Silicones are Versatile Solutions to Protect Hair
Silicones provide strengthening properties, protect against heat damage from curling irons and blow dryers, and aid color retention in hair care products.
Sabrina Marchioretto and Suzanne van Doorn
Dow Corning SA
In today’s competitive hair care market, innovative products continue to proliferate, resulting in endless variations on grooming and styling. The most successful rely on a combination of documented performance and response to market opportunity.
Hair protection has become an important function of many shampoos, conditioners and styling aids. Through daily washing and styling regimens, hair can be stripped of its moisture and natural oils, becoming brittle and dull. Hair that has been temporarily or permanently dyed is especially in need of protection to maintain its condition and color. Other factors, such as the environment, stress, poor diet and hormonal changes, can also detrimentally affect the beauty of hair—as can heated grooming tools, chlorine from pools, seawater and even sweat. Some hair may be naturally prone to damage because of its thin or fine nature.
This article reviews studies by Dow Corning designed to evaluate the protective properties of silicones for hair care products. Silicone materials were screened as agents to provide strengthening properties, protect against heat damage from curling irons and blow dryers, and aid color retention.
Protection and More
Demand for protective hair care products is on the rise and a significant number currently offer protection claims. Out of 1,155 hair care products recorded in 2007 and early 2008, 31% claimed strengthening, 20% color protection, and 13% heat protection.1
However, consumer buying habits reveal that a range of additional expectations, some subtle, also influence product selection. Besides protection and pure performance, consumers want a distinctive sensory experience, whether in terms of texture, feel during application, fragrance or visual characteristics. They also want products that connote freshness and purity, soothe the senses or link with environmental responsibility.
Silicones were initially incorporated into hair care formulations for their conditioning properties and perception of moisturization. With evolving technology, a number of silicone materials are now recognized for their essential sensory and functional benefits in shampoos, conditioners, stylers and fixatives. Silicones can give a soft and smooth feel, aid detangling and combing, add shine, enhance straightening or encourage curl formation, provide volume and act as anti-frizz agents.
Specialty silicones enable formulators to create products that help protect hair. In this series of studies, quantitative methods were used to assess three primary protective strategies: strengthening, and heat and color protection.
Qualitative observations were conducted to complement quantitative protection studies. Scanning electronmicroscopy (SEM) photographs were used to evaluate the condition of hair with and without various treatments. Sensory evaluations were correlated to analytical results for parameters such as shine, feel and color intensity, to ensure consumer-perceivable benefits.
Enhancing Hair Strength
With their inherent conditioning ability, many silicones address the physical and environmental causes of hair damage. They can be especially useful in products designed for curly or kinky hair or for weakened or damaged hair. In these cases, the ability of silicones to impart a perception of moisturization and control frizziness contributes to the conditioning effect.
In this series of evaluations, single-fiber tensile testing was the chosen study method for several reasons. It is both scientific and quantitative, and it is a recognized measure of hair strength.2-5 In addition, results are not affected by friction resistance; silicones are already known for reducing friction.
Nonfunctional and functional silicones were evaluated for their ability to enhance hair strength, both dilute and in a leave-on hair formulation. Work at 20% elongation was calculated, and statistical comparisons were made between untreated and treated fibers. Two silicones in particular demonstrated properties to enhance hair strengthening, making it less prone to damage and breakage:
• Silicone quaternium-16 (and) undeceth-11 (and) butyloctanol (and) undeceth-5; and
• Aminopropyl phenyl trimethicone
Two possible mechanisms suggest how a silicone film contributes to hair strength. It may protect the hair cuticle during elongation, and it also may help seal the cuticle, reducing moisture penetration into and out of the hair cortex to maintain an optimal moisture level for hair strength. The hair strengthening properties of these materials, combined with other functional benefits of silicones for hair care, differentiate silicones from other ingredients promoted for enhancing hair strength.
