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13 June 2011 2012 25: 115 originally published onlineJournal of Thermoplastic Composite Materials

D. Merinska, H. Kubisova, A. Kalendova, P. Svoboda and J. HromadkovaNanocomposites

Processing and Properties of Polyethylene/Montmorillonite  

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Processing and Properties ofPolyethylene/Montmorillonite

Nanocomposites

D. Merinska,* H. Kubisova, A. Kalendova, and P. Svoboda

Faculty of Technology in Zlin, Tomas Bata University,762 72 Zlin, Czech Republic

J. Hromadkova

Institute of Macromolecular Chemistry, Prague, Heyrovskeho nam. 2

ABSTRACT: Polyethylene (PE) nanocomposite samples were prepared withCloisite 25A, 30B, and 93A and Nanofil 5 and 3000 nanofillers. The amount ofmodified Naþ montmorillonite (MMT Naþ) was fixed to 5 wt%. For thecompounding of PE matrix and nanofillers, two different compounding equipmentswere used, KO Kneader Buss and APV twin-screw extruder. In all samples, maleicanhydride-modified PE (PEMa) was added as a compatibilizer. The content ofPEMa in mixtures was always 5wt%. The level of MMT exfoliation in thenanocomposite systems was studied by X-ray diffraction and by transmissionelectron microscopy observations. The properties of samples were evaluated bydynamical mechanical analysis (E* modulus at 308C) and by the measurement oftensile properties (stress and strain at break). Because of the possibility of usage ofprepared materials in packaging industry, barrier properties were measured withfocus on oxygen, carbon dioxide, and water vapor permeability. The influence of twodifferent used compounding equipments on the prepared nanocomposite samples ofPE nanocomposites was discussed.

KEY WORDS: polymer nanocomposite, montmorillonite, PE, compounding,Cloisite, Nanofil.

Journal of THERMOPLASTIC COMPOSITE MATERIALS, Vol. 25—February 2012 115

0892-7057/12/01 115–17 $10.00/0 DOI: 10.1177/0892705711404939� The Author(s), 2011. Reprints and permissions:

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*Author to whom correspondence should be addressed.E-mail: merinska@ft.utb.cz

INTRODUCTION

NOWADAYS, IT IS very well known that nanocomposite materialrepresents a material in a polymer area, where filler particles in nano-

sizes are homogenously dispersed in a polymer matrix. This type of fillershas taken a considerable position in preparing filled materials in the lastdecade either in polar polymer or in nonpolar, such as polyethylene (PE) orpolypropylene. A study of polymer nanocomposites involves either the ownpreparation process or an observation of phenomena, which take place innanocomposite properties. What is very important for filled materialsgenerally is in the same way important for nanocomposites. Interactionsbetween polymer matrix and filler particles have a significant importance inpolymer/clay nanocomposites. Clay nanoparticles can have a function, forexample, as a novel flame retardant [1–4] or they can cause increaseddimensional stability. Later, also barrier properties are being studied [5–7].Although some of these improvements can be achieved with conventionalfillers (mineral or glass), the content has to be quite high (20–40wt%)compared to typical 3–5% charge in case of nanoclay.

Layered nanoparticles come especially from minerals of the group oflayered clays. Currently, one of the widely studied and used one ismontmorillonite (MMT). Montmorillonite belongs to the group of thestructure 2:1 minerals. Its structure is being described as one aluminiumoxide octahedral layer sandwiched by two layers of silicon oxide [8,9].

The natural montmorillonite disintegration into individual platelets innanoscale demands two main steps. The first one is a modification of MMTby an intercalation (organofilization) of certain organic compounds.Intercalation is an organic compound (i.e., octadecylamine [10,11] orquaternary ammonium salts [12]) insertion between MMT layers. When thedistance of layers becomes higher, the molecular bonds become weaker andanother step – compounding with polymer – than the energy for a layerseparation is lower [13].

Two main modes of organofilization were published. The first one isbased on the ion-exchange reaction where the inter-layer cations (usuallysodium) are replaced with onium organocations. The most common oniumions are quartenary ammonium salts or acid salts of amines [14,15]. Anothermode is based on the ion–dipole treatment, where organic moleculescontaining a sufficiently strong dipole (or more dipoles) coordinate to thesodium in the gallery space [16,17].

