Abstract:
To provide a friction material with an excellent balance for both service brake and parking brake usage that satisfies the required performance for the service brake and improves the static coefficient of friction for the parking brake. The friction material comprising a fibrous base material, a friction modifier, and a binder, wherein said friction modifier comprises 2-20 volume % of cashew dust, 10-25 volume % of a rubber composition, and 2-5 volume % of a solid lubricant, a total volume of the cashew dust and the rubber composition is 30 volume % or less, and said friction modifier does not use graphite because it can cause the reduction of the coefficient of friction during the parking brake operation.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) AND CLAIM OF PRIORITY 
       [0001]    The present application claims the benefit under 35 U.S.C. section 119(a) of Japanese Patent Application filed in the Japan Patent Office on Jul. 4, 2007 and assigned serial number  2007 - 176029 , the disclosure of which is incorporated herein by reference. 
       FIELD OF INVENTION 
       [0002]    This invention relates to a friction material for use as a disc brake pad and a brake lining for automobiles. 
       BACKGROUND OF THE INVENTION 
       [0003]    A friction material mounted on a rear brake of an automobile is often used for both service brakes and parking brakes. The required performance for the friction material to be used in the service brake is different from the required performance for the friction material to be used in the parking brake. The service brake needs such as excellent and stable brake effectiveness, good fading resistance and good wearing resistance, high mechanical strength, and low incidence of noise. The parking brake needs high static coefficient of friction and a small amount of thermal expansion and contraction so as to securely hold an automobile on a slope. 
         [0004]    For the friction material with high static coefficient of friction when using the parking brake, the patent document 1 (Japanese Unexamined Patent Publication No. 9-67449) discloses a friction material for a parking brake made by entirely or partially coating a rubber or solution of rubber compounds on a friction surface of the friction material and by heating and vulcanizing the same, and the patent document 2 (Japanese Unexamined Patent Publication No. 9-67450) discloses a hybrid friction material for a parking brake made by forming as placing a high dumping raw material plate made of rubber or rubber compounds, when pre-forming the friction material, on agitation compounds for the friction material manufacturing to be embedded therein to coexist surfaces with different characteristics on the friction surface. Also, the patent document 3 (Japanese Unexamined Patent Publication No. 2005-233283) discloses a friction material for a parking brake which uses 25-40 volume % of a hard inorganic particle with an average particle diameter of between 200 and 400 μm and Mohs hardness of 7-9 as the abrasive. 
         [0005]    However, these friction materials are used solely for parking brakes, and they cannot satisfy the required performance when used for service brakes. Even if a material with a high dumping capacity is applied on the friction surface of the friction material, just like the patent documents 1 and 2, effective performance does not last because the high dumping material becomes worn out after continuous use of the same for the service brake, and the friction material includes abrasive largely just like the patent document 3 and thus causes a problem of increasing the abrasiveness against a mating member (such as a disc brake rotor and a brake drum) when the service brake is applied. 
         [0006]    On the other hand, study regarding the friction material used to secure both service brake and parking brake performances has been done, and the patent document 4 (Japanese Unexamined Patent Publication No. 2002-275452) discloses a friction material that offers superior brake effectiveness for the service brake and at the parking brake operation restricts the noise generation due to oscillation of a vehicle body when the driver gets off the vehicle after applying the parking brake. This friction material includes 1-15 volume % of an inorganic fiber with Mohs hardness of lower than 4.5 and 1-15 volume % of cashew dust. However, no study has ever been conducted as to the improvement of the static coefficient of friction during the parking brake and restriction of the thermal expansion and contraction of the friction material. 
         [0007]    Patent Documents
   [Patent Document 1] Japanese Unexamined Patent Publication No. 9-67449.   [Patent Document 2] Japanese Unexamined Patent Publication No. 9-67450.   [Patent Document 3] Japanese Unexamined Patent Publication No. 2005-233283.   [Patent Document 4] Japanese Unexamined Patent Publication No. 2002-275452.   
