Closed-cell carbon foams from diphenolic acid-based polybenzoxazine

Carbon 95 (2015) 919e929

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Carbon journal homepage: www.elsevier.com/locate/carbon

Closed-cells carbon foams from diphenolic acid-based polybenzoxazine ~ iga Ruiz a, Andrzej Szczurek b, Alicia Martínez de Yuso Arisa b, Camilo Javier Zún diz a, Vanessa Fierro b, Alain Celzard b, * Juan Carlos Ronda a, Virginia Ca a b

nica, Universitat Rovira i Virgili, Campus Sescelades, Marcel.lí Domingo 1, 43007 Tarragona, Spain Departament de Química Analítica i Química Orga Institut Jean Lamour, UMR CNRS e Universit e de Lorraine n
7198, ENSTIB, 27 rue Philippe S eguin, CS 60036, 88026 Epinal Cedex, France

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 June 2015 Received in revised form 29 August 2015 Accepted 3 September 2015 Available online 8 September 2015

Highly porous polybenzoxazine-based vitreous carbon foams prepared by a self-blowing process followed by pyrolysis at 900
C are presented. Both organic and carbon foams were described in terms of porosity and thermal and mechanical properties, but the investigations mainly focused on carbon foams, with additional Raman, mercury porosimetry and SEM studies. It is clearly shown that the special foaming process, unlike others previously reported for preparing the same kind of materials, allowed obtaining a significant fraction of closed cells. This feature, although rather common for many organic foams, is unique for carbon foams who had to endure significant release of volatile matters and shrinkage during pyrolysis. As a consequence, the lowest thermal conductivities ever reported for cellular vitreous carbon foams were measured. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Carbon foam Closed porosity Characterisation Polybenzoxazine

1. Introduction Carbon foams are highly porous materials, presenting advantageous properties such as low density, high thermal resistance, high modulus and strength with respect to their high porosity, and controllable thermal and electrical conductivity, depending on the degree of graphitization [1]. Hence, carbon foams have found a broad range of engineering applications such as high temperature thermal insulation, electrodes for energy storage, lightweight structural components, filters for corrosive fluids or molten metals, radar absorbing materials, and conductive heat sinks, among others. Many examples have recently been reviewed elsewhere [2]. Carbon foams can be produced from different precursors, such as pitches and coals at high pressure, leading to cellular materials. In contrast, pyrolysis of thermoset reticulated polymer foams lead to reticulated materials. The structures of the two kinds of materials differ in the way the porosity can be described: either based on spherical pores connected with each other for cellular carbon foams, or based on a network of straight struts for reticulated carbon foams. However, cellular carbon foams can also be prepared from thermoset foams, such as those described from tannin-based

* Corresponding author. E-mail address: [email protected] (A. Celzard). http://dx.doi.org/10.1016/j.carbon.2015.09.012 0008-6223/© 2015 Elsevier Ltd. All rights reserved.

resins [3,4]. The carbon they are made of is highly disordered and called vitreous carbon, as the latter is indeed shiny, smooth, and brittle with a conchoidal fracture with sharp edges like glass, i.e., definitely different from graphite. Vitreous carbon is traditionally obtained by pyrolysis of polyfurfuryl alcohol [5 and refs. therein] and of phenolic resins in general (phenol-formaldehyde, resorcinol-formaldehyde, resorcinol-furfural, tannin-formaldehyde) [3,6]. Vitreous carbon is characterised by having low thermal expansion, high modulus, low thermal conductivity and high resistance to temperature in nonoxidising environments [7]. Conventional phenolic materials are cross-linked products from low molecular weight precursors, typically formed through condensation reactions between phenols and aldehydes. Such materials are abundantly produced industrially worldwide, due to the versatility of their structures and the fact that they display desirable properties, especially good thermal insulating properties and excellent fire retardance [8e10 and refs. therein]. However, there are some drawbacks associated with these materials, such as poor toughness, poor shelf life, and production of by-products during their process of polymerization that requires, in many cases, the use of a strong acid or base catalyst to cure. Polybenzoxazine is a newly developed class of phenolic resins which can overcome the drawbacks of conventional phenolic resin synthesis [11]. Furthermore, these thermosetting materials have

