INFLUENCE OF SILICA PROPERTIES ON PERFORMANCE OF ANTIFOAMS IN PULP AND PAPER APPLICATIONS .2. IN-SITU HYDROPHOBING

Precipitated silica antifoam formulations used in pulp and paper appli cations typically consist of oil dispersions of silica particles that have been surface treated to render them hydrophobic. In an earlier pa per [1] I studied precipitated silicas hydrophobed by the dry-roast pr ocess as defoamers for paper mill black liquors. In this process, the dry silica powder is impregnated with polydimethylsiloxane, cured by h eating, and later dispersed in a mineral oil carrier. However, there i s another common process, called in-situ hydrophobing, in which the si lica is reacted directly in a mineral oil carrier with a more reactive silicone, such as an hydroxyl-terminated polydimethylsiloxane, usuall y in the presence of an alkaline promoter. In this work I show that th e mechanisms of hydrophobing are different in these two processes, and so the optimum silica properties for maximum black liquor defoaming a re also different.