Organo-montmorillonite barrier layers formed by combustion: Nanostructure and permeability

Organo-montmorillonite barrier layers formed by combustion: Nanostructure and permeability

Applied Clay Science 49 (2010) 213–223 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 e...

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Applied Clay Science 49 (2010) 213–223

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

Organo-montmorillonite barrier layers formed by combustion: Nanostructure and permeability James B. Fox a,1, Preejith V. Ambuken a,1, Holly A. Stretz a,⁎, Roberta A. Peascoe b, E. Andrew Payzant c a b c

Department of Chemical Engineering, Tennessee Technological University, 1020 Stadium Drive, Box 5013, Cookeville, TN, 38505-0001, United States Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996-2200, United States Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN, 37831-6062, United States

a r t i c l e

i n f o

Article history: Received 3 September 2009 Received in revised form 13 May 2010 Accepted 19 May 2010 Available online 31 May 2010 Keywords: Montmorillonite High temperature X-ray diffraction Permeability Organo-montmorillonite

a b s t r a c t Self-assembly of nanoparticles into barrier layers has been the most cited theoretical explanation for the significant reduction in flammability often noted for polymer/montmorillonite nanocomposites. Both mass and heat transport reductions have been credited for such improvements, and in most cases a coupled mechanism is expected. To provide validation for early transport models, the structure of model barrier layers was investigated, these being produced by combustion of a homologous series of organo-montmorillonites. One model barrier layer was subjected to novel permeability analysis to obtain a flux, which will be useful in the evaluation of transport models. The effects of compatibilizer structure, temperature and pressure on barrier layer structure were examined. XRD versus TGA results suggest that the onset of chemical degradation and the onset of physical collapse on heating are correlated. Addition of pressure as low as 7 kPa affected the onset of structural collapse; for the case of a “two-tailed” dimethyl dialkyl quaternary ammonium ion compatibilized organo-montmorillonite this meant expansion of the basal spacing rather than the expected densification. Permeability of Ar through the ash was found to be a sensitive measure of structural change of high aspect ratio MMT nanoparticles. Actual fluxes ranged from 0.139 to 0.151 mol (m2 s)−1 for 0.5 mm thick samples. © 2010 Elsevier B.V. All rights reserved.

1. Introduction The incorporation of organo-montmorillonites into nylon-6 and polyolefin-based materials has been shown to improve the fire performance of such materials (Gilman et al., 1997; Qin et al., 2005). This high temperature performance has been attributed to the formation of a layered assembly of high aspect ratio nanoparticles at the surface of the part (Gilman et al., 2006, 2000; Jang and Wilkie, 2005; Kashiwagi et al., 2004; Paul and Robeson, 2008), referred to as the “barrier layer.” Some researchers have suggested that the process of assembly involves migration of the filler particles to the surface of the melt (Gilman et al., 2006; Kashiwagi et al., 2004; Lewin et al., 2005, 2006; Tang and Lewin, 2007; Tang et al., 2006), though a traditional view holds that the filler particles simply compact (or ablate) as organic components volatilize away (Bocchini et al., 2006; Gilman et al., 2006). In all cases, however, the montmorillonite (MMT) particles

⁎ Corresponding author. Tel.: + 1 931 372 3495; fax: + 1 931 372 6352. E-mail addresses: [email protected] (H.A. Stretz), [email protected] (R.A. Peascoe), [email protected] (E.A. Payzant). 1 Tel.: + 1 931 372 3495; fax: + 1 931 372 6352. 0169-1317/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2010.05.009

do appear to assemble at the surface of the part, exhibiting new structural orders, with characteristic peaks appearing in the X-ray diffraction scan. Barrier layers formed from different MMT-based nanocomposites, however, produce a variety of X-ray diffraction signatures on combustion/pyrolysis. The overall objective of the following research was to identify and characterize a few of these structural variations as “model” barrier layers for future fundamental studies of mass and heat transport properties. To this end, the structures discussed were formed at high temperature from pristine organoclays, rather than formation from nanocomposites, where morphological heterogeneities exist at multiple scales and are difficult to control. For instance, MMT in a multi-phase polymer-based melt can migrate and concentrate at interfaces (micron-scale heterogeneity) (Fenouillot et al., 2009; Koo et al., 2003; Lee et al., 2005; Stretz and Paul, 2005), and MMT-based char is known to crack and form visible islands (Castrovinci and Camino, 2007; Kashiwagi et al., 2004). This report will describe only barrier layers formed from organo-montmorillonites, thereby isolating the formation of only nano-scale morphologies of interest. On what basis might the developing barrier layer serve to improve flame retardance of the composite? Several proposed mechanisms have been discussed in recent literature, and these results have been thoroughly reviewed by Leszczynska et al. (2007) These mechanisms

