Carbon fibers derived from oleic acid-functionalized lignin via thermostabilization accelerated by UV Irradiation
Lignin is a biorenewable precursor source suitable for the production of low-cost carbon fibers. In this work, softwood lignin was chemically modified with fatty acids to employ UV-triggered oxidative reactions occurring at long alk(en)yl chains as a pretreatment prior to thermostabilization. The chemical transformation of oleic acid-functionalized lignin (OAFL) under the UV irradiation was the free radical-based cross-linking reaction assisted by atmospheric oxygen, which successfully formed oxidatively cross-linked networks of lignin derivatives at the surface of the fiber. We observed that OAFL showed more rapid oxidative cross-linking than stearic acid-functionalized lignin (SAFL) because the presence of carbon？carbon double bond in OAFL contributed to the formation of more stable allyl radicals in addition to alkyl radicals under the UV irradiation. This UV pretreatment played a pivotal role in the conversion of flexible pristine OAFL fibers into infusible fibers during the subsequent thermostabilization step at the elevation rate of 2 °C/min without melt deformation. As the elevation rate of 2 °C/min is among the fastest thermostabilization process for lignin-derived carbon fibers, the duration of the thermostabilization of the OAFL-derived fibers in this study (∼2 h) is considerably less than previously reported ones. In addition, the UV irradiation process in this study requires less power (150 W) and exposure time (8 min), which can be appreciated by carbon fiber manufacturers aiming to reduce the fabrication cost. Therefore, the chemical functionalization with fatty acids envisages the possibility to produce carbon fibers from lignin precursors via a rapid stabilization step with spending less amount of time and energy.