Applied Clay Science 45 (2009) 185–193

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Applied Clay Science

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Effect of heating of organo-montmorillonites under different atmospheres

R. Scaffaro a,⁎, M.C. Mistretta a, F.P. La Mantia a, A. Frache b

a Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Università di Palermo, Viale delle Scienze, Ed.6, 90128 Palermo, Italyb Centro per l'Ingegneria delle Materie Plastiche, Politecnico di Torino sede di Alessandria, INSTM research unit, Via Teresa Michel 5, 1500 Alessandria, Italy

⁎ Corresponding author.E-mail address: scaffaro@dicpm.unipa.it (R. Scaffaro

0169-1317/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.clay.2009.06.002

a b s t r a c t

a r t i c l e i n f o

Article history:Received 29 September 2008Received in revised form 14 May 2009Accepted 8 June 2009Available online 13 June 2009

Keywords:MontmorilloniteOrganic modifierDegradationOxidation

We studied the influence of heating on the behaviour of two organo-montmorillonites paying particularattention to the possible degradation effects of the organic modifier under different atmospheres. Hoffmannelimination and anucleophilic substitution on the nitrogen led to rapid degradation of the modifier. Asconfirmed by deconvoluted FTIR spectra, the presence of oxygen accelerated the degradation rate. Thedegradation products of the modifier (α-olefins transforming into various carboxyl compounds if oxygen ispresent) initially increased the basal spacing, followed by a collapse of the particle layers when thedecomposition products migrated toward the surface and eventually volatilized.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

In the past two decades, there has been a growing interest in thepreparation of hybrids based on a polymer matrix and clays as filler.The final aim was to achieve a dispersion of the filler at a nanometricscale by the intercalation of polymer chains into the interlayer spacesor, under appropriate conditions, the total destruction of the claymineral particles with complete exfoliation. This process shouldimprove properties such as barrier, mechanical and thermomechani-cal values, without affecting other important features like transpar-ency, density and processability (Kojima et al., 1985; Okada et al.,1988; Lan and Pinnavaia, 1994; Messersmith and Giannelis, 1994;Pinnavaia and Beall, 2000).

The incompatibility between these hydrophilic clay minerals andhydrophobic polymer matrices can inhibit the development ofnanostructures, especially when the materials are prepared in themelt. To improve the affinity between the two materials, the claysmust be made organophilic by ion exchange with cationic surfactantslike alkylammonium salts (Hendricks, 1941; Earnest, 1980, 1988, 1991;Ray and Okamoto, 2003).

Unfortunately, alkylammonium surfactants are thermally unstableat temperatures adopted for processing the most common thermo-plastic polymers. This feature can affect the exfoliation of the particles,the interface interactions and the effectiveness of additives, likecompatibilizers or stabilizers. In addition, the products of thedegradation reaction may cause undesired colour change, promotethe degradation of the matrix and induce microcracks that reduce themechanical resistance (Delozier et al., 2002; Fornes et al., 2003;

).

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Osman et al., 2003; Yoon et al., 2003; Shah and Paul, 2006; He et al.,2006; Scaffaro et al., 2008).

Some authors (Xie et al., 2001a,b; Osman et al., 2003) studied thethermal non-oxidative degradation of modified montmorillonites:important degradation phenomena, according to the Hoffmannelimination (March, 1985), were observed above 155 °C. The presenceof acidic sites, of the clay mineral, catalyzed the degradation of theorganic modifier that occurs in three steps: the first related to the freeorganic modifier; the second, involving the physically absorbedorganic modifier; the last, extended to the chemically bound modifier.Bellucci et al. (2007) confirmed these results. In particular, they foundthat the degradation of the free modifier occurred in a single step,while the bound modifier degraded in two steps at temperatureslower and higher if compared with the neat salt. This was explainedconsidering that, in the earliest phases of the reaction, the catalysis ofthe clay minerals accelerates the degradation and later, the mineralclay acts like a barrier trapping the volatile degradation products andreducing the degradation kinetic.

Cervantes-Uc et al. (2007) and Edwards et al. (2005) studied thedegradation products of some commercially available modified clayminerals observing that the degradation mechanism and thetemperature of beginning degradationwere different for each sample.The degradation products were mainly water, aldehydes, carboxylicacids, various aliphatic compounds, carbon dioxide and aromaticcompounds, if originally present in the modifier.

Francowski et al. (2007) studied the change in the morphology ofthe clay mineral. Depending on the degradation conditions, theyfound an increase or a decrease of the basal spacing. The first situationoccurred when the volatile products could not escape, thus expandingthe particles. When the degradation products were easily eliminated,the particles collapsed.

