Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
Z~)06493
ACRYLIC SYNTHETIC FIBER AND PROCESS
FOR PREPARATION THEREOF
BACKGROUND OF THE lNV ~:N~l~IoN
1. Field of the Invention
This invention relates to an acrylic synthetic
fiber, especially an acrylic synthetic fiber having
vein-like or straw-like voids extending substantially in
parallel to the longitudinal axis of the fiber, which
can be easily split into finer fibers by an external
force, and a process for the preparation of this acrylic
synthetic fiber. Furthermore, the present invention
relates to a pulpy acrylic synthetic fibrous article
having excellent properties as the starting material for
a friction material, paper or nonwoven fabric.
Moreover, the present invention relates to a friction
material comprising this pulpy acrylic synthetic fibrous
article as a base material.
2. Description of the Related Art
Hollow acrylic fibers are known, for example,
from Japanese Unexamined Patent Publication No.
51-149922 and Japanese Unex~mined Patent Publication
No. 57-89612. The conventional hollow acrylic fibers
include a fiber having cell-like independent voids in
the interior thereof and a tubular fiber having in the
interior thereof a hole continuous along the fiber axis.
The hollow acrylic fiber having cell-like
independent voids has only a few large voids in the
transverse section thereof, as disclosed in, for
example, Japanese Unex~mined Patent Publication
No. 51-149922.
The hollow acrylic fiber disclosed in Japanese
Unex~ined Patent Publication No. 57-89612 has in the
transverse section thereof several of relatively large
voids, as shown in the drawings of the patent
publication, and this fiber cannot be easily split.
~.~
ZOC)~493
Voids in these known hollow fibers are cell-
like voids or long voids extending along the
longitudinal direction of the fiber, and the length of
these voids is about 40 to about 50 ~m at most.
The objects of forming voids in fibers in the
conventional techniques are to decrease the weight,
improve the heat-insulating property, impart a water-
absorbing property, give a soft touch, and give a dry
touch. To attain these objects, acrylic synthetic
fibers having voids as disclosed in the above-mentioned
patent publications provide excellent results.
Recently, the need for a fiber having a
variety of greatly improved properties has increased,
and attention is now focused on a fiber having
characteristics such that, after a formation of a fiber
structure or after a further formation of the fiber
structure into a fibrous product such as a knitted or
woven fabric, the fiber can be split into finer fibers
by various means. Fibers having such properties are
characterized in that the freedom of processability is
increased, whereby the fibers can be split into finer
fibers at an optional processing stage after a formation
of fiber structures, and fibrous products having
excellent properties not attainable from conventional
fibers can be provided.
From this viewpoint, the hollow acrylic
synthetic fibers disclosed in the above-mentioned patent
publications have problems in that they cannot be split
into finer fibers by an external force, for example, by
beating and rubbing.
The main reasons why the fibers disclosed in
the above-mentioned patent publications cannot be easily
split into finer fibers by an external force are that
(1) the proportion of voids in the transverse section of
fiber is small and (2) the voids are cell-like voids and
do not extend far in the longitudinal direction of the
fiber.
2!~06493
Fibers that can be split into finer fibers by
an external force are known from, for example, Japanese
Unexamined Patent Publication No. 47-32122 and Japanese
Unex~mined Patent Publication No. 55-30460.
The fiber disclosed in Japanese Unex~mined
Patent Publication No. 47-32122 is a conjugate fiber in
which, in the transverse section of a single filament, a
water-insoluble polymer is separated into several parts
by a water-soluble polyamide extending in radial
directions. The fiber disclosed in Japanese Unex~mined
Patent Publication No. 55-30460 is a fibrilated
conjugate fiber composed of a polyamide and a polymer
having no affinity with the polyamide.
The costs of these fibers are inevitably high,
mainly for the following reasons. Namely, since the
fibers are formed by bonding at least two polymers
having different characteristics, different polymers
must be used, and a special spinneret must be used for
the conjugation. Moreover, it is difficult to maintain
a constant ratio between the two components, and when
both components are made finer and bonded together, an
advanced technique is necessary for adjusting the ratio
between the two components.
Prior to the present invention, a fiber
composed of an acrylic polymer, that can be easily split
into finer fibers by an external force, was not known.
SUMMARY OF THE INVENTION
The primary object of the present invention is to
provide an acrylic polymer fiber that can be easily
split into fine fibers by an external force.
It was found that, if openings having an
indeterminate shape are formed in the transverse section
of the fiber and each opening has a vein-like or straw-
like void extending substantially in parallel to the
longitudinal axis of the fiber in the interior of the
fiber, this fiber can be easily split into finer fibers
by an external force.
2006~3
More specifically, in accordance with one aspect of
the present invention, there is provided an acrylic
synthetic fiber having in the transverse section thereof
a multiplicity of openings having an indeterminate shape
and a size of 0.l to l.6 ~m, wherein in the interior of
the fiber, each opening forms a vein-like or straw-like
void extending substantially in parallel to the
longitudinal axis of the fiber and having a length of at
least 60 ~m.
