Materials
Plastics
Introduction
What are Plastics
• Polymer
– “Poly” – many
– “mer” – unit
– Many Units
• Carbon based, high molecular weight,
versatile synthetic materials that are built up
from monomeric units
How plastics are made
• Addition or Condensation Reaction
• Addition
– A simple combining of molecules without
generating byproducts
– Vinyls
• PE
• PP
• PS
Addition Reaction - Polyethylene
How plastics are made
• Condensation
– Involves removing certain atoms from each
molecule, allowing the molecules to combine
– Byproducts are generated that must be removed
– Nylons
– PC
Condensation Reaction - Polycarbonate
Types of Plastics
• Thermoplastic
– Soften with heated, then solidify when cooled
– Only physical changes
• Thermoset
– Polymers that chemically react when heated to
form a cross-linked polymer chain network
– Not reformable with heating
Thermoplastics
• Amorphous
– Random Structure
– Tg
– Polystyrene, Polycarbonate
• Semi-Crystalline
– Organized Molecular Arrangement
– Tg, Tm
– Polyethylene, Polypropylene
Crystallinity
Semi-crystalline
Amorphous
Thermoplastics
• The ability of plastics to form crystals is
largely dependent on the structure of the
plastic molecule
– Linear plastics with small side groups can form
crystalline regions
– HDPE, LDPE, Acetals, Nylon and PET
Structure Property Relationship
• The Property of a Plastic Material
formulation can be tailored to meet most
end use applications
• The properties are dependent on
– The chemical composition of the polymer
– Additives
Structure Property Relationship
• Chemical Composition varies by
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Structure of the repeat unit
Average molecular weight
Molecular weight distribution
Linear, branched or cross-linked
Structure Property Relationship
• PMMA and PS are
very different in
behavior and
properties because
their repeat units are
different
Molecular Distribution
Structure Property Relationship
• Number-Average Molecular Weight (Mn)
– Mn = NiMi / Ni
• where Ni is the number of molecules of the ith
species of molecular weight Mi.
– Measured from colligative properties such as:
• freezing point depression for low molecular weight
• osmotic pressure for higher molecular weight
• gel permeation or size exclusion chromatography
Structure Property Relationship
• Weight-Average Molecular Weight (MW)
– MW= NiMi2/ NiMi
• where Ni is the number of molecules of the ith
species of molecular weight Mi.
– Measured using techniques such as:
• light scattering
• gel permeation or size exclusion chromatography.
Structure Property Relationship
• Polydispersity(MWD) = MW / Mn
– A measure of the distribution of molecular
weights of polymer chains.
Effect of Mw on Viscosity
Log 
Low
Shear
Medium
Shear
High
Shear
Log 
Log shear rate
• Low shear – lots of
entanglements, Mw has
direct effect on viscosity
• Medium shear – reduced
entanglements Mw has less
effect on viscosity
• High shear – few
entanglements, Mw has no
effect on viscosity
Effects of MWD on Viscosity
Viscosity
Narrow MWD
Broad MWD
Shear Rate
Structure Property Relationship
• Additives
– Used to enhance specific properties
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Combustion modifiers
Release agents
Blowing Agents
UV stabilizers
Fillers
Reinforcements
Colors
– Additives are like medications, they have side effects
Plastics Behavior and Properties
Plastics Behavior and Properties
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Mechanical Behavior
Flow Behavior
Short Term Mechanical Properties
Long Term Mechanical Properties
Thermal Properties
Electrical Properties
Environmental Properties
Other Properties
Mechanical Behavior
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Viscoelasticity
Creep
Stress Relaxation
Recovery
Loading Rate
Viscoelasticity
• Elastic
– The material returns to original shape after the load has
been removed
– Linear stress strain response
• Viscous
– The material will deform or flow under load
– Nonlinear stress-strain response
• Plastics show both responses
– Short term load
• elastic
– Long term load
• viscous
Creep
• One of the most important results of
plastics’ viscoelastic behavior
• Deformation over time when a material is
subjected to a constant stress
• The polymer chains slip past one another
• Some of the slippage is permanent
Creep
Stress Relaxation
• Gradual decrease in stress at constant strain
• Same polymer chain slippage as in creep
Recovery
• The degree to which a plastic returns to its
original shape after a load is removed
Temperature and Loading Rate Effects
• Loading Rate
– The rate at which the part is stressed or strained
• Thermoplastics become stiffer and fail at
smaller strain levels as the strain rate
increases
• At higher temperatures plastics lose their
stiffness and become more ductile
Temperature and Loading Rate Effects
Flow
Types of Flow
• Drag Flow
– Caused by the relative motion of one boundary
with respect to the other boundary that contains
the fluid
– Two major boundaries in injection unit are the
barrel and screw surfaces
– Since the screw is rotating in a stationary
barrel, one boundary is moving relative to the
other boundary
– This causes drag flow to occur
Types of Flow
• Pressure Flow
– Caused by the presence of pressure gradients
– Pressure flow is what occurs downstream of the
injection unit
• Sprue, runner, gate and cavity
– Flow occurs because the pressure is higher at
the injection unit discharge than in the mold
Types of Flow
• For the overall system
– The injection unit uses drag flow to move the
material and build pressure
– This pressure buildup at the discharge of the
injection unit results in pressure flow through
the mold
Shear Flow Induced by Drag Flow
• Different layers of plastics move at different
velocities with the maximum velocity being
at the moving boundary and zero velocity at
the wall
Shear Flow Induced by Pressure Flow
• Different layers of plastics move at different
velocities with the maximum velocity being
at the centerline of flow and zero velocity at
the walls
pressure
velocit
y
diameter
Shear Rate
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Difference in velocity per normal distance
The change in shear strain with time
Units of seconds-1
Drag Flow

