The key requirements for calcined petroleum coke in brake pad manufacturing

The key requirements for calcined petroleum coke in brake pad manufacturing center on high purity, a specific physical structure, and a particle size that matches the formulation. Its purpose is to serve as a friction stabilizer and functional filler in the formulation, enhancing braking performance by optimizing the coefficient of friction, heat dissipation, and noise reduction.

Key Chemical Specifications

Chemical purity is the primary consideration, as it directly affects the performance, service life, and environmental friendliness of brake pads.

Fixed Carbon: The content is typically required to be ≥ 98.5%. A high carbon content is essential for ensuring the material’s good lubricity and high-temperature resistance.

Ash Content: The content must be strictly controlled at ≤ 0.5%; high-performance applications may even require ≤ 0.2%. Low ash content effectively reduces abrasive wear at high temperatures and minimizes wear on the brake rotor.

Sulfur Content: Typically required to be between ≤ 0.5% and ≤ 0.8%. Sulfur is a key corrosive element; excessively high sulfur content can corrode metal components in the braking system at high temperatures, so low sulfur content is the prevailing trend.

Volatile Matter: Required to be ≤ 0.3% to ≤ 0.5%**. Low volatile matter ensures the material’s thermal stability, reduces gas release at high braking temperatures, and prevents the formation of a gas cushion at the friction interface, which could impair braking performance.

Trace Metals: Attention should be paid to the content of heavy metals such as nickel and vanadium. These elements are considered potentially harmful to the environment, and their levels are one of the key indicators for distinguishing between different grades of petroleum coke. Comparative studies have shown that the nickel and vanadium content in traditional petroleum coke is significantly higher than in certain new types of non-petrochemical-based cokes.

Key Physical and Structural Properties

Physical properties determine how calcined petroleum coke functions within the friction material matrix.

True Density: Typical values range from 2.05 to 2.10 g/cm³, with some products ranging from 1.95 to 2.2 g/cm³. True density affects the packing efficiency of the filler and the structural strength of the final product.

Particle Size Distribution: This is a key parameter that must be customized according to the formulation. Common mesh sizes include 20–40 mesh, 40–60 mesh, and 60–80 mesh, corresponding to a D50 of approximately 50–200 µm. For heavy-duty or high-performance braking applications, finer particles (D50 ≤ 75 µm) are recommended to achieve better performance; For commercial vehicle drum brake pads, a medium particle size can be selected to balance cost considerations.

Hardness: The Mohs hardness is approximately 2.5–3.0 or 3–4. This hardness range is designed to balance the material’s wear resistance with its compatibility with the brake rotor, preventing excessive scratching of the mating surface.

Thermal Conductivity: Excellent thermal conductivity (typically 90–120 W/(m·K)) is one of its key properties, enabling rapid dissipation of friction-generated heat and reducing the temperature of the friction surface, thereby significantly delaying “thermal fade” at high temperatures.

Porosity and Structure: Ideal calcined petroleum coke should have a loose, porous texture. This structure helps absorb braking noise and enhances the stability of the coefficient of friction. An intuitive criterion for evaluation is that the cross-section of the particles should exhibit a metallic luster, which distinguishes them from coal-based particles that have high hardness but poor porosity.

Performance Implications and Selection Criteria

The combined effect of these indicators ultimately manifests in the following performance characteristics of brake pads:

Stable Coefficient of Friction: By forming a uniform transfer film, it helps maintain the coefficient of friction within a stable range of 0.35–0.45 μ, reducing fluctuations.

Enhanced Resistance to Thermal Fade: Relying on its high thermal conductivity, it effectively dissipates heat, enabling the brake pad to withstand braking temperatures as high as 600°C.

Improved NVH (Noise, Vibration, and Harshness): Through its porous structure and appropriate particle size distribution, it effectively suppresses brake squeal and vibration, enhancing ride comfort.

Reduced Wear: Minimizes wear on the brake rotor while maintaining the structural integrity of the brake pad itself.

Selection and Dosage Recommendations:

Dosage: The typical addition rate is approximately 6–8% in disc brake pads and 8–12% in drum brake pads; specific adjustments should be made based on the formulation system.

Selection: For heavy-duty braking applications requiring high performance, prioritize fine-particle products with low sulfur, low ash, and high true density. At the same time, avoid using coal-based particles with high hardness and poor porosity, as they may scratch the brake rotor and provide poor noise reduction and heat dissipation.

Overall, selecting calcined petroleum coke for brake pads involves striking the optimal balance between purity (low carbon, low sulfur, low ash) and structure (porous, with specific hardness and particle size) to meet the comprehensive requirements for friction stability, heat dissipation, and comfort under specific braking conditions.