CAB-O-SIL® fumed silica is a highly effective gelling agent for the sulfuric acid electrolyte in lead acid storage batteries. It provides the following benefits in this application:

High Thickening Efficiency

The most common grades used in this application are CAB-O-SIL M-5 and M-7D. The choice between these two grades is determined by the type of dispersion equipment being used to incorporate the silica into the acid solution. If a planetary mixer is used, grade M-5 is the better choice. When using a high-speed disperser, either grade M-5 or M7-D can be used. Both grades will gel 27 wt.% sulfuric acid (specific gravity 1.36) at concentrations between 3.2 and 4.0 wt.%.

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Low Reactivity

Silica is inert to sulfuric acid. There are no concerns about any gradual reactions with the electrolyte or any other components of the battery. The performance of the battery will not change due to the presence of the silica. 

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High Purity

The high purity of fumed silica high ensures no contamination ions or other chemical species are introduced into the electrolyte, which can affect stability, power output and the service life of a battery.

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Viscosity Stability

The thickening action of fumed silica is affected by the dispersion conditions used in dispersing the silica into the acid solution. Once this is done and the acid solution has set up into a gel, it is unaffected by the environmental conditions experienced in the battery, such as temperature variations, and the charging and discharging of the battery.

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Low Color Effects

The index of refraction of silica is 1.46. Silica renders the acid solution a milky white color. It does not impart any off colors, such as yellow or red, which could give the perception of contamination. Since the white color is due to the mismatch of the index of refraction, it has no hiding power and will not mask any color due to contamination.

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Ease of Incorporation

Fumed silica has a hydrophilic surface and is easily wet by the aqueous sulfuric acid solution. The low bulk density of M-5 can create some dusting during dumping of the powder, which can easily be controlled by appropriate engineering controls.

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Delayed Initial Thickening

The acid solution's thickening response to the addition of fumed silica can be delayed by constant vigorous agitation of the mixture until just prior to filling the battery. This ensures a low viscosity electrolyte, which is able to completely wet and fill the entire cell, ensuring maximum capacity of the battery. The acid mixture then will gel within 4 to 18 hours after mixing had stopped.

Recommendations for Dispersion Equipment
The optimal mixer for this application is a twin shaft disperser. The disperser should be equipped with a variable, low-speed, center-mounted shaft. A side and bottom scraper blade should be mounted on this shaft. Typical speed ranges for this shaft are 10 to 60 rpm. When fumed silica is initially added to the acid solution the solution will thicken and this blade ensures that all the mixture is circulated into the mixing zone of the second blade. The second shaft is mounted off-center and is a variable, high-speed shaft. A saw tooth (or "Cowles" type) blade is mounted on this shaft. This blade disperses the silica into the acid. The speed range of this shaft should be from 800 to as high as 1800 rpm. Proper dispersion ensures that the silica will not settle out and syneresis of the acid from the gel is minimized.

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