Plastics and Properties Important in Extrusion Chapter 4

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Plastics and Properties Important in
ExtrusionChapter 4
Professor Joe Greene CSU, CHICO
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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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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 281000 or 28,000 g/mole .

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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

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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

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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

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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

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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

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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

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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.

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Elastomers

• Elastomers are materials capable of large elastic
  deformations with elastic elongation gt 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

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Flow Behavior of Plastic Melts

• Viscosity
• Defined as the materials 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 (Englishlb/ft2-sec)
• Viscosity can be reduce by
• flowing faster (increasing shear rate)
• increasing temperature

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Melt Index

• 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
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Melt Index and Viscosity

• Melt index for common materials
• Material Temp Mass
• Polyethylene 190C 10 kg
• Nylon 235C 1 kg
• Polystyrene 200C 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

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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 3.1 Melt Index and Molecular Weight of PS
• Mn Melt Index (g/10min)
• 100,000 10.00
• 150,000 0.30
• 250,000 0.05
• T200C with mass 5 kg

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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
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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 ?

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Effect of Shearing

• 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

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Viscosity

• Viscosity is defined as a fluids 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
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Viscosity

• 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.

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Viscosity

• 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

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Viscosity

• 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

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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

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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

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Shear Thinning or Pseudoplastic Behavior

• 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

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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
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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

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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

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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.

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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

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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

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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

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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

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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

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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

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Thermal Stability and Induction Time

• 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.

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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.

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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

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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

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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

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PVT Data for Flow Analysis

• 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
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PVT Data for Flow Analysis

• 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
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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