Patent Publication Number: US-2016245353-A1

Title: Friction material composition and friction material

Description:
TECHNICAL FIELD 
     The present invention relates to a friction material composition and a friction material. More specifically, the present invention relates to a friction material composition capable of reducing squeal (brake noise) after leaving a friction material in a cold environment, and the friction material obtained from the friction material composition. 
     BACKGROUND ART 
     Conventionally, a friction material such as brake is produced by using a fiber base material, a friction modifier and a binder, blending them, and performing a production process including steps such as preforming, thermoforming and finishing. In the friction material such as brake, a fiber base material, e.g., an organic fiber such as aramid fiber, an inorganic fiber such as glass fiber, or a metal fiber such as copper fiber, an organic friction modifier such as rubber dust and cashew dust, an inorganic friction modifier such as calcium carbonate, barium sulfate, metal particle, ceramic particle and graphite, and a binder such as phenol resin, are used. 
     The fiber base material forming the framework of a friction material supplements the strength, heat resistance and wear resistance of the friction material, and among metal fibers to be used as the fiber base material, a copper fiber is soft and highly ductile, leading to excellent formability, and therefore, the copper fiber is generally used. The copper fiber has a function of diffusing heat generated on the frictional surface due to its high thermal conduction during friction braking and in turn, enhancing the fade resistance. The copper fiber is sometimes used in the powder form, i.e., as a copper powder. 
     A fading phenomenon is caused by a decomposition gas generated resulting from decomposition of an organic substance contained in the friction material upon exposure of the friction material to high temperature and high load, and when copper having large thermal conductivity is added to the friction material, the heat radiation property of the friction material itself is increased, so that the generation of decomposition gas can be suppressed. 
     It has been heretofore disputed whether the friction material could achieve both of assurance of braking property and enhancement of anti-squeal performance or fading resistance. In order to improve the anti-squeal performance, for example, Patent Document 1 describes a technique of using, for a brake disc pad, a friction material having blended therein a fluorine-based polymer such as polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP) and polyvinylidene fluoride (PVDF). In addition, each of Patent Documents 2 and 3 describes a friction material composition containing an active alumina as an inorganic friction modifier and a fluorine-based polymer as an organic friction modifier, and a friction material using the friction material composition. 
     Brake noise is generated under various conditions, but particularly when low-speed braking with low deceleration is performed after leaving the friction material under low-temperature and high-humidity conditions, the friction material generates squeal. For example, when we start an automobile parked overnight in the morning and a first braking is performed, a squeal is likely to be generated. 
     As a technique for reducing such squeal of a friction material after leaving it under low-temperature and high-humidity conditions, Patent Document 4 describes a technique of blending, in a friction material composition, a fibrous fluorine-based polymer fiber having a fiber diameter of 5 μm to 30 μm and a fiber length of 0.5 mm to 80 mm in order to obtain a friction material in which the change in brake output between the case at low temperature and the case at high temperature is lessened and noise generation at low temperature is reduced. 
     On the other hand, in the case of a friction material containing a heavy metal such as copper, the heavy metal is released as wear debris into air due to braking, and its impact on the natural environment is pointed out. With such concern, a friction material containing no heavy metal such as copper has been developed, and a move toward removing a heavy metal is recently accelerated on an international basis. As a friction material in which copper is substantially removed, for example, Patent Document 5 describes a non-asbestos friction material containing neither a heavy metal nor a heavy metal compound and containing graphite and coke in an amount of 5% by volume to 25% by volume in total. 
