1999 Volume 78 Issue 1 Pages 33-41
The viscoelastic properties of coals in the thermoplastic phase during carbonization were measured with dynamic mechanical oscillation instrumentation. The temperature dependence and frequency dependence of viscoelastic properties of the coals were similar to those of polymers and thermosetting resins and this method proved promising in studying the coal structural changes during thermoplastic phase from the analogy to polymers.
As the coals started to soften, the storage modulus (G') and the loss modulus (G'') decreased and the loss tangent (tan δ=G''/G') increased. While for the caking and slightly caking coals tanδ exceeded unity which showed that the bulk structure of the coal was flowing, for the non-caking coal it was lower than unity in the whole temperature range. Near the temperature of maximum fluidity, tanδ showed a peak while G' and G'' minimum (104-105 Pa) and the complex viscosity (η*) also minimum (104-105Pa·s). Further increase in temperature decreased tanδ and the gel point where tanδ=1 was reached near theresolidification temperature. There was a close relationship between the caking characteristic parameters measured with conventional method and the rheological moduli obtained here. The difference in G' near the resolidificationtemperature among the coals suggested that the compressibility of semi-coke layer could be evaluated by this method.
The frequency dependence of the viscoelastic properties of coal was measured at 450°C. As cure reaction proceeded, the crossover point of G' and G'' shifted toward low frequency side. This shift might be correlated to the molecular weight and molecular weight distribution of the coal in plastic state. Furthermore η*decreased as the frequency increased which suggests that the Gieseler plastometer overestimates the fluidity in high fluidity range due to the high rotation speed.