Abstract:
A substantially copper-free friction material for a brake pad includes a binder forming approximately 8-18% by volume, a fiber forming 5-14% by volume, at least one lubricant forming up to 6% by volume, at least one abrasive forming 10-27% by volume, at least one carbon content component forming 36-51% by volume, and at least one filler forming 6-21% by volume. The friction material can be applied to the backing plate of a brake pad.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a low-steel friction material and in particular to a friction material for a disc brake pad to be used for disc brakes of vehicles. The friction material is formed without any copper-based materials while maintaining the excellent frictional performance across a wide range of temperatures and high mechanical strength of low-steel friction materials containing copper. 
       BACKGROUND OF THE INVENTION 
       [0002]    Traditionally, friction materials such as brake pads were made using an asbestos-based friction material. As certain negative factors associated with the use of asbestos became known, many manufacturers switched to non-asbestos-based friction materials. For disc brake pads, steel fiber is also used into friction material to improve the matrix strength. 
         [0003]    Copper in low-steel friction materials provides many useful properties and performance characteristics like increased friction coefficient at high temperatures and excellent heat transfer properties. In addition, copper provides an improvement of the longevity of the friction material and reduces brake dust. 
         [0004]    While copper and copper alloys provide many positive properties for use in low-steel disc brake pads, it is advantageous to limit the use of copper in friction material for economic reasons. Copper is among the most important industrial metals. It is used in electrical power cables, data cables, electrical equipment, automobile radiators, cooling and refrigeration tubing, heat exchangers etc. Since copper is a finite resource, the price of copper fluctuates dramatically depending on the copper consume demand based on the economic condition. 
         [0005]    It is also advantageous to limit the emission of copper from friction material into the environment. Copper is toxic to many aquatic organisms in both marine and freshwater environments, e.g. impairs the sensory systems of salmon, reducing their ability to elude predators and hindering their return to spawning streams. Potentially harmful impacts of copper leaking into aquatic environments have been well documented, as in the case of San Francisco Bay area water studies. 
         [0006]    Even though each piece of friction material gives off only small amounts of copper, due to the sheer number of vehicles on the roads and in industrial applications using such friction materials, regulatory authorities have been looking for ways to reduce the input of copper into the environment. Therefore, it is desirable to produce a low-steel disc brake pad without the use of copper or alloys thereof, the pad having similar attributes and positive qualities as brake pads including copper-based metals and alloys. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention is directed to a friction material that is free of copper and copper alloys. The current invention discloses compositions of friction materials that contain steel fiber and other matrix strengthening materials, commonly referred to as low-steel in industry parlance. The compositions described in this invention are unique in that they provide the same level of friction, pad life, noise and other performance characteristics of a conventional pad that uses copper based materials, while employing neither of copper and copper containing materials. The compounding ingredients described herein, when mixed appropriately as described using the suggested proportions, provide a family of materials that provide outstanding friction materials that match all performance aspects of a copper containing material, without the use of copper and copper alloys. 
         [0008]    According to a first aspect of the invention a friction material for a brake pad is provided, comprising:
       a binder forming approximately 8-18% by volume;   a fiber forming 5-14% by volume;   at least one lubricant forming up to 6% by volume;   at least one abrasive forming 10-27% by volume;   at least one carbon content component forming 36-51% by volume; and   at least one filler forming 6-21% by volume;
 
wherein the friction material is substantially free of copper.
       
 
         [0015]    According to an exemplary embodiment said binder comprises 
         [0000]    a modified phenolic resin,
 
an unmodified phenolic resin,
 
a non-phenolic resin,
 
a mixture of a phenolic resin and a non-phenolic resin, or
 
a mixture of one or more straight or modified phenolic resin systems including unmodified phenolic resin, silicon modified resin, NBR modified resin, phosphorous modified resin, and boron modified resin.
 
         [0016]    According to an exemplary embodiment said fiber is chosen from a group comprising 
         [0000]    polyacrylonitrile, PAN,
 
polyaramides,
 
cellulose fibers, and
 
steel fibers.
 
