Sustainability: Trends in Polymers and Surfactants for Hair Care

The present article considers some of the latest trends in sustainable surfactant and polymer technologies for hair care. It is not intended to be comprehensive but more of a look at a few bright spots in this emerging field.

Current Perspectives on Natural Surfactants

The growth of the sustainability and natural products megatrends has led to ever-increasing consumer appeal for greener products in the personal cleansing category.^{1, 2} Whether motivated by a sense of environmental and social responsibility or by the perception—sometimes false—that such products are safer and healthier, consumers are continuously seeking and preferentially purchasing so-called natural cleansing products. Accordingly, formulators must identify and employ functionally equivalent natural alternatives to the traditional surfactants used in cleansers, to provide aesthetically pleasing lather and detergency without irritation.

Unfortunately, there is no clear definition of what constitutes a natural surfactant, and in the absence of a standard definition, ingredient suppliers, finished goods manufacturers and nongovernmental organizations have been left to devise their own definitions of natural. However, there are common themes among these various definitions and most tend to hold that surfactants for natural cleansers should be: mostly or wholly derived from renewable, non-petrochemical feedstocks; environmentally benign and readily biodegradable; and absolutely free of any by-products or impurities that could pose potential health risks.

Natural surfactants: In the strictest sense, a natural surfactant should be a naturally occurring compound that does not undergo any synthetic chemical modification to achieve its surface-active properties. Such natural surfactants are produced by living organisms, i.e., plants or microbes, and are isolated and purified following extraction or excretion from the organism. Perhaps the most well-known natural surfactants are the saponins^{3, 4}— complex glycosides of triterpenoids or steroids extracted from plants such as Quillaja saponaria (soapbark tree), Yucca schidigera (Mohave yucca), Sapindus mukurossi (soapnut) and Sapindus saponaria (soapberry). Another notable example of natural surfactants are the so-called biosurfactants,⁵ which include sophorolipids and rhamnolipids. These surfactants are produced by yeasts and bacteria, respectively, via fermentation of biomass feedstocks, e.g., sugars, triglycerides, fatty acids, etc. Saponins and biosurfactants, with INCI names including Hydrolyzed Candida Bombicola Sophorolipids, Rapeseed Sophorolipids and Rhamnolipids, represent truly natural small molecule surfactants, yet these compounds often lack the performance and efficiency of man-made surfactants that are required for successful products. Additionally, they are difficult to produce sustainably at the scales required for widespread adoption. Given the energy- and water-intensive separation processes typically required to obtain these surfactants, it is doubtful they would contribute positively to the overall life cycle assessment of cleansing products. Ultimately, these uber-natural surfactants are generally relegated to use as secondary or tertiary surfactants in premium natural products due to their high cost and limited availability.

Naturally derived surfactants: Another market driven approach to formulating natural cleansers is the adoption of naturally derived surfactants. In this sense, natural indicates that the surfactant is prepared mostly or entirely from non-petrochemical, preferentially renewable feedstocks of plant, microbial or inorganic origin. By this definition, the most natural (i.e., 100%) surfactants are those whose hydrophilic head groups and lipophilic tail groups are naturally derived. Examples include sugars, sugar alcohols, polyfunctional organic acids, amino acids, hydrolyzed proteins and (poly)glycerols, which are then covalently linked to naturally derived lipophiles such as fatty acids or fatty alcohols to yield amphipathic molecules. Chemical reactions that may be employed to synthesize such naturally derived surfactants are generally limited to simple acid- or base-catalyzed hydrolysis or condensation reactions that produce benign small molecules (e.g. H2O, NaCl) as by-products and include (trans)esterification, (trans)amidation and etherification.⁶ Whether the cosmetic chemist works with natural or naturally derived surfactants, these materials behave just like petrochemically sourced surfactants in formulations.

Current Perspectives on Sustainable Polymers

As companies move toward more sustainable technologies, there is also significant interest in developing and using natural and naturally derived polymers to replace synthetic polymers. Currently this change is mostly concentrated in the hair styling and hair care area; perhaps skin care could come next.

