WO1995020005A1

WO1995020005A1 - Polyimide composition having improved properties

Info

Publication number: WO1995020005A1
Application number: PCT/US1995/000378
Authority: WO
Inventors: Daniel Eugene George
Original assignee: E.I. Du Pont De Nemours And Company
Priority date: 1994-01-21
Filing date: 1995-01-19
Publication date: 1995-07-27
Also published as: JPH09508161A; CA2179487A1

Abstract

Polyimide compositions containing natural graphite powder, along with carbon or graphite fiber, exhibit an unusual combination of low wear and friction and low coefficient of thermal expansion.

Description

TITLE

POLY IMIDE COMPOSITION HAV I NG IMPROVED PROPERTI ES

BACKGROUND OF THE INVENTION Polyimide compositions, such as those described in Edwards, U.S.

Patent 3,179,614, can be used in a wide variety of commercial applications. The outstanding performance characteristics of polyimide compositions 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. It is often desirable to incorporate various additives in such polyimide compositions before fabrication into their final form. Accordingly, graphite has been incorporated to improve the wear characteristics of such compositions in bearing applications. Diamonds have been incorporated for abrasive applications. Fluoropolymers have been incorporated in the past for lubricity in forming and extrusion of shapes.

Despite the variety of polyimide compositions and additives that have previously been available, a continuing need for polyimide compositions, particularly when processed into the shape of bushings and bearings, is a reduction in the thermal expansion of such materials. In bushings and bearings, close clearances to adjacent metal surfaces are needed, in combination with excellent wear characteristics. U.S. patent 5,284,904 discloses polyimide compositions containing graphite powder and wollastonite, said compositions having both low wear and friction performance and a low coefficient of thermal expansion. However, it is still desirous to continue to develop other and better polyimide compositions having both low wear and friction performance and a low coefficient of thermal expansion.

In the present invention, it was found that a polyimide composition containing natural graphite powder, along with carbon or graphite fiber filler, exhibits greatly reduced thermal expansion compared to the same composition containing no fiber filler. The composition also was found to exhibit low wear and friction performance against a steel mating surface, superior to the same composition containing no fibrous carbon or graphite filler. In contrast, the polyimide compositions of the above- referenced U.S. patent application are taught to be, with respect to wear and friction performance against a steel mating surface, only equivalent to the same compositions without wollastonite.

SUMMARY OF THE INVENTION The present invention provides a polyimide composition comprised of complemental quantities of (a) about 8-69 parts by weight of at least one polyimide, (b) about 30-80 parts by weight of natural graphite powder, and (c) about 1-12 parts by weight of carbon or graphite fibrous filler. DETAILED DESCRIPTION OF THE INVENTION

The compositions of the present invention contain about 8-69 parts by weight of at least one polyimide and preferably about 30-56 parts by weight. Parts by weight, as used herein, are indicated as parts per 100 parts of the total weight of the polyimide, the natural graphite powder, and the carbon or graphite fibrous filler. A wide variety of polyimides can be used, including those described in Edwards, U.S. Patent 3,179,614, hereby incorporated by reference. The polyimides described therein are prepared from at least one diamine and at least one anhydride. Preferred diamines which can be used include m-phenylene diamine (MPD), p-phenylene diamine (PPD), oxydianiline (ODA), methylene dianiline (MDA) and toluene diamine (TDA). Preferred anhydrides which can be used include benzophenone tetracarboxylic dianhydride (BTDA), biphenyl dianhydride (BPDA), trimellitic anhydride (TMA), pyromellitic dianhydride (PMDA), maleic anhydride (MA) and nadic anhydride (NA). Preferred polyimides which can be used in the present invention include those prepared from the following combinations of anhydride and diamine: BTDA-MPD, MA-MDA, BTDA-TDA-MDA, BTDA-MDA-NA, TMA-MPD & TMA-ODA, BPDA-ODA & BPDA-PPD, BTDA-4,4'- diaminobeπzophenone, and BTDA-bis (p-aminophenoxy)-p,p'-biphenyl. An especially satisfactory polyimide useful in the present invention is that prepared from pryomellitic dianhydride and 4,4 '-oxydianiline.

