IMPROVEMENT OF NOISE BEHAVIOR OF NON-ASBESTOS FRICTION MATERIALS THROUGH USE OF FLUOROPOLYMERS

Abstract: Friction materials which contain at least one fluoropolymer material for use in friction linings for applications such as, but not limited to brake disk pads, are described. The fluoropolymer material, such as, but not limited to PTFE, FEP, and PVDF, is preferably present in an amount of from about 0.1 to about 5 weight percent, based on the total weight of the friction material matrix.

Description: IMPROVEMENT OF NOISE BEHAVIOR OF NON-ASBESTOS FRICTION MATERIALS THROUGH USE OF FLUOROPOLYMERS BACKGROUND OF THE INVENTION 1. Technical Field The present invention relates generally to friction materials, and more particularly, to friction materials containing at least one fluoropolymer for use in friction linings for applications such as, but not limited to brake disk pads. 2. Discussion Friction materials, such as those typically employed in brake linings, are usually comprised of either asbestos fibers, mixtures of asbestos fibers and other heat resistant inorganic or organic fibers, asbestos-free mixtures of heat resistant inorganic or organic fibers, or metal powders such as iron powder, copper powder, steel powder or mixtures thereof, in combination with an organic monomeric or polymeric binder system (e.g., phenolic or cresylic resin). Because asbestos has been alleged to be the cause of certain health problems and is no longer environmentally acceptable, most modern friction materials are made without asbestos. Thus, most current friction materials are made from synthetic and steel fibers, and iron, ceramic, and metallic powders. A typical friction material formulation may optionally contain one or more of the following components: thermosetting resinous binders (e.g., phenolic resins such as phenol-formaldehyde resins, epoxies, and the like) present in conventional amounts; reinforcing fibers (e.g., aramid, steel, acrylic, and the like) present in conventional amounts; metal powders (e.g., iron, copper, brass, zinc, aluminum, antimony, and the like) present in conventional amounts; solid lubricants (e.g., molybdenum disulfide, graphite, coke, stannic sulfide, antimony trisulf.de, and the like) present in conventional amounts; abrasives (e.g., tin oxide, magnesia, silica, iron oxide, alumina, rutile, and the like) present in conventional amounts; organic fillers (e.g., rubber particles, cashew nut shell particles, nitrile rubber particles, and the like) present in conventional amounts; and inorganic fillers (e.g., barytes, gypsum, mica, and the like) present in conventional amounts. Other materials may be added as well, as is known in the art. In practice, friction materials are used in automotive brakes to slow (i.e., decelerate) and stop vehicles, by substantially converting kinetic energy into heat. In operation, the formed friction materials run against the cast iron mating surfaces of brake rotors or drums, depending on the configuration of the brake system. Friction materials are typically comprised of thermoset composites containing several different materials, as previously described. The important end-use performance aspects of friction materials which are of interest to automotive manufacturers are the friction behavior, wear life, and noise levels. Unsatisfactory noise and vibration behavior of brake systems is an important source of customer complaints and dissatisfaction. Customer dissatisfaction results in large warranty costs to automotive manufacturers. During use, the physiochemical changes at the interface between the friction material and the cast iron rotors/drums of the brake system govern the performance behavior of the friction couple. When friction materials run against the cast iron surfaces in a friction couple, transfer films (also referred to as a glaze) with complex chemical compositions are formed on the cast iron and friction material surfaces. The transfer films generated during brake use are thermal reaction products of the cast iron and the chemical ingredients present in the friction material. Transfer film formation is a dynamic process, i.e., the film is continuously generated and destroyed during brake use. The chemical and physical characteristics of the transfer film govern brake performance. The formation of a thin and uniform transfer film is desirable in brake operation. Conversely, uneven transfer films are thought to produce undesirable levels of noise and/or vibration in brake systems, especially during braking operations. Therefore, there exists a need for a friction material that is capable of continuously generating a thin and uniform transfer film, and which can be universally incorporated in a variety of compositions for the preparation of friction materials, especially in asbestos-free, semi-metallic, and/or low- metallic friction materials, and above all, offers low-noise and low-vibration levels during braking operations. SUMMARY OF THE INVENTION In accordance with one embodiment of the present invention, a friction material matrix comprises at least one fluoropolymer and at least one binder system. In accordance with another embodiment of the present invention, a friction material matrix comprises at least one fluoropolymer present in an amount up to about 2 weight percent based on the total weight of the friction material matrix, and at least one binder system. In accordance with still another embodiment of the present invention, a friction material matrix comprises at least one fluoropolymer present in an amount from about 0.1 to about 5 weight percent based on the total weight of the friction material matrix, wherein