WEAR RESISTANT ARTICLE MADE FROM A THERMOSETTING POLYMER AND A FLUOROCARBON ADDITIVE

Abstract: A wear resistant article comprising a thermosetting polymer-fluorocarbon composition and method for making same.

Description: TITLE WEAR RESISTANT ARTICLE MADE FROM A THERMOSETTING POLYMER AND A FLUOROCARBON ADDITIVE BACKGROUND OF THE INVENTION Technical Field 5 This invention relates to a wear resistant article comprising a thermosetting polymer-fluorocarbon composition and to a method for making said article. DESCRIPTION OF RELATED ART There is abundant literature on the use of additives or surface coatings to improve the performance in a variety of applications of articles manufactured from 10 polymers. Although surface treatments have long been used to improve wear resistance, the incorporation of fluorocarbons into polymers and elastomers to produce a composition from which wear resistant articles can be made is not known to the art. U.S. Patent No. 5,061,759 relates to rubber compositions vulcanizable by 15 means of peroxides and containing processing-coadjuvating additives which are perfluoropolyethers in amounts ranging from 0.5 to 1 part by weight of the rubber composition. The additives improve processability in extrusion and give better detachability of a vulcanized article from a molding die without adversely affecting the vulcanizing system or the properties of the vulcanized article. The only articles 20 described in this patent are test pieces for evaluation of physical properties. There is no reference to wear resistance of the articles. Japanese Patent Application Pub. No. 60-104161 describes an abrasion resistant molding material comprising a thermosetting resin or a thermoplastic resin with a fluorinated oil, preferably 1-10%, uniformly adhered to, impregnated or 25 dispersed jn the molding material. Suitable fluorinated oils include perfluoropolyethers (such as, hexafluoropropylene oxide oligomers), and fluorocarbon oils (such as, tetrafluoroethylene oligomers). The molding material is used to make more wear resistant sliding components for electrical contact switches, electromagnetic switches and the like. U.S. Patent No. 5,143,963 and U.S. Patent No. 5,286,773 relate to a composition of matter formed by melt blending a thermoplastic polymer and from 0.01% to less than 1% of a fluorocarbon additive and to a method of forming the composition. The fluorinated additive is a fluorocarbon oil, gum or grease including fluorinated hydrocarbons and fluorinated hydrocarbon polyether oils. The compositions are said to enhance and accelerate molding and extrusion operations for films and fibers. The basic bulk mechanical, physical and chemical properties of the thermoplastic polymers are retained or even enhanced. Due to the concentration of the fluorocarbon additive at the surface, articles made from the composition acquire fluorocarbon-like surface properties. There is no disclosure that the articles are wear resistant. The composition is useful for medical products e.g., vascular grafts, mammary or ocular implants, electronic equipment, electro-optical or electro-mechanical components and as molds for precision parts for such devices. U.S. Patent No. 4,481,333 describes a thermoplastic composition which comprises mixtures of vinyl chloride polymers and chlorinated polyethylene and which contains up to 0.4% of finely divided fluoropolymers, such as polytetrafluoroethylene, polytrifluoroethylene, polyvinyl fluoride, polyvinylidene fluoride and fluorinated polyethers e.g., perfluorovinylpolyether. The composition has a markedly improved processability, particularly when shaped into articles by extrusion. The use of such compositions for making shaped articles such slabs, tubes, and sheeting is described. There is no mention of such articles having improved wear resistance. There is a continuing need to improve performance in use, simplify production method and improve cost effectiveness for articles made from polymers. An object of the present invention is to make available articles which are useful in applications where minimum wear is required. Articles made from thermosetting polymer-fluorocarbon compositions by the procedures of this invention are found to show improved wear resistance in a wide range of applications. Incoφoration of a fluorocarbon with the polymer greatly increases the longevity or permanence of the beneficial effect compared to surface treatment of the polymeric article with a fluorocarbon. SUMMARY OF THE INVENTION It has now been found that a wear resistant article can be made from a thermosetting polymer (or mixed thermosetting polymers) and about 0.01% to less than 1%, typically about 0.1% to less than 1%, bulk concentration by weight of an additive consisting of a fluorocarbon (or mixture of fluorocarbons), said fluorocarbon additive having a lower surface energy than that of said polymer and the concentration of said fluorocarbon being substantially higher at the surface of said article than the nominal bulk concentration. By "wear resistant" it is meant that the article