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Thermochimica Acta 552 (2013) 37– 45

Contents lists available at SciVerse ScienceDirect

Thermochimica Acta

jo ur n al homepage: www.elsev ier .com/ locate / tca

hermo-oxidative ageing of an organo-modified clay and effectsn the properties of PA6 based nanocomposites

. Scaffaroa,∗, L. Bottaa, A. Fracheb, F. Bellucci c

Dipartimento di Ingegneria Civile, Ambientale, Aerospaziale, dei Materiali, Università di Palermo, Viale delle Scienze, 90128 Palermo, ItalyDipartimento di Scienza Applicata e Tecnologia, Politecnico di Torino – sede di Alessandria, Via T. Michel, 5, 15121 Alessandria, ItalyProplast – Consorzio per la promozione della cultura plastica, Strada Comunale Savonesa 9, 15057 Rivalta Scrivia, AL, Italy

r t i c l e i n f o

rticle history:eceived 11 July 2012eceived in revised form 8 November 2012ccepted 9 November 2012vailable online 20 November 2012

eywords:ontmorilloniterganic modifieregradationanocompositesGA–FTIR

a b s t r a c t

In this work a careful investigation on the degradation of an organically modified montmorillonitesample (Cloisite 15A) thermal treated under different atmospheres, namely nitrogen, air and oxygenenriched atmosphere was performed. The exposure time of the thermal treatment ranged between 5and 300 min. The chemical composition evolution as function of the thermal treatment conditions ofthe clay organic modifier was monitored by means of combined thermogravimetry/Fourier transforminfrared spectroscopy (TGA–FTIR). Moreover, the morphological behaviour of treated Cloisite 15A sam-ples as function of the time and of the decomposition atmosphere was investigated by X-ray diffractionanalysis (XRD). In order to understand the possible different interactions of neat and degraded modifiedclay with a polymeric matrix, PA6 based composites containing 5% of clay were prepared in the melt andfully characterized by a rheological, mechanical and morphological point of view.

The results showed that during the thermal treatment the organic modifier in the interlamellar spaces

undergoes a rearrangement, while the external unbound modifier degrades. At long exposure times,the degradation gradually extends to the clay galleries involving the internal modifier. These effectsare more intense, as expected, when increasing oxygen concentration of the treatment atmosphere. Asexpected, the properties of the prepared nanocomposites were influenced by the degradation level of theincorporated clay. In particular, the morphology and mechanical properties of the composites worsenedby increasing the time of the thermal treatment of the clay.

. Introduction

Organically modified montmorillonites are widely used for thereparation, in the melt, of nanocomposites with thermoplastics well thermosetting polymers as a matrix. Unfortunately, manyurfactants used as organic modifiers are thermally unstable athe temperatures commonly adopted for processing. Many paperseported studies about the thermal stability of organoclays andheir influence on polymer based nanocomposites [1–23].

The thermal non-oxidative degradation of modified montmo-illonites was already studied. In particular, it was observed thatmportant degradation phenomena of the organic modifier, accord-ng to the Hoffmann elimination, occur at above 155 ◦C [1,2]. Theresence of clay acidic sites catalysed the degradation of the organic

odifier that occurs in three steps: (i) degradation of the free

rganic modifier, (ii) degradation of the physically absorbed organicodifier and (iii) degradation extended to the chemically bounded

∗ Corresponding author. Tel.: +39 091 23863723; fax: +39 091 23860841.E-mail address: roberto.scaffaro@unipa.it (R. Scaffaro).

040-6031/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.tca.2012.11.007

© 2012 Elsevier B.V. All rights reserved.

modifier. In other works [3], it was found that the degradation ofthe free modifier occurred in a single step, while the bounded mod-ifier degraded in two steps at temperatures respectively lower andhigher if compared with the neat salt. This was explained consid-ering that, in the earliest phases of the reaction, the catalysis of themineral clay accelerates the degradation. Later, the mineral clayacts like a barrier trapping the volatile degradation products thusreducing the degradation kinetic.

