RHEOLOGY MODIFIERS
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Rheology is the science of flow.
Every time a lotion is poured, a cream squeezed
from a tube or a lipstick applied, rheology is
involved.
Even when products are at rest, it plays an
important part in controlling stability.
The rheology of a system is described in terms of its
viscosity
Viscosity is the resistance to flow
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Shear stress is the force per unit area applied and
shear rate is the resulting velocity gradient.
The way the viscosity changes at varying shear
rates greatly influences the overall product stability
and performance.
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A material which maintains a constant viscosity, regardless of shear rate,
has Newtonian flow
Dilatant flow is a form where viscosity increases with shear, sometimes
encountered in highly pigmented/filled systems.
Pseudoplastic flow ,also known as shear thinning behaviour, and typical of
many commercial systems, is when viscosity decreases with increasing shear
rate.
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At low shear rates, i.e., near at rest
conditions, non-Newtonian fluids
exhibit high viscosities that are
relatively insensitive to shear rate
and characterized by zero shear
viscosity.
The zero shear viscosity is known to
be highly sensitive to the molecular
weight and concentration of the
rheological additives.
The rates of deformation
associated with this region include
sedimentation and levelling forces,
and one can tailor the zero shear
viscosity to combat these forces.
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•At moderate shear rates the decrease in
viscosity versus shear rate helps when pouring
and pumping these fluids.
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•At high shear rates it is found that a second
Newtonian plateau in viscosity is reached
usually characterised by the so-called infinite
viscosity.
•The shear forces in this area are close in
magnitude to forces developed during
rubbing and spraying exercises.
•The low viscosities exhibited by the
rheological additives in this region imply low
resistance to rubbing and thus a smooth
sensation of the substance during its
application.
•Non-Newtonian fluids also exhibit elastic
properties, i.e., when subjected to high shear
rates, non-Newtonian fluids will exhibit elastic
stresses.
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The desirability of the elastic response will vary with the
intended use of the personal care product.
In the case of toothpaste, an elastic force is needed to
increase extrudate spring back during the tube filling
operation in toothpaste production or while dispensing it at
home.
However, excessive elasticity might not be desirable, as it may
make the toothpaste too stringy.
High elasticity is needed to stabilize foams, for example in
shaving creams, as it provides strength to film at the air/liquid
interface in the matrix of bubbles.
In the case of creams and lotions, a short texture with less
elasticity may be desired.
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Examples of substances that impart viscosity as well as elasticity to a
fluid are cellulose ethers, xanthan gum, and crosslinked polyacrylic
acids.
Clays can impart viscosity without elasticity.
A variety of additives are available in the marketplace that allow
formulators to create a range of viscosities and elasticities in the final
product.
Interacting particulates such as smectic, hydrophilic and organoclays
represent one class of materials used in personal care products that
can impart non-Newtonian characteristics to formulations.
At a very low concentration, they are known to impart significant
viscosity enhancement to the base fluid without any significant
elasticity.
They typically exhibit a flow curve similar to fluid
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These materials cause gelling if used at higher concentrations.
In the case of polymers, their zero shear viscosity, shear-
thinning, and elasticity characteristics are a function of their
structural characteristics.
The rigidity of the polymer, its weight average molecular
weight, polydispersity, and degree of branching each play a
part in determining these properties.
Water-soluble cellulose ether derivatives such as
carboxymethylcellulose (CMC), hydroxyethylcellulose (HEC),
hydroxypropyl cellulose, and methylcellulose impart
pseudoplastic or shear-thinning rheology to formulations.
This characteristic makes these polymers attractive candidates
as thickening agents in personal care products.
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Classification of rheology modifiers
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Viscosity imparting agents : These agents are used when it is desirable
to increase or decrease the viscosity of a liquid either to serve as adjacent
for palatability or to improve pour ability. They are also called thickening
agents.
Viscosity imparting agents are of two types
a) Viscosity modifier-Viscosity modifiers decrease the viscosity of a liquid to
improve pour ability and make it more palatable.
b) Viscosity enhancer- Viscosity enhancers increase the viscosity of a liquid to
improve pour ability and make it more palatable.