     PRIOR ART DOCUMENTS 
     Patent Documents 
     Patent Document 1: JP-T-2004-501271 (the term “JP-T” as used herein means a published Japanese translation of a PCT patent application) 
     Patent Document 2: JP-A-2011-17016 
     Patent Document 3: WO 2004/001162 
     Patent Document 4: JP-A-H08-326805 
     Patent Document 5: JP-A-2008-179806 
     SUMMARY OF THE INVENTION 
     Problems That the Invention is to Solve 
     In such a copper-free friction material containing no copper, even when a fluorine-based polymer fiber is used as in Patent Document 4 in order to solve the squeal problem of the friction material after leaving it at low temperature, a lubricating function due to malleability and ductility of copper is not provided and therefore, if the friction material absorbs moisture after leaving it in a cold environment, the friction coefficient increases, and the squeal is readily generated. In addition, since copper is not contained, the thermal conductivity lowers, and the formability of the friction material is reduced. Reduction in the formability may bring about a decrease in the strength of the friction material and an adverse change in the wear amount during the use. Furthermore, since the fluororesin fiber exhibits poor wettability with a phenol resin, an increase in the amount thereof is disadvantageous for the formability of the friction material. 
     Accordingly, with respect to a friction material composition containing neither copper nor a copper-containing metal and a friction material using the composition, a problem to be solved by the present invention is to provide a friction material in which an increase in the friction coefficient after leaving it in a cold environment is suppressed and not only the wear resistance is enhanced but also the noise resistance and brake output for fitting to a wide range of vehicle types are improved. 
     Means for Solving the Problems 
     As a result of intensive studies to solve the above-described problem, the present inventors have found that, with regard to a copper-free friction material, when a fluorine-based polymer particle is contained in a friction material composition, a friction material having good thermoformability can be obtained due to enhanced dispersibility of the fluorine-based polymer particle in the friction material composition, and the obtained friction material provides stable friction coefficient and wear resistance and can reduce squeal after leaving it in a cold environment. The present invention has been accomplished based on this finding. The generation of squeal after leaving the friction material in a cold environment is caused by intrusion of water in air into a pore of the friction material. It is considered that when a fluorine-based polymer powder is blended, the water repellent function of the powder produces an effect of preventing water from being absorbed in a pore of the friction material. In addition, the fluorine-based polymer powder is likely to be uniformly dispersed in the friction material, as compared with the case of a fluorine-based fiber, and therefore, the expected effect is obtained even when a small amount thereof is used. Furthermore, the amount of the fluorine-based polymer powder to be used is small relative to the case of the fluororesin fiber, so that formability can also be ensured. 
     That is, the present invention could be achieved by the following (1) to (5). 
     (1) A friction material composition comprising a fiber base material, a binder and a friction modifier, wherein a content of copper in the friction material composition is 0.5% by mass or less and the friction material composition comprises a fluorine-based polymer particle. 
     (2) The friction material composition according to (1), wherein the fluorine-based polymer particle is a polytetrafluoroethylene particle. 
     (3) The friction material composition according to (1) or (2), wherein a content of the fluorine-based polymer particle is from 0.2% by mass to 3.5% by mass. 
     (4) A friction material formed by molding the friction material composition as described in any one of (1) to (3). 
     (5) A method for producing a friction material, comprising thermoforming a friction material composition under a thermoforming surface pressure of 50 MPa to 70 MPa at 140° C. to 170° C. for 3 minutes to 10 minutes, wherein the friction material composition comprises a fiber base material, a binder and a friction modifier and further comprises a fluorine-based polymer particle, and a content of copper in the friction material composition is 0.5% by mass or less. 
     Advantageous Effects of the Invention 