The polyimide compositions of the present invention also contain about 30-80 parts by weight of natural graphite powder. Less than about 30 parts by weight of the natural graphite powder could significantly diminish the frictional properties required for bushing and bearing applications. Greater than about 80 parts by weight of the natural graphite powder may result in a depreciation of the structural integrity of the final product and loss of the outstanding mechanical properties for which polyimides are noted. Preferably, the natural graphite powder comprises about 40-70 parts by weight. The graphite powder used in the compositions of the present invention is naturally-occurring. Natural graphite powder, in combination with carbon or graphite fiber in the specified concentration, provides reduced wear and friction. For contrast, when all synthetically-produced graphites are used in combination with carbon or graphite fiber in a polyimide composition, the wear and friction properties of the composition are increased significantly.

A central feature of the compositions of the present invention is the incorporation of about 1-12 parts by weight of chopped or milled carbon fiber or graphite fiber. As little as 1 part by weight of the carbon or graphite fiber incorporated into the polyimide composition will significantly reduce the coefficient of linear thermal expansion of the composition, as compared to a comparable graphite powder/polyimide composition. In addition, the composition containing graphite powder along with the carbon or graphite fiber, when tested against a steel mating surface, shows wear and friction performance superior to a comparable graphite powder/polyimide composition, even at high PV (pressure x velocity) conditions. Greater than 12 parts by weight of the carbon or graphite fiber can cause variable friction behavior and can depreciate the overall mechanical properties of parts made from these compositions. Preferably, about 4-10 parts by weight carbon or graphite fiber is used herein.

The present compositions can further contain up to about 10 parts by weight of other additives, fillers, and dry lubricants which do not depreciate the overall performance characteristics of the finished polyimide parts, as will be evident to those skilled in the art. Typical of such additional additives are tungsten disulfide and molybdenum disulfide.

In the preparation of the present compositions, the order of additional of components is not critical. The three basic components (_.&, the polyimide, the natural graphite powder, and the carbon or graphite fiber) can be blended using conventional milling techniques in the required quantities. In the alternative, some commercially available polyimides contain encapsulated graphite, to which additional graphite can be added by conventional blending techniques, if desired. The natural graphite powder and carbon or graphite fiber can also be conveniently incorporated into the polyimide, as an alternative to milling techniques, by blending into a polymer solution of polyimide precursors prior to precipitation as the polyimide. This lattermost preparation technique is preferred.

The polyimide compositions of the present invention, when processed into parts, are suitable for providing wear surfaces in the form of bushings and bearings where close clearances to adjacent metal surfaces are needed. These bearings include a multitude of small motor bearings. Parts formed from the present compositions exhibit a reduced coefficient of linear thermal expansion by as much as 25% compared to a comparable graphite powder/polyimide composition. In addition, the composition, when tested against a steel mating surface, shows wear and friction performance superior to a comparable graphite powder/polyimide composition, even at high PV (pressure x velocity) conditions. The reduction of expansion coefficient exhibited by the compositions of the present invention, while improving the wear and friction characteristics, appears to be unique in the graphite or carbon fiber/natural graphite powder/polyimide compositions of the present invention. Other fibrous fillers, when incorporated into the relatively hard graphite powder/polyimide composition, also lower thermal expansion by equivalent amounts, but often they are abrasive in nature, causing high friction and excessive wear to the polyimide composition and to the steel mating surface. It is surprising, therefore, that the carbon fiber performs well when used in the range within the scope of this invention. EXAMPLES

In each of the examples below, polyimide resins were prepared from pyromellitic dianhydride and 4,4 '-oxydianiline, according to the procedures of U.S. Patent 3,179,614 or U.S. Patent 4,622,384. The indicated quantity of graphite powder was incorporated into the polymer solution prior to precipitation as the polyimide. Subsequently, carbon fiber (Toray MLD 300, average length, 130 um, diameter, 7 um) was added to the graphite-filled resin by dry blending. The carbon fiber may also be added to the polymer solution prior to precipitation as the polyimide. The fiber was added to the resin as 5 weight percent, 10 weight percent, etc., so that the final concentration was less than the added amount. For example, 5 grams of fiber added to 100 grams of a resin composition containing 60 weight percent graphite yielded a final product of composition 38.1% polyimide, 57.1% graphite, and 4.8% carbon fiber.

