Weathering Test Methods: Natural Weathering
Organizing the scope of weathering test methods into logical order helps the engineer visualize tools available for weathering tests. One order organizes exposures showing those with slowest rate of degradation on one extreme through the exposures showing the fastest rates of degradation on the opposite extreme. An example of this order may place the end use environment of an automobile in rural Michigan towards one end, at latitude exposures in Florida and Arizona farther along, sun tracking and EMMAqua exposures farther still, up to harsh artificial laboratory methods such as high irradiance xenon or metal halide exposures towards the other end. Organizing exposures in this manner points out a primary consideration for the engineer; a trade off exists between acceleration (speed of obtaining testing results) and confidence (types and rates of degradation are what is actually observed in the end use environment). Figure 1 presents one such organization.
The product engineer must weigh alternatives and tradeoffs and decide on an appropriate test matrix. Three general guidelines may assist the engineer. First, relying on several test methods rather than a single method provides robust results. Initiating several smaller tests along different points of the testing continuum in Fig. 1, rather than one large complex test program using a single methodology, reduces testing risk and does not put all the eggs in one basket. Comparisons between several test methods and multiple replicates within methods also results in a higher level of information. Secondly, prudent development engineers precede accelerated testing with identical specimens on natural - real-time exposures. Development engineers often need to show how accelerated results directly relate to natural exposures. Only direct comparison between identical specimens exposed to natural and accelerated methods can achieve confidence in the accelerated methods for specific material types, formulations and lots. Thirdly, after initial product approval by a manufacturer, drifts in formulation, manufacturing process or handling often occur. "Surveillance testing" involves regular sampling from production lots and places samples on natural exposure using a quality control approach. For example, several manufacturers sample production lines once a year and place the samples on natural exposures with a very limited frequency of evaluation. In the unlikely event of customer complaints sometime in the future, reference data from "surveillance testing" is readily at hand. Surveillance data also offers opportunities to anticipate customer dissatisfaction before it arises in the market. Inexpensive, simple, regular surveillance testing provides a level of assurance throughout the product life cycle.
From a design and product engineers standpoint, it may be important to consider materials weathering behavior as an interaction between the formulation, processing and end use environment. A Venn diagram illustrates this interplay in Fig. 2.
The test methods presented here within represent a collection of tools for the product engineer. These analytical tools - like all others - if used improperly, can result in erroneous decisions with catastrophic results. Likewise, if used with skill, can enhance product performance and customer satisfaction. This treatment only reviews a number of tools available to the engineer. This treatment does not present application and methodology details. Proper use of these tools includes reviewing procedures, cautions and warnings as specified in appropriate standards.
Natural Weathering Factors
Solar irradiance on materials causes fading, color change, surface erosion, loss of gloss and numerous other deteriorations. Increasing the levels of irradiance represents a primary accelerating factor for different weathering test methods. Solar irradiance is a critical weathering variable for many materials.
A solar spectral power distribution is simply a measure of sunlight intensity over a continuum of wavelengths. Spectral radiometers measure solar spectral power distributions. The geometry of the measurement including elevation, angle, diffuse component, direct component as well as atmospheric conditions effect the power of light (irradiance in W/m2) striking a material's surface or entering a spectral radiometer. Standards bodies publish standard solar spectral power distributions such as ASTM E 892, E 893. Proper reference to spectral power distributions includes reference of geometric condition as well as wavelength region. Earth's atmosphere filters the solar spectral power distribution to approximately 300 - 3000nm bandwidth. This represents the effective solar spectrum for most cases.
Use of spectral radiometers for daily measurement of exposure conditions requires expensive, sensitive instruments. Exposure laboratories typically measure "narrow bands" of solar spectra. The 300 - 800 nm narrow band includes both high energy UV light considered important for photo degradation, and visible light that may have an independent or synergistic effect on materials photo degradation. By accounting for the amount of time a surface is exposed to irradiance, a "radiant exposure" (in MJ/m2) can be calculated. Often, exposure laboratories record the 300 - 400 nm or 295 - 385 nm UV region and report radiant exposure in MJ/m2 total UV. Total UV timing often provides better correlation between exposures than total solar or other irradiance bands. Two identical materials exposed under different irradiances may have similar levels of degradation given similar radiant exposure of total solar UV. Some testing efforts utilize even narrower bands to time exposures. A 10 nm band pass filter centered at 340 or 420 nm covering simple detectors also provides a timing mechanism for natural and artificial exposures.
Words of caution; problems often occur when trying to reconcile data between two exposures timed using different bands of irradiance - for instance one vendor representing materials performance after exposure to "X" MJ/m2 using a narrow band radiation at 420 nm while a competitive vendor presents results timed using 300-820 nm radiant exposure - begs the question; " How many MJ/m2 at 300 -820 nm equals how many MJ/m2 at 420 nm?". Engineers must carefully convert from one radiant exposure type to another. Figure 10.3 shows a spectral power distribution measured in Phoenix, AZ from 280 to 800 nm, September 1998, global normal, near solar noon. The following reduced values were obtained:
|Spectral Band||Irradiance W/cm2|
|300-800 nm||6.51 X 10-2|
|300-400 nm||6.14 X 10-3|
|At 340±5 nm||6.28 X 10-4|
|At 420±5 nm||1.50 X 10-3|
Temperature's effect on material weathering includes thermal oxidation degradations, subsequent reaction rates and accelerating other weathering reactions. Increasing levels of temperature represents a primary accelerating technique for different weathering test methods. Temperature represents a critical weathering variable for many materials. In wet chemistry, a rise in 10° C often results in a doubling of reaction rates . Photolytic and hydrolytic weathering reaction rates also accelerate with rising temperatures. Engineers must consider the effect of temperature on weathering mechanisms. Significant misunderstanding may result from conducting weathering exposures at temperatures causing different weathering mechanisms than encountered in end use environments.
Engineers consider several different temperatures important to weathering test methods. Exposure laboratories generally report one or several critical temperatures of exposure.
Exposure laboratories typically measure ambient or air temperatures using WMO or NOAA documented techniques. Local and regional geography, ecosystems and atmospheric conditions influence a shaded measurement device such as thermometer, thermocouple or RDT sensor enclosed in a ventilated shelter. Measurements obtained indicate the level of energy in the mass of air surrounding the sensor.
By contrast, materials surface characteristics: reflectance, transmittance, absorptance, along with convection, conduction and emittance of the material interact with ambient temperatures and solar irradiance to determine temperature of a materials surface . Anyone picking up a piece of metal pipe warming in the summer's sun can attest to the significant differences between ambient and surface temperatures. Exposure laboratories typically measure surface temperature by attaching thermocouples to or just below the surface absorbing radiation. Measurements obtained indicate the level of energy at the absorbing surface.
Bulk temperature indicates the amount of energy inside the material surrounding the sensor. Environmental, surface and intrinsic properties influence bulk temperatures of the material. Thermocouples or other sensors can be cast or implanted inside materials to measure this property as a function of weathering.
Exposure laboratories typically employ two main types of moisture measurement; relative humidity and wet time. Exposure laboratories typically measure relative humidity using WMO or NOAA documented techniques. As in ambient temperature, local and regional geography, ecosystems and atmospheric conditions influence a shaded measurement device such as wet bulb/dry bulb or solid state sensor enclose in a ventilated shelter. Measurements obtained indicate the amount of water vapor in the air mass relative to the air's maximum capacity for the air temperature. Exposure laboratories measure wet time with a variety of techniques. Wet time is the amount of time liquid water is present on a material's surface due to condensation and precipitation. Sensors use change in resistivity of wick like material, liquid bridging between conductive leads and activation of voltaic cells to measure presence of liquid water. Water acts both as a chemical reagent in many hydrolytic and galvanic weathering reactions and as a physical stressor in materials degradation. Moisture is a critical weathering variable for many materials.
Moisture cycling by humidity or liquid water creates mechanical stress cyclic loading in many materials. Absorbed water results in compressive stresses on the outside and tensile stresses in the bulk. Drying creates bulk compressive and surface tensile stress gradients. The daily fatigue due to the night's condensation and day's solar drying, in subtropical and tropical environments may interact with hydrolytic, thermal and photo degradations causing mechanisms significantly different than those observed in arid exposures.
Engineers often underestimate effects of condensation in subtropical environments. ISO 9223 offers an estimate of wet time by measuring the amount of time relative humidity is above 80% with temperatures above freezing. Exposure tests, however, should utilize direct measurement data for critical considerations. The black body properties of the night sky further enhance condensation by causing materials surface temperatures to drop significantly below surrounding air mass temperatures. This phenomena causes materials to act as condensers in the night air.
The effects of rain may be similar to condensation. Whereas condensation often occurs at regular daily intervals, rain can play a more infrequent, seasonal role. The frequency and duration of rain is also closely linked with irradiance and temperature excursions. One example of catastrophic excursions occurs in Arizona's arid climate when materials heated close to heat deflection temperatures in the summer afternoon undergo dramatic thermal shocks due to sudden afternoon seasonal monsoon showers.
The effects of relative humidity, like liquid water, depend on material characteristics. Relative humidity often receives undue attention simply because exposure facilities report this data. In fact, the amount of water molecules available for materials degradation at the highest possible terrestrial relative humidities (vapor pressures) may represent order of magnitudes less than amount of water molecules available in liquid state depending on how the calculation is performed. Test development should consider effects of water vapor with consideration to presence of liquid water in the exposure environment. In sub-tropical environments, high relative humidities often effect biological degradation (algae, mildew, etc.). Relative humidity microclimates present in full scale testing often result in bio degradation not observed in small coupon exposures
After considering the most critical weathering variables, engineers should characterize the end use environment in terms of these variables. This usually leads to the question "Which end use environment?" An automobile used in metro Detroit, MI area experiences considerably different weathering variables than the same vehicle in Los Angeles, CA or Orlando, FL. Engineers should design components for robust performance in worst-case end use markets. By using this design criteria, the probability of satisfactory performance in milder end use environments (those with milder levels of critical weathering variables) usually increases. Typically components fail fastest in "worst case" end use environments. Exposure in these areas accelerates failures due to increased levels of critical weathering variables including irradiance, temperature and moisture. For these two primary reasons; 1) Characterizing performance in "worst case" end use environments and 2) Accelerating the rate of failure due to increased levels of critical weathering variables, industries focus material natural weathering exposure tests on two major US locations; South Florida and Desert Arizona termed "Reference" or "Standard" Environments.
Materials encounter dramatically increased levels of critical weathering variables between the traditional large automotive markets of the Mid-Atlantic, New England and Mid-Western states and the reference environments of South Florida and Arizona. For example, solar radiant exposures in Boston, MA average approximately 4749 MJ/m2 per year total solar while Phoenix, AZ averages approximately 7941 per year (under similar conditions), a 60% increase. Mean temperatures in Columbus, OH average approximately 11° C compared to 24° C in Miami, FL area...a 118% increase. Relative humidity averages for New York, NY average approximately 65% for the year compared to approximately 74% measured in Miami...a 9% increase. These increased levels of critical weathering variables in reference environments accelerate degradation and failure [5,6].
