Rheology Notes PDF

## Rheology ### Unite 4 **Introduction to Polymer Rhealogy:** The flow process in manufacturing polymer products can be represented in the following way: - Raw Material Activities: - Molecular and compositional modification and enhancement - Processing Activities: - Rheology and equipmen...

## Rheology ### Unite 4 **Introduction to Polymer Rhealogy:** The flow process in manufacturing polymer products can be represented in the following way: - Raw Material Activities: - Molecular and compositional modification and enhancement - Processing Activities: - Rheology and equipment design studies - Final Products: - Product design and end properties investigation **Rhealogy:** Rhealogy is the science of deformation and flow of matter. A very high-performance polymer pellet (raw material) is useless if it can't be transformed into practically useable products. **Polymers Rhealogy:** Polymers Rhealogy studies give initial information on how polymers behave during actual polymer processing. - Transformation means "deformation" and flow of polymer raw materials into a specified and required shape. - The Rhealogy of polymer powder OR pellet is important in the first section (melts). - However, the Rhealogy of polymers is much more complex because the fluid shows non-ideal behavior. **All these rheological properties depend upon the rate of shear, the molecular weight, structure of polymers, the concentration of additives and temperature.** **Dependence of Viscosity with Temperature:** - The viscosity of liquid will decrease when temperature will increase and the velocity of gas will increase when temperature will decrease. - **In liquid, viscosity α 1/T** - **In gas, viscosity α T** **(i) Dependence of viscosity with temperature in gases:** - In gases, the cause of viscosity is collision. - Temperature is nothing but the root mean square velocity of the molecules in the gas. - High temp. cause higher velocity, increasing the number of collisions, increasing velocity. - Velocity has a relation with the mean free path of the gases: - μ = 1/2 * λ * ρ * ν Where: - ρ = density of the gas - λ = mean free path - ν = root mean square velocity of molecules **(ii) Dependence of viscosity with temperature in liquid:** - In liquid, the cause of viscosity is cohesive. - Rise in temperature causes higher molecular vibration. - Higher molecular vibration causes cohesive bonds to break, reducing the cohesion. - Reduction in cohesive forces results in reduction of viscosity. - **Black colored bonds = cohesive bonds** - **Blue colored bonds = adhesive bonds** **Zero Shear Viscosity:** - Zero shear viscosity is the viscosity of a material when it is effectively at rest. - Zero shear viscosity describes a plateau value, which you can sometime see when you use a high-powered rheometer to measure viscosity under extremely low shear conditions. ### Unit-5 **Measurement of Viscosity:** Measurement of viscosity typically involves dynamic viscosity and kinematic viscosity **(i) Dynamic Viscosity:** Dynamic viscosity, often referred to as viscosity, is a measure of a fluid's internal resistance to flow under an applied force or stress. - It is denoted by μ - μ = τ / (dν/dy) - Where: - τ = shear stress - dν = change in velocity - dy = change in distance **(ii) kinematic Viscosity** Kinematic viscosity is the ratio of dynamic viscosity to the density of fluid. - It is denoted by ν - ν = μ / ρ - Where: - μ = dynamic viscosity - ρ = density of fluid **Normal Stress:** Stress is said to be Normal stress when the direction of the deforming force is perpendicular to the cross-sectional area of body. The length of the wire OR the volume of the body changes stress will be at normal. **Normal stress is classified into two types:** - **Longitudinal Stress:** - **Bulk stress OR Volumetric stress** **(i) Longitudinal Stress:** When two cross-sectional areas of the cylinder are subjected to equal and opposite forces, the stress experienced by the cylinder, in this experiment, is called Longitudinal Stress. - **Longitudinal stress = F / A** When the body is under longitudinal stress: - The deforming force will be acting along the length of the body. - Longitudinal stress results in the change in the length of the body hence, thereby it affects slight change in diameter. Further, it is classified into two parts. - **Tensile stress:** If the applied force results in the increase in the object's length, then the resulting stress is termed as tensile stress. - **Compressive stress:** If the applied force results in the decrease in the object's length, then the resulting stress is termed as compressive stress. **(ii) Bulk stress OR Volumetric stress:** When the applied force acts from all directions, resulting in the change of volume of the object, then such stress is called volumetric stress OR Bulk stress. - It is denoted by K. - K = - (∂P/∂V) - Where: - P = pressure - V = volume **Time Dependent Fluid Responses:** The time-dependent fluid response refers to how fluids, such as liquid or gases, change over time in response to an external force... It involves studying dynamic aspects like flow rates, pressures and velocities, considering time