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Progress in Organic Coatings 77 (2014) 1045–1052

Contents lists available at ScienceDirect

Progress in Organic Coatings

j o ur na l ho me pa ge: www.elsev ier .com/ locate /porgcoat

reparation and properties of hydrophobic layered silicate-reinforcedV-curable poly(urethane acrylate) nanocomposite films forackaging applications

owan Kima, Mijin Lima, Insoo Kima, Jongchul Seoa,∗, Haksoo Hanb

Department of Packaging, Yonsei University, 1 Yonseidae-gil, Wonju-si, Gangwon-do 220-710, Republic of KoreaDepartment of Chemical and Biomolecular Engineering, Yonsei University, 262 Seongsanno, Seodaemun-gu, Seoul 120-749, Republic of Korea

r t i c l e i n f o

rticle history:eceived 29 September 2013eceived in revised form 2 January 2014ccepted 13 March 2014vailable online 3 April 2014

eywords:oly(urethane acrylate)loisite 15Aanocomposite film

a b s t r a c t

A series of poly(urethane acrylate)/Cloisite 15A (PUA/C15A) nanocomposite films were successfully pre-pared via a UV-curing system, and their physical and barrier properties were investigated as a functionof clay content. The physical properties were strongly dependent upon the chemical and morphologicalstructures originating from differences in Cloisite 15A content. With high clay content, the PUA/C15Ananocomposite films displayed an intercalation/exfoliation combined structure. However, no stronginterfacial interactions occurred between the PUA and clay, possibly leading to poor dispersion withrelatively high clay content. The thermal stability displayed some enhancement with the introductionof clay into PUA, while the gas and moisture barrier properties showed significant enhancement. Theoxygen transmission rate (OTR) and water vapor transmission rate (WVTR) decreased with increasing

3 2 2

arrier propertiesnterfacial interaction

contents of Cloisite 15A, and varied within the range of 714.0–71.1 cm /m day and 29.9–13.9 g/m day,respectively. Thus the enhanced gas and moisture barrier properties of PUA/C15A nanocomposite filmsmake them promising candidates for food and pharmaceutical packaging applications. However, furtherstudies will be performed to increase the compatibility and dispersion of clay particles in the PUA polymermatrix.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

Growing concerns for environmental protection have drivenndustry toward the use of solvent-free polymerization systems,

here ultraviolet (UV)-curing has become a viable alternative toonventional thermal curing of solvent-containing polymer for-ulations. Added benefits of UV-curable materials such as fast

uring speed, low energy consumption, high cost efficiency, andess pollution have led to their increased use in various industries,

here applications include paints, thin film coatings, adhesives,ackaging overcoat films, and biomedical materials [1–3]. Low vis-osity and high curing speed are two desirable properties pursued

y industry for UV-curable oligomers. Of the UV-curable poly-ers, poly(urethane acrylate) (PUA) is one of the most widely used

olymers due to its high solubility, low melting viscosity and three-

∗ Corresponding author at: Department of Packaging, Yonsei University, 1onseidae-gil, Wonju-si, Gangwon-do 220-710, Republic of Korea.el.: +82 33 760 2697; fax: +82 33 760 2954.

E-mail address: jcseo@yonsei.ac.kr (J. Seo).

ttp://dx.doi.org/10.1016/j.porgcoat.2014.03.007300-9440/© 2014 Elsevier B.V. All rights reserved.

dimensional architectures, though efforts to improve propertiesand expand applications in various areas continues [3,4]. Despitethese advantageous properties, issues related to low barrier andmechanical properties remain when applying PUA films to foodand pharmaceutical packagings [5,6].

In the food and pharmaceutical packaging industries, goodgas and moisture barrier properties are critical for achieving along protective period for packaged products [7,8]. The reduc-tion of oxygen, moisture sorption, and diffusion can suppressinternal damage and improve long-term performance. Therefore,barrier films must prevent or at least decrease the gas/moisturetransfer between the product and the surrounding atmosphere.Recently, the combination of organic polymers and inorganic fillershas become an exciting subject, receiving considerable researchattention for the improved thermal stability, mechanical strength,vapor permeability, and optical and electrical properties cre-ated by the organic–inorganic hybrid materials [1,2,4,9–12]. To

achieve enhanced physical properties and processability in theinorganic–organic hybrid materials, it is necessary to homoge-neously disperse inorganic filler into the organic polymer matrix[1,3,9–11].

