Oxidatively stable polyimide compositions

Graphite-filled polyimide compositions of excellent high temperature stability obtained through use of graphite having low reactive impurity content.

BACKGROUND OF THE INVENTION 
Polyimides, such as those prepared according to Edwards, U.S. Pat. No. 
3,179,614, are useful in a wide variety of commercial applications. The 
outstanding performance characteristics of these polymers under stress and 
at high temperatures have made them useful in the form of bushings, seals, 
electrical insulators, compressor vanes and impellers, pistons and piston 
rings, gears, thread guides, cams, brake linings, and clutch faces. While 
basically non-melt fabricable, these polyimide resins can be molded into 
the desired final shape by specialized fabrication techniques. 
It is often desirable to incorporate fillers in such polyimide compositions 
before fabrication into their final form. For example, the admixture of 
graphite in a polyimide intended for a bearing surface gives a lubricating 
effect which improves the wear characteristics of the final product. The 
graphite is typically incorporated in the course of preparation of the 
polyimide by precipitation of the polyimide resin on the graphite 
particles. 
While the addition of graphite to polyimides has contributed significantly 
to the wear characteristics of the final polyimide product, incorporation 
of graphite also has generally resulted in a depreciation of physical 
properties under prolonged exposure to high temperatures. Specifically, 
the polyimide exhibits an undesirable weight loss, shrinkage, and loss of 
tensile strength and elongation. 
SUMMARY OF THE INVENTION 
The present invention provides a graphite-filled polyimide composition 
which exhibits improved physical properties when subjected to higher 
temperatures for extended periods of time. 
Specifically, the instant invention provides, in a non-melt fabricable 
polyimide composition containing about from 5 to 75 volume percent 
graphite, the improvement wherein the graphite contains less than about 
0.15 weight percent of at least one reactive impurity selected from the 
group consisting of ferric sulfide, barium sulfide, calcium sulfide, 
copper sulfide, barium oxide, calcium oxide and copper oxide. 
DETAILED DESCRIPTION OF THE INVENTION 
The present invention is applicable to those polyimide compositions 
described, for example, in U.S. Pat. Nos. 3,179,614 and 3,179,631, both 
hereby incorporated by reference. Graphite, commercially available in a 
wide variety of forms as a fine powder, is typically admixed with a 
polymer solution before precipitation of the polyimide. The particle size 
of the graphite can vary widely, but is generally in the range of about 
from 5 to 75 microns. Preferably, for particularly good oxidative 
stability, the average particle size is about from 5 to 25 microns. The 
total concentration of the graphite introduced into the resin varies, of 
course, with the final wear properties desired and the particular end use 
application. However, in general, the graphite concentration is about from 
5 to 75 percent by volume. 
The present invention is based on the discovery that the depreciation of 
physical properties on high temperature aging previously encountered was 
due to the presence of reactive impurities in the graphite that had an 
adverse effect on the oxidative stability of the final polymeric blend. 
Specifically, it has been found that markedly improved physical properties 
can be obtained using graphite having less than about 0.15 weight percent 
reactive impurities, and preferably less than about 0.10 weight percent. 
Particularly deleterious reactive impurities include iron sulfide and the 
oxides and sulfides of barium, calcium, and copper. 
The level of total inorganic impurities can be measured as the weight of 
ash residue of pyrolyzed graphite. The presence and quantity of reactive 
or catalytically active impurities can be determined by emission 
spectroscopy or X-ray fluorescence. In general, the reactive impurities in 
graphite constitute about one-half of the total inorganic impurities. 
The unusually pure graphite used in accordance with the instant invention 
can be either naturally occurring graphite or synthetic graphite. Natural 
graphite generally has a wide range of impurity concentrations, while 
synthetically produced graphite is commercially available having low 
reactive impurity concentrations. Graphite containing an unacceptably high 
concentration of impurities can be purified by chemical treatment with a 
mineral acid. For example, treatment of the impure graphite with sulfuric, 
nitric or hydrochloric acid at elevated or reflux temperatures can be used 
to reduce the impurities to an acceptable level. Alternatively, commercial 
graphite compositions are available that typically satisfy the purity 
levels required in the instant invention, such as "Dixon Airspun KS-5" 
commercially available from The Joseph Dixon Crucible Co. 
