Composite Structures 174 (2017) 26–32

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Composite Structures

journal homepage: www.elsevier .com/locate /compstruct

The influence of curing agents in the impact properties of epoxy resinnanocomposites

http://dx.doi.org/10.1016/j.compstruct.2017.04.0500263-8223/� 2017 Published by Elsevier Ltd.

⇑ Corresponding author.E-mail addresses: ana.amaro@dem.uc.pt (A.M. Amaro), lfb@ubi.pt (L. Bernardo),

deesy.pinto@staff.uma.pt (D.G. Pinto), sergio@dec.uc.pt (S. Lopes), joaor@uma.pt(J. Rodrigues).

Ana M. Amaro a, Luís Bernardo b,⇑, Deesy G. Pinto c, Sérgio Lopes d, João Rodrigues c

aUniversity of Coimbra, Center for Mechanical Engineering, Materials and Processes (CEMMPRE), Department of Mechanical Engineering, 3030-788 Coimbra, PortugalbUniversity of Beira Interior, Centre of Materials and Building Technologies (C-MADE), 6201-001 Covilhã, PortugalcCQM – Centro de Química da Madeira, University of Madeira, Campus da Penteada, 9020-105 Funchal, PortugaldUniversity of Coimbra, Center for Mechanical Engineering, Materials and Processes (CEMMPRE), Department of Civil Engineering, 3030-788 Coimbra, Portugal

a r t i c l e i n f o

Article history:Received 30 January 2017Revised 17 April 2017Accepted 20 April 2017Available online 21 April 2017

Keywords:A. NanocompositesB. Impact behaviorD. Mechanical testingE. Cure

a b s t r a c t

This study investigates the impact properties (impact strength (IS) and impact energy (IE)) of epoxy resinnanocomposites (EPNCs) manufactured with different curing agents and reinforced with aluminananoparticles (NPs). The NPs consisted on alpha alumina with irregular shapes (100 nm maximum size)pretreated with a silane agent. The weight fractions of alumina NPs were 1, 3 and 5 wt(%). Two differentepoxy (EP) resins were studied and compared. The first one was cured and post cured with bis (4-aminophenyl) methane (DDM) and the second one was cured with 3-Dodec-2-enyloxolane-2,5-dione(DDSA) + 8-methyl-3a,4,7,7a-tetrahydro-4,7-methano-2-benzofuran-1,3-dione (MNA). Comparatively tothe neat EP, and among the three NPs loadings used in this study, the EPNCs with 1 wt(%) showed themaximum improvement in IS and IE, around 78%(IE)-89%(IS) for DDM and 82%(IS)-93%(IE) for DDSA+ MNA. EPNCs with 1 wt(%) cured with DDM present the best results for IS and IE, when compared withEPNCs cured with DDSA + MNA. IS and IE of EPNCs highly reduce at 3–5 wt(%).

� 2017 Published by Elsevier Ltd.

1. Introduction

Thermosetting EP resin systems are widely used as matrices inEPNCs for metal substitution in several engineering applications,such as aircraft, automotive, aerospace, shipbuilding, electronicdevices, textiles, machinery, civil construction, and many others[1,2]. EPNCs are also extensively used under harsh working condi-tions because they offer a unique combination of properties thatare unattainable with others thermosetting polymer systems [3–8]. EP resins are available in a wide variety of physical forms, fromlow-viscosity liquid to high-melting solids and allow simplest for-mulations to combine a single EP resin with several curing agents.After curing, EP resins offer better mechanical strength, low shrink-age, excellent adhesion to many substrates, high-temperature per-formance, effective electrical insulation, chemical, and manyothers properties to the final cured product [8–10]. The propertiesof EPNCs are directly dependent on the properties of the matrix.For this reason, a considerable amount of research has been carriedout [11,12] to improve even more the performance of EP resins that

