Cellulose Case Study

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Extracellular or bacterial cellulose (BC) synthesized by nonpathogenic aerobic bacteria in a culture medium containing carbon and nitrogen sources can be promising biodegradable nanofibers for composite materials because of their excellent mechanical properties [1]. Although BC is chemically identical to plant-based cellulose, it exists in extremely pure form and does not contain any hemicellulose, lignin, pectin or wax materials [2]. In addition, it has higher crystallinity, degree of polymerization, specific surface area, and significantly higher mechanical properties compared to the plant-based cellulose [3,4]. It has some other unique properties, such as low density, low thermal expansion as well as high optical transparency [1,5]. As a …show more content…
Two limiting factors, i.e., cross-head speed and draw ratio were optimized to obtain the highest possible alignment of the nanofibrils during the hydrogel stretching. High cross-head speed and draw ratio typically lead to nanofibrillar fracture [ ]. Hence, the draw ratio and cross-head speed for largest inelastic deformation to align TBC in the stretching direction were optimized by conducting preliminary experiments. Figure 2 shows typical stretching behavior of TBC hydrogel at a cross-head speed of 0.03 mm/min. The optimum stretching of TBC hydrogel was obtained at a draw ratio of 1.2 without any initiation of fracture in TBC. Stretching higher than 1.2 draw ratio showed initiation of fracture in the hydrogel for nearly all cases. On the other hand, lower cross-head speed was found to be the best for controlled stretching up to the maximum draw ratio. The cross-head speed was set to 0.03 mm/min based on the preliminary experiments. Lower cross-head speed gives sufficient time for maximum rearrangement and reduces the stress on nanofibrils.[9,22] However, extremely low cross-head speed such as <0.05). This significant enhancement in tensile properties of stretched TBC is simply due to the nanofibrillar alignment which was confirmed by the calculation of orientation …show more content…
After resin impregnation in the TBC, the gel was stretched in the same way as the BC and the mechanical properties of the final composites were investigated to understand the efficacy of the stretching process. It is well-known that porosity should be reduced as much as possible in a composite material since the stress transfer between fiber and resin is negatively influenced by the pores.[24] The porosity in the composites was calculated according to the equation: Porosity=(1-ρ_c/(w_f ρ_f+w_r ρ_r ))×100% , where ρc, ρf, ρr, wf, and wr are the bulk density of composite, density of TBC, density of SPI, weight fraction of TBC, and weight fraction of SPI, respectively.[25] The porosities in both composites were calculated to be in an acceptable range of 2.0-3.5%. The fiber content in both composites was maintained at about 42 ± 2 wt% based on the previous studies by controlling the viscosity of the resin. Figure 8 shows the FESEM images of surfaces of control and stretched TBC-SPI composites. As can be seen from Figure 8, the resin filled the pores effectively in both composites. Also, the surface of control TBC-SPI composites showed random undulations (Figure 8a) while stretched TBC-SPI composites had a much smoother surface (Figure 8b). The formation of smoother surface can be

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