will have a longer life in use compared to an article that is identical except for the absence of the fluorocarbon additive. The wear resistant article may also contain conventional compounding additives such as fillers, reinforcing agents colorants and the like. A valuable property of an article of this invention is that its wear resistant character extends throughout its useful life. In a further aspect this invention relates to a method for making a wear resistant article which consists of mixing a thermosetting polymer, a fluorocarbon and other compounding additives and producing the desired shape by an appropriate procedure such as molding or extrusion. DETAILED DESCRIPTION OF THE INVENTION A wear resistant article of this invention comprises a thermosetting polymer, or mixed thermosetting polymers, and an additive consisting of fluorocarbon or mixture of fluorocarbons, the proportion of the latter ranging from about 0.01% to less than 1%, typically from about 0.1%, to less than 1% bulk concentration by weight of the amount of polymer, said fluorocarbon additive having a lower surface energy than that of said polymer and the concentration of said fluorocarbon being substantially higher at the surface of said article than the nominal bulk concentration. The wear resistant article may also contain conventional compounding additives such as fillers, reinforcing agents, colorants and the like. A valuable property of an article of this invention is that its wear resistant character is retained throughout its useful life. This invention also relates to a method for incoφorating a thermosetting polymer with a fluorocarbon and other compounding additives and for forming a wear resistant article of this invention from such a composition. Wear Resistant Article As used herein the term "gasket" includes (1) articles inserted between two relatively hard surfaces to cushion their contact, and (2) articles that contact against a hard surface to form a seal, such as weather-stripping or lubricant seals; and the term "blade" includes articles that move across a hard surface to remove a substance, such as windshield wipers and squeegees. Preferred wear resistant articles of this invention are stationary or moving parts which are in sliding contact with stationary or moving surfaces, such as wear pads, windshield wiper blades, conveyer line sections, engine drive belts, gears, contact switches, magnetic switches, luggage straps and buckles, footwear (particularly footwear for athletic puφoses), window hardware components, seals between flanges such as gaskets for engines, for lines that carry gases or liquids, for pumps, for reaction vessels and the like, seal caps and boots for lubricated joints, wire harness connectors, spark plug caps, oriented bumpers, bumper suspensions, tail light assemblies, parts for domestic and commercial appliances. The wear resistant articles of this invention are suitable in applications where some flexibility, and the maintenance of a high level of performance when exposed to adverse conditions over extended periods, are desirable. They meet these requirements when they are used as sealing elements or as items for aesthetic or functional puφoses. The recent trend towards using harder materials was prompted by deficiencies inherent in softer materials known to the prior art. The softer materials do not provide as good or as durable a seal when used as gaskets because they tend to develop shape deformities on prolonged exposure to adverse conditions. Such deformities may also evidence themselves as unsightly bulges and irregularities which is undesirable in accessories intended to have esthetic appeal, such as window hardware and articles used in architectural applications. The harder materials currently used largely overcome these problems. However there is a need for improvement in applications where components are subjected to shifts, torques or vibrations, such as in a vehicle, travelling on bumpy road surfaces or when changing speed. To some extent this can be overcome by using surface coatings. Surface coatings are often lacking in durability so that the rate of wear tends to increase over the life of the treated article. A particular advantage of the wear resistant articles of this invention derives from compounding the fluorocarbon material and thermosetting polymer components prior to fabrication of the desired shape. Migration of the fluorocarbon towards the surface of the article occurs during the fabrication step so that the concentration of fluorocarbon is much greater at the surface than the nominal bulk concentration. The high concentration of fluorocarbon additive near the surface gives rise to fluorocarbon-like surface properties such as, lower friction, hydrophobicity, chemical inertness and non-adherent surfaces. These properties are often beneficial and in particular the extent to which wear occurs is considerably decreased and it can be virtually eliminated in many situations. An important additional benefit which derives from mixing the fluorocarbon component with