A predictive framework for the elastic modulus of recycled carbon-fiber needlepunched nonwoven materials: Integrating process and structure

Recycled carbon fibers, destined for landfills or incineration, can be redirected and redeployed for numerous applications, thus reducing the demand for virgin fibers. Needlepunched nonwovens made from recycled carbon fibers opened pathways to second-life opportunities and a broad range of industrial applications, including composite materials. Despite this progress, unraveling the framework that establishes the design rules for predicting the elastic properties of these nonwoven materials-three to four orders of magnitude lower than their constituent fibers-remains a formidable challenge. Herein, we propose a systematic theoretical framework that hinges on the 'process-structure-property' relationship, integrating the properties of recycled carbon fibers, the three-dimensional (3D) morphological features of the nonwoven, and the proportion of fiber length present in the thru-thickness direction. The analytical framework devised for the elastic modulus of recycled carbon-fiber (rCF) based needlepunched nonwoven materials extended the 'Paper Physics' approach into three dimensions by featuring 3D fiber orientation and fiber length distributions. Quantitative descriptors of the 3D orientation states of two distinct needlepunched nonwovens made from recycled carbon fibers, influenced by process conditions, have been obtained using X-ray micro-computed analysis. A reasonably good agreement between the predicted and experimental elastic modulus values in both the machine and cross-machine directions, coupled with a reduction from the carbon fiber modulus in the range of hundreds of gigapascals to a few tens of megapascals in nonwovens, justifies the robustness of the predictive model.