3. Measurement methods for the study of the primary skin feeling
To correlate the primary skin feeling with rheological material constants, sensory assessments are compared with the onset of flow and maximum viscosity measured for the products. The onset of flow of a sample is determined by means of a shear stress ramp test. In this test the torque is increased from zero to a pre-defined end value and the shear stress determined at which the sample begins to flow. Stationary flow curves are unsuitable for accurate determination of the onset of flow, since the product will start to flow with even the slightest rate of shear 5 and 6. If a shear stress ramp is used, the onset of flow τF can be determined from the maximum of the viscosity curve ηmax ( Fig. 1).
Full-size image (25 K)
Fig. 1.
Viscosity curves of O/W lotions assessed as good (C and D) and not good (A and G).
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The shear stress is increased logarithmically with time in order to expand the region of flow onset while still being able to utilize and plot the entire measuring range of the instrument. In these tests the change of the shear stress and shear rate in the sample is unknown. Tests to determine the flow onset and maximum viscosity are performed with the shear stress-controlled dynamic stress rheometer (DSR) from Rheometric Scientific. Since this dynamic test procedure depends on how quickly the shear stress is increased, the optimal rate of increase permitting easy recognition of the flow onset must first be determined. Furthermore, the creep recovery of the samples used must be investigated to determine the time needed by the sample to recover from the stress of filling into the measuring instrument.
Correlation of the assessments by sensory testing panels with the values measured for flow onset and maximum viscosity gave the initial “window of measured values” recognizable as a square in Fig. 1. The boundaries of the window are determined by the results for the two lotions receiving a poor sensory assessment that have values above and below those for the lotions C and D receiving the best assessment (Table 1).
Table 1.
Comparison of flow onset with the sensory assessment for O/W lotions A to G
Product Shear stress (flow onset) τF (Pa) Dyn. maximum viscosity ηmax (Pa s) Sensory assessmenta
Lotion A 13 500 4
Lotion B 12 510 3
Lotion C 12 460 2
Lotion D 9.5 250 2
Lotion E 10 300 3
Lotion F 12 570 3
Lotion G 6,5 120 4
a
1, Very good…5, unsatisfactory.
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The values measured with this method for the shear stress and viscosity at flow onset provide the upper and lower limits for the respective measured variable. This is depicted graphically as a window.
The limits are values that include the values measured for the lotions assessed as good. Lotions C and D, which received the best evaluation, have measured values that are all within these limits. Lotion E conforms to the limits for flow onset and maximum viscosity but not for minimum viscosity. Lotions A, B, F and G do not conform to at least one of the limits. A lotion that is supposed to have an optimal skin feeling assessment must, like Lotions C and D, conform to all limits.
The limits for the W/O/W creams were determined according to the same method as for the lotions. The assessment of the W/O/W creams were obtained from a market research test, in which both the creams D and F received a score of 2 for the primary and secondary skin feeling. The shear stress values at yield stress determined from the viscosity curves (>8.5τ<15 Pa) do not differ significantly from those of the lotions (>6.5τ<13 Pa). However, the viscosity values η at yield stress are higher by a factor of 10. This can be attributed to the thicker consistency of a cream compared with a lotion and explains why the type of product determines the skin feeling to be expected when applying an emulsion. Consequently, a viscosity and shear stress range can be determined for the flow limits for different product classes (e.g. milk, lotion, cream, gel) and emulsion types (W/O, O/W, W/O/W) that can be correlated with the assessment of primary skin feeling in sensory panel tests.
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