Effect of β-resin of binderless coke on mechanical properties of high-density carbon blocks at high molding pressure

High-density carbon blocks have excellent mechanical, thermal, and electrical properties. In particular, these blocks are applied in various fields while maintaining excellent physical properties even in harsh environments. In this study, binderless coke manufactured under certain conditions was used to form green bodies (GBs) under various pressure conditions of 50 to 250 MPa, and the bodies were carbonized to form a high-density carbon block (CB). Then, the effect of the β-resin and oxygen functional groups of binderless coke on the mechanical properties of the high-density carbon block according to molding pressure was considered. When molding at a pressure of under 200 MPa, the ratio of O and C (O/C) has a greater effect, and the larger the O/C, the higher the mechanical properties. On the other hand, when molding at a high pressure of 250 MPa, the β-resin content has a greater effect and steadily increases when the β-resin content is low and when the mechanical properties are sufficiently reduced. In particular, in the case of CB-N7A3–250, which has the highest β-resin content of 3.7 wt%, the density was 1.79 g/cm3, the flexural strength was 106 MPa, and the shore hardness was 99 HSD.

www.nature.com/scientificreports/ done to prevent this problem. Mochida et al. reported that the raw material was oxidatively stabilized before molding, the test piece was not deformed even at high temperatures, volatile matter was removed so that swelling was suppressed, and then molding and heat treatment were performed 4,[25][26][27] . In addition, Ragan et al. oxidized needle-coke to give a hydroxyl group, a carbonyl group, a carboxyl group, etc. that can contribute to the bonding strength, mixed with a coal tar binder pitch and subjected to molding and heat treatment. Then, it was reported that the amount of oxygen functional groups and the amount of escaped oxygen were compared according to the degree of oxidation of needle coke, and high mechanical properties appeared when the needle coke was molded with the most oxygen functional groups 28 . In addition, volatile components of low molecule substances can be removed by vacuum heat treatment 29 .
In previous study, in the process of manufacturing binerless coke to suppress the swelling phenomenon, a gas mixed with nitrogen and air was blown as a carrier gas to increase the molecular weight through cross-linking of low molecular weight substances. An increase in molecular weight was indirectly shown through changes in the solubility of toluene and quinoline and the fixed carbon content through proximate analysis. In addition, using XPS, it was confirmed that oxygen was taken up and the O function groups were developed, and it was confirmed that the swelling phenomenon was suppressed as the flow rate ratio of air increased by SEM. Then, the density, flexural strength and shore hardness are measured, and it is reported that the mechanical properties increase 30 .
In this study, binderless coke, which is one of the priority materials, was produced by heat treatment at 470 °C while flowing a mixed gas of air and nitrogen from a coal tar pitch. Then, using this material, a high-density carbon block was produced by cold pressing at 50 to 250 MPa. After that, the β-resin content contained in the binderless coke was measured, and the atomic % of O and C of the binderless coke was measured using X-ray photoelectric spectroscopy (XPS) to derive the O/C value. Then, the changes in the contents of fixed carbon and volatiles were investigated via proximate analysis (PA). Finally, the effect of β-resin and oxygen functional groups of binderless coke on the mechanical properties of high-density carbon blocks was analyzed according to the molding pressure.

Materials and methods
Binderless coke. The physical characteristics of the binderless coke used in this study are shown in Table 1 30 .
As shown in the table, as the flow rate of air increases, the content of volatiles decreases and the content of fixed carbon gradually increases, showing the typical characteristics of coke produced through air blowing 31 . In addition, β-resin peaked at 3.7 wt% in N7A3 and then decreased from 1.2 wt% to 0.4 wt% in the order of N5A5, N3A7, and N0A10.

