460
ISSN 1229-9197 (print version)
ISSN 1875-0052 (electronic version)
Fibers and Polymers 2018, Vol.19, No.2, 460-470
Mechanical Properties of Alumina Nanofilled Polymeric
Composites Cured with DDSA and MNA
Ana M. Amaro1*, Deesy G. Pinto
2, Luís Bernardo
3, Sérgio Lopes
4,
João Rodrigues2,5
, and Cristina S. Louro1
1CEMMPRE, Department of Mechanical Engineering, University of Coimbra, Rua Luís Reis Santos,
Pinhal Marrocos, Coimbra 3030-788, Portugal2CQM – Centro de Química da Madeira, University of Madeira, Campus da Penteada, Funchal 9020-105, Portugal
3University of Beira Interior, Centre of Materials and Building Technologies (C-MADE), Covilhã 6201-001, Portugal4CEMMPRE, Department of Civil Engineering, University of Coimbra, Rua Luís Reis Santos, Coimbra 3030-788, Portugal5School of Materials Science and Engineering/Center for Nano Energy Materials, Northwestern Polytechnical University,
Xi’an 710072, China
(Received August 23, 2017; Revised December 29, 2017; Accepted January 2, 2018)
Abstract: Reinforced concrete is widely used in structures. New materials to replace both the steel and the concrete havebeen studied in many research centres. One of the possibilities for the reinforcement is the partial or total replacement of thesteel bars by new composite materials. Nano composites are very promising, and an investigation line was developed to thisend by an interdisciplinary team. On this work, the mechanical properties of epoxy resin nanocomposites (EPNCs) filled withα-Al2O3 nanoparticles (NPs) with irregular shape and approximately 100 nm maximum diameter size was investigated. Thevariable study was the alumina NPs contents: 1, 3 and 5 wt.%. The NPs were previously pretreated with a silane agent(APTES). Two hardeners, 3-dodec-2-enyloxolane-2,5-dione (DDSA) and 8-methyl-3a,4,7,7a-tetrahydro-4,7-methano-2-benzofuran-1,3-dione (MNA), frequently used in epoxy resin embedding tissues, were used simultaneously for this study.Unlike other hardeners, DDSA does not need curing treatment, constituting a novel application and a saving time-energyduring the manufacturing process. Considering the mechanical behaviour, it was observed that the EPNCs filled with 5 wt.%of alumina NPs showed the maximum improvement in flexural modulus, around 14 % when compared to the pristine EPsample. No relevant effect was observed on the flexural strength by adding alumina NPs. Additionally, the maximumincrease observed for hardness, and Young’s modulus were about 13 % and 28 %, respectively (the maximum increase wasobserved at 3 wt.%).
Keywords: Polymer-matrix composites (PMCs), Nano-alumina, DDSA/MNA, Mechanical properties, Cure
Introduction
Nowadays polymer-matrix composites (PMCs), including
both classes thermosetting and thermoplastic matrix materials,
are extensively utilized as metal substitutes in several
engineering applications (automotive, electronic packaging,
aircraft, aerospace, shipbuilding, new energy and civil
construction, etc.), including under hard working conditions
[1-3]. Among many polymer materials, epoxy (EP) resins
integrate a primary class of thermoset materials, due to their
low shrinkage during cure, low weight, excellent adhesion to
many substrates, high mechanical strength, high-temperature
performance, high resistance to chemicals-corrosion, effective
electrical insulation, good chemical and solvent resistance,
etc. [4-9]. Moreover, EP resins present higher reactivity, can
be used on a broad range of temperature and allow simplest
formulations to combine a single EP resin with several
chemical curing agents to obtain the final cured product [6,8].