In accordance with another aspect of the present
invention, there is provided a process for the
preparation of an acrylic synthetic fiber, which
comprises a) dissolving in a suitable solvent an acrylic
polymer comprising at least 60% by weight of an
acrylonitrile unit and 5% to 20% by weight, based on the
weight of the acrylic polymer, of a polyalkylene glycol
having a number average molecular weight of 5,000 to
50,000, b) aging the formed spinning solution for at
least 4 hours, and c) extruding the spinning solution
into a coagulating medium through a spinneret.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure l is an electron microscope photograph of
the longitudinal section of the acrylic synthetic fiber
prepared in Example l according to the process of the
present invention;
Fig. 2 is a similar photograph of the transverse
section of the acrylic synthetic fiber shown in Fig. l;
Fig. 3 is a similar photograph of the fiber
obtained by splitting the acrylic synthetic fiber shown
in Fig. l;
Fig. 4 is an electron microscope photograph of the
transverse section of the acrylic synthetic fiber
prepared in Example 2 according to the present
invention;
Fig. 5 is a similar photograph of the fiber
obtained by splitting the acrylic synthetic fiber shown
in Fig. 4;
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Figs. 7 through g are electron microscope
photographs of the transverse sections of the acrylic
synthetic fibers prepared in Example 3 according to the
process of the present invention;
Figs. 6 and 10 are similar photographs of the
transverse sections of the comparative acrylic synthetic
fibers obtained in Example 3;
Figs. 12 and 13 are electron microscope photographs
of the transverse sections of the acrylic synthetic
fibers prepared in Example 4 according to the process of
the present invention;
Fig. 11 is similar photograph of the transverse
section of the comparative acrylic synthetic fiber
obtained in Example 4;
Fig. 14 is an electron microscope photograph (100
magnifications) of the shape and construction of the
acrylic synthetic fibrous article prepared in Example 6;
Fig. 15 is an electron microscope photograph (5,000
magnifications) of the longitudinal section of the
acrylic synthetic fiber prepared in Example 5; and
Fig. 16 is a similar photograph (3,000
magnifications) of the transverse section of the acrylic
synthetic fiber shown in Fig. 15; and
Fig. 17 is an electron microscope photograph (200
magnifications) of the fiber obtained by splitting the
acrylic synthetic fiber shown in Figs. 15 and 16.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The acrylic synthetic fiber of the present
invention will now be described in detail.
The acrylic polymer constituting the acrylic
synthetic fiber of the present invention is a
homopolymer of acrylonitrile or is a copolymer comprised
of at least 60% by weight (all of "%" given hereinafter
are by weight unless otherwise indicated) of
acrylonitrile and up to 40% of an ethylenic monomer
copolymerizable with acrylonitrile, or a mixture of two
or more of such polymers.
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Ethylenic monomers copolymerizable with
acrylonitrile are known monomers. For example, there
can be mentioned acrylic acid, methacrylic acid, esters
thereof (such as methyl acrylate, ethyl acrylate, methyl
methacrylate and ethyl methacrylate), vinyl acetate,
vinyl chloride, vinylidene chloride, acrylamide,
methacrylamide, methacrylonitrile, allylsulfonic acid,
methallylsulfonic acid, styrenesulfonic acid,
vinylpyridine, 2-methyl-5-vinylpyridine and N,N-
dimethylaminoethyl methacrylate.
As pointed out hereinbefore, the acrylic syntheticfiber of the present invention is characterized in that,
in the section of the fiber cut orthogonally to the
longitudinal axis of the fiber (hereinafter referred to
as ~the transverse section of the fiber"), many openings
having an indeterminate shape are formed and each
opening forms a vein-like or straw-like void extending
substantially in parallel to the longitudinal axis of
the fiber.
In the transverse section of the fiber of the
present invention, the sectional shape of the void of
each opening is indeterminate. More specifically, the
sectional shapes of voids of the openings include
substantially circular shapes, flat shapes, shapes of
repeated bends having acute edges, shapes having a large
section, shapes having a small section and the like, as
shown in Fig. 2 of the accompanying drawings, and the
shape and size of the voids are not constant but
irregular. Since many such indeterminate voids are
present, splitting can be easily performed by an
external force. It i5 especially preferred that
sectional shapes of the voids be defined by repeated
bends having acute edges. If the voids have such a
sectional shape, the fiber can be split more easily.
The size (diameter) of the voids is not
particularly critical, so long as the requirements
described hereinafter are satisfied, but to split the
2C~0~93
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fiber easily and obtain fine fibers by splitting,
preferably many fine voids are present. Note, even if
relatively large voids are present, the intended objects
can be attained if fine voids are present around these
relatively large voids.
As shown in Fig. l of the accompanying drawings,
each of the above-mentioned openings forms a vein-like
or straw-like void extending substantially in parallel
to the longitudinal axis of the fiber in the interior of
the fiber.
The length of the voids along the longitudinal axis
of the fiber (hereinafter referred to as "the void
length") should be such that the fiber can be easily
split. These slender voids are distinguishable over the
conventional voids which are formed to have a relatively
large and independent cell-like shape for imparting a
soft touch and attaining a heat-insulating effect. In
the fiber of the present invention, the void length is
at least 60 ~m. If the void length is smaller than
60 ~m, splitting of the fiber is very difficult even if
the void number is increased.
