Dynamic properties of ultraviolet-exposed polyurea

Polyurea is used in military and civilian applications, where exposure to the sun in long durations is imminent. Extended exposure to ultraviolet radiation from the sun can deteriorate its mechanical performance to suboptimal levels. This study reports on the dynamic mechanical properties of polyurea as a function of ultraviolet radiation exposure duration. Six sets of samples were continuously exposed to ultraviolet radiation for different durations up to 18 weeks. Control samples were also tested that did not receive ultraviolet exposure. The dynamic properties were measured using a dynamic mechanical analyzer. Exposed samples exhibited significant color changes from transparent yellow to opaque tan after 18 weeks of exposure. Changes of color were observed as early as 3 weeks of exposure. The dynamic properties showed an initial increase in the dynamic modulus after 3 weeks of exposure, with no further significant change in the stiffness thereafter. The ultraviolet exposure had a significant impact at relatively short loading times or low temperature, for example, up to 6 decades of time. As loading time increases or polyurea operates at high temperature, the effect of ultraviolet exposure and temperature on the performance become highly coupled.