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Effect of composition and processing parameters on the characteristics of tannin-based rigid foams. Part II: Physical properties
Materials Chemistry and Physics 123 (2010) 210–217
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Effect of composition and processing parameters on the characteristics of tannin-based rigid foams. Part II: Physical properties W. Zhao a,b , V. Fierro a , A. Pizzi c , G. Du b , A. Celzard a,∗ a Institut Jean Lamour - UMR CNRS 7198, CNRS - Nancy-Université - UPV-Metz, Département Chimie et Physique des Solides et des Surfaces, ENSTIB, 27 rue du Merle Blanc, BP 1041, 88051 Épinal Cedex 9, France b Department of Wood Science and Technology, South West Forestry University, Kunming, Yunnan, People’s Republic of China c ENSTIB-LERMAB, Nancy-University, 27 rue du Merle Blanc, BP 1041, 88051 Epinal Cedex 9, France
a r t i c l e
i n f o
Article history: Received 19 October 2009 Received in revised form 16 March 2010 Accepted 31 March 2010 Keywords: Rigid foams Mechanical properties Permeability Electrical conductivity Solvent absorption
a b s t r a c t The present work is the continuation of the previous one published in the same issue of this journal, but now focuses on some selected physical properties of tannin-based rigid foams and derived glasslike carbon foams. Such materials are new, lightweight, cellular solids, prepared from 95% natural precursors: bark extracts and furfuryl alcohol, as detailed in the companion paper. After a few structural characteristics are briefly recalled, physical properties like compressive strength, permeability to fluids, solvent absorption, and electrical conductivity are measured, discussed and modelled. The effects of changing a few experimental parameters that have been varied in the synthesis of the foam: amounts of blowing agent, strengthener and nanofillers, shape of the moulds and restricted foaming are discussed in relation with the pore structure observed in the companion paper. Slightly anisotropic properties are evidenced, in agreement with the orientation of the cells, as expected for foams grown vertically in cylindrical moulds. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Tannin-based rigid foams and their carbonaceous counterparts are new, easily prepared, cellular solids based on renewable resources [1,2]. Prepared from mimosa bark extracts as a major component, the interest for such cheap, lightweight, cellular materials is increasingly growing, given that they are able to compete with more expensive synthetic polymer rigid foams [3]. The use of furfuryl alcohol as a strengthener, also having a vegetable origin, substantially lessens the friable nature of all phenolic foams. Thus, tannin-based foams may be obtained within a wide range of apparent densities: from 0.04 to 0.16 g cm−3 . Their outstanding heat-insulating properties and fire resistance were already reported in a previous work of the authors [4]. From a more academic point of view, their pore structure was thoroughly investigated in the companion paper published in the same issue of this journal [5]. A few preliminary studies of the same authors were made on the same materials [1,6,7], but no detailed and systematic investigation of the effects of processing and composition parameters on physical properties has been published so far. The present paper is thus the first thorough study of tanninbased rigid foams whose properties are accounted for through the
∗ Corresponding author. Tel.: +33 329 29 61 14; fax: +33 329 29 61 38. E-mail address: [email protected] (A. Celzard). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.03.084
detailed examination of their porosity characteristics. Additionally, these properties are now methodically modelled by simple power laws. Modelling the physical properties of tannin-based rigid foams is of interest for two main reasons. First, the behaviours reported here should apply to a number of materials of similar open cell structure (e.g., metallic or ceramic foams whose industrial applications are increasingly growing), and might have a predictive character. Secondly, these cellular materials have potential uses as thermal insulators, acoustic absorbers, cores for sandwich panels, filters for corrosive matters, etc. In other words, tannin foams have the same applications as commercial synthetic phenolic foams [3], so their physical properties are worth studying. Once pyrolysed, the resultant reticulated carbon foams are interesting materials that can be used as porous electrodes, high-temperature insulators, catalytic filters, EMI shields or as base of new carbon–carbon composites [4,8]. Thus, again, observing the effect of synthesis on the physical behaviour and modelling the latter is of great interest. In the following, selected properties directly related to the structural characteristics evidenced in the companion paper are described: compressive strength and electrical conductivity, both accounting for the solid phase, and permeability and liquid absorption, both related to the pore space. The surface area was considered as a characteristic of the pore structure in the companion paper, and was estimated from cells’ size and packing. In the present work, surface area is also considered as a property which is
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now estimated from the permeability of the foams. In all the cases, these properties are measured and modelled as a function of apparent density, i.e., as a function of porosity. In a few situations, the effect of additional processing and composition parameters on several physical properties, like mould shape, amount of nanofiller and strengthener, and restricted foaming, are reported. 2. Experimental 2.1. Materials The preparation of rigid foams based on tannins derived from Mimosa (Acacia mearnsii formerly mollissima, De Wildt) was already described in several previous works, and notably in the companion paper [5]. Briefly, these materials are based on a self-blowing mixture of commercial flavonoids (i.e., condensed tannins), furfuryl alcohol, formaldehyde, water, para-toluene sulphonic acid, and diethyl ether. Tannins are cross-linked with formaldehyde, leading a hard, dark, resin. This reaction and the auto-condensation of furfuryl alcohol as well are catalysed by acid. Since both reactions are strongly exothermic, diethyl ether evaporates, leading to the porosity of the material. At the same time, the latter hardens. The formulation is optimised for a simultaneous foaming and hardening, so neither cracks nor pore collapse occur (see [1,6]). Standard foams were prepared using the following amounts of each component—tannin: 30 g; furfuryl alcohol: 10.4 g; diethyl ether: from 1.5 to 5 g; water: 6 g; formaldehyde (37% water solution): 7.4 g; para-toluene-4-sulphonic acid (65% water solution): 11 g. Varying the amount of diethyl ether (blowing agent) allowed tuning the apparent density of the resultant foams. After mixing, these reagents were let to foam in an open 500 mL beaker, the latter being as wide as high (diameter and height 9 cm). Non-standard foams were obtained as soon as the aforementioned conditions were not obeyed exactly: • either using the same composition as before but foamed in other moulds: a narrower and a broader one, of diameters 6 and 12 cm, respectively; • or using the same composition as before, foamed in standard mould but in restricted conditions (limiting the volume to ca 75% by application of a load over the surface during rising); • or using nanofillers: from 0.5 to 3 g of nanoclay in the formulation, using standard mould; • or finally using different levels of furfuryl alcohol (strengthener): from 6 to 15 g, in standard mould. Carbon foams derived from these polymeric rigid cellular materials were prepared through a simple pyrolysis of the latter under nitrogen flow up to 900 ◦ C. Porous materials before and after pyrolysis were referred to as “organic” and “carbon” foams, just like in the companion paper. Given that the true densities of tannin-based resin and tannin-derived vitreous carbon are 1.59 and 1.98 g cm−3 , respectively [3,8], porosities within the ranges 90–97.5% and 92–98% were typically obtained for organic and for carbon foams, respectively. All the materials, as expected for foams raised vertically, presented slightly elongated cells along the vertical direction. The latter is referred to as z, whereas the orthogonal (horizontal) plane is called xy. Such a feature, extensively visualised and discussed in the companion paper, needs to be carefully taken into account as far as physical properties are measured. 2.2. Measurement of physical properties 2.2.1. Structural anisotropy The structural anisotropy was estimated from a number of measurements of cell dimensions that have been carried out on the basis of plenty of SEM images of carbon foams [5]. Given that organic and carbon foams strictly presented the same cell structure, with the exception that pyrolysis-induced shrinkage led to cells typically 9% smaller in carbon foams, only the results for carbons materials were reported. The aspect ratio, A, of the cells, defined as the ratio of cell lengths (measured along z-direction) to diameters (measured along xy), is plotted in Fig. 1 as a function of density. The values of A are consistent with those observed for rigid polyurethane foams grown in the same conditions [9]. It can be seen from Fig. 1 that the materials are increasingly anisotropic when the density increases. Such behaviour is expected to have a strong influence on the physical properties of the foams, and was indeed already observed for mechanical resistance and permeability to water [3]. 2.2.2. Mechanical resistance The compression strength of foam samples of dimensions 3 cm × 3 cm × 1.5 cm was measured with an Instron 4206 universal testing machine equipped with a 1 kN head, and using a load rate of 2.0 mm min−1 . The complete strain–stress characteristics were recorded for a broad range of bulk densities, but only the mechanical resistance – or compressive strength, also called fracture stress – was reported here. The strength was defined as the highest height of the long serrated plateau
Fig. 1. Aspect ratio of the cells of carbon foams as a function of their apparent density.
