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  Super absorbent polymers: Highlights on PAC & PAM

Due to the extraordinary water absorption properties the super absorbent polymers (SAPs) are used widely in diapers, training pants, adult incontinence products and external feminine hygiene products, as well as agricultural and gardening applications, says
R Rathinamoorthy.

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 liquid, water.

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.

Mechanism

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

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 polymers(7), whereas 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.

Sodium Polyacrylate

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 was,

Raw materials like Acrylic Acid

Methyl Acrylate and
NaOH

Polymerisation in water

Polymer Extrusion:

Evaporation of water produces cylindrical fibres of precise diameter & length

Finishing: Thermal & conditioning treatments. Product Packaging

Application

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).

Medical dressing

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.

Hydraulic Mulching

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.

Coal dewatering

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 of separation(23).

Nurseries

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.

Others

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.

Conclusion

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.

Reference

1. Buchholz and Graham: Modern Superabsorbent Polymer Technology, Wiley, 1997, ISBN 0471194115.

2.Yan WL and Bai RB: Water Res 39:688, 2005.

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.

5. http://www.functionalpolymers.basf.com/portal/streamer?fid=291074.

6. Owens DE, Jian Y, Fang JE, Slaughter BV, Chen YH, Peppas NA: Macromolecules 40:7306, 2007.

7. www.pslc.ws.

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 Information Brochures.

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 (51)

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.

24. http://chemistry.org/portal/a/c/s/1/wondernetdisplay.html?DOC=wondernet/activities/polymers/ diapers.html.

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.

26. www.greenmcs.com.

27. www.made-in-china.com.

28. www.koboproducts.com.

Department of Textile Technology,
PSG College of Technology,
Coimbatore 641 004.
Email: r.rathinamoorthy@gmail.com.

published February , 2009
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