         [0017]    The fiber is preferably a steel fiber. The fiber is free of the aramid fiber family for environmental reasons. 
         [0018]    According to an exemplary embodiment said lubricant comprises at least one of metal sulfides, organic lubricants and metal lubricants. Alternatively the lubricant comprises a metal sulfide complex. Still further the lubricant may be selected from the group comprising tin sulfides, zinc sulfide, iron sulfides, molybdenum sulfide, tin powder, and zinc powder. 
         [0019]    The lubricant is preferably free of antimony trisulfide and antimony trioxide for environmental reasons. 
         [0020]    According to an exemplary embodiment said abrasive is chosen from a group comprising mineral fibers, 
         [0000]    zirconia,
 
zircon,
 
zirconium silicate,
 
mica,
 
alumina,
 
ceramic fibers,
 
calcium magnesium silicate,
 
calcium magnesium zirconium silicate,
 
calcium magnesium aluminum silicate,
 
magnesium aluminum silicate,
 
synthetic mineral fibers such as hardwool, slagwool and rockwool, silica, silicon dioxide, sand, silicon carbide, iron oxide, iron chromite and magnesium oxide.
 
         [0021]    The friction material further includes abrasives. Abrasives are typically classified by their Mohs hardness. 
         [0022]    Carbon contents include natural graphite, synthetic graphite, petroleum coke, desulfurised petroleum coke, and carbon black. 
         [0023]    The friction material may include fillers forming approximately 6-21% by volume of the material. Fillers may include at least one material selected from the group comprising lime, calcium oxide, calcium hydroxide, talc, calcium carbonate, calcium silicate, barites, rubber such as rubber powder or recycled rubber crumbs and various types of friction dusts. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  is a perspective view of an exemplary brake pad incorporating a friction material according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0025]    The present invention is a new friction material for disc brake pads and other brake materials and includes a binder, a fiber, at least one lubricant, abrasives, a carbon component and optionally other materials. The brake material is substantially free of copper or alloys of copper such as brass and bronze. 
         [0026]    The present invention is directed to a friction material and in particular, a disc brake pad for vehicles and other applications that is substantially free of copper. The friction material is a low-steel material that is formed from a binder, fibers, a lubricant, abrasives or friction modifiers and other materials. 
         [0027]    Friction materials for disc brake pads must have consistent, reliable performance over a wide range of operating conditions. As disc brake pads for vehicles require reliable stops and long life while being subjected to extreme and harsh conditions, manufacturers have consistently tried to improve the performance of their brake pads. At the same time, due to new or expected regulations, brake pad manufacturers are attempting to remove copper and copper alloys from brake pads. While some manufacturers have been attempting to partially remove copper or copper alloys, they have not been able to successfully replace and thus remove copper completely while maintaining sufficient performance and characteristics including stopping capability, excellent high temperature resistance, minimal high temperature pad wear, minimal rotor wear and excellent pad integrity at high service temperature. 
         [0028]    As stated above, the disc brake pad or friction material is formed from a material having binder, fibers, a lubricant, abrasives, a carbon component, and optionally other materials. The copper-free low-steel brake material is also substantially free of asbestos, antimony, brass, bronze, and copper alloys. 
         [0029]    The friction material generally includes a binder such as phenolic resin which may be either straight or modified phenolic resin. Examples of modified binders may include silicone, acrylic, epoxy, and nitrite modified binders. The binder generally forms approximately 8-18% by volume of the final composition. The binder is typically used to develop the matrix that holds all other ingredients together in a friction material. Many manufacturers mix various binders to improve performance characteristics. The binder system may also comprise a mixture of two types of systems, at least one of which is a phenolic type binder. 
         [0030]    The friction material also includes steel fibers that generally form approximately 5-14% by volume of the final friction material composition. The fibers are typically used to provide integrity and structural strength in the friction material. Many manufacturers mix various fibers and fiber lengths to improve the performance of the brake characteristics of the friction material. The fibers used are free of aramid material for environmental reasons. 
         [0031]    The friction material also includes at least one lubricant. Lubricants are included in the friction material to reduce pad and disc wear during service. The friction material includes approximately less than or equal to 6% by volume of the final friction material composition. The lubricant may include metal sulfides, metal lubricants or a combination thereof. Examples of metal sulfides include, but are not limited to, tin sulfides, zinc sulfide, iron sulfides, molybdenum sulfide. An example of metal lubricants includes tin and zinc powders. Various metal sulfide complexes are also available in the market, which typically include tin sulfide as one of the main ingredients. The lubricant is free of antimony trisulfide and antimony trioxide for environmental reasons. 
         [0032]    The friction material further includes abrasives. The abrasives generally form approximately 10-27% by volume of the final friction material composition. Abrasives are typically classified by their Mohs hardness. Examples of abrasives include alumina, magnesium oxide, zirconium silicate, silica, silicon dioxide, sand, silicon carbide, mullite, mica, iron chromite, complex mineral silicates such as calcium magnesium silicate, calcium magnesium zirconium silicate, calcium magnesium aluminum silicate, and magnesium aluminum silicate. 
         [0033]    The hard abrasives—those with higher values on the Mohs hardness scale—are generally used in low concentrations while the mild abrasives—those with lower values on the Mohs hardness scale—are typically used in higher concentrations to achieve the same desired friction level. 
         [0034]    Other ingredients included in the friction material generally form approximately 42-72% of the final composition of the friction material. The other ingredients generally are used as fillers or modifiers to create specific performance characteristics. For example, the other ingredients generally provide bulk to the formulation, reduce cost, provide noise reduction and help with coating the rotor surface with a uniform transfer layer. 
         [0035]    Examples of other ingredients are carbon contents including graphite, petroleum coke, desulfurised petroleum coke and fillers including lime, calcium oxide, calcium hydroxide, calcium silicate, barium sulphate, rubber including various powder rubbers and recycled rubber and friction dust including brown, black, straight, modified or other grades of friction dust. 
         [0036]    The following Example 1 provides some exemplary friction materials using the present invention that have sufficient performance characteristics. The example is evaluated for the mixing, pressing, physical compression, bonding to back plate at room temperature, and costs. The composition amounts described below have been rounded off to the nearest number for sake of simplicity. 
       Example 1 
       [0037]      
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Fiber 
                 10% 
               