The mixing of modified montmorillonite with the polymer (compounding)is usually performed in melt in a screw compounder. To evaluate thenanocomposite morphology, two main expressions are used – intercalationand delamination, exfoliation, respectively. The intercalated level is the state

116 D. Merinska et al.

wherein the nanocomposite well-ordered multi-layered structuresare noticeable. The extended polymer chains are inserted into aninterlayer. The delaminated or exfoliated state represents the structurewith the individual silicate layers. They are homogenously dispersed in apolymer. Actually, in a polymer matrix, both structures can co-existtogether [18].

Generally, the compounding equipment has a big influence on themontmorillonite particles exfoliation in either polar polymer matrix (PA) ornonpolar one (polyolefins). Unfortunately, the MMT exfoliation is not thatsimple, as it was expected. And the connection of highly polar nanofillerlayers with nonpolar PE matrix brings extra problems. It is the reason,why studies concerning this topic have been published during severalpast year [7,19,20]. The better level of MMT exfoliation can be achievedby higher amount of energy brought in a system. This can be achieved bythe starting of compounding in a single screw when one can study theeffect of increasing screw speed. The next step could be twin-screwextruder or even special types of very high efficient extruders (with conicscrews, etc.) [21].

Preparation of PE nanocomposites because of their high hydrophobicityalso should be performed with a compatibilizer in order to improve theaffinity of the modified MMT and a polymer matrix. Currently, maleicanhydride-modified polypropylene is being studied and widely used [20–23].

The aim of this study was to investigate the process of MMT exfoliationby transmission electron microscopy (TEM) and X-ray diffraction (XRD) inPE matrix when the two different types of compounding equipments werebeing used. Then, the mechanical properties were evaluated with focus onmodulus by DMA, tensile strength, and barrier properties.

EXPERIMENTAL

Commercial modified MMT nanofiller, Cloisite 25A, 30B, and 93A, fromSouthern Clay Products, Inc., and Nanofil 5 from Sudchemie Germany wereused as nanofillers. Differences among individual nanofillers lie in thedifferent types of intercalant agents (details can be found on the websites ofabove-mentioned companies). All samples were prepared with 5wt% ofMMT. The polymer/clay composites were compounded with KO Busskneader D¼ 30mm, L/D¼ 18, at the temperature of 1808C and in twin-screw laboratory extruder APV MP19-25TC from APV Baker, D¼ 19mm,L/D¼ 25 at the same conditions. After pelletization of the strands, thepellets of mixtures were pressed into the sheets. The achieved level ofexfoliation of the prepared sheets after a compounding process was

Polyethylene/Montmorillonite Nanocomposites 117

measured by XRD. Compounded samples were analyzed by XRDpowder diffractometer (INEL) equipped with the curved position sensitivedetector CPS 120 (1208 2�), reflection mode with a germanium mono-chromator (Cua1 radiation). Samples were placed in a holder and exposedfor 2000 s.

From mechanical properties, E* modulus, loss angle, and tensile strengthwere evaluated. The measurement of E* modulus was carried out by DMADX 04T (R.M.I., Czech Republic) at 308C. The specimens were cut out fromthe pressed sheets. The DMA values are summarized in Table 1.

Transmission Electron Microscopy

For the transmission electron microscopy (TEM; JEM 200CX), thespecimens were cut using Leica cryo-ultramicrotome at sample temperature�1008C and knife temperature �508C to obtain ultra-thin sections with thethickness approximately 50 nm and an acceleration voltage of 100 kV wasused.

Barrier Properties

PE/Cloisite nanocomposite-molded films with the thickness about 50 mmwere used for the measurement carried using the device for gas permeabilitywith a pressure pickup LMP 150 (made in Tesla Roznov, Czech republic), atthe temperature 258C and the hypertension 0.4–0.6MPa. On this device,oxygen and carbon dioxide permeability coefficient measurements werecarried out.

The water vapor permeability was measured in the agreement with theASTM E 95-96 – Standard Test Methods for Water Vapor Transmission ofMaterials. Out measurement was carried out at 308C and 50% relativehumidity.

Table 1. The data of DMA measurement.

