Unveiling latent effects of continuous densification process on the properties of carbon nanotube fiber

Carbon nanotube fibers (CNTFs) translate the outstanding properties of individual CNTs into bulk materials. Although direct spinning enables continuous fiber production, it typically yields poorly aligned and loosely packed structures—especially for high-linear-density fibers—limiting their properties and applicability. To overcome this limitation, this study evaluates the characteristics of CNTFs treated by a continuous chlorosulfonic acid (CSA) densification method compared with batch process, which is one of the most efficient post-treatment methods for directly spun CNTFs. Despite a reduced stretching ratio from 80 % to 30 % to prevent fiber breakage during the process, continuously densified CNTF achieves a specific strength of 2.3 N/tex and conductivity of 1593 S·m 2 /kg, which are far superior to those of batch-densified CNTF. These improvements are attributed to collapsed CNT structures formed under compressive stress naturally induced during the continuous process, thereby increasing inter-tube contacts. Molecular dynamics simulations confirm that the presence of CSA between CNTs or CNT bundles facilitates effective load transfer, promoting structural evolution of CNTs under compressive stress. These results demonstrate that the continuous CSA densification can produce high-performance and high-linear-density CNTFs suitable for industrial-scale production, outperforming commercial aramid fibers such as Kevlar 49 in both strength and electrical conductivity.