What Is Void Volume?
Void volume refers to the interstitial void space that remains between particles when carbon black aggregates are compressed under a given pressure. Carbon black forms complex three-dimensional structures (structure) through the fusion and bonding of primary particles, and the amount of voids changes depending on the size and shape of this structure.
Void volume is an indicator that quantifies not the volume of the particles themselves, but rather the "space created by the particles." Carbon black with more developed structure has larger interparticle voids and correspondingly higher void volume values. This characteristic directly affects reinforcement, conductivity, and dispersibility when compounded into rubber or resins, making it an extremely important parameter in materials design.
Traditionally, the structure of carbon black has been indirectly evaluated through DBP oil absorption (dibutyl phthalate oil absorption). However, void volume has been gaining increasing attention in recent years because it captures the spatial structure more directly.
Differences from DBP Oil Absorption
Both DBP oil absorption and void volume are methods for evaluating carbon black structure, but there are fundamental differences in their measurement principles and the information they provide.
DBP oil absorption is a method that measures the amount of oil absorbed by dripping DBP (dibutyl phthalate) oil into a carbon black sample while kneading, until the point where torque rises sharply (the endpoint). It is standardized by JIS K 6217-4 and ASTM D 2414, and is widely adopted. In contrast, void volume directly measures the interparticle voids as a volume through compression, offering the advantage of being less susceptible to wettability and surface chemistry effects.
Comparison Item | DBP Oil Absorption | Void Volume |
|---|---|---|
Measurement principle | Torque change through oil dripping and kneading | Direct measurement of void space under compression |
Primary standards | JIS K 6217-4 / ASTM D 2414 | ASTM D 6086, etc. |
Measured quantity | Oil absorption volume (mL/100g) | Interparticle void volume (cm³/g) |
Surface chemistry influence | Susceptible (depends on oil affinity) | Less susceptible |
Structure evaluation | Indirect | Direct |
Reproducibility | Depends on endpoint determination | High reproducibility through compression control |
Thus, while DBP oil absorption indirectly evaluates structure through oil absorption behavior, void volume quantifies the spatial structure itself, providing information closer to the essential characteristics of the material.
Why Void Volume Is Gaining Attention
In recent years, the growing attention on void volume in powder evaluation of carbon black is driven by the following reasons.
High Correlation with Actual Dispersion Behavior
When compounding carbon black into rubber or resins, the physical properties of the final product (tensile strength, conductivity, wear resistance, etc.) are heavily influenced by the dispersion state of the carbon black. Because void volume directly reflects the void structure under compression, many studies have reported a high correlation between void volume and ease of dispersion in polymers.
Evaluation Free from Surface Treatment and Oxidation Effects
DBP oil absorption is affected by the chemical properties of the sample surface (presence of oxidation treatment, type and amount of functional groups), whereas void volume evaluates only the physical spatial structure, enabling pure comparison of structure. This is particularly advantageous when comparing carbon black grades with different surface treatments.
Demand for Evaluating Conductive Additives for Lithium-Ion Batteries
For carbon black used as a conductive additive in lithium-ion battery electrodes, dispersibility in electrode slurry and conductive path formation capability determine performance. Void volume contributes to improving material selection accuracy for these high-performance applications.
Measurement Principles and Methods
Void volume measurement generally follows three main steps:
- Sample filling and compression: A known mass of carbon black powder is loaded into a dedicated cell and compressed by applying pressure in stages.
- Volume measurement: The apparent volume (particles + voids) of the sample is precisely measured at each pressure stage, calculated from piston displacement or internal cell volume.
- Void volume calculation: The void volume is obtained by subtracting the true particle volume from the apparent volume. Results are expressed in cm³/g.
By repeating measurements at varying compression pressures, a pressure–void volume curve can be obtained. Comparing the shape of this curve and values at specific pressures enables quantitative evaluation of structural differences between different carbon black grades.
To ensure measurement accuracy, precise weighing of the sample amount, uniform filling within the cell, and control of compression speed are essential. Manual measurement makes managing these conditions cumbersome, making this a field where the benefits of automation are significant.
Automated Measurement with the S-500
Asahi Souken's S-500 Oil Absorption Tester is an automated measurement instrument that, while primarily designed for DBP oil absorption measurement, also supports data acquisition necessary for void volume calculation. High-precision torque sensors and liquid delivery pump control ensure highly reproducible measurements.
The S-500 can perform both oil absorption measurement compliant with JIS K 6217-4 and ASTM D 2414 and void volume evaluation via compression on a single platform, unifying sample preparation processes and significantly improving evaluation efficiency. Automatic digital data recording also minimizes inter-operator variability.
Key Advantages of Automated Measurement with the S-500
- Full automation from sample loading to measurement completion, reducing operator workload
- Precise control of compression speed and pressure improves batch-to-batch reproducibility
- Measurement data exportable in CSV and PDF formats, eliminating the need for manual transcription to QC reports
- Continuous measurement of multiple carbon black grades for efficient comparative structure evaluation
Even skilled technicians are prone to variation in endpoint determination and compression operations during manual measurement. The S-500's automated control eliminates these human errors, delivering reliable results that conform to ISO/ASTM standards.