407
Acta Chim. Slov. 1998, 45(4), pp. 407-418
VISCOELASTIC PROPERTIES OF WELAN SYSTEMS -
PRACTICAL APPLICATIONS
Andreja Zupanèiè
University of Ljubljana, Faculty of Chemistry and Chemical Technology, Aškerèeva 5,
1001 Ljubljana, Slovenia
Abstract
A detailed  rheological characterization of aqueous welan systems was performed in
the concentration range from 0.1 to 0.5 %w/w of the polymer. In order to illustrate the
practical application of aqueous welan systems, the rheological investigations were
extended to alumina dispersions in aqueous welan matrices where the polymer
concentrations in the disperse medium were of the same range (0.1-0.5 %w/w). The
dependence of the viscoelastic properties of aqueous welan systems and alumina-welan
dispersions on polymer concentration was examined and discussed in the light of
fractional constitutive equations and in the view of practical applications. The
experimental data of aqueous welan systems, as well as the results of modeling were
compared with those obtained for alumina-welan dispersions.
1. INTRODUCTION
Aqueous polymer solutions, especially those of natural biopolymers, are very
important material source in various industrial branches because they exhibit a high
stability level and good compatibilty and because they are biodegradable. In recent years,
the production and the application of such enviromentally friendly materials has
increased. Industrial uses of polysaccharide systems centre on their ability to increase
their weight in the water by several times when it thickens or structures, thus controlling
the rheology of hydrated systems. The solution properties of these carbohydrates are of
408
considerable interest for a number of practical commercial applications such as
thickening, suspending and gelling agents[1]. Such properties can be profitably
exploited for many industrial applications, in particular in the food, pharmaceutical and
ceramic industries.
Hydrophilic polymers and their aqueous dispersions exhibit some specific
rheological properties.  The structural conditions of these systems, which are influenced
by various environmental conditions and polymer concentration, can be determined from
detailed rheological examination. Rheological  characterization of aqueous
polysaccharide systems deals with a wide range of mechanical responses, from  pure
liquids to elastic solids. These  systems in general exhibit the rheological behavior
characteristic for polymers or polymer solutions, but under certain circumstances also
some specific rheological properties. The transport properties and, specifically, the
rheological behavior of real and complex materials such as aqueous polysaccharide
systems can be significantly affected by several factors, mainly related to molecular and
supermolecular features. Most of these factors are common to all polymeric systems,
others are peculiar to carbohydrate polymers. Since the mechanical properties of aqueous
polysaccharide dispersions depend on their physical and chemical conditions, their
network structural formations depend on polymer concentration, ionic strengths,
presence of organic solvents, pH, temperature,  etc. In general, hydrophilic polymers are
able to form three-dimensional network structures or gel structure already at very low
polymer concentrations. The gel structure exhibits elastic solid behavior under the
conditions of small deformations. Through a rheological study it is possible to evaluate
not only the time evolution of the macroscopic properties of a given system, but also its
structural and /or molecular characteristics.
The structural conditions of aqueous polysaccharide dispersions can be classified
as: dilute polymer solutions, weak gels, micro gels and strong gels. Above the critical
polymer concentration, overlapping and associations of polymer molecules result in the
formation of extended junction zones, or in other words, in the formation of a
continuous three-dimensional network, where the junction zones play the part of  point
crosslinks in covalent gels[2]. Strong gel behavior can be observed when the polymer
association is strong. Under small deformation conditions the mechanical spectra of
strong gels closely approximate solid-like behavior. When subjected to large deformation
or continuous flow conditions, the gel network ruptures into small gel regions. The flow
conditions then become heterogeneous. This is not the case for weak gels, which may
flow homogeneously, even under continuous shear conditions. They exhibit marked non-
409
Newtonian properties, strictly connected to the progressive disruption of the network
into smaller flow units as the shear rate increases. Also weak gels exhibit elastic solid
behavior under the conditions of small deformations, but the elastic contribution to the
viscoelastic response in this case is not so predominant as for strong gels. When polymer
aggregation is favored, segregation into two phases may take place or interchain
association may be restricted to small microgel clusters dispersed in a macromolecular
solution[2]. The rheological behavior of the microgels depends on the polymer
concentration. At a high concentration of the aggregates they exhibit solid like behavior
at low stresses and their rheological properties cannot be easily distinguished from those
of weak gels. Correlation of rheological data and modeling constitutes a complementary
step, necessary for the  exploitation of the experimental results.
