Polyimide resin composition for sliding members

A polyimide resin composition for sliding members which has high heat resistance and sliding properties and excels in wear resistance under sliding conditions at high PV values and which can keep low the rate of shrinkage due to crystallization treatment. The composition is made up of 100 parts by weight of resin composition comprising 50-90% by weight of a thermoplastic polyimide resin and 50-10% by weight of graphite obtained by graphitizing a non-phenolic resin material and containing 97% or more of fixed carbon; 5-20 parts by weight of tetrafluoroethylene resin; and 5-30 parts by weight of a powdery hardened phenolic resin.

This invention relates to a polyimide resin composition for sliding members 
used under conditions where dynamically and thermally high loads are 
applied. 
Polyimide resins have high heat resistance and their properties have been 
improved so that they can be used as sliding members under harsh 
conditions where they are subjected to high-load, high-speed sliding 
contacts at high temperatures. 
It is disclosed in Unexamined Japanese Patent Publication 63-8455 to 
improve the sliding properties of polyimide resins by adding 
tetrafluoroethylene resin thereto. Also, Unexamined Japanese Patent 
Publication 63-314274 discloses that the wear resistance of polyimide 
resins is improved by adding thereto hardened phenolic resin as well as 
tetrafluoroethylene resin. 
But such a conventional polyimide resin composition for sliding members, 
comprising a polyimide resin, tetrafluoroethylene resin and hardened 
phenolic resin, tends to be low in wear resistance at temperatures near 
the glass transition temperature (Tg=240.degree. C.) of polyimide resin if 
it works at a high PV rate, e.g. 640 kg/cm.sup.2 m/min. 
In order to improve the wear resistance under the above-described harsh 
conditions it is known, to subject a thermoplastic polyimide resin to heat 
treatment to increase its crystallinity to 25%. But the resin article 
shrinks at a rate of as much as 2-5% when subjected to crystallization 
treatment. Thus, it is difficult to control the dimensions of the end 
products with high accuracy. 
It is disclosed in Unexamined Japanese Patent Publications 4-175373 and 
4-202470 to reduce the rate of shrinkage due to crystallization treatment 
by adding a thermotropic liquid crystal polymer. The liquid crystal 
polymer serves to reduce the coefficient of linear expansion by being 
highly orientated and thus improve the dimensional accuracy of the end 
product. 
But since a composition comprising a thermotropic liquid crystal polymer 
and a thermoplastic polyimide resin is non-compatible, its wear resistance 
scarcely improves. 
It is an object of this invention to provide a polyimide resin composition 
for sliding members which has high heat resistance and excellent sliding 
properties, which has high wear resistance under sliding conditions at 
high PV values and which can keep the rate of shrinkage due to 
crystallization treatment low so that the dimensional accuracy of the 
article can be controlled easily. 
According to the present invention, there is provided a polyimide resin 
composition for sliding members comprising 100 parts by weight of resin 
composition comprising 50-90% by weight of a thermoplastic polyimide resin 
represented by the following formula (1) and 50-10% by weight of graphite, 
the graphite can be obtained by graphitizing a non-phenolic resin material 
or natural graphite can be used. The graphite contains 97% or more of 
fixed carbon; 5-20 parts by weight of tetrafluoroethylene resin; and 5-30 
parts by weight of a powdery hardened phenolic resin. 
##STR1## 
(wherein X is a member selected from the group consisting of a direct 
bond, a hydrocarbon group having a carbon number of 1-10, a 
hexafluorinated isopropylidene group, a carbonyl group, a thio group and a 
sulfone group; R1-R4 may be the same or different and are selected from at 
least one of hydrogen, a lower alkyl group, a lower alkoxy group, chlorine 
or bromine and; Y is a quadrivalent group selected from the group 
consisting of an aliphatic group having a carbon number of two or more, a 
cyclic aliphatic group, a monocyclic aromatic group, a condensed 
polycyclic aromatic group, and a non-condensed polycyclic aromatic group 
in which aromatic groups are bonded together directly or through 
crosslinking agent. 
The above-mentioned graphite containing 97% or more of fixed carbon should 
preferably be scaly natural graphite. 
