Abstract:
Composition railroad friction materials having low wear rates are characterized by the use of synthetic fiber and by the absence of lead and asbestos. These friction materials are particularly suitable for use in railroad brake shoes and contain, by approximate weight, 0.5-11% non-asbestos fiber, of which at least 0.5% is synthetic fiber, 66-81% filler and 14-21% organic binder.

Description:
This is a continuation of application Ser. No. 815,637, filed July 14, 1977, now abandoned. 
    
    
     This invention relates generally to composition type railroad friction materials and, more particularly, to such materials which feature the use of synthetic fibers and the absence of asbestos and lead. 
     BACKGROUND OF THE INVENTION 
     Most of the composition type railroad friction materials in use today include asbestos and lead. An example of this type of material is shown in U.S. Pat. No. 3,168,487--Spokes et al. Some environmentalists have warned of possible problems caused by the use of lead in friction materials. As a result, demand has developed for a lead-free friction material. Examples of some composition friction materials which exclude lead are U.S. Pat. Nos. 3,492,262--Griffith and 3,959,194--Adelmann. The former patent discloses a composition which has no lead and includes up to 16.5% by weight asbestos fiber, while the latter patent discloses some compositions which delete lead and have up to 11.1% by weight asbestos fiber. 
     More recently, certain environmentalists have pointed out that possible problems may be caused by the use of asbestos fiber in friction materials. Thus, it may be desirable to eliminate asbestos as well as lead from composition friction materials. U.S. Pat. No. 3,959,194 has some examples which utilize cellulosic fiber in a range of 3.5-8.0% by weight as a substitute for asbestos, although the patent is not concerned with eliminating asbestos in friction materials. 
     Asbestos has traditionally been used in friction materials because of its high heat resistance and strength and its low cost. A direct substitution of other types of fiber for asbestos is expensive, since other fibers cost much more than asbestos, and difficult, since no known fiber combines all of the desirable qualities of asbestos noted above. It is known to use high-content carbonized or graphitized fibers in aircraft friction materials, as shown in U.S. Pat. No. 3,552,533--Nitz. The use of glass fiber in friction materials is disclosed in several patents. U.S. Pat. No. 3,743,069--Barnett relates a clutch facing consisting almost entirely of bundles of continuous glass filaments. U.S. Pat. No. 3,627,606--Bentz teaches a glass-filament-reinforced fabric clutch facing impregnated with a cement containing lead (litharge). U.S. Pat. No. 3,713,934--Morton discloses a clutch facing composed of glass and asbestos. None of these friction materials would be suitable for railroad brake shoe use, since they are either too expensive and/or contain lead or asbestos. 
     It is, therefore, an object of this invention to provide lead-free, asbestos-free, high coefficient of friction, composition friction materials which feature a low fiber content including synthetic fiber and low wear rates. 
     SUMMARY OF THE INVENTION 
     I have discovered that railroad brake shoes that meet A.A.R. (Association of American Railroads) standards for brake shoes can be made from compositions having a low, non-asbestos fiber content which includes synthetic fiber. The friction materials according to this invention comprise, by weight, 0.5-11.0% non-asbestos fiber, including at least 0.5% synthetic fiber, 66-81% filler and 14-21% organic binder. A preferred embodiment of this composition friction material for railroad car use contains 2.7-3.5% synthetic fiber, 74-82% filler and 16-19% organic binder. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The criterion for suitability of a composition friction material for railroad car brake shoe use is the ability of the friction material to pass the standards set forth in the A.A.R. Specification M-926-72, Feb. 13, 1973 Revision. Some of the pertinent dynamometer performance test criteria called for in this A.A.R. Specification are as follows: 
     1. Instantaneous Retarding Force During 45 Minute Dynamometer Drag Test: 
     
         ______________________________________Drag               Retarding Force______________________________________Heavy load         400 lbs. min.Light load         300 lbs. min.______________________________________ 
    