Protection from Heat
Moisture content is crucial to hair’s healthy appearance and feel. Blow drying and curling with hot irons can rapidly reduce moisture content below its normal level, leading to damage. Heating appliances soften hair keratin. If the appliances are too hot, they can cause the water in the hair to boil, forming minute bubbles of steam inside the softened hair shaft, weakening the fiber and potentially leading to total fracture. In general, silicones have good thermodynamic properties to help protect hair and maintain its moisture.
Thermodynamic studies measure heat flow through a material and can demonstrate the importance of keeping moisture in hair. High thermal conductivity implies fast heat transfer. Because silicones have very low thermal conductivity, one might expect that because heat flow is slow, the presence of silicone along the hair shaft could reduce hair damage from heat.6 Heat protection can also be determined using thermogravimetric analysis (TGA) to evaluate weight (moisture) loss in hair during heat treatments. Differential scanning calorimetry (DSC) can measure the thermal behavior of hair in terms of its heat absorption during the heat treatment. Heat flow is measured as hair is exposed to increasing temperatures from 40°C to 180°C.
Analytical assessments using dynamic TGA helped establish the possible role silicones could play in reducing water loss from hair fibers. Tresses were treated with various silicones applied at a level of 0.125g silicone per gram of hair, and the treated hair was subjected to various heating cycles in a nitrogen atmosphere. Heating progressed at a rate of 10°C per minute, ranging from 25 to 300°C.
Figure 2 compares the results of tresses treated with three silicones versus no silicone. All the silicones helped retain moisture in the hair over a broad temperature range, with the silicone quat microemulsion (represented by the right-hand set of bars in the figure) performing best. In the DSC studies, all tresses treated with silicones allowed more heat to be absorbed. The silicone film on hair makes it more difficult for water to evaporate, which translates to better heat protection.
Qualitative analysis using SEM helped validate the other heat protection studies. SEM images were used to compare untreated hair and hair treated with neat aminopropyl phenyl trimethicone. After three thermal/mechan-ical stress cycles, the hair protected with silicone had a smooth, unabraded cuticle. In contrast, the untreated hair had a scaled appearance due to desquamation of the hair cuticle.
Sensory evaluations are another qualitative approach that correlate analytical results to consumer-perceivable benefits. Silicones provide an improved sensory profile after heat treatments, particularly after a shampoo cycle. Sensory results of the tests were shown in two ways. Five individual attributes of dry hair (feel, combability, detangling, static formation and shine) were evaluated.
Before shampooing, and with or without silicone treatment, sensory profiles of heat-treated hair were better than hair that had not received the heat treatment. These results reflect a common observation: immediately after thermal treatment, hair typically feels better and is free of tangles, easier to comb and most shiny (Figure 3), possibly because thermal treatments temporarily flatten the hair cuticle.
When tresses were treated with silicone, positive sensory effects were sustained through three shampoos, showing that silicones provided protection from heat damage and improved the sensory profile of the hair tresses. Figure 4 illustrates the improvement with silicone treatment. These studies suggest that silicones can protect hair during heat treatments and against the loss of positive sensory characteristics, even after shampooing.
Silicones Protect Color
The use of hair colorants is increasing on a global scale, and with that growth, the importance of maintaining long term performance. Today’s market is mainly composed of rinse-off conditioners and shampoos, and claims of color protection are linked to hair beauty, such as shine, softness and suppleness. Because color fading is increased by shampooing, there are benefits to using rinse-off or leave-on products that offer color protection.
Spectrophotometry is an analytical method that measures the influence of repeated shampooing and UV exposure on color depth, yellowing and loss of redness in color-treated hair.7 It determines three parameters that define color:
• L* values indicate the level of darkness (color depth);
• a* values indicate the redness intensity (from green to red); and
• b* values indicate yellowing level (from blue to yellow)
Overall difference in color is expressed by the value ΔE, which comprises the three color parameters:
As part of this study, a series of silicones were tested and showed color retention properties when used at 2% silicone active level in a rinse-off conditioner. Figure 5 illustrates one series of results, expressed by ΔE value, of two silicones that significantly reduced the loss of hair color compared to the control (the same rinse-off conditioner without silicone):
• Amodimethicone; and
• Silicone quaternium-16 (and) un- deceth-11 (and) butyloctanol (and) undeceth-5.
