Abrasion-Resistant Fluorocarbon Polymer Composites

The present invention relates to abrasion-resistant fluorocarbon polymer composites, such as polytetrafluoroethylene ("PTFE") composites, having high frictional efficiency over a wide range of temperature and load conditions. More particularly, the present invention relates to abrasion-resistant, anti-friction tubing.

Fluorocarbon polymers, such as PTFE resins, are well known in the art and have heretofore been utilized in extruded tubular products. Although PTFE resins in their pure form exhibit excellent frictional efficiencies, they generally have unacceptably low abrasion resistance, that is, they wear too rapidly. Attempts have been made to improve the abrasion resistance of PTFE resins by the addition of fillers, both inorganic and organic.

The wear resistance of PTFE extruded tubular products has traditionally been enhanced by the inclusion of inert, inorganic fillers such as glass fibers, carbon, asbestos fibers, mica, metals and metal oxides. See, for example, U.S. Pat. No. 3,409,584. While a measure of improvement in wear resistance has thus been achieved, PTFE composites comprising inorganic fillers nevertheless have several disadvantages. For example, such composites generally exhibit rapid deterioration in frictional efficiency after relatively short periods of use. Moreover, the use of such composites as liners for externally lubricated push-pull cable assemblies is not generally recommended because the inorganic fillers have been found to separate from the composite and form an abrasive slurry with the lubricant. This abrasive slurry not only decreases frictional efficiency, but it can also cause catastrophic and rapid failure of the liner. As a practical result, therefore, it has previously not been possible to successfully use inorganically filled PTFE composites in lubricated push-pull cable assemblies.

Fluorocarbon polymers have also been modified to include organic fillers. See, for example, U.S. Pat. Nos. 3,652,409 and 4,362,069. Generally, such organically filled fluorocarbon polymers, and particularly those filled with polyamide resins, do not lend themselves readily to extrusion, being more adapted to molding techniques. On the other hand, it has been found that prior art polymeric composites found suitable for producing tubular products by extrusion generally suffer from early deterioration under severe temperature and load conditions.

U.S. Pat. No. 4,451,616, issued to Kawachi et al, discloses a process for the preparation of a composite comprising PTFE and an organic filler is disclosed. Kwachi et al teach the use of a filler selected from the group consisting of polyimide resins, polyamide-imide resins, polyamide resins and carbon fiber powders. The Kawachi process involves coagulation of PTFE and one of the above mentioned fillers from an aqueous dispersion of these two components. The weight proportion of PTFE and the filler in their aqueous dispersion is disclosed as being from 100:5 to 80. Although the patent discloses that the abrasion resistance of PTFE can be enhanced by the incorporation of the above mentioned fillers, there is no indication that any one of those fillers is preferred over another, or that a particular concentration of filler in the composite is preferred.

U.S. Pat. No. 3,391,221, issued to Gore et al, discloses fluorocarbon polymer molding compositions containing from about 10 to about 55 volume percent of what are called "permanent lubricant modifiers" selected from the class consisting of (a) nonvolatile liquids which remain thermally stable and liquid at the sintering temperatures of the fluorocarbon polymer, and have lower vapor pressures at those temperatures and (b) materials which are liquid during the forming of the fluorocarbon polymer article and are transformed into a solid in the final shaped article. One important function of the lubricant modifiers of Gore is to act as a lubricating agent during shaping of the polymer. A variety of materials are disclosed as lubricant modifiers, including: aromatic polyamides formed by the reaction of aromatic dicarboxylic acids such as terephthalic acid with aromatic amines such as phenyl diamine or biphenyl diamine; the aromatic polyimides formed by the reaction of such acid dianhydrides as pyromellitic dianhydride with the stated aromatic diamine; the polyamide, polyimide copolymers from the above named components; aromatic polyesters formed from the aromatic dicarboxylic acids and aromatic diols; polybenzimidiazoles formed from the aromatic tetracarboxylic acids such as pyromellitic acid and aromatic tetramines; aromatic polyethers; and Novolac epoxy resins. The only guidance that the patent provides with respect to the selection of modifiers for the enhancement of frictional efficiency is that phenyl silicone lubricants are said to provide high lubricity under high unit loads, and that polymerizable monomers and prepolymers that are polymerized in situ provide molded articles that have a low coefficient of friction. The patent provides no indication that any particular concentration of filler is preferred over another.