can be influenced, for instance, by modifying the molecular archi-tecture and structure, i.e., by increasing the crosslink density togenerate high stiffness and strength [13] and, simultaneously, tosolve some disadvantages of EP resins such as, low flow, lowtoughness (weak resistance to crack initiation and propagation),brittle failure (because plastic deformation is constrained [14]),high coefficient of linear thermal expansion, shrinkage, low ther-mal conductivity and many others [1,2]. One of the most widelyused thermosetting EP resin to obtain EPNCs is Bisphenol A digly-cidyl ether (BADGE or DGEBA – 2-[[4-[2-[4-(Oxiran-2-ylmethoxy)phenyl]propan-2-yl]phenoxy]methyl]oxirane) [8,15,16]. Somestudies show that DDM [17–20] or DDSA and MNA [21] are suit-able as curing agents for EP resins systems. However, it shouldbe noted that DDM is listed as a nocive substance for humanhealth, namely as carcinogenic (Directive 67/548/EEC).

For EP resin systems, the properties of the matrix can beimproved with toughening agents, such as liquid, rigid or hybridrigid rubbers, rubbery particles, thermoplastics, unreactive tough-eners and many others [1,22–24]. Unreactive tougheners canimprove, for example, the toughness of the final EPNC but, often,reduce other properties such as elastic modulus, strength, andthermo-mechanical properties (for instance, glass transition tem-perature) [1,25]. These drawbacks affect the properties of the finalpolymer composite. For these reasons, much research has been

nprev

ious

stud

ies.

Variablestudy

Parameter

study

Results

harde

ner

+Al 2O3NPs

,46.73

nm

and

ent.

Variation

ofthetough

ener

content:

0–3wt(%).

IEIE

was

directly

prop

ortion

alto

wt(%)co

ntent;

IEincrea

seswiththead

dition

ofAl 2O3NPs

and

reaches

themax

imum

at3wt(%)withan

improv

emen

tof

80%morethan

that

ofnea

tEP

resin.

1)+am

ineba

sedharde

ner

(PH-861

)etran

sformation.

Variation

ofthetough

ener

content:

0–5wt(%)

IEof

notch

edsamples

TheIE

ofEP

NCsattainsitsop

timum

valueat

acritical

concentrationof

2wt(%)withan

improv

emen

tof

84%morethan

that

ofnea

tEP

resin.

andE4

4)+4,40

-e(DDM)an

dChinawoo

doilacid

+Al 2O3NPs

withamea

ndiam

eter

ofnm

andwithsu

rfacetrea

tmen

t(K

H57

0,KH59

0,an

dA15

1)).

Untrea

tedan

dtrea

tedNPs

witha

variationof

thetough

ener

content:

0–40

wt(%).

ISof

unnotch

edsamples.

ISincrea

seswithsize

redu

ction.T

hemax

imum

increa

sewas

foundfortrea

tedAl 2O3NPs

(KH57

0)with20

nm

insize

andfor5wt(%).

in,a

ndbisp

hen

ol-A

(DER

331)

+a

rden

er(H

Y29

54)+

Al 2O3NPs

,13nm

atmen

t.

Variation

ofthetough

ener

content:

0–10

vol.(%).

IEof

unnotch

edsamples.

Al 2O3NPs

werean

effectivetough

ener

atlow

contentof

only

0.5vo

l.(%)an

dreached

the

max

imum

intherange

of1to

2vo

l.(%).

Athigher

vol.(%),theIE

decrea

sesgrad

ually.

,4-diaminod

iphen

ysulfon

e(D

DS)

0.4nm,u

ntrea

tedan

dpre-grafted

(PS)

orpo

lyarcrylam

ide(PAAM).

Untrea

tedan

dgraftedAl 2O3NPs

fora

fixe

dco

ntentof

0.98

vol.(%).

ISof

unnotch

edsamples

Inco

mpa

riso

nwithnea

tEP

NCs,

untrea

tedan

dgraftedAl 2O3NPs

,increa

setheIS

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%an

d55

%,

resp

ective

ly.

