understanding the mechanical
TRANSCRIPT
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Understanding the Mechanical
Properties of Filled Polyolefins
J M Adams, C D Paynter & S Ritchie
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IntroductionPolylefins are the most
used polymers today
Use of a variety of
virgin polyolefin typesand additives generates
wide property envelope
‘Green’ implicationsImpact Strength
F l e x u r a
l M o d u l u s
Filled grades
Polymer Market %
PE 32
PP 19
PVC 16PUR 8
PS 6
PET 6
PA 2
PC 1
Other 10
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Background
Inorganic fillers affect composite properties:
Polyme
r
$/litre Filler $/litre
PE 0.70 Fine 2.80
PP 0.79 Ultrafine 14.50
Increase:
•Density
•Thermal conductivity•Electrical conductivity
•Stiffness
Decrease:
•Thermal expansion
•Warpage•Surface finish
•Colour
•Tensile strength
•Flammability
It Depends:
•Permeability
Fillers are expensive but also:
Remember the compounding cost!
Inorganic fillers forfunctionality, not cost
Filler (kt) 1972 1987 2002
ATH 4 35 200
Asbestos 60 6 0
Carbonate 282 854 1200
Kaolin 24 24 24
Silica 13 8 6
Talc 6 80 160
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Filler Particles
Talc Kaolin
Calcium Carbonate Bentonite1 μm
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Stiffness – 1 – Basic Data
0 10 20 30 40 50
0
1
2
3
4
5
6
7
Filler Loading (wt %)
Y o u n g s
M o d u l u s
( G P a
)
calciumcarbonate
kaolin
talc
mica
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Stiffness - 2
What is the origin of the different stiffeningeffects shown by different fillers?
Could be:• Aspect Ratio (Ec =Ef.Ø.MRF + Em(1-Ø); MRF= f(AR))
• Delamination in processing (increasing AR)
• Particle alignment in processing (increasing MRF)
• Relative stiffness of filler particles
• Interfacial properties
• (Induced) polymer morphology
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Stiffness – 3 – Aspect Ratio
20 40 60 80 100 120 140
4.5
5.5
6.0
6.5
0
5.0
4.0
3.5
3.0
kaolin
talc
Particle Aspect Ratio
F l e x u r a l
M o d u l u s
( G P a )
40 wt% filled PP
So, Aspect ratio is important.BUT it is not the only factor.
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Stiffness – 4 – Effects during Processing
Delamination during processing does NOT happen. Trial with PP filled(40 wt%) with talc. AR before compounding/moulding = 29, and 27 afterwards.
Depth (mm)2 4
I n
t e n s
i t y
R a t
i o
FTIR – Orientation vs Depth
0
kaolin
talc
Talc-filled
Kaolin-filled0
1
Orientation effects arethe SAME for differentplatey fillers.
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Stiffness – 5 – Modelling for Filler Modulus
(Padawar & Beecher: Ec=Ef.Ø.MRF + Em(1-Ø)
20 40 60 80 100 120 140
4.5
5.5
6.0
6.5
0
5.0
4.0
3.5
3.0
kaolin
talc
Particle Aspect Ratio
F l e x u r a l
M o d u l u s
( G P a )
40 wt% filled PP
Ef = 35 GPa
Ef = 20 GPa
Particle stiffness seems important.
BUT, is this the (full) answer?
T
OT
T O
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Stiffness – 6 – Interfacial Properties
F l e x u r a l
M o d u l u s
( G P
a )
2.5
3.5
4.5
u n t r e a t e d
W i t h A H T
W i t h P C 1 A
W i t h
P C 1 A / P C 1 B
W i t h 3 - A P S
W i t h
P o l y b o n d
W
i t h 3 - A P S +
P o
l y b o n d
PP 40 wt% filled with kaolin
The chemistry of the interfacedoes make a difference, BUT thereasons are not crystal clear.
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Stiffness – 7 – Polymer MicrostructureTrans-
crystalline
Spherulitic
Youngs Modulus (GPa) 1.09 0.67
Tensile Yield Strength (MPa) 25.0 18.6
Elongation to Break (%) 4 >300
Failure Energy (kJ m2) 28.0 28.5
We know that
for unfilled PP thepolymer microstructure
is critical to performance.
Optical
Micrographs
Spherulite
Transcrystalline
Region nearNucleating surface
SpheruliticStucturein unfilled PP
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Stiffness – 8- NucleationModelling the shape of the DSC crystallisation curve => Nucleation Properties
20 wt% filled PP crystallised at 135 oC
Nf = NA1.5(filled) – NA
1.5(unfilled)
NA = [2√3 (δro /δt)2 tm
2]-1
temperature
H e a t o u t p u t
tm
Sample tm (min) Number ofnucleating sites per
SA of filler Nf (x 106
m-2)
Unfilled PP 12.2 -
PP + Calcium
Carbonate
3.2 65
PP + StearicAcid coated
Calcium
Carbonate
3.4 60
PP + Talc 0.9 1300
AND we know that fillers
affect the crystallisationof the polymer (Example 1).
