Amodimethicone and other Amine-functionalized Silicones

For people with curly hair, silicones elicit many emotions. For some, they are a holy grail ingredient, while others shy away from them for fear they will dry out their hair.

It is evident that there exists a considerable amount of confusion in the curly community in regard to whether silicones are good for curly hair and compatible with shampoo free hair care routines.

In this column, we’ll take a look at amodimethicone and other similar molecules, such as bis-aminopropyl dimethicone and trimethyl silylamodimethicone. And we’ll talk about the pros and cons of using them, especially if you shy away from sulfates.

These are being used more often by chemists as conditioning agents in hair product formulations. They are popular because of their ease of use in processing and manufacturing products as well as for their many benefits to the hair.

Amodimethicone is an abbreviation of “amine-functionalized silicone,” which is a family of silicones modified to have specific properties. The simplest, and perhaps most well-known silicone, polydimethylsiloxane (dimethicone, by INCI naming standards), consists of methyl groups (-CH3) as the pendant group along the backbone of the polymer chain (Figure 1). Amine-functionalized silicones have been chemically modified so that some of the pendant groups along the backbone have been replaced with various alkylamine groups (-R-NH2). These amine groups become positively charged in aqueous solutions because of their electron-donating (basic) tendencies, yielding an inorganic, cationic polymer.

These inorganic cationic polymers deposit onto the hair because of the electrostatic attraction between the polymer and the negatively-charged protein surface of the cuticle. In this manner, they behave much like polyquaternium materials (organic cationic polymers), which are excellent conditioning agents as well. The charge density of the polymer can be varied by changing the placement and quantity of the amine groups. A polymer with greater charge density will be more substantive to the hair than one with lesser charge density.

One interesting property of these polymers is that they provide selective conditioning to the areas most in need of it. The mechanism by which they accomplish this is, again, electrostatic attraction. Highly damaged areas of the hair cuticle possess higher negative charge density, which enhances the affinity of the cationic polymer to that specific area. These polymers can provide a targeted beneficial effect.

Once the amine-functional silicone is deposited onto the surface of the hair, it spreads out and forms a cross linked film when it dries. This cross linked film can last through several washings, which is considered to be advantageous in most applications. A unique property of these polymers is that once in place on the surface of the hair, they repel further deposition of amine-functional polymers on top of the existing layer, preventing buildup. This cross-linked film seals moisture inside the hair shaft, holding the cuticle flat and providing excellent wet and dry comb-ability. An additional benefit of these silicones over other cationic polymers (such as polyquats) is their high refractive index, which gives the hair a high degree of gloss and shine.

Silicones, including amodimethicone, also protect from thermal damage resulting from styling tools, such as hot rollers, curling irons, and blow dryers. This phenomenon is due to their very low thermal conductivity — much lower than water, glycerin, or mineral oil. This reduces heat transfer through the hair surface to the cortex of the hair. Very high temperatures found when styling or processing hair (sometimes as high as 100°C to 160°C) are capable of vaporizing water contained within the cortex. It is extremely important to maintain proper hydration of the hair because water has a very high specific heat which helps protect the hair from getting too hot. Hair that reaches too high temperatures can suffer permanent damage to the delicate keratin fibrils in the cortex. A protective layer of amodimethicone on the surface can help prevent or reduce damage done in this manner. One frequent question that arises is whether these amine-functional silicones — amodimethicone in particular — are water soluble. This question is most relevant for those on a shampoo free routine who wash with conditioners. They fear that the only way to prevent buildup of these silicones is to use a traditional surfactant such as sodium lauryl or laureth sulfate, cocamidopropyl betaine, or the ammonium lauryl or laureth sulfates.

The short answer is that these polymers are not water soluble. The silicone is provided to product manufacturers as a mixture of silicone/cationic surfactant/nonionic surfactant, which enables it to be readily dispersed into an aqueous formulation because this mixture is water soluble. However, once the product is used and the amodimethicone is deposited onto the surface of the hair and forms a film, it is not water soluble.

Conclusions

Amodimethicone and other similarly modified silicone polymers are considered to be among the best high-performance conditioning polymers currently available to the hair-care product formulator. They provide many unique benefits, including the following:

• Provide deep conditioning
• Provide targeted conditioning to areas of particularly damaged hair
• Protect from thermal damage
• Increase color retention
• Resist build up
• Impart gloss and shine

These modified silicones seem to be of particular benefit for those of us with damaged hair, permanently colored hair or those concerned about the buildup of conditioning agents. It would be necessary to use a shampoo containing one of the lauryl or laureth sulfates or cocamidopropyl betaine to completely remove this silicone from the hair, which may be of concern to those who prefer to use only conditioner-cleansing methods.

References
[i] Urrutia, Adriana, Silicone: The Basis of a Perfect Formulation for Hair Care, Dow Corning de Mexico S.A. de C.V.
[ii] Wacker-Belsil corporate literature
[iii] Heat Protection for Hair Care, Dow Corning