Ch7. Rheology

Bule Hora University
College of Health and Medical Sciences
CHAPTER 7
Department Of Pharmacy
INTEGRATED PHYSICAL PHARMACY AND PHARMACEUTICS I
RHEOLOGY
By: Aliyi Gerina [B.pharm]
4/5/2022
1
Rheology by Aliyi G. Bule Hora University

Outline
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 Introduction
 Newtonian and Non-Newtonian systems
 Thixotropy
 Determination of viscosity
 Different factors affecting rheological
properties
 Pharmaceutical applications of rheology

Objectives
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At the end of this lesson the students should be able to:
 Define rheology
 Define and understand Newton's law of flow and its application
 Differentiate flow properties and corresponding rheogram between
Newtonian and Non-Newtonian materials
 Appreciate the fundamentals of determination of rheological properties
 Recognize different factors affecting rheological properties of materials
 Provide examples of fluid pharmaceutical products exhibiting various
rheological behaviors

Introduction
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o The term “rheology” from the Greek rheo (“to flow”) and
logos (“science”).
o It is defined as the science concerned with deformation of
matter under the influence of stress.
• Definition:
• the flow of fluids (liquids and gases) and
• the deformation of solids under the influence of shearing stress.

Introduction…
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 The deformation of any pharmaceutical system can be
arbitrarily divided into two types:
• Elastic Deformation:
• it is a spontaneous and reversible deformation
• Plastic Deformation:
• it is a permanent and irreversible deformation
 The plastic deformation: flow and exhibited by viscous bodies.
 Great importance in any liquid dosage forms like
suspensions, solutions, emulsions etc

Introduction…
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 Ideal solids deform elastically
 The energy required for the deformation
 is fully recovered when the stresses are removed
 Ideal fluids such as liquids and gases deform irreversibly they flow
 The energy required for the deformation
 is dissipated within the fluid in the form of heat and
 cannot be recovered simply by removing the stresses.

The deformation is recoverable if
the material returns to its initial
shape when the stress is removed.
Deformation is permanent if
the material remains deformed
when the stress is removed.
While elastic deformation is recoverable, plastic deformations are not
Recoverable versus permanent
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Importance of Rheology
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o In liquid, and semisolid DF: In preparation,
development and evaluation of suspensions, emulsions,
pastes, suppositories, tablets coating.
o The manufacturer of medicinal and cosmetic creams ,
pastes, and lotions must be able to pour the products
with an acceptable
o consistency and
o smoothness for each batch.

Importance of Rheology …
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o Rheology involved in
 Mixing and flow of materials
 Packaging into containers
 Removal prior to use.
 Whether this is achieved by
 pouring from a bottle
 extrusion from a tube or
 passage through a syringe needle.
 The product rheology must be optimized:
 by controlling the viscosity of the product.

Importance of Rheology …
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o In acceptability to the patient, physical stability, and
even
biological availability (i.e., viscosity affect the
absorption rate of drugs from GIT).
o Rheology properties of pharmaceutical systems can influence
the selection of equipment used in its manufacture
 Inappropriate equipment from rheological perspective may
result in an undesirable product, at least in terms its flow
characteristics.

Classification of rheological systems
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o Materials are classified according to type of flow in to two
categories
 Newtonian systems
 Simple fluids which obey Newtonian law of flow
 Non-Nowtonian systems
 do not obey Newtonian law of flow

Newtonian systems : Viscosity
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Measure of the resistance of liquid
to deformation under shear stress
A fluid’s internal resistance to flow
and may be thought of as a measure
of fluid friction
The resistance offered when one part
of the liquid flows past another.
Viscosity
 The flow of liquids or semisolids is described by
viscosity.

