Aluminium Design and Construction John Dwight

are, and the designer/fabricator must rely closely on information provided
by the adhesive maker. Tables 11.7 and 11.8 in Chapter 11 provide data
on selected adhesives supplied by one particular manufacturer (Ciba).
3.6.2 Surface preparation
The success of a bonded joint stands or falls on the care with which the
mating surfaces are prepared. Possible levels of such preparation in
increasing order of efficacy (and cost) are:
1. degrease only;
2. degrease, abrade and remove loose particles;
3. degrease and pretreat.
The appropriate level must be specified by the designer. The object of
using level (2), or best of all (3), instead of (1), is to make the joint more
reliable and durable in humid environments.
(a) Degreasing
The prime requirement in preparing surfaces for bonding is to remove
all traces of oil and grease, even though the surfaces may appear clean.
The following are acceptable degreasing methods.
Vapour degreasing. The parts are suspended in the vapour of a halocarbon
solvent, such as trichloroethylene (‘trike’). This calls for a special
vapour degreasing unit.
Immersion degreasing. The parts are successively immersed in two
tanks of the same solvent in liquid form, the first tank acting as a
wash and the second as a rinse.
Wiping. The surfaces are degreased using a clean brush or cloth soaked
in halocarbon or other degreasing solvent.
Detergent degreasing. The surfaces are scrubbed in a solution of liquid
detergent, and then washed with clean hot water, followed by drying
in air.
A simple test to find whether a part has been properly degreased is to see
if distilled water wets the surface and spreads, rather than forms droplets.
(b) Abrading
Light abrasion of the surfaces after degreasing gives a better key to the
adhesive. The preferred method is to grit blast, after which all loose
particles must be removed, using a clean soft brush or compressed air.
Another way to remove loose bits is to repeat the degrease operation.
If grit-blast equipment is unavailable, or the part is too thin, abrasion
can be carried out using a wire brush, abrasive cloth or a water-proof
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