Various additives have been included in such compositions. Graphite has been incorporated to improve wear characteristics in bearing applications.

Fluoropolymers have been incorporated for lubricity. Unfortunately, a continuing problem of obtaining uniform distribution of the additives in such molding compositions exists. As a result, large quantities of expensive additives have been required to provide acceptable properties in shaped articles. Reproducibility of components for wear and bearing materials as well as high cost through inclusion of large quantities of expensive ingredients such as polytetrafluoroethylene has limited use of such compositions.

SUMMARY OF THE INVENTION The present invention solves the aforementioned problems by providing a polymeric composition having uniformly dispersed therein controlled amounts of zinc oxide and non-fibrillating tetrafluoroethylene homopolymers and copolymers of tetrafluoroethylene, collectively referred to herein as PTFE, in finely divided form. The zinc oxide must be of a selected particle size, and the PTFE must have a controlled molecular weight. The two ingredients are present in the composition in a selected weight ratio. When used in accordance with the present invention, these additives disperse uniformly throughout the polymer matrix and give molding compositions of exceptional uniformity. Shaped articles prepared therefrom exhibit improved wear properties.

Polymeric blends of this invention comprise from about 40 to 98 weight percent of at least one thermoplastic polymer which is melt processible at temperatures of less than about 400°C., from about 1 to 38 weight percent of zinc oxide having a mean particle size of less than 3. 7m, preferably at least 25% having a particle size of less than 2p, and from about 38 to 1 weight percent of

PTFE in finely divided powder form having a molecular weight in the range from about 80,000 to 1, 000,000. The PTFE has a mean particle size of from about 1.8p m to about 1 50pm. Preferably the size is from 4to 20pm. The zinc oxide and PTFE particles are present in a ratio of from about 95 parts PTFE/5 parts of zinc oxide to about 10 parts PTFE/90 parts zinc oxide. It has been found that by controlling the ingredients of the composition as described above the components can be blended in commercially available equipment such as a screw extruder to provide blends of exceptional uniformity. It is believed that when the additives are used as described herein they exhibit a synergistic effect in providing composition uniformity. When used separately improved results are not observed.

High performance shaped articles are formed from the polymeric compositions of this invention by a suitable molding operation such as injection or compression molding. Adding zinc oxide to thermoplastic resins containing PTFE in accordance with the present invention significantly improves the reproducibility of the articles as wear and bearing materials. Also improved is the potential operating range for these wear and friction materials. An additional advantage is potential cost reduction since less of the expensive ingredient PTFE is required.

The compositions of this invention may also be comprised of blends of the thermoplastic polymers with other polymers such as polyimides and polyimide precursor resins. These blends may contain from about 40 to 93weight percent of thermoplastic polymer and from about 5 to 40 weight percent of a polyimide or polyimide precursor resin, preferably 10-30 weight percent of the polyimide or polyimide precursor resin is used. The polyimide may or may not be melt processible.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a photomicrograph of a polymeric composition which is outside to scope of the present invention and is described in Sample VI of Example 3 showing poor dispersion of PTFE in the sample.

FIG. 2 is a photomicrograph of a polymeric composition of this invention , Sample VIII, described in Example 3 showing substantially uniform dispersion of PTFE in the sample.

DETAILED DESCRIPTION A wide variety of melt processible polymers can be blended with the zinc oxide and PTFE particles. An individual resin or various types of alloys or a

mixture of a plurality of resins of polypropylene, polyethylene, chlorinated polyethylene, polyethylene terephthalate, polybutylene terephthalate, polyamides, polysulfones, polyetherimides, polyether sulfones, polyphenylene sulfone, polyphenylene sulfide, polyether ketones, polyether ether ketones, ABS resins, polybutadiene, polymethyl methacrylate, polyacrylonitrile, polyacetal, polycarbonate, ethylene-vinyl acetate copolymers, polyvinyl acetate, ethylene- vinyl acetate, ethylene-tetrafluoroethylene copolymers, aromatic polyesters, grafted polyphenylene ether resin, and liquid crystal polymers. Polyamides which can be used include nylon 6, nylon 6,6, nylon 610, nylon 612 and aromatic polyamides. Polyesters include polybutylene terephthalate and polyethylene terephthalate.

