One-Component Polyurethane Adhesives: CAB-O-SIL® Fumed Silica

Prepared by: Nihan Yaman - Composite Chemicals Sales Manager - Univar Kimya

One-component moisture-cured polyurethane adhesives used in construction, industrial assembly and automotive applications must be both high-strength and flexible. Cabot's treated fumed silicas impart thixotropy to adhesives and, as a result, provide shear thinning, spread and ease of application, while also giving the adhesive flow resistance and a stable cure profile. Treated fumed silicas improve the mechanical properties of adhesives by increasing the tensile strength, elongation and shear resistance of the cured adhesive. Untreated fumed silicas are hydrophilic and therefore absorb undesirable amounts of water under ambient conditions. For this reason, the use of surface-treated fumed silicas is recommended for good stability and shelf life in moisture-cured adhesives.

Product Performance - Rheology Control

Properties imparted by treated fumed silica to one-component PU adhesives: • Sag and flow resistance, • Prevention of filler settling, • Easy application due to shear thinning, • Viscosity increase, • Rapid viscosity recovery. The surface treatment applied to silica significantly affects rheological performance. Selection of a highly hydrophobic silica such as CAB-O-SIL TS-720 provides thixotropy and shear thinning to one-component polyurethane adhesives. Conversely, a partially hydrophobic fumed silica such as CAB-O-SIL TS-610, even when used at high levels, does not impart thixotropy and shear thinning to the formulation, providing only a minor increase in viscosity. In Cabot's product range, highly hydrophobic silicas treated with PDMS are available for rheology control in 1K PU adhesives. Among these, CAB-O-SIL ULTRABOND provides the highest viscosity and, as evident from the yield stress above, the highest sag resistance. Precipitated coated calcium carbonate (PCC) excessively increases viscosity and thixotropy in 1K PU adhesives, while PDMS silicas are more effective for these systems. PDMS-treated fumed silica provides sag resistance and rheology control without the weight increase seen with PCC use. ULTRABOND fumed silica is one of the best thixotropic agents for 1K PU adhesives. ULTRABOND distinctly differs from equivalent PDMS silicas through excellent sag resistance achieved at lower usage levels. *Equivalents A, B and C are fumed silicas modified with polydimethylsiloxane. For more detailed information and inquiries, you can contact Univar Kimya, Cabot's representative in Turkey.

Sources:

Application Guide CAB-O-SIL Fumed Silica in One Component Polyurethane Adhesives.pdf, 2015 Cabot Corporation.