Hair colorant technology advances further

While the popularity of colouring hair continues to grow, the number of brands of hair colorants has also increased correspondingly. Colorant products for different consumer groups such as Asian, Caucasian and African have become commonplace in the market. There are also a plethora of product types for various desirable results, such as temporary colour, semi-permanent colour, demipermanent colour, permanent colour, progressive colour, highlights and bleaches. This article will give an overview of the technology required to deliver such results to the consumer.

Types of colorants

To achieve desirable and unique results of each type of hair colorant, specific colour molecules are chosen.

Temporary colorants
Temporary colorants are typically a leaveon type of product sold in the form of foam (mousse), spray-on or styling gel. The colouring ingredients are pigments with intense colours, although acid dyes and basic dyes are said to be used in some cases.2 The molecular sizes of these colouring materials are relatively large (in the range of 0.40 to 50 µ or 400 to 50,000 nm). As a result, it is not possible for them to diffuse inside the hair shaft; they remain on the surface of the hair fibres and are held in place by the polymeric film from the product composition. This fixative film is quite “protective” film is removed, the pigment molecules detach from the hair surface readily by the rinse of water. A formula of this type of product is generally very mild. It simply adheres to the hair surface through physical interactions and does not cause damage to hair.

Semi-permanent colorants
Consumers who choose to keep the colour on their hair for a longer period of time may prefer semi-permanent colour over temporary dye. Generally, semi-permanent colorants are used to enrich and highlight the natural colour of hair, and sometimes to hide the first signs of grey hair. However, this class of products is probably not suitable for grey coverage, especially when the end user has more than 30% grey hair. The primary reason is because semipermanent dyes deposit indiscriminately to both grey and pigmented hair. While both types of hair acquire the colour to a similar extent, the lighter-colored grey hair will remain lighter and continue to be evident. As a result, the contrast between the grey hair and the pigmented hair stays noticeable.

Semi-permanent colorants are generally presented to the consumer in liquid or cream formulations. Alkalinity in these formulations is usually higher than in temporary colorants. These products are actually formulated at high pH levels (between 9 to 10). Dyestuffs used in these products are mostly direct dyes, such as HC dyes, disperse dyes and D&C dyes. Although indo dyes have been claimed to be semi-permanent dyes, they are rarely used in this class of product.3 The molecular size of the most commonly used semi-permanent dyes is usually less than 1 nm. Their small molecular size allows them to diffuse inside the hair and provide some degree of affinity to hair through Van der Waals and dipole interactions. These semi-permanent colour formulations also contain amines as alkalising agents to promote dye diffusion inside hair.

Shade formulation in this colorant class is relatively straightforward. One may simply apply the colour principle when developing different shades. For instance, by combining a yellow dye and a red dye, the final colour on the hair will be a red colour with a gold tone. Similarly, a red and a blue dye will impart a mahogany colour on the hair.

It is noteworthy to mention that the advantage of semi-permanent dyes is the relatively mild nature of the formula as it does not require the incorporation of certain ingredients necessary for permanent colorants which may cause undesirable damage to hair.

Permanent colorants
Permanent colour is by far the most popular hair colorant in the market. In addition to its long lasting feature, permanent colour has the ability to make hair lighter or darker, add highlights and completely cover grey. It utilises a technology very different from that of the semi-permanent colorants. Instead of engaging actual dyes in the colouring process, the formulation contains a combination of primary intermediates and couplers.

Immediately prior to application to hair, this combination is added to a lotion of hydrogen peroxide. This final mixture is able to lighten natural colour and deposit new colours on the hair during the process.

By definition, a primary intermediate, when in its pure form, is a colourless molecule that can be readily oxidised. In its oxidised state, the molecule can further react with another primary intermediate to form a colour molecule or cross-react with a coupler, also colourless, to produce a different colour.

Chemistry of permanent colorants
A more thorough account of the colour formation process using hydrogen peroxide as oxidant has been reported in a previous publication.4 In its simplified version, this process can be represented by the equations shown in Figure 1.

