Materials and Design 62 (2014) 158–167

Contents lists available at ScienceDirect

Materials and Design

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

Improvement of epoxy resin properties by incorporationof TiO2 nanoparticles surface modified with gallic acid esters

http://dx.doi.org/10.1016/j.matdes.2014.05.0150261-3069/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +381 11 3303 683; fax: +381 11 3370 387.E-mail address: edzunuzovic@tmf.bg.ac.rs (E.S. Dzunuzovic).

Tijana S. Radoman a, Jasna V. Dzunuzovic b, Katarina B. Jeremic c, Branimir N. Grgur c, Dejan S. Milicevic d,Ivanka G. Popovic c, Enis S. Dzunuzovic c,⇑a Innovation Center, Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, Belgrade 11120, Serbiab Institute of Chemistry, Technology and Metallurgy (ICTM), Center of Chemistry, University of Belgrade, Studentski trg 12–16, 11000 Belgrade, Serbiac Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11120 Belgrade, Serbiad Laboratory for Radiation Chemistry and Physics ‘‘Gamma’’, Institute of Nuclear Science Vinca, University of Belgrade, Mike Petrovica Alasa 12-14, 11001 Belgrade, Serbia

a r t i c l e i n f o a b s t r a c t

Article history:Received 18 February 2014Accepted 12 May 2014Available online 16 May 2014

Keywords:Epoxy/TiO2 nanocompositesSurface modificationGlass transition temperatureWater vapor permeabilityElectrochemical impedance spectroscopyCorrosion current

Epoxy resin/titanium dioxide (epoxy/TiO2) nanocomposites were obtained by incorporation of TiO2 nano-particles surface modified with gallic acid esters in epoxy resin. TiO2 nanoparticles were obtained by acidcatalyzed hydrolysis of titanium isopropoxide and their structural characterization was performed byX-ray diffraction and transmission electron microscopy. Three gallic acid esters, having different hydro-phobic part, were used for surface modification of the synthesized TiO2 nanoparticles: propyl, hexyl andlauryl gallate. The gallate chemisorption onto surface of TiO2 nanoparticles was confirmed by Fouriertransform infrared and ultraviolet–visible spectroscopy, while the amount of surface-bonded gallateswas determined using thermogravimetric analysis. The influence of the surface modified TiO2 nanopar-ticles, as well as the length of hydrophobic part of the gallate used for surface modification of TiO2

nanoparticles, on glass transition temperature, barrier, dielectric and anticorrosive properties of epoxyresin was investigated by differential scanning calorimetry, water vapor transmission test, dielectricspectroscopy, electrochemical impedance spectroscopy and polarization measurements. Incorporationof surface modified TiO2 nanoparticles in epoxy resin caused increase of glass transition temperatureand decrease of the water vapor permeability of epoxy resin. The water vapor transmission rate ofepoxy/TiO2 nanocomposites was reduced with increasing hydrophobic part chain length of gallate ligand.Dielectric constant of examined nanocomposites was influenced by gallate used for the modification ofTiO2 nanoparticles. The nanocomposites have better anticorrosive properties than pure epoxy resin,because the surface modified TiO2 nanoparticles react as oxygen scavengers, which inhibit steel corrosionby cathodic mechanism.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Polymer nanocomposites represent hybrid materials whichhave synergistic properties of polymer matrix and applied nano-scale reinforcement. The smaller size of nanoparticles, their extre-mely high surface to volume ratio, strong interactions between thepolymer and nanofiller and formation of large interface betweenthem are the key factors for the distinctly different and betterproperties of nanocomposites than that of the conventional poly-mer composites containing micrometer-sized fillers [1,2]. Thebasic concepts for the preparation of polymer nanocompositesand change of the polymer properties after addition of inorganic

nanoparticles are covered in numerous review papers [3–5]. Thecombination of the inherent performances of inorganic nano-sizedparticles with properties of polymers leads to the significantimprovement of mechanical, thermal, optical, barrier, electrical,magnetic and other properties of polymer matrix. In order toachieve the desired properties of the polymer nanocomposite,nanoparticles must be uniformly dispersed within the polymermatrix. However, the uniform incorporation of nanofillers in poly-mer matrix is difficult to accomplish, due to their high surfaceenergy and consequently, great tendency to agglomerate. Ade-quate chemical treatment of the surface of nanoparticles is provedto be very effective method which can be used to prevent theiragglomeration, decrease surface tension, and improve interactionof nanoparticles with polymer matrix [6]. Silane modified halloy-site nanotubes were better dispersed in unsaturated polyester

T.S. Radoman et al. / Materials and Design 62 (2014) 158–167 159

resin than unmodified, causing toughness improvement and bettermechanical properties [7]. Hamming et al. showed that biomimeticsurface modification of TiO2 nanoparticles increased their interfa-cial interaction with poly(methyl methacrylate) (PMMA) matrix,leading to the increase of the glass transition temperature [8].According to Špírková et al., the application of organically modifiedlayered nanosilicates improves their dispersion in the polyepoxy-functional polysiloxanes cured with organic diamines [9]. Also,modified multi-walled carbon nanotubes can be better dispersedwithin the polymer matrix than unmodified ones, leading to thesignificant improvements of mechanical properties [10]. Further-more, Dzunuzovic et al. have observed that thermal properties ofpolystyrene (PS) can be considerably improved by incorporationof TiO2 nanoparticles surface modified with 6-palmitate ascorbicacid [11] and alkyl gallates [12]. These authors have also investi-gated the influence of TiO2 nanoparticles surface modified withgallates [13] and 6-palmitate ascorbic acid [14] on the propertiesof PMMA matrix. The authors concluded that the length of ali-phatic part of the used gallates has great effect on dispersabilityof surface modified TiO2 nanoparticles and on the properties ofboth matrices, especially on thermo-oxidative stability.

Epoxy resins are thermosetting polymers which have beenwidely used as anticorrosive coatings, adhesives, paints, electronicdevices, in automotive and aerospace industry, etc., because oftheir good performances such as electrical and corrosion resis-tance, excellent chemical, moisture and solvent resistance, gooddimensional stability, good adhesion to many substrates, easy ofcure and processing. Properties of epoxy resins are mainly influ-enced by their molecular structure, amount and type of the appliedhardener and curing conditions [15]. However, epoxy resins arebrittle materials due to the crosslinked structure [16] and thereforehave low impact and fracture strength [17]. Properties of epoxyresins can be significantly improved using different nanoparticlesas fillers. Nanoparticles can fill up the cavities present in resin,decrease total free volume, leading to the significant tougheningand improvement of the polymer properties [18]. Uniform disper-sion of small amount of SiO2, Fe2O3 and halloysite clay nanoparti-cles improves homogeneity, barrier properties and corrosionresistance of the epoxy-based coatings [19,20]. Incorporation ofZnO nanoparticles in epoxy-based coatings enhances corrosionresistance, resistance against hydrolytic degradation and adhesionof coatings [21]. The incorporation of Al2O3 nanoparticles in epoxyresin simultaneously improving stiffness, strength and toughnessof epoxy, without sacrificing thermo-mechanical properties [22].

