Polymer Degradation and Stability 87 (2005) 183e189

www.elsevier.com/locate/polydegstab

Mechanical, thermal and flammability properties ofpolyethylene/clay nanocomposites

Chungui Zhao, Huaili Qin, Fangling Gong, Meng Feng,Shimin Zhang, Mingshu Yang*

CAS Key Laboratory of Engineering Plastics, Center for Molecular Sciences, Institute of Chemistry,

Chinese Academy of Sciences, Beijing 100080, China

Received 10 July 2004; received in revised form 8 August 2004; accepted 11 August 2004

Abstract

Polyethylene/clay composites were prepared by melt compounding polyethylene and montmorillonite clay organically modifiedwith different intercalating agents. Wide-angle X-ray diffraction (WAXD) and transmission electron microscopy (TEM) revealed

that PE/clay nanocomposites formed when using a reactive intercalating agent, while only conventional microcomposites wereobtained from the common alkylammonium intercalated clay. Mechanical testing indicated that the reinforcement in thenanocomposites was more effective than that in their conventional counterparts. The thermal stability and flammability of the PE/

clay nanocomposites were measured by thermogravimetric analysis (TGA) and cone calorimetry. By adding only 2 phr organoclay,the peak heat release rate of the nanocomposite was reduced by 54%. Further studies indicated that the improvement offlammability is due to the formation of a clay-enriched protective char during combustion.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: Polyethylene; Clay; Nanocomposites; Mechanical; Flammability

1. Introduction

Polymer/clay nanocomposites have generated con-siderable interests in the past decade because adding justa tiny amount of clay to the polymer matrix couldproduce a dramatic enhancement in physical, thermal,and mechanical properties [1e3]. Recently, cone calori-metric experiments have demonstrated that the flameretardancy of many polymer/clay nanocomposites,including polyamide 6 (PA6) [4,5], polyamide 66(PA66) [6], polypropylene (PP) [4,5], polystyrene (PS)[7,8], and poly (ethylene-co-vinyl acetate) (EVA) [9,10]were improved; concretely, the peak heat release rate

* Corresponding author. Tel.: C86 10 82615665; fax: C86 10

62559373.

E-mail address: yms@iccas.ac.cn (M. Yang).

0141-3910/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymdegradstab.2004.08.005

decreased significantly during the combustion. Poly-ethylene (PE) is the most widely used polymer, whichpossesses a fortuitous combination of many usefulproperties: light weight, low cost, high chemical re-sistance, low dielectric constant and losses, goodprocessability, etc. [11]. However, owing to its inherentchemical nature, PE has not enough stiffness and caneasily catch fire. Inspired by their excellent overallproperties, PE/clay nanocomposites have been studied.

Because PE is a non-polar polymer, it is difficult toprepare PE/clay nanocomposites. In general, twoeffective ways are used to fabricate PE/clay nano-composites, in-situ polymerisation [12e14] and meltintercalation [15e17]. Melt intercalation has beenconsidered the most desirable for the thermoplasticpolymers, by which the clay layers are directly dispersedin the molten polymers by using conventional meltprocessing techniques. The melt intercalation method to

184 C. Zhao et al. / Polymer Degradation and Stability 87 (2005) 183e189

prepare non-polar polyolefin/clay nanocomposites isbased on the introduction of a compatibilising agent tomatch the polarity between the hydrophilic clay surfaceand the hydrophobic polymers [18]. In PE, homoge-neous dispersion of clay layers was realized by usingmaleic anhydride-grafted polyethylene [15e17]. How-ever, introduction of maleic anhydride would weakensome properties, such as environmental stability [19,20].Zhang and Wilkie [21] studied the flammability of PE/clay nanocomposites, which were prepared by meltintercalation using several organoclays, the reduction inpeak heat release rate was 30e40% by adding 3 wt%organoclay, but the mechanical properties were notreported.

