natural exposure testing of coatings

Prog. Polym. Sci. 25 (2000) 1337–1362

Accelerated and outdoor/natural exposure testing of coatings L.F.E. Jacques* DuPont Performance Coatings, Florida Weathering and Testing Laboratory, 3500 W. 9th Avenue, Hialeah, FL 33012, USA

Abstract This article reviews the current state of the art of weathering test design. It describes the primary environmental and procedural variables involved in the design of a weathering test. The relationship between the environmental variables and thecurrenttechniquesavailable toacceleratethe degradation process eitheroutdoorsorinthe laboratoryisdiscussed. Primaryenvironmental variables includelight,heat and moisture; proceduralvariablesinclude exposurecycle,exposure time and test initiation. Both laboratory accelerated and outdoor weathering procedures are addressed in the paper. Each of the variables is briefly examined and examples of practical solutions given. The most commonly used exposure racks, boxes and cabinets as well as laboratory accelerated testing devices and instrumentation for monitoring and reporting climatological data are described. 䉷 2000 Published by Elsevier Science Ltd. Keywords: Service life; Weathering; Testing; Environment; Durability; Photodegradation

Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1338 2. Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1338 3. Environmental variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1340 3.1. Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1341 3.2. Heat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1349 3.3. Moisture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1352 3.4. Air pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1353 3.5. Other environmental variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1355 4. Procedural variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1355 4.1. General test fixtures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1355 4.2. Selection of exposure angle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1355 4.3. Specimen mounting and special test fixtures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1356 4.4. Time of exposure and length of the test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1358 4.5. Specimen replication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1360 5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1360 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1360 * Tel.: ⫹1-305-822-0171; fax: ⫹1-305-447-4293. E-mail address: [email protected] (L.F.E. Jacques). 0079-6700/00/$ - see front matter 䉷 2000 Published by Elsevier Science Ltd. PII: S0 0 7 9 - 6 7 0 0 ( 0 0 ) 0 0 03 0 - 7

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1. Introduction Weathering is a science considered by many to be the exposure of polymeric materials to either a natural environment or a weathering chamber of some sort. In fact weathering is a much more complex topic and should be thought of as the process of sample production, measurement, test design, exposure and evaluation as a whole. The science of weathering is moving from low-technology exposure methods, to a complex maze of mathematical models, environmental monitoring and photodegradation mechanisms. Traditional weathering techniques involve the exposure of materials together with materials of known weathering performance as a comparative technique, or merely measuring the property changes of a material on exposure. Vast archives of data have been built based on these techniques. As the expected lifetime of a coating has increased, traditional methods of testing have become increasingly unwieldy in determining whether newly developed products are sufficiently durable. The time required to bring a newly developed product to the market is becoming too long, especially in today’s competitive environment. A second drawback to the traditional approach is that weathering is an inherently variable science. The weather is variable and thus materials exposed to it are exposed to a constantly changing environment. Burroughs [1] has published work based on climatological statistics concluding that the outlook is gloomy for any of the observed climate data to contain predictable cycles that could be useful in terms of influencing a decision in the service life prediction field. Both the exposure and measurement portions of a weathering test contribute to the variability of the end result. The state of the art in realistic modern weathering tests is made up of a combination of the traditional techniques and the newer service life prediction techniques. The result has been that there is an increased awareness of weathering as a whole science and an increase in appreciation of the influencing factors of weathering test design. This paper will examine the commonly cited variables, practical difficulties of handling these variables, and their influence on the test design. Practical considerations in the implementation of short term weathering tests will be addressed. The test design will be reviewed from a specimen exposure standpoint. The resulting service life prediction must be understood to be as precise as the measured data applied to the models, hence the need to understand the sources of variability.

2. Methodology Changes in the results expected from a weathering test have been fueled by the worldwide need to develop new ‘environmentally friendly’ products, and increased consumer expectation of quality and durability. As the coating lifetimes increase, the need to predict service life by weather testing is becoming the focus of many research groups and government organizations. It is interesting to review the approaches taken in the field of building materials testing. In this field, expected lifetimes may be greater than several decades, thus it is not even possible to plan a real time outdoor exposure test. The international Council for Building Research Studies and Documentation (CIB) sponsored a Working Commission W80 on Prediction of Service life of Building Materials and Components. The activities of the commission are linked to the RILEM Committee 140 TSL (International Union of Testing and Research Laboratories for Materials and Structures) on the prediction of service life of building material and components. The ensuing movement will be towards incorporation of the work onto an international standard throughout the International Standards Organization (ISO). In the United States new links are

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Fig. 1. Diagram to show critical components of standard test design.

being forged between the government and industry in the form of a consortium of companies. The consortium is coordinated and supported with research facilities and manpower, by the National Institute of Standards and Technology. New committees within ASTM (American Society of Testing and Materials) have been formed, such as ASTM G03.08 on the Service Life Prediction of Organic Materials. The focus of these new committees is the development of improved methodology for prediction of service life through weathering The initial work on the design of short-term test methods [2] outlined in a proposed approach test program design, to predict service life of a material. The proposal is summarized in Fig. 1. The testing program should include the following: (a) consideration of the factors which might affect durability; (b) accelerated testing to define the most significant factors causing degradation, and the mechanism of that degradation; (c) evaluation of the significant causes of degradation in the proposed end use environment; (d) construction of models and damage functions for the materials response to the environment based on the accelerated test results and the environmental measurement. The experimental variables involved in the test design arise from two sources, the inherent environmental variability, and the variability introduced by procedure. Within the environmental grouping there are two levels. Firstly, the primary level encompassing the three main factors, which cause weather degradation, light, heat and moisture. These primary factors can be used to describe the end use climate in a broad sense. For every weathering test, either in the laboratory or outdoors, at least one of the three factors plays a role in material degradation. At the secondary level are the factors that are influenced by specific local climatic conditions or special materials property requirements, e.g. stress caused by movement such as expansion and contraction of different substrates. Typically the end use environment is influenced by, but not exactly matched by the general climate, the influence of the secondary environmental variables. Preparation of a model to predict service life, or test the relative performance of materials must

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Fig. 2. Variables encountered in weathering tests.

then take into consideration both the primary and secondary conditions as necessary. Fig. 2. Shows a schematic diagram outlining the main test variables. Ultimately it is desirable to study the response of a material to the appropriate environmental variables by accelerated testing, describe the response mathematically and then substitute values for these variables with measurements of the end use climate of the material’s application. Outdoor or site testing may then be used to characterize the actual specimen exposure conditions, define realistic material degradation modes and ultimately verify the model or damage function. While this is the final goal, there are many practical influences that must be taken into consideration and compromises to be made.

