Page 307 - Handbook of Adhesives and Sealants

Primers and Adhesion Promoters 271

TABLE 7.7 Effect of Substrate on Silane
Adhesion Effectiveness 27

Silica
Quartz
Glass
Aluminum
Copper
Alumina
Inorganics
Alumino-Silicates (Clays)
Silane Effectiveness None Slight Good Excellent Mica
Talc
Inorganic Oxides
Steel, Iron
Asbestos
Nickel
Zinc
Lead
Chalk (Calcium Carbonate)
Gypsum (Calcium Sulfate)
Barytes (Barium Sulfate)
Graphite
Carbon Black

that smooth, high energy substrates are excellent substrates for silane
attachment. Rough, discontinuous substrates show very little benefit.
Silane adhesion promoters increase the initial bond strength and
also stabilize the surface to increase the permanence of the joint in
moist aging environments. 21,24 The effect of a silane adhesion promoter
on the durability of mild steel joint bonded with an epoxy adhesive is
shown in Fig. 7.4. Silane based coupling agents are also capable of
increasing the environmental resistance of aluminum, 29 titanium, 30
and stainless steel 31 joints.
Although the best results can be obtained in using silanes as sub-
strate primers, they can also be added to the adhesive with some ef-
fect. The lap-shear values in Table 7.8 show the improvement in bond
strength when silane coupling agents are incorporated into the ad-
hesive formulation, specifically nitrile phenolics. The integral blend
method of applying the silane involves adding 0.1 to 2.0% by weight
of the silane to the polymer matrix prior to application. The advantage
of this method is that it does not require a separate substrate coating