Aging of Poly(3-Hydroxybutyrate-Co-3-Hydroxyhexanoate)-Based Biocomposites under Accelerated Photo-Oxidation

In recent decades, the use of polyhydroxyalkanoate biopolymers has drawn increasing attention. Still, one of the biggest challenges that hinders their wide applications is the fine-tuning of their photooxidative aging. Therefore, the objective of this study is to investigate the effect of Agave Americana fibers (AFs) before and after surface treatment with sodium bicarbonate (NaHCO₃) on the accelerated photooxidation of PHBHHx using a SEPAP 24-48 enclosure. Biocomposite samples are prepared by melt compounding at a filler content of 30 wt.%. Colorimetric analysis reveals a yellowing effect of neat PHBHHx after 216h of exposure to UV rays, which is attributed to the formation of carbonyl groups. For the untreated biocomposites (PAF), they show a browning in color, while the PHBHHx-treated AF (PAFB) shows a lighter surface. The former process is attributed to the presence of lignin, whereas the latter is owing to the successful removal of lignin and hemicellulose by NaHCO₃. Changes in the chemical structure are assessed via FTIR-ATR spectroscopy, which shows a decrease in the 1741 cm^-1 band intensity for PHBHHx and PAF (after photooxidation) due to chain scission. However, no noticeable change is observed considering the same peak in the PAFB spectra. Furthermore, the evaluation of the hydroxyhexanoate (HHx) content in the different formulations is evaluated by ¹H NMR spectroscopy. The results show a slight decrease in HHx molar % after exposure to 216 h under accelerated photooxidation. Nevertheless, the decrease seems to level off after the addition of AF, particularly the treated ones. The overall findings highlight the beneficial effect of the incorporation of AF into the PHBHHx matrix, especially when treated with NaHCO₃ which confers a better photostability.