Plastics Mechanical Properties

1. 1

2. Mechanical Properties
 Mechanical Properties of Viscoelastic
Materials
 Stress / Strain Behavior
 Creep
 Toughness
 Reinforcement, Fillers, Modifiers
2

3. 3
Properties and product
performance
MaterialsMaterials
•PropertiesProperties
•AvailabilityAvailability
•CostCost
The successful
design of a
product depends
on the synergy
of the design,
manufacturing
and materials
3

4. Why look at polymer chemistry,
structures and properties ?
What we really want to learn is ….
How to make a plastic part that delivers the
required life cycle performance at the best
cost and meets regulations
4
Product
Performance
GOAL =

5. Plastics – from chemistry to performance
5
Polymer Polymer
ChemistryChemistry
MaterialMaterial
microstructuremicrostructure
MaterialMaterial
propertiesproperties
ProductProduct
PerformancePerformance

6. Intermolecular Attraction Forces
The performance of a plastic part depends
on the attraction forces between polymeric
chains
These forces increase as chain length
increases
These forces are stronger as the chain to
chain distance decreases
Force  1/ d & Force  n
6

7. ‹#›
Bonding Energy &
Distance
7
Bond
energy
Bond length
7

8. Intermolecular Attraction Attraction
ForcesForces
Force  1/ d
8
I
n
t
e
r
m
o
le
c
u
la
r

f
o
r
c
e
s
Intermolecular distance, d
Intermolecular
distance, d
d

9. Intermolecular Attraction
Forces
orce 
n
9
I
n
t
e
r
m
o
le
c
u
la
r

f
o
r
c
e
s
Degree of polymerization, n

10. Mechanical Properties & M
W
Higher MW means stronger intramolecular interactions
which means better mechanical properties
10
Intermolecular
forces
Molecule length to molecular weight

11. Differences in Properties
 Leathery
 More soluble
 Transparent
 Low shrinkage
 Tough
11
Amorphous
 Melt
 Less soluble
 Opaque
 High shrinkage
 Rigid
Crystalline

12. Mechanical Properties -
stiffness
12
Amorphous
s
t
if
f
n
e
s
s
temperature
T
g
T
g
– glass transition temperature

13. Mechanical Properties -
stiffness
13
Temperature
st
iffn
ess
Semi crystalline
T
g
T
m
– melt temperature
T
m

14. Mechanical Properties -
stiffness
14
T
m
T
m
T
g
s
ti
f
fness
Temperature
Amorphous Amorphous
plasticplastic
Semi crystalline
plastic

15. 15
Physical Physical
PropertiesProperties
15%GF
Polyester, PBT
1400 nylon 6/6
13%GF
DensityDensity 0.0509 lb/in³ 0.0444 lb/in³
Water Water
AbsorptionAbsorption
0.1 % 1.1 %
Linear Mold Linear Mold
ShrinkageShrinkage
0.005 in/in 0.006 in/in

16. Plastic Mechanical Properties
16

17. Secondary Bonds
These bonds are physical in nature,
there are no chemical changes
happening and they are weaker than
chemical bonds
Hydrogen Bonds
Entanglement
Van der Waals
17
Increasing
strength

18. Intermolecular Attraction Forces
The performance of a plastic part depends on the
attraction forces between polymeric chains
These forces increase as chain length increases
These forces are stronger as the chain to chain
distance decreases
Force d & Force n

19. Elastic Behavior of Solids
Stress: Applied force per unit area
Strain: Displacement of sample
Stress/Strain = E (Young’s Modulus)
Large modulus E : Stiff materials
Constant modulus--linear S/S curve:
Hookean material (like most metals or
ceramics)
19

20. Stress/Strain Curve
Linear Elastic Material
20

21. Types of Forces
 Pulling on the end: Tensile
21
F
F

22. Types of Forces
 Rotational: Torsion
22
T
T

23. Types of Forces
Pushing and sliding: Shear
23


24. Stress Testing
 Tensile Test
24
A
F
L
Stress,  = F / A
Strain,  = L / L

25. 25

26. 26
Tensile Testing Results Stress vs Strain
For plastics
the rate of stress applied affects the material’s response

27. 27

28. 28

29. Elastic Behavior of Solids
Stress: Applied force per unit area
Strain: Displacement of sample
Stress/Strain = E (Young’s Modulus)
Large modulus E : Stiff materials
Constant modulus--linear S/S curve: Hookean
material (like most metals or ceramics)
29

