Parent Pitch Research Articles

Contribution of the isotropic phase to the rheology of partially anisotropic coal-tar pitches

A commercial impregnating coal-tar pitch was thermally treated at 430°C for 2–5 h in nitrogen. Resultant pitches, which contained mesophase ranging from 10 to 46 vol.%, were filtered to separate the isotropic phases, in order to study their contribution to the rheology of the whole pitches. The parent pitch, thermally treated pitches and the individual isotropic phases were characterized by measuring their elemental composition, softening point, carbon yield, solubility in toluene and in N-methyl-2-pyrrolidone, mesophase content (by optical microscopy) and X-ray diffraction parameters. The rheological behavior of the pitches and the isotropic phases was studied using steady shear rheometry. Results showed that not only had the whole pitches, which were partially anisotropic, a non-Newtonian behavior, but also the single isotropic phases.

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

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.

Influence of QI on mesophase structure

The influence of quinoline insoluble (QI) particles on the transformation of isotropic pitch to mesophase has been studied using light microscopy, scanning electron microscopy, and transmission electron microscopy. The authors conclude that QI does not significantly affect either the nucleation of mesophase spheres or the rate at which mesophase forms. However, QI does profoundly affect the structure of the pitch at all stages of conversion and the structure of carbons derived from mesophase. QI particles appear to “pin” the mesophase structures in their vicinity, making the mesophase behave as a much more viscous material than would otherwise be the case; this is consistent with the many changes of layer plane orientation observed within QI-rich mesophase. The complex structure caused by the QI persists through to the stage of solidification so that some properties of the carbon product may be much modified by controlling the occurrence of QI in the parent pitch. All as-received pitch samples, including nominally “QI-free pitches” contain significant numbers of QI particles.

Pyrolysis of coal-tar pitches: kinetics of conversion to coke and iodine adsorption by mesophase

Pitch is the parent material for cokes of major commercial importance. These cokes are graphitizable as a result of formation of liquid crystals (mesophase) during pitch pyrolysis. The concept of molecular cohesion energy to create mesophase is described. Extents of formation of mesophase from blends of different pitches do not follow an additive rule. Mesophase has chemical properties similar to the parent pitch. Iodine adsorption (up-take) by mesophases from different pitches indicates that they solidify differently, HTT 500° to 600°C.

Modification of mesophase pitch by blending Part 2 Modification of mesophase pitch fibre precursor with thermoresisting polyphenyleneoxide (PPO)

Polyphenyleneoxide was blended in amounts of 5 or 10 wt% into petroleum-derived mesophase pitch to reinforce the pitch fibre before the oxidative stabilization to achieve better handling properties. Although polyphenyleneoxide was fusible but hardly soluble in the mesophase pitch even at a spinning temperature of ∼ 350° C, blended pitch could be smoothly spun into pitch fibre 10μm diameter, as could the parent pitch. Fibrous polyphenyleneoxide of less than 1μm diameter was homogeneously dispersed in the pitch fibre, being arranged along the fibre axis. Such fibrous polyp henyleneoxide reinforced the pitch fibre considerably. The fibrous substances at the centre of the fibre disappeared in the carbonized fibre at 1300° C after oxidation at 250° C, although some short ones were observed in the skin region of the fibre, suggesting that polyphenyleneoxide was co-carbonized to be assimilated with mesophase pitch at the centre of the fibre, where the effects of oxidation may be rather limited. The oxidation reactivity and its mechanical strength after carbonization were slightly lower in comparison with those of the parent mesophase pitch.

Electrical resistivity of pitch during heat treatment

The d.c. electrical resistivity of coal-tar pitch has been measured from ambient temperature to 450 °C. The change in resistivity with temperature up to 200 °C has been found to be consistent with viscosity-controlled diffusion of current carriers. In an isothermal study at 450 °C, the start of mesophase formation was found to be accompanied by a rapid rise in the pitch resistivity, which subsequently levelled off and remained nearly constant until the onset of mesophase coalescence and phase inversion. The resistivity of the resultant mesophase pitch was found to be less temperature susceptible than that of the parent pitch.

Hydrogen donor ability of pitch and e.s.r. characterization of iodine complexes

This study examines properties of pitches by complexing molecular constituents with iodine and relating e.s.r. data with hydrogen donor ability and X-ray analyses. Spin concentrations were measured for the parent pitches and for complexes containing 10 wt% iodine. Hydrogen donor abilities were measured by formation of 9,10-dihydroanthracene from anthracene. Stacking relations between molecules were deduced from X-ray data. A correlation was found between the hydrogen donor of the pitches and the ratio of spin concentration in the complexes and the parent pitch.

The effects of pressure on the carbonization of pitch and pitch/carbon fibre composites

Three pitches which give carbons of varying optical texture have been carbonized singly and with different carbon fibres at pressures in the range 0.1 to 200 MPa. The effect of pressure on the carbonization system is to retard growth and coalescence of the growth units of mesophase, thus reducing the size of the optical texture of the resultant carbon. With increasing pressure botryoidal (spherical) structures are formed. On co-carbonization of pitches with carbon fibres the alignment of the basal planes of the matrix carbon parallel to the fibre length at the fibre/matrix interface is improved within a given pressure range. This range is dependent upon the parent pitch used and is experimentally determined. This effect is seen for all fibre types.

Unusual structures in mesophase from a petroleum pitch

Cokes from the carbonization of a petroleum pitch possess an optical texture containing anisotropic spherules with an unusual structure. These spherules (10 to 20 μm diameter) were found to be present in the original pitch and could be removed as a quinoline-insoluble fraction. This suggests that the spherules are formed during the modification of the pitch after initial fractionation of the petroleum feed-stock. The optical texture of these spherules, examined by polarized light optical microscopy, reveals a complicated ‘rosette’ structure quite unlike the structure of the spheres of mesophase now known as ‘Brooks and Taylor’ spheres. An EDAX analysis of polished segments of the spherules indicated no unusually high concentration of inorganic matter, e.g. vanadium, suggesting that the spherules did not originate from asphaltenes. The influence of such spherules or inclusions in the parent pitch upon properties of resultant cokes is briefly commented upon.