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DOI: 10.1007/s00339-004-2932-
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].
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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/cm 3 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.
96 Applied Physics A – Materials Science & Processing
compressing pressures and then allowed a lower maximum compressing pressure. All samples exhibited similar densities, values of 1.44– 1 .53 g/cm 3 for BC-based composites and 1. 44 – 1 .49 g/cm 3 for MFC-based composites. The clearest difference between the two composites was in Young’s modulus. BC composites revealed an outstandingly higher modulus than did MFC composites at any compressing pressure and resin content. However, the bending strength of BC composites, though higher, was not as high in proportion to the Young’s modulus. In addition, MFC-based composites had exhibited increasing modulus and strength with increased compressing pressure, whilst BC-based composites seemed not to undergo significant changes in mechanical properties relative to the compressing pressure or resin content. To gain a better comprehension of these differences in mechanical properties, the stress–strain curves of BC-based composites were compared with those of MFC-based com- posites at different resin-content levels, as shown in Fig. 5. In the case of MFC-based composites, the Young’s modulus, de- picted as the slope of the linear portion of the stress–strain curve, decreased with reduced resin contents, and as a con- sequence the bending strength, i.e., the maximum allowable stress, was also decreased. In the meantime, the strain at yield increased with decreasing resin contents, reaching 0.06 of yield strain for MFC composites with 2 .7% resin content. The higher elongation seems to be a consequence of defor- mation in the form of sliding or straightening of the elements that are not strongly adhered because of the low resin con- tent. On the other hand, BC-based composites exhibited small variations as a function of resin content or compressing pres- sure and showed a brittle behavior compared to MFC-based composites. The disparity between BC-based and MFC-based com- posites seemed to be attributable to the micro-order morph-
FIGURE 5 Stress–strain curves of MFC-based and BC-based composites. The percentages correspond to the resin-content values. All samples were compressed at 100 MPa, except the BC 21.9%, which was compressed at 50 MPa
ology of the fibers in the materials, as implied from the scanning electron microscopy (SEM) images of Figs. 1a and c. Thus, we compared the stress–strain curves of non- impregnated sheets of BC and MFC (Fig. 6). The densities of the sheets were not significantly different from each other, 1 .1 g/cm 3 for BC and 0 .9 g/cm 3 for MFC. Looking at the stress–strain curves, we see that BC sheets have significantly higher modulus and strength than do MFC sheets. Further- more, in contrast to the deforming behavior of MFC sheets, BC sheets deformed almost linearly against the applied stress until failure, as shown in Fig. 6, curve A. According to Ya- manaka et al. [6], the high modulus of BC sheets could be attributed to a high planar orientation of the ribbon-like elem- ents when compressed into sheets and to the ultra-fine struc- ture of the elements, which allows more extensive hydrogen bonds. Moreover, as compared in the SEM images of 1a and c, the relative straightness, continuity, and uniformity of the di- mensions of the elements of BC might contribute to the high modulus as well. Taking into account the stress–strain curve of BC sheets, the deforming behavior of the BC composites is strongly af- fected by the deforming behavior of the BC sheets itself, rather than by deformations in the form of sliding or straight- ening of individual fibers or elements as in MFC-based com- posites. The difference in deforming behavior between BC- based and MFC-based composites can be compared to that between plastics reinforced with woven fibers and plastics re- inforced with randomly oriented fibers. In order to verify the relevance of the uniform and con- tinuous network of BC in obtaining a high Young’s modulus, we performed an additional experiment in which BC sheets were prepared from disintegrated BC pellicles and compos- ites were produced following the same procedure. As can be seen in the SEM pictures at the same magnifications, BC pellicles have a networked structure of extremely fine and straight interconnected ribbon-like elements (Fig. 7a). After disintegration, the BC fragments seem to have entangled elements that clustered, forming bundles of ribbon-like fibrils (Fig. 7b), which very much resemble the appearance of MFC fibril bundles when examined from a micro-scale perspective (Fig. 7c). We observed that the Young’s modulus and strength of composites based on disintegrated BC (curve B, Fig. 8)
FIGURE 6 Typical stress–strain curves of BC ( curve A ) and MFC ( curve B ) sheets. The density of the BC sheet was 1.1 g/cm 3 and that of the MFC sheet was 0.9 g/cm 3
NAKAGAITO et al. Bacterial cellulose: production of high-strength composites 97
FIGURE 7 Scanning electron micrographs of a a BC pellicle, b disinte- grated BC, c MFC
neared the values of MFC composites (curve C, Fig. 8). The change in micro-scale morphology most likely prevented the orientation of the ribbon-like elements when converted to sheets, causing the fragmented BC to have a structure similar to that of MFC sheets. These results strongly suggest that the high Young’s mod- ulus of BC composites derives from the planar and straight orientation of the interconnected, continuous, and dimension- ally uniform ribbon-shaped microfibril bundles, which until now could only be produced in nature.
FIGURE 8 Stress–strain curves of MFC-based and BC-based composites of similar PF resin contents and compressed at 100 MPa
4 Conclusion
BC pellicles compressed into sheets were impreg- nated with phenolic resin to produce high-strength compos- ites. The Young’s modulus of the composites was significantly higher when compared to that of MFC-based composites, 28 GPa against 19 GPa, respectively. The higher modulus of BC composites was credited to the extremely fine, pure, and dimensionally uniform ribbon-like cellulose microfibril bun- dles, arranged in a network of relatively straight and contin- uous alignment, and also to the planar orientation of these elements obtained through the compression of the BC pelli- cles into sheets.
ACKNOWLEDGEMENTS The authors would like to thank Dr. Y. Kuwana, Fujiko Co., Ltd., Kobe, Japan, for kindly providing the BC pel- licles; Kimura Chemical Plants Co., Ltd., for support in disintegrating BC; and Gun Ei Chemical Co., Ltd., for furnishing the PF resin. We would also like to thank Dr. H. Hori and Dr. J. Sugiyama, RISH – Research Institute for Sustainable Humanosphere, Kyoto University, and Mr. T. Nishiwaki, Mit- subishi Chemical Group Science and Technology Research Center, Inc., for support in taking the SEM photographs of the MFC samples. The first author is indebted to the support by a grant-in-aid from the Ministry of Education, Science, and Culture of Japan.
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