 
       SUMMARY OF THE INVENTION 
       [0012]    This invention was made in consideration of the above-situations and provides a friction material with an excellent balance for both service brake and parking brake usages which satisfies the required performance for the service brake and maintains the high static coefficient of friction during parking brake operation; and restricts the thermal expansion or contraction of the friction material to restrain reduction of the pressing force against the mating member. 
         [0013]    The inventors of the present application conducted a thorough study to achieve the above-described objects and then found that eliminating a factor of reducing the brake force is more effective than positively increasing the static coefficient of friction during the parking brake operation for the friction material generally used for both service brake and parking brake and that (1) the pressing force against the mating member (e.g., disc brake rotor or brake drum) is reduced by contracting the friction material by naturally cooling the same when the friction material, which is expanded due to the frictional heating during the service brake operation, is used for the parking brake, and (2) graphite generally applied as the solid lubricant for the friction material reduces the static friction coefficient when a high humidity atmosphere exists, such as during the night time when the parking brake is frequently used. 
         [0014]    &lt;1&gt; This invention discloses a friction material comprising a fibrous base material, a friction modifier, and a binder, wherein said friction modifier includes 5-20 volume % of cashew dust, 10-25 volume % of a rubber composition, and 2-5 volume % of a solid lubricant, a total volume of said cashew dust and said rubber composition is less than 30 volume %, and said friction modifier is without graphite. 
         [0015]    &lt;2&gt; This invention also discloses the friction material according to &lt;1&gt;, wherein the cashew dust is manufactured using furfural as a curing agent 
         [0016]    &lt;3&gt; This invention also discloses the friction material according to &lt;1&gt; and &lt;2&gt; wherein a part of the rubber composition is a vulcanized acrylic rubber powder, and the friction material includes at least 5 volume % or more of a vulcanized acrylic rubber powder as the rubber composition 
         [0017]    &lt;4&gt; This invention also discloses the friction material according to &lt;1&gt;-&lt;4&gt;, wherein the solid lubricant is molybdenum disulfide. 
         [0018]    This invention can provide a friction material that:
   (1) gives a superior braking effect, good fading and wearing resistance, high mechanical strength, and low incidence of noise when used as a service brake;   (2) can maintain a high static friction coefficient and at the same time minimizes and reduces the pressing force required to brake by restricting the thermal expansion and contraction of the friction material against the surface of the mating member and also securely keeps the vehicle on the slope when used as a parking brake, and   (3) can be used as both service brake and parking brake.   
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The above and other objects of the present invention will become readily apparent by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein: 
           [0023]      FIG. 1  is a table showing evaluation results of embodiments 1-8 and comparative examples 1-4; 
           [0024]      FIG. 2  shows the evaluation method and standards; and 
           [0025]      FIG. 3  shows the evaluation method for the static coefficient of friction during parking brake operation. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0026]    The friction material of the present invention is prepared by forming a friction material granulation which is comprised of a fibrous base material, a friction modifier, and a binder and then curing the same thereafter. Here, in the friction material of the present invention, the friction modifier is characterized to include cashew dust and rubber composition but does not include graphite. 
         [0027]    An inorganic fiber and organic fiber without asbestos, which is generally used for the friction material, may be used as the above-addressed fibrous base material. Examples of the inorganic fibers are a steel fiber, stainless fiber, copper fiber, brass fiber, bronze fiber, aluminum fiber, potassium titanate fiber, glass fiber, rockwool, wollastonite, and more, and examples of the organic fibers are aramid fiber, carbon fiber, polyimide fiber, cellulose fiber, acrylic fiber, and more. Among these fibrous base materials they can be used alone or as a combination of two or more types. An amount of overall fibrous base materials included in the friction material may be adjusted within a range that does not lose the advantages of the present invention. 
