The Effect of Ultraviolet Radiation on the Hyperelastic Behavior of Polyurea

Abstract

Polyurea is widely used in military and civilian applications, where long durations of exposure to the sun is common. Extended ultraviolet exposure can deteriorate the mechanical performance of the material to suboptimal levels. This study reports on the hyperelastic behavior of polyurea as a function of exposure duration to ultraviolet radiation. Initially, five sets of samples were continuously exposed to ultraviolet radiation for different durations up to 15 weeks. An additional set of samples were kept unexposed to report on the properties of an as-cast polyurea control. The samples were then loaded uniaxially in tension under a quasi-static loading rate (ϵ˙ = 0.0127 s- 1) using a standard load frame. All exposed samples exhibited incremental color changes, which ranged from transparent yellow in virgin samples to opaque tan after 15 weeks of exposure. Changes in color were observed as early as 3 weeks of ultraviolet exposure. In addition to discoloration, crazing developed in all samples for exposure durations of 3 weeks and beyond. The size of the cracks increased as the duration of exposure continued. Postmortem examination indicated the initial microcracks grew due to loading and were visible to the naked eye. Discoloration and crazing were attributed to the formation of a thin ultraviolet-degraded layer due to the interaction of oxygen and ultraviolet light with the chromophoric groups in the polyurea macromolecule. The hyperelastic behavior was studied by fitting the stress–strain data into a four-term Ogden model. The overall hyperelastic behavior of polyurea remained unchanged even after 15 weeks of continuous ultraviolet exposure, where no permanent failure (except the ultraviolet-induced cracks) was recorded at the gauge length up to 250% strain. The values of the fitting parameters were nearly identical, supporting the conclusion of the insensitivity of polyurea hyperelastic behavior to ultraviolet radiation.