One of the most widely used thermosetting polymer to obtain
EPNCs is the 2-[[4-[2-[4-(Oxiran-2-ylmethoxy)phenyl]propan-
2-yl]phenoxy]methyl]oxirane, a D.E.R.TM 332 liquid epoxy
resin based on DGEBA [9-11]. An extensive research has
been carried out to improve even more the performance of
EP resins and, simultaneously, to solve some disadvantages
of this material which are responsible to limit its applications
to high-performance coating and adhesives, namely: low
toughness and thermal conductivity, brittle failure, high
coefficient of linear thermal expansion (CTE) and poor
thermal stability and viscoelastic properties [5,12-15]. For
this, several new types of EP resins have been developed,
such as Bisphenol A, Bisphenol F, Novolac (a phenol-
formaldehyde resin based), Aliphatic and Glycidylamine
[16,17]. Despite of this, in the last years one of the most
common studied technique to improve the performance of
EP resins consists of the addition of modifiers/fillers, such as
soft rubbers particles, diluents, flame retardants, pigments,
dyes and unreactive fillers, into the EP resins to enhance
specific physical and mechanical performances to the final
EPNCs [5,6,12,18]. The observed enhancements to the final
EPNCs strongly depend on the characteristics related to the
used filler, such as: particle size and shape variation, the
ratio of the surface area to volume, pretreatment of NPs
before use, filling content, as well as the mixing technique
[12,19]. Among many types of fillers that can be utilized,*Corresponding author: [email protected]
DOI 10.1007/s12221-018-7664-7
Alumina Nanofilled Cured with DDSA and MNA Fibers and Polymers 2018, Vol.19, No.2 461
several studies [7,12,20,21] have reported that the incorpora-
tion of inorganic NPs, such as nanosized alumina (Al2O3),
into different thermoset systems, can significantly improve
the properties of the EPNCs, without sacrificing their core
properties. This is observed even with low contents of
inorganic NPs. In addition to the low cost of alumina NPs
(when compared to other ceramics NPs), high density
(~3.69 g cm-3), low CTE and dielectric constant, excellent
thermal conductivity (~20 Wm-1K-1) and high elastic
modulus, previous studies show that EP resins reinforced
with alumina (Al2O3) NPs constitute a viable solution for a
large variety of applications [3,4,22].
For the sake of clarity, we summarized in this article some
results from previous works, which studied the influence of
the addition of Al2O3 NPs on the flexural and hardness
properties of EPNCs.
By using non-treated Al2O3 NPs, with 100 nm size and 0.5
to 10 wt.% content, Kurahatti et al. [23] observed that the
maximum increase in the flexural properties of the reinforced
EPNCs was reached with 0.5 wt.% loading. However, no
significant effect was seen in the hardness of the reinforced
EPNCs. Baskaran et al. [24] studied the flexural properties
of EPNCs reinforced with non-treated spherical alumina
NPs (with 60-70 nm diameter and with 0-10 wt.%). The
authors observed that the flexural strength increased up to
5 wt.(%) loading. Hiremath et al. [25] studied the flexural
properties of EPNCs filled with 0.5-1.5 wt.% of rod shape
alumina NPs (with 10 nm diameter and 50 nm maximum
length). For contents up to 1 wt.%, the authors observed and
increase in the flexural modulus of EPNCs. In 2011, Coelho
[26] observed modification in the hardness of EPNCs filled
with 2.5 and 5 wt.% of non-treated spherical alumina (with
80 to 100 nm diameters). The author observed that, when
compared with the net EPs, the hardness drops for 2.5 wt.%
but increases at 5 wt.%. Coelho also observed that the results
depend on the degree of moisture of the NPs.
As stated in a previous review article [12], relevant
differences on the influence of alumina NPs in important
mechanical properties of EPNCs are still reported in the
literature. For this reason, experimental studies on the effect
of alumina NPs in the flexural and hardness properties of
EPNCs are still needed.
When compared with other hardeners used in the literature
(such as DDM), DDSA+MNA don’t require post-cure,
allowing to skip the post-curing cycle and also to save time-
energy during the manufacturing process. This aspect
constitutes an important advantage for thermoset manufacture.
Moreover, information obtained directly from the curing
agent’s suppliers shows that DDSA and MNA have less
adverse effects on the environment and for human health, in
comparison with others hardeners used in the literature. For
instance, DDM has been considered a substance of very high
concern for workers and consumers by the European
Chemicals Agency (ECHA).
The bibliography review showed that DDSA and MNA
have already been used as curing agents for EP resin
embedding tissues for diagnostic transmission electron
microscopy [27]. Also in a previous study, the authors
evaluated the influence of curing agents in the impact
properties of EPNCs using DDM and DDSA+MNA [20].
The authors concluded that DDM was the one that presents
the best results concerning the impact performance of
EPNCs, although the observed improvement is not very high
when compared with EPNCs cured with DDSA+MNA. For
this reason, the authors concluded that the use of DDSA+
MNA could be considered a good option regarding thermoset
manufacture for impact performance.