The primary intermediate, paminophenol, shown in equation (1) is first oxidised by hydrogen peroxide to its benzoquinone-imine. The benzoquinoneimine can react with other p-aminophenols, and with further oxidation. This reaction path leads to the formation of the trimeric dye molecule as illustrated in equation (2). However, in the presence of a coupler such as m-aminophenol, the reaction between the quinone-imine and the coupler will take preference to form the corresponding indo dye molecule given in equation (3). This is because the rate of reaction in equation (3) is faster than that of equation (2).5 When couplers are simultaneously present in the dye formulation, one can anticipate indo dyes of different colours forming during the colour development process. The resultant colour on hair will therefore be a combination of colours from all the indo dyes developed during the dyeing process.

The above reaction scheme may mislead us to treat these intermediate/coupler pairs as direct colours. That is, one may be tempted to apply the colour principle in formulating different shades as discussed for semipermanent colour. We may speculate that if we know the colours from the combinations of a primary intermediate and a coupler, we will be able to develop hair colorant formulas of any desirable shade by applying the colour theory.

For instance, we anticipate the combination of a pair of primary intermediate/couplers for red indo dye and a pair of primary intermediate/couplers for blue indo dye to form a purple colour. In practice, however, the resultant colour may not be purple if the “wrong” pairs are chosen. Experienced formulators will probably be able to achieve the desired colour by attempting a few different combinations. For formulators with less experience, this may prove to be more difficult and frustrating.

To address this issue, Lowenstein has recently launched its new Mix n’ Colour line of products. This product line was developed based on the rates of reaction between primary intermediates and couplers. The scientists who developed this product are able to assemble certain primary intermediates and couplers into eight groups such that each combination will produce a definite colour on hair on oxidation (see Table 1). The name of each collection is simple and self-explanatory. If two of these groups are chosen in a formulation, this new group-pair will yield a resultant colour which appears to be the combination of the two colours from the two independent groups, just as the colour principle predicts. Using combinations of these eight groups of dye precursors, one should be able to develop most colours of different shades and intensities on hair. This is the first product of its kind to reach such a milestone. It removes the trial-anderror from permanent colour formulation, replacing the hit-and-miss method in colour formulation with a more logical and systematic approach.

Colour deposition
Permanent colorant is different from temporary and semi-permanent colorant in many ways. As the name implies, the colour developed in the hair is much more resistant to shampooing. This is mainly due to the penetration of the small intermediates and couplers (also less than 1 nm) inside the hair and the subsequent reaction forming larger dye molecules, which can either be trapped in the matrix of the cortex layer of the hair or bind covalently to the hair keratin. The latter conclusion can be supported by the observation that a portion of colour will stay in the hair and refuse extensive and repeated solvent extractions. In addition, the penetration of the intermediates and couplers can be facilitated by ammonia or other amine alkalisers from the formulation. It is generally hypothesised that ammonia will swell the hair and allow dye precursors to diffuse into the hair. However, we observed no swelling of a Caucasian light brown hair fibre placed in a 10% concentrated ammonia solution for over an hour under an electronic microscope. In light of this result, the amine alkaliser may not be a swelling agent as is generally believed. Instead, it most likely reacts with the lipid layers situated between cuticle layers6, subsequently opening up the diffusion path in the hair to allow more efficient penetration of dye intermediates and other reactive molecules inside the hair shaft. In fact, the hair does swell in another alkaline solution such as NaOH solution.7

Bleaching
In addition to the ability to deposit new colours on hair, permanent colorant has an additional unique feature. The combination of hydrogen peroxide and ammonia lightens the background of hair and allows the new colour to be more perceptible and vibrant. It has now been established by Perm, et al.8 that the bleaching is accomplished by the action of ammonia in breaking down the melanosomal membrane, releasing the melanin nanoparticles, and finally decolorising these nanoparticles through oxidation. We observe that replacing ammonia with another alkaliser such as sodium hydroxide in a hair bleaching composition does not lighten the hair. This is consistent with the finding from Perm, et al. that ammonia is a definite requirement in the bleaching process.

It is also known that monoethanolamine can be used in lieu of ammonia to induce lightening of hair. However, the quantity has to be significantly increased. The mechanism of bleaching with monoethanolamine is not apparent.

