How to Measure Thixotropy for Pharmaceutical and Cosmetic
s e p e r uipm o rh o eq& he • w tsy rs ics vmi i rvrk d ra a e ggr p s o n oo e & w a mr r s r r p r d Tim’s Top Tips - How To Measure Thixotropy For Pharmaceutical And Cosmetice Industries• i n a h p s a n t o t p a i h n re r & s a m k e e r • n e c m i s o s n u t e w r n m c i e i t e f c d s s i s n n m c i c e a u i e c r f r g t i p a s c s f a n i n e r n i i t y u s p r s g s t m e t n s • e i y r s r t u t g e s d c s n i e u n i f mt i t i ry d s c l • e n e e i t o p mp g • s s y n l i d • i y g t o s l r r n s p o t i e e s s t t • s u s m • t e d r y t c l s e n
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Company Profile Contact Details
Rheology Solutions Pty Ltd is a specialist sales and service Rheology Solutions Pty Ltd organisation dedicated to the science of materials Address: 15 -19 Hillside Street, Bacchus Marsh, Victoria, 3340 characterisation and are the exclusive Australian distributor for PO Box 754, Bacchus Marsh, Victoria, 3340 the brand names HAAKE, NESLAB, PRISM and CAHN from Phone: 03 5367 7477 Thermo Fisher Scientific, Optical Control Systems, Schleibinger Fax: 03 5367 6477 Gerate and Marimex Industries Corporation range of equipment Email: [email protected] and instruments, and they also distribute the Shimadzu range of Website: www.rheologysolutions.com tensile testers and texture analysers.
Rheology Solutions recognises the importance of specialisation Disclaimer and dedication to a specification science and as such provides full technical support and service throughout Australia. The The information contained in this report is not intended for direct company goal is to integrate industry experience and materials use as a tool for process development. It is a guide only. This characterisation techniques to provide practical solutions for document remains the property of Rheology Solutions Pty Ltd, customers. and may not be reproduced or altered in any way without the written permission of the owners. Rheology Solutions has an established applications laboratory equipped with a comprehensive range of instruments to meet the Written by Dr. T. Kealy requirements of material characterisation. Specialist contract Technical Manager testing services are also available and contracts can be tailored Rheology Solutions Pty Ltd to suit discrete tests or protracted testing requirements involving a series of tests over a period of weeks or months. © Rheology Solutions Pty Ltd 2007
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Page 2 (4) [email protected] p k se ent p e r uipm o rh o eq & he • w tsy rs ics vmi i rvrk d rra a eo n gg p s oo e & w a r r s Tim’s Top Tips - How To Measure Thixotropy For Pharmaceuticalr And Cosmeticr mpIndustriesr d e • i n a h p s a n t o t p a i h n e r & s r a m k e e r • n e m i s c o s n u t m c e w r n i i t e e f c s s i d s n m c n i c e a u i e c r f r g t i p a s c s f a n Tim's Top Tips - Rheology Solutions for the Pharmaceuticalr i n e n t i y i u s p r s g s t m e t n s • e i y r s r t and Cosmetic Industries. u t g e s d c s n i e u n i f mt i t i ry d s c l • e n e e o i t
g p mp • s s y n l • i d i y g t o s l r r n s p How To Measure Thixotropy Foro t i e e s s t t • s u s m
t • e d r y t l c e s n
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t Pharmaceutical And Cosmetic Industries• y a n l r n i r
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s t n t s t Key Words: Rheology, rotational, liquid, viscosity, thixotropy, yield stress. s t c
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About The Author Introduction e
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Tim has a background in Often the cosmetics and l pharmaceutical industries must
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engineering and specifically in n
overcome problems related to (and often dominatedu by) the flow
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rheology, with a B.Eng and Ph.D. in n properties of their product, thought the relationships between
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r Chemical Engineering and has held c
these properties and production relatedt issues are not always
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postdoctoral research positions in i
immediately apparent. It is the purposec of this series of articles,
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engineering rheology. Tim's r
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“Rheology Solutions for the Pharmaceuticaln and Cosmetic
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research has continued for the last Industries”, to help illuminate the issues faced a by the industry,
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a b
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seven years and recent interests l
how they relate to the flow propertiesa of problem materials and
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and publications include the r
how they can be successfully measured and controlled witha a
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application of rheology and rheometry to mineral, food, polymer a
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and surface coatings systems. His current position encompasses i i
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the managment of customer contract testing and also includes s e
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customer focussed education and training. Additionally he is v
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available to provide technical input for existing or proposed s
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materials characterisation systems for both laboratory and Thixotropy is a form of time dependent behavioura describing a
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production. material whose viscosity decreases over time while it is subjected t
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to shearing forces. After some timee the material recovers r
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completely to its original state. s c
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Table Of Contents ts
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Thixotropy should not be confused with rheopexy, a rare c m v
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Introduction...... 3 t t
phenomenon, (rheopexy is also time dependent,t but the viscosity
a
a u
s
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r
r r
a t
n
m
e
e i
increases with time). The following techniques and discussionsr
Definitions ...... 3 x
t
f
d a
e
g a
o h
for measuring thixotropy apply for rheopectic measurementsu n r c
i
m
Background and discussion ...... 3 w t e
t
s
e c x
f
s l
also. r
e n
o a e
c
Measurement techniques and pitfalls - CS or CR? 4 i t e
s
r i
e
w
r
e c
c
n
t e
s
e
i
r
Experimental procedure n
a
c
e Rheodestruction is also a time related phenomenon,m but
e w
i
s
m t y
h
l
c
t
1. The thixotropy loop test ...... 5 f
s
n
•
rheodestroyed materials do not recover their initialo state. The
i
o
o p
s e
t
w
e
e
2. The constant shear test ...... 7 following techniques and discussions for measuring thixotropy h t •
c
s t
d
r
s
n
•
e
r
o
apply for rheodestructive measurements also. u
t
3. The shear and recovery test ...... 8 e
i s
t e
a o
c
e
c c
s
m
d s
i
r
y
Summary ...... 11 e
d g l
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p
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Other information for pharmaceutical c n
Background and Discussion i
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r
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n
d
t g a o
o
e r f
and cosmetic industries ...... 12 n i
t
i
t d d
s n e a
•
i t n
Thixotropy is a relative measurement and as such depends s on Pharmaceutical and cosmetic dictionary ...... 13 m i l
a
a o
r
e i
o n c
t
i t t r
the experimental conditions and technique used to measurea it. s t a
Information request form ...... 17 d • g y n s
r e i There are several such techniques, the simplest beings modifiedl o
o d n o c r a C flow curves, or constant shear rate or shear t stresse n e g
Q c n i r o r
i l o v u t o r a c t a
c e a e s i e s r i c t i r e n v p d u p r t a o n e g a a m s r r • e e s t od s p e t l n www.rheologysolutions.com (5) Page 3• e a n m a c s t i e e r s t v t o e r m s l i p e u • l a s r t a p s t r i u t d s o c s s e i n np l Tim’s Top Tips - How To Measure Thixotropy For Pharmaceutical And Cosmetic Industries
measurements. A more complicated, but perhaps more Measurement Techniques intuitively understood method is the shear and recovery method. and Pitfalls In order to attain repeatability the most appropriate technique should be selected and the sample handling and experimental CS or CR? procedures defined fully. These should be unchanged for all Thixotropy can be collected either in Controlled Rate, CR, tests, so that thixotropy for different materials can be properly (impose a shear rate and measure the shear stress) or in compared. Controlled Stress, CS, (impose a shear stress and measure the resultant shear rate). In theory, for materials with no time Thixotropy is a relative measure of the extent and speed of dependent properties both CS and CR flow curves should yield recovery of the internal structure of a material during and after identical results. For thixotropic materials one should choose a shear. It is useful because it allows an estimate of the effects of technique and stick to it, so that results will be comparable from agitation, pumping etc for prolonged periods, and also the one test to the next. The main difference lies in the sensitivity of effects of ceasing the agitation etc (i.e. how quickly the structure the instrument at low shear rates. CS instruments generally will rebuild, and how difficult it will be to restart the process as combine an air bearing with a high-end motor to provide good a result). control and measurement of very small deflections, and also of high rpm measurements. CR instruments generally do not have Note: Working definitions are provided at the end of the paper. the same level of control and detection at low rpm.
