Post Harvest Processing
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Post Harvest Processing 78 Enzyme/Zinc Chloride Pretreatment of Short- Staple Cotton Fibres for Energy Reduction during Nano-Fibrillation by Refining Process N. Vigneshwaran1*, Vilas Karande1, G.B. Hadge1, S.T. Mhaske2 and A.K. Bharimalla1 1Nanotechnology Research Lab, Chemical and Biochemical Processing Division, Central Institute for Research on Cotton Technology, 2Department of Polymer and Surface Engineering, Institute of Chemical Technology, Matunga, Mumbai–400019, India e-mail: [email protected] Abstract—Cellulose is a renewable, biodegradable and the most abundant biopolymer available on the Earth. Natural Cellulosic fibers are synthesized mainly in plants and cellulose constitutes 40-50% of wood, 80% of flax and 90% of cotton fibers. Microfibrils are defined as the fibers of 0.1-1.0 μm diameter, with high aspect ratio and nanofibrils are at least one dimension in nanometer scale (1-100 nm). Nanofibrils of cellulose have potential use in high efficiency filters, tissue scaffolds and as reinforcing agent in composites. In this work, we have processed short-staple cotton fibres through refining process for the production of nanofibrils. The refining process, through shear force, pumps water into the secondary layer and loosens the compactness of fibrillar structure by disrupting the hydrogen bonding. To enhance the efficiency of refining process, pretreatments using enzyme / zinc chloride were developed to open up the primary layer. Cellulase enzyme pretreatment hydrolyzed the surface molecules in cellulose while zinc chloride act as a swelling agent thereby increasing the accessibility to secondary layer. Since the refining is a continuous process with very low residence time, minimum of 30 passes were required for complete fibrillation resulting in huge energy consumption. With pretreatments, the required number of passes for nano-fibrillation reduced drastically to fifteen only. Degree of polymerization of cotton fibres (11188) significantly reduced to 8144 in the case of fibrillation without pretreatment while it was 5147 and 6949 in the case of fibrillation with enzymatic and zinc chloride pretreatments, respectively. Energy required (for 20 g of cotton fibres) for initial 5 passes for fibrillation without pretreatment and enzyme and zinc chloride pretreatments are 1.346, 0.6764 and 0.8053 MJ, respectively. In subsequent passes, no significant difference was noticed. The pretreated fibres showed more than 50% reduction in energy consumption during refining process. The refining of the cotton fibers without pretreatment required at least 30 passes to achieve a fibril diameter of 400 nm whereas still smaller size (~100nm) could be achieved only in 15 passes using enzymatic / zinc chloride pretreatments. Nanofibrils of cellulose thus produced are now being evaluated for their use as fillers in biopolymer nanocomposites for use in food packaging. INTRODUCTION Cellulose is a renewable, biodegradable and most abundant biopolymer available in the biosphere (Lee et al., 2009) and is produced in nature at an annual rate of 1011-1012 tons (Zhao et al., 2007). Cellulose is the main constituent of the plants serving to maintain their structure. The properties of cellulose like good tensile strength, low density, biodegradability etc. leads to rising research interest. Cellulose is the structural material of the fibrous cells with high level of strength and stiffness per unit weight and has a straight carbohydrate polymer chain consisting of β-1-4 glucopyranose units and a degree of polymerization of about 10,000 (Kamel, 2007). The molecules aggregate and are present in the form of microfibrils (Hult et al., 2003). The hydroxyl (-OH) groups in the cellulose structure play a major role in governing the reactivity and physical property of the cellulose. Natural Cellulosic fibers are synthesized mainly in plants and cellulose constitutes 40-50% of wood, 80% of flax and 90% of cotton fiber. In recent years, many researchers and manufacturers use natural fibers to replace man-made fibers as reinforcement material and fillers to make environmentally safe products. Cellulose fibers can be mechanically disintegrated to the structural nanoscale fibrils (Ahola et al., 2008). 472 World Cotton Research Conference on Technologies for Prosperity The word fibril has been described by various researchers to describe relatively long and very thin pieces of cellulosic material. Microfibrils are defined as the fibers of cellulose of 0.1-1μm in diameter (Chakraborty et al., 2005), with corresponding minimum length of 5-50 μm and nanofibrils are at least one dimension in nanometer scale (1-100 nm). The micro/nanofibrils isolated from the natural fibers have much better mechanical properties (Cao and Tan, 2002). Therefore much attention has been given in the last decade to study how to make micro/nano fibrils and how to combine them with the different polymers to make composites. In the present study, effect of enzymatic and zinc chloride pretreatments of cellulose