Imperial College London

Imperial College London

‘Aligning Carbon Nanofibres and Graphene Nanoplatelets using an AC External Electric-Field to Improve the Multifunctional Properties of Epoxy-Nanocomposites’ Anthony J. Kinloch1, Ehsan Bafekrpour2, Kamran Ghorbani3, Raj B. Ladani3, Adrian P. Mouritz3, Chun H. Wang3, Shuying Wu3 and Jin Zhang4 1Department

of Mechanical Engineering, Imperial College London, London SW7 2AZ,

U.K. 2School

of Fashion and Textiles, RMIT University, Brunswick, Melbourne, 3056, Australia.

3Sir

Lawrence Wackett Aerospace Research Centre, School of Aerospace, Mechanical & Manufacturing Engineering, RMIT University, GPO Box 2476, Melbourne, VIC 3001, Australia.

4Institute

for Frontier Materials, Deakin University, Geelong Waurn Ponds Campus, VIC 3220, Australia.

3rd Progress Meeting

Imperial College London Outline of Talk 1. Introduction 2. Aims 3. Materials 4. Specimen Preparation Methods 5. Characterisation of the Structure of the Epoxy Nanocomposites 6. Properties of the Epoxy Polymer Nanocomposites 7. Toughening Mechanisms and Modelling Studies 8. Concluding Remarks 3rd Progress Meeting

Imperial College London 1. Introduction There is a need to improve the mechanical and electrical properties crosslinked (i.e. thermosetting) epoxy polymers.

• • •

They tend to be very brittle polymers. Are electrical insulators. And have poor thermal conductivity.

These may be major disadvantages for many application such as:

• •

Adhesives for ‘chip bonding & underfills’ in circuit boards.

•

Conductive coatings, e.g. for NDT or lightening-strike protection.

Adhesive and matrices for composites where electrical conductivity and good toughness are required.


Imperial College London 2. Aims Main aims are:

• To study the inclusion of carbon nanofibres (CNFs) or graphene nanoplatelets (GnPs) in a thermosetting, epoxy polymer.

• To study the epoxy polymer nanocomposite as an adhesive layer bonding substrates of carbon-fibre reinforced plastic (CFRP).

• To measure the toughness, electrical conductivity and the thermal conductivity as a function of the content of CNFs or GnPs.

• To study the use of an AC electric-field to align the CNFs or GnPs.

• To assess the effects of aligning the CNFs or GnPs.



3. Materials


Imperial College London Materials: Carbon Nanofibres (CNFs)

n nanofibres • Pyrograf®III PR-24-

•

HHT (Applied Sciences Inc.). Diameter: 70-200 nm; Length : 10-100 µm.

Graphene Nanoplatelet


Imperial College London Materials: Graphene Nanoplatelets (GnPs)

• Multi-layer graphene flakes (thickness=20 nm, diameter=5 µm). • XG-Science Inc.


Imperial College London Materials: Epoxy resin system and CFRP substrates The epoxy resin system was: • A bisphenol A/F resin mixture cured with a blend of aliphatic amines.

• •

Cured at 25oC for 48 h. 0 to 1.6 wt% of CNFs or GnPs were used.

The substrate: • CFRP used, which was ‘T700’ carbon fibre with an epoxy matrix.

• •

Cured at 120oC for 1h prior to bonding. The substrates were lightly abraded and solvent cleaned prior to being bonded. 3rd Progress Meeting


4. Specimen Preparation Methods


Imperial College London Mixing the CNFs or GnPs into the epoxy + hardener

• CNFs or GnPs were dispersed in the epoxy resin and hardener using a three-roll mill.

• Then alignment was achieved via an AC electric-field, before curing the epoxy resin.

• Property measurements:  Fracture energy, GIc.  Electrical conductivity.  Thermal conductivity.


Imperial College London Alignment procedure for the double-cantilever beam tests

•

Epoxy adhesive layer: Bisphenol-A/F based epoxy resin with a blend of amine curing agents.