A.M. Amaro et al. / Composite Structures 174 (2017) 26–32 27

performed on the incorporation of unreactive nano tougheners invery low NPs content (as a function of the weight fractions wt(%)or volume fractions vol.(%)). Such tougheners are characterizedby their specific shape and size (nanoscale), high surface areas, sur-face pretreatment and a good degree of dispersion into the EP resinmatrix [1,2,4,11,12]. These nano tougheners have already demon-strated their capability to improve the stiffness, toughness, flexuralmodulus and strength, hardness and many others properties of thefinal EPNCs [1,2,11,12]. One of the most used nano tougheners forEPNCs is alumina (Al2O3) NPs. These inorganic NPs can signifi-cantly improve the mechanical properties of the EPNCs, withoutsacrificing their basic properties, even with a low percentage ofloading [1,4,12,26–33].

The results published in the past years, combined with the lowcost of alumina, in comparison with others metal oxides NPs(information’s obtained directly from the suppliers show that alu-mina NPs are cheaper than, for example, titanium NPs), show thatalumina NPs are a viable solution to be used as tougheners forEPNCs.

EP resin matrices are very sensitive to notches and local hetero-geneities. For instance, NPs, which may act as stress concentratorsand reduce the absorbed IE of the EPNC when more NPs are incor-porated. The IE of notched samples is generally much lower thanthat of unnotched samples [34]. Unnotched samples emphasizethe measurement of the energy to initiate a crack which adds tothe energy required to propagate a crack through the EPNC [26].Riley et al. [35] confirmed this effect when they found that theimpact properties (for instance, IS) of polymers composites aremainly increased by NPs with low aspect ratio since large NPscan act as crack initiation sites and high aspect ratio NPa are ableto induce large stress concentrations near their edges.

Another fact that can influence the final impact properties ofEPNCs is the state of the NPs dispersion. If the NPs are inadequatelydispersed into the EP resin matrix, the NPs agglomerates remain asstress concentrators within the matrix [26,34]. However, if the NPsare well dispersed into the EP resin matrix, the impact propertiesof the final can certainly be enhanced [35,36].

The main objective of this study is to evaluate the IS and IE ofunnotched EPNCs with the incorporation of alumina NPs (1, 3and 5 wt(%)) into the EP resin matrix. Related to this objective,some recent research works show that the influence induced bythe addition of alumina NPs on the impact properties of EPNCsare pointed out in Table 1. The results of these works are con-fronted with the results obtained in the present study. Addition-ally, the effect of different curing agents in the impact propertiesof EPNCs is also investigated in this article. For that, the resultsof two different thermosetting EP resin systems were studied.Namely one cured and post cured with DDM and another onecured with DDSA + MNA.

Table1

Impa

ctprop

erties

asafunc

tion

ofalum

inaNPs

conten

tsi

Referen

ceCom

poundmaterials

Aiet

al.2

015[37]

EPresin+Nitofi

llEP

LVwithou

tsu

rfacetrea

tm

Moh

anty

etal.2

015[38]

EPresin(Bon

dtitePL

-41

+Al 2O3(<50

nm)by

siz

Fuet

al.201

0[39]

EPresin(bismaleimide

diam

indiph

enylmethan

anhyd

ride

asharde

ners

20,5

0,80

,100

and50

0(silan

eco

uplingag

ents

Wetze

let

al.(20

03,200

5)[26,34

]EP

resin,e

pich

lorohyd

rcy

cloa

liph

atic

amineha

andwithou

tsu

rfacetre

Jiet

al.2

004[40]

EPresin(typ

eE-51

)+4

harde

ner

+Al 2O3NPs

,1witheither

polystyren

e

2. Material and experimental procedure

2.1. Material

The polymeric matrix used was 2-[[4-[2-[4-(Oxiran-2-ylmethoxy)phenyl]propan-2-yl]phenoxy]methyl]oxirane, a D.E.R.TM 332based on DGEBA, with uniform performance and exceptionallylow viscosity, low chloride content and light color. The used curingagents were Bis (4-aminophenyl)methane (DDM), 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). In one of theprocedures (DDSA + MNA), a catalyst was applied during curingreaction, the N,N-dimethyl-1-phenylmethanamine (BDMA). All ofthese mentioned raw chemicals were purchased from Sigma-Aldrich Co.

a)

