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Materials Science and Engineering A 445–446 (2007) 141–147

Processing and characterization of polyethylene/Brazilianclay nanocomposites

E.M. Araujo a,∗, R. Barbosa a, A.W.B. Rodrigues a, T.J.A. Melo a, E.N. Ito b

a Department of Materials Engineering, Federal University of Campina Grande, Campina Grande/PB, Brazilb Department of Materials Engineering, Federal University of Sao Carlos, Sao Carlos, SP, Brazil

Received 4 April 2006; received in revised form 16 August 2006; accepted 10 September 2006

bstract

Nanocomposites containing polyethylene (PE) and montmorillonite clay organically modified (OMMT) with quaternary ammonium salts werebtained via direct melt intercalation. A montmorillonite sample from the Brazilian state of Paraıba was treated with four different types of quaternarymmonium salts. After the treatment, the powder was characterized by X-ray diffraction and scanning electron microscopy. The dispersion andorphologies of OMMT within PE were investigated by X-ray diffraction and transmission electron microscopy. The results revealed the formation

f intercalated montmorillonite layers in the PE matrix. The thermal stability and flammability of the PE/montmorillonite clay nanocomposites wereeasured by thermogravimetry and horizontal burning tests for HB classification, Underwrites Laboratories (UL 94), respectively. It was shown

hat the samples do not degrade at the processing temperature. By adding only 3 wt.% montmorillonite, the burning rate of the nanocompositesas reduced by 17%, and, in general, the tensile test showed that the yield strength and modulus of the nanocomposites are close to the pure PE.owever, for a selected salt for MMT treatment the mechanical properties of nanocomposites were improved.2006 Elsevier B.V. All rights reserved.

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eywords: Nanocomposites; Polyethylene; Montmorillonite clay; Thermal stab

. Introduction

In the majority of conventional polymer composites, i.e.,mmiscible systems, the weak interaction between organic andnorganic phase leads to not enhanced mechanical and thermalroperties. On the other hand, strong interaction between theolymer and organically modified clay (organoclay) leads theispersion on the inorganic phase in nanometer scale and thisavor the nanocomposites formation. As a result, the nanocom-osites present unique properties that not were observed inonventional composites. The small amounts used of organoclay<10 wt.%) in the polymer matrices enhance several proper-ies significantly such as: mechanical, thermal, optical, electric,imensional, flammability and barrier due to the large contactrea between polymer and clay on a nanoscale as reported on

iterature [1–10]. The incorporation of organoclays into polymer

atrices has been known for 50 years and one of the pioneeringorks was from Toyota as reported by Cho and Paul [2]. Con-

∗ Corresponding author. Tel.: +55 83 3310 1178/58109 970.E-mail address: edcleide@dema.ufcg.edu.br (E.M. Araujo).

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921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2006.09.012

Flammability

idering the dimensions of polymer chains and their crystallinessemblies, it can be said that all polymer have structure on theanometer size scale and, further that the mechanical propertiesf polymer are governed by the interactions of these nanos-ructures to another. The most commonly used clay to prepareanocomposite is from the smectite group, such as montmoril-onite (MMT). In this clay mineral the silicate layers are joinedhrough relatively weak dipolar Van der Waals forces and theations Na+ and Ca2+ are located in the interlayer [11]. Theodium and calcium ions usually can be replaced by organications through an ion exchange reaction by exchange cationsresent on the surface and in the basal spacing of the clay withurfactants cations such as primary, secondary, tertiary and qua-ernary alkylammonium ions to convert an organophilic silicate1,12]. This will improve the interaction with many polymers.owever, the organic salt molecule should have a large organicroup (at least C12) and a cationic polar group (hydrophilic). Theddition of the organic salt molecules of large sizes increases

he basal spacing (d0 0 1) of the clay and then the organoclaynd the thermoplastic matrix will have better interaction [2]. Asentioned above, these polymer-clay nanocomposites exhibit

uperior strength, modulus, heat distortion temperature, optical

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42 E.M. Araujo et al. / Materials Science a

roperties, water and gas barrier properties, flame retardancynd with comparable impact strength as neat polymer. Moreover,eforcing character of the organoclay particles within of polymeranocomposite is due too polymer chains mobility restrictionhen they are contact with the clay layers. In the nanocom-osites of polymer/organoclay the disperse phase is in the formamellar of typically 1 nm in thickness and hundreds of nanome-er in width and length [2,3,5–7,11,13].

