Delicate microstructures found in most consumer products such as toiletries/cosmetics, coatings, foodstuffs rupture easily when transported, and may not regain their original structure until a defined time, or in certain cases, not at all.
This, in turn, affects end-product stability and performance owing to the fact that product separation may occur due to structural breakdown in dispersion.
It is possible to avoid such instabilities in the case of materials with a yield stress that is large enough to handle the stresses experienced during transportation. Assuming that the structure is broken when the yield stress is exceeded, the period over which the structure regains its original structure becomes the key parameter influencing product stability.
The mechanical stability of such products can be identified with a rheological test, in which the sample is subjected to a pre-shear history for simulating the structure breakdown and then monitored for structural re-build using an oscillatory time sweep at low strains and stresses.
The structural integrity can be measured from the storage modulus, G'. Therefore, it is possible to use the recovery rate of the storage modulus subsequent to a period of perturbation as a measure of structural re-build.
This experiment measured the structural recovery of a commercial moisturizing cream. A Kinexus rheometer equipped with a Peltier plate cartridge and a serrated plate-plate measuring system was employed to perform rotational rheometer measurements at 25°C, using standard pre-configured sequences in rSpace software.
The use of a standard loading sequence ensured that a consistent and controllable loading protocol was applied onto the samples. The length of the Linear Viscoelastic Region (LVER) was measured by performing a stress-controlled amplitude sweep at 1Hz in order to identify a suitable stress value to be applied in the subsequent time sweep test.
Pre-shearing of the sample was done at 10s-1 for 3 min prior to returning to oscillatory mode and applying the pre-determined stress at 1 Hz frequency for 10min to observe recovery of the storage modulus.
The G' recovery curve for the moisturizing cream is depicted in Figure 1, revealing the high thixotropic behavior of the sample as more time is required for full recovery (more than 600 s) even though most of the recovery takes place within the first 40 s. Such a long time delay is good enough for instabilities to occur when exposed to intermittent stresses for a longer period.
Figure 1. Oscillatory time sweep after pre-shear to follow structure re-build
This experiment involved pre-shearing of a moisturizing lotion to simulate transportation stresses and monitoring of its structural rebuild as a function of time through G’.
The results revealed that the sample is highly thixotropic and therefore is susceptible to stability issues during transportation.
This information has been sourced, reviewed and adapted from materials provided by Malvern Panalytical.
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