Among different nanoparticles used to improve properties ofepoxy resins, TiO2 is probably the most interesting and the mostinvestigated metal-oxide. Titanium dioxide is prepared in anataseand rutile forms, it is inexpensive and quite efficient and hasunique properties such as non-toxicity, chemical, corrosion andphoto stability, good electrical properties, good compatibility withvarious materials, high photocatalytic activity, high refractiveindex and ability to absorb ultraviolet (UV) light [23]. Due to theirexcellent properties, TiO2 nanoparticles have been used to prepareantibacterial, anticorrosive, transparent and self-cleaning coatings,as photocatalysts, in skin care products and nanomedicine, in foodpackaging, for UV protection, to built solar cells, for water purifica-tion, in gas sensors, for lithium-ion batteries, etc. Different authorsreported that the presence of TiO2 nanoparticles in epoxy resinscan improve mechanical [1,22,24–26], viscoelastic [24] and ther-mal properties [24] and corrosion resistance [27,28], increasehydrophilicity on the surface of the coatings [29] and provideexcellent optical transparency of the epoxy based coatings [30].

One of the greatest issues present in urban areas today, whichproduce serious costs to the mankind, is corrosion of metal. Thestrategy usually used to prevent the appearance of the corrosionon the metallic substrates is application of protective coatings with

anticorrosion ability. However, conventional anticorrosive coatingsare not permanently impenetrable and cannot fully protect metalon which they are applied from the attack of corrosive species(mainly oxygen and water). Namely, this type of coatings permitsslow diffusion of oxygen within the film, especially if defects existin the coating, which eventually leads to the localized corrosion ofmetal and causes delamination of the applied coating. In addition,conventional anticorrosive coating formulas sometimes includetoxic carcinogens as corrosion inhibitors, such as hexavalent chro-mium compounds, which use is restricted. Therefore, the moderncoating technology requires development of materials which willimprove safety and reduce environmental impact and, at the sametime, will have excellent anticorrosive, thermal and mechanicalperformances in order to overcome harmful environmental condi-tions. The use of nanofillers in anticorrosion coatings has beenextensively explored in order to prepare effective anticorrosiveenvironmentally friendly coatings.

In this paper, the gallic acid esters surface modified TiO2 nano-particles were used to prepare three nanocomposites based onepoxy resin. The TiO2 nanoparticles were synthesized by acid cat-alyzed hydrolysis of titanium isopropoxide. The average particlesize, size distribution and crystal structure of the synthesizedTiO2 nanoparticles was estimated by transmission electron micros-copy (TEM) and X-ray diffraction. Surface modification of TiO2

nanoparticles was performed with three amphiphilic esters of gal-lic acid. The hydrophobic part of the used gallates is represented byalkyl chains of different length, propyl (C3), hexyl (C6) and lauryl(C12). Surface modified TiO2 nanoparticles were characterizedusing Fourier transform infrared (FTIR) and ultraviolet–visible(UV–Vis) spectroscopy. The influence of the surface modifiedTiO2 nanoparticles, as well as the chain length of hydrophobic partof the gallate used for surface modification of TiO2 nanoparticles onglass transition temperature, barrier, dielectric and anticorrosiveproperties of epoxy resin was investigated.

2. Materials and methods

2.1. Materials

Titanium isopropoxide was obtained from TCI Europe. Gallicacid, propyl gallate, dodecyl (lauryl) gallate, 1-hexanol and 2-pro-panol were obtained from Sigma–Aldrich. Epoxy resin, CHS-EPOXY210 � 75, was purchased from Spolchemie. The curing agent, EPIK-URE 3115 � 70, was purchased from Momentive. All chemicalswere used as received without further purification.

2.2. Synthesis of hexyl gallate

Hexyl gallate (HG) was synthesized by esterification of gallicacid with hexyl alcohol, according the procedure described else-where [31]. The reaction was performed in the reaction flask con-nected to a Soxhlet apparatus, containing 10 g of sodium sulfate asa drying agent. Gallic acid (50 g), hexyl alcohol (136 g and 80 mlextra to fill Soxhlet apparatus) and sulfuric acid (1 ml) were placedinto the flask. The reaction mixture was stirred with magnetic stir-rer at 165 �C. The water formed during the reaction made azeo-trope with hexyl alcohol and was captured by drying agent inSoxhlet apparatus. After 8 h, reaction mixture was placed in Rota-vapor and hexyl alcohol was distilled until a mixture of 75 wt% ofhexyl gallate in hexyl alcohol was reached. The mixture was thenpoured into methylene chloride with stirring in order to crystallize.The obtained suspension was washed with water and hexyl gallatewas placed between two layers. The crude product was separatedby filtration, washed with water and methylene chloride and driedin vacuum oven at 60 �C.

160 T.S. Radoman et al. / Materials and Design 62 (2014) 158–167

2.3. Synthesis and modification of colloidal TiO2

The colloidal TiO2 dispersion was synthesized by hydrolysis oftitanium isopropoxide at 80 �C under a stream of dry nitrogen for8 h, according to procedure described in the literature [32]. A drop-ping funnel, containing 2.0 ml of 2-propanol, was loaded with12.5 ml of titanium isopropoxide, the mixture was added to75 ml of deionized water and vigorously stirred. During the hydro-lysis, a white precipitate was formed. Within 10 min of the alkox-ide addition, 0.57 ml of 65% nitric acid was added to the hydrolysismixture. The mixture was stirred at 80 �C, alowing 2-propanol toevaporate during that time. Finally, approximately 70 ml of stableTiO2 colloidal solution was obtained.

