Principles of Shampoo Formulation
Modern hair care has seen an increase in demand and interest in recent years, driven by continued consumer desires for products that address their individual hair needs, as well as conditioning and scalp care.
Formulas have expanded to incorporate greener ingredients, along with a consciousness of skin barrier health and other developing hair health aspects. This article describes the components of two basic hair product types, shampoo and conditioner, for modern needs. Specific considerations include scalp and barrier health, greener ingredients and hair shine. To illustrate how such products interact with hair, however, a brief review of hair biology is first described.
Hair Biology Basics
As many readers know, hair is structured much like a cylinder composed of coaxial layers and consists of an outer visible part, the shaft and an internal part rooted in the skin, formed by the bulb and matrix. The shaft comprises three sections with cells containing different types of keratins; these basic proteins form hair as well as the nails and the stratum corneum of the epidermis. Keratin fibers elongate by 0.3 mm per day, which amounts to one centimeter per month, multiplied on average by a staggering 100,000 hairs.
All products of sebaceous and sweat secretions converge along the hair shaft and, as sebum ascends, it also absorbs odorous substances and pollutants from the environment. Continuous processes of epithelial cell desquamation occur, sometimes imperceptible to the naked eye and other times manifesting as dandruff.
The matrix and bulb are made up of cells in reproductive activity, multiplying as they move upward, giving rise to the shaft. Surrounding the bulb are capillary blood vessels, from which the cells derive nutrients. Melanocytes are also scattered around, embedded in the hair scaffolding, and are responsible for color.
Hair has a life cycle alternating between phases of activity and reproductive rest. The cycle of each bulb appears to be independent of those surrounding it, and unlike the seasonal molts of animals, humans do not shed their hair all at once under normal conditions.
The well-being of the hair derives not only from a healthy organism, but also from the balance of the scalp, its secretions, blood circulation and resident microflora (bacteria, yeasts and fungi). The properties of the shaft, composed of non-vital cells, depend on both environmental exposure and cosmetic treatments.
Hair Care Types
Trichological products can be grouped into three main classes: hair cleansers, treatments for hair and scalp anomalies, and aesthetic and styling products. Regarding the first class, today’s formulation research focuses on the use of surfactants with low irritant power — e.g. sulfosuccinates.
The second class includes mainly lotions for the treatment of dandruff, seborrhea and alopecia. These are formulated to ensure the high transdermal absorption of active ingredients.
Finally, the third class encompasses a wide range of combing and styling products. Examples include conditioners, masks, lotions, fixatives and gels.
A different approach to classifying hair products is to divide them by their intended rinse-off or leave-on application. As one might expect, rinse-off formulas would include traditional shampoos and some types of conditioners; leaveon formulas, intended to remain in contact with the hair and scalp for an extended period, could comprise lotions, hair oils, fixatives, etc. The present article delves into the structure and function of basic rinse-off shampoos and conditioners.
Hair and Scalp Cleansing
Today, hair and scalp cleansing products are oriented toward mildness to better maintain cutaneous structures. The ability for a product to visibly improve the mane from the first application, giving it a natural and healthy appearance, is indispensable.
Washing is no longer just about removing dirt; people desire easily combed, soft, shiny hair. Product formulation must consider the continuous stress endured by the hair due to various treatments and environmental conditions (e.g., sun, heat, washing, chlorine in tap water and swimming pools, styling, coloring, etc.) disrupts and alters its keratin, depleting useful lipids and minerals and replacing them with pollutants.
Shampoo properties and sensory effects: An ideal shampoo must move away dirt and sebum without being too aggressive and degreasing to the scalp while respecting the skin’s pH. It must have good foaming properties, possibly with a low surfactant content. The foam must be fine, dense, soft and sufficiently stable, yet easy to rinse off. Indeed, poor, incomplete rinsing is the main cause of scalp irritation, itching and intolerance.
The hair, once dried, should be soft, voluminous and shiny, and have good dry combability and an antistatic effect. A good shampoo must also scent the environment and deodorize the hair for an extended period.