3. Environmental variables The modeling process requires a detailed knowledge of the actual end use environment, and the test environment. Modern electronic data acquisition techniques allow such detailed data to be accumulated and enhance the suitability of the results for data manipulation purposes. Even though the response or a material to light heat and moisture can be effectively documented in the laboratory this response cannot be predicted for the end use environment unless the data applied to the relationship is truly representative of that environment. For practical purposes it is often necessary to substitute specific end use environment tests with several carefully selected general test sites and programs. In addition, the end use environment is itself variable and it should be recognized that the precision of the service life prediction is relative to this variability.

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Fig. 3. Eppley laboratories, precision spectral pyranometer.

Three facets of the primary environmental variables should be taken in to consideration: (a) measurement of the end use environment to produce data to substitute into a final model or estimate; (b) measurement of the test environment at the outdoor test site; (c) simulation and control of the environment in accelerated testing.

3.1. Light Sunlight, particularly the ultraviolet portion of the solar spectrum is very frequently responsible for the initiation of weathering degradation. A material must absorb light in order to affect that material. The shorter wavelengths possessing higher photon energies are more strongly absorbed in most polymeric materials, and have a greater potential to break chemical bonds in that material. In some cases such as with poly-vinyl chloride, which does not absorb short wavelength light, the absorption can occur through the impurities and additives present in the material. In the test environment knowledge on the distribution of the wavelengths present and their intensities is paramount. Sunlight varies in intensity and spectral energy distribution, with season, location and atmospheric effects such as aerosols or pollutants. Data from end use climates, test climates and accelerated tests must be recorded consistently, and expressed consistently in order to be applied to the development and application of a service life prediction model.

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Fig. 4. Eppley laboratories, total ultraviolet radiometer.

Unfortunately many of the different sources of information give data expressed in different units, older databases (in particular) give light information in Langleys. The use of SI units is recommended. To compare the quantities of energy present in different light sources in one system, or similar light sources in different systems it is necessary to select a single unit of energy in which all may be expressed. The International System of Units (SI) notation employed in thermochemical and energetics studies is the Joule (J). Radiant energy in joules per square meter can be derived from integrating the W m ⫺2 with time since 1 W ˆ 1J s⫺1 : In atmospheric exposure reporting the values of megajoules per square meter are used to express radiant energy quantities. Prior to the adoption of SI, the common unit of energy used in solar radiation measurement was the Langley. This unit is a measure of the incident thermodynamic energy and is defined as 1 Langley ˆ 1 cal cm⫺2 : Direct conversion from cal cm ⫺2 to MJ m ⫺2 nm ⫺1 is possible by multiplying by a factor of 0.4184. Conversions can only be made, provided the wavelengths measured can be assumed to be the total spectrum. It is important to note that the Langley can only be used to express energy for the total sunlight spectrum. The instrument most commonly used to continuously monitor radiant energy at weathering sites for total sunlight spectrum data is the pyranometer. This instrument is sensitive to wavelengths in the range of 285–3000 nm, which fully encompasses the sunlight spectrum (ultraviolet, visible and infrared range). The spectral response of a pyranometer generally corresponds to the full range of wavelengths in sunlight, but may be modified by covering the sensor with different filter glasses to select certain parts of the visible and infrared spectrum. A representative model of pyranometer is shown in Fig. 3. It is a standard practice to use an instrument that is sensitive to the total hemisphere of incoming radiation for weathering tests. This includes both the direct and the diffuse components of sunlight. Wavelength

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Fig. 5. Total solar energy recorded in Phoenix Arizona at different exposure angles.

distributions in the diffuse portion of sunlight may vary slightly from the wavelengths in the direct portion, due to selective reflection from cloud cover. By using this type of instrument the data recorded is as close as possible to that impinging on the sample surface. A pyrheliometer records only direct beam radiation and therefore is not usually used to monitor light for weathering with the exception of some special applications. Practices for the application of these instruments can be obtained from the World Meteorological Association [3]. In some circumstances it may be desirable to monitor a specific part of the spectrum, for example the ultraviolet wavelengths. Ultraviolet light is known to be the most damaging to paint and coating systems, due to its higher photon energy and may therefore be of more interest. A typical Total Ultraviolet Radiometer is shown in Fig. 4, and can be used to monitor wavelengths from 300 to 385 nm. Research groups are currently moving towards the monitoring of the very short wavelengths [4] based on the philosophy that not only are these wavelengths in general, the most destructive, they are also indicative of the variability of the most damaging wavelengths in the sunlight spectrum. Specialized equipment is necessary to continuously monitor these short wavelengths. Even though the TUVR (Total ultraviolet Radiometer) is sensitive in the 295 nm range, the signal representing these wavelengths is very small in relation to the signal representing the much larger quantity of longer wavelength ultraviolet. Therefore any fluctuation in the very short wavelengths is masked by the response of the instrument to the total bandwidth of the incident sunlight. The best solution to the problem of selecting the required wavelengths is to monitor wavelength by wavelength on a continuous basis. Such an instrument has been used by Correll [5] for long-term studies of spectral ultraviolet-B radiation (290–315 nm. Zerlaut [6] recommends the monitoring of wavelengths according to the specific ultraviolet response of the material being investigated. This response is called the activation spectrum of a material and is defined as the photo degradation of a material plotted as a function of the wavelength [7,8] At outdoor exposure test stations it is of paramount importance that the radiant energy for the location is known. This information may be available through site monitoring as in the case of a commercial test facility or it may be available through a large organization such as the National Oceanographic and Atmospheric Administration, Government and military sources [9]. Comparison of solar data from different locations for weathering purposes requires a basic knowledge of the data collection conditions and instruments used. The technical explanations regarding the measurement of light and the variables involved will not be included in this text. If the reader is interested in a more detailed and theoretical approach then the text by Zerlaut [6] is very informative.

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Fig. 6. ACUVEX SM Fresnel reflector test machine.

When an outdoor test is planned, the angle of exposure is closely related to the amount of sunlight impinging on the surface. For this reason the exposure angle has a very profound effect on the test results. If sunlight is being monitored to provide radiant dose information for the test, then the sensor must be mounted at the same angle as the specimens. Fig. 5 shows data illustrating the effect of the exposure angle on the quantities of radiant energy received by specimens. The difference in sunlight energy deposited on the sample surface also varies seasonally as the sun changes its azimuth angle throughout the year. In the winter months the more vertical angles such as 45⬚ facing south, receive more sunlight energy since the sun passes closer to the horizon. In the summer months the angles closer to horizontal, such as 5⬚ receive the largest radiation doses since the sun’s azimuth angle at this time of year is much larger. Some test methods specify the angle to be used such as SAE J1976 [10], the standard for automobile coatings tests, requires a 5⬚ angle. There are additional methods that can be employed to increase the amount of solar radiation deposited on the surface of the test rack. The angle of the rack can be adjusted according to the time of year. A typical schedule for this adjustment is 5⬚ in the summer, latitude angle during April and September followed by 45⬚ during the winter months. Even higher levels of radiant energy can be achieved by using a rack that continuously tracks the sun during the day. A unique outdoor accelerated test that uses high levels of irradiance is the Fresnel Reflector Device

Fig. 7. Average monthly radiant dose for ACUVEX.

