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Plastics and Properties Important in Extrusion Chapter 4

Plastics and Properties Important in Extrusion Chapter 4. Professor Joe Greene CSU, CHICO. Chapter 4 Objectives. Topics Main types of plastics Flow properties Thermal properties Help Select appropriate machines for extrusion Set proper processing conditions Analyze extrusion probelms.

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Plastics and Properties Important in Extrusion Chapter 4

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  1. Plastics and Properties Important in ExtrusionChapter 4 Professor Joe Greene CSU, CHICO

  2. Chapter 4 Objectives • Topics • Main types of plastics • Flow properties • Thermal properties • Help • Select appropriate machines for extrusion • Set proper processing conditions • Analyze extrusion probelms

  3. Polymer Chains • Average Molecular Weight • Polymers are made up of many molecular weights or a distribution of chain lengths. • The polymer is comprised of a bag of worms of the same repeating unit, ethylene (C2H4) with different lengths; some longer than others. • Example, • Polyethylene -(C2H4)-1000 has some chains (worms) with 1001 repeating ethylene units, some with 1010 ethylene units, some with 999 repeating units, and some with 990 repeating units. • The average number of repeating units or chain length is 1000 repeating ethylene units for a molecular weight of 28*1000 or 28,000 g/mole .

  4. Main Type of Plastics • Polymers are carbon-based materials made up of very long molecules • Polymers • Thermoplastic: Melt and flow upon heating • Can be reheated and flow again • When cooled behaves as a solid • Very suitable for recycling • Thermoset: React and cross-link (set-up) upon heating • Can be heated only once. • Material is not easily recycled

  5. Amorphous and Crystalline Plastics • Thermoplastics are further classified based upon molecular arrangement of polymer chains • Amorphous: (without shape) • Polymer chains are random arrangement • Crystalline • Polymer chains form regular pattern

  6. States of Thermoplastic Polymers • Amorphous- Molecular structure is incapable of forming regular order (crystallizing) with molecules or portions of molecules regularly stacked in crystal-like fashion. • A - morphous (with-out shape) • Molecular arrangement is randomly twisted, kinked, and coiled

  7. States of Thermoplastic Polymers • Crystalline- Molecular structure forms regular order (crystals) with molecules or portions of molecules regularly stacked in crystal-like fashion. • Very high crystallinity is rarely achieved in bulk polymers • Most crystalline polymers are semi-crystalline because regions are crystalline and regions are amorphous • Molecular arrangement is arranged in a ordered state

  8. Factors Affecting Crystallinity • Cooling Rate from mold temperatures • Barrel temperatures • Injection Pressures • Drawing rate and fiber spinning: Manufacturing of thermoplastic fibers causes Crystallinity • Application of tensile stress for crystallization of rubber

  9. Types of Polymers • Amorphous and Semi-Crystalline Materials • PVC Amorphous • PS Amorphous • Acrylics Amorphous • ABS Amorphous • Polycarbonate Amorphous • Phenoxy Amorphous • PPO Amorphous • SAN Amorphous • Polyacrylates Amorphous • LDPE Crystalline • HDPE Crystalline • PP Crystalline • PET Crystalline • PBT Crystalline • Polyamides Crystalline • PMO Crystalline • PEEK Crystalline • PPS Crystalline • PTFE Crystalline • LCP (Kevlar) Crystalline

  10. Liquid Crystalline Plastics (LCPs) • The molecules of LCPs are rod-like structures organized in large parallel domains, not only in the solid state but also in the melt state.

  11. Elastomers • Elastomers are materials capable of large elastic deformations with elastic elongation > 200% • Conventional: vulcanizable • polyisoprene, polybutadiene, polychloroprene, polyisobutylene • Thermoset elastomers: cross-linking reaction • polyurethane, silicone • Thermoplastic elastomers: physical linking • olefinic, TPO • urethane, TPU • etherester, TPE • copolyester, TPE • styrenic, TPR

  12. Flow Behavior of Plastic Melts • Viscosity • Defined as the material’s resistance to flow • Most important property of plastics for processing • Low viscosity materials flow easily: e.g. water, syrup, olive oil • High viscosity materials flow very slowly when heated: most plastics, e.g., LDPE, HDPE, PP, PS, PU, Nylon, PET, PBT, etc. • Units are Pascal-seconds (Metric= N/m2-sec), Poise (English=lb/ft2-sec) • Viscosity can be reduce by • flowing faster (increasing shear rate) • increasing temperature

