Properties of Typical Pyrolysis Blacks
Carbon blacks are complex materials with a larger number of important characteristics than most other fillers. These include particle size, specific surface area, “structure” as measured by oil absorption, surface chemistry, and presence of impurities, especially larger sized grit.
There are also a large number of different grades, each optimized for a particular application. Various nomenclatures are used to describe carbon black fillers, of which ASTM D1765-14 is the most important today. Tires use a variety of carbon black types, depending on the tire part involved. The main types are the higher specific area, highly reinforcing types with ASTM numbers 399 and below. Often referred to as high or super abrasion furnace blacks, they are found especially in tire treads, where the highest reinforcement is required. Carbon blacks with ASTM numbers above 500 (Fast extrusion furnace, general purpose furnace, and semireinforcing furnace) are the other main type, mainly used in carcass and inner liner applications. The properties of the pyrolysis blacks vary with the rubber source and pyrolysis conditions, but do not currently equal those of the virgin carbon blacks present, even when tread rubber containing the most reinforcing types is used in their production.
Most manufacturers’ literature is fairly vague but usually gives specific surface area and oil absorption values which suggest that they are similar to the less reinforcing ASTM 500 and above blacks. An independent laboratory, ARTIS www.artis.uk.com, have published more informative data (Norris and Bennett 2014). According to their independent analysis, these claims may be optimistic. They analyzed five commercially available pCBs from different producers, with the results shown in Table 3.
They also tested them against a range of carbon blacks in an SBR elastomer compound. They found that the dispersion was invariably poorer for the pCBs and that the property levels obtained were closer to the weakly reinforcing N700 blacks than to the N500 series usually claimed. Given the more promising surface area and oil absorption numbers reported above, this is probably due to the high ash content and poor dispersion.