Retrogradation in radiation-synthesized cassava starch/acrylic acid super water absorbent and its effect on gel stability

Abstract

The effects of long-term storage and retrogradation on the stability of the gel properties of radiation-synthesized super water absorbents (SWA) made from cassava starch and acrylic acid were explored in this study. Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM) analyses were used to explore the retrogradation phenomenon. TEM and scanning area electron diffraction (SAED) images revealed the presence of starch nanocrystallites and increase of crystalline state in SWA matrix after 24 months storage. FT-IR peak ratios and enthalpy changes in DSC analysis also confirmed retrogradation of aged SWA samples. However, no sharp peak was observed in XRD diffractograms which is mainly due to the nanostarches present in the SWA. The most critical property affected by starch retrogradation in SWA is the degree of swelling. The degree of swelling of SWA was greatly reduced from 148 to 30 g/g while the gel fraction remained stable after 24 mos. The aged SWA samples with low degree of swelling due to retrogradation returned to their high and stable soil water content after 15 d in unsterile pure garden soil. Despite being exposed to high energy gamma radiation of starch with acrylic acid to form SWA, wherein possible oxidation and chemical modification of starch may take place, retrogradation can still occur. Lowering the storage temperature evidently suppressed the effect of starch retrogradation in SWA. Suppression of starch retrogradation in the optimized SWA is crucial to prolong its marketable life.