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j. Cosmet. Sci., 57, 441-454 (November/December 2006)
A facial mask comprising Dead Sea mud
BASIM ABU-JDAYIL, Department of Chemical and Petroleum
Engineering, United Arab Emirates University, P.O. Box 17555,
ALAin, UAE, and HAZIM A. MOHAMEED, Department of
Chemical Engineering, Jordan Univemity of Science and Technology, P.O.
Box 3030, 22110 Irbid, Jordan.
Accepted for publication June 27, 2006.
Synopsis
Many investigators have proved that Dead Sea salt and mud are useful in treating skin disorders and skin
diseases. Therefore, the black mud has been extensively used as a base for the preparation of soaps, creams,
and unguents for skin care. This study concerns a facial mask made mainly of Dead Sea mud. The effects
of temperature and shearing conditions on the rheological behavior of the facial mask were investigated. The
mud facial mask exhibited a shear thinning behavior with a yield stress. It was found that the apparent
viscosity of the mask has a strong dependence on the shear rate as well as on the temperature. The facial mask
exhibited a maximum yield stress and very shear thinning behavior at 40øC, which is attributed to the
gelatinization of the polysaccharide used to stabilize the mud particles. On the other hand, the mud mask
exhibited a time-independent behavior at low temperatures and shear rates and changed to a thixotropic
behavior upon increasing both the temperature and the shear rate. The shear thinning and thixotropic
behaviors have a significant importance in the ability of the facial mask to spread on the skin: the Dead Sea
mud mask can break down for easy spreading, and the applied film can gain viscosity instantaneously to
resist running. Moreover, particle sedimentation, which in this case would negatively affect consumer
acceptance of the product, occurs slowly due to high viscosity at rest conditions.
INTRODUCTION
The Dead Sea region is the major spa area in the Middle East for patients with various
types of arthritis. The unique climatic conditions in this area and balneologic therapy--
which is based primarily on mud packs and bathing in sulfur baths and in Dead Sea
water--combine to alleviate the symptoms of arthritis (1).
The Dead Sea has a salt content of about 320 g/L, of which potassium chloride, mag-
nesium chloride, calcium chloride, and sodium chloride (with their respective bromides)
are the major components, comprising 98% of the salts on a dry weight basis. Another
mineral-rich constituent of the Dead Sea is its "black mud" (rich in organic substances),
also known as "bituminous tar." The therapeutic effect of processed Dead Sea mud is
Address all correspondence to Basira Abu-Jdayil.
441
442 JOURNAL OF COSMETIC SCIENCE
related to its high content of minerals and its ability to retain heat for many hours, thus
stimulating blood circulation and clearing the skin of dead epidermal cells (2). It has
been shown that Dead Sea salt and mud are useful in treating skin disorders and skin
diseases such as psoriasis (3), seborrheic dermatitis, xerosis, artopic dermatitis, stage I
skin burns, and sensitive skin (4). In addition, black mud has been extensively used as
a base for the preparation of soaps, creams, and unguents for skin care. The manufac-
turers of those products claim that the mud has major effects on revitalizing and toning
the skin. Dead Sea mud deep cleanses; it removes impurities by deep washing of the skin.
It penetrates pores to absorb accumulated dirt, makeup residue, and excess fatty secre-
tions like hardened sebum.
The demand for Dead Sea (DS) cosmetics is increasing. Dead Sea cosmetics include
shampoos, creams, lotions, masks . .. etc. They have Dead Sea salt and/or mud in their
formulas. Consumer acceptance of Dead Sea cosmetics depends on the stability of the
products and their ability to spread on the skin, which is directly related to flow
behavior. Semisolid systems are used widely in the formulation of topical pharmaceutical
and cosmetic preparations. Rheological properties of semisolids are highly important
physical parameters in technical manufacturing (filling, storage) and in aesthetic terms.
