KR19980702524A - Thermoplastic Compositions with Enhanced Abrasion - Google Patents

Abstract

A self-lubricating polymeric composition suitable for forming a low friction shaped article, which comprises 70 to 99.5 wt% of a thermoplastic polymer and 30 to 0.5 wt% of a melt blend of a lubricating system comprising ultra high molecular weight polyethylenes, pentaerythritol stearate and a calcium carbonate.

Description

Thermoplastic Compositions with Enhanced Abrasion

Thermoplastic polymers such as polyamides, polyesters, polyphenylene sulfides, polyoxymethylenes, polyolefins, styrene polymers and polycarbonates are characterized by polymers exhibiting exceptional mechanical and electrical properties as well as good casting performance and chemical resistance. It is done. However, these polymers may exhibit insufficient friction properties when utilized in some frictional environments, such as plastic to metal and plastic to plastic contact surfaces. Although many lubricating compositions have been applied to thermoplastic polymers to promote the friction and wear of moldings made from thermoplastic polymers, some applications have banned the use of some lubricants because of possible contamination, such as food processing, garment manufacturing and volatile environments.

Attempts have been made to integrate lubricating oil directly into the polymer matrix prior to manufacture of the moldings in order to enhance the frictional performance and reduce the surface wear of the materials made from the thermoplastic polymers. Solid lubricants and fibers (eg graphite, mica, silica, talc, boron nitride and molybdenum sulfide), paraffin wax, petroleum and synthetic lubricating oils, and other polymers (eg polyethylene and polytetrafluoroethylene Many materials, including), have been added to thermoplastic polymers to enhance friction and wear properties. However, the addition of these additives in various combinations into thermoplastic polymers has enhanced frictional properties while reducing other desirable physical and mechanical properties. Some additives have been found to be satisfactory for a short period of time at low speeds and during degradation. However, the frictional properties of most of these lubricants deteriorate considerably over time under increased load.

It would be desirable to provide a non-toxic, self-lubricating thermoplastic composition having surface wear resistance and low friction properties even under increased load over long periods of time. When made into moldings, suitable compositions must retain the desired mechanical and physical properties associated with thermoplastic polymers for a long time and must be safely utilized in the food processing and garment manufacturing industries.

Summary of the Invention

FIELD OF THE INVENTION The present invention relates to a self-lubricating composition suitable for forming low friction, moldings, characterized by a melt mixture of about 70 to about 99.5 weight percent thermoplastic polymer and about 30 to about 0.5 weight percent lubrication system. Silver is characterized by ultra high molecular weight polyolefins, pentaerythritol tetrastearate (PETS) and calcium carbonate. Process aids that do not deteriorate from features of the present invention can be added to the composition to improve physical properties and processes, such as dispersion of the lubrication system in the polymer matrix.

The composition can be formed from bearings, gears, cams, rollers, sliding plates, pulleys, levers, guides, conveyor links, etc., which are self-lubricating moldings that exhibit good friction and are useful in many applications, where low friction and reduced Parts showing wear properties are preferred.

The present invention relates to self-lubricating, low wear compositions containing thermoplastic polymers and lubrication systems suitable for use as molding resins for making moldings. Moldings made from this composition show not only reduced surface wear under load but also low friction.

The present invention relates to self-lubricating compositions that can be made into moldings that exhibit good frictional properties. Generally, the composition may be characterized by a mixture of about 70 to about 99.5 weight percent thermoplastic polymer and about 30 to about 0.5 weight percent lubrication system. Typically, the composition may contain about 85 to about 99 weight percent thermoplastic polymer and about 15 to about 1 weight percent lubrication system. Preferably, the composition contains about 98% by weight thermoplastic polymer and about 2% by weight lubrication system based on the total weight of the composition.