Several industrial products and biomaterials are composed of solid particles
dispersed in polymeric gel matrices. The rheological properties of filled gels are strictly
governed by those of the polymeric matrix. The influence of particle addition can become
very important, depending on the effective volume concentration of the disperse phase
and the type and extent of particle-gel matrix interactions. Weak gel matrices are
frequently preferred for many practical purposes, particularly when remarkable
concentration levels of the disperse phase must be reached in the filled gel, since their
viscous and elastic properties are favorably combined to ensure easy manipulation and
transportation, as well as good stability. Among several polysaccharides giving rise to
more or less weakly structured systems when dissolved in water under proper conditions,
welan can be considered a good candidate to form weak gel matrices, since the sol/gel
transition can be attained at low polymer concentrations and their gel properties are only
slightly dependent on environmental conditions. Welan gum is produced by Alcaligenes
spp. ATCC 31555 and has the same basic backbone repeating units as gellan gum,
another microbial polysaccharide of widespread industrial use, but with a single a-L-
rhamnose or a-L-mannose as side groups in the ratio of 2:1 per repeating unit[2]. Even
if the primary structure of these two bacterial polysaccharides differ basically only for the
side chains, significant differences can be noted in their viscoelastic properties in aqueous
media. Thus, whereas gellan can form stable gels in salt aqueous solutions, welan
systems often behave as weak gels, so standing between solutions and chemical gels, and
their thermal stability represents an important characteristic for oilfield applications.
The present paper reports the results of experimental studies carried out on
aqueous welan systems under small and large deformation conditions. Furthermore, the
paper deals with the problems related to the preparation of concentrated aqueous
410
dispersions of ceramic powders in welan matrices. High purity alumina powder is
selected in order to prepare ceramic dispersion in gel matrices. The specific objective of
this work is to examine the influence of welan concentration on the structural conditions
of aqueous alumina-welan systems.
2. EXPERIMENTAL
The materials used for the preparation of polysaccharide matrices, alumina
suspensions, and alumina dispersions in welan matrices were: welan K1A96 (Monsanto)
high purity alumina powder A16 SG (Alcoa) (d50 = 0.5 mm), distilled water and anionic
dispersant Duramax D3021 (modified polyacrylic acid, Mw 5000 Rohm&Haas).
The powdered polymer (welan)  was  dissolved at different concentrations (0.1 - 1
%w/w) in distilled water by mechanical stirring at ambient temperature and stored for
two days in order to ensure a complete wetting of the polymer. Alumina-welan
dispersions were prepared in two steps. Highly concentrated aqueous alumina
suspensions were used as a source of particle addition in the gel matrices. Alumina
suspensions were prepared at the same solid volume fraction of 0.55 by dispersing the
alumina  powder in aqueous - dispersant  solution. The dispersant  concentration was 0.3
%w/w as calculated on the alumina powder, solid dry weight basis. The suspensions
were prepared by ball milling in 0.5 litre polypropylene jars filled with 500 g of Si3N4
milling balls for 16 hours at a speed of 70 rpm. The two components (alumina
suspensions and welan systems) were blended together in proper quantities by hand
mixing. For all the systems the final solid volume fractions were, 0.38 and 0.48 with
welan concentration ranging from 0 to 0.5 %w/w. The rheological characterization was
carried out two days after the preparation.
