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Rheology and Plasticity

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Rheology and Plasticity Rheology and Plasticity The National Brick Research Center January 2017 Webinar What is Rheology or Plasticity? • Rheology is the study of deformation and flow. – In other words, rheology is the study of how the material moves through the extruder in reaction to the force that is applied. • Plasticity is the property of a material that allows it to be deformed (shaped) when a sufficient force is applied and to retain the new shape when the force is removed. – Plasticity is the property of the material that allows us to shape material into the desired form. – Plasticity also allows the material to maintain the new shape through, texturing/decorating, cutting, setting and drying. – A high plasticity material requires more force to deform and deforms to a greater degree without cracking than a low plasticity material. What is Shear? • Shear is the movement (sliding) of one plane or layer of material relative to an adjacent plane or layer due to an applied force. • Shear Stress (τ) is the force applied over a unit area. • Shear Rate ( ) is the change in velocity of one layer relative to another due tȯ an applied force. • Viscosity (η) is the resistance to flow. = ̇ Shear and Extrusion • In extrusion we apply a force that results in flow. • For piston extrusion, the ram movement results in shear and the force required to maintain the shear rate and the resulting flow are characteristics of the material. • For auger extrusion the shear rate is determined by the auger speed. The power required to maintain the auger speed and flow rate is analogous to shear stress. Typical Flow Behavior - I • There are several “characteristic” types of flow. • For Newtonian fluids, there is a linear relationship between shear stress and shear rate. • For dilatant or shear thickening material, the shear stress increases non-linearly as the shear rate is increased. – This type of behavior is potentially dangerous in extrusion. • For shear thinning materials, the shear stress decreases as the shear rate increases. – This type of behavior is more common in materials with a high clay content and is thought to be the result of alignment of the platy clay particles. Typical Flow Behavior - II • For our materials, a minimum stress is required to initiate flow. – This minimum stress is known as the yield stress. – Yield stress is related to what we call “green strength” – This type of flow is sometimes called “Bingham” plastic. • There is also shear thinning and shear thickening behavior with yield stress. Plastic Limit and Liquid Limit • For clay based materials, a minimum amount of water is required for the material to have cohesion and flow. • When we add water to a dry raw material (with sufficient clay content), the material goes through several stages. • In the first stage the material agglomerates. – The balls get larger until we reach something known as the “plastic limit.” – Once we achieve the plastic limit, the material has a certain working range until we exceed the liquid limit of the material-the material becomes too soft for stiff extrusion. • The plastic limit, working range and liquid limit are a unique to each raw material. • A material that has a wide working range of moisture content is called “fat” while a material with narrow range is called “short”. • The bottom line is that plasticity is a function of moisture content and that this optimum moisture content is a characteristic of the material. Where does plasticity come from? • Resistance to shear depends on the viscosity of liquid and the interparticle forces. • Clay particles tend to have a lot of surface charge. – Some clay minerals like montmorillonite have a higher surface charge than other clay minerals like kaolin. – The amount of clay mineral (think sand, silt, clay from particle size analysis) is also import Water molecule attached to surface for plasticity. • Water molecules are also polar and interact with the clay’s surface charge • This interaction between the clay surface Clay surface and surrounding water results in cohesion between the two that we call plasticity. • Plasticity is typically higher for materials with higher clay mineral contents. Variables related to plasticity • Type of Clay Mineral • Amount of Clay Mineral in the mix • Water Content • Organic Mater • Soluble Salts • Additives • Mixing/Weathering/Ageing Extrusion Mechanics • There are two typical flow patterns in the extruder. – Due to wall slip, plug flow occurs where material moves at a uniform rate. – This is more characteristic in the shaper cap. • Laminar or differential flow occurs where there is excessive drag on the walls or around a restriction. Flow