Kenaf, the fast-growing hibiscus relative celebrated as a low-carbon alternative to glass fiber, has a dirty secret: lignin. The phenolic polymer that cements cellulose microfibrils together absorbs ultraviolet light, fractures into quinones and turns bright bast strands an unsightly brownish-yellow within days of outdoor exposure. Conventional fixes—UV-absorber coatings, hindered-amine stabilizers or nano-oxide barriers—add cost, chemicals and recycling headaches.
Now a joint academic-industry group led by Seoul National University and Hyundai Motor Company has flipped the problem on its head. By immersing 8 mm kenaf snippets in a 4:1 pyridine–benzoyl chloride bath at 105 °C for two hours, the researchers achieve two goals at once: hydroxyl groups along cellulose and hemicellulose are esterified, while roughly half of the chromophore-rich lignin dissolves away. The resulting "benzoylated kenaf" (BKF) emerges hydrophobic, thermally more stable and, most surprisingly, photobleaching.
Under accelerated weathering (65.5 W m⁻² UVA, 300–400 nm) the fibers behave unlike any natural material the authors have tested. During the first 48 h the sample's lightness index L* jumps from 74 to 84, scattering light through a network of nano-cracks that form as the surface rearranges. Once this "stabilized BKF" (sBKF) state is reached, further colorimetry, electron-spin resonance and X-ray photoelectron data flat-line: no new radicals, no additional oxygen uptake, no extra yellowing. After the full 500 h cycle the total color difference ΔE* is statistically identical between 48 h and 500 h, whereas unmodified kenaf continues to darken, culminating in a 15-unit ΔE* swing and a 96 % plunge in tensile strength.
Microscopy reveals why. Initial benzoylation roughens the outer cell wall, but the inner core remains intact. Subsequent UV merely etches established micro-cracks without deepening them, a self-limiting process the team attributes to the radical-quenching resonance of grafted aromatic esters. Density-functional calculations show that benzoyl-capped phenolic sites in residual lignin require 104 kcal mol⁻¹ to abstract hydrogen—well above the 71–95 kcal mol⁻¹ energy window of UVA photons—effectively shutting down the semiquinone radical chain.
Automotive engineers are paying attention. "We need natural fibers that survive 2 000 h of xenon weathering for exterior trim," said Seunghwan Ko, Hyundai materials researcher and co-corresponding author. "BKF passes the 500 h milestone with flying colors and we see a clear path to 1 000 h by tweaking reaction time and liquor ratio." Because the chemistry uses the same wet-processing lines that turn hemp or kenaf into pulp, scale-up cost is estimated below USD 0.30 per kg of fiber.
Life-cycle analysts note an extra benefit: removed lignin can be recovered as a low-molecular aromatic feedstock, avoiding the yield penalty that plagues totally delignified cellulose. The team is now molding BKF into polybutylene-succinate composites for dashboard panels and trunk liners, targeting 40 % bio-content and full UV stability for 15 years.
If the strategy proves compatible with continuous roll-to-roll reactors, Hyundai believes benzoylated bast fibers could reach commercial volumes by 2028, giving carmakers, furniture designers and even fashion houses a drop-in route to sustainable, light-proof biocomposites that grow, rather than drill, out of the ground.
See the article:
DOI
https://doi.org/10.1016/j.jobab.2025.11.002
Original Source URL
https://www.sciencedirect.com/science/article/pii/S2369969825000817
Journal
Journal of Bioresources and Bioproducts
