Super absorbent polymer is a new functional SAP powder material with wide application and broad market prospect. This paper introduces its structural characteristics, water absorption mechanism, characteristics and classification to help us understand this new SAP powder material.
(1) Structural features
Super absorbent polymer is prepared by crosslinking hydrolysis technology on the basis of water-soluble polymer. It has the characteristics of low crosslinking degree, high expansion rate and insoluble in water. From the point of view of chemical structure, the main chain or side chain of superabsorbent resin contains hydrophilic genes such as hydroxyl, amido, carboxyl and sulfonic groups; From the point of view of physical structure, it is a three-dimensional network with low cross-linking.
(2) Water absorption mechanism
The water absorption of super absorbent polymer is divided into several stages. In the initial stage, the water absorption is very slow, because the water absorption is achieved through capillary adsorption and dispersion. Then, the water molecules are separated from the hydrophilic groups of the resin by hydrogen bonds. Anions are fixed on the super absorbent polymer chain, and cations can move freely. With the further decomposition of hydrophilic groups, the electrostatic repulsion between the resin networks increases, and the cations cannot diffuse to the external solvent, resulting in the increase of cation concentration in the resin network, which makes the inside and outside of the network osmotic pressure and further permeates water. With the increase of water absorption, the osmotic pressure difference inside and outside the network tends to zero; The expansion of the network increases its elastic contraction force, gradually offsets the electrostatic repulsion of anions, and finally achieves water absorption equilibrium.
(1) High water absorption
The water absorption capacity of commonly used water absorbent materials such as sponge is about 20 times of its own weight, and starch resin can absorb hundreds of times of its own weight.
(2) Good water retention
After water absorption, the common water absorbing material is under pressure and easy to drain, but the super absorbent resin is under pressure and water is not easy to flow out of the resin. In other words, it still has good water retention under external pressure.
(3) Good thermal stability
Different water absorbent resins have different thermal stability. When the starch was heated at 150 ℃ for 1 hour, it began to turn black and the water absorption capacity decreased.
If the super absorbent polymer is stored in a sealed container, it can be stored for 3-4h, and its water absorption capacity remains unchanged.
The super absorbent resin is in the form of water coagulation after absorbing water. Its viscosity is higher than the general water solubility, and the thickening effect is obvious.
Starch is a kind of natural polyhydroxy compound, which has a wide range of raw materials, a wide variety and low price. The monomers grafted with starch are mainly hydrophilic and hydrolyzed ethylene monomers. At present, free radical graft copolymerization is usually used to synthesize superabsorbent resins.
(2) Synthetic resin system
With the in-depth study, there are more and more kinds of them. Acrylic acid and polyacrylic alcohol are the most important types.
(3) Fiber series
Because of the appearance of starch super absorbent resin, people think of using cellulose to prepare super absorbent resin. Fiber materials come from a wide range of sources and can react with various low molecules, which is an aspect of the development of new SAP powder materials in recent ten years.
(4) Organic-inorganic composite super absorbent polymer.
In recent years, super absorbent polymer has developed rapidly in China. After years of efforts, China has developed a series of low-cost, salt resistant, alkali resistant and multifunctional new organic-inorganic composite high-efficiency water absorbent resins. This paper explains its structural characteristics, water absorption mechanism, characteristics and classification to help people understand this new SAP powder material.