To make a CR or CS measurement you will need:
• A viscometer or rheometer In general a viscometer can make only CR measurements - in other words flow and viscosity curves are the main purpose of the instruments. As a result thixotropy loops, and steady shear measurements can also be made with these instruments. A rheometer is capable of much more, including viscoelastic measurements, creep and recovery measurements and so on. CS instruments have an air bearing so that these extra measurements can be made. The air bearing also allows shear and recovery measurements to be made because they can probe the structure of the material without influencing the rate at which it rebuilds.
• A suitable sensor system Viscometric geometries should be used to make measurements to generate flow and viscosity curves. Viscometric geometries include cone and plate, cup and bob, plate and plate (all for rotational instruments) and capillaries (for a flow-through device such as a capillary viscometer). Typically rotational devices are used and the viscometric geometries that apply are shown in Figure 1.
.
A
Page 4 (4) [email protected] p k se ent p e r uipm o rh o eq & he • w tsy rs ics vmi i rvrk d rra a eo n gg p s oo e & w a r r s Tim’s Top Tips - How To Measure Thixotropy For Pharmaceuticalr And Cosmeticr mpIndustriesr d e • i n a h p s a n t o t p a i h n e r & s r a m k e e r • n e m i s c o s n u t m c e w r n i i t e e f c s s i d s n m c n i c e a u i e c r f maximum shear rate back to zero shear againr (calledg t the down-i p a s c s f a n i n e r n t i y i curve). The rate of change of shear rateu for the sup- and down-p r s g s t m e t n curves and the time at maximums shear• erate are useri definedy and r s r t u t g e s d c s the number of data points is usuallyn highi so thate a continuous u n i f mt i t i ry d s c l curve is generated. Plotting• the results as sheare rate n vs. shear e e o i t
g p mp • stress or viscosity (Figure 3), if thes up-curves liesy above the down- n l • i d i y g t o s l r r n curve and the measurements can be repeatedp after some rest o t i e e s s t t • s time, then the material is thixotropic.u s m t • e d r y t l c e s n
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Figure 1: Standard viscometric geometries (L to R) Cup & Bob, b
a
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•
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Cone & Plate, Plate & Plate i
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1.0 The thixotropy loop test f s l
r
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c
i
t e
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w
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c
n
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The following discussion is based on a CR ramp test, but the a
c
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i
s
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h
l
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arguments hold equally for CS ramp tests. Similarly, step tests t f
s
n
•
o
i
o
o p
s e
t
could be used rather than ramp tests (see “How to measure flow w
e
e
h t •
c
s t
d
r
s
n
•
and viscosity curves for the pharmaceutical and cosmetics e
r
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i s
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industries” for further discussion on these subjects). As e
c c
s
m
d s
i
r
y
e
d g l
e t
mentioned before, once a technique is settled upon, it should be u
e
o
n
h
a
o i
t
d
p
c
c n
used for every test. Figure 3: Thixotropy loop. i i
r
o
n
d
t g a o
o
e r
f
n i
t
i
t d d
s n e a
•
i t n
This thixotropy loop technique involves a transition from zero s m i l
a
a o
r
e i
o n c
t
i t t r
shear rate to a maximum designated shear rate (called the up- a s t
a d
• g y n s
r e i curve), holding the shear rate constant for a time (this step is s l o
o d n o c r a C sometimes left out), and then ramping smoothly from the t e n e g
Q c n i r o r
i l o v u t o r a c t a
c e a e s i e s r i c t i r e n v p d u p r t a o n e g a a m s r r • e e s t od s p e t l n www.rheologysolutions.com (5) Page 5• e a n m a c s t i e e r s t v t o e r m s l i p e u • l a s r t a p s t r i u t d s o c s s e i n np l Tim’s Top Tips - How To Measure Thixotropy For Pharmaceutical And Cosmetic Industries
• The reaction of the material to the imposed shear rate • Sensor inertia (reaction = shear stress) or shear stress (reaction = shear Sensors, in particular concentric cylinders and large diameter rate) is measured and the viscosity calculated according to cones or plates may be quite heavy. As the rate of rotation equation (1). changes through the ramp test, this weight causes the sensors to accelerate to a higher speed than expected by the • The result of the experiment is a thixotropy loop, as control software in the viscometer or rheometer and so the illustrated in Figure 3. shear rate experienced by the material is higher than that 'imposed' by the controller. This results in a shift in shear • The thixotropy of different materials can be compared by stresses and viscosities for the flow curve from the actual comparing the area bounded by the thixotropy loop, as ones. This effect can be reduced or removed by allowing illustrated in Figure 3. sufficient time for the ramp. A general rule of thumb is to allow at least 1s of test time for every 1s-1 of shear rate in the 1.2 Benefits of the thixotropy loop test ramp. So, a ramp from 0-100s-1 should take at least 100s to complete. • The complete picture A ramp test can provide a complete picture of how the • Time material will behave between the set experimental limits. Because of the potential problems with inertia, ramp tests covering very wide ranges of shear rate can take • Simple measurement and simple analysis considerable time to complete. On the other hand, because The measurement is quite simple to execute and the data is the test is a relative one, the maximum shear rate can be straightforward to process. defined to shorten the duration of all tests if necessary.
• Other data • Temperature control Since viscosity data can also be generated when viscometric At high shear rates, shear heating can be an issue. Shear geometries are used, other interesting and useful data is heating is caused by internal frictional heat generated as the available to the user. lamina of fluid move over each other. When the measuring gap is small this can usually be successfully controlled, but prolonged exposure to high shear rates can still be a 1.3 Potential problems with the thixotropy loop test problem.