In our previous works [22,23], chlorosilane-modifiedmontmorillonites were investigated, and the resultsconfirmed that the hydroxyl groups at the edge of claylayers could be reacted. Further study [24] showed thatdiminishing of hydroxyl groups at the edge of clay layerscan improve the intercalation of PE into interlayers, andthe intercalated PE nanocomposites were obtained bymelt intercalation using common alkylammonium in-tercalated clay which was pre-treated by chlorosilane.However, chlorosilane modification is a time-consumingprocess and involves troublesome solvent disposal. Inthis paper, a reactive intercalating agent was directlyused to modify the montmorillonite clay, so that thechemical reaction with hydroxyl groups at the edge ofclay layers and the interlayer ion-exchange were carriedout in one step. PE/clay nanocomposites were thendirectly prepared by melt intercalating PE and the abovemodified clay. The nanostructure was verified by wide-angle X-ray diffraction (WAXD) and transmissionelectron microscopy (TEM), and their mechanical,thermal and flammability properties were measuredand discussed.

2. Experimental

2.1. Materials

A high-density polyethylene (PE, 1600J, density:0.958 g/cm3, melting flow index: 18 g/10 min) waspurchased from Yanshan Petrochemical Co., (China)and used as composite matrix resin. The pristine clayused was Na-montmorillonite (Na-MMT, ZhangjiakouQinghe Chemical Factory, China) with a cation ex-change capacity (CEC) of 90 meq/100 g. WAXDshowed that the interlayer spacing (d001) was 0.98 nm.Na-MMT was dried at 100 �C in vacuum for 8 h priorto use.

Two intercalating agents were used for the modifica-tion of Na-MMT. One was the commonly-used dio-ctadecyldimethylammonium chloride (industrial grade,Beijing Chemical Reagent Co., China; abbreviated as

DOAc), and the other was the reactive (N-g-trimethox-ylsilanepropyl)octadecyldimethylammonium chloride(industrial grade, Jiangsu Institute of Chemical Engi-neering, China, abbreviated as JSAc). They were used asreceived.

N (CH2)17CH3

CH3

CH3(CH2)17+

Cl-CH3

N (CH2)17CH3

CH3

(CH3O)3Si(CH2)3+

Cl-CH3

JSAc DOAc

2.2. Preparation of organoclays

Na-MMT (100 g) was mixed with anhydrous ethanol(1000 mL) with stirring to form a uniformly dispersedsuspension, and then the intercalating agent equivalentto 1.2 CEC of Na-MMT was added into the dispersion.The admixture was stirred at 75 �C for 8 h, and thenfiltered to get organoclays. The organoclays were driedin vacuum at 100 �C for 8 h, and then ground intopowder. The products obtained with DOAc and JSAcare named as DM and JS, respectively.

2.3. Melt processing

PE and organoclays were melt compounded in a co-rotating twin-screw extruder (FZ 30 mm, L/FZ 24).The barrel temperature was maintained at 180 �C, andthe screw speed was 200 rpm. The samples are markedwith the composition and the weight fraction oforganoclays, e.g., PE/JS5 represents the compositecontaining 5 phr of JS (5 parts JS per 100 parts PE).

The extruded pellets were injection-moulded intoplates with thickness of 2 mm by Mini-Max 183 moulder(CSI Co., USA) for the measurements of WAXD andTEM.

Standard tensile (ASTM D638), flexural (ASTMD790), notched Izod (ASTM D256) and cone calori-metric (100! 100! 10 mm3) specimens were injection-moulded using an injection-moulding machine. Thebarrel temperature was 180 �C, and injection pressurewas 80 MPa. After moulding, the specimens were placedin a desiccator for 48 h prior to the test.

2.4. Characterization

The structure of PE/clay composites was character-ized by WAXD and TEM. WAXD was performed ona Rigaku D/max 2400 diffractometer using Cu Karadiation (lZ 0.154 nm) at a generated voltage of40 kV and current of 120 mA at room temperature.

185C. Zhao et al. / Polymer Degradation and Stability 87 (2005) 183e189

The diffractograms were scanned from 1.5 � to 40 � in the2q range in 0.02 � steps with a scanning rate of 8 �/min.TEM was carried out on a Hitachi (Japan) H-800 withan acceleration voltage of 200 kV. The ultrathin sliceswere prepared by sectioning the injection-mouldedsamples along the direction perpendicular to the in-jection under cryogenic condition.