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Newtonian law of flow
o The phenomenon of viscosity is best understood by a consideration
of a hypothetical cube of fluid made up of infinitely thin layers
o which are able to slide over one another like a pack of playing cards.
o When a tangential force is applied to the uppermost layer
 each subsequent layer will move at progressively decreasing
velocity and the bottom layer will be stationary.
Newtonian fluids: Viscosity…

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 The bottom layer is considered to be fixed in place
 The adhesive force between the wall and the flowing layers
 Inter-molecular cohesive forces
 Inter-molecular force (viscous forces) : viscosity
 Viscosity is the opposing force to flow,
 it is characteristic of the medium.
 In other words, viscosity describes the internal friction of a
moving fluid.

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o A velocity gradient will therefore exist, and
o this will be equal to the velocity (ms-1) of the upper layer in
divided by the height (m) of the cube.
o Fig. Representation of the shearing force required to produce a definite velocity
gradient between the parallel planes of a block of material.

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 The difference in velocity (dv) b/n two planes
of a liquid separated by an infinitesimal
distance (dx) is the velocity gradient or a rate of shear
(dv/dx or G).
o The resultant velocity gradient is the rate of shear,
G, will have units s-1.
Shear rate (G) = dv/ dx

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 The force required for one layer of a liquid to slip
past another layer with a given velocity depends
 Directly
 on the viscosity of the liquid and
 on the areas of layers exposed to each other and
 Inversely on the distance separating the two
surfaces.
 A fluid with high viscosity resists motion
 because its molecular make up gives it a lot of
internal friction.
18

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o The applied stress,the shear stress,
o S , is derived by dividing the applied force, F’, by the
area of the upper layer.
o It have units of N m-2.
Shear stress (S) = F’/ A

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 Newton recognized that: the higher the viscosity of a liquid, the
greater the force per unit area required to produce a certain rate of
shear
 Thus, the rate of shear is  to the shearing stress
Newtonian law
dx
dv
A
F

'
dx
dv
A
F


'
Shear
of
Rate
Stress
Shearing
G
S
dx
dv
A
F



'

Where  is a constant known as coefficient of viscosity,
usually referred to simply as viscosity or absolute
viscosity.
Where
S is the shearing stress (dyne.cm-2)
G is the rate of shear (sec-1)
Units: dynes-sec/cm 2
Absolute viscosity or dynamic viscosity

Newtonian fluids: Kinematic
Viscosity
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 It is the absolute viscosity divided by the density of liquid
at a specified temperature.
KV = η/p
The preferred unit when the shear stress and shear rate of the
fluid
 are influenced by the density.
 Units: Stock (s) or centistokes .

Newtonian fluids
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Liquids that obey Newton’s law of flow:
Newtonian liquids
 Newton’s equation for the flow of a liquid is
S = η * G
 Plot of shear stress Vs shear rate:
 the slope gives the viscosity.
 The curve always passes through the origin.
 The reciprocal of viscosity is called fluidity: 1/η =
Slope

Characteristics of Newtonian flow
o The passage through the origin indicates
that
o even a mild force can induce flow in
these systems.
o The linear nature of the curve
shows that the viscosity (η) of a
Newtonian liquid
o is a constant unaffected by
the value of the rate of shear.
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Newtonian fluids…
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o Examples of Newtonian fluids include:
 Water
 Air
 Glycerin and most mineral oil
 True solutions
 Very dilute suspension and emulsions
 Liquid paraffin

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o Most pharmaceutical fluids do not follow Newtonia
law
o because the viscosity of the fluid varies with the rate of
shear.
o These materials are non-Newtonian fluids.
o The viscosities of non-Newtonian fluids vary with
shear rate stating a single viscosity is misleading :
apparent viscosity
Non-Newtonian systems

Non-Newtonian system…
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o It can be seen in liquid and solid heterogeneous dispersions
o such as colloids, emulsions, liquid suspensions and ointments.
o there are three classes of non-Newtonian flow
plastic,
pseudoplastic
dilatant

Plastic flow (Bingham bodies)
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 A plastic material does not flow
 until a certain minimum shearing stress, the yield
value, is applied.
 The substance initially behaves like an elastic body
and fails to flow when less amount of stress is
applied.
 Further increase in the stress leads to a nonlinear
and increase in the shear rate which then turns to
linearity