No additional previous studies were found in the literature
about the manufacturing of EPNCs with DDSA and MNA
as curing agents.
This article presents an experimental study which aims to
explore the reinforcing effects of pretreated alpha alumina
NPs (with non-spherical shapes and with 100 nm maximum
size) on bending (modulus and strength) and hardness
properties of EPNCs cured with DDSA and MNA. The
EPNCs were filled with three different concentration of
Al2O3 NPs, namely: 1, 3 and 5 wt.%. These contents were
also adopted in a previous authors work [21], in which the
EPNCs were obtained with a different curing agent (DDM)
and manufacturing process.
Experimental
Materials
The elementary characteristics of the pristine chemicals
used in this study, without extra treatment, and the nano-
Al2O3 reinforcement, are presented in Table 1 and 2,
respectively. Both materials, EP resin, and ceramic NPs are
similar to those described in a previous author’s work [21].
Two curing catalysts or initiators were selected, the 3-dodec-
2-enyloxolane-2,5-dione (DDSA) and the 8-methyl-3a,4,7,7a-
tetrahydro-4,7-methano-2-benzofuran-1,3-dione (MNA). The
catalyst applied during curing reaction was the N, N-
dimethyl-1-phenylmethanamine (BDMA), purchased from
Sigma-Aldrich Co. The nano-ceramic powder was submitted
to a pretreatment with a silane agent, the 3-aminopropyl
triethoxysilane (APTES) by the supplier, in order to ensure
its uniform dispersion into the EP resin during processing.
The size dimension of the as-received functionalized Al2O3
NPs was confirmed by using using Scanning Electron
Microscopy, SEM (FEI Quanta 400 FEG E SEM). A
representative micrograph is shown in Figure 1(a). It is
possible to observe that the shape of the alumina NPs is
irregular. The particle size distribution of the α-Al2O3
powder, Figure 1(b), confirms the nano-scale-dimension of
the NPs. Its particle size is less than 100 nm, as requested.
462 Fibers and Polymers 2018, Vol.19, No.2 Ana M. Amaro et al.
Nanocomposite Preparation
The successive steps used in this study for the EPNCs
fabrication are summarized in Figure 2. The stoichiometric
amount of the thermoset system constituents, presented in
Table 1, was the same as recommended by Martinet et al.
[27]: 1(D.E.R.332):0.85(DDSA):0.15(MNA):0.04(BDMA).
The functionalized ceramic NPs, as-received from the
supplier, were added into the thermoset mixture always at
the same stage, that is, after the preheating of the EP matrix
and before the addition of the curing agents (DDSA+MNA,
also preheated) into the mixture. Thus, three nanocomposite
samples were prepared to contain 1, 3 and 5 wt.% NPs. A
pure neat EP resin samples were also prepared and will be
referenced as samples control. The curing process was
carried out in a Precision Scientific Napco Vacuum Oven
(Model 5831), under 20 mm Hg at 60oC for 24 hours [28-
30]. After thermal treatment, the samples were cool down to
room temperature. The samples were cut in order to obtain
the final dimensions requested by the standards.
Nanocomposite Characterization
The effect of nano-Al2O3 content up to 5 wt.% on the
EPNCs mechanical response was evaluated by applying
three different techniques, that is, static three-point bending
(3PB), Vickers Hardness (HV) and Dynamic Sensing Indentation
(DSI). Bending properties, namely bending stiffness modulus
(Eb) and bending strength (σ) were obtained by performing
3PB tests on the EPNCs specimens cut nominally to
84×12×4 mm and with a span of 64 mm, according to the
standard ASTM D-790-2. The flexural properties were
obtained using a Shimadzu AG-10 universal testing machine
equipped with a 5 kN load cell and TRAPEZIUM software
at a displacement rate of 5 mm/min. Experimental details
about this technique can be found in the literature [31,32].
Five replicates of each nanocomposite sample were used,
and all the tests were carried out at 23 oC (room temperature).
The bending strength (σ) was calculated as the nominal
stress at middle span section obtained using the maximum
value of the load according to equation (1).
(1)
where P is the load [N], L the span length (m), b the width
(m) and h the thickness of the sample test (m).
