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Title: Plastics and Properties Important in Extrusion Chapter 4


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 281000 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 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

12
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

13
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
14
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

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 3.1 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

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
  • 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
  • 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
20
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.

21
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

22
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

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
  • 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
  • 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
  • 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
  • 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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