Experts Dissect Challenges of Accelerated Weathering Alternatives during Annual ConferenceFebruary 29, 2012
The ability to accurately test for weathering and meaningfully predict the durability of polymers and coatings intended for outdoor exposure is critical to product development and to certification. Outdoor weathering sites in Arizona, Florida and Ohio routinely expose samples to the conditions of their respective climates. But these tests take up to five years, and this can pose a problem due to today’s fast-evolving technology and competitive markets. Rapid change requires faster decisions, which in turn calls for rapid evaluation of new formulations.
“Outdoor testing is best,” as experts put it during the Alternative Weathering Task Group meeting at AAMA’s 75th Annual Conference in Naples, FL, “…but who can wait?”
Sean Fowler of Q-Lab Corporation (Westlake, OH) and Oscar Cordo, Manager of Technical Standards for Atlas Material Testing Technology (Chicago, IL) updated the Alternative Accelerated Weathering Task Group on the status of accelerated laboratory testing developments and correlation with outdoor weather testing. Correlation means agreement of results between outdoor and accelerated tests. Yet, even with good correlation, there can be variability among lab test results—lab-to-lab and sample-to-sample—, as well as due to apparatus and human factors.
Laboratory testing does offer control, as well as monitoring, of all test parameters, such as spectral distribution and irradiance of light, surface temperature of the exposed sample, relative humidity in the test environment and frequency and duration of light/dark and rain cycles. But, accelerated weathering poses risks of poor correlation with actual experience, as there is an inherent conflict between speed and realism. As a result, some subjects can pass an accelerated test under current protocols but fail in actual outdoor use and vice versa.
A Look at the Variables
To achieve acceptable correlation, Fowler and Cordo noted that one must first understand and accurately characterize the service environment. This means defining the three key elements involved: solar radiation, water exposure and temperature.
Radiation. Radiation (sunlight) has two essential parameters:wavelength andspectral power distribution (SPD), which is the intensity of illumination (a.k.a., irradiance) at each different wavelength across the spectrum of short to long wavelengths (i.e., ultraviolet, visible and infrared - the wavelength ranges for each being defined by the International Commission on Illumination (CIE)).
Well known to cause degradation of materials and coatings due to photochemical reactions, UV can be further subdivided into three subspectra: UV-A - wavelengths of 315-400 nanometers (nm), a billionth of a meter; UV-B - 280-315 nm and UV-C - 100-200 nm. UV-C radiation is found only in outer space, being screened out by earth’s ozone layer. Short wavelengths cause polymer degradation; longer wavelengths cause fading and color change.
The key problem in simulated or accelerated testing is that light sources (i.e., the output from lamps and filters) have a hard time matching natural sunlight. To achieve accurate and reliable predictive test results, there needs to be good correlation by wavelength with the incident full-spectrum light energy. The challenge to defining full-spectrum lighting—that which mimics daylight—lies in the fact that there is no single SPD associated with natural daylight, since the spectrum of daylight varies constantly depending upon atmospheric composition, particularly the amount of water vapor. This is further complicated by seasonal variations and solar angles at different locations.
Water. The relevant characteristics of water exposure include the amount, frequency of exposure, temperature of the exposed surface, purity and the phase of the water contact (vapor, liquid or solid ice). Various combinations of these affect hydrolysis, plasticization, moisture stresses, material removal (erosion) and molecular mobility of the exposed surface.
Water can have both physical and chemical effects on materials. Physical effects include absorption, freeze/thaw cycles, erosion, thermal shock (as when a summer thunderstorm deposits rain on a previously sun-heated panel) and impact (as from hail). In terms of chemical effects, water may act as a solvent to accelerate degradation mechanisms that are occurring due to UV exposure. It may act as a reagent in the hydrolysis process, which causes chalking. In the form of acid rain, it can also change the pH, which can cause an etching effect on coatings.
