It's not easy to predict how a part will stand up to the ravages of nature. This may be especially true for parts made from plastics.
Vice President Technical Services
Q-Panel Lab Products
EDITED BY Amy Higgins
It's no secret that accelerated development schedules now land new products on the showroom floor at break-neck speed. Gone are the days when designs had the luxury of long in-depth testing programs.
Accelerated development schedules present a conundrum of sorts when it comes to the effects of weathering. The forces of nature haven't changed. It still takes months or even years to thoroughly test new designs. Accelerated tests can provide a few insights. But they are not a substitute for extended exposure to real conditions.
Without field tests, there is no good way of understanding exactly how products are likely to handle weather. Designers who lack the time for such tests should know what they may be missing. They should also understand what information accelerated tests can and can't provide.
The process of weathering depends greatly on atmospheric elements such as solar radiation, high temperatures, and any kind of moisture. Other agents such as microorganisms, ozone, pollution, and oxygen are also destructive and must be reckoned with. In addition, there are often synergistic effects between sunlight and moisture that may not be readily identifiable from properties listed in material data sheets.
Weathering causes millions of dollars of product damage annually. Damage ranges from aesthetic alterations in color and gloss, to surfaces that craze, peel, delaminate, chalk, and oxidize. Weathering frequently contributes to the deterioration of a plastic's mechanical strength as well.
In today's international economy, few companies only sell locally. Differences in climate can lead to drastic differences in how products perform. Consequently, scientists often use "Florida Weathering" and "Arizona Weathering" as benchmarks for durability testing. These locations typically produce faster degradation than exposures in more temperate climates.
No laboratory test can replicate all of the things which can occur outdoors. Durability testing programs should include natural weathering exposures in both Florida and Arizona in conjunction with accelerated tests conducted in a laboratory.
Florida has high-intensity sunlight, high year-round temperatures, abundant rainfall, and high humidity. Likewise, the Arizona desert exposes samples to brutal, yet realistic conditions: high UV, high temperatures, large daily temperature fluctuations, and low moisture. For many materials, Arizona's harsh climate produces even faster degradation than Florida's. Particularly affected are coating color and gloss. Many materials, and particularly plastics are prone to heat aging and fading there.
There are various ways of mounting and exposing samples in natural outdoor tests. Each has advantages and limitations, depending on the material's intended application.
KNOW YOUR ENEMY
The electromagnetic energy from sunlight can be categorized as ultraviolet (UV), visible, and infrared. IR is most important because it heats test specimens. Darker specimens will absorb more heat than light specimens. Heat aging alone can be a significant cause of degradation. In addition, high temperatures may also greatly accelerate the photochemical reactions caused by visible and UV light. Expansion and contraction from thermal cycling may also result in deformation or delamination.
Visible light is also responsible for some of the heat build up in materials exposed outdoors. More important, it can be a factor in fading, especially for nondurable dyes, inks, and pigments. Most fading from weathering comes from the short wavelength visible light and longer wavelength UV.
Although it makes up only about 5% of sunlight, short wavelength UV light accounts for most physical property degradation to durable materials. Photochemical degradation comes from photons of light-breaking chemical bonds. For each type of chemical bond there is a critical threshold wavelength of light with enough energy to cause a reaction. Light of any wavelength shorter than the threshold can break the bond, but longer wavelengths of light can't regardless of their intensity or brightness. Typically, only short-wavelength, high-energy UV light has enough energy to break polymer bonds.
The short-wavelength cutoff of any light source is critically important. For example, if a particular polymer is only sensitive to UV below 295 nm (the solar cutoff point), it will never experience photochemical deterioration outdoors. If the same polymer sees a light source with a spectral cutoff of 280 nm in a laboratory, it will deteriorate.
THE RIGHT EXPOSURE
To maximize sunlight dosage, researchers normally mount test specimens facing the equator. Any object will receive more solar energy when sunlight strikes it dead on rather than at an angle. In Florida and Arizona the sun climbs high above the horizon during summer time. With the sun at its highest point, samples exposed at 5° from the horizontal get more solar energy than those exposed at an angle of 45°.
The angle of exposure is called the tilt angle. It significantly affects a sample's response to the environment. Tilt angle determines how much solar radiation the sample receives as well as how fast it heats or cools. Tilt angle also influences how long the sample stays wet from dew formation and rainfall.
