The current building blocks of permanent hair colorant were identified in the 1860s. In 1863, German chemist August Wilhelm von Hofmann, Ph.D., discovered p-phenylene diamine (PPD), a key synthetic dye.1 Then in 1867, London chemist E.H. Thiellay and Parisian hairdresser Leon Hugot demonstrated the use of hydrogen peroxide to lighten hair. By 1907, the first commercially available permanent hair colorant was created by Eugene Schueller2 and by 1956, the products were available in an in-home kit using a one-step process for lightening and color formation.
Since that time, new hair dyes have been added to the color palette but the basic chemistry remains unchanged. The color transformation these products can deliver, i.e., covering unwanted gray hair or changing hair to a totally new shade, has grown this category to US $7+ billion worldwide, according to internal research, with markets continuing to expand. While the transformational color benefits are compelling, women also recognize that these products ultimately lead to hair damage. Internal consumer testing has shown that women surveyed about what causes hair damage choose permanent colorants among their top answers, along with use of high temperature implements such as flat irons.3
This article explores the main changes to hair after regular use of permanent hair colorants and how these changes impact noticeable hair properties such as feel, styling, breakage, etc. It then considers several strategies to reduce the damage caused during hair coloring, or minimize the impact of damage post-coloring. All of these interventions have a positive impact on women, allowing them to achieve the color they desire but with minimal consequences to hair health.
How Hair Changes During Coloring
Hair color and bleach products are divided into four different categories depending on the technology used, as shown in Table 1. These chemistries give different levels of color transformation in terms of both the level of lightening of the underlying hair color and amount of synthetic color delivered; and how long this transformation lasts. Importantly, the chemistry used also determines the severity of damage to the hair fibers.
This review is focused on permanent hair color, or Level 3 products. These products contain two bottles that are mixed together immediately before use. One bottle contains hydrogen peroxide, typically either 6% or 9% active, at a pH of 2-4, and one bottle contains dye precursors and an alkalizer, typically ammonia or ethanolamine, at a pH of 10-11. The final mixed pH is 10. The mixed product is applied to dry hair and left for 25-40 min before rinsing. Figure 1 shows a schematic of the two processes that occur to form the final color.
The first step involves the bleaching of hair melanin by perhydroxyl anions, which are formed by increasing the pH of hydrogen peroxide to pH 10. This lightens the underlying color of hair. The second step is the diffusion of dye precursors into hair and the coupling of these dyes, which is activated by hydrogen peroxide, to form an extended chromophore, which is the new color. A range of shades can be achieved by altering the degree of lightening and changing hydrogen peroxide levels, or by changing the amount and combination of dye precursors. Many hair color products contain four to eight different dye precursor structures that can couple together to give a range of shades including blondes, reds, browns and blacks.
The same oxidative chemistry responsible for creating the final hair color is also responsible for changing hair properties. There are two key reactive species formed by hydrogen peroxide at pH 10: the perhydroxyl anion, as shown in Equation 1; and hydroxyl radicals formed catalytically in the presence of redox active metals—mainly copper and to a lesser extent, iron, as shown in Equation 2.
It is known that low levels of these metals already exist in hair. Some are endogenous, incorporated during follicle growth, and some are exogeneous, typically from washing with tap water containing these metals.4 Table 2 summarizes the important reactivity of these species with hair, their consequent impact on single hair properties, and how they impact properties that are noticeable to the consumer after using hair dyes. Perhaps the most important reaction is the removal of the F-Layer from each cuticle surface, shown in the schematic in Figure 2.
The F-Layer is a branched-lipid, 18-methylicosanoic acid that is chemically bound to each cuticle surface.5 It provides hair with a hydrophobic coating, i.e., having a water contact angle of 70-90 degrees,6 which imparts a silky feeling when wet, and shine, combability and manageability when dry. Its removal and the oxidation of the formed thiols to cysteic acid creates a more hydrophilic surface, i.e., a water contact angle of 50-70 degrees. This has an impact on the look and feel of hair, and a significant impact on the deposition of actives such as silicones and quats from rinse-off products. This is especially true for silicone-based conditioning actives, which have much lower deposition on colored hair than uncolored hair, as described later in this article.