The surface of TiO2 nanoparticles was modified with three gallicacid esters: propyl (TiO2-PG), hexyl (TiO2-HG), and lauryl (TiO2-LG)gallate. Modification of colloidal particles with all three gallateswas performed in the same manner. As an example, the modifica-tion with hexyl gallate is described. Hexyl gallate (0.0772 g) wasdissolved in 15 ml of chloroform and methanol mixture (4:1 vol./vol.). TiO2 colloid solution (2 ml) was diluted ten times with deion-ized water and mixed with prepared hexyl gallate solution in aseparation funnel. After shaking for a short time, a dark-red chloro-form phase, containing TiO2 particles surface modified with hexylgallate (TiO2-HG), separated from the aqueous phase. The obtainedred chloroform solution was drop-wise added to a 100 times largervolume of methanol to remove residual free hexyl gallate mole-cules. The surface modified TiO2 particles were separated by cen-trifugation and redispersed in chloroform or xylene for furtherusage.

2.4. Preparation of epoxy/TiO2 nanocomposite films

The commercially available epoxy resin, CHS-EPOXY 210 � 75,was used as polymer matrix and cured with EPIKURE 3115 � 70hardener, using a weight ratio of 100:35. The nanocomposites with1 wt% of TiO2, calculated with respect to the total mass of epoxyresin and curing agent, were prepared by adding appropriateamount of surface modified TiO2 nanoparticles, dispersed inxylene, to epoxy resin followed by mixing in ultrasonic bath (Son-orex Digitec) for 10 min. After that, corresponding amount of cur-ing agent, EPICURE 3115 � 70, was added and obtained mixturewas additionally mixed in ultrasonic bath for 10 min. In order toachieve fully cured nanocomposite films of uniform thickness,the dispersions were drawn in films on 10 � 10 cm glass and7 � 15 cm steel plates with wire-wound rods, and cured at roomtemperature for 21 days.

2.5. Characterization of nanoparticles

X-ray diffraction (XRD) measurements were performed on aPhillips PW1710 powder diffractometer with Ni filtered Cu Karadiation. The particle diameter was obtained using the Scherrerequation.

The average particle size and size distribution of the TiO2 nano-particles were determined by a transmission electron microscopy(JEOL-1200EX).

FTIR spectra of dry, unmodified and modified, TiO2 nanoparti-cles were recorded on a Bomem MB 100 FTIR spectrophotometerin the form of KBr pellets. The absorption spectra of the TiO2 colloidin water and TiO2 surface modified nanoparticles in chloroformwere recorded using a Perkin–Elmer Lambda 35 UV/Visspectrometer.

The contents of gallates grafted on the surface of TiO2 weredetermined by thermogravimetric analysis (TGA) using a SetaramSetsys Evolution-1750 instrument. The measurements wereperformed at a heating rate of 10 �C/min, in dynamic argon

atmosphere (flow rate 20 cm3/min). Before measurements sampleswere dried in vacuum oven for 12 h, at the temperature of 60 �C.

2.6. Characterization of nanocomposites

Differential scanning calorimetry (DSC) measurements wereperformed on a Perkin Elmer DSC-2 instrument in a nitrogen atmo-sphere, at a heating rate of 20 �C/min. The samples for DSC mea-surements were cut from the cured films pulled off from theglass plates. Coated glass plates were submerged overnight inwater. The films were pulled off and dried in vacuum oven for2 h at 100 �C.

Water vapor transfer through the epoxy films was measuredusing standard (ASTM: D1653) permeability cup (BYK-Gardner).The cup was filled with a desiccant, dry calcium chloride, and thenthe film was clamped and sealed across the open end of the cup.The cup was placed in an atmosphere of controlled relative humid-ity (85%) provided by saturated potassium chloride solutions. Dur-ing the test, vapor was allowed to pass from a solution through thetest film to a desiccant within the permeability cup.

Dielectric properties were measured on Agilent 4284A PrecisionLCR Meter in the temperature range 20–140 �C. The readings werecarried out at increments of 2 �C during a heating run, with a heat-ing rate of 1.7 �C/min between equilibrated temperatures. At eachequilibrated temperature, measurements of capacitance and tandwere taken at several frequencies from 50 Hz to 1 MHz. The sizeof the test samples was 13 mm in diameter.

Polarization measurements were carried out using Gammry PC3potentiostat, in three-compartment glass cell (100 cm3) with satu-rated calomel electrode (SCE) as the reference and Pt plate as acounter electrode. The mild steel (MS, AISI 1212) electrodes(A = 2 cm2) were mechanically polished with fine emery papersdegreased in acetone and pickled in hydrochloric acid. The elec-trode was submersed perpendicularly into 1 wt% solution of sur-face modified TiO2 nanoparticles (TiO2-PG, TiO2-HG and TiO2-LG)in chloroform and the coatings was obtained by slowly evaporationof chloroform. The obtained film was dried in an oven at 35 �C for20 min.

Electrochemical impedance spectroscopy (EIS) measurementswere carried out in 0.1M H2SO4 solution at room temperature,using a PAR 273 Potentiostat/Galvanostat/PAR 5301 lock-in ampli-fier integrated with a PC system. A nonconductive PMMA-cell, with8 holes for coated specimens (diameter size 1 cm), with three elec-trodes was used. The coated steel specimen was the working elec-trode (WE), a Pt mesh counter electrode was used and referenceelectrode (RE) was a saturated calomel electrode. The cell assemblywas located at a Faraday cage to prevent electrical interferences.Experiments were performed under the open circuit potential, overa frequency range from 105–10 Hz, using a 10 mV amplitude sinu-soidal voltage. A sample was immersed in the test cell withoutpresoaking in a solution, and the impedance measurement wascarried out immediately.

3. Results and discussions

3.1. Characterization of the synthesized bare TiO2 nanoparticles

Nanosized TiO2 colloids were obtained by acid catalyzed hydro-lysis of titanium isopropoxide. The average particle size and sizedistribution were estimated by TEM. The TEM image of the synthe-sized TiO2 nanoparticles and size distribution histogram are shownin Fig. 1. One can see that the particles are roughly spherical inshape with average particle size of 3.9 ± 0.9 nm.

The crystal structure of the obtained TiO2 nanoparticles wasestimated by X-ray diffraction. In the diffraction pattern of the

Fig. 1. (a) TEM image of TiO2 and (b) TiO2 nanoparticle diameter size distribution.

T.S. Radoman et al. / Materials and Design 62 (2014) 158–167 161

synthesized TiO2, shown in Fig. 2, characteristic peaks of anataseTiO2 crystal form at 25.28�, 37.89�, 48.04�, 54.20�, 55.09�, 63.16�,63.53�, 69.95�, and 75.20� were observed. The average crystallinesize of TiO2 nanoparticles, calculated from XRD data usingDebye–Scherrer’s formula, is 4.2 nm. The obtained value is in goodagreement with the value estimated from TEM analysis.