• Understanding the meaning of the data • Settling materials Sometimes, the real physical meaning of the changes in the If a material tends to settle, long test times tend to reduce thixotropy loop area, are not apparent. The areas do the likelihood of successful test outcomes. The longer the however offer a good comparison technique for different test takes, the more likely the solid fraction is to have settled materials, or to gauge the effects of different rheology out of the measurement space, or at least created a modifiers on the system. concentration gradient through it. Sometimes it is possible to ramp from high to low shear rates first, rather than the • Relative data other way around, allowing the material to be kept in The results generated are relative only, and not absolute. suspension longer, and if possible to reduce ramp times as Changing experimental procedure or handling could change much as practicable. Alternatively using a modified step test the results significantly. A single loop does not fully describe with high shear steps to resuspend materials between whether or not the sample is thixotropic, or if descending 'measurement' steps can also be successful. The rheodestruction has taken place. Consecutive curves after key is to keep the technique consistent once it has been fixed rest times are necessary to assess the recovery of the decided upon, so that all data is comparable. Often settling material. materials will appear to be thixotropic as the shear stress generally drops while the material settles. • Solid fraction size For multi-phase systems, often there is a solid fraction, which • Chaotic flow has particles of considerable size. If this size is close to the One of the key assumptions for rheological measurements is size of the measuring gap, then one or more particles may that the flow in the measuring gap is laminar. Too high shear 'bridge' the gap and cause an artificially high shear stress. rates can cause the flow regime to become turbulent and the The problem can be solved by using particles no more than measurements are unreliable. The onset of chaotic flow can 1/3 of the gap size (1/10 for concentrated pastes). Also, be overcome or delayed by changing the measuring often larger particles contribute little to the overall flow geometry or the measurement gap. behaviour of the material and can be removed without large penalties for the applicability of the measurement.
Page 6 (4) [email protected] p k se ent p e r uipm o rh o eq & he • w tsy rs ics vmi i rvrk d rra a eo n gg p s oo e & w a r r s Tim’s Top Tips - How To Measure Thixotropy For Pharmaceuticalr And Cosmeticr mpIndustriesr d e • i n a h p s a n t o t p a i h n e r & s r a m k e e r • n e m i s c o s n u t m c e w r n i i t e e f c s s i d s n m c n i c e a u i e c r f • Slip • The reaction of the material to ther imposedg t shear i rate p a s c s f a n i n e r n t i y i Multi-phase systems, tend to slip at the boundary of the (reaction = shear stress) or shearu stress (reactions = p shear r s g s t m e t n measuring geometry. A major assumption for rheological rate) is measured and s the viscosity• e calculatedi accordingy to r s r t u t g e s d c s measurements is that the first layer of material 'sticks' to the equation (1). n i e u n i f mt i t i ry d s c l walls of the measuring geometry. Plate & plate or concentric • e n e e o i t
g p mp • cylinder geometry walls can be roughened or serrated to • The results of the steady shears rates experimenty are viscosity n l • i d i y g t o s l r r n reduce or remove the slip phenomenon. and shear stress curves,s as illustrated inp Figure 5. o t i e e s s t t • s u s m
t • e d r y t l c e s n • Time dependent materials The thixotropy of differenta r materialsi cano be comparedt by r m c a p
t • y a n l r Sample handling and experimental technique can be crucial comparing the times takenn toi reach a certain % rof the initial g o t
o o
m f
n p n c i for repeatably measuring time dependent (eg thixotropic) shear stress, or some fixede value of the shear stress, or an o t r s
s • s c o
m e f materials. The same handling procedure (pouring, mixing, equilibrium value (see Figures • 5). •
t t e
s
s t n t s t s
t
c resting, loading into the test equipment etc) and e c m y
n a
e s u
m r t e
experimental procedure (rest time, ramp time and upper and d t s
u
C r
o n y m t r
s
Q
o u
lower limits) is critical for repeatable measurements, and it s
p r
c l
n t
C
i
a s
g
• must be remembered that for time dependent materials the c
Q
n i
g n
i
s i t
l
t
n
t
results are relative only - they depend on the technique used i a p
g
s
n
t
c
o
e i
s n
e
i
t t
e
t to generate them. •
t
m
p
r s
s
u
o
e e
k
t
r
t
c
t
n
• m
s a
k e
l
o
s
n
c
t
i t b
2.0 The constant shear test m a s
l n
u
a g n
l e b r
o n
t
e
i
s
m
b n t
r
s
n u
o &
The following discussion is based on a CR test, but the i a
r
e
b
t r
c t
r
g
s
e
a
arguments hold equally for CS tests, where the shear rate is •
k n
n
b
i
c i
c
t y
b
a
r s
•
l monitored as a function of time. g
u
e o
r
b n
t y t
i
r
t a
l
• n
r
s o
a
o
t i s
o e
r r
t
a
The constant shear technique involves imposing a single, b b
r r e e
l
a
t t
a l
o
a
a
e
c
constant shear rate (of shear stress) on the material and i
b
s
r
a
m
m
n
•
e l
t
o
monitoring the response. The results are usually compared by an o
• y
i
s
a
e
r t
n
s
h
a o
assessment of the time taken to reach a certain predetermined m
r o
t p c
i
i
a
l t o •
•
r
a p
shear stress or viscosity, or to achieve a predetermined % of the h
s
o
c s
e
p
i
p
b
c l k
Figure 5: Results from a constant shear testa
i
r
a
initial or final value, or by the time taken to achieve an o
p
l v
•
o
h
r p
s
s
e
a
equilibrium value w
n k s
on r
o • • i
i
o t
&
2.2 Benefits of the constant shear testt s
a
a
r
w s
s
on
c
r i
e
i
i
i
r
t • l t
a
e
e
p a
s t
p
s
r p
c
i m
• Steady shear rates e
r
a e
a
r
o
t
r
e
c
•
t
e
•
Tests can be chosen to cope better with sheara heating. s
c
i
h
m
a v
c ts
on r
o
i
n
s •
a c t
l
e
s
a h
s a
i
i
s
• Sensor inertia r
c
m v
i r
e
r
u s
e
• l e
d
r
Because the shear rate is unchanged for the durationt of the
t
t
a
a u
s
i c s
r
r r
a t
n
m
test, the controller has time to adjust the speed e of rotation e
i r
x
t
f
d a
e
g a
o
h
of the sensor so that it is exactly as specified. u
n r c
i m
w t e
t
s
e c x
f s l
r
e n
o a e
c
i
t e
s
r
Figure 4: Input for constant shear test i
• Intuitive interpretation of the data e
w
r
e c
c
n
t e
s
e
i
r
n
The meaning of the data is easily understood and applieda
c
e m
e w
i
s
m t y
h
l c
t
once the appropriate process shear rates or shear stressesf s
n
•
2.1 Experimental procedure o
i
o
o p
s e
t
w
are known. e
e
h t •
c
s t
d
r
s
n
•
e
r
o u t The experimental layout can be one shown in Figure 1. e
i s
t e
a o
c
e
c
• Time c s
m
d s
i
r
y
e
d g l
e
t
u
Appropriate shear rates and % of original values can e be
o n
• The material is loaded and the measuring geometry closed. h
a
o i
t
d
p
c
c
n i
chosen to enable the testing to be relatively quick. i r
o
n
d
t g a o
o
e r
f
n i
t i
• A constant shear rate (CR) or shear stress (CS) is chosen, as t d d
s n e a
•
i t n
s • Simple measurement and simple analysis m
i l
a is the number of measurement points. a o
r
e i
o n c
t
i t t r
The measurement is quite simple to execute and the dataa is s t
a d
• g y n s
r e i
s l