28 A.M. Amaro et al. / Composite Structures 174 (2017) 26–32

The alumina NPs (99.99% purity) present an average size lessthan 100 nm, a specific surface area of � 20 m2/g and almost allthe NPs are of irregular shape [2]. The as-received alumina NPsincorporated a surface pretreatment (functionalization) in orderto ensure a better dispersion into the EP resin. This pretreatmentwas performed by the manufacturer (NanoshellTM LLC) with asilane agent (3-Aminopropyl)triethoxysilane, also called APTES.The weight fractions of alumina NPs were as follows: 1, 3 and5 wt(%). More details about the fundamental characteristics ofthe raw chemicals and the aluminum oxide powders used in thisstudy can be found in a preceding paper [2].

b) c)

15mm

60mm

Fig. 1. Charpy Equipment: a) general view; b) hammer view; c) supports view.

Fig. 2. Specimens geometry (dimensions in mm).

2.2. Nanocomposite preparation

Two different manufacturing processes were considered for thestudied EPNCs: manufacturing process 1 (MP1) and manufacturingprocess 2 (MP2). In MP1, the stoichiometric amount of the ther-moset system constituents were the same from Belleri and Bar-della [20,41]: 3.51(DER332):1(DDM). In MP2, the stoichiometricamount of the thermoset system constituents were the same asrecommend by Martinet et al. [21]: 1(DER332):0.85(DDSA):0.15(MNA):0.04(BDMA). In MP1 and MP2 the functionalized aluminaNPs, as-received from the supplier, were added to the mixture,with various weights fractions (1, 3 and 5 wt(%)), always at thesame stage, after the preheating of the EP matrix and before theaddition of the curing agents (DDM – preheated and in the liquidstate – MP1, DDSA + MNA – preheated – MP2) into the mixture.For MP1 and MP2 the neat EP samples (control group) were pre-pared similarly but without the addition of alumina NPs. ForMP1 and MP2, the curing stage was carried out in an oven (Preci-sion Scientific Napco Vacuum Oven Model 5831), under vacuum(20 mmHg) at 60 �C for 24 h [20,42–44].

Only for MP1, the cured samples were subjected to a post curingcycle with two steps: 2 h at 100 �C and more 3 h at 180 �C [20,39–41,45–47]. All the resulting EPNCs were allowed to cool to roomtemperature naturally and then detached after 24 h. The sampleswere cut in order to obtain the final dimensions requested by thestandards. More details about the successive steps used in MP1and MP2 to fabricate the EPNCs can be found in previous worksof the authors [2].

2.3. Experimental procedure

The impact behavior of the manufactured EPNCs was evaluatedby Charpy impact tests. The impact tests intend to measure theresistance to failure of material to a suddenly applied force. TheCharpy impact tests constitute high-speed fracture tests measuringthe energy needed to break the sample under bending conditions.The samples are deformed within a short time and thereforeexposed to high strain rates [26,34]. These tests were done accord-ing to EN ISO 179-1 [48] standard by using a CEAST 9050 pendu-lum testing machine, connected to a software CEAST DAS 8000Junior in order to store the data. Unnotched samples with rectan-gular dimensions of 80 � 12 � 4 mm3 were fractured by a Charpyimpact hammer (reference 2129, code 7601.005.1) with an inci-dent energy of 5 J and an impact speed of 2.9 m/s at room temper-ature and normal atmospheric condition. Measurement dataacquisition was implemented with the Visual IMPACT Software.Fig. 1 (a) shows the Charpy equipment used in these tests. Thehammer is positioned at 150� and then released so as to impactthe specimen. In Fig. 1 (b) and (c) the hammer dimension andthe supports are displayed, respectively. Fig. 2 illustrates thedimension of the samples.

The impact strength (IS) was calculated using the followingequation:

IS ¼ Ebreak

w � t ð1Þ

where Ebreak is the impact energy at break point, w and t are thewidth and thickness of the EPNC specimen, respectively. The impactenergy (IE) was calculated by using Eq. (2).