Increased solar radiation as latitude or cloud cover decreases provides higher probabilities for photolytic degradation effecting form, fit and function of automotive components. Increased temperatures in both the surrounding air and due to materials solar absorptance provide kinetic energy to accelerate photolytic and hydrolytic reaction rates. Increased concentration of water molecules and ions both as vapor and condensation accelerates degradation in reference environments over traditional end use environments. As the levels of individual critical weathering variables increase, the probability of interactions between these variables increases. Interactions can accelerate deterioration rates beyond what is expected by increased levels of critical weathering factors alone.
Exposures in reference environments represent a first step in accelerating degradation from traditional end use markets (see Fig. 1). Comparison between South Florida and Phoenix, AZ offers an effective technique for understanding these environments with respect to solar radiant exposure, temperature and moisture.
South Florida - Subtropical
The local climate of the Miami, FL Area provides increased levels of three critical weathering variables above most end-use environments simultaneously; higher solar radiant exposure, increased temperatures and more moisture.
Figure 4 shows a graph of sliding mean ambient air temperatures in the Miami area. Interaction of lower latitude warmth coupled with moderating effect of Caribbean moisture results in the elevated, constant temperature trends between approximately 20 and 27° C throughout the year. More instructive than air temperature, however, is the temperature materials achieve. Black panel temperatures more closely represent material temperatures under solar irradiance than do mean ambient temperatures. The approximate 40 - 60° C maximum black panel temperature range represents an important critical weathering variable characteristic of the South Florida reference environment.
Figure 10.5 shows yearly trends for total solar radiant exposure in the Miami area. Major influences on these solar trends include elevation of the sun (almost directly overhead at summer solstice) and occurrence of a rainy season starting in June. The graph of UV only in Fig. 10.6 (considered critical wavelengths for most solar degradation reactions) follows the same trends. The 12 - 24 MJ/m2 per day total solar (6200 MJ/m2 in 1997) total solar and 0.6 - 1.1 MJ/m2 per day UV (310 MJ/m2 in 1997) approximately, represent important critical weathering variable characteristics of the South Florida reference environment.
Figure 7 is a graph of average high and low relative humidities indicating the saturating effect of the Caribbean moisture in South Florida. Almost every night, the air mass reaches saturation with average daytime humidities rarely dropping below 50%. Figure 10.8 shows average daily time wet time. The 6 to 10 hours materials spend each day soaking in liquid water represents an important critical weathering variable characteristic of the South Florida reference environment.
Arizona - Arid Desert
The local climate of the Phoenix, AZ area provides increased levels of two critical weathering variables above most US end use markets; higher radiant exposure and increased temperatures.
Figure 4 shows a graph of sliding mean air temperatures in the Phoenix, AZ area. Lack of atmospheric moisture causes increased irradiance levels, driving summer ambient temperatures higher. Summer to winter temperature swings indicate the importance of exposure test start time on some materials degradation. This 10° C to 33° C yearly temperature swing clearly differentiates Arizona's temperatures from South Florida's. The approximate 28° to 60° C maximum black panel temperature range represents a critical weathering variable characteristic of the Phoenix, Arizona reference environment.
Figure 5 shows yearly trends for solar radiant exposure in the Phoenix area. Lack of atmospheric moisture, resulting clear skies, and low latitudes result in increased radiant exposure well above Miami areas in summer months. Winter month radiant exposures are more similar. The 10 to 30 MJ/m2 per day (7800 MJ/m2 in 1997) total solar and 0.5 to 1.5 MJ/m2 UV (390 MJ/m2 in 1997) approximate values represent important critical weathering variable characteristics of the Phoenix, AZ reference environment. Figure 6 shows yearly trends for UV radiant exposure in Phoenix.
Figure 7 shows average high and low relative humidities in the Phoenix area. The daily saturation effect observed in South Florida is absent in Phoenix. In fact, on average the atmosphere is rarely saturated. Figure 8 shows average daily wet time. The fact that any time is spent with condensation is a testimony to the black body absorbing power of the desert night sky to drop surface temperatures far enough below ambient for surfaces to act as condensers in this arid environment. Seasonal rains play a large part in increasing levels of wet time in the winter.
High irradiances coupled with high temperatures in a narrow time period of Arizona's summer can create a bump in material degradation curves in Arizona that may not be observed in Florida. Temperature - irradiance interactions can create unique degradation mechanisms during summer months in Arizona. The Florida reference environment, in contrast, provides less variation: Temperature - irradiance interactions have a higher probability of continued influence throughout the year. Adding the moisture variable to these interacting factors further differentiates these two reference environments. These interactions in critical weathering variables result in dramatically different environments and ecologies. It is not surprising that materials degradation mechanisms should differ widely between these exposure environments. With understanding of these characteristics, engineers tailor exposure test methods effecting key mechanisms specific to materials, processes and end use environments. By exposure in both reference environments and analysis of empirical weathering data, the engineer obtains an understanding of materials performance in worst case end use environments and extrapolates this understanding to proper development and interpretation of accelerated exposures.
Natural exposures are conducted under natural environmental conditions rather than inside a laboratory under artificial conditions.
Full System Exposures
By placing an entire automobile on exposure, engineers simulate the complex interactions of materials and weather with complex exposure angles, effects of assembly, production stresses, interactions with other materials, etc. and achieve best simulation of end use performance. This is especially important for components such as automotive interiors. A hot, enclosed automotive interior exposes any single component to a virtual soup of plasticizers, blowing agents, stabilizers, absorbers, process aids, lubricants and other volatile reagents effecting performance. The complex thermal energy balance due to solar absorbtion, heat sink properties, exterior color, insulation, filtering effects of single layer and laminated glass in concert with daily and seasonal sun angles, relative humidities, ambient temperatures, etc. represent an extremely complex and dynamic exposure environment that can not be simulated except by system exposure. The important influence of the full system exposure on exposure test method development can explain attributes of many exposure test methods including; under glass, black box, IPDP, CTH, etc. These and other methods attempt to standardize, simplify and enhance critical weathering variables observed in full system exposures. Other industries extensively use full system exposures as base lines for exposure development. The building industry's use of full building system exposures for characterizing degradation represents one example .
Full system exposure designs typically employ all or most of the end use system such as an automobile, automobile without engines, partially assembled, partial sections of automobiles or wrecked automobiles, depending on the degradation under investigation. Exposure laboratories often own a number of automobiles providing full system exposure space for a number of different tests. These exposure designs are typically not standardized - being agreed on by the individual researcher and exposure facility. Automobiles generally face south and are often black to maximize exposure temperatures.
Temperatures in or on automotive full system exposures depend highly on the system, components and local environment. An example of some approximate interior temperatures observed include (note different thermocouple locations and auto types between Florida and Arizona):
|Location in system||Arizona Maximum||Florida Maximum|
|Dash||95° C||105° C|
|Head liner||75° C||75° C|
|Seat||90° C||89° C|
Material exposures in full systems provide engineers with good opportunities for measuring actual performance temperatures. Exposure laboratories often offer thermocouple placement and data logging capabilities providing the design engineer with invaluable insights into performance criteria. Exterior body temperatures are highly dependent on full system design, orientation and local conditions. Engineers often get indications of exterior body temperatures from standard black and white panels mounted independently at the exposure facility.
System design influences exterior and interior radiant exposure. Exposure laboratories usually measure exterior radiant exposure directly with reference to angle (see Fig. 5, Fig. 10 and Fig. 11). Interior irradiances, in contrast, depend highly on glass filtering, angles and shading, etc. Well-equipped exposure facilities offer spectral radiometer measurements of interior locations in full system exposures. These measurements offer engineers the ability to characterize and compare full system exposure characteristics to those of other test methods. Examples of some interior irradiances observed in September, 1998 near solar noon include:
|Location in system||Irradiance in W/cm2 300-800 nm|
|Center instrument panel-horizontal, south||3.2 X 10-2|
|Driver's air bag||2.3 X 10-2|
|Hat shelf||2.4 X 10-2|
Care must be taken to insure the effects of solarization of glass do not affect the exposure results. Condensation of volatile materials on glass must be kept to a minimum by regular inspections and cleaning. Exterior and interior full system moisture characteristics, like temperature and irradiance, depend on system design and environmental characteristics. Daily temperature cycles as well as system ventilation effect interior relative humidities.
Often, engineers try to solve simple research questions, such as which component, formulation, vendor, or process gives better performance under a simple set of exposure conditions. Also, engineers often wish to simplify exposures from complex, dynamic end use or full system exposures. Rack exposures simply hold a collection of specimens for a period of time in the same orientation.
Unbacked racks expose materials so that the portion of the specimen being evaluated is subject to the effect of weathering on all sides. Exposure laboratories should only use non-corroding (typically aluminum alloy) materials for rack structures and fasteners. A variety of clamping devices, slots, mechanical fasteners, etc. attach specimens allowing expansion and contraction due to temperature and moisture. Both backed and unbacked racks are usually oriented due south with popular exposure angles including 90° , 45° , 5° and at latitude angles from horizontal. Specific research questions may require exposure orientation angles other than due south. The area beneath the rack should be typical of the climatological area; grass in subtropics and temperate, gravel in desert. Albedo should be of low reflectance. Racks should maintain the lowest position of the specimens 18 inches above ground to prevent contact with vegetation and maintenance damage. Racks should be placed in clear areas - no shadows when the sun is 20° or higher above the horizon. ASTM G7, GM9163(2) and SAE J1976 cover specific design details [8-10].
Rack angle affects radiant exposure. Figures 5, 10 and 11 show radiant exposure values for various exposure angles in the reference environments. Figure 8 shows average times of wetness for unbacked black metal panels oriented at 5° in the reference environments.
Engineers discovered that backing test specimens with various substrates may result in mild acceleration over unbacked exposures.
In contrast to unbacked exposure racks, specimens are mounted on a substrate, typically exterior grade plywood, with specimens attached to the skyward surface. Often, plywood with overlays and painting extend the life of the rack and prevent delamination. Only the front surface of the specimen is exposed. Proper specimen arrangement prevents contamination from water run off from specimens mounted higher on the fixture. Insulated exposures differ from backed exposures; insulated exposures attached specimens to a skyward facing surface with insulating material attached to the back and usually results in higher exposure temperatures. Occasionally, some backed exposures utilize black painted substrates to absorb solar energy for increased specimen exposure temperatures. ASTM G7 covers specific exposure details.
Temperature differences observed on backed and unbacked racks can significantly impact long term exposure results. Specimens in the backed exposure achieve higher surface temperatures since they do not receive air circulation on the backside. Increases in the critical weathering variable of temperature often result in increased rates of materials degradation (acceleration). The Fischer paper provides an excellent overview of temperature consistency issues for backed exposures .
Rack angle effects radiant exposure for backed, as well as unbacked exposures. Figures 5, 9 and 10 show radiant exposure values for various exposure angles in the reference environments.