U.S. Pat. No. 3,356,759, issued to Gerow, discloses compositions of aromatic polypyromellitimides and a polyfluorocarbon resin. Although this patent broadly refers to the presence of from about 10 to about 90% by weight of fluorocarbon resin in the composite, it expressly teaches that the composite preferably have no more than 50% by weight of the fluorocarbon resin. Accordingly, the Gerow reference teaches composites in which the polyfluorocarbon components preferably constitute a minor proportion of the composite.

Composites comprising a mixture of PTFE and polyarylene sulfide have heretofore been used in fabricating flexible liner or tubing for push-pull cable assemblies. For example, U.S. Pat. No. 4,362,069, issued to Gitras and assigned to the assignee of the present invention, describes a fluorocarbon composite fabricated from a mixture of PTFE resin and a polymer of arylene sulfide. The composite described in this patent has exceptional anti-friction, anti-abrasion characteristics across a relatively wide range of load and temperature conditions. However, as explained in the brochure entitled Abrasion Resistant Anti-friction Tubing in Push-pull Assemblies by the Markel Corporation, these composites must be used with an external lubricant in order to realize a performance advantage over unfilled PTFE products. When used without an external lubricant, these composites exhibit performance characteristics that are no better than conventional unfilled PTFE. Such composites not only have the disadvantage of requiring a lubricant, but also of precluding the use of significant amounts of inorganic filler in the organically filled composite. See, for example, col 5, lines 8-33 of the Gitras patent described above.

The compositions of the present invention are particularly well adapted for use as liners in push-pull cable assemblies and the like. Push-pull cable assemblies are typically used for the transmission of force or other mechanical control commands from one location to another in apparatus such as automobiles, aircraft, motorcycles, boats and bicycles. Such cable assemblies typically comprise a wire cable for transmitting the appropriate force and an abrasion-resistant, anti-friction liner surrounding the wire cable. Since the anti-friction, abrasion-resistant liner is the primary bearing surface in push-pull cable assemblies, it is subjected to unidirectional, reciprocating and/or rotary contact with the internal wire cable. In order to achieve superior or even acceptable liner life under these conditions, push-pull cable assemblies have heretofore typically required the application of lubricant between the liner and the wire cable.

The abrasion resistant, high efficiency composites of the present invention comprise a major proportion by weight of a resin of fluorocarbon polymers, and from about 2% to less than about 23% by weight of polyamide-imide resin filler. More particulary, it has been found that a critical range exists in the amount of polyamide-imide filler used in the abrasion resistant, high efficiency compositions of the present invention. Applicants have unexpectedly discovered that the abrasion resistance and frictional efficiency of the present composites vary with the amount of the polyamide-imide included in the composite. At the same time, the desirability of providing composites that are readily paste extrudable places additional constraints upon the amount of polyamide-imide filler to be used in conjunction with the fluorocarbon polymers in accordance with this invention. Accordingly, it has been found that amounts of polyamide-imide filler of from about 2% to less than about 23% by weight are required to meet both of these requirements. It is even more preferred that amounts of polyamide-imide filler of from about 5% to about 20% by weight be so included.

Applicants have found that composites formulated according to the present invention have exceptional frictional efficiencies in the dry state, that is, in the absence of an external lubricant. Moreover, the present composites have exceptional abrasion-resistance over extended periods of use. The ability of the present composites to maintain high frictional efficiencies in the dry state is not only unexpected, it satisfies an important need in the art. As discussed above, tubular products formulated from heretofore used fluorocarbon polymer composites are typically only used in push-pull cable assemblies in conjunction with an external lubricant. These cable assemblies are frequently responsible for throttle, choke and transmission control in automobiles, trucks and the like. If the external lubricant is properly applied to such assemblies, their performance is generally satisfactory. If, on the other hand, the lubricant is omitted from or improperly applied to the cable assembly, the incidence of liner wear-through and resultant control cable failure increases dramatically. Thus, liners that require lubricants to maintain high frictional efficiencies and prolong useful life are not only disadvantageous, they are potential safety hazards.

