Fumed Silica Filler ? Why so Popular?

Fumed silica is a popular filler that is used in many adhesive and sealant formulations. Even though fumed silica is not a very good extender for minimizing cost, modifying thermal expansion coefficients, or significantly improving cured strength, it is one of the most widely used fillers in a variety of applications from structural adhesives to pressure sensitive adhesives.

Fumed silica, an amorphous silicon dioxide, is a versatile, efficient additive used in adhesive and sealant formulations primarily for flow control and thixotropy. Fumed silica has long been the dominant thixotrope employed in the adhesive and sealant industry.

This article will explore why fumed silica is so widely used and valued by the formulator. The origin and various types of fumed silica will also be described. The property modifications that are possible will be identified with the focus being on thixotropy and rheological properties. Starting formulations will be provided for several adhesive and sealant products.

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Origin and Characteristics

Silica is an abundant mineral found in crystalline form (quartz) and amorphous form (diatomaceous silica). Diatomaceous silica is used more extensively than quartz because it is a softer material providing less machining and abrasive problems. There is also concern over respiratory problems possibly associated with inhalation of finely divided quartz. Although it is an excellent additive for increasing viscosity, diatomaceous silica has a low oil adsorption and very large surface area so that it is not a particularly good extender since it cannot be easily incorporated into the formulation in high concentrations.

Fumed silica is produced by the vapor-phase hydrolysis of silicon chlorides in a hydrogen-oxygen flame or an electric arc furnace. Fumed silica is sometimes also referred to as pyrogenic silica. The high temperature creates silicon dioxide molecules which then condense to form discrete particles which can attach themselves to one another due to the high temperature of the process. The result is a three-dimensional branched aggregate.

Fumed silica is naturally hydrophilic due to the silanol groups on the particle surface. This accounts for its high surface energy and good wetting properties. However, fumed silica particle can be "treated" to provide a number of hydrophobic grades. In these processes the silanol groups on the surface are generally replaced with organosilicon groups.

The isolated and the hydrogen bonded hydroxyl groups are hydrophilic sites, whereas the siloxane is a hydrophobic site. Thus, fumed silica grades are generally characterized by their surface area and whether they are hydrophilic (standard grade) or hydrophobic. The hydrophilic silica is most effective in nonpolar and medium polar media. The hydrophobicity of treated fumed silica results in lower adsorbed moisture on the silica. This makes it ideal for use in systems where moisture sensitivity is important and must be maintained low (e.g., moisture cure urethanes and silicones).

The performance of hydrophilic fumed silica is often improved by adding a polar substance such as ethylene glycol, glycerin, or some secondary amines to the formulation. In medium polar to polar media, hydrophobic fumed silica is a more efficient thickening agent and generally preferred. Comparative sag resistance properties of various commercial types of fumed silica in a liquid epoxy adhesive system are shown in Table 1.

Fumed silica is typically available with sizes in the 7-40 nanometer range 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 it