The Role of Thermal Expansion Mismatch on the Drying of Refractory Castables Containing Polymeric Fibers - Preprint

The most common additive for improving the efficiency and security of refractory castables drying is the incorporation of polymeric fibers. However, the fundamental mechanisms by which they enhance water removal remain unresolved. Two main hypotheses are currently considered in the literature: (i) the formation of microcracks in the matrix surrounding the fibers during heating, due to the coefficient of thermal expansion mismatch between these two phases, and (ii) polymer melt viscous flow by the pressurized water vapor, which generates open channels increasing the permeability. The current work investigates the potential role of the thermal expansion mismatch between polymer fibers and a refractory matrix during heating, aiming to assess the feasibility of the first hypothesis. An analytical thermomechanical model was used to estimate the interfacial tangential stresses induced by thermal expansion differences, using temperature-dependent material properties compiled from the literature for five polymers: linear low density polyethylene (LLDPE); ultrahigh molecular weight polyethylene (UHMWPE); polypropylene (PP); polyamide 6,6 (PA 6,6); and polyethylene terephthalate (PET). The results indicate that the tangential stresses can reach significantly higher levels than the maximum vapor pressure induced during the heating for all polymers considered, with the highest stresses observed for PP and PA 6,6. These values suggest that the thermal expansion mismatch plays a non-negligible role in the permeability evolution during refractory castables drying, regardless of the mechanisms of permeability enhancement. A sensitivity analysis revealed that the tangential stresses are significantly influenced by the polymer’s thermal expansion coefficient and the modulus of elasticity of the fiber, and of the refractory castable, which enable the search for optimal fiber properties for drying applications. When compared with experimental results in the literature, these findings support the hypothesis that thermal expansion mismatch can contribute to microcrack formation around polymeric fibers during drying, enhancing water removal from refractory castables, although more experimental investigations are under way at the moment in the authors’ research group to further validate this mechanism.