CompositesWorld
Issue link: https://cw.epubxp.com/i/632301
CompositesWorld.com 31 NEWS N E W S N S N E W S E N W S W looked at before, many times," confrms Clif Eberle, tech- nology development manager for carbon and composites at Oak Ridge National Laboratory (ORNL, Oak Ridge, TN, US) and the director, materials and processing technology, for the Institute for Advanced Composites Manufacturing Innova- tion (IACMI, Knoxville, TN). "But market pull and changed conditions are making it important to revisit these alterna- tive ideas, so that we can address critical needs for expanded material choices." ORNL is one of several entities involved in investigating how materials other than PAN can be transmuted into usable fber forms for lightweighting applications. It has investi- gated several options with numerous industry and univer- sity partners and, in concert with independent researchers in Sweden, Finland, Canada, Brazil, Australia, Korea and Japan, ORNL says that lignin holds promise. Lots of trees = lots of lignin Lignin is currently in the spotlight, says Eberle, because it meets key criteria. "It's carbon-rich, it's low in cost, the tech- niques exist to process it and the scale is right — there's plenty available and it's renewable." In terms of scale, "plenty" could be an understatement: As the second most abundant organic polymer on the planet, after cellulose, and the most abundant aromatic bio-polymer, lignin is found in virtually all terrestrial plants. Its complex, branched and non-linear molecules give a plant its structure: Cellulose fbers act as load-bearing elements while lignin acts as the matrix, bonding with the cellulose to create a structural bio-composite that supports the plant for upright growth. Tree types of lignin monomers, sometimes called phenyl- propanoids, exist, often mixed together in the same plant: cumaryl alcohol, typical of annual plants and grasses; coniferyl alcohol, found in softwood trees; and sinapyl alcohol, found mostly in hardwoods. Lignin is obtained from pulp and paper operations, or bio-refneries. Wood chips are placed in a high-temperature solution of sodium hydroxide and sodium sulfde, called white liquor, to dissolve the lignin and separate it from cellulosic fbers. At a bio-refnery, the cellulose is used to make ethanol; at a kraft pulping plant, the cellulosic pulp goes on to become paper. Te lignin-containing waste liquid, or black liquor, from either process can be burned for fuel, or processed to extract the lignin, using a precipitation process. Precipitated lignin is dried to a powder form that must be treated. Treatment includes purifcation, melt-spinning or melt-blowing stabilization (in an oxidative atmosphere), and carbonization. Hardwood lignin reportedly melt-spins well but stabilizes slowly; softwood lignin is said to stabilize well but does not melt spin. Tese liabilities, however, could prove to be "myths" — processing limitations that can be overcome with some technical creativity. Key to the lignin fber production process is keeping the temperature low enough to draw a fber without crosslinking the lignin before the stabilization step. Lignin-based Carbon Fiber? SIDE STORY In North America, Oak Ridge National Laboratory (ORNL, Oak Ridge, TN, US) has been at the forefront of research into alternatives to the polyacrylonitrile (PAN) precursor used to make aerospace-grade carbon fber. For more than 15 years, the group has investigated low-cost carbon fber production using alternatives that have included textile-grade PAN, polyolefn and lignin. Following years of bench- and laboratory-scale projects, ORNL is now home to the US Department of Energy's (DoE) Carbon Fiber Technology Facility (CFTF), where, in 2013, a fexible and fully instrumented carbon fber production line manufactured by Harper International (Lancaster, NY, US) was installed. The new fber line, with low- and high-temperature slot furnaces, is fexible enough to handle non-PAN-derived fbers at a semi-production scale, says ORNL's Connie Jackson, who manages fber production. The line could accommodate, as part of future studies, nontraditional processing technolo- gies, such as plasma- and microwave-based heat treatment and stabilization, which, so far, has occurred at a smaller scale. The group's current work involves carbonization of the lowest-cost PAN available, normally used for apparel, acquired from multiple suppliers. Fibers are carbonized in ultra-large-tow format (up to 610K). Jackson says that produced tows have been shown to have a "modest" tensile strength of about 500 ksi. Part of the CFTF's activities involve developing ways to treat the fbers, including plasma treatment, sizings for compatibility with thermoplas- tics and tow splitting for more manageable fber products. In 2011, ORNL established the Oak Ridge Carbon Fiber Composites Consortium to accelerate the development and deployment of new, lower cost carbon fber composite materials for lightweighting, to enhance the economic competitiveness of US-based manufacturers. So far, more than 50 consortium members are participating in projects and technology transfer initiatives. The consortium is currently merging with the Institute for Advanced Composites Manufacturing Innovation (IACMI, Knoxville, TN, US). Clif Eberle, the technology development manager for carbon and composites at ORNL and now IACMI's director of materials and processing technology, says, "Our thesis is driven by trying to achieve energy security for US taxpayers and lower energy costs. In the US alone, we use 19 million barrels of oil per day, and about 70% of that is for transportation. If we can reduce that energy usage with lighter weight vehicles made with low-cost carbon fber, it will make a huge diference to our country, and take us closer to energy independence." Alternative precursor R&D;: Oak Ridge National Laboratory ORNL: Investigating multiple options Textile-PAN carbon fbers are shown here during a production run on ORNL's alternative- precursor-capable CFTF production line, the fruit of 15 years of R&D; (see photo of the full line on p. 30). ORNL also is investigating low-cost carbon fber derived from polyolefn and lignin precursors. Source | ORNL