Thermal degradation and rheological behaviour ofEVA/montmorillonite nanocomposites

A. Rivaa,1, M. Zanettia, M. Bragliab, G. Caminoa,*,1, L. Falquic

aUniversita di Torino, via P. Giuria 7, 10125 Turin, ItalybTILAB, via Reiss Romoli 274, 10148 Turin, Italy

cIstituto di Studi Chimico-Fisici di Macromolecole Sintetiche e Naturali, IMAG-CNR, Via De Marini 6, 16149 Genova, Italy

Received 6 September 2001; received in revised form 6 December 2001; accepted 10 December 2001

Abstract

The study is focused on the co-polymer poly(ethylene-co-vinylacetate) (EVA) used in telecommunication cable manufacturing.Different modified fillosilicates and EVA polymer were mixed at 120 �C in a Brabender mixer AEW330 in order to obtain polymerlayered silicate nanocomposites(PLSN). Exfoliated nanocomposites were obtained with montmorillonite exchanged with methyl

tallow bis(2-hydroxyethyl) ammonium and fluorohectorite exchanged with octadecylammonium, whereas with montmorilloniteexchanged with dimethyl dehydrogenated tallow ammonium and fluorohectorite exchanged with amino dodecanoic acid an inter-calated nanocomposite and a microcomposite were obtained respectively. It has been demonstrated that a low silicate percentage

(10%) nanodispersed in the polymer matrix is capable of dramatically reducing the influence of oxygen during thermooxidation,and to substantially increase the storage modulus of the material. The acid catalysis of acetic acid elimination observed heatingEVA nanocomposites in nitrogen is more effective in the case of exfoliated morphologies. # 2002 Elsevier Science Ltd. All rightsreserved.

Keywords: Nanocomposite; EVA; Montmorillonite; Melt compounding; Thermal degradation; Rheology

1. Introduction

Polymer layered silicate nanocomposites (PLSN)constitute a new class of materials characterized by anultrafine silicate phase dispersion, with a particle aver-age size of few nanometers and high aspect ratio. PLSNexhibit a different properties combination comparedwith conventional filled polymers, thus opening newtechnological and economic perspectives [1–3].Poly(ethylene-co-vinylacetate) is a copolymer used for

many applications, including telecommunication cablemanufacturing that is our main interest. An importantobjective pursued by manufacturers is to produce aflame retardant polymer, without affecting the mechan-ical properties of the material.

Nanocomposites have been used since their firstapplications to improve the mechanical properties ofpolymers, in particular they were found having superiorstrength and modulus and comparable impact strength,if compared to unfilled polymers [4]. A most recentapplication involves the study of nanocomposites as anew class of fire retardant materials [1].The objective of the study was to produce fire retar-

dant nanocomposites with poly(ethylene-co-vinylace-tate) copolymer (EVA) and modified phyllosilicates.

2. Experimental

2.1. Materials

The materials used to prepare the composites were:poly(ethylene-co-vinylacetate) (EVA) containing 19wt.% of vinylacetate ESCORENE UL0019, EXXONChemical, Synthetic fluoroectorite (Somasif ME100),Co-Op Chemical Co, Somasif ME100 exchanged with

0141-3910/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.

PI I : S0141-3910(02 )00065-4

Polymer Degradation and Stability 77 (2002) 299–304

www.elsevier.com/locate/polydegstab

* Corresponding author at present address: Tel.: +39-0131-229-

324; fax: +39-0131-229-331.

E-mail address: gcamino@mail.polial.polito.it (G. Camino).1 Present address: Centro di Cultura per l’Ingegneria delle Materie

Plastiche, c/o Politecnico di Torino, sede di Alessandria, Viale T.

Michel 5, 15100 Alessandria, Italy.

octadecilammonium (Somasif ME100/ODA) or withaminododecanoic acid (Somasif ME100/ADA) [5].Montmorillonite exchanged with methyl tallow bis 2hydroxyethyl ammonium or dimethyl dihydrogenatedtallow ammonium (Cloisite 30B and Cloisite 6Arespectively), Southern Clay products Inc.

