super absorbent polymers

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Superabsorbent Polymers

Introduction:

Superabsorbent polymers (SAPs) are materials that have the ability to absorb and retain largevolumes of water and aqueous solutions. This makes them ideal for use in water absorbing

applications such as baby nappies and adults incontinence pads to absorbent medical

dressings and controlled release medium.

Early superabsorbents were made from chemically modified starch and cellulose and other

polymers like poly(vinyl alcohol) PVA, poly(ethylene oxide) PEO all of which are

hydrophilic and have a high affinity for water. When lightly cross-linked, chemically or

physically, these polymers became water-swellable but not water-soluble.

Todays superabsorbent polymers are made from partially neutralised, lightly cross-linked

poly(acrylic acid), which has been proven to give the best performance versus cost ratio. The

polymers are manufactured at low solids levels for both quality and economic reasons, and

are dried and milled in to granular white solids. In water they swell to a rubbery gel that in

some cases can be up to 99wt% water.

Mechanisms of swelling in superabsorbent polymers:

Before discussing the synthesis of superabsorbent polymers it is first necessary to understand

the reasons why they swell. There are several mechanisms to the process of swelling, all of

which contribute to the final swelling capacity (or centrifuge retention capacity CRC which

is the amount of 0.9 wt% saline solution that a SAP can retain under free swelling conditions

when surface water has been removed in a centrifuge).


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Figure 1 is a diagrammatic representation of part of the polymer network. The polymer backbone

in SAP is hydrophilic i.e. water loving because it contains water loving carboxylic acid groups

(COOH). When water is added to SAP there is a polymer/solvent interaction; hydration1

and the

formation of hydrogen bonds are two of these interactions.

Hydration:

This is the interaction of ions of a solute with molecules of a solvent i.e. COO-

and Na+

ions

attract the polar water molecules (figure 2).

Hydrogen Bonds:

Hydrogen bonds are electrostatic interactions between molecules, occurring in molecules that

have hydrogen atoms attached to small electronegative atoms such as N, F and O. The hydrogen

atoms are attracted to the non-bonding electron pairs (lone pairs) on other neighbouring

electronegative atoms (figure 3)

In water the electronegative atom is oxygen which pulls the hydrogens electrons towards itself

setting up a dipole in the molecule. The positive hydrogen atoms are attracted to the oxygen lone

pairs on other water molecules. Oxygen has two lone pairs of electrons and each is capable of

hydrogen bonding to two other water molecules.

These effects decrease the energy and increase the entropy of the system. Due to the hydrophilic

nature of SAP the polymer chains have a tendency to disperse in a given amount of water (i.e.


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they are trying to dissolve in the water), which leads to a higher number of configurations for the

system and also increases entropy.

Why doesnt SAP dissolve in water?

Cross-links between polymer chains form a three-dimensional network and prevent the polymer

swelling to infinity i.e. dissolving. This is due to the elastic retraction forces of the network, and

is accompanied by a decrease in entropy of the chains, as they become stiffer from their

originally coiled state (figure 4).


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There is a balance now between the forces of retraction and the tendency for the chains to swell

to infinite dilution. The degree of cross-linking has a direct effect on the level of swelling of the

polymer and the strength of the network i.e. Increased cross-link density = decreased swelling

capacity = increased gel strength. Cross-linking is discussed in more detail later.

Examples of Super Absorbent Polymers:

Most common synthetic polymers are hydrophobic, or water-hating, which means that they do

not absorb water. Everyday examples are: raincoats, plastic cups, bags, plastic tabletops, etc.

Some polymers, though, are hydrophilic, or water loving. They readily absorb water. An

excellent example is sodium polyacrylate or polyacrylamide. These super absorbent polymers

(SAP) contain high concentrations of sodium and potassium ions, which encourage the

absorption of water through a process known as osmosis. The SAP provided by Prof Bunsen

Science is an anionic polyacrylamide polymer. It is made up of crosslinked copolymers of

acrylamide and potassium acrylate. It is a chemical relative of cyanoacrylate which is the

hardworking polymer in Super Glue.

