Significato e scopo
5.1 Come con qualsiasi test accelerato, the increase in weathering rate compared to in-service exposure depends on the material. Perciò, there is no acceleration factor that can be used to correlate two different types of outdoor weathering exposure. When comparing coatings with different compositions, the weatherability rankings of the coatings provided by these two programs may not be consistent. The two processes should not be used interchangeably.

5.2 The procedures described in this practice are designed to provide a higher degradation rate of coatings than those provided by fixed Angle, open, outdoor exposure racks. For many products, fixed-angle exposure will yield a higher degradation rate than the normal end use of the material.

5.2.1 For materials with higher temperature end-use conditions, using procedure A (black box) instead of open rack direct exposure is a more realistic test. For many coatings, this process provides degradation rates higher than 5° that would be provided by an open exposure facing the equator, as the black box produces higher sample temperatures and longer sample humidity times during daylight exposure. The temperature of the black box sample is comparable to that on the hood, roof and deck cover of a car parked in the sun. ASTM STP 781 gives the relative rates of gloss loss and color change resulting from exposure to procedure A in certain automotive coatings. four

5.2.2 The weathering accelerated degradation described in procedure C is generated by reflecting sunlight from ten mirrors onto the air-cooled sample area. Approximately 1400 MJ/m2 in the central Arizona climate received UV radiation exposure (295 to 385nm) during a typical year when the samples were exposed to these devices. In comparison, approximately 333 MJ/m2 central Arizona latitude exposure and 280 MJ/m2 Southern Florida latitude exposure to UV radiation during the same time period. Tuttavia, the test described in Procedure C only reflects the beam radiation onto the test sample. The reflected direct beam of sunlight contains a lower percentage of shortwave UV radiation than global daylight, because shortwave UV is more easily scattered by the atmosphere, and because mirrors are generally less efficient at shorter UV wavelengths. The level of UV radiation exposure should not be used to calculate the acceleration coefficient because the acceleration depends on the material.

5.3 Due to differences in ultraviolet (UV) radiazione, wet time, temperatura, contaminanti, e altri fattori, the weather resistance of coatings for outdoor use can vary considerably depending on the geographical location of exposure. Perciò, it cannot be assumed that the results of a single exposure at a single location will help determine the relative weatherability of different locations. Exposure is recommended at multiple locations with different climates that represent a wide range of expected use conditions to determine weather resistance and/or service life.

ASTM D 4141-2022 Darkbox ed esposizione solare concentrata per rivestimenti

5.4 Due to annual climate variability, the results of a single exposure test cannot be used to predict the absolute rate of material degradation.

Nota 3: Repeated exposures of three or more years, starting at different times of the year, are typically required to obtain “average” test results for a given location.

5.4.1 The degradation curves of many coatings are not linear functions of exposure time or radiation exposure. When short exposures are used as an indicator of weather resistance, the results obtained may not be representative of those obtained for long exposures.

Nota 4: Guidance G141 provides information to address variability in exposure testing for non-metallic materials. Guidance G169 provides information on applying statistics to exposure test results.

5.5 It is recommended that at least one control material be used in any exposure assessment. The control material is used to compare the performance of the test material relative to the control when the materials are not ranked against each other. The reference material used shall have a similar composition and structure to the test material and shall have known weather resistance. Two control materials are recommended, one with relatively good weather resistance and one with poor weather resistance.

ASTM D 4141-2022 Darkbox ed esposizione solare concentrata per rivestimenti

Ambito di applicazione
1.1 The practice covers two accelerated outdoor exposure procedures for assessing the external weather resistance of coatings applied to substrates.

1.2 The two processes are as follows:

1.2.1 ProceduraA — Black box Exposure.

1.2.2 Program C — Fresnel reflector rack exposure.

Nota 1: Procedure B describes the heating black box subprocedure, which is no longer in common use and has been removed from the 2014 revision of this standard.

1.3 This standard does not cover all procedures that can be used by users to accelerate the outdoor exposure of coatings. Other procedures are used to provide specific effects; Tuttavia, the two processes described here are widely used.

1.4 I valori espressi in unità SI o unità libbra di inchiostro devono essere trattati separatamente come valori standard. I valori specificati in ciascun sistema non sono necessariamente gli stessi; Perciò, per garantire il rispetto degli standard, ciascun sistema dovrebbe essere utilizzato indipendentemente l'uno dall'altro e i valori dei due sistemi non dovrebbero essere combinati.

ASTM D 4141-2022 Darkbox ed esposizione solare concentrata per rivestimenti

1.5 Questo standard non è destinato ad affrontare tutti i problemi di sicurezza, se presente, legati al suo utilizzo. È responsabilità dell'utente di questo standard stabilire la sicurezza adeguata, pratiche sanitarie e ambientali e per determinare l'applicabilità delle restrizioni normative prima dell'uso.

1.6 La presente norma internazionale è stata sviluppata in conformità con i principi di standardizzazione riconosciuti a livello internazionale stabiliti nella Decisione sui principi per lo sviluppo di norme internazionali, Linee guida e raccomandazioni emesse dal Comitato sugli ostacoli tecnici al commercio dell'Organizzazione mondiale del commercio.