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Table 1 Basic variables to be considered for the use of solar data in a weathering test Select wavelengths to be monitored Placement of sensor Calibration Cosine response Data collection Behind glass

(a) Total spectrum; (b) broad band ultraviolet; (c) UVB; (d) selected wavelengths Sensor should be place at the same angle as the specimens, since different placement angles will give different amounts of radiation Calibration of the sensor to a standard reference source and Calibration of chart recorder or electronic data acquisition system Ensure the sensor being used has an accurate adherence to the Lambert Cosine law Determine interval for individual data readings in electronic systems Mount the sensor behind the same type of glass as the specimens, if the specimens are exposed behind glass

often known as Emmaqua 䉸. This test method is used for a variety of materials, and utilizes sunlight, concentrated by a Fresnel array of highly efficient reflectors onto the surface of the sample. The Fresnel concentrator is most efficiently used in a desert environment where it concentrates ultraviolet light to approximately 500% of its natural level. The Fresnel array contains 10 reflectors. The efficiency of the reflector results in approximately 500% increase in the ultraviolet region of the spectrum and at least 800% increase in the visible and longer wavelength regions. The entire device tracks the sun throughout the day in order to maintain the focus of the sun’s rays on the surface of the samples. The reflectors need to be washed daily in order to maintain the high level of reflection efficiency. The temperature can be controlled and spray cycles may be added to the test. Newer developments in equipment allow this type of test method to become more efficient. For example, the use of dual axis tracking and computer control to increase the amount of sunlight deposited on the specimens per unit time when compared to the more traditional devices that only employ single axis tracking [11] (such a device is shown in Fig. 6). Fig. 7 shows the average monthly radiant dose for the instrument illustrated in Fig. 6. The value of this type of test is in the use of concentrated sunlight to achieve acceleration thus ensuring that only wavelengths found in sunlight are present. The same light measurement parameters are of interest in laboratory accelerated testing. The wavelengths present and their intensities can be modified depending on the device used. In order to apply different accelerated testing stresses to the materials. The photodegredation of organic materials is a result of the absorbed radiant energy causing scission of chemical bonds and subsequent formation of reactive groups; the short wavelengths available in the UVB region can break bonds with higher energy than can the wavelengths found in sunlight, as shown in Table 1. Therefore the absorption of shortwavelengths that do not exist in sunlight and the degradation they produce may ultimately lead to alternative chemical degradation pathways. This may in turn lead to possible reversals of material performance rankings [12]. In addition, if there is a deficiency in a wavelength when compared to sunlight, a similar type of effect may occur. The philosophy behind the use of UVB Fluorescent lamps is to achieve greater acceleration, at a lower cost. Unfortunately, with this method, the risk of producing erroneous results is increased. If the materials’ mode of degradation is well understood fluorescent may be a suitable choice for, comparative types of weathering test where the goals are related to quality control procedures and consistency is being tested [1]. These types of quality control tests are quick and cheap with a knowledge of the material, climate and in-service performance being essential. Light sources available consist mainly of xenon arc, carbon arc and fluorescent. Metal halide

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Fig. 8. Fluorescent F40 UVB lamps vs Miami average optimum daylight.

and mercury lamps are also used, but their application is much less common. Fig. 8 shows a typical fluorescent lamp spectrum. For most applications the xenon arc spectrum modified with specific filters gives a good simulation of sunlight, such as the test practices outlined in ASTM G26 [13] and ASTM G155 [14] Figs. 9 and 10 show different modifications of the xenon arc spectrum and its relationship to the sunlight spectrum. Fig. 9 Shows the spectral power distribution obtained using xenon filtered through borosilicate inner and outer filters. Fig. 10 Shows the spectral power distribution obtained using xenon filtered through quartz inner and borosilicate outer filters When a light source is used that can be compared to sunlight, it is possible to consider radiant energy dosage and intensity as comparable to the data collected from the natural environment. As an example, the building code in Dade County Florida specifies the xenon arc for roofing materials; as a special protocol for ‘Accelerated Exposure for Roofing Materials Using A Controlled Irradiance Water Cooled Xenon Arc Apparatus’ [15]. The length of this test has been calculated according to environmental site measurements, with the radiant energy dosage used as the controlling factor.

Fig. 9. Xenon arc with borosilicate inner and outer filters vs miami, average optimum daylight.

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Fig. 10. Xenon arc, quartz inner and type S. Borosilicate outer filters vs Miami, average optimum daylight.

Two factors are of main importance when selecting a light source for accelerated testing. Firstly the spectrum should at best have a cut on at the same or very similar wavelength to that of the end use environment, since short wavelengths present in an artificial source can induce unnatural chemical degradation and provide an erroneous test result. These unnatural degradation processes may lead to performance reversals of different materials when comparing laboratory to outdoor exposures [16]. In addition it is also desirable to make sure that the full sunlight spectrum is simulated. Secondly, the intensity of the irradiance level should be controlled and maintained at the set level. The ‘average optimum daylight’ for Miami has been measured using a spectroradiometer. Measurements were made at periods throughout the day at the spring equinox, and the average data was taken. Only scans for a clear sky were used for the averaging process. If the Miami average optimum daylight curve is used as a baseline reference for sunlight, a setting of 0.35Wmsq.nm can be used to simulate sunlight intensity. In general for automotive coating tests such as SAE J1960 [17], a higher level of 0.55w.msq.nm is used. In addition to the use of a higher irradiance setting, SAE J 1960 also specifies the more severe xenon filter combination of quartz inner and borosilicate outer filters (see Fig. 11). This combination produces a spectrum that includes some wavelengths that are shorter than those found in sunlight and therefore are more damaging. The idea is to increase the acceleration rate of the test but the price paid for this acceleration rate is the increased risk that a totally different degradation pathway may occur that would never actually occur in the end use environment. Crewdson [16] has reported higher correlation to outdoor test results when the borosilicate inner and outer filter combination is used rather than the quartz inner and borosilicate outer filter system. Occasionally, the use of SAE J1960 is often modified by agreement between buyer and seller when qualification testing is being performed. Acceleration of photodegradation has been achieved by even further increase in the irradiance level, however it has been found that this acceleration technique may also lead to unnatural chemical degradation if the level is too high [18,19]. The Japanese automobile industry has several requirements for irradiance levels to be set at three times the irradiance level of the US SAE J1960 specification at 1.65 W m ⫺2 nm ⫺1. This massive increase in irradiance level has been found to accelerate the degradation of some materials but not others. In most cases the threefold increase in irradiance level does not increase the acceleration rate by a factor of three [18]. Searle [20] has also reported that an increase in

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Fig. 11. Black panel as specified by ASTM. Resistance temperature detector (bolted to the front of the panel).

