Bacterial Cellulose: Production of High-Strength Composites, Notas de estudo de Engenharia de Produção

Baixe Bacterial Cellulose: Production of High-Strength Composites e outras Notas de estudo em PDF para Engenharia de Produção, somente na Docsity! DOI: 10.1007/s00339-004-2932-3 Appl. Phys. A 80, 93–97 (2005) Materials Science & Processing Applied Physics A a.n. nakagaito s. iwamoto h. yano
Bacterial cellulose: the ultimate nano-scalar cellulose morphology for the production of high-strength composites Research Institute for Sustainable Humanosphere, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan Received: 13 May 2004 /Accepted: 17 May 2004 Published online: 6 July 2004 • © Springer-Verlag 2004 ABSTRACT High-strength composites were produced using bacterial cellulose (BC) sheets impregnated with phenolic resin and compressed at 100 MPa. By utilizing this unique material synthesized by bacteria, it was possible to improve the mechan- ical properties over the previously reported high-strength com- posites based on fibrillated kraft pulp of plant origin. BC-based composites were stronger, and in particular the Young’s mod- ulus was significantly higher, attaining 28 GPa versus 19 GPa of fibrillated pulp composites. The superior modulus value was attributed to the uniform, continuous, and straight nano-scalar network of cellulosic elements oriented in-plane via the com- pression of BC pellicles. PACS 81.05.Lg; 81.05.Qk 1 Introduction Cellulose is one of the most copious polymers on the planet Earth. It is the main cell-wall component of just about every plant. In a previous work [1], the present authors produced high-strength plant-fiber composites by ex- ploiting the strength of the microfibrils, which are the smallest structural unit of plant-cell walls and are made of stretched cellulose chains. The composites were based on a form of expanded high-volume cellulose known as microfibrillated cellulose (MFC) obtained through fibrillation of kraft pulp (Fig. 1a and b), and they were compared with composites based on non-fibrillated kraft pulp. Both materials, in the form of sheets, were impregnated with phenolic resin, stacked in layers, and compressed. The bending strength of composites based on MFC achieved remarkable values of up to 370 MPa, which is comparable to the strength of a commercial magne- sium alloy. In addition, the effect of the degree of microfib- rillation on the mechanical properties of the final composites was evaluated [2]. Despite the good mechanical properties of MFC-based composites such as strength and toughness, the Young’s modulus was relatively low, exhibiting values around 19 GPa, which is quite a long way from the possibilities of- fered by the high modulus of microfibrils, estimated to be around 140 GPa [3].
Fax: +81-774/38-3600, E-mail: yano@rish.kyoto-u.ac.jp Besides being the cell-wall component of plants, cellu- lose is also secreted extracellularly as synthesized cellulose fibers by some bacterial species. Bacterial cellulose (BC) is produced by Acetobacter species cultivated in a culture medium containing carbon and nitrogen sources. It presents unique properties such as high mechanical strength and an extremely fine and pure fiber network structure, as shown in Fig. 1c and d. This network structure is in the form of a pellicle made up of a random assembly of ribbon- shaped fibrils, less than 100-nm wide, which are composed of a bundle of much finer microfibrils, 2 to 4 nm in diam- eter [4]. Instead of being obtained by fibrillation of fibers, BC is produced by bacteria in a reverse way, synthesizing cellulose and building up bundles of microfibrils. These bun- dles are somewhat straight, continuous, and dimensionally uniform (Fig. 1c and d). Current applications for BC in- clude use as a dietary food (nata-de-coco), as medical pads for skin burns, as reinforcement in high-strength papers, as binding or thickening agents, and as diaphragms of elec- troacoustic transducers. For the last application, Nishi et al. [5] reported a strikingly high dynamic Young’s modu- lus, close to 30 GPa, for sheets obtained from BC pellicles when adequately processed. Due to this remarkable modulus, BC sheets seemed to be an ideal candidate as raw mate- rial to further enhance the Young’s modulus of high-strength composites. In this study, we produced BC-based composites and com- pared their mechanical properties with those of MFC-based composites. The bending strength increased to values up to 425 MPa, and the Young’s modulus increased from 19 GPa of MFC composites to 28 GPa, nearly retaining the mod- ulus of the BC sheets. The mechanical properties are due to the uniqueness of the uniform nano-scalar networked BC structure, which orients bi-dimensionally when compressed and of which, so far as we know, bacteria has been the sole producer. 2 Experimental 2.1 Preparation of BC sheets The BC pellicles were furnished by Fujicco Co., Ltd., Kobe, Japan. The bacterial strain, Acetobacter xylinum FF-88, was incubated for ten days in a static culture contain- ing 5% (V/V) coconut milk (nitrogen content: 0.8%, lipid: 94 Applied Physics A – Materials Science & Processing FIGURE 1 Scanning electron micro- graphs (micro-scale order) of a MFC and c a BC pellicle. Atomic force mi- crographs in tapping mode (nano-scale order) of b an MFC sheet and d a BC sheet 30%) and 8% (W/V) sucrose, adjusted to pH 3.0 by acetic acid. BC fiber content in the pellicles was approximately 1% (V/V). The gel-like pellicles of BC about 10-mm thick were washed in running water for one week. The pellicles were cut into pieces of 8 cm by 10 cm and boiled in a 1% (W/W) aque- ous solution of NaOH for 3 h to remove bacterial cell debris. After that the pieces were washed again in running water for one week. BC sheets were prepared by compressing the pel- licle pieces between porous metal plates (approximately 30- to 50-µm-diameter pores) under a slight pressure of 0.3 MPa to squeeze out water. After that, BC sheets separated by fil- ter paper were sandwiched between two metal plates and oven dried at 70 ◦C for 48 h. In order to assure complete dry- ing, they were further vacuum dried at 70 ◦C for 5 h, after which the oven-dried weight was measured. The obtained sheets were approximately 50-µm thick and 1.1 g/cm3 in density. BC sheets were also prepared from disintegrated BC pel- licles obtained by means of three passes through a grinder (KMI-10, Kurita Kikai Co. Ltd.). Sheets were obtained by fil- tration and were dried following the same procedure as just described. Sheets of disintegrated BC were approximately 80-µm thick. 2.2 Preparation of BC composites The dried BC sheets were immersed in phenol- formaldehyde (PF) resin diluted in methanol, at concentra- tions of 1%, 8%, and 15% (W/W), which delivered resin contents for the impregnated sheets of 2.7%, 12.4%, and 21.9%, respectively. Immersed mats were maintained in re- duced pressure at 0.03 MPa for 12 h and kept at ambient pressure at 20 ◦C over 96 h. Impregnated sheets were taken out of the solutions, air dried for 48 h, cut into smaller pieces of 3 cm by 4 cm, put in a vacuum oven at 50 ◦C for 6 h, and then weighed again. PF resin contents were calculated from the oven-dried weights before and after impregnation. Finally, the small impregnated pieces were stacked in layers of about 25 sheets, put in a metal die, and hot pressed at 160 ◦C for 30 min under compressing pressures of 15, 30, 50, 80, 100, and 150 MPa, depending on the resin content of the sample.