ENVIRONMENTALLY BENIGN CONVERSION OF RICE STRAW INTO VALUE-ADDED PRODUCTS

Abstract

Rice straw, a leftover vegetative part of rice plant, is neither efficiently composted nor used as a feed in India. Moreover, due to the need for quick disposal, operational difficulties in small-scale pulp and paper mills and lack of logistical convenience rice straw doesn’t find other large-scale commercial applications, and therefore is burnt openly in fields causing tremendous air pollution in parts of India. In the present work, a prospective commercially viable small-scale on-field utilization of rice straw was identified for the first time through an environmentally benign process. Refiner mechanical pulping was success- fully used for converting rice straw into paperboard and a food-serving bowl. The prepared rice straw paperboard was found equivalent to grade-III Kraft paper as specified in IS 1397:1990, when compared for tensile and burst index. The moulded bowl could become an environment friendly alternative for the presently used disposable food-serving containers made of plastics. Three different chemical treatments were attempted to impart water- and grease-resistance to the rice straw paperboard and bowl as demanded by the conceived application. Water absorption was reduced from 387 to 3 g/m2 as measured by Cobb60 test and grease-resistance was improved from 0 to 10 as measure by Kit test. Moreover, the sizing process didn’t render the bowls harmful to consumers’ health in terms of toxicity, the utilization of rice straw pulp for making value-added products holds promise in mitigating air pollution, reducing plastic-based disposables, and in promoting entrepreneurship in rural areas. A unique approach was used to utilize the rice straw. It was converted into a paperboard of 200 g/m2 and was coated with up to 10 g/m2 cellulose nano-fibrils (CNF) extracted from the rice straw. The CNF of 50 nm size could be extracted via the process which involved a chemical process followed by a two-step mechanical process – 1) high speed homogenization and 2) probe sonication. Subsequent steps of homogenization and probe sonication saved electrical energy by 11.45% than using probe sonication alone. The adapted process did not alter the crystalline arrangement of the cellulose as shown by infrared spectroscopy and x-ray diffractometry, making the process acceptable. Various properties of the coated paperboard were tested, and it was found that the coating played a great role toward reducing Bendtsen roughness, water absorption (Cobb60), and porosity; while improving water vapor barrier and grease-resistance. i Extraction of nano-silica from Rice straw was done. The nano-silica was characterized through FTIR (Fourier Transform Infrared Spectrophotometer), XRD (X-Ray Diffraction), FESEM (Field Emission Scanning Electron Microscopy), BET (Brunauer–Emmett–Teller), and was tested for its ethylene gas scavenging property. Duplex paper board in which mechanically pulped rice straw paperboard was sandwiched between two softwood paper. Further duplex paper board was coated by Nano-silica and PVA (polyvinyl alcohol) for improving the barrier, mechanical, surface properties and ethylene gas scavenging of duplex paper board. The barrier and Cobb properties of paper board decreased significantly. Lastly, characterization of the liquor obtained from refiner mechanical pulping was done for total dissolved salts, total solids, total reducing sugar, and total phenolic content. The liquor was fermented by employing Pichia stipitis for the conversion into bioethanol and quantified with the help of HPLC (high performance liquid chromatography). Keywords: Rice straw; stubble burning; eco-friendly pulping; cellulose nano-fibrils; energy saving; nano-silica; poly (vinyl alcohol); coating; ethylene scavenging; food-serving containers; packaging; bioethanol.