Abstract:
A friction material is disclosed, which is characterized in that, in the friction material comprising reinforcing material, friction modifier, solid lubricant and thermosetting organic binder, said thermosetting organic binder is a resin having less hydroxyl group content compared with phenol resin.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a friction material with decreased variation in friction coefficient. 
     The friction material such as brake pad, brake lining, clutch facing or the like used for the brake and the clutch of motorcars, railroad vehicles, industrial machines, etc. is generally made by combining and molding principal raw materials comprising reinforcing material such as asbestos, metallic fiber, iron powder or the like, friction modifier such as organic dust, metal oxide powder or the like and wear resistant solid lubricant such as graphite, molybdenum disulfide or the like with phenol resin being a thermosetting organic binder. 
     The friction material containing the phenol resin as a binder adsorbs, however, the moisture in atmosphere during the use because of the moisture adsorption property of phenol resin resulting in significant changes in friction and wear characteristics, which has been a problem. 
     As a result of investigations to solve said problem, a friction material of hardly subjected by moisture in atmosphere has been developed by the invention. 
     SUMMARY OF THE INVENTION 
     The gist of the invention lies in a friction material characterized in that, in the friction material comprising reinforcing material, friction modifier, solid lubricant and thermosetting organic binder, said thermosetting organic binder is a resin having less hydroxyl group content compared with phenol resin, wherein, as the resin containing no hydroxyl group or the resin with small content of hydroxyl group, the use of condensed polycyclic polynuclear aromatic resin or xylene resin is effective. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a chart showing the variation of friction coefficient according to the absolute humidity of friction materials. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The friction material of the invention can contain 10 to 50 vol. % of reinforcing material, 5 to 10 vol. % of friction modifier, 10 to 30 vol. % of solid lubricant, 5 to 20 vol. % of filler and 10 to 30 vol. % of resin containing no hydroxyl group or resin with small content of hydroxyl group. 
     As the reinforcing material aforementioned, one of steel fibers, brass fibers and steel fibers provided with copper coating can be used in combination with iron powder. 
     As the friction modifier aforementioned, one or more of rubber powder, cashew powder, alumina powder, etc. can be used. 
     As the solid lubricant aforementioned, graphite, molybdenum disulfide, etc. can be used. 
     Moreover, as the filler, barium sulfate, calcium silicate, etc. can be used. 
     The phenol resin being a thermosetting organic binder, which is widely used for the friction materials at present, contains a plenty of hydroxyl groups and, to these hydrophilic groups, the water molecules in atmosphere are adsorbed. Such adsorption of water molecules to hydroxyl groups is considered to be a predominant cause of changes in friction and wear characteristics of friction materials. 
     Hence, decreasing the hydrophilic groups of thermosetting organic binder as few as possible serves sufficiently for making the changes in friction and wear characteristics of friction materials smaller. 
     As the thermosetting organic binder to be used in the invention, a resin containing no hydroxyl group or having small content of hydroxyl group such as condensed polycyclic polynuclear aromatic resin or xylene resin is designated. 
     In following, the illustration will be made about the examples of the invention. 
     EXAMPLE 
     Respective friction materials were manufactured as described below using a condensed polycyclic polynuclear aromatic resin with a hydroxyl group content of 30% (referred to as resin B), xylene resin with a hydroxyl group content of 0% (referred to as resin C) and a condensed polycyclic polynuclear aromatic resin with a hydroxyl group content of 0% (referred to as resin D) when assuming the content of hydroxyl group of phenol resin (referred to as resin A) to be 100%. 
     Namely, 25 vol. % of graphite, 20 vol. % of steel fibers provided with copper coating, 20 vol. % of iron powder, 6.5 vol. % of rubber powder, 0.5 vol. % of alumina powder and 8 vol. % of barium sulfate were mixed, then 20 vol. % of resin A, B, C or D were added to this mixture, and the whole was molded under heat to produce conventional friction material No. 5 and the friction materials of the invention No. 1, No. 2 and No. 3. 
     Further, 20 vol. % of graphite, 20 vol. % of steel fibers, 20 vol. % of iron powder, 6.5 vol. % of rubber powder, 0.5 vol. % of alumina powder and 13 vol. % of barium sulfate were mixed and, after the addition of 20 vol. % of resin D to this mixture, the whole was molded under heat to obtain the friction material of the invention No. 4. 
     At this time, since acidic catalyst is used in the cases of said resins B, C and D, it is preferable to use after the surface of barium sulfate used as a filler is coated with silane coupling agent of epoxy type, chlorine type or the like or acid-treated with a solution dissolved 10% p-toluenesulfonic acid into methanol. 
     Next, of these friction materials No. 1 through No. 5, the friction coefficient was measured, respectively, in moistened atmosphere, the results of which are shown in FIG. 1. Moreover, the results of the calculation of final variation rate of friction coefficient are shown in Table 1. 
     
                       TABLE 1______________________________________                   Ratio of  Reduction rate inFriction                hydroxyl group                             friction coefficientmaterial No.    Binder  (%)       (%)______________________________________Friction 1      Resin   30        56material of     Bthe inventionFriction 2      Resin   0         58material of     Cthe inventionFriction 3      Resin   0         44material of     Dthe inventionFriction 4      Resin   0         27material of     Dthe inventionConventional    5      Resin   100       83friction        Amaterial______________________________________ 
    
     As shown, with the conventional friction material No. 5 used phenol resin as a thermosetting organic binder, the friction coefficient was decreased by 83% in moistened atmosphere. In comparison with this, the decrease in the friction coefficient of friction materials of the invention No. 1, No. 2, No. 3 and No. 4 used condensed polycyclic polynuclear aromatic resin or xylene resin as thermosetting organic binder was only 27 to 58% or so. 
     As described above, the friction material, wherein the hydrophilic groups of thermosetting resin were decreased as few as possible, has conspicuous effects that the stabilized friction characteristics can be exhibited and the like, since it is hardly effected by the amount of moisture in atmosphere, that is, the humidity even if this may change.