ISSN Original Article Synthesis and swelling properties of superabsorbent composite based on guar gum and bentonite

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1 Available online at Advances in Polymer Science and Technology: An International Journal Universal Research Publications. All rights reserved ISSN Original Article Synthesis and properties of superabsorbent composite based on guar gum and bentonite Kartika Rathore 1* and Sangeeta Loonker 2 1) Research Scholar: Applied and Environmental Chemistry Laboratory, Department of Chemistry, Jai Narain Vyas University, Jodhpur (Rajasthan) India. address: kartika.rathore2012@gmail.com Mobile number: ) Professor: Applied and Environmental Chemistry Laboratory, Department of Chemistry, Jai Narain Vyas University, Jodhpur (Rajasthan) India. Received 20 December 2016; accepted 09 January 2017 Abstract Novel PMMA/(guar gum modified bentonite) superabsorbent composite was synthesized by graft copolymerization in complete aqueous solution using boric acid as a cross linker and hydrogen peroxide as an initiator. Reinforcing of the polymer chains with bentonite sheets was confirmed by Fourier transform infrared (FTIR) spectra. SEM and TGA were used to study morphology and thermal stability of composite respectively. The effect of factors such as monomer concentration, cross-linker concentration, initiator concentration, guar gum concentration, bentonite concentration, salt concentration, ph and temperature on was investigated. Water absorption capacity of composite was investigated in distilled water as a function of percentage and found to be 4030% Universal Research Publications. All rights reserved Key Words: Superabsorbent, guar gum, bentonite, methyl methacrylate,. 1. Introduction Superabsorbents are three dimensional networks of hydrophilic polymers held together by crosslinks of covalent bonds or ionic and secondary forces in the form of hydrogen bonds or hydrophobic interactions [1-3]. They can absorb and retain large quantities of aqueous fluids while preserving their shapes. Because of the unique characteristics they are extensively applied in various fields such as agriculture and horticulture [4,5], hygienic products as disposable diapers and feminine napkins[6], wastewater treatment[7 9], bio-medical area as antibacterial materials[10] and drug delivery [11,12]. However the traditional superabsorbents are mainly based on fully organic petroleum based polymers with relatively higher production cost and poor environmental friendly properties [13]. Polymerization of acrylate monomers onto polysaccharides is an efficient route for the preparation of a superabsorbent polymer. The superabsorbent forming ability through polymerization of monomers onto polysaccharides such as starch, chitosan, sodium alginate, carrageenan, cellulose and xanthan gum, has been well documented [14 18]. GG is a hydrophilic, nonionic polysaccharide extracted from the endospermic seed of the plant Cyamopsis tetragonalobus. It consists of a linear backbone of - (1-4)- linked D-mannose units and is solubilised by the presence of randomly attached -(1-6)- linked galactose units as side chains. GG and its derivatives form valuable ingredients for foods, cosmetics and pharmaceuticals [19]. GG has better reactivity and can be facilely modified by grafting vinyl monomers onto its backbone to derive new materials with improved structure and performance [20]. The structure of guar gum is shown in figure 1. Fig.1. Structure of guar gum 7