V
 
H
• Pressure Flow


V

D/2
Shear Stress
• The stress required to achieve a shearing
type flow
• Force divided by the area over which it acts
• Units of Pascal or psi
• Drag Flow
F

A
• Pressure Flow
  pressure
Shear Viscosity
• Internal resistance to shear flow
• Ratio of shear stress to shear rate
• Units of poise or Pa-sec




Shear Heat
• Viscous heat generation
• Heat generated due to shear flow
• Conversion of mechanical energy to heat
through friction
• Amount is equal to the product of the
viscosity and the shear rate squared

Q   * 2
Effect of Temperature on Viscosity
Viscosity
Temperature
Types of Fluids
• Newtonian
– A fluid whose viscosity is independent of shear rate
• Shear thinning(pseudo-plastic)
– A fluid whose viscosity decreases with increasing shear
rate
• Shear thickening(dilatants)
– A fluid whose viscosity increases with increasing shear
rate
Flow Behavior
Power law Fluids
• Polymer melts are shear thinning fluids
• The fact that the viscosity reduces with
shear rate is of great importance in the
injection molding process
• Important to know the extent of the change
of viscosity with shear rate
– m is the consistency index
– n is the power law index

  m * ( )
n 1
Mechanical Properties
Mechanical Properties
• Important in all applications
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Stiffness
Hardness
Toughness
Impact Strength
The ability to support loads
Mechanical Properties
• Mechanical property data is used to
– Select materials
– Estimate part performance
– Predict deformation and stresses from applied
loads
Mechanical Properties
• Most data have been derived from laboratory tests
and may not directly apply to your application
• Data should be used for comparison purposes only
because
– Difference between testing and end use conditions
– Material and processing variability
– Unforeseen environmental or loading conditions
Types of Forces
• There are four fundamental forces we deal with in
the testing of mechanical properties of plastics
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Tensile
Compressive
Shear
Torsion
• These forces are tested alone and in combinations
Tension and Compression Forces
• Tension
– Pulling force
• Compression
– Pushing force
Shear and Torsion Force
• Shear
– Opposing forces at the
same point
• Torsional Force
– Turning force
Stress and Strain
• Stress is the force per area that is applied to
the specimen
F
 
A
– Units of psi or Pa
• Strain is the change is dimension divided by
the original dimension
– No units
L