Accordingly, one aspect of the present invention is a push-pull cable assembly having an extruded tubular liner PG,9 comprising a composite comprising a major proportion by weight of resin of fluorocarbon polymers, and from about 2% to less than about 23% by weight of a polyamide-imide resin filler. It has been discovered that such push-pull cable assemblies can be exposed to relatively long periods of constant use without a substantial decrease in frictional efficiency and without wear-through of the tubular liner. Moreover, it has been found that push-pull cable assemblies according to the present invention do not require the presence of an external lubricant to achieve this result. These characteristics are not only unexpected, but they also satisfy an important need for increased safety in the operation of push-pull cable assemblies for use as throttle, gear box, clutch, brake, choke and transmission cables, and the like.

Accordingly, it is an object of the present invention to provide fluorocarbon polymer compositions useful to provide composite having high frictional efficiencies in the dry state.

It is a further object of the present invention to provide a fluorocarbon polymer composite having a high degree of abrasion resistance in the dry state.

It is also an object of the present invention to provide a fluorocarbon polymer composite adapted for fabricating extruded tubular products.

It is a further object to provide high efficiency, anti-friction, abrasion-resistant, extruded tubing for use as liners in unidirectional, reciprocating, or rotary cable assemblies.

It is yet another object to provide abrasion-resistant, anti-friction tubing for push-pull cable assemblies, said tubing operating efficiently over a wide range of temperatures and load conditions.

It is still a further object to provide fluorocarbon polymer composites which perform efficiently and effectively at temperatures ranging from about room temperature to 250.degree. F. and which obviate the effects of stress relaxation generally occurring over a period of use.

These and other objects and advantages are attained through employment of one or more embodiments of the present invention.

FIG. 1 is a fragmentary view of an exemplary extruded tubular product used in push-pull cable assemblies of the present invention.

The abrasion-resistant, high efficiency compositions of the present invention comprise a major proportion by weight of a resin of fluorocarbon polymer, and less than about 23% by weight of polyamide-imide resin filler. Such compositions may optionally include inorganic fillers, lubricants, pigments and other modificants as will be appreciated by those skilled in the art. According to preferred embodiments, the composites consist essentially of from about 77% to about 98% of a single fluorocarbon polymer or a mixture of two or more fluorocarbon polymers, and from about 2% to less than about 23% polyamide-imide resin.

Fluorocarbon polymers suitable for use according to the present invention include a wide variety of fluorocarbon polymers but preferably comprise polytetrafluoroethylene ("PTFE"). PTFE polymers useful in the practice of the present invention preferably comprise a major proportion of PTFE homopolymer, although it is contemplated that copolymers of tetrafluoroethylene with other halocarbon monomers may also be used according to some embodiments. The PTFE polymers suitable for use in the composites of the present invention include conventional PTFE polymers obtained by conventional means, for example, by the polymerization of tetrafluoroethylene under pressure using free radical catalysts such as peroxides or persulfates.

According to one preferred aspect of this invention, the PTFE polymer resins are paste extrudable polymer resins. Such resins are generally in the form of extrusion grade powders, fine powders, and the like. The preferable PTFE powders are dispersion grade and not granular. Techniques for the production of fine PTFE powders are well known, and the use of polymers produced by any of these techniques is within the scope of this invention. For example, fine PTFE powder may be produced by coagulating colloidal PTFE particles as disclosed in U.S. Pat. No. 4,451,616, which is incorporated herein by reference.

The polyamide-imide resins of the present invention are polymers having both an amide bond and an imide bond in the molecule. Techniques for fabricating such polymers are well known and readily available to those skilled in the art. For example, polyamide-imide resins can be obtained by reacting an aromatic diamine having an amide group with an aromatic tetracarboxylic acid, such as pyromellitic acid. Polyamide-imide resins can also be obtained by reacting a tricarboxylic acid, such as anhydrous trimellitic acid with a diamine, such as 4,4'-diaminodiphenyl ether. Polyamide-imide resins are generally represented by the following structural formula: ##STR1##

Polyamide-imide resins of the type suitable for use in the practice of the invention are preferably high temperature, thermoplastic polyamide-imide resins such as those available commercially from Amoco Chemicals Corporation under the general designation "TORLON". Although most grades of polyamide-imide resins are generally adaptable for use in the present compositions, TORLON resins having the grade designation "4000TF" are preferred. TORLON-400TF is a high molecular weight, high performance, thermoplastic material with exceptional thermal resistance, creep resistance and superior mechanical stress properties.

