Sizing up natural fibers

A bio-fiber’s status as “natural,” as opposed to synthetic, has consequences. The two of most concern to composites manufacturers are its hydrophilic nature, and its poor bonding properties with thermoplastic and thermoset polymers. While both are problematic, the solution to potential moisture absorption, and release, by fibers in the matrix is relatively straightforward: Dry the fibers, in pellet, chopped or mat/fabric formats, with heat, a desiccant, or both, and keep them dry until just before use in the molding process.

Solving the problem of natural fibers’ inherently poor adhesion to polymers has been a bit more technically challenging. One of the most common, and simplest, approaches is to dose the resin with coupling agents to chemically induce a stronger resin/fiber bond. Sunstrand’s (Louisville, KY, US) Dr. Trey Riddle, however, says this approach is quantitatively flawed and minimally effective.

According to Riddle, the concentration of coupling-agent additives is typically in the range of 3%, or 3g per 100g of resin. A coupling-agent additive functions by modifying the polymer with functional groups that enhance its potential to bond with fibers. It does not modify the fiber directly. If the composite has a fiber content of 25%, there is three times more resin than fiber. This means the vast majority of additive-modified polymer is not coming into contact with the fibers, says Riddle, but is simply “floating” in the resin-rich areas between the fibers. “While these additives have proven effective at providing modest improvements in performance, they are not able to migrate to the bond areas and must be mixed uniformly throughout the resin.  Thus, significantly more additive must be mixed into the resin to ensure bonding is enhanced, driving up material usage and cost.”

A better approach, and the standard process used with glass fiber, Riddle contends, is to put the coupling agent where it is needed by modifying the fibers directly.  Improving bonding properties by modifying the fibers is more efficient because all of the fiber surface is in contact with
the polymer. The problem is that not a lot of work has been done to develop sizing technology specifically for natural fibers. “Traditional sizing treatments do achieve modest performance improvements,” Riddle notes, but contends that “modest” isn’t enough. The chemistries simply don’t react as well with natural fibers as they do with the glass fiber for which they were specifically formulated. 

Riddle says Sunstrand has a program to develop and improve sizing for natural fibers, and says it entails chemical and mechanical methods. He reports some of the methodologies Sunstrand has tried have already yielded improvements in fiber/resin bonding. “It is our hope that in the near future we’ll actually be able to commercially supply value-added fibers that perform better as a result of the treatment programs we
apply to them.”

This short article is a Side Story to a CW feature article titled, "Natural fiber composites: Market share, one part at a time." To read this feature article, click on its title under "Editor's Picks" at top right.