Another important change to hair is the oxidation of cystine to cysteic acid, shown in Equation 3. Cystine is a key amino acid in keratin proteins as it acts as a cross-linker between protein chains, imparting strength and thus protection from damage. After hair coloring, cystine groups are oxidized, which can lead to a decrease in break stress of up to 25% and thus increased fiber breakage.7 Hair swelling also is increased, which causes increased water-induced dye loss and ultimately faster color fade. In addition, the creation of negatively charged cysteic acid groups increases the uptake of water hardness ions such as calcium and magnesium, and copper and iron from tap water. In fact, calcium ions can increase from 2,000-3,000 µg/g in uncolored hair up to 12,000 µg/g in colored hair.
Minimizing oxidative damage to hair during coloring is a clearly the preferred strategy to reduce fiber damage but obviously, the same reactive species responsible for damage, e.g., perhydroxyl anions, are the same species responsible for achieving the desired hair color. Thus, it is also important to consider strategies to minimize the impact to fibers; for example, by adding materials that “restore” hair’s surface hydrophobicity. These strategies are discussed in the next two sections, along with select examples of solutions.
Strategies to Prevent Oxidative Damage
Reducing the levels of perhydroxyl anion in hair dyes in order to decrease oxidative damage is not effective, as the consequence will be reduced melanin bleaching. Demi-permanent products, for example, use lower hydrogen peroxide levels, i.e., 1% vs. the 3% used in Level 3 dyes, which lowers the associated hair fiber changes but also reduces the lightening of hair. As consequence, the level of gray coverage or color transformation also is reduced—especially when moving to lighter or vibrant shades.
Another strategy to directly reduce oxidative damage is to identify a different oxidant system to replace hydrogen peroxide at pH 10. This has proven challenging, as any change to the oxidant system must not only maintain melanin-bleaching performance, but also not significantly impact dye formation kinetics and chemistry. One system that has been commercialized in the last ten years is the combination of ammonium carbonate and hydrogen peroxide at pH 9.8 The lower pH reduces the concentration of perhydroxyl anion by tenfold and with the addition of glycine to the system, reduces the formation of reactive radicals. In this system, the species responsible for bleaching melanin is the peroxymonocarbonate anion, as shown in Figure 3.
The replacement of monoethanolamine for ammonia as an alkalizer also has been proposed as less damaging. It certainly reduces the odor of monoethanolamine products, which can be perceived by the user as less damaging. However, recent work indicates the underlying chemistry leading to cuticle removal and protein loss is not significantly different between the alkalizers.9
On the other hand, reducing redox metal-induced hydroxyl radical formation is a viable strategy—especially by targeting exogenous copper in tap water. Such redox metals typically are coordinated to charged groups in hair such as carboxylates and sulfonates, i.e., oxidized cysteic acid, and located in the cuticle region.10 There are two main approaches to reducing metal-induced damage: either prevent the copper from participating in radical formation during coloring, or remove these metals from hair prior to coloring with a rinse-off or leave-on product.
For the first approach, several chelants (see Figure 4) have been identified for incorporation with the colorant. These include (N, Nʹ-ethylenediamine disuccinic acid (EDDS) and diethylenetriamine penta(methylene phosphonic acid) (DTPMP).11 For the second approach, chelants such as EDDS and histidine are effective when added to shampoos, conditioners or other treatments12 but they must readily be delivered into hair as these products are used for only ~30 sec vs. ~30 min in the case of dyes. The difference is, in the latter scenario, the chelant mainly is removing copper whereas in the former scenario, it is preventing hydroxyl radical formation. Regularly using these products will also keep copper levels low before colorant is applied.