The larger the void length, the more easily split
the fiber, as long as the void length is at least 60 ~m.
Therefore, most preferably the voids are continuous
substantially along the entire length of the fiber.
In the transverse section of the fiber, voids
should be present in a large number such that the fiber
can be easily split, but the necessary number of voids
depends on the void length and cannot be easily
stipulated. If the void length is large, the fiber can
be easily split even when the number of voids is
relatively small, but, in general, preferably at least
lO0 voids are present. Where the number of voids is
smaller than lO0, splitting of the fiber is very
difficult even if the voids are continuous voids having
a length of at least 60 ~m. If at least lO0 voids are
present in the transverse section, the more easily split
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the fiber, and further, the finer the split fibers.
To obtain fine split fibers, preferably the voids
are uniformly dispersed in the transverse section of the
fiber.
In the acrylic synthetic fiber of the present
invention, the void ratio, that is, the ratio of the
sectional area of the void to the total area of the
transverse section of the fiber, is preferably 5 to 80%.
If the void ratio is lower than 5%, the void number is
small and splitting of the fiber is difficult. If the
void ratio is higher than 80%, the preparation of the
fiber per se is difficult.
The void ratio referred to herein is defined by the
following formula:
true denier
(1 - ) x 100 (%)
apparent denier
The apparent denier is calculated from the
sectional area of the single fiber and the true denier
is calculated by the weight method. The determination
is performed with respect to 10 sample fibers of one
lot, and the mean value is calculated.
The size (diameter) of the voids in the transverse
section of the fiber cannot be clearly specified because
the voids have an indeterminate shape, but preferably
the average diameter of the circumscribed circle of the
voids is at least about 0.1 ~m.
As pointed out hereinbefore, the acrylic synthetic
fiber of the present invention is characterized by the
void length, the number of voids, and the cross-
sectional shape of the voids. The fiber can be easily
split by an external force due to the combination of
these characteristic features, and the split fiber can
be used in the form of an assembly of fine fibers or a
dispersion of fine fibers.
In the present invention, the external force means
the stress imposed on the fiber at the fiber-processing
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g
step, for example, by a disk refiner used in the paper-
making industry or a columnar stream punching of
high-pressure water adopted in the nonwoven fabric-
manufacturing process.
The acrylic synthetic fiber of the present
invention can be used in the fields of clothing nonwoven
fabrics, paper products and the like while the foregoing
characteristic properties are utilized.
The process for the preparation of the acrylic
synthetic fiber of the present invention will now be
described.
As pointed out hereinbefore, the acrylic polymer
used in the present invention is a polymer comprising at
least 60% of acrylonitrile. If the amount of
acrylonitrile is smaller than 60%, the softness and
wooly touch inherently possessed by the acrylic
synthetic fiber are lost. The upper limit of the amount
of acrylonitrile is not critical. The acrylic polymer
used in the present invention can be a mixture
comprising at least two kinds of acrylic polymers. In
this case, the content of acrylonitrile should be at
least 60% based on the total weight of the polymer
mixture.
The polymer is dissolved in a known solvent for
acrylic polymers, for example, an organic solvent such
as dimethylformamide, dimethylacetamide or dimethyl-
sulfoxide, a concentrated aqueous solution of an
inorganic salt such as a rhodanate, zinc chloride, or a
concentrated aqueous solution of an inorganic acid such
as nitric acid, whereby a spinning solution is prepared.
An optimum concentration of the polymer in the spinning
solution depends on the kind of the solvent, but in
general, preferably the polymer concentration is lO
to 30%.
A polyalkylene glycol is added to this spinning
solution. The addition of the polyalkylene glycol is
one of important requirements for the preparation of the
Z006~q93
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acrylic synthetic fiber of the present invention.
Especially, the molecular weight and amount of the
polyalkylene glycol added make great contributions to
formation of voids.
The polyalkylene glycol is preferably a random or
block copolymer comprising ethylene oxide and propylene
oxide at a weight ratio of from 80/20 to 20/80. The
number average molecular weight of the polyalkylene
glycol is 5,000 to 50,000, preferably lO,000 to 20,000.
If the number average molecular weight of the
polyalkylene glycol is lower than 5,000, voids
continuously extending in the longitudinal direction of
the fiber cannot be formed, and a microporous fiber
having very fine, substantially spherical voids is
formed. If the number average molecular weight of the
polyalkylene glycol is higher than 50,000, a fiber
having large vein-like voids is obtained, and in the
transverse section of the obtained fiber, only a few of
the needed of voids are present. This fiber cannot be
split into fine fibers by an external force such as a
columnar stream of a liquid. Especially, if the number
average molecular weight of the polyalkylene glycol is
lO,000 to 20,000, a fiber having fine and slender voids
which are continuous along the longitudinal direction of
the fiber and having an indeterminate cross-sectional
shape in the transverse section of the fiber can be
obtained.
