The science between this popular ingredient commonly found in sun-protective hair products.
Polymer scientists continually collaborate with cosmetic chemists and formulators to develop new molecules designed to overcome limitations that exist with currently available ingredients. One example of such work is Polyquaternium-59 (Crodasorb UV-HPP), a cationic polymer which is now being used in a number of commercially-available skin and hair care formulations, including some products by Ouidad. Since the structure, function, and performance of the polymers in this category can vary so widely, as each is specifically tailored to meet a targeted need, it is worthwhile to take the time to examine this particular polymer to gain an understanding of its potential benefits to both the consumer and the chemist in a formulation.
What is it?
Polyquaternium-59 is a polyester molecule that has quaternized ammonium sites (positively-charged) along both its backbone as well as in pendant groups attached to the chain.
(IUPAC name: Poly(20,25-dioxo-2,5,10,15,18-pentamethyl-10-(2-hydroxy-3-(3-(3-phenyl-2-propenamido)propyldimethylammonio)propyl)-10-azonia-1,4,7,13,16,19-hexaoxapentacosanediyl) chloride)
Compared to many cationic polymers, it is of relatively low molecular weight, averaging 5000 grams per mole, and according to the manufacturer, Croda, it is 65% active, which means it has a high level of charge density relative to other polyquaternium materials used in cosmetic applications. This enhances the water solubility of these polymers and also increases substantivity to hair, and thus improves conditioning properties. The high degree of water solubility also means this polymer is suitable for cold mix processes, which is appealing to formulators from both a cost-saving perspective and from an environmentally friendly manufacturing paradigm. [i]
The novel twist to the polymer structure in PQ-59 is the inclusion of groups capable of absorbing UV radiation at the ends of the pendant groups. (For those interested in the specific organic chemistry, these groups are a carbonyl group conjugated diene/ aromatic moieties ). These portions of the molecule transform the harmful, high energy UV radiation into a lower energy form (infrared) that is emitted as heat. This sun protection quality is perhaps the most valuable contribution this polymer makes to any personal care product formula.
Sun Protection for Hair
The surface of human hair is highly hydrophobic, which helps to seal moisture into the hair shaft, protect it from the environment and mitigate effects from fluctuations in humidity that can cause structural damage.
Exposure to ultraviolet radiation occurs whenever we go outside. We are all well aware of the importance of limiting skin exposure to these harmful rays, but it is less well known the extent of damage they do to hair as well. UV-B radiation (280-320 nm) cleaves disulfide bonds (S-S) in the cuticle, depletes cystine in the structure, and thus damages the protein structure of the protective cover of the hair strand. This increases surface roughness and porosity, which results in frizz, tangling, and ultimately, breakage. Breakage of disulfide bonds can also lead to frizz and unmanageability as it disrupts curl structures. [ii]
UV also depletes the protective lipids found on the surface of the cuticle. This increases combing forces necessary to detangle hair, which generally results in formation of split ends and breakage.
The surface of human hair is highly hydrophobic, which helps to seal moisture into the hair shaft, protect it from the environment and mitigate effects from fluctuations in humidity that can cause structural damage. UV-B breaks down tryptophan found in the protein structure of the hair and creates a more highly negatively charged surface, which becomes more hydrophilic and less capable of moisture retention and more susceptible to ill effects from the environment.
Ultraviolet radiation also penetrates into the cortex of the hair where it breaks down protein structures within the hair strand as well, compromising the mechanical integrity of the hair. This results in a lower tensile strength for the hair, and so it breaks more easily. UV-A radiation in the cortex reacts with both natural melanin pigments and chemical dye molecules, causing photobleaching and yellowing, both definitely undesirable effects.
Clearly, protecting hair from damage from ultraviolet radiation is a desirable goal, for health, strength, luster, color retention and overall beauty of our tresses. This is especially true for longer hair, as damage is cumulative. Since wearing a hat everywhere does not seem like a fashionable solution, chemists and formulators have been experimenting with different ingredients for a while. Some of the limitations of common sun protective ingredients in current use are that the smaller molecules are not very substantive to hair, and they are often very greasy. For this reason, polyquaternium-59 was developed to overcome some of the deficiencies of other options. Its greater charge density as a cationic polymer enhances its substantivity to the surface of hair, which carries a slight negative charge. Also, as a lighter weight, water-soluble polymer, it has no greasy tactile sensation.
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