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

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 in the initial stage of adsorption - the higher the humidity, the higher the weight gain. Pads cured at 150°C adsorb less moisture than pads cured at 220°C. 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. Static modulus of compression, dynamic modulus of compression and hardness measurements are compared, and they all show the same trend. A rate constant of adsorption is proposed to define and compare moisture sensitivity of friction material