To obtain the bending stiffness modulus (Eb) a linear
regression of the load-displacement curves was performed,
with a correlation factor greater than 95 % for the interval in
σ3PL
2bh2
-----------=
Figure 1. α-Al2O3 nanoparticles with APTES surface modification; (a) FESEM morphology (×60957) and (b) particle size distribution.
Figure 2. Flow chart of the EPNC samples fabrication.
Alumina Nanofilled Cured with DDSA and MNA Fibers and Polymers 2018, Vol.19, No.2 463
the linear segment. The Eb value was determined by equation
(2).
(2)
being I the moment of inertia of the cross-section (m4). The
parameters ∆P and ∆u are, respectively, the load range and
the flexural displacement range of the middle span.
Besides ultramicrohardness (H) and Young’s Modulus (E),
also the Reduced Modulus (Er) was derived by depth-
sensing indentation (DSI) test (equation (3)).
(3)
where Eo/E and νo/ν are the Young’s modulus and the
Poisson’s ratio of the indenter and the material, respectively
(Eo=1050 GPa; νo=0.07). An approximate value of 0.41 [29]
was used for the EPNCs Poisson’s ratio, independently of
the Al2O3 content. Thus, the reported Er values are, therefore,
estimates.
An ultramicrohardness equipment from Fisher Instruments
(Fischerscope H100) was used by applying a maximum load
of 1000 mN. Measurement of the indentation depth was
achieved with a capacitance displacement gauge of 2-nm
accuracy. During the test, the load P was increased in 60
steps until the nominal test load was reached, the same steps
being used during unloading. The time between consecutive
steps was 0.5 s. At each step, the indentation depth h was
measured and stored on the computer in the form of h versus
P. Two creep periods of 30 s were allowed during the tests:
at the maximum load (1000 mN) and at the lowest load
during unloading (0.4 mN).
The ultramicrohardness (H) was calculated as the ratio of
the applied load to the projected area of contact between the
indenter and the sample. By knowing the indenter geometry
(Vickers), the projected area can be evaluated from the
plastic depth, which was obtained by computer fitting a
straight line tangent to the unloading curve at the point of
Eb
PL3
Δ
48 uIΔ---------------=
1
Er
-----1 ν
2–
E------------
1 νo
2–
Eo
------------+=
Table 1. Main characteristics of the chemicals (supplied by Sigma-Aldrich Co.)
D.E.R.™ 332 DDSA MNA BDMA
IUPAC name 2-[[4-[2-[4-(oxiran-2-
ylmethoxy) phenyl]
propan-2-yl]
phenoxy]methyl] oxirane
3-dodec-2-enyloxolane-
2,5-dione
8-methyl-3a,4,7,7a-tetra-
hydro-4,7-methano-2-
benzofuran-1,3-dione
N,N-dimethyl-1-phenyl-
methanamine
Chemical name bisphenol A diglycidyl
ether resin
2-dodecen-1-ylsuccinic
anhydride
4,7-methanoisobenzofuran-
1,3-dione, 3a,4,7,7a-
tetrahydromethyl-
N,N-dimethylbenzylamine
Empirical formula
(hill notation)
C21H24O4 C16H26O3 C10H10O3 C6H5CH2N(CH3)2
Molecular weight (g/mol) 340.41 266.38 178.18 135.21
Epoxide equivalent weight
(g/eq)
171-175 - - -
Epoxide percentage (%) 24.6-25.1 - - -
Epoxide group content
(mmol/kg)
5710-5850 - - -
Viscosity @ 25 oC (mPa·s) 4000-6000 - - -
Density (g/ml) 1.16 (25 oC) 1.00 (20 oC) 1.232 0.9 (25 oC)
Melting point (ºC) 40-44 41-43 < 180 -75 oC
Boiling point (ºC) 210 (1 mm Hg) 180-182 (5 mm Hg) 140 (10 mm Hg) 183-184 (765 mm Hg)
Vapor pressure (mm Hg) 3.66E-09 (25 oC) < 1 (20 oC) 5 (120 oC) -
Vapor density (vs. air) - 9.2 6.1 (20 oC) -
Refractive index 1.569-1.574 1.477-1.479 n20/D 1.506 n20/D 1.501
Appearance - clear yellow viscous
liquid
clear light yellow oily
liquid
-
464 Fibers and Polymers 2018, Vol.19, No.2 Ana M. Amaro et al.
maximum load and then extrapolating to zero. Due to
geometrical imperfections of the diamond indenter and to
the thermal drift and zero position, the depth values were
corrected [33].