Fig. 1. ATR–FTIR of the two modified montmorillonites.

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The aim of this work is to study the influence of temperature on themodifier degradation. Organo-montmorillonites with different con-tent of the same organic modifier were exposed to differentatmospheres for different times. In the view of application of theseorgano-bentonites in the preparation of intercalated/exfoliatednanocomposites by melt processing, it is important to understand indetail how the degradation products may interact with other additivesor with the polymer matrix itself.

2. Experimental

2.1. Materials and preparations of the samples

Two samples of organically modified montmorillonite (Cloisite15A and Cloisite 20A, Southern Clay Product) were used. Bothmontmorillonites were modified with the quaternary ammoniumsalt shown in Scheme 1 (HT = hydrogenated tallow), withapproximate composition (by mass) of 65% C18, 30% C16 and 5% C14.

Cloisite 15A contained an excess of modifier (1.25 meq/g) whileCloisite 20A contained about the CEC amount (0.95 meq/g).

Cylindrical samples of both organo-montmorillonites were pre-pared by compressing about 3 g of powder at 150 bar and roomtemperature for about 10 s as described before (Scaffaro et al., 2008).

Organo-montmorillonite samples were exposed to air, oxygen andnitrogen at 240 °C for 5–300 min.

2.2. Characterization

The surface of the heated samples was analyzed by an AutoImageFTIR microscope Perkin Elmer equipped with a Micro-ATR objective.To get more information on the characteristics of some parts of thespectrum, some spectra were deconvoluted using an appropriatesoftware. Contact angle measurements were performed on a FTA1000, (TenAngstrom) equipment using deionized water as liquid.Wide angle X-ray diffraction (WAXD) patterns were recorded using aPhilips PW 1830 powder diffractometer (Ni-filtered Cu Kα radiation).

3. Results and discussion

Apart from the typical peaks of the montmorillonite at 1012 cm−1

and 912 cm−1 (Fig.1 and Table 1), the characteristic absorption bands ofthe organic modifier were observed: the symmetric and asymmetricstretching vibration of CH2 at 2848 cm−1 and 2920 cm−1, the bendingvibration of CH2 at 1475 cm−1 (shoulder) and the C–N stretchingvibration at 1468 cm−1. 15A presented a higher intensity of themethylene group vibrations (2848 cm−1, 2920 cm−1 and 1475 cm−1)confirming that this sample contained a higher amount of modifier. Thespectra as a function of exposure time in air are reported in Fig. 2. After5 min, there were no significant variations on the surface of both 15Aand 20A but new absorption bands appeared after 10 min in the region1600–1900 cm−1. In this region the absorptionbands of carbonyl groupslike carboxylic acids, ketons, aldehydes, esters, periesters, peracids,lactons are found and this is the reason why the band is broadened.

In Fig. 3 the deconvoluted spectra are reported for materialsexposed to air at three different times.

After 5 min, no significant changes in the spectra were observedwhile after 10 min several new peaks appeared. For both samples, a

Scheme 1. Quaternary ammonium salt used for the modification of the montmorillonite.

band was observed at 1705 cm−1 and it was assigned to γ-ketoacids.It is worth noting that, at this time, this was the only band in thecarbonyl region and the other absorption bands are related to differentcompounds. The band around 1640 cm−1 was assigned to C_C(Geuskens and Kabama, 1982; Rjieb et al., 2000), of α-olefins formedby the Hoffman reaction, overlapping the bending vibration ofstructural –OH groups (Francowski et al., 2007).

At 300 min, the intensity of the absorption bands in the carbonylregionwas higher, and higher for 15A than for 20A, reasonably due to thehigher amount of modifier of the former. All the bands were broad, andsome of them were very close to each other thus evidencing a greatheterogeneity of the degradation products. Some of these compoundscould be identified (Geuskens and Kabama, 1982; Lacoste et al., 1993;Rjieb et al., 2000; Salvalaggio et al., 2006): the peak around 1710 cm−1

(both in 15A and in 20A) was assigned to carboxylic acids, the peakaround1737 cm−1 (20A) to ester/aldehydes, thepeakaround1772 cm−1

was assigned to periesters. The appearance of these compounds afterlonger times was also observed by Cervantes-Uc et al. (2007).

The organic modifier undergoes the Hoffman elimination consist-ing in the formation of a tertiary amine and free α-olefins. In thepresence of oxygen, the free olefins can form various carboxylcompounds (Cervantes-Uc et al., 2007); a possible reaction mechan-ism is reported in Scheme 2.