IE ¼ DEw � t ð2Þ

where DE is the IE absorbed during the impact.Cross section morphology was analyzed by scanning electron

microscope (SEM) using a ZEISS MERLIN compact/VPCompact fieldemission scanning electron microscope (FESEM). A scanning elec-tron microscopy with X-ray spectroscopy (SEM-EDS) was also per-formed in order to obtain a chemical characterization of thesample. All the specimens were previously sputtered coated withgold, approximately 10 nm thickness, using an Edwards EXC witha source Huttinger PFG 1500 DC.

A minimum of five unnotched samples of each category (0, 1, 3and 5 wt(%)) was used for all the tests, and the average value wasconsidered for the analysis.

3. Results and discussion

The experimental results concerning the impact properties ofthe tested samples, for the three EPNCs filled with alumina NPsand also for the reference neat samples, are presented in Table 2

Table 2Experimental results of the tested samples.

Mechanical Property Curing agents Nano Al2O3 content (wt(%))

0 1 3 5

IS (kJ/m2) DDSA + MNA 11.382 20.692 11.729 12.140Std Dev 1.446 2.574 1.544 1.899IS (kJ/m2) DDM 15.596 29.399 6.741 9.070Std Dev 1.379 1.257 0.882 1.503IE (kJ/m2) DDSA + MNA 11.637 22.453 10.887 12.032Std Dev 1.252 1.868 1.611 2.047IE (kJ/m2) DDM 16.176 28.738 6.894 9.341Std Dev 0.121 3.300 0.703 0.832

Fig. 4. Impact energy for different curing agents (DDSA + MNA and DDM) as afunction of alumina nanoparticles content.

A.M. Amaro et al. / Composite Structures 174 (2017) 26–32 29

and in Figs. 3 and 4. Table 2 presents the average values and therespective standard deviations values for IS and IE, calculated fromEqs. (1) and (2), respectively. Figs. 3 and 4 show graphically theevolution of the average IS and IE values, for the different curingagents used in this study (DDSA + MNA and DDM), as a functionof the alumina NPs content, respectively. The referred figures alsopresent the corresponding dispersion bands.

Table 2 and Figs. 3 to 4 show that the trends observed for IS andIE agree with each other. The results also demonstrate that, for theneat samples, higher averages values for IS and IE were obtainedwith DDM as the curing agent (around + 37% for IS and + 39% forIE), when compared to the samples with DDSA + MNA. This seemsto show that EP resins cured with DDM have higher crosslink den-sity, which generates higher strength and higher energy absorptioncapacity.

From Table 2 and Figs. 3 to 4 it is also possible to observe thatthe 1 wt(%) alumina NPs content promotes higher IS and IE for boththe used curing agents (DDSA + MNA and DDM) when comparedwith the other percentage loadings. The relative change isaround + 82% (DDSA + MNA) and + 89% (DDM) for IS, and + 93%(DDSA + MNA) and + 78% (DDM) for IE, respectively, when com-pared with the neat samples. These results are in agreement withthe results obtained by Wetzel et al. [26]. As for the neat EP resins,for 1 wt(%) the results also show that for samples cured with DDM,higher averages values for IS and IE were obtained when comparedto the same ones obtained for samples cured with DDSA + MNA(around +42% for IS and +28% for IE).

Fig. 3. Impact strength for different curing agents (DDSA + MNA and DDM) as afunction of alumina nanoparticles content.

Ji et al. 2004 [40] observed for EPNCs filled with untreated andtreated alumina NPs an increase in IS around 19% and 55%,respectively. However, for such results, these authors requiredmore content of alumina NPs (0.98 vol.(%) �3 wt(%)). Fu et al.2010 [39] found the higher increase in IS at 5 wt(%) for all rangeof size of NPs until 100 nm. In addition, the treated NPs improvemore the IS than untreated ones. The authors state that thiscan be attributed to the good dispersion of NPs when they are

Fig. 5. SEM images for some samples: (a) DDSA – 1 wt(%); (b) DDM – 3 wt(%).

30 A.M. Amaro et al. / Composite Structures 174 (2017) 26–32

modified with a surface treatment which induces interactions andcompatibilities between the NPs and matrix [12]. This aspect canalso explain the tendencies observed in the present study.

