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Colloids and Surfaces

A: Physicochemical and Engineering Aspects 152 (1999) 8994

Rheological studies to objectify sensations occurring whencosmetic emulsions are applied to the skin

R. Brummer *, S. Godersky

Beiersdorf AG, Unnastrae 48, 20245 Hamburg, Germany

Received 22 January 1998; accepted 7 July 1998

Abstract

When cosmetic emulsions are applied to the skin, different flow conditions arise because the film thickness of theemulsion decreases as it is rubbed in. The sensations experienced during application the so-called skin feeling

therefore result in part from the flow properties of an emulsion subjected to varying conditions of stress. To take this

into account when investigating the skin feeling, a more discriminating approach is developed in this study than

simple correlation of the skin feeling with a stationary rheological parameter. In this study, the skin feeling is divided

into two parts, a primary and secondary skin feeling, which are each correlated with different rheological variables.

The primary skin feeling describes the sensations at the start of application, whereas the secondary skin feeling

describes the sensations at the end of application when the product has been almost completely rubbed into the skin.

The primary skin feeling correlates with the shear stress at the onset of flow tF

and the dynamic viscosity gdyn

. Ranges

are defined for these variables DtF

and Dgdyn

that encompass the values for products assessed as good in sensory

panel tests. The secondary skin feeling correlates with the value of the stationary viscosity g for the rate of shear

prevailing at the end of application to the skin. Ranges of the stationary viscosity Dg are determined for this rate of

shear that will include all values corresponding to a good assessment of the secondary skin feeling. The applicationshear rate for the creams studied is in the magnitude of cAppl=500 s1 and for the lotions c

Appl.=5000 s1.

Furthermore, the results of this study show that the shear stress at the onset of flow alone tF

is not an unambiguous

criterion for distinguishing between types of products, as has been described in the literature. In the lotions and

creams studied, the characteristic difference is the height of the maximum gmax

of a dynamic viscosity curve. The

viscosity maximum for creams is in the order of magnitude ofgmax=1000 Pa s and for lotions g

max=100 Pa s.

Barry [1] correlated results of sensory testing of cosmetic products with the stationary viscosity for a defined rate

of shear. The results of this work show, however, that sensory testing does not correlate with the stationary viscosity

alone. To assess the primary skin feeling by means of rheological measurements, a dynamic viscosity curve is needed

that must be evaluated at the characteristic points.

The results of rheological measurements are correlated with those of field tests. For this we carefully considered the

most important criteria of sex, age and skin type, so that the results of the field tests are representative of human

skin. 1999 Elsevier Science B.V. All rights reserved.

Keywords: Rheology; Cosmetic emulsions; Skin feeling

* Corresponding author. Tel:+49 40 49 09 3902; Fax: +49 40 49 09 6192.

E-mail address:[email protected] ( R. Brummer)

0927-7757/99/$ see front matter 1999 Elsevier Science B.V. All rights reserved.

PII: S 0 9 2 7 - 7 7 5 7 ( 9 8 ) 0 0 6 2 6 - 8


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90 R. Brummer, S. Godersky/Colloids Surfaces A: Physicochem. Eng. Aspects 152 (1999) 8994

1. Introduction Every measuring system needs a certain time (rest

time) to attain a stationary state, and stationary

measurements are impossible unless the rest timeManufacturers of cosmetic emulsions set high

standards for product development and pro- is known [2,3]. When interpreting the results of

dynamic measurements, the time scales of theduction in order to meet customers expectations

of the quality of their products. The prerequisite experiment must be taken into account.

Also affecting the reproducibility of the resultsfor good customer acceptance is products of con-sistently high quality. One criteria for assessing is the pretreatment of the sample, that is, the

mechanical stress placed on the sample when it isquality is stability during transport. To guarantee

quality is not impaired during transport, methods removed from the storage container or filled into

the measuring instrument. Differences arising fromof evaluation are needed that detect changes, for

example, due to mechanical stress and temperature pretreatment can be minimized by uniform hand-

ling of the sample and maintaining a recovery timefluctuations. Consumers judge the quality of pro-

ducts in part according to sensory criteria, the prior to the measurement. The recovery time of a

sample is determined in a creep test. All measure-so-called cosmetic elegance. To determine cos-

metic elegance a series of tests is conducted with ments are performed at room temperature of

250.5C. The viscosity of non-Newtonian fluidstest persons who assess the product according to

defined criteria such as color, odor, appearance, depends on the rate of shear [4 ]. Measurement ata single rate of shear is, therefore, inadequate toabsorption capacity and general skin feeling when

applying it to the skin. Unfortunately these tests characterize these fluids and they must be mea-

sured over a range of shear rates.are costly, time-consuming and by nature

subjective.

