Review

Superabsorbent Polymers: A Diminutive Review

Madhuja Manoj Katkar

UG Student, Institute of Chemical Technology Mumbai

Abstract

Super absorbency is superior swelling abilities originate to absorb and retain large amounts of aqueous solutions. Superabsorbent polymers are cross-linked networks of hydrophilic polymer chains. With today’s demand for a one-click solution, the development of superabsorbent technology has been largely directed by the needs of disposable hygiene segment, agriculture industry etc. Over the years, starch-grafted superabsorbent polymer turned into cross-linked acrylic homopolymers. This paper discusses concept, mechanism and major application areas of superabsorbent.

Keywords: Absorbent, Agriculture, Industrial, Medical, Polymer, Water

Introduction

A Polymer is a large molecule composed of several repeating units. 3-D crosslinked network of polymer molecules forms polymer gels.  Superabsorbent polymer (SAP) is chemically or physically cross-linked polymer gels. When water comes into contact with one of these chains, it is drawn into the molecule by osmosis. Water rapidly migrates into the interior of the polymer network where it is stored.  Type and degree of crosslinkers governability of the superabsorbent polymer to absorb and retain large volumes of water and aqueous solutions [1]. superabsorbent polymer, hydrogel, absorbent polymers, absorbent gels, super soakers, super slurpers, water gel are the few other appellations for superabsorbent polymers. These polymers have the ability to swell in water and retaining a significant fraction (>20%) of water within their structure, without dissolving in water [2]. water uptake potential of these materials goes as high as 100,000% of its own weight in a short period of time [3]. The maximum volume of superabsorbent polymers covers the synthetic or petrochemical source and mostly acrylic acid and its sodium or potassium salts, and acrylamide is used in the industrial production of superabsorbent polymers. Nevertheless, superabsorbent polymers produced from proteins and polysaccharides like chitin, cellulose, starch, and natural gums etc., are the greener alternatives [4].

Super absorbent polymers are classified based on polymer type, crosslinking and type of absorbent, as follows figure 1.

classification-of-superabsorbent-polymer-jtcs-online
classification-of-superabsorbent-polymer-jtcs-online

Currently, synthetic polymers, especially acrylic, are used as for manufacturing of superabsorbent. This is because of cost effectiveness of the process and good absorption efficiency of these superabsorbent. However, today’s market is demanding for “greener” alternatives to replace these synthetics partially or fully. Carbohydrate polymers, polysaccharides, such as Chitin, cellulose, starch, and natural gums such as xanthan, guar and alginates are polymerized to arrive at the solution. These polysaccharides are cheapest and most abundant, available, and renewable organic materials.

Mechanisms of Swelling in Superabsorbent Polymers

The polymer backbone in superabsorbent polymer is hydrophilic i.e. ‘water loving’ because it contains water loving carboxylic acid groups (–COOH) as shown in figure 2 [1].

Structure of SAP
Figure 2 Structure of SAP [1]

When water is added to superabsorbent polymer there is a polymer-solvent interaction of ions of a solute with molecules of a solvent i.e. COO- and Na+ ions attract the polar water molecules, as shown in figure 3, called as hydration and the formation of hydrogen bonds are two of these
interactions.

Polymer-solvent interaction
Figure 3 Polymer-solvent Interaction

Super absorbent polymers are polymerized by one of the following techniques [4],

  • Bulk Polymerization
  • Solution Polymerization
  • Inverse Suspension Polymerization

Bulk polymerization is the simplest technique with high concentration monomer and monomer soluble initiators and it produces high molecular weight polymer with high purity. In solution co-polymerization reactions, the ionic or neutral monomers are mixed with the multifunctional cross-linking agent while in inverse suspension polymerization, the monomer solution is dispersed in the non-solvent forming fine monomer droplets, which are stabilized by the addition of stabilizer

Evaluation Methods [2]

Super absorbents are evaluated for the following properties;

  • Free-absorbency Capacity: This test indicates uptake of distilled water while the sample is freely swollen without any load on the testing sample. Free absorbency capacity is tested by either of the three methods viz; Tea-bag Method, Centrifuge method and sieve method.
  • Absorbency Under Load : This test indicates an uptake of 0.9% NaCl solution while the testing sample is pressurized by some loads, usually,  specified to be pressures 0.3, 0.6, or 0.9 psi.
  • Wicking Capacity and Rate: Wicking properties of superabsorbent polymer are evaluated by preparing filter paper cone with superabsorbent polymer at tip and holding it in petri dish containing water, for specified time.
  • Swelling Rate: Rate of swelling is evaluated by vortex method or swelling time profile test.
  • Swollen Gel Strength: The mechanical strength or modulus of swollen superabsorbent polymers is evaluated by rotational rheometry to quantify the swollen gel strength of superabsorbent polymer materials.
  • Soluble Fraction: The sol content, i.e. sum of all water-soluble is measured by extraction of superabsorbent polymer sample in distilled water.
  • Residual Monomer: High performance liquid chromatography (HPLC) is often taken as a preferred method to quantify the residua monomer.
  • Ionic Sensitivity: To achieve a comparative measure of sensitivity of the superabsorbent polymer materials towards particular aqueous fluid, a dimensionless swelling factor is calculated measuring absorption in a given fluid and absorption in distilled water.

