ScienceAsia 33 Supplement 1 (2007): 39-43

Superabsorbent Polymers and SuperabsorbentPolymer Composites

Suda Kiatkamjornwong*

Department of Imaging and Printing Technology, Faculty of Science, Chulalongkorn University,Phyathai Road, Wangmai, Pathumwan, Bangkok 10330, Thailand.

* Corresponding author, E-mail: ksuda@sc.chula.ac.th

ABSTRACT: This article briefly describes the general aspects and cross-linking polymerization of superabsorbentpolymers (SAPs) and superabsorbent polymer composites (SAPCs). Research and development of SAPs hasbecome very active after the Northern Region Laboratory of the United States Department of Agricultureinvented the first SAP, used as a soil conditioner. Based on the advancements in organic/inorganic syntheses,SAPs are produced and used in a wide range of applications for fluid absorption. The rapid growingnanotechnology has led to more explorations of SAPs and SAPCs for applications in biomedical, biotechnologyand advanced technologies. Examples of research work of SAPs and SAPCs published in refereed, reviewedarticles are introduced.

KEYWORDS: Superabsorbent polymers; Superabsorbent polymer composites; Water absorption capacity; Cross-linking reaction; Ionic functional groups.

INTRODUCTION

Superabsorbent polymers (SAPs) or hydrogels areloosely cross-linked, three-dimensional networks offlexible polymer chains that carry dissociated, ionicfunctional groups. They are basically the materials thatcan absorb fluids of greater than 15 times their owndried weight, either under load or without load, suchas water, electrolyte solution, synthetic urine, brines,biological fluids such as urine sweat, and blood. Theyare polymers which are characterized by hydrophilicitycontaining carboxylic acid, carboxamide, hydroxyl,amine, imide groups and so on, insoluble in water, andare cross-linked polyelectrolytes. Because of their ionicnature and interconnected structure, they absorb largequantities of water and other aqueous solutions withoutdissolving by solvation of water molecules via hydrogenbonds, increasing the entropy of the network to makethe SAPs swell tremendously. The factors that supplyabsorbing power to polymers are osmotic pressure,based on movable counter-ions, and affinity betweenthe polymer electrolyte and water. The factor thatsuppresses absorbing power, in contrast, is found inthe elasticity of the gel resulting from its networkstructure. Not only are they of high fluid absorbingcapacity, but the absorbed fluid is hard to release, asthey merely immobilize the fluid by entrapment ratherthan by holding it in the structure

SAPs possess a number of attributes that makethem attractive in many different applications. SAPshave supplanted much of the traditional absorbents ininfant diapers and have made significant improvementsin the performance of feminine hygiene products and

doi: 10.2306/scienceasia1513-1874.2007.33(s1).039

adult incontinence products as a result of superiorwater-absorbing properties. Because of their excellentcharacteristics, SAPs are widely used in many fields,such as agriculture, horticulture, sanitary goods, andmedicine. The basic property of water absorption hassuggested the use of SAPs in many other applications,including paper towels, surgical sponges, meat trays,disposable mats for outside doorways and inbathrooms, household pet litter, bandages and wounddressings. The ability of the swollen gels to release thewater to the surroundings as vapor has also been usedin various ways, for example, as humidity-controllingproducts or as soil conditioners. SAPs may also be usedto release water-soluble substances from within thenetwork structure into the surroundings as a solution.For example, pharmaceuticals and fertilizers may beincorporated into SAPs to yield controlled releaseproducts. Another characteristic of the swollen polymeris its rubbery nature, which has been used to controlthe consistency of products as diverse cosmetics orconcrete or to contribute a soft, yet dry, feel to a productlike hot or cold packs for sore muscles. The soft, rubberynature may also be employed to impact sealingproperties to products that are in contact with wateror aqueous solutions, for example, underground wiresand cables. Since they are widely applied not only in thefields of personal care products, bio-sorbent, bio-material, pharmaceutical, drug delivery systems, butalso in agriculture, forestation, industrial, construction,communication industries, and environmentalapplications, SAPs provide ways for water managementfor both wanted or unwanted water depending on itsparticular purpose. Therefore, water absorption

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capacity (WAC) is the most important characteristicsof SAPs and SAPCs. This can be measured by volumetric,gravimetric, spectroscopic and microwave methods.