3.2. Characterization of gallate surface modified TiO2 nanoparticles

Surface modification and hydrophobication of the synthesizedTiO2 nanoparticles was performed by three different alkyl gallates:propyl, hexyl and lauryl gallate. Charge transfer complex betweenthe TiO2 nanoparticles and the alkyl gallates was formed, whichwas confirmed by FTIR and UV–Vis spectroscopy. FTIR spectra ofthe TiO2 nanoparticles, hexyl gallate (HG) and TiO2 nanoparticlesmodified with hexyl gallate (TiO2-HG) are given in Fig. 3.

Characteristic bands at 3450 cm�1 and 3348 cm�1, observed inthe FTIR spectrum of HG, originate from OH stretching vibrations.The bands at 2953 and 2869 cm�1 are assigned to the asymmetricand symmetric CAH stretching in methyl group, while bands at2931 and 2854 cm�1 are ascribed to the asymmetric and symmet-ric CAH stretching vibrations in methylene groups, respectively.Band at 1668 cm�1 corresponds to the ester carbonyl stretchingvibrations and bands at 1608, 1533, and 1408 cm�1 are assignedto the aromatic ring stretching vibrations. Peak which is observed

Fig. 2. XRD pattern of the synthesized TiO2 nanoparticles.

at 1465 cm�1 is ascribed to the aliphatic CAH bending and aro-matic ring stretching vibrations, while band at 1379 cm�1 corre-sponds to the CAO stretching vibrations of phenolic group. Bandassigned to in-plane OH bending vibrations was overlapped (shoul-der at 1330 cm�1) with the band at 1304 cm�1, which originatesfrom the CAO stretching vibrations of phenolic group. Other char-acteristic bands can be found at 1257 cm�1 (C(@O)AO stretchingvibration from ester group), 1196 cm�1 (in-plane bending vibra-tions of phenolic group) and 1030 cm�1 (OACAC stretching vibra-tion from ester group). FTIR spectrum of dried TiO2 colloids showsbroad adsorption between 3700 and 3000 cm�1, corresponding tothe stretching vibrations of surface hydroxyl groups and symmet-ric and asymmetric stretching vibrations of surface adsorbedwater. The bands which can be observed at 2956, 2922 and2853 cm�1 correspond to organic residues that arise due to theused production method for colloids. Two bands at 1630 and1385 cm�1 are assigned to the bending vibrations of adsorbedwater and nitrate anion, respectively. In the FTIR spectrum of sur-face modified TiO2 nanoparticles, obtained after adsorption of HGon the surface of TiO2 nanoparticles, the bands at 3450, 3348 and1196 cm�1 completely disappeared. The bands corresponding tothe C(@O)AO and OACAC stretching vibration from ester groupbecame broader and shifted to lower and higher frequency, respec-tively. At the same time, the band at 1379 cm�1, corresponding to

Fig. 3. FTIR spectra of the TiO2 nanoparticles, hexyl gallate (HG) and TiO2

nanoparticles modified with hexyl gallate (TiO2-HG).

Fig. 5. The TG curves of TiO2-PG, TiO2-HG and TiO2-LG, obtained in argonatmosphere.

162 T.S. Radoman et al. / Materials and Design 62 (2014) 158–167

CAO stretching vibrations of phenolic group, with unchangedintensity, became broader and shifted to the lower frequency (at1360 cm�1). Also, the intensity of band at 1304 cm�1 drasticallydecreased, but this band did not completely disappear. This indi-cates that the third phenolic AOH group remains unbounded andit could be concluded, according to the results obtained in thisand our previous works, that HG was bonded to the surface Tiatoms through the adjacent AOH groups, forming binuclear (bridg-ing) complex.

The formation of charge transfer complex between the TiO2

nanoparticles and used alkyl gallates was also confirmed by UV–Vis spectroscopy. Absorption spectra of bare TiO2 and TiO2 nano-particles surface modified with propyl, hexyl and lauryl gallateare shown in Fig. 4. The absorption spectrum of the surface modi-fied TiO2 nanoparticles is red-shifted (absorption onset is around650 nm) compared to the unmodified TiO2 nanoparticles (absorp-tion onset is around 380 nm). The binding of gallate molecules topentacoordinated surface Ti atoms in square pyramidal positionleads to the charge transfer complex formation, causing red shiftof absorption spectra of TiO2 nanoparticles upon modification withgallates [11,12].

The amount of gallates adsorbed on the surface of TiO2 was cal-culated from the TGA results. TGA curves of TiO2-PG, TiO2-HG andTiO2-LG obtained in argon atmosphere are shown in Fig. 5. The firststage of mass loss observed in the TG curves of surface modifiedTiO2 corresponds to the loss of the absorbed water. The mass losswhich occurred between 210 and 800 �C was correlated to themass of gallates grafted on the surface of TiO2 nanoparticles.Experimentally determined mass losses of samples TiO2-PG,TiO2-HG and TiO2-LG in this temperature region are 14.8, 18.1and 22.5 wt%, respectively. According to the results obtained byTGA, the amount of surface adsorbed propyl gallate, hexyl gallateand lauryl gallate is 0.85, 0.90 and 0.89 mmol per gram of TiO2,respectively. The obtained values indicate that the graft densityfor examined samples is almost the same. It is known, from the lit-erature, that the concentration of the Ti surface sites can be calcu-lated using the equation: [Tisurf] = [TiO2] 12.5/D, where [TiO2] is themolar concentration of TiO2 and D is the diameter of particles inangstroms [33]. In the present and our previous works, we haveshown that gallic acid and its esters are bonded to the surface Tiatoms through the adjacent AOH groups, forming binuclear (bridg-ing) complex, i.e. molar ratio between Tisurf atoms and gallateligands is 2:1 [12,34]. In the present work, [Tisurf] is 0.0178 mol/dm3, so the concentration of gallates required for covering all Tisurface sites (100% coverage) should be 0.0089 mol/dm3 or

Fig. 4. The absorption spectra of aqueous TiO2 colloid solution and solutions ofTiO2-PG, TiO2-HG and TiO2-LG in chloroform.

1.86 mmol of gallate per gram of TiO2. Calculated coverage basedon the results obtained by TGA is about 46% of maximal (theoret-ical) value for TiO2-PG and about 48% for TiO2-HG and TiO2-LGnanoparticles.