(see below). Samples were tested along the two aforementioned orthogonal axes: growing z-direction and flat xy-direction. 2.2.3. Permeability Cubic samples of side 17 mm were installed and fixed with hot-melt glue into cylindrical copper tubes of diameter 26 mm and length 30 mm. The glue neither intruded the porosity nor covered the opposite faces of the foam through which water was forced to flow. Before starting the experiment, two or three pressure–vacuum cycles were repeated in order to saturate the porosity with water. The permeability, k, of these samples was then determined by measuring the amount of water passing throughout during times ranging from 3 to 10 min, and applying Darcy’s law: k=Q
L A
P
(1)
Q is the flow rate of water, L and A are sample thickness and cross-sectional area, respectively, is the dynamic viscosity of water at room temperature (10−3 Pa s), and P is the pressure drop over the sample. 2.2.4. Electrical conductivity This property could be measured for carbon foams only. The dimensions of samples of strictly parallelepiped shape and known density were measured, and two copper wires were glued with silver paint on each of their opposite faces. Depending on the latter, the electrical conductivity was measured along xy- or along z-direction. The four-probe method was applied: a known current, I, was forced into the sample (in one direction then in the opposite one) via two wires, and the corresponding voltage drops, V+ and V− , were measured with the two other copper wires. The real voltage drop across the sample was taken as being the arithmetic mean of these two values, in order to correct the results from (very) low thermo-electric contributions. Current supply and voltage measurement were simultaneously achieved by a Keithley 237 source-measure unit. In each experiment, several values of current were tested in order to check Ohm’s law, which was then applied for determining the electrical conductivity, : =
L 2I |V + | + |V − | S
(2)
where L and S are the thickness and the cross-sectional area of the sample, respectively. 2.2.5. Liquid absorption Absorption of various liquids at saturation and as a function of time was already reported for tannin-based organic foams having different densities [1,3]. In the present work, the same was repeated for both organic and carbon foams, having the same densities, and soaked into the same liquids. Twelve cubic 2 cm × 2 cm × 2 cm samples of density 0.065 g cm−3 were soaked in the following liquids: water, methylene chloride, chloroform, benzene, cyclohexane, and diesel oil. These liquids were chosen because they present different polarities and different molecular sizes; nonoxygenated liquids were preferred for this study, due to the expected hydrophobic nature of vitreous carbon, and because oxygenated solvents had already been studied in the case of organic foams [3]. The increase of weight of the foams was thus measured for various soaking times. In the case of two selected, very different, liquids: water and diesel oil, the absorption kinetics of both organic and carbon foams presenting three different densities: 0.05, 0.065 and 0.1 g cm−3 , was also investigated.
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Fig. 3. Compressive strength of organic foams (density 0.065 g cm−3 , z-direction) as a function of the amount of nanoclay added to the initial composition. The line is just a guide for the eye.
Fig. 2. Mechanical properties of organic and carbon foams of same density (0.061 g cm−3 ), undergoing compression along the two directions z and xy: (a) strain–stress characteristics; (b) compressive strength versus density of the same materials, plotted in a double logarithmic scale. The straight lines correspond to the application of Eq. (3).
3. Results and discussion 3.1. Compressive strength 3.1.1. Standard foams Whether they were organic or carbon, tannin-derived foams all presented the same typical brittle behaviour. Strain–stress characteristics are given as example in Fig. 2a for organic and carbon foams of density 0.061 g cm−3 , both measured according to the two xyand z-directions. The three usual regions: linear elastic, horizontal plateau, and densification, are typical of brittle foams [10]. The serrated character of the plateau, due the co-existence of collapsed and uncollapsed zones, is better seen in the case of carbon foams because vitreous carbon is much harder than tannin-based resin. Whatever the density of the foams, the compressive strength is always the highest: (a) along z-direction, whether the foam is organic or not; (b) for carbon foams. Compressive strengths measured as the highest stress of the plateau, , are plotted as a function of density in Fig. 2b, using a double-log scale. The straight lines evidence that the following equation, already confirmed by many analytical, numerical and experimental works (see for example [10]), holds: ∝ da
(3)
The exponent a is associated to the structure and deformation mechanics of the cellular material. According to Sanders and Gibson [11], the theoretical values of a are 1.5, 2 and 1.36 if the foams are based on open-cells, closed-cells and hollow spheres, respectively.
From Fig. 2b, it can be observed that the exponent is close to 1.5 in most cases (1.55 and 1.65 for organic foams along xy- and zdirections, respectively, and 1.58 and 1.51 for carbon foams along xy- and z-directions, respectively). The value of the compression exponents is near that of open-celled foam. This finding is in agreement with SEM observations (see companion paper) as well as with permeability and absorption experiments reported below. Small differences between exponents measured along different directions are related to the anisotropy of foams grown vertically, and were also noticed by Auad et al. [12] who measured the mechanical properties of phenolic foams along the same axes. Although low, anisotropy may indeed be seen for each kind of foam: organic and carbon. The mechanical resistance is always higher along the direction of foaming (z), and might be explained by the preferential orientation of the struts. Moreover, the aspect ratio of the cells increases with density, as seen in Fig. 1. These results fully agree with the observation, in other slightly anisotropic foams [9,13], of higher collapse stresses in the direction of rising. Such a minor anisotropy was also evidenced by electrical conductivity measurements (see below). 3.1.2. Modified foams The natural scattering of the results of both elastic modulus and compressive strength could not evidence any significant effect of the mould diameter. Such result was also found for polyurethane foams grown in various cylindrical moulds presenting the same ratio of extreme diameters [14]. The role of nanofillers on mechanical resistance is now examined. Various amounts (from 0.2 to 3 g) of nanoclay were incorporated to the typical composition of an organic foam of density 0.065 g cm−3 (i.e., based on 3 g of diethyl ether and 10.5 g of furfuryl alcohol, see Table 2 of the companion paper for more details). The compressive strength of the resultant composite materials is presented in Fig. 3 along z-direction. Despite the natural scattering of the results, a maximum is evidenced when 1 g of nanoclay was introduced. Such an amount corresponds to typically 1.3 vol.% of nanoclay inside the organic foam. Lower volume fractions have a lower effect on the mechanical resistance, but higher amounts possibly lead to cracks and defects in the structure. Adding nanoclay may thus reinforce the foam. At the optimum concentration, increase of compressive strength as high as ca 45% is achieved. This effect is even higher than that reported for copolymer foams [15].
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Fig. 5. Permeability of organic and carbon foams depending on density and measurement direction: (a) in linear scale (lines are power laws fitted to the experimental points); (b) in double logarithmic scale for checking Eq. (4) (straight lines). Fig. 4. Strain–stress characteristics of organic foams grown in restricted conditions (see text for details).
Strain–stress characteristics of tannin-based rigid foams grown under restricted conditions are presented in Fig. 4. Samples 1 and 3 correspond to the upper parts of the material raised with a load applied on its top surface, whereas samples 2 and 4 are the bottom parts of the same foam. Samples 5 and 6 are vertical slices on this foam, thus including both upper and bottom parts (see Fig. 5 of the companion paper [5] for additional details). A number of sharp inverted peaks may be seen all along the plateaus, which are explained by the catastrophic collapse of cell groups. Rather than being crushed layer by layer as before, assemblies of inclined cells are broken simultaneously, leading to the spectacular snaps seen in Fig. 4. This is especially true for samples 2, 5 and 6, for which the highest proportion of inclined cells has been observed [5]. Obviously, these effects vanish when densification begins (sharp increase of compressive stress beyond 50% strain). 3.2. Permeability Fig. 5a compares the permeability, k, of organic and carbon foams, measured along the two xy- and z-orthogonal directions. Carbon foams present permeabilities that are typically 10 times lower than those of organic foams. This finding may be explained by the shrinkage occurring during pyrolysis, inducing both smaller cells and possibly pore closure, so only part of the porosity is available for fluid flow. As expected, low-density foams, i.e. having the biggest cells and the widest pore windows, are much more perme-
able than the ones having higher density. At very low density, k is expected to tend towards infinity as the porosity becomes 100%. Carbon foams, having a narrower porosity due to pyrolysis-induced shrinkage, present a lower permeability than their organic counterparts of similar density. However, the same power law: k ∝ dn
(4)
is found to fit the data very correctly, whatever the measurement direction and whatever the chemical nature of the foam. As seen in Fig. 5b, the exponent n is close to −2: −2.03 and −2.41 for organic foams along xy and z, respectively, and −2.00 and −2.31 for carbon foams along xy and z, respectively. The value of n may be recovered from the expression of the permeability. According to Brace [16]: k ∝ D2 ˚3 , where D and ˚ are average cell diameter and porosity, respectively. This equation was shown to be derived from the theory of Kozeny and Carman that is even more commonly used (see [3]). Given the extremely high porosities considered here, always higher than 90%, it can be approximated that ˚ remains roughly constant in the investigated range of apparent densities, thus k ∝ D2 . Given that the proportionality D ∝ 1/d was demonstrated in the companion paper [5], one finally gets: k ∝ d−2
(5)
Even if the anisotropy is very low, permeability along the xydirection is higher than along z-direction. Interconnection between cells is indeed better when the cells have higher contact areas, which was demonstrated by the wider windows measured along xy (see companion paper). At the lowest densities, anisotropy tends to vanish.
