Copper Free Formulations
The role of copper (Cu) in modern brake disc pads is essential due to its contribution to heat transfer and dissipation during braking while reinforcing the thermal resistance over time. It also strongly contributes to the performance in terms of noise, vibration and harshness (in short NVH) due to its lubrication properties. Since 2010, when the states of Washington and California adopted legislation to reduce the usage of Cu in brake pads to < 0.5% in weight by 2023, the global brake pad industry has been looking for alternatives.
Graphite in general is well known for its thermal conductivity properties and is therefore considered a potential replacement for copper. Several primary properties of graphite, such as morphology, purity, degree of graphitization, porosity or particle size distribution, contribute to thermal dissipation (conductivity), which is a secondary property as described in Figure 1. Therefore, not all graphite types are suitable to replace copper. In addition, it is generally acknowledged by the industry that graphite alone cannot be a 1:1 replacement for copper, as some other adjustments are required to reach the desired final properties.
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In order to preserve braking performance during severe conditions, especially at high temperatures, brake pads must dissipate temperature in directions parallel to the friction layer, but not in the perpendicular direction, which would cause heating of the back plate, pistons, and the brake fluid causing failure of the entire system.
The thermal conductivity of natural flake graphite can vary from 200-500 W/mK along the axes parallel to the graphite layers, and from 5-15 W/mK along perpendicular axis, showing a clear anisotropic behavior. In contrast, copper exhibits a perfect isotropic behavior with thermal conductivity values averaging around 300-350 W/mk in all 3 directions. The data indicates that an anisotropic graphite would be beneficial in achieving the desired thermal conductivity in specific directions. During the mixing and pressing process of brake pads, an alignment parallel to the friction layer can be obtained while using strong anisotropic morphologies of ingredients.
Superior Graphite conducted several tests in collaboration with the University of Southern Illinois Carbondale supporting the idea that anisotropic graphitic material would positively contribute to heat dissipation in parallel directions compared to a fully isotropic material obtained in a similar electro-thermal treatment, as can be seen in the figures below:
The modified pads using only anisotropic graphitic materials shows an increase in thermal conductivity of around 10% in both the X and Y directions parallel to the brake pad layer whereas no change was detected in the Z direction through thickness of the pad. The isotropic material used in comparison has a similar influence in the thermal behavior in all directions.
Superior Graphite developed resilient and other graphitic grades to address these challenges. The resilient products have a unique morphology and porosity created during Electro-Thermal Treatment/Purification. Other graphitic grades like PNC have been developed in response to the copper ban challenges, in anticipation of new upcoming regulations and limits on emissions which will emphasize usage of clean materials.
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