2.2. Sample preparation

The polymer and filler were mixed in a Brabenderinternal mixer AEW330 at a temperature of 120�5 �C,60 rpm for 10 min. The composites were then pressed at120�3 �C and 100 bar for 2 min to obtain 120�120�3mm and 100�100�5 mm slabs or 23�10�0.5 mmfilms.

2.3. Characterization

2.3.1. X ray diffraction (XRD)XRD analyses were performed using a Philips PW

1830 X-ray generator with a 57Co anode as the X-raysource (l=0.179 nm).

2.3.2. Transmission electron microscopy (TEM)Bright field TEM analyses performed on the samples

obtained from EVA, Cloisite 6A and Cloisite 30B wereobtained at 80 kV, with a Zeiss EM 900. The sampleswere cooled at �80 �C and then microtomed with adiamond knife cooled at �60 �C to give sections with anominal thickness of 50 nm and 1 mm2 of superficialarea. The sections were collected on the surface of asolution of dimetylsulfoxide and water, 60 and 40 partsrespectively, and than transferred on Cu grids of 200mesh.

2.3.3. Thermogravimetry (TGA)The thermogravimetries were carried out in nitrogen

or air (60 cc/min) in a TA 2050 thermobalance using 15mg samples, either heating from 50 to 550 �C at 10 �C/min or in air in two successive isothermal conditions,heating from 50 to 330 �C at 20 �C/min, keeping at330 �C for 120 min to complete deacetyilation, thanheating at 20 �C/min from 330 to 400 �C, keeping at400 �C for 13 h.

2.3.4. Differential scanning calorimetry (DSC)DSC analyses were performed on 5 mg samples

heated from �100 to 200 �C at 10 �C/min in a N2 flow,using a TA Instruments DSC 2920.

2.3.5. RheologyRheological measurements were carried out at 120 �C

using 1 Hz frequency and 0.05% strain with parallelplates geometry on a Dynamic Analyzer RheometerRDA II from Rheometrics. The plates diameter were 25mm and the sample thickness was 3.5�0.5 mm.

2.3.6. Dynamic mechanical analysis (DMA)The measurements were performed with RSA II

Solids analyzer using a tension tool on films havingdimensions of 23�10�0.5 mm from �50 to 50 �C at 1Hz frequency and 0.05% strain.

3. Results and discussion

3.1. Structural characterization

The formation of an exfoliated nanocomposite in thesample prepared with EVA 19 and ME100/ODA issuggested by XRD analysis as shown in Fig. 1 where thecomplete loss of order is represented by the disappear-ing of the peak relative to the interlayer spacing ofME100/ODA (2.0 nm) after melt compounding. In aprevious work, Zanetti et al. [6] obtained an exfoliatednanocomposite from the same polymer and phyllosili-cate, by melt compounding the polymer and the fillerinto a DACA twin screw miniextruder. An identicalXRD pattern was reported, and the TEM images con-firmed the silicate exfoliation. The different mixing con-ditions did not affect the dispersion of the silicate layersinto the polymer matrix, so that nanocomposites can beprepared even mixing the filler and the polymer withlow shear rates.Similar results were suggested by XRD analysis for

the composite prepared with EVA 19 and Cloisite 30B.The XRD curve of the composite did not show the peakcorresponding to the main interlayer spacing of 1.8 nm,but a weak signal due to a smaller interlayer distancewas still observed, probably caused by a small amountof not-exchanged montmorillonite present as an impur-ity into the commercial product, corresponding to aninterlayer spacing of about 1.5 nm.The TEM image of this composite shows the presence

of well-dispersed single silicate layers and confirms thehypothesis of exfoliation suggested by XRD analysiswith the formation of a nanocomposite. Some particles

Fig. 1. XRD patterns of ME100/ODA and the relative composite

with EVA polymer.

