Page 372 - Handbook of Adhesives and Sealants

328 Chapter Nine

TABLE 9.3 Thixotropic Fillers (Fumed Silica and Reinforcing Thixotropic Fibers) in
Epoxy Adhesive Formulations 9
Control Experimental
Formulation
Epoxy resin 1 47 40
Epoxy resin 2 22 18.7
Dicyandiamide 3.7 3.1
Cure accelerator 1.3 1.1
Ground limestone 20 28.4
Fumed silica, 6 3
surface treated
Cellulose — 5.6
reinforcing fibers
Appearance–Relative
Surface 8.5 8
Gloss 9 9
Uniformity 9 8
Viscosity–Relative
Initial 86 91
Aged 105 120
Slump resistance No movement No movement
1
( ⁄2 ⁄2 4 in.)
1
Shear overlap
strength 1770 (100% cohesive failure) 1801 (100% cohesive failure)
Initial 1579 (62% cohesive failure) 1727 (95% cohesive failure)
25 h humidity 1288 (62% cohesive failure) 1592 (97% cohesive failure)
250 h salt spray

applied to cause the adhesive to flow so that the ‘‘shims’’ meet the
substrate surface to provide a positive stop. Paper, mat, and other
carrier materials may also be used for this purpose. Glass or polymeric
microballoons, incorporated directly into the adhesive formulation,
can also provide the shimming function. Here the diameter of the mi-
croballoons are used to determine the thickness of the bondline. An-
other option is to design mechanical shims into the joint itself. The
parts to be assembled are designed with lips or stops so that the ad-
hesive cannot flow out of the joint area or that a certain predetermined
adhesive thickness is always maintained.
In certain cases it is necessary for the adhesive formulator to reduce
the viscosity of the adhesive system to achieve better wetting char-
acteristics. Wetting, as measured by the contact angle that the adhe-
sive makes on the surface, is not governed by the viscosity of the ad-
hesive. However, the rate and ease which the adhesive wets the
surface of the substrate and fills in the peaks and valleys on the sur-
face is a function of viscosity. Highly viscous adhesives could require
an impractical amount of time to adequately wet the surface of a sub-
strate.