irradiance does not always produce the expected increase in acceleration. It has been proposed that this is the result of the ‘cage effect’ in which more radicals are formed by photo-initiated reactions than can be used up as part of the normal degradation pathway; therefore, excess radicals may recombine reducing the efficiency of the high irradiance exposure. In contrast, new research at NREL [21] (National Renewable Energy Laboratory, Denver, Colorado) has indicated that an ultra accelerated exposure test using light at an intensity of between 50 and 100 times that of natural sunlight can be used to achieve the degradation caused by 12 months exposure in a period of 3–10 days. This work requires adequate control of sample temperatures. The exposure equipment used is based on a modification of the NREL high flux solar furnace. Variability in test results from machine to machine, lab to lab and test run to test run, are strongly influenced by the precision of light source control. All lamps and their filters (if used) age with use [22,23]. The mechanisms of this aging process are reduced efficiency and solarization of the envelope and filters [23]. The weathering test must include certain protocols to ensure that the light source does not fluctuate. Both fluorescent and xenon weathering devices should have the capability of light source intensity and spectrum control. This can be achieved by lamp swapping routines where the light intensity is monitored; when the intensity drops below a certain level the lamps are changed in the case of fluorescent or the input power to the light source is increased in the case of xenon arcs. Modern xenon weathering devices utilize an automatic feedback loop so that as the intensity drops the input power is increased automatically adjusted until the desired set level of light intensity is achieved. Periodic calibration of the light monitoring system is required. The best wavelength at which to monitor the xenon lamp intensity has been a subject of debate, In general either 340 nm is used or broad band ultraviolet is used. Both types of monitoring are effective in keeping the light source at a stable level. Scott [23] has reported that the system of monitoring at 340 nm is more effective in maintaining the spectral output of the xenon arc lamp than the broad band method.

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3.2. Heat At first glance it appears that heat is a less complex test parameter than light. The same types of variation apply to the environmental monitoring of heat, in that it varies with location, season and atmospheric effects such as sudden changes form rain or snow. NOAA (National Oceanographic and Atmospheric Administration, in USA) provides specifications for monitoring ambient air temperature that are designed to promote consistency between measurements for site to site and season comparisons. Specified instrument housings are all wooden boxes with louvered sides, a double roof and a slotted bottom. The boxes must be supported at 48 in. above the ground. The box construction is designed to ensure that the sensor to be used is screened from sunlight, and has a free circulation of air around it. These types of structures are also used for housing humidity sensors intended to measure ambient climatic humidity. Most data available for different locations is based on ambient air temperature, however it is necessary to verify method of recording, in order to use the data on an accurate basis. The measurement of ambient temperature can be used as a climatic descriptor, however ambient temperature is not coincidental with specimen surface temperature. Weathering is mainly a surface effect; therefore specimen surface temperature is more influential on the weathering properties of the material than the ambient temperature. If precise data is needed then the specimen surface temperature should be monitored. The radiation source has a profound heating effect on the surface temperature of a sample. The precise effect of radiation on a sample depends on the source itself, and the specific absorbtive and conductive properties of the sample. Thus, each different material is unique in its response to weathering. Kockott [24] defines the following equation as a first approximation to defining the absorbed radiation contributing to the heating of the specimen. Z Eabs ˆ El e…l† dl where e (l ) is the absorbance and El the thermal conductivity. Thus the theoretical ideal black body has absorptance of 100%, and the theoretical ideal white body has absorbtance of 0%. Saunders and Martin have developed techniques for predicting the surface temperature of a coating based on physical parameters of the coating, and additional solar and atmospheric information [25]. Work of this type may reduce the need to monitor the actual specimen surface, provided other information is available. The ‘Black Panel Temperature’ is a frequently reported parameter. This measurement is important since it theoretically represents the maximum surface temperature achieved during exposure; in fact this is not always the case. Different black coatings have different absorption coefficients and therefore are affected uniquely by solar energy as described above. In addition the substrate and mounting of the coating and sensor affect the temperature. When the black panel temperature is measured a specified instrument must be used if the data is to be considered comparable. At this time no specified outdoor weathering black panel exists, thus it is difficult to compare weathering tests done at different laboratories. A standard is currently underdevelopment by ASTM to define the requirements for black panel sensors in outdoor weathering tests. The most commonly used outdoor black panel sensor is a black coated steel panel with a thermocouple fixed to either the front or the back of the panel. Both of these designs have drawbacks. The back mounted thermocouple does not strictly measure the surface temperature. If the thermocouple is mounted on the front, of the panel the measured temperature is

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Table 2 Black panel temperatures in ⬚C, recorded in Miami for June 1998 Day

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 Average Total YTD Total

Ambient temperature

Black panel temperature

Relative Humidity

Wet time

Max

Avg

Max

Avg

Max

Avg

Hours

34 35 36 36 35 36 36 33 32 35 35 35 36 36 36 36 36 36 36 35 37 35 35 35 35 34 34 35 34 35

28 29 29 29 30 30 30 28 28 29 28 29 30 29 30 30 30 29 29 29 28 27 30 28 28 26 26 27 26 28

53 49 27 54 54 55 52 45 45 55 55 55 52 51 51 56 55 56 58 54 54 54 51 51 53 51 50 56 56 54

31 31 32 32 32 33 33 30 30 32 31 32 32 32 33 33 32 32 32 31 32 31 33 31 31 28 28 29 28 32

100 100 100 100 100 100 98 98 99 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100

81 79 79 80 77 76 71 80 80 79 80 77 76 74 72 75 78 78 78 80 79 83 78 82 85 87 88 88 88 79

11.7 12.4 10.8 12.4 10.0 11.3 10.5 10.8 7.5 12.8 13.5 11.8 8.9 9.9 9.6 7.8 9.7 2.7 8.9 5.2 13.2 10.2 9.2 11.5 14.9 16.3 16.3 19.0 19.2 9.5

35 N/A N/A

29 N/A N/A

52 N/A N/A

31 N/A N/A

100 N/A N/A

80 N/A N/A

11.3 337.5 2137.0

affected by the epoxy used for attachment. Some black panels are constructed using resistance temperature detectors, such as those used for accelerated weathering. Because solar energy has a great deal of influence on black panel temperature, it is possible to obtain different temperatures depending on the orientation of identical black panel sensors. For this reason attention should be paid to the orientation of a specimen on exposure outdoors, and in the end use environment. Table 2 shows black panel temperature measured in Florida, using an non-insulated black panel mounted at 45⬚ facing South. The benefit of using the black panel temperature as an outdoor measurement is that it is more realistic as a worst case specimen condition. If measured correctly the data can be

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Fig. 12. European standard black panel. The sensor is mounted behind the black panel, which is (in turn) mounted on an insulating plastic plate.

