The aluminium industry has always insisted that its material does not need painting, as it has its own built-in protective surface coating of aluminium oxide, which forms immediately on exposure to the atmosphere. However; in order to compete with the colours of paint finishes available for steel cladding, manufacturers have produced a number of organic-coated colour finishes for aluminium sheets. Colour anodizing is
limited in that it only really offers black, grey, gold or bronze finishes.
Thus profiled aluminium sheet is available with mill finish, anodized, or with an organic coating. Mill finish is a natural oxidized surface, which darkens with age. Anodized finishes are generally about 25 mm thick according to BS 3987: I 964 and BS 1615: 1972. There are five different methods of colour anodizing:
- organic dyes, which do not penetrate deeply into the anodic film and are therefore vulnerable to abrasion; many of these are as light fast as inorganic pigments, but some start to fade within two years;
- inorganic pigments, which are used for gold and bronze finishes;
- electrolytic colour, which is deposited deeper than organic dyes to produce black, bronze and grey finishes that resist ultraviolet radiation and abrasion;
- integral or hard colour, where organic acids produce results similar to the electrolytic process;
- special alloys, which produce a grey or gold finish during anodizing.
Aluminium sheeting can be either supplied as pre-coated fluoropolymers or post-coated as alkyd finish.
The important thing to realize is that the stoved-on alkyd finish has to be applied after forming. Because PVF2 and acrylics are precoated, they are supplied to the fabricator as finished sheets for forming. Alkyd finishes have to be stoved on after forming.
The safe temperature range for aluminium is -80°C to +I00°C, above which gradual loss of strength occurs. Above these temperature ranges the colour coating will also discolour and deteriorate.
Some colours reflect or absorb heat more than others, and dark colours are more likely to reach much higher surface temperatures than light colours. It is important to obtain assurances from the manufacturer on the relative durability, colour fastness and temperature limitations of the various colours available.
For both aluminium and steel profiled sheeting, colour matching between batches of sheet and flash-
5.20 Test rigs for measurement of the strength of metal profiled sheeting.
ings is often a problem. Samples of the limits of colour change can be approved before ordering, but some mismatch of colours on a large job is inevitable.The selection of neutral colours for sheeting or contrasting colours for flashings is a way of overcoming this difficulty.
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