The lower ΔE value for hair treated with the two silicones indicates that these silicones help to maintain color intensity and color depth.
In a similar series of tests, colored hair treated with a rinse-off conditioner containing bis-isobutyl PEG/PPG-20/35/ amodimethicone copolymer (and) cetyl ethylhexanoate (and) polysorbate 80 (and) butylene glycol showed the ability of this silicone emulsion to help maintain redness. A sensory evaluation using 18 panelists also demonstrated that tresses treated with eight shampoo and conditioner cycles retained more red with this material.
In another test, bis-hydroxy/methoxy amodimethicone also contributed positively to a* value. After eight shampoo and conditioner cycles on hair tresses, use of this silicone fluid resulted in less red color loss than the control and matched the commercial benchmark at the 95% confidence level. A sensory evaluation using 14 panelists confirmed these results.
Additional tests demonstrated that the addition of silicone quaternium-16 (and) undeceth-11 (and) butyloctanol (and) undeceth-5, or amodimethicone (and) C11-15 pareth-7 (and) laureth-9 (and) glycerin (and) trideceth-12, can improve color protection properties from rinse-off conditioners.
Several hypotheses explain why silicones protect against color fading. Although most colorant does not penetrate the hair cuticle, a portion remains on the upper layers of cuticle and can be removed when hair is washed. When deposited on hair, silicone forms an external, homogeneous, water-insoluble film that helps color redeposit on the hair during washes.
Additionally, silicone could form a complex with the colorant, which could redeposit on the hair shaft after the hair is rinsed. This leads to higher retention of colorant in the presence of silicone when hair is washed.
Silicones have very low surface tension and help colorant spread on hair and form a homogeneous film. In addition, because of this spreading behavior, silicone may help the colorant further penetrate the hair cuticle. Based on these studies, several silicones showed potential for providing protective characteristics along with versatility in formulation for hair care products. Table 1 summarizes their protective properties.
The silicones described here provide a range of hair protection propertieswhich, when combined with their functional and sensory benefits, offer greater flexibility for formulators. Additionally, these specialty silicones can be used with organic materials, including natural oils, butters or waxes to provide synergistic effects for conditioning, shine and sensory properties. These silicone materials offer a wide range of innovative technology and formulation concepts for hair protection. Information from these studies can also serve as a screening tool to help formulators determine which silicones are most appropriate for specific applications or chemistries.8 The result can be shorter test times and faster development of highly differentiated products to protect hair and help create and maintain its healthy appearance.
Silicones Offer Versatility for Formulating
Specialty silicones can provide multifunctional, individualized properties in hair care products for consumers who seek performance and protection, along with distinctive sensory characteristics.
1. Dow Corning survey based on Global New Products Database, Mintel Group (2007, 2008)
2. T Scharfeld, C D’Arrigo, and KH Lim, A single hair fiber tensile tester for operation under a scanning electron microscope, Massachusetts Institute of Technology, 1-5
3. AN Syed and H Ayoub, Correlating porosity and tensile strength of chemically modified hair, Cosmet Toil, 117:11, 57-64 (November 2002)
4. AN Syed, H Ayoub and A Kuhajda, Recent advances in treating excessively curly hair, Cosmet Toil, 113:9, 47-56 (September 1998)
5 B Johnson, K Quackenbush and B Swanton, Silicones for hair strengthening, Cosmet Toil, 122:3, 59-64 (2007)
6. A Gomes, “O uso da Technologia Cosmética no Trabalho do profissional cabeleireiro,” 102-105, Ed. Senac – Brasil, (1999)
7. A Gomes, et al., The use of silicones in hair colorant formulations, Dow Corning white paper (2000)
8. For additional details, see www.dowcorning.com/personal