the thermosetting polymer and other additives prior to or during fabrication is the greater longevity of performance compares with that obtained with post applied surface coatings. Thermosetting Polymer Resin Thermosetting polymer resins are used in the compositions from which the wear resistant articles of this invention are made. Thermosetting resins are converted to infusible material during the fabrication process and thus the finished product is no longer susceptible to molten reprocessing without severe property losses. Both hard and soft categories of thermosetting resins are useful to make compositions suitable for fabricating the wear resistant articles of this invention. In general hard polymer resins can be classified as having a Rockwell R scale hardness in the range of from about 50 to about 115, as measured by ASTM method D-785-89. Soft polymer resins are those having a Durometer Shore A scale hardness in the range of from about 10 to about 90, as measured by ASTM method D-2240-91. There is some degree of overlap between the top of the range on the Shore D scale and the bottom of the range on the Rockwell R scale. Although measured Shore D scale hardness values above 90 and Rockwell R scale hardness values above 115 are not thought to be reliable; polymers having such hardness values fall within the purview of this invention. A polymer resin having a hardness that lies within both the designated ranges can be used in compositions from which the wear resistant articles of this invention are made. Thermosetting polymer resins are used in the form of molding granules or powders and are mixed with fluorocarbons and other compounding additives prior to or during the fabrication step. Thermosetting hard and soft polymer resins which are suitable for making of the wear resistant articles of the present invention, include, but are not necessarily limited to: allyl molding compounds, bis-maleimides, epoxy resins, phenolic resins, polyesters, ethylene-propylene-diene teφolymers (e.g., EPDM rubber), polyimides, ionomers (e.g., Surlyn®), polyurethanes, segmented polyurea/urethanes, silicones and urea-melamine formaldehyde resins. Thermosetting resins are also meant to include thermoplastic resins which have been cross-linked to a high enough degree such that molten reprocessing is no longer possible without severe property losses. Mixtures of thermoplastic and thermosetting polymers, such as EPDM rubber "alloys" with polypropylene (Santoprene®) and with PVC (Alcryn®) are also suitable for making the wear resistant articles of this invention. Articles comprising foamed polymer-fluorocarbon compositions are also within the purview of this invention. Articles fabricated from the above compositions possess excellent wear resistance in a wide variety of uses and many of them are attractive from the standpoint of availability and cost. Additives For Polymer Resin The polymer resins used in the wear resistant articles of this invention may also contain one or more of the following conventional resin compounding ingredients, such as but are not necessarily limited to: organic or inorganic fillers, reinforcing agents, colorants, thermal stabilizers, antioxidants, antiozonants, antistatic agents, antimicrobial agents, plasticizers, lubricants, antifogging agents, coupling agents, flame retardants, foaming agents, fragrances, heat stabilizers, impact modifiers, mold release agents, titanates, ultraviolet stabilizers, thermally conductive fillers, electrically conductive fillers, curing agents, cross-linking agents catalysts, and the like. Fluorocarbon Additives Suitable fluorocarbon additives are oils, gums or greases comprising fluorinated hydrocarbons or fluorinated hydrocarbonpolyethers having six or more carbon atoms, including linear, branched and cyclic compounds. Examples of suitable compositions are perfluoroalkylpolyethers, ( e.g. Krytox® Fomblin®, Demnum® and Aflunox®), perfluoropolyethylene oxide, perfluoropolypropylene oxide, polytetrafluoroethylene (Teflon®), perfluoropolyethylene-polypropylene, perfluoropolybutadiene, polyvinylidene fluoride perfluorocarbons and fluorohydrocarbons including ethers containing functional group(s) such as but not limited to alcohols, amines, amides, esters, nitriles, thiols, acids, acid halides, including chlorine , bromine and iodine. Higher molecular weight homologous linear and branched fluorohydrocarbons and fluorinated cyclic hydrocarbons may also be used, as well as partially fluorinated additives, such as those more than 50% fluorinated, preferably more than 75% fluorinated. For example, polychlorotrifluoroethylenes, polytetrafluoroexetanes, or other highly fluorinated compounds may be selected. Fluorocarbon or mixed fluorocarbon additives compounded with thermosetting polymer resins preferably have a surface energy substantially lower than that of the polymers. This leads to migration of the fluorocarbon to the surface of the wear resistant article during the fabrication process. Generally it is preferred that a fluorocarbon have a surface energy at least 5 