Tensile and flexural tests were conducted on anInstron universal material test machine Model 1122.Tensile strength was determined at a crosshead speed of50 mm/min, and the results reported here are theaverages of five successful tests. Flexural propertieswere measured at a crosshead speed of 2 mm/min, andthe results reported here are the averages of threesuccessful tests. Notched Izod impact tests wereperformed at room temperature using a CSI-137C(CSI Co., USA) impact tester, the values reported hererepresent the averages of 10 specimens.

The thermal stability was tested by thermogravimet-ric analysis (TGA) using PerkineElmer TGA-7 at a scanrate of 20 �C/min in nitrogen.

Flammability properties were measured using a conecalorimeter (Fire Test Technology, UK) accordingto ASTM E 1354. The heat flux was maintained at35 kW/m2, and the spark was continuous until thesample ignited. Heat release rate (HRR), total heatreleased, effective heat of combustion (EHC), mass lossrate (MLR), CO yield, and CO2 yield data wererecorded, and the values reported here were the averagesof two duplicate experiments.

PE/clay nanocomposites and its residue after com-bustion were characterized using Fourier-transformedinfrared spectroscopy (FTIR) and WAXD. FTIRspectra were recorded with KBr pellets on a PerkineElmer System 2000 Fourier-transformed infrared spec-trometer.

3. Results and discussion

3.1. Structure of PE/clay composites

The polarity of clay mainly consists in its interlayersurface. Through the ion-exchange with common alkylammonium, the surface of the clay would change fromhydrophilic to hydrophobic, and can be used tofabricate most of polymer/clay nanocomposites. Butfor non-polar polyethylene, the hydroxyl groups at theedge of clay layers impair the formation of nano-composites [25]. Only conventional microcompositescan be obtained by directly melt compounding PE andcommon alkyl ammonium modified clay, and therefore,we chose a reactive intercalating agent for the modifi-cation of clay surface. The molecule of the reactiveintercalating agent JSAc contains both exchangeablecations that act as ammonium cations to replace the

interlayer NaC and reactive methoxylsilane groups thatcould react with hydroxyl groups at the edges of claylayers. The latter reaction could further reduce theinterlayer attraction and improve the wettability be-tween PE and organoclay, and ultimately enhance theintercalation of PE into interlayers of clay [24].

WAXD patterns for the organoclays and PE/claycomposites are shown in Fig. 1. The characteristic peaktoward low angle corresponds to the (001) planediffraction of clay layers. The diffraction peak of PE/DM5 is almost the same as DM, substantiating that thecompatibility of DM and PE was poor and insufficientto form a nanocomposite. So, the PE/DM compositesare conventional microcomposites. For JS, the ion-exchange and the chemical reaction with hydroxylgroups at the edge of clay layers were carried onsimultaneously. Secondly, the reactive methoxylsilanegroups between JSAc molecules attached to the sameclay layer surfaces can react with each other throughhydrolysis (distance of Si and N in JSAc is about 0.6 nm,but interlayer spacing of JS is 2.6 nm. So, methoxylsi-lane groups of JSAc attached to different clay layersurfaces cannot contact and react), by which the polargroups in the clay surface would be overlaid morecompletely and the order degree of (001) of the claydecreased. Both these effects promote the formation ofnanocomposites. As shown in Fig. 1, the WAXDpattern of JS reveals two broad diffraction peaks atabout 3.4 � and 6.4 �. After compounded with PE, thosecharacteristic peaks disappeared, suggesting that theinterlayer spacing of clay was not less than 6 nm(corresponding to 2qZ 1.5 �, which was the limit ofthe WAXD test in this study.) and the nanocompositeswere formed. The results indicated that PE/clay nano-composites could be prepared by using the reactive

0 10 20 30

PE/JS5

JS

PE/DM5

DM

Arb

itrary In

ten

sity (cp

s)

2 (degree)

3.4o

6.4o

Fig. 1. WAXD patterns of organoclays and PE/clay composites.

