The Mechanism of Moisture Transport in a Gel Cylinder

In drying of gels, the analysis of the process must encompass both heat and mass transfer aspects as well as deformation, usually very strong, of the material in the process. Deformation is almost always assumed to be linearly dependent on moisture content without considering an internal stress or pressure build-up. Since there is a great commercial interest in production of non-deformed dry gels by convective ambient pressure drying, it was interesting to investigate the behavior of such materials in drying. Convective drying would eliminate the expensive supercritical. In this work several types of gels were synthesized and dried convectively in a lab scale. Since drying induces the internal stress formation responsible for shrinkage and cracking, great care was taken to measure deformation and internal pressure development in gel during drying. Cylindrical gel samples 35 mm in diameter were dried convectively in air at 30, 40, and 50 °C. The hydrogels were made of PVP, agar-agar, and silica. Different types of behavior of the gels during drying was observed: almost uniform deformation in the case of PVP, case-hardening for agar-agar gels, and cracking for silica hydrogels. This behavior directly results from different mechanical properties of the investigated gels. It was also observed that the internal pressure changes from initially overpressure to underpressure in the course of drying which is also a result of internal stress build up. The experimental data may be used for validation of mechanical models of drying of non-linear visco-elastic solids.