Catalytic graphitization of novolac resin for refractory applications

This study investigated how to induce graphite generation from the carbonization process of novolac resins using conditions that can be adopted for carbon-containing refractories (CCRs) production. The effect of boron oxide or boric acid (graphitizing agents), cross-linking additive (hexamethylenetetramine) and some processing parameters (mixing technique, vacuum degassing, heating rate and thermal treatments) on carbon graphitization from a commercial novolac resin were evaluated. The X-ray diffraction (XRD) technique was selected to measure the graphitization level and crystal parameters of the prepared samples. Based on the attained results, adding graphitizing agents prior to the pyrolysis of resin resulted in carbon crystallization. The best graphitization level was obtained when the mixtures containing 6 wt% B₂O₃ or 10 wt% H₃BO₃ were fired up to 1000 °C for 5 h using a heating rate of 3 °C/min. Although the reproducibility of the obtained results was ascertained, heterogeneous graphitization could be observed based on the XRD profiles, as well as some discrepancies in the calculated graphitization level values. This phenomenon was attributed to the additives susceptibility to agglomeration, preferential graphitization starting from lower binding energy sites and heat treatment temperature, among others.     

Phenolic resins: 100 Years and still going strong

After 100 years phenolic resins continue to be a prominent resin system with an impressive worldwide volume of nearly 6 million tons/year. It is a ubiquitous adhesive for a diverse spectrum of materials such as wood, glass, metal, paper and rubber with several of these applications being developed by Baekeland during the early stages of his resin commercialization. Many recent technical conferences have been held and were identified with important early Baekeland advances such as Baekeland 2007, Baekeland 2009 and the more recent Baekeland 2011 – all commemorating different initial activities of Baekeland and centennial recognition of his 1907 patent, first production of phenolic resin in Erkner, Germany in 1909 and the centennial celebration of the production of phenolic resin in Japan in 1911. This presentation provides an overview and evaluation of large volume application markets for resole and novolak resins in 2011 and comments related to anticipated greater growth of novolak resins over resole resins. Both resole and novolak resins are viewed as reactive intermediates that undergo a variety of chemical transformations into various improved and in many instances upgraded resin systems that lead to both recognizable and newly reactive resins for value added products. New areas involving phenolic resins are described and consist of Phenol Resorcinol Formaldehyde (PRF) resins, Nanomodification, Novel Novolak Process, ionic liquids, Phenolic Hybrids, and Poly Aryl Ether Amide based on novolak and phenylene bisoxazoline (PBO).     

Comparison between carbonization of wood charcoal with Al-triisopropoxide and alumina

A comparison was made between the catalytic carbonization of biomass carbon suspended in Al-triisopropoxide and in biomass carbon mixed with 40 μm sized Al₂O₃ particles. Both types of samples were plasma sintered during 5 min under an argon pressure of 50 MPa at temperatures up to 2200 °C. Plate-like catalytic graphitization develops by formation and dissociation of plate-like Al₄C₃. Plasma sintering under the proper CO partial pressure and heat treatment temperature is instrumental in forcing the Al₂O₃ to react with the carbon, forming first Al₄C₃ and subsequently graphite. The difference between Al-triisopropoxide and Al₂O₃ is a matter of intensity of the graphite reaction versus the size of the graphite patches.     

Synthesis of structure-controlled carbon nano spheres by solution plasma process

Structure-controlled carbon nanospheres (CNSs) were synthesized by an innovative plasma in liquid method, termed solution plasma processing (SPP). CNSs were formed by using benzene as a carbon precursor. Typically, 500 mg of CNSs were obtained from 100 ml of benzene with 20 min of treatment. The average diameters of CNSs increased from 20 to 100 nm when the pulse frequency of the bipolar power supply adjusted from 25 to 65 kHz. The TEM images showed that CNSs synthesized at 25–50 kHz consisted of amorphous carbon, while CNSs generated at 65 kHz were composed of continuous short range graphite with turbostratic structure. X-ray diffraction (XRD) patterns and TEM images showed that CNSs synthesized at 25–50 kHz had a low graphitization degree, while CNSs synthesized at 65 kHz consisted of graphite sheets with regular and ordering structure in basal planes. The increase of D/G intensity ratio in Raman spectroscopy confirmed the transition from amorphous carbon to nanocrystalline-graphite (NCG) with increasing pulse frequency. The resistivity of CNSs also increased with increasing pulse frequency. CNSs synthesized at 65 kHz have shown similar degree of resistivity as other commercial carbon black materials.     