  13. Melt Index Mass Temp • Melt index test • Measures the flow of a material at a temperature and under a load or weight. • Procedure (ASTM D 1238) • Set the temperature per the material type. • Add plastic pellets to chamber. Pack with rod. • Place mass (5Kg) on top of rod. • Wait for the flow to stabilize and flow at constant rate. • Start stop watch • Measure the flow in a 10 minute interval • Repeat as necessary Plastic Plastic Resin

  14. Melt Index and Viscosity • Melt index for common materials Material Temp Mass • Polyethylene 190°C 10 kg • Nylon 235°C 1 kg • Polystyrene 200°C 5 kg • Melt Index is indication of Viscosity • Viscosity is resistance to flow • Melt index flow properties • High melt index = high flow = low viscosity • Low melt index = low flow = high viscosity

  15. Melt Index and Molecular Weight • Melt Index is indication of length of polymer chains • Molecular Weight is a measurement of the length of polymer chains • Melt index MW properties • High melt index = high flow = short chains • Low melt index = low flow = long chains • Table 3.1Melt Index and Molecular Weight of PS Mn Melt Index* (g/10min) • 100,000 10.00 • 150,000 0.30 • 250,000 0.05 * T=200°C with mass =5 kg

  16. Stresses, Pressure, Velocity, and Basic Laws • Stresses: force per unit area • Normal Stress: Acts perpendicularly to the surface: F/A • Extension • Compression • Shear Stress,  : Acts tangentially to the surface: F/A • Very important when studying viscous fluids • For a given rate of deformation, measured by the time derivative d /dt of a small angle of deformation , the shear stress is directly proportional to the viscosity of the fluid F Cross Sectional Area A A F A F   = µd /dt Deformed Shape F

  17. Some Greek Letters • Nu:  • xi:  • omicron:  • pi:  • rho:  • sigma:  • tau:  • upsilon:  • phi: • chi:  • psi:  • omega: • Alpha: • beta: • gamma:  • delta: • epsilon: • zeta: • eta: • theta: • iota: • kappa: • lamda: • mu:

  18. Effect of Shearing Wall Wall Velocity, v F Fluid Fluid H shear rate = v/H Wall Wall Before: Wall at Rest After: Top Wall Set in motion induces shear stress • Shear flows are present in plastic processing • In shear flow (tangential flow), layers of the plastic move at different velocities. • Rate of shearing is called the shear rate • shear rate = velocity/thickness • Thin gaps = high shear rates • High flow rates = high shear rates

  19. Viscosity V Moving, u=V Y= h y Y= 0 x Stationary, u=0 • Viscosity is defined as a fluid’s resistance to flow under an applied shear stress, Fig 2.2 • The fluid is ideally confined in a small gap of thickness h between one plate that is stationary and another that is moving at a velocity, V • Velocity is u = (y/h)V • Shear stress is tangential Force per unit area,  = F/A P

  20. Viscosity Ln 0.01 0.1 1 10 100 Ln shear rate, • For Newtonian fluids, Shear stress is proportional to velocity gradient. • The proportional constant, , is called viscosity of the fluid and has dimensions • Viscosity has units of Pa-s or poise (lbm/ft hr) or cP • Viscosity of a fluid may be determined by observing the pressure drop of a fluid when it flows at a known rate in a tube.

  21. Viscosity Ln 0.01 0.1 1 10 100 Ln shear rate, • For non-Newtonian fluids (plastics), Shear stress is proportional to velocity gradient and the viscosity function. • Viscosity has units of Pa-s or poise (lbm/ft hr) or cP • Viscosity of a fluid may be determined by observing the pressure drop of a fluid when it flows at a known rate in a tube. Measured in • Cone-and-plate viscometer • Capillary viscometer • Brookfield viscometer

  22. Viscosity T=200 T=300 Ln T=400 0.01 0.1 1 10 100 Ln shear rate, • Kinematic viscosity, , is the ratio of viscosity and density • Viscosities of many liquids vary exponentially with temperature and are independent of pressure • where, T is absolute T, a and b • units are in centipoise, cP

  23. Viscosity Models • Models are needed to predict the viscosity over a range of shear rates. • Power Law Models (Moldflow First order) • Moldflow second order model • Moldflow matrix data • Ellis model

