NN5

Effect of Fabrication and Electrical Testing on

the Measured Performance of Thermoplastic

CNT Composites

Jane M. Spikowski

PolyOne Corporation

ACCE 2013

Novi, MI

September 11-13, 2013

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Formulation Polymer Science Chemistry Processing

Inputs Inputs

Expertise Expertise

Sheet, Film, and Roll Stock Sheet, Film, and Roll Stock

Advanced Composites Advanced Composites

Measurement Challenges in CNT Composites

Multi-phase materials

Dispersion

Distribution

High sensitivity due to low filler concentration

Processing-dependent orientation of conductive particles

Injection molding vs. compression molding

Potential for large effect due to high aspect ratio of CNT

Dispersion

Processing

Product

Forming

Categories of Electrical Testing Methods

1. Standardized methods reported on datasheets

2. Methods to understand electrical behavior mechanistically

3. Application-driven testing the true test of a materials performance

Two ASTM D257 test configurations MIL-DTL-83528C

Two-Probe vs. Four-Probe Measurement Techniques

Current flows through

measurement circuit

Contact resistance and

measured resistance are not

independent

Two-Probe

V

Four-Probe

Test Specimen

Current Source

Resistance

Meter Voltmeter

Contact

Resistance

Current does not flow through

measurement circuit

Contact resistance is

independent of measured

resistance

V

Two-Probe Silver Paint End Probe Method

Common variation of ASTM D4496

Best opportunity for current to travel through the entire cross-section of

the part

Purpose: Eliminate the potentially insulative skin layer from the electrical

circuit

Two-probe method susceptible to contact resistance

Two-Probe Silver Paint Surface Probe Method

Available current path is not restricted to the surface of the part -

opportunity for an in-plane volume resistance measurement

MIL-DTL-83528C Basis for interpretation as a volume resistance method (ASTM D257 assumes current travels across surface only)

Current must pass through insulative skin layer, if present

Same measurement area as end probe method

Effective measurement area

(MIL-DTL-83528C)

Current and

voltage electrodes

Effective measurement area

Materials

Two formulations: 3% and 5% multi-walled CNT in PC

Compounded using 18mm co-rotating intermeshing twin-screw extruder

22 Factorial DOE based on previously identified key factors:

Melt temperature (as controlled by varying the barrel temperatures) Molding temperature

Injection speed

Injection molded into disk, rectangular bar, and tensile bar:

Two-Probe Silver Paint Surface vs. End Probe

Lowest resistivity at high temperature and low speed as measured by

both methods

Statistically significant difference between surface vs. end probe

measurements for 3% CNT molded using fast speed

[Low, Low] [Low, High] [High, High] [High, Low]0

2

4

6

8

10

Lo

g(V

olu

me

Resis

tivity [

-cm

])

Molding Condition Set [Temperature, Speed]

3% Surface

3% End

5% Surface

5% End

Silver Paint Method Customization

Both silver paint electrode configurations can be used for bulk or localized

testing

Opportunity to reveal different features of electrical performance

Highly customizable test techniques applicable to complex parts

BULK LOCALIZED

Localized Silver Paint Electrode Results

End vs. surface probe difference increases away from gate

As residence time in mold , uniformity in thickness direction

Relaxation of shear induced stresses introduced by high injection speed

FAST

Injection

Speed

Localized Silver Paint Electrode Results

No significant difference between end vs. surface probes

Less sensitive to residence time effects due to lower shear-induced

stresses

SLOW

Injection

Speed

Design Space Using Two-Probe Test Methods

From the same starting material, varying the molding conditions and test

method provides a wide range of measured properties:

Effective specification requires more than just a target resistivity value

Formulation provides coarse tuning

Processing provides fine tuning

Insulating

Anti-Static 109 1012

Static Dissipative 106 109

Conductive

Conclusions:

Electrical Testing and Injection Molding Effects

Each test configuration revealed that increasing the melt temperature and

decreasing the injection speed minimizes the resistivity

Different methods can result in drastically different absolute

measurements of the same specimen

Using a combination of methods enables the detection of more detailed

features

Residence time effects

Variation in the thickness direction

Silver paint electrode methods are highly customizable and compatible

with complex geometries

Secondary Heating (Annealing) Effects

Annealing Effects Bulk Measurements

Heated injection molded parts above Tg using a press low-strain

For all molding conditions, resistance decreased as annealing time

increased

Bulk silver paint

surface resistance

method

Annealing Effects Local Measurements

Annealing eliminated the discrepancy between surface and end probe

measurements enhancement of conductive CNT network

Suggests relaxation of molded-in strain and nano-scale rearrangement

High Speed Low Speed

Gate End

Conclusions

Each forming process after initial compounding can affect conductive

network

Changes can occur in the quiescent state in the melt without additional

mixing or CNT dispersion

Relaxation of strains introduced before last cooling step

Nano-scale CNT rearrangement and association

Heat history can have a significant effect on properties of thermoplastic

CNT composites

Acknowledgements and Contact Information

The authors would like to acknowledge:

Glenn Evans

Rachel Winkelman

Polymer Diagnostics, Inc.

Ohio Department of Development

Contact Information:

[email protected]

440.930.1127

NN5

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