the fluoropolymer is selected from the group consisting of fluorinated ethylene propylene, polyvinylidene fluoride, polytetrafluoroethylene, and combinations thereof, and at least one binder system. A more complete appreciation of the present invention and its scope can be obtained from the following brief description of the drawings, detailed description of the invention, and the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a graphical illustration of the results of a dynamometer test of the noise behavior characteristics of a pair of brake pads having a brake lining formulation containing no fluoropolymer material; and Figure 2 is a graphical illustration of the results of a dynamometer test of the noise behavior characteristics of a pair of brake pads having a brake lining formulation containing at least one fluoropolymer material, in accordance with one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION The present invention proposes the use of fluoropolymers in friction materials to reduce or prevent unacceptable levels of brake noise during routine operation of the automobile's brake system. As that term is used herein, "friction material matrix" means at least one binder system (e.g., phenolic resin), and optionally, additives such as, but not limited to, reinforcing fibers, metal powders, abrasives, lubricants, organic fillers, inorganic fillers, and the like. As that term is used herein, "fluoropolymer" means at least one polymeric material containing fluorine. Fluoropolymers are composed basically of linear polymers in which some of or all the hydrogen atoms are replaced with fluorine, and they are characterized by relatively high crystallinity and molecular weight. As a class, fluoropolymers rank among the best of the plastics in chemical resistance and elevated-temperature performance. Their maximum service temperature ranges up to about 500°F (260°C). They also have excellent frictional properties and cannot be wet by many liquids. Their dielectric strength is high and is relatively insensitive to temperature and power frequency. Mechanical properties, including tensile creep and fatigue strength, are only fair, although impact strength is relatively high. There are three major classes of fluoropolymers. In order of decreasing fluorine replacement of hydrogen, they are fluorocarbons, chlorotrifluoroethylene, and fluorohydrocarbons. There are two fluorocarbon types of note: polytetrafluoroethylene (PTFE) and fluorinated ethylene propylene (FEP). PTFE, a perfluorinated straight-chain high polymer having the chemical formula (CF 2 CF 2 ) n , is made by polymerizing the tetrafluoroethylene (TFE) monomer. The white-to-translucent solid polymer has an extremely high molecular weight, i.e., in the 10 6 -10 7 range, and, consequently, has a viscosity of about 10 GPa»s (10 11 P) at 380°C. Its high thermal stability results from the strong carbon-fluorine bond and characterizes PTFE as a very useful high temperature polymer. Its heat resistance, chemical inertness, electrical insulation properties, and its low coefficient of friction in a very wide temperature range make PTFE the most outstanding plastic in the industry. PTFE is readily commercially available from Honeywell, Inc. (under the tradename HALON), Daikin Kogyo (under the tradename POLYFLON), DuPont (under the tradename TEFLON), Hoechst (under the tradename HOSTAFLON), and ICI (under the tradename FLUON). One particular grade of TEFLON that is particularly preferred in the practice of the present invention is available under the tradename TEFLON PTFE K-10 (Du Pont). TEFLON PTFE K-10 is a free-flowing white powder having an average particle size of 560 micrometers, a bulk density of 570 g/L, a standard specific gravity of 2.16, a melting range of 320-340 C (608-644 F), is insoluble in all common solvents, and is stable to all common reagents at ordinary temperatures, and reacts with alkali metals and fluorine or reactive agents yielding fluorine. FEP is a semi-crystalline perfluorinated polymer closely related to PTFE being a copolymer of tetrafluoroethylene and hexafluoropropylene. Its properties are similar, but generally a little inferior to, those of PTFE; but, FEP has the great practical advantage of being melt-processable, albeit at greater expense. Compared to PTFE, FEP has similarly excellent chemical resistance and electrical properties (up to very high frequencies) and good weathering resistance. FEP has better radiation resistance and much higher impact strength than PTFE, but has lower maximum use and heat deflection temperatures and is even a little less stiff and strong. FEP is readily commercially available from DuPont (under the tradename TEFLON FEP) and Hoechst (under the tradename HOSTAFLON FEP). The fluorohydrocarbons are of two kinds: polyvinylidene fluoride (PVF 2 , or PVDF) and polyvinyl fluoride (PVF). While similar to the other fluoropolymers, fluorohydrocarbons have somewhat lower heat resistance and considerably higher tensile and cbmpressive strength. PVDF is the polymer of 1 ,1-difIuoroethylene. PVDF is a semi- crystalline polymer containing 59.4% fluorine. The symmetrical arrangement of the hydrogen and fluorine atoms in the chain contributes to the unique polarity which influences the polymer's dielectric properties and solubility. PVDF is readily commercially available from Pennwalt Corp., (under the tradename KYNAR). Accordingly, fluoropolymers such as, but not limited to PTFE, FEP, and PVDF can be used for the purposes of this invention. Fluoropolymers, in either a powder or dispersion form, can be incorporated, in varying amounts, into the friction material during a wet