in an extruder • Clay materials exhibit a phenomena know as wall slip which makes the actual measurement of viscosity difficult. • With wall slip a low viscosity film forms due to dewatering and or alignment of the clay particles near the extruder wall. • Wall slip allows the material to flow at pressures much lower than would be expected. How do you measure plasticity and rheology 3-Point Bend Test of Lab Extruded Bars 50 Atterberg Plasticity and Liquid Limit Maximum Load @ Failure 40 Yield Point Plastic Deformation 30 20 Green Strength (psi) Strength Green Elastic Deformation 10 0 0.00 0.05 0.10 0.15 0.20 Displacement (in.) Pfefferkorn Plasticity Test Capillar Check Rheometer Capabilities • Measure the flow behavior of a raw material while simulating the shear rates that the material would experience in the plant. • Look for problem rheologies like dilatency. • Find the optimum moisture content for a particular raw material. • Compare replacement materials or materials from different stockpiles. • Study the effect of additives on flow behavior. Capillar Check Rheometer Simulating High Shear Rates • To simulate plant shear rates, a combination of ram speed and die opening are used as shown in the table below. • We currently have three die (3 mm, 4.9 mm, and 6 mm). Extrusion Speed (ft/min) Ram Speed 3 mm die 4.9 mm die 6 mm die (mm/min) 50 46 17 11 100 91 34 23 200 182 68 46 250 228 85 57 300 273 102 68 350 319 120 80 Simulating shear rate • For a given ram speed, we can increase the extrudate speed by restricting the die opening. – We are essentially forcing a given volume of material to flow at a faster rate as we reduce the die opening at a given ram speed. Extrusion Speed (ft/min) Ram Speed 3 mm die 4.9 mm die 6 mm die (mm/min) 300 273 102 68 Typical Rheometer Data 7000 0.8 6000 • The radial (black) and 0.6 axial pressure (red) are 5000 recorded at regular 4000 intervals during an 3000 0.4 extrusion run. Anisotropy Pressure (kPa) Pressure 2000 The anisotropy (green) is 1000 0.2 the ratio of the radial to 0 axial pressure. 0.0 The axial pressure typically 0 100 200 300 400 500 600 decreases as a function of Time (Seconds) ram speed (less material Radial Pressure to push) Axial Pressure Anisotropy Pressure Measurement Radial pressure measurement Axial pressure measurement from torque on drive for ram movement Clay Shale Mix • The slope of these curves give us information about the behaviors 5000 mentioned previously. • A downward slope, after an 4000 initial peak indicates shear 3000 thinning behavior. • An upward slope would indicate 2000 dilatency or dewatering and would Radial Pressure (kPa) Pressure Radial be a red flag for extrusion 1000 • For this material the shear thinning behavior is not very pronounced. 0 0 500 1000 1500 2000 2500 • The noise is caused by larger Time (seconds) particles flowing through the die 17 ft/min 34 ft/min and would be minimized with a 68 ft/min 102 ft/min larger die. Shale • This material is reportedly more difficult to extrude than the previous material. • The drop seen after initial flow is characteristic of shear thickening or gel formation along the wall where the particles align and pack until the agglomerate gets large enough that it is dislodged from the wall. • This material also required less pressure to extrude which indicates lower plasticity than the previous material. • In this comparison, we have not compared moisture contents yet, but would need to test the flow behavior for several moisture content for this to be a full comparison. Radial Pressure Comparison • This is a comparison of the peak radial pressures for each material as a function of ram/extrudate speed. • Both materials show a slight increase in pressure as a function of extrudate speed. Axial Pressure Comparison • The peak axial pressures 8000 7000 show a different 6000 relationship with an 5000 4000 initial dip followed by an (kPa) Pressure Axial upward slope. 3000 2000 0 20 40 60 80 100 120 Extrudate Speed (ft/min) Clay Shale Mix Shale What is Anisotropy 1.0 • The anisotropy is the 0.8 ratio of the radial to 0.6 axial pressure. Radial Pressure (kPa) Pressure Radial 0.4 • Anisotropy gives us an indication of how much 0.2 0 20 40 60 80 100 120 Extrudate Speed (ft/min) wall slip is taking place Clay Shale Mix Shale in the material. Further Work • Further extrusions with these materials to look at the effect of moisture content. • Look at the particle size distribution and soluble salt content of the materials and look for correlation to observed flow behavior. • Study more materials • Work with additives like deflocculants, lubricants and binders. Further Reading Important Dates May 9-11, 2017 NBRC Spring Meeting March 20-23, 2017 NBRC Spring Short Course .
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