This document summarizes research on incorporating polyamide 6 nanofibrous structures into glass fiber-reinforced epoxy composites to improve their mechanical properties. Experiments showed that the nanofibers had a catalytic effect on the curing of the epoxy resin, lowering its activation energy. Composites with nanofibers deposited on or between fiber layers showed increased stress at failure compared to a control without nanofibers. The deposited nanofibers in particular improved load transfer to the glass fibers. While nanofibers did not significantly increase modulus, their inclusion helped prevent delamination cracks in the composites. The research demonstrates that nanofibers can enhance some mechanical properties of glass fiber-ep

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THE USE OF NANOFIBRE STRUCTURES IN FIBRE-REINFORCED
COMPOSITES: CHALLENGES AND OPPORTUNITIES
K. De Clerck1*
, S. Van der Heijden1
, B. De Schoenmaker1
, I. De Baere2
, W. Van
Paepegem2
and H. Rahier3
1
Department of Textiles, Ghent University
Technologiepark-Zwijnaarde 907, 9052 Zwijnaarde, Belgium
2
Department of Materials Science, Ghent University
Technologiepark-Zwijnaarde 903, 9052 Zwijnaarde, Belgium
3
Vrije Universiteit Brussel, Department Materials and Chemistry, Pleinlaan 2, B-1050 Brussels,
Belgium
karen.declerck@ugent.be
Abstract: Owing to their light weight and high stiffness and strength, fibre-reinforced epoxy resin composites
are widely used in industry. However, an epoxy matrix is a brittle material, so an improvement of the crack
resistance of the interlaminar space would be interesting. Therefore, secondary (sub-)micron reinforcements
are often incorporated in the matrix. Since it is difficult to obtain a homogeneous dispersion of these
nanoparticles with common techniques, the mechanical improvement of the composites is only moderate.
Thermoplastic nanofibrous structures can tackle this dispersion issue, as they can be inserted by other
means. Therefore, this study investigated the effect of polyamide 6 nanofibrous structures on the mechanical
properties of a glass fibre/epoxy composite. In addition to the mechanical properties the effect of the
nanofibrous structures on the epoxy matrix is looked at through a curing study. The nanofibres are produced
using a multi-nozzle electrospinning set-up.
Keywords: Nanofibres, fibre-reinforced composites, mechanical properties.
1. Introduction
Fibre reinforced epoxy resin composites are widely used in industry, due to their light weight and high
stiffness and strength [1,2]. However these composites still face a serious problem, the epoxy matrix is a
brittle material, which could lead to unexpected failure of the composite.
A fair amount of research has been done to improve the mechanical properties of the epoxy matrix, one
possible solution consists of adding nanoparticles, like nanoclays and carbon nanotubes (CNT) to the matrix.
In CNT nanocomposites, one aims to improve the resin and fibre/resin interface properties, by mixing in
carbon nanotubes with a theoretically exceptionally high stiffness and strength [3-5]. However the overall
improvement in mechanical properties (stiffness, fracture toughness) of the epoxy matrix is only moderate in
most cases [6-8]. Furthermore the toughness of the matrix can also be improved by incorporating rubber
particles or embedding thermoplastic inclusions [9,10]. At present both techniques are still facing problems.
An important issue in mixing (nano)particles or a thermoplastic in the epoxy matrix is the need for special
mixing methods to obtain a homogeneous dispersion in the resin [3-5]. Also the high production cost of CNT
and even more importantly the potential health hazard involved with the use of nanoparticles need to be
tackled [11].
Thermoplastic nanofibres have the potential to provide a solution for these problems. Nanofibrous webs can
be readily embedded in the resin, they have the large benefit of their inherent nanoscale distribution which
may improve the traditional limitations in (nano)particle dispersion. Owing to their macro scale length, no
health hazards are involved in the use of electrospun nanofibres. Recent literature indicates that nanofibres
may contribute substantially to the ductility and fracture toughness of the composite [12-15].
The present paper describes the results of a running research project in which the potentials of nanofibrous
materials are looked at to improve the mechanical performance of glass fibre reinforced epoxy composites.
Focus is given to the toughening of the epoxy matrix as the envisaged effect is similar to toughening of
epoxies through the addition of thermoplastics.

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Both the electrospinning of the nanofibres as well as their effect on the epoxy matrix behaviour is to be
discussed. The effect on the epoxy matrix will be looked at through a detailed curing study and their
toughening potential.
2. Materials and Methods
The PA6 nanofibres are electrospun from a 1:1 formic acid/acetic acid solution on a multi-nozzle set-up [16].
