Thermoplastic polymers, e.g. polyamides, polyesters, polyphenylene sulfide, polyoxmethylene, polyolefins, styrene polymers, and polycarbonates, are characterized as polymers that exhibit exceptional mechanical and electrical properties, as well as good moldability and chemical resistance. However, these polymers may exhibit inadequste tribological properties when utilized in some friction environments, e.g. plastic to metal, and plastic to plastic nterfsces. While many hrhicatins compositions have been applied to thermoplastic @olymers to improve fiction and wear properties of shaped articles @repared tJxiom, some applications prohibited the use of certain lubricants because of possible @ontamination, e.g. @ood handling,

clothing preparation and volatile environments.

Attempts have been made to improve the friction properties and reduce the suriice wear of articles prepared from thermoplastic polymers by incorporating lubricants directly into the polymer matrix prior to the fabrication of shaped articles thereform Many materials, including solid lubricaus and fibers (e.g. graphite, mica, silica, talc, boron nitride and moybdenum sulfide), paradm waxes, petroleum and synthetic lubricating oils, and other polymers (e.g. polyethylene and polytetrafluoroethylene) have been added to thermoplastic polymers to improve friction and wear properties. However, the addition of many of these additives in various combinations to thermoplastic pours, while improving tribological properties have reduced other desirabie physical and mechanical properties. Some additives have proven -iiictory for short terms at bw speeds and loads. However, friction characteristics of many of these lubricants significantly deteriorate over long periods of time under increased loads.

There is a deire for a nofrtoxic, bw wear thermoplastic compositions @ossessing surface wear resistance and low friction properties under increasing loads over bng periods of time. A suitable composition, when @abricated into a shaped @rticle, should maintain the desired mechanical and physical properties long associated with hermoplastic polymers, and be @afely utilized in bod handling and cbthing nanufacturing industries.

SUMMARY OF THE INVENTION The present invention relates to a low wear polymeric composition suitable for forming a bw friction, shaped article, characterized as a melt blend of from about 70 to about 99.5 weight weight of a atbemoplastic polymer, and from about 30 to about 0.5 weight percent of polytetrafluorocthylene, pentaerythritol tetrastearate (PETS0, and fine particle, stearate coated calcium carbonate. Processing aids that do not detract from the characteristics of the invention may be added to the composition to enhance physical properties and processing, e.g. dispersion of the lubricating system within the polymer matrix.

The composition may be formed into low wear shaped articles, e.g. bearings, gears, cams, rollers, sliding plates, pulleys, levers, guides, conveyor links, etc., which exhibit good friction properties and are useful in numerous applications wherein parts exhibiting bw friction and reduced wear properties are desirable.

The ilion is directed to a bw wear polymuic composition which may be fabricated into shaped articles exhibiting good friction properties. Generally, the composition may be characterized as an admixture of from about 70 to about 99.S weight percent of a thermoplastic polymer and from about 30 to about 0.5 weight percent of a lubricating system. Typically, the composition may contain from about

85 to about 99 weight percent of the thermoplastic polymer and from about 15 to about I weight percent of the tubricating system Preferably, the composition contains about 98 weigh@ percent of the thermoplasti@ polymer and about 2 weight percent of the lubricating system, based on the total weight of the composition.

Thermoplastic polymer useful in the bw wear composition of the present invention may be, generally, selected from polyamides, polyesters, polyphenylene sulfides, polyolefins, polyoxymethylenes, styrene polymers, and polycarbonates. particular preferred thermoplastic polymer of the invention is polyoxymethylenes, i.e. polymeric acetals or oxymethylene polymers. Polyoxymethylenes exhibit physical and mechanical properties that make them suitable for many industrial applications.