1 nic Coatings 77 (2014) 1045–1052

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2

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Wavelength(nm)

200 300 400 500 600 700 800

Abs

orba

nce(

Arb

itary

Uni

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0.0

0.5

1.0

1.5

2.0

2.5

3.0

PUA/C15A-0PUA/C15A-2PUA/C15A-4

046 D. Kim et al. / Progress in Orga

There have been some studies on UV-curable polymer/clayanocomposites [1,2,4,9,13]; however, most have focused on the

mprovement of physical properties of the nanocomposites, suchs thermal and mechanical properties. To expand the use of UV-urable PUA into food and pharmaceutical packaging industries,mprovements in the gas and moisture barrier properties of filmsre essential factors in addition to thermal and mechanical proper-ies [10,11].

In this study, a commercially available, organically modified,ayered silicate with a relatively high hydrophobic surface, Cloisite5A [14,15], was selected, and poly(urethane acrylate)/Cloisite 15APUA/C15A) nanocomposite films were prepared with 0, 0.5, 1, 3nd 5 wt% clay loadings. As a reactive monomer, methyl methacry-ate (MMA), having high dipole moment value, was used to enhanceispersion and exfoliation of Cloisite 15A in PUA matrix. Chemicaltructures were confirmed via Fourier-transform infrared (FT-IR)pectrum, while the morphologies were corroborated via X-rayiffraction (XRD), scanning electron microscope (SEM), and trans-ission electron microscopy (TEM). Further effects of organoclay

n thermal and mechanical properties have been studied on theasis of thermal gravimetric analyzer (TGA), differential scanningalorimeter (DSC), and nanoindentation. The barrier properties ofhe as-prepared nanocomposite films were investigated by mea-uring the oxygen transmission rate (OTR) and the water vaporransmission rate (WVTR), and were interpreted by relating to thehemical structure and morphology of the films.

. Experimental

.1. Materials

Polycaprolactone triol (PCLT, average Mn: 900 g/mol, CAS No:7625-56-2) as a polyol, 2-hydroxyethyl acrylate (HEA, Mw:16.12 g/mol, CAS No: 37625-56-2) as a reactive monomer, andibutyltin dilaurate (DBT, Mw: 631.56 g/mol, CAS No: 77-58-7)s a reaction catalyst were purchased from Aldrich Chemi-al Co. Isophorone diisocyanate (IPDI, Mw: 222.28 g/mol, CASo: 4098-71-9) as a diisocyanate was purchased from TCIorea. Trimethylolpropane triacrylate (TMPTA, Mw: 296.32 g/mol,AS No: 15625-89-5) and methyl methacrylate (MMA, Mw:00.12 g/mol, CAS No: 80-62-6) as reactive diluents were pur-hased from Miwon Commercial Co. Ltd (Anyang City, Korea) andldrich Chemical Co., respectively. 1-Hydroxycyclohexyl phenyletone (Irgacure 184D, Mw: 204.26 g/mol, CAS No: 947-19-3) as

photoinitiator was purchased from Ciba Specialty Chemicals Co.loisite 15A (CAS No: 68953-58-2), a natural montmorillonite mod-

fied with a dimethyl dehydrogenated quaternary ammonium saltith an interlayer spacing of 21.5 A, was kindly supplied from

outhern Clay Products Inc. All the chemicals were of reagent gradend used without further purification.

.2. Preparation of urethane acrylate oligomer and PUA/C15Aanocomposite films

Our method for preparing crosslinkable urethane acrylateligomer PCLT–IPDI was described in our previous studies [10–12];CLT and IPDI (1:2.5 by mole) were mixed in a 500 mL four-neckedask in an oil bath equipped with a mechanical stirrer, a thermome-er, a dropping funnel, and a reflux condenser with a drying tube,nd thoroughly mixed. Approximately 200 ppm of DBT was then

dded. After the urethane-forming reaction proceeded at 80 ◦C forver 3 h, the reaction mixture was cooled to 60 ◦C, and HEA wasdded dropwise. Tipping the NCO-terminated prepolymer withEA was done for 1 h at a temperature below 60 ◦C.

