Progress in Organic Coatings 62 (2008) 376–381

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Progress in Organic Coatings

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Inorganic–organic hybrid materials with zirconium oxoclusters as protectivecoatings on aluminium alloys

Fabrizio Girardia, Francesco Graziolaa,c, Paolo Aldighierib,Lorenzo Fedrizzib, Silvia Grossa, Rosa Di Maggioc,∗

a ISTM-CNR, Dipartimento di Scienze Chimiche, Universita di Padova, and INSTM UdR Padova, via Marzolo, 1, Padova 35131 (PD), Italyb Dipartimento di Scienze e Tecnologie Chimiche, Universita di Udine, via Cotonificio, 108, Udine 33100 (UD), Italyc Dipartimento di Ingegneria dei Materiali e Tecnologie Industriali, Universita di Trento, via Mesiano, 77, Trento 38100 (TN), Italy

a r t i c l e i n f o

Article history:Received 16 September 2007

a b s t r a c t

Inorganic–organic hybrid materials are attracting a strong scientific interest mainly for their outstand-and tc comthermre OM

ratioand

rder tA105e as bes wratchespecating

Accepted 7 February 2008

Keywords:Anticorrosion propertiesHybrid inorganic–organic materialsOxocluster

ing inherent mechanicalboth inorganic and organisynthesis, chemically andcluster Zr4O4(OMc)12, whealloys.

In particular, the molaramount of photo-initiatorment, were optimised in oto coat aluminium alloy (Athe hybrid coatings behav

Several coated samplmicroscopy (ESEM) and sctheir scratch resistance, rorder to evaluate if the co

In order to measure shear stobulk samples were also obtaineanalysis (DMTA) was performepolymer for comparison. The vThe length and mass of all theshrinkage of the materials in th

1. Introduction

Aluminium alloys have good resistance to corrosion and arewidely used in several industrial applications, ranging from drink-cans to aircraft. One route to improve their resistance to corrosionis by a conversion coating, typically applied as a pre-treatmentfor achieving a better adhesion of the subsequent applied organicpaint. Conventionally, conversion layers are applied by dipping sub-strates in a tank of the treatment chemicals (when the coating isformed), followed by rinsing and finally painting [1].

The failure of organic coatings on metal alloy upon exposure toelectrolytes depends on the corrosive environment, on the surface

∗ Corresponding author. Tel.: +39 0461 882419; fax: +39 0461 881977.E-mail address: rosa.dimaggio@unitn.it (R. Di Maggio).

0300-9440/$ – see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.porgcoat.2008.02.001

hermal properties, which can be traced back to the intimate coupling ofponents. By carefully choosing the experimental parameters used for theirally stable acrylate-based hybrid material embedding the zirconium oxo-c CH2 C(CH3)C(O)O, can be deposited as UV-cured films on aluminium

s between the oxocluster and the monomer, the polymerisation time, thethe deposition conditions, by using an home-made spray-coating equip-o obtain the best performing layers in terms of transparency and hardness0, AA6060 and AA2024) sheets. Furthermore, it was also evaluated whetherarrier to corrosion.ere prepared and characterised. Environmental scanning electronictest were used to investigate the morphology of the films and to evaluatetively. Electrochemical impedance spectroscopy (EIS) was performed in

s actually protect the metallic substrate from corrosion.rage modulus (G′) and loss modulus (G′′) of the materials used for coatings,d by UV-curing of the precursors solution. Dynamical mechanical thermald in shear mode on cured disks of both the hybrid materials and pristinealues of Tg were read off as the temperatures of peak of loss modulus.

samples were measured before and after the DMTA analysis, so that theat temperature range was exactly evaluated.

