Page 374 - Handbook of Adhesives and Sealants

330 Chapter Nine

properties in the cryogenic range ( 150 F) dictate specially selected
primary resins (see Chapter 17). Flexibilizers are also recommended
for bonding materials having differing rates of thermal expansion.
They provide a degree of elongation in the joint, which allows the
adhesive to move with the substrate. This is particularly important
when the rate of heating or cooling of the joint is fast.
For high temperature resistance, the selection of the base resin and
curing agent are crucial. The cured system must provide a high glass
transition temperature. Unfortunately, this usually means that the
formulation requires an elevated temperature cure. Also, peel
strength is generally low because the high temperature resins tend to
have little elongation. Resins and curing agents that provide high
crosslinking density and a high degree of molecular aromaticity are
usually chosen. Ideally, elevated temperature resistant adhesives
should be processed in such a way as to minimize oxygen entrapment
during application; they should be as dense as possible; and they
should contain antioxidants to improve thermal stability.
Sometimes it is not the high or low temperature itself that causes
degradation of the bond, but it is the excursion or process of getting
to these temperatures that causes the degradation. Because of differ-
ences in thermal expansion coefficient and thermal conductivity, the
adhesive does not expand or contract evenly with the substrate as the
environmental temperature changes. Thus, it is important that the
formulator either modify the thermal expansion coefficient of the ad-
hesive to better match the substrates or provide sufficient flexibility
within the adhesive to survive the temperature excursions and the
resulting stresses caused within the joint. For adjusting the thermal
expansion coefficient, fillers are normally used as discussed in a later
section. To provide greater flexibility, the adhesive formulator uses
flexibilizers or blends the base resin with an elastomeric resin.
It also should be noted that high temperature adhesives provide
somewhat lower room temperature strengths than adhesives formu-
lated specifically for lower temperature service. This is primarily due
to the greater toughness of adhesives that have been formulated to
cure at room temperature and their resulting higher resistance to de-
formation in a tensile shear joint (see Chapter 3).

9.3.3 Improving toughness
The ability of an adhesive to absorb energy without catastrophic fail-
ure can be increased through toughening. This results in enhanced
resistance to fracture, impact, and thermal stress with minimal
change in the gross properties of the matrix resin. Improved toughness
also results in higher peel strength. The adhesive formulator can im-
prove toughness of the adhesive formulation by various means.