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Effect of heating of organo-montmorillonites under different atmospheres
Applied Clay Science 45 (2009) 185–193
Contents lists available at ScienceDirect
Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y
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 b
Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Università di Palermo, Viale delle Scienze, Ed.6, 90128 Palermo, Italy Centro per l'Ingegneria delle Materie Plastiche, Politecnico di Torino sede di Alessandria, INSTM research unit, Via Teresa Michel 5, 1500 Alessandria, Italy
a r t i c l e
i n f o
Article history: Received 29 September 2008 Received in revised form 14 May 2009 Accepted 8 June 2009 Available online 13 June 2009 Keywords: Montmorillonite Organic modifier Degradation Oxidation
a b s t r a c t We studied the influence of heating on the behaviour of two organo-montmorillonites paying particular attention to the possible degradation effects of the organic modifier under different atmospheres. Hoffmann elimination and anucleophilic substitution on the nitrogen led to rapid degradation of the modifier. As confirmed by deconvoluted FTIR spectra, the presence of oxygen accelerated the degradation rate. The degradation products of the modifier (α-olefins transforming into various carboxyl compounds if oxygen is present) initially increased the basal spacing, followed by a collapse of the particle layers when the decomposition 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 the preparation of hybrids based on a polymer matrix and clays as filler. The final aim was to achieve a dispersion of the filler at a nanometric scale by the intercalation of polymer chains into the interlayer spaces or, under appropriate conditions, the total destruction of the clay mineral particles with complete exfoliation. This process should improve properties such as barrier, mechanical and thermomechanical values, without affecting other important features like transparency, 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 and hydrophobic polymer matrices can inhibit the development of nanostructures, especially when the materials are prepared in the melt. To improve the affinity between the two materials, the clays must be made organophilic by ion exchange with cationic surfactants like alkylammonium salts (Hendricks, 1941; Earnest, 1980, 1988, 1991; Ray and Okamoto, 2003). Unfortunately, alkylammonium surfactants are thermally unstable at temperatures adopted for processing the most common thermoplastic polymers. This feature can affect the exfoliation of the particles, the interface interactions and the effectiveness of additives, like compatibilizers or stabilizers. In addition, the products of the degradation reaction may cause undesired colour change, promote the degradation of the matrix and induce microcracks that reduce the mechanical resistance (Delozier et al., 2002; Fornes et al., 2003; ⁎ Corresponding author. E-mail address: [email protected] (R. Scaffaro). 0169-1317/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2009.06.002
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 the thermal non-oxidative degradation of modified montmorillonites: important degradation phenomena, according to the Hoffmann elimination (March, 1985), were observed above 155 °C. The presence of acidic sites, of the clay mineral, catalyzed the degradation of the organic modifier that occurs in three steps: the first related to the free organic modifier; the second, involving the physically absorbed organic modifier; the last, extended to the chemically bound modifier. Bellucci et al. (2007) confirmed these results. In particular, they found that the degradation of the free modifier occurred in a single step, while the bound modifier degraded in two steps at temperatures lower and higher if compared with the neat salt. This was explained considering that, in the earliest phases of the reaction, the catalysis of the clay minerals accelerates the degradation and later, the mineral clay acts like a barrier trapping the volatile degradation products and reducing the degradation kinetic. Cervantes-Uc et al. (2007) and Edwards et al. (2005) studied the degradation products of some commercially available modified clay minerals observing that the degradation mechanism and the temperature of beginning degradation were different for each sample. The degradation products were mainly water, aldehydes, carboxylic acids, various aliphatic compounds, carbon dioxide and aromatic compounds, if originally present in the modifier. Francowski et al. (2007) studied the change in the morphology of the clay mineral. Depending on the degradation conditions, they found an increase or a decrease of the basal spacing. The first situation occurred when the volatile products could not escape, thus expanding the particles. When the degradation products were easily eliminated, the particles collapsed.
