Beyond the Tube: Unlocking the Fascinating Science Behind Your Lipstick's Performance

Dr. Xuzi Kang, TRI Princeton

Postdoctoral Fellow

So, you've got your favorite lipstick, but have you ever wondered what makes it glide on your lips as smooth as a buttered penguin sliding down an icy slope? In the realm of cosmetics, lipstick stands as an iconic symbol of elegance and beauty. However, behind the simplicity of its application lies a complex world of scientific analysis. Today, we venture into the science of lipstick testing, unearthing how texture analyzers assess the smoothness and hardness of lipsticks and how the field of Differential Scanning Calorimetry (DSC) decrypts the mysteries of their melting behaviors.

The intricacies of lipstick texture

At TRI Princeton, Dr. Xuzi Kang has developed methods to measure the texture of lipsticks with a texture analyzer setup that is close to real life. The methods exploit a domain known as tribology, the science of interacting surfaces in relative motion, or in simpler terms, the study of friction, wear, and lubrication. Here, the texture analyzer's task is to emulate the contact between the lipstick and the lips.

In the tribology study, 1). the lipstick is applied onto a special substrate that mimics human lips’ texture and characteristics. A lipstick held by a tubal probe glides across the surface, recording the friction force. Lower friction suggests a smoother application, providing a pleasant, effortless user experience. 2). A needle probe penetrates the lipstick, recording the hardness force and adhesiveness force. Higher forces show the lipstick is harder and more adhesive, and lower forces show the lipstick is softer and less adhesive. A lipstick too firm might result in a difficult application process, and too much adhesiveness might result in a heavy or sticky application experience. 3). A transfer test is also performed to provide essential insights into its performance and wearability.

Figure 1. Lipstick friction setup and sample graph. The friction fixture is designed to drag the lipstick left and right along the special substrate. A tube holder is used to secure the lipstick. To mimic real-life usage, the lipstick is set at an angle of 60 degrees, and 80.0 g weight is added. Each stroke is 60.0 mm in length and three cycles were performed. The resulting graph is generated simultaneously.

Table 1. The table shows the net payout, initial stiction, friction, and friction reduction when layering different types of products. We can see from the data that most types of lipsticks offer decent payouts except the two special ingredient ones. Creamy 1 has the least initial stiction, which indicates that it is the easiest to begin sliding on the lips. Matte lipsticks show more friction than other types, which indicates a less smooth application. For most lipsticks, friction decreases when laying, but special-ingredient #2 shows increasing friction when laying, which is rare and not a desirable quality when using a substrate that mimics human lips.

Figure 2. Lipstick needle penetration setup and sample graph. A 1.0 mm diameter needle was used to puncture each lipstick three times, and the resulting graph is generated simultaneously.

Figure 3. Hardness and adhesiveness are shown for different types of lipsticks. The special-ingredient #2 lipstick is the firmest and the most adhesive, followed by the matte 1 lipstick; while shimmery 2 is the softest and least adhesive.

Figure 4. Lipstick color transfer setup and transfer results. Clean substrates were put on the application samples with the smooth side facing each other with a 300 g weight on top for 10 seconds to mimic kissing. Weight transfer is measured. Matte lipsticks transfer very minimally, while creamy and shimmery lipsticks transfer the most.

The melting mystery: unraveled by DSC

DSC stands as the key that unlocks the secret of lipstick’s thermal behavior. This thermoanalytical technique measures how a material's heat capacity alters with temperature, thereby shedding light on lipstick's melting characteristics.

During a DSC test, the lipstick sample is heated at a controlled rate in a furnace. As the lipstick starts melting, it absorbs heat, resulting in a difference in heat flow relative to a reference. This difference, charted against the temperature, manifests as a DSC curve.

The onset of this curve's peak signifies the beginning of the melting process. The peak's highest point denotes the lipstick's melting point, an essential indicator of the product's stability and its behavior under varying conditions. The energy required for the melting process, known as the enthalpy of fusion, is calculated from the area under the curve.

Intriguingly, DSC also unveils the degree of crystallinity in the lipsticks, which shows the uniformity of the polydisperse molecules within the composition. A lipstick with a high crystalline structure will present a sharp DSC peak and a higher melting point, signaling a robust structure. In contrast, lipstick with less crystallinity structure will showcase a broader peak and a lower melting point, indicating a softer, more malleable composition.

Figure 5. The sample DSC melting curves of lipstick show different thermal behaviors of the lipstick when it is melted, cooled down, and melted again.

Table 2. The melting behaviors of the lipsticks are shown. After melting, cooling, and re-melting, the enthalpy decreases which means less energy is required for the re-melting process. So, if your lipstick is accidentally left in the car and it melted, when you bring it back to life, there is an inevitable alteration in the lipstick's original properties.

Figure 6. There is a correlation between hardness and melting point despite different brands of lipsticks being used. Shimmery 2 has the lowest melting point, which indicates it has a softer composition. The special-ingredient #2 has the highest melting point which indicates it is firmer.

In conclusion

Lipsticks are not only skin deep, they are a combination of art and science — a product of creative color theory and meticulous chemical engineering. Behind each glamorous shade is a series of tests and measurements ensuring that it's pretty and also performs well. By utilizing the methods above, it is easy to assess different characteristics of your lipsticks, such as smoothness, hardness, wearability, and melting.

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Looking to launch a new lipstick line or enhance your current line? For more information about these and other relevant lipstick testing methods, contact us at TRI.