POLYIMIDE COMPOSITIONS WITH IMPROVED WEAR RESISTANCE

Abstract: Polyimide compositions can be substantially improved by incorporating in the composition an inorganic, low hardness, thermally stable, sheet silicate, such as muscovite mica, talc, and kaolinite, which results in improved wear resistance and reduced coefficient of friction.

Description: TITLE POLYIMIDE COMPOSITIONS WITH IMPROVED WEAR RESISTANCE This application claims the benefit of U.S. Provisional Application No. 60/001,062, filed July 11, 1995. BACKGROUND OF THE INVENTION Polyimide compositions, such as those described in U.S. Patent 3, 179,614, may be used in a wide variety of commercial applications. The unique 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, clutch faces, and thrust plugs. Furthermore, blending polyimide compositions with polyamide and polyester resin compositions, such as those described in U.S. Patent 5,346,969, are useful to form parts having a wider range of physical properties, such as improved high temperature performance, with the added benefit that they can be produced through injection molding. Additionally, it is often desirable to incorporate various additives in such polyimide compositions and blends before fabrication into their final form. Accordingly, graphite has been incoφorated into polyimides to improve the wear characteristics of such compositions in bearing applications, diamonds have been incorporated for abrasive applications, and fluoropolymers have been incoφorated for lubricity in forming and extrusion of shapes. Despite the variety of polyimide compositions and additives that have previously been available, there is a continuing need for polyimide compositions and blends which exhibit improved wear resistance and friction at conditions of high pressure and velocity, particularly when processed into the shape of bushings and bearings. Specifically, even with the incoφoration of graphite as a lubricant in a polyimide, its friction at high PV (pressure x velocity) conditions can be too high to allow utility without excessive wear or catastrophic failure. In the present invention, it was found that a polyimide composition containing an inorganic, low hardness, thermally stable, sheet silicate, even at low concentrations, greatly reduced its wear and friction against a steel mating surface at moderate PV values, compared with the same composition which contained no sheet silicate additive. Furthermore, it was found that the "PV limit", that is, the maximum PV value which a composition can tolerate without catastrophic failure, was increased several-fold, compared with the same composition which contained no sheet silicate. Likewise, it was found that blends of polyimide with polyamide and polyester resins exhibited greatly reduced wear and friction characteristics when a sheet silicate was incoφorated into the composition. SUMMARY OF THE INVENTION The present invention resides in the discovery that the wear resistance and coefficient of friction at high pressure x velocity conditions of a polyimide composition can be substantially improved by incoφorating in the composition from about 0.1 weight percent up to about 30 weight percent of at least one of an inorganic, low hardness, thermally stable, sheet silicate. The present invention, therefore, provides an improved polyimide composition which contains (a) about 70-99.9 weight percent of at least one polyimide and (b) about 0.1-30 weight percent of at least one of 0.1-30 weight percent of at least one of an inorganic, low hardness, thermally stable, sheet silicate sheet silicate. The present invention further provides an improved polyimide composition which is a blend of at least one polyimide with at least one other polymer which is melt processible at a temperature of less than 400°C and is selected from polyamide and polyester resins, and the blend includes from 0.1 weight percent up to 30 weight percent of at least one of an inorganic, low hardness, thermally stable, sheet silicate. According to another aspect, the present invention is a method for improving wear resistance and reducing coefficient of friction of a polyimide composition, or a polyimide composition which is a blend of a polyimide with at least one other polymer as defined above selected from polyamide and polyester resins, wherein the method comprises incoφorating into the composition from 0.1 weight percent up to 30 weight percent of an inorganic, low hardness, thermally stable, sheet silicate. In a preferred embodiment ofthe invention, the sheet silicate is present in the composition in the range of from 1 weight percent to 20 weight percent and the silicate is selected from the group consisting of muscovite mica, talc, and kaolinite. DETAILED DESCRIPTION OF THE INVENTION The composition ofthe present invention contains (a) from about 70-99.9 weight percent of at least one polyimide, but generally about 90-99 weight percent, and (b) and from 0.1-30 weight percent of at least one of an inorganic, low hardness, thermally stable, sheet silicate, said weight percents being based solely upon the weight of components (a) and (b). The present invention further contains a polyimide composition which is a blend of from 20-30 weight percent of at least one polyimide, from 45-79.9 weight percent of at least one polymer which is melt processible at a temperature of less than about 400°C and is selected from polyamide and polyester resins, and from 0.1-30 weight percent of at least one of an inorganic, low hardness, thermally stable, sheet silicate. A wide variety of polyimides are suitable for use according to the invention, including those described in U.S. Patent 3,179,614, the teachings of which are incoφorated herein 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 include those prepared from the following combinations of anhydride and diamine: BTDA-MPD, MA-MDA, BTDA-TDA- MPD, BTDA-MDA-NA, TMA-MPD & TMA-ODA BPDA-ODA, BPDA-MPD, BPDA-PPD, BTDA-4,4'-diaminobenzophenone, and BTDA-bis(p-phenoxy)-p,p'- biphenyl. An especially satisfactory polyimide useful in the present invention is that prepared from pyromellitic dianhydride and 4,4-oxy dianiline (PMDA-ODA). The polyimide compositions ofthe present invention contain from about 0.1 to 30 weight percent of an inorganic, low hardness, thermally stable, sheet silicate, such as muscovite mica [KAl3Si3θ]o(OH)2], talc [Mg3Si4θ j o(OH)2], and kaolinite [Al2Si2θ5(OH)4], and mixtures thereof. Sheet silicates of this kind have strong two-dimensional bonding within the silicate layers, but weak inter-layer bonding, which gives rise to lubricating characteristics of a platey compound such as graphite. For the puφose of this invention, the term inorganic is meant to include sheet silicates which occur naturally as well as those which may be synthesized in a lab. Low hardness is desirable to preclude abrasiveness toward the mating surface. Hardness is a mineral's ability to resist scratching of its smooth surface. Mohs Scale of Hardness is known to those skilled in the art to be the scale wherein talc has a hardness of 1 (least hard), and a diamond has a hardness of 10 (most hard). For the compositions of this invention, low hardness is understood to be less than 5. In addition, maintaining phase stability of crystal structure ofthe sheet silicates is critical, as is maintaining thermal stability ofthe sheet silicates' structural water at temperatures of up to 450 °C, as shown by thermogravimetric analysis (TGA). Thermal loss ofthe structural water during processing ofthe polyimide composition can result in harm to polyimide integrity, and possibly change the crystal structure ofthe sheet silicate, giving a harder, more abrasive compound. Examples of sheet silicates which are not stable enough to be included in this invention are montmorillonite [(l/2Ca,Na)0.35(Al 3 Mg)2(Si,Al)4O 10 (OH)2.nH 2 O], vermiculite [(Mg,Ca)0.35(Mg,Fe,Al) 3 (Al,Si)4θι o(OH) 2 .4H2θ], and pyrophyllite [A-2S-4O1 o(OH)2]. Also, inorganic compounds which have a three-dimensional structure rather than a sheet structure, such as silica (S1O2), barite (BaSO4) ^d calcite (CaCOβ), do not have the beneficial effects ofthe compounds included in this invention. Dramatic improvements in the wear and friction characteristics of the polyimide have been seen with about 1 weight percent of one ofthe sheet silicates. At amounts above about 30 weight percent, wear resistance can be affected because of an overall reduction in physical properties. Preferably, the compositions comprise about 0.1-20 weight percent of sheet silicate. Most preferably, the compositions comprise about 1-10 weight percent of sheet silicate. The polyimide compositions ofthe present invention can also contain a blend of at least one polyimide with at least one other polymer which is melt processible at a temperature of less than about 400°C and is selected from polyamide and polyester resin and may be present in a concentration of from about 45 to 79.9 weight percent. Melt processible is used in its conventional sense, that the polymer can be processed in extrusion apparatus at the indicated temperatures without substantial degradation ofthe polymer. Such polymers include polyamides or polyesters. A wide variety of polyamides and/or polyesters can be blended with polyimides. For example, polyamides which can be used include nylon 6, nylon 6,6, nylon 610 and nylon 612. Polyesters which can be used include polybutylene terephthalate and polyethylene terephthalate. A fusible or melt processible polyamide or polyester can be, and preferably is, in the form of a liquid crystal polymer (LCP). LCP's are generally