502 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS PRECIPITATED SILICAS PROCESS VARIABLES (3) Reactant Concentration (2) Rates Of Addition Of Reactants (3) Fraction Of Theoretical Silicate In The Reactor (4) Reaction Temperature Silicate Acid Stirred Reactor Fraction Of Theoretical Silicate Filtration Step Washing Step Drying • Milling Step Step WET CAKE MOISTURE =STRUCTURE Figure 4. Precipitated silica manufacturing steps Finished Product reactor and then simultaneously adding the acidulating agent and the remaining silicate at predetermined rates until the silica precipitation is completed. At this point the precipitate is filtered, washed to remove the sodium sulfate by-product, dried and milled to the desired degree of fineness. It turns out that the amount of water associated with the washed silica precipitate is the structural water, and it is a very important property which "tags" the product properties of the resulting dry finished product. This property is referred to as the % wet cake moisture (WCM). Under a given set of manufacturing and process conditions, % WCM will vary with the structure level of the dry product. A product with a very high % WCM content is defined as a high-structure silica and a product with a rela- tively low water content is defined as a low-structure silica. In this context, we will use % WCM or structure synonymously. B. WET CAKE MOISTURE VERSUS STRUCTURE INDEX As mentioned above, the water content of the precipitated product prior to drying is an important property. This water is present between the particles and inside the pard-

PROPERTIES AND STRUCTURE OF SILICAS 503 10.0 1.0 WATER PORE VOLUME STRUCTURE INDEX 6.5 ml/g VHS 5.0 ml/g HS 3.0 ml/g MS 1.5 ml/g LS 1. 40 50 60 70 80 90 Percent Wet Cake Moisture Figure 5. Plot of water pore volume versus WET cake moisture cles. Therefore, at a fixed set of reaction conditions, the water associated with the precipitate or the filter cake is called the structural water. The amount of water associated with one part of the dry, recoverable product is defined as the water pore volume. The amount of water associated with 100 parts of the dry recoverable product is defined as Structure Index (SI) (15 a, b). Thus the water pore volume (PV) and SI are related to the % WCM by the following equation. % WCM ) (1) PV= 100 - % WCM SI ( %WCM ) = x 100 (2) 100 - % WCM A plot of water pore volume versus the % WCM is shown in Figure 5. Thus a product with water pore volume above 6.5 or SI values above 650 is arbitrarily defined as a VHS (very high structure silica). A low structure silica (LS) is one which exhibits a water pore volume above 1.5 or SI values above 150. These definitions are clearly explained in Figure 5. It must be pointed out that the log of the water pore volume or the log SI is linearly related to the % WCM. The SI values of products between the % WCM values of 80 to 85 are given in Table II (15). C. STRUCTURE CONTROL One of the key variables which can be used for controlling the properties and the struc- ture of silica is the fraction of the theoretical silicate which is added to the reaction me-