Piston seal member and disc brake using the piston seal member

1. A piston seal member which holds a cylinder bore and a piston that slides in the cylinder bore in a fluid-tight and slidable manner,

the piston seal member being formed of a crosslinked rubber composition, the rubber composition including 100 parts by weight of an ethylene-propylene rubber, 20 to 60 parts by weight of a first carbon black, 50 to 100 parts by weight of a second carbon black, and 5 to 20 parts by weight of carbon nanofiber having an average diameter of 0.5 to 500 nm, the total amount of the first carbon black and the second carbon black being 70 to 160 parts by weight,

the first carbon black having an average particle diameter of 35 to 100 nm and a DBP absorption of 50 to 200 ml/100 g, and

the second carbon black having an average particle diameter that is greater than the average particle diameter of the first carbon black and ranges from 60 to 500 nm and a DBP absorption of 5 to 50 ml/100 g.

2. The piston seal member as defined in claim 1 ,

wherein the carbon nanofibers are uniformly dispersed in the ethylene-propylene rubber; and

wherein the rubber composition in uncrosslinked form has a first spin-spin relaxation time (T 2 n ), measured at 150° C. by a Hahn-echo method using a pulsed NMR technique, of 100 to 3000 microseconds, and a fraction (fnn) of components having a second spin-spin relaxation time (T 2 nn ) of less than 0.2.

3. The piston seal member as defined in claim 1 , wherein the rubber composition has a hardness (JIS-A) of 80 to 95.

4. The piston seal member as defined in claim 1 ,

wherein the rubber composition has a dynamic modulus of elasticity at 10 Hz and 30° C. of 60 to 90 MPa and a dynamic modulus of elasticity at 10 Hz and 200° C. of 30 to 40 MPa.

5. The piston seal member as defined in claim 1 ,

wherein the rubber composition has a compression set at 200° C. after 24 hours of 14% or less and a compression set at 230° C. after 24 hours of 24% or less.

6. The piston seal member as defined in claim 1 , wherein the rubber composition has an instantaneous strain of 2% or less and a steady-state strain of −2000 ppm/hour to +2000 ppm/hour in a creep test conducted at a temperature of 200° C. and a load of 250 KPa.

7. The piston seal member as defined in claim 1 , the piston seal member being used for a caliper body of a disc brake.

8. A disc brake comprising:

the piston seal member as defined in claim 1 ;

a cylinder having the cylinder bore; and

the piston inserted into the cylinder bore,

the piston seal member being fitted into a ring-shaped groove formed in an inner wall of the cylinder bore, the piston seal member causing the piston inserted into the cylinder bore to come in contact with the cylinder in a fluid-tight and movable manner and rolling back the piston which has been moved forward due to hydraulic pressure.