The tip-to-collector distance and flow rate are set at 7 cm and 2 mL/h, respectively. Furthermore, the voltage
is adjusted between 25 and 30 kV, in order to obtain steady state conditions [17].
A Jeol Quanta 200F Field emission gun Scanning electron microscope (FE-SEM) is used to examine the
fibre diameter of the electrospun nanofibres and the composite structure. Prior to SEM analysis, the samples
are coated by a gold/platinum alloy using a sputter coater (Balzers Union SKD 030). The average fibre
diameter and their standard deviations are based on 50 measurements, using Cell
D
software from Olympus.
An optical microscope, a Olympus BX51 with a Olympus UC30 camera, is used to visualize crack
propagation on the polished edges of the composites.
The (MT)DSC measurements are performed using a TA Instruments Q2000 Tzero™ DSC, purged with a
constant nitrogen flow of 50 mL/min. The instrument is calibrated using sapphire (Tzero calibration), indium
(heat flow rate and temperature), and tin (temperature). The modulation amplitude for the MTDSC
measurements is chosen at 0.5°C with a period of 60 seconds.
For the curing experiments Epikote resin 828 LVEL (diglycidyl ether of bisphenol A, DGEBA, difunctional)
from Hexion is used as epoxy. It is combined with the tetrafunctional methylenedianiline (MDA) from Sigma-
Aldrich. Both products are used as received.
The matrix of the larger scale composite plates was EPIKOTE resin MGS RIMR 135 with EPIKURE curing
agent MGS RIMH 137, both purchased from Hexion (Currently Momentive). The composite plates were
reinforced with unidirectional E-glass fabric (Roviglas R17/475). In the fibre direction the reinforcement was
475 g/m², while in the perpendicular direction the reinforcement was 17 g/m².
The composite plates were manufactured by vacuum assisted resin transfer moulding (VARTM) using a
closed steel mould. The epoxy is first cured at room temperature for 24 hours and, thereafter, post cured for
15 hours at 80 °C, as described by the supplier. For all composite plates the stacking sequence was
[0°,90°]2s. The samples were cut to dimensions on a water-cooled diamond saw. All specimens had a
thickness of 3.0 mm and a nominal width of 30 mm, as described in the ASTM D3039/D3039M-00. End tabs
were used to avoid failure at the clamps.
The nanofibres were incorporated in the glass fibre/epoxy composites in two different ways. On one hand,
they were inserted in between the various glass fibre mats as a stand-alone structure. On the other hand
they were directly deposited on one side of the glass fibre reinforcement. Of course, also a reference
composite without nanofibres was manufactured, to be able to investigate the improvement of the nanofibre
addition.
The tensile tests on the composites were performed on an electromechanical Instron 5800R machine with a
load cell of 100 kN following the ASTM D3039/D3039M-00 standard. The tests were displacement controlled
with a speed of 2 mm/min and both displacement and load were recorded. All specimens were instrumented
with two strain gauges to measure the longitudinal and transversal strain, εxx and εyy respectively. Not all
samples were loaded till failure, since needed for other experiments.
3. Results
3.1 Curing Experiments
Small composite samples were produced by adding resin to nanofibrous structures in the DSC crucibles.
SEM analysis confirmed the sample preparation was optimised to ensure the resin is totally impregnated in
between the fibres. If it would remain on top of the nanofibrous structure, mainly the reaction of the bulk
would be measured and investigated.
A non-isothermal study of the curing behaviour of pure resin and resin impregnated within nanofibres was
performed [18]. This already revealed an earlier onset of curing on the addition of nanofibres. It seems from
that the nanofibres have a catalytic effect on the curing reaction of DGEBA/MDA. Therefore the activation

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energy (Ea) is analysed using the Friedman method [19]. The activation energy indeed shows to be lower
when nanofibres are added to the resin, which confirmes that the nanofibres have a catalytic effect on the
curing reaction. Moreover, this Ea is not only lower in the beginning, at lower conversion degrees, but for all
the different degrees of conversion. Next to non-isothermal experiments, a curing reaction can also be
investigated with quasi-isothermal MTDSC experiments. These also revealed the curing of the resin
accelerates when PA 6 nanofibres are added to the resin system.
The influence of different nanofibre fractions was examined by comparing the curing behaviour of samples
containing 0, 5, 15, 27 and 42 wt% PA 6 nanofibres, Figure 1 [18]. Incorporating higher nanofibre fractions,
the reaction accelerates: the initial rate is higher and the maximum of the heat flow rate curve shifts to
shorter times.
