GB2127423A - Oxidatively stable polyimide compositions - Google Patents

Abstract

Graphite-filled polyimide compositions of excellent high temperature stability are obtained through use of graphite having less than 0.15 weight percent of any of the reactive impurities ferric sulphide and the oxides and sulphides of barium, calcium and copper. There may be from 5 to 75 volume percent of graphite in the polyimide composition.

Description

SPECIFICATION Oxidativelystable polyimide compositions Polyimides, such as those prepared according to Edwards, U.S. Patent 3,179,614, are useful in a wide variety of commercial applications. The outstanding performance characteristics ofthese 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 ofthefinal product. The graphite is typically incorporated in the course of preparation of the polyimide by precipitation ofthe polyimide resin on the graphite particles.

While the addition of graphite to polyimideshas contributed significantly to the wear characteristics of the final polyimide product, incorportation 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 oftensile strength and elongation.

The present invention provides a graphite-filled polyimide composition which exhibits improved physical properties when subjected to highertemper aturesforextended periods of time.

Specifically, the instant invention provides, in a non-meltfabricable 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 ferric sulfide and oxides and sulfides of barium, calcium, and copper.

The present invention is applicable to those polyimide compositions described, for example, in U.S. Patents 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 ofthe polyimide. The particle size of the graphite can vary widely, but is generally in the range of aboutfrom 5to 75 microns. Preferably, for particularly good oxidative stability, the average particle size is about from 5 to 25 microns. The total concentration ofthe 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 aboutfrom 5 to 75 percent by volume.

The present invention is based on the discovery thatthe 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 oftotal 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 ofthetotal 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, commercialgraphite compositions are availablethat 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 afterexposure to elevated temperatures of 200 to 400"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 permitthe 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 ofthe examples, polyimide resins were prepared from pyromellitic dianhydride and 4,4oxydianiline according to the procedures of U.S.

Patent3,179,614. The indicated quantities of graphite powder were incorporated into the polymer solution priorto precipitation. The resulting filled resin powderwasthen 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 art 40000 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 barswere tested for high temperature oxidative stability by treating at 3600C, 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 600-700"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 COMPARATIVE 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 ?5 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 Ato 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"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 summarize in Table I.

TABLE I TS, MPa/E, % Vol % After Example 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 COMPARATIVE EXAMPLES D, E, & F The procedure of Examples 1 and 2 and Compara30 tive ExamplesAto Cwas repeated, exceptthatthe 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.

TaiuLEll TS, MPa/E, % Vol % After Example 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 COMPARATIVE EXAMPLES G & The procedure of Example 2 was repeated, using 40 Dixon Airspun KS-5 synthetic graphite in all examples. The percentage oftotal inorganic impurities in the graphite varied as summarized in Table III. The tensile bars were treated for 200 hours at360 C and the bars tested before and after heat treatment. The test results are also summarized in Table Ill.

TABLE 111 TS, MPa/E, % Vol % Graphite After Example Graphite % Ash Initial 200 hrs Wt Loss Shrinkage 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 thatthe tensile bars were tested for 200 hours, and the total inorganic impurity content ofthe graphite varied from 0.13 percent to 0.044 percent.

The test results are summarized in Table lV.

TABLE lIV TS, MPa/E, % Vol % Graphite After Example Graphite % Ash Initial 200 hrs Wt Loss Shrinkage 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 ofthe molded compositions with a variation in total inorganic impurity content within the range ofaboutfrom 0.04to 0.13.

EXAMPLES 16 TO 18 AND COMPARATIVE EXAMPLE I The procedure of Examples 5to 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 to 100 C with six normal acid concentrations for a period of two hours. The results are summarized in Table V.

TABLE V TS, MPa/E, /O Graphite Acid Wt After Example Treatment % Ash Initial 200 hrs Wt Loss Shrink None 2.52 55.60/4.2 31.3/1.5 12.2 1.6 16 HCI 1.74 68.9/4.1 54.7/1.8 1.8 0.4 17 H2S04 1.87 68.2/3.6 46.7/1.3 1.8 0.4 18 HCL-HNO3 1.63 70.0/4.8 49.7/2.2 1.3 0.3 EXAMPLES 19 TO 23 The procedure of Examples 5to 8was 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 ofthe polyimide composition.

TABLE VI TS/E - MPa/% Vol % After % Wt Example 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

Claims (4)

1. In a non-meltfabricable 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 ferric sulfide and oxides and sulfides of barium, calcium, and copper.

2. The composition of Claim 1 wherein the graphite contains lessthan about 0.10 weight percent ofthe reactive impurities.

3. The composition of Claim 1 or Claim 2 wherein the polyimide is prepared from pyromellitic dianhydride and 4,4'-oxydianiline.

4. A non-meltfabricable polyimide composition substantially as hereinbefore described in any one of the foregoing Examples.