Further, 1-5 parts by weight of a thermotropic liquid crystal polymer 
expressed by the following formula may be added to the aforementioned 
polyimide resin composition for sliding members. 
##STR2## 
The thermoplastic polyimide resin used in the present invention and 
expressed by the formula (1) can be obtained by cyclodehydrating a 
polyamide acid obtained by reacting an aromatic ether diamine expressed by 
the following formula (3) with one or more tetracarbonic acid-anhydrodes. 
##STR3## 
wherein X is a member selected from the group consisting of a direct bond, 
a hydrocarbon group having a carbon number of 1-10, a hexafluorinated 
isopropylidene group, a carbonyl group, a thio group and a sulfone group; 
and R1-R4 are the same or different and are selected from at least one of 
hydrogen, a lower alkyl group, a lower alkoxy group, chlorine or bromine. 
One of such polyimide resins is sold by Mitsui Toatsu Chemical Co. under 
the name of AURUM (in which R1-R4 in the formula (1) are all hydrogen). 
The graphite used in this invention, which contains 97% or more of fixed 
carbon, may be scale-like natural graphite that is dug out of the ground 
or artificial graphite. It was found out by experiments that among natural 
graphites, scale-like graphite having an average diameter of about 10 
.mu.m is especially preferable in attaining the object of this invention. 
Artificial graphite is preferable which is formed by solidifying coke 
originating from pitch with tar or pitch, calcining it at about 
1200.degree. C. and growing graphite crystals at about 2300.degree. C. in 
a graphitizing furnace. Artificial graphite should be formed, not from a 
phenolic resin, but from pitch, coal tar, coke, wooden material, furan 
resin or polyacrylonitrile. This is because it is not preferable to use 
graphite formed from a phenolic resin in combination with a hardened 
phenolic resin which has been added to the resin composition according to 
the present invention. 
The fixed carbon in the graphite is the component that remains when the 
water, ash and volatile contents have been measured and removed by an 
industrial analysis in a coal testing method. Its main component is carbon 
with trace amounts of hydrogen, oxygen and nitrogen contained therein. If 
the content of fixed carbon is less than 97%, the end product obtained 
will not be satisfactory both in wear resistance and the rate of shrinkage 
due to crystallization treatment. 
The graphite, which contains 97% or more of fixed carbon, should be added 
in an amount of 50-10% by weight in combination with 50-90% by weight of 
thermoplastic polyimide resin to make up the 100 parts of resin 
composition. If over 50% by weight, the melt viscosity of the composition 
would be too large for melt forming. If less than 10% by weight, the wear 
resistance would not be improved sufficiently. 
The tetrafluoroethylene used in this invention should preferably be in a 
powdery form so that it can be homogeneously mixed in the composition. For 
example, it may be in the form of molding powder, fine powder or powder 
obtained by pulverizing a molded and calcined resin by irradiation with 
electron beams or as gamma rays. 
The tetrafluoroethylene resin should be added in an amount of 5-20 parts by 
weight with respect to 100 parts by weight of the composition comprising 
thermoplastic polyimide resin and graphite. If less than 5 parts, it would 
not impart sufficient sliding properties to the thermoplastic polyimide 
resin composition. If over 20 parts, the mechanical strength inherent in 
the thermoplastic resin would be hampered. 
The hardened phenolic resin in a powdery form used in this invention may be 
produced by heating a novolak or resol type phenolic resin, which is 
produced by adding a formalin-producing compound to a phenol, after adding 
known fillers as necessary, with or without crosslinking agents such as 
hexamine, and then pulverizing the thus hardened resin. Methods for 
producing such powder are disclosed in Unexamined Japanese Patent 
Publications 57-17701 and 58-17114. One of such resins is commercially 
available under the name of BELL-PEARL made by Kanebo. 
The phenolic resin should be a heat-unmeltable powdery resin. Preferably, 
such resins should have an average particle diameter of 50 .mu.m or less 
and 80% or more of them should have a particle diameter not exceeding 150 
.mu.m. If over 150 .mu.m, the adhesion between particles would be 
insufficient, so that the mechanical strengths of the formed article such 
as wear resistance and bending strength would decrease. 