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many advantageous characteristics compared with more traditional phenolic resins, such as high thermal stability, excellent mechanical properties, easy processability, low water absorption, and near zero shrinkage after polymerization [12,13]. Thus, polybenzoxazines are becoming excellent candidates to replace traditional raw materials for organic and carbon foam preparation. To date, limited work has been devoted to the preparation and characterisation of polybenzoxazine foams. In most cases, the foamed materials were produced from bisphenol A-based benzoxazine (BPA-Bz) with either a chemical blowing agent [14,15] or glass microballoons [16,17]. In addition, a polybenzoxazine foam (BPA-Bz) was used only once as organic precursor for the preparation of carbon foam [14]. In the latter work, mainly density, electrical conductivity and compressive properties were examined after pyrolysis of one single sample. Therefore, further investigations are needed for analysing more in-depth the physical properties of polybenzoxazine-derived carbon foams, as well as their correlations with structural variables to better understand their performances. Rigid foams derived from diphenolic acid-based benzoxazine (DPA-Bz) have been proposed as competitive alternatives to traditional polybenzoxazine foams. They indeed present significant advantages such as the use of a renewable compound for the monomer preparation, the in situ generation of CO2 as blowing agent, and the control of the related decarboxylation reaction. Therefore, the resultant properties can be tuned through the control of foaming time or temperature [18]. Thus, taking advantage of the good performance displayed by these rigid foams, DPA-Bz has been used to produce flame retardant foams [19]. The present work is focused on the conversion of DPA-Bz organic foams into carbon foams by pyrolysis under nitrogen atmosphere. Precursor organic foams with different densities were prepared through a self-induced foaming process. Cellular structure as well as compressive and thermal properties of both types of foams were determined and correlated to density. Whereas finding closed-cells carbon foams was highly unlikely, because gases must escape the material undergoing pyrolysis, the present materials based on DPA-Bz foams unquestionably proved to have closed cells as revealed by pycnometry, SEM and porosimetry studies. 2. Experimental 2.1. Foams' preparation 2.1.1. Materials The following chemicals were used as received from their suppliers: ammonium sulphate (Scharlau), paraformaldehyde (Probus) and 4,40 -bis(4-hydroxyphenyl) pentanoic acid (DPA) (Aldrich). 1,3,5-Triphenylhexahydro-1,3,5- triazine and 4,40 -bis-[6-(3-phenyl3,4-dihydro-2H-1,3-benzoxazine)] pentanoic acid (DPA-Bz) were synthesised according to procedures reported elsewhere [20]. 2.1.2. Crosslinking reaction About 9 g of DPA-Bz were degassed in an oven for 10 min at 140
C, and then compressed in a rectangular steel mould (cavity dimension: 32 mm  15 mm  15 mm) under a pressure of 9.2 MPa using a manual hydraulic press (15-ton sample pressing (SPECAC) equipped with water cooled/heated platens). Next, the material was heated at 140
C for 6 h, then at 160
C for 2 h in order to obtain a rigid polymer sample. 15 samples (3 similar specimens for 5 different foaming conditions, see below) were prepared according to such process. 2.1.3. Foaming process The aforementioned polymerized DPA-Bz specimens were

placed in a conventional oven and heated from 30
C up to various foaming temperatures: 190, 200, 210, 220 and 230
C, at a heating rate of 2.0
C min1 for 4.5 h. After foaming, the samples were allowed to cool slowly down to room temperature. Their surface was peeled off before analysis. These foams were simply labelled 1, 2, 3, 4 and 5, respectively.

2.1.4. Carbonisation Cubic samples about 24 mm of edge were carbonised in a tubular furnace at a final temperature of 900
C, using a heating rate of 4
C min1 in a stream of high-purity nitrogen. The carbonisation temperature was maintained during 2 h before the furnace was switched off for cooling under the nitrogen flow. The resultant carbon foams were labelled 1C, 2C, 3C, 4C and 5C.

2.2. Foams' characterisation 2.2.1. Porosity The apparent density, or bulk density rb (g cm3), of the samples was calculated as the weight/volume ratio of parallelepiped samples of accurately known dimensions. The true density, or skeletal density rs (g cm3), defined as the density of the solid from which the considered material is made of, was estimated by helium pycnometry using an Accupyc II 1340 (Micromeritics, USA) apparatus after the samples were thoroughly ground for destroying any closed cell. From the values of rb and rs, the overall porosity F of organic and carbon foams was calculated according to:

F ¼ 1  rb =rs :

(1)

But before such measurement was carried out, the samples were also tested as raw blocks in the pycnometer. Then, the fraction of closed porosity Fc (dimensionless) was calculated as:

Fc ¼ 1 

skeletal density of raw monoliths rs

(2)

Mercury porosimetry was also tested with the initial aim of determining the distribution of diameters of the pores connecting the cells, using an AutoPore IV 9500 (Micromeritics, USA) porosimeter. The experiments were performed in two steps: low (0.001e0.24 MPa) and high (0.24e414 MPa) pressure.

