Polymers are substances whose molecules
have high molar masses and are composed of a large number of repeating
units. Polymers are formed
by chemical reactions in which a large number of molecules called monomers
are joined sequentially, forming a chain. Super absorbent polymers (SAP) and
Hydrogels are very unique water-absorbing and water-holding materials. They
are solid, granular or powder cross linked polymers that rapidly absorb and
retain large volumes of aqueous solutions. It can absorb up to 500 times of
its own weight of pure water and even under pressure resist release of the
absorbed water(1). The absorptive properties of SAP are ideally suited for
the absorption and solidification of various types of liquids like water,
sludge and blood.
These are growing in use in the medical
industry as coverings for wounds; In the plant industry as a way to deliver
water to plants slowly over an extended period of time; With forest fire
fighting groups as a protective covering for houses at risk; In the space
program as giant diapers for astronauts during takeoff and space walks, and;
In the disposable baby and adult diaper industry. In each of the uses, the
important characteristic of hydrogels is its ability to serve as a super
absorber and to keep the surrounding area damp but not wet. This is done by
forming a gel with the polymer as the solid net surrounding the suspended
Gels are formed by one of the class of
matter (liquid, gas or solid) enclosing and suspending a different class.
Solids in gas, gas in solids, liquid in gases, solids in liquids or in this
case liquids suspended in a solid. The hydrogels containing amide, amine,
carboxylic acid, and ammonium groups, can bind heavy metal ions and dyes
through the polar functional groups, and be used as good absorbents for
water purifications(2,3). The absorption rate of the polymers differs
according to their mechanism used for preparation.
A superabsorbent is a material that acts
like a super-sponge. As a superabsorbent absorbs water it forms a gel. In
addition to the amazing ability of these material is to increase moisture
availability. When water comes into contact with a superabsorbent, an
electrical repulsion takes place within the particles. When this happens,
water is drawn into the particles resulting into a swelling of each
particle. At maximum absorption capacity each particle will expand to over
thirty times its original volume. The water absorbent property of these
polymers due to the presence of sodium or potassium molecules that form
bridges between the long hydrocarbon chains. These bridges -- known as
cross-linking enable the polymer to form into a huge single super-molecule
including its ability to degrade in the environment and breakdown into
simpler molecules, and hold significant amounts of water.
The absorption capacity of super
absorbents is affected by acidity and alkalinity (pH), conductivity, and
other variables that inhibit expansion of the gel particles. The pH of the
absorbed fluid should not present a problem in most plant related
applications since the pH and the growing environment is normally within the
ideal range for optimum absorption. Superabsorbent polymers, such as
polyacrylamide (PAM) and polyacrylate (PAC), have a typical
three-dimensional network structure with a suitable crosslinking density(4).
In this article, the super absorbable
polymers (SAP) like Polyacrylate, Sodium Polyacrylate and Polyacrylamide
polymers and their applications are discussed.
Polyacrylate is an example of a type of
chemical called hydrogels or super absorbents. They are called as because
they have the ability to absorb
and hold a tremendous amount of water(5).
Hydrogels use cross-linked polymers to absorb many times their weight of
water. The superabsorbent material in PampersŪ brand disposable diapers is
sodium polyacrylate(5). At the present time only three superabsorbent gels
are used commercially. They are sodium polyacrylate, potassium polyacrylate,
and polyacrylamide. The potassium polyacrylate and polyacrylamide are widely
used as fire-blocking gels. The superabsorbent gel commonly used to absorb
chemical spills is potassium polyacrylate. Polyacrylamide gel is widely used
in horticulture to retain moisture around root systems of seedlings.
Acrylate monomers are esters, which
contain vinyl groups, two carbon atoms double bonded to each other, directly
attached to the carbonyl carbon. Polymers with bound positive or negative
charges are referred to as polyelectrolytes, macroions, or polyions.
Polyelectrolytes can be polyanions or polycations and are water soluble
polymers if their structure is linear which is shown in Figure 2. Water
interacts strongly with polyanions, such as PAC, via, hydrogen bonding to
the anionic groups (such as the carboxylate oxygens in PAC) aiding in the
dissolution of the polymer. But it is identified that the mechanical
strength of single PAC is feeblish, which cannot meet the application
requirement as a water treater(6).
Synthesis of Polyacrylate
PAC can function as both an antiscalant
and a dispersant. Polymeric antiscalants are generally low molecular weight
polymeric dispersants consist of higher molecular
weight fractions. Dispersants do not stop the formation of scale, but
instead are able to keep the scale particles suspended in the bulk fluid by
imparting a negative charge to the particles. These negatively charged repel
one another and prevent aggregation (and precipitation) into larger
particles of scale. PAC comprises about 5% of many laundry detergent
formulations because of its dispersant properties.
