Mesophase pitch

The following natural precursors have been selected for KOH activation coal (C), coal semi-coke (CS), pitch semi-coke (PS) and pitch mesophase (PM). An industrial activated carbon (AC) was also used. Activation was performed at 800°C in KOH with 4 1 (C KOH) weight ratio, for 5 hours, followed by a careful washing of the samples with 10% HC1 and distilled water. The activation process supplied highly microporous carbons with BET specific surface areas from 1900 to 3150 m2/g. The BET surface area together with the micro and the total pore volume of the KOH-activated carbons are presented in Table 1. The mean micropore width calculated from the Dubinin equation is designed as LD. 

Materials used in the activation with KOH include high volatile bituminous coal C, coal semi-coke CS, pitch mesophase PM, pitch semicoke PS and commercial activated carbon AC. The semi-cokes CS and PS were produced by the heat treatment of corresponding parent materials at 520°C with a heating rate of 5°C/min and 2 hours soaking time. The preparation of mesophase PM comprised the soaking of coal-tar pitch at 450°C for 7 h with a continuous stirring. All the treatments were performed under argon in a vertical Pyrex retort of 45 mm diameter. 

Mesophase pitch Mesophase-pitch-based carbon fibers... 

The Pitch-Mesophase-Coke Transformation As Studied by Thermal Analytical and Rheological Techniques... 

Mesophase Pitch. Mesophase pitch is a generic term for those products of pyrolysis that contain mesophase, or are entirely mesophase, but which on reheating still pass through a fluid phase prior to the formation of semi-coke. The following are areas of scientific or industrial interest where the rheological properties of mesophase pitch are of interest. ... 

Figure 6, Schematic diagram describing the pitch-mesophase-coke transformation.

The preferred orientation of the matrix parallel to the fiber bundles appears to be affected by processing pressure during pyrolysis (400-500°C), becoming more random as impregnation pressures are increased. Even transverse lamellar orientations can result (Figure lb). Because pitch mesophase coalescence has been inhibited by the pressure, grains of matrix may be transversely oriented to the filaments. 

Lately, potassium hydroxide (KOH) activation has been used to produce activated carbons from cheap and available natural precursors, such as coals and pitch-derived carbonaceous materials. This method efficiently develops the micropores, and aUows to get various pore size distributions depending on the kind of precursor and activation conditions [82—85]. The nitrogen adsorption isotherms (Fig. 23.11) at 77 K of activated carbons obtained from coal (C), coal semicoke (CS), pitch semicoke (PS), pitch mesophase (PM), and a commercial activated carbon (AC), using KOH and carbon in the 4 1 ratio, reveal that a... 

Figure 23.11 Nitrogen adsorption isotherms at 77 K for potassium hydroxide (KOH) activated carbons (A-) from various precursors. C = coal CS = coal semicoke PM = pitch mesophase PS = pitch semicoke AC = commercial activated carbon. (Adapted from Ref. [84].)...

Cyclic hydrocarbons with six equivalent substituent chains emanating from several carbon atoms about the ring can again adopt conformations in which the molecular shape is disk-like, but the aspect ratio is much larger. If the chains are somewhat flexible and the molecules possess an appropriate balance between order and disorder and attractive and repulsive intermolecular forces, the creation of new mesophase types can be envisioned - and several have been found [102,104]. The two general classes of aggregation in liquid crystalline phases of discotic molecules are lenticular nematic (N ) and columnar discotic (D) [100-104]. Carbonaceous pitch mesophases, discussed in section 1.4.5, resemble phases. Only those discotic mesophases with benzene, cyclohexane and shape-related cores having primarily alkyl chains as substituents will be discussed here. 

In another paper [15], these authors have prepared the first inorganic mesophase by low-temperature thermolysis of the borazine. Following the synthesis procedure reported by Sneddon etal. [11-13] (Eq. 14.1), the prediction made on the formation of biphenyl and naphtalen-type units and requirements for the formation of pitch mesophases [16], the authors have modified the thermolysis rate to maintain a certain mobility in as-formed molecules, and therefore obtain optically anisotropic phases during thermolysis of borazine. The formation of a liquid-crystalline phase during thermolysis provides an efficient way to produce a final BN material with a high degree of crystalline order by heating to 1800 °C. 

Pyrolysis of pitch fibers yields low cost fibers with low strength (1 GPa) and low modulus (<40 GPa). An advance in the 1970s afforded ultrahigh modulus carbon fibers (>380 GPa) from mesophase pitch. Mesophase is a liquid crystal phase formed in the transformation of organic precursors to carbon. The chemical pretreatment of the pitch is complex and the potential cost advantage of using a pitch precursor is still not fully realized [1-8]. 

The mesophase prepared by this route is mainly insoluble in solvents like pyridine or quinoline. Thus, extraction techniques have been extensively used to determine the degree of the isotropic pitch/mesophase conversion and then the reaction kinetics. At 350 C, the formation of the mesophase is extremely slow. One week is required to achieve a 40 wt.% concentration of mesophase. At 400-450°C, one to forty hours are required to achieve a 50 wt.% concentration of mesophase. Finally, temperatures above 500 C are considered undesirable due to the risk of coke formation. The formation of pyridine and quinoline extractables, i.e., mesophase, has been reported to follow first order kinetics. 

Matsumoto and Mochida [33], using NMR with a hydrogenated coal tar pitch mesophase carbon fiber, showed that the initial attack by oxygen was on -CH3 and -CH2- groups, with the gradual formation of carboxyls, esters and aryl carbonyls. Fairly stable cross-links were formed via phenols, ethers and esters. The workers also found that slower heating rates (0.5°C/min instead of 2.0°C/min) produced better mechanical properties and that the final choice would be controlled by the desired target properties and economics. 

Polynuclear compounds basically in the form of flat diitics are synthesized in heat treatment of heavy petroleum or coal residues. Such oligomeric and polymeric molecules can be considered anisometric formations with a certain ratio of the diameter (analog of /) to the thickness (analog of d). Beginning with certain values of Hd, such formations are stacked with respect to each other in a certain order, forming nematic columns distributed in an isotropic matrix of smaller molecules. From this point of view, pitch mesophases are close to lyotropic systems, but the irrevCTsibility of their properties with respect to the temperature and concentration scales preserves their special status. The role of these phases in the mechanism of formation of carbon fibers is well known but far from totally elucidated. 

---