186 C. Zhao et al. / Polymer Degradation and Stability 87 (2005) 183e189

intercalating agent, but using common intercalatingagent would form conventional microcomposites.

Fig. 2 is the TEM image of PE/JS5, which contains5 phr of the organoclay JS. There exist exfoliated claylayers; meanwhile some aggregates of clay layers are stillpresent in the PE matrix. Therefore, the obtained PE/JScomposites are partially exfoliated nanocomposites.

3.2. Mechanical properties

The mechanical properties of composites depend onmany factors, including the aspect ratio of the filler, thedegree of dispersion of the filler in the matrix resin andthe adhesion at the fillerematrix interface. Table 1 liststhe mechanical properties of PE and PE/clay compo-sites. With increasing clay loading, the strength andmodulus of PE/clay composites increase, while thenotched impact strength decreases. The strength andmodulus of PE/JS nanocomposites are obviously higherthan their relative PE/DM counterparts. As discussedabove, the dispersion of the organoclay in PE/JSnanocomposites is better than that in PE/DM compo-sites, so PE/JS nanocomposites are more effective thanPE/DM microcomposites in reinforcement.

PE/JS nanocomposites exhibit excellent mechanicalproperties. Here, we are especially concerned with theirthermal and flammability properties, which are reportedand discussed in the following sections.

3.3. Thermal properties

Clay layers have good barrier action, which canimprove the thermal stability of polymer/clay nano-

Fig. 2. TEM image of PE/clay nanocomposite containing 5 phr of JS.

composites. On the other hand, the alkylammoniumcations in the organoclay could suffer decompositionfollowing the Hofmann elimination reaction [26,27], andits product would catalyse the degradation of polymermatrixes [28]. Thirdly, the clay itself can also catalysethe degradation of polymer matrixes [29,30]. The lattertwo actions would reduce the thermal stability ofpolymer/clay nanocomposites.

The thermal stability of PE and PE/clay nano-composites are tested by TGA, and their TGA andDTG curves in nitrogen atmosphere are shown in Fig. 3.At the initial stage of the degradation (before 400 �C),due to the Hofmann elimination reaction and the clay-catalysed degradation, the PE/clay nanocomposites de-grade faster than pure PE (the residual of PE and PE/JS5at 400 �C are 99.2% and 97.0%, respectively). At

Table 1

Mechanical properties of PE and PE/clay composites

Sample Tensile

strength

(MPa)

Flexural

strength

(MPa)

Flexural

modulus

(MPa)

Izod impact

strength

(J/m)

PE 22G 1.2 26G 1.1 710G 54 20G 4.1

PE/JS5 25G 1.4 28G 0.8 780G 61 16G 4.6

PE/JS10 27G 1.1 33G 2.0 1050G 63 16G 3.4

PE/JS15 28G 1.4 38G 1.3 1330G 108 12G 2.9

PE/DM5 21G 2.0 26G 2.1 750G 65 22G 4.5

PE/DM10 23G 0.9 31G 0.9 980G 78 16G 3.6

PE/DM15 24G 1.8 30G 1.2 1030G 99 14G 3.1

300 350 400 450 500 550

-4

-3

-2

-1

0

DT

G (%

/°C

)

Temperature (oC)

PE

PE/JS2

PE/JS5

PE/JS10

PE/JS15

0

20

40

60

80

100

PE

PE/JS2

PE/JS5

PE/JS10

PE/JS15

Weig

ht (%

)

Fig. 3. TGA and DTG curves of PE and PE/clay nanocomposites in

nitrogen atmosphere.

187C. Zhao et al. / Polymer Degradation and Stability 87 (2005) 183e189

temperature higher than 400 �C, PE/clay nanocompo-sites are more stable than pure PE. The onset temper-atures of PE/clay nanocomposites are all higher thanthat of pure PE (onset temperature of pure PE, PE/JS2,PE/JS5, PE/JS10, and PE/JS15 is 428.0, 461.5, 453.5,450.5, and 451.0 �C, respectively). With increasing clayloading, the onset temperature of PE/clay nanocompo-site decreases. From DTG curves, the maximum de-composition temperature (Tmax) is obtained, which is484, 492, 491, 484, and 479 �C for pure PE, PE/JS2, PE/JS5, PE/JS10, and PE/JS15, respectively.

