Page 243 - Handbook of Adhesives and Sealants

212 Chapter Six

TABLE 6.4 Surface Treatments for the Adhesive Bonding of Aluminum 12
Surface treatment Type of bond
Solvent wipe (MEK, MIBK, trichloroethylene)............ Low to medium strength
Abrasion of surface, plus solvent wipe (sandblasting,
coarse sandpaper, etc.) ................................. Medium to high strength
Hot-vapor degrease (trichloroethylene)................... Medium strength
Abrasion of surface, plus vapor degrease................. Medium to high strength
Alodine treatment ..................................... Low strength
Anodize ............................................... Medium strength
Caustic etch* .......................................... High strength
Chromic acid etch (sodium dichromate–sulfuric acid)† .... Maximum strength
*A good caustic etch is Oakite 164 (Oakite Products, Inc., 19 Rector Street, New York, N.Y.).
†Recommended pretreatment for aluminum to achieve maximum bond strength and weath-
erability:
1. Degrease in hot trichloroethylene vapor (160 F).
2. Dip in the following chromic acid solution for 10 min at 160 F:
Sodium dichromate (Na 2 Cr 2 O 7 2H 2 O ............................... 1 part/wt.
Conc. sulfuric acid (sp. gr. 1.86)....................................10 parts/wt.
Distilled water ..................................................30 parts/wt.
3. Rinse thoroughly in cold, running, distilled, or deionized water.
4. Air-dry for 30 min, followed by 10 min at 150 F.

The sequence of these surface treating steps is important. The sub-
strate is initially degreased to remove gross organic contamination. It
is then subjected to mechanical abrasion to remove strongly attached
weak boundary layers. A second degreasing step is then performed to
clean the substrate of residue and possible contaminants leftover from
the abrasion processes. Note that the first degreasing step is neces-
sary; otherwise, the contaminants would be driven further into the
substrate by the mechanical abrasion process. The abrasive medium
could itself become contaminated and spread a weak boundary layer
from article to article. Once the substrate is clean, the final step, if
necessary, is usually one that acts on the pure substrate surface. This
process is intended to alter the physical or chemical nature of the
substrate. Its goal is to provide better wetting, to passivate the surface
so that weak boundary layers cannot develop, or to simply give the
substrate more ‘‘teeth’’ for mechanical interlocking with an adhesive.
Table 6.5 shows the effect of various combinations of aluminum sur-
face preparations on lap shear strength. With this particular combi-
nation of adhesive, adherend, and processing conditions, optimum
bond strength (3,000 psi) on aluminum occurs when a treatment con-
sisting of vapor degreasing, abrading, alkaline cleaning and acid etch-
ing is used. However, simple solvent wiping and abrasion results in
moderate and relatively consistent bond strength (1,500–2,000 psi).