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Table 1 Materials description. Organoclay designation

Quaternary ammonium ion

Commercial designation

CECa (meq/100 g)

Percent loss on Ignition (TGA)

d001 spacing at room temp. (Å)

M3(HT) M2(HT)2 M(HT)3 M2(Alk)2

Trimethyl, hydrogenated tallow Dimethyl, di-hydrogenated tallow Methyl, tri-hydrogenated tallow Dimethyldialkyl ammonium halide 70% C18, 26% C16, 4%C14b

Experimental Cloisite 20A Experimental Nanomer I-44P

95 95 95 145c

29 39 49 38

19.0 26.1 36.8 27.8

a b c

CEC = cation exchange capacity, the milliequivalents of charge on the surface. Goodarzi V, Jafari S-H, Khonakdar HA, Monemian S-A, Hassler R, Jehnichen D. Journal of Polymer Science B 2009;47(7):674–684. Rohlmann CO, Horst MF, Quinzani LM, Failla MD. European Polymer Journal 2008; 44:2749–2760.

may be classified as either limiting mass transfer theories or limiting heat transfer theories. Regarding limiting mass transfer, the flux of volatilized fuel is significantly reduced (encountering a tortuous path of clay platelets) compared to the rate of buildup of carbonaceous char in the barrier layer (Lyon, 1998; Stretz et al., 2005). Indeed, for some polymers the rate of char production might be increased by catalysis at the clay surface. (Song et al., 2007) Jang and Wilkie (2005) have reported evidence that the limiting mass transfer mechanism is important for polymers which produce a stable radical on degradation. Others have reported that mass transfer is limiting when oxygen diffusion to the inner condensed phase is reduced, important in polypropylene (PP) for instance (Fina et al., 2008; Grassie and Scott, 1985). Pastore et al. (2004) have verified that the barrier layer structure for poly (ethylene-co-vinyl acetate)/MMT nanocomposites differs if the composite is degraded in air versus nitrogen. If limiting mass transfer is an important mechanism, one might assume that high aspect ratio of the MMT particles would be a relevant structural factor in determining flammability. A second group of mechanisms could be described as limiting heat transfer theories. Some authors have discussed the possibility that the thermal conductivity in the barrier layer was low (Lewin et al., 2007; Zhang et al., 2008). However, Mohaddespour et al. (2008) have shown that for high density polyethylene and PP the thermal conductivity of the nanocomposite increased versus the virgin polymer. A similar finding was reported by Zhao et al. (2009) in nylon-based nanocomposites when the MMT was at high loadings or was intercalated. Therefore, if limiting thermal conductivity is noted in some cases, this would likely be due to the carbonaceous char buildup, and not due to the clay mineral itself. It has also been suggested that the surface layer insulates the polymer from the external radiant flux, changing the mode of transmission from thermal conduction to radiative transfer (Bocchini et al., 2007). This mechanism is most closely associated with carbon nanotube-based nanocomposites. Catalysis at the surface has another contribution to the energy balance in that it would lead to a localized increase in the rate of endothermic pyrolysis, effectively cooling the surface. The contribution of a barrier layer towards thermal protection is not yet well understood, though significant changes in the temperature profile of the degrading nanocomposite have certainly been observed (Koo et al., 2004; Stretz et al., 2004). Further, it is difficult to separate the contributions of mass transfer versus heat transfer limitations for a given nanocomposite experimentally, particularly if the mass and heat transfer reductions are coupled. To help resolve this issue, Stoliarov and Lyon (2008) have reported a computational model involving both contributions. Interestingly, application of nanocomposites in other high temperature material fields such as ablation is also seeing early computational model development (Ho et al., 2008). If there is an optimal barrier layer structure for limiting transport, what variables affect the formation of structure of this layer under combustion or pyrolytic conditions? The present work was aimed at developing model barrier layers for further study and subsequently examining in at least one case the effect of layer structure on mass

