Superabsorbent polymer

Superabsorbent polymers (SAP) are polymers that can absorb and retain extremely large amounts of a liquid relative to its own mass. [1]

Water absorbing polymers, classified as hydrogels, absorb aqueous solutions through hydrogen bonding with the water molecule. So an SAP's ability to absorb water is a factor of the ionic concentration of an aqueous solution. In deionized and distilled water, SAP may absorb 500 times its weight, but when put into a 0.9% saline solution, the absorbency drops to maybe 50 times its weight. The presence valent cations in the solution will impede the polymers ability to bond with the water molecule.

Superabsorbent polymers are most commonly made from the polymerization acrylic acid blended with sodium hydroxide in the presence of an an intiator to form a poly-acrylic acid, sodium salt. This polymer is the most common type of SAP made in the world today. Other materials are also used to make a superabsorbent polymer, such as, polyacrylamide copolymer, ethylene maleic anhydride copolymer, cross-linked carboxy-methyl-cellulose, polyvynyl alcohol copolymers, cross-linked polyethylene oxide, and starch grafted copolymer of polyacrylonitrile to name a few. The latter is one of the oldest SAP forms created.

Today superabsorbent polymers are made using one of two primary methods; suspension polymerization or solution polymerization. Both processes have their advantages over the other and both yield consist quality of product.

Solution based polymerization is the most common process used today for SAP manufacture. This process is efficient and generally has a lower capital cost base. The solution process uses a water based monmer solution to produce a mass of reactant polymerized gel. The polymerization's own reaction energy (exothermic) is used to drive much of the process, helping reduce manufacturing cost. The reactant polymer gel is then chopped, dried and ground to its final granule size. Any treatments to enhance performance characteristics of the SAP is usually accomplished after the final granule size is created.

The suspension process is practiced by a few companies because it offers a higher degree of production control and product engineering during polymerization step. Rather than react water-based monomer solution, this process suspends the water based reactant in a hydrocarbon based solvent. The net result is that the suspension polymerization creates the primary polymer particle in the reactor rather than mechanically in post-reactions stages. Performance enhancements can also be during or just after the reaction stage.

The total absorbency and swelling capacity are controlled by the type and degree of cross-linking to the polymer. Low density cross-linked SAP generally has a higher absorbent capacity and swell to a larger degree. These types of SAPs also have a softer and more cohesive gel formation. High cross-link density polymers exhibit lower absorbent capacity and swell. The gel strength is firmer and can maintain particle shape even under modest pressure.

The largest use of SAP is found in personal disposable hygiene products, such as baby diapers, adult protective underwear and feminine napkins. (SAP is not used in tampons as some have reported) They are also used for blocking water penetration in underground power or communications cable, horticultural water retention agents, control of spill and waste aqueous fluid, artificial snow for motion picture and stage production. The first commercial use was in 1978 for use in feminine napkins in Japan, disposable bed liners for nursing home patients in the USA.

Copolymer Chemistry
Polyacrylate/polyacrylamide copolymers 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 & cable waterblocking

Solution Polymers
Offers the absorbency of a granular polymer supplied in solution form. Solutions and can be diluted with water prior to application. Can coat most substrates or used to saturated. After drying at a specific temperature for a specific time, the result is a coated substrate with superabsorbent functionality. For example, this chemistry can be applied directly onto wires & cables, though it is especially optimized for use on components such as rolled goods or sheeted substrates.

Uses

 * Wire & Cable Waterblocking
 * Filtration Applications
 * Spill Control
 * Hot & Cold Therapy Packs
 * Composites & Laminates
 * Medical waste solidification
 * Mortuary pads
 * Motionless waterbeds
 * Candles
 * Diapers and Incontinence Garments
 * Fragrance carrier
 * Wound dressings
 * Fire protection
 * Surgical pads
 * Water retention for supplying water to plants