The purpose of this work was to obtain organoclays and withhem to produce PE nanocomposites. The organoclays were pre-ared using a montmorillonite sample from the Brazilian statef Paraıba and testing four different types of quaternary ammo-ium salts to investigate their performance. The morphologynd the mechanism of formation of the resulting PE/organoclaysanocomposites were investigated.

. Experimental

.1. Materials

A high density polyethylene (PE, JV-060U, density:.957 g/cm3, melting flow index: 7.0 g/10 min) was suppliedy Braskem/Brazil and used as composite matrix. The usedristine clay was Na-montmorillonite (MMT, Brasgel PA,oa Vista/Paraıba, Northeast of Brazil) supplied by Bentonitniao do Nordeste with a cation exchange capacity (CEC) of0 meq/100 g and the interlayer spacing (d0 0 1) obtained by XRDas 12.5 A. MMT was dried at 60 ◦C for 48 h prior to use.araiba state is responsible for majority of the montmorillonitelay national reserves around 62%. This clay was not yet testedo be used in polymer nanocomposites.

The quaternary ammonium salts used for the modificationf MMT were: alkyldimethylbenzylammonium chloride (Dodi-en; C19H32N+(CH3)2Cl−; abbreviated as D), cethyltrimethy-ammonium chloride (Genamin; C16H33N+(CH3)3Cl−; abbre-iated as G), esthearildimethylammonium chloride (Praepa-en; (C16H33)2N+(CH3)2Cl−; abbreviated as P), all industrialrades supplied by Clariant/Brazil and cethyltrimethylammo-ium bromide (Cetremide; C16H33N+(CH3)3Br−; abbreviateds C), industrial grade from Vetec/Brazil. These salts presenthe same groups used to make organoclays according to the lit-rature [14–17] and were used as received.

.2. Preparation of organoclays

The preparation of the organophilic montmorilloniteOMMT) with the Dodigen, Genamin and Praepagen salts wasimilar. The Na-MMT (32 g) was mixed in distilled water withtirring to form a uniformly dispersed suspension. The suspen-ion was stirred for 20 min after all the clay has been added. Andhen the salt equivalent to 1.0 CEC of Na-MMT was added intohe dispersion. The mixture was stirred for 20 min. After 24 h,he mixture of montmorillonite and the salt was washed with

istilled water for several times to remove excess salts, dried at0 ◦C for 48 h, and finally, passed in a sieve 200 mesh, accord-ng to the procedure described by Araujo et al. [4,11,18]. Theroducts obtained with Dodigen, Genamin and Praepagen were

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gineering A 445–446 (2007) 141–147

amed as D-OMMT, G-OMMT and P-OMMT, respectively. Thereparation of the organophilic montmorillonite (OMMT) withhe Cetremide salt was as follows: the Na-MMT (32 g) and thealt equivalent to 1.0 CEC of Na-MMT were mixed in distilledater at 80 ± 5 ◦C and they were kept for 20 min with stirring

o form a uniformly dispersed suspension. After 24 h, the samerocedure above was made for mixture of montmorillonite andhe salt. The product obtained with Cetremide was named as-OMMT.

.3. Nanocomposite preparation

PE/organoclay composites, containing 3 wt.% clay, were meltompounded in a counter-rotating twin-screw extruder (Torqueheometer Haake) operating at 170–200 ◦C and 60 rpm. In order

o assure a better dispersion of the fine clay powder in the highiscosity PE polymer, a 1:1 PE/organoclay master was previ-usly produced in a Torque Rheometer Haake with internalixer, at 190 ◦C and 60 rpm for 7 min.Standard tensile (ASTM D638), notched Izod impact (ASTM

256) and flammability UL94 HB samples were injectionoulded in a Fluidmec machine at 200 ◦C. After moulding, the

amples were placed in a desiccator for 48 h prior to the test.