In summary, the organoclay has two opposingfunctions in the thermal stability of the polymer/claynanocomposite, one is its barrier effect which shouldimprove the thermal stability, and the other is thecatalysis effect towards the degradation of the polymermatrix which would decrease the thermal stability. Whenadding a low fraction to the polymer matrix, the claylayers should be well dispersed, the barrier effect ispredominant, but with increasing loading, the catalysingeffect rapidly rises and becomes dominant, so that thethermal stability of the nanocomposites decreases. Forthe PE/JS nanocomposites, the thermal stability of PE/JS2 is the best; when the clay loading exceeds 10 phr, itsTmax is already lower than that of pure PE.

3.4. Flammability properties

Cone calorimetry is an effective method to measurethe flammability of materials under fire-like conditions.The heat release rate (HRR) curves for PE and PE/claynanocomposites are shown in Fig. 4. Comparing withthe pure PE, the peak HRR values of the PE/claynanocomposites are significantly reduced, and the valuedecrease with increasing loading of clay, indicating thatflammability resistance of PE/clay nanocomposites wasimproved. In Fig. 4, one can also see that addition of JSchanges the induction time of burning (ti), and for PE/clay nanocomposites the HRR curves after ignitionexhibit two typical stages: one is the rapid increase stage,and another is the plateau stage as a function of time. tiof PE, PE/JS2, PE/JS5, PE/JS10, and PE/JS15 is 113,184, 157.5, 114, and 103 s, respectively. Combining theabove analysis for thermal stability, organoclay also hastwo opposite effects on ti of PE/clay nanocomposites,one is the barrier effect to improve ti and another iscatalysis to decrease ti. The integrated result is that ti ofPE/JS2 is the largest; with increasing clay loading, ti isreduced. At the initial stage after ignition, HRR of PE/clay nanocomposites increase more rapidly as a functionof time than PE due to the catalysis by organoclay, andthen PE/clay nanocomposites turn into the plateau stagedue to some enhancement of flame resistance.

Fig. 5 presents the mass loss rate (MLR) curves forPE and PE/clay nanocomposites during combustion.The shape of curves is similar to that of HRR,

suggesting that the lower HRR of nanocomposites iscaused by the reduction of MLR and that the enhancedflame retardancy of PE/clay nanocomposites was due tosomething changing in the condensed phase but not inthe gas phase. Table 2 lists some data of PE and PE/claynanocomposites in cone calorimetric experiments. InPE/clay nanocomposites, with increasing clay loading,the average MLR decreases, but there is no significantdifference in the total heat released and average effectiveheat of combustion (EHC) between pure PE and the PE/clay nanocomposites. In addition, the PE/clay nano-composites do not show any increase in CO and CO2

yield. The results demonstrate that the improvement offlammability for PE/clay nanocomposites is not caused

0 500 1000 1500

0

300

600

900

1200

1500

PE/JS15

PE/JS10

PE/JS5

PE/JS2

PE

Heat release rate (kW

/m

2)

Time (S)

Fig. 4. Heat release rate for PE and PE/clay nanocomposites.

0 500 1000 1500

0

22

44

PE/JS15

PE/JS10

PE/JS2

PE

Ma

ss

L

os

s R

ate

(g

/s

)

Time (s)

Fig. 5. Mass loss rate for PE and PE/clay nanocomposites, the curve of

PE/JS5 which lies between PE/JS2 and PE/JS10 was omitted for the

sake of clarity.

188 C. Zhao et al. / Polymer Degradation and Stability 87 (2005) 183e189

by a change of the combustion process in the condensedphase, but should be a physical barrier action.

