Combining Silanes and Mineral Fillers

Several filler surface pretreatment procedures are commercially used the neat silane (dry method I) or silane solution (slurry method II) are added uniformly onto the filler 

In the dry procedure, the silane is sprayed onto well-agitated filler. In order to obtain maximum efficiency, uniform silane dispersion is essential through the shear rates provided by the mixing equipment, for example, kneaders, Banbury, Hauschild, Primax, and Plowshare mixers, two-roll mills, or extruders [19b]. Most important commercial silane coating processes are continuous and have high-throughput rates. Silane addition control, dwell time, and exact temperature control within the system are essential. All parameters need adjustment depending on the type of silane employed. 

Silanes need, in general, a certain time and temperature to react with the filler surface. Therefore, the treated filler is heated (commonly at about 30-150 °C) after the addition of the silane to remove reaction by-products (especially the alcohol resulting from hydrolysis), solvents, and water, and bond the silane completely and permanently to the filler surface. Explosive limits and concentrations for the evolved alcohol should be considered and special collection systems can be installed to reduce the risk of explosions. 

In an industrial setting, dwell times of only 2-3 min are common. Since the main covalent reaction is completed after about 15-30 min, an additional heating step is recommended to assure fixation of the silane on the mineral. In some cases, a catalyst can help activate a slowly reacting silane or mineral [20]. Typical silane loadings are between 0.7 and 2 wt% relative to the filler and depend on filler surface area and chemistry and application procedure. 

The neat silane can also be added during compounding and is expected to migrate to the filler surface. The in situ treatment procedure provides the opportunity to coat freshly formed filler surfaces, for example, during silica/rubber compounding. 

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