Sunlight
Light energy falling on the earth includes light we can see, from red to blue, and light we can’t, such as Ultraviolet (UV) and infrared. In general, any form of electromagnetic energy can be characterised by its wavelength.
The visible region of the electromagnetic radiation spectrum runs from about 400 to 700 nm. UV light falling on the earth has a wavelength between 300 and 380 nm and infrared between 780 and 3000 nm.
The two particular wavebands that affect the long term performance of pre-finished steel roof and wall cladding are ultraviolet, and infrared light.
Infrared light - temperature effects
The most destructive light to materials whether human skin, plastics or pre-finished steel is UV light. This light is directly absorbed at the atomic and molecular level within materials, leading to the breaking of chemical bonds. For all organic coatings, this eventually leads to a loss of gloss, fading, chalking, brittleness and longterm loss of coating adhesion.
Absorption of UV light depends heavily on the pigmentation of a particular coating. Different colours will behave better or worse in this respect. For this reason, guarantees of durability often give different periods for different colours – in the case of the Confidex® guarantee for Colorcoat HPS200® Ultra, colours are grouped into two categories, CD1 and CD2, depending on their pigmentation and reaction to light.
It is well known that the seasons bring changes in our exposure to UV light. Furthermore, worldwide climates differ in their degree of weathering effects due to different light energies around the world. Naturally, the durability of any material, including pre-finished steel, will be affected by this wide range of solar energy levels such that expected lifetimes will be longer in the less severe locations.
Annual incident solar energy levels in a selection of worldwide locations:
| Location | Latitude (oN) | Annual incident solar energy levels |
| Central Sahara | 25 | 2500 |
| Haifa, Israel | 33 | 2000 |
| Tamani, Italy | 38 | 1800 |
| Freiberg, Germany | 48 | 1100 |
| Helsinki, Finland | 60 | 950 |
| Lerwick, Scotland | 60 | 775 |
Infrared light is absorbed by all forms of matter, causing an increase in surface temperature that can become markedly greater than the surrounding air temperature. Colour has a significant effect on the absorption of infrared radiation. For example, on a hot summer day in the UK, a black roof could reach a surface temperature of 80°C, while a white one would only reach 40°C.
Furthermore, colour affects the thermal radiation from products. Thermal radiation from black or dark coloured surfaces is understandably greater than from lightcoloured ones. Exterior surfaces are continuously subjected to thermal changes, which can be rapid and of considerable magnitude. On cold, clear nights with no wind, thermal radiation from external surfaces of buildings can make the envelope temperature more than 5°C lower than the air temperature. This can cause the formation of condensation on the external sheet of a roof.
Higher surface temperatures increase the rate of chemical reaction and these cause the weathering of pre-finished steel cladding. These changes are similar to those caused by UV light. Additionally, climates which can cause extreme temperature cycling tend to add to the stress factors experienced by a pre-finished steel product, such as coating expansion and contraction and moisture condensation.
The combination of UV light and cyclic temperature changes provide a severe test to a pre-finished steel product. This is why the Period to Re-paint Decision (PRDs) and Confidex® Guarantee period for Colorcoat® products exposed in hot, higher UV climates are lower than for more temperate regions such as Northern Europe.