Thermoplastic polymers useful in the self-lubricating compositions of the present invention may generally be selected from polyamides, polyesters, polyphenylene sulfides, polyolefins, polyoxymethylenes, styrene polymers and polycarbonates. Particularly preferred thermoplastic polymers of the invention are polyoxymethylene, ie acetal polymers or oxymethylene polymers. Polyoxymethylenes exhibit physical and mechanical properties that make them suitable for many industrial applications.

Polyoxymethylene, ie polyacetal or oxymethylene polymers useful in the present invention are generally characterized as having repeating oxymethylene units of formula (I).

-O-CH _2-

Polyoxymethylenes useful in making the compositions of the present invention generally have a fairly high content of oxymethylene units, ie generally at least 85%. These materials are homopolymers, copolymers, terpolymers and the like, which are commercially available from numerous preparations. These high crystalline acetals, which will be briefly described below, are well known in the art and have been extensively reviewed. For example, a review of acetal polymers under the heading Acetal Resins, written by TJ Dolce and JA Grates, can be found in the Second Edition of Encyclopedia of Polymer Science and Engineering , John Wiley and Sons, New York, 1985, Vol. 1, pp. 42-61. Further information on acetal polymers can be found in French Patent No. 1,221,148 and US Patent No. 3,027,352, 3,072,069, 3,147,234 and 3,210,318.

Typically, acetal homopolymers can be prepared by polymerizing anhydrous formaldehyde or trioxane. Oxymethylene homopolymers are end-capping or incorporating stabilizing compounds into homopolymers with those derived from ester or ether groups such as alkanoic anhydrides (eg acet anhydride) or dialkyl ethers (eg dimethyl ether) By application, they are typically stable against thermal decomposition. Commercially available acetal homopolymers are made by polymerizing anhydrous formaldehyde in the presence of an inhibitor followed by end-capping the polymer by acetylation of the hemiacetal end group with acet anhydride in the presence of a sodium acetate catalyst. Methods for preparing end-capped acetal homopolymers are described in US Pat. 2,786,994 and 2,998,409. Acetal homopolymers are well been known in the art and are commercially available under the trademark DELRIN ^R and ^R TENAC.

Acetal polymers found to be particularly suitable for use in the compositions of the present invention are crystalline oxymethylene copolymers with repeating units consisting essentially of oxymethylene groups mixed with oxy (higher alkylene) groups of formula (2).

Wherein R ₁ and R ₂ are halogen, each substituted with hydrogen, a lower alkyl group or a lower alkyl group, and R ₃ is methylene, oxymethylene or lower alkyl or haloalkyl substituted with methylene or lower substituted with oxymethylene Alkyl or haloalkyl, n is an integer of 0 or 1 or more and 3 or less. Each lower alkyl group preferably contains one or two carbon atoms. The oxymethylene group will generally consist of 85 to 99.9% of the repeat units of the copolymer, and are generally incorporated by ring-open polymerization of trioxane in the presence of an acid catalyst. The oxy (higher alkylene) group is incorporated into the polymer by copolymerizing at least two adjacent carbon atoms in the ring by adding cyclic ether or cyclic formal to trioxane. Cyclic ethers and formal are integrated by breaking the oxygen-carbon linkage. Preferred oxy (higher alkylene) groups are oxyethylene having the structure of formula (3).

-O-CH ₂ -CH _2-

Oxyethylene can be incorporated into the polymer by copolymerizing ethylene oxide or 1,3-dioxolane into trioxane.

Preferred crystalline acetal copolymers described above having a structure consisting essentially of oxymethylene and oxyethylene groups are thermoplastics having a melting point of at least 150 ° C. They are normally castable or processable at temperatures ranging from about 175 ° C to 230 ° C. These copolymers are normally highly crystalline and exhibit a polymer crystallinity of about 40% to about 90% or more.