Proper water exposure should not be underestimated as a factor in achieving correlation, as it changes the rate of reactions and changes the mode of degradation. While the technical emphasis in laboratory weathering simulation is usually on the UV light aspects of weathering, moisture could be the more important factor for many materials. For example, in one test while gloss stayed constant in radiant exposure with no moisture, there was a remarkable loss of gloss when moisture was introduced.
In particular, it has been found that test panels are wet for a longer time outdoors than one might expect (e.g., 8+ hours a day in the Florida environment), with dew being the primary form of outdoor moisture. Significant water is needed to correctly reproduce gloss loss, but simulation testing rarely reaches the temperature/humidity conditions found outdoors.
Although water exposure is difficult to accelerate, reproducing an accurate moisture exposure profile over time is a key to accuracy of results in terms of performance predictability.
Temperature. Temperature-related variables include the high and low extremes, exposure time and the rate of change or cycling. Heat accelerates secondary photodegradation reactions, and thermal cycling causes physical stress. Differences in coefficients of thermal expansion can impart stress into assemblies and painted parts, potentially leading to delamination and cracking. Darker colors, of course, reach higher temperatures under the same exposure (e.g., a 16°C difference can be measured between the surface temperatures of equally exposed white and black panels).
Any of these effects can occur synergistically, with temperature, moisture and radiation working together to degrade materials. For a chance at correlation, it is necessary to account for all of these factors when designing test protocols.
Additionally, presenters during the task group meeting reviewed current accelerated testing in both outdoor and laboratory environments.
Outdoor tests can be accelerated through use of Fresnel natural sunlight concentrators, timed water spray and proper humidity. The presentation specifically covered the evolution of such testing devices and recent enhancements to testing protocols. Natural sunlight concentration is a relatively low-cost testing option with the most realistic spectral content, but it has limited capacity to regulate temperature and moisture. Research studies have shown that for accelerated outdoor testing, solar concentrating followed by a nighttime wetting cycle provided the best correlation.
Accelerated laboratory weathering technologies reviewed included fluorescent UV and Xenon arc lamp light sources.
Fluorescent UV, such as provided by the UVA-340 fluorescent lamp, is cost effective and gives good SPD correlation in the 280-360 nm wavelengths, especially near the upper end of the range. The spectrum is stable, which affords good repeatability and reproducibility, and the testing apparatus can simulate aggressive and realistic moisture attack. However, fluorescent UV does not produce visible light and is deficient in long wave output (the lack of infrared creates unrealistic temperature vs. color response data).
The Xenon arc lamp was recommended as offering the best realistic simulation of short-wave UV and visible light when properly filtered. The irradiance spectrum of filtered Xenon in the UV range offers the closest match to natural sunlight than any other artificial light source, according to Fowler and Cordo. The program went on to describe the functional elements of the Xenon arc lamp testing device and how it operates.
Newer filter systems are now available that provide spectrally the closest match to sunlight in the critical UV and visible regions of the spectrum. Filter technology enhancements have been key to improving the correlation of characteristics of lab Xenon lamps with those of natural sunlight.
ASTM test methods employing Xenon lamps are ASTM D6695 for coatings and ASTM D2566 and D4459 for plastics. Research shows that Xenon lamp exposure with proper filtering is effective, but the presenters assert that testing per ASTM cycles provides inadequate moisture. Using test methods for automotive finishes, such as SAE J2527, appears to be superior to these methods, although it does not provide adequate moisture for good correlation with outdoor testing. A significant research effort is aimed at improving the moisture portion of Xenon test cycles to mimic the amount of moisture uptake in automotive coatings under natural Florida weathering exposures. A new ASTM test method is being proposed that utilizes the protocol developed during this research effort.
Efforts continue to devise accelerated testing methods that accurately simulate and correlate to outdoor exposure over time. The reliability of the resulting predictive performance depends on carefully defining test protocols and conditions, and research continues to pinpoint and fine tune the variables that affect correlation. Primarily, these are the accumulation of photons vs. time and enhanced control of moisture exposures.