polyesters, including, but not limited to, polyesteramides and po-yesterimides. LCP's are described by Jackson et al., for example, in U.S. Patent 4,169,933, 4,242,496 and 4,238,600, as well as in "Liquid Crystal Polymers: VI Liquid Crystalline Polyesters of Substituted Hydroquinones." The specific LCP used in the present invention is not critical, so long as the basic amide or ester moiety is present. The present composition can further include other additives, fillers and dry lubricants which do not depreciate the overall characteristics ofthe finished polyimide parts, as would be evident to those skilled in the art. The additives may be present in an amount of up to about 60 weight percent based upon the total weight ofthe composition. In particular, the incoφoration of graphite into the composition can extend the range of its utility as a wear resistant material. Another beneficial additive is carbon fiber for the puφose of reducing coefficient of thermal expansion. In the preparation ofthe present compositions, the order of addition of components is not critical. The two basic components, the polyimide and the inorganic, sheet silicate may be blended in the required quantities using conventional milling techmques. The sheet silicate may also be conveniently incoφorated into the polyimide, as an alternative to milling, by blending into a polymer solution of polyimide precursors prior to precipitation as the polyimide. The lattermost preparation technique is preferred. Similar preparation methods may be used for blends ofthe polyimide, sheet silicate, and polyamide or polyester resin. The polyimide compositions ofthe present invention, when processed into parts, are suitable for providing wear surfaces in the form of bushings, seals, thrust washers, compressor vanes and impellers, pistons and piston rings, gears, and cams, especially where the operating conditions involve high PV (pressure x velocity) conditions. The addition of 1-8 weight percent of a sheet silicate to a polyimide reduced its wear at a PV of 100,000 psi-fpm (3.5 MPa-m/s) by at least 10-fold, and as great as 35-fold, compared with the same polyimide without the sheet silicates. Such compositions also exhibited greatly reduced friction at this PV and higher so that the maximum PV which the pure polyimide could tolerate without failure was increased by at least 6-fold. The blends ofthe present invention are useful in a wide variety of physical configurations, including, for example molded articles, films and fibers. The blends can be injection molded using conventional techniques which substantially broadens the applicability ofthe polyimides. The present invention is further illustrated by the following specific Examples and Comparative Examples. EXAMPLES In each ofthe 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 inorganic, sheet silicate and other additives were incorporated into the polymer solution prior to precipitation as the polyimide. 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°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 ofthe wear/friction test block was machined to such a curvature that it conformed to the outer circumference ofthe 1.375" (34.9 mm) diameter x 0.375" (9.5 mm) wide metal mating ring. The blocks were oven dried and maintained dry over desiccant 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. After weighing, the dry polyimide block was mounted against the rotating metal ring and loaded against it with the selected test pressure. Rotational velocity ofthe ring was set at the desired speed. No lubricant was used between the mating surfaces. The rings were SAE 4620 steel, Rc 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 friction force was recorded continuously. At the end ofthe test time, the block was dismounted, weighed, and the wear calculated by the following calculation: weight loss (grams) wear volume (cc/hr.) = material density (g/cc) x test duration (brs.) PV (pressure x velocity) limit tests were performed using the same Falex No. 1 Ring and Block Wear and Friction Tester. In these tests, wear blocks and rings already tested at a PV of 100,000 psi-fpm (3.5 MPa-m/s) were restarted at this same PV. At intervals of 15-20 minutes, the PV was increased in increments by increasing the velocity to a maximum of 1365 fpm (6.93 m/s), after which the load was increased until failure was achieved. Failure was defined as the rapid and uncontrollable rise in friction. The friction force was recorded continuously. EXAMPLES 1 TO 15 AND COMPARATIVE EXAMPLES A AND B In Examples 1 to 15, a polyimide resin prepared from pyromellitic dianhydride and 4,4'-oxydianiline, as described in U.S. Patent No. 4,360,626, was precipitated in the presence ofthe sheet silicate additive present in such quantity as to yield the percentage sheet