Page 373 - Handbook of Adhesives and Sealants

Adhesive Composition and Formulation 329

Wetting speed can be improved by incorporating reactive or non-
reactive diluents into the system. Diluents are liquid resins that usu-
ally lower the viscosity of the adhesive system to make a thin uniform
coating possible. The diluent must be compatible with the base resin
and not degrade the inherent wetting characteristics of the adhesive.
Diluents reduce viscosity and reduce the time it takes for the adhe-
sives to effectively wet the substrate. The reduced viscosity also aids
the removal of entrapped air and aids the capillary action of the ad-
hesive in filling pores and cavities that may be on the substrate sur-
face. However, the addition of diluents, especially unreacted liquid res-
ins, usually results in lowered crosslink density which in turn results
in reduced elevated temperature strength and environmental resis-
tance.
Solvents can also be used to lower the viscosity of the adhesive to
achieve faster wetting of the substrate. However, solvents must be
completely evaporated before the adhesive hardens, or else voids could
develop in the joint area due to the solvent trying to escape at a later
time.

9.3.2 Extending temperature range
Often the adhesive formulator must provide a formulation that must
resist either high or low temperatures during service (and sometimes
both high and low temperatures). In these cases, the trade-offs that
must be faced are very formidable. It is difficult to achieve high tem-
perature resistance without a system that cures at elevated temper-
atures. So certain processing advantages provided by room tempera-
ture curing adhesives must be sacrificed if a high degree of
temperature resistance is needed. High temperature resistant adhe-
sives are generally brittle and lack high peel strength. The develop-
ment of an adhesive formulation that has high temperature resistance
and high peel strength has long been a ‘‘holy grail’’ to adhesive for-
mulators. With the arrival of certain elastomer modified hybrid sys-
tems, this goal has been achieved for the most part, as will be de-
scribed in the next section.
Sometimes it is necessary to provide a formulation that endures
both high and low temperatures. The thermal tiles on the NASA space
shuttle are an example. In this application, silicone elastomer based
adhesives are used. Shear strength is not as good as high temperature
epoxies, for example, but the application does not require excessively
high shear strength and a willing trade-off was made to get higher
peel strength and resistance to both high and low temperatures.
Flexibilizers can be used to lower the glass transition temperature
and improve the low temperature bond strengths of many adhesives.
Good properties can be obtained down to 40 F; however, adhesive