J. Bermejo

Chemical and physical properties of carbon as related to brake performance

Abstract The two objectives of this paper are: (a) to review the literature relevant to the structure, properties and applications of carbon-carbon (C/C) disc brakes, and (b) to present aspects of carbon science relevant to an understanding of the operational behaviour of such brakes. The literature describes numerous studies relating structure and composition of (C/C) disc brakes to wear mechanisms and performance. C/C brakes are manufactured from carbon fibres and a matrix carbon, usually an isotropic carbon from a resin, an anisotropic carbon from coal-tar pitch, or an isotropic carbon from pyrolysis of methane (CVD or CVI). Coefficients of friction depend upon the weave of the fibres, the matrix carbon, heat treatment temperatures, ambient gases such as air, water vapour and carbon dioxide, the testing conditions used (low- or high-speed), and the generation of wear dust. Wear generates debris (wear dust) from the surface which is then further comminuted during braking. Wear rates are also associated with gasification of both the brakevsurface and of wear dust. Mechanisms of generation of wear dust are summarised, and dull and lustrous wear dust surfaces are reported. Structures in the various forms of carbons are described and the special characteristics of C/C disc brakes are set out. The physical properties of graphite of dominant importance to brake performance are cleavage of the graphite crystal, and its thermal conductance and thermal capacity. Operational changes to braking performance are modelled. Carbon gasification reactions are explained, as well as the role of surface oxygen complexes and their influence on the absorption of water vapour which then, as a film, acts as a lubricant. The roles of wear dust during braking are elaborated upon.

Modification of coal-tar pitch by air-blowing — I. Variation of pitch composition and properties

Abstract Pitch oxidation by air-blowing has been studied as a possible method to modify pitch properties for composite preparation. Two commercial coal-tar pitches, an impregnating pitch and a binder pitch, were air-blown at temperatures between 250–300 °C, for 18 and 14 h, respectively, to produce pitches of similar softening point ranging from 148 to 223 °C. Parent and treated pitches were characterized by elemental analysis, FTIR, thermal analysis (TMA, TGA, and DTA), solubility in toluene and quinoline, XPS, X-ray diffraction, and optical microscopy. Additionally, parent pitches were also characterized by extrography. Pyrolysis products of parent and treated pitches, obtained at 450 °C, were characterized by optical microscopy to monitor the influence of air-blowing on mesophase formation. Results show that pitch modification by air-blowing produces a significant increase in carbon yield, without restricting the fluidity necessary to pitch for impregnation. During air-blowing, the reactions of dehydrogenative polymerization and crossing-linking of oligomers occur.

A comparative study of air-blown and thermally treated coal-tar pitches

Abstract A commercial impregnating coal-tar pitch was thermally treated at 430°C for times varying between 2 and 6 h and the same pitch was air-blown at 275°C for between 10 and 30 h. Both series of pitches were characterized and their properties compared. The pitches from the thermal treatments contained mesophase, the amount increasing from 10 vol.% to 65 vol.% with increasing time of treatment, whereas those from the air-blowing were completely isotropic. The study of the pitches revealed that the thermally treated pitches still contained a considerable amount of light compounds together with the large planar molecules generated. On the contrary, light components polymerized to a considerable extent during air-blowing to form cross-linked molecules. Evidence of cross-linked structures is provided by a decrease in the iodine uptake and also by the X-ray diffraction results. Furthermore, extensive removal of aliphatic hydrogen took place during pitch air-blowing, as confirmed by the increase in the aromaticity indices of the pitches.

Application of extrography for characterization of coal tar and petroleum pitches

Extrography has been used for fractionation of coal tar and petroleum pitches into six fractions of increasing polarity by a given sequence of solvents. Sample recovery, composition of the fractions and reproducibility of separation into distinct classes of compounds were determined. The results of several samples show the efficiency of extrography for the characterization of pitches of different origin. FT-i.r. and gas chromatography were used to evaluate the separation. Extrography is a rapid, simple and reproducible technique for the characterization of pitch materials.

Densification of unidirectional C/C composites by melted pitch impregnation

Abstract Four unidirectional pitch-based C/C composites were densified by multiple melted pitch impregnation under vacuum followed by recarbonization to 1273 K. The pitches used for the impregnation were the same pitches used as carbon matrix precursors in the preparation of the C/C composites. Recarbonization steps in one of the composites were also performed under pressure in order to study the effect of pressure during the densification process. The efficiency of the impregnation was evaluated in terms of the voids accessible to the impregnant and was followed by the weight gain during the process and polarized-light microscopy of impregnated composites. Wide differences in the densification process were observed dependent primarily on the physical and chemical properties of the pitch, especially the carbon yield and pitch fluidity. The composites at different stages of densification were characterized by standard procedures (bulk density and porosity), and microscopical techniques (polarized-light microscopy, image analysis and scanning electron microscopy). On the basis of results a mechanism describing the whole process involved in the densification of unidirectional C/C composites is proposed.