Fig. 6. SEM-EDS spectra for DDM 5 wt(%) (a)

However, for the two mentioned studies [39,40] the authorsrequired more content of alumina NPs, in comparison with the pre-sent study. Here, it is only required a content of 1 wt(%) to obtain

(a)

(b)

(c)

(d)

SEM; (b) zone 1; (c) zone 2; (d) zone 3.

A.M. Amaro et al. / Composite Structures 174 (2017) 26–32 31

larger increments in IS and this is an advantage in the final cost ofthe EPNCs.

For IE, the same trend was found by Mohanty et al. 2015 [38].These authors observed that with an increase in wt(%) of aluminaNPs in the EP matrix, the IE increases up to a critical concentrationand decreases thereafter. These authors observed, for 5 wt(%) load-ing of alumina NPs and for the thermoset system cured with DDM,a drastic reduction in IE (�42%). However, in such study, it is for3 wt(%) loading of alumina NPs that the drastic reduction in theIE is higher (�57%).

In the present study, for percentage loading above 1 wt(%), ISand IE seems to be less influenced by the curing agents as the alu-mina NPs content increases. Furthermore, the results show that forsamples cured with DDM, smaller averages values for IS and IEwere obtained when compared to the same ones obtained for sam-ples cured with DDSA + MNA. These results doesn’t agree with theresults previously observed for the neat EP resins and for EPNCswith 1 wt(%) loading. However, the differences seems to decreaseas the alumina NPs content increases. The influence of agglomera-tions of NPs for percentage loading above 1 wt(%) can probablyexplain these differences. This aspect is dicussed later.

Both IS and IE decreases as the percentage loading increasesabove 1 wt(%), for both thermoset systems (DDM and DDSA+ MNA). However, for thermoset systems cured with DDM, thereduction is higher. In this case, the relative change for IS is around�57% and �42% for 3 wt(%) and 5 wt(%), respectively, with respectto the neat samples. For IE, the same relative changes are very sim-ilar. On the other hand, for thermoset systems cured with DDSA+ MNA, the relative changes with respect to the neat samples arenegligible.

Haupert and Wetzel [34] justify the decrease in the IE with theincrease in the filler contents with the small stress concentrationeffects around the inclusions. The results in the present study arein agreement with the obtained by these authors for both curingagents studied. In this study, the different trends observed forthe EPNCs cured with DDM and DDSA + MNA can also be explaineddue to an increasing of the NPs agglomeration as the NPs weightfraction increases. Such agglomerations of NPs act as additionalstress concentrators and contributes to less formation of covalentbonds between the nanofiller and the polymeric matrix and alsoto the worse dispersion of the NPs [2]. This can be responsible,according to some authors [38,49], for the decrease in the impactproperties of EPNCs. These aspects certainly contribute to reduceboth IS and IE of EPNCs when more NPs are incorporated and canexplain the tendencies observed in the present study.

Wetzel et al. [26,34] and Mohanty and Srivastava [38] studiedthe impact behavior (IB) of EPNCs with different thermoset sys-tems and have also reported a strong improvement on the IB ofthe EPNCs up to a certain critical loading of alumina NPs into theEP resin matrix. At higher NPs contents, IB (for instance, IE)decreases gradually. According to these authors, this is because,for higher NPs contents, the NPs agglomerations induce failure justlike large particles. Wetzel et al. [36] explain these results due to:the geometrical properties of the NPs, poor interfacial adhesion,the inhomogeneous dispersion state (agglomeration of the NPs),energy dissipating fracture mechanisms (for instance: matrix shearyielding, plastic deformation, particle pull out, microcracking, voidformation, residual stress fields and crack pinning or crack fronttrapping [26,39,40]) and many others. In the present study andin terms of IS and IE, the alumina NPs and the NPs agglomerationsact as failure sites [26] and induces the drastic reductions observedat 3 and 5 wt(%) for the thermoset systems, in particular for theone cured with DDM, and for the unnotched tested samples. Asmentioned by Wetzel et al. [12,26], NPs agglomeration has thesame effect than the notches in specimens.

