Flow process in manufacturing polymer products can be represented as follows: Introduction
Introduction • Rheology = Science of deformation and flow of matter • A very high performance polymer granules or pellets (raw materials) is useless if it cannot be transformed into a practically useable products • Transformation means deformation and flow of polymer raw materials into a specified and required shapes • The rheology of polymer powder or pallet is importance in first section melts or liquids.
Introduction • In melt processing of thermoplastics polymers rheological studies give initial information on how these polymer behave during actual polymer processing. • e.g: effect of temperature, pressure & geometry on polymer flow behaviour in processes such as extrusion & injection moulding • Flow behaviour is important in injection molding, compression moulding, blow moulding, calendering cold forming and spinning of fibres • It is also importance in the formulation of polymeric materials in preparing for fabrication process especially extrusion and mill rolling
For many simple fluids the study of rheology involves the measurement of viscosity the viscosity depends primarily on temperature and hydrostatic pressure • However the rheology of polymers is much more complex because the fluid shows non ideal behaviour
All these rheological properties depend upon the rate of shear, the molecular weight, structure of polymers the concentration of additives and temperature. • In most cases, flow is involved in the processing and fabrication of the plastics.
The degree of orientations is determined by rheological behaviour of the polymer and nature of the flow in fabrication process • Molecular orientation hence influence the mechanical properties of moulded object films and fibres
Importance of rheology • Mechanical properties that shown by any polymer products is the most importance factors considered by manufactured and user. • In actual conditions the optimum mechanical properties is not importance if the product could not be process as faster, simple or easier and relatively low cost • Flow involved is rheological studies that also involved: • types and degree of orientation • Flow properties in actual processing
The importance of rheological studies are: • Can identify the behaviour of flow during flowing together with factors that influenced the flow of polymers. • Can predict the real complex processing condition through easier component and predict the final properties of polymer • Can relate the qualitative and quantitative parameters such as output and used of materials properties
Can choose the suitable polymer for specific processing conditions and services • To produce a product with optimum processing properties. importance in real processing to produce maximum output with minimum input • In some cases, factors as • Molecular structure, • morphology, • Polymer melt, • Blends and polymer modification Can be studies by relationship between the rheological properties and materials structure.
Flow • Flow is the continuous deformation under an influenced of constant force any particle of materials will not back to the original positions after the force of deformation been released • All the body in the nature will flow if given a period of time and appropriate temperature even with very low applied force
Flow • Ability to flow for a molten materials depends on the molecular chain mobility that hold molecule together. • Low mobility with high degree of chain entanglement will influenced the ability to flow and the process ability of polymeric materials
Plate of area A Force =F Velocity = V/U Direction of flow Fluids Stationary Plate Example of flow
Starting position of the fluid particles Force = F Velocity = V Direction of flow Velocity Profile Stationary plate
Viscosity • Consider 2 plates (A= area of the plate), • separated by distance, D • The space between them is occupied by • the liquid • One plate moves relatively to the other • with velocity U • The movement is resisted by the viscous • reaction in the fluid • Since the movement is in shear, the • Reaction is the shear viscosity F S A θ D Shear stress, ζ = Shear force/Area of the shear face = F/A Nm-2 Shear strain,γ = Amount of shear displacement, S/Distance between shearing surfaces (D) = Tan θ Viscosity, η = Shear stress/Rate of shear strain = ζ / (d γ/dt) = ζ / γ
Viscosity • The unit of viscositiy was poise, P, or centipoise, cP. 1 mPa·s = 1 cP. • ηrapidly decreases as temperature increases. • Ideal fluids are called Newtonian. The viscosity is independent of the rate of shear Shear rate is a measure of the rate of sheardeformation Rheogram for Newtonian liquids. A - high viscosity, B - low viscosity.
Newtonian Liquid • Newtonian liquid, where shear stress is proportional to shear rate, with the proportionality constant being the viscosity • A Newtonian fluid (named for Isaac Newton) is a fluid that flows like water • For example, water is Newtonian, because it continues to exemplify fluid properties no matter how fast it is stirred or mixed. • If the liquid is not Newtonian, a plot of shear vs. the rate of shear is not a straight line but a curve
Viscosity • - Most polymer melts & rubber compound • behave in pseudoplastic. • How can we relate the pseudoplastic • behavior to the morphology of the polymer • (long chain & coiled in complex structure)??? • Dilatant behavior can cause processing • difficulties Newtonian and non-Newtonian bahavior Variation of apparent viscosity with shear rate
Viscosity • Thixotropy • Thixotropy is the property of some non-newtonianpseudoplastic fluids to show a time-dependent change in viscosity . • Viscosity decreases as the material is stirred until some minimum value is reached. It increases again when the substance is no longer agitated. • Many gels and colloids are thixotropic materials, exhibiting a stable form at rest but becoming fluid when agitated Thixotropic substance at different shear rates.
