505.7-77 interpretation of results

The materiel specification should indicate the permitted changes in the external conditions and/or performance of the test item after exposure to the required level of irradiation for certain durations. In addition, the following aspects of interpretation may be considered:

7.1 Comparison with Field Experience

The effects of exposing material to solar radiation are well documented (see also paragraphs 7.2 and 7.3 below). Investigate any marked differences between the expected effects and the behavior under test conditions, and the basic cause established, i.e., whether caused by the test equipment or procedure, or by some peculiarity in the test item.

7.2 Thermal Effects

• a. The maximum surface and internal temperatures attained by materiel will depend on:
• (1) the temperature of the ambient air.
• (2) the intensity of radiation.
• (3) the air velocity.
• (4) the duration of exposure.
• (5) the thermal properties of the materiel itself, e.g., surface reflectance, size and shape, thermal conductance, and specific heat.
• b. Materiel can attain temperatures in excess of 60 °C (140 °F) if fully exposed to solar radiation in an ambient temperature as low as 35 to 40 °C (95-104 °F). The surface reflectance of an object affects its temperature rise from solar heating to a major extent; changing the finish from a dark color, for example, to a gloss white will effect a considerable reduction in temperature. Conversely, a pristine finish designed to reduce temperature can be expected to deteriorate in time resulting in an increase in temperature. Most materials are selective reflectors, i.e., their spectral reflectance changes with wavelength. For instance, paints, in general, are poor infrared reflectors although they may be very efficient in the visible region. Furthermore, the spectral reflectance of many materials changes sharply in the visible (producing a color sensation to the human eye) and in the near infrared. It is important, therefore, that the spectral energy distribution of the radiation source(s) used in any simulated test should closely duplicate that of natural radiation.

7.3 Degradation of Materials

The combined effects of solar radiation, atmospheric gases, temperature, humidity changes, etc., are often collectively termed “weathering,” and result in the “ageing” and ultimate destruction of most organic materials (e.g., plastics, rubbers, paints, timber, etc.). Many materials that give satisfactory service in temperate regions have been found to be completely unsuitable for use under more adverse conditions. Typical effects 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 ultraviolet portion, resulting in photodegradation 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 ultraviolet radiation on polyvinyl chloride, where the apparent effects of ultraviolet radiation alone are 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 that do not occur under weathering. This can be often attributed to one or more of the following causes:

• a. Many laboratory sources of ultraviolet radiation differ considerably from natural solar radiation in spectral energy distribution.
• b. When the intensity of ultraviolet, temperature, humidity, etc., are increased to obtain accelerated effects, the rate of the individual reactions (that occur under normal exposure conditions), are not necessarily increased to the same extent. In some cases, e.g., fluorescent lamps, the infrared energy of the source is significantly less than that of true solar loading, resulting in a surface test temperature that is lower than would be experienced out-of-doors.
• c. The artificial tests, in general, do not simulate all the natural weathering factors.