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Progress in Organic Coatings 75 (2012) 386– 391

Contents lists available at SciVerse ScienceDirect

Progress in Organic Coatings

j ourna l ho me p ag e: www.elsev ier .com/ locate /porgcoat

ffect of nano-sized mesoporous silica MCM-41 and MMT on corrosionroperties of epoxy coating

a Wanga,b, Keqi Chenga, Hang Wuc, Cheng Wangc, Qunchang Wangc, Fuhui Wangb,c,∗

College of Materials Science and Engineering, Shenyang University of Chemical Technology, Shenyang 110142, ChinaCollege of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, ChinaState Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, 110016 Shenyang, China

r t i c l e i n f o

rticle history:eceived 12 January 2012eceived in revised form 8 July 2012ccepted 17 July 2012

a b s t r a c t

This study investigated the effect of co-incorporation of two different kinds of nano materials with dif-ferent forms, layers (Na-MMT) and mesoporous silica particles (MCM-41), into the polymer matrix onthe corrosion performance of epoxy resin. Correspondingly corrosion performance of the coatings wasstudied by electrochemical impedance spectroscopy (EIS) in 3.5% NaCl aqueous solution and salt spray

vailable online 16 August 2012

eywords:anocompositesISolymer coatingsild steel

test. The X-ray diffraction (XRD) measurement showed that the Na-MMT layers were exfoliated and thehexagonal framework structure of MCM-41 was retained during and after the composite preparation.The co-incorporation of Na-montmorillonite (Na-MMT) and MCM-41 into the epoxy coating possessedthe best corrosion resistance than incorporating either Na-MMT or MCM-41 particles separately due todifferent interfacial structures between the fillers and the matrix.

© 2012 Elsevier B.V. All rights reserved.

. Introduction

Epoxy has been widely used as a coating material to protect theteel reinforcement in concrete structures, because of its outstand-ng processability, excellent chemical resistance, good electricalnsulating properties, and strong adhesion to heterogeneous mate-ials. However, the major disadvantage of pure epoxy resins is theirrittleness and low fracture toughness. Nonetheless, the successfulpplication of epoxy coatings is often hampered by their suscep-ibility to damage by surface abrasion and wear. They also showoor resistance to the initiation and propagation of cracks [1–4].

ncorporation of nano-sized fillers to coatings can help in improv-ng many properties of the coatings such as UV resistance, corrosionesistance and mechanical properties like scratch and abrasion. Thearrier properties of organic coatings can be improved by inclusionf proper fillers. There are various reports concerning improv-ng corrosion resistance of coatings using nano-particles such asi [5,6], TiO2 [7], SiO2 [7], ZrO2 [8], and Zn [9]. Nanoclay is alsontroduced into epoxy matrix and endowed epoxy/clay composite

ignificantly improved physical and chemical properties [10]. It wasound that the toughness and stiffness were improved, togetherith low water absorption, lower cure shrinkage, moderate glass

∗ Corresponding author at: College of Materials Science and Chemical Engineer-ng, Harbin Engineering University, Harbin 150001, China. Tel.: +86 24 23915900.

E-mail address: fhwang@imr.ac.cn (F. Wang).

300-9440/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.porgcoat.2012.07.009

transition temperature (Tg) and higher tensile strength throughthe incorporation of nano-clay modified with siloxane [11–13]. Theimprovement of the corrosion resistance of carbon steel with epoxyresin reinforced with organically modified clay has been clearlydemonstrated [14]. More recently study by Hang et al. [15,16] hasshowed that IBA-modified clay allows the corrosion performanceof epoxy coatings to be improved by an increase of the barrier prop-erties of the film and by the inhibitive action of IBA at the carbonsteel/coating interface.