Typically, the oxymethylene copolymer is stabilized after preparation by decomposition of the unstable molecular ends of the polymer chain to such an extent that the degradation of the polymer chain ends is prevented by relatively stable carbon-carbon bonds. Degradation of the labile molecule ends is by hydrolysis, for example by Berardinelli in US Pat. Generally affected as disclosed in 3,219,623. The oxymethylene copolymer can again be stabilized by end-capping, for example, acetylation to acet anhydride in the presence of a sodium acetate catalyst, which is well known to those skilled in the art.

Particularly preferred families of oxymethylene copolymers are commercially available acetal copolymers under the CELCON ^R trademark. CELCON acetal copolymers are typically a copolymer of about 98% by weight trioxane and about 2% dioxolane. CELCON is a registered trademark of Hoechst Celanese Corporation. Compositions of the present invention can be prepared using any commercial grade of CELCON acetal, including U-10, M-25, M-90, M-270 and M-450. CELCON M-25 acetal copolymer has a melt index of about 2.5 g / 10 min when tested according to ASTM D1238-82. CELCON M-90 acetal copolymers have a lower molecular weight and melt viscosity than CELCON M-25.

Oxymethylene terpolymers can also be used to make the self-lubricating compositions of the present invention. These terpolymers contain oxymethylene groups, oxy (higher alkylene) groups such as those corresponding to formula (2) and different third groups polymerized with oxymethylene and oxy (higher alkylene) groups.

Formula 2

The terpolymers described above are typically made by reacting trioxane with a third monomer which is a bifunctional compound such as cyclic ether or cyclic acetal and diglysid of formula (4).

Wherein Z is a carbon-carbon bond, an oxygen atom, an oxyalkoxy of 1 to 8 carbon atoms, preferably 2 to 4 carbon atoms, an oxycycloalkoxy group of 4 to 8 carbon atoms or oxypoly (lower alkoxy) Group, preferably having 2 to 4 repeated lower alkoxy groups each having 1 to 2 carbon atoms. Examples of suitable difunctional compounds include diglycidyl ethers of ethylene glycol, 1,2-propanediol and 1,4-butanediol, with diglycidyl ethers of 1,4-butanediol. Generally, when preparing the terpolymer, 99.89 to 89.0 weight percent trioxane, 0.1 to 10 weight percent cyclic ether or cyclic acetal and 0.01 to based on the total weight of monomers used to form the terpolymer Preference is given to 1% by weight bifunctional compound. Particularly preferred oxymethylene terpolymers contain from about 0.05%, 2.0% and 97.95% by weight of 1,4-butanediol diglycidyl ether crosslinker, dioxolane and trioxane, respectively, based on the total weight of the terpolymer Acetal polymers under the name CELCON U10 made commercially are commercially available from Hoechst Celanese Corporation. Oxymethylene-based terpolymers are prepared and stabilized by methods well known to those skilled in the art, such as the addition of antioxidants and formaldehyde and acid carriers. More detailed descriptions of the oxymethylene-based terpolymers and methods of preparing their compositions can be found in the previously cited patents.

These oxymethylene polymers can be combined in various ratios by melt mixing in an extruder or similar apparatus forming a suitable polymer for the preparation of the self-lubricating compositions of the present invention. Generally, polyoxymethylene polymers are easily mixed with the lubrication system and process aids in the molten state of the polymer, ie at a temperature of at least about 170 ° C.