SampleE* modulus(MPa) KO

E modulus(MPa) APV

Lossangle KO

Loss angleAPV

Unfilled PE 286.4 0.161325A 459.9 471.2 0.1224 0.111530B 469 478.2 0.1284 0.115693A 514.9 526.1 0.1324 0.1292Nanofil 5 466.27 470.1 0.1187 0.1056

118 D. Merinska et al.

DISCUSSION

In order to compare the influence of different compounding devices onthe level of exfoliation, the XRD technique was used. Figure 1 shows XRDpatterns of samples prepared by KO Buss kneader. In all cases, it is possibleto notice that the exfoliation of MMT particles did not happen completely.In the comparison with the pattern of unfilled PE, all other curves exhibitpeaks with various heights and peak positions, which belong to thenonexfoliated montmorillonite agglomerates. The patterns of samples filledby Cloisite 93A and Nanofil 5 even exhibit the next broadening peak. Thus,as it was said, already from this first observation, it can be concluded thatMMT here is not well exfoliated. Figure 2 shows the patterns ofnanocomposite samples prepared on APV twin-screw extruder. The shapeof patterns here is completely different from those in Figure 1. Thenoticeable maxima are present only in two shown patterns. However, they

2 4 6 8 10 122 theta

0

10000

20000

30000In

ten

sity

(u

.a.)

unfilled PE

PE + Cloisite 25 A

PE + Cloisite 93A

PE + Cloisite 30B

PE+Nanofil 5

Figure 1. XRD patterns of PE/clay samples (5% of MMT loading) prepared in KO kneaderBuss.

Polyethylene/Montmorillonite Nanocomposites 119

are not high and sharp. Moreover, in the area between 2.58 and 78, the curveis flat without any significant peaks. It indicates that the level of exfoliationafter the compounding in APV achieved much higher level. To show thedifference more clearly, two selected fillers are plotted in one graph(Figure 3). The curves belonging to samples prepared in APV device arealmost without any peak – there are no measurable multilayer regularlyarranged structures. XRD analysis has indicated better mixing in twin-screwextruder. To confirm these results, we had carried out also TEMobservations of the structures.

TEM allowed us to obtain the images of the sample morphology intwo different magnifications. These images are shown in Figures 4(a),

2 4 6 8 10 122 theta

0

400

800

1200

1600In

tens

ity (

a.u.

)

unfilled PE

PE+Cloisite 25A

PE+Cloisite 30B

PE+Cloisite 93A

PE+Nanofil 5

Figure 2. XRD patterns of PE/clay samples (5% of MMT loading) prepared in APV twin-screw extruder.

120 D. Merinska et al.

Figure 4. TEM images of the sample PE/Cloisite 25A prepared in: (a) KO Buss and (b) APVtwin-screw extruder.

2 4 6 8 10 122 theta

0

5

10

15

20

25In

tens

ity (

a.u.

)

PE+Nanofil 5 (KO Buss)

PE+Cloisite 30B (KO Buss)

PE+Nanofil 5 (APV)

PE+Cloisite 30B (APV)

Figure 3. XRD patterns of PE/clay samples (5% of MMT loading) prepared in KO kneaderBuss and APV twin-screw extruder – the comparison.

Polyethylene/Montmorillonite Nanocomposites 121

(b) and 5(a), and (b) for PE/Cloisite 25A composite. The first pair of TEMimages (Figures 4 and 5) belongs to the samples of nanocomposite PE/Cloisite 25A, (a) shows the sample prepared by compounding in KO Buss,(b) shows the images of sample prepared by APV extruder. It is possible tosee clearly the bigger MMT agglomerates in case of KO Buss kneaderpreparation. The size of montmorillonite particles in figure b is noticeablylower. The same trend confirms the second pair of TEM images with highermagnification (Figure 5(a) and (b)). The thickness of MMT particle in figureb (APV extruder) is significantly smaller; it is close to tens of nanometer.The similar situation is possible to describe in the case of the next usednanofillers, Cloisite 93 A and Nanofil 5 (Figures 6 and 7(a) and (b)).In both cases, it is possible to recognize in the images the presence of

Figure 6. TEM images of the sample PE/Cloisite 93A prepared in: (a) KO Buss and (b) APVtwin-screw extruder (higher magnification).

Figure 5. TEM images of the sample PE/Cloisite 25A prepared in: (a) KO Buss and (b) APVtwin-screw extruder (higher magnification).

122 D. Merinska et al.

bigger and thicker (with more MMT layers together, only intercalated, notexfoliated) particles of clay nanofiller for the samples prepared in KOkneader Buss. In contrast, the particles in the samples compounded in APVdevice, the particles are smaller with the thickness going to the particularMMT layers dispersed in the PE matrix.

To sum up, the results of TEM observation support the results from XRDmeasurement. The level of exfoliation was higher for the samples preparedby APV twin-screw extruder.