Polyoxymethylenes, i.e. polyacetals or oxymethylene polymers useful in the present invention are generally characterized as having recurring oxymethylene units of the general formula: -(O-CH2-)- @olyoxymethylenes that are useful in mainng composition of the invin @generally have a fairy high content of oxymethylene units, ie. generally @reater than about 85 percent. These materials are @ommercially available from a number of

manufacturers a@ as homopolymers copolymers, terpolymers and the like. These highly crystalline acetals, described briefly hereinbelow, are well known in the art and have been reviewed extensively. For example, a review of polymeri@ aces entitled, "Acetal Resins," by T. J Dolce and 1. A Grates, may be found in the Second Edition of Encyclopedia of Polymer Science and Engineering John Wiley and Sons, New York, 1983, Vol. 1, pp. 42-61. Additional information on acetal polymers can be fount in French Paent No. 1,221,148 as well as commonly assigned U.S. Patent Nos.

3,027,352, 3,072,069, 3,147,234, and 3,210,318.

Typically, acetal homopolymers may be prepared by polymerizing anhydrous formaldehyde or trioxane. Oxymethylene homopolymers are typically stabilised against thermal degradation by cndwcapping with, for cxampk, ester or ether groups, such as those derived from alkanoic anhydrides (e.g. acetic anhydride) or dialkyl ethers, (e.g. di-thyl ether) or by incorporating stabilizer compounds into the homopolymer. Commercially available acetal homopolymer is made by polymerizing @nhydrous fruni-de in the presence of an initiator after which the polymer is end. capped by @cetylation of the @emiacetal end group. with acetic @nhydride in the @resence of sodium acetate catalyst. Methods for making end-capped acetal @omopolymers are @aught in U.S. Patent Noa 2,786,994 and 2,998,409. Acetal homopolymers are @ell know in the art and are @ommercially available sunder the rademarks Delrin# and Tenac#.

Polymeri@ acctals which have been found to be especially suitable for use in the composition of the present invention are crystailie oxymethylene copolymers having repeat units which consist essentially of oxymethylene groups interspersed with oxy(higher alkylene) groups of the general formula wherein each R1 and R2 is hydrogen, a lower alkyl group, or a halogen substituted lower alkyl group, each R, is a methylene, oxymethylene, bwer alkyl or haloalkyl substituted methylene or lower alkyl or haloalkyl substituted oxymethylene group, and n is zero or an integer from one to thru, inclusive. Each bwer alkyl group preferably contains one or two carbon atom. Oxymethylene groups generally will constitute from about 85 to 99.9 pew of the recurring units in such copolymers and are generally incorporated by ring-opening polymerization of trioxane in the presence of an acidic catalyst. The oxy(higher alkylene) groups are incorporated into the polymer by copolymerizing a cyclic ether or cyclic formal having at least two adjacent carbon atoms in the ring in addition @o @rioxane. The cyclic ether or formal is incorporated by the breaking of an oxygen-to-carbon linkage. The preferred oxy(higher alkylene) group is oxyedyis, having the @ormula:

Oxyethylene may be incorporated into the polymer by copolymerization of ethylene oxide or 1,3-dioxolane with trioxane The preferred csysullie acetal copolymers, as described above which have a structure consisting essentially of oxymethylene and oxyethylene groups, are thermoplastic materials having a melting point of at least 150°C. They normally are millable or processible at temperatures ranging from about 175°C to about 230°C.

These copolymers are normally highly crystalne and exhibit a polymer crystallinity from about 40 percent to about 90 percent or greater.

Typically, oxymethylene copolymers are stabilized after manufacture by degradation of unstable molecular ends of the polymer chains to a point where a relatively stable carbon-to-carbon linkage prevents further degradstion of each end of the polymer chain. Such degradation of unstable molecular ends is generally effected @y hydrolysis, as disclosed, for example, in U.S. Patent No. 3,219,623 to Berardinelli.

Oxymethylene copolymer may also be stabilized by end-capping, again using techniques well known to hose skilled in the art, as for @xample by @cetylation with acetic @nhydride in the present of a sodium acetate catalyst.

A particularly preferred class of oxymethyinc copolymers is commercially availabk under the trade name Celcon acetal copolymer from Hoechst Celanese Corporation, Hoechst Technical Polymers. Celcon acetal copolymers typically are copolymers of about 98 weight percent oftrioxanc and about 2 percent of dioxolane.