The lubrication system of the present invention is characterized by containing ultra high molecular weight polyolefin, pentaerythritol tetrastearate and calcium carbonate. Ultra high molecular weight polyolefins have an inherent crystallinity of about 40%, a weight-average molecular weight of at least about 3x10 ⁶ (typically from about 5x10 ⁶ to about 6x10 ⁶ ), at least about 28 dl / g (measured by ASTM No. D4020) Straight chain polyethylene exhibiting a viscosity, a bulk density of at least about 0.5 g / cm 3 (measured by ASTM No. D1895) and a specific gravity of about 0.93 g / cm 3. Particularly preferred is Ultra High Molecular Weight (UHMW) polyethylene encountered with FDA / USDA compliance, Hostalen ^R GUR 415 UHMW polyethylene distributed by Hoechst Celanese Corporation of Somerville, NJ. In general, the lubrication system of the present invention is characterized by containing at least about 0.1% by weight UHMW polyethylene, 0.25% by weight PETS and at least about 0.25% by weight calcium carbonate, based on the total weight of the composition. Typically, the lubrication system comprises from about 0.2 to about 10.0 weight percent UHMW polyethylene, from about 0.25 to about 2.0 weight percent PETS and from about 0.25 to about 4.0 weight percent calcium carbonate based on the total weight of the composition do. Preferably, the lubrication system is characterized by containing about 1.5% by weight UHMW polyethylene, 1.0% by weight PETS and about 1.0% by weight calciumate, based on the total weight of the composition.

Several additional elements may be added to the compositions of the present invention to assist in lubrication and processing. In general, the additive may be proportionally combined with the lubrication system and mixed as a package with the thermoplastic polymer or may be added directly to the composition. These additives may comprise (a) at least about 0.25 weight percent polyoxymethylene terpolymer, based on the total weight percent of the composition; (b) at least about 0.25 weight percent sterically hindered phenol; (c) at least about 0.05% by weight of calcium ricinoleate or calcium hydroxystearate. Typically, these additives comprise (a) about 0.25 to about 2.0 weight percent polyoxymethylene terpolymer, based on the total weight percent of the composition; (b) about 0.25 to 0.75 weight percent of hindered phenols; (c) can be admixed with the self-lubricating composition in an amount selected from about 0.05 to 0.3 weight percent calcium ricinoleate or calcium hydroxystearate. Preferably, these additives comprise (a) about 0.5 weight percent polyoxymethylene terpolymer, based on the total weight of the composition; (b) about 0.4 weight percent sterically hindered phenol; (c) can be admixed with the composition in an amount of 0.01% by weight of calcium ricinoleate or calcium hydroxystearate. The addition of these process aids will typically be settled in accordance with the amount of thermoplastic resin. Other processing aids known to those skilled in the art, such as silicone or fluoropolymer molding sprays, can also be used as auxiliary molding agents.

Calcium carbonates useful in the present invention are characterized by showing a particle size of about 0.6 μg, a surface area of about 7 m 2 / gm, a bulk density of about 25 lb / ft ³ , and a specific gravity of about 2.7. Preferred calcium carbonate is Pfizer, Inc. Super-Pflex ^R 200 available from.

The hindered phenols useful in the present invention are generally known as antioxidants or free radical inhibitors. 2,2'-methylenebis (4-methyl-6-t-butylphenol), hexamethylene glycol-bis (3,5-di-t-butyl-4-hydroxyhydrocinnamate), tetrabis [methylene ( 3,5-di-t-butyl-4-hydroxyhydrocinnamate)] methane, triethyleneglycol-bis-3- (3-t-butyl-4-hydroxy-5-methylphenyl) propionate, 1 , 3,5-trimethyl-2,4,6-tris (3,5-di-t-butyl-4-hydroxy-benzyl) -benzene, p-octadecyl-3- (4'-hydroxy-3 ', 5'-di-t-butyl-phenol) propionate, 4,4'-methylenebis (2,6-di-t-butylphenol), 4,4'-butylidene-bis- (6 -t-butyl-3-methylphenol), 2,2'-thiodiethyl-bis- [3- (3,5-di-t-butyl-4-hydroxyphenol)] propionate, di-ste Aryl-3,5-di-t-butyl-4-hydroxybenzylphosphonate and 2-t-butyl-6- (3-t-butyl-5-methyl-2-hydroxybenzyl) -4-methylphenyl At least one of the acrylates may be used. However, useful hindered phenols are not limited to these compounds. Other steric hindrances or spatially hindered phenols of the same kind as those described above are also effective. Among them, hexamethylene glycol-bis (3,5-di-t-butyl-4-hydroxyhydrocinnamate), for example Irganox ^R 259, tetrabis [methylene (3,5-, available from Ciba-Geigy) Di-t-butyl-4-hydroxyhydrocinnamate)] methane, for example Irganox 1010 and triethylene glycol-bis-3- (3-t-butyl-4-hydroxy- produced by Ciba-Geigy 5-methylphenol) propionate, for example Irganox 245 manufactured by Ciba-Geigy, is effective. Preferred hindered phenols are hexamethylene glycol-bis (3,5-di-t-butyl-4-hydroxyhydrocinnamate).