As previously stated for this study, EP resins cured with DDMseems to have higher crosslink density when compared with EPresins cured with DDSA + MNA. EP resins cured with DDM are stif-fer when compared with the one cured with DDSA + MNA. For thisreason, the negative influence of NPs agglomerations for higherNPs loadings is probably higher for EP resins cured with DDM.

In this study, among the three NPs loadings considered, theresistance to impact loading of the EPNCs is highest for 1 wt(%)of NPs for both thermoset systems studied. This can be attributedto previous results from the authors and for the same EPNCs usedin this study [2]. In this previous study, the better dispersion of alu-mina NPs was achieved for 1 wt(%). This was confirmed with SEMimages of the samples.

Fig. 5 show some examples of the obtained SEM images. FromFig. 5 (b), for 3 wt(%) (DDM), it is possible to visualize NPs agglom-erations. Fig. 5 (a), for 1 wt(%) (DDSA + MNA), shows good NPs dis-persion and no NPs agglomerations are visualized.

Fig. 6 shows the SEM_EDS spectra for EPNCs with DDM, whichconfirms the bad dispersion for 5 wt(%). These figures are represen-tative for all the studied cases.

From Fig. 6 it is possible to confirm that for higher values of NPscontents the dispersion in the composite is not so good. In fact,even in near regions, like the regions studied in this case, the dis-tribution of alumina NPs is not uniform. In zone 2, Al2O3 wasdetected, while in zone 3 it did not.

The results of this study show that the degree of enhancementof a specific property is highly dependent on the type of the matrix– tougheners thermosetting system used, the extent of adhesion ofthe toughener to the matrix (quality of the interphase), and thestate of the dispersion (which depends on the surface treatmentof the NPs) of the toughener throughout the EP resin matrix. Thisresults confirm the observations obtained in previous studies(see for instance, [50]).

4. Conclusions

This article presented an experimental study on the reinforcingeffects of pretreated alpha alumina NPs (with non-spherical shapesand with 100 nm maximum size) used as nanofillers in EP resins.The impact properties of EPNCs cured with DDSA + MNA andDDM curing agents, namely IS and IE, were studied. The two differ-ent EP resin matrices were filled with three different weight frac-tions of alumina NPs: 1, 3 and 5 wt(%). From the obtained results,the following conclusions can be drawn:

- Among the three NPs loadings considered in this study, maxi-mum values for the IS and IE of the tested EPNCs were observedfor 1 wt(%) content of alumina NPs. The increase is around 82%(IS)-93%(IE) for DDSA + MNA and 78%(IE)-89%(IS) for DDM, incomparison with the neat samples;

- For percentage loading above 1 wt(%), namely for 3 wt(%) and5 wt(%), IS and IE decreases as the percentage loading of alu-mina NPs increases. For the thermoset system cured withDDM, the decrease is around �57% at 3 wt(%) and �42% at5 wt(%) loading of alumina NPs, with respect to the neat sam-ples. For the thermoset system cured with DDSA + MNA andfor 3–5 wt(%), the IS and the IE does not change significantlyin comparison with the neat samples;

- Although the thermoset system cured with DDM presents thebest results for the impact performance at the critical percent-age loading of NPs, when compared with the thermoset systemcured with DDSA + MNA, the improvement is not very high. Forthis reason, and since for the curing agent DDSA + MNA no post-cure is required, the use of DDSA + MNA can be a good option interms of thermoset manufacture.

e Structures 174 (2017) 26–32

Acknowledgements

This research is sponsored by UID/EMS/00285/2013 – and bynational funds through FCT – Fundação para a Ciência e a Tecnolo-gia –, under grants PEst-C/EME/UI0285/2013, PEst-OE/QUI/UI0674/2013 and SFRH/BPD/85049/2012. DP and JR acknowledgethe support of LREC – Laboratório Regional de Engenharia Civilda Madeira and Dr. César Fernandes, for having borrowed the castsused in the preparation of EPNCs specimens.

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