Each H, E and Er value is a result of at least twenty
indentation tests along the samples surface area.
Following similar experimental procedures of previous
works [21], all samples were indented using a Karl Frank
GMBH hardness machine. For that, a load of 9.8 N was
applied during 30 s. Large indentations marks, covering the
EPNCs surface response (matrix and reinforcement) were
attained. After removing the indenter, considering dimensions
close to the ideal, the Vickers microhardness (HV1) which
reflects only the irreversible plastic deformation, was derived
according to equation (4). The lengths of the diagonals of the
indents were measured with an optical microscope. Each
hardness HV1 reported represents the average of ten randomly
marks on each sample to ensure accurate results.
(MPa) (4)
where P is the maximum load (9.8 N) and d is the Vickers
diagonal length (m).
Both fracture cross-section and surface morphologies of
the EPNCs samples were analyzed using a ZEISS MERLIN
Compact/VP FEG-SEM equipment. The analysis made by
energy-dispersive X-ray spectrometer (EDS), couplet to
SEM, allow to evaluate the reinforcement Al2O3 dispersion
into the epoxy matrix. All specimens were previously sputtered
coated with gold before microstructural investigation
(approximately 10 nm thickness) using an Edwards EXC
with a source Trumpf Huttinger PFG 1500 DC.
Results and Discussion
As mentioned before, three contents of nano-Al2O3 powder,
1, 3 and 5 wt.%, were selected to increase the mechanical
response of the epoxy resin composites, cured with both
DDSA and MNA. The results are summarized in Table 3,
which includes, also, the mechanical behaviour of the neat
EP samples. The volume fractions of the ceramic particles
reinforcement were determined from equation (5) and were
also included in Table 3. It is important to note that the
polymeric matrix density, ρm, was calculated considering the
mass ratio used on the EPMCs preparation, that is: 1D.E.R.:
0.85DDSA: 0.15MNA: 0.04BDMA, meaning ρm=1.086 g/
cm3.
(5)
where Wr is the weight fraction of particles; ρr is the density
of the α-Al2O3 powder (3.97 g/cm3, see Table 2) and ρm is
the density of the matrix.
Figure 3 shows the typical load versus displacement
curves for the EPNCs obtained from the 3PB tests. In order
to allow the identification of all the curves, horizontal offsets
were performed (as indicated in the graph). These curves
HV11.854P
d2
-----------------=
Vr
Wr
Wr 1 Wr–( )ρr
ρm
------
--------------------------------=
Figure 3. Typical load-displacement curves obtained by 3PB
method (data are horizontally offset, but not otherwise scaled).
Table 2. Main properties of functionalized alumina powder nano
grade (supplied by Nanoshell LLC and Intelligent Materials Pvt ltd)
Empirical formula
(Hill Notation)Al2O3
Cristal form alpha
Particle size < 100 nm
Molecular weight 101.96 g/mol
Melting point 2040 oC
Boiling point 2980 oC
Vapor pressure 1 hPa at 2158 oC
Particle shape Non spherical (irregular shape)
pH 9.4-10.1 at 20 oC
Relative density 3.97 g/cm3
SSA 15-20 m2/g
Color White
Al2O3 content 99.99 %
Impurities (ppm)
Si/Na/K/Fe/Cu/Ti/Mn
10.8/9.01/10.6/9.75/0.12/0.86/0.72
Surface modification APTES (3-aminopropyl triethoxysilane)
Alumina Nanofilled Cured with DDSA and MNA Fibers and Polymers 2018, Vol.19, No.2 465
have a nearly linear elastic behaviour, in an early stage, with
a non-linear region that starts around 195 N. After the peak,
the load decreases and the drop observed is similar to all
curves. From this figure, it is possible to observe that, when
compared with the pristine EP sample, the maximum load
increases for EPNCs. However, no significant differences in
the maximum load occur in function of the NPs content for
EPNCs.
Figure 4 and Table 3 show the average flexural properties
results as a function of the Al2O3 NPs content. From this
figure and table, it is possible to conclude that the average
bending stiffness modulus presents a general tendency to
increase slightly with the NPs content. This small increase,
when compared to the neat EP, is around 9 % and 14 %, for
Al2O3 contents of 3 and 5 wt.%, respectively. Then, the
EPNCs filled with 5 wt.% of alumina NPs showed the
maximum improvement for the flexural modulus. On the
other hand, no relevant effect was observed on the flexural
strength by adding alumina NPs.
