Technical Note:
Weathering

LS-170a
Figure 1: Solar spectrum.

Weathering is a complex topic. In some circumstances, combined attack, for example by UV and ozone, is more serious than separate exposures to the same aggressors. Moisture level often plays an important role. In some polymer materials, thin, UV induced, cross-linked surface layers provide partial protection against further damage. In others, the UV creates "color centers" that enhance the effect of subsequent radiation. Absorbing "UV stabilizers" can afford some protection for plastics.

Oriel® Solar Simulators for Weathering

In all cases, the collimated and carefully spectrally characterized output of our solar simulators helps you unravel the complex interplay. The round-the-clock availability of high intensity levels allows accelerated testing. You can subject a test sample to the equivalent of many years of solar UV in a matter of days. Simple dose control using the shutter and shutter timer allows determination of the relationships between exposure dose and color change or change in mechanical properties.

Reciprocity

Most accelerated weathering tests rely on the response (the weathering) being independent of irradiation rate; the response must depend linearly only on the total radiant exposure (or "dose").

Effects that depend on dose rather than dose rate are said to follow the law of reciprocity. Accelerated tests are simplest for materials with reciprocity; you can easily relate the results of these tests to expected lifetime outdoors. (In principle, characterization of the dose rate dependence of degradation for a sample that does not exhibit reciprocity will also allow estimation of lifetime, but you need extensive data on the variation of the solar irradiation level).

Fig. 2 illustrates two of the simplest types of departure from reciprocity. The top graph shows an effect that "saturates" at high flux levels; this may be due to sample heating. This type of behavior limits the rate of acceleration. For other processes the rate increases, i.e. the curve swings upwards, at higher fluxes, due to increasing sample temperature.

The lower graph is typical of a weathering change where a simple conversion takes place. The upper curve and the left axis show the cumulative dose as time progresses. (The UV flux from this simulator is ca. 143 W m-2. In one minute the dose is 60s x 143 W m-2 = 8580 J m-2, or 8.58 kJm-2.) The cumulative dose eventually becomes high enough to have converted a significant fraction of the material and subsequent irradiation has lowered efficiency.

We have used these two simple examples for clarification. Identification of the cause of reciprocity failure may not be so simple. For example, saturation at high flux levels may also be due to cumulative dose saturation shown in the lower figure.

LS-170b
Figure 2: Two different types of reciprocity failure.

Be Careful of Other Effects

Effects other than the one under study may set a limit on accelerated testing. The maximum irradiation rate must not damage the sample (or cause non-linear response, Fig. 2). Absorbed visible and infrared radiation in intense solar radiation or simulator beams can quickly char dark fabrics, even though the ultraviolet induced weathering effects is still following the law of reciprocity. We offer simulators that have greatly reduced visible and infrared output. This allows realization of high ultraviolet intensities without complications from visible and infrared heating.

The Importance of Spectral Matching

When the action spectrum for the weathering effect is not precisely known, as is often the case, it is important that the test spectrum closely simulates the expected spectrum for deployment. Using a total ultraviolet level for tests can be misleading if the simulator spectrum isn't a reasonable approximation to a time weighted outdoor spectrum. This is particularly true for highly variable action spectra.

Any weathering effect following reciprocity will be proportional to the total effective dose; the wavelength integrated product of the action spectrum and the dose spectrum. The appropriate dose spectrum should take into account the diurnal and annual dependence of the outdoor spectrum and may be expressed as "worst case" annual irradiance, or for materials like those making up patio furniture, the local spectrum for total summertime dose. Reliable information is gradually becoming more available on UV dose spectra for various sites throughout the world.

The integrated effectiveness spectrum indicates how effective each source is in producing weathering. In the example, we are able to compare the sources because we used a known action spectrum. The simulator based on the metal halide lamp produces 10% more weathering than expected from power measurement.

Example

For our example, we use the summer noon UV spectrum and a “weathering action spectrum” based on the ultraviolet absorption of polycarbonate resin. We compute the effect at each wavelength (in arbitrary units) for the solar spectrum by multiplying the value of the solar irradiance by the value for the action spectrum. Integrating the values gives the total weathering effect for this action spectrum and that solar spectrum.

We repeat this process for the simulator with atmospheric attenuation (AA) filter, and a metal halide based simulator. For both of these we scale the output to match the total solar irradiance from 280 - 400 nm. The results are tabulated.

Table 1

Source UV Spectrum Integrated Effect (Relative)
Sun Noon, summer Spectrum, see Photochemistry & Photobiology 1
91260 UV Simulator with AA Filter Scaled to 1 sun, see 150 - 300 W Oriel® Solar Simulators 0.97
Metal Halide Simulator Scaled to 1 sun, see Simulation of Solar Irradiation 1.1

Broadband Power of Energy Meters

When the action spectrum is not known, the closer the spectral match between the simulator and the average solar irradiance, the easer it is to extrapolate simulator results to outdoor reality. This is particularly important when comparing new product formulations with possible differences in action spectra.

Broadband power or energy meters are sensitive to a broadband of wavelengths. Broadband UV meters are useful when working with a single source or two sources with similar spectral content. Using a meter to measure solar UV and then comparing this value with simulator output can lead to serious errors. In biological applications, Sayre1 showed errors of factors of twenty!

For valid comparison of sources with different spectral outputs using a broadband meter either:

The sources must have similar spectral content

or

The meter spectral response must have the same shape as the action spectrum


References

  1. Sayre, R. and Kligman L., Photochem. Photobiol. 55:1:141