polymers
Review
Mechanical, Thermal, and Electrical Properties of
Graphene-Epoxy Nanocomposites—A Review
Rasheed Atif, Islam Shyha and Fawad Inam *
Department of Mechanical and Construction Engineering, Faculty of Engineering and Environment,
Northumbria University, Newcastle upon Tyne NE1 8ST, UK; aatif.rasheed@northumbria.ac.uk (R.A.);
islam.shyha@northumbria.ac.uk (I.S.)
* Correspondence: fawad.inam@northumbria.ac.uk; Tel.: +44-191-227-3741
Academic Editor: Antonio Pizzi
Received: 11 June 2016; Accepted: 25 July 2016; Published: 4 August 2016
Abstract:
Monolithic epoxy, because of its brittleness, cannot prevent crack propagation and is
vulnerable to fracture. However, it is well established that when reinforced—especially by nano-fillers,
such as metallic oxides, clays, carbon nanotubes, and other carbonaceous materials—its ability to
withstand crack propagation is propitiously improved. Among various nano-fillers, graphene has
recently been employed as reinforcement in epoxy to enhance the fracture related properties of
the produced epoxy–graphene nanocomposites. In this review, mechanical, thermal, and electrical
properties of graphene reinforced epoxy nanocomposites will be correlated with the topographical
features, morphology, weight fraction, dispersion state, and surface functionalization of graphene.
The factors in which contrasting results were reported in the literature are highlighted, such as the
influence of graphene on the mechanical properties of epoxy nanocomposites. Furthermore, the
challenges to achieving the desired performance of polymer nanocomposites are also suggested
throughout the article.
Keywords:
mechanical properties; thermal properties; electrical properties; graphene;
epoxy; nanocomposites
1. Introduction
Polymer Matrix Composites (PMCs) have found extensive applications in aerospace, automotive,
and construction, owing to ease of processing and high strength-to-weight ratio, which is an important
property required for aerospace applications [
1
]. Among different polymers, epoxy is the most
commonly used thermosetting polymer matrix in PMCs [
2
]. The damage tolerance and fracture
toughness of epoxy can be enhanced with the incorporation of (nano-) reinforcement, such as metallic
oxides [
3
–
5
], clays [
6
–
8
], carbon nanotubes (CNTs) [
9
–
11
], and other carbonaceous materials [
12
–
14
].
After the groundbreaking experiments on the two-dimensional material graphene by Nobel Laureates
Sir Andre Geim and Konstantin Novoselov [
15
] from the University of Manchester, graphene came into
the limelight in the research community, mainly because of its excellent electrical [
16
], thermal [
17
], and
mechanical properties [
18
]. Graphene found widespread applications in electronics [
19
], bio-electric
sensors [
20
], energy technology [
21
], lithium batteries [
22
], aerospace [
23
], bio-engineering [
24
], and
various other fields of nanotechnology [
25
]. There is an exponential rise in the use of graphene in
different research areas, mainly because of the properties inherited in, and transferred by, graphene to
the processed graphene-based materials.
To summarize the research trends related to graphene-based nanocomposites, multiple review
articles were recently published in which various aspects of graphene-based nanocomposites were
discussed. There are numerous ways to produce and characterize graphene-based materials [
26
].
Graphene-based materials were studied for different properties, such as thermal properties [
27
],
Polymers 2016, 8, 281; doi:10.3390/polym8080281 www.mdpi.com/journal/polymers
Polymers 2016, 8, 281 2 of 37
mechanical properties [
28
], electrical properties [
29
], rheological properties [
30
], microwave
adsorption [
31
,
32
], environmental and toxicological impacts [
33
], effect of preparation [
34
], and
gas barrier properties [
35
]. These materials have found biological applications, especially related
to toxicity [
36
], and in other applications like electrically-conductive adhesives [
37
] and selective
photoredox reactions [
38
]. Because of their hierarchical pore structures, these materials were found
suitable for gas sorption, storage, and separation [
39
]. Various factors influence the mechanical
properties of graphene-based materials—e.g.,
γ
-ray irradiation was found to have a strong influence
on the structure–property relationship [
40
]. Various theoretical models were developed to predict
the mechanical properties of epoxy–graphene nanocompsites and correlated with interphases and
interfacial interactions [
41
]. It was presented that continuum mechanics can be used to predict the
minimum graphene sheet dimensions and optimum number of layers for good reinforcement [
42
].
Graphene was compared with other reinforcements, such as clays [
43
] and CNTs [
44
], and was shown
to have properties superior to the other nano-fillers. Various surface modifications were employed to
improve interfacial interactions, and their influence on the performance of polymer nanocomposites
was studied [45].
To date, eclectic reviews on graphene composites are covering a broad range of graphene-related
issues; it can, however, be observed that there is an obvious gap in the lack of a review article discussing
the mechanical, thermal, and electrical properties of epoxy–graphene nanocomposites. Therefore, this
review article discusses the correlation between graphene structure, morphology, weight fraction,
dispersion, surface modifications, and the corresponding mechanical, thermal, and electrical properties
of epoxy–graphene nanocomposites.