     2. Static Coefficient of Friction: 
     9 Test Average--0.38 min. 
     3. Stop Distances From 90, 70, 50, 30, 10 mph Under Light and Heavy Brake Shoe Loads (all stop distances must be within varying tolerances). 
     4. Wear Loss: 
     Drag Tests (total)--0.60 in 3  max. 
     Test Stops (total per sequence)--1.20 in 3  max. 
     As used herein, the term &#34;synthetic fiber&#34; means fiber made from a substance which does not naturally occur in a fibrous state and includes glass, polyester and kaowool. The term &#34;synthetic fiber&#34; excludes all forms of cellulose, which naturally occurs in a fibrous state, but which can also be processed into a different fibrous form (e.g., rayon). Some example mixes used a single type of synthetic fiber, while others used several types of synthetic fibers in combination. Other example mixes contained a mixture of synthetic and cellulose fibers. All mixes were totally free of asbestos and lead. 
     Organic binders, such as styrene butadiene rubber (SBR), nitrile butadiene rubber (NBR) and modified phenolic and cashew resins were used. Curing agents for the organic binders included sulfur, zinc oxide and hexamethylene tetramine. 
     A number of filler materials were used in varying combinations to produce the necessary low wear rate, hardness and high coefficient of friction. The fillers used were cast iron grit, kyanite, cashew nut particles, red iron oxide (hematite), black iron oxide, powdered alumina, graphite, barytes, coke, kaolin, cryolite, carbon black and zinc powder. 
     Examples of the mixes formulated and tested are shown below and denoted Mixes A-R. The compositions of ingredients are expressed in weight percentages: 
     
         ______________________________________                  Weight %______________________________________MIX AGlass Fiber                 3.02Organic Binder              18.63rubber            15.74resin             2.89Curative Agents             1.64Filler Materials            76.71iron grit         12.11kyanite           12.11powdered alumina  0.27black iron oxide  8.46barytes           21.21coke              22.55MIX BGlass Fiber                 2.91Organic Binder              17.25rubber            14.47resin             2.78Curative Agents             1.62Filler Materials            78.22iron grit         23.37kyanite           11.68powdered alumina  0.26graphite          7.90barytes           20.42coke              11.68cryolite          2.91MIX CGlass Fiber                 1.61Organic Binder              16.98rubber            12.03resin             4.95Curative Agents             1.54Filler Materials            79.85iron grit         24.54kyanite           13.99powdered alumina  0.48graphite          3.92barytes           17.11coke              14.99cryolite          4.82MIX DGlass Fiber                 1.62Organic Binder              17.18rubber            14.59resin             2.59Curative Agents             3.08Filler Materials            78.13iron grit         24.75kyanite           14.11powdered alumina  0.26graphite          7.65barytes           16.22coke              15.12MIX EPolyester Fiber             0.71Organic Binder              18.93rubber            10.98resin             7.95Curative Agents             1.24Filler Materials            79.11iron grit         13.36kyanite           20.60powdered alumina  0.53graphite          8.73barytes           19.10coke              11.44cryolite          5.35MIX FGlass Fiber                 3.20Organic Binder              19.74rubber            16.68resin             3.06Curative Agents             1.74Filler Materials            75.32kyanite           12.84hematite          16.04cashew particles  1.28powdered alumina  0.28graphite          6.40barytes           22.44coke              16.04MIX GGlass Fiber                 2.87Organic Binder              17.69rubber            14.95resin             2.74Curative Agents             1.56Filler Materials            77.90iron grit         23.02kyanite           11.51powdered alumina  0.26graphite          5.74barytes           20.12coke              14.38cryolite          2.87MIX HFiber                       3.59glass             3.06cellulose         0.53Organic Binder              17.81rubber            6.64resin             11.17Curative Agents             2.39Filler Materials            76.22graphite          6.99barytes           13.87coke              7.31kaolin            3.62powdered alumina  0.53iron grit         22.37kyanite           13.17hematite          8.36MIX JFiber                       2.16glass             1.45cellulose         0.71Organic Binder              17.82rubber            15.06resin             2.76Curative Agents             1.57Filler Materials            78.45kyanite           11.59powdered alumina  0.26iron grit         23.19graphite          5.73barytes           20.26coke              14.48cryolite          2.89 MIX LFiber                       3.32cellulose         0.50polyester         0.33kaowool           2.49Organic Binder              19.57rubber            2.95resin             16.62Curative Agents             2.65Filler Materials            74.48iron grit         31.08kyanite           12.44graphite          6.70barytes           15.56coke              4.98powdered alumina  0.50kaolin            3.22MIX MGlass Fiber                 3.24Organic Binder              16.97rubber            9.46resin             7.51Curative Agents             1.17Filler Materials            78.64iron grit         24.71kyanite           14.09powdered alumina  1.01graphite          1.98barytes           16.81coke              10.26cryolite          4.86carbon black      4.92MIX NGlass Fiber                 3.19Organic Binder              16.90rubber            14.35resin             2.55Curative Agents             1.60Filler Materials            78.31iron grit         24.34kyanite           13.88powdered alumina  0.26graphite          7.78barytes           17.16coke              10.11cryolite          4.78MIX OGlass Fiber                 6.21Organic Binder              16.99rubber            4.92resin             12.07Curative Agents             3.79Filler Materials            73.01iron grit         31.12kyanite           12.46graphite          6.63barytes           12.12coke              7.62zinc powder       3.06MIX PFiber                       6.85glass             6.34cellulose         0.51Organic Binder              17.36rubber            5.02resin             12.34Curative Agents             2.40Filler Materials            73.40iron grit         31.75kyanite           12.71graphite          6.85barytes           13.89coke              5.08zinc powder       3.12MIX RFiber                       9.56glass             3.18cellulose         6.38Organic Binder              18.32rubber            10.21resin             8.11Curative Agents             4.51Filler Materials            67.61iron grit         26.67kyanite           15.21barytes           6.38coke              11.17powdered alumina  1.08carbon black      7.10______________________________________ 
    
