Processing Challenges

NDSU’s research incorporates fiber volumes from 5 to 50 percent, along with a multitude of chemical treatments and a range of processes, including pultrusion, layup, mat production and extrusion. “Bast fibers require drying, fiber milling and either chemical or mechanical processing to remove naturally occurring pectin. Untreated, the fibers tend to bind or clump when combined with resin,” says Ulven. Removing the pectin results in a shorter length/diameter ratio than the fiber has in its bundle form, reducing the load transfer capability. But the shorter fibers allow resin to penetrate more thoroughly so that the fibers wet out more easily. “Overall, we’re trying to balance fiber separation while maintaining high aspect ratios so that we can compete with fiberglass,” Ulven adds.

bicycle frame hybrid flax fiber carbon fiber model

Students at North Dakota State University developed a bicycle frame using a hybrid flax fiber/carbon fiber model as a technology demonstrator.

RheTech’s biggest challenge determining the correct fiber length. “We want the longest fiber to create the best strength,” Preston says. “But the length of the fiber is restricted by the capacity of the extruding compounder. Fibers that are too long cross each other, forming a bridge that blocks output.” Processing the composite matrix also is limited by a melting point of approximately 400 degrees F. Above that, biobased fibers tend to burn. “The good news is that in running the equipment at lower temperatures, the cooling time is reduced and the total energy expended by the process is lower,” Preston says.

Gradient Engineering found that the length of the natural fiber is a critical parameter. “There are two aspects at play here, bondability and fiber length, and they are somewhat intertwined,” says Riddle. “We’ve had an easier time developing the combination of right surface treatment and fiber length for thermosets than thermoplastics. In an ideal composite failure, you have fiber breakage after the matrix begins to fail. However, if the fiber is not treated properly or is too short, it can pull out of the resin when the bonding fails. The longer the fiber, the more surface area is available for engagement and bonding.”

Developers of bio-fiber composites also must address water absorption and resin compatibility. Natural fibers are more absorbent, which can deform surfaces by swelling and creating voids, making it particularly challenging where exposure to inclement weather and water is unavoidable. Matching the right resin with the right natural fiber is one way to address the challenge of moisture uptake. “Theoretically, if the fiber is fully encapsulated, moisture uptake will be limited,” says Ulven. But for marine or wind energy applications, when the boat hits a rock or a rock hits a blade, water ingress can become a problem.