Mechanical Properties of Alumina Nanofilled Polymeric ... · 03.05.2018 · end by an interdisciplinary team. On this work, the mechanical properties of epoxy resin nanocomposites

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.


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

-


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)


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


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.


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.





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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