Page 485 - Handbook of Adhesives and Sealants

Selection of Adhesives 421

Inflammability and toxicity of the adhesive and accessory materials

Of these factors probably the most important are the method chosen
for application of the adhesive and curing parameters such as tem-
perature and pressure.
The method chosen for the application of an adhesive to the work-
piece is determined by the size and shape of the parts being bonded
and the number of components to be bonded. This consideration will
often lead to the form of adhesive that is most appropriate for the
application. For example, films and spray coatings are very common
for large parts. Pastes are commonly used for joints requiring narrow
beads. UV cured adhesives may be best for high volume parts where
strength is not critical and the adhesive can be directly exposed to a
light source.
Curing temperature and pressure is often a critical parameter in
the bonding process. Sub-components may not be resistant to the tem-
peratures and pressures that are required for certain adhesives. High
curing temperatures may cause internal stress that will crack or dis-
tort delicate parts. Low temperature cures may require too long for
practical assembly operations. The main problem is that high strength
and temperature resistance usually coincide with high temperature
cures.
Another critical processing parameter is the surface preparation
that may be required. The degree of preparation will depend on the
type of substrate, its condition, and the adhesive chosen in addition
to the joint strength and durability required for the application.

11.2.2.2 Joint design. Once the method of bonding and the substrates
are chosen, the specific joint design can be addressed. The most im-
portant consideration in the joint design may be the ease in which the
substrates can be fabricated. For some substrates, it may be easy and
inexpensive to design a joggle overlap joint. With other substrates
(composites, for example) it may be very difficult. Joint designs should
consider:
The type of substrate
The cost associated with manufacturing the joint design
Ease at which the joint can be fabricated and assembled
The ease in which the joint can be inspected after bonding is com-
plete
Most importantly, the type of stress that the joint will see in service

One must consider the type of stress that the joint is expected to
encounter in service. Make certain that peel and cleavage stresses will