Welcome to our comprehensive series about solid cosmetics and formulations. This blog post covers the following elements:
  1. Solid product formats: The old, the new and the differences between formulations
  2. How to differentiate solid formulations?
  3. Hydrophilic and lipophilic sticks, bars and balms
  4. Loose powders, pressed powders and individual piece solid formulations
  5. Bulk vs individual dose dispensed formulations
  6. Challenges of formulating solid products
  7. Packaging and preservation: Two aspects that are on consumers´ minds
  8. Product formats to look out for

 

Water and cosmetics: From anhydrous formulations to water usage

Cosmetic products can be viewed from many angles in the context of water. Not only is the solvent a key ingredient which can be present to over 80% in formulations for e.g. toners or moisturizing mists, but it is also used during agriculture for many natural ingredients, extraction, steam distillation of essential oils, cleaning during and after manufacturing and many products due to their rinse-off format require water for their intended use as cleansers, conditioners, etc.

In the wider context of environmental aspects, focusing on the absence of water from product formulations in itself bears aspects that can be alluring.

While being anhydrous does not automatically translate into being solid, many water-free products come in a solid format. This opens up the possibility for individual dosage and pre-dispensed bulk formulation as well as a potential for substantial (plastic) packaging reduction. Many consumers are now (questionably) seeking products without or with less preservation which is a possibility for products that are formulated without water.

For the scope of this article, we will focus on solid product formats and only make mention of anhydrous, liquid formulations while it should be noted already here that not all anhydrous products are automatically solid in nature.

Solid product formats: The old, the new, and the differences between formulations

Many cosmetic products that have their historical origin as a solid have gone through an evolution into a liquid format, many of which are now experiencing somewhat of a revival as a solid.

Bar soap, mattifying and coloring, deodorizing or perfuming face, body, and hair, as well as cleansing tooth powders, are some historical examples which might be more commonly known to have evolved into various types of formats like shower gel, shampoo, liquid foundation, roll-on or spray deodorants and fragrances or toothpaste.

However, many more products found their way onto the cosmetic market in a solid format, which today are available in various liquid or semi-solid states. Mascara, which started as a solid cake formulation with pigments dispersed in a matrix of waxes and butters is one example and so is nail color, which used to be a mixture of pigment and abrasive powders sometimes dispersed in an oily or waxy matrix which could be used to impart color and shine onto the nail plate by buffing it with a brush in its early days.

Some product formats, like lipstick, which represents a very common formulation type for solid cosmetics, where an active ingredient or pigment is dispersed or dissolved (in the case of an active) in an at room-temperature solid, oily base, has endured over centuries and is still a popular format for solid as well as anhydrous cosmetic products today.

Many products can be transformed into a solid formulation, with the nowadays available variety of functional ingredients offering an ever-growing space for formulators to explore.

How to differentiate solid formulations?

The main differences for solid cosmetics lie in the type of their matrix or base (lipophilic or hydrophilic) in their coherence i.e. loose powders vs. e.g. bars, sticks, or cakes as well as their availability as bulk or pre dispensed units like e.g. single-dose sachets, tablets, sheets, pods or pellets.

Solid is a state of matter, in which the cohesive forces between particles constituting the material are strong enough for the material to have a stable, defined shape and volume when at rest.

Water, with its main function as a polar solvent in cosmetics, has the ability to impart liquid properties to or support the development of a liquid crystal state in a formulation. Its absence however does not automatically translate into a solid formulation as other, non-aqueous, polar solvents like for example Propanediol can be used in its place. As such it is possible to create products without water, but yet with a polar base, which can assume either a solid or liquid state depending on the type of formulation.

Hydrophilic and lipophilic sticks, bars, and balms

The defining boundaries between the following categories are fluid and always depend on individual formulations. Formulators will certainly find overlaps and association with different groups depending on their recipe.

Solid formats with a polar base e.g. a stick can be formulated with the help of wax-like polar ingredients such as higher molecular weight Polyethylene Glycols, which provide a solid structure while bringing along their hydrophilic nature.

Traditional soap bars as well as syndets, which are defined as “synthetic detergents” as opposed to soaps, derived from the saponification of lipids, are often comprised of a majority of surfactant materials, which allows for the relatively easy incorporation of oils like Almond oil, butters as well as humectants like glycerin. Formats such as the recently more and more popular shampoo and conditioner bars can fall into this category. This type of product usually requires the co-use with water in order to generate a lather or an emulsion to be used for cleansing and conditioning purposes.

More traditional, lipid-based stick-type, bar and balm formats often utilize combinations of waxes (Wax flakes & Candelilla wax), oils and butters, to create a solid texture which once applied to the skin melts at body temperature to create a soft film on the application surface.

This very popular format is used for many product types including solid fragrances, skincare sticks, and balms including SPF formulations, lotion bars, lip-balm, cake mascara, stick foundations, and most famously traditional lipstick.

Loose powders, pressed powders, and individual piece solid formulations

Another way of looking at solid products is their degree of aggregation. We can distinguish between loose powders, which can vary according to their particle size and particle size distribution from very fine, homogenous powders like baby powder to more coarse aggregate materials like bath salts. Loose powders have been used for many years for various purposes including, deodorants, fragranced powders, cleansers, makeup, nail color, hair styling, oral care, and even depilatory powders.

They are often mixtures of active ingredients, fragrances or pigments blended with a matrix or bulking base to provide a vehicle. Common bases in loose powders include Talc, Kaolin, Calcium Carbonate, Magnesium Carbonate, Metallic soaps like Zinc Stearate, Magnesium Stearate or Starch. Different types of polymers like Nylon-12, or silica beads can be used as texture modifiers or to alter the free-flowing properties of a powder bulk. Coarse products with the intent to be dissolved (like bath salts) are often formulated with the help of inorganic salt-crystals like Sodium Chloride or Sodium Phosphate.

