Primary Particle Size and Porosity
The next consideration is the primary particle diameter (fineness) which is used to describe the primary particle dimension. Carbon black primary particle size is usually determined by electron microscopy. Specific surface area (SSA) is also widely used as a descriptor of particle size. The primary particle size can be approximately derived from the specific surface area assuming ideally spherical particle shapes and neglecting any particle porosity. A linear correlation between primary particle size and specific surface area is valid for carbon black grades with a specific surface area lower than about 150 m2g-1 as usually no porosity occurs in the primary particles of such materials.
Porosity is an important parameter for carbon blacks and needs careful definition. In the carbon black context, it refers to the presence of small holes, cavities, or channels in the particles themselves, as illustrated in Fig. 4. It does not refer to the space between the particles, despite this often being called void volume, which can give the impression that it refers to porosity. The first subdivision of porosity is into closed and open pores. Closed pores are entirely within the solid phase with no outlet to the exterior, while open pores do have some external outlet and may be at the surface or internal. Closed pores mainly affect the specific gravity of the particles and have little effect on other properties. They do not contribute to specific surface area. Open pores may or may not affect the specific gravity, depending on the method of determination. They do contribute to specific surface area, although this depends on the pore opening size and the size of the absorbing species being used for the measurement. Pores are conventionally divided into three types according to size: macropores with sizes >50 nm, mesopores with sizes between 2 and 50 nm, and micropores with sizes <2 nm.
Gas adsorption techniques such as the nitrogen adsorption method based on the theories of Langmuir and of Brunauer, Emmett, and Teller (BET) are applied to measure the total surface area (ASTM D6556). Nitrogen adsorption allows
Fig. 4 HRTEM images of primary particles of extraconductive ENSACO® 350G (BET SSA of 780 m2/g) indicating pores in the primary particles (arrow)
measurement of the total specific surface area comprised of the geometrical surface area of the primary particles, open porosity, and surface morphology. Alternative techniques are the adsorption of iodine (ASTM D1510) or cetyltrimethylammonium bromide (CTAB) (ASTM D3765). The iodine number is expressed in milligrams of iodine per gram of carbon black and hence is not a true surface area value. However, the iodine number and nitrogen gas adsorption provide the same surface area as long as the iodine adsorption is not affected by porosity, surface chemistry, and impurities. Unlike nitrogen, the larger iodine molecules do not penetrate into small pores. In addition, oxide groups and hydrocarbon impurities at the carbon surface depress the iodine number. Due to the huge steric extension of the CTAB molecule, the CTAB method basically eliminates any contributions to the measured surface area from porosity but normally remains unaffected by the surface chemistry and therefore gives the best correlation with the primary particle size. For nonporous carbon black, the ratio of the specific BET surface area (BET SSA) or iodine absorption and the specific CTAB surface area (CTAB SSA) is roughly unity. An iodine/CTAB ratio of less than unity indicates surface impurities or surface oxides and greater than unity indicates the existence of open porosity (Kuhner and Voll 1993). Real density measurements with different media can be used to distinguish internal closed porosity from open porosity.