The evaluation of semisolid cosmetic structure and consistency is, therefore, essential in
order to determine, adjust, and perhaps predict the performance of newly designed
products (5). The rheological properties of a semisolid system significantly determine its
quality, usefulness, and purpose. Therefore, rheology has always played and will play a
role in the preparation, development, and manufacture of any formulation. For that
matter, rheological determinations are indispensable in the analysis of its properties. The
importance of rheological properties in semisolid pharmaceutical and cosmetic forms is
such that theological and thixotropic studies have become crucial tools from both
pharmacotechnical and galenic points of view. In a similar way, rheology can elucidate
the possible modifications of the system, expressed as a function of time and tempera-
ture, from the variation in the hysteresis loops of the apparent viscosity (area under the
curve) (6). Thus, pharmacotechnical tests that include the determination of organoleptic
properties, pH, sign, and macroscopic and microscopic examination allow us to evaluate
the evolution of the properties of the formulations mentioned, according to the time,
temperature, and gravity. As a rule, the rheological study and, more precisely, the
evaluation of thixotropic properties, allow us to obtain a correct picture of the physical
properties and structural stability of semisolid systems (7,8).
This study aimed to use theological measurements in the evaluation of a commercial
facial mask sample made mainly of Dead Sea mud.
MATERIALS AND METHODS
MATERIALS
The facial mask samples were supplied by Ammon for Dead Sea Salts and Soap Products
(Amman, Jordan). The components of the mask used were Dead Sea mud (solids) 67.0
wt%, glycerin 7.0 wt%, and stabilizer (with a trade name ofpolysaccharide) 1.0-1.5%.
The remainder was deionized water. The chemical identity of Dead Sea mud is natural
sediment. It is a mixture of solid mineral clays with an interstitial solution of inorganic
FACIAL MASK OF DEAD SEA MUD 443
salts and sulfide compounds originated from microbiological activity (4). The particle
size distribution of the mud solids is 86-98% <5 pm; 2-9%: 5-20 pm; and 0-7% >
20 pm.
The stabilizer "polysaccharide" is a modified starch containing glucose as the sole
monomer with a molecular mass of 5 to 6 million daltons. It is obtained by fermentation
of Sc/erotium ro/•/3ii on a glucose-enriched medium. The fermentation medium is filtered.
After being washed with alcohol, the product is again dissolved, filtered, and dried. The
type of linkages found in the molecule gives it a high stability; polysaccharide aqueous
solutions show therefore a good resistance to aging and most enzymatic degradations.
Polysaccharide displayed a good ability to stabilize the mud suspension due to its
capacity to increase in a significant and stable way the viscosity of the medium. Poly-
saccharide can be used in suspensions at a recommended dosage level of 1.0-1.5 wt%.
RHEOLOGICAL MEASUREMENTS
The rheological properties of facial mud were measured with a concentric-cylinder
Haake-VT 500 viscometer, which has an inner cylinder rotating in a stationary outer
cylinder. Three different measuring systems were used: MV2, MV3, and SV1. MV2 and
MV3 used the same cup, with a radius of 21.0 mm, and different bobs, with radii of 18.4
and 15.2 ram, respectively. On the other hand, the cup radius of the SV1 system is 11.55
mm, while its bob radius is 10.1 min. Samples were allowed to relax (more than 10 min)
prior to measurement of their viscosity. It should be pointed out that the viscometer
operated in the range where the laminar flow is dominant. The viscometer was ther-
mostatically controlled with a water circulator (Haake D8) at the desired temperature
with a precision of + 0.1 øC.
METHODOLOGY
The experiments performed to characterize the shear-, time- and temperature depen-
dency of the flow behavior of Dead Sea mud consisted of a series of two measurements:
Apparent viscosity versus shear rate. A fresh sample was loaded into the annular gap of the
concentric -cylinder viscometer. Samples were left to reach the desired temperature. The
apparent viscosities of facial mud were measured in the temperature range between 5.0 ø
and 60.0øC by continuous increasing (forward measurements) and continuous decreasing
(backward measurements) of the shear rate. The values of the shear rate and apparent
-I
viscosity were recorded every 30 sec. The shear rate was varied from 2.200 to 159.80 s
The flow curves of the facial mud was modeled using the Herschel-Bulkley (H-B) model:
(1)
where ß is the shear stress, 'r o is the yield stress, m is the consistency coefficient, and n
is the flow behavior index. Typically, the Herschel-Bulkley model is used for many
materials, as the NewtonJan, shear thinning, shear thickening and Bingham plastic may
be considered as special cases.