silicate shown in Table I. No other additives were present. In Comparative Examples A and B, a polyimide resin was prepared from pyromellitic dianhydride and 4,4'-oxydianiline, but contained no sheet silicate or other additives. The samples were tested for wear and friction as described in the procedure above at PV's (pressure x velocity) of 100,000 psi-fpm (3.5 MPa-m/s) and 50,000 psi-fpm (1.75 MPa-m/s). At 100,000 psi-fpm, Examples 1 through 9, containing amounts of sheet silicate ranging from 1.0 to 7.4 weight percent , show an improvement in wear rate of 10 to 35 times, compared with Comparative Example A containing no sheet silicate tested at the same conditions. The coefficient of friction of Examples 1 through 9 is less than one-half that of Comparative Example A. Moreover, the coefficient of friction range is much smaller, that is, more constant than that of Comparative Example A. Similarly, at a PV of 50,000 psi-fpm, Examples 10 through 15, containing amounts of sheet silicate of 1.0 to 3.8 weight percent, show an improvement in wear rate of 6 to 20 times compared with Comparative Example B containing no sheet silicate tested under the same conditions. Again, the coefficient of fπction of Examples 10 through 15 is less than that of Comparative Example B and the friction range is much smaller The test results are summarized in Table I TABLE I Polyimide Coefficient of Ex Sheet Silicate Wt Wear Volume Friction No Additive % ccx 10-4/hour Range PV = = 100,000 psi-fpm (3 5 MPa-m/s) 256 psi, 390 fp (177 MPa, 198 m/s 1 Kaolinite 74 87 007-008 2 Kaolinite 38 87 009-011 3 Kaolinite 10 56 007-009 4 Talc 74 115 005-007 5 Talc 38 100 007-009 6 Talc 10 95 006-008 7 Muscovite 74 194 007-010 8 Muscovite 38 159 006-010 9 Muscovite 10 72 007-009 A None 00 2007 025-038 PV = = 50,000 psi-fpm (175 MPa-m/s) 190 psi, 268 φm (130 MPa, 135 ms; 10 Kaolimte 38 49 011-012 11 Kaolinite 10 41 009-013 12 Talc 38 125 013-017 13 Talc 10 74 012-020 14 Muscovite 38 104 011-013 15 Muscovite 10 41 012-016 B None 00 829 015-049 EXAMPLES 16 TO 29 AND COMPARATIVE EXAMPLES C TO H The procedure for preparation of Examples 1-15 and Comparative Examples A and B was repeated for Examples 16-29 and Comparative Examples C-H, starting with pyromellitic dianhydride and 4,4'-oxydianiline monomers and including graphite and carbon fiber in some ofthe examples in addition to the sheet silicates. Samples were tested for PV (pressure x velocity) limit, (denoted as "PV of Failure" in the tables below), as described in the procedure above. The polyimide containing no additives was not included as a Comparative Example since when tested at 100,000 psi-φm (3.5 MPa-m/s), its high and variable fπction showed that it was already at or above its PV limit. Examples 16-20 show that the inclusion of one ofthe sheet silicates, kaolinite, talc, and muscovite, gives a consistently low friction at high PV's and a PV limit of at least 700,000 psi-φm (24.5 MPa-m/s). Examples 21 and 22 compared with Comparative Example C show that the sheet silicate addition to a composition containing 15 weight percent graphite increased its PV limit by about 2-fold. Examples 23-27 compared with Comparative Example D show that addition of a sheet silicate to a composition containing 37 weight percent graphite increased its PV limit by at least 3 -fold. Examples 28 and 29 compared with Comparative Example E show that when carbon fiber is included as one ofthe components together with graphite, the PV limit is increased about 2-fold by the inclusion of a sheet silicate. Comparison of Examples 23-27 with Comparative Examples F-H show that the performance of compositions containing the thermally stable sheet silicates, kaolinite, talc, or muscovite is greatly superior to that of compositions containing one ofthe less stable sheet silicates, pyrophyllite and montmorillonite. The test results are summarized in Table II. TABLE H PV of < -.oefficient of Friction Failure Ex. Silicate Wt. Other Wt. Range Before kpsi-φm No Additive % Additive % Failure (MPa-m/s) 16 Kaolinite 8 None 0 0.09-0.13 > 700 (> 24.5) 17 Kaolinite 4 None 0 0.10-0.14 > 785 (> 27.5) 18 Kaolinite 1 None 0 0.09-0.11 700 (24.5) 19 Talc 8 None 0 0.06-0.11 700 (24.5) 20 Talc 4 None 0 0.08-0.14 > 875 (> 30.6) 21 Muscovite 20 Graphite 13 0.05-0.11 875 (30.6) 22 Kaolinite 8 Graphite 14 0.08-0.12 > 875 (> 30.6) 23 Kaolinite 20 Graphite 31 0.03-0.08 > 700 (> 24.5) 24 Talc 20 Graphite 31 0.03-0.11 > 875 (> 30.6) 25 Muscovite 20 Graphite 31 0.03-0.06 > 700 (> 24.5) 26 Kaolinite 8 Graphite 34 0.03-0.12 > 875 (> 30.6) 27 Talc 8 Graphite 34 0.10-0.13 > 875 (> 30.6) 28 Talc 8 Graphite 56 0.06-0.08 785 (27.5) C Fiber 5 29 Muscovite 5 Graphite 57 0.06-0.13 700 (24.5) C Fiber 5 C None 0 Graphite 15 0.16-0.18 440 (15.4) D None 0 Graphite 37 0.11-0.25 250 (8.8) E None 0 Graphite 60 0.04-0.07 