Surface Properties
The morphology of the surface is the final important property variable. Hereby, the surface activity describes the surface properties and is broadly defined as the tendency of the carbon black particles to interact with themselves and their surroundings. In polymers, more specifically elastomers where reinforcing effects are important, the surface activity dominates the polymer-filler interaction, filler-aggregate interaction, as well as the filler-ingredient interactions. In liquid systems like paints and inks, surface activity is more closely associated with the stability of the final coating dispersion and its rheology.
The surface activity in a chemical sense is linked to the surface group chemistry and surface microstructure. Surface heterogeneities given by graphitic planes at the surface, amorphous carbon, crystallite edges, and slit-shaped cavities all representing adsorption sites of different energies describe the surface microstructure. Also the nature and amount of surface functional groups attached to surface carbon control the interaction with the surrounding media. Oxygen functional groups at the surface being mainly carboxylic, quinonic, phenolic, or lactonic groups are subject to acidic reactions in organic solvents or aqueous media and affect the dispersion of carbon black during mixing. Such surface oxides can react as Lewis bases which may act as anchor atoms for Lewis acids. Higher molecular weight organic molecules may become adsorbed as impurities on the carbon surface during production. The qualitative and quantitative analysis of the surface groups is rather difficult especially if their amount at the surface is low. Chemical titration techniques, thermal desorption coupled with mass spectrometry, photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry are methods frequently applied for this purpose. The volatile content measured at 950 °C includes both the total amount of functional groups and the absorbed organics (Accorsi and Romero 1995). The solvent extractable content (ASTM D4527-99) measures the amount of the latter.
In a physical sense, the surface activity is linked to variations in surface energy that determine the absorptive capacity of the filler and its energy of absorption (Bansal and Donnet 1993b; Wang and Wolf 1993). The surface energy is the work being necessary to create a unit new surface of liquid or solid. This energy is caused by different types of cohesive forces, such as dispersive, dipole-dipole, induced dipole, and hydrogen bond forces. Inverse gas chromatography is today the most accurate way to measure the surface energy of carbon black.
The surface chemistry can, to some extent, be controlled by the process conditions, including additive additions. Some success has been achieved in increasing the extent and strength of polymer physical adsorption. This has been achieved by modifying the production process to increase the surface roughness and the amount of exposed crystallite edges. These blacks are known as nanostructure blacks. Many attempts have also been made to beneficially change the surface chemistry of carbon blacks by the use of modifiers. Historically, these have been largely unsuccessful, but have received recent impetus by the competition from silica in some tire formulations. Progress has now been reported, including polymer grafting of the surface. A related development is the dual-phase blacks where silica has been introduced into the structure thereby providing a surface receptive to modification.