Fig. 1. UV–vis absorbance spectra of the PUA/C15A nanocomposite films.

Cloisite 15A was first dispersed in MMA using a magnetic stir-rer and ultrasonic cleaner for 24 h [16,17]. And then, the MMAand clay dispersion, urethane acrylate oligomer and TMPTA wereput into the vial glass and the mixture solution was sonicatedusing an ultrasonicator (Ultra-cell Vcx 750, power: 750 W; fre-quency: 20 kHz) for 4 h. Finally, the oligomer and photoinitiatorwere put into the mixture solution and stirred with ultrasonica-tion for 0.5 h. Each mixture solution was then bar-coated on aglass substrate. The coated films were exposed to UV irradiationfrom a medium-pressure 1.2 kW mercury lamp (main wavelength:365 nm) for 10 min. The film thickness was kept at approximately30 �m to aid in the evaluation of physical properties. The clay load-ings in the nanocomposites were 0 wt%, 0.5 wt%, 1 wt%, 3 wt%, and5 wt%, respectively, and the compositions and sample codes for thenanocomposite films are summarized in Table 1. Fig. 1 shows theUV–vis absorbance spectra of the PUA/C15A nanocomposite films.Although the absorbance slightly increases with increasing claycontent, the difference of absorbance at the main wavelength of365 nm is not big, which indicates that the clay incorporation doesnot affect the degree of photopolymerization by our UV curing.

2.3. Characterization

FTIR spectra of the as-prepared PUA/C15A nanocomposite filmswere recorded with a Spectrum 65 FTIR spectrometer in attenuatedtotal reflection (ATR) mode (PerkinElmer Co. Ltd., Massachusetts,USA). X-ray diffraction (XRD) was used to identify the dispersedstate over a fairly large sample volume. In this study, XRD patternswere collected on a Siemens Bruker-AXS D5005 diffractometerwith a nickel-filtered CuK� radiation source (� = 1.5418 A). Theradiation source was operated at 40 kV and 45 mA, and data werecollected in the 2� range from 1◦ to 9◦ at 0.02◦ intervals, and witha scan speed of 2.0◦/min.

The fractured surfaces of the PUA/C15A nanocomposite filmswere investigated using a Quanta 250 SEM (FEI Co. Ltd., Hillsboro,OR, USA). Prior to the examination, all of the samples were coatedwith a thin layer of platinum (Pt). The morphology of the PUA/C15Ananocomposite films was also examined via TEM with a Hitachielectron microscope. The samples for the TEM analysis were sec-tioned with an ultramicrotome.

The decomposition temperature and thermal behaviors of the

PUA/C15A nanocomposite films were investigated by using a Q10differential scanning calorimeter (TA Instrument Co. Ltd., Delaware,USA) and a TGA 4000 thermogravimetric analyzer (PerkinElmer Co.

D. Kim et al. / Progress in Organic Coatings 77 (2014) 1045–1052 1047

Table 1Compositions for the PUA/C15A nanocomposite films with different clay contents.

Sample code Clay (wt%) Composition (g)

Cloisite 15A Oligomer TMPTA MMA Irgacure 184D

PUA/C15A-0 0 0.00 14.00 7.00 9.00 0.90PUA/C15A-1 0.5 0.15 13.93 6.96 8.96 0.90

13.813.513.3

Ln

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smooth and dense fractured surface. However, the nanocompos-ite films reveal the presence of many small and homogenouslydistributed domains, and are getting rough and complicated with

%T

rans

mitt

ance

Cloisite 15A

PUA/C15A-0

PUA/C15A-1

PUA/C15A-2

PUA/C15A-3

PUA/C15A-4

N-HC=O

-NCOC=C

PUA/C15A-2 1.0 0.30

PUA/C15A-3 3.0 0.90PUA/C15A-4 5.0 1.50

td., Massachusetts, USA) at a heating rate of 20 ◦C/min under aitrogen atmosphere.