The rheological tests were performed under steady and oscillatory shear
conditions at 20, 25 and 30°C using mainly the controlled stress rheometer Haake,
RheoStress RS 150, equipped with a double cone sensor system (DC 6/4). Some
additional tests were done by a controlled rate rheometer Haake CV100, equipped with
coaxial cylinder sensor system ZB15. The rheological properties were studied under
small and large deformations, as well as in flow conditions, by applying different
procedures: stress and frequency sweeps, stress ramps and multistep sequences.
411
3. RESULTS AND DISCUSSION
Flow curves - large deformation conditions
The rheological behavior of aqueous welan systems strongly depends on the
polymer concentration. At 0.1 wt % of welan the linear and non-linear properties are
quite similar to those of polymer solutions. Shear thinning behavior and negligible time
dependent effects are observed in continuous shear. With increasing polymer
concentration, above 0.2 wt % of welan, more pronounced shear thinning behavior and
detectable thixotropic time dependent effects are observed. The rheological properties in
this case resemble those peculiar for structured liquids or weak gels. Figure 1.
demonstrates the influence of welan concentration on the flow properties of aqueous
welan systems.
The rheological investigation is extended to examination of alumina dispersions in
aqueous welan matrices where the solid volume fraction (Φ) of alumina powder is 0.38.
The most significant changes in the flow properties of the dispersions are observed
already at the lowest polymer concentration (0.1 %w/w of welan) in the disperse
medium. At higher welan concentrations the shape of the flow curves, as well as the
viscosity level, become similar to those of aqueous welan matrices.
1,E-02
1,E-01
1,E+00
1,E+01
1,E+02
1,E+03
1,E+04
1,E-02 1,E-01 1,E+00 1,E+01 1,E+02
shear stress (Pa)
V
is
co
si
ty
 (
P
a.
s)
0.1 %
0.2 %
0.3 %
0.5 %
1,E-02
1,E-01
1,E+00
1,E+01
1,E+02
1,E+03
1,E+04
1,E+05
1,E-02 1,E-01 1,E+00 1,E+01 1,E+02
shear stress (Pa)
vi
sc
os
ity
 (
P
a.
s)
Al 38
0.1 %
0.2 %
0.3 %
0.5 %
Fig.1: Flow curves of aqueous welan systems
at different welan concentrations
Fig.2: Flow curves of alumina-welan
dispersions at Φ = 0.38 and different welan
concentrations
Viscoelastic properties - small deformation conditions
The results obtained from stress sweeps at a frequency of 1 Hz show that the
critical strain for the linear viscoelastic regime decreases with increasing polymer
concentration. Figure 3 illustrates such peculiar features for some aqueous welan systems
412
at different polymer concentrations. The limit of linear viscoelastic response of all
aqueous welan systems ranged up to 10% of strain amplitude, whereas the critical strain
amplitude of alumina-welan dispersions is shifted toward lower values of strain amplitude
(between 0.7 and 2%) as shown in Figure 4. The numbers beside the symbols in Figures
3 and 4 indicate welan concentrations in %w/w.
Some preliminary tests were performed to check the influence of environmental
conditions on the rheological properties of aqueous welan systems. The effects of pH by
addition of HCl or NH4OH, temperature (8 - 45°C) and the presence of dispersant were
0,1
1
10
100
0,1 1 10 100 1000
γ (%)
G' [Pa]
0.5
0.3
0.1
1
10
100
1000
0,1 1 10 100 1000
γ [%]
G' [Pa]
0.5
0.3
0.2
0.1
Fig. 3: Stress sweep tests: aqueous welan
systems at different welan concentrations
Fig. 4: Stress sweep tests: alumina-welan
dispersions at Φ = 0.38 and different welan
concentrations
checked in the range of polymer concentration between 0.3 and 1 %w/w of welan. It was
found that the systems were almost insensible to the investigated environmental
conditions. This is confirmed in Figure 5 which illustrates the mechanical spectra of
aqueous welan systems at different pH (2 – 9.5) and the same polymer concentration of
0.5 %w/w.
