| |
| Chemical and Physical Properties |
| Stability and Compatibility |
Stability is an important consideration in formulations containing SIL-POX and other component. The time required for gelation is usually used a measure of stability. For some application however, gelation is required step during processing.
The stability of the system that contain SIL-POX depends on several variables : silica content, temperature, size or surface area of silica particles, pH particles charge salt concentration and the compatibility of various additives such as surfacetants, freeze thaw stabilisers and organic solvents. The electrical properties of sols prevent this form occuring. Colloidal particals are usually electrically charged as are SIL-POX. In a aquasol , the surface of the colloidal particles is hydrates forming silanol groups. SioH when alkaline in pH as are most SIL-POX some of these groups become ionized and produce an SiO2 group at the surface. This gives the particle a negative charge. The negative charges on the surface of the particles attracts ions of opposite charge and form an electrical "double layer" of charge which is responsible for the stability of the colloidal dispersion. particles of like charge will repel each other and thus the electrostatic repulsion of the particles for each other prevents the particles from coming together, colloiding and agglomerating. |
| |
| Temperature |
| As might be expected, since gelation involves molecular movement the rate of gelling increases with increasing temperatures. |
| |
| Gel Phase |
Before a sol solidifies only slow increase in viscosity can be observed with little change in properties as to the point where viscosity begins to increase rapidly and solidification occures at the gel point.
One way of determining gel point is to observe when the miniscus of a sol in a test tube no larger remains level when the tube is titled. |
| |
| Freezing |
| freezing temperatres cause crystals of ice to form SIL-POX, which increases the concentration of silica in the unfrozen portion. It is therefore extremely important to keep SIL-POX from freezing. |
| |
| Particle Size |
| The relative stability of various grades of SIL-POX increases with increasing particle size other variables remaining constant. |
| |
| Effect of pH |
| The tendency of SIL-POX to gel is greatest at pH 5-6. As the further reduced these sols become more stable and consequently can be effectively used in acidic media. To acidify SIL-POX, acid should be added with agitation in such a manner that the PH moves rapidly through the critical range 5-6 above about 10.5, the silica in SIL-POX becomes increasingly solubilized and the alkali silicate acts like any other soluable salt in destablizing the remaining colloidal silica. However high concentration of sodium hydroxide also cause coagulation of silica therefore in adding sodium hydroxide to SIL-POX vigrous mixing must be maintained during addition to prevent local gelation even though high pH is not finally reached. |
| |
| Salt concentration |
| SIL-POX have higher salt concentration would cause gelling. As salt is added, the counter ions move closer to particle surfaces, thereby reducing the distance through which repelling forces act. Salt induced gelation is frequently adopted by adding sodium chloride or Ammonium chloride, where presence of sodium is not desired, ammonium salts. Such chlorides, acetates and nitrates are preferred. |
| |
| Surfactants |
| SIL-POX is compatiable with an ionic and many non-ionic surfactants. Such additions improve wetting proporties of SIL-POX. Cationic Surfactants are not desired as they cause silica particle in SIL-POX to precipitate. Surfacetant addition may cause some foaming in SIL-POX solutions. These may be reduced by adding of antifoam agents. |