transport. The controlled production of these model barrier layers was examined by varying organo-montmorillonite structure and process pressure while characterizing structural changes after an isothermal equilibration at high temperatures. Thin samples of pristine organomontmorillonite were used in this study (rather than nanocomposites). The MMT loading was initially high, and the MMT particles were quite crowded throughout the process of assembly. Therefore, the possibility of net migration of MMT platelets or formation of gross cracks in the bulk sample was limited and monolithic model layers were ensured. Changes in ceramic layer nanostructure were characterized using real-time high temperature X-ray diffraction (HTXRD) and thermal analysis. The real-time XRD was an important tool to protect the formed fragile ash samples from alteration arising from a manual transfer-type procedure. Select model “barrier layers” were then subjected to permeability analysis and the effects of the ash structure on the diffusion of a model gas, Ar, are discussed. 2. Experimental Four different organo-montmorillonites were used. M(HT)3, M2 (HT)2 (Cloisite 20A), and M3HT were obtained from Southern Clay Products, and M2(Alk)2 (Nanomer I-44P) was obtained from Nanocor Co. These organo-montmorillonites consisted of MMT modified with quaternary alkylammonium ions. A description of the materials is given in Table 1. Here M refers to the methyl substituent, (HT) refers to a hydrogenated tallow substituent, and (Alk) refers to an alkyl substituent. A homologous series of alkylammonium ions with one, two and three alkyl tails is seen. M2(Alk)2 is similar to the M2(HT)2 organo-montmorillonite (comparing the loss on ignition and 001 reflection position in Table 1) except for the reported higher cation exchange capacity (CEC) of the native MMT. TGA analysis was performed on a TA Instruments model SDT 2960 Simultaneous DSC-TGA. The simultaneous DSC feature was turned off. The samples were analyzed in air at a temperature ramp rate of 10 °C/ min.

Fig. 1. Apparatus to determine permeability.

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Fig. 2. M3(HT) HTXRD data. Summary of XRD scans for the “one-tailed” compatibilized organo-montmorillonite followed by details of scans at 25 °C, 200 °C, 300 °C, 450 °C and 800 °C.

All of the high temperature XRD samples were analyzed at Oak Ridge National Labs using a Philips X'Pert Pro MPD diffractometer with Cu Kα radiation at 45 kV and 40 mA. The beam geometry

consisted of 0.04 radian soller slits, a parabolic multilayer mirror with a 1/2° fixed slit on the incident beam side, a multipurpose sample stage, a diffracted-beam parallel-plate collimator (0.09°), and a

