Easily Processable Thermoplastic Microcomposites with Enhanced UV Shielding Performance Based on Anionic Polyamide 6 and Sulfonated Lignin

Anionic polyamide 6 (aPA6) thermoplastic composites are known for their excellent mechanical and functional properties, including high strength, toughness, good processability, and gas-barrier capabilities. However, when exposed to the environment, they seem to lack proper mechanisms to resist degradation from radiation and atmospheric agents. For this reason, in this paper, the incorporation of a microscopic bio-based sulfonated lignin filler was investigated, aiming to improve UV shielding properties, which are expected to prolong the life cycle of the material and to decrease microplastic formation with extensive environmental benefits. In this work, anionic ring-opening polymerization (AROP) of ε-caprolactam (CL) was employed for the first time to produce aPA6/lignin microcomposites containing up to 5 wt% lignin in a simple in situ one-pot process. A marked increase in attenuation coefficient was observed in the UV range, up to 894% at 346 nm. Nuclear magnetic resonance (NMR) and microscopic characterization excluded chemical bonding between biofiller and matrix, resulting in a moderate decrease in mechanical performance (−21.4% in yield strength and −16.3% in stress at break). However, the absence of lignin–PA6 hybrids may be beneficial to the recyclability of the material. Overall, this study proved the benefits of introducing a widely available and environmentally friendly filler into recyclable high-performance composite materials, imparting further functional properties and anticipating novel applications.