L
Stress-Strain
Terms and Definitions
• Proportional Limit
– The end of the region where
the plastic shows linear
stress-strain behavior
• Elastic Limit
– The point after which the
plastic will permanently
deform
– Applications that cannot
tolerate permanent
deformations must stay
under the elastic limit
Terms and Definitions
• Yield Point
– Marks the beginning of the
region in which the ductile
plastic continues to deform
without a corresponding
increase in stress
– Elongation at yield gives
the upper limit for
application that can tolerate
a small deformation
Terms and Definitions
• Break Point
– Shows the strain value
at which the test bar
breaks
• Ultimate Strength
– Measures the highest
stress value
– Used for general
strength comparisons
Terms and Definitions
• Elastic Modulus
– The slope of the linear
region of the stressstrain curve
– Ratio of stress-strain
response
– Used to compare
materials and make
structural calculations
– Units of psi or Pa
Short Term Mechanical Test
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Tensile
Flexural
Compressive
Impact
Hardness
Coefficient of Friction
Tensile Tester
• Measures a plastics
stiffness
• After the test bar is
clamped in the jaw,
the jaws then move at
a constant rate of
separation
• The force required for
movement is recorded
Tensile Test Data
• Tensile Modulus measure a plastics
stiffness
– Used for comparisons and structural
calculations
– The higher the modulus the greater the stiffness
• Tensile stress at yield establishes an upper
limit for applications that can tolerate a
small permanent deformation
Tensile Test Data
• Elongation at yield is the strain value at the
yield point
– Determines the upper limit for application that
can tolerate small permanent deformations
• Tensile Stress at Break is the stress applied
at the time of fracture
– Establishes an upper limit for
• One time use applications that fail due to fracture
• Parts that can still function with large deformations
Tensile Test Data
• Elongation at Break measures the strain at fracture
as a percentage of elongation
– Useful for applications that fail by fracture
• Ultimate Strength measures the highest stress
value during the tensile test
– Useful for comparing general strengths between
plastics
• Ultimate Elongation is the elongation at the
breaking point
Stress Strain Curves
Stress Strain Curves
Poisson’s Ratio
• Parts subjected to
tensile or compressive
stress deform in two
directions
• Poisson’s Ratio
measures the lateral to
longitudinal strains
Poisson’s Ratio
• Usually between 0.35
to 0.42 for plastics
• Required for many
structural analysis
calculations
Flexural Tester
Flexural Test Data
• Flexural Modulus is the ratio of stress to
strain in the elastic region of the stress
strain curve
– Measures the plastics stiffness in bending
– Compressive and tensile forces are both
measured as a result of bending
– Used in bending structural calculations
– Test values for tensile modulus correspond well
with flexural modulus for solid plastics
Flexural Test Data
• Ultimate Flexural Stress is the highest value
of stress on the stress-strain curve
– Measures the level after which severe
deformation or failure will occur
Flexural Properties
Compressive Tester
• Measures a materials
hardness
• The test specimen is
compressed at a
constant strain rate
between two parallel
platens until it
ruptures or deforms by
a certain percentage
Compressive Test Data
• Shows a materials hardness and load
capabilities
• Compressive Strength measures the
maximum compressive stress recorded
during the test
– Useful in structural calculations for load
bearing applications
Compressive Properties
Shear Strength
• Measures the shearing force required to
make holes or tears in the plastic
• Useful in structural calculations for parts
that may fail in shear
• Data does not account for stress
concentrations or mold-in stresses
Tear Strength
• The force required to rip the plastic divided
by the thickness
• Provides relative data for comparing
materials
Impact Tester
Impact Test
• Impact Strength measures a plastics ability
to absorb and dissipate energy
• Hard to relate at actual part performance
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Part geometry
Temperature
Stress concentrations
Molding stresses
Impact speed
Impact Tests
• Izod is most widely used
– Uses horizontally notched sample to
concentrate impact
• Charpy uses a vertically notched sample
• Use for comparing materials relative impact
strength
Tensile and Impact
• Impact Strength and Tensile Modulus
provide insight into a plastics mechanical
nature
– High impact strength and large tensile modulus
suggest a tough material
– High impact strength and small tensile modulus
indicates a ductile, flexible material
– Low Impact strength and a large tensile
modulus typify a brittle material
Hardness Tester
• A load is applied to an
indentor, which
presses against the
plastic
Hardness Data
Abrasion Resistance
• Abrasion Resistance is measured by
applying a Taber Abrader with 250gr
weight and a CS 10-F textured abrader to a
test specimen for a set number of cycles
– Then measuring the changes in volume and
transparency
Abrasion Resistance Data
Coefficients of Friction
• Ratio of the friction
force, the force needed
to initiate sliding, to
the normal force, the
force perpendicular to
the contact surface
Coefficients of Friction (Static) Ranges
for Various Materials
Long Term Mechanical
Properties
• Creep
• Stress Relaxation
• Fatigue
Creep
• Short Term testing gives us data for periodic
loading
• It is not unusual for plastic parts to be subjected to
continuous loading or loads that last a long time
• The viscous nature of plastics make these long
term loading to be of interest even if small
• Creep is the deformation or strain due to viscous
or cold flow
• To design parts that are subjected to long term
loading, the designer must utilize creep data
Examples of Creep
Creep
• The time and temperature dependent creep
modulus of a polymer is
Ec ( t , T ) 
0
 (t , T )
• Manufacturers generate creep data by subjecting
molded test specimen to varying stress level and
measuring the change in dimension over time
Creep Data
Creep Sample Problem
How much would the
material be strained
after 1000 hours at a
constant stress of 2800
psi?

