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Closing the Loop on Automotive Carbon Fiber Prepreg Manufacturing Scrap...

Publication Type
ORNL Report
Publication Date

The project demonstrated how to “close the loop” oncarbon fiber by integrating industrial carbon fiber scrap into new functional components in an automotivelightweighting application. The project serves as a validation of discontinuous recycled carbon fiber in a commercial context, while generating comprehensivematerial data throughout the production chain.
To this end, the project exhibited increasing complexity asmaterial evaluation progressed from benchtop to commercial scale through full-scale part production, with key material properties thoroughly characterized throughout the process. Of particular focus was the form of the fiber that was fed into compounding, as recycled fiber has historically been problematic to feedat commercial-scale. Carbon fiber is energy-intensive to manufacture, so reuse of existing fiber materialcan reduce costs and increase sustainability. Additionally, by integrating recovered short fiber into a thermoplastic, regrind processes can be used to providefeedstock for later generations of product. While regrind plastics are not “infinitely recyclable”, reusing themanufacturing scrap over several generations of pro
ducts can greatly increase material sustainability andlower the fractional embodied energy of each successive product. As such, this project supports the IACMI technical goals of (1) 25% lower carbon fiber-reinforced polymer (CFRP) cost, (2) 50% reduction in CFRP embodied energy, and (3) 80% composite recyclability into useful products. The initial stage of the project involved downselecting surface treatment (sizing) chemistries. Sizing evaluations were performed on Vartega’s chemically recycled intermediate modulus fiber along with standard modulus dry scrap whichwas oversized with sizing provided by Michelman. More dramatic improvements from sizing were found on the standard rather than the intermediate modulusfibers. The strength of the chemically recycled individual fibers were evaluated by Michelman and ORNLthrough single fiber testing and found to be comparable to similarly evaluated virgin fibers. UDRI’s mechanical testing on injection molded test specimens identified similar mechanical properties and fiber distribution relative to benchmark specimens. Additional surface chemistry tests and visualizations were performed by the Colorado School of Mines to confirm closeconformance between the benchmark and recycled-fiber specimens. As the mechanical test results exceeded the 80% threshold established as the go/no-go(GNG), the project scale was increased to use commercial-scale equipment that would both better characterize the manufacturing utility of the target product format and allow qualitative assessment of a complex commercial part.
An upscaled compounding evaluation was performed with a 27 mm twin-screw compounding extruding using oversized standard modulus fibers that were formatted to improve bulk solids transfer. The project team anticipated that milestonemechanical benchmarks could be achieved given the favorable performance of the sized standard modulus material identified in the initial micro-compounding trials. While the mechanical performance did meetthe milestone target for that phase of the project, mechanical properties for this standard modulus-basedcompound were still less than those of the Ford specification. To compare the performance, the project team oversized intermediate modulus dry fibers and c
ompounded them with the project resin at BASF using a 40 mm compounder. Test specimen mechanicalperformance exceeded the targets laid out in both the project milestone and the Ford specification. A series of prototype parts were successfully molded, albeit with instances of short shot components due to thehigh thermal conductivity of the carbon fiber compared to glass fiber for which the prototype tool was designed for.
The project demonstrated that recycled carbon fiber is a viable option in fiber reinforced compound, providing greatly increased strength and modulus for applications that require them. The “agglomerated” format that facilitated effective bulk solids transfer of recovered fiber showed promise for industrial application.