Numerous publications have been devoted to the preparationand characterization of the properties of polymer/mesoporous sil-ica MCM-41 composites. Epoxy resin, polyethylene, polypropylenenanocomposites with enhanced thermal stability and mechanicalproperties were obtained in previous study [17–22]. Meso-porous materials are used as the reinforcing materials toenhance the mechanical, thermal properties of polymer mate-rials, due to its unusual characters, such as extended inorganicor inorganic–organic hybrid arrays with exceptional long-rangeordering, highly tunable textural and large surface area properties,controlled pore size and shape. Also, the reinforcing and toughingeffect of co-incorporation of two different kinds of nanomaterialswith different forms, layers (Na-montmorillonite (Na-MMT)) andparticles (MCM-41), into the polypropylene were obtained [19].

However, research about the use of mesoporous silica materialsin paints has not previously been reported. Few papers have pre-sented results concerning the corrosion protection of metals. In thispaper, the corrosion resistance of the epoxy coatings modified by

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tion peak and two additional small reflections (1 1 0) and (2 0 0)at low reflection angles (Fig. 1(a)), with the characteristics of

N. Wang et al. / Progress in Or

ifferent contents of nano-sized mesoporous silica MCM-41 andMT is studied by electrochemical impedance spectroscopy (EIS)

nd salt spray test.

. Experimental

.1. Materials

In the experiments, commercial available epoxy resin (E44) washosen as main component of the paint. The nano-sized meso-orous silica MCM-41 particles were prepared which was reported

n the literature [17]. In this study, nano-sized mesoporous MCM-1 particles with uniform diameters in the order of 80–100 nm,pherical shape and mono-disperse were synthesized by our groupn order to exclude the size and shape effects of fillers on com-osites. The particle size distribution measured by particle sizenalyzer and TEM, IR analysis were reported in previous paper17]. MMT was purchased from Fenghong Co. Ltd., Zhejiang. Thislay consisted of a 2/1 ratio of silica to alumina and CEC value was15 mequiv./100 g. Epoxy of bisphenol A type (E-44) used was pur-hased from WuXi Epoxy Co. Ltd. All other chemicals and solventsere of analytical grade and used without further purification.

.2. Preparation of epoxy nanocomposite coatings

MCM-41 and MMT nanoparticles were kept in a vacuum ovent 80 ◦C for 1 h to remove physically absorbed moisture and thenirectly added to the epoxy resin with butyl alcohol, dimethyl ben-ene solution as solvent. The ratio between the epoxy resin andolvents were 2:1. A ball mill was used as a mixing machine. Theall mill time was 40 min and the rotation speed was 500 r/min.s curing agent, polyamine (650#) was used. The content of

he MMT or MCM-41 power in the paint was 0 wt.%, 0.3 wt.%,.5 wt.% and 1 wt.%. For MMT/MCM-41 adding, they were 0 wt.%,.15 wt.%/0.15 wt.%, 0.25 wt.%/0.25 wt.%, 0.5 wt.%/0.5 wt.%, respec-ively. The weight ratio of MMT to MCM-41 was 1:1.

Steel substrates (50 mm × 50 mm × 1 mm, UNS G 10190) withounded corners and edges were polished with fine emery paper,ashed with acetone and dried for further use. The liquid paints

pure epoxy and epoxy nano-composites) with 30 ± 3 �m werehen applied by using a model XB-120 coater and cured at roomemperature. After solidification, the coating thickness was mea-ured by a Qnix4500 digital meter. The coated samples were keptn desiccator for a week before testing.

.3. XRD and DSC test

The identification of MCM-41, MMT, and the epoxy nanocom-osites was carried out by X-ray diffraction (D/max-2500PC, usingu Ka radiation at 50 kV and 200 mA with a scanning rate 1◦/miny 0.01 steps).

Differential scanning calorimeter (DSC) was obtained using aETZSCHSTA 449C thermal analysis system from −30 to 220 ◦C at

he heating rate of 10◦/min under nitrogen atmosphere. The dataas analyzed by Perkin-Elmer 7 series thermal analysis system

o obtain glass transition temperature. Each sample was less than mg.

