NEW CARBON MATERIALS Volume 28, Issue 2, Feb 2013 Online English edition of the Chinese language journal

Cite this article as: New Carbon Materials, 2013, 28(2):127–133.

Received date: 05 January 2013; Revised date: 01 April 2013 *Corresponding author. E-mail: deniz.baran@aksa.com Copyright©2013, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-5805(13)60071-2

RESEARCH PAPER

Synthesis of carbon foam with high compressive strength from an asphaltene pitch Deniz Baran1,*, Mehmet Ferhat Yardim2, Hüsnü Atakül2, Ekrem Ekinci3

1AKSA, Denizcalı Koyu, Yalova, Turkey; 2Istanbul Technical University, Maslak, 34469 Istanbul, Turkey; 3Tdinamik, Energy,Industry & Foreign Trade Co. Bulgurlu Uskudar, 34696 Istanbul, Turkey

Abstract: Carbon foams were synthesized using an asphaltene pitch as a carbon precursor. The effects of foaming conditions and carbonization on the pore structures and physical properties of the carbon foams were investigated. Results indicate that the average pore size and density of as-synthesized carbon foams were about 150 µm and 800 kg/m3, respectively. The compressive strength of the carbon foam increased from 10 to 18.7 MPa after carbonization at 1 323 K. The high ash content (41.76%) of the pitch plays an important role in determining the density and compressive strength of the carbon foams.

Key Words: Carbon foam; Asphaltite pitch; Compressive strength

1 Introduction

Carbon foam is a high porous material made up of spherical cellular voids in its main carbon matrix, characterized by low density (200-800 kg/m3), high temperature tolerance (up to 3 300 K in inert atmosphere), high strength (up to 20 MPa, compression), large external surface area with open cell structure (up to 98%), and adjustable thermal and electrical conductivity[1,2]. It is one of the important types of new carbon materials and has many important applications. Carbon foams are often classified as either closed or open cell foams [3]. Various precursors for the preparation of carbon foams have been known since the 1960’s. Applications of carbon foams depend on their precursors, synthesis methods, structure and properties.

Polyacrylonitrile (PAN)[4], vinylidine chloride polymer[5], polyurethane[6], phenolic polymer[7], pyrolizable organic compound such as sugar or cellulose[8], coal tar and petroleum pitch[3], coal[9] and synthetic mesophase pitches[10-13] are the different precursors of carbon foam. One of the more frequently used materials suitable for foaming is Mitsubishi AR pitch derived from naphthalene by the catalytic polymerization using HF-BF3 as catalyst[14]. This pitch is a 100% anisotropic mesophase, cost of which is more than the total cost of carbonization and graphitization[15].

Low cost nature-occurring pitches suitable for carbon foam have been reported, such as that from coal [16]. In this study, an asphaltite pitch from Southeastern Anatolian Avgamasya asphaltites in Turkey is reported to prepare carbon

foam. In general, natural asphaltic materials were formed by migration and solidification of petroleum. Petroleum lost its light components and underwent a series of complicated chemical and biological reactions and physical changes under the influence of time, heat and pressure during migration, leading to the formation of pitch [16, 17].

Asphaltite pitch can be characterized by different physical and chemical properties. Asphaltite pitches have a melting point between 120 and 315 °C which is one of the important properties related to foaming. Turkish asphaltite pitch can be classified between asphaltite and asphaltic pyrobitumen with respect to thermal maturation[18]. The pitches from Turkish asphaltites are composed of 1-5.3mass% moisture, 4-40 mass% volatile matter, 33-45 mass% ash, 4.1-6.4 mass% sulfur, 47-59 mass% fixed carbon and 3.2-5.6 mass% hydrogen. Their solubility in CS2 is between 4.9 and 30 mass%[19]. It can be found that the Asphaltite pitches have ash contents as high as 40 mass%, which were composed of inorganic compounds such as carbonates, sulphates, silicates, and clay. These inorganic compounds have a negative effect on the thermal properties of the asphaltites[20]. In this study, the natural asphaltite pitch was used without any pretreatment, therefore the carbon foam incorporated mineral matter in its matrix.

