The contact angle that decides whether rain cleans a façade

Fan Lee takes on both halves of a sixty year constraint, that anodised aluminium comes in natural or bronze outdoors because the dyes which colour it indoors fade in sunlight: a set of UV stable colours including a grey nobody had managed in coil, and an oxide layer that breaks pollutants down and lets rain sheet them off.

Anodised aluminium has been used on façades worldwide for more than sixty years, and Fan Lee is candid that for most of that time the exterior palette has been dull: natural tone or bronze, and little else. The reason is not aesthetic conservatism. The wide range of colours available indoors comes from organic anodising dyes, and those fade fairly quickly in sunlight, so they were never an option on an elevation.

Two mechanisms, one oxide layer

A sheet, not a bead. Below 60 degrees a surface is hydrophilic and gains some self-cleaning; at 8.9 degrees water spreads as a film instead of forming drops, and lifts the dirt with it. — **A sheet, not a bead.** Below 60 degrees a surface is hydrophilic and gains some self-cleaning; at 8.9 degrees water spreads as a film instead of forming drops, and lifts the dirt with it.

The more interesting half of his account is about dirt. The technique forms a single phase titanium crystal oxide on the aluminium, a purely inorganic self-cleaning oxide layer, and it works in two ways at once.

The first is photocatalytic. Under ultraviolet light in daylight, titanium dioxide releases electrons, and the electrons and the holes they leave behind react with oxygen and water in the air to produce singlet oxygen and hydroxyl radicals. Both are strongly oxidising, and they break the organic and inorganic pollutants sitting on the surface down into carbon dioxide and water. Water can then permeate beneath what remains of the dirt and lift it away. Lee claims that effect for over 25 years, which is a claim about the oxide layer rather than about a coating.

The second is wetting, and this is where the number matters. A drop of water on a surface makes a contact angle; below about 60 degrees the surface is hydrophilic and gains a degree of self-cleaning, and the smaller the angle the more effectively rain carries dirt away. Their measured figure, certified by a national architectural materials test centre, is 8.9 degrees, which puts it in super hydrophilic territory. In practice water does not bead at all: it spreads as a thin film, and the film removes the residue that photocatalysis has already broken up. Lee is explicit that this distinguishes the approach from the self-cleaning paints offered by the large coatings companies, which rely on wettability alone and have no mechanism for decomposing anything.

Methyl red under forced UV. Against conventional sulphuric acid anodising, which barely changes, the treated panel has largely cleared at six hours and completely at thirty. — **Methyl red under forced UV.** Against conventional sulphuric acid anodising, which barely changes, the treated panel has largely cleared at six hours and completely at thirty.

The evidence he offers is straightforward. Under ultraviolet light some ten to twenty times standard intensity, with methyl red as an indicator, the treated panel has largely cleared after six hours and completely after thirty, while conventional sulphuric acid anodised material shows virtually no change. A project in Chengdu makes the same point after a year in the open: the treated panels remain clean while the fluoropolymer coated aluminium alongside has greyed and lost gloss, and on a large Shanghai building the coated panels have collected dirt where the angle lets it settle.

The colours, and why grey was hard

Colours that hold outdoors. Electrolytic colouring gives bronze through copper to aged brass and black, and, unusually, a UV stable grey. — **Colours that hold outdoors.** Electrolytic colouring gives bronze through copper to aged brass and black, and, unusually, a UV stable grey.

On colour, Lee separates what is easy from what is not. Organic dyeing gives any colour you like, indoors only. Inorganic dyeing works outdoors but only in the yellow family, from light champagne to dark gold, and holds for decades. The difficult route is UV stable electrolytic colouring, which is where the range now runs from bronze through copper and warm copper to aged brass and a permanent black.

Grey is the one he dwells on, because it had not been available to the façade industry at all. The only other producer he knows of works in batches, and batch anodising varies in shade from panel to panel, which is precisely the problem on an elevation. Their process runs in coil form, which holds colour consistently across a run. Gloss is a separate axis, from 10 gloss units up to 800, where 800 is effectively a mirror. Work on a stable white is still in progress.

A finish that waits for the weather. The foggy mirror reads as an ordinary panel most of the year and, in the fogs of late autumn and winter, makes the building appear to float. — **A finish that waits for the weather.** The foggy mirror reads as an ordinary panel most of the year and, in the fogs of late autumn and winter, makes the building appear to float.

Two smaller ideas are worth noting because both are about getting more from less. On one job they cut the sheet in a diamond rather than a square, which raised material efficiency by around 10 per cent, more wall for the same aluminium. And the finish called foggy mirror is a piece of opportunism: in a location that fogs in the mornings of late autumn, early spring and winter, the panels read as ordinary cladding for most of the year, and when the fog arrives the building looks like an airship floating between its neighbours.