Concurrently, the progressive decrease of the methylene and of theC–N absorption bands was observed. This is consistent with the abovestated hypotheses of the progressive conversion of the modifier intocarbonyl compounds and the eventual escape of the low molecularmass volatile compounds from the sample surface.

To better understand the role of oxygen in this degradationprocess, the organo-montmorillonite samples were exposed to pureoxygen and pure nitrogen (Figs. 4 and 5). Heating the organo-montmorillonite in nitrogen, the spectra remained unchanged. In thepresence of oxygen, a broad band appeared in the region 1600–1900 cm−1, even after 5 min, and became more pronounced at highertesting times due to the faster degradation. It is worth noting that thetemperature of 240 °C is in the range of the typical processingtemperatures of several thermoplastic polymers.

At high testing times, the peak assigned to the symmetric andasymmetric stretching vibration bands of CH2 at 2848 cm−1 and

Table 1Peak assignments of the spectra reported in Fig. 1.

1012 cm−1 Stretching vibration Si–O–Si912 cm−1 Stretching vibration Si–O–M (M=Al, Mg, Fe)2848 cm−1, 2920 cm−1 Symmetric, asymmetric stretching vibration of CH2

1475 cm−1 Bending vibration of CH2

1468 cm−1 Stretching vibration of C–N3616 cm−1, 1636 cm−1 Bending, stretching vibration OH

Fig. 2. ATR–FTIR spectra of Cloisite 15A (a) and 20A (b) as a function of the exposure time in air.

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2920 cm−1, to the bending vibration band of CH2 at 1475 cm−1 and tothe C–N stretching vibration band at 1468 cm−1, Fig. 5, practicallydisappeared indicating the complete decomposition of the modifier.

The area of the region 1550–1900 cm−1, which includes thecarbonyl region, and the area of the region 2750–3050 cm−1 as afunction of time are shown in Figs. 6 and 7. The absorption bandareas did not change in nitrogen while, in the presence of oxygen,the area between 1550 and 1900 cm−1 increased and that between2750 and 3050 cm−1 decreased. Both organo-montmorillonitesshowed an induction time for degradation, about 5 min for 15Aand about 30 min for 20A due to the different modifier content.Considering that 20A has an approximately stoichiometric amount ofmodifier, there is reasonably a very scarce amount of free modifierand therefore the oxidative degradation will start later. In addition,the final value of the band area was lower for 20A than for 15A,confirming the direct relationship between the degradation extentand the band area.

In oxygen, 15A presented no induction time for degradation or, atleast, less than 5 min; 20A a time of about 5 min. As expected, theoxidative processes were more efficient in the presence of pure

oxygen and consequently the degradation rate was faster for bothorgano-montmorillonites. In contrast to 15A, the band area of 20A didnot show a maximum: due to the different modifier amount, thisphenomenon could be reasonably observed only at longer times.

Themass loss curves in oxygen (Fig. 8) again indicated that the freeorganic modifier undergoes a fast degradation/volatilization.

Considering that the mass loss of unmodifiedmontmorillonite wasnegligible (at 240 °C up to 300 min), the mass loss of the organo-montmorillonite corresponded to the content of the organic modifier.Therefore, the amount of the remnantmodifier could be evaluated as afunction of the oxidation time (Fig. 8b), considering the initialamounts of organic modifier (43% by mass for 15A, 38% by mass for20A). As expected, 15A lost the modifier more rapidly than 20A but, atlonger times, the slope of the curves became similar. In both samplesabout 30% of modifier was lost after 300 min.

The contact angle (Figs. 9 and 10) initially increased. One reasonmay be the removal of surface humidity but the difference betweenthe two organo-montmorillonites suggested some other mechanismsinvolving the migration of non-polar compounds to the surface. For15A, these non-polar compounds could be molecules of the excess of

Fig. 3. Deconvoluted spectra in the region 1900–1550 cm−1 for 15A (a–d) and 20 A (e–h) exposed to air.

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the modifier. However, the increase of the contact angle observed alsofor 20A suggested that another mechanism must also be taken intoaccount. In particular, due to the initial and non-oxidative degradationstep (Scheme 2) some α-olefins are formed. The migration of thesecompounds to the surface can be considered responsible of theincrease of the contact angle in the first minutes of treatment. Innitrogen, no oxidative degradation may occur but only thermaldegradation reactions. For 15A, the increase of the contact angle, i.e.decrease of polarity (about 10°), was more significant than for 20A(about 5°) because of modifier migration together with α-olefins

formation and migration: in 20A, only the second process couldproceed.