30. Solid Materials
A Solid can be defined as a state of the material where
the deformation of the part is a function of the load
applied to it
= f (force)
Elastic behavior - Small deformations then return to
original shape
Virtually all applied energy retained and used to
rebound
Forces typically normalized for sample area
30

31. Mechanical Response as a function of
Time
‹#›
F
F
F
time
input

time
displacement
31

32. Elastic Solid Model
‹#›
F
F


k
Spring constant
or stiffness
k
32

33. Elastic Solid –
Microstructural Behavior
‹#›
The applied force straightens polymer chain
segments
FF
The polymer chain segments return back to a more
disorder and stable configuration when force is
removed
33

34. Viscous Behavior
 A fluid can be defined as a state of the material
where the RATE of DEFORMATION of is a function
of the load applied
d dt = f (force)
34

35. Viscous Behavior
Typically applied to liquids; arises from
entanglement
Flow Resistance = Viscosity
Stress causes velocity gradient with time and
distance: Shear Rate
Stress = Viscosity * Shear rate, for a Newtonian
liquid
35

36. Viscous Behavior
Typically applied to liquids; arises from
entanglement
Flow Resistance = Viscosity
Stress causes velocity gradient with time and
distance: Shear Rate
Stress = Viscosity * Shear rate, for a
Newtonian liquid
36

37. 37
Cone and Plate
cone
polymer

38. Stress and Shear Rate
Polymer melt
stationary plate
moving plate
force
Shear stress
38

39. Newtonian Fluid
 Linear shear-rate with stress:


Slope = 
viscosity
Example – water
=shear stress
=shear rate
39

40. Newtonian / NonNewtonian
 Non-Newtonian (non-linear) types
Pseudoplastic: Shear-thinning , most plastics
Dilatant: Shear-thickening


pseudoplasticpseudoplastic
dilatantdilatant
40

41. 41
Rheometry Experiments
Experiment H Re-Grind PC Melt Rheology at 550
o
F

42. Effects of Time and
Temperature
Compared to other materials, the properties of plastic
are more sensitive to the time (how long) at which
they are observed and measured
The properties are also sensitive to the temperature
they are being observed and measured
42

43. Elastic Solid –
Microstructural Behavior
One of the most microstructural features of polymers
or plastics is that they try to keep the level of
disorder or entropy as high as possible
The preference for high entropy is the driving force
for the polymer chains to spring back once the force
is removed
43

44. Mechanical Response as a function of
Time
 Fluid Like Behavior
F
time

time
input
response
No recovery
44

45. Viscous Fluid Model
F
F

ddt
C
Viscous Damping
Constant
45

46. force
force
The polymer chain rub against the nearby chains.
This frictional is proportional to the rate of
deformation
Heat is
generated
46

47. The plastic part is subjected to a tensile force
F F
L
o
, original length
F F
L
new
= L
o
+ L
The plastic part is increases its length L, when
the force is removed it will not spring back –
this a permanent deformation
The plastic part is increases its length L
L
new
= L
o
+ L
permanent
47

48. Mechanical Response as a function of
Time
 Viscoelastic Like Behavior
F
time

time
input response
recovery
48

49. Viscoelastic Solid Model
F

(t)
time
49

50. Viscoelastic Solid,
Microstructural Behavior
Polymer chain segments are stretched by the
force, this is the elastic element of the model
Heat is
generated
As the Polymer chain segments are stretched
there is friction between these chain segments –
this is the viscous damping element
When the force is removed, the chains return to
the original state – during this motion, there is
also friction
50

51. General Viscoelastic
Model
51
F
C
E
K
E
C
P
K
E
– elastic stretching of
chain segments
C
E
– friction between chain
segments (very small)
C
P
– friction between
complete polymer chains

52. Maxwell Viscoelastic
Model
52
F
K
E
C
P
K
E
– elastic stretching of
chain segments
C
P
– friction between
complete polymer chains

53. Viscoelastic Behavior
Continuum of liquids and solids is continuum of
viscous to elastic behavior
Disentanglement is time dependent
Elastic and Viscoelastic materials tend to be
stiffer at high shear rates (short time)
Viscous properties: energy dissipation in the mass--
long range, long time
Elastic properties: Molecular stretching, bending--short
range, short time
53

54. Mechanical Response & Intermolecular
Forces
 The same plastic can have the mechanical response of
An Elastic Solid
A Viscoelastic Solid
A Viscoelastic Fluid
A Viscous Fluid
 The particular mechanical response depends on the
intermolecular forces
54

