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Study of the Molecular-Weight Distribution of Binder Pitches for Carbon Blocks.

ABSTRACT: The present study aimed to identify the required characteristics of binder pitches in the filler-binder mixing process to effectively manufacture graphite blocks. To this end, a binder pitch was separated into pitch fractions of varying molecular-weight segments. The role and effectiveness of each pitch fraction were then analyzed with respect to their molecular-weight distribution. As a result, the optimal molecular-weight distribution was determined. More specifically, a coal-tar pitch was separated into solvent-soluble and solvent-insoluble fractions. The molecular-weight distribution was determined according to this classification, and the characteristics of each pitch fraction were examined. The pitch separation process was conducted using three solvents: hexane, toluene, and quinoline. The resulting pitch was separated into the following pitch fractions: hexane-soluble (HS), hexane-insoluble-toluene-soluble (HI-TS), toluene-insoluble-quinoline-soluble (TI-QS), and quinoline-insoluble (QI). Fourier transform infrared (FT-IR) spectrum, matrix-assisted laser desorption ionization-time of flight (MALDI-TOF), and softening point of each pitch fraction were measured. Also, pitch samples were refabricated while varying the mixing ratio of these pitch fractions, and carbon blocks were then prepared using them. The compressive strength and porosity of these blocks were measured and compared. The P154_B pitch with a high content of TI-QS was used to fabricate a green block. Due to the high viscosity of the binder used, the fluidity was not sufficiently high, and thus, the green block made of this pitch had relatively low strength. The other blocks had similar levels of strength. After the carbonization process, the carbon block with a high content of HS (P352_B-C) and the carbon block with the HS content removed (P073_B-C) showed lower compressive strength than their respective green-block counterparts (P352_B and P073_B). However, their strength was higher compared to those of the other carbon blocks. In the case of carbon block P073_B-C, the HS content was completely removed, and thus, the content of TI-QS (β-resin) was relatively high. Accordingly, this carbon block ended up with large amounts of components that had high coking values (CVs), and this contributed to limiting the formation of pores. Therefore, the compressive strength of this carbon block was high. In the case of the carbon block with a high content of HS (P352_B-C), a suitable level of viscosity was achieved because the HS components ensured high fluidity. As a result, blocks with higher density and compressive strength could be fabricated. The major findings of the present study confirm that producing carbon blocks with high mechanical properties requires binder pitches with a balanced combination of suitable viscosity to ensure sufficiently high fluidity and a proper level of CV to effectively suppress the formation of pores in the mixing and molding process.

SUBMITTER: Cho JH

PROVIDER: S-EPMC8153676 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:The present study focused on the development of a binder pitch to allow for low-temperature forming processes when fabricating coke-based artificial graphite blocks while increasing the density of the resultant blocks. To this end, high-softening-point (200 °C) pitches were fabricated. The pitch and byproducts obtained from the pitch synthesis were then used as binders to fabricate blocks with high mechanical strength and low porosity. Pitches were fabricated using pyrolyzed fuel oil (PFO), a petroleum residue. A high-softening-point (200 °C) pitch synthesized at 420 °C for 3 h was used as a binder pitch, and conventional pitch (124 °C) was synthesized at 400 °C for 1 h and then used. Pitch byproducts were extracted according to the boiling point of naphthalene (two rings) and anthracene (three rings) with varying numbers of aromatic rings by distillation. The largest amount of pitch byproduct was obtained in the temperature range from 220 to 340 °C, and the content of naphthalene in the byproduct was the highest over the entire temperature range. The fabricated pitches at 420 °C and byproducts were mixed to form modified pitches. It was found that their softening point and coking value (CV) decreased with the increasing content of the pitch byproduct. Low-boiling point components of the byproducts were removed from the modified pitches at the kneading process temperature (200 °C), and the mass-loss rate observed in the carbonization process temperature range (200-900 °C) was comparable to that of the high-softening-point pitch. The kneading rate of the pitch and byproduct was determined and selected based on the mass-loss rate described above, and blocks were then fabricated using a hot press. Subsequently, the fabricated blocks were subjected to heat treatment for carbonization (900 °C) and graphitization (2700 °C). After the heat treatment, the true density and apparent density of the blocks were measured, and the porosity of the blocks was calculated based on these values. The porosity of the graphite block fabricated using the pitch with a softening point of 120 °C was 21.84%, while the porosity of the graphite block fabricated using the modified pitch was 14.9%. For mechanical strength analysis, their compressive strength was measured. The compressive strength of the graphite block made of the conventional pitch (CP) was measured to be 47.59 MPa, while the compressive strength of the graphite block made of pitch mixed with a byproduct distilled at 220-340 °C was 58.79 MPa. This result suggested that a decrease in the porosity resulted in increased mechanical strength. The application of the modified pitches developed in the present study temporarily decreased the softening point of the high-softening-point pitch due to the effect of the added byproducts, allowing for a low-temperature forming process. It was also possible to fabricate artificial graphite blocks with low porosity due to the high CV of the high-softening-point pitch. As a result, blocks with high mechanical strength could be obtained.

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Study of the Molecular-Weight Distribution of Binder Pitches for Carbon Blocks.

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