To prepare the acrylic synthetic fiber of the
present invention, the spinning solution formed by
dissolving the polyalkylene glycol must be aged for at
least 4 hours.
By the term "aging" as used herein is meant that
the spinning solution formed by dissolving the acrylic
polymer and polyalkylene glycol is not violently stirred
or shaken but, for example, the spinning solution is
allowed to stand or is gently moved, for example, gently
delivered through a pipe.
ZO0~0~93
The reason why an acrylic synthetic fiber having
the above-mentioned voids can be obtained by thus aging
the spinning solution in-the present invention has not
been elucidated, but it is considered that the reason is
probably as follows. Namely, if the spinning solution
is aged for at least 4 hours, cohesion of the
polyalkylene glycol occurs, and when the spinning
solution passes through a pipe and is spun into a
coagulating medium from a spinneret, the shearing force
acts on the spinning solution and fine streaks of the
polyalkylene glycol are formed. Then, a phase
separation occurs between the two polymers, because of a
difference of coagulating characteristics, that is,
coagulation of the acrylic polymer and non-coagulation
of the polyalkylene glycol, whereby voids having a
complicated shape as mentioned above are formed.
Thus, the spinning solution must be aged for at
least 4 hours before the spinning.
In the foregoing points, the process and fiber of
the present invention are essentially distinguishable
over the process and fiber disclosed in Japanese
Unexamined Patent Publication No. 57-89612. More
specifically, in the process disclosed in Japanese
Unex~mined Patent Publication No. 57-89612, a
polyalkylene oxide having a number average molecular
higher than 100,000 is used. If a polyalkylene oxide
having such a high molecular weight is used, the
polyalkylene oxide is dispersed in the form of spheres
in the spinning solution, as taught in the above-
mentioned patent publication. Accordingly, when thisspinning solution is spun in a coagulating bath, spheres
of the polyalkylene oxide are present in the fiber, and
the polyalkylene oxide is eluted in the coagulating
bath, water-washing bath or drawing bath and there
remain spherical voids or voids elongated in the
longitudinal direction of the fiber according to the
degree of drawing.
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In contrast, in the present invention, since the
polyalkylene glycol used has a low molecular weight such
as a number average molecular weight of 5,000 to 50,000,
the polyalkylene glycol is dissolved in the spinning
solution to form a homogeneous solution, and if this
solution is aged, cohesion of the polyalkylene glycol
occurs in the spinning solution. By aging the spinning
solution for at least 4 hours, fine streaks are formed
by the cohesion of the polyalkylene glycol. When this
spinning solution is extruded into the coagulating bath,
in the coagulated fiber, a phase separation into the
acrylic polymer and the streak-like polyalkylene glycol
occurs, and simultaneously, by elution of the
polyalkylene glycol, fine voids are formed in the
coagulated fiber.
The aging time is at least 4 hours and the upper
limit of the aging time is not critical, but preferably
6 to lO hours.
In the present invention, the amount of the
polyalkylene glycol added is 5 to 20%, preferably lO to
15%, based on the acrylic polymer. If the amount of the
polyalkylene glycol added is smaller than 5%, the number
of voids present in the transverse section of the fiber
is small, and a fiber having many voids, for example, at
least lO0 voids, cannot be obtained. If the amount of
the polyalkylene glycol added is larger than 20%, the
number of openings increases but the number of openings
is too large and the fiber is split during the
preparation or the spinning cannot be carried out
stably. If the amount of the polyalkylene glycol added
is lO to 15%, the best balance between the number of
openings and the spinning stability is maintained.
In the foregoing description, the polyalkylene
glycol is added after the preparation of the spinning
solution but the mixing method is not limited to this
method, and the spinning solution can be prepared
according to a method in which the polyalkylene glycol
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is mixed with the acrylic polymer and the mixture is
dissolved in a solvent for the polymer, or a method in
which the polyalkylene glycol is dissolved in advance in
a solvent for the acrylic polymer and the acrylic
polymer is then dissolved in the solution.
The spinning solution is extruded into a
coagulating medium for the spinning solution through a
spinneret, and the extrudate is passed through water-
washing, drawing and drying steps and is heat-set
according to need.
In this preparation process, the polyalkylene
glycol is eluted from the coagulated fiber during the
coagulating, water-washing and drawing steps. Steps
subsequent to the spinning step, as adopted in the
conventional process for the preparation of acrylic
synthetic fibers, can be directly adopted in the present
invention.
Namely, in the present invention, as the means for
spinning the spinning solution, there can be adopted the
wet spinning method comprising extruding the spinning
solution into a dilute aqueous solution of a solvent,
the dry spinning method comprising extruding the
spinning solution into an inert gas such as air or
nitrogen gas, and the dry-wet spinning method comprising
extruding the spinning solution into the above-mentioned
inert gas and then introducing the extrudate into a
dilute aqueous solution of a solvent. The coagulated
fiber obtained by the spinning is washed with water and
then drawn, water-washed and simultaneously drawn, or
drawn and then water-washed, whereby the solvent is
removed.