2 Bentonite is a kind of silicate mineral of aluminum phyllosilicate. Polymer/bentonite composites are novel composites based on the intercalation of polymer chains into bentonite sheets. These systems evoke intense research interest due to outstanding mechanical, thermal and molecular barrier properties as compared with conventional composites [21]. The preparation of the biopolymer-based superabsorbent composites can also improve the mechanical properties of materials and can lower the cost of the finished product compared with the synthetic counterparts as well as providing biodegradable characteristics [22]. In this paper, we attempt to synthesize and characterize new superabsorbent composites based on guar gum grafted PMMA in presence of bentonite particles. 2. Experimental 2.1 Materials Guar gum (GG, Shri Ram Gum Industries Basni Jodhpur), Bentonite (ASES Chemicals Jodhpur), Methyl methacrylate (MMA stabilised for synthesis, Merck), Boric acid (H 3BO 3, Qualigens Fine Chemicals), Acetone (Qualigens), Hydrogen peroxide (H 2O 2, Emplura 30% w/v) and Ethanol (C 2H 5OH, Emsure). All of these are used without further purification. 2.2 Synthesis of PMMA/(guar gum modified bentonite) superabsorbent composites Bentonite (0.3 g) was added to 30 ml of distilled water and magnetically stirred for half an hour which resulted in good bentonite dispersion. GG (0.5g) was added in 50 ml of distilled water and magnetically stirred for half an hour. The resulting guar gum solution was added to bentonite dispersion and vigorously stirred for two hours resulting in formation of guar gum modified bentonite mixture. The monomer MMA (8mL/100mL) was added in above mixture and stirred for half an hour. Then required volume of initiator hydrogen peroxide (1mL/10mL) was added to generate radicals. The aqueous solution of cross-linker boric acid (0.1g/10 ml) was added in the mixture. The reaction mixture was continuously stirred and heated to 60ºC for about 3 hours. After that it was precipitated using excess of acetone and kept overnight. Then it was washed several times with mixture of distilled water and ethanol (60:40) to remove homopolymer and unreacted mass. The grafted product was dried in oven at 100ºC. Finally, the resulting dried product was pulverized into powder. 2.3 Absorbency or Swelling Measurement Absorbency of superabsorbent composite is measured by the free method and calculated in terms of percentage. A 0.1 g of dry sample was immersed in distilled water at room temperature for 4 h to reach equilibrium. The swollen gel was taken out, dried between folds of filter paper (blotting) and weighed. After weighing the swollen samples, the equilibrium water absorbency of the superabsorbent was calculated using Eq. (1) W 1 and W 2 are the weight of the dry sample and water swollen sample respectively. 3. Results and discussion 3.1 SEM Analysis SEM analysis was carried out for morphological study of composite. Figure 2 shows that prepared superabsorbent composite has an uneven and coarse surface. It has porous and broad network structure. These observations are in good agreement with our water-absorbency observations. Fig.2. SEM micrograph of PMMA/guar gum modified bentonite 3.2 FTIR analysis To study the functional groups and bonds characteristic of prepared superabsorbents, FTIR was carried out on sample made by pelleting with KBr. FTIR spectra was collected in Agilent Cray 600 series spectrophotometer in the range cm -1. A peak at 3640 cm -1 is due to the stretching vibrations of structural OH groups of bentonite. Broad peak at 3440 cm -1 is due to OH stretching of water in bentonite. A sharp peak at 2920 cm -1 is attributed to C-H stretching vibrations of guar gum. The absorption peak at 1720 cm -1 is due to C=O stretching vibrations of PMMA. A peak at 1640 cm -1 is due to OH bending vibrations of guar gum. A peak at 1040 cm -1 is attributed to stretching vibrations of Si-O groups. The bands corresponding with the AlAlOH and AlMgOH bending vibrations were observed at 920 and 840 cm -1 respectively. A peak at 880 cm -1 is due to Al-OH- Fe bending. A peak at 1400 cm -1 can be attributed to bending vibration of C-H bonds of -CH3 group. The result obtained by FTIR analysis showed that the reaction of both GG and bentonite with MMA monomer took place during polymerization reaction. The FTIR spectra of composite is shown in figure TGA Analysis Figure 4 shows the thermogram of PMMA/guar gum modified bentonite. From the figure it can be seen that there is no appreciable weight loss at 100ºC. The onset of thermal degradation occurs at 241ºC with a weight loss of % Swelling = W 2 - W 1 x (1) 11% which is the result of desorption of water. The W 1 composite shows 46% weight loss at 331ºC. There is about 55% weight loss at 450ºC. At 882ºC composite loses 61% % Swelling is the equilibrium water absorbency of sample of its weight whereas guar gum loses almost all its weight which are the averages of three measurements. at 600ºC [23] and bentonite looses about 84% weight at 690ºC [24]. The result showed that introduction of GG 8

3 Fig.3. FTIR spectra of PMMA/guar gum modified bentonite Fig.4. Thermogram of PMMA/guar gum modified bentonite modified bentonite into PMMA polymeric network enhanced the thermal stability of superabsorbent composite. 3.4 Effect of reaction parameters Effect of monomer concentration The influence of monomer (methyl methacrylate) concentration has been studied by synthesizing series of composites with varied volume of methyl methacrylate. The effect of monomer concentration on water absorbency is shown in figure 5. It is clear from the figure that initially water absorbency increases by increasing monomer concentration and reaches maximum at 8 % (v/v) and thereafter it reduces. Co-polymerization between MMA and guar gum modified bentonite increases the percentage thereafter it decreases due to formation of homopolymer of MMA. Fig.5. Effect of monomer concentration on percentage 9