2 Experimental

2.1 Precursor

An asphaltite pitch from Avgamasya in the open mines of

Deniz Baran et al. / New Carbon Materials, 2013, 28(2): 127–133

Sirnak was used in this study. Sample was homogenized and crushed to 50-100 mesh for foaming experiments. Proximate analyses of the pitch were performed using ASTM D 3174 for ash, ASTM D 3173 for moisture, ASTM D 3175 for volatile matter and fixed carbon was determined by subtracting the three ultimate analysis results from 100%. Ultimate analysis of samples was conducted by instrumental analysis. The results of the proximate and ultimate analyses of the pitch are presented in Table 1. These characterization results are in good agreement with the literature [21].

2.2 Foaming process

Foaming experiments were carried out in an autoclave. 20 g crushed pitch was placed in a cylindrical aluminum mould (Φ37×100 mm) that was put into the autoclave. After leaking test, the autoclave was purged with nitrogen under 1 MPa to displace air.

The autoclave was fed with nitrogen to an initial pressure, then heated to a desired temperature at a heating rate of 2–2.5 K/min. At the desired temperature, the autoclave was held for a desired time. After that the final pressure of the autoclave was released to atmospheric pressure at an rate set according to the processing parameters. Finally, the autoclave was left to cool down to room temperature and green carbon foam was obtained. The green carbon foams were kept in a furnace at 368-378 K to avoid absorbing moisture. The values of the initial nitrogen pressure (P), the final temperature (T), the pressure release time (tpr) and the soak time (tsk) during the foaming were changed from 1 to 9.8 MPa[16,22], 673 to 872 K, 5 to 650 s and 15 to 60 min, respectively.

The green carbon foams were carbonized in a horizontal furnace (Φ50×900 mm). The furnace was first heated to 823 K with 2.5 K/ min under a nitrogen flow rate of 500 cm3/min and kept at this temperature for 1 h. Second, the temperature was raised to 1 123 K at 1.5 K/min and held at this temperature for 1 h. Then, the temperature was raised again to 1 323 K at 0.5 K/min and held at this temperature for 2.5 h. Finally, the furnace was cooled down to room temperature at a rate of 5 K/min.

2.3 Characterization

The surface and the microstructure of the carbonized carbon foams were studied by scanning electron microscopy (SEM, Japan, Jeol JSM-6390LV model). Compressive strength of the samples was measured by mechanical test equipment (Autograph AGS J model). The density of the carbon foams was determined by their dimension and weight.

3 Results and discussion

3.1 Initial nitrogen pressure

The carbon foams were synthesized from asphaltite pitch by varying the initial nitrogen pressure from 1 to 9.8 MPa

under 773 K for 60 min and a pressure release time 5s. The SEM images of the foams produced at 2.5 and 7.8 MPa are shown in Fig. 1. It is found that the cells have been developed for all pressures investigated. However, the cell sizes are larger at lower pressures than those at higher pressure. At low pressures joints and ligaments are not fully developed. The sizes of the cells and pores decrease and porosity increases with the initial nitrogen pressure. As manifested in SEM images of carbon foams obtained at 2.5 and 7.8 MPa under the same magnification of 200 shown in Fig. 1, the cell sizes are reduced by 2- 3 fold and the number of cells and pores is increased by 2-4 times with an increase of the initial nitrogen pressure from 2.5 to 7.8 MPa. The distribution of sizes of cells and pores became narrow with the pressure. The ligaments and junctions are formed at high initial pressures. So it can be concluded that the increase in the initial nitrogen pressure is favorable for foaming. With increase of the initial nitrogen pressure, the stress on the bubble wall also increases, resulting in a formation of more pores while keeping the sizes of pores small.

