Kinetics of Moisture Adsorption and Its Impact on Brake Pad Compression Modulus and Compressibility: Compression Modulus vs. Dynamic Modulus and Friction Variation

The kinetics of moisture adsorption is studied for copper-free brake pads. The pad weight gain is found to increase linearly with the square root of exposure time to humidity at a given temperature – the higher the humidity, the higher the weight gain. As the moisture content in the pad increases, the tangent modulus increases while the secant modulus decreases, resulting in decreasing compressibility associated with the tangent modulus of compression and increasing compressibility associated with the secant modulus of compression – compressibility defined as a reciprocal of compression modulus. Compression modulus and dynamic modulus are compared, and implications for friction coefficient variation are discussed. A rate constant of adsorption is proposed to define and compare moisture sensitivity of friction materials.