Page 349 - Handbook of Adhesives and Sealants

Adhesive Classifications 305

only a gap filler but also helps to distribute the stress within the joint
area, thus, improving the ultimate strength of the joint. These tapes
offer bond strengths up to 10 times those of conventional pressure
sensitive adhesive while maintaining enough cohesive strength to sup-
port modest loads for long periods of time. 14
Pressure sensitive adhesives provide a relatively low strength bond
upon the brief application of slight pressure (usually by hand). They
may be applied to any clean, dry surface. Since they are capable of
sustaining only light loads because of creep, they are not considered
structural adhesives. Surface cleanliness is very important in utilizing
pressure sensitive adhesives.
Pressure sensitive adhesives build a stronger bond over time. One
should not test pressure sensitive adhesive immediately after they are
applied and the joint is made. Full strength may take several days to
develop as shown in Fig. 8.7. Pressure sensitive tapes require rela-
tively close mating substrates. The pressure sensitive adhesive film

Figure 8.7 Bond strength development at room temperature of various
pressure sensitive adhesives. 12