300 A. Riva et al. / Polymer Degradation and Stability 77 (2002) 299–304

of non-exfoliated silicate were still present justifying thepresence of the weak XRD signal at 7.0 2� angle.The XRD analysis carried out on the samples pre-

pared mixing the polymer with ME100/ADA did notshow changes of the silicate structure after the meltcompounding. Both the XRD patterns of the pure sili-cate and the composite have a sharp signal correspond-ing to an interlayer spacing of 1.7 nm.The absence of structural modifications of the silicate

suggests the formation of a microcomposite. By meltcompounding the polymer and the filler into a DACAtwin screw miniextruder, Zanetti et al. [6] obtained amicrocomposite from the same polymer and phyllosili-cate. An identical XRD pattern was reported, and theTEM images confirmed that no changes in the phyllosi-licate structure were obtained. This shows that the mix-

ing conditions do not affect the morphology of theEVA19–ME100/ADA microcomposite.Concerning the compound prepared from EVA and

Cloisite 6A, an intercalated nanocomposite has beenobtained, as suggested by the XRD analysis. Three dif-fraction peaks were obtained from the XRD analysis ofthe pure silicate, and three peaks were detected too inthe case of the composite, showing a larger interlayerspacing caused by the polymer intercalation, as shownin Fig. 3.TEM analysis confirmed this hypothesis, as shown by

the images reported in Fig. 2, where small (about600�50 nm) and well dispersed silicate particles arevisible into the polymer matrix.

3.2. Thermogravimetry

EVA undergoes two degradation steps as shown inFig. 4 where the TG curves of EVA polymer and somecomposites are reported. The first decomposition step isdue to acetic acid and other products loss, with differ-ences observed if the degradation occurs in nitrogen [7]or air [8]. These reactions mainly involve the vinyl ace-tate units of the polymeric chains. The second degrada-tion step involves the polymeric chain and leads to thecomplete polymer volatilization, without the formationof stable residues above 600 �C.

Fig. 3. XRD patterns of Cloisite 6A and the relative composite with

EVA polymer.

Fig. 2. TEM photo of the EVA and 10% Cloisite 6A composite. The

dark lines represent the silicate particles well dispersed into the poly-

mer matrix (white-grey).

Fig. 4. TGA results for experiments carried out in nitrogen, heating at

10 �C/min.

A. Riva et al. / Polymer Degradation and Stability 77 (2002) 299–304 301

The analyses carried out in N2 are reported in Fig. 4,where the curves of the weight loss and the weight lossderivative vs. temperature are reported for all the repre-sentative samples. The behavior of the EVA–ME100/ODA and EVA–ME100/ADA during the analyses wasthe same found by Zanetti et al. [6] for these compositesobtained by melt compounding in a twin screw mini-extruder. In particular a strong catalytic acceleration fordeacetylation was observed in ME100/ODA nano-composites whereas ME100/ADA behaves as pure EVA.The results for the exfoliated nanocomposite obtained

from EVA and Cloisite 30B show that the first degra-dation step takes place without a strong catalytic effectas it was found for EVA–ME100/ODA (no increase inthe weight loss speed was found), but at a temperatureactually a bit lower than for all the other composites(330 and 346 �C respectively).The catalytic effect reported in literature [6] could be

less effective in this case, as ME100/ODA undergoes thedegradation of the octadecylammonium salt used as theorganic modifier (that is responsible for producing thecatalytic H+ sites on the silicate surface) at lower tem-peratures than the tetra alkyl ammonium salt used inCloisite30B, as the former presents the second degradationstep, in which the formation of the acidic site takes place,beginning at about 300 �C, the latter at about 320 �C [9].A further role could be played by exfoliation fine

morphological details, contributing to lower the acidcatalysis effect by lowering the contact between thepolymer and the protonic sites in Cloisite30B comparedto ME100/ODA.The second degradation step for this nanocomposite

is very similar to the one observed for the other com-posites, but the onset temperature is lower than the oneobserved for the EVA–ME100/ODA nanocomposite,which is more stable than pure EVA during this step.The results for the intercalated nanocomposite