© 2008 Elsevier B.V. All rights reserved.

substrate reactivity and especially on the coating properties [2,3].The protection is typically based on barrier effect, which reducesoxygen and water flow to metal/coating interface. Whatever causesstress and strain into the coating also affects interface stability, thusinducing blistering one of the most important phenomenon lead-ing to coating failure and to a loss of protection of the substrate. Thephenomena leading to blisters formation and growth in the coat-ing on steels have been intensively studied on steel, much less onaluminium alloys. The electrochemical processes occurring in theearly corrosion steps at the metal surface under the blisters, strictlydepend on the microstructure and chemical heterogeneities of themetal alloy. Furthermore, their formation is often due to wateruptake, swelling and thermal degradation of the organic coating.Hybridisation of organic polymer with organic compounds repre-sents an effective strategy to improve the properties of the coatingsand to widen their application as protective materials [4]. Polymeric

rganic

F. Girardi et al. / Progress in O

nanocomposites based on metal oxide nanoparticles or clustersare therefore replacing conventional composites, characterised byreinforcing inorganic fillers whose size is in the order of microns.An added value comes from the easy processing without excessiveweight increase. To this aim, the use of organically functionalizedmetal oxoclusters, which can be grafted to the forming polymermatrix through formation of covalent bonds, has been proven to bea suitable approach to achieve a better control of the dispersion ofthe inorganic building blocks into the organic backbone [5].

PMMA shows interesting characteristics for protective coatingapplication through barrier mechanism, such as easy processability,resistance to stress cracking, good thermal stability and water per-meability. Many efforts to modify such a matrix have been alreadyperformed by several routes [4,6–9].

In this framework, this study was devoted to investigateif PMMA-based hybrid materials, obtained by photo-initiatedpolymerisation of methacrylate monomers and the methacrylate-modified zirconium oxocluster, enable to improve the barrierproperties of the neat alloys. By carefully optimising the exper-imental parameters used for the synthesis, chemically andthermally stable methacrylate-based hybrid materials embeddingthe zirconium oxocluster were deposited as films and UV-curedon aluminium alloys. In this paper a first preliminary study on thebehaviour of these films in a corrosive environment is reported. Theaim of the investigation was to clarify how the organically modi-fied oxocluster modifies PMMA matrix and whether these hybridmaterials show enhanced properties with respect to PMMA for bar-rier protection of aluminium alloys, especially the very challengingAA2024, with very thin coating and without any preliminary con-version coating.

2. Experimental

2.1. Sample preparation and instrumentation

Zirconium butoxide (Zr(OBu)4) 80 wt.% in butanol and(Zr(OPr)4) 70 wt.% in propanol were purchased from Aldrich.Methacrylic acid and vinyl acetic acid 99%, purchased from Aldrich,were distilled under reduced pressure. All the chemicals werestored under argon, while the solvents were additionally stored onmolecular sieves.

IRGACURE 819, kindly gifted by Ciba Speciality Chemicals Inc.(Switzerland), was employed as photo-initiator at 4 wt.% amount

with respect to the monomer. Benzoyl peroxide (BPO) was pur-chased from Aldrich.

The zirconium oxocluster Zr4O2(OMc)12, Zr(4), bearing twelvemethacrylic functionalities was synthesised according to the pre-viously reported procedure [6] starting from zirconium butoxideand methacrylic acid, as reported in the quoted reference.

The zirconium oxocluster of formula [Zr6O4(OH)4(OOCCH2CHCH2)12(n-PrOH)]2·4(CH2CHCH2COOH), Zr(6), wasprepared by reaction between vinylacetic acid and zirconiumpropoxide in the molar ratio 7:1 [7]. Bulk samples have been pre-pared both with photo- and thermal-initiator (benzoyl peroxide,BPO), provided in the same percentage with respect the amount ofmonomer.

Hereafter, zirconium atom in Zr(4) and Zr(6) will be labelled Zr4and Zr6, respectively.

The used substrates were aluminium alloys: AA1050, AA6060and AA2024. Before deposition, they were cleaned with detergentand rinsed in doubly distilled water and finally etched by usingcommercial products. Indeed the substrates of AA1050 and AA6060alloys were treated with Gardoclean 515, Gardoclean 62 and Gar-dobond 620 HP purchased by ChemMetal, whereas those of AA2024

Coatings 62 (2008) 376–381 377

alloy were treated with Metaclean T2001, Turco Liquid Aluminetchand Turco Liquid Smutgo NC of Henkel.