• •

Substrates: 12 plies of cured CFRP. For example: O (a) mi t

Epoxy/xGnPs/hardener i AC Power Supply

Cured CFRP Aluminum substrates Electrodes

Silicon rubber mold/Spacer 3rd Progress Meeting

Imperial College London Mode I: The double cantilever beam (DCB) test

This is ISO 25217: 2009 test method for measuring the toughness, GIc, of an adhesive.

Failure always occurred through the centre of the epoxy polymer nanocomposite layer. 3rd Progress Meeting


5. Characterisation of the Structure of the Epoxy Nanocomposites


Imperial College London Electric-field alignment: Model calculations for GnP epoxy-resin nanocomposites

50

(e)

40

900

Distance (µm)

Rotation time (s)

1200 (d)

600 300 0 0

20 40 60 o 80 Initial angle θo ( )

The rotation time (to align) as a function of the initial angle (away from the alignment direction) for the GnPs.

30 20 10 0 0

50

100 150 200 250 Time (s)

The distance from two oppositely charged GnP ends as a function of time. (So, the time for ‘chain’ formation is shown.)

And good agreement between the above predictions and the 3rd Progress Meeting microscopy measurements was observed.

Imperial College London Electric-field alignment: GnP epoxy nanocomposites Applied an AC field of 30 V/mm at 10 KHz - before the epoxy resin cured.

CNFs

Note the ‘chain’ formation of the GnPs.

GnPs: 0.1 wt%


Imperial College London Electric-field alignment: CNF epoxy nanocomposites

0 minutes

CNFs

5 minutes

1

5 minutes

CNFs: 0.1 wt% Applied an AC field of 30 V/mm at 10 KHz before the epoxy resin cured. 3rd Progress Meeting


6. Properties of the Epoxy Polymer Nanocomposites



6.1 Electrical and Thermal Properties


Imperial College London AC conductivity of the epoxy polymer nanocomposites 2

(a)10

1

10

AC Conductivity (S/m)

100 10-1

0.1 Random 0.4 Random 0.7 Random 1.0 Random 1.6 Random Neat Epoxy

0.1 Aligned 0.4 Aligned 0.7 Aligned 1.0 Aligned 1.6 Aligned Model Equation (1)

10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 1 10

102

103 Frequency (Hz)

104

105

CNF-epoxy polymer nanocomposites

GNP-epoxy polymer nanocomposites 3rd Progress Meeting

Imperial College London Electrical conductivity - conductive network formation A: At low nanofiller content: B: Increasing the nanofiller content: low electrical conductivity no complete conductive path, but similar to polymer matrix. electrical conductivity increases gradually due to tunnelling effects among the neighbouring CNTs.

Polymer CNFs

C: At the percolation threshold concentration: the first complete electrically-conductive path forms, electrical conductivity of the nanocomposite increases remarkably, following a percolation power law.

Tunnelling effects

Complete electrically conductive path formed 3rd Progress Meeting

Imperial College London Thermal conductivity of the GnP epoxy polymer nanocomposites

Thermal conductivity (W/mK)

During alignment

0.50

Randomly-oriented Parallel to alignment Transverse to alignment

0.45 0.40 0.35 0.30

0.0 has0.5 1.0 1.5 2.0 Frequency of AC field little effect Graphene3content (Wt %) Progress Meeting rd


6.2 Fracture Energy, GIc


Imperial College London Values of GIc for the CNF epoxy polymer nanocomposites Random Aligned

2500

2

GIc (J/m )

2000

1500

1000 Neat Resin 500

0

0

0.1

0.4

0.7

1.0

1.6

CNF wt% For the aligned nanocomposites, the crack grows transverse to the direction of alignment. 3rd Progress Meeting

Imperial College London Values of GIc for the GnP epoxy nanocomposites

1800 1500

Randomly-oriented Transverse to alignment Parallel to alignment

GIc/(J/m2)

1200

During alignment For the aligned nanocomposites, the crack may grow either transverse, or parallel, to the direction of alignment.

900 600 300

0 Frequency0.0of AC has little 0.5field 1.0 1.5 effect 2.0 Graphene content (Wt %) 3 Progress Meeting rd


7. Toughening Mechanisms and Modelling Studies



The main toughening mechanisms identified were:

• Interfacial debonding of the nano-reinforcements. • Plastic void growth which initiates from the microvoids created by the debonded nano-reinforcements.