E

2.2 x10 psi 
5
  0.013
2800psi

Stress Relaxation
• Stress relaxation data is used for
applications where strain levels remain
constant over a long period of time
• When plastics are stretched, compressed,
bent or sheared to a fixed value of strain,
the stress value decrease with time due to
the viscous effects(molecular relaxation)
Stress Relaxation Examples
Stress Relaxation
• The time and temperature dependent relaxation
modulus of a polymer is
 (t , T )
E r (t , T ) 
0
• Stress relaxation data is generated by applying a
fixed strain to molded samples and measuring the
gradual decrease in stress with time
Stress Relaxation Data
Stress Relaxation Sample Problem
• What is the stress of
the polycarbonate after
104 hours at a 2%
constant strain?
  4000psi
Fatigue
• Fatigue properties are used when designing
parts that are subjected to repeated or cyclic
loadings
• Tests are ran in bending, torsion and
tension
Fatigue Curves
Fatigue Example Problem
• What is the amount of
stress that will lead to
failure after 1 million
cycles for
– Tensile = 34N/m2
– Bending = 38N/m2
Thermal Properties
Thermal Properties
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Glass Transition Temperature
Melting Temperature
Coefficient of Thermal Expansion
Deflection Under Load
Thermal Conductivity
Specific Heat
Vicat Softening Temperature
Glass Transition and Melting
Temperature
• Specific volume vs
temperature provides
o Tm = melting
temperature
• Tg = glass transition
temperature
Melting Temperature
• While cooling the melt,
the specific volume of the
melt sharply drops at a
temperature which is
termed as Tm.
• This is due to the
crystalline regions
forming
• Only for semi-crystalline
plastics
Glass Transition Temperature
• While cooling noncrystalline polymer melt
there is no sharp drop in
specific volume and the
melt becomes highly
viscous and it appears like
solid.
• Since the glass behaves in
this manner the
temperature at which the
specific volume curve
changes its slope is called
Tg- glass transition
temperature.
Glass Transition Temperature
• Polymer becomes :
– hard, stiff and brittle
below Tg
– highly viscous but
solid at Tg
– rubbery, flexible and
softer above Tg
• Both amorphous and
semi-crystalline
plastics have Tg
Coefficient Of Linear Thermal
Expansion
• Measures the change in length per unit
length of a material per unit change in
temperature
• Expressed in in/in/°F or cm/cm/°C
• Used to calculate the dimensional change
resulting from thermal expansion
• Very important when components of an
assembly are made of different materials
Heat Deflection Under Load
• Used to compare elevated temperature
performance of plastics under load
• Temperature requirements often limit
plastics choice more than any other factor
• Does not represent the upper temperature
limit
• Molding factors, sample preparation and
thickness significantly affects the values
Heat Deflection Under Load
• The test bar is loaded
on a support, the
temperature raises
until the applied load
causes the bar to
deflect
Vicat Softening Temperature
• Ranks the thermal performance of plastics
according to the temperature that causes a
specified penetration by a lightly loaded probe
• Used as a general indicator of short term, high
temperature performance
• Less sensitive to sample thickness and molding
effects
• Often used as the ejection temperature
Vicat Softening Temperature
• A flat ended probe
contacts a plastic
specimen submerged in a
heated oil bath
• A specified load is applied
and the temperature is
increased
• Temperature of the oil
bath when penetration
takes place
Thermal Conductivity
• Indicates a materials ability to conduct heat
energy
• Measured in Btu*in/(hr*ft2*°F) or
W/(°K*m)
• Used to calculate heating and cooling
requirements in mold filling, thermal
insulation or heat dissipation analysis
Thermal Conductivity Data
Specific Heat
• Reflects the heat required to cause a one
degree temperature change in a unit mass of
material
• Measured in Btu/lb/°F or KJ/kg/°C
• Used in heat transfer calculations from
mold filling and cooling analysis
Electrical Properties
Electrical Properties
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Resistivity
Dielectric
Dissipation
Arc Resistance
Resistivity
• Measure of a plastics electrical insulating
properties
• Used to compare plastics as electrical
insulators