Gilman et al. [4] put forward a mechanism by whichpolymer/clay nanocomposites promote the formation ofa char, which could act as a barrier to mass and heattransfer. During the experiments, we observed that thepure PE had a boiling surface at the base of the flameand no char formed during the combustion. However, inthe case of PE/clay nanocomposites, black char formedrapidly before ignition even in PE containing onlya small fraction of clay (e.g., 2 phr in PE/JS2). At theend of combustion, pure PE left no residue, but the PE/clay nanocomposites left a solid char-like residue shapedlike the original specimen. From Table 2, one could seethat the reduction of the peak HRR was 54% on adding2 phr JS, but a further increase of clay resulted only ina marginal improvement on the peak heat release. Thatis to say, as long as the nanocomposite is formed, theprotective char can be rapidly formed during thecombustion and the peak HRR reduces dramatically,suggesting that the formation of the protective char isinduced by the presence of the clay.

To clarify the composition and structure of the charformed during combustion, the PE/clay nanocompositesand their char were characterized by FTIR and WAXD,and the results are shown in Figs. 6 and 7. The IRabsorption peaks at 2920 cm�1 and 2850 cm�1 representCH2 stretching, and peaks at 1460 cm�1 and 720 cm�1

correspond to the bending and rocking of CH2, re-spectively. The peaks at 1090 cm�1and 1040 cm�1 are thestretching of SieO, and the peak at 460 cm�1 is related toabsorption of AleO. It can be seen that the char residuehas almost no hydrocarbon absorption and containsa great amount of clay. The WAXD peak of the (001)plane of the clay in the char exhibits a broad shouldershape (pointed out by an arrow in Fig. 7), indicating thatthe clay layers in the char are randomly arranged.

Briefly, organoclays have catalysis and barrier effectson the flammability of PE/clay nanocomposites. Thesetwo effects change the induction time of burning. At theinitial stage after ignition, HRR of PE/clay nano-composites increase more rapidly as a function of timethan PE due to the catalysis by organoclay, and thenPE/clay nanocomposites turn into the plateau stage

Table 2

Data recorded in cone calorimeter experiments

Samples PE PE/JS2 PE/JS5 PE/JS10 PE/JS15

Peak HRR (kW/m2) 1470 670 620 540 390

Average HRR (kW/m2) 510 440 450 380 280

Total heat

released (mJ/m2)

310 320 320 310 300

Average EHC (mJ/kg) 0.30 0.29 0.30 0.29 0.29

Average MLR (g/s) 17.2 15.5 15.1 13.6 9.6

CO (kg/kg) 0.0002 0.0002 0.0002 0.0002 0.0003

CO2 (kg/kg) 0.02 0.02 0.02 0.02 0.02

because the incorporation of the clay into the PE matrixinduces the formation of clay-enriched protective charsat the sample surface during the combustion. Thesechars retard the heat and mass transfer, and result in theimprovement of flammability properties of PE/claynanocomposites.

4. Conclusions

Reactive intercalating agent was used to modifymontmorillonite clay, and the PE/clay nanocompositeswere then prepared by melt intercalation. When usingcommon intercalating agent, only conventional micro-composites were obtained. The nanocomposites provedmore effective than conventional composites in re-inforcement. Organoclay has two opposed functions in

3000 2000 1000 0

Ab

so

rb

an

ce

(cm-1

)

B

A

Fig. 6. FTIR spectra of PE/JS5 (A) and its char (B).

0 10 20 30

Arb

itrary In

ten

sity (cp

s)

A

B

2 (degree)

Fig. 7. WAXD patterns of PE/JS5 (A) and its char (B).

189C. Zhao et al. / Polymer Degradation and Stability 87 (2005) 183e189

the thermal stability of the PE/clay nanocomposite, oneis the barrier effect to improve the thermal stability, andanother is catalysis, leading to a decrease of the thermalstability. The flammability of the PE/clay nanocompo-sites was greatly improved due to the formation of theclay-enriched protective char during the combustion.

Acknowledgement

The authors would like to thank Prof. Zhongjun Shuand Prof. Shousheng Yang (Chinese People’s ArmedPolice Force Academy) for their collaboration in conecalorimetric experiments.

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