The melt processible polyesters are preferably in the form of liquid crystalline polymers (LCPs). The LCPs are generally polyesters including, but not limited to polyesteramides and polyesterimides. LCPs are described in Jackson et al. in U. S. Pat. Nos. 4,169,933,4,242,496, and 4,238,600, as well as in "Liquid Crystal Polymers: VI Liquid Crystalline Polyesters of Substituted Hydroquinones", Contemporary Topics in Polymer Science, 1984, Vol. 5, pp.

177-208. Others are described in Calundann, U. S. Pat. No. 4,219,461.

Particularly desirable LCPs are prepared from monomers such as phenyl hydroquinone, hydroquinone, t-butyl hydroquinone, 1,4-benzene dicarboxylic acid, 1,3-benzene dicarboxylic acid, 4-hydroxybenzoic acid, terephthalic acid and 2,6-napthalene dicarboxylic acid in varying ratios. Preferred LCP compositions are disclosed in U. S. Pat. Nos. 4,664,972 and 5,110,896 which is incorporated by reference herein. The compositions of the first-mentioned patent consist essentially of (I) an aromatic diol consisting essentially of 95 to 55 mole % t- butylhydroquinone and 5 to 45 mole % of one or more polyaromatic diols, (II) a dicarboxylic acid component selected from"para"or"meta"oriented aromatic dicarboxylic or 1,4-cyclohexane dicarboxylic acid or mixtures thereof, provided that no more than 80 mole % of the dicarboxylic acid component comprises a naphthalene dicarboxylic acid, and (III) an aromatic hydroxycarboxylic acid component selected from 4- (4"-hydroxyphenyl) benzoic acid or mixtures thereof; where said copolyester contains equal chemical equivalents of components (I) and (II) and contain about 20 to 60 mole %, based on total moles (I) + (II) + (III) of component (III). The compositions of the second mentioned patent consist essentially of recurring units derived from (I) hydroquinone, (II) 4,4'-

dihydroxyphenyl, (III) terephthalic acid, (IV) 2,6-naphthalene dicarboxylic acid, and (V) 4--hydroxybenzoic acid wherein the mole ratio of (I) : (II) is from 65: 35 to 40: 60, preferably 60: 40 to 40: 60, wherein the molar ratio of (III) to (IV) is from 85: 15 to 50: 50, preferably 85: 15 to 60: 40, wherein the molar ratio of (I) to (II) to the total of (III) to (IV) is substantially 1 : 1, preferably 0.95-1.05: 1. 00, and wherein there are 200 to 600, preferably 200 to 450 moles of (V) per 100 moles of (I) plus (II). Other LCPs useful in this invention consist essentially of repeating units derived from (I) t-butylhydroquinone and terephthalic acid, (II) t- butylhydroquinone and 2,6-naphthalene carboxylic acid, and (III) 4- hydroxybenzoic acid, the molar ration of (I) : (II) being about 4: 1 to about 1: 4, more preferably about 3: 1 to 1: 3 with the molar ratio of (I) + (II) : (III) being about 3: 1 to about 2 : 3.

A wide variety of polyimide resins can be used in the present invention.

Aromatic polyimides, such as those described in Edwards, U. S. Pat. No.

3,179,614, and Manwiller and Anton, U. S. Pat. No. 4,755,555, can be used.

Certain of those polyimides have been found to be particularly satisfactory in the present invention, namely, those having a rigid polymeric structure.

