07) 2575–2578www.elsevier.com/locate/matlet

Materials Letters 61 (20

Comparison of flammability behavior of polyethylene/Brazilian claynanocomposites and polyethylene/flame retardants

Renata Barbosa a, Edcleide M. Araújo a,⁎, Tomas Jeferson A. Melo a, Edson N. Ito b

a Department of Materials Engineering, Federal University of Campina Grande, Campina Grande/PB, Av. Aprígio Veloso, 882, Bodocongó, CEP. 58109-970, Brazilb Department of Materials Engineering, Federal University of São Carlos, São Carlos/SP, Brazil

Received 6 April 2006; accepted 28 September 2006Available online 16 October 2006

Abstract

Polyethylene (PE)/Brazilian clay nanocomposites and PE/commercial flame retardant systems were produced via direct melt intercalation. Amontmorillonite sample from the Brazilian state of Paraíba was organically modified with esthearildimethylammonium chloride (Praepagen)quaternary ammonium salt and has been tested to be used in polymer nanocomposites. The dispersion analysis and the interlayer distance of theclay particles were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The flammability behavior of theobtained systems was investigated by horizontal burning tests for HB classification, Underwriters Laboratories (UL94). It was observed that theburning rate of PE/Brazilian clay nanocomposites was significantly reduced in relation to pure PE and PE/flame retardant systems, indicating thatthe PE/Brazilian clay system was more efficient.© 2006 Elsevier B.V. All rights reserved.

Keywords: Nanocomposite; Brazilian clay; X-ray technique; Flammability properties

1. Introduction

Due to the great interest in modern materials of engineering,much attention has been given to polymer-layered silicatenanocomposites as a result of the potentially superior propertiesof these materials compared to conventional composites. Minimaladdition levels (b10 wt.%) of organoclays enhance manyproperties such as mechanical, thermal stability, dimensionaland gas barrier, flame retardancy and ionic conductivity propertiessignificantly [1–5]. This new class of materials, according toKomarneni [6], is defined as nanocomposites. To obtain com-patible clays with polymer matrices, quaternary ammonium saltswith at least 12 carbons atoms have been used in aqueous dis-

⁎ Corresponding author. Tel./fax: +55 83 3310 1178.E-mail addresses: rrenatabarbosa@yahoo.com (R. Barbosa),

edcleide@dema.ufcg.edu.br (E.M. Araújo), tomas@dema.ufcg.edu.br(T.J.A. Melo), pgenito@iris.ufscar.br (E.N. Ito).

0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2006.09.055

persions of sodium smectite clays. In these dispersions, the clayparticles or layers must be separated from one another and not bestacked, in order to facilitate the introduction of the organiccompounds. As a result, the clay exchange cations are replaced bythe organic cations of the quaternary ammonium salt that wereadsorbed on the negative sites of the clay surfaces. So, theobtained clay known as organophilic or organoclay is notanymore soluble in water and it will be compatible with polymermatrices, if the organic quaternary ammonium ions were properlychosen [7–10]. The clay layers display a high barrier action andthe large thermal stability is related to the lowering of thediffusion of oxygen molecules into the nanocomposites, due tothe barrier property of the clay. So, at a lower level of oxygen,which is the main factor for the deterioration of the polymer, thenanocomposite is stronger toward the oxidative decomposition[11–14].

In this work, Brazilian clay and two types of commercialflame retardants were used in PE matrix to evaluate theflammability behavior of PE/clay nanocomposites and PE/flame retardants systems.

2576 R. Barbosa et al. / Materials Letters 61 (2007) 2575–2578

2. Experimental

2.1. Materials

A high density polyethylene (PE, JV-060U) was suppliedby Braskem/Brazil and used as a composite matrix. Na-montmorillonite (MMT, Brasgel PA, Boa Vista/PB, Northeastof Brazil) supplied by Bentonit União do Nordeste with acation exchange capacity (CEC) of 90 meq/100 g and aninterlayer spacing d001=12.5 Å was used as a nanofiller. Thequaternary ammonium salt used for the modification of MMTwas esthearildimethylammonium chloride — Praepagen (P)with industrial grade supplied by Clariant/Brazil and it wasused as received. The commercial flame retardants usedwere: antimonium trioxide (Sb2O3) as a white solid, suppliedby Chemtra Chemical/Brazil, abbreviated as AT, andchlorinated paraffin as powder solid, abbreviated as Chlorez700 (CL).

