UV absorber, Light stabilizers can effectively alleviate the photoaging behavior of cured coatings. If the organic coating used outdoors has high quality and service life requirements, it is a mature operation method to add various suitable UV absorbers and light stabilizers to the coating formulation. UV absorbers GSSORB®UV-1, GSSORB®UV-326 and other products are very classic anti-aging products.
There are many ways to characterize the photoaging performance of coatings. For polymers without carbonyl structure, the photoaging behavior can be characterized by the change of carbonyl signal in infrared absorption spectrum. However, some coatings contain ester carbonyl groups, urethane bonds, etc., including photocurable UV coatings, and it is not suitable to use carbonyl values to characterize their photoaging behavior. At this time, we can use the yellowness index (yellowing index, YI) to characterize. International standard methods follow ASTM D1925. Generally, the cured film is exposed to ultraviolet light. After a period of time, the transmittance of the cured film at several specific wavelengths is measured with a UV-visible spectrophotometer. The irradiation light source can be 320nm filtered by Pyrex glass. Medium-pressure mercury lamps for short-wave ultraviolet light below.
The yellowness index is defined as:
The aging test using artificial light source is called accelerated aging, and accelerated aging is often accompanied by humidity, temperature, salt spray, etc. Additional device, in order to comprehensively evaluate the anti-aging performance of the coating more objectively. In addition to the ultraviolet light source, the xenon lamp with a power of several hundred watts has a continuous spectrum starting from 250nm, which is similar to the distribution of sunlight reaching the ground, and is often used as a light source for artificial aging machines. Other artificial aging light sources are QUVA and QUVB. QUVA is a long-wave ultraviolet output, and the output wavelength of QUVB is 280-360nm, and the center is located at 310mm. The photoaging test that is relatively primitive but fully consistent with natural aging is carried out under direct sunlight, such as the Florida 45-degree angle exposure test adopted in the United States.
Due to the light-absorbing effect of ozone in the earth’s atmosphere, after sunlight passes through the atmosphere and reaches the ground, the high-energy short-wave ultraviolet rays below 295nm are basically filtered out. Therefore, the wavelength band where ground sunlight has a strong effect on the photoaging of polymer coatings is mainly concentrated at 295-400nm. The shorter the wavelength and the higher the energy, the stronger the photoaging effect on polymer coatings will be. Affected by many factors such as season, time period, altitude, latitude, weather, air pollution, etc., the intensity of ultraviolet rays in the ground sunlight may change. The ozone layer absorbs ultraviolet light below 320nm, but with air pollution and ozone layer destruction, the intensity of short-wave ultraviolet light reaching the ground is increasing year by year. The photoaging results obtained by using different light sources may be inconsistent, mainly because the wavelength distribution, intensity, ambient humidity, temperature and other conditions of various light sources 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 changes in the absorption spectrum, gloss, haze, adhesion, mechanical properties, and thermodynamic mechanical behavior of the cured film. Due to the universality of photooxidation reaction in the photoaging process, the change of hydroxyl index or peroxide index in the coating is often used as a parameter to reflect photoaging. Polymers with different structures can also characterize the photoaging behavior according to the concentration change of sensitive groups.
Tag:   UV Absorber Light Stabilizer