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Fig. 6. Surface area of carbon foams calculated from Eq. (6) using the data of Fig. 5a. The line is just a guide for the eye.
3.3. Surface area The surface area of the carbon foams may be recalculated on the basis of the experimental permeability values presented in Fig. 5 and from the Kozeny–Carman equation (see [17] and references therein): k=
˚3 ς(dSA )
2
(6)
in which is the so-called Kozeny constant ( = 5 in many porous materials). Eq. (6) obviously has the same form as the aforementioned Brace’s equation, and its application leads to a curve of surface area versus density which presents a maximum at a value of d = 0.065 g cm−3 , see Fig. 6. It is extremely interesting to see that, even if the surface area calculated from Eq. (6) is higher than the one estimated from cell sizes seen by electron microscopy, the same behaviour is obtained (see companion paper [5]), with a maximum at a very similar density (i.e., near 0.07 g cm−3 ). The parameters used for making these calculations (cell diameters and permeability) are indeed totally independent from each other. 3.4. Electrical conductivity The electrical conductivity, , of carbon foams along the two usual axes is given in Fig. 7a. As expected, increases with density. Given the structure of the foam, such an increase might be accounted for by the Bruggeman asymmetric model, which considers that the insulating phase is completely coated by the conducting one. However, SEM revealed that 3–3 connectivity exists between both conducting and insulating phases, thus the generalised effective medium (GEM) equation should be better. GEM considers an intermediate situation between symmetric model (for which the permutation of the phases does not change the connectivity) and asymmetric one [18]. Close to the percolation threshold of the solid, conducting, phase (i.e. at very low density in the present case), the GEM equation reads: = s [(f − fc )/(1 − fc )]u , where s is the electrical conductivity of the (non-porous) solid, u an exponent whose value depends on the geometry of the phases, f the volume fraction of solid, and fc the critical volume fraction at which the conductivity of the material vanishes. Given the structure of the foam, its conductivity is expected to vanish when the thickness of the pore walls tends to zero, in other words when f → 0. Therefore, it is a good approximation to write fc = 0. Since f = d/ds , the GEM equation
Fig. 7. Electrical conductivity of carbon foams depending on density and measurement direction: (a) in linear scale; (b) in double logarithmic scale at densities below 0.09 g cm−3 for checking Eq. (7) (straight lines).
thus reduces to: = s
d u ds
(7)
or, more simply, to ∝ du . The exponent u is such that: u = m(1 − fc ) ≈ m, where m is a parameter whose value is directly related to the shape of the phase having the lowest conductivity (i.e. voids in the present case). m = 3/2 for spherical voids, and tends towards ∞ and towards 5/3 for infinitely flattened ellipsoids (discs) or elongated ellipsoids (needles), respectively [18]. Log–log plot of conductivity versus apparent density evidences the relevance of Eq. (7) at low density (i.e., below 0.09 g cm−3 ), see Fig. 7b. Eq. (7) has the same form as the well-known percolation equation describing the electrical conductivity of heterogeneous systems based on a random mixture of conducting and insulating grains [19]. Near the percolation threshold (i.e., close to a density of zero in our case), percolation theory predicts an exponent close to 2 in any three-dimensional medium. The slopes of the straight lines observed in Fig. 7b are indeed 1.83 and 1.64 for conductivities measured along z and xy directions, respectively. Such values are in good agreement with percolation theory, but can be understood in a different way if the GEM theory is now considered. The exponents, very close to 3/2, correspond to spherical voids (see [20] and refs. therein), as expected for tannin-based carbon foams. SEM observation did not evidence cells having aspect ratios higher than 2.5 (see Fig. 1). Such anisotropy would lead to an exponent u of 1.56
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However, the very high uptake of chlorinated solvents, especially methylene chloride, is astonishing. Besides, given the extremely low surface area and the correspondingly low amounts of moieties at the surface of carbonaceous materials, the observed water absorption is rather high. This finding should be assigned to the level of oxygen present in the carbon foams (typically from 5 to 10 wt.%), and originating from the furanic structures that survived pyrolysis [22,23]. In organic foams, molecules of low polarity intrude the porosity much slower than more polar ones, due to their lower interaction with the surface. Bigger molecules like alkanes probably cannot permeate throughout pore walls, so absorption finally stops in the absence of pressure. A higher viscosity may also contribute to a lower uptake. As far as carbonaceous materials are concerned, for which the impact of the surface chemistry is the lowest, it can be observed that the higher the viscosity, the lower the uptake. No proportionality, however, may be evidenced, given the following dynamic viscosities: 0.44, 0.57, 0.65, 1, 1.02 and 7.5 cP for CH2 Cl2 , CHCl3 , C6 H6 , H2 O, C6 H12 and diesel oil, respectively. The absorption kinetics of two very different liquids: water and diesel oil, are worth studying as a function of the nature (organic or carbonaceous) of foams of different densities. The corresponding absorption data plotted as a function of time (not shown, but presenting obviously higher uptakes at lower foam densities) may be modelled according to various equations available from the literature (see [24] for details). Three models have been successfully tested here, and bring information about the affinity of the foams for the different liquids. The pseudo first-order equation of Lagergren is one of the most widely used for the adsorption of solute from a liquid solution. It is assumed here that it can be also used for absorption in a porous medium. Once rearranged and linearised, it reads:
Fig. 8. Absorption kinetics of various liquids (indicated on the plot) by foams having the same density: 0.065 g cm−3 : (a) organic foams; (b) carbon foams.
log(qe − qt ) = log(qe ) − (see [20,21] for details of calculation). It thus appears that electrical conductivity is not an accurate method for determining the shape of the cells within conducting foam, especially when the scattering of the experimental points is considered (see again Fig. 7).
t 2.303
(8)
where is the kinetic constant of pseudo first-order ad/absorption (min−1 ), and qe and qt (mg/g of foam) represent the amounts of absorbed liquid at equilibrium and at time t (min), respectively. Elovich’s equation describes activated ad/absorption, and can be expressed as follows:
3.5. Absorption of liquids
qt =
Absorption kinetics of various liquids by organic and carbon foams having the same density: 0.065 g cm−3 , are presented in Fig. 8a and b, respectively. At the highest soaking time, organic foams always present higher uptakes as far as water and halogenated molecules are concerned. The opposite is observed with hydrocarbons (benzene and alkanes) for which the affinity is higher for carbon foams. These findings are not surprising, given the expected higher hydrophobic character of carbon materials.
