Feedback

Architects memo no. 78: November 2004
cool eh!


In Memo 73 we discussed the often, much hyped role that paint can play in temperature control in buildings, and advised that, for all of the hyperbole associated with this subject, it all came down to colour. The same memo did however have a lightly veiled reference to the fact that something was brewing in the Resene labs, which might change the paradigm.

To recapitulate, the sun emits its energy over a wide band of wavelengths. About 5% of this energy is radiated in the ultra-violet range - not a lot but it is very damaging to organic matter, due to its very short wavelengths.

44% of the sun's energy is emitted in the visible wavelengths. It is the ability to reflect this 44% that makes white appear white; and the ability to absorb it that makes black appear black.

Although the intensity of the sun's energy peaks in this visible range (actually at about the wavelength of blue light), the majority, 51% is emitted in the infra-red range.

The ability of white to reflect visible light extends through into the infra-red and, because of this, white surfaces remain relatively cool to touch even in direct sunlight. The converse is true of black and dark colours, which absorb in this infra-red area, resulting in significant heat build up of the surface. As the emissivity of paints is not particularly good, this surface heat is conducted into the substrate and radiated into the building or vehicle.

Not all dark colours behave like this however, and once again Mother Nature beat us by a few million years. Most plants have leaves of a very high chroma green and some of them are very dark indeed. If the leaves reached the same temperature when exposed to solar radiation that a similar coloured paint would, they would shrivel and die. The fact that they don't is because the pigment they contain, chlorophyll, absorbs in the visible range in order to photosynthesise its requirement, but reflects in the infra-red range, keeping the plant cool.

Over the past few decades there has been vigorous industrial effort developing high heat resistant pigments for the ceramics industry. Most paint pigments will simply break down at the very high temperatures at which ceramics are fired. A different strategy is needed and the ceramics pigment industry has found novel ways of doping refractory metal oxides of titanium, zirconium, chromium and so on with other metal ions to produce a wide range of high temperature colours.

More recently the behaviour of these mixed metal oxides has been examined in the infra-red range and it became apparent that some have anomalous behaviour showing partial reflectance over these wavelengths. Focussing on this aspect, further refinements have been made to maximise this behaviour to the extent that useful products have been commercialised.

So what does this mean? In a practical example we studied the immensely popular COLORSTEEL® colour Karaka. This colour has a light reflectance value of about 5% (i.e. it absorbs 95% of all visible light) and, when formulated with traditional pigments, adsorbs strongly in the infra-red. Under a standard Resene test of irradiation with infra-red lamps, the surface temperature rises 28.5°C after 5 minutes exposure. The same colour produced with infra-red reflecting pigments rose only 16.2°C in the same test.

This 12°C difference can make a tremendous difference to the stresses exerted upon the substrate and can have a telling effect on its stability as well as the heat gain of it.

The initial launch of this technology will be in Resene Hi-Glo acrylic roof paint where it will appear as a 'CoolColour™' option for a limited range of dark colours.

As well as the new pigments being inherently durable in themselves, it is expected that the resulting lower temperatures will give added longevity to the coating.

Our utilisation of this technology is clearly in its infancy and as experience is gained, one can readily envisage many areas it could be used on. Imagine dark trim enamels that did not flake off windowsills in two years? Cool eh!

View the Resene CoolColour brochure. (You will need Acrobat Reader).

Follow
View Videos
 

Resene Paints Ltd

Choose your Resene paint colours

Our vision is to be respected as an ethical and sustainable company and acknowledged as the leading provider of innovative paint and colour technology. View Resene’s profile and Environmental Policy.

Any personal data, such as name, address, phone, email details, supplied to Resene for the purposes of receiving newsletters, orders etc will be retained by Resene Paints Ltd and used only by Resene Paints Ltd and its trusted agencies for the purposes of communication/promotion/providing service to you. We do not sell, trade or otherwise transfer your personal data to outside third parties.. If you wish to opt out of communication from Resene please complete our contact us form. View our full Privacy Policy.

Colours shown on this website are a representation only. Please refer to the actual paint or product sample. Resene colour charts, testpots and samples are available for ordering online.   See measurements/conversions for more details on how electronic colour values are achieved.

Australia:
PO Box 924, Beenleigh, Queensland 4207
P: 1800 738 383. F: 1800 064 960
advice@resene.com.au
www.resene.com.au

New Zealand:
PO Box 38242, Wellington Mail Centre, Lower Hutt 5045
P: +64 (4) 577 0500, 0800 RESENE (737 363). F: (04) 577 0600
advice@resene.co.nz
www.resene.co.nz