Apparent viscosity measurements as a f•nction of time at constant shear rate. In transient
measurements, a fresh sample was sheared at constant shear rates, namely at 2.20, 10.21,
28.38, 47.43, 79.02 and/or 131.90 s -•, and the apparent viscosity was measured as a
444 JOURNAL OF COSMETIC SCIENCE
function of shearing time until an equilibrium state was reached. Most of the samples
reached the equilibrium state in 30 min. The procedure was then repeated with other
fresh samples at other shear rates and temperatures. The time-dependent flow properties
could be modeled by applying the structural kinetic (SK) model, which is adopted by
using the analogy with chemical reactions. The final form of the model (9) is:
= + (2)
where qqo is the initial apparent viscosity at t = 0 (structured state), qFl• is the equilibrium
apparent viscosity as t --> oo (equilibrium state), t is the shearing time, k -- k (•/) is the
rate constant of structure breakdown, and o• is the order of the structure breakdown
reaction. Details and assumptions of this model are reported by Abu-Jdayil (9).
Rheological experiments were carried out in triplicate, and the reproducibility was + 5 %
on average. The average values were used for analysis.
RESULTS AND DISCUSSION
FLOW CURVES
It should be pointed out that no surface slip was observed in the viscometer systems
used. Figure 1 shows the flow curves of the facial mask measured with different systems,
which have different gap widths. It is clear that the shear stress values (which represent
also the apparent viscosity values) of the mud mask are independent of the measuring
250
• Facial Mask•
T = 25 øC •
I .w I v
200 i [--I MV2[ ...................................................... ß ............... : .......................
• V svu
•7
150 ......................................................................................................................
.......... ........................................................ ! ...............................................
,
50 - I , i
...................... 5 .............................................. • ...............................................
,
I I I • I I
0 40 B0 120 160
• {1/$)
Figure 1. Flow curves of the facial mask measured with different measuring systems.
200
FACIAL MASK OF DEAD SEA MUD 445
Facial Mask : ':
:
:
J A T = lS øC :
---, ..... , ............................... : ......... : ...... • .... • ...............
--
6--
,,
........................................ : ..............................................................................
• ,,
,,
,
,,
,,
.... I I i I i III i I I I I is[
2 4 6 8 2 4 6 8
i 10 100
• (x/s)
Figure 2. Flow curves of the facial mask fitted to the Herschel-Bulkley model.
system. Since the slip conditions encountered in a viscometer are a function of the gap
width, the data points presented in Figure 1 show that the slip conditions in our system
are not clear.
On the other hand, the shear stress-shear rate curves of the facial mask shown in Figure
1 indicate a shear-thinning flow with yield stresses. Figure 2 shows the flow curves of
the mud mask at different temperatures, fitted to the Herschel-Bulkley model (equation
1). The regressed values of 7o, m, and n for the forward measurements are presented in
Table I. It is clear that the parameters of the H-B model are temperature-dependent,
Table I
Regressed Parameters of the Herschel-Bulkley Model
Temperature (øC) •r o (Pa) m (Pa s") n
5 40.0 11.30 0.63
10 38.0 10.43 0.62
15 35.0 9.96 0.62
20 29.5 9.17 0.62
25 30.6 14.66 0.49
30 40.7 21.92 0.38
35 60.0 25.16 0.32
40 70.0 30.34 0.22
45 67.0 29.58 0.18
50 56.1 8.34 0.42
55 54.5 7.20 0.45
60 52.3 5.53 0.45
446 JOURNAL OF COSMETIC SCIENCE
which is a reflection of the dependence of the apparent viscosity of the mud mask on
temperature. This aspect will be addressed later.
The parameters shown in Table I indicate that the facial mask exhibited two major
rheological properties: a yield stress and a shear-thinning behavior at stresses above the
yield stress, where the ,-values at different temperatures are less than unity. It has been
found that cohesive sediments, such as concentrated clay water suspension or aqueous
clays/grains mixtures, show the same rheological behavior (10,11).
The yield stress, which is the minimum stress required to start flow, originates in a
percolating network of strong interactions of colloidal forces (van der Waals, double-
layer, etc) or direct contact forces (friction and collision) between the small particles of
the Dead Sea mud (12). This network was broken during flow. The yield stress is an
important material property in various aspects involved in the transport and the end use
of the mask. To guarantee that quality is not impaired during transport, a high-yield
stress is required. On the other hand, a high-yield stress requires high pumping pres-
sures and makes difficulties in the end use of the mask. Thus for a given mud mask, an
optimum yield stress is always desirable, and this may be achieved simply by adjusting
the solids concentration (13).
