Characterization of polyurethanes containing different silicas

Four silicas, two fumed (one hydrophilic and one hydrophobic) and two precipitated silicas (one hydrophilic and one hydrophobic), were added to a thermoplastic polyurethane (PU). The rheological, mechanical and adhesion properties of the PU–silica composites were considered in this study. In general, the addition of silica increased the viscosity, the storage and loss moduli of the PU–silica composites in solution but only the hydrophilic fumed silica imparted pseudoplasticity and thixotropy. The rheological and mechanical properties imparted by adding fumed silicas to PU were always more important than for precipitated silicas. Interactions between the hydrophilic fumed silica, the polyurethane and/or the solvent seemed to be responsible for the improved rheological properties of the composites. Addition of silica did not modify the glass transition temperature but increased the softening temperature of PU composites. On the other hand, the immediate (green) T-peel strength of roughened or (roughened + chlorinated with 2 wt% trichloroisocyanuric acid solutions in 2-butanone) styrene–butadiene rubber/PU composite joints was greatly improved if the PU contained silica, mainly fumed silicas irrespective of their hydrophilic or hydrophobic nature. Similar T-peel strength, 72 h after bond formation was found in the joints produced with PU composites with and without silica.