Celco is a tradelnark of Hoechst Celanese Corporation. The compositions of the current jiMnion may be nude using any commercial grade of Celcon acetal, inching Celcon grades U-10, M-25, M-90TM, M.270Th and M4SO. Celcon M-25 acetal copolymer has a melt index of about 2.5 g/10 min when tested in accordance with ASTM D123S82. Ceicon M-90 acetal copolymer has a lower molecular weight and melt viscosity than Celcon M-25. Celcon M-270 has an even lower molecular weight and melt viscosity than Celcon M-25.

Oxymethylene terpolymers may also be used in making the low wear polymeric compositions of the pment inventioa These terpolynurs contain oxymethylene groups, oxy(higher alkylene) groups such as those corresponding to the general formula: and a different third group which has been nterpolymerized with the oxymethylene and oxy(higher @lkylene) groups A erpolymer as described above is typically made

by reacting trioxane with a cyclic ether or cyclic acetal and a third monomer which is a bifunctional compounds, such as a diglycide of the formula: wherein Z represents a carbon-to-carbon bond, an oxygen atom, an oxyalkoxy group of 1 to 8 carbon atoms, inchisive, preferably 2 to 4 carbon atoms, an oxycycloalkoxy group of 4 to 8 carbon atoms, inclusive, or an oxypoly(lower alkoxy) group, preferably one having from 2 to 4 recurring bwer alkoxy groups each with I or 2 carbon atoms. Examples of suitable bifimctlonal compounds include the diglycidyl ethers of ethylene blycol, 1,2-propanediol, and 1,4-butanediol with the diglycidyl ether of 1,4-butanediol being preferred. Generally, when preparing such terpolymers, ratios of from 99.89 to 89.0 weight percent trioxane, 0.1 to 10 weight percent of the cyclic ether or cyclic acetal, and 0.01 to 1 weight percent of the bifunctional compound are pEd, based on the total weight of monomers used in forming the terpolymer. A particularly preferred oxymethylene terpolymer is commercially available from Hoechst Celanese Corporation, Hoechst Technical Polymers under the name Celcon U10 acetal polymer, made from 1,4-butanediol diglycidyl ether crosslinker, dioxolane and rioxane contaiing about 0.0S weight percent, 2.0 weight percent, and 97.95 weight percent, espectivley, of repeating @nits derived from these three monomers,

based on the total weight ofthe terpolymer, The oxymethylene-based terpolymers are made and stabilized by methods well known in the art, such as by the addition of antioxodants and formaldehyde and acid scavengers. More detailed descriptions of the methods for making oxymethylene-based terpolymers and their compositions can be found in previously cited patents.

These oxymethylene polymers may be combined in various proportions by melt blending in extruders or similar apparatus to form suitable polymers for preparation of the low wear composition of the present invention. Generally, polyoxymethylene polymers are readily blended with the lubricating system and processing aids when the polymer is in the molten state, i.e. at temperatures of at least about 1700C.

The luliny system of the present invention may be characterized as containing a lubricating amount, sufficient to reduce fiction and wear, of polytetrafluoroethylene (PTFE), pentaerythritol tetrastearate (PETS), and fine particle, stearate coated calcium carbonate. The components of the lubricating system may be blondod together with the desind polymer to form the lubricating composition or separately to form a lubricating system package which subsequently may be combined with the desired polymer to produce a thermoplastic composition exhibiting low wear properties. Generally, the lubricating system may be characterized as containing at least about 0.5 weight percent of PTFE, at least about 0.25 weight

percent of PETS, and at least about 0.25 weight percent of fine, particle, stearate coated calcium carbonate, based on the total weight of the composition. Typically, the lubricating system will contain from about 0.5 to about 5.0 weight percent of EwTFE, from about 0.25 to about 2.0 weight percent of PETS, and from about 0.25 to about 2.0 weight pereent of fine particle, stearate coated calcium carbonate, based on the total weight of the composition. Preferably, the composition will contain about 0.8 weight percent of PTFE, about 0.5 weight percent of PETS, and about 1.0 weight percent of fine partick, stearate coated calcium carbonate, based on the total weight of the composition.

