Superabsorbent resin is a kind of macromolecule containing hydrophilic groups and crosslinking structures. It was first prepared by Fanta and others by grafting polyacrylonitrile onto starch and then saponification. According to the raw materials, there are starch system (graft, carboxymethylation, etc.), cellulose system (carboxymethylation, graft, etc.), and synthetic polymer system (polyacrylic acid system, polyvinyl alcohol system, polyoxyethylene system, etc.). Compared with starch and cellulose, polyacrylic acid superabsorbent resin has a series of advantages such as low production cost, simple process, high production efficiency, strong water absorption capacity and long product shelf life, and has become a hot research topic in this field.

At present, the polyacrylic acid system accounts for 80% of the world's super absorbent resin production. Superabsorbent resins are generally polymer electrolytes containing hydrophilic groups and crosslinked structures. Before water absorption, the polymer chains are close to each other and intertwined, and cross-linked to each other to form a network structure, so as to achieve overall fastening. When in contact with water, water molecules penetrate into the resin through capillary action and diffusion, and the ionizing groups on the chain are ionized in water. The polymer chain expands and swells due to the electrostatic repulsion between the same ions on the chain. Due to the requirement of electrical neutrality, the counter ions cannot migrate to the outside of the resin, and the ion concentration difference between the internal and external solutions of the resin forms the reverse osmosis pressure. Water further enters the resin under the action of reverse osmosis pressure to form hydrogel. At the same time, the crosslinked network structure and hydrogen bonding of the resin limit the infinite expansion of the gel. When there is a small amount of salt in the water, the reverse osmosis pressure decreases, and the polymer chain shrinks due to the shielding effect of counter ions, resulting in a great decrease in the water absorption capacity of the resin. Generally, the water absorption capacity of superabsorbent resin in 0.9% NaCl solution is only about 1 / 10 of that in deionized water. Water absorption and water retention are two aspects of the same problem, which Lin runxiong and others discussed thermodynamically. At a certain temperature and pressure, the super absorbent resin can absorb water spontaneously, and water enters the resin, so that the free enthalpy of the whole system decreases until equilibrium. If water escapes from the resin and causes the free enthalpy to rise, it is not conducive to the stability of the system. Differential thermal analysis showed that 50% of the water absorbed by the superabsorbent resin was still enclosed in the gel network above 150 ° C. Therefore, even if pressure is applied at normal temperature, water will not escape from the superabsorbent resin, which is determined by the thermodynamic properties of the superabsorbent resin.