2.2.2. Morphology The structure of carbon foams, including cell morphology and average size, was evaluated by Scanning Electron Microscope (SEM) observations using a FEI Quanta 600 FEG microscope at various magnifications. For that purpose, samples were installed on a carbon-coated sample holder for ensuring a good electrical contact with the latter. Prior to the observations, samples were covered with carbon in a metallisation system under vacuum. For all samples, two modes of observations using detectors of secondary electrons (SE) and backscattered electrons (BSE) were used for obtaining the most detailed observations. It is indeed well known that secondary electrons best show the topological contrast, and are the most relevant for estimating average cell diameters since cells and pore walls are best defined using SE. But it was also clearly evidenced [21] that backscattered electrons, normally used for picturing the chemical contrast, can find application here for observing cell walls, and may therefore allow visualising closed cells. Average cell diameters were estimated using Image Pro-Plus 6.0 software, based on more than 100 selected cells in each measurement. After calibration, the software automatically gave the average cell diameters.

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2.2.3. Carbon texture Raman spectroscopy was carried out using a Horiba XploRa Raman spectrometer without sample preparation. The spectra were collected under a microscope using a 100  objective. The Raman-scattered light was dispersed by a holographic grating with 1200 lines/mm and detected by a CCD camera. A laser of wavelength 532 nm, filtered at 10% of its nominal power, was used. The corresponding incident power, around 1.8 mW, was low enough to avoid any heating or damage of the samples. Each spectrum was obtained by accumulation of 2 spectra recorded from 800 to 2200 cm1 over 120 s. Samples of different densities were investigated in various places and, as expected, no tangible differences were observed. Therefore, only one spectrum was investigated in detail, and was fitted by using five mixed GaussianeLorentzian profiles for the bands D4, D1, D3, G and D2 appearing at increasingly high wavenumbers (see for example [22]). The in-plane ‘‘crystallite’’ size La was calculated from the widely accepted, following, equation [23]:

La ðnmÞ ¼

  560 ID1 1 IG El4

(3)

where El (eV) is the energy of the laser line (here 2.33 eV at 532 nm), and ID1 and IG are the intensities of D1 and G bands, respectively 2.2.4. Thermal conductivity The thermal conductivity was measured at room temperature by the transient plane source method with a thermal conductivity analyser Hot Disk TPS 250. The method is based on a transiently heated plane sensor used as heat source and a dynamic temperature sensor. This sensor consists of an electrically conducting pattern in the shape of a double spiral, which has been etched out of a thin nickel foil and sandwiched between two thin sheets of Kapton®. For the measurement of thermal conductivity, the plane sensor was fitted between two identical pieces of cubic samples with 12 mm and 20 mm edges for organic and carbon foams, respectively. The corresponding values of thermal conductivity were calculated with the Hot Disk 6.1 software. 2.2.5. Mechanical properties Mechanical tests were carried out in compression using an Instron 5944 universal testing machine equipped with a 2 kN load cell at a constant load rate of 0.5 mm min1, with which the full strain e stress compression curves were recorded. Cubic samples with 12 mm and 20 mm edges for organic and carbon foams, respectively, were always tested along the growing direction of the foam despite no obvious anisotropy was observed. Due to the high amount of debris produced during the stressstrain experiments of carbon foams, prone to modify the crosssection of the samples during the test and therefore to induce errors in the calculation of stress, two metallic plates were glued to the top and bottom opposite faces before starting the compression [24]. Young's modulus, E, was defined as the slope of the linear elastic phase in the initial part of the curve that presents the steepest slope [25]. After the elastic deformation, a strain maximum was sometimes followed by a decrease before reaching the long serrated plateau, typical of elastic-brittle foams [26]. The compressive strength, spl, has thus been taken as the highest height of the long serrated plateau (hence the subscript “pl”) for all the samples.