2. Epoxy as Matrix
There are various types of epoxy which have a wide range of applications because of their
superior attributes, such as improvement in composite mechanical properties, acceptable cost, and
processing flexibility [
2
]. Phenolic glycidyl ethers are formed by the condensation reaction between
epichlorohydrin and a phenol group. Within this class, the structure of the phenol-containing molecule
and the number of phenol groups per molecule distinguish different types of resins and the final
properties of monolithic epoxies and nanocomposites [
2
]. The epoxies have found some “high-end”
applications, including aerospace, marine, automotive, high-performance sports equipment (such
as tennis rackets), electronics, and industrial applications [
46
]. Due to the superior properties of
carbonaceous materials, such as high strength and stiffness, they are most widely used at present as
reinforcement in advanced Epoxy Matrix Composites (EMCs) [47–50].
Epoxy resins are of particular interest to structural engineers because these resins provide a unique
balance of chemical and mechanical properties combined with extreme processing versatility [
51
].
When a composite is produced from epoxy-carbon using hand lay-up process, a great flexibility
in aligning the fraction of fibers in a particular direction is available, which is dependent upon
the in-service load on the composite structural member. In-plane isotropy can also be achieved by
stacking the resin-impregnated fiber layers at equal numbers of 0
˝
, +45
˝
,
´
45
˝
, and 90
˝
. There are
also other stacking sequences that can be used to achieve in-plane isotropy. The specific stiffness
of quasi-isotropic epoxy–graphite laminated composite is higher than many structural metals. The
highest specific strength achieved in epoxy–graphite is higher than common structural metals, with
the exception of ultrahigh-strength steels and some
β
-titanium alloys. For example, the epoxy-carbon
crutch is 50% lighter and still stronger than the aluminium crutch [2].
3. Graphene as Reinforcement
Graphene—a densely packed honey-comb crystal lattice made of carbon atoms having a thickness
equal to the atomic size of one carbon atom—has revolutionized the scientific parlance due to
its exceptional physical, electrical, and chemical properties. The graphene now found in various
applications was previously considered only a research material and a theoretical model to describe
Polymers 2016, 8, 281 3 of 37
the properties of other carbonaceous materials such as fullerenes, graphite, Single-Walled Carbon
Nanotubes (SWNTs), and Multi-Walled Carbon Nanotubes (MWNTs). It was believed that the real
existence of stand-alone single layer graphene would not be possible because of thermal fluctuations,
as the stability of long-range crystalline order found in graphene was considered impossible at finite
(room) temperatures. This perception was turned into belief by experiments when the stability of
thin films was found to have direct relation with the film thickness; i.e., film stability decreases with
a decrease in film thickness [
52
]. However, graphene can currently be found on a silicon substrate
or suspended in a liquid and ready for processing. Although its industrial applications are not
ubiquitous, it is widely used for research purposes (e.g., as reinforcement in PMCs) and has shown
significant improvement in different (mechanical, thermal, electrical etc.) properties of produced
nanocomposites [52–56].
The ability of a material to resist the propagation of an advancing crack is vital to the prevention
of failure/fracture [
57
]. Graphene can significantly improve fracture toughness of epoxy at very low
volume fraction by deflecting the advancing crack in the matrix. The details of the influence of various
kinds of graphene/graphite nanoplatelets (GNPs) on the fracture toughness of epoxy nanocomposites
are listed in Table 1. In all the composite systems mentioned in Table 1, epoxy was used as matrix
and the nanocomposites were produced using solution casting technique, except [
58
] where the resin
infiltration method was employed. The incorporation of graphene in epoxy can increase its fracture
toughness by as much as 131% [
59
]. It can also be observed that graphene size, weight fraction, surface
modification, and dispersion mode have strong influence on the improvement in fracture toughness
values of the produced epoxy–graphene nanocomposites. Monolithic epoxy shows brittle fracture and
beeline crack propagates, which results in straight fracture surfaces. The advancing crack in epoxy
interacts with the graphene sheets. Initially, the crack propagates through the epoxy matrix as there
are no significant intrinsic mechanisms available in monolithic epoxy to restrict crack propagation.
However, no sooner than the crack faces strong graphene sheets ahead, it surrenders and subdues.
Nevertheless, the extent of matrix strengthening and crack bridging provided by graphene strongly
depends upon its dispersion state and interfacial interactions with the epoxy matrix [60,61].
4. Fracture Toughness
The successful employment of epoxy-based nanocomposites relies on the ability of the composite
system to meet design and service requirements. The epoxy-based nanocomposites have found
applications in aerospace, automotive, and construction due to ease of processing and high
strength-to-weight ratio. In many applications, the composite system undergoes external loadings.
The relationship between loads acting on a system and the response of the system towards the
applied loads is studied in terms of mechanical properties. Therefore, epoxy-based nanocomposites
are supposed to have superior mechanical properties. There are various tests to measure mechanical
properties, such as tensile testing, bend testing, creep testing, fatigue testing, and hardness testing,
to name a few. These tests usually take specimens of specific geometries and subject to loading at
certain rate. In general, the industrial scale samples contain porosity and notches which act as stress
concentrators and are deleterious to the mechanical properties of nanocomposites. Sometimes, it
becomes difficult to control the maximum flaw size. The shape of the flaw is another very important
parameter, as pointed notch (V-notch) is more detrimental than round notch (U-shaped) [62].
