A Study of Moisture Sorption-Desorption and Its Influence on the Dynamic Modulus and Friction Coefficient of Copper-Free Brake Pads

The moisture sorption-desorption kinetics of copper-free brake pads was studied in detail. The sorption-desorption behavior is dependent on the environmental temperature and humidity. At 24 °C under 54% RH, the sorption increases rapidly for a week or so identified as the first stage of sorption, enters the second stage of negligible weight gain for a month and then the third stage of rapid sorption again. With increasing moisture sorption, the pad thickness increases through the 3 stages and the dynamic modulus also increases through the 3 stages. Friction materials lose moisture rapidly at 130°C and behave like desiccants. The sorption-desorption phenomenon significantly influences the friction coefficient -- a higher moisture content leading to lower friction coefficients. It is demonstrated that the rising friction coefficient for the half a dozen braking stops at the beginning of every brake testing in general is due to moisture desorption caused by rising pad temperatures. The pads become a reservoir for releasing moisture for a long period of time during testing. As many standardized brake test procedures do not specify the moisture content of the pads, brake test repeatability and reproducibility become problematic. Also, friction material property measurements for quality control becomes problematic as the moisture content is not specified or considered. For modeling and simulation of brake friction, vibration and noise, the moisture sorption-desorption effects will have to be incorporated for reliable outcomes.