55. Deborah’s Number
The Deborah number is a dimensionless number,
used in rheology to characterize how "fluid" a material
is.
It is defined as the ratio of a relaxation time,
characterizing the intrinsic fluidity of a material, and the
characteristic time scale of an experiment
The smaller the Deborah number, the more fluid the
material appears.
De = relaxation time / observation time
55

56. Effects of Time and
Temperature – Silly Putty
When heated, it becomes more like a viscous fluid
The longer a load is applied, the more it will act like a
viscous fluid
 When the temperature is lowered, it becomes more
like a solid
The shorted the load is applied, the more it will act as
a solid
56

57. 57
time
Tension in a part
Relaxation curves
Increasing
temps
Initial
Tension
t
o
Length after time t
o
1
2
3
3
2
1

58. 58
ll
Stress Relaxation

59. Mechanical Response and
Intermolecular Forces
 The same plastic can have the mechanical response
of
An Elastic Solid
A Viscoelastic Solid
A Viscoelastic Fluid
A Viscous Fluid
 The particular mechanical response depends on the
intermolecular forces
59

60. Mechanical Response and
Intermolecular Forces
When the intermolecular forces are very
high, the chains are held together tightly
The only possible motion is the stretching
and spring back of short chain segments
Therefore the plastic acts as an Elastic
Solid
60

61. Mechanical Response
and Intermolecular Forces
When the intermolecular forces are not so
strong, the chains are held together less
tightly and they are more separated
When force is applied, longer segments
can stretch and there is friction between
these chains – Viscoelastic Solid
61

62. 62
Mechanical Response
The mechanical response a plastic part
depends on intermolecular forces
% of crystallinity
Temperature
Hydrogen Bonds
Molecular Weight
Chain to chain distance
Entanglements

63. Temperature and Mechanical Response
 Mechanical Response & T
g
63

64. Load Rate
& Mechanical Response
 Stress / Strain Curve
64

Increase of
Strain rate


65. 65

66. 66
Dynamic Mechanical
Analysis
From TA Instruments

67. Dynamic Mechanical
Analysis
67

68. 68
Dynamic Mechanical
Analysis
From TA Instruments

69. 69
Dynamic Mechanical
Analysis
From TA Instruments

70. 70
Dynamic Mechanical
Analysis
Viscous elastic response
From TA Instruments

71. 71
Viscous elastic response

72. 72
Viscous elastic response

73. 73

74. Dynamic Mechanical
Analysis
74

75. 75

76. Creep
Small, constant load, long time
Results from stretching and uncoiling
/disentanglement
Opposed by strong intermolecular forces and
crosslinking
It’s a function Temperature, time, load
76
forceforce
Heat
Chains flow by each other

77. 77
From TA Instruments

78. 78
From TA Instruments

79. Creep Viscoelastic Model
79
(t)
time
Critical time
F
C
E
K
E
Permanent
Deformation –
creep
For this load, the
viscous motion has
started
For this load, there
has not been enough
time to start the
viscous motion

80. Creep –
Temperature, Time and Load
The critical time is f(temperature, load)
The higher the Temp, the lower t
critical
–
this is because the higher temp makes
the material more viscous like
The higher the Load, the lower t
critical
–
This is because higher loads can start
whole displacement in shorter periods
of time
80

81. 81

82. 82

83. 83

84. 84

85. 85
Creep – time, temp loads

86. 86

87. 87

88. Impact Strength and
Toughness
Toughness: Absorb energy without
breaking
Related to area under stress/strain curve
Toughness experiments mostly short time,
e.g., impact strength
88

89. Plastic Toughness
The amorphous region can deform more and
absorb more energy
89
crystalline
amorphous

90. 90
Depends on material
ability to absorb energy
Stress/strain curve
Area underneath
Stress/strain curve is
the measure of impact
 strain
 stress

91. Toughness is not Strength
Tough: High elongation, low modulus.
High M
W
Low Intermolecular Strength
Rubber, slight cross linking
Brittle: Low elongation, high modulus
Crystalline
High degree of Xlinked rigid
High Intermolecular Strength
91

92. 92
Degree of Crosslinking & Toughness
ToughTough
StrongStrong

93. 93
Small Additives
Get between
Polymer chains
This increases d and
can make the degrade
properties
Example – excess of colorant can weaken a plastic partExample – excess of colorant can weaken a plastic part

94. Reinforcements
Different polymer chains
are attracted to the
reinforcement
Works as a bridge to
attract polymer chains that
normally would not interact
This makes the properties
better
94