The following examples are general illustrations of the process for preparing the polymer compositions of the present invention. They are provided solely for the purpose of supporting the detailed description.

Example 1

To prepare a mixture of self-lubricating compositions containing 7.9% by weight of lubrication system, the following elements are used:

a) 90 lbs of polyoxymethylene copolymer labile flakes;

b) 4.9 lbs of UHMW polyethylene;

c) 1.5 lbs polyoxymethylene terpolymer;

d) 1.0 lbs of pentaerythritol tetrastearate (PETS);

e) 2.0 lbs calcium carbonate;

f) 0.1 lbs of calcium ricinoleate;

g) 0.5 lbs of preferred hindered phenol.

The compound is mixed in a vat and then mixed quickly by a high speed mixing in a Henschel mixer for 30 seconds to form a mixture. The mixture is extruded into strands in a Werner and Pfleiderer twin screw ZSK extruder, heated and purged with polyacetal pellets. The extruder zone is operated at 372 to 387 ° F, the melting temperature is 415 ° F, under vacuum of 27 in mercury and the rotational speed is 150 rpm. The strand is then quenched in cold water and cut into pellets. Pellets have conventional pressure, speed and rotational settings, nozzle temperature settings from 360 to 420 ° F, and barrel temperature settings from 350 to 420 ° F, which form 1.25 diameter discs weighing 7 gm each for mechanical and friction analysis. Injection molded in.

The disc is prepared for surface abrasion resistance and torque analysis at a weight of 0.1 mg by washing in methanol bath and drying in air. These discs are tested according to the Pin-on-Disk Wear Test. In performing this test, a mechanical Nylatron Nylon pin with a spherical tip of 0.187 inch radius was mounted on the upper layer of the Falex Friction and Wear Test Machine, Model Multi-Specimen, 0.469 inch from the center of the test disc mounted on the lower spindle. Mount on the spindle. A 20 pound load is applied to the test disc by means of an air cylinder applied to the disc against the spherical pin tip. The rotation speed is 425 rpm (104.3 ft / min). During the test, the air stream is directed against the disk surface at a distance of 40 standard cubic feet per hour and 2 inches to remove debris. Test times range from 0.5 to 65 hours. After the test, the pin tip and the disk are separated from the contact and the disk is removed and shaken in air to remove debris, and the lost weight, ie surface wear, is measured.

The torque (Γ) measured during the test is converted into a friction coefficient f by applying the following equation.

[Equation 1]

f = Γ (2.137 / 20)

2.137 The factor is a special factor on this machine. Results for surface wear and friction coefficients are shown in Table 1.

Comparative Example 2

As a comparative example, in place of the lubrication system of Example 1, the polymer composition was prepared by substituting 1.5% by weight of polytetrafluoroethylene (PTPE), the following elements being used:

a) 213.84 lbs polyoxymethylene copolymer labile layer (97.2 wt%);

b) 1.1 lbs polyoxymethylene terpolymer pellets (0.5 wt%);

c) 199.76 gm of N, N'-ethylene bis-stearamide (0.2 wt%);

d) 1.1 lbs of preferred sterically hindered phenol (0.5 wt%);

e) 99.88 gm of calcium ricinoleate (0.1 wt%);

f) 3.3 lbs. PTFE (1.5 wt% of total weight).

