Note: Descriptions are shown in the official language in which they were submitted.
7~057-21
FRICTION MATERIAli COMPOSITION AND PROCESS FOR PP~O~UCTION
THER~3OF
Field of ~he Invention
The present invention relates to a friction material
composition which i5 suitably mounted on the contact fric-
~ion surface of an ultrasonic motor, as well as to a process
for producing said friction material composition.
Background of the Invention
Ultrasonic motors are cons-tituted and operated as
follows. A piezoelectric ceramic is attached to an elastlc
vibrator, an ultrasonic voltage is applied to the pieæoelec-
tric ceramic to excite the elastic vibrator to allow the vi-
brator to give rise to ultrasonic vibration; and the ultra-
sonic vibration is ~ransmitted hy a contact-friction force,
to a movement pres~ure-contacted with the elastic vibrator,
to obtain a driving force.
There~ore, the state of the contact friction between
the ela~tic vibrator and the movement is an important factor
in determining the properties of ultrasonic motor, such as
output torque, number of rotations, input/output efficiency,
~0 life and the like.
It was proposed in Japanese Patent Publication
Kokai(Laid-Open) No. 23585/1987 that an ultrasonic motor
having a ~table state of contact friction can be obtained by
mounting a fric~ion material composition on the contact-
friction surface of the elastic vibrator or movement. Such
,, ~ . .. . .......
. .
. .. ~ .
-
: . . : . .
.
" , ' ' :
2~90~59 72057-21
a friction material composi~ion mu~t have properties such
as:
1. ~mall in the abrasion caused by contact friction
(larg~ abrasion resistance),
2. the frictional coefficient ~ i5 stable and large,
3. low in property deterioration and stable over a
long period of time, and
4. no ~riction sound is generated.
In order to improve the abrasion resistance of a
friction material composition, addition of a carbon fiber to
the friction material composition was propo~ed in, for exam-
ple, ~apanese Patent Publication ~okai (Laid-Open) ~os.
147978/1987 and 255483/1989. When ~uch a conventional fric-
tion material composition containing a carbon fiber is used
lS in an ultrasonic motor and the re~ulting ultrasonic motor is
~ubjected to continuous operation, however, there occurs a
phenomenon in which the number of rotations of the ultra-
sonic motor is significantly reduced for few to several min-
utes a~ the particular ~imings determined by the kind of the
carbon fiber.
An object of the present invention is to provide
a friction material composition which iB free from the above
problem~ of the prior art and, when used in an ultrasonic
motor, is superior in abrasion resistance and gives no sharp
reduction in num~er of rotations in continuous operation of
the motor, and a process for producing ~uch a friction mate-
rial composition.
~n o.rder to develop ~he above friction material com-
position, tha pre~ent inventor made an extensive study on
,
:
~ ~9 ~ ~ 9 72057-21
various compositions. As a result, the present inventor
found that a composition comprising a polyacrylonitrile-
based carbon fiber as a component gives good results. Thi~
finding has led to the completion of the present invention.
Summary of the Invention
Accor~ing to the present invention, there is pro-
vided a friction material composition which comprises at
least a polyacrylonitrile-based carbon fiber, a fiber compo-
nen~ other than the polyacrylonitrile-based carbon fiber,
and a thermosetting resin.
The pre~ent invention further provides a process for
producing a friction material composition, which comprises
dispersing, in a dispersing medium, at lea;~st a polyacryloni-
trile-based carbon fiber, a fiber component other than the
polyacrylonitrile-based carbon fiber, and a thermasetting
resin, filtering the dispersion through a filter material,
and subjecting the sheet-shaped material remaining on the
filter material to moldin~ under heating and pressure.
Brief Description of the Drawings
Fig. 1 is a graph showing the change of number of
rotations with ~ime when the friction material composition
of Example 1 was used in an ultrasonic motor and the motor
~5 wa~ subjected to a continuous operation of 300 hours.
Fig. 2 is a graph ~howing the change of number of
rotations with time when the friction material composition
of Comparative E~ample 1 wa~ used in an ultrasonic motor and
.
,
2 ~ 9 ~ ~ ~ 9 720~7-21
the motor was subjected to a continuous operation of 300 hours.
Fig. 3 is a graph showing the change of number of
rotations with time when the friction material cornposition of
Comparative Example 2 was used in an ultrasonic motor and the
motor was subjected to a continuous operation of 300 hours.