Other authors [4,5] studied the degradation products of somecommercially available modified clays observing that the degra-dation mechanism and the temperature at which the degradationstarts is different for each sample.

In our previous work [7], we studied the influence of thermaltreatments on the behaviour of clays, paying particular atten-tion to the possible degradations effects of the organic modifier.For this purpose, it was studied the degradation under differentatmospheres of two different clay samples with the same organic

modifier at different concentration. The results indicated thatdegradation of organo-modified montmorillonites proceeded withinitial formation of �-olefins that eventually transformed into var-ious carboxyl compounds. In the first steps of the degradation, the

3 chimica Acta 552 (2013) 37– 45

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Table 1Nanocomposites prepared in the frame of this work.

Sample code PA6 (wt%) C15A (wt%) Thermal treatmenttime (min)

PA6/C15A 95 5 0

8 R. Scaffaro et al. / Thermo

roducts caused an expansion of the interlayer spaces. On increas-ng the reaction time, these compounds likely diffused towards theurface of the sample and eventually volatilized. This caused a col-apse of the clay mineral particles that presented interlayer spacingery similar to those measured for unmodified montmorillonite.

higher amount of organic modifier increased the degradationinetics independently of the atmosphere used.

The thermal instability of the organic modifiers can affect thexfoliation of the particles, the interface interactions, and the effec-iveness of additives like compatibilizers or stabilizers. In addition,he degradation products may cause undesired colour change, pro-

ote the degradation of the matrix and induce microcracks thateduce the mechanical resistance [24–32].

In particular, Scaffaro et al. found [32] that the degradation ofhe organic modifier during processing, promoted the degradationf the PET during the preparation of PET/clay nanocomposites. Con-equently, the slight enhancement of the mechanical performanceas a balance between the positive effect of introducing the organ-

clays and the negative effect of degradation phenomena inducedy the presence of these fillers.

Moreover, in a polymer blend nanocomposite system [31],espite the good morphology of filled compatibilized blends theighest increments were surprisingly observed in the binarylend. This phenomenon was attributed to the possible interactionetween the compatibilizing system and the degradation productsf the modified clay caused by thermo-oxidation during processing.

Aim of this work was a deeper investigation of the degrada-ion of an organically modified montmorillonite sample, thermalreated under different atmospheres and for different times, by spe-ific spectroscopic methods and in particular by using a combinedhermogravimetry/Fourier transform infrared analysis. Moreover,n order to understand the possible different interactions of neatnd degraded modified clay, composites containing 5% of clay witholyamide 6 as a matrix were prepared in the melt and fully char-cterized by a rheological, mechanical and morphological point ofiew.

. Experimental

.1. Materials

The polyamide 6 (PA6) used in this work was a sample (S35 NAT)indly supplied by Radicinova (Italy) with an intrinsic viscosity of.4 dL/g measured in sulphuric acid.

The organically modified clay was Cloisite 15A (C15A), suppliedy Southern Clay Products (USA). It is a sample of montmoril-

onite modified with 1.25 meq/g of quaternary ammonium withwo methyl substituents and two C18 hydrogenated tallow tails2M2HT).

.2. Clay treatment

The study of decomposition behaviour of Cloisite 15A was car-ied out by applying a thermal treatment at 240 ◦C in differenttmospheres, namely under nitrogen flux (N2) air flux (Air) andxygen enriched air (70–30% nitrogen–oxygen) flux (Ox). The ther-al treatment exposure time ranged between 5 and 300 min.The clay organic modifier chemical composition evolution as

unction of the thermal treatment conditions has been moni-ored by means of combined thermogravimetry/Fourier transformnfrared (TGA–FTIR), using a Perkin Elmer Pyris 1 thermogravi-

etric analyzer interfaced to a Perkin Elmer Spectrum GX Fourierransform infrared spectrophotometer at a scan rate of 10 ◦C/minnder both inert (nitrogen) and oxidative synthetic air (70–30%itrogen–oxygen) atmosphere. Spectra have been collected in

PA6/C15A-60 min 95 5 60PA6/C15A-300 min 95 5 300

correspondence of DTG peaks, while the release profile of alchilcompounds and CO2 have been monitored through profiles associ-ated to FT-IR peak at 2940, 2350 cm−1, respectively as function ofthe temperature.