1 1 ln(rs) + ln(t) s s
(9)
where r is the initial ad/absorption rate (mg (g min)−1 ), and the parameter 1/s (mg g−1 ) is related to the number of sites available for ad/absorption. Finally, the pseudo-second-order equation may be expressed in the following form: 1 t t = + qt qe q2e
(10)
Table 1 Kinetic parameters for the absorption of water and diesel oil in organic and carbon foams of different densities. System
Density
Pseudo first order qe (mg g
−1
)
Elovich’s equation −1
(min
)
2
R
r (mg g
−1
min
−1
)
Pseudo second order −1
1/s (mg g
)
R
qe (mg g−1 )
(g mg−1 min−1 )
R2
2
Water/organic
0.05 0.065 0.1
6274.8 5468.9 4271.7
5.33 × 10−3 4.64 × 10−3 4.92 × 10−3
0.97888 0.99446 0.95217
15656.9 1107.6 3168.4
748.1 840.6 523.9
0.97625 0.97098 0.99363
8555.1 6669.8 4996.8
2.77 × 10−6 2.63 × 10−6 4.17 × 10−6
0.99569 0.99503 0.99288
Water/carbon
0.05 0.065 0.1
5517.0 4921.5 3131.8
4.91 × 10−3 4.85 × 10−3 4.28 × 10−3
0.99314 0.97147 0.98861
2844.6 2293.6 1116.7
816.5 577.6 501.6
0.97768 0.97233 0.99225
7322.8 5570.4 4215.9
2.82 × 10−6 2.99 × 10−6 4.78 × 10−6
0.99579 0.99160 0.99505
Diesel/organic
0.05 0.065 0.1
2149.3 1306.8 950.8
4.11 × 10−3 4.32 × 10−3 3.59 × 10−3
0.98635 0.99626 0.99510
42718.4 7664.0 3183.2
393.2 238.2 171.8
0.99681 0.98276 0.97501
4688.0 2582.1 1802.9
9.08 × 10−6 1.35 × 10-5 1.61 × 10−5
0.99819 0.99815 0.99744
Diesel/carbon
0.05 0.065 0.1
2819.0 2827.5 1968.3
4.49 × 10−3 5.26 × 10−3 4.83 × 10−3
0.99033 0.98561 0.98628
11.89 × 106 5287.6 15155.1
420.8 415.6 288.6
0.97721 0.99218 0.99038
7599.9 4218.0 3371.3
7.19 × 10−6 6.74 × 10−6 9.55 × 10−6
0.99883 0.99622 0.99664
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Fig. 9. Fits of three absorption models (Eqs. (8)–(10)) to the absorption data of water ((a), (c) and (e)) and diesel oil ((b), (d) and (f)) in organic and carbon foams having densities: 0.05, 0.065 and 0.1 g cm−3 . (a) and (b) Lagergren’s equation; (c) and (d) Elovich’s equation; (e) and (f) pseudo-second-order equation.
in which qe and qt have the same meaning as before, and is the corresponding kinetic constant (g (mg min)−1 ). Fig. 9 presents the fits of Eqs. (8)–(10) to the absorption data of water and diesel oil in organic and carbon foams having densities: 0.05, 0.065 and 0.1 g cm−3 . Most of times, excellent agreements were found, so it is uneasy to deduce which model worked better. The corresponding parameters of each equation are gathered in Table 1, as well as their correlation coefficients, R2 . Concerning the absorption of water, first, application of Lagergren’s equation leads to kinetic constants, , that are almost the same, whatever the density and the chemical nature of the foam. Their value range from 4.3 × 10−3 to 5.3 × 10−3 min−1 , typically 10 times higher than what is observed in activated carbons which present a tortuous, narrow, and well-developed porosity. Logically, the absorbed amount
at equilibrium decreases with density, due to the correspondingly lower porosity (see Fig. 1 of the companion paper). The trend that can be derived from the parameter 1/s from Elovich’s equation is that the number of sites available for absorption decreases with density, for both organic and carbon foams. At equal density, more absorption sites are available for water in organic than in carbon foams. The equilibrium absorbed amounts, qe , derived from the pseudo-second-order equation are close to those obtained from Lagergren’s model. At similar densities, the values of qe are higher in organic foams than in carbon foams, thus corroborating the results of Elovich’s equation. The kinetic constant, , increases with density, whatever the nature of the foam. From the point of view of the correlation coefficient, the pseudo-second-order model is the one leading to the best fit to the experimental data.
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The same conclusions can be drawn if the absorption of diesel oil is now considered. The kinetic constants from Lagergren’s equation are again very close to each other, but are slightly higher when diesel oil is absorbed by carbon. The number of available sites for absorption decreases with density, more sites existing for diesel oil absorption on carbon than on organic foam. The initial absorption rate, r, is also most of times higher on carbon. The values of the absorbed amounts at equilibrium, qe , present more discrepancies than in the previous case of water absorption when results from Lagergren’s and pseudo-second-order equations are compared. However, all decrease with density, as expected, and qe is unquestionably always higher for carbon foams. The aforementioned statements all lead to the conclusion of a higher affinity of diesel oil for carbon materials, and of water for organic foams.
applications for which other kinds of foams are presently used. The performances of such materials as electrodes for electrochemical storage, thermal insulators, acoustic absorbers, filters, catalyst supports, etc. are the purpose of our present works.
4. Conclusion
[1] G. Tondi, A. Pizzi, Ind. Crops Prod. 29 (2009) 356–363. [2] G. Tondi, S. Blacher, A. Léonard, A. Pizzi, V. Fierro, J.M. Leban, A. Celzard, Microsc. Microanal. 15 (2009) 384–394. [3] G. Tondi, W. Zhao, A. Pizzi, G. Du, V. Fierro, A. Celzard, Biores. Technol. 100 (2009) 5162–5169. [4] G. Tondi, V. Fierro, A. Pizzi, A. Celzard, Carbon 47 (2009) 2761. [5] W. Zhao, A. Pizzi, V. Fierro, G. Du, A. Celzard, Mater. Chem. Phys. 122 (2010) 175–182. [6] N.E. Meikleham, A. Pizzi, Appl. Pol. Sci. 53 (1994) 1547–1556. [7] G. Tondi. Ph.D. Thesis, ENSTIB, University of Nancy, 2009. [8] G. Tondi, V. Fierro, A. Pizzi, A. Celzard, Carbon 47 (2009) 1480–1492. [9] M.C. Hawkins, B. O’Toole, D. Jackovich, J. Cell. Plast. 41 (2005) 267–285. [10] L.J. Gibson, M.F. Ashby, Cellular Solids: Structure and Properties, second ed., Cambridge University Press, Cambridge, 1977. [11] W.S. Sanders, L.J. Gibson, Mater. Sci. Eng. A 347 (2003) 70–85. [12] M.L. Auad, L. Zhao, H. Shen, S.R. Nutt, U. Sorathia, J. Appl. Pol. Sci. 104 (2007) 1399–1407. [13] A.T. Huber, L.J. Gibson, J. Mater. Sci. 23 (1988) 3031–3040. [14] D. Jackovich, B. O’Toole, M.C. Hawkins, L. Sapochak, J. Cell. Plast. 41 (2005) 153–168. [15] K.W. Park, S.R. Chowdhury, C.C. Park, G.H. Kim, J. Appl. Pol. Sci. 104 (2007) 3879–3885. [16] W. Brace, J. Geophys. Res. 82 (1977) 3343–3349. [17] A. Celzard, J.F. Marêché, G. Furdin, Prog. Mater. Sci. 50 (2005) 93–179. [18] D.S. McLachlan, M. Blaszkiewicz, R.E. Newnham, J. Am. Ceram. Soc. 73 (1990) 2187–2203. [19] S. Kirkpatrick, Rev. Mod. Phys. 45 (1973) 574–588. [20] A. Celzard, J.F. Marêché, F. Payot, G. Furdin, Carbon 40 (2002) 2801–2815. [21] A. Celzard, J.F. Marêché, G. Furdin, S. Puricelli, J. Phys. D: Appl. Phys. 33 (2000) 3094–3101. [22] G. Tondi, A. Pizzi, H. Pasch, A. Celzard, K. Rode, Eur. Pol. J. 44 (2008) 2938–2943. [23] G. Tondi, A. Pizzi, H. Pasch, A. Celzard, Polym. Degrad. Stabil. 93 (2008) 968–975. [24] V. Fierro, V. Torné-Fernández, D. Montané, A. Celzard, Micropor. Mesopor. Mater. 111 (2008) 276–284.