Less damaging colorant
Although consumers like the long lasting effect of permanent colour in the hair, one of the concerns of using such colorant is the hair damage imposed during the colouring process. Such damage usually results in a rougher tactile feel and weaker tensile strength of the finished hair. Formulation and research scientists are both working hard to reduce the damage caused by this type of product. In the market, the most common method of “reducing” the damage is to either incorporate conditioning agents in the colorant composition, or provide a postcolouring conditioner to mask the result of the damage. However, such remediation only provides relief to the rough surface and allows for easier detangling of the treated hair fibres. There is no actual reduction in the degree of damage to the hair.

In an effort to impart permanent colour with lightening and minimal damage, we have now discovered that this objective can be achieved by engaging a proprietary oxidation system. The bleaching is probably achieved by a mechanism which involves slow release of the active oxidising species to perform its function. Using the appropriate level of the oxidant in a suitable formulation, we found that the composition will allow equivalent colour deposition and equal lightening of natural colour as the current permanent colorant products in the market, but with less damage. This is evidenced in the decrease in the formation of cysteic acid detected on the surface of the hair (Fig. 2).

There are two IR absorptions, namely, amide III protein band (~1230 cm-1) and cysteic acid stretching (~1040 cm-1), scientists in the field of hair research use to identify hair damage. While the amide III band is fairly constant regardless of the treatment the hair has experienced, the cysteic acid absorption intensity, on the other hand, increases when the fibre is subjected to oxidation. This increase is a result of the oxidative cleavage of disulfide bonds in the hair, and usually corresponds to the weakening of tensile strength of the hair. The ratio of the cysteic acid peak versus the amide III band is, therefore, a commonly accepted method used to measure the degree of damage of hair due to oxidation.9

In Figure 2, the IR absorption of an intact hair swatch (Swatch #1) suggests that this hair exhibits minimal damage as the ratio of the integrated area of the cysteic acid absorption, B, to that of the amide III band, A, is relatively small with a value of 0.119. When the hair was treated with a conventional permanent colour composition from the market, significant damage is imposed on the hair and an increase in cysteic acid results (Swatch #2). The integrated area ratio of Band B to Band A augments to 0.164. Meanwhile, using the permanent colour formulation developed at Lowenstein delivers equivalent colour and lightening results to the hair (Swatch #3) as the commercial product, but the damage is much minimised with the area ratio of Band B to Band A at 0.124. This result strongly suggests that our technology is successful in satisfying the market need for a permanent colour with reduced damage.

Colour fading
Answers to colour fading remain to be found. Many fade resistant ingredients are said to improve the colour longevity on hair due to sun exposure. Unfortunately, in reality, not many can actually deliver such benefit when incorporated into the dye formulation. Taking Beer’s Law into consideration, we believe the difficulty of finding a solution is due to the fact that it is highly unlikely to block the majority of sunlight by keeping it away from the dyes inside the hair, with only a thin layer of sunscreen molecules. An alterative idea is to use a quencher molecule, which may serve the purpose of stabilising the dyes if one can find such a molecule with suitable properties: colourless in visible wavelengths, molecular size small enough to diffuse into the hair together with the dye precursors, and low energy level to quench the energy from the dye molecule at its excited state. This latter strategy may be more achievable in our opinion.

Demi-permanent colorants
Both permanent colorant and demipermanent colorant products employ the same primary intermediates and couplers to generate colour molecules inside the hair and the dye formation goes through the same steps. That is, it begins from the diffusion of small molecules into inner layers of the hair shaft and forms larger coloured molecules there. One should expect and indeed observe that the colour is definitely more resistant to shampoo than the smaller size semi-permanent dyes.

The class of demi-permanent colorant was invented as a result of clever formulation by chemists who realised that hair dyeing actually depends on two critical steps. The first is the obvious colour formation step. The other is the diffusion of dye molecules into the hair shaft. Unable to alter the reaction path of the dye precursors to develop colours with novel properties, they discovered certain active ingredients in the permanent colorant formula could be changed. This led to the invention of demi-permanent colorants to-date.

