Bacterial cellulose the ultimate nano-scalar

Bacterial cellulose the ultimate nano-scalar

(Parte 1 de 2)

Materials Science & Processing AppliedPhysicsA 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 611-01, Japan

Received: 13 May2004/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 mechanical properties over the previously reported high-strength composites based on fibrillated kraft pulp of plant origin. BC-based composites were stronger, and in particular the Young’s modulus 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 compression 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 exploitingthestrength of the microfibrils, which arethe 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 basedon non-fibrillatedkraftpulp. Bothmaterials, inthe 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 magnesium alloy. In addition, the effect of the degree of microfibrillation 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’smoduluswasrelatively low,exhibitingvaluesaround 19 GPa, which is quite a long way from the possibilities offered by the high modulus of microfibrils, estimated to be around140 GPa [3].

Fax: +81-774/38-3600, E-mail: yano@rish.kyoto-u.ac.jp

Besides being the cell-wall component of plants, cellulose 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 ribbonshaped fibrils, less than 100-nm wide, which are composed of a bundle of much finer microfibrils, 2 to 4n m in diameter [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 bundles are somewhat straight, continuous, and dimensionally uniform (Fig. 1c and d). Current applications for BC include 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 electroacoustic transducers. For the last application, Nishi et al. [5] reported a strikingly high dynamic Young’s modulus, 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 material to further enhance the Young’s modulus of high-strength composites.

Inthisstudy,weproducedBC-basedcompositesandcompared 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 modulus 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 F-8, was incubated for ten days in a static culture containing 5% (V/V) coconut milk (nitrogen content: 0.8%, lipid:

94 Applied Physics A – Materials Science & Processing

FIGURE 1 Scanning electron micrographs (micro-scale order) of a MFC and c a BC pellicle. Atomic force micrographs in tapping mode (nano-scale order) of b an MFC sheet and d aB C sheet

30%)a nd 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-m thick were washed in running water for one week. The pellicles were cut intopiecesof8c m by10cm andboiledina 1%(W/W) aqueous 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 pellicle pieces between porous metal plates (approximately 30- to 50-µm-diameter pores) under a slight pressure of 0.3M Pa to squeeze out water. After that, BC sheets separated by filterpaperweresandwichedbetweentwometalplatesandoven dried at 70 ◦C for 48 h. In order to assure complete drying, 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 1.1g /cm3 in density.

BC sheets were also prepared from disintegrated BC pellicles obtained by means of three passes through a grinder (KMI-10,Kurita Kikai Co. Ltd.). Sheets were obtained by filtration 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 phenolformaldehyde (PF) resin diluted in methanol, at concentrations of 1%, 8%,a nd 15% (W/W), which delivered resin contents for the impregnated sheets of 2.7%, 12.4%,a nd 21.9%, respectively. Immersed mats were maintained in reduced pressure at 0.03 MPa for 12 h and kept at ambient pressureat20 ◦Cover96h.Impregnatedsheetsweretakenout of the solutions, air dried for 48 h, cut into smaller pieces of 3c m by 4c m, 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 pressedat 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.

NAKAGAIT Oet al. Bacterial cellulose: production of high-strength composites 95

ThePFresinusedwasPL-2340,Mn=3,351,producedby GunEi Chemical IndustryCo., Ltd.

2.3 Bending test

Specimens having dimensions of about 1-m thick, 35-m long, and 6-m wide were subjected to a threepoint bending test using an Instron 41. The span was set to 25mmandthecrossheadspeedto5m m/min.Young’smodu- lus (E) and bending strength (σb) were thus determined.

Dried BC and MFC sheets were cut into rectangular strips 35-m long and 5-m wide and subjected to a tensile test using the same Instron 41 at a strain rate of 1mm/minoveraspanof25mm.

3 Results and discussion

Young’s modulus (E) and bending strength (σb) as a function of compressing pressure of BC-based compos- ites are compared with those of MFC-based composites [1] in Figs. 2, 3, and 4, for three resin-content conditions. The compressing pressures and resin contents were settled based on our previous study with MFC-based composites [1,2]. In the case of BC composites, a particular range of compressing pressures was chosen for each PF resin content condition, because higher resin contents induced cracks under higher

FIGURE 2 Young’s modulus (E) and bending strength (σb) against compressing pressure of © BC-based composites and ∆ MFC-based composites.

The resin content of all BC composites was 2.7%; that of MFC composites was 2.5–2.7%

FIGURE 3 Young’s modulus (E) and bending strength (σb) against compressing pressure of © BC-based composites and ∆ MFC-based composites.

The resin content of all BC composites was 12.4%; that of MFC composites was 10.3–12.5%

FIGURE 4 Young’s modulus (E) and bending strength (σb) against compressing pressure of © BC-based composites and ∆ MFC-based composites.

The resin content of all BC composites was 21.9%; that of MFC composites was 19.4–21%

96 Applied Physics A – Materials Science & Processing compressing pressures and then allowed a lower maximum compressing pressure.

Theclearestdifferencebetweenthetwocompositeswasin

Young’s modulus. BC composites revealed an outstandingly highermodulusthandidMFCcompositesatanycompressing 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 hadexhibited increasingmodulus and strengthwithincreased 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 composites at different resin-content levels, as shown in Fig. 5. In thecaseofMFC-basedcomposites,theYoung’smodulus,depicted as the slope of the linear portion of the stress–strain curve, decreased with reduced resin contents, and as a consequence 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 deformation in the form of sliding or straightening of the elements that are not strongly adhered because of the low resin content. Onthe other hand,BC-basedcompositesexhibitedsmall variations as a function of resin content or compressing pressure and showed a brittle behavior compared to MFC-based composites.

The disparity between BC-based and MFC-based composites 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 50MPa 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 nonimpregnated sheets of BC and MFC (Fig. 6). The densities of the sheets were not significantly different from each other, 1.1g /cm3 for BC and 0.9g /cm3 for MFC. Looking at the stress–strain curves, we see that BC sheets have significantly higher modulus and strength than do MFC sheets. Furthermore, 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 Yamanaka et al. [6], the high modulus of BC sheets could be attributed to a high planar orientation of the ribbon-like elements when compressed into sheets and to the ultra-fine structure of the elements, which allows more extensive hydrogen bonds.Moreover,ascomparedintheSEMimagesof1aandc, the relative straightness, continuity, and uniformity of the dimensions 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 affected by the deforming behavior of the BC sheets itself, rather than by deformations in the form of sliding or straightening of individual fibers or elements as in MFC-based composites. 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 reinforced with randomly oriented fibers.

In order to verify the relevance of the uniform and continuous 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 composites 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 1.1g /cm3 and that of the MFC sheet was 0.9g /cm3

NAKAGAIT Oet al. Bacterial cellulose: production of high-strength composites 97

FIGURE 7 Scanning electron micrographs of a a BC pellicle, b disintegrated 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 modulus of BC composites derives from the planar and straight orientationof the interconnected, continuous, and dimensionally 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

4C onclusion

(Parte 1 de 2)

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