Further, the hardened phenolic resin used in this invention should be 
hardened sufficiently. For example, if the degree of hardness is 
represented in terms of the solubility in methanol, the solubility should 
be 20% by weight or less, preferably 15% by weight or less and most 
preferably 5% by weight. If the solubility is higher than 20% by weight, 
foaming would occur while molding. The product thus formed will develop 
gaps and small cracks therein. 
The hardened phenolic resin should be added in an amount of 5-30 parts by 
weight with respect to 100 parts by weight of the composition comprising 
thermoplastic polyimide resin and graphite. If less than 5 parts by 
weight, the wear resistance would scarcely improve. If more than 30 parts 
by weight, the melt viscosity of the composition would be too high for 
melt forming. Also, if too much hardened phenolic resin is added, it is 
impossible to lower the friction coefficient. 
The thermotropic liquid crystal polymer (hereinafter abbreviated as LCP) 
used in the present invention should have a basic structure (II)-(IV) as 
shown in Formula (4). For example, commercially available XYDAR: made by 
Japan Petrochemical, or SUMIKA SUPER: made by Sumitomo Chemical Co., Ltd. 
may be used. 
If 1-5 parts by weight of LCP is added into the composition, the 
flowability while molding can be improved and the rate of shrinkage during 
crystallization can be reduced. If less than 1 part by weight is added, no 
effect such as improvement in flowability or prevention of shrinkage 
during crystallization would appear. If over 5 parts by weight are added, 
the wear resistance would be damaged markedly. 
Various known additives including the following additives may be added 
provided their amounts are controlled so as not to impair achieving the 
object of the present invention. 
The following additives may be added: 
1) Reinforcing agents such as glass fiber, carbon fiber, boron fiber, 
silicon carbide fiber, carbon whisker, asbestos, metallic fiber and rock 
wool; 
2) Flame retardancy improvers such as antimony trioxide, magnesium 
carbonate and calcium carbonate; 
3) Electrical property improvers such as clay and mica; 
4) Crack resistance improvers such as asbestos, silica and graphite; 
5) Thermal conductivity improvers such as iron, zinc, aluminum, copper and 
other metallic powders; and 
6) Other fillers such as glass beads, glass baloons, calcium carbonate, 
alumina, talc, diatomaceous earth, alumina hydrate, shirasu balloons and 
other metallic oxides and inorganic pigments, namely natural or artificial 
compounds which are stable at temperatures above 300.degree. C. 
Means for mixing the above-mentioned component materials is not limited. 
The materials may be fed separately into a melting mixer or two or more of 
the materials may be mixed together beforehand using a general purpose 
mixer such as a Henschel mixer, ball mixer or ribbon blender. In this 
case, they should be mixed at 250.degree.-420.degree. C., preferably at 
300.degree.-400.degree. C. The composition may be molded by compression 
molding or sintering molding. Otherwise, it may be injection-- or 
extrusion-molded after forming a homogeneous molten blend.

The polyimide resin composition for sliding members according to this 
invention has a heat-resistant polyimide resin as its matrix and 
tetrafluoroethylene resin, which is excellent in reducing the friction 
coefficient. Thus, the composition shows high heat resistance and 
excellent frictional properties. Further, by adding predetermined amounts 
of powdery hardened phenolic resin and graphite containing a predetermined 
amount of fixed carbon, the wear resistance improves and the rate of 
shrinkage during crystallization can be reduced. Thus, its dimensions can 
be controlled easily and with high accuracy. 
The raw materials used in Examples and Comparative Examples are listed 
below. The contents are in weight percent. Their abbreviations are shown 
in brackets. 
(1) Thermoplastic polyimide resin (TPI) made by Mitsui Toatsu Chemical Co.: 
AURUM #450. 