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3. Results and discussion 3.1. Synthesis, curing and foaming process of the DPA-Bz DPA-Bz was synthesised by the reaction of renewable diphenolic acid (DPA) with 1,3,5-triphenylhexahydro-1,3,5 triazine and paraformaldehyde in high yield and purity [20]. The corresponding molecular structure is shown in Fig. 1(i). It is well known that thermal curing of benzoxazine leads to cross-linked materials through oxazine ring opening, giving phenolic structures with Mannich bridges, see Fig. 1(ii). In the case of DPA-based benzoxazine, additional crosslinking takes place due to esterification reactions between carboxylic and phenolic hydroxyl groups formed during the opening of the oxazine ring (Fig. 1(ii)) [20]. Upon heating over 200
C, these materials undergo decarboxylation by evolution of the dangling carboxylic acid groups present in the structure. CO2 is therefore released and, in the absence of external pressure, acts as in situ foaming agent, leading to rigid polybenzoxazine foams [18], see Fig. 1(iii), having a rather uniform structure based on more or less polyhedral cells. Foam morphology and properties were found to strongly depend on both prepolymer preparation and foaming conditions, because foaming and crosslinking processes occur simultaneously [18,19]. For instance, it was found that density shows a significant dependence on foaming temperature, and decreases as the temperature increases. Thus, through the careful selection of appropriate foaming temperatures and times, polybenzoxazine foams with different characteristics could be produced. In the present study, five foaming temperatures were selected to produce organic foams with different densities and resultant structural, thermal and mechanical properties. 3.2. Foams' structure Bulk density, as well as the skeletal density measured before and after grinding of the foams, are reported in Tables 1 and 2 for organic and carbon foams, respectively. Skeletal densities of finely ground powders of polybenzoxazine resin and derived carbon were 1.22 g cm3 and 1.86 g cm3, respectively, in agreement with the theoretical density reported elsewhere for a similar resin, 1.19 g cm3 [14], and for carbon derived from phenolic precursors, 1.88e2.14 g cm3 [3,4,27,28]. Comparing the data of bulk and skeletal densities (without grinding) first suggested us that there was some problem with the measurements. The latter were therefore repeated with other samples, and the same results were recovered within a few % of difference only. In contrast, after grinding, the results were quite stable, showing that these foams, both before and after carbonization, present a significant part of closed porosity. The corresponding data are also given in Tables 1 and 2. The total porosities are huge: 93e97% and 95e99% for organic and carbon foams, respectively. The fraction of closed porosity seems to vary randomly, and takes average values of 48 and 45% for organic and carbon foams, respectively. Such values are lower but still in agreement with data reported for flame retardant polybenzoxazine foams, ranging from 51 to 61% with an average of 55%. As for the latter foams, less amount of CO2 was generated during the foaming process, affecting the final cellular structure and therefore the physical properties [19]. In the present work, finding different fractions of closed porosity from one sample to another, without any obvious correlation with the overall porosity, is explained by the small size of the samples. Small sizes indeed intensify the effect

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Fig. 1. Synthesis, curing, foaming and carbonisation processes of DPA-Bz. (A colour version of this figure can be viewed online.)

Table 1 Densities and porosities of polybenzoxazine-derived organic foams. Sample#

Bulk density (g cm3)

Skeletal density (g cm3) Before grinding

After grinding

1 2 3 4 5

0.0836 0.0645 0.0593 0.0453 0.0352

0.42 0.49 0.66 0.44 1.14

1.22 1.22 1.22 1.22 1.22

Overall porosity (%)

Fraction of closed porosity (%)

93.2 94.7 95.1 96.3 97.1

65.6 59.8 45.9 63.9 6.6

Overall porosity (%)

Fraction of closed porosity (%)

95.0 96.7 98.2 98.3 98.6

46.8 28.3 60.9 40.9 48.0

Table 2 Same as Table 1 but for carbon foams. Sample#

1C 2C 3C 4C 5C

Bulk density (g cm3)

0.0939 0.0620 0.0343 0.0314 0.0261

Skeletal density (g cm3) Before grinding

After grinding

0.99 1.33 0.73 1.10 0.97

1.86 1.86 1.86 1.86 1.86

of heterogeneities, always present in this kind of materials, and any surface defect or damage due to handling and sample cutting may also locally but significantly increase the fraction of open cells. Many commercial organic foams have fractions of closed cells higher than 90%, especially those designed for thermal insulation purposes such as extruded polystyrene or rigid polyurethane foams. However, finding so high fraction of closed cells in the present phenolic foams was unexpected, as their chemical nature makes them intrinsically brittle materials so that cell wall fracture is very likely to occur. Even more unexpected was to find that closed cells survived the pyrolysis process. Indeed, weight losses ranging from 57 to 63 wt.%, with an average of 61 wt.%, were measured during carbonisation, corresponding to a significant release of volatile matter. The latter is, however, much lower than that reported for similar materials, for which only 25e30% of the

initial foam mass remained at 800
C [14]. Moreover, the foams shrank by 30e51 vol.%, with an average value of 45 vol.%, suggesting high dimensional changes, hence the surprise to find closed cells in the resultant carbon foams. However, this result is unquestionable as such closed cells were clearly observed by SEM, as seen in Fig. 2. Comparing the left part (secondary electrons) and the right part (backscattered electrons) of Fig. 2 unmistakably shows that most cells are closed. Backscattered electrons indeed allow seeing the bottom of the cells, unlike conventional secondary electrons, rather emphasising the cell edges and cell walls. Additionally, SEM pictures show that most cells indeed have a polyhedral shape with rather flat cell wall, which is a typical feature of many closed-cell foams such as expanded polystyrene [29]. Examination of many SEM pictures allowed to conclude that the cell walls in the materials of highest

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Fig. 2. SEM pictures of carbon foams obtained with secondary electrons (left) and backscattered electrons (right). (a) Sample 1C; (b) sample 2C; (c) sample 3C; (d) sample 4C; (e) sample 5C. The scale bar is 2 mm in all pictures.

