ASTM G90-17

4.1 Results obtained from this practice can be used to compare the relative durability of materials subjected to the specific test cycle used. Results obtained from this practice can be considered as representative of natural weathering only when the degree of comparative performance has been established for the specific materials being tested.

4.2 The acceleration factor relating the rate of degradation in this accelerated exposure to the rate of degradation in a natural weathering exposure varies with the type and formulation of the material. Each material and formulation may respond differently to the increased level of irradiance and differences in temperature and humidity. Thus an acceleration factor determined for one material may not be applicable to other materials. For this reason, the use of a single acceleration factor is not recommended. Also, a different acceleration factor may be obtained by using different mirror types and configurations. Because of variability in test results for both accelerated and natural weathering exposures, results from a sufficient number of tests must be obtained to determine an acceleration factor for a material. Further, the acceleration factor is applicable to only one exposure location because results from natural weathering will vary due to seasonal or annual differences in climatic factors.

4.3 The relative durability of materials determined by this practice can be used to determine the relative durability of the materials exposed under natural weathering conditions provided the materials have similar acceleration factors. However, even if results from a specific accelerated test condition are found to be useful for comparing the durability of materials exposed in a particular exterior location, it cannot be assumed that they will be useful for determining the relative durability for a different location. The relative durability of materials in natural weathering exposure can be very different depending on the location of the exposure because of differences in important climatic factors, such as sunlight, time of wetness, temperature, pollutants, etc.

4.4 Variations in results may be expected when operating conditions vary within the limits of this practice. For example, there can be large differences in the amount of degradation in a single material between separate, although supposedly identical, exposures carried out for the same duration or number of exposure cycles. This practice is best used to compare the relative performance of materials tested at the same time in the same fresnel reflector device. Because of possible variability between the same type of exposure device and variability in irradiance, temperature and moisture levels at different times, comparing the amount of degradation in materials exposed for the same duration or radiant energy at separate times is not recommended.

4.5 This practice should not be used to establish a “pass/fail” approval of materials after a specific period of exposure unless performance comparisons are made relative to a control material exposed simultaneously. It is strongly recommended that at least one control test specimen be exposed with each test. It is preferable to use two control test specimens, one with relatively good durability and one with relatively poor durability. Alternatively, the variability in the test can be defined so that statistically significant pass/fail judgements can be made.

4.6 The use of at least three replicates of each control test specimen and each material being evaluated is recommended. Consult Guide G169 for performing statistical analysis.

1.1 Fresnel-reflecting concentrators using the sun as source are utilized in the accelerated outdoor exposure testing of materials.

1.2 This practice covers a procedure for performing accelerated outdoor exposure testing of materials using a Fresnel-reflector accelerated outdoor weathering test machine. The apparatus (see Fig. 1 and Fig. 2) and guidelines are described herein to minimize the variables encountered during outdoor accelerated exposure testing.