The mechanical spectra of aqueous welan systems at different polymer
concentrations (at 25 °C) are reported in Figure 6. For the lowest polymer concentration
(0.1 %w/w) the viscous component exceeds the elastic one under low amplitudes of
oscillatory shear strains, below i.e. 10%. At >0.2 %w/w of welan the elastic component
becomes predominant and both moduli are less sensitive to the applied frequency. When
polymer concentration exceeds 0.1%, a net change is observed in the profiles of both
moduli, suggesting the appearance of a sol/gel  transition.
413
Influence of pH 
1
10
100
0,1 1 10 100
frequency ω (rad/s)
G
'  
 G
" 
  (
P
a)
8,7 8,7
9,1 9,1
9,5 9,5
7 7
2 2
G'  
 G" 
0.5 wt % of welan
Fig. 5: The influence of pH on the mechanical spectra of aqueous welan systems
At above 0.1 % of welan the increase of polymer concentration (C ), G’ and G’’
scale as C 1.9-2.3 and C 1.7-1.95, respectively (both exponents increase with decreasing
frequency). A C 2 dependence of the storage modulus (G’) is often observed for gelling
systems, in accordance with theoretical models proposed for biopolymers with high
functionality[3].
Let us now consider the effects of welan concentration on viscoelastic properties
of alumina-welan dispersions at a solid volume fraction of 0.38. The mechanical spectra
of alumina-welan dispersions at different polymer concentrations in the disperse medium
are shown in Figure 7. As already mentioned, the most significant changes in the
rheological properties are observed for the systems containing 0.1 %w/w of welan.
Under linear oscillatory shear conditions the elastic component appreciably predominates
over the viscous one for all alumina-welan systems and both moduli strongly increase in
the presence of polymer in the disperse medium. When polymer concentration exceeds
0.1%, the dispersions behave like structured liquids or weak gels. With increasing
polymer concentration (> 0.2 %w/w) in the disperse medium the traces of G’(ω) and
G’’(ω)  become parallel to each other and G’ is at least one order of magnitude higher
than G’’. Such a condition corresponds to an apparent sol/gel transition according to the
criterion suggested by Winter and Chambon [4].
414
0,01
0,1
1
10
100
0,01 0,1 1 10 100
ω [rad/s]
G
', 
G
'' 
[P
a]
G' 0.5 G'' 0.5
G' 0.3 G'' 0.3
G' 0.2 G'' 0.2
G' 0.1 G'' 0.1
welan
Fig. 6: Frequency dependence of G’ and G’’ for aqueous welan systems, numbers beside the
symbols indicate welan concentrations in %w/w and curves calculated from the Friedrich
model.
0,1
1
10
100
1000
0,01 0,1 1 10 100
ω [rad/s]
G
', 
G
'' 
[P
a]
0.0 wt %
0.1 wt %
0.5 wt %
0.3 wt %
alumina-welan
Fig. 7: Frequency dependence of G’ (filled symbols) and G’’ (open symbols) for alumina-welan
dispersions with Φ=0.38 at 25°C, and curves calculated from the Friedrich model
415
In the case of such structured fluids the analysis of experimental data must
necessarily be performed by resorting to phenomenological models instead of molecular
approaches, particularly when we aim to describe effects of polymer concentration or
sol/gel transitions. Satisfactory data fitting of mechanical spectra is provided by the
fractional derivative (per time) Maxwell model suggested by Friedrich and Braun [5].