.4. Corrosion performance tests

To verify the effect of MCM-41 and MMT nanoparticles on the

orrosion performance of epoxy coating on the mild steel sub-trates, EIS, salt spray test were carried out. EIS measurementsere performed with 84362 Autolab using ZSimpwin software.ll EIS measurements were carried out at room temperature in

Coatings 75 (2012) 386– 391 387

3.5% NaCl solution. For the impedance measurement, the coat-ings studied were 30 ± 3 �m thick. The area of 9 cm2 was used fortesting and the data normalized for 1 cm2. Test system consistedof a three-electrode cell, in which a saturated calomel electrode(SCE), a stainless steel electrode and a coated coupon were usedas reference, counter, and working electrodes, respectively. Exper-iments were performed under the open circuit potential. Threereplications were performed to ensure repeatability. Impedancespectra of coupons in different immersion times were recorded in10−2–105 Hz frequency range, with sinusoidal alternating potentialsignal of 10 mV.

Finally, the corrosion performance of the coated specimens wasevaluated in a neutral salt spray test, following the procedure ofASTM B 117 and employing 5 wt.% NaCl solution at 35 ± 2 ◦C for500 h. Prior to exposure, the backs and edges of the specimens werecovered with hot melt mixture of beeswax and colophony resin. Thespecimens were removed from the salt spray chamber after 500 hand representative areas were imaged with a digital camera. Theimages were then used to evaluate the corrosion performance ofthe coated specimens.

3. Results and discussion

3.1. Characterization of epoxy nano-composites

Fig. 1. X-ray patterns of all samples. (a) MCM-41; (b) epoxy nano-composite.

3 ganic Coatings 75 (2012) 386– 391

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Table 1Glass transition temperature, �H, and Tonset characteristics of epoxy resin nano-coatings.

Sample Tg �H (J/gepoxy) Tonset (◦C)

Epoxy 84.44 5.66 96.10Epoxy/MMT (0.5 wt.%) 85.57 9.58 98.45Epoxy/MCM-41

(0.5 wt.%)89.08 17.50 97.96

Epoxy/MMT/MCM-41(0.25 wt.%/0.25 wt.%)

93.32 18.33 95.33

Fc

88 N. Wang et al. / Progress in Or

ell-aligned hexagonal ordering [17]. Pristine MMT showed aiffraction peak of the (0 0 1) plane at 4.0◦ in a 2� value, and its basalpacing was 2.21 nm (Fig. 1(b) curve (a)). Curve b was the resultbtained for epoxy/MMT nano-coating. The MMT layers were inter-alated as indicated by the diffraction peak which started to shift tomaller angle position and became broader. Curves (c) and (d) gaveypical XRD patterns of epoxy/MCM-41 and epoxy/MMT/MCM-41ano-coating and the appearance of the two curves was very simi-

ar. Only one reflection with a lower intensity remained in the smallngle 2� region for the MCM-41 after the composite preparationFig. 1(b) curves (c and d)). The retention of the MCM-41 charac-eristic diffraction peaks in the XRD pattern of the epoxy/MCM-41nd epoxy/MMT/MCM-41 nano-coating indicated that the hexago-al framework structure of MCM-41 was retained during and afterhe composite preparation. The diffraction peak of MMT maybe dis-ppear eventually or overlap with that of MCM-41, which indicatedhat the layers of MMT were exfoliated or intercalated to somextent (Fig. 1(b) curve (d)), which will be further confirmed by ouresearch in next step. This result was similar to our previous studyn PP/MMT/MCM-41 [19].

