Analysis of radical content on carbon black pigments by electron spin resonance: influence of functionality, thermal treatment and adsorption of acidic and basic probes

The radical content and its change under thermal treatment of different types of carbon black (CBs) are examined using electron spin resonance (ESR) through calibration via stable free radicals. In order to characterise a range of different CBs, Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) have been used in an attempt to determine the nature of the functional groups present on their surface. A linear correlation was found between the oxygen content on the CB surface obtained by XPS analysis and the radical content for several grades of pigment. Increasing surface oxygen content resulted in minor changes in radical concentration between the different CB grades analysed. An overall decrease in radical content occurred with exposure of the CBs at higher treatment temperatures accompanied by a recovery between 450 and 750°C. Depending on the CB grade there were also some structural changes, which resulted in an increase in the specific surface area, as well as, a progressively more extensive localisation of the itinerant basal plane electron states associated with chemisorptions of atmospheric oxygen after the thermal treatment. The change of the radical content of the CBs with the thermal treatment was coincident with parallel FMC studies using basic and acidic probes, which revealed changes in acidic and basic character throughout the treatment. These features could be related to changes in surface morphology and chemistry of the modified blacks. Other techniques like thermogravimetric (TGA), BET analyses and Karl Fisher test were also performed in order to determine the differences in specific surface, water and volatile content of the CB pigments and their influence on the radical content. Inter-relationships are shown and discussed