obtained from EVA and Cloisite 6A show that no cata-lytic effect is observed on the first degradation step ofthe polymer. This may be due to the less efficient dis-persion of the silicate layers caused by the intercalation.The second degradation step shows no differences

with pure EVA. The different dispersion that char-acterizes the intercalated nanocomposite does not affectthe composite behavior during the thermal degradation.The experiments carried out on EVA polymer in air,

reported in Fig. 5, show that the first degradation step isvery similar to that observed in nitrogen while the seconddegradation step takes place at a temperature 50 �C lowerthan in nitrogen. The behavior of the EVA–ME100/ODAand EVA–ME100/ADA during the analyses, reportedin Fig. 5, was again the same found by Zanetti et al. [6]for the composites obtained by melt compounding in atwin screw miniextruder. The different mixing condi-tions do not affect the thermo-oxidative behavior of thenanocomposite and of the microcomposite, while a

lower catalytic effect on deacetylation than in nitrogenwas observed, because in air a fraction of acetate groupsare eliminated through oxidative processes [8].During the analyses carried out on the nanocompo-

sites obtained from EVA, Cloisite30B and Cloisite6A,the first degradation step was found to be identical inboth cases, and we had no more evidence of the weakcatalytic effect found in nitrogen. During the seconddegradation step, the composites obtained with Cloisite6A and Cloisite 30B have the maximum weight lossspeed at a temperature (about 486 �C) very similar tothe one observed for EVA–ME100/ODA.Isothermal thermogravimetries were also carried out

in air, to investigate the influence of the structureobtained after the melt compounding on a slowerdegradation, when oxygen has more time to diffuse inthe polymer matrix. The first isothermal segment at330 �C had the aim to complete the first degradationstep, to focus the analysis on the second degradationstep. This temperature was chosen as it was near to thepeak of the weight loss speed for all the composites. Thesecond isothermal segment at 400 �C had the aim toobtain a slow thermooxidation, in order to investigatethe differences between the various nanocomposites.The results of these experiments are reported in Fig. 6,

where the curves of the weight loss are reported vs. time.The aim of the experiment was to investigate the second

Fig. 5. TGA results for experiments carried out in air, heating at

10 �C/min.

302 A. Riva et al. / Polymer Degradation and Stability 77 (2002) 299–304

degradation step, but some differences between thecomposites are evident even after the first degradationstep. At the end of the first isothermal segment, at 134min, the samples show different residual weights, com-paring the three nanocomposites with the pure polymerand the microcomposite. The former have a residualweight, at 134 min, of 85% (exfoliated nanocomposites)and 82% (intercalated nanocomposite), while themicrocomposite has a residual weight of 78%. As thefiller concentration is the same for all the filled compo-sites, this difference is attributable to the barrier effectcaused by the dispersion of the silicate layers, that mayalso inhibit the influence of oxygen during the degrada-tion. The second degradation step put on evidence thedifferences between the various nanocomposites, depend-ing on the dispersion of the silicate layers. The better isthe silicate layers dispersion into the polymer matrixgoing from the microcomposite (ME100/ADA) to thenanocomposites (Cloisite 6A, ME100/ODA, Cloisite30B), the slower is the degradation obtained at a tem-perature of 400 �C. In particular the intercalated mor-phology obtained with Cloisite 6A seems to be less efficientthan the exfoliated structure achieved with ME100/ODAand Cloisite 30B. This fact confirms the barrier effectcaused by an homogeneous silicate layers dispersion intothe polymer matrix, especially on a slow degradation.