Figure 1. Influence of the PA 6 nanofibre fraction on the quasi-isothermal curing of the DGEBA-MDA
system: heat flow as a function of time (Tcure=80°C) [18].
In addition to nanofibres also the effect of microfibers is looked at. Despite the huge difference in
concentration of amide groups at the surface, the curing kinetics of the DGEBA-MDA system did not change
very explicit by adding similar weight fractions of macrofibres instead of nanofibers. This suggests that the
extra amide groups available on the nanofibres do not influence significantly the curing kinetics. It has to be
noticed that the moisture absorption of both nonwoven structures is similar, which suggests that especially
water molecules present inside the polyamide nanofibre and released during cure at more elevated
temperature (80 °C) may cause the catalytic effect.
3.2 Mechanical Behaviour
For studying the mechanical behaviour of glass fibre-epoxy composites upon the addition of a secondary
nanofibrous reinforcement, larger scale composite samples were produced [20]. Again the nanofibres are
well dispersed within the resin-rich region, Figure 2. Further the SEM-images show the epoxy/nanofibre
regions are to be considered as extra separated plies.
Figure 2. Cross section of the secondary nanofibre reinforced glass fibre composites (A.) and a zoom of the
resin rich region filled with nanofibres between two plies with different fibre orientation (B.)[20] .
Table 1 summarizes the tensile properties of different composite plates. Glass fibre composite plates with a
secondary reinforcement of interlayered and deposited nanofibres are compared to samples without a

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secondary reinforcement. No substantial increase in Young’s modulus is observed. Indeed in the 0°-direction
the mechanical properties of the glass fibres are dominant compared to the ones of the thermoplastic
nanofibres. Glass fibres have a lot higher strength and stiffness compared to the used PA 6 nanofibres.
Furthermore, the stiffness of thermoplastic nanofibres is around 1 to 2 GPa [21], while the applied epoxy
matrix has a stiffness of 3.27 ± 0.07 GPa.
On the other hand adding nanofibres in between the glass fibre plies increased the stress at failure.
Interlayered nanofibres increased the stress at failure from 550 MPa to 581 MPa. When the nanofibres are
directly deposited onto the glass fibres the stress at failure increases further to 611 MPa. Thus, even though
the mechanical properties of the nanofibres are inferior compared to the properties of the glass fibres, they
improve pronouncedly the failure stress. It seems that the deposited nanofibres even further facilitate the
transfer of the load to the primary reinforcement. This can be understood by taking into account that there is
a direct contact between the deposited nanofibres and the glass fibres, which improves the load transfer to
the glass fibres. This is further confirmed through optical microscopy.
By testing the composites under 45° the mechanical properties of the glass fibres are less dominant
compared to the other components. So, the advantages of nanofibres should be more pronounced. This is
confirmed in Table 1 illustrating the shear modulus.
Table 1. Tensile properties of the different composite plates [20].
Stress at failure
[MPa]
Young’s modulus
[GPa]
Shear modulus
[GPa]
Glass fibre composite 550 ± 16 26.0 ± 0.6 4.0 ± 0.4
Interlayered nanofibres 581 ± 13 26.0 ± 0.2 4.7 ± 0.3
Deposited nanofibres 611 ± 18 27.2 ± 0.6 4.7 ± 0.2
4. Conclusions
Addition of polyamide nanofibres into the resin resulted in a catalytic effect on the DGEBA-MDA cure. This
was proven by the lower activation energy when PA 6 nanofibres were added to the resin. However, as the
difference in catalytic behaviour between conventional and nanofibres of PA 6 was very small, the amide
groups at the fibre surface are not thought to be responsible for the acceleration. As the moisture absorption
of nanofibres and microfibres was similar, and as the acceleration could be correlated with the moisture
content of the fibres, the catalysing effect can most probably be attributed to water diffusing from the
nanofibres into the polymer matrix. This will be further elaborated in future work.
The incorporation of polyamide 6 nanofibrous structures in the [0°,90°]2s-composite increases the stress at
failure, deposited nanofibres are slightly better than the interlayered nanofibrous structures. It is found that
nanofibres prevent or minimize the formation of delamination cracks between two glass fibre plies. The
tensile experiments under 45° also demonstrate that the deposited nanofibres facilitate the load transfer to
the glass fibres. Thus, it can be concluded that deposited nanofibres improve significantly some mechanical
properties of a glass fibre composite.
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