     A friction material formed from the friction material composition in the present invention, when used as a friction material such as disc brake pad or brake lining for automobiles, imposes a small environmental load because of substantially no presence of copper in the wear debris to be produced during braking, exerts stable friction coefficient, noise resistance and wear resistance, and can maintain the squeal characteristics of the friction material after leaving it in a cold environment at the same level as that of a friction material containing copper. In addition, a friction material having the above-described properties can be provided by using the friction material composition in the present invention. 
     Mode for Carrying Out the Invention 
     The mode for carrying out the present invention is described in detail below, but the following embodiment is only illustrative, and the present invention is not limited thereto. 
     In the present invention, the “copper-free” means that a copper component is not substantially contained in the friction material and specifically indicates that the content of the copper component is 0.5% by mass or less based on the whole amount of the friction material composition. 
     In this description, the “mass” means “weight”. 
     The friction material composition in the present invention contains a fiber base material, a binder and a friction modifier, in which a content of copper in the friction material composition is 0.5% by mass or less and the friction material composition contains a fluorine-based polymer particle. 
     The fiber base material is used for reinforcement of a friction material to be produced and as the fiber base material, for example, a heat-resistant organic fiber, an inorganic fiber, or a metal fiber is used. Examples of the heat-resistant organic fiber include an aromatic polyamide fiber (aramid fiber) and a flame-resistant acrylic fiber; examples of the inorganic fiber include a potassium titanate fiber and a ceramic fiber (biosoluble fibers are preferably used), a glass fiber, a carbon fiber, a rock wool and the like; and examples of the metal fiber include a steel fiber. These may be used individually or in combination of two or more kinds thereof. In order to ensure sufficient mechanical strength, the content of the fiber base material is preferably from 1% by mass to 15% by mass, more preferably from 2% by mass to 10% by mass, based on the whole amount of the friction material composition. 
     The binder is composed of a thermosetting resin, and examples of the thermosetting resin include a phenol resin, an epoxy resin, a resin obtained by modifying such a thermosetting resin with cashew oil, silicone oil, various elastomers, etc., and a resin obtained by dispersing, in the thermosetting resin above, various elastomers, a fluorine-based polymer, etc. These may be used individually or in combination of two or more kinds thereof. In order to ensure sufficient mechanical strength and wear resistance, the content of the binder is preferably from 7% by mass to 12% by mass, more preferably from 8% by mass to 11% by mass, based on the whole amount of the friction material composition. 
     The fluorine-based polymer for use in the present invention is used as a friction modifier and may be a homopolymer of tetrafluoroethylene (TFE) or may be a copolymer with a copolymerizable monomer except for TFE, such as fluorine-containing monomer having an ethylenically unsaturated group. Some fluorine-based polymers have a molecular weight of several million, and a polymer in any form such as granular, plate-like or amorphous shape may be appropriately used, but the fluorine-based polymer is usually used in the powder form. 
     Accordingly, the fluorine-based polymer as used in the present invention encompasses not only a homopolymer of TFE (PTFE) but also a modified polytetrafluoroethylene that is modified by copolymerizing other monomer(s) in such a small amount as not imparting melt flowability. 
     Examples of the fluorine-containing monomer having an ethylenically unsaturated group include hexafluoropropylene, perfluorobutene-1, perfluorohexene-1, perfluorononene-1, perfluoro(methyl vinyl ether), perfluoro(ethyl vinyl ether), perfluoro(propyl vinyl ether), perfluoro(heptyl vinyl ether), (perfluoromethyl)ethylene, (perfluorobutyl)ethylene, and chlorotrifluoroethylene. Among them, hexafluoropropylene, perfluoro(n-propyl vinyl ether) and (perfluoro-n-butyl)ethylene are preferred. These fluorine-containing monomers may be used individually or in combination of two or more kinds thereof. The amount of the copolymerizable monomer is, usually, preferably 1 mol % or less, more preferably 0.5 mol % or less. 