At longer times, all the curves presented maxima at lowertemperatures passing from nitrogen to air to oxygen. The presenceof these maxima can be explained considering the degradation pathproposed in Scheme 2. Under nitrogen, only α-olefins are formed asdegradation products. These compounds migrate to the surface,reduce the polarity and eventually escape as volatile products. Atlong times, as fewer molecules are present on the surface the initialvalue of the contact angle is approached. Under air and oxygen, the α-

Scheme 2. Possible reaction path of the degradation of the organic modifier of the montmorillonite.

Fig. 4. ATR–FTIR spectra of Cloisite 15A (a) and 20A (b) as a function of the exposure time in nitrogen.

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Fig. 5. ATR–FTIR spectra of Cloisite 15A (a) and 20A (b) as a function of the exposure time in oxygen.

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olefins will progressively transform into carbonyl compounds that,having a higher polarity, cause a decrease of the contact angle. Themaxima under oxygen occurred earlier than those under air and, inaddition, the final value of contact angles was the lowest.

Fig. 6. Area of the bands at 1550–1900 cm−1.

For 15A, the maxima under air and oxygen were at lowertemperature than 20A, due to the different amount of modifier inthe two organo-montmorillonites. The free modifier present in 15Acan easily reach the sample surface and react with oxygen. In 20A, the

Fig. 7. Area of the bands at 2750–3050 cm−1.

Fig. 8. Reaction of 15A and 20A at 240 °C under oxygen: (a) thermogravimetric curves; (b) residual organic modifier.

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oxidation reactions may start only after the first step of α-olefinproduction and migration to the surface.

These results allowed elaborating the thermal behaviour oforgano-montmorillonites under heating. Initially, the free modifier

Fig. 9. Contact angle as a function of time fo

(if present) migrates to the sample surface, immediately followed bythe bounded modifier that underwent the Hoffman elimination. Onceon the surface, these non-polar compounds may either escape or reactwith oxygen (if present). In the latter case, carbonyl compounds are

r 15A exposed to different atmospheres.

Fig. 10. Contact angle as a function of time for 20A exposed to different atmospheres.

Fig. 11. WAXS diffraction pattern of 15A exposed to air.

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formed and an increase of the polarity of the surface is observed. Atlong reaction times, reasonably, large parts of the free modifier (ifinitially present) and of the bound modifier are degraded.

In the WAXS pattern (Fig. 11), 15A presented two main reflectionsat 3.2° and 7.1°, corresponding to basal spacings of 2.76 nm and1.25 nm (Table 2). This put into evidence that, despite the excess, theorganic modifier was not intercalated into all clay mineral particles.The broad reflection at 2θ=3.2° evidenced a strong heterogeneity, thesecond broad reflection at higher angles indicated that some particlesremained unmodified or scarcely modified as 1.25 nm is the basalspacing corresponding to the unreacted clay mineral (Sposito, 1981,1984).

After 10 min heating, basal spacing increased to 3.14 nm. Thisfeature can be explained considering that in the first stages of the

Table 2Reflections and basal spacing of 15A exposed to air at 240 °C.

Time, min 2θ, degrees Interlayer distance, nm

0 3.2 2.767.1 1,25

10 2.8 3.146.6 1.34

30 6.3 1.4160 6.5 1.35100 6.6 1.34

degradation, the volatile products tend to increase the interlayerdistance. Similar results were found by Shah and Paul (2006).

The reflection was also sharper than that of the corresponding theneat sample: evidently, the interlayer distances were more homo-geneous. The 1.25 nm-reflection remained unchanged.

After 30 min heating, the reflection at lower angles practicallydisappeared. The one at higher angles increased in intensity andbroadness due to the progressive escape of the volatile products andcollapse of the structure. This structure was no longer expandable likeanhydrous clay minerals (Shah and Paul, 2006).

At longer times, the organo-montmorillonites presented onereflection only, at around 6.5–6.6°, which became more pronouncedwith time; the corresponding basal spacing was about 1.25 nm. All thevolatile products were escaped and most of the clay mineral particlescollapsed.

4. Conclusions

Degradation of organo-montmorillonites proceeded with initialformation of α-olefins that eventually transformed into variouscarboxyl compounds. In the first steps of the degradation, theproducts caused an expansion of the interlayer spaces. On increasingthe reaction time, these compounds likely diffused toward the surfaceof the sample and eventually volatilize. This caused a collapse of theclay mineral particles that presented interlayer spacings very similarto unmodified montmorillonite. A higher amount of organic modifierincreased the degradation kinetics independently of the atmosphereused.

Ackowledgements

This work was financially supported by University of Palermo(Fondi ex 60% 2006).

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