Bacterial Cellulose and Its Polymeric Nanocomposites - The Nanoscience and Technology of Renewable Biomaterials - page 234

The washing procedure of the BC pellicle, was also found to impact the performance
of BC sheets (8). When BC was washed with varying concentrations of NaOH or
NaClO (0-1% wt/wt, as active Cl), the tensile modulus varied with the treating solution
concentration and passed through a maximum suggesting competition of two factors. The
maximum Young modulus was measured (24 GPa) at 0.5% NaClO% and at 5% NaOH.
In the case of the NaClO treatment, the noncellulosic components, i.e. proteinaceous
cell debris, were effectively removed with increasing concentration thus allowing better
interfiber bonding. At the same time NaClO caused a disintegration of the cellulose
chains thus lowering the overall stiffness. These two competing effects converged to
provide the highest modulus at 5% concentration. Similarly NaOH which also removed
proteinaceous components thus enhancing interfacial bonding also induced the fibers to
curl, an effect that was more pronounced at high concentrations and impaired the stiffness
of the BC fibers and sheets. Again these two competing effects converged to define
5% NaOH as the optimum concentration for mechanical performance. Interestingly an
even higher modulus was obtained when the BC sheets were consecutively treated with
NaOH and NaClO. In this case a modulus of 30 GPa was obtained. Further rheological
and morphological characterization of BC sheets showed that two transitions occured,
one at 50 ◦ C and 230 ◦ C that were ascribed to water desorption and cellulose degradation
respectively. The BC sheets also exhibited no preferential orientation of fibers (9).
In order to enhance various properties of BC mats, many researchers have combined
BC with synthetic polymers, thermoplastic or thermosetting, utilizing various manufac-
turing approaches for these fiber reinforced nanocomposites.

9.3 BC Nanocomposites by in situ Polymerization

9.3.1 BC Nanocomposites with Thermosetting Phenolic and Epoxy Resins

The first report on BC reinforced thermosetting composites used phenol-formaldehyde
(PF) resins (10) and clearly demonstrated that the nanoscalar structure of BC con-
tributes significantly to the fiber reinforcing potential. In this study, BC sheets dried
between metal plates at 70 ◦ C were impregnated with PF resins and cured under pres-
sure (15-150 MPa) and temperature to deliver composites with resin concentrations of
2.7%, 12.4% and 21.9%. Comparison of the bending and tensile properties of the BC/PF
nanocomposites with those of microfibrillated cellulose, MFC/PF composites (11) with
similar density (1.5 g/cm 3 ) indicated that BC was a better reinforcement than MFC.
For example, when pressed at 100 MPa, the BC/PF nanocomposites reached a Young
modulus of 28 GPa well above that of the MFC/PF composite at 15 Gpa (10). Bending
strength was also higher for the BC/PF nanocomposite compared to the MFC/PF com-
posite although differences were not as marked as for the Young modulus. The high
modulus of the BC nanocomposites was ascribed to the straight and planar orientation of
the BC fibers and to the fact that the fibers are continuous, uniform and intertwined, not
the case of MFC. In contrast PF resin content and pressure did not affect the mechanical
properties of the nanocomposites. In fact, the Young moduli reached by adding the
phenolic resin (28 GPA) were not as high as those reached in neat BC sheets (30 GPa)
after proper washing and hot-pressing as previously reported (8).