The constitution and structure of coal‐tar pitch

Abstract

Five commercial coal‐tar pitches, an air‐blown pitch and a straight‐run bitumen have been fractionated by solvent extraction and precipitation, and the molecular weights and elementary composition of the products have been determined. All five normal pitches are similar as regards the lower end of their molecular‐weight‐distribution curves, 50% by weight ranging from 150 to 350. The distributions of the upper 50% are also continuous, but the top molecular weight varies from 1400 in the low‐temperature carbonization pitch to about double that figure in the horizontal‐retort pitch. The molecular weights of bitumen and air‐blown pitch extend to above 5000. The coke‐oven pitch fractions all consist predominantly of condensed aromatic molecules, mainly hydrocarbon in nature, exhibiting only a limited degree of methyl substitution. The horizontal‐retort pitch is slightly less aromatic. The vertical‐retort pitch, though containing low‐molecular‐weight paraffins, increases in aromaticity with increasing molecular weight. The low‐temperature carbonization pitch fractions all have lower carbon/hydrogen ratios than those of the other pitches. Ultracentrifuging of tars and their solutions has failed to reveal any significant amount of disperse phase. Pitch is considered to be a supercooled liquid whose physical properties can be adequately explained without recourse to a colloid theory.