Page 69 - Handbook of Adhesives and Sealants

An Introduction to Adhesives and Sealants 37

4. Forming a ‘‘joint’’ structure (adhesive or sealant material, inter-
phase regions, and adherends) that is resistant to the operating
stress and environment
5. Design of the joint
6. Selection and control of materials and manufacturing processes.

1.6.1.1 Surface condition. Above all else, one must start with a clean
surface. Foreign materials such as dirt, oil, moisture, and weak oxide
layers must be removed from the substrate surface, or else the adhe-
sive or sealant will bond to these weak boundary layers rather than
to the substrate in question.
Various surface preparations remove or strengthen the weak bound-
ary layer. These treatments generally involve physical or chemical pro-
cesses or a combination of both. The choice of surface preparation pro-
cess will depend on the adhesive or sealant, the substrate, the nature
of the substrate before bonding, the required bond strength and du-
rability, and the production processes, time, and budget available to
the user. Surface preparation methods for specific substrates will be
discussed in a later chapter.

1.6.1.2 Wetting the substrate. Initially, the adhesive or sealant must
be either a liquid or a readily deformed solid so that it can be easily
applied and formed to the required geometry within the assembled
joint. It is necessary for the adhesive or sealant to flow and then con-
form to the surfaces of the adherends on both micro- and macro-
scales. Small air pockets caused by the roughness of the substrate at
the interface must be easily displaced with adhesive or sealant. While
it is in the liquid state, the material must ‘‘wet’’ the substrate surface.
The term wetting refers to a liquid spreading over and intimately con-
tacting a solid surface as shown in Fig. 1.10. The causes of good and
poor wetting will be explained in the following chapter. One result of
good wetting is greater contact area between adherend and adhesive
over which the forces of adhesion can act.

1.6.1.3 Solidification of the adhesive or sealant. The liquid adhesive or
sealant, once applied, must be converted into a solid. Solidification
occurs in one of three ways: chemical reaction by any combination of
heat, pressure, curing agent, or other activator such as UV light, ra-
diation, etc.; cooling from a molten liquid to a solid; and drying due
to solvent evaporation. The method by which solidification occurs de-
pends on the choice of adhesive or sealant material.
When organic resins solidify, they undergo volumetric shrinkage due
to the crosslinking reaction, loss of solvent, or thermal expansion co-
efficient (contraction on cooling from an elevated temperature cure).