In the dry powdered state, the chains of
the polymer are coiled and lined with carboxyl groups or (-COON). When
hydrated with water, the carboxyl groups dissociate into negatively charged
carboxylate ions (COO). These ions repel one another along the polymer
chain, thereby widening the polymer coils allowing water to move into
contact with more carboxyl groups. As the polymer continues to uncoil the
ultimate swelling forms a gel-like consistency. The polymer does not form a
solution because it still has cross linking due to hydrogen bonding where
hydrogen atoms in water are being attracted to the oxygen atom in the
carboxylate ions between chains.
Disposal of absorbed water -- To
reliquify the gelled polymer for disposal purposes, common table salt is
added. When salt is added, each ion of sodium (Na-) and each ion of chlorine
(Cl-) is quickly surrounded by six molecules of water which is drawn out
from the polymer-water complex. The presence of sodium chloride in the
solution greatly decreases the ability of polyacrylate to absorb and retain
water. Other ionic compounds such as baking soda or vinegar can also be
used. Sodium polyacrylate is considered to be non-toxic, but inhalation of
airborne particles of the powder or contact with the eve can have some
serious adverse reactions.
Synthesis from acrylic acid may proceed
by polymerisation followed by the neutralisation of some or all of the
carboxylate groups. Or the acrylic acid may be partially or completely
neutralised and then polymerised8 as in Figure 3.
Polyacrylamide -- A ghost polymer
Polyacrylamides (often known as PAM)
absorb about 300 to 400 times its own weight in water. Because of the lower
absorbency and longer time to breakdown it cost less than polyacrylate. An
interesting physics application created by polyacrylamide is its ability to
effectively mimic the optical qualities of water. Because the crystals of
the polyacrylamide when hydrated are made up almost entirely of water, they
essentially look like water and light will pass from the surrounding water
into the crystal without being refracted at all.
These crystals are sometimes referred to
as ghost crystals because when hydrated the crystal is almost invisible in a
glass of water. A fully-grown hydrated crystal contains so much water that
optically behaves like water. However, when the crystal is lifted into air,
it becomes visible because the speed of light in the crystal is different
from the speed of light in air.
Synthesis of polyacrylamide
Polyacrylamide gels are formed by the
polymerisation of acrylamide in aqueous solution in the presence of small
amounts of a bi functional crosslinker. The crosslinker is usually
methylenebisacrylamide, The copolymerisation of acrylamide with
methylenebisacrylamide produces a mesh-like network in three dimensions,
consisting of acrylamide chains with interconnections formed from the
methylenebisacrylamide(9). Among variety of method, the polymerisation of a
polyacrylamide matrix with ethylenebisacrylamide cross-linking is important
one as shown in Figure 4. For discussions of the composition of
polyacrylamide gels, a standard nomenclature has been widely adopted.
Upon the introduction of catalyst, the
polymerisation of acrylamide and methylene bisacrylamide proceeds via, a
free-radical mechanism. The most common system of catalytic initiation
involves the production of free oxygen radicals by ammonium persulfate in
the presence of the tertiary aliphatic amine N, N, N', N'-tetramethylethylenediamine.
Another catalytic system involves the generation of free radicals via, a
photochemical process using a very small amount of riboflavin in the
presence of N, N, N', N'-tetramethylethylenediamine. In both catalytic
systems, the presence of excess oxygen will inhibit the polymerisation
elongation process and can lead to shorter average chain length.
Super absorbent fibre
Super-absorbent fibre has the property
of absorbing moisture up to several thousand times its original weight,
undergoing significant expansion, and eventually becoming a gel(10). Super
absorbent fibres are manufactured as follows: Discontinuous fibres are
coated with a binder material and with the binder material adhering the
fibres to super absorbent particles, fibres in the product are substantially
unbounded except to the super absorbent particles. The binder may be present
at an amount which is sufficient to substantially and continuously coat the
fibres. Plural coatings of various binder materials may be used. The binder
material may be heat fusible or heat curable and the treated fibres mixed
with other fibres for use in producing a wide variety of products.
Super-absorbent polymers are primarily
used as an absorbent for water and aqueous solutions for diapers, adult
incontinence products, feminine hygiene products, and similar
applications(11). Mē Polymer Technologies has recently begun to represent
the Superabsorbent Polymer fibre (OASISŪ SAF) products for Technical
Absorbents Limited (TAL) of Grimsby, North East Lincolnshire, United
Kingdom. The raw materials are mixed together with water and reacted using a
polymerisation initiator. The very high conversion of monomer raw materials
to polymer is achieved. The aqueous polymer solution is then extruded into a
hot air stream and dried. The filaments are then heated to dry and cure the
polymer, producing filaments, which are insoluble in water. Moisture may be
added to the fibres to aid processing.