Backed exposure relative humidities and wet times in the arid reference environment typically follow seasonal variations (see Fig.s 7 and 8). In subtropical Florida, however, a short, but possibly significant, wet time effect occurs on backed and insulated exposures for many specimens. At night, colder than ambient specimens condense moisture onto surfaces. As the sun rises, condensation evaporates more slowly from backed surfaces resulting in slightly longer wet times than for unbacked surfaces. This slight extension in wet time coincides with solar irradiation. The possibility of interacting critical weathering variables of solar irradiation and wet time is higher for backed specimens than for unbacked specimens.
Many automotive materials are not designed for end use applications involving direct exposure to outdoor environmental weathering. Automotive interior products require weathering test methods simulating conditions found in their specific end use environment or under glass.
Both backed and unbacked configurations support specimens beneath glazings of appropriate glass material, parallel to and 76mm or more from the glass. The entire glass, specimen, substrate assembly faces south at an appropriate angle. Gaps around edges of backing substrates provide ventilation in the backed configuration. The type and quality of glass used represents a critical consideration of these exposures. Figure 11 shows solar energy transmittance of 3 glass types often specified. Materials exposed under laminated windshield type glass usually degrade at significantly slower rates than under single strength window glass due differences in transmitted solar UV. Even within a single type glass, significant variations in solar transmittance often occur. 300% variations in solar transmittance characteristics have been reported between lots of the same glass type. ASTM STP 1202 reports solarization considerations for glass material. Most test methods recommend a 3-month minimum pre-aging exposure of glass before use in under glass exposures. Maintenance considerations include cleaning of glass both inside and outside. Additionally, most under glass practices recommend relative measures of material degradation. Standard or reference specimens with well-characterized degradation characteristics are exposed side by side with test specimens under the same piece of glass. Performance ratings relative to reference materials compare degradations in a blocked experimental fashion. Most practices also condemn comparisons of materials performance in under glass exposures conducted at separate times or under separate exposure glazing. ASTM G 24 (A), ISO 877, ISO 105-B01 and Ford BI-160-01 cover specific exposure details.
Figures 12 and 13 compare daily mean total solar and UV radiant exposure, under glass and direct, in Arizona. In general, non-laminated, under glass exposures receive about 25% less total solar and UV than direct exposure.
Under glass exposures essentially eliminate time of wetness. Even in subtropical environments, precipitation and condensation typically occur on the glass rather than on the specimen. Well ventilated under glass exposures, however, often accumulate significant condensation on the underside of the glazing, which can drip onto specimens resulting in spots, especially in near horizontal orientations.
Modern rack designs position the unbacked, backed and under glass exposure types at any angle relative to the horizontal. Engineers choose specific angles for two major reasons; 1) to simulate end conditions and 2) to achieve additional acceleration. The 90° from horizontal orientation more closely represents orientations of car doors, fenders, seat backs, etc. Due to the angle of incidence of solar irradiance and scattering of light through the atmosphere, the 90° angle achieves significantly less radiant exposure than a 45° angle under the same conditions. 90° exposure angles cut off the north half of the diffuse sky light. Ground albedo also becomes an important consideration. ASTM G-7 covers specific exposure details.
In the past, exposure racks were typically positioned and fixed at a single angle. Modern exposure facilities now employ racks attached by a single axle to fixed vertical members. The axle allows rack pivoting to the appropriate angels from horizontal as needs arise. Unbacked, backed, under glass or other exposure enhancements then clamp to the pivoted frame. Although simple in retrospect, the pivoting frame advancement provided basis for considerable development of natural weathering test methods. This humble, simple improvement in rack design has allowed more advances in conventional weathering technology than any other single advancement.
Because of the reduced angle of incidence of solar irradiance throughout the year, 90° exposures generally run cooler than other angles. Direct convection and wind turbulence also effects the 90° angles slightly. The effect of ground albedo is greatest on 90° surfaces so ground reflections and re-radiation from asphalt surfaces may significantly affect 90° temperatures.
Due to the angle of repose, liquid water from condensation and precipitation runs directly off specimen's surfaces. Rain may not even strike the exposure surface under certain wind conditions even though the exposure laboratory reports accumulated precipitation.
Engineers quickly discovered changing the angle from 90° to 45° dramatically effects degradation rates. The popularity of the 45° exposure angle shows a predominant interest in many weathering tests for acceleration rather than simulation of end use conditions.
Modern 45° racks typically employ the same designs as the 90° or horizontal racks with the exposure area simply being pivoted and locked to 45° from the horizontal. Exposure laboratories use suitable measuring devices such as inclinometers to certify the angles from horizontal with periodic checks to insure shifting has not occurred due to wind loading, soil settlement or other changes. ASTM G7 and GM 9163 (2) cover specific exposure details.
Figures 14 and 15 show total solar radiant exposure for the 45° angle. In general, conducting exposures at 45° accelerates photodegradation rates beyond 90° exposures due to increases in the critical weathering variable of irradiance.
At Latitude Angle
Orientating racks at latitude angles provides optimum orientation to the sun's direct irradiance throughout the year. Twice each year (at equinox), the sun's irradiance strikes perpendicular (normal) to the specimens surface. This exposure angle again increases levels of critical weathering factors and accelerates degradation for many materials over 45° exposures.
At latitude exposure rack designs follow the same considerations for other fixed angle rack designs and specific exposure details are covered in ASTM G7.
Figures 14 and 15 show radiant exposures for the at latitude angles in the reference environments.
Horizontal and 5° South
As the variety of exposure angles became more widespread in weathering testing, tests demonstrated the effects of moisture and the diffuse component of solar as important critical weathering variables. The horizontal and 5° exposure angles may increase some of these weathering variables more than the 90° , 45° and at latitude angles.
Again, horizontal and 5° racks employ the same basic considerations as other exposure angles discussed so far and specific exposure details are covered in ASTM G7.
Horizontal and 5° angles expose specimens to essentially the entire sky dome; 180° from north horizon to south, 180° east horizon to west. In arid Arizona environments, approximately 70% of the solar UV energy is in the direct beam from the sun. In subtropical Miami, however, approximately 50% of the UV can be received from other areas of the sky, referred to as the diffuse component of solar radiation. As a surface's exposure angle increases, specimens receive less of this diffuse component (this affects radiant exposure levels most at greatest solar elevations). Orientations at horizontal or 5° can maximize critical weathering variables near summer solstice in the Miami, FL area. Figures 14 and 15 show irradiances for the 5° angle.
A 5° angle (instead of true horizontal) allows run off of precipitation and condensation liquid and allows residue accumulated on the specimen's surface to be washed off. Blowing dust, dirt, weathering end products and bio contaminates can accumulate on horizontal surfaces and significantly reduce or alter solar exposure effects on the material's surface. Subsequent evaporation of contaminated water can etch or mark the surface confusing weathering results. Figure 9 shows wet time averages for 5° unbacked black metal panels.
Data from the static exposure angles discussed so far shows the effect of angle on increasing critical weathering variables. A dynamic exposure, which varies orientation in response to the seasonal variation in the sun's path, can increase critical weathering variables throughout the year.
The variable angle rack design utilizes the pivoting axle discussed previously. At four times of the year, crews manually readjust variable angle racks to a new angle as the sun's path shifts higher or lower in the sky. Racks lock into position utilizing pre-drilled positioning arms. The following schedule shows typical variable rack angles;
|Dates||Variable Rack Angles Near Phoenix, AZ||Variable Rack Angles Near Miami, FL|
|09/01-10/31||34° C||26° C|
|11/01-02/28||54° C||45° C|
|03/01-04/30||34° C||26° C|
|05/01-08/31||14° C||5° C|
Variable angle exposures increase the yearly average temperature over static orientations for most materials. In Miami, FL, the variable angle exposures present specimens to the scattered UV portion of the sun's energy reflected from the north sky during critical summer months. Compare the radiant exposure of variable angles to the at latitude static angle as shown in Fig. 16.
Accelerating Natural Exposures.
Weathering test methods presented so far easily relate back to full system end use exposures. The discussion has focused on how simple increases in critical weathering variables can significantly affect the rate at which materials degrade. Simple optimization of location, backing and angles can modestly accelerate degradation rates. Methods of acceleration and increasing critical weathering variables beyond those discussed so far may present considerable risks for correlating testing results back to full system end use exposures. Engineers should only use accelerated test methods in conjunction with test methods presented so far. ASTM G 90 clearly states "No accelerated exposure test can be specified as a total simulation of natural field exposures".
ASTM E 632 provides procedures for calculating acceleration factors for accelerated exposures. Specific acceleration factors must be developed for specific materials types, formulations, processing conditions, etc.
The methodology and approaches for accelerating natural exposures follow the general approach outlined so far; by increasing levels of individual natural weathering variables (temperature, irradiance, moisture) test engineers can expect faster degradation of materials. Successful test programs take great care to insure the same mechanisms occurring in end use conditions occur at increased levels of critical weathering variables, including interactions between critical weathering variables .
Accelerated natural exposures often increase temperatures significantly above those materials experience in other natural exposures. Often, however, increased temperatures result from increases in other critical weathering variables. Many accelerated test designs include processes for reducing temperatures back to appropriate levels for materials (below melting, heat deflection or glass transition points). Designs often include safety mechanisms to prevent degradation mechanisms caused by exposure to inappropriate temperature levels.
Increased levels of irradiance often dictate critical design elements in natural accelerated exposures. Collection and concentration of solar irradiance offers use of the natural spectral power distribution of the sun as well as the cyclic variations experienced by materials in end use markets. As the levels of total solar radiation increase, however, so do levels of solar IR and IR re-radiated from specimens, enclosures and other sources. Accelerated exposures can quickly exceed temperature limits of materials and significantly alter degradation mechanisms. Use of natural solar irradiance, however, represents one of the most attractive characteristics of natural accelerated exposure methods.
Elevation of moisture for natural acceleration can present difficult goals for accelerated exposure designs. Diffusion rates and mechanisms often limit moisture available for degradation reactions. Elevated temperatures and limited wetting cycles can also limit availability of this important weathering reaction reagent.
Previously, data compared and contrasted levels of critical weathering variables in sub-tropical Florida and desert Arizona. Design and selection of appropriate natural accelerated exposure tests must be made with the reference environment in mind: degradation mechanisms often widely differ in these two reference environments. Only by conducting accelerated testing in concert with subtropical and desert natural exposures, will test engineers obtain empirical data to link accelerated exposures to naturally occurring degradation phenomena.
ASTM G 7, ASTM D 4141 and SAE J1976 detail black box exposures. Black box exposures simulate the air heat sink characteristics of an automotive body. Boxes are constructed of metal coated with flat black paint. Clamps attach test specimens over the opening in the skyward facing surface forming the top surface of the box. Black painted sheet metal panels fill the open areas so the box is completely closed. Pivoting axles typically allow any angle but a 5° exposure is typical. Specimens include automotive coatings as well as landau or other flexible automotive roof materials.