These outcomes are in good agreement with those
observed by others authors [23,34], who concluded that the
reinforcement by Al2O3 NPs can lead to better mechanical
properties of the EPNCs. In particular, some previous
studies show that Al2O3 NPs can improve the flexural
modulus without losing flexural strength [12,23,34-37], as
also observed in the present study. According to Rallini et al.
[35], the presence of Al2O3 NPs also slightly affected the
flexural properties, namely the maximal stress and the
elastic modulus of the EPNCs. These authors analysed two
different filler concentrations, 1 and 5 wt.%, which are in
accordance with the composition used in this study.
The increasing in flexural modulus can be attributed to the
higher stiffness of alumina NPs and to the restriction of
chain mobility in the matrix [32,37]. Thus, the results
obtained in this study can be considered reliable, because it
is widely accepted that the addition of the rigid filler
increases the flexural modulus of the EPNCs by following
the rule of mixtures [38].
However, by a careful analysis of the results presented in
Figure 4, one can conclude that the addition of 1 wt.% NPs
leads to a small decrease of the flexural modulus, around
4 % when compared to the standard neat EP. This behaviour
is in agreement with those obtained by Kurahatti et al. [23]
and can be attributed to the interfacial interaction between
the EP matrix and the NPs. This seems to show that, in some
cases, a minimum content of Al2O3 NPs is need to improve
effectively the flexural modulus of EPNCs.
As previously stated, considering bending strength, from
Figure 4 and Table 3 it is possible to conclude that adding
Al2O3 NPs up to 5 wt.% does not seem significant to
influence the thermoset system used in this study. The same
conclusions were obtained by Naous et al. [36]. Those
authors studied the effect of Al2O3 NPs in DGEBA-D.E.R.
331 matrix and they concluded that the flexural stress shown
a very small increase with the increase of Al2O3. The
observation that no relevant effect is observed on the flexural
strength by adding and increasing Al2O3 NPs content is
explained in some previous studies due to the detrimental
effect on the flexural strain to break [12,37]. This effect is
due to the inherent stiffness of Al2O3 NPs and the restriction
of polymer chain mobility in the EP matrix. In fact, it was
shown that the actual deformation observed for the net
matrix is much larger than the observed deformation of theFigure 4. Dependence of the bending stiffness modulus and the
bending strength as a function of alumina nanoparticles content.
Table 3. Mechanical properties of the EPNCs cured with DDSA and MNA
Nano Al2O3 content, wt.% (vol.%)
0 (0) 1 (0.3) 3 (0.9) 5 (1.5)
Flexural strength, σ (MPa) 105.7±2.04 105.0±3.49 100.9±11.94 107.8±2.66
Flexural modulus, Eb (GPa) 2.43±0.05 2.34±0.14 2.64±0.13 2.78±0.09
Hardness, HV1 (MPa; (kg/mm2)) 208±5 (21) 183±5 (19) 248±70 (25) 180±5 (18)
Ultramicrohardness, H (MPa) 132±8 156±8 160±17 157±15
Young’s modulus, E (GPa) 2.6±0.2 3.1±0.1 3.2±0.2 3.1±0.13
Reduced modulus, ER (GPa) 3.1±0.1 3.7±0.2 3.9±0.2 3.7±0.2
466 Fibers and Polymers 2018, Vol.19, No.2 Ana M. Amaro et al.
EPNC, with the result that the polymer matrix reaches the
flexural strain to break limit at a lower total deformation
[37]. Hence, the total EPNC strain to break decreases, as
observed in Figure 3.
It should be also referred that in other previous studies,
such as the one performed by Wetzel et al. [37], with uncoated
Al2O3 NPs with smaller average size (13 nm), it was
observed that the flexural strength tends rather do slightly
higher values as the filler content increases up to certain
limit (4 vol. %). The authors state that this increase suggests
that, in certain conditions, the NPs are able to introduce
some additional mechanisms in failure without decreasing
the matrix deformation. These results show the influence of
the Al2O3 NPs size and pretreatment on the flexural properties
of the studied EPNCs.
