Industrial Carbon and Graphite Materials. Группа авторов. Читать онлайн. Newlib. NEWLIB.NET

Автор: Группа авторов
Издательство: John Wiley & Sons Limited
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Жанр произведения: Техническая литература
Год издания: 0
isbn: 9783527674053
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      Only a few materials are used as binders on an industrial scale, such as CTP for synthetic graphite and other not graphitized carbon products. Polymer resins are used as binders and impregnation materials in a wide variety of applications (thermosetting resins).

      6.1.1.1.4.1 Coal‐Tar Pitch

      CTP is the most common binder material for industrial carbons.

Binder pitch Impregnation pitch Special binder pitch
Softening point (Mettler) (°C) 100–140 65–90 180
QI (%) 6–13 2–5 10–16
TI (%) 24–33 14–20 38–45
Coking value (%) 50–60 42–48 >70
Viscosity (150 °C), (mPa s) 1206–4500 300–1200

      CTP is a highly complex mixture of hundreds of different aromatic compounds. Even with most modern analytical methods like matrix‐assisted laser desorption/ionization time‐of‐flight (MALDI‐TOF) spectrometry, it is not possible to obtain additional information to explain differences in pitch behavior during mixing, forming, and baking [25].

      Pitch specifications used in industry are a compromise of the most important characteristics. A pitch classification according to the solubility in various solvents has been developed [26]. Today the most common characteristic is the solubility in quinoline (quinoline insolubles [QIs]) and in toluene (toluene insolubles [TIs]). QIs contain all the solid contaminants and, if existing, also mesophase spheres (secondary QI), which can result from the distillation conditions or from thermal treatment of the pitch [27].

      The content of primary and secondary QI influences the production behavior and the physical properties of the synthetic carbon artifact. High content in primary QI (>12 wt%) gives hard and isotropic carbon after baking [28, 29]. Consequently, the strength of the carbon material is improved, but the thermal expansion behavior and the thermal and electrical conductivity are deteriorated. The thermal stress resistance of such a carbon artifact suffers as improved mechanical properties may not compensate the poor electrical and thermal properties.

      Secondary QIs negatively impact the interaction between the solid carbon filler and the binder pitch (wettability) during the mixing process. This can lead to losses in the apparent density of the artifact. Secondary QIs possess good graphitizability and give a binder coke with good electrical and thermal conductivity. The strength tends to lower values. The binder pitch demand during mixing increases with increasing QI content [30].

      TIs are large condensed aromatic molecules that are important for the carbon yield of the pitch and the gluing of the carbon granules. The difference between TI and QI is the β‐resin content of TI with a molecular mass of the molecules above 400 (5–20 ring aromatics). Other specification properties are the softening point, the carbon yield, viscosity, and ash content. The viscosity–temperature curve is an important factor for mixing and forming and contains additional information that cannot be drawn from the correlation between softening point and viscosity [31].

      Due to the soft distillation conditions of impregnation pitch, it remains rather reactive and changes during storage and transportation under temperatures of around 460 K and under impregnation conditions in the process. These polymerization and polycondensation reactions, so‐called aging, change the quality of the pitch and make special control necessary.

      6.1.1.1.4.2 Petroleum Pitch

      Petroleum pitch can be obtained from the lube oil process, from asphaltic residues from vacuum distillation, and from deasphalting units [34]. Improved petroleum pitch qualities can be obtained by treatment of decant oils, e.g. by dealkylation and dehydrogenation followed by condensation and polymerization reactions [35]. Condensation reactions can be enhanced by the addition of sulfur or Lewis acids [36, 37]. Today the main sources of petroleum pitch are decant oils from the catalytic cracker unit. The availability of commercial petroleum pitch is, however, limited to few producers.

      The interest in petroleum binder pitch in the graphite industry is rather limited. The reason is the poor artifact properties resulting from its use, particularly due its low carbon yield. This low carbon yield is due to a lower degree in aromaticity compared to CTPs. Thus petroleum‐derived pitch is used for binder application only in blends with CTP. Petroleum pitch is a niche product with currently one producer only.