PMC

Schematic diagram of moisture absorption process of hygroscopic exothermic fibers.

a, b Moisture adsorption kinetics of the hygroscopic layer and the hygroscopic/ evaporative bilayer structure. c Schematic of mass transfer between the two layers. Baseline: The hygroscopic layer and the evaporative layer was separated by a separator to prevent mass transfer between them. Device 1: The outside surface of the evaporative layer was covered by a separator to prevent evaporation. Device 2: Without any additional refitment. The letters I-VI represent the hygroscopic layer and the evaporative layer of the three devices, respectively. The arrows represent the transfer of water. d Mass change in different parts of the devices at 25 °C, 70% RH. e Water content map of the SSEG after moisture adsorption in 60% RH for over 30 h. The water content map was depicted by gravimetrically analyzing water absorbed by each layer. The resolution corresponds to the film thickness, which is 0.25 mm for the hygroscopic layer, and 0.33 mm for the evaporative layer. f Mass change in the bilayer structures of the hygroscopic layer/cellulon paper and the hygroscopic layer/evaporative layer versus time under one-sun illumination.

Effect of moisture content on the net isosteric heat of adsorption for different hygroscopic exothermic fibers.

As shown in , during the investigated period, the weight gain of SIM was very small, that is, the moisture content of the SIM sample was very low. Thus, SIM is very poorly hygroscopic. The weight gains of the tablets (T-1 and T-2) after 10 days storage at 25°/90% RH were about 3%, correspondingly, those of T-4 and T-5, T-3 and T-6 were about 6 and 8%, respectively. This result suggested that a small amount (5%) of the hygroscopic excipient (citric acid, NaCMC, PVPP or sorbitol) in tablet would lead to obvious increase of moisture content in tablet. The increase of moisture content could be mainly attributed to the moisture sorption of citric acid, PVPP, NaCMC or sorbitol in tablet. Sorbitol and citric acid were most hygroscopic (), the weight gains of T-3 and T-6 containing citric acid and sorbitol were also the most. During the experiment, we also observed that after 10 days storage at 25°/90% RH the tablets (T-6) containing 5% sorbitol became incompact. This indicated that the moisture sorption of T-6 would lead to the decrease of cohesion of the tablet.