Finaly, when compared with a previous study by the
authors which uses identical EPNCs using DDM as hardener
[21], the results from Figure 4 and Table 3 are somewhat
different. In such previous study, maximum values for the
bending strength and bending stiffness modulus were
observed for 1 wt.% of alumina NPs. The increase was
around 59 % and 27 %, respectively when compared with
the neat samples. For percentage loading above 1 wt.%, both
bending strength and bending stiffness highly decreases as
the percentage loading of NPs increases. These results show
the influence of the curing agent and manufacturing process
on the flexural properties of the studied EPNCs.
Pondering on the DSI method, Figure 5(a) exhibit the
typical load (P) - penetration depth (h) curves for the control
EP and EPNCs samples. The shape of the obtained curves
in Figure 5(a) agree well with other ones reported in previous
studies [21,33,39]. The general analysis of this figure allows
to conclude that non-linear incidents, as cracking or
delamination, did not occur during ultramicroindentation
testes since the shape of the curve are continuous and similar
for all samples. An overlapping for the three EPNCs
composites curves is evident, anticipate similarities of
hardness, H and elastic, E and Er, values (see Table 3).
Taking into account the depth residual indentation mark
values, (hr), in Figure 5(a) the plastic deformation of neat EP
samples is, as expected, higher than the three composite
samples.
It is also clear that the addition of Al2O3 NPs up to 5 wt.%
(H~160 MPa) enhances the hardness of the epoxy resin
almost 21 % (H~132 MPa). This result agrees with the fact
that stronger and harder nanosized particles improves the
polymer matrix hardness [40,41]. Nevertheless, due to the
low nano-Al2O3 content, no significant variation of the
mechanical property between EPNCs samples cured with
DDSA and MNA can be noticed. These results are in
agreement with those presented in other research works [42],
who concluded that the hardness of the EPNCs was almost
unchanged as the alumina NPs content increases. It is also
important to take into account that, in some cases, an
opposite behaviour could occur, as mentioned by Coelho
[26].
From Table 3, comparing the elastic properties trends, ER
and E, as a function of nano-Al2O3 content, an acceptable
correlation between 3PB and DSI tests can be inferred,
despite the small variations observed in the tendencies and
of the inherent processes differences. Nevertheless, the
hardness evolution HV1, from Vickers hardness test, is
entirely different from that obtained for hardness H measured
by DSI. Figure 5(b) illustrates the hardness results as a
function of the volumetric Al2O3 content. Since the hardness
of composite materials has been predicted by the rule of
mixtures, equation (6) was also included in Figure 5(b).
(6)
where Hc, Hr, and Hm represent the hardness of the
Hc HrVr HmVm+=
Figure 5. (a) Typical load-displacement curves obtained by DSI
method (hr-depth of residual mark; hmax-maximum depth beneath
the sample surface) and (b) hardness evolution as a function of the
nano-Al2O3 content.
Alumina Nanofilled Cured with DDSA and MNA Fibers and Polymers 2018, Vol.19, No.2 467
composite, the reinforcement, and the matrix, respectively.
As can be seen from Figure 5(b), while the ultramicrohardness
shows a favourable trend to the NPs reinforcement, i.e.,
there is an increase followed by a plateau, the HV1 does not
present a similar tendency. This unexpected result seems to
contradict the predictable standard behaviour and could be
related to the morphological characteristics of the EPNCs
samples.
The results from Figure 4 and Table 3 show some differences
from the ones reported by the authors in a previous study
[21]. As previously referred, identical EPNCs using DDM as
hardener were used. In this study, maximum values for the
ultramicrohardness are obtained for 1 wt.% (with an increase
around 5 % when compared with the neat samples). For
higher percentage loading, the ultramicrohardness decreases.
These results are similar to the ones previously referred and
related to the flexural properties of the EPNCs. They show
again the influence of the curing agent and manufacturing
process on the hardness properties of the studied EPNCs.