In this work, physical properties of tannin-based foams and their carbonaceous counterparts: strain–stress characteristics, electrical conductivity, permeability, absorption of fluids and surface area, have been described and modelled. It was shown that all the measured behaviours can be predicted by rather simple laws, whose parameters can bring information about the cellular structure of the foams. Especially, power laws were found to account very suitably for the changes of electrical conductivity, permeability and compressive strength as a function of apparent density. The corresponding exponents were found to be in complete agreement with what was expected for a material based on almost spherical, connected, cells. On the other hand, absorption experiments evidenced different affinities of tannin and carbon for liquid molecules of different viscosities and polarities. Carbon foams have the highest affinity for alkanes, whereas organic materials can absorb high amounts of water and halogenated solvents. The results presented in this article not only support what is known from the behaviour of cellular solids, but also demonstrate the interest of such cheap, biosourced materials. The properties reported here are similar to those of commercial phenolic foams as far as organic materials are concerned, and their carbonaceous counterparts also present good electrical conductivity and even higher compressive strengths. Preliminary results (not presented here) also evidenced exceptionally high thermal insulating properties. All these data are thus of great interest for exploring potential
Acknowledgements The authors gratefully acknowledge the financial support of the CPER 2007–2013 “Structuration du Pôle de Compétitivité Fibres Grand’Est” (Competitiveness Fibre Cluster), through local (Conseil Général des Vosges), regional (Région Lorraine), national (DRRT and FNADT) and European (FEDER) funds. References
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Effect of composition and processing parameters on the characteristics of tannin-based rigid foams. Part II: Physical properties W. Zhao a,b , V. Fierro a , A. Pizzi c , G. Du b , A. Celzard a,∗ a Institut Jean Lamour - UMR CNRS 7198, CNRS - Nancy-Université - UPV-Metz, Département Chimie et Physique des Solides et des Surfaces, ENSTIB, 27 rue du Merle Blanc, BP 1041, 88051 Épinal Cedex 9, France b Department of Wood Science and Technology, South West Forestry University, Kunming, Yunnan, People’s Republic of China c ENSTIB-LERMAB, Nancy-University, 27 rue du Merle Blanc, BP 1041, 88051 Epinal Cedex 9, France
a r t i c l e
i n f o
Article history: Received 19 October 2009 Received in revised form 16 March 2010 Accepted 31 March 2010 Keywords: Rigid foams Mechanical properties Permeability Electrical conductivity Solvent absorption
a b s t r a c t The present work is the continuation of the previous one published in the same issue of this journal, but now focuses on some selected physical properties of tannin-based rigid foams and derived glasslike carbon foams. Such materials are new, lightweight, cellular solids, prepared from 95% natural precursors: bark extracts and furfuryl alcohol, as detailed in the companion paper. After a few structural characteristics are briefly recalled, physical properties like compressive strength, permeability to fluids, solvent absorption, and electrical conductivity are measured, discussed and modelled. The effects of changing a few experimental parameters that have been varied in the synthesis of the foam: amounts of blowing agent, strengthener and nanofillers, shape of the moulds and restricted foaming are discussed in relation with the pore structure observed in the companion paper. Slightly anisotropic properties are evidenced, in agreement with the orientation of the cells, as expected for foams grown vertically in cylindrical moulds. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Tannin-based rigid foams and their carbonaceous counterparts are new, easily prepared, cellular solids based on renewable resources [1,2]. Prepared from mimosa bark extracts as a major component, the interest for such cheap, lightweight, cellular materials is increasingly growing, given that they are able to compete with more expensive synthetic polymer rigid foams [3]. The use of furfuryl alcohol as a strengthener, also having a vegetable origin, substantially lessens the friable nature of all phenolic foams. Thus, tannin-based foams may be obtained within a wide range of apparent densities: from 0.04 to 0.16 g cm−3 . Their outstanding heat-insulating properties and fire resistance were already reported in a previous work of the authors [4]. From a more academic point of view, their pore structure was thoroughly investigated in the companion paper published in the same issue of this journal [5]. A few preliminary studies of the same authors were made on the same materials [1,6,7], but no detailed and systematic investigation of the effects of processing and composition parameters on physical properties has been published so far. The present paper is thus the first thorough study of tanninbased rigid foams whose properties are accounted for through the
∗ Corresponding author. Tel.: +33 329 29 61 14; fax: +33 329 29 61 38. E-mail address: [email protected] (A. Celzard). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.03.084
detailed examination of their porosity characteristics. Additionally, these properties are now methodically modelled by simple power laws. Modelling the physical properties of tannin-based rigid foams is of interest for two main reasons. First, the behaviours reported here should apply to a number of materials of similar open cell structure (e.g., metallic or ceramic foams whose industrial applications are increasingly growing), and might have a predictive character. Secondly, these cellular materials have potential uses as thermal insulators, acoustic absorbers, cores for sandwich panels, filters for corrosive matters, etc. In other words, tannin foams have the same applications as commercial synthetic phenolic foams [3], so their physical properties are worth studying. Once pyrolysed, the resultant reticulated carbon foams are interesting materials that can be used as porous electrodes, high-temperature insulators, catalytic filters, EMI shields or as base of new carbon–carbon composites [4,8]. Thus, again, observing the effect of synthesis on the physical behaviour and modelling the latter is of great interest. In the following, selected properties directly related to the structural characteristics evidenced in the companion paper are described: compressive strength and electrical conductivity, both accounting for the solid phase, and permeability and liquid absorption, both related to the pore space. The surface area was considered as a characteristic of the pore structure in the companion paper, and was estimated from cells’ size and packing. In the present work, surface area is also considered as a property which is
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now estimated from the permeability of the foams. In all the cases, these properties are measured and modelled as a function of apparent density, i.e., as a function of porosity. In a few situations, the effect of additional processing and composition parameters on several physical properties, like mould shape, amount of nanofiller and strengthener, and restricted foaming, are reported. 2. Experimental 2.1. Materials The preparation of rigid foams based on tannins derived from Mimosa (Acacia mearnsii formerly mollissima, De Wildt) was already described in several previous works, and notably in the companion paper [5]. Briefly, these materials are based on a self-blowing mixture of commercial flavonoids (i.e., condensed tannins), furfuryl alcohol, formaldehyde, water, para-toluene sulphonic acid, and diethyl ether. Tannins are cross-linked with formaldehyde, leading a hard, dark, resin. This reaction and the auto-condensation of furfuryl alcohol as well are catalysed by acid. Since both reactions are strongly exothermic, diethyl ether evaporates, leading to the porosity of the material. At the same time, the latter hardens. The formulation is optimised for a simultaneous foaming and hardening, so neither cracks nor pore collapse occur (see [1,6]). Standard foams were prepared using the following amounts of each component—tannin: 30 g; furfuryl alcohol: 10.4 g; diethyl ether: from 1.5 to 5 g; water: 6 g; formaldehyde (37% water solution): 7.4 g; para-toluene-4-sulphonic acid (65% water solution): 11 g. Varying the amount of diethyl ether (blowing agent) allowed tuning the apparent density of the resultant foams. After mixing, these reagents were let to foam in an open 500 mL beaker, the latter being as wide as high (diameter and height 9 cm). Non-standard foams were obtained as soon as the aforementioned conditions were not obeyed exactly: • either using the same composition as before but foamed in other moulds: a narrower and a broader one, of diameters 6 and 12 cm, respectively; • or using the same composition as before, foamed in standard mould but in restricted conditions (limiting the volume to ca 75% by application of a load over the surface during rising); • or using nanofillers: from 0.5 to 3 g of nanoclay in the formulation, using standard mould; • or finally using different levels of furfuryl alcohol (strengthener): from 6 to 15 g, in standard mould. Carbon foams derived from these polymeric rigid cellular materials were prepared through a simple pyrolysis of the latter under nitrogen flow up to 900 ◦ C. Porous materials before and after pyrolysis were referred to as “organic” and “carbon” foams, just like in the companion paper. Given that the true densities of tannin-based resin and tannin-derived vitreous carbon are 1.59 and 1.98 g cm−3 , respectively [3,8], porosities within the ranges 90–97.5% and 92–98% were typically obtained for organic and for carbon foams, respectively. All the materials, as expected for foams raised vertically, presented slightly elongated cells along the vertical direction. The latter is referred to as z, whereas the orthogonal (horizontal) plane is called xy. Such a feature, extensively visualised and discussed in the companion paper, needs to be carefully taken into account as far as physical properties are measured. 2.2. Measurement of physical properties 2.2.1. Structural anisotropy The structural anisotropy was estimated from a number of measurements of cell dimensions that have been carried out on the basis of plenty of SEM images of carbon foams [5]. Given that organic and carbon foams strictly presented the same cell structure, with the exception that pyrolysis-induced shrinkage led to cells typically 9% smaller in carbon foams, only the results for carbons materials were reported. The aspect ratio, A, of the cells, defined as the ratio of cell lengths (measured along z-direction) to diameters (measured along xy), is plotted in Fig. 1 as a function of density. The values of A are consistent with those observed for rigid polyurethane foams grown in the same conditions [9]. It can be seen from Fig. 1 that the materials are increasingly anisotropic when the density increases. Such behaviour is expected to have a strong influence on the physical properties of the foams, and was indeed already observed for mechanical resistance and permeability to water [3]. 2.2.2. Mechanical resistance The compression strength of foam samples of dimensions 3 cm × 3 cm × 1.5 cm was measured with an Instron 4206 universal testing machine equipped with a 1 kN head, and using a load rate of 2.0 mm min−1 . The complete strain–stress characteristics were recorded for a broad range of bulk densities, but only the mechanical resistance – or compressive strength, also called fracture stress – was reported here. The strength was defined as the highest height of the long serrated plateau
Fig. 1. Aspect ratio of the cells of carbon foams as a function of their apparent density.