Morphological behaviour as function of decomposition atmo-sphere and time of treated Cloisite 15A samples was investigatedby means of X-ray diffraction using an ARL X’TRA 48 X ray diffrac-tometer with Cu K� radiation (� = 1.54062 A). More specifically, thefirst and second order clay interlayer distance diffraction peaks in2� range comprised between 1◦ and 10◦ have been monitored.

2.3. Nanocomposites

Prior to use and/or processing both PA6 and C15A were driedunder vacuum overnight at 120 ◦C.

Degraded clay was obtained by thermally treating the samplein air at 240 ◦C for 60 min or 300 min prior to use.

The nanocomposites were obtained by melt extrusion using aco-rotating twin screw extruder (L/D = 35; OMC, Saronno Italia)with a thermal profile of 180–200–210–220–230–240–240 ◦C,extrusion speed 220 rpm and feeding speed 12 rpm. PA6 and neator thermally treated C15A (95/5 by weight) were initially premixedat the solid state and therefore fed to extruder trough the feedingsystem. The composition and the sample codes are summarized inTable 1.

Morphological analysis was performed by scanning electronmicroscopy (SEM) FEI QUANTA 200F on samples fractured underliquid nitrogen and then and sputter-coated with a thin layer ofgold to avoid electrostatic charging under the electron beam.

The transmission electron microscopy (TEM) images wereobtained with a Philips EM 208 TEM with 100 keV accelerating volt-age. Ultrathin sections (50 nm thickness) of the specimens, cooledat −80 ◦C, were obtained by cryoultramicrotomy with a diamondknife cooled at −60 ◦C.

Dynamic rheological characterization was performed on cylin-drical samples (D = 25 mm, thickness = 2 mm) by using a parallelplate rheometer (SR5, Rheometrics) at 240 ◦C in the frequencyrange 0.1–100 rad/s. According to the strain sweep tests, here notreported for sake of brevity, the strain was set at 10%, withinthe linear viscoelastic range. The specimens were prepared bycompression moulding using a Carver laboratory press (T = 240 ◦C,P = 100 bar).

Tensile test were performed on an Instron (UK) 3365dynamometer on specimens 10 mm × 0.8 mm × 90 mm cut off fromcompression moulded sheets prepared as above described for therheological tests. The tensile speed was 50 mm/min, the grip dis-tance was 50 mm. All the results here presented are the average ofat least 10 replicates with a scattering of ±7% from the average.

3. Results and discussion

3.1. Cloisite 15A characterization

The comparison of thermal decomposition behaviour, both ininert and oxidative atmosphere, of the Cloisite 15A and its organicmodifier 2M2HT is shown in Fig. 1a and b. Moreover, in Fig. 1 thereare shown the normalized curves of the Cloisite 15A describing

R. Scaffaro et al. / Thermochimica Acta 552 (2013) 37– 45 39

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ig. 1. TGA and DTG of Cloisite 15A and its organic modifier 2M2HT in (a) nitrogen

dotted line) are the TGA normalized curves of Cloisite 15A.