Preparation of green body and carbonization.
To analyze the change in the mechanical properties of the green body due to the molding pressure, the green body is manufactured from 50 to 250 MPa at room temperature. At this time, a mold of 35 × 35 × 40 mm size was used. The prepared green body was carbonized at 1200℃ for 1 h in a nitrogen atmosphere at a heating rate of 10 °C/min. Analysis. Proximate analysis. Proximate analysis was measured using thermogravimetric analysis (TGA, STA409PC, Netzsch Corp, Germany), and the analysis was performed by quoting the international standard KS E ISO 1171 30 . The content of β-resin was checked by ASTM D2318-15 32 and ASTM D4072-98 33 .
O/C ratio analysis. Hydroxy group (-OH), carbonyl group (-C=O), and carboxyl group (-COOH), etc. are generated when air is added to the coal tar pitch for reaction 34,35 . Using X-ray photoelectron spectroscopy (XPS, K-alpha + , Thermo Scientific, USA), the atomic % of O and C from the hydroxy group (-OH), carbonyl group (-C=O), and carboxy group (-COOH) imparted to the surface was measured, and the O/C ratio was determined 30 .
Mechanical properties. Flexural strength was measured using a Universal Testing Machine (UTM, WL2100, WITHILAB ltd, Korea) with reference to ASTM D790-17 36 . Shore hardness was measured using a shore hardness tester (SH, Type-D, Kobunshi Keiki, Japan) with ASTM D2240 37 . www.nature.com/scientificreports/ Porosity analysis. Apparent density and true density were measured to measure the porosity. Apparent density was measured using the Archimedes method, and true density was measured using a pycnometer (BEL pycno, MicrotracBEL). And porosity is calculated by equation 1 where ρ is density.

Results and discussion
Effect of molding pressure on green density. Figures 1 and 2 shows the change in green density due to molding pressure. The green body is density of green body before carbonization. The green density due to pressure is clearly displayed in the two regions. In region I, 50-200 MPa, the green density gradually increases to 1.38 g/cm 3 as the pressure increases, but at 250 MPa, the green density decreases or the rate of increase is very weak. Table 2 and Fig. 1 show the XPS results of analyzing the oxygen functional groups of the binderless cokes used in this research 29 . As shown in Table 2 and Fig. 1, the oxygen content in the binderless cokes increases as the proportion of injected air increases, and the O1s/C1s value increases from 6.97 to 11.20, with an increase of 38%. Figure 3 shows     www.nature.com/scientificreports/ is over 7.5, the green density value is generally slightly increased. However, when the manufacturing pressure is 250 MPa, the green density decreases as the O/C ratio increases. This means that the green density is very closely related to the molding pressure and O/C and β-resin contents 13,30,38-40 . Effect of β-resin content on green density at high molding pressure. Figure 4 graphs the changes in β-resin and O/C of the binderless coke produced according to the air flow rate analyzed in Tables 1 and 2.
The β-resin of the binderless coke increases to 3.7 wt% until the air ratio reaches 0.3 and then decreases to 0.4 wt%. On the other hand, the O/C steadily increases as the proportion of air increases, and it is often shown using graphs that the relationship between β-resin and O/C is trade-off. This is because as the flow rate of air increases, oxygen excessively crosslinks the small molecule substance, and the content of the β-resin having an intermediate molecular weight decreases so that it is present at a high molecular weight. These properties have a great influence on the physical properties of high-density carbon blocks and show the highest mechanical strength values when the air ratio is 1.0 at a molding pressure of 150 MPa, which has been discussed in previous studies 30 . Figure 5 shows the change in green density in region II in Fig. 2 according to the β-resin content. N10A0 and N7A3, which have β-resin contents of 2.7 and 3.7 wt%, respectively, have increased green density. On the other hand, in the case of N5A5, N3A7 and N0A10 having a low β-resin content of 1.2 wt% or less, the green density decreases when molded at high pressure. In particular, GB-N0A10 decreased by 2.63%. This is because the β-resin existing in the binderless coke leaked to the outside and did not act as a binder when molded at 250 MPa. However, in the case of GB-N10A0-250 and GB-N7A3-250, the densities increase by 1.0% and 1.2%, respectively, despite molding at 250 MPa. This is because, as shown in Fig. 4, there is sufficient β-resin in the binderless coke, so it plays a sufficient role as a binder. From this, it can be judged that the β-resin content, not the O/C, has a great influence on the mechanical characteristics in the high-pressure region of over 200 MPa. Figure 6 shows changes in density, flexural strength and shore hardness after carbonization. Similar to Fig. 1, Fig. 6 can be divided into two areas, one in which each mechanical property increases and the other in which it does not, based on 200 MPa. Figure 7 shows the changes in the mechanical properties of the carbon block after carbonization of region I in Fig. 6 by O/C. In the   www.nature.com/scientificreports/ low molding pressure region I of 200 MPa or less, the mechanical properties after carbonization increase as the molding pressure increases. This shows almost the same tendency as the tendency of green density. From this tendency, it can be seen that when the green density is high, high mechanical properties after carbonization can be ensured 11,41 . At a pressure of 200 MPa or less, the mechanical properties increase after carbonization as the oxygen content increases. These trends are consistent with previous papers 30 .