Black boxes lead to increased temperatures during exposure periods and alter rates of cool off and warm up during evening and morning periods. Black boxes may achieve significantly higher temperatures during the day than unbacked and backed exposures at the same angle. Additionally, the insulating effect of the warmed air trapped in the box dampens out many bulk temperature variations due to passing clouds, intermittent breezes, etc.
Black box radiant exposure depends on the angle of exposure and environment. Figures 14 and 15 show radiant exposures achieved by panels at various angles of exposure.
By far, the greatest number of black box exposures are conducted in the subtropical environments. At day break, specimens warm more slowly due to trapped cool air in the box. Black box exposures maintain surface condensation longer into the day than backed and unbacked samples. This provides a slightly increased probability for interactions between irradiance and moisture than other exposures.
Black Box Under Glass
Often, engineers wish to subject interior automotive materials to exposures simulating heat sink characteristics of an automotive body. ASTM G24 B and Ford BI-106-01 describe black box under glass exposures. In this exposure, mesh screens support specimens inside below glass enclosing the box. Blowers inside the boxes circulate enclosed air for better temperature distribution. Small filtered vents provide some air ventilation and although the box is not truly sealed, considerably less air is exchanged with the outside than in regular under glass exposures. Typical black box under glass exposure angles include 45° , 5° or variable angle orientations. As in normal black box designs, black paint covers the outside of the box while the inside surfaces are typically left as bare metal.
Figures 12 and 13 show typical irradiance for under glass exposure. Glass filtering considerations reviewed in section 10.3.4 also apply to black box under glass exposures including solarization and cleaning.
Due to green house effect, absorbance and interior ambient conditions, it is not unusual for air and specimen temperatures to exceed 80° C in Arizona black box under glass exposures. The increase in the critical weathering factor of temperature results in an acceleration of weathering degradation mechanisms for interior automotive components.
Black box under glass exposures are designed to simulate and accelerate interior automotive conditions. Liquid water is typically prevented from accumulating on the specimen's surface by the cover glass
Heated Black Box
The heated black box exposure design provides heat to the interior air of a black box during day light hours. Thermostat controllers connected to simple electrical "base board" style heating elements and blowers maintain elevated temperatures from approximately 9 am to 3 pm each day. ASTM D 4141 specifies details of the heated black box exposure.
As in regular black box exposures, radiant exposure depends on the angle of exposure and environment (see Fig.s 14 and 15)
Heating of panels by backside air during daylight hours allows the accelerating effect of elevated temperatures to coincide with periods of highest irradiance. On cloudy days or days with cooler than normal ambient temperatures, specimens still achieve accelerating effects of the warmer backside temperatures due to the heating elements. Heaters also decrease the variability of daytime specimen temperatures due to clouds, wind, precipitation, etc.
Turning off heating elements at night allows specimens to return to ambient or cooler temperatures necessary for optimal condensation formation. Maintaining heating at night would prevent formation of condensation and possibly decrease levels and effects of the critical weathering variable, temperature.
Just as black box exposures attempt to accelerate degradation by increasing the level of the temperature, spray racks attempt to accelerate degradation rates by increasing the level of the critical weathering variable, moisture. This approach is especially noteworthy in arid desert conditions found in the Phoenix, AZ reference environment.
Spray racks typically employ a regular backed or unbacked rack. Spray nozzles mounted to the rack provide an even distribution of water spray over the specimen's surface. Plumbing connects nozzles to a very pure water supply. Electrical timers control the frequency and duration of the spray cycles. Selection of rack exposure angle affects drying rate as well as radiant exposures and temperatures.
Under non-spray conditions, specimen temperatures can be expected to maintain normal exposure temperatures. During periods of spray, however, specimens can undergo considerable thermal shocks as surface temperatures rapidly drop to or below ambient conditions. This thermal cycling can provide fatigue effects on some materials and composites.
The effect of moisture is highly dependent on the frequency, duration and timing of the spray cycles. A typical spray cycle is a 30 second wetting spray every 2 hours during daylight hours. Engineers should not overlook custom spray cycles for accelerating the effect of this critical weathering variable.
Water quality for accelerated exposures cannot be over emphasized. Modern exposure laboratories typically supply 1 meg ohm or greater purity water to specimens by using a combination of de-ionization, reverse osmosis and filtration in conjunction with strict quality assurance systems. Process control and monitoring for pH, total dissolved solids, resistivity and dissolved silicates is critical for meaningful results for this test method and all others employing direct application of water to test specimens.
The evolution in exposure methods from static to variable angle exposure racks dramatically increased levels of radient energy deposited on specimens. A similar jump in exposure development occurred with automatic tracking mounts that follow the sun's path from sunrise to sunset. These sun tracking mechanisms dramatically increase solar irradiance and represent the next milestone in natural weathering acceleration methods.
Track Rack's often employ two axis of movement. Changing the angle from horizontal four times per year accommodates the elevation of the sun. A second pivot attaches the specimen mounting frame 90° to the pivot for elevation. Slow stepping motors drive the azimuth pivot orienting the specimen's surface perpendicular to the sun's rays throughout the day. The alignment with the sun is typically accomplished by clock mechanisms or sensitive photo detector systems that feed back alignment information to the azimuth stepping motor. A variety of exposure configurations including those discussed so far mount on top of this tracking device for significantly elevated levels of the critical weathering variable- irradiance.
Figure 16 shows the average solar irradiance received on a Track Rack in Phoenix, AZ compared to variable and fixed angle exposures. Perpendicular orientation to the sun's direct irradiance results in substantially increased radiant exposure. Use in an environment with high direct solar radiation component represents one important consideration. Arid desert environments like Phoenix provide a majority of solar radiation in direct beam from the solar disk. Subtropical environments, however, typically include diffuse reflected sun light as a significant portion of the total irradiance. Track Rack exposure efficiency is maximized with direct beam. The majority of Track Rack exposures are conducted in arid environments.
Although specimens exposed on aTrack Rack generally do not exceed maximum temperatures achieved on variable angle exposures, average temperatures throughout the day will be higher on Track Racks. Occasionally a phenomena in Arizona arises where maximum ambient temperatures occur in the afternoon combined with high solar irradiance. In these periods, Track Rack exposures may slightly exceed variable angle exposure temperatures.
Typically, racks are re-oriented and locked pointing due south for night time conditions and receive similar levels of condensation as variable angle exposures. Regular track rack exposures do not increase moisture above levels encountered in variable angle exposures.
Track Rack with Water Spray
The Track Rack with water spray exposure utilizes the same follow-the-sun mount as normal Track Rack exposures. Spray nozzles attached to the rack and provide intermittent wetting of specimens using a highly pure water supply.
Radiant exposure for the Track Rack with water is essentially the same as for regular Track Rack exposures shown in Fig. 16. Temperature for this exposure typically runs the same as for normal Track Rack exposures; however materials are subjected to the thermal shocks as detailed in Spray Rack exposures. By combining increased levels of irradiance and temperature achieved in regular Track Rack exposures with increased levels of moisture from spray systems, three critical weathering variables - irradiance, temperature and moisture - interact at increased levels with this exposure. A typical wetting cycle includes a 30 minute water spray once every hour from 8:00 am to 4:00 pm. Engineers often develop custom spray cycles to adjust frequency and duration for specific end uses and materials.
Test engineers should not overlook the advantages of Track Rack with water spray. This exposure often represents a prudent balance between comparison to end use conditions and moderate acceleration. In this exposure, increased levels of the critical weathering variables may still remain close enough to real world conditions that degradation mechanisms may accelerate but may not change from end use mechanisms. Track Rack exposures increase weathering variables enough to provide considerable accelerations beyond real time degradation rates for many materials.
CTH Glass Track
Weathering tests often utilize simple under glass and black box racks mounted on follow-the-sun trackers. Interior automotive components, however, often require specific treatment for accelerated exposures. Engineers designed the CTH (Controlled Temperature and Humidity) Glass Track specifically for accelerated testing of interior automotive materials.
The CTH Glass Track utilizes true dual axis tracking with two independently pivoting axles oriented vertically and horizontally. The vertical axle is connected to a stepping motor - photodetector system and aligns with the sun's azimuth throughout the day. The horizontal axle is connected to an independent motor -photodetector system and aligns with the sun's elevation throughout the day. True dual axis tracking mounts maintain the specimen surface oriented perpendicular (± 1° ) to the sun's rays from morning to evening.
The dual axis tracking mechanism supports a metal cabinet. A standard 3mm thick clear tempered safety glass (Herculite) covers the sun facing side of the cabinet. Fixtures support specimens in the box with in 76mm of the cover glass. Blowers mounted to the upper edge circulate air within the sealed box to maintain temperature uniformity throughout the exposure area with in ± 5° C of the set point. Thermostatic controllers and timers activate heaters. A heating element in an open water filled container provides humidity during night time. Temperature and humidity sensors connected to simple controllers and timers allow accurate setting of these critical weathering factors. SAE J2230 details this test method.
The SAE standard requires radiometric data measurements in conjunction with CTH exposures. Figure 10.23 shows radiant energy deposited on CTH exposures for Phoenix, AZ and Miami, FL (see Tony's data). Often, Standard reference materials with known degradation rates (Blue Wool, Polystyrene, etc.) are included with specimens. As with other under glass exposures, maintenance considerations include cleaning of glass both inside and outside.
Contrary to irradiance, which varies as a function of natural solar conditions, CTH artificially elevates and maintains the temperature critical weathering variable at 70° C during the day and at 38° C at night, ± 5° C.
CTH exposures only control humidity at night. Sensors and controllers connected to heating elements submerged in water pans (or in some cases ultrasonic humidity sources) maintain night time relative humidities at 75 ± 10%. Since this simulates car interior variables, no condensation should occur on specimen's surfaces.
Engineers developed the CTH exposure method to test small flat specimens of simple materials or composites. Automotive interior materials researchers realized that complex geometry, fastening systems, foam substrates, etc. caused significant degradation influences. Engineers developed IP/DP exposures (Instrument Panel, Deck Panel Exposure) to expose entire sub assemblies of interior automotive systems in such a way as to simulate the interior automotive environment. IP/DP exposures include entire sub-assemblies such as instrument consoles, steering assemblies, arm rests, center consoles, etc. rather than simple material plaques. Manufacturers find IP/DP provides an effective method for fastener and adhesive integrity as well as systems degradation. Small plaques of materials are typically only exposed as screening pre tests for full assembly exposures.
IP/DP exposure fixtures utilize sealed metal enclosures with glass covers facing the sun. Moderate insulation covers the metal sides and bottom. GM 9538P includes IP/DP design requirements. Glazing material options include clear tempered and laminated glass. Thermostatically controlled blowers mounted inside the boxes recirculate interior air with uniform flow. A black metal panel mounted parallel and adjacent to the glass senses temperatures within the enclosure. External racks orient IP/DP enclosures at 45° or 51° angles from the horizontal.
