IEC 60068-2-5:2010 pdf – Environmental testing- Part 2-5:Tests -Test Sa:Simulated solar radiation at ground level and guidancefor solar radiation testing.
A5 Degradation of materials
The combined effects of solar radiation, atmospheric gases, temperature and humidity changes, etc. are often collectively termed weathering and result in me ageing and ultimate destruction of most organic materials (e.g. plastics, rubbers, paints, limber, etc.).
Many materials which give satisfactory service in temperate regions have been found to be completely unsuitable for use under the more adverse conditions of the tropics. Typical defects are the rapid deterioration and breakdown of paints, the cracking and disintegration of cable sheathing and the fading of pigments.
The breakdown of a material under weathering usually results not from a single reaction, but from several individual reactions of different types occurring simultaneously, often with interacting effects. Although solar radiation, principally the ultra-violet — resulting in photo- degradation — is often the major factor, its effects can seldom be separated In practice from those of other weathering factors. An example is the effect of ultra-violet radiation on polyvinyl chloride, where the apparent effects of ultra-violet radiation alone is small but its susceptibility to thermal breakdown, in which oxygen probably plays a major role, is markedly increased.
Unfortunately, artificial tests occasionally produce abnormal defects which do not occur under natural weathering. This can often be attributed to one or more of the following causes:
a) many laboratory sources of ultra-violet radiation differ considerably from natural solar radiation in spectral energy distribution;
b) when the intensity of ultra-violet radiation, temperature. humidity. etc. are increased to obtain an accelerated effect, the rate5 of the individual reactions which occur under normal exposure conditions are not necessarily increased to the same extent;
C) the artificial tests, in general. do not simulate all the natural weathering factors.
8.1 General
The radiation source may comprise one or more lamps and their associated optical components. e.g. reflectors, filters, etc.. to provide the required spectral distribution and irradiance.
Depending on place, time. irradiance. spectral distribution and power of radiation, different lamps with different filters can be used.
8.2 Filters
The choice of filters depends on the source, the equipment and Spectral distribution. The present preference Is therefore for glass filters to be used, although fundamentally a glass is not as accurately reproducible as a chemical solution. Some trial and error may be necessary to compensate for different optical densities by using different plate thicknesses. Glass filters are proprietary articles and manufacturers should be consulted concerning the choice of filters suitable for particular purposes. The choice will depend on the source and its method of use.
Some glass filters may be prone to rapid changes in spectral characteristics when exposed to excessive ultra-violet radiation. This deterioration may be largely prevented by interposing the ultra-violet tiller between the source and the infra-red filter. Interference type filters, which function by reflecting instead of absorbing the unwanted radiation, thus resulting In reduced heating of the glass, are generally more stable than absorption filters.
8.3 Uniformity of irrad lance
Owing to the distance of the sun from the earth, solar radiation appears at the earth’s surface as an essentially parallel beam. Artificial sources are relatively close to the working surface and means of directing and focusing the beam shall be provided with the aim of providing a uniform irradiance at the measurement plane within specification limits (i.e. 1 120 W1m2 ± 10 %). Uniform irradiation is more readily achieved with a long-arc lamp mounted in a parabolic ‘trough’ type reflector. By employing very elaborate mounting techniques, it is possible to Irradiate, with some degree of uniformity, a large surface by a number of lamps. It Is also possible using a turntable.
It is generally advisable to locate radiation source(s) outside the test chamber. This avoids possible degradation of the optical components, e.g. by high humidity conditions and contamination of test specimens by ozone generated by some types of lamps. In this case, the spectral transmittance of the window material shall be taken into account.
Precise collimation of the radiation beam is not normally necessary except for testing special equipment such as solar cells, solar tracking devices, etc.