used to compare exposure sites and different times of year. Some weathering test methods require measurement of both black panel and white panel. In addition, this test parameter can be related to a commonly used accelerated test setting for a weathering device. Two dominant types of black panel sensor are currently in use for accelerated testing and are primarily designed for use in a Weather-Ometer 䉸, but can be easily used to monitor the outdoor environment to obtain comparative data. One is mounted on a thermally insulating plastic material, and the second is mounted with no backing. As expected the insulated panel produces higher temperature measurements than the non-insulated panel under the same radiant energy source. The black panel is merely a metal panel, painted with a specified type of black coating. On the insulated panel a platinum resistance temperature detector is mounted on the rear surface of the panel between the panel and the insulating material. On the non-insulated panel the sensor is mounted on the front and coated with the same specified black coating. Figs. 11 and 12 illustrate the black panel thermometers. The primary function of the black panel in a weathering device is to be used to control the temperature in that device. This gives the ability to set instrument temperature conditions based on the outdoor black panel measurements which more realistically describe the specimen condition than if the ambient temperature were used. In some test methods which use precise environmental control, both the ambient temperature and black panel temperature are controlled. It should be noted that the function of the black panel in a Weather-Ometer 䉸 is to ensure control of the instrument, and is not used to accurately represent the surface temperature of the individual specimens. The actual specimen temperatures are a function of the artificial light source used the machine control settings and the physical properties of the specimens themselves. Fisher [26] has reported that there are differences in surface temperatures depending on the

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light source used within the weathering device, and the intensity setting of that source. Different colors achieve different surface temperatures and the relationship between colors can be distorted in comparison to outdoor measurement if unrealistic light sources or intensities are used. 3.3. Moisture The presence of water may be necessary for some degradation reactions to occur, depending on the material being tested. For those materials that require the presence of water it is an extremely important environmental parameter. Water causes damage to materials by both physical and chemical processes. Absorption of water into a material, from humid air or surface deposition causes a volume expansion causing stresses within the material. A wet period followed by a dry period then causes a volume contraction of the surface layers setting up more stresses within the material. Cycling of such stresses can eventually result in fatigue, setting the stage for further chemical and mechanical change or degradation. An example of the role of water in chemical decomposition is the chalking of TiO2 pigmented coating and synthetic materials. It has now been long known that TiO2 pigmented systems chalk, i.e. the binding material or the plastic materials are decomposed under influence of the weather and the TiO2 pigment particles are set free on the surface where they create a dull layer that can be easily wiped off. It has been observed that the chalking is strongest where more water is available on the surface. No chalking can be observed in dry atmospheres. The individual photochemical process was discovered in detail several years ago [27]. An electron hole pair is created in the TiO2 lattice through radiation by short wavelength light. The electron hole pair immediately continues to react with the hydroxide groups present on the surface and the Ii ions. A hydroxide and a perhydroxy radical are formed through the conversion of an oxygen and a water molecule, whereby the TiO2 surface again assumes the hydratized (original) form, the so-called chalking cycle. …Ti4⫹ ·OH⫺ † ⫹ O2 ⫹ H2 O ) …Ti4⫹ ·OH⫺ † ⫹ OH ⫹ HO2 The OH and HO2 radicals cause oxidation decomposition of the organic binding material or the synthetic material, which then leads to the exposure of the TiO2 pigment particles, causing the chalking effect. Water may be delivered to the surface of a specimen in several forms; water represented as relative humidity; rainwater; water formed at the surface of the specimen as dew or condensation. Ambient relative humidity can be monitored using the same standard specified NOAA box used for monitoring ambient temperature. In the past a hair hygrometer was used to measure ambient humidity. Nowadays humidity probes are offered, even combination temperature and humidity probes are available, which produce a signal that can be easily used by electronic data acquisition systems. Other useful parameters include the hours of wetness; this measurement gives data denoting he number of hours wherein water is present on a surface. The water may be present due to rainfall, dew formation or condensation due to other factors, but it is useful as an indicator of the total time of wetness to which a specimen is exposed. Sensors are also available which measure the number of hours of rainfall alone, and as combination of time of rain and total time of wetness give number of hours the specimen has been exposed to dew. The Sereda meter one of the most well know devices for measuring time of wetness and was developed by Sereda at the National Research council of Canada [28]. The Sereda meter is specified by ASTM G-84 [16] for use in the measurement of time of wetness of surfaces, a detailed description of its use can be found therein. In view of the information that South

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Fig. 13. Hours of surface wetness recorded in Miami.

Florida Data shows an unusually high number of hours of wetness, it is interesting to note that the South Florida environment is specified by ASTM as a standard region for outdoor weathering and is specified by many industries as a weather testing site. Fig. 13 shows the number of hours of wetness for southern Florida. The actual time of wetness of a specimen surface varies from specimen to specimen due to both the absorptive properties of the specimen, and its color and orientation. Darker colored specimens attain higher surface temperatures, which will have a drying effect and may reduce the wet time compared to light colored specimens of the same construction. Taking the effect of surface temperature a step further, note that the specimen mounting and rack angle may also affect he surface temperature and therefore the time of wetness. In accelerated weathering water is applied to the panel in similar processes. Relative humidity should be controlled at the appropriate level. For example in order to simulate Miami Florida the relative humidity should be set at 98% when the lamp is off, and as high as possible when the lamp is on. Spray cycles may be used to simulate rainfall, and condensation my also be possible depending on the desired simulation. Many accelerated test results are considered variable because relative humidity is not controlled during the exposure. Of course the resulting chamber humidity varies with the ambient laboratory humidity which is not always repeated from one laboratory to another. Thus for interlaboratory comparisons and repeatability testing when the humidity is a critical degradation factor, it must be controlled within the device. 3.4. Air pollutants It is not realistic to assume that air contains only N2, O2, CO2, H2O and inert gases. There are sources of natural pollution, and of course man made pollution that result in other gases being present in the atmosphere. Generally sources of pollution that are significant are CO3, SO2, NOx, HCL, HF and O3. Other pollutants may be formed from these in the atmosphere such as H2SO4, HNO3, etc. [29,30]. Important factors to be considered are (a) the availability of the pollutant; (b) bulk transportation of the pollutant to a surface;

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(c) interaction of the pollutant with the surface; (d) the action of other climatic pollutants and other environmental factors; (e) the susceptibility of the material to the pollutant. Dry and wet depositions are the processes for bulk transportation of the pollutant to a surface. The deposition processes can limit the atmospheric lifetime of a pollutant, limit the atmospheric concentration and affect the distance traveled before deposition. When gaseous and particulate species accumulate directly on a surface this is called dry deposition. Wet deposition is when the pollutants are incorporated into a cloud droplet form and then deposited as falling precipitation. Two of the main factors in pollutant transportation are the gas concentration and wind velocity. A combination of these factors determines the presentation rate to the material surface; e.g. assuming an average wind velocity of 2 m s ⫺1 and a SO2 concentration of 60mg ⫺3 m, the presentation rate to a sample area of 1 cm 2 can be calculated as shown below.