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MECHANISM
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Associative thickeners – mechanism of action
Associative thickeners increase the viscosity of aqueous
systems through a system of molecular associations and
interactions.
The thickeners consist of water soluble polymeric chains
modified with hydrophobic caps.
The hydrophobic caps of the polymer associate, that is
build a structured network with other hydrophobes in
the formula.
This results in micelle type structures that form a network
resulting in significant viscosity build.
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Hydrophilic Clays
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Applications
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Widely used in shampoos, conditioners, hair spray, and
hair-styling gels, toothpastes, and denture adhesives
pseudoplastic rheology is particularly beneficial in
surfactant-based haircare formulations like shampoos
where cellulose ethers can be used to reduce or
eliminate inorganic salt added for thickening
Cellulosic thickeners can be used to achieve viscosities
higher than possible with salt or even salt combined
with alkanolamide.
In many cases, even the alkanolamide can be replaced
by the cellulose ether
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incorporation of 1% hydroxyethylcellulose into a
TEA-lauryl sulfate luxury shampoo increased the
formulation viscosity from a Brookfield viscosity of
460 cps to a gel with a viscosity of 5300 cps.
Unlike salt, cellulose ethers do not influence
surfactant cloud points, and they can be used to
viscosify surfactant systems that are difficult to
thicken, such as imidazolidine- derived amphoterics,
sulfosuccinates, and highly ethoxylated alkyl ether
sulfates
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In other haircare applications, such as conditioning hair
rinses, addition of a low level of hydroxyethylcellulose
polymeric thickener can significantly increase finished
product viscosity and improve shelf stability
Cellulose ethers in general have this effect on product
viscosity and shelf stability.
Methylhydroxypropylcellulose effectively thickens sodium
lauryl sulfate; a surfactant commonly used in surfactant-
based haircare formulations, yielding solutions with
excellent high temperature freeze/thaw stability.
Cellulosics achieve the enhanced shelf stability by
maintaining the viscosity of the formulation at room
temperature, and during freeze/thaw cycling.
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As SUSPENDING AGENTS
rheological additives have been used widely to prevent
sedimentation of solid particulates, prevent coalescence in
emulsions, and halt collapse of foams.
Rheological substances can impart suspending power to the
base fluid.
The polymer’s yield stress or high viscosity at low shear rates
are both used for this purpose.
Fluids that possess a yield stress may experience flow only
when the imposed stress on the fluid surpasses its yield
stress.
Below the yield stress the fluid displays solid like properties.
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Among the polysaccharides, xanthan gum has been
widely used as a suspending aid.
Xanthan gum has a double helical structure and
undergoes significant hydrogen bonding in solution.
At rest or when subjected to very low deformations, a
weak three dimensional network structure is the
prevailing structure which gives rise to the yield stress
When subjected to higher deformations, this structure
can easily be broken down to give rheological
behaviour similar to fluid
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Other polysaccharides that exhibit yield stresses
are kappa and iota carrageenans.
These polysaccharides will also form weak gels and
are used in personal care products for stabilization.
Carboxymethylcellulose imparts a high viscosity at
low shear to formulations, enabling it to effectively
suspend solids
CMC has a high capacity for water-binding, and it
is generally used to effect rheology and prevent
syneresis in high solids formulations
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Methylhydroxypropylcellulose has been shown to
enhance shampoo lather by way of the water-
binding, surface activity, and thermal gelation
properties of this cellulose ether.
This polymer can stabilize lather by a mechanism
known as interfacial gelation
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Hydrophobically modified cellulose ethers, such as
modified hydroxyethylcellulose, viscosify aqueous
phases through both hydrogen-bond network formation
and through the formation of three-dimensional
networks due to hydrophobic interactions.
This dual thickening mechanism makes modified
hydroxyethylcellulose particularly effective at
suspending solids.
The hydrophobic moieties may also associate with
surfactant micelles, making modified hydroxyethyl
cellulose a particularly efficient thickener for surfactant
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Modified hydroxyethylcellulose finds use in many
applications, including viscosity and structure development in
shampoos, conditioners, and in hand and body lotions.
