Significance and Use
5.1 As with any accelerated test, the increase in rate of weathering compared to in-service exposure is material dependent. Therefore, no single acceleration factor can be used to relate two different types of outdoor weathering exposures. The weather resistance rankings of coatings provided by these two procedures may not agree when coatings differing in composition are compared. These two procedures should not be used interchangeably.
5.2 The procedures described in this practice are designed to provide greater degradation rates of coatings than those provided by fixed-angle, open-rack, outdoor exposure racks. For many products, fixed angle exposures will produce higher degradation rates than the normal end use of the material.
5.2.1 The use of Procedure A (Black Box) instead of an open-rack direct exposure is a more realistic test for materials with higher temperature end use service conditions. For many coatings, this procedure provides greater rates of degradation than those provided by 5°, equator-facing, open-rack exposures because the black box produces higher specimen temperatures during irradiation by daylight and longer time of specimen wetness. The black box specimen temperatures are comparable to those encountered on the hoods, roofs, and deck lids of automobiles parked in sunlight. The relative rates of gloss loss and color change produced in some automotive coatings by exposures in accordance with Procedure A are given in ASTM STP 781.
5.2.2 The acceleration of degradation by weathering as described in Procedure C is produced by reflecting sunlight from ten mirrors onto an air-cooled specimen area. Approximately 1400 MJ/m2 of ultraviolet radiant exposure (295 to 385 nm) is received over a typical one-year period when samples are exposed on these devices in a central Arizona climate. This compares with approximately 333 MJ/m2 of ultraviolet radiant exposure from a central Arizona at-latitude exposure and 280 MJ/m2 of ultraviolet radiant exposure from a southern Florida at-latitude exposure over an equivalent time period. However, the test described by Procedure C reflects only direct beam radiation onto test specimens. The reflected direct beam of sunlight contains a lower percentage of short wavelength ultraviolet radiation than global daylight because short wavelength ultraviolet is more easily scattered by the atmosphere, and because mirrors are typically less efficient at shorter ultraviolet wavelengths. Ultraviolet radiant exposure levels should not be used to compute acceleration factors since acceleration is material dependent.
5.3 The weather resistance of coatings in outdoor use can be very different depending on the geographic location of the exposure because of differences in ultraviolet (UV) radiation, time of wetness, temperature, pollutants, and other factors. Therefore, it cannot be assumed that results from one exposure in a single location will be useful for determining relative weather resistance in a different location. Exposures in several locations with different climates that represent a broad range of anticipated service conditions are recommended to determine weathering resistance and/or service life.
5.4 Because of year-to-year climatological variations, results from a single exposure test cannot be used to predict the absolute rate at which a material degrades.
Note 3: Three or more years of repeat exposures, starting at various times of the year, are typically needed to get an “average” test result for a given location.
5.4.1 The degradation profile for many coatings is not a linear function of exposure time or radiant exposure. When short exposures are used as indications of weather resistance, the results obtained may not be representative of those from longer exposures.
Note 4: Guide provides information for addressing variability in exposure testing of nonmetallic materials. Guide provides information for applying statistics to exposure test results.
5.5 It is recommended that at least one control material be part of any exposure evaluation. Control materials are used for comparing the performance of the test materials relative to the controls when materials are not being ranked against one another. The control material used should be of similar composition and construction to the test materials and be of known weather resistance. It is preferable to use two control materials, one with relatively good weather resistance and one with poor weather resistance.
1.1 This practice covers two accelerated outdoor exposure procedures for evaluating the exterior weather resistance of coatings applied to substrates.
1.2 The two procedures are as follows:
1.2.1 Procedure A—Black Box Exposure.
1.2.2 Procedure C—Fresnel Reflector Rack Exposure.
Note 1: Procedure B described a Heated Black Box procedure that is no longer in common use and has been removed as of the 2014 revision of this standard.
1.3 This standard does not cover all the procedures that are available to the user for accelerating the outdoor exposure of coatings. Other procedures have been used in order to provide a particular effect; however, the two procedures described here are widely used.
1.4 The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each system are not necessarily exact equivalents; therefore, to ensure conformance with the standard, each system shall be used independently of the other, and values from the two systems shall not be combined.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.