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 DOI: 10.1177/0021998312460711

2013 47: 2987 originally published online 3 October 2012Journal of Composite MaterialsAhmad Ramazani SA, M Shafiee, H Abedsoltan and A Shafiee

Gas barrier and mechanical properties of crosslinked ethylene vinyl acetate nanocomposites  

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JOURNAL OFC O M P O S I T EM AT E R I A L SArticle

Gas barrier and mechanical properties ofcrosslinked ethylene vinyl acetatenanocomposites

Ahmad Ramazani SA, M Shafiee, H Abedsoltan and A Shafiee

Abstract

In this research, the effects of crosslinking agent and clay content on the morphology, barrier and mechanical properties

of ethylene vinyl acetate-organoclay nanocomposites prepared by solution method were studied. Dicumyl peroxide has

been used as crosslinking agent. The morphology of the prepared nanocomposites was investigated using wide-angle

X-ray diffraction and transmission electron microscopy. Wide-angle X-ray diffraction and transmission electron micro-

scopyindicated that the prepared nanocomposites had predominantly intercalated morphologies. The obtained results of

permeability tests showed that the permeability of ethylene vinyl acetate films dramatically decreases with addition of

organoclay and dicumyl peroxide. Mechanical tests showed that tensile modulus and tensile strength of ethylene vinyl

acetate increase with addition of organoclay. Furthermore, the mechanical properties of ethylene vinyl acetate nano-

composites significantly improved in presence of crosslinking agent (dicumyl peroxide).

Keywords

Polymer/clay nanocomposites, ethylene vinyl acetate, organoclay, dicumyl peroxide, barrier

Introduction

In recent years, with the advent of polymer/clay nano-composites (PCN), a new age in material science andtechnology has begun. These novel synthetic materialsexhibit superior mechanical and gas barrier propertiesin comparison with conventional composites.1–11

The most common mineral used in nanocompositesis montmorillonite (MMT). The extra-ordinary reinfor-cing effect of this material that provided the interactionbetween polymer and clay improves by means of achemical treating method. This is usually performedby the aim of ammonium alkyl compounds and thetreated clay by this method is called organoclay. Themorphology of the nanocomposites can range fromintercalated to exfoliated. The intercalated nanocompo-sites are structures where the extended polymer chain isinserted into the gallery space between parallel individ-ual clay sheets while the exfoliated ones result from theweak interaction with the adjacent layers’ gallery cat-ions, where the individual layers are not parallel to eachother.12–17 The aim of this study is to prepare and inves-tigate properties of nanocomposite based on ethylenevinyl acetate (EVA).

Zanetti et al.18 prepared EVA/clay nanocompositesusing melt-blending and studied the thermal and flame-retardant properties of these materials. Alexandreet al.19,20 also obtained EVA/montmorillonite nano-composites with a semi-intercalated and semi-exfoliatedstructure by melt-blending. They found that theYoung’s modulus and the thermal stability of thesenanocomposites were enhanced by MMT. However,to the best of our knowledge, no publications havebeen devoted to study the effects of chemical crosslink-ing on the barriers properties of these nanocomposites.In this work, the effects of crosslinking reactions on gasbarrier and mechanical properties of EVA nanocompo-site films are studied. Among several ways of crosslink-ing, thermochemical method involving organicperoxides as well as dicumyl peroxide (DCP) is

Department of Chemical and Petroleum Engineering, Sharif University of

Technology, Iran

Corresponding author:

Ahmad Ramazani SA, Department of Chemical and Petroleum

Engineering, Sharif University of Technology, P.O. Box: 11365-8639,

Tehran, Iran.

Email: ramazani@sharif.edu

Journal of Composite Materials

47(23) 2987–2993

! The Author(s) 2012

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DOI: 10.1177/0021998312460711

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chosen because of its controlled decomposition rate,minimal side products, and economical process.21

In other words, the present study covers the effectsof organophilic montmorilollonite and crosslinkingagent (DCP) on the morphology, gas barrier and mech-anical properties of EVA nanocomposites.