Colloidal size materials, like fumed silica, are also used for
stabilization
Fumed silica can be processed to develop aggregate
particles, and thus form weak three-dimensional structures.
Stabilization can also be achieved directly by milling the
materials to be used in the formulation to colloidal sizes to
take advantage of colloidal forces for stabilization.
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THIXOTROPIC AGENTS
Thixotropy, however, is a time-dependent
phenomenon, defined as:
• The ability of the substance to exhibit lower
viscosities as a function of shear rate and duration.
• And its ability to have its structure reformed over a
period of time.
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For toothpaste, a great effort has been directed towards
optimisation of toothpaste physical attributes.
These attributes are strongly dependent on rheological
characteristics of the toothpaste system, such as viscosity,
pseudoplasticity, thixotropy and low shear yield stress.
The major rheological additive types are currently used in
toothpaste systems.
They are generally classified into four main categories:
1) natural, 2) modified natural, 3) synthetics, and 4)
inorganic.
These classes are represented respectively by 1) xanthan
gum, carrageenan; 2) cellulose ethers; 3) crosslinked
polyacrylic acids; 4) clays and amorphous silicone dioxide.
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Carrageenan has a higher thixotropic index than seen with
xanthan gum, but it also recovers its initial structure very
quickly.
Interacting fillers such as clay, fumed silica, and aluminum-
magnesium hydroxide are also used as thixotropic modifiers in
personal care products
GELLING AGENTS
Gel-
1) coherent, two-component systems formed by a solid substance
finely dispersed or dissolved in a liquid phase;
2) exhibit solid-like behaviour under the action of mechanical
forces;
3) both the dispersed component and the solvent should extend
continuously throughout the whole system, each phase being
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Gels into two major classes, strong and weak gels.
Strong gels possess the canonical features of true
gels.
They manifest typical behaviour of viscoelastic
solids and rupture beyond a certain deformation
value rather than flow.
Weak gels resemble strong gels at low deformation
rates but their three dimensional networks get
progressively broken down at higher deformation
rates and they flow as a dispersed system.
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Gelatine, agar, pectins, alginates, and kappa
carrageenans will undergo gelation to yield strong
gels
Salts of crosslinked polyacrylic acid, iota-
carrageenan, and cellulose ethers, will also form
gels and are used in personal care formulations that
exploit weak gel properties.
They are highly useful in skin creams, shaving gels,
hair styling gels, and gel toothpaste formulations.
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Silica gels and fumed silica can yield weak gels.
They are used in toothpaste where it serves a dual
role as an abrasive and a rheology modifier.
water-soluble organic binder is necessary to modify
the toothpaste rheology and to prevent water
separation.
Carboxymethylcellulose and carrageenans are often
combined with silica for this purpose.
Thus rheological additives significantly influence the
mechanical, textural, stability, and ultimately the
quality of personal care products.
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Product Application and Selection
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Choosing the right rheological additive
Cosmetic products range from pressed dry powders
to suspensions in liquids; from single-phase solid or
liquid formulations to emulsions.
The choice of rheological additive and level of use
depends on the system and the desired
performance criteria
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The properties imparted by the rheological additive
benefit different cosmetics in different ways.
Stick products (lipstick and antiperspirant)
• maintain homogeneity in molten and setting stages
• gain improved pay-out
• increase high-temperature integrity
• eliminate oil migration
• reduce “creasing” around lips and eyes
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Mascara
• improve film-build
• increase water-resistance
• eliminate oil migration
Emulsions
• elevate drop-point temperature of water-in-oil creams
• eliminate syneresis
• improve emulsion stability
• enable cold-process emulsification
• create novel viscosity effectwsww.DuloMix.com
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UV Sunscreen
• eliminate ultrafine Ti02 skin-whitening problems
• enhance sun protection factor (SPF)
• optimise use of sunscreen actives
• reduce separation and settlement
Antiperspirant Aerosols
• uniform distribution of active ingredients
• soft and dry skin feel
• significant less whitening
• reduced separation and settlement
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