.4. Characterization

The structure of PE/organoclay composites was characterizedy XRD and TEM. XRD measurement was performed usingXRD-6000 Shimadzu diffractometer, operating at 40 kV and0 mA, with 2θ scan range of 2–30◦ at room temperature, at acanning speed of 2◦/min, using Cu K� radiation (λ = 0.154 nm).EM was carried out on a Philips CM120 with an accelerationoltage of 120 kV. Samples were cryogenically microtomed intoltrathin sections (25–50 nm thick) from Izod impact bars withdiamond knife using a RMC MT-7000 under cryogenic con-itions (−80 ◦C) inside the microtoming chamber. A cuttingpeed of 0.1 mm s−1 was maintained throughout the microtom-ng operation. The ultrathin sections were collected on a copperrid. XRD and SEM (SSX-550 Shimadzu) measurements wereade directly from MMT and organoclay powders.Tensile tests were conducted using a LLoyd LR/10KN Uni-

ersal Machine at room temperature and crosshead speed of0 mm min−1 for the determination of tensile modulus (TM),ield strength (YS) and elongation at yield (EY) measurements,ccording to the standard ASTM D638. Izod impact tests werearried out on notched specimens at room temperature usingCeast Resil 5,5 equipment, according to the standard ASTM256. Typically, five or more samples were used for each deter-ination.The thermal stability was investigated by thermogravimetry

TG) using TGA S1H Shimadzu at a scan rate of 12.5 ◦C min−1,rom room temperature up to 900 ◦C under a nitrogen and airtmosphere with a flow rate of 50 mL min−1. The sample mass

as 5.0 ± 0.5 mg. The TG curves were analyzed with the aid of

he TASYS software from Shimadzu.Flammability properties were measured using horizontal

urning tests for HB classification, according to UL 94 [19].

E.M. Araujo et al. / Materials Science and Engineering A 445–446 (2007) 141–147 143

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ig. 1. Visual illustration of injection molded bars for horizontal burning testor HB classification.

his test is made for materials that continues to burn and spreadame, after removal of the test flame. The aim of this test is tolassify the material as 94 HB, i.e., the rate of the material isown of certain minimal value. The dimension of the samples 125 mm × 13 mm × 3 mm. Fig. 1 illustrates the specimens ofhe PE and its nanocomposites.

. Results and discussion

.1. Structure of PE/clay nanocomposites

To investigate the effect of organic modifier on the mor-hology of clay it was showed in the Fig. 2 the X-ray powderiffraction patterns for the MMT clay, either unmodified or mod-

ig. 2. XRD patterns of unmodified montmorillonite clay (MMT) and mod-fied with the salts: Cetremide (C-OMMT), Dodigen (D-OMMT), GenaminG-OMMT) and Praepagen (P-OMMT).

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Fig. 3. SEM photomicrograph of C-OMMT organoclay powder.

fied with the quaternary ammonium salts Dodigen, Genamin,raepagen and Cetremide. It can be observed that the unmodifiedlay (MMT) has a peak corresponding to an interlayer spacing0 0 1 = 12.5 A. These interlayer spacings for the treated samplesith quaternary ammonium salts, obtained from the correspond-

ng XRD patterns, are 20.4 A for Cetremide (C-OMMT), 20.7 Aor Dodigen (D-OMMT) and 21.4 A for Genamin (G-OMMT).n the case of the treated montmorillonite with Praepagen (P-MMT) two peaks were observed, ascribed to the interlayer

pacings d0 0 1 = 29.2 A and d0 0 2 = 18.5 A, in which occurredhe intercalation of the salt between the layers of organoclay.nother peak corresponding to interlayer spacings of 12.5 A isrobably due to an incomplete ion exchange, with some residuala-MMT remaining in the material. The interlayer distance isetermined by the diffraction peak in the X-ray method, usinghe Bragg equation. The results indicated that all the quaternarymmonium salts were intercalated between two basal planes ofMT, leading to an expansion of the interlayer spacing. As aatter of fact, all the d0 0 1 spacings had raised upon the treat-ent with the quaternary ammonium salts. Fig. 3 shows a SEM

hotomicrograph of C-OMMT organoclay particles. The parti-le shape of the organoclay powder indicates a layer structureith an average particle diameter of about 8 �m. As indicated

n Fig. 2, the basal spacing of this organoclay is about 20.4 A.he others organoclays presented similar morphology.