Carbonization characteristics according to O/C.
Effect of β-resin on mechanical properties at high molding pressure. Figure 8 is a graph showing the mechanical properties of region II of Fig. 6 according to the β-resin content. As shown in Fig. 8a, in the case of density, the lower the β-resin content, the greater the decrease in density when molded at 250 MPa, and in the case of CB-N0A10, the decrease is 1.98%. On the other hand, in the case of CB-N10A0 and CB-N7A3, it increased by 2.45% and 3.53%, respectively. This is because the β-resin content is sufficient, and β-resin does not escape to the outside even when molded under high pressure and is also present inside to increase the density. In particular, CB-N7A3-250 showed the highest density at 1.79 g/cm 3 . As shown in Fig. 8b, the lower the β-resin content was, the greater the decrease in flexural strength, which was 44% in the case of CB-N0A10. On the other hand, in the case of CB-N10A0 and CB-N7A3, they increased by 25% and 28%, respectively, and in particular, CB-N7A3-250 had the highest flexural strength of 106 MPaAs, as shown in Fig. 8c. The lower the   www.nature.com/scientificreports/ β-resin content was, the greater the decrease in shore hardness, which was 6.38% in the case of CB-N0A10. On the other hand, CB-N10A0 and CB-N7A3 increased by 4.17% and 10.00%, respectively, and CB-N7A3-250 showed the highest 99HSD shore hardness. Figure 9a and b show changes in porosity depending on the β-resin content of the high-density carbon blocks molded at 200 MPa and 250 MPa, respectively. CB-N0A10, which has the lowest β-resin content of 0.4 wt%, increased its porosity from 4.9% to 6.7%. On the other hand, the porosity of CB-N7A3, which had the highest β-resin content of 3.7 wt%, decreased from 3.3% to 0.6%. From this, it was found that the β-resin content has a great influence on the porosity of the carbon block, and this porosity has a great influence on the mechanical properties. The above-mentioned changes in mechanical properties can also be explained by this change in porosity. These trends in mechanical properties can be explained through the relationship between β-resin and O/C shown in Fig. 3. At relatively low pressures below 200 MPa, higher mechanical properties are exhibited when the O/C ratio is high, whereas at high pressures above 200 MPa, the β-resin content plays an important role. Therefore, even if the O/C is low, it can be judged that the high-density carbon block with a high β-resin content exhibits higher mechanical properties. As expected in Fig. 3, when the β-resin content is low, the amount that protrudes to the outside when molding at a low pressure of under 200 MPa is small, and it can play a sufficient role in the O-functional group. However, when molding at a high pressure over 200 MPa, β-resin, which acts as a binder, protrudes to the outside, make higher porosity and the mechanical properties deteriorate. On the other hand, when the β-resin content is high, β-resin is sufficiently present inside the carbon particles even at a high pressure exceeding 200 MPa and make lower porosity. As a result, β-resin content determine the porosity of high-density carbon blocks and that affects change of mechanical properties at high molding pressure.
Based on the above results, when the molding pressure is varies based on 200 MPa, the schematic diagram of the mechanical properties of the high-density carbon block with β-resin content can be represented as in Fig. 10. When the molding pressure is under 200 MPa, carbon particles are bonded by acting as a sufficient binder without outflow of β-resin, and the mechanical properties increase as the molding pressure increases.

Conclusion
In this study, carbon blocks were produced using binderless coke produced in a previous study, and the effects of the O/C ratio and β-resin content due to molding pressure on the mechanical properties were analyzed.
By up-taking oxygen into the binderless coke, the cross-linking reaction of volatile matter was induced to increase the molecular weight. As a result, the content of volatile matter can be reduced, the occurrence of swelling after carbonization can be prevented. It was found that β-resin and O/C have a cross-sectional relationship with each other, which has a great influence on the mechanical properties. The tendency of mechanical properties is displayed in the same way, and singular points appear, especially when molding at a pressure exceeding 200 MPa. This illustrates the fact that the β-resin content has a greater effect than the O/C ratio when molding at high pressure. As a result, the density of CB-N7A3-250, which appeared to have the best physical characteristics, was 1.79 g/cm 3 , the flexural strength was 106 MPa, and the shore hardness was 99 HSD. This is because the β-resin content is the highest at 3.7 wt%, and the physical properties steadily increase even when molded at high pressure. www.nature.com/scientificreports/ Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.