o straightforward to process. d • The material is subjected to a shear rate profile, like in Figure o n o c r a C
t e n e g
Q c
4. o n i r r
i l o v u t o r a c t a
c e a e s i e s r i c t i r e n v p d u p r t a o n e g a a m s r r • e e s t od s p e t l n www.rheologysolutions.com (5) Page 7• e a n m a c s t i e e r s t v t o e r m s l i p e u • l a s r t a p s t r i u t d s o c s s e i n np l Tim’s Top Tips - How To Measure Thixotropy For Pharmaceutical And Cosmetic Industries
2.3 Potential problems with the constant shear test • Chaotic flow One of the key assumptions for rheological measurements is A single test does not fully describe whether or not the sample that the flow in the measuring gap is laminar. Too high shear is thixotropic, or if rheodestruction has taken place. Consecutive rates can cause the flow regime to become turbulent and the curves after fixed rest times are necessary to assess the recovery measurements are unreliable. The onset of chaotic flow can of the material. be overcome or delayed by changing the measuring geometry or the measurement gap. • Solid fraction size For multiphase systems, often there is a solid fraction, which • Slip has particles of considerable size. If this size is close to the Multi-phase systems tend to slip at the boundary of the size of the measuring gap, then one or more particles may measuring geometry. A major assumption for rheological 'bridge' the gap and cause an artificially high shear stress. measurements is that the first layer of material 'sticks' to the The problem can be solved by choosing a measurement walls of the measuring geometry. Plate & plate or concentric geometry so that maximum particle size is no more than 1/3 cylinder geometry walls can be roughened or serrated to of the gap size (1/10 for concentrated pastes). Sometimes reduce or remove the slip phenomenon. larger particles contribute little to the overall flow behaviour of the material and can be removed without large penalties • Time dependent materials for the applicability of the measurement. Sample handling and experimental technique can be crucial for repeatably measuring time dependent (eg thixotropic) • Early warning of problems with technique or materials. The same handling procedure (pouring, mixing, measurement resting, loading into the test equipment etc) and Having data for the complete range of shear rates allows the experimental procedure (rest time, ramp time and upper and user to investigate the possibility of settling, chaotic flow in lower limits) is critical for repeatable measurements, and it the measuring gap and slip at the measurement geometry must be remembered that for time dependent materials the walls. Data from a single shear rate test may not reveal this results are relative only - they depend on the technique used information readily, especially if only a few data points are to generate them. taken. Often in these cases settling materials will appear to be thixotropic as the shear stress generally drops while the 3.0 The shear and recovery test material settles. The following discussion is based on a CS/CR/CS test, but the • Temperature control arguments hold equally for CS/CS/CS tests, where the viscosity of At high shear rates, shear heating can be an issue. Shear the middle segment is dictated by the imposition of a high shear heating is caused by internal frictional heat generated as the stress for a short time. lamina of fluid move over each other. When the measuring gap is small this can usually be successfully controlled, but The shear and recover technique involves non-destructive prolonged exposure to high shear rates can still be a evaluation of the initial structure, using an oscillatory problem. To reduce this, the steady shear chosen for the test measurement, followed by a highly destructive rotation segment can be reduced to reduce or remove the likelihood of shear at high shear rate or shear stress. These two are immediately heating. followed by a non-destructive oscillatory measurement identical in its settings to the first segment, observing structure rebuild. • Settling materials The results are usually compared by an assessment of the time If a material tends to settle, long test times tend to reduce taken to finally regain a certain predetermined viscosity, or to the likelihood of successful test outcomes. The longer the achieve a predetermined % of the initial value, or by the time test takes, the more likely the solid fraction is to have settled taken to achieve an equilibrium value. out of the measurement space, or at least created a concentration gradient through it. Sometimes it is possible to step from high to low shear rates, rather than the other way around, allowing the material to be kept in suspension longer, and if possible to reduce step times as much as practicable. Alternatively using a modified step test with high shear steps to resuspend materials between descending 'measurement' steps can also be successful. A settling material will appear to be thixotropic as the shear stress generally drops while the material settles. Figure 6: Input for shear and recovery test
Page 8 (4) [email protected] p k se ent p e r uipm o rh o eq & he • w tsy rs ics vmi i rvrk d rra a eo n gg p s oo e & w a r r s Tim’s Top Tips - How To Measure Thixotropy For Pharmaceuticalr And Cosmeticr mpIndustriesr d e • i n a h p s a n t o t p a i h n e r & s r a m k e e r • n e m i s c o s n u t m c e w r n i i t e e f c s s i d s n m c n i c e a u i e c r f • Shear heating r g t i 3.1 Experimental procedure p a s c s f a n i n e r n t i y i These tests, including the high shearu segments can be pset up r s g s t m e t n to reduce the likelihoods of shear• heating.e i y The experimental layout can be one shown in Figure 1. r s r t u t g e s d c s n i e u n i f mt i t i ry d • Simple analysis s c l • The material is loaded and the measuring geometry closed. • e n e e o i t
g p mp The data is straightforward to process. • s s y n l • i d i y g t o s l • The parameters (determined before) for the oscillatory and r r n s p o t i e e s s t t • s high shear segments are chosen. u s m e 3.3 Potential problemst with• the shear and recovery d r y t l c e s n
a r i o t
r m c test a p t • y • The material is subjected to a shear rate profile, like in Figure a n l r n i r
g o t
o o
m f n p n 6. c i e o t r s
s • More complicated• measurement s c o
m
e f s •
• t The measurement ist not as straightforwarde to set up, as s
• The reaction of the material to the imposed deformation (for s t n t s t s
t c
e c m some preliminary work needs to be doney to find the Linear
n a
e s oscillatory measurements it is convenient to use the complex u
m r t e
d t
s u
Viscoelastic Envelope (LVE) for the C oscillatory segments. r
viscosity), shear rate (reaction = shear stress) or shear stress o n y m t r
s
Q
o u
s
p r
c
Experienced operators can quickly achievel this.
n t
(reaction = shear rate) is measured and the viscosity C
i
a s
g
• c
Q
n i
g n
i
s i t
calculated according to equation (1). l
t
n
t
i
a p
g
• Instrument s n
t
c
o
e i
s n
e
i
t t
e t
A CS instrument is required for this type• of measurement,
t
m
p r
• The results of the steady shear rate experiment are viscosity s
s
u
o
e e
k
t r
which is usually more costly, and also involves thet need for
c
t
n
•
vs. time curves, as illustrated in Figure 7. m
s a
k e
l
o
s
n
c
a supply of clean, oil-freet compressed air.