The elastic modulus and hardness of the PUA/C15A nanocom-osite films was measured using a MTS XP nanohardness testerMTS Co. Ltd., Minnesota, USA) equipped with a standard Berkovichthree-sided pyramid) diamond indenter. During the indenter isriven into the nanocomposite films and withdrawn from them,he hardness was computationally calculated on the indentationoad divided by the projected contact angle and the modulus cane calculated based on the relationships developed by Sneddon18,19]. The load-displacement curve was recorded at five dif-erent locations, from which the effective modulus and hardnessould be obtained by averaging the indentation depth range of000–5000 nm into the surface.

The OTR of the PUA/C15A nanocomposite films was measuredith an OTR 8001 Oxygen Permeability Tester (Systech Instrumentso. Ltd., Illinois, USA). The OTR test was carried out at 23 ◦C and% relative humidity. The WVTR of the nanocomposite films wasetermined using a WVTR 7001 water vapor permeation analyzerSystech Instruments Co. Ltd.), according to ASTM F1249. The WVTRests were carried out at 90% relative humidity and 37.5 ◦C, andhe data were directly obtained from the phosphorous pentoxideP2O5) moisture sensor. The water contact angles on the PUA/C15Aanocomposite films were measured using a Phoenix 300 contactngle goniometer (SEO Co. Ltd., Suwon, Korea). A droplet of wateras gently placed on the film and observed through the eyepiece by

pplying the �/2 method. Contact angles were measured with def-nite time intervals for a single drop, and the measurements wereecorded as snapshots. To minimize the effect of evaporation onhe water contact angle, all measurements were conducted in ahamber controlled in the range of 80–90% relative humidity.

. Results and discusssion

.1. Preparation of PUA/C15A nanocomposite films

FTIR analysis was employed to verify the successful preparationf the PUA/C15A nanocomposite films during the UV-curing sys-em, as well as to identify interfacial interactions between PUA andlay, a critical factor in the improvement of physical performance12,20]. Fig. 2 shows the FTIR spectra of clay (a) and the PUA/C15Aanocomposite films with various clay contents (b)–(f). In the spec-rum of the Cloisite 15A (Fig. 2(a)), the absorption peaks at 2926,850, and 1470 cm−1 contribute to the absorbance of methylene’ssymmetric stretching, symmetric stretching, and bending vibra-ions, respectively, which indicates the typical IR spectra of Cloisite5A [1,11,21]. The 3627 and 1637 cm−1 peaks are caused by thetretching vibration of Al–OH in silicate, and the absorbed H O Hending vibration, respectively. Fig. 2(b) and (c)–(f) shows the FTIRpectra of the UV-cured pure PUA film and PUA/C15A nanocom-osite films with various clay contents. All of the films exhibited

haracteristic absorption peaks at 3381 and 1720 cm−1, whichre caused by N H and C O stretching in the urethane linkages.urthermore, the absorption bands near 1636 and 2276 cm−1 cor-esponding to the C C double bonds of the methacrylate groups

6 6.93 8.91 0.898 6.79 8.73 0.870 6.65 8.55 0.86

and the NCO of the diisocyanate groups, respectively, disappearedin pure PUA film and all of the nanocomposite films, indicating thatall monomers, regardless of clay contents, were completely poly-merized with urethane acrylate oligomers under our UV-curingsystem [1,11,21]. However, the PUA/C15A nanocomposite filmsshowed similar IR spectra to pure PUA, and no apparent changeor shift in the characteristic peaks of pure PUA was observed withthe addition of Cloisite 15A particles. This indicates poor interfacialinteraction between the PUA and Cloisite 15A [12,20].

Changes in the interlayer distance of the clay as well as thedispersion state can be identified via XRD analysis. A shift tolower angles of the first characteristic peak represents the for-mation of an intercalated structure, whereas the disappearanceof the peak signals the probable existence of an exfoliated struc-ture [1–4,21,22]. The XRD pattern of Cloisite 15A and those ofPUA/C15A nanocomposite films are shown in Fig. 3(a) and (b)–(f),respectively. The Cloisite 15A showed two peaks; one apparentpeak at 2.6◦ and a slight indication at 7.1◦. The first peak cor-responds to a gallery distance of 3.395 nm, which is consistentwith other literatures [13,22]. In the case of the nanocompositefilms, however, this peak is apparently shifted to a lower angleand becomes broader than that of Cloisite 15A, implying the for-mation of intercalated/exfoliated nanocomposite films. The secondpeak, corresponding to the basal distance of 1.24 nm, disappearedin the nanocomposite films, which may come from a relativelysmall amount of clay in polymer matrix and/or perfect exfoliationof the polymer matrix into the clay galleries [16,17,22].

