Page 246 - Handbook of Adhesives and Sealants

Surfaces and Surface Preparation 215

6.3.4 Substrate equilibrium
After the surface-preparation process has been completed, the sub-
strates may have to be stored before bonding. Typical storage lives for
various metals subjected to different treatments are shown in Table
6.6.
The maximum allowable time between surface treatment and ap-
plication of the adhesive will be dependent on how soon the substrate
surface can change in the shop environment and how strong the sub-
sequent reformed surface is attached to the base material. For ex-
ample, after its natural surface is exposed, copper alloys can form
oxide layers relatively quickly. In the case of copper, this boundary
layer is weakly attached to the base metal. On the other hand, alu-
minum oxide layers are formed very fast: however, they are generally
tightly attached to the base metal and do not provide a significant
problem for adhesion. Certain polymeric surface treatments lose their
effectiveness very quickly because of the reactive and mobile nature
of the polymer molecules. Because of the relatively short storage life
of many treated materials, the bonding operation should be conducted
as soon as possible after the surface preparation process.
If prolonged storage is necessary, either (1) the parts should be care-
fully protected and stored in a controlled, clean environment, or (2) a
compatible organic primer may be used to coat the treated substrates
immediately after surface preparation. The primer will protect the
treated surface during storage and interact with the adhesive during
bonding. Many primer systems are sold together with adhesives for
this purpose. Certain primers have also been formulated specifically
for corrosion resistance and, thereby, continue to protect the surface
after the joint is placed in service. Such primers are described in the
next chapter.

6.4 Passive Surface Preparation Methods
The surface preparation processes described in this section are clas-
sified as passive processes. They do not actively alter the chemical
nature of the surface. Passive processes only clean the substrate and
remove weak boundary layers in the form of contamination. Solvent
washing, chemical cleaning, and mechanical abrasion are considered
passive processes. Depending on the degree of adhesion and perma-
nence required, passive processes may be used as either the only sur-
face preparation or as the initial step in a more detailed surface treat-
ment process.