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924

density (1C and 2C in Fig. 2(a) and (b), respectively) are totally smooth, whereas those of all the other foams have a much higher extension and are somewhat blistered. In other words, cell walls are porous in samples 3C, 4C and 5C, but many of these pores are also closed (see especially the right part of Fig. 2(d) and (e)). From many other pictures than those presented in Fig. 2, the cell size distributions were obtained, and therefore the average cell diameters of carbon foams, DC (mm), could be calculated. The results are given in Fig. 3. As it can be expected for most foams, the cells were increasingly bigger as the bulk density decreased [26]. Despite the existence of two populations of cells in the lightest foams, especially well seen in Fig. 2(c), the data points could be fitted, as usual [30], by a power law such as:

1C 2C 3C 4C 5C

-1

Cumulative intrude volume (cm g )

DC frx b

14 3

12

(4)

The absolute value of the exponent x, 2.12, is much higher than in carbon foams prepared by physical foaming: 0.7e1 [21], mixed physical-chemical foaming: 1.74 [4], or resin frothing: 0.4 [30]. In other words, the changes of bulk density in polybenzoxazine-based carbon foams had a much higher impact than in other vitreous cellular carbon foams. Closed cells were finally confirmed by mercury intrusion experiments, see Fig. 4. The curves of cumulated intruded volumes indeed presented several changes of slope, which is typical of materials undergoing collapse during the test [31,32]. This finding was clearly observed for all samples, but especially for those of low bulk density, even showing stepwise curves related to successive collapses of gradually smaller cells, immediately followed by sudden mercury penetration. Open pores would have led to smooth curves instead. Big cells were also most probably destroyed from the lowest mercury pressures used by the device, explaining the fact that intruded volumes much lower than the expected values

10 8 6 4 2 0 0.001

0.01

0.1

1

10

100

1000

P (MPa) Fig. 4. Mercury intrusion curves of polybenzoxazine-based carbon foams. (A colour version of this figure can be viewed online.)

were measured. In these conditions, no pore-size distribution should be calculated from Fig. 4. In the latter, the extrusion curves were not shown for clarity, but all were straight horizontal lines, suggesting that no mercury went out the porosity after intrusion. This result also agrees with the irreversible collapse of the materials during the experiment, all samples being indeed recovered as debris after the tests. Carbon foams derived from polybenzoxazine are very similar to tannin-based carbon materials previously reported [3,4], being

(a) Sample 1C

Sample 2C

1600

1450

1300

1300

1300

1125

1125

1125

800 650 500

Cell size (μm)

1450

950

950 800 650 500

800 650 500

375

375

300

300

300

200

200

200

125

125 5

10

15

20

25

125 0

5

10

% of occurences

1125

1125

950 800 650 500

500 300

200

200

125

125

% of occurences

25

10

15

20

25

30

35

1400 1200

650 375

20

(b)

800

300

15

5

% of occurences

950

375

10

0

25

Average cell size (μm)

1300

Cell size (μm)

1450

1300

5

20

Sample 4C

1600

1450

0

15

% of occurences Sample 3C

1600

Cell size (μm)

950

375

0

Sample 5C

1600

1450

Cell size (μm)

Cell size (μm)

1600

1000 800 600 400 200

0

5

10

15

20

% of occurences

25

30

35

0 0.03

0.04

0.05

0.06

0.07

0.08

0.09

Bulk density (g cm-3)

Fig. 3. (a) Cell size distributions, and (b) average cell diameters derived from SEM pictures analysis of polybenzoxazine-based carbon foams. The curve in (b) was calculated from Eq. (4). (A colour version of this figure can be viewed online.)

~ iga Ruiz et al. / Carbon 95 (2015) 919e929 C.J. Zún

shiny, rigid and brittle, and emitting a typical high-pitched sound when broken. It can definitely be expected that the carbon they are made of is also glasslike, given the non-graphitisable nature of carbon derived from phenolic resins. The first-order region (600e2000 cm1) of the Raman spectrum of 3C carbon foam sample is presented in Fig. 5 and indeed exhibits all the features of a typical disordered carbon derived from a phenolic precursor [33,34], with an intense and broad D1 band at 1349 cm1 and a much less intense G band at 1593 cm1. For adjusting correctly the spectrum envelope, components D4, D3 and D2 appearing at 1197, 1534 and 1603 cm1, respectively, have to be taken into account. The broad but little D4 band accounts for the shoulder of the left part of the spectrum, and is characteristic of very poorly organised carbons. The D3 band (also called D00 sometimes) is also present as a very wide but more intense band in poorly crystallised carbons [35], and is attributed to defects outside the plane of aromatic layers such as tetrahedral carbons [36]. Finally, and although not leading to a clear shoulder on the right side of the spectrum, the D2 band (also called D0 sometimes) was looked for. This band is attributed to second first order zone boundary phonon [37] and indeed contributed to a better adjustment of the overall spectrum, as seen in Fig. 5. From the fits, the calculated ratio ID1/IG, used as a graphitisation indicator since the 70's [37e40], was 1.92, suggesting a crystallite size La of 9.91 nm according to Eq. (3). Although such calculation is never absolute nor accurate, at least it provides a representative order of magnitude of La [41e43], roughly in agreement, on average, with other values reported for glasslike carbon prepared around 900
C (although the scattering is quite large): ~5.5 nm [44]; ~10 nm (calculated from Eq. (4) with a ID1/IG ratio of 1.65 obtained with a laser working at 514.5 nm) [45]; ~16.9 nm [33]; ~21 nm (calculated from Eq. (4) with a ID1/IG ratio of 0.8 obtained with a laser working at 514.5 nm) [46].