GA of the organic fraction (dotted line, nC15A). The normalizedurves were calculated taking into account the wt% loss on igni-ion of Cloisite 15A (43%) declared in the technical data sheet ofhe supplier. It is worth noting that this weight loss can not be

ttributed only to the fraction of the organic modifier. Indeed,ontmorillonite contains water to some extent (adsorbed and

ncluded in crystalline structure) thus the total weight loss can belso attributed to water loss. So, ultimately, the normalized curves

Fig. 2. FT-IR analysis of volatiles released du

phere and (b) oxidative atmosphere (70–30% nitrogen–oxygen). The nC15A curves

of C15A show the thermal decomposition behaviour of the organicmodifier fraction together with the water contained in the mont-morillonite fraction.

The clay organic modifier 2M2HT decomposes in a temperature

range between 250 ◦C and 420 ◦C under inert atmosphere (Fig. 1ablack line) with a maximum decomposition rate at 390 ◦C, whereasthe first DTG is ascribed to the low molecular weight stabilizingagent of 2M2HT. Under oxidative atmosphere (Fig. 1b black line)

ring thermo-oxidation of Cloisite 15A.

40 R. Scaffaro et al. / Thermochimica Acta 552 (2013) 37– 45

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All these results, further, are fully according with a degrada-tion path in which the a-olefins in the presence of oxygen and/orwater oxidize to form various carbonyl compounds and, eventually,carbon dioxide (see Scheme 1) [7].

Fig. 3. XRD patterns of Cloisite 15A thermal treated for differen

ts decomposition occurs in two steps. During the first decomposi-ion event, between 250 ◦C and 480 ◦C, about 90 wt% of the volatilesroducts are released, while a stable char residue of 10 wt% decom-oses in the temperature range between 480 ◦C and 620 ◦C.

When intercalated in the clay galleries and under inert atmo-phere, the organic modifier decomposes at higher temperatureith respect to the pristine 2M2HT and in multiple steps (Fig. 1a

rey line). These differences are clearer by comparing the curvef neat 2M2HT with the normalized curve of Cloisite 15A (dot-ed line, nC15A). This behaviour is according with the studieslready reported in literature [1–3]. Even in oxidative atmospherehe organic modifier decomposition differs to that of the neat saltFig. 1b grey line and dotted line) and, in particular, when it isncluded inside the clay, its thermal stability is higher. In fact,bserving the TGA curves of 2M2HT and nC15A, it is worth not-ng that after the main decomposition event the residual amountf salt in Cloisite 15A is higher than that observed for the corre-ponding neat salt. For instance, at the temperature of about 480 ◦Che residue of 2M2HT is about 10% while the amount in nC15A isbout 32%. Even considering that the residual amount is the sumf the organic modifier and water in the montmorillonite – esti-ated in the scientific literature at 6.5% [20] – the residual organicodifier is at least about 26%. This is ascribed to the acidic sites

resent onto the clay surface which promote the formation of a sta-le char phase that decompose at temperatures ranging from 400 ◦Co 800 ◦C releasing only CO2 [3]. In order to verify this hypothesis,T-IR spectra of the volatile part evolved during the TGA of Cloisite5A were collected under oxidative atmosphere and reported inig. 2a–c.

It can be observed that the most relevant degradation productsre carbon dioxide and alkanes likely coming from the volatiliza-ion of the �-olefins of the organic modifier. In particular, in Fig. 2at is reported the DTG curve of Cloisite 15A while in Fig. 2b therere reported the corresponding absorbances as a function of tem-erature at 1750 cm−1 (related to the stretching of C O in carbonylompounds), 2360 cm−1 (related to the stretching of CO in carbon

ioxide) and at 2940 cm−1 (related to the asymmetric stretching ofethylene groups in alkanes).The elimination of alkanes starts at about 250 ◦C, Fig. 2b, and

s complete at about 350 ◦C. At the same time, carbon dioxide is

es in (a) nitrogen (N2), (b) air and (c) oxygen enriched air (Ox).

produced in the range 260–380 ◦C with a maximum at about 330 ◦C.This is more evident by analyzing Fig. 2c where there are reportedthe FTIR spectra of Cloisite 15A at three different temperatures, cor-responding to the peaks observed in the DTG, Fig. 2a. At 255 ◦C thereis the maximum formation of alkanes confirming their degradationby Hoffmann elimination. Moreover, it can be observed a band cor-responding to the stretching of CO in CO2 at 2360 cm−1 and anotherat 1750 cm−1 assigned to carbonyls. By increasing the tempera-ture these three bands are always present but with a progressivelydecrease of the alkanes band and a corresponding increase of theband of CO2 that is maximum at 330 ◦C. This latter observationssuggests that the olefins are almost completely volatilized abovethis temperature.