SYNTHESIS TECHNIQUES

From a material resource point of view, SAPs canalso be divided into natural macromolecules, semi-synthetic polymers, and synthetic polymers. From apreparation point of view, they can be synthesized bygraft polymerization, cross-linking polymerization,networks formation of water-soluble polymer andradiation cross-linking, etc. There are many types ofSAPs presently in the market. Mostly, they are lightlycross-linked copolymers of acrylate and acrylic acid,and grafted starch-acrylic acid polymers prepared byinverse suspension, emulsion polymerization, andsolution polymerization. The polymerizationtechniques have been described1.

Bulk polymerizationBulk polymerization is the simplest technique which

involves only monomer and monomer-solubleinitiators. High rate of polymerization and degree ofpolymerization occur because of the highconcentration of monomer. However, the viscosity ofreaction increases markedly with the conversion whichgenerates the heat during polymerization. Theseproblems can be avoided by controlling the reaction atlow conversions. The advantage of bulk polymerizationis that it produces high molecular weight polymer withhigh purity. Polyacrylate SAPs are prepared by thistechnique.

Solution polymerization/cross-linkingIn solution co-polymerization/cross-linking

reactions, the ionic or neutral monomers are mixedwith the multifunctional cross-linking agent. Thepolymerization is initiated thermally, by UV-irradiation,or by a redox initiator system. The presence of solventserving as a heat sink is the major advantage of thesolution polymerization over the bulk polymerization.The prepared SAPs need to be washed with distilledwater to remove the unreacted monomers, oligomers,cross-linking agent, the initiator, the soluble andextractable polymer, and other impurities. Phaseseparation occurs and the heterogeneous SAP is formedwhen the amount of water during polymerization ismore than the water content corresponding to theequilibrium swelling. The best example is preparationof poly(2-hydroxy ethyl methacrylate) SAPs fromhydroxyl ethyl methacrylate, using ethylene glycoldimethacrylate as a cross-linking agent. Using thismethod, a great variety of hydrogels has beensynthesized. The SAPs can be made pH-sensitive or

temperature-sensitive by incorporating methacrylicacid or N-isopropyl acrylamide as monomers.

Suspension polymerization or inverse suspensionpolymerization

Suspension polymerization is a method to preparespherical SAP microparticles with size range of 1 µm to1 mm. In suspension polymerization, the monomersolution is dispersed in the non-solvent forming finemonomer droplets, which are stabilized by the additionof stabilizer. The polymerization is initiated by radicalsfrom thermal decomposition of an initiator. The newlyformed microparticles are then washed to removeunreacted monomers, cross-linking agent, and initiator.Some SAPs microparticles of poly(hydroxy ethylmethacrylate) have been prepared by this method.Recently, the inverse suspension technique has beenwidely used for polyacrylamide-based SAPs because ofits easy removal and management of the hazardous,residual acrylamide monomer in the polymer.

Polymerization by irradiationIonizing high energy radiation, like gamma rays2

and electron beams3, has been used as an initiator toprepare the SAPs of unsaturated compounds. Theirradiation of aqueous polymer solution results in theformation of radicals on the polymer chains. Also,radiolysis of water molecules results in the formationof hydroxyl radicals, which also attack the polymerchains, resulting in the formation of macroradicals.Recombination of the macroradicals on different chainsresults in the formation of covalent bonds, so finally across-linked structure is formed. Examples of polymerscross-linked by the radiation method are poly(vinylalcohol), poly(ethylene glycol) and poly(acrylic acid).The major advantage of the radiation initiation over thechemical initiation is the production of relatively pureand initiator-free SAPs.

Cross-linking in Superabsorbent PolymersThere are two main types of cross-linking, bulk and

surface cross-linking in most advanced SAPs, especiallythose used in diapers and napkins. The differencebetween the surface cross-linking and bulk cross-linking is shown below. Network formation is causedby post-polymerization cross-linking or curing in thecase of using a UV source1. A bi-functional or multi-functional monomer is first mixed with the pre-formedpolymer chains and a coupling reaction between thecross-linker and the functional groups on the pre-formed polymer is triggered by low temperature mixing,followed by heating. Ionic cross-linking and covalentcross-linking are the two different types of post-polymerization cross-linking4.