Polymers 2016, 8, 281 4 of 37
Table 1. A brief record of epoxy-based nanocomposites studied for improvement in fracture toughness values.
Sr. Authors Year Reinforcement/(wt %)
Dispersion
method
% Increase in
K
1C
(MPa¨m
1/2
)
Remarks Ref.
1
Wan et al.
2014
GO (0.25 wt %)
Sn + BM
25.6
K
1C
drops after 0.25 wt %
of reinforcement
[
63]
DGEBA-f-GO (0.25 wt %) 40.7
2 Sharmila et al. 2014 MERGO (0.25 wt %) MS + USn 63
K
1C
drops after 0.25 wt %
of reinforcement
[64]
3
Zhang et al.
2014
GnPs (0.5 wt %)
Sn
27.6 Trend still increasing
[
65]
fGnPs (0.3 wt %) 50.5
K
1C
drops after 0.3 wt % of
reinforcement
4
Moghadam et al.
2014
UG (0.5 wt %)
3RM
55
K
1C
drops after 0.5 wt % of
reinforcement
[
66]
GO (0.5 wt %) 57
G-NH
2
(0.5 wt %) 86
G-Si (0.5 wt %) 86
5 Ma et al. 2014 m-GnP (1 wt %) MS + Sn 131
K
1C
drops after 1 wt % of
reinforcement of m-GnP
[59]
6
Chandrasekaran et al.
2014
TRGO (0.5 wt %)
3RM
44.5 Trend still increasing
[
67]
GNP (1 wt %) 49 K
1C
drops after 1 wt %
MWCNTs (0.5 wt %) 12.7 Trend still increasing
7
Wan et al.
2014
GO (0.1 wt %)
Sn + BM
24
K
1C
improves with silane
functionalization
[
68]
Silane-f-GO (0.1 wt %) 39
8
Zaman et al.
2014
m-clay (2.5 wt %)
MS
38
K
1C
drops after 2.5 wt %
m-clay
[
69]
m-GP (4 wt %) 103 Trend still increasing
9 Jiang et al. 2014 SATPGO (0.5 wt %) USn 92.8
K
1C
drops after 0.5 wt % of
reinforcement
[70]
10
Shokrieh et al.
2014
GPLs (0.5 wt %)
Sn
39
K
1C
drops after 0.5 wt % of
reinforcement
[
71]
GNSs (0.5 wt %) 16
Polymers 2016, 8, 281 5 of 37
Table 1. Cont.
Sr. Authors Year Reinforcement/(wt %)
Dispersion
method
% Increase in
K
1C
(MPa¨m
1/2
)
Remarks Ref.
11 Jia et al. 2014 GF (0.1 wt %) (resin infiltration) None 70
K
1C
did not change much between 0.1 to 0.5 wt %
[58]
12
Tang et al.
2013
Poorly dispersed RGO (0.2 wt %) Sn 24
Trend still increasing
[
72]
Highly dispersed RGO (0.2 wt %) Sn + BM 52
13
Wang et al.
2013 GO
10.79 µm (0.5wt %)
USn
12
K
1C
drops after 0.5 wt % of reinforcement
[
57]
1.72 µm (0.5 wt %) 61
0.70 µm (0.1 wt %) 75
14
Chandrasekaran et al.
2013 GNPs* (0.5 wt %) 3RM 43
Dispersion and K
1C
improved
with three roll milling
[73]
15
Li et al.
2013
APTS-GO (0.5 wt %)
USn
25 Trend still increasing
[
74]
GPTS-GO (0.2 wt %) 43 K
1C
drops after 0.2 wt % of reinforcement
16
Shadlou et al.
2013
ND (0.5 wt %)
USn
No effect
Fracture toughness improvement is higher by
CNF and GO (high aspect ratio) compared with
that by spherical ND
[
75]
CNF (0.5 wt %) 4.3
GO (0.5 wt %) 39.1
17
Jiang et al.
2013
GO (0.1 wt %)
Sn
31
Trend remains same after 1 wt % of reinforcement
[76]
ATS (1 wt %) 58.6 K
1C
drops after 0.1 wt % of reinforcement
ATGO (1 wt %) 86.2
The maximum improvement is achieved with
functionalization
18
Liu et al.
2013
p-CNFs (0.4 wt %)
Sn
41
Trend still increasing
[
77]
m-CNFs (0.4 wt %) 80
19
Wang et al.
2013
ATP (1 wt %)
Sn
14 K
1C
drops after 0.1 wt %
[
78]
GO (0.2 wt %) 19 Trend still increasing after 0.2 wt %
ATP (1 wt %) + GO (0.2 wt %) 27
K
1C
drops with the further increase in ATP of
reinforcement
20
Alishahi et al.
2013
ND (0.5 wt %)
Sn
´26.9
Trend still increasing
[
79]
CNF (0.5 wt %) 19
GO (0.5 wt %) 23
CNT (0.5 wt %) 23.8