As is generally known, styling products provide temporary benefits to retain shape and style in hair. They may be applied to damp hair after shampooing and/or conditioning, or to dry hair, and should be readily removed by water or shampoo. In this category, synthetic polymers offer advantages to the formulator such as water and/or ethanol solubility, good hold and film-formation. However, from a green chemistry perspective, the disadvantages of synthetic polymers are most notably their lack of sustainability, renewability and biodegradability. And while natural styling polymer alternatives, including starches, polysaccharides and gums, are well-known to formulators in the industry, so are their limitations; i.e., lack of solubility, high viscosity or other undesirable properties. In addition, synthetic polymers cover a wider variety of properties than the currently available naturally derived options, making some synthetic polymers irreplaceable in existing styling product formulations.

Starches: Current natural and naturally derived alternatives to traditional styling products are largely based on corn starch and modified corn starch but again, these materials are not soluble in ethanol, they form an opaque film, and they impart a stiff feeling on the hair. Additionally, concerns have been raised over growing practices diverting food sources to non-food applications. Furthermore, as formulators are likely aware, in shampoos and conditioners, high molecular weight cationic starches do not provide an appropriate level of conditioning or detangling to wet hair.

However, researchers have recently divulged that coacervates formed from certain cationic starches, having molecular weights in the range of approximately 850,000 to 15 million and charge densities in the range 0.2 to 5 milliequivalents/gram, as well as anionic surfactants in the presence of zinc pyrithione—i.e., in anti-dandruff shampoos—provide enhanced deposition of conditioning agents on hair without undesirable buildup on repeated shampoo applications.⁷ Other physical and/or chemical modifications can be applied to native starches to change or enhance their properties for specific cosmetic applications. Physical treatments mainly affect the granule structure, allowing the starch to become cold water-dispersible for manufacturing ease. This reduces the amount of total energy required to formulate, as starches not modified in this manner would require an extended cook time at 80–100°C to become dispersible in water. Chemical treatments may be used to lower the molecular weight of the starch molecule and/or to add functional substituent groups such as esters or ethers, to enhance solubility, stability and aesthetic attributes.

Gums: Early two-in-one shampoos were based upon the cationic cellulose derivative polyquaternium-10 and the cationic galactomannan guar hydroxypropyltrimonium chloride but recent patent applications disclose that guar gum is relatively costly. As a result, non-guar galactomannans have been introduced to two-in-one shampoos and also to conditioners.^8–13 In this context, another cationic galactomannan touted for its rheology modification and deposition is cationic Cassia.^{14, 15}

Galactomannans are polysaccharides that consist of a poly(mannan) backbone with galactose side groups. Galactomanans from different sources differ in the pattern of placement of the galactose side groups. For example, guar gum has approximately one galactose side group for every two mannose backbone units, whereas Cassia has only one galactose for every five mannose backbone units, and the galactose units are bonded to the C-6 position of the mannose unit. Higher galactose substitution leads to better water solubility; consequently, guar gum is soluble in cold water but Cassia gum is only sparingly soluble in cold and hot water.

Cationic modification occurs primarily at the 6-position of the mannose unit (Figure 1). The galactose side groups tend to shield the mannose C-6 position from cationic modification. The lower graft density of galactose in Cassia makes it easier to cationically modify Cassia galactomannan than galactomannan from guar. Paradoxically, cationic modification of the sparingly soluble Cassia galactomannan can render it to be more soluble than cationically modified guar galactomannan.

Cationic hydrophobically modified galactomannan ethers (specifically, guars) are water-soluble when the degree of cationic substitution is from 0.01 to 0.5 and the degree of C12–C32 alkyl substitution is below 0.001. If the hydrophobic substitution is raised from this tiny amount, the solubility in water is compromised.¹⁶

Biopolymers: Biopolymers including chitan, chitosan, xanthan gum and pullulan represent naturally derived materials that either in their natural or chemically modified state offer greater potential than starch to provide sustainable alternatives to existing synthetic polymers and enhanced functionality. These polymers are viable options for hair care formulations based on their solubility properties and potential for binding with hair keratin. Chitin is a polysaccharide comprised of β-1→4 linked N-acetylglucosamine units. It is found in many places throughout the natural world as the main component of the cell walls of fungi and the exoskeletons of crustaceans—e.g., crabs, lobsters, shrimps—and insects. Chitin is one of the most abundant naturally derived materials in the world. However, it is generally insoluble in most cosmetic solvents.