dynes/cm lower than the surface energy of the polymer with which it is compounded. The amount of fluorocarbon which may be compounded with a polymeric material is in the range of about 0.01% to less than 1%, typically from about 0.1% to less than 1%, bulk concentration on a weight basis. The beneficial effects of the fluorocarbon additive are evident at the lowest end of the range, due to the concentration at the surface being much higher than the nominal bulk concentration. At bulk loading levels of 0.1% and 1% the concentration of fluorocarbon in a surface layer 10 nm thick is respectively about 35% and about 88%. It is preferred that at least 0.1% of fluorocarbon be used in many of the shaped articles of the invention in order to obtain a desirable level of performance in most applications. The optimum amount of fluorocarbon additive will vary with the selected polymer, processing conditions, and intended use, and is readily determined by routine experimentation. Surface Concentration of Fluorocarbon Additive The relationship between the surface concentration and the nominal bulk loading of a fluorocarbon additive in a polymer is ascertained by determining the former by conducting the ESCA analytical procedure on test specimens, "poker chips", compounded with different levels of perfluoroalkylpolyether oil, (Krytox®). This procedure, which quantitatively analyzed the atomic concentration of elements in a surface layer 5 to 10 nanometers (nm) thick, is calibrated by applying a film of Krytox® oil to a glass slide and determining the level of fluorine in the film. This amount of fluorine, about 65wt%, corresponds to 100% coverage, with Krytox® oil. By determining the fluorine level in the surface layer of a PVC "poker chip" the level of Krytox® oil in the surface layer is ascertained. The data given in Table 1 are from a series of samples in which the Krytox® additive ranged from about 0.1 wt% to about 1.0 wt%. The level of Krytox® is clearly much higher in the surface layer than in the bulk of the sample. The degree to which fluorocarbon surface enrichment occurs depends upon the particular polymer and fluorocarbon components of a composition and the procedure used to mix them and fabricate an article of this invention. Table 1 Compounding the Polymer Resin Fluorocarbon Composition At some point during fabrication of a wear resistant article comprising a thermosetting polymer-fluorocarbon composition, the thermosetting polymer- fluorocarbon composition must be processed as a liquid. The fluorocarbon is completely or to a large degree immiscible with the polymer and other components. Sufficient mixing must be provided to disperse the fluorocarbon throughout the polymer-fluorocarbon composition. The fluorocarbon wets the processing equipment surfaces because of its low surface tension. Processing a thermosetting polymer-fluorocarbon composition from the liquid state to a solid article should include an essentially non-turbulent (i.e., not well mixed) flow field and adequate quench time to allow the fluorocarbon enriched surface layer to develop. Antithetically, if the liquid state were in a highly turbulent (i.e., well mixed) flow field and instantaneously quenched to the solid state considerably less fluorocarbon surface enrichment would occur. In order to maximize the fluorocarbon surface enrichment when the thermosetting polymer-fluorocarbon composition contains other additives which remain as solids when the polymer-fluorocarbon composition is being processed as a liquid, these solid ingredients should be wetted with molten polymer or molten composition ingredients other than the fluorocarbon prior to addition of the fluorocarbon. Otherwise, the fluorocarbon having a very low surface tension would wet the solid additive surfaces resulting in decreased fluorocarbon surface enrichment for the thermosetting polymer-fluorocarbon phase. Thermosetting polymer-fluorocarbon compositions can be made via two different routes: 1) Involves molten processing of additives and fluorocarbons with an already formed polymer with simultaneous or subsequent curing (i.e., cross-linking) to affect its thermoset character; 2) Involves contacting monomer, additive and fluorocarbon to form the thermosetting polymer directly; the fluorocarbon will need to be well mixed in the reactants as they contact each other, to form the enriched surface layer as the thermoset polymerization occurs. In either case, it is unlikely that the fluorocarbon will interfere with polymerization or curing reactions. In the case where molten processing of the thermosetting polymer-fluorocarbon occurs, some of the possible (but not inclusive) processing methods are: 1) dry mixing a polymer powder, a plasticizer, and other additives so as to wet the ingredients which remain solid during the molten processing, adding the fluorocarbon last to the dry blend, and feeding to a molten mixing device; 2) dry blending all the ingredients except the fluorocarbon, using the dry blend to feed the molten mixing device, and liquid injecting the fluorocarbon into a