Resistance Mechanisms of Biomodified Binders against Ultraviolet Exposure

This paper investigates the effect of ultraviolet exposure on the physicochemical properties of biomodified binders as well as their resistance mechanisms against aging. Biomodifiers were extracted from four biomass sources (wood pellet, miscanthus, corn stover, and animal waste) and blended with asphalt binder to produce biomodified binders. The effect of ultraviolet exposure on a thin film of a biomodified binder was simulated via conditioning each specimen in an ultraviolet irradiation machine for up to 400 h. The evolution of material properties during the above exposure was studied utilizing UV-vis, optical microscopy, Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) analysis, and density functional theory (DFT) analysis. The study results showed that during ultraviolet exposure the compositional and surface properties of the materials gradually change, with the change being most notable beyond 50 h exposure time. FTIR results showed that peaks associated with ketones and carboxylic acids continuously increased during the aging process. Alkanes were reduced with aging in all biomodified binders, indicating a loss of volatiles and lighter molecules such as saturates. The latter reduction was also verified by EDX. Biomodified binders showed significantly lower values of an aging index compared to nonmodified binders, with the biomodifier extracted from miscanthus having the highest resistance to ultraviolet exposure and showing the lowest aging index. This was attributed to the presence of carbonaceous particles (as evidenced in optical microscopy) as well as the specific nature of the biomodifiers' molecules having the lowest affinity to react with other molecular species and/or oxygen in their surrounding environment. UV-vis results confirmed that these carbonaceous particles can adsorb ultraviolet light and function as a built-in resistance mechanism against aging to delay the progress of aging in biomodified binders. In addition, our DFT analysis showed that biomodifiers such as miscanthus have polarizability ranging from 57.3 to 111.2 Bohr(3); this is significantly lower than that of an asphalt binder, which ranges from 251.5 to 8753 Bohr(3). The lower polarizability can be another contributor to a biomodified binders' lower propensity to aging compared to nonmodified binders. The study results provide an in-depth understanding of the reinforcement mechanisms that biomodifiers contribute to a biomodified binders' improved resistance against ultraviolet aging. This enables formulators and manufacturers to incorporate such built-in reinforcement mechanisms to extend the sustainability and durability of binders used in outdoor applications.