.1.2. DSC measurementThe exothermic peaks in DSC analysis of different epoxy nano-

oatings reveal the possibilities that chemical bonding at interfaceith epoxy matrix may be formed when curing the composites. For

he precise investigation into the exothermic peaks, MMT, MCM-41

nd MMT/MCM-41 were mixed stoichiometrically with polyamineo obtain the data such as the onset temperature (Tonset), Tg (glassransition temperature) and the heat of reaction (�H) evolved inhe curing analyzed by system software, as shown in Table 1. The

ig. 2. Nyquist plots of epoxy nanocomposites coatings immersion in 3.5% NaCl electrolyontaining MCM-41; (d) epoxy coating containing MMT/MCM-41.

maximum exothermic heat was given off in the combination ofepoxy and MMT/MCM-41 particles. This may result from the rea-son that the MMT was found to be exfoliated and the formationof epoxy chain in the mesopore channels of the MCM-41 (see Sec-tion 3.1.1), thus improving cross-linking extent. However, in caseof single MMT and single MCM-41 particles adding, epoxy resincannot wet their surfaces sufficiently. Shi et al. [7] reported that Tg

(glass transition temperature) of high solid epoxy coatings is closelyrelated to the cross-linking degree: the higher cross-linking is, thehigher Tg is. The shift of the glass temperature to a higher tem-perature of epoxy/MMT/MCM-41 was probably due to the stronginteraction between MMT/MCM-41 and epoxy resin. Moreover,combining two nano-materials with different shapes may generatemore effectively enhanced effect due to the expected synergisticeffect [19]. More compact structure of coatings can be obtainedthrough improving cross-linking extent, which leads to better bar-

rier performance against water permeation.

te (a) epoxy varnish coating; (b) epoxy coating containing MMT; (c) epoxy coating

ganic Coatings 75 (2012) 386– 391 389

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.2. Corrosion performance tests

.2.1. EIS studyNyquist plots were displayed in Fig. 2 for the coatings with MMT,

CM-41 and MMT/MCM-41 nano-particles after various immer-ion times in 3.5% NaCl electrolyte.

Fig. 2(a) showed the electrochemical impedance spectra of theoating without nano-particles (varnish coating). The impedanceesponse revealed initial behaviour that was dominated by theoating capacitance at high frequencies and coating resistancen the low frequency region with a resistive component greaterhan 1.8 × 106 � cm2 after 24 h immersion in the electrolyte. Withncreasing immersion time (168 h), the resistance value decreased,ue to the penetration of water and movement of ionic specieshrough the coating layer, increasing the coating conductivity [23].

ith additional immersion time up to 408 h, the second semicir-le at low frequencies immerged in the EIS spectra and the barrierroperties of the coating decreased further. It can be ascribed to

ncreasing of corrosion rate, possibly through the presence of fur-her pores in the coating or an increase in the area exposed at thease of the existing pores or flaws [24].

However, the spectra of the coatings with 0.5 wt.% MMT poweras different from that of the varnish coating (Fig. 2(b)). In the

mmersion time of 408 h, the resistance value remained above × 106 � cm2, almost the same as vanish epoxy coating after 24 h

mmersion time. The capacitive arc changed as a semicircle, andhe resistance of the coating was detected. At the same time, thereas a short line appearing at the end of the semicircle, which didot disappear until 624 h when the tail transformed to be a semi-ircle. After 624 h, there are two time constants, the corrosion ofhe substrate was occurring [25].

The spectra of the coatings with 0.5 wt.% MCM-41 power wasifferent from that of the varnish coating and the coating with.5 wt.% MMT (Fig. 2(c)). After 24 h immersion, there was onlyne capacitive arc, which implied that the coating acted as anntact capacitor prohibiting permeation of corrosive species such as

ater, oxygen, and other ions towards the surface of the metal sub-trate [26]. The coating capacitance at high frequencies and coatingesistance in the low frequency region were with a resistive com-onent greater than 6 × 106 � cm2 after 168 h immersion in thelectrolyte.