As in most other under glass exposures, pyranometers mounted under similar glass filters measure total and UV radiant exposure. Due to the geometry of many assemblies, including curved or sloped surfaces, different specimen areas may receive different radiant exposure. Figures 12 and 13 show typical radiant exposures for assembly surfaces oriented parallel to the glass covers at 45° . Again, maximizing irradiances requires appropriate glass maintenance and cleaning.
IP/DP exposures do not control temperatures as do CTH exposures. IP/DP fixtures monitor temperature and if the black panel temperature exceeds a target setting, the interior fan activates to circulate air from irradiated areas across non-irradiated sides and bottom surfaces of the metal enclosure. This usually cools the interior environment sufficiently to prevent catastrophic atypical failures of the assemblies. Standards allow for removal of some of the insulation for proper temperature adjustment. Should the black panel temperatures continue to rise due to system failures, a curtain falls across the glass, stopping the exposure. Figure 10.19 shows typical IP/DP standard black panel minimum and maximum temperatures for the IP/DP exposures for selected times of year in Arizona.
Standard maximum temperature limits include 85, 93, 102, or 110° C depending on the test method specified. Calculations combine temperature settings, reference environment characteristics and fixture orientation to obtain a "seasonally adjusted solar radiation" or "SASR" factor. This factor normalizes measured radiant exposure in an attempt to compensate for exposures conducted at different times of the year or different temperatures.
As with CTH, exposures in IP/DP fixtures simulate automotive interior conditions and therefore preclude condensation on specimen's surfaces. One configuration of this test method allows for placement of a water tray inside the exposure cabinet within 100 mm of the glass. Including the water supply typically raises the interior relative humidity to between 35 ± 5% at the hottest times and 85 ± 5% at the coolest times of the day.
IP/DP with Tracking
IP/DP with tracking simply utilizes a single axle and drive motor to orient an IP/DP box towards the azimuth of the sun throughout the day. Trackers do not change the angle of elevation of the box (45° or 51° from horizontal ) daily or seasonally. Typically, a photo detector coupled to a stepping motor drive mechanism maintains the standard IP/DP box oriented towards the sun's azimuth. GM 9538P covers exposure details for this test method.
Radiant exposure levels on tracking IP/DP represent fairly unique levels: On one hand, the boxes follow the sun through the daily azimuth while on the other hand, they are not adjusted for daily or seasonal changes in the sun's elevation.
Temperature control for tracking IP/DP follows the same procedures as for non-tracking exposures. Internal blower circulation truncates temperature rise at maximum settings as shown in Fig. 19; however, the tracking configuration maintains the maximum temperatures for longer periods each day. Tracking IP/DP exposures reference separate "SASR" factors in the standard to adjust for increased radiation and temperatures due to the tracking characteristics.
The same relative humidity enhancements for non-tracking IP/DP exposures apply to tracking IP/DP exposures.
Sun Tracking Carousel
Engineers developed IP/DP boxes to expose subassemblies of automotive interiors to increased levels of critical weathering variables. Sun tracking carousels similarly expose full system automobiles by directing the same side of an automobile towards the sun throughout the day.
[The sun tracking carousel utilizes a turn-table platform on which the automobile is mounted. Casters or full wheel assemblies support the load of the car. Wheels align tangentially to the center of rotation. A stepping motor and a low drive gear ratio slowly rotate the platform and car, aligning with solar azimuth throughout the day. A similar photocell alignment sensor used for other solar tracking equipment directs rotation of the carousel. After sunset, crews rotate the fixture back to it's morning start up orientation.
Similar irradiance considerations for full system exposures apply to sun tracking carousel full system exposures. Whereas in full system exposures, significant shading occurs, shading in sun tracking carousel can be somewhat controlled. By specifying orientations, the engineer can maximize irradiances for specific interior and exterior target components.
The engineer can maximize system or component temperatures by orienting the automobile on the carousel. The opportunity for gathering baseline temperature data throughout the exposure should not be overlooked and can be applied to the development of other test methods or performance criteria.
After obtaining maximum acceleration from normal incidence, sun tracking exposures, Engineers often want to accelerate UV degradation beyond the methods discussed so far using NATURAL sunlight. Inventors developed an elegant solution to concentrate the image of several suns onto a single target area of the test material. This method became known as the "EMMA", an acronym for Equatorial Mount with Mirrors for Acceleration. Standards for this test method include ASTM G90, ASTM D4364 A, ISO 877 and SAE J1961.
ASTM G 90 thoroughly discusses design constraints of the EMMA device. The following description presents only a summary of the design: Direct solar radiation reflects off flat mirrors towards a target area. Ten flat mirrors measuring approximately 15 cm by 142 cm are laid side by side on a follow the sun track rack. The ten mirrors are adjusted so the reflected solar image from each mirror coincides on the target area suspended above the track rack. The mirror bed tracks the sun throughout the day keeping the reflected solar image on the target. A blower forces air across the target to cool test specimens. The optical system is often described as a Fresnel reflecting solar concentrator with mirrors positioned as tangents to an imaginary parabolic trough.
Obviously, with ten images of the sun focused at a single target, the tendency for specimen heat build is considerable. Blowers force air across the specimens to enhance cooling. The target board, located along the centerline of the mirrors, lies next to a wind tunnel assembly which deflects cooling air across the specimens. The test specimen's optical properties (solar reflectance, absorptance and emittance), thermal conduction, ambient temperature and irradiance determine the actual temperature a material reaches. Specimen mounting on the target also significantly affects exposure temperature. Specimens attach directly to insulating backing (plywood) or mount in frames that raise specimens slightly from the backing, providing backside cooling from forced air. Temperature measurements indicate painted metal panels along with other specimens of high conductivity or coefficient of heat exchange undergo the most effective back side cooling in the uninsulated configuration. The more insulative and absorptive the test specimens, the greater the excursion from ambient temperatures due to solar concentration. Materials with low heat failure mechanisms such as low heat distortion temperatures, should receive special consideration. Materials with low HDT may distend out of the cooling air flow and result in catastrophic failure under solar concentration.
Use of proper materials for reflective mirrors represents a primary critical consideration for the EMMA test method. Mirrors must be highly reflective throughout the entire solar spectrum in order to properly concentrate both UV and visible light onto targets. The reflective material must possess robust weathering characteristics since it will undergo considerable track rack exposure. The reflective material used must maintain high specular reflectance from 300 nm to 2500 nm throughout its use on the exposure device. Regular mirror maintenance (washing, measurement and replacement) is key to the quality and quantity of the irradiance on the target.
EMMA methods use normal incidence radiometric measurement of total solar or UV multiplied by mirror solar reflectance times number of mirrors to calculate radiant exposure. Correction factors also include the cosine error associated with the altitude of the sun for that particular season, if necessary. Raising and lowering the tracking bed on a mast at the north end accommodates seasonal sun altitude performed on a regular basis throughout the year. Alternatively, a dual axis tracking mechanism may also orient the mirror bed at the sun without need for the cosine error factor to calculate solar radiation dosage on target boards. Figure 20 compares EMMA radiant exposure to Track Rack radiant exposure.
EMMA methods do not artificially introduce moisture to test specimens. Only ambient relative humidities supply moisture for material degradation.
EMMA under Glass
EMMA exposures with glass filters inserted into the optical path between specimens mounted in the target area and concentrating mirrors provide simulated spectral power distribution for automotive interiors. This configuration results in an under glass exposure with considerably increased levels of irradiance. Simple frames hold glass parallel to and above irradiated specimens. Care must be taken not to interrupt cooling air flow across specimens. Similar considerations for glass choice and maintenance used in normal under glass exposures apply to EMMA under glass exposures. ASTM G 90 and ASTM D 4364 cover details of this exposure type.
Normal incidence (tracking) pyranometers mounted under the same type of glass measure radiant exposure for EMMA under glass. Alternatively, normal solar transmittance measurements of the glass provide factors for converting irradiance measured from normal exposures.
EMMA under glass temperatures can often run slightly cooler than normal EMMA temperatures due to the filtering effects of the glass. The glass cover can also enhance air flow across the specimen, which results in additional cooling.
As in normal EMMA exposures, EMMA under glass methods do not artificially introduce moisture to test specimens.
The EMMA exposure configuration increases levels of irradiance (concentrated by mirrors) and temperature (due to concentrated irradiance). The next logical step in EMMA development increases levels of the critical weathering variable of moisture; hence EMMAqua = EMMA + Water. ASTM G 90 and ASTM D 4364 cover exposure details of this test method.
The EMMAqua exposure utilizes the same basic design as the EMMA exposure except that spray nozzles suspended below the target area apply a water spray to the target areas. The location of spray nozzles and plumbing must not block irradiance, yet remain close enough or powerful enough to uniformly and adequately wet the target area.
The irradiance considerations for EMMA apply to EMMAqua exposures.
The same temperature conditions on EMMA and EMMAqua exist during exposure periods with outwater spray. During periods of water spray, however, dramatic thermal shocks occur on the surface of the specimens in EMMAqua exposure. The cool water in conjunction with forced air flow can suddenly drop specimen surface temperatures below ambient.
The effect of EMMAqua's moisture depends highly on the material. A typical EMMAqua spray schedule includes 8 minutes of continuous wetting spray every hour. Consideration of water quality cannot be over emphasized. Water contamination under elevated levels of critical weathering variables in this exposure quickly leads to erroneous results. Very high levels of water quality control minimize non-typical water spotting and etching of specimen surfaces. Standards recommend water quality levels less than 1 PPM solids and 0.2 PPM silica. Frequent application of contaminated water will quickly reduce reflectivity of mirrors. Sudden specimen wetting, rapid evaporation of water due to specimen surface temperature with forced air, and irradiation of deposits on a specimen's surface may quickly alter weathering mechanisms and produce meaningless results.
EMMA with Night Time Wetting
The EMMA with Night Time Wetting utilizes the same fixture design as the normal EMMAqua machine. Only timing of water sprays differentiate the two exposure methods. ASTM G 90 and ASTM D4364 cover exposure details for this test method.
The same temperature conditions exist on EMMA with night time wetting and normal EMMA during exposure periods without water spray. By not applying water spray during the day under irradiance, night time wetting exposures eliminate dramatic thermal shocks and water spotting problems occasionally encountered in EMMAqua exposures.
The irradiance considerations for EMMA apply to EMMAqua with night time wetting exposures.
A typical night time wetting spray cycle includes 4 wetting sprays each hour of the night for a 3 minute period each cycle. The night time wetting procedure more closely simulates the night time wetting condensation observed in South Florida 5° exposures. For some materials, maintaining wet conditions throughout the night allows diffusion of water into the material surface. The presence of water is necessary for hydrolytic degradation mechanisms that may not occur under EMMAqua exposure conditions. As in other exposures, however, the requirement for highly pure water cannot be over emphasized.
The EMMAqua plus represents recent enhancements of the basic EMMAqua design, primarily for better control of critical weathering variables.
EMMAqua Plus utilizes true dual axis tracking. The dual axis mount allows horizontal orientation of the target area for specimen maintenance. The mounting also allows the target area to be fully rotated to a 5° from horizontal angle for night time water spray. A programmable controller allows custom spray cycles.
