Aim

Processing and performance of polymer-clay

nanocomposites: implications for processability and

performance in medical devices and packaging

Prof. Eileen Harkin-Jones

School of Mechanical and Aerospace Engineering

Aim

• To investigate interactions between process-structure-performance in polymer-clay nanocomposites

• To examine the implications for the processing and performance of medical devices and packaging

• To highlight areas for future research

Polymer nanocomposites

• Composites in which reinforcing particles have

at least one dimension (i.e. length, width, or thickness)

on the nanometer scale [1]

Surface area: 125 x (6 sides of area 1 x1 ) = 750 units

6 sides of area 5 x 5 = 150 units

In going from micro to nano scale the specific surface area increases significantly leading to enhancement of material properties[1] Nanotschnology for engineers-polymer nanocomposites-EPFL-Google

Layered silicates or nanoclays

• The dimensions of a clay platelet are typically 200-1000nm in lateral dimension and 1 nm thick.

TEM of mmtCan exist in a number of forms

Advantages of polymer-clay nanocomposites

• strength and stiffness

• permeation resistance

• flame retardancy

• heat deflection temperature

• because of the large surface area of the nanofiller, only small quantities need be used

• there should be no need for new processing equipment to mix these fillers into the polymer

• The composite is recyclable

Specific advantages of nanoclays in medical devices and

packaging

• Controlled permeation rates of therapeutic agents in a device

• Controlled degradation behaviour of devices, packaging [e.g tissue scaffolds, shedding of surface biofilms from tubing]

• Better high-temperature performance and thus improved performance in sterilisation of packs/devices

• Extended property range of medical polymers

Manufacturing of medical devices and packaging

Different processes

• Catheters – tube extrusion

• Flexible packaging – blown film extrusion

• Rigid packaging – thermoforming, stretch blow moulding

• Biodegradable screws– injection moulding

In each case the polymer will experience a particular

thermal and deformation history which in turn can be

expected to influence structuring and performance

Research at QUB

• Current research at QUB investigating the relationship between processing-structure-performance of polymer nanocomposites

• Processes of interest include thermoforming and injection stretch blow moulding (ISBM)

• Focus on PET-clay and PP-clay systems today

ISBM & Thermoforming

Essentially biaxial deformation processes

plug

sheet

mould

(a) (b) (c)evacuated air

air pressurepressure belland clamp

cutting tool

Preform= injection moulded

Amorphous PET tube

Preform = extruded sheet

Important material parameters for processing

Thermoforming

• Sag resistance of sheet at forming temperature

• Sheet modulus and yield stress

• Strain hardening for stability and uniform stretching

ISBM

• Strain hardening behaviour

• Tcc-Tg temperature processing window

Materials & Methods

• PET + Somasif synthetic nanoclay - ISBM applications

• PP + MMT (Cloisite 15A) + MAH - Thermoforming applications

• Materials compounded on twin screw extruder to form pellets

• Pellets compression moulded into sheet

• Sheet biaxially stretched

• Structure characterised using TEM, XRD, DSC, POM

• Performance measured using Tensile tests, O2 gas barrier, DMTA

Biaxial Stretching

grips

‘preform’

top heater temperature sensor

Capable of duplicating the deformation behaviour of polymeric materials inISBM and thermoforming processes.

Strain rate to 32 s-1

Temperature to 200 ºC

Stretch ratio to 4.5

Various stretching modes Constant width, Simultaneous equibiaxial, Sequential

PP-clay

Sample(°C) Xc Crystallinity

(%)

Tm (⁰C) Tc (⁰C)

PP 1.0 62 165 121

PPNC5- 1.0 61 164 116

• No change in Xc – shrinkage same

• Tc is lower – longer demould times

• Longer in melt state – possibly more molecular relaxation but clay is likely to have opposite effect on relaxation

Preform: Crystallization behaviour

Preform: modulus versus temperature

• Room temperature modulus is higher for the pp-clay nanocomposite

• However, close to forming temperatures the nanocomposite has a lower modulus – likely to cause problems with sheet sagging