A wide range of grafting combinations were attempted including work with acrylic acid,

acrylamideandpolyvinyl alcohol(PVA). Other materials are also used to make a superabsorbent

polymer, such as polyacrylamide copolymer,ethylenemaleic anhydridecopolymer, cross-linked

carboxymethylcellulose, polyvinyl alcohol copolymers, cross-linked polyethylene oxide, and

starch grafted copolymer ofpolyacrylonitrileto 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.

Domestic & Industrial Applications:

Hydrophilic polymers have numerous applications. To name a few:

Fuel filters in aircraft contain SAP as they have to remove all traces of water from the

fuel.

Telecommunication cables are kept dry with SAP fillers.
http://en.wikipedia.org/wiki/Acrylic_acidhttp://en.wikipedia.org/wiki/Acrylic_acidhttp://en.wikipedia.org/wiki/Acrylamidehttp://en.wikipedia.org/wiki/Acrylamidehttp://en.wikipedia.org/wiki/Polyvinyl_alcoholhttp://en.wikipedia.org/wiki/Polyvinyl_alcoholhttp://en.wikipedia.org/wiki/Polyvinyl_alcoholhttp://en.wikipedia.org/wiki/Ethylenehttp://en.wikipedia.org/wiki/Ethylenehttp://en.wikipedia.org/wiki/Maleic_anhydridehttp://en.wikipedia.org/wiki/Maleic_anhydridehttp://en.wikipedia.org/wiki/Maleic_anhydridehttp://en.wikipedia.org/wiki/Carboxymethylcellulosehttp://en.wikipedia.org/wiki/Carboxymethylcellulosehttp://en.wikipedia.org/wiki/Polyethylene_oxidehttp://en.wikipedia.org/wiki/Polyethylene_oxidehttp://en.wikipedia.org/wiki/Polyacrylonitrilehttp://en.wikipedia.org/wiki/Polyacrylonitrilehttp://en.wikipedia.org/wiki/Polyacrylonitrilehttp://en.wikipedia.org/wiki/Suspension_polymerizationhttp://en.wikipedia.org/wiki/Suspension_polymerizationhttp://en.wikipedia.org/wiki/Solution_polymerizationhttp://en.wikipedia.org/wiki/Solution_polymerizationhttp://en.wikipedia.org/wiki/Solution_polymerizationhttp://en.wikipedia.org/wiki/Suspension_polymerizationhttp://en.wikipedia.org/wiki/Polyacrylonitrilehttp://en.wikipedia.org/wiki/Polyethylene_oxidehttp://en.wikipedia.org/wiki/Carboxymethylcellulosehttp://en.wikipedia.org/wiki/Maleic_anhydridehttp://en.wikipedia.org/wiki/Ethylenehttp://en.wikipedia.org/wiki/Polyvinyl_alcoholhttp://en.wikipedia.org/wiki/Acrylamidehttp://en.wikipedia.org/wiki/Acrylic_acid


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Disposable baby nappies contain SAP to keep mom's pet dry and happy.

In agriculture as water retainers in soil, e.g. AquaSorb and Water Grabber . The polymer

retains huge quantities of water and releases it to the plants over time to ensure a constant

supply of moisture and nutrients.

Novelty toys that grow in size.

Candles

Composites and laminates

Controlled release of insecticides and herbicides

Diapersand 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

Magical effects

Medical waste solidification

YinCheng's pads

Motionless water beds

Spill control

Surgical pads

Potting soil

Waste stabilization and environmental remediation

Water retention for supplying water to plants

Wire and cable water blocking

Wound dressings

Fuel monitor systems in aviation

Fuel monitor systems in vehicles

Artificial snow
http://en.wikipedia.org/wiki/Diaperhttp://en.wikipedia.org/wiki/Diaperhttp://en.wikipedia.org/wiki/Fire-retardant_gelhttp://en.wikipedia.org/wiki/Fire-retardant_gelhttp://en.wikipedia.org/wiki/Grow_monstershttp://en.wikipedia.org/wiki/Grow_monstershttp://en.wikipedia.org/wiki/Potting_soilhttp://en.wikipedia.org/wiki/Potting_soilhttp://en.wikipedia.org/wiki/Potting_soilhttp://en.wikipedia.org/wiki/Grow_monstershttp://en.wikipedia.org/wiki/Fire-retardant_gelhttp://en.wikipedia.org/wiki/Diaper


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Orbeez

A SAP can absorb as much as 500 times its own mass in water. The best absorption ratios are

achieved with de-ionised water due to the osmotic process.