One of the aims of research on cosmetic elegance

is to objectify assessment by finding and being 3. Measurement methods for the study of the

primary skin feelingable to measure physical parameters that determine

the skin feeling of cosmetic products. It is suspected

that the rheological properties of the products To correlate the primary skin feeling with rheo-

logical material constants, sensory assessments arehave a strong influence, since flow properties deter-

mine consistency and spreadability. Therefore the compared with the onset of flow and maximum

viscosity measured for the products. The onset ofpurpose of this study is to demonstrate whetherthere is any correlation between the skin feeling flow of a sample is determined by means of a shear

stress ramp test. In this test the torque is increasedand rheological properties of cosmetic products.

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 for2. General experimental conditions

accurate determination of the onset of flow, since

the product will start to flow with even the slightestIn rheological studies, the deformation and flow

of substances are determined as a function of rate of shear [5,6 ]. If a shear stress ramp is used,

the onset of flow tF

can be determined from theapplied loads. Either the rate of shear is measured

for a given stress or the shear stress is measured maximum of the viscosity curvegmax

( Fig. 1).

The shear stress is increased logarithmically withfor a given rate of shear. The results are indepen-dent of the substance, temperature, pressure, meas- time in order to expand the region of flow onset

while still being able to utilize and plot the entireuring time or previous loads. These influences must

be determined or excluded in preliminary measuring range of the instrument. In these tests

the change of the shear stress and shear rate inexperiments.

For instance the influence of the time of meas- the sample is unknown. Tests to determine the

flow onset and maximum viscosity are performedurement on test results can be observed for thixo-

tropic fluids and may be found for other fluids. with the shear stress-controlled dynamic stress


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91R. Brummer, S. Godersky/Colloids Surfaces A: Physicochem. Eng. Aspects 152 (1999) 8994

Fig. 1. Viscosity curves of O/W lotions assessed as good (C and D) and not good (A and G).

rheometer (DSR) from Rheometric Scientific. measured values that are all within these limits.

Lotion E conforms to the limits for flow onset andSince this dynamic test procedure depends on how

quickly the shear stress is increased, the optimal maximum viscosity but not for minimum viscosity.

Lotions A, B, F and G do not conform to at leastrate of increase permitting easy recognition of the

flow onset must first be determined. Furthermore, one of the limits. A lotion that is supposed to have

an optimal skin feeling assessment must, likethe creep recovery of the samples used must be

investigated to determine the time needed by the Lotions C and D, conform to all limits.The limits for the W/O/W creams were deter-sample to recover from the stress of filling into the

measuring instrument. mined according to the same method as for the

lotions. The assessment of the W/O/W creamsCorrelation of the assessments by sensory testing

panels with the values measured for flow onset were obtained from a market research test, in

which both the creams D and F received a scoreand maximum viscosity gave the initial window

of measured values recognizable as a square in of 2 for the primary and secondary skin feeling.

The shear stress values at yield stress determinedFig. 1. The boundaries of the window are deter-

mined by the results for the two lotions receiving from the viscosity curves (>8.5t6.5t


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Table 1

Comparison of flow onset with the sensory assessment for O/W lotions A to G

Product Shear stress (flow onset)tF

( Pa) Dyn. maximum viscosity gmax

( 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 good5, unsatisfactory.

emulsion types (W/O, O/W, W/O/W ) that can be Stern [7] cites shear rate estimates of different

correlated with the assessment of primary skin authors in the range 104c105 s1. Shear rates

feeling in sensory panel tests. of up to 2500 s1 can be achieved with the DSR

and RDA rotary rheometers from Rheometric

Scientific (One Possuntown Road, Piscataway,4. Investigation of the secondary skin feeling NJ 08854/Gottfert GmbH, Siemensstrasse 2, 74711

Buchen/Odw.). To obtain higher shear rates ofFor the correlation of the secondary skin feeling 104c105 s1 the high-pressure capillary vis-

with the rheological variables, the sensory assess- cometer ( HKV ) Rheomat 2000 from Gottfertment of the products is compared with their sta- must be used.tionary viscosity curves for shear rates up to The study on the correlation of the secondary105s1. This maximum rate of shear is estimated skin feeling with the viscosity curve was performedassuming that the spreading rate isv=1 m s1and with the same products as those used for the studythe film thickness of the cream x=0.01 mm: on primary skin feeling [8] . To determine experi-

mental values for the shear rate occurring on

application of a cosmetic emulsion to the skin, thecdv

dx#

Dv

Dx=

1 m s1

105 m=105 s1

Fig. 2. Viscosity curves for the oils AG (mean values and confidence intervals) measured with different instruments.


