Ulven’s NDSU team has found that bio-fibers bond well with biobased vinyl ester or epoxy-like resins developed from sources such as vegetable oil. “We see greater interlaminar and tensile strength and better resistance to moisture,” he notes. “We’ve also achieved success in combining natural fibers with polyurethane. The isocyanate in polyurethane reacts with water fairly violently. As a result, the inherent moisture in natural fibers leads to outstanding bonding.” NDSU discovered this when researching sandwich structures, such as flooring for mass transit.
“The range of resin systems – for example, just the different grades of epoxy alone – will require time to optimize the varying chemistries. It will be trial and error, manufacturing and retesting,” says Riddle. But off-the-shelf chemistry for biobased resin systems with material specifications in place are on the way.
Keys to Gaining Commercial Acceptance
By their nature, bio-fibers are variable. There is color variation as well as disparities in moisture content, depending on where the fiber is grown, the amount of rainfall and the age of the fiber when harvested. “We have information about the strength and stiffness of most bio-fibers,” says Riddle. “But since interdependence is so critical, we emphasize material qualification on the actual composite system rather than the individual fiber. Now we’re talking about composite tensile, compressive, flexural and impact performance.”
At this stage in the industry’s development, projects frequently move into uncharted territory. “The hardest job I have is creating the market,” RheTech’s Preston admits. To help specifiers compare materials, the company created a reference sheet showing typical glass fiber characteristics versus comparable natural fiber characteristics.
Efforts are underway to create a robust grading system for assessing the properties of natural fibers. “We are working with colleagues in Australia and Canada, as well as with ASTM International, to establish that grading criteria so that part designers will be able to specify grades of fiber just as they do resin,” says Ulven.
One approach to gaining commercial acceptance is hybridizing natural fibers with glass or carbon fibers. “We’ve taken a critical look to see if the micromechanics still apply when adding a known, traditional structural fiber,” says Ulven. “A mix as aggressive as 90 percent flax and 10 percent carbon fiber reduces the variability of the composite systems’ performance. Introducing a known fiber to the mix may facilitate acceptance by the design community.”
Creating a Value Proposition
“We’d like natural fiber composites to offer a value proposition of reduced costs, but at this stage, it’s a goal. In practice, the verdict isn’t in yet,” says Riddle. E-glass fabrics represent a mature market with off-the-shelf items. Until the market for bio-fiber composites matures and is as readily available as E-glass, costs will be a limiting factor, he adds.