The water absorption principle of water-retaining agents is the same as that of general SAPs. The high molecular electrolyte molecular chain repels the amide group and carboxyl group in water, causing the molecular chain expansion force and the interaction due to the restriction of the molecular chain expansion force caused by the cross-linking points. Taking PAM polyacrylamide as an example, the water-retaining agent will have a large number of amide and hydrophilic groups of carboxyl, which can use the osmotic pressure generated by the difference in concentration between the ions and groups inside the resin and the components of the aqueous solution, as well as the affinity between the high molecular electrolyte and water, to absorb a large amount of water until the concentration difference disappears. The balance point between the rubber elasticity, which controls the water retention agent to reach the desired water absorption level, is where its apparent water absorption capacity lies.
Due to the cross-linking of the molecular structure, the water-absorbing part of the molecular network cannot be squeezed out by general physical methods, which plays a role in water retention. Therefore, for the same composition of polyacrylamide cationic polymer, the lower the cross-linking degree, the higher the water absorption rate, and the lower its water retention, stability, and solidification strength, and vice versa. Therefore, for long-term use of water-retaining agents, a higher cross-linking degree is required internationally, and the pursuit of high water absorption rate and speed is not the ultimate goal. For example, for flocculant polyacrylamide PMA, its apparent rate is not high, and the water absorption rate varies greatly depending on the particle size. The high-strength water-retaining agent after water absorption has a certain shape, is not easy to disintegrate, and is conducive to soil ventilation. The absorption and release of water are reversible.
Water-retaining agents are divided into two main categories: one is PAM polyacrylamide-acrylic acid copolymer cross-linked materials (PAM polyacrylamide, sodium polyacrylate, potassium polyacrylate, ammonium polyacrylate, etc.), and the other is starch-grafted acrylic acid salt copolymer cross-linked materials (starch-grafted acrylic acid salt).
PAM polyacrylamide is a white granular crystal, mainly composed of: acrylamide 65%-66% + potassium acrylate 23%-24% + water 8%-10% + cross-linking agent 0.5%- 1.0%. In international markets, most of the water-retaining agents produced in France, Germany, Japan, the United States, and Belgium are products of this type of composition. This product has the characteristics of a long service life and a long-term water retention capacity in soil for about 4 years, but its water absorption capacity will gradually decrease year by year. According to observations of afforestation experiments in loess areas, the water absorption rate of afforestation with this type of water-retaining agent maintained at 100-120 times in the first year, decreased by 20%-30% in the second year, decreased by about 40%-50% in the third year, and decreased more in the fourth year.
Starch-grafted acrylic acid salt copolymers
Starch-grafted acrylic acid salt copolymers are white or light yellow granular crystals, mainly composed of: starch 18%-27% + acrylic acid salt 62%-71% + water 10%+ cross-linking agent 0.5%-1.0%. When used for water retention in afforestation areas, the service life of this type of product can generally only be maintained for more than one year, but it has excellent properties such as high water absorption rate and speed. According to comparative experiments on water absorption of loess extract, this type of water-retaining agent can absorb 150-160 times its own weight of water within 15-20 minutes after encountering water.