Climate and solar irradiation are the primary causes of damage to coatings, plastics, inks, and other polymer materials. This damage includes loss of gloss, fading, yellowing, cracking, peeling, embrittlement, reduced strength, and delamination. Since solar radiation has dual effects of thermal radiation and photochemical reactions, high-temperature tests cannot substitute for light exposure aging tests, and neither can pure ultraviolet (UV) radiation nor infrared (IR) radiation replace solar radiation tests.
Introduction to UV Aging Test
UV aging falls under solar irradiation aging, where "UV" stands for Ultraviolet. It is typically used to assess the rate at which products degrade under UV exposure. Solar irradiation aging is the main type of aging damage suffered by materials used outdoors. Even for indoor materials, they may still experience some degree of solar irradiation aging or aging caused by UV radiation from artificial light sources (such as UV lamps and the UV bands in fluorescent lights). Solar radiation is mainly concentrated in the visible light spectrum (0.4–0.76 μm), with smaller portions in infrared (>0.76 μm) and ultraviolet (<0.4 μm) wavelengths. Over 99% of the total radiation energy falls within the 0.15–4 μm range, primarily distributed in the visible, infrared, and ultraviolet regions. The visible light region accounts for approximately 50% of the total solar radiation energy, the infrared region about 43%, and the ultraviolet region only about 7%. Ultraviolet light, which ranges from 0.01–0.40 μm on the electromagnetic spectrum, is invisible to the human eye. It is classified into UVA, UVB, and UVC rays with wavelength ranges of 400-315 nm, 315-280 nm, and 280-190 nm, respectively. According to ASTM G154, UV radiation cannot replicate the full spectrum of sunlight. Its principle lies in the fact that for durable materials exposed outdoors, the short-wave UV wavelengths (300–400 nm) are the primary cause of aging damage. In the short-wave UV region, from 365 nm down to the lowest wavelength of sunlight, UV fluorescent lamps can effectively simulate sunlight.
Modes of UV Aging Test
The UV aging test can be configured with three aging modes: light exposure, condensation, and spray.
- Light Exposure Phase: Simulates daylight duration (typically between 0.35 W/m² and 1.35 W/m², with midday summer sunlight around 0.55 W/m²) and test temperatures (50°C–85°C) to replicate various usage environments and meet testing requirements across different regions and industries.
- Condensation Phase: Simulates nighttime surface condensation on samples. During this phase, UV fluorescent lamps are turned off (darkness), and only the test temperature (40–60°C) is controlled, with sample surface humidity maintained at 95–100% RH.
- Spray Phase: Simulates rainfall by continuously spraying water onto the sample surface.
Due to the harsher conditions of accelerated UV aging tests compared to natural environments, aging damage that would take years to occur outdoors can be replicated within days or weeks.
UV Aging Lamps
- UVA-340: Primarily used to simulate the mid-to-short UV wavelengths in sunlight, typically for outdoor product light aging tests.
- UVA-351: Primarily used to simulate the mid-to-short UV wavelengths of sunlight filtered through window glass, typically for indoor products.
- UVB-313: Emits significant radiation below 300 nm, a wavelength that is nominally the cutoff for solar radiation. This can induce aging phenomena that do not occur outdoors. The use of this lamp is not recommended for simulating sunlight. Instead, it is mainly used for accelerated durability testing of materials and for qualitative comparisons between two or more materials.
The irradiance levels of UVA-340 are as follows:
- 0.69 W/m² @ 340 nm: Equivalent to midday summer sunlight, yielding rapid results.
- 1.38 W/m² @ 340 nm: Equivalent to solar maximum, yielding rapid results.
- 0.35 W/m² @ 340 nm: Equivalent to March/September daylight, suitable for general testing or low UV intensity tests.
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