2.2. Preparation of organoclays

The Na-MMT clay was modified organically with quater-nary ammonium salt according to the procedure described byAraújo et al. [9,16,17] and Barbosa [15]. The product ob-tained with Praepagen was named as P-OMMT.

Fig. 1. XRD patterns of montmorillonite clay modified with the Praepagen salt(P-OMMT) and for the PE systems with 1 and 3 wt.% of organoclay.

Fig. 2. TEM photomicrographs of PE systems with (a) 1 wt.% and (b) 3 wt.% oforganoclay.

2.3. Preparation of composites

PE/P-OMMT composites and PE/flame retardants systems,containing 1 and 3 wt.% of clay or flame retardants, were meltcompounded in a counter-rotating twin-screw extruder (TorqueRheometer Haake) operating at 170–200 °C and 60 rpm.Flammability UL94 HB samples were injection-moulded in aFluidmec machine at 200 °C.

2.4. Characterization of dispersibility of the clay in polymermatrix

The structure of PE/organoclay composites was character-ized by XRD and TEM. XRD measurement was performedusing a XRD-6000 Shimadzu diffractometer (40 kV, 30 mA)with 2θ scan range of 2–30° at room temperature, at a scanning

Fig. 3. Burning rate for PE and PE/commercial flame retardant systems with 1and 3 wt.%.

Fig. 5. Burning rate for PE, PE/commercial flame retardants systems and PE/P-OMMT with 1 wt.%.

2577R. Barbosa et al. / Materials Letters 61 (2007) 2575–2578

speed of 2°/min with Cu (λ=0.154 nm). TEM was carried outon a Philips CM120 with 120 kV. Samples were cryogenicallymicrotomed into ultrathin sections (25–50 nm thick) with adiamond knife using a RMC MT-7000 under cryogenicconditions (−80 °C) inside the microtoming chamber.

Fig. 4. Burning characteristics at the beginning of the test for: (a) PE matrix and(b) PE/flame retardant.

2.5. Flammability test

Flammability properties were measured using horizontalburning tests for HB classification according to UL94 [14]. Thedimension of the standard bar samples is 125×13×3 mm. Theflammability data reported here are the averages of fivesamples.

3. Results and discussion

3.1. Structure of PE/clay nanocomposites

Fig. 1 presents X-ray diffraction patterns for the P-OMMT clay andfor the PE systems with 1 and 3 wt.% of organoclay. The organoclay(P-OMMT) presents two peaks, with interlayer spacings of 29.2 Å and18.5 Å, in which the intercalation of the salt between the layers oforganoclay occurred. Another peak (12.5 Å) is probably due to anincomplete ion exchange; with some residual MMT remaining in thematerial. For the X-ray diffraction pattern of the nanocomposite of PEwith 3 wt.% of P-OMMT organoclay, the main diffraction peak pointsout to an interlayer spacing of 36.12 Å, due to the intercalation of thepolymer chains between the layers of the organoclay. A second broadpeak can be attributed to a small part of montmorillonite layers thatwere not intercalated by PE molecules. A third broad diffraction peak

Fig. 6. Burning rate for PE, PE/commercial flame retardants systems and PE/P-OMMT nanocomposite with 3 wt.%.

2578 R. Barbosa et al. / Materials Letters 61 (2007) 2575–2578

(12.5 Å) is probably due to an incomplete ion exchange, with thepresence of some residual MMT based in the literature [18]. A similarbehavior is shown for the PE with 1 wt.% of organoclay with two peaksaround of 20 and 40 Å. This indicates that even increasing the contentof organoclay (1 to 3 wt.%) it is still possible to obtain partiallyintercalated nanocomposites.