The drawing is carried out in water, a solvent-
containing aqueous solution or steam at 50 to 150C at a
draw ratio of several to ten-odd times. The drawing can
be performed in a single stage or a plurality of stages.
Moreover, several drawing media can be used in
combination. The drawn fiber is dried, and if desired,
Z00~493
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the dried fiber is subjected to the secondary drawing or
to the heat treatment, whereby the acrylic synthetic
fiber of the present invention can be obtained.
The acrylic synthetic fibrous article of the
present invention will now be described.
By the term "acrylic fibrous article" used herein
is meant a pulpy fibrous article prepared from the
above-mentioned acrylic synthetic fiber. More
specifically, the acrylic fibrous article of the present
invention is characterized as having as a trunk an
acrylic synthetic fiber having in the transverse section
thereof a multiplicity of openings having an
indeterminate shape and a size of 0.1 to 1.6 ~m, wherein
in the interior of the fiber, each opening forms a
vein-like or straw-like void extending substantially in
parallel to the longitudinal axis of the fiber and
having a length at least 60 ~m, and the surface of the
trunk has a multiplicity of fine fibrils branched from
the trunk and the trunk is partially split in the
longitudinal direction of the trunk and separated into a
plurality of fibers.
The acrylic synthetic fibrous article of the
present invention can be easily prepared by applying an
external force to the above-mentioned acrylic synthetic
fiber, for example, by beating the acrylic synthetic
fiber by a disk refiner customarily adopted in the
paper-making industry or by punching the acrylic
synthetic fiber by a high-pressure water columnar stream
~ adopted in the nonwoven fabric-preparing process. At
this step, the amount of generated fibrils, the fineness
of the fibrils and the frequency of splitting of the
trunk can be adjusted by appropriately selecting the
conditions of the external force applied to the fiber.
Figure 14 is an electron microscope photograph (100
magnifications) of the fibrous article obtained by
beating the fiber shown in Fig. 2. As is seen from
Fig. 14, a multiplicity of fine fibrils branched from
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the fiber are formed on the surface of the fiber, and it
is seen that the acrylic synthetic fiber constituting
the trunk is partially split into a plurality of finer
fibers.
The fine fibrils branched from the acrylic
synthetic fiber or trunk may have many voids extending
along the longitudinal axis of the fiber, as well as the
acrylic synthetic fiber or trunk, or the fine fibrils
may not have such voids.
The acrylic synthetic fiber is split in a plurality
of finer fibers at an optional position in the
longitudinal direction of the fiber, but this splitting
position is not critical.
The fact that the trunk fiber is split in a
plurality of fine fibers at an optional position means
that the fiber has an improved softness and pliability,
and a paper or sheet product or nonwoven fabric having
high elasticity and bulkiness can be obtained from this
fiber.
The friction material of the present invention will
now be described.
This frictional material is prepared from the
above-mentioned acrylic synthetic fiber.
More specifically, the frictional material of the
present invention is characterized as comprising an
acrylic synthetic fibrous article, a resin and a filler,
said acrylic synthetic fibrous article having as a trunk
an acrylic fiber having in the transverse section
thereof a multiplicity of openings having an
indeterminate shape and a size of 0.1 to 1.6 ~m, wherein
in the interior of the fiber, each opening forms a
vein-like or straw-like void extending substantially in
parallel to the longitudinal axis of the fiber and
having a length of at least 60 ~m, and the surface of
the trunk has a multiplicity of fine fibrils branched
from the trunk and the trunk is partially split in the
longitudinal direction of the trunk and separated into a
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plurality of fibers.
By using the fiber having the above-mentioned
specific shape as the substrate of the friction
material, the friction coefficient and abrasion
resistance are highly improved in the obtained friction
material.
The reason for this improvement has not been
elucidated, but it is considered that the reason is
probably as follows. Namely, since the acrylic
synthetic fiber of the present invention has many
vein-like voids in the trunk, the resin and filler are
allowed to intrude into these voids, and since many fine
fibrils are present on the surface of the trunk, the
resin and filler intrude into spaces defined by the
trunk and fibrils. Accordingly, the fibrous article,
resin and filler are very closely integrated as a whole.
The transverse section and longitudinal section of
each of the fine fibers formed at the split portion of
the acrylic fiber or trunk depend on the splitting
degree, but if the splitting degree is low, for example,
if the trunk fiber is split into 2 to 10 fine fibers,
these sections are substantially the same as those of
the original fiber (trunk) except that the number of
openings in the transverse section should naturally be
smaller than the number of openings in the transverse
section of the original fiber (trunk). If the splitting
degree is very high, openings are not found in
transverse sections of some of fine fibers formed by
splitting.
In the transverse sections of fine fibrils branched
from the trunk, openings are found or not found, and the
presence or absence of openings depends on the branching
degree. Namely, where the fibrils are relatively thick,
openings are found, and where the fibrils are very fine,
openings are not found. Generally, these fine and thick
fibrils are mingled.
