The effect of ultraviolet radiation on ultrasonic 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 magnitude of the surface crack increases. It should be noted that the effect of long-term ultraviolet radiation on acoustic properties is minimal and that the modulus and shear rate monotonically decrease after the first increase during the first three weeks of exposure. The damping characteristics were measured at 23 ° C. and 40 ° C. The p-wave attenuation does not show a significant change and as the temperature and the UV exposure time increase, the s-wave attenuation monotonically decreases.

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

Effects of gamma rays and ultraviolet irradiation on hardness, elastic modulus and viscoelasticity of castor oil (PU) derived polyurethane were investigated by nanoindentation test. Modification of the radiation induced surface morphology was observed with atomic force microscopy. Polyurethane derivatives derived from castor oil have little change in hardness, modulus of elasticity, viscoelasticity and contact angle, and exhibit good resistance to gamma rays. Due to UVA radiation, the hardness of PU increases in the region near the surface, and after UVC radiation the hardness of PU decreases. Although the roughness increases markedly, the contact angle of water droplets decreases after UVC radiation but does not decrease after gamma radiation. These results are due to the different reaction of polyurethane to radiant energy. The increase in hardness caused by UVA is due to higher crosslinking at shallow depths and the reduction in mechanical properties may be due to chain scission.

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