Module 5: Physical Rheology PDF

Module 5: RHEOLOGY KNOWLEDGE OBJECTIVES 1. Differentiate Newtonian to Non-Newtonian Systems. 2. Differentiate flow properties and corresponding rheograms between Newtonian and Non- Newtonian materials. 3. Discuss the application of rheology in pharmaceutical formulation and analyses...

Module 5: RHEOLOGY KNOWLEDGE OBJECTIVES 1. Differentiate Newtonian to Non-Newtonian Systems. 2. Differentiate flow properties and corresponding rheograms between Newtonian and Non- Newtonian materials. 3. Discuss the application of rheology in pharmaceutical formulation and analyses of emulsions, pastes, suppositories and creams. MODULE OUTLINE 1. Types of flow 2. Determination of rheologic properties 3. Techniques for rheological measurements The term “rheology,” from the Greek rheo (‘to flow”) and logos (“science”), was suggested by Bingham and Crawford \ to describe the flow of liquids and the deformation of solids. “Rheology is the study of the flow of the materials that behave in an interesting or unusual manner. Oil and water flow in familiar, normal ways, whereas mayonnaise, peanut butter, chocolate, bread dough, in silly putty flow in complex and unusual ways”. – In rheology, we study the flows of unusual materials “All normal or Newtonian fluids (air, water, oil, honey) follow the same scientific laws. On the other hand, there are also fluids that do not follow the Newtonian flow laws. These Non-Newtonian fluids, for example mayonnaise, paint, molten plastics, foams, clays, and many other fluids, behave in a wide variety of ways”. – The science of studying these types of unusual materials is called rheology Viscosity is an expression of the resistance of a fluid to flow; the higher the viscosity, the greater is the resistance. In pharmaceutical sciences, rheology is widely used to study polymers. An understanding of the viscosity of liquids, solutions, and dilute and concentrated colloidal systems. Rheology is involved in the mixing and flow of materials, their packaging into containers, and their removal prior to use, whether this is achieved by pouring from a bottle, extrusion from a tube, or passage through a syringe needle. Rheologic properties of a pharmaceutical system can influence the selection of processing equipment used in its manufacture. Scott-Blair recognized the importance of rheology in pharmacy and suggested its application in the formulation and analysis of the following pharmaceutical products: - Emulsions - Pastes - Suppositories - Tablet coatings Examples of Complex Fluids Foods - Emulsions (mayonnaise, ice cream) - Foams (ice cream, whipped cream) - Suspensions (mustard, chocolate) - Gels (cheese) Biofluids - Suspension (blood) - Gel (mucin) - Solutions (spittle) Personal Care Products - Suspensions (nail polish, face scrubs) - Solutions/Gels (shampoos, conditioners) - Foams (shaving cream) Electronic and Optical Materials - Liquid crystals (monitor displays) - Melts (soldering paste) Pharmaceuticals - Gels (creams, particle precursors) - Emulsions (creams) - Aerosols (nasal sprays) Polymers TERMS: Deformation → change of the shape and the size of a body due to applied forces (external forces and internal forces). Flow → irreversible deformation (matter is not reverted to the original state when the force is removed) Elasticity → reversible deformation (matter is reverted to the original form after stress is removed) Strain: deformation in term of relative displacement of the particles composing the body Stress: measure of internal forces acting within a (deformable) body Shear: deformation of a body in one direction only (resulting from the action of a force per unit area τ=shear stress) and having a given perpendicular gradient (γ=shear strain). Classification of Materials According to Types of Flow and Deformation: 2 Categories 1. Newtonian Systems 2. Non-Newtonian Systems NEWTONIAN SYSTEMS Newton’s Law of Flow Newton was the first to study flow properties of liquids in a quantitative way. He recognized that the higher the viscosity of a liquid, the greater is the force per unit area (shearing stress) required to produce a certain rate of shear. Newtonian Flow: Ex. water, ethanol, acetone, glycerine, benzene - Has linear relationship between shear rate and shear stress - Constant viscosity with increasing rate of shear Shearing Stress (F) is the force per unit area required to bring about flow. F = F′/A Rate of Shear (G) or Velocity Gradient is the difference of velocity (dv) between two planes of liquid separated by an infinitesimal distance (dr). G = dv/dr Rate of shear should be directly proportional to shearing stress, or: F′ = η dv A dr η- coefficient of viscosity; simply viscosity Unit: poise (cgs unit dyne sec/cm-2 or g/cm sec) Useful Conversion/Equation: - A more convenient unit for most work is the centipoise (cp) – 1 cp=0.01 poise - Fluidity (ø) – reciprocal of viscosity ø = 1/η Kinematic Viscosity Is the absolute viscosity divided by the density of the liquid at a specific temperature; kinematic viscosity = η/ƿ Units: stoke (s), centistoke (cs) A. Shear Dependent Viscosity - Plastic - Pseudoplastic - Dilatant B. Time Dependent Viscosity - Thixotropy - Rheopexy Non-Newtonian Systems The majority of fluid pharmaceutical products are not simple liquids and do not follow Newton’s law of flow. Generally exhibited by liquid and solid heterogeneous dispersions such as colloidal solutions, emulsions, liquid suspensions, and ointments. When non-Newtonian materials are analyzed in a rotational viscometer and results are plotted, various consistency curves, representing three classes of flow (shear dependent) are recognized: a) Plastic - Bingham bodies - A yield value must be overcome before the system begins to flow b) Pseudoplastic - Shear thinning systems - Curve begins at the origin - No yield value - Viscosity decreases with increasing shear rate Ex. polymer solution, Na alginate, Pentylcellulose, PEG c) Dilatant - Shear thickening system - Reverse effect to pseudoplastic flow - Viscosity increases with increasing rate of shear - Suspensions containing a high concentration (about 50% or greater) Ex. starch in water, zinc oxide Thixotropy – (gel-sol transformation) fluid after agitation but resume their solid state after being undisturbed for a period of time. Thixotropic preparations have the advantage that the particles remain in more or less permanent suspension during storage and yet when required for use, the paste are readily made fluid by tapping or shaking. Ex. some suspensions, gels, bentonite magma Measurement of Thixotropy Most apparent characteristic of a thixotropic system is the hysteresis loop (up and down curves of the rheogram). Area of Hysteresis – measure of thixotropic breakdown obtained by planimeter or suitable technique. Negative Thixotropy/Antithixotropy - Represents an increase rather than a decrease in consistency on the down-curve (sol- gel formation). Dilatant vs Antithixotropy  ❖ Dilatant systems are deflocculated and ordinarily contain greater than 50% by volume of solid dispersed phase while Antithixotropy systems have low solid content (1%-10%) and are flocculated. Rheopexy – rare phenomenon in which the longer the fluid undergoes shearing forces, the higher the viscosity. Rheopectic system – gel is the equilibrium form Antithixotropic system – sol is the equilibrium form Viscoelasticity The property of materials wherein this exhibit both the viscous properties of liquids as well as the elastic property of solids. When materials are subjected to stress for a period of time they undergo slight deformation under the influence of the stress but once the stress is removed, they exhibit their behavior by returning to their original shape with little permanent deformation. Application of Viscoelasticity - Creams - Lotions - Ointments - Suspensions - Colloidal dispersions Determination of Rheologic Properties I. Single-shear Rate Instrument Suitable for use only with Newtonian materials a) Capillary Viscometer b) Falling-Sphere Viscometer II. Multipoint, Rotational Instruments Can be used both for Newtonian and non-Newtonian materials a) Cup-and-bob Viscometer b) Cone-and-Plate Viscometer