i t b
m
a s
l n
u
a g n
l e b r
o
The thixotropy of different materials can be compared by n
t
e
i
s
m
b n t
• Solid fraction size r
s
n u
o &
comparing the times taken to reach a certain % of the initial i
a
r
e
b
t r
c t
r
For multiphase systems, often there is a solidg fraction, which s
e
shear stress, or some fixed value of the shear stress, or an a
•
k n
n
b
i
c i
c
t y
has particles of considerableb size. If this size is close to the
a
r s
equilibrium value (see Figure 5). •
l g
u
e o
r
b n
t
y t
size of the measuring gap, then onei or more particles may
r
t a
l
• n
r
s o
a
o
t
i s
'bridge' the gap and cause an artificiallyo high shear stress. e
r r
t
a b
b
r r e e
l
a
t
t
a The problem can be solved byl choosing a measuring
o
a
a
e c i
b
s
r
a
m m
n
geometry so that maximum particle size• is no more than 1/3 e l
t
o
o
• y
i
s
a
e
r
of the gap size (1/10 for concentratedt pastes). Sometimes
n
s
h
a o
m
r o
t p c
i
i
a
l t o larger particles contribute little to the overall• flow behaviour
•
r
a p
h
s
o
c s
e
p
i
of the material and can be removed withoutp large penalties
b
c l k
a
i
r a o
p
l v
•
o
h
r for the applicability of the measurement. p
s
s
e
a w
n k s
on r
o • • i
i
o t
&
t
s
a
a
• Early warning of problems with technique orr w s
s
on
c
r i
e
i
i
i
r
t • l t
a
measurement e
e
p a
s t
p
s
r p
c
i
m
e
r a
e
a
Having data for the complete range ofr shear rates allows the
o
t
r
e
c
•
t e • a
s
c
user to investigate the possibility of settling, chaotic flowi in h
m
a v
c ts
on r
o
i
n
s
•
the measuring gap and slip at the measurement geometry a c t
l
e
s
a h
s a
i
i
s
r
c
m v i
walls. Data from a single shear rate test may notr reveal this
e r
u
s
Figure 7: Results from a shear and recovery test e
• l
e d r t
t t
information readily. Often in these cases settling material a a u
s
i c s
r
r r
a t
n
m
e
e i r
will appear to be thixotropic as the shear stress generallyx
t
f
d a
e
g
3.2 Benefits of the shear and recovery test a
o h u
n drops while the material settles. r c
i m
w t e
t
s
e c x
f s l
r
e n
o a e
c
i
t e
s
• Monitors recovery r i
e
• Settling materials w
r
e c
c
n
t e
s
e
i
r
Of the tests mentioned this is the only one that immediately n a
If a material tends to settle, long test times tend toc reduce
e m
e w
i
s
m t y
h
l
informs about recovery. c t
f
the likelihood of successful test outcomes. The longer the s n
•
o
i
o
o p
s e
t
w
test takes, the more likely the solid fraction is to have settlede
e
h t •
c
s t
• Intuitive interpretation of the data d
r
s n •
out of the measurement space, or at least createde a r
o u t e
i s
t e
a The meaning of the data is easily understood and applied o
c
e
c
concentration gradient through it. Settling materials will c s
m
d s
i
r
y
e
once the appropriate process shear rates or shear stresses d g l
e t
appear to be thixotropic as the shear stress generally u drops e
o
n
h
a
o i
t
d p
are known. c
c
n
i
while the material settles. i
r
o
n
d
t g a o
o
e r
f
n i
t
i
t d d
s • Time n e a
•
i t n
s m i l
a
a o
r
Appropriate shear rates and % of original values can be e i
o n c
t
i t t r
a s t
a d
• g
chosen to enable the testing to be relatively quick. y n s
r e i
s l o
o d n o c r a C
t e n e g
Q c n i r o r
i l o v u t o r a c t a
c e a e s i e s r i c t i r e n v p d u p r t a o n e g a a m s r r • e e s t od s p e t l n www.rheologysolutions.com (5) Page 9• e a n m a c s t i e e r s t v t o e r m s l i p e u • l a s r t a p s t r i u t d s o c s s e i n np l Tim’s Top Tips - How To Measure Thixotropy For Pharmaceutical And Cosmetic Industries
• Chaotic flow One of the key assumptions for rheological measurements is that the flow in the measuring gap is laminar. Too high shear rates can cause the flow regime to become turbulent and the measurements are unreliable. The onset of chaotic flow can be overcome or delayed by changing the measuring geometry or the measurement gap.
• Slip Multi-phase systems tend to slip at the boundary of the measuring geometry. A major assumption for rheological measurements is that the first layer of material 'sticks' to the walls of the measuring geometry. Plate & plate or concentric cylinder geometry walls can be roughened or serrated to reduce or remove the slip phenomenon.
• Time dependent materials Sample handling and experimental technique can be crucial for repeatably measuring time dependent (eg thixotropic) materials. The same handling procedure (pouring, mixing, resting, loading into the test equipment etc) and experimental procedure is critical for repeatable measurements, and it must be remembered that for time dependent materials the results are relative only - they depend on the technique used to generate them.
Page 10 (4) [email protected] p k se ent p e r uipm o rh o eq & he • w tsy rs ics vmi i rvrk d rra a eo n gg p s oo e & w a r r s Tim’s Top Tips - How To Measure Thixotropy For Pharmaceuticalr And Cosmeticr mpIndustriesr d e • i n a h p s a n t o t p a i h n e r & s r a m k e e r • n e m i s c o s n u t m c e w r n i i t e e f c s s i d s n m c n i c e a u i e c r f r g t i p a c Summary n s s f a i n e r n t i y i Determining the most suitable typeu of measurements p or r s g s t m e t n instrument is not simply a s matter of• addinge up ithe rankingy for r s r t Table 1 summarises the possibilities for measuring a flow or u t g e s d c s each. Rather, identify which measurementn i technique,e variable etc u n i f mt viscosity curve using the techniques discussed. Each of the i t i ry d s c l is most relevant and appropriate• for your application/product.e n e e o i techniques is ranked between 0 and 5 for each of the potential t
g p mp • Often, more than one techniques s is requiredy to ensure n l • issues and solutions, where: i d i y g t o s l r r n consistency, reproducibilitys and accuracy is achieved. p o t i e e s s t t • s u s m
