in accordance with the present invention , an elastomer , preferably a fluoroelastomer or another elastomer mixed with a fluoroelastomer , is used as the primary binder or base for a wet friction material . the elastomer is compounded to contain an effective amount of wollastonite , which is an amount required to reduce the static relative to the dymanic coefficient of friction in the resultant material . other additives such as fibers and fillers may be included but are considered optional for the purpose of the present invention . the term &# 34 ; fluoroelastomer &# 34 ; as used herein include all presently known materials classified as elastomers and containing fluoride groups . fluoroelastomers currently known and available include copolymers of vinylidene fluoride and hexafluoropropylene , terpolymers of vinylidene fluoride , hexafluoropropylene and tetrafluoroethylene , and alternating copolymers of propylene and tetrafluoroethylene . such fluoroelastomers are available commercially under the trademarks &# 34 ; viton &# 34 ;, &# 34 ; fluorel &# 34 ;, &# 34 ; aflas &# 34 ; and the like . such fluoroelastomers have excellent solvent and oil resistance and also have a relatively high temperature resistance compared to other available elastomers . the fluoroelastomer is preferably the only binder in the system , but other binders , such as other elastomers and / or curable organic resins or inorganic binders , may be employed . amounts of components listed herein will be in terms of percentage by volume rather than the more commonly used percentage by weight . this is done to make the description more aptly reflective of the actual characteristics of the friction material in terms of the various components , irrespective of specific gravities of individual components , since the volume , rather than the weight , of each component is more meaningful in terms of the geometry and exposed working surface of the friction material . the amount of elastomer employed is preferably above the minimum required to mechanically bond all of the other solid materials together in the finished product , and a minimum of at least 20 percent by volume elastomer will normally be required for this purpose . the other significant material used in the present composition is wollastonite , which is a mineral otherwise called tabular spar defined as calcium silicate ( casio 3 ), having a specific gravity in the order of 2 . 9 and a hardness of approximately 4 . 5 moh . preferably , the wollastonite is used in powdered or particulate form , wherein the particles are smaller than 300 mesh , and the average aspect ratio ( length to diameter ) is in the order of from about 3 to one to as high as 15 to one , with most grades having aspect ratios of from about 3 - 5 to one . as stated herein before , the amount of wollastonite included in the composition is that amount desired to produce the desired altered friction characteristics of the final material . in general , the total composition will contain from about 5 to about 35 percent by volume , and preferably in the order of from about 10 to about 25 percent by volume , of the wollastonite . it has been found that a suitable friction material may be prepared from a compound of the elastomer and wollastonite alone . in order to improve durability and strength of the material , however , and to decrease the total volume of relatively expensive elastomer required , however , other fillers and reinforcing materials will be incorporated into the friction material composition , typically in the form of fibers and / or powdered or particulate fillers . certain fibers or fillers are known to augment the overall friction properties of the material , for example , a relatively hard or sharp particle or fiber , although such conventional additives are not known or used to significantly alter the ratio of the static to dynamic coefficient of friction . suitable fibers that may be incorporated into the material include , for example , organic and inorganic fibers , such as carbon fibers , carbon fiber precursers , other polymer fibers , and cotton , asbestos , glass , ceramic , mineral , metallic and others . fibers serve to provide a reinforcing matrix and generally improve useful service life of the material . if fiberous materials are employed , they may be present in an amount of from about 5 to about 40 percent by volume . various types of fillers may also be incorporated in the composition , and numerous types of fillers are available for this purpose . furthermore , it is known that some fillers may be used as lubricants , some may be used as abrasives , while others are relatively neutral and serve primarily as fillers . typical fillers , for example , which may be included in the present composition include barium sulfate , molybdenum disulfide , alumina , silica , petroleum coke , graphite , carbon black , powdered metals , powdered polymers , mixtures thereof , and the like . in general , up to at least 60 to 70 percent of the volume of the material , including the wollastonite , may comprise fillers , whereas the other fillers used as abrasives would normally be used in lesser amounts , i . e ., less than 20 percent . in addition , minor amounts of conventional accelerators , stabilizers , processing aids and curing agents suitable for the elastomer used may be incorporated into the material . the uncured fluoroelastomer , wollastonite , and any fibers , fillers and curatives are then thoroughly mixed together by a suitable technique , such as by compounding in a banbury mixer , such that the additives are uniformly dispersed in the elastomer . following the mixing procedure , the mixture is formed into the shape of a friction member , typically a pad , a grooved disc or ring shape , and the member is then cured under heat and pressure . several methods available for this purpose . the material may first be sheeted on a conventional sheeter , cut into the desired shape , and subsequently cured . in the alternative , the material may be extruded or may be molded , such as by injection , compression or transfer molding techniques . grooves