     Among the fluorine-based polymers for use in the present invention, polytetrafluoroethylene (PTFE, melting point: 327° C.) is preferred. 
     The fluorine-based polymer particle for use in the present invention can be usually produced by suspension polymerization. The suspension polymerization can be performed by using TFE alone or using TFE and a copolymerizable monomer, in an aqueous medium containing a dispersant and a polymerization initiator. The polymerization temperature is usually from 50° C. to 120° C., preferably from 60° C. to 100° C. The polymerization pressure may be appropriately selected. 
     The fluorine-based polymer particle for use in the present invention is obtained by pulverizing the particle having an average particle diameter of several mm obtained by the suspension polymerization. In particular, when a polytetrafluoroethylene (hereinafter, referred to as PTFE) powder having an average particle diameter of about 20 μm or less is pulverized and classified, the fluorine-based polymer particle can be easily adjusted to have a desired particle diameter. In the present invention, a fluorine-based polymer particle having an average particle diameter of 10 μm or less is preferably used, and a fluorine-based polymer particle having an average particle diameter of 0.5 μm to 8 μm is more preferably used. If the average particle diameter exceeds 10 μm, the effect on the blending amount is reduced, and thus, a fluorine-based polymer particle having an average particle diameter of 10 μm or less is preferably used. 
     The blending amount of the fluorine-based polymer particle for use in the present invention is preferably from 0.2% by mass to 3.5% by mass, more preferably from 0.2% by mass to 3.0% by mass, based on the whole amount of the friction material composition. When the fluorine-based polymer particle is blended so as to fall within the range above, the friction coefficient after leaving the friction material in a cold environment can be restrained from rising, and the squeal can be advantageously reduced. 
     In the friction material composition in the present invention, materials usually employed are used as long as the gist of the present invention is observed. Specifically, examples of other friction modifiers include an organic filler such as cashew dust, rubber dust (pulverized powder of tire tread rubber), various unvulcanized rubber particles and various vulcanized rubber particles, an inorganic filler such as barium sulfate, calcium carbonate, calcium hydroxide, vermiculite, mica, plate-like potassium titanate, scale-like lithium potassium titanate or magnesium potassium titanate, and amorphous potassium titanate, an abrasive such as silicon carbide, alumina, magnesia, chromite, triiron tetroxide, zirconium oxide and zirconium silicate, a lubricant such as molybdenum disulfide, tin sulfide, zinc sulfide and iron sulfide, and a non-ferrous metal particle other than copper and a copper alloy, such as zinc powder and tin powder. These may be used individually or in combination of two or more thereof. The content of the friction modifier may be appropriately adjusted according to the desired frictional properties. The content of the friction modifier is, according to the desired friction coefficient, preferably from 60% by mass to 90% by mass, more preferably from 65% by mass to 85% by mass, based on the whole amount of the friction material. 
     In addition, other blending materials may be used, if desired. 
     The production of a friction material using the friction material composition in the present invention may be performed by a usual production process, but the friction material is preferably produced by setting the surface pressure high during thermoforming. Specifically, the friction material can be produced through steps of mixing of raw materials, preforming, thermoforming, heating, grinding, etc. A powdered mixture obtained by mixing raw materials by a blender, etc. is put in a preforming mold and preformed, and the preformed body is put in a thermoforming mold and a metal plate is laminated thereon, followed by thermoforming under a thermoforming surface pressure higher by 10% to 40% than usual cases, specifically, at 50 MPa to 70 MPa, preferably from 55 MPa to 70 MPa, more preferably from 55 MPa to 65 MPa. When the thermoforming surface pressure falls within the range above, a frictional strength comparable to that of a conventional product can be ensured. The thermoforming temperature is preferably from 140° C. to 170° C., and the thermoforming time is preferably from 3 minutes to 10 minutes. 
    