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Fig. 6. Thermal conductivity of polybenzoxazine-based foams, plotted as a function of bulk density. The straight lines are calculated from Eq. (5). (A colour version of this figure can be viewed online.)

3.3. Foams' physical properties Thermal properties of raw and pyrolysed polybenzoxazine foams are presented in Fig. 6. Both sets of values could be satisfactorily adjusted by straight lines, which is a very typical behaviour in this (narrow) range of high porosities [4,47,48]. Carbonisation produced an increase of thermal conductivity with respect to organic foams, due to the intrinsically higher conductivity of

Fig. 5. First-order spectrum of 3C carbon foam sample, and its corresponding deconvolution into D4, D1, D3, G and D2 bands (from left to right). The inset shows the laser spot (in green) at the edge between three cells. (A colour version of this figure can be viewed online.)

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carbon. The higher slope of the linear fit corresponding to carbon foams, 44% higher than that of organic foams, is in agreement with the average lower fraction of open cells formerly calculated and reported in Tables 1 and 2. Extrapolating the two straight lines to zero bulk density led to almost the same value of 0.0336e0.0350 W m1 K1, which is significantly higher than the conductivity of air at room temperature (0.026 W m1 K1). A more accurate fit was thus carried out, based on the following equation [26,49]:

 k¼

2 fe  3 3



  rb r ks þ 1  b kg þ krad rs rs

(5)

where fe is the fraction of solid located at cell edges, i.e., fe ¼ 1 for purely reticulated foams, fe ¼ 0 for purely closed cell foams, and 0 < fe < 1 for any intermediate situation. k is the overall thermal conductivity originating from conduction through the solid cell walls and edges (conductivity ks), through the gas contained in the cells (conductivity kg), and from the radiative transfer (krad). In the

very narrow range of densities explored here at room temperature, krad may be assumed to be constant and kg can be set to 0.026 W m1 K1, which is the value for air at 20
C. In these conditions, Eq. (5) can be adjusted as a straight line to the data of Fig. 6, having a slope ranging from ks/3rs e kg (for completely open cells) to 2ks/3rs e kg (for completely closed cells), and an intercept kg þ krad. The corresponding parameters obtained from the linear fits lead to 0.33  ks  0.66 W m1 K1 and krad ¼ 0.0076 W m1 K1 for organic foams, and to 0.69  ks  1.39 W m1 K1 and krad ¼ 0.0090 W m1 K1 for carbon foams. Organic polybenzoxazine foams are therefore quite thermally insulating materials among phenolic foams. Their conductivity values are indeed as low as, if not lower than, those of other synthetic phenolic foams (0.04e0.08 W m1 K1 for densities 0.04e0.2 g cm3 [50], 0.057 W m1 K1 at 0.12 g cm3 [51]), and definitely lower than those presented by natural phenolic foams: lignophenolic rigid foams (0.072 W m1 K1 at 0.45 g cm3 [51], tannin-based foams prepared by different methods (physical foaming: 0.037e0.07 W m1 K1 at 0.03e0.20 g cm3 [[52] and

Fig. 7. Stressestrain compression curves of: (a) organic polybenzoxazine-derived foams; (b) carbon polybenzoxazine-derived foams measured as such; (c) same as (b) but measured with metal plates glued to the sample ends. For clarity, only 0.1% of the data points are materialised by open and full symbols for organic and carbon foams, respectively. (A colour version of this figure can be viewed online.)

