Stronger, stiffer, greener: Carbon-negative natural-fiber composites

Stronger, stiffer, greener: Carbon-negative natural-fiber composites

University of Michigan Department of Mechanical Engineering 2019/2020 Annual Report

BY: Kim Roth, Scientific Writer

Lightening the weight of the machines that move people by land, sea and air is beneficial for many reasons, including improved fuel efficiency and the ability to carry higher payloads or added safety systems. A 10% reduction in the weight of a passenger car, for instance, leads to about a 6% improvement in fuel economy.

One key strategy to reduce weight is to design structures using lighter materials with improved properties, such as those with greater specific strength and stiffness. Given their light weight, neat polymers are attractive materials for weight reduction, but they’re neither strong enough nor stiff enough for structural applications. As a result, manufacturers across industries add reinforcing fibers, such as glass and, more recently, carbon fiber to add strength and stiffness to the material.

But producing the fibers — whether glass or carbon — for use in today’s advanced structural composites requires energy, and generating this energy releases greenhouse gases into the atmosphere.

“It takes a lot of energy to make these reinforcing materials,” said ME Professor Alan Taub. “Unfortunately, this makes many types of fiber-reinforced lightweight composites carbon positive and compels us to ask how we can do better. How can we produce advanced materials that are carbon neutral or, better yet, carbon negative?”

Taub, in collaboration with ME Research Associate Professor Mihaela Banu, are looking beyond carbon neutrality and toward nature for a greener alternative: natural fibers from plants, since they absorb — rather than release — CO2 as they grow.

Others, too, have looked at natural fibers to improve the properties of polymer composites, “but the improvements to date haven’t been enough to enable widespread use in industrial applications,” said Taub, who holds a joint appointment in Materials Science and Engineering and also directs the University’s recently launched Michigan Materials Research Institute. “Our challenge is to find new ways to improve the strength and stiffness of these natural fibers.”

A triple challenge
Taub’s group is working with bamboo, hemp and flax, adding an environmentally-friendly nanomaterial to particular cells in the plants during growth or after harvesting to improve strength and stiffness. The cells form fibers, and these would then be extracted and used to reinforce the polymer composites.

Part of the challenge, noted Taub, is that plant transport mechanisms are extremely complex, which makes getting even nanoscale material to the targeted cells difficult. And therein lies another challenge — the need to work simultaneously at multiple scales. Extracting the strengthened fibers presents yet another hurdle since current methods can cause the fibers to undergo damage. The team is developing new ways to extract them from the plant to avoid this.

Abundant applications
With three decades of auto industry experience, Taub first is looking at automotive applications for the natural-fiber reinforcements under development. “But we’re certainly not restricted to one application — everything from appliances and sporting goods to commercial aircraft and ocean-going vessels could make use of natural-fiber-reinforced composites if we’re successful,” he said.

And although the process for producing the strong natural fibers will be different than for glass or carbon fibers, the processing and equipment used for forming the resulting polymer composite into structures remains for the most part the same from an industrial perspective.

“What we’re looking at is a material substitution that fits into existing processing capabilities,” Taub said. This means the natural fibers have potential for large-scale implantation, which is key to positively impacting the environment.

“To have a real impact, we have to replace tons of material. Fortunately, enough of this plant material is already being grown, so what we’re focused on now is improving the properties, improving the extraction methods and making these advances with minimal cost increases,” he said.

The project draws upon expertise across the University and includes collaborator Regina Baucom, associate professor of Ecology and Evolutionary Biology. Students from the fall 2019 semester of MSE489, the Materials Science and Engineering senior capstone design course, will conduct an environmental lifecycle and cost analysis.

The project grew out of work by graduate student Amy Langhorst, who earned her bachelor’s degree in U-M Materials Science and Engineering in 2013 and took the senior design course with Taub. She now works at Ford Motor Company and is pursuing a doctoral degree with Taub as her advisor.

The project is part of U-M’s Global CO2 Initiative, which supports development of sustainable and commercially viable carbon-negative technologies. Funding for early-stage, exploratory work is provided through the U-M College of Engineering Blue Sky Initiative, designed to help faculty develop high-risk, high-reward concepts.

The project has high-reward potential indeed. “When we’re successful, we’ll have a CO2 negative material,” Taub said. “Not only does using plant fibers prevent CO2 emissions during fiber production; our methods also reclaim CO2 as the plants grow. We should be able to achieve lighter-weight structures, enabling better fuel economy using materials that are CO2 negative. Our challenge is to improve the mechanical properties of the fibers while maintaining low cost.”