Synergetic effect of carbon nanofibers and short carbon fibers on the ...

4290 CARBON 48 (2010) 4289– 4300
considerable attention. Different dispersing techniques for
CNTs were documented in recent review papers [12,13].
CNFs <strong>and</strong> CNTs were proven to stiffen <strong>and</strong> toughen EPs even
at their extremely low contents [14–17]. To reinforce EP with
low-loading CNFs or CNTs is <strong>of</strong> practical interest because
the viscosity <strong>of</strong> the resin can be kept on an acceptable level.
The large aspect ratios <strong>and</strong> huge specific surface areas <strong>of</strong>
CNTs or CNFs guarantee an efficient matrix/filler stress
transfer. Available literatures to date <strong>of</strong>fer evidence strong
CNT–polymer interactions. Efficient stress transfer between
CNTs <strong>and</strong> polymer matrix was evidenced in [18–20] <strong>and</strong> simulated
in [21–23]. The stress transfer efficiency between
nanotube <strong>and</strong> matrix was estimated to be at least one order
<strong>of</strong> magnitude larger than in conventional fiber-based composites
[20].
Direct comparisons <strong>of</strong> the mechanical properties between
micr<strong>of</strong>iller- <strong>and</strong> nan<strong>of</strong>iller-reinforced composites are scarce.
A definite answer is still lacking to the basic question: do
nan<strong>of</strong>iller-reinforced composites always display superior
mechanical properties to micr<strong>of</strong>iller-reinforced composites
Microsized particles or <strong>short</strong> <strong>fibers</strong> (micro-scale in diameter
or/<strong>and</strong> in length) were proven to <strong>effect</strong>ively stiffen <strong>and</strong>
toughen EP matrix. However, they strongly lower the ductility
due to the highly constrained deformability <strong>of</strong> the EP matrix
<strong>and</strong> stress concentration occurring near the microsized
filler [7–9,24,25]. Obvious stress concentration near the fillers
can induce critical failure <strong>of</strong> the composite system. Thus, in
many cases, the strength <strong>of</strong> EP can only be moderately improved
<strong>and</strong> even decreased with the addition <strong>of</strong> microsized
fillers. Unlike micr<strong>of</strong>illers, the much smaller size <strong>of</strong> nan<strong>of</strong>illers
might guarantees a good integration with the matrix
<strong>and</strong> avoid obvious stress concentration [25]. The incorporation
<strong>of</strong> low-loading nan<strong>of</strong>illers makes it possible a simultaneous
improvement in E-modulus, strength <strong>and</strong> toughness
<strong>of</strong> EP [15,25,26]. Despite the fact that nan<strong>of</strong>iller-reinforced
polymer composites attracted great attentions in the last
two decades, with low-loading nan<strong>of</strong>illers such as nanoparticles
<strong>and</strong> CNTs, the mechanical properties <strong>of</strong> EP matrix can
only be moderately improved [15,25,26]. However, our previous
work [27] revealed that a combined utilization <strong>of</strong> microsized
<strong>short</strong> <strong>carbon</strong> <strong>fibers</strong> (SCFs) <strong>and</strong> low-loading nano-silica
particles leads to significant synergisms on the mechanical
<strong>and</strong> fracture toughness <strong>of</strong> EP. Combined utilization <strong>of</strong> differently
scaled fillers seems to be a promising route to greatly
enhance the modulus, strength <strong>and</strong> fracture toughness <strong>of</strong>
EP simultaneously.
In this work, we systematically compare the mechanical
<strong>and</strong> fracture properties <strong>of</strong> three series <strong>of</strong> epoxy composites,
i.e. conventional composites reinforced with microsized SCFs
(SCF/EP composites), composites reinforced with CNFs at low
concentrations (CNF/EP composites), <strong>and</strong> composites reinforced
with both SCFs <strong>and</strong> CNFs (SCF/CNF/EP composites).
The main aims <strong>of</strong> the present work are to deepen the underst<strong>and</strong>ing
on the structure–property relationship <strong>of</strong> EP composites
<strong>and</strong> to investigate how the combination <strong>of</strong> the
multiscale <strong>carbon</strong>s, i.e. combination <strong>of</strong> SCFs <strong>and</strong> CNFs, affects
the mechanical properties <strong>and</strong> the fracture toughness <strong>of</strong> the
EP. By revealing the synergetic action <strong>of</strong> the multiscale <strong>carbon</strong>
fillers, we propose a promising composite formulation route
for high-performance applications.