  24. Viscosity Models • Models are needed to predict the viscosity over a range of shear rates. • Power Law Models (Moldflow First order) where m and n are constants. If m =  , and n = 1, for a Newtonian fluid, you get the Newtonian viscosity, . • For polymer melts n is between 0 and 1 and is the slope of the viscosity shear rate curve. • To find constants, take logarithms of both sides, and find slope and intercept of line

  25. Shear Thinning or Pseudoplastic Behavior Power law approximation Actual Log viscosity Log shear rate • Viscosity changes when the shear rate changes • Higher shear rates = lower viscosity • Results in shear thinning behavior • Behavior results from polymers made up of long entangles chains. The degree of entanglement determines the viscosity • High shear rates reduce the number of entanglements and reduce the viscosity. • Power Law fluid: viscosity is a straight line in log-log scale. • Consistency index: viscosity at shear rate = 1.0 • Power law index, n: slope of log viscosity and log shear rate • Newtonian fluid (water) has constant viscosity • Consistency index = 1 • Power law index, n =0

  26. Effect of Temperature on Viscosity • When temperature increases = viscosity reduces • Temperature varies from one plastic to another • Amorphous plastics melt easier with temperature. • Temperature coefficient ranges from 5 to 20%, • Viscosity changes 5 to 20% for each degree C change in Temp • Barrel changes in Temperature has larger effects • Semicrystalline plastics melts slower due to molecular structure • Temperature coefficient ranges from 2 to 3% Viscosity Temperature

  27. Viscous Heat Generation • When a plastic is sheared, heat is generated. • Amount of viscous heat generation is determined by product of viscosity and shear rate squared. • Higher the viscosity = higher viscous heat generation • Higher the shear rate = higher viscous heat generation • Shear rate is a stronger source of heat generation • Care should be taken for most plastics not to heat the barrel too hot due to viscous heat generation

  28. Thermal Properties • Important is determining how a plastic behaves in an extruder. Allows for • selection of appropriate machine selection • setting correct process conditions • analysis of process problems • Important thermal properties • thermal conductivity • specific heat • thermal stability and induction time • Density • Melting point and glass transition

  29. Thermal Conductivity • Most important thermal property • Ability of material to conduct heat • Plastics have low thermal conductivity = insulators • Thermal conductivity determines how fast a plastic can be processed. • Non-uniform plastic temperatures are likely to occur. • Long times are needed to equalize temperatures • Channel is 20 mm in diameter, it may take 5 to 10 minutes for temperatures to equalize • Typical residence is 30 seconds. • Results in high temperature melt stream persists all through the die and causes non-uniform flow at the die exit and a local thick spot in extruded product.

  30. Specific Heat and Enthalpy • Specific Heat • The amount of heat necessary to increase the temperature of a material by one degree. • Most cases, the specific heat of semi-crystalline plastics are higher than amorphous plastics. • The amount of heat necessary to raise the temperature of a material from a base temperature to a higher temperature is determined by the enthalpy differences between two temperatures. • If you know the starting temperature (room T) and the ending temperature (die exit) then we can determine the energy required to heat plastic material. • Enthalpy to heat of PVC from Room T to 175C is 150 kW.hr/kg or for 100 kg/hr (220lbs/hr) the minimum power is 5 kW (6.7 HP) • LDPE is much higher enthalpy than PVC, or it takes more energy to heat up and cool down than PVC

  31. Specific Heat and Enthalpy • Specific Heat • The amount of heat necessary to increase the temperature of a material by one degree. • Most cases, the specific heat of semi-crystalline plastics are higher than amorphous plastics. • If an amount of heat is added Q, to bring about an increase in temperature, T. • Determines the amount of heat required to melt a material and thus the amount that has to be removed during injection molding. • The specific heat capacity is the heat capacity per unit mass of material. • Measured under constant pressure, Cp, or constant volume, Cv. • Cp is more common due to high pressures under Cv

  32. Specific Heat and Enthalpy • Specific Heat Capacity • Heat capacity per unit mass of material • Cp is more common than Cv due to excessive pressures for Cv • Specific Heat of plastics is higher than that of metals • Table

  33. Thermal Stability and Induction Time • Plastics degrade in plastic processing. • Variables are: • temperature • length of time plastic is exposed to heat (residence time) • Plastics degrade when exposed to high temperatures • high temperature = more degradation • degradation results in loss of mechanical and optical properties • oxygen presence can cause further degradation • Induction time is a measure of thermal stability. • Time at elevated temperature that a plastic can survive without measurable degradation. • Longer induction time = better thermal stability • Measured with TGA (thermogravimetric analyzer), TMA

