Page 106 - Handbook of Adhesives and Sealants

74 Chapter Two

Approximate
Cure
Temperature

Shear
Strength, psi

Test Temperature

Figure 2.14 Plot of the strength of an aluminum joint (bonded with an
elevated temperature curing epoxy adhesive) as a function of test tem-
perature.

of the adhesive, and highly deformable adhesives usually have low
cohesive strength. Another approach is to adjust the expansion coef-
ficient of the adhesive to a value nearer to that of the substrate. This
is generally accomplished by selection of a different adhesive or by
formulating the adhesive with specific fillers to ‘‘tailor’’ the thermal
expansion coefficient. A third possible solution is to coat one or both
substrates with a primer or coupling agent. This substance can pro-
vide either resiliency or an intermediate thermal expansion coefficient
that will help reduce the overall stress in the joint.

2.4.2.3 Setting stresses due to shrinkage of the adhesive or sealant.
Nearly all polymeric materials (including adhesives and sealants)
shrink during solidification. Sometimes they shrink because of escap-
ing solvent, leaving less mass in the bond line. Even 100% reactive
adhesives, such as epoxies and urethanes, experience some shrinkage
because their solid polymerized mass occupies less volume than the
liquid reactants. Table 2.3 shows typical volume shrinkage for various
reactive adhesive systems during cure.
The result of such shrinkage is internal stresses and the possible
formation of cracks and voids within the bond-line itself. Elastic ad-
hesives deform when exposed to such internal stress and are less af-
fected by shrinkage. Formulators are often able to adjust the final
hardness of the adhesive or sealant to minimize stress during shrink-
age.