Scheme 1. Degradation path of organic modifier of Cloisite 15A.

R. Scaffaro et al. / Thermochimica Acta 552 (2013) 37– 45 41

Fig. 4. TGA and DTG in oxidative atmosphere of Cloisite 15A thermal treated for different times in (a) nitrogen (N2) and (b) oxygen enriched air (Ox).

Fig. 5. FTIR profiles at (a) 2940 cm−1 and (b) 2350 cm−1 of the product released during TGA of the samples treated in nitrogen atmosphere.

42 R. Scaffaro et al. / Thermochimica Acta 552 (2013) 37– 45

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Fig. 6. FTIR profiles at (a) 2940 cm−1 and (b) 2350 cm−1 of the product

.2. Thermal treated Cloisite 15A characterization

The X-ray diffraction patterns of treated Cloisite 15A as functionf decomposition atmosphere (nitrogen, air or oxygen enriched air)nd exposure time are shown in Fig. 3a–c. It can be seen that inter-ayer distances in the original Cloisite 15A are grouped in threeifferent families. Indeed, behind the most relevant peak, locatedt low angles and corresponding to an interlayer distance of 2.8 nm,here are also a shoulder at higher angles (2� = 5◦, interlayer dis-ance 1.8 nm) and a peak at 2� = 7.6◦ which corresponds to thenterlayer distance of pristine montmorillonite (Cloisite Na+).

A 5 min thermal treatment at 240 ◦C induces the rearrangementf the organic modifier within the galleries, indeed the shouldert 2� = 5◦ disappears whatever the treatment atmosphere is and,oreover, the intensity of the peak at lower angle increases.Moreover, in the case of the samples thermally treated in air

r oxygen-enriched air, Fig. 3b and c, the basal spacing increasedo 3.1 nm. This result can be explained considering that in the firsttages of the degradation, the volatile products tend to increase thenterlayer distance. Similar results were found by Shah and Paul27].

On increasing the exposure time, the intensity of the peakt lower angles decreases progressively, while the higher angleseak intensity increases, more rapidly if the decomposition occurs

n the presence of oxygen. This behaviour suggests that twoain phenomena occur during the clay thermal treatment: (i)

he decomposition of organic modifier provokes the decrease ofhe interlamellar distance, as faster as the oxidative nature of thetmosphere increases and (ii) the morphology evolves to a more

isordered state (related to the broadness and loss of the intensityf the main diffraction peaks) with the exposure time.

The thermogravimetric analyses carried out on the same sam-les examined by X-ray diffraction, confirm such hypotheses. In

ed during TGA of the samples treated in oxygen enriched atmosphere.

fact, it can be noted that the decomposition of the thermally treatedclay, Fig. 4a and b, starts at a temperature higher than that observedfor the untreated one. Actually, it is missing the first degradationevent of the organic modifier (between 230 ◦C and 280 ◦C witha volatile products maximum release rate at 255 ◦C) essentiallydue to the fraction of the organic modifier that can be consid-ered free, i.e. not intercalated or resident in peripheral zones ofthe clay galleries. Also in this case, it is possible to put into evidencetwo concurrent phenomena: (i) decomposition, during the thermaltreatment, of the non intercalated fraction of the organic part and(ii) rearrangement of part of the organic modifier resident in theperipheral zone of the interlamellar spaces that leads to a higherintercalation. This last feature was evidenced by XRD analysis andby the increment of the threshold temperature for the release ofthe degradation products.