(see below). Samples were tested along the two aforementioned orthogonal axes: growing z-direction and flat xy-direction. 2.2.3. Permeability Cubic samples of side 17 mm were installed and fixed with hot-melt glue into cylindrical copper tubes of diameter 26 mm and length 30 mm. The glue neither intruded the porosity nor covered the opposite faces of the foam through which water was forced to flow. Before starting the experiment, two or three pressure–vacuum cycles were repeated in order to saturate the porosity with water. The permeability, k, of these samples was then determined by measuring the amount of water passing throughout during times ranging from 3 to 10 min, and applying Darcy’s law: k=Q
L A
P
(1)
Q is the flow rate of water, L and A are sample thickness and cross-sectional area, respectively, is the dynamic viscosity of water at room temperature (10−3 Pa s), and P is the pressure drop over the sample. 2.2.4. Electrical conductivity This property could be measured for carbon foams only. The dimensions of samples of strictly parallelepiped shape and known density were measured, and two copper wires were glued with silver paint on each of their opposite faces. Depending on the latter, the electrical conductivity was measured along xy- or along z-direction. The four-probe method was applied: a known current, I, was forced into the sample (in one direction then in the opposite one) via two wires, and the corresponding voltage drops, V+ and V− , were measured with the two other copper wires. The real voltage drop across the sample was taken as being the arithmetic mean of these two values, in order to correct the results from (very) low thermo-electric contributions. Current supply and voltage measurement were simultaneously achieved by a Keithley 237 source-measure unit. In each experiment, several values of current were tested in order to check Ohm’s law, which was then applied for determining the electrical conductivity, : =
L 2I |V + | + |V − | S
(2)
where L and S are the thickness and the cross-sectional area of the sample, respectively. 2.2.5. Liquid absorption Absorption of various liquids at saturation and as a function of time was already reported for tannin-based organic foams having different densities [1,3]. In the present work, the same was repeated for both organic and carbon foams, having the same densities, and soaked into the same liquids. Twelve cubic 2 cm × 2 cm × 2 cm samples of density 0.065 g cm−3 were soaked in the following liquids: water, methylene chloride, chloroform, benzene, cyclohexane, and diesel oil. These liquids were chosen because they present different polarities and different molecular sizes; nonoxygenated liquids were preferred for this study, due to the expected hydrophobic nature of vitreous carbon, and because oxygenated solvents had already been studied in the case of organic foams [3]. The increase of weight of the foams was thus measured for various soaking times. In the case of two selected, very different, liquids: water and diesel oil, the absorption kinetics of both organic and carbon foams presenting three different densities: 0.05, 0.065 and 0.1 g cm−3 , was also investigated.
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Fig. 3. Compressive strength of organic foams (density 0.065 g cm−3 , z-direction) as a function of the amount of nanoclay added to the initial composition. The line is just a guide for the eye.
Fig. 2. Mechanical properties of organic and carbon foams of same density (0.061 g cm−3 ), undergoing compression along the two directions z and xy: (a) strain–stress characteristics; (b) compressive strength versus density of the same materials, plotted in a double logarithmic scale. The straight lines correspond to the application of Eq. (3).
3. Results and discussion 3.1. Compressive strength 3.1.1. Standard foams Whether they were organic or carbon, tannin-derived foams all presented the same typical brittle behaviour. Strain–stress characteristics are given as example in Fig. 2a for organic and carbon foams of density 0.061 g cm−3 , both measured according to the two xyand z-directions. The three usual regions: linear elastic, horizontal plateau, and densification, are typical of brittle foams [10]. The serrated character of the plateau, due the co-existence of collapsed and uncollapsed zones, is better seen in the case of carbon foams because vitreous carbon is much harder than tannin-based resin. Whatever the density of the foams, the compressive strength is always the highest: (a) along z-direction, whether the foam is organic or not; (b) for carbon foams. Compressive strengths measured as the highest stress of the plateau, , are plotted as a function of density in Fig. 2b, using a double-log scale. The straight lines evidence that the following equation, already confirmed by many analytical, numerical and experimental works (see for example [10]), holds: ∝ da
(3)
The exponent a is associated to the structure and deformation mechanics of the cellular material. According to Sanders and Gibson [11], the theoretical values of a are 1.5, 2 and 1.36 if the foams are based on open-cells, closed-cells and hollow spheres, respectively.
From Fig. 2b, it can be observed that the exponent is close to 1.5 in most cases (1.55 and 1.65 for organic foams along xy- and zdirections, respectively, and 1.58 and 1.51 for carbon foams along xy- and z-directions, respectively). The value of the compression exponents is near that of open-celled foam. This finding is in agreement with SEM observations (see companion paper) as well as with permeability and absorption experiments reported below. Small differences between exponents measured along different directions are related to the anisotropy of foams grown vertically, and were also noticed by Auad et al. [12] who measured the mechanical properties of phenolic foams along the same axes. Although low, anisotropy may indeed be seen for each kind of foam: organic and carbon. The mechanical resistance is always higher along the direction of foaming (z), and might be explained by the preferential orientation of the struts. Moreover, the aspect ratio of the cells increases with density, as seen in Fig. 1. These results fully agree with the observation, in other slightly anisotropic foams [9,13], of higher collapse stresses in the direction of rising. Such a minor anisotropy was also evidenced by electrical conductivity measurements (see below). 3.1.2. Modified foams The natural scattering of the results of both elastic modulus and compressive strength could not evidence any significant effect of the mould diameter. Such result was also found for polyurethane foams grown in various cylindrical moulds presenting the same ratio of extreme diameters [14]. The role of nanofillers on mechanical resistance is now examined. Various amounts (from 0.2 to 3 g) of nanoclay were incorporated to the typical composition of an organic foam of density 0.065 g cm−3 (i.e., based on 3 g of diethyl ether and 10.5 g of furfuryl alcohol, see Table 2 of the companion paper for more details). The compressive strength of the resultant composite materials is presented in Fig. 3 along z-direction. Despite the natural scattering of the results, a maximum is evidenced when 1 g of nanoclay was introduced. Such an amount corresponds to typically 1.3 vol.% of nanoclay inside the organic foam. Lower volume fractions have a lower effect on the mechanical resistance, but higher amounts possibly lead to cracks and defects in the structure. Adding nanoclay may thus reinforce the foam. At the optimum concentration, increase of compressive strength as high as ca 45% is achieved. This effect is even higher than that reported for copolymer foams [15].
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Fig. 5. Permeability of organic and carbon foams depending on density and measurement direction: (a) in linear scale (lines are power laws fitted to the experimental points); (b) in double logarithmic scale for checking Eq. (4) (straight lines). Fig. 4. Strain–stress characteristics of organic foams grown in restricted conditions (see text for details).
Strain–stress characteristics of tannin-based rigid foams grown under restricted conditions are presented in Fig. 4. Samples 1 and 3 correspond to the upper parts of the material raised with a load applied on its top surface, whereas samples 2 and 4 are the bottom parts of the same foam. Samples 5 and 6 are vertical slices on this foam, thus including both upper and bottom parts (see Fig. 5 of the companion paper [5] for additional details). A number of sharp inverted peaks may be seen all along the plateaus, which are explained by the catastrophic collapse of cell groups. Rather than being crushed layer by layer as before, assemblies of inclined cells are broken simultaneously, leading to the spectacular snaps seen in Fig. 4. This is especially true for samples 2, 5 and 6, for which the highest proportion of inclined cells has been observed [5]. Obviously, these effects vanish when densification begins (sharp increase of compressive stress beyond 50% strain). 3.2. Permeability Fig. 5a compares the permeability, k, of organic and carbon foams, measured along the two xy- and z-orthogonal directions. Carbon foams present permeabilities that are typically 10 times lower than those of organic foams. This finding may be explained by the shrinkage occurring during pyrolysis, inducing both smaller cells and possibly pore closure, so only part of the porosity is available for fluid flow. As expected, low-density foams, i.e. having the biggest cells and the widest pore windows, are much more perme-
able than the ones having higher density. At very low density, k is expected to tend towards infinity as the porosity becomes 100%. Carbon foams, having a narrower porosity due to pyrolysis-induced shrinkage, present a lower permeability than their organic counterparts of similar density. However, the same power law: k ∝ dn
(4)
is found to fit the data very correctly, whatever the measurement direction and whatever the chemical nature of the foam. As seen in Fig. 5b, the exponent n is close to −2: −2.03 and −2.41 for organic foams along xy and z, respectively, and −2.00 and −2.31 for carbon foams along xy and z, respectively. The value of n may be recovered from the expression of the permeability. According to Brace [16]: k ∝ D2 ˚3 , where D and ˚ are average cell diameter and porosity, respectively. This equation was shown to be derived from the theory of Kozeny and Carman that is even more commonly used (see [3]). Given the extremely high porosities considered here, always higher than 90%, it can be approximated that ˚ remains roughly constant in the investigated range of apparent densities, thus k ∝ D2 . Given that the proportionality D ∝ 1/d was demonstrated in the companion paper [5], one finally gets: k ∝ d−2
(5)
Even if the anisotropy is very low, permeability along the xydirection is higher than along z-direction. Interconnection between cells is indeed better when the cells have higher contact areas, which was demonstrated by the wider windows measured along xy (see companion paper). At the lowest densities, anisotropy tends to vanish.