The acrylic synthetic fibrous article of the
20(~193
- 17 -
present invention is contained preferably in an amount
of 10 to 70%, more preferably 20 to 60%, in the friction
material. If the content of the fiber is lower than
10%, no substantial improvement of the friction
coefficient or abrasion resistance is obtained even by
using the fiber as base material. If the fiber content
exceeds 70%, the resulting product is not suitable as
the friction material because the amount of the fiber is
too large. Preferably, the content of the resin is 20
to 70%, and the content of the filler is 10 to 50%.
A resin customarily used for friction materials can
be used in the present invention. For example, phenolic
resins, epoxy resins, polyimide resins, melamine resins,
natural rubbers and synthetic rubbers can be used.
In the present invention, the filler is used for
improving the characteristics of the friction material.
In general, at least one member selected from the group
consisting of metal powders, silica, clay, wollastonite,
mica, talc, diatomaceous earth, calcium carbonate,
cashew dust and graphite can be used as the filler.
For improving the characteristics of the friction
material, other fibrous material such as a glass fiber,
a metal fiber, a carbon fiber, a flame-retardant fiber,
a polyvinyl alcohol fiber, a polyamide fiber, a
polyester fiber, an acrylic fiber or cotton can be
incorporated.
In the acrylic synthetic fibrous article of the
present invention, the acrylic fiber as the trunk has
many vein-like or straw-like voids. Accordingly, the
resin and filler are allowed to intrude easily into
these voids. Moreover, the resin and filler intrude
into spaces formed by partial splitting of the trunk and
the fine fibrils branched from the trunk. Accordingly,
the fibrous article, resin and filler are very closely
and intimately mingled and integrated with one another.
Therefore, the friction material has a high friction
coefficient and an excellent abrasion resistance.
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The acrylic synthetic fibrous article used for the
friction material of the present invention can be
prepared by beating the above-mentioned acrylic
synthetic fiber by a disk refiner customarily used in
the paper-making industry. At this step, the degree of
formation of fine fibrils and the degree of splitting of
the trunk are changed according to the properties
required for the friction material. In general, these
degrees are preferably such that the freeness, used for
indicating the beating degree of pulp in the paper-
making industry, is about 600 to about 200 cc, but the
freeness is not limited to within this range, and may be
larger than 600 cc or smaller than 200 cc. Namely, the
freeness can be appropriately set according to the
properties required for the friction material. Note,
the freeness referred to herein is the value determined
according to the method of JIS P8121-1976.
The acrylic synthetic fibrous article is mixed with
the resin and filler and molded into a friction
material.
The friction material can be prepared according to
a process in which a substrate comprising the acrylic
synthetic fibrous article and filler, for example, a
paper-like sheet or a nonwoven fabric, is prepared and
the substrate is impregnated with the resin, molded and
then cured, a method in which the acrylic synthetic
fibrous article is mixed with the resin and filler, and
the mixture is molded and then curing, and other known
methods.
The present invention will now be described in
detail with reference to the following examples that by
no means limit the scope of the invention.
Acrylic synthetic fiber and process for preparation
thereof
Example 1
A polymer comprising 95.0% of acrylonitrile, 4.5%
of methyl acrylate and 0.5% of sodium methallylsulfonate
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and a polyethylene oxide/polypropylene
oxide/polyethylene oxide block polyether (number average
molecular weight = 10,000, polyethylene
oxide/polypropylene oxide ratio = 70/30) were dissolved
in dimethylformamide to form a spinning solution
containing 23% of the acrylic polymer and 2.3% of the
block polyether. The spinning solution was allowed to
stand for 6 hours and then was extruded into a
coagulating bath maintained at 35C and having a
dimethylformamide concentration of 75% through a
spinneret. The extrudate was washed with water, drawn
at a draw ratio of 12 in boiling water and dried in hot
air at 80C to obtain a fiber having a fineness of
1.5 d.
An electron microscope photograph (4,000 magnifica-
tions) of the longitudinal section of the fiber cut in
the longitudinal direction (hereinafter referred to as
"longitudinal section") is shown in Fig. 1, and a
similar photograph of the transverse section of the
fiber is shown in Fig. 2.
In Fig. 1, black portions are spaces, and it is
seen that these spaces are continuous streak-like spaces
extending substantially in parallel to one another along
the longitudinal axis of the fiber. When the
longitudinal section of the fiber was observed, these
spaces were found to have a length of at least 60 ~m.
In Fig. 2, black portions are openings, and it is
seen that these openings include substantially circular
openings, flat openings, openings having a shape of
repeated bends having acute edges, openings having a
large section, openings having a small section, and
various openings having an indeterminate shape, are
irregularly mingled with one another.
The void ratio of the obtained fiber was 35%.
The fiber was treated with a high-pressure water
stream jetted under a pressure of 50 kg/cm2 from a
nozzle having an orifice diameter of 0.15 mm 10 times,
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whereby the fiber was split into fine fibers to form an
assembly of fine-denier fibers.
Figure 3 is an electron microscope photograph (200
magnifications) of the obtained assembly. As is seen
Fig. 3, the fiber of the present invention can be easily
split by an external force to form an assembly of finer
continuous fibers.