as a variable. Time dependent fluid is considered as non-Newtonian and divided into two parts: - **Thixotropic fluid:** - **Rheopectic fluid:** **Thixotropic & Rheotropic fluids:** - Thixotropic fluids are those fluids whose viscosity decrease with time. - Ex: printer ink - Rheopectic fluids are those fluids whose viscosity increase with time. - Ex: Gypsum paste **Viscometer:** - A viscometer is an instrument used to measure the viscosity of a fluid. - For liquids with viscosities which vary with flow conditions, an instrument called a rheometer is used. - Thus, a rheometer can be considered as a special type of viscometer. **Cone and Plate viscometer:** - It is a multipoint viscometer which is used to measure the viscosity of non-Newtonian fluid. - It consists of a cone which is attached to a stationary shaft of motor and a fixed lower plate in lower part. And dial gauges are attached to measure the τ Torque. **Test Procedure:** - Sample is placed at the center of plate. - After that the cone is driven at a variable speed, and the sample is sheared in the narrow gap. - Torque produced on the is read on the indicator scale. - η = (C T)/U - Where: - C = constant - T = Torque - U = RPM **Mooney Viscometer:** - The Mooney viscometer is a device used to measure the viscosity of rubber and other polymers in an unvulcanized state. - It involves the torque required to rotate a cylindrical sample at a constant temperature, providing insights into the material's processing characteristics. **Working Principle:** - Mooney viscometer consists in the measurement of the torque necessary to rotate a disc in a cylindrical chamber filled with the rubber compound to be vulcanized. - The rubber compound is introduced under pressure into the test chamber, made up of two halves. Inside the test chamber, a disc is rotated by means of a motor. - To avoid rubber slippage during the determination, both the chamber walls and the rotor surface are striated. **Test Procedure:** - The rubber compound, including the vulcanizing system, is shaped on has 6-8 mm thick sheets. - Round-shaped samples with 45 mm diameter are cut from the sheets. - The samples are placed in the middle of the motor shaft. - Before the beginning of the measurement, the instrument is heated up to 110°C. - After the sample is introduced, it takes a minute for the sample to reach the thermal equilibrium, and then the motor is started. - The value of Mooney viscosity (MU) decreases at the beginning, due to the decrease of the compound viscosity as temperature rises. - After about 4 mins a minimum value is reached, which stays constant for a while. This value is indicated as MU. - After a certain period of time, vulcanization starts, and the Mooney viscosity increases. **Rheometer:** - A Rheometer is a scientific or laboratory device used to measure the flow and deformation characteristics of materials, especially fluids and soft solids. - It is used for those fluids of which viscosity can't be defined by a single value of viscosity and therefore require more parameters to be set and measured than is the case for a viscometer. **Parallel- Plate Rheometer:** - A parallel-plate rheometer is a type of rheometer that consists of two parallel plates b/w which the material/specimen sample is placed. - The plates can be moved relative to each other to apply shear stress to the material, to measure the rheological properties, such as viscosity and shear rate. - This setup is particularly useful for studying the flow behavior of liquids, polymers and other materials. **When to Use Parallel Plates:** - Low/medium/High viscosity liquids - Soft Solids/Gels - Thermosetting materials - Samples with large particles - Samples with long relaxation time - Temperature Ramps/Sweeps - Materials that may slip - Small sample volume **Note:** - As diameter decreases, shear stress increases. - As gap height decreases, shear rate increases. **Oscillating Disc Rheometer:** - An oscillating disc rheometer is a rheological instrument used to analyze the viscoelastic properties of materials. - It typically involves a disc that oscillates back and forth, applying controlled shear stress to the sample. - This allows researchers to study how the material responds to varying stress and strain conditions, providing insights into its behavior under different circumstances. - The data obtained from an oscillating disc rheometer helps in understanding the material’s viscoelasticity, including factors like storage modulus, loss modulus and complex viscosity. **Consisting of an oscillating disc, enclosed in an unsealed, stationary cavity:** - Disc oscillates at a fixed frequency and amplitude (1° or 3°) and operates in the same range of temperatures and pressure as the Mooney viscometer. - Test specimen of vulcanizable rubber compound is inserted into the cure meter test cavity and after a closure action forms a sealed cavity under positive pressure. - Cavity maintained at elevated vulcanization temp - Rubber totally surrounds a biconical disk after the dies are closed. - Oscillation of disc exerts a shear strain on the test specimen. - Force required to oscillate or rotate the disk to maximum amplitude