The composition is mixed, extruded and molded according to the process of Example 1 to form a 7 gm disc for weight loss and coefficient of friction analysis. The analysis results are shown in Table 1.

Comparative Example 3

As a comparative example, instead of the lubrication system of Example 1, the polymer composition was prepared by substituting 3.0 wt% PTPE, and the following elements were used:

a) 210.54 lbs polyoxymethylene copolymer labile layer;

b) 1.1 lbs polyoxymethylene terpolymer pellets (0.5 wt%);

c) 199.76 gm of N, N'-ethylene bis-stearamide (0.2 wt%);

d) 1.1 lbs of preferred sterically hindered phenol (0.5 wt%);

e) 99.88 gm of calcium ricinoleate (0.1 wt%);

f) 6.6 lbs. PTFE (3.0 wt% of gross weight).

The composition is mixed, extruded and molded according to the process of Example 1 to form a 7 gm disc for weight loss and coefficient of friction analysis. The analysis results are shown in Table 1.

The analytical results show that after 0.5 hour of pin-on-disk test, the sample of Example 1 exhibited an average weight loss of 1.1 mg; After a 17 hour test, the disks exhibited an average weight loss of 4.3 mg and a coefficient of friction of 0.075; After a total of 65 hours of testing, the discs show an average weight loss of 5.4 mg.

Abrasion test results for discs made from compositions containing PTFE as a lubrication system show significantly higher surface wear and coefficient of friction than the present invention.

In order to show the wear performance of the composition in the presence and absence of lubrication system components, several formulated examples are prepared according to the method of Example 1 above, which are as follows:

Example 4

ingredient wt%

Polyoxymethylene Copolymer 95.5

Polyoxymethylene Terpolymer0.5

Preferred sterically hindered phenol 0.4

Calcium Ricinoleate 0.1

UHMW Polyethylene 1.5

PETS 1.0

Calcium Carbonate 1.0

Example 5

ingredient wt%

Polyoxymethylene Copolymer 97

Polyoxymethylene Terpolymer0.5

Preferred sterically hindered phenol 0.4

Calcium Ricinoleate 0.1

UHMW Polyethylene 0

PETS 1.0

Calcium Carbonate 1.0

Example 6

ingredient wt%

Polyoxymethylene Copolymer 96.5

Polyoxymethylene Terpolymer0.5

Preferred sterically hindered phenol 0.4

Calcium Ricinoleate 0.1

UHMW Polyethylene 1.5

PETS 0

Calcium Carbonate 1.0

Example 7

ingredient wt%

Polyoxymethylene Copolymer 96.5

Polyoxymethylene Terpolymer0.5

Preferred sterically hindered phenol 0.4

Calcium Ricinoleate 0.1

UHMW Polyethylene 1.5

PETS 1.0

Calcium Carbonate 0

Example 8

ingredient wt%

Polyoxymethylene Copolymer 98.7

Polyoxymethylene Terpolymer0.5

Preferred sterically hindered phenol 0.5

Calcium Ricinoleate 0.1

N, N'-ethylene bis- stearamide 0.2

UHMW Polyethylene 0

PETS 0

Calcium Carbonate 0

Example 9

ingredient wt%

Polyoxymethylene Copolymer 96.6

Polyoxymethylene Terpolymer0.5

Preferred sterically hindered phenol 0.4

Calcium Ricinoleate 0.1

UHMW Polyethylene 0.4

PETS 1.0

Calcium Carbonate 1.0

Example 10

ingredient wt%

Polyoxymethylene Copolymer 96.2

Polyoxymethylene Terpolymer0.5

Preferred sterically hindered phenol 0.4

Calcium Ricinoleate 0.1

UHMW Polyethylene 0.8

PETS 1.0

Calcium Carbonate 1.0

The compositions of Examples 4-10 were molded into test disks and tested for wear rate according to the Pin-on-Disk Wear Test in Example 1. The test results are shown in Table 1.