Phenolic resin binder for the production of metallurgical quality briquettes from coke breeze: Part III the effect of the type of acidic hardeners on the quality of the formed coke and the possibility to avoid the curing stage to produce metallurgical briquettes with enough strength

In order to avoid the curing stage in formed coke production, the effects of hardeners, on the tensile strength of the briquettes bonded with resole binders of F / P = 2.0 and N / P ratios ranging 0.1–0.5, were studied. The raw briquettes were produced by mixing the resole binders containing these hardeners, with the coke breeze. They were hardened at room temperature for 24 h and also at 200 °C for 2 h. The briquettes were also produced from the resole binders of the same N / P but containing no hardener, and cured at 200 °C for 2 h. The tensile strengths of the briquettes were measured. H₃PO₄ resulted in the briquettes having the lowest tensile strength under any conditions investigated. H₂SO₄ and p-TSA hardeners were found to be unsuitable for the aim of this investigation, because they could not fully harden the briquettes at room temperature. ATP resulted in the briquettes of the highest tensile strength of 52.13 MPa, with the resole binder of N / P = 0.1 after 24 h hardening at room temperature but resulted in briquettes of relatively lower tensile strength when the N / P ratio of resoles was increased. By blending ATP and p-TSA it became possible to produce formed coke with sufficient tensile strength by hardening them at room temperature.     

Formation of hollow MgO-rich spinel whiskers in low carbon MgO–C refractories with Al additives

MgO–C refractories with different carbon contents have been developed to meet the requirement of steel-making technologies. Actually, the carbon content in the refractories will affect their microstructure. In the present work, the phase compositions and microstructure of low carbon MgO–C refractories (1 wt% graphite) were investigated in comparison with those of 10 wt% and 20 wt% graphite, respectively. The results showed that Al₄C₃ whiskers and MgAl₂O₄ particles formed for all the specimens fired at 1000 °C. With the temperature up to 1400 °C, more MgAl₂O₄ particles were detected in the matrix and AlN whiskers occurred locally for high carbon MgO–C specimens (10 wt% and 20 wt% graphite). However, the hollow MgO-rich spinel whiskers began to form locally at 1200 °C and grew dramatically at 1400 °C in low carbon MgO–C refractories, whose growth mechanism was dominated by the capillary transportation from liquid Al at these temperatures.     

Nanoscopic observations of stress-induced formation of graphitic nanocrystallites at amorphous carbon surfaces

The formation of graphitic nanocrystallites at the surface of amorphous carbon under large mechanical stresses was examined by using micro-Raman spectrometry, transmission electron microscopy and in-situ compressions. In the Raman analyses of severely deformed (above a strain energy density criterion of 5.9 J/m²) surface regions of nanoscratched and nanoindented amorphous carbon films, two additional sharp and narrow peaks, D_Gr and G_Gr at 1330 and 1580 cm⁻¹, appeared from the main unchanged broad spectra, revealing the transformation of some small-range amorphous carbon to nanocrystalline graphite. Transmission electron microscopic images presented the formation of surface shear layer within which dispersed graphitic nanocrystallites (a size of about 3 nm) were formed in the remaining amorphous matrix. The in-situ nanoscopic observation of amorphous carbon nanopillars under compressions confirmed the formation of graphitic nanocrystallites at pillar edge surfaces. The formed graphite (0 0 1) and (1 0 0) lattices were well oriented along maximum resolved shear stresses, being an evidence of lattice reconstruction and suggesting a possibility of stress-induced graphitization of amorphous carbon in the absence of heat.     

The influence of the formation of Fe3C on graphitization in a carbon-rich iron-amorphous carbon mixture processed by Spark Plasma Sintering and annealing

A promising fabrication method of bulk porous graphitic materials is based on consolidation of metal-amorphous carbon powder mixtures, in which the metal serves as both a graphitization catalyst and a removable space holder. In this work, iron was evaluated for this purpose. The phase formation and evolution in a carbon-rich iron-amorphous carbon mixture during Spark Plasma Sintering (SPS) and subsequent annealing was studied to reveal the peculiarities of the low-temperature catalytic graphitization process determined by the transformations of the iron catalyst. Mixtures of carbon black with iron of the Fe-20 wt%C composition were ball milled, Spark Plasma Sintered at 600–900 °C for 5 min and further annealed at 800 °C for 2 h. During the SPS, iron carbide Fe₃C formed, while the free carbon remained poorly graphitized. In the compact sintered at 900 °C, Fe₃C was the only iron-containing phase and metallic iron was not detected. For conducting structural studies of the free carbon by X-ray diffraction and Raman spectroscopy, iron was dissolved from the sintered compacts in HCl solution. It was found that during annealing, the graphitization degree increased only in the compacts that still contained free (metallic) iron. These results suggest that Fe₃C does not catalyze graphitization in a carbon-rich mixture of iron and carbon black making the presence of residual (metallic) iron crucial for the advancement of catalytic graphitization during annealing.     