is continuously recorded as - a function of time, with the force being proportional to the shear modulus (stiffness) of the test specimen at the test temperature. **Test procedure:** - Bring the temperature of both dies to the temperature of test with the disk in place and the dies in the closed position. Set the running time and torque level. - Open the dies and placed the test specimen on top of the disk and close the dies. This operation must be completed within 90s. - Start the recorder at the instant the dies are closed. - The disc may be oscillating at zero time or oscillation may be started not later than 1 min after the dies are closed. **Rheological Properties:** Rheological properties describe how materials deform and flow under stress. Key parameters include viscosity, elasticity and yield stress. **Rheological measurements are vital in industries like food processing, cosmetics and material science to understand and optimize product behaviour. Rheometry helps to quantify these properties.** **Common types of Rheological properties and the measurement:** - **Viscosity:** Describes a material's resistance to flow. It can be either dynamic and kinematic and can be calculated by using Newton's law of viscosity. - **Elasticity:** Indicates a material's ability to return to its original shape after deformation, often measured through storage modulus. - In other words, elasticity refers to the ratio of the proportional rate of change in one variable and the proportional rate of change in another variable. - Elasticity (E) = Proportionate rate of change in X / Proportionate rate of change in y E = dX / dy - **Yield stress:** The minimum stress required to initiate flow in a material is known as yield stress. - The modulus crossover point, the stress at a phase angle 45°, and the onset point are the three methods to measure the yield stress. - **Creep:** It is the gradual deformation of a material under a constant load over time. It is measured by using Extensometers. - **Strain rates:** The rate at which the material is deformed is known as strain rate. It’s a purely geometrical quantity, a way to quantify the amount of deformation in a given material and has no units. - **Strain rate = change in length of fluid particle / original length of fluid particle** - **Shear rates:** The rate at which adjacent layers of material move relative to each other, influencing viscosity. It is measured by using a viscometer. - **Capillary Viscometer:** It is the most common and simplest device for measuring viscosity. - Its main component is a straight tube. - A capillary viscometer has a pressure-driven flow, for which the velocity gradient or strain rate and also the shear rate will be maximum at the wall and zero at the center of the flow, making it a non-homogeneous flow. - It is widely used because they are relatively inexpensive to build and simple to operate. Despite their simplicity, long capillary viscometers provide the most accurate viscosity data available. Another major advantage is that the capillary viscometer has no free surface in the test region, unlike the types of rheometers such as the cone and plate rheometer. - This is important for processes with higher rates of deformation such as mixing, extrusion and injection molding. - **Other advantages of the capillary viscometer:** - Capillary flows and geometries are very similar to those encountered in seal processing equipment. - A capillary viscometer can be adapted to on-line measurement. - The system allows the study of flow anomalies such as extrudate swell, melt fracture, or stick-slip conditions. - A capillary viscometer can be used to study the pressure dependence of viscosity. - **Working:** In a capillary viscometer, the material is fed into a cylinder where the temperature is maintained within a very narrow range (about T ± 0.5°C). Once the material is molten, the piston traveling at a well-controlled speed pushes the material though the capillary. The pressure is measured at the inlet of the circular capillary, and for rectangular capillaries, the pressure can be measured inside the capillary using the piston speed. The dimensions of the piston, and using the apparent shear rate can be calculated, and using the pressure and the capillary the apparent shear stress can be computed. **Types of Piston Used in Capillary:** - **Circular capillary:** This capillary works best for higher shear rates so it is useful to define the pseudoplastic behavior of polymer melts. - **Rectangular capillary:** This capillary is used for lower shear rates, so it is useful to measure the Newtonian plateau of a polymer melt. This capillary has the advantage that the pressure can be measured directly inside the cappillary. Therefore, inlet pressure corrections such as Bagley corrections are not required. This type of capillary is easier to clean and operate. - **Annular capillary:** This capillary allows the alteration of the length and the radius by changing only the core and doesn’t have entrance effects. The main disadvantages are that it is difficult to control the temperature of the core and that a considerably higher amount of polymer melt is required.