Let me introduce the SAP that you are more interested in recently!Super Absorbent Polymer (SAP) is a new type of functional polymer material. It has a high water absorption function that absorbs water several hundred to several thousand times heavier than itself, and has excellent water retention performance. Once it absorbs water and swells into a hydrogel, it is difficult to separate the water even if it is pressurized. Therefore, it has a wide range of uses in various fields such as personal hygiene products, industrial and agricultural production, and civil engineering.
Super absorbent resin is a kind of macromolecules containing hydrophilic groups and cross-linked structure. It was first produced by Fanta and others by grafting starch with polyacrylonitrile and then saponifying. According to the raw materials, there are starch series (grafted, carboxymethylated, etc.), cellulose series (carboxymethylated, grafted, etc.), synthetic polymer series (polyacrylic acid, polyvinyl alcohol, polyoxy Ethylene series, etc.) in several categories. 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. It has become the current research hotspot in this field.
What is the principle of this product? At present, polyacrylic acid accounts for 80% of the world’s super absorbent resin production. The super absorbent resin is generally a polymer electrolyte containing a hydrophilic group and a cross-linked structure. Before absorbing water, the polymer chains are close to each other and entangled together, cross-linked to form a network structure, so as to achieve the overall fastening. When in contact with water, water molecules penetrate into the resin through capillary action and diffusion, and the ionized groups on the chain are ionized in the water. Due to the electrostatic repulsion between the same ions on the chain, the polymer chain stretches and swells. Due to the requirement of electrical neutrality, counter ions cannot migrate to the outside of the resin, and the difference in ion concentration between the solution inside and outside the resin forms a reverse osmotic pressure. Under the action of reverse osmosis pressure, water further enters the resin to form a hydrogel. At the same time, the cross-linked network structure and hydrogen bonding of the resin itself limit the unlimited expansion of the gel. When the water contains a small amount of salt, the reverse osmotic pressure will decrease, and at the same time, due to the shielding effect of the counter ion, the polymer chain will shrink, resulting in a great decrease in the water absorption capacity of the resin. Generally, the water absorption capacity of super absorbent resin in 0.9% NaCl solution is only about 1/10 of that of deionized water. Water absorption and water retention are two aspects of the same problem. Lin Runxiong et al. discussed them in thermodynamics. Under a certain temperature and pressure, the super absorbent resin can absorb water spontaneously, and the water enters the resin, reducing the free enthalpy of the entire system until it reaches equilibrium. If water escapes from the resin, increasing the free enthalpy, it is not conducive to the stability of the system. Differential thermal analysis shows that 50% of the water absorbed by the super absorbent resin is still enclosed in the gel network above 150°C. Therefore, even if pressure is applied at normal temperature, water will not escape from the super absorbent resin, which is determined by the thermodynamic properties of the super absorbent resin.
Next time, tel the specific purpose of SAP.
Post time: Dec-08-2021