Fig. 4 presents X-ray diffraction patterns for the PE/nmodified clay and PE/modified clay composites. The diffrac-ogram of the PE/unmodified clay composite has a peak corre-ponding to an interlayer spacing of 13.3 A, which is close to theistance of 12.5 A (see Fig. 2) of the unmodified clay, indicatinghat the increase of the basal spacing did not occur. On the otherand, the sample of the nanocomposite of PE with the modifiedlay with Cetremide, PE/C-OMMT, presents the displacementf the XRD peak toward a lower angle values, what represents anncrease to 30.02 A in the basal spacing. It was thus noticed thatith the presence of the organoclay, the peak related to the origi-

al PE/unmodified clay interlayer spacing disappeared, and twoew broad diffraction peaks appeared, related to the distances of0.02 and 15.57 A. The first peak can be due to the intercalationf the polymer between the layers of organoclay and the second

144 E.M. Araujo et al. / Materials Science and En

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an be attributed to a small amount of montmorillonite layershat were not intercalated by PE molecules. The X-ray diffrac-

ion pattern for the sample of the nanocomposite of PE with the

odified clay with Dodigen, PE/D-OMMT is almost the sames the one of treated montmorillonite with Dodigen (see Fig. 2),ut it presents a broader peak. This may be due to a poor com-

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Fig. 5. TEM photomicrographs of PE/clay com

gineering A 445–446 (2007) 141–147

atibility between the organoclay and PE, what was insufficiento form a nanocomposite. In other words, this can evidence theresence of residual layered clay tactoids in this sample. The X-ay diffraction pattern for the sample of the nanocomposite ofE with the modified clay with Genamin, PE/G-OMMT, shows

hat the intercalation of the polymer chains increases the inter-ayer spacing to 34.61 A, leading to a shift of the diffraction peakowards lower angles. A second broad peak, corresponding ton interlayer spacing of 19.53 A, which is close to the distancef 21.4 A of the clay modified with Genamin, can be ascribedo a small amount of montmorillonite layers that were not inter-alated by PE molecules. For the X-ray diffraction pattern forhe sample of the nanocomposite of PE with the modified clayith Praepagen, PE/P-OMMT, the main diffraction peak pointsut to an interlayer spacing of 36.12 A, due to the intercalationf the polymer chains between the layers of the organoclay. Aecond broad peak, related to a distance of 19.41 A, similarly tohe case of Genamin, can be attributed to a small part of mont-

orillonite layers that were not intercalated by PE molecules.third broad diffraction peak conforms to an interlayer spacing

f 12.5 A and it is probably due to an incomplete ion exchange,ith the presence of some residual MMT.TEM photomicrograph of PE/modified clay composite is pro-

ided in Fig. 5, which contain 3 wt.% of the P-OMMT. In thishotomicrograph exist intercalated clay layers but it can be seenhat some aggregated of clay layers are still present in the PE

atrix. Therefore, the PE/P-OMMT composites are partiallyntercalated nanocomposites. This agrees with the XRD patternbtained for the P-OMMT sample.

Based on results, it seems that only conventional microcom-osites can be obtained by directly melt compounding PE andommon alkylammonium modified clay due to the non-polarature polyethylene, i.e., the hydroxyl groups at the edge of clay

ayers impair the formation of nanocomposites. As the moleculef the reactive alkylammonium salts contains exchangeableations, the surface of the clay can change from hydrophilic toydrophobic improving the wettability between PE and organ-

posite containing 3 wt.% of P-OMMT.

E.M. Araujo et al. / Materials Science and Engineering A 445–446 (2007) 141–147 145

Table 1Mechanical properties of PE and PE/clay composites

Specimens Tensile modulus (MPa) Yield strength (MPa) Elongation at yield (%) Izod impact strength (J/m)

PE 939.46 ± 77.1 19.28 ± 0.6 7.4 ± 4.3 128.2 ± 5.4PE/MMT 903.8 ± 110.3 18.8 ± 0.5 8.1 ± 3.0 96.5 ± 13.5PE/D-OMMT 879.1 ± 78.9 18.7 ± 0.4 10.0 ± 0.2 96.4 ± 6.7P 10.0 ± 0.4 98.1 ± 5.9P 8.6 ± 3.5 92.2 ± 5.3P 6.7 ± 4.2 99.3 ± 9.9

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E/G-OMMT 895.8 ± 44.4 19.1 ± 0.2E/C-OMMT 943.5 ± 92.9 18.9 ± 0.4E/P-OMMT 1011.5 ± 103.2 19.9 ± 0.8

clay and this enhance the intercalation of PE into interlayers oflay [8].

