Influence of Graphite and Carbon Nanotubes on the Mechanical and Electrical Properties of Cast Epoxy Composites

Clarissa F. M. de Souza 1, a, Janaina L. Leite 1,2,b , Gean V. Salmoria 1,c and Antonio Sergio Pouzada 3,d

11Universidade Federal de Santa Catarina (UFSC), CIMJECT – Dep. Eng. Mecânica, 88040-900, Florianópolis, SC, Brazil

2 Instituto Superior Tupy - Sociedade Educacional de Santa Catarina (SOCIESC), Mestrado Profissional em Engenharia Mecânica , Joinville 89206-001, SC, Brazil

3 Institute for Polymers and Composites/I3N, University of Minho, Campus de Azurém, 4800-058 Guimarães, Portugal

a clarissafmdesouza@yahoo.com.br, b janaina.leite@sociesc.org.br, c gsalmoria@cimject.ufsc.br, d asp@dep.uminho.pt (corresponding author)

Keywords: Epoxy resin; graphite; carbon nanotubes; composites.

Abstract. This study evaluates the influence of graphite and multi-wall carbon nanotubes on the

mechanical and electric properties of cast epoxy resin. The epoxy resin based composites were

prepared with various graphite and MWNCT content up to 5.0%. Specimens were characterized by

DMA, SEM and electric resistivity tests. The observation of fracture surfaces showed a reasonable

dispersion of graphite and MWCNT into the epoxy matrix. The graphite and MWCNT have almost

the same effect in the electric conductivity of the epoxy composites at low content (0.2 and 0.5 %).

The MWCNT composites seem to reach percolation at concentrations near 0.5 % whereas graphite

composites reach it at 2%. Higher concentration of graphite and MWCNT have limited effect in the

electric conductivity but reduces mechanical properties.

Introduction

In rapid manufacturing technology (RM), the process of casting resin composites has been widely

used for making functional parts and tools because it can be done easily and quickly [1, 2]. The

casting process based on thermosetting resin composites is an economic alternative for the

production of structural parts and tools comparing to other rapid tooling process [1-7]. The use of

graphite and carbon nanotubes (CNT) in composites has been attracting scientific and industrial

interest by virtue of their characteristics, i.e., superior thermal, electric and mechanical properties

[8-13]. Resulting from the carbon nanotube structure (tiny concentric graphene cylinders) they have

a high thermal conductivity that can be either semi-conducting or metal-like. Their extremely high

strength-to-weight ratio, very high Young modulus and high flexibility and toughness, make them

candidate materials for future reinforced polymer matrix composites [8, 9, 12].

The epoxy resins are well established as thermosetting matrices of advanced composites, with

promising characteristics for a wide range application. They have excellent electrical insulation and

mechanical properties. The incorporation of graphite and multi wall carbon nanotubes (MWCNT)

into epoxy resin bulk material surely enhances the electric, thermal and mechanical properties of the

epoxy resin but also modifies their processing. In epoxy composites with graphite and carbon

nanotube reinforcements, the dispersion of the carbon nanotubes and the interfacial adhesion

between the phases, matrix and reinforcement, are important factors that influence the properties of

the composite [4, 12]. This work evaluates the incorporation of graphite and MWCNT on the

mechanical and electric properties of cast epoxy composites.

Materials Science Forum Vols. 730-732 (2013) pp 909-914Online available since 2012/Nov/12 at www.scientific.net© (2013) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.730-732.909

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Experimental

Raw Materials. The epoxy system used in this study was TCR 550 based on bisphenol-A and XR

435 as hardener from Central Fiber Glass (Brazil). The graphite and MWNT were obtained from

MER Corp. (USA). The graphite has an average particle size of 50 µm. The MWNT were

synthesized by chemical vapour deposition. The properties of the MWCNT are: purity over 90 %,

average diameter of 140±30 nm, and average length of 7 ± 2µm.