     The performance of these composition friction materials against the A.A.R. standards is shown below, where P=passed test and F=failed test: 
     
         ______________________________________TEST RESULTS-EXAMPLE MIXESStatic    Wear Loss Drag Tests  Stop DistancesCoeff. of         (in.sup.3)                   Light Heavy Light HeavyMix  Friction Drag/Stops                   Drag  Drag  BSL   BSL______________________________________A    .347     .16/.33   P     P     P     PB    .514     .09/.33   P     P     P     PC    .570     .11/.26   P     P     P     PD    .480     .23/.56   P     P     P     PE    .390     .17/.57   P     P     P     PF    .563     .33/.18   P     P     P     PG    .515     .11/.33   P     P     P     PH    .397     .23/.56   P     P     P     PJ    .597     .10/.32   P     P     P     PL    .494      .17/1.03 P     P     P     PM    .457     .15/.59   P     P     P     PN    .544     .12/.31   P     P     P     P______________________________________ 
    
     All of the above mixes passed all of the A.A.R. performance tests, with the exception of Mix A which exhibited slightly low coefficients of friction. Mix L showed a relatively high wear rate, although within the A.A.R. standards. 
     It can be seen from the above test results that friction materials made from compositions including various synthetic fibers along, in combination with other synthetic fiber, or in combination with cellulose fiber can meet rigorous A.A.R. standards and exhibit low wear rates. The best wear rates were exhibited by Mixes A, B, C, G and N, which contained from 1.61-3.19% of a single synthetic fiber, and by Mix J which contained a low total content fiber mixture of synthetic and cellulose fibers. 
     Overall fiber content ranged from 0.71% polyester (Mix E) to 9.56% glass and cellulose (Mix R). Binder content ranged from 16.90% (Mix N) to 19.74% (Mix F), while filler content was from 67.61% (Mix R) to 79.85% (Mix C). The amount and type of curative agents is mainly dependent on the amount and composition of the organic binder used. 
     In summation, I have discovered that composition friction materials suitable for railroad use can be formulated, without the use of lead or asbestos, by using a relatively low content of various synthetic fibers alone, in combinations, or in combination with cellulose fiber.