In order to prepare the deposition formulation, the zirconiumoxocluster Zr(4) was dispersed in the liquid methylmethacrylatemonomer in a Zr4:MMA molar ratio of 1:50 and 1:25, respectively.After that, Irgacure 819 photo-initiator (4.2 wt.% with respect to themonomer) was added. On the MMA and Zr4:MMA solutions viscos-ity measurements by means of the torsion viscometer (VISCOLOG)were performed.

The liquid formulation was spray-coated onto the different sub-strates using a home-made spray-coating equipment consisting in aair-brush Paasch Model VLS. The operating pressure was 700 mbar.The films were in operando polymerised under a UV lamp (HeliosItalquarz s.r.l. 125 W, 230 V) for 1 h, a time interval optimised on thebasis of the time-resolved IR measurements, elsewhere reported.The distance between the lamp and the substrate was set to 19 cm.No thermal curing was performed after polymerisation.

In order to have an estimate of the coatings thickness, a DEK-TAK3 profilometer was used to measure height difference betweencoated and uncoated regions. The same procedure was unapplica-ble to AA6060 coated substrates because of the small thickness ofthe layer with respect to surface roughness.

The morphology of the samples was observed by a Philips XL30ESEM (environmental scanning electron microscopy).

Impedance measurements were performed with a PAR 283potentiostat connected to a Solartron 1255 Frequency ResponseAnalyser, using a 10 mV sinusoidal signal superimposed to the freecorrosion potential; the signal frequencies ranged between 100 kHzand 10 mHz. In this work the electrochemical impedance mea-surements were performed, as a function of immersion time ina 0.3 wt.% Na2SO4 solution. The tests were performed in a con-ventional three electrodes cell, using a calomel electrode (SCE) asreference and platinum as counter-electrode. The working elec-trode area was about of 6 cm2: the samples were inserted in a sealedsample-holder uncovering only the testing area.

The scratch tests were performed on all the coated samplesaccording to ASTM D 3363–05, the method for film hardness eval-uation by pencil test refers to following classification, from max tomin, 5H–4H–3H–2H–H–F–HB–B–2B–3B–4B–5B–6B.

Bulk samples for thermal and mechanical characterisationwere obtained also in bulk form by polymerizing the solutionin polypropylene tubes with outer diameter of 4 mm. Similarly,also bulk samples of 2-hydroxyethyl methacrylate and methylmethacrylate with Zr(6) in 1:25 and 1:10 molar ratio were prepared.

Dynamic mechanical thermal analysis (DMTA) was performed inshear mode on cured prism having planar and parallel faces 12 mm2

large and 3.5 mm thick, by using a Seiko DMS 6100 instrument at afrequency of 1 Hz, with a displacement 0.005 mm along the diam-eter direction. The scan maximum temperature was 200 ◦C withheating rate 2 ◦C min−1. Shear storage modulus (G′), loss modulus(G′′) and tan ı were measured. The Tg was read as the temperaturesof the peak of loss modulus (G′′).

Differential scanning Calorimetry (DSC) analyses were per-formed by DSC92 SETARAM from 30 to 200 ◦C in N2 with heatingrate 10 ◦C/min.

Gravimetric liquid sorption were measured by weighing tinybulk cylinders of the same size, previously immersed in distilledwater at 25 ◦C, removed periodically and dried with filter paperusing a precision balance. The volume fraction (�) of water is cal-culated by

� = �s(mt − m0)�wm0

rganic Coatings 62 (2008) 376–381

378 F. Girardi et al. / Progress in O

where �s and �w are the specific densities of samples, measured byArchimedes’s method, and water, respectively.

3. Results and discussion

The UV curing technique is achieving increasing importance inthe field of coatings [10], because it induces a fast polymer for-mation with an effective transformation of the liquid monomerinto a solid film. It can be considered an environmental friendlytechnique, since in several cases it is a solvent free process,and it is usually carried out at room temperature [11]. In thisstudy, the hybrid coatings were prepared by an optimised syn-thesis procedure in which the photo-polymerisation is carried outsimultaneously to the coating deposition, thus ensuring a betterhomogeneity of the deposited films.

A plurality of different transition metal oxoclusters, contain-ing different functional groups has been prepared and structurallycharacterised [5–7,12]. In this study, Zr(4) has been chosen onaccount of its size and functionalization as well.

