The kinetics of non-isothermal dehydration of equilibrium swollen Ca-alginate hydrogel

Non-isothermal dehydration of Ca-alginate hydrogel was investigated for the first time. Thermogravimetric dehydration curves were recorded at heating rates from 5 to 20 K min⁻¹ in the temperature range from 300 to 450 K. The dependence of apparent activation energy on the degree of dehydration was determined by the Friedman's differential isoconversion method. The experimental conversion curves of dehydration are fully fitted with the statistical model of hydrogel dehydration. The values of the parameters of the statistical model and their changes with the heating rate were determined. The rate constant as the function of temperature was calculated at all heating rates, and it was found that these dependencies obey the Arrhenius equation. From the values of rate constant, the changes in the kinetic parameters (activation energy and pre-exponential factor) of non-isothermal dehydration of Ca-alginate hydrogel with temperature and dehydration degree were calculated. The obtained results indicate the existence of two phase states of water absorbed on the hydrogel.