Polyamide-imide resins are available in a wide variety of particle sizes, and resins having all such particle sizes are believed to be readily adaptable for use according to the present invention. Applicants have found, however, that the particle size of the polyamide-imide resins used in the present composites has an impact on the properties of the shaped article produced from the composite. For example, when the composites are processed as hereinafter described to produce extruded tubular products, polyamide-imide powders having average particle sizes up to about 50 microns are typically preferred. Applicants have found that powders having average particle sizes in excess of about 50 microns tend to result in extruded products that have relatively low frictional efficiency and abrasion resistance. Moreover, the smaller particle sizes tend to facilitate the paste extrusion process.

In accordance with the present invention, the polyamide-imide resins are blended with the PTFE resin in amounts sufficient to attain a composite that is at once readily extrudable by paste extrusion, has a relatively high frictional efficiency and exhibits excellent abrasion resistance over extended periods of use. It has been discovered that polyamide-imide resins are preferably present in amounts from about 2% to less than about 23% by weight of the composite. Composites formulated within this range contain sufficient polyamide-imide resin to ensure that abrasion resistance and high frictional efficiency are imparted to the composite while leaving the composite in a condition such that it is readily extrudable by the paste extrusion process. It has further been discovered that it is generally more preferred that the polyamide-imide resins be incorporated in the composites of the present invention in amounts from about 5% to about 20% by weight of the composite, with about 10% to about 15% being the most preferred.

The composites of the present invention may optionally include further additives such as lubricating fluids, inorganic fillers, pigments and other modificants generally known to those skilled in the art. Useful inorganic fillers include glass, metal and metal oxide components. These and other inorganic fillers can generally be employed in the form of beads, fibers, powders, liquids and the like, as is well understood by those skilled in the art. Inorganic fillers may be incorporated in amounts sufficient to impart the desired increase in tensile strength, as is well understood by those skilled in the art. In typical prior art composites utilizing polypheylene sulfide as the organic filler, the amount of inorganic filler was limited to about 4% by weight of the composite. Since the present compositions do not rely on external lubricants for their superior performance, it is contemplated that inorganic fillers can advantageously be added to the present composites in amounts up to about 10%.

Methods for formulating polymer composites are well known to those skilled in the art and may be used in the formulation of the composites of the present invention. One preferred method of formulating such composites comprises mixing a fine PTFE powder resin with a fine polyamide-imide powder resin. Any well known mixing process that achieves homogeneous and uniform mixing may be employed, although mixing by tumbling in a suitable commercial blender such as Patterson-Kelly Twin-Shell at temperatures up to about 68.degree. F. for a period of about 3 minutes is generally preferred. In formulating the blend, it has been found that the PTFE and the polyamide-imide resins are preferably in powder form and have an average particle size estimated to be from about 2 to about 50 microns.

It is contemplated that the fluorocarbon-based polymeric composites of the present invention may be processed using various fabricating methods, including extrusion, to produce abrasion resistant shaped articles having high frictional efficiencies in the dry state. Although it is contemplated that the present composites may be processed by any one of various well known extrusion techniques, the present composites are particularly well adapted for processing by paste extrusion to fabricate tubes, rods, wire coatings, liners, and the like. In the paste extrusion process, the fluorocarbon-based polymeric composite is compressed into a cylindrical preform by techniques well known in the art. An extrusion aid, typically a volatile lubricant such as naphtha or other volatile paraffinic hydrocarbon, is optionally added to the preformed composite in an amount of from 10% to 25% by weight. The preformed composite is then shaped into the desired form by cold flow extrusion. After extrusion, the extrusion aid is substantially removed from the shaped article. According to one embodiment, removal of the extrusion aid comprises heating the shaped composite for a time and at a temperature sufficient to effect removal of the extrusion aid, typically for about 15 seconds at about 350.degree. F. The shaping process further preferably comprises a sintering step in which the extruded composite is heated for a time and at a temperature sufficient to fuse or sinter the compressed powders into a homogeneous product, typically for about 20 seconds at about a temperature of at least about 647.degree. F. (342.degree. C.). The shaping process may be, and preferably is, carried out continuously.

The shaped articles of the present invention may be further treated after extrusion by post curing at temperatures from about 500.degree. F. to about 900.degree. F. for time periods from about 5 minutes to about 24 hours, and preferably at from about 500.degree. F. to about 527.degree. F. for at least about 16 hours, depending on the state of the cure of the polyamide-imide prior to blending. It also should be understood that the polyamide-imide can be precured and then ground prior to blending and fabrication.