Experimental

Materials

EVA, containing 18% of vinyl acetate monomer man-ufactured by Hyundai Petrochemical of Korea(MFI¼ 2.2 g/10min and density¼ 0.94 g/cm3) andClosite 15A (C15A) with a cation exchange capacity(CEC) value of about 125meq/100 g from SouthernClay Inc. (Gonzales, Texas, USA) were used as rawmaterials in this project. Density of organoclay as thefiller was 1.66 g/cm3. Thermal gravimetric analysis(TGA) curve of Cloisite 15A nanoclay in nitrogenatmosphere are shown in Figure 1. Thermal decompos-ition of the alkyl quaternary ammonium modiEedmontmorillonite has taken place in two regions. TheErst-stage degradation at around 200�C refers to theevolution of interlayer absorbed water, CO2, and longchain alkyl fragments whereas second stage of thermaldegradation was shown in between 200�C and 500�C,which is due to the evolution of water from dehydrox-ylation of CO2 and organic substances. Dicumyl perox-ide (DCP) with 99% purity and a density of 1.02 g/cm3

was also used as the crosslinking agent accompanied byorganoclay as the filler with 1.66 g/cm3 density.The solvent which is used for the preparation of

nanocomposite films is the commercial xylene withhigh purity. Due to high boiling point of xylene com-pared to other solvents such as chloroform, tempera-ture increases up to about 138�C, hence the crosslinkingreaction increases significantly.

Preparation

Crosslinked and uncrosslinked EVA nanocompositesfilled with different amounts (3%, 5% and 7% w/w)of the organophilic clay (C15A) were prepared via solu-tion intercalation method. In this method, mixture oforganoclay and xylene was kept under reflux and stir-red for 24 h at 140�C. In the next step, weighted amountof polymer was added to the solutions under 3 h ofstirring and reflux to obtain necessary exfoliation oforganoclay by diffusion of EVA chains into clay inter-layer spaces. In order to prepare crosslinked EVAnanocomposites, the above mentioned process was fol-lowed by addition of 2wt% DCP as a curing agent. Themixing was continued for 20min. Finally the solventwas evaporated in a vacuum oven and the preparednanocomposites were roll milled at room temperature.All nanocomposite films (with and without DCP) wereprepared by compression molding with thin wall mouldat 150�C. Samples which contained crosslinking agent(DCP) were cured at the resident time of 15min.

Characterization

Wide-angle X-ray diffraction. To investigate the structureand degree of dispersion of prepared nanocomposites, aPhilips X’pert wide-angle X-ray diffraction (WXRD)

Figure 1. TGA of the virgin organoclay.

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system (40 kW, 30mA) was utilized. This instrumentevaluates the evolution of clay d001 reflection. The gal-lery spacing was obtained according to Bragg’s law22

d ¼ n�=2 sin � ð1Þ

where d is the spacing between layers of clay, �, thewavelength of X-ray, equals to 0.154 nm, y, the anglewherein first peak in spectra takes place and n corres-ponds to the order of diffraction, which is assumed 1 incalculations.

Transmission electron microscopy. To evaluate the morph-ology of the nanocomposites, transmission electronmicrographs (TEM) were taken from 60 to 80 nmthick, ultramicrotomed sections. The TEM employedwas a ZEISS/EM 900 with an acceleration voltage of100 kV.

Permeability measurement. In order to determine oxygenpermeability, an apparatus based on a modified versionof ASTM 1434D method was utilized. Pure oxygen wasfed to a stainless steel cell via steel tubing with regulatedcontrol valves. The internal diameter of the cell was90mm. The composite layer placed between the cellplates and fastened with pairs of nuts and bolts.

Mechanical properties. Prepared films were cut intodumbbell-shape specimen along longitudinal direction.The specimen had a gauge length of 12.5mm and awidth of 3mm. The tensile tests were performed usinga Hounsfield-H10KS machine with a load cell of 100N.The tensile speed was 12.5mm/min.

Results and discussion

Morphological characterization

Wide angle X-ray diffraction. Each prepared nanocompo-site samples were analyzed by WXRD in order toevaluate whether intercalation or exfoliation occurred.