3.3. DSC analysis of the synthesized epoxy/TiO2 nanocomposites

The influence of TiO2 nanoparticles surface modified with gal-lates on the glass transition temperature (Tg) of nanocompositeswas determined by DSC. DSC thermograms of epoxy sample andsamples of epoxy nanocomposites with TiO2-PG, TiO2-HG andTiO2-LG nanoparticles are shown in Fig 6. Values of the glass tran-sition temperature were taken as the midpoint of the glass transi-tion event (calculated as the peak maximum in the first derivativeof heat flow) and listed in Table 1. Obtained results show thatincorporation of surface modified TiO2 nanoparticles causes theincrease of Tg of epoxy resin. All samples were treated in the sameway and postcuring reactions were not observed by DSC, indicatingthat maximal curing degree was reached in the examined samples.Thus, the increase of Tg with the addition of surface modified TiO2

nanoparticles indicates the attractive interactions between nano-particles and polymer matrix, leading to the reduced polymer seg-mental mobility at the interface of polymer/nanoparticles[7,35,36]. Also, during the curing reactions, aminolysis of gallate

Fig. 6. DSC thermograms of the examined epoxy and epoxy nanocompositesamples.

Table 1Values of the glass transition temperature and water vapor transmission rate of theexamined samples.

Sample Tg (DSC) (�C) WVTR (g/m2 h)

Epoxy 79 0.519Epoxy/TiO2-PG 87 0.496Epoxy/TiO2-HG 87 0.477Epoxy/TiO2-LG 87 0.431

T.S. Radoman et al. / Materials and Design 62 (2014) 158–167 163

ester groups could have partly occurred together with amide for-mation, which led to chemical bonding of nanoparticles to polymermatrix, causing increase of Tg. Since all examined nanocompositeshad the same amount of the surface modified TiO2 nanoparticleswith almost the same grafting density of gallate ligands (the onlydifference is the length of hydrophobic part of the gallate), andsince the obtained shift in Tg values is almost the same for all sam-ples (Table 1), it can be concluded that in this case the chain lengthof hydrophobic part of the gallate has no influence on Tg value.

3.4. Water vapor permeability of the synthesized epoxy/TiO2

nanocomposites

Organic coatings of different composition are required to pro-tect a great variety of substrates against numerous aggressiveagents, especially against water. A diffusion of water through anorganic coating is considered to be the primary cause for corrosionof metallic substrates, the development of micro-organisms, dam-age due to frosting and loss of insulation properties. Incorporationof inorganic particles in polymer is one of the three mainapproaches to improve the barrier properties of polymer. Incorpo-rated inorganic nanoparticles significantly prolong and complicatethe paths of water molecules through the polymeric material. Thewater impermeability can be further improved by surface modifi-cation of nanoparticles which leads to better distribution of nano-particles in polymer matrix and better interfacial interactionbetween nanoparticles and polymer matrix. Good adhesive inter-face interactions give compact and pore free microstructure, whichleads to the reduction of the water vapor penetration. Also, thestrong attractive interactions at interface increase Tg and reducefree volume, contributing in this manner to the better barrier prop-erties. Further reduction of free volume and improvement of bar-rier properties can be achieved by chemical bonding ofnanoparticles to the polymer matrix [37,38]. The influence of sur-face modified TiO2 nanoparticles on the water vapor permeabilityof epoxy resin coating was examined using permeability cup

Fig. 7. The dependence of water transmitted through the examined films on time.

method. The dependence of water weight, passed through exam-ined films, on time is shown in Fig. 7. Water vapor transmissionrate (WVTR) of the coatings was calculated using the slope of linearfit of change in passed water weight vs. time. The obtained valuesare represented in Table 1. Incorporation of surface modified TiO2

nanoparticles in epoxy resin decreases the water vapor transmis-sion rate of epoxy coatings. The water vapor transmission rate ofepoxy/TiO2 nanocomposites decreases with increasing alkyl chainlength of gallate ligand due to the increase of hydrophobicity ofsurface modified TiO2 nanoparticles.

3.5. Dielectric properties of the synthesized epoxy/TiO2

nanocomposites

In order to examine the influence of the surface modified TiO2

nanoparticles with gallates on the dielectric properties of the pre-pared nanocomposites, dielectric spectroscopy measurementswere performed. The permittivity in nanocomposites depends onthe polarizations associated with applied nanoparticles and poly-mer matrix and interfacial polarizations at the interface betweenthe nanoparticles and polymer. Due to the presence of externalelectric field, rotational diffusions of segments and small groupspresent in epoxy resin occurs, leading to the orientation of dipolargroups along with respect to the applied electric field, i.e. to theappearance of dipolar polarization. This type of polarization hasthe major influence on the value of dielectric constant of epoxyresin in the frequency range applied during the dielectric measure-ments. Ionic polarization, caused by displacement of charged ionspresent in the impurities formed during the synthesis of epoxyresin, can be neglected [39]. The temperature dependences ofdielectric constant (e0) and dissipation factor (tand) of the pureepoxy resin and prepared nanocomposites, measured at 1000 Hzare shown in Fig 8. From these results it can be observed that forthe pure epoxy resin e0 remains almost constant up to the�60 �C, while rapid increase occurred at higher temperatures. Sim-ilar behavior was observed for the e0 of nanocomposites, exceptthat in these cases plateau is extended up to approximately 80 �C(Fig. 8a). The increase of the dielectric constant with temperatureis attributed to the increased segmental mobility of epoxy resinat higher temperatures [40]. On the other hand, dielectric constantdecreases with increasing frequency (Fig. 9). This behavior is theresults of the reduction of dipolar polarization caused by impossi-bility of dipolar groups in epoxy resin to orient themselves underan applied electric field. This was observed for other nanocompos-ites based on epoxy resin [41,42]. The appearance of atomic, elec-tronic and dipolar polarizations caused by orientation ordisplacement of bound charge carriers can also be reason for thisparticular change in dielectric constant value [43].