As a general rule, the exposure angle should be representative of what the material will see in normal use. There are a few commonly used angles, measured from the horizontal:
45° South is regarded by many as the typical outdoor exposure angle and is the angle of choice for many industries. It's typically selected as the standard for materials without a specific end-use angle.
5° South is for automotive products and other materials whose end use is at or near the horizontal. Compared to 45°, this is harsher because samples get more annual solar radiation. In summer months the effects are enhanced by higher temperatures. And although the 5° tilt lets moisture run off the test samples, they experience longer wet time than those tested at a 45° tilt.
0° Horizontal exposures are rarely used for flat specimens or panels. The slight 5° angle is preferred because it lets water easily run off. But, horizontal mounting is useful for many three-dimensional parts.
90° South or vertical greatly reduces solar radiant exposure, lowers exposure temperatures, and lets samples dry off more quickly. It is mostly used for residential coatings.
Latitude-angle exposures test specimens to the most solar radiation of any fixed tilt angle. For example, test facilities in Florida are at a latitude of 25° have specimens mounted at 25° tilt angles. Arizona facilities sit at 34° North latitude and put specimens at 34° South tilt angles. These exposure angles maximize the radiant energy. Latitude angle is also widely used to compare solar devices such as power cells and solar heaters.
Variable-angle exposures are adjusted seasonally to maximize solar radiation by constantly aligning the specimens at normal incidence to the sun. This method increases the total radiant exposure by as much as 10% compared to other fixed angle exposures.
Natural weathering exposures typically take place in standard frames or racks that hold test specimens in place. The fixtures are such that they don't interfere with the test. They generally are made from wood, aluminum, or stainless steel.
Direct exposure is the most widely used test method. Specimens mount on an exposed rack so they face the sun. They get the full impact of rain, dew formation, hail, abrasion from blowing sand or dirt, and mildew or fungus buildup.
Samples on direct exposure frames may be exposed either with or without backing. Open-backed exposure is most common for rigid panels, glass, free-films, coil coatings, and plastic lenses such as taillight assemblies. Samples rest on an open framework, usually facing south, and are open to the elements at both top and bottom.
Nonrigid specimens, three-dimensional parts, and samples requiring higher temperatures such as elastomers and automotive coatings are often mounted over a plywood backing. The solid backing typically gives the samples greater wet time and higher temperatures than open back mounting. Standard procedures ASTM G7, D1435, and SAE J1960 cover direct exposure methods.
Another setup employs a Black Box configuration designed to imitate exposure on the trunk and hood of an automobile. Typical exposure frames are a 5 12-ft aluminum box 9-in. deep. The test panels form the top surface. Because the top surface must be completely enclosed, blank or "dummy" panels are positioned in any empty spaces. Black-box setups generally boost exposure temperatures and lengthen wet times. The Black Box exposure method is covered by ASTM D4141.
A number of Under Glass exposures serve as tests for interior parts. One method tests samples inside a ventilated framework, 3 in. below a glass cover. The glass filters out part of the sunlight spectrum and protects samples from direct rainfall and most condensation. But samples still see fluctuations in humidity. The glass cover must be cleaned once a month in accordance to ASTM G24 to prevent dirt buildup which might prevent solar transmittance.
The automotive interior materials (AIM) Box was developed 15 years ago by GM and was customized for the automotive industry because typical under glass exposures did not realistically simulate the end use of automotive interior materials. As before, the glass cover keeps out direct rainfall and filters the solar energy. The AIM Box can operate at either a fixed, static-exposure angle or can track the sun throughout the day to maximize radiation.
The sealed AIM Box lets researchers expose automotive materials to higher temperatures than conventional under-glass equipment. A blower recirculates ambient air within the cabinet to lower the temperature limit below the setpoint. This is important because many specifications for automotive interior materials require testing at different temperatures. Limits depend on where the material sits in an automobile and temperatures it sees in service. For example, 93°C is the usual limit for door panels or lower instrument panel assemblies; a 102°C limit is used for testing center consoles.
GM9538P, "Weathering Exposure Tests for Interior Trims," is a widely used automotive specification for AIM Box testing. GM9538P includes "Seasonally Adjusted Solar Radiation" (SASR) Factors. A SASR Factor is a mathematical calculation that helps compensate for variability in radiation and temperature throughout the year. Different SASR Factors may be used depending on temperature requirements.
AIM Box testing was once limited to the peak radiation months of April through October, to maximize UV dosage. For example, an AIM Box test in the winter may take months to accumulate a particular radiation dosage. AIM Box tests in the summer may hit the same radiation level in a few weeks. The variation in solar irradiance comes from differences in the sun's position and angle throughout the year.