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The aim of this work is to study the influence of temperature on the modifier degradation. Organo-montmorillonites with different content of the same organic modifier were exposed to different atmospheres for different times. In the view of application of these organo-bentonites in the preparation of intercalated/exfoliated nanocomposites by melt processing, it is important to understand in detail how the degradation products may interact with other additives or with the polymer matrix itself. 2. Experimental 2.1. Materials and preparations of the samples Two samples of organically modified montmorillonite (Cloisite 15A and Cloisite 20A, Southern Clay Product) were used. Both montmorillonites were modified with the quaternary ammonium salt shown in Scheme 1 (HT = hydrogenated tallow), with approximate composition (by mass) of 65% C18, 30% C16 and 5% C14. Cloisite 15A contained an excess of modifier (1.25 meq/g) while Cloisite 20A contained about the CEC amount (0.95 meq/g). Cylindrical samples of both organo-montmorillonites were prepared by compressing about 3 g of powder at 150 bar and room temperature for about 10 s as described before (Scaffaro et al., 2008). Organo-montmorillonite samples were exposed to air, oxygen and nitrogen at 240 °C for 5–300 min. 2.2. Characterization The surface of the heated samples was analyzed by an AutoImage FTIR microscope Perkin Elmer equipped with a Micro-ATR objective. To get more information on the characteristics of some parts of the spectrum, some spectra were deconvoluted using an appropriate software. Contact angle measurements were performed on a FTA 1000, (TenAngstrom) equipment using deionized water as liquid. Wide angle X-ray diffraction (WAXD) patterns were recorded using a Philips 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 of the organic modifier were observed: the symmetric and asymmetric stretching vibration of CH2 at 2848 cm− 1 and 2920 cm− 1, the bending vibration of CH2 at 1475 cm− 1 (shoulder) and the C–N stretching vibration at 1468 cm− 1. 15A presented a higher intensity of the methylene group vibrations (2848 cm− 1, 2920 cm− 1 and 1475 cm− 1) confirming that this sample contained a higher amount of modifier. The spectra as a function of exposure time in air are reported in Fig. 2. After 5 min, there were no significant variations on the surface of both 15A and 20A but new absorption bands appeared after 10 min in the region 1600–1900 cm− 1. In this region the absorption bands of carbonyl groups like 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 materials exposed to air at three different times. After 5 min, no significant changes in the spectra were observed while after 10 min several new peaks appeared. For both samples, a
Fig. 1. ATR–FTIR of the two modified montmorillonites.
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 the carbonyl region and the other absorption bands are related to different compounds. The band around 1640 cm− 1 was assigned to C_C (Geuskens and Kabama, 1982; Rjieb et al., 2000), of α-olefins formed by the Hoffman reaction, overlapping the bending vibration of structural –OH groups (Francowski et al., 2007). At 300 min, the intensity of the absorption bands in the carbonyl region was higher, and higher for 15A than for 20A, reasonably due to the higher amount of modifier of the former. All the bands were broad, and some of them were very close to each other thus evidencing a great heterogeneity of the degradation products. Some of these compounds could 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 peak around 1737 cm− 1 (20A) to ester/aldehydes, the peak around 1772 cm− 1 was assigned to periesters. The appearance of these compounds after longer times was also observed by Cervantes-Uc et al. (2007). The organic modifier undergoes the Hoffman elimination consisting in the formation of a tertiary amine and free α-olefins. In the presence of oxygen, the free olefins can form various carboxyl compounds (Cervantes-Uc et al., 2007); a possible reaction mechanism is reported in Scheme 2. Concurrently, the progressive decrease of the methylene and of the C–N absorption bands was observed. This is consistent with the above stated hypotheses of the progressive conversion of the modifier into carbonyl compounds and the eventual escape of the low molecular mass volatile compounds from the sample surface. To better understand the role of oxygen in this degradation process, the organo-montmorillonite samples were exposed to pure oxygen and pure nitrogen (Figs. 4 and 5). Heating the organomontmorillonite in nitrogen, the spectra remained unchanged. In the presence of oxygen, a broad band appeared in the region 1600– 1900 cm− 1, even after 5 min, and became more pronounced at higher testing times due to the faster degradation. It is worth noting that the temperature of 240 °C is in the range of the typical processing temperatures of several thermoplastic polymers. At high testing times, the peak assigned to the symmetric and asymmetric stretching vibration bands of CH2 at 2848 cm− 1 and Table 1 Peak assignments of the spectra reported in Fig. 1.
Scheme 1. Quaternary ammonium salt used for the modification of the montmorillonite.
1012 cm− 1 912 cm− 1 2848 cm− 1, 2920 cm− 1 1475 cm− 1 1468 cm− 1 3616 cm− 1, 1636 cm− 1
Stretching vibration Si–O–Si Stretching vibration Si–O–M (M = Al, Mg, Fe) Symmetric, asymmetric stretching vibration of CH2 Bending vibration of CH2 Stretching vibration of C–N Bending, stretching vibration OH
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Fig. 2. ATR–FTIR spectra of Cloisite 15A (a) and 20A (b) as a function of the exposure time in air.
