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Holnazarov Bakhodir Azamovich, Senior Research Fellow, Termez State University E-mail: [email protected] Turaev Khayit Khudainazarovich, Doctor of Chemical Sciences, Professor, Termez State University Dzhalilov Abdulakhat Turapovich, Member of the Academy of Sciences of the Republic of Uzbekistan,
Professor, Doctor of Chemical Sciences, Director of the Tashkent Research Institute of Chemical Technology, Republic of Uzbekistan
SYNTHESIS OF STARCH, ACRYLAMIDE, ACRYLIC ACID AND MONTMORILLONITE-BASED SUPERABSORBENT POLYMER COMPOSITE
Abstract. This article studies the synthesis of a high swelling hydrogel based on starch, acrylamide, acrylic acid and montmorillonite. The reaction temperature and the effect of reaction time on the properties of the product are studied. The product of the copolymerization reaction was characterized by IR spectroscopy. The surface structures of the hydrogel were studied with an electron scanning microscope.
Keywords: Starch, acrylamide, acrylic acid, montmorlonite, superabsorbent hydrogel, binding agent, initiator, swelling kinetics.
Introduction: Hydrogels are three-dimensional hydrophilic polymeric networks capable of absorbing large amounts of water or biological fluids. They are widely used in gardening and agriculture. In agriculture, they are used for water retention [2], packaging materials, oil extraction, heavy metal absorbents and drug release carriers [3]. Starch-based hydrogels, copolymer of acrylic acid and acryamide, are widely used because of their high water absorption and low cost. However, these types of superabsorbent (SA) usually have a low salt tolerance and a slow rate of water absorption. In addition, after water absorption, they exhibit poor strength, dispersing and elastic properties of the gel. These disadvantages severely limit the quality of the product and the scope of this type of SA. To improve the
properties of such a SA material, researchers used such methods as the formation of interpenetrating networks and mixing with inorganic clays [4]. These methods help to improve the mechanical properties of hydrogels. Adding inorganic clay is a relatively effective way to improve the properties of this type of hydrogel [5]. Montmorillonite, bentonite, kaolin are commonly used to modify hydrogel with high water absorption. The results showed that the addition of suitable amounts of inorganic clays can improve the mechanical strength and water-absorbing ability of the hydrogel [6]. The gel prepared by researchers [7] showed an improvement in salt tolerance and strength, which compared to a gel without kaolin and the addition of 5-10% kaolin to a composite hydrogel increases its water absorption coefficient by
20% and its ability to conserve water by 25%, which is the addition of modified montmorillonite not only increased the water absorption coefficient, but also improved the ability of the gel to conserve water.
Bentonite is a clay mineral whose main component is montmorillonite. Its crystal structure consists of two layers of an oxygen-silicon tetrahedron with one layer of an octahedral sheet of alumina. This clay has good properties of expansion, absorption and caking. Kaolin is a clay mostly composed of ka-olinite. It has a relatively high ductility and sintering [8]. It is easily dispersed in water and has good fire resistance. Many researchers have recently reported compound hydrogels made from bentonite and kaolin. Researches have shown that bentonite and kaolin have a different effect on the properties of the hydrogel. Kaolin significantly increases the tensile strength and mechanical properties of composite gels, and bentonite significantly increases their water-absorbing ability [9]. To date, most studies have focused on the effect of one particular type of clay on the effectiveness of gels. However, it is not clear how the performance of a hydrogel changes when two different types of clay are added at the same time and whether two different types of gels can be syner-gistically applied to achieve a good effect.
Therefore, in this research, inorganic fillers starch, acrylic acid and acrylamide, bentonite were used as a water-absorbing monomers a, N, N'-methylene bi-sacrylamide (MBAm) as a crosslinking agent and potassium persulfate (KPS) as an initiator for the preparation ofcomposite hydrogel based on starch, acrylic acid, acrylamide and bentonite. The effect ofconcentrations of initiator, crosslinking agent, bentonite on the water-absorbing ability of the hydrogel has been studied. In addition, the effect of the degree of neutralization of acrylic acid and the reaction temperature on the ability of the hydrogel to absorb water was also studied. The thermal stability, water-saving capacity, multiple water absorption and salt tolerance of the composite hydrogel were analyzed. The micromorphology and chemical structure of the gel were also analyzed.