The SEM images for the fractured surface of neat PUA and thenanocomposite films are shown in Fig. 4, with the gray-coloredregions and bright spots indicating the bulk of the polymer matrixand the distribution of clay particles, respectively. PUA exhibits

Wavenumber (cm-1)

40080012001600200024002800320036004000

Fig. 2. FTIR spectra of Cloisite 15A and PUA/C15A nanocomposite films.

1048 D. Kim et al. / Progress in Organic Co

2 (Degree)1 2 3 4 5 6 7 8 9

Inte

nsity

(a.

u.)

(a) Cloisite 15A

(b) PUA/C15 A-0

(d) PUA/C1 5A-2

(e) PUA/C15A-3

(f) PUA/C15A-4

(c) PUA/C1 5A-1(001 )

(002 )

*

iiwsPtdo

o

Fig. 3. XRD patterns of Cloisite 15A and PUA/C15A nanocomposite films.

ncreasing clay content [22]. These issues imply a weak interfacialnteraction between PUA and clay, which may be attributed to the

eak chemical interactions between the relatively nonpolar clayurface and polar urethane bonds presented in the segments of theUA [16,17]. This corresponds well with the FTIR result. However,he phenomena of exfoliation, intercalation, and aggregation, are

ifficult to study from SEM analysis conclusively, but can be easilybserved from TEM and XRD studies.

Since XRD and SEM analysis give the macroscopic conformationf a sample, to further investigate the clay dispersion in the polymer

Fig. 4. SEM images for the fractured surface of the PUA/C15A nanoc

atings 77 (2014) 1045–1052

matrix, TEM analysis was performed. The results are depicted inFig. 5. The low magnification TEM images (a-1) and (b-1) revealthat the clay was dispersed uniformly and randomly during theUV-curing process in the polymer matrix. In the high magnificationimages, PUA/C15A-2 (1 wt% clay loading) showed a well-exfoliatedstructure, whereas PUA/C15A-4 (5 wt% clay loading) resulted in anintercalated and exfoliated structure with some aggregates in thepolymer matrix, further supporting the SEM analysis [16,17].

3.2. Thermal properties

DSC and TGA analyses were performed in a nitrogen atmosphereto investigate how the clay content affects the thermal behaviorsand thermal stability of PUA and its nanocomposite films. In gen-eral, the introduction of inorganic fillers into polymers can increasethermal stability by decreasing the mobility of the polymer chainsclose to the filler surface [11,23]. The physical confinement andstrong interactions between the polymer chains and the filler sur-face are two significant variables which affect chain mobility. DSCcurves of pure PUA and PUA/C15A nanocomposite films are shownin Fig. 6, which represents the first heating cycle.

The pure PUA exhibits a glass transition temperature (Tg) of53.0 ◦C driven by the hard segment [24]. As shown in Fig. 6and Table 2, the Tg values of the PUA/C15A nanocomposite filmschanged in the range of 52.3–53.5 ◦C, showing no apparent changeby the addition of the clay. The Tg value is strongly associated withthe segmental mobility of the polymer chains. Therefore, Cloisite

15A, with a relatively high aspect ratio, can serve to increase theTg value of the PUA by intercalating polymer chains into the claygalleries and decreasing the mobility of the chains close to thefiller surface. As evidenced by the FTIR and SEM analyses, however,

omposite films: (a) 0 wt%, (b) 1 wt%, (c) 3 wt%, and (d) 5 wt%.