References:

Buchholz , FL (Editor) & Graham, AT - Modern Superabsorbent Polymer Technology

John Wiley & Sons, November 1997, 0471194115

K. Horie, M. Bron, R. B. Fox, J. He, M. Hess, J. Kahovec, T. Kitayama, P. Kubisa, E.

Marchal, W. Mormann, R. F. T. Stepto, D. Tabak, J. Vohldal, E. S. Wilks, and W. J.

Work (2004)."Definitions of terms relating to reactions of polymers and to functional

polymeric materials (IUPAC Recommendations 2003)".Pure and Applied Chemistry76

(4): 889906.doi:10.1351/pac200476040889.

Katime Trabanca, Daniel; Katime Trabanca, Oscar; Katime Amashta, Issa Antonio

(September 2004).Los materiales inteligentes de este milenio: Los hidrogeles

macromoleculares. Sntesis, propiedades y aplicaciones.(1 ed.). Bilbao: Servicio

Editorial de la Universidad del Pas Vasco (UPV/EHU).ISBN84-8373-637-3.

Buchholz, Fredric L; Graham, Andrew T, ed. (1997).Modern Superabsorbent Polymer

Technology(1 ed.). John Wiley & Sons.ISBN0-471-19411-5.
http://en.wikipedia.org/w/index.php?title=Orbeez&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Orbeez&action=edit&redlink=1http://www.iupac.org/publications/pac/2004/pdf/7604x0889.pdfhttp://www.iupac.org/publications/pac/2004/pdf/7604x0889.pdfhttp://www.iupac.org/publications/pac/2004/pdf/7604x0889.pdfhttp://www.iupac.org/publications/pac/2004/pdf/7604x0889.pdfhttp://en.wikipedia.org/wiki/Digital_object_identifierhttp://en.wikipedia.org/wiki/Digital_object_identifierhttp://dx.doi.org/10.1351%2Fpac200476040889http://dx.doi.org/10.1351%2Fpac200476040889http://dx.doi.org/10.1351%2Fpac200476040889http://en.wikipedia.org/wiki/International_Standard_Book_Numberhttp://en.wikipedia.org/wiki/International_Standard_Book_Numberhttp://en.wikipedia.org/wiki/Special:BookSources/84-8373-637-3http://en.wikipedia.org/wiki/Special:BookSources/84-8373-637-3http://en.wikipedia.org/wiki/Special:BookSources/84-8373-637-3http://en.wikipedia.org/wiki/International_Standard_Book_Numberhttp://en.wikipedia.org/wiki/International_Standard_Book_Numberhttp://en.wikipedia.org/wiki/Special:BookSources/0-471-19411-5http://en.wikipedia.org/wiki/Special:BookSources/0-471-19411-5http://en.wikipedia.org/wiki/Special:BookSources/0-471-19411-5http://en.wikipedia.org/wiki/Special:BookSources/0-471-19411-5http://en.wikipedia.org/wiki/International_Standard_Book_Numberhttp://en.wikipedia.org/wiki/Special:BookSources/84-8373-637-3http://en.wikipedia.org/wiki/International_Standard_Book_Numberhttp://dx.doi.org/10.1351%2Fpac200476040889http://en.wikipedia.org/wiki/Digital_object_identifierhttp://www.iupac.org/publications/pac/2004/pdf/7604x0889.pdfhttp://www.iupac.org/publications/pac/2004/pdf/7604x0889.pdfhttp://en.wikipedia.org/w/index.php?title=Orbeez&action=edit&redlink=1

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