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miniprop point detector. The geometry and optics used provide a nearly monochromatic and pseudo-parallel X-ray beam. The pseudoparallel beam optics makes the diffraction scans nearly insensitive to the displacement of the sample within the chamber, a consideration for samples which are degrading during the test. XRD patterns were collected in air from 25 °C to 850 °C with a temperature ramp of 20 °C per min and a 30 s hold at temperature prior to initiating the XRD scan. Temperature was controlled using an Anton Paar XRK900 reaction chamber attachment. The pressurized samples were analyzed at Tennessee Technological University Center for Manufacturing Research on a Rigaku Ultima IV diffractometer in parallel beam mode using CuKα radiation at 40 kV and 44 mA. The beam geometry consisted of 1.0 mm divergent slit, height limiting slit 10 mm, 0.5oH PSA box component, 0.2 mm scattering slit and receiving slit. The pressure experiments were conducted on a Carver 3850 hot press. The samples were pressed at 250 °C and 7 kPa between sheets of aluminum foil and exposed to pressure for 1, 5 and 10 min time periods. After the time elapsed for each sample, they were promptly removed from the press and placed in a desiccator until they were ready for XRD analysis. The permeability apparatus is shown in Fig. 1, and has been described elsewhere. (Kannan et al., 2010) Ash samples were compacted as if for XRD analysis in a sample holder/metal ring, and then transferred carefully to a sandwich of compacted ash between two track-etched polycarbonate membranes (TEPC). The permeability of the membranes, as they have multiple through thickness holes in them, is considered to be negligible. The purpose of the membranes was to provide structural support for the ash sample as well as to help seal the diffusion apparatus. This sandwich was then fitted into a sealed chamber with nitrogen purge on both sides at room temperature and leak-checked. Ar gas at a specified mass fraction/ flow rate was introduced on one side of the sandwich membrane, and the presence of Ar which had diffused through the sample detected in the purge stream on the other side using an online mass spectrophotometer, Monitor Instruments model MG 2100. Membranes used were a product of Sterlitech Corp, with 0.03 µm holes. Area for all − 2 membranes was 9.78 × 10 4 m , and carrier gas (N2) flowrate was 9.5 sccm on the sample side. The flowrate on the purge side was 30 sccm of Ar. 3. Results and discussion 3.1. Effect of surfactant on temperature-induced physical structure HTXRD was used to follow the change in the basal spacing during heating of four organo-montmorillonites. Selected HTXRD scans for M3HT at a range of temperatures are given in Fig. 2, and a graph summarizing the onset of developing phases for all three organomontmorillonites is presented in Fig. 3. The room temperature scan of this organo-montmorillonite revealed a single reflection corresponding to d = 19.0 Å. Upon isothermal equilibration in air at 300 °C for 10 min, a second phase was indicated by a shifted reflection, with a small shoulder corresponding to the original phase still present at lower angle. This new higher angle phase at 16.9 Å is present throughout the range of higher temperatures. As the temperature is increased (up to 800 °C) an additional new peak at 15.2 Å appears, broadens and shifts towards 9.9 Å, and eventually becomes too small and too broad to resolve. Upon returning to room temperature, only the small peak at 16.9 Å remains. The structure of the organo-montmorillonite has collapsed irreversibly at high temperatures, but the stable phase at 16.9 Å is not the expected phase of pristine MMT (which would have a basal spacing of ∼10 Å (Fornes and Paul, 2003)). Note also that the low angle background often seen in literature XRD scans is fairly low, and this is attributed to the parallel beam geometry used. The HTXRD scans for the two-tailed M2(HT)2 are given in Fig. 4. In the room temperature scan of as-received M2(HT)2 there are three

Fig. 3. (A) HTXRD results for the d-values of major reflections versus temperature for four organo-montmorillonites. (B) TGA for corresponding organo-montmorillonites. Effect of organo-montmorillonite compatibilizer structure on onset of physical (XRD) collapse is compared to the onset of chemical thermal degradation (TGA). The TGAbased degradation events noted at about 220 °C are attributed to degradation of the organic compatibilizer for each organo-montmorillonite; the events above 500 °C are attributed to loss of structural water from the inorganic MMT.

reflections. The phase at 26.1 Å is interpreted to be the 001 reflection, with two other reflections corresponding to the 16.9 Å phase and the 002 reflection. At 250 °C the 001 reflection has shifted to 32.6 Å, and a new phase has appeared at 15.4 Å. At 350 °C another phase is first noted at 14.0 Å, broadening and eventually disappearing. Clearly the behavior of the M2(HT)2 is the same as that described for the M3(HT) within the detection limits, and no effect of the variable stabilizer structure is seen. The data in Table 1 confirm this. Pastore et al. (2004) have noted the onset of a collapsing phase in a nanocomposite at about 300 °C using HTXRD (poly(ethylene-co-vinyl acetate)/Nanofil 15©), which is in good agreement with the current results. The HTXRD scans for M(HT)3 are shown in Fig. 5. The room temperature as-received scan for this organo-montmorillonite

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Fig. 4. M2(HT)2 HTXRD data. Summary of XRD scans for a “two-tailed” compatibilized organo-montmorillonite followed by details of scans at 25 °C, 200 °C, 250 °C, 450 °C and 800 °C.

indicates a 001 reflection at 36.8 Å. When the organo-montmorillonite reaches 300 °C the d-spacing has reduced to 25.9 Å, and the new phase at 16.1 Å has begun to appear. At 500 °C another peak

emerges at 13.7 Å which broadens and shifts at higher temperatures to become the 10.1 Å collapsed phase seen for the other organomontmorillonites. The onset temperature of this new collapsing

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Fig. 5. M(HT)3 HTXRD data. Summary of XRD scans for the “three-tailed” compatibilized organo-montmorillonite followed by details of scans at 25 °C, 200 °C, 300 °C, 450 °C and 650 °C.

