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The accelerated weathering of flax and miscanthus fibers possessing distinct chemical compositions was investigated. The chosen fibers included raw, extractive-free (EF) and delignified samples (x3), alone and used as fillers in a stabilized polypropylene blue matrix (PP). Modifications in both color and the chemical composition of the fibers throughout the weathering process under ultraviolet (UV) light were meticulously tracked and analyzed by spectrophotometry and attenuated total reflectance with Fourier-transform infrared spectroscopy (ATR-FTIR). The inherent nature and composition of the selected fibers led to varied color-change tendencies. Raw and EF flax fibers exhibited lightening effects, while raw and EF miscanthus fibers demonstrated darkening effects. Extractives exhibited negligible influence on the color alteration of both flax and miscanthus fibers. This disparity between the fibers correlates with their respective lignin content and type, and the significant formation of carbonyl (C=O) groups in miscanthus. Better stability was noted for delignified flax fibers. A comparative study was achieved by weathering the PP matrix containing these various fibers. Contrary to the weathering observations on individual fibers, it was noted that composites containing raw and EF flax fibers exhibited significant color degradation. The other fiber-containing formulations showed enhanced color stability when compared to the pure PP matrix. The study highlights that the UV stability of composites depends on their thermal history. As confirmed by thermogravimetric analysis (TGA), fiber degradation during extrusion may affect UV stability, a factor that is not apparent when fibers alone are subjected to UV aging. [ABSTRACT FROM AUTHOR]

Flax fabric textile is of great interest as bio‐fiber reinforcement in composites, which has gained popularity due to the high demand for sustainable materials. Despite its sustainability, flax is inherently flammable. This study demonstrates a two‐step polyelectrolyte complex (PEC) coating that renders flax fabric self‐extinguishing. The PEC coating consists of polyethylenimine (PEI) and sodium hexametaphosphate (PSP), cured with a citric acid buffer. Sonication employed during the curing process improves the infiltration of the coating into the flax fibers and maintains the fabric's hand. When comparing non‐sonicated and sonicated flax samples, the latter achieves self‐extinguishing behavior and passes the vertical flame test. Pyrolysis‐combustion flow calorimetry reveals that this PEI/PSP coating (with sonication) reduces the peak heat release rate and total heat release by 40% and 67%, respectively. The intumescent mechanism is predominantly observed in the condensed phase, as suggested by differential scanning calorimetry and X‐ray photoelectron spectroscopy. The results demonstrate that the two‐step PEC treatment with sonication effectively reduces the flammability of flax fabric, making it a promising option for the development of functional fibers, particularly for bio‐composite reinforcement. [ABSTRACT FROM AUTHOR]