Silicones have been a very popular ingredient in hair care products for several decades. One notable product was called “Sudden Date”, which was touted for its ability to add shimmer to the hair and to revive a tired hairstyle in the event that there was no time for a proper washing. Their popularity has grown due to their unique ability to condition the hair without the build-up associated with many of the more traditional oils and fatty alcohols. According to a recent publication by Dow Corning, 82% of new hair care products introduced in the USA contain silicones.
The reason for the popularity of silicones in products for the skin and hair lies in their molecular structure. Rather than being made up of a carbon-based backbone (organic), silicones (inorganic) are made up of a backbone of repeating units of silicon bonded to oxygen, with small organic molecules forming a sheath around the outside of the molecule. This unique structure allows the silicone molecule to be very flexible and also to spread very easily and evenly onto the surface of a hair strand. The flexibility of the molecule allows for the passage of gaseous molecules through its structure. This makes the films formed on the surface of the hair very “breathable.” The films that are formed are noted for their lightweight, emollient and silky feel, and thus these materials are used as conditioning agents in many products. Silicones also have a high refractive index which makes light reflect off the surface of the hair, making it appear shiny and glossy.
Silicones are used as conditioning agents in shampoos, where they have been found to deposit at high rates onto the surface of the hair, especially if combined in the product with a cationic (positively-charged) polymer (referred to on labels as Polyquaterniums). This mechanism of conditioning is known as “dilution deposition” or the “Lochhead Effect.” Due to this property, they played a major role in the innovation of two-in-one shampoos, and are still used in those formulations today.
Silicones are also used in rinse-off conditioners, intensive treatment conditioners and leave-in conditioners, where they reduce combing friction, provide an emollient effect, impart gloss and reduce static charge between hair strands. In styling products, their primary role is to add a softening effect (called plasticization) to the sometimes brittle polymers used to hold the style. Some forms have been found to aid in color retention, to boost foaming of shampoos and to enhance curl retention.
There are many different forms of silicones as the backbone lends itself to chemical modifications which can influence the final properties of the molecule. Also, the number of repeat units present in the molecule (known as the molecular weight) will affect the performance of the ingredient, depending upon the final application of the product. It should be mentioned for practitioners of the “Curly Girl method” that only the PEG-modified ones or the dimethicone copolyols are water soluble.