a) Bulk or core/bulk cross-linking: Such a cross-

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linking of the polymer normally takes place during thepolymerization stage of the monomer to form a networkin which a cross-linking agent is actually a co-monomerwith a higher functionality than the main monomer.The reactivity ratio of the cross-linker and the monomeris very important. If the reactivity ratio of the cross-linker is higher than that of the monomer, it will reactat low monomer conversion. On the other hand, if thereactivity ratio of the cross-linker is lower than that ofthe monomer, it will react at a high monomerconversion. Extractable product contains lowmolecular weight polymer chains that are notincorporated in the polymer network and can be readilyextracted in excess liquid. The degree of soluble polymeris important in determining the optimum cross-linkinglevel, since too low a cross-linking level gives high WACwith low strength and high extractable product, whiletoo high a cross-linking level induces low extractableproduct with non-tacky gel and having low WAC. Toobtain an acceptable level for most SAP users, a balanceis required with a small amount of extractable, high gelstrength and high WAC.

b) Surface cross-linking: This type of surface cross-linking is a new process that improves the absorptionagainst a pressure profile of the polymer gel, such as forfeminine napkins. For the surface cross-linking reaction,surface treatment is necessary. Because high swellingcapacity is obtained, but poor absorption againstpressure occurs due to low elastic gel strength, causedby the low core cross-linking level. Swollen particlesare easily deformed and clump together under load.The liquid supplying voids are closed. When the swellingunder load is increased, the capacity under load isdecreased. Therefore, gel blocking happens, so post-treatment is necessary to solve this problem This takesplace during the final stages of SAP manufacture, andcan be used to tune the desired properties of the SAP.Surface treatment is also a necessary step to providebetter SAP products. A compound having at least twofunctional groups, capable of reacting with thecarboxylate groups on the polymer backbone, such aspolyhydric alcohol (monopropylene glycol), diglycidylethers (ethylene glycol diglycidyl ether), or quaternaryamines, is usually used as a cross-linking agent. Thecross-linking agent can be incorporated to the SAP bydissolving it into a solution containing water and aswelling suppressant solvent. Generally, the cross-linking reaction of the coated SAP particles can beperformed by heating the polymer for a predeterminedtime. The amount of the cross-linking agent and heatingtime can be varied to give SAPs products with differentswelling properties, such as absorption against pressureand saline flow conductivity. By the surface treatment,a highly cross-linked shell results with increased rigidityand a lower level of cross-linking core. The shell is of

finite thickness, which can be fully controlled byadjusting the coating solvents and particle size of SAPs.The harder surface prevents gel blocking or fish eyesand allows liquid to flow freely to the SAP particles forfluid absorption.

Particle size and particle size distribution (PSD) areone of the important parameters for performancequality of the final products. Generally particle sizeand PSD have a direct effect on a free swell rate of theSAPs, where the small particle size can absorb fasterthan the larger particle size. The PSD controls theextent of fluid absorption; inter-particle voids resultingfrom a broad PSD hold unabsorbed water, which maycause problems with the products. Therefore, controlsin polymerization technique, optimization of grindingmachine and particle size classification are the keys tosuccess in industrial production of diapers or napkins.

Many kinds of SAPs have been commercializedsince the hydrolyzed starch-polyacrylonitrile graftcopolymer; the super slurper was first developed bythe Northern Regional Laboratory of the United StatesDepartment of Agriculture in 19614. Since thentremendous amounts of research and developmenthave been made in an attempt to modify SAPs structureand morphology to enhance WAC, gel strength andabsorption rate. New types of SAPs and superabsorbentpolymer composites (SAPCs) are emerging for moreadvanced applications, especially in thenanotechnology field. A well-known example of SAP isan acrylate family which is widely used in many industrialapplications especially in personal care products.

Polyacrylate superabsorbent polymers are one ofthe largest families of the polymer. They are preparedby free-radical initiated polymerization of acrylic acidand its salts with a cross-linker in aqueous solution oras suspension drops of aqueous solution in ahydrocarbon solvent. Furthermore, copolymerizationof acrylic acid with other hydrophilic vinyl monomerssuch as acrylamide has expanded widely the applicationsof SAPs. The types of co-monomer and cross-linkermonomer play an important role in designing SAPsproperties and applications. Bulk, solution, andsuspension polymerizations are the major productionprocesses for polyacrylate SAPs. In addition,polymerization by radiation has also gained muchattention for future clean technology production. Themonomer and cross-linker concentrations, the initiatortype and concentrations, pre-neutralization or post-neutralization of the carboxylic acid groups, the relativereactivities of the monomer pairs and cross-linkerpolymerization additives, the medium pH, the reactiontemperature, in conjunction with the types of initiator,are all significant factors in both polymerizationprocesses. Several initiating sources for a particularmonomers and special additives include thermal