Pitch-based carbon composites with granular reinforcements for frictional applications

Abstract Four pitches (a commercial impregnating coal-tar pitch, an air-blown pitch and two thermally treated coal-tar pitches) and four granular carbons (graphite, anthracite, petroleum coke and foundry coke) were used to prepare carbon composites. A preliminary study of the main parameters involved in the preparation process of the carbon composites was carried out for the selection of the most suitable conditions. Mixtures with 30 wt.% of pitch and a moulding pressure of 80 MPa were selected and used for the preparation of all carbon composites. The behaviour of the resultant materials during carbonisation influenced their final properties. Composites obtained from the commercial pitch and air-blown pitch combined with anthracite and foundry coke deformed on carbonisation, presenting a high porosity and low mechanical properties. Materials obtained from the thermally treated pitches, foundry coke and anthracite were the ones with the highest compressive strength. The frictional behaviour of the materials was mainly governed by the type of granular carbon used.

Suitability of thermogravimetry and differential thermal analysis techniques for characterization of pitches

Abstract The pyrolysis of a number of coal tar and petroleum pitches was studied by means of thermogravimetry, derivative thermogravimetry and differential thermal analysis (DTA). Acenapthylene was used as a model compound to interpret the results of pitch pyrolysis. DTA exo- and endothermal peaks were assigned to different types of phenomena such as devolatilization, polymerization, condensation and cracking. It was found that endothermal phenomena (devolatilization) are prevalent for binder coal tar pitches, whereas exothermal phenomena predominate for petroleum pitches. Results obtained with various pitch fractions provided evidence of interactions among constituents of fractions.

Catalytic polymerization of anthracene oil with aluminium trichloride

Abstract A commercial anthracene oil was polymerized with anhydrous aluminium trichloride in the temperature range of 250–325°C. The kinetics of the process and the conversion of anthracene oil were monitored by thermogravimetric analysis, solubility parameters and carbon yield. The reactivity of the major anthracene oil components was monitored by gas chromatography. The cokes obtained at 900°C from reaction products were characterized in terms of optical texture and air reactivity. The results showed that the treatment of anthracene oil with aluminium trichloride led to pitch-like materials, temperature and aluminium trichloride concentration being the controlling parameters of anthracene oil conversion. The residue obtained by thermogravimetric analysis at 350°C was used as a measure of the degree of anthracene oil conversion. Cokes obtained by carbonization of reaction products, in which aluminium trichloride had previously been removed, showed an optical texture of a larger size and a lower air reactivity than cokes from reaction products where aluminium trichloride was present.

Mass spectrometric characterization of polynuclear aromatic nitrogen compounds in coal tar pitches separated by extrography

Abstract Extrography is a rapid technique for fractionating relatively large quantities of coal tar and petroleum pitches. Mass spectrometry (m.s.) and gas chromatography (g.c.)-m.s. have confirmed that the technique separates non-basic heterocyclic nitrogen compounds from aza bases in coal tar pitches. However, the nitrogen-containing fractions also contain higher molecular weight (MW) polynuclear aromatic compounds. Azabenzopyrenes are the major low-MW bases present. A range of mixed heterocyclic compounds were identified, including basic quinones.

Chemical and rheological characterization of air-blown coal-tar pitches

A commercial impregnating coal-tar pitch was air-blown at 275 °C for 10, 18, 25 and 30 hours. Isotropie pitches were produced with softening points ranging from 140 to 210 °C. The parent and treated pitches were characterized by elemental analysis, Fourier transform infrared, solubility in toluene and N-methylpyrrolidone, iodine adsorption and reflected light optical microscopy. The viscoelasticity of the pitches also was investigated using transient shear and controlled strain oscillatory rheometry. The chemical characterization of the pitches suggested that, as air-blowing proceeded, larger molecules formed through dehydrogenative polymerization (aromatization/condensation) and cross-linking of the pitch molecules. This is consistent with the rheological characterization of the pitches. The results obtained from transient shear and oscillatory rheometry showed that the parent impregnating pitch exhibited a viscous behavior (typical of fluids composed of small molecules). In contrast, the air-blown pitches, which contained large aromatic molecules, showed viscoelastic properties.