The combination of all these properties (i.e., sensory, technological and functional) can be achieved through the careful selection of ingredients and appropriate balancing of their properties (see Typical Shampoo Formula). The purity of the raw materials used is also essential; metallic impurities and common allergens must be minimized.
Surfactant and ingredient selections: To build a shampoo formula, first, deionized water is typically used to avoid any coloring or pro-oxidant effects due to iron salts and any precipitation due to calcium and magnesium ions in hard water. The main surfactant, generally anionic, acts by carrying out the dissolution or micro-emulsion of surface lipids and environmental dirt deposited on hair and scalp. It should also contribute to the formation of abundant foam during massaging into wet hair. This is the reason why one of the most widely used primary surfactants is sodium lauryl ether (laureth) sulfate (SLES).
Sulfate-free formulas: In fact, in sulfate-free combinations, obtaining the desirable viscosity with low surfactant content is more complex, and modern formulas usually contain a mix of three or four corrective surfactants.
Skin barrier health: A balance among anionic, amphoteric and non-ionic surfactants seems to be the best formulation strategy in terms of maintaining skin barrier health and reducing protein damage and irritation potential. Indeed, some surfactants (i.e., anionics) are harsh toward proteins such as skin keratin, while others are harsh toward lipids (non-ionics and amphoterics). In practice, the addition of an amphoteric surfactant at about 20–30% with respect to the anionic will reduce charge density, thereby stabilizing the micelles.
Foam stabilizers: Foam stabilizers are needed to give volume, stability and firmness to a product’s foam. Additional useful effects include increasing viscosity, skin conditioning and hair shine. Ingredients like Coconut fatty acid diethanol amide, Coconut fatty acid mono ethanol amide, and soya fatty acid diethanol amide help to stabilize the foam in formulation.
pH Balance: The formulation must maintain a pH level close to that of the scalp (approximately 5-6) to preserve its natural barrier and prevent irritation.
Rheological additives: Rheological additives are added when sufficient viscosity has not been achieved with the mixture between surfactants and foam boosters. The proper viscosity is interpreted by the consumer as a richness and high concentration of ingredients in a formula, but also the suitable distribution of the product from the container to the hand thanks to easy dilution with water and massaging onto the hair — without dripping into the eyes.
The cheapest is sodium chloride, which is already contained as a reaction by-product in the manufacture of many surfactants. It is not advisable to exceed 1.5% of total salt content in cleansers, however, due to possible risks of precipitation at low temperatures and insolubilization of fragrances and active substances over time. Furthermore, the viscosity/salt content curve often has a maximum beyond which values collapse.
PEG-150 distearate has been used extensively as a thickener, despite the need of preparing a pre-solution of it before adding it to the formula. One main advantage is the high transparency and brilliance it gives to the cleanser, even if it does not enhance foam.
A note on hydrophilic polymers: Hydrophilic polymers that are swellable in water are rarely used because they can interfere with manufacturing time and they often compete with surfactants for water coordination. Some of them, e.g., xanthan gum, are used to suspend insoluble particles or optical additives such as opacifiers (acrylic or acryloyl-styrene polymers) or pearlizers (surfactant mixtures with crystallized stearic acid derivatives); optical additives also include all soluble dyes.
Packaging, fragrance and additional properties: Often packaged in transparent containers, shampoos must be light-stable, a task charged to UV filters added either in the formula or in the plastic material.
Perfumes also play a key role in the success of commercial products. The scent notes released by warm water in the shower send a sensory message of pleasantness and enchantment to the user. Generally, shampoo fragrances do not require solubilizers because the surfactants are able to micro-emulsify them in the formula. However, to improve solubilization or ease of addition during the production process, they can be pre-mixed with foam boosters. Mixtures of odorants must be prepared in consideration of the hydrolytic effect of the surfactant system. Often, their introduction causes significant changes in the rheology of the system by decreasing the viscosity.
Lastly, one or more conditioning ingredients may be incorporated into today’s cleansers. These act by depositing on the hair, forming protective and shiny films capable of repairing the surface of keratin structures.
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