Nano carbon containing MgO-C refractory: Effect of graphite content

Development of low carbon containing MgO-C refractories has been studied containing a fixed 0.9 wt% of nano carbon and 1–9 wt% of flake graphite. Refractory compositions were prepared and processed as per the conventional manufacturing techniques of MgO-C refractory. Properties of the different compositions were evaluated and also compared against the conventional MgO-C refractory containing 10 wt% graphite prepared under exactly similar conditions. Addition of 3 wt% of flake graphite in combination with 0.9 wt% of nano carbon black was found to be optimum and resulted in better/comparable properties to that of conventional MgO-C refractory.     

Low temperature graphitization of interphase polyacrylonitrile (PAN)

Ordered polyacrylonitrile (PAN) interphase structures were formed in solution-cast PAN/carbon nanotube (CNT) composite films by enhancing polymer crystallization conditions and processing parameters for five types of CNTs. All film samples were heat-treated using similar stabilization and carbonization (up to 1100 °C) processes. Both the precursor and carbonized materials were characterized by electron microscopy and X-ray spectroscopy. Highly ordered graphitic structure was formed predominantly in the carbonized materials at 1100 °C (i.e., ∼1500 °C lower than the temperature used in a commercial graphitization process). The ordering of the graphite structure formed at 1100 °C was further improved by heat treatment up to 2100 °C. Multiple characterization results indicate that the early onset of PAN conversion to graphite is directly related to the polymer interphase formation as well as the CNT type. Based on the stabilization and carbonization parameters used in this study, PAN/single-wall carbon nanotube (SWNT) samples showed more prevalent graphite formation at 1100 °C. This work demonstrates the influence of CNT type regarding interfacial confinement toward this low-temperature polymer-to-graphite conversion process.     

Phase and microstructural evolution based on Al,Si and TiO2 reactions with a MgO-C resin-bonded refractory

Adding carbon to refractory products (i.e. MgO-C) usually results in unique properties that allow these materials to attain the performance level required for the steel-making process. Nevertheless, C oxidation is still a big concern and using antioxidant additives has become an important alternative to prevent carbon loss and induce the generation of further compounds (carbides, nitrides, etc.) in the fired microstructure, which will affect the overall thermo-mechanical behavior of the refractories. Aiming to better understand the main phase transformations derived from the interaction of polymeric binders with antioxidants and other refractory components in the MgO-C system, this work addresses the evaluation of mixtures containing novolak resin + antioxidants (Al, Si, and/or TiO₂) and: (i) fine MgO, (ii) ferrocene (catalyst for carbon graphitization), and (iii) coal-tar-based binder. X-ray diffraction, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analyses were carried out in order to identify the main phases comprising the fired samples (kept at 1000 °C and 1400 °C for 5 h in a reducing environment) and observe their morphology and distribution in the resultant microstructure. Based on the attained results, a great variety of whiskers with different shapes, dimensions and distributions were also observed on the fractured surface of the prepared samples containing antioxidants. Furthermore, not only the carbon graphitization, but also the presence of MgO and other binders (Carbores® P) affected the phase evolution at a high temperature, which highlights the complexity of phase transformations and the many likely interactions derived from the combination of various raw materials.     

Strengthening of Al2O3-C slide gate plate refractories with microcrystalline graphite

The novel carbon sources (including nano-carbon black, carbon nanotubes and graphene oxide nanosheets, etc.) have been extensively researched in low carbon Al₂O₃-C refractory systems. In the present work, ultrafine microcrystalline graphite (UMCG) and nickel-loaded ultrafine microcrystalline graphite (NMCG) were added into low carbon Al₂O₃-C slide gate plate refractories to partially replace graphite flake (GF), respectively. The mechanical properties, phase compositions and microstructures were investigated by three-point bending test, X-ray diffraction, scanning electron microscopy, transmission electron microscopy and energy dispersive X-ray spectroscopy. Also, the reaction mechanisms of in-situ formed ceramic phases were discussed by thermodynamic analysis. The results indicate that the existence of UMCG powders can facilitate the in-situ formation of intertwined ceramic whiskers, leading to increased densification and mechanical properties of low carbon Al₂O₃-C slide gate plate. Moreover, multi-walled carbon nanotubes and ceramic phases intensively interlock with each other in the Al₂O₃-C refractories containing NMCG powders, which results in their better mechanical properties; the cold modulus of rupture are 36.03±0.12 MPa and 32.14±0.17 MPa for the specimens after coking at 1200 °C and 1350 °C, respectively. This work puts forward a practical application for the microcrystalline graphite as a candidate carbon source in Al₂O₃-C slide gate plate refractories.     