The shaped articles prepared according to the present invention are abrasion-resistant extruded products having superior frictional efficiency and wear resistance in the dry state, excellent resistance to cold flow, and extended useful life. Unlike composites which contain inert inorganic materials as fillers or additives, it has been found that increased wear resistance is achieved with the composites of the present invention without attendant decrease in frictional efficiency and/or shortened life cycles.

Applicants have found that the present composites are particularly well adapted for use as tubular liners for push-pull cable assemblies. A exemplary push-pull cable assembly is illustrated in FIG. 1. The assembly of FIG. 1 is indicated generally by the reference numeral 1 and comprises a central core 5. As is well understood by those skilled in the art, central core 5 is typically a standard braided steel rope or any other means for transmitting a force along the length of the assembly. A tubular liner 2 comprising the composite of the present invention surrounds the central core 5. More particularly, tubular liner 2 forms a material chamber 4 having a mating surface with core 5. The outside surface 3 of the tubular liner 2 is, in most cases, covered by steel ribbon armor or wire serve (not shown) which is in turn placed in a metal supporting conduit or jacket (not shown).

Shaped articles, particularly tubular liners for use in push-pull cable assemblies, comprising the present composites exhibit excellent and altogether unexpected results. For example, one important aspect of the present composites and the shaped articles produced therefrom is the dry state characteristic of the material. The present shaped articles exhibit substantially reduced coefficients of friction in the dry state and this frictional efficiency is generally maintained over a relatively extended product life. It will be appreciated by those skilled in the art, however that the addition of external lubricants to push-pull cable assemblies of the present invention may nevertheless produce even further performance improvements. Accordingly, push-pull cable assemblies that include external lubricants are also within the scope of the present invention.

The following examples, set forth by way of illustration but not limitation, depict the improvements which are achieved utilizing the fluorocarbon-based polymeric composites of the present invention.

A series of tests that illustrate the effect of polyamide-imide ("PAI") concentration on the compositions of the present invention were performed. Each of the formulas, labeled as A through E in Table I, were prepared according to the following procedure. A polyamide-imide resin was mixed in a Patterson Kelly Twin Shell mixer with a PTFE resin according to the proportions indicated in Table I. The polyamide-imide resin was a product of the Amoco Chemicals Company sold under the designation "TORLON 4000TF". The PTFE resin was a product of the E.I. duPont de Nemours Company sold under the designation "Grade 6C". The resins were mixed for about 3 minutes at a temperature of about 65.degree. F. to produce a uniform and homogeneous blend. A volatile extrusion lubricant was added to the blend in amount sufficient to constitute about 18% by weight of the composite. The composite was removed from the mixer and compressed into a cylindrical preform. The preformed composite was then paste extruded into a tubular product. After extrusion, the extrusion lubricant was removed from the tubular product by heating at a temperature of about 350.degree. F. for about 15 seconds. The tubular product was then sintered for about 20 seconds at a temperature of above about 650.degree. F. An extruded tubular product having an inside diameter of about 0.095" (2.4 mm) and a wall thickness of about 0.013" (3.3 mm) was produced.

Tests were performed by securing the tubular product over an "S" shaped routed fixture wherein the curvilinear portions of the inner radii of the "S" fixture extend about 120.degree. and wherein a flexible cable is drawn through the liner, reciprocating at 60 cycles per minute, each cycle consisting of a forward travel of 11/2 inches and a like return. Frictional efficiency and abrasion resistance of the composite are determined by applying a variable load to the fixture. The variable load is applied by springs which may be adjusted over a range of 0 to 18 pounds-force. As the term is used herein, "low-load frictional efficiency" refers to a frictional efficiency determined with the load springs set to range from about 0 to about 6 pounds-force. As the term is used herein, "high-load frictional efficiency" refers to a frictional efficiency determined with the load springs set to range from about 6 to about 18 pounds-force. Frictional efficiency measurements are taken at various intervals of cycles by employing a load cell (transducer) and recording the actual load necessary to move the cable over the surface of the liner at 4 cycles per minute. For the actual measurement, the spring is replaced by a 5 pound dead-weight. The frictional efficiency is calculated as a percentage by dividing the measured force into the five pound dead-weight. In the test results, the letter "F" following a given calculated efficiency at a given number of cycles indicates a failure of the liner, i.e., a wearing through of the liner by the cable. Such a failure is determined by the cable contacting a base metal after wear-through and closing an electrical circuit which stops the tester. The frictional efficiencies, as measured according to the technique described above, have been found to accurately predict the properties of the composites and the tubular products made therefrom.

