Use of Lithium Aluminum Hydride in the Study of Surface Chemistry of Carbon Black

Considerable interest has been focused in recent years on the effects of carbon black surface chemistry on its properties in rubber and other polymer systems. Methods have been developed in our laboratory for the quantitative characterization and description of the surface chemistry of carbon blacks. The total surface acidity as given by the active (acidic) hydrogen is determined by reaction with strongly basic lithium aluminum hydride in diethyl carbitol, or by equilibration with aqueous alkali under nitrogen. Exclusion of oxygen is essential since carbon blacks readily undergo base catalyzed oxidation. Selective neutralization techniques are employed to further differentiate between strong and weak surface acids. For example, using aqueous sodium bicarbonate solution, only those acids stronger than carbonic acid will be neutralized. Strong acids, as determined by bicarbonate neutralization, are shown to be aromatic carboxyl groups, while weak acids determined by difference between total acidity and strong acid content are phenols and hydroquinone derivatives. Neutral quinone and lactone surface groups are quantitatively determined by chemical reduction with lithium aluminum hydride, or alternatively by analysis of their thermal decomposition products (carbon monoxide and dioxide, hydrogen, and water) obtained on vacuum pyrolysis of carbon black. The distribution of neutral and acidic surface groups on representative channel, furnace, and thermal black is discussed in terms of the state of oxidation of the individual carbons.