Dielectric constant of nanocomposite prepared using TiO2

nanoparticles surface modified with propyl gallate is almostthrough the whole temperature region higher than the valuesof dielectric constant for the pure epoxy resin (Fig. 8a). On theother hand, nanocomposites synthesized using TiO2 nanoparticlessurface modified with esters of the gallic acid having longer alkylchain lengths (TiO2-HG and TiO2-LG), have lower values of dielec-tric constant than the value of the pure resin. It appears that forthese two samples, epoxy/TiO2-HG and epoxy/TiO2-LG, nanoparti-cles restrict movement of the epoxy chains, reducing in this mannerorientation of dipolar groups, i.e. dipolar polarization. However,according to the DSC results, all three nanocomposites have thesame glass transition temperature, which is higher than the glasstransition temperature of pure epoxy resin. This indicates that theTiO2-PG nanoparticles also decrease the mobility of the polymerchains. Bearing in mind that all samples contain the same amountof TiO2, the reasons for the different permittivity of the

Fig. 8. The temperature dependences of: (a) dielectric constant (e0) and (b) dissipation factor (tand) of the pure epoxy resin and prepared nanocomposites, measured at1000 Hz.

Fig. 9. The frequency dependences of dielectric constant of the pure epoxy resinand prepared nanocomposites, measured at 40 �C.

Fig. 10. Slow potentiodinamic polarization curves of the investigated samples.

164 T.S. Radoman et al. / Materials and Design 62 (2014) 158–167

nanocomposites should be sought in the different contributions ofinterfacial polarization.

Due to the shortest alkyl chain of propyl gallate, interaction ofhydrophilic bare TiO2 surface and polar groups from epoxy resin(especially OH groups) is much more pronounced between TiO2-PG and epoxy resin than between TiO2-HG or TiO2-LG and epoxyresin. Thus, the accumulation of space charges at epoxy/TiO2-PGinterface boundary is high, leading to the high interfacial polariza-tion and consequently to the increase of e0. Also, the obtainedresults indicate that the polarization processes at the epoxy/TiO2-HG and epoxy/TiO2-LG interfaces are reduced, due to reducedhydrophilicity of nanoparticles surface, caused by the presence oflonger alkyl chain lengths of the gallates used for the surface mod-ification of TiO2 nanoparticles. The decrease of the dielectric con-stant for nanocomposites compared to the value of pure polymermatrix was also observed by Singha and Thomas for TiO2/epoxyand Al2O3/epoxy nanocomposites at relatively low nanofiller con-centrations and by Nelson and Fothergill for TiO2/epoxy nanocom-posites [44–46].

A single peak, ascribed to the glass transition temperature of theepoxy resin, is observed in the temperature dependences of e00 andtand (Fig. 8b). Values of the glass transition temperature of the pre-pared samples, determined as tand peak, are 121, 128, 125 and126 �C for pure epoxy resin, epoxy/TiO2-PG, epoxy/TiO2-HG and

epoxy/TiO2-LG, respectively. The obtained trend of Tg values is con-sistent with results from the DSC measurements. Furthermore, itcan be observed that values of tand for the nanocomposites pre-pared with TiO2-HG and TiO2-LG nanoparticles are lower than cor-responding values for the pure epoxy resin. The value of tanddepends on the electrical conductivity of polymer nanocomposite,which further depends on the number of charge carriers, theirrelaxation time and applied frequency. Dissipation factor valuepoints out to the possible dielectric losses in the examined insulat-ing material. Therefore, for the dielectric material it is desired tohave low tand values. Lower values of tand for investigated nano-composites are probably caused by their lower electrical conduc-tivity due to the lower mobility of charge carriers [44]. Thehindrances of the charge transport can occur probably because ofincreasing hydrophobicity at epoxy/TiO2 interface. On the otherhand, tand values of epoxy/TiO2-PG nanocomposite are somewhathigher than the values for the unfilled epoxy resin at temperatureshigher than glass transition temperature of the nanocomposite.According to the obtained results, it can be concluded that theinvestigated dielectric parameters strongly depend on the pro-cesses which occur at the epoxy/TiO2 interfaces.

3.6. Anticorrosive properties of the synthesized epoxy/TiO2

nanocomposites coatings

Corrosion behavior was investigated in two corrosion media.Polarization measurements of the dip-coated samples were

T.S. Radoman et al. / Materials and Design 62 (2014) 158–167 165

performed in 3% NaCl aerated solution, c(O2) � 0.25 mM, after sta-bilization of corrosion potential, and results are shown in Fig. 10.Corrosion of the bare mild steel is characterized with anodic disso-lution of the mild steel given with the following reaction:

Fe ¼ Fe2þ þ 2e ð1Þ

Cathodic reaction on bare mild steel is associated with diffusioncontrolled oxygen reduction reaction:

O2 þ 2H2Oþ 4e ¼ 4OH� ð2Þ

Corresponding polarization curve can be given by followingequation:

j ¼ j0ðFe2þjFeÞ exp2:303

baE� ErðFe2þjFeÞh i� �

� jLðO2Þ ð3Þ

where j0(Fe2+|Fe) is exchange current density, E(SCE) is actual elec-trode potential, ba is anodic Tafel slope, Er(Fe2+|Fe) is reversiblepotential of mild steel electrode and jL(O2) is limiting diffusion cur-rent density of the oxygen reduction reaction. For conditions thatE = Ecorr, corrosion current density is:

jcorr ¼ j0ðFe2þjFeÞ exp2:303

ba½Ecorr � ErðFe2þjFeÞ�

� �¼ jLðO2Þ ð4Þ

Rearranging Eq. (4), the corrosion potential can be obtained:

Fig. 11. Dependence of impedance magnitude on frequency of epoxy and epo

Ecorr ¼ ErðFe2þjFeÞ þ 2:303ba

lnjLðO2Þ

j0ðFe2þjFeÞð5Þ

Determined values for corrosion current density of 57 lA/cm2,obtained by extrapolating anodic Tafel slopes of 134 mV/dec on cor-rosion potential of –575 mV, were in good agreement with cathodiclimiting current density.

In the case of deep coated samples, corrosion potential was atmore negative potentials, and cathodic current was smaller thanin the case of bare mild steel. Corrosion current density was�22 lA/cm2 for TiO2-LG and TiO2-HG samples, while for TiO2-PGit was 12 lA/cm2. Lower corrosion potentials and smaller corrosioncurrent density could indicate that smaller corrosion can be con-nected with oxygen scavenger mechanism. The main property ofoxygen scavengers is to inhibit the corrosion by the cathodic mech-anism, which is based on the lowering of oxygen concentration inthe diffusion layer. As proposed by Favre and Landolt, in aqueoussolutions tannins (or in our case gallic acid esters) are oxidizedby oxygen into quinones, the rate of oxidation increasing withthe pH [47]. Because of this property, tannins have been proposedas oxygen scavengers to be used in closed cooling systems. Due tothe vicinity of hydroxyl groups on the aromatic rings, tannins areable to form chelates with iron and with other metallic cations.Ferrous complexes are colorless, very soluble and extremely sensi-tive to oxidation. In the presence of oxygen, they are converted into

xy nanocomposite samples for different immersion times in 0.1 M H2SO4.