    
     EXAMPLES 
     The present invention is described more specifically below by referring to Examples, but the scope of the present invention is not limited only to these Examples. 
     Test Example 1 
     Examples 1 to 6 and Comparative Examples 1 to 3 
     &lt;Production of Friction Material&gt; 
     The blending materials shown in Table 1 were collectively put in a mixing stirrer, followed by mixing at room temperature for 2 minutes to 10 minutes, thereby obtaining a friction material composition. Each of the obtained friction material compositions was processed through steps of preforming (1), thermoforming (2), heating and firing (3), etc., thereby preparing a friction material. 
     (1) Preforming 
     The friction material composition above was put in the mold for preforming press, followed by forming at room temperature under 20 MPa for 5 seconds, thereby preparing a preformed article (sample size: 45×80 mm). 
     (2) Thermoforming 
     This preformed article was put in a thermoforming mold, and a metal plate (pressure plate: P/P) previously coated with an adhesive was laminated thereon, followed by thermoforming at 150° C. under 50 MPa for 5 minutes. 
     (3) Heating and Firing 
     This thermoformed product was heat-treated at 250° C. for 2 hours and grinding was performed so as to have a predetermined thickness of 15.5 mm. Subsequently, the surface of the thermoformed product was subjected to a scorching treatment, followed by finishing coating, thereby obtaining a friction material (brake pad). 
     &lt;Friction Material Evaluation Test&gt; 
     Using the prepared friction materials of Examples 1 to 6 and Comparative Examples 1 to 3, evaluation of formability of the friction material, general performance test, wear amount of friction material, and squeal test were performed. The evaluation results obtained are shown in Table 1. 
     (1) Evaluation of Formability of Friction Material 
     The thermoformed product was measured for the hardness on an HRS scale by means of a Rockwell hardness meter at predetermined 4 points in total, i.e., 2 points on the outer circumference and 2 points on the inner circumference, where the hardness could be usually measured. 
     An average value of the hardness at the 4 measured potions above was calculated, and when the average value had a variation within ±20% relative to the preset target hardness value, the sample was rated as “A”. Furthermore, the sample was rated as “B” when the variation of the average value is more than 20% and 30% or less, and rated as “C” when the variation exceeds 30%. Although a sample rated as “C” in this evaluation was formable, the friction material was soft and insufficient in the strength and was judged to be incapable of withstanding a friction test, and therefore, such a sample was not subjected to the friction test evaluation. 
     (2) General Performance Test 
     In conformity with JASO·C406, μ at the time of second effect with 2.94 m/s 2  and 5.88 m/s 2  at 50 km/h and 100 km/h, respectively, was defined as μ in the normal region. 
     (3) Measurement of Wear Amount of Friction Material 
     The thickness of the friction material (brake pad) was measured by a micrometer before and after carrying out the general performance test above, and the thickness difference therebetween was calculated as the wear amount. 
     (4) Squeal Test 
     A squeal test was carried out under the following conditions by using a dynamo tester, and the squeal generation rate was evaluated. 
     &lt;Ambient Condition: temperature: 10° C., humidity: 40%&gt; 
     Burnishing: 200 times at 65 km/h, 3.43 m/s 2  and 120° C. 
     Matrix: once under each of the conditions of 10 km/h, 20 km/h and 30 km/h, 0.98 m/s 2 , 1.96 m/s 2  and 2.94 m/s 2 , and 50° C. to 200° C. (rise or drop in steps of 50° C.) 
     &lt;Ambient Condition: temperature: 10° C., humidity: 80%&gt; 
     Leaving: 4 hours 
     Braking: 3 times at 10 km/h and 0.98 m/s 2    
     These tests were carried out 10 cycles in total. However, burnishing after second cycle was changed to 35 times. The sample was rated as “A” when the squeal generation rate was 5% or less, rated as “B” when it was more than 5% and less than 10%, and rated as “C” when it was 10% or more. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                   
                   