Detailed Description of the Invention
The present invention is hereinafter described in
detail.
Since the present friction material composition is
characterized by comprising a polyacrylonitrile-based carbon
fiber, the polyacrylonitrile-based carbon fiber is explained.
The polyacrylonitrile-based carbon fiber is produced by first
producing a polyacrylonitrile fiber by a known method and then
subjecting the fiber to a treatment for flame resistance and
subsequent carbonization.
The polyacrylonitrile-based carbon fiber preferably
has a fiber length of 1 mm or more. A fiber length up to few
to several tens of meters can be used, but preferably the length
is up to about 40 mm. The carbon fiber has no restriction for
the diameter which is generally 30 ~m or less. A more preferred
range of the length of the polyacrylonitrile-based carbon fiber
is l to 10 mm.
, , . . , : ' , ,
:. . : .,
2 ~ ~ O ~ ~ 9 72057-21
The present friction material composition further
comprises a fiber component other than the polyacrylonitrile-
based carbon fiber. The other fiber componen-ts may be any
known fibers such as aromatic polyamide fiber, phenolic fiber,
polyacrylonitrile fiber, rock wool fiber and the like.
Fihrillated fibers obtained by grinding and crushing such fiber
may also be used. Of these fibers, an aroma-tic polyamide fiber
is particularly preferable in
4a
: ' ' , . ~ ' ' ' ,,;,: ,
', ~ ' :'' ' '
,~ 20~0éj59
view of the preventability for ~he disintegration during
handling.
The present friction material composition further-
more comprises a thermoset~ing resin. As the thermosetting
resin, there i9 mentioned at least one resin selected from
known thermosetting resins such as polyimide resin, phenolic
resin, rubber-modified phenolic resin, epoxy resin, urea
resin, polyurethane resin and the like. ~he thermose~ting
resin has no particular restriction, but particularly
preferable are a polyimide resin, a phenolic resin and a
rubber~modified phenolic resin.
In the present friction material composition, there
can be used, as necessary, a friction coefficient modifier
such as carbon black, yraphite, fluorinated graphite, sulfu-
rized molybdenuml melamine powder, cashew dust or the like.
The proportions of the polyacrylonitrile-based car-
bon fiber, the fiber component other than the polyacryloni-
trile-based carbon fiber, and the thermosetting resin in the
present friction material composition are, for example, 5-
60~ by volume, preferably 10-~0% by volume, 2-75% by volume,
preferably 3-20% by volume, and 20-80% by volume, preferably
30-50% by volume, respectively.
When the proportion of the polyacrylonitrile-based
carbon fiber is less than 5% by volume, there can be ob
tained no effect brought about by the use of the polyacry-
lonitrile-based carbon fiber. When the proportion is more
than 60% by volume, the re~ulting friction material compo~i-
tion has voids in which the resin i~ not present suffi-
ciently. When the proportion o the fiber component other
.
'
.... .
~09~9
than the polyacrylonitrile-based carbon fiber i5 less than
2% by ~olume, the sheet-shaped material obkained by ~ilter-
ing may be disintegrated during the handling. Meanwhile,
the maximum proportion of said fiber component is 75% by
volume in view of the reguired proportions of ~he other com-
ponents. When the propor-tion of the thermosetting resin is
less than 20% by volume, the resulting friction material
compo~ition has voids in which the resin is not present suf-
ficiently. When the proportion of the resin is more than
80% by volume, the resin leaks through the interface between
male die and female die, making it difficult to obtain the
required composition.
The present friction material composi~ion having the
above-mentioned constitution can be produced by the present
process described below.
First, the raw materials are weighed so as to give
the above-mentioned component proportions. Then, they are
dispersed in an appropriate dispersing medium. In this
case, there is no restriction as to the stirrer.
Thereafter, the dispersion is filtered through a filter ma-
terial to obtain a sheet-shaped material. As the filter ma-
terial, there can be used an appropriate material such as
filter cloth, filter paper or the like, but there is pre-
ferred a filter material of low filtering loss, capable of
collecting thereon the raw materials in the dispersion, in a
high recovery ratio. It i8 possible to adopt reduced-pres~
sure filtration in order to shorten the filtering time.
The dispersing medium is required to be a liquid at
room temperature and not to dissolve the components of the
~ .
