Latex emulsions less stable at higher temperatures
Many cosmetic products contain natural rubber latex particles which aid good film-formation and high elasticity. The products are shipped, stored, bought, and used at varying temperatures. Just think of the sun lotion you took on your last holiday, which sat on the dashboard of a car in the blazing sun. Even under such extreme circumstances, those emulsions should keep their intended look and feel
, or, in another word, their stability. Schaubach explains: It is therefore advisable to study the stability of cosmetic products at different temperatures to draw conclusions for optimal processability and performance of the formulation.
In this example, a stability study was carried out with two different latex emulsions (Latex A, Latex B) at 25 °C and 40 °C, respectively. They were measured for six days, being scanned every hour. Schaubach says: With the MultiScan MS 20, all four measurements could be carried out simultaneously, as up to six independent ScanTowers can be used at once with one device. For long-term measurements, this yields significant time savings for researchers.
After shaking the mixtures to create homogeneous samples, they were poured into transparent glass vials, which were then put in the ScanTowers and scanned from the bottom to the top fill level.
Since the latex emulsions appear completely turbid, no transmission was measured. However, the backscattering intensities can be used for analysis (see Figure 5). Again, the color-coding of the curves indicates the passage of time, from red (start of the experiment, t = 0 s) to purple (end of the experiment, t = 6 d), with every curve representing one individual measurement. In this study, the relative backscattering intensities are plotted, i.e., the intensity change with respect to the first measurement. This approach reveals even slightest variations in intensity
, explains Schaubach.
In Figure 5, you can see that the studied emulsions show differently pronounced signal changes over time when measured at 25 °C or 40 °C, respectively. Looking at the different position zones, it is also possible to deduce the occurring destabilization mechanisms. At 25 °C both latex emulsions show hardly any changes in both the middle and the bottom zone. The top zone, however, looks very much different with time. The seen reduction of the backscattering signal in the top zone is typical for a sedimentation process
, knows Schaubach.