The Effect of Ultraviolet Radiation on the Hyperelastic Behavior of Polyurea

Polyurea is widely used for military and civilian use and is generally exposed to sunlight for a long time. Long-term UV exposure can reduce the mechanical properties of the material to sub-optimal levels. This study reports the relationship between the superelastic behavior of polyurea and the duration of ultraviolet rays. Initially, five groups of samples were continuously exposed to ultraviolet radiation for different periods up to 15 weeks. Other sets of samples were not exposed to report the properties of the as-cast polyurea control. Then uniaxially stretch the sample with a quasi-static loading speed (\ (\ dot {\ epsilon} = 0.0127 {s} ^ {- 1} \)) using a standard loading frame. All exposed samples showed an increase in color change in the range from clear yellow in the first sample to opaque tan after 15 weeks exposure. Changes in color were observed as early as 3 weeks after UV irradiation. In addition to discoloration, silver stripes appeared in all samples for more than 3 weeks. As the exposure time increases, the size of the crack increases. Postmortem examination shows that initial microcracks grow with load and can be seen with the naked eye. Discoloration and cracking are due to the formation of a thin layer of UV degradation due to the interaction of the chromophore in the polyurea polymer with oxygen and ultraviolet radiation. The superelastic behavior was investigated by applying stress-strain data to a four-point Ogden model. Even after 15 weeks of continuous UV irradiation, the overall superelastic behavior of the polyurea did not change and no permanent failure was recorded at 250% strain gauge length (except UV induced cracking). The values ​​of the fitting parameters are about the same, which supports the conclusion that the polyurea superelastic behavior is not sensitive to ultraviolet radiation.

This study shows that the mechanical properties of polyurea's low strain rate are a function of the duration of ultraviolet irradiation. The polyurea sample was cast and continuously exposed to UV radiation for 15 weeks to simulate direct sunlight exposure of about 9,200 hours. Each sample (unexposed and exposed) was uniaxially tested according to ASTM D 638-8 with semi-static tension. Stress-strain data were used to calculate the elastic modulus of polyurea, 0.2% offset yield stress, shear modulus and stress at 250% strain. In addition, the optical properties and surface topology change of polyurea were investigated using ultraviolet - visible spectroscopy (UV - Vis) and scanning electron spectroscopy (SEM), respectively. Finally, we apply stress-strain data to Ogden's superelastic model. The change in Ogden's fitting parameter is due to the duration of UV exposure

Polyurea is widely used for military and civilian use and is generally exposed to sunlight for a long time. Long-term UV exposure can reduce the mechanical properties of the material to sub-optimal levels. This study reports the relationship between the superelastic behavior of polyurea and the duration of ultraviolet rays. Initially, five groups of samples were continuously exposed to ultraviolet radiation for different periods up to 15 weeks. Other sets of samples were not exposed to report the properties of the as-cast polyurea control. Then uniaxially stretch the sample with a quasi-static loading speed (\ (\ dot {\ epsilon} = 0.0127 {s} ^ {- 1} \)) using a standard loading frame. All exposed samples showed an increase in color change in the range from clear yellow in the first sample to opaque tan after 15 weeks exposure. Changes in color were observed as early as 3 weeks after UV irradiation.