Precipitated silica properties

In summary, the processing parameters that have the greatest impact on precipitated silica properties, and consequently on suitability for various end uses, are the following ... 

The typical range of properties for commercial coUoidal silicas, silica gels, precipitated silicas, and pyrogenic silicas is given in Table 1. 

Journal of Applied Polymer Science 83, No.2, 10th Jan.2002, p.357-66 MORPHOLOGY AND PHYSICAL PROPERTIES OF CLOSED CELL MICROCELLULAR ETHYLENE-OCTENE COPOLYMER EFFECT OF PRECIPITATED SILICA FILLER AND BLOWING AGENT Nayak N C Tripathy D K Indian Institute of Technology... 

Nonblack fillers such as the precipitated silicas can reduce both rate and state of cure. The mechanism appears to be one of a competitive reaction between rubber and filler for the zinc oxide activator. Use of materials such as diethylene glycol or triethanolamine prevents this competition thereby maintaining the desired cure characteristics. Neutral fillers such as calcium carbonate (whiting) and clays have little or no effect on the cure properties. 

Precipitated silicas having properties matching those from the conventional sodium silicate route are said to be possible (79). The hydrofluosilicic acid that is generated can be recycled in the system, or can be used to manufacture sodium silicofluoride or ammonium fluoride. 

The diversity of silica forms and their properties leads to a broad range of applications. Reviews on usage include those on crystalline and vitreous silicas (94), on silica sols (95), on silica gels and precipitated silicas (96), and on pyrogenic silicas, precipitated silicas, and silica gels (97). 

Property Colloidaln silica Silica gels Precipitated silica Pyrogenic silica... 

The precipitated silicas include a wide range of silicas with a variety of structural characteristics. Most of the preparation methods are patented. In general the formation involves a coagulation and precipitation from silica solutions. Properties may therefore be supposed to be similar to those of the gels. For these silicas however, preparation conditions are such as to avoid gel growth and stimulate precipitation. As an overall definition, Barby proposed dry silicas with no long or short distance characteristic structure. 

Mix 0.5 gram of the precipitated silica with 1 gram of powdered calcium fluoride. Place the mixture in a test tube, moisten it with 36 N H2S04, and warm it gently under the hood. Dip a stirring rod in water and lower it, with a drop adhering, into the gas in the test tube. Note the precipitate that forms in the drop of water. Write equations for all the reactions, and state what rather unusual properties are shown by this experiment to be possessed by hydrofluoric acid and by silicon tetrafluoride. 

Fumed silica is typically available with sizes of 7 to 40 nm and surface areas ranging from 50 to 380 m2/g. Unlike precipitated silica, fumed silica has no internal surface area. The specific gravity of fumed silica is approximately 2.2. Because of its high surface area to weight ratio, formulations generally require only a minor amount of fumed silica (1 to 5 percent by weight) to achieve thixotropic properties. 

Properties of precipitated silica BET specific surface area 15 mVg [1861], Reference [1860] reports the specific surface areas of four silicas from Rhone Poulenc A 175 in7g, B 320 m7g, C 90 mVg and D 160 m7g. 

The introduction to the section Silica Gels and Powders by W. Welsh constitutes an introduction to the study of silica powders. Detailed accounts of the synthetic processes and applications of fumed silicas, silica gels, and precipitated silicas are given by Ferch (Chapter 24) and Patterson (Chapter 32). For scientists, silica powders are of special interest because they offer the opportunity of working with very pure systems with well-controlled ultimate particle size and specific surface area. One of the most important aspects of silica powders is their adsorptive properties. These properties are the subject of the work by Kenny and Sing (Chapter 25), which includes the crystalline zeolitic silica known as silicalite. 

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