or dry mixing process. In accordance with a preferred embodiment of the present invention, the fluoropolymer material is present in the friction material matrix in an amount of from about 0.1 to about 5 weight percent, based on the total weight of the friction material matrix. In accordance with a highly preferred embodiment of the present invention, the fluoropolymer material is present in the friction material matrix in an amount of up to about 2 weight percent, based on the total weight of the friction material matrix. By way of a non-limiting example, a typical formulation of a friction material matrix containing at least one fluoropolymer material, in accordance with one embodiment of the present invention, is presented in Example I, below: EXAMPLE I By way of a non-limiting example, a typical formulation of a friction material matrix containing at least one fluoropolymer material, in accordance with another embodiment of the present invention, is presented in Example II, below: EXAMPLE II The friction materials containing at least one fluoropolymer material of the present invention reduce the tendency for brakes to generate noise (as well as vibration) by ensuring, for example, that a thin and uniform transfer film is continuously being generated at the friction couple between the brake pad and the rotor or drum. In order to evaluate the noise behavior characteristics of brake pads having a friction lining formulation containing at least one fluoropolymer material at levels in accordance with the present invention, a comparison test was performed. A first pair of brake pads having a friction lining formulation containing no fluoropolymer material (designated HFM-1) was subjected to a dynamometer test in order to evaluate the total noise produced at various frequencies (see Figure 1). A second pair of brake pads having a friction lining formulation containing at least one fluoropolymer material present at levels in accordance with the present invention, i.e., from about 0.1 to about 5 weight percent, was also subjected to a dynamometer test in order to evaluate the total noise produced at various frequencies (see Figure 2). As can be determined from comparing Figure 1 with Figure 2, the total noise of the HFM-1 formulation, especially at 5000 Hz, was significantly higher than the HFM-2 formulation, thus indicating that brake pads containing at least one fluoropolymer material in accordance with the present invention will produce less noise, especially less high frequency noise (i.e., squeal). Also noteworthy was the fact that the HFM-2 formulation did not produce any noise whatsoever above the 4000 Hz frequency. The foregoing description is considered illustrative only of the principles of the invention. Furthermore, because numerous modifications and changes will readily occur to those skilled in the an, it is not desired to limit the invention to the exact construction and process shown as described above. Accordingly, all suitable modifications and equivalents that may be resorted to that fall within the scope of the invention as defined by the claims that follow.

Claims: What is claimed is: 1. A friction material matrix, comprising: at least one fluoropolymer; and at least one binder system. 2. The friction material matrix of claim 1, wherein the fluoropolymer is present in an amount from about 0.1 to about 5 weight percent based on the total weight of the friction material matrix. 3. The friction material matrix of claim 1, wherein the fluoropolymer is present in an amount up to about 2 weight percent based on the total weight of the friction material matrix. 4. The friction material matrix of claim 1, wherein the fluoropolymer is selected from the group consisting of fluorinated ethylene propylene, polyvinylidene fluoride, polytetrafluoroethylene, and combinations thereof. 5. The friction material matrix of claim 1 , further comprising any one of the following components: at least one reinforcing fiber; at least one metal powder; at least one abrasive; at least one lubricant; at least one organic filler; and at least one inorganic filler. 6. A friction material matrix, comprising: at least one fluoropolymer present in an amount up to about 2 weight percent based on the total weight of the friction material matrix; and at least one binder system. 7. The friction material matrix of claim 6, wherein the fluoropolymer is selected from the group consisting of fluorinated ethylene propylene, polyvinylidene fluoride, polytetrafluoroethylene, and combinations thereof. 8. The friction material matrix of claim 6, further comprising any one of the following components: at least one reinforcing fiber; at least one metal powder; at least one abrasive; at least one lubricant; at least one organic filler; and at least one inorganic filler. 9. A friction material matrix, comprising: at least one fluoropolymer present in an amount from about 0.1 to about 5 weight percent based on the total weight of the friction material matrix, wherein the fluoropolymer is selected from the group consisting of fluorinated ethylene propylene, polyvinylidene fluoride, polytetrafluoroethylene, and combinations thereof; and at least one binder system. 10. The friction material matrix of claim 9, further comprising any one of the following components: at least one reinforcing fiber; at least one metal powder; at least one abrasive; at least one lubricant; at least one organic filler; and at least one inorganic filler.

Inventors: KESAVAN SUNIL KUMAR, KESAVAN, SUNIL, KUMAR
Applicants: HONEYWELL INT INC, HONEYWELL INTERNATIONAL INC.
Application year: 2001
Main classification: F16D69
IPC codes: C09K3, F16D69