The Effect of Thermal Processing in Vacuum on the Water Adsorption Characteristics of Carbon Black

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THE EFFECT OF THERMAL PROCESSING IN VACUUM ON THE WATER ADSORPTION CHARACTERISTICS OF CARBON BLACK

KURT D. SCHACHNER AND PAUL E. THOMA Johnson Controls, Inc., Central Research, P.O. Box 591, Milwaukee, WI 53201 USA

ABSTRACT Commercially available carbon black contains oxygen complexes on its surface that affect the surface properties of the carbon. Water adsorption on the surface of carbon black is influenced by the amount and type of oxygen complexes present. When carbon black is heated in vacuum at sufficiently high temperatures, removal of the oxygen complexes occurs and the surface of the carbon particles is modified. The amount of water adsorbed by the carbon is dependent on the vacuum heat treatment temperature. As the heat treatment temperature increases, water adsorption on the carbon decreases. Commercially available electrically conductive carbon black adsorbs fr'om 1.25% to 2.50% water when exposed to 50% relative humidity for 24 hours at 25'C. This variation in water adsorption is due to a difference in the amount of oxygen complexes on the surface of the carbon. The carbon with more oxygen complexes adsorbs more water. However, when this carbon black is heat treated at 1200'C for 4 hours in a vacuum of lx 10-5 torr or better, the water adsorbed by the carbon is 0.4% when exposed to 50% relative humidity. Data showing the dependence of water adsorption on vacuum thermal processing are presented and discussed. INTRODUCTION Carbon black is produced by thermal cracking of a hydrocarbon. The principal method employed today is the oil furnace process in which a heavy aromatic tar fraction of petroleum distillate is atomized into a preheated closed furnace [1]. In this process, the atomized oil particles cluster and fuse together before pyrolysis is complete. The resulting carbon black consists of spherical particles which are fused into an aggregate structure. The properties of carbon black are controlled by a number of factors which include particle size, surface area, shape and size of the aggregate structure, surface oxidation and chemistry, surface morphology, and degree of graphitization [ 1-6]. Carbon black, as produced, contains various amounts and types of chemisorbed oxygen complexes on the surface of the carbon particles, which are adsorption sites for water [1,3]. To decrease the amount of water adsorbed by a particular carbon black, the oxygen complexes must be removed and the surface of the carbon particles modified. Thermal processing of carbon black in vacuum or reducing environments decreases the amount of surface oxidation and changes surface characteristics [5,6]. Most literature on vacuum thermal processing of carbon and its influence on water adsorption describe processing conditions up to 1200'C in vacuum [3,5,7]. Our results indicate that considerable water adsorption still occurs at high humidities (>80%RH) on an electrically conductive carbon black (VulcanĀ® XC72) after thennal processing at 1200'C in vacuum. The data presented in this paper show the effect of vacuum thermal processi

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