Composites Preparation. The graphite and MWNTs were added into epoxy resin with 0.5%, 2%

and 5% (w/w). The mixtures (epoxy/graphite and epoxy/MWCNT) were carefully stirring by hand,

so the mixtures were kept in a vacuum oven for 4h to remove the air bubbles. The hardener was

added into the mixtures at the resin-to-hardener weight ratio of 2:1 and was hand stirred. The

mixtures (resin, fillers and hardener) were cast into a silicon mould. The mouldings were cured at

room temperature for 20 h and after that post-cured in an oven at 100ºC during 8 h.

Dynamical-mechanical Thermal Analysis (DMA). The mechanical analysis of the epoxy

composites was carried out by dynamical-mechanical analysis using Q800 TA Instruments

equipment. The clamping method used for the analysis was single cantilever. The test was carried

out at 30ºC with a force rate of 2 N/min up to 18 N. The specimen dimensions were 35×5×1.4 mm.

The test to obtain the loss tangent (tan δ) curves was performed from 30ºC up to 140

ºC. The heating

rate was 3°C/min, the frequency: 1Hz, and the maximum strain: 0.5%.

Scanning Electron Microscope (SEM). The epoxy/graphite and epoxy /MWCNT composites were

cryogenic fractured for observation of the microstructure by SEM using a Phillips XL30 scanning

electron microscope. The specimens were coated with gold in a Bal-Tec Sputter Coater SCD005.

Electric Resistivity Measurements. The electrical conductivity of the epoxy composites was

determined using a bi-directional two probe test. With this method it is possible to obtain a direct

measure of the electrical resistivity from the voltage and the DC current that flows through the

sample [14]. The test was done with an electrometer Keithley Instruments 6517. The voltage data

were recorded by varying the electrical current in the range from 5×10-9

to 10-3

A. The test samples

were square plates of 10 mm and thickness of 1 mm.

Results and Discussion

Mechanical and Viscoelastic Behaviour. The mechanical behaviour of the graphite/epoxy

composites is described by the stress-strain curves in Fig. 1 and the data in Table 1. These data were

obtained by DMA at constant strain rate of 2 N.m-1

in single-cantilever flexural testing mode. The

graphite/epoxy composites show increasing tensile strength and flexural modulus with the increase

in graphite fraction up to 2.0%. The decrease in the tensile strength and flexural modulus in

composites with 5.0% graphite can be related to the inefficient dispersion observed with larger

graphite fraction, resulting in the formation of clusters that act as solid lubricants in the composite.

The stress-strain curves for the epoxy/MWNT composites with various MWNT fractions (0%,

0.5%, 2.0% and 5.0%), obtained by flexural mechanical analysis are shown in Fig. 2 and Table 1.

The curves showed a slight increase of the ultimate strength with the increase in MWCNT. This is

probably due to the weak interfacial bonding between MWCNT and the epoxy resin, and also to the

inefficient dispersion of MWCNT. The mechanical properties showed the small increase in ultimate

strength with the increase of the MWCNT fraction. The mechanical properties were expected to be

better, but the inefficient dispersion and weak interfacial bonding contributed to those results.

910 Advanced Materials Forum VI

0 1 2 3 4 5 6 7

0

20

40

60

80

100

Str

ess (

MP

a)

Strain (%)

Fig. 1 – Stress - strain curves for

graphite/epoxy composites with (▲) 0%,

(■)0,5%, (●)2,0% and (�) 5.0% of graphite

(w/w).

0 2 4 6 8 10

0

20

40

60

80

100

Str

ess (

MP

a)

Strain (%) Fig. 2 – Stress versus strain curves for epoxy

specimens/MWCNT with (■)0%, (●)0.5%,

(▲)2.0%, (�)5.0% of MWCNT (w/w).

Table 1 – Mechanical properties of the epoxy composites.

