CompositesWorld
Issue link: https://cw.epubxp.com/i/632301
FEBRUARY 2016 32 CompositesWorld WORK IN PROGRESS Weyerhaeuser (Federal Way, WA, US), using US Department of Energy (DoE) funding to investigate addition of PAN polymer to lignin to increase fber quality. Te study showed that, as more PAN was added, the fber properties improve, but the larger percentage of PAN drove costs up and the resulting hybrid fbers tended to be porous, perhaps due to immiscibility problems between the PAN and lignin during the spinning stage. Unlocking lignin's potential Although there are only a handful of lignin suppliers capable of producing high-purity fber today, interest in lignin fber appears SIDE STORY GrafTech International (Independence, OH, US) is working with Oak Ridge National Laboratory (ORNL, Oak Ridge, TN, US) and the US Department of Energy's Advanced Manufacturing Ofce (DoE AMO) in a collaborative efort to develop lignin fber manufacturing and demonstrate it in viable products. The company is investigating lignin-based carbon fber for GrafTech's trademarked GRAFSHIELD and FiberForm high-temperature industrial furnace insulation products (FiberForm was formerly produced by Fiber Materials Inc., now part of GrafTech). Ryan Paul, innovation and technology manager at GrafTech, explains that his company's current insulation products are made with pitch or rayon-based carbon fber, but that the company wanted a US-based and lower cost fber source. That led GrafTech to work with ORNL several years ago, when a prototype lignin fber insulation product was produced and tested, using hardwood-sourced lignin precursor. GrafTech was awarded a cooperative agreement by DoE AMO in late 2014 to advance the technology readiness level (TRL) of lignin technology by overcoming some of the key technical challenges associated with lignin precursors and fber production scale-up. GrafTech prefers pure lignin, with minimal addition of polymers, at least for now, explains Paul. "High purity is key, with little ash and other contaminants, so that the fbers can be melt spun. We're also targeting a specifc range of molecular weight in the precursor." A big challenge, says Paul, is that each lignin source varies, and a stable supply chain consisting of multiple suppliers is needed: "We are identifying a range of US lignin suppliers and trying to determine what source, and even what species of plant, is best." The concept is that fbers are melt-blown into a web, and the entire web is stabilized at a temperature between 250°C and 300°C, and then carbonized. ORNL's carbon fber line is now fully operational; GrafTech plans to scale up the technology to the next level in 2016 to 2017, and as the project moves forward and meets its milestones, GrafTech will assess bringing the product to market. The mat form is expected to work well for GrafTech's products, because fbers are milled, sized and processed into rigid insulation panels. The fbers ultimately are bonded together and aligned parallel to the wall of a furnace when the insulation is installed, providing through-plane thermal conductivity of 0.45 W/mK at 1500°C, yet ofering enough strength to withstand handling and machining. Adds Paul, "We expect to get good Alternative precursor R&D;: First commercial use of lignin carbon fber? Slightly off the composites path GrafTech International's GRI high-temperature furnace insulation (large image at bottom) is a target future application for lignin-based carbon fber that the company is developing with Oak Ridge National Laboratory (ORNL) and the DoE, a project to move lignin fber into viable products. The inset image at right shows an insulation "stack" that combines several layers of the bonded carbon fber GRI insulation layers. Source (both photos) | GrafTech International fber quality, but we can also get away with some fber porosity and some shorter fbers, and still get the functionality we need." "It's a matter of understanding the performance requirements for an application, and maximizing the value of the fber product for that applica- tion. We don't need aerospace-grade, perfect fber for many of these kinds of high-volume uses," asserts ORNL's technology development manager for carbon and composites, Clif Eberle. Paul adds that "we're going for the 'low-hanging fruit' here, with a sustainable, and ultimately customizable, carbon fber solution that's half the price of our current product." ORNL has reported that the lignin fbers with the best mechan- ical properties are from softwood sources and exhibit 155 ksi strength (about one-third that of pitch fber) and 12 Msi modulus (slightly better than E-glass). Typically, the fber properties range much lower. "Lignin does not readily produce structural fbers, since aligned crystalline morphology doesn't occur," says Mark Downing, a former manager in ORNL's Bioenergy Program. Several investigators have looked at "doping" or mixing other polymers with lignin to improve it as a fber precursor. One example from several years ago was Zoltek Corp.'s (St. Louis, MO, US, now part of Toray Carbon Fibers) partnership with