Page 131 - Handbook of Adhesives and Sealants

100 Chapter Three

ommended for bonding plastic or elastomeric surfaces. The physical
nature of these substrates often results in cleavage or peeling forces
acting on the adhesive, and generally, there are significant differences
in thermal expansion coefficients. Brittle adhesives would provide
joints having high stress concentrations; whereas, flexible adhesives
would provide more uniform stress distribution.
When an application requires both high temperature resistance and
high peel strength the adhesive user faces a difficult compromise.
Usually, the more heat resistant polymers are also more densely cross-
linked and brittle. Flexibilized polymers have better resistance to peel,
impact, and fatigue forces but reduced heat resistance. As we will see
in later chapters, modern adhesive formulations, notably temperature
resistant thermoset base resins with a discrete elastomeric phase,
have come a long way in solving this dilemma.
In summary, usually a tough, flexible adhesive or sealant is pre-
ferred over a brittle stiff one as long as the adhesive has sufficient
cohesive strength for the application. With these adhesives, it is much
easier to have a joint with uniformly distributed stress. However,
there are disadvantages in using tough, flexible adhesives that must
be overcome. They usually have lower cohesive strength. Since the
forces that hold the internal molecules together are lower, the tem-
perature capability and environmental resistance also suffer. As a re-
sult, tough, flexible adhesives are commonly used on substrates such
as plastics and elastomers where the environmental service conditions
are usually not extreme and the physical properties of the adhesive
closely match those of the substrates. More brittle adhesives are more
densely cross-linked and usually employed in structural applications
that are likely to see elevated temperatures and aggressive environ-
mental conditioning.

3.3.2 Adhesive thickness
The most important aspects of adhesive thickness, or bond-line thick-
ness, are its magnitude and its uniformity or homogeneity. Generally,
one tries to have as thin an adhesive layer as possible without any
chance of bond starvation. In practice, this translates into bond-line
thicknesses from 0.002 in. to 0.008 in. Adhesive strength does not vary
significantly in this range. It is usually best to attempt to have a con-
stant bond-line thickness of about 0.005 in. With thicker adhesive
bond-lines, one runs the risk of incorporating higher void concentra-
tions into the joint. In addition, stresses at the corner of the
adhesive-adherend tend to be larger because it is difficult to keep the
loads axial with a very thick bond-line. It should also be remembered
that adhesives are generally formulated to cure in thin sections.