Example 2
The same acrylic polymer as used in Example 1 and
an ethylene oxide/propylene oxide random copolymer
polyether (number average molecular weight = 10,000,
ethylene oxide/propylene oxide ratio = 75/25) were
dissolved in a 67% aqueous solution of nitric acid to
form a spinning solution having an acrylic polymer
concentration of 16% and a random copolymer polyether
concentration of 2.4%. The spinning solution was
allowed to stand for 4 hours and then was extruded into
a 37% aqueous solution of nitric acid maintained at 0C
through a spinneret. The extrudate was washed with
water, drawn at a draw ratio of 10 and dried by hot air
at 70C to obtain a fiber having a fineness of 3.5 d.
The void ratio of the fiber was 40%.
An electron microscope photograph (1,000 magnifica-
tions of the transverse section of the obtained fiber is
shown in Fig. 4. When the longitudinal section of the
fiber was observed, continuous streak-like spaces having
a length of at least 60 ~m were found.
The obtained fiber was treated with a high-pressure
water stream 5 times in the same manner as described in
Example 1 to obtain an assembly of finely split fibers.
An electron microscope photograph (200 magnifications)
of the obtained assembly is shown in Fig. 5.
For comparison, the above procedures were repeated
in the same manner except that the aging time of the
spinning solution was changed to 3 hours. The
transverse section of the obtained fiber and several
large and long voids. This fiber could not be split by
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a high pressure water stream.
Example 3
An acrylic polymer comprising 97% of acrylonitrile,
2.5% of methyl acrylate and 0.5% of sodium
allylsulfonate and an ethylene oxide/propylene oxide
random copolymer polyether having an ethylene
oxide/propylene oxide copolymerization ratio of 75/25
and a number average molecular weight of 3,000, 5,000,
30,000, 50,000 or 60,000 were dissolved in a 67% aqueous
solution of nitric acid at 0C to form a spinning
solution. In the spinning solution, the acrylic polymer
concentration was 18% and the polyalkylene glycol
concentration was 1.8%.
The spinning solution was slowly delivered in a
pipe over a period of 5 hours and then was extruded into
a 38% aqueous solution of nitric acid at 0C through a
spinneret. The extrudate was washed with water, drawn
at a draw ratio of 8 in boiling water and dried in hot
air at 70C to obtain a fiber.
Electron microscope photographs of the transverse
sections of the obtained fibers are shown in Figs. 6
through 10, and the obtained fibers are summarized in
Table 1. In Table 1, the amount of the polyalkylene
glycol added is expressed in terms of % based on the
acrylic polymer. When the longitudinal sections of the
fibers shown in Figs. 7, 8 and 9 were observed,
continuous streak-like spaces having a length of at
least 60 ~m were found.
These fibers were treated with a high-pressure.
water stream in the same manner as described in
Example 1. The fibers of the present invention were
split into finer fibers, but little splitting of the
comparative fibers occurred.
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Example 4
An acrylic polymer comprising 96% of acrylonitrile,
3.5% of vinyl acetate and 0.5% of sodium styrene-
sulfonate and an ethylene oxide/propylene oxide random
copolymer polyalkylene glycol (ethylene oxide/propylene
oxide ratio = 75/25, number average molecular weight =
20,000) were dissolved in a 67% aqueous solution of
nitric acid at 0C to form a spinning solution. In the
spinning solution, the concentration of the acrylic
polymer was 16% and the concentration of the
polyalkylene glycol was 3, 5, 20 or 24%. The obtained
spinning solution was allowed to stand for 5 hours and
extruded into a 38% aqueous solution of nitric acid at
0C through a spinneret. The extrudate was washed with
water, drawn at a draw ratio of 10 in boiling water, and
dried in hot air at 70C to form a fiber.
When the polyalkylene glycol concentration was 24%,
yarn breakage occurred during the spinning operation and
it was difficult to carry out a stable spinning
operation.
Electron microscope photographs of the transverse
sections of the obtained fibers are shown in Figs. 11
through 13. The obtained fibers are summarized in
Table 2. When the longitudinal sections of the fibers
shown in Figs. 12 and 13 were observed, continuous
streak-like spaces having a length of at least 60 ~m
were found.
Then the fibers were treated with a high-pressure
water stream in the same manner as described in
Example 1. The fibers of the present invention were
split into finer fibers, but when the polyalkylene
glycol concentration was 3%, the number of voids was
small and the fiber could not be split into finer
fibers.
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Example 5
A polymer comprising 95.0% of acrylonitrile, 4.5%
of methyl acrylate and 0.5% of sodium methallylsulfonate
and a polyethylene oxide/polypropylene
oxide/polyethylene oxide block copolymer polyether
(number average molecular weight = 10,000, polyethylene
oxide/polypropylene oxide ratio = 70/30) were dissolved
in dimethylformamide to form a spinning solution having
an acrylic polymer concentration of 23% and a block
copolymer polyether concentration of 2.3%.
The spinning solution was allowed to stand for 4
hours and then was extruded into a coagulating bath
maintained at 24C and having a dimethylformamide
concentration of 76% through a spinneret. The
coagulated fiber was washed with water, drawn at a draw
ratio of 9 in boiling water and dried in hot air at 80C
to obtain a fiber having a fineness of 1.5 denier.