[ ] [ ] [ ]{ } [ ]τ τ γ γ γ+ = + +∞D G D D G Dc c d0 ∆ (1)
where τ  and γ  are the stress and strain tensors, respectively, and c and d are the
derivation  orders. When c = d = 1,  the equation corresponds to the ‘solid model’ for
G∞ > 0, whereas the canonical Maxwell ‘liquid model’ is obtained for G∞ = 0. The
fractional derivative model was selected because it is able to describe a wide range of
viscoelastic responses, from viscous liquid to elastic solid, by a limited number of
parameters. Under oscillatory shear conditions the linear viscoelastic response can be
expressed by the following equations:
( )
G G G
d d c d c
c c c
'
( ) ( )
( ) ( )
( ) =
cos (
2
) cos (
2
)
cos (
2
)
ω
λω
π
λω
π
λω
π
λω
∞ +
+ −




+ +
∆
1 2 2
(2)
( )
G G
d in d c in d c
c c c
' '
( ) ( )
( ) ( )
( ) =
s (
2
) s (
2
)
cos (
2
)
ω
λω
π
λω
π
λω
π
λω
∆
+ −




+ +1 2 2
(3)
where G∞ represents the equilibrium modulus when the frequency tends to zero, and
hence the model describes liquid-like responses if G∞ = 0. Parameter λ0 is a characteristic
time of a material which determines the role of the relative contribution of elastic and
viscous components in the viscoelastic response. Figures 6 and 7 report a comparison
between experimental data and the curves calculated from equations (2) and (3).
Apparently, the model provides an adequate correlation of loss and storage moduli as the
function of frequency in the whole range of polymer concentration for both aqueous
welan systems and alumina-welan dispersions. In order to obtain reasonable values of the
parameters in equations (2) and (3), the yielding conditions: 0 < c ≤ d ≤ 1 and G∞ ≥ 0
should be satisfied. As an objective function in the minimization procedure the mean
square relative deviation was used. Evaluation of the model parameters by applying the
fitting procedure simultaneously for both G’(ω) and G’’(ω) showed that for almost all
studied systems exponent d converged to the limiting conditions d = 1. Consequently, the
influence of polymer concentration on the frequency dependence of the dynamic moduli
416
is described through the variation of only four parameters: G∞ , ∆G, λ0 and c. The
variation of relaxation time λ0 and exponent c with increasing welan concentration in
aqueous welan systems and alumina-welan dispersions is shown in Figure 8. Increase of
λ0 with polymer concentration in aqueous welan systems indicates that the elastic
contribution becomes more important. The value of λ0 evaluated for the reference
aqueous alumina suspension is quite different from those of the alumina-welan
dispersions (Figure 8). Noticeable increase of exponent c is observed only at low
polymer concentrations. At higher welan concentrations c values ranges around 0.8 for
both welan and alumina-welan systems. The most significant differences in parameter
∆G, which control the level of the dynamic moduli, is observed between 0.1 and 0.2
%w/w,  where sol-gel transition can be expected for aqueous welan systems, whereas
∆G exhibits a strong increase as soon as welan is present in the alumina-welan
dispersions (Figure 9). The differences in structural conditions between aqueous welan
systems and alumina-welan dispersions are indicated also with the appearance of G∞, (Ge
in Figure 9) for the alumina-welan dispersions. For aqueous welan systems the values of
G∞ converge to limit conditions, G∞ = 0. When sol-gel transition is observed for the
aqueous systems, alumina dispersions exhibit ‘solid like’ response under non-destructive
conditions of small deformations.
0
30
60
90
120
150
180
0 0,1 0,2 0,3 0,4 0,5
conc of welan (wt%)
λλ 0
 (
se
c.
)
0
0,2
0,4
0,6
0,8
1
c 
(-
)
λο λο c c
alumina-welan welan
0,01
0,1
1
10
100
1000
0 0,2 0,4 0,6
conc of welan (wt%)
∆G
, G
e 
(P
a)
Ge alum.-wel.
alumina-welan
welan
Fig. 8: Parameters λ0 and c of the Friedrich
model vs. welan concentration for aqueous
systems and alumina-welan dispersions at 25°C
   Fig. 9: Parameters ∆G and G∞ of the Friedrich
model vs. welan concentration for aqueous
systems and alumina-welan dispersions
417
0,01
0,1
1
10
0,1 1 10 100
shear stress [Pa]
vi
sc
os
ity
 [P
a.
s]
20
25
30
Φ = 0.38 + welanΦ = 0.48
Φ = 0.48 + welan
Fig. 10: Flow curves of aqueous alumina suspension (Φ = 0.48) and alimina welan dispersions
(Φ = 0.38 and 0.48) at 20 °C, 25 °C and 30 °C.
