Superabsorbent polymers are polymers that can absorb and retain extremely
large amounts of a liquid relative to their own mass.
Water-absorbing polymers, which are classified as hydrogels when
cross-linked, absorb aqueous solutions through hydrogen bonding with water
molecules. A SAP's ability to absorb water is a factor of the ionic
concentration of the aqueous solution. In deionized and distilled water, a SAP
may absorb 500 times its weight and can become up to 99.9% liquid, but when put
into a 0.9% saline solution, the absorbency drops to maybe 50 times its
weight. 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 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, 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, with advancements made
quickly with the process breakthrough of proper product placement at the optimal
depth in the media. History
Until the 1980s, water absorbing materials were cellulosic or 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, but most of it is lost under moderate pressure.
In the early 1960s, the United States Department of Agriculture 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. 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. 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
water blocking 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. 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 three primary methods: gel
polymerization, suspension polymerization or solution
polymerization. Each of the processes have their respective advantages but all
yield a consistent quality of product. = Gel polymerization =
A mixture of frozen acrylic acid, water, cross-linking agents and UV initiator
chemicals are blended and placed either on a moving belt or in large tubs. The
liquid mixture then goes into a "reactor" which is a long chamber with a
series of strong UV lights. The UV radiation drives the polymerization and
cross-linking reactions. The resulting "logs" are sticky gels containing 60-70%
water. The logs are shredded or ground and placed in various sorts of driers.
Additional cross-linking agent may be sprayed on the particles' surface; this
"surface cross-linking" increases the product's ability to swell under
pressure—a property measured as Absorbency Under Load or Absorbency
Against Pressure. The dried polymer particles are then screened for proper
particle size distribution and packaging. The gel polymerization method
is currently the most popular method for making the sodium polyacrylate
superabsorbent polymers now used in baby diapers and other disposable hygienic
articles. = Solution polymerization =
Solution polymers offer the absorbency of a granular polymer supplied in
solution form. Solutions can be diluted with water prior to application, and can
coat most substrates or used to saturate them. After drying at a specific
temperature for a specific time, the result is a coated substrate with
superabsorbency. For example, this chemistry can be applied directly onto
wires and cables, though it is especially optimized for use on
components such as rolled goods or sheeted substrates.
Solution-based polymerization is commonly used today for SAP manufacture
of co-polymers, particularly those with the toxic acrylamide monomer. 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 exothermic reaction energy 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 are 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 the 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-reaction
stages. Performance enhancements can also be made during, or just after, the
reaction stage. Uses
Fukushima I nuclear accidents In the aftermath of the Fukushima I
nuclear accidents, in order to stop the highly radioactive water leak near the
reactor 2, TEPCO attempted to plug a trench leading to a damaged storage pit
with a combination of superabsorbent polymer, sawdust and shredded newspaper.
However, this attempt failed and an injection of sodium silicate was still
required to effectively stop the leak. See also
Sodium polyacrylate Citations
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. "Definitions of
terms relating to reactions of polymers and to functional polymeric materials".
Pure and Applied Chemistry 76: 889–906. doi:10.1351/pac200476040889.
Katime Trabanca, Daniel; Katime Trabanca, Oscar; Katime Amashta, Issa
Antonio. Los materiales inteligentes de este milenio: Los hidrogeles
macromoleculares. Síntesis, propiedades y aplicaciones.. Bilbao: Servicio
Editorial de la Universidad del País Vasco. ISBN 84-8373-637-3.
Buchholz, Fredric L; Graham, Andrew T, ed.. Modern Superabsorbent Polymer
Technology. John Wiley & Sons. ISBN 0-471-19411-5.
External links Nonwovens Containing Immobilized
Superabsorbent Polymer Particles Super Absorbent Polymer Production with
LIST Kneader Reactors