Applications of Superabsorbent Polymers

In general, superabsorbent polymers are white sugar-like materials, started to be practically applied in sanitary products, now they have been widely used not only for hygiene products such as disposable diapers for children, etc., but also for water retaining soil products for gardening, water stop materials for the civil engineering and construction, sheets for raising seedling, fresh-keeping agents for food distribution fields, materials for poultice etc. (5). However, this paper emphases major applications areas like agriculture, industrial grade and medical sector, specific towards wound dressing, diapers and sanitary pads.

Superabsorbents in Agriculture

In agriculture and agroforestry, Super Absorbent Polymers are used as soil water retention agent, seed coating, soil-less cultivation, artificial turf, etc.

Super Absorbent Polymers act as micro water reservoirs at plant roots. They absorb natural and supplied water 400-500 times their own weight and release it slowly on account of root capillary suction mechanism, thus preventing water loss in soil by leaching and evaporation.  Superabsorbent polymers form a consistent cyclic process of absorption and release of water; the water so released can provide optimum moisture for quick germination and seedling maturation as shown in figure 4.

Figure 4 Working of SAP at Plant Roots [6]

Thus, superabsorbent polymers can help in saving water and labor by reducing irrigation frequency, help to overcome drought conditions and act as soil conditioners, prevent leaching in sandy soils, runoffs in mountainous and sloping fields, restore soil biota, reduce soil erosion by surface run-offs, fertilizer and pesticide leaching to groundwater, reduce cost of water and irrigation and increases yields of crops [6].

Because of the superabsorbent polymer coating, the seed emergence quickly and give plants an early, health start. It also exhibits strong and good disease resistance. Such superabsorbent polymer coated seeds are very useful to plants in the drought area.

SAP Coated Seeds
Figure 5 SAP Coated Seeds [6]

Application of superabsorbent polymers in golf and lawn can requires less water and fertilizer, with faster, healthier grow of lawn and the green last longer. Thus, it can reduce the cost of maintenance and causes less pollution [6].

Industry Grade applications

Superabsorbent polymers have numerous scope in various applications such as Functional coatings, Fire extinguishing gel, Cooling material, Anti-flood bag, Additive in concrete, Air fresher, Additives of cosmetics, etc. 

To prevent water penetration, tape consisting of superabsorbent polymer is wrapped around the cable to provide protection against damage and corrosive effect of water. Such cables are used in waterproof tape and waterproof ointment for using in optical fiber cables.

Superabsorbent polymer anti-flood bag can be a good alternative to than traditional sandbags. Superabsorbent anti-flood bag absorb water completely in 3 to 5 minutes when meets water and the bag will inflates quickly. Comparatively low weight of these bags facilitates easy storage and transportation.

Anti-flood bag before and after water absorption
Figure 6 Anti-flood Bag Before and After Water Absorption [6]

Super absorbent polymer for concrete, in a controlled way, significantly change their properties in response to an external stimulus. When it is exposed to water, they swell, and when subsequently subjected to drying, they reversibly shrink. These key properties can actively be used in relation to concrete.

Super absorbent when used in fire extinguisher gel, layer of gel adheres the surface. This gel, thus, contain water invariably. And fire cannot, therefore, spread rapidly since it has to overcome the quantity of water present in the gel.

Superabsorbents in medical Field

The largest use of superabsorbent polymers is in personal hygiene products. Superabsorbent polymers are primarily used as an absorbent for water and aqueous solutions for diapers, adult incontinence products, feminine hygiene products, and similar applications as a replacement of traditional absorbent materials such as cloth, cotton, paper wadding, and cellulose fiber. Superabsorbent polymer for diaper owns good permeability, strong absorptive capacity and maintenance ability, with no return infiltration phenomenon under a certain pressure. Superabsorbent polymer for sanitary napkin shows fast suction of blood, high absorption capacity, and good gel strength and moisture resistance. The strong absorption capacity and water retention ability in women`s sanitary napkin can keep dry and comfortable.

Diaper with SAP
Figure 7 Diaper with SAP

Wound dressing with super absorbent material consists of polymer based core absorbs extrudes from wound for longer time at higher rate and are suitable as dressing for pressure ulcers, venous leg ulcers, postoperative wounds. Another type of dressing used as primary dressing for dermal wounds like pressure ulcers, skin tears, burns etc., intends to maintain a moist environment and thus, enhances healing and prevents wound desiccation promoting autolytic debridement of a dry wound.

Wound dressing with SAP
Figure 8 Wound Dressing with SAP

To maintain the dry atmosphere in operation theatres, polymeric textile consisting sheath of cellulose and superabsorbent polymer absorbs and retains the liquids coming in contact.

Conclusion

Superabsorbent polymers have shaped an eye-catching space in the perspective of super-swelling behaviour, chemistry, and crafting the variety of end products, ranged from personal care products to agriculture.  However, further research is necessary to widen the application horizon of super absorbent polymers. Especially, for environment sustainability, superabsorbent polymer prepared from natural resources would greatly increase its potential in various products.

References

  1. Superabsorbent Polymers, Mark Elliott, http://chimianet.zefat.ac.il
  2. Superabsorbent Polymer Materials: A Review, https://www.researchgate.net/publication/242582443
  3. Applications of Super Absorbent Polymers in Agriculture, http://vikaspedia.in/agriculture/crop-production
  4. https://nptel.ac.in/courses/Absorbent Fibres
  5. Super absorbent polymer and preparation method thereof, US 2016/0030921 A1
  6. Super Absorbent Polymer, https://www.socochem.com
Please cite this article as: Madhuja Katkar (2019) Superabsorbent Polymers: A Diminutive Review. Journal of Textile and Clothing Science. https://www.jtcsonline.com/super-absorbent-polymers/
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