                   
                   
                 Example 
                 Comparative Example 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 1 
                 2 
                 3 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 Blending 
                 Fiber base material 
                 Aramid fiber 
                 5.0 
               
               
                 composition 
                   
                 Copper fiber 
                 0.0 
               
               
                 (% by mass) 
                   
                 Biosoluble inorganic fiber 
                 5.0 
               
               
                   
                 Binder 
                 Phenol resin 
                 9.0 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Friction modifier 
                 PTFE powder (average particle  
                 0.2 
                 0.5 
                 1.0 
                 2.0 
                 3.0 
                 3.5 
                 0.0 
                 0.0 
                 0.0 
               
               
                   
                   
                 diameter: 4 μm) 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                   
                 PTFE fiber (average fiber  
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 1.0 
                 5.0 
               
               
                   
                   
                 length: 5 mm) 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Cashew dust 
                 5.0 
               
               
                   
                   
                 Rubber dust 
                 4.5 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Barium sulfate 
                 24.3 
                 24.0 
                 23.5 
                 22.5 
                 21.5 
                 21.0 
                 24.5 
                 23.5 
                 19.5 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Plate-like potassium titanate 
                 24.0 
               
               
                   
                   
                 Chromite 
                 0.5 
               
               
                   
                   
                 Triiron tetroxide 
                 8.0 
               
               
                   
                   
                 Graphite 
                 7.0 
               
               
                   
                   
                 Calcium hydroxide 
                 3.0 
               
               
                   
                   
                 Tin sulfide 
                 1.5 
               
               
                   
                   
                 Mica 
                 1.5 
               
               
                   
                   
                 Zinc powder 
                 1.5 
               
            
           
           
               
               
               
            
               
                   
                 Total (% by mass) 
                 100.0 
               
            
           
           
               
               
            
               
                 Thermoforming surface pressure [MPa] 
                 50 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Friction material formability 
                 A 
                 A 
                 A 
                 A 
                 A 
                 A 
                 A 
                 B 
                 C 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Friction test 
                 JASO Performance, 
                  50 km/h 
                 2.94 m/s 2   
                 0.34 
                 0.33 
                 0.32 
                 0.31 
                 0.30 
                 0.29 
                 0.35 
                 0.36 
                 incapable  
               
               
                   
                 second effect 
                   
                 5.88 m/s 2   
                 0.33 
                 0.33 
                 0.32 
                 0.30 
                 0.30 
                 0.28 
                 0.35 
                 0.35 
                 of 
               
               
                   
                   
                   
                 2.94 m/s 2   
                 0.37 
                 0.36 
                 0.35 
                 0.34 
                 0.32 
                 0.29 
                 0.38 
                 0.39 
                 evaluation 
               
               
                   
                   
                 100 km/h 
                 5.88 m/s 2   
                 0.36 
                 0.35 
                 0.35 
                 0.32 
                 0.31 
                 0.29 
                 0.37 
                 0.38 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Wear amount of pad after JASO performance test [mm] 
                 0.74 
                 0.70 
                 0.68 
                 0.65 
                 0.63 
                 0.61 
                 0.78 
                 1.15 
                   
               
               
                   
                 Score of squeal generation rate 
                 B 
                 A 
                 A 
                 A 
                 A 
                 A 
                 C 
                 C 
                   
               
               
                   
               
            
           
         
       
     
     It has been found from the results in Table 1 that, with regard to the copper-free friction material, in Comparative Example 1 not containing PTFE particle, the wear amount of the friction material was as large as 0.78 mm and the score in the evaluation of squeal generation rate was 10% or more, whereas in Examples 1 to 6 containing a PTFE particle, the wear amount of the friction material is small as compared with Comparative Example 1 and the score in the evaluation of squeal generation rate was excellent. In Comparative Examples 2 and 3 using a PTFE fiber, the formability was seriously reduced. In addition, in Comparative Example 2, the reduction of the formability is considered to have brought about a decrease in the friction material strength and an increase in the wear amount, as compared with Examples 1 to 6. Furthermore, in Comparative Example 2, the effect on squeal was not obtained, as compared with Examples 1 to 6. 
     Test Example 2 
     Examples 7 to 10 and Comparative Example 4 
     &lt;Production of Friction Material&gt; 
     Friction materials were produced by using the blending materials shown in Table 2 by the same method as in Test Example 1, but the thermoforming surface pressure in thermoforming (2) was changed so as to fall within the range of 50 MPa to 70 MPa. 
     &lt;Friction Material Evaluation Test&gt; 
     Using the prepared frictions materials of Examples 7 to 10 and Comparative Example 4 and the friction material of Example 2 prepared in Test Example 1, general performance test, measurement of wear amount of friction material, and squeal test were performed. The test methods are the same as in Test Example 1. The evaluation results obtained are shown in Table 2. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                   
                   