While both demi-permanent and permanent colorants are still utilising the same dye precursors, Table 2 represents the major changes in the active ingredient lists between these two colorant products. As a result of the differences shown in Table 2, demi-permanent colour is able to create a “tone-on-tone” colour on hair, a terminology coined by L’Oreal marketing experts. This is accomplished by the use of monoethanolamine and a lower concentration of hydrogen peroxide than in the permanent colour formula. This arrangement limits the degree of lightening of the natural hair colour and possibly the depth of penetration of the dye precursors inside the hair shaft as well. Due to these two important factors, the colour comes off the hair somewhat faster than the conventional permanent dyes, and the hair will gradually reach a colour close to its original shade in the end.

Another advantage of the demipermanent colour is the lower degree of damage to hair. Since this class of product uses a smaller quantity of hydrogen peroxide, the low damage claim is not so unreasonable.

Because of these unique characteristics, home users choose demipermanent colour for subtle but noticeable colour enhancement and to blend away small amounts of grey. People who choose this kind of product usually do so over concerns about the hair damage regular permanent colour may cause. Professional salon stylists may apply both permanent and demi-permanent colorants to one single patron. They treat the new growth with permanent colorant and use demi-permanent colour as a colour refresher for the ends or processed part of the hair. This is a very intellectual manipulation of two technologies. First of all, the stylist will lighten the natural colour in the new growth and colour it with permanent dyes. Meanwhile, if the stylist continues to treat the rest of the hair length with the same formula, this will further weaken the already damaged hair.

The shift to demi-permanent colour in the second half of the treatment does not only allow for the refreshment of colour from a previous process. It actually does so with minimal damage to the fibres and with colours of reasonable likeness.

Miscellaneous colorants

In addition to the four most common categories of hair colorants in the market, there are a few more products that should not be overlooked.

Henna dye
Henna is a plant-based dye of which the major component is lawsone, which can bind to keratin to provide permanent colour. Henna is also known to have a conditioning effect on the treated hair. Despite its natural plant-derived concept, there are a few disadvantages in Henna dye. This dye contains a significant number of metal ions. That is why Henna treated hair should not be curled (permanent wave) and coloured with permanent colour later. Henna dye also requires a longer processing time to achieve the same intensity as that of synthetic permanent colour.

Highlights
This is the product that adds highlights or streaks to hair. It utilises a mixture of hydrogen peroxide and persulfates to bleach the melanin in hair. This blend of oxidants can provide much more efficient lightening to hair than peroxide alone. As an extension of this technology, we have discovered that dyes derived from diazonium coupling10 can be incorporated in the highlight formulation. Such combination does not only allow high lift to hair, but deposits vibrant colours at the same time. This is an improvement from the former technology which requires bleaching and colouring to be performed in two separate steps.

Progressive colours
These are gradual hair colour products targeting male customers. The major dyeing ingredient in this type of product is lead acetate, which, when left in hair, will be oxidised by atmospheric oxygen to form lead sulfide.

The major advantage of this type of product is that it introduces little damage to the hair due to the peroxide or the alkalising agent. The consumer is also able to control the colour intensity by applying the product daily to the hair in the beginning. Once the desirable colour is attained, the product needs to be used only once or twice a week for maintenance. However, end users should be clear that this product does not show obvious colour change immediately after application, and should not form a negative opinion on the product without giving it a chance to perform its science in the hair. In addition, one should remember not to use conventional permanent colour soon after application of this progressive colour product.

Looking forward
There is never a shortage of requests for new products from the market and R&D scientists of every discipline are always engaged with inventing new technologies to meet these demands. We believe natural products or products that are friendly to the environment will eventually win the hearts of consumers. This conclusion applies also to hair colorants. Being natural is a strong concept because consumers believe that if such a product is good for the environment, it is also good for themselves.

As a matter of fact, as we prepare this manuscript, there are already hair colorants in the market declaring the use of natural ingredients in the formulation. Clairol has introduced a water-based colorant. Antica is selling its herbal hair colour gel under its herbal banner. As exciting as these marketing concepts sound, the acceptance of these products by consumers has been, at best, moderate. Apparently, educated consumers and critics have raised their standards and requirements to a higher level and demand some measurable benefits.