Nonwoven textile inlays containing super
absorbents can be used for clothing elements, such as absorbent vests, tops,
and single sweat-absorbing elements, as well as for inlays placed inside
protective gloves and footwear(12). Telem Gok Sadikoglu(13) reported that
adding 3.5% SAF is enough to improve the thermal properties of the nonwoven
fabric and the perception of dampness. The general manufacturing sequence
Raw materials like Acrylic Acid
Methyl Acrylate and
Polymerisation in water
Evaporation of water produces
cylindrical fibres of precise diameter & length
Finishing: Thermal & conditioning
treatments. Product Packaging
Global demand for SAPs totaled an
estimated 1.05 million tons in 2003, and the demand is expected to increase
an average of 3.6 per cent per year from 2003 to 2008(14). Globally, baby
diapers account for an estimated 81 per cent of SAP demand. Adult
incontinence garments make up the next largest segment of the SAP market (8
per cent) followed by other applications (6 per cent) and feminine hygiene
products (5 per cent).
The "Other applications"
category includes de-watering agents for sewage sludge, drying agents for
china-clay slurries and pulverised-coal slurries(15), and a wide range of
other uses. Based on superabsorbent polymers, some kinds of superabsorbent
hydrogels have been researched, many biomedical and technological
applications such as artificial implants, contact lens, pharmaceutical,
biosensors, and metal ions reclamation have been resolved and novel
conducting hydrogel have been developed(16).
The absorbent core of the general
medical wound dressing consists of three components, a thin sheet of
polyurethane foam, and a piece of nonwoven fabric and a layer of
superabsorbent polyacrylate fibres. The core is located centrally upon a
larger piece of polyurethane film and is held in place by the perforated
silicone adhesive layer that extends to the outer margins of the dressing.
This gentle adhesion also tends to prevent maceration by inhibiting the
lateral movement of exudate from the wound on to the surrounding skin(17).
Reduction in soil erosion
PAM is highly effective in reducing soil
erosion off fields and can increase water infiltration into irrigated
furrows(18,19). PAM has been shown to significantly reduce soil erosion by
90 - 95 per cent when applied to irrigation water. PAM's three most common
forms are dry granules, solid blocks (cubes), and emulsified liquids. The
application method of PAM chosen depends on the form of PAM selected. The
use of dry granular PAM into irrigation water is facilitated by the use of
an augured metering system and excellent mixing and thorough dissolving
before the PAM reaches the irrigated furrows(20). PAM blocks (or cubes) are
usually placed in wire baskets that need to be secured to the edge of the
ditch to avoid washing of the blocks down the ditch(21). Liquid PAM can be
metered directly from the container into the irrigation ditch, directly into
the furrow, or through a pipe line or injector pump.
Super absorbents are mixed with water,
seed and other additives to promote and speed germination. In addition, the
use of a superabsorbent improves fluid flow since the material acts as a
lubricant. Super absorbents are used in hydro mulching to promote emergence
and growth. Super absorbents are not appropriate for all applications. Where
there is ample rainfall or the area receives adequate, uniform irrigation
the use of super absorbents may not be justified.
In most coal preparation processes,
water is a necessary medium, but the presence of water in coal after it has
been cleaned has a negative impact on transportation costs, handling and
specific energy values. The utilisation of super absorbent polymers (SAP) in
the nappy application is well-known. The concept of utilising these polymers
was investigated by Venter et al(22) for the purposes of dewatering coal and
other fines, generated by preparation processes such as flotation. SAPs are
granular highly cross-linked synthetic copolymers with excellent
water-absorbing properties. Using a water-permeable cloth as a barrier
between the slurry and the polymer was a novel idea that solved the problem
A major application for super absorbents
in the nursery industry is to improve the moisture holding capacity of soil
and soil-less mixes. In addition, using super absorbents can increase
aeration and drainage. Increased aeration reduces the likelihood of root rot
since there is a better opportunity for root gas exchange. The increase in
aeration and expansion of the growing media occurs as superabsorbent
particles expand. In expanding they push surrounding soil particles apart,
loosening the media, increasing pore space.
Further there are many uses for polymers
including use in infant diapers(24), adult incontinence products, feminine
hygiene products, paper towels, surgical sponges, meat trays, disposable
mats for outside doorways and bathrooms, household pet litter, bandages and
wound dressings, controlled drug delivery, humidity-controlling products,
water purification(25), soil conditioners, controlled release of fertilisers,
thickening agents for cosmetics to concrete, sealing of underground cables,
artificial snow, sensors, aqueous waste management and gelling agents.
