Silicone based defoamers are silicone compounds emulsified in water. The emulsifier system ensures a good spreading of the active components in the foaming medium and also gives synergy effects on drainage.
A silicone compound has low surface tension and can cover a foam lamella. Due to the hydrophobisity of the silicone oil in the compound, it will bridge the lamellaes and make oil lenses that cause the foam bubbles to leak the contained gas/air. The compound also contains silica particles that have affinity to the surface of the silicone oil. This helps puncture and rupture the lamellaes. Smaller bobbles leak gas/air to larger bubbles and larger bubbles are broken down. This affects and reduces the problems both with entrained air and surface foam. The silicone droplets of the emulsifier are broken down to even smaller droplets by the energy release of bubble rupture, causing them to become colloidal and too small to work as defoamers. These small colloidal drops follow the liquid stream and have little affinity to other surfaces.
In a pulp mill, a silicone defoamer improves drainage of filters and presses and controls the surface foam. Silicone has low affinity to fibres and process equipment and follows the used black liquor to incineration in the recovery boiler. All of these things contribute to a cleaner pulp.