Presentation rate ˆ

…Wind velocity × concentration† …60 mg m4 × 2 m s⫺1 † ˆ 104 104

ˆ 0:012 mg cm⫺3 s⫺1 ˆ 1:2 × 10⫺5 mg cm⫺3 s⫺1 The physico-chemical processes relating to the pollutant/surface interface most probably involve the diffusion of the pollutants to the material surface, the formation of a product and additional diffusion through the product layer. For example, the SO2 deposition on to a dry calcareous stone is dependent on humidity but is also an order of magnitude smaller than deposition onto a wet surface [31,32]. The cause of this is probably the transportation of the soluble product away from reaction sites leaving a ‘fresh’ surface for further reaction. This indicates the importance of time of wetness when formulating models and damage functions for material degradation. The time of wetness has to measured, or calculated from the frequency and duration of actual rain events. Using a substitute such as a critical humidity value for a given length of time is not a valid approach. In the presence of multiple pollutants, synergistic interactions may occur. For example, SO2 and NO2 degrade calcareous materials individually but when combined the effect is greater than the sum of the individual degradation rates [33]. Another example is the effect of SO2 and O3 on calcareous materials where a linear relationship between degradation rate and the concentration of the two pollutants was found. The ozone would not be expected to have a direct effect, but if the damage function was extrapolated to low SO2 environments inaccurate results will be obtained indicating that degradation could be attributed just to O3. Care must be taken in relating environmental data directly to degradation rates when the mechanisms are not fully understood. A striking example of the effect of multiple pollutants is that of acid etch on automotive coatings. This is known to be caused by atmospheric acid precipitations such as acid rain, fog, dew and acid dust. ‘Environmental etch damage’ is manifested as microscopic blisters, material erosion, clearcoat cracking, and non-removable water spots [34]. This etching has been shown to be the result of a combination of acidic atmospheric pollution and ultraviolet light. Work carried out to develop accelerated test methods for this type of weathering effect illustrate the varying effects of different pollutants and concentrations, and the synergistic effect of ultraviolet radiation.

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Table 3 Other environmental factors Biological attack, such as fungus, algae, damage from insects etc. End use of Material such as weight of building components, movement creating tensile and compression stress etc. Physical Factors such as erosion

3.5. Other environmental variables Many climates and materials applications include other degradation forces such as specific biological factors and physical stresses, but these are not exclusive to every climate or material. Hence their influences on a material may need to be studied separately, but should not be ignored. For example, the degradation of rubber by ozone only occurs when a tensile stress is applied [35]. The proposed end use environment of the material should be evaluated in terms of any potential problems to which the material is susceptible. Table 3 gives a listing of such factors. It is very typical to expose building sealant materials under stress. Various methods are used to adjust the amount of stress the sealant is subjected to in a cyclic manner, while on exposure. Typically the sealant is applied between two appropriate substrates, then after curing is placed under stress by inserting specific size shims between the sandwich of the two substrates. Different size shims can be used to vary the amount of stress. Such a test has been reported by Marechal [36] who reports the use of this type of weathering as a means to develop a service life prediction model. 4. Procedural variables 4.1. General test fixtures ASTM G07 [37] gives information on specified test racks where test fixtures may be constructed of any material which will not interfere with the test, and which is suitable for the geographical area in which they will be used. It is usually necessary to ensure the entire rack surface is at the correct angle by using an inclinometer, and a level. The distance above ground for the lowest section of the rack is dependent on the location. However the test specimens should be mounted at a sufficient height to avoid contact with vegetation and to prevent damage which might occur during area maintenance. The area surrounding the test site should be free of objects likely to shade the specimens during exposure. Ground cover in the immediate vicinity of the racks must be representative of the location; Gravel for desert areas (to reduce abrasion caused by blowing sand), and low-cut grass for most other areas. Roof top exposure setups are excluded from ground cover requirements. 4.2. Selection of exposure angle Selection of the most appropriate exposure angle is a common problem in designing an outdoor weathering test. Theoretically there are many different angles to choose from, ranging from 0⬚ (horizontal) to 90⬚ (vertical) facing South, North or any other direction. Test racks normally face towards the sun in order to achieve the maximum solar effect. Angles of exposure which face away from the sun

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are generally used when this simulation is more appropriate or when exposure to sunlight hinders the particular effect of weathering that is under investigation. Frequently more algal growth and other fungal growth is found on test racks which do not face the sun. This may be due to a combination of factors, the micro-organisms may simply have an intolerance to extreme exposure to sunlight or may have an intolerance to the higher temperatures and reduced wet times of a sun facing test surface. There are some angles which are traditionally used such as 45⬚ facing South for general testing, 5⬚ facing South for many automobile coatings and 90⬚ facing South for many building products such as window frames or siding material. There are some general characteristics to remember when selecting the test angle. The first is that the end use of the product or material should be kept in mind, and this is true of many of the other exposure variables such as specimen mounting. If a material is to be used as a covering or decoration for the sides of buildings the most realistic test result will be obtained at 90⬚. This does not necessarily mean that the fastest test result can be obtained at 90⬚ facing South. The speed of the test is determined by the combined effect of the environmental variables that can be enhanced or decreased by adjusting the exposure angle. The most dramatic effect of exposure angle can be seen when the amount of sunlight incident on the surface is measured at different angles (refer to Fig. 5) If it is desired to increase the amount of sunlight deposited on the specimen surface the rack should be positioned at a normal incidence to the sun at all times. This is achieved through the use of a motorized solar tracking rack. Thus by adjusting the exposure angle the amount of sunlight exposure can be altered, however it must be kept in mind that the amount of sunlight may affect other environmental factors such as surface temperature and time of wetness. Increasing the time at which the sun’s angle is incident on a rack will increase the temperature. This increase of temperature, in turn, decreases the time of wetness. 4.3. Specimen mounting and special test fixtures The test environment can also be changed dramatically by changing the method of specimen mounting. A specimen which is exposed so that the air can flow freely both at the front and back side of the specimen will normally achieve a much lower surface temperature than a specimen that is exposed on top of a thermally insulating backing. Untreated plywood is commonly used a specimen backing material. To ensure a realistic test the end use of the material should be considered when a specific backing or absence of a backing is selected. For example, a coating may be used to cover siding material that is itself mounted in use with a backing, and is therefore more realistically exposed with a backing. Surface temperatures vary considerably with test fixture type as shown in Fig. 14. In some cases the samples may not be self-supporting, for example, coatings applied to rubber or flexible plastic substrates. In this case, if a lower temperature, open back exposure is required then the specimens are often mounted onto a backing of expanded aluminum. This substrate is a highly conductive non-corroding, open mesh to keep the temperature as low as possible, yet support the specimen. Many test racks are equipped with a flap that is used to cover the top 2 in. of the panel. Thus there is a masked area which does not get exposed to sunlight (see Fig. 15). It is important to note that even though this part of the panel has been exposed to heat and humidity, it may not be appropriate as an accurate experimental control. A special type of test fixture is called the ‘Black Box’, shown in Fig. 15. This is an empty box painted black and mounted as a normal rack fixture and is generally oriented at 5⬚ facing South. This type of rack is used in the test method ASTM D-4141 [38]. The specimens themselves form the top of the box. This

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Fig. 14. Surface temperatures of different colored automotive panels.

type of fixture is designed to induce high specimen temperatures as the air inside the box heats during the day. In general, on sunny days the black box temperature is not as high as that achieved on an insulated rack such as black painted plywood. The box contains an enclosed air space inside. The air in this space heats during the day, but is not directly affected by solar radiation thus its temperature fluctuates more slowly and increases more slowly than the surface temperature on the insulated rack. The black box

Fig. 15. Black box testing fixture.

