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Comparison of two cosmetic lotions using frequency sweeps


Comparison of two cosmetic lotions using frequency sweeps

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									Application Note Physica Rheometers

Comparison of two cosmetic lotions using frequency sweeps
Problem                                                          Results and Discussion
The new, improved version of a lotion and it’s                   Fig. 1 shows the frequency sweeps. The storage
predecessor will be tested at room temperature for               modulus G’ represents the elastic part and the loss
differences. Various criteria relating to the                    modulus G’’ the viscous part of the deformation
application properties of the lotions will be                    behaviour of viscoelastic substances.
assessed using the measurement results.                          It holds that:
Information is required on the following:
Consistency, stability of the shape, spreadability,              • Both lotions show the relationship G’ > G’’ in the
creaminess when applied to the skin and long-term                  frequency range ω = 0.25 to 63 1/s .
stability during storage.
                                                                 • For ω = 10 1/s it holds that:
                                                                                  G’           tanδ
Solution                                                             Lotion 1     1350 Pa      0.44
                                                                     Lotion 2     140 Pa       0.19
Equipment and Method
Two skin care lotions were tested using oscillatory                Here the G’-values for L1 are almost 10 times
tests with amplitude and frequency sweeps.                         higher than those for L2. The loss factor tanδ =
Measurements were taken with a UDS 200                             G’’/G’ is significantly higher for L1 than for L 2.
rheometer and temperature control system                         • At very low frequencies L1 displays G“ > G’. At a
TEK150P with Peltier heating.                                      higher frequency (= at a rapid load) L1 displays a
The plate-plate measuring system MP 32 (∅                          gel character (with G’ > G’’); at slower movement
75 mm) with a plate distance of d = 1 mm was                       or at rest, however, a liquid character (G’’ > G’).
used. The measuring temperature was T = 20°C.                      For L2 the slopes of the G’- and G’’-curves
                                                                   remain almost unchanged, which means that the
a) Amplitude sweep (AS): A deformation
                                                                   gel character with G’ > G’’ is not dependent on
controlled AS test at a constant angular frequency
                                                                   the speed of movement and is also present in
of ω = 10 1/s was performed as a pre-test. By doing
this, the limit of the linear-viscoelastic (LVE)                   the state of rest.
deformation range γL was found out for every lotion              At this point it becomes obvious that the amplitude
L. Results: For L1 γL = 0.3% and for L2 γL = 5%. L2              sweep at ω = 10 1/s is not sufficient to judge the
has a comparatively higher limiting value for the                behaviour at rest. For both lotions a measurement
LVE range, which means the structure can be                      of G’ > G’’ is taken from which we can assume a
deformed up to larger deflections without being                  gel structure. However, this is not true for L1 at rest,
destroyed.                                                       as described above.
b) Frequency sweep (FS): Shear stress controlled                 Consistency: Resistance to a finger pressed down
frequency sweeps at a constant shear stress                      slowly. Recommended test: FS, analysis at ω = 10
amplitude of τ = 1.2 Pa were then carried out in                 1/s or below. The G’ value is the „structural
order to study the time-dependent behaviour and                  strength“; L1 is „stiffer“.
structural character of the substance. In these tests
the τ-amplitudes corresponded to the γ-amplitudes                Stability of the shape: Behaviour of the surface
0.05 to 0.6% for L1 and 0.7 to 1.2% for L2, with                 after a finger has been pressed into the substance
increasing γ values from the high to the low                     and then removed. Recommended test: FS,
frequencies.                                                     analysis at the lowest ω values (= behaviour at
                                                                 rest). L2 remains deformed, because G’ > G’’ (gel
                                                                 character). L1 levels again, because G’’> G’.

Anton Paar Germany GmbH           Tel.: +49 (0)711 720 91-0                                                     Page 1 of 2
Web:           Mail:                                            anco99041e-b.doc
Application Note Physica Rheometers
Spreadability: Constant flow at medium and high                   Long-term        stability      during       storage:
shear load. Recommended test: FS, analysis at ω =                 Recommended test: FS, analysis at the lowest ω
10 to 100 1/s (or rotational test at shear rates from             values (= under conditions of quasi-rest). L2 is
γ = 10 to 100 1/s). Higher values for G’ or for the
 &                                                                stable, because G’ > G’’ (stable gel structure). L1
complex viscosity |η*| (or the shear viscosity η)                 displays here G’’ > G’. For this reason the lotion
suggest that L1 is more difficult to spread and L2                tends towards phase separation, syneresis or to
seems smoother, creamier and less solid which                     free flowing on a level surface (unstable
gives it a nicer feeling when applied to the skin.                superstructure) after a period of time at rest.
Due to the lower tanδ values, L2 seems richer but
not stiffer.
            4                                                                               4
      10                                                                               10

     Pa·s                                                                              Pa

            3                                                                               3
      10                                                                               10

                                                                                                      Lotion (1)

                                                                                                G'           G'
            2                                                                               2
 |η *| 10                                                                              10                    G''

                                                                                                G''   Lotion (2)


            1                                                                               1
      10                                                                               10                    G''

            0                                                                               0
      10                                                                               10
                0.1                  1                           10       1/s       100
                             Angular Frequency ω

Fig. 1 Frequency sweeps. Lotion 2 displays almost constant values for G’ and G’’, also at small
       frequencies. In contrast, both curves for Lotion 1 fall with an increasing slope. They then even display
       G’’ > G’.

Text:        Thomas Mezger
Measurement: Klaus Wollny
Date:        30. March 2000

Anton Paar Germany GmbH           Tel.: +49 (0)711 720 91-0                                                               Page 2 of 2
Web:           Mail:                                                      anco99041e-b.doc

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