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phase, noted in Fig. 3, is comparable to the two previous organomontmorillonites. Results shown in Fig. 3 summarizing the onset of gross physical collapse are discussed next, and the more subtle behavior of the organo-montmorillonites at a slightly lower temperature range will be discussed afterwards. 3.2. Correlation of physical structure with chemical degradation of surfactant Might this surfactant-dependent physical change be related to the onset of chemical degradation? To answer this question, the results of thermal gravimetric analysis (TGA) are presented in Fig. 3 with direct comparison to the graph of HTXRD results detailing onset of physical collapse. No large changes in the onset of chemical degradation for these three organo-montmorillonites are noted; all of the organomontmorillonites appear to begin to chemically degrade at around 220 °C (similar results for Cloisite 20A (M2(HT)2) were obtained by Wang et al. (2001)). The mass loss behavior for the three organomontmorillonites is different and these differences are the expected result of the pristine organo-montmorillonites having more organic content as the number of tails is increased for a given cation exchange ratio. The onset of physical collapse is noted in the HTXRD graph at temperatures in the same range as chemical degradation. The fourth organoclay studied had the same organic content by TGA as M2(HT)2 (TGA results not shown here) and the HTXRD results were quite similar as well (compare Fig. 6 for M2(Alk)2 to Fig. 4 for the M2(HT)2). Tang and Lewin (2007), who have processed this exact material, reported collapse to the degraded phases similar to what is shown here. Lan (2007) has reported that for pellets heated to 850 °C in a muffle furnace for 1 h and undisturbed (at 8% loading), the physical “scaffolding” does not visibly collapse. The subtle increase in intensity of the 001 reflection at temperatures leading up to physical collapse for all four organo-montmorillonites is presented in Fig. 7. All of the organo-montmorillonites display an increase in intensity on increasing temperature from room temperature to about 250 °C. However the organo-montmorillonites with a two-tailed surfactant both demonstrate considerable swelling, which does not occur to the same degree for either M(HT)3 or M3(HT). Note that this increase in 001 reflection for the two-tailed variety cannot be the result of “intercalation” since there is neither polymer nor excess compatibilizer present in these experiments. Further, it cannot be attributed readily to production of off-gases, since each of the organo-montmorillonites demonstrated a similar onset of chemical degradation in the TGA results, and only the two-tailed varieties have a significant shift in 001 reflection prior to full chemical and physical collapse. We therefore attempt here only to describe the noted effect, and caution authors to consider this intrinsic behavior of the organo-montmrillonite prior to attributing such swelling to either intercalation or gas production when observed during nanocomposite formation. 3.3. Effect of pressure on physical structure To test the effect of process pressure on the structure, M2(HT)2 and M2(Alk)2 were pressed in a Carver press at 250 °C and 7 kPa for different periods of time and subsequently loaded for room temperature XRD analysis. Fig. 8 shows the scans that were obtained from the unpressed and pressed samples of M2(HT)2. The unpressed sample has a reflection at d = 24.9 Å, with a 002 reflection with d = 12.1 Å. Pressing the sample at 1 min yielded a less intense 001 reflection, shifted slightly to 25.9 Å. At 5 min, both reflections shifted even further to 29.3 Å and 13.5 Å, respectively. The sample that was pressed for 10 min has phases at d = 37.2 Å and at d = 14.0 Å. Note that adding pressure is not leading to a collapse, but instead to an expansion for the M2(HT)2! This expansion was also noted by Yoon et

