JEC COMPOSITES MAGAZINE - Issue #99 - September 2015 - 43

Fig. 3: FGS (functionalised Graphene Sheets) improves the Young's Modulus of
Epoxy up to a certain point ~ 0.1wt%. But adding more than this does not make
any more improvements [9]

A similar trend is also seen with another property called fracture
toughness (the resistance of the material to cracking). Structural polymers such as epoxies typically have low fracture toughness and are
inherently brittle. By modifying plastics using graphene, the polymer
could have a better fracture resistance. The theory behind this is that
cracks growing through a polymer will be blocked or diverted around
flakes of graphene. Since several million flakes of graphene can be
crammed into a cubic centimetre of polymer, the overall diversion
a crack path could take would be massive. This would create a more
damage tolerant material.
In [9], 0.125 wt% of oxygen-functionalised graphene increased the
fracture toughness of epoxy by 65%. In another paper, using 0.5
wt% of oxygen-functionalised graphene produced a 40% increase in
fracture toughness which outperformed both GNP and MWNT [10].
The best results regarding mechanical property improvements have
been when graphene is functionalised with oxygen atoms. The
oxygen atoms are thought to provide a better adhesion to the matrix
and a better stress transfer. One problem with oxygen functionalisation is that it disrupts the electrical conductivity of graphene,
which no longer becomes highly conducting. Therefore, creating
an electrically-conductive composite at the same time as improving
mechanical properties may be difficult. However, a compromise can
be reached if the amount of functionalisation can be controlled.
Electrically-conductive composites can be created by making the
flakes of graphene form a conductive pathway through the polymer.
To do this, the graphene flakes must touch at least one other to carry
the charge from one flake to another. At the same time, the flakes
must resist agglomeration. The conductivity of polymers can be
enhanced by several orders of magnitude, from 10-14 S m-1 to ~ 10-1 S
m-1, using up to 1 wt% GNPs. But the conductivity reaches a plateau
when more GNPs are added [11]. Research is required to overcome
this barrier.
Unlike electrical conductivity, thermal conductivity is one area
where the improvements continue even when using higher additions
of graphene [12] (see Figure 5). This seems a promising area with
graphene outperforming graphite and carbon black.

Fig. 4: Improvement of electrical conductivity in a polycarbonate matrix. Conductivity increases quickly at first but soon levels off. This trend is seen in all polymers [11]

Fig. 5: Thermal conductivity improves linearly when you add more graphene to
epoxy. MLG = MultiLayer Graphene (up to 5 layers of graphene stacked on top of
one another) [12]

No99 September 2015 /

jec composites magazine 43

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #99 - September 2015