Yarn blend for friction applications

Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Staple length fiber

Reexamination Certificate

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Reexamination Certificate

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06506491

ABSTRACT:

This invention relates to a spun staple yarn having improved frictional characteristics.
BACKGROUND OF THE INVENTION
Fluoropolymer fibers are known as friction modifiers in many different end uses. However, fluoropolymer fibers have low tensile modulus and are characterized by low adhesion to other resins; they are relatively high cost products, and some fluoropolymer are characterized by a tendency to cold flow. When used as a friction modifier in industrial applications, such as bearings, fluoropolymer fibers typically take one of the following forms: continuous fluoropolymer filament yarn wound around the surface of an article or part, continuous fluoropolymer filament yarn knitted or woven into fabrics, or continuous fluoropolymer filament yarn chopped into very short floc; this floc is then mixed with a resin and molded into articles or parts.
Although a fluoropolymer staple yarn offer advantages compared to the continuous filament yarn, heretofore, it has too difficult to make a spun fluoropolymer staple yarn. With a commercially viable spun staple fluoropolymer yarn, yarns and fabrics which use staple fibers having various cut lengths could be designed to meet the needs of industrial friction reducing applications.
Japanese Patent Application HEI 1[1989]-139833 discloses a fiber material having good flexibility made by mixing less than 30% polytetrafluoroethylene fibers or strands with natural and/or synthetic fibers. Fabrics and clothing made with the fiber material of this publication have both good draping and improved anti-pilling properties.
SUMMARY OF THE INVENTION
The present invention is a staple yarn comprising a blend of 25 to 90 weight percent fluoropolymer fiber and 75 to 10 weight percent of one or more types of blend fiber. The yarn or fabrics made from the yarn of the present invention function as friction modifiers in load-bearing products such as fabric liners for rubber parts, composite and fabric bearings, and sealing materials.
DETAILED DESCRIPTION
The present invention relates to a spun staple yarn having improved frictional characteristics and the load-bearing and other products products which may be made using this yarn as a component. The spun staple yarns of this invention are a combination of fluoropolymer fibers and blend fibers.
By “spun staple yarn” with reference to a synthetic fiber, it is meant yarn which is made by cutting continuous filament yarn or a continuous tow to a specified length to make a staple fiber and then processing it through common cotton system equipment to form a yarn from the staple. Common method used to make staple yarn include ring-spinning, open-end spinning, and air-jet spinning.
By “fluoropolymer” it is meant polymers such as poly(tetrafluoroethylene), abbreviated “PTFE” herein, and polymers generally known as fluorinated olefinic polymers, for example, copolymers of tetrafluoroethylene and hexafluoropropene (FEP), copolymers of tetrafluoroethylene and perfluoroalkyl-vinyl esters such as perfluoropropyl-vinyl ether (PFA) and perfluoroethyl-vinyl ether, fluorinated olefinic terpolymers including those of the above-listed monomers and other tetrafluoroethylene based copolymers. For the purposes of this invention the preferred fluoropolymer is PTFE.
Fluoropolymer continuous filament may be spun by a variety of means, depending on the exact fluoropolymer composition desired. The fibers may be spun by dispersion spinning; that is, a dispersion of insoluble fluoropolymer particles is mixed with a solution of a soluble matrix polymer, and this mixture is then coagulated into filaments by extruding the mixture into a coagulation solution in which the matrix polymer becomes insoluble. The insoluble matrix material may later be sintered and removed if desired. Alternatively, if melt viscosities are amenable, filament may also be spun directly from a melt. Fibers may also be produced by mixing fine powdered fluoropolymer with an extrusion aid, forming this mixture into a billet and extruding the mixture through a die to produce fibers which may have either expanded or un-expanded structures.
After the spinning process, the continuous filament fibers are then cut to staple lengths in the range of 0.5 to 3.0 inches. For the purposes of this invention the preferred staple length is from 1.5 to 2.0 inches. Denier per filament may range from 0.1 to 6.0.
By “blend fiber” is meant commonly available fibers including polyesters, polyamides, aromatic polyamides, polypropylenes, polyethylenes and their copolymers; natural fibers such as cotton, rayon, and wool; fibers made from high performance polymers including poly(para-phenylene terephthalamide(PPD-T), poly(para-phenylene benzobisoxazole (PBO), ultrahigh molecular weight polyethylene(UHMWPE), anisotropic melt polyesters; and high modulus fibers such as carbon fibers, metal fibers and glass fibers and any of the previously mentioned classes or types of fibers with special coatings, such as metal-coated para-aramid fibers. The cut staple lengths of the blend fiber may be any length which is compatible with the fluoropolymer staple. For the purposes of this invention, the preferred staple length is equal to or less than that of the fluoropolymer staple and is the range of 0.5 to 2.0 inches.
The fluoropolymer staple fiber and the blend staple fiber are combined and processed into a staple yarn using conventional methods. For example, the required amounts of each staple fiber can be blended together and then opened and carded to produce a carded sliver. The carded sliver can then be drawn and further attenuated into a roving and finally ring spun into a staple yarn.
The yarns of the present invention contain 25 to 90 weight percent fluoropolymer fiber and 75 to 10 weight percent blend fiber. A preferred weight distribution is 35 to 75 weight percent fluoropolymer content; 65 to 25 weight percent blend fiber content. A more preferred weight distribution is 35 to 65% fluoropolymer, 65 to 35% blend fiber. The practical frictional benefits of the fiber are reduced when the fluoropolymer content is below 35 weight percent for most industrial end uses. Above 90 weight percent fluoropolymer content, a usable staple fiber yarn is more difficult to produce since filament to filament cohesion in the yarn bundle is low. Also the yarn becomes less cost effective. A preferred yarn of the present invention is a 50 weight percent PTFE and 50 weight percent blend fiber. The preferred blend fiber is polyester fiber.
The yarns of this invention have a low yarn-to-metal coefficient of friction and allow the tailoring of both yarn structure and fiber content for the particular load-bearing application. For example, the blend fiber may be selected to increase an inherent deficiency in fluoropolymer fiber, such low modulus by selecting as the blend fiber a high modulus fiber. If desired, continuous filament yarns can be added to the staple yarn of this invention to provide additional strength or wear properties. For example, one or more continuous filament yarns can be used as core yarns with the blended staple fibers substantially wrapped or overspun around these yarns. If very high tenacity yarns with improved frictional or noise reduction properties are desired, a high tenacity continuous filament yarn could by wrapped by the staple fiber composition of the present invention. Alternatively, the blended staple fiber can first be made and then plied with the continuous filament yarns. It is also possible for one or more continuous filament yarns to substantially wrap the blended staple yarns. For example, a continuous filament thermoplastic yarn could wrap a fluoropolymer and thermoplastic blended staple yarn wherein the filament yarn would act as a sacrificial covering for the staple yarn.
Likewise, selection of blend fiber and/or the actual yarn structure, for example, the length of the staple fluoropolymer or blend fibers, the placement of the staple fibers in the body of the yarn or the yarn twist, may be optimized to compensate for shrinkage or adhesion problems known to exist with fluoro

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