ON THE MECHANICAL PROPERTIES OF POLYUREA

Polyurea is used in military and civilian applications where exposure to the sun is usually very long, which can reduce its mechanical properties to sub-optimal levels. This study reports the relationship between ultrasonic properties of polyurea and duration and temperature of ultraviolet irradiation. Six groups of samples were constantly exposed to ultraviolet radiation for different periods up to 15 weeks. Control samples that did not receive UV irradiation were also tested. Ultrasonic properties were measured in a pitch capture configuration using a high frequency contact ultrasonic transducer. All the exposed samples showed significant color change from clear yellow to opaque light brown. Changes in color were observed to be exposed to ultraviolet radiation as early as 3 weeks. Nevertheless, cracks were observed only in samples exposed for 6, 9, 12 and 15 consecutive weeks. As the exposure time increases, the size of the surface crack increases

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