Page 429 - Handbook of Adhesives and Sealants

Adhesives Families 371

caulking compounds, encapsulants, potting compounds, conformal
coatings and molding compounds for rubbery parts. 16
Several other approaches have been developed for improving the
toughness and flexibility of epoxy adhesives. Modification of epoxy
resin with alkyl resin is reported as a means of flexibilizing epoxies
for adhesive applications. 17 Modification of epoxy resins through the
direct addition of engineering thermoplastic, such as polysulfone, has
also been proposed to enhance the toughness of epoxy adhesives. 18 It
is likely that the search for toughened epoxy adhesives will continue
in the future.

10.3.2.2 Epoxy-phenolic. Epoxy-phenolic adhesives are made by
blending epoxy resins with phenolic resins to improve the high tem-
perature capabilities of the standard epoxy resins. Adhesives based on
epoxy-phenolic blends are good for continuous high-temperature ser-
vice in the 350 F range or intermittent service as high as 500 F. They
retain their properties over a very high temperature range, as shown
in Fig. 10.4. Shear strengths of up to 3,000 psi at room temperature
and 1,000 to 2,000 psi at 500 F are available. Resistance to weather-
ing, oil, solvents, and moisture is very good. Because of the rigid na-
ture of the constituent materials, epoxy-phenolic adhesives have rel-
atively low peel and impact strength and limited thermal-shock
resistance.
These adhesives are available as pastes, solvent solutions, and film
supported on glass fabric. The films generally give better strengths
than do liquid systems. Cure requires a temperature of 350 F for 1 hr
under 15–50 psi pressure. Most adhesives of this type have limited
storage life at room temperature.

Figure 10.4 The effect of temperature on the tensile
shear strength of modified epoxy adhesives (sub-
strate is aluminum). 3