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One low-copper formulation and one copper-free formulation were made into disc pads, and both of them were cured under 4 different conditions. These pads had no backing layer and no scorched layer. Pad thickness, dynamic modulus and natural frequencies were continuously monitored over a period of 12 months. After 12 months at room temperature, pad thickness, dynamic modulus and natural frequencies all increased to higher values. The low-copper formulation increased relatively rapidly during the first 60 days and the copper-free formulation increased relatively rapidly for the first 90 days, and then slowly thereafter. Two competing processes are found to be taking place simultaneously; internal stress relief leading to pad expansion and cross-linking of the resin leading to pad shrinkage. As the pad properties are changing continuously, the timing of property measurement becomes an important issue for quality assurance.

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.