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Stiffness – 9 – Polymer Morphology
Nucleation and crystalgrowth in PP much reduced
by modification of the
mineral surface
Untreated Mineral
Surface Treated with 3-APS
Transcrystallisation
Residual Spherulitic Growth
AND we know that fillers
affect the crystallisation of
the polymer (Example 2).
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Stiffness – 10 – CrystallinitySample % Crystallinity
(DSC)
%
Crystallinity
(IR)
CrystallinityIndex
(XRD)
α-phase
Orientation
Index (XRD)
β-phase
Index
(XRD)
Unfilled PP 60 68 3.7 0.88 0.05
PP + 40 wt%
Calciumcarbonate
59 66 3.6 0.80 0.07
PP + 40 wt%
Stearatecoatedcalciumcarbonate
60 68 4.0 0.83 0.12
PP + 40 wt%
Talc
55 - 11.5 0.92 0.02
AND we know that fillers affect the
crystallisation of the polymer (Example 3).
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Stiffness – 11 - Summary
Composite stiffness is determined by:
•Filler loading•Stiffness of the filler
•Aspect Ratio of the filler
•Important effects from:(i) nucleation/crystallisation properties of fillers
(ii) surface treatment which modifies these properties
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Impact – 1 - Background
Impact strength
S t i f f n e s s
- F l e x
M o d u l u s
‘Everyone knows’ that fillers thatgive high stiffness have poor impact
properties
Talc and clay filled PP
0 40 80 120 160
Aspect Ratio
F a l l i n g
W t
I m p a c t
S t r e n g t
h ( J )
0
5
10x
x
x
x
x
x
x x
x
x
‘Everyone knows’ that this
is a result of the high
stresses at the edgesof a high AR inclusion
(Higher AR = sharper edges)
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Impact – 2 – Key Information - 1
Talc Calcium Carbonate
0
10
20
30
C h a r p y
N o t c h e d
I m p a c t
S t r e
n g t h
( k J m
- 2 )
0 10 20 30 40 50 0 10 20 30 40 50
Uncoated
Coated
•At loadings from 10-50 wt%, coated carbonate gives notched IS > unfilled.
•Below 20 wt% loading talc gives notched IS = unfilled, but declines thereafter.
•Even with excellent dispersion given by twin screw compounder, uncoatedcarbonate (low AR) performs as poorly as talc (which has high AR).
Filler Loading (wt%) Filler Loading (wt%)
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Impact – 3 – Key Information - 2
Talc Calcium Carbonate
0
0 10 20 30 40 50 0 10 20 30 40 50
Uncoated
Coated
At loadings from 10-50 wt%:• coated carbonate gives un-notched IS >> unfilled.
• uncoated carbonate gives un-notched IS = unfilled.• talc gives un-notched IS = 2* unfilled.
Filler Loading (wt%) Filler Loading (wt%)
F a l l i n g W e i g h t I m p a c t S t
r e n g t h ( J )
5
10
15
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Impact – 4 – Important Factors• Particle shape – High AR = sharp edges => stress foci
• Poor Particle Dispersion => aggregates => Griffith flaws
• Crystallisation & Microstructure
• Mechanics of crack propagation
BUT REMEMBER that THE MOST IMPORTANT FACT is that
PROPERTIES CAN IMPROVE when you add filler particles!
Poor dispersion
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Impact – 5 – Unfilled PP
Measured IS as a function of sampledistance from the gate and T.
Deduced that High IS corresponds to:
•Low crystallinity
•High β-phase content•Low α-phase alignment
Question:•Does this translate to the filled case?
Distance from the gate (mm)
Distance from the gate (mm)
Distance from the gate (mm)
β - p
h a s e
i n d
e x -
B
α
- p h a s e o r i e n t a t
i o n
i n d e x -
A
C r y s t a l
l i n i t
y i n d e x -
C
220 oC
0
0
080
80
80
250 oC
280 oC
1
2.2
0
0.25
0.5
0.7
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Impact – 6 – Nucleation/Crystallisation
We do have data that demonstrate the importance of nucleation
and crystallisation in determining the impact performance of filled
polyolefins. Data for Polypropylene is given below.
