Mesophase development in petroleum and coal-tar pitches and their blends

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Abstract

The behaviour of mixtures of an impregnating coal-tar pitch and a petroleum pitch during the early stages of carbonisation was studied, focusing on the formation and development of mesophase and the role played by primary quinoline insolubles (QIs). The blends were prepared in different proportions ranging from 20 to 80% of the impregnating pitch. The parent pitches and their blends were thermally treated at 430 °C for 3 h and the resultant products were characterized by thermal analysis and optical microscopy. The results showed that primary QIs not only control the coalescence of mesophase spheres but also the nucleation of new spheres in both parent pitches and their blends. Moreover, only the petroleum pitch was able to generate a massive amount of small spheres throughout the isotropic phase.

Introduction

Carbon materials form a vast and diverse family of materials with applications varying from the lead of a pencil to the high-technology components used in the aerospace industry [1]. However, some of these materials are restricted to high-technology applications because of their high cost. Nowadays many research groups are focusing on ways to reduce production costs by searching for simplified processes or cheaper raw sources.

Pitches are the main precursor for the production of graphitizable carbons. There are several kinds of pitches that are generally classified according to their origins. Coal-tar pitches (CTPs) and petroleum pitches (PPs) are the most common. This is mainly because of their high volume of production and the existence of a well established consumers market for the aluminium and steel industry [2], [3], although the advent of new synthetic pitches has stimulated the development of new applications [4], [5]. CTPs and PPs are both composed of hydrocarbons but with different functionalities. Whereas PPs contain more aliphatic chains and are richer in hydrogen, CTPs are highly aromatic with very little aliphatic hydrogen. Another important difference is the presence of solid particles in CTPs, the so called primary quinoline insolubles (QIs). These differences in composition affect the behaviour of the pitches during carbonisation and determine the development of mesophase, which is an intermediate in the formation of the pregraphitic structure of cokes [6], [7].

The use of commercial pitches as precursors of carbon materials requires the improvement of some of their properties. In short, commercial pitches require pre-treatments prior to use. Several processes were developed by the authors to isolate the isotropic and anisotropic phases (mesophase) that coexist in thermally treated CTPs and PPs [8], [9]. The separation procedures are different in each case due to the properties displayed by each phase after thermal treatment. Each isolated phase has been successfully used to prepare carbon materials. The isotropic phase obtained from a thermally treated CTP has been applied in the preparation of general purpose carbon fibres (GPCF) [10], while the anisotropic phase of the same CTP has been employed as self-sintering material for high density graphites [11], [12]. Furthermore, the mesophase obtained from a thermally treated PP has been used to prepare graphitic carbon fibres [9]. No application has yet been found for the isotropic phase obtained from the PP, as it contains a significant amount of tiny mesophase spheres.

The thermal treatment of blends of PP and CTP could lead to the development of new precursors in two ways: (i) by facilitating the separation process and (ii) by improving the properties of the new precursors. Precursors that are richer in hydrogen due to the presence of PP may be obtained after an adequate separation of the isotropic phase from the thermally treated pitches. It is generally assumed that these pitches would be more fluid and more reactive in air, thereby facilitating the spinning and stabilisation of GPCF. Furthermore, the mesophase obtained after separation may be tested as a precursor for high density graphites or electrode materials in supercapacitors, after chemical activation [13].

As a preliminary step, this paper studies the early stages of carbonisation of blends of PP A-240 and an impregnating CTP (BI) with special attention being paid to mesophase formation and development, and to the role that primary QIs play in the process. To clarify this point, blends were also prepared with filtered BI (QI free, labelled BIf) and the same PP, A-240.

Section snippets

Raw materials

An impregnating CTP (BI) supplied by Industrial Quı́mica del Nalón and the PP Ashland 240 (A-240) were selected as raw materials. Blends were prepared in the solid state by mixing BI and A-240 in proportions 80:20, 60:40, 40:60 and 20:80 wt.%. The blends obtained were labelled B1, B2, B3 and B4, respectively.

Softening point

The SP of each pitch was measured using a Mettler Toledo FP90 following the ASTM D3104 standard procedure.

Solubility

The solubility of the pitch in toluene was determined using the Pechiney B-18

Characterization of parent pitches and their blends

The main properties of the parent pitches and their blends are summarized in Table 1. The elemental analysis indicates that the C/H atomic ratio is higher in BI than in A-240 (1.92 vs. 1.40), due to the higher aliphatic hydrogen content of the PP. Whereas the softening point of BI is around 30 °C lower than that of A-240 (95 vs. 127 °C), the toluene insoluble content is higher in BI. These results indicate that BI is made up of a heavy fraction, which is responsible of the high TI content,

Conclusions

The blend of the impregnating CTP with the PP A-240 accelerates pyrolysis and caused a higher amount of mesophase to be developed. This mesophase was found to be more soluble in NMP, as compared to that developed in the impregnating pitch, this being interesting for the potential applications of this mesophase as precursor of carbon materials.

The presence of primary QIs in the pitches delays pyrolysis, as proved by the higher mesophase content developed in the samples obtained from the filtered

Acknowledgements

This work was supported by MICYT-FEDER (project 1FD1997-1657MAT). E. Mora is grateful to CSIC for a research grant I3P-BPD2001-1.

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