.2. Mechanical properties

Table 1 shows yield strength, tensile modulus, elongationt yield and Izod impact strength of pure PE and PE/clayanocomposites. Generally, it can be observed that the mechan-cal properties of the nanocomposite samples are close to the

echanical properties of pure PE. The samples PE/P-OMMTnd PE/C-OMMT present higher values for the tensile modulushen compared with pure PE. Probably, these samples displayhigher rigidity than the PE matrix. The yield strength valuesf the nanocomposites do not change significantly in relation toure PE, while the Izod impact strength decreases slightly, whats a typical characteristic of polymer/clay nanocomposites. Xut al. [10] reported that the poorer properties of nanocompos-tes can be attributed to the weak interactions between polymernd clay, due to non-polar nature polymer matrix. It is inter-sting to observe that the PE/P-OMMT system exhibits betterechanical properties than the others systems. This fact can

e related to the size of the organic modifier that will dictatehe amount of interlayer spacing, in other words, the larger therganic species, the greater the separation of layers [20]. Then,sufficient level of layer separation must be achieved to reduce

ayer–layer attraction and favor the intercalation/exfoliation ofhe polymer into organoclay. The Praepagen salt presents in itstructure two alkyl tails that can have allowed the access of theolyethylene to the silicate surface and for this reason the sys-em presented strength and modulus higher than their relativether composites. However, we are especially concerned withheir thermal and flammability properties.

.3. Thermal stability of PE/clay nanocomposites

In order to investigate the effect of the organoclay on thehermal stability of PE/clay nanocomposite, we present the TGnalysis of the systems. The clay layers display a high barrierction and the large thermal stability is related to the lowering ofhe diffusion of oxygen molecules into the nanocomposites, dueo barrier property of the clay. So, at a lower level of oxygen,hich is main factor for the deterioration of the polymer, the

anocomposite is stronger toward the oxidative decomposition.n the other hand, the alkylammonium cations in the modifiedrganoclays could suffer decomposition, following the Hofmannlimination reaction. It should be remarked that the reaction

ig. 6. TG curves of PE and PE/clay nanocomposites in nitrogen atmosphere.

roducts, as well the clay itself, can catalyze the degradationf the polymer. As pointed out by Ray and Okamoto and Zhaot al. [8,21], it is hence very important to analyze the thermaltability of the PE/clay nanocomposites.

The thermal stability of PE/clay nanocomposites is improvedith the presence of the quaternary ammonium salts in nitrogen

tmosphere, as pointed out in Fig. 6. Before 400 ◦C takes place

Fig. 7. TG curves of PE and PE/clay nanocomposites in air atmosphere.

146 E.M. Araujo et al. / Materials Science and Engineering A 445–446 (2007) 141–147

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Fig. 8. Samples during the horizontal burning test for HB c

emperatures higher than 400 ◦C the PE/clay nanocompositesre more stable than pure PE. This suggests that at the range ofhe processing temperatures the degradation of the system doesot occur, i.e., the nanocomposites are stable up to 340 ◦C.

Fig. 7 shows TG curves using air atmosphere, in this condi-ions we can see the best thermal enhanced properties of PE/clayanocomposites. It can be seen that the composites containingodified clay present thermal stability inferior than PE/MMT

ystem. In spite of the initial temperatures of decompositionas 334, 286, 277, 257 and 254 ◦C for PE/MMT, PE/C-OMMT,E/D-OMMT, PE/G-OMMT, and PE/P-OMMT, respectively,

hey are already superior than processing temperature. This canndicate that at the range of the processing temperatures theegradation of the system does not occur. As mentioned above,ccording to Zhao et al. [8], the organoclay has two opposingunctions in the thermal stability of the nanocomposites, one isue to barrier property to the oxygen and the other is the cataly-is effect towards the degradation of the polymer, but the barrierffect is predominant with addition low fraction of clay to theolymer matrix. Only with increasing loading, the catalyzingffect rapidly rises and becomes dominant and the thermal sta-ility of the nanocomposites decreases.