Several additional components may be added to the composition of the present invention to aid lubricity and processing. Generally, the additives may be combined proportionally with the lubricating system and admixed as a package for addition to the thermoplastic polymer or they may be blended directly with the polymer.

Generally, these additives may be selected from (a) at least about 0.25 weight percent of a polyoxymethylene terpolyma*, (b) at least about 0.1 weight percent of a hindered phenol; (c) at least about 0.05 weight percent of calcium ricinoleate; and, optionally, (d) at least about 0.1 weight percent of N,N'-ene bis-stearamide, based on the total weight percent of the composition. Typically, these additives may be admixed with the aeff-lubricating composition in amounts selected from: (a) from about 0.25 to about 2.0 weight percent of a @olyoxymethylene terpolyms, (b) from about 0.1 to about 0.75 weight perocat of a hindered phenol; (c) from about 0.05 to

about 0.2 weight percent of calcium ricinoleate; and, optionally, (d) from about 0.1 to about 0.5 weight percent of N,N'-ethylene bis-stearamide, based on the total weight percent of the composition. Preferably, these additives are admixed with the composition in amounts of (a) about 0.5 weight percent of a polyoxyrnethylene terpor, (b) about 0.5 weight percent of a hindered phenol; (c) about 0.1 weight percent of calcium ricinoleate; and, optionally, (d) about 0.2 weight percent of N,N"- ethylene bis-stearamide, based on the total weight percent of the composition. The addition of these processing aids will typically resuk in a concomitant adjustment in the amount of thermoplastic resin. Other processing aids known to those skieed in the art which do not detract from the improved wear properties of the composition, such as silicone and fluoropolymer mold sprays may be ueed to aid in mold release when preparing shaped articles.

A particular preferred polytetrafluoroethylene (PTFE) which meets FDA/USDA compliance, is Hostaflon# TF9203 distributed by Hoechst Celanese Corporation of Somerville, New Jersey. A preferred pentserythritol tetrastearate (PETS) l Glycolube PQD distributed by Lonaa, Inc. The fine particle, stearate coated calcium carbonate useful in the invention is charecterleed as exhibiting a particle size of about 0.6 pm, a surface area of about 7 m2/gm, a bulk density of about 25 lb/ft3, and a specific gravity of about 2.7. A pressed fine particle, stearate coated calcium carbonate is Super-Pflex# 200 available from Pfizer, Inc. The hindered phenol useflil in the present imn are generally known as antioxidants or free radical inhibitors.

At least one of 2,2'-methylenebis(4-methyl-6-t-butylphenol) hexamethyleneglycol bis(3,5-di-t-butyl-4-hydroxyhydrocinnamate), tetrabis[methylene(3,5-di-t-butyl-4- hydroxydrocinnamate)]methane, triethyleneglycol-bis-3-(3-t-butyl-4-hydroxy-5- methylphenyl) propionate, 1,3,5-trimethyl-2,4,6-tris(3,5-di-butyl-4-hydroxy-benzyl)- benzene, p-octadecyl-3«4'-hydroxy-3',S'-di-t-butyl-phenol)propiona te, 4,4'- methylenebis(2,6-di-t-butylpehnol), 4,4'-butylidene-bis-(6-t-butyl-3-methylphenol), 2,2'-thiodiethyl-bis-[3-(3,5-di-t-butyl-4-hydroxyphenol)]pro pionate, di-stearyl-3,5-di- t-butyl-4-hydroxybenzylphosphonate and 2-t-butyl-6-(3-t-butyl-5-methyl-2- hydroxybenzyl)-4-methylphenylacrylate may be used. However, the useful hindered phenols are not limited to these compounds. Other hindered or stereo-obstructing phenols of the same kind as the above described ones are effective. Of these, hexamethyleneglycol-bis(3,5-di-t-butyl-4-hydroxyhydrocinnama te), for example Irganox# 259 available from Ciba-Geigy, tetrakis[methylene(3,5-di-t-butyl-4- hydroxyhydrocinnamate)]methane, for exampk Irganox 1010 made by Ciba-Geigy and triethyleneglycol-bis-3-(3-t-butyl-4-hydroxy-5-methylphenyl) propionate, for example box 245 made by CIba-Geigy are efctive. A prefred hindered phenol <BR> <BR> <BR> <BR> <BR> <BR> is b-1-bis(3 ,5-di-t-butyl-4-hydroxyhydrocinnamate). The N,N- ethykne bia-stearanide useflil in the invention is marketed under the trade name of AcrawaxS C by Lonza, Inc.

