Page 488 - Handbook of Adhesives and Sealants

424 Chapter Eleven

available within this family with different characteristics regarding
end-properties, application requirements, curing processes, and so
forth.
There is also no general-purpose adhesive. There is no perfect so-
lution for any particular adhesive problem. However, with the great
many types of adhesives available and with the many available vari-
ations that can be provided, a ‘‘good’’ solution can usually be found.
However, these solutions generally result in a compromise where some
lesser requirements may have to be sacrificed for other more critical
requirements.
The best adhesive for a particular application will depend on the
materials to be bonded, the service and assembly requirements, and
the economics of the bonding operation. By using these factors as cri-
teria for selection, the many commercially available adhesives can be
narrowed down to a few possible candidates.
Choose the appropriate adhesive in cooperation with an adhesive
supplier. Once the general product requirements, joint design, and ser-
vice requirements are known, adhesive systems with good durability
can be selected. The desired form of the adhesive and method of ap-
plication can then be chosen based on the formulation possibilities
available within the class of adhesive chosen and on the availability
of production equipment and scheduling requirements.
The tendencies to over specify the adhesive requirements must be
avoided. Requirements for higher strength or greater heat resistance
than is actually necessary for a specific application will exclude from
consideration many formulations that may be equally acceptable for
the job, at lower cost, and with simpler production capabilities. Usu-
ally, the objective in specifying an adhesive is to select a material that
will cause the joint to fail in cohesion rather than adhesion. Ideally,
the substrate fails before the adhesive. Low strength materials such
as paper, fabric, etc. may be weaker than most structural adhesives,
and the use of high strength adhesives could be an expensive over-
specification. Lower cost and more easily applied adhesives may result
in the same failure mode—cohesive failure of the substrate.
Re-evaluate the preceding stages in the bonding process to deter-
mine that the selection of adhesive does not violate any of the critical
requirements. Make appropriate changes in the plan to accommodate
the adhesive of choice, or select another adhesive.

11.4 Substrates

Generally, the first consideration in making an adhesive selection is
the type of substrates that must be held together. Often a two-