What is the light aging protection of UV absorbers for coatings?
UV absorbers and light stabilizers are effective in slowing down the light aging behavior of cured coatings. The addition of various suitable UV absorbers and light stabilizers to coating formulations is a well-established method of operation for organic coatings for outdoor use where high quality and longevity are required. UV absorbers such as EVERSORB 80, EVERSORB 93, etc. are classic anti-aging products.
There are many ways to characterize the light aging performance of coatings. For polymers without carbonyl structure, their light aging behavior can be characterized by the carbonyl signal change in the infrared absorption spectrum. However, some coatings contain ester carbonyl, ammonia ester bond, etc., including light-curing UV coatings, which are not suitable to use carbonyl value to characterize their light aging behavior. In this case, we can use the yellowing index (YI) to characterize them. The international standard method follows ASTM D1925, in which the cured film is exposed to UV light, and after each period of exposure, the transmittance of the cured film at several specific wavelengths is measured by UV-visible spectrophotometer, and the source of irradiation can be a medium-pressure mercury lamp with Pyrex glass filtering out the short-wave UV light below 320 nm.
The yellowness index is defined as.
The aging test using artificial light source is called accelerated aging, accelerated aging is often matched with additional devices such as humidity, temperature, salt spray, etc., in order to more objectively and comprehensively evaluate the anti-aging performance of the coating. In addition to ultraviolet light source, the power of a few hundred watts of xenon lamp, its spectral output from 250nm continuous spectrum, and reach the ground of the sun’s spectral band distribution is similar to that commonly used in artificial aging machine light source. Other artificial aging light sources include QUVA and QUVB. QUVA is a long-wave ultraviolet output, and the output wavelength of QUVB is 280-360nm, with the center located at 310mm. more primitive, but fully consistent with the natural aging of the light aging test is carried out in direct sunlight, such as the United States, the use of the Florida 45-degree angle exposure test.
Due to the earth’s atmospheric ozone absorption, sunlight through the atmosphere to the ground, 295nm below the high-energy short-wave ultraviolet light is basically filtered out. Therefore, the ground sunlight on the polymer coating light aging has a strong effect on the wavelength band is mainly concentrated in the 295 ~ 400nm, the shorter the wavelength, the higher the energy, the polymer coating light aging effect will be more intense. Influenced by season, time, altitude, latitude, weather, air pollution and many other factors, the ground sunlight UV intensity may change. The ozone layer absorbs ultraviolet light below 320nm, but with atmospheric pollution, ozone layer destruction, the intensity of short-wave ultraviolet light arriving at the ground is increasing year by year. Using different light sources to get the light aging results may not be consistent, mainly because of a variety of light source wavelength distribution, intensity, environmental humidity, temperature and other conditions are not completely consistent.
In addition to the yellowness index, the anti-aging performance of the cured film can also be characterized by examining the absorption spectrum, gloss, haze, adhesion, mechanical properties, thermodynamic mechanical behavior and other indicators of change. Due to the prevalence of photo-oxidation in the light aging process, the change of hydroxyl index or peroxide index in the coating is often used as a parameter to reflect the light aging. Different structures of polymers can also be characterized according to the changes in the concentration of sensitive groups.