Raman Spectroscopic Characterization of Environmentally Friendly Binder Systems for Carbon‐Bonded Filters

Abstract

A contribution to the production of zero‐defect materials is the development of carbon‐bonded alumina foam filters for removing nonmetallic inclusions from molten metals. Herein, such filters containing various mixtures of Carbores P, lactose, and tannin as binder are investigated by Raman spectroscopy. The filter samples differ in whether or not they are fired and whether or not n‐Si is added to them. Raman measurements are also performed on samples of the individual binder substances, which have been preannealed at temperatures up to 1400 °C. The variation of the sp2 carbon cluster sizes due to the heat treatment is estimated using the intensity ratio of the D and G bands as determined from these measurements. The conversion of the samples from hydrogen‐rich amorphous carbon systems to nanocrystalline graphite is observed. The results show no significant differences between the binders in regard to the material properties that can be studied with Raman spectroscopy.

A theoretical investigation into gallic acid pyrolysis

“…In recent years, there has been a pronounced research interest in environmentally friendly binders for carbon-bonded refractories based on magnesia 1 and alumina. [2][3][4] Carbon-bonded foam filters are commonly used to remove non-metallic inclusions from steel melts, ensuring favorable mechanical properties in the final product, which includes an increase of tensile strength, ductility, and fracture toughness as well as a decrease of fatigue compared to unfiltered steel. [1][2][3][4][5][6][7] Due to the enhanced durability of the steel castings after filtration, castings have to be fixed or discarded less often, leading to an economic advantage.…”

“…[2][3][4] Carbon-bonded foam filters are commonly used to remove non-metallic inclusions from steel melts, ensuring favorable mechanical properties in the final product, which includes an increase of tensile strength, ductility, and fracture toughness as well as a decrease of fatigue compared to unfiltered steel. [1][2][3][4][5][6][7] Due to the enhanced durability of the steel castings after filtration, castings have to be fixed or discarded less often, leading to an economic advantage. 5 Although steel filtration can also be performed with the help of sintered oxidic filters, [8][9][10][11] carbon-bonded filters exhibit excellent hightemperature behavior, such as a marked resistance to thermal shock and corrosion, and they do not suffer as much from creep as other filter materials.…”

“…1,[3][4][5][6][7]12,13 These desirable features of carbon-bonded filters are rooted in the use of carbon-containing binders, which form a stabilizing carbonaceous matrix between the filter particles after heat treatment. [1][2][3][4]12,14 The extent of this matrix, and thus the thermomechanical characteristics of the filter material, is partially determined by the carbon yield of the binder during the aforementioned heating. [1][2][3][4]14 A major problem of conventional binders in the form of pitches, resoles, and novolaks, even for state-ofthe-art binders such as Carbores ® P, lies in the emission of carcinogenic polycyclic aromatic hydrocarbons, primarily benzo[a]pyrene, or phenol during production and operation, making them an ecological liability and a health risk.…”

“…[1][2][3][4]12,14 The extent of this matrix, and thus the thermomechanical characteristics of the filter material, is partially determined by the carbon yield of the binder during the aforementioned heating. [1][2][3][4]14 A major problem of conventional binders in the form of pitches, resoles, and novolaks, even for state-ofthe-art binders such as Carbores ® P, lies in the emission of carcinogenic polycyclic aromatic hydrocarbons, primarily benzo[a]pyrene, or phenol during production and operation, making them an ecological liability and a health risk. [1][2][3][4]14 In contrast, binders based on mixtures of lactose and tannins are completely environmentally friendly in this sense.…”

“…This matrix resulting from environmentally friendly binders was described as amorphous or glassy, and while its general features mimic those of the Carbores®P matrix according to SEM images and Raman spectroscopy, it exhibited thinner struts and more cracks by comparison 1–4 . This issue is related to the fact that the carbon yield of lactose/tannins is generally lower than that of Carbores®P 1–4 . These differences in the carbonaceous matrix lead to unfavorable mechanical properties of the lactose/tannin‐based filters, as demonstrated in the form of a significantly lower cold crushing strength compared to Carbores®P 2 .…”