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External surface area refers to the surface of carbon black that is accessible to rubber molecules. Since carbon black is commonly used as a reinforcing agent in rubber compounds, its effectiveness depends on its specific surface area. However, when the pore size of carbon black is smaller than a certain threshold (typically around 2 nm), rubber molecules cannot penetrate into these pores. As a result, the inner surfaces of such small pores do not contribute to reinforcement and should be excluded. The external surface area, therefore, represents the surface area that remains after excluding these inaccessible internal areas.
Total surface area, on the other hand, includes both the external surface and the internal surface of all pores within the carbon black particles. This measurement accounts for the entire surface available for interaction, making it a more comprehensive indicator of the material's properties. The BET method, which uses nitrogen adsorption, is widely recognized as the standard technique for measuring total surface area.
There are two primary methods used to determine the surface area of carbon black:
1. **CTAB Method**: This involves liquid-phase adsorption using cetyltrimethylammonium bromide (CTAB) solution. The CTAB molecules are smaller than rubber molecules, so this method can measure surfaces that may still include some internal pore areas that are not accessible to rubber. As a result, the surface area measured by this method tends to be larger than the actual external surface. Additionally, the CTAB method is more complex and requires high precision, which has limited its widespread use.
2. **STSA Method (Single-Temperature Surface Area)**: Also known as the nitrogen adsorption t-plot method, this technique measures surface area at low temperatures using physical adsorption. It is standardized under GB/T 10722 - 2003 and is considered more accurate for determining the external surface area of carbon black.
According to the De Boer theory and the carbon black model, the relationship between the relative pressure (P/Po) of nitrogen adsorption and the statistical thickness (t) of the adsorption layer at liquid nitrogen temperature can be expressed as:
**t = 0.088(P/Po)² + 0.645(P/Po) + 0.298**
This formula helps estimate the thickness of the nitrogen layer adsorbed onto the surface of carbon black, providing valuable insights into its surface characteristics. By understanding these parameters, researchers and manufacturers can better evaluate the performance and application potential of different types of carbon black.
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