CONCLUSIONS
Aqueous welan systems exhibit weak gel properties even at low polymer
concentration and their behavior does not change appreciably when pH and other
environmental conditions are changed. At 0.1 %w/w of welan the rheological properties
are similar to those of polymer solution, whereas at higher polymer concentrations the
systems exhibit behavior characteristic for structured systems or weak gels. Such
rheological properties of aqueous welan systems enable different practical applications
under wide range of environmental conditions.
Experimental tests performed with alumina-welan dispersions showed that even a
small addition of welan to the disperse medium produces significant changes in the
rheological properties of aqueous  alumina suspensions. As soon as welan is present in
the disperse medium, the rheological properties of alumina-welan dispersions changes
significantly.  Such  systems exhibit  the  behavior  of structured  systems. At  above  0.1
%w/w of welan in the disperse phase the rheological properies of alumina-welan systems
are governed by welan concentration (weak gel behavior). The biopolymer under such
conditions provides an essential contribution to particle stabilisation.
The experimental results demonstrate that welan can be used in ceramics as
matrices for particle dispersions also in the case when high solid content should be
418
achieved. We need some additional tests to support these perspectives. Only for
illustration, alumina-welan dispersions are possible to prepare at least to the solid volume
fractions of 0.48. The effects of temperature (20 - 30 °C) on steady shear viscosity,
evaluated from sequential shear rate steps, are studied for aqueous alumina suspensions
at Φ = 0.48 and alumina - welan (0.25 %w/w) dispersions at Φ = 0.38 and 0.48. Figure
10 shows that the presence of welan in the disperse medium reduces the temperature
effects.
REFERENCES
[1] P. A. Sandford, I. W. Cottrell, D. J. Pettitt, Pure & Appl. Chem. 1984, 56 (7),  879 - 892.
[2] R. Lapasin and S. Pricl, "Rheology of industrial polysaccharides, Theory and Applications"
Chapman&Hall, Blackie Academic & Professional, London, 1995,  Chapter. 1.
[3]  R. Lapasin, A. Martorana, M. Grassi, S. Pricl, Proc. of  5th European Rheology Conference,
 Ed. I. Emri, Portorož , Slovenia 1998, 579 -580.
[4] H. H. Winter, F. Chambon, J. Rheol. 1987, 31, 683 - 697.
[5] Chr. Friedrich, H. Braun., Rheol. Acta 1992, 31, 309 - 322.
POVZETEK
Obširna reološka karakterizacija vodnih gelskih sistemov velana je bila izvedena v
obmoèju koncentracij 0.1 do 0.5 utežnih % polimera v vodi. Da bi prikazala praktièno
uporabo vodnih sistemov velana, sem raziskave razširila na disperzije glinice v vodnih
gelskih matricah velana, pri katerih je bila koncentracija polimera v disperznem mediju v
enakem obmoèju kot pri vodnih sistemih velana. Odvisnost viskoelastiènih lastnosti
vodnih gelskih struktur velana in vodnih disperzij glinice v velanu od koncentracije
polimera sem v predstavljenem delu analizirala z uporabo delnih odvodov konstitutivnih
enaèb. Primerjava eksperimentalnih podatkov in rezultatov modeliranja, pridobljenih na
vodnih sistemih velana, z rezultati doloèenimi za vodne disperzije glinice v gelskih
matricah velana omogoèa predvideti pomen vodnih sistemov velana v tudi razliènih
drugih aplikacijah.