molten mixing device; 3) compounding the polymer and additives without the fluorocarbon and feeding the compounded polymer to a molten mixing device with liquid injection of the fluorocarbon into a molten mixing device. Liquid injection of the fluorocarbon into a molten mixing device is the preferred manner of adding fluorocarbon to the polymer-fluorocarbon composition. Some examples of molten mixing devices which can be used to mix the polymer-fluorocarbon composition are continuos mixers, extruders, kneaders and injection molding machines. When the thermosetting polymer is directly formed from the reactants, the fluorocarbon will need to be well mixed in with the reactants as they contact each other, and will need to form the enriched surface layer as the thermoset polymerization occurs. Fabrication of a Wear Resistant Article Thermosetting polymer-fluorocarbon articles, the concentration of said fluorocarbon being in the range from about 0.01% to less than 1% bulk concentration by weight can be formed by a number of established fabrication processes. The fabrication process will usually be determined by cost effectiveness, for example the elimination of topical application labor costs, and/or the need to fit existing process equipment. In turn, the fabrication process used to make the thermosetting polymer- fluorocarbon article will determine the nature of the polymer and fluorocarbon feedstocks to the process. If the fabrication process involves little or no mixing, e.g., compression molding, then a pre-compounded polymer-fluorocarbon composition would best serve as the feedstock. However, if the fabrication process employs a molten mixing device with sufficient mixing, the process feeds could be a post-compounded polymer-fluorocarbon composition, polymer with fluorocarbon liquid injection, or blends of polymer and a pre-compounded polymer-fluorocarbon composition. If the thermosetting polymer-fluorocarbon article is comprised of a polymer foam, then the maximum outer surface enrichment by the fluorocarbon will be obtained if the fluorocarbon surface enrichment largely occurs prior to the foam expansion and freezing step. Conversely, if the foamed polymer-fluorocarbon composition is processed molten as an expanded foam the fluorocarbon, due to its low surface tension, would enrich the surfaces of the interior foam cells decreasing enrichment of the outer surface of the article. Some examples of thermosetting polymer-fluorocarbon article fabrication processes include, but are not limited to: extrusion, injection molding, injection blow molding, extrusion blow molding, calendaring, pultrusion, reactive injection molding, and reactive extrusion. Migration of the fluorocarbon towards the surface of the article also occurs during the fabrication step so that the concentration of fluorocarbon is much greater at the surface than the nominal bulk concentration. The high concentration of fluorocarbon additive near the surface gives rise to fluorocarbon-like surface properties such as, lower friction, hydrophobicity, chemical inertness and non- adherent surfaces. These properties result in performance improvements such as decreased wear and noise reduction. An important additional benefit which derives from mixing the fluorocarbon component with polymer and other additives prior to or during fabrication is the greater longevity of performance compared with that obtained with post applied surface coatings. Some examples of thermosetting polymer-fluorocarbon article shapes that can be fabricated include, but are not limited to: fibers, rods, tubes, films, sheets, profiles and intricately shaped parts.

Claims: WHAT IS CLAIMED IS: 1. A wear resistant article comprising a thermosetting polymer and a fluorocarbon additive having a nominal bulk concentration in the range of from 0.01% to less than 1% by weight based on the polymer, said fluorocarbon additive having a lower surface energy than that of said polymer, the concentration of said fluorocarbon. 2. The article of claim 1 wherein the fluorocarbon is higher at the surface of said article than the nominal bulk concentration. 3. The article of claim 2 wherein the fluorocarbon additive having a nominal bulk concentration in the range of 0.1% to less than 1% by weight based on the polymer, the concentration of said fluorocarbon in an outer layer of said article having a thickness of 10 nanometers being respectively between 35% to 88% by weight. 4. The article of claim 3 wherein the polymer is from a polyurethane resin. 5. The article of claim 1, 2, 3 or 4 in the form of a gasket. 6. The article of claim 1, 2, 3 or 4 in the form of a blade. 7. The article of claim 3 wherein said fluorocarbon additive has a surface energy at least five dynes/cm lower than the surface energy of said polymer.

Inventors: DU PONT BENJAMIN F, DUCH MICHAEL WILLIAM, KOSINSKI LEONARD EDWARD RAYMON, DU PONT, BENJAMIN, F., DUCH, MICHAEL, WILLIAM, KOSINSKI, LEONARD, EDWARD, RAYMOND
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1998
Main classification: C08K5
IPC codes: C08K5, C08L101