         [0028]    The friction modifier is added to adjust the coefficient of friction, suppress the noise, and to prevent rust and has such as an inorganic filler, an organic filler, and a solid lubricant as appropriate, where the organic filler may be such as cashew dust and rubber composition. The cashew dust is added to stabilize the coefficient of friction during the service brake operation. If the friction material frictionally engages the surface of the mating member during the service brake operation, the cashew dust in the friction material melts due to the frictional heating, and a thin organic film uniformly covers the surface of the mating member. Covering the surface with the film stabilizes the coefficient of the friction material. However, the cashew dust expands and contracts in great extent due to the heat, which causes the friction material to thermally expand and contract. Here, adding the rubber composition helps to restrict the thermal expansion of the friction material. The rubber composition, functioning as an elastic member, contracts the same, or in equal proportion to, (equally) the amount of expansion of the cashew dust due to the frictional heating so that the thermal expansion of the friction material itself is restricted. 
         [0029]    Also, the addition of the rubber composition increases the elasticity of the friction material, and therefore the amount of contraction of the friction material increases when pressing the friction material against the mating member. If the amount of contraction of the rubber composition is equal to or more than the amount of expansion of the friction material, then when the friction material is contracted by cooling, no radical reduction of the pressing force can be seen, thereby restraining a decrease of the pressing force against the mating member. 
         [0030]    Also, the solid lubricant is used to secure the wearing resistance during the service brake operation. Adding a solid lubricant that is something other than graphite to the friction material can effectively restrict the reduction of the static coefficient of friction during parking brake operation. This is true because graphite is characterized by the fact that the coefficient of friction thereof becomes lower in humid air than in dry air. 
         [0031]    The amount of cashew dust is preferably 5-20 volume %. If the amount of cashew dust is less than 5 volume %, it is difficult for the cashew dust film to form on the mating member. If the cashew dust film is not formed on the mating member, iron composition of the mating member is transferred to stick on the friction material, which causes an iron to iron grinding type friction to increase the abrasiveness against the mating member and to damage the wearing resistance. Also, if the amount of cashew dust is more than 20 volume %, the cashew dust film formed on the mating member becomes too thick, which tends to reduce the coefficient of friction during the service brake operation. 
         [0032]    The cashew dust is made by curing cashew nut shell liquid, or polymer of the same, by curing agents such as furfural, formaldehyde (as one type of an aldehyde), or hexamethylenetetramine, then cooling and crushing. In this invention, the cashew dust, which is obtained by using such as formaldehyde and hexamine as a curing agent, and the cashew dust, which is obtained as using furfural as a curing agent, can be used independently or in combination; however, the cashew dust, which is obtained as using the furfural as the curing agent is preferable. Comparing the cashew dust which is obtained using the furfural as the curing agent, and the cashew dust which is obtained using such as the formaldehyde and hexamine as the curing agent, the former cashew dust, shows high heat resistance and can restrict the thermal expansion. 
         [0033]    The amount of rubber composition is preferably 10-25 volume %. If the amount of the rubber composition is less than 10 volume %, the thermal expansion of the friction material itself becomes larger and the elasticity of the friction material itself becomes smaller, and therefore the static coefficient of friction, during the parking brake operation, tends to decrease. Also, the vibration damping effect becomes less effective, and a squealing noise tends to be caused during the service brake operation. If the amount of the rubber composition is more than 25 volume %, the wearing resistance is decreased. 
         [0034]    The rubber composition may be any one of or any combination of natural rubber, polyisoprene rubber (IR), acrylonitrile butadiene rubber (NBR), styrene butadiene rubber (SBR), butadiene rubber (BR), chloroprene rubber (CR), butyl rubber (IIR), ethylene propylene rubber (EPM or EPDM), urethane rubber, silicone rubber, fluoro rubber, powder of vulcanized rubber or unvulcanized rubber of acrylic rubber, and crushed powder of tire tread rubber or wiper rubber or window strip rubber. Also, as a part of the rubber composition, vulcanized acrylic rubber powder with high heat resistance is preferably included at 5 volume % or more relative to the total amount of friction material. 