For the practical applications of prepared nanocomposite materials,mechanical properties are very important. Dynamical mechanical analysis(DMA) is a powerful method for obtaining values of modulus and loss angletan �. The data from DMA are listed in the Table 1 and in two graphs(Figures 8 and 9). Figure 6 represents the data of E* modulus. It is clear thatall prepared samples, prepared either by KO Buss or by APV extruder, showabout 50% increase in E* (the improvement of stiffness). Moreover, thisgraph brings the information that there is no significant difference betweenvalues obtained for both observed devices.

Next data obtained from DMA measurement focus on loss angle tan �.Values of tan � are calculated as the ratio of loss modulus E0 and storagemodulus E00 (tan �¼E0/E00). This tan � value describes the level of materialelasticity. Many times higher elasticity is considered as a positive changewith exception to shock absorbing application when high tan � is required.And the elasticity is influenced by the level of MMT exfoliation in a polymermatrix, better exfoliation causes higher elasticity. So, it can be considered tobe one of the possible evaluations of MMT exfoliation. Figure 9 showsvalues of tan � of observed samples. We can say, that the results are in goodagreement with the results of E*. All values of filled samples are lower

Figure 7. TEM images of the sample PE/Nanofil 5 prepared in: (a) KO Buss and (b) APVtwin-screw extruder (higher magnification).

Polyethylene/Montmorillonite Nanocomposites 123

Loss angle

0

0,02

0,04

0,06

0,08

0,1

0,12

0,14

0,16

0,18

unfilledPE

25A 30B 93A Nanofil 5

nanofillers

loss

an

gle

KO APV

Figure 9. Graph of DMA – loss angle (tan �).

E*modulus (at 30°C)

–100

0

100

200

300

400

500

600

Nanofil 593A30B25Aunfilled PE

nanofillers

E*m

odul

us (M

Pa)

APV KO

Figure 8. Graph of DMA – E* modulus.

124 D. Merinska et al.

(we have achieved better elasticity), than for unfilled PE and there is also thesame tendency, APV samples are better than KO Buss ones. To sum up, theuse of twin-screw extruder brings the better level of MMT particlesexfoliation.

The data from tensile stress measurement are plotted in graph inFigure 10. The tendency here is quite different from the DMA results.Values of tensile stress vary in higher scale and not all of them are betterthen unfilled PE. The best results were achieved for the sample PE/Nanofil 5and not significantly lower the value of sample PE filled with Cloisite 93A.In addition, here the differences between values related to both usedcompounding devices have better visibility. Samples prepared by APVextruder show much higher improvement compared to the case of KO Buss.The tensile test represents a mechanical behavior of samples during adestruction process. The explanation of lower values of destructionproperties can be that bigger MMT agglomerates (compare Figures 4 and5 – TEM images) cause stress concentration during the tensile measurement.Better dispersion of the nanolayer causes increase in tensile strength such asit was shown for example for PE/Nanofil 5 (compare Figures 7 and 10).These results are in very good agreement with the TEM observation.

PE nanocomposites have been studied for longer period of time asmaterials that would be possible used in a packaging industry [24]. That is

Tensile stress

0

5

10

15

20

25

30

35

unfilledMMT

25A 30B 93A Nanofil 5

nanofillers

Ten

sile

str

ess

(MP

a)KO APV

Figure 10. Graph of mechanical properties measurement – tensile strength.

Polyethylene/Montmorillonite Nanocomposites 125

why also in our work, we were interested in the measurement of permeabilityproperties. Figures 11–13 represent three graphs showing permeabilitycoefficients for oxygen, carbon dioxide, and water vapor. Figure 11 exhibitsthe observation of oxygen permeability. The values plotted here show thatthe sample PE/Nanofil 5 is the worse one, which is exactly the oppositeresult to the tensile strength measurement. All composites containingCloisites exhibit lower permeability coefficient in comparison with theunfilled PE matrix. When we focus on the differences between samplesprepared by different compounding devices, the tendency is not the same,but almost in all cases, the values are lower for samples prepared by APVextruder.

unfilled

PE

25A

30B

93A

N5

0,00E+00

1,00E-17

2,00E-17

3,00E-17

4,00E-17

5,00E-17

6,00E-17

7,00E-17

8,00E-17

9,00E-17

Oxy

gen

per

m.c

oef

.

nanofillers

Oxygen permeability coeficient

KO APV

Figure 11. Graph of permeability – oxygen permeability coefficient.