.4. Horizontal flammability tests, UL94 HB

Horizontal flammability test standardized as UL94 HB [19]as used to investigate the effect of the organoclay on theammability properties of PE/clay nanocomposites. Flammabil-

ty of the nanocomposites is improved due to formation of a layerhermal insulating from of organoclay nanoparticles that servess a barrier to both mass and energy transport. It is generallyonsidered that the nanocomposite structure increases barrierroperties to the volatile products available from the polymerombustion preventing that these products escape and maintain-ng the flame, and reduce also the amount of volatile products.t is very difficult to obtain flammability valid values for allhe materials in all applications because the real conditions of

emanding change a lot. Each safety organism or test of materialave registered a large methods number far example, minimalgnition temperature, flame spread rate, smoke density, mass loss8,22,23].

cation test for the PE matrix (a) and its nanocomposite (b).

The photos in Fig. 8a and b illustrate the samples during theest for the PE matrix and PE/clay nanocomposite, respectively.he photos show the behavior for all the materials studied. Com-aring with the nanocomposite, it can be observed that for theure PE occurs a continue drip of material, higher emission ofmoke and height of the flame, after the ignition source has beenemoved. Fig. 9 presents the burning rate (mm/min) results forhe pure PE and PE/clay nanocomposites. It can be seen that theurning rate values of the PE/D-OMMT, PE/C-OMMT, PE/G-MMT and PE/P-OMMT systems are significantly reduced

n relation to PE matrix and PE/MMT, i.e., the burning rateecreased with the presence of quaternary ammonium salts in thelay. The burning rate reduction of PE/P-OMMT, PE/G-OMMT,E/C-MMT and PE-D-OMMT is 17, 9, 7 and 4%, respectively,ompared to PE matrix. As expected, the flammability resistancef PE/clay nanocomposites was improved. The results indicatehat the improvement of flammability of the PE/clay nanocom-osites can be related to barrier action of organoclay and thathe national clay organically modified is an efficient alterna-ive for the use in nanocomposites. As reported by Valera et al.

Fig. 9. Burning rate for PE and PE/clay nanocomposites.

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E.M. Araujo et al. / Materials Science a

barrier for heat diffusion. And moreover, the nanoclays areame retardants and burning characteristics should be consid-red associated with two possibilities, the gas barrier propertiesf nanolayers which impede gas diffusion, and the retardingombustion nature of the silicate layers as reported too by Pre-ton et al. [25]. Tang et al. [26] also reported the good gas barrierroperties of nanocomposites based on EVA that were assignedo an ablative reassembly of the reticular layers of the silicate inhe nanocomposites during thermal-oxidation.

. Conclusions

The effect of organoclay and four different types of quater-ary ammonium salts on thermal, mechanical and flammabilityroperties of polyethylene/Brazilian clay nanocomposites weretudied. The results indicated that all the quaternary ammo-ium salts were intercalated between two basal planes of MMT,eading to an expansion of the interlayer spacing and that, thebtained PE/P-OMMT nanocomposites are partially interca-ated. Incorporation of the organoclay improved the thermal sta-ility of PE. As expected, the flammability resistance of PE/clayanocomposites was improved due to the barrier effect of therganoclay during the burning. The data showed the efficiencyf the quaternary ammonium salts in the organophilization oflay and that the Brazilian clay can be used as nanoparticle inE nanocomposites.

cknowledgments

The authors thank to PhD Elias Hage Junior-DEMa/UFSCar/razil, to Braskem for donating the PE, to Bentonit Uniaoordeste for donating of the clay, to Clariant for donating of the

alts, to FAPESQ/MCT/CNPq (Fundacao de Amparo a Pesquisao Estado da Paraıba), to RENAMI (Rede de Nanotecnolo-ia Molecular e de Interfaces), CNPq and CAPES (Brazilianesearch Council) for the financial support.

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