D. Kim et al. / Progress in Organic Coatings 77 (2014) 1045–1052 1049

and (b) 5 wt% (1 and 2 indicate low and high magnifications, respectively).

nchtc

puptnPnpaapoptot

Temperature (oC)

-50 0 50 100 150

Hea

t Flo

w (

w/g

)

PUA/C15A-0

PUA/C15A-1

PUA/C15A-2

PUA/C15A-3

PUA/C15A-4

53.0oC

53.1oC

52.3oC

53.5oC

52.5oC

TT

Fig. 5. TEM images of the PUA/C15A nanocomposite films: (a) 1 wt%

o strong interactions were observed between the PUA polymerhains and Cloisite 15A, in addition to poor dispersion in relativelyigh clay content. This may have hindered the enhancement ofhe thermal properties of the PUA/C15A nanocomposite films inomparison to other researches [2,4,11].

By means of TGA, the mass loss of pure PUA and its nanocom-osites was studied as a function of temperature from 50 to 600 ◦Cnder a nitrogen atmosphere, as shown in Fig. 7. The nanocom-osite films showed two stages of decomposition, which is similaro the pure PUA, indicating that the presence of the clay doesot significantly influence the thermal degradation patterns of theUA/C15A nanocomposite films. The first stage encompassed a sig-ificant weight loss occurring at approximately 320–430 ◦C, and isrimarily attributed to the less-stable urethane functional groups inliphatic polyurethane acrylates, which can be decomposed to formn alcohol and isocyanate group [1,10,11]. The second stage, at tem-eratures above 400 ◦C, may result from the further decompositionf the remainders [1,23]. As summarized in Table 2, all nanocom-osite films containing Cloisite 15A exhibited higher decomposi-

ion temperatures than pure PUA. The nanocomposite formationf PUA and Cloisite 15A shows a notable 18.0–23.1 ◦C increase inhe PUA thermal stability at T3%. The Cloisite 15A particles have

able 2hermal and mechanical properties of the PUA/C15A nanocomposite films.

Sample code DSC TGA

Tga (◦C) T3%

b (◦C)

PUA/C15A-0 53.0 288.4

PUA/C15A-1 53.1 306.4

PUA/C15A-2 52.3 309.2

PUA/C15A-3 53.5 311.5

PUA/C15A-4 52.5 307.3

a The glass transition temperature (Tg) was measured during the 1st heating.b Decomposition temperature at 3% weight loss of sample.c Decomposition temperature at 10% weight loss of sample.

Fig. 6. DSC thermograms of the PUA/C15A nanocomposite films.

Nanoindentation

T10%c (◦C) Hardness (GPa) Modulus (GPa)

341.3 0.114 2.446355.9 0.102 2.443360.4 0.110 2.671355.8 0.112 2.731354.5 0.109 2.560

1050 D. Kim et al. / Progress in Organic Coatings 77 (2014) 1045–1052

o

100 200 300 400 500 600

Wei

ght R

eten

tion

(%)

0

20

40

60

80

100

PUA/C15A-0PUA/C15A-1PUA/C15A-2PUA/C15A-3PUA/C15A-4

gcct

3

aaawlseoataaiosntfi

0 1 2 3 4 5

OT

R (

cm3/ m

2da

y)

0

200

400

600

800

WV

TR

(g

/ m2 d

ay)

0

10

20

30

40

50

OTRWVTR

Temperature ( C)

Fig. 7. TGA thermograms of the PUA/C15A nanocomposite films.

reater thermal stability than the polymer matrix and can physi-ally confine the polymer chains, resulting in a decrease in polymerhain mobility. In addition, the well-dispersed clay nanolayers inhe polymer matrix may act as a barrier to heat transfer [21].

.3. Nanoindentation

It is well known that mechanical properties such as modulusnd hardness of polymer/clay nanocomposite films increase withdded clay, due to the reinforcing effect of intercalated or exfoli-ted organoclay. Specifically, nanointentation is a useful techniquehich can provide mechanical properties at the very thin surface

ayers and the data can then be compared to that of classical ten-ile tests [1,25]. In this study, the modulus and hardness values ofach film were averaged in the surface indentation depth rangef 4000–5000 nm, and the values of all samples were plotted as

function of clay, as shown in Fig. 8. From Fig. 8, it is observedhat with an increase in clay loading from 1 to 5 wt%, the hardnessnd elastic modulus of PUA changed from 0.114 GPa to 0.110 GPand 2.446 GPa to 2.560 GPa, respectively. There was no big changen hardness with the incorporation of clay in the content rangef 1–5 wt%, whereas the modulus increased to 3 wt%, and then

lightly decreased. This decrease in 5 wt% nanocomposite film witho enhancement in hardness from the added clay may be relatedo the morphology and chemical structure of the nanocompositelms [16,17]. When the clay content exceeds a specific value, clay

