The hunt for white aluminium

(Nanowerk News) Designers worldwide have for many years been on the lookout for a technology which would make it possible to create the first pure white aluminium products—that is without the need for a traditional surface treatment. And Bang & Olufsen’s designers are no exception.
The challenge of bringing out the white colour is fundamentally related to the size of the white crystals producing the white colour. The crystals disperse all colours in all directions, resulting in what scientists call perfect dispersion and consequently the colour white. While other colour molecules are quite small, the white colour is a different matter altogether, which means that the existing technology used to colour aluminium cannot be used to produce the colour white.
Bang & Olufsen BeoSound 8
Bang & Olufsen BeoSound 8
All depends on the eye that sees
The physical world really has no colour. When we perceive a telephone, a car or an apple as being red, it is because the light reflected by the surface of the apple primarily contains those wavelengths of the visible spectrum that evoke a sensation of red.
Black absorbs all wavelengths and is therefore perceived as black. White, on the other hand, reflects all colours and therefore has what scientists call perfect dispersion.
Visible colours are divided into seven spectral colours: red, orange, yellow, green, blue, indigo and violet. The colour of an object depends on the light reflected by it, or the contrasting colour which it absorbs.
The light coming from the object can be direct or can result from what is called transmission, for example if the object is transparent.
The combination of the reflection of the reflective layer and the reflection of the underlying layer is something which the scientists are working with in this project.
The solution has always been to make products in the palest grey aluminium colour possible: the colour which is now known from Apple’s products. As an alternative, some manufacturers have chosen to spray their products white. However, spray-painting has its problems. For example, paint is susceptible to scratching and denting, while also producing a more artificial and non-organic look. Also, any sharp edges invariably become rounded as a result of the application of the paint.
“But why all this talk about design, surfaces and appearance? Does it really matter so much? Do customers not primarily buy products for their functions and functionality, i.e. what’s inside?” asks Senior Manager Jørgen Dam, Bang & Olufsen.
“In actual fact, the appearance of the product is equally important; you have to feel like getting close to it, touching it and lifting it up! Being the first to produce white aluminium would take Bang & Olufsen into the lead, quite apart from it being a fantastic ‘story'. It simply means that we will be able to do things which have never been possible before for us or for anybody else in the world,” Jørgen Dam says.
Millions of coloured pores
Today, Bang & Olufsen sells its products in colours such as red, blue, gold and black. The colours are not painted on, but are organic and integrated into the aluminium surface. This is achieved through a special chemical hardening process known as anodizing.
Professor Rajan Ambat, DTU Mechanical Engineering, explains: “Today, we exploit the fact that the anodized surface of aluminium is full of millions of microscopic ‘nanopores'. These ‘nanopores’ are filled with traditional colour pigments which after a chemical hardening process sit well-protected inside the transparent and hard-wearing anodized surface layer. After polishing, the surface of the aluminium takes on a glass-like appearance and becomes extremely durable. The result is a surface with what we call organic qualities and a far more natural look.”
However, this method cannot be used with white as the ‘white colour molecules’ are far too big to fit into the holes.
“It would require a quantum leap for this to be realistic. Together with DTU and the Danish Technological Institute, we therefore decided to take a different approach. Rather than using colour pigments, we try to build up the top layer and design it so that it becomes white in the process. What is quite magical is that with this technology, the product is aluminium-coloured before anodizing, but will appear white or white-like afterwards, without ‘white pigmentation’,” Jørgen Dam explains.
Anodizing—durable and decorative
The raw aluminium surface in itself is soft and susceptible to corrosion from the atmospheric air, and untreated aluminium loses its sheen and becomes grey with time.
An anodized surface, on the other hand, lasts for many years without requiring much maintenance at all. Anodizing aluminium is an electrochemical process which increases the thickness of the natural, transparent oxide film. This causes a porous oxide film to form, which is an integrated part of the aluminium surface.
Standard anodizing increases the thickness of the oxide firm by anything between 5 and 25 micrometres. With this treatment, new protective properties can be added as well as decorative elements in the form of colours. In brief, the purpose of anodizing is to
  • – improve corrosion resistance
  • – harden the surface to increase mechanical wear resistance
  • – create an electrically isolating surface
  • – produce a decorative surface (for example using colours)
  • Taking the lead with aluminium
    “We are working on a coating of the layer of aluminium which sits just under the glass-like anodized surface. Depending on how it is treated, and the nanoparticles added, we can achieve perfect dispersion of the light hitting the surface—and consequently, it will be perceived as white by the human eye. This is done, among other things, by systematically combining advanced coating techniques, including the incorporation of nano and microparticles in the aluminium phase prior to anodizing,” Rajan Ambat explains.
    Pastels, pastels, pastels
    “Another interesting perspective is that if we manage to include the colour white in our colour palette, we will be able to develop a whole host of other colours such as, for example, the popular pastels,” says Jørgen Dam. He stresses that a technological partnership such as this requires an extremely high level of knowledge among all partners:
    “At DTU we have access to some very specific competencies which I cannot imagine that we would ever be able to build up here at Bang & Olufsen. Why don’t we just take on people who can solve this problem, one might ask. However, the fact of the matter is that building up the necessary competencies to the level which DTU is contributing to this project would take several years. I’m not sure at all that we would ever be able to establish a sufficiently strong environment for it to ever work.”
    Source: DTU
    Subscribe to a free copy of one of our daily
    Nanowerk Newsletter Email Digests
    with a compilation of all of the day's news.