t • e d r y 5 = Excellent 4 = Good 3 = Adequate t l c e s n
a r i o t
r m c 2 = Possible 1 = Difficult 0 = Not Possible a p
t • y a n l r n i r
g o t
o o
m f
n p n c i e o t r s
s • s c Table 1: Assessment of strengths/weaknesses for each technique o m
e f s •
•
t t e
s
s t n t s t s
t c
e c m y
n a
e s Technique: Thixotropy Loop Constantu Shear Shear & Recovery
m r t e
d t
s
u
C r
o n y m t r
s
Measurement Q
o u
s
p r
c l
n t
C
i
a s
g
• c
Rapid 3 4Q 4
n i
g n
i
s i t
l
t
n
t
i
a p
g
s
n
t
c
Easy 4 4o 3
e i
s n
e
i
t t
e
t •
t
m
p
r s
s
u
o
e
Accurate 4 4 5e
k
t
r
t
c
t
n
• m
s a
k e
l
o
s
n
Small sample volume 4 4 4c t
i t b
m
a s
l n
u
a g n
l e b r
o n
Temperature control 4 5t 5 e
i
s
m
b n t
r
s
n u
o &
i
a
r
e
b
t r
Measuring system c t
r
g
s
e
a
•
k n
n
b
i
c i
c
t
y
Rheometer or viscometer Both Bothb Rheometer
a
r s
•
l g
u
e o
r
b n
t y t
i
r
t
a l
Large variety of sensors 5 5• 5 n
r
s o
a
o
t i s
o e
r r
t
a b
b
r r e e
Structural disruption on loading avoidable 2 2l 2
a
t t
a l
o
a
a
e c i
b
s
r
a
m
m n •
e Slip avoidable 4 4 4 l
t
o
o
• y
i
s
a
e
r t
n
s
h
a o
Number of Participants m
r o
t p c
i
i
a
l t o •
•
r
a p
h
s
o
c
Single operator 5 5 5s e
p
i
p
b
c l k
a
i
r a o
p
l v
•
o
h
r
Experimental p
s
s
e
a w
n k s
on r
o • • i
i
o t
Measures materials with large particles & agglomerates 1 1& 1 t
s
a
a
r
w s
s
on
c
r i
e
i
i
i
r
t
• l
Settling suspension measurements 2 2 1 t
a
e
e
p a
s t
p
s
r p
c
i
m
e
r
a e
a r
Direct determination of recovery from measurement No No Yes o
t
r
e
c
•
t e • a
s
c
i
h
m
a v
c
Shear heating reduced 5 4ts 5
on r
o
i
n
s •
a c t
l
e
s
a h
s a
i
i
s
r
Detection of slip, turbulence, shear heating etc 4 5 1 c
m v
i r
e
r
u s
e
• l
e d r t
t
t
a
a u
s
Inertia avoidable 4 4 4 i c s
r
r r
a t
n
m
e e
i r
x
t
f
d a
Results e g a
o h u
n r c
i m
w t e
t
s
e c x
f
s
Intuitively comprehended 4 5 5 l r
e n
o a e
c
i
t e
s
r i
e
w
r
e c
c
n
t e
s
e
i
r
n
a
c
e m
e w
i
s
* Depending on the test, these parameters may be viewed m t y
h
l c
t
f
s
n
• o
i
alternatively as either a strength or as a weakness o
o p
s e
t
w
e
e
h t •
c
s t
d
r
s
n
•
e
r
o u t e
i s
t e
a o
c
e
c c
s
m
d s
i
r
y
e
d g l
e
t
u
e
o
n
h
a
o i
t
d
p
c
c
n
i
i
r
o
n
d
t g a o
o
e r
f
n i
t
i
t d d
s n e a
•
i t n
s m i l
a
a o
r
e i
o n c
t
i t t r
a s t
a d
• g y n s
r e i
s l o
o d n o c r a C
t e n e g
Q c n i r o r
i l o v u t o r a c t a
c e a e s i e s r i c t i r e n v p d u p r t a o n e g a a m s r r • e e s t od s e t p n www.rheologysolutions.com (5)l Page 11• e a n m a c s t i e e r s t v t o e r m s l i p e u • l a s r t a p s t r i u t d s o c s s e i n np l Tim’s Top Tips - How To Measure Thixotropy For Pharmaceutical And Cosmetic Industries
Contact Details Other Notes Available in the Tim's Tips - Rheology Solutions for the Pharmaceutical and Cosmetic Industries Series are:
• How To Measure Yield Stress (Rheo356)
• How To Measure Flow and Viscosity Curves (Rheo352)
Other Information Available for the Pharmaceutical and Cosmetic Industries include:
Head Office Rheology Solutions Pty Ltd • Rheology Solutions for Pharmaceutical, Cosmetics and Allied Industries Information Kit Address: 15 -19 Hillside Street, Bacchus Marsh, Victoria, 3340 PO Box 754, Bacchus Marsh, Victoria, 3340 • Applications Laboratory and Contract Testing Capabilities Phone: 03 5367 7477 Statement for General Manufacturing Industries Fax: 03 5367 6477 Email: [email protected] • Technical Literature for Pharmaceutical and Cosmetic Website: www.rheologysolutions.com Industries
Managing Director – Pat Griffin Email: [email protected]
Technical Manager – Dr Tim Kealy Email: [email protected]
Service Engineer – Richard Donaldson Email: [email protected] [email protected]
Focused on providing our customers with materials characterisation solutions through knowledge, experience and support.
Page 12 (4) [email protected] p k se ent p e r uipm o rh o eq & he • w tsy rs ics vmi i rvrk d rra a eo n gg p s oo e & w a r r s Tim’s Top Tips - How To Measure Thixotropy For Pharmaceuticalr And Cosmeticr mpIndustriesr d e • i n a h p s a n t o t p a i h n e r & s r a m k e e r • n e m i s c o s n u t m c e w r n i i t e e f c s s i d s n m c n i c e a u i e c r f r g t i p a s c s f a n i n e r n t i y i Pharmaceutical & Cosmetic Dictionaryu s p r s g s t m e t n s • e i y r s r t u t g e s d c s n i e u n i f mt i t i ry d s c l • e n e e o i t
Industry Term: Capillary Rheometer. g p mp • s s y n l • i d i y g Definition: A rheometer which measures flow properties throught a capillary. o s l r r n s p o t i e e s s t Governing Properties: The pressure on the liquid and the pressure dropt of the liquid through• the capillary. The s u s m
t • e d r y t l capillary geometry dictates the shear forces experiencedc by the liquide as if flows. s n
a r i o t
r m c a p
t • Rheology Solutions Instrument: HAAKE RheoCap. y a n l r n i r
g o t
o o
m f
n p n c i e o t r s
s • s c o
m
e f s •
•
t t e
s
s t n t s t Industry Term: Complex Viscosity. s
t c
e c m y
n a
e s u
m r t
Definition: The viscosity measured by dynamic rheometry, related to both the viscouse and elastic
d t
s
u
C r
o n y m t r
portions of flow for a viscoelastic fluid. s
Q
o u
s
p r
c l
n t
C
i
a Governing Properties: This is a property governed by the viscoelastic properties of the material s - elastic and
g
• c
Q
n i
g n
i
s i t
l
t
viscous moduli (G' and G''). It is measured on a CS n rheometer using a frequency sweep. t
i
a p
g
s
n
t
c
o
e i s n
e
Rheology Solutions Instrument: HAAKE RheoStress, HAAKE MARS. i
t t
e
t •
t
m
p
r s
s
u
o
e e
k
t
r
t
c
t
n
• m
s a
k e
l
o
s
n
c
t
i t b
m
a s
l n
u
a
g
Industry Term: Controlled Rate. n
l e b r
o n
t
e
i
s
m b
n
Definition: Mode of operation for a rheometer or viscometer. Controls thet shear rate imposed on the
r
s
n u
o &
i
a
r
e
b
t
r
c
sample. t r
g
s
e
a