  34. Thermal Conductivity Q T+T T • Most important thermal property • Ability of material to conduct heat • Plastics have low thermal conductivity = insulators • Thermal conductivity determines how fast a plastic can be processed. • Non-uniform plastic temperatures are likely to occur. • Where, k is the thermal conductivity of a material at temperature T. • K is a function of temperature, degree of crystallinity, and level of orientation • Amorphous materials have k values from 0.13 to 0.26 J/(msK) • Semi-crystalline can have higher values

  35. Thermal Stability and Induction Time • Plastics degrade in plastic processing. • Variables are: • temperature • length of time plastic is exposed to heat (residence time) • Plastics degrade when exposed to high temperatures • high temperature = more degradation • degradation results in loss of mechanical and optical properties • oxygen presence can cause further degradation • Induction time is a measure of thermal stability. • Time at elevated temperature that a plastic can survive without measurable degradation. • Longer induction time = better thermal stabilty • Measured with TGA (thermogravimetric analyzer), TMA

  36. Thermal Stability and Induction Time Temperature (degrees C) 10. 260 240 220 200 HDPE 1 Induction Time (min) EAA .1 .0018 .0020 .0022 Reciprocal Temp (K-1) • Plastics degrade in plastic processing. • Induction time measured at several temperatures, it can be plotted against temperature. Fig 4.13 • The induction time decreases exponentially with temperature • The induction time for HDPE is much longer than EAA • Thermal stability can be improved by adding stabilizers • All plastics, especially PVC which could be otherwise made.

  37. Density • Density is mass divided by the volume (g/cc or lb/ft3) • Density of most plastics are from 0.9 g/cc to 1.4 g/cc_ • Table 4.2 • Specific volume is volume per unit mass or (density)-1 • Density or specific volume is affected by temperature and pressure. • The mobility of the plastic molecules increases with higher temperatures (Fig 4.14) for HDPE. PVT diagram very important!! • Specific volume increases with increasing temperature • Specific volume decrease with increasing pressure. • Specific volume increases rapidly as plastic approaches the melt T. • At melting point the slope changes abruptly and the volume increases more slowly.

  38. Melting Point • Melting point is the temperature at which the crystallites melt. • Amorphous plastics do not have crystallites and thus do not have a melting point. • Semi-crystalline plastics have a melting point and are processed 50 C above their melting points. Table 4.3 • Glass Transition Point • Point between the glassy state (hard) of plastics and the rubbery state (soft and ductile). • When the Tg is above room temperature the plastic is hard and brittle at room temperature, e.g., PS • When the Tg is below room temperature, the plastic is soft and flexible at room temperature, e.g., HDPE

  39. Thermodynamic Relationships • Expansivity and Compressibility • Equation of state relates the three important process variables, PVT • Pressure, Temperature, and Specific Volume. • A Change in one variable affects the other two • Given any two variables, the third can be determined • where g is some function determined experimentally. • Reference: MFGT242 Polymer Flow Analysis Book

  40. Thermodynamic Relationships • Coefficient of volume expansion of material, , is defined as: • where the partial differential expression is the instantaneous change in volume with a change in Temperature at constant pressure • Expansivity of the material with units K-1 • Isothermal Compressibility, , is defined as: • where the partial differential expression is the instantaneous change in volume with a change in pressure at constant temperature • negative sign indicated that the volume decreases with increasing pressure • isothermal compressibility has units m2/N

  41. PVT Data for Flow Analysis Polypropylene Pressure, MPa 1.40 0 20 60 100 Specific Volume, cm3/g 160 1.20 1.04 100 200 • PVT data is essential for • packing phase and the filling phase. • Warpage and shrinkage calculations • Data is obtained experimentally and curve fit to get regression parameters • For semi-crystalline materials the data falls into three area; • Low temperature • Transition • High temperature Temperature, C

  42. PVT Data for Flow Analysis Polystyrene Pressure, MPa 1.40 0 20 60 100 Specific Volume, cm3/g 160 1.20 1.04 100 200 • Data is obtained experimentally and curve fit to get regression parameters • For amorphous there is not a sudden transition region from melt to solid. There are three general regions • Low temperature • Transition • High temperature Temperature, C

  43. PVT Data for Flow Analysis • The equations fitted to experimental data in previous PVT Figures 2.11 and 2.12 are: • Note: All coefficients are found with regression analysis • Low Temperature region • High Temperature Region • Transition Region

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