The compositions made in accordance with the present invention exhibit 
improved physical properties after exposure to elevated temperatures of 
200.degree. to 400.degree. C., both at atmospheric and elevated pressures. 
The improved properties include markedly reduced weight loss and shrinkage 
and significantly higher tensile strength and elongation at break after 
high temperature aging than are found using conventional graphite fillers. 
These improved physical properties permit the use of the present polyimide 
compositions in a variety of high temperature applications, such as 
aircraft jet engines, in which outstanding, long-term, high temperature 
performance is required.

The present invention is further illustrated by the following specific 
examples. 
In each of the examples, polyimide resins were prepared from pyromellitic 
dianhydride and 4,4'-oxydianiline according to the procedures of U.S. Pat. 
No. 3,179,614. The indicated quantities of graphite powder were 
incorporated into the polymer solution prior to precipitation. The 
resulting filled resin powder was then converted into standard ASTM-E8 
tensile bars having a nominal thickness of 0.10" by direct forming at a 
pressure of 100,000 psi. The resulting molded test bars were sintered for 
three hours at 400.degree. C. under nitrogen at atmospheric pressure. 
After cooling to room temperature, the test bars were marked for 
identification, weighed and measured in width and thickness. 
The tensile bars were tested for high temperature oxidative stability by 
treating at 360.degree. C., either at atmospheric pressure or elevated 
pressures of 70 psia. 
The total inorganic impurity concentration of the graphite was measured by 
burning the graphite at atmospheric pressure at a temperature of 
600.degree.-700.degree. C. and weighing the inorganic ash residue. 
The tensile bars were tested for tensile strength and elongation according 
to ASTM-E-8. 
EXAMPLES 1 TO 2 AND COMATIVE EXAMPLES A TO C 
In Examples 1 and 2, tensile bars were prepared using a commercial 
polyimide resin prepared from pyromellitic dianhydride and 
4,4'-oxydianiline combined with 10 and 25 volume percent of Dixon KS-5 
graphite having less than 0.15 weight percent total inorganic impurities, 
about half of which are reactive impurities. In Comparative Examples A to 
C, tensile bars were prepared from the same resin using 0, 10 and 27 
volume percent of Dixon 200-09 graphite containing about 2 percent total 
inorganic impurities. The test bars were heated at 360.degree. C. with one 
atmosphere of flowing air (1.5 liters/min) for a period of 120 hours. The 
tensile bars were tested before and after heat treatment and the results 
are summarized in Table I. 
TABLE I 
______________________________________ 
TS, MPa/E, % 
Ex- Vol % After % % 
ample Graphite Initial 120 hrs Wt Loss 
Shrinkage 
______________________________________ 
1 10 81.4/10.0 
57.2/2.8 
1.16 0.73 
2 25 69.2/5.3 56.5/2.6 
0.85 0.36 
A 0 74.5/8.0 38.5/1.9 
1.55 0.46 
B 10 66.3/7.1 36.4/1.7 
5.58 1.13 
C 27 51.2/3.6 30.8/1.4 
9.56 1.96 
______________________________________ 
EXAMPLES 3 & 4 AND COMATIVE EXAMPLES D, E, & F 
The procedure of Examples 1 and 2 and Comparative Examples A to C was 
repeated, except that the testing was carried out at a pressure of 70 psia 
in air and the tensile bars were heated for 100 hours instead of 120 
hours. The tensile bars were tested before and after heat treatment and 
the results are summarized in Table II. 
TABLE II 
______________________________________ 
TS, MPa/E, % 
Ex- Vol % After % % 
ample Graphite Initial 100 hrs Wt Loss 
Shrinkage 
______________________________________ 
3 10 81.3/16.0 
45.2/1.7 
3.01 0.60 
4 25 69.2/5.3 45.4/1.6 
1.96 0.10 
D 0 74.5/8.0 24.1/0.8 
3.73 0.50 
E 10 66.3/7.1 23.1/0.8 
13.41 0.64 
F 27 51.2/3.6 14.2/0.4 
20.00 0.68 
______________________________________ 
EXAMPLES 5 to 8 AND COMATIVE EXAMPLES G & H 
The procedure of Example 2 was repeated, using Dixon Airspun KS-5 synthetic 
graphite in all examples. The percentage of total inorganic impurities in 
the graphite varied as summarized in Table III. The tensile bars were 
treated for 200 hours at 360.degree. C. and the bars tested before and 
after heat treatment. The test results are also summarized in Table III. 