Analyzing the data of the residue at 800 ◦C it is possible to notehow the heat exposure time under inert atmosphere has low influ-ence on the degradation of the organic modifier, Fig. 4a. In fact,between the residue of neat Cloisite and that of Cloisite treated for300 min, there is a difference of only 5%, corresponding to 12.5% ofthe organic fraction. On the contrary, under oxidative atmosphere,the thermal treatment leads to a difference of 13% in the residue,corresponding to the degradation of 32.5% of the organic fraction(see table insets of Fig. 4a and b).

In Figs. 5 and 6a and b there are reported the FTIR profiles at2940 cm−1 and 2350 cm−1 of the product released during thermo-gravimetric test of the samples treated, respectively, in nitrogenand oxygen enriched atmosphere.

Also in this case, it possible to note that the products ini-

tially released are basically alkyl compounds while, on increasingtemperature, CO2 is eventually formed. These results allowconcluding that during the treatment the organic modifier inthe interlamellar spaces undergoes a rearrangement, while the

R. Scaffaro et al. / Thermochimica Acta 552 (2013) 37– 45 43

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xternal modifier degrades. The degradation, at long exposureimes, gradually extends to the clay galleries involving the inter-al modifier. These effects are more intense, as expected, when

ncreasing oxygen concentration (N2 < air < O2 enriched air).

.3. Nanocomposites

In Fig. 7a–d there are reported the SEM micrographs ofA6/C15A (a and b) and PA6/C15A-300 min at two different

Fig. 8. TEM micrographs of PA6/C15A

5A-300 min (c and d) at two different magnifications.

magnifications. The morphology of PA6/C15A, Fig. 7a and b, ischaracterized by a good dispersion of the clay inside the matrixtogether with a good interfacial adhesion. The situation is differentfor PA6/C15A-300 min, Figs. 7c and d. In this case, the homogene-ity and the dispersion are poor. The clay is clearly present under

the form of aggregates with a scarce interfacial adhesion with thematrix. This is reasonably due to the degradation undergone byC15A during the thermal treatment and, in particular, to the degra-dation of the organic modifier.

(a) and PA6/C15A-300 min (b).

44 R. Scaffaro et al. / Thermochimica Acta 552 (2013) 37– 45

Table 2Tensile properties of the materials prepared in this work.

Materiali E (MPa) TS (MPa) EB (%)

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PA6/C15A 1875 ± 110 52.3 ± 3.1 9 ± 1.5PA6/C15A-60 min 1735 ± 101 43.8 ± 2.7 14 ± 1.8PA6/C15A-300 min 1640 ± 95 37.5 ± 2.6 4 ± 1

The morphology of these two samples was also studied byEM analysis. In Fig. 8, TEM micrographs of PA6/C15A (a) andA6/C15A-300 min (b) are reported. In PA6/C15A the clay appearsell dispersed. Moreover, small tactoids and even some single layer

an be detected. On the contrary, the TEM image of PA6/C15A-00 min shows that the clay is present under the form of aggregatesnd collapsed tactoids, confirming the results of the SEM analysis.

In Table 2 there are reported the elastic modulus (E), the tensiletress (TS) and the elongation at break (EB) of all the samples. Thelastic modulus of the materials is worsens from neat (1875 MPa)o 60 min treated (1735 MPa) to 300 min (1640 MPa) thermallyreated clay. These results are not surprising considering the drastichange of morphology observed in the composites containing thereated clay. As expectable, a similar behaviour can be observedor TS, while EB presents a maximum for the sample PA6/C15A0 min. This apparently strange behaviour can be explained consid-ring the degradation phenomena of the organic modifier of thelay during the thermal treatment. Initially, the thermal degrada-ion starts with a typical Hoffmann elimination that degrades therganic modifier into �-olefins and substituted amines. The form-rs, in the presence of humidity and oxygen, continue to degrade toarious carbonyl and carboxyl compounds [1]. In particular, in therevious section it was demonstrated that the thermal treatment

nduces the release of �-olefins in the earliest degradation stages.n the following, it can be hypothesized their oxidation to carbonylnd carboxyl compounds and, finally to CO2.