166 T.S. Radoman et al. / Materials and Design 62 (2014) 158–167

ferric complexes (tannates), which have a dark blue color and areinsoluble.

The corrosion behavior of real epoxy systems was investigatedby the EIS. In the sodium chloride solution pure epoxy coatingshowed no indication of corrosion, even after 100 days of immer-sion, so the corrosion was investigated in more aggressive 0.1 MH2SO4. From Fig. 11, where the dependence of the impedance mag-nitude over frequency for different immersion times is shown, itcould be seen that all prepared nanocomposites showed superiorcharacteristics in comparison to the pure epoxy. Since main catho-dic reaction in this case is the reduction of hydrogen ion, the samescavenger mechanism could be applied, keeping in mind that cor-rosion media was not deaerated, so present oxygen can convertferrous into ferric complexes, improving the corrosion resistanceby filling the pores with insoluble ferric gallate.

4. Conclusions

TiO2 nanoparticles, obtained by acid catalyzed hydrolysis oftitanium isopropoxide, were surface modified with three gallic acidesters, having different chain lengths of hydrophobic part: propyl,hexyl and lauryl gallate. Surface modified TiO2 nanoparticles wereincorporated in epoxy resin and the influence of the surface mod-ified TiO2 nanoparticles, as well as the length of hydrophobic partof the gallate used for surface modification of TiO2 nanoparticles,on glass transition temperature, barrier, dielectric and anticorro-sive properties of the so obtained epoxy/TiO2 nanocompositeswas investigated.

The formation of a charge transfer complex between the TiO2

nanoparticles and the gallates was confirmed by FTIR and UV spec-troscopy. The amount of surface-bonded gallates, determined bythermogravimetric analysis, is almost the same. The epoxy/TiO2

nanocomposites have higher Tg than pure epoxy resin. The increaseof Tg of nanocomposites indicates the attractive interactionsbetween nanoparticles and polymer matrix, leading to the reducedpolymer segmental mobility at the interface of polymer/nanoparti-cles. The water vapor impermeability of epoxy resin was improvedby incorporation of surface modified TiO2 nanoparticles. The watervapor transmission rate of epoxy/TiO2 nanocomposites decreaseswith increasing alkyl chain length of gallate ligand due to theincrease of hydrophobicity of surface modified TiO2 nanoparticles.

Due to different contributions of interfacial polarization, dielec-tric constant of epoxy/TiO2-PG nanocomposite is higher, whilenanocomposites synthesized using TiO2 nanoparticles surfacemodified with esters of the gallic acid with longer alkyl chainlengths of hydrophobic part (TiO2-HG and TiO2-LG) have lower val-ues of dielectric constant than the value of the pure resin. Incorpo-ration of TiO2 nanoparticles surface modified with gallates inepoxy resin improves its anticorrosive properties because the sur-face modified TiO2 nanoparticles react as oxygen scavengers,which inhibit steel corrosion by cathodic mechanism.

Acknowledgements

This work was financially supported by the Ministry of Educa-tion, Science and Technological Development of the Republic ofSerbia (research Project Number: 172062).

References

[1] Ng CB, Scbadler LS, Siegel RW. Synthesis and mechanical properties of TiO2-epoxy nanocomposites. Nanostruct Mater 1999;12:507–10.

[2] Kovacevic V, Lucic S, Leskovac M. Morphology and failure in nanocomposites.Part I: structural and mechanical properties. J Adhesion Sci Technol2002;16:1343–65.

[3] Kickelbick G. Concepts for the incorporation of inorganic building blocks intoorganic polymers on a nanoscale. Prog Polym Sci 2003;28:83–114.

[4] Jordan J, Jacob KI, Tannenbaum R, Shart MA, Jasiuk I. Experimental trends inpolymer nanocomposites – a review. Mater Sci Eng A 2005;393(1–2):1–11.

[5] Krumpfer JW, Schuster T, Klapper M, Müllen K. Make it nano-Keep it nano.Nano Today 2013;8:417–38.

[6] Kango S, Kalia S, Celli A, Njuguna J, Habibi Y, Kumar R. Surface modification ofinorganic nanoparticles for development of organic–inorganic nanocomposites– a review. Prog Polym Sci 2013;38:1232–61.

[7] Albdiry MT, Yousif BF. Role of silanized halloysite nanotubes on structural,mechanical properties and fracture toughness of thermoset nanocomposites.Mater Des 2014;57:279–88.

[8] Hamming LM, Qiao R, Messersmith PB, Brinson LC. Effects of dispersion andinterfacial modification on the macroscale properties of TiO2 polymer–matrixnanocomposites. Compos Sci Technol 2009;69:1880–6.

[9] Špirkova M, Duchek P, Strachota A, Poreba R, Kotek J, Baldrian J, et al. The roleof organic modification of layered nanosilicates on mechanical and surfaceproperties of organic–inorganic coatings. J Coat Technol Res 2011;8:311–28.

[10] Hadavand BS, Javid KM, Gharagozlou M. Mechanical properties of multi-walled carbon nanotube/epoxy polysulfide nanocomposite. Mater Des2013;50:62–7.

[11] Dzunuzovic E, Vodnik V, Jeremic K, Nedeljkovic JM. Thermal properties of PS/TiO2 nanocomposites obtained by in situ bulk radical polymerization ofstyrene. Mater Lett 2009;63:908–10.

[12] Dzunuzovic ES, Dzunuzovic JV, Radoman TS, Marinovic-Cincovic MT, NikolicLjB, Jeremic KB, Nedeljkovic JM. Characterization of in situ preparednanocomposites of PS and TiO2 nanoparticles surface modified with alkylgallates: effect of alkyl chain length. Polym Compos 2013;34:399–407.

[13] Dzunuzovic ES, Dzunuzovic JV, Marinkovic AD, Marinovic-Cincovic MT,Jeremic KB, Nedeljkovic JM. Influence of surface modified TiO2 nanoparticlesby gallates on the properties of PMMA/TiO2 nanocomposites. Eur Polym J2012;48:1385–93.

[14] Dzunuzovic E, Marinovic-Cincovic M, Vukovic J, Jeremic K, Nedeljkovic JM.Thermal properties of PMMA/TiO2 nanocomposites prepared by in-situ bulkpolymerization. Polym Compos 2009;30:737–42.