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fixture also cools more slowly than the insulated rack since the air must cool inside the box thus the box will hold higher temperatures much later in the day than the insulated rack. Black boxes are used frequently for the testing of automotive coatings and are cited as an option in SAE J1976. For some applications testing is carried out in an under glass frame. Typical test methods outlining this procedure are ASTM G-24 [39] and AATCC 16-C [40] and ISO 877 [41], ISO 105-B01 [42]. This type of test is frequently used for automobile interior trim plastics and textiles, furnishing textiles, etc. Namely, it is used in applications where the end use environment is not that of direct weathering, such as draperies, or car seats. The samples are placed in a frame behind a layer of glass. The exposure area is decreased so that the edges of the frame are not used, since there is increased shading in this area. The main consideration in this type of testing is the glass used. ASTM and AATCC prescribe single strength window glass. There are vast differences in the UV transmission properties of different glasses. These are most profound in the 300–320 nm range. Ketola [43] reports up to 300% differences in degradation rate in materials that are sensitive in this waveband. The transmission properties of single strength window glass can vary significantly from lot to lot, and can also solarize on exposure. The major solarization changes occur in the first six months of exposure and then appear to stabilize [43,44]. Thus if pre-aged glass is used for the test, then this variable can be eliminated. It is recommended that if material comparisons are to be made from this type of exposure, that the materials be exposed simultaneously under the same glass. Variability in temperature may also arise from differences in exposure position on the test rack. Fischer has reported temperature and gloss differences between identical specimens arising from different exposure locations on the test rack [45]. In general Fischer reports that the center of the rack is insulated by the edges of the rack, thus heating the air above the rack center, which causes convection currents from the edge to the middle of the rack, thus increasing the temperature differential across the entire exposure area. It should be noted that these studies were carried out using an identical specimen color over the entire rack area, thus enhancing the uniformity of the effect. If different colors were used over the entire rack, the effect of color as the cause of different surface temperatures would interfere with the gradient and thus reduce the convection effect. In general, specimen rotation during outdoor exposure is not a common practice.

4.4. Time of exposure and length of the test The length of the test is usually a function of the goal of the experiment and the test parameter being measured. The test may be timed by a fixed calendar exposure, a fixed dose of radiation, or the amount of degradation measured. However; before test length can be discussed it is necessary to address the problem of ‘when to start the test’. In general, since the weather is incapable of repeating itself, then any outdoor exposure test is unique. In addition, the greater the seasonal differences at the test location the greater the variability due to exposures at different times of the year. If a true representation of a material’s durability is to be obtained, then exposure start times need to be staggered either randomly or periodically, according to a specific statistical test design. Allowing for multiple start dates and exposure periods is the only practical way to address seasonal or general environmental variation. The most commonly used method is the timing of exposure by fixed calendar increments. This does not necessarily mean that it is the best method to achieve the goal of the test. The disadvantage of timing the test by time increments is the variability of the test environment discussed above, no two-year period

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is the same as another two-year period. Thus with this method it is of paramount importance that at least the primary environmental parameters are monitored during the exposure. It is a common practice to expose materials until they have been subjected to a specific amount of sunlight. The underlying principle is that the length of exposure time will vary, and seasonal difference in sunlight will be minimized. For example, a much shorter exposure may be required during the summer, to achieve 1000 MJ m ⫺2 nm ⫺1 of total solar radiation than during the winter months. To some extent this technique offsets the seasonal solar radiation inequalities, but it should be noted that the spectral distribution of sunlight also changes during the year particularly in the short wavelengths. Hence, this technique would be more effective if sunlight were monitored in terms of the short wavelength ultraviolet in addition to total radiation. Allowing the test length to be governed by the amount of degradation is primarily used as part of a reliability analysis for service life prediction studies. The ability of the reliability technique to consider test variability as part of a formal distribution makes it a very valuable tool for weathering studies where data typically has a large spread. Using this technique specimens are exposed until they reach a certain predetermined amount of degradation. Once this level has been reached they are removed from exposure. Since there is more interest in the first few failures there is no necessity to continue exposure of the entire population of samples once the percentage of interest has already failed, and they may then be removed from exposure. This technique is called censoring and can be used to cut exposure costs. Drawbacks arise from the fact that many replicate specimens are needed to construct the failure curve and a specific and point (i.e. level of degradation) must be defined at the outset of the test. This technique is normally used for accelerated testing, however it may be adapted to outdoor testing as reported by Crewdson [46]. It should also be noted that it remains of paramount importance that the primary environmental parameters are known and monitored. A failure curve constructed for an exposure started in June, may not be the same as a failure curve constructed from an exposure started in December. There are examples in the literature that demonstrate successful results using these techniques. Schutyser [47] developed a model showing that the service life of a pigmented powder coating, with respect to color change as a function of temperature when subjected to accelerated weathering, can be described by means of an Arrhenius–Weibull model. This work also illustrated the important concept that these models are material specific. In this case the modeling technique produced different results which were dependent on the Tg of the coating, the weathering temperature range and the service temperature for which the prediction is being made. Owing to the long lifetimes of 20–30 years that are expected from the reflector materials for use in photovoltaic modules there has been a concerted effort by researchers at NREL, to develop predictive models. Kim et al. [48] have published work in which the weathering stress factors of UV light, temperature and relative humidity are ascertained in terms of their relative significance in the degradation of optical properties of reflector materials. The results of this work show that the cumulative dosage of ultraviolet radiation between 290 and 320 nm, together with the synergistic effects of temperature and relative humidity can be built into the framework of a stress time performance model. The model developed for accelerated lab data was found to have a high level of agreement with field data obtained from outdoor sites. Timing the exposure by degradation occurrence can also allow for shortened testing periods due to the nature of the measurement parameters. For example, if a microscopic examination is made at set intervals degradation will be apparent after a shorter time than if just the naked eye is used. Thus there may be a savings or competitive advantage involved in early detection of specimen degradation. Despite this fact most exposures are still evaluated visually.