2920 cm− 1, to the bending vibration band of CH2 at 1475 cm− 1 and to the C–N stretching vibration band at 1468 cm− 1, Fig. 5, practically disappeared indicating the complete decomposition of the modifier. The area of the region 1550–1900 cm− 1, which includes the carbonyl region, and the area of the region 2750–3050 cm− 1 as a function of time are shown in Figs. 6 and 7. The absorption band areas did not change in nitrogen while, in the presence of oxygen, the area between 1550 and 1900 cm− 1 increased and that between 2750 and 3050 cm− 1 decreased. Both organo-montmorillonites showed an induction time for degradation, about 5 min for 15A and about 30 min for 20A due to the different modifier content. Considering that 20A has an approximately stoichiometric amount of modifier, there is reasonably a very scarce amount of free modifier and 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 extent and the band area. In oxygen, 15A presented no induction time for degradation or, at least, less than 5 min; 20A a time of about 5 min. As expected, the oxidative processes were more efficient in the presence of pure
oxygen and consequently the degradation rate was faster for both organo-montmorillonites. In contrast to 15A, the band area of 20A did not show a maximum: due to the different modifier amount, this phenomenon could be reasonably observed only at longer times. The mass loss curves in oxygen (Fig. 8) again indicated that the free organic modifier undergoes a fast degradation/volatilization. Considering that the mass loss of unmodified montmorillonite was negligible (at 240 °C up to 300 min), the mass loss of the organomontmorillonite corresponded to the content of the organic modifier. Therefore, the amount of the remnant modifier could be evaluated as a function of the oxidation time (Fig. 8b), considering the initial amounts of organic modifier (43% by mass for 15A, 38% by mass for 20A). As expected, 15A lost the modifier more rapidly than 20A but, at longer times, the slope of the curves became similar. In both samples about 30% of modifier was lost after 300 min. The contact angle (Figs. 9 and 10) initially increased. One reason may be the removal of surface humidity but the difference between the two organo-montmorillonites suggested some other mechanisms involving the migration of non-polar compounds to the surface. For 15A, these non-polar compounds could be molecules of the excess of
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Fig. 3. Deconvoluted spectra in the region 1900–1550 cm− 1 for 15A (a–d) and 20 A (e–h) exposed to air.
the modifier. However, the increase of the contact angle observed also for 20A suggested that another mechanism must also be taken into account. In particular, due to the initial and non-oxidative degradation step (Scheme 2) some α-olefins are formed. The migration of these compounds to the surface can be considered responsible of the increase of the contact angle in the first minutes of treatment. In nitrogen, no oxidative degradation may occur but only thermal degradation 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 could proceed. At longer times, all the curves presented maxima at lower temperatures passing from nitrogen to air to oxygen. The presence of these maxima can be explained considering the degradation path proposed in Scheme 2. Under nitrogen, only α-olefins are formed as degradation products. These compounds migrate to the surface, reduce the polarity and eventually escape as volatile products. At long times, as fewer molecules are present on the surface the initial value of the contact angle is approached. Under air and oxygen, the α-
R. Scaffaro et al. / Applied Clay Science 45 (2009) 185–193
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.
olefins will progressively transform into carbonyl compounds that, having a higher polarity, cause a decrease of the contact angle. The maxima under oxygen occurred earlier than those under air and, in addition, the final value of contact angles was the lowest.
For 15A, the maxima under air and oxygen were at lower temperature than 20A, due to the different amount of modifier in the two organo-montmorillonites. The free modifier present in 15A can easily reach the sample surface and react with oxygen. In 20A, the
Fig. 6. Area of the bands at 1550–1900 cm− 1.
Fig. 7. Area of the bands at 2750–3050 cm− 1.
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Fig. 8. Reaction of 15A and 20A at 240 °C under oxygen: (a) thermogravimetric curves; (b) residual organic modifier.
oxidation reactions may start only after the first step of α-olefin production and migration to the surface. These results allowed elaborating the thermal behaviour of organo-montmorillonites under heating. Initially, the free modifier
(if present) migrates to the sample surface, immediately followed by the bounded modifier that underwent the Hoffman elimination. Once on the surface, these non-polar compounds may either escape or react with oxygen (if present). In the latter case, carbonyl compounds are
Fig. 9. Contact angle as a function of time for 15A exposed to different atmospheres.