Fig. 2(a) and (b) shows the TEM images of the PE systems. Thereexist intercalated clay layers but it can be seen too that some aggregatedclay layers are still present in the PE matrix. Therefore, the obtainedPE/P-OMMT composites are partially intercalated nanocompositesaccording to the XRD pattern (Fig. 1). Similar results were presentedby Wang et al. [19].

3.2. Horizontal flammability tests, UL94 HB

Horizontal flammability test standardized as UL94 HB [14] wasused to investigate the flammability properties of the systems. FromFig. 3 analysis, it shows that the systemswith 3wt.% of additive presenta reduction on the burning rate in relation to PE matrix, specifically thePE/CL (3 wt.%) system. In this study, probably the used flameretardants volatilized during their decomposition, diluting the volatilecombustibles of the flame and suggesting the formation of a protectiveoxide char of the product surface, reducing the oxygen diffusion for thereactive centre. These chars retard the heat transfer, and result in theimprovement of flammability properties [13,15,20,21].

Fig. 4(a) and (b) presents the burning characteristics at thebeginning of the test for PE matrix and PE/flame retardant systems,respectively. It can be observed that the PE/flame retardant systemsshowed a lower rate of combustion in comparison with the purepolymer and also a lower tendency of dripping.

In Fig. 5, it can be observed that the burning rate of the systems withflame retardants (1 wt.%) is reduced dramatically in relation to the PE/P-OMMT (1 wt.%) nanocomposite. However, it is close to pure PE.With the increase of the amount of clay to 3 wt.% (see Fig. 6), theburning rate value is significantly reduced as compared to PE matrixand the systems with flame retardants. The results demonstrate that theflammability resistance of PE/clay nanocomposite was improved. Thisis most probably due to the organoclay contributing to reduce theflammability of the systems, suggesting the formation of a barrierproperty that acts to retard the heat and mass transfer during thecombustion and moreover, that the Brazilian clay organically modifiedis an efficient alternative for the use in nanocomposites. As reported byValera et al. [22], the thermal degradation of the aliphatic chains in thePP and EP/EVA matrix can be retarded by an improvement in thedispersion and exfoliation of the silicate layers. And moreover, thenanoclays are flame retardants and burning characteristics should beconsidered to be associated with two possibilities, the gas barrierproperties of nanolayers which impede gas diffusion, and the retardingcombustion nature of the silicate layers as reported too by Preston et al.[24]. Tang et al. [23] also reported the good gas barrier properties ofnanocomposites based on EVA that were assigned to an ablativereassembly of the reticular layers of the silicate in the nanocompositesduring thermal-oxidation.

4. Conclusions

Polyethylene/Brazilian clay nanocomposites and polyethyl-ene/commercial flame retardant systems were produced viadirect melt intercalation. The obtained PE/organoclay nano-composites were partially intercalated. As expected, theflammability resistance of PE/Brazilian clay nanocomposites

was improved due to the barrier effect of the organoclay duringthe combustion and the nanocomposite is more effective thanconventional PE/flame retardants systems. By adding only 3 wt.% montmorillonite, the burning rate of the nanocomposites wasreduced by 17%. This also indicates that the Brazilian clay canbe used as a nanoparticle in PE nanocomposites.

Acknowledgments

The authors express their thanks to PhD Elias Hage Júnior-DEMa/UFSCar/Brazil, to PhD Helio de Lucena Lira and toPhD Marcelo Silveira Rabello-DEMa/UFCG/Brazil, to Bras-kem, to Bentonit União Nordeste, to Clariant, to RENAMI(Rede de Nanotecnologia Molecular e de Interfaces), toFAPESQ/MCT/CNPq (Fundação de Amparo à Pesquisa doEstado da Paraíba), to CNPq and CAPES (Brazilian ResearchCouncil) for the financial support.

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