Page 378 - Handbook of Adhesives and Sealants

334 Chapter Nine

Figure 9.4 The coefficients of thermal expansion of filled epoxy resins compared with
those of common metals. 13

9.3.5 Reducing shrinkage
All resins will shrink on hardening or curing. This shrinking process
could cause internal stresses within the joint that degrade its
strength. Depending on the primary base resin, the adhesive formu-
lator may need to reduce the amount of shrinkage when the adhesive
hardens. This can be accomplished in several ways.
Fillers reduce the rate of shrinkage by bulk displacement of the
resin in the adhesive formulation. Fillers may improve operational
bond strength by 50 to 100%. The effect of various fillers and loading
ratios on the strength properties of epoxy adhesive formulations is
indicated in Fig. 9.5. The effect of different fillers loaded at a constant
100 phr is indicated in Table 9.4 for shear strength on phenolic lam-
inates and aluminum.
Another way of reducing shrinkage is by blending the base resin
with one that does not shrink as much on cure. In fact, there are
certain monomers that have been developed (although expensive and
not commercially available) that will expand on curing. 16 These mon-
omers when blended with more common resins can provide a resulting
adhesive system with either zero net shrinkage or actual volume ex-
pansion during cure. It is believed that when formulated into adhesive
systems, these expanding monomers will provide exceptional bond
strength and mechanical interlocking capability that previously was
not realized with conventional adhesive technology.