Abstract
Biomass such as agricultural crops and residues, forest resources and residues, animal and municipal wastes is the largest source for cellulose in the world. These materials can be utilised as cheap cellulosic materials for possible medical-textile applications. In the current work, lignocellulosic materials like sugarcane bagasse and rice straw were grafted with acrylamide, acrylic acid and their mixtures. The grafted materials were characterized using FTIR spectroscopy and tested for water retention values. The grafted material showed higher water retention values, which increased after alkali treatment.
Keywords: Sugarcane bagasse; Rice straw; Grafting
1. Introduction
Absorbent polymers are materials that have the ability to absorb and retain large volumes of water and aqueous solutions. This makes them ideal for use in water-absorbing applications such as babies’ nappies and adult incontinence pads, in absorbent medical dressings and as a controlled-release medium.
Lignocellulosic agricultural by-products are an abundant and cheap source for cellulose fibres. Agro-based biofibres have a composition, properties and structure that make them suitable for uses such as composite, textile, pulp and paper manufacture. In addition, biofibres can also be used to produce fuel, chemicals, enzymes and food. By-products produced from the cultivation of corn, wheat, rice, sorghum, barley, sugarcane, pineapple, banana and coconut are the major sources of agro-based biofibres.
Depleting natural resources, regulations on using synthetic materials, growing environmental awareness and economic considerations are the major driving forces to utilise annually renewable resources such as biomass for various industrial applications. Biomass, such as agricultural crops and residues, forest resources and residues, animal and municipal wastes, is the largest source for cellulose in the world. (1,2,3)
Biopolymers, being renewable raw materials, are gaining considerable importance because of the limited existing quantities of fossil supplies and the recent environment conservative regulations. In this regard, cellulose-rich biomass acquires enormous significance as chemical feedstock, since it consists of cellulose, hemicelluloses and lignin, which are biopolymers containing many functional groups suitable to chemical derivatisation.(4)
Graft copolymerisation of cellulose is a process in which attempts have been made to combine synthetic polymers with cellulose, to produce material with best properties of both. This process is known as grafting, usually done by modifying the cellulose molecules through creation of branches of synthetic monomers that confer certain desirable properties on the cellulose without destroying its intrinsic properties.(5) By chemical modification of cellulose through graft copolymerisation with synthetic monomers, many different properties, including water sorbency, elasticity, ionexchange capabilities, thermal resistance and resistance to microbiological attack, can be improved.(6)
In the current work, the lignocellulosic materials such as sugarcane bagasse and rice straw, with and without alkali treatment, were grafted with acrylic acid, acrylamide and their mixtures using potassium persulfate (KPS) as an initiator. The grafted biopolymers were evaluated using FTIR, water-retention test methods.
2. Materials and Methods
2.1. Materials
Sugarcane bagasse and rice straw obtained from local agricultural fields. All other chemicals used were of laboratory grade.
2.2. Methods
2.2 1. Alkali Treatment of Lignocellulosic Materials
Alkali treatment reduces the lignin and hemicellulose content in biomass, increases the surface area, allowing penetration of water molecules to the inner layers, and breaks the bonds between hemicellulose and lignin-carbohydrate. Dilute sodium hydroxide is usually used for alkali treatment. (15) In the current work, the lignocellulosic materials were treated with 40gpl NaOH at 70°C for 2 hours. They were then filtered and washed with water till they became free of alkali.
2.2.2. Determination of Lignin Content of Lignocellulosic Materials
The content of lignin in both the lignocellulosic materials (Bagasse and rice straw) was determined using the methods recommended by the National Energy Laboratory. (3,6)
2.2.3. Grafting of Lignocellulosic Materials
The grafting reaction was carried out in a similar assembly as mentioned in starch water in a 500ml three neck flask. The initiation process to produce free radicals on to the lignocellulose chain was carried out using potassium persulfate and it was added (1% on weight of material) 15 min. before the addition of monomer. The reactions (initiation and propagation) were carried out at 70°C under nitrogen atmosphere and the reaction mixture was continuously stirred during these reactions. At the end of reaction (2 hours after addition of monomer) the product was precipitated in methanol, filtered using a sintered glass filter of porosity 2, and then dried at 60°C in an oven to a constant weight.
The homopolymers of acrylamide were removed using Soxhlet extraction with morpholine, the acrylic acid homopolymers were removed using ethanol water mixture and their mixtures using ethanol.
