Carbon black total surface and external surface relationship - Database & Sql Blog Articles

1206 side full color side light

The external surface area of carbon black is a key parameter in determining its performance as a reinforcing agent in rubber. This concept refers to the surface that is accessible to rubber molecules. When the pores within carbon black are very small (typically less than 2 nm), rubber molecules cannot penetrate into these pores. As a result, the inner surfaces of such pores do not contribute to the reinforcement and should be excluded from the measurement. Therefore, the term "external surface area" is used to describe the surface area that remains after subtracting the internal surfaces of pores that are too narrow for rubber molecules to enter. On the other hand, the total surface area includes both the external surface and the internal surfaces of all pores, regardless of their size. This value is typically measured using the BET method, which involves nitrogen adsorption at liquid nitrogen temperature. The BET specific surface area is widely accepted as the standard measure of the total surface area of carbon black. There are two primary methods for measuring the surface area of carbon black: (1) CTAB Method: This is a liquid-phase adsorption technique using cetyltrimethylammonium bromide (CTAB). It measures the surface area by the adsorption of CTAB molecules onto the carbon black. However, since CTAB molecules are smaller than rubber molecules, this method may still include some internal pore surfaces that are inaccessible to rubber. As a result, the CTAB method tends to overestimate the external surface area. Additionally, the procedure is complex and requires high precision, which has limited its widespread use. (2) STSA Method: Also known as the nitrogen adsorption t-plot method, this is a low-temperature physical adsorption technique. It follows the GB/T 10722-2003 standard and is considered more accurate for determining the true external surface area of carbon black. Using 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 equation helps in calculating the effective surface area by accounting for the actual adsorption behavior of nitrogen on carbon black.