BTF3823 Rheology PDF

RHEOLOGY (BTF3823) TOPIC OUTCOMES 1. Differentiate between type of rheological systems. 2. Analyze the behavior on flow properties of colloidal dosage forms.  DEFINITION OF RHEOLOGY Rheology is the science/physics that concerns with the flow of liquids and the deformation of solids...

RHEOLOGY (BTF3823) TOPIC OUTCOMES 1. Differentiate between type of rheological systems. 2. Analyze the behavior on flow properties of colloidal dosage forms.  DEFINITION OF RHEOLOGY Rheology is the science/physics that concerns with the flow of liquids and the deformation of solids under the influence of stress. The term rheology was derived from two Greek words: rheo – to flow logos – science Study of flow properties of liquids is important for pharmacist working in the manufacture of several dosage forms, viz., simple liquids, gels, ointments, creams, and pastes. These systems change their flow behavior when exposed to different stress conditions.  FUNDAMENTALS OF RHEOLOGY 4 i. Manufacturing of dosage forms: Materials undergo process such as mixing, flowing through pipes, filling into the containers etc. Flow related changes influence the selection of mixing equipment. ii. Handling of drugs for administration: The syringeability of the medicines, the pouring of the liquids from containers, extrusion of ointment from tubes, all depend on the changes in flow behavior of dosage forms.  The stress may be: Tensile stress: Stress applied perpendicularly to the surface of the body Shearing stress: Stress applied tangentially to the surface of a body The DEFORMATION that results from the application of a stress may be divided into two types: i) Elastic Deformation : It is a spontaneous and reversible Exhibited by elastic bodies (ex. Rubber) ii) Plastic Deformation : It is a permanent or irreversible deformation. Plastic deformation is exhibited by viscous bodies.  VISCOSITY Viscosity is qualitatively defined as the resistance of a liquid to flow due to internal friction between different layers of liquid when it flows. Coefficient of Viscosity, ɳ = shearing stress, F/rate of shear, G The unit of viscosity is poise or dyne.sec.cm-2 Representation of shearing force acting on a block of material RHEOGRAM Rheological properties of a system are usually expressed in the form of a graph wherein shearing stress (on the x axis) is plotted against rate of shear (on the y axis) and this is called rheogram.  TYPES OF RHEOLOGICAL SYSTEMS Rheology Non – Newtonian Newtonian Plastic Pseudoplastic Dilatant  NEWTONIAN SYSTEMS If a system shows same values of ɳ (coefficient of viscosity) at different shearing rates at a given temperature, it is said to be a Newtonian system. Shearing Shear rate Coefficient of viscosity stress X1 Y1 X1/Y1 = ɳ X2 Y2 X2/Y2 = ɳ......... Xn Yn Xn/Yn = ɳ The linear curve passes through the origin indicating that even a mild force can induce a flow. The curve also indicates that the values of ɳ are constant at different shear rates. Examples Water, simple organic liquids, dilute suspensions and emulsions  NON - NEWTONIAN SYSTEMS ▪ If a system shows different values of ɳ (coefficient of viscosity) at different shearing rates at a given temperature, it is said to be a non - Newtonian system. ▪ Therefore the viscosity is called apparent viscosity. ▪ Non - Newtonian bodies are those substances, which fail to follow Newton's law i.e. liquid & solid , heterogeneous dispersions such as colloidal solutions, emulsions, liquid suspensions and ointments. Shearing Shear rate Coefficient of viscosity stress X1 Y1 X1/Y1 = ɳ1 X2 Y2 X2/Y2 = ɳ2......... Xn Yn Xn/Yn = ɳn Types of non-Newtonian systems: 1. Plastic 2. Pseudoplastic 3. Dilatant  PLASTIC FLOW: ▪ Plastic flow curve does not pass through the origin. ▪ As increase the stress, leads to non-linear increase in shear rate but after that curve is Slope = mobility linear. ▪ The linear portion extrapolated intersects the x axis at the point called as yield value. ▪ Curve A to B is not linear. The slope of the curve gradually increases until the point B and it indicates a gradual decrease in the B viscosity of the system. Yield ▪ The system which shows gradual decrease A value in viscosity on increasing the shear stress is called shear thinning system.  Plastic flow explained by flocculated particles in concentrated suspensions, ointments, pastes and gels. F/A Yield value Increase stress Flow Flocculated Individual Particles particles EXAMPLES: ZnO in mineral oil, certain pastes, paints and ointments. The equation describing plastic flow is, U=F–f/G Where, f = Yield value F = Shearing stress G = Rate of shear  PSEUDOPLASTIC FLOW Many pharmaceutical products liquid dispersion of natural and synthetic gums shows pseudoplastic flow. eg. 1. Tragacanth in water 2. Sod. Alginate in water 3. Methyl cellulose in water 4. Sodium CMC in water Graph for pseudo plastic flow is like this Rate of shear, G Shearing stress, F In which curve is passing from origin (Zero shear stress), so no yield value is obtained. As shear stress increases, shear rate increases but not linear.  With increase in the shearing stress the disarranged molecules orient themselves in the direction of flow, thus reducing friction and allows a greater rate of shear at each shearing stress. Some of the solvent associated will be released resulting in decreased viscosity. This type of flow behavior is also called as shear thinning system.  Pseudoplastic flow can be explained by Long chain molecules of polymer. Polymer & water molecules align on direction of force Water Stress Polymer long chain with water molecules In storage condition, arrange randomly in dispersion. The exponential equation shows this flow FN = η G N = no. of given exponent η = Viscosity coefficient G = Rate of shear In case of pseudoplastic flow, N > 1. i.e. More N >1, the greater pseudoplastic flow of material. If N = 1, the flow is Newtonian.  