The WXRD pattern of uncrosslinked EVA nano-composite films is presented in Figure 2. The d001peak of closite 15A at 2y¼ 2.8 corresponds to3.15 nm interlayer spacing (Bragg’s equation). WXRDpatterns for EVA18% reveal that the nanocompositeswith 3, 5 and 7wt% clay content have 6.8, 5.86 and 4.62interlayer spacing respectively (Figure 2). The clay dif-fraction peaks of all three systems shift to lower angles,suggesting intercalated structures in the three nanocom-posite films. Figure 3 shows that crossklinked EVAnanocomposite films with 3, 5 and 7wt% organoclayhave 5.92, 5.35 and 5.04 nm interlayer-spacing, respect-ively. In this figure the small peaks are observed at 9�

which corresponds to the presence of DCP molecules.23

The structures of all prepared EVA nanocompositeswith crosslinking agent are also intercalated.Therefore, WXRD patterns demonstrate that theEVA chains diffuse into the clay galleries, and causethe formation of intercalated structures.

Transmission electron microscopy. To make an indubitableexamination about the extent of exfoliation in preparednanocomposites, the transmission electron microscopy(TEM) was utilized. This test is a supplementary onefor WXRD results to confirm about the extent ofexfoliation. TEM micrographs of EVA18/5%clay andEVA18/DCP/5%clay nanocomposites are shown inFigures 4 and 5, respectively. Very small stacks of sili-cate layers (2–4 sheets) can be observed in these images.

Figure 2. WXRD patterns of closite 15A and EVA18/closite 15A nanocomposites with various clayloading for uncrosslinked EVA.

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WXRD patterns and TEM images confirm that thestructures of prepared nanocomposites are highly inter-calated. TEM images also illustrate uniform distribu-tion of organoclay in both crosslinked anduncrosslinked systems.

Gas barrier properties

A one-dimensional flux through an isotropic media J isdescribed by Fick’s first law of diffusion

J ¼ D�C=l ð2Þ

where C is the concentration of the permeant, D and lare diffusion coefficient and thickness of homogeneous

film, respectively. For gases, the concentration isexpressed in terms of partial pressure. Analogous toequation (2) we obtain

J ¼ p�P=l ð3Þ

Permeability of gases calculated from the rearrange-ment of equation (3):

p ¼ lJ=�P ð4Þ

where J is the gas volumetric flow rate per unit area(cm3 (STP)/(s cm2)), l is the thickness of composite(cm), �P is the pressure difference across the composite

Figure 3. WXRD patterns of closite 15A and EVA18/DCP/closite 15A nanocomposites with various clayloading for crosslinked EVA.

Figure 4. TEM microphotograph of EVA18/5 wt% closite 15A

nanocomposite.

Figure 5. TEM microphotograph of EVA18/DCP/5 wt% closite

15A nanocomposite.

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(cmHg) and p is the permeability (cm3 (STP) cm/(scm2cmHg).

Permeability is usually expressed in barrer(1 barrer¼ 7.5005� 10�18m2/s Pa or 10�10cm3 (STP)cm/(s cm2cmHg)). Permeability coefficient is plotted asa function of the wt% of clay for EVA in Figure 6. TheEVA/clay nanocomposites with a small fraction ofC15A exhibit a reduction in the permeability ofoxygen. The reduction of permeability arises from thelonger diffusive path that the penetrants must travel inthe presence of the filler. A sheet-like morphology is par-ticularly efficient in maximizing the pathway length dueto possessing the large length-to-width ratio. Moreover,Figure 6 also shows that the amount of permeability isfurther decreased in the presence of crosslinking agent.This was anticipated because this decrease in permeabil-ity is mainly affected by micro-scale reduction of meshsize of polymer, which is needed for passing of gasthrough polymer material. For a nanocomposite filmwith a volume fraction of flakes, ’, and aspect ratio ofa, the ratio of nanocomposite permeability to pure poly-mer permeability, is obtained by Nielsen and Cussler(random and regular array) models.24–26 The details ofthese models are presented in Table 1, where pc and pmare the permeability coefficients of a polymer filled withparticles and pure polymer film, respectively. Thevolume fraction of organoclay is obtained from weightpercent using the following equation

’ ¼1

1þ 100�wtwt ð�f=�mÞ

ð5Þ

where rf and rm are the densities of filler andmatrix, respectively. The density of organoclay isabout 1.66 g/cm3 and that of polymeric matrix isabout 0.93 g/cm3.

