Superabsorbent polymer

Superabsorbent polymers (SAP) (also called slush powder) are polymers that can absorb and retain extremely large amounts of a liquid relative to their own mass.

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 (from 30–60 times its own volume), but when put into a 0.9% saline solution, the absorbency drops to maybe 50 times its weight. The presence of valent cations in the solution will impede the polymers ability to bond with the water molecule.

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 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, 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.

History

"Until the 1980’s, water absorbing materials were cellulosic or fiber-based products. Choices were tissue paper, cotton, sponge, and fluff pulp. The water retention capacity of these types of materials is only 20 times their weight – at most.

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 grating combinations were attempted including work with acrylic acid, acrylamide and polyvinyl alcohol (PVA)."

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 & cable waterblocking

Current synthesis

Copolymer chemistry

Superabsorbent polymers are now commonly made from the polymerization of acrylic acid blended with sodium hydroxide in the presence of an initiator to form a poly-acrylic acid sodium salt (sometimes referred to as sodium polyacrylate). 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 carboxymethylcellulose, polyvinyl 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 a consistent quality of product.

Solution polymerization

Solution polymers offer 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.

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 monomer 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.

Suspension polymerization

The suspension process is practiced by only a few companies because it requires a higher degree of production control and product engineering during polymerization step. 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.

Uses

• Candles
• Composites & laminates
• Controlled release of insecticides & herbicides
• Diapers and incontinence garments
• Drown-Free water source for feeder insects
• Filtration Applications
• Fire-retardant gel
• Fragrance carrier
• Frog Tape (high tech masking tape designed for use with latex paint).
• Grow-In-Water toys
• Hot & Cold Therapy Packs
• Medical waste solidification
• Mortuary pads
• Motionless waterbeds
• Spill control
• Surgical pads
• Potting soil
• Waste Stabilization & Environmental Remediation
• Water retention for supplying water to plants
• Wire & Cable Waterblocking
• Wound dressings
• Fuel monitor systems in aviation
• Fuel monitor systems in vehicles

See also

• Sodium polyacrylate

References

• K. Horie, M. Báron, R. B. Fox, J. He, M. Hess, J. Kahovec, T. Kitayama, P. Kubisa, E. Maréchal, W. Mormann, R. F. T. Stepto, D. Tabak, J. Vohlídal, E. S. Wilks, and W. J. Work (2004). "Definitions of terms relating to reactions of polymers and to functional polymeric materials (IUPAC Recommendations 2003)" (PDF). Pure and Applied Chemistry. 76 (4): 889–906. doi:10.1351/pac200476040889. CS1 maint: Multiple names: authors list (link)
• Katime Trabanca, Daniel; Katime Trabanca, Oscar; Katime Amashta, Issa Antonio (September 2004). Los materiales inteligentes de este milenio: Los hidrogeles macromoleculares. Síntesis, propiedades y aplicaciones (1 ed.). Bilbao: Servicio Editorial de la Universidad del País Vasco (UPV/EHU). ISBN 8483736373. CS1 maint: Multiple names: authors list (link)

• Buchholz, Fredric L; Graham, Andrew T, ed. (1997). Modern Superabsorbent Polymer Technology (1 ed.). John Wiley & Sons. ISBN 0471194115. CS1 maint: Multiple names: editors list (link)

External links

• Nonwovens Containing Immobilized Superabsorbent Polymer Particlesde:Superabsorber

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