Thermogravimetry/mass spectrometry study of weakly basic starch-based ion exchanger

Water-soluble starch was cross-linked with epichlorohydrin in the presence of NH4OH. The weakly basic ion exchanger prepared this way was made into a salt form with HCl, H2SO4, and H3PO4. The samples obtained were analyzed with dynamic and isothermal thermogravimetry/mass spectrometry (TG/MS) method in an inert environment. From the dynamic TG/MS plots, it is evident that dehydration is the most important primary reaction observed by this method. The formation of CO2 as a predominant secondary product has lower intensity by one order of magnitude than water. The third most abundant ion CO (m/z 28) has intensity of the same order as CO2, while methane, methyl, formyl, formaldehyde and other less important species follow. The calculated rate constants of residue formation as well as gasification from the isothermal experiments fit first-order kinetics, and indicate that the thermal stability of starch is improved by cross-linking with epichlorohydrin. The rate constants of gasification are three orders of magnitude larger than for residue formation, while the activation energies of gasification are smaller than for residue formation. The introduction of amino groups results in a slightly increased rate of thermolysis, while the activation energies are close to unmodified material. Compared to free amine form of the modified starch, the salt form increases the rate of dehydration and decreases the activation energies of gasification especially for H2SO4, where SO2 is formed.