, :- . ' ' ~ ' " ' : ;
,~. . . . .
: .: . .
- 2 ~ 9 72057-21
pre~ent friction material compo~ition. For ~xample, water
is u~ed as the dispersing medium.
The sheet-~haped material obtained by filtration is
then dried. Since in this step it is sufficient if water iq
removed, drying, for example, at 60C for about one night is
sufficient. The dried sheet-shaped material is subjected to
molding under heating and pressure, whereby a friction mate-
rial composi~ion according to the present invention can be
obtained. The conditions of heating and pressure are, for
example, 200-220C, 10-30 minutes and 200 kgf~cm2 when the
thermosetting resin is a polyimide resin, and 150-170C, 10-
30 minutes and 200 kgf/cm2 when the thermosetting resin is a
phenolic resin.
The present invention i~ hereinafter described in
more detail referring to Example~.
Example 1
In water was dispersed a compo~ition comprising 20%
by volume of a polyacrylonitrile-based carbon fiber [TORAYCA
CHOPPED FIBER T010 (trade-mark ), a product of TORAY
INDUSTRIESI INC., fiber length = 6 mm], 20% by volume of an
aromatic polyamide fiber pulp [REVLAR (trade-mark), a prod~
uct of TORAY DuPont] and 60% by volume of a polyimide
[ TECHMIGHT (trade-mark), a product of Mitsui P~trochemical
Industries,Ltd.]o The resulting dispersion was filtered to
obtain a shee~shaped material. The sheet-shaped material
was dried at 60C for one night. The dried material was
treated at 200 kgf/cm2 at 220C for 10 minutes to produce a
friction material composition of the present invention.
,
,;
- ~ .
.
,
72057-21
Compara~ive Example 1
A composition was subjected to the same treatment as
in E~ample 1 except that the polyacrylonitrile-based carbon
fiber used in the composition of E~ample 1 was replaced by a
pitch-based carbon fiber [RURECA CHOP C103S (trade-mark), a
product of ~ureha Chemical Industry Co., Ltd~, fiber length
= 3 mm]~ to obtain a friction material compo~ition for com-
parison.
Comparative E~ample 2
A compo~ition was subjected to the ~ame treatment as
in E~ample 1 except that the polyacrylonitrile-based carbon
fiber used in the composition of Example 1 was replaced by a
phenol-based carbon fiber lRYNO~ (trade mark), a product of
Gun-ei Ragaku, fiber length - 6 mm], to obtain a friction
material composition for comparison.
Each of the above-obtained fric~ion material compo-
~i~ions was used in an ultrasonic motor [USM 40D, a product
of Matsushita Electric Industrial Co., ~td.] and tested.
That i~, the motor was subjected to 20-hour aging
(load-free operation) and then to 300-hour continuous opera-
tion under a load of 200 gf-cm. The changes of number of
rotation~ with time are shown in Figs. 1 ~o 3.
A~ is clear from Fig. 1, when the friction material
composition of E~ample 1 wa~ used, the number o~ rotations
showed no sharp reduction for 100 hour~ and was stable.
,.,.. ",
. . ;
.
. . .,, .. . ~ :,
. "
2~ jS9
In con~rast, when the friction material compositions
of Comparative E~amples 1 and 2 were used, as is clear from
Figs. 2 and 3, the number of rotations showed sharp reduc-
tion several times each for several minutes in khe time pe-
riod of 10 hours in the case of the friction material compo-
~ition of Comparative Example 1, and there was frequent al-
ternation of fast rotation and slow rotation even in the
~ime period of one hour in the case of the friction material
composition of Comparative Example 2.
The friction material composition of the present in-
vention comprises at least a polyacrylonitrile based carbon
fiber, a fiber component other than said polyacrylonitrile-
based carbon fiber, and a thermosetting resin, and has ex-
cellent abrasion resistance and yet gives rise to no sharp
reduction in number of rotations during continuous opera-
tion.
The reasons for these effects are considered to be
that while a pitch-based carbon fiber or the like, when
worn, genera~e3 a hard powder and invites reduction in num-
ber of rotations by acting like rollers, the polyacryloni-
trile-based carbon fiber, when worn, generates a very fine
or soft powder and invites no reduction in number of rota-
tions .
... .
. . ,
'
,