3.3. Rheology

Elastic properties versus temperature were measuredon films at a frequency of 1 Hz and strain value of

0.05% in the region of linear visco-elastic response. Thecurve of the microcomposite EVA–ME100/ADA,reported in Fig. 7, shows an elastic modulus slightlyhigher than the pure polymer as expected because of thelow filler concentration, while the exfoliated nano-composite EVA–ME100/ODA with the same silicateconcentration shows a considerable increase. The high-est elastic modulus difference between microcompositeand nanocomposite is observed above the glass transi-tion temperature of the EVA polymer (�28 �C), wherethe increased mobility of the macromolecules is hin-dered by the well dispersed exfoliated silicate layers thatcharacterize the nanocomposite structure. Similarresults were obtained for the loss modulus.To investigate the influence of the structure on the

elastic properties at a temperature above the meltingpoint of EVA polymer (86 �C), experiments with theparallel plate geometry were carried out. The curves ofthe storage modulus correlated with the elastic proper-ties of the investigated materials are reported in Fig. 8.

Fig. 6. Isothermal TGA results for experiments carried out in air at a

temperature of 330 and 400 �C.

Fig. 7. Elastic modulus of EVA, EVA+10%ME100/ODA,

EVA+10%ME100/ADA. The measurements were carried out at 1 Hz

frequency and 0.05% strain.

Fig. 8. Storage modulus (G0) curves versus frequency at a temperature

of 110 �C and 0.05% strain.

A. Riva et al. / Polymer Degradation and Stability 77 (2002) 299–304 303

The curves of the microcomposite and the pure poly-mer are overlapped and show a different trend com-pared to the nanocomposites sample curves, in the lowfrequency region (<1 rad/s). The latter are parallel inthe whole investigated angular frequency range from0.01 to 500 rad/s. In previous references [10,11] werereported the same experiments, above described, con-ducted on intercalated polystyrene nanocomposites, andit was evidenced that the slope of the storage moduluscurve vs. frequency was constant in the whole frequencyrange investigated (non terminal behavior).The trend of the storage modulus of the polymer and

of the microcomposite reported in Fig. 8 does not showa sudden variation of the curve slope at low frequenciesas reported in literature for polystyrene, probably becausethe molecular weights distribution of the commercial EVAis much broader compared to the polymer used in theseprevious works [10,11]. Even if in our graph the curvesslope variation for the polymer and the microcomposite isgradual, it is possible to observe the different behavior ofthe nanocomposites curves that show approximately aconstant slope in the whole frequency range.The non-terminal frequency behavior observed for

polystyrene intercalated nanocomposites [10], wasattributed to the retardation of molecular relaxationprocesses produced by the confined geometric effect.The same evidence is obtained for our exfoliated nano-composites. The different storage modulus measured onnanocomposites samples could be due to a differenthomogenization of the silicate layers into the polymer,as shown by TEM images, corresponding to differentsurface interactions. In fact we obtained the lowestmodulus for the intercalated nanocomposite, while theexfoliated nanocomposites show a higher modulus. Thedifference observed between the two exfoliated nano-composites may be due to the different morphology andaspect ratio presented by the fluorohectorite and mon-tmorillonite layers.

4. Conclusions

The study has demonstrated the possibility of produ-cing exfoliated nanocomposites in EVA polymer with

the following silicates: Somasif ME100/ODA and Cloi-site 30B. Cloisite 6A instead, gave an intercalatednanocomposite.The other silicate (Somasif ME100/ADA) gave the

typical structure of the filled polymer where the particlesdimensions are micrometrics.In the nanocomposite samples, low silicate percentage

(10%) added to the polymer, is capable of reducingdramatically the influence of oxygen during thermo-oxidation, independently on whether the morphology isintercalated or exfoliated, whereas acid catalyzed dea-cetylation is mostly observed in nitrogen and more evi-dent in exfoliated structures.The effect of the composite structure on the material

rheological behaviour, is a dramatic increase of the sto-rage and loss modules in the nanocomposite, comparedwith the non filled polymer and with the micro-composite.Rheological analysis conducted on the composites at

110 �C, over the melting temperature of the polymer,show a non terminal behavior at low frequencies for allthe nanocomposites, while the value of the storagemodulus showed differences between the various nano-composites, depending on the dispersion of the inor-ganic phase.

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