Like skin, hair is susceptible to damage from exposure to the sun. This damage affects its aesthetics and mechanical and physical properties, resulting in dull, flat hair. Research indicates that both UVA and UVB radiation are responsible for this damage. Hoting,1 for example, studied hair tresses dyed red by exposing them to UVA and UVB radiation, finding that the tresses underwent more fading after UVA irradiation.
Artificial hair color based on pyrazole intermediates also has shown significant fading after shampoo washing and UV exposure.2 Hoting thus concluded that exposure to UVA radiation resulted in a significant and perceivable change in the dyed red hair color. According to his research, the red pigment introduced into the hair fiber by the dyeing process acts as a photoreceptor, absorbing photons and photochemically degrading after UVA and visible light irradiation.
The effects of UVB radiation on hair also can be severe, resulting in the breakdown of disulfide bonds both inside the hair fiber and on the surface of the cuticle.3 Such disruptions to the disulfide linkages of the hair fiber can have a major impact on hair’s mechanical properties, resulting in the loss of tensile strength and an increase in porosity and irregularities on the hair surface.
To protect hair from UV damage, UV filters deposited on the hair surface absorb incident UV light. As noted, since both UVA and UVB radiation cause severe and perceivable damage, broad-spectrum UV absorbers provide the best protection from fading and sun damage. In addition to broad spectrum coverage, deposition and absorption onto the hair surface are crucial factors to consider when designing materials for rinse-off systems.
In the present article, the authors introduce a quaternized diestera designed to provide broad-spectrum protection to hair in rinse-off systems. An absorption spectrum of the diester solution (20.0 mg/L active in SDA-40), shown in Figure 1, ranges from the UVB region (280–320 nm) to the UVA region (320–400 nm). The absorption peak, located at 310 nm, falls within the UVA region, thus researchers concluded the material should provide efficacious color protection for dyed hair. Additional studies (data not shown) examined the material’s stability after five days of UV exposure, showing no change in absorbance or net weight for the diester.
Thus, further studies were warranted to examine its ability to protect hair fibers from cuticle damage and reduced tensile strength resulting from degradation of tryptophan and proteins on the hair surface and inside the hair fiber. The ability of the diester to protect hair from colorfade in the presence of UV radiation also was examined.
Materials and Methods
Various protocols were used to evaluate the diester’s ability to protect hair fibers from UV damage. Bleached European brown hair tresses were dyed with either a commercially available medium brown hair color or medium auburn color for 30 min following the instructions provided in color kits. For testing of UVB damage, virgin dark brown hair was used. Some tresses were treated with control and diester-containing shampoos and conditioners (see Formula 1 and Formula 2). A subset of treated tresses were then irradiatedb for 3 hr on each side, totaling 6 hr-equivalent to one day of UV exposure4-with UV radiation (200–400 nm) at 50 W/m1 using a quartz inner filter and type “s” borosilicate outer filter. At the start of the study and at intervals equivalent to every five days of UV exposure-e.g., after every 30 hrs-tresses were treated again with control and diester-containing shampoos and conditioners. Irradiated tresses were exposed to the total equivalent of 15 days of UV exposure. Test conditions were maintained at 40°C and 65% relative humidity.
Different types of tresses were compared, including those that were dyed and not exposed to treatment or UV exposure; dyed and treated with the control shampoos and conditioners and exposed to UV; and dyed and treated with shampoos and conditioners containing the diester and exposed to UV. Comparative color loss was determined both objectively and subjectively. The total color loss (∆E) was calculated by assessing the changes in L*, a*, b* readings on the treated tresses using a spectrocolorimeterc. The equation used to calculate the total color loss was:
∆E = [(∆L)2 + (∆a)2 + (∆b)2]1/2
The changes in hair color indexes were calculated using the respective values after the shampoo and conditioner treatments and before UV exposure; and after the equivalent of 15 days of UV exposure. The results of the instrumental evaluations were confirmed via subjective testing. The swatches were photographed and compared to dyed but untreated swatches, which represent the initial dye color, to allow for visual comparisons. Panelists were also asked to rate the color protection effects in simple comparisons of the control and the diester swatches to the untreated control.
To determine the amount of damage on the hair fiber due to the UVB irradiation, several parameters of the hair fiber were measured. As noted, UV exposure to hair can break down the disulfide bonds in the hair fiber. Tryptophan is a key amino acid found on the hair surface and it decomposes under UVB irradiation.2
Surface tryptophan degradation is thus an indicator of damage to the disulfide bonds and it can be quantified through comparative fluorescence intensities of hair tresses using a spectrofluorometerd. The excitation wavelength was set at 295 nm and the tryptophan emission intensity was measured at 340 nm. The emission intensity was assumed to be directly proportional to the surface tryptophan concentration.
Using surface tryptophan intensities of the hair sample before UV exposure as the baseline, the tryptophan protection factors of hair samples treated with diester shampoo or the control shampoo after UV exposure can be calculated using the equation:
F (Tr) = [If / Io] X 100%
In this equation, F(Tr) is the tryptophan protection factor, calculated as a percentage; If is the surface tryptophan fluorescence intensity of the hair sample after UV exposure; and Io is the surface tryptophan fluorescence intensity of the same hair sample before UV exposure.
