Meaning and use
4.1 Activation spectrum identification of the spectral regions of specific exposure sources used, which may be the primary cause of changes in the appearance and/or physical properties of the material.

4.2 Spectroscopic techniques use a prism or grating spectrometer to determine the effect on the material of an isolated narrow spectral band of a light source, each band in the absence of other wavelengths.

4.3 The sharp-cut filter technique uses a specially designed set of sharp-cut UV/visible transmission glass filters to determine the relative actinic effects of each spectral band of a source when simultaneously exposed to a longer wavelength than the target spectral band.

4.4 Both spectroscopy and filtering techniques provide activation spectra, but they differ in several ways:

4.4.1 Spectral technology generally provides better resolution because it determines the impact of the narrower spectral portion of the light source, rather than filter technology.

4.4.2 Filtration techniques are more representative of polychromatic radiation, and samples are usually exposed to different, sometimes antagonistic, photochemical processes, often occurring simultaneously. However, since the filters only transmit wavelengths longer than the cutoff wavelength of each filter, the antagonistic process of wavelengths shorter than the cutoff wavelength is eliminated.

ASTM G178-2016 Standard Practice for Determination of Material Activation Spectra (Sensitivity to Exposed Source Wavelength) Using Long Pass Filters or Spectroscopic Techniques

4.4.3 In filter technology, a separate sample is used to determine the effect of the spectral band, and the sample is large enough to measure mechanical and optical changes. In spectroscopic techniques, a single small sample is used to determine the relative effects of all spectral bands, except for spectrometers as large as Okazaki Type (1) 3. Thus, property changes are limited to those changes that can be measured on a very small cross-section of the specimen.

4.5 The information provided by activation spectroscopy about the spectral region of the light source responsible for degradation can theoretically be used for stability and stability testing of polymer materials (2).

4.5.1 Light shielding requirements are determined based on the activation spectrum of the unstable material exposed to solar radiation, thus determining the type of UV absorber for better shielding protection. The closer the absorption spectrum of the UV absorber is to the activation spectrum of the material, the better the screening effect is. However, a good match between the UV absorption spectrum and the activation spectrum of the UV absorber does not necessarily guarantee adequate protection, as it is not a criterion for selecting an effective UV absorber. Factors such as dispersion, compatibility, and migration can have a significant impact on the effectiveness of UV absorbers (see note 3). Activation spectra are determined using a light source that simulates the spectral power distribution of the light source to which the material will be exposed under conditions of use.

Note 3: In A study by ASTM G03.01, based on activation spectral predictions of copolyesters exposed to borosilicate glass to filter xenon arc radiation, UV absorber A would be superior to UV absorber B for outdoor use due to stronger absorption of harmful wavelengths of solar simulated radiation. However, both additives provide the same degree of protection to copolyesters when exposed to xenon arc radiation or outdoors.

ASTM G178-2016 Standard Practice for Determination of Material Activation Spectra (Sensitivity to Exposed Source Wavelength) Using Long Pass Filters or Spectroscopic Techniques

4.5.2 Comparing the activation spectra of a stable material to that of an unstable material provides information on the integrity of the screening and identifies any spectral regions that are not sufficiently screened.

4.5.3 Comparison of material activation spectra based on solar radiation with those based on exposure to other types of light sources provides useful information for selecting suitable manual test sources. The latter needs to be adequately matched to the harmful wavelengths of solar radiation to simulate the effects of outdoor exposure. The difference in wavelength between natural and artificial light sources that cause degradation may lead to different degradation mechanisms and types.

4.5.4 Published data suggest that better correlations can be obtained between natural weathering tests under different seasonal conditions when exposure time is timed only in terms of solar UV radiation exposure rather than total solar radiation exposure. Timing exposure based only on the UV portion of the material that is harmful as determined by the activation spectrum can further improve the correlation. However, while it is an improvement over the current temporal approach to exposure, it does not take into account the effects of moisture and temperature.

4.6 Over a longer test period, the activation spectrum will record the effects of different spectral power distributions caused by lamp or filter aging or daily or seasonal variations in solar radiation.

4.7 Theoretically, the activation spectrum may vary with the sample temperature. However, using the same type of radiation source, similar activation spectra were obtained at ambient temperatures (by spectroscopic technique) and at about 65°C (by filtration technique).

ASTM G178-2016 Standard Practice for Determination of Material Activation Spectra (Sensitivity to Exposed Source Wavelength) Using Long Pass Filters or Spectroscopic Techniques

Radius
1.1 This practice describes the relative actinic effects of the various spectral bands that determine the exposure source on the material. The activation spectrum is specific to the light source to which the material is exposed in order to obtain the activation spectrum. Light sources with different spectral power distributions will produce different activation spectra.

1.2 This practice describes two processes for determining the activation spectrum. One uses a sharp cut-off UV/visible transmission filter, and the other uses a spectrometer to determine the relative degradation caused by a single spectral region.

Note 1: Other techniques can be used to isolate the effects of the various spectral bands of the light source, such as interference filters.

1.3 This technique is suitable for the determination of spectral effects of solar radiation and laboratory accelerated test devices on materials. They are described for the ultraviolet region, but can be extended to the visible region using different cutoff filters and appropriate spectrometers.

1.4 These technologies are applicable to a variety of transparent and opaque materials, including plastics, paints, inks, textiles, etc.

1.5 Changes in the optical and/or physical properties of materials can be determined by various appropriate methods. Evaluation methods are beyond the scope of this practice.

1.6 This standard is not intended to address all safety concerns, if any, associated with its use. It is the responsibility of users of this standard to establish appropriate safety and health practices and to determine the applicability of regulatory restrictions prior to use.

Note 2: There is no ISO standard equivalent to this standard.