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Rheology Modifiers Selection for Adhesives

Rheology modifiers improve the viscosity and have a great impact on the application characteristics. Adding rheology modifiers to your adhesive formulation will prevent settling down of fillers during settling and transportation. The guidelines in this selection guide apply to rheology modifiers and thickeners which are used to adjust the flow properties of adhesive and sealants. Read on to select the right rheology modifier for your formulation and easily adjust the flow characteristics of your final adhesive.

Inorganic Rheology Modifiers


Fumed Silica


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

Fumed silica rheology modifierFumed silica is typically available with sizes in the 7-40 nanometer range and surface areas ranging from 50 to 380 m2/g. Unlike precipitate 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 little fumed silica (1-5% by weight) to achieve thixotropic properties.

The surface chemistry of fumed silica is extremely important because of its influence on the rheological behavior of the formulation. Thus, fumed silica grades are generally characterized by their surface area and whether they are hydrophilic (standard grade) or hydrophobic.

  • Hydrophilic silica: Most effective in nonpolar and medium polar media. 
  • Hydrophobic silica: In medium polar to polar media, hydrophobic fumed silica is a more efficient thickening agent and generally preferred. Also noted for providing superior moisture resistance in adhesives, sealants, and coatings.

The thixotropic characteristics provided by fumed silica are due to its ability to develop a loosely woven, lattice-like network by hydrogen bonding between particles. This network raises the apparent viscosity of the system, increases the cohesive forces, and contributes to the suspension of solid components.

Because the hydrogen bonds themselves are relatively weak, they are easily disrupted through the action of an applied stress or shearing force and quickly reformed when the stress or shearing force is removed.

Precipitated Calcium Carbonate

  

Omya Calcium Carbonates
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