Representative of such rigid polymeric materials are those prepared using aromatic diamines and anhydrides such as m-phenylene diamine (MPD); bis- 4,4' (3 aminophenoxy) biphenyl; 3,4-oxydianiline (3,4-ODA); oxydianiline (ODA); p-phenylene diamine (PPD); benzophenone-3,3', 4,4'-tetracarboxylic dianhydride (BTDA); bis phenol-A-diphthalicanhydride (BPADA); pyromellitic dianhydride (PMDA); and 3,3'4,4'-biphenyltetracarbocyclic dianhydride (BPDA). The dianhydride and the aromatic diamine may be reacted in substantially equimolar quantities. However, excesses of dianhydride or diamine can be used to beneficially modify the properties of the final polyimide. The reaction product of the dianhydride and the aromatic diamine is a polyimide precursor resin, containing polyamic acid which can be thermally or chemically converted to polyimide according to known techniques. Recently, Kaku, in U. S. Pat. No.

5,346,969, described blends of polyimide precursor resin such as described in Endrey, U. S. Pat. No. 3,179,631 and Gall, U. S. Pat. No. 3,249,588, with at least one polyamide or polyester, preferably in the form of liquid crystalline polymer (LCP). Blends of polyimide precursor with at least one polymer which is melt processible at a temperature of less than about 400°C. are combined to provide polyimides with injection molding capability. At least one polymer which is melt

processible at temperatures of less than about 400°C. is blended with the polyimide resin. Melt processible is used in its conventional sense, that the polymer can be processed in extrusion apparatus at the indicated temperatures without substantial degradation of the polymer. The zinc oxide and PTFE particles are included in the blending operation.

The zinc oxide particles may be prepared from commercially available crystalline zinc oxide by grinding and screening the oxide to obtain the required particle size distribution.

Nonfibrillating tetrafluoroethylene polymers and copolymers such as tetrafluoroethylene/hexafluoropropylene are commercially available in micropowder form. E. I. du Pont de Nemours and Company sells such materials under its trademark, Teflon MP. Preparation of such copolymers is described in Morgan, U. S. 4,879,362. Such copolymers have recurring units of tetrafluoroethylene and a comonomer selcted from the group consisting of hexafluoroproplyene, perfluoro (alkyl vinyl ether) where the alkyl group has 1-4 carbons and mixtures of the comonomers.

The present polymeric compositions may include additives in addition to zinc oxide and PTFE, such as molybdenum disulfide, glass fibers, carbon fibers and carbonaceous fillers such as graphite. The particular additive selected will depend on the effect desired.

In certain applications it has been found to be advantageous to coat the zinc oxide particles before incorporating them in the polymeric composition.

Suitable materials include coupling agents such as titanate-, silane-, zirconaluminate-, and alumina-type coupling agents, silylating agents, silanol- modified silicone oil, alkoxy-modified silicone oil, and SiH-modified silicone oil.

Less than 2 weight percent of the coating material is generally used with from about 0.2 to about 1 weight percent based on the weight of the zinc oxide being preferred.

The present invention is further illustrated by the following Examples in which parts and percentages are by weight unless otherwise indicated. In the Examples wear specimens were prepared by machining test blocks of the composition described. A 6.35mm (0.25") wide contact surface of a wear/friction test block was machined to such a curvature that it conformed to the outer circumference of a 35mm (1.38") diameter X 8.74mm (0.34") wide metal mating ring. The blocks were oven dried and maintained 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 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, Rc 58-63,6-12 RMS. A new ring was used for each test.

Test time was 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 of the test time, the block was dismounted, weighed, and the wear calculated by the following calculation: Wear Volume Calculation wear volume (cc/hr) = weight loss (grams) material density (g/cc) X test duration (hr) 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 were started at a given PV. At intervals of 10-20 minutes, the PV was increased in increments by increasing the velocity to a maximum of 2.67 mps (525fpm) 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. A low number is desired for Wear Volume and a low number or narrow range for Coefficient of Friction. PV limit is an indication of the range of operability for a given formulation. A large number is desired. The Wear Volume and Coefficient of Friction are correlated to the uniformity of dispersion of the zinc oxide and PTFE in the polymer matrix.

