Organo-functional silicones – choices for formulations

Silicone compounds have been known since the 1860s, but it was not until the pioneering of Rochow in the 1940s that this important class of compounds achieved commercial viability. This was due in large part to the development of a process which was called the direct process, and now bears Rochow’s name. Silicone chemistry provides the polymer chemist with the ability to construct precise molecules having desirable nanotechnology properties.

Silicone polymers are derived from SiO2, a naturally occurring mineral that makes up 25% of the earth’s crust. SiO2 is converted to Si at high temperature in the presence of a carbon source. Figure 1 shows the two materials.

The resulting Si metal is then crushed and reacted in a fluidised bed reactor to produce chlorosilanes, using the Rochow process. The reaction is shown in Figure 2. The preparation of chlorosilanes is practised by a small number of manufacturers who grind up silicon metal and react it in a tubular reactor with methyl chloride. The manufacturers of chlorosilanes are affectionately referred to as “crushers”.

Chlorosilanes are hydrolysed in water to make intermediates used to make silicone derivatives. The reaction product of water and chlorosilanes is referred to as hydrolysate.

The chlorosilanes are placed into water, HCl stripped off and after distillation and a variety of clean-up processes, a series of silicone building blocks emerge. The most important of these include hexamethyldisloxane (MM), cyclomethicone (D4) and silanic hydrogen compound. That is just the start of the story. These materials are combined in a variety of ways to make silicone polymers of interest. The unit operations are shown in Figure 3.

Construction

Highly specialised activities often create a jargon or language that facilitates improved communication among practitioners, and keeps people outside the field from feeling comfortable in these specialised activities. Chemistry, law and government are but a few examples. Silicone chemistry is also an example. The language makes use of the letters M, D, T and Q to specify structural groups placed into a molecule by its construction. The construction step is the process in which the length of the polymer chain, its branching and its positions for insertion of organic groups is determined. The shorthand for construction is shown in Figure 4.

Clearly, there is no real 1⁄2 O. This nomenclature is used so that when two or more groups are linked together a single oxygen exists between them. For example MM (hexamethyldisiloxane) a key material is often referred to as 0.65 viscosity silicone fluid or hexamethyl disiloxane. Its structure is shown in Figure 5. M units are chain terminators since they are mono-functional.

Silicone fluids – dimethicone

To show the importance of construction, one needs only look at silicone fluids (dimethicone). Reaction of M and D will result in silicone fluids – an example is shown in Figure 6. D units are linear chain extenders. They provide larger and larger molecules having higher and higher viscosity. The correct name for this molecule is MD80M Silicone fluids, also called silicone oils, or simple silicone are sold by their viscosity and range from 0.65 cst to 1,000,000 cst. If the product is not made by blending two different viscosity fluids the viscosity is related to molecular weight. The viscosity allows for an approximate calculation of the value of “n”. The data is shown in Figure 7. The differences in viscosity, feel and cushion going from a low viscosity to a high viscosity silicone fluid is an important effect. This effect is strictly a function of construction. Silicone fluids are insoluble in oil and water.

Blending silicone of different viscosity results in products with very different cosmetic aesthetics. The high viscosity fluid (generally 60,000 or above) is generally sticky when applied to skin. Addition of low viscosity silicone (generally 100 cps of below), provides wetting and spreading on the skin.

Functionalisation

Up to now we have only considered silicone homo-polymers. This class is best understood and an important class of compounds.

It would indeed be a sad situation if the organo-functional materials were not available, or if the formulation chemist was not made aware of the advantageous of such materials.

The preparation of a silanic hydrogen containing polymer by the construction process does not alter solubility. The silanic hydrogen pre-polymer assumes its altered solubility only after the functionalisation reaction is run. For this reason silanic hydrogen containing polymers are considered precursors to organo-functional products. A single silanic hydrogen polymer can give rise to an entire family of analogues depending on which functional group is placed onto the backbone in the functionalisation reaction.

In order to make these products more easily formulated, organo-functional dimethicone compounds have been developed. These include dimethicone compounds with improved oil solubility called alkyl dimethicone compounds; and dimethicone compounds with improved water solubility, called PEG/PPG dimethicone. There is also a series of compounds in which surfactant groups are grafted onto the backbone to improve virtually all surfactant properties including detergency, conditioning, wetting, and emulsification. This ability to provide silicone products with improved applicability in personal care products not only opens the possibility of many high performance products, but also can be a source of frustration to many formulators who have not been given the necessary structure/function relationships to make intelligent choices in picking products. We refer to properly selected silicone compounds as “formulator friendly™”.

Often the formulator is left to use products recommended by suppliers, rather than to be a participant in choosing the optimised product for an application. The key to avoid this situation is to learn the rules of structure/function related to silicones and apply them to new products, resulting in the most cost-effective products possible. The reaction used to place organofunctionality into silicone compounds is called hydrosilylation. This process is used in the construction part of silicone preparation. The key reaction is one in which a silanic hydrogen (Si-H) is reacted with a terminal double bond. This results in a stable Si-C bond (Fig. 8).

These materials are reacted with in the equilibration reaction to make reactive intermediates which are hydrosilylated in the functionalisation reaction. The vinyl containing groups that are reacted include

The comparison of products within a class shows the importance of the construction.