Agro residues are produced in considerable amounts worldwide which are dumped to landfills or burnt for heating with very low energy recovery. On the contrary, these agro residues have very high potential to be used as feedstock in some of the high value applications which includes the fabrication of acoustic materials in the form of microfibrils composed holocellulose. However, the complex arrangement of lignin, cellulose, and hemicellouse in lignocellulosic biomass should be broken before their utilization in acoustic materials. Therefore, present study also focusses on the delignification of rice straw to produce microfibrils for acoustic applications. Production of microfibrils for acoustic applications provide an alternate high value, innovative and cost effective technique for rice straw utilization compared to conventional agro residue conversion processes. The efficacy of the proposed technique could be quantified on the basis of cost competitiveness of the final product compared to commercially available alternatives, and reduction in propensity of waste generation during conversion of rice straw into acoustical materials.
In this study, NADES were derived from biorenewable resources that needs low-cost recycling, shows high reusability, non-toxicity, and mild acidic properties, have been synthesized and used for the delignification of rice straw. Firstly, NADES was prepared by incubating a mixture of lactic acid (LA), choline chloride (CC) and water in 9:1:0.11 molar ratio at 60-80℃ for 30 minutes in a reactor. After that, synthesized NADES was characterized using melting point temperature (Tm), and glass transition temperature (Tg) analysis to validate the formation of desired NADES. Further, a clear solution from incubating LA-CC-water at aforementioned conditions also indicated NADES formation. On the other hand, rice straw was grinded to powder form and sieved to obtain the particles of less than 0.5 mm size. Post this, delignification of powdered and sieved rice straw was carried out using natural deep eutectic solvents (NADES). In general, 5-10 wt% of powdered and sieved rice straw was pre-treated with NADES (LA, CC and water) at 120℃ for 60-200 minutes to produce cellulosic microfibrils. Notably, NADES due to their mild acidity need to be removed from the microfibrils before fabrication of the final product, therefore, microfibrils were washed by distilled water until the neutral pH (6.8-7.2) was obtained. Eventually, obtained microfibrils were used for acoustic materials fabrication via different techniques and the efficacy of the sheets has been examined on impedance tube set-up.
The results suggest that the sheet made from rice straw microfibrils could be utilized in a range of acoustical applications and have effectiveness comparable to commercially available sheets made of synthetic materials. It has been found that the acoustical performance of sheets made from rice straw improves significantly after conversion into microfibrils due to reduced fiber diameter, and increased tortuosity. Further, reusability of NADES for multiple cycles of operation without loss of pretreatment efficiency leads to significant reduction in pretreatment cost and ultimately enhances the profitability of the entire process.