•
k n
n
b
i
c i
Governing Properties: CR mode is usually available using a CS rheometer or a CR viscometer.c t y
b
a
r s
•
l g
u
e o
r
b
Rheology Solutions Instrument: HAAKE ViscoTester 550, HAAKE RotoVisco, HAAKE RheoStress, HAAKEn MARS. t y t
i
r
t a
l
• n
r
s o
a
o
t i s
o e
r r
t
a b
b
r r e e
l
a
t t
a l
o
a
a
e c i
b
s
r
a
m
m n •
e
Industry Term: Controlled Stress. l
t
o
o
• y
i
s
a
e
r t
Definition: n Mode of operation for a rheometer or viscometer. Controls the shear stresss imposed on
h
a o
m
r o
t p c
i
i
a
l
t o
the sample. • •
r
a p
h
s
o
c s
e
p
i
p
Governing Properties: CS mode is usually available using a CS rheometer but not on a CR viscometer.b c l k
a
i
r a o
p
l v
•
o
h
r
Rheology Solutions Instrument: HAAKE RheoStress, HAAKE MARS. p
s
s
e
a w
n k s
on r
o • • i
i
o t
&
t
s
a
a
r
w s
s
on
c
r i
e
i
i
i
r
t • l t
a
e
e
p a
s t
Industry Term: Flow Curve. p
s
r p
c
i
m
e
r
a e
a
r
o
t
r
Definition: A flow curve is a plot showing the relationship between shear rate and shear stress. e
c
•
t e • a
s
c
i
h
m
a
Governing Properties: It can be measured using a CS rheometer or a CR viscometer. v c ts
on r
o
i
n
s •
a c t
l
e
Rheology Solutions Instrument: HAAKE ViscoTester 550, HAAKE RotoVisco, HAAKE RheoStress, HAAKE MARS.s
a h
s a
i
i
s
r
c
m v
i r
e
r
u s
e
• l
e d r t
t
t
a
a u
s
i c s
r
r r
a t
n
m
e e
i r
x
t
f
d a
e
g
Industry Term: Linear Viscoelastic Envelope (LVE). a
o h u
n r c
i m
w t e
t
Definition: The LVE is the region in which the internal structure of a material remains unchangeds as e c x
f s l
r
e n
o a e
c
i
t
e
the imposed stress or deformation is gradually increased. s r i
e
w
r
e c
c
n
t e
s e
i
Governing Properties: r Measured on a CS rheometer using a stress sweep or a strain sweep. n
a
c
e m
e w
i
s
m t y
h
l
Rheology Solutions Instrument: HAAKE RheoStress, HAAKE MARS. c
t
f
s
n
•
o
i
o
o p
s e
t
w
e
e
h t •
c
s t
d
r
s
n
•
e
r
o u t e
i s
t e
a o c
Industry Term: Rheodestruction. e
c c
s
m
d s
i
r
y
e
d g l
e
t
Definition: The irrecoverable changes in the structure of a material due to the action of shearingu
e
o
n
h
a
o i
t
d
p
c
c n
forces. i
i
r
o
n
d
t g a o
o
e r f
Governing Properties: It can be measured using a thixotropy loop on a CS rheometer or a CR viscometer. n i
t
i
t d d
s n e a
• i
Rheology Solutions Instrument: HAAKE ViscoTester 550, HAAKE RotoVisco, HAAKE RheoStress, HAAKE MARS. t n
s m i l
a
a o
r
e i
o n c
t
i t t r
a s t
a d
• g y n s
r e i
s l o
o d n o c r a C
t e n e g
Q c n i r o r
i l o v u t o r a c t a
c e a e s i e s r i c t i r e n v p d u p r t a o n e g a a m s r r • e e s t od s e t p n www.rheologysolutions.com (5)l Page 13• e a n m a c s t i e e r s t v t o e r m s l i p e u • l a s r t a p s t r i u t d s o c s s e i n np l Tim’s Top Tips - How To Measure Thixotropy For Pharmaceutical And Cosmetic Industries
Industry Term: Rheology. Definition: The flow and deformation of matter. Governing Properties: N/A Rheology Solutions Instrument: HAAKE ViscoTester 550, HAAKE RotoVisco, HAAKE RheoStress, HAAKE MARS.
Industry Term: Rheometer (Controlled Stress). Definition: An instrument designed for the measurement of viscous and viscoelastic flow properties at specified temperature and atmospheric conditions, by measuring the force required to move one layer over another without turbulence. Governing Properties: Rheometers often have air bearings, making them highly sensitive to small variations in load or displacement and can operate in rotation or oscillation for Controlled Rate or Controlled Stress modes. Some rheometers have mechanical bearings, but in general they do not have the required sensitivity to make good use of CS mode in these cases and can not run oscillatory measurements well (or at all). Rheology Solutions Instrument: HAAKE RheoStress, HAAKE MARS.
Industry Term: Rheometer. Definition: An instrument designed for the measurement of viscous and viscoelastic flow properties at specified temperature and atmospheric conditions, by measuring the force required to move one layer over another without turbulence. Governing Properties: Rheometers often have air bearings, making them highly sensitive to small variations in load or displacement and can operate in rotation or oscillation for Controlled Rate or Controlled Stress modes. Some rheometers have mechanical bearings, but in general they do not have the required sensitivity to make good use of CS mode in these cases and can not run oscillatory measurements well (or at all). Rheology Solutions Instrument: HAAKE RheoStress, HAAKE MARS.
Industry Term: Rheopexy. Definition: Rheopectic fluids show shear thinning behaviour combined with a time dependency. The viscosity of a rheopectic fluid increases when subjected to a constant shear rate for a period of time. The viscosity of rheopectic fluids often recovers substantially over a period of time after the shearing forces have been removed. Governing Properties: Rheopexy depends on the rate of structural recovery in the material. It can be measured using a flow curve on a CR or CS instrument, or by measuring the recovery of the moduli after shearing on a CS rheometer. Rheology Solutions Instrument: HAAKE ViscoTester 550, HAAKE RotoVisco, HAAKE RheoStress, HAAKE MARS.
Industry Term: Shear Rate. Definition: The rate of change of displacement resulting from an imposed shear stress. Governing Properties: N/A Rheology Solutions Instrument: HAAKE ViscoTester 550, HAAKE RotoVisco, HAAKE RheoStress, HAAKE MARS.
Industry Term: Shear Stress. Definition: This is the force per unit area imposed on an element of fluid. Governing Properties: The shear stress is dependent on the geometry of the fluid element and can be measured by a CR viscometer and may be imposed by a CS rheometer. Rheology Solutions Instrument: HAAKE ViscoTester 550, HAAKE RotoVisco, HAAKE RheoStress, HAAKE MARS.
