The pros and cons of silicones

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.

There are several main categories for silicones approved for use in hair care products:

  1. Cyclomethicones: These are low molecular weight silicones that are ring-shaped. They have been found to provide very light conditioning effects as well as to speed drying time after a wash. These molecules are volatile and will thus evaporate from the surface of the hair, leaving behind no residue. This volatility may perhaps make delicate, curly hair feel drier, but that is just my own speculation
  2. Dimethicones: These have been the most commonly used silicones in conditioning products until recent years. They spread easily onto the hair, provide gloss and substantivity (lasting conditioning effects), and provide a soft, silky feel to the hair. They also reduce static and fly-away hair. All of these effects are influenced by the molecular weight of the molecule, which is not usually disclosed on the product package. Due to their extremely hydrophobic nature (lack of water solubility), these products may build up on the hair over time if a traditional surfactant-containing shampoo is not used.
  3. Dimethiconols: These silicones are either dimethicones or cyclomethicones combined with very high molecular weight dimethicones that possess a hydroxy-functionality (an alcohol group) at the end of the molecule. These molecules provide significant conditioning effects to the hair and also build the viscosity (thickness) of the product. These are not water soluble.
  4. Phenyl Trimethicones: These are also not water soluble and are used for medium conditioning effects as well as a very high gloss and shine.
  5. Dimethicone Copolyols or PEG-modified dimethicones: These are the only silicones that are water-dispersible or water soluble. They are made by chemically adding groups to the silicone molecule that are water soluble. This unique structure enables these silicones to not only provide excellent conditioning benefits, but also to act as nonionic surfactants. They can provide foam boosting and facilitate good wetting of the hair in a shampoo. They provide lubrication, reduce tackiness (sticky-feel), can go into clear formulations due to their water solubility, and do not show as much tendency to stick to the hair. Since they don’t have as much substantivity (the ability to stick to a surface), they are primarily used only for light conditioning.
  6. Amodimethicones: These silicone molecules are modified by adding amine-functional groups to the structure. This makes them more polar and highly attracted to the negatively charged surface of the hair. Thus amodimethicones are noted for their high rate of deposition onto the surface of the hair, their extreme substantivity, and for great reductions in combing friction in both wet and dry hair. These silicones are considered to be the most useful for extremely dry or damaged hair due to their strong conditioning effects. These silicones are also not water soluble, so due to their high level of substantivity there may be some build-up if hair is not regularly shampooed. However, a preliminary study of this by Dow Corning showed only slight build-up after 3 uses.

Removal of build-up X-ray refraction studies performed at Dow Corning have shown that silicone molecules are almost 100% removed from the surface of the hair when a shampoo containing sodium lauryl sulfate, sodium lauryl ether sulfate, ammonium lauryl sulfate, ammonium lauryl ether sulfate, or cocamidopropyl betaine was used. This is excellent news for those who enjoy the benefits of silicone additives in the products they use and who do not mind using a shampoo on an occasional basis. However, if one plans to use the method of conditioner-washing one’s hair, these water insoluble and organic insoluble materials seem to be something that should possibly be avoided, with the exception of the dimethicone copolyols or PEG-modified variety.

Bottom-line studies show that silicones actually minimize scalp irritation caused by the surfactants used in many hair care products. Fifty years of studies and data demonstrate that these are one of the safest materials we use in personal care products. There is no (current) scientific evidence that silicones are drying or otherwise cause dull hair, although some curlies report anecdotal evidence of such. Not properly washing silicones out of hair may result in them coating the hair, preventing hydration. Some curlies obtain the best results when they use silicone-containing products in cycles: they use them for a while, then lay off for a while, then come back to them. Finally, if you follow a shampoo free method and like the results you get with silicones, you may obtain best results if you use products only containing the Dimethicone Copolyols or PEG-modified dimethicones.

So what's silicon? A nonmetallic element occurring extensively in the earth's crust in silica and silicates, having both an amorphous and a crystalline allotrope, and used doped or in combination with other materials in glass, semiconducting devices, concrete, brick, refractories, pottery, and silicones. Atomic number 14; atomic weight 28.086; melting point 1,410°C; boiling point 2,355°C; specific gravity 2.33; valence 4. Symbol: Si