The following examples are general illustrations of neethods for preparing the polymeric composition of the nvention. Thy are provided for purposes of

exemplification only, as should be appreciated from the foregoing discussion.

Ramble 1 To prepare a blend of the low wear polymeric composition, containing 3.4 weight percent, based on the total weight of the composition, of the lubricating system, the following components were utilized: a) 96.1 wt.% of polyoxymethylene copolymer unstabilized flake; b) 0.5 wt.% of polyoxymethylene terpolymer; c) 0.8 wt.% of PIFE; d) 1.0 wt.% of PETS; e) 1.0 wt.% of calcium carbonate; f) 0.1 wt.% of calcium ricinoleate; and g) 0.5 wt.% of prefored hindered phenol The components were tumbled in a barrel followed by high speed mixing for 30 sec. in a Henschel mixer to form a mixture. The mixture was fed into a Werner and Pfleiderer twin screw ZSK extruder and extruded into strands. The extruder zones were operated at 372C to 387°F, the melt temperature was 415°F and under a vacuum of 27 n. Hg, and the screw speed was 150 pm. Strands of extrudate were produced at a rate of 38 Whr. Thereafter, the strands were quenched in cold water and cut into pellets. The pellets were nejction molded at conventional pressure,

velocity and cycle time settings, a nozzle teture setting of 360° to 4200 F, and barrel temperature setting of 350° to 420°F to form 1.25 in diameter disks, each weighing about 7 gm, for mechanical and tniological analyses.

The disks were prepared for surface wear resistance and torque analyses by cleaning in a bath of isopropanol, drying in air, and weighing to about one tenth (1/10) of a milligram These disks were tested according to a Pin-on-Dislc Wear Test. In performing the tests, a machined Nylatron pin with a spherical tip having a radius of 0.187 inches was mounted on the upper spindle of a Falex Friction and Wear Test Machine, Model Multi-Specimen at a distance of 0.469 inches from the center of the test disks, which was mounted on the lower spindle. A bad of 20 pounds was applied to the test disks by means of an air cylinder which pressed the disk against the spherical pin tip. The rotational veloccy was 425 rpm (104.3 ft/min). During the test, a stream of air at 40 standard cubic feet per hour (SCFH) and a distance of 2 inches from the disk was directed against the disk surface to remove debris Testing times ranged from 0.5 to 65 hours. After testing, the pin tip and disk were separated from concact and the disk was removed, air bruhed to remove loose debris, and weighed for weight loss, i.e., surface wear.

Torque (#), mewed during the test, was converted into a coefficient of friction ~) by application ofthe equation: ~=#(2.137/20)

The factor 2.137 is a specific coefficient for this imchine. Results of surice wear and coefficients of friction are in Table II Example 2 In another example, a bw wear composition was prepared containing 3.0 wt% polytetrafhioroethylene (PTFE) in the lubricating system, wherein the lubricating system comprised 4.6 weight percent of the composition, the following components were utilized: Components a) 94.9 wt.% of polyoxymethylene copolymer unstabilized flake; b) 0.5 wt.% of polyoxymethylene terpolymer, c) 3.0 wt.% of PTFE; d) 0.5 % of PETS; e) 0.5 wt.% of calcium carbonate; f) 0.1 wt.% of calcium ricinoleate; and g) 0.5 wt% of preferred hindered phenol The components were mixed, extruded and molded according to the process of Example 1 to form 7 gm disks for weight loss and coefficient of fiction analyses.

