Page 514 - Handbook of Adhesives and Sealants

444 Chapter Eleven

Other methods than adhesive bonding may also be considered at
this stage in the process. Heat welding (e.g., ultrasonic assembly)
methods for bonding thermoplastic parts may provide fast bonds and
the elimination of adhesive mixing or application. Solvent welding is
also a fast method of bonding that uses the resin in the substrate itself
as the adhesive.
Factors that must be considered in selecting an adhesive relative to
the assembly operation include: form of the adhesive; method of prep-
aration and use; shelf (storage) life; working life; method or machinery
necessary for bonding; and processing variables. The last factor in-
cludes the time allowed between coating and bonding; drying time and
temperature; temperature required for application and curing; bond-
ing pressure required; rate of strength development at various tem-
peratures; and other properties such as odor, flammability, and toxicity
of the adhesive which may call for extra equipment or precautionary
measures on the shop floor. These safety and environmental factors
are discussed more fully in Chapter 18.
Adhesives require time, pressure, and heat singly or in combination
to achieve hardening. Curing conditions are often a severe restricting
factor in the selection of an adhesive. In some applications, the curing
temperature influences the selection of an adhesive. A number of ther-
mosetting adhesives require heat and pressure to form the bond.
These tend to be adhesives having high degrees of strength, temper-
ature resistance, and environmental stability. Where such elevated
temperature cures cannot be used, room temperature setting adhe-
sives, such as two component epoxies or urethanes, can be used.
Several production factors must be taken into account when select-
ing an adhesive for an assembly operation. These factors include:
The process and type of equipment necessary for metering and mix-
ing of the adhesive
The processes and equipment necessary for applying the adhesive
to the substrate
The processes required for curing the adhesive and associated
equipment whether it be jigs and fixturing, ovens, or pressure de-
vices
The amount and type of pressure required to mate the substrates
and to force the adhesive to flow as with the case of pressure sen-
sitive or contact adhesives
The availability of substrate cleaning and treatment facilities may
also be a factor in selecting adhesives (certain adhesives such as the
hybrid acrylics and epoxy formulations are more resistant to oily
substrates than other adhesives