Utilization of hollow kapok fiber for the fabrication of a pH-sensitive superabsorbent composite with improved gel strength and swelling properties

A pretreated kapok fiber (PKF) with a hollow tube structure was introduced into a poly(sodium acrylate) network by a simultaneous free-radical graft copolymerization and crosslinking reaction to fabricate a novel kapok fiber-g-poly(sodium acrylate) (PNaA/PKF) superabsorbent composite. The network characteristics and surface morphologies of the PNaA/PKF superabsorbent composite were investigated by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM), as well as by determination of mechanical properties, swelling and stimuli responses to salts, and pH. The results showed that the incorporation of a specific amount of PKF not only improved the water absorption, but also the gel content and gel strength. The composite with 10 wt% PKF showed the best water absorption, gel content and gel strength. The swelling kinetics of the composite followed Schott's pseudo-second-order kinetics model, and the swelling rate constant was enhanced 2.63 fold after adding 10 wt% PKF. The swelling and deswelling behaviors in various saline and pH solutions revealed the stimuli-sensitivity of the PNaA/PKF composite to salt concentration, ionic charge and external pH, and a remarkable time-dependent swelling process with an overshooting characteristic was observed in pH 2 solutions.