Infra Red Reflectance Coating

Reducing the amount of heat entering a railway carriage from the roof reduces the discomfort of the passengers during summer and the energy needed to run the air conditioning. With global warming increasing, such energy savings are going to be ever more important.

It is well known that a white or light coloured surface remains cooler than a black, or darker surface thanks to the interaction of the painted roof with the infra-red radiation emitted by the Sun. As infra-red radiation accounts for approximately 50% of the total energy of solar radiation, reducing the amount absorbed by the paint is clearly beneficial. Work by NK Bansal and others found that a black painted enclosure recorded a maximum of 7^oC higher temperature than a corresponding white painted enclosure during the hours of maximum solar radiation.

Carriage roofs are not painted white or very light colours because they will very quickly become dirty through use, resulting in both a poor looking carriage and a now darker surface.

By modifying the pigments and additives within a standard rail approved paint, Blocksil can improve the Total Solar Reflectance (TSR) of the paint, even with darker colours. Depending on the paint colour and the levels of brightness, laboratory tests suggest a temperature reduction of between 9^oC and 14^oC, with an improvement in the TSR of between 50% and 32%.

If a standard dark coated railway carriage increases the temperature by some 7^oC and our InfraRed Reflectance (IRR) paint can reduce the transmitted temperature by between 9^oC and 14^oC, then the effect of the sunlight will effectively be neutralised or even reversed. This would allow the air conditioning to focus on reducing the relative humidity.

An additional benefit of having an improved TSR is an enhanced lifespan of the paint. The reduced surface temperature will reduce the thermal degradation of the polymeric matrix. Temperature differences between day and night, direct sunlight and shadowed areas might also level out. This in turn would lead to reduced thermal warping and stress.

Attached is an image taken with a thermal imaging camera. It has an IRR coated panel to the top of the image (orange in colour) and a standard grey coated panel at the bottom (purple in colour). The panel edges can just be made out as black lines. A heat source was in place above the panels.

The temperature where the camera is spotted (white target circle at the centre of the image) is 20.8^oC. The temperature range within the total image area of the camera is 18.8^oC to 32.8^oC. Thus the hottest spot, the orange / white area at the top of the image, is 32.8^oC. So the IRR painted panel is reflecting more heat back to the camera than the darker, standard paint which has absorbed the heat. Put it another way, the Blocksil IRR paint has prevented 12^oC (32.8 – 20.8) of heat from entering the coated surface.

Demonstrating the Infra Red Reflectance properties of our IRR coating