Plastic flow,…
 The slope of the rheogram is termed
mobility, analogous to fluidity in
Newtonian systems and its reciprocal
is known as the Plastic viscosity, U
 Where u is the plastic viscosity
and f is the Bingham yield
stress or value
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Plastic flow,…
 Extrapolations of the linear
plot gives ‘x’ intersect: yield
value.
 This curve does not pass
through the origin
 Intersects the shear stress
axis at a particular point, yield
value.
 As the curve above yield value
tends to be straight,
 the plastic flow is similar to the
Newtonian flow above yield
value. 4/5/2022
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Plastic flow,…
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 Flocculated particles in a concentrated
suspensions usually show plastic flow.
 The yield value represents the stress required to break the
inter-particular contacts so that particles behave
individually.
 Thus it is indicative of the forces of flocculation
 increased by the increased concentration of the
dispersed phase.

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Plastic flow,…
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o Other examples of plastic flow:
 Solid powder materials
 Topical ointments
 Pastes

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 Pseudo plastic flow is exhibited by polymer
dispersions like:
Tragacanth in water
Sodium alginate in water
Methyl cellulose in water
Sodium carboxy methyl cellulose in water
Pseudo-plastic Flow (shear thinning)

Pseudo-plastic Flow…
 The relationship between shear
stress and the shear rate
 is not linear and the curve
starts from origin.
 The viscosity of a pseudo-
plastic substance decreases
with increasing G (shear-
thinning systems).
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Pseudo-plastic flow behavior; Structural reasons
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 At rest, the linear polymers dispersed at random in
the dispersion medium.
 As shearing stress is applied, the macromolecules
become aligned with the long axis parallel to the
direction of flow.
 With this ordered alignment the molecules pass one
another with less frictional resistance, and the
viscosity is decrease.

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 If the S is decreased, the orientation of the
macromolecules becomes more random.
 Greater frictional resistance to the flow is reflected
in an increased viscosity.
 Since only a molecular alignment is involved
 Emulsions and dispersions of many types: shear-
thinning

Dilatant Flow(Shear
thickening)
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 Resistance to flow (viscosity) increases with
increase in shear stress.
 When S is applied their volume increases and
 hence they are called Dilatant.
 This property is also known as shear thickening
 Dilatant flow is observed in suspensions
containing
 more than 50%W/V of solids,
 deflocculated particles .

Dilatant Flow,…
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Dilatant Flow,…
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 At rest, the particles are closely packed with a
minimum interparticle volume, or voids.
 The vehicle is sufficient to fill this volume.
 As the shear stress is increased, the bulk of the
system expands or dilates.
 Increase in the inter-particle void volume
 Vehicle become insufficient to fill the increased voids.
• The resistance to flow increases:
 This process is reversible

Dilatant Flow,…
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o The effect is reversible and removal of the shear stress
results in the re-establishment of the fluid nature.

Dilatant Flow,…
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o Dilatancy can be a problem during the processing
of dispersions and the granulation of tablet masses
when high-speed blenders and mills are employed.
 If the material being processed becomes dilatant in nature
then the resultant solidification could overload and
damage the motor.

summary
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o Non-Newtonian,
 Time dependent behavior.
o Definition
o Thixotropy: It is the decrease in viscosity as a function
of time upon shearing, then recovery of original viscosity
as a function of time without shearing.
o Thixotropic material is
o any material that exhibits a reversible time-dependent decrease
in apparent viscosity.
Thixotropy

Thixotropy,...
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 Thixotropic substances on applying shear stress
convert to sol (fluid) and on standing they
slowly turn to gel (semisolid).

Reason for thixotropic property
 Thixotropic systems contain
 asymmetric particles which
set up a loose three
dimensional structure which
is
 rigid and resembles a gel
which is broken down on
applying shear: Solution
 On removing the applied
stress, the material reform its
original structure of state: Gel
4/5/2022
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Thixotropy,...
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o Rheogram : show a hysteresis where, as the S
is increased an up-curve is obtained, on
reducing the S gradually a down curve shifted to
the left side is obtained.
o In the case of
o plastic and
o pseudoplastic materials the downcurve will be
the right of the upcurve.