4 3.4.2 Effect of initiator concentration Initiator has greater influence on rate of polymerization as it involves in initiating free radicals. Figure 6 depicts that continuous increase in water absorbency is observed till an optimum value of 10% (v/v) which indicates that initiation of free radicals increases the polymerization which in turn increases absorbency and decreases with further increase in content of initiator. At higher levels of initiator concentration, grafting is not favourable and this behaviour is attributed to the formation of excess number of free radical sites leading to shorter polymer chains. The molecular weight in free radical polymerization will decrease with an increase of initiator concentration. With the decrease of molecular weight, the relative amount of polymer chain ends increases. The polymer chain ends do not contribute to water absorbency [25].Therefore, the increase of initiator content is responsible for decrease in water absorbency Effect of bentonite content Figure 8 shows the effect of bentonite concentration on water absorbency of superabsorbent composite. It is clear from the figure that percentage increases when bentonite concentration increases from 0.33 to 1 % (w/v). On further increase in bentonite concentration there is decrease in water absorbency.the increase in percentage from introduction of bentonite can be because of the fact that bentonite particles act as cross-linking agent. According to literature [27], ultrafine clay mineral powder act as additional cross linking points in polymer networks.the rigid bentonite prevent the tangle of graft polymeric chains and weaken the hydrogen-bonding interaction among hydrophilic groups. Fig.6. Effect of initiator concentration on percentage Effect of cross-linker content In our study the water absorbency decreases with increase of cross-linker content from 2 to 10 wt%. as shown in figure 7. According to Flory s theory (26), higher crosslinker concentration will induce the generation of more crosslink points and increases cross linking density. As a result, the network voids for holding water is minimized and water absorption is decreased. However, the insufficient crosslinker makes the 3-D hydrophilic network fail to form and results in increase of soluble portion concentration. Fig.8. Effect of bentonite concentration on percentage Effect of guar gum content The effect of GG content on water absorbency of the superabsorbents is shown in Figure 9. The water absorbency increased with increasing the amount of GG from 0.2 to 1 wt %.When the amount of GG was low, the homopolymer content increases which cannot contribute to water absorbency. The homopolymer content decreased with the increase of GG content at a fixed cross-linking density [20]. Fig.7. Effect of crosslinker concentration on percentage Fig.9. Effect of guar gum concentration on percentage Effect of reaction temperature Figure 10 shows that percentage of superabsorbent composite increase as the reaction temperature increases from 30 to 60ºC and decreases with further increase in reaction temperature. When the reaction temperature is decreased, the rate of polymerization also decreases, which results in the enhancement of cross-linking efficiency [28]. 10

5 At the same crosslinker content, the water absorbency decreases with increase of cross linking efficiency according to Flory s network theory [26]. With the increase of reaction temperature, the polymerization reaction rate increases and the molecular weight decreases, so the water absorbency is also decreased Effect of ph The percentage of superabsorbent composite was studied at various ph values between 2.0 and 13.0 at room temperature. To prepare ph media, the stock NaOH (ph 13.0) and HCl (ph 2.0) solutions were diluted with distilled water to reach desired basic and acidic ph respectively. Under acidic ph, most of carboxylate groups are protonated, thus the main anion-anion repulsive forces are eliminated and as a result capacity values are considerably lower. The loss in highly basic solutions can be attributed to charge screening effect of excess Na+ in media, which in turn shields the carboxylate anions and prevents effective anion-anion repulsion [32]. In our study the prepared composite showed maximum percentage 3780 %) at ph 9 as shown in figure 12. Fig.10. Effect of temperature on percentage Effect of saline concentration on percentage The effect of NaCl and CaCl 2 concentration on percentage was studied. The percentage is less than in distilled water. Different concentration of NaCl and CaCl 2 solutions were prepared in order to study effect of ion charge and ion concentration on water absorption [29].The water absorption decreases with increasing the ionic strength of saline solution as cited in Flory equation [26].This can be attributed to change in osmotic pressure in the environment. The same reason results in the decrease in as the concentration of salt is increased. At low salt concentration the rate of composite is faster than at higher concentrations. The factors that caused the water absorption in the polymer included the osmotic pressure difference between the interior and exterior of polymer. The increasing osmotic pressure with increase of salt concentration led to decrease in osmotic pressure difference between the interior and exterior of the superabsorbent polymer [30]. In addition multivalent cations (Ca 2+ ) can neutralize several charges inside the gel by complex formation with carboxylate groups, leading to increased ionic crosslinking degree and consequently loss of [31].The effect of cation charge and its concentration on is shown in figure 11. Fig.11. Effect of saline concentration on percentage Fig.12. Effect of ph on percentage. 4. Conclusion Novel superabsorbent composites of PMMA/guar gum modified bentonite were prepared in an aqueous solution by graft copolymerization reaction of methyl methacrylate on guar gum modified bentonite using boric acid as a crosslinker and hydrogen peroxide as an initiator. The composites were characterized by SEM, FTIR and TGA. The effects of initial monomer concentration, reaction temperature, amount of crosslinker, initiator concentration, bentonite concentration and guar gum concentration on water absorbency were investigated. Under our experimental conditions, prepared superabsorbent composites has water absorbency of 4030% which has composition of 1% (w/v) crosslinker, 8% (v/v) of methylmethacrylate, 1% (w/v) of guar gum, 10 % (v/v) of initiator, 1% (w/v) of bentonite and reaction temperature of 60ºC. The effect of salt concentration and ph on water absorbency has also been studied. The prepared superabsorbent composite can be used in various applications like agriculture, horticulture, removal of toxic dyes etc. 5. Acknowledgements The authors are thankful to Defence Lab, DRDO, Jodhpur and Department of Chemistry, Jai Narain Vyas University, Jodhpur for providing testing facilities. References 1. Buchhulz FL, Peppas NA. Superabsorbent Polymer Scienceand Technology. ACS Symposium Series 573, 11

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7 31. Hussam Aldeen Kalaleh, Mohammed Tally and Yomen Atassi, Research and Reviews in polymer, 4 no 4, (2013) Mohammad Sadeghi and Hossein Hosseinzadeh, Turk J Chem, 32, (2008) Source of support: Nil; Conflict of interest: None declared 13