Super absorbent materials are the
versatile, natural, biodegradable and renewable ones that have many
commercial applications. SAPs have led world to the development of a wide
range of products for day-to-day life. Further the use of super absorbent
polymer (SAP) in waste stabilisation offers many advantages over more
conventional absorbents. These advantages include minimal increase in waste
volume or weight, lower transport fees due to less loads, lower total
landfill disposal fees due since waste volume has not expanded, faster
processing time and consummate labour savings, etc. Moreover the use of SAP
is appropriate and cost effective. Undoubtedly, the effectiveness of SAPs
shows that superabsorbent materials will replace traditional absorbent
materials such as cloth, cotton, paper wadding, and cellulose fibre and play
a vital role in forthcoming decades.
1. Buchholz and Graham: Modern
Superabsorbent Polymer Technology, Wiley, 1997, ISBN 0471194115.
2.Yan WL and Bai RB: Water Res 39:688,
3.Ali AE, Shawky HA, Rehim HA: Eur Polym
J 39:2337, 2003.
4.Tang QW, Lin JM, Wu JH: J Appl Polym
Sci 104:735, 2007.
6. Owens DE, Jian Y, Fang JE, Slaughter
BV, Chen YH, Peppas NA: Macromolecules 40:7306, 2007.
8. David Cash: Superabsorbent Polymers,
Chem 13 News, May 2007, pp 6.
9. Super Absorbent Cellulosic Fibre and
Method of Making Same: US Patent 6844066.
10.Technical Absorbent Limited SAF
11. Grazyna Bartkowiak: Liquid Sorption
by Nonwovens Containing Superabsorbent Fibres, Fibres & Textiles in
Eastern Europe, January/March 2006, Vol 14, No: 1 55-61.
12.Telem Gok Sadikoglu: Effect on
Comfort Properties of Using Superabsorbent Fibres in Nonwoven Interlinings,
Fibres & Textiles in Eastern Europe July/September 2005, Vol 13, No: 3
13.Qunwei Tang, Jihuai Wu, Jianming Lin,
Qinghua Li, Shijun Fan: Two-step Synthesis of Polyacrylamide/Polyacrylate
Interpenetrating Network Hydrogels and Its Swelling/Deswelling Properties, J
Mater Sci (2008) 43:5884-5890.
14. Sanjay V Malhotra: Vineet Kumar,
Anthony East, Michael Jaffe, Applications of Corn-Based Chemistry, National
Academy of Engineering Publication, Volume 37, Number 4 - Winter 2007.
15. Farrar D, P Flesher, M Skinner, and
D Marshall (1995): Water Absorbing Polymers, US Patent 5, 384, 343.
16.Tang QW, Lin JM, Wu JH: Carbohydr
Polym 67:332, 2007.
17. Dykes P J, Heggie R, Hill S A:
Effects of Adhesive Dressings on the Stratum Corneum of the Skin, J Wound
Care 2001; 10(2): 7-10.
18.Trenkel J, D Burton, and C Shock: PAM
and/or Low Rates of Straw Furrow Mulching to Reduce Soil Erosion and
Increase Water Infiltration in a Furrow Irrigated Field, 1995 trial, OSU,
Malheur Experiment Station Special Report 964:167-175, 1996.
19. Shock C, J Trenkel, D Burton, M
Saunders, and E Feibert: Season-long Comparative Effectiveness of
Polyacrylamide and Furrow Mulching to Reduce Sediment Loss and Improve Water
Infiltration in Furrow Irrigated Onions, OSU, Malheur Experiment Station
Special Report 964:176-185, 1996.
20. Lentz R D, I Shainberg, R E Sojka
and D L Carter: Preventing Irrigation Furrow Erosion with Small Applications
of Polymers, Soil Sci Soc Am J 1992, 56:1926-1932.
21. Rout T J, R E Sojka, and R D Lentz:
Polyacrylamide Effect on Furrow Erosion and Infiltration, American Society
of Agricultural Engineers, Spokane, Washington, 1993.
22. Peer F and Venter T: Dewatering of
Coal Fines Using a Super Absorbent Polymer, The Journal of The South African
Institute of Mining and Metallurgy July/August 2003, 403-410.
23. Dzinomwa G P T, Wood C J, and Hill D
J T: Fine Coal Dewatering Using pH-and Temperature-sensitive Superabsorbent
Polymers, Polymers forAdvanced Technologies, Vol 8, pp 767-772.
25. Daniel Burton, Jan Trenkel, and
Clint Shock: Effects of Polyacrylamide Application Method on Soil Erosion
and Water Infiltration, Malheur Experiment Station Oregon State University
Ontario, Oregon, 1995.
Department of Textile Technology,
PSG College of Technology,
Coimbatore 641 004.