Brake Pad Moisture Adsorption and Its Effect on Brake Pad Compression Strain/Modulus and Friction Coefficient: Effect of Pad Cure Temperatures

The moisture adsorption kinetics of copper-free brake pads was studied to confirm an earlier finding that the adsorption weight gain follows a logarithmic relationship with respect to the square root of humidity exposure time and the relationship is linear in the beginning. When the pad cure temperature was raised from 120 to 180 and 240 °C, the adsorption rate increased. The 180 °C cure produced the highest pad modulus. With increasing moisture adsorption, the pad compression modulus increased just like the pad dynamic modulus, meaning decreasing compression/compressibility while the ISO ‘compressibility’ determined after 3 compressions under 160 bars increased in contradiction. It is concluded that the ISO ‘compressibility’ is a destructive hardness measurement like the Gogan or Rockwell hardness: the key difference is the indenter covers the entire surface of the pad. The true compressibility must be determined as an inverse function of bulk modulus. It is recommended that the pad compression modulus should be measured under low pressures like 10 bars for the purpose of determining the initial quality of virgin pads if the current ‘compressibility’ machines are to be used. Compression tangent modulus and dynamic modulus measurements produce the same results, meaning that one could replace the other. Pad cure temperature affects friction coefficient and moisture adsorption influences friction coefficient in opposite direction for moderate-speed snubs of 80 km/h vs. low-speed stops of 5 km/h.