December 6, 2022

Stability Testing of Solid Cosmetics


Solid cosmetics are already a beauty industry staple: soaps and lipsticks are probably the most common examples. But their use is accelerating. For certain applications, solid cosmetics offer a few advantages over liquid formulations: these include convenience, higher concentrations, and compatibility with low-waste, plastic-free, or fully recyclable packaging.

With consumers increasingly choosing to consider the ecological credentials of products, cosmetics manufacturers are now rolling out new solid formulations for a wide range of cosmetic products, including shampoos, make-up, perfumes, deodorants, and even toothpaste.1

The Importance of Stability Testing for Solid Cosmetics

Stability – or a lack thereof – is a primary concern when developing solid cosmetics. It goes without saying that lipsticks that fade or solid perfume that smells worse over time are not desirable products.2

Personal care products that are not stable can deteriorate on the shelf in several ways: for wax- or oil-based solid cosmetics such as lipstick, the two most common problems are the exudation of oil droplets and the formation of crystals of waxy compounds on the surface.

Instabilities such as these can be critical: cosmetic products must not only fulfill the same function each time they are used but must also look, smell, and feel the same. Fast deterioration of solid cosmetic products can render them unusable or unappealing to consumers – both of which are unacceptable outcomes for the manufacturer.

To prevent this, new solid cosmetic products must generally be subjected to rigorous stability and shelf-life testing before they are brought to market.

Traditionally, this is carried out by mimicking the worst-case conditions that the products might be exposed to on the shelf or in storage – for example, being left on a shelf exposed to direct sunlight. This is typically achieved through slow temperature cycling, repeated over long periods of time.

Common stability testing procedures include heating products such as lipsticks to 40 °C over the course of 12 hours, then cooling them to 4 °C over the next 12 hours, and repeating this cycle for between 1 and 6 months before the product is sold.

A new cosmetic product’s journey from initial research and development to quality testing and approval is already lengthy. However, this type of slow stability testing over weeks and months represents a serious bottleneck. The longer a product undergoes stability testing, the greater the chance of the manufacturer losing a critical market advantage. On the other hand, ending stability testing prematurely runs the unacceptable risk of mass-producing and selling an unstable product.

Accelerating Stability Testing

It is commonly assumed that stability testing of solid cosmetics can only be achieved by imitating the conditions under which they might deteriorate: that is, thermally cycling them over relatively long time periods. However, the process can be accelerated dramatically using the right technology. While it is impossible to speed up time, it is, in fact, possible to accurately anticipate how solid cosmetics will behave over longer periods.

This is where the Rheolaser Crystal – a unique microstructural thermal analyzer that enables accurate long-term stability analysis in a matter of hours – comes into play.

The Rheolaser Crystal relies on the Diffusing Wave Spectroscopy (DWS) principle: observing the scattering pattern produced when a laser is incident on the surface of a sample enables the device to monitor nanometer-size variations in structure. With minute changes in the structure having a great impact on the scattering pattern, the analyzer can accurately characterize the structural changes in the sample in terms of a characteristic timescale.


Learn More About Rheolaser Crystal

The Rheolaser Crystal combines DWS capabilities with a precise temperature control unit that enables thermal cycling of the sample, typically between 4 C and 40 C. Stable formulations respond to thermal cycling with an equal signal across multiple temperature cycles – while unstable formulations show a variation in the signal which can be easily detected. Unlike conventional solid cosmetic stability testing, the Rheolaser Crystal enables measurements to be obtained in just a matter of hours rather than months; and samples can be quickly ranked in order of stability.

As the sample size is sufficiently big, the method is robust against inhomogeneities in your sample. The Rheolaser Crystal is engineered with usability in mind: analysis can be launched in a single click, and virtually any solid or semi-solid sample can be analyzed in easily adaptable sample holders. The analyzer works with samples as big as 5 g and as small as 0.05 g and provides temperature control from 4 C to 90 C.

The Rheolaser Crystal removes the quality control bottleneck for solid cosmetics, enabling manufacturers to accelerate their products’ journey to market. With nanometer sensitivity and precision temperature control, this is the fastest and most accurate way to monitor microstructure evolution in solid cosmetics.

At Formulaction, we are changing the ways in which manufacturers and researchers analyze the end-use properties of products. To find out more about the Rheolaser Crystal or to book a free demo, get in touch with a member of the Formulaction team today.

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