In this framework, the UV-activated photo-polymerisation wasalso used to induce the copolymerisation of the methacrylategroups of the zirconium oxocluster with those of MMA, and tocovalently anchor the cluster to the polymer backbone.

3.1. Viscoelastic properties of bulk samples

Although the properties of thin polymer films are expectedto be different from those of the bulk state (the Tg value of thepolymer in form of thin film is greater or lower than the corre-sponding bulk value, according to the higher or lower interfacialenergy, respectively) [13,14], we used bulk samples for studyingthe effect of oxoclusters incorporation. The temperature variationof storage modulus (G′), loss modulus (G′′) and tan ı (loss angle)for neat PMMA and two Zr(4) hybrid systems are shown in Fig. 1.After Tg (108 ◦C) was exceeded, PMMA flows, whereas the hybridsamples show a rubbery plateau. Meanwhile, they shrink by about5.5 and 4.7 vol% for Zr4:MMA/1:25 and 1:50, respectively, with anegligible mass loss. This suggests that a densification process tookplace during analysis and that samples are thermally stable [15].

The Tg value, evaluated from the maxima of G′′ curve, showsa decrease going from PMMA (108 ◦C) to the hybrid systems. Themeasured value of Tg for PMMA is comparable to that of commer-

cial PMMA. Actually, a peak at lower temperature (about 80 ◦C) ispresent and ascribed to the evaporation of residual monomer, asdetermined by DSC analysis. Instead, the Tg of the hybrid mate-rials are 95 and 91 ◦C, for Zr4:MMA/1:50 and 1:25, respectively.This can be related to an increasing amount of Zr(4) oxocluster,which widens the distance between polymeric chain, thus havinga plasticizing effect and depressing Tg.

On the other side, the storage modulus values slightly increaseat low temperatures by incorporating the oxocluster in the matrixand the detected values are 3.8 × 10+8 Pa and 3.2 × 10+8 Pa forZr4:MMA/1:50 and 1:25, respectively. Moreover, the nanocompos-ites are very reliable at high temperature. The hybrid samples havehigher G′ values in the rubbery region as compared to the purePMMA. The rate of decrease of G′ is considerably lower with greateroxocluster loading, thus suggesting that the oxocluster are chemi-cally bonded to the matrix: the crosslinking hampers the segmentalmotions of the polymeric chains. Consistently with this interpreta-tion, the magnitude of the tan ı peak decreases with the increasingamount of oxoclusters. This indicates that the oxocluster restrictsthe high temperature mobility of the polymers chains, inducingmore elastic behaviour and less viscous flow.

Fig. 1. G′ (a), G′′ (b) and tan ı (c) for PMMA, Zr4:MMA/1:50 and Zr4:MMA/1:25sample.

3.2. Morphological study

The scanning electron micrographs of the coatings are given inFigs. 2–4. They show for different oxocluster content, the effect ofthe microstructure, composition and roughness of the substrateson the morphology of the coatings.

The PMMA coatings are homogeneous and transparent (Fig. 2).Indeed, all the features of the etched substrates appear underthe coating, especially the typical “bowl-shaped hole” surface ofAA2024 (see Fig. 2c), due to the detachment of some cathodicAl–Cu intermetallics compounds occurred during etching. Thecoatings Zr4:MMA/1:50 are slightly less transparent, though the“bowl-shaped hole” microstructure of AA2024 is still visible and

F. Girardi et al. / Progress in Organic

and 38 days from the percentage of adsorbed water with respect tothe dry weight of samples, are reported in Table 2, along with thedensities of the bulk samples and volume fraction (of water). Theoxocluster favours a higher uptake of water with respect to pris-tine PMMA. This finding can be ascribed to the increase of the freevolume of the materials, induced by the presence of the oxocluster,

Fig. 2. Scanning electron micrographs for PMMA coating on AA1050 (a), AA6060 (b)and AA2024 (c).