Moreover, the fact that , is less than unity indicates that the mud facial mask is a
shear-thinning material, regardless of temperature. This means that the apparent vis-
cosity of the facial mask decreases with increasing the shear rate. The weak bondings
between the particles of mud explain the strength drop observed when the shear rate
increases. When these bonds are destroyed under an increase of the tangential stresses,
the structure breaks down and the water earlier contained in the network becomes
available to the flow. The amount of free water increases in the slurry and both the
viscosity and the shear stress temporarily decrease (14). It should be pointed out here
that the presence of water between the particles increases the slip conditions between the
particles, which results in ease of flow.
In addition, the shear thinning often evident in mineral suspensions is attributed to the
alignment of particles or riocs. An increase in the shear rate from rest results in the
alignment of particles in the direction of shear, and therefore provides a lower resistance
to flow (15).
TEMPERATURE EFFECT
The dependence of the rheological behavior of the stabilizer "polysaccharide" on tem-
perature was first investigated. As can be seen in Figure 3, at relatively low shear rates
(below 300 s-1), the apparent viscosity of the stabilizer increases with temperature. On
heating the starch in excess water, the branched-chain, water-imbibing polymer of
glucose (the monomer of the polysaccharide) melts, and the granules swell in a process
known as gelatinization. The degree of structural gelatinization of starch depends upon
the heating process and the type of starch utilized (16). However, by increasing the
temperature of the starch system, the degree of gelatinization increases, which results in
an increase in the apparent viscosity. The high viscosity of the polysaccharide and the
formation of the gel structure lead to stabilization of the suspension of the Dead Sea
mud.
FACIAL MASK OF DEAD SEA MUD 447
0.100
2 m
0.010 --
i
8 --
6 --
i
4 i
2 i
•olysaccharids solution %
I wtø/o
I T: 5øC
A T = 15 øC
I T: 25 øC
ß [] • T: 35 o½
.............. ..... _i ...................................................... ..... c_ .....
I
I
I
ø
0.001 I I I I I I I I iiiii i I I I I IIII
2 4 6 2 4 6 8 2 4 6 8
10 100 1000 10000
Figure 3. Temperature dependence of the theological behavior of polysaccharide.
On the other hand, Figure 3 shows that at high shear rates (above 300 s-•) the apparent
viscosity of the polysaccharide decreases with temperature, which means that the gel
structure is destroyed under the effect of high shearing. The high temperature softens the
granules of the polysaccharide, and the stresses imposed on them are large enough for
deformation and flow, which in turn results in the decrease in viscosity with tempera-
ture.
The effect of temperature on the rheological behavior of the Dead Sea facial mask is
shown in Figures 4-6. The investigated facial mask demonstrates an unexpected behav-
ior with temperature. This behavior can be divided into three stages. In the first stage,
which covers the temperature range of 5øC to 20øC, the apparent viscosity of mud
behaves like the normal liquid, i.e., the apparent viscosity decreases as the temperature
increases (see Figure 4).
However, an interesting behavior has been observed in the second stage, which covers
the temperature range of 20øC to 40øC. As shown in Figure 5, the apparent viscosity of
the facial mask increases with temperature. Above 40øC, the mud mask behaves typi-
cally in that the apparent viscosity decreases with temperature. This stage is demon-
strated in Figure 6.
It seems that the presence of the stabilizer "polysaccharide" is responsible for the unusual
behavior of the second stage. It should be stated here, that the rheological measurements
on the facial mask were carried out in the low region of shear rate (below 200 s -1)
(compare Figures 4-6). In this shear-rate region, it has been shown that the polysac-
charide viscosity increases with temperature (see Figure 3). This explains the atypical
behavior of the facial mask with temperature in the second stage. It can be concluded
448 JOURNAL OF COSMETIC SCIENCE
lOO
4 6 8 2 4 6 8
1 10 100
• (1/s)
Figure 4. Effect of temperature on the apparent viscosity of the facial mask (5ø-20øC).
lOO
lO
Figure 5. Effbct of temperature on the apparent viscosity of the facial mask (20ø•40øC).
FACIAL MASK OF DEAD SEA MUD 449
lOO
Facial Hask
--
- i• © T: 40 øC
-
- •:: •[ T: 45 øC
- : •j 0 T: 50 oC
• • T: 60 øC
-
-
..................................................................................................... .......
2 4 6 8 2 4 6
1 lO lOO
Figure 6. Effect of temperature on the apparent viscosity of the facial mask (40ø-60øC).
here that this type of starch was modified to start gelatinization at a low temperature,
compared to natural starches. For example, the initial gelatinization temperature of
wheat starch in water was found to be in the range of 55 ø to 66øC and for corn starch
in water was found to be in the range of 65 ø to 76øC (17). This gives an advantage for
the suspensions utilizing this polysaccharide: the system will be highly stable at room
temperature.