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Fig. 10. Contact angle as a function of time for 20A exposed to different atmospheres.
degradation, the volatile products tend to increase the interlayer distance. Similar results were found by Shah and Paul (2006). The reflection was also sharper than that of the corresponding the neat sample: evidently, the interlayer distances were more homogeneous. The 1.25 nm-reflection remained unchanged. After 30 min heating, the reflection at lower angles practically disappeared. The one at higher angles increased in intensity and broadness due to the progressive escape of the volatile products and collapse of the structure. This structure was no longer expandable like anhydrous clay minerals (Shah and Paul, 2006). At longer times, the organo-montmorillonites presented one reflection only, at around 6.5–6.6°, which became more pronounced with time; the corresponding basal spacing was about 1.25 nm. All the volatile products were escaped and most of the clay mineral particles collapsed. 4. Conclusions Fig. 11. WAXS diffraction pattern of 15A exposed to air.
formed and an increase of the polarity of the surface is observed. At long reaction times, reasonably, large parts of the free modifier (if initially present) and of the bound modifier are degraded. In the WAXS pattern (Fig. 11), 15A presented two main reflections at 3.2° and 7.1°, corresponding to basal spacings of 2.76 nm and 1.25 nm (Table 2). This put into evidence that, despite the excess, the organic modifier was not intercalated into all clay mineral particles. The broad reflection at 2θ = 3.2° evidenced a strong heterogeneity, the second broad reflection at higher angles indicated that some particles remained unmodified or scarcely modified as 1.25 nm is the basal spacing corresponding to the unreacted clay mineral (Sposito, 1981, 1984). After 10 min heating, basal spacing increased to 3.14 nm. This feature can be explained considering that in the first stages of the
Degradation of organo-montmorillonites proceeded with initial formation of α-olefins that eventually transformed into various carboxyl compounds. In the first steps of the degradation, the products caused an expansion of the interlayer spaces. On increasing the reaction time, these compounds likely diffused toward the surface of the sample and eventually volatilize. This caused a collapse of the clay mineral particles that presented interlayer spacings very similar to unmodified montmorillonite. A higher amount of organic modifier increased the degradation kinetics independently of the atmosphere used. Ackowledgements This work was financially supported by University of Palermo (Fondi ex 60% 2006). References
Table 2 Reflections and basal spacing of 15A exposed to air at 240 °C. Time, min
2θ, degrees
Interlayer distance, nm
0
3.2 7.1 2.8 6.6 6.3 6.5 6.6
2.76 1,25 3.14 1.34 1.41 1.35 1.34
10 30 60 100
Bellucci, F., Camino, G., Frache, A., Sarra, A., 2007. Catalytic charring–volatilization competition in organoclay nanocomposites. Polym. Deg. Stab. 92 (3), 425–436. Cervantes-Uc, J.M., Cauich-Rodriguez, J.V., Vásquez-Torres, H., Garfias-Mesías, L.F., Paul, D.R., 2007. Thermal degradation of commercially available organoclays studied by TGA–FTIR. Thermoch. Acta 457 (1–2), 92–102. Delozier, D.M., Orwoll, R.A., Caboon, J.F., Johnston, N.U., Smith, J.G., Connell, J.W., 2002. Preparation and characterization of polyimide/organoclay nanocomposites. Polymer 43 (3), 813–822. Earnest C.M., 1980. Perkin-Elmer Thermal Analysis Application Study, 31, Pt. 1. Earnest, C.M., 1988. Composition analysis by thermogravimetry, ASTM STP 497. ASTM, Philadelphia, PA. 272. Earnest, C.M., 1991. Thermal analysis in geosciences. Springer, Berlin. 288.