Weight loss, mg ¹ / coefficient of friction Example Lubrication system, wt% Test time (unit; time) 0.5 1.5 7 65 One 7.9 1.1 1.4 4.3 / 0.075 5.4 2 1.5 PTFE 8.1 15.9 122 / 0.14 181 3 3.0 PTFE 3.4 15.9 87 / 0.13 155 4 3.5 1.8 2.1 4.2 8.6 5 2.0 n / d ² 2 45 n / d 6 2.5 n / d 4.1 167 n / d 7 2.5 n / d 2.4 64 n / d 8 0 n / d 113 260 n / d 9 2.4 n / d 3.0 6.9 30.1 10 2.8 n / d 2.8 7.8 12.4 Load at ¹ @ 20 lbs; Speed of 104.3 ft / min ² n / d: no data

The table shows that the compositions of Examples 4, 9 and 10 containing UHMW polyethylene, calcium carbonate and PETS provide good abrasion properties after 17 and 65 hours of abrasion testing as compared to compositions containing less of these three components. .

Claims (20)

Forming a low friction molding, consisting of a melt mixture of from about 70 to about 99.5 weight percent thermoplastic polymer and from about 30 to about 0.5 weight percent lubrication system comprising ultra high molecular weight polyolefin, pentaerythritol tetrastearate, and calcium carbonate Self-lubricating composition suitable for. The composition of claim 1 wherein the thermoplastic polymer is selected from the group consisting of polyamides, polyesters, polyphenylene sulfides, polyoxymethylenes, styrene polymers and polycarbonates. The composition of claim 2 wherein the thermoplastic polymer is polyoxymethylene. 4. The composition of claim 3 wherein the polyoxymethylene is selected from the group consisting of (i), (ii), (iii) and (iv) below. (i) oxymethylene homopolymers; (ii) an oxymethylene copolymer consisting of from about 85 to about 99.9% of oxymethylene repeating units mixed with repeating units of formula (2); Formula 2 Wherein R ₁ and R ₂ are each selected from the group consisting of hydrogen, lower alkyl radicals and halogen-substituted lower alkyl radicals, each lower alkyl radical having 1 to 2 carbon atoms, and R ₃ each being methylene , Oxymethylene, lower alkyl and haloalkyl-substituted methylene, and lower alkyl and haloalkyl-substituted oxymethylene radicals, and n is an integer from 0 to 3, inclusive. (iii) an oxymethylene terpolymer that is the reaction product of trioxane and cyclic ethers and / or cyclic acetals, and diglycidyl ether crosslinkers of formula (4); Formula 4 (Wherein Z is selected from the group consisting of carbon-carbon bonds, oxygen, oxyalkoxy units of 1 to 8 carbon atoms, and oxypoly (lower alkoxy) units.) (iv) a mixture of (i), (ii) and (iii). The composition of claim 4 wherein the polyolefin is a straight chain, ultra high molecular weight polyethylene. The composition of claim 5 wherein the polyethylene exhibits at least about 40% crystallinity, a molecular weight of at least about 3 × 10 ⁶ , an intrinsic viscosity of at least about 28 dl / g, a bulk density of about 0.5 g / cm 3 and a specific gravity of at least about 0.93. . 7. The composition of claim 6 consisting of about 85 to about 99 weight percent polyoxymethylene and about 15 to about 1 weight percent lubrication system. 8. The composition of claim 7, wherein the lubrication system comprises at least about 0.1 weight percent ultra high molecular weight polyethylene, at least about 0.1 weight percent pentaerythritol tetrastearate and at least about 0.1 weight percent calcium carbonate. 