How is the electrical conductivity of recovered carbon black measured

The electrical conductivity of recovered carbon black (rCB) describes its ability to conduct electric current, a key parameter for conductive masterbatch, antistatic materials and lithium-ion battery conductive additive applications. Unlike metal conductors, rCB conductivity depends on carbon aggregate network formation, BET surface area, porosity, ash content and dispersion quality, as referenced at rcb-mill.com.

Two primary test categories: powder conductivity and compound conductivity

1. Powder volume resistivity (direct test on rCB powder, common for factory QC)

Volume resistivity is the most widely used test for raw rCB powder. Conductivity is the reciprocal of resistivity (σ = 1/ρ).
Test principle & procedure:

1. A defined mass of dried rCB powder is loaded into a cylindrical conductive test cell with two electrodes at top and bottom.
2. Apply controlled mechanical compression (fixed pressure, critical for repeatability) to compact the powder to a consistent density. Pressure is standardized because contact between carbon aggregates dominates powder conductivity.
3. Apply DC voltage and measure resistance across the compressed powder bed.
4. Calculate volume resistivity:
   ρ=R×LA​

• ρ = volume resistivity (Ω·cm)
• R = measured resistance
• A = cross-sectional area of the sample cell
• L = thickness of compressed powder bed
  Conductivity (S/cm) = 1 / ρ.

Key notes:

• Compression pressure must be kept identical between samples; loose powder will show much higher resistivity (lower conductivity).
• Ash and mineral impurities act as insulators and increase powder resistivity.
• Thermal devolatilization improves inter-aggregate contact and reduces surface tar, lowering resistivity (higher conductivity).

2. Compound resistivity / conductivity (test within polymer matrix, application-oriented test)

This method better reflects real end-use performance for rubber, plastic masterbatch or battery electrodes.

1. Disperse rCB at a fixed loading percentage into a non-conductive polymer binder (e.g. EVA, PP) or battery electrode slurry.
2. Compression mold or cast a uniform test sheet / coating film with defined thickness.
3. Use a 4-point probe method to measure sheet surface resistivity or volume resistivity.
4. Calculate conductivity of the filled composite.

4-point probe is preferred for thin films, as it eliminates contact resistance errors between probes and sample surface.

Standard test methods

• ASTM D4496: DC resistance of conductive and antistatic plastic materials (for polymer composites filled with rCB).
• ASTM D257: Standard test methods for DC resistance or conductance of insulating materials.
• For battery electrode coatings: customized 4-point probe testing on dried electrode films is commonly used.

Factors affecting measured electrical conductivity of rCB

1. Carbon aggregate network (DBP structure): Higher DBP means more branched aggregates. They form connected conductive networks more easily, giving higher conductivity at the same loading.
2. Ash content: Inorganic minerals (ZnO, silica, calcium salts) are non-conductive. Higher ash disrupts the conductive carbon network and increases resistivity. This is why battery-grade rCB requires ultra-low ash.
3. Dispersion / de-agglomeration: Hard sintered agglomerates reduce effective contact points between aggregates. Proper de-agglomeration grinding unlocks aggregates and improves conductivity.
4. Surface tar and volatile matter: Tar is semi-insulating and coats carbon surfaces. Thermal devolatilization removes tar and improves inter-aggregate electrical contact.
5. Porosity and moisture: Pores and adsorbed moisture change contact resistance; samples must be fully dried before testing.

Typical value reference

• High-quality battery-grade rCB powder (compressed): 0.1–10 Ω·cm (high conductivity)
• Standard dry-processed rubber/masterbatch rCB powder: 10–1000 Ω·cm
• Crude pyrolysis char (high ash and tar): >1000 Ω·cm, poor conductivity

Virgin conductive furnace carbon black often shows lower resistivity than untreated rCB; high-purity processed rCB can reach comparable conductivity.

Practical application meaning

• Battery conductive additive: Low volume resistivity enables fast electron transport in the electrode, improving rate capability.
• Antistatic rubber/plastic: Controlled conductivity prevents static charge accumulation.
• Rubber reinforcement grades: Conductivity is not the main target, but consistent resistivity can act as an indirect QC indicator of aggregate structure and ash variation.

rCB electrical conductivity is measured in two main ways: compressed powder volume resistivity for raw material QC, and 4-point probe resistivity on polymer or electrode composite films to simulate real application conditions. Conductivity is heavily influenced by aggregate structure, ash content, de-agglomeration quality and residual tar. Since conductivity is the inverse of resistivity, lower measured resistivity equals higher electrical conductivity.