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al. (2003) and in the absence of pressure at 240 °C by Scaffaro et al. (2009). (Note that XRD results from Scaffaro et al. are reported for Cloisite 15A, which is the same surfactant chemical structure as for Cloisite 20A, except the organic content was higher for Cloisite 15A.) Fig. 9 shows the results for the pressure experiments on M2(Alk)2. In the pressed samples the 001 reflection from the pristine organomontmorillonite has shifted to lower angle with direct correlation to the results seen for the M2(HT)2. Figs. 10 and 11 show the same analysis for the M3(HT) and M(HT)3 organo-montmorillonites. Here we see the more intuitive result that the 001 reflection decreases at longer pressurization times, and draw the reader's attention to the previously described thermal behavior in Fig. 7, where M3(HT) and M (HT)3 also exhibited a pattern differing from that of M2(HT)2 and M2 (Alk)2. In summary, the subtle structure of the formed barrier layer appears to be sensitive to both temperature and pressure. Two patterns have emerged, one in which the M2(HT)2 materials swell with temperature and slight pressure, and one in which the remaining materials are not very temperature sensitive prior to onset of physical degradation but then on addition of pressure they collapse. With these “model” structures now in our “library”, we can briefly examine the effect of variable physical structure of the barrier layer on permeability of a model gas, Ar. 3.4. Permeability of model barrier layer Permeability is a bulk-scale measurement, and structural change by HTXRD is a meso-scale measurement. Relating the two can give some insight into how meso- or nano-scale structural changes can affect bulk-scale properties of the ash. It is a straightforward calculation that with only 1% (v/v) of a filler whose aspect ratio is as high as 200 (and with good orientation), permeability of the composite layer can be reduced by as much as 50%. (Adame and Beall, 2009; Lan, 2007; Nielson, 1967; Paul and Robeson, 2008) Therefore it is reasonable that a structural change in which one dimension of the particle collapses or expands significantly might produce a change in effective aspect ratio, and in turn bulk permeability might also change. The permeabilities measured for the M2(HT)2 organo-montmorillonite and its ash (one of the model barrier layers with a useful variability in physical structure) are shown in Fig. 12. The plateau concentration in such an experiment can be related to the steady state flux of the permeant Ar. The molar flux of Ar (JAr) was calculated as the product of molar concentration (CAr) and volumetric flowrate of the carrier gas (VN2) per unit cross-sectional area of the membrane (Aash). JAr =

CAr *VN2 Aash

ð1Þ

The relevant fluxes calculated from data such as that shown in Fig. 12 are presented in Table 2. Note that in this particular experimental setup, a concentration gradient is driving diffusion, not a pressure gradient. This corresponds to the case found in an oxidizing nanocomposite sample, in which the gas produced at steady state inside the nanocomposite is nearly atmospheric pressure, the backside of the sample is at atmospheric pressure, and the flux of volatile organics from the condensed phase to the gas phase is driven by concentration differences. The fragile ash samples were sandwiched between two track-etched polycarbonate (TEPC) membranes for mechanical stabilization, so permeability of these membranes is described first. The flux for a single track-etched membrane (TEPC) was 0.162 mol/m2 s and the double membrane produced a flux of 0.152 mol/m2 s. This change is consistent with the concept that the flux should decrease as the thickness of the sample increases. In the next experiment, M2(HT)2 organo-montmorillonite was formed into a

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Fig. 6. M2(Alk)2 HTXRD data. Summary of XRD scans for a “two-tailed” (higher charge density) compatibilized organo-montmorillonite followed by details of scans at 25 °C, 200 °C, 250 °C, 450 °C and 800 °C.

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Fig. 10. XRD scans of pressed M3(HT). (a) unpressed (b) pressed for 1 min at 250 m °C (c) pressed for 5 min (d) pressed for 10 min. XRD scans are taken on cooled product ash at room temperature.

Fig. 7. Details of the temperature-dependence of the basal spacing at lower temperatures for four organo-montmorillonites.

Fig. 11. XRD scans of pressed M(HT)3. (a) unpressed (b) pressed for 1 min at 250 m °C (c) pressed for 5 min (d) pressed for 10 min. XRD scans are taken on cooled product ash at room temperature.

Fig. 8. XRD scans of pressed M2(HT)2. (a) unpressed (b) pressed for 1 min at 250 oC (c) pressed for 5 min (d) pressed for 10 min. XRD scans are taken on the cooled product ash at room temperature.

monolith by hand using an aluminum ring mold, similar to the manner by which most powder XRD samples are packed (M2(HT)2 neat in Fig. 12). The monolith was then deposited between the two

Fig. 9. XRD scans of pressed M2(Alk)2. (a) unpressed (b) pressed 1 min (c) pressed 10 min. XRD scans are taken on cooled product ash at room temperature.