On the other hand, another global hair colorant manufacturer once had a product claiming the use of 5,6-dihydorxyindole (DHI) to colour hair. Presumably this molecule is a precursor for melanin, the pigment responsible for the colour of our hair (Fig. 3). The dihydroxyindole, once inside the hair, will undergo an oxidation reaction similar to the natural process of melanin formation and finally give pigmentation to the hair.11 This product stimulated a good degree of excitement in the market and news media proclaimed this to be the breakthrough of hair colour technology. Consumers had high expectations that this product would finally release them from a multitude of shortcomings of current products in the market. However, the product was unfortunately introduced before the technology was ready, and was discontinued shortly after the launch. It should be no surprise that the company will re-introduce this product to the market when the technology is improved. Not only has the enthusiasm for a truly natural colorant not diminished, the call for such a product continues to strengthen everyday.

Along with the same concept, but taking a different approach, is the use of enzyme as an active reagent to dye hair in lieu of peroxide. The substantial research activity in this area is apparent from the quantity of patents issued. Claims in these patents cover a wide variety of enzymes such as reductive oxidases12, peroxidase13 and laccase.14 Evolution of enzyme technology has come a long way from its initial experimental stage in the 1980s when the enzyme had to be kept at low temperature to a much more stable and practical product prototype. The key to the success of such a product relies on the development of a delivery system from which the enzyme is able to perform its function.

Moreover, there are cosmeceutical concepts which envisage the use of certain ingredients such as polypeptides or polynucleotides to introduce hair colour change before the hair emerges from the scalp15, and the use of tyrosinase to promote melanin formation inside hair follicles to eliminate grey hair from the root level.16 Another approach is to deposit melanin directly to the follicle17 such that this pigment can enter the hair shaft and mingle with the keratogenic hair matrix cells; and eventually, the hair shaft containing the “injected” melanin molecules will grow past the skin surface.

Conclusion

As the hair colorant market continues to grow globally, manufacturers and suppliers alike will invest in new products and new technologies. The forward-looking examples discussed, if successful, will certainly represent “step-changes” to the hair colorant industry. Meanwhile, there are also efforts to create incremental changes with the development of new dyes, more efficient delivery systems, innovative dyeing methods and even novel product applicators. Although titles of these research programs may not be striking, their impact will be important to the overall advancement of the hair colorants in the market in the short term. The innovations from these research programs will allow us to have more efficient, more proficient and safer products for our loyal customers in the pipeline.

Acknowledgements

The authors would like to thank Professor Xuechuan Wang at the Shaanxi University of Science & Technology for the FTIR work, and Michael Lowenstein at Jos H. Lowenstein for the experiments on slow-release mechanism.

References
1 FR patent 158,558, February, 1884.
2 Corbett J.F. Hair Colouring Rev. Prog. Coloration, 15, 1985, 52-61.
3 For examples, see US 3,956,342, May, 1976; US 4,248,591, February, 1981.
4 Feng Y., Chan A. J.S.C.C., 45, 1994, 299-308.
5 Corbett J.F. J. Chem. Soc. (B), 1970, 1502-9.
6 Schueller R., Romanowski P. eds. Conditioning Agents for Hair and Skin. ed. N.Y.: Marcel Dekker, Inc., 1999.
7 When a Caucasian light brown hair fiber was placed in a 2% NaOH solution, it was observed under an electronic microscope that the diameter of the hair increased steadily over the first 0.1 – 0.5 hour. Weissman J., Chan A. unpublished results.
8 Perm P., Dube K.J., Madison S.A., Bartolone J. J. Cosmet. Sci., 54, 2003, 395-409.
9 Signori V., Lewis D.M. Int. J. Cosmet. Sci., 19, 1997, 1-13.
10 Chan A., Kung S. Unpublished results.
11 Brown K., Prota G. Cosmetics & Toiletries, 109, 1994, 59-60, 63-4.
12 US patent 6,958,080, October, 2005.
13 US patent 5,849,041, December, 1998.
14 US patent 7,060,112, June, 2006.
15 US patent 7,026,447, April, 2006.
16 US patent 6,372,489, April, 2002.
17 US patent 6,224,901, May, 2001.