The resulting filled polyimide resin powder was converted into test specimens by direct forming at a pressure of 100,000 psi (689 MPa) at room temperature. The resulting parts were sintered for three hours at 400 degrees C under nitrogen at atmospheric pressure. After cooling to room temperature, the parts were machined to final dimensions for test specimens. The 0.25" (6.35 mm) wide contact surface of the wear/friction test block was machined to such a curvature that it conformed to the outer circumference of the 1.375" (34.9 mm) diameter x 0375" (95 mm) wide metal mating ring. The blocks were oven dried and maintained dry over dessicant until tested.

Wear tests were performed using a Falex No. 1 Ring and Block Wear and Friction Tester. The equipment is described in ASTM Test method D2714. The polyimide block was mounted against the rotating metal ring and loaded against it with the selected test pressure. Rotational velocity of the ring was set at the desired speed. No lubricant was used between the mating surfaces. The rings were SAE 4620 steel, Re 58-63, 6-12 RMS. A new ring was used for each test. Test time was usually 24 hours, except when friction and wear were high, in which case the test was terminated early.

The coefficient of linear thermal expansion was determined by a thermomechanical analyzer (ASTM E831). On each specimen it was measured in the direction perpendicular to the direction of forming pressure when the part was made.

Examples 1 to 6 and Comparative Examples A to J (Table 1.

Wear test results for these examples are at 256 psi (1.77 MPa) pressure and 390 fpm (1.98 m/s) velocity for a PV of approximately 100,000 psi-fpm (3.5 MPa-m/s). Examples 1-5 show the effect of adding carbon fiber on the wear, friction, and coefficient of linear thermal expansion (CTE) of a natural graphite-filled polyimide. Specifically, Example 1 and 2 are compositions from the addition of 5 and 10 weight percent, respectively, carbon fiber to the composition of Comparative Example A. All three properties were improved significantly. Example 3 shows that further improvement results from an even higher natural graphite content.

Examples 4 and 5 show similar improvement over the parent composition in Comparative Example B at lower natural graphite content. Example 6 demonstrates good results using a natural graphite from a second source.

The addition of carbon fiber to a synthetic graphite in the polyimide composition in Comparative Example C similarly improved the CTE, but resulted in very high wear and friction compared with its parent composition, Comparative Example D. Synthetic graphite from a second source, Comparative Example E, gave excellent wear and friction results, but these were degraded significantly with the addition of carbon fiber in Comparative Example F. Comparative Examples G through J show that the addition of higher concentrations of carbon fiber can result in either variable friction or very high friction with reduced wear resistance, even when the graphite is natural grade.

