Lecture 5 - Viscosity Control Flashcards
Two primary purposes of rheology additives
Enhancement of product form
- Aesthetics, texture, processing, packaging control
- Flow & application properties
Product stability
- Preventing colloidal systems from breaking down
- Retard coalescence and creaming
functions of thickeners
Control phase separation Prevent syneresis Suspend particulate materials Form gels Retard or eliminate crystal growth Extend shelf life Have a positive effect on product application
The flow properties of cosmetic solutions, emulsions, and dispersions are a result of colloidal structures and/or thickening agents such as:
Particle-particle interaction
Association structure disturbances
Polymer behavior in solution
define rheology
how materials deform or flow in response to external forces
“the branch of physics that studies the deformation and flow of matter”
define shear stress
the external force applied over an area. Unit are Pascals (Pa) = Newtons per meter2 (1Pa= 10 dynes/cm2 )
define shear rate
ratio of velocity of material to its distance from a stationary object -the velocity gradient (1/sec) –HOW FAST
define shear strain
relative displacement of the faces of a sheared body (for example a layer of fluid)divided by the distance between them- (no units)-HOW FAR (relative measure of deformation)
define viscosity
a measure of a material’s resistance to flow measured in Pascal seconds = 1 poise (P) in cgs system = 100 centipoise (cps)
define newtonian flow
not variable with shear linear response of flow rate to shear stress
define pseudoplastic flow
shear thinning
define dilatant flow
shear thickening
define thixotropic flow
shear thinning with time dependence (slower return to original state)
4 Factors Governing O/W emulsion viscosity
Presence of thickening agents in external water phase (hydrocolloids)
Presence of thickening agents in lipid phase and dispersed in water phase (co-emulsifiers & solid amphiphiles that form liquid crystalline gel networks)
Volume ratio of internal- dispersed phase to external-continuous phase (phase volume)
Lipid composition of internal phase
6 sources of thickeners
Synthetic polymers – acrylates, carbomers, acrylamides, polyurethanes etc…
Direct nature sourced carbohydrates: algal, plant exudates and seed gums
Fermentation derived microbiological
by-products - exopolysaccharides
Cellulosics - cotton linters, wood fiber
Smectite Clays – mined & purified silicate minerals – bentonites, hectorites
Inorganic silicas
what is the Concentration- Response Function
The viscosity/thickness response of the solvent media versus concentration is dependent on interaction of gum molecules/particles with each other , the solvent and other ingredients
what is the Critical Overlap Concentration
concentration above which dramatic viscosity increase occurs due to interaction of polymer molecules to form interpenetrating networks in the solvent.