Tensile properties were measured according to ASTM D638, and flexural properties were measured according to ASTM D-790.

Scanning Electron Microscopy/Energy Dispersive X-ray (SEM/EDX) analysis was used in determining dispersion of PTFE in the samples of the Examples. This analytical technique provides morphological and elemental composition information from the sample surface. Using commercially available microscopes and x-ray analyzers, the sample is placed in a vacuum chamber, and a primary beam of electrons is rastered across the surface. Images are obtained from the remission of electrons. Elemental information is derived from the emission of x-rays whose energy levels are characteristic of each element.

Mapping of individual elements is achieved by limiting detection to desired

energy level. The analyzer then applies a light dot on a grid location each time an x-ray of that energy is detected. As the primary beam rasters across the sample surface, a map of the x-ray sources, representing the corresponding element, is generated. The resolution of the elemental map can be improved by multiple passes over the sample surface and averaging the acquired signals.

EXAMPLES EXAMPLE 1 : 56 parts of a liquid crystalline polyester (DuPont Zenite 6000) and 24 parts of polyimide resin prepared from pyromellitic dianhydride and 4,4'- oxydianiline (present as its precursor, polyamic acid) were blended with zinc oxide additive (obtained from Matsushita as WZ-511 powder) having a starting mean particle size of 2. 111m and PTFE (DuPont Teflon MP-1600 micropowder) in the amounts documented in the table below as to yield the desired formulation.

This was accomplished using a 30mm twin screw extruder with barrels set to 290°C and the die at 335°C. Quenching was accomplished using a water spray.

The strand was cut into pellets using a standard rotating blade cutter. The pellets were molded into standard 6.4 mm thick ASTM (D638) tensile test bars using a 170 g capacity, 145 ton clamping pressure injection molding machine. The profile was as follows: Rear 313°C, Center 334°C, Front 335°C and Nozzle 332°C ; Boost 1 sec, Injection 20 sec, Hold 20 sec, Injection Pressure 3.4 MPa, Ram Speed fast, Screw Speed 107 rpm and Back Pressure minimum.

The samples were made into the test specimens by machining. Wear testing was done at a PV of 1.75 MPa-m/s (1. 28 MPa, 1.36m/s) Sample No. Parts Parts Wear Coefficient PTFE ZnO Volume of Friction cc x 1 0~4/hr (Comparative) I 20 0 39.3 0. 11-0. 38 II 10 10 2.0 0.25-0.40

EXAMPLE 2: 56 parts of a liquid crystalline polyester (DuPont Zenite 7000) and 24 parts of polyimide resin prepared from pyromellitic dianhydride and 4,4'- oxydianiline (present as its precursor, polyamic acid) were blended with zinc oxide additive (obtained from Matsushita as WZ-511 powder) having a starting mean particle size of2. 1um and PTFE (DuPont Teflon MP-1600 micropowder) in the amounts documented in the table below as to yield the desired formulation.

This was accomplished using a 30mm twin screw extruder with barrels set to 320°C and the die at 335°C. Quenching was accomplished using a water spray.

The strand was cut into pellets using a standard rotating blade cutter. The pellets were molded into standard 6.4 mm thick ASTM (D638) tensile test bars using a 170 g capacity, 145 ton clamping pressure injection molding machine. The profile was as follows: Rear 313°C, Center 334°C, Front 335°C and Nozzle 332°C ; Boost 1 sec, Injection 20 sec, Hold 20 sec, Injection Pressure 3.4 MPa, Ram Speed fast, Screw Speed 107 rpm and Back Pressure minimum.