on fibrillation of cotton fibers by refining process have been studied. The purpose of these pretreatments is to loosen the structure of the fiber either by reducing the secondary forces such as hydrogen bonding and van der Waals forces or by swelling the fibers. Pretreatments will also be helpful in obtaining cellulose nanofibrils in an energy efficient way. Nanofibrils from cotton fiber were prepared by top down approach using the Lab-Disc Refiner. The refining is a pulping method in which the fibers are separated from the matrix by means of mechanical forces. The main objective of the process is to loosen and separate the fibers from the matrix, to break the fiber layer, to peel the fiber cell wall to some extent, and to fibrillate fibers to the desired quality. MATERIALS AND METHODS For enzyme pretreatment; about 20 g of cotton fibers were dispersed in 2 l of acetate buffer (pH 4.8) along with 1% of cellulase enzyme and stirring was done using mechanical stirrer at 45ºC for 30 min. After this, 10 ml of 1 M NaOH solution was added into the suspension to deactivate enzyme followed by washing with distilled water. For zinc chloride pretreatment; 71.5% solution of zinc chloride in water was prepared and allowed to cool as the exothermal reaction raised the temperature. After cooling, 20 g of cotton fibers were added into it and stirred for 1 h at 35ºC. Finally, the fibres were washed four times using distilled water to remove the zinc chloride completely. Cotton fibers before pretreatment and after the enzymatic / zinc chloride pretreatments were subjected to the refining process for fibrillation in a lab disc refiner. The output (fibrillated cotton fibers) from the fibrillation zone of refiner was collected in a vessel and one such process was completed in 2 minutes and considered as one pass. The sample was passed through the refiner up to 30 passes and characterization was done after every 5 passes. The schematic of entire process is given in figure 1. Fig. 1: Typical Process for Preparation of Cellulose Nanofibrils The nanofibrils obtained by this process were analyzed by scanning electron microscopy, atomic force microscopy, and their degree of polymerization (DP) was analyzed by viscometric method. Simultaneously, the energy consumption was analyzed using the energy meter attached with the refiner. Enzyme/Zinc Chloride Pretreatment of Short-Staple Cotton Fibres for Energy Reduction During Nano-Fibrillation 473 RESULTS AND DISCUSSION The product obtained by refining process was analyzed by scanning electron microscopy and the obtained micrographs were subjected to image analysis as given in table 1. After the SEM analysis, diameter of the cellulose fibril was measured at 15 different locations of various images and an average diameter was reported. TABLE1: DIAMETER OF THE FIBRILLATED COTTON (IN NM ± SD) FIBERS BEFORE PRETREATMENT AND AFTER ENZYMATIC, ZINC CHLORIDE PRETREATMENTS MEASURED FROM SEM IMAGES No. of Passes Control Cotton Fibres Enzymatic Pretreated Cotton Fibres Zinc Chloride Pretreated Cotton Fibres 5 809±0.53 339±0.18 272±0.014 10 709±0.46 334±0.20 204±0.14 15 617±0.50 142±0.07 206±0.22 20 533±0.47 146±0.14 168±0.10 25 473±0.38 154±0.10 170±0.12 30 452±0.35 152±0.09 174±0.11 From table 1 it has been observed that the diameter of the fibrillated before pretreatment has been reduced to 453 nm after 30 passes from an initial diameter of 21 µm. It is also observed that the diameter of the fibrillated cotton fibers after Enzymatic and Zinc Chloride pretreatments has been reduced down to 152 and 175 nm, respectively. The cotton fibers fibrillated even after 30 passes sample without any pretreatment has an average diameter of ~ 453 nm whereas less than this was achieved after 5 passes after enzymatic and zinc chloride pretreatments. Figure 2 shows the SEM micrographs of the fibrillated cotton fibres (enzyme pretreated) after every 5 passes; ‘a’ and ‘b’ corresponds to initial fibre while the figures from ‘c’ to ‘h’ represents stage after every 5 passes. Fig. 2: SEM Micrographs of the Fibrillated Cotton Fibers After Enzymatic Pretreatment Figure 3 shows the SEM micrographs of the fibrillated cotton fibres (zinc chloride pretreated) after every 5 passes; ‘a’ and ‘b’ corresponds to initial fibre while the figures from ‘c’ to ‘h’ represents stage after every 5 passes. Also, the swelling of fibrils due to zinc chloride treatment is clearly visible in the SEM micrographs. 474 World Cotton Research Conference on Technologies for Prosperity Fig. 3: SEM Micrographs of the Fibrillated Cotton Fibers After Zinc Chloride Pretreatment The AFM analysis of the fibrillated fibrils after 30 passes was carried out using silicon tip in a tapping mode. After the AFM analysis, diameter of the cellulose fibril was measured by image analysis and reported in table 2. Enzyme/Zinc Chloride Pretreatment of Short-Staple Cotton Fibres for Energy Reduction During Nano-Fibrillation 475 TABLE 2: DIAMETER OF THE FIBRILLATED COTTON FIBERS BEFORE PRETREATMENT AND AFTER ENZYMATIC, ZINC CHLORIDE PRETREATMENTS MEASURED FROM AFM IMAGES Sample Avg.