• Source of reduction – early melting of smaller spherulites in pp-clay sheet

0

500000000

1000000000

1500000000

2000000000

2500000000

3000000000

-40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140

Temperature(C)

Stor

age

Mod

ulus

(Pa) Cross G'

Preform: Deformation behaviour

0

1

2

3

4

5

6

7

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1

Nominal Strain (mm/mm)

Tru

e S

tre

ss

(N

/mm

2)

PPNC5-3.0

PP3.0

No difference in strain hardening behaviour

Preform: Temperature sensitivity

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

6

6.5

7

7.5

8

8.5

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6


Tru

e S

tress (

N/m

m2)

PPNC5-sr16-SR2.5-T150

PPNC5-sr16-SR2.5-T145

PP-sr16-SR2.5-T150

PP-sr16-SR2.5-T145

• At 145 C the yield stress for the pp-clay is 25% greater than the unfilled pp- will require greater forming forces at this temperature

• At 150 C no obvious difference in yield behaviour

SR=1.0 SR=1.5 SR=2.0

SR=2.5SR=3.0SR=3.5

Effect of stretching on structure

Stretching helps delaminate clay stacks and causes orientation of platelets

Platelet thickness distribution

SR=1.0SR=1.5 SR=2.0

SR=3.5 SR=3.0 SR=2.5

Orientation distribution

The exfoliation number (N) is defined as the percentage of the total clay interfacial area that is exposed to the polymer matrix. It is a dimensionless quantity, which ranges from 0 to 100, with 0 indicating no exfoliation and 100 indicating complete exfoliation.

Exfoliation number - N

Exfoliation number

Large increase between SR=3 and SR=3.5

Mechanical & barrier properties of stretched sheet

Stretching ratio

Effect on Modulus (%)

Effect on Yield Strength (%)

Effect on stress at Break (%)

Effect on O2 barrier (%)

N

1.0 0 -27. -19 - 10

1.5 0 -24 -40 - 21

2.5 +4 +9 -17 +11 30

3.0 +10 +12 +4 +24 31

3.5 +15 +44 +15 +46 48

• Increase in exfoliation as SR increases

• Main improvement is in barrier and yield strength

• Improvement in yield may be connected to crystallite size modification

High temperature performance

Cross-over=70 0C

Cross-over=100 0C

As N increases the reduction

in nanocomposite modulus

due to early crystallite melting

is compensated for by the

greater contribution of the

clay. Cross over temperature

Increases.

PET-Clay

Sample(°C) Xc Crystallinity

(%)

Tg (⁰C) Tcc(⁰C)

PET 11 79 128

PET + 5% Clay 10 78 125

Preform : Crystallization behaviour

• Tcc-Tg = temperature processing window.

• Slight reduction with incorporation of clay (2 C⁰ )

Preform : modulus versus temperature

6.5

7.0

7.5

8.0

8.5

9.0

9.5

10.0

20 40 60 80 100 120 140 160

Temperature (C)

Lo

g E

' (P

a)

virgin PETPET+1%MAEPET+2%MAEPET+5%MAE

•Addition of clay enhances high temperature modulus•Less likely to have problems with sag (extrusion blow moulding)•Unlike behaviour of PP-clay system

Preform : deformation behaviour

Equi-biaxial stretching, strain rate 8/s, T = 100 °C

0

5

10

15

20

25

30

35

0 0.5 1 1.5 2


Tru

e S

tre

ss (

MP

a)

virgin PET

PET+1%MAE

PET+2%MAE

PET+5%MAE

• Clay inclusion leads to large increase in strain hardening even at 1wt%

• At 5wt% and nominal strain =1.8 would need 90% more work energy to deform the PET-clay material

Note: SR=Nominal +1

Effect of stretching on structure

Unstretched SR=3.0

Stretching

Tactoid folding and bending

Effect of stretching on tactoid thickness

•Stretching causes increase in the concentration of tactoids having thickness 1-2 nm .

•Tactoids having 5-10 and 10-15 nm thickness are higher for the unstretched sheet.

