Addition of Silica to Polyurethane Adhesives

The influence of the addition of silica (Aerosil-200) (5-25 wt%) to polyurethane adhesives on their adhesion properties with non-chlorinated and surface-chlorinated rubbers has been studied. The chlorinating agent was Trichloroisocyanuric acid (TIC) in 2-butanone solution at a concentration of between 1 and 9 wt%. In general, silica produced an increase in the adhesive viscosity and an improvement of green (immediate) peel strength (especially with chlorinated rubber). The best results were obtained for a silica content of 10-20 wt%. However, the addition of silica did not improve the peel strength after a thermal ageing process. Polyurethane adhesives containing silica undergo an improvement in their resistance to degradation by chlorine on the rubber surface. On the other hand, the chlorination of silica produces the rupture of Si-O bonds and the formation of Si-H and Si-Cl groups. Furthermore, the stirring speed (directly related to the dispersion) of silica into the adhesive is an important parameter which affects the viscosity and peel strength. A stirring speed of 1000 rpm gives the best silica dispersion.