Page 14 (4) [email protected] p k se ent p e r uipm o rh o eq & he • w tsy rs ics vmi i rvrk d rra a eo n gg p s oo e & w a r r s Tim’s Top Tips - How To Measure Thixotropy For Pharmaceuticalr And Cosmeticr mpIndustriesr d e • i n a h p s a n t o t p a i h n e r & s r a m k e e r • n e m i s c o s n u t m c e w r n i i t e e f c s s i d s n m c n i c e a u i e c r f r g t i Industry Term: Slip. p a s c s f a n i n e r n t i y i Definition: The liquid does not adhere to the wall of the measuring geometry.u s p r s g s t m e t n s • e i y Governing Properties: N/A r u s r t s t g e s d c e Rheology Solutions Instrument: HAAKE ViscoTester 550, HAAKE RotoVisco, HAAKE RheoStress,n i HAAKE MARS. (Serrated u n i f mt i t i ry d s c l sensors should be used.) • e n e e o i t
g p mp • s s y n l • i d i y g t o s l r r n s p o t i e e s s t t • s Industry Term: Thixotropy Loop. u s m t • e d r y t l c e s n Definition: Thixotropy loop is a technique to measure the thixotropya r ofi a liquid o and consistst of two r m c a p
t • y a n l r consecutive flow curves. The difference in the areasn belowi the flow curve is ar measure of g o t
o o
m f
n p n c i the thixotropy fluid and the loop is called a thixotropye loop. o t r s
s • s c o
m e f Governing Properties: Measuring using flow curves on a CS rheometer ors CR• viscometer. •
t t e
s
s t n t s t s
Rheology Solutions Instrument: HAAKE ViscoTester VT550, HAAKE RotoVisco, HAAKE RheoStress, HAAKE MARS.t c
e c m y
n a
e s u
m r t e
d t
s
u
C r
o n y m t r
s
Q
o u
s
p r
c l
n t
C
i
a s
g
Industry Term: Thixotropy. • c
Q
n i
g n
i
s i t
l
t
n
t
i
Definition: Thixotropic fluids show shear thinning behaviour combined with a timep dependency. The a
g
s
n
t
c
o
e i
s n
e
i
t t
viscosity of a thixotropic fluid drops when subjected e to a constant shear rate for a period
t •
t
m
p
r s
s
u
o
e e
k
of time. The viscosity of thixotropic fluids often recovers substantiallyt over a period of time r
t
c
t
n
• m
s a
k e
l
o
after the shearing forces have been removed. s
n
c
t
i t b
m
a s
l n
u
a
g
Governing Properties: Thixotropy depends on the rate of structural recovery inn the material. It can be measured
l e b r
o n
t
e
i
s
m
b
n
using a flow curve on a CR or CS instrument, or by measuringt the recovery of the moduli
r
s
n u
o &
i
a
r
e
b
t
r
c
after shearing on a CS rheometer. t r
g
s
e
a
•
k n
n
b
i
c i
Rheology Solutions Instrument: HAAKE ViscoTester 550, HAAKE RotoVisco, HAAKE RheoStress,c HAAKE MARS. t y
b
a
r s
•
l g
u
e o
r
b n
t y t
i
r
t a
l
• n
r
s o
a
o
t i s
o e
r r
t
a b
b
r r e e
l
a
Industry Term: Viscoelastic Measurements. t t
a l
o
a
a
e c i
b
s
r
Definition: Materials, which are partly elastic (i.e. solid) and partly viscous (i.e. fluid). When theya are
m
m
n
•
e l
t
o
o
• y
i
s
a
e r
deformed some of the energy is stored (solid) while the remaindert is lost through flow
n
s
h
a o
m
r o
t p c
i
(fluid). i
a
l t o •
•
r
a p
h
s
o
c s e
Governing Properties: N/A. p
i
p
b
c l k
a
i
r a o
p
l v
•
o
Rheology Solutions Instrument: N/A. h
r p
s
s
e
a w
n k s
on r
o • • i
i
o t
&
t
s
a
a
r
w s
s
on
c
r i
e
i
i
i
r
t • l t
a
Industry Term: Viscometer. e
e
p a
s t
p
s
r p
c
i
m
e
r a e
Definition: An instrument for measuring the viscosity of a liquid at specified temperaturea and r
o
t
r
e
c
•
t e • a
s
c
atmospheric conditions, by measuring the force required to move one layer over anotheri
h
m
a v
c ts
on r
o
i
n
s
without turbulence; also referred to as viscometer. •
a c t
l
e
s
a h
s a
i
i
s
r
c
m v
Governing Properties: Viscometers usually have mechanical bearings in their motor and generally operatei in r
e
r
u s
e
• l
e d r t
t t
rotational mode only. a
a u
s
i c s
r
r r
a t
n
m
e e i r
Rheology Solutions Instrument: HAAKE ViscoTester 550, HAAKE RotoVisco. x
t
f
d a
e
g a
o h u
n r c
i m
w t e
t
s
e c x
f s l
r
e n
o a e
c
i
t e
s
r i
e
w
Industry Term: Viscosity Curve. r e c
c
n
t e
s
e
i
r
n
a
c
e Definition: A viscosity curve is the (usually non-linear) relationship between viscositym and shear rate
e w
i
s
m t y
h
l c
t
f
s
n
•
derived from a flow curve on a CS rheometer or CR viscometer. o
i
o
o p
s e
t
w
e
Governing Properties: Viscosity is the shear stress divided by the shear rate. These are measured on a CR e
h t •
c
s t
d
r
s
n •
e
viscometer or CS rheometer using a flow curve. r o u t e
i s
t e
a o
c
e
c
c
s
Rheology Solutions Instrument: HAAKE ViscoTester VT550, HAAKE RotoVisco, HAAKE RheoStress, HAAKE MARS.m d s
i
r
y
e
d g l
e
t
u
e
o
n
h
a
o i
t
d
p
c
c
n
i
i
r
o
n
d
t g a o
o
e r
f
n i
t
i
t d d
s n e a
•
i t n
s m i l
a
a o
r
e i
o n c
t
i t t r
a s t
a d
• g y n s
r e i
s l o
o d n o c r a C
t e n e g
Q c n i r o r
i l o v u t o r a c t a
c e a e s i e s r i c t i r e n v p d u p r t a o n e g a a m s r r • e e s t od s e t p n www.rheologysolutions.com (5)l Page 15• e a n m a c s t i e e r s t v t o e r m s l i p e u • l a s r t a p s t r i u t d s o c s s e i n np l Tim’s Top Tips - How To Measure Thixotropy For Pharmaceutical And Cosmetic Industries
Industry Term: Viscosity. Definition: The resistance to flow of a fluid. Governing Properties: Viscosity is the shear stress divided by the shear rate. These are measured on a CR viscometer or CS rheometer using a flow curve. Rheology Solutions Instrument: HAAKE ViscoTester VT550, HAAKE RotoVisco, HAAKE RheoStress, HAAKE MARS.
Industry Term: Yield Stress. Definition: The minimum shear stress required to initiate flow in a fluid. Governing Properties: Governed by the structural properties of the material at rest, measured by extrapolation using a flow curve, or using the vane technique, both on a CR or CS instrument. It can also be measured using a CS rheometer by a stress ramp. Rheology Solutions Instrument: HAAKE ViscoTester 550, HAAKE RotoVisco, HAAKE RheoStress, HAAKE MARS.
Notes:
• o ViscoTester 550 and RotoVisco are controlled rate viscometers, RheoStress is a controlled stress rheometer, MARS is a modular R&D Controlled Stress Rheometer, all of which are HAAKE brand names of Thermo Fisher Scientific (Karlsruhe, Germany) GmbH.
Disclaimer
The information contained in this report is not intended for direct use as a tool for process development. It is a guide only. This document remains the property of Rheology Solutions Pty Ltd, and may not be reproduced or altered in any way without the written permission of the owners.
© Rheology Solutions Pty Ltd 2007
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