For the systems included with 0.25 wt.% MMT and 0.25 wt.%CM-41 nano-particles together, it was clearly seen that only one

pparent time constant was observed for 408 h of immersion as

hown in Fig. 2(d). It was characterized by a single capacitive loopepresentative of resistance of coating. The coating capacitancet high frequencies and coating resistance in the low frequencyegion were with a resistive component greater than 6 × 106 � cm2

Scheme 1. Model analogy novel

Fig. 3. Equivalent electrical circuits.

after 408 h immersion in the electrolyte. The impedance valuesrecorded for these coatings were clearly higher than impedancedata obtained for adding only MMT or MCM-41 nano-particles tothe epoxy coatings, indicating the barrier properties and high ohmicresistance of co-incorporation of MMT and MCM-41. After 960 himmersion, there was an arc appearing at the end of the semicir-cle, which implied that there were two time constants. The metalsubstrate began to react with the corrosive aqueous media [27].

From the facts mentioned above, there were two time con-stants in the spectrum of the coating for varnish epoxy after 408 himmersion in the NaCl solution. However, in the spectra of the coat-ing with 0.5 wt.% MMT, 0.5 wt.% MCM-41 and co-incorporation of0.25 wt.% MMT and 0.25 wt.% MCM-41, the second time constantappeared after immersion for 624 h, 624 h and 960 h, respectively.This indicated effectiveness of MCM-41 and MMT nano-particlesfor improving barrier properties of coating layer. MCM-41 and MMTnano-particles tended to occupy small hole defects formed fromlocal shrinkage during curing of the epoxy resin and acted as abridge interconnecting more molecules. This resulted in a reducedtotal free volume as well as an increase in the cross-linking den-sity. This novel nano-network composite with the fully exfoliated

MMT and dispersed MCM-41 was seen in Scheme 1. This result wascoincided with our previous DSC results.

Analyses of Nyquist plots suggested that different equivalentcircuit models were required to fit the results (Fig. 3), which

nano-network composite.

390 N. Wang et al. / Progress in Organic

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after salt spray for 500 h (Fig. 5(d)). The corrosion results coincided

ig. 4. Time dependence of coating resistance for epoxy nano-coatings containingifferent contents of MMT, MCM-41, MMT/MCM-41 nanoparticles.

epresented initial times of corrosion process, water saturatedpoxy coating on the mild steel substrate, and finally accumulationf corrosion products at metal/coating interface (diffusion process)28–30]. During the initial period of immersion, Fig. 3(a) was used tot the impedance data, where Rs was the solution resistance, Cc andc were the coating capacitance and the coating resistance, respec-ively. As the immersion time increases, EIS data can no longer beatisfactorily fitted with the model in Fig. 3(a). Therefore, the longime degradation of polymer-coated metals may be described byhe general circuit shown in Fig. 3(b). Here, Cd1 is the capacity ofouble layer, and Rt is the charge transfer resistance [31,32].

The coating resistance of all the coatings studied was plot-ed as a function of immersion time in Fig. 4. It was showedlearly that the coating resistance always decreased with increasing

Coatings 75 (2012) 386– 391

immersion time at first, and then attained a plateau. It wasalso clearly observed that the coating resistance of epoxy coat-ings containing single MMT, single MCM-41 or co-incorporationof MMT/MCM-41 was higher than for the neat epoxy coatedspecimen. This face may be attributed to the higher barrier prop-erties and ionic resistance of single MMT, single MCM-41 orco-incorporation of MMT/MCM-41 nano-particles embedded in theepoxy coating samples.

For single MMT and single MCM-41 filling epoxy coating, it canbe found that the coating resistance with 0.5 wt.% MMT alone andwith 0.5 wt.% MCM-41 alone was the best among the others. Forthe epoxy coating with single MMT, as the XRD results indicated,the layers of MMT were only intercalated (Fig. 1(b) curve (b)). Theimprovement in coating resistance may be presumably due to theinteraction between the epoxy matrix and the clay layers.

For single nano-sized MCM-41 filling, also as the XRD result indi-cated, the chain of epoxy was formed in the mesopore channelsof the MCM-41 which restrained the agglomeration of MCM-41particles on one hand. The epoxy phase in the nano-sized poresextending along the channels to the openings could enhance theinteraction through the entanglement and inter-diffusion betweenthe matrix and the particulate on the other hand.

