Evaluation of the thermoplasticity of Upper Freeport coal and its extracts using dynamic viscoelasticity

The viscoelastic properties of Upper Freeport coal and extracts, including carbon disulfide/N-methyl-2-pyrrolidinone mixed solvent (1:1 by volume) solubles and pyridine solubles, were characterized by controlled strain oscillatory rheometry to evaluate the thermally induced relaxation of coal molecules. The dynamic viscoelastic modulus of the pelletized sample was measured at temperatures where the samples plasticize, while the frequency was increased from 0.05 to 5 Hz, to yield a data set suitable for analysis by time-temperature superposition. Although the applicability of the time-temperature superposition principle to the coal samples may be doubtful, it allows the empirical construction of master curves at reduced temperatures, yielding data for the frequency dependencies of the modulus over a wide range, i.e., 10^-3-10³ Hz. However, the temperature dependencies of the shift factor are not explainable by means of the assumptions that underlie the William-Landall-Ferry equation. Arrhenius-type plots of the shift factors show that the apparent activation energy (ΔH_a) is temperature dependent. The approximate distribution ranges of ΔH_a are from 50 to 230 kJ/mol for pyridine solubles, and 50 to 370 kJ/mol for raw coal. The multiexponential behaviors of the shift factors suggest that the thermorheological characteristics of coal molecules in the plastic phase are not simple, but involve multiple viscoelastic mechanisms with different temperature dependencies.