2.2.4. FTIR analysis
The FTIR spectra of bagasse (alkali treated), grafted bagasse, rice straw (alkali treated) and grafted rice straw were recorded using Shimadzu FTIR spectrophotometer using ATR mode of operation and scanning of the FTIR spectrophotometer was carried out from 4000 to 600 cm-1
2.2.5. Measurement of Water Absorbency
The dry sample was weighed (0.2 gm) and immersed in water for 24h to reach absorption equilibrium. The fully swollen fibres were separated from the unabsorbed water with a 65-mesh screen. Then, the fibre mass was weighed. The relative water absorbency was calculated by formula (9) as follows:
Water absorbency (gm/gm) = Mass of fully swollen sample – Mass of dry sample/Mass of dry sample
3. Results and Discussion
The FTIR spectrum of bagasse grafted with acrylamide (AAm-g-B), showed the peak for N-H stretching vibration of NH2 group at 3417.63cm-1. In the case of bagasse grafted with acrylic acid (AA-g - B), the peak for COOH group was observed at 1716.53cm-1, while in the case of bagasse grafted with a mixture of monomers, the peaks at 3332.76 and 1712.67cm-1 confirm the presence of both NH2 and COOH groups (Fig.1). The FTIR spectrum of rice straw grafted with acrylamide showed a peak at 3328.91cm-1, which confirms the presence of NH2 group of acrylamide in the graft. The presence of a peak at 1712.67cm-1 in the case of rice straw grafted with acrylic acid confirms grafting and, in the case of rice straw grafted with a mixture of acrylamide and acrylic acid, the peaks at 3392.55 and 1747.39cm-1 confirm the presence of NH2 and COOH groups in the grafts (Fig.2).
The grafting of vinyl monomers on bagasse and rice straw was carried out. Since the lignin content of these lignocellulosic materials is high, purification of these materials was done. The delignification was by NaOH treatment. The lignin content of these materials was found to be lowered after caustic treatment. (Table 1).
The graft add-on and graft yield were found to be distinctly low for both the materials (Table 2).
This might be attributed to the presence of lignin even after caustic treatment, in both the cases. The graft addon and graft yield were slightly higher for rice straw than that of bagasse, irrespective of the monomer used. This may be due to higher lignin content in bagasse, after caustic treatment than that of rice straw. The lignin seems to be reacting with initiator, making less amount of initiator available for grafting, which results in a decrease in graft add-on with increasing concentration of lignin. (10) Lignin also causes physical resistance, hindering grafting of vinyl monomers on cellulosic substrates. The overall effect is seen in the drastic reduction in graft add-on values. Reaction of substrates with monomers takes place in heterogeneous medium and thus physical hindrance caused by lignin becomes an important aspect in causing reduction in level of grafting. The graft add-on levels were found to be higher when both cellulosic materials were grafted with a mixture of acrylamide and acrylic acid (50:50) than when grafted with individual monomers. The reason behind this is explained in an earlier publication from our laboratory.(11)
Acrylamide (AAm) and acrylic acid (AA) monomer molecules are present in solution with some kind of association between the two, which increases or decreases depending upon their relative proportions in the bath. It is, therefore, possible that the Am and AA molecules form a liable complex, and the extent of its formation seems to have considerable influence in changing the rates of reaction during the grafting process.
1. Due to the complex formation, mobility of the reacting species in the solution is reduced, thereby retarding the rate of homopolymerisaion.
2. When one monomer molecule diffuses inside the polymer structure it automatically carries another monomer present in the complex, thus increasing the monomer concentration in the fibre phase – a very favourable situation for higher graft-copolymer formation.
3. When the monomer molecule reacts with the free radical on the backbone of the polymer-chain molecule, the chain propagation is enhanced due to the complex, and hence a higher number of molecules is utilised, resulting in synergistic influence.
The water absorbency in case of AAm and AA grafted rice straw and bagasse was found to be higher for grafting with mixture of monomers than that for individual monomers. But the overall water absorbency was lower as compared to that for starch. This is because of low level of grafting in both bagasse and rice straw (Table 3).
The water absorbency was found to be increasing with alkali treatment of the grafts. The absorbency of grafted rice straw was higher than that of grafted bagasse for all the monomer combinations and both before and after alkali treatment. This may be due to the higher level of grafting in rice straw than in bagasse. The absorbency of grafted rice straw with a mixture of Am and AA (50:50) and then alkali treated was found to be the best (20.7725 times wt/wt) among all grafted lignocellulosic materials.
4. Conclusion
The lignocellulosic materials were successfully grafted with vinyl monomers using KPS as initiator. The graft add-on was found to be distinctly lower for both the materials as lignin acts as inhibitor in grafting. Graft add-on was higher using mixture of monomers than when used as individual monomers. The waterretention values increased after grafting and saponification of grafted product showed further enhancement in water absorption. The maximum water-retention value was obtained for the alkali-treated rice straw, which was grafted with a mixture of acrylic acid and acrylamide and later saponified.
References
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