Use of a dual axis tracking slightly improves the irradiance collection efficiency by eliminating small cosine losses. These increases may be on the order of 1 - 4% over standard EMMAqua manual elevation tracking systems.
The EMMAqua Plus system utilizes the same temperature control system as described for the normal EMMA exposure. ASTM G 90 and ASTM D 4364 cover this exposure type.
Designers included several enhancements for control of the critical weathering variable of moisture in the EMMAqua Plus exposure. A programmable controller allows easy set up of almost unlimited spray cycles to satisfy test engineers' specific requirements. Additionally, target rotation to a near horizontal position at night allows water from night time spray to remain on the specimen's surface instead of dripping off. This configuration more closely simulates standing condensation conditions in the Miami, FL reference environment.
EMMAqua Soak Freeze Cycles
Test engineers soon began to couple EMMAqua exposures that elevate critical weathering variables of temperature, irradiance and moisture with increased levels of other critical weathering factors. The EMMAqua soak freeze cycle represented the first hybrid exposure of this type to gain widespread popularity.
The EMMAqua Soak Freeze Cycle utilizes a standard EMMAqua or EMMA exposure during day light hours. Each night, however, crews manually remove specimens from the EMMAqua target area to inside laboratories. Samples soak for a 1 to 4 hour period in warm purified water followed by placement in a freezer for the remainder of the night. The next morning, crews re-mount specimens on the EMMAqua target board and repeat the cycle.
The irradiance considerations for EMMA apply to EMMAqua soak freeze cycles.
The daily temperature cycling may present considerable fatigue stresses to paint systems especially clear coat, base coat systems. Increasing the temperature during the soak phase allows higher diffusion rates to occur. It is unclear how the sudden freeze affects the system beyond the thermomechanical stresses. This type of environmental cycling, although probably not correlatable with the reference environments of Phoenix, AZ and Miami, FL, may accelerate degradation mechanisms and failure types observed in temperate conditions of traditional automotive markets.
Immersion in warm water may considerably accelerate the uptake of water in materials through diffusion and absorbtion. Absorbed water may then be available for hydrolytic reactions independently or in concert with irradiation on EMMAqua exposure. The mechanical stresses of drying on EMMAqua exposure, re-hydration in warm water soak and crystallization in deep freeze may increase the levels of the critical weathering variable of mechanical stress depending on the material.
No discussion of natural accelerated weathering test methods would be complete without discussion of a concept called "Super Maq". Super Maq attempts to combine higher levels of the critical weathering variables of temperature, irradiance and moisture with large component or assembly exposures.
Super Maq utilizes the same basic design as normal EMMAqua devices except scaled up to a size allowing large specimens to be mounted on the target area. Super Maq increases the dimensions of a typical EMMAqua approximately 4 times. 10 mirrors measuring approximately 61 cm wide by 6 meters long concentrate solar irradiance onto a 61 cm by 6 meters wide target area. A large blower forces cooling air across the target area and retractable spray nozzles provide programmed wetting cycles. The large target area allows exposure of large assemblies such as composite bumper systems, fender assemblies and other large automotive components. Super Maq represents an attempt to combine acceleration considerations with full system exposure approaches.
Due to critical air flows over complex geometry, some Super Maq exposures require considerable pre-testing to insure proper specimen cooling. Increased blower capacity, both air velocity and air volume, provide additional cooling for the increased target sizes.
Azimuth tracking is the same on Super Maq exposures as for normal EMMAqua exposures. The increased size and weight of the Super Maq structure requires an automated hoist system for elevation tracking. This results in slightly better alignment than for typical EMMAqua exposures, less consine losses and slightly higher irradiances. The mirror system utilizes the same design as EMMAqua, only larger.
Due to the height of the target area above the concentrating mirrors, it is not practical to mount spray nozzles permanently using the same design as in EMMAqua without significantly interrupting the optical path on Super Maq. During periods of spray, nozzles automatically rotate towards the target area. At the end of the spray cycle, nozzles withdraw leaving the target area unshaded.
EMMAqua Special Cycles.
Test Engineers should not overlook the opportunity to develop special cycles of EMMAqua exposure for specific material investigations. The ability to tailor spray cycles and to perform night time environmental cycling may naturally lead to other evolutions of niche exposure methods not found in standards. The ability to confidently develop special cycles or hybrid exposures is only meaningful if the engineer has characterized the materials performance in the reference environments under natural exposure conditions. The next quantum leap in evolution of natural accelerated weathering test methods, like the pivoting rack, follow the sun trackers and EMMAqua concentrators, may come from hybrid test methods; utilizing combinations of different natural, accelerated and artificial weathering test methods.
The design for special exposures is only limited by engineer's ability to link back or correlate to the reference environments, imagination and funding. Remaining within the paradigms represented by standards is important for comparisons of performance between different vendors, materials, processes, quality control issues and the like. For research and development of materials and processes, however, understandings often come from experimentation outside the standards requirements and utilize novel test method approaches.
Future expectations include new test methods that not only increase the level of the critical weathering variables of temperature, irradiance and moisture, but that also affect the amplitude and frequency of variable cycles.
Subsequent sections of this chapter discuss alternate sources of radiant exposure besides the sun. Hybrid exposures to these sources, in addition to the natural solar spectral power distributions seem to represent a natural extension to current test methods.
As for temperature, moisture cycles continue to develop in frequency and duration.
Laboratory Accelerated Weathering
Due to the need for more rapid evaluations of the resistance of coatings to weathering than can be obtained by outdoor exposure tests plus the need for controlled conditions, test devices with artificial light sources are generally used to accelerate the degradation. The light sources in these devices include two types of filtered carbon arcs, filtered xenon arcs, and metal halide or fluorescent UV lamps. Although the light source is a critical factor in the weathering of materials, heat and moisture play a significant role in the effect of weather on materials through their effect on the secondary reactions following bond breakage by the absorbed radiation and are an important component of the laboratory accelerated tests.
The acceleration over "real time" weathering may occur for several reasons: (1) the tests often run continuously, uninterrupted by the diurnal cycle, seasonal variations and weather conditions, (2) specimens can be exposed to irradiance levels and spectral energies that are only encountered during peak daily or seasonal conditions outdoors, and (3) temperatures, thermal cycles, humidity and water exposure can be manipulated to maximum, but not unrealistic stress levels.
In addition to the ability to manipulate and accelerate weathering conditions on demand, a fundamental benefit of a laboratory test is the repeatability compared to what is essentially an uncontrolled and variable phenomena, the actual weather. In addition, each of the weather factors can be manipulated independently. Thus, research can be conducted into the specific response of the materials to various weather factors, experiments that would be impossible to conduct outdoors.
Accelerating Degradation by Intensification of Weather Factors
While the weatherability of a material depends on its resistance to all weather factors, the radiation of the sun, particularly the ultraviolet (UV) portion is mainly responsible for limiting the lifetime of materials exposed to the environment. Thus, the single most significant component of simulated weather is the nature of the radiation source. The type and intensity of actinic radiation to which the materials are exposed are the determinant factors in accelerating degradation.
The type of radiation is described by the wavelengths emitted and their intensities, i.e by the spectral power distribution of the light source. Both the absorption of light, a necessary first step in the interaction of light with materials, and bond breakage, a critical primary effect of the absorbed radiation, are wavelength dependent. Generally, short UV wavelengths are absorbed by materials to a greater extent than longer wavelengths. In addition, the shorter the wavelength, the larger the energy of the photon associated with that wavelength and the greater the propensity to break the higher energy chemical bonds. Therefore, while the presence of shorter wavelengths than those present in sunlight can accelerate degradation, the mechanism and type of degradation will be altered compared with effects of solar radiation. Acceleration involving change in mechanism and type of degradation cannot be relied on to predict lifetimes under environmental conditions.
If the emission of the accelerated test source differs from that of solar radiation in the UV and visible regions, it can also distort the stability ranking of materials compared with their ranking when exposed outdoors. Stability ranking depends not only on the relative sensitivity of the materials to the radiation absorbed, but to the relative amount of light they absorb. The latter will vary with the light source because of differences in absorption properties of materials.
The intensity of light of each wavelength falling on the surface of a material, i.e., the irradiance level of exposure, is determined by the number of photons associated with each wavelength of light. The number of incident photons absorbed by a material capable of absorbing at that wavelength increases with increasing intensity. However, the dependence of rate of degradation on irradiance level is complex. It varies with the type of material, type of stabilizers present and wavelength of light and is rarely a linear relationship. For example, the rates of photooxidation of both polypropylene and polyethylene have been shown to be proportional to various fractional powers of the light intensity ranging from the square root to the first power. At high intensities of light, the quantum yield of degradation, i.e., the amount of degradation per photon absorbed, will often be less than at low intensities. This is explained, in part, by the "cage effect" in a free radical process. As a result of the high concentrations of free radicals formed a the high irradiance levels, recombination occurs to a large extent so that reaction with oxygen or other molecules is reduced. Thus, doubling the intensity does not necessarily double the rate of degradation. In photooxidation reactions, oxygen diffusion may become the rate limiting step at high irradiance levels.
The temperature of materials exposed to solar radiation can have a significant influence on the effect of that absorbed radiation. The destructive effects of light are usually accelerated at elevated temperatures, primarily by increasing the rate of secondary reactions. At high temperatures molecules have a greater mobility. Therefore, the rate of oxygen diffusion increases and free radical fragments formed from the primary photochemical processes are more readily separated. Thus, the chance of recombination is reduced and secondary reactions are promoted. Also, reactions may take place at higher temperatures that would occur at a low rate, or not at all, at lower temperatures. For example, increase in the rate of hydroperoxide decomposition with temperature can change the mechanism of degradation.
A general "rule of thumb" for the effect of temperature on reaction rates is that the latter doubles for each 10 ° C increase in temperature. However, for photochemical degradation, this relationship generally not observed. Initially, change in property depends very little on temperature, but starting at a temperature which is material specific, the reaction rate increases rapidly. Often, a different mechanism of degradation is triggered at a specific temperature. The temperature of samples exposed to solar radiation or simulated solar radiation is usually considerably higher than the ambient air temperature. Solar absorptivity is closely linked to color, with white absorbing only about 20% of the incident energy and black about 90%. Thus, samples under the same light will reach different on-exposure temperatures, and this can affect the rate of degradation. Because the thermal conductivity of polymers is generally low, the temperatures at the surface can be considerably higher than those reached in the bulk of the material. This can exert physical stresses and effect chemical reactions.
Differences in ambient temperatures can affect the stability ranking of materials because the effect of temperature on the secondary reaction depends on the material and the specific degradation process. When the testing temperature is close to the glass-transition temperature, small differences in temperature during weathering cause great differences in the results of weathering. This can account for the effect of temperature on ranking in the case of amorphous and semicrystalline polymers with small differences in their glass-transition temperatures.