may be imparted in the outside surface of the friction element either by molding or by cutting the grooves in the cured piece . regardless of the method used to process the material , it is also necessary to secure the friction element to a rigid backing member such as a steel plate . this may be done either by compression molding the element against the backing member with the addition of a small amount of an adhesion promoter and / or by securing the friction element to the backing member with a suitable adhesive . the elastomer is cured under conventional conditions depending on the type of elastomer and the curatives employed . generally , heat and pressure are applied simultaneously , for example , at temperatures of about from 350 to 475 degrees f . and at pressures of from about 50 to about 4 , 000 pounds per square inch until the elastomer has fully cured . the cured friction material of the present invention , after being mounted on a backing plate , is typically incorporated into a mechanical assembly having oil circulating therein . the friction plate is brought into and out of engagement with a relatively rotating reaction plate , which may occur at various speeds and pressures . the friction material of the present invention exhibits substantially uniform levels of friction over wide pressure ranges , with static and dynamic coefficients of friction being in the order of from about 0 . 05 to 0 . 25 . moreover , however , the addition of wollastonite uniquely serves to lower the static coefficient of friction relative to the dynamic coefficient over the entire range of operating conditions , such that with proper compounding , the ratio between the two values approaches one . this allows the friction member to be capable of smoother engagements with the reaction member without excessive noise or chatter . it will be appreciated that the measured levels of static and dynamic coefficients of friction vary with pressure , speed , and other factors . also , even though a standard measurement technique is employed , the numerous variables involved make it difficult to quantify these values with great or reproducible exactitude . nevertheless , comparative testing under similar conditions clearly indicates that the addition of wollastonite causes the level of static friction to more closely resemble the level of dynamic friction , with the attendant beneficial results hereinbefore described . the following comparative example is illustrative of the friction material of the present invention and its unique properties . the following two batches of materials were prepared by identical methods and were processed into finished friction discs of identical size and groove pattern . in general , each batch was mixed in a banbury mixer , the mixture was then sheeted to a given thickness , and the sheet was cut into rings of equal size . the rings were applied against a steel backing plate and bonded and cured under 100 psi for 15 minutes at 350 ° f . and post cured at 400 ° f . for 24 hours . ______________________________________ sample 1 sample 2 vol . % weight % vol . % weight % ______________________________________fluoroelastomer 48 . 81 43 . 46 48 . 82 47 . 34 ( viton e - 60 ) graphite fiber 29 . 33 28 . 55 29 . 33 31 . 10 ( thornel vmd ) wollastonite 15 . 19 21 . 56 -- --( nyad 400 ) carbon black -- -- 15 . 19 14 . 57 ( n - 990 ) bonding agents and 6 . 66 6 . 43 6 . 66 6 . 99curatives______________________________________ it may be seen that the above formulations were substantially identical , except that sample 1 contained wollastonite and sample 2 contained carbon black as a filler instead of wollastonite . the friction elements from each of samples 1 and 2 were subjected to identical testing on a dynamometer to determine the static and dynamic coefficients of friction over a range of speeds and unit pressures . the results of the tests on samples 1 and 2 are illustrated in fig1 and 2 , respectively . the dotted lines indicate the static coefficient , and the solid lines indicate the dynamic coefficient . the numbers within each graph indicate that measurements were made at 3 , 000 , 5 , 000 , 7 , 000 and 9 , 000 feet per minute . in comparing fig1 and 2 , it will be noted that in the case of the material containing wollastonite ( fig1 ), the static and dynamic coefficients of friction at the various speeds are very close in value . in contrast , the respective coefficients for sample 2 ( fig2 ) at the various speeds are more widely separated , with the static coefficient being significantly higher than the dynamic value . the values shown in fig1 will allow for a smoother engagement than the values shown in fig2 the latter being typical of conventional fluoroelastomer friction materials . fig3 and 4 are representative torque traces for sample 1 , and fig5 and 6 are torque traces , under the same conditions , for example 2 . in fig3 and 5 , the evaluation was made at 7 , 000 feet per minute and 200 psi ; and in fig4 and 6 , the evaluation was at 5 , 000 fpm and 200 psi . the figures illustrate the torque , in foot pounds , over a period of time , as the friction material is brought into frictional engagement with a relatively stationary mating member . for example , in fig3 the torque increases to about 400 as the members are first engaged and later decreases to zero at about 0 . 8 seconds when the members are fully engaged . fig5 and 6 illustrate torque tracings that would be typical of a conventional fluoroelastomer friction material . the area of interest in the tracing is shortly before full engagement . if there is a disparity in the dynamic and static coefficients of friction , i . e ., dynamic is lower than static , there will be an abrupt increase in torque shortly before full engagement , as illustrated . this so - called &# 34 ; rooster tail &# 34 ; effect may be very undesirable where smooth engagements are required , as discussed previously . in connection with the friction material of the present invention , as represented in fig3 and 4 there is no similar increase in torque shortly before engagement , thus allowing for smooth engagements .