Page 506 - Handbook of Adhesives and Sealants

438 Chapter Eleven

still affected by them. Where minimum stress between adherends of
the same materials is the objective, it is desirable to choose an adhe-
sive that is similar with respect to rheological properties, thermal
expansion, and chemical resistance. This assumes that all other
adhesive requirements are met. Frequently, these other adhesive prop-
erties are more critical for the joint than ultimate bond strength.
Generally, rigid adhesives are selected for applications requiring
high shear strength where the joint is designed so that the load is
usually in shear. These resins are generally highly crosslinked ther-
mosets such as epoxies, acrylics, phenolics and other such polymers.
Often an elevated temperature cure is required. These adhesives have
very high lap shear strengths but at the expense of low peel and im-
pact strength.
Peel and impact strengths of adhesives are related in that adhesives
with good peel properties generally will have good impact strength.
Thermosetting adhesives tend to form rigid, relatively nonyielding
bonds and, therefore, peel strength and impact strength will be lower
than for tough rubber/elastomer based and thermoplastic adhesives.
Of the epoxies, the polysulfide-epoxies, epoxy-nylons, and polyamide-
epoxies have excellent peel properties with strengths ranging from 20
to 100 pwi for solvent wiped aluminum bonded to itself and nearly
150 pwi for a chromic acid etched aluminum bonded to itself. Ure-
thanes also have very high peel strength and flexibility.
Soft mastic adhesives should be selected for applications where
large areas are likely to move, warp, or twist. Mastics can include
epoxies, urethanes, silicones, and rubber based materials that, de-
pending on hardener and type, can cure to a flexible joint. The soft
adhesive flows to relieve the stresses created when the substrates
move.

11.5.2 Relationship between adhesive
selection, bondline thickness, and viscosity
Thickness of the adhesive in the joint (i.e., bond-line thickness) can
be a significant parameter in applications requiring a high strength
requirement. The highest tensile and shear strength are obtained with
high modulus adhesives when the film thickness is a minimum. As
was indicated in Chapter 3, optimum strength is usually achieved
2
when the bondline thickness is between 0.002 in. and 0.006 in. For
bond-line thicknesses above 0.006 in., the stress distribution is such
that the adhesive may experience cleavage type forces. Below 0.001
in, bond strengths usually decrease, depending on the smoothness and
gap tolerance of the substrates. Starvation of the adhesive in the joint
is always a potential hazard when one tries for a very thin bond thick-