The coating resistance of the co-incorporation of MCM-41 andMMT filler was one order of magnitude higher than either the purenano-sized MCM-41 filler or MMT filled epoxy coating. The MMTand MCM-41 was found to be exfoliated and the formation of epoxychain in the mesopore channels of the MCM-41, respectively asrevealed by the X-ray diffraction measurements (Fig. 1(b) curve(d)). The co-incorporation of MMT and MCM-41 improved interfa-cial interaction in the nano-coatings owing to the inter-diffusionand entanglement between the epoxy chains and the nano mate-rials. Normally, exfoliated nanocomposite should exhibit superioranti-corrosion properties because of the stronger interfacial inter-action between the matrix and the exfoliated clay platelets [14].Because of the different shapes of MCM-41 and MMT, they togethercaused a reduction of the total free volume and an enhancement ofthe cross-linking density of the cured epoxy. For the epoxy coat-ing with MMT/MCM-41, the MCM-41 particles dispersed well inthe epoxy matrix because of the formation of epoxy chain in thenano-sized pores. The dual nano-structured MCM-41 played animportant role as the bridges in the interconnected matrix. Addi-tion, the MCM-41 and MMT nanoparticles may act to prevent epoxydisaggregation during curing and result in a more homogenouscoating.

In addition, epoxy coatings containing MMT/MCM-41 offeredsignificant barrier properties for corrosion protection and reducethe trend for the coating to blister or delaminate.

3.2.2. Salt spray testThe corrosion resistance of various epoxy coating specimens

was evaluated by the rusts and blistering along the coating’s sur-face on the mild steel substrate. The aspects of epoxy resin, epoxyresin with MMT and with MCM-41 and with MMT/MCM-41, afterexposure in salt fog for 500 h, were shown in Fig. 5. For the var-nish epoxy resin, serious rusting appeared along the surface forneat epoxy coating with diameter of rusts almost 2–4 mm wasobserved (Fig. 5(a)). A few rusts with diameter 1–2 mm could beobserved for MMT incorporated epoxy resin (Fig. 5(b)), while veryfew rusts could be observed for MCM-41 incorporated epoxy resin(Fig. 5(c)). However, no apparent rusting along the surface wasobserved on the MMT/MCM-41 incorporated epoxy resin surface

with the impedance spectra, implying that the co-incorporation ofMMT and MCM-41 can effectively prevent the epoxy resin fromblistering and delamination.

N. Wang et al. / Progress in Organic Coatings 75 (2012) 386– 391 391

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ig. 5. Aspects of (a) epoxy varnish, (b) varnish with MMT (0.5 wt.%), (c) varnish witn salt spray for 500 h.

For further research, it is also necessary to make the transitionrom solvent-based to waterborne epoxy. More in-depth study ofhe effect of nanoparticles on epoxy-curing dynamics and kineticsould further advance the knowledge base of such nanocomposite

oating systems.

. Conclusions

EIS results showed that the co-incorporation of MMT.25 wt.%/MCM-41 0.25 wt.% significantly improved the corrosionesistance of epoxy varnish via increasing barrier properties, whichade water and ion species hard to transport and reduced the trend

or substrate rusting and blistering of coating film.The mechanism for the improvement of corrosion resistance of

poxy resin/MMT/MCM-41 can be ascribed to the formation of aano-network composite. The MMT and MCM-41 was found to bexfoliated and the formation of epoxy chain in the mesopore chan-els of the MCM-41. The structure of cross-linked MCM-41 andMT particles contributed to the compact nanocomposite coatings,

o that the corrosion resistance was improved.

cknowledgements

The authors gratefully acknowledge the financial supportf the National Natural Science Foundation of China (Grantos.: 51103086 and 51173110), Distinguished Young Scholarsf Liaoning Province Higher Growth Plans, China (Grant No.:JQ2011040), and China Postdoctoral Science Foundation (Granto.: 2012M510922).

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