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93R. Brummer, S. Godersky/Colloids Surfaces A: Physicochem. Eng. Aspects 152 (1999) 8994

Table 2

Different oils and combinations of oils

Components and composition in parts by volume Viscosityg(Pa s) Verbal sensory assessments of skin feeling

Oil A Oil 1=100% 0.674 No assessment

Oil B Oil 1/oil 2=50%/50% 0.0878 Oily, greasy, neutral skin feeling

Oil C Oil 1/oil 3=50%/50% 0.0357 Spreads well, somewhat oily and dull

Oil D Oil 1/oil 2/oil 3=33%/33%/33% 0.0242 Somewhat oily, disappears slowly,

very good skin feeling

Oil E Oil 2=100% 0.0161 No assessment

Oil F Oil 2/oil 3=50%/50% 0.00643 Watery, dry skin feeling

Oil G Oil 3=100% 0.00298 No assessment

viscosity of a series of Newtonian oils was mea- intervals (95%). Oils C to G could not be measured

with the HKV because of the low viscosity. Onsured and the corresponding skin feeling deter-

mined by a test panel. The aim is to determine the the one hand the accuracy of the available pressure

transducer was insufficient for the pressure differ-viscosity of the oil considered to have the optimal

skin feeling. ences to be measured Dp


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capacity perceptible on the skin increases with ing (Fig. 3). Knowledge of both windows during

product development will make it possible to opti-decreasing viscosity. The oils can be clearly distin-

guished in the sensory test. Since most cosmetic mize new products, reduce development times and

submit only selected products to extensive fieldemulsions show non-Newtonian flow behavior, it

is possible to find a rate of shear at which the tests.

viscosity is g=0.0280.005 Pa . This shear rate is

ca c=5000 s1 for the O/W lotions and ca

c=500 s1for the W/O/W creams. The shear rates Acknowledgments

measured by this method are clearly lower than

the estimated value of 105s1. This is due to the The authors thank their colleagues at Beiersdorf

AG for supporting this work, especially Mrdependence of the rate of shear of product applica-

tion on the type of product as well as to the fact Nielsen, who helped in product selection, Ms

Cailloux and all those participating in the sensorythat the sensory skin feeling is product-specific.

This is understandable if one considers how each panel tests. Special appreciation is expressed to the

rheology team for their help in the measurementstype of product is used. A lotion is applied to

large areas of the skin like the arms, legs and and interpretation of the results. We also thank

Professor Dr Schluter of the Technical College oftrunk. A cream is usually applied to a smaller

area, for example on the face, and rubbed in with Hamburg for stimulating discussions.a lower shear rate than a lotion.

References

[1] B.W. Barry, Sensory testing of spreadability investigation5. Results

of rheological conditions operative during application of

topical preparations, J. Pharm. Sci. 61 (3) (1972) 335341.

[2] M. Pahl, Praktische Rheologie der Kunststoffe undComparison of the flow onset values measuredElastomere, VDI-Verlag, Dusseldorf, 1991.for both product types shows that the onset of

[3] W.-M. Kulicke, Flieverhalten von Stoffen undflow is not a distinguishing criterion for the typeStoffgemischen, Huthig and Wepf Verlag, Basel, 1986.

and consistency of a product. The shear stress at[4] R. Brummer, G. Hamer, Rheologische Memethoden zur

the onset of flow tF varies more among different Charakterisierung kosmetischer Produkte; AppliedRheology; Volume 7, Vincentz Verlag, 1997, pp. 1924.products of the same type than between two

[5] J. Haag, Praktische Rheologie-Bestimmung der Fliegrenze,different types of product. The perceptible consis-Firmenschrift 2/92 der Firma Bohlin Instruments,tency of a product is determined by the maximumMuhlacker, 1992.

viscosity of the product. This can be explained by[6] S. Schaugsdat, Rheologische Untersuchungen zur Stabilitat

the fact the when cosmetic products are applied to kosmetischer Emulsionen; Diplomarbeit im StudiengangChemieingenieurwesen der Fachhochschule Hamburg,the skin, the product starts to flow quickly, makingHamburg, 1995.it impossible to detect the onset of flow in sen-

[7] P. Stern, Die Rheologie in der Kosmetik; Zweitessory tests.Rheologiesymposium der DDR, Vortragsband 2,

By measuring dynamic viscosity curves for eachTabarz/Thuringen, 1987.

product it is possible to obtain the corresponding [8] L.E. Pena, Secondary structural rheology of a model cream,J. Soc. Cosmet. Chem. 45 (2 ) ( 1994) 7784.windows for the primary and secondary skin feel-

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