Thixotropy,...
 Rheogram of thixotropic materials depends on
 Applied shear rate or shearing force
 The length of the time a sample is subjected to
shearing or kinematic history.
 The previous history of the sample has
significant effect on
 the rheologic properties of a thixotropic system.
50
4/5/2022
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Thixotropy,...
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 Thixotropic substances are now a day’s more used in
suspensions to give stable suspensions.
 On storage turn to gel: viscosity increases infinitely, do
not allow the dispersed particles to settle down:
 stable suspension
 When shear stress is applied they turn to solution and
 thus are easy to pour and measure for dosing.
 Solve the problems,
 stability and
 pourability.

Thixotropy,...
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 Examples
 Gels that show thixotropic flow
 include aluminum hydroxide gel and bentonite magma
 Paints are also another example of thixotropic fluids
 When modern paints are applied the shear created by the
brush or roller will allow them to thin and wet out the
surface evenly
 Once applied the paints regain their higher viscosity
which avoids drips and runs.

Irreversible Thixotropy
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 Application of shear stress causes breakdown of
structure within the system but
 The structure does not reform on removal of the
shear stress, or
 The time lag is so long that from a practical point of
view the effect is irreversible.
 Example: gels produced from higher M.Wt
polysaccharides
 On application of high shear,
 The 3D structure of the polysaccharides is reduced
to a 2D one and
 The original gel-like structure is never

Negative thixotropic or Antithixotropy
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 Antithixotropy is believed to result from an increased collision
frequency of the dispersed particles in suspensions which
 results in an increased inter-particle bonding with time.
 This changes the original state of the system from a network of a
large number of individual particles and small floccules to an
equilibrium state consisting of a small number of relatively large
floccules.
 Which helps the dispersed particles to acquire a random orientation
and the network is established: Gel

Negative thixotropic…
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 At rest the large floccules break up and gradually return to
the original state of small floccules and individual particles.
viscosity decreases: Solution
Negative thixotropic or Antithixotropy…

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 Rheopexy is the conversion of solids to gel up on
application of shearing stress.
 Substances with low melting point
 shows rheopectic property.
 Example: polymers with low melting point
 Rheopectic fluids are rarely encountered

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o Different equipments called viscometers are used to
measure rheological properties
o such as viscosity of different fluids and semisolids.
o Successful determination and evaluation of rheologic
properties of any particular system depend on choosing the
correct instruments
 For Newtonian system
 Single point instruments can be used
 Instruments that operate at a single rate of shear.
 For no-Newtonian system,
 multipoint instruments should be used
 instruments that can operate at different rates of shear
Determination of the rheological
properties

Determination of the rheological properties…
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Ostwald Viscometer
 It is a type of capillary viscometer
 This is ‘U’ shape tube with two bulbs and two
marks .
 It is used to determine the viscosity of
Newtonian liquids.

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Principle:
 When a liquid flows by gravity, the time required for
the liquid to pass between two marks, upper mark
and lower mark,through a vertical capillary tube is
determined.
 The time of flow of the liquid under test is compared
with the time required for a liquid of known viscosity
(usually water).

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 The viscosity of unknown liquid η1 can be
determined using the equation
η1 = (ρ 1t1 ) η2
ρ 2t2
ρ1= Density of unknown liquid
ρ2= Density of known liquid
t1= Time of the unknown liquid
t2= Time of the known liquid
η 2= Viscosity of known liquid

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Falling-sphere viscometer
 Consists of cylindrical transparent tube having
graduated section near the middle of its length and
generally a steel ball that is allowed to fall through
the tube.