In our case, the increase of EB observed for PA6/C15A 60 mins therefore explainable with the increase of the concentrationf �-olefins (early degradation stage). In particular, while dur-ng the thermal treatment the decomposition products are free tovolve into the atmosphere, during processing they are confinednd trapped in the surrounding of the polymer thus acting as plas-icizer. The further decrease of EB for the sample PA6/C15A 300 mins due to the further degradation of �-olefins and to the collapse ofhe tactoids into large disordered aggregates – as demonstrated byRD analysis – that act as defects inducing the premature break of

he specimen.These results are corroborated by the rheological analysis. In

ig. 9 it is reported the complex viscosity as a function of fre-uency for all the materials. PA6/C15A displays the highest values

ig. 9. Viscosity as a function of frequency for all the nanocomposites prepared inhis work.

Fig. 10. Conservative (G’) and dissipative (G”) moduli as a function of frequency forall the nanocomposites prepared in this work.

of viscosity in the whole frequency range, but especially in the lowfrequency region. This is evidence of good clay dispersion insidethe matrix, with the formation of structured material typical ofclay basic nanocomposites [31–36]. Surprisingly, the lowest vis-cosity values are displayed by PA6/C15A 60 min while PA6/C15A300 min viscosity curve is intermediate between the two. This isin accordance with the interpretation of the mechanical results. Infact, the presence of free �-olefins in the PA6/C15A 60 min is notonly responsible of a higher EB but also of a melt plasticization thatcauses a sharp decrease of viscosity.

Further confirmation can be found by analyzing the conserva-tive (G’) and dissipative (G”) shear moduli, Fig. 10 G” is practicallycomparable for all the materials. As regards G’, PA6/C15A shows thehighest values of in the whole frequency range, thus confirming theformation of a structured material in the melt [31–36]. Similarly towhat observed for the viscosity, PA6/C15A 60 min shows the lowestvalues of G’ and this agrees with the plasticizing effect of �-olefinsinvoked above to interpret the mechanical and the viscosity data.

4. Conclusions

The degradation of an organically modified montmorillonitesample, i.e. Cloisite 15A, was studied under different atmospheresand for different thermal treatment exposure time (up to 300 min).

The evolution of the chemical composition as function of thethermal treatment conditions (environmental atmosphere andtime) of the clay organic modifier was monitored by combiningTGA and FTIR analysis. Moreover, the morphological evolution asfunction of the environmental atmosphere and time of Cloisite 15Asamples was investigated by XRD.

The results indicated that the decomposition of organic modi-fier provokes the decrease of the interlamellar distance, as faster asthe oxidative nature of the atmosphere increases. Concurrently, themorphology evolves to a more disordered state with the exposuretime. In particular, during the thermal treatment the organic mod-ifier in the interlamellar spaces undergoes a rearrangement, whilethe external unbound modifier degrades. The degradation, at longexposure times, gradually extends to the clay galleries involvingthe internal modifier.

The properties of PA6 based composites containing 5% of ther-mal treated clay prepared by melt compounding were influencedby the level of degradation, i.e. the exposure time, of the incorpo-rated clay. In particular, the morphology and mechanical properties

of the composites worsened by increasing the time of the thermaltreatment of the clay. The composites incorporating the clay treatedfor 60 min showed the highest elongation at break and the low-est melt viscosity. This behaviour was attributed to the presence

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f free �-olefins in early degradation stage that, even during thehort extrusion process, remain trapped in the surrounding of theolymer thus acting as plasticizer for the composite.

cknowledgement

The work has been financially supported by MIUR (PRIN 2008).

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