Results of the analyses are nn Tabb II

Example 3 In another example, a low wear polymeric composition was prepared containing 3.0 wt. % of PTFE and 4.1 wt. % of the lubricating system. the components were blended in accordance with Example 1, as follows: Components a) 94.5 wt.% of polyoxymethylene copolymer unstabilized flake; b) 0.5 wt.% of polyoxymethylene terpolymer, c) 3.0 wt.% ofPTFE; d) 0.25 wt.% of PETS; e) 0.25 wt.% of calcium carbonate; f) 0.1 wt.% ofcalciumricinoleate; and g) 0.5 wt.% of preirred hindered phenol The components were mixed, extruded and molded according to the process of Example 1 to form 7 gm disks for weight bss and coefficient of fiction analyses.

Results ofthe analyses are in Table II.

To demonstrate the wear performance ofthe composition in the presence of a PTFE lubricating system containing no pentserythritol tetrastearate and calcium carbonate, two (2) formulations were prepared in accordance with the method of Example 1, above, as follows:

Comparative Example 4 Components a) 97.2 wt.% of polyoxymethylene copolymer unstabilized flake; b) 0.5 wt.% of polyoxymethylene terpolymer, c) 1.5 wt.% ofPTFE; d) 0 wt.% of PETS; e) 0 wt.% of calcium carbonate; f) 0.1 wt.% of calcium ricinoleate; 8) 0.5 wt.% of prefored hindered phenol; and h) 0.2 wt.% of N,N'-ethylene bis-stearamide Comparative Example 5 nents a) 95.7 wt.% of polyoxymethylene copolymer unstabilized flake; b) 0.5 wt% of polyoxymthylene terpolymer; c) 3.0 wL% of PTFE; d) 0 wt.% of PETS; e) 0 wt.% of calcium carbonate; f) 0.1 wt.% of calcium ricinoleate; 8) 0.5 wt.% of preferred hindered phenol; and 1) 0.2 wt.% of N,N'-ethylene bis-stearamide

The weight percentages of the components of the compositions of Examples 1 through 5 are summarized in Table I. Comparative Examples 4 and 5 contain PTFE in the lubricating system, but do not contain PETS and calcium carbonate.

TABLE I Exampk 1 Example 2 Example 3 Comparative Comparative Example 4 Exampk 5 Components POM- 96.1 94.9 95.4 97.2 95.7 Copolymer POM- 0.5 0.5 0.5 0.5 0.5 Terpolymer PTFE 0.8 3.0 3.0 1.5 3.0 PETS 1.0 0.5 0.25 0 0 Calcium 1.0 0.5 0.2S 0 0 Carbonate Calcium 0.1 0.1 0.1 0.1 0.1 Ricinoleate Irganox 259 0.5 0.5 0.5 0.5 0.5 Acrawax C 0 0 0.2 0.2 Results of wear testing of disks prepared from compositions containing PTFE, as the lubricating system, exhibited suffice wear as well as coefficients of friction significantly higher than the invention. Whereas the lubricating system

containing a combination of PTFE, pentaerythritol tetrastearate and calcium carbonate exhibited superior wear properties after several hours of testing. Table II illustrates the results of wear testing results for the examples. After 0.5 hours of pin-on-disk testing, the composition of Example 1 exhibited an average weight loss of 2.6 mg after 1.5 hours of wear testing, 17 mg of weight loss after 17 hours of wear testing, and 44.8 mg of weight bss after 65 hours oftesting. Similarly, Example 2 exhibited 9 mg of weight bss after 1.5 hours oftesting, and Example 3 exhibited 19.6 mg of weight loss after 1.5 hours of wear testing. However, Comparative Examples 4 and 5 exhibited more significant weight losses of 29 and 24.3 mgs, respectively, after 1.5 hours of wear testing.

TABLE II Wt. Loss, mg. (Coefficient of Friction) Example Lubrication Test Time, hrs. No. System, wt% 1.5 17 65 1 3.4 2.6 (0.075) 17 (0.085) 44.8 (**) 2 4.6 9 (0.064) 34 (0.12) 70 (**) 3 4.1 19.6 (0.11) 78.1 (0.13) ** Comp. 4 1.5 PTFE 29 (0.13) 133 (0.15) 181 (**) Comp. 5 3.0 PTFE 24.3 (0.12) 120 (0.17) 201 (**) ** no data