Mechanical Characterization of Microfibrillated Cellulose (MFC)-poly(lactic acid)

nanocomposites

Jie Ding, Lech Muszyński , John Simonsen

Department of Wood Science and Engineering

Application of the Concept of the Essential Work of Fracture (EWF)

Poly(lactic acid) PLA is a versatile polymer made from

renewable agricultural raw materials and is compostable.

Background

Poly(lactic acid) (PLA) and its products

Applications

• disposable cups,

plates, containers

• plastic bags

• food wraps

http://www.ecothefriendlyfrog.co.uk/pla.shtm

Known issues

• weak & brittle

Needs reinforcement

Potential reinforcement: Microfibrillated Cellulose (MFC)

Background

MFC are cellulosic fibrils

disintegrated from plant cell walls

(usually aggregates of microfibrils).

Typical thickness range: 20-40 nm

(aggregates), could be as small as

3-10 nm (individual fibrils)

(Svagan et al. 2007)

Structure and appearance of MFC

by SEM (by Jie Ding)

Background

Advantages:

Both components derived from renewable materials

Both are environmentally friendly

carbon neutral

compostable

Small addition of MFC improves strength and elastic

modulus of PLA (Mathew & Oksman 2006)

No satisfactory formulation commercialized to-date

MFC/Poly(lactic acid)(PLA) Composites

Many formulations are generated in the search of “the perfect one “

Prototype formulations are generated in small amounts of thin transparent films

There is a need for a quick and efficient way of evaluating mechanical properties of new formulations

Properties of interest

• Strength

• Elastic modulus

• Toughness

Background

Develop a quick and efficient method for

evaluating the

strength,

elastic modulus and

toughness

in thin transparent polymer films.

Objective

0.016

0.008

0

0.024

0.032

εyy

Approach

We have successfully applied non-contact optical methods for full-field measurement of deformations and strains in thin transparent films

0.016

0.008

0

0.024

0.032

εyy

Approach

Optical methods also allow analysis of failure modes, work to failure and fracture mechanisms.

Approach

Fracture toughness concept OK for brittle materials

work to failure work of fracture

Not true for ductile materials:

work to failure

essential work of fracture

+ work of plastic deformation

Wf = We + Wp Strain

Str

ess

Str

ess

Strain

We

Wp

• Represents the energy consumed within the

fracture zone where new surface is generated (Kwon and Jar, 2007)

• Well correlated to fracture toughness for ductile

polymers (Barany et al, 2003)

• Therefore it is a material constant, independent of

sample geometry (Wu and Mai, 1996)

Essential Work of Fracture (EWF)

Approach

Measurement of EWF

Wf = We + Wp

Liu & Nairn (1998) used

double-edge notched

tension (DENT) specimens

Wf= welt + wpVp

Wf/lt=wf= we + βwpl

Approach

Plastic

Deformation

Zone

Shape factor

Typical experimental results for measuring the essential

work of fracture.

Theory of the EWF Method

βwp

we

wf

Schematic drawing of the relationship between specific total fracture work wf and ligament l

l

Large amount of samples needed

Approach

0 W

• Large amount of samples

• Assumes knowledge of the shape of the plastic deformation zone (β factor)

• Assumes uniform level of plastic deformation within the zone

Opportunities

Drawbacks of the traditional EWF experimental Method

Solution: optical measurement of strains

• The actual distribution of plastic deformation can be

readily measured

• No need to make assumptions regarding the shape of

plastic deformation zone

• No need for multiple tests

Materials & Methods

P

P

tension

Strain

mapping

After

failure

Permanent

strain

Evaluate EWF using Digital Image Correlation (DIC)

Case 1

Materials & Methods

Case 2

Case 3S

tre

ss

Strain

Strain

Str

ess

Strain

Str

ess

Wp

Wp + We

Case 1

Case 2

Case 3

Materials & Methods

Polyester film

• Ductile and transparent (to substitute for MFC/PLA composite)

• Identical speckle pattern printed on all specimens

• Use double-edge notched specimens and calculate we in both ways

Tensile tests on thin film

specimens

(Modified ASTM D 882-09)

• 1 kN Instron (ElectroPuls

E1000) testing frame

• Optical measurement of

deformations and strains:

Digital Image Correlation

(DIC), precision ± 0.4 μm

specimen

Evaluate EWF using Digital Image Correlation (DIC)

Aj

Plastic

deformation

Materials & Methods

0.016

0.008

0

0.024

0.032

εyy

Future work

No need to notch the specimens because we can trace back the strain concentrations leading to failure anywhere in the specimen

Preliminary Conclusion

It is possible to measure

• Strength

• Elastic modulus

• Toughness

On a small set of specimens subjected to a simple tensile test

Acknowledgments

CSREES/USDA NRI CGP #2008-01522 competitive

grant

Lech Muszyński

John Nairn

John Simonsen

All graduate students in my project group

jie.ding@oregonstate.edu