Content of Clay (wt %)

0 1 2 3 4 5

Har

dnes

s (G

Pa)

0.00

0.04

0.08

0.12

0.16

0.20

Mod

ulus

(G

Pa)

2.0

2.2

2.4

2.6

2.8

3.0

HardnessModulus

Fig. 8. Elastic modulus and hardness of the PUA/C15A nanocomposite films.

Content of Clay (wt %)

Fig. 9. WVTR and OTR of the PUA/C15A nanocomposite films.

particles are not dispersed uniformly in the nanocomposites, pos-sibly inducing reduction in the mechanical properties of the filmswith high clay content. As indicated by the FTIR and SEM anal-yses, however, there is no strong interfacial interaction betweenthe PUA matrix and Cloisite 15A, and the existing weak interfacialinteraction can be destroyed by high clay content, which makesthe bond matrix discontinuous and leads to catastrophic failureof the PUA/C15A nanocomposite films [11,12,20,26,27]. Furtherstudies on the prevention of clay aggregation and the induction ofstronger interfacial interactions between PUA and clay are requiredto improve mechanical properties [4,9,14,26,27].

3.4. Barrier properties

Specific gas and moisture barrier properties are critical forachieving a long protective period for packaged products [7,8].The reduction in moisture sorption and diffusion can suppressinternal damage and improve long-term performance. Therefore,barrier films must prevent or at least decrease the gas/moisturetransfer between the product and the surrounding atmosphere. Inthis study, organic–inorganic nanocomposite films incorporatingCloisite 15A with relatively high aspect ratio and high hydropho-bicity were prepared via a solution blending and UV-curing method.Water vapor and oxygen permeability were investigated as a func-tion of Cloisite 15A content. The results are depicted in Fig. 9 andsummarized in Table 3.

For the PUA/C15A nanocomposite films, the WVTR varied in therange of 29.9–13.9 g/m2 day and can be observed to decrease withincreasing Cloisite 15A content. The OTR initially changed from714.0 to 71.1 cm3/m2 day and decreased substantially at 1 wt% clayloading, and then decreased slightly with increasing content ofCloisite 15A. Overall, the WVTR and OTR decreased as the Cloisite15A content increased, indicating that the barrier properties of PUAwere greatly enhanced by the PUA/C15A nanocomposite systems.Both oxygen and moisture diffusion in the pure PUA film wereretarded by the incorporation of Cloisite 15A. It is generally under-stood that barrier properties of materials indicate their resistanceto diffusion and sorption of substances, and are highly dependentupon both their chemical and morphological structures [7,27–30].

The degree of surface hydrophobicity is an important factorin moisture barrier properties from the standpoint of chemicalstructure. Of the commercially available Cloisite clays, Cloisite

15A is generally accepted as exhibiting the highest hydrophobicity[15,22]. It is therefore anticipated that blending Cloisite 15A intothe PUA matrix will produce increased hydrophobicity in the PUAwith a relatively hydrophilic amide group in the repeating unit.

D. Kim et al. / Progress in Organic Coatings 77 (2014) 1045–1052 1051

Table 3Barrier properties and water contact angles for the PUA/C15A nanocomposite films.

Sample code WVTR (g/m2 day) OTR (cm3/m2 day) Contact angle (◦)

At 0 s At 1100 s �a

PUA/C15A-0 29.9 714.0 61.1 36.4 24.7PUA/C15A-1 28.3 220.0 62.9 46.9 15.8PUA/C15A-2 15.3 84.2 67.7 55.1 12.6PUA/C15A-3 18.6 87.3 68.3 59.2 9.1

Tist[iwC

cpmb1cm

tthCrort(fiCttdt

Ffi

PUA/C15A-4 13.9 71.1

a Difference between the contact angle at 0 s and 1100 s, respectively.

he variation in contact angle caused by the Cloisite 15A load-ng will give insight into the behavior of the nanoparticles on theurface of the PUA/C15A nanocomposite films, as well as the rela-ion to the hydrophobicity or hydrophilicity of the polymer matrix26,27,29]. To investigate the surface properties of films originat-ng from variations in Cloisite 15A content, the water contact angles

ere measured at 23 ◦C and 39% relative humidity; dependence onloisite 15A content and time are depicted in Fig. 10.