• Indicates current leakage through an
insulating body
• Should be at least 108 ohm*cm to be
considered an insulating material
Volume Resistivity Data
Dielectric Strength and Constant
• Dielectric Strength measures the voltage an
insulating material can withstand before
electrical breakdown occurs
– Best indicator of a material’s insulating
capabilities
– Measured in volts per mil of thickness
– Higher values indicate better insulating
characteristics
Dielectric Strength Data
Dielectric Strength and Constant
• The Dielectric Constant is the ratio of the
capacitance of a plate electrode system to a
test specimen
– Lower values indicated better insulating
characteristics
Dissipation Factor
• Measures a plastics tendency to convert
current into heat
• Important in applications such as radar and
microwave equipment that run at high
frequencies
• Lower values indicate less power loss and
heat generation
Arc Resistance
• Measures the number of seconds a plastics
surface will resist forming a continuous
conductive path while being exposed to
high voltage electric arc
• Plastics with higher values are used in
closely spaced conductors, circuit breaker
and distributor cap applications
Environmental Properties
Environmental Properties
• Pay close attention to the environment to
which the part will be exposed during
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Processing
Secondary Operations
Assembly
End Use
• Chemical exposure and weather conditions
may determine which plastic you choose
Environmental Properties
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Water Absorption
Hydrolytic Degradation
Chemical Resistance
Weatherability
Gas Permeability
Water Absorption
• Plastics absorb water to varying degrees,
depending on their molecular structure,
fillers and additives
• Adversely affects both mechanical and
electrical properties and causes swelling
• Measures the amount of water absorbed as a
percent of total weight
Hydrolytic Degradation
• Exposing plastics to moisture at elevated
temperature can lead to hydrolysis
– A chemical process that severs polymer chains by
reacting with water
– Reduces the molecular weight and degrades the plastic
• Degree of degradation depends on
– Exposure time
– Temperature
– Stress levels
Chemical Resistance
• Chemical Resistance of a plastic depends on
– The chemical
– Exposure time and temperature
– Stress level
• Type of chemical attack varies with the plastic and the
chemical
– Degradation
– Stress cracking
– Swelling
• Consider all substances a part will encounter
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Manufacturing
Assemble
Storage
End Use
Weatherability
• Plastics in outdoor use are exposed to
weather that can affect the performance of
the part
• Ultraviolet radiation can cause
embrittlement, fading and surface cracking
• Actual and accelerated testing
• Additives and higher molecular weight can
improve stability
Gas Permeability
• Measures the amount of gas that can pass
through a plastic in a given time
• Used in packaging and medical
applications, where the plastic forms a
barrier
Other Properties
Other Properties
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Density
Specific Gravity
Specific Volume
Transmittance
Refractive Index
Flammability
Density
• Mass per unit volume
• Useful in converting volume into part
weight and cost calculations
• Expressed in lb/ft3 or Kg/m3
Specific Gravity
• The ratio of a material's density to the
density of water
• Used in a variety of calculations and
comparisons when relative weight matters
Specific Volume
• The reciprocal of density
• Measured in ft3/lb or m3/Kg
Density and Specific Volume Data
Transmittance
• Measures a material’s transparency
• Haze is the percentage of transmitted light
passing through a plastic that is scattered
• Luminous transmittance is the ratio of
transmitted light to incident light
Transmittance Data
Refractive Index
• Ratio of light’s velocity in a vacuum to its
velocity as it passes through a plastic
• Important in optical lens and light-pipe
calculations
Refractive Index Data
Flammability
• Most Plastics need an additive to meet
flame resistance ratings
– Oxygen Index measures the percentage of
oxygen need to support flame in a plastic
sample
– UL 94 Classes
• Established by Underwriter Laboratories to classify
the burning behavior of plastics