few very thin cracks are present (Fig. 3). The morphology of theZr4:MMA/1:25 coatings appears completely different from the oth-ers, due to noticeable opacity and especially to the presence of welldetectable cracks on the coatings (Fig. 4). This can appear surpris-ing, but the difference of thickness should be taken into account toexplain this finding. Indeed, the solution Zr4:MMA/1:25 has higherviscosity and consequently thicker coatings were obtained. Becauseof this, a greater and differential strain was experienced by the coat-

Table 1Scratch hardness values

Alloy coating AA1050 AA6060 AA2024

PMMA F 2H 3HZr4:MMA/1:50 F 3H 3HZr4:MMA/1:25 F 2H F

Coatings 62 (2008) 376–381 379

Fig. 3. Scanning electron micrographs for Zr4:MMA/1:50 coating on AA2024.

ing, which, along with the higher storage modulus, makes stressesto be enough great to overcome the intrinsic resistance of fracture.The stresses produced at the coating/substrate interface can neg-atively affect also the coating adhesion. Nevertheless, as generalfinding, the pieces seem to adhere quite well to substrate.

The data in Table 1 are the results of the pencil tests, according tothe classification ASTM D 3363–05, and indicate the scratch hard-ness is quite good, but the coating having higher oxocluster contentshows a slightly worse behaviour with respect to the others.

3.3. Barrier properties of coatings

The measured swelling indices in water (Isw), evaluated after 3

consistently with the above observed Tg trend. Furthermore, thewater uptake increases, increasing the amount of cluster. Althoughthe water uptake of the hybrid materials is doubled comparedwith neat PMMA, it does not compromise the general hydrophobicbehaviour of the hybrid coating, which is one of the most impor-tant property for a corrosion barrier. In order to ascertain whetherthe type of cluster contributes on its own to the water absorption,a further polymer composite was prepared, containing Z(6) oxo-cluster in the Zr6:Monomer/1:25 molar ratio. Indeed, this kind ofoxocluster does not swell in water, whereas is soluble in organicsolvents. The water uptake of this composite, Table 2, is slightlylower, suggesting that the nature of the organic moieties can affectthis property of the sample as well as the amount of oxocluster.

Furthermore, many authors have reported the greater wateruptake of nanocomposites prepared modifying PMMA with zir-

Table 2Swelling index, density and fraction volume of absorbed water

Coating Isw (%) Bulk density(g/cm2)

�

3 days 38 days 3 days 38 days

PMMA 0.65 0.72 1.06 0.69 0.84Zr4:MMA/1:50 1.22 1.24 1.18 1.44 1.46Zr4:MMA/1:25 1.42 1.50 1.20 1.70 1.80Zr6:MMA/1:25 1.33 1.37 1.23 1.64 1.69PHEMA 40.2 41.9 1.28 51.5 53.6Zr6:HEMA/1:25 30.3 29.9 1.28 38.8 38.3Zr6:HEMA/1:10 27.9 27.1 1.29 36.0 35.0

380 F. Girardi et al. / Progress in Organic

impedance, which can be related to the different barrier proper-ties of the two coatings (PMMA and Zr4:MMA/1:25). The fittingof the data was not performed, because the attention was mainlyfocused on the measurements of the impedance modulus |Z| vs.time, reported in Fig. 6a–c for all the considered samples.

Upon immersion in aqueous solutions, the coated samplesshowed values of impedance modulus greater than those ofuncoated substrates, but lower than modulus usually measured onsubstrates coated with thick organic paint layer (50–100 �m), asused in the industrial practice. Taking into account that the thick-ness of the coatings of the present work ranges between a fewmicrons of PMMA to about 10–12 �m of Zr4:MMA/1:25, the valuesappear therefore quite interesting.

As a general trend, the corrosion resistance of the samplesdecays initially with time, even though each sample with a dif-ferent rate. Indeed, the resistance of the samples AA1050 and AA6060 maintains a quite constant value for a long period. In both thecases, the hybrid coatings behave better than neat PMMA, even ifthe cracks evidenced by SEM analysis represent a preferential pathfor electrolytes and oxygen diffusion to metal. However, the greater

Fig. 4. Scanning electron micrographs for Zr4:MMA/1:25 coating on AA1050 (a),AA6060 (b) and AA2024 (c).