350 (12.3) C Fiber 5 F Pyrophyllite 20 Graphite 31 0.11-0.11 150 (5.3) G Pyrophyllite 8 Graphite 34 0.12-0.20 300 (10.5) HMontmorillonite 20 Graphite 34 0.09-0.11 200 (7.0) EXAMPLE 30 AND COMPARATIVE EXAMPLES I TO K For Example 30 and Comparative Examples I-K, a liquid crystallme polyester (i.e., Zenite 6000 as sold by E. I. du Pont de Nemours and Company) was blended with a sheet silicate additive and/or polyimide resin prepared from pyromellitic dianhydride and 4,4'-oxydianiline monomers, (present as its precursor, polyamic acid), in such quantity as to yield the percentages shown in Table III. The composition was formed into a strand using a 30 mm twin screw extruder with baπels set to 290°C and the die at 335°C. The strand was quenched using a water spray. The quenched strand was then cut into pellets using a standard rotating blade cutter. The pellets were molded into standard 6.4 mm thick tensile test bars as specified in ASTM D638 using a 160 ton injection molding machine with a baπel size of 170g capacity. The molding profile was as follows: Rear 313°C Center 334°C Front 335°C Nozzle 332°C Boost 1 sec Injection 20 sec Hold 20 sec Injection Pressure 3.4 MPa Ram Speed fast Screw Speed 107 φm Back Pressure minimum The samples were made into the test specimens described above by machining. Testing for friction and for PV (pressure x velocity) limit was conducted as described in the procedure above, except that the samples were not previously tested specimens and the test was started at a PV of 12,500 psi-φm. Example 30 shows that the inclusion of a sheet silicate into the polyimide/polyester blend gives a much improved (at least 2x) coefficient of friction over the polyester resin alone (Comparative Example I), the polyester/sheet silicate composition (Comparative Example J), and the polyester/polyimide blend with no sheet silicate (Comparative Example K). Additionally, the PV limit of 43,000 psi-φm (1.51 MPa-m/s) ofExample 30 is approximately 2x better than the Comparative Examples. The test results are summarized in Table EQ. TABLE ffl PV of Coefficient of Friction Failure Ex. Polyimide Silicate Wt Range Before psi-φm No Wt % Additive % Failure (MPa-m s) 30 28.5 Kaolinite 0.41-0.7 43,000 (1.51) I 0 Kaolinite 0 0.75-0.80 25,000 (0.88) J 0 Kaolinite 5 <12,300 (<0.43) K 30 Kaolinite 0 0.91-1.0 24,900 (0.88)

Claims: CLAIMS What is claimed is: 1. A polyimide composition, comprising: (a) about 70-99.9 weight percent of at least one polyimide, and (b) about 0.1-30 weight percent of at least one inorganic, low hardness, thermally stable, sheet silicate, said weight percents being based upon the weight of components (a) and (b) only. 2. A composition according to Claim 1, which is a blend wherein said polyimide is present in a range of about 20-30 weight percent and said blend further comprises about 45-79.9 weight percent of at least one other polymer which is melt processible at a temperature of less than about 400°C selected from the group consisting of polyamide and polyester resins. 3. A composition according to Claim 1 or Claim 2 in which said sheet silicate is present in an amount of from 1 - 20 weight percent ofthe composition and is selected from the group consisting of muscovite mica, talc and kaolinite. 4. A composition according to Claim 1 further comprising at least one of additives, fillers or dry lubricants. 5. A method for improving wear resistance and reducing coefficient of friction of a polyimide composition which comprises incoφorating into said composition from about 0.1 weight percent up to 30 weight percent of at least one inorganic, low hardness, thermally stable, sheet silicate. 6. The method of Claim 5 in which said sheet silicate is present in a concentration of from 1 weight percent up to 20 weight percent and is selected from the group consisting of muscovite mica, talc and kaolinite. 7. A molded article having improved wear resistance and reduced coefficient of friction made from the composition according to Claim 1 or Claim 2. 8. An injection molded article having improved wear resistance and reduced coefficient of friction made from the composition according to Claim 1 Claim 2. 9. A molded article having improved wear resistance and reduced coefficient of friction made from the composition according to Claim 1 or Claim 2 selected from the group consisting of a bushing, seal, electrical insulator, thrust washer, compressor vane, impeller, piston, piston ring, gear, thread guide, cam, brake lining, and thrust plug.

Inventors: BLOOM JOY SAWYER, FEIST THOMAS PAUL, GEORGE DANIEL EUGENE, BLOOM, JOY, SAWYER, FEIST, THOMAS, PAUL, GEORGE, DANIEL, EUGENE
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1996
Main classification: C08K3
IPC codes: C08K3, C08K7, C08L67, C08L77, C08L79, F16C33, F16D69, F16H53, F16H55, F16J1, F16J9