Understanding the Mechanical

8/17/2019 Understanding the Mechanical

1/31

Understanding the Mechanical

Properties of Filled Polyolefins

J M Adams, C D Paynter & S Ritchie


2/31

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


3/31

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


4/31

Filler Particles

Talc Kaolin

Calcium Carbonate Bentonite1 μm


5/31

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


6/31

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


7/31

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.


8/31

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.


9/31

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


10/31

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.


11/31

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


12/31

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).


13/31

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).


14/31

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).


15/31

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


16/31

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)


17/31

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%)


18/31

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


19/31

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


20/31

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)



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


21/31

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.


22/31

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


23/31

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


24/31

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


25/31

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.


26/31

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.


27/31

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.


28/31

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.


29/31

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


30/31

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


31/31

Acknowledgements• Imerys Minerals Ltd

- Phil McGenity

- Andy Riley

- Mike Hancock • EU – Interreg – MNAA

• Universities of Exeter & Bristol - MCSW

Understanding the Mechanical

Documents

Transcript of Understanding the Mechanical