UVB rays not only decompose the surface tryptophan but also penetrate the hair cuticle and cause damage inside the cortex. This damage affects hair’s mechanical properties and can be measured in terms of its tensile strength (stress-to-break). Using a tensile testere, the tensile strength of single fibers from the treated tresses was measured before and after UV exposure.
UV Protection with Diester
UVA protection in shampoo: Auburn-colored tresses were treated with the control and diester shampoo and exposed to 15 equivalent days of UV exposure (see Figure 2). The treated tresses were visually compared to one another and to a dyed tress that was not exposed to UV irradiation
(Figure 2a). The tresses treated with the control shampoo (Figure 2b-c) lost a significant amount of color while the hair tresses washed with the shampoo containing the diester (Figure 2d-e) showed less color fade.
The total color loss (∆E) of the tresses treated with the shampoos was also assessed after 15 days of UV exposure. Tresses that demonstrated better color protection had a smaller ∆E. The quantitative color loss data confirms what the visual assessment illustrates—that the shampoo containing the diester had a significantly smaller ∆E, and the diester-treated tress maintained more color. The diester thus demonstrates color protection with a ∆E of 10.5, compared to the control shampoo with a ∆E of 12.4, a difference of almost 2 units (see Figure 3).
To confirm the correlation between visual inspection of images, color index measurement and consumer perception, 17 panelists selected the tresses they thought had the most and least change in color after tresses were exposed to 15 days of UV. Subjective evaluation results indicated that 100% of the panelists perceived that the diester shampoo gave superior color protection on the medium auburn treated hair (data not shown).
A similar study was performed with the shampoos on hair dyed with medium brown color. The tresses shown in Figure 4 were treated with the control and diester shampoo. Hair tresses washed with the diester shampoo (Figure 4e-f) showed less colorfade than the control shampoo (Figure 4c-d). The left two tresses were dyed and not exposed to UV (Figure 4a-b), to serve as a reference.
Figure 5 shows the quantitative ∆E total color changes of hair tresses colored medium brown that were treated with both shampoos and exposed to
15 days of UV exposure. Hair tresses treated with the diester shampoo showed less total color change, almost 3 units, than the control shampoo, indicating the diester shampoo provided better color protection.
Again, the panel evaluation demonstrated a strong correlation between the visual results and the instrumental results. On medium brown hair, 94.1% of panelists chose the tresses treated with the diester shampoo as having retained the most color.
UVA protection from a conditioner: Another set of auburn-colored tresses was treated with a rinse-off conditioner containing the diester (Formula 2) and the same conditioner without the UV filter to serve as a control.
Figure 6 shows that hair tresses washed with the diester conditioner (Figure 6d-e) showed less colorfade on auburn-colored hair than the control conditioner (Figure 6b-c). The left tress was dyed but not exposed to UV radiation (Figure 6a) to serve as a reference.
Although not as definitive as the shampoo studies, 64.7% of the panelists preferred the diester conditioner on auburn-colored hair.
UVB protection from a shampoo: To assess for surface tryptophan damage, the surface fluorescence intensities of virgin brown tresses treated with either the control or diester shampoo were determined after 15 equivalent days of UV irradiation (see Figure 7) and compared to the fluorescence intensities of the hair before UV exposure.
The shampoo containing the diester showed 97.48% tryptophan protection after the 15 equivalent days of exposure, while the control shampoo showed 91.42% tryptophan protection. The addition of the diester to the shampoo improved the protection against degradation of amino acids in the hair fiber. The tryptophan degradation testing was not performed on the conditioner series; however, researchers anticipate similar results would be seen.
Testing UVB Protection with Tensile Strength
Fibers showing a decrease in stress-to-break forces after UV exposure have undergone a loss in mechanical strength primarily due to the breaking of disulfide bonds inside the hair cortex. A comparison of the tensile strength of virgin brown hair treated with the diester shampoo was made to hair similarly treated with the control rinse-off product. Note that disulfide bonds inside the cortex have been broken in the tresses treated with the control shampoo, while tresses treated with the diester have no disulfide bond breakage (see Figure 8). The diester shampoo protected the hair from UVB damage and preserved the hair’s natural strength.
Using the same methodology, virgin brown hair tresses were treated with the control and diester conditioner, respectively, for tensile strength measurements. The diester conditioner also provided protection against damage to the fiber (see Figure 9).
While protection of hair from the sun is not a new concept, technology continues to improve to offer consumers better protection as part of their daily regimen. As shown, the diester is substantive to the hair and offers broad-spectrum protection in rinse-off systems. Although the diester has not yet been tested in leave-in systems, the authors expect the diester to provide good hair protection.
The addition of the diester in wash-off systems provides protection against UVA-induced colorfade as well as protection from UVB-induced disulfide bond breakage that manifests as a loss of hair strength. The diester also protects from the degradation of amino acids on the hair surface. This broad-spectrum protection gives consumers the freedom to enjoy the sun without the concern for hair damage. In addition, it offers protection for artificial hair color and fiber integrity.
Acknowledgments: The authors would like to acknowledge Marni Dexter, Sety Britton and
Abel Pereira for their hard work and contributions to this project.
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