2. Experimental
2.1. Materials <strong>and</strong> preparation
All materials used in the present work were prepared on the
basis <strong>of</strong> a commercially available epoxy resin (DER331 by
DOW) with an epoxide equivalent weight 182–192 g/equiv.
Cycloaliphatic amine hardener (HY 2954; Huntsman) was used
to cure the epoxy materials. Milled PAN-based <strong>carbon</strong> <strong>fibers</strong>
(Tenax â A-385) were supplied by Tenax GmbH (Germany).
The diameter <strong>and</strong> the length <strong>of</strong> the <strong>fibers</strong> are, respectively,
7 lm <strong>and</strong> 40–70 lm. The SCFs were supplied without sizing
treatment <strong>and</strong> were used as-received. CNFs (GANF1) were supplied
by Grupo Antolin (Spain). The CNFs have diameters in
the range <strong>of</strong> 20–80 nm <strong>and</strong> lengths larger than 30 lm. The theoretical
E-modulus <strong>of</strong> the CNFs is 230 GPa <strong>and</strong> the theoretical
strength is 2.7 GPa. Fumed spherical silica nanoparticles
(Aerosil R8200) with hexamethyldisilazane modification were
supplied by Evonik GmbH (Germany).
Three series <strong>of</strong> epoxy composites, i.e. CNF-filled EP composites
(CNF/EP), composites reinforced with the combined
nan<strong>of</strong>illers, i.e. nano-SiO 2 <strong>and</strong> CNFs (CNF/nano-SiO 2 /EP), <strong>and</strong>
composites reinforced with the combined multiscale <strong>carbon</strong>s,
i.e. SCFs <strong>and</strong> CNFs (SCF/CNF/EP), were prepared in the present
work. Pure EP <strong>and</strong> conventional composites filled uniquely
with SCFs were used for references. The loading <strong>of</strong> the CNFs
varied from 0.125 to 0.75 vol.%. The detailed compositions <strong>of</strong>
the materials are disclosed in Table 1. The volume fractions
<strong>of</strong> all the fillers were calculated by considering their weights
<strong>and</strong> (bulk) densities. For simplification purpose, the composite
materials were referenced according to the type <strong>and</strong> the
fraction <strong>of</strong> the fillers as shown in the left column <strong>of</strong> Table 1.
For example, 10CF0.5NF refers to the composite filled with
10 vol.% SCFs <strong>and</strong> 0.5 vol.% CNFs.
A master batch <strong>of</strong> CNF-filled epoxy resin was firstly prepared
by dispersing a CNF/epoxy mixture with a three roll calender
(Exakt 80 E, Exakt GmbH, Germany). Three roll calender
was firstly introduced for dispersing CNTs by Gojny et al. in
2004 [14] <strong>and</strong> received intensive attentions in recent years.
Interestingly, this dispersion technique is well established in
the praxis – for example nano-SiO 2 is traditionally dispersed
in dental composites by such calendering process. The small
gap between the rolls <strong>and</strong> a mismatch in rolling speed can result
in enormous shear forces, which can <strong>effect</strong>ively disentangle
CNTs or CNFs agglomerates without significantly reducing
their aspect ratios. The mismatch between the angular velocity
<strong>of</strong> adjacent rolls was fixed at x 1 :x 2 :x 3 = 1:3:9. The gap distance
between the rolls was adjusted to be 50 lm for
beginning <strong>and</strong> was reduced to 25 lm for the second pass <strong>and</strong>
finally to 5 lm for the third pass. With regard to the SCF/
CNF/EP composites, required amounts <strong>of</strong> SCFs were mixed
into the diluted CNF master batch using a vacuum dissolver
(Dispermat, VMA-Getzmann GmbH, Germany). The CNF/
nano-SiO 2 /EP composites were prepared by mixing <strong>and</strong> diluting
the CNF/EP master batch with the nano-silica/EP master
batch formulation. The nano-silica master batch is prepared
by using the dissolver with an extremely high rotation speed
(5800 rpm). Afterwards, the compounds were blended
with the curing agent HY2954 by stirring in the dissolver
for 15 min. Finally the mixture was poured into release