~ iga Ruiz et al. / Carbon 95 (2015) 919e929 C.J. Zún

refs. therein.] or frothing of liquid resin (0.04e0.06 W m1 K1 at 0.02e0.12 g cm3 [53]), and tannin-based polyHIPEs (0.062e0.077 W m1 K1 at 0.12e0.25 g cm3 [54]. The values of Fig. 6 are, however, very difficult to compare with those reported recently for other polybenzoxazine foams (0.069e0.108 W m1 K1 at 0.415e0.720 g cm3 [15]). The latter materials had indeed much higher densities, and no accurate extrapolation to our range of densities could be done because of the scattering and the low number of conductivity data published in Ref. [15]. As for the values of 0.33  ks  0.66 W m1 K1 and krad ¼ 0.0076 W m1 K1, they are quite near those obtained for tannin-based foams obtained by resin frothing (0.655 and 0.011 W m1 K1, respectively) [53]. The present, low, value of krad may again easily be explained by the closed cells that the latter foams do not have. The corresponding carbon foams are also quite good thermal insulators, especially at low densities where the conductivity is as low as 0.04 W m1 K1 at 0.026 g cm3. This is the lowest value we could find for phenolic-derived carbon foam. The slope with which the conductivity increased with density, 0.23 W m1 K1 g1 cm3, is also the lowest ever reported for this kind of materials, as values of 0.6, 0.24, 0.39 and 0.26 W m1 K1 g1 cm3 were found for tanninbased foams prepared by physical foaming, mixed physicalchemical foaming, resin frothing, and emulsion-templating, respectively [4 and refs. therein,28,30]. As for the values of 0.69  ks  1.39 W m1 K1 and krad ¼ 0.0090 W m1 K1, the former is lower than expected for a pure, non-porous, glassy carbon (3.5e3.8 W m1 K1 [55,56]). This may be due to the too simplistic form of Eq. (5), used for fitting a too narrow range of high porosities, thereby leading to a poor extrapolation at zero porosity. On the other hand, the value of krad is consistent with the one obtained for organic foam and remains quite low, in agreement with a significant amount of closed cells. Stress-strain compression curves of organic polybenzoxazine foams are given in Fig. 7(a), and unambiguously show that the behaviour of these materials is elastic-brittle, according to the GibsoneAshby classification [26]. The corresponding moduli and compressive strengths are presented in Fig. 8. These values could not be directly compared with those of the literature, as only much denser polybenzoxazine foams were reported so far, e.g., having densities typically 10 times higher than here, within the ranges

Fig. 8. Compressive moduli and strengths of organic and carbon foams, adjusted by Eqs. (6) and (7). (A colour version of this figure can be viewed online.)

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0.415e0.720 g cm3 (with modulus and strengths of 465e1065 MPa and 12e63 MPa, respectively [15]), 0.293e0.512 g cm3 (with strengths of 1.8e4.8 MPa [17]), 0.273e0.407 g cm3 (with modulus and strengths of 267e679 MPa and 5.2e12.4 MPa, respectively [14]). However, values of the same order of magnitude are found as soon as the values presented in Fig. 8 are extrapolated to such higher densities using Eqs. (6) and (7), see below. Fig. 7(b) shows the compression curves of only two carbon foam samples, and suggests that such kind of materials should not be tested mechanically without preparation. This is especially clear for sample 2C, which presents an exaggeratedly long linear part, up to 35% strain, and whose slope can definitely never correspond to the modulus of the material. The latter is indeed so brittle that it breaks from the first moment it is deformed, and therefore this linear part can in no way be attributed to elasticity. The rupture is actually localised at the contact between sample and compression platens, and is therefore progressive until complete collapse of the porous structure. No modulus can be determined in such conditions. This serious problem has been discussed elsewhere [24,57,58] and can be avoided by using stiff plates preliminary fixed onto the sample faces submitted to compression. Such bonding of the specimen ends indeed leads to a much more homogeneous distribution of the load through the section [59] and allows a sudden fracture in the bulk of the material from very low strains. The high slope of the very initial linear part, such as the one presented by sample 1C in Fig. 7(c), is then really representative of the modulus. The corresponding values of moduli and compressive strengths are presented in Fig. 8. They compare quite well, at similar rb/rs ratios, with values published for other cellular vitreous carbon foams [30 and refs. therein] and for reticulated vitreous carbon foams (see Ref. [4] for a critical review of such kind of data). Power laws were very satisfactorily fitted to modulus E and strength spl, according to [26]:

Ef

 m rb rs

spl f

 n rb rs

(6)

(7)

where m and n are two exponents whose values depend on the foam structure. For organic foams, the fits of Eqs. (6) and (7) to the data of Fig. 8 led to m ¼ 0.95 and n ¼ 1.2, i.e., m z n z 1. For carbon foams, m and n were found to be 1.03 and 1.45, respectively. Values of modulus exponent close to 1 correspond to the lower theoretical limit but have been also reported elsewhere [15,59]. It is therefore possible that, despite all experimental precautions, especially in the case of carbon foams, the modulus was still underestimated. Using Eq. (6), a modulus of 116 MPa is indeed predicted at a density of 0.48 g cm3, i.e., significantly lower than what was measured at such density for a polybenzoxazine-based carbon foam: 829 MPa [14]. Introducing the latter data in Fig. 8 and repeating the fit with Eq. (6) would lead to a value of m close to 2, in perfect agreement with GibsoneAshby’ theory [26]. As for the compressive strength, Eq. (7) allowed predicting a value of 10.6 MPa at 0.48 g cm3, in excellent agreement with the experimental data reported for a polybenzoxazine-based carbon foam at such density: 9.5 MPa [14]. The value of the exponent n, close to 1.5, is very common in carbon foams [25], and is also the theoretically expected value for open cell foams [26]. The present exponent n is lower than the value of 2 corresponding to closed cell foams [60], but the mechanical behaviour is most possibly controlled by the most fragile cells, i.e., the open ones. Being in