Formulas A through E were tested in the dry state at room temperature and found to have the low-load frictional efficiencies shown in Table II. As illustrated by the data provided in Table II, applicants have found that at least about 2% by weight of PAI must be included in the present composites in order to obtain acceptable abrasion resistance in the dry state. Unexpectedly, the data also illustrate that there is a decrease in frictional efficiency when the weight concentration of PAI in the composite is increased from about 13% to about 20%, although both formulations satisfy the objects of the invention.

Applicants have attempted to paste extrude a composite comprising about 23% by weight of PAI according to the procedure described above. It has been found, however, that paste extrusion of composites comprising about 23% PAI by weight or more is not generally possible. The composite formulation labeled as formulation F in Tables I and II was formulated according to the procedures described above, except that the formula was extruded according to the well known hot melt extrusion process. The data in Table II illustrates that the hot melt extruded product has properties generally inferior to composites made according to the present invention.

A series of tests that illustrate the effect of PAI resin particle size on composite performance were performed. In particular, formulations G and H were prepared according to the same procedure used to prepare formulations A through E of Example 1, except that formula H was not sifted prior to blending. Formulations G and H each consisted essentially of about 10% by weight PAI and about 90% by weight PTFE. Thus, formulations G and H are essentially identical except that the PAI resin included in formula G had a maximum particle size of about 150 micron while the maximum particle size for formula H is estimated to be about 300 micron.

Tests were performed on the above described formulas by securing the tubular product over an "S" shaped routed fixture as described above, except that the high-load frictional efficiencies were measured. Applicants believe that high-load efficiency tests more accurately simulate the conditions experienced by tubular liners in actual push-pull cable assembly operation than do low-load tests. The temperature conditions under which the various formulations were tested, together with the frictional efficiency of each during testing, are set forth in Table III.

As illustrated by the data provided in Table III, applicants have found that the particle size of the PAI resin used in the present compositions has an affect on the properties of the resulting extruded tubular product. In particular, applicants have found that when the PAI resin is sifted to exclude substantially all those particles greater than about 150 micron, an increase in the initial frictional efficiency of about 17% is realized. Moreover, an advantage in frictional efficiency is maintained over the life of the product. A comparison of abrasion resistance based upon life cycle data cannot be made for formulas G and H because the testing apparatus malfunctioned at about 330,000 cycles. However, a comparison of the weight loss data indicates that the abrasion resistance of formula G is superior to that of formula H.

A series of tests were performed to illustrate the superior performance of the present composites in comparison to two of the most commonly used materials for tubular liners in push-pull cable assemblies. In particular, PTFE composites sold under the designation "AR-500" and "AR-425" were tested under load conditions described above with respect to Example 2. AR-500 is a PTFE composite manufactured by the assignee of the present invention and consists essentially of about 10% by weight of a resin of polyphenylene sulphide and about 90% by weight of PTFE resin. AR-425 is a PTFE composite manufactured by the assignee of the present invention and consists essentially of about 6% by weight of glass beads and about 94% by weight of PTFE resin.

The temperature conditions under which the AR-425 and AR-500 products were tested, together with the frictional efficiencies of each during testing, are set forth in Table IV. For comparison purposes, the test results from formula G are repeated in Table IV. As illustrated by the data provided in Table IV, the present composites possess dry state abrasion resistance that is dramatically superior to two of the most widely used prior art products. Importantly and unexpectedly, the present composites also exhibit truly extraordinary properties at high temperature conditions. For example, the present composites are capable of withstanding 500,000 cycles at 250.degree. F. without breakthrough, while the state of the art AR-500 material wore through at about 100,000 cycles. Moreover, the frictional efficiency of the present composite is substantially higher at 250.degree. F. than either AR-425 or AR-500. These results are especially important with respect to the use of the present composites in tubular liners for push-pull cable assemblies. Push-pull cable assemblies are typically exposed to engine temperatures that range from below freezing to above 250.degree. F. The present composites provide liners that exhibit long life and high frictional efficiencies in the dry state over a wide range of temperature conditions.