Research objects and methods: The experiment used Golden Corn corn starch produced by the Tashkent starch factory, acrylamide (AA), acrylic acid (AAc), a crosslinking agent (CA) - N, N'-meth-ylenebisacrylamide with main component content of 98% or more, potassium persulfate (KPS), sodium metabisulfite (initiator) and used montmorillonite (MT) from the Navbahor deposit (Uzbekistan).
Research objects and methods: In a four-necked round-bottomed flask with a mechanical stirrer, a reflux condenser, a thermometer and a nitrogen line, 8.1 g of dried potato starch was dispersed in 90 ml of distilled water. After purging with nitrogen for 10 minutes, dissolved oxygen was removed from the solution, the starch solution was heated to 80 °C in a water bath for 30 minutes with stirring to form a starch suspension. The ammonium persulfate initiator was added in amount of 0.5 to 1.5% to the starch suspension and the reaction was continued at 60 °C for 10 minutes. After that, in the process, a mixed solution of acrylamide, acrylic acid, crosslinking agent and mineral ultrafine powder was prepared by mixing acrylamide monomer -7.1 g, acrylic acid - 3.5 ml, crosslinking agent N, N-methylenebisacrylamide in amount of 0.25 to 1%, distilled water - 30 ml and ultrafine mineral powder of 1 to 5% at room temperature and the reaction mixture is stirred for another 3 hours to ensure that the graft polymerization is completed. The nitrogen atmosphere was maintained throughout the reaction period. The graft copolymer was added to a 3% sodium hydroxide solution and left for the saponification reaction at 95 °C for 2 hours.
The saponified product was filtered and washed several times with distilled water to remove unreacted starting material, the monomer and washing were continued until the basic pH of the solution was equal to 7. The saponified product was dehydrated with methanol and the residual methanol was removed with anhydrous ethanol. The dehydrated sample was dried in vacuum at 60 °C until the weight of the sample
became constant. After grinding and subsequent Results and discussion: IR spectra were used filtration through a sieve, a powdered superabsor- to identify the groups involved in the reaction, the
bent composite is obtained. spectra of montmorillonite (a) and SA (b) are shown
in (Fig. 1).
a) b)
Figure 1. Infrared Spectra (a) of montmorillonite, (b) starch/ copolymer based SA (AA-AAc /montmorillonite
Figure 2. Effect of crosslinking agent concentration on hydrogel swelling
Figure 3. Effect of initiator concentration on hydrogel swelling
IR spectra of corn starch, acrylamide, acrylic acid trum One FTIR using KBr. The IR spectrum ofthe montmorillonite obtained granulated highly swell- highly swellable hydrogel shows absorption bands ing hydrogel were recorded on a Perkin Elmer Spec- corresponding to the functional groups attached to
the monomeric units. In a layered silicate structure, OH groups can be absorbed at 3624-3390 cm-1. The peak at 1001 cm-1, due to the stretching of Si-O in MT, was not detected in nanocomposite MT hydrogels. The absorption bands in the regions 2931 cm-1 correspond to the asymmetric and symmetric stretching of the - CH2 groups. Stretching the -C= = acrylamide group and acrylic acid with a frequen-
cy of 1653 cm-1 manifests itself in all spectra of hydrogel composites. Absorption bands at 1404 cm-1 lead to symmetrical and asymmetric stretching of -COO-acrylate (acrylic acid, neutralized with NaOH).
The (Figure 2 (b)) shows the uniform distribution of the unreacted portions of montmorillonite in the polymer composite.
Figure 4. Effect of montmorillonite concentration on hydrogel swelling
Figure 5. Effect of salt concentration on hydrogel swelling
a) b)
Figure 6. SEM image showing surface structures of montmorillonite (a) and a starc /copolymer based SA (AA-AAc) of montmorillonite 3% (b)
Conclusion: In summary, the inclusion of hy- ization method. Multifunctional crosslinkers, such drophilic substances containing hydrophilic groups, as IBA, were used in the polymerization process. such as acrylic acid, polymers, such as starch, and Hydrogel systems based on starch, copolymer (AA-clay, such as montmorillonite, in AA hydrogels can AAc) and montmorillonite showed high water abbe sequentially obtained by a free-radical polymer- sorption.
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