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The mechanism of overbasing of additives used in lubricating oils is studied. Overbased calcium sulphonates, the most widely used, are examined. Although the geometrical and physical nature of colloidal carbonate particles has been well studied, little is known about the fundamental aspects and mechanism of the overbasing process. In this paper, assessment of the physical and/or chemical nature of the reaction is made from equations related to the chemical engineering model of the shrinking spherical particle. The results show that the overbasing process is under diffusion control, and the chemical reaction is not determining.

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