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 typ es of carbon black (CBs) are examined using electron spin resonance (ESR) t hrough calibration via stable free radicals. In order to characterise a ran ge of different CBs, Fourier transform infrared spectroscopy (FTIR) and X-r ay photoelectron spectroscopy (XPS) have been used in an attempt to determi ne the nature of the functional groups present on their surface. A linear c orrelation was found between the oxygen content on the CB surface obtained by XPS analysis and the radical content for several grades of pigment. Incr easing surface oxygen content resulted in minor changes in radical concentr ation between the different CB grades analysed. An overall decrease in radi cal content occurred with exposure of the CBs at higher treatment temperatu res accompanied by a recovery between 450 and 750 degreesC, Depending on th e CB grade there were also some structural changes, which resulted in an in crease in the specific surface area, as well as, a progressively more exten sive localisation of the itinerant basal plane electron states associated w ith chemisorptions of atmospheric oxygen after the thermal treatment. The c hange of the radical content of the CBs with the thermal treatment was coin cident with parallel FMC studies using basic and acidic probes, which revea led changes in acidic and basic character throughout the treatment. These f eatures could be related to changes in surface morphology and chemistry of the modified blacks. Other techniques like thermogravimetric (TGA), BET ana lyses and Karl Fisher test were also performed in order to determine the di fferences in specific surface, water and volatile content of the CB pigment s and their influence on the radical content. Inter-relationships are shown and discussed. (C) 2000 Elsevier Science Ltd. All rights reserved.