Page 339 - Handbook of Adhesives and Sealants

Adhesive Classifications 297

toinitiators are able to crosslink with other components in the adhe-
sive formulation. The adhesive that is not directly exposed to light or
UV will not cure. Therefore, the substrates are generally limited to
those that are transparent or to where there is only one substrate (e.g.,
wirebonding). Figure 8.4 illustrates bonding concepts where UV cured
adhesives have been successful.
Aerobic acrylic adhesives are generally used in UV curing applica-
tions. These adhesives can cure by either direct exposure to UV or by
using a preapplied activator to the substrate. Figure 8.5 shows that
complete curing using the preapplied activator requires 24–48 hrs at
room temperature. UV cure, however, reaches the same degree of bond
strength in only seconds.
The promise of light curing adhesive has not been fully realized
because some clear substrates do not transmit sufficient spectral light.
Special resins are available that cure synergistically by combining UV
and visible curing mechanisms. These products also have faster and
deeper cures. They have the ability to cure through to deep (1/4 in.)
9
layers to form structural strength bonds (3,000 psi) on steel. They
also can cure completely in shadowed areas.
8.2.3.1.5 Adhesives catalyzed by the substrate. Certain single component
adhesives cure via a chemical reaction mechanism that uses the active
metal ions (e.g., iron and copper) on the substrate as a catalyst. These
adhesive systems are called anaerobic adhesives because they cure
only in the absence of oxygen. Anaerobic is a term borrowed from
biology that refers to life forms that live in the absence of oxygen.

Figure 8.4 Bonding categories using UV aerobic adhesives. 10