120 124 128 132 136
Temperature of onset of crystallisation (oC)
F a l
l i n g
W e i g h t I m p a c t
S t r e n g t h
( J )
0
6
12
Unfilled PP
Uncoated Carbonate
Coated Carbonate
1. Crystallisation does determine Impact Strength.
2. Unfilled point is NOT on the line
=> Mechanism for good impact performance is different in filled and unfilled cases.
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Impact – 7 – Fracture Mechanics
b
w a
Umes-Uke = G (b w Z)
Z is a function of (a/w)
(a)crack length
plastic zone
radius rp
bwZ (mm2)
F a i l e n e r g y
( J )
Sample Notched IS(kJ/m2)
Gc(kJ/m2)
rp(mm)
Unfilled PP 11.5±1.0 3.2 0.08
Carbonate filled 9.5±2.0 3.1 0.26
Coatedcarbonate filled 13.0±4.5 5.0 0.31
Filled materials have:
- higher Gc values
- much bigger plastic (deformation) zones
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Impact – 8 – Critical Ligament Theory
Interparticle Ligament Thickness
0.1 μm 1 μm 10 μm
I z o d I m p a c t
E n e r g y
( J / m
)
0.1 μm 1 μm 10 μm
F W I
m p a c t
S t r e n g t h
( J )
Bartczak’s Critical Thickness
0.6 μm particles
0.9 μm particles
0.75 μm particles
We found no
critical ligament thickness
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Impact - 9 – Instrumented Charpy Impact Test
0
50
100
150
200
250
0.0 1.1 2.2 3.3 4.4
Displacement (mm)
L o a d ( N )
GT
GIC
GPP
Piezoelectric load cell
Striker
Generalised curve
Post peak energy
ie
plastic or ductile
region
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Impact - 10 -Total energy results
Perpendicular G1G2G3G4 G6G5
Flow direction + molecular orientation
0
5
10
15
20
25
30
perpendicular g1 g2 g3 g4 g5 g6
flow orientation and position in mould
G T k J
/ m 2
neat 5% cc 10% cc 30%cc 20%cc 25%cc5 vol% 10 vol% 30 vol% 20 vol% 25 vol%
G1 G2 G3 G4 G5 G6
Impact performance is
very dependent upon:
- flow & molecular orientation- position in the mould
especially for filled materials.
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0
50
100
150
200
250
0.0 2.0 4.0 6.0
displacement (mm)
l o a d ( N )
25%-g1
5%-g1
Impact – 11 - Thin orientated injection moulded samples (G1)
5 vol% filler
25 vol% filler
brittle
ductile
More highly filled system has the highest impact strength.
The initiation energy remains constant BUT
the post-peak energy absorption is increased by the increased filler volume.
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0
50
100
150
200
250
0 . 0
0 . 5
1 . 0
1 . 5
2 . 0
2 . 5
3 . 0
3 . 5
4 . 0
4 . 5
displacement (mm)
l o a d ( N )
25%-g1
25%-perpendicular
Impact - 12 - Impact properties as a function of flow orientation
25 vol% filler (Perpendicular)
25 vol% filler (G1)
brittle
ductile
We see large differences between samples having different orientation:
-the initiation of the crack is the same
- but we lose almost all of the post-peak toughness when the flow andmolecular orientation is perpendicular to the direction of fracture.
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PP - 5 vol% Calcium Carbonate PP – 25 vol% Calcium Carbonate
Impact – 13 - Fracture Surface
The more ductile deformation in the 25% filled case can clearly be seen.
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Directly under
fracture surface
100 m under
fracture surface
400 m under
fracture surface
Impact - 14 - SEM of the stress whitened plastc deformation
zone - microtomed surface perpendicular to the crack face
under the fracture surface
PP – 20 vol% Calcium Carbonate
Large scale voidingVoiding still present.
Cavitation extends ~ 130 μminto the bulk, allowing plastic
deformation and dissipationof energy.
Voiding/cavitation does
not extend this far into
The bulk.
F l S
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Final Summary
Composite stiffness – critical properties:• Filler Loading
• Stiffness of the filler
• Aspect Ratio of the filler
• (i) nucleation/crystallisation properties of fillers(ii) surface treatment which modifies these properties
Impact Performance – governed by:
• Filler loading• Processing conditions, which determine:
(i) filler dispersion
(ii) geometric/orientation effects
• Aspect Ratio of the filler• (i) nucleation/crystallisation properties of fillers
(ii) surface treatment which modifies these properties
• Degree of plastic flow, determined by filler size and loading, and
by the interparticle ligament geometry
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Acknowledgements• Imerys Minerals Ltd
- Phil McGenity
- Andy Riley
- Mike Hancock • EU – Interreg – MNAA
• Universities of Exeter & Bristol - MCSW