Quantifying the performance of hair colorants is essential in developing new products and confirming claims made about their properties. This article describes an approach that can be used not only to compare hair dye colour but also allow the response of colorants to washing and exposure to light.
As an example, the difference in colour of some blue dyes within the Jarocol range have been explored and their resistance to washing tracked using the technique. It is also demonstrated how the photostability of the dyes can be successfully quantified.
The structure of hair
The hair fibre is made up of three parts, the cuticle, the cortex and the medulla (see Fig. 1). Within the cortex of the hair are melanin particles, and it is these pigment particles that are responsible for the colour of our hair. There are two types of melanin; eumelanin and pheomelanin. Eumelanin is largely responsible for brown and black coloration, while red hair contains mainly pheomelanin.1,2 The size, shape, distribution and concentration of melanin particles within the hair determine the overall colour. Changing the colour of hair can be a temporary, semi-permanent or permanent process, which will utilise different classes of dyes, depending upon the desired effect. Basic dyes are high molecular weight molecules carrying a positive charge. The positive charge of the dye forms a weak bond with the negative charge present on the surface of the hair. As these molecules are too large to penetrate into the hair shaft, they only coat the surface of the fibre and therefore are washed off easily.3 Direct hair dyes are smaller coloured molecules that are able to penetrate into the hair. However due to their small size they can also be washed out of the hair gradually during shampooing, hence their classification as semi-permanent. Oxidative hair dyes are small colourless molecules, which fall into two categories: primary intermediates (bases) and couplers. These small molecules penetrate the hair shaft. The primary intermediate is oxidised upon addition of an oxidant, typically hydrogen peroxide; the oxidised molecule can then react with the coupler to form larger coloured molecules within the hair shaft. Because of their size, these larger coloured molecules are trapped within the hair, resulting in a permanent colour.2,4,5 When formulating with hair dyes one important factor to consider is the longevity of the colour on hair and whether it meets customer expectations. Fading of hair colour due to UV exposure and washing can occur when using dyes from any of the three classifications: basic, direct, and oxidative. While in some instances, fading is actually desirable, for example in temporary applications, in others, such as permanent vibrant red shades, it is seen as a negative effect. The cosmetic industry accepts that colour stability is an important consideration when formulating hair colourants and hair care products. This has given rise to the launch of many raw materials specifically aimed at reducing colour fade. As a result, claims such as ‘fade resistant’ and ‘longer-lasting colour’ now regularly appear on product packaging. It is therefore very important for manufacturers to be able to substantiate these claims with analytical data. Analysis of longevity can be performed visually. However, the perception of colour is very subjective, and therefore it is important to use analytical methods to allow discrete measurements to be taken. One of the most important mathematical approaches to make use of the concept of colour space for colour measurement and specification was introduced in 1976. The CIELAB colour space system enables colour to be quantified as points within a visually uniform three-dimensional colour space illustrated in Figure 2. The dimensions are specified in terms of L* for lightness and a* and b* for hue. The L* axis has a fixed scale of 100 to 0 running from top to bottom, where 100 represents white (lightness) and 0 represents black (darkness). The a* and b* axes have no specific numerical limit and represent greenness/redness and blueness/yellowness respectively; values of higher magnitude represent more saturated colour.6,7 CIELAB measurements can be taken using a reflectance spectrophotometer, also known as a colorimeter. The machine measures the intensity of visible light reflected from a sample, typically over the wavelengths 400 nm to 700 nm, using a standard light source.6 The colorimeter converts these measurements into the CIELAB system, expressing the colour of the sample in terms of L*, a* and b*. It is important to use an appropriate substrate for colour measurement to ensure that readings are accurate and consistent. Hair and wool were both investigated as they are similar protein based structural materials which allow dye uptake. However, when hair swatches were analysed they were found to produce inconsistent results due to their uneven cuticle surface. Worsted gabardine wool was found to be the most suitable. It is a tightly woven fabric providing a smoother, flatter, uniform surface from which to take analytical measurements. Being able to quantify colour is a useful tool for quality control and colour matching purposes, but it can also be used as the basis for producing stability data for the wash and light fastness of a dye or finished formulation. This article focuses on the methods used for obtaining L*a*b* readings to quantify wash fastness and light fastness. It also gives examples of how to interpret the results obtained.
Preparation of initial substrates
In order to demonstrate the shift in colour most effectively, simple aqueous solutions of dye were used. UV stabilisers or materials to improve wash fastness were not added. Direct and basic dyes are known to fade at a faster rate than oxidative dyes. Analysis was therefore undertaken on dyes selected from these categories. Jarocol Blue 12 (HC Blue 12), Jarocol Blue 2 (HC Blue 2), Jarocol Blue 15 (HC Blue 15) and Jarocol Steel Blue (C.I. Basic Blue 99) were chosen for the study to show the range of available blue dyes. Due to the different chemical compositions of the dyes involved in this article, each dye was used at its optimum stable pH level. Jarocol Blue 12 and Jarocol Blue 2 were analysed at pH 8 and Jarocol Blue 15 and Jarocol Steel Blue at pH 4. A 1% aqueous solution of each dye was prepared and pH adjusted as required using either citric acid or monoethanolamine. The solutions were then applied to equally sized squares of worsted gabardine wool for 60 minutes at 40°C. After 60 minutes, the wool squares were rinsed in warm water and allowed to dry. Once dry, L*a*b* measurements were taken using a Datacolor SP600 machine, with a D65 illuminant at a 10° angle of observer. This produced initial colour readings (Table 1). These initial results were plotted onto the colour wheel to show the position of each dye with respect to colour space (Fig. 3).