95. 95

96. 96

97. 97
Melt Flow Rate
(ASTM D1238)
Given a resin's MFR,will the part fill properly?
•Test conditions are not real world processing conditions
•Different weights and temperatures used for different resins
-Comparison of different resins is not 1 to 1
-Only relative comparisons are possible
•Single-Point Data vs. Rheological Curves
Len Czuba
August 2006

98. 98
Melt Flow Rate
Test Apparatus:
Resin

99. 99
Melt Flow Index
# grams of flow per 10
minutes
Weighted Plunger
Barrel
Molten
Pellets
Extrudate
Orifice
Heater Band
Dynisco LMI 4000
Len Czuba
August 2006

100. 100
Tensile Strength
(ASTM D638)
•Cross-head speed not standardized
•Specimen thickness can be anything up to 0.55"
•Specimen gating not standardized
•How did the specimen fail:
-Ductile ?
-Brittle ?
Len Czuba
August 2006

101. 101
Tensile Strength
(ASTM D638)
•Ultimate tensile strength
•Tensile modulus
•Tensile elongation

102. 102
Tensile Strength
Test Apparatus:

103. 103
Impact Resistance
ASTM D256
Is this a relevant impact test
for your device?
•Five Different Methods
-Izod (Methods A, C, and D)
-Charpy (Method B)
-Unnotched (Method E)
•Cannot correlate results from different methods
•Specimen toughness highly dependent on notch size
•Specimen preparation not standardized

104. 104
Impact Resistance
Test Apparatus:
Len Czuba
August 2006

105. 105

106. 106
Impact Resistance
Test Specimen:
Len Czuba
August 2006

107. 107

108. Summary
Elastic, Viscoelastic, Viscous
Stretch/bend vs entanglement
Tensile, Compressive, Flexural, Torsional, Shear
Stress/strain performance
Strength/toughness
Effect of modification on properties
108

109. Summary
 Plastic materials behave as elastic solids, viscous
fluid or a combination of both
 The mechanical behavior of plastics depends on
factors such as:
Intermolecular forces
Temperature
Time load is applied
109

110. Summary
There are many important mechanical properties that
must be considered for processing and use, such as
Tensile strength
Impact Resistance
Creep
Use Temperature
Processing Temperature
110

112. Design Example
Reduce Costs of the
system without
reducing quality or
compromising safety
This part works
mostly in bending
 = Mc / I
old

113. Load and Material Interaction
Normally, we want the material property
to be higher than the value actually
applied to the material – example yield
stress
 material yield stress > applied stress
 Safety Factor = material yield / load
Caution – loads and property values are
probabilistic not deterministic

114. Load and Material Interaction
Failure
probability

115. Load ave = 6000, std dev = 1000
PET ave = 12000, std dev = 1000
Example -
Material Properties & Loads

116. Load ave = 6000, std dev = 2000
PET ave = 12000, std dev = 1000
Solution 1 - use the same part for another application

117. Load ave = 6000, std dev =1000
PET ave = 12000, std dev 1500
Solution 2 - get cheaper materials

118. Load ave = 6000, std dev = 1000
Regrind PET ave = 9500, std dev = 1200
Solution 3 - get cheaper materials, use regrind

119. 25% regrind 75% virgin
Solution 4 - get cheaper materials, use regrind + virgin

120. Solution 4 - get cheaper materials, use regrind + virgin

121. Solution 4 - get cheaper materials, use regrind + virgin

122. Solution 5 - Redesign Part
The stress depends on the MC / I
where C - is the distance from the neutral axis & I is
the moment of inertia of area
We can redesign the part to reduce the C / I ratio so
that even with the if M is the same, we reduce the
stress

123. Solution 5 - Redesign Part
Existing cross section New cross section

125.  = Mc / I
new

126. Possible Solutions
Add a great % of virgin material, or use 100 %
virgin
Good Quality Control
Increases Solid Waste Problem
Increases Production Costs, not
added value
No new jobs are created by importing
resin

127. Possible Solutions
Increase the use of regrind and redesign the part
Reduces Solid Waste Volume
Reduces Costs
Can generate more jobs in PR
Reduces production time (material
sources are closer by )

128. Design Summary
Most recycled materials will have lower properties
than virgin materials
Virgin material can be combined with recycled
material to improve properties
Increasing the use of regrind reduces costs

129. Design Summary
The best way to take advantage of the low cost of
recycled and compensate for the lower performance
is by redesign of the part

130. 130