Filler

[%] Flexural modulus

[MPa] Tensile strength

[MPa] Elongation at break

[%]

Graphite

0.0 1778.3 ± 79.50 91.46 ± 1.35 6.65 ± 0.28

0.5 1923.0 ± 109.30 92.45 ± 2.30 5.80 ± 0.70

2.0 2719.5 ± 77.10 101.4 ± 1.80 4.20 ± 0.24

5.0 2056.0 ± 35.35 72.90 ± 4.80 4.40 ± 0.75

MWCNT

0.0 1778.3 ± 79.50 91.46 ± 1.35 6.65 ± 0.28

0.5 1724 ± 27.57 94.49 ± 0.28 9.20 ± 0.70

2.0 1764 ± 33.85 95.66 ± 3.51 8.40 ± 0.99

5.0 1882 ± 60.81 94.74 ± 7.31 7.71 ± 0.53

Figure 3 shows how the loss tangent of graphite/epoxy composites varies with various graphite

fractions and temperatures. The neat epoxy showed a transition at 92.0ºC (when tan δ reached the

maximum in the curve, corresponding to the glass transition, Tg). The Tg of composites with 0.5%

graphite fraction was 89.2ºC, and for 2.0 and 5.0% graphite fraction were 91.5ºC. The rise of Tg

with the increasing graphite fraction, confirms the rigidity increment in composites with higher

graphite fraction. The tan δ curves of the graphite/epoxy composites have a narrower distribution

when the graphite fraction increases. This is caused by the mobility of the polymer chains being

reduced by the rigid graphite particles.

Figure 4 shows tan δ curves as a function of temperature for epoxy/MWCNT composites with

various MWCNT fractions (0%, 0.5%, 2.0% and 5.0%). The Tg of the neat epoxy resin when tan δ

reached the maximum is ca. 92ºC. The Tg of epoxy/MWCNT composites with 0.5, 2.0 and 5.0% of

MWCNT fraction were 86ºC, 85

ºC and 86

ºC, respectively. The addition of 0.5% and 5.0% of

MWCNT narrowed the distribution in comparison the neat epoxy resin. These results are relatable

to the change in polymer molecules mobility caused by the nanotubes.

Materials Science Forum Vols. 730-732 911

20 40 60 80 100 120 140

0,0

0,2

0,4

0,6

0,8

1,0T

an δ

Temperature (0C)

Fig. 3 – Tan δ curves for graphite/epoxy

composites with variable graphite

concentration. (▲) 0%, (■) 0,5%, (●) 2,0%,

(�) 5.0%.

30 60 90 120 150

0,0

0,2

0,4

0,6

0,8

Tanδ

Temperature (0C)

Figure 4 – Tan δ curves for MWCNT/epoxy

composites specimens with (■)0%, (●)0.5%,

(▲)2.0%, (�)5.0% of MWCNT (w/w).

Microstructure. The observation of fractured surface of graphite/epoxy composites show a surface

irregularity increasing when the graphite fraction increases as exemplified in Fig. 5 for the extreme

percentages of filling (0% and 5%). The white arrows indicate the voids caused by the

polymerisation of the epoxy system. The reaction gases are difficult to remove even keeping the

mixture under vacuum during the reaction. The black arrows show the dispersion of graphite

agglomerates and particles in the matrix. The inefficient dispersion in the cases of high graphite

fraction can be confirmed in the Figure 5b. The agglomerates have a detrimental effect on the

mechanical properties of the composites with high graphite fraction.

Fig. 5 - Fractured surface of graphite/epoxy composites with 0% of graphite (50× magnification) (a)

and 5.0% (120× magnification) (b).

Examples of micrographs of the cryogenic fractured surfaces of the epoxy/MWCNT composites

with various MWCNT fractions are shown in Fig. 6. These micrographs showed a slight change on

the fracture surface that becomes rougher due to the addition the MWCNT. The addition of

MWCNT into epoxy resin by mechanical mixing methods permits the formation of agglomerates

and some air bubbles, as can observed in the micrographs (indicate by the narrows, Fig. 6 b). This

fact affects the mechanical properties as mentioned previously.