DILATANT FLOW ▪ Certain suspensions with high concentration of dispersed solids shows an increase in resistance to flow with increasing rates of shear. ▪ This system increase in volume when sheared, such system called as dilatant flow. ▪ Also, called as “Shear thickening system” i.e. when stress is removed, dilatant system return to its original position  23 Graph for dilatant flow is like this Rate of shear, G Shearing stress, F ❖ In which curve is passing from origin (Zero shear stress), so no yield value is obtained. ❖ Non-linear increase in rate of shear. ❖ Increase resistance to flow on increase rate of shear  In which, particles are closely packed with less voids spaces, also amount of vehicle is sufficient to fill the void volume. Increase rate of shear At rest: close packed Open packed: Less void volume High void volume Sufficient vehicle Insufficient vehicle Low consistency High consistency  So 25 therefore, dilatant suspension can be poured from bottle because in these condition it is fluid. When stress is increased, the particles shows the open packing and bulk of system (void volume is increase) is increased. Finally system show the paste like consistency. RHEOGRAMS OF DIFFERENT FLUIDS Newtonian flow Plastic flow Pseudoplastic flow Dilatant flow  THIXOTROPIC BEHAVIORS ▪ DEFINITION: It is a comparatively slow recovery, on standing of a material which lost its consistency through shearing. ▪ Thixotropy is only applied to shear-thinning systems. This indicates a breakdown of structure (shear-thinning), which does not reform immediately when the stress is removed or reduced.  THIXOTROPIC BEHAVIORS Newtonian systems If the shearing stress was reduced once the desired maximum rate had been reached, the down curve would be identical with & superimposed on the up-curve. Non Newtonian systems With shear-thinning systems (i.e., plastic & pseudoplastic), the down - curve is frequently displaced to the left of the up-curve and for shear thickening system (dilatant), the down curve is displaced to the right of the up curve. This phenomenon is known as Thixotropy.  INSTRUMENTATION CHOICE OF VISCOMETER Single/One point Multipoint At a single rate of shear one point Several rates of shear many points on the curve. on the curve Equipment Equipment 1) Ostwald viscometer 1) Cup and bob 2) Falling sphere viscometer 2) Cone and plate Applications: Applications: Newtonian fluids Non-Newtonian fluids Newtonian fluids  Viscosity of some products Products Absolute viscosity (Centipoise, cp) Cough syrup 190 Detergents 1470 Face cream 10,000 Hair cream 5,000 Hand cleaner 2,000 Hand cream 780 Latex emulsion 200 Paraffin emulsion 3000 Shampoo 3000 Soap solution 82 Toothpaste 70,000 20 cP to 2 M cP  Rheology properties in pharmaceutical products Rheology properties in suspension ▪ The flow properties of suspensions depend upon rheological properties. ▪ The presence of dispersed particles in the liquid continuous medium causes streamlines of liquid around the particles to be distorted and as a result the viscosity.  During storage, the suspensions exhibit gel like structure & lowers the rate of settling. On moderate shaking, the product assumes sol-like behaviour, which permit pouring of the product from the bottle. The sol like behavior also helps in uniform spreading of dermatological preparations. Therefore, in order to maintain these properties the flow properties should be studied. Dispersion medium is largely responsible for determining the rheology of the suspensions. The pre-formulation studies include evaluation of vehicles used in the suspension. An optimum viscosity of this medium should be selected through experimentation.  Rheological evaluation is used as a quality control parameter for comparing products. The consistency of suspensions are evaluated using cup & bob or cone & plate viscometers. Rheology properties in emulsions: The following flow related attributes are desirable for the overall performance of an emulsion Removal of an emulsion from a bottle or tube Flow of an emulsion through a hypodermic needle Spreadability of an emulsion on the skin Stress induced flow changes during manufacture  In general, dilute emulsions exhibit “Newtonian flow”. As the viscosity of the emulsion increases, flocculation of globules will be reduced because the mobility of globules is restricted, leads to creaming. Due to this antagonistic effect, an optimum viscosity is desirable for good stability. Concentrated emulsions exhibit “non-Newtonian flow”. Multipoint viscometers are used for viscosity analysis.  RHEOLOGY IN SEMISOLIDS Viscosity of the semisolid dosage forms may directly influence the diffusion rate of the drug. Therefore, the product behavior must be monitored at the time of application. This also helps to monitor batch-to-batch consistency. Examples of semisolids: creams, ointments, pastes, gels. Most topical semisolids, when applied on the surface of the skin, show non-Newtonian behaviour. The structures formed within semisolid drug products during manufacturing can show a wide range of behaviours including, thixotropy & irreversible or reversible structure damage.  Semisolids do not flow at low shear stresses but undergo reversible deformation like elastic solids. When a characteristic shear stress is exceeded, they flow like liquids. Best instrument for determining the rheological properties of pharmaceutical semisolids is rotational viscometer. E.g. cone-plate viscometer, cup & bob viscometer, Brookfield viscometer. Ointments and Creams: Ointments and creams shows plastic and thixotropic behavior. Determination of rheological behavior and thixotropic coefficients → serve as a guide for quality control measures, for stability, spreadability, performance and elegance of the preparation. Thixotropic pharmaceutical preparations have high consistency in the container under storage and hence more physical stability. Yet, they are easily pourable and spreadable for use. ▪ The viscosity of creams and lotions may affect the rate of absorption of the products by the skin. ▪ A greater release of active ingredients is generally possible from the softer, less viscous bases. ▪ The viscosity of semi-solid products may affect absorption of these topical products due to the effect of viscosity on the rate of diffusion of the active ingredients. THANK YOU