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Chemical structure of polysiloxanes

Silicone additives, often also referred to as “silicones,” can be used without understanding their basic underlying chemistry. However, it is helpful to outline and understand several of the underlying principles of silicone chemistry. In this way, one can readily learn how to understand and characterize silicone additives.
Additionally, the relationship between structural features and resultant properties will become more understandable.

Polydimethylsiloxane

All silicone additives are derived from the basic structure of polydimethylsiloxanes (see figure).

Varying the chain length creates products with very different properties. While short-chained silicones are relatively compatible in coating systems and have the typical silicone properties, such as low surface tension, the long-chained molecules are very incompatible, and cause a very definite cratering (hammertone effect).  Accordingly, pure polydimethylsiloxanes (silicone oils) are rarely ever used in modern paints.


Polyether modification

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.

Silicone macromers

In the synthesis of “silicone macromers” (see figure), the chain length of the silicone block can be very precisely adjusted as compared to “organically modified polysiloxanes”. Using the functional group, which might be, for example, an amino, methacryl, vinyl or hydroxyl group, these special components can be further transformed to become active substances with a defined structure. The additives created in this manner have tailor-made properties.

Polymethylalkylsiloxanes

Another method of changing the silicone structure is to replace one of the two methyl groups of the dimethyl structures with longer alkyl chains, resulting in polymethylalkylsiloxanes (see figure). Compared with polydimethylsiloxanes, such products clearly demonstrate higher surface tensions, and exhibit less influence on the surface slip. Such silicones are often used as the active substance in defoamers. Just like polydimethylsiloxanes, the polymethylalkylsiloxanes can also be organically modified with polyether chains. In addition to typical silicone characteristics, such silicone additives also exhibit slight defoaming properties.

Other types of modification (e.g. thermostable modified polysiloxane)

Polyether-modified polysiloxanes are thermally stable up to about 150 °C; the polyether chains will decompose at higher temperatures. Through the use of non-polyether structural elements such as polyester and aralkyl groups (see figure), various thermally stable products can be produced. These modified polysiloxanes are thermally stable up to 220 °C and can be used at much higher baking temperatures.

Reactive silicones

Generally speaking, silicone additives are non-reactive, i.e. they do not interfere with the cross-linking reaction of the binder. For special applications, it can however be desirable for the silicone additive to be incorporated into the binder matrix. Reactive products have (at the terminal end of the organic modification) either primary hydroxyl groups (for reaction with isocyanates or other OH-reactive systems) or double bonds (for reaction with UV-curable systems).

Silicone surfactants

Silicone surfactants are polyether-modified dimethylsiloxanes, but in this case the molecular weights are considerably lower than the usual molecular weights of most other silicone additives.. The siloxane chain consists of just a few Si-O units, and on average contains just around one polyether chain. For this reason, they have a quite pronounced surfactant structure (polar/non-polar).  In aqueous systems, such products significantly lower the surface tension without simultaneously increasing the slip of the coating. If a greater slip is required, the silicone surfactants can easily be combined with other silicone additives which are suitable for aqueous systems.

Compared with the fluorosurfactants, which are also frequently used when low surface tensions are required in  coatings, the silicone surfactants have the advantage of not increasing the system’s tendency to foam.