Strategies to Repair and Minimize Oxidative Damage
Many strategies to reduce the damage caused by oxidative hair dyes have been in the area of surface materials. These return the hydrophilic surface properties of hair, caused by removal of the F-Layer lipid, closer to the hydrophobic state of uncolored hair. Such materials include silicones, quats and even F-Layer lipid itself.13
Silicones are the most commonly used materials to deliver this surface benefit. They generally are modified to have some polarity so they are attracted to the negative charge of colored hair. Amodimethicones, for example, incorporate amino groups or silicone quats that include quaternary charged groups.14 By restoring hydrophobic surface properties, both wet and dry combing forces are decreased and flyaways caused by static are improved.15 These materials also protect hair from breakage by reducing tangling, and many provide a degree of wash-fade protection, slowing the rate whereby the formed dye is removed from hair during washing.16 The mechanism of action for these materials is likely related to the formation of a hydrophobic surface coating, which moderates the diffusion of hydrophilic dyes out of hair.
An alternative strategy is to alter the surface hydrophobicity of hair by adding a material such as a polymer that deposits on hair and then further enhances the deposition of a non-polar silicone, such as polydimethylsiloxanes (PDMS). An example of this strategy would be the incorporation of a high charge density cationic polymer, poly(diallyldimethyl ammonium chloride) (poly(DADMAC)), combined with anionic surfactants in a shampoo.17 This combination forms a liquid crystal structure that on dilution, deposits from the shampoo onto hair to: provide “slip planes” along the hair surface for wet conditioning purposes; and form a hydrophobic layer that changes the surface energy of the fibers. This increased hydrophobicity allows for enhanced silicone deposition on previously colored or bleached hair, as shown in Table 3.
In summary, the hair color category is likely to continue growing, and consumers will seek the desired transformations of covering gray or achieving a shade different from their own. With the current oxidative chemistry, which delivers both lightening and color-formation, there will also be associated changes to hair structure signifying poor hair health. These include loss of shine, poor feel, breakage, etc. However, there are opportunities to reduce the damage that occurs during coloring and prevent further damage by protecting hair after coloring.
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- S Godfrey, W Staite, P Bowtell and J Marsh, Metals in female scalp hair globally and its impact on perceived hair health, Int J Cosmet Sci 35(3) 264-271 (2013)
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- AD Bailey, G Zhang and BP Murphy, Comparison of damage to human hair fibers caused by monoethanolamine and ammonia based hair colorants, J Cosmet Sci 65 1-9 (2014)
- JM Marsh et al, Preserving fiber health: Reducing oxidative stress throughout the life of the hair fiber, Int J Cosmet Sci (37) supplement S2 16-24 (2015)
- JM Marsh et al, Role of copper in photochemical damage to hair, Int J Cosmet Sci 36(1) 32-38 (2014)
- JM Marsh et al, Advanced hair damage model from ultra-violet radiation in the presence of copper, Int J Cosmet Sci 37 532-541 (2015)
- M Okamoto, N Tanji, T Habe, S Inoue, S Tokunaga and H Tamamachi, ToF-SIMS characterization of the lipid layer on the hair surface. II: Effect of the 18-MEA lipid layer on surface hydrophobicity, Surf Interface Anal 43(1-2) 298-301 (2011)
- S Herrwerth, I Ulrich-Brehm, U Kortemeier, P Winter, M Ferenz and B Gruening, Silicone quaternium-22: New silicone technology for premium hair conditioning with additional benefits, SÖFW 135(6) 11-12, 14-18 (2009)
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- A Schlosser, Silicones used in permanent and semi-permanent hair dyes to reduce the fading and color change process of dyed hair occurred by wash-out or UV radiation, J Cosmet Sci 55(suppl) S123-131 (2004)
- MA Brown, TA Hutchins, CJ Gamsky, MS Wagner, SH Page and JM Marsh, Liquid crystal colloidal structures for increased silicone deposition efficiency on color-treated hair, Int J Cosmet Sci 32(3) 193-203 (2010)