By relating moisture barrier properties and contact angles, itan be observed that water molecules are difficult to dissolve in aolymer matrix, and often only a relatively small amount of waterolecules diffuse. This may originate from the enhanced hydropho-

icity in PUA, caused by the incorporation of hydrophobic Cloisite5A. In addition, introducing high-aspect-ratio Cloisite 15A parti-les into a PUA film significantly complicates the path of the waterolecules [2–4,8,9,26,27].The initial contact angles are strongly dependent upon the con-

ent of Cloisite 15A. The initial contact angle increased from 61.3◦

o 74.6◦ as the Cloisite 15A content increased, implying that theydrophobicity of the pure PUA film was enhanced by the increasedloisite 15A content. In general, changes in contact angle withespect to time stem from the combined effect of the partial evap-ration and spreading/wetting process of the liquid drop, whichesults in a decrease in the contact angle [31]. As the environmen-al condition was controlled to maintain relatively high humidity80–90% relative humidity), the changes in contact angle may ariserom spreading and wetting process of the water drop. As shownn Fig. 10, the nanocomposite films containing various contents ofloisite 15A showed decreases in contact angles with increasingime. Depending on the content of Cloisite 15A, the changes in con-

act angle with respect to time varied from 24.7 to 9.1, apparentlyecreasing as Cloisite 15A content increased. This indicates thathe loading of relatively hydrophobic Cloisite 15A will induce an

Time (sec)

0 200 400 600 800 1000 1200

Con

tact

Ang

le (o )

20

30

40

50

60

70

80

PUA/C15A-0PUA/C15A-1PUA/C15A-2PUA/C15A-3PUA/C15A-4

ig. 10. Time dependence of water contact angles for the PUA/C15A nanocompositelms.

74.6 64.0 10.6

increased hydrophobic surface property and, as a result, enhancewater resistance in pure PUA film.

The WVTR and OTR values decreased to 1 wt%, unexpectedlyfollowed by a slight increase in WVTR at 3 wt%. The OTR did notdecrease with increasing clay content. This can be explained by apoor dispersion of clay particles at relatively high clay contents,and weak interfacial interaction between relatively hydropho-bic Cloisite 15A and hydrophilic urethane linkages. As shown inthe SEM and TEM results, the low content clay is well dispersedthroughout the matrix, whereas the high content clay induces poordispersion and small separated domains in the PUA matrix. Thispoor dispersability may result in a slight increase in WVTR forthe PUA/C15A-3 nanocomposite film, with no apparent decreasein OTR. Further studies are needed to improve the performance ofthe moisture barrier properties via chemical modification of theclay surface, and mixing methods.

4. Conclusions

The PUA/C15A nanocomposite films prepared via a UV-curingsystem showed intercalation/exfoliation combined structures,whereas no strong interfacial interactions formed between thePUA and clay, resulting in poor dispersion in relatively high claycontent. The thermal stability was slightly enhanced by the intro-duction of clay into PUA, while the oxygen and moisture barrierproperties were greatly enhanced. The OTR and WVTR decreasedfrom 714.0 cm3/m2 day to 71.1 cm3/m2 day and 29.9 g/m2 day to13.9 g/m2 day, respectively, with increasing content of clay par-ticles. Thus, incorporating Cloisite 15A particles with a relativelyhigh aspect ratio and hydrophobicity into the PUA matrix is anefficient way to improve the gas and moisture barrier propertiesof PUA. Further studies are required to increase the compatibilityand dispersion of clay particles in the PUA polymer matrix, max-imizing their performance in nanocomposite films and expandingapplications into packaging.

Acknowledgments

This work was supported by the National Research Foundation(NRF) of Korea Grant founded by the Korean Government (MEST)(No. NRF-2013R1A1A2057674).

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