EFFECT OF UV RADIATION ON HAIR STRUCTURE 105 well as daily shampooing and grooming practices, contributes to what is defined as hair damage. Although different irradiation methodologies were used, as well as different analytical tests, the results of the studies herein reviewed indicate very similar findings. All results are summarized in the Conclusion section of this paper. OPTIONS FOR PHOTOPROTECTION ON HUMAN HAIR In the previous section the effect of radiation on the structure of human hair was discussed in detail. It is apparent that there is unanimous consensus on the fact that UV or VIS radiation is capable of altering the chemical composition, morphology, and appearance of human hair. The following discussion will describe a few options for the photoprotection of human hair through the use of UV absorbers formulated in hair care products. Pande and Jachowicz (13) proposed two approaches to photoprotection of hair: (i) reducing the amount of rays reaching the hair structure (via use of UV filters) and (ii) changing the chemical environment that favors the photodegradation of Trp to one that quenches photochemical intermediates or retards the excited-state decomposition. The authors agree with the model according to which Try sensitizes the decomposition of other amino acids. In their studies it was found that a leave-on styling formulation containing 0.2% w/w of octylmethoxycinnamate (OMC) was successful at reducing the extent of Trp photo­ degradation up to 40%.f Also, the problem of delivering the UV filter to the hair from rinse-off formulations was discussed. The oil-soluble UV filters do not normally have affinity for the hair, and the deposition of oily ingredients or of oil-soluble ingredients from rinse-off products is very formulation-dependent. This paper concluded that in many cases UV filtersg will only deposit in minimum amounts from cleansing or conditioning rinse-off products, mainly due to their poor affinity for the substrate and the short contact time (e.g., hair-to-shampoo). This topic is of much importance since it is one of the determining factors in whether or not a formulation is capable of providing photoprotection to the hair. Other aspects of this discussion will be explored in the next few paragraphs. Bernhard et al. ( 15) described their work that involved five UV absorbers and two cosmetic formulations (hair-setting lotion and hairshine spray). The irradiation ener f y used was up to 29,880 Whm - 2 (Heraeus, Xenon test) (29,880 Whm - 2 = 107 x 10 J m- 2 ), and the total irradiation time was up to 30 days (RH 50%, 25°-45°C). Before these authors tested the efficacy of the sunscreens as applied to hair from a cosmetic formulation, they tested the stability of the sunscreens themselves as they were exposed to the radiation. The results indicate that after ten days of irradiation Benzophenone-3 was the most stable (90%), Benzophenone-4 was less stable (60%), and octyl dimethyl PABA and phenylbenzimidazole sulphonic acid were even less stable. The photostability f The protocol followed for the irradiation was 10 hr, followed by a shampoo. This was repeated three times to give 30 hr total irradiation time. g No cationically modified sunscreens were tested in this study these compounds are now widely available in the market and are designed for hair care, rinse-off application.
106 JOURNAL OF COSMETIC SCIENCE of UV absorbers is another determining factor in providing photoprotection. The con­ centration of the sunscreens tested in this work was 5 % solids, and the formulations were applied by immersing the hair in the formulations, to aid in the uniformity of appli­ cation. The test protocol included application of the formulation, drying, irradiation for three days, washing with 1 % SLS, rinsing, and drying. This cycle was repeated ten times during the duration of the experiment. Benzophenone-3 and -4 were shown to provide better protection than the other UV absorbers tested in terms of protection from melanin photobleaching and retention of tensile properties. The ability of UV absorbers to penetrate the hair shaft was investigated. Although none of the UV absorbers tested were shown to have diffused through the hair fiber, the distribution of the UV filters on the hair fiber surface was even greater in the case of the oily-shine spray formulations. Two different formulations, carrying the same UV ab­ sorber, showed different overall effectiveness towards protection of the hair from the radiation. A number of mechanisms may be responsible for that, including H-bonding, interaction of the UV absorber with PVP, and other possibilities that are discussed in the article. One may conclude that when different formulations are used as vehicles for UV absorbers for the protection of hair, overall efficacy does not solely depend on the UV absorber used and on its concentration, but is the result of complex interactions between the active material, the vehicle used, and the substrate. A very good point was made in this study, to investigate the stability of UV absorbers under UV irradiation. This factor could be a determining one when prolonged irradiation times are used and the test formulation is not reapplied to the hair samples. The impact of the choice of UV absorber on the formulation's appearance and perfor­ mance (other than photoprotection) is eloquently presented by Georgalas (16). This author comments on the challenges of formulating with sunscreens in hair care. The paper covers a variety of chemical classes of sunscreen and discusses a few points that are very relevant to formulation strategy: 1. In rinse-off formulations (shampoos and conditioners), oil-soluble sunscreens can be treated as if they were fragrances (need to be solubilized/emulsified). Water-soluble sunscreens can sometimes be more easily incorporated, although incompatibilities and pH-dependent solubility may be very important. 2. In styling aids (carbomer gels), some water-soluble sunscreens may have a salt­ thinning effect and the oil-soluble sunscreens may require solubilizers/emulsifiers, which may have a sharp impact on the thickness of the gel and its clarity (causing a haze or imparting a yellow coloration to the gel). 3. With hairsprays, the large amount of solvent, ethanol, makes it easy to incorporate sunscreens into these formulations however, during application, hairsprays deposit droplets onto the hair shaft. A large portion of these droplets stay as such and dry at the intersection of the hair fibers, and consequently a very small portion of the hair shaft surface is covered by the hairspray formulation. In summary, hairs prays do not form a continuous film onto the hair fibers therefore, they do not offer uniform surface protection. 4. Some sunscreens, Benzophenone-3 and -4, for instance, are beige, powdery sub­ stances there is a need to make sure that the sunscreen active does not recrystallize once the solvent from which they were deposited onto the hair (e.g., ethanol or water) has evaporated.
Previous Page Next Page