroethylene
short fiber did not function as a binder sufficiently, and thus, it
was not possible to obtain a favorable paper-like shape and to
obtain a satisfactory papyraceous material A. TABLE-US-00001
Average PI.sup.1) PTFE.sup.2) Basis expansion Water Breaking (Part
by (Part by weight Thickness coefficient absorption length
Dielectric mass) mass) (g/m.sup.2) (.mu.m) (.mu.m/m.degree. C.) (%)
(km) constant Example 1 0.8 3.2 256 154 -1.5 0.1 2.1 2.15 Example 2
1.6 2.4 260 182 -4.7 0.1 4.1 2.34 Example 3 2.4 1.6 249 227 -4.8
0.2 5.6 2.35 Example 4 3.2 0.8 268 274 -3.6 0.3 5.6 2.36 Example 5
0.4 3.6 294 162 20 0.1 1.3 2.10 Comparative 4.0 0 -- -- -- -- -- --
example 1 Comparative 0 4.2 220 100 168 0.0 0.3 1.95 example 2
Comparative 0.8 3.2 -- -- -- -- -- -- example 3 .sup.1)Fibrous
polyimide; .sup.2)Polytetrafluoroethylene
Evaluation of Oxidation Resistance
EXAMPLE 6
[0089] The composite papyraceous material B obtained in Example 2
and cut into pieces of 100 mm in length and 10 mm in width was used
as a sample. The sample was immersed and left in an oxidizing agent
solution prepared by dissolving iron sulfate (II) to a
concentration of 20 ppm in an aqueous 30 percent by mass hydrogen
peroxide solution at 70.degree. C. for 80 hours. Then, the sample
was removed, washed with water, and dried, and the breaking length
thereof was determined.
[0090] The breaking length of the sample immersed in the oxidizing
agent solution relative to 100% of the breaking length of the
sample immersed in pure water instead of the oxidizing agent
solution and processed similarly was used as an indicator of the
oxidation resistance. As a result, the composite papyraceous
material B had a high oxidation resistance of 95%. It is useful as
a base material for electrolyte films, for example, in polymer
electrolyte fuel cell, that demand high oxidation resistance.
COMPARATIVE EXAMPLE 4
[0091] A composite papyraceous material B was prepared in a manner
similar to Example 2, except that the polyimide short fiber was
replaced with an aramide fiber having a fiber diameter of
approximately 15 .mu.m and an average fiber length of 6 mm (trade
name: Twaron, manufactured by Japan Aramid).
[0092] The basis weight of the composite papyraceous material
obtained was 296 g/m.sup.2; the thickness, 295 .mu.m; and the
breaking length, 1.9 km. The oxidation resistance of the composite
papyraceous material B obtained, as determined similarly to Example
6, was 63%, indicating that the composite papyraceous material was
inferior in oxidation resistance.
Evaluation of Sliding Property
EXAMPLE 7
[0093] Two composite papyraceous materials A obtained in Example 1
and cut to 500.times.500 mm in size were heated to 350.degree. C.
under 3 MPa pressure and additionally at 350.degree. C. for 15
minute under the same pressure by using a pressing machine having a
pressurizing plate equipped with a heating apparatus, to give a
densified composite papyraceous material.
[0094] The density of the composite papyraceous material obtained
was 1.60 g/m.sup.2, and the thickness was 322 .mu.m. A sliding test
was performed by using the composite papyraceous material and cut
to a size of 30.times.30 mm as a test sample.and a carbon steel
S45C jig having a sliding face of a ring-shaped cross section
having an outer diameter 25 mm and an inner diameter 20 mm as a
counter material, in a frictional abrasion tester (EMFIII-E,
manufactured by Toyo Baldwin). The surface roughness Ra of the
sliding face of the counter material was 0.5 a, and the test was
performed without use of a lubricant. The abrasion amount and the
friction coefficient of the sample were determined after slid at
peripheral velocity of 100 m/minute, pressure of 0.49 MPa, and
normal temperature for 20 hours. The abrasion amount was evaluated
with the change in weight of the sample between before and after
the test. The friction coefficient was calculated from the stress
(torque) generated in the rotation direction of the sample during
measurement, as determined by the load cell attached to the
apparatus. The results obtained are summarized in Table 2.
COMPARATIVE EXAMPLE 5
[0095] A composite papyraceous material A was prepared in a manner
similar to Example 1, except that an aramide fiber having a fiber
diameter of approximately 15 .mu.m and an average fiber length of 6
mm (trade name: Twaron; manufactured by Japan Aramid) was used
instead of the polyimide short fiber. Then, two composite
papyraceous materials A were laminated in a manner similar to
Example 7, to give a composite papyraceous material. The density of
the composite papyraceous material obtained was 1.45 g/m.sup.2, and
the thickness was 353 .mu.m. The composite papyraceous material
obtained was evaluated in the sliding test, similarly to Example 7.
The results obtained are summarized in Table 2. TABLE-US-00002
Density Thickness Abrasion Friction (g/cm.sup.3) (.mu.m) amount
(mg) coefficient Examples 7 1.60 322 0.9 0.20 Comparative 1.45 353
3.1 0.23 example 5
[0096] As apparent from Table 2, the composite papyraceous
materials according to the present invention are superior in
sliding property and resistant to abrasion even under high sliding
conditions, and have a smaller friction coefficient.
* * * * *