Page 483 - Handbook of Adhesives and Sealants

Selection of Adhesives 419

can also be used as the adhesive. An example is the manufacture of a
urethane roller skating wheel with a metallic hub. The liquid ure-
thane casting resin is cast into a mold that already contains the hub.
The urethane fills the mold and cures, thereby forming the roller
wheel and bonding directly to the hub. The urethane that is used to
fabricate the wheel is also the adhesive. A primer may be needed on
the insert to obtain maximum adhesion. With insert bonding there
often is no reason to select a ‘‘third party’’ adhesive; however, an ad-
hesive or primer must sometimes be applied to the shaped substrate.
There are other types of bonding where a ‘‘third party’’ adhesive is
not necessary. These are solvent cementing and various forms of ther-
mal welding. These methods are commonly used on certain thermo-
plastics. In such cases, the polymeric material in one of the substrates
becomes the adhesive. The thermoplastic substrate material is made
fluid by either dissolving the surface with a solvent or by using ele-
vated temperatures to create a melt zone. The resulting fluid polymer
is used as the ‘‘adhesive’’ to make the bond. Once the solvent evapo-
rates or the melt zone cools and solidifies, a bonded joint is formed.
Whether bonding conventionally with adhesives or using insert
bonding, solvent cementing, or heat sealing, the principles of adhe-
sives bonding hold true. Close attention must be given to surface prep-
aration, to entrapped air in the bond line, to stresses within the bond,
and to weak boundary layers.

11.2.2 Information regarding product and
processing requirements
One should first carefully gather all of the information possible about
the product being bonded and the processing capabilities that are
available. This is a fact-finding step to accumulate critical information
on the application and the restrictions that may be placed on the bond-
ing process. The question of joint design, service environment, or spe-
cific adhesive is not considered directly at this time. This first step in
the assembly process asks the following:

1. What materials are to be joined? Are there alternative materials
available?
2. What surface preparation is required? This may depend on the
choice of adhesive and the service environment that is expected.
3. What are the overriding cost factors that must be considered?
4. What methods of adhesive application are suitable and workable?
5. What methods are available to harden or cure the adhesive? Will
holding fixtures be available? Ovens? Other special equipment?