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.
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.
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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