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~ iga Ruiz et al. / Carbon 95 (2015) 919e929 C.J. Zún

presence of a significant part of open cells (see again Table 2) may therefore prevent the observation of the exponent foreseen for purely closed cell foams. 4. Conclusion Polybenzoxazine foams were prepared from renewable diphenolic acid, using a self-foaming procedure without using an external physical blowing agent. The resultant materials were investigated both as organic foams and as carbon foams after the former were pyrolysed at 900
C. Given the phenolic nature of the organic precursors, the carbon foams were made of glasslike carbon, presenting the same usual features as many other cellular vitreous carbon foams prepared from other thermoset resins. The self-induced foaming process, however, allowed obtaining organic materials containing a significant fraction of closed cells, up to 66%, which was rather unexpected with respect to the brittleness which characterises ultra-low density polybenzoxazine foams as these ones. Even more unexpected is that such closed cells survived carbonisation, despite the extensive weight loss and shrinkage which occurred accordingly. As far the authors know, no carbon foams with so many closed cells, up to 60%, had ever been reported before. Closed cells were especially well evidenced by SEM and confirmed by helium pycnometry and mercury porosimetry. As a consequence, whereas having mechanical properties similar to those of other phenolic-based vitreous carbon foams at comparable densities, the present polybenzoxazine-derived foams exhibited the lowest thermal conductivity ever reported for this kind of material. Acknowledgements The French team gratefully acknowledges the financial support ^ le de Compe titivite  of the CPER 2007e2013 ‘‘Structuration du Po Fibres Grand’Est’’ (Competitiveness Fibre Cluster), through local ne ral des Vosges), Regional (Re gion Lorraine), National (Conseil Ge (DRRT and FNADT) and European (FEDER) funds. The Spanish authors also express their thanks to MICINN (Ministerio de Ciencia e  n) (Spain) (MAT2011-24823). Innovacio References [1] C. Chen, E.B. Kennel, A.H. Stiller, P.G. Stansberry, J.W. Zondlo, Carbon foam derived from various precursors, Carbon 44 (8) (2006) 1535e1543. [2] M. Inagaki, J. Qiu, Q. Guo, Carbon foam: preparation and application, Carbon 87 (2015) 128e152. [3] G. Tondi, V. Fierro, A. Pizzi, A. Celzard, Tannin-based carbon foams, Carbon 47 (6) (2009) 1480e1492. [4] X. Li, M.C. Basso, F.L. Braghiroli, V. Fierro, A. Pizzi, A. Celzard, Tailoring the structure of cellular vitreous carbon foams, Carbon 50 (5) (2012) 2026e2036. [5] A.J.G. Zarbin, R. Bertholdo, M.A.F.C. Oliveira, Preparation, characterization and pyrolysis of poly(furfuryl alcohol)/porous silica glass nanocomposites: novel route to carbon template, Carbon 40 (13) (2002) 2413e2422. [6] K. Kinoshita, Carbon, Electrochemical and Physicochemical Properties, WileyInterscience, New York, 1988. [7] J.M. Friedrich, C. Ponce-de-Leon, G.W. Reade, F.C. Walsh, Reticulated vitreous carbon as an electrode material, J. Electroanal. Chem. 561 (2004) 203e217. [8] A. Gardziella, L.A. Pilato, A. Knop, Phenolic Resins: Chemistry, Applications, Standardization, Safety and Ecology, Springer Berlin, 2000. [9] L. Pilato (Ed.), Phenolic Resins: A Century of Progress, Springer-Verlag, Berlin Heidelberg, 2010. [10] G. Tondi, W. Zhao, A. Pizzi, G. Du, V. Fierro, A. Celzard, Tannin-based rigid foams: a survey of chemical and physical properties, Biores Technol. 100 (2009) 5162e5169. [11] N.N. Ghosh, B. Kiskan, Y. Yagci, Polybenzoxazinesdnew high performance thermosetting resins: synthesis and properties, Prog. Polym. Sci. 32 (11) (2007) 1344e1391. [12] Y. Yagci, B. Kiskan, N.N. Ghosh, Recent advancement on polybenzoxazineda newly developed high performance thermoset, J. Polym. Sci. A Polym. Chem. 47 (21) (2009) 5565e5576. [13] H. Ishida, Overview and historical background of polybenzoxazine research, in: I. Hatsuo, A. Tarek (Eds.), Handbook of Benzoxazine Resins, Elsevier,

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