R. Scaffaro et al. / Applied Clay Science 45 (2009) 185–193 Edwards, G., Halley, P., Kerven, G., Martin, D., 2005. Thermal stability analysis of organosilicates, using solid phase microextraction techniques. Thermoch. Acta 429 (1), 13–18. Fornes, T.D., Yoon, P.J., Paul, D.R., 2003. Polymer matrix degradation and color formation in melt processed nylon 6/clay nanocomposites. Polymer 44 (24), 7545–7556. Francowski, D.J., Capracotta, M.D., Martin, J.D., Khan, S.A, Spontak, R.J., 2007. Stability of organically modified montmorillonites and their polystyrene nanocomposites after prolonged thermal treatment. Chem. Mater. 19 (11), 2757–2767. Geuskens, G., Kabama, M.S., 1982. Photo-oxidation of polymers—part V: a new chain scission mechanism in polyolefins. Polym. Deg. Stab. 4 (1), 69–76. He, H., Duchet, J., Galy, J., Gerard, J.F., 2006. Grafting of swelling clay materials with 3aminopropyltriethoxysilane. J. Coll. Int. Sci. 228 (1), 171–176. Hendricks, S.B., 1941. Base exchange of the clay mineral montmorillonite for organic cations and its dependence upon adsorption due to van der waals forces. J. Phys. Chem. 45, 65. Kojima, Y., Usuki, A., Kawasumi, M., Okada, A., Kurauchi, T., Kamigaito, O., 1985. J. Mater. Res. 8. Lacoste, J., Vaillant, D., Carlsson, D.J., 1993. Gamma-, photo-, and thermally-initiated oxidation of isotactic polypropylene. J. Polym. Sci. 31 (3), 715–722. Lan, T., Pinnavaia, T.J., 1994. Clay-reinforced epoxy nanocomposites. Chem. Mater. 6 (5), 2216–2219. March, J., 1985. Advanced organic chemistry, reactions, mechanisms and structures, 3rd ed. John Wiley and Sons, New York, pp. 387–906. Messersmith, P.B., Giannelis, E.P., 1994. Synthesis and characterization of layered silicate–epoxy nanocomposites. Chem. Mater. 6 (10), 1719–1725. Okada A., Fukushima Y., Kawasumi M., Inagaki S., Usuki A., Sugiyama S., Kurauchi T., Kamigaito O., 1988. United States Patent No 4 739, 007.
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Contents lists available at ScienceDirect
Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y
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 b
Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Università di Palermo, Viale delle Scienze, Ed.6, 90128 Palermo, Italy Centro per l'Ingegneria delle Materie Plastiche, Politecnico di Torino sede di Alessandria, INSTM research unit, Via Teresa Michel 5, 1500 Alessandria, Italy
a r t i c l e
i n f o
Article history: Received 29 September 2008 Received in revised form 14 May 2009 Accepted 8 June 2009 Available online 13 June 2009 Keywords: Montmorillonite Organic modifier Degradation Oxidation
a b s t r a c t We studied the influence of heating on the behaviour of two organo-montmorillonites paying particular attention to the possible degradation effects of the organic modifier under different atmospheres. Hoffmann elimination and anucleophilic substitution on the nitrogen led to rapid degradation of the modifier. As confirmed by deconvoluted FTIR spectra, the presence of oxygen accelerated the degradation rate. The degradation products of the modifier (α-olefins transforming into various carboxyl compounds if oxygen is present) initially increased the basal spacing, followed by a collapse of the particle layers when the decomposition 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 the preparation of hybrids based on a polymer matrix and clays as filler. The final aim was to achieve a dispersion of the filler at a nanometric scale by the intercalation of polymer chains into the interlayer spaces or, under appropriate conditions, the total destruction of the clay mineral particles with complete exfoliation. This process should improve properties such as barrier, mechanical and thermomechanical values, without affecting other important features like transparency, 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 and hydrophobic polymer matrices can inhibit the development of nanostructures, especially when the materials are prepared in the melt. To improve the affinity between the two materials, the clays must be made organophilic by ion exchange with cationic surfactants like alkylammonium salts (Hendricks, 1941; Earnest, 1980, 1988, 1991; Ray and Okamoto, 2003). Unfortunately, alkylammonium surfactants are thermally unstable at temperatures adopted for processing the most common thermoplastic polymers. This feature can affect the exfoliation of the particles, the interface interactions and the effectiveness of additives, like compatibilizers or stabilizers. In addition, the products of the degradation reaction may cause undesired colour change, promote the degradation of the matrix and induce microcracks that reduce the mechanical resistance (Delozier et al., 2002; Fornes et al., 2003; ⁎ Corresponding author. E-mail address: [email protected] (R. Scaffaro). 0169-1317/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2009.06.002
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 the thermal non-oxidative degradation of modified montmorillonites: important degradation phenomena, according to the Hoffmann elimination (March, 1985), were observed above 155 °C. The presence of acidic sites, of the clay mineral, catalyzed the degradation of the organic modifier that occurs in three steps: the first related to the free organic modifier; the second, involving the physically absorbed organic modifier; the last, extended to the chemically bound modifier. Bellucci et al. (2007) confirmed these results. In particular, they found that the degradation of the free modifier occurred in a single step, while the bound modifier degraded in two steps at temperatures lower and higher if compared with the neat salt. This was explained considering that, in the earliest phases of the reaction, the catalysis of the clay minerals accelerates the degradation and later, the mineral clay acts like a barrier trapping the volatile degradation products and reducing the degradation kinetic. Cervantes-Uc et al. (2007) and Edwards et al. (2005) studied the degradation products of some commercially available modified clay minerals observing that the degradation mechanism and the temperature of beginning degradation were different for each sample. The degradation products were mainly water, aldehydes, carboxylic acids, various aliphatic compounds, carbon dioxide and aromatic compounds, if originally present in the modifier. Francowski et al. (2007) studied the change in the morphology of the clay mineral. Depending on the degradation conditions, they found an increase or a decrease of the basal spacing. The first situation occurred when the volatile products could not escape, thus expanding the particles. When the degradation products were easily eliminated, the particles collapsed.