Sustainable phenolic thermosets coatings derived from urushiol

The over-exploitation of finite fossil resources and/or the increased environmental and sustainable awareness inspire scientists and technologists to search for inexpensive alternatives from renewable chemicals. Phenol formaldehyde (PF) resins, the oldest type of synthetic polymers with good mechanical properties and heat resistance, are widely used in the production of coatings, laminates, molding compositions, and glues. Here, biobased urushiol-derived PF resins were synthesized from the alkali-catalyzed reaction between urushiol and formaldehyde. The chemical compositions and molecular structures of resole resins were characterized by carbon-13 nuclear magnetic resonance and Fourier transform infrared spectroscopy, and their curing behaviors were studied by differential scanning calorimetry. The as-prepared urushiol-derived resole resins had methylol (Ph−CH₂OH), ortho- and para-hemiformal groups (Ph−CH₂OCH₂OH), and the para−para/ortho−para/ortho−ortho links of methylene groups (Ph−CH₂−Ph), whereas the resole resins had low curing temperatures at about 100–113°C. Additionally, given the long side alkyl group moiety on the aromatic rings of urushiol, the films of cured urushiol-derived resole resins had low glass transition temperatures of 132 ± 2°C. Furthermore, the as-prepared urushiol-derived coatings exhibited excellent physical and mechanical properties.     

Graphitization at low temperatures (600–1200 °C) in the presence of iron implications in planetology

Our objective is to understand how graphite can be formed at “low” temperatures (<1200 °C) in contrast to the high temperature of the industrial processes (∼3000 °C), and from precursors which are non-graphitizable by a thermal treatment alone. Blends of iron and saccharose char were heated between 650 and 1600 °C. The carbons obtained were characterized by SEM, TEM and Raman microspectrometry. Our work confirms that graphite can be formed from non-graphitizable carbons during a heat-treatment in the presence of iron. Carbon and iron migrations, below the eutectic temperature (1150 °C), appear to be a key factor for carbon transformation. Iron migration and graphitization could be favored by nucleation of Fe nanoparticles and surface melting, detected as soon as 900 °C. This allows formation of turbostratic macroporous carbons. Above the eutectic, all iron is liquid and graphitization occurs; it is complete at 1600 °C. Heat-treatment duration, observed over 4 orders of magnitude, favors the structural improvement. Concerning applications in planetology these experimental samples are pertinent experimental analogues of natural carbons from differentiated parent-bodies (with an iron core), and explain how graphite can be formed at temperatures below 1200 °C in these environments.     

High rate performance activated carbons prepared from ginkgo shells for electrochemical supercapacitors

Partially graphitized ginkgo-based activated carbon (GGAC) is fabricated from ginkgo shells by pyrolysis, KOH activation and heat treatment using cobalt nitrate as graphitization catalyst. The graphitization temperature is 900 °C. The GGAC has a microporous structure and its specific surface area is 1775 m² g⁻¹. XRD patterns show that the carbon becomes more graphitic after heat treatment. The specific capacitance of the GGAC reaches to 178 F g⁻¹ at a potential scan rate of 500 mV s⁻¹, which is superior to that of commercial activated carbons and ordered mesoporous carbons. The high electrochemical performance of the GGAC is attributed to its good electronic conductivity and high surface area. Partially graphitized activated carbon is a promising electrode material for electrochemical supercapacitors with high rate performance.     

Large-scale synthesis of hollow highly-graphitic carbon nanospheres by the reaction of AlCl3·6H2O with CaC2

Hollow carbon nanospheres (HCNSs) were prepared on large scale by the reaction of AlCl₃·6H₂O and CaC₂ in the temperature range of 250–500 °C. Characterization of the products by X-ray diffraction, Raman spectroscopy and transmission electron microscopy indicates that they have a uniform size of about 30 nm and a high degree of graphitization. Based on a series of comparative experiments and the systematic characterization of the products, both the water of crystallization and AlCl₃ in AlCl₃·6H₂O play crucial roles in preparing the HCNSs. It was found that AlCl₃ exhibits a strong graphitization behavior on solid carbon nanospheres even at low temperatures, and the graphitization contributes to their evolution to HCNSs.     

