BULETINUL INSTITUTULUI POLITEHNIC DIN IAŞI - Universitatea ...

Bul. Inst. Polit. Iaşi, t. LVI (LX), f. 2, 2010 109
3. The surface of Anodized Aluminium
Aluminium alloys and anodized aluminium will theoretically have a high
surface tension, so wetting of the surface with an adhesive would not be a
problem, because the adhesives will have a surface tension much lower.
Aluminium and anodized aluminium will react with CO2 and possible
contaminations in the air and will there fore in practical have a surface tension,
which is so low that wetting with the adhesives is not possible, because the
actually surface tension is lower for aluminium surface than for the adhesive.
If the surface is new created the surface tension will still be high and wetting
will not be a problem. For solving the problem with not anodized aluminium the
aluminium surface is often chromatized or phosphatized to obtain a good wetting.
The surface of anodized aluminium itself is possible to wet with an adhesive
especially if it is new treated and that will say that as soon after the anodizing the
wetting will take place the better adhesion will be obtained. The bigger surface
the better wetting is possible and that means also that if the anodized surface is
used for bonding before sealing. The fresh anodized aluminium surface is the best
surface for adhesive bonding, because the surface there has reactive groups,
which will give possibility for chemical bonding in the interphase. The reactive
groups give a bigger contribution to the surface tension because of more hydrogen
bonds and the physical adsorption in the interphase will increase. The mechanical
interlocking will also increase because of the porousity in the anodized surface.
After sealing the surface is like un-anodized aluminium and the advantages
obtained by the anodizing will disappear for the bonding.
4. Surface Treatments
Before bonding the surface need to be clean. The surface shall as mentioned
above be with so many reactive groups as possible and the surface tension shall
be as big as possible. The possibility for mechanical interlocking shall be
present if possible.
If the surface is contaminated with oil or grease from production or cutting
process the surface tension will be the same as the surface tension of oil and
grease, which will say very low, because those materials will almost have a
contribution form the dispersion forces, very few if any from the polar forces
and no from the hydrogen forces. Such a surface is impossible to bond on. The
grease and oil shall be removed with an organic solvent as f.ex. isopropyl
alcohol or CO2. The surface can if it is non anodized aluminium or is a sealed
anodized aluminium be treated with a Scotch Brite for obtaining a bigger
surface for mechanical interlocking. The treatment with Scotch Brite shall take
place before the cleaning for removing dust from the surface.