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Fig. 6. Surface area of carbon foams calculated from Eq. (6) using the data of Fig. 5a. The line is just a guide for the eye.
3.3. Surface area The surface area of the carbon foams may be recalculated on the basis of the experimental permeability values presented in Fig. 5 and from the Kozeny–Carman equation (see [17] and references therein): k=
˚3 ς(dSA )
2
(6)
in which is the so-called Kozeny constant ( = 5 in many porous materials). Eq. (6) obviously has the same form as the aforementioned Brace’s equation, and its application leads to a curve of surface area versus density which presents a maximum at a value of d = 0.065 g cm−3 , see Fig. 6. It is extremely interesting to see that, even if the surface area calculated from Eq. (6) is higher than the one estimated from cell sizes seen by electron microscopy, the same behaviour is obtained (see companion paper [5]), with a maximum at a very similar density (i.e., near 0.07 g cm−3 ). The parameters used for making these calculations (cell diameters and permeability) are indeed totally independent from each other. 3.4. Electrical conductivity The electrical conductivity, , of carbon foams along the two usual axes is given in Fig. 7a. As expected, increases with density. Given the structure of the foam, such an increase might be accounted for by the Bruggeman asymmetric model, which considers that the insulating phase is completely coated by the conducting one. However, SEM revealed that 3–3 connectivity exists between both conducting and insulating phases, thus the generalised effective medium (GEM) equation should be better. GEM considers an intermediate situation between symmetric model (for which the permutation of the phases does not change the connectivity) and asymmetric one [18]. Close to the percolation threshold of the solid, conducting, phase (i.e. at very low density in the present case), the GEM equation reads: = s [(f − fc )/(1 − fc )]u , where s is the electrical conductivity of the (non-porous) solid, u an exponent whose value depends on the geometry of the phases, f the volume fraction of solid, and fc the critical volume fraction at which the conductivity of the material vanishes. Given the structure of the foam, its conductivity is expected to vanish when the thickness of the pore walls tends to zero, in other words when f → 0. Therefore, it is a good approximation to write fc = 0. Since f = d/ds , the GEM equation
Fig. 7. Electrical conductivity of carbon foams depending on density and measurement direction: (a) in linear scale; (b) in double logarithmic scale at densities below 0.09 g cm−3 for checking Eq. (7) (straight lines).
thus reduces to: = s
d u ds
(7)
or, more simply, to ∝ du . The exponent u is such that: u = m(1 − fc ) ≈ m, where m is a parameter whose value is directly related to the shape of the phase having the lowest conductivity (i.e. voids in the present case). m = 3/2 for spherical voids, and tends towards ∞ and towards 5/3 for infinitely flattened ellipsoids (discs) or elongated ellipsoids (needles), respectively [18]. Log–log plot of conductivity versus apparent density evidences the relevance of Eq. (7) at low density (i.e., below 0.09 g cm−3 ), see Fig. 7b. Eq. (7) has the same form as the well-known percolation equation describing the electrical conductivity of heterogeneous systems based on a random mixture of conducting and insulating grains [19]. Near the percolation threshold (i.e., close to a density of zero in our case), percolation theory predicts an exponent close to 2 in any three-dimensional medium. The slopes of the straight lines observed in Fig. 7b are indeed 1.83 and 1.64 for conductivities measured along z and xy directions, respectively. Such values are in good agreement with percolation theory, but can be understood in a different way if the GEM theory is now considered. The exponents, very close to 3/2, correspond to spherical voids (see [20] and refs. therein), as expected for tannin-based carbon foams. SEM observation did not evidence cells having aspect ratios higher than 2.5 (see Fig. 1). Such anisotropy would lead to an exponent u of 1.56
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However, the very high uptake of chlorinated solvents, especially methylene chloride, is astonishing. Besides, given the extremely low surface area and the correspondingly low amounts of moieties at the surface of carbonaceous materials, the observed water absorption is rather high. This finding should be assigned to the level of oxygen present in the carbon foams (typically from 5 to 10 wt.%), and originating from the furanic structures that survived pyrolysis [22,23]. In organic foams, molecules of low polarity intrude the porosity much slower than more polar ones, due to their lower interaction with the surface. Bigger molecules like alkanes probably cannot permeate throughout pore walls, so absorption finally stops in the absence of pressure. A higher viscosity may also contribute to a lower uptake. As far as carbonaceous materials are concerned, for which the impact of the surface chemistry is the lowest, it can be observed that the higher the viscosity, the lower the uptake. No proportionality, however, may be evidenced, given the following dynamic viscosities: 0.44, 0.57, 0.65, 1, 1.02 and 7.5 cP for CH2 Cl2 , CHCl3 , C6 H6 , H2 O, C6 H12 and diesel oil, respectively. The absorption kinetics of two very different liquids: water and diesel oil, are worth studying as a function of the nature (organic or carbonaceous) of foams of different densities. The corresponding absorption data plotted as a function of time (not shown, but presenting obviously higher uptakes at lower foam densities) may be modelled according to various equations available from the literature (see [24] for details). Three models have been successfully tested here, and bring information about the affinity of the foams for the different liquids. The pseudo first-order equation of Lagergren is one of the most widely used for the adsorption of solute from a liquid solution. It is assumed here that it can be also used for absorption in a porous medium. Once rearranged and linearised, it reads:
Fig. 8. Absorption kinetics of various liquids (indicated on the plot) by foams having the same density: 0.065 g cm−3 : (a) organic foams; (b) carbon foams.
log(qe − qt ) = log(qe ) − (see [20,21] for details of calculation). It thus appears that electrical conductivity is not an accurate method for determining the shape of the cells within conducting foam, especially when the scattering of the experimental points is considered (see again Fig. 7).
t 2.303
(8)
where is the kinetic constant of pseudo first-order ad/absorption (min−1 ), and qe and qt (mg/g of foam) represent the amounts of absorbed liquid at equilibrium and at time t (min), respectively. Elovich’s equation describes activated ad/absorption, and can be expressed as follows:
3.5. Absorption of liquids
qt =
Absorption kinetics of various liquids by organic and carbon foams having the same density: 0.065 g cm−3 , are presented in Fig. 8a and b, respectively. At the highest soaking time, organic foams always present higher uptakes as far as water and halogenated molecules are concerned. The opposite is observed with hydrocarbons (benzene and alkanes) for which the affinity is higher for carbon foams. These findings are not surprising, given the expected higher hydrophobic character of carbon materials.
1 1 ln(rs) + ln(t) s s
(9)
where r is the initial ad/absorption rate (mg (g min)−1 ), and the parameter 1/s (mg g−1 ) is related to the number of sites available for ad/absorption. Finally, the pseudo-second-order equation may be expressed in the following form: 1 t t = + qt qe q2e
(10)
Table 1 Kinetic parameters for the absorption of water and diesel oil in organic and carbon foams of different densities. System
Density
Pseudo first order qe (mg g
−1
)
Elovich’s equation −1
(min
)
2
R
r (mg g
−1
min
−1
)
Pseudo second order −1
1/s (mg g
)
R
qe (mg g−1 )
(g mg−1 min−1 )
R2
2
Water/organic
0.05 0.065 0.1
6274.8 5468.9 4271.7
5.33 × 10−3 4.64 × 10−3 4.92 × 10−3
0.97888 0.99446 0.95217
15656.9 1107.6 3168.4
748.1 840.6 523.9
0.97625 0.97098 0.99363
8555.1 6669.8 4996.8
2.77 × 10−6 2.63 × 10−6 4.17 × 10−6
0.99569 0.99503 0.99288
Water/carbon
0.05 0.065 0.1
5517.0 4921.5 3131.8
4.91 × 10−3 4.85 × 10−3 4.28 × 10−3
0.99314 0.97147 0.98861
2844.6 2293.6 1116.7
816.5 577.6 501.6
0.97768 0.97233 0.99225
7322.8 5570.4 4215.9
2.82 × 10−6 2.99 × 10−6 4.78 × 10−6
0.99579 0.99160 0.99505
Diesel/organic
0.05 0.065 0.1
2149.3 1306.8 950.8
4.11 × 10−3 4.32 × 10−3 3.59 × 10−3
0.98635 0.99626 0.99510
42718.4 7664.0 3183.2
393.2 238.2 171.8
0.99681 0.98276 0.97501
4688.0 2582.1 1802.9
9.08 × 10−6 1.35 × 10-5 1.61 × 10−5
0.99819 0.99815 0.99744
Diesel/carbon
0.05 0.065 0.1
2819.0 2827.5 1968.3
4.49 × 10−3 5.26 × 10−3 4.83 × 10−3
0.99033 0.98561 0.98628
11.89 × 106 5287.6 15155.1
420.8 415.6 288.6
0.97721 0.99218 0.99038
7599.9 4218.0 3371.3
7.19 × 10−6 6.74 × 10−6 9.55 × 10−6
0.99883 0.99622 0.99664
216
W. Zhao et al. / Materials Chemistry and Physics 123 (2010) 210–217
Fig. 9. Fits of three absorption models (Eqs. (8)–(10)) to the absorption data of water ((a), (c) and (e)) and diesel oil ((b), (d) and (f)) in organic and carbon foams having densities: 0.05, 0.065 and 0.1 g cm−3 . (a) and (b) Lagergren’s equation; (c) and (d) Elovich’s equation; (e) and (f) pseudo-second-order equation.