SASR Factors compensate for how long it takes a test to reach a particular dose of radiation. It also lets researchers test during any month of the year. A SASR Factor is calculated by multiplying the measured total radiation by an adjustment factor specified for a particular temperature limit and month of test. Different SASR Factors handle peak temperature limits appropriate for certain types of material testing.
Air, cloud cover, atmospheric moisture, and pollution all filter out UV. Consequently there are seasonal differences in the intensity and spectral distribution of natural light. During winter, for example, the sun is low in the sky and its light must travel through denser air to reach the earth's surface.
The shortest most damaging UV rays are filtered out during the winter. The intensity of UV at 320 nm, for example, changes about eight-to-one from summer to winter. This is especially significant for polymeric materials such as PVC. From summer to winter, the solar cut-off also shifts from about 295 nm to nearly 310 nm. Consequently, materials sensitive to UV below 310 nm degrade little, if at all during the winter months.
Seasonal differences tend to average out during longer-term exposure tests exceeding two years. But, these differences may affect test results for less-durable products and materials designed for shorter service lives. Items in this category include bumper stickers, decals, and some printing inks or textiles. The shorter the exposure, the more significant the potential effects of seasonal variation. Seasonal differences can affect the time to failure, the mode of failure, and even the ranking of a specimen relative to an array of similar materials.
The best way to test materials which have a short service life is to expose a number of different test sample groups at each season of the year. If there is an urgent need for information, tests should take place immediately, regardless of the season, but understanding that there is potential for seasonal differences. Normally, tests should take place during summer, the season of greatest severity.
It is possible to accelerate a natural exposure artificially. For example, test panels on a plywood backer will see higher temperatures and stay damp longer than on an unbacked panel. Similarly, a specimen mounted at Latitude Angle will see more radiant energy than one mounted on a horizontal rack. Depending on the intended enduse, these could be accelerated exposures.
Laboratory simulations may give accelerated results, but sometimes at the expense of accuracy. There will probably always be controversy about whether lab tests correlate with natural exposure data. Accelerated light sources with short wavelength UV give fast tests, but may not always be accurate. Usually, where they are wrong, they are wrong on the safe side they are too severe. Light sources that eliminate wavelengths below the solar cutoff of 295 nm will give better, more accurate results, but at the price of reduced acceleration.
Users can program any accelerated test chamber for various levels of UV spectrum, moisture, humidity, temperature, and test cycle. The chosen parameters are, to a certain extent, arbitrary. No single test cycle or device can reproduce all variables found outdoors in different climates, altitudes, and latitudes. Consequently, even the most elaborate test chamber is really just a screening device. The real usefulness of accelerated testers is that they can give reliable indications of which material performs best under a specific set of conditions.
SASR factors for Arizona AIM Box testing
|The monthly seasonally adjusted solar radiation (SASR) factors for an Arizona AIM Box test give a method for mathematically compensating for the variability in solar radiation and temperature throughout the year. The test used a tracking fixture and a 93°C temperature limit.|
WHICH APPROACH IS BEST?
The weathering industry offers a range of techniques to gauge weathering and light stability. Unfortunately, no single testing technique suits all materials and applications. The best approach depends on goals, time frame, and budget. Each technique has inherent strengths and weaknesses.
Natural Florida weathering is the world standard for sunlight and moisture, but tests can take years to complete.
Arizona desert weathering has even more sunlight and higher temperatures than Florida, but lacks moisture.
Natural sunlight concentrators intensify natural sunlight on test specimens, but they are subject to seasonal variations in weather.
Fluorescent UV testers are fast and economical. Fluorescent UV lamps provide the best simulation of solar UV. But they lack the longer wavelengths necessary for testing certain materials.
Xenon arc tests reproduce the full spectrum of sunlight, including UV, visible, and infrared. They are especially useful for testing dyes, pigments, textiles, inks, and indoor materials. But, xenon arcs are more expensive and inherently less stable than fluorescent lamps.
No laboratory test can replicate all conditions found outdoors. It's advisable to institute testing based on natural exposures in both Florida and Arizona. These types of exposures can be remarkably inexpensive. Also recommended is at least one accelerated test. The test chosen should be optimized for the material and the end use. Natural out-door exposures provide a solid baseline, while accelerated tests give fast data on new developments.