126 D. Merinska et al.

Similarly to the first-described graph (Figure 11), the next one deals withCO2 coefficient (Figure 12). However, here the values for Nanofil 5 arebetter in comparison with unfilled PE; surprisingly, it is the lowest from allobserved samples. This result can be explained by different sizesand polarities of O2 and CO2 molecules, which also influence thetransmission of these gases through the PE film. Nevertheless, the resultsof APV samples exhibited here also have the lowest values of CO2

permeability coefficient.The water vapor permeability was the last-studied property. The

measurement is not the same as in cases mentioned above (it was describedin ‘Experimental’ section). The data are presented in Figure 13. The results

clear

PE

25A

30B

93A

N5

0,00E+00

1,00E-17

2,00E-17

3,00E-17

4,00E-17

5,00E-17

6,00E-17

7,00E-17

8,00E-17

9,00E-17

1,00E-16

Per

m. c

oef

.

nanofillers

Carbon dioxide permeability coefficient

KO APV

Figure 12. Graph of permeability – carbon dioxide permeability coefficient.

Polyethylene/Montmorillonite Nanocomposites 127

show that the permeability after 288 h of observation was lower for almostall studied samples in comparison with a clean PE, except of PE/Cloisite30B. It indicates that all other samples can be used for the decreasing ofwater vapor permeability. The big influence of different preparationmethods was obtained in case of nanofiller Cloisite 25A and Nanofil 5.

Any product (in our case nanocomposite) has to be evaluated manyangles and not just from one point of view. This overall summary is shownin Table 2. The best composite in our case was PE/Cloisite 93A, second

clea

r PE

25A

30B

93A

N5

0

0,05

0,1

0,15

0,2

0,25

0,3

0,35

0,4

0,45

0,5

wei

gh

t lo

ss (

kg)

nanofillers

Water steam permeability after 288 hours

KO APV

Figure 13. Graph of permeability – water vapor permeability.

128 D. Merinska et al.

being PE/Cloisite 25 A prepared by twin-screw extruder. The worst in ourstudy was PE/Cloisite 30 B. This shows the importance of modification atthe clay manufacture and the selection for the particular polymer matrix.

CONCLUSIONS

In this study, the influence of different compounding devices on the PE/nanofiller properties was observed. The samples with choice type ofcommercial nanofiller were prepared either by the use of KO Buss kneaderor APV twin-screw extruder. The morphology, mechanical properties, andpermeability of three gases were studied.

Generally, almost in all cases, the results obtained for samples preparedby twin-screw extruder were better than those obtained for KO Busssamples. This phenomenon was different only in the case of E* modulusmeasurement, where all values increased in the same level disregarding theway of preparation or the type of nanofiller. A little bit different is thesituation in case of values tan �, where the improvement was observed alsofor all samples, but it is possible to notice the influence of compoundingdevices. By contrast, the tensile stress did not exhibit the improvement for allsamples; here PE with nanofillers Cloisite 93A and Nanofil 5 can be used forthe increasing tensile stress. To lower the permeability for observedsubstances, oxygen, carbon dioxide, and water vapor samples withCloisite 25A and 93 A seem to be the most suitable.

Table 2. The summary evaluation of measured properties.

Nanofiller Cloisite 25 A Cloisite 30 B Cloisite 93 A Nanofil 5

Modification(www.pages)

APV KO APV KO APV KO APV KO

E* modulus þ þ þ þ þ þ þ þ

tan � þ þ þ þ þ þ þ þ

Tensile strength 0 � � � þ þ þ þ

Oxygen permeability þ þ þ þ þ þ � �

Carbon dioxide permeability þ þ þ þ þ þ þ þ

Water steam permeability þ þ � � þ þ þ þ

Total 0 1 2 2 0 0 1 1

þ, positive effect; 0, almost no effect; and �, negative effect.

Polyethylene/Montmorillonite Nanocomposites 129

To sum up, the best results were obtained for the sample of PE/Cloisite93A prepared by APV extruder.

The future work will follow presented work and it will be focused mainlyto the elaboration of influence of other modified conditions used for PEnanocomposite preparation on its properties.

ACKNOWLEDGMENTS

This project was supported by the Academy of Sciences of the CzechRepublic (project AVOZ4050913). This study has been also partiallysupported by the Ministry of Education of The Czech republic as a part ofthe project no. VZ MSM 7088352102.

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