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The aim of this work is to study the influence of temperature on the modifier degradation. Organo-montmorillonites with different content of the same organic modifier were exposed to different atmospheres for different times. In the view of application of these organo-bentonites in the preparation of intercalated/exfoliated nanocomposites by melt processing, it is important to understand in detail how the degradation products may interact with other additives or with the polymer matrix itself. 2. Experimental 2.1. Materials and preparations of the samples Two samples of organically modified montmorillonite (Cloisite 15A and Cloisite 20A, Southern Clay Product) were used. Both montmorillonites were modified with the quaternary ammonium salt shown in Scheme 1 (HT = hydrogenated tallow), with approximate composition (by mass) of 65% C18, 30% C16 and 5% C14. Cloisite 15A contained an excess of modifier (1.25 meq/g) while Cloisite 20A contained about the CEC amount (0.95 meq/g). Cylindrical samples of both organo-montmorillonites were prepared by compressing about 3 g of powder at 150 bar and room temperature for about 10 s as described before (Scaffaro et al., 2008). Organo-montmorillonite samples were exposed to air, oxygen and nitrogen at 240 °C for 5–300 min. 2.2. Characterization The surface of the heated samples was analyzed by an AutoImage FTIR microscope Perkin Elmer equipped with a Micro-ATR objective. To get more information on the characteristics of some parts of the spectrum, some spectra were deconvoluted using an appropriate software. Contact angle measurements were performed on a FTA 1000, (TenAngstrom) equipment using deionized water as liquid. Wide angle X-ray diffraction (WAXD) patterns were recorded using a Philips 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 of the organic modifier were observed: the symmetric and asymmetric stretching vibration of CH2 at 2848 cm− 1 and 2920 cm− 1, the bending vibration of CH2 at 1475 cm− 1 (shoulder) and the C–N stretching vibration at 1468 cm− 1. 15A presented a higher intensity of the methylene group vibrations (2848 cm− 1, 2920 cm− 1 and 1475 cm− 1) confirming that this sample contained a higher amount of modifier. The spectra as a function of exposure time in air are reported in Fig. 2. After 5 min, there were no significant variations on the surface of both 15A and 20A but new absorption bands appeared after 10 min in the region 1600–1900 cm− 1. In this region the absorption bands of carbonyl groups like 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 materials exposed to air at three different times. After 5 min, no significant changes in the spectra were observed while after 10 min several new peaks appeared. For both samples, a
Fig. 1. ATR–FTIR of the two modified montmorillonites.
