Adhesives and sealants used for elastic bonding in construction, industrial assembly, automotive and other applications require strength and flexibility. Cabot Corporation’s line of treated fumed silicas impart thixotropy in adhesives, enabling the adhesives to shear-thin for easy dispensing and application, yet be non-sagging and have stable bond lines until cured. Treated fumed silica can enhance mechanical properties of the cured adhesive, resulting in greater tensile strength, elongation and shear strength.

Application Information

Product Performance – Reinforcement

The addition of even low loadings of fumed silica will increase the tensile strength. At higher loading levels, CAB-O-SIL TS-610 DiMeDi treated silica provides greater reinforcement than the PDMS treated silicas.

Coated Precipitated Calcium Carbonate (PCC) has only a modest effect on Lap Shear Adhesion Strength, while fumed silica significantly increases this strength.

While both coated Precipitated Calcium Carbonate (PCC) and treated fumed silica can provide thixotropy, enabling formulation of sag-resistant adhesives, coated PCC has only a modest impact on mechanical properties.

Product Performance – Rheology Control

Silica surface treatment strongly influences rheological performance. Selecting a highly hydrophobic silica, (CAB-O-SIL TS-720) imparts thixotropic or shear-thinning behavior in one-component polyurethane adhesives. In contrast, a moderately hydrophobic silica, (CAB-O-SIL TS-610), imparts small increases in viscosity without thixotropy or shear-thinning behavior, even at increased loadings.

Cabot offers a range of highly hydrophobic, PDMS-treated silicas for rheology control of 1K PU adhesives. Of these, CAB-O-SIL ULTRABOND fumed silica provides the greatest increase in viscosity and sag resistance as predicted by yield stress.

While coated Precipitated Calcium Carbonates (PCC) do increase viscosity and impart thixotropy in 1K PU adhesives, PDMS-treated fumed silicas are more efficient. Using a PDMS-treated fumed silica can provide sag resistance and rheology control without the significant weight (specific gravity) increase encountered with use of coated PCCs.

ULTRABOND fumed silica is one of the best thixotropes on the market for 1K PU adhesives. With ULTRABOND fumed silica, significantly lower loadings are required to achieve excellent sag resistance when compared with competitive PDMS-treated silica.

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