The building products market has seen unprecedented growth in demand for coloured fenestration products in the last few years. Driven by manufacturer innovation and consumer demand, more than 70% of windows and doors in Continental Europe are now coloured and the UK is fast following their lead.
One in four UK windows and doors is now coloured, and a significant 35% of PVC-U products are now anthracite grey. Moving away from traditional white or woodgrain, the move towards contemporary design has also accelerated growth in darker coloured composite doors, which are now popular in dark shades of painted green, blue and black.
This ‘colour explosion’ may be a great sign of a vibrant industry, but as with every emerging market, there can be contra-indications. People may remember the problem of warping rosewood PVC panels several years ago – bad for the industry, but due to lower volumes, that problem was on a tiny scale compared with the hundreds of thousands of coloured doors we now have in the market.
The threat facing us that the industry cannot ignore has its roots in basic physics. Put simply, the darker the colour (anthracite greys, blacks, dark greens and blues) the higher the absorption of incident sunlight (Solar Heat Gain) which is converted into heat (Heat Build Up), while the titanium dioxide present in white, cream and lighter colours reflects more of the incoming energy and the infrared waves that do the damage. The higher the Heat Build Up, the higher the material expansion rate, which ultimately affects the product’s integrity and stability.
Expounding the problem is the complexity of different substrates with different expansion rates which are housed together – take doors, where for example a black GRP composite product could be assembled into a white PVC frame. The different rates of expansion under temperature changes can lead to serious product failure, even to the point where hardware slippages mean doors won’t lock or open. In parallel, additional complexities are arising from a migration towards new techniques and applications including mechanical joints and increased use of thermal reinforcements.
This report will not come as no surprise to industry which has seen the introduction of various attempts to the try to address the problem, especially by thermal reinforcements. But these solutions (which could potentially exacerbate the problem) are only remedial – they do not address the problem of heat build-up at source, before it happens.
In this Special Report, we look at the physics behind the threat, the impact on different substrates and thanks to testing, what next generation painting solutions are available.
The Power of Solar Radiation
We all know that feeling when you get into a hot car that has been sitting in direct sunlight, when the black seats and steering wheel are too hot to touch. But how many of us understand the science behind this?
Sunlight is not only comprised of Ultraviolet (accounting for 5% of the sun’s energy reaching earth’s surface) and the Visible light spectrum (50% which makes up the wavelengths that give us the perception of colour, but there is also the Infrared spectrum which is not visible to the human eye. Infrared radiation does not influence the tone of a colour but is absorbed by the object and converted into heat.
To appear black, a pigment must absorb practically the entire spectrum of visible light. During the absorption process, light energy is converted into heat energy, causing the temperature of the surface to rise. Two of the most extreme examples of this are carbon black and copper chromite. These materials not only absorb virtually all the visible light, but continue out onto the infrared region as well. In contrast, a white object reflects all wavelengths of light, so the light is not converted into heat and the temperature of the object does not increase noticeably.
CAPTION: Roughly 50% of solar radiation is absorbed at the earth’s surface. Black surfaces usually absorb up to 90% of this energy and therefore get hot. White surfaces, on the other hand, absorb only up to 25% and tend to stay much cooler.
Total Solar Reflectance
Total Solar Reflectance (TSR) is the percentage of irradiated energy that is reflected by an object. The effectiveness of the reflectance can also be expressed in terms of Heat Build Up or HBU, as determined using ASTM* D-8403 test method, which shows the rise in temperature above the ambient. As a benchmark our tests show that a standard white UK window profile will have a reflectance of around 88% and Heat Build Up of between 20°c and 22°c .
The graph below shows the % of reflectance generated from a white frame.
*The scope of this test method covers the HBU in rigid and flexible PVC building products above ambient air temperature relative to black, which occurs due to absorption of the sun’s energy
A black coated surface shows the complete opposite, only 5% of all variants of the spectrum are reflected (95% absorbed) and the HBU is 44°c above ambient temperature.
Expansion Rates of Different Substrates
With some substrates, like wood, temperatures of up to 700C (1580F) do not significantly influence the substrate. However, with PVC-U this is a different story, as high temperature can cause critical damage, such as softening, deforming and changes to the mechanical properties of the substrate.
Heat Build Effects on PVC-U
PVC-U profile has an expansion rate 0.00006/oc which equates to 1.2mm per metre for every 20oC change in temperature. This may at first look insignificant, but the chart below shows the dimensional changes in typical two metre PVC-U window frame from a baseline temperature of 10 oc.
The above numbers are for non-reinforced PVC-U and the use of aluminium or steel reinforcement systems will have a positive effect on these findings.
Use of Cool Colour Infra-Red Reflective (IRR) Pigments
To counter the problem of heat expansion with dark colours, specialist building product paint manufacturer RegaLead has developed a range of Cool Colours to complement its existing range. These Cool Colours use specialised Infra-Red-Reflective (IRR) Pigments which substantially reduce any heat build-up. The IRR pigments reflect 44% of the infrared wavelength and the heat build-up is only marginally over that of standard white profile. The graph below shows the reflectance properties of Cool Colour Black across the spectrum of wavelengths.
In testing, the HBU of white PVC-U profile was 22.33oc. With the IRR black paint under the same conditions, the HBU was 24.91oc. In contrast, with standard black paint the HBU was 42.22oc – more than enough to warp a frame.
RegaLead can provide TSR (total solar response) and HBU (Heat Build Up) values for any Cool Colour formulation, but here’s a sample.
Standard Paint Versus Infra-Red- Reflective Paint
|PVC FRAME COLOUR (Using white as the benchmark)|
|White not painted||TSR||81.51||–|
|Cream not painted||TSR||76.18||–|
- The white sample frame had the highest TSR value, as expected
- The cream samples all had high TSR values again as expected due to the high amount of white in the colour
- The slate blue and anthracite colours performed significantly better with the IRR paint when to HBU
- The HBU on the white control sample was 22.33 oc compared with 42.44 on painted black, which reduced to 24.91 when IRR pigments were used.
The popularity of coloured windows and doors continues to grow at a significant rate, and recent colour trends for grey, moving into deep, rich colours in the future, will only accelerate this growth. However, there is a risk that the UK fenestration industry is sleepwalking into a future issue, as dark coloured windows and doors absorb heat and suffer as a result of HBU.
We are already seeing moves, particularly from within the reinforcement industry, to counter this with a return to metal reinforcement. This will help, but it deals with the effects of the problem, not the problem itself.
In order to prevent the problem rather than just cure it, the solar reflectance of coloured windows and doors needs to be raised. Thanks to RegaLead’s experience in the North American markets, where IRR paints are now commonplace and based on our extensive laboratory testing, the company will now be offering IRR pigments in all colours across ColorSpray GRP and Aqua paints.