(2) Scaly natural graphite (scale graphite) provided by Japan Graphite: ACP 
(containing 99.5% fixed carbon) 
(3 ) Artificial graphite (round graphite) made by LONZA JAPAN: KS10 
(containing 99.5% fixed carbon) 
(4) Earth-particle-like graphite (EPL graphite) made by Japan Graphite: 
Blue P (containing 92.5% fixed carbon) 
(5) Thermotropic liquid crystal polymer (LCP) made by Sumitomo Chemical 
Co., Ltd.: SUMIKA SUPER E5000 
(6) Powdery hardened phenolic resin (PF-1) made by Kanebo: BELL-PEARL C2000 
(average particle diameter: 48 .mu.m) 
(7) Powdery hardened phenolic resin (PF-2) made by Kanebo: BELL-PEARL R900 
(average particle diameter: 22 .mu.m) 
(8) Tetrafluoroethylene resin (PTFE) made by Kitamura: KTL610 
(EXAMPLES 1-8 AND COMATIVE EXAMPLES 1-8) 
Raw materials were added at the rates shown in Tables 1 and 2. After 
dry-blending, they were granulated by extruding at 370.degree.-400.degree. 
C. using a twin-screw melt extruder. The pellets thus obtained were fed 
into an injection molder and injection-molded under the injection pressure 
of 1000 kg/cm.sup.2, keeping the cylinder temperature at 
370.degree.-400.degree. C. and the metal mold temperature at 
150.degree.-200.degree. C. The test pieces thus obtained were measured for 
(1) friction coefficient, (2) wear coefficient, (3) limit PV value, (4) 
flexural modulus and (5) change in dimensions due to crystallization 
treatment in the following ways. The results are shown in Tables 3 and 4. 
(1) Friction coefficient 
Friction coefficients of the test pieces were measured using a thrust type 
friction/wear tester (made by the applicant), causing them to slide on a 
mating member made of SUJ2 at the surface pressure of 5.0 kg/cm.sup.2 and 
the sliding speed of 128 m/minute with no lubrication for 60 minutes. 
Wear coefficient.times.10.sup.-10 
We used the same tester as used in the measurement of friction coefficient. 
The wear coefficients (cm.sup.3 /kgf.m) of the test pieces were measured 
from the results of the wear test in an amorphous state and in the state 
after crystallization treatment when the test pieces were slid on a mating 
member made of SUJ2 at the surface pressure of 5.0 kg/cm.sup.2 and the 
sliding speed of 128 m/minute with no lubrication for 100 hours. 
(3) Limit PV value 
In the wear test, the limit PV values were measured after crystallization 
treatment when the test pieces were slid on a mating member made of SUJ2 
at the sliding speed of 128 m/minute with no lubrication for 100 hours. 
The limit PV values (kg/cm.sup.2 --m/min) are indicated in terms of the 
surface pressures when the friction coefficients exceeded 
100.times.10.sup.-10 cm.sup.3 /kgf m. 
(4) Flexural modulus 
Flexural moduli (kgf/cm.sup.2) were measured under ASTM -D790 at normal 
temperature and at 230.degree. C. 
(5) Change in dimensions due to crystallization treatment 
Twenty thrust washer test pieces of 66.5 mm in outer diameter, 37 mm in 
inner diameter and 2 mm thick (obtained by disc gate forming with the gate 
diameter being 2.5 mm as measured from the center of the inner diameter 
and cutting the inner periphery of the pieces obtained) were subjected to 
crystallization treatment in which they were heated to 320.degree. C. in 
steps for two hours. The test pieces were checked for (a) standard 
deviations in outer diameter, (b) rates of shrinkage and (c) presence of 
warpage as observed with naked eye, before and after the treatment. 
As will be apparent from the results shown in Tables 3 and 4, Comparative 
Examples 1-3, which contained no powdery hardened phenolic resin, showed 
high friction coefficients, and Comparative Example 4, which contained 
graphite containing 97% or more of fixed carbon in the amount exceeding 
the predetermined range, was not moldable. Comparative Example 5, in which 
the content of fixed carbon in the graphite was less than 97% and 
Comparative Example 6, which contained no graphite, showed remarkable 
changes in dimensions when subjected to crystallization treatment. 
Comparative Examples 7 and 8, which contained a liquid crystal polymer, 
were poor in limit PV value and wear coefficient. 
In contrast, Examples 1-8 according to this invention, which satisfy the 
required conditions both in terms of the kinds of materials added and 
their contents, showed sufficiently large limit PV values higher than 
1500. Their other properties such as wear coefficient, mechanical strength 
(flexural modulus), friction coefficient were also satisfactory as sliding 
parts. Change in dimensions after crystallization treatment was small. 