Using these models, the effective aspect ratios of theorganoclay platelets in nanocomposites at 3, 5 and 7%clay loading were calculated and reported in Tables 2and 3. The most obvious conclusion that can be madefrom Tables 2 and 3 is that the clay flakes are welldispersed and intercalated within the matrix.

Mechanical properties of the preparednanocomposite films

Tensile tests were performed on the nanocompositefilms to investigate the effects of organoclay content(Closite 15A) and crosslinking agent on the mechanicalproperties. The tensile properties of the prepared filmsare listed in Table 4. Addition of organoclay and cross-linking agent was found to have progressive effects onthe tensile properties of EVA18. Results show that thetensile modulus and stress at the break of the nanocom-posites improve by adding organoclay content. Theimprovement of mechanical properties by the addition

Figure 6. The effects of clay content and addition of cross-

linking agent on the permeability coefficient of prepared films.

Table 1. Details and formulas for each model used in the text

Model

Filler

type Formulas

Nielsen Ribbon (pm/pc) (1�f)¼ 1þ af/2

Cussler (Regular array) Ribbon (pm/pc) (1�f)¼ 1þ (af)2/4

Cussler (Random array) Ribbon (pm/pc) (1�f)¼ (1þ af/3)2

Table 2. Oxygen permeability coefficient and aspect ratio

measurement in EVA18/Clay nanocomposites with various

composite theories

Organoclay

weight

percent

O2 permeability

coefficient

(Barrer)

Aspect ratio (a)

Nielsen

Cussler

(Regular

array)

Cussler

(Random

array)

0 10.84 – – –

3 6.62 71 91 47

5 5.87 55 62 35

7 4.53 63 56 38

Table 3. Oxygen permeability coefficient and aspect ratio

measurement in EVA18/DCP/Clay nanocomposites with various

composite theories

Organoclay

weight

percent

O2 permeability

coefficient

(Barrer)

Aspect ratio (a)

Nielsen

Cussler

(Regular

array)

Cussler

(Random

array)

0 8.57 – – –

3 5.88 50 76 34

5 4.3 65 67 40

7 3.26 74 60 43

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of organoclay may be a result of rich interfacial bond-ing due to long aliphatic chains in C15A. Also thecrosslinking agent enhances mechanical propertiesbecause of the formation of chemical bonds betweenpolymer molecular chains to form a three-dimensionalnetwork.

Conclusions

In this research work, morphology, gas barrier andmechanical properties of EVA nanocomposites pre-pared by solution method were studied. The effects ofaddition of DCP as a crosslinking agent were alsoinvestigated. In this method, EVA chains are moreeasily entered into the clay sheets and form the inter-calated nanocomposites. The polarity of the EVA andthe basal spacing of organoclay are of importance tomorphology and gas barrier properties of the finalnanocomposites. Observed results from permeabilitytests showed that a small fraction of organoclay andcrosslinking agent (DCP) caused the barrier propertiesof the prepared nanocomposites to enhance remark-ably. EVA-based nanocomposite films exhibit improve-ment in tensile properties especially by the addition ofDCP. For instance, an addition of 2wt% of DCPmakes the barrier properties to improve by 20% andthe Young’s modulus further to increase by up to 58%.Results also showed that tensile modulus and stress atthe break of the nanocomposites were enhanced byincreasing the organoclay content. Different modelswere used to model the oxygen permeability of pre-pared nanocomposite films. The results indicated thatthe clay stacks homogeneously intercalated in the EVAmatrix.

As it is obvious in permeability modelling tests, pres-ence of nanoclays increase the tortuosity, therefore per-meability has been reduced significantly. On the otherhand, addition of DCP leads to creation of a densestructure hence decreasing the permeability, which isobvious. But the higher effect of presence of nanoclaywould be more important than DCP. Also it must bementioned that the improvement in the mechanicalstrength of structure is because of creation of crosslink-ing between polymer chains.

Funding

This research received no specific grant from any fundingagency in the public, commercial, or not-for-profit sectors.

Conflict of interest

None declared.

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