Field-emission scanning electron microscope images of the surface of a control BC sample (a, c) and stretched BC sample (b, d) at different magnifications.Tiny fibers of bacterial cellulose (BC) could strengthen a new generation of ‘green’ nanocomposites, according to researchers from Cornell University [Rahman and Netravali, Composites Science & Technology 136 (2016) 85]. Replacing current non-degradable composites with green alternatives could substantially reduce carbon footprints.
Although BC nanofibers are intrinsically very strong and stiff, they tend to form in a random, tangled web. To make the most of their attributes as reinforcements in a composite,the BC nanofibers need to be lined up–or oriented. Now researchers Anil N. Netravali and Muhammad M. Rahman have come up with a straightforward way to line up these nanofibers in an orderly manner before they are incorporated into a composite material.
The simple approach relies on stretching out the tangled fibers into a more oriented arrangement. A hydrogel, containing 95% water and 5% BC nanofibers, is stretched and then dipped into a soy protein isolate (SPI) resin. The resin impregnates the hydrogel, replacing the water. When the sample is dried and hot pressed (or ‘cured’), a membrane-like composite of oriented BC fibers in an SPI matrix is created.
“Although the orientation was far from perfect, the composites made using SPI-based resin and stretched BC resulted in much higher tensile properties,” says Netravali.
The Young’s modulus of the composites increased by around 90% from 0.9 GPa to 3.6 GPa, while the fracture stress went up by approximately 66% from 25.3 MPa to 66.3 MPa.
“Our research also showed that there is significant scope to obtain further orientation of the nanofibers so much higher composite properties can be achieved,” he adds.
In fact, the researchers have developed alternative means of aligning the fibers by growing the BC in PDMS templates with either narrow channels or small-diameter tubes.
“Our effort will be to further stretch them to obtain even higher orientation and then use them to fabricate stronger composites,” explains Netravali. “Once perfect orientation of nanofibers is obtained, these composites could be very strong, reflecting the BC nanofiber tensile properties.”
If that aim can be achieved, these fully ‘green’ nanocomposites could find applications in many areas of aerospace and transportation. Because the BC nanofibers are only around 55 nm in diameter, they have the potential to make transparent composites, which could enable the fabrication of dramatically lighter windows for cars and airplanes or screens for computers, mobile phones, and other electronic devices, suggests Netravali.
Ian Hamerton of the University of Bristol believes the approach is intriguing.
“There is potential for these materials to be examined in their own right as more environmentally benign composites or introduced as interleaf toughening layers between plies in more conventional composite systems,” he comments.
Although the process uses alkaline conditions, it does appear to be more environmentally benign–although a full life cycle assessment of the new materials would need to be undertaken, points out Hamerton.
“The long term ageing/longevity under different environmental conditions is one aspect that I’d also be interested to see tested,” he adds.
This article was originally published in Nano Today (2016), doi: 10.1016/j.nantod.2016.12.007