TABLE II 
______________________________________ 
Example 
Content Number 1 2 3 4 5 6 7 8 
______________________________________ 
% by TPI (1) 50 70 90 70 70 70 70 70 
weight 
Scale (2) 50 30 10 -- 30 30 30 30 
graphite 
Round (3) -- -- -- 30 -- -- -- -- 
graphite 
EPL (4) -- -- -- -- -- -- -- -- 
graphite 
Part LCP (5) -- -- -- -- -- -- -- 3 
by PF-1 (6) 20 20 20 20 -- 5 30 20 
weight 
PF-2 (7) -- -- -- -- 20 -- -- -- 
PTFE (8) 10 10 10 10 10 5 20 10 
______________________________________ 
TABLE 2 
__________________________________________________________________________ 
Comparative Example 
Content Number 
1 2 3 4 5 6 7 8 
__________________________________________________________________________ 
% by TPI (1) 50 
100 
70 
40 
70 
100 
100 
100 
weight 
Scale graphite 
(2) 50 
-- 30 
60 
-- 
-- -- -- 
Round graphite 
(3) -- 
-- -- 
-- 
-- 
-- -- -- 
EPL graphite 
(4) -- 
-- -- 
-- 
30 
-- -- -- 
Part LCP (5) -- 
-- -- 
-- 
-- 
-- 5 20 
by PF-1 (6) -- 
-- -- 
20 
20 
20 20 20 
weight 
PF-2 (7) -- 
-- -- 
-- 
-- 
-- -- -- 
PTFE (8) -- 
20 10 
10 
10 
5 5 5 
__________________________________________________________________________ 
TABLE 3 
__________________________________________________________________________ 
Example 
Item Number 
1 2 3 4 5 6 7 8 
__________________________________________________________________________ 
(1) Friction coefficient 
0.15 
0.10 
0.10 
0.10 
0.10 
0.15 
0.15 
0.10 
(2) Friction 
Amorphous 
15 10 30 50 30 70 70 10 
coefficient .times. 
Crystallization 
15 10 30 40 30 60 60 10 
10.sup.-10 
(3) Limit PV value 
&gt;2000 
&gt;2000 
1500 
1500 
&gt;2000 
1500 
1500 
&gt;2000 
(4) Flexurous 
At normal temp. 
70000 
55000 
40000 
50000 
55000 
54000 
50000 
55000 
modulus 
At 230.degree. C. 
50000 
38000 
23000 
33000 
38000 
36000 
31000 
38000 
(5) Change in 
(a) Outer 
0.010 
0.010 
0.015 
0.015 
0.010 
0.015 
0.020 
0.010 
dimension 
diameter SD 
(b) Rate of 
0.6 0.7 0.8 0.7 0.7 0.7 1.0 0.7 
shrinkage 
(c) Warpage 
No No No No No No No No 
__________________________________________________________________________ 
TABLE 4 
__________________________________________________________________________ 
Comparative Example 
Item Number 
1 2 3 *4 5 6 7 8 
__________________________________________________________________________ 
(1) Friction coefficient 
0.25 
0.40 
0.10 
-- 0.25 
0.40 
0.25 
0.25 
(2) Friction 
Amorphous 
&gt;1000 
&gt;1000 
450 -- 300 150 200 650 
coefficient .times. 
Crystallization 
&gt;1000 
&gt;1000 
300 -- 150 50 100 300 
10.sup.-10 
(3) Limit PV value 
300 300 500 -- 500 1000 
500 300 
(4) Flexurous 
At normal temp. 
70000 
30000 
55000 
-- 35000 
35000 
38000 
42000 
modulus 
At 230.degree. C. 
50000 
15000 
38000 
-- 19000 
19000 
19000 
30000 
(5) Change in 
(a) Outer 
0.010 
0.280 
0.020 
-- 0.065 
0.200 
0.098 
0.040 
dimension 
diameter SD 
(b) Rate of 
0.6 4.3 1.0 -- 1.8 3.8 2.3 1.1 
shrinkage 
(c) Warpage 
No Yes No -- Yes Yes Yes No 
__________________________________________________________________________ 
*Not moldable