The samples were made into the test specimens by machining. Wear testing was done at a PV of 1.75 MPa-m/s (1. 28MPa, 1.36m/s) Sample No. Parts Parts Wear Coefficient PTFE ZnO Volume of Friction cc x 10'4/hr (Comparative) III 20 0 9.9 0. 22-0. 26 IV 10 10 2.8 0.09-0.17 EXAMPLE 3: The same method for sample preparation as used in Example 2 was utilized except the polyimide resin prepared from pyromellitic dianhydride and 4,4'-oxydianiline was individual component is documented in the table below.

Sample No. Parts Parts Parts Parts PV Wear* Coefficient LCP Polyimide PTFE ZnO Limit Volume of Friction MPA-m/s cc x 10'4/hr vi 63 27 10 0 4.6 15. 2 0.26-0.40 Vil 53 27 15 5 6.8 1.2 0.21-0.27 VIII 53 27 10 10 5.7 4.2 0.25-0.33 IX 53 27 5 15-5.0 0.23-0.32

The resulting parts from Samples VI and VIII were analyzed for PTFE dispersion using SEM/EDX analysis. The fluorine map for Sample VI is shown in FIG. 1 which shows numerous dense areas where the fluorine is concentrated (i. e. poorly dispersed). This is contrasted to Sample VIII shown in FIG. 2 where the fluorine is uniformly dispersed. Wear Volume for Sample VI as over 3X for Sample VIII, and the range for Coefficient of Friction was almost double that for Sample VIII.

EXAMPLE 4: Samples were prepared as in Eample 1.

Sample No. Parts Parts Wear Coefficient PTFE ZnO Volume of Friction cc x 10'4/hr X 20 0 41.5 0. 20-0. 32 xi 10 10 7. 2 0.20-0. 33 XI ! 0 20 104.9 0.26-0.48 EXAMPLE 5: Samples were prepared as in Example 2 where 280 parts of the liquid crystalline polymer were used and the 120 parts of the polyimide were used in addition to the materials listed in the table below.

Sample No. Parts Parts PV-Limit Wear Coefficient PTFE ZnO (MPa-m/s) Volume of Friction cc x 10~4Er XIII* 100 0 4.6 5.1 0.21-0.28 XIV 95 5 5.6 1.7 0.19-0.28 XV 25 75 5.6 5.4 0.25 XVI 5 95-14.0** 0.28-0.41 * (Comparative) **Sample failed after 3 hours rather than the 24 hr. that all the other samples were run.

EXAMPLE 6: Samples XVII, XVIII, and XIX were prepared as in Example 1 using 56 parts of Zeniteg 6000 and 24 parts of polyamic acid, except that the zinc oxide used was 503R obtained from Zinc Corporation of America having a mean particle size of 3.7um after coating and no discernible unique structure. Sample XVIII utilized a zinc oxide coated according to the following procedure: To a 5 L round bottom flask 2L of distilled water and 500 g zinc oxide were added with agitation. The dispersion was heated to 75°C, and the pH was adjusted to 8.7. To

this dispersion 100ml of a solution containing 40 ml of alumina and 60ml of distilled water was added over a 20 minute period while maintaining a pH of 8.2 with a 17.5% solution of hydrochloric acid. The dispersion was stirred for 30 minutes then filtered, washed with distilled water and placed in an oven at 110°C until dried. The sample was sonified to the desired particle size.

Sample XIX utilized a zinc oxide coated according to the following procedure: To a 5 L round bottom flask 3L of distilled water and 500 g zinc oxide was added with agitation. The dispersion was heated to 75°C and the pH was adjusted to 9.5 with 30% sodium hydroxide solution. 42.5g Kasil (Vinnings VSA-38) diluted to 100 ml with distilled water was added over a 30 minute period keeping the pH at 9.5 with 17.5% HC1 solution. The dispersion was stirred for 30 minutes. The pH was adjusted to 8.2 with 17.5% HCl solution, and 40 ml of alumina was added over a 30 minute period while maintaining a pH of 8.2 with a 17.5% solution of hydrochloric acid. The dispersion was stirred for 30 minutes then filtered, washed with distilled water and placed in an oven at 110°C until dried. The sample was sonified to the desired particle size.