Chitosan is a polysaccharide comprised of β-1→4 linked D-glucosamine units. It is produced commercially by the deacetylation of chitin in varying degrees (60–100%) but can be found naturally, also in the cell walls of some fungi. The amino group in chitosan has a pKa value of ~6.5, thus, chitosan is positively charged and soluble in acidic to neutral solutions, and will bind with negatively charged surfaces.

Xanthan gum is a microbial polymer prepared commercially via the pure culture fermentation of Xanthomonas campestris. It is an anionic polyelectrolyte with a β-(1→4)-D-glucopyranose glucan (as cellulose) backbone with side chains of -(3→1)-α-linked D-mannopyranose-(2→1)-β-D-glucuronic acid- (4→1)-β-D-mannopyranose on alternating residues. Xanthan gum has a relatively reproducible specification as it is produced by fermentation. Each molecule consists of ~7,000 pentamers and the gum is less polydisperse than most hydrocolloids.

Pullulan is a polysaccharide polymer consisting of maltotriose units—three glucose units connected by α-1,4 glycosidic bonds—connected to each other by an α-1,6 glycosidic bond. It is produced from starch by the fungus Aureobasidium pullulans and differs from most polysaccharides in that it is easily water-soluble as a result of the low degree of hydrogen bonding in its crystal form. In a recent patent application,¹⁷ pullulan with up to 35% ethanol in water is used to provide naturally derived hair fixative agents with enhanced drying times. In relation, it was found that drying times decreased with an increase in alcohol.

Although the naturally derived polymers discussed here have potential in hair styling applications, none are a perfect replacement for synthetic polymers. There is opportunity in the industry for the identification of new chemical approaches to add cosmetic functionality to naturally derived polymers—specifically in areas where no naturally derived polymer alternative for styling formulations has been found. Perhaps the greatest opportunity is to develop naturally derived hair care polymers that can be considered not only commodity formulation ingredients, but also that are truly fun.

Looking Ahead: Oil Gellants

One interesting and useful class of naturally derived materials is oil gellants, which are derived from the plentiful resource of pine tree extract. These materials are offered under the INCI names: Ethylenediamine/Stearyl Dimer Dilinoleate Copolymer, Ethylenediamine/Hydrogenated Dimer Dilinoleate Copolymer Bis-Di-C14-18 Alkyl Amide, Bis-stearyl Ethylenediamine/Neopentyl Glycol/Stearyl Hydrogenated Dimer Dilinoleate Copolymer, Polyamide-3, Polyamide-4 and Polyamide-6. These vegetable-derived polyamides, produced using naturally occurring fatty acids, offer a broad range of functionality to personal care formulations as film-formers, water repellent agents, emulsion stabilizers, structuring agents and wetting agents. They also improve the compatibility of actives, specifically in sunscreen formulations. The versatility of these polymers is demonstrated by their ability to produce personal care products in many different forms, ranging from sprays to gels to aqueous emulsions and crystal clear sticks.

Figure 2 depicts the generalized structure of the polyamide polymer. The origin of the different classes of chemistry lies in the selection of diamines and terminators used in the condensation reaction.^18–26 Hence, some polyamides are relatively nonpolar and some are nearly water-soluble. When formulating, one’s polymer selection can be made based on its solubility parameter relative to the other ingredients in the personal care formulation. In this respect, it important to consider all three components of the Hansen Solubility Parameter²⁷ in order to optimize for best compatibility—i.e., the energy from dispersion bonds between molecules, from dipolar intermolecular forces and from hydrogen bonds between molecules.

These polyamide polymers have the ability to form shear-thinning, oil-based gels in which the modulus of the gel, from deformable gel up to a clear, hard stick form, is proportional to the concentration of polyamide used. This gelation mechanism (see Figure 3) is postulated to arise from a supramolecular hydrogen-bonded network.²⁸

Conclusion

As a concluding observation, within efforts toward sustainable, greener materials, hydrogen-bonded supramolecular structures are the focus of emerging self-healing material technologies.²⁹ Thus, self-healing, easily restructured hair fixatives are a distant possibility as these technologies advance. Moreover, hydrogen-bonded supramolecular structures should be easily removed by shampoo.

References

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