Viscosity • When the relationship of shear stress t versus shear rate g is non-linear two types of viscosity at any value of shear rate can be obtained: • Apparent viscosity from slope taken from a line that connect the value of shear stress with shear rate at any point of shear rate from the origin • Constant viscosity from slope taken from a line at particular value of shear rate for materials that showed non newtonian behaviour
Viscosity • When the curve is nonlinear, the viscosity • May be defined in two ways; • Calculating apparent viscosity, ηa • Calculating consistency viscosity, ηc ηo – viscosity at a very low shear Rate, which behave like Newtonian behavior ηa – is the slope of the secant line from the origin to the shear stress at the given value of shear rate ηc ηc – the slope of the line at the chosen value of Rate of shear ηo ηa The ηa is greater than ηc
Non- newtonian flow • Most of the polymer systems not follow Newtonian law. • Non Newtonian flow can be classified into 3 parts as: • Non time dependence flow, • Time dependence flow • Viscoelastic flow
Behaviour of viscous material • Materials will demonstrate behaviour: • At low strain rate – behave according to the Newtonian relationship • Totally dependent with time. • Stress being function of strain rate • Stress independent of strain h= viscosity de/dt = strain rate
Non time dependence flow • Shear rate for non time dependence flow can represents mathematically the shear stress as: • In rheological studies there are 4 types of flow that not dependence with time • Bingham body flow, • Pseudoplastic flow, • Newtonian flow • Dilatant flow
Bingham Body Pseudoplastic fluid Shear Stress Newtonian fluid Dilatant fluid Shear Rate
Body Bingham flow • Body Bingham is elastic solid ideal materials that their structure will collapse when the stress applied greater than their yield stress ty, • Shear stress for body Bingham are proportional with shear rate given as: • where h plastic viscosity that reach a infinity when shear rate almost zero (g 0) and reach a value h when shear rate approach infinity value (g no limits). • Materials that represents model Bingham including emulsion and suspension with high concentration such as paint, printing ink, clay slurry and plastic emulsion.
Pseudoplastic flow • Viscosity of pseudoplastic flow decreased with the increased in shear rate it showed the shear thinning behaviour • During real processing that involved a higher range of shear rate no problems of flowing for pseudoplastic materials • At suppressed condition molecule has higher entanglement and will have random conformation or orientation • Under the applications of shear force uncoiled of molecule chain occur and the orientation of molecule increased even though the occurrence of Brownian movement will try to gives the original conformation (the condition where no force occurred) • At very high shear rate the almost Newtonian behaviour was observed for materials with pseudoplastic flows
Pseudoplastic • Pseudoplastic, or shear-thinning fluids have a lower apparent viscosity at higher shear rates. Pseudo-plastic substance with yield value Pseudo-plastic substance.
Viscosity Newtonian Shear Thinning Shear Rate Newtonian & Pseudoplastic Flow
Dilatant Flow • Viscosity value for Dilatant flow increased with increasing shear rate its enable the polymer to be process at high shear rate due to the ability to flow polymer is low. • Dilatant behaviour normally shown by polymer with high suspension such as PVC and materials with non uniform particles shape materials that difficult to be compressed under high shear rate. • Dilatant behaviour is hardly shown for molten polymer except under a special condition where the melt crystallization occurred during flow.
Dilatant • A dilatant material is one in which viscosity increases with the rate of shear (also termed shear thickening). • The dilatant effect can be seen more readily with a mixture of corn starch and water
Time dependence flow • Flow properties that dependence with time are dependence on: • Types of shear flow, • Flow history • Moulding time. • This types of flow showed a reversible conditions
Viscoelastic Flow • This flow are shown by materials that has the dominant viscous behaviour but has the elastic recovery after the deformation. • Viscoelastic flow has a properties in between the solid and liquid behaviour. ** Please refer the viscoelastic behaviour (viscoelasticity)
Viscoelastic behaviour • Polymer is called viscoelastic because: • Showing both behaviour elastic & viscous behaviour • Instantaneously elastic strain followed by viscous time dependent strain
Influenced of temperature on viscosity • Understanding the influenced of temperature with the melt viscosity is importance in: • Polymer processing • To estimate the thermal resistance of particular materials • Big variation in viscosity with range of temperature represent the materials need a higher activation energy • polymer molten viscosity that dependence on temperature have a higher temperature from glass transition temperature Tg or their melting temperature Tm.
The Andrade or Arrhenius equations can relate the activation energy during chain mobility as Where h= viscosity of polymer melt AEa = activation energy R = Universal gas constant T = Temperature (°K) A = Arrhenius constant
When taking the logarithm plot from log h against log (1/T) will given one straight line where the slope is the same activation energy according to this equations: • If viscosity at various temperature taken at constant shear stress activation energy is supposed to be constant and not dependence on shear stress where it been taken. • If the viscosity at constant temperature at various shear rate activation energy dependence on shear rate example activation energy decreased with increasing shear rate • However the flow according to Arrhenius equations activation energy almost not dependence on temperature.
Instruments for rheology measurements • A very popular types of instruments to measure viscosity is capillary rheometer or viscometer • It function in conditions of load and forced is constant or at constant volume rate • In conditions of constant shear stress measurement of flow rate was taken based on the speed of piston • Pressure at the outer layer of die is measured using the pressure transducer
Viscometers • are employed to measure viscosity. • Capillary viscometer • Rotational rheometer • Simple shear viscometer • Cone & plate rheometer • Parallel plate viscometer • Tensile & extensional viscometer Schematic diagram of a cone and plate viscometer. Schematic diagram of a rotational viscometer
Instruments for viscosity measurements Piston Polymer melt Barrel Constant shear rate Rheometer Pressure Transducer Atmosphere pressure Extrudate
Example of exams question • What are the importance of rheological studies in polymer processing. • Discuss the non-newtonian behaviour of polymeric materials. • What are the influenced of pseudoplastic flow towards polymer processing? • Most polymers melt exhibit pseudoplastic characteristics under shear conditions. How these differ from those of Newtonian fluids
Students Activity • Discuss with the person next to you what you understand on the importance of rheology in polymer processing