Stretching conditions – stretch ratio 3; strain rate 8/s; temp. 100 deg C)

Orientation distribution

SR=2

SR=2.5SR=3.0

SR=1.0


Stretchratio

Effect onmodulus (%)

Effect onYield

strength (%)

Effecton stressat break

(%)


N

1.0 30 0 ? 40 9

2.0 41 12 -20 12

2.5 50 26 2 13

3.0 27 23 6 33 15

• Even in the unstretched state and at low exfoliation level the clay has a significant effect on modulus and barrier enhancement. This may be due to the high aspect ratio of this clay N [particle length 1200nm compared with 200nm for Cloisite 15A].

• Increasing the SR increases the particle alignment which contributes further to modulus enhancement


Stretchratio

Effect onmodulus (%)

Effect onYield

strength (%)

Effecton stressat break

(%)


N

1.0 30 0 ? 40 9

2.0 41 12 -20 12

2.5 50 26 2 13

3.0 27 23 6 33 15

• At SR=3 strain induced crystallinity and molecular orientation increase the modulus of the matrix and the contribution of the clay is less important.

• O2 barrier enhancement better than pp-clay system in unstretched state – probably due to higher aspect ratio.

• At SR=3 strain induced crystallinity and molecular orientation increase the barrier of the matrix and the contribution of the clay is less important

High temperature performance

8

8.2

8.4

8.6

8.8

9

9.2

9.4

9.6

9.8

0.0 50.0 100.0 150.0 200.0 250.0

Temperature (C)

Lo

g E

' (M

Pa

)

virgin PET PET+1%MAE PET+2%MAE PET+5%MAE

• Significant enhancement in storage modulus at high temperatures

Equi-biaxial stretching, strain rate 8/s, T = 100 °C

Implications for processing and performance of medical devices and

packaging

Processing

• Be aware that addition of clay may alter temperature processing windows and forming forces required

• Sag resistance of preforms can be improved or reduced depending on the base polymer and the influence of clay on crystallite perfection

Processing

• Should achieve more uniform wall thickness in PET-clay products due to increased strain hardening

• Pre-orientation of clay in a preform will further significantly increase forming forces required for deformation in that direction make preforms with as little orientation as possible (difficult in injection moulded preforms, ok for sheet)

Processing

• For the materials studied in QUB to date (PP, PET, HDPE) the incorporation of clay does not alter the crystallinity

levels so shrinkage should not be different for the clay

filled systems.

• Demould times may be longer as Tc tends to be reduced [may be different for other materials]

Performance

• It is possible to attain good levels of O2 barrier enhancement with low addition levels of clay in PP and PET

• Modulus enhancement is much better (at both room and higher temperatures) in the PET system at the same SR – possibly due to the larger aspect ratio of clay used in PET since exfoliation levels are actually higher in PP

• Varying stretch ratio provides a means to control exfoliation levels and hence performance e.g drug release rates could be controlled.

Performance

• Clay: aspect ratio, degree of alignment, potential for bending and twisting

• Changes in crystallite size and size distribution due to presence of clay

• Changes in % Xc due to nucleating effect of clay

• Increased capacity for molecular entanglement due to presence of nanoscale particles – more ‘crosslink’ points

• Strength of interaction between clay and polymer

• Matrix modulus relative to clay modulus – greater effect in a softer matrix

• Degradation of matrix

Possible sources of property change

Performance

• In the composites made to date we are still not close to full exfoliation.

- will incorporate more extensional flow in the actual

compounding stages to try and improve this.

- will also look at higher SRs to determine if exfoliation

continues to increase

Potential future work

• Incorporation of conductive particles – use of different stretching regimes to impart tailored anisotropic electrical properties

• Determine influence of structuring on release of therapeutic agents

• Determine effects of sterilising environments on performance of polymer-clay devices/packaging

• Incorporate clay into polymers with Tg close to 37 C and examine switching capacity

Acknowledgements

Academic Staff

• Prof. C. Armstrong• Prof. P. Hornsby• Dr M. McAfee• Dr T. McNally• Dr P.Martin• Dr G.Menary

Research Fellows

• Dr J. Hill• Dr R.Rajeev• Dr S. Xie• Dr Richard

O’Shaughnessey

PhD students

• R.Abu-Zurak• Y.Shen• K.Soon

Documents

Aim