Transcript Plastics and Properties Important in Extrusion Chapter 4 Professor Joe Greene

Plastics and Properties Important
in Extrusion
Chapter 4
Professor Joe Greene
CSU, CHICO
1
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
2
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 .
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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
HDPE
PP
PET
PBT
Polyamides
PMO
PEEK
PPS
PTFE
LCP (Kevlar)
Crystalline
Crystalline
Crystalline
Crystalline
Crystalline
Crystalline
Crystalline
Crystalline
Crystalline
Crystalline
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.
Mechanical Properties
Density, g/cc
Tensile Strength,
psi
Tensile Modulus,
psi
Tensile
Elongation, %
Impact Strength
PEEK
1.30-1.32
LCP Polyester
1.35 - 1.40
Nylon 6,6
1.13-1.15
10,000 – 15,000
16,000 – 27,000
14,000
500K
1,400K - 2,800K
230K – 550K
30% - 150%
1.3%-4.5%
15%-80%
0.6 – 2.2
2.4 - 10
0.55 – 1.0
R120
R124
R120
40 - 47
25-30
80
320 F
356F -671F
180F
ft-lb/in
Hardness
CLTE
10-6 mm/mm/C
HDT
264 psi
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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
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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
Material
Air
Water
Olive Oil
Plastic melts
Pitch
Viscosity (Pascal-second)
0.00001
0.001
0.1
100 to 1,000,000
1,000,000,000
Viscosity (Poise)
0.0001
0.01
1
10 to 100,000
100,000,000
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Melt Index
Mass
• Melt index test
Plastic
– 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
Temp
Plastic
Resin
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Melt Index and Viscosity
• Melt index for common materials
Material
Temp
Mass
• Polyethylene
• Nylon
• Polystyrene
190°C
235°C
200°C
10 kg
1 kg
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
• 150,000
• 250,000
* T=200°C with mass =5 kg
10.00
0.30
0.05
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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
Cross Sectional A
Area A
A
F
F
– 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

Deformed Shape
F
 = µd /dt
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Some Greek Letters
• Alpha: 
• Nu: 
• beta: 
• xi: 
• gamma: 
• omicron: 
• delta: 
• pi: 
• epsilon: 
• rho: 
• zeta: 
• sigma: 
• eta: 
• tau: 
• theta: 
• upsilon: 
• iota:
• phi:

• kappa: 
• chi: 

• psi: 
• lamda:
• mu: 
• omega:
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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
Wall
Wall
Fluid
Wall
F
H
Wall
Before: Wall at Rest
Velocity, v
Fluid
shear rate = v/H
After: Top Wall Set in motion
induces shear stress
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Viscosity
• Viscosity is defined as a fluid’s resistance to flow under an
applied shear stress, Fig 2.2
Moving, u=V
y
Y= h
V
P
x
Stationary, u=0
Y= 0
• 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
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Viscosity
• For Newtonian fluids, Shear stress is proportional to velocity
gradient.
Ln 
du
 yx  
 
dy
0.01
0.1 1
10
100
Ln shear rate,

• The proportional constant, , is called viscosity of the fluid
and has dimensions
M
 
LT
• 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
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tube.
Viscosity
• For non-Newtonian fluids (plastics), Shear stress is
proportional to velocity gradient and the viscosity function.
du
 yx  
  Ln 
dy
0.01
0.1 1
10
100
Ln shear rate,

• 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
e
a b lnT
Ln
T=200

T=300
T=400
0.01
0.1
1
Ln shear rate,
10

100
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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, .
  m
n 1
• 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
ln   n 1ln   ln m
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Shear Thinning or Pseudoplastic Behavior
• Viscosity changes when the shear rate changes
Power law
approximation
Actual
– Higher shear rates = lower viscosity
Log
viscosity
– Results in shear thinning behavior
Log shear rate
– 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
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Temperature
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
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–
–
–
–
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
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spot in extruded product.
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
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heat up and cool down than PVC
Specific Heat and Enthalpy
• Specific Heat
 dQ 
 dQ 
CP  
 ; CV  

dT
dT

P

V
– 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.
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– Cp is more common due to high pressures under Cv
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
Material
ABS
Acetal
PA66
PC
Polyethylene
PP
PS
PVC
Steel (AISI
1020)
Steel (AISI
P20)
Specific Heat Capacity
(J/(kgK))
1250-1700
1500
1700
1300
2300
1900
1300
800-1200
460
460
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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
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• Measured with TGA (thermogravimetric analyzer), TMA
T+T Q
T
Thermal Conductivity
• Most important thermal property
 dQ 
 dT 


kA




 dt 
 dx 
– 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)
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– Semi-crystalline can have higher values
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
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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.
10.
Temperature (degrees C)
260 240 220 200
Induction 1
Time
(min)
.1 .0018
HDPE
EAA
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.0020 .0022
-1
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 37
increases more slowly.
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
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flexible at room temperature, e.g., HDPE
Thermodynamic Relationships
• Expansivity and Compressibility


f p, Vˆ , T  0
– 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
Vˆ  f  p, T 
– 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:
1  Vˆ 




V  T  p
• 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:
1  Vˆ 

   
ˆ
V  p 
T
• 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
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2
• isothermal compressibility has units m /N
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
Polypropylene
Pressure, MPa
1.40
0
three area;
20
– Low temperature
– Transition
– High temperature
Specific
Volume,
cm3/g
60
100
160
1.20
1.04
100
200
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
1.40
Specific
Volume,
cm3/g
Polystyrene
0
Pressure, MPa
20
60
100
160
1.20
1.04
100
200
Temperature, C
42
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
Vˆ 
a1
aT
 2  a5 e a6T a7 p
a 4  p a3  p
– High Temperature Region
Vˆ 
a1
a 2T

a 4  p a3  p
– Transition Region
p  b1  b2T
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