As the theological parameters are concerned, the results of Table I demonstrate that the
facial mask mud yield stress is strongly dependent on temperature. The values of the
yield stress reflect the behavior of the apparent viscosity with temperature. The three
stages of temperature effect on the yield stress can be distinguished easily in Table I.
On the other hand, the shear-thinning behavior, which can be assessed by inspecting the
values of the flow index, n, is the most pronounced at the end of the second stage and
at the beginning of third stage (40ø-45øC).
SHEARING TIME EFFECT
As mentioned above, the apparent viscosity of the facial mask was measured by increas-
ing (forward measurement) and decreasing (backward measurement) the shear rate in
order to test the presence of a time-dependent behavior. The flow curves (•r versus 4/) of
the mud mask at different temperatures are shown in Figure 7. There are hysteresis loops
between the forward and backward curves, indicating a time-dependent theological
450 JOURNAL OF COSMETIC SCIENCE
100 •
8 --
6 --
4 •
Facial Mask
Forward measurement /
Back-ward measurement)
T: 5 øC
.
2 4 6 8 2 4 6 8
1 10 100
Figure 7. Temperature effect on the hysteresis loops of the flow curves of the facial mask.
behavior. As shown in Figure 7, at low temperatures the direction of the hysteresis loops
is counterclockwise, indicating an anti-thixotropic behavior, which means that there is
an increase in the mud viscosity with shearing tinhe. In some conditions, the right kind
of attraction between particles of mud is given; shearing can then promote temporary
aggregation rather than breakdown, due to the collision of these attractive particles. This
results in anti-thixotropy (18). Like other similar suspensions, there is a range of flow
conditions under which shear-enhanced collisions make structure rather than break it
(18). However, this anti-thixotropic behavior is relatively small (according to the size of
the hysteresis loop) and disappears gradually with increasing temperature.
Above 25øC, the facial mask shows hysteresis loops with a clockwise direction, indi-
cating a thixotropic behavior. The size of the hysteresis loops becomes wider as the
temperature increases from 25 ø to 60øC (see Figure 7).
It should be pointed out that the shear-thinning and thixotropic behaviors have indus-
trial and commercial significance. For example, since the viscosity decreases with shear
rate and shearing time during the mixing process, this will lead to less power consump-
tion. Moreover, particle sedimentation, which in this case would negatively affect the
consumer acceptance of the product, will occur slowly due to high viscosity at rest
conditions. On the other hand, the shear-thinning and thixotropic behaviors have a
significant importance in the ability of the facial mask to spread on the skin, where the
Dead Sea mud mask can break down for easy spreading and the applied film can gain
viscosity instantaneously to resist running. Newtonian materials do not behave in this
way, because when spread on the skin they run very quickly, reducing the thickness of
the required film.
FACIAL MASK OF DEAD SEA MUD 451
In order to evaluate the effect of shearing time on the rheological behavior of the mud
mask, the viscosity-shear rate relationship was determined at different times of shearing.
Dead Sea mud mask samples were sheared at different values of constant shearing rate
and at different temperatures for 40 min. At 5øC the mud mask samples exhibit a
time-independent behavior at low shear rate and a weak thixotropic behavior at high
shear rate (see Figure 8). The weak bonding between particles could explain the strength
drop observed when the temperature and shear rate increase. However, the rate and
extent of viscosity decay depend on both the applied shear rate and the temperature.
Typical thixotropic behavior obtained at different shear rates for the Dead Sea mud mask
at 45øC is shown in Figure 9.
The observed time-dependent flow behavior of the mud mask was modeled using the
structural kinetics approach (9). This model postulates that the change in the rheological
properties is associated with shear-induced breakdown of the internal fluid structure in
the Dead Sea mud. Using the analogy with chemical reactions, the final form of the
structural breakdown process can be expressed as in equation 2. For all mud mask
samples investigated, it was found that their apparent viscosity data at constant shear
rates could be correlated with equation 2, using o• -- 2, i.e., with a 2nd order irreversible
kinetic model. A good comparison between the model fitted results (solid lines) and the
experimental apparent viscosity/time data for the mud mask can be seen in Figures 8
and 9.