Results
The results in Figure 3 show HC Blue 15 to give a much deeper ‘true’ blue colour than the other dyes investigated. This is illustrated by the combination of the lowest L* value, a near zero a* value and a large negative b* value. HC Blue 15 is found much further into the blue region of colour space than the other dyes. By comparison HC Blue 12 is more violet in tone as indicated by its higher a* value coupled with a negative b* value. These results can be seen visually in Figure 4.
Wash fastness
Dyed worsted gabardine wool squares were washed using a 12 % active ammonium laureth sulfate solution and dried in an oven at 50°C. Once dry, L*a*b* measurements were taken. This process was repeated for 12 cycles and L*a*b* measurements taken after completion of 12 cycles, the results are shown in Table 2.
Results
The results in Table 2 show that all four dyes undergo some degree of wash out. As shown by the lower L* and more negative b* values, HC Blue 15 remains the darkest and most blue dye after 12 washes, the change in a* indicates that it becomes slightly more green after washing. In comparison an increase in L* value and the less negative b* value show that HC Blue 12 and HC Blue 2 become much lighter and more yellow in tone after washing. This can be seen visually in Figure 4.
Light fastness
A Xenon arc light chamber was used, exposing the dyed wool squares to UV and visible light (300 nm-800 nm) at an intensity of 550 Watts/m2 for 24 hours. This equates to 24 days of exposure to UK/Northern European sunlight assuming that:
• One day in Miami sunlight = 9.5 MJ/m2 (300 nm-800 nm).
An irradiance level of 550 W/m2 (300-800nm) = “Miami Peak Sunlight”
• One hour exposure in Xenon arc light chamber at 550 W/m2 (300 nm-800 nm) = 1.98 MJ/m2.*
• One day exposure = 9.5/1.98 = 4.8 machine hours. [*Q (radiation in J/m2) = I (irradiance in W/m2) x T (secs)].
One year exposure to Miami peak sunlight is equal to five years exposure to UK/Northern Europe sunlight we can calculate that:
• One day UK/Northern European sunlight – 4.8/5 = 1 machine hour.8
L*a*b* measurements were taken after 24 hours’ exposure (equivalent to 24 days UK/Northern European sunlight) and the results are shown in Table 3.
Results Results in Figure 6 indicate that all four dyes undergo some colour shift when exposed to the Xenon arc light chamber. HC Blue 12 shows the largest change in lightness (L* value). In addition, a reduction in b* means that more purple tone is the result of exposure for 24 hours. The worsted gabardine wool results are shown in Figure 4.
Conclusions
Colour interpretation can often be very subjective. There are many factors affecting visual assessment including those depending on the viewer and the lighting conditions. Changes in colour strength and tone, related to dye performance cannot easily be visually qualified in different locations and by different operators. By quantifying the colour shade and strength values in a 3D-colour space such as CIELAB, a finite value can be assigned to a particular dye shade, and mapped or graphed over a period of light exposure and wash cycles. This can remove the ambiguities in interpretation and allow true appreciation of dye performance and benefits. Using the Vivimed Jarocol blue hair dyes as an example, this article shows how the different dyes available to a formulator can be quantified and how final colour can be affected by exposure to light and washing. This in turn can help guide the formulator to specific choices of dye intermediates that will answer the product brief with respect to initial colour, light and washfastness, as well as any formulation costing requirements. Such quantification will allow a formulator to more accurately track performance and assess colour matches. Improving hair colour uptake and increasing longevity of colour continue to be researched. As new raw materials are launched, it is important for formulator to be able to quantify these improvements in order to substantiate product claims – the approach described here will allow the formulator to do just that.
References
1 Swift JA. Fundamentals of human hair science (Cosmetic Science Monographs No 1) 1997; 41-57. PPCC 2 Morel OJX, Christie RM. Current trends in the chemistry of permanent hair dyeing (Chemical Reviews) 2010. 3 Corbett JF. Hair colorants: chemistry and toxicology (Cosmetic Science Monographs No. 2) 1998; 41-42. 4 Williams DF, Schmitt WH. Chemistry and technology of the cosmetics and toiletries industry 2nd edn 1996; 93-94. 5 Corbett JF. Hair colorants: chemistry and toxicology (Cosmetic Science Monographs No. 2) 1998; 1998; 7-8. 6 Christie RM. Colour Chemistry 2001, 21 7 Hunter Lab Applications Notes, Vol 8 No 7. 8 Atlas Suntest CPS+ Unit – Real time versus machine hours.