Thus, while degradation can be accelerated by testing at temperatures higher than materials are exposed to under use conditions, caution must be exercised to avoid obtaining unrealistic test data. If the temperature exceeds that at which the mechanism changes, it precludes simulation of results of natural exposures. The closer the accelerated test temperature is to the temperature under natural exposure conditions, the more likely that the effects of natural conditions will be simulated.
Moisture, in combination with radiation, is often a key contributor to material weathering. Moisture, in the form of condensation, water spray, or immersion on humidity, can contribute physically to degradation through the mechanical forces imposed when moisture is absorbed or desorbed. Moisture also can participate in photochemical reactions. The frequency and duration of the exposure to moisture is often a critical parameter and high humidity can be as effective as liquid water.
Water absorption by synthetic materials such as polymers and coatings from both humidity and direct wetness is a diffusion-controlled process. The hydration of the surface layers can produce swelling, resulting in mechanical stress on the drier subsurface layers. In time, an equilibrium may be reached as moisture diffuses into the interior. A subsequent drying out of the surface layer results in a volume contraction; the inner hydrated layers resist this contraction, and surface stress cracking may result.
Moisture may participate in chemical degradation as well. It may act as a solvent or carrier, e.g., in leaching away plasticizers or in carrying dissolved oxygen. Water may also participate as a reactant in hydrolysis reactions. For example, titanium dioxide (TiO2) pigmented coatings and building products such as vinyl siding can "chalk" on outdoor exposure. This chalk is a friable white powder which can be wiped off the surface. The chalk results from the degradation of the binder material and a release of TiO2 particles at the surface. Experience shows that chalking is strongest where more water is available at the surface; little to no chalking occurs in dry environments.
Carbon Arc Weathering Devices
The first Fade-Ometer®, introduced in 1919, used an enclosed carbon arc (ECA) light source. It was used to evaluate the lightfastness of synthetically dyed textiles. Today, enclosed carbon arc weathering devices are available in single and twin arc versions with capabilities for periodic water spray on the samples and condensation during a dark period as well as with humidity and temperature control. The arc is enclosed in a Pyrexä globe to provide some optical filtering and an oxygen-deficient atmosphere to enhance the arc efficiency. Carbon rods are replenished daily.
The ECA spectral emission in UV and visible regions bears little resemblance to daylight. The emission spectrum is shown in Fig.10.21 in comparison to solar radiation. Two strong cyanogen emission bands, peaking at 358 nm and 386 nm, are about 4 and 20 times daylight intensity, respectively. This type of light source can be expected to have a weaker effect than solar radiation on materials that absorb only short wavelength UV radiation, but a stronger effect on materials that also absorb long wavelength UV and visible light. Therefore, in evaluating the relative light stabilities of materials, some of which absorb only short wavelength UV and others that also absorb long wavelength UV, the enclosed carbon arc could distort the rankings when compared with samples exposed to natural solar radiation.
The introduction in the 1930's of the open flame, or "Sunshine" carbon arc, with Corex® filters provides more UV < 300 nm than sunlight but gives a much better match in the 300-340 nm region and deviates less than the enclosed carbon arc at longer wavelengths. Its emission spectrum is given in Fig. 10.21 along with that of the ECA. When used without filters for faster testing, stability rankings of some materials have been shown to be distorted when compared with outdoor exposures. The open flame arc uses three pairs of cored carbon rods and operates in a free flow of air; the arc is rotated among the pairs to provide approximately a day's operation per set and the arc is usually filtered by flat Corex filters arrayed around the arc.
Both carbon arc technologies typically require daily replacement of the carbon rods and cleaning of the filters or globes. The transmission characteristics of the filters and globes change with exposure to UV radiation and they must be periodically replaced. Accumulated carbon soot must also be removed. There is a large volume of historical data using carbon arcs, and a number of test methods still specify their use. While good correlation with outdoor exposures has been reported for some materials, such as fluoropolymer coatings whose weathering mechanisms may be appropriate for these limited spectrum sources, this technology has largely been replaced with xenon arc systems.
Xenon Arc Weathering Devices
Introduced in the 1950's, the xenon long arc source, when properly filtered, more closely simulates full spectrum solar radiation than any other artificial light source. Unlike the electrically burning carbon arcs, the xenon arc is a gas discharge lamp in a sealed quartz envelope. Long arc lamps, where the arc length is greater than the arc diameter, are more stable than short arc discharge lamps.
Figures 22 and 23 show the SPD through various filter combinations compared with Miami average optimum sunlight. The unfiltered radiation of a xenon arc contains considerable radiation in the short wavelength UV region. The spectral distribution can be adjusted through optical filtering to simulate solar radiation ranging from above-the-atmosphere through to sunlight-as-filtered-through-window-glass conditions. The CIRA (near infrared absorbing) filter is used in conjunction with the soda lime glass filter (C) for applications requiring cooler sample temperatures. The combination also provides an optimum match to the cut-on of solar radiation in the UV region. The xenon arc is strongly preferred as a light source when the material to be tested will be exposed to natural sunlight. Xenon has been extensively adopted by the automotive, polymer additive and textile industries in particular, and is generally replacing carbon arc technology.
Unlike carbon arc exposure devices which had minimal control over the light intensity, modern xenon apparatus exercise control over both the spectral distribution of energy through optical filtering and irradiance control through electrical power management. In the course of xenon arc development, two related systems have emerged; air-cooled and water-cooled xenon lamp technologies. See Fig. 24 and Fig. 25 for Xenon Weatherometers. The type of cooling has some influence on the overall design and on the optical filtering system.
As lamps and filters age, the quartz envelope of the burner and the filters themselves undergo a UV degradative process called "solarization" which decreases the optical transmission. The shorter wavelengths are decreased most. Additionally, electrode material in the burner partially evaporates and is deposited on the inside of the quartz envelope, also decreasing transmission. The automatic light monitoring and control systems of these exposure devices adjusts for this light fall-off until such time as the lamps and filters need replacing.
Fluorescent UV Weathering Devices
Figure 26 shows the spectral irradiance of two commonly used fluorescent UV lamps in comparison with daylight radiation. These sources are incorporated into fluorescent condensation devices in which dark periods with condensation and temperature during the light and dark periods are varied. They may also incorporate water spray on the exposed surface of the samples. The emission of the fluorescent UVB-313 lamp, formerly in common use, has very little resemblance to solar radiation. The irradiation peaks at 313 nm and nearly all of its energy is concentrated between 280 nm and 360 nm. A large fraction is at wavelengths shorter than those present in sunlight and it has very little energy at wavelengths longer than 360 nm. Due to the predominance of short wavelength UV and deficiency of long wavelength UV and visible radiation, reversals in stability ranking of materials have often been reported between laboratory accelerated and outdoor tests when the accelerated test uses fluorescent UVB lamps. They cannot be recommended for aging tests on polymers.
For example, evaluations of TiO2 pigmented polyurethane and acrylic paints have shown that polyurethane is superior to the acrylic paint when they are exposed outdoors, but performs much worse than the acrylic based on UVB radiation. It was also found in this study that the discrimination between different grades of TiO2 pigments by solar radiation could not be duplicated by tests using UVB lamps.
The new UVA-340 lamps provide a good simulation of solar radiation in the spectral range between 300 and 350 nm. However, it only simulates global radiation at the short wavelength end of the ultraviolet spectrum and contains only small amounts of radiation in the visible and near infrared regions. Fluorescent UVA lamps with peak emissions at longer wavelengths have more long wavelength UV energy, but less short wavelength energy. The UVA-351 lamp is used to simulate sunlight through window glass at the short wavelength end.
Simulation of the long wavelength UV and visible radiation can be just as important as simulation of the short wavelength UV. For example, wavelengths longer than 350 nm cause photobleaching of yellowing produced by shorter wavelengths as well as by thermal effects. Fluorescent UVA sources can distort the stability ranking of colored materials compared with their ranking by solar radiation through window glass. While short wavelength UV generally causes the greatest degree of color change in pigments, longer wavelength UVA and visible wavelengths can contribute significantly as well. Therefore, it is important to simulate the full spectral distribution of the end use environment for accurate lightfastness results. For example, chrome yellow pigments which are very sensitive to blue light and pigments used in inks for exterior automotive trim decals, limited-edition lithographic prints, commercial billboards, fine art oil paintings and retail packaging can be expected to have vastly different performances under fluorescent UVA exposure and solar radiation.
In a study of TiO2 pigmented resins, their stability rankings based on gloss measurements when exposed in a fluorescent UV condensation device to fluorescent UV lamps with peak emissions between 313 nm and 370 nm were compared with ranking by natural weathering. Contrary to the results of the natural weathering, which showed the polyurethane paint superior to the other four coatings, the thermosetting acrylic paint exhibited superior durability tests under all fluorescent sunlamp exposures; natural weathering ranked this coat third. Chalk and mass loss measurements also showed discrepancies in the fluorescent sunlamp rankings. Further ranking of the four distinct pigment types in the same resin showed major distortions in the performance ranking based on fluorescent sunlamp tests compared to the natural weathering.
Thus, the effects of sunlight exposure can only be reproduced in an artificial test in which the source closely simulates the spectral power distribution of solar radiation over the full range of active wavelengths. Because of the lack of visible and near infrared radiation, the samples are only slightly warmer than the surrounding air in contrast to conditions outdoors. This has the advantage of keeping the samples moist for a longer period of time after they have been covered with dew, for instance, which is especially important with respect to the corrosion of the undersurface of a coating. On the other hand, the mechanical stresses resulting outdoors from the heating and cooling of the upper absorbent layer do not appear, which is a definite disadvantage. This is important in relation to cracking, among other things.
Fluorescent condensation devices do have a role. However, given both the limited spectral distribution of their sources and their more limited simulation of the other weathering factors, the results obtained by their use must be carefully evaluated and qualified within the constraints of the test. As results obtained with them for a sample may or may not correlate with outdoor tests, in-service history, or more complete accelerated laboratory weathering results, their relatively low cost may be partially offset by the need for more rigorous and frequent correlation studies to verify the relevancy of the results.
Metal Halide Devices
The emission curve of the borosilicate filtered HMI metal halide lamp is shown in Fig. 23. It has a multiline spectrum which can be considered to be a continuum for purposes of material testing. It gives a relatively good simulation of terrestrial solar radiation in the UV region above 300 nm, but requires additional filtering of the short wavelength radiation for better simulation of sunlight. However, metal halide lamps have technical problems which must be taken into account when operating weathering systems. One is the dependence of the spectral distribution of the radiation on the temperature of the lamp requiring as constant a temperature as possible in the vicinity of the lamp. Because of the effect of temperature on the SPD, the latter changes with change in the power. Thus the ability to alter the level of irradiance by changing the power is limited to about 5 to 10%. Reduction in irradiance relies on either close-meshed wire filters or increasing the distance of the lamp from the object. The other problem is the variation in SPD from one lamp to another of the same type. It is remedied by measuring the SPD of each lamp and selecting one accordingly.