               
                   
                 Lubricant 
                  2% 
               
               
                   
                 Total Abrasive 
                 18% 
               
               
                   
                 Carbon Content 
                 45% 
               
               
                   
                 Fillers 
                 12% 
               
               
                   
                 Binder 
                 13% 
               
               
                   
                 Total 
                 100%  
               
               
                   
                   
               
             
          
         
       
     
         [0038]    The ingredients for the mixture of Example 1 are generally prepared in a standard mixer for approximately 10 minutes and are then pressed into a press cure mould and later baked as is well-known in the art. The above friction material composition in this Example 1 was found to have good all-around characteristics for all of the above desirable characteristics. A material made using the above formulation had performance characteristics that are comparable to friction formulations that typically contain copper or copper alloys in them. 
       Properties of Copper-Free Friction Material 
     Performance 
       [0039]    The copper free friction material has comparable performance as conventional friction formulations that typically contain copper or copper alloys in them with global performance test schedule. The new material has better speed sensitivity, especially at high temperature. The compressive elastic modulus of copper free formulations is quite high (see Table 1), which is important for the function of the braking system. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Compressive E-Modulus comparison 
               
             
          
           
               
                   
                 Copper-free 
                 Formulation 
               
               
                   
                 formulation 
                 with copper 
               
               
                   
                   
               
             
          
           
               
                   
                 Compressive E-Modulus MPa 
                 750-1100 
                 ~590 
               
               
                   
                   
               
             
          
         
       
     
       Wear 
       [0040]    Copper free friction material has a good pad and disc wear behaviour in the temperature range from 100° C. to 300° C. On vehicles in different service conditions it has the same level of pad service life as friction formulations that typically contain copper or copper alloys, meanwhile a better service life of the brake disc. 
       Disc Cracking 
       [0041]    Generally a lower compressibility of a brake pad results in impaired disc cracking. The new material keeps the same level in disc cracking as friction formulations that typically contain copper or copper alloys in them, however with a lower compressibility. Lower compressibility is a positive factor for the function e.g. of an air disc brake.