Synthesis and characterization of phosphorylated cellulose nanocrystals for enhanced UV stability of epoxy nanocomposite films

Epoxy resins offer excellent chemical resistance, low-temperature curing, and low shrinkage. Despite these excellent properties, they are susceptible to UV degradation, which compromises their durability in outdoor applications. Herein, we fabricated a nanocomposite epoxy film based on phosphorylated cellulose nanocrystal (P-CNC) to improve the UV stability of cured epoxy resin by incorporating liquid ammonia-treated P-CNC into an epoxy matrix using integral blending method. Ammonium phosphate groups ((NH4)2HPO4) were formed by nucleophilic attack of phosphates by ammonia on the P-CNC surface. The functional groups, morphology, crystallinity index, and surface charges of P-CNC were characterized using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X-ray diffraction (XRD), and electrophoretic light scattering (ELS), respectively. The effect of P-CNC on the UV stability of the fabricated epoxy nanocomposite film was investigated using FTIR and UV-Vis before and after exposure to UV photons. The results showed that the incorporation of 1.0 wt.% of surface-treated P-CNC into the epoxy matrix maintained the UV stability of the nanocomposite film up to 800 h. Finally, a mechanism for UV stability of P-CNC epoxy nanocomposite film was proposed whereby the (NH4)2HPO4 groups grafted onto the P-CNC surface absorb UV photons and convert them into low-energy thermal energy or short wavelength electromagnetic waves, which are not sufficient to initiate photodegradation of the nanocomposite film. Thus, this new film provides resistance against UV degradation for extended service life for outdoor applications of epoxy resin.