However, because of their high efficiency and low infrared output which eliminates the need for water cooling, these sources are ideally suited for use in large scale multiple source arrays and are effective for thermal loading studies. They are used in some European test methods, e.g., DIN 75 220, "Aging of Automotive Components in Solar Simulation Units", and comply with certain military testing requirements (MIL-STD 810). Versions of these metal halide systems also meet the specialized lighting requirements for the ultra-high speed photography used in crash sled and SRS airbag inflation testing
Laboratory Accelerated Versus Natural Weathering
There are two fundamental issues which must be considered in sequence when selecting a laboratory accelerated weathering method. These are Correlation and Acceleration. The term correlation refers to the ability of the accelerated test to produce results which agree with real-time outdoor results. Correlation, as applied to a weathering test, generally refers to the agreement of the accelerated weathering techniques with outdoor testing in rating performance differences of various samples with different durabilities. The term correlation has also been used with reference to the type of degradation produced and to the profile of the plot of physical property versus time or radiant exposure.
Acceleration is a measure of how rapidly the test can be conducted using an accelerated weathering device compared with outdoor weathering. Only if agreement exists with outdoor results is it valid to estimate the acceleration of the laboratory test. The acceleration factor is the time to failure (or change in property) under the accelerated test condition divided by the time to failure (or change in property) under natural exposure, both evaluated by the same technique.
Acceleration factors determined on the basis of irradiance levels only are erroneous for a number of reasons. For most polymeric materials, the rate of degradation is not simply a linear function of the level of irradiance. Also, it does not take into account the effect of temperature, moisture and other weather factors. Because of the complex nature of the interaction of the combination of weather stresses with materials, there is presently no simple way to estimate acceleration factors. Thus, there is no substitute for determining the acceleration factor for a given material experimentally. Further, and most importantly, acceleration factors are material specific and vary with formulation. Therefore, it is not possible to establish a single acceleration factor for extrapolating test results to predict lifetimes under natural weathering conditions for a variety of materials and formulations.
The acceleration factor for any sample will depend not only on the accelerated test, but on the geographical, seasonal and environmental conditions of outdoor exposure. It can vary over a range of values from 1 to 100 or larger. However, for accelerated test conditions that correlate with outdoor exposure, the factors are generally between 2 and 10. Generally, the greater the acceleration the poorer the correlation. Figure 10.27 shows the relation between acceleration and correlation of seven weathering test methods with Florida weathering results in a correlation study of gel coats.
The needs of the worldwide automotive industry have driven much of modern weathering research. Vehicles today are more complex assemblies of dozens of different chemistries involving coatings, polymers and copolymer blends, synthetic dyestuffs, woven and non-woven textiles, glazing systems, leathers, pigments, etc., all of which have specific lightfastness and weatherability patterns. The major auto manufacturers worldwide have for decades conducted outdoor natural and accelerated weathering tests in a variety of climates. The tests have been carefully refined and the database on the weathering of automotive materials is large. While the goal of the manufacturers may be to offer a lightfastness or weatherability warranty, they face the dilemma of very short concept-to-market product cycles in which real time weathering tests for material and chemistry selection is not a viable option.
Major automakers have worked with their suppliers, the Society of Automotive Engineers (SAE), Industrial Fabrics Association International (IFAI) and other organizations to standardize on laboratory accelerated weathering test conditions that show generally good correlation with both their historical outdoor tests and actual service use for a wide variety of materials. Today the majority of automotive suppliers test samples from each production lot of every component that will be exposed to sunlight as a quality control check; additional samples are tested in research for initial material selection. These tests are predominately based on the SAE J1885 interior or SAE J1960 exterior component test methods utilizing the xenon arc. These tests are checked by simultaneously exposing standard reference materials (such as polystyrene or American Association of Textile Chemists and Colorists "blue wool" lightfastness standards) to help ensure calibration of the weathering devices and inter-laboratory agreement. Additional outdoor weathering tests are generally conducted to validate the laboratory testing. The automotive manufacturers and their suppliers continue to experiment with new technology and testing approaches. This is exemplified by efforts to expose large components such as assembled instrument panels, door assemblies and bumper fascia systems to gauge weathering effects on stressed and formed multi-component systems. Large component xenon and metal halide exposure systems can accommodate test specimens ranging up to full size automobiles.
Because these methods, or similar versions, have been developed to show good correlation to actual weathering, manufacturers now have the ability to do not only relative rank comparison tests on material, but to make material service lifetime predictions, if only on a limited basis, predicted on a relatively short accelerated laboratory test. It is important to note that no artificial test can ever precisely duplicate all elements of an outdoor exposure. While test methods such as SAE J1885 and J1960 have shown good correlation to outdoor weathering (e.g., south Florida and Arizona) for many materials, they do not purport to be 100% equivalent. Their main value lies in providing repeatable conditions for material comparison.
When problems related to reproducing the results of accelerated weathering occur, the blame is often placed solely on the weathering test. However, the uniformity of the test specimens is a very important consideration. It is often forgotten that preparation of the samples and their treatment can have a substantial effect on the test results. Even if the specimens are guaranteed to come from the same batch, it is worth investigating how they were stored or treated between the time of sampling and the start of testing. Non-uniform conditions during production of the specimens can also make the results of weathering tests difficult to reproduce. Specimens cut from the same sheet often vary significantly. Non-uniformity in response to weathering is particularly applicable to aliphatic type polymers since absorption of radiation longer than 300 nm is mainly due to impurities which are non-uniformly distributed. Degradation of many aromatic type polymers is also largely due to the presence of impurities.
Before drawing any final conclusions concerning the ability of a polymer to withstand outdoor environment based on artificial weathering tests, outdoor exposure tests should be conducted for a reasonable length of time. Artificial weathering does not replace natural exposure; it is a complimentary technique, the nature of which largely depends on how intelligently it is used. Accurate predictions of durabilities depend on valid accelerated tests. All important weathering factors must be adequately simulated by the accelerated test. The accelerated test be designed to simulate as closely as possible the worst case in-service conditions. The guiding principle should be: "simulate, then accelerate".
 Boxhammer, J. and Schonlein, A. Paper presented at Polymer Testing '96 Conference at Rapra Technology Limited, Shawbury, UK, September (1996)
 Hardcastle, H.K. Journal of Vinyl and Additive Technology. (1988) 4, No. 3 p. 169
 Grossman, P.R. Investigation of Atmospheric Exposure Factors that Determine Time-of-Wetness of Outdoor Structures - ASTM STP No. 646 (1978) ASTM, Philadelphia, PA
 Wootton, A.B., Paper presented at Polymer Testing '96 Conference at Rapra Technology Limited, Shawbury, UK, September (1996)
 Climates of the States - National Oceanic and Atmospheric Administration Summaries (1985) Gale Research Co.
 George,O.G. World Distribution of Solar Radiation (1996) Univ. Wisconsin
 Sjostrom, C. Feedback From Practice of Durability Data Inspection of Buildings (1990) Joint CIB/RILEM Committee W80/100-TSL Report p. 19
 ASTM 1998 Annual Book of ASTM Standards (1998) various volumes
 General Motors Engineering Materials and Processes Standards (1997) Various Volumes, General Motors Corp. Warren, MI
 1998 SAE Handbook (1998) 1, Society of Automotive Engineers, Warrendale, PA
 Fischer, R.M., Murray, W.P. and Ketola, W.D. Progress in Organic Coatings (1991) 19 p.151-163
 Martin et. al. Methodologies for Predicting the Service Lives of Coatings Systems (1996) Federation of Societies for Coatings Technology, Blue Bell, PA
List of Figures:
Fig. 1 The Weathering Test Method Continuum
Fig. 2 Venn Diagram of Formulation, Processing and Environment
Fig. 3 Arizona Solar Spectral Distribution
Fig. 4 Ambient Temperature - Sliding Average in Florida and Arizona
Fig. 5 Near Horizontal Surface - Average Radiant Exposure per Day in Florida and Arizona - Total Solar
Fig. 6 Near Horizontal Surface - Average Radiant Exposure per Day in Florida and Arizona - UV only
Fig. 7 Percent Relative Humidity - High and Low in Florida and Arizona
Fig. 8 Wet Time - Hours Each Day - Florida and Arizona
Fig. 9 Tilted 45° Surface - Average Radiant Exposure per Day - Florida and Arizona
Fig. 10 Tilted At Latitude Surface - Average Radiant Exposure per Day - Florida and Arizona
Fig. 11 Solar Spectral Transmittance of 5 Types of Glass
Fig. 12 Tilted 45° Surface Direct and Under Glass - Average Radiant Exposure per Day in Arizona - Total Solar
Fig. 13 Tilted 45° Surface Direct and Under Glass - Average Radiant Exposure per Day in Arizona - UV only
Fig. 14 Tilted 45° , 26° , 5° Surfaces - Average Radiant Exposure per Day in Florida - Total Solar
Fig. 15 Tilted 45° , 34° , 5° Surfaces - Average Radiant Exposure per Day in Arizona - Total Solar
Fig. 16 Comparison of Track Rack to Variable Angle Rack - Average Radiant Exposure per Day in Arizona - Total Solar
Fig. 17 Comparison of Black Box to Unbacked Black Metal Panels - Near Horizontal - June in Florida
Fig. 18 CTH Glas Trac - Average Radiant Exposure per Day - Arizona and Florida - Total Solar
Fig. 19 51° IPDP Standard Black Panel - Maximum and Minimum Temperatures - Near Summer and Winter Solstice in Arizona
Fig. 20 Comparison of EMMA to Track Rack - Average Radiant Exposure per Day in Arizona
Fig. 21 Spectral Power Distribution for the Enclosed and Sunshine Carbon Arc
Fig. 22 Xenon Arc with Q/Q, S/S and S/C Filters Vs. Miami Average Optimum Sunlight
Fig. 23 Xenon Arc with C/CIRA and Filtered HMI Vs. Miami Average Optimum Sunlight
Fig. 24 Xenon Weather-Ometer® Atlas Ci5000 (Courtesy of Atlas Electric Devices Company)
Fig. 25 Xenon Weather-Ometer® Atlas Suntest XLS (Courtesy of Atlas Electric Devices Company)
Fig. 26 UVA-340 and FS-40 UVB Fluorescent Sunlamps Vs. Miami Average 45°S Daylght
Fig. 27 Correlation and Acceleration Characteristics of the Weathering Devices
compared with Miami Weathering Tests for Gel Coats. (Reproduced with Permission from the SPI 51st Annual Conference Session 22-B, 1996)
List of Photographs:
1 Full System Exposure
2 Unbacked Exposure Rack
3 Backed Exposure Rack
4 Under Glass Exposure Rack
5 90° South Backed Rack
6 45° South Backed Rack
7 Near Horizontal Unbacked Rack
8 Black Box
9 Black Box Under Glass
10 Track Rack
11 CTH Glas Trac
12 IPDP Fixture
13 Tracking IPDP Fixture
14 Sun Tracking Carrousel
16 EMMAqua Plus
17 Super Maq