Sample XX was prepared according to Example 1 using 56 parts of Zenite 6000 and 24 parts of polyamic acid precursor. WZ-511 Zinc oxide powder was used as received from the supplier. Sample No. Parts Parts Coating Wear Coefficient Tensile Elong Flexural Flexural PTFE ZnO Type Volume of Friction Strength (%) Strength Modulus zc x 10~4Alr at Break MPa) (GPa) (MPa) XVII10 ! 0 Uncoated 16.7 0.3-0.35 34.5 1.7 53.8 0. 38 XVIII 10 10 amorphous 12.4 0.27-0.37 35.8 1.9 57.9 0.39 almina XIX 10 10 alumina 13.6 0.29-0.35 43.4 2.2 70.3 0.39 silica XX 10 10 organic 13.0 0. 28-0. 35 45.5 2. 5 70. 3 0.41 EXAMPLE 7: Samples were prepared according to Example 1 without the incorporation of the polyimide. Wear testing was done at a PV of 0.34 MPa-m/s (3.4MPa, O.lm/s)

Sample No. Parts Parts Parts PV Limit Wear Coefficient LCP PTFE ZnO (MPa-m/s) Volume'of Friction cc x 10'4/hr XXI 70 15 15 3.42 6.5 0.12-0.13 XXII* 70 30 0 0.7 0.2 0.16-0.21 * (Comparative) EXAMPLE 8: Sample XXIII was prepared from 56 parts Polyamide (DuPont Zytel HTN) blended with 10 parts zinc oxide additive (obtained from Matsushita as WZ-511 powder) having a starting mean particle size of2. 1um, 10 parts PTFE (DuPont Teflon) MP-1600 micropowder) and 24 parts polyimide resin prepared from pyromellitic dianhydride and 4,4'-oxydianiline (present as its precursor, polyamic acid) in such quantity as to yield the percentages shown in the table below. This was accomplished using a 30mm twin screw extruder with barrels set to 320°C and the die at 335°C. Quenching was accomplished using a water spray.

The strand was cut into pellets using a standard rotating blade cutter. The pellets were molded into standard 6.4 mm thick ASTM (D638) tensile test bars using a 170 g capacity, 145 ton clamping pressure injection molding machine. The profile was as follows: Rear 315°C, Center 335°C, Front 335°C and Nozzle 335°C ; Boost 0.5 sec, Injection 20 sec, Hold 20 sec, Injection Pressure 4.8 MPa, Ram Speed fast, Screw Speed 120 rpm and 0.34 MPa Back Pressure.

Sample XXIV utilized imidized polyimide powder resin prepared from pyromellitic dianhydride and 4,4'-oxydianiline.

The samples were made into the test specimens by machining. Wear testing was done at a PV of 1.75 MPa-m/s (1. 28MPa, 1.36m/s) Sample No. Parts Parts Wear Coefficient PTFE ZnO Volume of Friction cc x 10'4/her XXIII 10 10 10.0 0. 16-0. 28 XXIV 10 10 8.1 0.20-0.27 EXAMPLE 9: Samples were prepared as in Example 1 using 10 parts of zinc oxide to 10 parts of PTFE. The compounded resin was injection molded into a 1. 5mm thick by 50.8mm diameter disk for use as a hysterisis washer. The conditions

utilized to accomplish this were as follows: Barrel temperatures 320°C. rear, 335°C. front, 335°C. nozzle: Mold 100°C. ; Cycle 1 sec boost, 15 sec inject, 20 sec cool; Pressure 0.3MPa back, 2.7MPa boost, 2.4MPa Inject; Ram speed, fast; Screw 84rpm. The washer performed as required with little evidence of wear after 10,000 kilometers of service.