Figure 15 shows an electron microscope photograph
(5,000 magnifications) of the longitudinal section of
the fiber cut along the longitudinal axis of the fiber,
and Fig. 16 shows a similar photograph (3,000
magnifications) of the transverse section of the fiber.
In Fig. 15, the two photographs are linked together
at the respective alternate long and short dash
lines (a-a). In Fig. 15, the black portions are spaces,
and it is seen that these spaces are continuous
streak-like spaces extending substantially in parallel
to one another along the longitudinal axis of the fiber.
These spaces had a length of at least 60 ~m.
In Fig. 16, black portions are openings, and it is
seen that these openings include substantially circular
openings, flat openings, openings having a shape of
repeated bends having acute edges, openings having a
large section, openings having a small section, and
various openings having an indeterminate shape are
irregularly mingled with one another.
The void ratio of the obtained fiber was 37~.
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Then the fiber was treated with a high-pressure
water stream jetted under a pressure of 50 kg/cm2 from a
nozzle having an orifice diameter of 0.15 mm 10 times,
whereby the fiber was split into finer fibers to form an
assembly of fine-denier fibers.
Figure 17 is an electron microscope photograph (100
magnifications) of the obtained assembly of fine-denier
fibers.
Acrylic synthetic fibrous article
Example ~
The acrylic synthetic fiber prepared in Example 1
was cut to 15 mm, and 10 parts of the cut fiber was
dispersed in 90 parts of water, and the fiber dispersion
was treated by a paper-making disk refiner having a disk
clearance adjusted to 0.1 mm and beaten so that the
freeness was 450 cc.
Since the fiber of the present invention had voids,
the fiber was easily split at the beating step and
fibrils were formed very easily.
Figure 14 shows an electron microscope photograph
(100 magnifications) of the fibrous article formed by
the beating. From this photograph, it is seen that many
fine fibrils branched from the thick fiber (the original
acrylic synthetic fiber as the trunk) were formed on the
surface of the trunk, and that the trunk fiber was
partially split in the longitudinal direction and
separated into the fibers.
The aqueous dispersion containing the beaten fiber
was passed through an ordinary paper-making machine and
then dried by hot air at 85C to obtain a sheet-like
product having a basis weight of 45 g/m2. The obtained
sheet-like product was a pliable and elastic paper-like
nonwoven fabric having a soft touch.
When the sheet-like product was dried after the
paper-making operation, no substantial shrinkage was
observed in the longitudinal or transverse direction of
the sheet-like product, and the paper-like sheet product
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had a very uniform plane.
Friction material
Example 7
In a Henschel mixer, 50% of the acrylic synthetic
fibrous article prepared in Example 6, 25% of a phenolic
resin and 25% of calcium carbonate as the filler were
sufficiently mixed, and the mixture was compression-
molded in a mold at 150C under 5 kg/cm2 for 10 minutes
to form a pad of a disk brake for an automobile.
When this pad was subjected to the constant-speed
friction test according to method of JIS D-4411, it was
found that the friction coefficient at 250C was 0.45
and the abrasion loss was 1.48 x 10 7 cm3/kg.m.
Example 8
The procedures of Example 7 were repeated in the
same manner except that the fiber was cut into 5 mm and
the fiber concentration in the dispersion was changed to
1%, whereby a brake pad was prepared.
When the brake pad was tested in the same manner as
described in Example 7, it was found that the friction
coefficient was 0.45 and the abrasion loss was 1.47 x
10 7 cm3/kg.m. Friction materials having the same
performances were obtained with a good reproducibility.
Example 9
A brake pad was prepared by treating a mixture
comprising 40% of the acrylic synthetic fiber prepared
in Example 6, 15% of a glass fiber, 22% of diatomaceous
earth and 23% of a phenolic resin in the same manner as
described in Example 7.
When the obtained brake pad was subjected to the
abrasion test, it was found that the friction
coefficient was 0.45 and the abrasion loss was 1.46 x
10-7 cm3/kg m
As apparent from the foregoing description, the
following effects can be obtained according to the
present invention.
1. The acrylic synthetic fiber of the present
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invention can be easily split into fibers continuous in
the longitudinal direction by an external force such as
a high-pressure water stream.
Furthermore, according to the preparation
process of the present invention, by spinning a spinning
solution containing a specific polyalkylene glycol after
aging, a fiber having many voids having an indeterminate
shape in the transverse section of the fiber and being
continuous in the longitudinal direction of the fiber
can be easily obtained.
2. The acrylic synthetic fibrous article of the
present invention is suitable for the production of a
nonwoven fabric or paper-like product having a high
elasticity and bulkiness, and the acrylic synthetic
fibrous article of the present invention has
characteristics suitable for a resin reinforcer and the
like.
3. Since the friction material of the present
invention is formed by using the fibrous article
comprising fibers having a specific shape and structure,
uniform mixing and close and integral conjugation can be
accomplished, and therefore, the friction coefficient
and abrasion resistance are greatly improved in the
friction material of the present invention.