Table 1

Coeff

Carbon Wear Coeff of Exp

Graphite Graphite Fiber ccxlO-4 Friction um/m-deg C

No. Type Wt. % Wt. % per hr. Range 35-300 C

1 NAT-A 57.1 4.8 7.4 0.06-0.10 12

2 NAT-A 54.5 9.1 7.8 0.06-0.07 11.5

3 NAT-A 66.7 4.8 6.6 0.05-0.06 10

4 NAT-A 47.6 4.8 7.0 0.06-0.08 14

5 NAT-A 45.5 9.1 8.7 0.10-0.12 14

6 NAT-B 57.1 4.8 6.9 0.09-0.12 15.5

A NAT-A 60.0 0.0 10.8 0.11-0.17 15

B NAT-A 50.0 0.0 11.1 0.18-0.21 15

C SYN-A 57.1 4.8 75.0 >0.60 11.5

D SYN-A 60.0 0.0 19.6 0.16-0.23* 14

E SYN-B 60.0 0.0 6.0 0.06-0.09 16.5

F SYN-B 57.1 4.8 73.3 >0.60 13.5

G NAT-A 43.5 13.0 NM >0.50 11.5

H NAT-A 24.0 20.0 NM >0.52 16

I NAT-A 29.6 20.0 162.0 >0.65 16.5

J NAT-A 52.2 13.0 11.0 0.07-0.38* 12.5

Variable friction

NM Not Measured

NAT-A Natural graphite powder, Southwestern Microcrystal Graphite, grade 200-09

NAT-B Natural graphite powder, Asbury Graphite Mills

SYN-A Synthetic graphite powder, Lonza, KS-10

SYN-B Synthetic graphite powder, Conoco XP 5u Micronized

Example 7 and Comparative Examples K and L (Table 2)

Wear test conditions for the tests in Table 2 are the same as for Table 1 (PV = 100,000 psi-fpm). For Example 7 and Comparative Examples K and L, a chopped graphite fiber (Kureha grade 2007S, approximately 200 um average length and 20 um diameter) was used in place of the carbon fiber. Example 7 shows that the CTE can be reduced and good wear and friction test results maintained when the graphite fiber is combined with natural graphite. Comparative Example K shows that in combination with synthetic graphite, the graphite fiber gives very high friction. Comparative Example L shows that at a higher concentration of graphite fiber, poor wear and friction results, even when the graphite is the natural grade.

Table 2

Coeff

Graphite Wear Coeff of Exp

Graphite Graphite Fiber ccxlO-4 Friction um/m-deg C

No. Type Wt. % Wt. % per hr. Range 35-300 C

7 NAT-A 57.1 4.8 11.9 0.14-0.16 13

K SYN-A 57.1 4.8 32.0 >0.63 11.5 L NAT-A 43.5 13.0 141.5 >0.59 13.5

Examples 8 and 9 and Comparative Examples M and N (Table 3.

Wear and friction test results for the examples in Table 3 are at 192 psi (1.32 MPa) pressure and 134 fpm (0.68 m/s) for a PV of approximately 25,000 psi-fpm (0.9 MPa-m/s). Examples 8 and 9 show that even at a lower PV, friction performance is improved compared with the parent composition in Comparative Example M. Wear and friction are both superior compared with Comparative Example N, in which carbon fiber is combined with synthetic graphite.

Table 3

Coeff

Carbon Wear Coeff of Exp

Graphite Graphite Fiber ccxlO-4 Friction um/m-deg C

No. Type Wt. % Wt. % per hr. Range 35-300 C

8 NAT-A 57.1 4.8 1.0 0.13-0.14 12 9 NAT-A 54.5 9.1 0.8 0.11-0.12 11.5

M NAT-A 60 0.0 1.0 0.23-0.27 15 N SYN-A 57.1 4.8 2.7 0.19-0.42 11.5

Claims

CLAIMS I CLAIM;

1. A polyimide composition comprised of complemental quantities of (a) about 8-69 parts by weight of at least one polyimide,

(b) about 30-80 parts by weight of natural graphite powder,

(c) and about 1-12 parts by weight of carbon or graphite fiber, wherein the parts by weight given above are based upon the total weight of components (a), (b), and (c) only.

2. The polyimide composition of Claim 1 wherein the polyimide is a single polyimide.

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

4. The polyimide composition of Claim 1 further containing up to about 10 weight percent of other additives.

5. The polyimide composition of Claim 1 wherein the parts by weight of the component (b) ranges from 40-70.

6. The polyimide composition of Claim 1 wherein the parts by weight of the component (c) range from 4-10.

7. The polyimide composition of Claim 1 comprised of about 30-56 parts by weight of the component (a), about 40-70 parts by weight of the component (b), and about 4-10 parts by weight of the component (c).