Formulators using hydrophilic fumed silicas in moisture curing systems, such as STP or polyurethane, must address premature curing of these systems due to the large water adsorption capacity of hydrophilic silica. Several options are available to solve this challenge, including adding process steps (e.g., pre-drying hydrophilic silica or removing water during compounding via heat and vacuum) or adding additional components (e.g., chemical dryers). A better option is replacing the hydrophilic silica with hydrophobic silica, which can greatly reduce or eliminate the need for these extra process steps. Two hydrophobic fumed silica additives* were compounded in a model formulation of MS Polymer™ SAX 400 silyl-terminated polyether, diisononyl phthalate, and an organotin catalyst. The effect of fumed silica surface chemistry on rheology is observed in this system. The less hydrophobic fumed silica additive, with stronger particle-matrix interaction, provides a Newtonian rheology.
In contrast, the more hydrophobic fumed silica additive, with weaker particle-matrix and hence stronger particle-particle interaction, provides thixotropic rheology, resulting in a system with yield stress or sag resistance and viscosity that shear-thins. With either of these fumed silicas, the particle-matrix interaction is sufficient to increase the system's tensile strength and a greater than the three-time increase in elongation at break. With increased loading of fumed silica, both tensile strength and elongation at break increase, but usage at very high loadings is limited by the corresponding increase in viscosity. If you are looking for high quality, high purity and cost-effective Fumed Silica, or if you require the latest price of Fumed Silica, please feel free to email contact mis-asia.
