Synthesis of fast-swelling superabsorbent hydrogels: effect of crosslinker type and concentration on porosity and absorption rate

Fast-swelling highly porous superabsorbent hydrogels were synthesized through a rapid solution polymerization of concentrated partially neutralized acrylic acid under normal atmospheric conditions. Acetone and sodium bicarbonate were used as porosity generators (porogens) during polymerization process for porosity generation. N,N^′-methylenebisacrylamide (MBA) and 1,4-butanedioldiacrylate were used as the water- and the oil-soluble crosslinkers, respectively. The temperature changing of the reaction mixture during polymerization and foam formation process was monitored and investigated in details. Time and sequence of addition of the porogens and gelation time were recognized to be important to increase efficiency of the porogens. The concentration of the crosslinkers on gelation time was optimized to achieve highly porous products. It was found that higher crosslinker concentration, especially in the case of MBA, causes decreased gelation times. Shorter gelation time resulted in more porogen bubbles trapped in the viscose reaction mixture led to products with higher porosity. The effect of type and concentration of the crosslinking agents on the process and swelling behavior of the hydrogels (in water and saline solutions) were investigated. Power law relationships were found for the variation of swelling in terms of either crosslinker or saline concentration. Less sensitivity to the change of salinity was achieved by employing higher amount of crosslinker.