         [0035]    The amount of the cashew dust and the rubber composition combined is preferably 30 volume % in total or less. If the total amount of the cashew dust and the rubber composition is more than 30 volume %, the amount of organic substance involved in the friction material becomes too much, which worsens the fading resistance (tends to fading). 
         [0036]    Next, the solid lubricant can be one of or a combination of metallic sulfide, without graphite, such as molybdenum disulfide, stannic sulfide, and tungsten sulfide; however, the molybdenum disulfide, which is especially not likely to be affected by humidity, is preferably used independently. The amount of the solid lubricant relative to the entire amount of the friction material is preferably 2-5 volume %. If the amount of the solid lubricant is less than 2 volume %, lubrication of the friction material agency is insufficient, which damages the wearing resistance during the service brake operation. Also, if the amount of solid lubricant is more than 5 volume %, the lubrication agency becomes too much, which reduces the coefficient of friction during the service brake operation, and therefore sufficient brake effectiveness cannot be obtained. 
         [0037]    An inorganic filler may be zirconium silicate, zirconium oxide, magnesium oxide, silicon carbide, silicon dioxide, aluminum oxide, barium sulfate, calcium carbonate, calcium hydroxide, mica, vermiculite, triiron tetroxide, a metal powder such as steel, stainless, copper, brass, bronze, aluminum, tin, and zinc. The above-listed examples of metal powders can be used alone or in combination of two or more types. 
         [0038]    A binder that can generally be provided for the friction material can be used. For example, one of or a combination of two or more of such as phenol resin, acrylic rubber denatured phenol resin, NBR denatured phenol resin, phenol alkyl resin, melamine resin, epoxy resin, and benzoxazine resin may be used. The amount of binder relative to the entire amount of the friction material is preferably 10-30 volume %. 
         [0039]    The manufacturing method of the friction material of the present invention is to mix the above-identified compositions evenly using mixers such as a Henschel mixer, a Loedige mixer, an Eirich mixer, to pre-mold in a mold, and to form the molded product by forming the pre-molded product at 140-180° C., 20-50 MPa for 5-15 minutes. Next, the obtained molded product is heat-treated (postcured) at 140-250° C. for 2-48 hours and then is spray-painted, baked, and grinded on the surface as necessary to achieve a final product. When manufacturing a disc brake pad, the disc brake pad is manufactured by placing the pre-molded product on a steel or aluminum plate which is cleaned, surface-treated, and adhesive-coated in advance, and in this situation, molding by the mold for molding, heat-treating, painting, baking, and grinding. 
       Embodiments 
       [0040]    Embodiments and comparative examples are shown in the following sections to explain the present invention concretely, but the present invention is not limited to the following embodiments. 
         [0041]    The composition of the friction material as shown in  FIG. 1  is mixed with the Loedige mixer for 5 minutes and is pressed in the metal mold for molding at 10 MPa for 1 minute so as to pre-form. The pre-molded product is mounted on the iron plate which is cleaned, surface-treated, and adhesive-coated in advance, and in this situation, molding by the mold for molding at the molding temperature of 150° C., the molding pressure of 40 MPa for 10 minutes, heat-treated (postcured) at 200° C. for 5 hours, and grinded to manufacture the friction material (disc brake pad for passenger cars) (as appears in the embodiments 1-8 and the comparative examples 1-4). The friction material is evaluated with respect to the dynamic coefficient of friction during the service brake operation, the static coefficient of friction during the parking brake operation, the wearing resistance, and the fading resistance.  FIG. 1  shows the evaluation results;  FIG. 2  shows the evaluation method and standards; and  FIG. 3  shows the evaluation method for the static coefficient of friction during parking brake operation. 
         [0042]    While the embodiments of the present invention disclosed herein are presently considered to be preferred embodiments, various changes and modifications can be made without departing from the spirit and scope of the present invention. The scope of the invention is indicated in the appended claims, and all changes that come within the meaning and range of equivalents are intended to be embraced therein.