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4.5. Specimen replication The number of replicate specimens which must be used cannot be given as a predetermined number since the requirement depends on the statistical test design, homogeneity of specimens, method of evaluation, exposure schedule and many other factors. However, it is certain that the commonly used practice of exposing a single specimen will produce unacceptable results. Variabilities reported by Crewdson [44] can give a coefficient of variance as much as 40%. In this work, textile samples were exposed under glass frames as specified in ASTM G24 and AATCC 16 C. Replicate specimens were exposed at regular intervals within the glass frames, such that an even distribution of the entire useable exposure area was covered. The degradation parameter of interest was color change. Fischer [45] has reported variability in gloss of as many as 40 gloss units between replicate PVC film specimens exposed simultaneously on the same test rack. An interesting phenomenon observed in this work is that variability between the replicate specimens increases with exposure time, yet as the samples reach the limit of degradation the variability begins to decrease. Based on this evidence, the exposure of multiple specimens should be considered mandatory.

5. Summary Development of weathering techniques is traditionally focused on achieving faster and faster test results, while the expected lifetime of a coating is increasing. The newer modeling research for accelerated weathering is being developed specifically to combat this problem. The key to the success of the service life prediction approach is to develop accelerated weathering techniques that can be used to identify the weathering stresses that are most significant in the degradation process, thus making the development of a degradation model a realistic goal. In addition Many of the items discussed above as a part of this technique, such as increased sample replication, simulation of end use environment, environmental monitoring are very useful when applied to the traditional testing methods. There are multiple reasons for performing weathering tests, such as research and development of new materials, quality control, meeting customer testing requirements and protection from liability for materials already in service. Not all of the variables mentioned in this review apply to the exposure testing of every material and test design. However, it is important to consider each variable before dismissing it as irrelevant. At a minimum, the test environment should be known and measured in as much detail as is practical, and the exposure method, mounting, schedule and number of replicate specimens should be carefully considered as an integral part of the test design.

References [1] Burroughs W. Journal of the American Chemical Society. September 1998. [2] Lewry AJ, Crewdson LFE. Construction and building materials 1900;8(4):211–22. [3] Guide to meteorological instruments and methods of observation. World Meteorological Organization, Geneva, Switzerland. [4] Martin JW, Saunders SC, Floyd LF, Wineburg JP. Methodologies for prediction of the service lives of coatings systems. United States Department of Commerce, National Institute of Standards and Technology. NIST Building Science Series 172.

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[5] Correll DL, Clark CO, Goldberg B, Goodrich VR, Hayes D, R , Klein WH, Schecher WD. Spectral ultraviolet-B radiation fluxes at the earth’s surface: long term variations at 39⬚N, 77⬚W. Journal of Geophysical Research 1992;97(d7):7579–91. [6] Zerlaut G. Solar ultraviolet radiation: aspects of importance to the weathering of materials. In: Ketola WD, Grossman D, editors. Accelerated and outdoor durability testing of organic materials, STP# 1202. Philadelphia: ASTM, 1994. [7] Searle ND. Spectral factors in photodegradation: activation spectra using the sharp cut filter technique. In: Kockott D, editor. International Symposium on Natural and Accelerated Weathering of Organic Materials. Atlas SFTS BV. Netherlands: Lochem. [8] Trubiroha P. The spectral sensitivity of polymers in the spectral range of solar radiation. In: Patsis AV, editor. Advances in the stabilization and controlled degradation of polymers, vol. 1. Lancaster, PA, USA: Technomic Publishing, 1989. [9] Bennett I. Monthly maps of mean daily insolation for the United States. US Army Natick laboratories, Natick, Massachusetts. Solar Energy Volume IX, no. 3, July–September 1965. [10] SAE J1976. Outdoor weathering of exterior materials. Society of Automotive Engineers Test Method. [11] AZTEST technical bulletin No 1. 21212 West Patton Rd, Wittmann, AZ 85361, USA. [12] Lemaire J, et al. Polymer Degradation and Stability 1986;15:1–13. [13] Operating light-exposure apparatus (xenon-arc type) with and without water for exposure of nonmetallic materials. ASTM Book of Standards, vol 14.02. p 1258–67. [14] Operating xenon arc light apparatus for exposure of non-metallic materials. ASTM Book of Standards, vol 14.02. p. 1523–29. [15] Dade County Florida. Accelerated exposure of roofing materials using a controlled irradiance water cooled xenon arc apparatus. Metro Dade Center, Building and Zoning Department, Miami, 1974. [16] Verma M, Crewdson L. A study of the color change of automotive coatings subjected to accelerated and natural SAE weathering tests for exterior materials durability. Society of Automotive Engineers, 1994 Annual Congress, P.O. Box 6410000, Pittsburgh, PA 15624-1000, USA. [17] SAE J1060 Jun80. Accelerated exposure of automotive exterior materials using a controlled irradiance water cooled xenon arc apparatus. [18] Crewdson LFE. Correlation of outdoor and laboratory accelerated weathering tests at currently used and higher irradiance levels — Part II. Materials life society. First International Symposium on Weathering, May 1992, Tokyo, Japan. [19] Crewdson LFE, Scott K. A comparison of experimental high irradiance and standard SAE weathering tests for automotive exterior materials. Society of Automotive Engineers, International Congress and Exposition, Detroit Michigan, 28 February–3 March, 1994. [20] Searle N. Private communication. [21] Jorgensen G, Bingham C, Netter J, Goggin R, Lewandowski A. A unique facility for ultra-accelerated natural sunlight exposure testing of materials. American Chemical Society 1999. [22] Ketola WD, Skogland TS, Fischer RM. Effects of filter and burner aging on the spectral power distribution of xenon arc lamps. Durability Testing on Nonmetallic Materials, ASTM STP 1996:1294. [23] Scott KP. Proceedings of Materials Life Society, First International Symposium on Weathering, May 1992, Tokyo, Japan. [24] Kockott D. Factors influencing the reliability of results in accelerated weathering tests. Durability Testing on Nonmetallic Materials, ASTM STP 1996:1294. [25] Saunders SC, Jensen MA, Martin JW. A study of meteorological processes important in the degradation of materials through surface temperature. NIST Technical Note 1275, 1990. [26] Fisher, R.M., Surface temperatures of materials in exterior exposures and artificial accelerated tests, accelerate and outdoor durability testing organic materials, ASTM STP1202. [27] Volz HG, Kampf G, Klaeren A. Farbe ⫹ Lack 1976;82:805 (Defazet 1978). [28] Sereda PJ, Cross SG, Slade HF. Measurement of time of wetness by moisture sensors and their calibration. Atmospheric Corrosion of Metals, ASTM STP 1982;7676:267–85. [29] Wayne RP. Chemistry of atmospheres. New York: Oxford University Press, 1985. [30] Franey JP, Gradedel TE. JAPCA 1985;35:644–8. [31] Lewry AJ, Asidev-Dompreh J, Bigland DJ, Butlin RM. The effect of humidity on the dry deposition of sulphur dioxide onto calcareous stones. Construction and Building Materials 1994;8:97–100. [32] Lewry AJ, Bigland DJ, Butlin RN. The effects of sulphur dioxide on calcareous stone: a chamber study. Construction and Building Materials 1994;8:261–5.

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