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 the carbonyl region and the other absorption bands are related to different compounds. The band around 1640 cm− 1 was assigned to C_C (Geuskens and Kabama, 1982; Rjieb et al., 2000), of α-olefins formed by the Hoffman reaction, overlapping the bending vibration of structural –OH groups (Francowski et al., 2007). At 300 min, the intensity of the absorption bands in the carbonyl region was higher, and higher for 15A than for 20A, reasonably due to the higher amount of modifier of the former. All the bands were broad, and some of them were very close to each other thus evidencing a great heterogeneity of the degradation products. Some of these compounds could 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 peak around 1737 cm− 1 (20A) to ester/aldehydes, the peak around 1772 cm− 1 was assigned to periesters. The appearance of these compounds after longer times was also observed by Cervantes-Uc et al. (2007). The organic modifier undergoes the Hoffman elimination consisting in the formation of a tertiary amine and free α-olefins. In the presence of oxygen, the free olefins can form various carboxyl compounds (Cervantes-Uc et al., 2007); a possible reaction mechanism is reported in Scheme 2. Concurrently, the progressive decrease of the methylene and of the C–N absorption bands was observed. This is consistent with the above stated hypotheses of the progressive conversion of the modifier into carbonyl compounds and the eventual escape of the low molecular mass volatile compounds from the sample surface. To better understand the role of oxygen in this degradation process, the organo-montmorillonite samples were exposed to pure oxygen and pure nitrogen (Figs. 4 and 5). Heating the organomontmorillonite in nitrogen, the spectra remained unchanged. In the presence of oxygen, a broad band appeared in the region 1600– 1900 cm− 1, even after 5 min, and became more pronounced at higher testing times due to the faster degradation. It is worth noting that the temperature of 240 °C is in the range of the typical processing temperatures of several thermoplastic polymers. At high testing times, the peak assigned to the symmetric and asymmetric stretching vibration bands of CH2 at 2848 cm− 1 and Table 1 Peak assignments of the spectra reported in Fig. 1.
Scheme 1. Quaternary ammonium salt used for the modification of the montmorillonite.
1012 cm− 1 912 cm− 1 2848 cm− 1, 2920 cm− 1 1475 cm− 1 1468 cm− 1 3616 cm− 1, 1636 cm− 1
Stretching vibration Si–O–Si Stretching vibration Si–O–M (M = Al, Mg, Fe) Symmetric, asymmetric stretching vibration of CH2 Bending vibration of CH2 Stretching vibration of C–N Bending, stretching vibration OH
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Fig. 2. ATR–FTIR spectra of Cloisite 15A (a) and 20A (b) as a function of the exposure time in air.
2920 cm− 1, to the bending vibration band of CH2 at 1475 cm− 1 and to the C–N stretching vibration band at 1468 cm− 1, Fig. 5, practically disappeared indicating the complete decomposition of the modifier. The area of the region 1550–1900 cm− 1, which includes the carbonyl region, and the area of the region 2750–3050 cm− 1 as a function of time are shown in Figs. 6 and 7. The absorption band areas did not change in nitrogen while, in the presence of oxygen, the area between 1550 and 1900 cm− 1 increased and that between 2750 and 3050 cm− 1 decreased. Both organo-montmorillonites showed an induction time for degradation, about 5 min for 15A and about 30 min for 20A due to the different modifier content. Considering that 20A has an approximately stoichiometric amount of modifier, there is reasonably a very scarce amount of free modifier and 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 extent and the band area. In oxygen, 15A presented no induction time for degradation or, at least, less than 5 min; 20A a time of about 5 min. As expected, the oxidative processes were more efficient in the presence of pure
oxygen and consequently the degradation rate was faster for both organo-montmorillonites. In contrast to 15A, the band area of 20A did not show a maximum: due to the different modifier amount, this phenomenon could be reasonably observed only at longer times. The mass loss curves in oxygen (Fig. 8) again indicated that the free organic modifier undergoes a fast degradation/volatilization. Considering that the mass loss of unmodified montmorillonite was negligible (at 240 °C up to 300 min), the mass loss of the organomontmorillonite corresponded to the content of the organic modifier. Therefore, the amount of the remnant modifier could be evaluated as a function of the oxidation time (Fig. 8b), considering the initial amounts of organic modifier (43% by mass for 15A, 38% by mass for 20A). As expected, 15A lost the modifier more rapidly than 20A but, at longer times, the slope of the curves became similar. In both samples about 30% of modifier was lost after 300 min. The contact angle (Figs. 9 and 10) initially increased. One reason may be the removal of surface humidity but the difference between the two organo-montmorillonites suggested some other mechanisms involving the migration of non-polar compounds to the surface. For 15A, these non-polar compounds could be molecules of the excess of
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Fig. 3. Deconvoluted spectra in the region 1900–1550 cm− 1 for 15A (a–d) and 20 A (e–h) exposed to air.
