Hydrophobic fumed silica coatings

Fumed silica, or fumed silicon dioxide, is produced by the vapor phase hydrolysis of silicon tetrachloride in an H2/O2 flame. The reactions are shovm in Chapter 19. Hydrophilic fumed silica bearing hydroxyl groups on its surface is produced by this process. Hydrophobic fumed silica is made by processing fumed hydrophilic silica through in-line hydrophobic treatments such as with silanes, siloxanes, silazanes, and so on [1]. Examples of different types of hydrophobic fumed silica coatings include DMDS (dimethyldichlorosilane), TMOS (trimethoxyoctylsilane), HMDS (hexamethyldisilazane). 

Hydrophobic Silicas. Because foaming is a surface phenomenon, any antifoam used must concentrate at the surface (or gas—liquid interface). Hydrophobic silicas, which are silicas that have been treated with a compound that causes them to float on the top of water, have been used to fulfill this function for almost 30 years. U.S. Patent 3 408 306 (5) discloses the use of a hydrophobic silica dispersed in a hydrocarbon oil. Hydrophobic silica for this composition, which is still in use today, is made either by continuous ( dry roast ) or batch process. In either process, precipitated silicas rather than silica gels or fumed silicas are typically used to make antifoams. During a continuous process, silicone oils, usually poly(dimethylsiloxane), are sprayed onto a bed of hydrophilic silica. The bed is heated to temperatures ranging up to 300 °C, and reaction times are up to 20 h. At these temperatures and reaction times, bond formation between the silica particle and silicone oil may occur in addition to simple coating of the particle. 

Precipitated silicas but also fumed silicas are widely used as carrier for liquids and semiliquids and as free flow agent for powder formulations, particularly of hygroscopic and caking substances. Hydrophobic pyrogenic silicas are used in technical products like fire extinguishing powders, pigments or powder coatings, but also in... 

Chem. Descrip. Fumed silica, surf, treated Uses Rheology control agent, reinforcing agent, tlow aid, hydrophobing agent for powd. coatings, elastomers, and toners Properties Wh. fine powd., bulk dens. 6 Ib/ff surf, area 212 028 m /g ... 

Retention of flow control properties of fumed silica can be improved dramatically by applying a hydrophobic surface coating to the silica. Accelerated aging of epoxy adhesives containing hydrophobic fused silica (4 weeks at 60°C) showed no increase in adhesive sag, while untreated silicas, asbestos, and clay all showed some sag increase. Colloidal silica is also claimed to possess good sag resistance over time. ... 

---