The rate constant, k, is a measure of the rate of thixotropic breakdown. Meanwhile the
ratio of the initial to equilibrium viscosity, qqo/q%, can be considered as a relative
measure of the amount of structural breakdown, or in other words as a relative measure
lOO
2 i
I=,½i,I M,$k "•] ............................. :: ............................. 'i- q- 2.20 1/s
k ............................. :: ............................. '71
• 47.43 1/•
,
,
............................. 4 ............................. : ............................. • ............................
••• ............ • ............. i ............. • ............ _i ............. • .............
. _•>•• ,,_ .................... :: .......
• • ....... •k - >!< ....... X- ......... • ........... X ...........
I I I I
5 15 25 35
0 10 20 30
Shearing time (min)
Figure 8. Dependence of the facial mask's apparent viscosity on shearing time at 5øC.
452 JOURNAL OF COSMETIC SCIENCE
lOO
........................... ............................. [ /__i__ /
! • S-K modem i 28 38 1 s
,-----' _• ............. : .............. ?•---S-Kmodel)___!._. . /
' " 131.9 1/s
2
15 25
o lO 2o 3o
Shearing time (rain)
5 35
4O
Figure 9. Dependence of the facial mask's apparent viscosity on shearing time at 45øC.
of the extent of thixotropy. The values of k and Xlo/Xl• as a function of the applied shear
rate and the temperature are reported in Table II. As one expected for a thixotropic
structured material, k generally increases with increasing shear rate and temperature.
Thixotropy is the result of structural breakdown under shear and manifests itself as a
decrease in the apparent viscosity with time. As time of shear elapses, the rate of
breakdown will decrease, as a fewer structural bonds are available for breakdown. Struc-
tural reformation may take place and the rate of this process will increase with time of
shear due to the increasing number of bonding sites available (15). Table II shows also
that the amount of structural breakdown (Xlo/Xl•) increases also with temperature and
shear rate.
CONCLUSIONS
The temperature and shearing conditions dependency of the apparent viscosity were
investigated for a facial mask made mainly of Dead Sea mud. The mud facial mask
behaved like a shear-thinning material with a yield stress and generally exhibited a
thixotropic behavior in the temperature range of 5 ø to 60øC. This behavior has a
practical significance that decelerates particle sedimentation due to high viscosity at rest
conditions. In addition, the shear-thinning and thixotropic behaviors have a significant
importance in the ability of the facial mask to spread on the skin with a controllable film
thickness. The Herschel-Bulkley model fitted well the flow curves of the mud facial
mask. The effect of temperature on the facial mask's apparent viscosity was divided into
three stages. In the first stage, 5ø-20øC, the viscosity decreased, as expected, with
FACIAL MASK OF DEAD SEA MUD 453
Table II
Degree and Extent of Thixotropy of Dead Sea Mud Mask, Evaluated at Different Shear Rates
and Temperatures
T] o
Temperature (øC) 'y' (s •) k x 10 3 (rain -•) 'qo/qq•, (Pas)
5 2.20 0.0 1.00 24.4
10.21 47 1.05 12.7
28.38 121 1.16 6.2
47.43 190 1.23 4.5
15 2.20 28 1.03 22.3
10.21 79 1.05 10.6
28.38 125 1.17 5.2
47.43 361 1.28 2.7
25 2.20 39 1.08 23.5
10.21 139 1.13 10.1
28.38 209 1.20 4.9
47.43 404 1.35 2.8
35 2.20 82 1.34 41.4
10.21 270 1.43 12.2
28.38 280 1.50 5.5
79.02 489 1.51 3.8
45 2.20 150 1.90 47.8
10.21 328 1.93 12.9
28.38 555 1.97 5.2
131.90 618 2.76 2.7
temperature. But increasing the temperature from 20 ø to 45øC led to an increase in
viscosity. This behavior was attributed to the gelatinization of the stabilizer. In the third
stage, 45ø-60øC, the mud mask regained normal behavior and its viscosity decreased
with temperature. As far as the effect of steady shearing on the flow properties of a Dead
Sea mud mask is concerned, the second order structural kinetic model described its
thixotropic behavior well. The rate of structural breakdown increased with both shear
rate and temperature.
ACKNOWLEDGMENTS
The authors are grateful to Dr. Hussam EI-Haffar and Mrs. Aida Frehatt from Aremort
Co. for their kind cooperation and supply of materials.
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454 JOURNAL OF COSMETIC SCIENCE
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