the modifier. However, the increase of the contact angle observed also for 20A suggested that another mechanism must also be taken into account. In particular, due to the initial and non-oxidative degradation step (Scheme 2) some α-olefins are formed. The migration of these compounds to the surface can be considered responsible of the increase of the contact angle in the first minutes of treatment. In nitrogen, no oxidative degradation may occur but only thermal degradation 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 could proceed. At longer times, all the curves presented maxima at lower temperatures passing from nitrogen to air to oxygen. The presence of these maxima can be explained considering the degradation path proposed in Scheme 2. Under nitrogen, only α-olefins are formed as degradation products. These compounds migrate to the surface, reduce the polarity and eventually escape as volatile products. At long times, as fewer molecules are present on the surface the initial value of the contact angle is approached. Under air and oxygen, the α-
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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.
olefins will progressively transform into carbonyl compounds that, having a higher polarity, cause a decrease of the contact angle. The maxima under oxygen occurred earlier than those under air and, in addition, the final value of contact angles was the lowest.
For 15A, the maxima under air and oxygen were at lower temperature than 20A, due to the different amount of modifier in the two organo-montmorillonites. The free modifier present in 15A can easily reach the sample surface and react with oxygen. In 20A, the
Fig. 6. Area of the bands at 1550–1900 cm− 1.
Fig. 7. Area of the bands at 2750–3050 cm− 1.
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Fig. 8. Reaction of 15A and 20A at 240 °C under oxygen: (a) thermogravimetric curves; (b) residual organic modifier.
oxidation reactions may start only after the first step of α-olefin production and migration to the surface. These results allowed elaborating the thermal behaviour of organo-montmorillonites under heating. Initially, the free modifier
(if present) migrates to the sample surface, immediately followed by the bounded modifier that underwent the Hoffman elimination. Once on the surface, these non-polar compounds may either escape or react with oxygen (if present). In the latter case, carbonyl compounds are
Fig. 9. Contact angle as a function of time for 15A exposed to different atmospheres.
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Fig. 10. Contact angle as a function of time for 20A exposed to different atmospheres.
degradation, the volatile products tend to increase the interlayer distance. Similar results were found by Shah and Paul (2006). The reflection was also sharper than that of the corresponding the neat sample: evidently, the interlayer distances were more homogeneous. The 1.25 nm-reflection remained unchanged. After 30 min heating, the reflection at lower angles practically disappeared. The one at higher angles increased in intensity and broadness due to the progressive escape of the volatile products and collapse of the structure. This structure was no longer expandable like anhydrous clay minerals (Shah and Paul, 2006). At longer times, the organo-montmorillonites presented one reflection only, at around 6.5–6.6°, which became more pronounced with time; the corresponding basal spacing was about 1.25 nm. All the volatile products were escaped and most of the clay mineral particles collapsed. 4. Conclusions Fig. 11. WAXS diffraction pattern of 15A exposed to air.
formed and an increase of the polarity of the surface is observed. At long reaction times, reasonably, large parts of the free modifier (if initially present) and of the bound modifier are degraded. In the WAXS pattern (Fig. 11), 15A presented two main reflections at 3.2° and 7.1°, corresponding to basal spacings of 2.76 nm and 1.25 nm (Table 2). This put into evidence that, despite the excess, the organic modifier was not intercalated into all clay mineral particles. The broad reflection at 2θ = 3.2° evidenced a strong heterogeneity, the second broad reflection at higher angles indicated that some particles remained unmodified or scarcely modified as 1.25 nm is the basal spacing corresponding to the unreacted clay mineral (Sposito, 1981, 1984). After 10 min heating, basal spacing increased to 3.14 nm. This feature can be explained considering that in the first stages of the
Degradation of organo-montmorillonites proceeded with initial formation of α-olefins that eventually transformed into various carboxyl compounds. In the first steps of the degradation, the products caused an expansion of the interlayer spaces. On increasing the reaction time, these compounds likely diffused toward the surface of the sample and eventually volatilize. This caused a collapse of the clay mineral particles that presented interlayer spacings very similar to unmodified montmorillonite. A higher amount of organic modifier increased the degradation kinetics independently of the atmosphere used. Ackowledgements This work was financially supported by University of Palermo (Fondi ex 60% 2006). References
Table 2 Reflections and basal spacing of 15A exposed to air at 240 °C. Time, min
2θ, degrees
Interlayer distance, nm
0
3.2 7.1 2.8 6.6 6.3 6.5 6.6
2.76 1,25 3.14 1.34 1.41 1.35 1.34
10 30 60 100
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