                   
                   
                 Example 
                 Comparative 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                 2 
                 7 
                 8 
                 9 
                 10 
                 Example 4 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 Blending 
                 Fiber base material 
                 Aramid fiber 
                 5.0 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 composition 
                   
                 Copper fiber 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 10.0 
               
            
           
           
               
               
               
               
            
               
                 (% by mass) 
                   
                 Biosoluble inorganic fiber 
                 5.0 
               
               
                   
                 Binder 
                 Phenol resin 
                 9.0 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 Friction modifier 
                 PTFE powder (average particle 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
                 0.0 
               
               
                   
                   
                 diameter: 4 μm) 
                   
                   
                   
                   
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Cashew dust 
                 5.0 
               
               
                   
                   
                 Rubber dust 
                 4.5 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Barium sulfate 
                 24.0 
                 24.0 
                 24.0 
                 24.0 
                 24.0 
                 14.5 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Plate-like potassium titanate 
                 24.0 
               
               
                   
                   
                 Chromite 
                 0.5 
               
               
                   
                   
                 Triiron tetroxide 
                 8.0 
               
               
                   
                   
                 Graphite 
                 7.0 
               
               
                   
                   
                 Calcium hydroxide 
                 3.0 
               
               
                   
                   
                 Tin sulfide 
                 1.5 
               
               
                   
                   
                 Mica 
                 1.5 
               
               
                   
                   
                 Zinc powder 
                 1.5 
               
            
           
           
               
               
               
            
               
                   
                 Total (% by mass) 
                 100.0 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Thermoforming surface pressure [MPa] 
                 50 
                 55 
                 60 
                 65 
                 70 
                 50 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Friction test 
                 JASO Performance, 
                  50 km/h 
                 2.94 m/s 2   
                 0.33 
                 0.32 
                 0.32 
                 0.31 
                 0.30 
                 0.35 
               
               
                   
                 second effect 
                   
                 5.88 m/s 2   
                 0.33 
                 0.33 
                 0.33 
                 0.31 
                 0.28 
                 0.35 
               
               
                   
                   
                 100 km/h 
                 2.94 m/s 2   
                 0.36 
                 0.36 
                 0.35 
                 0.34 
                 0.30 
                 0.39 
               
               
                   
                   
                   
                 5.88 m/s 2   
                 0.35 
                 0.35 
                 0.34 
                 0.32 
                 0.28 
                 0.39 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 Wear amount of pad after JASO performance test [mm] 
                 0.70 
                 0.61 
                 0.54 
                 0.48 
                 0.45 
                 0.48 
               
               
                   
                 Score of squeal generation rate 
                 A 
                 A 
                 A 
                 A 
                 A 
                 A 
               
               
                   
               
            
           
         
       
     
     It has been found from the results in Table 2 that when the surface pressure during thermoforming is raised by 10 to 40% (Examples 7 to 10), the squeal is improved and the strength of the friction material, i.e., wear resistance, substantially comparable to that of Comparative Example 4 containing copper, can be ensured. 
     While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope of the invention. This application is based on Japanese Patent Application No. 2013-234271 filed on Nov. 12, 2013, the contents of which are incorporated herein by way of reference. 
     INDUSTRIAL APPLICABILITY 
     The friction material composition in the present invention, when used for a friction material such as disc brake pad and brake lining, imposes a small environmental load because of substantially no presence of copper in the wear debris produced during braking, exerts stable friction coefficient, noise resistance and wear resistance, and in particular can reduce squeal after leaving the friction material in a cold environment. Accordingly, a friction material formed by molding the friction material composition in the present invention is suitable for a brake pad, etc. of an automobile.