in which qe and qt have the same meaning as before, and is the corresponding kinetic constant (g (mg min)−1 ). Fig. 9 presents the fits of Eqs. (8)–(10) to the absorption data of water and diesel oil in organic and carbon foams having densities: 0.05, 0.065 and 0.1 g cm−3 . Most of times, excellent agreements were found, so it is uneasy to deduce which model worked better. The corresponding parameters of each equation are gathered in Table 1, as well as their correlation coefficients, R2 . Concerning the absorption of water, first, application of Lagergren’s equation leads to kinetic constants, , that are almost the same, whatever the density and the chemical nature of the foam. Their value range from 4.3 × 10−3 to 5.3 × 10−3 min−1 , typically 10 times higher than what is observed in activated carbons which present a tortuous, narrow, and well-developed porosity. Logically, the absorbed amount
at equilibrium decreases with density, due to the correspondingly lower porosity (see Fig. 1 of the companion paper). The trend that can be derived from the parameter 1/s from Elovich’s equation is that the number of sites available for absorption decreases with density, for both organic and carbon foams. At equal density, more absorption sites are available for water in organic than in carbon foams. The equilibrium absorbed amounts, qe , derived from the pseudo-second-order equation are close to those obtained from Lagergren’s model. At similar densities, the values of qe are higher in organic foams than in carbon foams, thus corroborating the results of Elovich’s equation. The kinetic constant, , increases with density, whatever the nature of the foam. From the point of view of the correlation coefficient, the pseudo-second-order model is the one leading to the best fit to the experimental data.
W. Zhao et al. / Materials Chemistry and Physics 123 (2010) 210–217
217
The same conclusions can be drawn if the absorption of diesel oil is now considered. The kinetic constants from Lagergren’s equation are again very close to each other, but are slightly higher when diesel oil is absorbed by carbon. The number of available sites for absorption decreases with density, more sites existing for diesel oil absorption on carbon than on organic foam. The initial absorption rate, r, is also most of times higher on carbon. The values of the absorbed amounts at equilibrium, qe , present more discrepancies than in the previous case of water absorption when results from Lagergren’s and pseudo-second-order equations are compared. However, all decrease with density, as expected, and qe is unquestionably always higher for carbon foams. The aforementioned statements all lead to the conclusion of a higher affinity of diesel oil for carbon materials, and of water for organic foams.
applications for which other kinds of foams are presently used. The performances of such materials as electrodes for electrochemical storage, thermal insulators, acoustic absorbers, filters, catalyst supports, etc. are the purpose of our present works.
4. Conclusion
[1] G. Tondi, A. Pizzi, Ind. Crops Prod. 29 (2009) 356–363. [2] G. Tondi, S. Blacher, A. Léonard, A. Pizzi, V. Fierro, J.M. Leban, A. Celzard, Microsc. Microanal. 15 (2009) 384–394. [3] G. Tondi, W. Zhao, A. Pizzi, G. Du, V. Fierro, A. Celzard, Biores. Technol. 100 (2009) 5162–5169. [4] G. Tondi, V. Fierro, A. Pizzi, A. Celzard, Carbon 47 (2009) 2761. [5] W. Zhao, A. Pizzi, V. Fierro, G. Du, A. Celzard, Mater. Chem. Phys. 122 (2010) 175–182. [6] N.E. Meikleham, A. Pizzi, Appl. Pol. Sci. 53 (1994) 1547–1556. [7] G. Tondi. Ph.D. Thesis, ENSTIB, University of Nancy, 2009. [8] G. Tondi, V. Fierro, A. Pizzi, A. Celzard, Carbon 47 (2009) 1480–1492. [9] M.C. Hawkins, B. O’Toole, D. Jackovich, J. Cell. Plast. 41 (2005) 267–285. [10] L.J. Gibson, M.F. Ashby, Cellular Solids: Structure and Properties, second ed., Cambridge University Press, Cambridge, 1977. [11] W.S. Sanders, L.J. Gibson, Mater. Sci. Eng. A 347 (2003) 70–85. [12] M.L. Auad, L. Zhao, H. Shen, S.R. Nutt, U. Sorathia, J. Appl. Pol. Sci. 104 (2007) 1399–1407. [13] A.T. Huber, L.J. Gibson, J. Mater. Sci. 23 (1988) 3031–3040. [14] D. Jackovich, B. O’Toole, M.C. Hawkins, L. Sapochak, J. Cell. Plast. 41 (2005) 153–168. [15] K.W. Park, S.R. Chowdhury, C.C. Park, G.H. Kim, J. Appl. Pol. Sci. 104 (2007) 3879–3885. [16] W. Brace, J. Geophys. Res. 82 (1977) 3343–3349. [17] A. Celzard, J.F. Marêché, G. Furdin, Prog. Mater. Sci. 50 (2005) 93–179. [18] D.S. McLachlan, M. Blaszkiewicz, R.E. Newnham, J. Am. Ceram. Soc. 73 (1990) 2187–2203. [19] S. Kirkpatrick, Rev. Mod. Phys. 45 (1973) 574–588. [20] A. Celzard, J.F. Marêché, F. Payot, G. Furdin, Carbon 40 (2002) 2801–2815. [21] A. Celzard, J.F. Marêché, G. Furdin, S. Puricelli, J. Phys. D: Appl. Phys. 33 (2000) 3094–3101. [22] G. Tondi, A. Pizzi, H. Pasch, A. Celzard, K. Rode, Eur. Pol. J. 44 (2008) 2938–2943. [23] G. Tondi, A. Pizzi, H. Pasch, A. Celzard, Polym. Degrad. Stabil. 93 (2008) 968–975. [24] V. Fierro, V. Torné-Fernández, D. Montané, A. Celzard, Micropor. Mesopor. Mater. 111 (2008) 276–284.
In this work, physical properties of tannin-based foams and their carbonaceous counterparts: strain–stress characteristics, electrical conductivity, permeability, absorption of fluids and surface area, have been described and modelled. It was shown that all the measured behaviours can be predicted by rather simple laws, whose parameters can bring information about the cellular structure of the foams. Especially, power laws were found to account very suitably for the changes of electrical conductivity, permeability and compressive strength as a function of apparent density. The corresponding exponents were found to be in complete agreement with what was expected for a material based on almost spherical, connected, cells. On the other hand, absorption experiments evidenced different affinities of tannin and carbon for liquid molecules of different viscosities and polarities. Carbon foams have the highest affinity for alkanes, whereas organic materials can absorb high amounts of water and halogenated solvents. The results presented in this article not only support what is known from the behaviour of cellular solids, but also demonstrate the interest of such cheap, biosourced materials. The properties reported here are similar to those of commercial phenolic foams as far as organic materials are concerned, and their carbonaceous counterparts also present good electrical conductivity and even higher compressive strengths. Preliminary results (not presented here) also evidenced exceptionally high thermal insulating properties. All these data are thus of great interest for exploring potential
Acknowledgements The authors gratefully acknowledge the financial support of the CPER 2007–2013 “Structuration du Pôle de Compétitivité Fibres Grand’Est” (Competitiveness Fibre Cluster), through local (Conseil Général des Vosges), regional (Région Lorraine), national (DRRT and FNADT) and European (FEDER) funds. References