Figures 6 to 8 display the representative SEM surface
micrographs of the three EPNCs samples. Several EDS
analysis, which spectrum are within these figures, were
made in order to monitoring the nano-reinforcement
dispersion quality. It is evident, by selecting an analysis area
less than 1×1 μm (typically the samples area was ~ 48 mm2)
that the NPs seems to be uniformly distributed in all EP
matrixes. Moreover, a good adhesion between NPs and
epoxy resin occur during composite processing, as can be
seen for the higher volume fraction of nano-Al2O3 (5 wt.%
or 1.5 vol.%) in Figure 8. This is an indication that with the
selected thermoset system (D.E.R.332/DDSA/MNA/BDMA),
following the particular manufacturing process (Figure 2)
in addition to the pretreatment performed with APTES by
the manufacturer, it is possible to achieve a good NPs
dispersion in the EPNCs. These results are in agreement
with those in which the authors [43,44] have observed
improvements in the filler dispersion due to the silane
treatment. In any case, the reinforcement concentration used
in this study does not go beyond 2 vol.% reducing the
susceptibility for agglomeration.
Hence, since no morphological evidence justifies the HV1
and H trends, one possible explanation is the size of the
indentation area. While the hardness, by microidentation
test, is achieved by using a higher residual area, integrating
the composite response as a whole, in the ultramicroindentation
test the contact area is much smaller, being the hardness
values achieved by the elastic deformation response.
Nevertheless, taking into consideration the average deviation,
Figure 6. SEM micrographs of the EPNC with 1 wt.% Al2O3 and the respective EDS spectrums of the identified surface zones.
468 Fibers and Polymers 2018, Vol.19, No.2 Ana M. Amaro et al.
Figure 7. SEM micrographs of the EPNC with 3 wt.% Al2O3 and the respective EDS spectrums of the identified surface zones.
Figure 8. SEM micrographs of the EPNC with 5 wt.% Al2O3 and the respective EDS spectrums of the identified surface zones.
Alumina Nanofilled Cured with DDSA and MNA Fibers and Polymers 2018, Vol.19, No.2 469
it is no mistake to say that the hardness HV1 is almost
similar for all the samples (see horizontal bar in Figure 5(b)).
When the previous results are compared with same ones
reported in the literature, some differences are observed. For
instance, in the study from Amaro et al. [21], the best
dispersion of NPs was achieved for 1 wt.%. For 3 wt.% and
5 wt.% some aggregates were observed, which can justify
the decrease observed for the values of the studied mechanical
properties above 1 wt.% (as previously referred). Since
identical EPNCs using DDM as hardener were used in [21],
these results show that the curing agent and manufacturing
process can influence the dispersion of the NPs and, hence,
the final mechanical properties of the EPNCs.
Conclusion
This article presented an experimental study on the
reinforcing effects of pretreated α-Al2O3 NPs (with non-
spherical shapes and with 100 nm maximum size) on the
mechanical properties of EPNCs cured with both DDSA and
MNA hardeners. Flexural and hardness properties were
evaluated according to the concentrations NPs fill: 1, 3 and
5 wt.%.
From this study, the main findings are:
1. DDSA and MNA is shown to be adequate EP resin
hardeners (D.E.R.TM 332) to fabricate the EPNCs reinforced
with ceramic NPs;
2. Average flexural modulus of the tested EPNCs generally
increases with the Al2O3 NPs loading. When compared to
the neat EP, a maximum increase of about 14 % for 5 wt.%
was observed. As also seen in previous studies, a minimum
content of alumina NPs, around 3 wt.%, seems to be
needed to improve the flexural modulus of the EPNCs;
3. No significant influence on the flexural strength of the
EPNCs was observed by adding Al2O3 NPs up to 5 wt.%;
4. The average values of the EPNCs ultramicrohardness
increased around 21 % when compared to the neat EP.
5. No significant variation occurs for Young’s and reduced
modulus in comparison to neat EP;
6. SEM/EDS analysis validate the mechanical behaviour, by
showing uniform distribution and random orientation of
the nano-Al2O3 particles into EP matrix.
7. The results from this study show that the curing agent and
manufacturing process also influences the final mechanical
properties of the EPNCs. Hence, the curing agent and
manufacturing process also constitute an important
variable study and must also be considered for future
research works.
Acknowledgements
This research is sponsored by UID/EMS/00285/2013 –
and by national funds through FCT – Fundação para a
Ciência e a Tecnologia, under grants PEst-C/EME/UI0285/
2013, PEst-OE/QUI/UI0674/2013 (CQM) and SFRH/BPD/
85049/2012. DP and JR acknowledge the support of LREC
– Laboratório Regional de Engenharia Civil da Madeira and
Dr. César Fernandes, for having borrowed the casts used in
the preparation of EPNCs specimens.
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