A more elegant method than controlling compatibility by means of chain length is to modify the silicone basic structure by adding side chains.
The majority of silicone additives used in coatings today are such “organically modified polysiloxanes.” In the majority of cases, the modification involves polyether chains (see figure) which lead to an improvement of compatibility. The degree of compatibility can be controlled by the number of these side chains (i.e. ratio of dimethylsiloxane units to polyether modifications (x;y). At the same time, this also influences the surface tension: Generally speaking, the more dimethylsiloxane units, the lower the surface tension. Furthermore, the structure of the polyether chains themselves can also be varied; the key factor here is the polarity.
Polyethers consist of ethylene oxide units (EO) and/or propylene oxide units (PO). Polyethylene oxide is very hydrophilic (polar), whereas polypropylene oxide by comparison, is rather hydrophobic (non-polar).
Therefore, the polarity of the entire silicone additive can be controlled via the ratio EO/PO: A greater proportion of EO increases the polarity and the additive is water-soluble and more compatible in polar coating systems. At the same time, however, the tendency toward foam stabilization increases. On the other hand, a greater proportion of PO reduces both the water solubility and foaming tendency.