(a) (b)

912 Advanced Materials Forum VI

Fig. 6 - Micrographs of the surface cryogenic fracture of epoxy/MWCNT composites with 0% (a)

and 5.0% (b) of MWCNT (w/w) at 50× magnification.

Electrical Conductivity. The electrical conductivity of the graphite and MWCNT epoxy

composites are affected by the filler system used as shown in Fig. 7. The measurements on the

compositions that were studied showed that this graphite and the MWCNT have almost the same

effect in the conductivity of the epoxy composites when low fractions (0.2 and 0.5 %) are used. The

MWCNT composites seem to reach the percolation at concentrations near 0.5 % and graphite

composites near of 2% of filler content. Higher concentration of MWCNT and graphite seems to

have limited effect in the electric conductivity as they had in the mechanical properties.

Fig.7- Electrical conductivity of composites with graphite and MWCNT epoxy composites.

Conclusions

The mechanical properties of the epoxy/graphite composites increase when the graphite fraction

goes up to 2.0%. The micrographs of these composites showed voids resulting from the

polymerisation of the epoxy system. The graphite reinforcement tended to agglomerate in the epoxy

matrix, indicating the inefficient dispersion especially when high graphite fractions are used.

The mechanical performance of the epoxy/MWCNT composites is not very much dependent on the

MWCNT content. The mechanical properties were expected to be higher, but the poor dispersion

and weak interfacial bonding contributed to those results. The microscopy analysis of the composite

structure showed voiding and the formation of MWCNT agglomerates.

The graphite and MWCNT have a similar effect on the electric conductivity of the composites at

low content (0.2 and 0.5 %). The MWCNT composites seem to reach the percolation at near 0.5%

whereas the graphite composites reach that at 2% of filler content. Higher concentration of graphite

and MWCNT seems to have limited effect in electric conductivity and mechanical properties.

(a) (b)

Materials Science Forum Vols. 730-732 913

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Advanced Materials Forum VI 10.4028/www.scientific.net/MSF.730-732 Influence of Graphite and Carbon Nanotubes on the Mechanical and Electrical Properties of Cast

Epoxy Composites 10.4028/www.scientific.net/MSF.730-732.909

DOI References

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Injeção, Polímeros 18 (2008) 262-269.

http://dx.doi.org/10.1590/S0104-14282008000300013 [3] S. I. Chung, Y. G. Im, H. D. Jeong and T. Nakagawa, The effects of metal filler on the characteristics of

casting resin for semi-metallic soft tools, Journal of Materials Processing Technology 134 (2003) 26-34.

http://dx.doi.org/10.1016/S0924-0136(02)00275-3 [8] I. Novák and I. Krupa, Electro-conductive resins filled with graphite for casting applications, European

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(2005) 1555-1561.

http://dx.doi.org/10.1016/j.compositesa.2005.02.006 [10] J. Li, J. -K. Kim and I. M. L. Sham, Conductive graphite nanoplatelet/epoxy nanocomposites: Effects of

exfoliation and UV/ozone treatment of graphite, Scripta Materialia 53 (2005) 235- 240.

http://dx.doi.org/10.1016/j.scriptamat.2005.03.034 [11] H. Chen, O. Jacobs, W. Wu, G. Rüdiger and B. Schädel, Effect of dispersion method on tribological

properties of carbon nanotube reinforced epoxy resin composites, Polymer Testing 26 (2007) 351-360.

http://dx.doi.org/10.1016/j.polymertesting.2006.11.004 [12] J. Shen, W. Huang, L. Wu, Y. Hu and M. Ye, Thermo-physical properties of epoxy nanocomposites

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http://dx.doi.org/10.1590/S1516-14392008000300019