The effect of the pressure on the density of the foam is shown in Fig. 2. More dense carbon foam formation is directly reflected by the compressive strength, which shows the similar changing trend of density with the pressure as shown in Fig. 3. It is clear that both the density and compressive strength increase substantially with pressure up to 6.8 MPa then level off. Comparing the density and compressive strength of asphaltite carbon foams with those of the foams from coal, coal tar pitch, petroleum pitch, and AR mesophase pitches[4,5,23,24], which have densities of 160-800, 560-670, 340 and 200-600 kg/m3 compressive strengths of 2.5-18.7, 8-18.2, 3.9 and 1-4 MPa, respectively, Asphaltite-derived carbon foam has higher density and compressive strength. The inorganic matrix is suspected to be well incorporated into the foam as an reinforcing filler, which plays a positive role in the improvement of density and strength.

Table 1 Proximate and ultimate analysis

of asphaltite (as received)

Proximate analysis w/%

Ultimate analysis w/%

Moisture 0.70

Ash 41.76

Volatile matter 33.28

Fixed carbon 24.26

C 42.5

H 4.06

O 2.18

N 2.80

S 6.00

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Fig. 1 SEM images of carbon foams synthesized at initial pressure of (a) and (b) 2.5 MPa, (c) and (d) 7.8 MPa (T = 773 K, tpr = 5 s, tsk = 60 min).

Fig. 2 Effect of pressure on the density of carbon foams(T = 773 K, tpr = 5 s, tsk = 60 min).

3.2 Temperature

The carbon foams were synthesized from asphaltite pitch by varying the foaming temperature from 673 to 873 K at the initial nitrogen pressure of 6.8 MPa, holding time of 60 min at the final foaming temperature and a pressure release time of 5 s. The cell size distribution is wider and the cell size is larger at low foaming temperatures than at high temperatures.

Furthermore, the ligaments and junctions are not well developed and the spherical geometry is not the overwhelming structures at low foaming temperatures as compared with the structure at high foaming temperatures. In general, increasing the pressure and temperature could result in similar trends in foaming of asphaltite. However, the difference in the changing trend for density and compressive strength with varying pressure and temperature indicated that role of pressure and temperature in foaming is different. The effect of temperature on the density of the foams is shown in Fig. 4. Experimental results indicate a slight increase of density with the temperature between 683 and 883 K. The changes in compressive strength of the foams with temperature are shown in Fig. 5. The compressive strength of the foam is increased by 10 fold with temperature from 683 K to 823 K. The compressive strength sharply increases with temperature and levels off. The improved compressive strength with temperature in the low temperature range could be caused by a decreasing of viscosity with temperature. At high temperatures, this effect becomes weakened. Therefore, it may be concluded that when the carbon precursor is foamed at low viscosity the bubbles are formed under high stress, causing compaction of cells and leading to improved ligament, junction and wall.

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Fig. 3 Effect of pressure on the compressive strength of carbon foams (T = 773 K, tpr = 5 s, tsk = 60 min).

Fig. 4 Effect of temperature on the density of carbon foams (P = 6.8 MPa , tpr = 5 s, tsk = 60 min).

Fig. 5 Effect of temperature on compressive strengths of carbon foams (P = 6.8 MPa, tpr = 5s, tsk = 60 min).

3.3 Pressure release time

Pressure release is one of the most critical steps in foam formation. Volatiles released at high temperatures are dissolved in pitch at high pressures, lowering the viscosity of pitch and resulting in a complete melting of pitch[11]. When the pressure is vented down rapidly in about 3-5 s to atmospheric pressure, volatile components are released from the pitch, leading to the formation of bubbles, rapid movement of which

causes breaking of cells and as a results opens the cell structure[10,12,22,25]. An extraordinarily-high open porosity up to 98% is reported in the literature[26] by controlling pressure release times and other foaming parameters. In order to investigate the effect of the pressure release times on structure and properties of the foams, foaming was carried out at 6.8 MPa and 823 K for 1 h by varying pressure release time from 5 to 650 s.

