Sharpness STP type adhesives and sealants are the first choice for consumers in the "self-service" (DIY) market. In Europe, the production of these products has been changed to transparent packaging to highlight this advantage. In the formulation of the product, wax, clay and calcium carbonate are used as additives to achieve rheology control and reinforcement of STP adhesives and sealants. These reinforcing agents are opaque due to the mismatch of refractive index and polymer matrix, and fumed silica Instead, its refractive index is very close to the polymer matrix without significantly affecting the transparency of the product. Fumed silica is therefore widely used as a reinforcing agent in transparent STP adhesives and sealants.
Hydrophilic fumed silica is used in a moisture-curing system such as STP or polyurethane, which may cause premature curing of the product due to the rapid water adsorption capacity of the hydrophilic silica. There are several options for solving this problem, including adding process steps (pre-drying hydrophilic silica, or removing water during mixing by heat and vacuum); or adding additional components (chemical drying agents); one is better The choice is to use hydrophobic rather than hydrophilic fumed silica, which can greatly reduce or even eliminate these additional processing steps.
Two hydrophobic fumed silica additives, CAB-O-SIL® TS-610 or CAB-O-SIL®TS-720, compounded in a polyether containing MS polymer TMSAX400 silyl terminated, respectively In the formulation model of the oxime ester and organotin catalysts, the rheological effects imparted by the surface chemistry of fumed silica in this system can be clearly observed. Fumed silica CAB-
The weaker hydrophobic properties of O-SIL® TS-610 enhance particle-matrix interactions, providing rheology close to Newtonian fluids. In contrast, CAB-O-SIL® TS-720 has strong hydrophobic properties, which weakens the particle-matrix interaction, and the particle-particle interaction is enhanced, which makes the system have thixotropic properties; the system has a certain yield stress. Resistance to sagging and shear thinning viscosity as in Figure 1.


In any of the above fumed silicas, the particle-matrix interaction is sufficient to increase the tensile strength of the system and the elongation at break greater than 3 times (Fig. 2). As the amount of fumed silica increases, the tensile strength and elongation at break increase, but the viscosity also increases substantially, thereby limiting the amount of fumed silica added.