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 The tube is filled with the liquid whose viscosity is to be
determined and the ball is allowed to fall.
 The velocity of the falling ball is measured and viscosity is
calculated using stoke’s law
V= d2 (ρ s- ρ o ) g
18
where d= Diameter of the falling ball; ρ s =Density of the
ρ l=Density of liquid; g= Gravitational acceleration; v =
settling velocity
 As d2g/18 is constant, can be replaced by another
constant ‘K' Therefore, the equation will be
 = K(ρ s- ρ 1 )
V

Factors affecting rheological properties
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Chemical factors
Extent of Polymer Hydration
 In hydrophilic polymer solution the molecules are
regarded as completely surrounded by
immobilized water molecules forming a solvent
layer.
 Such hydration of hydrophilic polymers gives rise
to an increased viscosity.

Factors affecting rheological,…
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Impurities, Trace Ions and Electrolytes
 Chemical impurities are the major factors in
changing the viscosity of natural polymers.
 At high conc. =>compete for adsorbed water
molecules surrounding the hydrated polymer =>
the polymer become dehydrated => the
viscosity of the dispersion decreased =>
precipitation occurs.

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Physical factors
Temperature
 A temperature increase usually produces a rapid
viscosity decrease.
 Prolonged heating may produce drastic decrease in
viscosity
 due to decomposition of the polymer,
e.g., gelatin.

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Concentration
In concentrated suspensions:
 a decrease in particle size or an increase in the
surface area of the solid phase produce an
increase in the viscosity of the system.
 This due to immobilization of the vehicle with an
increase in
the fraction of the suspension volume occupied by the
solid.

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Rheology of emulsion
o All except dilute medicinal emulsions exhibit
o non-Newtonian behavior.
o The fluid emulsions
o are usually pseudoplastic.
o Those approaching a semisolid nature behave
o plastically.
o The semisolid creams
o are usually viscoelastic.
Significance of rheology

Significance of rheology …
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Rheology of suspension
o Most exhibit plasticity and pseudoplasticity along
with thixotropy.
o The rheological properties of suspensions are
markedly affected by
 the degree of flocculation
 The types and quantities of suspending and
 thickening agent.
o Preferred rheological properties
 Pseudoplasticity along with thixotropy

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o Flocculated suspensions will exhibit
o plastic or
o pseudoplastic (more usually) behaviour.
o If the breakdown and reformation of the bonds between
floccules is time dependent then thixotropic behaviour
will also be observed.
o Concentrated deflocculated suspensions exhibit
o dilatancy.

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Rheology of semisolids
o Rheological characteristics of semisolids
affect
 The stability and their extrudability from tubes
 The capacity to take up solids or liquids
 The spreadabilty on the skin and
 Release of active from the base.

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o Most of the topical semi-solids
o show plastic flow behaviour.
o Hydrocarbon bases (e.g petrolatum) exhibit
o plastic flow with varying degree of thixotropy..
o Some paste exhibit
o dilatancy when subject to shear where as others demonstrate
typical
o pseudoplastic flow with thixotropy

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Rheology of powders
o Good flow property of powder are required to ensure
uninterrupted flow from hopper and into the die
cavities
 in high speed tabletting machines
 during encapsulation and
 during the packaging
o Factors affecting the flow properties of particulate
solid are
 the surface characteristics and
 Stickiness.

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o Improvement in the flow characteristics can be
achieved by
 reducing the surface roughness of particle
reduced by the presence of fines in powder
 which fill the irregularities.
 reducing the stickiness b/n particles
reduced by the addition of lubricant
 such as talc, starch and magnesium strearate

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Rheology and processing
o The rheological considerations of the material are
important
o to ensure proper mixing.
o Large impellar operating at a low shear rate is required
o for shear thinning systems.
o Semisolid material exhibiting dilatant properties
o should be processed with low shear mixer.

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o Low shear mixing is sometimes required in certain
polmeric dispersion
o in order to prevent the breakdown of struture due to
depolymerisation at high shear rates.
o Rheological considerations are also important
o during fluid transfer including pumping from one vessel
to another and flow though pipes.

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Thank You!!!

Ch7. Rheology

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