Page 497 - Handbook of Adhesives and Sealants

430 Chapter Eleven

11.4.1 Adhesives for metal
The chemical types of structural adhesives for metal bonding were
described in the preceding chapter. Adhesives for bonding metals may
be divided into two main groups:

Structural—for use in continuously stressed structures
Non-structural—for use in low strength applications

The heat and environmental resistance are often of great importance,
especially in the case of structural adhesives.
Since organic adhesives readily wet most metallic surfaces, the ad-
hesive selection does not depend as much on the type of metal sub-
strate as on other requirements. However, virtually all commonly
bonded metal surfaces exist as hydrated oxides. This situation is dis-
cussed fully in Chapter 6. Adhesives used for bonding metal are really
bonding metal oxide surfaces. They, therefore, must be compatible
with the firmly bonded layer of water attached to the surface of most
metal oxide crystals. The base metal greatly influences the properties
of the hydrated oxide surface. Thus, certain metals will possess sur-
faces that interact more effectively with one type of adhesive than
another.
Thermal and oxidative stability as well as corrosion and water re-
sistance, depend on the adherend surface as well as on the adhesive
itself. Epoxy and phenolic based adhesives degrade less rapidly at el-
evated temperatures when in contact with aluminum or glass than
when in contact with copper, nickel, magnesium or zinc surfaces. The
2
divalent metals (Zn, Cu, Fe , Ni, Mg, etc.) have a more basic oxide
3
surface than the higher valence metal oxides (Al, Fe , Si, etc.) and
hence promote dehydrogenation reactions which lead to faster degra-
dation.
Selecting a specific adhesive from a table of general properties is
difficult because formulations within one class of adhesive may vary
widely in physical properties. General physical data for several com-
mon structural metal adhesives are presented in Table 11.2. This table
may prove useful in making preliminary selections or eliminating ob-
viously unsuitable adhesives. Once the candidate adhesives are re-
stricted to a few types, the designer can search more efficiently for the
best bonding system.
Nonstructural adhesives for metals include elastomeric and ther-
moplastic resins. These are generally used as pressure-sensitive, con-
tact, or hot-melt adhesives. They are noted for fast production, low
cost, and low to medium strength. Typical adhesives for nonstructural
bonding applications were described previously. Most nonstructural