Laboratoryaccelerated Weathering Tests

Laboratory-accelerated test devices that utilize artificial light sources have been used for more than 80 years to support the development of progressively more weatherable formulations as well as for quality control and specification tests. The devices allow control of the three main test parameters, that is, radiation, heat, and moisture, for more consistent exposure conditions compared with their variability outdoors. The consistency of exposure conditions is often monitored with a weathering reference material, such as the AATCC blue wool standards [123] or the polystyrene chips [124] to improve reproducibility of laboratory tests. Often, problems related to reproducibility can be traced to differences in replicate specimens because of nonuniformity in materials or differences among polymer batches. Specimens cut from the same sheet can vary in sensitivity because of differences in content of UV-absorbing impurities or in nonuniform distribution of stabilizers. Aliphatic-type polymers are particularly prone to effects of nonuniform distribution of impurities since only the impurities are capable of absorbing terrestrial solar radiation. Acceleration over real-time weathering is realized by continuous exposure to defined conditions, uninterrupted by diurnal and seasonal cycles or variations in local weather conditions. Further acceleration can be achieved by intensification of irradiance, temperature, and moisture. However, the tests are only useful if they simulate the effects of outdoor exposure along with accelerating the rate of degradation. The validity of the tests requires that the chemical and physical changes induced in materials and the stability rankings of materials are representative of the effects of the weather. For this reason, the relative intensities of the stress factors that influence the degradation process must be similar to those that exist in the natural environment so that the complex interactions of their effects are reproduced.

The single most important consideration when conducting laboratory-accelerated weathering tests is the spectral power distribution (SPD) of the radiation source. Both the absorption of light, which is a prerequisite to degradation, and bond breakage, the primary photochemical step following absorption of light, are wavelength dependent. The wavelengths absorbed and the amount of radiation absorbed at each wavelength are dependent on the relation between the absorption properties of the material and the SPD of the light source. Therefore, matching the SPD of terrestrial solar radiation, that is, the incident wavelengths and their relative intensities, is critical to reproducing the degradation caused by exposure to natural weather conditions. If the spectral emission properties of the accelerated test source differ from that of solar radiation in the UV and visible regions, the mechanism and type of degradation as well as the stability ranking of materials can be distorted compared with the effects of the natural environment. A close match to the short wavelength cut-on of solar radiation is essential, particularly when testing polymers such as polycarbonate or isophthalate-based esters and others that are very sensitive to small changes in radiation in this region. It is now an established fact that differences in the ratio of long- to short-wavelength UV irradiance can alter the mechanism of degradation of many aromatic-type polymers, as well as the stability ranking of these materials. It can also distort the antagonistic reactions produced in some polymers by these spectral regions (see Section 8.6.2). Misleading information is often obtained when the full range of actinic solar UV and visible radiation is not adequately reproduced in laboratory-accelerated tests [125]. Nevertheless, the SPD of some of the light sources commonly used in artificial weathering devices has very little resemblance to the SPD of hemispherical solar radiation on Earth's surface, referred to as "daylight."

Due to differences in mechanism of degradation with initiating wavelengths, the effectiveness of light stabilizers that work by interfering with the mechanism of degradation will depend on the light source used for testing it. For example, hindered amine stabilizers (HALS) have been shown [48] to protect polycarbonate and aromatic polyurethanes only against photooxidation by long-wavelength UV. These stabilizers are ineffective against the photo-Fries reactions promoted by short wavelengths. Therefore, their effectiveness in protecting these polymers against solar radiation cannot be reliably determined using a light source that has shorter wavelength radiation than sunlight.

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