TABLE III 
______________________________________ 
Ex- TS, MPa/E, % % 
am- Vol % Graphite After % Wt Shrink- 
ple Graphite % Ash Initial 
200 hrs 
Loss age 
______________________________________ 
5 25 0.12 59.3/3.3 
42.2/2.1 
3.5 1.12 
6 " 0.12 57.9/3.4 
45.0/2.2 
3.4 0.76 
7 " 0.12 60.1/3.8 
43.1/2.3 
3.8 0.76 
8 " 0.12 58.6/3.5 
40.0/2.1 
3.7 0.72 
G " 0.96 60.7/3.2 
28.5/1.2 
21.6 3.50 
H " 0.24 58.2/3.6 
29.4/1.2 
11.6 1.68 
______________________________________ 
EXAMPLES 9 to 15 
The procedure of Examples 1 and 2 was repeated, except that the tensile 
bars were tested for 200 hours, and the total inorganic impurity content 
of the graphite varied from 0.13 percent to 0.044 percent. The test 
results are summarized in Table IV. 
TABLE IV 
______________________________________ 
Ex- TS, MPa/E, % % 
am- Vol % Graphite After % Wt Shrink- 
ple Graphite % Ash Initial 
200 hrs 
Loss age 
______________________________________ 
9 25 0.130 68.9/3.9 
44.7/1.6 
2.63 0.63 
10 25 0.080 58.0/3.6 
40.6/1.4 
1.81 0.40 
11 25 0.046 60.6/4.9 
42.2/1.6 
1.70 0.45 
12 25 0.044 64.3/4.0 
44.7/1.3 
1.37 0.31 
13 10 0.046 77.2/8.6 
41.6/1.9 
2.00 0.63 
14 10 0.081 72.7/6.5 
38.2/1.8 
1.96 0.63 
15 10 0.044 76.3/9.7 
41.2/1.9 
1.66 0.50 
______________________________________ 
The test results indicate no significant effect on the oxidative stability 
of the molded compositions with a variation in total inorganic impurity 
content within the range of about from 0.04 to 0.13. 
EXAMPLES 16 TO 18 AND COMATIVE EXAMPLE I 
The procedure of Examples 5 to 8 and Comparative Examples G and H was 
repeated. In Comparative Example I, a graphite was used which contained an 
unacceptably high level of impurities. In Examples 16 to 18, the same 
graphite was treated with acids to remove impurities. The treatment was 
carried out at temperatures of 80.degree. to 100.degree. C. with six 
normal acid concentrations for a period of two hours. The results are 
summarized in Table V. 
TABLE V 
______________________________________ 
Ex- Graph- TS, MPa/E, % 
% 
am- Acid ite Wt After Wt % 
ple Treatment % Ash Initial 
200 hrs 
Loss Shrink 
______________________________________ 
I None 2.52 55.60/4.2 
31.3/1.5 
12.2 1.6 
16 HCl 1.74 68.9/4.1 
54.7/1.8 
1.8 0.4 
17 H.sub.2 SO.sub.4 
1.87 68.2/3.6 
46.7/1.3 
1.8 0.4 
18 HCl--HNO.sub.3 
1.63 70.0/4.8 
49.7/2.2 
1.3 0.3 
______________________________________ 
EXAMPLES 19 TO 23 
The procedure of Examples 5 to 8 was repeated, except that increased 
graphite concentrations were used. The results, which are summarized in 
Table VI, indicate that graphite loadings as high as 70 volume percent do 
not affect oxidative stability of the polyimide composition. 
TABLE VI 
______________________________________ 
TS/E - MPa/% 
Ex- Vol % After % Wt % 
ample Graphite Original 200 hrs 
Loss Shrinkage 
______________________________________ 
19 40 6.14/3.0 45.1/1.2 
1.37 0.31 
20 50 49.9/2.0 37.6/0.9 
1.41 0.44 
21 60 43.6/1.5 31.4/0.7 
1.30 0.31 
22 63 44.3/1.5 30.4/0.6 
1.25 0.27 
23 70 38.5/0.8 26.8/0.5 
1.49 0.22 
______________________________________