9. The composition of claim 8, comprising (a) about 0.1 weight percent polyoxymethylene terpolymer, based on the total weight percent of the composition; (b) about 0.1% by weight of sterically hindered phenols; And (c) about 0.05% by weight calcium ricinoleate or calcium hydroxystearate. Molds made from the self-lubricating composition according to claim 9, exhibiting a weight loss of about 4.2 mg and a coefficient of friction of about 0.075 due to abrasion after 17 hours at a rotational speed of 104.3 ft / min and an applied load of about 21 lbs. . (a) preparing a melt mixed composition consisting of about 30 to about 0.5 weight percent lubrication system comprising about 70 to about 99.5 weight percent thermoplastic polymer and ultra high molecular weight polyolefin, pentaerythritol tetrastearate and calcium carbonate; (b) preparing the composition into a molding which exhibits an improved coefficient of friction and surface wear resistance. 12. The process of claim 11 wherein the thermoplastic polymer is selected from the group consisting of polyamides, polyesters, polyphenylene sulfides, polyoxymethylenes, styrene polymers and polycarbonates. 13. The method of claim 12, wherein the polyoxymethylene is selected from the group consisting of (i), (ii), (iii) and (iv) below. (i) oxymethylene homopolymers; (ii) an oxymethylene copolymer consisting of from about 85 to about 99.9% of oxymethylene repeating units mixed with repeating units of formula (2); Formula 2 Wherein R ₁ and R ₂ are each selected from the group consisting of hydrogen, lower alkyl radicals and halogen-substituted lower alkyl radicals, each lower alkyl radical having 1 to 2 carbon atoms, and R ₃ each being methylene , Oxymethylene, lower alkyl and haloalkyl-substituted methylene, and lower alkyl and haloalkyl-substituted oxymethylene radicals, and n is an integer from 0 to 3, inclusive. (iii) an oxymethylene terpolymer that is the reaction product of trioxane and cyclic ethers and / or cyclic acetals, and diglycidyl ether crosslinkers of formula (4); Formula 4 (Wherein Z is selected from the group consisting of carbon-carbon bonds, oxygen, oxyalkoxy units of 1 to 8 carbon atoms, and oxypoly (lower alkoxy) units.) (iv) a mixture of (i), (ii) and (iii). The method of claim 13, wherein the composition consists of about 85 to about 99 weight percent polyoxymethylene and about 15 to about 1 weight percent lubrication system. 15. The method of claim 14, wherein the polyoxymethylene is an oxymethylene copolymer consisting of about 85 to about 99.9% of oxymethylene repeating units mixed with repeating units of formula (2). Formula 2 Wherein R ₁ and R ₂ are each selected from the group consisting of hydrogen, lower alkyl radicals and halogen-substituted lower alkyl radicals, each lower alkyl radical having 1 to 2 carbon atoms, each of R ₃ Methylene, oxymethylene, lower alkyl and haloalkyl-substituted methylene, and lower alkyl and haloalkyl-substituted oxymethylene radicals, n being an integer from 0 to 3, inclusive. The method of claim 15, wherein the lubrication system comprises at least about 0.25 weight percent ultra high molecular weight polyethylene, at least about 0.25 weight percent pentaerythritol tetrastearate and at least about 0.25 weight percent calcium carbonate. The method of claim 16, wherein the lubrication system comprises about 0.1 wt% ultra high molecular weight polyethylene, about 0.1 wt% pentaerythritol tetrastearate, and about 0.1 wt% calcium carbonate. 18. The composition of claim 17, wherein the composition comprises (a) about 0.5 weight percent polyoxymethylene terpolymer, based on the total weight percent of the composition; (b) about 0.4 weight percent sterically hindered phenol; And (c) about 0.1% by weight of calcium ricinoleate or calcium hydroxystearate. A molding made according to the method of claim 18, exhibiting a weight loss of about 4.3 mg and a friction coefficient of less than about 0.05 after 17 hours at a rotational speed of 104.3 ft / min and an applied load of about 21 lbs. 20. The molding according to claim 19 selected from the group consisting of bearings, gears, cams, rollers, sliding plates, pulleys, levers, guides and conveyor belt links.