Page 338 - Handbook of Adhesives and Sealants

296 Chapter Eight

moisture to completely cure. As the adhesive cures on the outer edges
it provides a barrier for the moisture to diffuse to the inner regions of
the joint.
Moisture curing adhesives generally can cure completely at room
temperature. The cure rate depends on the base polymer and its re-
action chemistry and on the humidity of the curing environment. Gen-
erally, moisture cured adhesives cure more slowly than their two com-
ponent reactive counterparts. Moisture cured adhesives can be
accelerated by curing in a high humidity enclosure, and some types
can be accelerated by heating.
Polyurethanes and silicones are common single component adhe-
sives that cure by reacting with environmental moisture. These are
available as pastes and liquids. For certain applications requiring un-
usually high bond strength and setup time, reactive polyurethane hot
melt adhesives have been developed. 7,8 These systems are applied like
a hot melt adhesive, and the parts can be rapidly assembled. As a hot
melt, this adhesive has a high degree of early handling strength. The
unique feature, however, is that these adhesives will continue to cure
and crosslink by reacting with the moisture in the air so that when
completely cured they have the strength and durability typical of most
structural adhesives.

8.2.3.1.4 UV/light curing adhesives. Adhesives that cure by exposure to
radiation such as a UV or light have become essential to many assem-
bly operations where high throughput or delicate components are
prevalent. The features and benefits of UV curing are described in
Table 8.5. These adhesives require the presence of the correct wave-
lengths of light at sufficient intensity to form durable structural
bonds. UV and light cure adhesives use photoinitiators to activate the
free radical curing mechanism. The free radicals formed by the pho-

TABLE 8.5 Features and Benefits of UV Curing 9
Features Benefits
cures in seconds increase productivity
cures only upon exposure to light increased design and process control
structural strength improves integrity of the total assembly
100% solvent free reduced regulatory cost
USP Class VI certification medical grades available
‘‘worker friendly’’ longwave UV light easy to introduce
range of viscosities for easy dispensing meet most production requirements
range of properties—rigid to flexible meet many design requirements
1
1
deep section curing for potting ⁄8– ⁄4 in. typical for clear grades
optically clear for easy inspection
grades available with secondary cure cures where not exposed to light
mechanism