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initiator, redox initiator, and photochemical initiator,such as non-ionizing radiation or ionizing radiation.The types of cross-linking agent modify the finalproperties of SAPs. Without a cross-linking agentincorporated in the SAPs, soluble polymers are alwaysobtained. Cross-linking polymerization plays asignificant role in the properties of superabsorbentpolymers. Three principal bonding types, namely,covalent, ionic and physical cross-links are used tobind the polymer chains together. The copolymerizingcross-linkers used in SAPs range from di-functionalcompounds such as N, N’-methylenebisacrylamide ordiacrylate esters to tri-functional compounds like 1,1,1-trimethyolpropanetricarylate to tetra-functionalcompounds such as tetraallyloxyethane. In addition,covalently cross-linked SAPs can also be produced.The preformed polymers with the above-mentionedfunctional monomers react with the carboxylic acidgroups by means of a condensation polymerization.Ionic cross-links are formed in which a polyvalent ionof opposite charge reacts with the charged polymerchains. The cross-linking points are thus formed as aresult of charge association of the unlike charges.Physical cross-links are formed by means of hydrogenbonding of segments of one polymer chain with thesegments of another polymer chain. This type of cross-link is not quite stable, because it is easily destroyedunder heat or pressure.

Although there are a few researchers in the area ofSAPs in Thailand, SAPs research work carried out herefalls into the categories of redox initiation, radiationinitiation and polymerization, either bycopolymerization or graft copolymerization as follows.Polysaccharide can be a very good substrate forpreparing biodegradable SAPs. Starch and chitosanare the most suitable substrates for graft co-polymerization5-10 to produce SAPs. Syntheses of SAPsby hydrophilic vinyl monomers by inverse suspensionpolymerization have been widely carried out11-12 andby solution polymerization13-15, via radiation initiationand polymerization16-22.

SUPERABSORBENT POLYMER COMPOSITES (SAPCS)

Theoretically, there is a wide range of inorganicmaterials with expandable layers available for utilizationfor the preparation of SAPCS. In recent years, the studyof organic-inorganic nanocomposites has become avery important field. Currently, reinforcing polymerswith small amounts of smectite clays has attractedincreasing interest, because the derived hetero-structural materials exhibit impressive mechanical,thermal, optical, and other properties that increasetheir technological values. The organic/inorganicphases are quite different in nature and incompatible,

leading to phase separation. Interfacial bonding andadhesion is introduced to avoid phase separation bycovalent bonding. Phyllosilicate is an appropriatematerial to prepare SAPCs. Clays have sandwich typesof structure with an octahedral Al sheet and twotetrahedral Si sheets. There are many types ofphyllosilicate: kaolinite, montmorillonite, hectrite,saponite and, synthetic mica, etc. The commonly usedlayered silicate for the preparation of polymer/layeredsilicate nanocomposites belongs to the same generalfamily of 2:1 layered phyllosilicate. Their crystalstructure consists of layers made up of two tetrahedrallycoordinated silicon atoms fused to an edge-sharedoctahedral sheet of aluminium or magnesiumhydroxide. The layer thickness is around 1 nm, and thelateral dimensions of these layers may vary from 30 nmto several microns or larger, depending on the particularlayered silicate. Stacking of the layers leads to a regularvan der Waals gap between the layers called theinterlayer or gallery. Isomorphic substitution withinthe layers (for example, Al3+ is replaced by Mg2+ or Fe2+,or Mg2+ is replaced by Li+) generates negative chargesthat are counterbalanced by alkali and alkali earthcations situated inside the galleries. In the case oftetrahedrally substituted layered silicates, the negativecharge is located on the surface of silicate layers, andhence, the polymer matrices can react interact morereadily with these than with octahedrally-substitutedmaterial. Therefore, intercalation structures can beproduced in SAPCs. The SAPCs find many newapplications beyond those of SAPS, such asnanocomposite materials, for example, fabrication ofsilver or zinc nanoparticles in SAP or SAPC networks.The proven applications are for catalysis, optics,electronics, bio-medicals and quantum-sized domainapplications. Some potential application of SAPCs inwater treatments have been described23-26.

FUTURE PROSPECTS

SAPs and SAPCs are very dynamic research areasin polymer sciences due to the advances in polymersyntheses resulting in better WAC, higher gel strengthand faster absorption rate, as well as other materialsproperties for comfort and safety. Nanotechnologywill likely play a major role in the production of thehigher performance SAPs and SAPCs.

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