Results indicate that the changes in the pressure release time have not a noticeable impact on the foam characteristics. Foaming carried out with long release times have resulted in carbon foams with relatively uniform structure small pore sizes and improved ligament and junctions. Increase in the pressure release time from 5 s to 600s slightly increases the foam density from 830 to 845 kg/m3. This is in line with the reported observations of an increase in quality of foaming with increased pressure release times. The increase in pressure release time, however, has resulted in a substantial increase in the foam compressive strength up to about 50% as seen in Fig. 6. Density increases rapidly with pressure release time up to 300 s and levels off. Variation of pressure release time appears to have different effects on the foaming of asphaltite and mesophase pitches. In case of mesophase, foaming characteristics improve with the release time. The foam density and compressive strength decrease from 560 to 240 kg/m3 and 3.31 to 2.16 MPa, respectively with pressure release times from 5 s to 600 s[27]. These results indicate that the two materials behave inversely with pressure release times in the foaming process. The difference between the foams produced from natural and synthetic pitches may originate partially from the high mineral content of the former, which introduces an additional factor affecting the viscosity of the pitch precursor and consequently cell formation and evolution mechanism under bubble expansion.

3.4 Soak time

The foaming experiments were conducted at 6.8 MPa and 823 K for different soak times from 15 to 60 min with a pressure release times of 5 s. SEM images reveal that the cells Fig. 6 Effect of pressure release time on compressive strength of

carbon foams (P = 6.8 MPa, T = 823 K, tsk = 600 s).

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Fig. 7 SEM images of carbon foams: (a) and (b) green carbon foam, (c) and (d) carbonized carbon foams (P = 6.8 MPa, T = 823 K, tpr = 5 s, tsk = 60 min).

Table 2 Characteristics of green and carbonized carbon foam samples

Mass m/g Density ρ/kg·m-3 Compressive strength σ/MPa Mass loss w/%

Green foam 2.84 830.50 10.50 Carbonized foam 2.24 829.50 18.70 21.20

become more uniform in size, more spherical in shape, and smaller in diameter with an increased soak time. The ligaments and junction are more developed and the number of cracks of the cell walls is decreased with increasing soak time. The density of the foams increases, from 820 to 832 kg/m3 and the compressive strength increases from 8.8 to 10.5 MPa with the soak time from 15 to 60 min. The increase in soak time results in compact and strong asphaltite foams.

3.5 Carbonization

Asphaltite pitch based carbon foam prepared at 6.8 MPa and 823 K for 60 min with a pressure release times of 5 s was subjected to carbonization described above. SEM images of the carbon foam before and after carbonization are shown in Fig. 7 with 25 and 200 magnifications. One of the most distinctive differences in the SEM images of green and carbonized foams is migration of some inorganic constituents as a result of high temperatures for carbonization. Of course,

this is important for the utilization aspect of the carbon foam and furthermore demineralization is needed for certain applications. The SEM images of green and carbonized foams reveals that the former has a more uniform structure. Both foams are characterized and the results are presented in Table 2. Carbonization has resulted in a 21% mass loss, and density stays the same but the compressive strength has increased by 80%. The mass loss has been caused by both organic and inorganic components. The loss of organic volatiles and reordering of graphenic sheets as well as changes in the inorganic structure due to melting and migration are believed to affect the foam characteristics. Asphaltite foams have much higher compressive strengths than mesophase pitch carbon foams. In mesophase pitch foams introduction of organic and inorganic constituents to the precursor resulted in a decrease in compressive strength[28], the sharp increase for asphaltite foams compared with mesophase foam is partially attributed to the incorporated organic/inorganic matrix of asphaltite pitch.

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The inorganic components act as reinforcement in asphaltite foams, which are associated with organic parts and do not disturb the foaming mechanism.

4 Conclusions

Carbon foams were synthesized from a natural pitch originated from Avgamasya in Turkey. The initial nitrogen pressure, foaming temperature, pressure release time and the inorganic content of the natural pitch were found to affect the microstructure and physical properties of carbon foams considerably. The increasing the initial nitrogen pressure results in an increase in the porosity, density and compressive strength of the foams. Furthermore, improvements in the foam structure in ligament, junction and wall are also observed. Similar role is also observed with the foam temperature. Pressure release time does not have a significant effect on density but has a substantial effect on the compressive strength of carbon foams. Soak time has a similar but less effect on the asphaltite foams than the pressure and temperature. The inorganic components of the asphaltite may serve as filler and are responsible for the improvement in some physical properties such as density and compressive strength. Carbonization causes a substantial increase in the compressive strength of the asphaltite foams.

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