Synthesis and properties of 1,3,4-oxadiazole-containing high-performance bismaleimide resins

Two novel bismaleimide (BMI) monomers containing 1,3,4-oxadiazole, i.e., 5-tert-butyl-1,3-di[5-(4-maleimidophenyl)-1,3,4-oxadiazole-2-yl]benzene (Buoxd) and 4,4′-[5-(4-maleimidophenyl)-1,3,4-oxadiazole-2-yl]diphenyldimethylsilane (Sioxd), were designed, synthesized and copolymerized with 4,4′-bismaleimidodiphenylmethane (BMDM) to yield a new series of high-performance bismaleimide resins. Both monomers obtained are readily soluble in common organic solvents, such as dichloromethane and chloroform, enabling an easy solution processing. The thermal properties of the two monomers were carefully studied by the differential scanning calorimetry (DSC), optical microscopy and thermogravimetric analysis (TGA, simultaneous DSC). The BMI resins based on a mixture of Buoxd (or Sioxd) and BMDM in a weight ratio of 10% were prepared. DSC investigations showed that the thermal curing of the BMI resins could be accomplished at a lower temperature than the thermal curing temperatures of Buoxd and Sioxd, and the thermal processing window, i.e., the temperature range between the melting transition and thermal curing process, was over 26 °C. The thermal properties and thermal mechanical properties of the resulting BMI resins were investigated by DSC, thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA). No glass transition temperature was found in the range of 50–350 °C, and very good thermal stability (T_d > 490 °C in nitrogen) and high thermo-oxidative stability (T_d > 460 °C in air) were revealed. Composites composed of the above BMI resins and glass cloth were also prepared, which showed high bending modulus (>1.6 GPa) at a very high temperature (e.g., 400 °C).     

Electron microscope study of the formation of graphitic nanostructures in nickel-loaded wood char

We investigated the graphitization of carbonized larch wood chars impregnated with aqueous solution of nickel acetate, using scanning electron microscopy (SEM), both in secondary and transmission modes, and high-resolution transmission electron microscopy (HRTEM). Heat treatment of the chars at 500 °C brought about homogeneous distribution of metallic Ni particles about 5 nm in size in the amorphous carbon matrix. Graphitization sporadically started at this temperature, and some of the Ni particles are aggregated. SEM observations on chars heat-treated at 900 °C suggested that graphitic nanoshells about 50–200 nm in diameter, formed by catalytic effects of the Ni particles, grow in a “meandering” manner inside the amorphous carbon matrix. Some graphitic protrusions are found to grow outwards. Upon removal of the residual amorphous carbon matrix, long chains of the graphitic nanoshells exhibited a three-dimensionally intertwined structure, while transmission SEM showed that the interior of the shells is empty. HRTEM images exhibited not only stacked graphitic layers, but also cross-sectional contrasts expected from the hexagonal net of the graphite structure. These findings are discussed from the viewpoints of processing parameters, such as the use of aqueous solutions and atmosphere, specific to the catalytic graphitization of lignocellulosic materials.     

Acceleration of graphitization on carbon nanofibers prepared from bacteria cellulose dispersed in ethanol

Bacteria cellulose is composed of pure cellulose nanofibrils with about 30–60 nm width. The original conglomerate of bacteria cellulose was dispersed in ethanol or distilled water and filtered, and paper like sheets were prepared. The conglomerate itself and the sheets were carbonized and then heat-treated at 3100 °C. The development of graphite structure by the heat treatment was observed for these sheets, especially for that derived from the nanofibrils dispersed in ethanol, while the heat-treated conglomerate exhibited the nature of nongraphitizing carbon. The difference in graphitizability seems to be attributed to graphitization behavior on the surface of nongraphitizing carbon.     

Synthesis,processing and properties of TaC–TaB2–C ceramics

Multi-phase ceramics in the TaC–TaB₂–C system were prepared from TaC and B₄C mixtures by reactive pressureless sintering at 1700–1900 °C. The pressureless densification was promoted by the use of nano-TaC and by the presence of active carbon in the reaction products. The presence of TaB₂ inhibited grain growth of TaC and increased the hardness compared to pure TaC. If a coarse TaC powder was used, the compositions did not densify. In contrast, pure nano-TaC was pressureless sintered at 1800 °C by the addition of 2 wt.% carbon introduced as carbon black or graphite. The introduction of carbon black resulted in fully dense TaC ceramics at temperatures as low as 1500 °C. The grain size of nominally pure TaC ceramics was a strong function of carbon stoichiometry. Enhanced grain size in sub-stoichiometric TaC, compared to stoichiometric TaC, was observed. Additional work is necessary to optimize processing parameters and evaluate the properties of ceramics in the TaC–TaB₂–C system.