• The energy dissipated by pull-out and rupture of the nanoreinforcements.

CNFs

GnPs


Imperial College London The pull-out of CNFs in the epoxy nanocomposites no

Progress Meeting 1.5 wt%.3rdaligned

Imperial College London SEM images of CNFs bridging the crack in the epoxy nanocomposites

0.4 wt%. random

0.7 wt%. random

0.4 wt%. aligned

0.7 wt%. aligned


Imperial College London SEM image of CNFs bridging the crack in the epoxy nanocomposites

pp

1.0 wt% aligned CNFs 3rd Progress Meeting

Imperial College London Modelling the toughness of the CNF epoxy nanocomposites - 1

GIc GCU

GIc = GCU + ∆Grupture + ∆Gpull−out + ∆Gdb + ∆Gv : the fracture energy of the CNF epoxy nanocomposite. : fracture energy of the ‘unmodified’ epoxy polymer.

∆Grupture : the contribution from the rupture of the CNFs. ∆Gpull-out : the contribution from the pull-out of the CNFs. ∆Gdb

: the contribution from the debonding of the CNFs from the matrix.

∆Gv

: the contribution from plastic void growth of the matrix around the debonded CNF.

• Equations are derived in Comp. Sci. Tech. 17, 2015, 146-158. • The above contributions are defined by: 3rd Progress Meeting

Imperial College London Modelling the toughness of the CNF epoxy nanocomposites - 2 ∆Grupture =

Vfpo σf lf εmax 2

Vfpo σ2f df ∆∆∆Gpull−out = 8τi ∆Gdb =

=

Vfpo σ2f lf 2 Ef

Vf lpo Gi df

∆Gv = 1 +

µm

3

2

(Vvoid − Vfpo )σy ryu K 2vm

Where: Vfpo is the volume fraction of CNFs which are pulled out; σf is the tensile strength of the CNF; lf is the length of the CNF; εmax is the tensile failure strain of the CNF; E is the tensile modulus of the CNF; df is the diameter of the CNF; τi is the interfacial shear strength between the CNF and the matrix; Vf is the volume fraction of CNFs; lpo is the CNF pull-out length; Gi is the interfacial fracture energy; µm is a constant which allows for the pressure dependency of the yield stress of the epoxy; Vvoid is the volume fraction of voids; σy is the yield stress of the epoxy polymer; ryu is the radius of the plastic zone on the ‘control’ epoxy; Kvm is the maximum stress concentration for the von Mises stress around a debonded CNF. rd Progress Meeting All these parameters may be3directly measured or calculated.

Imperial College London Modelling the toughness of the CNF epoxy nanocomposites - 3

(b) 1

Neat Epoxy Random Experimental Aligned Experimental Model Random N1 counted po

3.0

GIc (kJ/m2)

2.5

Model Aligned Napo counted

2.0 1.5 1.0 0.5 0.0 0.1

0.4

0.7 1 1.3 rd 3 Progress Meeting CNF (wt%)

1.6


8. Concluding Remarks



• CNFs or GNPs were dispersed in a liquid epoxy resin using a threeroll mill to give a good degree of dispersion.

• An AC electric-field aligned the CNFs or GnPs to form a chain-like structure in the epoxy resin, which was then cured to give the epoxy polymer nanocomposite.

• CNFs are more effective than GnPs in improving toughness and conductivity.

• Alignment has a significant effect at low concentration levels, but the improvement diminishes at relatively high concentrations.

• The highest increases in toughness and conductivity were for the epoxy nanocomposites containing 1.6 wt% of aligned CNFs.

• The fracture energy and the electrical conductivity of this CNF nanocomposite were increased by about 1600% (from 134 to 2345 J/m2) and seven orders of magnitude compared to the unmodified epoxy, respectively.

• The main toughening mechanisms have been identified and modelled.



Many thanks to the Australian Research Council for their funding of this work. And thank you for your attention!
