Powder Coatings – the Sustainable Choice
Readers will be well aware of the green credentials of aluminium, with its impressive recycling rates based on the fact that recycling takes only 5% of the energy of virgin aluminium production. Fewer may be aware that the use of powder coatings on aluminium gives more than simply a limitless range of colour options but enhanced green credentials. Powder coating users are also adopting ever greener technologies, from the installation of more efficient processes to the development of combined wood/aluminium windows, a perfect use of 2 sustainable materials.
Beginning of life
The US ‘Whole Building Design Guide’ recommends “factory finishing rather than field-coating where possible…Powder coating is preferable to solvent based coating application systems.”1. Powder coating of extruded aluminium is a very efficient process, especially on modern vertical coating lines. The aluminium extrusions, up to 8m in length, are hung vertically and very close together before being pre-treated (often now with organic pre-treatments rather than the older Chrome-based systems) in an efficient spray process. Powder is then applied in specially designed booths, with all over-sprayed powder collected and re-sprayed. When the colour needs to be changed, the booth is sealed and pressurised air knives used to drive any excess powder down to a recycling system for collection and later use. This ensures 99%+ usage of the powder coating material, which is then baked in an oven to melt and form a tough durable coating.
An example of the high rating given to powder coating as a green process is given by Metalliform, a UK based manufacturer of classroom and stadium seating. They attracted finance from the UK’s Carbon Trust, a government body dedicated to reducing use of fossil fuels, for the installation of new, more efficient powder coating equipment2. Leading powder supplier AkzoNobel are also showing the way in the field of reducing waste in the coating process with their “onLine” coating control process. The system monitors the total coating process, alerting operators and remote AkzoNobel technicians instantly in the event of a problem so that interventions can be made to minimise waste and rejects.
The powder coating protects the aluminium substrate from the weather and thus from oxidation or Filiform corrosion. But nowadays it also offers an added benefit – reflection of solar radiation, helping to keep the metal, and thus the building, cool. This is recognised by the US Green Building council’s “Leadership in Energy and Environmental Design” (LEED) programme in 2 ways – with credits both for the reduction in heat-island effects, and the impact on energy efficiency of the building3. A new innovation for aluminium extrusions, such techniques have been used on roofing for several years. The effect is achieved through the use of novel infra-red (IR) reflecting pigments. IR reflection rates can be increased by up to 6 times, depending on the shade. In a similar vein, the Time Warner building in New York – which was designed to very high green credentials - utilised powder coated fins to keep in winter sun but block summer sun; these were coated in a blue metallic powder coating (though not an IR-reflecting version – surely an oversight that would be remedied if the building were constructed in 2008?).
The lifetime of modern powder coatings also makes them a green choice. Ultra-durable polyesters and fluorocarbon powders are now available, with expected lifetimes on a building in excess of 30 years. AkzoNobel’s Interpon D2000 product surpassed 15 years on a California hospital (Mission Hospital, in Viejo, pictured) in 2006. Measurements of its gloss and colour showed they had hardly changed since installation in 1991, making re-painting or re-cladding a far-away prospect.
Specifiers are increasingly interested in colour, and the palette available from a powder coating is constantly increasing. Even 10 years ago, most powder coating suppliers offered solid colours only, yet today all have a range of metallic and other effects. This colour availability has led architects to begin specifying different colours inside and out, as in a recent library building in Otley, Yorkshire, UK4, where the exterior of the profiles was a metallic grey but white was used on the inside. This project additionally used blue glass which was accented by the grey powder coating, and is lit by blue lighting at night to dramatic effect. Such innovative ideas from architects force the supply chain – powder coating suppliers, applicators and fabricators – constantly to invest in new technology to satisfy the vision. The challenge is to create these new colours in a way that is not detrimental to – or maybe even enhances – the environment.
End of life
A misconception of coating aluminium is that this somehow damages its recyclability at end-of-life. In fact the recycling process easily removes the organic coating with no detrimental effect on the aluminium so produced. It is true that painted aluminium attracts slightly lower prices from recyclers than uncoated aluminium, but when looking at the total environmental and energy savings from recycling the metal this is negligible. The recycled aluminium can then be utilised in another building project. Around 90% of construction aluminium is recycled at end-of-life, and new construction uses around 45% recycled aluminium – and rising5. In the USA, one leading window manufacturer, Keymark Corporation, uses “an aluminium billet consisting of a recycled content of 64% (41% Pre-Consumer & 23% Post-Consumer)”6 in their window profiles, and are now moving to a powder coating process from liquid paint to further enhance their green credentials.
While it is widely accepted that PVCu is a poor material for sustainable windows and doors, there is much debate about the relative merits of aluminium and wood. Wood costs very little in energy to engineer into the required shape, and is of course biodegradable at end-of-life. However, it takes much maintenance over its lifetime to protect it, and old wooden windows cannot generally be re-used. One way around this is to utilise aluminium on the exterior for strength and durability, with engineered wood on the inside for its warm feel and aesthetically pleasing qualities. Such an approach is gaining ground, the only downside being that today the wood has to be coated with a liquid paint, which is not as environmentally friendly as powder coating – but a suitable powder is surely around the corner, since it is already available for medium density fibreboard (MDF). This approach has been taken by Senior Aluminium Systems in the UK, they of the Otley library fame (above) in their new “Hybrid” range7 (pictured), which won an award for best new window product among trade magazine journalists at a recent show.
Specifiers are increasingly seeing aluminium as a sustainable choice for construction, and its use with a powder coating for decoration is supported by a raft of new standards and environmental rating agencies. As engineers and firms continue to innovate in areas such as the shapes possible and the materials that aluminium can be combined with, the possibilities for aluminium in construction in the future will continue to open up.
1. http://www.wbdg.org/design/greenspec_msl.php?s=050500, accessed June 25 2008
2. Finishing magazine May/June 2008
3. Journal of Coatings Technology, April 2004
4. SAS in landmark attraction for Otley, Building Talk, November 3 2006
5 European Aluminium Association 2004
6. www.keymarkcorp.com, accessed June 25 2008
7. http://www.seniorhybrid.co.uk – accessed June 25 2008
*The author has a bachelor’s degree in colour chemistry from the University of Leeds, UK, and is Market Manager at AkzoNobel for Architectural Powder Coatings.