conium alkoxides [4]. It was also reported for nanocompositesprepared with poly-2-hydroxy ethyl methacrylate (PHEMA) andzirconium alkoxides that the behaviour switched from hydrophilicfor neat PHEMA to utterly hydrophobic for the nanocomposites[16]. On account of this, it is very intriguing to study how pre-formed oxoclusters would act when incorporated into a hydrophilicpolymer, as an example PHEMA. To this aim, further swellingexperiments were carried out on bulk PHEMA and nanocompositeprepared with HEMA and Zr(6) oxocluster in Zr6:HEMA molar ratio1:25 or 1:10. Although the pure PHEMA in water is a hydro-gel, theincorporation of oxocluster leads to a decrease of Isw in a propor-tional way with its content: the oxocluster hinders the formation ofhydrogen bonds, the true responsible of the hydrophilicity of purePHEMA.

Coatings 62 (2008) 376–381

Electrochemical impedance spectroscopy (EIS) is an importanttechnique in order to evaluate water uptake of the coating, by fittingimpedance data with an equivalent circuit, in which a capacitanceis associated to the coating [3]. As an example, Fig. 5a and b showBode Modulus Impedance spectra obtained on coated AA2024 alloy.These data clearly indicate a different evolution with time of the

Fig. 5. Impedance modulus vs. frequency, measured at four times for PMMA (a) andZr4:MMA/1:25 (b) on AA2024 substrate.

F. Girardi et al. / Progress in Organic

[5] (a) U. Schubert, Chem. Mater. 13 (2001) 3487;(b) Y. Gao, F.R. Kogler, U. Schubert, J. Polym. Sci., Part A: Polym. Chem. 43 (2005)

Fig. 6. Impedance modulus, measured at 1 Hz vs. time for PMMA, Zr4:MMA/1:50and Zr4:MMA/1:25 coatings on AA1050 (a), AA6060 (b) and AA2024 (c).

thickness and the volume expansion due to the water uptake ofhybrid coating make them resistant against corrosion even afterlong time. On the contrary, PMMA coatings show the best corrosionresistance on AA2024 substrate. Giving that pure organic monomerhas the lowest viscosity (0.6 cP) upon spraying, it is likely to bespread out and to better fit the substrate roughness with respectto hybrids. At this regard, the peculiarity of the surface microstruc-ture of this alloy is the main cause of failure of the hybrid coating,especially of Zr4:MMA/1:25 samples. In this case the cracks driveimmediately electrolytes into the holes under the coatings, wherethe corrosion starts.

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Coatings 62 (2008) 376–381 381

4. Conclusions

The analyses performed on bulk samples are consistent with thehypothesis that zirconium oxoclusters strongly interacts with thepolymeric host matrix, thus increasing the spacing among the poly-mer chains and the free volume: Tg decreases whereas the waterabsorption increases with the oxoclusters content. These featuresare related to the greater water uptake of hybrid materials, evenif the hydrophobic character of the PMMA matrix is maintained.Moreover, the oxocluster also hamper the segmental motion of thechains themselves when the glass transition was exceeded: thecrosslinking of the polymeric chains is also effective. This in turnimproves greatly the mechanical properties (G′) at high tempera-ture.

The hybrid coatings appear promising as barrier against cor-rosion. They generally behave better than pure PMMA, whendeposited on AA1050 and AA6060 substrates. Although the wateruptake of hybrids is greater than that of pure PMMA, the impedancemodulus |Z| measured for the sample coated by hybrids is greaterespecially at long time.

The hybrids do not protect AA2024 substrates in the presentform, because the cracks on the hybrid coatings drive electrolytes tothe cavities derived from the etching process, where the corrosionis severe. At this regard, a second layer of hybrid materials depositedsuccessively could improve performance, sealing the cracks on thefirst coating.

Acknowledgments

The University of Padova, Italy, ISTM-CNR, INSTM and the Uni-versity of Trento, Italy are acknowledged for the financial support.F. Graziola thanks the University of Trento (Italy) for a researchscholarship. The authors thank Lorenzo Dainese and StefanoMercanzin (Workshop of the Dipartimento di Scienze Chimiche,University of Padova) and Antonio Ravazzolo for the helpfultechnical support.

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