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Determination of the Mechanical and Thermal Stability of Skin Cream with Oscillation and Freeze-Thaw Cycle Tests
페이스북 트위터 구글 싸이월드 마이피플 
작성자 : 사이언스21 | 조회: 3 | 날짜: 2018-10-08 13:52:15



Relevant for: Cosmetics, Pharmaceuticals, Cream, Stability, Freeze-Thaw Cycle Tests


1. Introduction
The feel and longtime stability of skin cream, both in cosmetic and pharmaceutical applications, are important points for the acceptance of a product by the consumer. To a large amount, these properties are influenced by the manufacturing process but also by the ingredients. The replacement of ingredients with functional properties such as emulsifiers or stabilizers may be necessary because they are suspected to cause allergies or are not accepted by the consumer anymore. But the quality of the product needs to stay the same. As a rheometer is an excellent tool for the characterization of the physical properties of an emulsion, it is suitable for measuring and quantifying the influence of a change in composition or manufacturing process on the properties of the final product. The mechanical and thermal stability of the final product can be tested on a small scale in relatively short times.


2. Experimental Setup
All measurements in this application report were carried out with an Anton Paar MCR rheometer equipped with a PP25 measuring system, a Peltier lower plate (P-PTD 200) and a Peltier heated hood (H-PTD 200) as shown in Fig. 1. H-PTD 200 ensures an even temperature distribution in the sample. To avoid the formation of skin on the sample surface due to drying during longtime tests, an 'Evaporation Blocker' was mounted into the Peltier hood. This accessory reduces the volume of air around the sample and allows saturating this volume with additional water. The samples were three variations of a rich skin cream which were to be tested for differences in mechanical stability, flow point and thermal stability.


2.1 Amplitude and Frequency Sweep
For the analysis of the mechanical properties and for the measurement of the flow point, the samples were tested in an amplitude sweep as well as in a frequency sweep at a temperature of +25 ºC. The amplitude sweep was carried out at an angular frequency ω = 10 rad/s at strains γ = 0.01 to 100 %. For the frequency sweep a strain γ = 0.05 % was used and a frequency range = 0.1 to 100 rad/s.


2.2 Thermal Loop Test
To simulate the thermal stresses during storage and use (e. g. indoors, car, handbag) the samples were subjected to fifteen thermal cycles from +5 °C to 45 °C. During the cycles, the samples were measured at an angular frequency ω = 10 rad/s and a strain γ = 0.1 %. The heating and cooling rate was set to 5 K/min. The relative change of the value of the storage modulus G' compared to the value at the start of the thermal cycles was used as a measure of the structural changes in the material. All necessary calculations were performed with the rheometer software. The thermal cycles can be carried out with the rheometer software using the internal loop. The parameters have to be set for one cycle only. The temperature is checked at the start of the test to ensure that the measurement is started only after a thermal equilibrium has been reached.


3. Results

3.1 Mechanical Stability and Flow Point
The results of the amplitude sweeps are shown in Fig. 2 and Fig. 3. The samples 'cream 2' and 'cream 3' show very similar values in the LVE range, the storage and the loss modulus. The values for 'cream 1' are about 20 % higher; this cream gives a richer feel than the two other products.
The limiting strain of the LVE range is about 0.7 % for all. They show a similar mechanical stability because they are of the same emulsion type.


The yield point (or yield stress) is calculated as the stress value where the value of the G‘ has decreased by 5 % compared to the value in the LVE range. The flow point (or flow stress) is the stress value at the crossover of G‘ and G“. Fig. 3 shows G‘ and G“ as a function of the shear stress as well as the calculated flow point values. The enlarged detail shows the order of the samples with regard to the flow point values. This value is for example reflected in the force that is required to squeeze out the cream from a tube.The results of the frequency sweeps (Fig. 4), all three samples have a so-called gel-like behavior: G’ and G“ change little with the frequency, the relation between both values (i.e. the loss factor tanδ) remains more or less constant. The loss factor (tanδ) is calculated as: tanδ = G″ / G′ The order of the samples from low to high G' and G'' values is identical to the order found in the amplitude sweeps. The curve shapes are very similar. The tanδ shows a very slight increase at the lower frequencies. This might indicate inferior long term stability.




3.2 Thermal Stability
Fig. 5 shows the change of temperature and storage modulus over time in a part of the thermal cycle test. Both the minimum and the maximum values increase from cycle to cycle due to structural changes in the sample.



The maxima of all cycles are determined by the rheometer software and then normalized by dividing the value of each cycle by the value from the first cycle. The relative structural change Δ is then calculated as:



The structural change of all three samples is shown in Fig. 6. It can be seen that the value increases continuously over the duration of the measurement.


A sample that shows a small value for the relative structural change at the end of the thermal cycles has the smallest increase in the storage modulus and hence the highest thermal stability. In this test, sample 'cream 3' shows a significantly smaller structural change than the other two samples. It can therefore be expected to have the highest thermal stability in everyday use as well.


자료제공 : 안톤파코리아(주)
발표자 : 조윤형(info.kr@anton-paar.com)

제조사 : Anton Paar GmbH
원산지 : Austria

모델명 : MCR xx2a




- 2018년 10월호 기사 -


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