The downside of loose powders

Loose powders can be difficult to dose and handle by the consumer, which is why specific packaging considerations with respect to opening and closing as well as dispensing mechanisms are often required to be taken into account during product development. The addition of sieve or an additional snap-cap can be necessary or at least of an advantage. Depending on the formulation loose powders can be sensitive to humidity and show aggregation or caking tendencies if hygroscopic ingredients are used to a substantial degree.

Pressed powders can provide an easier to handle alternative to loose powders as they stay put in their pan upon opening without the risk of spillage or dust development. Compared to loose powders they require additional binding ingredients that aid the cohesion of the pressed powder in its pan for it not to crumble and disintegrate- turning into a loose powder- during use. Fatty soaps as mentioned above, Kaolin or Synthetic Wax can be used for this purpose.

Pressed powders require an applicator such as a sponge or a brush in order to be delivered. This poses two limiting factors: the need for an applicator itself as well as the limited amount of product that can be used at once. For the application of larger amounts of formula such as e.g. baby powder a pressed powder would potentially deliver too little product. Also, the repeated contact with the specific application site (an infants´ nappy area) with a brush or applicator could pose substantial microbiological challenges and risks that need to be considered.

Solid products that are formulated as individual pieces, such as the previously mentioned soap and syndet bars, shampoo bars, lotion bars as well as the popular format of bath bombs provide the opportunity to minimize packaging materials. Many of these product formats are sold “naked” (without any packaging) which can potentially lead to stability challenges as well as difficulties providing the (per regulation) required labeling and product information.

Bath bombs or bath fizzers are often based on a combination of a Carbonate (Sodium Bicarbonate) and an acid, which when in contact with water, engage in an acid-base reaction, leading to the release of Carbon dioxide which results in the disintegration of the fizzer. The addition of colorants, fragrances as well as foaming agents such as SLS can enhance the fizzing bath experience.

Bulk vs individual dose dispensed formulations

We have already discussed individually packaged (sticks, pressed powders, balms) and potentially unpackaged (soap bars, lotion bars, conditioning bars, bath bombs) solid product formats as well as briefly highlighted the limitations of loose powders.

Some of these limitations can be overcome with pre-dispensing (often used in the pharmaceutical industry) in order to circumvent dosage and handling challenges that come with bulk powders.

Available product formats that enable a pre-dispensed formulation include individual powder sachets (e.g. for at-home face-mask preparations), tablets (oral care, bath tablets), pearls, or spheres (face cleansers), and paper-thin sheets (soap sheets). Individually dispensed formulations still require packaging, but they also hold the advantage to be able to be taken on travel as individual units.

Challenges of formulating solid products

Each previously discussed product format poses its own individual challenge. It is difficult to generalize what challenges apply for all solid formulations as their constitution can vary greatly depending on the format. Products that are formulated with combinations of liquid and solid ingredients to provide a balm or a stick base can potentially experience “blooming” the recrystallization of solid components as well as liquid “sweating”- the formation of small liquid droplets on the surface of a stick formulation. Products intended to melt at body temperature will be sensitive to heat, while powder products, tablets, or bath bombs can be sensitive to humidity. Powder sedimentation in formulations that include dispersions as well as inadequate cohesion for pressed powders and tablets can pose further challenges that heavily depend on the product format.

Packaging and preservation: Two aspects that are on consumers´ minds

As mentioned before, the possibility for reduced packaging that lends itself to some solid product formats is an attractive aspect for consumers and the industry alike.

Another aspect that consumers sometimes (questionably) find attractive is the possibility to reduce the need for preservatives in solid products that have a low water activity. Microbial growth requires the presence of water. With the formulation of anhydrous products, the need for preservation decreases, which makes it possible for some products to be formulated without the addition of preservatives. The intended use, application area, and product type however sometimes still require preservation for safety reasons, even if no water is added to the formulation.

Product formats to look out for

This article gave a brief overview of the possible classification, some challenges, and advantages for the formulation of solid product formats as well as highlighting some key raw materials. Solid cosmetic formats are currently experiencing popularity partly due to the above-discussed possibilities for packaging material minimization as well as potentially reduced use of preservatives. The absence of water from anhydrous formulations is an additional seemingly relevant benefit that consumers are attracted to. We have however not discussed here, how water is used during the manufacturing of these product types as well as how these products impact the consumption of water during use. A product that contains water, but requires less water during manufacturing and due to the use of technology that is easily rinsed off during use by the consumer, might have a much bigger impact on overall water use, than simply its absence from the formulation.

Another type of product we have not discussed here are products that can be reconstituted into aqueous formulations with the addition of water by the consumer.

Water use in the context of cosmetic products is a complex topic and requires a multi-angle review in order to pass judgement on the real impact that a product may or may not have on the consumer and on the environment. Its presence or absence in the actual product is only one of the aspects that need to be considered.

Easy rinse and water-use conscious products

Reconstitution product

 

 

Literature

  • Cosmetics Business, Blue Gold: Water in Cosmetics, July 15, 2019 viewed November 2, 2020
  • Spitz, L. Soap history, marketing and advertising. In Soap Technology for the 1990s. Champaign, IL USA: American Oil Chemists’ Society 1-47.
  • Flick, E. Cosmetic and Toiletry Formulation, Volume 8, 2nd Edition, Noyes Publications/William Andrew Publishing, LLC 2001
  • Barel, A., Paye M., Maibach H., Handbook of Cosmetic Science and Technology Marcel Dekker, Inc. New York • Basel, 2001 pp 485, pp 645

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