TEPC membranes and installed in the permeability chamber. Thus the Ar must transport through two layers of TEPC and one layer of M2 (HT)2 of 0.5 mm thickness. Given the additional resistance offered by the organo-montmorillonite, the flux should go down from experiment B to experiment C, and it did. In experiment D, the M2(HT)2 was heated to 250 °C. The flux increased by a small value, perhaps

Fig. 12. Evolution of Ar concentration on sample side through M2(HT)2 organomontmorillonite and ash. The Ar concentration has been normalized by the steady state concentration of Ar on the sample side through a single TEPC membrane, where TEPC = track-etched polycarbonate. The TEPC membranes sandwiched the ash to provide structural stability.

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Acknowledgements

Table 2 Fluxes calculated from permeability data. Expt. Membrane A B C D E F a

TEPCa single TEPC double M2(HT)2 (unpressed) M2(HT)2 (heated to 250 °C in air, 15 min) M2(HT)2 (heated to 850 °C in air, 15 min) M2(HT)2 (heated to 250 °C, 74.1 kPa, 10 min)

CAr JAr (mol L− 1) (mol)(m2 s)− 1 0.0279 0.0260 0.0238 0.0247 0.0240 0.259

0.162 0.152 0.139 0.144 0.140 0.151

Track-etched membrane for structural integrity.

indicating a structural change. If indeed there was degradation of the organic material in the organo-montmorillonite (which is expected at that temperature) there would likely be void spaces produced during degradation, and an increased flux is consistent with this idea. In experiment E, the Cloisite 20A was heated to an even higher temperature, 850 °C, and we see the flux was nearly what it was in the case of the unheated organo-montmorillonite. These small changes in flux represent only the reproducibility for this technique. In experiment F, where the M2(HT)2 was pressed during heating, the flux is considerably higher than any of the three previous values. Clearly the morphology of the sample has changed such that a lesser resistance to gaseous transport is seen. A pressed, densified sample might be expected to produce more resistance to transport, not less. As the permeability results show reduced resistance, the physical changes noted by HTXRD might offer a better explanation for the increased permeability. Indeed the HTXRD-measured structural changes match the permeability results, given the fact that the spacing between galleries has increased (expanded). This expansion would lead to a nano-scale reduction in the effective aspect ratio, a concept consistent with the bulk-scale increase in permeability which was observed.

4. Conclusions We report the production of several model barrier layers with variable physical structure as verified by high temperature X-ray diffraction (HTXRD). Four organo-montmorillonites were equilibrated at temperatures between room temperature and 800 °C in air. All of the organo-montmorillonites showed identical onset of physical collapse within detection limits which correlated with chemical collapse as observed by TGA. Addition of subtle pressure at moderate temperatures altered the barrier layer structure, but not uniformly. M2(HT)2 and M2(Alk)2 subjected to moderate pressure produced a new “expanded” phase, where M(HT)3 and M3(HT) both exhibited a collapsed phase under the same pressure. This pressure-dependent pattern of behavior correlated with subtle changes seen in the 001 reflections at moderate temperatures prior to physical collapse. Given this library of model barrier layers, early results of novel permeability testing for the ash are reported. The pressure-degraded M2(HT)2 ash showed a higher flux of Ar permeant than the pristine M2 (HT)2. This result is consistent with XRD information showing that the interlayer spacings have actually expanded under pressure, which would lead to a lower MMT effective aspect ratio, and lower resistance to transport. The higher Ar flux is counterintuitive to the concept that higher processing pressure should have densified the ash. Instead structural changes provide the most consistent explanation, and permeability was shown to be a bulk-scale measure of nano-structural change for MMT nanoparticle assemblies. The fluxes ranged from 0.139 to 0.151 mol (m2 s)−1 for 0.5 mm thick layers, values which will provide useful limits in modeling to attempt to decouple the contributions of mass and heat transfer to flammability.

Funding for this project was provided by the National Institute of Standards and Technologies Grant Number 70NANB7H6006 and the Tennessee Technological University Center for Energy Systems Research. We thank Southern Clay Products for donating materials for this project. The real-time high temperature X-ray diffraction measurements were conducted at the Oak Ridge National Laboratory's High Temperature Materials Laboratory which is sponsored by the U. S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Program. Other X-ray diffraction measurements were conducted courtesy of the Tennessee Technological University Center for Manufacturing Research.

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