POLYMER BEADS
Water-absorbing polymers, which are classified as hydrogels when mixed, absorb aqueous solutions through hydrogen bonding with water molecules. A SAP's ability to absorb water depends on the ionic concentration of the aqueous solution. In deionized and distilled water, a SAP may absorb 300 times its weight (from 30 to 60 times its own volume) and can become up to 99.9% liquid, but when put into a 0.9% saline solution, the absorbency drops to approximately 50 times its weight.[citation needed] The presence of valence cations in the solution impedes the polymer's ability to bond with the water molecule. The total absorbency and swelling capacity are controlled by the type and degree of cross-linkers used to make the gel. Low-density cross-linked SAPs generally have a higher absorbent capacity and swell to a larger degree. These types of SAPs also have a softer and stickier gel formation. High cross-link density polymers exhibit lower absorbent capacity and swell, but the gel strength is firmer and can maintain particle shape even under modest pressure.
The largest use of SAPs is found in personal disposable hygiene products, such as baby diapers, adult diapers and sanitary napkins. SAP was discontinued from use in tampons due to 1980s concern over a link with toxic shock syndrome. SAP is also used for blocking water penetration in underground power or communications cable, in self-healing concrete, horticultural water retention agents, control of spill and waste aqueous fluid, and artificial snow for motion picture and stage production. The first commercial use was in 1978 for use in feminine napkins in Japan and disposable bed liners for nursing home patients in the USA. Early applications in the US market were with small regional diaper manufacturers as well as Kimberly Clark.
HISTORY
Until the 1920s, water absorbing materials were fiber-based products. Choices were tissue paper, cotton, sponge, and fluff pulp. The water absorbent capacity of these types of materials is only up to 11 times their weight and most of it is lost under moderate pressure.
In the early 1960s, the United States Department of Agriculture (USDA) was conducting work on materials to improve water conservation in soils. They developed a resin based on the grafting of acrylonitrile polymer onto the backbone of starch molecules (i.e. starch-grafting). The hydrolyzed product of the hydrolysis of this starch-acrylonitrile co-polymer gave water absorption greater than 400 times its weight. Also, the gel did not release liquid water the way that fiber-based absorbents do. The polymer came to be known as “Super Slurper”.
The USDA gave the technical know-how to several USA companies for further development of the basic technology. A wide range of grafting combinations were attempted including work with acrylic acid, acrylamide and polyvinyl alcohol (PVA). Today's research has proved the ability of natural materials, e.g. polysaccharides and proteins, to perform super absorbent properties in pure water and saline solution (0.9%wt.) within the same range as synthetic polyacrylates do in current applications. Soy protein/poly(acrylic acid) superabsorbent polymers with good mechanical strength have been prepared. Polyacrylate/polyacrylamide copolymers were originally designed for use in conditions with high electrolyte/mineral content and a need for long term stability including numerous wet/dry cycles. Uses include agricultural and horticultural. With the added strength of the acrylamide monomer, used as medical spill control, wire and cable water blocking.