[15] May CA, Tanaka Y. Epoxy Resins. New York: Marcel Dekke; 1973.[16] Hodgkin JH, Simon GP, Varley RJ. Thermoplastic toughening of epoxy resins: a

critical review. Polym Adv Technol 1998;9:3–10.[17] Ratna D, Banthia AK. Rubber toughened epoxy. Macromol Res 2004;12:11–21.[18] Hanus MJ, Harris AT. Nanotechnology innovations for the construction

industry. Prog Mater Sci 2013;58:1056–102.[19] Huang T-C, Su Y-A, Yeh T-C, Huang H-Y, Wu C-P, Huang K-Y, et al. Advanced

anticorrosive coatings prepared from electroactive epoxy–SiO2 hybridnanocomposite materials. Electrochim Acta 2011;56:6142–9.

[20] Shi X, Nguyen TA, Suo Z, Liu Y, Avci R. Effect of nanoparticles on theanticorrosion and mechanical properties of epoxy coating. Surf Coat Technol2009;204:237–45.

[21] Ramezanzadeh B, Attar MM. Studying the effects of micro and nanosized ZnOparticles on the corrosion resistance and deterioration behavior of anepoxypolyamide coating on hot-dip galvanized steel. Prog Org Coat2011;71:314–28.

[22] Wetzel B, Rosso P, Haupert F, Friedrich K. Epoxy nanocomposites – fractureand toughening mechanisms. Eng Fract Mech 2006;73:2375–98.

[23] Chen X, Mao SS. Titanium dioxide nanomaterials: synthesis, properties,modifications, and applications. Chem Rev 2007;107:2891–959.

[24] Chatterjee A, Islam MS. Fabrication and characterization of TiO2–epoxynanocomposite. Mater Sci Eng A 2008;487:574–85.

[25] Al-Turaif HA. Effect of nano TiO2 particle size on mechanical properties ofcured epoxy resin. Prog Org Coat 2010;69:241–6.

[26] Bittmann B, Haupert F, Schlarb AK. Preparation of TiO2/epoxy nanocompositesby ultrasonic dispersion and their structure property relationship. UltrasonSonochem 2011;18:120–6.

[27] Balaskas AC, Kartsonakis IA, Tziveleka L-A, Kordas GC. Improvement ofanticorrosive properties of epoxy-coated AA 2024-T3 with TiO2

nanocontainers loaded with 8-hydroxyquinoline. Prog Org Coat 2012;74:418–26.

[28] Zhang X, Wang F, Du Y. Effect of nano-sized titanium powder addition oncorrosion performance of epoxy coatings. Surf Coat Technol 2007;201:7241–5.

[29] Sangermano M, Malucelli G, Amerio E, Bongiovanni R, Priola A, Di Gianni A,et al. Preparation and characterization of nanostructured TiO2/epoxypolymeric films. Macromol Mater Eng 2006;291:517–23.

[30] Yu Y-Y, Rao Y-C, Chang C-C. Preparation and characterization of highlytransparent epoxy/inorganic nanoparticle hybrid thin films. Thin Solid Films2013;546:236–41.

[31] Sas B, Coppens B, Van Hemel J. Method of synthesizing alkyl gallates. WIPOPatent Publication Number WO 01/30299 A2 (03.05.2001).

[32] O’Regan B, Moser J, Anderson M, Graetzel M. Vectorial electron injectioninto transparent semiconductor membranes and electric field effects onthe dynamics of light-induced charge separation. J Phys Chem 1990;94:8720–6.

[33] Chen LX, Rajh T, Wang Z, Thurnauer MC. XAFS studies of surface structures ofTiO2 nanoparticles and photocatalytic reduction of metal ions. J Phys Chem B1997;101:10688–97.

[34] Jankovic IA, Šaponjic ZV, Dzunuzovic ES, Nedeljkovic JM. New hybridproperties of TiO2 nanoparticles surface modified with catecholate typeligands. Nanoscale Res Lett 2010;5:81–8.

[35] Omrani A, Afsar S, Safarpour MA. Thermoset nanocomposites using hybridnano TiO2–SiO2. Mater Chem Phys 2010;122:343–9.

T.S. Radoman et al. / Materials and Design 62 (2014) 158–167 167

[36] Cangialosi D, Boucher VM, Alegría A, Colmenero J. Physical aging in polymersand polymer nanocomposites: recent results and open questions. Soft Matter2013;9:8619–30.

[37] Perera DY. Effect of pigmentation on organic coating characteristics. Prog OrgCoat 2004;50:247–62.

[38] Shreepathi S, Naik SM, Vattipalli MR. Water transportation through organiccoatings: correlation between electrochemical impedance measurements,gravimetry, and water vapor permeability. J Coat Technol Res 2012;9:411–22.

[39] Zhao H, Li RKY. Effect of water absorption on the mechanical and dielectricproperties of nano-alumina filled epoxy nanocomposites. Composites: Part A2008;39:602–11.

[40] Psarras G, Manolakaki E, Tsangaris GM. Electrical relaxations in polymericparticulate composites of epoxy resin and metal particles. Composites A2002;33:375–84.

[41] Ramajo L, Reboredo M, Castro M. Dielectric response and relaxationphenomena in composites of epoxy resin with BaTiO3 particles. CompositesA 2005;36:1267–74.

[42] Amendola E, Scamardella AM, Petrarca C, Acierno D. Epoxy-nanocompositeswith ceramic reinforcement for electrical insulation. J Appl Polym Sci2011;122:3686–93.

[43] Tsangaris GM, Kouloumbi N, Kyvelidis S. Interfacial relaxation phenomena inparticulate composites of epoxy resin with copper or iron particles. MaterChem Phys 1996;44:245–50.

[44] Singha S, Thomas MJ. Permittivity and tan delta characteristics of epoxynanocomposites in the frequency range of 1 MHz–1 GHz. IEEE Trans DielectrElectr Insul 2008;15:2–11.

[45] Singha S, Thomas MJ. Dielectric properties of epoxy nanocomposites. IEEETrans Dielectr Electr Insul 2008;15:12–23.

[46] Nelson JK, Fothergill JC. Internal charge behaviour of nanocomposites.Nanotechnology 2004;15:586–95.

[47] Favre M, Landolt D. The influence of gallic acid on the reduction of rust onpainted steel surfaces. Corros Sci 1993;34:1481–94.